publicationDate,title,abstract,id
2022-09-09,Magnetization dynamics and reversal of two-dimensional magnets,"Micromagnetics simulation based on the classical Landau-Lifshitz-Gilbert
(LLG) equation has long been a powerful method for modeling magnetization
dynamics and reversal of three-dimensional (3D) magnets. For two-dimensional
(2D) magnets, the magnetization reversal always accompanies the collapse of the
magnetization even at low temperatures due to intrinsic strong spin
fluctuation. We propose a micromagnetic theory that explicitly takes into
account the rapid demagnetization and remagnetization dynamics of 2D magnets
during magnetization reversal. We apply the theory to a single-domain magnet to
illustrate fundamental differences in magnetization trajectories and reversal
times for 2D and 3D magnets.",2209.04483v1
2016-08-16,Power Spectral Density of Magnetization Dynamics Driven by a Jump-Noise Process,"Random magnetization dynamics driven by a jump-noise process is reduced to
stochastic magnetic energy dynamics on specific graphs using an averaging
technique. An approach to analyzing stochastic energy dynamics on graphs is
presented and applied to the calculation of power spectral density of random
magnetization dynamics. An eigenvalue technique for computing the power
spectral density under specific cases is also presented and illustrated by
numerical results.",1608.04733v1
2024-01-19,Modelling and dynamics of pendulum systems subjected to a nonstationary magnetic field,"The prepared doctoral dissertation focuses on studying dynamics of systems
composed of magnetic pendulums subjected to a non-stationary magnetic field. A
magnetic pendulum is a physical pendulum with a magnet attached to its end and
is placed in an external magnetic field. The non-stationary external magnetic
field is generated by an electric coil placed under the pendulum and powered by
a time-varying current signal. The presented research mainly concerns the
oscillations in one potential well of a single magnetic pendulum, as well as
the control of the energy flow between two torsionally coupled magnetic
pendulums.",2401.10594v1
2024-03-20,Molecular Dynamics Simulations of Microscopic Structural Transition and Macroscopic Mechanical Properties of Magnetic Gels,"Magnetic gels with embedded micro/nano-sized magnetic particles in
crosslinked polymer networks can be actuated by external magnetic fields, with
changes in their internal microscopic structures and macroscopic mechanical
properties. We investigate the responses of such magnetic gels to an external
magnetic field, by means of coarse-grained molecular dynamics simulations. We
find that the dynamics of magnetic particles are determined by the interplay of
between magnetic dipole-dipole interactions, polymer elasticity and thermal
fluctuations. The corresponding microscopic structures formed by the magnetic
particles such as elongated chains can be controlled by the external magnetic
field. Furthermore, the magnetic gels can exhibit reinforced macroscopic
mechanical properties, where the elastic modulus increases algebraically with
the magnetic moments of the particles in the form of
$\propto(m-m_{\mathrm{c}})^{2}$ when magnetic chains are formed. This
simulation work can not only serve as a tool for studying the microscopic and
the macroscopic responses of the magnetic gels, but also facilitate future
fabrications and practical controls of magnetic composites with desired
physical properties.",2403.13481v1
2020-09-24,Magnetic winding: what is it and what is it good for?,"Magnetic winding is a fundamental topological quantity that underpins
magnetic helicity and measures the entanglement of magnetic field lines. Like
magnetic helicity, magnetic winding is also an invariant of ideal
magnetohydrodynamics. In this article we give a detailed description of what
magnetic winding describes, how to calculate it and how to interpret it in
relation to helicity. We show how magnetic winding provides a clear topological
description of magnetic fields (open or closed) and we give examples to show
how magnetic winding and helicity can behave differently, thus revealing
different and imporant information about the underlying magnetic field.",2009.11708v1
2014-03-13,Monte Carlo simulated dynamical magnetization of single-chain magnets,"Here, a dynamical Monte-Carlo (DMC) method is used to study
temperature-dependent dynamical magnetization of famous Mn2Ni system as typical
example of single-chain magnets with strong magnetic anisotropy. Simulated
magnetization curves are in good agreement with experimental results under
typical temperatures and sweeping rates, and simulated coercive fields as
functions of temperature are also consistent with experimental curves. Further
analysis indicates that the magnetization reversal is determined by both
thermal-activated effects and quantum spin tunnelings. These can help explore
basic properties and applications of such important magnetic systems.",1403.3188v2
2023-09-22,Electric control of optically-induced magnetization dynamics in a van der Waals ferromagnetic semiconductor,"Electric control of magnetization dynamics in two-dimensional (2D) magnetic
materials is an essential step for the development of novel spintronic
nanodevices. Electrostatic gating has been shown to greatly affect the static
magnetic properties of some van der Waals magnets, but the control over their
magnetization dynamics is still largely unexplored. Here we show that the
optically-induced magnetization dynamics in the van der Waals ferromagnet
Cr$_2$Ge$_2$Te$_6$ can be effectively controlled by electrostatic gates, with a
one order of magnitude change in the precession amplitude and over 10% change
in the internal effective field. In contrast to the purely thermally-induced
mechanisms previously reported for 2D magnets, we find that coherent
opto-magnetic phenomena play a major role in the excitation of magnetization
dynamics in Cr$_2$Ge$_2$Te$_6$. Our work sets the first steps towards electric
control over the magnetization dynamics in 2D ferromagnetic semiconductors,
demonstrating their potential for applications in ultrafast opto-magnonic
devices.",2309.12776v1
2010-06-21,Dynamics of magnetic charges in artificial spin ice,"Artificial spin ice has been recently implemented in two-dimensional arrays
of mesoscopic magnetic wires. We propose a theoretical model of magnetization
dynamics in artificial spin ice under the action of an applied magnetic field.
Magnetization reversal is mediated by domain walls carrying two units of
magnetic charge. They are emitted by lattice junctions when the the local field
exceeds a critical value $H_c$ required to pull apart magnetic charges of
opposite sign. Positive feedback from Coulomb interactions between magnetic
charges induces avalanches in magnetization reversal.",1006.4075v2
2024-02-05,Bifurcation to complex dynamics in largely modulated voltage-controlled parametric oscillator,"An experimental demonstration of a parametric oscillation of a magnetization
in a ferromagnet was performed recently by applying a microwave voltage,
indicating the potential to be applied in a switching method in non-volatile
memories. In the previous works, the modulation of a perpendicular magnetic
anisotropy field produced by the microwave voltage was small compared with an
external magnetic field pointing in an in-plane direction. A recent trend is,
however, opposite, where an efficiency of the voltage controlled magnetic
anisotropy (VCMA) effect is increased significantly by material research and
thus, the modulated magnetic anisotropy field can be larger than the external
magnetic field. Here, we solved the Landau-Lifshitz-Gilbert equation
numerically and investigated the magnetization dynamics driven under a wide
range of the microwave VCMA effect. We evaluated bifurcation diagrams, which
summarize local maxima of the magnetization dynamics. For low modulation
amplitudes, the local maximum is a single point because the dynamics is the
periodic parametric oscillation. The bifurcation diagrams show distributions of
the local maxima when the microwave magnetic anisotropy field becomes larger
than the external magnetic field. The appearance of this broadened distribution
indicates complex dynamics such as chaotic and transient-chaotic behaviors,
which were confirmed from an analysis of temporal dynamics.",2402.02742v1
2014-12-17,Revealing the role of orbital magnetism in ultrafast laser-induced demagnetization in multisublattice metallic magnets by terahertz spectroscopy,"Simultaneous detection of THz emission and transient magneto-optical response
is employed to study ultrafast laser-induced magnetization dynamics in
multisublattice magnets NdFeCo and GdFeCo amorphous alloys with in-plane
magnetic anisotropy. A satisfactory quantitative agreement between the dynamics
revealed with the help of these two techniques is obtained for GdFeCo. For
NdFeCo the THz emission reveals faster dynamics than the magneto-optical
response. This indicates that in addition to spin dynamics of Fe ultrafast
laser excitation triggers faster magnetization dynamics of Nd.",1412.5396v1
2006-05-18,Small-scale magnetic buoyancy and magnetic pumping effects in a turbulent convection,"We determine the nonlinear drift velocities of the mean magnetic field and
nonlinear turbulent magnetic diffusion in a turbulent convection. We show that
the nonlinear drift velocities are caused by the three kinds of the
inhomogeneities, i.e., inhomogeneous turbulence; the nonuniform fluid density
and the nonuniform turbulent heat flux. The inhomogeneous turbulence results in
the well-known turbulent diamagnetic and paramagnetic velocities. The nonlinear
drift velocities of the mean magnetic field cause the small-scale magnetic
buoyancy and magnetic pumping effects in the turbulent convection. These
phenomena are different from the large-scale magnetic buoyancy and magnetic
pumping effects which are due to the effect of the mean magnetic field on the
large-scale density stratified fluid flow. The small-scale magnetic buoyancy
and magnetic pumping can be stronger than these large-scale effects when the
mean magnetic field is smaller than the equipartition field. We discuss the
small-scale magnetic buoyancy and magnetic pumping effects in the context of
the solar and stellar turbulent convection. We demonstrate also that the
nonlinear turbulent magnetic diffusion in the turbulent convection is
anisotropic even for a weak mean magnetic field. In particular, it is enhanced
in the radial direction. The magnetic fluctuations due to the small-scale
dynamo increase the turbulent magnetic diffusion of the toroidal component of
the mean magnetic field, while they do not affect the turbulent magnetic
diffusion of the poloidal field.",0605444v1
2015-02-27,RC-circuit-like dynamic characteristic of the magnetic domain wall in ferromagnetic nanowires,"We have investigated dynamic behaviors of the magnetic domain wall under
perpendicular magnetic field pulses in ferromagnetic nanowires using
micromagnetic simulations. It has been found that the perpendicular magnetic
field pulse can trigger the magnetic domain wall motion, where all the field
torques are kept to be on the plane of nanowire strip. The magnetic domain wall
speed faster than several hundreds meters per second is predicted without the
Walker breakdown for the perpendicular magnetic driving field stronger than
$200~\mathrm{mT}$. Interestingly, the dynamic behavior of the moving magnetic
domain wall driven by perpendicular magnetic field pulses is explained by
charging- and discharging-like behaviors of an electrical RC-circuit model,
where the charging and the discharging of ""magnetic charges"" on the nanowire
planes are considered. The concept of the RC-model-like dynamic characteristic
of the magnetic domain wall might be promising for spintronic functional device
applications based on the magnetic domain wall motion.",1502.07807v1
2022-04-26,Slow spin dynamics and quantum tunneling of magnetization in the dipolar antiferromagnet DyScO$_3$,"We present a comprehensive study of static and dynamic magnetic properties in
the Ising-like dipolar antiferromagnet (AFM) DyScO$_3$\ by means of DC and AC
magnetization measurements supported by classical Monte-Carlo calculations. Our
AC-susceptibility data show that the magnetic dynamics exhibit a clear
crossover from an Arrhenius-like regime to quantum tunneling of magnetization
(QTM) at $T^* = 10$ K. Below $T_{\mathrm{N}} = 3.2$ K DyScO$_3$ orders in an
antiferromagnetic $GxAy$-type magnetic structure and the magnetization dynamics
slow down to the minute timescale. The low-temperature magnetization curves
exhibit complex hysteretic behavior, which depends strongly on the magnetic
field sweep rate. We demonstrate that the low-field anomalies on the
magnetization curve are related to the metamagnetic transition, while the
hysteresis at higher fields is induced by a strong magnetocaloric effect. Our
theoretical calculations, which take into account dipolar interaction between
Dy$^{3+}$ moments, reproduce essential features of the magnetic behavior of
DyScO$_3$. We demonstrate that DyScO$_3$ represents a rare example of inorganic
compound, which exhibits QTM at a single-ion level and magnetic order due to
classical dipolar interaction.",2204.12398v1
2013-02-08,Investigation on the critical dynamics of real magnetics models by computational physics methods,"The critical dynamics of classical 3D Heisenberg model and complex model of
the real antiferromagnetic Cr2O3 is investigated with use of the method of
molecular dynamics. The dynamic critical exponent z are determined for these
models on the basis of the theory dynamic finite-size scaling.",1302.1964v1
2022-06-12,Exchange-enhancement of the ultrafast magnetic order dynamics in antiferromagnets,"We theoretically demonstrate that the ultrafast magnetic order dynamics in
antiferromagnets is exchange-enhanced in comparison to their ferromagnetic
counterparts. We provide an equation of motion for the magnetic order dynamics
validated by computer simulations using atomistic spin dynamics methods. The
exchange of angular momentum between sublattices speeds up the dynamics in
antiferromagnets, a process absent in ferromagnets.",2206.05783v1
2018-07-24,Asymptotic Dynamics of Magnetic Micro-Swimmers,"Micro-swimmers put into motion by a rotating magnetic field have provided
interesting challenges both in engineering and in mathematical modelling. We
study here the dynamics of a permanent-magnetic rigid body submitted to a
spatially-uniform steadily-rotating magnetic field in Stokes flow. This system
depends on two external parameters: the Ma- son number, which is proportional
to the angular speed of the magnetic field and inversely proportional to the
magnitude of the field, and the conical angle between the magnetic field and
its axis of rotation. This work focuses on asymptotic dynamics in the limits of
low and high Mason number, and in the limit of low conical angle. Analytical
solutions are provided in these three regimes. In the limit of low Mason
number, the dynamical system admits a periodic solution in which the magnetic
moment of the swimmer tends to align with the magnetic field. In the limit of
large Mason number, the magnetic moment tends to align with the average
magnetic field, which is parallel to the axis of rotation. Asymptotic dynamics
in the limit of low conical angle allow to bridge these two regimes. Finally,
we use numerical methods to compare these analytical predictions with numerical
solutions.",1807.09059v1
2016-01-23,Nonlinear magnetization dynamics of antiferromagnetic spin resonance induced by intense terahertz magnetic field,"We report on the nonlinear magnetization dynamics of a HoFeO3 crystal induced
by a strong terahertz magnetic field resonantly enhanced with a split ring
resonator and measured with magneto-optical Kerr effect microscopy. The
terahertz magnetic field induces a large change (~40%) in the spontaneous
magnetization. The frequency of the antiferromagnetic resonance decreases in
proportion to the square of the magnetization change. A modified
Landau-Lifshitz-Gilbert equation with a phenomenological nonlinear damping term
quantitatively reproduced the nonlinear dynamics.",1601.06213v1
2023-07-04,Phonon-induced magnetization dynamics in Co-doped iron garnets,"The developing field of strain-induced magnetization dynamics offers a
promising path toward efficiently controlling spins and phase transitions.
Understanding the underlying mechanisms is crucial in finding the optimal
parameters supporting the phononic switching of magnetization. Here, we present
an experimental and numerical study of time-resolved magnetization dynamics
driven by the resonant excitation of an optical phonon mode in iron garnets.
Upon pumping the latter with an infrared pulse obtained from a free-electron
laser, we observe spatially-varying magnetization precession, with its phase
depending on the direction of an external magnetic field. Our micromagnetic
simulations effectively describe the magnetization precession and switching in
terms of laser-induced changes in the crystal's magneto-elastic energy.",2307.01907v1
2013-02-14,Atomistic Coupling between Magnetization and Lattice Dynamics from First Principles,"We formulate a new scheme to study the combined magnetization and lattice
dynamics in magnets, so-called magnetoelastics. The coupling between
magnetization and lattice are considered through an expansion of
electron-phonon coupling, while the magnetization is coupled to electrons
through sd-like interaction. We show the that the time-scale of the
magnetization dynamics due to coupling between magnetic degrees of freedom and
electronic degrees of freedom can be transferred to lattice degrees of freedom
and therefore can give rise to lattice dynamics on the same time-scale. This
opens a new route to understand and treat ultrafast lattice dynamics induced by
magnetization dynamics. We also show that all the parameters necessary to
simulate this coupled lattice and magnetization dynamics can be obtained from
first-principles.",1302.3333v2
2015-07-22,Spin-current emission governed by nonlinear spin dynamics,"Coupling between conduction electrons and localized magnetization is
responsible for a variety of phenomena in spintronic devices. This coupling
enables to generate spin currents from dynamical magnetization. Due to the
nonlinearity of magnetization dynamics, the spin-current emission through the
dynamical spin-exchange coupling offers a route for nonlinear generation of
spin currents. Here, we demonstrate spin-current emission governed by nonlinear
magnetization dynamics in a metal/magnetic insulator bilayer. The spin-current
emission from the magnetic insulator is probed by the inverse spin Hall effect,
which demonstrates nontrivial temperature and excitation power dependences of
the voltage generation. The experimental results reveal that nonlinear
magnetization dynamics and enhanced spin-current emission due to magnon
scatterings are triggered by decreasing temperature. This result illustrates
the crucial role of the nonlinear magnon interactions in the spin-current
emission driven by dynamical magnetization, or nonequilibrium magnons, from
magnetic insulators.",1507.06081v1
2023-08-25,Coexistence of non-trivial van der Waals magnetic orders enable field-free spin-orbit torque switching at room temperature,"The discovery of van der Waals (vdW) materials exhibiting non-trivial and
tunable magnetic interactions at room temperature can give rise to exotic
magnetic states, which are not readily attainable with conventional materials.
Such vdW magnets can provide a unique platform for studying new magnetic
phenomena and realising magnetization dynamics for energy-efficient and
non-volatile spintronic memory and logic technologies. Recent developments in
vdW magnets have revealed their potential to enable spin-orbit torque (SOT)
induced magnetization dynamics. However, the deterministic and field-free SOT
switching of vdW magnets at room temperature has been lacking, prohibiting
their potential applications. Here, we demonstrate magnetic field-free and
deterministic SOT switching of a vdW magnet (Co0.5Fe0.5)5GeTe2 (CFGT) at room
temperature, capitalizing on its non-trivial intrinsic magnetic ordering. We
discover a coexistence of ferromagnetic and antiferromagnetic orders in CFGT at
room temperature, inducing an intrinsic exchange bias and canted perpendicular
magnetism. The resulting canted perpendicular magnetization of CFGT introduces
symmetry breaking, facilitating successful magnetic field-free magnetization
switching in the CFGT/Pt heterostructure devices. Furthermore, the SOT-induced
magnetization dynamics and their efficiency are evaluated using 2nd harmonic
Hall measurements. This advancement opens new avenues for investigating tunable
magnetic phenomena in vdW material heterostructures and realizing field-free
SOT-based spintronic technologies.",2308.13408v1
2009-07-23,Dynamic switching of magnetization in driven magnetic molecules,"We study the magnetization dynamics of a molecular magnet driven by static
and variable magnetic fields within a semiclassical treatment. The underling
analyzes is valid in a regime, when the energy is definitely lower than the
anisotropy barrier, but still a substantial number of states are excited. We
find the phase space to contain a separatrix line. Solutions far from it are
oscillatory whereas the separatrix solution is of a soliton type. States near
the separatrix are extremely sensitive to small perturbations, a fact which we
utilize for dynamically induced magnetization switching.",0907.4105v1
2004-09-25,Nano-mechanical magnetization reversal,"The dynamics of the ferromagnetic order parameter in thin magnetic films is
strongly affected by the magnetomechanical coupling at certain resonance
frequencies. By solving the equation of motion of the coupled mechanical and
magnetic degrees of freedom we show that the magnetic-field induced
magnetization switching can be strongly accelerated by the lattice and
illustrate the possibility of magnetization reversal by mechanical actuation.",0409681v1
2008-05-18,Effect of spin diffusion on spin torque in magnetic nanopillars,"We present systematic magnetoelectronic measurements of magnetic nanopillars
with different structures of polarizing magnetic layers. The magnetic reversal
at small magnetic field, the onset of magnetic dynamics at larger field, and
the magnetoresistance exhibit a significant dependence on the type of the
polarizing layer. We performed detailed quantitative modeling showing that the
differences can be explained by the effects of spin-dependent electron
diffusion.",0805.2706v1
2016-08-19,Vesicles in magnetic fields,"Liposome vesicles tend to align with an applied magnetic field. This is due
to the directional magnetic susceptibility difference of the lipids which form
the membrane of these vesicles. In this work a model of liposome vesicles
exposed to magnetic field is presented. Starting from the base energy of a
lipid membrane in a magnetic field, the force applied to the surrounding fluids
is derived. This force is then used to investigate the dynamics of vesicle in
the presence of magnetic fields.",1608.05587v1
2004-12-09,"On ""Pulsar dynamics: magnetic dipole model revisited""","Comments on the article ""Pulsar dynamics: magnetic dipole model revisited"".",0412218v1
1992-10-04,On the quantisation of SU(2) magnetic monopole dynamics,"We argue that there is no consistent quantisation of the two BPS SU(2)
magnetic monopole dynamical system compatible with the correspondence
principle.",9210019v1
2019-12-29,Synchronized excitation of magnetization dynamics via spin waves in Bi-YIG thin film by slot line waveguide,"We have studied magnetization dynamics of a single Bi-YIG thin film by means
of the high frequency power response induced by a slot line waveguide. Multiple
absorption peaks that correspond to excitement states in magnetization dynamics
appeared without the ferromagnetic resonance (FMR) condition. The peaks were
strongly influenced by a waveguide line width and a distance between the lines.
Micromagnetics simulation reveals that each line induces a local magnetization
dynamics oscillation and generates spin waves. The spin wave that propagates
from one of the lines interferences with the other side of local magnetization
dynamics oscillation around the other line, resulting in an amplification of
the oscillation when they are in synchronization with each other. This
amplification occurs at both sides of the lines by the interference. Thus, the
possible mechanism of the excitation in the magnetization dynamics oscillation
is the synchronization of mutual magnetization dynamics oscillation via spin
waves. This technique resonantly excites the local magnetization dynamics
without the FMR condition, which is applicable as a highly coherent spin waves
source.",1912.12663v1
2017-01-16,Dynamics of magnetic nano particles in a viscous fluid driven by rotating magnetic fields,"The rotational dynamics of magnetic nano particles in rotating magnetic
fields in the presence of thermal noise is studied both theoretically and by
performing numerical calculations. Kinetic equations for the dynamics of
particles with uniaxial magnetic anisotropy are studied and the phase lag
between the rotating magnetic moment and the driving field is obtained. It is
shown that for large enough anisotropy energy the magnetic moment is locked to
the anisotropy axis so that the particle behaves like a rotating magnetic
dipole. The corresponding rigid dipole model is analyzed both numerically by
solving the appropriate Fokker-Planck equation and analytically by applying an
effective field method. In the special case of a rotating magnetic field
applied analytic results are obtained in perfect agreement with numerical
results based on the Fokker-Planck equation. The analytic formulas derived are
not restricted to small magnetic fields or low frequencies and are therefore
important for applications. The illustrative numerical calculations presented
are performed for magnetic parameters typical for iron oxide.",1702.04673v1
2019-01-10,Ultrafast magnetization dynamics in uniaxial ferrimagnets with compensation point. GdFeCo,"We derive an effective Lagrangian in the quasi-antiferromagnetic
approximation that allows to describe the magnetization dynamics for uniaxial
f-d (rare-earth - transition metal) ferrimagnet near the magnetization
compensation point in the presence of external magnetic field. We perform
calculations for the parameters of GdFeCo, a metallic ferrimagnet with
compensation point that is one of the most promising materials in ultrafast
magnetism. Using the developed approach, we find the torque that acts on the
magnetization due to ultrafast demagnetization pulse that can be caused either
by ultrashort laser or electrical current pulse. We show that the torque is
non-zero only in the non-collinear magnetic phase that can be acquired by
applying external magnetic field to the material. The coherent response of
magnetization dynamics amplitude and its timescale exhibits critical behavior
near certain values of the magnetic field corresponding to a spin-flop like
phase transition. Understanding the underlying mechanisms for these effects
opens the way to efficient control of the amplitude and the timescales of the
spin dynamics, which is one of the central problems in the field of ultrafast
magnetism.",1901.03072v2
1997-08-06,Symmetry and Dynamics of A Magnetic Oscillator,"We consider a permanent magnetic dipole in an oscillating magnetic field.
This magnetic oscillator has two dynamical symmetries. With increasing the
amplitude $A$ of the magnetic field, dynamical behaviors associated with the
symmetries are investigated. For small $A$, there exist symmetric states with
respect to one of the two symmetries. However, such symmetric states lose their
symmetries via symmetry-breaking pitchfork bifurcations and then the
symmetry-broken states exhibit period-doubling transitions to chaos.
Consequently, small chaotic attractors with broken symmetries appear. However,
as $A$ is further increased they merge into a large symmetric chaotic attractor
via symmetry-restoring attractor-merging crisis.",9708005v1
2020-01-08,Non-equilibrium spin dynamics in the temperature and magnetic field dependence of magnetization curves of ferrimagnetic Co$_{1.75}$Fe$_{1.25}$O$_4$ and its composite with BaTiO$_3$,"A comparative study of the non-equilibrium magnetic phenomena (magnetic
blocking, memory, exchange bias and aging effect) has been presented for
ferrimagnetic Co$_{1.75}$Fe$_{1.25}$O$_4$ (CFO) and its composite with
non-magnetic BaTiO$_3$ (BTO). Synchrotron X-Ray diffraction patterns have
confirmed coexistence of CFO and BTO structures in composite, but magnetic spin
dynamics have been remarkably modified. The blocking phenomenon of
ferrimagnetic domains below the room temperature has been studied by different
modes of (zero field cooled and field cooled) magnetic measurements in
collaboration with magnetic fields ON and OFF modes and time dependent
magnetization. The applications of unconventional protocols during time
dependent magnetization measurement at different stages of the temperature and
field dependence of the magnetization curves have been useful to reveal the
non-equilibrium dynamics of magnetic spin order. The applying of off-field
relaxation experiments has made possible to tune the magnetic state and
coercivity of the systems. The role of interfacial coupling between magnetic
and non-magnetic particles has been understood on different magnetic phenomena
(meta-stable magnetic state, exchange bias and memory effect) by comparing the
experimental results of Co$_{1.75}$Fe$_{1.25}$O$_4$ spinel oxide and its
composite with BaTiO$_3$ particles.",2001.02602v2
2023-05-16,Non-periodic input-driven magnetization dynamics in voltage-controlled parametric oscillator,"Input-driven dynamical systems have attracted attention because their
dynamics can be used as resources for brain-inspired computing. The recent
achievement of human-voice recognition by spintronic oscillator also utilizes
an input-driven magnetization dynamics. Here, we investigate an excitation of
input-driven chaos in magnetization dynamics by voltage controlled magnetic
anisotropy effect. The study focuses on the parametric magnetization
oscillation induced by a microwave voltage and investigates the effect of
random-pulse input on the oscillation behavior. Solving the
Landau-Lifshitz-Gilbert equation, temporal dynamics of the magnetization and
its statistical character are evaluated. In a weak perturbation limit, the
temporal dynamics of the magnetization are mainly determined by the input
signal, which is classified as input-driven synchronization. In a large
perturbation limit, on the other hand, chaotic dynamics are observed, where the
dynamical response is sensitive to the initial state. The existence of chaos is
also identified by the evaluation of the Lyapunov exponent.",2305.09151v1
2004-12-27,Magnetization suppression of Type-II Superconductors by external alternating magnetic field,"The effect of suppression of static magnetization of an anisotropic hard
superconductor by alternating magnetic field is analyzed theoretically. The
magnetic moment suppression dynamics is described with respect to the
magnetization loop of the superconductor. It is found that in some cases the
magnetic moment varies nonmonotonically with the growth in amplitude
$\mathrm{h}$ of the alternating field. Effect of transition, induced by
$\mathrm{\mathbf{h}}(t)$, of superconductor form paramagnetic into the
diamagnetic state is considered. The amplitude of alternating magnetic field
$\mathrm{h}_{c}(\delta,\vartheta)$ for which the complete suppression of the
magnetization occurs is calculated as the function of anisotropy parameter
$\delta$ and it orientation angle $\vartheta$ with respect to the
crystallographic axes of the sample.",0412695v1
2015-06-05,On the magnetization process of ferromagnetic materials,"The present article concludes that a ferromagnetic sample could be considered
like a paramagnetic system where the role of magnetic moments plays magnetic
domains. Based on this conclusion and taking into account presence of an
anisotropic field the formula which describes magnetization dependence on the
external magnetic field is derived. Expressions for a remanent magnetization
and a coercive force are presented. The new parameter to characterize a
magnetic stiffness of a material is introduced. A physical expression for a
dynamic magnetic susceptibility as a function of material's characteristics,
external magnetic field, and a temperature is given.",1506.01805v3
2020-09-14,Voltage-driven Magnetization Switching via Dirac Magnetic Anisotropy and Spin--orbit Torque in Topological-insulator-based Magnetic Heterostructures,"Electric-field control of magnetization dynamics is fundamentally and
technologically important for future spintronic devices. Here, based on
electric-field control of both magnetic anisotropy and spin--orbit torque, two
distinct methods are presented for switching the magnetization in topological
insulator (TI)/magnetic-TI hybrid systems. The magnetic anisotropy energy in
magnetic TIs is formulated analytically as a function of the Fermi energy, and
it is confirmed that the out-of-plane magnetization is always favored for the
partially occupied surface band. Also proposed is a transistor-like device with
the functionality of a nonvolatile magnetic memory that uses voltage-driven
writing and the (quantum) anomalous Hall effect for readout. For the
magnetization reversal, by using parameters of Cr-doped
Bi_{1-x}Sb_{x})_{2}Te_{3}, the estimated source-drain current density and gate
voltage are of the orders of $10^4$--$10^5$~A/cm$^2$ and 0.1~V, respectively,
below 20~K and the writing requires no external magnetic field. Also discussed
is the possibility of magnetization switching by the proposed method in
TI/ferromagnetic-insulator bilayers with the magnetic proximity effect.",2009.06187v1
2016-12-12,Symbolic Dynamics of Magnetic Bumps,"For n convex magnetic bumps in the plane, whose boundary has a curvature
somewhat smaller than the absolute value of the constant magnetic field inside
the bump, we construct a complete symbolic dynamics of a classical particle
moving with speed one.",1612.03670v1
2021-01-12,Modeling of Thermal Magnetic Fluctuations in Nanoparticle Enhanced Magnetic Resonance Detection,"We present a systematic numerical modeling investigation of magnetization
dynamics and thermal magnetic moment fluctuations of single magnetic domain
nanoparticles in a configuration applicable to enhancing inductive magnetic
resonance detection signal to noise ratio (SNR). Previous proposals for
oriented anisotropic single magnetic domain nanoparticle amplification of
magnetic flux in MRI coil focused only on the coil pick-up voltage signal
enhancement. Here we extend the analysis to the numerical evaluation of the SNR
by modeling the inherent thermal magnetic noise introduced into the detection
coil by the insertion of such anisotropic nanoparticle-filled coil core. We
utilize the Landau-Lifshitz-Gilbert equation under the Stoner-Wohlfarth single
magnetic domain (macrospin) assumption to simulate the magnetization dynamics
in such nanoparticles due to AC drive field as well as thermal noise. These
simulations are used to evaluate the nanoparticle configurations and shape
effects on enhancing SNR. Finally, we explore the effect of narrow band
filtering of the broadband magnetic moment thermal fluctuation noise on the
SNR. Our results provide the impetus for relatively simple modifications to
existing MRI systems for achieving enhanced detection SNR in scanners with
modest polarizing magnetic fields.",2101.04649v1
2021-12-02,Heisenberg representation of nonthermal ultrafast laser excitation of magnetic precessions,"We derive the Heisenberg representation of the ultrafast inverse Faraday
effect that provides the time evolution of magnetic vectors of a magnetic
system during its interaction with a laser pulse. We obtain a time-dependent
effective magnetic operator acting in the Hilbert space of the total angular
momentum that describes a process of nonthermal excitation of magnetic
precessions in an electronic system by a circularly polarized laser pulse. The
magnetic operator separates the effect of the laser pulse on the magnetic
system from other magnetic interactions. The effective magnetic operator
provides the equations of motion of magnetic vectors during the excitation by
the laser. We show that magnetization dynamics calculated with these equations
is equivalent to magnetization dynamics calculated with the time-dependent
Schr\""odinger equation, which takes into account the interaction of an
electronic system with the electric field of light. We model and compare
laser-induced precessions of magnetic sublattices of an easy-plane and an
easy-axis antiferromagnetic systems. Using these models, we show how the
ultrafast inverse Faraday effect induces a net magnetic moment in
antiferromagnets and demonstrate that a crystal field environment and the
exchange interaction play essential roles for laser-induced magnetization
dynamics even during the action of a pump pulse. Using our approach, we show
that light-induced precessions can start even during the action of the pump
pulse with a duration several tens times shorter than the period of induced
precessions and affect the position of magnetic vectors after the action of the
pump pulse.",2112.01350v1
2007-01-17,Projected single-spin flip dynamics in the Ising Model,"We study transition matrices for projected dynamics in the
energy-magnetization space, magnetization space and energy space. Several
single spin flip dynamics are considered such as the Glauber and Metropolis
canonical ensemble dynamics and the Metropolis dynamics for three
multicanonical ensembles: the flat energy-magnetization histogram, the flat
energy histogram and the flat magnetization histogram. From the numerical
diagonalization of the matrices for the projected dynamics we obtain the
sub-dominant eigenvalue and the largest relaxation times for systems of varying
size. Although, the projected dynamics is an approximation to the full state
space dynamics comparison with some available results, obtained by other
authors, shows that projection in the magnetization space is a reasonably
accurate method to study the scaling of relaxation times with system size. The
transition matrices for arbitrary single-spin flip dynamics are obtained from a
single Monte-Carlo estimate of the infinite temperature transition-matrix, for
each system size, which makes the method an efficient tool to evaluate the
relative performance of any arbitrary local spin-flip dynamics. We also present
new results for appropriately defined average tunnelling times of magnetization
and compute their finite-size scaling exponents that we compare with results of
energy tunnelling exponents available for the flat energy histogram
multicanonical ensemble.",0701408v1
2005-12-08,Physics of Complex Magnetic Materials: Quasiparticle Many-Body Dynamics,"A brief survey of the author's works on the quantum theory of magnetism.
Theoretical foundation and applications of the generalized spin-fermion (sp-d)
exchange lattice model to various magnetic systems, e.g., rare-earth metals and
compounds, and magnetic and diluted magnetic semiconductors are discussed
briefly. The main emphasis is put on the dynamic behavior of two interacting
subsystems, the localized spins and spin density of itinerant carriers. A
nonperturbative many-body approach was used to describe the quasiparticle
dynamics. The approach permits to investigate and clarify the role of various
interactions and disorder effects in combined quantum models of magnetism in
unified and coherent fashion.",0512183v1
2010-04-14,Dynamics of strongly magnetized ejecta in Gamma Ray Bursts,"We consider dynamical scales in magnetized GRB outflows, using the solutions
to the Riemann problem of expanding arbitrarily magnetized outflows (Lyutikov
2010). For high ejecta magnetization, the behavior of the forward shock closely
resembles the so-called thick shell regime of the hydrodynamical expansion. The
exception is at small radii, where the motion of the forward shock is
determined by the dynamics of subsonic relativistic outflows. The behaviors of
the reverse shock is different in fluid and magnetized cases: in the latter
case, even for medium magnetization, sigma ~ 1, the reverse shock forms at
fairly large distances, and may never form in a wind-type external density
profile.",1004.2429v1
2018-12-17,Dynamically generated magnetic moment in the Wigner-function formalism,"We study how the mass and magnetic moment of the quarks are dynamically
generated in nonequilibrium quark matter. We derive the equal-time transport
and constraint equations for the quark Wigner function in a magnetized quark
model and solve them in the semi-classical expansion. The quark mass and
magnetic moment are self-consistently coupled to the Wigner function and
controlled by the kinetic equations. While the quark mass is dynamically
generated at the classical level, the quark magnetic moment is a pure quantum
effect, induced by the quark spin interaction with the external magnetic field.",1812.06684v1
2023-09-27,Regulating spin dynamics in magnetic nanomaterials,"Magnetic nanomaterials can be used in the construction of devices for
information processing and memory storage. For this purpose, they have to enjoy
two contradictory properties, from one side being able of keeping for long time
magnetization frozen, hence information stored, and from the other side
allowing for quick change of magnetization required for fast erasing of memory
and rewriting new information. Methods of resolving this dilemma are suggested
based on triggering resonance, dynamic resonance tuning, and on quadratic
Zeeman effect. These methods make it possible to realize effective regulation
of spin dynamics in such materials as magnetic nanomolecules and magnetic
nanoclusters.",2309.15568v1
2006-01-27,Current-controlled magnetization dynamics in the spin-flip transistor,"The current driven magnetization dynamics of a thin-film, three magnetic
terminal device (spin-flip transistor) is investigated theoretically. We
consider a magnetization configuration in which all magnetizations are in the
device plane, with source-drain magnetizations chosen fixed and antiparallel,
whereas the third contact magnetization is allowed to move in a weak anisotropy
field that guarantees thermal stability of the equilibrium structure at room
temperature. We analyze the magnetization dynamics of the free layer under a dc
source-drain bias current within the macrospin model and magneto-electronic
circuit theory. A new tunable two-state behavior of the magnetization is found
and the advantages of this phenomenon and potential applications are discussed.",0601632v1
2017-07-11,Magnetophononics: ultrafast spin control through the lattice,"Using a combination of first-principles and magnetization-dynamics
calculations, we study the effect of the intense optical excitation of phonons
on the magnetic behavior in insulating magnetic materials. Taking the
prototypical magnetoelectric \CrO\ as our model system, we show that excitation
of a polar mode at 17 THz causes a pronounced modification of the magnetic
exchange interactions through a change in the average Cr-Cr distance. In
particular, the quasi-static deformation induced by nonlinear phononic coupling
yields a structure with a modified magnetic state, which persists for the
duration of the phonon excitation. In addition, our time-dependent
magnetization dynamics computations show that systematic modulation of the
magnetic exchange interaction by the phonon excitation modifies the
magnetization dynamics. This temporal modulation of the magnetic exchange
interaction strengths using phonons provides a new route to creating
non-equilibrium magnetic states and suggests new avenues for fast manipulation
of spin arrangements and dynamics.",1707.03216v3
2023-01-09,X-ray detected ferromagnetic resonance techniques for the study of magnetization dynamics,"Element-specific spectroscopies using synchrotron-radiation can provide
unique insights into materials properties. The recently developed technique of
X-ray detected ferromagnetic resonance (XFMR) allows studying the magnetization
dynamics of magnetic spin structures. Magnetic sensitivity in XFMR is obtained
from the X-ray magnetic circular dichroism (XMCD) effect, where the phase of
the magnetization precession of each magnetic layer with respect to the
exciting radio frequency is obtained using stroboscopic probing of the spin
precession. Measurement of both amplitude and phase response in the magnetic
layers as a function of bias field can give a clear signature of spin-transfer
torque (STT) coupling between ferromagnetic layers due to spin pumping. Over
the last few years, there have been new developments utilizing X-ray scattering
techniques to reveal the precessional magnetization dynamics of ordered spin
structures in the GHz frequency range. The techniques of diffraction and
reflectometry ferromagnetic resonance (DFMR and RFMR) provide novel ways for
the probing of the dynamics of chiral and multilayered magnetic materials,
thereby opening up new pathways for the development of high-density and
low-energy consumption data processing solutions.",2301.03256v1
2003-07-10,Tunable Magnetic Relaxation In Magnetic Nanoparticles,"We investigate the magnetization dynamics of a conducting magnetic
nanoparticle weakly coupled to source and drain electrodes, under the
assumption that all relaxation comes from exchange of electrons with the
electrodes. The magnetization dynamics is characterized by a relaxation time
$t_1$, which strongly depends on temperature, bias voltage, and gate voltage.
While a direct measure of a nanoparticle magnetization might be difficult, we
find that $t_1$ can be determined through a time resolved transport
measurement. For a suitable choice of gate voltage and bias voltage, the
magnetization performs a bias-driven Brownian motion regardless of the presence
of anisotropy.",0307256v1
2005-03-31,Boundary conditions for magnetization in magnetic nano-elements,"We show that the dynamic magnetization at the edges of a thin magnetic
element with finite lateral size can be described by new effective boundary
conditions that take into account inhomogeneous demagnetizing fields near the
element edges. These fields play a dominant role in the effective pinning of
the dynamic magnetization at the boundaries of mesoscopic and nano-sized
magnetic elements. The derived effective boundary conditions generalize
well-known Rado-Weertman boundary conditions and are reduced to them in the
limiting case of a very thin magnetic element.",0503745v1
2006-01-17,Electric field control of magnetization dynamics in ZnMnSe/ZnBeSe diluted-magnetic-semiconductor heterostructures,"We show that the magnetization dynamics in diluted magnetic semiconductors
can be controlled separately from the static magnetization by means of an
electric field. The spin-lattice relaxation (SLR) time of magnetic Mn2+ ions
was tuned by two orders of magnitude by a gate voltage applied to n-type
modulation-doped (Zn,Mn)Se/(Zn,Be)Se quantum wells. The effect is based on
providing an additional channel for SLR by a two-dimensional electron gas
(2DEG). The static magnetization responsible for the giant Zeeman spin
splitting of excitons was not influenced by the 2DEG density.",0601383v1
2008-10-28,"Anomalous magnetization behavior in Ce(Fe,Si)2","We report the effect of Si doping on the magnetization behavior of CeFe2. It
is found that Si stabilizes the dynamic antiferromagnetic state of CeFe2.
Multi-step magnetization behavior, unusual relaxation effect, thermal and
magnetic history dependence, which are signatures of martensitic scenario, are
found to be present in this system. We also show that one can induce the
magnetization steps with the help of appropriate measurement protocol. Detailed
magnetization relaxation studies have been carried out to understand the
dynamics of magnetic phase transition.",0810.5017v1
2014-06-18,Spin-lattice coupling induced weak dynamical magnetism in EuTiO_3 at high temperatures,"EuTiO_3, which is a G-type antiferromagnet below T_N = 5.5 K, has some
fascinating properties at high temperatures, suggesting that macroscopically
hidden dynamically fluctuating weak magnetism exists at high temperatures. This
conjecture is substantiated by magnetic field dependent magnetization
measurements, which exhibit pronounced anomalies below 200 K becoming more
distinctive with increasing magnetic field strength. Additional results from
muon spin rotation (${\mu}$SR) experiments provide evidence for weak
fluctuating bulk magnetism induced by spin-lattice coupling which is strongly
supported in increasing magnetic field.",1406.4915v1
2015-03-27,Spin superradiance by magnetic nanomolecules and nanoclusters,"Spin dynamics of assemblies of magnetic nanomolecules and nanoclusters can be
made coherent by inserting the sample into a coil of a resonant electric
circuit. Coherence is organized through the arising feedback magnetic field of
the coil. The coupling of a magnetic sample with a resonant circuit induces
fast spin relaxation and coherent spin radiation, that is, superradiance. We
consider spin dynamics described by a realistic Hamiltonian, typical of
magnetic nanomolecules and nanoclusters. The role of magnetic anisotropy is
studied. A special attention is paid to geometric effects related to the mutual
orientation of the magnetic sample and resonator coil.",1503.08036v1
2020-02-17,Periodic Magnetic Geodesics on Heisenberg Manifolds,"We study the dynamics of magnetic flows on Heisenberg groups. Let $H$ denote
the three-dimensional simply connected Heisenberg Lie group endowed with a
left-invariant Riemannian metric and an exact, left-invariant magnetic field.
Let $\Gamma$ be a lattice subgroup of $H,$ so that $\Gamma\backslash H$ is a
closed nilmanifold. We first find an explicit description of magnetic geodesics
on $H$, then determine all closed magnetic geodesics and their lengths for
$\Gamma \backslash H$. We then consider two applications of these results: the
density of periodic magnetic geodesics and marked magnetic length spectrum
rigidity.",2002.06982v1
2011-11-28,Dynamical friction in a gaseous medium with a large-scale magnetic field,"The dynamical friction force experienced by a massive gravitating body moving
through a gaseous medium is modified by sufficiently strong large-scale
magnetic fields. Using linear perturbation theory, we calculate the structure
of the wake generated by, and the gravitational drag force on, a body traveling
in a straight-line trajectory in a uniformly magnetized medium. The functional
form of the drag force as a function of the Mach number (V_0/c_s, where V_0 is
the velocity of the body and c_s the sound speed) depends on the strength of
the magnetic field and on the angle between the velocity of the perturber and
the direction of the magnetic field. In particular, the peak value of the drag
force is not near Mach number 1 for a perturber moving in a sufficiently
magnetized medium. As a rule of thumb, we may state that for supersonic motion,
magnetic fields act to suppress dynamical friction; for subsonic motion,
magnetic fields tend to enhance dynamical friction. For perturbers moving along
the magnetic field lines, the drag force at some subsonic Mach numbers may be
stronger than it is at supersonic velocities. We also mention the relevance of
our findings to black hole coalescence in galactic nuclei.",1111.6632v1
2016-01-21,Vortex Dynamics-Mediated Low-Field Magnetization Switching in an Exchange-Coupled System,"A magnetic vortex has attracted significant attention since it is a
topologically stable magnetic structure in a soft magnetic nanodisk. Many
studies have been devoted to understanding the nature of magnetic vortex in
isolated systems. Here we show a new aspect of a magnetic vortex the dynamics
of which strongly affects the magnetic structures of environment. We exploit a
nanodot of an exchange-coupled bilayer with a soft magnetic Ni81Fe19
(permalloy; Py) having a magnetic vortex and a perpendicularly magnetized
L10-FePt exhibiting a large switching field (Hsw). The vortex dynamics with
azimuthal spin waves makes the excess energy accumulate in the Py, which
triggers the reversed-domain nucleation in the L10-FePt at a low magnetic
field. Our results shed light on the non-local mechanism of a reversed-domain
nucleation, and provide with a route for efficient Hsw reduction that is needed
for ultralow-power spintronic devices.",1601.05521v1
2018-01-17,Magnetic effect on dynamical tide in rapidly rotating astronomical objects,"By numerically solving the equations of rotating magnetohydrodynamics (MHD),
the magnetic effect on dynamical tide is studied. It is found that magnetic
field has a significant impact not only on the flow structure, i.e. the
internal shear layers in rotating flow can be destroyed in the presence of a
moderate or stronger magnetic field (in the sense that the Alfv\'en velocity is
at least of the order of 0.1 of the surface rotational velocity), but also on
the dispersion relation of waves excited by tidal force such that the range of
tidal resonance is broadened by magnetic field. A major result is that the
total tidal dissipation scales as square of the field strength, which can be
used to estimate the strength of internal magnetic field in the astronomical
object of a binary system. Moreover, with a moderate or stronger field the
ratio of the magnetic dissipation to the viscous dissipation is almost
inversely proportional to magnetic Prandtl number (i.e. the ratio of viscosity
to magnetic diffusivity), and therefore in the astrophysical situation at small
magnetic Prandtl number magnetic dissipation dominates over viscous dissipation
with a moderate or stronger field.",1801.05552v2
2018-04-27,Excitation and coherent control of magnetization dynamics in magnetic tunnel junctions using acoustic pulses,"We experimentally study magnetization dynamics in magnetic tunnel junctions
driven by femtosecond-laser-induced surface acoustic waves. The acoustic pulses
induce a magnetization precession in the free layer of the magnetic tunnel
junction through magnetoelastic coupling. The frequency and amplitude of the
precession shows a pronounced dependence on the applied magnetic field and the
laser excitation position. Comparing the acoustic-wave-induced precession
frequencies with precession induced by charge currents and with micromagnetic
simulations we identify spatially non-uniform magnetization modes localized
close the edge regions as being responsible for the optically induced
magnetization dynamics. The experimental scheme even allows us to coherently
control the magnetization precession using two acoustic pulses. This might
prove important for future applications requiring ultrafast spin manipulation.
Additionally, our results directly pinpoint the importance of acoustic pulses
since they could be relevant when investigating optically-induced temperature
effects in magnetic structures.",1804.10503v1
2021-12-02,Hamilton-Jacobi Equations of Nonholonomic Magnetic Hamiltonian Systems,"In order to describe the impact of different geometric structures and
constraints for the dynamics of a Hamiltonian system, in this paper, for a
magnetic Hamiltonian system defined by a magnetic symplectic form, we first
drive precisely the geometric constraint conditions of magnetic symplectic form
for the magnetic Hamiltonian vector field.which are called the Type I and Type
II of Hamilton-Jacobi equation. Secondly, for the magnetic Hamiltonian system
with nonholonomic constraint, we first define a distributional magnetic
Hamiltonian system, then derive its two types of Hamilton-Jacobi equation.
Moreover, we generalize the above results to nonholonomic reducible magnetic
Hamiltonian system with symmetry. We define a nonholonomic reduced
distributional magnetic Hamiltonian system, and prove two types of
Hamilton-Jacobi theorem. These research work reveal the deeply internal
relationships of the magnetic symplectic structure, nonholonomic constraint,
the distributional two-form, and the dynamical vector field of the nonholonomic
magnetic Hamiltonian system.",2112.00961v2
2014-07-04,Mixed spin (1/2-1) hexagonal Ising nanowire: some dynamic behaviors,"The dynamic behaviors of a mixed spin (1/2-1) hexagonal Ising nanowire (HIN)
with core-shell structure in the presence of a time dependent magnetic field
are investigated by using the effective-field theory with correlations based on
the Glauber-type stochastic dynamics (DEFT). According to the values of
interaction parameters, temperature dependence of the dynamic magnetizations,
the hysteresis loop areas and the dynamic correlations are investigated to
characterize the nature (first- or second-order) of the dynamic phase
transitions (DPTs). Dynamic phase diagrams, including compensation points, are
also obtained. Moreover, from the thermal variations of the dynamic total
magnetization, the five compensation types can be found under certain
conditions, namely the Q-, R-, S-, P-, and N-types.",1407.1092v1
2021-09-24,Fluidic Endogenous Magnetism and Magnetic Monopole Clues from Liquid Metal Droplet Machine,"Magnetism and magnetic monopole are classical issues in basic physics.
Conventional magnets are generally composed of rigid materials with shapes and
structures unchangeable which may face challenges sometimes to answer the above
questions. Here, from an alternative other than rigid magnet, we disclosed an
unconventional way to generate endogenous magnetism and then construct magnetic
monopole through tuning liquid metal machine. Through theoretical
interpretation and conceptual experiments, we illustrated that when gallium
base liquid metal in solution rotates under actuation of an external electric
field, it forms an endogenous magnetic field inside which well explains the
phenomenon that two such discrete metal droplets could easily fuse together,
indicating their reciprocal attraction via N and S poles. Further, we conceived
that the self-driving liquid metal motor was also an endogenous magnet owning
the electromagnetic homology. When liquid metal in solution swallowed aluminum
inside, it formed a spin motor and dynamically variable charge distribution
which produced an endogenous magnetic field. This finding explains the
phenomena that there often happened reflection collision and attraction fusion
between running liquid metal motors which were just caused by the dynamic
adjustment of their N and S polarities. Finally, we conceived that such
endogenous magnet could lead to magnetic monopole and four technical routes
were suggested as: 1. Matching the interior flow field of liquid metal
machines; 2. Superposition between external electric effect and magnetic field;
3. Composite construction between magnetic particles and liquid metal motor; 4.
Chemical ways such as via galvanic cell reaction. Overall, the fluidic
endogenous magnet and the promising magnetic monopole it enabled may lead to
unconventional magnetoelectric devices and applications in the near future.",2111.02867v1
2017-12-20,Optical Manipulation of Magnetic Vortex Visualized in situ by 4D Electron Microscopy,"Understanding the fundamental dynamics of topological vortex and antivortex
naturally formed in micro/nanoscale ferromagnetic building blocks under
external perturbations is crucial to magnetic vortex based information
processing and spintronic devices. All previous studies have focused on
magnetic vortex-core switching via external magnetic fields, spin-polarized
currents, or spin waves, which have largely prohibited the investigation of
novel spin configurations that could emerge from the ground states in
ferromagnetic disks and their underlying dynamics. Here, we report in situ
visualization of femtosecond laser quenching induced magnetic vortex change in
various symmetric ferromagnetic Permalloy disks by Lorentz phase imaging using
4D electron microscopy. Besides the switching of magnetic vortex chirality and
polarity, we observed with distinct occurrence frequencies a plenitude of
complex magnetic structures that have never been observed by magnetic field or
current assisted switching. These complex magnetic structures consist of a
number of newly created topological magnetic defects (vortex and antivortex)
strictly conserving the topological winding number, demonstrating the direct
impact of topological invariant on the magnetization dynamics in ferromagnetic
disks. Their spin configurations show mirror or rotation symmetry due to the
geometrical confinement of the disks. Combined micromagnetic simulations with
the experimental observations reveal the underlying magnetization dynamics and
formation mechanism of the optical quenching induced complex magnetic
structures. Their distinct occurrence rates are pertinent to their
formation-growth energetics and pinning effects at the disk edge. Based on
these findings, we propose a paradigm of optical-quenching-assisted fast
switching of vortex cores for the control of magnetic vortex based information
recording and spintronic devices.",1712.07280v1
2004-11-11,Dynamical Symmetry Breaking on a Cylinder in Magnetic Field,"We study dynamical symmetry breaking on a cylinder in external magnetic field
parallel to the axis of cylinder when magnetic field affects the dynamics of
fermions only through the Aharonov-Bohm phase. We find that unlike other
previously studied cases magnetic field in our case counteracts the generation
of dynamical fermion mass which decreases with magnetic field. There exists
also a purely kinematical contribution to the fermion gap which grows linearly
with magnetic field. Remarkably, we find that the total fermion gap, which
includes both the dynamical and kinematical contributions, always increases
with magnetic field irrespectively the values of coupling constant and the
radius of cylinder. Thus, although the dynamical mass is suppressed, external
magnetic field does enhance the total fermion gap in the spectrum.",0411157v2
2020-01-22,Imaging three-dimensional nanoscale magnetization dynamics,"The ability to experimentally map the three-dimensional structure and
dynamics in bulk and patterned three-dimensional ferromagnets is essential both
for understanding fundamental micromagnetic processes, as well as for
investigating technologically-relevant micromagnets whose functions are
connected to the presence and dynamics of fundamental micromagnetic structures,
such as domain walls and vortices. Here, we demonstrate time-resolved magnetic
laminography, a technique which offers access to the temporal evolution of a
complex three-dimensional magnetic structure with nanoscale resolution. We
image the dynamics of the complex three-dimensional magnetization state in a
two-phase bulk magnet with a lateral spatial resolution of 50 nm, mapping the
transition between domain wall precession and the dynamics of a uniform
magnetic domain that is attributed to variations in the magnetization state
across the phase boundary. The capability to probe three-dimensional magnetic
structures with temporal resolution paves the way for the experimental
investigation of novel functionalities arising from dynamic phenomena in bulk
and three-dimensional patterned nanomagnets.",2001.07940v1
2003-04-04,Dynamic exchange coupling and Gilbert damping in magnetic multilayers,"We theoretically study dynamic properties of thin ferromagnetic films in
contact with normal metals. Moving magnetizations cause a flow of spins into
adjacent conductors, which relax by spin flip, scatter back into the
ferromagnet, or are absorbed by another ferromagnet. Relaxation of spins
outside the moving magnetization enhances the overall damping of the
magnetization dynamics in accordance with the Gilbert phenomenology. Transfer
of spins between different ferromagnets by these nonequilibrium spin currents
leads to a long-ranged dynamic exchange interaction and novel collective
excitation modes. Our predictions agree well with recent
ferromagnetic-resonance experiments on ultrathin magnetic films.",0304116v1
2007-09-27,Dynamical Electron Mass in a Strong Magnetic Field,"Motivated by recent interest in understanding properties of strongly
magnetized matter, we study the dynamical electron mass generated through
approximate chiral symmetry breaking in QED in a strong magnetic field. We
reliably calculate the dynamical electron mass by numerically solving the
nonperturbative Schwinger-Dyson equations in a consistent truncation within the
lowest Landau level approximation. It is shown that the generation of dynamical
electron mass in a strong magnetic field is significantly enhanced by the
perturbative electron mass that explicitly breaks chiral symmetry in the
absence of a magnetic field.",0709.4427v3
2011-02-14,Application of Resonance Perturbation Theory to Dynamics of Magnetization in Spin Systems Interacting with Local and Collective Bosonic Reservoirs,"We apply our recently developed resonance perturbation theory to describe the
dynamics of magnetization in paramagnetic spin systems interacting
simultaneously with local and collective bosonic environments. We derive
explicit expressions for the evolution of the reduced density matrix elements.
This allows us to calculate explicitly the dynamics of the macroscopic
magnetization, including characteristic relaxation and dephasing time-scales.
We demonstrate that collective effects (i) do not influence the character of
the relaxation processes but merely renormalize the relaxation times, and (ii)
significantly modify the dephasing times, leading in some cases to a
complicated (time inhomogeneous) dynamics of the transverse magnetization,
governed by an effective time-dependent magnetic field.",1102.2847v1
2012-03-24,Dynamics of a dielectric droplet suspended in a magnetic fluid in electric and magnetic fields,"The behavior of a microdrop of dielectric liquid suspended in a magnetic
fluid and exposed to the action of electric and magnetic fields is studied
experimentally. With increasing electric field, the deformation of droplets
into oblate ellipsoid, toroid and curved toroid was observed. At the further
increase in the electric field, the bursting of droplets was also revealed. The
electrorotation of deformed droplets was observed and investigated. The
influence of an additional magnetic field on the droplet dynamics was studied.
The main features of the droplet dynamics were interpreted and theoretically
examined.",1203.5434v1
2015-09-09,Inertial terms to magnetization dynamics in ferromagnetic thin films,"Inertial magnetization dynamics have been predicted at ultrahigh speeds, or
frequencies approaching the energy relaxation scale of electrons, in
ferromagnetic metals. Here we identify inertial terms to magnetization dynamics
in thin Ni$_{79}$Fe$_{21}$ and Co films near room temperature. Effective
magnetic fields measured in high-frequency ferromagnetic resonance (115-345
GHz) show an additional stiffening term which is quadratic in frequency and
$\sim$ 80 mT at the high frequency limit of our experiment. Our results extend
understanding of magnetization dynamics at sub-picosecond time scales.",1509.02836v1
2018-10-04,Domain wall dynamics in easy-cone magnets,"We theoretically and numerically investigate magnetic domain wall dynamics in
a nanowire of easy-cone magnet. The easy-cone domain wall exhibits several
distinguishing dynamic features in comparison to the easy-axis domain wall. The
features of easy-cone domain wall are related to the generation of additional
chiral spin textures due to the domain wall precession, which is common for
various driving sources such as magnetic fields and spin-transfer torques. The
unique easy-cone domain wall dynamics could enrich magnetic domain wall study
and find use in device applications based on easy-cone domain walls.",1810.02216v1
2020-05-12,Analysis in k-space of Magnetization Dynamics Driven by Strong Terahertz Fields,"Demagnetization in a thin film due to a terahertz pulse of magnetic field is
investigated. Linearized LLG equation in the Fourier space to describe the
magnetization dynamics is derived, and spin waves time evolution is studied.
Finally, the demagnetization due to spin waves dynamics and recent experimental
observations on similar magnetic system are compared. As a result of it, the
marginal role of spin waves dynamics in loss of magnetization is established.",2005.05742v1
1999-09-27,Topological Constraints on the Relaxation of Complex Magnetic Fields,"Newly emerging magnetic flux can show a complicated linked or interwoven
topology of the magnetic field. The complexity of this linkage or knottedness
of magnetic flux is related to the free energy stored in the magnetic field.
Magnetic reconnection provides a process to release this energy on the time
scale of the dynamics. At the same time it approximately conserves the total
magnetic helicity. Therefore the conservation of total magnetic helicity is a
crucial constraint for the relaxation of complex magnetic fields. However, the
total magnetic helicity is only the first, most elementary, quantity of an
infinite series of topological invariants of the magnetic field. All these
invariants are strictly conserved in ideal magnetohydrodynamics. As an example
a preliminary set of these invariants is derived. The relevance of these higher
order invariants for the final state of relaxation under magnetic reconnection
and their implications for the release of magnetic energy are discussed.",9909443v1
2014-08-05,Tunable asymmetric magnetoimpedance effect in ferromagnetic NiFe/Cu/Co films,"We investigate the magnetization dynamics through the magnetoimpedance effect
in ferromagnetic NiFe/Cu/Co films. We observe that the magnetoimpedance
response is dependent on the thickness of the non-magnetic Cu spacer material,
a fact associated to the kind of the magnetic interaction between the
ferromagnetic layers. Thus, we present an experimental study on asymmetric
magnetoimpedance in ferromagnetic films with biphase magnetic behavior and
explore the possibility of tuning the linear region of the magnetoimpedance
curves around zero magnetic field by varying the thickness of the non-magnetic
spacer material, and probe current frequency. We discuss the experimental
magnetoimpedance results in terms of the different mechanisms governing the
magnetization dynamics at distinct frequency ranges, quasi-static magnetic
properties, thickness of the non-magnetic spacer material, and the kind of the
magnetic interaction between the ferromagnetic layers. The results place
ferromagnetic films with biphase magnetic behavior exhibiting asymmetric
magnetoimpedance effect as a very attractive candidate for application as probe
element in the development of auto-biased linear magnetic field sensors.",1408.0992v1
2020-09-03,Coincidence inelastic neutron scattering for detection of two-spin magnetic correlations,"Inelastic neutron scattering (INS) is one powerful technique to study the
low-energy single-spin dynamics of magnetic materials. A variety of quantum
magnets show novel magnetic correlations such as quantum spin liquids. These
novel magnetic correlations are beyond the direct detection of INS. In this
paper we propose a coincidence technique, coincidence inelastic neutron
scattering (cINS), which can detect the two-spin magnetic correlations of the
magnetic materials. In cINS there are two neutron sources and two neutron
detectors with an additional coincidence detector. Two neutrons from the two
neutron sources are incident on the target magnetic material, and they are
scattered by the electron spins of the magnetic material. The two scattered
neutrons are detected by the two neutron detectors in coincidence with the
coincidence probability described by a two-spin Bethe-Salpeter wave function.
Since the two-spin Bethe-Salpeter wave function defines the momentum-resolved
dynamical wave function with two spins excited, cINS can explicitly detect the
two-spin magnetic correlations of the magnetic material. Thus, it can be
introduced to study the various spin valence bond states of the quantum
magnets.",2009.01680v2
2021-06-28,Stability of a Magnetically Levitated Nanomagnet in Vacuum: Effects of Gas and Magnetization Damping,"In the absence of dissipation a non-rotating magnetic nanoparticle can be
stably levitated in a static magnetic field as a consequence of the spin origin
of its magnetization. Here we study the effects of dissipation on the stability
of the system, considering the interaction with the background gas and the
intrinsic Gilbert damping of magnetization dynamics. At large applied magnetic
fields we identify magnetization switching induced by Gilbert damping as the
key limiting factor for stable levitation. At low applied magnetic fields and
for small particle dimensions magnetization switching is prevented due to the
strong coupling of rotation and magnetization dynamics, and the stability is
mainly limited by the gas-induced dissipation. In the latter case, high vacuum
should be sufficient to extend stable levitation over experimentally relevant
timescales. Our results demonstrate the possibility to experimentally observe
the phenomenon of quantum spin stabilized magnetic levitation.",2106.14858v3
2004-11-11,Magnetic Soret effect: Application of the ferrofluid dynamics theory,"The ferrofluid dynamics theory is applied to thermodiffusive problems in
magnetic fluids in the presence of magnetic fields. The analytical form for the
magnetic part of the chemical potential and the most general expression of the
mass flux are given. By employing these results to experiments, global Soret
coefficients in agreement with measurements are determined. Also an estimate
for a hitherto unknown transport coefficient is made.",0411308v1
2009-11-11,Wavelets spectra of magnetization dynamics in geometry driven magnetic thin layers,"Squared cobalt thin layers of different thickness and width were investigated
by numerical simulations. Using zero-valued externally applied magnetic field
(geometry driven regime) and different initial conditions the magnetization
dynamics were examined. The wavelet-based spectral analysis was applied.
Transient states of different types were identified.",0911.1994v1
2014-05-26,Magneto-Optical Spectrum Analyzer,"We present a method for the investigation of gigahertz magnetization dynamics
of single magnetic nano elements. By combining a frequency domain approach with
a micro focus Kerr effect detection, a high sensitivity to magnetization
dynamics with submicron spatial resolution is achieved. It allows spectra of
single nanostructures to be recorded. Results on the uniform precession in soft
magnetic platelets are presented.",1405.6551v1
2016-03-30,Distributed chaos and solitons at the edges of magnetically confined plasmas,"It is shown, using results of measurements of ion saturation current in the
plasma edges of different magnetic fusion confinement devices (tokamaks and
stellarators), that the plasma dynamics in the edges is dominated by
distributed chaos with spontaneously broken translational symmetry at low
magnetic field, and with spontaneously broken reflexional symmetry (by helical
solitons) at high magnetic field.",1603.09186v2
2014-11-26,The dynamic magnetic behaviors of the Blume-Capel Ising bilayer system,"The dynamic magnetic behaviors of the spin-1 Blume-Capel Ising bilayer system
(BCIS) are studied in an oscillating external magnetic field on a two-layer
square lattice by utilizing the mean field theory based on Glauber-type
stochastic dynamics (DMFT). The dynamic equations describing the
time-dependencies of the average magnetizations are obtained with the Master
equation. The dynamic phases in this system are found by solving these dynamic
equations. The temperature dependence of the dynamic order parameters is
examined to characterize the nature (continuous or discontinuous) of the phase
transitions and to obtain the dynamic phase transition points (DPT). The
dynamic phase diagrams are shown for ferromagnetic / ferromagnetic,
antiferromagnetic / antiferromagnetic, antiferromagnetic / antiferromagnetic
interactions in the plane of the reduced temperature versus magnetic field
amplitude and they display dynamic tricritical and reentrant behavior as well
as the dynamic triple point.",1411.7146v1
2002-04-09,Local Circumstellar Magnetic Fields Around Evolved Stars,"I argue that the presence of magnetic fields around evolved stars, e.g.,
asymptotic giant branch stars, and in PNe, does not necessarily imply that the
magnetic field plays a global dynamical role in shaping the circumstellar
envelope. Instead, I favor magnetic fields with small coherence lengths, which
result from stellar magnetic spots or from jets blown by an accreting
companion. Although the magnetic field does not play a global role in shaping
the circumstellar envelope, it may enhance local motion (turbulent) via
magnetic tension and reconnection. The locally strong magnetic tension may
enforce coherence flow which may favor the masing process.",0204157v1
2016-02-29,Gyrotropic skyrmion modes in ultrathin magnetic circular dots,"We calculate low-frequency gyrotropic spin excitation modes of the skyrmion
ground state ultrathin cylindrical magnetic dots. The skyrmion is assumed to be
stabilized at room temperature and zero external magnetic field due to an
interplay of the isotropic and Dzyaloshinskii-Moriya exchange interactions,
perpendicular magnetic anisotropy and magnetostatic interaction. We consider
Bloch- and Neel-type magnetic skyrmions and assume that the dot magnetization
does not depend on the thickness coordinate. The skyrmion gyrotropic
frequencies are calculated in GHz range as a function of the skyrmion
equilibrium radius, dot radius and the dot magnetic parameters. Recent
experiments on magnetic skyrmion gyrotropic dynamics in nanodots are discussed.",1603.00072v1
2016-07-19,MHD flow and heat transfer due to the axisymmetric stretching of a sheet with induced magnetic field,"The full MHD equations, governing the flow due to the axisymmetric stretching
of a sheet in the presence of a transverse magnetic field, can be cast in a
self similar form. This allows evaluation of the induced magnetic field and its
effect on the flow and heat transfer. The problem involves three parameters-
the magnetic Prandtl number, the magnetic interaction number, and the Prandtl
number. Numerical solutions are obtained for the velocity field, the magnetic
field, and the temperature, at different values of the magnetic Prandtl number
and the magnetic interaction number. The contributions of the viscous
dissipation, Joule heating, and streamwise diffusion to the heat flux toward
the sheet are assessed.",1607.05722v1
2017-02-27,Magnetization reversal by superconducting current in $\varphi_0$ Josephson junctions,"We study magnetization reversal in a $\varphi_0$ Josephson junction with
direct coupling between magnetic moment and Josephson current. Our simulations
of magnetic moment dynamics show that by applying an electric current pulse, we
can realize the full magnetization reversal. We propose different protocols of
full magnetization reversal based on the variation of the Josephson junction
and pulse parameters, particularly, electric current pulse amplitude, damping
of magnetization and spin-orbit interaction. We discuss experiments which can
probe the magnetization reversal in $\varphi_0$-junctions.",1702.08394v4
2022-04-11,Deformation of a magnetic liquid drop and unsteady flow inside and outside it in a non-stationary magnetic field,"Small steady-state deformational oscillations of a drop of a viscous magnetic
liquid in a non-stationary uniform magnetic field are theoretically
investigated for low Reynolds numbers. The drop is suspended in another viscous
magnetic liquid immiscible with the former. The non-stationary magnetic field
causes flow inside and outside the drop. The inertia is taken into account by
regarding the flow as essentially unsteady. The variation of the magnetic field
is so slow that the approximation of quasi-stationary magnetic field may be
used.",2204.05387v2
2019-11-21,Coupled spin-charge dynamics in magnetic van der Waals heterostructures,"We present a phenomenological theory for coupled spin-charge dynamics in
magnetic van der Waals heterostructures. The system studied consists of a
layered antiferromagnet inserted into a capacitive vdW heterostructure. It has
been recently demonstrated that charge doping in such layered antiferromagnets
can modulate the strength, and even the sign, of exchange coupling between the
layer magnetizations. This provides a mechanism for electrically generating
magnetization dynamics. The central result we predict here is that the
magnetization dynamics reciprocally results in inducing charge dynamics. Such
dynamics makes magnetic van der Waals heterostructures interesting candidates
for spintronics applications. To this end, we also show that these systems can
be used to convert sub-THz radiation induced magnetization dynamics into
electrical signals.",1911.09688v2
2021-04-14,Non-resonant circles for strong magnetic fields on surfaces,"We study non-resonant circles for strong magnetic fields on a closed,
connected, oriented surface and show how these can be used to prove the
existence of trapping regions and of periodic magnetic geodesics with
prescribed low speed. As a corollary, there exist infinitely many periodic
magnetic geodesics for every low speed in the following cases: i) the surface
is not the two-sphere, ii) the magnetic field vanishes somewhere.",2104.06733v1
2020-07-26,Effect of the anomalous magnetic moment of quarks on magnetized QCD matter and meson spectra,"We systematically investigate the effect of the anomalous magnetic
moment(AMM) of quarks on the magnetized QCD matter, including the magnetic
susceptibility, the inverse magnetic catalysis around the critical temperature
and the neutral/charged pion and rho meson spectra under magnetic fields. The
dynamical AMM of quarks, its coupling with magnetic field causes Zeeman
splitting of the dispersion relation of quarks thus changes the magnetism
properties and meson mass spectra under magnetic fields. It is found that
including the AMM of quarks cannot fully understand lattice results of the
magnetized matter: The AMM of quarks reduces the dynamical quark mass thus
causes the inverse magnetic catalysis around $T_c$. The neutral pion mass is
very sensitive to the AMM, it decreases with magnetic field quickly, and the
charged pion mass shows a nonlinear behavior, i.e., firstly linearly increases
with the magnetic field and then saturates at strong magnetic field. For rho
meson, it is observed that AMM reduces the mass of neutral rho meson mass with
different $s_z$, and reduces the mass of $s_z=+1,0$ component charged rho meson
mass but enhances the $s_z=-1$ component charged rho meson mass. The magnetic
susceptibility at low temperature can be either positive or negative with
different AMM.",2007.13122v3
2023-07-28,Reversible magnetic domain reorientation induced by magnetic field pulses with fixed direction,"Nanoscale magnetic domains with controllable configurations could be used for
classical and quantum applications, where the switching of magnetization
configurations is an essential operation for information processing. Here, we
report that the magnetic domain reorientation in a notched ferromagnetic
nanotrack can be realized and effectively controlled by applying uniform
magnetic field pulses in a fixed in-plane direction perpendicular to the
nanotrack. Our micromagnetic simulation results show that the configurations of
magnetic domains in the notched nanotrack can be switched between a
head-to-head state and a tail-to-tail state in a reversible manner driven by
magnetic field pulses, while it is unnecessary to reverse the direction of the
magnetic field. Such a unique magnetic domain reorientation dynamics is found
to depend on magnetic parameters and nanotrack geometries. The reorientation
dynamics of magnetic domains also depends on the strength and length of the
applied magnetic field pulse. In addition, we point out that the notches at the
center of the nanotrack play an important role for the stabilization of the
head-to-head and tail-to-tail states during the magnetic domain reorientation.
We also qualitatively explain the field-induced reorientation phenomenon with a
simplified two-dimensional macrospin model. Our results may make it possible to
build spintronic devices driven by a fixed magnetic field. Our findings may
also motivate future studies to investigate the classical and quantum
applications based on nanoscale magnetic domains.",2307.15263v1
2006-06-02,Magnetically tuned spin dynamics resonance,"We present the experimental observation of a magnetically tuned resonance
phenomenon resulting from spin mixing dynamics of ultracold atomic gases. In
particular we study the magnetic field dependence of spin conversion in F=2
87Rb spinor condensates in the crossover from interaction dominated to
quadratic Zeeman dominated dynamics. We discuss the observed phenomenon in the
framework of spin dynamics as well as matter wave four wave mixing. Furthermore
we show that the validity range of the single mode approximation for spin
dynamics is significantly extended in the regime of high magnetic field.",0606046v1
2007-12-07,Short-time critical dynamics at perfect and non-perfect surface,"We report Monte Carlo simulations of critical dynamics far from equilibrium
on a perfect and non-perfect surface in the 3d Ising model. For an ordered
initial state, the dynamic relaxation of the surface magnetization, the line
magnetization of the defect line, and the corresponding susceptibilities and
appropriate cumulant is carefully examined at the ordinary, special and surface
phase transitions. The universal dynamic scaling behavior including a dynamic
crossover scaling form is identified. The exponent $\beta_1$ of the surface
magnetization and $\beta_2$ of the line magnetization are extracted. The impact
of the defect line on the surface universality classes is investigated.",0712.1087v1
2021-04-24,Theory of magnetic inertial dynamics in two-sublattice ferromagnets,"The magnetic inertial dynamics have been investigated for one sublattice
ferromagnets. Here, we develop the magnetization dynamics in two-sublattice
ferromagnets including the intra- and inter-sublattice inertial dynamics.
First, we derive the magnetic susceptibility of such a ferromagnet. Next, by
finding the poles of the susceptibility, we calculate the precession and
nutation resonance frequencies. Our results suggest that while the resonance
frequencies show decreasing behavior with the increasing intra-sublattice
relaxation time, the effect of inter-sublattice inertial dynamics is
contrasting.",2104.11878v2
2023-09-08,Emergence of Chaos in Magnetic-Field-Driven Skyrmions,"We explore magnetic-field-driven chaos in magnetic skyrmions. Oscillating
magnetic fields induce nonlinear dynamics in skyrmions, arising from the
coupling of the secondary gyrotropic mode with a non-uniform, breathing-like
mode. Through micromagnetic simulations, we observe complex patterns of
hypotrochoidal motion in the orbital trajectories of the skyrmions, which are
interpreted using bifurcation diagrams and local Lyapunov exponents. Our
findings demonstrate that different nonlinear behaviors of skyrmions emerge at
distinct temporal stages, depending on the nonlinear dynamic parameters.
Investigating the abundant dynamic patterns of skyrmions during the emergence
of chaos not only enhances device reliability but also provides useful
guidelines for establishing chaos computing based on skyrmion dynamics.",2309.04243v1
2007-07-16,Magnetodipolar interlayer interaction effect on the magnetization dynamics of a trilayer square element with the Landau domain structure,"We present a detailed numerical simulation study of the effects caused by the
magnetodipolar interaction between ferromagnetic (FM) layers of a trilayer
magnetic nanoelement on its magnetization dynamics. As an example we use a
Co/Cu/Ni80Fe20 element with a square lateral shape where the magnetization of
FM layers forms a closed Landau-like domain pattern. First we show that when
the thickness of the non-magnetic (NM) spacer is in the technology relevant
region h ~ 10 nm, magnetodipolar interaction between 90o Neel domain walls in
FM layers qualitatively changes the equilibrium magnetization state of these
layers. In the main of the paper we compare the magnetization dynamics induced
by a sub-nsec field pulse in a single-layer Ni80Fe20 (Py) element and in the
Co/Cu/Py trilayer element. Here we show that (i) due to the spontaneous
symmetry breaking of the Landau state in the FM/NM/FM trilayer its domains and
domain walls oscillate with different frequencies and have different spatial
oscillation patterns; (ii) magnetization oscillations of the trilayer domains
are strongly suppressed due to different oscillation frequencies of domains in
Co and Py; (iii) magnetization dynamics qualitatively depends on the relative
rotation sense of magnetization states in Co and Py layers and on the
magnetocrystalline anisotropy kind of Co crystallites. Finally we discuss the
relation of our findings with experimental observations of magnetization
dynamics in magnetic trilayers, performed using the element-specific
time-resolved X-ray microscopy.",0707.2344v1
2009-05-11,Dynamically Generated Anomalous Magnetic Moment in Massless QED,"In this paper we investigate the non-perturbative generation of an anomalous
magnetic moment for massless fermions in the presence of an external magnetic
field. In the context of massless QED in a magnetic field, we prove that the
phenomenon of magnetic catalysis of chiral symmetry breaking, which has been
associated in the literature with dynamical mass generation, is also
responsible for the generation of a dynamical anomalous magnetic moment. As a
consequence, the degenerate energy of electrons in Landau levels higher than
zero exhibits Zeeman splitting. We explicitly report the splitting for the
first Landau level and find the non-perturbative Lande $\textit{g}$-factor and
Bohr magneton. We anticipate that a dynamically generated anomalous magnetic
moment will be a universal feature of theories with magnetic catalysis. Our
findings can be important for condensed planar systems as graphene, as well as
for highly magnetized dense systems as those forming the core of compact stars.",0905.1733v2
2015-07-24,Coherent zero-field magnetization resonance in a dipolar spin-1 Bose-Einstein condensate,"With current magnetic field shielding and high precision detection in dipolar
spinor Bose-Einstein condensates, it is possible to experimentally detect the
low or zero field nonsecular dipolar dynamics. Here we analytically investigate
the zero-field nonsecular magnetic dipolar interaction effect, with an emphasis
on magnetization dynamics in a spin-1 Bose-Einstein condensate under the single
spatial mode approximation within the mean field theory. Due to the biaxial
nature of the dipolar interaction, a novel resonance occurs in the condensate
magnetization oscillation, contrast to the previous assumption of a conserved
magnetization in strong magnetic fields. Furthermore, we propose a
dynamical-decoupling detection method for such a resonance, which cancels the
stray magnetic fields in experiments but restores the magnetization dynamics.
Our results shed new lights on the dipolar systems and may find potential
applications beyond cold atoms.",1507.06729v1
2019-03-17,"Dynamical symmetry breaking, magnetization and induced charge in graphene: Interplay between magnetic and pseudomagnetic fields","In this paper, we investigate the two competing effects of strains and
magnetic fields in single-layer graphene to explore its impact on various
phenomena of quantum field theory, such as induced charge density, magnetic
catalysis, symmetry breaking, dynamical mass generation and magnetization. We
show that the interplay between strains and magnetic fields produces not only a
breaking of chiral symmetry, as it happens in QED$_{2+1}$, but also parity and
time-reversal symmetry breaking. The last two symmetry breakings are related to
the dynamical generation of a Haldane mass term. We find that it is possible to
modify the magnetization and the dynamical mass independently for each valley,
by strain and varying the external magnetic field. Furthermore, we discover
that the presence of a non-zero pseudomagnetic field, unlike the magnetic one,
allows us to observe an induced ""vacuum"" charge and a parity anomaly in
strained graphene. Finally, because the combined effect of real and
pseudomagnetic fields produces an induced valley polarization, the results
presented here may provide new tools to design valleytronic devices.",1903.07180v2
2020-03-28,Spin transport and dynamic properties of two-dimensional spin-momentum locked states,"Materials with spin-momentum locked surface or interface states provide an
interesting playground for studying physics and application of charge-spin
current conversion. To characterize their non-equilibrium magnetic and
transport properties in the presence of a time-dependent external magnetic
field and a spin injection from a contact, we introduce three macroscopic
variables: a vectorial helical magnetization, a scaler helical magnetization,
and the conventional magnetization. We derive a set of closed dynamic equations
for these variables by using the spinor Boltzmann approach with the collision
terms consistent with the symmetry of spin-momentum locked states. By solving
the dynamic equations, we predict several intriguing magnetic and transport
phenomena which are experimentally accessible, including magnetic resonant
response to an AC applied magnetic field, charge-spin conversion, and spin
current induced by the dynamics of helical magnetization.",2003.12700v1
2022-11-15,Nonlinear sub-switching regime of magnetization dynamics in photo-magnetic garnets,"We analyze, both experimentally and numerically, the nonlinear regime of the
photo-induced coherent magnetization dynamics in cobalt-doped yttrium iron
garnet films. Photo-magnetic excitation with femtosecond laser pulses reveals a
strongly nonlinear response of the spin subsystem with a significant increase
of the effective Gilbert damping. By varying both laser fluence and the
external magnetic field, we show that this nonlinearity originates in the
anharmonicity of the magnetic energy landscape. We numerically map the
parameter workspace for the nonlinear photo-induced spin dynamics below the
photo-magnetic switching threshold. Corroborated by numerical simulations of
the Landau-Lifshitz-Gilbert equation, our results highlight the key role of the
cubic symmetry of the magnetic subsystem in reaching the nonlinear spin
precession regime. These findings expand the fundamental understanding of
laser-induced nonlinear spin dynamics as well as facilitate the development of
applied photo-magnetism.",2211.08048v2
2002-06-16,Nonlinear Turbulent Magnetic Diffusion and Mean-Field Dynamo,"The nonlinear coefficients defining the mean electromotive force (i.e., the
nonlinear turbulent magnetic diffusion, the nonlinear effective velocity, the
nonlinear kappa-tensor, etc.) are calculated for an anisotropic turbulence. A
particular case of an anisotropic background turbulence (i.e., the turbulence
with zero mean magnetic field) with one preferential direction is considered.
It is shown that the toroidal and poloidal magnetic fields have different
nonlinear turbulent magnetic diffusion coefficients. It is demonstrated that
even for a homogeneous turbulence there is a nonlinear effective velocity which
exhibits diamagnetic or paramagnetic properties depending on anisotropy of
turbulence and level of magnetic fluctuations in the background turbulence.
Analysis shows that an anisotropy of turbulence strongly affects the nonlinear
mean electromotive force. Two types of nonlinearities (algebraic and dynamic)
are also discussed. The algebraic nonlinearity implies a nonlinear dependence
of the mean electromotive force on the mean magnetic field. The dynamic
nonlinearity is determined by a differential equation for the magnetic part of
the alpha-effect. It is shown that for the alpha-Omega axisymmetric dynamo the
algebraic nonlinearity alone cannot saturate the dynamo generated mean magnetic
field while the combined effect of the algebraic and dynamic nonlinearities
limits the mean magnetic field growth. Astrophysical applications of the
obtained results are discussed.",0206261v1
2020-01-13,Gate-tunable spin waves in antiferromagnetic atomic bilayers,"The emergence of two-dimensional (2D) layered magnetic materials has opened
an exciting playground for both fundamental studies of magnetism in 2D and
explorations of spinbased applications. Remarkable properties, including spin
filtering in magnetic tunnel junctions and gate control of magnetic states,
have recently been demonstrated in 2D magnetic materials. While these studies
focus on the static properties, dynamic magnetic properties such as excitation
and control of spin waves have remained elusive. Here we excite spin waves and
probe their dynamics in antiferromagnetic CrI3 bilayers by employing an
ultrafast optical pump/magneto-optical Kerr probe technique. We identify
sub-terahertz magnetic resonances under an in-plane magnetic field, from which
we determine the anisotropy and interlayer exchange fields and the spin damping
rates. We further show tuning of antiferromagnetic resonances by tens of
gigahertz through electrostatic gating. Our results shed light on magnetic
excitations and spin dynamics in 2D magnetic materials, and demonstrate their
unique potential for applications in ultrafast data storage and processing.",2001.04044v1
2020-09-02,Magnetized Decaying Turbulence in the Weakly Compressible Taylor-Green Vortex,"Magnetohydrodynamic turbulence affects both terrestrial and astrophysical
plasmas. The properties of magnetized turbulence must be better understood to
more accurately characterize these systems. This work presents ideal MHD
simulations of the compressible Taylor-Green vortex under a range of initial
sub-sonic Mach numbers and magnetic field strengths. We find that regardless of
the initial field strength, the magnetic energy becomes dominant over the
kinetic energy on all scales after at most several dynamical times. The
spectral indices of the kinetic and magnetic energy spectra become shallower
than $k^{-5/3}$ over time and generally fluctuate. Using a shell-to-shell
energy transfer analysis framework, we find that the magnetic fields facilitate
a significant amount of the energy flux and that the kinetic energy cascade is
suppressed. Moreover, we observe nonlocal energy transfer from the large scale
kinetic energy to intermediate and small scale magnetic energy via magnetic
tension. We conclude that even in intermittently or singularly driven weakly
magnetized systems, the dynamical effects of magnetic fields cannot be
neglected.",2009.01331v2
1995-05-03,Magnetic Field Effect in a Two-dimensional Array of Short Josephson Junctions,"We study analytically the effect of a constant magnetic field on the dynamics
of a two dimensional Josephson array. The magnetic field induces spatially
dependent states and coupling between rows, even in the absence of an external
load. Numerical simulations support these conclusions.",9504017v1
2017-06-13,The Dynamics of Magnetic Vortices in Type II Superconductors with Pinning Sites Studied by the Time Dependent Ginzburg-Landau Model,"We investigate the dynamics of magnetic vortices in type II superconductors
with normal state pinning sites using the Ginzburg-Landau equations. Simulation
results demonstrate hopping of vortices between pinning sites, influenced by
external magnetic fields and external currents. The system is highly nonlinear
and the vortices show complex nonlinear dynamical behaviour.",1706.04002v1
2018-12-18,Dynamical response of the Ising model to the amplitude modulated time dependent magnetic field,"The dynamical Ising model under the effect of the amplitude modulated time
dependent periodic magnetic field has been solved by using EFT with Glauber
type of stochastic process. Several cases with amplitude modulation have been
investigated. It has been shown that, amplitude modulation could display
dynamical phase transition on the magnetic system.",1812.07610v1
2009-05-28,Hydrodynamic theory of coupled current and magnetization dynamics in spin-textured ferromagnets,"We develop the hydrodynamical theory of collinear spin currents coupled to
magnetization dynamics in metallic ferromagnets. The collective spin density
couples to the spin current through a U(1) Berry-phase gauge field determined
by the local texture and dynamics of the magnetization. We determine
phenomenologically the dissipative corrections to the equation of motion for
the electronic current, which consist of a dissipative spin-motive force
generated by magnetization dynamics and a magnetic texture-dependent
resistivity tensor. The reciprocal dissipative, adiabatic spin torque on the
magnetic texture follows from the Onsager principle. We investigate the effects
of thermal fluctuations and find that electronic dynamics contribute to a
nonlocal Gilbert damping tensor in the Landau-Lifshitz-Gilbert equation for the
magnetization. Several simple examples, including magnetic vortices, helices,
and spirals, are analyzed in detail to demonstrate general principles.",0905.4544v2
2011-05-02,Magnetization Oscillation of a Spinor Condensate Induced by Magnetic Field Gradient,"We study the spin mixing dynamics of ultracold spin-1 atoms in a weak
non-uniform magnetic field with field gradient $G$, which can flip the spin
from +1 to -1 so that the magnetization $m=\rho_{+}-\rho_{-}$ is not any more a
constant. The dynamics of $m_F=0$ Zeeman component $\rho_{0}$, as well as the
system magnetization $m$, are illustrated for both ferromagnetic and polar
interaction cases in the mean-field theory. We find that the dynamics of system
magnetization can be tuned between the Josephson-like oscillation similar to
the case of double well, and the interesting self-trapping regimes, i.e. the
spin mixing dynamics sustains a spontaneous magnetization. Meanwhile the
dynamics of $\rho_0$ may be sufficiently suppressed for initially imbalanced
number distribution in the case of polar interaction. A ""beat-frequency""
oscillation of the magnetization emerges in the case of balanced initial
distribution for polar interaction, which vanishes for ferromagnetic
interaction.",1105.0280v1
2018-02-28,Roles of chiral renormalization on magnetization dynamics in chiral magnets,"In metallic ferromagnets, the interaction between local magnetic moments and
conduction electrons renormalizes parameters of the Landau-Lifshitz-Gilbert
equation such as the gyromagnetic ratio and the Gilbert damping, and makes them
dependent on the magnetic configurations. Although the effects of the
renormalization for nonchiral ferromagnets are usually minor and hardly
detectable, we show that the renormalization does play a crucial role for
chiral magnets. Here the renormalization is chiral and as such we predict
experimentally identifiable effects on the phenomenology of magnetization
dynamics. In particular, our theory for the self-consistent magnetization
dynamics of chiral magnets allows for a concise interpretation of domain wall
creep motion. We also argue that the conventional creep theory of the domain
wall motion, which assumes Markovian dynamics, needs critical reexamination
since the gyromagnetic ratio makes the motion non-Markovian. The non-Markovian
nature of the domain wall dynamics is experimentally checkable by the chirality
of the renormalization.",1803.00017v2
2018-06-12,"Dynamical and current-induced Dzyaloshinskii-Moriya interaction: Role for damping, gyromagnetism, and current-induced torques in noncollinear magnets","Both applied electric currents and magnetization dynamics modify the
Dzyaloshinskii-Moriya interaction (DMI), which we call current-induced DMI
(CIDMI) and dynamical DMI (DDMI), respectively. We report a theory of CIDMI and
DDMI. The inverse of CIDMI consists in charge pumping by a time-dependent
gradient of magnetization $\partial^2 M(r,t)/\partial r\partial t$, while the
inverse of DDMI describes the torque generated by $\partial^2 M(r,t)/\partial
r\partial t$. In noncollinear magnets CIDMI and DDMI depend on the local
magnetization direction. The resulting spatial gradients correspond to torques
that need to be included into the theories of Gilbert damping, gyromagnetism,
and current-induced torques (CITs) in order to satisfy the Onsager reciprocity
relations. CIDMI is related to the modification of orbital magnetism induced by
magnetization dynamics, which we call dynamical orbital magnetism (DOM), and
spatial gradients of DOM contribute to charge pumping. We present applications
of this formalism to the CITs and to the torque-torque correlation in textured
Rashba ferromagnets.",1806.04782v3
2020-11-10,Ultrafast high-harmonic nanoscopy of magnetization dynamics,"Light-induced magnetization changes, such as all-optical switching, skyrmion
nucleation, and intersite spin transfer, unfold on temporal and spatial scales
down to femtoseconds and nanometers, respectively. Pump-probe spectroscopy and
diffraction studies indicate that spatio-temporal dynamics may drastically
affect the non-equilibrium magnetic evolution. Yet, direct real-space magnetic
imaging on the relevant timescale has remained challenging. Here, we
demonstrate ultrafast high-harmonic nanoscopy employing circularly polarized
high-harmonic radiation for real-space imaging of femtosecond magnetization
dynamics. We observe the reversible and irreversible evolution of nanoscale
spin textures following femtosecond laser excitation. Specifically, we map
quenched magnetic domains and localized spin structures in Co/Pd multilayers
with a sub-wavelength spatial resolution down to 16 nm, and strobosocopically
trace the local magnetization dynamics with 40 fs temporal resolution. Our
approach enables the highest spatio-temporal resolution of magneto-optical
imaging to date. Facilitating ultrafast imaging with an extreme sensitivity to
various microscopic degrees of freedom expressed in chiral and linear
dichroism, we envisage a wide range of applications spanning magnetism, phase
transitions, and carrier dynamics.",2011.05450v1
2022-05-13,Precession dynamics of a small magnet with non-Markovian damping: Theoretical proposal for an experiment to determine the correlation time,"Recent advances in experimental techniques have made it possible to
manipulate and measure the magnetization dynamics on the femtosecond time scale
which is the same order as the correlation time of the bath degrees of freedom.
In the equations of motion of magnetization, the correlation of the bath is
represented by the non-Markovian damping. For development of the science and
technologies based on the ultrafast magnetization dynamics it is important to
understand how the magnetization dynamics depend on the correlation time. It is
also important to determine the correlation time experimentally. Here we study
the precession dynamics of a small magnet with the non-Markovian damping.
Extending the theoretical analysis of Miyazaki and Seki [J. Chem. Phys. 108,
7052 (1998)] we obtain analytical expressions of the precession angular
velocity and the effective damping constant for any values of the correlation
time under assumption of small Gilbert damping constant. We also propose a
possible experiment for determination of the correlation time.",2205.06399v1
2023-05-26,Magnetized dusty plasma: On issues of its complexity and magnetization of charged dust particles,"It is possible to excite various linear and non-linear low-frequency modes in
dusty plasma which is an admixture of electrons, ions, gas atoms, and
negatively charged solid particles. The experimental as well as theoretical
study of these low-frequency dynamical modes in dusty plasma is very complex
because of the involvement of dynamics of electrons, ions, and neutrals. If the
external magnetic field is introduced to dusty plasma then the dynamics of it
will be more complex. The complexity of magnetized dusty plasma where plasma
species are magnetized is discussed by keeping the experimental observations in
magnetized dusty plasma devices in mind. The requirement of theoretical
modeling, as well as computation experiments in understanding the dynamics of
dusty plasma in the presence of a strong magnetic field, is highlighted in the
context of experimental findings. The major challenges to magnetizing charged
massive particles in experiments and some expected solutions are discussed in
this report.",2305.16898v1
2006-09-15,Superconducting pipes and levitating magnets,"Motivated by a beautiful demonstration of the Faraday's and Lenz's law in
which a small neodymium magnet falls slowly through a conducting
non-ferromagnetic tube, we consider the dynamics of a magnet falling through a
superconducting pipe. Unlike the case of normal conducting pipes, in which the
magnet quickly reaches the terminal velocity, inside a superconducting tube the
magnet falls freely. On the other hand, to enter the pipe the magnet must
overcome a large electromagnetic energy barrier. For sufficiently strong
magnets, the barrier is so large that the magnet will not be able to penetrate
it and will be suspended over the front edge. We calculate the work that must
done to force the magnet to enter a superconducting tube. The calculations show
that superconducting pipes are very efficient at screening magnetic fields. For
example, the magnetic field of a dipole at the center of a short pipe of radius
$a$ and length $L \approx a$ decays, in the axial direction, with a
characteristic length $\xi \approx 0.26 a$. The efficient screening of the
magnetic field might be useful for shielding highly sensitive superconducting
quantum interference devices, SQUIDs. Finally, the motion of the magnet through
a superconducting pipe is compared and contrasted to the flow of ions through a
trans-membrane channel.",0609141v1
2008-05-26,Magnetic field induced incommensurate resonance in cuprate superconductors,"The influence of a uniform external magnetic field on the dynamical spin
response of cuprate superconductors in the superconducting state is studied
based on the kinetic energy driven superconducting mechanism. It is shown that
the magnetic scattering around low and intermediate energies is dramatically
changed with a modest external magnetic field. With increasing the external
magnetic field, although the incommensurate magnetic scattering from both low
and high energies is rather robust, the commensurate magnetic resonance
scattering peak is broadened. The part of the spin excitation dispersion seems
to be an hourglass-like dispersion, which breaks down at the heavily low energy
regime. The theory also predicts that the commensurate resonance scattering at
zero external magnetic field is induced into the incommensurate resonance
scattering by applying an external magnetic field large enough.",0805.3922v2
2012-06-29,Enhancement of critical current density in superconducting/magnetic multi-layers with slow magnetic relaxation dynamics and large magnetic susceptibility,"We propose to use superconductor-magnet multi-layer structure to achieve high
critical current density by invoking polaronic mechanism of pinning. The
magnetic layers should have large magnetic susceptibility to enhance the
coupling between vortices and magnetization in magnetic layers. The relaxation
of the magnetization should be slow. When the velocity of vortices is low, they
are dressed by nonuniform magnetization and move as polarons. In this case, the
viscosity of vortices proportional to the magnetic relaxation time is enhanced
significantly. As velocity increases, the polarons dissociate and the viscosity
drops to the usual Bardeen-Stephen one, resulting in a jump in the I-V curve.
Experimentally the jump shows up as a depinning transition and the
corresponding current at the jump is the depinning current. For Nb and proper
magnet multi-layer structure, we estimate the critical current density $J_c\sim
10^{9}\ \rm{A/m^2}$ at magnetic field $B\approx 1$ T.",1206.6929v2
2018-09-13,On the role of magnetic fields in star formation,"Magnetic fields are observed in star forming regions. However simulations of
the late stages of star formation that do not include magnetic fields provide a
good fit to the properties of young stars including the initial mass function
(IMF) and the multiplicity. We argue here that the simulations that do include
magnetic fields are unable to capture the correct physics, in particular the
high value of the magnetic Prandtl number, and the low value of the magnetic
diffusivity. The artificially high (numerical and uncontrolled) magnetic
diffusivity leads to a large magnetic flux pervading the star forming region.
We argue further that in reality the dynamics of high magnetic Prandtl number
turbulence may lead to local regions of magnetic energy dissipation through
reconnection, meaning that the regions of molecular clouds which are forming
stars might be essentially free of magnetic fields. Thus the simulations that
ignore magnetic fields on the scales on which the properties of stellar masses,
stellar multiplicities and planet-forming discs are determined, may be closer
to reality than those which include magnetic fields, but can only do so in an
unrealistic parameter regime.",1809.04921v1
2015-12-30,Magnetization dynamics and spin pumping induced by standing elastic waves,"The magnetization dynamics induced by standing elastic waves excited in a
thin ferromagnetic film is described with the aid of micromagnetic simulations
taking into account the magnetoelastic coupling between spins and lattice
strains. The simulations have been performed for the 2 nm thick Fe81Ga19 film
dynamically strained by longitudinal and transverse standing waves with various
frequencies, which span a wide range around the resonance frequency nu_res of
coherent magnetization precession in unstrained Fe81Ga19 film. It is found that
standing elastic waves give rise to complex local magnetization dynamics and
spatially inhomogeneous dynamic magnetic patterns. The spatio-temporal
distributions of the magnetization oscillations in standing elastic waves have
the form of standing spin waves with the same wavelength. Remarkably, the
amplitude of magnetization precession does not go to zero at the nodes of these
spin waves, which cannot be precisely described by simple analytical formulae.
In the steady-state regime, the magnetization oscillates with the frequency of
elastic wave, except for the case of longitudinal waves with frequencies well
below nu_res, where the magnetization precesses with a variable frequency
strongly exceeding the wave frequency. The precession amplitude at the
antinodes of standing spin waves strongly increases when the frequency of
elastic wave becomes close to nu_res. The results obtained for the
magnetization dynamics driven by elastic waves are used to calculate the spin
current pumped from the dynamically strained ferromagnet into adjacent
paramagnetic metal. Importantly, the transverse charge current created by the
spin current via the inverse spin Hall effect is high enough to be measured
experimentally.",1512.09051v1
2016-05-18,Standing magnetic wave on Ising ferromagnet: Nonequilibrium phase transition,"The dynamical response of an Ising ferromagnet to a plane polarised standing
magnetic field wave is modelled and studied here by Monte Carlo simulation in
two dimensions. The amplitude of standing magnetic wave is modulated along the
direction x. We have detected two main dynamical phases namely, pinned and
oscillating spin clusters. Depending on the value of field amplitude the system
is found to undergo a phase transition from oscillating spin cluster to pinned
as the system is cooled down. The time averaged magnetisation over a full cycle
of magnetic field oscillations is defined as the dynamic order parameter. The
transition is detected by studying the temperature dependences of the variance
of the dynamic order parameter, the derivative of the dynamic order parameter
and the dynamic specific heat. The dependence of the transition temperature on
the magnetic field amplitude and on the wavelength of the magnetic field wave
is studied at a single frequency. A comprehensive phase boundary is drawn in
the plane described by the temperature and field amplitude for two different
wavelengths of the magnetic wave. The variation of instantaneous line
magnetisation during a period of magnetic field oscillation for standing wave
mode is compared to those for the propagating wave mode. Also the probability
that a spin at any site, flips, is calculated. The above mentioned variations
and the probability of spin flip clearly distinguish between the dynamical
phases formed by propagating magnetic wave and by standing magnetic wave in an
Ising ferromagnet.",1605.05643v2
2022-02-15,The effect of inter-particle hydrodynamic and magnetic interactions in a magnetorheological fluid,"A magnetorheological fluid, which consists of magnetic particles suspended in
a viscous fluid, flows freely with well-dispersed particles in a the absence of
a magnetic field, but particle aggregation results in flow cessation when a
field is applied. The mechanism of dynamical arrest is examined by analysing
interactions between magnetic particles in a magnetic field subject to a shear
flow. An isolated spherical magnetic particle undergoes a transition between a
rotating state at low magnetic field and a static orientation at high magnetic
field. The effect of interactions for spherical dipolar and polarisable
particles with static orientation is examined for a dilute viscous suspension.
There are magnetic interactions due to the magnetic field disturbance at one
particle caused by the dipole moment of another, hydrodynamic interactions due
to the antisymmetric force moment of a non-rotating particle in a shear flow,
and a modification of the magnetic field due to the particle magnetic moment
density. When there is a concentration variation, the torque balance condition
results in a disturbance to the orientation of the particle magnetic moment.
The net force and the drift velocity due to these disturbances is calculated,
and the collective motion generated is equivalent to an anisotropic diffusion
process. When the magnetic field is in the flow plane, the diffusion
coefficients in the two directions perpendicular to the field direction are
negative, implying that concentration fluctuations are unstable in these
directions. This instability could initiate field-induced dynamical arrest in a
magnetorheological fluid.",2202.07488v1
2019-03-26,Dynamical evolution of magnetic fields in the intracluster medium,"We investigate the evolution of magnetic fields in galaxy clusters starting
from constant primordial fields using highly resolved ($\approx \rm 4 ~kpc$)
cosmological MHD simulations. The magnetic fields in our sample exhibit
amplification via a small-scale dynamo and compression during structure
formation. In particular, we study how the spectral properties of magnetic
fields are affected by mergers, and we relate the measured magnetic energy
spectra to the dynamical evolution of the intracluster medium. The magnetic
energy grows by a factor of $\sim$ 40-50 in a time-span of $\sim 9$ Gyr and
equipartition between kinetic and magnetic energy occurs on a range of scales
($< 160 \rm ~kpc$ at all epochs) depending on the turbulence state of the
system. We also find that, in general, the outer scale of the magnetic field
and the MHD scale are not simply correlated in time. The effect of major
mergers is to shift the peak magnetic spectra to it smaller scales, whereas the
magnetic amplification only starts after $\lesssim$ 1 Gyr. In contrast,
continuous minor mergers promote the steady growth of the magnetic field. We
discuss the implications of these findings in the interpretation of future
radio observations of galaxy clusters.",1903.11052v1
2020-11-20,An Investigation of Commercial Iron Oxide Nanoparticles: Advanced Structural and Magnetic Properties Characterization,"Magnetic nanoparticles (MNPs) have been extensively used as tiny heating
sources in magnetic hyperthermia therapy, contrast agents in magnetic resonance
imaging (MRI), tracers in magnetic particle imaging (MPI), carriers for
drug/gene delivery, etc. There have emerged many magnetic
nanoparticle/microbeads suppliers since the last decade, such as Ocean
NanoTech, Nanoprobes, US Research Nanomaterials, Miltenyi Biotec, micromod
Partikeltechnologie GmbH, and nanoComposix, etc. In this paper, we report the
physical and magnetic characterizations on iron oxide nanoparticle products
from Ocean NanoTech. Standard characterization tools such as Vibrating-Sample
Magnetometer (VSM), X-Ray Diffraction (XRD), Dynamic Light Scattering (DLS),
Transmission Electron Microscopy (TEM), and Zeta Potential Analyzer are used to
provide magnetic nanoparticle customers and researchers with an overview of
these iron oxide nanoparticle products. In addition, the dynamic magnetic
responses of these iron oxide nanoparticles in aqueous solutions are
investigated under low and high frequency alternating magnetic fields, giving a
standardized operating procedure for characterizing the MNPs from Ocean
NanoTech, thereby yielding the best of magnetic nanoparticles for different
applications.",2011.10215v1
2023-12-05,THz-Driven Coherent Magnetization Dynamics in a Labyrinth Domain State,"Terahertz (THz) light pulses can be used for an ultrafast coherent
manipulation of the magnetization. Driving the magnetization at THz frequencies
is currently the fastest way of writing magnetic information in ferromagnets.
Using time-resolved resonant magnetic scattering, we gain new insights to the
THz-driven coherent magnetization dynamics on nanometer length scales. We
observe ultrafast demagnetization and coherent magnetization oscillations that
are governed by a time-dependent damping. This damping is determined by the
interplay of lattice heating and magnetic anisotropy reduction revealing an
upper speed limit for THz-induced magnetization switching. We show that in the
presence of nanometer-sized magnetic domains, the ultrafast magnetization
oscillations are associated with a correlated beating of the domain walls. The
overall domain structure thereby remains largely unaffected which highlights
the applicability of THz-induced switching on the nanoscale.",2312.02654v1
2023-12-18,Enhancing thermal stability of optimal magnetization reversal in nanoparticles,"Energy-efficient switching of nanoscale magnets requires the application of a
time-varying magnetic field characterized by microwave frequency. At finite
temperatures, even weak thermal fluctuations create perturbations in the
magnetization that can accumulate in time, break the phase locking between the
magnetization and the applied field, and eventually compromise magnetization
switching. It is demonstrated here that the magnetization reversal is mostly
disturbed by unstable perturbations arising in a certain domain of the
configuration space of a nanomagnet. The instabilities can be suppressed and
the probability of magnetization switching enhanced by applying an additional
stimulus such as a weak longitudinal magnetic field that ensures bounded
dynamics of the perturbations. Application of the stabilizing longitudinal
field to a uniaxial nanomagnet makes it possible to reach a desired probability
of magnetization switching even at elevated temperatures. The principle of
suppressing instabilities provides a general approach to enhancing thermal
stability of magnetization dynamics.",2312.11293v1
1997-09-19,Zero-temperature dynamic transition in the random field Ising model: A Monte Carlo study,"The dynamics of a random (quenched) field Ising model (in two dimension) at
zero temperature in the presence of an additional sinusoidally oscillating
homogeneous (in space) magnetic field has been studied by Monte Carlo
simulation using the Metropolis single spin flip dynamics. The instantaneous
magnetisation is found to be periodic with the same periodicity of the
oscillating magnetic field. For very low values of amplitude of oscillating
field and the width of randomly quenched magnetic field, the magnetisation
oscillates asymmetrically about a nonzero value and the oscillation becomes
symmetric about a zero value for higher values of amplitude of oscillating
field and the width of the quenched disorder. The time averaged magnetisation
over a full cycle of the oscillating magnetic field defines the dynamic order
parameter. This dynamic order parameter is nonzero for very low values of
amplitude of oscillating magnetic field and the width of randomly quenched
field. A phase boundary line is drawn in the plane formed by the amplitude of
the oscillating magnetic field and the width of the randomly quenched magnetic
field. A tricritical point has been located, on the phase boundary line, which
separates the nature (discontinuous /continuous) of the dynamic transition.",9709217v1
2009-12-18,Stochastic resonance of a nanomagnet excited by spin transfer torque,"Spin transfer torque from spin-polarized electrical current can excite
large-amplitude magnetization dynamics in metallic ferromagnets of nanoscale
dimensions. Since magnetic anisotropy energies of nanomagnets are comparable to
the thermal energy scale, temperature can have a profound effect on the
dynamics of a nanomagnet driven by spin transfer torque. Here we report the
observation of unusual types of microwave-frequency nonlinear magnetization
dynamics co-excited by alternating spin transfer torque and thermal
fluctuations. In these dynamics, temperature amplifies the amplitude of
GHz-range precession of magnetization and enables excitation of highly
nonlinear dynamical states of magnetization by weak alternating spin transfer
torque. We explain these thermally activated dynamics in terms of non-adiabatic
stochastic resonance of magnetization driven by spin transfer torque. This type
of magnetic stochastic resonance may find use in sensitive nanometer-scale
microwave signal detectors.",0912.3842v1
2016-05-02,Electrical Detection of Magnetization Dynamics via Spin Rectification Effects,"The purpose of this article is to review the current status of a frontier in
dynamic spintronics and contemporary magnetism, in which much progress has been
made in the past decade, based on the creation of a variety of micro- and
nano-structured devices that enable electrical detection of magnetization
dynamics. The primary focus is on the physics of spin rectification effects,
which are well suited for studying magnetization dynamics and spin transport in
a variety of magnetic materials and spintronic devices. Intended to be
intelligible to a broad audience, the paper begins with a pedagogical
introduction, comparing the methods of electrical detection of charge and spin
dynamics in semiconductors and magnetic materials respectively. After that it
provides a comprehensive account of the theoretical study of both the angular
dependence and line shape of electrically detected ferromagnetic resonance
(FMR), which is summarized in a handbook formate easy to be used for analyzing
experimental data. We then review and examine the similarity and differences of
various spin rectification effects found in ferromagnetic films, magnetic
bilayers and magnetic tunnel junctions, including a discussion of how to
properly distinguish spin rectification from the spin pumping/inverse spin Hall
effect generated voltage. After this we review the broad applications of
rectification effects for studying spin waves, nonlinear dynamics, domain wall
dynamics, spin current, and microwave imaging. We also discuss spin
rectification in ferromagnetic semiconductors. The paper concludes with both
historical and future perspectives, by summarizing and comparing three
generations of FMR spectroscopy which have been developed for studying
magnetization dynamics.",1605.00710v2
2005-09-11,Orbital magnetic dynamics in chiral p-wave superconductors,"We present a theory of orbital magnetic dynamics for a chiral p-wave
superconductor with broken time-reversal symmetry. In contrast to the common
Landau-Lifshitz theory for spin ferromagnets, the case of orbital magnetism
cannot be described in terms of local magnetization density. Hence it is
impossible to define unambiguously the spontaneous magnetic moment: the latter
would depend on conditions of its experimental investigation.
  As an example of this we consider orbital magnetization waves and the domain
structure energy.",0509277v2
2008-05-15,Dynamical Casimir effect for magnons in a spinor Bose-Einstein condensate,"Magnon excitation in a spinor Bose-Einstein condensate by a driven magnetic
field is shown to have a close analogy with the dynamical Casimir effect. A
time-dependent external magnetic field amplifies quantum fluctuations in the
magnetic ground state of the condensate, leading to magnetization of the
system. The magnetization occurs in a direction perpendicular to the magnetic
field breaking the rotation symmetry. This phenomenon is numerically
demonstrated and the excited quantum field is shown to be squeezed.",0805.2210v1
2008-06-29,Dynamical Regimes Induced by Spin Transfer in Magnetic Nanopillars,"We demonstrate the predicted out-of-plane precession induced by spin transfer
in magnetic nanostructures with in-plane magnetic field. We show that other
magnetic excitations have a significant effect on the stability of the out-of
plane precession, making it extremely sensitive to the orientation of the
applied magnetic field. The data are supported with micromagnetic simulations.
Our results elucidate the relation between the excitation spectrum and the
specific dynamical behaviors of nanoscale magnets.",0806.4731v2
2009-11-14,Symmetry-breaking magnetization dynamics of spinor dipolar Bose-Einstein condensates,"Symmetry-breaking magnetization dynamics of a spin-1 Bose-Einstein condensate
(BEC) due to the dipole-dipole interaction are investigated using the
mean-field and Bogoliubov theories. When a magnetic field is applied along the
symmetry axis of a pancake-shaped BEC in the m = 0 hyperfine sublevel,
transverse magnetization develops breaking the chiral or axial symmetry. A
variety of magnetization patterns are formed depending on the strength of the
applied magnetic field. The proposed phenomena can be observed in 87Rb and 23Na
condensates.",0911.2741v1
2011-08-01,A non isothermal phase-field model for the ferromagnetic transition,"We propose a model for non isothermal ferromagnetic phase transition based on
a phase field approach, in which the phase parameter is related but not
identified with the magnetization. The magnetization is split in a paramagnetic
and in a ferromagnetic contribution, dependent on a scalar phase parameter and
identically null above the Curie temperature. The dynamics of the magnetization
below the Curie temperature is governed by the order parameter evolution
equation and by a Landau-Lifshitz type equation for the magnetization vector.
In the simple situation of a uniaxial magnet it is shown how the order
parameter dynamics reproduces the hysteresis effect of the magnetization.",1108.0378v1
2013-12-13,"Quantum states, symmetry and dynamics in degenerate spin s=1 magnets","The article deals with spin s=1 magnets. The symmetry conditions for normal
and degenerate equilibrium states are defined and types of magnetic ordering
found out. For each type of symmetry breaking the structure of source in the
Gibbs statistical operator has been obtained and additional thermodynamic
parameters introduced. The algebra of Poisson bracket for magnetic degrees of
freedom has been established and nonlinear dynamic equations have been derived.
Using the models of the exchange interaction, we have calculated the spectra of
collective excitations for two degenerate states whose order parameters have
digerent signature under the time reversal transformation.",1312.3799v2
2015-08-15,On an integrable magnetic geodesic flow on the two-torus,"We completely integrate the magnetic geodesic flow on a flat two-torus with
the magnetic field $F = \cos (x) dx \wedge dy$ and describe all contractible
periodic magnetic geodesics. It is shown that there are no such geodesics for
energy $E \geq 1/2$, for $E< 1/2$ simple periodic magnetic geodesics form two
$S^1$-families for which the (fixed energy) action functional is positive and
therefore there are no periodic magnetic geodesics for which the action
functional is negative.",1508.03745v2
2017-11-08,Charge pumping induced by magnetic texture dynamics in Weyl semimetals,"Spin-momentum locking in Weyl semimetals correlates the orbital motion of
electrons with background magnetic textures. We show here that the dynamics of
a magnetic texture in a magnetic Weyl semimetal induces a pumped electric
current that is free from Joule heating. This pumped current can be regarded as
a Hall current induced by axial electromagnetic fields equivalent to the
magnetic texture. Taking a magnetic domain wall as a test case, we demonstrate
that a moving domain wall generates a pumping current corresponding to the
localized charge.",1711.03135v1
2024-03-11,Magnetic vortex polarity reversal induced gyrotropic motion spectrum splitting in a ferromagnetic disk,"We investigate the gyrotropic motion of the magnetic vortex core in a chain
of a few micron-sized Permalloy disks by electrical resistance measurement with
amplitude-modulated magnetic field. We observe a distinctive splitting of the
resistance peak due to the resonant vortex-core motion under heightened radio
frequency (RF) magnetic field excitation. Our micromagnetic simulation
identifies the splitting of the resonant peak as an outcome of vortex polarity
reversal under substantial RF amplitudes. This study enhances our understanding
of nonlinear magnetic vortex dynamics amidst large RF amplitudes and proposes a
potential pathway for spintronic neural computing thanks to their unique and
controllable magnetization dynamics.",2403.06491v1
2003-08-15,Current-Driven Magnetization Dynamics in Magnetic Multilayers,"We develop a quantum analog of the classical spin-torque model for
current-driven magnetic dynamics. The current-driven magnetic excitation at
finite field becomes significantly incoherent. This excitation is described by
an effective magnetic temperature rather than a coherent precession as in the
spin-torque model. However, both the spin-torque and effective temperature
approximations give qualitatively similar switching diagrams in the
current-field coordinates, showing the need for detailed experiments to
establish the proper physical model for current-driven dynamics.",0308320v2
2004-03-03,Current-Induced Nanomagnet Dynamics for Magnetic Fields Perpendicular to the Sample Plane,"We present electrical measurements of high-frequency magnetic dynamics
excited by spin-polarized currents in Co/Cu/Ni80Fe20 nanopillar devices, with a
magnetic field applied perpendicular to the sample layers. As a function of
current and magnetic field, the dynamical phase diagram contains several
distinguishable precessional modes and also static magnetic states. Using
detailed comparisons with numerical simulations, we provide rigorous tests of
the theory of spin-transfer torques.",0403100v1
2004-07-07,Roles of non-equilibrium conduction electrons on magnetization dynamics of ferromagnets,"The mutual dependence of spin-dependent conduction and magnetization dynamics
of ferromagnets provides the key mechanisms in various spin-dependent
phenomena. We compute the response of the conduction electron spins to a
spatial and time varying magnetization ${\bf M} ({\bf r},t)$ within the
time-dependent semiclassical transport theory. We show that the induced
non-equilibrium conduction spin density in turn generates four spin torques
acting on the magnetization--with each torque playing different roles in
magnetization dynamics. By comparing with recent theoretical models, we find
that one of these torques that has not been previously identified is crucial to
consistently interpret experimental data on domain wall motion.",0407174v2
2005-02-08,Microwave spectroscopy on magnetization reversal dynamics of nanomagnets with electronic detection,"We demonstrate a detection method for microwave spectroscopy on magnetization
reversal dynamics of nanomagnets. Measurement of the nanomagnet anisotropic
magnetoresistance was used for probing how magnetization reversal is resonantly
enhanced by microwave magnetic fields. We used Co strips of 2 um x 130 nm x 40
nm, and microwave fields were applied via an on-chip coplanar wave guide. The
method was applied for demonstrating single domain-wall resonance, and studying
the role of resonant domain-wall dynamics in magnetization reversal.",0502197v2
2006-06-21,Current-excited magnetization dynamics in narrow ferromagnetic wires,"We investigate the current-excited magnetization dynamics in a narrow
ferromagnetic Permalloy wire by means of Lorentz microscopy, together with the
results of simultaneous transport measurements. A detailed structural evolution
of the magnetization is presented as a function of the applied current density.
Local structural deformation, bidirectional displacement, and magnetization
reversal are found below the Curie temperature with increasing the current
density. We discuss probable mechanisms of observed features of the
current-excited magnetization dynamics.",0606547v1
2008-06-21,Limiting Dynamics for Spherical Models of Spin Glasses with Magnetic Field,"We study the Langevin dynamics for the family of spherical spin glass models
of statistical physics, in the presence of a magnetic field. We prove that in
the limit of system size N approaching infinity, the empirical state
correlation, the response function, the overlap and the magnetization for these
N-dimensional coupled diffusions converge to the non-random unique strong
solution of four explicit non-linear integro-differential equations, that
generalize the system proposed by Cugliandolo and Kurchan in the presence of a
magnetic field. We then analyze the system and provide a rigorous derivation of
the FDT regime in a large area of the temperature-magnetization plane.",0806.3519v1
2009-01-28,Ultrafast Magnetization Dynamics in Diluted Magnetic Semiconductors,"We present a dynamical model that successfully explains the observed time
evolution of the magnetization in diluted magnetic semiconductor quantum wells
after weak laser excitation. Based on the pseudo-fermion formalism and a second
order many-particle expansion of the exact p-d exchange interaction, our
approach goes beyond the usual mean-field approximation. It includes both the
sub-picosecond demagnetization dynamics and the slower relaxation processes
which restore the initial ferromagnetic order in a nanosecond time scale. In
agreement with experimental results, our numerical simulations show that,
depending on the value of the initial lattice temperature, a subsequent
enhancement of the total magnetization may be observed within a time scale of
few hundreds of picoseconds.",0901.4541v1
2010-06-30,Supercurrent-Induced Magnetization Dynamics,"We investigate supercurrent-induced magnetization dynamics in a Josephson
junction with two misaligned ferromagnetic layers, and demonstrate a variety of
effects by solving numerically the Landau-Lifshitz-Gilbert equation. In
particular, we demonstrate the possibility to obtain supercurrent-induced
magnetization switching for an experimentally feasible set of parameters, and
clarify the favorable condition for the realization of magnetization reversal.
These results constitute a superconducting analogue to conventional
current-induced magnetization dynamics and indicate how spin-triplet
supercurrents may be utilized for practical purposes in spintronics.",1007.0004v1
2012-10-15,Optimal Control of Stochastic Magnetization Dynamics by Spin Current,"Fluctuation-induced stochastic magnetization dynamics plays an important role
in magnetic recording and writing. Here we propose that the magnetization
dynamics can be optimally controlled by the spin current to minimize or
maximize the Freidlin-Wentzell action functional of the system hence to
increase or decrease the happening probability of the rare event. We apply this
method to study thermal-driven magnetization switching problem and to
demonstrate the advantage of the optimal control strategy.",1210.4036v1
2014-03-26,Driving ferromagnets into a critical region of a magnetic phase diagram,"Exciting a ferromagnetic sample with an ultrashort laser pulse leads to a
quenching of the magne- tization on a subpicosecond timescale. On the basis of
the equilibration of intensive thermodynamic variables we establish a powerful
model to describe the demagnetization dynamics. We demonstrate that the
magnetization dynamics is mainly driven by the equilibration of chemical
potentials. The minimum of magnetization is revealed as a transient electronic
equilibrium state. Our method iden- tifies the slowing down of ultrafast
magnetization dynamics by a critical region within a magnetic phase diagram.",1403.6885v2
2015-06-18,Dynamical systems study in single-phase multiferroic materials,"Electric field induced magnetization switching in single-phase multiferroic
materials is intriguing for both fundamental studies and potential
technological applications. Here we develop a framework to study the switching
dynamics of coupled polarization and magnetization in such multiferroic
materials. With the coupling term between the polarization and magnetization as
an invariant dictated by the Dzyaloshinsky-Moriya vector, the dynamical systems
study reveals switching failures and oscillatory mode of magnetization if the
polarization and magnetization relax slowly during switching.",1506.07864v1
2018-05-10,Spatial dynamics analysis of polarized atom vapor,"We analyze the spatial dynamics of polarized atom vapor and present a
mathematical method to eliminate the diffusion effect partially. It is found
that the diffusion effect of polarized atoms can be regarded as a low pass
filter in spatial frequency domain and fits well with a Butterworth filter. The
fitted spatial filter can be used to restore the original magnetic image before
being blurred by the diffusion, thus improving the magnetic spatial resolution.
The results of spatial dynamics simulation and magnetic image restoration show
the potential usage of this method in magnetic gradiometer and atomic magnetic
microscopy.",1805.03823v1
2019-11-19,On the Dynamics of Inverse Magnetic Billiards,"Consider a strictly convex set $\Omega$ in the plane, and a homogeneous,
stationary magnetic field orthogonal to the plane whose strength is $B$ on the
complement of $\Omega$ and $0$ inside $\Omega$. The trajectories of a charged
particle in this setting are straight lines concatenated with circular arcs of
Larmor radius $\mu$. We examine the dynamics of such a particle and call this
inverse magnetic billiards. Comparisons are made to standard Birkhoff billiards
and magnetic billiards, as some theorems regarding inverse magnetic billiards
are consistent with each of these billiard variants while others are not.",1911.08144v1
2020-03-10,Spin dynamics of a hydrogen atom in a periodic magnetic structure,"The spin dynamics of a hydrogen atom during the passage of a periodic
magnetic structure is discussed. The occupation numbers of the components of
the hyperfine structure are considered as a function of time. The
characteristic low-frequency oscillations are visible, which have a direct
analogue in the effect of nuclear magnetic resonance. An envelope forms of
these oscillations are found using the Krylov-Bogolyubov-Mitropol'skii method.
The dependence of spin dynamics on the parameters of the magnetic structure is
investigated. It is shown that this dependence is very sensitive to the
structure of the magnetic field.",2003.04653v1
2021-01-15,Spin dynamics from a constrained magnetic Tight-Binding model,"A dynamics of the precession of coupled atomic moments in the tight-binding
(TB) approximation is presented. By implementing an angular penalty functional
in the energy that captures the magnetic effective fields self-consistently,
the motion of the orientation of the local magnetic moments is observed faster
than the variation of their magnitudes. This allows the computation of the
effective atomic magnetic fields that are found consistent with the
Heisenberg's exchange interaction, by comparison with classical atomistic spin
dynamics on Fe, Co and Ni magnetic clusters.",2101.06121v2
2023-08-11,Dynamical Majorana Ising spin response in a topological superconductor-magnet hybrid by microwave irradiation,"We study a dynamical spin response of surface Majorana modes in a topological
superconductor-magnet hybrid under microwave irradiation. We find a method to
toggle between dissipative and non-dissipative Majorana Ising spin dynamics by
adjusting the external magnetic field angle and the microwave frequency. This
reflects the topological nature of the Majorana modes, enhancing the Gilbert
damping of the magnet, thereby, providing a detection method for the Majorana
Ising spins. Our findings illuminate a magnetic probe for Majorana modes,
paving the path to innovative spin devices.",2308.05955v2
2014-01-08,Generating buoyant magnetic flux ropes in solar-like convective dynamos,"Our Sun exhibits strong convective dynamo action which results in magnetic
flux bundles emerging through the stellar surface as magnetic spots.
Global-scale dynamo action is believed to generate large-scale magnetic
structures in the deep solar interior through the interplay of convection,
rotation, and shear. Portions of these large-scale magnetic structures are then
believed to rise through the convective layer, forming magnetic loops which
then pierce the photosphere as sunspot pairs. Previous global simulations of 3D
MHD convection in rotating spherical shells have demonstrated mechanisms
whereby large-scale magnetic wreaths can be generated in the bulk of the
convection zone. Our recent simulations have achieved sufficiently high levels
of turbulence to permit portions of these wreaths to become magnetically
buoyant and rise through the simulated convective layer through a combination
of magnetic buoyancy and advection by convective giant cells. These buoyant
magnetic loops are created in the bulk of the convective layer as strong
Lorentz force feedback in the cores of the magnetic wreaths dampen small-scale
convective motions, permitting the amplification of local magnetic energies to
over 100 times the local kinetic energy. While the magnetic wreaths are largely
generated the shearing of axisymmetric poloidal magnetic fields by axisymmetric
rotational shear (the $\Omega$-effect), the loops are amplified to their peak
field strengths before beginning to rise by non-axisymmetric processes. This
further extends and enhances a new paradigm for the generation of emergent
magnetic flux bundles, which we term turbulence-enabled magnetic buoyancy.",1401.1826v1
2016-06-29,Dynamic Elastic Moduli in Magnetic Gels: Normal Modes and Linear Response,"In the perspective of developing smart hybrid materials with customized
features, ferrogels and magnetorheological elastomers allow a synergy of
elasticity and magnetism. The interplay between elastic and magnetic properties
gives rise to a unique reversible control of the material behavior by applying
an external magnetic field. Albeit few works have been performed on the
time-dependent properties so far, understanding the dynamic behavior is the key
to model many practical situations, e.g. applications as vibration absorbers.
Here we present a way to calculate the frequency-dependent elastic moduli based
on the decomposition of the linear response to an external stress in normal
modes. We use a minimal three-dimensional dipole-spring model to theoretically
describe the magnetic and elastic interactions on the mesoscopic level.
Specifically, the magnetic particles carry permanent magnetic dipole moments
and are spatially arranged in a prescribed way, before they are linked by
elastic springs. An external magnetic field aligns the magnetic moments. On the
one hand, we study regular lattice-like particle arrangements to compare with
previous results in the literature. On the other hand, we calculate the dynamic
elastic moduli for irregular, more realistic particle distributions. Our
approach measures the tunability of the linear dynamic response as a function
of the particle arrangement, the system orientation with respect to the
external magnetic field, as well as the magnitude of the magnetic interaction
between the particles. The strength of the present approach is that it
explicitly connects the relaxational modes of the system with the rheological
properties as well as with the internal rearrangement of the particles in the
sample, providing new insight into the dynamics of these remarkable materials.",1606.09104v2
2022-09-06,Temperature- and field angular-dependent helical spin period characterized by magnetic dynamics in a chiral helimagnet $MnNb_3S_6$,"The chiral magnets with topological spin textures provide a rare platform to
explore topology and magnetism for potential application implementation. Here,
we study the magnetic dynamics of several spin configurations on the monoaxial
chiral magnetic crystal $MnNb_3S_6$ via broadband ferromagnetic resonance (FMR)
technique at cryogenic temperature. In the high-field forced ferromagnetic
state (FFM) regime, the obtained frequency f vs. resonance field Hres
dispersion curve follows the well-known Kittel formula for a single FFM, while
in the low-field chiral magnetic soliton lattice (CSL) regime, the dependence
of Hres on magnetic field angle can be well-described by our modified Kittel
formula including the mixture of a helical spin segment and the FFM phase.
Furthermore, compared to the sophisticated Lorentz micrograph technique, the
observed magnetic dynamics corresponding to different spin configurations allow
us to obtain temperature- and field-dependent proportion of helical spin
texture and helical spin period ratio L(H)/L(0) via our modified Kittel
formula. Our results demonstrated that field- and temperature-dependent
nontrivial magnetic structures and corresponding distinct spin dynamics in
chiral magnets can be an alternative and efficient approach to uncovering and
controlling nontrivial topological magnetic dynamics.",2209.02266v1
2010-10-21,Nonequilibrium phase transition in the kinetic Ising model driven by propagating magnetic field wave,"The two dimensional ferromagnetic Ising model in the presence of a
propagating magnetic field wave (with well defined frequency and wavelength) is
studied by Mone Carlo simulation. This study differs from all of the earlier
studies done so far, where the oscillating magnetic field was considered to be
uniform in space. The time average magnetisation over a full cycle (the time
period) of the propagating magnetic field acts as the dynamic order parameter.
The dynamical phase transition is observed. The temperature variation of the
dynamic order parameter, the mean square deviation of the dynamic order
parameter, the dynamic specific heat and the derivative of the dynamic order
parameter are studied. The mean square deviation of the dynamic order
parameter, dynamic specific heat show sharp maxima near the transition point.
The derivative of dynamic order parameter shows sharp minimum near the
transition point. The transition temperature is found to depend also on the
speed of propagation of the magnetic field wave.",1010.4366v4
2007-01-29,Scaling laws of turbulent dynamos,"We consider magnetic fields generated by homogeneous isotropic and parity
invariant turbulent flows. We show that simple scaling laws for dynamo
threshold, magnetic energy and Ohmic dissipation can be obtained depending on
the value of the magnetic Prandtl number.",0701319v1
2010-09-09,Closed magnetic geodesics on closed hyperbolic Riemann surfaces,"We prove the existence of Alexandrov embedded closed magnetic geodesics on
closed hyperbolic surfaces. Closed magnetic geodesics correspond to closed
curves with prescribed geodesic curvature.",1009.1723v1
2019-04-01,Mechanism of Electron Spin Relaxation in Spiral Magnetic Structures,"Spin dynamics in spiral magnetic structures has been investigated. It has
been shown that the internal spatially dependent magnetic field in such
structures produces a new mechanism of spin relaxation.",1904.00564v1
2019-06-12,Dynamics of magnetic flux tubes in accretion discs of T Tauri stars,"Dynamics of slender magnetic flux tubes (MFT) in the accretion discs of T
Tauri stars is investigated. We perform simulations taking into account
buoyant, aerodynamic and turbulent drag forces, radiative heat exchange between
MFT and ambient gas, magnetic field of the disc. The equations of MFT dynamics
are solved using Runge-Kutta method of the fourth order. The simulations show
that there are two regimes of MFT motion in absence of external magnetic field.
In the region $r<0.2$ au, the MFT of radii $0.05 \leq a_0 \leq 0.16\,H$ ($H$ is
the scale height of the disc) with initial plasma beta of 1 experience thermal
oscillations above the disc. The oscillations decay over some time, and MFT
continue upward motion afterwards. Thinner or thicker MFT do not oscillate. MFT
velocity increases with initial radius and magnetic field strength. MFT rise
periodically with velocities up to 5-15 km s$^{-1}$ and periods of $0.5-10$ yr
determined by the toroidal magnetic field generation time. Approximately 20 %
of disc mass and magnetic flux can escape to disc atmosphere via the magnetic
buoyancy over characteristic time of disc evolution. MFT dispersal forms
expanding magnetized corona of the disc. External magnetic field causes MFT
oscillations near the disc surface. These magnetic oscillations have periods
from several days to 1-3 months at $r < 0.6$ au. The magnetic oscillations
decay over few periods. We simulate MFT dynamics in accretion discs in the
Chameleon I cluster. The simulations demonstrate that MFT oscillations can
produce observed IR-variability of T Tauri stars.",1906.05133v1
2019-08-23,Massively parallel atomistic simulation of ultrafast thermal spin dynamics of a permalloy vortex,"Ultrafast magnetization dynamics probes the most fundamental properties of
magnetic materials, exploring questions about the fundamental interactions
responsible for magnetic phenomena. Thermal effects are known to be extremely
important for laser-induced dynamics in metallic systems, but the dynamics of
topological magnetic structures are little understood. Here we apply a
massively parallel atomistic spin dynamics simulation to study the response of
a permalloy vortex to a 50 fs laser pulse. We find that macroscopically the
short timescale dynamics are indistinguishable from the bulk, but that strong
edge spin waves lead to a complex time evolution of the magnetic structure and
long-lived oscillations on the nanosecond timescale. In the near future such
simulations will provide unprecedented insight into the dynamics of magnetic
materials and devices beyond the approximations of continuum micromagnetics.",1908.08885v1
2020-11-14,Generic Theory of the Dynamic Magnetic Response of Ferrofluids,"Ferrofluids belong to an important class of highly functional soft matter,
benefiting from their magnetically controllable physical properties. Therefore,
it is of central importance to quantitatively predict the dynamic magnetic
response of ferrofluids. Traditional dynamic theories, however, are often
restricted to the near-equilibrium regime and/or only apply to nearly ideal
ferrofluids that are monodisperse, dilute enough, and weakly interacting. In
this paper I develop a self-consistent and nonperturbative dynamical mean field
theory for typical ferrofluids which are often polydisperse, concentrated, and
strongly interacting, possibly driven far from equilibrium. I obtain a general
nonperturbative expression for the dynamic magnetic susceptibility,
quantitatively agreeing with the spectra obtained from Brownian Dynamics
simulations on both mono- and bidisperse samples. Furthermore, I derive a
generic magnetization relaxation equation (MRE) for both mono- and polydisperse
ferrofluids by employing the projection operator technique in nonequlibrium
statistical mechanics. This MRE is in simple closed form and independent of
which model is employed to approximate the equilibrium magnetization curve.
Existing models can be recovered as low-order approximations of my generic and
nonperturbative MRE. My theory can play a key role in studying the dynamics of
ferrofluids and other polar fluids. It may also have substantial and immediate
consequences to various ferrofluid applications.",2011.07277v1
2021-03-12,Dynamically induced magnetism in KTaO$_3$,"Dynamical multiferroicity features entangled dynamic orders: fluctuating
electric dipoles induce magnetization. Hence, the material with paraelectric
fluctuations can develop magnetic signatures if dynamically driven. We identify
the paraelectric KTaO$_3$ (KTO) as a prime candidate for the observation of the
dynamical multiferroicity. We show that when a KTO sample is exposed to a
circularly polarized laser pulse, the dynamically induced ionic magnetic
moments are of the order of 5\% of the nuclear magneton per unit cell. We
determine the phonon spectrum using ab initio methods and identify T$_{1u}$ as
relevant soft phonon modes that couple to the external field and induce
magnetic polarization. We also predict a corresponding electron effect for the
dynamically induced magnetic moment which is enhanced by several orders of
magnitude due to the significant mass difference between electron and ionic
nucleus.",2103.07244v1
2009-09-28,Magnetic properties and spin dynamics in single molecule paramagnets Cu6Fe and Cu6Co,"The magnetic properties and the spin dynamics of two molecular magnets have
been investigated by magnetization and d.c. susceptibility measurements,
Electron Paramagnetic Resonance (EPR) and proton Nuclear Magnetic Resonance
(NMR) over a wide range of temperature (1.6-300K) at applied magnetic fields,
H=0.5 and 1.5 Tesla. The two molecular magnets consist of
CuII(saldmen)(H2O)}6{FeIII(CN)6}](ClO4)38H2O in short Cu6Fe and the analog
compound with cobalt, Cu6Co. It is found that in Cu6Fe whose magnetic core is
constituted by six Cu2+ ions and one Fe3+ ion all with s=1/2, a weak
ferromagnetic interaction between Cu2+ moments through the central Fe3+ ion
with J = 0.14 K is present, while in Cu6Co the Co3+ ion is diamagnetic and the
weak interaction is antiferromagnetic with J = -1.12 K. The NMR spectra show
the presence of non equivalent groups of protons with a measurable contact
hyperfine interaction consistent with a small admixture of s-wave function with
the d-function of the magnetic ion. The NMR relaxation results are explained in
terms of a single ion (Cu2+, Fe3+, Co3+) uncorrelated spin dynamics with an
almost temperature independent correlation time due to the weak magnetic
exchange interaction. We conclude that the two molecular magnets studied here
behave as single molecule paramagnets with a very weak intramolecular
interaction, almost of the order of the dipolar intermolecular interaction.
Thus they represent a new class of molecular magnets which differ from the
single molecule magnets investigated up to now, where the intramolecular
interaction is much larger than the intermolecular one.",0909.5063v1
2013-01-03,Coherent spin dynamics of nanomolecules and magnetic nanoclusters,"Spin dynamics of nanomolecules and nanoclusters are analyzed. The nanosizes
of these objects make it possible to consider them as single-domain magnets
with a large total spin, where the motion of the spins of all atoms, composing
a nanocluster, occurs in a coherent way. Another meaning of coherence in spin
dynamics is the coherent spin motion of several nanomolecules or nanoclusters.
Different approaches for treating spin dynamics are compared and the main
mechanisms influencing the spin motion are studied. Spin dynamics of separate
magnetic nanomolecules and nanoclusters are investigated, as well as the spin
dynamics of the ensembles of these nano-objects.",1301.0421v1
2005-03-31,Spectroscopy of Magnetic Excitations in Magnetic Superconductors Using Vortex Motion,"In magnetic superconductors a moving vortex lattice is accompanied by an ac
magnetic field which leads to the generation of spin waves. At resonance
conditions the dynamics of vortices in magnetic superconductors changes
drastically, resulting in strong peaks in the dc I-V characteristics at
voltages at which the washboard frequency of vortex lattice matches the spin
wave frequency $\omega_s({\bf g})$, where ${\bf g}$ are the reciprocal vortex
lattice vectors. We show that if washboard frequency lies above the magnetic
gap, peaks in the I-V characteristics in borocarbides and cuprate layered
magnetic superconductors are strong enough to be observed over the background
determined by the quasiparticles.",0504006v1
2005-08-26,Kinetic arrest of the first order ferromagnetic to antiferromagnetic transition in Ce(Fe$_{0.96}$Ru$_{0.04}$)$_2$ : formation of a magnetic-glass,"We present results of dc magnetization and magnetic relaxation study showing
the kinetic arrest of a first order ferromagnetic to antiferromagnetic
transition in Ce(Fe$_{0.96}$Ru$_{0.04}$)$_2$. This leads to the formation of a
non-ergodic glass-like magnetic state. The onset of the magnetic-glass
transformation is tracked through the slowing down of the magnetization
dynamics. This glassy state is formed with the assistance of an external
magnetic field and this is distinctly different from the well known
'spin-glass' state.",0508625v1
1995-09-27,Does Magnetic Charge Imply a Massive Photon ?,"In Abelian theories of monopoles the magnetic charge is required to be
enormous. Using the electric-magnetic duality of electromagnetism it is argued
that the existence of such a large, non-perturbative magnetic coupling should
lead to a phase transition where magnetic charge is permanently confined and
the photon becomes massive. The apparent masslessness of the photon could then
be used as an argument against the existence of such a large, non-perturbative
magnetic charge. Finally it is shown that even in the presence of this
dynamical mass generation the Cabbibo-Ferrari formulation of magnetic charge
gives a consistent theory.",9509157v1
2006-08-03,Magnetization of rotating ferrofluids: the effect of polydispersity,"The influence of polydispersity on the magnetization is analyzed in a
nonequilibrium situation where a cylindrical ferrofluid column is enforced to
rotate with constant frequency like a rigid body in a homogeneous magnetic
field that is applied perpendicular to the cylinder axis. Then, the
magnetization and the internal magnetic field are not longer parallel to each
other and their directions differ from that of the applied magnetic field.
Experimental results on the transverse magnetization component perpendicular to
the applied field are compared and analyzed as functions of rotation frequency
and field strength with different polydisperse Debye models that take into
account the polydispersity in different ways and to a varying degree.",0608033v1
2010-09-22,Direct Measurement of Effective Magnetic Diffusivity in Turbulent Flow of Liquid Sodium,"The first direct measurements of effective magnetic diffusivity in turbulent
flow of electro-conductive fluids (the so-called beta-effect) under magnetic
Reynolds number Rm >> 1 are reported. The measurements are performed in a
nonstationary turbulent flow of liquid sodium, generated in a closed toroidal
channel. The peak level of the Reynolds number reached Re \approx 3 10^6, which
corresponds to the magnetic Reynolds number Rm \approx 30. The magnetic
diffusivity of the liquid metal was determined by measuring the phase shift
between the induced and the applied magnetic fields. The maximal deviation of
magnetic diffusivity from its basic (laminar) value reaches about 50% .",1009.4333v1
2015-04-05,Magnetic states in multiply-connected flat nano-elements,"Flat magnetic nano-elements are an essential component of current and future
spintronic devices. By shaping an element it is possible to select and
stabilize chosen metastable magnetic states, control its magnetization
dynamics. Here, using a recent significant development in mathematics of
conformal mapping, complex variable based approach to the description of
magnetic states in planar nano-elements is extended to the case when elements
are multiply-connected (that is, contain holes or magnetic anti-dots). We show
that presence of holes implies a certain restriction on the set of magnetic
states of nano-element.",1504.01162v1
2018-03-19,Dynamics of a Magnetic Needle Magnetometer: Sensitivity to Landau-Lifshitz-Gilbert Damping,"An analysis of a single-domain magnetic needle in the presence of an external
magnetic field ${\bf B}$ is carried out with the aim of achieving a high
precision magnetometer. We determine the uncertainty $\Delta B$ of such a
device due to Gilbert dissipation and the associated internal magnetic field
fluctuations that gives rise to diffusion of the magnetic needle axis direction
${\bf n}$ and the needle orbital angular momentum. The levitation of the
magnetic needle in a magnetic trap and its stability are also analyzed.",1803.10064v2
2020-12-23,Magnetic Moment Tensor Potentials for collinear spin-polarized materials reproduce different magnetic states of bcc Fe,"We present the magnetic Moment Tensor Potentials (mMTPs), a class of
machine-learning interatomic potentials, accurately reproducing both
vibrational and magnetic degrees of freedom as provided, e.g., from
first-principles calculations. The accuracy is achieved by a two-step
minimization scheme that coarse-grains the atomic and the spin space. The
performance of the mMTPs is demonstrated for the prototype magnetic system bcc
iron, with applications to phonon calculations for different magnetic states,
and molecular dynamics simulations with fluctuating magnetic moments.",2012.12763v2
2022-06-21,Magnetic Forces in Paramagnetic Fluids,"An overview of the effect of a magnetic field gradient on fluids with linear
magnetic susceptibilities is given. It is shown that two commonly encountered
expressions, the magnetic field gradient force and the concentration gradient
force for paramagnetic species in solution are equivalent for incompressible
fluids. The magnetic field gradient and concentration gradient forces are
approximations of the Kelvin force and Korteweg-Helmholtz force densities,
respectively. The criterion for the appearance of magnetically induced
convection is derived. Experimental work in which magnetically induced
convection plays a role is reviewed.",2206.10529v4
2023-07-02,Unveiling Stable One-dimensional Magnetic Solitons in Magnetic Bilayers,"We propose a novel model which efficiently describes the magnetization
dynamics in a magnetic bilayer system. By applying a particular gauge
transformation to the Landau-Lifshitz-Gilbert (LLG) equation, we successfully
convert the model into an exactly integrable framework. Thus the obtained
analytical solutions allows us to predict a 1D magnetic soliton pair existed by
tunning the thickness of the spacing layer between the two ferrimagnetic
layers. The decoupling-unlocking-locking transition of soliton motion is
determined at various interaction intensitiy. Our results have implications for
the manipulation of magnetic solitons and the design of magnetic soliton-based
logic devices.",2307.00471v1
1997-12-11,Global three-dimensional simulations of magnetic field evolution in a galactic disk,"The evolution of three-dimensional, large-scale, magnetic fields, in a
galactic disk is investigated numerically. The N-body simulations of galactic
dynamics are incorporated into the kinematic calculations of induction
equations to study the influence of non-axisymmetric gas flows on the galactic
magnetic field. The time-dependent gas velocity fields are introduced as input
parameters for the MHD-simulations. Our principal concern is to check how
dynamical evolution of the galactic gas affects the global magnetic field
structure and intensity. We have found that the magnetic field responds
sensitively to changes in the gas velocity field, and even slight variations of
the dynamical parameters, such as the gas mass/total mass ratio results in
nonuniform intensity structures, i.e. magnetic arms. The magnetic lines of
force are well aligned with spiral arms and bar due to compressional flows in
these features. In the inter-arm regions the areas with magnetic vectors going
opposite to the main magnetic spirals are present.",9712148v1
1999-04-07,Magnetic field correlations in kinematic two-dimensional magnetohydrodynamic turbulence,"The scaling properties of the second order magnetic structure function
D_2^{(B)}(r) and the corresponding magnetic correlation function C_2^{(B)}(r)
are derived for two-dimensional magnetohydrodynamic turbulence in the kinematic
regime where the ratio of kinetic energy to magnetic energy is much larger than
one. In this regime the magnetic flux function $\psi$ can be treated as a
passive scalar advected in a two-dimensional turbulent flow. Its structure
function $D_2^{(\psi)}(r)$ and the one for the magnetic field D_2^{(B)}(r) are
connected by an exact relation. We calculate D_2^{(\psi)}(r) and thus
D_2^{(B)}(r) within geometric measure theory over a wide range of scales r and
magnetic Prandtl numbers Pr_m. The magnetic field correlations follow a
r^{-4/3} - scaling law and show an anticorrelation at the beginning of the
Batchelor regime indicative of the formation of strongly filamented current
sheets. Differences to the full dynamic regime, where the ratio of kinetic to
magnetic energies is smaller than in the kinematic case, are discussed.",9904014v2
1997-09-10,Dynamical symmetry breaking in the external gravitational and constant magnetic fields,"We investigate the effects of the external gravitational and constant
magnetic fields to the dynamical symmetrybreaking. As simple models of the
dynamical symmetry breaking we consider the Nambu-Jona-Lasinio (NJL) model and
the supersymmetric Nambu-Jona-Lasinio (SUSY NJL) model non-minimally
interacting with the external gravitational field and minimally interacting
with constant magnetic field. The explicit expressions for the scalar and
spinor Green functions are found up to the linear terms on the spacetime
curvature and exactly for a constant magnetic field. We obtain the effective
potential of the above models from the Green functions in the magnetic field in
curved spacetime. Calculating the effective potential numerically with the
varying curvature and/or magnetic fields we show the effects of the external
gravitational and magnetic fields to the phase structure of the theories. In
particular, increase of the curvature in the spontaneously broken chiral
symmetry phase due to the fixed magnetic field makes this phase to be less
broken. On the same time strong magnetic field quickly induces chiral symmetry
breaking even at the presence of fixed gravitational field within nonbroken
phase.",9709077v1
2013-11-25,Magnetic catalysis in flavored ABJM,"We study the magnetic catalysis of chiral symmetry breaking in the ABJM
Chern-Simons matter theory with unquenched flavors in the Veneziano limit. We
consider a magnetized D6-brane probe in the background of a flavored black hole
which includes the backreaction of massless smeared flavors in the ABJM
geometry. We find a holographic realization for the running of the quark mass
due to the dynamical flavors. We compute several thermodynamic quantities of
the brane probe and analyze the effects of the dynamical quarks on the
fundamental condensate and on the phase diagram of the model. The dynamical
flavors have an interesting effect on the magnetic catalysis. At zero
temperature and fixed magnetic field, the magnetic catalysis is suppressed for
small bare quark masses whereas it is enhanced for large values of the mass.
When the temperature is non-zero there is a critical magnetic field, above
which the magnetic catalysis takes place. This critical magnetic field
decreases with the number of flavors, which we interpret as an enhancement of
the catalysis.",1311.6265v3
2014-03-25,Simulations of magnetic nanoparticle Brownian motion,"Magnetic nanoparticles are useful in many medical applications because they
interact with biology on a cellular level thus allowing microenvironmental
investigation. An enhanced understanding of the dynamics of magnetic particles
may lead to advances in imaging directly in magnetic particle imaging (MPI) or
through enhanced MRI contrast and is essential for nanoparticle sensing as in
magnetic spectroscopy of Brownian motion (MSB). Moreover, therapeutic
techniques like hyperthermia require information about particle dynamics for
effective, safe, and reliable use in the clinic. To that end, we have developed
and validated a stochastic dynamical model of rotating Brownian nanoparticles
from a Langevin equation approach. With no field, the relaxation time toward
equilibrium matches Einstein's model of Brownian motion. In a static field, the
equilibrium magnetization agrees with the Langevin function. For high frequency
or low amplitude driving fields, behavior characteristic of the linearized
Debye approximation is reproduced. In a higher field regime where magnetic
saturation occurs, the magnetization and its harmonics compare well with the
effective field model. On another level, the model has been benchmarked against
experimental results, successfully demonstrating that harmonics of the
magnetization carry enough information to infer environmental parameters like
viscosity and temperature.",1403.6427v1
2015-01-28,Superconductor Dynamics,"Superconductors used in magnet technology could carry extreme currents
because of their ability to keep the magnetic flux motionless. The dynamics of
the magnetic flux interaction with superconductors is controlled by this
property. The cases of electrical transport in a round wire and the
magnetization of wires of various shapes (circular, elliptical, plate) in an
external magnetic field are analysed. Resistance to the magnetic field
penetration means that the field produced by the superconducting magnet is no
longer proportional to the supplied current. It also leads to a dissipation of
electromagnetic energy. In conductors with unequal transverse dimensions, such
as flat cables, the orientation with respect to the magnetic field plays an
essential role. A reduction of magnetization currents can be achieved by
splitting the core of a superconducting wire into fine filaments; however, new
kinds of electrical currents that couple the filaments consequently appear.
Basic formulas allowing qualitative analyses of various flux dynamic cases are
presented.",1501.07159v1
2015-05-26,Vacancy defects and monopole dynamics in oxygen-deficient pyrochlores,"The idea of magnetic monopoles in spin ice has enjoyed much success at
intermediate temperatures, but at low temperatures a description in terms of
monopole dynamics alone is insufficient. Recently, numerical simulations were
used to argue that magnetic impurities account for this discrepancy by
introducing a magnetic equivalent of residual resistance in the system. Here we
propose that oxygen deficiency is the leading cause of magnetic impurities in
as-grown samples, and we determine the defect structure and magnetism in
Y2Ti2O(7-\delta) using diffuse neutron scattering and magnetization
measurements. These defects are eliminated by oxygen annealing. The
introduction of oxygen vacancies causes Ti4+ to transform to magnetic Ti3+ with
quenched orbital magnetism, but the concentration is anomalously low. In the
spin-ice material Dy2Ti2O7 we find that the same oxygen-vacancy defects
suppress moments on neighbouring rare-earth sites, and that these magnetic
distortions dramatically slow down the long-time monopole dynamics at
sub-Kelvin temperatures.",1505.07035v1
2018-04-16,Triple Exponential Relaxation Dynamics in a Metallacrown-Based {$Dy^{III}Cu^{II}_5$} 3d-4f Single-Molecule Magnet,"The interplay of strong single-ion anisotropy and magnetic interactions often
give rise to novel magnetic behavior and can provide additional routes for
controlling magnetization dynamics. However, novel effects arising from
interactions between lanthanide and transition-metal ions are nowadays rarely
observed. Herein, a {$Dy^{III}Cu^{II}_5$} 3d-4f single-molecule magnet (SMM) is
constructed as a rigid and planar [15-MC-5] metallacrown (MC), where the
$Dy^{III}$ ion is trapped in the central pseudo-$D_{5h}$ pocket. A strong axial
crystal field (CF) imbues the $Dy^{III}$ ion with large Ising-type magnetic
anisotropy, and we are able to observe and model the magnetic interactions
between the $Cu^{II}-Cu^{II}$ and $Dy^{III}-Cu^{II}$ pairs. Butterfly-shaped
magnetic hysteresis shows clear steps at $\pm$0.4 T, coincident with level
crossings in our model exchange Hamiltonian between the {$Cu^{II}_5$} and
$Dy^{III}$ spin systems. Most intriguingly, this air-stable SMM exhibits three
distinct regimes in its magnetic relaxation dynamics, all clearly displaying an
exponential dependence on temperature.",1804.05818v1
2019-03-27,Real-space imaging and flux noise spectroscopy of magnetic dynamics in Ho$_2$Ti$_2$O$_7$,"Holmium titanate (Ho$_2$Ti$_2$O$_7$) is a rare earth pyrochlore and a
canonical example of a classical spin ice material. Despite the success of
magnetic monopole models, a full understanding of the energetics and relaxation
rates in this material has remained elusive, while recent studies have shown
that defects play a central role in the magnetic dynamics. We used a scanning
superconducting quantum interference device (SQUID) microscope to study the
spatial and temporal magnetic fluctuations in three regions with different
defect densities from a Ho$_2$Ti$_2$O$_7$ single crystal as a function of
temperature. We found that the magnetic flux noise power spectra are not
determined by simple thermally-activated behavior and observed evidence of
magnetic screening that is qualitatively consistent with Debye-like screening
due to a dilute gas of low-mobility magnetic monopoles. This work establishes
magnetic flux spectroscopy as a powerful tool for studying materials with
complex magnetic dynamics, including frustrated correlated spin systems.",1903.11465v1
2020-07-15,Magnetic Charge Propagation upon a 3D Artificial Spin-ice,"Magnetic charge propagation in bulk frustrated materials has yielded a
paradigm-shift in science, allowing the symmetry between electricity and
magnetism to be studied. Recent work is now suggesting magnetic charge dynamics
upon the spin-ice surface may have important implications in determining the
ordering and associated phase space. Here we detail a 3D artificial spin-ice, a
3D nanostructured array of magnetic islands which captures the exact geometry
of bulk systems, allowing field-driven dynamics of magnetic charge to be
directly visualized upon the surface. Using magnetic microscopy, we observe
vastly different magnetic charge dynamics along two principle directions. These
striking differences are found to be due to the surface-termination and
associated coordination which yields different energetics and interaction
strengths for magnetic charges upon the surface.",2007.07618v2
2021-09-28,Dynamical Switching of Magnetic Topology in Microwave-Driven Itinerant Magnet,"We theoretically demonstrate microwave-induced dynamical switching of
magnetic topology in centrosymmetric itinerant magnets by taking the
Kondo-lattice model on a triangular lattice, which is known to exhibit two
types of skyrmion lattices with different magnetic topological charges of |Nsk|
= 1 and 2. Our numerical simulations reveal that intense excitation of a
resonance mode with circularly polarized microwave field can switch the
magnetic topology, i.e., from the skyrmion lattice with |Nsk| = 1 to another
skyrmion lattice with |Nsk| = 2 or to a nontopological magnetic order with
|Nsk| = 0 depending on the microwave frequency. This magnetic-topology
switching shows various distinct behaviors, that is, deterministic irreversible
switching, probabilistic irreversible switching, and temporally random
fluctuation depending on the microwave frequency and the strength of the
external magnetic field, the variety of which is attributable to different
energy landscapes in the dynamical regime. The obtained results are also
discussed in the light of time-evolution equations based on an effective model
derived using perturbation expansions.",2109.13511v1
2022-06-24,All-optical non-linear chiral ultrafast magnetization dynamics driven by circularly polarized magnetic fields,"Ultrafast laser pulses provide unique tools to manipulate magnetization
dynamics at femtosecond timescales, where the interaction of the electric field
-- such as excitation of spin carriers to non-equilibrium states, generation of
localized charge currents, demagnetization, or inverse Faraday effect --
dominates over the magnetic field. Recent proposals using structured laser
beams have enlightened the possibility to generate intense femtosecond magnetic
fields, spatially isolated from the electric field. Here we demonstrate the
relevance of this novel scenario to femtomagnetism, unveiling the purely
precessional, non-linear, chiral response of the magnetization when subjected
to circularly polarized magnetic fields. This fundamental result not only opens
an avenue in the study of laser-induced ultrafast magnetization dynamics, but
also sustains technological implications as a route to promote all-optical
non-thermal magnetization switching both at shorter timescales -- towards the
attosecond regime -- and at THz frequencies.",2206.12238v2
1997-12-02,Rotational Dynamics of the Magnetic Particles in Ferrofluids,"A new theory for the dynamics of the magnetic particles and their magnetic
moments in ferrofluids is developed. Based on a generalized Lagrangian
formulation for the equations of motion of the colloidal particle, we introduce
its interaction with the solvent fluid via dissipative and random noise
torques, as well as the interactions between the particle and its magnetic
moment, treated as an independent physical entity and characterized by three
generalized coordinates, its two polar angles and its modulus. It has been
recognized recently that inertial effects, as well as the particle's rotational
Brownian motion, may play important roles on the dynamic susceptibility of a
class of magnetic fluids. No satisfactory theory existed, up to now, that takes
this effects into account. The theory presented here is a first-principles
3-dimensional approach, in contrast to some phenomenological 2-dimensional
approaches that can be found in the recent literature. It is appropriate for
superparamagnetic, non-superparamagnetic and mixed magnetic fluids. As a simple
application, the blocked limit (magnetic moment fixed in the particle) is
treated numerically. The rotational trajectory of the particles in presence of
a magnetic field, as well as the response functions and dynamic susceptibility
matrices are explicitly calculated for some values of the parameters",9712025v1
2007-10-12,Dynamics of Magnetized Spherical Accretion Flows,"Transonic accretion flow with self-consistent treatment of random magnetic
field is presented.",0710.2543v1
2009-07-17,Current-induced dynamics in non-collinear dual spin-valves,"Spin-transfer torque and current induced spin dynamics in spin-valve
nanopillars with the free magnetic layer located between two magnetic films of
fixed magnetic moments is considered theoretically. The spin-transfer torque in
the limit of diffusive spin transport is calculated as a function of magnetic
configuration. It is shown that non-collinear magnetic configuration of the
outermost magnetic layers has a strong influence on the spin torque and spin
dynamics of the central free layer. Employing macrospin simulations we make
some predictions on the free layer spin dynamics in spin valves composed of
various magnetic layers. We also present a formula for critical current in
non-collinear magnetic configurations, which shows that the magnitude of
critical current can be several times smaller than that in typical single spin
valves.",0907.3094v2
2012-09-06,Field-driven femtosecond magnetization dynamics induced by ultrastrong coupling to THz transients,"Controlling ultrafast magnetization dynamics by a femtosecond laser is
attracting interest both in fundamental science and industry because of the
potential to achieve magnetic domain switching at ever advanced speed. Here we
report experiments illustrating the ultrastrong and fully coherent light-matter
coupling of a high-field single-cycle THz transient to the magnetization vector
in a ferromagnetic thin film. We could visualize magnetization dynamics which
occur on a timescale of the THz laser cycle and two orders of magnitude faster
than the natural precession response of electrons to an external magnetic
field, given by the Larmor frequency. We show that for one particular
scattering geometry the strong coherent optical coupling can be described
within the framework of a renormalized Landau Lifshitz equation. In addition to
fundamentally new insights to ultrafast magnetization dynamics the coherent
interaction allows for retrieving the complex time-frequency magnetic
properties and points out new opportunities in data storage technology towards
significantly higher storage speed.",1209.1280v1
2016-02-10,The inverse thermal spin-orbit torque and the relation of the Dzyaloshinskii-Moriya interaction to ground-state energy currents,"Using the Kubo linear-response formalism we derive expressions to calculate
the heat current generated by magnetization dynamics in magnets with broken
inversion symmetry and spin-orbit interaction (SOI). The effect of producing
heat currents by magnetization dynamics constitutes the Onsager reciprocal of
the thermal spin-orbit torque (TSOT), i.e., the generation of torques on the
magnetization due to temperature gradients. We find that the energy current
driven by magnetization dynamics contains a contribution from the
Dzyaloshinskii-Moriya interaction (DMI), which needs to be subtracted from the
Kubo linear response of the energy current in order to extract the heat
current. We show that the expressions of the DMI coefficient can be derived
elegantly from the DMI energy current. Guided by formal analogies between the
Berry phase theory of DMI on the one hand and the modern theory of orbital
magnetization on the other hand we are led to an interpretation of the latter
in terms of energy currents as well. Based on \textit{ab-initio} calculations
we investigate the heat current driven by magnetization dynamics in Mn/W(001)
magnetic bilayers. We predict that fast domain walls drive strong ITSOT heat
currents.",1602.03319v1
2017-03-06,Ultra-low-energy Electric field-induced Magnetization Switching in Multiferroic Heterostructures,"Electric field-induced magnetization switching in multiferroics is intriguing
for both fundamental studies and potential technological applications. Here, we
review the recent developments on electric field-induced magnetization
switching in multiferroic heterostructures. Particularly, we study the dynamics
of magnetization switching between the two stable states in a shape-anisotropic
single-domain nanomagnet using stochastic Landau-Lifshitz-Gilbert (LLG)
equation in the presence of thermal fluctuations. For magnetostrictive
nanomagnets in strain-coupled multiferroic composites, such study of
magnetization dynamics, contrary to steady-state scenario, revealed intriguing
new phenomena on binary switching mechanism. While the traditional method of
binary switching requires to tilt the potential profile to the desired state of
switching, we show that no such tilting is necessary to switch successfully
since the magnetization's excursion out of magnet's plane can generate a
built-in asymmetry during switching. We also study the switching dynamics in
multiferroic heterostructures having magnetoelectric coupling at the interface
and magnetic exchange coupling that can facilitate to maintain the direction of
switching with the polarity of the applied electric field. We calculate the
performance metrics like switching delay and energy dissipation during
switching while simulating LLG dynamics. The performance metrics turn out to be
very encouraging for potential technological applications.",1706.03039v1
2017-12-01,Relativistic Dynamics of Point Magnetic Moment,"The covariant motion of a classical point particle with magnetic moment in
the presence of (external) electromagnetic fields is revisited. We are
interested in understanding Lorentz force extension involving point particle
magnetic moment (Stern-Gerlach force) and how the spin precession dynamics is
modified for consistency. We introduce spin as a classical particle property
inherent to Poincare\'e symmetry of space-time. We propose a covariant
formulation of the magnetic force based on a \lq magnetic\rq\ 4-potential and
show how the point particle magnetic moment relates to the Amperian (current
loop) and Gilbertian (magnetic monopole) description. We show that covariant
spin precession lacks a unique form and discuss connection to $g-2$ anomaly. We
consider variational action principle and find that a consistent extension of
Lorentz force to include magnetic spin force is not straightforward. We look at
non-covariant particle dynamics, and present a short introduction to dynamics
of (neutral) particles hit by a laser pulse of arbitrary shape.",1712.01825v1
2018-03-05,Generation and stability of dynamical skyrmions and droplet solitons,"A spin-polarized current in a nanocontact to a magnetic film can create
collective magnetic oscillations by compensating the magnetic damping. In
particular, in materials with uniaxial magnetic anisotropy, droplet solitons
have been observed a self-localized excitation consisting of partially reversed
magnetization that precesses coherently in the nanocontact region. It is also
possible to generate topological droplet solitons, known as \emph{dynamical
skyrmions}. Here we study the conditions that promote either droplet or
dynamical skyrmion formation and describe their stability in magnetic films
without Dzyaloshinskii-Moriya interactions. We show that Oersted fields from
the applied current as well as the initial magnetization state can determine
whether a droplet or dynamical skyrmion forms. Dynamical skyrmions are found to
be more stable than droplets. We also discuss electrical characteristics that
can be used distinguish these magnetic objects.",1803.01750v1
2022-11-28,Superfluid-like spin transport in the dynamic states of easy-axis magnets,"The existing proposals for superfluid-like spin transport have been based on
easy-plane magnets where the U(1) spin-rotational symmetry is spontaneously
broken in equilibrium, and this has been limiting material choices for
realizing superfluid-like spin transport to restricted class of magnets. In
this work, we lift this limitation by showing that superfluid-like spin
transport can also be realized based on easy-axis magnets, where the U(1)
spin-rotational symmetry is intact in equilibrium but can be broken in
non-equilibrium. Specifically, we find the condition to engender a
non-equilibrium easy-cone state by applying a spin torque to easy-axis magnets,
which dynamically induces the spontaneous breaking of the U(1) spin-rotational
symmetry and thereby can support superfluid-like spin transport. By exploiting
this dynamic easy-cone state, we show theoretically that superfluid-like spin
transport can be achieved in easy-axis magnets under suitable conditions and
confirmed the prediction by micromagnetic simulations. We envision that our
work broadens material library for realizing superfluid-like spin transport,
showing the potential utility of dynamic states of magnets as venue to look for
spin-transport phenomena that do not occur in static magnetic backgrounds.",2211.15091v1
2023-05-25,Crystallization dynamics of magnetic skyrmions in a frustrated itinerant magnet,"We investigate the phase ordering kinetics of skyrmion lattice (SkL) in a
metallic magnet. The SkL can be viewed as a superposition of magnetic stripes
whose periods are determined by the quasi-nesting wave vectors of the
underlying Fermi surface. An effective magnetic Hamiltonian that describes the
electron-mediated spin-spin interaction is obtained for a two-dimensional s-d
model with the Rashba spin-orbit coupling. Large-scale Landau-Lifshitz-Gilbert
dynamics simulations based on the effective spin Hamiltonian reveal a two-stage
phase ordering of the SkL phase after a thermal quench. The initial fast
crystallization of skyrmions is followed by a slow relaxation dominated by the
annihilation dynamics of dislocations, which are topological defects of the
constituent magnetic stripe orders. The late-stage phase ordering also exhibits
a dynamical scaling symmetry. We further show that the annihilation of
dislocations follows a power-law time dependence with a logarithmic correction
that depends on magnetic fields. Implications of our results for SkL phases in
magnetic materials are also discussed.",2305.16182v1
2019-10-17,Magnetic textures and dynamics in magnetic Weyl semimetals,"Recent theoretical and experimental attemps have been successful in finding
magnetic Weyl semimetal phases, which show both nodal-point structure in the
electronic bands and magnetic orders. Beyond uniform ferromagnetic or
antiferromagnetic orders, nonuniform magnetic textures, such as domain walls
and skyrmions, may even more enrich the properties of the Weyl electrons in
such materials. This article gives a topical review on interplay between Weyl
electrons and magnetic textures in those magnetic Weyl semimetals. The basics
of magnetic textures in non-topological magnetic metals are reviewed first, and
then the effect of magnetic textures in Weyl semimetals is discussed, regarding
the recent theoretical and experimental progress therein. The idea of the
fictitious ""axial gauge fields"" is pointed out, which effectively describes the
effect of magnetic textures on the Weyl electrons and can well account for the
properties of the electrons localized around magnetic domain walls.",1910.07774v1
2020-03-09,Magnetic Noise Enabled Biocompass,"The discovery of magnetic protein provides a new understanding of a
biocompass at the molecular level. However, the mechanism by which magnetic
protein enables a biocompass is still under debate, mainly because of the
absence of permanent magnetism in the magnetic protein at room temperature.
Here, based on a widely accepted radical pair model of a biocompass, we propose
a microscopic mechanism that allows the biocompass to operate without a finite
magnetization of the magnetic protein in a biological environment. With the
structure of the magnetic protein, we show that the magnetic fluctuation,
rather than the permanent magnetism, of the magnetic protein can enable
geomagnetic field sensing. An analysis of the quantum dynamics of our
microscopic model reveals the necessary conditions for optimal sensitivity. Our
work clarifies the mechanism by which magnetic protein enables a biocompass.",2003.13816v2
2020-04-25,Pulse-assisted magnetization switching in magnetic nanowires at picosecond and nanosecond timescales with low energy,"Detailed understanding of spin dynamics in magnetic nanomaterials is
necessary for developing ultrafast, low-energy and high-density spintronic
logic and memory. Here, we develop micromagnetic models and analytical
solutions to elucidate the effect of increasing damping and uniaxial anisotropy
on magnetic field pulse-assisted switching time, energy and field requirements
of nanowires with perpendicular magnetic anisotropy and yttrium iron
garnet-like spin transport properties. A nanowire is initially magnetized using
an external magnetic field pulse (write) and self-relaxation. Next, magnetic
moments exhibit deterministic switching upon receiving 2.5 ns-long external
magnetic pulses in both vertical polarities. Favorable damping
({\alpha}~0.1-0.5) and anisotropy energies (10^4-10^5 J m^-3) allow for as low
as picosecond magnetization switching times. Magnetization reversal with fields
below coercivity was observed using spin precession instabilities. A
competition or a nanomagnetic trilemma arises among the switching rate, energy
cost and external field required. Developing magnetic nanowires with optimized
damping and effective anisotropy could reduce the switching energy barrier down
to 3163kBT at room temperature. Thus, pulse-assisted picosecond and low energy
switching in nanomagnets could enable ultrafast nanomagnetic logic and cellular
automata.",2004.12243v1
2023-11-10,Exchange stiffness proportional to power of magnetization in permalloy co-doped with Mo and Cu,"The exchange stiffness of magnetic materials is one of the essential
parameters governing magnetic texture and its dynamics in magnetic devices. The
effect of single-element doping on exchange stiffness has been investigated for
several doping elements for permalloy (NiFe alloy), a soft magnetic material
whose soft magnetic properties can be controlled by doping. However, the impact
of more practical multi-element doping on the exchange stiffness of permalloy
is unknown. This study investigates the typical magnetic properties, including
exchange stiffness, of permalloy systematically co-doped with Mo and Cu using
broadband ferromagnetic resonance spectroscopy. We find that the exchange
stiffness, which decreases with increasing doping levels, is proportional to a
power of magnetization, which also decreases with increasing doping levels. The
magnetization, $M_{\rm s}$, dependence of the exchange stiffness constant, $A$,
of all the investigated samples, irrespective of the doping levels of each
element, lies on a single curve expressed as $A\propto M_{\rm s}^n$ with
exponent $n$ close to 2. This empirical power-law relationship provides a
guideline for predicting unknown exchange stiffness in non-magnetic
element-doped permalloy systems.",2311.05852v2
2011-03-23,Nonlinear Dynamics of Magnetic Islands Imbedded in Small-Scale Turbulence,"The nonlinear dynamics of magnetic tearing islands imbedded in a pressure
gradient driven turbulence is investigated numerically in a reduced
magnetohydrodynamic model. The study reveals regimes where the linear and
nonlinear phases of the tearing instability are controlled by the properties of
the pressure gradient. In these regimes, the interplay between the pressure and
the magnetic flux determines the dynamics of the saturated state. A secondary
instability can occur and strongly modify the magnetic island dynamics by
triggering a poloidal rotation. It is shown that the complex nonlinear
interaction between the islands and turbulence is nonlocal and involves small
scales.",1103.4443v1
2012-05-29,Magnetization dynamics at elevated temperatures,"By using the quantum kinetic approach with the instantaneous local
equilibrium approximation, we propose an equation that is capable of addressing
magnetization dynamics for a wide range of temperatures. The equation reduces
to the Landau-Lifshitz equation at low temperatures and to the paramagnetic
Bloch equation at high temperatures. Near the Curie temperature, the
magnetization reversal and dynamics depend on both transverse and longitudinal
relaxations. We further include the stochastic fields in the dynamic equation
in order to take into account fluctuation at high temperatures. Our proposed
equation may be broadly used for modeling laser pump-probe experiments and heat
assisted magnetic recording.",1205.6506v1
2013-09-14,Dynamics of the polarization of a pinned domain wall in a magnetic nanowire,"We consider the dynamics of polarization of a single domain wall in a
magnetic nanowire, which is strongly pinned by impurities. In this case the
equation of motion for the polarization parameter does not include any other
dynamical variables and is nonlinear due to magnetic anisotropy. We calculated
numerically the magnetization dynamics for different choices of parameters
under short current pulses inducing polarization switching. Our results show
that the switching is most effective for very rapid current pulses. Damping
also enhances the switching probability.",1309.3687v1
2014-08-27,On planar quantum dynamics of a magnetic dipole moment in the presence of electric and magnetic fields,"The planar quantum dynamics of a neutral particle with a magnetic dipole
moment in the presence of electric and magnetic fields is considered. The
criteria to establish the planar dynamics reveal that the resulting
nonrelativistic Hamiltonian has a simplified expression without making
approximations, and some terms have crucial importance for system dynamics.",1408.6578v2
2019-07-29,Magnetization Dynamics in Holographic Ferromagnets: Landau-Lifshitz Equation from Yang-Mills Fields,"We introduce a new approach to understand magnetization dynamics in
ferromagnets based on the holographic realization of ferromagnets. A
Landau-Lifshitz equation describing the magnetization dynamics is derived from
a Yang-Mills equation in the dual gravitational theory, and temperature
dependences of the spin-wave stiffness and spin transfer torque appearing in
the holographic Landau-Lifshitz equation are investigated by the holographic
approach. The results are consistent with the known properties of magnetization
dynamics in ferromagnets with conduction electrons.",1907.12306v1
2020-06-25,Novel dynamic critical phenomena induced by superfluidity and the chiral magnetic effect in Quantum Chromodynamics,"In this thesis, we study the novel dynamic critical phenomena induced by
superfluidity and the chiral magnetic effect in Quantum Chromodynamics (QCD),
respectively. In particular, we find the new dynamic universality class of the
high-density QCD critical point between the nuclear and quark superfluid phases
beyond the conventional Hohnenberg and Halperin's classification. We also
consider the second-order chiral phase transition under an external magnetic
field and find that the inclusion of the chiral magnetic effect changes the
dynamic universality class from the model E into model A within the
conventional classification.",2006.14127v1
2005-05-12,Dynamics of magnetization coupled to a thermal bath of elastic modes,"We study the dynamics of magnetization coupled to a thermal bath of elastic
modes using a system plus reservoir approach with realistic magnetoelastic
coupling. After integrating out the elastic modes we obtain a self-contained
equation for the dynamics of the magnetization.
  We find explicit expressions for the memory friction kernel and hence, {\em
via} the Fluctuation-Dissipation
  Theorem, for the spectral density of the magnetization thermal fluctuations.
  For magnetic samples in which the single domain approximation is valid, we
derive an equation for the dynamics of the uniform mode.
  Finally we apply this equation to study the dynamics of the uniform
magnetization mode in insulating ferromagnetic thin films.
  As experimental consequences we find that the fluctuation correlation time is
of the order of the ratio between the film thickness, $h$, and the speed of
sound in the magnet and that the line-width of the ferromagnetic resonance peak
should scale as $B_1^2h$ where $B_1$ is the magnetoelastic coupling constant.",0505331v3
2009-03-22,Quantum Spinodal Phenomena,"We study the dynamical magnetization process in the ordered ground-state
phase of the transverse Ising model under sweeps of magnetic field with
constant velocities. In the case of very slow sweeps and for small systems
studied previously (Phys. Rev. B 56, 11761 (1997)), non-adiabatic transitions
at avoided level-crossing points give the dominant contribution to the shape of
magnetization process. In contrast, in the ordered phase of this model and for
fast sweeps, we find significant, size-independent jumps in the magnetization
process. We study this phenomenon in analogy to the spinodal decomposition in
classical ordered state and investigate its properties and its dependence on
the system parameters. An attempt to understand the magnetization dynamics
under field sweep in terms of the energy-level structure is made. We discuss a
microscopic mechanism of magnetization dynamics from a viewpoint of local
cluster flips and show that this provides a picture that explains the size
independence. The magnetization dynamics in the fast-sweep regime is studied by
perturbation theory and we introduce a perturbation scheme based on interacting
Landau-Zener type processes to describe the local cluster flip dynamics.",0903.3739v1
2011-09-07,A theoretical analysis of inertia-like switching in magnets: applications to a synthetic antiferromagnet,"The magnetization dynamics of a synthetic antiferromagnet subject to a short
magnetic field pulse, has been studied by using a combination of
first-principles and atomistic spin dynamics simulations. We observe switching
phenomena on the time scale of tens of picoseconds, and inertia-like behavior
in the magnetization dynamics. We explain the latter in terms of a dynamic
redistribution of magnetic energy from the applied field pulse to other
possible energy terms, such as the exchange interaction and the magnetic
anisotropy, without invoking concepts such as inertia of an antiferromagnetic
vector. We also demonstrate that such dynamics can also be observed in a
ferromagnetic material where the incident field pulse pumps energy to the
magnetic anisotropy.",1109.1414v1
2018-06-04,Monte Carlo simulation of equilibrium and dynamic phase transition properties of an Ising bilayer,"Magnetic properties of an Ising bilayer system defined on a honeycomb lattice
with non-magnetic interlayers which interact via an indirect exchange coupling
have been investigated by Monte Carlo simulation technique. Equilibrium
properties of the system exhibit ferrimagnetism with $P$-, $N$- and $Q$- type
behaviors. Compensation phenomenon suddenly disappears with decreasing strength
of indirect ferrimagnetic interlayer exchange coupling. Qualitative properties
are in a good agreement with those obtained by effective field theory. In order
to investigate the stochastic dynamics of kinetic Ising bilayer, we have
introduced two different types of dynamic magnetic fields, namely a square
wave, and a sinusoidally oscillating magnetic field form. For both field types,
compensation point and critical temperature decrease with increasing amplitude
and field period. Dynamic ferromagnetic region in the presence of square wave
magnetic field is narrower than that obtained for sinusoidally oscillating
magnetic field when the amplitude and the field period are the same for each
type of dynamic magnetic fields.",1806.01002v2
2021-06-24,Modeling Magnetic Particle Imaging for Dynamic Tracer Distributions,"Magnetic Particle Imaging (MPI) is a promising tracer-based, functional
medical imaging technique which measures the non-linear magnetization response
of magnetic nanoparticles to a dynamic magnetic field. For image
reconstruction, system matrices from time-consuming calibration scans are used
predominantly. Finding modeled forward operators for magnetic particle imaging,
which are able to compete with measured matrices in practice, is an ongoing
topic of research. The existing models for magnetic particle imaging are by
design not suitable for arbitrary dynamic tracer concentrations. Neither
modeled nor measured system matrices account for changes in the concentration
during a single scanning cycle. In this paper we present a new MPI forward
model for dynamic concentrations. A static model will be introduced briefly,
followed by the changes due to the dynamic behavior of the tracer
concentration. Furthermore, the relevance of this new extended model is
examined by investigating the influence of the extension and example
reconstructions with the new and the standard model.",2106.13102v2
2022-11-14,Magnetization Dynamics in Synthetic Antiferromagnets with Perpendicular Magnetic Anisotropy,"Understanding the rich physics of magnetization dynamics in perpendicular
synthetic antiferromagnets (p-SAFs) is crucial for developing next-generation
spintronic devices. In this work, we systematically investigate the
magnetization dynamics in p-SAFs combining time-resolved magneto-optical Kerr
effect (TR-MOKE) measurements with theoretical modeling. These model analyses,
based on a Landau-Lifshitz-Gilbert approach incorporating exchange coupling,
provide details about the magnetization dynamic characteristics including the
amplitudes, directions, and phases of the precession of p-SAFs under varying
magnetic fields. These model-predicted characteristics are in excellent
quantitative agreement with TR-MOKE measurements on an asymmetric p-SAF. We
further reveal the damping mechanisms of two procession modes co-existing in
the p-SAF and successfully identify individual contributions from different
sources, including Gilbert damping of each ferromagnetic layer, spin pumping,
and inhomogeneous broadening. Such a comprehensive understanding of
magnetization dynamics in p-SAFs, obtained by integrating high-fidelity TR-MOKE
measurements and theoretical modeling, can guide the design of p-SAF-based
architectures for spintronic applications.",2211.07744v2
2003-03-15,Dynamical magnetic relaxation: A nonlinear magnetically driven dynamo,"A non-linear, time-dependent, magnetically driven dynamo theory which shows
how magnetically dominated configurations can relax to become helical on the
largest scale available is presented. Coupled time-dependent differential
equations for large scale magnetic helicity, small scale magnetic helicity,
velocity, and the electromotive force are solved. The magnetic helicity on
small scales relaxes to drive significant large scale helical field growth on
dynamical (Alfv\'en crossing) time scales, independent of the magnitude of
finite microphysical transport coefficients, after which the growing kinetic
helicity slows the growth to a viscously limited pace. This magnetically driven
dynamo complements the nonlinear kinetic helicity driven dynamo; for the
latter, the growing magnetic helicity fluctuations suppress, rather than drive,
large scale magnetic helicity growth. A unified set of equations accommodates
both types of dynamos.",0303354v2
2001-10-04,Dynamics of Magnetization Reversal in Models of Magnetic Nanoparticles and Ultrathin Films,"We discuss numerical and theoretical results for models of magnetization
switching in nanoparticles and ultrathin films. The models and computational
methods include kinetic Ising and classical Heisenberg models of highly
anisotropic magnets which are simulated by dynamic Monte Carlo methods, and
micromagnetics models of continuum-spin systems that are studied by
finite-temperature Langevin simulations. The theoretical analysis builds on the
fact that a magnetic particle or film that is magnetized in a direction
antiparallel to the applied field is in a metastable state. Nucleation theory
is therefore used to analyze magnetization reversal as the decay of this
metastable phase to equilibrium. We present numerical results on magnetization
reversal in models of nanoparticles and films, and on hysteresis in magnets
driven by oscillating external fields.",0110103v1
2008-10-01,Lattice disorder and Ferromagnetism in La0.67Ca0.33MnO3 nanoparticle,"We study the ferromagnetism of La0.67Ca0.33MnO3 in bulk polycrystalline,
nanocrystalline and amorphous phase. The structural change from crystalline
phase to amorphous phase exhibited a systematic decrease of TC(paramagnetic to
ferromagnetic transition temperature) and spontaneous magnetization (MS). The
experimental results suggested few more features, e.g., appearance of large
magnetic irreversibility in the temperature dependence of magnetization, lack
of magnetic saturation at high magnetic field, blocking of magnetization below
TB, and enhancement of coercivity. In addition, the magnetic phase transition
near to TC has changed from first order character in bulk sample to second
order character in nanocrystalline and amorphous samples. We understand the
observed magnetic features as the effects of decreasing particle size and
increasing magnetic (spin- lattice) disorder. We noted that magnetic dynamics
of amorphous samples is distinctly different from the nanocrystalline samples.
The ferromagnetism of amorphous samples are comparable with the properties of
reported amorphous ferromagnetic nanoparticles. We also demonstrate the effect
of disorder shell in controlling the dynamics of ferromagnetic cores.",0810.0090v1
2011-03-24,Spontaneous demagnetization of a dipolar spinor Bose gas at ultra-low magnetic field,"Quantum degenerate Bose gases with an internal degree of freedom, known as
spinor condensates, are natural candidates to study the interplay between
magnetism and superfluidity. In the spinor condensates made of alkali atoms
studied so far, the spinor properties are set by contact interactions, while
magnetization is dynamically frozen, due to small magnetic dipole-dipole
interactions. Here, we study the spinor properties of S=3 $^{52}$Cr atoms, in
which relatively strong dipole-dipole interactions allow changes in
magnetization. We observe a phase transition between a ferromagnetic phase and
an unpolarized phase when the magnetic field is quenched to an extremely low
value, below which interactions overwhelm the linear Zeeman effect. The BEC
magnetization changes due to magnetic dipole-dipole interactions that set the
dynamics. Our work opens up the experimental study of quantum magnetism with
free magnetization using ultra-cold atoms.",1103.4819v1
2012-05-25,Dynamics of the Chiral Magnetic Effect in a weak magnetic field,"We investigate the real-time dynamics of the chiral magnetic effect in
quantum electrodynamics (QED) and quantum chromodynamics (QCD). We consider a
field configuration of parallel (chromo)electric and (chromo)magnetic fields
with a weak perpendicular electromagnetic magnetic field. The chiral magnetic
effect induces an electromagnetic current along this perpendicular magnetic
field, which we will compute using linear response theory. We discuss specific
results for a homogeneous sudden switch-on and a pulsed (chromo)electric field
in a static and homogeneous (chromo)magnetic field. Our methodology can be
easily extended to more general situations. The results are useful for
investigating the chiral magnetic effect with heavy ion collisions and with
lasers that create strong electromagnetic fields. As a side result we obtain
the rate of chirality production for massive fermions in parallel electric and
magnetic fields that are static and homogeneous.",1205.5679v1
2013-06-05,Magnetic chains: From self-buckling to self-assembly,"Spherical neodymium-iron-boron magnets are perman-ent magnets that can be
assembled into a variety of structures due to their high magnetic strength. A
one-dimensional chain of these magnets responds to mechanical loadings in a
manner reminiscent of an elastic rod. We investigate the macroscopic mechanical
properties of assemblies of ferromagnetic spheres by considering chains, rings,
and chiral cylinders of magnets. Based on energy estimates and simple
experiments, we introduce an effective magnetic bending stiffness for a chain
of magnets and show that, used in conjunction with classic results for elastic
rods, it provides excellent estimates for the buckling and vibration dynamics
of magnetic chains. We then use this estimate to understand the dynamic
self-assembly of a cylinder from an initially straight chain of magnets.",1306.1014v2
2015-02-23,Torsion induced effects in magnetic nanowires,"Magnetic helix wire is one of the most simple magnetic systems which manifest
properties of both curvature and torsion. There exist two equilibrium states in
the helix wire with easy-tangential anisotropy: a quasi-tangential
magnetization distribution in case of relatively small curvatures and torsions,
and an onion state in opposite case. In the last case the magnetization is
close to tangential one, deviations are caused by the torsion and curvature.
Possible equilibrium magnetization states in the helix magnet with different
anisotropy directions are studied theoretically. The torsion also essentially
influences the spin-wave dynamics, acting as an effective magnetic field.
Originated from the curvature induced effective Dzyaloshinskii interaction,
this magnetic field leads to the coupling between the helix chirality and the
magnetochirality, it breaks mirror symmetry in spin-wave spectrum. All
analytical predictions on magnetization statics an dynamics are well confirmed
by the direct spin lattice simulations.",1502.06482v3
2017-02-15,Triad interactions and the bidirectional turbulent cascade of magnetic helicity,"Using direct numerical simulations we demonstrate that magnetic helicity
exhibits a bidirectional turbulent cascade at high but finite magnetic Reynolds
numbers. Despite the injection of positive magnetic helicity in the flow, we
observe that magnetic helicity of opposite signs is generated between large and
small scales. We explain these observations by carrying out an analysis of the
magnetohydrodynamic equations reduced to triad interactions using the Fourier
helical decomposition. Within this framework, the direct cascade of positive
magnetic helicity arises through triad interactions that are associated with
small scale dynamo action, while the occurrence of negative magnetic helicity
at large scales is explained through triad interactions that are related to
stretch-twist-fold dynamics and small scale dynamo action, which compete with
the inverse cascade of positive magnetic helicity. Our analytical and numerical
results suggest that the direct cascade of magnetic helicity is a finite
magnetic Reynolds number $Rm$ effect that will vanish in the limit $Rm \to
\infty$.",1702.04787v2
2017-04-12,Ultra-fast magnetization manipulation using single femtosecond light and hot-electrons pulse,"Current induced magnetization manipulation is a key issue for spintronic
application. Therefore, deterministic switching of the magnetization at the
picoseconds timescale with a single electronic pulse represents a major step
towards the future developments of ultrafast spintronic. Here, we have studied
the ultrafast magnetization dynamics in engineered Gdx[FeCo]1-x based structure
to compare the effect of femtosecond laser and hot-electrons pulses. We
demonstrate that a single femtosecond hot-electrons pulse allows a
deterministic magnetization reversal in either Gd-rich and FeCo-rich alloys
similarly to a femtosecond laser pulse. In addition, we show that the limiting
factor of such manipulation for perpendicular magnetized films arises from the
multi-domain formation due to dipolar interaction. By performing time resolved
measurements under various field, we demonstrate that the same magnetization
dynamics is observed for both light and hot-electrons excitation and that the
full magnetization reversal take place within 5 ps. The energy efficiency of
the ultra-fast current induced magnetization manipulation is optimized thanks
to the ballistic transport of hot-electrons before reaching the GdFeCo magnetic
layer.",1704.03749v1
2019-10-02,Magnetic Scattering Chapter,"The present chapter reviews current neutron and x-ray scattering techniques
employed to elucidate the magnetic structures and spin dynamics of magnetic
materials. Both techniques provide measurements as a function of the energy and
the momentum transferred from the spin system to the probe particles, in terms
of five-dimensional data sets as a function of various thermodynamic fields at
the control of the experimenter. These scattering techniques yield fundamental
information about the equal-time correlations such the magnetic configuration
and symmetry, as well as the dynamics that determine the exchange interactions
for prototypical systems that behave as linear, planar, or three-dimensional
systems. Historically, neutron scattering has been the magnetic scattering
technique of choice for such investigations, but the extraordinary advances in
resonant x-ray scattering techniques have enable new types of magnetic
scattering measurements. The type of information obtained with the two
techniques is largely complementary and depends on the interests of the
investigators. We discuss these possibilities and provide numerous examples of
the techniques applied to different classes of magnetic systems.",1910.01218v1
2022-08-02,Emergence of low temperature glassy dynamics in Ru substituted non magnetic insulator CaHfO3,"Non magnetic insulators/semiconductors with induced magnetism introduced via
transition metal substitution are one of the promising materials in the field
of spintronic, magnetoelectronics and magneto optical devices. In this context,
here, we focus on magnetism induced in a non magnetic insulator CaHfO3, by the
substitution of 4d element Ru, at Hf site. Structural investigations indicate
that substitution of Ru4+ (up to 50%) does not affect the original crystal
structure of the parent compound. Magnetic studies divulge a crossover from a
diamagnetic to paramagnetic state with 20% Ru substitution. Further replacement
of Hf results in a glassy magnetic state in CaHf1-xRuxO3 (0.3 < x < 0.5). The
nature of the low temperature glassiness (below 20 K) in these compositions is
confirmed through Vogel Fulcher and Power law, along with, magnetic memory
effect and relaxation dynamics. The observed glassiness is explained through
the phenomenological hierarchical model. Our studies indicate that the presence
of competing short range interactions among randomly arranged Ru cations in non
magnetic insulator CaHfO3 are responsible for the observed low temperature
magnetic state in this series with compositions > 0.25.",2208.01282v1
2022-11-02,A study of global magnetic helicity in self-consistent spherical dynamos,"Magnetic helicity is a fundamental constraint in both ideal and resistive
magnetohydrodynamics. Measurements of magnetic helicity density on the Sun and
other stars are used to interpret the internal behaviour of the dynamo
generating the global magnetic field. In this note, we study the behaviour of
the global relative magnetic helicity in three self-consistent spherical dynamo
solutions of increasing complexity. Magnetic helicity describes the global
linkage of the poloidal and toroidal magnetic fields (weighted by magnetic
flux), and our results indicate that there are preferred states of this
linkage. This leads us to propose that global magnetic reversals are, perhaps,
a means of preserving this linkage, since, when only one of the poloidal or
toroidal fields reverses, the preferred state of linkage is lost. It is shown
that magnetic helicity indicates the onset of reversals and that this signature
may be observed at the outer surface.",2211.01356v1
2020-05-19,Numerical Analysis of the Screening Current-Induced Magnetic Field in the HTS Insert Dipole Magnet Feather-M2.1-2,"Screening currents are field-induced dynamic phenomena which occur in
superconducting materials, leading to persistent magnetization. Such currents
are of importance in ReBCO tapes, where the large size of the superconducting
filaments gives rise to strong magnetization phenomena. In consequence,
superconducting accelerator magnets based on ReBCO tapes might experience a
relevant degradation of the magnetic field quality in the magnet aperture,
eventually leading to particle beam instabilities. Thus, persistent
magnetization phenomena need to be accurately evaluated. In this paper, the 2D
finite element model of the Feather-M2.1-2 magnet is presented. The model is
used to analyze the influence of the screening current-induced magnetic field
on the field quality in the magnet aperture. The model relies on a coupled
field formulation for eddy current problems in time-domain. The formulation is
introduced and verified against theoretical references. Then, the numerical
model of the Feather-M2.1-2 magnet is detailed, highlighting the key
assumptions and simplifications. The numerical results are discussed and
validated with available magnetic measurements. A satisfactory agreement is
found, showing the capability of the numerical tool in providing accurate
analysis of the dynamic behavior of the Feather-M2.1-2 magnet.",2005.09467v2
2014-07-08,Competing magnetic phases and field-induced dynamics in DyRuAsO,"Analysis of neutron diffraction, dc magnetization, ac magnetic
susceptibility, heat capacity, and electrical resistivity for DyRuAsO in an
applied magnetic field are presented at temperatures near and below those at
which the structural distortion (T_S = 25 K) and subsequent magnetic ordering
(T_N = 10.5 K) take place. Powder neutron diffraction is used to determine the
antiferromagnetic order of Dy moments of magnitude 7.6(1) mu_B in the absence
of a magnetic field, and demonstrate the reorientation of the moments into a
ferromagnetic configuration upon application of a magnetic field. Dy magnetism
is identified as the driving force for the structural distortion. The magnetic
structure of analogous TbRuAsO is also reported. Competition between the two
magnetically ordered states in DyRuAsO is found to produce unusual physical
properties in applied magnetic fields at low temperature. An additional phase
transition near T* = 3 K is observed in heat capacity and other properties in
fields greater than about 3 T. Magnetic fields of this magnitude also induce
spin-glass-like behavior including thermal and magnetic hysteresis, divergence
of zero-field-cooled and field-cooled magnetization, frequency dependent
anomalies in ac magnetic susceptibility, and slow relaxation of the
magnetization. This is remarkable since DyRuAsO is a stoichiometric material
with no disorder detected by neutron diffraction, and suggests analogies with
spin-ice compounds and related materials with strong geometric frustration.",1407.2184v1
2020-03-18,On the saturation mechanism of the fluctuation dynamo at ${\text{Pr}_\mathrm{M}} \ge 1$,"The presence of magnetic fields in many astrophysical objects is due to
dynamo action, whereby a part of the kinetic energy is converted into magnetic
energy. A turbulent dynamo that produces magnetic field structures on the same
scale as the turbulent flow is known as the fluctuation dynamo. We use
numerical simulations to explore the nonlinear, statistically steady state of
the fluctuation dynamo in driven turbulence. We demonstrate that as the
magnetic field growth saturates, its amplification and diffusion are both
affected by the back-reaction of the Lorentz force upon the flow. The
amplification of the magnetic field is reduced due to stronger alignment
between the velocity field, magnetic field, and electric current density.
Furthermore, we confirm that the amplification decreases due to a weaker
stretching of the magnetic field lines. The enhancement in diffusion relative
to the field line stretching is quantified by a decrease in the computed local
value of the magnetic Reynolds number. Using the Minkowski functionals, we
quantify the shape of the magnetic structures produced by the dynamo as
magnetic filaments and ribbons in both kinematic and saturated dynamos and
derive the scalings of the typical length, width, and thickness of the magnetic
structures with the magnetic Reynolds number. We show that all three of these
magnetic length scales increase as the dynamo saturates. The magnetic
intermittency, strong in the kinematic dynamo (where the magnetic field
strength grows exponentially) persists in the statistically steady state, but
intense magnetic filaments and ribbons are more volume-filling.",2003.07997v1
2002-08-20,Dynamics of QCD in a Strong Magnetic Field,"QCD in a strong magnetic field yields an example of a rich, sophisticated and
controllable dynamics.",0208180v1
2012-03-27,A Generic Property of Exact Magnetic Lagrangians,"We prove that for the set of Exact Magnetic Lagrangians the property ""There
exist finitely many static classes for every cohomology class"" is generic. We
also prove some dynamical consequences of this property.",1203.6095v1
2017-02-21,Numerical simulations of magnetic billiards in a convex domain in $\mathbb{R}^2$,"We present numerical simulations of magnetic billiards inside a convex domain
in the plane.",1702.06309v1
2003-06-03,Local Relaxation and Collective Stochastic Dynamics,"Damping and thermal fluctuations have been introduced to collective normal
modes of a magnetic system in recent modeling of dynamic thermal magnetization
processes. The connection between this collective stochastic dynamics and
physical local relaxation processes is investigated here. A system of two
coupled magnetic grains embedded in two separate oscillating thermal baths is
analyzed with no \QTR{it}{a priori} assumptions except that of a Markovian
process. It is shown explicitly that by eliminating the oscillating thermal
bath variables, collective stochastic dynamics occurs in the normal modes of
the magnetic system. The grain interactions cause local relaxation to be felt
by the collective system and the dynamic damping to reflect the system
symmetry. This form of stochastic dynamics is in contrast to a common
phenomenological approach where a thermal field is added independently to the
dynamic equations of each discretized cell or interacting grain. The dependence
of this collective stochastic dynamics on the coupling strength of the magnetic
grains and the relative local damping is discussed.",0306047v1
2016-11-09,Simultaneous imaging of strain waves and induced magnetization dynamics at the nanometer scale,"Changes in strain can be used to modify electronic and magnetic properties in
crystal structures, to manipulate nanoparticles and cells, or to control
chemical reactions. The magneto-elastic (ME) effect--the change of magnetic
properties caused by the elastic deformation (strain) of a magnetic
material--has been proposed as an alternative approach to magnetic fields for
the low power control of magnetization states of nanoelements since it avoids
charge currents, which entail ohmic losses. Multiferroic heterostructures
\cite{Zheng2004} and nanocomposites have exploited this effect in search of
electric control of magnetic states, mostly in the static regime. Quantitative
studies combining strain and magnetization dynamics are needed for practical
applications and so far, a high resolution technique for this has been lacking.
Here, we have studied the effect of the dynamic strain accompanying a surface
acoustic wave on magnetic nanostructures. We have simultaneously imaged the
temporal evolution of both strain waves and magnetization dynamics of
nanostructures at the picosecond timescale. The newly developed experimental
technique, based on X-ray microscopy, is versatile and provides a pathway to
the study of strain-induced effects at the nanoscale. Our results provide
fundamental insight in the coupling between strain and magnetization in
nanostructures at the picosecond timescale, having implications in the design
of strain-controlled magnetostrictive nano-devices.",1611.02847v1
2020-11-16,Magnetic Dynamic Polymers for Modular Assembling and Reconfigurable Morphing Architectures,"Shape morphing magnetic soft materials, composed of magnetic particles in a
soft polymer matrix, can transform shapes reversibly, remotely, and rapidly,
finding diverse applications in actuators, soft robotics, and biomedical
devices. To achieve on-demand and sophisticated shape morphing, the
manufacturing of structures with complex geometry and magnetization
distribution is highly desired. Here, we report a magnetic dynamic polymer
composite composed of hard-magnetic microparticles in a dynamic polymer network
with thermal-responsive reversible linkages, which permit functionalities
including targeted welding, magnetization reprogramming, and structural
reconfiguration. These functions not only provide highly desirable structural
and material programmability and reprogrammability but also enable the
manufacturing of structures with complex geometry and magnetization
distribution. The targeted welding is exploited for modular assembling of
fundamental building modules with specific logics for complex actuation. The
magnetization reprogramming enables altering the morphing mode of the
manufactured structures. The shape reconfiguration under magnetic actuation is
coupled with network plasticity to remotely transform two-dimensional
tessellations into complex three-dimensional architectures, providing a new
strategy of manufacturing functional soft architected materials such as
three-dimensional kirigami. We anticipate that the reported magnetic dynamic
polymer provides a new paradigm for the design and manufacturing of future
multifunctional assemblies and reconfigurable morphing architectures and
devices.",2011.07736v1
2024-02-08,Balancing a 3D Inverted Pendulum using Remote Magnetic Manipulation,"Remote magnetic manipulation offers wireless control over magnetic objects,
which has important medical applications, such as targeted drug delivery and
minimally invasive surgeries. Magnetic manipulation systems are categorized
into systems using permanent magnets and systems based on electromagnets.
Electro-Magnetic Navigation Systems (eMNSs) are believed to have a superior
actuation bandwidth, facilitating trajectory tracking and disturbance
rejection. This greatly expands the range of potential medical applications and
includes even dynamic environments as encountered in cardiovascular
interventions. In order to highlight the dynamic capabilities of eMNSs, we
successfully stabilize a (non-magnetic) inverted pendulum on the tip of a
magnetically driven arm. Our method employs a model-based design approach,
where we capture the dynamics that describe the interaction of the pendulum
system and the magnetic field through Lagrangian mechanics. Using system
identification we estimate the system parameters, the actuation bandwidth, and
characterize the system's nonlinearity. We design a state-feedback controller
to stabilize the inherently unstable dynamics, and compensate for errors
arising from the calibration of the magnetic field and the angle measurement
system. Additionally, we integrate an iterative learning control scheme that
allows us to accurately track non-equilibrium trajectories while concurrently
maintaining stability of the inverted pendulum. To our knowledge, this is the
first effort to stabilize a 3D inverted pendulum through remote magnetic
manipulation.",2402.06012v1
2013-01-14,Polarised Electromagnetic wave propagation through the ferromagnet: Phase boundary of dynamic phase transition,"The dynamical responses of ferromagnet to the propagating electromagnetic
field wave passing through it are modelled and studied here by Monte Carlo
simulation in two dimensional Ising ferromagnet. Here, the electromagnetic wave
is linearly polarised in such a way that the direction of magnetic field is
parallel to that of the magnetic momemts (spins). The coherent propagating mode
of spin-clusters is observed. The time average magnetisation over the full
cycle (time) of the field defines the order parameter of the dynamic
transition. Depending on the value of the temperature and the amplitude of the
propagating magnetic field wave, a dynamical phase transition is observed. The
dynamic transition was detected by studying the temperature dependences of the
dynamic order parameter, the variance of the dynamic order parameter, the
derivative of the dynamic order parameter and the dynamic specific heat. The
phase boundaries of the dynamic transitions were drawn for two different values
of the wave lengths of the propagating magnetic field wave. The phase boundary
was observed to shrink (inward) for lower speed of propagation of the EM wave.
The divergence of the releavant length scale was observed at the transition
point.",1301.3071v2
2013-08-19,Dynamical bar-mode instability in rotating and magnetized relativistic stars,"We present three-dimensional simulations of the dynamical bar-mode
instability in magnetized and differentially rotating stars in full general
relativity. Our focus is on the effects that magnetic fields have on the
dynamics and the onset of the instability. In particular, we perform
ideal-magnetohydrodynamics simulations of neutron stars that are known to be
either stable or unstable against the purely hydrodynamical instability, but to
which a poloidal magnetic field in the range of $10^{14}$--$10^{16}$ G is
superimposed initially. As expected, the differential rotation is responsible
for the shearing of the poloidal field and the consequent linear growth in time
of the toroidal magnetic field. The latter rapidly exceeds in strength the
original poloidal one, leading to a magnetic-field amplification in the the
stars. Weak initial magnetic fields, i.e. $ \lesssim 10^{15}$ G, have
negligible effects on the development of the dynamical bar-mode instability,
simply braking the stellar configuration via magnetic-field shearing, and over
a timescale for which we derived a simple algebraic expression. On the other
hand, strong magnetic fields, i.e. $\gtrsim 10^{16}$ G, can suppress the
instability completely, with the precise threshold being dependent also on the
amount of rotation. As a result, it is unlikely that very highly magnetized
neutron stars can be considered as sources of gravitational waves via the
dynamical bar-mode instability.",1308.3989v1
2023-04-19,Real-time observation of magnetization and magnon dynamics in a two-dimensional topological antiferromagnet MnBi2Te4,"Atomically thin van der Waals magnetic materials have not only provided a
fertile playground to explore basic physics in the two-dimensional (2D) limit
but also created vast opportunities for novel ultrafast functional devices.
Here we systematically investigate ultrafast magnetization dynamics and spin
wave dynamics in few-layer topological antiferromagnetic MnBi2Te4 crystals as a
function of layer number, temperature, and magnetic field. We find
laser-induced (de)magnetization processes can be used to accurately track the
distinct magnetic states in different magnetic field regimes, including showing
clear odd-even layer number effects. In addition, strongly field-dependent
antiferromagnetic magnon modes with tens of gigahertz frequencies are optically
generated and directly observed in the time domain. Remarkably, we find that
magnetization and magnon dynamics can be observed in not only the time-resolved
magneto-optical Kerr effect but also the time resolved reflectivity, indicating
strong correlation between the magnetic state and electronic structure. These
measurements present the first comprehensive overview of ultrafast spin
dynamics in this novel 2D antiferromagnet, paving the way for potential
applications in 2D antiferromagnetic spintronics and magnonics as well as
further studies of ultrafast control of both magnetization and topological
quantum states.",2304.09390v2
2009-06-09,Unified nonequilibrium dynamical theory for exchange bias and training effects,"We investigate the exchange bias and training effects in the FM/AF
heterostructures using a unified Monte Carlo dynamical approach. This real
dynamical method has been proved reliable and effective in simulating dynamical
magnetization of nanoscale magnetic systems. The magnetization of the
uncompensated AF layer is still open after the first field cycling is finished.
Our simulated results show obvious shift of hysteresis loops (exchange bias)
and cycling dependence of exchange bias (training effect) when the temperature
is below 45 K. The exchange bias fields decrease with decreasing the cooling
rate or increasing the temperature and the number of the field cycling. With
the simulations, we show the exchange bias can be manipulated by controlling
the cooling rate, the distributive width of the anisotropy energy, or the
magnetic coupling constants. Essentially, these two effects can be explained on
the basis of the microscopical coexistence of both reversible and irreversible
moment reversals of the AF domains. Our simulated results are useful to really
understand the magnetization dynamics of such magnetic heterostructures. This
unified nonequilibrium dynamical method should be applicable to other exchange
bias systems.",0906.1753v1
2017-10-26,Dynamical spin accumulation in large-spin magnetic molecules,"The frequency-dependent transport through a nano-device containing a
large-spin magnetic molecule is studied theoretically in the Kondo regime.
Specifically, the effect of magnetic anisotropy on dynamical spin accumulation
is of primary interest. Such accumulation arises due to finite off-diagonal in
spin components of the dynamical conductance. Here, employing the Kubo
formalism and the numerical renormalization group (NRG) method, we demonstrate
that the dynamical transport properties strongly depend on magnetic
configuration of the device and intrinsic parameters of the molecule.
Specifically, the effect of dynamical spin accumulation is found to be greatly
affected by the type of magnetic anisotropy exhibited by the molecule, and it
develops for frequencies corresponding to the Kondo temperature. For the
parallel magnetic configuration of the device, the presence of dynamical spin
accumulation is conditioned by the interplay of ferromagnetic-lead-induced
exchange field and the Kondo correlations.",1710.09623v1
2019-03-20,Nonlinear magnetization dynamics driven by strong terahertz fields,"We present a comprehensive experimental and numerical study of magnetization
dynamics triggered in a thin metallic film by single-cycle terahertz pulses of
$\sim20$ MV/m electric field amplitude and $\sim1$ ps duration. The
experimental dynamics is probed using the femtosecond magneto-optical Kerr
effect (MOKE), and it is reproduced numerically using macrospin simulations.
The magnetization dynamics can be decomposed in three distinct processes: a
coherent precession of the magnetization around the terahertz magnetic field,
an ultrafast demagnetization that suddenly changes the anisotropy of the film,
and a uniform precession around the equilibrium effective field that is relaxed
on the nanosecond time scale, consistent with a Gilbert damping process.
Macrospin simulations quantitatively reproduce the observed dynamics, and allow
us to predict that novel nonlinear magnetization dynamics regimes can be
attained with existing table-top terahertz sources.",1903.08395v2
2023-03-29,General-relativistic simulations of the formation of a magnetized hybrid star,"Strongly magnetized neutron stars are popular candidates for producing
detectable electromagnetic and gravitational-wave signals. A rapid density
increase in a neutron star core could also trigger the phase transition from
hadrons to deconfined quarks and form a hybrid star. This formation process
could release a considerable amount of energy in the form of gravitational
waves and neutrinos. Hence, the formation of a magnetized hybrid star is an
interesting scenario for detecting all these signals. These detections may
provide essential probes for the magnetic field and composition of such stars.
Thus far, a dynamical study of the formation of a magnetized hybrid star has
yet to be realized. Here, we investigate the formation dynamics and the
properties of a magnetized hybrid star through dynamical simulations. We find
that the maximum values of rest-mass density and magnetic field strength
increase slightly and these two quantities are coupled in phase during the
formation. We then demonstrate that all microscopic and macroscopic quantities
of the resulting hybrid star vary dramatically when the maximum magnetic field
strength goes beyond a threshold of $\sim 5 \times 10^{17}$ G but they are
insensitive to the magnetic field below this threshold. Specifically, the
magnetic deformation makes the rest-mass density drop significantly,
suppressing the matter fraction in the mixed phase. Therefore, this work
provides a solid support for the magnetic effects on a hybrid star, so it is
possible to link observational signals from the star to its magnetic field
configuration.",2303.16820v1
2022-07-14,Attosecond magnetization dynamics in non-magnetic materials driven by intense femtosecond lasers,"Irradiating solids with ultrashort laser pulses is known to initiate
femtosecond timescale magnetization dynamics. However, sub-femtosecond spin
dynamics have not yet been observed or predicted. Here, we explore ultrafast
light-driven spin dynamics in a highly non-resonant strong-field regime.
Through state-of-the-art ab-initio calculations, we predict that a non-magnetic
material can be transiently transformed into a magnetic one via dynamical
extremely nonlinear spin-flipping processes, which occur on attosecond
timescales and are mediated by a combination of multi-photon and spin-orbit
interactions. These are non-perturbative non-resonant analogues to the inverse
Faraday effect that build up from cycle-to-cycle as electrons gain angular
momentum. Remarkably, we show that even for linearly polarized driving, where
one does not intuitively expect any magnetic response, the magnetization
transiently oscillates as the system interacts with light. This oscillating
response is enabled by transverse anomalous light-driven currents in the solid,
and typically occurs on timescales of ~500 attoseconds. We further demonstrate
that the speed of magnetization can be controlled by tuning the laser
wavelength and intensity. An experimental set-up capable of measuring these
dynamics through pump-probe transient absorption spectroscopy is outlined and
simulated. Our results pave the way for new regimes of ultrafast manipulation
of magnetism.",2207.06711v1
2002-04-08,Determining the Saddle Point in Micromagnetic Models of Magnetization Switching,"A numerical model of single-domain nanoscale iron magnets fabricated using
scanning-microscope-assisted chemical vapor deposition is simulated using
finite-temperature micromagnetics. A Projective-dynamics method is used to
determine the magnetization at the saddle point as a function of temperature.
This magnetization is found to decrease linearly as the temperature is raised.",0204180v1
2004-01-08,On Effective Hamiltonians for Adiabatic Perturbations of Magnetic Schrodinger Operators,"We construct almost invariant subspaces and the corresponding effective
Hamiltonian for magnetic Bloch bands. We also discuss the question of the
dynamics related to the effective Hamiltonian. We assume that the magnetic and
electric potentials are slowly varying perturbations of the potential of a
constant magnetic field and a periodic lattice potential, respectively.",0401017v1
2003-10-01,Compressible dynamics of magnetic field lines for incompressible MHD flows,"It is demonstrated that the deformation of magnetic field lines in
incompressible magnetohydrodynamic flows results from a compressible mapping.
Appearance of zeroes for the mapping Jacobian correspond to the breaking of
magnetic field lines, associated with local blowup of the magnetic field. The
possibility of such events is found to be unlikely in two dimensions but not in
three dimensions.",0310006v1
2009-04-21,Chaotic magnetic field reversals in turbulent dynamos,"We present direct numerical simulations of reversals of the magnetic field
generated by swirling flows in a spherical domain. In agreement with a recent
model, we observe that coupling dipolar and quadrupolar magnetic modes by an
asymmetric forcing of the flow generates field reversals. In addition, we show
that this mechanism strongly depends on the value of the magnetic Prandtl
number.",0904.3343v1
2011-11-22,On magnetic leaf-wise intersections,"In this article we introduce the notion of a magnetic leaf-wise intersection
point which is a generalization of the leaf-wise intersection point with
magnetic effects. We also prove the existence of magnetic leaf-wise
intersection points under certain topological assumptions.",1111.5115v2
2014-11-27,The critical relaxation of the model of iron-vanadium magnetic superlattice,"The critical relaxation of iron-vanadium magnetic superlattice in case of the
equality between interlayer and intralayer exchange interactions is
investigated. The dynamic and static critical exponents of the model are
calculated. A value of the critical temperature is evaluated.",1411.7505v1
2019-09-30,Integrable magnetic flows on the two-torus: Zoll examples and systolic inequalities,"In this paper we study some aspects of integrable magnetic systems on the
two-torus. On the one hand, we construct the first non-trivial examples with
the property that all magnetic geodesics with unit speed are closed. On the
other hand, we show that those integrable magnetic systems admitting a global
surface of section satisfy a sharp systolic inequality.",1909.13821v1
2022-05-11,Homotopy transitions and 3D magnetic solitons,"This work provides a concept for three-dimensional magnetic solitons based on
mapping the homotopy path between various two-dimensional solutions onto the
third spatial axis. The representative examples of statically stable
configurations of that type in the model of an isotropic chiral magnet are
provided. Various static and dynamic properties of such three-dimensional
magnetic solitons are discussed in detail.",2205.05379v1
2022-07-25,Spin Coulomb drag by non-equilibrium magnetic textures,"Interaction between local magnetization and conduction electrons is
responsible for a variety of phenomena in magnetic materials. We have shown
that the spin-dependent motive force induced by magnetization dynamics in a
conducting ferromagnet lead to the spin Coulomb drag effect. The spin Coulomb
drag an intrinsic friction mechanism which operates whenever the average
velocities of up-spin and down-spin electrons differ.",2207.11978v1
1997-08-03,A Vorticity-Magnetic Field Dynamo Instability,"We generalize the mean field magnetic dynamo to include local evolution of
the mean vorticity in addition to the mean magnetic field. The coupled
equations exhibit a general mean field dynamo instability that enables the
transfer of turbulent energy to the magnetic field and vorticity on larger
scales. The growth of the vorticity and magnetic field both require helical
turbulence which can be supplied by an underlying global rotation. The dynamo
coefficients are derived including the backreaction from the mean magnetic
field to lowest order. We find that a mean vorticity field can actually seed
exponential growth of mean magnetic field from only a small scale seed magnetic
field, without the need for a seed mean magnetic field. The equations decouple
when the fluctuating velocity and magnetic field cross-correlations vanish,
resulting in the separate mean field magnetic and mean field vorticity dynamo
equations.",9708019v1
2006-09-22,Buoyant magnetic flux ropes in a magnetized stellar envelope: Idealized numerical 2.5-D MHD simulations,"Context: The context of this paper is buoyant toroidal magnetic flux ropes,
which is a part of flux tube dynamo theory and the framework of solar-like
magnetic activity. Aims: The aim is to investigate how twisted magnetic flux
ropes interact with a simple magnetized stellar model envelope--a magnetic
""convection zone""--especially to examine how the twisted magnetic field
component of a flux rope interacts with a poloidal magnetic field in the
convection zone. Method: Both the flux ropes and the atmosphere are modelled as
idealized 2.5-dimensional concepts using high resolution numerical
magneto-hydrodynamic (MHD) simulations. Results: It is illustrated that twisted
toroidal magnetic flux ropes can interact with a poloidal magnetic field in the
atmosphere to cause a change in both the buoyant rise dynamics and the flux
rope's geometrical shape. The details of these changes depend primarily on the
polarity and strength of the atmospheric field relative to the field strength
of the flux rope. It is suggested that the effects could be verified
observationally.",0609630v1
2002-11-21,Temperature-dependent magnetization in diluted magnetic semiconductors,"We calculate magnetization in magnetically doped semiconductors assuming a
local exchange model of carrier-mediated ferromagnetic mechanism and using a
number of complementary theoretical approaches. In general, we find that the
results of our mean-field calculations, particularly the dynamical mean field
theory results, give excellent qualitative agreement with the experimentally
observed magnetization in systems with itinerant charge carriers, such as
Ga_{1-x}Mn_xAs with 0.03 < x < 0.07, whereas our percolation-theory-based
calculations agree well with the existing data in strongly insulating
materials, such as Ge_{1-x}Mn_x. We comment on the issue of non-mean-field like
magnetization curves and on the observed incomplete saturation magnetization
values in diluted magnetic semiconductors from our theoretical perspective. In
agreement with experimental observations, we find the carrier density to be the
crucial parameter determining the magnetization behavior. Our calculated
dependence of magnetization on external magnetic field is also in excellent
agreement with the existing experimental data.",0211496v1
2014-12-29,Magnetic balltracking: Tracking the photospheric magnetic flux,"Context: One aspect of understanding the dynamics of the quiet Sun is to
quantify the evolution of the flux within small-scale magnetic features. These
features are routinely observed in the quiet photosphere and were given various
names, such as pores, knots, magnetic patches. Aims: This work presents a new
algorithm for tracking the evolution of the broad variety of small-scale
magnetic features in the photosphere, with a precision equal to the
instrumental resolution. Methods: We have developed a new technique to track
the evolution of the individual magnetic features from magnetograms, called
""magnetic balltracking"". It quantifies the flux of the tracked features, and it
can track the footpoints of magnetic field lines inferred from magnetic field
extrapolation. The algorithm can detect and quantify flux emergence, as well as
flux cancellation. Results: The capabilities of magnetic balltracking are
demonstrated with the detection and the tracking of two cases of magnetic flux
emergence that lead to the brightening of X-ray loops. The maximum emerged flux
ranges from 10^{18} Mx to 10^{19} Mx (unsigned flux) when the X-ray loops are
observed.",1412.8294v1
2015-09-02,Dynamo model for the inverse transfer of magnetic energy in a nonhelical decaying magnetohydrodynamic turbulence,"The inverse cascade of magnetic energy occurs when helicity or rotational
instability exists in the magnetohydrodynamic (MHD) system. This well known
phenomenon has been considered as a basis for the large scale magnetic field in
universe. However nonhelical magnetic energy in a decaying MHD system also
migrates toward the large scale, which holds vital clues to the origin of large
scale magnetic field in a quiescent astrophysical system. Zeldovich's rope
dynamo model is considered as a basic and symbolistic model of magnetic field
amplification. However, the rope model assuming specific external forces like
buoyancy or Coriolis force is not appropriate for a decaying turbulent system
without any external force. So we suggest a new dynamo model based on magnetic
induction equation excluding a forcing source. This model shows the expansion
and growth of magnetic field (flux) is basically the redistribution of energy
in the system. The transfer of magnetic energy is in fact a successive
induction of magnetic field resulted from the interaction between the fluid
motion and seed magnetic field. We also discuss about an analytic theorem based
on the scaling invariant MHD equation.",1509.00788v3
2020-01-27,Non-exponential magnetic relaxation in magnetic nanoparticles for hyperthermia,"Magnetic nanoparticle based hyperthermia emerged as a potential tool for
treating malignant tumours. The efficiency of the method relies on the
knowledge of magnetic properties of the samples; in particular, knowledge of
the frequency dependent complex magnetic susceptibility is vital to optimize
the irradiation conditions and to provide feedback for material science
developments. We study the frequency-dependent magnetic susceptibility of an
aqueous ferrite suspension for the first time using non-resonant and resonant
radiofrequency reflectometry. We identify the optimal measurement conditions
using a standard solenoid coil, which is capable of providing the complex
magnetic susceptibility up to 150 MHz. The result matches those obtained from a
radiofrequency resonator for a few discrete frequencies. The agreement between
the two different methods validates our approach. Surprisingly, the dynamic
magnetic susceptibility cannot be explained by an exponential magnetic
relaxation behavior even when we consider a particle size-dependent
distribution of the relaxation parameter.",2001.09774v1
2020-03-18,Thermally induced generation and annihilation of magnetic chiral skyrmion bubbles and achiral bubbles in Mn-Ni-Ga Magnets,"Magnetic chiral skyrmion bubbles and achiral bubbles are two independent
magnetic domain structures, in which the former with equivalent winding number
to skyrmions offers great promise as information carriers for further
spintronic devices. Here, in this work, we experimentally investigate the
generation and annihilation of magnetic chiral skyrmion bubbles and achiral
bubbles in the Mn-Ni-Ga thin plate by using the Lorentz transmission electron
microscopy (L-TEM). The two independent magnetic domain structures can be
directly controlled after the field cooling manipulation by varying the titled
angles of external magnetic fields. By imaging the magnetization reversal with
increasing temperature, we found an extraordinary annihilation mode of magnetic
chiral skyrmion bubbles and a non-linear frequency for the winding number
reversal. Quantitative analysis of such dynamics was performed by using L-TEM
to directly determine the barrier energy for the magnetization reversal of
magnetic chiral skyrmion bubbles.",2003.08035v1
2020-07-24,Can the dynamics of test particles around charged stringy black holes mimic the spin of Kerr black holes?,"We study the motion of electrically charged particles, magnetic monopoles,
and magnetic dipoles around electrically and magnetically charged stringy black
holes. From the analysis of the radius of the innermost stable circular orbit
(ISCO) of electrically charged particles, we show that the electric charge $Q$
of stringy black holes can mimic well the spin of Kerr black holes; the black
hole magnetic charge $Q_m$ can mimic spins up to $a_* \simeq 0.85$ for magnetic
dipoles; the magnetic charge parameter $g$ of a magnetic monopole can mimic
spins up to $a_* \simeq 0.8$. This is due to the destructive character of the
magnetic field and such a result excludes the existence of an appreciable black
hole magnetic charge in astrophysical black holes from the observation of
rapidly rotating objects with dimensionless spin up to $a_* \simeq 0.99$. We
then consider the magnetar SGR (PSR) J1745-2900 as a magnetic dipole orbiting
the supermassive black hole Sagittarius A* (Sgr A*). We show that Sgr A* may be
interpreted as a stringy black hole with magnetic charge $Q_{\rm m}/M \leq
0.4118$.",2007.12462v1
2020-10-10,Dynamics of charged and magnetized particles around cylindrical black holes immersed in external magnetic field,"This paper investigates the motion and acceleration of a particle that is
electrically and magnetically charged, which rotates around a cylindrical black
hole in the presence of an external asymptotically uniform magnetic field
parallel to the z axis. Basically, the work considers the circular orbits of
particles rotating around the central object and studies the dependence of the
most internal stable circular orbits (ISCO) on the so-called magnetic coupling
parameters, which are responsible for the interaction between the external
magnetic field and magnetized and charged particles. It is also shown that the
ISCO radius decreases with increasing magnetized parameter. Therefore,
collisions of magnetized particles around a cylindrical black hole immersed in
an external magnetic field were also studied, and it was shown that the
magnetic field can act as a particle accelerator around non-rotating
cylindrical black holes.",2010.04969v1
2021-07-02,Off-axis gyration induces large-area circular motion of anisotropic microparticles in a dynamic magnetic trap,"Magnetic tweezers are crucial for single-molecule and atomic
characterization, and biomedical isolation of microparticle carriers. The
trapping component of magnetic tweezing can be relying on a magnetic potential
well that can confine the relevant species to a localized region. Here, we
report that magnetic microparticles with tailored anisotropy can transition
from localized off-axis gyration to large-area locomotion in a rotating
magnetic trap. The microparticles, consisting of assemblies of magnetic cores,
are observed to either rotate about its structural geometric center or gyrate
about one of the magnetic cores, the switching of which can be modulated by the
external field. Raising the magnetic field strength above a threshold, the
particles can go beyond the traditional synchronous-rotation and
asynchronous-oscillation modes, and into a scenario of large-area circular
motion. This results in peculiar retrograde locomotion related to the
magnetization maxima of the microparticle. Our finding suggests the important
role of the microparticle's magnetic morphology in the controlled transport of
microparticles and developing smart micro-actuators and micro-robot devices.",2107.00869v1
2022-05-10,Non-hermitian off-diagonal magnetic response of Dirac fermions,"We perform a comparative study for the magnetization dynamics within linear
response theory of one and two dimensional massive Dirac electrons, after
switching on either a real (hermitian) or an imaginary (non-hermitian) magnetic
field. While hermitian dc magnetic fields polarize the spins in the direction
of the external magnetic field, non-hermitian magnetic fields induce only off
diagonal response. An imaginary dc magnetic field perpendicular to the mass
term induces finite magnetization in the third direction only according to the
right hand rule. This can be understood by analyzing the non-hermitian equation
of motion of the spin, which becomes analogous to a classical particle in
crossed electric and magnetic fields. Therein, the spin expectation value, the
mass term and imaginary magnetic field play the role of the classical momentum,
magnetic and electric field, respectively. The latter two create a drift
velocity perpendicular to them, which gives rise to the off-diagonal component
of the dc spin susceptibility, similarly to how the Hall effect develops in the
classical description.",2205.04795v2
2021-03-19,Spin-Injection-Generated Shock Waves and Solitons in a Ferromagnetic Thin Film,"Unsteady nonlinear magnetization dynamics are studied in an easy-plane
ferromagnetic channel subject to spin injection at one edge. The model
Landau-Lifshitz equation is known to support steady-state solutions, termed
dissipative exchange flows (DEFs) or spin superfluids. In this work, by means
of numerical simulations and theoretical analysis, we provide a full
description of the injection-induced, large-amplitude, nonlinear magnetization
dynamics up to the steady state. The dynamics prior to reaching steady state
are driven by spin injection, the perpendicular applied magnetic field, the
exchange interaction, and local demagnetizing fields. We show that the dynamics
take well-defined profiles in the form of rarefaction waves (RW), dispersive
shock waves (DSW), and solitons. The combination of these building blocks
depends on the interplay between the spin injection strength and the applied
magnetic field. A solitonic feature at the injection boundary, signaling the
onset of the magnetic ""supersonic"" condition at the injection edge, rapidly
develops and persists in the steady-state configuration of a contact soliton
DEF. We also demonstrate the existence of sustained soliton-train dynamics in
long time that can only arise in a nonzero applied magnetic field scenario. The
dynamical evolution of spin-injection-induced magnetization dynamics presented
here may help guide observations in long-distance spin transport experiments.",2103.10616v1
1996-07-16,Turbulent behaviour in magnetic hydrodynamics is not universal,"A short distance expansion method (SDE) that is well known in the quantum
field theory for analysis of turbulent behaviour of stochastic magnetic
hydrodynamics of incompressible conductive fluid is applied. As a result is
shown that in an inertial range the turbulent spectra of magnetic hydrodynamics
depend on a scale of arising of curls.",9607009v1
1998-10-30,Large Coherent Structure Formation by Magnetic Stretching Term in Two-Dimensional MHD Turbulence,"Using EDQNM approximation, it is shown that the magnetic stretching term is
responsible for turning the small-scale turbulent structures into the large
coherent magnetic structures in the 2-D MHD turbulence.",9810036v1
2006-03-05,"Magnetic Schroeodinger Operator: Geometry, Classical and Quantum Dynamics and Spectral Aymptotics","I study the Schroedinger operator with the strong magnetic field, considering
links between geometry of magnetic field, classical and quantum dynamics
associated with operator and spectral asymptotics.",0603120v3
2007-06-12,Triplet Josephson effect with magnetic feedback,"We study AC Josephson effect in a superconductor-ferromagnet heterostructure
with a variable magnetic configuration. The system supports triplet proximity
correlations whose dynamics is coupled to the magnetic dynamics. This feedback
dramatically modifies the behavior of the junction. The current-phase relation
becomes double-periodic at both very low and high Josephson frequencies
$\omega_J$. At intermediate frequencies, the periodicity in $\omega_J t$ may be
lost.",0706.1746v2
2008-06-23,A simple mechanism for the reversals of Earth's magnetic field,"We show that a model, recently used to describe all the dynamical regimes of
the magnetic field generated by the dynamo effect in the VKS experiment [1],
also provides a simple explanation of the reversals of Earth's magnetic field,
despite strong differences between both systems.",0806.3756v3
2009-04-22,Dynamic equations for three different qudits in a magnetic field,"A closed system of equations for the local Bloch vectors and spin correlation
functions of three magnetic qudits, which are in an arbitrary, time-dependent,
external magnetic field, is obtained using decomplexification of the
Liouville-von Neumann equation. The algorithm of the derivation of the dynamic
equations is presented. In the basis convenient for the important physical
applications structure constants of algebra su(2S+1) are calculated.",0904.3533v1
2020-05-06,Inverse-Magnetic Billiards on a Square,"We consider a polygon in a two-dimensional plane with a homogeneous constant
magnetic field orthogonal to such plane, but inside the polygon, the magnetic
field is zero. We study the dynamics of an electron with an initial velocity in
this setting. Problems arise because we have a boundary with corners. In the
square case, we generalize velocities with a rational slope and show numerical
evidence of a chaotic-like behavior with other initial conditions.",2005.02713v1
2020-10-31,Low-temperature asymptotics for the transverse dynamical structure factor for a magnetically polarized $XX$ chain at small and negative frequencies,"Using the truncated form factor expansion the low-temperature asymptotics for
the transverse dynamical structure factor of the magnetically polarized $XX$
chain is studied. Unlike the previous paper we do not use the representation of
structure factor in terms of the corresponding magnetic susceptibility. This
enables to obtain correct results at small and negative frequencies.",2011.00326v1
2021-06-10,Linear Stability of Periodic Trajectories in Inverse Magnetic Billiards,"We study the stability of periodic trajectories of planar inverse magnetic
billiards, a dynamical system whose trajectories are straight lines inside a
connected planar domain $\Omega$ and circular arcs outside $\Omega$. Explicit
examples are calculated in circles, ellipses, and the one parameter family of
curves $x^{2k}+y^{2k}=1$. Comparisons are made to the linear stability of
periodic billiard and magnetic billiard trajectories.",2106.05676v1
2011-11-30,Relation of Astrophysical Turbulence and Magnetic Reconnection,"Astrophysical fluids are generically turbulent and this must be taken into
account for most transport processes. We discuss how the preexisting turbulence
modifies magnetic reconnection and how magnetic reconnection affects the MHD
turbulent cascade. We show the intrinsic interdependence and interrelation of
magnetic turbulence and magnetic reconnection, in particular, that strong
magnetic turbulence in 3D requires reconnection and 3D magnetic turbulence
entails fast reconnection. We follow the approach in Eyink, Lazarian & Vishniac
2011 to show that the expressions of fast magnetic reconnection in Lazarian &
Vishniac 1999 can be recovered if Richardson diffusion of turbulent flows is
used instead of ordinary Ohmic diffusion. This does not revive, however, the
concept of magnetic turbulent diffusion which assumes that magnetic fields can
be mixed up in a passive way down to a very small dissipation scales. On the
contrary, we are dealing the reconnection of dynamically important magnetic
field bundles which strongly resist bending and have well defined mean
direction weakly perturbed by turbulence. We argue that in the presence of
turbulence the very concept of flux-freezing requires modification. The
diffusion that arises from magnetic turbulence can be called reconnection
diffusion as it based on reconnection of magnetic field lines. The reconnection
diffusion has important implications for the continuous transport processes in
magnetized plasmas and for star formation. In addition, fast magnetic
reconnection in turbulent media induces the First order Fermi acceleration of
energetic particles, can explain solar flares and gamma ray bursts. However,
the most dramatic consequence of these developments is the fact that the
standard flux freezing concept must be radically modified in the presence of
turbulence.",1112.0022v1
2012-10-13,Full-disk nonlinear force-free field extrapolation of SDO/HMI and SOLIS/VSM magnetograms,"Extrapolation codes in Cartesian geometry for modelling the magnetic field in
the corona do not take the curvature of the Sun's surface into account and can
only be applied to relatively small areas, e.g., a single active region. We
compare the analysis of the photospheric magnetic field and subsequent
force-free modeling based on full-disk vector maps from Helioseismic and
Magnetic Imager (HMI) on board solar dynamics observatory (SDO) and Vector
Spectromagnetograph (VSM) of the Synoptic Optical Long-term Investigations of
the Sun (SOLIS). We use Helioseismic and Magnetic Imager and Vector
Spectromagnetograph photospheric magnetic field measurements to model the
force-free coronal field above multiple solar active regions, assuming magnetic
forces to dominate. We solve the nonlinear force-free field equations by
minimizing a functional in spherical coordinates over a full disk excluding the
poles. After searching for the optimum modeling parameters for the particular
data sets, we compare the resulting nonlinear force-free model fields. We
compare quantities like the total magnetic energy content and free magnetic
energy, the longitudinal distribution of the magnetic pressure and surface
electric current density using our spherical geometry extrapolation code. The
magnetic field lines obtained from nonlinear force-free extrapolation based on
Helioseismic and Magnetic Imager and Vector Spectromagnetograph data have good
agreement. However, the nonlinear force-free extrapolation based on
Helioseismic and Magnetic Imager data have more contents of total magnetic
energy, free magnetic energy, the longitudinal distribution of the magnetic
pressure and surface electric current density compared to the one from Vector
Spectromagnetograph data.",1210.3668v1
2017-12-13,Studying the transfer of magnetic helicity in solar active regions with the connectivity-based helicity flux density method,"In the solar corona, magnetic helicity slowly and continuously accumulates in
response to plasma flows tangential to the photosphere and magnetic flux
emergence through it. Analyzing this transfer of magnetic helicity is key for
identifying its role in the dynamics of active regions (ARs). The
connectivity-based helicity flux density method was recently developed for
studying the 2D and 3D transfer of magnetic helicity in ARs. The method takes
into account the 3D nature of magnetic helicity by explicitly using knowledge
of the magnetic field connectivity, which allows it to faithfully track the
photospheric flux of magnetic helicity. Because the magnetic field is not
measured in the solar corona, modeled 3D solutions obtained from force-free
magnetic field extrapolations must be used to derive the magnetic connectivity.
Different extrapolation methods can lead to markedly different 3D magnetic
field connectivities, thus questioning the reliability of the
connectivity-based approach in observational applications. We address these
concerns by applying this method to the isolated and internally complex AR
11158 with different magnetic field extrapolation models. We show that the
connectivity-based calculations are robust to different extrapolation methods,
in particular with regards to identifying regions of opposite magnetic helicity
flux. We conclude that the connectivity-based approach can be reliably used in
observational analyses and is a promising tool for studying the transfer of
magnetic helicity in ARs and relate it to their flaring activity.",1712.04691v1
2021-05-31,Theoretical development in the viscosity of ferrofluid,"The viscosity of ferrofluid has an important role in liquid sealing of the
hard disk drives, biomedical applications as drug delivery, hyperthermia, and
magnetic resonance imaging. In the absence of a magnetic field, the viscosity
of ferrofluid depends on the volume concentration of magnetic nanoparticles
including surfactant layers. However, under the influence of a stationary
magnetic field, the viscosity of ferrofluid depends on the angle between the
applied magnetic field and vorticity in the flow. If this angle is 90o, then
there is a maximum increase in the viscosity. If the magnetic field and the
vorticity in the flow are parallel to each other, then there is no change in
the viscosity since the applied magnetic field does not change the speed of the
rotation of magnetic nanoparticles in the fluid. The viscosity of ferrofluid in
the presence of an alternating magnetic field demonstrates interesting
behavior. When field frequency matches with the relaxation time, known as
resonance condition, then there is no impact of an alternating magnetic field
in the viscosity of ferrofluid. If the frequency of an alternating magnetic
field is less than resonance frequency, then an alternating magnetic field
increases the viscosity of ferrofluid. Using higher frequency than resonance
condition reduces the viscosity of ferrofluid and researchers reported this
incident as the negative viscosity effect. If the frequency of an alternating
magnetic field tends to infinite, then ferrofluid ceases to feel a magnetic
field. In this case, there is no impact of an alternating magnetic field on the
viscosity of ferrofluid.",2105.14721v1
2022-06-28,Turbulent magnetic helicity fluxes in solar convective zone,"Combined action of helical motions of plasma (the $\alpha$ effect) and
non-uniform (differential) rotation is a key dynamo mechanism of solar and
galactic large-scale magnetic fields. Dynamics of magnetic helicity of
small-scale fields is a crucial mechanism in a nonlinear dynamo saturation
where turbulent magnetic helicity fluxes allow to avoid catastrophic quenching
of the $\alpha$ effect. The convective zone of the Sun and solar-like stars as
well as galactic discs are the source for production of turbulent magnetic
helicity fluxes. In the framework of the mean-field approach and the spectral
$\tau$ approximation, we derive turbulent magnetic helicity fluxes using the
Coulomb gauge in a density-stratified turbulence. The turbulent magnetic
helicity fluxes include non-gradient and gradient contributions. The
non-gradient magnetic helicity flux is proportional to a nonlinear effective
velocity (which vanishes in the absence of the density stratification)
multiplied by small-scale magnetic helicity, while the gradient contributions
describe turbulent magnetic diffusion of the small-scale magnetic helicity. In
addition, the turbulent magnetic helicity fluxes contain source terms
proportional to the kinetic $\alpha$ effect or its gradients, and also
contributions caused by the large-scale shear (solar differential rotation). We
have demonstrated that the turbulent magnetic helicity fluxes due to the
kinetic $\alpha$ effect and its radial derivative in combination with the
nonlinear magnetic diffusion of the small-scale magnetic helicity are dominant
in the solar convective zone.",2206.14152v2
1998-05-25,Magnetic Catalysis of Dynamical Symmetry Breaking and Aharonov-Bohm Effect,"The phenomenon of the magnetic catalysis of dynamical symmetry breaking is
based on the dimensional reduction $D\to D-2$ in the dynamics of fermion
pairing in a magnetic field. We discuss similarities between this phenomenon
and the Aharonov-Bohm effect. This leads to the interpretation of the dynamics
of the (1+1)-dimensional Gross-Neveu model with a non-integer number of fermion
colors as a quantum field theoretical analogue of the Aharonov-Bohm dynamics.",9805159v2
2009-11-12,Groebli solution for three magnetic vortices,"The dynamics of N point vortices in a fluid is described by the
Helmholtz-Kirchhoff (HK) equations which lead to a completely integrable
Hamiltonian system for N=2 or 3 but chaotic dynamics for N>3. Here we consider
a generalization of the HK equations to describe the dynamics of magnetic
vortices within a collective-coordinate approximation. In particular, we
analyze in detail the dynamics of a system of three magnetic vortices by a
suitable generalization of the solution for three point vortices in an ordinary
fluid obtained by Groebli more than a century ago. The significance of our
results for the dynamics of ferromagnetic elements is briefly discussed.",0911.2377v1
2014-07-11,"Dynamic phase transition properties for the mixed spin-(1/2, 1) Ising model in an oscillating magnetic field","We study the dynamic phase transition properties for the mixed spin-(1/2, 1)
Ising model on a square lattice under a time-dependent magnetic field by means
the effective-field theory (EFT) based on the Glauber dynamics. We present the
dynamic phase diagrams in the reduced magnetic field amplitude and reduced
temperature plane and find that the phase diagrams exhibit the dynamic
tricitical behavior, the multicritical and zero-temperature critical points as
well as reentrant behavior. We also investigate the influence of the frequency
(w) and observe that for small values of w the mixed phase disappears, but high
values it appears and the system displays reentrant behavior as well as
critical end point.",1407.3188v1
2019-07-03,Dynamic magnetic features of a mixed ferro-ferrimagnetic ternary alloy in the form of AB$_p$C$_{1-p}$,"Dynamic magnetic features of a mixed ferro-ferrimagnetic ternary alloy in the
form of AB$_p$C$_{1-p}$, especially. The effect of Hamiltonian parameters on
the dynamic magnetic features of the system are investigated. For this aim, an
AB$_p$C$_{1-p}$ ternary alloy system was simulated within the mean-field
approximation based on a Glauber type stochastic dynamic and for simplicity, A,
B and C ions as SA = 1/2, SB = 1 and SC = 3/2, were chosen respectively. It was
found that in our dynamic system the critical temperature was always dependent
on the concentration ratio of the ternary alloy.",1907.01850v1
2016-11-10,Magnetism and ultra-fast magnetization dynamics of Co and CoMn alloys at finite temperature,"Temperature-dependent magnetic experiments like pump-probe measurements
generated by a pulsed laser have become a crucial technique for switching the
magnetization in the picosecond time scale. Apart from having practical
implications on the magnetic storage technology, the research field of
ultrafast magnetization poses also fundamental physical questions. To correctly
describe the time evolution of the atomic magnetic moments under the influence
of a temperature-dependent laser pulse, it remains crucial to know if the
magnetic material under investigation has magnetic excitation spectrum that is
more or less dependent on the magnetic configuration, e.g. as reflected by the
temperature dependence of the exchange interactions. In this article, we
demonstrate from first-principles theory that the magnetic excitation spectra
in Co with fcc, bcc and hcp structures are nearly identical in a wide range of
non-collinear magnetic configurations. This is a curious result of a balance
between the size of the magnetic moments and the strength of the Heisenberg
exchange interactions, that in themselves vary with configuration, but put
together in an effective spin Hamiltonian results in a configuration
independent effective model. We have used such a Hamiltonian, together with
ab-initio calculated damping parameters, to investigate the magnon dispersion
relationship as well as the ultrafast magnetisation dynamics of Co and Co-rich
CoMn alloys.",1611.03541v2
2018-08-09,Effect of Magnetic Field Strength on Solar Differential Rotation and Meridional Circulation,"We studied the solar surface flows (differential rotation and meridional
circulation) using a magnetic element feature tracking technique by which the
surface velocity is obtained using magnetic field data. We used the
line-of-sight magnetograms obtained by the Helioseismic and Magnetic Imager
aboard the Solar Dynamics Observatory from 01 May 2010 to 16 August 2017
(Carrington rotations 2096 to 2193) and tracked the magnetic element features
every hour. Using our method, we estimated the differential rotation velocity
profile. We found rotation velocities of $\sim$ 30 and -170 m s$^{-1}$ at
latitudes of 0$^{\circ}$ and 60$^{\circ}$ in the Carrington rotation frame,
respectively. Our results are consistent with previous results obtained by
other methods, such as direct Doppler, time-distance helioseismology, or cross
correlation analyses. We also estimated the meridional circulation velocity
profile and found that it peaked at $\sim$12 m s$^{-1}$ at a latitude of
45$^{\circ}$, which is also consistent with previous results. The dependence of
the surface flow velocity on the magnetic field strength was also studied. In
our analysis, the magnetic elements having stronger and weaker magnetic fields
largely represent the characteristics of the active region remnants and solar
magnetic networks, respectively. We found that magnetic elements having a
strong (weak) magnetic field show faster (slower) rotation speed. On the other
hand, magnetic elements having a strong (weak) magnetic field show slower
(faster) meridional circulation velocity. These results might be related to the
Sun's internal dynamics.",1808.03005v1
2020-06-21,Rotational properties of annulus dusty plasma in a strong magnetic field,"The collective dynamics of annulus dusty plasma formed between a co-centric
conducting (non-conducting) disk and ring configuration is studied in a
strongly magnetized radio-frequency (rf) discharge. A superconducting
electromagnet is used to introduce a homogeneous magnetic field to the dusty
plasma medium. In absence of the magnetic field, dust grains exhibit thermal
motion around their equilibrium position. The dust grains start to rotate in
anticlockwise direction with increasing magnetic field (B $>$ 0.02 T), and the
constant value of the angular frequency at various strengths of magnetic field
confirms the rigid body rotation. The angular frequency of dust grains linearly
increases up to a threshold magnetic field (B $>$ 0.6 T) and after that its
value remains nearly constant in a certain range of magnetic field. Further
increase in magnetic field (B $>$ 1 T) lowers the angular frequency. Low value
of angular frequency is expected by reducing the width of annulus dusty plasma
or the input rf power. The azimuthal ion drag force due to the magnetic field
is assumed to be the energy source which drives the rotational motion. The
resultant radial electric field in the presence of magnetic field determines
the direction of rotation. The variation of floating (plasma) potential across
the annular region at given magnetic field explains the rotational properties
of the annulus dusty plasma in the presence of magnetic field.",2006.11710v1
2020-09-28,Magnetic domains and domain wall pinning in two-dimensional ferromagnets revealed by nanoscale imaging,"Magnetic-domain structure and dynamics play an important role in
understanding and controlling the magnetic properties of two-dimensional
magnets, which are of interest to both fundamental studies and
applications[1-5]. However, the probe methods based on the spin-dependent
optical permeability[1,2,6] and electrical conductivity[7-10] can neither
provide quantitative information of the magnetization nor achieve nanoscale
spatial resolution. These capabilities are essential to image and understand
the rich properties of magnetic domains. Here, we employ cryogenic scanning
magnetometry using a single-electron spin of a nitrogen-vacancy center in a
diamond probe to unambiguously prove the existence of magnetic domains and
study their dynamics in atomically thin CrBr$_3$. The high spatial resolution
of this technique enables imaging of magnetic domains and allows to resolve
domain walls pinned by defects. By controlling the magnetic domain evolution as
a function of magnetic field, we find that the pinning effect is a dominant
coercivity mechanism with a saturation magnetization of about 26~$\mu_B$/nm$^2$
for bilayer CrBr$_3$. The magnetic-domain structure and pinning-effect
dominated domain reversal process are verified by micromagnetic simulation. Our
work highlights scanning nitrogen-vacancy center magnetometry as a quantitative
probe to explore two-dimensional magnetism at the nanoscale.",2009.13440v1
2006-07-31,Dynamical and thermal effects in nanoparticle systems driven by a rotating magnetic field,"We study dynamical and thermal effects that are induced in nanoparticle
systems by a rotating magnetic field. Using the deterministic Landau-Lifshitz
equation and appropriate rotating coordinate systems, we derive the equations
that characterize the steady-state precession of the nanoparticle magnetic
moments and study a stability criterion for this type of motion. On this basis,
we describe (i) the influence of the rotating field on the stability of the
small-angle precession, (ii) the dynamical magnetization of nanoparticle
systems, and (iii) the switching of the magnetic moments under the action of
the rotating field. Using the backward Fokker-Planck equation, which
corresponds to the stochastic Landau-Lifshitz equation, we develop a method for
calculating the mean residence times that the driven magnetic moments dwell in
the up and down states. Within this framework, the features of the induced
magnetization and magnetic relaxation are elucidated.",0607820v1
2007-04-25,Dynamical models and the phase ordering kinetics of the s=1 spinor condensate,"The $s=1$ spinor Bose condensate at zero temperature supports ferromagnetic
and polar phases that combine magnetic and superfluid ordering. We investigate
the formation of magnetic domains at finite temperature and magnetic field in
two dimensions in an optical trap. We study the general ground state phase
diagram of a spin-1 system and focus on a phase that has a magnetic Ising order
parameter and numerically determine the nature of the finite temperature
superfluid and magnetic phase transitions. We then study three different
dynamical models: model A, which has no conserved quantities, model F, which
has a conserved second sound mode and the Gross-Pitaevskii (GP) equation which
has a conserved density and magnetization. We find the dynamic critical
exponent to be the same for models A and F ($z=2$) but different for GP ($z
\approx 3$). Externally imposed magnetization conservation in models A and F
yields the value $z \approx 3$, which demonstrates that the only conserved
density relevant to domain formation is the magnetization density.",0704.3440v2
2009-07-29,Interactions between magnetohydrodynamic shear instabilities and convective flows in the solar interior,"Motivated by the interface model for the solar dynamo, this paper explores
the complex magnetohydrodynamic interactions between convective flows and
shear-driven instabilities. Initially, we consider the dynamics of a forced
shear flow across a convectively-stable polytropic layer, in the presence of a
vertical magnetic field. When the imposed magnetic field is weak, the dynamics
are dominated by a shear flow (Kelvin-Helmholtz type) instability. For stronger
fields, a magnetic buoyancy instability is preferred. If this stably stratified
shear layer lies below a convectively unstable region, these two regions can
interact. Once again, when the imposed field is very weak, the dynamical
effects of the magnetic field are negligible and the interactions between the
shear layer and the convective layer are relatively minor. However, if the
magnetic field is strong enough to favour magnetic buoyancy instabilities in
the shear layer, extended magnetic flux concentrations form and rise into the
convective layer. These magnetic structures have a highly disruptive effect
upon the convective motions in the upper layer.",0907.5068v1
2011-03-23,Zero-temperature spin-glass freezing in self-organized arrays of Co nanoparticles,"We study, by means of magnetic susceptibility and magnetic aging experiments,
the nature of the glassy magnetic dynamics in arrays of Co nanoparticles,
self-organized in N layers from N=1 (two-dimensional limit) up to N=20
(three-dimensional limit). We find no qualitative differences between the
magnetic responses measured in these two limits, in spite of the fact that no
spin-glass phase is expected above T=0 in two dimensions. More specifically,
all the phenomena (critical slowing down, flattening of the field-cooled
magnetization below the blocking temperature and the magnetic memory induced by
aging) that are usually associated with this phase look qualitatively the same
for two-dimensional and three-dimensional arrays. The activated scaling law
that is typical of systems undergoing a phase transition at zero temperature
accounts well for the critical slowing down of the dc and ac susceptibilities
of all samples. Our data show also that dynamical magnetic correlations
achieved by aging a nanoparticle array below its superparamagnetic blocking
temperature extend mainly to nearest neighbors. Our experiments suggest that
the glassy magnetic dynamics of these nanoparticle arrays is associated with a
zero-temperature spin-glass transition.",1103.4492v1
2011-04-08,Magnetization Dissipation in Ferromagnets from Scattering Theory,"The magnetization dynamics of ferromagnets are often formulated in terms of
the Landau-Lifshitz-Gilbert (LLG) equation. The reactive part of this equation
describes the response of the magnetization in terms of effective fields,
whereas the dissipative part is parameterized by the Gilbert damping tensor. We
formulate a scattering theory for the magnetization dynamics and map this
description on the linearized LLG equation by attaching electric contacts to
the ferromagnet. The reactive part can then be expressed in terms of the static
scattering matrix. The dissipative contribution to the low-frequency
magnetization dynamics can be described as an adiabatic energy pumping process
to the electronic subsystem by the time-dependent magnetization. The Gilbert
damping tensor depends on the time derivative of the scattering matrix as a
function of the magnetization direction. By the fluctuation-dissipation
theorem, the fluctuations of the effective fields can also be formulated in
terms of the quasistatic scattering matrix. The theory is formulated for
general magnetization textures and worked out for monodomain precessions and
domain wall motions. We prove that the Gilbert damping from scattering theory
is identical to the result obtained by the Kubo formalism.",1104.1625v1
2011-10-21,Classical Orbital Magnetic Moment in a Dissipative Stochastic System,"We present an analytical treatment of the dissipative-stochastic dynamics of
a charged classical particle confined bi-harmonically in a plane with a uniform
static magnetic field directed perpendicular to the plane. The stochastic
dynamics gives a steady state in the long-time limit. We have examined the
orbital magnetic effect of introducing a parametrized deviation ($\eta$ -1)
from the second fluctuation-dissipation (II-FD) relation that connects the
driving noise and the frictional memory kernel in the standard Langevin
dynamics. The main result obtained here is that the moving charged particle
generates a finite orbital magnetic moment in the steady state, and that the
moment shows a crossover from para-to dia-magnetic sign as the parameter $\eta$
is varied. It is zero for $\eta = 1$ that makes the steady state correspond to
equilibrium, as it should. The magnitude of the orbital magnetic moment turns
out to be a non-monotonic function of the applied magnetic field, tending to
zero in the limit of an infinitely large as well as an infinitesimally small
magnetic field. These results are discussed in the context of the classic
Bohr-van Leeuwen theorem on the absence of classical orbital diamagnetism.
Possible realization is also briefly discussed.",1110.4749v2
2011-12-08,Dynamics of artificial spin ice: continuous honeycomb network,"We model the dynamics of magnetization in an artificial analog of spin ice
specializing to the case of a honeycomb network of connected magnetic
nanowires. The inherently dissipative dynamics is mediated by the emission,
propagation and absorption of domain walls in the links of the lattice. These
domain walls carry two natural units of magnetic charge, whereas sites of the
lattice contain a unit magnetic charge. Magnetostatic Coulomb forces between
these charges play a major role in the physics of the system, as does quenched
disorder caused by imperfections of the lattice. We identify and describe
different regimes of magnetization reversal in an applied magnetic field
determined by the orientation of the applied field with respect to the initial
magnetization. One of the regimes is characterized by magnetic avalanches with
a 1/n distribution of lengths.",1112.1857v3
2017-03-16,Relaxation dynamics of modulated magnetic phases in skyrmion host GaV4S8: an ac magnetic susceptibility study,"We report on the slow magnetization dynamics observed upon the magnetic phase
transitions of GaV4S8, a multiferroic compound featuring a long-ranged
cycloidal magnetic order and a N\'eel-type skyrmion lattice in a relatively
broad temperature range below its Curie temperature. The fundamental difference
between GaV4S8 and the chiral helimagnets, wherein the skyrmion phase was first
discovered, lies within the polar symmetry of GaV4S8, promoting a cycloidal
instead of a helical magnetic order and rendering the magnetic phase diagram
essentially different from that in the cubic helimagnets. Our ac magnetic
susceptibility study reveals slow relaxation dynamics at the field-driven phase
transitions between the cycloidal, skyrmion lattice and field-polarized states.
At each phase boundary, the characteristic relaxation times were found to
exhibit a strong temperature dependence, starting from the minute range at low
temperatures, decreasing to the micro- to millisecond range at higher
temperatures.",1703.10928v1
2018-03-19,"Magnetic susceptibility, nanorheology, and magnetoviscosity of magnetic nanoparticles in viscoelastic environments","While magnetic nanoparticles suspended in Newtonian solvents (ferrofluids)
have been intensively studied in recent years, the effects of viscoelasticity
of the surrounding medium on the nanoparticle dynamics are much less
understood. Here we investigate a mesoscopic model for the orientational
dynamics of isolated magnetic nanoparticles subject to external fields, viscous
and viscoelastic friction as well as the corresponding random torques. We solve
the model analytically in the overdamped limit for weak viscoelasticity. By
comparison to Brownian Dynamics simulations we establish the limits of validity
of the analytical solution. We find that viscoelasticity does not only slow
down the magnetization relaxation, shift the peak of the imaginary magnetic
susceptibility $\chi''$ to lower frequencies and increase the magnetoviscosity,
it also leads to non-exponential relaxation and a broadening of $\chi''$. The
model we study also allows to test a recent proposal for using magnetic
susceptibility measurements as a nanorheological tool using a variant of the
Germant-DiMarzio-Bishop relation. We find for the present model and certain
parameter ranges that the relation of the magnetic susceptibility to the shear
modulus is satisfied to a good approximation.",1803.07188v1
2018-04-11,Continuous Nucleation Dynamics of Magnetic Skyrmions in T-shaped Helimagnetic Nanojunction,"Magnetic skyrmions are topologically-protected spin textures existing in
helimagentic materials, which can be utilized as information carriers for
non-volatile memories and logic circuits in spintronics. Searching simple and
controllable way to create isolated magnetic skyrmions is desirable for further
technology developments and industrial designs. Based on micromagnetic
simulations, we show that the temporal dissipative structure can be developed
in the T-shaped helimagnetic nanojunction when it is driven to the
far-from-equilibrium regime by a constant spin-polarized current. Then the
magnetic skyrmions can be continuously nucleated during the periodic
magnetization dynamics of the nanojunction. We have systematically investigated
the effects of current density, Dzyaloshinskii-Moriya interaction, external
magnetic field, and thermal fluctuation on the nucleation dynamics of the
magnetic skyrmions. Our results here suggest a novel and promising mechanism to
continuously create magnetic skyrmions for the development of skyrmion-based
spintronics devices.",1804.04022v1
2018-04-26,Orbital quantum magnetism in spin dynamics of strongly interacting magnetic lanthanide atoms,"Laser cooled lanthanide atoms are ideal candidates with which to study strong
and unconventional quantum magnetism with exotic phases. Here, we use
state-of-the-art closed-coupling simulations to model quantum magnetism for
pairs of ultracold spin-6 erbium lanthanide atoms placed in a deep optical
lattice. In contrast to the widely used single-channel Hubbard model
description of atoms and molecules in an optical lattice, we focus on the
single-site multi-channel spin evolution due to spin-dependent contact,
anisotropic van der Waals, and dipolar forces. This has allowed us to identify
the leading mechanism, orbital anisotropy, that governs molecular spin dynamics
among erbium atoms. The large magnetic moment and combined orbital angular
momentum of the 4f-shell electrons are responsible for these strong anisotropic
interactions and unconventional quantum magnetism. Multi-channel simulations of
magnetic Cr atoms under similar trapping conditions show that their
spin-evolution is controlled by spin-dependent contact interactions that are
distinct in nature from the orbital anisotropy in Er. The role of an external
magnetic field and the aspect ratio of the lattice site on spin dynamics is
also investigated.",1804.10102v1
2018-06-06,Control of magnetization dynamics by spin Nernst torque,"Control of magnetization dynamics is one of the primary goals in spintronics.
It has been demonstrated using spin Hall effect i.e charge current to spin
current conversion in non-magnetic metal which has large spin-orbit coupling
such as Pt, W etc. Recently different groups have shown generation of spin
current in Pt, W while thermal gradient is created by virtue of spin Nernst
effect. In this work we show the evidence of magnetization control by spin
Nernst torque in Pt/Py bi-layer. We compared relative strength of spin Nernst
Torque and spin Hall torque by measuring the systematic variation of magnetic
linewidth on application of constant heat or charge current. Spin-torque
ferromagnetic resonance (ST-FMR) technique is adopted to excite the magnet and
to measure line-width precisely from the symmetric and anti-symmetric voltage
component. Control of magnetization dynamics by spin Nernst torque will emerge
as an alternative way to manipulate nano-magnets.",1806.01978v1
2018-06-07,Terahertz Emission from Compensated Magnetic Heterostructures,"Terahertz emission spectroscopy (TES) has recently played an important role
in unveiling the spin dynamics at a terahertz (THz) frequency range. So far,
ferromagnetic (FM)/nonmagnetic (NM) heterostructures have been intensively
studied as THz sources. Compensated magnets such as a ferrimagnet (FIM) and
antiferromagnet (AFM) are other types of magnetic materials with interesting
spin dynamics. In this work, we study TES from compensated magnetic
heterostructures including CoGd FIM alloy or IrMn AFM layers. Systematic
measurements on composition and temperature dependences of THz emission from
CoGd/Pt bilayer structures are conducted. It is found that the emitted THz
field is determined by the net spin polarization of the laser induced spin
current rather than the net magnetization. The temperature robustness of the
FIM based THz emitter is also demonstrated. On the other hand, an AFM plays a
different role in THz emission. The IrMn/Pt bilayer shows negligible THz
signals, whereas Co/IrMn induces sizable THz outputs, indicating that IrMn is
not a good spin current generator, but a good detector. Our results not only
suggest that a compensated magnet can be utilized for robust THz emission, but
also provide a new approach to study the magnetization dynamics especially near
the magnetization compensation point.",1806.02517v1
2019-06-15,Bursty magnetic friction between polycrystalline thin films with domain walls,"Two magnets in relative motion interact through their dipolar fields, making
individual magnetic moments dynamically adapt to the changes in the energy
landscape and bringing about collective magnetization dynamics. Some of the
energy of the system is irrevocably lost through various coupling mechanisms
between the spin degrees of freedom and those of the underlying lattice,
resulting in magnetic friction. In this work, we use micromagnetic simulations
to study magnetic friction in a system of two thin ferromagnetic films
containing quenched disorder mimicking a polycrystalline structure. We observe
bursts of magnetic activity resulting from repeated domain wall pinning due to
the disorder and subsequent depinning triggered by the dipolar interaction
between the moving films. These domain wall jumps result in strong energy
dissipation peaks. We study how the properties of the polycrystalline structure
such as grain size and strength of the disorder, along with the driving
velocity and the width of the films, affect the magnetization dynamics, average
energy dissipation as well as the statistical properties of the energy
dissipation bursts.",1906.06506v1
2019-08-19,Steady states of non-axial dipolar rods driven by rotating fields,"We investigate a two-dimensional system of magnetic colloids with anisotropic
geometry (rods) subjected to an oscillating external magnetic field. The
structural and dynamical properties of the steady states are analyzed, by means
of Langevin Dynamics simulations, as a function of the misalignment of the
intrinsic magnetic dipole moment of the rods with respect to their axial
direction, and also in terms of the strength and rotation frequency of an
external magnetic field. The misalignment of the dipole relative to their axial
direction is inspired by recent studies, and this is extremely relevant in the
microscopic aggregation states of the system. The dynamical response of the
magnetic rods to the external magnetic field is strongly affected by such a
misalignment. Concerning the synchronization between the magnetic rods and the
direction of the external magnetic field, we define three distinct regimes of
synchronization. A set of steady states diagrams are presented, showing the
magnitude and rotation frequency intervals in which the distinct self-organized
structures are observed.",1908.06718v1
2020-05-07,Ultrafast All Optical Magnetization Control for Broadband Terahertz Spin Wave Generation,"Terahertz spin waves could be generated on-demand via all-optical
manipulation of magnetization by femtosecond laser pulse. Here, we present an
energy balance model, which explains the energy transfer rates from laser pulse
to electron bath coupled with phonon, spin, and magnetization of five different
magnetic metallic thin films: Iron, Cobalt, Nickel, Gadolinium and Ni$_{2}$MnSn
Heusler alloy. Two types of transient magnetization dynamics emerge in metallic
magnetic thin films based on their Curie temperatures (T$_{C}$): type I (Fe,
Co, and Ni with T$_{C}$ > room temperature, RT) and type II films (Gd and
Ni$_{2}$MnSn with T$_{C}$ $\approx$ RT). We study the effect of laser fluence
and pulse width for single Gaussian laser pulses and the effect of metal film
thickness on magnetization dynamics. Spectral dynamics show that broadband spin
waves up to 24 THz could be generated by all-optical manipulation of
magnetization in these nanofilms.",2005.03493v1
2020-05-14,Magnetic field dependent 't Hooft determinant extended Nambu--Jona-Lasinio Model,"We study the implications of recent lattice QCD results for the magnetic
field dependence of the quarks dynamical masses on the 't Hooft determinant
extended Nambu--Jona-Lasinio Model in the light and strange quark sectors
(\emph{up}, \emph{down} and \emph{strange}). The parameter space is constrained
at vanishing magnetic field, using the quarks dynamical masses and the meson
spectra, whereas at non-vanishing magnetic field strength the dependence of the
dynamical masses of two of the quark flavors is used to fit a magnetic field
dependence on the model couplings, both the four-fermion Nambu--Jona-Lasinio
interaction and the six-fermion 't Hooft flavor determinant. We found that this
procedure reproduces the inverse magnetic catalysis, and the strength of the
scalar coupling decreases with the magnetic field, while the strength of the
six-fermion 't Hooft flavor determinant increases with the magnetic field.",2005.07049v2
2021-07-06,Static and dynamic magnetic properties of K3CrO4,"We report on the magnetic properties of geometrically frustrated K3CrO4, in
which Cr5+ cations are arranged on a distorted pyrochlore lattice. The crystal
structure, static and dynamic magnetic properties of the compound are
investigated in detail. A combination of DC and AC magnetic susceptibility
measurements together with thermoremanent magnetization decay measurements
reveal several magnetic transitions: the onset of glassy canted
antiferromagnetic order occurs at 36 K, followed by the appearance of
ferromagnetic/ferrimagnetic cluster glass behavior below the freezing
temperature of 20 K. Further field-induced, temperature-dependent transitions
are observed in the range 3-10 K. The frequency dependence of the freezing
temperature for the cluster glass state is analyzed on the basis of dynamic
scaling laws including the critical slowing down formula and the Vogel-Fulcher
law.",2107.02878v1
2022-01-12,The angular dependence of magnetization dynamics induced by a GHz range strain pulse,"The dynamics of magnetization is important in spintronics, where the coupling
between phonon and magnon attracts much attention. In this work, we study the
angular dependence of the coupling between longitudinal-wave phonon and magnon.
We investigated the magnetization dynamics using the time-resolved
magneto-optical Kerr effect, which allows measuring spin-wave resonances and
the magnetic echo signal. The frequency, mode number, and amplitude of the
spin-wave resonance change with the out-of-plane angle of the external magnetic
field. The amplitude of the magnetic echo signal caused by the strain pulse
also changes with the angle. We calculate these angular dependences based on
the Landau-Lifshitz-Gilbert equation and find that the angles of the external
field and magnetic moment are important factors for the phonon-magnon coupling
when phonon propagates in the thickness direction under the out-of-plane
magnetic field.",2201.04396v1
2022-03-30,Atomistic modeling of spin and electron dynamics in two-dimensional magnets switched by two-dimensional topological insulators,"To design fast memory devices, we need material combinations which can
facilitate fast read and write operation. We present a heterostructure
comprising a two-dimensional (2D) magnet and a 2D topological insulator (TI) as
a viable option for designing fast memory devices. We theoretically model
spin-charge dynamics between the 2D magnets and 2D TIs. Using the adiabatic
approximation, we combine the non-equilibrium Green's function method for
spin-dependent electron transport, and time-quantified Monte-Carlo for
simulating magnetization dynamics. We show that it is possible to switch the
magnetic domain of a ferromagnet using spin-torque from spin-polarized edge
states of 2D TI. We further show that the switching between TIs and 2D magnets
is strongly dependent on the interface exchange ($J_{\mathrm{int}}$), and an
optimal interface exchange depending on the exchange interaction within the
magnet is required for efficient switching. Finally, we compare the
experimentally grown Cr-compounds and show that Cr-compounds with higher
anisotropy (such as $\rm CrI_3$) results in lower switching speed but more
stable magnetic order.",2203.16008v1
2022-10-13,Influence of Ferromagnetic Interlayer Exchange Coupling on Current-induced Magnetization Switching and Dzyaloshinskii-Moriya Interaction in Co/Pt/Co Multilayer System,"This paper investigates the relationship among interlayer exchange coupling
(IEC), Dzyaloshinskii-Moriya interaction (DMI), and multilevel magnetization
switching within a Co/Pt/Co heterostructure, where varying Pt thicknesses
enable control over the coupling strength. Employing Brillouin Light Scattering
to quantify the effective DMI, we explore its potential role in magnetization
dynamics and multilevel magnetization switching. Experimental findings show
four distinct resistance states under an external magnetic field and spin Hall
effect related spin current. We explain this phenomenon based on the asymmetry
between Pt/Co and Co/Pt interfaces and the interlayer coupling, which, in turn,
influences DMI and subsequently impacts the magnetization dynamics. Numerical
simulations, including macrospin, 1D domain wall, and simple spin wave models,
further support the experimental observations of multilevel switching and help
uncover the underlying mechanisms. Our proposed explanation, supported by
magnetic domain observation using polar-magnetooptical Kerr microscopy, offers
insights into both the spatial distribution of magnetization and its dynamics
for different IECs, thereby shedding light on its interplay with DMI, which may
lead to potential applications in storage devices.",2210.07357v2
2023-02-27,Rotating hematite cube chains,"Recently a two-dimensional chiral fluid was experimentally demonstrated. It
was obtained from cubic-shaped hematite colloidal particles placed in a
rotating magnetic field. Here we look at building blocks of that fluid, by
analyzing short hematite chain behavior in a rotating magnetic field. We find
equilibrium structures of chains in static magnetic fields and observe chain
dynamics in rotating magnetic fields. We find and experimentally verify that
there are three planar motion regimes and one where the cube chain goes out of
the plane of the rotating magnetic field. In this regime we observe interesting
dynamics -- the chain rotates slower than the rotating magnetic field. In order
to catch up with the magnetic field, it rolls on an edge and through rotation
in the third dimension catches up with the magnetic field. The same dynamics is
also observable for a single cube when gravitational effects are explicitly
taken into account.",2302.13978v3
2023-05-31,Magnetization dynamics in a three-dimensional interconnected nanowire array,"Three-dimensional magnetic nanostructures have recently emerged as artificial
magnetic material types with unique properties bearing potential for
applications, including magnonic devices. Interconnected magnetic nanowires are
a sub-category within this class of materials that is attracting particular
interest. We investigate the high-frequency magnetization dynamics in a cubic
array of cylindrical magnetic nanowires through micromagnetic simulations based
on a frequency-domain formulation of the linearized Landau-Lifshitz-Gilbert
equation. The small-angle high-frequency magnetization dynamics excited by an
external oscillatory field displays clear resonances at distinct frequencies.
These resonances are identified as oscillations connected to specific geometric
features and micromagnetic configurations. The geometry- and
configuration-dependence of the nanowire array's absorption spectrum
demonstrates the potential of such magnetic systems for tuneable and
reprogrammable magnonic applications.",2306.00174v1
2002-02-20,A Laboratory Plasma Experiment for Studying Magnetic Dynamics of Accretion Discs and Jets,"This work describes a laboratory plasma experiment and initial results which
should give insight into the magnetic dynamics of accretion discs and jets. A
high-speed multiple-frame CCD camera reveals images of the formation and
helical instability of a collimated plasma, similar to MHD models of disc jets,
and also plasma detachment associated with spheromak formation, which may have
relevance to disc winds and flares. The plasmas are produced by a planar
magnetized coaxial gun. The resulting magnetic topology is dependent on the
details of magnetic helicity injection, namely the force-free state eigenvalue
alpha_gun imposed by the coaxial gun.",0202380v1
2004-12-15,Importance of magnetic helicity in dynamos,"Magnetic helicity is nearly conserved and its evolution equation provides a
dynamical feedback on the alpha effect that is distinct from the conventional
algebraic alpha quenching. The seriousness of this dynamical alpha quenching is
particularly evident in the case of closed or periodic boxes. The explicit
connection with catastrophic alpha quenching is reviewed and the alleviating
effects of magnetic and current helicity fluxes are discussed.",0412366v1
1997-07-08,Nonequilibrium Magnetization Dynamics of Nickel,"Ultrafast magnetization dynamics of nickel has been studied for different
degrees of electronic excitation, using pump-probe second-harmonic generation
with 150 fs/800 nm laser pulses of various fluences. Information about the
electronic and magnetic response to laser irradiation is obtained from sums and
differences of the SHG intensity for opposite magnetization directions. The
classical M(T)-curve can be reproduced for delay times larger than the electron
thermalization time of about 280 fs, even when electrons and lattice have not
reached thermal equilibrium. Further we show that the transient magnetization
reaches its minimum approx. 50 fs before electron thermalization is completed.",9707078v1
1998-04-19,Switching dynamics between metastable ordered magnetic state and nonmagnetic ground state - A possible mechanism for photoinduced ferromagnetism -,"By studying the dynamics of the metastable magnetization of a statistical
mechanical model we propose a switching mechanism of photoinduced
magnetization. The equilibrium and nonequilibrium properties of the Blume-Capel
(BC) model, which is a typical model exhibiting metastability, are studied by
mean field theory and Monte Carlo simulation. We demonstrate reversible changes
of magnetization in a sequence of changes of system parameters, which would
model the reversible photoinduced magnetization. Implications of the calculated
results are discussed in relation to the recent experimental results for
prussian blue analogs.",9804203v3
2000-11-13,Spin Excitations in Localized and Itinerant Magnets,"Collective excitations in magnetic materials can be investigated by means of
inelastic neutron scattering. We show that this experimental method gives
access to the complete spectrum of magnetic fluctuations through the energy-
and momentum-dependence of the dynamical susceptibility. We focus on the
dynamical properties of magnets with localized spin densities and of metals.
  From such studies, microscopic parameters like exchange integrals, spin-wave
stiffness and relaxation times can be determined. This is of great help to test
current theories in magnetism.",0011220v1
1996-11-25,Dynamical mass generation in 2+1-dimensional electrodynamics in an external magnetic field,"The influence of a magnetic field on the mass generation in 2+1 dimensional
QED is considered.It is shown that the magnetic field is a catalyst of the
generation of a fermion dynamical mass. The mass arises in the system with
arbitrary number of fermions, not only with $N\leq 4$, as it is in the system
without the magnetic field. The polarization tensor is calculated for a
constant magnetic field.",9611412v1
2006-04-11,Magnetic friction due to vortex fluctuation,"We use Monte Carlo and molecular dynamics simulation to study a magnetic
tip-sample interaction. Our interest is to understand the mechanism of heat
dissipation when the forces involved in the system are magnetic in essence. We
consider a magnetic crystalline substrate composed of several layers
interacting magnetically with a tip. The set is put thermally in equilibrium at
temperature T by using a numerical Monte Carlo technique. By using that
configuration we study its dynamical evolution by integrating numerically the
equations of motion. Our results suggests that the heat dissipation in this
system is closed related to the appearing of vortices in the sample.",0604092v1
2007-01-11,A non local shell model of turbulent dynamo,"We derive a new shell model of magnetohydrodynamic (MHD) turbulence in which
the energy transfers are not necessary local. Like the original MHD equations,
the model conserves the total energy, magnetic helicity, cross-helicity and
volume in phase space (Liouville's theorem) apart from the effects of external
forcing, viscous dissipation and magnetic diffusion. In the absence of magnetic
field the model exhibits a statistically stationary kinetic energy solution
with a Kolmogorov spectrum. The dynamo action from a seed magnetic field by the
turbulent flow and the non linear interactions are studied for a wide range of
magnetic Prandtl numbers in both kinematic and dynamic cases. The non locality
of the energy transfers are clearly identified.",0701141v1
2010-01-26,Spin-wave instabilities in spin-transfer-driven magnetization dynamics,"We study the stability of magnetization precessions induced in spin-transfer
devices by the injection of spin-polarized electric currents. Instability
conditions are derived by introducing a generalized, far-from-equilibrium
interpretation of spin-waves. It is shown that instabilities are generated by
distinct groups of magnetostatically coupled spin-waves. Stability diagrams are
constructed as a function of external magnetic field and injected
spin-polarized current. These diagrams show that applying larger fields and
currents has a stabilizing effect on magnetization precessions. Analytical
results are compared with numerical simulations of spin-transfer-driven
magnetization dynamics.",1001.4806v1
2010-07-09,Polarization and magnetization dynamics of a field-driven multiferroic structure,"We consider a multiferroic chain with a linear magnetoelectric coupling
induced by the electrostatic screening at the ferroelectric/ferromagnet
interface. We study theoretically the dynamic ferroelectric and magnetic
response to external magnetic and electric fields by utilizing an approach
based on coupled Landau- Khalatnikov and finite-temperature
Landau-Lifshitz-Gilbert equations. Additionally, we compare with Monte Carlo
calculations. It is demonstrated that for material parameters corresponding to
BaTiO3/Fe the polarization and the magnetization are controllable by external
magnetic and electric fields respectively.",1007.1543v1
2011-05-11,Dynamic fields in the partial magnetization plateau of Ca3Co2O6,"Fluctuation dynamics in magnetization plateaux are a relatively poorly
explored area in frustrated magnetism. Here we use muon spin relaxation to
determine the fluctuation timescale and associated field distribution width in
the partial magnetization plateau of Ca3Co2O6. The muon spin relaxation rate
has a simple and characteristic field dependence which we model and by fitting
to the data at 15 K extract a fluctuation timescale tau = 880(30) ps and a
field distribution width Delta = 40.6(3) mT. Comparison with previous results
on Ca3Co2O6 suggests that this fluctuation timescale can be associated with
short-range, slowly fluctuating magnetic order.",1105.2200v1
2011-05-24,Low-dimensionality and predictability of solar wind and global magnetosphere during magnetic storms,"The storm index SYM-H, the solar wind velocity v, and interplanetary magnetic
field Bz show no signatures of low-dimensional dynamics in quiet periods, but
tests for determinism in the time series indicate that SYM-H exhibits a
significant low-dimensional component during storm time, suggesting that
self-organization takes place during magnetic storms. Even though our analysis
yields no discernible change in determinism during magnetic storms for the
solar wind parameters, there are significant enhancement of the predictability
and exponents measuring persistence. Thus, magnetic storms are typically
preceded by an increase in the persistence of the solar wind dynamics, and this
increase is also present in the magnetospheric response to the solar wind.",1105.4763v1
2011-06-13,Quantum Dynamics of a Nanomagnet driven by Spin-Polarized Current,"A quantum theory of magnetization dynamics of a nanomagnet as a sequence of
scatterings of each electron spin with the macrospin state of the magnetization
results in each encounter a probability distribution of the magnetization
recoil state associated with each outgoing state of the electron. The quantum
trajectory of the magnetization contains the average motion tending in the
large spin limit to the semi-classical results of spin transfer torque and the
fluctuations giving rise to a quantum magnetization noise and an additional
noise traceable to the current noise.",1106.2359v1
2013-11-07,Dynamics of two-dimensional complex plasmas in a magnetic field,"We consider a two-dimensional complex plasma layer containing charged dust
particles in a perpendicular magnetic field. Computer simulations of both
one-component and binary systems are used to explore the equilibrium particle
dynamics in the fluid state. The mobility is found to scale with the inverse of
the magnetic field strength (Bohm diffusion) for strong fields. For bidisperse
mixtures, the magnetic field dependence of the long-time mobility depends on
the particle species providing an external control of their mobility ratio. For
large magnetic fields, even a two-dimensional model porous matrix can be
realized composed by the almost immobilized high-charge particles which act as
obstacles for the mobile low-charge particles.",1311.1668v1
2015-04-23,Magnetization damping in noncollinear spin valves with antiferromagnetic interlayer couplings,"We study the magnetic damping in the simplest of synthetic antiferromagnets,
i.e. antiferromagnetically exchange-coupled spin valves in which applied
magnetic fields tune the magnetic configuration to become noncollinear. We
formulate the dynamic exchange of spin currents in a noncollinear texture based
on the spindiffusion theory with quantum mechanical boundary conditions at the
ferrromagnet|normal-metal interfaces and derive the Landau-Lifshitz-Gilbert
equations coupled by the static interlayer non-local and the dynamic exchange
interactions. We predict non-collinearity-induced additional damping that can
be sensitively modulated by an applied magnetic field. The theoretical results
compare favorably with published experiments.",1504.06042v1
2016-02-09,A problem of Ulam about magnetic fields generated by knotted wires,"In the context of magnetic fields generated by wires, we study the connection
between the topology of the wire and the topology of the magnetic lines. We
show that a generic knotted wire has a magnetic line of the same knot type, but
that given any pair of knots there is a wire isotopic to the first knot having
a magnetic line isotopic to the second. These questions can be traced back to
Ulam in 1935.",1602.03142v1
2016-04-21,Non-equilibrium magnetic fields in ab initio spin dynamics,"Starting from the continuity equation for the magnetization in time-dependent
spin-density functional theory, we derive an expression for the effective
time-dependent magnetic fields driving the out-of-equilibrium spin dynamics in
magnetic systems. We evaluate these, so called, kinetic magnetic fields in the
ultrafast demagnetization response to optical pulse excitations of
ferromagnetic iron-based materials, namely Fe$_6$ cluster and bulk bcc Fe. We
identify spatial ""hot spots"" where the demagnetization is particularly enhanced
as a result of the increased kinetic torque.",1604.06262v2
2016-09-27,Sustained Turbulence in Differentially Rotating Magnetized Fluids at Low Magnetic Prandtl Number,"We show for the first time that sustained turbulence is possible at low
magnetic Prandtl number for Keplerian flows with no mean magnetic flux. Our
results indicate that increasing the vertical domain size is equivalent to
increasing the dynamical range between the energy injection scale and the
dissipative scale. This has important implications for a large variety of
differentially rotating systems with low magnetic Prandtl number such as
protostellar disks and laboratory experiments.",1609.08543v1
2018-04-15,Re-orientation of easy axis in $\varphi_0$ junction,"We study theoretically a dynamics of $\varphi_0$ junction with direct
coupling between magnetic moment and Josephson current which shows features
close to Kapitza pendulum. We have found that starting with oscillations along
$z$-axis, the character of magnetization dynamics changes crucially and stable
position of magnetic moment $\vec m$ is realized between $z-$ and $y$-axes
depending on parameters of the system. Changes in critical current and
spin-orbit interaction lead to the different stability regions for
magnetization. An excellent agreement between analytical and numerical results
is obtained for low values of the Josephson to magnetic energy ratio.",1804.05391v1
2018-04-19,Damping of magnetization dynamics by phonon pumping,"We theoretically investigate pumping of phonons by the dynamics of a magnetic
film into a non-magnetic contact. The enhanced damping due to the loss of
energy and angular momentum shows interference patterns as a function of
resonance frequency and magnetic film thickness that cannot be described by
viscous (""Gilbert"") damping. The phonon pumping depends on magnetization
direction as well as geometrical and material parameters and is observable,
e.g., in thin films of yttrium iron garnet on a thick dielectric substrate.",1804.07080v2
2018-07-13,On the classical dynamics of charged particle in special class of spatially non-uniform magnetic field,"Motion of a charged particle in uniform magnetic field has been studied in
detail, classically as well as quantum mechanically. However, classical
dynamics of a charged particle in non-uniform magnetic field is solvable only
for some specific cases. We present in this paper, a general integral equation
for some specific class of non-uniform magnetic field and its solutions for
some of them. We also examine the supersymmetry of Hamiltonians in
exponentially decaying magnetic field with radial dependence and conclude that
this kind of non-uniformity breaks supersymmetry.",1807.05216v1
2019-03-22,Learning magnetization dynamics,"Deep neural networks are used to model the magnetization dynamics in magnetic
thin film elements. The magnetic states of a thin film element can be
represented in a low dimensional space. With convolutional autoencoders a
compression ratio of 1024:1 was achieved. Time integration can be performed in
the latent space with a second network which was trained by solutions of the
Landau-Lifshitz-Gilbert equation. Thus the magnetic response to an external
field can be computed quickly.",1903.09499v1
2019-05-10,Lagrangian formulation for emergent elastic waves in magnetic emergent crystals,"Magnetic emergent crystals are periodic alignment of ""particle-like"" spin
textures that emerge in magnets. Instead of focusing on an individual spin or a
macroscopic magnetization field, we analyze the dynamical behaviors of these
novel states by taking a solid-state point of view. Based on variational
principles, the basic equations for lattice dynamics of any emergent
crystalline states appearing in magnetic materials is established. For small
amplitude emergent elastic waves propagating in emergent crystals, the basic
equations reduce to an eigenvalue problem, from which the dispersion relation
and vibrational patterns for all emergent phonons are determined at the long
wavelength limit.",1905.03991v1
2019-12-22,Observation of magnetic solitons in two-component Bose-Einstein condensates,"We experimentally investigate the dynamics of spin solitary waves (magnetic
solitons) in a harmonically trapped, binary superfluid mixture. We measure the
in-situ density of each pseudospin component and their relative local phase via
an interferometric technique we developed, and as such, fully characterise the
magnetic solitons while they undergo oscillatory motion in the trap. Magnetic
solitons exhibit non-dispersive, dissipationless long-time dynamics. By
imprinting multiple magnetic solitons in our ultracold gas sample, we engineer
binary collisions between solitons of either same or opposite magnetisation and
map out their trajectories.",1912.10513v2
2022-04-08,Particle dynamics in non-rotating Konoplya and Zhidenko black hole immersed in an external uniform magnetic field,"In this paper, we investigate the dynamics of particles in the background of
non-rotating Konoplya and Zhidenko black hole that is immersed in an external
uniform magnetic field. The work involves circular motion of electric and
magnetic particles and particle acceleration. First the motion of electric
charged particles is considered. The effective potential, energy and angular
momentum expressions are obtained along with their graphs. The analysis of ISCO
shows that radii of ISCO decrease with magnetic interaction parameter. Motion
of magnetically charged particles has also been studied.",2204.04195v1
2022-09-28,Unidirectional magnetic coupling,"We show that interlayer Dzyaloshinskii-Moriya interaction in combination with
non-local Gilbert damping gives rise to unidirectional magnetic coupling. That
is, the coupling between two magnetic layers -- say the left and right layer --
is such that dynamics of the left layer leads to dynamics of the right layer,
but not vice versa. We discuss the implications of this result for the magnetic
susceptibility of a magnetic bilayer, electrically-actuated spin-current
transmission, and unidirectional spin-wave packet generation and propagation.
Our results may enable a route towards spin-current and spin-wave diodes and
further pave the way to design spintronic devices via reservoir engineering.",2209.14179v1
2007-02-14,Dynamic Phase Transition in the Kinetic Spin-3/2 Blume-Capel Model: Phase Diagrams in the Temperature and Crystal-Field Interaction Plane,"We analyze, within a mean-field approach, the stationary states of the
kinetic spin-3/2 Blume-Capel model by the Glauber-type stochastic dynamics and
subject to a time-dependent oscillating external magnetic field. The dynamic
phase transition points are obtained by investigating the behavior of the
dynamic magnetization as a function of temperature and as well as calculating
the Liapunov exponent. Phase diagrams are constructed in the temperature and
crystal-field interaction plane. We find five fundamental types of phase
diagrams for the different values of the reduced magnetic field amplitude
parameter (h) in which they present a disordered, two ordered phases and the
coexistences phase regions. The phase diagrams also exhibit a dynamic double
critical end point for 0<h<1.44, one dynamic tricritical point for 1.44<h<5.06
and two dynamic tricritical points for h>5.06.",0702325v1
2013-11-14,Nonequilibrium dynamics of a mixed spin-1/2 and spin-3/2 Ising ferrimagnetic system with a time dependent oscillating magnetic field source,"Nonequilibrium phase transition properties of a mixed Ising ferrimagnetic
model consisting of spin-1/2 and spin-3/2 on a square lattice under the
existence of a time dependent oscillating magnetic field have been investigated
by making use of Monte Carlo simulations with single-spin flip Metropolis
algorithm. A complete picture of dynamic phase boundary and magnetization
profiles have been illustrated and the conditions of a dynamic compensation
behavior have been discussed in detail. According to our simulation results,
the considered system does not point out a dynamic compensation behavior, when
it only includes the nearest-neighbor interaction, single-ion anisotropy and an
oscillating magnetic field source. As the next-nearest-neighbor interaction
between the spins-1/2 takes into account and exceeds a characteristic value
which sensitively depends upon values of single-ion anisotropy and only of
amplitude of external magnetic field, a dynamic compensation behavior occurs in
the system. Finally, it is reported that it has not been found any evidence of
dynamically first-order phase transition between dynamically ordered and
disordered phases, which conflicts with the recently published molecular field
investigation, for a wide range of selected system parameters.",1311.3537v2
2014-06-25,Cylindrical Ising Nanowire in an Oscillating Magnetic Field and Dynamic Compensation Temperature,"The magnetic properties of a nonequilibrium spin-1/2 cylindrical Ising
nanowire system with core/shell in an oscillating magnetic field are studied by
using a mean-field approach based on the Glauber-type stochastic dynamics
(DMFT). We employ the Glaubertype stochastic dynamics to construct set of the
coupled mean-field dynamic equations. First, we study the temperature
dependence of the dynamic order parameters to characterize the nature of the
phase transitions and to obtain the dynamic phase transition points. Then, we
investigate the temperature dependence of the total magnetization to find the
dynamic compensation points as well as to determine the type of behavior. The
phase diagrams in which contain the paramagnetic, ferromagnetic,
antiferromagnetic, nonmagnetic, surface fundamental phases and tree mixed
phases as well as reentrant behavior are presented in the reduced magnetic
field amplitude and reduced temperature plane. According to values of
Hamiltonian parameters, the compensation temperatures, or the N-, Q-, P-, R-,
S-type behaviors in the Neel classification nomenclature exist in the system.",1406.6552v1
2018-03-03,"Dynamics in a one-dimensional ferrogel model: relaxation, pairing, shock-wave propagation","Ferrogels are smart soft materials, consisting of a polymeric network and
embedded magnetic particles. Novel phenomena, such as the variation of the
overall mechanical properties by external magnetic fields, emerge consequently.
However, the dynamic behavior of ferrogels remains largely unveiled. In this
paper, we consider a one-dimensional chain consisting of magnetic dipoles and
elastic springs between them as a simple model for ferrogels. The model is
evaluated by corresponding simulations. To probe the dynamics theoretically, we
investigate a continuum limit of the energy governing the system and the
corresponding equation of motion. We provide general classification scenarios
for the dynamics, elucidating the touching/detachment dynamics of the magnetic
particles along the chain. In particular, it is verified in certain cases that
the long-time relaxation corresponds to solutions of shock-wave propagation,
while formations of particle pairs underlie the initial stage of the dynamics.
We expect that these results will provide insight into the understanding of the
dynamics of more realistic models with randomness in parameters and
time-dependent magnetic fields.",1803.01225v1
2018-07-09,Hybrid-State Free Precession in Nuclear Magnetic Resonance,"The dynamics of large spin-1/2 ensembles in the presence of a varying
magnetic field are commonly described by the Bloch equation. Most magnetic
field variations result in unintuitive spin dynamics, which are sensitive to
small deviations in the driving field. Although simplistic field variations can
produce robust dynamics, the captured information content is impoverished.
Here, we identify adiabaticity conditions that span a rich experiment design
space with tractable dynamics. These adiabaticity conditions trap the spin
dynamics in a one-dimensional subspace. Namely, the dynamics is captured by the
absolute value of the magnetization, which is in a transient state, while its
direction adiabatically follows the steady state. We define the hybrid state as
the co-existence of these two states and identify the polar angle as the
effective driving force of the spin dynamics. As an example, we optimize this
drive for robust and efficient quantification of spin relaxation times and
utilize it for magnetic resonance imaging of the human brain.",1807.03424v1
2021-01-21,Effects of the dynamical magnetization state on spin transfer,"We utilize simulations of electron scattering by a chain of dynamical quantum
spins, to analyze the interplay between the spin transfer effect and the
magnetization dynamics. We show that the complex interactions between the
spin-polarized electrons and the dynamical states of the local spins can be
decomposed into separate processes involving electron reflection and
transmission, as well as absorption and emission of magnons - the quanta of
magnetization dynamics. Analysis shows that these processes are substantially
constrained by the energy and momentum conversation laws, resulting in a
significant dependence of spin transfer on the electron's energy and the
dynamical state of the local spins. Our results suggest that exquisite control
of spin transfer efficiency and of the resulting dynamical magnetization states
may be achievable by tailoring the spectral characteristics of the conduction
electrons and of the magnetic systems.",2101.08868v1
2021-11-26,Three-dimensional Resonant Magnetization Dynamics Unraveled by Time-Resolved Soft X-ray Laminography,"The imaging of magneto-dynamical processes has been, so far, mostly a
two-dimensional business, due to the constraints of the available experimental
techniques. In this manuscript, building on the recent developments of soft
X-ray magnetic laminography, we present an experimental setup where
magneto-dynamical processes can be resolved in all three spatial dimensions and
in time, with the possibility to freely tune the frequency of the dynamical
process. We then employ this setup to investigate the three-dimensional
dynamics of two resonant magneto-dynamical modes in a CoFeB microstructure
occurring at different frequencies, namely the fundamental vortex gyration mode
and a magnetic field-induced domain wall excitation mode. This new technique
provides much needed capabilities for the experimental investigation of the
magnetization dynamics of three-dimensional magnetic systems.",2111.13533v1
2022-05-04,Exceptional points as signatures of dynamical magnetic phase transitions,"One of the most fascinating and puzzling aspects of non-Hermitian systems is
their spectral degeneracies, i.e., exceptional points (EPs), at which both
eigenvalues and eigenvectors coalesce to form a defective state space. While
coupled magnetic systems are natural hosts of EPs, the relation between the
linear and nonlinear spin dynamics in the proximity of EPs remains relatively
unexplored. Here we theoretically investigate the spin dynamics of easy-plane
magnetic bilayers in the proximity of exceptional points. We show that the
interplay between the intrinsically dissipative spin dynamics and external
drives can yield a rich dynamical phase diagram. In particular, we find that,
in antiferromagnetically coupled bilayers, a periodic oscillating dynamical
phase emerges in the region enclosed by EPs. Our results not only offer a
pathway for probing magnetic EPs and engineering magnetic nano-oscillators with
large-amplitude oscillations, but also uncover the relation between exceptional
points and dynamical phase transitions in systems displaying non-linearities.",2205.02308v2
2013-05-22,Self-consistent calculation of spin transport and magnetization dynamics,"A spin-polarized current transfers its spin-angular momentum to a local
magnetization, exciting current-induced magnetization dynamics. So far, most
studies in this field have focused on the direct effect of spin transport on
magnetization dynamics, but ignored the feedback from the magnetization
dynamics to the spin transport and back to the magnetization dynamics. Although
the feedback is usually weak, there are situations when it can play an
important role in the dynamics. In such situations, self-consistent
calculations of the magnetization dynamics and the spin transport can
accurately describe the feedback. This review describes in detail the feedback
mechanisms, and presents recent progress in self-consistent calculations of the
coupled dynamics. We pay special attention to three representative examples,
where the feedback generates non-local effective interactions for the
magnetization. Possibly the most dramatic feedback example is the dynamic
instability in magnetic nanopillars with a single magnetic layer. This
instability does not occur without non-local feedback. We demonstrate that full
self-consistent calculations generate simulation results in much better
agreement with experiments than previous calculations that addressed the
feedback effect approximately. The next example is for more typical spin valve
nanopillars. Although the effect of feedback is less dramatic because even
without feedback the current can induce magnetization oscillation, the feedback
can still have important consequences. For instance, we show that the feedback
can reduce the linewidth of oscillations, in agreement with experimental
observations. Finally, we consider nonadiabatic electron transport in narrow
domain walls. The non-local feedback in these systems leads to a significant
renormalization of the effective nonadiabatic spin transfer torque.",1305.5087v1
2005-05-29,Dynamical Aspects of Photoinduced Magnetism and Spin-Crossover phenomena in Prussian Blue Analogs,"We present dynamical properties of spin crossover compounds with
photomagnetization, proposing a new model in which the spin-crossover phenomena
and magnetic ordering are incorporated in a unified way. By using this model,
the novel characteristics observed in Prussian blue analogs are qualitatively
well reproduced. We investigate the time evolution of the magnetization and
high spin fraction taking into account multi-time scales in a master equation
approach for the magnetic relaxation, the lattice (electronic) relaxation, and
the photoexcitation process. In particular, processes with different
temperature cycles starting from the photoinduced saturated magnetic state are
studied including the effect of photoirradiation. In the temperature cycle in
the low temperature region where the high spin state has a strong
metastability, the magnetization almost exactly follows the quasi-static
process. On the other hand, when the temperature is raised above the region,
the dynamics of the spin state and the magnetization couple and exhibit various
types of dynamical cooperative phenomena under time-dependent control of
temperature and photoirradiation.",0505702v1
2006-06-23,Dynamical spin chirality and spin anisotropy in gapped S=1/2 quantum systems,"We have studied the spin anisotropy in spin-singlet ground state compounds
and the magnetic chirality, as measured by inelastic polarized neutron
scattering techniques, in the chain-sublattice of Sr14Cu24O41. In-plane and out
of plane magnetic fluctuations are measured to be anisotropic and further
discussed in the light of the current hypothesis of spin-orbit coupling. We
show that under appropriate conditions of magnetic field and neutron
polarization, the \textit{trivial} magnetic chirality selects only one of the
Zeeman splitted triplet states for scattering and erases the other one that
posses opposite helicity. Our analysis pertains to previous studies on
dynamical magnetic chirality and chiral critical exponents, where the ground
state is chiral itself, the so-called \textit{non-trivial} dynamical magnetic
chirality. As it turns out, both \textit{trivial} and \textit{non-trivial}
dynamical magnetic chirality have identical selection rules for inelastic
polarized neutron scattering experiments and it is not at all evident that they
can be distinguished in a paramagnetic compound.",0606598v1
2004-05-10,Convective plan-form two-scale dynamos in a plane layer,"We study generation of magnetic fields, involving large spatial scales, by
convective plan-forms in a horizontal layer. Magnetic modes and their growth
rates are expanded in power series in the scale ratio, and the magnetic eddy
diffusivity (MED) tensor is derived for flows, symmetric about the vertical
axis in a layer. For convective rolls magnetic eddy correction is demonstrated
to be always positive. For rectangular cell patterns, the region in the
parameter space of negative MED coincides with that of small-scale magnetic
field generation. No instances of negative MED in hexagonal cells are found. A
family of plan-forms with a smaller symmetry group than that of rectangular
cell patterns has been found numerically, where MED is negative for molecular
magnetic diffusivity over the threshold for the onset of small-scale magnetic
field generation.",0405045v2
2007-05-03,Planar spin-transfer device with a dynamic polarizer,"In planar nano-magnetic devices magnetization direction is kept close to a
given plane by the large easy-plane magnetic anisotropy, for example by the
shape anisotropy in a thin film. In this case magnetization shows effectively
in-plane dynamics with only one angle required for its description. Moreover,
the motion can become overdamped even for small values of Gilbert damping. We
derive the equations of effective in-plane dynamics in the presence of
spin-transfer torques. The simplifications achieved in the overdamped regime
allow to study systems with several dynamic magnetic pieces (``free layers'').
A transition from a spin-transfer device with a static polarizer to a device
with two equivalent magnets is observed. When the size difference between the
magnets is less than critical, the device does not exhibit switching, but goes
directly into the ``windmill'' precession state.",0705.0406v1
2015-05-28,Dynamics of an ensemble of clumps embedded in a magnetized ADAF,"We investigate effects of a global magnetic field on the dynamics of an
ensemble of clumps within a magnetized advection-dominated accretion flow by
neglecting interactions between the clumps and then solving the collisionless
Boltzman equation. In the strong-coupling limit, in which the averaged radial
and the rotational velocities of the clumps follow the ADAF dynamics, the
averaged radial velocity square of the clumps is calculated analytically for
different magnetic field configurations. The value of the averaged radial
velocity square of the clumps increases with increasing the strength of the
radial or vertical components of the magnetic field. But a purely toroidal
magnetic field geometry leads to a reduction of the value of the averaged
radial velocity square of the clumps at the inner parts with increasing the
strength of this component. Moreover, dynamics of the clumps strongly depends
on the amount of the advected energy so that the value of the averaged radial
velocity square of the clumps increases in the presence of a global magnetic
field as the flow becomes more advective.",1505.07566v1
2016-05-17,Direct observation of dynamic modes excited in a magnetic insulator by pure spin current,"Excitation of magnetization dynamics by pure spin currents has been recently
recognized as an enabling mechanism for spintronics and magnonics, which allows
implementation of spin-torque devices based on low-damping insulating magnetic
materials. Here we report the first spatially-resolved study of the dynamic
modes excited by pure spin current in nanometer-thick microscopic insulating
Yttrium Iron Garnet disks. We show that these modes exhibit nonlinear
self-broadening preventing the formation of the self-localized magnetic bullet,
which plays a crucial role in the stabilization of the single-mode
magnetization oscillations in all-metallic systems. This peculiarity associated
with the efficient nonlinear mode coupling in low-damping materials can be
among the main factors governing the interaction of pure spin currents with the
dynamic magnetization in high-quality magnetic insulators.",1605.05211v1
2016-11-18,Propagating spin-wave normal modes: A dynamic matrix approach using plane-wave demagnetizating tensors,"We present a finite-difference micromagnetic approach for determining the
normal modes of spin-waves propagating in extended magnetic films and strips,
which is based on the linearized Landau-Lifshitz equation and uses the dynamic
matrix method. The model takes into account both short range exchange
interactions and long range dipole-dipole interactions. The latter are
accounted for through plane-wave dynamic demagnetization factors, which depend
not only on the geometry and relative positions of the magnetic cells, as usual
demagnetization factors do, but also on the wave vector of the propagating
waves. Such a numerical model is most relevant when the spin-wave medium is
spatially inhomogeneous perpendicular to the direction of propagation, either
in its magnetic properties or in its equilibrium magnetic configuration. We
illustrate this point by studying surface spin-waves in magnetic bilayer films
and spin-waves channelized along magnetic domain walls in perpendicularly
magnetized strips. In both cases, dynamic dipolar interactions produce
non-reciprocity effects, where counter-propagative spin-waves have different
frequencies.",1611.06153v1
2019-04-25,Electrical response of S-F-TI-S junctions on magnetic texture dynamics,"We consider a hybrid structure consisting of superconducting or normal leads
with a combined ferromagnet-3D topological insulator interlayer. We compare
responses of a Josephson junction and a normal junction to magnetic texture
dynamics. In both cases the electromotive force resulting from the
magnetization dynamics generates a voltage between the junction leads. For an
open circuit this voltage is the same for normal and superconducting leads and
allows for electrical detection of magnetization dynamics and a structure of a
given magnetic texture. However, under the applied current the electrical
response of the Josephson junction is essentially different due to the strong
dependence of the critical Josephson current on the magnetization direction and
can be used for experimental probing of this dependence. We propose a setup,
which is able to detect a defect motion and to provide detailed information
about the structure of magnetic inhomogeneity. The discussed effect could be of
interest for spintronics applications.",1904.11193v3
2019-05-29,Extracting the Dynamic Magnetic Contrast in Time-Resolved X-ray Transmission Microscopy,"Using a time-resolved detection scheme in scanning transmission X-ray
microscopy (STXM) we measured element resolved ferromagnetic resonance (FMR) at
microwave frequencies up to 10\,GHz and a spatial resolution down to 20\,nm at
two different synchrotrons. We present different methods to separate the
contribution of the background from the dynamic magnetic contrast based on the
X-ray magnetic circular dichroism (XMCD) effect. The relative phase between the
GHz microwave excitation and the X-ray pulses generated by the synchrotron, as
well as the opening angle of the precession at FMR can be quantified. A
detailed analysis for homogeneous and inhomogeneous magnetic excitations
demonstrates that the dynamic contrast indeed behaves as the usual XMCD effect.
The dynamic magnetic contrast in time-resolved STXM has the potential be a
powerful tool to study the linear and non-linear magnetic excitations in
magnetic micro- and nano-structures with unique spatial-temporal resolution in
combination with element selectivity.",1905.12497v2
2023-06-23,Spintronic reservoir computing without driving current or magnetic field,"Recent studies have shown that nonlinear magnetization dynamics excited in
nanostructured ferromagnets are applicable to brain-inspired computing such as
physical reservoir computing. The previous works have utilized the
magnetization dynamics driven by electric current and/or magnetic field. This
work proposes a method to apply the magnetization dynamics driven by voltage
control of magnetic anisotropy to physical reservoir computing, which will be
preferable from the viewpoint of low-power consumption. The computational
capabilities of benchmark tasks in single MTJ are evaluated by numerical
simulation of the magnetization dynamics and found to be comparable to those of
echo-state networks with more than 10 nodes.",2306.13270v1
2000-10-07,Properties of Simulated Magnetized Galaxy Clusters,"We study the evolution of magnetized clusters in a cosmological environment
using magneto-hydro dynamical simulations. Large scale flows and merging of
subclumps generate shear flows leading to Kelvin-Helmholtz instabilities,
which, in addition to the compression of the gas where the magnetic field is
frozen in, further amplify the magnetic field during the evolution of the
cluster. Therefore, well-motivated initial magnetic fields of
$<B^2>^{1/2}=10^{-9} {\rm G}$ reach the observed $\sim\mu{\rm G}$ field
strengths in the cluster cores at $z=0$. These magnetized clusters can be used
to study the final magnetic field structure, the dynamical importance of
magnetic fields for the interpretation of observed X-Ray properties, and help
to constrain further processes in galaxy clusters like the population of
relativistic particles giving rise to the observed radio halos or the behavior
of magnetized cooling flows.",0010149v1
2010-01-15,Control of the chirality and polarity of magnetic vortices in triangular nanodots,"Magnetic vortex dynamics in lithographically prepared nanodots is currently a
subject of intensive research, particularly after recent demonstration that the
vortex polarity can be controlled by in-plane magnetic field. This has
stimulated the proposals of non-volatile vortex magnetic random access
memories. In this work, we demonstrate that triangular nanodots offer a real
alternative where vortex chirality, in addition to polarity, can be controlled.
In the static regime, we show that vortex chirality can be tailored by applying
in-plane magnetic field, which is experimentally imaged by means of
Variable-Field Magnetic Force Microscopy. In addition, the polarity can be also
controlled by applying a suitable out-of-plane magnetic field component. The
experiment and simulations show that to control the vortex polarity, the
out-of-plane field component, in this particular case, should be higher than
the in-plane nucleation field. Micromagnetic simulations in the dynamical
regime show that the magnetic vortex polarity can be changed with
short-duration magnetic field pulses, while longer pulses change the vortex
chirality.",1001.2682v1
2014-05-09,Magnetization dynamics and damping due to electron-phonon scattering in a ferrimagnetic exchange model,"We present a microscopic calculation of magnetization damping for a magnetic
""toy model."" The magnetic system consists of itinerant carriers coupled
antiferromagnetically to a dispersionless band of localized spins, and the
magnetization damping is due to coupling of the itinerant carriers to a phonon
bath in the presence of spin-orbit coupling. Using a mean-field approximation
for the kinetic exchange model and assuming the spin-orbit coupling to be of
the Rashba form, we derive Boltzmann scattering integrals for the distributions
and spin coherences in the case of an antiferromagnetic exchange splitting,
including a careful analysis of the connection between lifetime broadening and
the magnetic gap. For the Elliott-Yafet type itinerant spin dynamics we extract
dephasing and magnetization times T_1 and T_2 from initial conditions
corresponding to a tilt of the magnetization vector, and draw a comparison to
phenomenological equations such as the Landau-Lifshitz or the Gilbert damping.
We also analyze magnetization precession and damping for this system including
an anisotropy field and find a carrier mediated dephasing of the localized spin
via the mean-field coupling.",1405.2347v1
2014-06-26,Spinmotive force due to motion of magnetic bubble arrays driven by magnetic field gradient,"Interaction between local magnetization and conduction electrons is
responsible for a variety of phenomena in magnetic materials. It has been
recently shown that spin current and associated electric voltage can be induced
by magnetization that depends on both time and space. This effect, called
spinmotive force, provides for a powerful tool for exploring the dynamics and
the nature of magnetic textures, as well as a new source for electromotive
force. Here we theoretically demonstrate the generation of electric voltages in
magnetic bubble array systems subjected to a magnetic field gradient. It is
shown by deriving expressions for the electric voltages that the present system
offers a direct measure of phenomenological parameter that describes
non-adiabaticity in the current induced magnetization dynamics. This spinmotive
force opens a door for new types of spintronic devices that exploit the
field-gradient.",1406.6964v2
2014-11-10,Induced magnetic moment in the magnetic catalysis of chiral symmetry breaking,"The chiral symmetry breaking in a Nambu-Jona-Lasinio effective model of
quarks in the presence of a magnetic field is investigated. We show that new
interaction tensor channels open up via Fierz identities due to the explicit
breaking of the rotational symmetry by the magnetic field. We demonstrate that
the magnetic catalysis of chiral symmetry breaking leads to the generation of
two independent condensates, the conventional chiral condensate and a spin-one
condensate. While the chiral condensate generates, as usual, a dynamical
fermion mass, the new condensate enters as a dynamical anomalous magnetic
moment in the dispersion of the quasiparticles. Since the pair, formed by a
quark and an antiquark with opposite spins, possesses a resultant magnetic
moment, an external magnetic field can align it giving rise to a net magnetic
moment for the ground state. The two condensates contribute to the effective
mass of the LLL quasiparticles in such a way that the critical temperature for
chiral symmetry restoration becomes enhanced.",1411.2493v1
2016-04-03,Spin relaxation signature of colossal magnetic anisotropy in platinum atomic chains,"Recent experimental data demonstrate emerging magnetic order in platinum
atomically thin nanowires. Furthermore, an unusual form of magnetic anisotropy
-- colossal magnetic anisotropy (CMA) -- was earlier predicted to exist in
atomically thin platinum nanowires. Using spin dynamics simulations based on
first-principles calculations, we here explore the spin dynamics of atomically
thin platinum wires to reveal the spin relaxation signature of colossal
magnetic anisotropy, comparing it with other types of anisotropy such as
uniaxial magnetic anisotropy (UMA). We find that the CMA alters the spin
relaxation process distinctly and, most importantly, causes a large speed-up of
the magnetic relaxation compared to uniaxial magnetic anisotropy. The magnetic
behavior of the nanowire exhibiting CMA should be possible to identify
experimentally at the nanosecond time scale for temperatures below 5 K. This
time-scale is accessible in e.g., soft x-ray free electron laser experiments.",1604.00626v1
2018-12-03,Engineering planar transverse domain walls in biaxial magnetic nanostrips by tailoring transverse magnetic fields with uniform orientation,"Designing and realizing various magnetization textures in magnetic
nanostructures are essential for developing novel magnetic nanodevices in
modern information industry. Among all these textures, planar transverse domain
walls (pTDWs) are the simplest and the most basic, which make them popular in
device physics. In this work, we report the engineering of pTDWs with arbitrary
tilting attitude in biaxial magnetic nanostrips by transverse magnetic field
profiles with uniform orientation but tunable strength distribution. Both
statics and axial-field-driven dynamics of these pTDWs are analytically
investigated. It turns out that for statics these pTDWs are robust again
disturbances which are not too abrupt, while for dynamics it can be tailored to
acquire higher velocity than Walker's ansatz predicts. These results should
provide inspirations for designing magnetic nanodevices with novel
one-dimensional magnetization textures, such as 360$^\circ$ walls, or even
two-dimensional ones, for example vortices, skyrmions, etc.",1812.00581v1
2019-07-23,Transverse magnetic field influence on wakefield in complex plasmas,"We present the results of an investigation of the wakefield around a
stationary charged grain in an external magnetic field with non-zero transverse
component with respect to the ion flow direction. The impact of the orientation
of magnetic field on the wake behavior in streaming complex plasmas is
assessed. In contrast to previously reported significant suppression of the
wake oscillations due to longitudinal magnetic field applied along flow, in the
presence of transverse to flow magnetic field the wakefield exhibits a long
range recurrent oscillations. Extensive investigation for a wide range of
parameters reveal that in the sonic and supersonic regimes the wake has strong
dependence on the direction of the magnetic field and exhibits sensitivity to
even a meager deviation of magnetic field from the longitudinal orientation.
The tool obtained with the study of impact of transverse component of magnetic
field on the wake around grain in streaming ions can be used to potentially
maneuver the grain-grain interaction to achieve controlled grain dynamics.",1907.09833v2
2021-03-24,Current-induced magnetization dynamics in single and double layer magnetic nanopillars grown by molecular beam epitaxy,"Molecular beam epitaxy is used to fabricate magnetic single and double layer
junctions which are deposited in prefabricated nanostencil masks. For all Co |
Cu | Co double layer junctions we observe a stable intermediate resistance
state which can be reached by current starting from the parallel configuration
of the respective ferromagnetic layers. The generation of spin waves is
investigated at room temperature in the frequency domain by spectrum analysis,
demonstrating both in-plane and out-of-plane precessions of the magnetization
of the free magnetic layer. Current-induced magnetization dynamics in magnetic
single layer junctions of Cu | Co | Cu has been investigated in magnetic fields
which are applied perpendicular to the magnetic layer. We find a hysteretic
switching in the current sweeps with resistance changes significantly larger
than the anisotropic magnetoresistance effect.",2103.13099v1
2021-06-10,Conserving Local Magnetic Helicity in Numerical Simulations,"Magnetic helicity is robustly conserved in systems with very large magnetic
Reynolds numbers, including most systems of astrophysical interest, and unlike
kinetic and magnetic energy is not dissipated at small scales. This plays a
major role in suppressing the kinematic large-scale dynamo and may also be
responsible for driving the large-scale dynamo through the magnetic helicity
flux. Numerical simulations of astrophysical systems typically lack sufficient
resolution to enforce global magnetic helicity over several dynamical times. In
these simulations, magnetic helicity is lost either through numerical errors or
through the action of an unrealistically large resistivity. Errors in the
internal distribution of magnetic helicity are equally important and typically
larger. Here we propose an algorithm for enforcing strict local conservation of
magnetic helicity in the Coulomb gauge in numerical simulations so that their
evolution more closely approximates that of real systems.",2106.06078v2
2021-11-24,Thermal generation of droplet soliton in chiral magnet,"Controlled creation of localized magnetic textures beyond conventional
$\pi$-skyrmions is an important problem in the field of magnetism. Here by
means of spin dynamics simulations, Monte Carlo simulations and harmonic
transition state theory we demonstrate that an elementary chiral magnetic
soliton with zero topological charge -- the chiral droplet -- can be reliably
created by thermal fluctuations in the presence of the tilted magnetic field.
The proposed protocol relies on an unusual kinetics combining the effects of
the entropic stabilization and low energy barrier for the nucleation of a
topologically-trivial state. Following this protocol by varying temperature and
the tilt of the external magnetic field one can selectively generate chiral
droplets or $\pi$-skyrmions in a single system. The coexistence of two distinct
magnetic solitons establishes a basis for a rich magnetization dynamics and
opens up the possibility for the construction of more complex magnetic textures
such as skyrmion bags and skyrmions with chiral kinks.",2111.12342v1
2022-02-03,Magnetic domain wall dynamics under external electric field in bilayer CrI$_3$,"Motivated by manipulating the magnetic order of bilayer CrI$_3$, we carry out
microscopic calculations to find the magnetic order and various magnetic
domains of the system in the presence of an electric field. Making use of
density functional simulations, a spin model Hamiltonian is introduced
consisting of isotropic exchange couplings, Dzyaloshinskii-Moriya (DM)
interaction, and on-site magnetic anisotropy. The spin dynamics of two
well-known states of bilayer CrI$_3$, low temperature (LT) and high temperature
(HT) phases, are obtained by solving the Landau-Lifshitz-Gilbert equation. We
show that the magnetic texture is stacking-dependent in bilayer CrI$_3$ and
stable magnetic domains can appear in the HT stack which are tunable by
external electric and magnetic fields. Therefore, we suggest that the HT phase
represents a promising candidate for data storage in the modern generation of
spintronic devices working on magnetic domain engineering.",2202.01394v2
2022-02-20,Magneto-optical Spectroscopy with RAMBO: A Table-Top 30 T Magnet,"Optically probing materials in high magnetic fields can provide enlightening
insight into field-modified electronic states and phases, while optically
driving materials in high magnetic fields can induce novel nonequilibrium
many-body dynamics of spin and charge carriers. While there are high-field
magnets compatible with standard optical spectroscopy methods, they are
generally bulky and have limited optical access, which prohibit performing
state-of-the-art ultrafast and/or nonlinear optical experiments. The Rice
Advanced Magnet with Broadband Optics (RAMBO), a unique 30-T pulsed mini-coil
magnet system with direct optical access, has enabled previously challenging
experiments using femtosecond optical pulses, including time-domain terahertz
spectroscopy, in cutting-edge materials placed in strong magnetic fields. Here,
we review recent experimental advances made possible by the first-generation
RAMBO setup. After summarizing technological aspects of combining optical
spectroscopic techniques with the mini-coil magnet, we describe results of
magneto-optical studies of a wide variety of materials, providing new insight
into the states and dynamics of four types of quasiparticles in solids -
excitons, plasmons, magnons, and phonons - in high magnetic fields.",2202.09732v1
2022-10-09,Method of dynamic resonance tuning in spintronics of nanosystems,"A method is advanced allowing for fast regulation of magnetization direction
in magnetic nanosystems. The examples of such systems are polarized
nanostructures, magnetic nanomolecules, magnetic nanoclusters, magnetic
graphene, dipolar and spinor trapped atoms, and quantum dots. The emphasis in
the paper is on magnetic nanomolecules and nanoclusters. The method is based on
two principal contrivances: First, the magnetic sample is placed inside a coil
of a resonant electric circuit creating a feedback field, and second, there is
an external magnetic field that can be varied so that to dynamically support
the resonance between the Zeeman frequency of the sample and the natural
frequency of the circuit during the motion of the sample magnetization. This
method can find applications in the production of memory devices and other
spintronic appliances.",2210.04262v1
2022-11-15,Nonlinear chiral photocurrent in parity-violating magnetic Weyl semimetals,"The strong correlation between the non-trivial band topology and the magnetic
texture makes magnetic Weyl semimetals excellent candidates for the
manipulation and detection of magnetization dynamics. The parity violation
together with the Pauli blocking cause only one Weyl node to contribute to the
photocurrent response, which in turn affects the magnetic texture due to the
spin transfer torque. Utilizing the Landau-Lifshitz-Gilbert equation and the
spin-transfer torque in non-centrosymmetric Weyl magnets, we show that the
chiral photocurrent rotates the magnetization from the easy c axis to the a or
b axis, which leads to an exotic current next to the photocurrent response. The
chiral photocurrent is calculated in the context of quantum kinetic theory and
it has a strong resonance on the order of mA/W near the Weyl nodes, the
magnitude of which is controlled by the momentum relaxation time. Remarkably,
we study the influence of magnetic texture dynamics on the topological
nonlinear photocurrent response, including shift and injection currents along
with the new chiral photocurrent, and show that both the magnitude and the
in-plane orientation of the chiral photocurrent are strongly correlated with
the direction of the magnetic moments.",2211.08521v1
2023-03-27,Magnetic manipulation of superparamagnetic colloids in droplet-based optical devices,"Magnetically assembled superparamagnetic colloids have been exploited as
fluid mixers, swimmers and delivery systems in several microscale applications.
The encapsulation of such colloids in droplets may open new opportunities to
build magnetically controlled displays and optical components. Here, we study
the assembly of superparamagnetic colloids inside droplets under rotating
magnetic fields and exploit this phenomenon to create functional optical
devices. Colloids are encapsulated in monodisperse droplets produced by
microfluidics and magnetically assembled into dynamic two-dimensional clusters.
Using an optical microscope equipped with a magnetic control setup, we
investigate the effect of the magnetic field strength and rotational frequency
on the size, stability and dynamics of 2D colloidal clusters inside droplets.
Our results show that cluster size and stability depend on the magnetic forces
acting on the structure under the externally imposed field. By rotating the
cluster in specific orientations, we illustrate how magnetic fields can be used
to control the effective refractive index and the transmission of light through
the colloid-laden droplets, thus demonstrating the potential of the
encapsulated colloids in optical applications.",2303.15336v1
2009-03-13,Spin-torque driven magnetization dynamics in a nanocontact setup for low external fields: numerical simulation study,"We present numerical simulation studies of the steady-state magnetization
dynamics driven by a spin-polarized current in a point contact geometry for the
case of a relatively large contact diameter (D = 80 nm) and small external
field (H = 30 Oe). We show, that under these conditions the magnetization
dynamics is qualitatively different from the dynamics observed for small
contacts in large external fields. In particular, the 'bullet' mode with a
homogeneous mode core, which was the dominating localized mode for small
contacts, is not found here. Instead, all localized oscillation modes observed
in simulations correspond to different motion kinds of vortex-antivortex (V-AV)
pairs. These kinds include rotational and translational motion of pairs with
the V-AV distance d ~ D and creation/annihilation of much smaller (satellite)
V-AV pairs. We also show that for the geometry studied here the Oersted field
has a qualitative effect on the magnetization dynamics of a 'free' layer. This
effect offers a possibility to control magnetization dynamics by a suitable
electric contact setup, optimized to produce a desired Oersted field. Finally,
we demonstrate that when the magnetization dynamics of the 'fixed' layer
(induced only by the stray field interaction with the 'free' layer) is taken
into account, the threshold current for the oscillation onset is drastically
reduced and new types of localized modes appear. In conclusion, we show that
our simulations reproduce semiquantitatively several important features of the
magnetization dynamics in a point contact system for low external fields
reported experimentally.",0903.2416v1
2006-09-10,Nonlinear shear-current dynamo and magnetic helicity transport in sheared turbulence,"The nonlinear mean-field dynamo due to a shear-current effect in a nonhelical
homogeneous turbulence with a mean velocity shear is discussed. The transport
of magnetic helicity as a dynamical nonlinearity is taken into account. The
shear-current effect is associated with the ${\bf W} {\bf \times} {\bf J}$ term
in the mean electromotive force, where ${\bf W}$ is the mean vorticity due to
the large-scale shear motions and ${\bf J}$ is the mean electric current. This
effect causes the generation of large-scale magnetic field in a turbulence with
large hydrodynamic and magnetic Reynolds numbers. The dynamo action due to the
shear-current effect depends on the spatial scaling of the correlation time
$\tau(k)$ of the background turbulence, where $k$ is the wave number. For
Kolmogorov scaling, $\tau(k) \propto k^{-2/3}$, the dynamo instability occurs,
while when $\tau(k) \propto k^{-2}$ (small hydrodynamic and magnetic Reynolds
numbers) there is no the dynamo action in a sheared nonhelical turbulence. The
magnetic helicity flux strongly affects the magnetic field dynamics in the
nonlinear stage of the dynamo action. Numerical solutions of the nonlinear
mean-field dynamo equations which take into account the shear-current effect,
show that if the magnetic helicity flux is not small, the saturated level of
the mean magnetic field is of the order of the equipartition field determined
by the turbulent kinetic energy. Turbulence with a large-scale velocity shear
is a universal feature in astrophysics, and the obtained results can be
important for elucidation of origin of the large-scale magnetic fields in
astrophysical sheared turbulence.",0609256v1
2015-11-24,Dynamical magnetoelectric phenomena of multiferroic skyrmions,"Magnetic skyrmions, vortex-like swirling spin textures characterized by a
quantized topological invariant, realized in chiral-lattice magnets are
currently attracting intense research interest. In particular, their dynamics
under external fields is an issue of vital importance both for fundamental
science and for technical application. Whereas observations of magnetic
skyrmions had been limited to metallic magnets so far, their realization was
discovered also in a chiral-lattice insulating magnet Cu2OSeO3 in 2012.
Skyrmions in the insulator turned out to exhibit multiferroic nature with
spin-induced ferroelectricity. Strong magnetoelectric coupling between
noncollinear skyrmion spins and electric polarizations mediated by relativistic
spin-orbit interaction enables us to drive motion and oscillation of magnetic
skyrmions by application of electric fields instead of injection of electric
currents. Insulating materials also provide an environment suitable for
detection of pure spin dynamics through spectroscopic measurements owing to
absence of appreciable charge excitations. In this article, we review recent
theoretical and experimental studies on multiferroic properties and dynamical
magnetoelectric phenomena of magnetic skyrmions in insulators. We argue that
multiferroic skyrmions show unique coupled oscillation modes of magnetizations
and polarizations, so-called electromagnon excitations, which are both
magnetically and electrically active, and interference between the electric and
magnetic activation processes leads to peculiar magnetoelectric effects in a
microwave frequency regime.",1511.07761v1
2018-07-21,Uniform and Nonuniform Precession of a Nanoparticle with Finite Anisotropy in a Liquid: Opportunities and Limitations for Magnetic Fluid Hyperthermia,"We focus on an in-depth study of the forced dynamics of a ferromagnetic
single-domain uniaxial nanoparticle placed in a viscous fluid and driven by an
external rotating magnetic field. The process of conversion of magnetic and
mechanical energies into heat is a physical basis for magnetic fluid
hyperthermia that is very promising for cancer treatment. The dynamical
approximation allows us to establish the limits of the heating rate and
understand the logic of selection of the system parameters to optimize the
therapy. Based on the developed analytical and numerical tools, we analyze from
a single viewpoint the synchronous and asynchronous rotation of the
nanoparticle or/and its magnetization in the following three cases. For the
beginning, we actualize the features of the internal magnetic dynamics, when
the nanoparticle body is supposed to be fixed. Then, we study the rotation of
the whole nanoparticle, when its magnetization is supposed to be locked to the
crystal lattice. And, finally, we realize the analysis of the coupled motion,
when the internal magnetic dynamics is performed in the rotated nanoparticle
body. In all these cases, we describe analytically the uniform mode, or
synchronous rotation along with an external field, while the nonuniform mode,
or asynchronous rotation, is investigated numerically.",1807.08120v1
2019-05-08,Flare Energy Release at the Magnetic Field Polarity Inversion Line During M1.2 Solar Flare of 2015 March 15. II. Investigation of Photospheric Electric Current and Magnetic Field Variations Using HMI 135-second Vector Magnetograms,"This work is a continuation of Paper I [Sharykin et al., 2018] devoted to
analysis of nonthermal electron dynamics and plasma heating in the confined
M1.2 class solar flare SOL2015-03-15T22:43 revealing energy release in the
highly sheared interacting magnetic loops in the low corona, above the polarity
inversion line (PIL). The scope of the present work is to make the first
extensive quantitative analysis of the photospheric magnetic field and
photospheric vertical electric current (PVEC) dynamics in the confined flare
region near the PIL using new vector magnetograms obtained with the
Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory
(SDO) with high temporal resolution of 135 s. Data analysis revealed sharp
changes of the magnetic structure and PVEC associated with the flare onset near
the PIL. It was found that the strongest plasma heating and electron
acceleration were associated with the largest increase of the magnetic
reconnection rate, total PVEC and effective PVEC density in the flare ribbons.
Observations and non-linear force-free field (NLFFF) extrapolations showed that
the magnetic field structure around the PIL is consistent with the
tether-cutting magnetic reconnection (TCMR) geometry. We gave qualitative
interpretation of the observed dynamics of the flare ribbons, magnetic field
and PVEC, and electron acceleration, within the TCMR scenario.",1905.03352v2
1999-06-17,Small-scale turbulent dynamo,"Kinematic dynamo theory is presented here for turbulent conductive fluids. We
describe how inhomogeneous magnetic fluctuations are generated below the
viscous scale of turbulence where the spatial smoothness of the velocity
permits a systematic analysis of the Lagrangian path dynamics. We find
analytically the moments and multi-point correlation functions of the magnetic
field at small yet finite magnetic diffusivity. We show that the field is
concentrated in long narrow strips and describe anomalous scalings and angular
singularities of the multi-point correlation functions which are manifestations
of field's intermittency. The growth rate of the magnetic field in a typical
realization is found to be half the difference of two Lyapunov exponents of the
same sign.",9906030v1
2008-07-28,Static and dynamic properties of vortices in anisotropic magnetic disks,"We investigate the effect of the magnetic anisotropy ($K_z$) on the static
and dynamic properties of magnetic vortices in small disks. Our micromagnetic
calculations reveal that for a range of $K_z$ there is an enlargement of the
vortex core. We analyze the influence of $K_z$ on the dynamics of the vortex
core magnetization reversal under the excitation of a pulsed field. The
presence of $K_z$, which leads to better resolved vortex structures, allows us
to discuss in more details the role played by the in-plane and perpendicular
components of the gyrotropic field during the vortex-antivortex nucleation and
annihilation.",0807.4546v1
2009-01-22,Monopole and Topological Electron Dynamics in Adiabatic Spintronic and Graphene Systems,"A unified theoretical treatment is presented to describe the physics of
electron dynamics in semiconductor and graphene systems. Electron spin fast
alignment with the Zeeman magnetic field (physical or effective) is treated as
a form of adiabatic spin evolution which necessarily generates a monopole in
magnetic space. One could transform this monopole into the physical and
intuitive topological magnetic fields in the useful momentum (K) or real spaces
(R). The physics of electron dynamics related to spin Hall, torque,
oscillations and other technologically useful spinor effects can be inferred
from the topological magnetic fields in spintronic, graphene and other SU(2)
systems.",0901.3411v1
2010-02-10,"Dynamics of Rotating, Magnetized Neutron Stars","Using a fully general relativistic implementation of ideal
magnetohydrodynamics with no assumed symmetries in three spatial dimensions,
the dynamics of magnetized, rigidly rotating neutron stars are studied.
Beginning with fully consistent initial data constructed with Magstar, part of
the Lorene project, we study the dynamics and stability of rotating, magnetized
polytropic stars as models of neutron stars. Evolutions suggest that some of
these rotating, magnetized stars may be minimally unstable occurring at the
threshold of black hole formation.",1002.2217v1
2010-10-22,Dipole-Quadrupole dynamics during magnetic field reversals,"The shape and the dynamics of reversals of the magnetic field in a turbulent
dynamo experiment are investigated. We report the evolution of the dipolar and
the quadrupolar parts of the magnetic field in the VKS experiment, and show
that the experimental results are in good agreement with the predictions of a
recent model of reversals: when the dipole reverses, part of the magnetic
energy is transferred to the quadrupole, reversals begin with a slow decay of
the dipole and are followed by a fast recovery, together with an overshoot of
the dipole. Random reversals are observed at the borderline between stationary
and oscillatory dynamos.",1010.4771v1
2011-03-05,Dynamical TAP equations for non-equilibrium Ising spin glasses,"We derive and study dynamical TAP equations for Ising spin glasses obeying
both synchronous and asynchronous dynamics using a generating functional
approach. The system can have an asymmetric coupling matrix, and the external
fields can be time-dependent. In the synchronously updated model, the TAP
equations take the form of self consistent equations for magnetizations at time
$t+1$, given the magnetizations at time $t$. In the asynchronously updated
model, the TAP equations determine the time derivatives of the magnetizations
at each time, again via self consistent equations, given the current values of
the magnetizations. Numerical simulations suggest that the TAP equations become
exact for large systems.",1103.1044v1
2011-06-12,Qutrit: entanglement dynamics in the finite qutrit chain in the consistent magnetic field,"Based on the Liouville-von Neumann equation, we obtain closed system of
equations for the description of a qutrit or coupled qutrits in arbitrary
time-dependent external magnetic field. The dependence of the dynamics on the
initial states and magnetic field modulation is studied analytically and
numerically. We compare the relative entanglement measure's dynamics in the
bi-qutrit system with permutation particle symmetry. We find the magnetic field
modulation which retains the entanglement in the system of two coupled qutrits.
Analytical formulas for entanglement measures in the chain from 2 to 6 qutrits
are presented.",1106.2297v1
2012-05-03,Dynamical Friction in a magnetized gas,"When a gravitating point mass moves subsonically through a magnetized and
isothermal medium, the dynamical structure of the flow is studied far from the
mass using a perturbation analysis. Analytical solutions for the first-order
density and the velocity perturbations are presented. Validity of our solutions
is restricted to the cases where the Alfven velocity in the ambient medium is
less than the accretor's velocity. The density field is less dense because of
the magnetic effects according to the solutions and the dynamical friction
force becomes lower as the strength of the magnetic field increases.",1205.0795v1
2013-02-01,Entanglement dynamics in finite qudit chain in consistent magnetic field,"Based on the Liouville-von Neumann equation, we obtain a closed system of
equations for the description of a qutrit or coupled qutrits in an arbitrary,
time-dependent, external magnetic field. The dependence of the dynamics on the
initial states and the magnetic field modulation is studied analytically and
numerically. We compare the relative entanglement measure's dynamics in
bi-qudits with permutation particle symmetry. We find the magnetic field
modulation which retains the entanglement in the system of two coupled qutrits.
Analytical formulae for the entanglement measures in finite chains from 2 to 6
qutrits or 3 quartits are presented.",1302.0105v1
2013-03-06,A Quantum Approach of Meso-Magnet Dynamics with Spin Transfer Torque,"We present a theory of magnetization dynamics driven by spin-polarized
current in terms of the quantum master equation. In the spin coherent state
representation, the master equation becomes a Fokker-Planck equation, which
naturally includes the spin transfer and quantum fluctuation. The current
electron scattering state is correlated to the magnet quantum states, giving
rise to quantum correction to the electron transport properties in the usual
semiclassical theory. In the large spin limit, the magnetization dynamics is
shown to obey the Hamilton-Jacobi equation or the Hamiltonian canonical
equations.",1303.1250v2
2013-09-18,Dynamical barrier for flux penetration in a superconducting film in the flux flow state,"The penetration of transverse magnetic flux into a thin superconducting
square film in the flux flow state is considered by numerical simulation. Due
to the film self-field, the governing equations are nonlinear, and in
combination with the finite viscosity of the moving vortices, this sets up a
dynamical barrier for flux penetration into the sample. The corresponding
magnetization loop is hysteric, with the peak in magnetization shifted from the
zero position. The magnetic field in increasing applied field is found to form
a well-defined front of propagation. Numerical estimates shows that the
dynamical barrier should be measurable on films with low volume pinning.",1309.4641v1
2015-08-09,Laser-induced spatiotemporal dynamics of magnetic films,"We present a theory for the coherent magnetization dynamics induced by a
focused ultrafast laser beam in magnetic films, taking into account nonthermal
(inverse Faraday effect) and thermal (heating) actuation. The dynamic
conversion between spin waves and phonons is induced by the magnetoelastic
coupling that allows efficient propagation of angular momentum. The anisotropy
of the magnetoelastic coupling renders characteristic angle dependences of the
magnetization propa-gation that are strikingly different for thermal and
nonthermal actuation.",1508.02094v2
2018-06-14,Boundary effects on the magnetic Hamiltonian dynamics in two dimensions,"We study the Hamiltonian dynamics of a charged particle submitted to a pure
magnetic field in a two-dimensional domain. We provide conditions on the
magnetic field in a neighbourhood of the boundary to ensure the confinement of
the particle. We also prove a formula for the scattering angle in the case of
radial magnetic fields.",1806.05395v1
2020-02-10,Collisionless particle dynamic in an axi-symmetric diamagnetic trap,"Particle dynamic in an axi-symmetric mirror machine with an extremely high
plasma pressure equal to pressure of vacuum magnetic field (so-called regime of
diamagnetic confinement) is investigated. Extrusion of magnetic field from
central region due to plasma diamagnetism leads to non-conservation of the
magnetic moment and can result in chaotic movement and fast losses of
particles. The following mechanisms can provide particle confinement for
unlimited time: absolute confinement of particles with high azimuthal velocity
and conservation of adiabatic invariant for particle moving in smooth magnetic
field. The criteria of particle confinement and estimations of lifetime of
unconfined particles are obtained and verified in direct numerical simulation.
Particle confinement time in the diamagnetic trap in regime of gas-dynamic
outflow is discussed.",2002.03535v1
2020-08-07,Anisotropic Ultrafast Spin Dynamics in Epitaxial Cobalt,"We investigate the ultrafast spin dynamics in an epitaxial hcp(1100) cobalt
thin film. By performing pump-probe magneto-optical measurements with the
magnetization along either the easy or hard magnetic axis, we determine the
demagnetization and recovery times for the two axes. We observe a 35% slower
dynamics along the easy magnetization axis, which we attribute to
magneto-crystalline anisotropy of the electron-phonon coupling, supported by
our ab initio calculations. This points towards an unambiguous and previously
undisclosed role of anisotropic electron-lattice coupling in ultrafast
magnetism.",2008.03119v1
2021-11-16,Dynamic alignment and plasmoid formation in relativistic magnetohydrodynamic turbulence,"We present high resolution 2D and 3D simulations of magnetized decaying
turbulence in relativistic resistive magneto-hydrodynamics. The simulations
show dynamic formation of large scale intermittent long-lived current sheets
being disrupted by the tearing instability into plasmoid chains. These current
sheets are locations of enhanced magnetic field dissipation and heating of the
plasma. We find magnetic energy spectra $\propto k^{-3/2}$, together with
strongly pronounced dynamic alignment of Elsasser fields and of velocity and
magnetic fields, for strong guide-field turbulence, whereas we retrieve spectra
$\propto k^{-5/3}$ for the case of a weak guide-field.",2111.08188v1
2021-11-12,Chaotic dynamics of a suspended string in a gravitational background with magnetic field,"We study the effects of a magnetic field on the chaotic dynamics of a string
with endpoints on the boundary of an asymptotically AdS$_5$ space with black
hole. We study Poincar\'e sections and compute the Lyapunov exponents for the
string perturbed from the static configuration, for two different orientations,
with position of the endpoints on the boundary orthogonal and parallel to the
magnetic field. We find that the magnetic field stabilizes the string dynamics,
with the largest Lyapunov exponent remaining below the
Maldacena-Shenker-Stanford bound.",2111.09441v1
2023-03-28,Role of intersublattice exchange interaction on ultrafast longitudinal and transverse magnetization dynamics in Permalloy,"We report about element specific measurements of ultrafast demagnetization
and magnetization precession damping in Permalloy (Py) thin films.
Magnetization dynamics induced by optical pump at $1.5$eV is probed
simultaneously at the $M_{2,3}$ edges of Ni and Fe with High order Harmonics
for moderate demagnetization rates (less than $50$%). The role of the
intersublattice exchange interaction on both longitudinal and transverse
dynamics is analyzed with a Landau Lifshitz Bloch description of
ferromagnetically coupled Fe and Ni sublattices. It is shown that the
intersublattice exchange interaction governs the dissipation during
demagnetization as well as precession damping of the magnetization vector.",2303.15837v1
2023-11-30,Excitation of the Gyrotropic Mode in a Magnetic Vortex by Time-Varying Strain,"We demonstrate excitation of the gyrotropic mode in a magnetostrictive vortex
by time-varying strain. The vortex dynamics is driven by a time-varying voltage
applied to the piezoelectric substrate and detected electrically by spin
rectification at subthreshold values of rf current. When the frequency of the
time-varying strain matches the gyrotropic frequency at given in-plane magnetic
field, the strain-induced in-plane magnetic anisotropy leads to a resonant
excitation of the gyration dynamics in a magnetic vortex. We show that
nonlinear gyrotropic dynamics can be excited already for moderate amplitudes of
the time-varying strain.",2311.18517v1
1999-02-05,Nonlinear Dynamics of A Damped Magnetic Oscillator,"We consider a damped magnetic oscillator, consisting of a permanent magnet in
a periodically oscillating magnetic field. A detailed investigation of the
dynamics of this dissipative magnetic system is made by varying the field
amplitude $A$. As $A$ is increased, the damped magnetic oscillator, albeit
simple looking, exhibits rich dynamical behaviors such as symmetry-breaking
pitchfork bifurcations, period-doubling transitions to chaos,
symmetry-restoring attractor-merging crises, and saddle-node bifurcations
giving rise to new periodic attractors. Besides these familiar behaviors, a
cascade of ``resurrections'' (i.e., an infinite sequence of alternating
restabilizations and destabilizations) of the stationary points also occurs. It
is found that the stationary points restabilize (destabilize) through
alternating subcritical (supercritical) period-doubling and pitchfork
bifurcations. We also discuss the critical behaviors in the period-doubling
cascades.",9902005v1
2002-11-26,Magnetic Z(N) symmetry in 2+1 dimensions,"This review describes the role of magnetic symmetry in 2+1 dimensional gauge
theories. In confining theories without matter fields in fundamental
representation the magnetic symmetry is spontaneously broken. Under some mild
assumptions, the low-energy dynamics is determined universally by this
spontaneous breaking phenomenon. The degrees of freedom in the effective theory
are magnetic vortices. Their role in confining dynamics is similar to that
played by pions and sigma in the chiral symmetry breaking dynamics.
  I give an explicit derivation of the effective theory in (2+1)-dimensional
weakly coupled confining models and argue that it remains qualitatively the
same in strongly coupled (2+1)-dimensional gluodynamics. Confinement in this
effective theory is a very simple classical statement about the long range
interaction between topological solitons, which follows (as a result of a
simple direct classical calculation) from the structure of the effective
Lagrangian. I show that if fundamentally charged dynamical fields are present
the magnetic symmetry becomes local rather than global. The modifications to
the effective low energy description in the case of heavy dynamical fundamental
matter are discussed. This effective lagrangian naturally yields a bag like
description of baryonic excitations. I also discuss the fate of the magnetic
symmetry in gauge theories with the Chern-Simons term.",0211248v1
2012-01-26,Dynamics of Magnetized Vortex Tubes in the Solar Chromosphere,"We use 3D radiative MHD simulations to investigate the formation and dynamics
of small-scale (less than 0.5 Mm in diameter) vortex tubes spontaneously
generated by turbulent convection in quiet-Sun regions with initially weak mean
magnetic fields. The results show that the vortex tubes penetrate into the
chromosphere and substantially affect the structure and dynamics of the solar
atmosphere. The vortex tubes are mostly concentrated in intergranular lanes and
are characterized by strong (near sonic) downflows and swirling motions that
capture and twist magnetic field lines, forming magnetic flux tubes that expand
with height and which attain magnetic field strengths ranging from 200 G in the
chromosphere to more than 1 kG in the photosphere. We investigate in detail the
physical properties of these vortex tubes, including thermodynamic properties,
flow dynamics, and kinetic and current helicities, and conclude that magnetized
vortex tubes provide an important path for energy and momentum transfer from
the convection zone into the chromosphere.",1201.5442v1
2017-09-27,Geometric Dynamics of Magnetization: Electronic Contribution,"To give a general description of the influences of electric fields or
currents on magnetization dynamics, we developed a semiclassical theory for the
magnetization implicitly coupled to electronic degrees of freedom. In the
absence of electric fields the Bloch electron Hamiltonian changes the Berry
curvature, the effective magnetic field, and the damping in the dynamical
equation of the magnetization, which we classify into intrinsic and extrinsic
effects. Static electric fields modify these as first-order perturbations,
using which we were able to give a physically clear interpretation of the
current-induced spin-orbit torques. We used a toy model mimicking a
ferromagnet-topological-insulator interface to illustrate the various effects,
and predicted an anisotropic gyromagnetic ratio and the dynamical stability for
an in-plane magnetization. Our formalism can also be applied to the slow
dynamics of other order parameters in crystalline solids.",1709.09513v3
2018-08-27,Dynamics of the antiferromagnetic skyrmion induced by a magnetic anisotropy gradient,"The dynamics of antiferromagnets is a current hot topic in condensed matter
physics and spintronics. However, the dynamics of insulating antiferromagnets
cannot be excited by an electric current, which is a method usually used to
manipulate ferromagnetic metals. Here, we propose to use the voltage-controlled
magnetic anisotropy gradient as an excitation source to manipulate insulating
antiferromagnetic textures. We analytically and numerically study the dynamics
of an antiferromagnetic skyrmion driven by a magnetic anisotropy gradient. Our
analytical calculations demonstrate that such a magnetic anisotropy gradient
can effectively drive an antiferromagnetic skyrmion towards the area of lower
magnetic anisotropy. The micromagnetic simulations are in good agreement with
our analytical solution. Furthermore, the magnetic anisotropy gradient induced
velocity of an antiferromagnetic skyrmion is compared with that of a
ferromagnetic skyrmion. Our results are useful for the understanding of
antiferromagnetic skyrmion dynamics and may open a new way for the design of
antiferromagnetic spintronic devices.",1808.08664v2
2019-10-03,Finite size effects on the ultrafast remagnetization dynamics of FePt,"We investigate the ultrafast magnetization dynamics of FePt in the L10 phase
after an optical heating pulse, as used in heat assisted magnetic recording. We
compare continuous and nanogranular thin films and emphasize the impact of the
finite size on the remagnetization dynamics. The remagnetization speeds up
significantly with increasing external magnetic field only for the continuous
film, where domain wall motion governs the dynamics. The ultrafast
remagnetization dynamics in the continuous film are only dominated by heat
transport in the regime of high magnetic fields, whereas the timescale required
for cooling is prevalent in the granular film for all magnetic field strengths.
These findings highlight the necessary conditions for studying the intrinsic
heat transport properties in magnetic materials.",1910.01413v1
2023-04-28,Competing signatures of intersite and interlayer spin transfer in the ultrafast magnetization dynamics,"Optically driven intersite and interlayer spin transfer are individually
known as the fastest processes for manipulating the spin order of magnetic
materials on the sub 100 fs time scale. However, their competing influence on
the ultrafast magnetization dynamics remains unexplored. In our work, we show
that optically induced intersite spin transfer (also known as OISTR) dominates
the ultrafast magnetization dynamics of ferromagnetic alloys such as Permalloy
(Ni80Fe20) only in the absence of interlayer spin transfer into a substrate.
Once interlayer spin transfer is possible, the influence of OISTR is
significantly reduced and interlayer spin transfer dominates the ultrafast
magnetization dynamics. This provides a new approach to control the
magnetization dynamics of alloys on extremely short time scales by fine-tuning
the interlayer spin transfer.",2304.14957v1
2023-05-24,Time-reversal Invariance Violation and Quantum Chaos Induced by Magnetization in Ferrite-Loaded Resonators,"We investigate the fluctuation properties in the eigenfrequency spectra of
flat cylindrical microwave cavities that are homogeneously filled with
magnetized ferrite. These studies are motivated by experiments in which only
small pieces of ferrite were embedded in the cavity and magnetized with an
external static magnetic field to induce partial time-reversal (T ) invariance
violation. We use two different shapes of the cavity, one exhibiting an
integrable wave dynamics, the other one a chaotic one. We demonstrate that in
the frequency region where only transverse-magnetic modes exist, the
magnetization of the ferrites has no effect on the wave dynamics and does not
induce T -invariance violation whereas it is fully violated above the cutoff
frequency of the first transverse-electric mode. Above all, independently of
the shape of the resonator, it induces a chaotic wave dynamics in that
frequency range in the sense that for both resonator geometries the spectral
properties coincide with those of quantum systems with a chaotic classical
dynamics and same invariance properties under application of the generalized T
operator associated with the resonator geometry.",2305.14753v1
2013-12-30,Dynamic behaviors of the hexagonal Ising nanowire,"By utilizing the effective-field theory based on the Glauber-type stochastic
dynamics, the dynamic behaviors of the hexagonal Ising nanowire (HIN) system in
the presence of a time dependent magnetic field are obtained. The time
variations of average order parameters and the thermal behavior of the dynamic
order parameters are studied to analyze the nature of transitions and to obtain
the dynamic phase transition points. The dynamic phase diagrams are introduced
in the plane of the reduced temperature versus magnetic field amplitude. The
dynamic phase diagrams exhibit coexistence phase region, several ordered phases
and critical point as well as a reentrant behavior.",1312.7786v1
2023-04-28,Persistent dynamic magnetic state in artificial honeycomb spin ice,"Topological magnetic charges, arising due to the non-vanishing magnetic flux
on spin ice vertices, serve as the origin of magnetic monopoles that traverse
the underlying lattice effortlessly. Unlike spin ice materials of atomic
origin, the dynamic state in artificial honeycomb spin ice is conventionally
described in terms of finite size domain wall kinetics that require magnetic
field or current application. Contrary to this common understanding, here we
show that thermally tunable artificial permalloy honeycomb lattice manifests a
perpetual dynamic state due to self-propelled magnetic charge defect relaxation
in the absence of any external tuning agent. Quantitative investigation of
magnetic charge defect dynamics using neutron spin echo spectroscopy reveals
sub-ns relaxation times that are comparable to monopole's relaxation in bulk
spin ices. Most importantly, the kinetic process remains unabated at low
temperature where thermal fluctuation is negligible. This suggests that dynamic
phenomena in honeycomb spin ice are mediated by quasi-particle type entities,
also confirmed by quantum Monte-Carlo simulations that replicate the kinetic
behavior. Our research unveils a new `macroscopic' magnetic particle that
shares many known traits of quantum particles, namely magnetic monopole and
magnon.",2305.00093v1
2023-07-12,Quasi-static magnetization dynamics in a compensated ferrimagnetic half-metal -- Mn$_2$Ru$_x$Ga,"Exploring anisotropy and diverse magnetization dynamics in specimens with
vanishing magnetic moments presents a significant challenge using traditional
magnetometry, as the low resolution of existing techniques hinders the ability
to obtain accurate results. In this study, we delve deeper into the examination
of magnetic anisotropy and quasi-static magnetization dynamics in \mrg\,(MRG)
thin films, as an example of a compensated ferrimagnetic half-metal, by
employing anomalous Hall effect measurements within a tetragonal crystal
lattice system. Our research proposes an innovative approach to accurately
determine the complete set of anisotropy constants of these MRG thin films. To
achieve this, we perform anomalous Hall voltage curve fitting, using torque
models under the macrospin approximation, which allow us to obtain out-of-plane
anisotropy constants $K_1=4.0\times10^4$ J m$^{-3}$ ($K_1/M=0.655$\,T) and
$K_2=2.54\times10^4$ J m$^{-3}$ ($K_2/M=0.416$\,T), along with a weaker
in-plane anisotropy constant $K_3=3.48\times10^3$ J m$^{-3}$
($K_3/M=0.057$\,T). By additionally employing first-order reversal curves
(FORC) and classical Preisach hysteresis (hysterons) models, we are able to
validate the efficacy of the macrospin model in capturing the magnetic behavior
of MRG thin films. Furthermore, our investigation substantiates that the
complex quasi-static magnetization dynamics of MRG thin films can be
effectively modelled using a combination of hysteronic and torque models. This
approach facilitates the exploration of both linear and non-linear quasi-static
magnetization dynamics, in the presence of external magnetic field and/or
current-induced effective fields, generated by the spin-orbit torque and spin
transfer torque mechanisms.",2307.06403v1
2000-12-20,Numerical Analysis of Magnetic Field Amplification by Turbulence,"We apply a Fourier spectral numerical method to 3D incompressible MHD
turbulence with a magnetic Prandtl number $Pr \geq 1$. We examine the processes
by which an initially weak, large-scale seed magnetic field and an initially
weak, small-scale, impulse-like seed magnetic field are amplified. We find that
in both cases the magnetic energy spectrum grows at all scales. The growth
rates at different amplification stages are analyzed. For a large-scale seed
magnetic field, the magnetic energy density grows as $\sim t^2$ for the first
few turbulence eddy turnover times, followed by a dynamic growth stage, where
nonlinear interactions between different scales of the turbulence contribute to
an exponential growth rate that is largely determined by the turbulence eddy
turnover time. For a seed magnetic field that is initially set up at a small
scale in the turbulence, during the kinematic development stage, the growth
rate of magnetic energy is $\propto 1/\tau_{max}$, where $\tau_{max}$ is the
eddy turnover time of the smallest eddies of the turbulence. The kinematic
growth stage is followed by a dynamic growth stage, where nonlinearity plays
important role. During such dynamic growth stage, the growth rate of total
magnetic energy is determined by both the magnetic energy amplification within
the turbulence inertial range and that within the turbulence dissipation range.",0012447v2
2015-02-23,Table-top Measurement of Local Magnetization Dynamics Using Picosecond Thermal Gradients: Toward Nanoscale Magnetic Imaging,"Recent advances in nanoscale magnetism have demonstrated the potential for
spin-based technology including magnetic random access memory, nanoscale
microwave sources, and ultra-low power signal transfer. Future engineering
advances and new scientific discoveries will be enabled by research tools
capable of examining local magnetization dynamics at length and time scales
fundamental to spatiotemporal variations in magnetic systems - typically 10 -
200 nm and 5 - 50 ps. A key problem is that current table-top magnetic
microscopy cannot access both of these scales simultaneously. In this letter,
we introduce a spatiotemporal magnetic microscopy that uses
magneto-thermoelectric interactions to measure local magnetization via the
time-resolved anomalous Nernst effect (TRANE). By generating a short-lived,
local thermal gradient, the magnetic moment is transduced into an electrical
signal. Experimentally, we show that TRANE microscopy has time resolution below
30 ps and spatial resolution limited by the thermal excitation area.
Furthermore, we present numerical simulations to show that the thermal spot
size sets the limits of the spatial resolution, even at 50 nm. The thermal
effects used for TRANE microscopy have no fundamental limit on their spatial
resolution, therefore a future TRANE microscope employing a scanning plasmon
antenna could enable measurements of nanoscale magnetic dynamics.",1502.06505v1
2019-02-04,Magnetic eddy viscosity of mean shear flows in two-dimensional magnetohydrodynamics,"Magnetic induction in magnetohydrodynamic fluids at magnetic Reynolds number
(Rm) less than~1 has long been known to cause magnetic drag. Here, we show that
when $\mathrm{Rm} \gg 1$ and the fluid is in a hydrodynamic-dominated regime in
which the magnetic energy is much smaller than the kinetic energy, induction
due to a mean shear flow leads to a magnetic eddy viscosity. The magnetic
viscosity is derived from simple physical arguments, where a coherent response
due to shear flow builds up in the magnetic field until decorrelated by
turbulent motion. The dynamic viscosity coefficient is approximately
$(B_p^2/2\mu_0) \tau_{\rm corr}$, the poloidal magnetic energy density
multiplied by the correlation time. We confirm the magnetic eddy viscosity
through numerical simulations of two-dimensional incompressible
magnetohydrodynamics. We also consider the three-dimensional case, and in
cylindrical or spherical geometry, theoretical considerations similarly point
to a nonzero viscosity whenever there is differential rotation. Hence, these
results serve as a dynamical generalization of Ferraro's law of isorotation.
The magnetic eddy viscosity leads to transport of angular momentum and may be
of importance to zonal flows in astrophysical domains such as the interior of
some gas giants.",1902.01105v3
2019-03-02,Microwave excitations and hysteretic magnetization dynamics of stripe domain films,"FeNi films with the stripe domain (SD) pattern are prepared by
electrodeposition and sputtering methods. The magnetic domain, static magnetic
parameters, and quality factor, as well as dynamic properties of the two films,
are respectively performed. The results show the magnetizations of the film
were dependent on the direction of SD, and the rotation of the SD is lagging
behind the magnetization reversal. The microwave properties of the SD emerge
dynamic hysteresis before the saturation magnetic field. These microwave
properties are selectively excited with acoustic mode, optical mode, and
spin-wave mode. The frequency and intensity of different resonance modes of
stripe domain are determined by the local magnetization. The magnetization
variations and the rotation of SD of different modes are further illuminated by
the micromagnetic simulation. The magnetic anisotropy and the resonance
intensity of permeability of different modes were finally described by the
modified resonance equations.",1903.00656v2
2023-10-25,Post-dynamical inspiral phase of common envelope evolution. The role of magnetic fields,"During common envelope evolution, an initially weak magnetic field may
undergo amplification by interacting with spiral density waves and turbulence
generated in the stellar envelope by the inspiralling companion. Using 3D
magnetohydrodynamical simulations on adaptively refined spherical grids with
excised central regions, we studied the amplification of magnetic fields and
their effect on the envelope structure, dynamics, and the orbital evolution of
the binary during the post-dynamical inspiral phase. About $95\%$ of magnetic
energy amplification arises from magnetic field stretching, folding, and
winding due to differential rotation and turbulence while compression against
magnetic pressure accounts for the remaining $\sim 5\%$. Magnetic energy
production peaks at a scale of $3a_\text{b}$, where $a_\text{b}$ is the
semimajor axis of the central binary's orbit. Because the magnetic energy
production declines at large radial scales, the conditions are not favorable
for the formation of magnetically collimated bipolar jet-like outflows unless
they are generated on small scales near the individual cores, which we did not
resolve. Magnetic fields have a negligible impact on binary orbit evolution,
mean kinetic energy, and the disk-like morphology of angular momentum
transport, but turbulent Maxwell stress can dominate Reynolds stress when
accretion onto the central binary is allowed, leading to an $\alpha$-disk
parameter of $\simeq 0.034$. Finally, we discovered accretion streams arising
from the stabilizing effect of the magnetic tension from the toroidal field
about the orbital plane, which prevents overdensities from being destroyed by
turbulence and enables them to accumulate mass and eventually migrate toward
the binary.",2310.16880v2
2024-01-24,Magnetism and spin dynamics of an S=3/2 frustrated trillium lattice antiferromagnet K2CrTi(PO4)3,"Competing magnetic interactions, frustration driven quantum fluctuations, and
spin correlations offer an ideal route for the experimental realization of
emergent quantum phenomena and exotic quasi particle excitations in 3D
frustrated magnets. In this context, trillium lattice, wherein magnetic ions
decorate a three-dimensional chiral network of corner-shared equilateral
triangular motifs, provides a viable ground. Herein, we present the crystal
structure, magnetic susceptibility, specific heat, electron spin-resonance,
muSR results on the polycrystalline samples of K2CrTi(PO4)3 wherein the Cr3+
ions form a perfect trillium lattice without any detectable anti-site disorder.
The Curie-Weiss fit of the magnetic susceptibility data above 100 K yields a
Curie-Weiss temperature of -23 K, which indicates the presence of dominant
antiferromagnetic interactions between Cr3+ (S=3/2) moments. The specific heat
measurements reveal the occurrence of two consecutive phase transitions, at
temperatures TL = 4.3 K and TH = 8 K, corresponding to two different magnetic
phases and it unveils the existence of short-range spin correlations above the
ordering temperature TH. The power-law behavior of ESR linewidth suggests the
persistence of short-range spin correlations over a relatively wide critical
region in agreement with the specific heat results. The muSR resultsprovide
concrete evidence of two different phases corresponding to two transitions,
coupled with thecritical slowing down of spin fluctuations above TL and
persistent spin dynamics below TL, consistent with the thermodynamic results.
Moreover, the muSR results reveal the coexistence of static and dynamic local
magnetic fields below TL, signifying the presence of complex magnetic phases
owing to the entwining of spin correlations and competing magnetic interactions
in this three-dimensional frustrated magnet.",2401.13445v1
2002-05-16,Helicity Balance and Steady-State Strength of the Dynamo Generated Galactic Magnetic Field,"We demonstrate that the inclusion of the helicity flux in the magnetic
helicity balance in the nonlinear stage of galactic dynamo action results in a
radical change in the magnetic field dynamics. The equilibrium value of the
large-scale magnetic field is then approximately the equipartition level. This
is in contrast to the situation without the flux of helicity, when the magnetic
helicity is conserved locally, which leads to substantially subequipartition
values for the equilibrium large-scale magnetic field.",0205266v1
2003-02-20,Turbulent magnetic Prandtl number and magnetic diffusivity quenching from simulations,"Forced turbulence simulations are used to determine the turbulent kinematic
viscosity, nu_t, from the decay rate of a large scale velocity field. Likewise,
the turbulent magnetic diffusivity, eta_t, is determined from the decay of a
large scale magnetic field. In the kinematic regime, when the field is weak,
the turbulent magnetic Prandtl number, nu_t/eta_t, is about unity. When the
field is nonhelical, eta_t is quenched when magnetic and kinetic energies
become comparable. For helical fields the quenching is stronger and can be
described by a dynamical quenching formula.",0302425v2
2003-10-28,On magnetic field generation in Kolmogorov turbulence,"We analyze the initial, kinematic stage of magnetic field evolution in an
isotropic and homogeneous turbulent conducting fluid with a rough velocity
field, v(l) ~ l^alpha, alpha<1. We propose that in the limit of small magnetic
Prandtl number, i.e. when ohmic resistivity is much larger than viscosity, the
smaller the roughness exponent, alpha, the larger the magnetic Reynolds number
that is needed to excite magnetic fluctuations. This implies that numerical or
experimental investigations of magnetohydrodynamic turbulence with small
Prandtl numbers need to achieve extremely high resolution in order to describe
magnetic phenomena adequately.",0310780v2
2005-04-27,Magnetic-field generation in helical turbulence,"We investigate analytically the amplification of a weak magnetic field in a
homogeneous and isotropic turbulent flow lacking reflectional symmetry (helical
turbulence). We propose that the spectral distributions of magnetic energy and
magnetic helicity can be found as eigenmodes of a self-adjoint,
Schr\""odinger-type system of evolution equations. We argue that large-scale and
small-scale magnetic fluctuations cannot be effectively separated, and that the
conventional alpha-model is, in general, not an adequate description of the
large-scale dynamo mechanism. As a consequence, the correct numerical modeling
of such processes should resolve magnetic fluctuations down to the very small,
resistive scales.",0504588v2
2004-01-28,Microscopic mechanisms of magnetization reversal,"Two principal scenarios of magnetization reversal are considered. In the
first scenario all spins perform coherent motion and an excess of magnetic
energy directly goes to a nonmagnetic thermal bath. A general dynamic equation
is derived which includes a tensor damping term similar to the
Bloch-Bloembergen form but the magnetization magnitude remains constant for any
deviation from equilibrium. In the second reversal scenario, the absolute value
of the averaged sample magnetization is decreased by a rapid excitation of
nonlinear spin-wave resonances by uniform magnetization precession. We have
developed an analytic k-space micromagnetic approach that describes this entire
reversal process in an ultra-thin soft ferromagnetic film for up to 90^{o}
deviation from equilibrium. Conditions for the occurrence of the two scenarios
are discussed.",0401590v1
2005-10-05,Magnetization dynamics using ultrashort magnetic field pulses,"Very short and well shaped magnetic field pulses can be generated using
ultra-relativistic electron bunches at Stanford Linear Accelerator. These
fields of several Tesla with duration of several picoseconds are used to study
the response of magnetic materials to a very short excitation. Precession of a
magnetic moment by 90 degrees in a field of 1 Tesla takes about 10 picoseconds,
so we explore the range of fast switching of the magnetization by precession.",0510121v2
2004-07-22,Anomalous magnetic moment in parity-conserving QED3,"In this article we derive the anomalous magnetic moment of fermions in
(2+1)-dimensional parity-conserving QED3, in the presence of an externally
applied constant magnetic field. We use a spectral representation of the photon
propagator to avoid infrared divergences. We also discuss the scaling with the
magnetic field intensity in the case of strong external fields, where there is
dynamical mass generation for fermions induced by the magnetic field itself
(magnetic catalysis). The results of this paper may be of relevance to the
physics of high-temperature superconductors.",0407265v1
2006-09-19,Magnetic Induced Axion Mass,"We study the effect of a uniform magnetic field on the dynamics of axions. In
particular, we show that the Peccei-Quinn symmetry is explicitly broken by the
presence of an external magnetic field. This breaking is induced by the
non-conservation of the magnetic helicity and generates an electromagnetic
contribution to the axion mass. We compute the magnetic axion mass in one loop
approximation, with no restriction on the intensity of the magnetic field, and
including thermal effects.",0609199v1
2006-04-08,On the photon anomalous magnetic moment,"It is shown that due to radiative corrections a photon having a non vanishing
component of its momentum perpendicular to it, bears a non-zero magnetic
moment. All modes of propagation of the polarization operator in one loop
approximation are discussed and in this field regime the dispersion equation
and the corresponding magnetic moment are derived. Near the first thresholds of
cyclotron resonance the photon magnetic moment has a peak larger than the
electron anomalous magnetic moment. Related to this magnetic moment, the
arising of some sort of photon ""dynamical mass"" and a gyromagnetic ratio are
discussed. These latter results might be interesting in an astrophysical
context.",0604059v1
2003-02-24,"Remarks on magnetic flows and magnetic billiards, Finsler metrics and a magnetic analog of Hilbert's fourth problem","We interpret magnetic billiards as Finsler ones and describe an analog of the
string construction for magnetic billiards. Finsler billiards for which the law
""angle of incidence equals angle of reflection"" are described. We characterize
the Finsler metrics in the plane whose geodesics are circles of a fixed radius.
This is a magnetic analog of Hilbert's fourth problem asking to describe the
Finsler metrics whose geodesics are straight lines.",0302288v1
2006-11-26,Optical Magnetometry,"Some of the most sensitive methods of measuring magnetic fields utilize
interactions of resonant light with atomic vapor. Recent developments in this
vibrant field are improving magnetometers in many traditional areas such as
measurement of geomagnetic anomalies and magnetic fields in space, and are
opening the door to new ones, including, dynamical measurements of bio-magnetic
fields, detection of nuclear magnetic resonance (NMR), magnetic-resonance
imaging (MRI), inertial-rotation sensing, magnetic microscopy with cold atoms,
and tests of fundamental symmetries of Nature.",0611246v1
2007-05-12,"Comment on six papers published by M.A. El-Hakiem and his co-workers in International Communications in Heat and Mass Transfer, Journal of Magnetism and Magnetic Materials and Heat and Mass Transfer","Comment on six papers published by M.A. El-Hakiem and his co-workers in
International Communications in Heat and Mass Transfer, Journal of Magnetism
and Magnetic Materials and Heat and Mass Transfer",0705.1803v1
2008-01-02,On scaling laws in turbulent magnetohydrodynamic Rayleigh-Benard convection,"We invoke the concepts of magnetic boundary layer and magnetic Rayleigh
number and use the magnetic energy dissipation rates in the bulk and the
boundary layers to derive some scaling laws expressing how Nusselt number
depends on magnetic Rayleigh number, Prandtl number and magnetic Prandtl number
for the simple case of turbulent magnetohydrodynamic Rayleigh-Benard convection
in the presence of uniform vertical magnetic field.",0801.0347v2
2008-01-23,Magnetic properties of the extended periodic Anderson model,"We study magnetic properties of the extended periodic Anderson model, which
includes electron correlations within and between itinerant and localized
bands. By combining dynamical mean-field theory with the numerical
renormalization group we calculate the sublattice magnetization and the
staggered susceptibility to determine the phase diagram in the particle-hole
symmetric case. We find that two kinds of magnetically ordered states compete
with the Kondo insulating state at zero temperature, which induces
non-monotonic behavior in the temperature-dependent magnetization. It is
furthermore clarified that a novel magnetic metallic state is stabilized at
half filling by the competition between Hund's coupling and the hybridization.",0801.3668v1
2008-11-06,Magnetic exchange interaction between rare-earth and Mn ions in multiferroic hexagonal manganites,"We report a study of magnetic dynamics in multiferroic hexagonal manganite
HoMnO3 by far-infrared spectroscopy. Low-temperature magnetic excitation
spectrum of HoMnO3 consists of magnetic-dipole transitions of Ho ions within
the crystal-field split J=8 manifold and of the triangular antiferromagnetic
resonance of Mn ions. We determine the effective spin Hamiltonian for the Ho
ion ground state. The magnetic-field splitting of the Mn antiferromagnetic
resonance allows us to measure the magnetic exchange coupling between the
rare-earth and Mn ions.",0811.0999v1
2009-06-30,Determination of a flow generating a neutral magnetic mode,"The problem of reconstruction of a flow of conducting incompressible fluid
generating a given magnetic mode is considered. We use the magnetic induction
equation to derive ordinary differential equations along the magnetic field
lines, which give an opportunity to determine the generating flow, if
additional data is provided on a two-dimensional manifold transversal to
magnetic field lines, and show that an arbitrary solenoidal vector field can
not be a neutral magnetic mode sustained by any flow of conducting fluid.",0906.5431v1
2009-07-25,Magnetic Nanoparticle Assemblies,"This chapter provides an introduction to the fundamental physical ideas and
models relevant to the phenomenon of magnetic hysteresis in nanoparticle
assemblies. The concepts of single-domain particles and superparamagnetism are
discussed. The mechanisms of magnetization by coherent rotation and the role of
temperature in the gradual decay of magnetization are analyzed in the framework
of simple analytical models. Modern numerical techniques (Monte Carlo
simulations, Magnetization Dynamics) used to study dense nanoparticle
assemblies are presented. An overview of the most common experimental
techniques used to measure the magnetic hysteresis effect in nanoparticle
assemblies are presented and the underlying principles are exposed.",0907.4417v2
2009-09-08,"Memory, Aging and Spin Glass Nature: A Study of NiO Nanoparticles","We report studies on magnetization dynamics in NiO nanoparticles of average
size 5 nm. Temperature and time dependence of dc magnetization, wait time
dependence of magnetic relaxation (aging) and memory phenomena in the dc
magnetization are studied with various temperature and field protocols. We
observe that the system shows memory and aging in field cooled and zero field
cooled magnetization measurements. These experiments show that the magnetic
behavior of NiO nanoparticles is similar to spin glasses. We argue that the
spin glass behavior originates from the freezing of spins at the surface of the
individual particles.",0909.1391v2
2010-01-11,Nonlocal effects on magnetism in the diluted magnetic semiconductor Ga_{1-x}Mn_{x}As,"The magnetic properties of the diluted magnetic semiconductor
Ga_{1-x}Mn_{x}As are studied within the dynamical cluster approximation. We use
the k-dot-p Hamiltonian to describe the electronic structure of GaAs with
spin-orbit coupling and strain effects. We show that nonlocal effects are
essential for explaining the experimentally observed transition temperature and
saturation magnetization. We also demonstrate that the cluster anisotropy is
very strong and induces rotational frustration and a cube-edge direction
magnetic anisotropy at low temperature. With this, we explain the
temperature-driven spin reorientation in this system.",1001.1716v1
2010-02-17,Isothermal remanent magnetization and the spin dimensionality of spin glasses,"The isothermal remanent magnetization is used to investigate dynamical
magnetic properties of spatially three dimensional spin glasses with different
spin dimensionality (Ising, XY, Heisenberg). The isothermal remanent
magnetization is recorded vs. temperature after intermittent application of a
weak magnetic field at a constant temperature $T_h$. We observe that in the
case of the Heisenberg spin glasses, the equilibrated spin structure and the
direction of the excess moment are recovered at $T_h$. The isothermal remanent
magnetization thus reflects the directional character of the
Dzyaloshinsky-Moriya interaction present in Heisenberg systems.",1002.3209v1
2011-05-22,Generalization of radiative jet energy loss to non-zero magnetic mass,"Reliable predictions for jet quenching in ultra-relativistic heavy ion
collisions require accurate computation of radiative energy loss. With this
goal, an energy loss formalism in a realistic finite size dynamical QCD medium
was recently developed. While this formalism assumes zero magnetic mass - in
accordance with the one-loop perturbative calculations - different
non-perturbative approaches report a non-zero magnetic mass at RHIC and LHC. We
here generalize the energy loss to consistently include a possibility for
existence of non-zero magnetic screening. We also present how the inclusion of
finite magnetic mass changes the energy loss results. Our analysis indicates a
fundamental constraint on magnetic to electric mass ratio.",1105.4359v1
2011-11-02,Dynamics of the magnetic flux penetration into type II superconductors,"The magnetic flux penetration into the half-space superconductor sample is
studied in the flux flow regime in parallel geometry assuming that an external
magnetic field increasing with time in accordance with the power law. Assuming
that the flux flow resistivity as a power-law function of the magnetic field
induction we found an exact analytical solution for the nonlinear local
magnetic flux diffusion equation. The obtained solution describes space-time
distribution of the magnetic induction in the sample.",1111.0397v2
2012-02-18,Measuring spectrum of spin wave using vortex dynamics,"We propose to measure the spectrum of magnetic excitation in magnetic
materials using motion of vortex lattice driven by both ac and dc current in
superconductors. When the motion of vortex lattice is resonant with oscillation
of magnetic moments, the voltage decreases at a given current. From transport
measurement, one can obtain frequency of the magnetic excitation with the wave
number determined by vortex lattice constant. By changing the lattice constant
through applied magnetic fields, one can obtains the spectrum of the magnetic
excitation up to a wave vector of order $10\rm{\ nm^{-1}}$.",1202.4048v1
2012-02-27,Fast magnetization reversal of nanoclusters in resonator,"An effective method for ultrafast magnetization reversal of nanoclusters is
suggested. The method is based on coupling a nanocluster to a resonant electric
circuit. This coupling causes the appearance of a magnetic feedback field
acting on the cluster, which drastically shortens the magnetization reversal
time. The influence of the resonator properties, nanocluster parameters, and
external fields on the magnetization dynamics and reversal time is analyzed.
The magnetization reversal time can be made many orders shorter than the
natural relaxation time. The reversal is studied for both the cases of a single
nanocluster as well as for the system of many nanoclusters interacting through
dipole forces.",1202.5873v1
2012-10-15,Dynamic Control of Magnetically Trapped Indirect Excitons by Using External Magnetic Bias,"We demonstrate an on demand spatial control of excitonic magnetic lattices
for the potential applications of excitonic-based quantum optical devices. A
two dimensional magnetic lattice of indirect excitons can form a transition to
one dimensional lattice configuration under the influence of external magnetic
bias fields. The transition is identified by measuring the spatial distribution
of two dimensional photoluminance for several values of the external magnetic
bias fields. The number of the trapped excitons is found to increase between
sites along a perpendicular direction exhibiting two to one dimensional lattice
transition. This work may apply for various controllable quantum simulations,
such as superfluid-Mott-insulators, in quantum optical devices.",1210.3933v1
2013-07-05,Magnetic properties of the nucleon in a uniform background field,"We present results for the magnetic moment and magnetic polarisability of the
neutron and the magnetic moment of the proton. These results are calculated
using the uniform background field method on 32^3 x 64 dynamical QCD lattices
provided by the PACS-CS collaboration as part of the ILDG. We use a uniform
background magnetic field quantised by the periodic spatial volume. We
investigate ways to improve the effective energy plots used to calculate
magnetic polarisabilities, including the use of correlation matrix techniques
with various source smearings.",1307.1509v1
2013-07-15,Quantum interference oscillations of the superparamagnetic blocking in an Fe8 molecular nanomagnet,"We show that the dynamic magnetic susceptibility and the superparamagnetic
blocking temperature of an Fe8 single molecule magnet oscillate as a function
of the magnetic field Hx applied along its hard magnetic axis. These
oscillations are associated with quantum interferences, tuned by Hx, between
different spin tunneling paths linking two excited magnetic states. The
oscillation period is determined by the quantum mixing between the ground S=10
and excited multiplets. These experiments enable us to quantify such mixing. We
find that the weight of excited multiplets in the magnetic ground state of Fe8
amounts to approximately 11.6%.",1307.3829v1
2014-01-18,Acceleration of magnetic dipoles by the sequence of current turns,"Acceleration of magnetic dipoles is carried out by the running gradient of
the magnetic field formed while sequent switching on the current turns.
Magnetic dipoles, with a diameter of sixty millimeters and full length one
meter, are pre-accelerated by using the gas-dynamic method to speed one
kilometer per second, corresponding to the injection rate into the main
accelerator. To prevent the turning of the dipoles by one hundred eighty
degrees in the field of the accelerating pulse and focus them, the magnetic
dipoles are accelerated inside the titanium tube. The magnetic dipoles have
mass two kilograms and acquire the finite speed five kilometers per second on
the acceleration length three hundreds meters.",1401.4518v1
2014-06-23,Magnetic energy dissipation and mean magnetic field generation in planar convection driven dynamos,"A numerical study of dynamos in rotating convecting plane layers is presented
which focuses on magnetic energies and dissipation rates, and the generation of
mean fields (where the mean is taken over horizontal planes). The scaling of
the magnetic energy with the flux Rayleigh number is different from the scaling
proposed in spherical shells, whereas the same dependence of the magnetic
dissipation length on the magnetic Reynolds number is found for the two
geometries. Dynamos both with and without mean field exist in rapidly rotating
convecting plane layers.",1406.5939v1
2015-08-12,Simultaneous electronic and the magnetic excitation of a ferromagnet by intense THz pulses,"The speed of magnetization reversal is a key feature in magnetic data
storage. Magnetic fields from intense THz pulses have been recently shown to
induce small magnetization dynamics in Cobalt thin film on the sub-picosecond
time scale. Here, we show that at higher field intensities, the THz electric
field starts playing a role, strongly changing the dielectric properties of the
cobalt thin film. Both the electronic and magnetic responses are found to occur
simultaneously, with the electric field response persistent on a time scale
orders of magnitude longer than the THz stimulus",1508.02921v1
2016-04-29,Ferroelectrics Manipulate Magnetic Bloch Skyrmions in a Composite Bilayer,"Theoretical investigation demonstrates that the composite bilayer (i.e.,
chiral-magnetic/ferroelectric bilayer) offers the possibility of
electric-induced magnetic Skyrmions [Phys. Rev. B \textbf{94}, 014311 (2016)].
In this Article, we propose a micromagnetic model to physically manipulate
magnetic Bloch Skyrmions propagating in a chiral-magnetic thin film with a
polarized ferroelectric essential to drive the system through the converse
magnetoelectric effect. Effects caused by different velocities of the
propagation, sizes of the thin film, and strength of the magnetoelectric
couplings strongly impact on quality and quantity of the magnetic Skyrmions.",1604.08780v2
2016-12-21,Critical current density of a spin-torque oscillator with an in-plane magnetized free layer and an out-of-plane magnetized polarizer,"Spin-torque induced magnetization dynamics in a spin-torque oscillator with
an in-plane (IP) magnetized free layer and an out-of-plane (OP) magnetized
polarizer under IP shape-anisotropy field ($H_{\rm k}$) and applied IP magnetic
field ($H_{\rm a}$) was theoretically studied based on the macrospin model. The
rigorous analytical expression of the critical current density ($J_{\rm c1}$)
for the OP precession was obtained. The obtained expression successfully
reproduces the experimentally obtained $H_{\rm a}$-dependence of $J_{\rm c1}$
reported in [D. Houssameddine $et$ $al$., Nat. Mater. 6, 447 (2007)].",1612.06951v1
2017-06-11,The contribution of kinetic helicity to turbulent magnetic diffusivity,"Using numerical simulations of forced turbulence, it is shown that for
magnetic Reynolds numbers larger than unity, i.e., beyond the regime of
quasilinear theory, the turbulent magnetic diffusivity attains an additional
negative contribution that is quadratic in the kinetic helicity. In particular,
for large magnetic Reynolds numbers, the turbulent magnetic diffusivity without
helicity is about twice the value with helicity. Such a contribution was not
previously anticipated, but, as we discuss, it turns out to be important when
accurate estimates of the turbulent magnetic diffusivity are needed.",1706.03421v2
2018-01-24,Voltage-induced precessional switching at zero-bias magnetic field in a conically magnetized free layer,"Voltage-induced magnetization dynamics in a conically magnetized free layer
with an elliptic cylinder shape is theoretically studied on the basis of the
macrospin model. It is found that an application of voltage pulse can induce
the precessional switching of magnetization even at zero-bias magnetic field,
which is of substantial importance for device applications such as
voltage-controlled nonvolatile memory. Analytical expressions of the conditions
for precessional switching are derived.",1801.07842v1
2017-10-11,"Diamagnetism of 2D-Fermions in the Strong Nonhomogeneous Static Magnetic Field $( {\bf B} =B( 0, 0, 1/cosh^{2}( \frac{x-x_{0} }{ δ})))$ : gas magnetization, ... and gas compressibility","We study diamagnetism of a gas of fermions moving in a nonhomogeneous
magnetic field $( {\bf B} =B( 0, 0, 1/cosh^{2}( \frac{x-x_{0} }{ \delta })))$
The gas magnetization, the static magnetic susceptibility, the chemical
potential and the gas compressibility are discussed and compared with the
uniform field case. General need to study dynamics of electrons in different
types of magnetic fields follows from a large number of experimental situations
in which its understanding enables physicists to obtain new information.",1802.01460v1
2019-01-22,Ultrashort magnetic impulses driven by coherent control with vector beams,"We introduce a new technique for the generation of magnetic impulses. This
technique is based on coherent control of electrical currents using cylindrical
laser beams with azimuthal polarization. When used to ionize a medium, in this
case atomic hydrogen is considered, an azimuthal current impulse is driven. The
spatial distribution of this current bears close resemblance to that of a
solenoid, and produces a magnetic field impulse. The excitation and relaxation
dynamics of this current temporally confine the resulting magnetic field to a
Tesla-scale, terahertz bandwidth impulse. Importantly, the magnetic fields are
spatially isolated from electric fields. This all-optical approach will enable
ultrafast time-domain spectroscopy of magnetic phenomena.",1901.07444v1
2019-04-29,Spontaneous Transport Barriers Quench Turbulent Resistivity in 2D MHD,"This Letter identifies the physical mechanism for the quench of turbulent
resistivity in 2D MHD. Without an imposed, ordered magnetic field, a
multi-scale, blob-and-barrier structure of magnetic potential forms
spontaneously. Magnetic energy is concentrated in thin, linear barriers,
located at the interstices between blobs. The barriers quench the transport and
kinematic decay of magnetic energy. The local transport bifurcation underlying
barrier formation is linked to the inverse cascade of $\langle A^2\rangle$ and
negative resistivity, which induce local bistability. For small scale forcing,
spontaneous layering of the magnetic potential occurs, with barriers located at
the interstices between layers. This structure is effectively a magnetic
staircase.",1904.12892v1
2019-10-08,On the rigidity of Zoll magnetic systems on surfaces,"In this paper we study rigidity aspects of Zoll magnetic systems on closed
surfaces. We characterize magnetic systems on surfaces of positive genus given
by constant curvature metrics and constant magnetic functions as the only
magnetic systems such that the associated Hamiltonian flow is Zoll, i.e. every
orbit is closed, on every energy level. We also prove the persistence of
possibly degenerate closed geodesics under magnetic perturbations in different
instances.",1910.03285v1
2021-01-26,Independent Control and Path Planning of Microswimmers with a Uniform Magnetic Field,"Artificial bacteria flagella (ABFs) are magnetic helical micro-swimmers that
can be remotely controlled via a uniform, rotating magnetic field. Previous
studies have used the heterogeneous response of microswimmers to external
magnetic fields for achieving independent control. Here we introduce analytical
and reinforcement learning control strategies for path planning to a target by
multiple swimmers using a uniform magnetic field. The comparison of the two
algorithms shows the superiority of reinforcement learning in achieving minimal
travel time to a target. The results demonstrate, for the first time, the
effective independent navigation of realistic micro-swimmers with a uniform
magnetic field in a viscous flow field.",2101.10628v2
2024-02-29,Quantum droplets with magnetic vortices in spinor dipolar Bose-Einstein condensates,"Motivated by the recent experimental realization of a Bose-Einstein
condensate (BEC) of europium atoms, we investigate the self-bound droplet state
of a europium BEC with spin degrees of freedom. Under a sufficiently weak
magnetic field, the droplet has a torus shape with circulating spin vectors,
which is referred to as a magnetic vortex. The ground state transforms from the
torus to cigar shape through bistability with an increase in the magnetic
field. Dynamical change of the magnetic field causes the torus to rotate due to
the Einstein-de Haas effect. The magnetic vortices form a supersolid in a
confined system.",2402.18885v1
2009-09-07,On the dynamic efficiency of internal shocks in magnetized relativistic outflows,"We study the dynamic efficiency of conversion of kinetic-to-thermal/magnetic
energy of internal shocks in relativistic magnetized outflows. We model
internal shocks as being caused by collisions of shells of plasma with the same
energy flux and a non-zero relative velocity. The contact surface, where the
interaction between the shells takes place, can break up either into two
oppositely moving shocks (in the frame where the contact surface is at rest),
or into a reverse shock and a forward rarefaction. We find that for moderately
magnetized shocks (magnetization $\sigma\simeq 0.1$), the dynamic efficiency in
a single two-shell interaction can be as large as 40%. Thus, the dynamic
efficiency of moderately magnetized shocks is larger than in the corresponding
unmagnetized two-shell interaction. If the slower shell propagates with a
sufficiently large velocity, the efficiency is only weakly dependent on its
Lorentz factor. Consequently, the dynamic efficiency of shell interactions in
the magnetized flow of blazars and gamma-ray bursts is effectively the same.
These results are quantitatively rather independent on the equation of state of
the plasma. The radiative efficiency of the process is expected to be a
fraction $f_r<1$ of the estimated dynamic one, the exact value of $f_r$
depending on the particularities of the emission processes which radiate away
the thermal or magnetic energy of the shocked states.",0909.1328v1
2015-05-17,"Equilibria, Dynamics and Current Sheets Formation in Magnetically Confined Coronae","The dynamics of magnetic fields in closed regions of solar and stellar
coronae are investigated with a reduced magnetohydrodynamic (MHD) model in the
framework of Parker scenario for coronal heating. A novel analysis of reduced
MHD equilibria shows that their magnetic fields have an asymmetric structure in
the axial direction with variation length-scale $z_\ell \sim \ell B_0/b$, where
$B_0$ is the intensity of the strong axial guide field, $b$ that of the
orthogonal magnetic field component, and $\ell$ the scale of $\mathbf{b}$.
Equilibria are then quasi-invariant along the axial direction for variation
scales larger than approximatively the loop length $z_\ell \gtrsim L_z$, and
increasingly more asymmetric for smaller variation scales $z_\ell \lesssim
L_z$. The $critical$ $length$ $z_\ell \sim L_z$ corresponds to the magnetic
field intensity threshold $b \sim \ell B_0/L_z$. Magnetic fields stressed by
photospheric motions cannot develop strong axial asymmetries. Therefore fields
with intensities below such threshold evolve quasi-statically, readjusting to a
nearby equilibrium, without developing nonlinear dynamics nor dissipating
energy. But stronger fields cannot access their corresponding asymmetric
equilibria, hence they are out-of-equilibrium and develop nonlinear dynamics.
The subsequent formation of current sheets and energy dissipation is
$necessary$ for the magnetic field to relax to equilibrium, since dynamically
accessible equilibria have variation scales larger than the loop length $z_\ell
\gtrsim L_z$, with intensities smaller than the threshold $b \lesssim \ell
B_0/L_z$. The dynamical implications for magnetic fields of interest to solar
and stellar coronae are investigated numerically and the impact on coronal
physics discussed.",1505.04370v2
2019-01-28,Study of Dynamo Action in Three Dimensional Magnetohydrodynamic Plasma with Arnold-Beltrami-Childress Flow,"For a three dimensional magnetohydrodynamic (MHD) plasma the dynamo action
with ABC flow as initial condition has been studied. The study delineates
crucial parameter that gives a transition from coherent nonlinear oscillation
to dynamo. Further, for both kinematic and dynamic models at magnetic Prandtl
number equal to unity the dynamo action is studied for driven ABC flows. The
magnetic resistivity has been chosen at a value where the fast dynamo occurs
and the growth rate shows no further variation with the change of magnetic
Reynold's number. The exponent of growth of magnetic energy increases,
indicating a faster dynamo, if a higher wave number is excited compared to the
one with a lower wave number. The result has been found to hold good for both
kinematic and externally forced dynamic dynamos where the backreaction of
magnetic field on the velocity field is no more negligible. In case of an
externally forced dynamic dynamo, the super Alfvenic flows have been found to
excite strong dynamos giving rise to the growth of magnetic energy of seven
orders of magnitude. The back-reaction of magnetic field on the velocity field
through Lorentz force term has been found to affect the dynamics of the
velocity field and in turn the dynamics of magnetic field, leading to a
saturation, when the dynamo action is very prominent.",1901.09610v1
2020-11-27,Vector Hamiltonian Formalism for Nonlinear Magnetization Dynamics,"Vector Hamiltonian formalism (VHF) for the description of a weakly nonlinear
magnetization dynamics has been developed. Transformation from the traditional
Landau-Lifshitz equation, describing dynamics of a magnetization vector
$\vec{m}(\vec{r}, t)$ on a sphere, to a vector Hamiltonian equation, describing
dynamics of a \emph{spin excitation vector} $\vec{s}(\vec{r}, t)$ on a plane,
is done using the azimuthal Lambert transformation that preserves both the
phase-space area and vector structure of dynamical equations, and guarantees
that the plane containing vector $\vec{s}(\vec{r}, t)$ is at each value of the
coordinate $\vec{r}$ perpendicular to the a stationary vector
$\vec{m}_0(\vec{r})$ describing the magnetization ground state of the system.
By expanding vector $\vec{s}(\vec{r}, t)$ in a complete set of linear magnetic
vector eigemodes $\vec{s}_\nu(\vec{r})$ of the studied system, and using a
weakly nonlinear approximation $|\vec{s}(\vec{r}, t)| \ll 1$, it is possible to
express the Hamiltonian function of the system in the form of integrals over
the vector eigenmode profiles $\vec{s}_\nu(\vec{r})$, and calculate all the
coefficients of this Hamiltonian. The developed approach allows one to describe
weakly nonlinear dynamics in micro- and nano-scale magnetic systems with
complicated geometries and spatially non-uniform ground states by numerically
calculating linear spectrum and eigenmode profiles, and semi-analytically
evaluating amplitudes of multi-mode nonlinear interactions. Examples of
applications of the developed formalism to the magnetic systems having
spatially nonuniform ground state of magnetization are presented.",2011.13562v1
2022-11-10,Electron dynamics in planar radio frequency magnetron plasmas: III. Comparison of experimental investigations of power absorption dynamics to simulation results,"In magnetized capacitively coupled radio-frequency discharges operated at low
pressure the influence of the magnetic flux density on discharge properties has
been studied recently both by experimental investigations and in simulations.
It was found that the Magnetic Asymmetry Effect allows for a control of the DC
self-bias and the ion energy distribution by tuning the magnetic field
strength. In this study, we focus on experimental investigations of the
electron power absorption dynamics in the presence of a magnetron-like magnetic
field configuration in a low pressure capacitive RF discharge operated in
argon. Phase Resolved Optical Emission Spectroscopy measurements provide
insights into the electron dynamics on a nanosecond-timescale. The magnetic
flux density and the neutral gas pressure are found to strongly alter these
dynamics. For specific conditions energetic electrons are efficiently trapped
by the magnetic field in a region close to the powered electrode, serving as
the target surface. Depending on the magnetic field strength an electric field
reversal is observed that leads to a further acceleration of electrons during
the sheath collapse. These findings are supported by 2-dimensional Particle in
Cell simulations that yield deeper insights into the discharge dynamics.",2211.05479v1
2023-10-13,Midpoint geometric integrators for inertial magnetization dynamics,"We consider the numerical solution of the inertial version of
Landau-Lifshitz-Gilbert equation (iLLG), which describes high-frequency
nutation on top of magnetization precession due to angular momentum relaxation.
The iLLG equation defines a higher-order nonlinear dynamical system with very
different nature compared to the classical LLG equation, requiring twice as
many degrees of freedom for space-time discretization. It exhibits essential
conservation properties, namely magnetization amplitude preservation,
magnetization projection conservation, and a balance equation for generalized
free energy, leading to a Lyapunov structure (i.e. the free energy is a
decreasing function of time) when the external magnetic field is constant in
time. We propose two second-order numerical schemes for integrating the iLLG
dynamics over time, both based on implicit midpoint rule. The first scheme
unconditionally preserves all the conservation properties, making it the
preferred choice for simulating inertial magnetization dynamics. However, it
implies doubling the number of unknowns, necessitating significant changes in
numerical micromagnetic codes and increasing computational costs especially for
spatially inhomogeneous dynamics simulations. To address this issue, we present
a second time-stepping method that retains the same computational cost as the
implicit midpoint rule for classical LLG dynamics while unconditionally
preserving magnetization amplitude and projection. Special quasi-Newton
techniques are developed for solving the nonlinear system of equations required
at each time step due to the implicit nature of both time-steppings. The
numerical schemes are validated on analytical solution for macrospin terahertz
frequency response and the effectiveness of the second scheme is demonstrated
with full micromagnetic simulation of inertial spin waves propagation in a
magnetic thin-film.",2310.09043v1
2017-06-19,Origin and Structures of Solar Eruptions II: Magnetic Modeling (Invited Review),"The topology and dynamics of the three-dimensional magnetic field in the
solar atmosphere govern various solar eruptive phenomena and activities, such
as flares, coronal mass ejections, and filaments/prominences. We have to
observe and model the vector magnetic field to understand the structures and
physical mechanisms of these solar activities. Vector magnetic fields on the
photosphere are routinely observed via the polarized light, and inferred with
the inversion of Stokes profiles. To analyze these vector magnetic fields, we
need first to remove the 180$^\circ$ ambiguity of the transverse components and
correct the projection effect. Then, the vector magnetic field can be served as
the boundary conditions for a force-free field modeling after a proper
preprocessing. The photospheric velocity field can also be derived from a time
sequence of vector magnetic fields. Three-dimensional magnetic field could be
derived and studied with theoretical force-free field models, numerical
nonlinear force-free field models, magnetohydrostatic models, and
magnetohydrodynamic models. Magnetic energy can be computed with
three-dimensional magnetic field models or a time series of vector magnetic
field. The magnetic topology is analyzed by pinpointing the positions of
magnetic null points, bald patches, and quasi-separatrix layers. As a well
conserved physical quantity, magnetic helicity can be computed with various
methods, such as the finite volume method, discrete flux tube method, and
helicity flux integration method. This quantity serves as a promising parameter
characterizing the activity level of solar active regions.",1706.05769v1
2019-11-14,Suspensions of magnetic nanogels at zero field: equilibrium structural properties,"Magnetic nanogels represent a cutting edge of magnetic soft matter research
due to their numerous potential applications. Here, using Langevin dynamics
simulations, we analyse the influence of magnetic nanogel concentration and
embedded magnetic particle interactions on the self-assembly of magnetic
nanogels at zero field. For this, we calculated radial distribution functions
and structure factors for nanogels and magnetic particles within them. We found
that, in comparison to suspensions of free magnetic nanoparticles, where the
self-assembly is already observed if the interparticle interaction strength
exceeds the thermal fluctuations by approximately a factor of three,
self-assembly of magnetic nanogels only takes place by increasing such ratio
above six. This magnetic nanogel self-assembly is realised by means of
favourable close contacts between magnetic nanoparticles from different
nanogels. It turns out that for high values of interparticle interactions,
corresponding to the formation of internal rings in isolated nanogels, in their
suspensions larger magnetic particle clusters with lower elastic penalty can be
formed by involving different nanogels. Finally, we show that when the
self-assembly of these nanogels takes place, it has a drastic effect on the
structural properties even if the volume fraction of magnetic nanoparticles is
low.",1911.06031v1
2020-07-14,"Inverse energy transfer in decaying, three dimensional, nonhelical magnetic turbulence due to magnetic reconnection","It has been recently shown numerically that there exists an inverse transfer
of magnetic energy in decaying, nonhelical, magnetically dominated,
magnetohydrodynamic turbulence in 3-dimensions (3D). We suggest that magnetic
reconnection is the underlying physical mechanism responsible for this inverse
transfer. In the two-dimensional (2D) case, the inverse transfer is easily
inferred to be due to smaller magnetic islands merging to form larger ones via
reconnection. We find that the scaling behaviour is similar between the 2D and
the 3D cases, i.e., the magnetic energy evolves as $t^{-1}$, and the magnetic
power spectrum follows a slope of $k^{-2}$. We show that on normalizing time by
the magnetic reconnection timescale, the evolution curves of the magnetic field
in systems with different Lundquist numbers collapse onto one another.
Furthermore, transfer function plots show signatures of magnetic reconnection
driving the inverse transfer. We also discuss the conserved quantities in the
system and show that the behaviour of these quantities is similar between the
2D and 3D simulations, thus making the case that the dynamics in 3D could be
approximately explained by what we understand in 2D. Lastly, we also conduct
simulations where the magnetic field is subdominant to the flow. Here, too, we
find an inverse transfer of magnetic energy in 3D. In these simulations, the
magnetic energy evolves as $ t^{-1.4}$ and, interestingly, a dynamo effect is
observed.",2007.07325v1
2023-11-16,Meridional Circulations of the Solar Magnetic Fields of Different Strength,"The meridional circulation of the solar magnetic fields in Solar Cycles 21-24
was considered. Data from both ground-based and space observatories were used.
Three types of time-latitude distributions of photospheric magnetic fields and
their meridional circulations were identified depending on the magnetic field
intensity. (i) low-strength magnetic fields. They were distributed evenly
across latitude and weakly depended on the magnetic fields of active regions
and their cycle variation; (ii) medium-strength magnetic fields. For these
fields a wave-like, pole-to-pole, antiphase meridional circulation with a
period of approximately 22 years was revealed. The velocities of meridional
flows were slower at the minima of solar activity, when they were at high
latitudes in the opposite hemispheres, and maximal at the solar maxima, when
the positive- and negative-polarity waves crossed the equator. The meridional
circulation of these fields reflects the solar global magnetic field dynamics
and determines the solar polar field reversal; (iii) high-strength (active
region) magnetic fields. They were distributed symmetrically in the Northern
and Southern hemispheres. Magnetic fields of both leading and following sunspot
polarity migrated from high to low latitudes. The meridional-flow velocities of
high-strength magnetic fields were higher at the rising and maxima phases than
at the minima. Some of the high-latitude active region magnetic fields were
captured by the second type meridional circulation flows and transported along
with them to the appropriate pole. But the magnetic fields of active regions
are not the main ones in the solar polar field reversal. The results indicate
that high-strength magnetic fields were not the main source of weak ones.",2311.09900v1
1997-03-12,Scaling picture of magnetism formation in the anomalous f-systems: interplay of the Kondo effect and spin dynamics,"Formation of magnetically ordered state in the Kondo lattices is treated
within the degenerate $s-f$ exchange and Coqblin-Schrieffer models. The Kondo
renormalizations of the effective coupling parameter, magnetic moment and spin
excitation frequencies are calculated within perturbation theory. The results
of one-loop scaling consideration of the magnetic state in Kondo lattices are
analyzed. The dependence of the critical values of the bare model parameters on
the type of the magnetic phase and space dimensionality is investigated.
Renormalization of the effective Kondo temperature by the interatomic exchange
interactions is calculated. An important role of the character of spin dynamics
(existence of well-defined magnon excitations, presence of magnetic anisotropy
etc.) is demonstrated. The regime of strongly suppressed magnetic moments,
which corresponds to magnetic heavy-fermion system, may occur in a rather
narrow parameter region only. At the same time, in the magnetically ordered
phases the renormalized Kondo temperature depends weakly on the bare coupling
parameter in some interval. The critical behavior, corresponding to the
magnetic transition with changing the bare $s-f$ coupling parameter, is
investigated. In the vicinity of the strong coupling regime, the spectrum of
the Bose excitations becomes softened. Thus on the borderline of magnetic
instability the Fermi-liquid picture is violated in some temerature interval
due to scattering of electrons by these bosons. This may explain the fact that
a non-Fermi-liquid behavior often takes place in the heavy-fermion systems near
the onset of magnetic ordering.",9703125v1
2008-07-10,Magnetic field distribution in the quiet Sun: a simplified model approach,"We simulate the dynamics and the evolution of quiet Sun magnetic elements to
produce a probability density function of the field strengths associated with
such elements. The dynamics of the magnetic field are simulated through a
numerical model in which magnetic elements are passively driven by an advection
field presenting spatio-temporal correlations which mimicks the granulation and
the mesogranulation scales observed on the solar surface. The field strength
can increase due to an amplification process which takes place where the
magnetic elements converge. Starting from a delta-like probability density
function centered on B=30 G, we obtain magnetic field strengths up to 2 kG (in
absolute value). To derive the statistical properties of the magnetic elements
several simulation runs are performed. The model is able to produce kG magnetic
fields in a time interval of the order of the granulation time scale. The mean
unsigned flux density and the mean magnetic energy density of the synthetic
quiet Sun reach respectively 100 G and 350 G in the stationary regime. The
derived probability density function of the magnetic field strength decreases
rapidly from B=30 G to B=100 G and presents a secondary maximum for B=2 kG.
  From this result it follows that magnetic fields >700 G dominate the unsigned
flux density and magnetic energy density although the probability density
function of the field strength presents a maximum for B=30 G.",0807.1625v2
2016-09-08,Influence of non-magnetic impurity scattering on the spin dynamics in diluted magnetic semiconductors,"The doping of semiconductors with magnetic impurities gives rise not only to
a spin-spin interaction between quasi-free carriers and magnetic impurities,
but also to a local spin-independent disorder potential for the carriers. Based
on a quantum kinetic theory for the carrier and impurity density matrices as
well as the magnetic and non-magnetic carrier-impurity correlations, the
influence of the non-magnetic scattering potential on the spin dynamics in DMS
after optical excitation with circularly polarized light is investigated using
the example of Mn-doped CdTe. It is shown that non-Markovian effects, which are
predicted in calculations where only the magnetic carrier-impurity interaction
is accounted for, can be strongly suppressed in the presence of non-magnetic
impurity scattering. This effect can be traced back to a significant
redistribution of carriers in $\mathbf{k}$-space which is enabled by the
build-up of large carrier-impurity correlation energies. A comparison with the
Markov limit of the quantum kinetic theory shows that, in the presence of an
external magnetic field parallel to the initial carrier polarization, the
asymptotic value of the spin polarization at long times is significantly
different in the quantum kinetic and the Markovian calculations. This effect
can also be attributed to the formation of strong correlations which
invalidates the semiclassical Markovian picture and it is stronger when the
non-magnetic carrier-impurity interaction is accounted for. In an external
magnetic field perpendicular to the initial carrier spin, the correlations are
also responsible for a renormalization of the carrier spin precession
frequency.",1609.02582v1
2018-06-13,Observational Evidence of Magnetic Reconnection Associated with Magnetic Flux Cancellation,"Using high spatial and temporal data from the \emph{Solar Dynamics
Observatory} (\emph{SDO}) and the \emph{Interface Region Imaging Spectrograph}
(\emph{IRIS}), several observational signatures of magnetic reconnection in the
course of magnetic flux cancellation are presented, including two loop-loop
interaction processes, multiple plasma blob ejections, and a sheet-like
structure that appeared above the flux cancellation sites with a Y-shaped and
an inverted Y-shaped ends. The \emph{IRIS} 1400 \AA\ observations show that the
plasma blobs were ejected from the tip of the Y-shaped ends of the sheet-like
structure. Obvious photospheric magnetic flux cancellation occurred after the
first loop-loop interaction and continued until the end of the observation.
Complemented by the nonlinear force-free field extrapolation, we found that two
sets of magnetic field lines, which reveal an X-shaped configuration, align
well with the interacted coronal loops. Moreover, a magnetic null point is
found to be situated at about $0.9$ Mm height right above the flux cancellation
sites and located between the two sets of magnetic field lines. These results
suggest that the flux cancellation might be a result of submergence of magnetic
field lines following magnetic reconnection that occurs in the lower atmosphere
of the Sun, and the ejected plasma blobs should be plasmoids created in the
sheet-like structure due to the tearing-mode instability. This observation
reveals detailed magnetic field structure and dynamic process above the flux
cancellation sites and will help us to understand magnetic reconnection in the
lower atmosphere of the Sun.",1806.04857v1
2019-12-05,Steering magnonic dynamics and permeability at exceptional points in a parity-time symmetric waveguide,"Tuning the low-energy magnetic dynamics is a key element in designing novel
magnetic metamaterials, spintronic devices and magnonic logic circuits. This
study uncovers a new, highly effective way of controlling the magnetic
permeability via shaping the magnonic properties in coupled magnetic waveguides
separated by current carrying spacer with strong spin-orbit coupling. The
spin-orbit torques exerted on the waveguides leads to an externally tunable
enhancement of magnetic damping in one waveguide and a decreased damping in the
other, constituting so a magnetic parity-time (PT) symmetric system with
emergent magnetic properties at the verge of the exceptional point where
magnetic gains/losses are balanced. In addition to controlling the magnetic
permeability, phenomena inherent to PT-symmetric systems are identified,
including the control on magnon power oscillations, nonreciprocal magnon
propagation, magnon trapping and enhancement as well as the increased
sensitivity to magnetic perturbation and abrupt spin reversal. These
predictions are demonstrated analytically and confirmed by full numerical
simulations under experimentally feasible conditions. The position of the
exceptional points and the strength of the spontaneous PT symmetry breaking can
be tuned by external electric and/or magnetic fields. The roles of the
intrinsic magnetic damping, and the possibility of an electric control via
Dzyaloshinskii-Moriya interaction are exposed and utilized for mode dispersion
shaping and magnon amplification and trapping. The results point to a new route
to designing optomagnonic waveguides, traps, sensors, and circuits.",1912.02500v1
2020-07-29,Impact of magnetic dopants on magnetic and topological phases in magnetic topological insulators,"A topological insulator doped with random magnetic impurities is studied. The
system is modelled by the Kane-Mele model with a random spin exchange between
conduction electrons and magnetic dopants. The dynamical mean field theory for
disordered systems is used to investigate the electron dynamics. The magnetic
long-range order and the topological invariant are calculated within the mean
field theory. They reveal a rich phase diagram, where different magnetic
long-range orders such as antiferromagnetic or ferromagnetic one can exist in
the metallic or insulating phases, depending on electron and magnetic impurity
fillings. It is found that insulator only occurs at electron half filling,
quarter filling and when electron filling is equal to magnetic impurity
filling. However, non-trivial topology is observed only in half-filling
antiferromagnetic insulator and quarter-filling ferromagnetic insulator. At
electron half filling, the spin Hall conductance is quantized and it is robust
against magnetic doping, while at electron quarter filling, magnetic dopants
drive the ferromagnetic topological insulator to ferromagnetic metal. The
quantum anomalous Hall effect is observed only at electron quarter filling and
dense magnetic doping.",2007.14687v1
2023-06-12,Modeling Magnetically Channeled Winds in 3D: I. Isothermal Simulations of a Magnetic O Supergiant,"In this paper we present the first set of 3D magnetohydrodynamic (MHD)
simulations performed with the riemann geomesh code. We study the dynamics of
the magnetically channeled winds of magnetic massive stars in full three
dimensions using a code that is uniquely suited to spherical problems.
Specifically, we perform isothermal simulations of a smooth wind on a rotating
star with a tilted, initially dipolar field. We compare the mass-loss, angular
momentum loss, and magnetospheric dynamics of a template star (with the
properties that are reminiscent of the O4 supergiant {\zeta} Pup) over a range
of rotation rates, magnetic field strengths, and magnetic tilt angles. The
simulations are run up to a quasi-steady state and the results are observed to
be consistent with the existing literature, showing the episodic centrifugal
breakout events of the mass outflow, confined by the magnetic field loops that
form the closed magnetosphere of the star. The catalogued results provide
perspective on how angular-momentum loss varies for different configurations of
rotation rate, magnetic field strength, and large magnetic tilt angles. In
agreement with previous 2D MHD studies, we find that high magnetic confinement
reduces the overall mass-loss rate, and higher rotation increases the mass-loss
rate. This and future studies will be used to estimate the angular-momentum
evolution, spin-down time, and mass-loss evolution of magnetic massive stars as
a function of magnetic field strength, rotation rate, and dipole tilt.",2306.06835v1
2022-09-21,Dynamics of photo-induced ferromagnetism in oxides with orbital degeneracy,"By using intense coherent electromagnetic radiation, it may be possible to
manipulate the properties of quantum materials very quickly, or even induce new
and potentially useful phases that are absent in equilibrium. For instance,
ultrafast control of magnetic dynamics is crucial for a number of proposed
spintronic devices and can also shed light on the possible dynamics of
correlated phases out of equilibrium. Inspired by recent experiments on
spin-orbital ferromagnet YTiO$_3$ we consider the nonequilibrium dynamics of
Heisenberg ferromagnetic insulator with low-lying orbital excitations. We model
the dynamics of the magnon excitations in this system following an optical
pulse which resonantly excites infrared-active phonon modes. As the phonons
ring down they can dynamically couple the orbitals with the low-lying magnons,
leading to a dramatically modified effective bath for the magnons. We show this
transient coupling can lead to a dynamical acceleration of the magnetization
dynamics, which is otherwise bottlenecked by small anisotropy. Exploring the
parameter space more we find that the magnon dynamics can also even completely
reverse, leading to a negative relaxation rate when the pump is blue-detuned
with respect to the orbital bath resonance. We therefore show that by using
specially targeted optical pulses, one can exert a much greater degree of
control over the magnetization dynamics, allowing one to optically steer
magnetic order in this system. We conclude by discussing interesting parallels
between the magnetization dynamics we find here and recent experiments on
photo-induced superconductivity, where it is similarly observed that depending
on the initial pump frequency, an apparent metastable superconducting phase
emerges.",2209.10567v1
2002-05-16,Anomalous Scalings and Dynamics of Magnetic Helicity,"It is demonstrated that the two-point correlation function of the magnetic
helicity in the case of zero mean magnetic field has anomalous scalings for
both, compressible and incompressible turbulent helical fluid flow. The
magnetic helicity in the limit of very high electrical conductivity is
conserved. This implies that the two-point correlation function of the
conserved property does not necessarily have normal scaling. The reason for the
anomalous scalings of the magnetic helicity correlation function is that the
magnetic field in the equation for the two-point correlation function of the
magnetic helicity plays a role of a pumping with anomalous scalings. It is
shown also that when magnetic fluctuations with zero mean magnetic field are
generated the magnetic helicity is very small even if the hydrodynamic helicity
is large. Astrophysical applications of the obtained results are discussed.",0205270v1
2000-06-01,"Magnetization plateaus of the Shastry-Sutherland model for SrCu_2(BO_3)_2: SDW, supersolid and bound states","We study the Heisenberg antiferromagnet on the Shastry-Sutherland lattice
under magnetic fields, to clarify the magnetic properties of SrCu_2(BO_3)_2.
Treating magnetic excitations promoted by the field as Bose particles and using
strong coupling expansion, we derive an effective Hamiltonian for the effective
magnetic particles. Anisotropic repulsive interactions between effective
particles induce `insulating' states with a stripe SDW structure at
magnetization m/m_{sat}=1/3 and a checkerboard structure at 1/2, and thereby
form magnetization plateaus. Supersolid phases appear around insulating SDW
phases by changing the magnetic field. Nature of these supersolid phases is
discussed in detail. We also demonstrate how the geometry of the
Shastry-Sutherland lattice affects dynamical properties of magnetic excitations
significantly and makes a novel type of quintuplet (S=2) boundstates condense
for very small magnetization.",0006020v2
2007-02-01,Negative magnetic susceptibility and nonequivalent ensembles for the mean-field $φ^4$ spin model,"We calculate the thermodynamic entropy of the mean-field $\phi^4$ spin model
in the microcanonical ensemble as a function of the energy and magnetization of
the model. The entropy and its derivative are obtained from the theory of large
deviations, as well as from Rugh's microcanonical formalism, which is
implemented by computing averages of suitable observables in microcanonical
molecular dynamics simulations. Our main finding is that the entropy is a
concave function of the energy for all values of the magnetization, but is
nonconcave as a function of the magnetization for some values of the energy.
This last property implies that the magnetic susceptibility of the model can be
negative when calculated microcanonically for fixed values of the energy and
magnetization. This provides a magnetization analog of negative heat
capacities, which are well-known to be associated in general with the
nonequivalence of the microcanonical and canonical ensembles. Here, the two
ensembles that are nonequivalent are the microcanonical ensemble in which the
energy and magnetization are held fixed and the canonical ensemble in which the
energy and magnetization are fixed only on average by fixing the temperature
and magnetic field.",0702004v1
2007-02-13,Inverse cascades and alpha-effect at low magnetic Prandtl number,"Dynamo action in a fully helical Beltrami (ABC) flow is studied using both
direct numerical simulations and subgrid modeling. Sufficient scale separation
is given in order to allow for large-scale magnetic energy build-up. Growth of
magnetic energy obtains down to a magnetic Prandtl number PM=RM/RV close to
0.005, where RV and RM are the kinetic and magnetic Reynolds numbers. The
critical magnetic Reynolds number for dynamo action RMc seems to saturate at
values close to 20. Detailed studies of the dependence of the amplitude of the
saturated magnetic energy with PM are presented. In order to decrease PM,
numerical experiments are conducted with either RV or RM kept constant. In the
former case, the ratio of magnetic to kinetic energy saturates to a value
slightly below unity as PM decreases. Examination of energy spectra and
structures in real space both reveal that quenching of the velocity by the
large-scale magnetic field takes place, with an inverse cascade of magnetic
helicity and a force-free field at large scale in the saturated regime.",0702109v1
2008-11-12,Magnetic Response of NiFe2O4 nanoparticles in polymer matrix,"We report the magnetic properties of magnetic nano-composite, consisting of
different quantity of NiFe2O4 nanoparticles in polymer matrix. The
nanoparticles exhibited a typical magnetization blocking, which is sensitive on
the variation of magnetic field, mode of zero field cooled/field cooled
experiments and particle quantity in the matrix. The samples with lower
particle quantity showed an upturn of magnetization down to 5 K, whereas the
blocking of magnetization dominates at lower temperatures as the particle
quantity increases in the polymer. We examine such magnetic behaviour in terms
of the competitive magnetic ordering between core and surface spins of
nanoparticles, taking into account the effect of inter-particle (dipole-dipole)
interactions on nanoparticle magnetic dynamics.",0811.1838v1
2013-01-24,Optimal conditions for magnetization reversal of nanocluster assemblies with random properties,"Magnetization dynamics in the system of magnetic nanoclusters with randomly
distributed properties are studied by means of computer simulations. The main
attention is paid to the possibility of coherent magnetization reversal from a
strongly nonequilibrium state with a mean cluster magnetization directed
opposite to an external magnetic field. Magnetic nanoclusters are known to be
characterized by large magnetic anisotropy and strong dipole interactions. It
is also impossible to produce a number of nanoclusters with identical
properties. As a result, any realistic system of nanoclusters is composed of
the clusters with randomly varying anisotropies, effective spins, and dipole
interactions. Despite this randomness, it is possible to find conditions when
the cluster spins move coherently and display fast magnetization reversal due
to the feedback action of resonator. By analyzing the influence of different
cluster parameters, we find their optimal values providing fast magnetization
reversal.",1301.5789v1
2013-08-10,Measurements of Coronal and Chromospheric Magnetic Fields using Polarization Observations by the Nobeyama Radioheliograph,"Coronal and chromospheric magnetic fields are derived from polarization and
spectral observations of the thermal free-free emission using the Nobeyama
Radioheliograph (NoRH). In magnetized plasma, the ordinary and extraordinary
modes of free-free emission have different optical depths. This creates a
circularly polarized component in an atmosphere with a temperature gradient. We
observed an active region on April 13, 2012 to derive its coronal and
chromospheric magnetic fields. The observed degree of circular polarization was
between 0.5 % and 1.7 %. The radio circular polarization images were compared
with ultraviolet images observed by the Atmospheric Imaging Assembly and the
photospheric magnetic field observed by the Helioseismic and Magnetic Imager,
both on board the Solar Dynamic Observatory. At the edge of the active region,
the radio circular polarization was emitted mainly from coronal loops, and the
coronal magnetic field was derived to be about 70 G. At the center of the
active region, the chromospheric and coronal components cannot be separated.
The derived magnetic field is about 20 % to 50 % of the corresponding
photospheric magnetic field, which is an emission-measure-weighted average of
the coronal and chromospheric magnetic fields.",1308.2283v1
2015-02-27,Magnetic catalysis and inverse magnetic catalysis in QCD,"We investigate the effects of strong magnetic fields on the QCD phase
structure at vanishing density by solving the gluon and quark gap equations,
and by studying the dynamics of the quark scattering with the four-fermi
coupling. The chiral crossover temperature as well as the chiral condensate are
computed. For asymptotically large magnetic fields we find magnetic catalysis,
while we find inverse magnetic catalysis for intermediate magnetic fields.
Moreover, for large magnetic fields the chiral phase transition for massless
quarks turns into a crossover. The underlying mechanisms are then investigated
analytically within a few simplifications of the full numerical analysis. We
find that a combination of gluon screening effects and the weakening of the
strong coupling is responsible for the phenomenon of inverse catalysis. In
turn, the magnetic catalysis at large magnetic field is already indicated by
simple arguments based on dimensionality.",1502.08011v2
2015-08-06,Near-Sun Speed of CMEs and the Magnetic Non-potentiality of their Source Active Regions,"We show that the speed of the fastest coronal mass ejections (CMEs) that an
active region (AR) can produce can be predicted from a vector magnetogram of
the AR. This is shown by logarithmic plots of CME speed (from the SOHO LASCO
CME catalog) versus each of ten AR-integrated magnetic parameters (AR magnetic
flux, three different AR magnetic-twist parameters, and six AR
free-magnetic-energy proxies) measured from the vertical and horizontal field
components of vector magnetograms (from the {\it Solar Dynamics Observatory's
Helioseismic and Magnetic Imager}) of the source ARs of 189 CMEs. These plots
show: (1) the speed of the fastest CMEs that an AR can produce increases with
each of these whole-AR magnetic parameters, and (2) that one of the AR
magnetic-twist parameters and the corresponding free-magnetic-energy proxy each
determine the CME-speed upper-limit line somewhat better than any of the other
eight whole-AR magnetic parameters.",1508.01532v1
2016-10-30,Cosmic ray acceleration via magnetic reconnection of magnetic islands/flux-ropes,"The dynamic processes of magnetic reconnection and turbulence cause magnetic
islands/flux-ropes generation. The in-situ observations suggest that the
coalescence or/and contraction of magnetic islands are responsible to the
charged particle acceleration (keV to MeV energy range). Numerical simulations
also support this acceleration mechanism. However, the most fundamental
question raise here is, does this mechanism contribute to the cosmic rays
acceleration? To answer this, we report, in-situ evidence of flux-ropes
formation, their magnetic re-connection and its manifestation as cosmic ray
(GeV charged particle) acceleration in interplanetary counterpart of coronal
mass ejection(ICME). Further, we propose that cosmic ray (high and/or
ultra-high energy) acceleration by Fermi mechanism is valid not only through
stochastic reflections of particles from the shock boundaries but also through
the boundaries of contracting magnetic islands or/and their merging via
magnetic re-connection. This has significant implications on cosmic ray origin
and their acceleration process.",1610.09628v2
2017-06-16,Observation of Various and Spontaneous Magnetic Skyrmionic Bubbles at Room-Temperature in a Frustrated Kagome Magnet with Uniaxial Magnetic Anisotropy,"Various and spontaneous magnetic skyrmionic bubbles are experimentally
observed for the first time, at room temperature in a frustrated kagome magnet
Fe3Sn2 with unixial magnetic anisotropy. The magnetization dynamics were
investigated using in-situ Lorentz transmission electron microscopy, revealing
that the transformation between different magnetic bubbles and domains are via
the motion of Bloch lines driven by applied external magnetic field. The
results demonstrate that Fe3Sn2 facilitates a unique magnetic control of
topological spin textures at room temperature, making it a promising candidate
for further skyrmion-based spintronic devices.",1706.05177v1
2019-08-18,Magnetic stochasticity and diffusion,"We develop a quantitative relationship between magnetic diffusion and the
level of randomness, or stochasticity, of the diffusing magnetic field in a
magnetized medium. A general mathematical formulation of magnetic stochasticity
in turbulence has been developed in previous work in terms of the ${\cal
L}_p$-norm $S_p(t)={1\over 2}|| 1-\hat{\bf B}_l.\hat{\bf B}_L||_p$, $p$th order
magnetic stochasticity of the stochastic field ${\bf B}({\bf x}, t)$, based on
the coarse-grained fields, ${\bf B}_l$ and ${\bf B}_L$, at different scales,
$l\neq L$. For laminar flows, stochasticity level becomes the level of field
self-entanglement or spatial complexity. In this paper, we establish a
connection between magnetic stochasticity $S_p(t)$ and magnetic diffusion in
magnetohydrodynamic (MHD) turbulence and use a homogeneous, incompressible MHD
simulation to test this prediction. Our results agree with the well-known fact
that magnetic diffusion in turbulent media follows the super-linear Richardson
dispersion scheme. This is intimately related to stochastic magnetic
reconnection in which super-linear Richardson diffusion broadens the matter
outflow width and accelerates the reconnection process.",1908.06474v2
2019-09-10,Are gravitating magnetic monopoles stable?,"The gravitating Julia-Zee dyon is a particle-like solution with both electric
and magnetic charge. It is found in the Einstein-Yang-Mills-Higgs system of
SU(2) with a scalar field in the adjoint representation coupled to gravity.
Within the magnetic ansatz this system is reduced from describing dyons to
describing the gravitating 't Hooft-Polyakov magnetic monopole. The stability
of the well-known static gravitating magnetic monopole solutions with respect
to perturbations within the magnetic ansatz--so-called magnetic
perturbations--is well studied, but their stability with respect to
perturbations outside the magnetic ansatz--so-called sphaleronic
perturbations--is not. I undertake a purely numerical study by adding
sphaleronic perturbations to gravitating magnetic monopole solutions and then
dynamically evolving the system. For large perturbations I find that the system
heads toward a dyon configuration, as expected. For sufficiently small
perturbations, however, the system oscillates about the magnetic ansatz in a
manner consistent with oscillations about a stable equilibrium.",1909.04205v1
2021-04-01,The Magnetic Field Across the Molecular Warped Disk of Centaurus A,"Magnetic fields are amplified as a consequence of galaxy formation and
turbulence-driven dynamos. Galaxy mergers can potentially amplify the magnetic
fields from their progenitors, making the magnetic fields dynamically
important. However, the effect of mergers on magnetic fields is still poorly
understood. We use thermal polarized emission observations to trace the
magnetic fields in the molecular disk of the nearest radio active galaxy,
Centaurus A, which is thought to be the remnant of a merger. Here, we detect
that the magnetic field orientations in the plane of the sky are tightly
following the $\sim3.0$ kpc-scale molecular warped disk. Our simple regular
large-scale axisymmetric spiral magnetic field model can explain, to some
extent, the averaged magnetic field orientations across the disk projected on
the sky. Our observations also suggest the presence of small-scale turbulent
fields, whose relative strength increases with velocity dispersion and column
density. These results have strong implications for understanding the
generation and role of magnetic fields in the formation of galaxies across
cosmic time.",2104.00688v1
2021-07-27,Updated Magnetized Transport Coefficients: Impact on Laser-Plasmas with Self-Generated or Applied Magnetic Fields,"Errors in the Epperlein & Haines [PoF (1986)] transport coefficients were
recently found at low electron magnetizations, with new magnetic transport
coefficients proposed simultaneously by two teams [Sadler, Walsh & Li, PRL
(2021) and Davies, Wen, Ji & Held, PoP (2021)]; these two separate sets of
updated coefficients are shown in this paper to be in agreement. The importance
of these new coefficients in laser-plasmas with either self-generated or
applied magnetic fields is demonstrated. When an external magnetic field is
applied, the cross-gradient-Nernst term twists the field structure; this
twisting is reduced by the new coefficients in the low magnetization regime.
For plasmas where only self-generated magnetic fields are present, the new
coefficients are found to result in the magnetic field moving with the
Righi-Leduc heat-flow, enhancing the impact of MHD. Simulations of Biermann
Battery magnetic fields around ICF hot-spot perturbations are presented, with
cross-gradient-Nernst transport increasing spike penetration.",2107.12988v1
2021-10-21,Magnetic hysteresis of individual Janus particles with hemispherical exchange-biased caps,"We use sensitive dynamic cantilever magnetometry to measure the magnetic
hysteresis of individual magnetic Janus particles. These particles consist of
hemispherical caps of magnetic material deposited on micrometer-scale silica
spheres. The measurements, combined with corresponding micromagnetic
simulations, reveal the magnetic configurations present in these individual
curved magnets. In remanence, ferromagnetic Janus particles are found to host a
global vortex state with vanishing magnetic moment. In contrast, a remanent
onion state with significant moment is recovered by imposing an exchange bias
to the system via an additional antiferromagnetic layer in the cap. A robust
remanent magnetic moment is crucial for most applications of magnetic Janus
particles, in which an external magnetic field actuates their motion.",2110.11158v1
2022-02-11,Mutual conversion between a magnetic Neel hopfion and a Neel toron,"Three-dimensional (3D) magnetic textures attract great attention from
researchers due to their fascinating structures and dynamic behaviors. Magnetic
hopfion is a prominent example of 3D magnetic textures. Here, we numerically
study the mutual conversion between a Neel-type hopfion and a Neel-type toron
under an external magnetic field. We also investigate the excitation modes of
hopfions and torons in a film with strong perpendicular magnetic anisotropy. It
is found that the Neel-type hopfion could be a stable state in the absence of
the external magnetic field, and its diameter varies with the out-of-plane
magnetic field. The Neel-type hopfion may transform to a Neel-type toron at an
out-of-plane magnetic field of about 20 mT, where the cross section structure
is a Neel-type skyrmion. The hopfion and toron show different excitation modes
in the presence of an in-plane microwave magnetic field. Our results provide a
method to realize the conversion between a Neel-type hopfion and a Neel-type
toron, which also gives a way to distinguish Bloch-type and Neel-type hopfions.",2202.05622v1
2022-03-04,Second-order magnetic responses in quantum magnets: Magnetization under ac magnetic fields,"We investigate second-order magnetic responses of quantum magnets against ac
magnetic fields. We focus on the case where the $z$ component of the spin is
conserved in the unperturbed Hamiltonian and the driving field is applied in
the $xy$ plane. We find that linearly polarized driving fields induce a
second-harmonic response, while circularly polarized fields generate only a
zero-frequency response, leading to a magnetization with a direction determined
by the helicity. Employing an unbiased numerical method, we demonstrate the
nonlinear magnetic effect driven by the circularly polarized field in the XXZ
model and show that the magnitude of the magnetization can be predicted by the
dynamical spin structure factor in the linear response regime.",2203.02145v2
2022-06-17,Theory of ultrafast magnetization of non-magnetic semiconductors with localized conduction bands,"The magnetization of a non-magnetic semiconductor by femtosecond light pulses
is crucial to achieve an all-optical control of the spin dynamics in materials
and to develop faster memory devices. However, the conditions for its detection
are largely unknown. In this work we identify the criteria for the observation
of ultrafast magnetization and critically discuss the difficulties hindering
its experimental detection. We show that ultrafast magnetization of a non
magnetic semiconductor can be observed in compounds with very localized
conduction band states and more delocalized valence bands, such as in the case
of a p-d charge transfer gap. By using constrained and time dependent density
functional theory simulations, we demonstrate that a transient ferrimagnetic
state can be induced in diamagnetic semiconductor V2O5 via ultrafast pulses at
realistic fluences. The ferrimagnetic state has opposite magnetic moments on
vanadium (conduction) and oxygen (valence) states. Our methodology outruns the
case of V2O5 as it identifies the key requirements for a computational
screening of ultrafast magnetism in non-magnetic semiconductors.",2206.08624v1
2022-09-23,Magnetic interactions in orbital dynamics,"The magnetic field of a host star can impact the orbit of a stellar partner,
planet, or asteroid if the orbiting body is itself magnetic or electrically
conducting. Here, we focus on the instantaneous magnetic forces on an orbiting
body in the limit where the dipole approximation describes its magnetic
properties as well as those of its stellar host. A permanent magnet in orbit
about a star will be inexorably drawn toward the stellar host if the magnetic
force is comparable to gravity due to the steep radial dependence of the
dipole-dipole interaction. While magnetic fields in observed systems are much
too weak to drive a merger event, we confirm that they may be high enough in
some close compact binaries to cause measurable orbital precession. When the
orbiting body is a conductor, the stellar field induces a time-varying magnetic
dipole moment that leads to the possibility of eccentricity pumping and
resonance trapping. The challenge is that the orbiter must be close to the
stellar host, so that magnetic interactions must compete with tidal forces and
the effects of intense stellar radiation.",2209.11574v1
2023-05-17,Material Parameters for Faster Ballistic Switching of an In-plane Magnetized Nanomagnet,"High-speed magnetization switching of a nanomagnet is necessary for faster
information processing. The ballistic switching by a pulsed magnetic filed is a
promising candidate for the high-speed switching. It is known that the
switching speed of the ballistic switching can be increased by increasing the
magnitude of the pulsed magnetic field. However it is difficult to generate a
strong and short magnetic field pulse in a small device. Here we explore
another direction to achieve the high-speed ballistic switching by designing
material parameters such as anisotropy constant, saturation magnetization, and
the Gilbert damping constant. We perform the macrospin simulations for the
ballistic switching of in-plane magnetized nano magnets with varying material
parameters. The results are analyzed based on the switching dynamics on the
energy density contour. We show that the pulse width required for the ballistic
switching can be reduced by increasing the magnetic anisotropy constant or by
decreasing the saturation magnetization. We also show that there exists an
optimal value of the Gilbert damping constant that minimizes the pulse width
required for the ballistic switching.",2305.10111v1
2023-08-24,Coercivity Mechanisms of Single-Molecule Magnets,"Magnetic hysteresis has become a crucial aspect for characterizing
single-molecule magnets, but the comprehension of the coercivity mechanism is
still a challenge. By using analytical derivation and quantum dynamical
simulations, we reveal fundamental rules that govern magnetic relaxation of
single molecule magnets under the influence of external magnetic fields, which
in turn dictates the hysteresis behavior. Specifically, we find that energy
level crossing induced by magnetic fields can drastically increase the
relaxation rate and set a coercivity limit. The activation of
optical-phonon-mediated quantum tunneling accelerates the relaxation and
largely determines the coercivity. Intra-molecular exchange interaction in
multi-ion compounds may enhance the coercivity by suppressing key relaxation
processes. Unpaired bonding electrons in mixed-valence complexes bear a
pre-spin-flip process, which may facilitate magnetization reversal. Underlying
these properties are magnetic relaxation processes modulated by the interplay
of magnetic fields, phonon spectrum and spin state configuration, which also
proposes a fresh perspective for the nearly centurial coercive paradox.",2308.12616v2
2024-01-03,Local distortion driven magnetic phase switching in pyrochlore $Yb_2(Ti_{1-x}Sn_x)_2O_7$,"While it is commonly accepted that the disorder induced by magnetic ion
doping in quantum magnets usually generates a rugged free-energy landscape
resulting in slow or glassy spin dynamics, the disorder/distortion effects
associated with non-magnetic ion sites doping are still illusive. Here, using
AC susceptibility measurements, we show that the mixture of Sn/Ti on the
non-magnetic ion sites of pyrochlore $Yb_2(Ti_{1-x}Sn_x)_2O_7$ induces an
antiferromagnetic ground state despite both parent compounds, $Yb_2Ti_2O_7$,
and $Yb_2Sn_2O_7$, order ferromagnetically. Local structure studies through
neutron total scattering reveals the local distortion in the non-magnetic ion
sites and its strong correlation with the magnetic phase switching. Our study,
for the first time, demonstrates the local distortion as induced by the
non-magnetic ion site mixture could be a new path to achieve magnetic phase
switching, which has been traditionally obtained by external stimuli such as
temperature, magnetic field, pressure, strain, light, etc.",2401.01807v1
2017-06-13,Dynamic phase transition features of the cylindrical nanowire driven by a propagating magnetic field,"Magnetic response of the spin-$1/2$ cylindrical nanowire to the propagating
magnetic field wave has been investigated by means of Monte Carlo simulation
method based on Metropolis algorithm. The obtained microscopic spin
configurations suggest that the studied system exhibits two types of dynamical
phases depending on the considered values of system parameters: Coherent
propagation of spin bands and spin-frozen or pinned phases, as in the case of
the conventional bulk systems under the influence of a propagating magnetic
field. By benefiting from the temperature dependencies of variances of dynamic
order parameter, internal energy and the derivative of dynamic order parameter
of the system, dynamic phase diagrams are also obtained in related planes for
varying values of the wavelength of the propagating magnetic field. Our
simulation results demonstrate that as the strength of the field amplitude is
increased, the phase transition points tend to shift to the relatively lower
temperature regions. Moreover, it has been observed that dynamic phase boundary
line shrinks inward when the value of wavelength of the external field
decreases.",1706.04089v1
2019-12-13,Magnetism in Massive Stars,"Massive stars are the drivers of star formation and galactic dynamics due to
their relatively short lives and explosive demises, thus impacting all of
astrophysics. Since they are so impactful on their environments, through their
winds on the main sequence and their ultimate supernovae, it is crucial to
understand how they evolve. Recent photometric observations with space-based
platforms such as CoRoT, K2, and now TESS have permitted access to their
interior dynamics through asteroseismology, while ground-based
spectropolarimetric measurements such as those of ESPaDOnS have given us a
glimpse at their surface magnetic fields. The dynamics of massive stars involve
a vast range of scales. Extant methods can either capture the long-term
structural evolution or the short-term dynamics such as convection, magnetic
dynamos, and waves due to computational costs. Thus, many mysteries remain
regarding the impact of such dynamics on stellar evolution, but they can have
strong implications both for how they evolve and what they leave behind when
they die. Some of these dynamics including rotation, tides, and magnetic fields
have been addressed in recent work, which is reviewed in this paper.",1912.06422v1
2020-01-02,Three-dimensional dynamics of magnetic hopfion driven by spin transfer torque,"Magnetic hopfion is three-dimensional (3D) topological soliton with novel
spin structure that would enable exotic dynamics. Here we study the current
driven 3D dynamics of a magnetic hopfion with unit Hopf index in a frustrated
magnet. Attributed to spin Berry phase and symmetry of the hopfion, the phase
space entangles multiple collective coordinates, thus the hopfion exhibits rich
dynamics including longitudinal motion along the current direction, transverse
motion perpendicular to the current direction, rotational motion and dilation.
Furthermore, the characteristics of hopfion dynamics is determined by the ratio
between the non-adiabatic spin transfer torque parameter and the damping
parameter. Such peculiar 3D dynamics of magnetic hopfion could shed light on
understanding the universal physics of hopfions in different systems and boost
the prosperous development of 3D spintronics.",2001.00417v1
2013-09-18,Tunable non-equilibrium dynamics: field quenches in spin ice,"We present non-equilibrium physics in spin ice as a novel setting which
combines kinematic constraints, emergent topological defects, and magnetic long
range Coulomb interactions. In spin ice, magnetic frustration leads to highly
degenerate yet locally constrained ground states. Together, they form a highly
unusual magnetic state -- a ""Coulomb phase"" -- whose excitations are pointlike
defects -- magnetic monopoles -- in the absence of which effectively no
dynamics is possible. Hence, when they are sparse at low temperature, dynamics
becomes very sluggish. When quenching the system from a monopole-rich to a
monopole-poor state, a wealth of dynamical phenomena occur the exposition of
which is the subject of this article. Most notably, we find reaction diffusion
behaviour, slow dynamics due to kinematic constraints, as well as a regime
corresponding to the deposition of interacting dimers on a honeycomb lattice.
We also identify new potential avenues for detecting the magnetic monopoles in
a regime of slow-moving monopoles. The interest in this model system is further
enhanced by its large degree of tunability, and the ease of probing it in
experiment: with varying magnetic fields at different temperatures, geometric
properties -- including even the effective dimensionality of the system -- can
be varied. By monitoring magnetisation, spin correlations or zero-field Nuclear
Magnetic Resonance, the dynamical properties of the system can be extracted in
considerable detail. This establishes spin ice as a laboratory of choice for
the study of tunable, slow dynamics.",1309.4676v2
1996-07-02,Generalized Dynamic Scaling for Critical Relaxations,"The dynamic relaxation process for the two dimensional Potts model at
criticality starting from an initial state with very high temperature and
arbitrary magnetization is investigated with Monte Carlo methods. The results
show that there exists universal scaling behaviour even in the short-time
regime of the dynamic evolution. In order to describe the dependence of the
scaling behaviour on the initial magnetization, a critical characteristic
function is introduced.",9607013v1
1996-08-29,Magnetic Field Symmetry of Dynamic Capacitances,"While the static capacitance matrix is always symmetric and thus an even
function of the magnetic field, the dynamic capacitance matrix of
multi-terminal samples obeys in general only the weaker Onsager-Casimir
symmetry relations. Our results are in accordance with recent experimental
observations of asymmetric dynamic capacitances in quantized Hall systems. We
predict quantization of the four-terminal resistances of an insulating Hall
sample.",9608154v1
1997-03-14,Geometric magnetism in classical transport theory,"The effective dynamics of a slow classical system coupled to a fast chaotic
environment is described by means of a Master equation. We show how this
approach permits a very simple derivation of geometric magnetism.",9703154v2
1996-07-05,Dynamical Magnetic Susceptibility for the $t$-$J$ Model,"We present results for the {\em dynamical}\/ magnetic susceptibility of the
$t$-$J$ model, calculated with the dynamical mean field theory. For $J=0$ we
find enhanced ferromagnetic correlations but an otherwise relatively
$\vec{q}$-independent dynamical magnetic susceptibility. For $J>0$ the explicit
antiferromagnetic exchange leads to a dynamic spin structure factor with the
expected peak at the antiferromagnetic Bragg point.",9607034v1
1997-11-05,Dynamics of topological magnetic solitons,"A direct link between the topological complexity of magnetic media and their
dynamics is established through the construction of unambiguous conservation
laws for the linear and angular momenta as moments of a topological vorticity.
As a consequence, the dynamics of topological magnetic solitons is shown to
exhibit the characteristic features of the Hall effect of electrodynamics or
the Magnus effect of fluid dynamics. The main points of this program are
reviewed here for both ferromagnets and antiferromagnets, while a
straightforward extension to the study of superfluids is also discussed
briefly.",9711035v1
2010-09-09,Hydrodynamic equation of a spinor dipolar Bose-Einstein condensate,"We introduce equations of motion for spin dynamics in a ferromagnetic
Bose-Einstein condensate with magnetic dipole-dipole interaction, written using
a vector expressing the superfluid velocity and a complex scalar describing the
magnetization. This simple hydrodynamical description extracts the dynamics of
spin wave and affords a straightforward approach by which to investigate the
spin dynamics of the condensate. To demonstrate the advantages of the
description, we illustrate dynamical instability and magnetic fluctuation
preference, which are expressed in analytical forms.",1009.1671v3
2011-04-03,Dynamical magnetoelectric effects in multiferroic oxides,"Multiferroics with coexistent ferroelectric and magnetic orders can provide
an interesting laboratory to test unprecedented magnetoelectric responses and
their possible applications. One such example is the dynamical and/or resonant
coupling between magnetic and electric dipoles in a solid. As the examples of
such dynamical magnetoelectric effects, (1) the multiferroic domain wall
dynamics and (2) the electric-dipole active magnetic responses are discussed
with the overview of recent experimental observations.",1104.0357v1
2012-10-27,Nonequilibrium quench dynamics in quantum quasicrystals,"We study the nonequilibrium dynamics of a quasiperiodic quantum Ising chain
after a sudden change in the strength of the transverse field at zero
temperature. In particular we consider the dynamics of the entanglement entropy
and the relaxation of the magnetization. The entanglement entropy increases
with time as a power-law, and the magnetization is found to exhibit
stretched-exponential relaxation. These behaviors are explained in terms of
anomalously diffusing quasiparticles, which are studied in a wave packet
approach. The nonequilibrium magnetization is shown to have a dynamical phase
transition.",1210.7328v2
2013-05-25,Thermally-Assisted Spin-Transfer Torque Dynamics in Energy Space,"We consider the general Landau-Lifshitz-Gilbert theory underlying the
magnetization dynamics of a macrospin magnet subject to spin-torque effects and
thermal fluctuations. Thermally activated dynamical properties are analyzed by
averaging the full magnetization equations over constant- energy orbits. After
averaging, all the relevant dynamical scenarios are a function of the ratio
between hard and easy axis anisotropies. We derive analytically the range of
currents for which limit cycles exist and discuss the regimes in which the
constant energy orbit averaging technique is applicable.",1305.5945v2
2022-10-03,Probing domain wall dynamics in magnetic Weyl semimetals via the non-linear anomalous Hall effect,"The magnetic textures of Weyl semimetals are embedded into their topological
structure and interact dynamically with it. Here, we examine electric
field-induced structural phase transitions in domain walls mediated by the spin
transfer torque, and their footprint in charge transport. Remarkably, domain
wall dynamics lead to a transient, local, non-linear anomalous Hall effect and
non-linear anomalous drift current, which serve as direct probes of the
magnetization dynamics and of the domain wall location. We discuss experimental
observation in state-of-the-art samples.",2210.01150v1
2023-02-10,Feedback Assisted Spin Dynamics in an Inhomogeneous Bias Magnetic Field,"We study dynamics of spins in the presence of a feedback magnetic field. The
inhomogeneity of the bias magnetic field results in that of the Larmor
frequencies of the spins. We find that the system exhibits rich nonlinear
dynamics: beyond limit cycles which have been the focus of previous studies,
quasi-periodic orbits and chaos emerge in a wide parameter space. The stable
regions of each phases are determined analytically or numerically. We establish
the relation between the synchronization frequency of limit cycles and the
field inhomogeneity. Our results have important implications to precision
measurement.",2302.05264v1
2001-11-21,Dynamics of Relativistic Magnetized Blast Waves,"The dynamics of a relativistic blast wave propagating through magnetized
medium is considered taking into account possible inhomogeneities of density
and magnetic field and additional energy supply. Under the simplifying
assumption of a spherically symmetric explosion in a medium with toroidal
magnetic field self-similar solutions for the internal dynamics of the flow are
derived. In the weakly magnetized case, when the bulk of the flow may be
described by the unmagnetized solutions, there is a strongly magnetized sheath
near the contact discontinuity (when it exists). Self-similar solutions inside
the sheath are investigated. In the opposite limit of strongly magnetized
upstream plasma new analytical self-similar solutions are found. Possible
application to the physics of Gamma-Ray Bursts is discussed.",0111402v1
2006-01-18,Magnetic domain structure and dynamics in interacting ferromagnetic stacks with perpendicular anisotropy,"The time and field dependence of the magnetic domain structure at
magnetization reversal were investigated by Kerr microscopy in interacting
ferromagnetic Co/Pt multilayers with perpendicular anisotropy. Large local
inhomogeneous magnetostatic fields favor mirroring domain structures and domain
decoration by rings of opposite magnetization. The long range nature of these
magnetostatic interactions gives rise to ultra-slow dynamics even in zero
applied field, i.e. it affects the long time domain stability. Due to this
additionnal interaction field, the magnetization reversal under short magnetic
field pulses differs markedly from the well-known slow dynamic behavior.
Namely, in high field, the magnetization of the coupled harder layer has been
observed to reverse more rapidly by domain wall motion than the softer layer
alone.",0601401v1
2007-01-15,Magnetization dynamics in the single-molecule magnet Fe8 under pulsed microwave irradiation,"We present measurements on the single molecule magnet Fe8 in the presence of
pulsed microwave radiation at 118 GHz. The spin dynamics is studied via time
resolved magnetization experiments using a Hall probe magnetometer. We
investigate the relaxation behavior of magnetization after the microwave pulse.
The analysis of the experimental data is performed in terms of different
contributions to the magnetization after-pulse relaxation. We find that the
phonon bottleneck with a characteristic relaxation time of 10 to 100 ms
strongly affects the magnetization dynamics. In addition, the spatial effect of
spin diffusion is evidenced by using samples of different sizes and different
ways of the sample's irradiation with microwaves.",0701324v1
2008-01-30,Femtosecond Control of the Magnetization in Ferromagnetic Semiconductors,"We develop a theory of collective spin dynamics triggered by ultrafast
optical excitation of ferromagnetic semiconductors. Using the density matrix
equations of motion in the mean field approximation and including magnetic
anisotropy and hole spin dephasing effects, we predict the development of a
light--induced magnetization tilt during ultra--short time intervals comparable
to the pulse duration. This femtosecond dynamics in the coherent temporal
regime is governed by the interband nonlinear optical polarizations and is
followed by a second temporal regime governed by the magnetic anisotropy of the
Fermi sea. We interpret our numerical results by deriving a
Landau--Gilbert--like equation for the collective spin, which demonstrates an
ultrafast correction to the magnetic anisotropy effective field due to second
order coherent nonlinear optical processes. Using the Lindblad semigroup
method, we also derive a contribution to the interband polarization dephasing
determined by the Mn spin and the hole spin dephasing. Our predicted
magnetization tilt and subsequent nonlinear dynamics due to the magnetic
anisotropy can be controlled by varying the optical pulse intensity, duration,
and helicity and can be observed with pump--probe magneto--optical
spectroscopy.",0801.4641v1
2008-12-07,Improved time-resolved magneto-optical Kerr effect technique and dynamic magnetization reversal mechanism of perpendicularly magnetized $L1_{\mathrm{0}}$ FePt films,"The dynamic coercivity cannot be measured rigorously by the conventional
time-resolved magneto-optical Kerr effect technique because the irreversible
deviation of the transient magnetization is accumulated. In order to remove the
accumulation effect, the alternating magnetic field is employed and
synchronized with the femtosecond laser pulse. Since the sample is reset before
each single laser pulse, the accumulation effect of the irreversible deviation
of the transient magnetization is removed. For perpendicularly magnetized
$L1_{\mathrm{0}}$ FePt films, the dynamic magnetization reversal process is
accomplished by the nucleation of reversed domains and the pinned domain wall
motion.",0812.1360v2
2009-11-03,Dynamical Bar-Mode Instability in Differentially Rotating Magnetized Neutron Stars,"This paper presents a numerical study over a wide parameter space of the
likelihood of the dynamical bar-mode instability in differentially rotating
magnetized neutron stars. The innovative aspect of this study is the
incorporation of magnetic fields in such a context, which have thus far been
neglected in the purely hydrodynamical simulations available in the literature.
The investigation uses the Cosmos++ code which allows us to perform three
dimensional simulations on a cylindrical grid at high resolution. A sample of
Newtonian magneto-hydrodynamical simulations starting from a set of models
previously analyzed by other authors without magnetic fields has been
performed, providing estimates of the effects of magnetic fields on the
dynamical bar-mode deformation of rotating neutron stars. Overall, our results
suggest that the effect of magnetic fields are not likely to be very
significant in realistic configurations. Only in the most extreme cases are the
magnetic fields able to suppress growth of the bar mode.",0911.0670v2
2012-10-22,Accelerating the switching of logical units by anisotropy driven magnetization dynamics,"In this work the magnetization dynamics of clusters supported on non-magnetic
substrates is shown to exhibit an unprecedented complex response when subjected
to external magnetic fields. The field-driven magnetization reversal of small
Co clusters deposited on a Cu(111) surface has been studied by means of
first-principles calculations and atomistic spin dynamics simulations. For
applied fields ranging from 1 Tesla to 10 Tesla, we observe a coherent
magnetization reversal with switching times in the range of several tenths of
picoseconds to several nanoseconds, depending on the field strength. We find a
non-monotonous dependence of the switching times with respect to the strength
of the applied field, which we prove to have its origin in the complex magnetic
anisotropy landscape of these low dimensional systems. This effect is shown to
be stable for temperatures around 10 K, and is possible to realize over a range
of exchange interactions and anisotropy landscapes. Possible experimental
routes to achieve this unique switching behaviour are discussed.",1210.5836v1
2013-01-28,Theoretical study of spin-torque oscillator coupled with a nano-magnet by dipole-dipole interaction,"The dynamics of a spin-torque-oscillator (STO) coupled with a nano-magnet
through dipole-dipole interaction was studied numerically by using the
macrospin model for the application of the STO as a read head sensor of hard
disk drives. We found that the current, which is required to induce the
oscillation of the free-layer (FL) of the STO, depends strongly on the distance
between the FL and the nano-magnet as well as on the relative orientation of
the magnetizations between them. To determine the dynamics of the STO it is
indispensable to consider the dynamics of the dipole-coupled nano-magnet. We
showed that we could detect the orientation of the magnetization of a
nano-magnet, or a recording-bit, by the modulation of the oscillation frequency
of the STO.",1301.6458v1
2013-04-09,Exchange randomness and spin dynamics in the frustrated magnetic Keplerate {W72V30},"The magnetic properties and spin dynamics of the spin frustrated
polyoxometalate {W72V30}, where 30 V^{4+} ions (s = 1/2) occupy the sites of an
icosidodecahedron, have been investigated by low temperature magnetization,
magnetic susceptibility, proton and vanadium nuclear magnetic resonance, and
theoretical studies. The field-dependent magnetization at 0.5 K increases
monotonically up to 50 T without any sign of staircase behavior. This
low-temperature behavior cannot be explained by a Heisenberg model based on a
single value of the nearest-neighbor exchange coupling. We analyze this
behavior upon assuming a rather broad distribution of nearest-neighbor exchange
interactions. Slow spin dynamics of {W72V30} at low temperatures is observed
from the magnetic field and temperature dependence of nuclear spin-lattice
relaxation rate 1/T_1 measurements.",1304.2603v1
2013-05-30,Guided self-assembly of magnetic beads for biomedical applications,"Micromagnetic beads are widely used in biomedical applications for cell
separation, drug delivery, and hypothermia cancer treatment. Here we propose to
use self-organized magnetic bead structures which accumulate on fixed magnetic
seeding points to isolate circulating tumor cells. The analysis of circulating
tumor cells is an emerging tool for cancer biology research and clinical cancer
management including the detection, diagnosis and monitoring of cancer.
Microfluidic chips for isolating circulating tumor cells use either affinity,
size or density capturing methods. We combine multiphysics simulation
techniques to understand the microscopic behavior of magnetic beads interacting
with Nickel accumulation points used in lab-on-chip technologies. Our proposed
chip technology offers the possibility to combine affinity and size capturing
with special antibody-coated bead arrangements using a magnetic gradient field
created by Neodymium Iron Boron permanent magnets. The multiscale simulation
environment combines magnetic field computation, fluid dynamics and discrete
particle dynamics.",1305.7072v1
2014-04-04,Evolution of magnetic dynamics in an artificially frustrated Fe nanoparticle system,"Frustrated lattices1-3, characterized by minor breakdown in local order in an
otherwise periodic lattice, lead to simultaneous possibilities of several
ground states which can trigger unique physical properties, in condensed matter
systems. In magnetic materials with atomic spins, frustration takes another
shape with added possibilities to construct various topological arrangements of
spins, whereby magnetic order is disturbed2,4. We have achieved a new approach
to introduce positional atomic disorder inside a Fe nanoparticle lattice,
forming domains without boundaries to study magnetic dynamics of the
constituent spins. This magnetism overrides the exchange bias derived magnetic
enhancement, appears only at temperatures around 200 K and is characterized by
a dynamic polarity, p = +/-1 (positive or negative) with a precise frequency.
The material otherwise behaves like a superparamagnet with characteristic
magnetization behaviour at room temperature and 2 K.",1404.1163v1
2014-12-18,Effective Equations for the Precession Dynamics of Electron Spins and Electron-Impurity Correlations in Diluted Magnetic Semiconductors,"Starting from a quantum kinetic theory for the spin dynamics in diluted
magnetic semiconductors, we derive simplified equations that effectively
describe the spin transfer between carriers and magnetic impurities for an
arbitrary initial impurity magnetization. Taking the Markov limit of these
effective equations, we obtain good quantitative agreement with the full
quantum kinetic theory for the spin dynamics in bulk systems at high magnetic
doping. In contrast, the standard rate description where the carrier-dopant
interaction is treated according to Fermi's golden rule, which involves the
assumption of a short memory as well as a perturbative argument, has been shown
previously to fail if the impurity magnetization is non-zero. The Markov limit
of the effective equations is derived, assuming only a short memory, while
higher order terms are still accounted for. These higher order terms represent
the precession of the carrier-dopant correlations in the effective magnetic
field due to the impurity spins. Numerical calculations show that the Markov
limit of our effective equations reproduces the results of the full quantum
kinetic theory very well. Furthermore, this limit allows for analytical
solutions and for a physically transparent interpretation.",1412.5898v1
2015-06-16,Visualization of superparamagnetic dynamics in magnetic topological insulators,"Quantized Hall conductance is a generic feature of two dimensional electronic
systems with broken time reversal symmetry. In the quantum anomalous Hall state
recently discovered in magnetic topological insulators, time reversal symmetry
is believed to be broken by long-range ferromagnetic order, with quantized
resistance observed even at zero external magnetic field. Here, we use scanning
nanoSQUID magnetic imaging to provide a direct visualization of the dynamics of
the quantum phase transition between the two anomalous Hall plateaus in a
Cr-doped (Bi,Sb)$_2$Te$_3$ thin film. Contrary to naive expectations based upon
macroscopic magnetometry, our measurements reveal a superparamagnetic state
formed by weakly interacting magnetic domains with a characteristic size of few
tens of nanometers. The magnetic phase transition occurs through random
reversals of these local moments, which drive the electronic Hall plateau
transition. Surprisingly, we find that the electronic system can in turn drive
the dynamics of the magnetic system, revealing a subtle interplay between the
two coupled quantum phase transitions.",1506.05114v1
2017-06-19,Magnetic Excitations and Continuum of a Field-Induced Quantum Spin Liquid in $α$-RuCl$_3$,"We report on terahertz spectroscopy of quantum spin dynamics in
$\alpha$-RuCl$_3$, a system proximate to the Kitaev honeycomb model, as a
function of temperature and magnetic field. An extended magnetic continuum
develops below the structural phase transition at $T_{s2}=62$K. With the onset
of a long-range magnetic order at $T_N=6.5$K, spectral weight is transferred to
a well-defined magnetic excitation at $\hbar \omega_1 = 2.48$meV, which is
accompanied by a higher-energy band at $\hbar \omega_2 = 6.48$meV. Both
excitations soften in magnetic field, signaling a quantum phase transition at
$B_c=7$T where we find a broad continuum dominating the dynamical response.
Above $B_c$, the long-range order is suppressed, and on top of the continuum,
various emergent magnetic excitations evolve. These excitations follow clear
selection rules and exhibit distinct field dependencies, characterizing the
dynamical properties of the field-induced quantum spin liquid.",1706.06157v1
2018-06-05,Electromagnetic effects induced by time-dependent axion field,"We studied the dynamics of the so-called $\theta$-term, which exists in
topological materials and is related to a hypothetical field predicted by
Peccei-Quinn in particle physics, in a magnetic superlattice constructed using
a topological insulator and two ferromagnetic insulators, where the
ferromagnetic insulators had perpendicular magnetic anisotropies and different
magnetic coercive fields. We examined a way to drive the dynamics of the
$\theta$-term in the magnetic superlattice through changing the inversion
symmetry (from an anti-parallel to a parallel magnetic configuration) using an
external magnetic field. As a result, we found that unconventional
electromagnetic fields, which are magnetic field-induced charge currents and
vice versa, are generated by the nonzero dynamics of the $\theta$-term.",1806.01548v1
2018-07-30,Dynamics of topological defects in a two-dimensional magnetic domain stripe pattern,"Two dimensional magnetic films with perpendicular magnetization spontaneously
form magnetic domain patterns that evolve or undergo symmetry transformations
as a function of temperature. When the system is driven from equilibrium by a
rapid change in temperature, topological pattern defects are the elementary
pattern excitations that affect this evolution. An elastic continuum model is
adapted to describe how a metastable population of topological defects alters
the domain density and the magnetic susceptibility of the ""stripe"" magnetic
domain pattern. Temporal changes in the susceptibility are interpreted using a
dynamical equation describing the defect population. Recent experiments provide
a quantitative verification of the model, and illustrate the use of the
magnetic susceptibility to follow the population dynamics of topological
defects in this system, and its potential role in investigating a pattern
melting phase transition.",1807.11424v1
2019-03-30,Dynamical sign reversal of magnetic correlations in dissipative Hubbard models,"In quantum magnetism, the virtual exchange of particles mediates an
interaction between spins. Here, we show that an inelastic Hubbard interaction
fundamentally alters the magnetism of the Hubbard model due to dissipation in
spin-exchange processes, leading to sign reversal of magnetic correlations in
dissipative quantum dynamics. This mechanism is applicable to both fermionic
and bosonic Mott insulators, and can naturally be realized with ultracold atoms
undergoing two-body inelastic collisions. The dynamical reversal of magnetic
correlations can be detected by using a double-well optical lattice or
quantum-gas microscopy, the latter of which facilitates the detection of the
magnetic correlations in one-dimensional systems because of spin-charge
separation. Our results open a new avenue toward controlling quantum magnetism
by dissipation.",1904.00154v3
2019-05-14,Rayleigh-Benard convection in a nonuniformly rotating electrically conductive medium in an external spiral magnetic field,"The research is devoted to the stability of convective flow in a nonuniformly
rotating layer of an electrically conducting fluid in a spiral magnetic field.
The stationary and oscillatory modes of magnetic convection are considered
depending on the profile of the angular rotation velocity (Rossby number
$\textrm{Ro}$) and on the profile of the external azimuthal magnetic field
(magnetic Rossby number $\textrm{Rb}$). The nonlinear dynamic system of Lorentz
type equations is obtained by using the Galerkin method. Numerical analysis of
these equations has shown the presence of chaotic behavior of convective flows.
The criteria of the occurrence of chaotic movements are found. It depends on
the parameters of convection: dimensionless numbers of Rayleigh $\textrm{Ra}$,
Chandrasekhar $\textrm{Q}$, Taylor $\textrm{Ta}$, and external azimuthal
magnetic field with the Rossby magnetic number $\textrm{Rb}=-1$ for Rayleigh
$(\textrm{Ro}=-1)$ and Kepler $(\textrm{Ro}=-3/4)$ profiles of the angular
rotation velocity of the medium.",1905.05472v1
2020-03-11,"Degenerate states, emergent dynamics and fluid mixing by magnetic rotors","We investigate the collective motion of magnetic rotors suspended in a
viscous fluid under an uniform rotating magnetic field. The rotors are
positioned on a square lattice, and low Reynolds hydrodynamics is assumed. For
a $3 \times 3$ array of magnets, we observe three characteristic dynamical
patterns as the external field strength is varied: a synchronized pattern, an
oscillating pattern, and a chessboard pattern. The relative stability of these
depends on the competition between the energy due to the external magnetic
field and the energy of the magnetic dipole-dipole interactions among the
rotors. We argue that the chessboard pattern can be understood as an
alternation in the stability of two degenerate states, characterized by striped
and spin-ice configurations, as the applied magnetic field rotates. For larger
arrays, we observe propagation of slip waves that are similar to metachronal
waves. The rotor arrays have potential as microfluidic devices that can mix
fluids and create vortices of different sizes.",2003.05082v1
2020-04-28,The photometric and polarimetric variability of magnetic O-type stars,"Massive star winds are important contributors to the energy, momentum and
chemical enrichment of the interstellar medium. Strong, organized and
predominantly dipolar magnetic fields have been firmly detected in a small
subset of massive O-type stars. Magnetic massive stars are known to exhibit
phase-locked variability of numerous observable quantities that is hypothesized
to arise due to the presence of an obliquely rotating magnetosphere formed via
the magnetic confinement of their strong outflowing winds. Analyzing the
observed modulations of magnetic O-type stars is thus a key step towards the
better understanding of the physical processes that occur within their
magnetospheres. The dynamical processes that lead to the formation of a
magnetosphere are formally solved utilizing complex MHD simulations. Recently,
an Analytic Dynamical Magnetosphere (ADM) model has been developed that can
quickly be employed to compute the time-averaged density, temperature and
velocity gradients within a dynamical magnetosphere. Here, we exploit the ADM
model to compute photometric and polarimetric observables of magnetic Of?p
stars, to test geometric models inferred from magnetometry. We showcase
important results on the prototypical Of?p-type star HD 191612, that lead to a
better characterization of massive star wind and magnetic properties.",2004.13594v1
2020-08-18,Dark soliton-like magnetic domain walls in a two-dimensional ferromagnetic superfluid,"We report a stable magnetic domain wall in a uniform ferromagnetic spin-1
condensate, characterized by the magnetization having a dark soliton profile
with nonvanishing superfluid density. We find exact stationary solutions for a
particular ratio of interaction parameters with and without magnetic fields,
and develop an accurate analytic solution applicable to the whole ferromagnetic
phase. In the absence of magnetic fields, this domain wall relates various
distinct solitary excitations in binary condensates through $\textrm{SO}(3)$
spin rotations, which otherwise are unconnected. Remarkably, studying the
dynamics of a quasi-two-dimensional (quasi-2D) system we show that standing
wave excitations of the domain wall oscillate without decay, being stable
against the snake instability. The domain wall is dynamically unstable to modes
that cause the magnetization to grow perpendicularly while leaving the domain
wall unchanged. Real time dynamics in the presence of white noise reveals that
this ""spin twist"" instability does not destroy the topological structure of the
magnetic domain wall.",2008.08175v2
2020-10-29,Collective spin dynamics under dissipative spin Hall torque,"Current-induced spin torques in layered magnetic heterostructures have many
commonalities across broad classes of magnetic materials. These include not
only collinear ferromagnets, ferrimagnets, and antiferromagnets, but also more
complex noncollinear spin systems. We develop a general Lagrangian-Rayleigh
approach for studying the role of dissipative torques, which can pump energy
into long-wavelength magnetic dynamics, causing dynamic instabilities. While
the Rayleigh structure of such torques is similar for different magnetic
materials, their consequences depend sensitively on the nature of the order
and, in particular, on whether there is a net magnetic moment. The latter
endows the system with a unipolar switching capability, while magnetically
compensated materials tend to evolve towards limit cycles, at large torques,
with chirality dependent on the torque sign. Apart from the ferromagnetic and
antiferromagnetic cases, we discuss ferrimagnets, which display an intricate
competition between switching and limit cycles. As a simple case for
compensated noncollinear order, we consider isotropic spin glasses, as well as
a scenario of their coexistence with a collinear magnetic order.",2010.15994v1
2021-01-15,The dynamics of local magnetic moments induced by itinerant Weyl electrons,"We derive the effective interactions between local magnetic moments which are
mediated by Weyl electrons in magnetic topological semimetals. The resulting
spin dynamics is governed by the induced Heisenberg, Kitaev and
Dzyaloshinskii-Moriya (DM) interactions with extended range and oscillatory
dependence on the distance between the spins. These interactions are realized
in multiple competing channels shaped by the multitude of Weyl nodes in the
electron spectrum. Microscopic spins need to be spatially modulated with a
channel-dependent wavevector in order to take advantage of the interactions.
The DM vector is parallel to the displacement between the two interacting
spins, and requires the presence of Weyl electron Fermi surfaces. We also
derive the Weyl-induced chiral three-spin interaction in the presence of an
external magnetic field. This interaction has an extended range as well, and
acts upon the spatially modulated spins in various channels. Its tendency is to
produce a skyrmion lattice or a chiral spin liquid which exhibits topological
Hall effect. Ultimately, the theory developed here addresses magnetic dynamics
in relativistic metals even when chiral magnetism is microscopically precluded.
We discuss insights into the ordered state of the magnetic Weyl semimetal
NdAlSi.",2101.06274v1
2021-02-05,Time-resolved imaging of Œrsted field induced magnetization dynamics in cylindrical magnetic nanowires,"Recent studies in three dimensional spintronics propose that the \OE rsted
field plays a significant role in cylindrical nanowires. However, there is no
direct report of its impact on magnetic textures. Here, we use time-resolved
scanning transmission X-ray microscopy to image the dynamic response of
magnetization in cylindrical Co$_{30}$Ni$_{70}$ nanowires subjected to
nanosecond \OE rsted field pulses. We observe the tilting of longitudinally
magnetized domains towards the azimuthal \OE rsted field direction and create a
robust model to reproduce the differential magnetic contrasts and extract the
angle of tilt. Further, we report the compression and expansion, or breathing,
of a Bloch-point domain wall that occurs when weak pulses with opposite sign
are applied. We expect that this work lays the foundation for and provides an
incentive to further studying complex and fascinating magnetization dynamics in
nanowires, especially the predicted ultra-fast domain wall motion and
associated spin wave emissions.",2102.03067v3
2021-10-12,Magnetization dynamics in the density matrix formalism,"Magnetization dynamics described by the Landau-Lifshitz-Gilbert-Slonczewski
(LLGS) equation can be formulated to have the form of the well-known
two-level-system (TLS) equations. Recently, we showed that a DC spin-transfer
torque (STT) term in the LLGS equation can be mapped to a modulation of the
carrier relaxation rates in the analogous TLS equations. Here, we extend the
analogy to the TLS dynamics by including the AC magnetic field, AC
demagnetization field, and AC STT excitation that we show constitute the
interaction term in the analogous TLS picture. Interestingly, we find that the
carrier injection rate in the TLS equations that is responsible for transitions
between the excited and ground states of the system naturally translates to an
intense short magnetic pulse that reverses the magnetization state.
Furthermore, we also show that the two helicities of circularly polarized
magnetic pulses correspond to the two carrier injection rates in the analogous
TLS picture. In the context of the highly debated all-optical helicity
dependent switching experiment, it offers a new explanation of the
magnetization reversal from first principles.",2110.06138v3
2022-05-10,A Machine-Learned Spin-Lattice Potential for Dynamic Simulations of Defective Magnetic Iron,"A machine-learned spin-lattice interatomic potential (MSLP) for magnetic iron
is developed and applied to mesoscopic scale defects. It is achieved by
augmenting a spin-lattice Hamiltonian with a neural network term trained to
descriptors representing a mix of local atomic configuration and magnetic
environments. It reproduces the cohesive energy of BCC and FCC phases with
various magnetic states. It predicts the formation energy and complex magnetic
structure of point defects in quantitative agreement with density functional
theory (DFT) including the reversal and quenching of magnetic moments near the
core of defects. The Curie temperature is calculated through spin-lattice
dynamics showing good computational stability at high temperature. The
potential is applied to study magnetic fluctuations near sizable dislocation
loops. The MSLP transcends current treatments using DFT and molecular dynamics,
and surpasses other spin-lattice potentials that only treat near-perfect
crystal cases.",2205.04732v1
2022-08-07,Transition state theory characterizes thin film macrospin dynamics driven by an oscillatory magnetic field: Inertial effects,"Understanding the magnetization switching process in ferromagnetic thin films
is essential for many technological applications. We investigate the effects of
periodic driving via magnetic fields on a macrospin system under explicit
consideration of inertial dynamics. This is usually achieved by extending the
Landau-Lifshitz-Gilbert equation with a term including the second time
derivative of the magnetization. The dynamics of the magnetization switching
can then be characterized by its switching rate. We apply methods from
transition state theory for driven systems to resolve the rate of magnetization
switching in this general case. In doing so, we find that magnetization
exhibits resonance-like behavior under certain driving conditions, and it can
be affected strongly by the system's relaxation rate.",2208.03613v1
2022-12-16,Dynamical magnetic fields in heavy-ion collisions,"The magnetic fields in heavy-ion collisions are important ingredients for
many interesting phenomena, such as the Chiral Magnetic Effect, Chiral Magnetic
Wave, the directed flow $v_1$ of $D^0$ mesons and the splitting of the spin
polarization of the $\Lambda$/$\bar{\Lambda}$. Quantitative studies of these
phenomena however suffer from limited understanding on the dynamical evolution
of these fields in the medium created by the collisions, which remains a
critical and challenging problem. The initial magnetic fields from the
colliding nuclei decay very fast in the vacuum but their lifetime could be
extended through medium response due to electrically conducting quarks and
antiquarks. Here we perform a detailed analysis of such medium effect on the
dynamical magnetic fields by numerically solving the Maxwell's equations
concurrently with the expanding medium described by viscous hydrodynamics,
under the assumption of negligible back reaction of the fields on the fluid
evolution. Our results suggest a considerable enhancement of late time magnetic
fields, the magnitude of which depends sensitively on the fireball expansion as
well as the medium electric conductivity both before and during hydrodynamic
stage.",2212.08579v2
2023-09-22,Laser-induced real-space topology control of spin wave resonances,"Femtosecond laser excitation of materials that exhibit magnetic spin textures
promises advanced magnetic control via the generation of ultrafast and
non-equilibrium spin dynamics. We explore such possibilities in ferrimagnetic
[Fe(0.35 nm)/Gd(0.40 nm)]$_{160}$ multilayers, which host a rich diversity of
magnetic textures from stripe domains at low magnetic fields, a dense
bubble/skyrmion lattice at intermediate fields, and a single domain state for
high magnetic fields. Using femtosecond magneto-optics, we observe distinct
coherent spin wave dynamics in response to a weak laser excitation allowing us
to unambiguously identify the different magnetic spin textures. Moreover,
employing strong laser excitation we show that we achieve versatile control of
the coherent spin dynamics via non-equilibrium and ultrafast transformation of
magnetic spin textures by both creating and annihilating bubbles/skyrmions. We
corroborate our findings by micromagnetic simulations and by Lorentz
transmission electron microscopy before and after laser exposure.",2309.12956v1
2023-11-07,"Magnetism in AV3Sb5 (A = Cs, Rb, K): Complex Landscape of the Dynamical Magnetic Textures","We have investigated the dynamical magnetic properties of the V-based kagome
stibnite compounds by combining the ab-initio calculated magnetic parameters of
a spin Hamiltonian like inter-site exchange parameters, magnetocrystalline
anisotropy and site projected magnetic moments, with full-fledged simulations
of atomistic spin-dynamics. Our calculations reveal that in addition to a
ferromagnetic order along the [001] direction, the system hosts a complex
landscape of magnetic configurations comprised of commensurate and
incommensurate spin-spirals along the [010] direction. The presence of such
chiral magnetic textures may be the key to solve the mystery about the origin
of the experimentally observed inherent breaking of the C6 rotational symmetry-
and the time-reversal symmetry.",2311.04099v1
2023-12-17,Magnetic Fluctuations in Niobium Pentoxide,"Using a spin-polarized muon beam we were able to capture magnetic dynamics in
an amorphous niobium pentoxide thin film. Muons are used to probe internal
magnetic fields produced by defects. Magnetic fluctuations could be described
by the dynamical Kubo-Toyabe model considering a time-dependent local magnetic
field. We state that observed fluctuations result from the correlated motion of
electron spins. We expect that oxygen vacancies play a significant role in
these films and lead to a complex magnetic field distribution which is
non-stationary. The characteristic average rate of magnetic field change is on
the order of 100~MHz. The observed dynamics may provide insight into potential
noise sources in Nb-based superconducting devices, while also highlighting the
limitations imposed by amorphous oxides.",2312.10697v1
2024-01-01,Calculation of Gilbert damping and magnetic moment of inertia using torque-torque correlation model within ab initio Wannier framework,"Magnetization dynamics in magnetic materials are well described by the
modified semiclassical Landau-Lifshitz-Gilbert (LLG) equation, which includes
the magnetic damping $\alpha$ and the magnetic moment of inertia $\mathrm{I}$
tensors as key parameters. Both parameters are material-specific and physically
represent the time scales of damping of precession and nutation in
magnetization dynamics. $\alpha$ and $\mathrm{I}$ can be calculated quantum
mechanically within the framework of the torque-torque correlation model. The
quantities required for the calculation are torque matrix elements, the real
and imaginary parts of the Green's function and its derivatives. Here, we
calculate these parameters for the elemental magnets such as Fe, Co and Ni in
an ab initio framework using density functional theory and Wannier functions.
We also propose a method to calculate the torque matrix elements within the
Wannier framework. We demonstrate the effectiveness of the method by comparing
it with the experiments and the previous ab initio and empirical studies and
show its potential to improve our understanding of spin dynamics and to
facilitate the design of spintronic devices.",2401.00714v1
2011-11-21,Multiscale dynamics of solar magnetic structures,"Multiscale topological complexity of solar magnetic field is among the
primary factors controlling energy release in the corona, including associated
processes in the photospheric and chromospheric boundaries. We present a new
approach for analyzing multiscale behavior of the photospheric magnetic flux
underlying this dynamics as depicted by a sequence of high-resolution solar
magnetograms. The approach involves two basic processing steps: (1)
identification of timing and location of magnetic flux origin and demise events
(as defined by DeForest et al., 2007) by tracking spatiotemporal evolution of
unipolar and bipolar photospheric regions, and (2) analysis of collective
behavior of the detected magnetic events using a generalized version of
Grassberger - Procaccia correlation integral algorithm. The scale-free nature
of the developed algorithms makes it possible to characterize the dynamics of
the photospheric network across a wide range of distances and relaxation times.
Three types of photospheric conditions are considered to test the method: a
quiet photosphere, a solar active region (NOAA 10365) in a quiescent
non-flaring state, and the same active region during a period of M-class
flares. The results obtained show (1) the presence of a topologically complex
asymmetrically fragmented magnetic network in the quiet photosphere driven by
meso- and supergranulation, (2) the formation of non-potential magnetic
structures with complex polarity separation lines inside the active region, and
(3) statistical signatures of canceling bipolar magnetic structures coinciding
with flaring activity in the active region. Each of these effects can represent
an unstable magnetic configuration acting as an energy source for coronal
dissipation and heating.",1111.5053v2
2015-02-12,A Multiscale Dynamo Model Driven by Quasi-geostrophic Convection,"A convection-driven multiscale dynamo model is developed in the limit of low
Rossby number for the plane layer geometry in which the gravity and rotation
vectors are aligned. The small-scale fluctuating dynamics are described by a
magnetically-modified quasi-geostrophic equation set, and the large-scale mean
dynamics are governed by a diagnostic thermal wind balance. The model utilizes
three timescales that respectively characterize the convective timescale, the
large-scale magnetic evolution timescale, and the large-scale thermal evolution
timescale. Distinct equations are derived for the cases of order one and low
magnetic Prandtl number. It is shown that the low magnetic Prandtl number model
is characterized by a magnetic to kinetic energy ratio that is asymptotically
large, with ohmic dissipation dominating viscous dissipation on the
large-scales. For the order one magnetic Prandtl number model the magnetic and
kinetic energies are equipartitioned and both ohmic and viscous dissipation are
weak on the large-scales; large-scale ohmic dissipation occurs in thin magnetic
boundary layers adjacent to the solid boundaries. For both magnetic Prandtl
number cases the Elsasser number is small since the Lorentz force does not
enter the leading order force balance. The new models can be considered fully
nonlinear, generalized versions of the dynamo model originally developed by
Childress and Soward [Phys. Rev. Lett., \textbf{29}, p.837, 1972]. These models
may be useful for understanding the dynamics of convection-driven dynamos in
regimes that are only just becoming accessible to direct numerical simulations.",1502.03856v2
2016-04-01,On the relationship between G-band bright point dynamics and their magnetic field strengths,"G-band bright points (GBPs) are regarded as good manifestations of magnetic
flux concentrations. We aim to investigate the relationship between the dynamic
properties of GBPs and their longitudinal magnetic field strengths. High
spatial and temporal resolution observations were recorded simultaneously with
G-band filtergrams and Narrow-band Filter Imager (NFI) Stokes I and V images
with Hinode /Solar Optical Telescope. The GBPs are identified and tracked in
the G-band images automatically, and the corresponding longitudinal magnetic
field strength of each GBP is extracted from the calibrated NFI magnetograms by
a point-to-point method. After categorizing the GBPs into five groups by their
longitudinal magnetic field strengths, we analyze the dynamics of GBPs of each
group. The results suggest that with increasing longitudinal magnetic field
strengths of GBPs correspond to a decrease in their horizontal velocities and
motion ranges as well as by showing more complicated motion paths. This
suggests that magnetic elements showing weaker magnetic field strengths prefer
to move faster and farther along straighter paths, while stronger ones move
more slowly in more erratic paths within a smaller region. The dynamic
behaviors of GBPs with different longitudinal magnetic field strengths can be
explained by that the stronger flux concentrations withstand the convective
flows much better than weaker ones.",1604.00152v1
2019-02-05,The Dynamics of Magnetism in Fe-Cr Alloys with Cr Clustering,"The dynamics of magnetic moments in iron-chromium alloys with different
levels of Cr clustering show unusual features resulting from the fact that even
in a perfect body-centred cubic structure, magnetic moments experience
geometric magnetic frustration resembling that of a spin glass. Due to the long
range exchange coupling and configuration randomness, magnetic moments of Cr
solutes remain non-collinear at all temperatures. To characterise magnetic
properties of Fe-Cr alloys, we explore the temperature dependence of
magnetisation, susceptibility, Curie temperature and spin-spin correlations
with spatial resolution. The static and dynamic magnetic properties are
correlated with the microstructure of Fe-Cr, where magnetisation and
susceptibility are determined by the size of Cr precipitates at nominal Cr
concentrations. The Curie temperature is always maximised when the solute
concentration of Cr in the $\alpha$ phase is close to 5 to 6 at.\%, and the
susceptibility of Fe atoms is always enhanced at the boundary between a
precipitate and solid solution. Interaction between Cr and Fe stimulates
magnetic disorder, lowering the effective Curie temperature. Dynamic simulation
of evolution of magnetic correlations shows that the spin-spin relaxation time
in Fe-Cr alloys is in the 20 to 40 ps range.",1902.01645v1
2020-05-25,Dynamics of ferromagnetic bimerons driven by spin currents and magnetic fields,"Magnetic bimeron composed of two merons is a topological counterpart of
magnetic skyrmion in in-plane magnets, which can be used as the nonvolatile
information carrier in spintronic devices. Here we analytically and numerically
study the dynamics of ferromagnetic bimerons driven by spin currents and
magnetic fields. Numerical simulations demonstrate that two bimerons with
opposite signs of topological numbers can be created simultaneously in a
ferromagnetic thin film via current-induced spin torques. The current-induced
spin torques can also drive the bimeron and its speed is analytically derived,
which agrees with the numerical results. Since the bimerons with opposite
topological numbers can coexist and have opposite drift directions, two-lane
racetracks can be built in order to accurately encode the data bits. In
addition, the dynamics of bimerons induced by magnetic field gradients and
alternating magnetic fields are investigated. It is found that the bimeron
driven by alternating magnetic fields can propagate along a certain direction.
Moreover, combining a suitable magnetic field gradient, the
Magnus-force-induced transverse motion can be completely suppressed, which
implies that there is no skyrmion Hall effect. Our results are useful for
understanding of the bimeron dynamics and may provide guidelines for building
future bimeron-based spintronic devices.",2005.11924v2
2022-05-18,Influence of Magnetic Fields on the Gas Rotation in the Galaxy $NGC\;6946$,"Magnetic fields can play an important role in the energy balance and
formation of gas structures in galaxies. However, their dynamical effect on the
rotation curve of galaxies is immensely unexplored. We investigate the
dynamical effect of the known magnetic arms of $NGC\;6946$ on its circular gas
rotation traced in HI, considering two dark matter mass density models, ISO,
and the universal NFW profile. We used a three-dimensional model for the
magnetic field structure to fit the modeled rotation curve to the observed data
via an $\chi$-squared minimization method. The shape of the HI gas rotation
curve is reproduced better including the effect of the magnetic field,
especially in the outer part, where the dynamical effect of the magnetic field
could become important. The typical amplitude of the regular magnetic field
contribution in the rotation curve is about $ 6 - 14 \; km s^{-1}$ in the outer
gaseous disk of the galaxy $NGC\;6946$. The contribution ratio of the regular
magnetic field to the observed circular velocity and to dark matter increases
with the galactocentric radius. Its ratio to the observed rotational velocity
is about five percent and to dark matter is about 10 percent in the outer
regions of the galaxy $NGC\;6946$. Therefore, the large-scale magnetic fields
cannot be completely ignored in the large-scale dynamics of spiral galaxies,
especially in the outer parts of galaxies.",2205.09069v2
2023-10-23,Nonlinear Optical Effects Due to Magnetization Dynamics In a Ferromagnet,"We theoretically consider magnetization dynamics in a ferromagnetic slab
induced by the magnetic field of a strong femtosecond laser pulse. The
longitudinal geometry, in which the initial magnetization lies in both the
plane of incidence and the sample plane, is studied. The magnetization
oscillations at the optical wave frequency are calculated with the use of the
Kapitza pendulum approach taking into account that the optical frequency is
much greater than the magnetization oscillation eigenfrequency. We study the
reflection of the electromagnetic wave from a ferromagnet %with such a dynamics
and show that this laser-induced low-frequency magnetization dynamics leads to
the appearance of the second-order nonlinearity in the Maxwell's equations,
which in turn gives rise to both the second harmonic generation (SHG) and
rectification effect. Although the amplitude of the magnetization oscillations
is small, the considered effect may be responsible for the SHG with the
efficiency comparable to that of nonmagnetic SHG from metal surfaces. Our
estimations show that the suggested mechanism may explain the recent
experiments on magnetization induced modulation of the SHG intensity in a
``forbidden'' $P_{in}P_{out}$ combination of incident and reflected waves in
cobalt/heavy metal systems, where it can be even more pronounced due to the
spin current flow through the ferromagnet/ heavy metal interface.",2310.14967v1
2002-12-19,Evolution of Magnetic Fields around a Kerr Black Hole,"The evolution of magnetic fields frozen to a perfectly conducting plasma
fluid around a Kerr black hole is investigated. We focus on the plunging region
between the black hole horizon and the marginally stable circular orbit in the
equatorial plane. Adopting the kinematic approximation where the dynamical
effects of magnetic fields are ignored, we exactly solve Maxwell's equations
with the assumptions that the geodesic motion of the fluid is stationary and
axisymmetric, the magnetic field has only radial and azimuthal components and
depends only on time and radial coordinates. We show that the stationary state
of the magnetic field in the plunging region is uniquely determined by the
boundary conditions at the marginally stable circular orbit. If the magnetic
field at the marginally stable circular orbit is in a stationary state, the
magnetic field in the plunging region will quickly settle into a stationary
state if it is not so initially, in a time determined by the dynamical time
scale. The radial component of the magnetic field at the marginally stable
circular orbit is more important than the toroidal component in determining the
structure and evolution of the magnetic field in the plunging region. Even if
at the marginally stable circular orbit the toroidal component is zero, in the
plunging region a toroidal component is quickly generated from the radial
component by the shear motion of the fluid. Finally, we show that the dynamical
effects of magnetic fields are unimportant in the plunging region if they are
negligible on the marginally stable circular orbit. This supports the
``no-torque inner boundary condition'' of thin disks, contrary to the claim in
the recent literature.",0212456v1
2011-02-28,First Flare-related Rapid Change of Photospheric Magnetic Field Observed by Solar Dynamics Observatory,"Photospheric magnetic field not only plays important roles in building up
free energy and triggering solar eruptions, but also has been observed to
change rapidly and permanently responding to the coronal magnetic field
restructuring due to coronal transients. The Helioseismic and Magnetic Imager
instrument (HMI) on board the newly launched Solar Dynamics Observatory (SDO)
produces seeing-free full-disk vector magnetograms at consistently high
resolution and high cadence, which finally makes possible an unambiguous and
comprehensive study of this important back-reaction process. In this study, we
present a near disk-center, GOES-class X2.2 flare occurred at NOAA AR 11158 on
2011 February 15 using the magnetic field measurements made by HMI. We obtained
the first solid evidence of an enhancement in the transverse magnetic field at
the flaring magnetic polarity inversion line (PIL) by a magnitude of 70%. This
rapid and irreversible field evolution is unequivocally associated with the
flare occurrence, with the enhancement area located in between the two
chromospheric flare ribbons. Similar findings have been made for another two
major flare events observed by SDO. These results strongly corroborate our
previous suggestion that the photospheric magnetic field near the PIL must
become more horizontal after eruptions.
  In-depth studies will follow to further link the photospheric magnetic field
changes with the dynamics of coronal mass ejections, when full Stokes inversion
is carried out to generate accurate magnetic field vectors.",1103.0027v1
2012-02-24,Dynamical Magnetic and Nuclear Polarization in Complex Spin Systems: Semi-magnetic II-VI Quantum Dots,"Dynamical magnetic and nuclear polarization in complex spin systems is
discussed on the example of transfer of spin from exciton to the central spin
of magnetic impurity in a quantum dot in the presence of a finite number of
nuclear spins. The exciton is described in terms of the electron and heavy hole
spins interacting via exchange interaction with magnetic impurity, via
hypeprfine interaction with a finite number of nuclear spins and via dipole
interaction with photons. The time-evolution of the exciton, magnetic impurity
and nuclear spins is calculated exactly between quantum jumps corresponding to
exciton radiative recombination. The collapse of the wavefunction and the
refilling of the quantum dot with new spin polarized exciton is shown to lead
to build up of magnetization of the magnetic impurity as well as nuclear spin
polarization. The competition between electron spin transfer to magnetic
impurity and to nuclear spins simultaneous with the creation of dark excitons
is elucidated. The technique presented here opens up the possibility of
studying optically induced Dynamical Magnetic and Nuclear Polarization in
Complex Spin Systems.",1202.5352v3
2012-07-20,Magnetic Catalysis: A Review,"We give an overview of the magnetic catalysis phenomenon. In the framework of
quantum field theory, magnetic catalysis is broadly defined as an enhancement
of dynamical symmetry breaking by an external magnetic field. We start from a
brief discussion of spontaneous symmetry breaking and the role of a magnetic
field in its a dynamics. This is followed by a detailed presentation of the
essential features of the phenomenon. In particular, we emphasize that the
dimensional reduction plays a profound role in the pairing dynamics in a
magnetic field. Using the general nature of underlying physics and its
robustness with respect to interaction types and model content, we argue that
magnetic catalysis is a universal and model-independent phenomenon. In support
of this claim, we show how magnetic catalysis is realized in various models
with short-range and long-range interactions. We argue that the general nature
of the phenomenon implies a wide range of potential applications: from certain
types of solid state systems to models in cosmology, particle and nuclear
physics. We finish the review with general remarks about magnetic catalysis and
an outlook for future research.",1207.5081v2
2017-12-20,Second-harmonic magnetic response characterizing magnetite-based colloid,"Nonlinear second-harmonic magnetic response (M2) was used to characterize an
aqueous colloidal solution of dextran-coated magnetite (Fe3O4) nanoparticles.
Data analysis with the formalism based on Gilbert-Landau-Lifshitz equation for
stochastic dynamics of superparamagnetic (SP) particles ensured extensive
quantifying of the system via a set of magnetic and magnetodynamic parameters,
such as the mean magnetic moment, the damping constant, the longitudinal
relaxation time, the magnetic anisotropy field and energy, and others. Combined
with transmission electron microscopy and dynamic light scattering, M2
technique allowed obtaining additional parameters, viz., the dextran-coating
thickness and the interparticle magnetic dipolar energy. Aggregated colloidal
nanoparticles were shown to be magnetically correlated inside the aggregate due
to magnetic dipole-dipole (d-d) coupling within the correlation radius ~50 nm.
With the d-d coupling account, the volume distribution of the aggregates
recovered from M2 measurements is well consistent with electron microscopy
results. From electron magnetic resonance, abrupt change of SP dynamics with
increasing external magnetic field was observed and explained. The presented
study exemplifies a novel M2-based procedure of comprehensive quantitative
characterization applicable for a wide variety of SP systems.",1712.07534v1
2019-10-15,Irregularly Shaped γ'-Fe4N Nanoparticles for Hyperthermia Treatment and T2 Contrast-Enhanced Magnetic Resonance Imaging with Minimum Dose,"Magnetic nanoparticles (MNPs) have been extensively used in drug/gene
delivery, hyperthermia therapy, magnetic particle imaging (MPI), magnetic
resonance imaging (MRI), magnetic bioassays, etc. With proper surface chemical
modifications, physicochemically stable and non-toxic MNPs are emerging
contrast agents and tracers for in vivo MRI and MPI applications. Herein, we
report the high magnetic moment, irregularly shaped {\gamma}'-Fe4N
nanoparticles for enhanced hyperthermia therapy and T2 contrast agent for MRI
application. The static and dynamic magnetic properties of {\gamma}'-Fe4N
nanoparticles are characterized by vibrating sample magnetometer (VSM) and
magnetic particle spectroscopy (MPS) systems, respectively. Compared to the
{\gamma}-Fe2O3 nanoparticles, {\gamma}'-Fe4N show at least 3 times higher
saturation magnetization (in emu/g), which, as a result, gives rise to the
stronger dynamic magnetic responses as proved in the MPS measurement results.
In addition, {\gamma}'-Fe4N nanoparticles are functionalized with oleic acid
layer by a wet mechanical milling process, the morphologies of as-milled
nanoparticles are characterized by transmission electron microscopy (TEM),
dynamic light scattering (DLS) and nanoparticle tracking analyzer (NTA). We
report that with proper surface chemical modification and tuning on
morphologies, {\gamma}'-Fe4N nanoparticles could be used as tiny heating
sources for hyperthermia and contrast agents for MRI applications with minimum
dose.",1910.06842v1
2021-11-26,Numerical Analysis of the Nanoparticle Dynamics in a Viscous Liquid: Deterministic Approach,"We study the deterministic dynamics of single-domain ferromagnetic
nanoparticles in a viscous liquid induced by the joint action of the gradient
and uniform magnetic fields. It is assumed that the gradient field depends on
time harmonically and the uniform field has two components, perpendicular and
parallel to the gradient one. We also assume that the anisotropy magnetic field
is so strong that the nanoparticle magnetization lies along the anisotropy
axis, i.e., the magnetization vector is ""frozen"" into the particle body. With
these assumptions and neglecting inertial effects we derive the torque and
force balance equations that describe the rotational and translational motions
of particles. We reduce these equations to a set of two coupled equations for
the magnetization angle and particle coordinate, solve them numerically in a
wide range of the system parameters and analyze the role of the parallel
component of the uniform magnetic field. It is shown, in particular, that
nanoparticles perform only periodic rotational and translational motions if the
perpendicular component of the uniform magnetic field is absent. In contrast,
the nanoparticle dynamics in the presence of this component becomes
non-periodic, resulting in the drift motion (directed transport) of
nanoparticles. By analyzing the short and long-time dependencies of the
magnetization angle and particle coordinate we show that the increase in the
parallel component of the uniform magnetic field decreases both the particle
displacement for a fixed time and its average drift velocity on each period of
the gradient magnetic field.",2111.13534v2
2022-04-25,Reduced effective magnetization and damping by slowly-relaxing impurities in strained $γ$-$\mathrm{Fe_2O_3}$ thin films,"We study the static and dynamic magnetic properties of epitaxially strained
$\gamma$-$\mathrm{Fe_2O_3}$ (maghemite) thin films grown via pulsed-laser
deposition on MgO substrates by SQUID magnetometry and cryogenic broadband
ferromagnetic resonance experiments. SQUID magnetometry measurements reveal
hysteretic magnetization curves for magnetic fields applied both in- and out of
the sample plane. From the magnetization dynamics of our thin films, we find a
small negative effective magnetization in agreement with a strain induced
perpendicular magnetic anisotropy. Moreover, we observe a non-linear evolution
of the ferromagnetic resonance-linewidth as function of the microwave frequency
and explain this finding with a model based on slowly relaxing impurities, the
so-called slow relaxor model. By investigating the magnetization dynamics in
our maghemite thin films as a function of frequency and temperature, we can
isolate the temperature dependent contribution of the slowly relaxing
impurities to the resonance linewidth and, in particular, observe a sign change
in the effective magnetization. This finding provides evidence for a transition
of the magnetic anisotropy from a perpendicular easy axis to an easy in-plane
anisotropy for reduced temperatures.",2204.11498v1
2020-07-14,Magnetic field transport in compact binaries,"Dwarf novae (DNe) and low mass X-ray binaries (LMXBs) show eruptions that are
thought to be due to a thermal-viscous instability in their accretion disk.
These eruptions provide constraints on angular momentum transport mechanisms.
We explore the idea that angular momentum transport could be controlled by the
dynamical evolution of the large scale magnetic field. We study the impact of
different prescriptions for the magnetic field evolution on the dynamics of the
disk. This is a first step in confronting the theory of magnetic field
transport with observations. We develop a version of the disk instability model
that evolves the density, the temperature and the large scale vertical magnetic
flux together. We take into account the accretion driven by turbulence or by a
magnetized outflow. To evolve the magnetic flux, we use a toy model with
physically motivated prescriptions depending mainly on the local magnetization.
We find that allowing magnetic flux to be advected inwards provides the best
agreement with DNe lightcurves. This leads to a hybrid configuration with an
inner magnetized disk, driven by angular momentum losses to an MHD outflow,
sharply transiting to an outer weakly-magnetized turbulent disk, where the
eruptions are triggered. The dynamical impact is equivalent to truncating a
viscous disk so that it does not extend down to the compact object, with the
truncation radius dependent on the magnetic flux and evolving as
$\dot{M}^{-2/3}$. Models of DNe and LMXBs lightcurves typically require the
outer, viscous disk to be truncated in order to match observations. There is no
generic explanation for this truncation. We propose that it is a natural
outcome of the presence of large-scale magnetic fields in both DNe and LMXBs,
the magnetic flux accumulating towards the center to produce a magnetized disk
with a fast accretion timescale.",2007.07277v1
2006-03-14,Spin dynamics and magnetic order in magnetically frustrated Tb2Sn2O7,"We report a study of the geometrically frustrated magnetic material Tb2Sn2O7
by the positive muon spin relaxation technique. No signature of a static
magnetically ordered state is detected while neutron magnetic reflections are
observed in agreement with a published report. This is explained by the
dynamical nature of the ground state of Tb2Sn2O7: the Tb3+ magnetic moment
characteristic fluctuation time is ~ 10^{-10} s. The strong effect of the
magnetic field on the muon spin-lattice relaxation rate at low fields indicates
a large field-induced increase of the magnetic density of states of the
collective excitations at low energy.",0603379v1
2006-12-15,Domain Nucleation and Annihilation in Uniformly Magnetized State under Current Pulses in Narrow Ferromagnetic Wires,"We investigate the current-driven magnetization dynamics in narrow Permalloy
wires by means of Lorentz microscopy and electron holography. Current pulses
are found to transform the magnetic structure in the uniformly magnetized state
below the Curie temperature. A variety of magnetic states including reversed
magnetic domains are randomly obtained in low probability. The dynamics of
vortices found in most of observed magnetic states seems to play a key role in
triggering the magnetization reversal.",0612383v2
2001-05-02,The Ising Model on a Dynamically Triangulated Disk with a Boundary Magnetic Field,"We use Monte Carlo simulations to study a dynamically triangulated disk with
Ising spins on the vertices and a boundary magnetic field. For the case of zero
magnetic field we show that the model possesses three phases. For one of these
the boundary length grows linearly with disk area, while the other two phases
are characterized by a boundary whose size is on the order of the cut-off. A
line of continuous magnetic transitions separates the two small boundary
phases. We determine the critical exponents of the continuous magnetic phase
transition and relate them to predictions from continuum 2-d quantum gravity.
This line of continuous transitions appears to terminate on a line of
discontinuous phase transitions dividing the small boundary phases from the
large boundary phase. We examine the scaling of bulk magnetization and boundary
magnetization as a function of boundary magnetic field in the vicinity of this
tricritical point.",0105002v1
2008-08-15,Alignment Dynamics of Single-Walled Carbon Nanotubes in Pulsed Ultrahigh Magnetic Fields,"We have measured the dynamic alignment properties of single-walled carbon
nanotube (SWNT) suspensions in pulsed high magnetic fields through linear
dichroism spectroscopy. Millisecond-duration pulsed high magnetic fields up to
56 T as well as microsecond-duration pulsed ultrahigh magnetic fields up to 166
T were used. Due to their anisotropic magnetic properties, SWNTs align in an
applied magnetic field, and because of their anisotropic optical properties,
aligned SWNTs show linear dichroism. The characteristics of their overall
alignment depend on several factors, including the viscosity and temperature of
the suspending solvent, the degree of anisotropy of nanotube magnetic
susceptibilities, the nanotube length distribution, the degree of nanotube
bundling, and the strength and duration of the applied magnetic field. In order
to explain our data, we have developed a theoretical model based on the
Smoluchowski equation for rigid rods that accurately reproduces the salient
features of the experimental data.",0808.2205v1
2008-11-13,Selection rules for Single-Chain-Magnet behavior in non-collinear Ising systems,"The magnetic behavior of molecular Single-Chain Magnets is investigated in
the framework of a one-dimensional Ising model with single spin-flip Glauber
dynamics. Opportune modifications to the original theory are required in order
to account for reciprocal non-collinearity of local anisotropy axes and the
crystallographic (laboratory) frame. The extension of Glauber's theory to the
case of a collinear Ising ferrimagnetic chain is also discussed. Within this
formalism, both the dynamics of magnetization reversal in zero field and the
response of the system to a weak magnetic field, oscillating in time, are
studied. Depending on the geometry, selection rules are found for the
occurrence of slow relaxation of the magnetization at low temperatures, as well
as for resonant behavior of the a.c. susceptibility as a function of
temperature at low frequencies. The present theory applies successfully to some
real systems, namely Mn-, Dy-, and Co-based molecular magnetic chains, showing
that Single-Chain-Magnet behavior is not only a feature of collinear ferro- and
ferrimagnetic, but also of canted antiferromagnetic chains.",0811.2118v1
2008-12-29,Temperature dependent dynamic and static magnetic response in magnetic tunnel junctions with Permalloy layers,"Ferromagnetic resonance and static magnetic properties of CoFe/Al2O3/CoFe/Py
and CoFe/Al2O3/CoFeB/Py magnetic tunnel junctions and of 25nm thick
single-layer Permalloy(Py) films have been studied as a function of temperature
down to 2K. The temperature dependence of the ferromagnetic resonance excited
in the Py layers in magnetic tunnel junctions shows knee-like enhancement of
the resonance frequency accompanied by an anomaly in the magnetization near
60K. We attribute the anomalous static and dynamic magnetic response at low
temperatures to interface stress induced magnetic reorientation transition at
the Py interface which could be influenced by dipolar soft-hard layer coupling
through the Al2O3 barrier.",0812.4953v1
2009-03-19,Magnetic Anisotropy and Magnetization Dynamics of Individual Atoms and Clusters of Fe and Co on Pt(111),"The recently discovered giant magnetic anisotropy of single magnetic Co atoms
raises the hope of magnetic storage in small clusters. We present a joint
experimental and theoretical study of the magnetic anisotropy and the spin
dynamics of Fe and Co atoms, dimers, and trimers on Pt(111). Giant anisotropies
of individual atoms and clusters as well as lifetimes of the excited states
were determined with inelastic scanning tunneling spectroscopy. The short
lifetimes due to hybridization-induced electron-electron scattering oppose the
magnetic stability provided by the magnetic anisotropies.",0903.3337v1
2010-05-21,Magnetization jumps and relaxation effect in doped CeFe2,"For the first time, we find that the dynamic antiferromagnetic phase present
in CeFe2 gets stabilized with Ga and Si substitutions. We find that phenomena
such as strain-induced first order jumps in the magnetization curves, asymmetry
between the M-H curves during the increasing and decreasing field cycles, the
envelope curve being inside the virgin curve, occur in these compounds.
Temperature and time dependences of magnetization show that the compounds
possess glassy behavior at low temperatures. Multi-step magnetization behavior,
unusual relaxation effect, thermal and magnetic history dependence, which are
signatures of the martensitic scenario due to the strong magneto-structural
coupling, are found to be present in this system. We also show that one can
induce the magnetization steps with the help of appropriate measurement
protocol. Detailed magnetization relaxation studies have been carried out to
understand the dynamics of magnetic phase transition.",1005.3931v1
2010-10-13,Paramagnetic Leidenfrost Drops,"We present a fluid dynamics video showing the behavior of drops of liquid
oxygen, at room temperature. Due to their low boiling point, these drops
levitate on a cushion of their own vapour. This property gives them a high
mobility, as known more generally in such Leidenfrost situations. But liquid
oxygen is also paramagnetic, and thus likely to be manipulated using a magnet.
It is first shown that the shape of the drop can be modified by changing the
drop/magnet distance; approaching the magnet acts as reinforcing gravity, so
that the drops get flattened by this action. The transformation is of course
reversible: as the magnet is withdrawn, the liquid recovers its quasi-spherical
shape. Magnets can also be used to trap the oxygen drops. As they pass above a
magnet, they slow down significantly, a consequence of their deformation:
despite a very low friction, the vibrations induced by the drop deformations
represent an important source of dissipation: below a well-defined velocity,
drops can even be stopped in the magnetic trap. Quicker ones can be decelerated
and then captured by series of magnets. Oberving the same events from the top
reveals the complex shapes adopted by the liquid as it crosses the traps.",1010.2611v1
2011-10-05,Self-organizing magnetic beads for biomedical applications,"In the field of biomedicine magnetic beads are used for drug delivery and to
treat hyperthermia. Here we propose to use self-organized bead structures to
isolate circulating tumor cells using lab-on-chip technologies. Typically blood
flows past microposts functionalized with antibodies for circulating tumor
cells. Creating these microposts with interacting magnetic beads makes it
possible to tune the geometry in size, position and shape. We developed a
simulation tool that combines micromagnetics and discrete particle dynamics, in
order to design micropost arrays made of interacting beads. The simulation
takes into account the viscous drag of the blood flow, magnetostatic
interactions between the magnetic beads and gradient forces from external
aligned magnets. We developed a particle-particle particle-mesh method for
effective computation of the magnetic force and torque acting on the particles.",1110.0983v1
2012-01-18,Dynamics of Current Induced Magnetic Superstructures in Exchange-Spring Devices,"Thermoelectric manipulation of the magnetization of a magnetic layered stack
in which a low-Curie temperature magnet is sandwiched between two strong
magnets (exchange spring device) is considered. Controllable Joule heating
produced by a current flowing in the plane of the magnetic stack (CIP
configuration) induces a spatial magnetic and thermal structure along the
current flow --- a magneto-thermal-electric domain (soliton). We show that such
a structure can experience oscillatory in time dynamics if the magnetic stack
is incorporated into an electric circuit in series with an inductor. The
excitation of these magneto-thermionic oscillations follow the scenario either
of ""soft"" of ""hard"" instability: in the latter case oscillations arise if the
initial perturbation is large enough. The frequency of the temporal
oscillations is of the order of $10^5 \div 10^7 s^{-1}$ for current densities
$j\sim 10^6 \div 10^7 A/cm^3$.",1201.3854v1
2012-08-30,Heavy and Light Monopoles in Magnetic Reversion in Artificial Spin Ice,"This work makes a theoretical study of the dynamics of emergent elemental
excitations in artificial spin ice systems with hexagonal geometry during the
magnetic reversion of the system. The magnetic and physical parameters of the
nanoislands that form the array are considered as variables in the study. The
parameters considered are: the energy barrier for the inversion of each
nanoisland, the magnetic moment of the nanomagnets and the possible disorder in
the sample. Our results show that the reversion dynamic presents two distinct
mechanisms of magnetic reversion, with different elemental excitations for each
mechanism. The first mechanism presents a reversion with the appearance of
magnetic monopoles that do not move in the samples (heavy monopoles) and the
absence of Dirac chains. In the other mechanism elemental magnetic excitations
(light monopoles) appear that move great distances in the sample, giving rise
to extensive Dirac chains during the magnetic reversion.",1208.6284v1
2012-11-06,"Spin dynamics of hard-soft magnetic multi-layer systems: Effect of Exchange, Dipolar and Dzyaloshinski-Moriya interactions","We investigate the effect of coupling (intensity and nature), applied field,
and anisotropy on the spin dynamics of a multi-layer system composed of a hard
magnetic slab coupled to a soft magnetic slab through a nonmagnetic spacer. The
soft slab is modeled as a stack of several atomic layers while the hard layer,
of a different material, is either considered as a pinned macroscopic magnetic
moment or as an atomic multi-layer system. We compute the magnetization profile
and hysteresis loop of the multi-layer system by solving the Landau-Lifshitz
equations for the net magnetic moment of each (atomic) layer. We study the
competition between the intra-layer anisotropy and exchange interaction,
applied magnetic field, and the inter-slab exchange, dipolar or
Dzyaloshinski-Moriya interaction. Comparing the effects on the magnetization
profile of the three couplings shows that despite the strong effect of the
exchange coupling, the dipolar and Dzyaloshinski-Moriya interactions induce a
slight (but non negligible) deviation in either the polar or azimuthal
direction thus providing more degrees of freedom for adjusting the spin
configuration in the multi-layer system.",1211.1217v1
2013-07-29,Energy transfers and magnetic energy growth in small-scale dynamo,"In this letter we investigate the dynamics of magnetic energy growth in
small-scale dynamo by studying energy transfers, mainly energy fluxes and
shell-to-shell energy transfers. We perform dynamo simulations for magnetic
Prandtl number $\mathrm{Pm}=20$ on $1024^3$ grid using pseudospectral method.
We demonstrate that the magnetic energy growth is caused by nonlocal energy
transfers from the large-scale or forcing-scale velocity field to small-scale
magnetic field. The peak of these energy transfers move towards lower
wavenumbers as dynamo evolves, which is the reason why the integral scale of
the magnetic field increases with time. The energy transfers $U2U$ (velocity to
velocity) and $B2B$ (magnetic to magnetic) are forward and local.",1307.7465v3
2013-08-28,The chiral magnetic nanomotors,"Propulsion of the chiral magnetic nanomotors powered by a rotating magnetic
field is in the focus of the modern biomedical applications. This technology
relies on strong interaction of dynamic and magnetic degrees of freedom of the
system. Here we study in detail various experimentally observed regimes of the
helical nanomotor orientation and propulsion depending on the actuation
frequency, and establish the relation of these two properties with remanent
magnetization and geometry of the helical nanomotors. The theoretical
predictions for the transition between the regimes and nanomotor orientation
and propulsion speed are in excellent agreement with available experimental
data. The proposed theory offers a few simple guidelines towards the optimal
design of the magnetic nanomotors. In particular, efficient nanomotors should
be fabricated of hard magnetics, e.g., cobalt, magnetized transversally and
have the geometry of a normal helix with a helical angle of 35-45 degrees.",1308.6115v1
2014-12-23,Current-induced spin torque resonance of a magnetic insulator,"Pure spin currents transport angular momentum without an associated charge
flow. This unique property makes them attractive for spintronics applications,
such as torque induced magnetization control in nanodevices that can be used
for sensing, data storage, interconnects and logics. Up to now, however, most
spin transfer torque studies focused on metallic ferromagnets, while magnetic
insulators were largely ignored, in spite of superior magnetic quality factors.
Here, we report the observation of spin torque-induced magnetization dynamics
in a magnetic insulator. Our experiments show that in ultrathin magnetic
insulators the spin torque induced magnetization dynamics can be substantially
larger than those generated by the Oersted field. This opens new perspectives
for the efficient integration of ferro-, ferri-, and antiferromagnetic
insulators into electronic devices.",1412.7460v1
2016-02-24,Synchrotron Magnetic Fields from Rayleigh-Taylor Instability in Supernovae,"Synchrotron emission from a supernova necessitates a magnetic field, but it
is unknown how strong the relevant magnetic fields are, and what mechanism
generates them. In this study, we perform high-resolution numerical gas
dynamics calculations to determine the growth of turbulence due to
Rayleigh-Taylor instability, and the resulting kinetic energy in turbulent
fluctuations, to infer the strength of magnetic fields amplified by this
turbulence. We find that Rayleigh-Taylor instability can produce turbulent
fluctuations strong enough to amplify magnetic fields to a few percent of
equipartition with the thermal energy. This turbulence stays concentrated near
the reverse shock, but averaging this magnetic energy throughout the shocked
region (weighting by emissivity) sets the magnetic fields at a minimum of 0.3
percent of equipartition. This suggests a minimum effective magnetic field
strength ($\epsilon_B > 0.003$) which should be present in all interacting
supernovae.",1602.07692v1
2016-02-25,Dependence of stellar magnetic activity cycles on rotational period in nonlinear solar-type dynamo,"We study turbulent generation of large-scale magnetic fields using nonlinear
dynamo models for solar-type stars in the range of rotational periods from 14
to 30 days. Our models take into account non-linear effects of dynamical
quenching of magnetic helicity, and escape of magnetic field from the dynamo
region due to magnetic buoyancy. The results show that the observed correlation
between the period of rotation and the duration of activity cycles can be
explained in the framework of a distributed dynamo model with a dynamical
magnetic feedback acting on the turbulent generation either from magnetic
buoyancy or magnetic helicity. We discuss implications of our findings for the
understanding of dynamo processes operating in solar-like stars.",1602.07815v1
2016-04-12,Voltage-driven magnetization switching and spin pumping in Weyl semimetals,"We demonstrate electrical magnetization switching and spin pumping in
magnetically doped Weyl semimetals. The Weyl semimetal is a new class of
topological semimetals, known to have nontrivial coupling between the charge
and the magnetization due to the chiral anomaly. By solving the
Landau-Lifshitz-Gilbert equation for a multilayer structure of a Weyl
semimetal, an insulator and a metal whilst taking the charge-magnetization
coupling into account, magnetization dynamics is analyzed. It is shown that the
magnetization dynamics can be driven by the electric voltage. Consequently,
switching of the magnetization with a pulsed electric voltage can be achieved,
as well as precession motion with an applied oscillating electric voltage. The
effect requires only a short voltage pulse and may therefore be more
energetically efficient for us in spintronics devices compared to conventional
spin transfer torque switching.",1604.03326v1
2016-08-02,Effect of temperature and magnetic field on two-flavor superconducting quark matter,"We investigate the effect of turning on temperature for the charge neutral
phase of two-flavor color superconducting (2SC) dense quark matter in presence
of constant external magnetic field. Within the Nambu-Jona-Lasinio model, by
tuning the diquark coupling strength, we study the interdependent evolution of
the quark Bardeen-Cooper-Schrieffer gap and dynamical mass as functions of
temperature and magnetic field. We find that magnetic field $B \gtrsim 0.02$
GeV$^2$ ($10^{18}$ G) leads to anomalous temperature behavior of the gap in the
gapless 2SC phase (moderately strong coupling), reminiscent of previous results
in the literature found in the limit of weak coupling without magnetic field.
The 2SC gap in the strong coupling regime is abruptly quenched at ultrahigh
magnetic field due to the mismatched Fermi surfaces of up and down quarks
imposed by charge neutrality and oscillation of the gap due to Landau level
quantization. The dynamical quark mass also displays strong oscillation and
magnetic catalysis at high magnetic field, although the latter effect is
tempered by nonzero temperature. We discuss the implications for newly born
compact stars with superconducting quark cores.",1608.00882v2
2016-10-21,Spin-torque switching in large size nano-magnet with perpendicular magnetic fields,"DC current induced magnetization reversal and magnetization oscillation was
observed in 500 nm large size Co90Fe10/Cu/Ni80Fe20 pillars. A perpendicular
external field enhanced the coercive field separation between the reference
layer (Co90Fe10) and free layer (Ni80Fe20) in the pseudo spin valve, allowing a
large window of external magnetic field for exploring the free-layer reversal.
The magnetization precession was manifested in terms of the multiple peaks on
the differential resistance curves. Depending on the bias current and applied
field, the regions of magnetic switching and magnetization precession on a
dynamical stability diagram has been discussed in details. Micromagnetic
simulations are shown to be in good agreement with experimental results and
provide insight for synchronization of inhomogenieties in large sized device.
The ability to manipulate spin-dynamics on large size devices could prove
useful for increasing the output power of the spin-transfer nano-oscillators
(STNOs).",1610.06959v1
2016-12-20,Nanosecond magnetization dynamics during spin Hall switching of in-plane magnetic tunnel junctions,"We present a study of the magnetic dynamics associated with nanosecond scale
magnetic switching driven by the spin Hall effect in 3-terminal nanoscale
magnetic tunnel junctions (3T-MTJs) with in-plane magnetization. Utilizing fast
pulse measurements in a variety of material stacks and detailed micromagnetic
simulations, we demonstrate that this unexpectedly fast and reliable magnetic
reversal is facilitated by the self-generated Oersted field, and the
short-pulse energy efficiency can be substantially enhanced by micromagnetic
curvature in the magnetic free layer. The sign of the Oersted field is
essential for this enhancement --- in simulations in which we artificially
impose a field-like torque with a sign opposite to the effect of the Oersted
field, the result is a much slower and stochastic switching process that is
reminiscent of the so-called incubation delay in conventional 2-terminal
spin-torque-switched MTJs.",1612.06463v3
2017-01-07,Dynamics of exciton magnetic polarons in CdMnSe/CdMgSe quantum wells: the effect of self-localization,"We study the exciton magnetic polaron (EMP) formation in (Cd,Mn)Se/(Cd,Mg)Se
diluted-magnetic-semiconductor quantum wells using time-resolved
photoluminescence (PL). The magnetic field and temperature dependencies of this
dynamics allow us to separate the non-magnetic and magnetic contributions to
the exciton localization. We deduce the EMP energy of 14 meV, which is in
agreement with time-integrated measurements based on selective excitation and
the magnetic field dependence of the PL circular polarization degree. The
polaron formation time of 500 ps is significantly longer than the corresponding
values reported earlier. We propose that this behavior is related to strong
self-localization of the EMP, accompanied with a squeezing of the heavy-hole
envelope wavefunction. This conclusion is also supported by the decrease of the
exciton lifetime from 600 ps to 200 - 400 ps with increasing magnetic field and
temperature.",1701.01824v1
2017-07-24,Magnetic order and spin dynamics in the helical magnetic system Fe$_3$PO$_4$O$_3$,"The 3$d$-electronic spin dynamics and the magnetic order in Fe$_3$PO$_4$O$_3$
were investigated by muon spin rotation and relaxation ($\mu$SR) and $^{57}$Fe
M\""ossbauer spectroscopy. Zero-field (ZF)-$\mu$SR and $^{57}$Fe M\""ossbauer
studies confirm static long range magnetic ordering below $T_{\mathrm{N}}$
$\approx$ 164\,K. Both transverse-field (TF) and ZF-$\mu$SR results evidence
100\% magnetic volume fraction in the ordered state. The ZF-$\mu$SR time
spectra can be best described by a Bessel function, which is consistent with
the helical magnetic structure proposed by neutron scattering experiments. The
M\""ossbauer spectra are described in detail by considering the specific angular
distribution of the local hyperfine field $B_{\mathrm{hyp}}$ with respect to
the local electric field gradient. The $\mu$SR spin-lattice relaxation rate
exhibits two peaks: One at the magnetic ordering temperature related to
critical magnetic fluctuations and another peak at 35\,K signaling the presence
of a secondary low energy scale in Fe$_3$PO$_4$O$_3$.",1707.07433v1
2018-02-19,Static properties and current-induced dynamics of pinned $90$ degree magnetic domain walls under applied fields: an analytic approach,"Magnetic domain walls are pinned strongly by abrupt changes in magnetic
anisotropy. When driven into oscillation by a spin-polarized current, locally
pinned domain walls can be exploited as tunable sources of short-wavelength
spin waves. Here, we develop an analytical framework and discrete Heisenberg
model to describe the static and dynamic properties of pinned domain walls with
a head-to-tail magnetic structure. We focus on magnetic domain walls that are
pinned by 90$^\circ$ rotations of uniaxial magnetic anisotropy. Our model
captures the domain wall response to a spin-transfer torque that is exerted by
an electric current. Model predictions of the domain wall resonance frequency
and its evolution with magnetic anisotropy strength and external magnetic field
are compared to micromagnetic simulations.",1802.06741v3
2019-02-20,Interplay of magnetization dynamics with microwave waveguide at cryogenic temperatures,"In this work, magnetization dynamics is studied at low temperatures in a
hybrid system that consists of thin epitaxial magnetic film coupled with
superconducting planar microwave waveguide. The resonance spectrum was observed
in a wide magnetic field range, including low fields below the saturation
magnetization and both polarities. Analysis of the spectrum via a developed
fitting routine allowed to derive all magnetic parameters of the film at
cryogenic temperatures, to detect waveguide-induced uniaxial magnetic
anisotropies of the first and the second order, and to uncover a minor
misalignment of magnetic field. A substantial influence of the superconducting
critical state on resonance spectrum is observed and discussed.",1902.07566v2
2020-02-14,Wide-field dynamic magnetic microscopy using double-double quantum driving of a diamond defect ensemble,"Wide-field magnetometry can be realized by imaging the optically-detected
magnetic resonance of diamond nitrogen vacancy (NV) center ensembles. However,
NV ensemble inhomogeneities significantly limit the magnetic-field sensitivity
of these measurements. We demonstrate a double-double quantum (DDQ) driving
technique to facilitate wide-field magnetic imaging of dynamic magnetic fields
at a micron scale. DDQ imaging employs four-tone radio frequency pulses to
suppress inhomogeneity-induced variations of the NV resonant response. As a
proof-of-principle, we use the DDQ technique to image the dc magnetic field
produced by individual magnetic-nanoparticles tethered by single DNA molecules
to a diamond sensor surface. This demonstrates the efficacy of the diamond NV
ensemble system in high-frame-rate magnetic microscopy, as well as
single-molecule biophysics applications.",2002.06237v3
2020-06-18,Large Surface Magnetization in Noncentrosymmetric Antiferromagnets,"Thin-film antiferromagnets (AFs) with Rashba spin-orbit coupling are
theoretically investigated. We demonstrate that the relativistic
Dzyaloshinskii-Moriya interaction (DMI) produces a large surface magnetization
and a boundary-driven twist state in the antiferromagnetic N\' eel vector. We
predict a magnetization on the order of $2.3\cdot 10^4$~A/m, which is
comparable to the magnetization of ferromagnetic semiconductors. Importantly,
the magnetization is characterized by ultra-fast terahertz dynamics and
provides new approaches for efficiently probing and controlling the spin
dynamics of AFs as well as detecting the antiferromagnetic DMI. Notably, the
magnetization does not lead to any stray magnetic fields except at corners
where weak magnetic monopole fields appear.",2006.10435v2
2020-09-09,Micromagnetic instabilities in spin-transfer switching of perpendicular magnetic tunnel junctions,"Micromagnetic instabilities and non-uniform magnetization states play a
significant role in spin transfer induced switching of nanometer scale magnetic
elements. Here we model domain wall mediated switching dynamics in
perpendicularly magnetized magnetic tunnel junction nanopillars. We show that
domain wall surface tension always leads to magnetization oscillations and
instabilities associated with the disk shape of the junction. A collective
coordinate model is developed that captures aspects of these instabilities and
illustrates their physical origin. Model results are compared to those of
micromagnetic simulations. The switching dynamics is found to be very sensitive
to the domain wall position and phase, which characterizes the angle of the
magnetization in the disk plane. This sensitivity is reduced in the presence of
spin torques and the spin current needed to displace a domain wall can be far
less than the threshold current for switching from a uniformly magnetized
state. A prediction of this model is conductance oscillations of increasing
frequency during the switching process.",2009.04376v1
2021-07-26,Sub-second Temporal Magnetic Field Microscopy Using Quantum Defects in Diamond,"Wide field-of-view magnetic field microscopy has been realised by probing
shifts in optically detected magnetic resonance (ODMR) spectrum of Nitrogen
Vacancy (NV) defect centers in diamond. However, these widefield diamond NV
magnetometers require few to several minutes of acquisition to get a single
magnetic field image, rendering the technique temporally static in it's current
form. This limitation prevents application of diamond NV magnetometers to novel
imaging of dynamically varying microscale magnetic field processes. Here, we
show that the magnetic field imaging frame rate can be significantly enhanced
by performing lock-in detection of NV photo-luminescence (PL), simultaneously
over multiple pixels of a lock-in camera. A detailed protocol for
synchronization of frequency modulated PL of NV centers with fast camera frame
demodulation, at few kilohertz frequencies, has been experimentally
demonstrated. This experimental technique allows magnetic field imaging of
sub-second varying microscale currents in planar microcoils with imaging frame
rates in the range of 50 to 200 frames per second (fps). Our work demonstrates
that widefield per-pixel lock-in detection of frequency modulated NV ODMR
enables dynamic magnetic field microscopy.",2107.12232v2
2023-09-06,Magnetic curvature and existence of closed magnetic geodesics on low energy levels,"To a Riemannian manifold ${(M, g)}$ endowed with a magnetic form ${\sigma}$
and its Lorentz operator ${\Omega}$ we associate an operator ${M^{\Omega}}$,
called the magnetic curvature operator. Such an operator encloses the classical
Riemannian curvature of the metric ${g}$ together with terms of perturbation
due to the magnetic interaction of ${\sigma}$. From ${M^{\Omega}}$ we derive
the magnetic sectional curvature ${\mathrm{Sec}^{\Omega}}$ and the magnetic
Ricci curvature ${\mathrm{Ric}^{\Omega}}$. On closed manifolds, with a
Bonnet-Myers argument, we show that if ${\mathrm{Ric}^{\Omega}}$ is positive on
an energy level below the critical value then, on that energy level, the
magnetic dynamics carries a contractible periodic orbit. In particular, when
${\sigma}$ is nowhere vanishing, this implies the existence of contractible
periodic orbits on every energy level close to zero. Finally, on closed
oriented even dimensional manifolds, we discuss about the topological
restrictions which appear when one requires ${\mathrm{Sec}^{\Omega}}$ to be
positive.",2309.03159v1
2024-02-06,Field-dependent magnetic relaxation times of magnetic nanoparticle systems: analytic approximations supported by numerical simulations,"Many estimates for the magnetic relaxation time of magnetic nanoparticle
systems neglect the effect of the applied field strength. This is despite many
applications of magnetic nanoparticles involving relaxation dynamics under the
influence of applied fields. Here, an analytic approximation for the
field-dependent Brownian relaxation time of single-domain, spherical magnetic
nanoparticles in an external applied field is developed mathematically. This
expression is validated by comparison with existing empirically-derived
expressions and by comparison to particle-level simulations that allow particle
rotations. Our approximation works particularly well for larger particles. We
then use the developed expression to analytically calculate the total magnetic
relaxation time when both Brownian and N\'eel relaxation mechanisms are at
play. Again, we show that the results match those found using particle-level
simulations, this time with both particle rotations and internal magnetization
dynamics allowed. However, for some particle parameters and for large field
strengths, our simulations reveal that the Brownian and N\'eel relaxation
mechanisms are decoupled and it is not appropriate to combine these to
calculate a total relaxation time.",2402.04427v1
2003-11-22,Influence of a Uniform Current on Collective Magnetization Dynamics in a Ferromagnetic Metal,"We discuss the influence of a uniform current, $\vec{j} $, on the
magnetization dynamics of a ferromagnetic metal. We find that the magnon energy
$\epsilon(\vec{q})$ has a current-induced contribution proportional to
$\vec{q}\cdot \vec{\cal J}$, where $\vec{\cal J}$ is the spin-current, and
predict that collective dynamics will be more strongly damped at finite ${\vec
j}$. We obtain similar results for models with and without local moment
participation in the magnetic order. For transition metal ferromagnets, we
estimate that the uniform magnetic state will be destabilized for $j \gtrsim
10^{9} {\rm A} {\rm cm}^{-2}$. We discuss the relationship of this effect to
the spin-torque effects that alter magnetization dynamics in inhomogeneous
magnetic systems.",0311522v1
2006-01-10,Engineering of spin-lattice relaxation dynamics by digital growth of diluted magnetic semiconductor CdMnTe,"The technological concept of ""digital alloying"" offered by molecular-beam
epitaxy is demonstrated to be a very effective tool for tailoring static and
dynamic magnetic properties of diluted magnetic semiconductors. Compared to
common ""disordered alloys"" with the same Mn concentration, the spin-lattice
relaxation dynamics of magnetic Mn ions has been accelerated by an order of
magnitude in (Cd,Mn)Te digital alloys, without any noticeable change in the
giant Zeeman spin splitting of excitonic states, i.e. without effect on the
static magnetization. The strong sensitivity of the magnetization dynamics to
clustering of the Mn ions opens a new degree of freedom for spin engineering.",0601194v1
2007-12-04,Microwave excitations associated with a wavy angular dependence of the spin transfer torque : model and experiments,"The spin transfer torque (STT) can lead to steady precession of magnetization
without any external applied field in magnetic spin valve where the magnetic
layer have very different spin diffusion length. This effect is associated with
an unusual angular dependence of the STT, called ""wavy"" (WAD-STT), predicted in
the frame of diffusive models of spin transfer. In this article, we present a
complete experimental characterization of the magnetization dynamics in the
presence of a WAD-STT. The results are compared to the prediction of the
magnetization dynamics obtained by single domain magnetic simulations
(macrospin approximation). The macrospin simulations well reproduced the main
static and dynamical experimental features (phase diagram, R(I) curves,
dependence of frequency with current and field) and suggest that the dynamical
excitations observed experimentally are associated with a large angle
out-of-plane precession mode. The present work validates the diffusive models
of the spin transfer and underlines the role of the spin accumulation and the
spin relaxation effects on the STT.",0712.0548v1
2008-07-29,Spin-Torque-Induced Rotational Dynamics of a Magnetic Vortex Dipole,"We study, both experimentally and by numerical modeling, the magnetic
dynamics that can be excited in a magnetic thin-film nanopillar device using
the spin torque from a spatially localized current injected via a
10s-of-nm-diameter aperture. The current-driven magnetic dynamics can produce
large amplitude microwave emission at zero magnetic field, with a frequency
well below that of the uniform ferromagnetic resonance mode. Micromagnetic
simulations indicate that the physical origin of this efficient microwave
nano-oscillator is the nucleation and subsequent steady-state rotational
dynamics of a magnetic vortex dipole driven by the localized spin torque. These
results show this novel implementation of a spintronic nano-oscillator is a
promising candidate for microwave technology applications.",0807.4660v2
2008-10-14,Theoretical study of the magnetization dynamics of non-dilute ferrofluids,"The paper is devoted to the theoretical investigation of the magnetodipolar
interparticle interaction effect on remagnetization dynamics in moderately
concentrated ferrofluids. We consider a homogeneous (without particle
aggregates) ferrofluid consisting of identical spherical particles and employ a
rigid dipole model, where magnetic moment of a particle is fixed with respect
to the particle itself. In particular, for the magnetization relaxation after
the external field is instantly switched off, we show that the magnetodipolar
interaction leads to the increase of the initial magnetization relaxation time.
For the complex ac-susceptibility we find that the this interaction leads to an
overall increase of the imaginary susceptibility part and shifts the peak on
its frequency dependence towards lower frequencies. Comparing results obtained
with our analytical approach (second order virial expansion) to numerical
simulation data (Langevin dynamics method), we demonstrate that the employed
virial expansion approximation gives a good qualitative description of the
ferrofluid magnetization dynamics and provides a satisfactory quantitative
agreement with numerical simulations for the dc magnetization relaxation - up
to the particle volume fraction c ~ 10% and for the ac-susceptibility - up to c
~ 5 %.",0810.2421v1
2009-04-18,Brownian Dynamics of charged particles in a constant magnetic field,"Numerical algorithms are proposed for simulating the Brownian dynamics of
charged particles in an external magnetic field, taking into account the
Brownian motion of charged particles, damping effect and the effect of magnetic
field self-consistently. Performance of these algorithms is tested in terms of
their accuracy and long-time stability by using a three-dimensional Brownian
oscillator model with constant magnetic field. Step-by-step recipes for
implementing these algorithms are given in detail. It is expected that these
algorithms can be directly used to study particle dynamics in various dispersed
systems in the presence of a magnetic field, including polymer solutions,
colloidal suspensions and, particularly complex (dusty) plasmas. The proposed
algorithms can also be used as thermostat in the usual molecular dynamics
simulation in the presence of magnetic field.",0904.2849v1
2010-05-26,Spin diffusion in the Mn2+ ion system of II-VI diluted magnetic semiconductor heterostructures,"The magnetization dynamics in diluted magnetic semiconductor heterostructures
based on (Zn,Mn)Se and (Cd,Mn)Te has been studied experimentally by optical
methods and simulated numerically. In the samples with nonhomogeneous magnetic
ion distribution this dynamics is contributed by spin-lattice relaxation and
spin diffusion in the Mn spin system. The spin diffusion coefficient of
7x10^(-8) cm^2/s has been evaluated for Zn(0.99)Mn(0.01)Se from comparison of
experimental and numerical results. Calculations of the giant Zeeman splitting
of the exciton states and the magnetization dynamics in the ordered alloys and
parabolic quantum wells fabricated by the digital growth technique show perfect
agreement with the experimental data. In both structure types the spin
diffusion has an essential contribution to the magnetization dynamics.",1005.4862v1
2012-03-29,Influence of a uniform magnetic field on dynamical chiral symmetry breaking in QED$_3$,"We study dynamical chiral symmetry breaking (DCSB) in an effective QED$_{3}$
theory of d-wave high temperature cuprate superconductors under a uniform
magnetic field. At zero temperature, the external magnetic field induces a
mixed state by generating vortices in the condensate of charged holons. The
growing magnetic field suppresses the superfluid density and thus reduces the
gauge field mass which is opened via the Anderson-Higgs mechanism. By
numerically solving the Dyson-Schwinger gap equation, we show that the massless
fermions acquires a dynamical gap through DCSB mechanism when the magnetic
field strength $H$ is above a critical value $H_{c}$ and the fermion flavors
$N$ is below a critical value $N_{c}$. Further, it is found that both $N_{c}$
and the dynamical fermion gap increase as the magnetic field $H$ grows. It is
expected that our result can be tested in phenomena in high temperature cuprate
superconductors.",1203.6423v1
2013-04-22,Vortex dynamics and second magnetization peak in PrFeAsO$_{0.60}$F$_{0.12}$ superconductor,"We have studied the vortex dynamics in the PrFeAsO$_{0.60}$F$_{0.12}$
superconducting sample by dc magnetization and dynamic magnetization-relaxation
rate $(Q)$ measurements. The field dependence of the superconducting
irreversible magnetization $M_s$ reveals a second magnetization peak or
fishtail effect. The large value of $Q$ is an indication of moderate vortex
motion and relatively weak pinning energy. Data analysis based on the
generalized inversion scheme suggests that the vortex dynamics can be described
by the collective pinning model. The temperature dependence of the critical
current is consistent with the pinning due to the spatial variation in the mean
free path near a lattice defect ($\delta l$ pinning). The temperature and field
dependence of $Q$ indicates a crossover from elastic to plastic vortex creep
with increasing temperature and magnetic field. Finally, we have constructed
the vortex phase diagram based on the present data.",1304.5978v1
2014-01-07,$α-γ$ transition in cerium: magnetic form factor and dynamic magnetic susceptibility in dynamical mean-field theory,"The nature of the elemental cerium phases, undergoing an isostructural volume
collapse transition, cannot be understood using conventional solid state
concepts. Using the combination of density functional theory and dynamical
mean-field theory, we study the magnetic properties of both the $\alpha$ and
the $\gamma$ phases. We compute the magnetic form factor, and show that it is
very close to free ion behavior in both the local moment $\gamma$ phase as well
as the more itinerant $\alpha$ phase, in agreement with neutron scattering
experiments. In sharp contrast, the dynamic local magnetic susceptibility of
the two phases is strikingly different. In the $\gamma$ phase, the sharp low
energy peak due to local moment formation and consequently low Kondo
temperature dominates the spectra. In the $\alpha$ phase two broad peaks can be
identified, the first is due to Kondo screening, and the second is due to
Hund's coupling. This shows that hybridization plays a central role in the
$\alpha-\gamma$ transition in cerium, and that from the point of view of
magnetic properties, the $4f$ electrons are strongly correlated in both phases.",1401.1245v1
2014-02-26,Improved Domain Wall Dynamics and Magnonic Torques using Topological Insulators,"We investigate the magnetization dynamics that arise when a thin-film
ferromagnet is deposited on a topological insulator (TI), focusing in
particular on domain-wall motion via current and the possibility of a spin-wave
torque acting on the magnetization. We show analytically that the coupling
between the magnetic domain wall and the TI removes the degeneracy of the wall
profile with respect to its chirality and topological charge. Moreover, we find
that the threshold for Walker breakdown of domain wall motion is substantially
increased and determined by the interaction with the TI, allowing for higher
attainable wall velocities than in the conventional case where the hard axis
anisotropy determines the Walker threshold. Finally, we show that the allowed
modes of spin-wave excitations and the ensuing magnetization dynamics in the
presence of a TI coupling enable a magnonic torque acting even on homogeneous
magnetization textures. Our results indicate that the TI-ferromagnet
interaction has a similar effect on the magnetization dynamics as an intrinsic
Dzyaloshinskii-Moriya interaction in ferromagnets.",1402.6701v2
2014-06-26,Tunable dynamic response of magnetic gels: impact of structural properties and magnetic fields,"Ferrogels and magnetic elastomers feature mechanical properties that can be
reversibly tuned from outside through magnetic fields. Here we concentrate on
the question how their dynamic response can be adjusted. The influence of three
factors on the dynamic behavior is demonstrated using appropriate minimal
models: first, the orientational memory imprinted into one class of the
materials during their synthesis; second, the structural arrangement of the
magnetic particles in the materials; and third, the strength of an external
magnetic field. To illustrate the latter point, structural data are extracted
from a real experimental sample and analyzed. Understanding how internal
structural properties and external influences impact the dominant dynamical
properties helps to design materials that optimize the requested behavior.",1406.6979v2
2015-10-07,Spin dynamics in paramagnetic diluted magnetic semiconductors,"Microscopic properties of low-energy spin dynamics in diluted magnetic
semiconductor are addressed in a framework of the Kondo lattice model including
random distribution of magnetic dopants. Based on the fluctuation-dissipation
theorem, we derive an explicit dependence of the spin diffusion coefficient on
the single-particle Green function which is directly evaluated by dynamical
mean-field theory. In the paramagnetic state, the magnetic scattering has been
manifested to suppress spin diffusion. In agreement with other ferromagnet
systems, we also point out that the spin diffusion in diluted magnetic
semiconductors at small carrier concentration displays a monotonic $1/T$-like
temperature dependence. By investigating the spin diffusion coefficient on a
wide range of the model parameters, the obtained results have provided a
significant scenario to understand the spin dynamics in the paramagnetic
diluted magnetic semiconductors.",1510.01807v1
2016-12-13,Dynamics in quantum Ising chain driven by inhomogeneous transverse magnetization,"We study the dynamics caused by transport of transverse magnetization in one
dimensional transverse Ising chain at zero temperature. We observe that a class
of initial states having product structure in fermionic momentum-space and
satisfying certain criteria, produce spatial variation in transverse
magnetization. Starting from such a state, we obtain the transverse
magnetization analytically and then observe its dynamics in presence of a
homogeneous constant field $\Gamma$. In contradiction with general expectation,
whatever be the strength of the field, the magnetization of the system does not
become homogeneous even after infinite time. At each site, the dynamics is
associated with oscillations having two different timescales. The envelope of
the larger timescale oscillation decays algebraically with an exponent which is
invariant for all such special initial states. The frequency of this
oscillation varies differently with external field in ordered and disordered
phases. The local magnetization after infinite time also characterizes the
quantum phase transition.",1612.04066v2
2017-06-21,Uniform magnetization dynamics of a submicron ferromagnetic coin driven by the spin-orbit coupled spin torque,"A simple model of magnetization dynamics in a ferromagnet/doped semiconductor
hybrid structure with Rashba spin-orbit interaction (SOI) driven by an applied
pulse of the electric field is proposed. The electric current excited by the
applied field is spin-polarized due to the SOI and therefore it induces the
magnetization rotation in the ferromagnetic layer via s-d exchange coupling.
Magnetization dynamics dependence on the electric pulse shape and magnitude is
analyzed for realistic values of parameters. We show that it is similar to the
dynamics of a damped nonlinear oscillator with the time-dependent frequency
proportional to the square root of the applied electric field. The
magnetization switching properties of an elliptic magnetic element are examined
as a function of the applied field magnitude and direction.",1706.06909v1
2017-09-29,Non-local Gilbert damping tensor within the torque-torque correlation model,"An essential property of magnetic devices is the relaxation rate in magnetic
switching which depends strongly on the damping in the magnetisation dynamics.
It was recently measured that damping depends on the magnetic texture and,
consequently, is a non-local quantity. The damping enters the
Landau-Lifshitz-Gilbert equation as the phenomenological Gilbert damping
parameter $\alpha$, that does not, in a straight forward formulation, account
for non-locality. Efforts were spent recently to obtain Gilbert damping from
first principles for magnons of wave vector $\mathbf{q}$. However, to the best
of our knowledge, there is no report about real space non-local Gilbert damping
$\alpha_{ij}$. Here, a torque-torque correlation model based on a tight binding
approach is applied to the bulk elemental itinerant magnets and it predicts
significant off-site Gilbert damping contributions, that could be also
negative. Supported by atomistic magnetisation dynamics simulations we reveal
the importance of the non-local Gilbert damping in atomistic magnetisation
dynamics. This study gives a deeper understanding of the dynamics of the
magnetic moments and dissipation processes in real magnetic materials. Ways of
manipulating non-local damping are explored, either by temperature, material's
doping or strain.",1709.10365v1
2017-10-09,Nanoscale mapping of ultrafast magnetization dynamics with femtosecond Lorentz microscopy,"Novel time-resolved imaging techniques for the investigation of ultrafast
nanoscale magnetization dynamics are indispensable for further developments in
light-controlled magnetism. Here, we introduce femtosecond Lorentz microscopy,
achieving a spatial resolution below 100 nm and a temporal resolution of 700
fs, which gives access to the transiently excited state of the spin system on
femtosecond timescales and its subsequent relaxation dynamics. We demonstrate
the capabilities of this technique by spatio-temporally mapping the
light-induced demagnetization of a single magnetic vortex structure and
quantitatively extracting the evolution of the magnetization field after
optical excitation. Tunable electron imaging conditions allow for an
optimization of spatial resolution or field sensitivity, enabling future
investigations of ultrafast internal dynamics of magnetic topological defects
on 10-nanometer length scales.",1710.03307v1
2018-08-19,Fast spin dynamics in hexagonal arrays of Fe atoms on metallic surface,"Understanding the nature of magnetic interactions in ultra-small magnetic
ensembles and their intrinsic properties is vital to uncover the dynamics
therein. In this study we reveal the spin dynamics of hexagonally arranged Fe
atoms on metallic surface that are triggered by magnetic pulse. The switching
process among various spin configurations and their relative magnetic order is
tuned by the amplitude and duration of the magnetic pulse. Even more we observe
a parity effect in the switching time as the size of the cluster varies in
which even number of Fe atoms shows faster dynamics. The changes in the
multistable magnetic states and switching times are explained by using the
relaxation of the exchange and anisotropy energies in time.",1808.06165v1
2019-11-05,Numerical methods for antiferromagnetics,"Compared with ferromagnetic counterparts, antiferromagnetic materials are
considered as the future of spintronic applications since these materials are
robust against the magnetic perturbation, produce no stray field, and display
ultrafast dynamics. There are (at least) two sets of magnetic moments in
antiferromagnets (with magnetization of the same magnitude but antiparallel
directions) and ferrimagnets (with magnetization of the different magnitude).
The coupled dynamics for the bipartite collinear antiferromagnets is modeled by
a coupled system of Landau-Lifshitz-Gilbert equations with an additional term
originated from the antiferromagnetic exchange, which leads to femtosecond
magnetization dynamics. In this paper, we develop three Gauss-Seidel projection
methods for micromagnetics simulation in antiferromagnets and ferrimagnets.
They are first-order accurate in time and second-order in space, and only solve
linear systems of equations with constant coefficients at each step.
Femtosecond dynamics, N\'{e}el wall structure, and phase transition in presence
of an external magnetic field for antiferromagnets are provided with the
femtosecond stepsize.",1911.01717v1
2020-10-13,A simulation framework for particle magnetization dynamics of large ensembles of single domain particles: Numerical treatment of Brown/Néel dynamics and parameter identification problems in magnetic particle imaging,"Magnetic nanoparticles and their magnetization dynamics play an important
role in many applications. We focus on magnetization dynamics in large
ensembles of single domain nanoparticles being characterized by either Brownian
or N\'{e}el rotation mechanisms. Simulations of the respective behavior are
obtained by solving advection-diffusion equations on the sphere, for which a
unified computational framework is developed and investigated. This builds the
basis for solving two parameter identification problems, which are formulated
in the context of the chosen application, magnetic particle imaging. The
functionality of the computational framework is illustrated by numerical
results in the parameter identification problems either compared quantitatively
or qualitatively to measured data.",2010.07772v1
2021-07-15,Wide-field magneto-optical microscope to access quantitative magnetization dynamics with femtosecond temporal and sub-micrometer spatial resolution,"We introduce a wide-field magneto-optical microscope to probe magnetization
dynamics with femtosecond temporal and sub-micrometer spatial resolution. We
carefully calibrate the non-linear dependency between the magnetization of the
sample and the detected light intensity by determining the absolute values of
the magneto-optical polarization rotation. With that, an analytical transfer
function is defined to directly map the recorded intensity to the corresponding
magnetization, which results in significantly reduced acquisition times and
relaxed computational requirements. The performance of the instrument is
characterized by probing the magnetic all-optical switching dynamics of GdFe in
a pump-probe experiment. The high spatial resolution of the microscope allows
for accurately subdividing the laser-excited area into different
fluence-regions in order to capture the strongly non-linear magnetization
dynamics as a function of the optical pump intensity in a single measurement.",2107.07375v2
2021-08-13,Optical signatures of the coupled spin-mechanics of a levitated magnetic microparticle,"We propose a platform that combines the fields of cavity optomagnonics and
levitated optomechanics in order to control and probe the coupled
spin-mechanics of magnetic dielectric particles. We theoretically study the
dynamics of a levitated Faraday-active dielectric microsphere serving as an
optomagnonic cavity, placed in an external magnetic field and driven by an
external laser. We find that the optically driven magnetization dynamics
induces angular oscillations of the particle with low associated damping.
Further, we show that the magnetization and angular motion dynamics can be
probed via the power spectrum of the outgoing light. Namely, the characteristic
frequencies attributed to the angular oscillations and the spin dynamics are
imprinted in the light spectrum by two main resonance peaks. Additionally, we
demonstrate that a ferromagnetic resonance setup with an oscillatory
perpendicular magnetic field can enhance the resonance peak corresponding to
the spin oscillations and induce fast rotations of the particle around its
anisotropy axis.",2108.06214v1
2021-11-29,Element selective ultrafast magnetization dynamics of hybrid Stoner-Heisenberg magnets,"Stoner and Heisenberg excitations in magnetic materials are inherently
different. The former involves an effective reduction of the exchange
splitting, whereas the latter comprises excitation of spin-waves. In this work,
we test the impact of these two excitations in the hybrid Stoner-Heisenberg
system FePd. We present a microscopic picture of ultrafast demagnetization
dynamics in this alloy, which represents both components of strong local
exchange splitting in Fe, and induced polarization in Pd. We identify
spin-orbit coupling and optical inter-site spin transfer as the two dominant
factors for demagnetization at ultrashort timescales. By tuning the external
laser pulse, the extrinsic inter-site spin transfer can be manipulated for site
selective demagnetization on femtosecond time scales providing the fastest way
for optical and selective control of the magnetization dynamics in alloys.
Remarkably, the drastic difference in origin of the magnetic moment of the Fe
and Pd species is not deciding the initial magnetization dynamics in this
alloy.",2111.14607v1
2024-01-19,Analysis of the nonlinear dynamics of a single pendulum driven by a magnetic field using the magnetic charges interaction model and the experimentally fitted interaction model,"In this work, we analyzed theoretically and experimentally the nonlinear
dynamics of a magnetic pendulum driven by a coil-magnet interaction. The force
between the magnetic elements and the resulting torque on the pendulum are
derived using both the magnetic charges interaction model and the
experimentally fitted interaction model. This enables the comparison between
the two models. The current in the coil is taken first as a sinusoidal current
and then as a square current. The comparison of the structure of each
interaction model is conducted and it appears that they give qualitatively
similar characteristics. The harmonic balance method is used to approximate the
frequency responses of the pendulum leading to both symmetric and asymmetric or
one-side (intrawell) oscillations. The two-parameters bifurcation diagrams are
plotted showing the different dynamical behaviors considering the current
amplitude and frequency as the control parameters. Good agreements are found
between our theoretical results and experimental ones.",2401.10957v1
2024-02-16,Magneto-Permeability Effect in Ferrofluid Flow through Porous Media studied via Multiparticle Collision Dynamics,"As more and more promising applications of magnetic nanoparticles in
complicated environments are explored, their flow properties in porous media
are of increasing interest. We here propose a hybrid approach based on the
Multiparticle Collision Dynamics Method extended to porous media via friction
forces and coupled with Brownian Dynamics simulations of the rotational motion
of magnetic nanoparticles' magnetic moment. We simulate flow in planar channels
homogeneously filled with a porous medium and verify our implementation by
reproducing the analytical velocity profile of the Darcy-Brinkman model in the
non-magnetic case. In the presence of an externally applied magnetic field, the
non-equilibrium magnetization and friction forces lead to field-dependent
velocity profiles that result in effective, field-dependent permeabilities. We
provide a theoretical expression for this magneto-permeability effect in
analogy with the magneto-viscous effect. Finally, we study the flow through
planar channels, where only the walls are covered with a porous medium. We find
a smooth crossover from the Poiseuille profile in the center of the channel to
the Brinkman-Darcy flow in the porous layers. We propose a simple estimate of
the thickness of the porous layer based on the flow rate and maximum flow
velocity.",2402.10799v1
2015-01-29,Dynamical evolution of the chiral magnetic effect: Applications to the quark-gluon plasma,"We study the dynamical evolution of the so-called chiral magnetic effect in
an electromagnetic conductor. To this end, we consider the coupled set of
corresponding Maxwell and chiral anomaly equations, and we prove that these can
be derived from chiral kinetic theory. After integrating the chiral anomaly
equation over space in a closed volume, it leads to a quantum conservation law
of the total helicity of the system. A change in the magnetic helicity density
comes together with a modification of the chiral fermion density. We study in
Fourier space the coupled set of anomalous equations and we obtain the
dynamical evolution of the magnetic fields, magnetic helicity density, and
chiral fermion imbalance. Depending on the initial conditions we observe how
the helicity might be transferred from the fermions to the magnetic fields, or
vice versa, and find that the rate of this transfer also depends on the scale
of wavelengths of the gauge fields in consideration. We then focus our
attention on the quark-gluon plasma phase, and analyze the dynamical evolution
of the chiral magnetic effect in a very simple toy model. We conclude that an
existing chiral fermion imbalance in peripheral heavy ion collisions would
affect the magnetic field dynamics, and consequently, the charge dependent
correlations measured in these experiments.",1501.07608v2
2023-11-03,Simulating Heavy-Hex Transverse Field Ising Model Magnetization Dynamics Using Programmable Quantum Annealers,"Recently, a Hamiltonian dynamics simulation was performed on a kicked
ferromagnetic 2D transverse field Ising model with a connectivity graph native
to the 127 qubit heavy-hex IBM Quantum architecture using ZNE quantum error
mitigation. We demonstrate that one of the observables in this Trotterized
Hamiltonian dynamics simulation, namely magnetization, can be efficiently
simulated on current superconducting qubit-based programmable quantum annealing
computers. We show this using two distinct methods: reverse quantum annealing
and h-gain state encoding. This simulation is possible because the 127 qubit
heavy-hex connectivity graph can be natively embedded onto the D-Wave Pegasus
quantum annealer hardware graph and because there exists a direct equivalence
between the energy scales of the two types of quantum computers. We derive
equivalent anneal pauses in order to simulate the Trotterized quantum circuit
dynamics for varying Rx rotations $\theta_h \in (0, \frac{\pi}{2}]$, using
quantum annealing processors. Multiple disjoint instances of the Ising model of
interest can be embedded onto the D-Wave Pegasus hardware graph, allowing for
parallel quantum annealing. We report equivalent magnetization dynamics using
quantum annealing for time steps of 20, 50 up to 10,000, which we find are
consistent with exact classical 27 qubit heavy-hex Trotterized circuit
magnetization dynamics, and we observe reasonable, albeit noisy, agreement with
the existing simulations for single site magnetization at 20 Trotter steps. The
quantum annealers are able to simulate equivalent magnetization dynamics for
thousands of time steps, significantly out of the computational reach of the
digital quantum computers on which the original Hamiltonian dynamics
simulations were performed.",2311.01657v1
2011-10-01,Spin-orbital phase synchronization in the magnetic field-driven electron dynamics in a double quantum dot,"We study the dynamics of an electron confined in a one-dimensional double
quantum dot in the presence of driving external magnetic fields. The orbital
motion of the electron is coupled to the spin dynamics by spin orbit
interaction of the Dresselhaus type. We derive an effective time-dependent
Hamiltonian model for the orbital motion of the electron and obtain a
synchronization condition between the orbital and the spin dynamics. From this
model we deduce an analytical expression for the Arnold tongue and propose an
experimental scheme for realizing the synchronization of the orbital and spin
dynamics.",1110.0114v1
2009-02-01,Neutron Studies of the Iron-based Family of High TC Magnetic Superconductors,"We review neutron scattering investigations of the crystal structures,
magnetic structures, and spin dynamics of the iron-based RFe(As,P)O (R=La, Ce,
Pr, Nd), (Ba,Sr,Ca)Fe2As2, and Fe1+x(Te-Se) systems. On cooling from room
temperature all the undoped materials exhibit universal behavior, where a
tetragonal-to-orthorhombic/monoclinic structural transition occurs, below which
the systems become antiferromagnets. For the first two classes of materials the
magnetic structure within the a-b plane consists of chains of parallel Fe spins
that are coupled antiferromagnetically in the orthogonal direction, with an
ordered moment typically less than one Bohr magneton. Hence these are itinerant
electron magnets, with a spin structure that is consistent with Fermi-surface
nesting and a very energetic spin wave bandwidth ~0.2 eV. With doping, the
structural and magnetic transitions are suppressed in favor of
superconductivity. Magnetic correlations are observed in the superconducting
regime, with a magnetic resonance that follows the superconducting order
parameter just like the cuprates. The rare-earth moments order
antiferromagnetically at low T like conventional magnetic-superconductors.
Pressure in CaFe2As2 transforms the system from a magnetically ordered
orthorhombic material to a collapsed non-magnetic tetragonal system. Tetragonal
Fe1+xTe transforms to a low T monoclinic structure at small x that changes to
orthorhombic at larger x, which is accompanied by a crossover from commensurate
to incommensurate magnetic order. Se doping suppresses the magnetic order.",0902.0091v2
2012-09-25,Magnetic Energy and Helicity Budgets in the Active-Region Solar Corona. II. Nonlinear Force-Free Approximation,"Expanding on an earlier work that relied on linear force-free magnetic
fields, we self-consistently derive the instantaneous free magnetic energy and
relative magnetic helicity budgets of an unknown three-dimensional nonlinear
force-free magnetic structure extending above a single, known lower-boundary
magnetic field vector. The proposed method does not rely on the detailed
knowledge of the three-dimensional field configuration but is general enough to
employ only a magnetic connectivity matrix on the lower boundary. The
calculation yields a minimum free magnetic energy and a relative magnetic
helicity consistent with this free magnetic energy. The method is directly
applicable to photospheric or chromospheric vector magnetograms of solar active
regions. Upon validation, it basically reproduces magnetic energies and
helicities obtained by well-known, but computationally more intensive and
non-unique, methods relying on the extrapolated three-dimensional magnetic
field vector. We apply the method to three active regions, calculating the
photospheric connectivity matrices by means of simulated annealing, rather than
a model-dependent nonlinear force-free extrapolation. For two of these regions
we correct for the inherent linear force-free overestimation in free energy and
relative helicity that is larger for larger, more eruptive, active regions. In
the third studied region, our calculation can lead to a physical interpretation
of observed eruptive manifestations. We conclude that the proposed method,
including the proposed inference of the magnetic connectivity matrix, is
practical enough to contribute to a physical interpretation of the dynamical
evolution of solar active regions.",1209.5606v1
2014-07-23,Magnetic Reconnection in Astrophysical Environments,"Magnetic reconnection is a process that changes magnetic field topology in
highly conducting fluids. Traditionally, magnetic reconnection was associated
mostly with solar flares. In reality, the process must be ubiquitous as
astrophysical fluids are magnetized and motions of fluid elements necessarily
entail crossing of magnetic frozen in field lines and magnetic reconnection. We
consider magnetic reconnection in realistic 3D geometry in the presence of
turbulence. This turbulence in most astrophysical settings is of pre-existing
nature, but it also can be induced by magnetic reconnection itself. In this
situation turbulent magnetic field wandering opens up reconnection outflow
regions, making reconnection fast. We discuss Lazarian \& Vishniac (1999) model
of turbulent reconnection, its numerical and observational testings, as well as
its connection to the modern understanding of the Lagrangian properties of
turbulent fluids. We show that the predicted dependences of the reconnection
rates on the level of MHD turbulence make the generally accepted Goldreich \&
Sridhar (1995) model of turbulence self-consistent. Similarly, we argue that
the well-known Alfv\'en theorem on flux freezing is not valid for the turbulent
fluids and therefore magnetic fields diffuse within turbulent volumes.
  This is an element of magnetic field dynamics that was not accounted by
earlier theories. For instance, the theory of star formation that was
developing assuming that it is only the drift of neutrals that can violate the
otherwise perfect flux freezing, is affected and we discuss the consequences of
the turbulent diffusion of magnetic fields mediated by reconnection.",1407.6356v1
2014-11-24,Manipulating Femtosecond Spin--Orbit Torques with Laser Pulse Sequences to Control Magnetic Memory States and Ringing,"Femtosecond (fs) coherent control of collective order parameters is important
for non--equilibrium phase dynamics in correlated materials. Here we propose a
possible scheme for fs control of a ferromagnetic order parameter based on
non--adiabatic optical manipulation of electron--hole ($e$--$h$)
photoexcitations between spin--orbit--coupled bands that are exchange--split by
magnetic interaction with local spins. We photoexcite fs carrier spin--pulses
with controllable direction and time profile without using
circularly--polarized light, via time--reversal symmetry--breaking by
non--perturbative interplay between spin--orbit and magnetic exchange coupling
of coherent photocarriers. We manipulate photoexcited {\em fs spin--orbit
torques} to control complex switching pathways of the magnetization between
multiple magnetic memory states. We calculate the photoinduced fs magnetic
anisotropy in the time domain by using density matrix equations of motion
rather than the quasi--equilibrium free energy. By comparing to pump--probe
experiments, we identify a ""sudden"" magnetization canting induced by laser
excitation, which displays magnetic hysteresis absent in static
magneto--optical measurements and agrees with switchings measured by Hall
magnetoresistivity. The fs magnetization canting switches direction with
magnetic state and laser frequency, which distinguishes it from nonlinear
optical and demagnetization longitudinal effects. By shaping two--color
laser--pulse sequences analogous to multi--dimensional Nuclear Magnetic
Resonance (NMR) spectroscopy, we show that sequences of clockwise or
counter--clockwise fs spin--orbit torques can enhance or suppress magnetic
ringing and switching rotation at any desired time. We propose protocols that
can provide controlled access to four magnetic states via consequative 90$^{o}$
switchings.",1411.6662v1
2014-12-01,Quantifying magnetic anisotropy dispersion: Theoretical and experimental study of the magnetic properties of anisotropic FeCuNbSiB ferromagnetic films,"The Stoner-Wohlfarth model is a traditional and efficient tool to calculate
magnetization curves and it can provides further insights on the fundamental
physics associated to the magnetic properties and magnetization dynamics. Here,
we perform a theoretical and experimental investigation of the quasi-static
magnetic properties of anisotropic systems. We consider a theoretical approach
which corresponds to a modified version of the Stoner-Wohlfarth model to
describe anisotropic systems and a distribution function to express the
magnetic anistropy dispersion. We propose a procedure to calculate the magnetic
properties for the anisotropic case of the SW model from experimental results
of the quadrature of magnetization curves, thus quantifying the magnetic
anisotropy dispersion. To test the robustness of the approach, we apply the
theoretical model to describe the quasi-static magnetic properties of amorphous
FeCuNbSiB ferromagnetic films. We perform calculations and directly compare
theoretical results with longitudinal and transverse magnetization curves
measured for the films. Thus, our results provide experimental evidence to
confirm the validity of the theoretical approach to describe the magnetic
properties of anisotropic amorphous ferromagnetic films, revealed by the
excellent agreement between numerical calculation and experimental results.",1412.0563v1
2017-09-28,Angular Momentum Transport in Thin Magnetically Arrested Disks,"In accretion disks with large-scale ordered magnetic fields, the
magnetorotational instability (MRI) is marginally suppressed, so other
processes may drive angular momentum transport leading to accretion. Accretion
could then be driven by large-scale magnetic fields via magnetic braking, but
large-scale magnetic flux can build-up onto the black hole and within the disk
leading to a magnetically-arrested disk (MAD). Such a MAD state is unstable to
the magnetic Rayleigh-Taylor (RT) instability, which itself leads to vigorous
turbulence and the emergence of low-density highly-magnetized bubbles. This
instability was studied in a thin (ratio of half-height H to radius R, $H/R
\approx 0.1$) MAD simulation, where it has a more dramatic effect on the
dynamics of the disk than for thicker disks. We find that the low-density
bubbles created by the magnetic RT instability decrease the stress (leading to
angular momentum transport) in the disk rather than increasing magnetic
torques. Indeed, we find that the dominant component of the stress is due to
turbulent magnetic fields, despite the suppression of the axisymmetric MRI and
the dominant presence of large-scale magnetic fields. This suggests that the
magnetic RT instability plays a significant role in driving angular momentum
transport in MADs.",1709.10113v2
2018-03-26,Nonkinematic solar dynamo models with double-cell meridional circulation,"Employing the standard solar interior model as input we construct a
dynamically-consistent nonlinear dynamo model that takes into account the
detailed description of the \Lambda- effect, turbulent pumping, magnetic
helicity balance, and magnetic feedback on the differential rotation and
meridional circulation. The background mean-field hydrodynamic model of the
solar convection zone accounts the solar-like angular velocity profile and the
double-cell meridional circulation. We investigate an impact of the nonlinear
magnetic field generation effects on the long-term variability and properties
of the magnetic cycle. The nonlinear dynamo solutions are studied in the wide
interval of the \alpha effect parameter from a slightly subcritical to
supercritical values. It is found that the magnetic cycle period decreases with
the increasing cycle's magnitude. The periodic long-term variations of the
magnetic cycle are excited in case of the overcritical \alpha effect. These
variations result from the hemispheric magnetic helicity exchange. It depends
on the magnetic diffusivity parameter and the magnetic helicity production
rate. The large-scale magnetic activity modifies the distribution of the
differential rotation and meridional circulation inside convection zone. It is
found that the magnetic feedback on the global flow affects the properties of
the long-term magnetic cycles. We confront our findings with solar and stellar
magnetic activity observations.",1803.09459v2
2021-04-05,Efficient determination of the true magnetic structure in a high-throughput ab initio screening: the MDMC method,"Finding the true magnetic structure at given external conditions is crucial
for describing magnetic materials and predicting their properties. This is
especially important for high-throughput screening of potentially good magnets
that without adequate description of the magnetic structure may result in an
enormous waste of computational resources. I introduce a method, which
accurately and efficiently treats magnetic ordering in the course of standard
first-principles calculations. The method is suitable for all temperatures and
is based on Monte Carlo (MC) technique. At high temperatures MC is coupled to
ab initio molecular dynamics, therefore treating atomic vibrations and magnetic
ordering simultaneously. The method takes care of the convergence to the true
ground-state magnetic structure at 0 K, coupling between spins and atomic
vibrations at high temperatures, and spin-spin interactions in non-Heisenberg
systems, i.e. it naturally goes beyond the usual pair-like effective exchange
interactions at all temperatures. I show that the method nicely reproduces the
magnetic structures at low and high temperatures even on relatively small
supercells and provides a ground for truly ab initio calculations of magnetic
systems in high-throughput studies. In its general formulation the MDMC method
deals with non-collinear magnetism, but its collinear version may be more
suitable for fast high-throughput calculations. The examples of the application
of the MDMC method addressed in this paper are $\alpha$-Mn; body-centered cubic
Fe, which is ferromagnetic at low and paramagnetic at high temperatures; and
MnB$_2$W$_2$, a new magnetic compound with non-trivial magnetism.",2104.02106v1
2021-12-11,Macroscopic Magnetic Monopoles in a 3D-Printed Mechano-Magnet,"The notion of magnetic monopoles has puzzled physicists since the
introduction of Maxwell's Equations and famously Dirac had hypothesized them in
the context of quantum mechanics. While they have proved experimentally elusive
as elementary particles, the concept has come to describe excitations or
topological defects in various material systems, from liquid crystals, to Hall
systems, skyrmion lattices, and Bose-Einstein condensate. Perhaps the most
versatile manifestation of magnetic monopoles as quasiparticles in matter has
been in so-called spin ice materials. There, they represent violations of the
ice rule, carry a magnetic charge, and can move freely unbound. Spin ice
emergent magnetic monopoles appear at the atomic scale in rare earth
pyrochlores or at the nano-scale in artificial spin ices systems. Here we
demonstrate for the first time that the notion of magnetic monopoles can be
transported at the macroscopic scale. We have built a mechano-magnet realized
via 3D-printing, that consists of mechanical rotors on which macroscopic
magnets can pivot. By controlling the relative height of the rotors we can
achieve different regimes for magnetic monopoles, including the free monopole
state. We then explore their driven dynamics under field. In the future,
integration of our proof of principle in an elastic matrix can lead to novel
macroscopic mechano-magnetic materials, to explore unusual piezomagnetism and
magnetostriction, with applications to actuators and soft-robotics.",2112.06058v2
2022-09-03,Core orientations and magnetic fields in isolated molecular clouds,"Molecular clouds are sites of star formation. Magnetic fields are believed to
play an important role in their dynamics and shaping morphology. We aim to
study any possible correlation that might exist between the magnetic fields
orientation inside the clouds and the magnetic fields at envelope scales and
their connection with respect to the observed morphology of the selected
clouds. We examine the magnetic field orientation towards the clouds L1512,
L1523, L1333, L1521E, L1544, L1517, L1780, and L183 using optical and
\textit{Planck} polarization observations. We also found the correlation
between the ambient magnetic field and core orientations derived using
\textit{Astrodendrogram} on the \textit{Herschel} 250 $\mu$m data. We find that
the magnetic fields derived from optical and \textit{Planck} agree with each
other. The derived magnetic fields are aligned along the observed emission of
each cloud as seen in \textit{Herschel} 250 $\mu$m data. We also find that the
relative orientation between the cores and the magnetic fields is random. This
lack of correlation may arise due to the fact that the core orientation could
also be influenced by the different magnetization within individual clouds at
higher densities or the feedback effects which may vary from cloud to cloud.
The estimated magnetic field strength and the mass-to-flux ratio suggest that
all the clouds are in a magnetically critical state except L1333, L1521E, and
L183 where the cloud envelope could be strongly supported by the magnetic field
lines.",2209.01509v1
2022-11-01,Effects of strong fringing magnetic fields on turbulent thermal convection,"We study the influence of fringing magnetic fields on turbulent thermal
convection in a horizontally extended rectangular domain. The magnetic field is
created in the gap between two semi-infinite planar magnetic poles, with the
convection layer located near the edge of the gap. We employ direct numerical
simulations in this setup for fixed Rayleigh and small Prandtl numbers, but
vary the fringe-width by controlling the gap between the magnetic poles and the
convection cell. The magnetic field generated by the magnets is strong enough
to cease the flow in high magnetic flux region of the convection cell. We
observe that as the local vertical magnetic field strength increases, the large
scale structures become thinner and align themselves perpendicular to the
longitudinal sidewalls. We determine the local Nusselt and Reynolds numbers as
functions of the local Hartmann number (based on the vertical component of the
magnetic field) and estimate the global heat and momentum transport. We show
that the global heat transport decreases with increasing fringe-width for
strong magnetic fields but increases with increasing fringe-width for weak
magnetic fields. In the regions of large vertical magnetic fields, the
convective motion becomes confined to the vicinity of the sidewalls. The
amplitudes of these wall modes show a non-monotonic dependence on the
fringe-width.",2211.00559v3
2023-04-22,A Multi-frequency Magnetic Particle Spectroscopy System for the Characterization of Magnetic Nanoparticles,"Magnetic particle spectroscopy (MPS) is one of the most versatile methods to
characterize the magnetic properties of magnetic nanoparticles (MNPs). The
excitation magnetic field is one of the most crucial factors that affects the
MPS signal of the MNPs. In this study, a multi-frequency MPS system is
developed to investigate the MPS signal of MNPs in different ac magnetic
fields. The MPS system consists of a multi-channel excitation module for the
generation of different-frequency ac magnetic fields and a detection module for
the measurement of the magnetic response of the MNPs. The MPS system allows to
generate ac magnetic fields with a frequency up to 32.6 kHz and amplitude up to
25 mT. The MPS signals of the MNPs in different ac magnetic fields are measured
to systematically evaluate the performance of the multi-frequency MPS system,
including the MNP spectra and its dynamic magnetization curve. In addition, the
signal-to-noise ratio (SNR) of the MPS system is quantitively assessed with
measured MPS signals of a given MNP sample and DI water. Furthermore, a series
of MNP samples with different iron concentrations are prepared and measured to
evaluate the limit-of-detection (LOD) in terms of iron concentration. The
influence of the excitation magnetic field, including frequency and amplitude,
is discussed based on the SNRs of the measured harmonics. Experimental results
show that the LOD is 2.3 ng in terms of iron.",2304.11290v1
2023-07-12,Magnetohydrodynamics simulation of magnetic flux rope formation in a quadrupolar magnetic field configuration,"Magnetic flux ropes (MFRs) play an important role in high-energetic events
like solar flares and coronal mass ejections in the solar atmosphere.
Importantly, solar observations suggest an association of some flaring events
with quadrupolar magnetic configurations. However, the formation and subsequent
evolution of MFRs in such magnetic configurations still need to be fully
understood. In this paper, we present idealized magnetohydrodynamics (MHD)
simulations of MFR formation in a quadrupolar magnetic configuration. A
suitable initial magnetic field having a quadrupolar configuration is
constructed by modifying a three-dimensional (3D) linear force-free magnetic
field. The initial magnetic field contains neutral lines, which consist of
X-type null points. The simulated dynamics initially demonstrate the oppositely
directed magnetic field lines located across the polarity inversion lines
(PILs) moving towards each other, resulting in magnetic reconnections. Due to
these reconnections, four highly twisted MFRs form over the PILs. With time,
the foot points of the MFRs move towards the X-type neutral lines and
reconnect, generating complex magnetic structures around the neutral lines,
thus making the MFR topology more complex in the quadrupolar configuration than
those formed in bipolar loop systems. Further evolution reveals the non-uniform
rise of the MFRs. Importantly, the simulations indicate that the pre-existing
X-type null points in magnetic configurations can be crucial to the evolution
of the MFRs and may lead to the observed brightenings during the onset of some
flaring events in the quadrupolar configurations.",2307.06025v1
2023-11-30,Properties of Mars' Dayside Low-Altitude Induced Magnetic Field and Comparisons with Venus,"Our research objective is to characterize Mars' low-altitude (250 km) induced
magnetic fields using data from NASA's MAVEN (Mars Atmosphere and Volatile
EvolutioN) Mission. We aim to assess how the induced magnetic fields behave
under different solar zenith angles and solar wind conditions, and
additionally, understand how planet-specific properties (such as Mars crustal
magnetism) alter the formation and structure of the magnetic fields. We then
use data from the Pioneer Venus Orbiter to compare induced magnetic fields at
Venus with those at Mars. At Venus, the vertical structure of the magnetic
field tends to exist in one of two states (magnetized or unmagnetized) but we
find the induced fields at Mars are more complicated, and we are unable to use
this simple classification scheme. We also find the low-altitude induced field
strength in the ionospheres of both Venus and Mars vary with as cosine of the
angle between solar wind velocity and the magnetic pileup boundary. The
low-altitude field strength at Venus tends to be higher than Mars. However,
Venus field strengths are lower than theoretical predictions assuming pressure
balance and negligible thermal pressure. For Mars, low-altitude field strengths
are higher than expected given these assumptions. Induced field strengths
exhibit a trend with solar wind dynamic pressure that is consistent with
pressure balance expectations at both planets, however there is significant
uncertainty in the Venus fit due to lack of upstream solar wind data. Our
results highlight major differences between the induced magnetic fields at
Venus and Mars, suggesting planet-specific properties such as size and the
presence of crustal magnetism affect the induced ionospheric magnetic fields at
non magnetized planets.",2311.18217v1
1999-01-05,On the Exchange of Kinetic and Magnetic Energy Between Clouds and the Interstellar Medium,"We investigate, through 2D MHD numerical simulations, the interaction of a
uniform magnetic field oblique to a moving interstellar cloud. In particular we
explore the transformation of cloud kinetic energy into magnetic energy as a
result of field line stretching. Some previous simulations have emphasized the
possible dynamical importance of a ``magnetic shield'' formed around clouds
when the magnetic field is perpendicular to the cloud motion (Jones et al.
1996, Miniati et al. 1998). It was not clear, however, how dependent those
findings were to the assumed field configuration and cloud properties. To
expand our understanding of this effect, we examine several new cases by varing
the magnetic field orientation angle with respect to the cloud motion (\theta),
the cloud-background density contrast, and the cloud Mach number.
  We show that in 2D and with \theta large enough, the magnetic field tension
can become dominant in the dynamics of the motion of high density contrast, low
Mach number clouds. In such cases a significant fraction of cloud kinetic
energy can be transformed into magnetic energy with the magnetic pressure at
the cloud nose exceeding the ram pressure of the impinging flow. We derive a
characteristic timescale for this process of energy ``conversion''. We find
also that unless the cloud motion is highly aligned to the magnetic field,
reconnection through tearing mode instabilities in the cloud wake limit the
formation of a strong flux rope feature following the cloud. Finally we attempt
to interpret some observational properties of the magnetic field in view of our
results.",9901048v1
2005-05-05,Current-driven magnetic rearrangements in spin-polarized point contacts,"A new method for investigating the dynamics of atomic magnetic moments in
current-carrying magnetic point contacts under bias is presented. This combines
the non-equilibrium Green's function (NEGF) method for evaluating the current
and the charge density with a description of the dynamics of the magnetization
in terms of quasistatic thermally-activated transitions between stationary
configurations. This method is then implemented in a tight-binding (TB) model
with parameters chosen to simulate the main features of the electronic
structures of magnetic transition metals. We investigate the domain wall (DW)
migration in magnetic monoatomic chains sandwiched between magnetic leads, and
for realistic parameters find that collinear arrangement of the magnetic
moments of the chain is always favorable. Several stationary magnetic
configurations are identified, corresponding to a different number of Bloch
walls in the chain and to a different current. The relative stability of these
configurations depends on the geometrical details of the junction and on the
bias, however we predict transitions between different configurations with
activation barriers of the order of a few tens of meV. Since different magnetic
configurations are associated to different resistances, this suggests an
intrinsic random telegraph noise at microwave frequencies in the I-V
characteristics of magnetic atomic point contacts at room temperature. Finally,
we investigate whether or not current induced torques are conservative.",0505134v1
2006-07-20,Dipolar interaction effects in the magnetic and magnetotransport properties of ordered nanoparticle arrays,"Assemblies of magnetic nanoparticles exhibit interesting physical properties
arising from the competition of intraparticle dynamics and interparticle
interactions. In ordered arrays of magnetic nanoparticles magnetostatic
interparticle interactions introduce collective dynamics acting competitively
to random anisotropy. Basic understanding, characterization and control of
dipolar interaction effects in arrays of magnetic nanoparticles is an issue of
central importance. To this end, numerical simulation techniques offer an
indispensable tool. We report on Monte Carlo studies of the magnetic hysteresis
and spin-dependent transport in thin films formed by ordered arrays of magnetic
nanoparticles. Emphasis is given to the modifications of the single-particle
behavior due to interparticle dipolar interactions as these arise in quantities
of experimental interest, such as, the magnetization, the susceptibility and
the magnetoresistance. We investigate the role of the structural parameters of
an array (interparticle separation, number of stacked monolayers) and the role
of the internal structure of the nanoparticles (single phase, core-shell).
Dipolar interactions are responsible for anisotropic magnetic behavior between
the in-plane and out-of-plane directions of the sample, which is reflected on
the investigated magnetic properties (magnetization, transverse susceptibility
and magnetoresistance) and the parameters of the array (remanent magnetization,
coercive field, and blocking temperature). Our numerical results are compared
to existing measurements on self-assembled arrays of Fe-based and Co
nanoparticles is made.",0607520v3
2008-12-31,Photospheric and Subphotospheric Dynamics of Emerging Magnetic Flux,"Magnetic fields emerging from the Sun's interior carry information about
physical processes of magnetic field generation and transport in the convection
zone. Soon after appearance on the solar surface the magnetic flux gets
concentrated in sunspot regions and causes numerous active phenomena on the
Sun. This paper discusses some properties of the emerging magnetic flux
observed on the solar surface and in the interior. A statistical analysis of
variations of the tilt angle of bipolar magnetic regions during the emergence
shows that the systematic tilt with respect to the equator (the Joy's law) is
most likely established below the surface. However, no evidence of the
dependence of the tilt angle on the amount of emerging magnetic flux, predicted
by the rising magnetic flux rope theories, is found. Analysis of surface plasma
flows in a large emerging active region reveals strong localized upflows and
downflows at the initial phase of emergence but finds no evidence for
large-scale flows indicating future appearance a large-scale magnetic
structure. Local helioseismology provides important tools for mapping
perturbations of the wave speed and mass flows below the surface. Initial
results from SOHO/MDI and GONG reveal strong diverging flows during the flux
emergence, and also localized converging flows around stable sunspots. The wave
speed images obtained during the process of formation of a large active region,
NOAA 10488, indicate that the magnetic flux gets concentrated in strong field
structures just below the surface. Further studies of magnetic flux emergence
require systematic helioseismic observations from the ground and space, and
realistic MHD simulations of the subsurface dynamics.",0901.0035v1
2012-11-13,Magnetic Wreaths and Cycles in Convective Dynamos,"Solar-type stars exhibit a rich variety of magnetic activity. Seeking to
explore the convective origins of this activity, we have carried out a series
of global 3D magnetohydrodynamic (MHD) simulations with the anelastic spherical
harmonic (ASH) code. Here we report on the dynamo mechanisms achieved as the
effects of artificial diffusion are systematically decreased. The simulations
are carried out at a nominal rotation rate of three times the solar value
(3$\Omega_\odot$), but similar dynamics may also apply to the Sun. Our previous
simulations demonstrated that convective dynamos can build persistent toroidal
flux structures (magnetic wreaths) in the midst of a turbulent convection zone
and that high rotation rates promote the cyclic reversal of these wreaths. Here
we demonstrate that magnetic cycles can also be achieved by reducing the
diffusion, thus increasing the Reynolds and magnetic Reynolds numbers. In these
more turbulent models, diffusive processes no longer play a significant role in
the key dynamical balances that establish and maintain the differential
rotation and magnetic wreaths. Magnetic reversals are attributed to an
imbalance in the poloidal magnetic induction by convective motions that is
stabilized at higher diffusion levels. Additionally, the enhanced levels of
turbulence lead to greater intermittency in the toroidal magnetic wreaths,
promoting the generation of buoyant magnetic loops that rise from the deep
interior to the upper regions of our simulated domain. The implications of such
turbulence-induced magnetic buoyancy for solar and stellar flux emergence are
also discussed.",1211.3129v1
2013-04-12,Comparison of force-free coronal magnetic field modeling using vector fields from Hinode and Solar Dynamics Observatory,"Photospheric magnetic vector maps from two different instruments are used to
model the nonlinear force-free coronal magnetic field above an active region.
We use vector maps inferred from polarization measurements of the Solar
Dynamics Observatory/Helioseismic and Magnetic Imager (HMI) and the Solar
Optical Telescope Spectropolarimeter (SP) aboard Hinode. Besides basing our
model calculations on HMI data, we use both, SP data of original resolution and
scaled down to the resolution of HMI. This allows us to compare the model
results based on data from different instruments and to investigate how a
binning of high-resolution data effects the model outcome. The resulting 3D
magnetic fields are compared in terms of magnetic energy content and magnetic
topology. We find stronger magnetic fields in the SP data, translating into a
higher total magnetic energy of the SP models. The net Lorentz forces of the
HMI and SP lower boundaries verify their force-free compatibility. We find
substantial differences in the absolute estimates of the magnetic field energy
but similar relative estimates, e.g., the fraction of excess energy and of the
flux shared by distinct areas. The location and extension of neighboring
connectivity domains differs and the SP model fields tend to be higher and more
vertical. Hence, conclusions about the magnetic connectivity based on
force-free field models are to be drawn with caution. We find that the
deviations of the model solution when based on the lower-resolution SP data are
small compared to the differences of the solutions based on data from different
instruments.",1304.3619v1
2014-03-15,Global evolution of the magnetic field in a thin disc and its consequences for protoplanetary systems,"The strength and structure of the large-scale magnetic field in
protoplanetary discs are still unknown, although they could have important
consequences for the dynamics and evolution of the disc. Using a mean-field
approach in which we model the effects of turbulence through enhanced diffusion
coefficients, we study the time-evolution of the large-scale poloidal magnetic
field in a global model of a thin accretion disc, with particular attention to
protoplanetary discs. With the transport coefficients usually assumed, the
magnetic field strength does not significantly increase radially inwards,
leading to a relatively weak magnetic field in the inner part of the disc. We
show that with more realistic transport coefficients that take into account the
vertical structure of the disc and the back-reaction of the magnetic field on
the flow as obtained by Guilet & Ogilvie (2012), the magnetic field can
significantly increase radially inwards. The magnetic-field profile adjusts to
reach an equilibrium value of the plasma $\beta$ parameter (the ratio of
midplane thermal pressure to magnetic pressure) in the inner part of the disc.
This value of $\beta$ depends strongly on the aspect ratio of the disc and on
the turbulent magnetic Prandtl number, and lies in the range $10^4-10^7$ for
protoplanetary discs. Such a magnetic field is expected to affect significantly
the dynamics of protoplanetary discs by increasing the strength of MHD
turbulence and launching an outflow. We discuss the implications of our results
for the evolution of protoplanetary discs and for the formation of powerful
jets as observed in T-Tauri star systems.",1403.3732v1
2017-10-03,"Long-range dynamical magnetic order and spin tunneling in the cooperative paramagnetic states of the pyrochlore analogous spinel antiferromagnets CdYb2X4 (X = S, Se)","Magnetic systems with spins sitting on a lattice of corner sharing regular
tetrahedra have been particularly prolific for the discovery of new magnetic
states for the last two decades. The pyrochlore compounds have offered the
playground for these studies, while little attention has been comparatively
devoted to other compounds where the rare earth R occupies the same
sub-lattice, e.g. the spinel chalcogenides CdR2X4 (X = S, Se). Here we report
measurements performed on powder samples of this series with R = Yb using
specific heat, magnetic susceptibility, neutron diffraction and
muon-spin-relaxation measurements. The two compounds are found to be
magnetically similar. They long-range order into structures described by the
\Gamma_5 irreducible representation. The magnitude of the magnetic moment at
low temperature is 0.77 (1) and 0.62 (1) mu_B for X = S and Se, respectively.
Persistent spin dynamics is present in the ordered states. The spontaneous
field at the muon site is anomalously small, suggesting magnetic moment
fragmentation. A double spin-flip tunneling relaxation mechanism is suggested
in the cooperative paramagnetic state up to 10 K. The magnetic space groups
into which magnetic moments of systems of corner-sharing regular tetrahedra
order are provided for a number of insulating compounds characterized by null
propagation wavevectors.",1710.01062v1
2018-04-09,Three Dimensional Aggregation of Magnetic Particles,"Magnetic drug delivery is a promising therapeutic because of magnetic fields'
ability to permeate unperturbed in human tissue. One of the long-standing
challenges in magnetic drug delivery is the inability to generate 3D
aggregation non-invasively within the interior of the body. Earnshaw's theorem,
which proves the impossibility of creating an energetic minimum in a curl-free
and divergence-free field such as a magnetic field. However, one of the
assumptions of Earnshaw's theorem is a static field. Here we show that it is
possible to utilize a dynamically changing field and a dissipative force such
as the drag, which is generally present, to create a stable aggregation point
for magnetic particles. We also introduce a theoretical framework for designing
the suitable magnetic fields for controlling a given magnetic particle in a
particular fluid. This framework enables accurate determination of the
necessary parameters for aggregation across a wide variety of magnetic
particles and across multiple biologically-relevant fluids. By coating magnetic
particles with desired therapeutic agents or attaching them to cells, a new
class of treatment methodologies can be created in therapies such as targeted
drug delivery and cell-based therapies. By dynamically changing the aggregation
point, agents can also be guided along a particular path in the body. This
technique of using dissipative forces to create a stable 3D aggregation point
for particles could possibly be extended to a broad range of applications such
as microscopic and macroscopic manipulation, robotics, guided self-assembly,
magnetic plasma confinement, tissue engineering, and ion traps for quantum
computers.",1804.02778v1
2019-09-30,"High-harmonic generation by electric polarization, spin current, and magnetization","High-harmonic generation (HHG), a typical nonlinear optical effect, has been
actively studied in electron systems such as semiconductors and
superconductors. As a natural extension, we theoretically study HHG from
electric polarization, spin current and magnetization in magnetic insulators
under terahertz (THz) or gigahertz (GHz) electromagnetic waves. We use simple
one-dimensional spin chain models with or without multiferroic coupling between
spins and the electric polarization, and study the dynamics of the spin chain
coupled to an external ac electric or magnetic field. We map spin chains to
two-band fermions and invoke an analogy of semiconductors and superconductors.
With a quantum master equation and Lindblad approximation, we compute the time
evolution of the electric polarization, spin current, and magnetization,
showing that they exhibit clear harmonic peaks. We also show that the
even-order HHG by magnetization dynamics can be controlled by static magnetic
fields in a wide class of magnetic insulators. We propose experimental setups
to observe these HHG, and estimate the required strength of the ac electric
field $E_0$ for detection as $E_0\sim100$kV/cm--1MV/cm, which corresponds to
the magnetic field $B_0\sim0.1$T--1T. The estimated strength would be relevant
also for experimental realizations of other theoretically-proposed nonlinear
optical effects in magnetic insulators such as Floquet engineering of magnets.",1910.00146v3
2020-02-20,Stoner-Wohlfarth switching of the condensate magnetization in a dipolar spinor gas and the metrology of excitation damping,"We consider quasi-one-dimensional dipolar spinor Bose-Einstein condensates in
the homogeneous-local-spin-orientation approximation, that is with
unidirectional local magnetization. By analytically calculating the exact
effective dipole-dipole interaction, we derive a Landau-Lifshitz-Gilbert
equation for the dissipative condensate magnetization dynamics, and show how it
leads to the Stoner-Wohlfarth model of a uni-axial ferro-magnetic particle,
where the latter model determines the stable magnetization patterns and
hysteresis curves for switching between them. For an external magnetic field
pointing along the axial, long direction, we analytically solve the
Landau-Lifshitz-Gilbert equation. The solution explicitly demonstrates that the
magnetic dipole-dipole interaction {\it accelerates} the dissipative dynamics
of the magnetic moment distribution and the associated dephasing of the
magnetic moment direction. Under suitable conditions, dephasing of the
magnetization direction due to dipole-dipole interactions occurs within time
scales up to two orders of magnitude smaller than the lifetime of currently
experimentally realized dipolar spinor condensates, e.g., produced with the
large magnetic-dipole-moment atoms ${}^{166} \textrm{Er}$. This enables
experimental access to the dissipation parameter $\Gamma$ in the
Gross-Pitaevski\v\i~mean-field equation, for a system currently lacking a
complete quantum kinetic treatment of dissipative processes and, in particular,
an experimental check of the commonly used assumption that $\Gamma$ is a single
scalar independent of spin indices.",2002.08723v2
2020-11-09,Quantum-torque-induced breaking of magnetic interfaces in ultracold gases,"A rich variety of physical effects in spin dynamics arises at the interface
between different magnetic materials. Engineered systems with interlaced
magnetic structures have been used to implement spin transistors, memories and
other spintronic devices. However, experiments in solid state systems can be
difficult to interpret because of disorder and losses. Here, we realize
analogues of magnetic junctions using a coherently-coupled mixture of ultracold
bosonic gases. The spatial inhomogeneity of the atomic gas makes the system
change its behavior from regions with oscillating magnetization -- resembling a
magnetic material in the presence of an external transverse field -- to regions
with a defined magnetization, as in magnetic materials with a ferromagnetic
anisotropy stronger than external fields. Starting from a far-from-equilibrium
fully polarized state, magnetic interfaces rapidly form. At the interfaces, we
observe the formation of short-wavelength magnetic waves. They are generated by
a quantum torque contribution to the spin current and produce strong spatial
anticorrelations in the magnetization. Our results establish ultracold gases as
a platform for the study of far-from-equilibrium spin dynamics in regimes that
are not easily accessible in solid-state systems.",2011.04271v2
2021-09-26,Disorder-Induced Complex Magnetization Dynamics in Planar Ensembles of Nanoparticles,"The magnetic relaxation characteristics are investigated in the
two-dimensional ($l^{}_x\times l^{}_y$) assembly of nanoparticles as a function
of out-of-plane positional disorder strength $\Delta(\%)$ using numerical
simulations. Such defects are redundantly observed in experimentally fabricated
nanostructures, resulting in unusual magnetization dynamics. The magnetization
decays exponentially for small and negligible dipolar interaction strength
$h^{}_d\leq0.2$. In such a case, the magnetization relaxation does not depend
on $\Delta$ and aspect ratio $A^{}_r=l^{}_y/l^{}_x$, as expected. In
square-like MNPs ensembles and perfectly ordered system ($\Delta(\%)=0$), the
magnetization relaxes rapidly with an increase in $h^{}_d$. Consequently, the
effective N\'eel relaxation time $\tau^{}_N$ decreases with $h^{}_d$. The
dipolar interaction of sufficient strength promotes antiferromagnetic coupling
in such a system, resulting in rapid magnetization decay. Remarkably, the
out-of-plane disorder instigates the magnetic moment to interact
ferromagnetically in the presence of large $h^{}_d$, even in the square-like
assembly of MNPs. As a result, magnetization relaxation slows down, resulting
in a monotonous increase of $\tau^{}_N$ with an increase in $\Delta$ and
$h^{}_d$ in such cases. Notably, there is a prolonged magnetization decay in
the highly anisotropic system with large $h^{}_d$. The dipolar interaction
induces ferromagnetic coupling along the long axis of the system in such cases.
Therefore, the magnetization ceases to relax as a function of time for large
$h^{}_d$, irrespective of disorder strength $\Delta(\%)$. The present work
could provide a concrete theoretical basis to explain the unexpected relaxation
behaviour observed in experiments. These results are also beneficial in digital
data storage and spintronics based applications where such nanostructures are
extensively used.",2109.12596v1
2021-10-11,Simulations of the line-driven instability in magnetic hot star winds,"Line-driven winds of hot, luminous stars are intrinsically unstable due to
the line-deshadowing instability (LDI). In non-magnetic hot stars, the LDI
leads to the formation of an inhomogeneous wind consisting of small-scale,
spatially separated clumps that can have great effects on observational
diagnostics. However, for magnetic hot stars the LDI generated structures, wind
dynamics, and effects on observational diagnostics have not been directly
investigated so far. We investigated the non-linear development of LDI
generated structures and dynamics in a magnetic line-driven wind of a typical
O-supergiant. We employed two-dimensional axisymmetric magnetohydrodynamic
(MHD) simulations of the LDI using the Smooth Source Function approximation for
evaluating the assumed one-dimensional line force. To facilitate the
interpretation of these magnetic models, they were compared with a
corresponding non-magnetic LDI simulation as well as a magnetic simulation
neglecting the LDI. A central result obtained is that the wind morphology and
wind clumping properties change strongly with increasing wind-magnetic
confinement. Most notably, in magnetically confined flows, the LDI leads to
large-scale, shellular sheets ('pancakes') that are quite distinct from the
spatially separate, small-scale clumps in non-magnetic line-driven winds. We
discuss the impact of these findings for observational diagnostic studies and
stellar evolution models of magnetic hot stars.",2110.05302v1
2023-07-12,Magnetic Fields in Multiphase Turbulence: Impacts on Dynamics and Structure,"Both multiphase gas and magnetic fields are ubiquitous in astrophysics.
However, the influence of magnetic fields on mixing of the different phases is
still largely unexplored. In this study, we use both turbulent radiative mixing
layer (TRML) and turbulent box simulations to examine the effects of magnetic
fields on cold gas growth rates, survival, and the morphology of the multiphase
gas. Our findings indicate that, in general, magnetic fields suppress mixing in
TRMLs while turbulent box simulations show comparatively marginal differences
in growth rates and survival of the cold gas. We reconcile these two seemingly
contrasting results by demonstrating that similar turbulent properties result
in comparable mixing -- regardless of the presence or absence of magnetic
fields. We, furthermore, find the cold gas clump size distribution to be
independent of the magnetic fields but the clumps are more filamentary in the
MHD case. Synthetic MgII absorption lines support this picture being marginally
different with and without magnetic fields; both cases aligning well with
observations. We also examine the magnetic field strength and structure in
turbulent boxes. We generally observe a higher mean magnetic field in the cold
gas phase due to flux freezing and reveal fractal-like magnetic field lines in
a turbulent environment.",2307.06411v1
2023-08-10,Spinning magnetized particles orbiting magnetized Schwarzschild black holes,"A way to test electromagnetic field and spacetime properties around black
holes is by considering the dynamics of test particles. In fact, in real
astrophysical scenarios, it is hard to determine spacetime geometry which is
dominating due to degeneracy gravitational effects in parameters of gravity
theories. In this work, we study for the first time the dynamics of spinning
particles that have magnetic dipole moments around Schwarzschild black holes
immersed in an external asymptotically uniform magnetic field using the
Mathisson-Papapetrou-Dixon (MPD) equation. There are two combined interactions:
gravitational interaction between the spin of the particle and
(electro)magnetic interaction between the external magnetic field and the
magnetic dipole moment of the particle to be taken into account. First, we
derive the effective potential of the test spinning magnetized particles in
motion around the black hole. We also study the combined effects of spin and
magnetic interactions on innermost stable circular orbits (ISCOs), the energy,
and angular momentum of the particles at ISCO together with superluminal
bounds. We investigated the collision of the particles and evaluated the
center-of-mass energy in the collisions. Finally, we consider various cases in
which neutron stars and rotating stellar mass black holes can be treated as
spinning magnetized particles, evaluating the effects of the spin and magnetic
moment of objects around supermassive and intermediate-mass black holes.",2308.05392v1
2011-07-01,Slow magnetic dynamics and hysteresis loops of a bulk ferromagnet,"Magnetic dynamics of a bulk ferromagnet, a new single crystalline compound
Co7(TeO3)4Br6, was studied by ac susceptibility and the related techniques.
Very large Arrhenius activation energy of 17.2 meV (201 K) and long attempt
time (2x10^(-4)s) span the full spectrum of magnetic dynamics inside a
convenient frequency window, offering a rare opportunity for general studies of
magnetic dynamics. Within the experimental window the ac susceptibility data
build almost ideally semicircular Cole-Cole plots. Comprehensive study of
experimental dynamic hysteresis loops of the compound is presented and
interpreted within a simple thermal-activation-assisted spin lattice relaxation
model for spin reversal. Quantitative agreement between the experimental
results and the model's prediction for dynamic coercive field is achieved by
assuming the central physical quantity, the Debye relaxation rate, to depend on
frequency, as well as on the applied field strength and sample temperature.
Cross-over between minor- to major hysteresis loops is carefully analyzed.
Low-frequency limitations of the model, relying on domain wall pinning effects,
are experimentally detected and appropriately discussed.",1107.0255v2
2013-06-04,Dynamic susceptibility and dynamic correlations in spin ice,"Here we calculate the dynamic susceptibility and dynamic correlation function
in spin ice using the model of emergent magnetic monopoles. Calculations are
based on a method originally suggested for the description of dynamic processes
in water ice (non-equilibrium thermodynamics approach). We show that for zero
temperature the dynamic correlation function reproduces the transverse dipole
correlations (static correlation function) characteristic of spin ice in its
ground state. At non-zero temperatures the dynamic correlation function
includes an additional longitudinal component which decreases as the
temperature decreases. Both terms (transverse and longitudinal) exhibit
identical Debye-like dependences on frequency but with different relaxation
times: the magnetic Coulomb interaction of monopoles reduces the longitudinal
relaxation time with respect to the transverse one. We calculate the dielectric
function for the magnetic monopole gas and discuss how the non-equilibrium
thermodynamics approach exposes corrections to the Debye-Huckel theory of
magnetic monopoles and the concept of ""entropic charge"".",1306.0653v1
2023-10-20,Heat equation from a deterministic dynamics,"We derive the heat equation for the thermal energy under diffusive space-time
scaling for a purely deterministic microscopic dynamics satisfying Newton
equations perturbed by an external chaotic force acting like a magnetic field.",2310.13338v2
2022-01-11,Coronal magnetic field evolution over the cycle 24,"The photospheric magnetic field vector is continuously derived from
measurements, while reconstruction of the three-dimensional (3D) coronal
magnetic field requires modelling with photospheric measurements as a boundary
condition. For decades the cycle variation of the magnetic field in the
photosphere has been investigated. To present, there is no study to show the
evolution of the coronal magnetic flux in the corona, nor the evolution of
solar cycle magnetic free energy. The paper aims to analyze the temporal
variation of the magnetic field and free magnetic energy in the solar corona
for the solar cycle 24 and how the magnetic field behaves in the two
hemispheres. We investigate if we can obtain better estimates of the magnetic
field at Earth using the nonlinear force-free field (NLFFF) extrapolation
method. To model the magnetic field over cycle 24 we apply the NLFFF
optimization method to the synoptic vector magnetic maps derived from the
observations of Heliospheric and Magnetic Imager (HMI) onboard Solar Dynamic
Observatory (SDO). We found that during the solar cycle 24, the maximum of the
Sun's dynamics is different from the sunspot number (SSN) maximum peak. The
major contribution to the total unsigned flux is provided by the flux coming
from the magnetic field structures other than sunspots (MSOS) within latitudes
between -30 and +30 degrees. The magnetic flux variation during the solar cycle
24 shows a different evolution in the corona than in the photosphere. We found
a correlation value of 0.8 between the derived magnetic energy from our model
and the flare energy index derived from observations. On average, cycle 24 had
a higher number of sunspots in the northern hemisphere (NH) but stronger flux
in the southern hemisphere (SH) which could more effectively reach the higher
layers of the atmosphere. The coupling between the hemispheres increases with
height.",2201.03853v1
2022-07-04,Three-dimensional decaying magnetic field belonging to Beltrami flow,"This study analysed a three-dimensional Taylor decaying vortex under an
applied magnetic field as a benchmark test problem to verify the calculation
method of an electromagnetic fluid flow and investigated the validity of the
decaying magnetic field model. First, we observed the flow structure of a
three-dimensional Taylor decaying vortex without an applied magnetic field. We
investigated the changes in the error between the calculation result and the
exact solution when the number of grid points and the Reynolds number varied
and showed the effectiveness of the benchmark test. Next, we analysed a
three-dimensional Taylor decaying vortex under an applied magnetic field and
clarified the characteristics of the decaying magnetic field. When a magnetic
field is applied, low magnetic pressure regions are connected in a mesh
pattern, and the magnetic pressure distribution with a distorted cubic
structure occurs to surround a high magnetic pressure region. In a stagnation
region, the magnetic energy becomes low, and the magnetic flux line is similar
to the streamline of the velocity field. High current densities occur in a grid
pattern, and the magnetic flux lines swirl around the high current density
region. The magnetic pressure and magnetic energy are high in the high current
density region. When the Reynolds number and the magnetic Reynolds number vary,
the decay trends of various energies agree well with the exact solution. The
transition to turbulent flow occurs at a high Reynolds number, and the kinetic
and total energies decrease rapidly. After the dissipation rate of kinetic
energy becomes maximum, the vortex structure decays, and the flow field
approaches a stationary state without magnetic fields. The three-dimensional
Taylor decaying magnetic field belonging to the Beltrami flow is a valuable
model for verifying the calculation method of electromagnetic fluid flows.",2207.01154v3
1995-09-22,Critical Dynamics of Magnets,"We review our current understanding of the critical dynamics of magnets above
and below the transition temperature with focus on the effects due to the
dipole--dipole interaction present in all real magnets. Significant progress in
our understanding of real ferromagnets in the vicinity of the critical point
has been made in the last decade through improved experimental techniques and
theoretical advances in taking into account realistic spin-spin interactions.
We start our review with a discussion of the theoretical results for the
critical dynamics based on recent renormalization group, mode coupling and spin
wave theories. A detailed comparison is made of the theory with experimental
results obtained by different measuring techniques, such as neutron scattering,
hyperfine interaction, muon--spin--resonance, electron--spin--resonance, and
magnetic relaxation, in various materials. Furthermore we discuss the effects
of dipolar interaction on the critical dynamics of three--dimensional isotropic
antiferromagnets and uniaxial ferromagnets. Special attention is also paid to a
discussion of the consequences of dipolar anisotropies on the existence of
magnetic order and the spin--wave spectrum in two--dimensional ferromagnets and
antiferromagnets. We close our review with a formulation of critical dynamics
in terms of nonlinear Langevin equations.",9509141v1
2004-08-04,Conservation laws for the voter model in complex networks,"We consider the voter model dynamics in random networks with an arbitrary
distribution of the degree of the nodes. We find that for the usual node-update
dynamics the average magnetization is not conserved, while an average
magnetization weighted by the degree of the node is conserved. However, for a
link-update dynamics the average magnetization is still conserved. For the
particular case of a Barabasi-Albert scale-free network the voter model
dynamics leads to a partially ordered metastable state with a finite size
survival time. This characteristic time scales linearly with system size only
when the updating rule respects the conservation law of the average
magnetization. This scaling identifies a universal or generic property of the
voter model dynamics associated with the conservation law of the magnetization.",0408101v1
2010-01-14,Interaction quench dynamics in the Kondo model in presence of a local magnetic field,"In this work we investigate the quench dynamics in the Kondo model on the
Toulouse line in presence of a local magnetic field. It is shown that this
setup can be realized by either applying the local magnetic field directly or
by preparing the system in a macroscopically spin-polarized initial state. In
the latter case, the magnetic field results from a subtlety in applying the
bosonization technique where terms that are usually referred to as finite-size
corrections become important in the present non-equilibrium setting. The
transient dynamics is studied by analyzing exact analytical results for the
local spin dynamics. The time scale for the relaxation of the local dynamical
quantities turns out to be exclusively determined by the Kondo scale. In the
transient regime, one observes damped oscillations in the local correlation
functions with a frequency set by the magnetic field.",1001.2418v2
2011-12-09,Influence of crystal anisotropy on the critical state stability and flux jumps dynamics in a single crystal of La(1.85)Sr(0.15)CuO(4),"We studied the critical state stability in a large cubic sample of a single
crystalline La(1.85)Sr(0.15)CuO(4) for different sample orientations with
respect to the external magnetic field as well as the dynamics of the flux
jumps. It is shown that thermomagnetic avalanches develop in dynamic conditions
characterized by significantly lower magnetic diffusivity than the thermal one.
In this case, critical state stability depends strongly on cooling conditions.
We compared predictions of the isothermal model and of the model for the weakly
cooled sample with experimental results. In both models, the field of the first
flux jump decreases with an increase of sweep rate of the external magnetic
field. We also investigated the influence of external magnetic field on the
dynamics of the following stages of the thermomagnetic avalanche. It is shown
that the dynamics of the flux jumps is correlated with the magnetic diffusivity
proportional to the flux flow resistivity.",1112.2111v1
2013-12-27,Spin Transfer Torque and Electric Current in Helical Edge States in Quantum Spin Hall Devices,"We study the dynamics of a quantum spin Hall edge coupled to a magnet with
its own dynamics. Using spin transfer torque principles, we analyze the
interplay between spin currents in the edge state and dynamics of the axis of
the magnet, and draw parallels with circuit analogies. As a highlighting
feature, we show that while coupling to a magnet typically renders the edge
state insulating by opening a gap, in the presence of a small potential bias,
spin-transfer torque can restore perfect conductance by transferring angular
momentum to the magnet. In the presence of interactions within the edge state,
we employ a Luttinger liquid treatment to show that the edge, when subject to a
small voltage bias, tends to form a unique dynamic rotating spin wave state
that naturally couples into the dynamics of the magnet. We briefly discuss
realistic physical parameters and constraints for observing this interplay
between quantum spin Hall and spin-transfer torque physics.",1312.7303v1
2014-04-12,Dynamical magnetic skyrmions,"Spin transfer torque (STT) affords magnetic nanodevices the potential to act
as memory, computing, and microwave elements operating at ultra-low currents
and at a low energy cost. Spin transfer torque is not only effective in
manipulating well-known magnetic structures, such as domain walls and vortices,
but can also nucleate previously unattainable nano-magnetic objects, such as
magnetic droplets and skyrmions. While the droplet and the skyrmion are both
solitons, the former is inherently dynamic and non-topological, whereas the
latter is static but topologically protected. Here we show that it is possible
to combine these properties into a novel topologically protected dynamical
skyrmion, which adds additional degrees of freedom, and functionality, to both
droplet and skyrmion based applications. Unlike static skyrmions, the dynamical
skyrmion can be nucleated and sustained without Dzyaloshinskii-Moriya
interaction (DMI) or dipole-dipole interaction (DDI), and is a generic soliton
solution independent of STT and damping once nucleated. In the presence of
large DMI, the dynamical skyrmion experiences strong breathing with particular
promise for skyrmion-based memory and microwave applications.",1404.3281v2
2014-11-27,Short-Time Dynamics of Fe2/V13 Magnetic Superlattice Models,"Critical relaxation from a low-temperature fully ordered state of Fe2/V13
iron-vanadium magnetic superlattice models has been studied using the method of
short-time dynamics. Systems with three variants of the ratio R of inter- to
intralayer exchange coupling have been considered. Particles with N = 262144
spins have been simulated with periodic boundary conditions. Calculations have
been performed using the standard Metropolis algorithm of the Monte Carlo
method. The static critical exponents of magnetization and correlation radius,
as well as the dynamic critical exponent, have been calculated for three R
values. It is established that a small decrease in the exchange ratio (from R =
1.0 to 0.8) does not significantly influence the character of the short-time
dynamics in the models studied. A further significant decrease in this ratio
(to R = 0.01), for which a transition from three-dimensional to
quasi-twodimensional magnetism is possible, leads to significant changes in the
dynamic behavior of iron-vanadium magnetic superlattice models.",1411.7546v1
2014-12-11,Deviation From the Landau-Lifshitz-Gilbert equation in the Inertial regime of the Magnetization,"We investigate in details the inertial dynamics of a uniform magnetization in
the ferromagnetic resonance (FMR) context. Analytical predictions and numerical
simulations of the complete equations within the Inertial
Landau-Lifshitz-Gilbert (ILLG) model are presented. In addition to the usual
precession resonance, the inertial model gives a second resonance peak
associated to the nutation dynamics provided that the damping is not too large.
The analytical resolution of the equations of motion yields both the precession
and nutation angular frequencies. They are function of the inertial dynamics
characteristic time $\tau$, the dimensionless damping $\alpha$ and the static
magnetic field $H$. A scaling function with respect to $\alpha\tau\gamma H$ is
found for the nutation angular frequency, also valid for the precession angular
frequency when $\alpha\tau\gamma H\gg 1$. Beyond the direct measurement of the
nutation resonance peak, we show that the inertial dynamics of the
magnetization has measurable effects on both the width and the angular
frequency of the precession resonance peak when varying the applied static
field. These predictions could be used to experimentally identify the inertial
dynamics of the magnetization proposed in the ILLG model.",1412.3783v1
2015-03-23,Local dynamics of topological magnetic defects in the itinerant helimagnet FeGe,"Chiral magnetic interactions induce complex spin textures including helical
and conical spin waves, as well as particle-like objects such as magnetic
skyrmions and merons. These spin textures are the basis for innovative device
paradigms and give rise to exotic topological phenomena, thus being of interest
for both applied and fundamental sciences. Present key questions address the
dynamics of the spin system and emergent topological defects. Here we analyze
the micromagnetic dynamics in the helimagnetic phase of FeGe. By combining
magnetic force microscopy, single-spin magnetometry, and
Landau-Lifschitz-Gilbert simulations we show that the nanoscale dynamics are
governed by the depinning and subsequent motion of magnetic edge dislocations.
The motion of these topologically stable objects triggers perturbations that
can propagate over mesoscopic length scales. The observation of stochastic
instabilities in the micromagnetic structure provides new insight to the
spatio-temporal dynamics of itinerant helimagnets and topological defects, and
discloses novel challenges regarding their technological usage.",1503.06622v2
2016-05-04,On the magnetism and dynamics of prominence legs hosting tornadoes,"Solar tornadoes are dark vertical filamentary structures observed in the
extreme ultraviolet associated with prominence legs and filament barbs. Their
true nature and relationship to prominences requires understanding their
magnetic structure and dynamic properties. Recently, a controversy has arisen:
is the magnetic field organized forming vertical, helical structures or is it
dominantly horizontal? And concerning their dynamics, are tornadoes really
rotating or is it just a visual illusion? Here, we analyze four consecutive
spectropolarimetric scans of a prominence hosting tornadoes on its legs which
help us shed some light on their magnetic and dynamical properties. We show
that the magnetic field is very smooth in all the prominence, probably an
intrinsic property of the coronal field. The prominence legs have vertical
helical fields that show slow temporal variation probably related to the motion
of the fibrils. Concerning the dynamics, we argue that 1) if rotation exists,
it is intermittent, lasting no more than one hour, and 2) the observed velocity
pattern is also consistent with an oscillatory velocity pattern (waves).",1605.01183v1
2016-06-22,Thermally induced magnetic relaxation in square artificial spin ice,"The properties of natural and artificial assemblies of interacting elements,
ranging from Quarks to Galaxies, are at the heart of Physics. The collective
response and dynamics of such assemblies are dictated by the intrinsic
dynamical properties of the building blocks, the nature of their interactions
and topological constraints. Here we report on the relaxation dynamics of the
magnetization of artificial assemblies of mesoscopic spins. In our model
nano-magnetic system - square artificial spin ice - we are able to control the
geometrical arrangement and interaction strength between the magnetically
interacting building blocks by means of nano-lithography. Using time resolved
magnetometry we show that the relaxation process can be described using the
Kohlrausch law and that the extracted temperature dependent relaxation times of
the assemblies follow the Vogel-Fulcher law. The results provide insight into
the relaxation dynamics of mesoscopic nano-magnetic model systems, with
adjustable energy and time scales, and demonstrates that these can serve as an
ideal playground for the studies of collective dynamics and relaxations.",1606.06899v1
2016-07-10,Magnetic Hamiltonian Monte Carlo,"Hamiltonian Monte Carlo (HMC) exploits Hamiltonian dynamics to construct
efficient proposals for Markov chain Monte Carlo (MCMC). In this paper, we
present a generalization of HMC which exploits \textit{non-canonical}
Hamiltonian dynamics. We refer to this algorithm as magnetic HMC, since in 3
dimensions a subset of the dynamics map onto the mechanics of a charged
particle coupled to a magnetic field. We establish a theoretical basis for the
use of non-canonical Hamiltonian dynamics in MCMC, and construct a symplectic,
leapfrog-like integrator allowing for the implementation of magnetic HMC.
Finally, we exhibit several examples where these non-canonical dynamics can
lead to improved mixing of magnetic HMC relative to ordinary HMC.",1607.02738v2
2017-06-23,Dynamic Simulation of Structural Phase Transitions in Magnetic Iron,"The occurrence of bcc-fcc ($\alpha$-$\gamma$) and fcc-bcc ($\gamma$-$\delta$)
phase transitions in magnetic iron stems from the interplay between magnetic
excitations and lattice vibrations. However, this fact has never been proven by
a direct dynamic simulation, treating non-collinear magnetic fluctuations and
dynamics of atoms, and their coupling at a finite temperature. Starting from a
large set of data generated by ab initio simulations, we derive non-collinear
magnetic many-body potentials for bcc and fcc iron describing fluctuations in
the vicinity of near perfect lattice positions. We then use spin-lattice
dynamics simulations to evaluate the difference between free energies of bcc
and fcc phases, assessing their relative stability within a unified dynamic
picture. We find two intersections between the bcc and fcc free energy curves,
which correspond to $\alpha$-$\gamma$ bcc-fcc and $\gamma$-$\delta$ fcc-bcc
phase transitions. The maximum fcc-bcc free energy difference over the
temperature interval between the two phase transition points is 2 meV, in
agreement with other experimental and theoretical estimates.",1706.07635v1
2018-03-20,Does the chiral magnetic effect change the dynamic universality class in QCD?,"In QCD matter under an external magnetic field, the chiral magnetic effect
(CME) leads to the collective gapless mode called the chiral magnetic wave
(CMW). Since dynamic universality class generally depends on low-energy gapless
modes, it is nontrivial whether the CME and the resulting CMW change that of
the second-order chiral phase transition in QCD. To address this question, we
study the critical dynamics near the chiral phase transition in massless
two-flavor QCD under an external magnetic field. By performing the dynamic
renormalization-group analysis within the epsilon expansion, we find that the
presence of the CME changes the dynamic universality class to that of model A.
We also show that the transport coefficient of the CME is not renormalized by
the critical fluctuations of the order parameter.",1803.07267v2
2018-10-10,Magnetization Dynamics,"Magnetism primarily describes the physics and materials science of systems
presenting a magnetization -- a macroscopic order parameter characterizing
electron angular momentum. The order parameter is associated with the
electronic exchange interactions, which is fundamentally quantum mechanical.
Its dynamic behavior bridges the macroscopic and the microscopic worlds. On
macroscopic length and time-scales, it interacts with electromagnetic fields
dictated by the Maxwells equations. On a microscopic scale, it involves the
quantum-mechanical electronic states both in spin-space and momentum space,
thus giving rise to a wide range of behavior that extend down to femtoseconds.
Thanks to the development of modern metrology, there have been many new and
noteworthy observations of magnetism-related phenomena across the entire range
-- from spin-torque induced antidamping dynamics, to ultrafast laser induced
femtosecond electron dynamics that involve spin current and angular momentum
conservation. In this review we introduce some observations on magnetodynamics,
and the scientific subjects these new results give rise to.",1810.04591v1
2018-12-20,Tunable Angle Dependent Magnetization Dynamics in Ni80Fe20 Nano-cross Structures of Varying Size,"We demonstrate a large angular dependence of magnetization dynamics in
Ni80Fe20 nano-cross arrays of varying sizes. By subtle variation of the
azimuthal angle of an in-plane bias magnetic field, the spin configuration and
the ensuing spin-wave dynamics, including mode softening, mode splitting, mode
crossover and mode merging, can be drastically varied to the extent that a
frequency minimum corresponding to mode softening converts to a mode crossover,
various mode splitting and mode crossover disappear and additional mode
splitting appears. Numerically simulated spin-wave spectra and phase profiles
revealed the nature of various spin-wave modes and the origin of above
variation of the dynamics with bias-field angle. All of these above
observations are further modified with the variation of dimensions of the
nano-cross. The numerically calculated magnetostatic field distributions
further supports the variation of dynamics with bias-field angle. These results
open a new avenue for engineering the nano-cross based magnetic devices such as
magnetic storage, spin-wave logic and on-chip data communication devices.",1812.08393v1
2021-07-23,Anomalous magnetic noise in imperfect flat bands in the topological magnet Dy2Ti2O7,"The spin ice compound Dy_2Ti_2O_7 stands out as the first topological magnet
in three dimensions, with its tell-tale emergent fractionalized magnetic
monopole excitations. Its real-time dynamical properties have been an enigma
from the very beginning. Using ultrasensitive, non-invasive SQUID measurements,
we show that Dy_2Ti_2O_7 exhibits a highly anomalous noise spectrum, in three
qualitatively different regimes: equilibrium spin ice, a `frozen' regime
extending to ultra-low temperatures, as well as a high-temperature `anomalous'
paramagnet. We show that in the simplest model of spin ice, the dynamics is not
anomalous, and we present several distinct mechanisms which give rise to a
coloured noise spectrum. In addition, we identify the structure of the
single-ion dynamics as a crucial ingredient for any modelling. Thus, the
dynamics of spin ice Dy_2Ti_2O_7 reflects the interplay of local dynamics with
emergent topological degrees of freedom and a frustration-generated imperfectly
flat energy landscape, and as such should be relevant for a broad class of
magnetic materials.",2107.11379v1
2021-07-24,Electron-Phonon Scattering governs both Ultrafast and Precessional Magnetization Dynamics in Co-Fe Alloys,"Recent investigations have advanced the understanding of how
structure-property relationships in ferromagnetic metal alloys affect the
magnetization dynamics on nanosecond time-scales. A similar understanding for
magnetization dynamics on femto- to pico-second time-scales does not yet exist.
To address this, we perform time-resolved magneto optic Kerr effect (TRMOKE)
measurements of magnetization dynamics in Co-Fe alloys on femto- to nano-second
regimes. We show that Co-Fe compositions that exhibit low Gilbert damping
parameters also feature prolonged ultrafast demagnetization upon
photoexcitation. We analyze our experimental TR-MOKE data with the
three-temperature-model (3TM) and the Landau-Lifshitz-Gilbert equation. These
analyses reveal a strong compositional dependence of the dynamics across all
time-scales on the strength of electron-phonon interactions. Our findings are
beneficial to the spintronics and magnonics community, and will aid in the
quest for energy-efficient magnetic storage applications.",2107.11699v1
2023-11-28,Machine learning force-field models for metallic spin glass,"Metallic spin glass systems, such as dilute magnetic alloys, are
characterized by randomly distributed local moments coupled to each other
through a long-range electron-mediated effective interaction. We present a
scalable machine learning (ML) framework for dynamical simulations of metallic
spin glasses. A Behler-Parrinello type neural-network model, based on the
principle of locality, is developed to accurately and efficiently predict
electron-induced local magnetic fields that drive the spin dynamics. A crucial
component of the ML model is a proper symmetry-invariant representation of
local magnetic environment which is direct input to the neural net. We develop
such a magnetic descriptor by incorporating the spin degrees of freedom into
the atom-centered symmetry function methods which are widely used in ML
force-field models for quantum molecular dynamics. We apply our approach to
study the relaxation dynamics of an amorphous generalization of the s-d model.
Our work highlights the promising potential of ML models for large-scale
dynamical modeling of itinerant magnets with quenched disorder.",2311.16964v1
2016-06-01,Large Deviations of the Finite-Time Magnetization of the Curie-Weiss Random Field Ising Model,"We study the large deviations of the magnetization at some finite time in the
Curie-Weiss Random Field Ising Model with parallel updating. While relaxation
dynamics in an infinite time horizon gives rise to unique dynamical
trajectories (specified by initial conditions and governed by first-order
dynamics of the form $m_{t+1}=f(m_t)$), we observe that the introduction of a
finite time horizon and the specification of terminal conditions can generate a
host of metastable solutions obeying \textit{second-order} dynamics. We show
that these solutions are governed by a Newtonian-like dynamics in discrete time
which permits solutions in terms of both the first order relaxation (""forward"")
dynamics and the backward dynamics $m_{t+1} = f^{-1}(m_t)$. Our approach allows
us to classify trajectories for a given final magnetization as stable or
metastable according to the value of the rate function associated with them. We
find that in analogy to the Freidlin-Wentzell description of the stochastic
dynamics of escape from metastable states, the dominant trajectories may switch
between the two types (forward and backward) of first-order dynamics.",1606.00316v1
2004-07-28,Gamma-ray burst internal shocks with magnetization,"(Please note that the abstract has been significantly shorted) We investigate
Gamma-ray Burst (GRB) internal shocks with moderate magnetization, with the
magnetization parameter $\sigma$ ranging from 0.001 to 10. Possible magnetic
dissipation in the stripped magnetized shells is also taken into account
through introducing a parameter $k$ ($0<k\leq 1$), which is the ratio of the
electric field strength of the downstream and the upstream. By solving the
general MHD jump conditions, we show that the dynamic evolution of the shock
with magnetic dissipation is different from the familiar one obtained in the
ideal MHD limit. In view of the possible high degree of linear polarization of
GRB 021206 and the identification of a possible highly magnetized flow in GRB
990123 and GRB 021211, we suggest that a mildly magnetized internal shock model
($0.01<\sigma<1$) with moderate magnetic dissipation is a good candidate to
explain the GRB prompt emission data.",0407581v2
2005-12-14,Turbulent Origin of the Galactic-Center Magnetic Field: Nonthermal Radio Filaments,"A great deal of study has been carried out over the last twenty years on the
origin of the magnetic activity in the Galactic center. One of the most popular
hypotheses assumes milli-Gauss magnetic field with poloidal geometry, pervading
the inner few hundred parsecs of the Galactic-center region. However, there is
a growing observational evidence for the large-scale distribution of a much
weaker field of B \lesssim 10 micro G in this region. Here, we propose that the
Galactic-center magnetic field originates from turbulent activity that is known
to be extreme in the central hundred parsecs. In this picture the spatial
distribution of the magnetic field energy is highly intermittent, and the
regions of strong field have filamentary structures. We propose that the
observed nonthermal radio filaments appear in (or, possibly, may be identified
with) such strongly magnetized regions. At the same time, the large-scale
diffuse magnetic field is weak. Both results of our model can explain the
magnetic field measurements of the the Galactic-center region. In addition, we
discuss the role of ionized outflow from stellar clusters in producing the long
magnetized filaments perpendicular to the Galactic plane.",0512373v1
2001-05-31,Dynamic Oscillations of Magnetization in High Spin Magnetic Clusters,"We have studied the time evolution of magnetization of a high spin magnetic
cluster in the ground state, in the presence of a sinusoidal axial magnetic
field and a static transverse field by explicitly solving time dependent
schr\""odinger equation. We observe oscillations of magnetization in the plateau
region which has all the characteristics reminiscent of the oscillation of
probability of occupation of a state in a two level lattice in the presence of
an oscillating electric field. The high spin of the ground state leads to a
large but finite two-level pseudo-lattice. The oscillations in magnetization
occur at amplitude of magnetic fields achievable in a laboratory and they also
persist for a wide range of spin-dipolar interactions.
  Thus, our study provides a magnetic analog of the optical two-level lattice.",0105616v1
2004-09-20,Switching of magnetization by non-linear resonance studied in single nanoparticles,"Magnetization reversal in magnetic particles is one of the fundamental issues
in magnetic data storage. Technological improvements require the understanding
of dynamical magnetization reversal processes at nanosecond time scales. New
strategies are needed to overcome current limitations. For example, the problem
of thermal stability of the magnetization state (superparamagnetic limit) can
be pushed down to smaller particle sizes by increasing the magnetic anisotropy.
High fields are then needed to reverse the magnetization that are difficult to
achieve in current devices. Here we propose a new method to overcome this
limitation. A constant applied field, well below the switching field, combined
with a radio-frequency (RF) field pulse can reverse the magnetization of a
nanoparticle. The efficiency of this method is demonstrated on a 20 nm cobalt
particle by using the micro-SQUID technique. Other applications of this method
might be nucleation or depinning of domain walls.",0409502v1
2005-05-06,Relaxation of thermo-remanent magnetization in Fe-Cr GMR multilayers,"The time decay of the thermo-remanent magnetization (TRM) in Fe-Cr giant
magnetoresistive (GMR) multilayers has been investigated. The magnetization in
these multilayers relaxes as a function of time after being cooled in a small
magnetic field of 100 Oe to a low temperature and then the magnetic field is
switched off. Low-field ($<$ 500 Oe) magnetization studies of these samples
have shown hysteresis. This spin-glass-like behavior may originate from
structural imperfections at the interfaces and in the bulk. We find that the
magnetization relaxation is logarithmic. Here the magnetic viscosity is found
to increase first with increasing temperature, then it reaches a maximum around
T$_g$, and then it decreases with increasing temperature. This behavior is
different from that of conventional spin glasses where the logarithmic creep
rate is observed to increase with temperature. Power law also gives good fits
and it is better than the logarithmic fit at higher temperatures. The dynamical
effects of these multilayers are related to the relaxation of thermally blocked
superparamagnetic grains and magnetic domains in the film layers.",0505145v1
2007-11-13,Dynamics of charged dust particles in protoplanetary discs,"We study the effect of an imposed magnetic field on the motion of charged
dust particles in magnetically active regions of a protoplanetary disc.
Assuming a power law structure for the vertical and the toroidal components of
the magnetic field for the regions beyond magnetically dead region of the disc,
the radial and the vertical velocities of the charged particles, in the
asymptotic case of small particles, are calculated analytically. While grains
with radii smaller than a critical radius significantly are affected by the
magnetic force, motion of the particles with larger radii is independent of the
magnetic field. The critical radius depends on the magnetic geometry and the
charge of the grains. Assuming that a grain particle has one elementary charge
and the physical properties of the disc correspond to a minimum-mass solar
nebula, we show that only micron-sized grains are affected by the magnetic
force. Also, charge polarity determines direction of the radial velocity. For
such small particles, both the radial and the vertical velocities increase due
to the magnetic force.",0711.2090v2
2007-12-15,Magnetized Ekman Layer and Stewartson Layer in a Magnetized Taylor-Couette Flow,"In this paper we present axisymmetric nonlinear simulations of magnetized
Ekman and Stewartson layers in a magnetized Taylor-Couette flow with a
centrifugally stable angular-momemtum profile and with a magnetic Reynolds
number below the threshold of magnetorotational instability. The magnetic field
is found to inhibit the Ekman suction. The width of the Ekman layer is reduced
with increased magnetic field normal to the end plate. A uniformly-rotating
region forms near the outer cylinder. A strong magnetic field leads to a steady
Stewartson layer emanating from the junction between differentially rotating
rings at the endcaps. The Stewartson layer becomes thinner with larger Reynolds
number and penetrates deeper into the bulk flow with stronger magnetic field
and larger Reynolds number. However, at Reynolds number larger than a critical
value $\sim 600$, axisymmetric, and perhaps also nonaxisymmetric, instabilities
occur and result in a less prominent Stewartson layer that extends less far
from the boundary.",0712.2543v2
2008-06-18,Selective Heating Mechanism of Magnetic Metal Oxides by Alternating Magnetic Field in Microwave Sintering Process,"The mechanism of rapid and selective heating of magnetic metal oxides under
the magnetic field of microwaves which continues beyond the Curie temperature $
T_{c} $ is identified by using the Heisenberg model. Monte Carlo calculations
based on the energy principle show that such heating is caused by non-resonant
response of electron spins in the unfilled 3d shell to the wave magnetic field.
Small spin reorientation thus generated leads to a large internal energy change
through the exchange interactions between spins, which becomes maximal around $
T_{c} $ for magnetite $ {\rm Fe}_{3}{\rm O}_{4} $. The dissipative spin
dynamics simulation yields the imaginary part of the magnetic susceptibility,
which becomes largest around $ T_{c} $ and for the microwave frequency around 2
GHz. Hematite $ {\rm Fe}_{2}{\rm O}_{3} $ with weak spontaneous magnetization
responds much less to microwaves as observed in experiments. The heating of
titanium oxide by microwave magnetic field only when oxygen defects are present
is also explained by our theory in terms of the absence of spontaneous
magnetization.",0806.3055v3
2008-11-29,Magnetic fingerprints of solar and stellar oscillations,"Waves connect all the layers of the Sun, from its interior to the upper
atmosphere. It is becoming clear now the important role of magnetic field on
the wave propagation. Magnetic field modifies propagation speed of waves, thus
affecting the conclusions of helioseismological studies. It can change the
direction of the wave propagation, help channeling them straight up to the
corona, extending the dynamic and magnetic couplings between all the layers.
Modern instruments provide measurements of complex patterns of oscillations in
solar active regions and of tiny effects such as temporal oscillations of the
magnetic field. The physics of oscillations in a variety of magnetic structures
of the Sun is similar to that of pulsations of stars that posses strong
magnetic fields, such as roAp stars. All these arguments point toward a need of
systematic self-consistent modeling of waves in magnetic structures that is
able to take into account the complexity of the magnetic field configurations.
In this paper, we describe simulations of this kind, summarize our recent
findings and bring together results from the theory and observations.",0812.0042v1
2010-10-29,Nonlinear dynamos at infinite magnetic Prandtl number,"The dynamo instability is investigated in the limit of infinite magnetic
Prandtl number. In this limit the fluid is assumed to be very viscous so that
the inertial terms can be neglected and the flow is slaved to the forcing. The
forcing consist of an external forcing function that drives the dynamo flow and
the resulting Lorentz force caused by the back reaction of the magnetic field.
The flows under investigation are the Archontis flow, and the ABC flow forced
at two different scales. The investigation covers roughly three orders of
magnitude of the magnetic Reynolds number above onset. All flows show a weak
increase of the averaged magnetic energy as the magnetic Reynolds number is
increased. Most of the magnetic energy is concentrated in flat elongated
structures that produce a Lorentz force with small solenoidal projection so
that the resulting magnetic field configuration was almost force-free. Although
the examined system has zero kinetic Reynolds number at sufficiently large
magnetic Reynolds number the structures are unstable to small scale
fluctuations that result in a chaotic temporal behavior.",1010.6276v1
2011-05-31,Magnetic field dependence of the quantum tunneling of normal-superconductor interfaces in a type-I Pb superconductor,"We report experimental evidence of the effect of an applied magnetic field on
the non-thermal magnetic relaxation in a disk-shaped type-I lead
superconductor. The time evolution of the irreversible magnetization proves to
be logarithmic for a wide range of temperatures and magnetic field values along
the descending branch of the hysteresis cycle. When the intensity of the
magnetic field increases, the crossover temperature separating the thermal and
non-thermal regimes of magnetic relaxation is found to decrease, whereas the
rate at which such relaxation occurs is observed to increase. These results are
discussed in the framework of a recent model for quantum tunneling of
normal-superconductor interfaces through the distribution of pinning energy
barriers generated by structural defects in the sample, considering that the
strength of the barriers decreases with the magnetic field. A phase diagram
describing the dynamics of interfaces during flux expulsion in the intermediate
state as a function of temperature and magnetic field is constructed.",1105.6222v1
2012-06-22,The MiMeS Survey of Magnetism in Massive Stars,"The Magnetism in Massive Stars (MiMeS) survey represents a high precision
systematic search for magnetic fields in hot, massive OB stars. To date, MiMeS
Large Programs (ESPaDOnS@CFHT, Narval@TBL, HARPSpol@ESO3.6) and associated PI
programs (FORS@VLT) have yielded nearly 1200 circular spectropolarimetric
observations of over 350 OB stars. Within this sample, 20 stars are detected as
magnetic. Follow-up observations of new detections reveals (i) a large
diversity of magnetic properties, (ii) evidence for magnetic wind confinement
in optical spectra of all magnetic O stars, (iii) the presence of strong,
organized magnetic fields in all known Galactic Of?p stars, and (iv) a complete
absence of magnetic fields in classical Be stars.",1206.5163v2
2012-08-26,Thermal Stability of the Magnetization in Perpendicularly Magnetized Thin Film Nanomagnets,"Understanding the stability of thin film nanomagnets with perpendicular
magnetic anisotropy (PMA) against thermally induced magnetization reversal is
important when designing perpendicularly magnetized patterned media and
magnetic random access memories. The leading-order dependence of magnetization
reversal rates are governed by the energy barrier the system needs to surmount
in order for reversal to proceed. In this paper we study the reversal dynamics
of these systems and compute the relevant barriers using the string method of
E, Vanden-Eijnden, and Ren. We find the reversal to be often spatially
incoherent; that is, rather than the magnetization flipping as a rigid unit,
reversal proceeds instead through a soliton-like domain wall sweeping through
the system. We show that for square nanomagnetic elements the energy barrier
increases with element size up to a critical length scale, beyond which the
energy barrier is constant. For circular elements the energy barrier continues
to increase indefinitely, albeit more slowly beyond a critical size. In both
cases the energy barriers are smaller than those expected for coherent
magnetization reversal.",1208.5233v1
2013-07-12,MHD Shock-Clump Evolution with Self-Contained Magnetic Fields,"We study the interaction of strong shock waves with magnetized clumps.
Previous numerical work focused on the simplified scenario in which shocked
clumps are immersed in a globally uniform magnetic field that extends through
both the clump and the ambient medium. Here we consider the complementary
circumstance in which the field is completely self-contained within the clumps.
This could arise naturally during clump formation via dynamical or thermal
instabilities for example as magnetic field pinches off from the ambient
medium. Using our AMR MHD code AstroBEAR, we carry out a series of simulations
with magnetized clumps that have different self-contained magnetic field
configurations. We find that the clump and magnetic evolution are sensitive to
the fraction of magnetic field aligned with versus perpendicular to the shock
normal. The relative strength of magnetic pressure and tension in the different
field configurations allows us to analytically understand the different cases
of post-shock evolution. We also show how turbulence and the mixing it implies
depends of the initial field configuration and suggest ways in which observed
shock-clump morphology may be used as a proxy for identifying internal field
topologies a posteriori.",1307.3316v1
2013-09-27,Constrained non-collinear magnetism in disordered Fe and Fe-Cr alloys,"The development of quantitative models for radiation damage effects in iron,
iron alloys and steels, particularly for the high temperature properties of the
alloys, requires understanding of magnetic interactions, which control the
phase stability of ferritic-martensitic, ferritic, and austenitic steels. In
this work, disordered magnetic configurations of pure iron and Fe-Cr alloys are
investigated using Density Functional Theory (DFT) formalism, in the form of
constrained non-collinear magnetic calculations, with the objective of creating
a database of atomic magnetic moments and forces acting between the atoms. From
a given disordered atomic configuration of either pure Fe or Fe-Cr alloy, a
penalty contribution to the usual spin-polarized DFT total energy has been
calculated by constraining the magnitude and direction of magnetic moments. An
extensive database of non-collinear magnetic moment and force components for
various atomic configurations has been generated and used for interpolating the
spatially-dependent magnetic interaction parameters, for applications in
large-scale spin-lattice dynamics and magnetic Monte-Carlo simulations.",1309.7183v1
2013-10-22,The Effect of Toroidal Magnetic Field on Thickness of a Viscose-Resistive Hot Accreting Flow,"By taking into account the effect of toroidal magnetic field and its
correspond heating, we determine the thickness of advection-dominated accretion
flows. We consider an axisymetric, rotating, steady viscous-resistive,
magnetized accretion flow under an advection dominated stage. The dominant
mechanism of energy dissipation is assumed to be turbulence viscosity and
magnetic diffusivity. We adopt a self-similar assumption in the radial
direction to obtain the dynamical quantities, that is, radial, azimuthal, sound
and Alfv\' en velocities. Our results show the vertical component of magnetic
force acts in the opposite direction of gravity and compresses the disc, thus
compared with the non-magnetic case, in general the disc half-thikness,
$\Delta\theta$, significantly is reduced. On the other hand, two parameters,
appearing due to action of magnetic field and reaction of the flow, affect the
disc thickness. The first one, $\beta_0$, showing the magnetic field strength
at the equatorial plane, decreases $\Delta\theta$. The other one, $\eta_0$ is
the magnetic resistivity parameter and when it increases, $\Delta\theta$
increases, too.",1310.6317v1
2014-04-29,The AC multi-harmonic magnetic susceptibility measurement setup at the LNF-INFN,"The AC magnetic susceptibility is a fundamental method in materials science,
which allows to probe the dynamic magnetic response of magnetic materials and
superconductors. The LAMPS laboratory at the Laboratori Nazionali di Frascati
of the INFN hosts an AC multi-harmonic magnetometer that allows performing
experiments with an AC magnetic field ranging from 0.1 to 20 Gauss and in the
frequency range from 17 to 2070 Hz. A DC magnetic field from 0 to 8 T produced
by a superconducting magnet can be applied, while data may be collected in the
temperature range 4.2-300 K using a liquid He cryostat under different
temperature cycles setups. The first seven AC magnetic multi-harmonic
susceptibility components can be measured with a magnetic sensitivity of 1x10-6
emu and a temperature precision of 0.01 K. Here we will describe in detail
about schematic of the magnetometer, special attention will be dedicated to the
instruments control, data acquisition framework and the user-friendly
LabVIEW-based software platform.",1404.7389v1
2014-10-15,The Magnetic Field in the Solar Atmosphere,"This publication provides an overview of magnetic fields in the solar
atmosphere with the focus lying on the corona. The solar magnetic field couples
the solar interior with the visible surface of the Sun and with its atmosphere.
It is also responsible for all solar activity in its numerous manifestations.
Thus, dynamic phenomena such as coronal mass ejections and flares are
magnetically driven. In addition, the field also plays a crucial role in
heating the solar chromosphere and corona as well as in accelerating the solar
wind. Our main emphasis is the magnetic field in the upper solar atmosphere so
that photospheric and chromospheric magnetic structures are mainly discussed
where relevant for higher solar layers. Also, the discussion of the solar
atmosphere and activity is limited to those topics of direct relevance to the
magnetic field. After giving a brief overview about the solar magnetic field in
general and its global structure, we discuss in more detail the magnetic field
in active regions, the quiet Sun and coronal holes.",1410.4214v1
2014-12-30,Structure and Evolution of Magnetic Fields Associated with Solar Eruptions (Invited Review),"This paper reviews the studies of solar photospheric magnetic field evolution
in active regions and its relationship to solar flares. It is divided into two
topics, the magnetic structure and evolution leading to solar eruptions and the
rapid changes of photospheric magnetic field associated with eruptions. For the
first topic, we describe the magnetic complexity, new flux emergence, flux
cancellation, shear motions, sunspot rotation, and magnetic helicity injection,
which may all contribute to the storage and buildup of energy and triggering of
solar eruptions. For the second topic, we concentrate on the observations of
rapid and irreversible changes of photospheric magnetic field associated with
flares, and the implication on the restructuring of three-dimensional magnetic
field. In particular, we emphasize the recent advances in observations of
photospheric magnetic field, as state-of-the-art observing facilities (such as
Hinode and Solar Dynamic Observatory) become available. The linkage between
observations and theories and future prospectives in this research area are
also discussed.",1412.8676v1
2015-01-05,Interfacial deformation and jetting of a magnetic fluid,"An attractive technique for forming and collecting aggregates of magnetic
material at a liquid--air interface by an applied magnetic field gradient was
recently addressed theoretically and experimentally [Soft Matter, (9) 2013,
8600-8608]: when the magnetic field is weak, the deflection of the liquid--air
interface has a steady shape, while for sufficiently strong fields, the
interface destabilizes and forms a jet that extracts magnetic material.
Motivated by this work, we develop a numerical model for the closely related
problem of solving two-phase Navier--Stokes equations coupled with the static
Maxwell equations. We computationally model the forces generated by a magnetic
field gradient produced by a permanent magnet and so determine the interfacial
deflection of a magnetic fluid (a pure ferrofluid system) and the transition
into a jet. We analyze the shape of the liquid--air interface during the
deformation stage and the critical magnet distance for which the static
interface transitions into a jet. We draw conclusions on the ability of our
numerical model to predict the large interfacial deformation and the consequent
jetting, free of any fitting parameter.",1501.01000v1
2015-04-16,Magnetized plasminos in cold and hot QED plasmas,"The complete quasi-particle spectrum of a magnetized electromagnetic plasma
is systematically explored at zero and nonzero temperatures. To this purpose,
the general structure of the one-loop corrected propagator of magnetized
fermions is determined, and the dispersion relations arising from the pole of
this propagator are numerically solved. It turns out that in the lowest Landau
level, where only one spin direction is allowed, the spectrum consists of one
positively (negatively) charged fermionic mode with positive (negative) spin.
In contrast, in higher Landau levels, as an indirect consequence of the double
spin degeneracy of fermions, the spectrum consists of two massless collective
modes with left- and right-chiralities. The mechanism through which these new
collective excitations are created in a uniform magnetic field is similar to
the production mechanism of dynamical holes (plasminos) at finite temperature
and zero magnetic fields. Whereas cold magnetized plasminos appear for moderate
magnetic fields and for all positive momenta of propagating fermions, hot
magnetized plasminos appear only in the limit of weak magnetic fields and soft
momenta.",1504.04268v2
2015-07-08,Enhancement of artificial magnetism via resonant bianisotropy,"All-dielectric ""magnetic light"" nanophotonics based on high refractive index
nanoparticles allows controlling magnetic component of light at nanoscale
without having high dissipative losses. The artificial magnetic optical
response of such nanoparticles originates from circular displacement currents
excited inside those structures and strongly depends on geometry and dispersion
of optical materials. Here a new approach for increasing magnetic response via
resonant bianisotropy effect is proposed and analyzed. The key mechanism of
enhancement is based on electric-magnetic interaction between two electrically
and magnetically resonant nanoparticles of all-dielectric dimer nanoantenna. It
was shown that proper geometrical arrangement of the dimer in respect to the
incident illumination direction allows flexible control over all vectorial
components of magnetic polarizability, tailoring the later in the dynamical
range of 100 % and enhancement up to 36 % relative to performances of
standalone spherical particles. The proposed approach provides pathways for
designs of all-dielectric metamaterials and metasurfaces with strong magnetic
response.",1507.02098v1
2015-09-18,Simulation of Stacks of High Temperature Superconducting Coated Conductors Magnetized by Pulsed Field Magnetization Using Controlled Magnetic Density Distribution Coils,"High temperature superconducting (HTS) stacks of coated conductors (CCs) can
work as strong trapped field magnets (TFMs) and show potential in electrical
applications. Pulsed field magnetization (PFM) is a practical method to
magnetize such TFMs, but due to heat generation during the dynamic process, it
cannot achieve a trapped field as high as field cooling can. In this work, we
construct a 2D electromagnetic-thermal coupled model to simulate stacks of HTS
CCs with realistic laminated structures magnetized by PFM. The model considers
temperature and anisotropic magnetic field dependent Jc of HTS and other
temperature dependent thermal and electrical material properties. Based on the
model, a configuration of controlled magnetic density distribution coils is
suggested to improve the trapped field compared to that obtained by ordinary
solenoids.",1509.05541v1
2015-10-01,Dynamos at extreme magnetic Prandtl numbers: Insights from shell models,"We present an MHD shell model suitable for computation of various energy
fluxes of magnetohydrodynamic turbulence for very small and very large magnetic
Prandtl numbers $\mathrm{Pm}$; such computations are inaccessible to direct
numerical simulations. For small $\mathrm{Pm}$, we observe that both kinetic
and magnetic energy spectra scale as $k^{-5/3}$ in the inertial range, but the
dissipative magnetic energy scales as $k^{-11/3}\exp(-k/k_\eta)$. Here, the
kinetic energy at large length scale feeds the large-scale magnetic field that
cascades to small-scale magnetic field, which gets dissipated by Joule heating.
The large-$\mathrm{Pm}$ dynamo has a similar behaviour except that the
dissipative kinetic energy scales as $k^{-13/3}$. For this case, the
large-scale velocity field transfers energy to the large-scale magnetic field,
which gets transferred to small-scale velocity and magnetic fields; the energy
of the small-scale magnetic field also gets transferred to the small-scale
velocity field, and the energy thus accumulated is dissipated by the viscous
force.",1510.00122v2
2015-11-11,Electric-field Controlled Magnetization Switching in Co/Pt thin-Film Ferromagnets,"A study of dynamic and reversible voltage controlled magnetization switching
in ferromagnetic Co/Pt thin film with perpendicular magnetic anisotropy at room
temperature is presented. The change in the magnetic properties of the system
is observed in a relatively thick film of 15 nm. A surface charge is induced by
the formation of electrochemical double layer between the metallic thin film
and non-aqueous lithium LiClO4 electrolyte to manipulate the magnetism. The
change in the magnetic properties occurred by the application of an external
electric field. As the negative voltage was increased, the coercivity and the
switching magnetic field decreased thus activating magnetization switching. The
results are envisaged to lead to faster and ultra-low power magnetization
switching as compared to spin-transfer torque (STT) switching in spintronic
devices.",1511.03717v1
2016-07-07,Understanding the focusing of charged particle for 2D sheet beam in a cusped magnetic field,"The requirement of axial magnetic field for focusing and transportation of
sheet beam using cusped magnets is less as compared to solenoid magnetic fields
which is uniform. There is often some confusion about how a cusped magnetic
field focuses high current density sheet beam because it is generally
understood that non-uniform magnetic field cannot guide the particle beam along
its axis of propagation .In this paper, we perform simple analysis of the
dynamics of sheet beam in a cusped magnetic field with single electron model
and emphasize an intuitive understanding of interesting features (as beam
geometry, positioning of permanent magnets, particle radius,particle
velocity,radius of curvature of particle inside cusped magnetic field)",1607.02492v1
2016-08-11,Chiral magnetic excitations in FeGe films,"Although chiral magnetic materials have emerged as a potential ingredient in
future spintronic memory devices, there are few comprehensive studies of
magnetic properties in scalably-grown thin films. We present growth, systematic
physical and magnetic characterization, and microwave absorption spectroscopy
of B20 FeGe thin films. We also perform micromagnetic simulations and
analytical theory to understand the dynamical magnetic behavior of this
material. We find magnetic resonance features in both the helical and
field-polarized magnetic states that are well explained by micromagnetic
simulations and analytical calculations. In particular, we show the resonant
enhancement of spin waves along the FeGe film thickness that has a wave vector
matching the helical vector. Using our analytic model, we also describe the
resonance frequency of a helical magnetic state, which depends solely on its
untwisting field. Our results pave the way for understanding and manipulating
high frequency spin waves in thin-film chiral-magnet FeGe near room
temperature.",1608.03582v1
2016-11-11,Intra-layer doping effects on the high-energy magnetic correlations in NaFeAs,"We have used Resonant Inelastic X-ray Scattering (RIXS) and dynamical
susceptibility calculations to study the magnetic excitations in
NaFe$_{1-x}$Co$_x$As (x = 0, 0.03, and 0.08). Despite a relatively low ordered
magnetic moment, collective magnetic modes are observed in parent compounds (x
= 0) and persist in optimally (x = 0.03) and overdoped (x = 0.08) samples.
Their magnetic bandwidths are unaffected by doping within the range
investigated. High energy magnetic excitations in iron pnictides are robust
against doping, and present irrespectively of the ordered magnetic moment.
Nevertheless, Co doping slightly reduces the overall magnetic spectral weight,
differently from previous studies on hole-doped BaFe$_{2}$As$_{2}$, where it
was observed constant. Finally, we demonstrate that the doping evolution of
magnetic modes is different for the dopants being inside or outside the Fe-As
layer.",1611.03621v1
2017-03-10,Magnetic micro-droplet in rotating field: numerical simulation and comparison with experiment,"Magnetic droplets obtained by induced phase separation in a magnetic colloid
show a large variety of shapes when exposed to an external field. However, the
description of shapes is often limited. Here we formulate an algorithm based on
three dimensional boundary-integral equations for strongly magnetic droplets in
a high-frequency rotating magnetic field, allowing us to find their figures of
equilibrium in three dimensions. The algorithm is justified by a series of
comparisons with known analytical results. We compare the calculated
equilibrium shapes with experimental observations and find a good agreement.
The main features of these observations are the oblate-prolate transition, the
flattening of prolate shapes with the increase of magnetic field strength and
the formation of star-fish like equilibrium shapes. We show both numerically
and in experiments that the magnetic droplet behaviour may be described with a
tri-axial ellipsoid approximation. Directions for further research are
mentioned, including the dipolar interaction contribution to the surface
tension of the magnetic droplets, account for the large viscosity contrast
between the magnetic droplet and the surrounding fluid.",1703.03654v1
2017-12-18,Complex magnetic behaviour and evidence of superspin glass state in a binary intermetallic compound Er5Pd2,"A binary intermetallic compound Er5Pd2 has been investigated using dc and ac
magnetic susceptibilities, magnetic memory effect, isothermal magnetization,
non-linear dc susceptibility, heat capacity and magnetocaloric effect studies.
Interestingly, even though the compound does not show geometrical frustration
it undergoes glassy magnetic phase transition below 17.2 K. Investigation of dc
magnetisation and heat capacity data divulged absence of long-ranged magnetic
ordering. Through magnetic memory effect, time dependent magnetization and ac
susceptibility studies it was revealed that the compound undergoes glass-like
freezing below 17.2 K. Analysis of frequency dependence of this transition
temperature through scaling and Arrhenius law; along with Mydosh parameter
indicate, that the dynamics in Er5Pd2 is due to the presence of strongly
interacting superspins rather than individual spins. This phase transition was
further investigated by non-linear dc susceptibility and was characterized by
static critical exponents {\gamma} and {\delta}. Our results indicate that this
compound show signature of superspin glass at low temperature. Additionally,
both conventional and inverse magnetocaloric effect was observed with large
value of magnetic entropy change and relative cooling power. Our results
suggest that Er5Pd2 can be classified as a superspin glass system with large
magnetocaloric effect.",1712.06388v1
2018-08-14,Interplay of uniform U(1) quantum spin liquid and magnetic phases in rare earth pyrochlore magnets : a fermionic parton approach,"We study the uniform time reversal invariant $U(1)$ quantum spin liquid (QSL)
with low energy fermionic quasi-particles for rare earth pyrochlore magnets and
explore its magnetic instability employing an augmented fermionic parton mean
field theory approach. Self consistent calculations stabilise an uniform $U(1)$
QSL with both gapped and gapless parton excitations as well as fractionalised
magnetically ordered phases in an experimentally relevant part of the phase
diagram near the classical phase boundaries of the magnetically ordered phases.
The gapped QSL has a band-structure with a non-zero $Z_2$ topological
invariant. The fractionalised magnetic ordered phases bears signature of both
QSL through fermionic excitations as well as magnetic order. Thus this provides
a possible way to understand the unconventional diffuse neutron scattering in
rare-earth pyrochlores such as Yb$_2$Ti$_2$O$_7$, Er$_2$Sn$_2$O$_7$ and
Er$_2$Pt$_2$O$_7$ at low/zero external magnetic fields. We calculate the
dynamic spin structure factor to understand the nature of the diffuse
two-particle continuum.",1808.04861v1
2019-03-14,Unidirectional propulsion of planar magnetic nanomachines,"Steering of magnetic nano-/microhelices by a rotating magnetic field is
considered as a promising technique for controlled navigation of tiny objects
through viscous fluidic environments. It has been recently demonstrated that
simple geometrically achiral planar structures can also be steered efficiently.
Such planar propellers are interesting for practical reasons, as they can be
mass-fabricated using standard micro/nanolithography techniques. While planar
magnetic structures are prone to in-plane magnetization, under the effect of an
in-plane rotating magnetic field, they exhibit, at most, propulsion due to
spontaneous symmetry breaking, i.e., they can propel either parallel or
anti-parallel to the rotation axis of the field depending on their initial
orientation. Here we demonstrate that actuation by a conically rotating
magnetic field (i.e., superposition of in-plane rotating field and constant
field orthogonal to it) can yield efficient unidirectional propulsion of planar
and magnetized in-plane structures. In particular, we found that a highly
symmetrical V-shape magnetized along its symmetry axis which exhibits no net
propulsion in in-plane rotating field, exhibits unidirectional in-sync
propulsion with a constant (frequency-independent) velocity when actuated by
the conically rotating field.",1903.06298v2
2019-05-10,Periodicity of magnetization reversals in $\varphi_0$ Josephson junction,"The magnetization reversal in ${\varphi_0}$-Josephson junction with direct
coupling between magnetic moment and Josephson current has been studied. By
adding pulse signal, the dynamics of magnetic moment components have been
simulated and the full magnetization reversal at different parameters of the
junction has been demonstrated. We obtain a detailed pictures representing the
intervals of the damping parameter $\alpha$, Josephson to magnetic energy
relation $G$ and spin-orbit coupling parameter $r$ with full magnetization
reversal. A periodicity in the appearance of magnetization reversal intervals
with increase in Josephson to magnetic energy relation is found. The obtained
results might be used in different fields of superconducting spintronics.",1905.03895v1
2019-08-07,Deformation of flexible ferromagnetic filaments under a rotating magnetic field,"Research on magnetic particles dispersed in a fluid medium, actuated by a
rotating magnetic field, is becoming increasingly active for both lab-on-chip
and bio-sensing applications. In this study, we experimentally investigate the
behaviour of ferromagnetic filaments in a rotating field. Filaments are
synthesized by linking micron-sized ferromagnetic particles with DNA strands.
The experiments were conducted under different magnetic field strengths,
frequencies and filament sizes, and deformation of the filaments was registered
via microscope and camera. The results obtained showed that the body
deformation is larger for longer filaments and higher frequencies and lower for
larger magnetic field. The angle between the filament tangent at the centre and
the magnetic field direction increases linearly with frequency at low-frequency
regime. A further increase in the frequency will result in filament movement
out of plane when the angle approaches 90 degrees. The experimental results
were used to estimate magnetic moment and the bending elasticity of the
filament.",1908.02604v2
2019-08-14,"Rivalry of diffusion, external field and gravity in micro-convection of magnetic colloids","Magnetic fields and magnetic materials have promising microfluidic
applications. For example, magnetic micro-convection can enhance mixing
considerably. However, previous studies have not explained increased effective
diffusion during this phenomenon. Here we show that enhanced interface smearing
comes from a gravity induced convective motion within a thin microfluidic
channel, caused by a small density difference between miscible magnetic and
non-magnetic fluids. This motion resembles diffusive behavior and can be
described with an effective diffusion coefficient. We explain this with a
theoretical model, based on a dimensionless gravitational Rayleigh number, and
verify it by numerical simulations and experiments with different cell
thicknesses. Results indicate the applicability and limitations for
microfluidic applications of other colloidal systems. Residual magnetic
micro-convection follows earlier predictions.",1908.05011v2
2019-10-25,The influence of crosslinkers and magnetic particle distribution along the filament backbone on the magnetic properties of supracolloidal linear polymer-like chains,"Diverse polymer crosslinking techniques allow the synthesis of linear
polymer-like structures whose monomers are colloidal particles. In the case
where all or part of these colloidal particles are magnetic, one can control
the behaviour of these supracolloidal polymers, known as magnetic filaments
(MFs), by applied magnetic fields. However, the response of MFs strongly
depends on the crosslinking procedure. In the present study, we employ Langevin
dynamics simulations to investigate the influence of the type of crosslinking
and the distribution of magnetic particles within MFs on their response to an
external magnetic field. We found that if the rotation of the dipole moment of
particles is not coupled to the backbone of the filament, the impact of the
magnetic content is strongly decreased.",1910.11607v1
2019-11-10,Dipolar-drift and collective instabilities of skyrmions in crossed nonuniform electric and magnetic fields in a chiral magnetic insulator,"Skyrmions dynamics in the approximation of point-particle skyrmions is
considered in the multiferroic insulators, where the spiral magnetic structure
is accompanied by finite electric dipole moment. The skyrmion motion is studied
in the gradients of perpendicular magnetic and electric fields. The nonuniform
magnetic field with a fixed gradient magnitude leads to the spiral motion,
where skyrmion circulates in the plane perpendicular to the direction of
magnetic field. The frequency of circulation is fixed by the derivative of the
external magnetic field and the magnitude of the dipole moment. Application of
additional electric field which perpendicular to the magnetic field, but their
gradients have same direction leads to the drift motion of skyrmions resulting
in the magneto-dipolar Hall effect. Hydrodynamics of the gas of interacting
skyrmions in crossed nonuniform electric and magnetic fields shows the new type
of hydrodynamical instability caused by the drift motion. Three regimes of
instability are described.",1911.03986v1
2020-01-04,Magnetic billiards: Non-integrability for strong magnetic field; Gutkin type examples,"We consider magnetic billiards under a strong constant magnetic field. The
purpose of this paper is two-folded. We examine the question of existence of
polynomial integral of billiard magnetic flow. We succeed to reduce this
question to algebraic geometry test on existence of polynomial integral, which
shows polynomial non-integrability for all but finitely many values of the
magnitude. In the second part of the paper we construct examples of magnetic
billiards which have the so called $\delta$-Gutkin property, meaning that any
Larmor circle entering the domain with angle $\delta$ exits the domain with the
same angle $\delta$. For ordinary Birkhoff billiard in the plane such examples
were introduced by E. Gutkin and are very explicit. Our construction of Gutkin
magnetic billiards relies on beautiful examples by F.Wegner of the so called
Zindler curves, which are related to the problem of floating bodies in
equilibrium, which goes back to S.Ulam. We prove that Gutkin magnetic billiard
can be obtained as a parallel curve to a Wegner curve. Wegner curves can be
written by elliptic functions in polar coordinates so the construction of
magnetic Gutkin billiard is rather explicit but much more complicated.",2001.02119v1
2020-02-04,Magnetically controlled vector based on E coli Nissle 1917,"In this paper, it is first discovered that EcN has natural magnetically
controlled properties, that is, E coli Nissle 1917 cells move in a gradient
magnetic field of permanent magnets without artificial magnetic labeling. It is
also shown in the paper that the cultivation of EcN in a medium enriched with
iron chelates and under the influence of an external magnetic field increases
several times the magnetophoretic mobility of E coli Nissle 1917 cells compared
to cultivation under standard conditions. Thus, the speed of movement of E coli
Nissle 1917 cells in a gradient magnetic field of laboratory magnets is
achieved of the order of several mm/s. It has also been shown for the first
time that the cultivation of E coli Nissle 1917 biomass is accelerated by
cultivation in an external magnetic field, while the change in the
concentration of iron chelates in the medium has no effect on the cultivation
dynamics of the E coli Nissle 1917 culture.",2002.01958v1
2020-03-26,Spin textures in chiral magnetic monolayers with suppressed nearest-neighbor exchange,"High tunability of two dimensional magnetic materials (by strain, gating,
heterostructuring or otherwise) provides unique conditions for studying
versatile magnetic properties and controlling emergent magnetic phases.
Expanding the scope of achievable magnetic phenomena in such materials is
important for both fundamental and technological advances. Here we perform
atomistic spin-dynamics simulations to explore the (chiral) magnetic phases of
atomic monolayers in the limit of suppressed first-neighbors exchange
interaction. We report the rich phase diagram of exotic magnetic
configurations, obtained for both square and honeycomb lattice symmetries,
comprising coexistence of ferromagnetic and antiferromagnetic spin-cycloids, as
well as multiple types of magnetic skyrmions. We perform a minimum-energy path
analysis for the skyrmion collapse to evaluate the stability of such
topological objects, and reveal that magnetic monolayers could be good
candidates to host the antiferromagnetic skyrmions that are experimentally
evasive to date.",2003.11933v1
2020-04-21,Magnetized particle motion around 4-D Einstein-Gauss-Bonnet Black Hole,"In this paper, we have investigated the dynamics of magnetized particles
around 4-D Einstein-Gauss-Bonnet black hole immersed in an external
asymptotically uniform magnetic field. We have shown that the magnetic
interaction parameter responsible for circular orbits decreases for negative
values of the Gauss-Bonnet parameter $\alpha$ and the range where magnetized
particle's stable circular orbits are allowed increases for the positive values
of the parameter $\alpha$. The study of the collisions of magnetized, charged
and neutral particles has shown that the center-of-mass energy of the particles
increases in the presence of positive Gauss-Bonnet parameter. Finally, we show
how the magnetic interaction and Gauss-Bonnet parameter may mimic the effect of
rotation of the Kerr black hole giving the same ISCO radius for magnetized
particles. Detailed analysis of the ISCO show that spin of Kerr black hole can
not be mimicked by the effects of magnetic interaction and the Gauss-Bonnet
parameters when $\alpha<-4.37$ and the spin parameter $a > 0.237$.",2004.10031v1
2020-04-28,A new 3D model for magnetic particle imaging using realistic magnetic field topologies for algebraic reconstruction,"We derive a new 3D model for magnetic particle imaging (MPI) that is able to
incorporate realistic magnetic fields in the reconstruction process. In real
MPI scanners, the generated magnetic fields have distortions that lead to
deformed magnetic low-field volumes (LFV) with the shapes of ellipsoids or
bananas instead of ideal field-free points (FFP) or lines (FFL), respectively.
Most of the common model-based reconstruction schemes in MPI use however the
idealized assumption of an ideal FFP or FFL topology and, thus, generate
artifacts in the reconstruction. Our model-based approach is able to deal with
these distortions and can generally be applied to dynamic magnetic fields that
are approximately parallel to their velocity field. We show how this new 3D
model can be discretized and inverted algebraically in order to recover the
magnetic particle concentration. To model and describe the magnetic fields, we
use decompositions of the fields in spherical harmonics. We complement the
description of the new model with several simulations and experiments.",2004.13357v1
2020-09-07,Time-Domain Terahertz Spectroscopy in High Magnetic Fields,"There are a variety of elementary and collective terahertz-frequency
excitations in condensed matter whose magnetic field dependence contains
significant insight into the states and dynamics of the electrons involved.
Often, determining the frequency, temperature, and magnetic field dependence of
the optical conductivity tensor, especially in high magnetic fields, can
clarify the microscopic physics behind complex many-body behaviors of solids.
While there are advanced terahertz spectroscopy techniques as well as high
magnetic field generation techniques available, combination of the two has only
been realized relatively recently. Here, we review the current state of
terahertz time-domain spectroscopy experiments in high magnetic fields. We
start with an overview of time-domain terahertz detection schemes with a
special focus on how they have been incorporated into optically accessible
high-field magnets. Advantages and disadvantages of different types of magnets
in performing terahertz time-domain spectroscopy experiments are also
discussed. Finally, we highlight some of the new fascinating physical phenomena
that have been revealed by terahertz time-domain spectroscopy in high magnetic
fields.",2009.03359v2
2020-10-12,Peculiarities of magnetic ordering in the S = 5/2 two-dimensional square-lattice antimonate NaMnSbO4,"An orthorhombic compound, NaMnSbO4, represents a square net of magnetic Mn2+
ions residing in vertex-shared oxygen octahedra. Its static and dynamic
magnetic properties were studied using magnetic susceptibility, specific heat,
magnetization, electron spin resonance (ESR), nuclear magnetic resonance (NMR)
and density functional calculations. Thermodynamic data indicate an
establishment of the long-range magnetic order with TN about 44 K, which is
preceded by a short-range one at about 55 K. In addition, a non-trivial
wasp-waisted hysteresis loop of the magnetization was observed, indicating that
the ground state is most probably canted antiferromagnetic. Temperature
dependence of the magnetic susceptibility is described reasonably well in the
framework of 2D square lattice model with the main exchange parameter J = -5.3
K, which is in good agreement with density functional analysis, NMR and ESR
data.",2010.05653v1
2020-10-14,Spin torque gate magnetic field sensor,"Spin-orbit torque provides an efficient pathway to manipulate the magnetic
state and magnetization dynamics of magnetic materials, which is crucial for
energy-efficient operation of a variety of spintronic devices such as magnetic
memory, logic, oscillator, and neuromorphic computing. Here, we describe and
experimentally demonstrate a strategy for the realization of a spin torque gate
magnetic field sensor with extremely simple structure by exploiting the
longitudinal field dependence of the spin torque driven magnetization
switching. Unlike most magnetoresistance sensors which require a delicate
magnetic bias to achieve a linear response to the external field, the spin
torque gate sensor can achieve the same without any magnetic bias, which
greatly simplifies the sensor structure. Furthermore, by driving the sensor
using an ac current, the dc offset is automatically suppressed, which
eliminates the need for a bridge or compensation circuit. We verify the concept
using the newly developed WTe2/Ti/CoFeB trilayer and demonstrate that the
sensor can work linearly in the range of 3-10 Oe with negligible dc offset.",2010.07158v1
2021-01-29,Data-driven MHD simulation of successive solar plasma eruptions,"Solar flares and plasma eruptions are sudden releases of magnetic energy
stored in the plasma atmosphere. To understand the physical mechanisms
governing their occurrences, three-dimensional magnetic fields from the
photosphere up to the corona must be studied. The solar photospheric magnetic
fields are observable, whereas the coronal magnetic fields cannot be measured.
One method for inferring coronal magnetic fields is performing data-driven
simulations, which involves time-series observational data of the photospheric
magnetic fields with the bottom boundary of magnetohydrodynamic simulations. We
developed a data-driven method in which temporal evolutions of the
observational vector magnetic field can be reproduced at the bottom boundary in
the simulation by introducing an inverted velocity field. This velocity field
is obtained by inversely solving the induction equation and applying an
appropriate gauge transformation. Using this method, we performed a data-driven
simulation of successive small eruptions observed by the Solar Dynamics
Observatory and the Solar Magnetic Activity Telescope in November 2017. The
simulation well reproduced the converging motion between opposite-polarity
magnetic patches, demonstrating successive formation and eruptions of helical
flux ropes.",2101.12395v1
2021-02-04,Nanoscale magnetization and current imaging using scanning-probe magneto-thermal microscopy,"Magnetic microscopy that combines nanoscale spatial resolution with
picosecond scale temporal resolution uniquely enables direct observation of the
spatiotemporal magnetic phenomena that are relevant to future high-speed,
high-density magnetic storage and logic technologies. Magnetic microscopes that
combine these metrics has been limited to facility-level instruments. To
address this gap in lab-accessible spatiotemporal imaging, we develop a
time-resolved near-field magnetic microscope based on magneto-thermal
interactions. We demonstrate both magnetization and current density imaging
modalities, each with spatial resolution that far surpasses the optical
diffraction limit. In addition, we study the near-field and time-resolved
characteristics of our signal and find that our instrument possesses a spatial
resolution on the scale of 100 nm and a temporal resolution below 100 ps. Our
results demonstrate an accessible and comparatively low-cost approach to
nanoscale spatiotemporal magnetic microscopy in a table-top form to aid the
science and technology of dynamic magnetic devices with complex spin textures.",2102.02792v1
2021-02-23,Magnetic charge's relaxation propelled electricity in two-dimensional magnetic honeycomb lattice,"Emerging new concepts, such as magnetic charge dynamics in two-dimensional
magnetic material, can provide novel mechanism for spin based electrical
transport at macroscopic length. In artificial spin ice of single domain
elements, magnetic charge's relaxation can create an efficient electrical
pathway for conduction by generating fluctuations in local magnetic field that
couple with conduction electrons spins. In a first demonstration, we show that
the electrical conductivity is propelled by more than an order of magnitude at
room temperature due to magnetic charge defects sub-picosecond relaxation in
artificial magnetic honeycomb lattice. The direct evidence to the proposed
electrical conduction mechanism in two-dimensional frustrated magnet points to
the untapped potential for spintronic applications in this system.",2102.11949v1
2022-01-27,Spin motive force by the momentum-space Berry phase in magnetic Weyl semimetals,"We show that the magnetic precession of ferromagnetic moments in a
noncentrosymmetric magnetic Weyl semimetal induces an electric current through
a mechanism analogous to the adiabatic charge pumping. The current is a
consequence of a Berry phase in the momentum space resulting from the circular
motion of Weyl nodes induced by the precession. This mechanism resembles the
Faraday effect, namely, induced magnetic field by circular electric current.
The circular motion of Weyl nodes induces magnetic charge current in the
momentum space, which results in a Berry phase that describes the adiabatic
pump. Experimentally, this phenomenon is similar to spin motive force, which is
an electric current induced by magnetic precision in the presence of the
spatial gradient of magnetization. However, unlike the conventional spin motive
force, this current occurs without a magnetization gradient. The result
demonstrates a nontrivial interplay between the topological electronic state
and magnetic dynamics.",2201.11568v2
2024-02-16,Nonlinear optics driven magnetism reorientation in semiconductors,"Based on nonlinear optics, we develop a band theory to elucidate how light
could manipulate magnetization, which is rooted by the quantum geometric
structure and topological nature of electronic wavefunctions. Their existence
are determined by the light polarization and specific material symmetry, based
on the magnetic group theory. In general, both circularly and linearly
polarized light could exert an effective magnetic field and torque effect, to
reorient the magnetization. They are contributed by spin and orbital angular
momenta simultaneously. Aided by group theory and first-principles
calculations, we illustrate this theory using a showcase example of monolayer
NiCl2, showing that light irradiation effectively generates an out-of-plane
effective magnetic torque, which lifts its in-plane easy magnetization.
According to magnetic dynamic simulations, the in-plane magnetization could be
switched to the out-of-plane direction in a few nanoseconds under a modest
light intensity, demonstrating its ultrafast nature desirable for quantum
manipulation.",2402.10518v1
2012-02-07,Nonequilibrium critical dynamics with domain wall and surface,"With Monte Carlo simulations, we investigate the relaxation dynamics with a
domain wall for magnetic systems at the critical temperature. The dynamic
scaling behavior is carefully analyzed, and a dynamic roughening process is
observed. For comparison, similar analysis is applied to the relaxation
dynamics with a free or disordered surface.",1202.1338v1
1998-03-15,Mean magnetic field renormalization and Kolmogorov's energy spectrum in MHD turbulence,"In this paper we construct a self-consistent renormalization group procedure
for MHD turbulence in which small wavenumber modes are averaged out, and
effective mean magnetic field at large wavenumbers is obtained. In this scheme
the mean magnetic field scales as $k^{-1/3}$, while the energy spectrum scales
as $k^{-5/3}$ similar to that in fluid turbulence. We also deduce from the
formalism that the magnitude of cascade rate decreases as the strength of the
mean magnetic field is increased.",9803021v1
1999-12-13,The boundary integral method for magnetic billiards,"We introduce a boundary integral method for two-dimensional quantum billiards
subjected to a constant magnetic field. It allows to calculate spectra and wave
functions, in particular at strong fields and semiclassical values of the
magnetic length. The method is presented for interior and exterior problems
with general boundary conditions. We explain why the magnetic analogues of the
field-free single and double layer equations exhibit an infinity of spurious
solutions and how these can be eliminated at the expense of dealing with
(hyper-)singular operators. The high efficiency of the method is demonstrated
by numerical calculations in the extreme semiclassical regime.",9912022v1
1997-05-22,Generalized Dynamic Scaling for Critical Magnetic Systems,"The short-time behaviour of the critical dynamics for magnetic systems is
investigated with Monte Carlo methods. Without losing the generality, we
consider the relaxation process for the two dimensional Ising and Potts model
starting from an initial state with very high temperature and arbitrary
magnetization. We confirm the generalized scaling form and observe that the
critical characteristic functions of the initial magnetization for the Ising
and the Potts model are quite different.",9705233v1
1998-11-11,Dynamical Mean Field Theory of the Antiferromagnetic Metal to Antiferromagnetic Insulator Transition,"We study the antiferromagnetic metal to antiferromagnetic insulator using
dynamical mean field theory and exact diagonalization methods. We find two
qualitatively different behaviors depending on the degree of magnetic
correlations. For strong correlations combined with magnetic frustration, the
transition can be described in terms of a renormalized slater theory, with a
continuous gap closure driven by the magnetism but strongly renormalized by
correlations. For weak magnetic correlations, the transition is weakly first
order.",9811144v2
1999-03-11,Localization by interference: Square billiard with a magnetic flux,"Eigenstates and energy levels of a square quantum billiard in a magnetic
field, or with an Aharonov-Bohm flux line, are found in quasiclassical
approximation, that is, for high enough energy. Explicit formulas for the
energy levels and wavefunctions are found. There are localized states, never
before noticed in this well studied problem, whose localization is due to phase
interference, even though there is no or negligible classical effect of the
magnetic field. These and related states account almost entirely for the
magnetic response in certain temperature ranges, and thus have a bearing on the
experiments of Levy et al.",9903193v1
1999-09-17,"Commensurate dynamic magnetic correlations in La2(Cu,Li)O4","When sufficient numbers of holes are introduced into the two-dimensional CuO2
square lattice, dynamic magnetic correlations become incommensurate with
underlying lattice in all previously investigated La_{2-x}A_xCu_{1-z}B_zO_{4+y}
(A=Sr or Nd, B=Zn) including high T_C superconductors and insulators, and in
bilayered superconducting YBa_2Cu_3O_{6.6} and Bi_2Sr_2CaCu_2O_8. Magnetic
correlations also become incommensurate in structurally related La_2NiO_4 when
doped with Sr or O. We report an exception to this so-far well established
experimental ""rule"" in La_2Cu_{1-z}Li_{z}O_4 in which magnetic correlations
remain commensurate.",9909256v2
2000-01-13,Quantum Dynamics in Nanoscale Magnets in Dissipative Environments,"In discrete energy structure of nanoscale magnets, nonadiabatic transitions
at avoided level crossings lead to fundamental processes of dynamics of
magnetizations. The thermal environment causes dissipative effects on these
processes. In this paper we review the features of the nonadiabatic transition
and the influence of the thermal environment. In particular we discuss the
temperature independent stepwise structure of magnetization at very low
temperatures (deceptive nonadiabatic transition), the alternate enhancement of
relaxation in the sequence of resonant tunneling points (parity effects), and
processes caused by combinations of nonadiabatic transitions and disturbance
due to external noises.",0001179v1
2001-06-22,Transition temperature of ferromagnetic semiconductors: a dynamical mean field study,"We formulate a theory of doped magnetic semiconductors such as
Ga$_{1-x}$Mn$_x$As which have attracted recent attention for their possible use
in spintronic applications. We solve the theory in the dynamical mean field
approximation to find the magnetic transition temperature $T_c$ as a function
of magnetic coupling strength $J$ and carrier density $n$. We find that $T_c$
is determined by a subtle interplay between carrier density and magnetic
coupling.",0106455v1
2002-12-05,Dynamic stiffness of spin valves,"The dynamics of the magnetic order parameters of
ferromagnet/normal-metal/ferromagnet spin valves and isolated ferromagnets may
be very different. We investigate the role of the nonequilibrium spin-current
exchange between the ferromagnets in the magnetization precession and
switching. We find a (low-temperature) critical current bias for a coherent
current-induced magnetization excitation in spin valves, which unifies and
generalizes previous ideas of Slonczewski and Berger. In the absence of an
applied bias, the effect of the spin transfer can be expressed as
magnetic--configuration-dependent Gilbert damping.",0212130v2
2003-03-18,Dynamics of magnetic moments of a nanoscopic array,"Dynamics of nanoscopic arrays of monodomain magnetic elements is simulated by
means of the Pardavi-Horvath algorithm. Experimental hysteresis loop is
reproduced for the arrays of Ni, with the period 100 nm and the mean coercive
field 710 Oe.We investigate the box-counting fractal dimension of a cluster of
elements with given orientation of magnetic moments. No fractal behavior is
found. Also, the damage spreading technique is applied to check the
criticality. We find that the consequences of a local flip of one magnetic
element remainlimited to a finite area. We conclude that the system does not
show a critical behavior.",0303330v2
2003-06-06,Projective Dynamics Analysis of Magnetization Reversal,"The computational Projective Dynamics method is used to analyze simulations
of magnetization reversal in nanoscale magnetic pillars. It is shown that this
method can be used to determine the magnetizations corresponding to the
metastable minimum and saddle point in the free energy, and the free-energy
barrier associated with those points. For the nanopillars studied here, entropy
is found to provide a significant contribution to the free-energy barrier which
determines the reversal time scale.",0306168v1
2003-07-18,Dynamics of a nanoparticle as a one-spin system and beyond,"We review some recent results beyond the now established theory of
magnetization switching of a nanoparticle within the single-spin approximation.
The first extension is that of the Stoner-Wohlfarth model for magnetization
static switching under applied magnetic field including the effect of
temperature at long-time scales. The second concerns a generalization of the
N\'eel-Brown model for thermoactivated dynamic magnetization switching to
include the effect of exchange interaction in the framework of Langer's theory
in the intermediate-to-high damping limit. We finally argue why the single-spin
approximation is not appropriate for very small nanoparticles.",0307457v1
2004-10-20,Static properties and spin dynamics of the ferromagnetic spin-1 Bose gas in magnetic field,"Properties of spin-1 Bose gases with ferromagnetic interaction in the
presence of a nonzero magnetic field are studied. The equation of state and
thermodynamic quantities are worked out with the help of a mean-field
approximation. The phase diagram besides Bose-Einstein condensation contains a
first order transition where two values of the magnetization coexist. The
dynamics is investigated with the help of the Random Phase Approximation. The
soft mode corresponding to the critical point of the magnetic phase transition
is found to behave like in conventional theory.",0410501v1
2005-03-02,Entropic sampling dynamics of the globally-coupled kinetic Ising model,"The entropic sampling dynamics based on the reversible information transfer
to and from the environment is applied to the globally coupled Ising model in
the presence of an oscillating magnetic field. When the driving frequency is
low enough, coherence between the magnetization and the external magnetic field
is observed; such behavior tends to weaken with the system size. The time-scale
matching between the intrinsic time scale, defined in the absence of the
external magnetic field, and the extrinsic time scale, given by the inverse of
the driving frequency, is used to explain the observed coherence behavior.",0503047v1
2005-07-29,Magnetic flux dynamics in critical state of one-dimensional discrete superconductor,"We give a theoretical description of avalanche-like dynamics of magnetic flux
in the critical state of ""hard"" type-II superconductors using a model of a
one-dimensional multijunction SQUID that well reproduces the main magnetic
properties of these objects. We show that the system under consideration
demonstrates the self-organized criticality. The avalanches of vortices
manifest themselves as jumps of the total magnetic flux in the sample. The
sizes of these jumps have a power-law distribution. Our results are in
qualitative agreement with experiments.",0507703v1
2005-08-07,Ultrafast light-induced magnetization dynamics in ferromagnetic semiconductors,"We develop a theory of the magnetization dynamics triggered by ultrafast
optical excitation of ferromagnetic semiconductors. We describe the effects of
the strong carrier spin relaxation on the nonlinear optical response by using
the Lindblad semigroup method. We demonstrate magnetization control during
femtosecond timescales via the interplay between circularly polarized optical
excitation, hole-spin damping, polarization dephasing, and the Mn-hole spin
interactions. Our results show a light-induced magnetization precession and
relaxation for the duration of the optical pulse.",0508178v1
2005-11-07,Current-induced switching in single ferromagnetic layer nanopillar junctions,"Current induced magnetization dynamics in asymmetric Cu/Co/Cu single magnetic
layer nanopillars has been studied experimentally at room temperature and in
low magnetic fields applied perpendicular to the thin film plane. In sub-100 nm
junctions produced using a nanostencil process a bistable state with two
distinct resistance values is observed. Current sweeps at fixed applied fields
reveal hysteretic and abrupt transitions between these two resistance states.
The current induced resistance change is 0.5%, a factor of 5 greater than the
anisotropic magnetoresistance (AMR) effect. We present an experimentally
obtained low field phase diagram of current induced magnetization dynamics in
single ferromagnetic layer pillar junctions.",0511144v1
2000-03-13,Hyperbolic Magnetic Billiards on Surfaces of Constant Curvature,"We consider classical billiards on surfaces of constant curvature, where the
charged billiard ball is exposed to a homogeneous, stationary magnetic field
perpendicular to the surface.
  We establish sufficient conditions for hyperbolicity of the billiard
dynamics, and give lower estimation for the Lyapunov exponent. This extends our
recent results for non-magnetic billiards on surfaces of constant curvature.
Using these conditions, we construct large classes of magnetic billiard tables
with positive Lyapunov exponents on the plane, on the sphere and on the
hyperbolic plane.",0003033v1
2004-11-24,Stochastic Dynamics of Magnetosomes in Cytoskeleton,"Rotations of microscopic magnetic particles, magnetosomes, embedded into the
cytoskeleton and subjected to the influence of an ac magnetic field and thermal
noise are considered. Magnetosome dynamics is shown to comply with the
conditions of the stochastic resonance under not-too-tight constraints on the
character of the particle's fastening. The excursion of regular rotations
attains the value of order of radian that facilitates explaining the biological
effects of low-frequency weak magnetic fields and geomagnetic fluctuations.
Such 1-rad rotations are effectively controlled by slow magnetic field
variations of the order of 200 nT.",0411225v1
2004-12-03,Numerical solutions of the three-dimensional magnetohydrodynamic alpha-model,"We present direct numerical simulations and alpha-model simulations of four
familiar three-dimensional magnetohydrodynamic (MHD) turbulence effects:
selective decay, dynamic alignment, inverse cascade of magnetic helicity, and
the helical dynamo effect. The MHD alpha-model is shown to capture the
long-wavelength spectra in all these problems, allowing for a significant
reduction of computer time and memory at the same kinetic and magnetic Reynolds
numbers. In the helical dynamo, not only does the alpha-model correctly
reproduce the growth rate of magnetic energy during the kinematic regime, but
it also captures the nonlinear saturation level and the late generation of a
large scale magnetic field by the helical turbulence.",0412025v1
2007-05-14,The dynamical response to the node defect in thermally activated remagnetization of magnetic dot array,"The influence of nonmagnetic central node defect on dynamical properties of
regular square-shaped 5 x 5 segment of magnetic dot array under the thermal
activation is investigated via computer simulations. Using stochastic
Landau-Lifshitz-Gilbert equation we simulate hysteresis and relaxation
processes. The remarkable quantitative and qualitative differences between
magnetic dot arrays with nonmagnetic central node defect and magnetic dot
arrays without defects have been found.",0705.1889v1
2008-01-11,Thermal Dynamics in Symmetric Magnetic Nanopillars Driven by Spin Transfer,"We study the effects of spin transfer on thermally activated dynamics of
magnetic nanopillars with identical thicknesses of the magnetic layers. The
symmetric nanopillars exhibit anomalous dependencies of switching statistics on
magnetic field and current. We interpret our data in terms of simultaneous
current-induced excitation of both layers. We also find evidence for coupling
between the fluctuations of the layers due to the spin transfer.",0801.1807v1
2008-08-26,Dynamical pinning of domain wall in magnetic nanowire induced by Walker breakdown,"Transmission probability of a domain wall through a magnetic nanowire is
investigated as a function of the external magnetic field. Very intriguing
phenomenon is found that the transmission probability shows a significant drop
after exceeding the threshold driving field, which contradicts our intuition
that a domain wall is more mobile in the higher magnetic field. The
micromagnetics simulation reveals that the domain wall motion in the wire with
finite roughness causes the dynamical pinning due to the Walker breakdown,
which semi-quantitatively explains our experimental results.",0808.3446v1
2008-09-03,Detection of coherent magnons via ultrafast pump-probe reflectance spectroscopy in multiferroic Ba0.6Sr1.4Zn2Fe12O22,"We report the detection of a magnetic resonance mode in multiferroic
Ba0.6Sr1.4Zn2Fe12O22 using time domain pump-probe reflectance spectroscopy.
Magnetic sublattice precession is coherently excited via picosecond thermal
modification of the exchange energy. Importantly, this precession is recorded
as a change in reflectance caused by the dynamic magnetoelectric effect. Thus,
transient reflectance provides a sensitive probe of magnetization dynamics in
materials with strong magnetoelectric coupling, such as multiferroics,
revealing new possibilities for application in spintronics and ultrafast
manipulation of magnetic moments.",0809.0672v1
2009-01-23,Non-equilibrium reversible dynamics of work production in four-spin system in a magnetic field,"A closed system of the equations for the local Bloch vectors and spin
correlation functions is obtained by decomplexification of the Liouville-von
Neumann equation for 4 magnetic particles with the exchange interaction that
takes place in an arbitrary time-dependent external magnetic field. The
analytical and numerical analysis of the quantum thermodynamic variables is
carried out depending on separable mixed initial state and the magnetic field
modulation. Under unitary evolution, non-equilibrium reversible dynamics of
power production in the finite environment is investigated.",0901.3637v3
2009-02-03,Dependence of critical current of spin transfer torque-driven magnetization dynamics on free layer thickness,"The dependence of the critical current of spin transfer torque-driven
magnetization dynamics on the free-layer thickness was studied by taking into
account both the finite penetration depth of the transverse spin current and
spin pumping. We showed that the critical current remains finite in the
zero-thickness limit of the free layer for both parallel and anti-parallel
alignments. We also showed that the remaining value of the critical current of
parallel to anti-parallel switching is larger than that of anti-parallel to
parallel switching.",0902.0420v1
2009-08-02,Nonlinear response of a thin metamaterial film containing Josephson junctions,"An interaction of electromagnetic field with metamaterial thin film
containing split-ring resonators with Josephson junctions is considered. It is
shown that dynamical self-inductance in a split rings results in reduction of
magnetic flux through a ring and this reduction is proportional to a time
derivative of split ring magnetization. Evolution of thin film magnetization
taking into account dynamical self-inductance is studied. New mechanism for
excitation of waves in one dimensional array of split-ring resonators with
Josephson junctions is proposed. Nonlinear magnetic susceptibility of such thin
films is obtained in the weak amplitude approximation.",0908.0103v1
2009-12-15,Trapped low magnetic field dynamics in YBCO single crystals,"Trapped low magnetic flux dynamics including local ones are investigated in
YBCO single crystals in the strong thermal fluctuations domain near the
superconducting phase transition temperatures. The essential difference from
quasi-logarithmic isothermal relaxation of a magnetization behavior (observed
earlier in high magnetic fields) was established. Within the framework of
classic model of a thermal activated vortices creep the estimation of an
effective pinning potential was made at these temperatures.",0912.2835v1
2010-01-15,Periodic magnetic geodesics on almost every energy level via variational methods,"For strong exact magnetic fields the action functional (i.e., the length plus
the linear magnetic term) is not bounded from below on the space of closed
contractible curves and the lower estimates for critical levels are derived by
using the principle of throwing out cycles. It is proved that for almost every
energy level the principle of throwing out cycles gives periodic magnetic
geodesics on the critical levels defined by the ""thrown out"" cycles.",1001.2677v1
2010-11-03,Convection and dynamo action in B stars,"Main-sequence massive stars possess convective cores that likely harbor
strong dynamo action. To assess the role of core convection in building
magnetic fields within these stars, we employ the 3-D anelastic spherical
harmonic (ASH) code to model turbulent dynamics within a 10 solar mass
main-sequence (MS) B-type star rotating at 4 times the solar rate. We find that
strong (900 kG) magnetic fields arise within the turbulence of the core and
penetrate into the stably stratified radiative zone. These fields exhibit
complex, time-dependent behavior including reversals in magnetic polarity and
shifts between which hemisphere dominates the total magnetic energy.",1011.1016v1
2011-03-30,Wave turbulence on the surface of a ferrofluid in a horizontal magnetic field,"We report observations of wave turbulence on the surface of a ferrofluid
submitted to a magnetic field parallel to the fluid surface. The magnetic wave
turbulence shows several differences compared to the normal field case reported
recently. The inertial zone of the magnetic wave turbulence regime is notably
found to be strongly increased with respect to the normal field case, and to be
well described by our theoretical predictions. The dispersion relation of
linear waves is also measured and differs from the normal field case due to the
absence of the Rosensweig instability.",1103.5847v1
2011-06-16,Thermomagnonic spin transfer and Peltier effects in insulating magnets,"We study the coupled magnon energy transport and collective magnetization
dynamics in ferromagnets with magnetic textures. By constructing a
phenomenological theory based on irreversible thermodynamics, we describe
motion of domain walls by thermal gradients and generation of heat flows by
magnetization dynamics. From microscopic description based on magnon kinetics,
we estimate the transport coefficients and analyze the feasibility of
energy-related applications in insulating ferromagnets, such as yttrium iron
garnet and europium oxide.",1106.3135v2
2011-07-18,Molecular-spin dynamics study of electromagnons in multiferroic RMn$_2$O$_5$,"We investigate the electromagnon in magnetoferroelectrics RMn$_2$O$_5$ using
combined molecular-spin dynamics simulations. We identify the origin of the
electromagnon modes observed in the optical spectra and reproduce the most
salient features of the electromagnon in these compounds. We find that the
electromagnon frequencies are very sensitive to the magnetic wave vector along
the $a$ direction. We further investigate the electromagnon in magnetic field.
Although the modes frequencies change significant under magnetic field, the
static dielectric constant electromagnon does not change much in the magnetic
field.",1107.3349v1
2011-12-06,Instability in magnetic materials with dynamical axion field,"It has been pointed out that the axion electrodynamics exhibits instability
in the presence of a background electric field. We show that the instability
leads to a complete screening of an applied electric field above a certain
critical value and the excess energy is converted into a magnetic field. We
clarify the physical origin of the screening effect and discuss its possible
experimental realization in magnetic materials where magnetic fluctuations play
the role of the dynamical axion field.",1112.1414v2
2012-01-11,Frequency domain studies of current-induced magnetization dynamics in single magnetic-layer nanopillars,"Spin transfer torque-induced high-frequency dynamics of single thin
cobalt-layer nanopillars of circular and elliptical shape have been observed
directly. Two types of precessional modes can be identified as a function of
magnetic field perpendicular to the layer plane, excited for negative current
polarity only. They are assigned to vortex-core and transverse spin-wave
excitations, which corroborate recent model predictions. The observed narrow
linewidth of 4 MHz at room temperature indicates the high coherence of the
magnetic excitations.",1201.2268v1
2012-03-19,Dynamics of Vortex Nucleation in Nanomagnets with Broken Symmetry,"We investigate fundamental processes that govern dynamics of vortex
nucleation in sub-100 nm mesoscopic magnets. We focus on a structure with
broken symmetry - Pacman-like nanomagnet shape - in which we study
micromagnetic behavior both by means of a simple model and numerically. We show
that it is possible to establish desired vortex chirality and polarity by
applying only quasi-static in-plane magnetic field along specific directions.
We identify the modes of vortex nucleation that are very robust against
external magnetic field noise. These vortex nucleation modes are common among
wide range of sub-100 nm magnets with broken rotational symmetry.",1203.4191v1
2012-04-23,Dynamics of a suspended nanowire driven by an ac Josephson current in an inhomogeneous magnetic field,"We consider a voltage-biased nanoelectromechanical Josephson junction, where
a suspended nanowire forms a superconducting weak-link, in an inhomogeneous
magnetic field. We show that a nonlinear coupling between the Josephson current
and the magnetic field generates a Laplace force that induces a whirling motion
of the nanowire. By performing an analytical and a numerical analysis, we
demonstrate that at resonance, the amplitude-phase dynamics of the whirling
movement present different regimes depending on the degree of inhomogeneity of
the magnetic field: time independent, periodic and chaotic. Transitions between
these regimes are also discussed.",1204.5020v1
2012-08-15,Inertia and chiral edge modes of a skyrmion magnetic bubble,"The dynamics of a vortex in a thin-film ferromagnet resembles the motion of a
charged massless particle in a uniform magnetic field. Similar dynamics is
expected for other magnetic textures with a nonzero skyrmion number. However,
recent numerical simulations revealed that skyrmion magnetic bubbles show
significant deviations from this model. We show that a skyrmion bubble
possesses inertia and derive its mass from the standard theory of a thin-film
ferromagnet. Besides center-of-mass motion, other low energy modes are waves on
the edge of the bubble traveling with different speeds in opposite directions.",1208.3123v2
2013-09-04,Dynamical Process of Liner Implosion in the Electromagnetic Flux Compression for Ultra-high Magnetic Fields,"The spatial distribution of magnetic fields that are generated by the
electromagnetic flux compression technique is investigated, with emphasis on
the dynamical processes of an imploding liner. By comparing with the results of
computer simulations, we found that the non-uniform implosion of a liner is
important in order to explain the magnetic field's distribution during the
liner's implosion. In addition, our results suggest that the initial inwards
compressing spool-like motion of the liner subsequently turns out to be
outwards stretching barrel-like motion along the magnetic field axis.",1309.1038v1
2014-05-28,Breathing modes of confined skyrmions in ultrathin magnetic dots,"The dynamics of individual magnetic skyrmions confined in ultrathin film dots
is studied theoretically. The systems considered are transition metal
ferromagnets possessing perpendicular magnetic anisotropy and particular
attention is given to the dynamic response of the skyrmions to perpendicular
driving fields. By using micromagnetics simulations, it is shown that breathing
modes can hybridize with geometrically-quantized spin wave eigenmodes of the
circular dots considered, leading to distinct features in the power spectrum
that differ to the behavior expected for uniformly magnetized systems. The
field dependence of the breathing modes offers a direct means of detecting and
characterizing such skyrmion states in experiment.",1405.7414v1
2014-09-26,Magnetic field reversals and long-time memory in conducting flows,"Employing a simple ideal magnetohydrodynamic model in spherical geometry,we
show that the presence of either rotation or finite magnetic helicity is
sufficient to induce dynamical reversals of the magnetic dipole moment. The
statistical character of the model is similar to that of terrestrial magnetic
field reversals, with the similarity being stronger when rotation is
present.The connection between long time correlations, $1/f$ noise, and
statistics of reversals is supported, consistent with earlier suggestions.",1409.7568v1
2015-05-10,Approaches for modeling magnetic nanoparticle dynamics,"Magnetic nanoparticles are useful biological probes as well as therapeutic
agents. There have been several approaches used to model nanoparticle
magnetization dynamics for both Brownian as well as N\'eel rotation. The
magnetizations are often of interest and can be compared with experimental
results. Here we summarize these approaches including the Stoner-Wohlfarth
approach, and stochastic approaches including thermal fluctuations.
Non-equilibrium related temperature effects can be described by a distribution
function approach (Fokker-Planck equation) or a stochastic differential
equation (Langevin equation). Approximate models in several regimes can be
derived from these general approaches to simplify implementation.",1505.02450v1
2015-08-19,Dynamical quenching of tunneling in molecular magnets,"It is shown that a single molecular magnet placed in a rapidly oscillating
magnetic field displays the phenomenon of quenching of tunneling processes. The
results open a way to manipulate the quantum states of molecular magnets by
means of radiation in the terahertz range. Our analysis separates the time
evolution into slow and fast components thereby obtaining an effective theory
for the slow dynamics. This effective theory presents quenching of the tunnel
effect. In particular, stands out its difference with the so-called coherent
destruction of tunneling. We support our prediction with numerical evidence
based on an exact solution of the Schr\""odinger's equation.",1508.04499v1
2015-08-24,Spin Dynamics Simulation of the Magneto-Electric Effect in a Composite Multiferroic Chain,"A composite multiferroic chain with an interfacial linear magneto-electric
coupling is used to study the magnetic and electric responses to an external
magnetic or electric field. The simulation uses continuous spin dynamics
through the Landau-Lifshitz-Gilbert equations of the magnetic spin and the
electric pseudo-spin. The results demonstrate an accurate description of the
distribution of the magnetisation and polarisation are induced by applied
electric and magnetic field, respectively.",1508.05693v1
2015-10-14,Stable determination of coefficients in the dynamical Schrödinger equation in a magnetic field,"In this paper we consider the inverse problem of determining on a compact
Riemannian manifold the electric potential or the magnetic field in a
Schr\""odinger equation with Dirichlet data from measured Neumann boundary
observations. This information is enclosed in the dynamical
Dirichlet-to-Neumann map associated to the magnetic Schr\""odinger equation. We
prove that the knowledge of the Dirichlet-to-Neumann map for the Schr\""odinger
equation uniquely determines the magnetic field and the electric potential and
we establish H\""older-type stability.",1510.04247v1
2015-11-14,Incoherent magnetization dynamics in strain mediated switching of magnetostrictive nanomagnets,"Micromagnetic studies of the magnetization change in magnetostrictive
nanomagnets subjected to stress are performed for nanomagnets of different
sizes. The interplay between demagnetization, exchange and stress anisotropy
energies is used to explain the rich physics of size-dependent magnetization
dynamics induced by modulating stress anisotropy in planar nanomagnets. These
studies have important implications for strain mediated ultralow energy
magnetization change in nanomagnets and its application in energy-efficient
nanomagnetic computing systems.",1511.04617v1
2015-12-14,Gauge and emergent electromagnetic fields for moving magnetic topological solitons,"We apply the general conception of non-Abelian gauge fields for description
of magnetic soliton excitations. We show that the component of the gauge field
along the soliton local magnetization (Abelian part of the gauge potential)
determines dynamics of spin fluctuations over the soliton background in a
ferromagnet. Assumption that the gauge field is a pure gauge allows calculating
the gauge field components and finding simple expressions for the emergent
electromagnet fields related to the soliton motion. The gauge field results in
soliton-magnon interaction leading to renormalization of the soliton and magnon
dynamics. The presented approach allows reaching more deep understanding of a
relationship of the field theory and condensed matter magnetism.",1512.04396v1
2016-02-12,Velocity Autocorrelation Function of Magnetized Two-Dimensional One-Component Plasma,"The velocity autocorrelation function (VAF) for a two-dimensional
one-component plasma (OCP) is investigated by employing molecular dynamics
techniques. The VAF exhibits well defined oscillations whose frequency is
independent of the dimensionless coupling parameter Gamma. However, the
presence of a uniform perpendicular external magnetic field increases the
height of the first peak. Molecular dynamics computer simulation results are
presented for a two-dimensional OCP in a perpendicular magnetic field as a
function of the Plasma parameter. Our simulation results clearly indicate that
at high temperature, the magnetic field affects the VAF in a substantial way.",1602.04176v1
2016-11-02,Heavy-Quark Diffusion Dynamics in Quark-Gluon Plasma under Strong Magnetic Fields,"We discuss heavy-quark dynamics in the quark-gluon plasma under a strong
magnetic field induced by colliding nuclei. By the use of the diagrammatic
resummation techniques for Hard Thermal Loop and the external magnetic field,
we show analytic results of heavy-quark diffusion coefficient and drag force
which become anisotropic due to the preferred spatial orientation in the
magnetic field. We argue that the anisotropic diffusion coefficient gives rise
to an enhancement/suppression of the heavy-quark elliptic flow depending on the
transverse momentum.",1611.00500v1
2016-11-16,Dynamic chiral magnetic effect and Faraday rotation in macroscopically disordered helical metals,"We develop an effective medium theory for electromagnetic wave propagation
through gapless non-uniform systems with dynamic chiral magnetic effect. The
theory allows to calculate macroscopic-disorder-induced corrections to the
values of optical, as well as chiral magnetic conductivities. In particular, we
show that spatial fluctuations of optical conductivity induce corrections to
the effective value of the chiral magnetic conductivity. Experimentally, these
corrections can be observed as sharp features in the Faraday rotation angle
near frequencies that correspond to the bulk plasmon resonances of a material.
Such features are not expected to be present in single crystal samples.",1611.05491v1
2017-10-23,Clustering of Magnetic Swimmers in a Poiseuille Flow,"We investigate the collective behavior of magnetic swimmers, which are
suspended in a Poiseuille flow and placed under an external magnetic field,
using analytical techniques and Brownian dynamics simulations. We find that the
interplay between intrinsic activity, external alignment, and magnetic
dipole-dipole interactions leads to longitudinal structure formation. Our work
sheds light on a recent experimental observation of a clustering instability in
this system.",1710.08339v2
2018-07-27,Thick Permalloy films for the imaging of spin texture dynamics in perpendicularly magnetized systems,"We demonstrated that thick Permalloy films exhibiting a weak growth-induced
perpendicular magnetic anisotropy can be employed as an ideal test system for
the investigation of gyration dynamics in topologically trivial and non-trivial
magnetic states ranging from an isolated magnetic skyrmion to more complex
n$\pi$ spin configurations.",1807.10654v1
2019-12-12,"Cluster glass behavior of frustrated birnessites AxMnO2*yH2O (A = Na, K)","We report on the synthesis and magnetic properties of frustrated
Na0.22MnO2*0.39H2O and K0.6MnO2*0.48H2O with the birnessite structure. The
structure, static and dynamic magnetic properties of the compounds are
investigated in detail. A combination of DC and AC magnetic susceptibility
measurements and magnetization decay measurements reveal cluster glass behavior
below the freezing temperature of 4 K for Na-birnessite and 6 K for
K-birnessite. The frequency dependence of the freezing temperature is analyzed
on the basis of dynamic scaling laws including the critical slowing down
formula and the Vogel-Fulcher law, which further confirm cluster glass
formation in both compounds.",1912.05997v1
2020-03-05,Unidirectional Pumping of Phonons by Magnetization Dynamics,"We propose a method to control surface phonon transport by weak magnetic
fields based on the pumping of surface acoustic waves (SAWs) by
magnetostriction. We predict that the magnetization dynamics of a nanowire on
top of a dielectric films injects SAWs with opposite angular momenta into
opposite directions. Two parallel nanowires form a phononic cavity that at
magnetic resonances pump a unidirectional SAW current into half of the
substrate.",2003.02519v2
2020-07-23,No feedback is possible in small-scale turbulent magnetic field,"Evolution of stochastically homogeneous magnetic field advected by
incompressible turbulent flow with large magnetic Prandtl numbers is considered
at the scales less than Kolmogorov viscous scale. It is shown that, despite
unlimited growth of the magnetic field, its feedback on the fluid's dynamics
remains negligibly small.",2007.11940v1
2020-12-15,Dynamically stabilized spin superfluidity in frustrated magnets,"We study the onset of spin superfluidity, namely coherent spin transport
mediated by a topological spin texture, in frustrated exchange-dominated
magnetic systems, engendered by an external magnetic field. We show that for
typical device geometries used in nonlocal magnetotransport experiments, the
magnetic field stabilizes a spin superflow against fluctuations, up to a
critical current. For a given current, the critical field depends on the
precessional frequency of the texture, which can be separately controlled. We
contrast such dynamic stabilization of a spin superfluid to the conventional
approaches based on topological stabilization.",2012.08340v1
2021-01-30,Nearly ideal memristive functionality based on viscous magnetization dynamics,"We experimentally demonstrate a proof-of-principle implementation of an
almost ideal memristor - a two-terminal circuit element whose resistance is
approximately proportional to the integral of the input signal over time. The
demonstrated device is based on a thin-film ferromagnet/antiferromagnet
bilayer, where magnetic frustration results in viscous magnetization dynamics
enabling memristive functionality, while the external magnetic field plays the
role of the driving input. The demonstrated memristor concept is amenable to
downscaling and can be adapted for electronic driving, making it attractive for
applications in neuromorphic circuits.",2102.00123v1
2022-02-12,Trace formula for the magnetic Laplacian on a compact hyperbolic surface,"We compute the trace formula for the magnetic Laplacian on a compact
hyperbolic surface of constant curvature with constant magnetic field for
energies above the Mane critical level of the corresponding magnetic geodesic
flow. We discuss the asymptotic behavior of the coefficients of the trace
formula when the energy approaches the Mane critical level.",2202.06055v3
2022-10-20,Numerical study of the Transverse Diffusion coefficient for a one component model of plasma,"In this paper we discuss the results of some Molecular Dynamics simulations
of a magnetized One Component Plasma, targeted to estimate the diffusion
coefficient $D_{\perp}$ in the plane orthogonal to the magnetic field lines. We
find that there exists a threshold with respect to the magnetic field strength
$|\vec B|$: for weak magnetic field the diffusion coefficients scales as
$1/|\vec B|^2$, while a slower decay appears at high field strength. The
relation of this transition with the different mixing properties of the
microscopic dynamics is investigated by looking at the behavior of the velocity
auto correlation.",2210.11125v1
2023-04-17,Skyrmion dynamics in moiré magnets,"We consider a twisted magnetic bilayer subject to the perpendicular electric
field. The interplay of induced Dzyaloshinskii - Moriya interaction and
spatially varying moir\'e exchange potential results in complex non-collinear
magnetic phases in these structures. We numerically demonstrate the coexistence
of intralayer skyrmions and bound interlayer skyrmion pairs and show that they
are characterized by distinct dynamics under the action of external in-plane
electric field. Specifically we demonstrate the railing behaviour of skyrmions
along the domain walls which could find applications in spintronic devices
based on van der Waals magnets.",2304.08306v2
2023-07-07,Time-dependent parameter identification in a Fokker-Planck equation based magnetization model of large ensembles of nanoparticles,"In this article, we consider a model motivated by large ensembles of
nanoparticles' magnetization dynamics using the Fokker-Planck equation and
analyze the underlying parabolic PDE being defined on a smooth, compact
manifold without boundary with respect to time-dependent parameter
identification using regularization schemes. In the context of magnetic
particle imaging, possible fields of application can be found including
calibration procedures improved by time-dependent particle parameters and
dynamic tracking of nanoparticle orientation. This results in reconstructing
different parameters of interest, such as the applied magnetic field and the
particles' easy axis. These problems are in particular addressed in the
accompanied numerical study.",2307.03560v1
2023-08-28,Laser-enhanced magnetism in SmFeO$_3$,"To coherently enhance inherent weak magnetic interactions in rare-earth
orthoferrite SmFeO$_3$ as a functional material for spintronic applications, we
simulate the dissipative spin dynamics that are linearly and quadratically
coupled to laser-driven infrared-active phonons. When linear coupling
dominates, we discover a magnetophononic dynamical first-order phase transition
in the nonequilibrium steady state which can inhibit strong enhancement of
magnetic interactions. By contrast, when quadratic spin-phonon coupling
dominates, no phase transition exists at experimentally relevant parameters. By
utilizing a chirp protocol, the phase transition can be engineered, enabling
stronger magnetic interactions. We also discuss the route for experimental
observation of our results.",2308.14939v2
2023-12-01,Magnetic-field-induced critical dynamics in magnetoelectric TbPO$_4$,"We report on the magnetoelectric dynamics in the linear magnetoelectric
antiferromagnet TbPO$_4$ studied by broadband dielectric spectroscopy. For the
phase transition into the magnetoelectric antiferromagnetic phase at $T_{N}
\approx 2.3$ K, a finite magnetic field $H$ induces critical behavior in the
quasi-static permittivity $\varepsilon'$. Plotting the corresponding anomaly as
function of $T/T_N(H)$, we observe the scaling behavior $\Delta \varepsilon'
\propto H^2$, a clear fingerprint of linear magnetoelectric antiferromagnets.
Above the phase transition, we find a critical slowing down of the ferroic
fluctuations in finite magnetic field. This behaviour can be understood via a
magnetic-field-induced relaxational response that resembles the soft-mode
behaviour in canonical ferroelectrics and multiferroics.",2312.00726v1
2023-12-22,Magnetic droplet solitons,"Magnetic droplets are nanoscale, non-topological, dynamical solitons that can
be nucleated in different spintronic devices, such as spin torque
nano-oscillators (STNOs) and spin Hall nano-oscillators (SHNOs). This chapter
first briefly discusses the theory of spin current driven dissipative magnetic
droplets in ferromagnetic thin films with uniaxial anisotropy. We then
thoroughly review the research literature on magnetic droplets and their
salient features, as measured using electrical, microwave, and synchrotron
techniques, and as envisaged by micromagnetic simulations. We also touch upon a
closely related soliton, the dynamical skyrmion. Finally, we present an outlook
of new routes in droplet science.",2312.14621v1
2024-02-05,Modelling nanomagnet vertex dynamics through Coulomb charges,"We investigate the magnetization dynamics in nanomagnet vertices often found
in artificial spin ices. Our analysis involves creating a simplified model that
depicts edge magnetization using magnetic charges. We utilize the model to
explore the energy landscape, its associated curvatures, and the fundamental
modes. Our study uncovers specific magnonic regimes and transitions between
magnetization states, marked by zero-modes, which can be understood within the
framework of Landau theory. To verify our model, we compare it with
micromagnetic simulations, demonstrating a noteworthy agreement.",2402.03036v1
2020-08-13,Magnetization Dynamics of Nanoscale Magnetic Materials: A Perspective,"Nanomagnets form the building blocks for a gamut of miniaturized
energy-efficient devices including data storage, memory, wave-based computing,
sensors and biomedical devices. They also offer a span of exotic phenomena and
stern challenges. The progress in the magnetization dynamics of single
nanomagnets and one- and two-dimensional arrays of nanostructures in the form
of dots, antidots, nanoparticles, binary and bicomponent structures and
patterned multilayers have been presented. Progress in unconventional and new
structures like artificial spin ice and three-dimensional nanomagnets and spin
textures like domain walls, vortex and skyrmions have been presented.
Furthermore, a huge variety of new topics in the magnetization dynamics of
magnetic nanostructures are rapidly emerging. An overview of the steadily
evolving topics like spatio-temporal imaging of fast dynamics of
nanostructures, dynamics of spin textures, artificial spin ice have been
discussed. In addition, dynamics of contemporary and newly transpired magnetic
architectures such as nanomagnet arrays with complex basis and symmetry,
magnonic quasicrystals, fractals, defect structures, novel three-dimensional
structures have been introduced. Effects of various spin-orbit coupling and
ensuing spin textures as well as quantum hybrid systems comprising of
magnon-photon, magnon-phonon and magnon-magnon coupling, antiferromagnetic
nanostructures are rapidly growing and are expected to dominate this research
field in the coming years. Finally, associated topics like nutation dynamics
and nanomagnet antenna are briefly discussed. Despite showing a great progress,
only a small fraction of nanomagnetism and its ancillary topics have been
explored so far and huge efforts are envisaged in this evergrowing research
area in the generations to come.",2008.05819v1
1999-07-12,Differential rotation and magnetic fields in stellar interiors,"The processes contributing to the evolution of an initially weak magnetic
field in a differentially rotating star are reviewed. These include rotational
smoothing (akin to convective expulsion) and a list of about 5 instabilities,
among them magnetorotational instability, byoyancy instability, and pinch-type
instabilities. The important effects of thermal and magnetic diffusion on these
instabilities are analyzed in some detail. The first instability to set in is a
pinch-type instability. It becomes important in modifying the field
configuration before magnetic buoyancy-driven instabilities set in. The
evolution of an initially strong field remains a more open question, including
the old problem whether dynamically stable magnetic equilibria exist in stars.",9907138v1
2001-01-17,The temperature-mass relation in magnetized galaxy clusters,"We use cosmological, magneto-hydrodynamic simulations of galaxy clusters to
quantify the dynamical importance of magnetic fields in these clusters. The
set-up of initial magnetic field strengths at high redshifts is chosen such
that observed Faraday-rotation measurements in low-redshift clusters are well
reproduced in the simulations. We compute the radial profiles of the
intracluster gas temperature and of the thermal and magnetic pressure in a set
of clusters simulated in the framework of an Einstein-de Sitter and a
low-density, spatially-flat CDM cosmological model. We find that, for a
realistic range of initial magnetic field strengths, the temperature of the
intracluster gas changes by less than $\approx5%$.",0101302v1
2006-11-17,Magnetic field vector retrieval with the Helioseismic and Magnetic Imager,"We investigate the accuracy to which we can retrieve the solar photospheric
magnetic field vector using the Helioseismic and Magnetic Imager (HMI) that
will fly onboard of the Solar Dynamics Observatory (SDO) by inverting simulated
HMI profiles. The simulated profiles realistically take into account the
effects of the photon noise, limited spectral resolution, instrumental
polarization modulation, solar p modes and temporal averaging. The accuracy of
the determination of the magnetic field vector is studied considering the
different operational modes of the instrument.",0611565v1
1995-11-02,"Berry phase, hyperorbits, and the Hofstadter spectrum: semiclassical dynamics in magnetic Bloch bands","We have derived a new set of semiclassical equations for electrons in
magnetic Bloch bands. The velocity and energy of magnetic Bloch electrons are
found to be modified by the Berry phase and magnetization. This semiclassical
approach is used to study general electron transport in a DC or AC electric
field. We also find a close connection between the cyclotron orbits in magnetic
Bloch bands and the energy subbands in the Hofstadter spectrum. Based on this
formalism, the pattern of band splitting, the distribution of Hall conduct-
ivities, and the positions of energy subbands in the Hofstadter spectrum can be
understood in a simple and unified picture.",9511014v1
1997-08-06,Macroscopic Quantum Coherence in Molecular Magnets,"We study macroscopic quantum coherence in antiferromagnetic molecular magnets
in the presence of magnetic fields. Such fields generate artificial tunnel
barriers with externally tunable strength. We give detailed semi-classical
predictions for the tunnel splitting in various regimes for low and high
magnetic fields. We show that the tunneling dynamics of the Neel vector can be
directly measured via the static magnetization and the specific heat. We also
report on a new quantum phase arising from fluctuations. The analytic results
are complemented by numerical simulations.",9708042v1
1998-01-13,Skyrmion in a real magnetic film,"Skyrmions are the magnetic defects in an ultrathin magnetic film, similar to
the bubble domains in the thicker films. Even weak uniaxial anisotropy
determines its radius unambiguously. We consider the dynamics of the skyrmion
decay. We show that the discreteness of the lattice in an isotropic 2D magnet
leads to a slow rotation of the local magnetization in the skyrmion and,
provided a small dissipation, to decay of the skyrmion. The radius of such a
skyrmion as a function of time is calculated. We prove that uniaxial anisotropy
stabilizes the skyrmion and study the relaxation process.",9801114v1
1998-06-05,Magnetic Particles,"We present an overview of the subject of Magnetic Particles, starting at a
level suitable for advanced high-school students and ending at a level suitable
for current practitioners in the field. The sub-topics covered include: Types
of Magnetic Materials, Relationships Between the Vector Fields, Magnetic
Energies, Dynamics of Magnetic Particles, and Magnetocaloric Effects.",9806082v1
1999-11-08,Feedback Effect on Landau-Zener-Stueckelberg Transitions in Magnetic Systems,"We examine the effect of the dynamics of the internal magnetic field on the
staircase magnetization curves observed in large-spin molecular magnets. We
show that the size of the magnetization steps depends sensitively on the
intermolecular interactions, even if these are very small compared to the
intra-molecular couplings.",9911106v2
2000-04-19,Statistical coarse-graining as an approach to multiscale problems in magnetism,"Multiscale phenomena which include several processes occuring simultaneously
at different length scales and exchanging energy with each other, are
widespread in magnetism. These phenomena often govern the magnetization
reversal dynamics, and their correct modeling is important. In the present
paper, we propose an approach to multiscale modeling of magnets, applying the
ideas of coarse graining. We have analyzed the choice of the weighting function
used in coarse graining, and propose an optimal form for this function. Simple
tests provide evidence that this approach may be useful for modeling of
realistic magnetic systems.",0004338v1
2001-01-29,Magnetic Properties for the One-Dimensional Multicomponent Spin-Gap System,"Magnetic properties for the one-dimensional multicomponent quantum spin
system with the excitation gap are studied based on the integrable spin model
introduced by Bariev {\it et al}. By exactly computing the magnetization, we
show how the characteristic structure with plateaus and cusps appears in the
magnetization process. To study low-energy dynamics of the system, we apply the
finite-size scaling analysis to the excitation spectrum, and thereby evaluate
the power-law exponent as well as the enhancement factor for the
low-temperature NMR relaxation rate $1/T_1$. We discuss the critical properties
of $1/T_1$ around plateaus and cusps in the magnetization curve.",0101423v1
2002-09-04,Correlation Effects on Magnetic Anisotropy in Fe and Ni,"We calculate magnetic anisotropy energy of Fe and Ni by taking into account
the effects of strong electronic correlations, spin-orbit coupling, and
non-collinearity of intra-atomic magnetization. The LDA+U method is used and
its equivalence to dynamical mean-field theory in the static limit is derived.
The effects of strong correlations are studied along several paths in $(U,J)$
parameter space. Both experimental magnitude of MAE and direction of
magnetization are predicted correctly near $U=1.9 eV$, $J=1.2 eV$ for Ni and
$U=1.2 eV$, $J=0.8 eV$ for Fe. The modified one-electron spectra by strong
correlations are emphasized in conjunction with magnetic anisotropy.",0209073v1
2002-11-05,Magnetic fluctuations and resonant peak in cuprates: a microscopic theory,"The theory for the dynamical spin susceptibility within the t-J model is
developed, as relevant for the resonant magnetic peak and normal-state magnetic
response in superconducting (SC) cuprates. The analysis is based on the
equations of motion for spins and the memory-function presentation of magnetic
response where the main damping of the low-energy spin collective mode comes
from the decay into fermionic degrees of freedom. It is shown that the damping
function at low doping is closely related to the c-axis optical conductivity.
The analysis reproduces doping-dependent features of the resonant magnetic
scattering.",0211090v1
2002-11-25,Electrical Manipulation of Nanomagnets,"We demonstrate a possibility to manipulate the magnetic coupling between two
nanomagnets with a help of ac electric field. In the scheme suggested the
magnetic coupling in question is mediated by a magnetic particle contacting
with both of the nanomagnets through the tunnel barriers. The electric field
providing a successive suppression of the barriers leads to pumping of
magnetization through the mediating particle. Time dependent dynamics of the
particle magnetization allows to to switch between ferro- and antiferromagnetic
couplings.",0211563v1
2003-05-21,From Bubble to Skyrmion: Dynamic Transformation Mediated by a Strong Magnetic Tip,"Skyrmions in thin metallic ferromagnetic films are stable due to competition
between the RKKY interaction and uniaxial magnetic anisotropy. We mimic the
RKKY interaction by the next-nearest-neighbors ferromagnetic and
antiferromagnetic exchange interactions. We demonstrate analytically and
numerically dissipative transformation of a bubble created by a strong
cylindrical magnetic tip into a stable Skyrmion.",0305487v1
2003-11-28,Rectangular quantum dots in high magnetic fields,"We use density-functional methods to study the effects of an external
magnetic field on two-dimensional quantum dots with a rectangular hard-wall
confining potential. The increasing magnetic field leads to spin polarization
and formation of a highly inhomogeneous maximum-density droplet at the
predicted magnetic field strength. At higher fields, we find an oscillating
behavior in the electron density and in the magnetization of the dot. We
identify a rich variety of phenomena behind the periodicity and analyze the
complicated many-electron dynamics, which is shown to be highly dependent on
the shape of the quantum dot.",0311639v1
2004-03-29,The origin of the glassy magnetic dynamics of the phase segregated state in perovskites,"In this paper we address many of the fundamental open questions regarding the
glassy behavior of the magnetic/electronic phase segregated state in rare earth
perovskites. In particular, magnetic relaxation experiments support that the
collective effects (memory, ageing, etc.) are due to interparticle
interactions, rather than the double-exchange vs. superexchange competition. A
careful study of the non-linear susceptibility in the critical region is
performed, and the critical exponents contrasted with those of conventional
spin-glasses and concentrated quenched ferrofluids. The phase segregated state
constitutes a sort of self-generated assembly of magnetic particles in which
magnetic interaction introduces collectivity among the clusters.",0403686v1
2004-05-13,Precessional dynamics of elemental moments in a ferromagnetic alloy,"We demonstrate an element-specific measurement of magnetization precession in
a metallic ferromagnetic alloy, separating Ni and Fe moment motion in Ni81Fe19.
Pump-probe X-ray magnetic circular dichroism (XMCD), synchronized with short
magnetic field pulses, is used to measure free magnetization oscillations up to
2.6 GHz with elemental specificity and a rotational resolution of < 2 deg.
Magnetic moments residing on Ni sites and Fe sites in a Ni81Fe19(50nm) thin
film are found to precess together at all frequencies, coupled in phase within
instrumental resolution of 90 ps.",0405295v1
2004-08-10,Dynamics of liquid crystalline domains in magnetic field,"We study microscopic single domains nucleating and growing within the
coexistence region of the Isotropic (I) and Nematic (N) phases in magnetic
field. By rapidly switching on the magnetic field the time needed to align the
nuclei of sufficiently large size is measured, and is found to decrease with
the square of the magnetic field. When the field is removed the disordering
time is observed to last on a longer time scale. The growth rate of the nematic
domains at constant temperature within the coexistence region is found to
increase when a magnetic field is applied.",0408208v2
2005-05-30,Dynamical Symmetry Breaking in Quasistatic Magnetic Oscillations,"Recent microwave experiments demonstrate the anapole-moment and
magnetoelectric properties in quasi-2D ferrite particles with
magnetic-dipolar-wave oscillating spectra. The theory developed in this paper
shows that there are the macroscopically quantum topological effects. Quantum
coherence for macroscopic systems refers to circumstances when large numbers of
particles can collectively cooperate in a single quantum state. These effects
are rarely observed through macroscopic measurements because statistical
averaging over many states usually masks all evidence of quantum discreteness.
Magnetic-dipolar oscillating modes in normally magnetized ferrite disks
demonstrate properties of a Hamiltonian system. The purpose of this paper is to
show that because of the adiabatic motion process for such a Hamiltonian system
one has macroscopic quantum effects of symmetry breaking, magnetic currents,
and eigen electric moments.",0505717v1
2006-05-15,Sculpting microscopic magnetic flux landscapes in a BSCCO2212 vortex lens,"We demonstrate experimentally that the micromagnetic profile of the
out-of-plane component of magnetic induction, B_z, in the crossing lattices
regime of layered superconductors can be manipulated by varying the in-plane
magnetic field, H_{||}. Moving Josephson vortices drag/push pancake vortex
stacks, and the magnetic profile, B_z(x), can be controllably sculpted across
the entire single crystal sample. Depending on the H-history and temperature we
can increase or decrease the flux density at the center and near the edges of
the crystal by as much as 40%, realising both 'convex' and 'concave' magnetic
flux lenses. Our experimental results are well described by molecular dynamics
simulations.",0605375v1
2006-09-01,Calculations of Magnetic Exchange Interactions in Mott--Hubbard Systems,"An efficient method to compute magnetic exchange interactions in systems with
strong correlations is introduced. It is based on a magnetic force theorem
which evaluates linear response due to rotations of magnetic moments and uses a
novel spectral density functional framework combining our exact diagonalization
based dynamical mean field and local density functional theories. Applications
to spin waves and magnetic transition temperatures of 3d metal mono--oxides as
well as high--T_{c} superconductors are in good agreement with experiment.",0609006v1
2006-11-02,Spin-phonon coupling in ZnCr2Se4,"Spin-phonon coupling and magnetodielectric effects of ZnCr2Se4 were
investigated by means of infrared (IR) spectroscopy as a function of
temperature and magnetic field. ZnCr2Se4 is dominated by ferromagnetic exchange
but undergoes antiferromagnetic order at T = 21 K. In the magnetically ordered
phase the low-frequency IR active phonon splits, indicating strong dynamic
anisotropy via magnetic exchange interactions. Antiferromagnetic order and
concomitantly the phonon splitting is wiped out by external magnetic fields.
Hence, ZnCr2Se4 is a prominent example of a spin-driven Jahn-Teller effect
which can be fully suppressed in external magnetic fields.",0611041v1
1992-12-03,Magnetic Fields and Passive Scalars in Polyakov's Conformal Turbulence,"Polyakov has suggested that two dimensional turbulence might be described by
a minimal model of conformal field theory. However, there are many minimal
models satisfying the same physical inputs as Polyakov's solution (p,q)=(2,21).
Dynamical magnetic fields and passive scalars pose different physical
requirements. For large magnetic Reynolds number other minimal models arise.
The simplest one, (p,q)=(2,13) makes reasonable predictions that may be tested
in the astrophysical context. In particular, the equipartition theorem between
magnetic and kinetic energies does not hold: the magnetic one dominates at
larger distances.",9212021v1
2004-11-04,Structure formation in electromagnetically driven granular media,"We report structure formation in submonolayers of magnetic microparticles
subjected to periodic electrostatic and magnetic excitations. Depending on the
excitation parameters, we observe the formation of a rich variety of
structures: clusters, rings, chains, and networks. The growth dynamics and
shapes of the structures are strongly dependent on the amplitude and frequency
of the external magnetic field. We find that for pure ac magnetic driving at
low densities of particles, the low-frequency magnetic excitation favors
clusters while high frequency excitation favors chains and net-like structures.
An abrupt phase transition from chains to a network phase was observed for a
high density of particles.",0411013v1
2000-10-19,Magnetic Field Saturation in the Riga Dynamo Experiment,"After the dynamo experiment in November 1999 had shown magnetic field
self-excitation in a spiraling liquid metal flow, in a second series of
experiments emphasis was placed on the magnetic field saturation regime as the
next principal step in the dynamo process. The dependence of the strength of
the magnetic field on the rotation rate is studied. Various features of the
saturated magnetic field are outlined and possible saturation mechanisms are
discussed.",0010047v1
2007-07-09,Magnetic strings in Lattice QCD as Nonabelian Vortices,"Lattice studies indicate existence of magnetic strings in QCD vacuum. We
argue that recently found nonabelian strings with rich worldsheet dynamics
provide a proper pattern for the strings observed on the lattice. In
particular, within this pattern we explain the localization of the
monopole-antimonopole pairs on the magnetic string worldsheet and the negative
contribution of the magnetic strings into the vacuum energy and gluon
condensate. We suggest the D2 brane realization of the magnetic string which
explains the temperature dependence of its tension.",0707.1284v1
2007-09-12,Two Ferromagnetic Phases in Spin-Fermion Systems,"We consider spin-fermion systems which get their magnetic properties from a
system of localized magnetic moments being coupled to conducting electrons. The
dynamical degrees of freedom are spin-$s$ operators of localized spins and
spin-1/2 fermi operators of itinerant electrons. We develop modified spin-wave
theory and obtain that system has two ferromagnetic phases. At the
characteristic temperature T* the magnetization of itinerant electrons becomes
zero, and high temperature ferromagnetic phase (T*<T<T_C) is a phase where only
localized electrons give contribution to the magnetization of the system. An
anomalous increasing of magnetization below T* is obtained in a good agrement
with experimental measurements of the ferromagnetic phase of UGe2.",0709.1759v2
2008-06-24,Reduction of velocity fluctuations in a turbulent flow of gallium by an external magnetic field,"The magnetic field of planets or stars is generated by the motion of a
conducting fluid through a dynamo instability. The saturation of the magnetic
field occurs through the reaction of the Lorentz force on the flow. In relation
to this phenomenon, we study the effect of a magnetic field on a turbulent flow
of liquid Gallium. The measurement of electric potential differences provides a
signal related to the local velocity fluctuations. We observe a reduction of
velocity fluctuations at all frequencies in the spectrum when the magnetic
field is increased.",0806.3872v1
2008-09-03,Magnetic dynamo action in astrophysical turbulence,"We investigate the structure of magnetic field amplified by turbulent
velocity fluctuations, in the framework of the kinematic Kazantsev-Kraichnan
model. We consider Kolmogorov distribution of velocity fluctuations, and assume
that both Reynolds number and magnetic Reynolds number are very large. We
present the full numerical solution of the model for the spectra and the growth
rates of magnetic fluctuations. We consider astrophysically relevant limits of
large and small magnetic Prandtl numbers, and address both helical and
nonhelical cases.",0809.0720v2
2008-11-13,Optical spin orientation of a single manganese atom in a quantum dot,"A hight degree of spin polarization is achieved for a Mn atom localized in a
semiconductor quantum dot using quasi-resonant optical excitation at zero
magnetic field. Optically created spin polarized carriers generate an energy
splitting of the Mn spin and enable magnetic moment orientation controlled by
the photon helicity and energy. The dynamics and the magnetic field dependence
of the optical pumping mechanism shows that the spin lifetime of an isolated Mn
atom at zero magnetic field is controlled by a magnetic anisotropy induced by
the built-in strain in the quantum dots.",0811.2165v1
2008-12-02,Local control of ultrafast dynamics in magnetic nanoparticles,"Using the local control theory we derive analytical expressions for magnetic
field pulses that steer the magnetization of a monodomain magnetic nanoparticle
to a predefined state. Finite-temperature full numerical simulations confirm
the analytical results and show that a magnetization switching or freezing is
achievable within few precessional periods and that the scheme is exploitable
for fast thermal switching.",0812.0584v1
2009-03-12,Self-organized patterns of macroscopic quantum tunneling in molecular magnets,"We study low temperature resonant spin tunneling in molecular magnets induced
by a field sweep with account of dipole-dipole interactions. Numerical
simulations uncovered formation of self-organized patterns of the magnetization
and of the ensuing dipolar field that provide resonant condition inside a
finite volume of the crystal. This effect is robust with respect to disorder
and should be relevant to the dynamics of the magnetization steps observed in
molecular magnets.",0903.2247v1
2009-06-13,Single-shot time-domain studies of spin-torque-driven switching in magnetic tunnel junctions,"We report single-shot measurements of resistance versus time for thermally
assisted spin-torque-driven switching in magnetic tunnel junctions. We achieve
sufficient sensitivity to resolve the resistance signals leading up to
switching, including the variations between individual switching events.
Analyses of pre-switching thermal fluctuations allow detailed measurements of
coherence times and variations in magnetization precession amplitude. We find
that with a small in-plane hard-axis magnetic field the magnetization dynamics
are more spatially coherent than for the case of zero field.",0906.2476v1
2009-10-10,Light-induced valley currents and magnetization in graphene rings,"We study the non-equilibrium dynamics in a mesoscopic graphene ring excited
by picoseconds shaped electromagnetic pulses. We predict an ultrafast buildup
of charge polarization, currents and orbital magnetization. Applying the light
pulses identified here, non-equilibrium valley currents are generated in a
graphene ring threaded by a stationary magnetic flux. We predict a finite
graphene ring magnetization even for a vanishing charge current; the
magnetization emerges due to the light-induced difference of the valley
populations.",0910.1950v1
2010-02-12,Real-time dynamics of the Chiral Magnetic Effect,"In quantum chromodynamics, a gauge field configuration with nonzero
topological charge generates a difference between the number of left- and
right-handed quarks. When a (electromagnetic) magnetic field is added to this
configuration, an electromagnetic current is induced along the magnetic field;
this is called the chiral magnetic effect. We compute this current in the
presence of a color flux tube possessing topological charge, with a magnetic
field applied perpendicular to it. We argue that this situation is realized at
the early stage of relativistic heavy-ion collisions.",1002.2495v1
2010-08-06,Ferromagnetic phases in spin-Fermion systems,"Spin-Fermion systems which obtain their magnetic properties from a system of
localized magnetic moments being coupled to conducting electrons are
considered. The dynamical degrees of freedom are spin-$s$ operators of
localized spins and spin-1/2 Fermi operators of itinerant electrons.
Renormalized spin-wave theory, which accounts for the magnon-magnon
interaction, and its extension are developed to describe the two ferrimagnetic
phases in the system: low temperature phase $0<T<T^{*}$, where all electrons
contribute the ordered ferromagnetic moment, and high temperature phase
$T^{*}<T<T_C$, where only localized spins form magnetic moment. The
magnetization as a function of temperature is calculated. The theoretical
predictions are utilize to interpret the experimentally measured
magnetization-temperature curves of $UGe_2$..",1008.1148v1
2010-09-04,Magnetization Reversal by Electric-Field Decoupling of Magnetic and Ferroelectric Domains Walls in Multiferroic-Based Heterostructures,"We demonstrate that the magnetization of a ferromagnet in contact with an
antiferromagnetic multiferroic (LuMnO3) can be speedily reversed by electric
field pulsing, and the sign of the magnetic exchange bias can switch and
recover isothermally. As LuMnO3 is not ferroelastic, our data conclusively show
that this switching is not mediated by strain effects but is a unique
electric-field driven decoupling of the ferroelectric and ferromagnetic domains
walls. Their distinct dynamics are essential for the observed magnetic
switching.",1009.0820v1
2011-05-05,Strains and pseudo-magnetic fields in circular graphene rings,"We demonstrate that circular graphene ring under a shear stress displays
strong pseudo-magnetic fields. We calculate the pseudo-magnetic field both from
continuum elasticity theory as well as molecular dynamics simulations. Stable
wrinkles are induced by shear deformations and lead to enhancement of the
pseudo-magnetic field. The strong pseudo-magnetic field found here can be
observed by imaging graphene flake at the atomic level e.g. through scanning
tunneling microscope.",1105.1026v2
2011-06-17,Current effect on magnetization oscillations in a ferromagnet - antiferromagnet junction,"Spin-polarized current effect is studied on the static and dynamic
magnetization of the antiferromagnet in a ferromagnet - antiferromagnet
junction. The macrospin approximation is generalized to antiferromagnets.
Canted antiferromagnetic configuration and resulting magnetic moment are
induced by an external magnetic field. The resonance frequency and damping are
calculated, as well as the threshold current density corresponding to
instability appearance. A possibility is shown of generating low-damping
magnetization oscillations in terahertz range. The fluctuation effect is
discussed on the canted antiferromagnetic configuration.",1106.3519v1
2011-11-15,Magnetization dynamics induced by in-plane currents in ultrathin magnetic nanostructures,"Ultrathin magnetic systems have properties qualitatively different from their
thicker counterparts, implying that different physics governs their properties.
We demonstrate that various such properties can be explained naturally by the
Rashba spin-orbit coupling in ultrathin magnetic systems. This work will be
valuable for the development of next generation spintronic devices based on
ultrathin magnetic systems.",1111.3422v2
2012-02-06,Possibility of superradiance by magnetic nanoclusters,"The possibility of realizing spin superradiance by an assembly of magnetic
nanoclusters is analyzed. The known obstacles for realizing such a coherent
radiation by magnetic nanoclusters are their large magnetic anisotropy, strong
dephasing dipole interactions, and an essential nonuniformity of their sizes.
In order to give a persuasive conclusion, a microscopic theory is developed,
providing an accurate description of nanocluster spin dynamics. It is shown
that, despite the obstacles, it is feasible to organize such a setup that
magnetic nanoclusters would produce strong superradiant emission.",1202.1115v1
2012-05-24,High Performance Calculation of Magnetic Properties and Simulation of Nonequilibrium Phenomena in nanofilms,"Images of surface topography of ultrathin magnetic films have been used for
Monte Carlo simulations in the framework of the ferromagnetic Ising model to
study the hysteresis and thermal properties of nanomaterials. For high
performance calculations was used super-scalable parallel algorithm for the
finding of the equilibrium configuration. The changing of a distribution of
spins on the surface during the reversal of the magnetization and the dynamics
of nanodomain structure of thin magnetic films under the influence of changing
external magnetic field was investigated.",1205.5588v1
2012-10-27,Magnet traveling through a conducting pipe: a variation on the analytical approach,"We present an analytical study of magnetic damping. In particular, we
investigate the dynamics of a cylindrical neodymium magnet as it moves through
a conducting tube. Owing to the very high degree of uniformity of the
magnetization for neodymium magnets, we are able to provide completely
analytical results for the EMF generated in the pipe, and the consequent
retarding force. Our analytical expressions are shown to have excellent
agreement with experimental observations.",1210.7796v1
2012-11-02,Magnetic power inverter: AC voltage generation from DC magnetic fields,"We propose a method that allows power conversion from DC magnetic fields to
AC electric voltages using domain wall (DW) motion in ferromagnetic nanowires.
The device concept relies on spinmotive force, voltage generation due to
magnetization dynamics. Sinusoidal modulation of the nanowire width introduces
a periodic potential for a DW the gradient of which exerts variable pressure on
the traveling DW. This results in time variation of the DW precession frequency
and the associated voltage. Using a one-dimensional model we show that the
frequency and amplitude of the AC outputs can be tuned by the DC magnetic
fields and wire-design.",1211.0372v1
2013-01-09,Onset of a Propagating Self-Sustained Spin Reversal Front in a Magnetic System,"The energy released in a magnetic material by reversing spins as they relax
toward equilibrium can lead to a dynamical instability that ignites
self-sustained rapid relaxation along a deflagration front that propagates at a
constant subsonic speed. Using a trigger heat pulse and transverse and
longitudinal magnetic fields, we investigate and control the crossover between
thermally driven magnetic relaxation and magnetic deflagration in single
crystals of Mn$_{12}$-acetate.",1301.1900v2
2013-03-12,In-Plane Magnetic Field Tolerance of a Dispersive Aluminum Nanobridge SQUID Magnetometer,"We characterize the flux sensitivity of a dispersive 3D aluminum nanobridge
SQUID magnetometer as a function of applied in-plane magnetic field. In zero
field, we observe an effective flux noise of 17 n$\Phi_0$/Hz$^{1/2}$ with 25
MHz of bandwidth. Flux noise increased by less than a factor of three with
parallel magnetic fields up to 61 mT. Operation in higher fields may be
possible by decreasing the dimensions of the shunt capacitor in the
magnetometer circuit. These devices are thus well suited for observing
high-speed dynamics in nanoscale magnets, even in the presence of moderate bias
magnetic fields.",1303.2737v1
2013-03-19,Magnetoelectric Resonances and Predicted Microwave Diode Effect of Skyrmion Crystal in Multiferroic Chiral-Lattice Magnet,"We theoretically discover that unique eigenmodes of skyrmion crystal (SkX)
are not only magnetically active to ac magnetic field (H^w) but also
electrically active to ac electric field (E^w) in a multiferroic chiral-lattice
magnet Cu2OSeO3, which amplifies the dynamical magnetoelectric coupling between
E^w and the spin texture. The resulting intense interference between their
electric and magnetic activation processes can lead to unprecedentedly large
diode effect on the microwave, i.e., its absorption by SkX changes up to ~20%
when the incident direction is reversed. Our results demonstrate that the
skyrmion can be a promising building block for microwave devices.",1303.4491v1
2013-05-21,Switching of a single ferromagnetic layer driven by spin Hall effect,"The magnetization switching of a thin ferromagnetic layer placed on top of a
heavy metal (such as Pt, Ta or W) driven by an in-plane current has been
observed in recent experiments. The magnetization dynamics of these processes
is studied in a full micromagnetic framework which takes into account the
transfer-torque from spin Hall effect due to the spin-orbit coupling.
Simulations indicate that the reversal occurs via nucleation of complex
magnetization patterns. In particular, magnetic bubbles appear during the
reversal of the magnetization in the perpendicular configuration while for the
in-plane configuration, nucleation of vortexes are observed.",1305.4806v1
2013-10-24,High-Speed DC Magnetricity in Spinor Polariton Condensates,"We investigate the spin dynamics of half-solitons in polariton Bose-Einstein
condensates. Half-solitons, which behave as magnetic monopoles, can be
accelerated in the presence of the effective magnetic field of the microcavity.
We study the generation of DC magnetic currents in a gas of half-solitons. At
low densities, the current is suppressed due to the dipolar oscillations. At
moderate densities, a magnetic current is recovered as a consequence of the
collisions between the carriers. We show a deviation from Ohm's law due to the
competition between magnetic dipoles and monopoles.",1310.6588v1
2013-10-28,Probing the structure of local magnetic field of solar features with helioseismology,"Motivated by the problem of local solar subsurface magnetic structure, we
have used numerical simulation to investigate the propagation of waves through
monolithic magnetic flux tubes of different size. A cluster model can be a good
approximation to simulate sunspots as well as solar plage regions which are
composed of an ensemble of compactly packed thin flux tubes. Simulations of
this type is a powerful tool to probe the structure and the dynamic of various
solar features which are related directly to solar magnetic field activity.",1310.7398v1
2014-03-25,Magnetic nanoparticle sensing: decoupling the magnetization from the excitation field,"Remote sensing of magnetic nanoparticles has exciting applications for
magnetic nanoparticle hyperthermia and molecular detection. We introduce,
simulate, and experimentally demonstrate an innovation---a sensing coil that is
geometrically decoupled from the excitation field---for magnetic nanoparticle
spectroscopy that increases the flexibility and capabilities of remote
detection. The decoupling enhances the sensitivity absolutely; to small amounts
of nanoparticles, and relatively; to small changes in the nanoparticle
dynamics. We adapt a previous spectroscopic method that measures the relaxation
time of nanoparticles and demonstrate a new measurement of nanoparticle
temperature that could potentially be used concurrently during hyperthermia.",1403.6432v1
2014-06-16,Applications of exchange coupled bi-magnetic hard/soft and soft/hard magnetic core/shell nanoparticles,"The applications of exchange coupled bi-magnetic hard/soft and soft/hard
ferromagnetic core/shell nanoparticles are reviewed. After a brief description
of the main synthesis approaches and the core/shell structural-morphological
characterization, the basic static and dynamic magnetic properties are
presented. Five different types of perspective applications, based on diverse
patents and research articles, are described: permanent magnets, recording
media, microwave absorption, biomedical applications and other applications.
Both the advantages of the core/shell morphology and some of the remaining
challenges are discussed.",1406.3966v2
2014-06-24,Atomistic calculation of the thickness and temperature dependence of exchange coupling through a dilute magnetic oxide,"The exchange coupling of two magnetic layers via a diffuse oxide interlayer
is studied with an atomistic spin model. We investigate the effect of magnetic
concentration and oxide layer thickness on the effective exchange coupling
strength and find an exponential dependence of the coupling strength on the
oxide thickness without the need for magnetic pinholes. Furthermore we show
that exchange coupling has a strong temperature dependence which is significant
for the reversal dynamics during heat assisted magnetic recording.",1406.6214v1
2015-05-01,Ising incommensurate spin resonance of CeCoIn$_{5}$: a dynamical precursor of the Q-phase,"It is shown by detailed inelastic neutron scattering experiments that the
gapped collective magnetic excitation of the unconventional superconductor
CeCoIn$_{5}$, the spin resonance mode, is incommensurate and that the
corresponding fluctuations are of Ising nature. The incommensurate peak
position of these fluctuations corresponds to the propagation vector of the
adjacent field induced static magnetic ordered phase, the so-called Q-phase.
Furthermore, the direction of the magnetic moment fluctuations is also the
direction of the ordered magnetic moments of the Q-phase. Hence the resonance
mode and the Q-phase share the same symmetry and this strongly supports a
scenario where the static order is realized by a condensation of the magnetic
excitation.",1505.00206v1
2015-12-08,Topological Entropy of Left-Invariant Magnetic Flows on 2-Step Nilmanifolds,"We consider magnetic flows on 2-step nilmanifolds $M = \Gamma \backslash G$,
where the Riemannian metric $g$ and the magnetic field $\sigma$ are
left-invariant. Our first result is that when $\sigma$ represents a rational
cohomology class and its restriction to $\mathfrak{g} = T_eG$ vanishes on the
derived algebra, then the associated magnetic flow has zero topological
entropy. In particular, this is the case when $\sigma$ represents a rational
cohomology class and is exact. Our second result is the construction of a
magnetic field on a 2-step nilmanifold that has positive topological entropy
for arbitrarily high energy levels.",1512.02612v1
2016-01-04,Generation of coherent radiation by magnetization reversal in graphene,"Local magnetic moments can be created in graphene by incorporating different
defects. The possibility of regulating dynamics of magnetization in graphene,
by employing the Purcell effect, is analyzed. The role of the system parameters
in magnetization reversal is studied. The characteristics of such a reversal
can be varied in a wide range, which can be used for various applications in
spintronics. It is shown that fast magnetization reversal generates coherent
radiation.",1601.00464v1
2016-02-22,Spintronics with magnetic nanomolecules and graphene flakes,"We show how the magnetization of nano-objects can be efficiently regulated.
Several types of nanosystems are considered: magnetics nanomolecules, magnetic
nanoclusters, polarized nanomolecules, and magnetic graphene. These
nano-objects and the structures composed of them enjoy many common properties,
with the main difference being in the type of particle interactions. The
possibility of governing spin dynamics is important for spintronics.",1602.06834v1
2016-03-12,Covariant Magnetic Connection Hypersurfaces,"In the single fluid, nonrelativistic, ideal-Magnetohydrodynamic (MHD) plasma
description magnetic field lines play a fundamental role by defining
dynamically preserved ""magnetic connections"" between plasma elements. Here we
show how the concept of magnetic connection needs to be generalized in the case
of a relativistic MHD description where we require covariance under arbitrary
Lorentz transformations. This is performed by defining 2-D {\it magnetic
connection hypersurfaces} in the 4-D Minkowski space. This generalization
accounts for the loss of simultaneity between spatially separated events in
different frames and is expected to provide a powerful insight into the 4-D
geometry of electromagnetic fields when ${\bf E} \cdot {\bf B} = 0$.",1603.03909v1
2016-07-12,Tailoring the topological details of the magnetic skyrmion by the spin configuration at the edges,"The magnetic skyrmion structure can be formed in the chiral magnets (CMs)
with strong Dzyaloshinskii-Moriya interactions. In this work, we propose a way
of artificially tailoring the topological details of the skyrmion such as its
radial and whirling symmetric patterns by external magnetic fields besieging
the CM slab. As long as the boundary magnetic fields are strong enough to fix
the boundary ferromagnetism, the attained skyrmion profile is stable over time.
The dynamics of spins is considered by numerically solving the non-equilibrium
Landau-Lifshitz-Gilbert equation.",1607.03268v2
2016-08-02,Topologically protected colloidal transport above a square magnetic lattice,"We theoretically study the motion of magnetic colloidal particles above a
magnetic pattern and compare the predictions with Brownian dynamics
simulations. The pattern consists of alternating square domains of positive and
negative magnetization. The colloidal motion is driven by periodic modulation
loops of an external magnetic field. There exist loops that induce
topologically protected colloidal transport between two different unit cells of
the pattern. The transport is very robust against internal and external
perturbations. Theory and simulations are in perfect agreement.",1608.00808v2
2017-01-01,Non-Abelian bootstrap of primordial magnetism,"We point out that a primordial magnetic field can be generated in the
electroweak phase transition by a non-Abelian bootstrap, where the field is
generated by currents of W's, which in turn are extracted from the vacuum by
the magnetic field. This magnetic field is produced as a vortex condensate at
the electroweak phase transition. It becomes stringy as a consequence of the
dynamical evolution due to magnetohydrodynamics.",1701.00245v3
2017-03-06,Controlling topological superconductivity by magnetization dynamics,"We study theoretically a chain of precessing classical magnetic impurities in
an $s$-wave superconductor. Utilizing a rotating wave description, we derive an
effective Hamiltonian that describes the emergent Shiba band. We find that this
Hamiltonian shows non-trivial topological properties, and we obtain the
corresponding topological phase diagrams both numerically and analytically. We
show that changing the precession frequency offers a control over topological
phase transitions and the emergence of Majorana bound states. We propose
driving the magnetic impurities or magnetic texture into precession by means of
spin-transfer torque in a spin-Hall setup, and manipulate it using spin
superfluidity in the case of planar magnetic order.",1703.01870v1
2017-08-21,Amplification of large-scale magnetic field in nonhelical magnetohydrodynamics,"It is typically assumed that the kinetic and magnetic helicities play a
crucial role in the growth of large-scale dynamo. In this paper we demonstrate
that helicity is not essential for the amplification of large-scale magnetic
field. For this purpose, we perform nonhelical magnetohydrodynamic (MHD)
simulation, and show that the large-scale magnetic field can grow in nonhelical
MHD when random external forcing is employed at scale $1/10$ the box size. The
energy fluxes and shell-to-shell transfer rates computed using the numerical
data show that the large-scale magnetic energy grows due to the energy
transfers from the velocity field at the forcing scales.",1708.06120v1
2017-11-14,Spin-Noise and Damping in Individual Metallic Ferromagnetic Nanoparticles,"We introduce a highly sensitive and relatively simple technique to observe
magnetization motion in single Ni nanoparticles, based on charge sensing by
electron tunneling at millikelvin temperature. Sequential electron tunneling
via the nanoparticle drives nonequilibrium magnetization dynamics, which
induces an effective charge noise that we measure in real time. In the free
spin diffusion regime, where the electrons and magnetization are in detailed
balance, we observe that magnetic damping time exhibits a peak with the
magnetic field, with a record long damping time of $\simeq 10$~ms.",1711.05142v1
2017-11-15,Reduction of electron heating by magnetizing ultracold neutral plasma,"Electron heating in an ultracold neutral plasma is modeled using classical
molecular dynamics simulations in the presence of an externally applied
magnetic field. A sufficiently strong magnetic field is found to reduce
disorder induced heating and three body recombination heating of electrons by
constraining electron motion, and therefore heating, to the single dimension
aligned with the magnetic field. A strong and long-lasting temperature
anisotropy develops, and the overall kinetic electron temperature is
effectively reduced by a factor of three. These results suggest that
experiments may increase the effective electron coupling strength using an
applied magnetic field.",1711.05617v1
2017-12-30,Chiral symmetry breaking in a semi-localized magnetic field,"In this work, I'm going to explore the pattern of chiral symmetry breaking
and restoration in a solvable magnetic distribution within Nambu--Jona-Lasinio
model. The special semi-localized static magnetic field can roughly simulate
the realistic situation in peripheral heavy ion collisions, thus the study is
important for the dynamical evolution of quark matter. I find that the magnetic
field dependent contribution from discrete spectra usually dominates over that
from continuum ones and chiral symmetry breaking is locally catalyzed by both
the magnitude and scale of the magnetic field.",1801.00134v1
2020-04-27,Nonreciprocal directional dichroism induced by a temperature gradient as a probe for mobile spin dynamics in quantum magnets,"Novel states of matter in quantum magnets like quantum spin liquids attract
considerable interest recently. Despite the existence of a plenty of candidate
materials, there is no confirmed quantum spin liquid, largely due to the lack
of proper experimental probes. For instance, spectrosocopy experiments like
neutron scattering receive contributions from disorder-induced local modes,
while thermal transport experiments receive contributions from phonons. Here we
propose a thermo-optic experiment which directly probes the mobile magnetic
excitations in spatial-inversion symmetric and/or time-reversal symmetric Mott
insulators: the temperature-gradient-induced nonreciprocal directional
dichroism (TNDD) spectroscopy. Unlike traditional probes, TNDD directly detects
mobile magnetic excitations and decouples from phonons and local magnetic
modes.",2004.13047v1
2020-09-03,Measurements of sub-nT dynamic magnetic field shielding with soft iron and mu-metal for use in linear colliders,"There is an increasing need to shield beams and accelerator elements from
stray magnetic fields. The application of magnetic shielding in linear
colliders is discussed. The shielding performance of soft iron and mu-metal is
measured for magnetic fields of varying amplitude and frequency. Special
attention is given to characterise the shielding performance for very
small-amplitude magnetic fields.",2009.01519v2
2020-09-29,Stationary solution for quasi-homogeneous small-scale magnetic field advected by non-Gaussian turbulent flow,"We consider fluctuations of magnetic field excited by external force and
advected by isotropic turbulent flow. It appears that non-Gaussian velocity
gradient statistics and finite region of pumping force provide the existence of
stationary solution. The mean-square magnetic field is calculated for arbitrary
velocity gradient statistics. An estimate for possible feedback of magnetic
field on velocity shows that, for wide range of parameters, stationarity
without feedback would take place even in the case of intensive pumping of
magnetic field.",2009.13746v1
2021-03-26,Linear damping and depletion in flowing plasma with strong sheared magnetic fields,"In this paper, we study the long-time behavior of the solution for the
linearized ideal MHD around sheared velocity and magnetic field under Stern
stability condition. We prove that the velocity and magnetic field will
converge to sheared velocity and magnetic field as time approaches infinity.
Moreover a new depletion phenomenon is proved: the horizontal velocity and
magnetic field at the critical points will decay to 0 as time approaches
infinity.",2103.14614v1
2023-10-02,Magnetization of Antiferromagnetic Cactus Graph Model with Exact Dimer Ground State,"We introduce and explore the magnetization behavior in a quantum spin system
on a cactus graph, deemed the Cactus Graph Model (CGM), featuring an exact
dimer singlet ground state. We analyze the singlet-triplet gap, interactions
among excited triplets, and correlated hopping under an external magnetic field
using a strong coupling expansion. Employing an effective hard-core boson
representation, we unveil density wave magnetization plateaus and supersolid
phases, revealing the intricate interplay between interactions and hopping
dynamics. Furthermore, we conjecture that correlated hopping gives rise to
multi-triplet bound states, and potentially engendering to the stabilization of
additional low-lying magnetization plateaus.",2310.01570v1
2023-10-30,Motions of a homopolar motor inside a conducting tube,"We analyze the physics of a type of homopolar motor comprising an AA battery
with two cylindrical neodymium magnets on each end that roll inside a metal
cylindrical tube. The motion of the motor results from the interaction between
the magnetic field of the magnets and the magnetic field created by the current
inside the magnets. We develop a model to describe the dynamics of the system,
including the calculation of the terminal velocity of the motor due to eddy
currents.",2310.19249v1
2023-11-01,Topological transport of vorticity on curved magnetic membranes,"In this work, we study the transport of vorticity on curved dynamical
two-dimensional magnetic membranes. We find that topological transport can be
controlled by geometrically reducing symmetries, which enables processes that
are not present in flat magnetic systems. To this end, we construct a vorticity
3-current which obeys a continuity equation. This continuity equation is immune
to local fluctuations of the magnetic texture as well as spatiotemporal
fluctuations of the membrane. We show how electric current can manipulate
vortex transport in geometrically nontrivial magnetic systems. As an
illustrative example, we propose a minimal setup that realizes an
experimentally feasible energy storage device.",2311.00323v1
2023-12-08,Decay and Fission of Magnetic Quivers,"In exploring supersymmetric theories with 8 supercharges, the Higgs branches
present an intriguing window into strong coupling dynamics. Magnetic quivers
serve as crucial tools for understanding these branches. Here, we introduce the
decay and fission algorithm for unitary magnetic quivers. It efficiently
derives complete phase diagrams (Hasse diagrams) through convex linear algebra.
It allows magnetic quivers to undergo decay or fission, reflecting Higgs branch
RG-flows in the theory. Importantly, the algorithm generates magnetic quivers
for the RG fixed points and simplifies the understanding of transverse slice
geometry with no need for a list of minimal transitions. In contrast, the
algorithm hints to the existence of a new minimal transition, whose geometry
and physics needs to be explored.",2312.05304v1
2023-12-24,A Novel Field-Free SOT Magnetic Tunnel Junction With Local VCMA-Induced Switching,"By integrating the local voltage-controlled magnetic anisotropy (VCMA)
effect, Dzyaloshinskii-Moriya interaction (DMI) effect, and spin-orbit torque
(SOT) effect, we propose a novel device structure for field-free magnetic
tunnel junction (MTJ). Micromagnetic simulation shows that the device utilizes
the chiral symmetry breaking caused by the DMI effect to induce a non-collinear
spin texture under the influence of SOT current. This, combined with the
perpendicular magnetic anisotropy (PMA) gradient generated by the local VCMA
effect, enables deterministic switching of the MTJ state without an external
field. The impact of variations in DMI strength and PMA gradient on the
magnetization dynamics is analyzed.",2312.15481v1
2002-01-02,A Coupled Dynamical and Chemical Model of Starless Cores of Magnetized Molecular Clouds: I. Formulation and Initial Results,"We develop a detailed chemical model for the starless cores of strongly
magnetized molecular clouds, with the ambipolar diffusion-driven dynamic
evolution of the clouds coupled to the chemistry through ion abundances. We
concentrate on two representative model clouds in this initial study, one with
magnetic fields and the other without. The model predictions on the peak values
and spatial distributions of the column densities of CO, CCS, N$_2$H$^+$ and
HCO$^+$ are compared with those observationally inferred for the well-studied
starless core L1544, which is thought to be on the verge of star formation. We
find that the magnetic model, in which the cloud is magnetically supported for
several million years before collapsing dynamically, provides a reasonable
overall fit to the available data on L1544; the fit is significantly worse for
the non-magnetic model, in which the cloud collapses promptly. The observed
large peak column density for N$_2$H$^+$ and clear central depression for CCS
favor the magnetically-retarded collapse over the free-fall collapse. A
relatively high abundance of CCS is found in the magnetic model, resulting most
likely from an interplay of depletion and late-time hydrocarbon chemistry
enhanced by CO depletion. These initial results lend some support to the
standard picture of dense core formation in strongly magnetized clouds through
ambipolar diffusion. They are at variance with those of Aikawa et al. (2001)
who considered a set of models somewhat different from ours and preferred one
in which the cloud collapses more or less freely for L1544.",0201019v1
2011-07-22,Interaction of Josephson and magnetic oscillations in Josephson tunnel junctions with a ferromagnetic layer,"We study the dynamics of Josephson junctions with a thin ferromagnetic layer
F [superconductor-ferromagnet-insulator-ferromagnet-superconductor (SFIFS)
junctions]. In such junctions, the phase difference $\phi$ of the
superconductors and magnetization $M$ in the F layer are two dynamic parameters
coupled to each other. We derive equations describing the dynamics of these two
parameters and formulate the conditions of validity. The coupled Josephson
plasma waves and oscillations of the magnetization $M$ affect the form of the
current-voltage ($I$-$V$) characteristics in the presence of a weak magnetic
field (Fiske steps). We calculate the modified Fiske steps and show that the
magnetic degree of freedom not only changes the form of the Fiske steps but
also the overall view of the $I$-$V$ curve (new peaks related to the magnetic
resonance appear). The $I$-$V$ characteristics are shown for different lengths
of the junction including those which correspond to the current experimental
situation. We also calculate the power $P$ absorbed in the system if a
microwave radiation with an ac in-plane magnetic field is applied (magnetic
resonance). The derived formula for the power $P$ essentially differs from the
one which describes the power absorption in an isolated ferromagnetic film. In
particular, this formula describes the peaks related to the excitation of
standing plasma waves as well as the peak associated with the magnetic
resonance.",1107.4493v2
2015-03-02,Spin-glass-like freezing of inner and outer surface layers in hollow γFe$_2$O$_3$ nanoparticles,"Disorder among surface spins largely dominates the magnetic response of
ultrafine magnetic particle systems. In this work, we examine time-dependent
magnetization in high-quality, monodisperse hollow maghemite nanoparticles with
a 14.8 $\pm$ 0.5 nm outer diameter and enhanced surface-to-volume ratio. The
nanoparticle ensemble exhibits spin-glass-like signatures in dc magnetic aging
and memory protocols and ac magnetic susceptibility. The dynamics of the system
slows near 50 K, and becomes frozen on experimental time scales below 20 K.
Remanence curves indicate the development of magnetic irreversibility
concurrent with the freezing of the spin dynamics. A strong exchange-bias
effect and its training behavior point to highly frustrated surface spins that
rearrange much more slowly than interior spins with bulk coordination. Monte
Carlo simulations of a hollow particle reproducing the experimental morphology
corroborated strongly disordered surface layers with complex energy landscapes
that underlie both glass-like dynamics and magnetic irreversibility. Calculated
hysteresis loops reveal that magnetic behavior is not identical at the inner
and outer surfaces, with spins at the outer surface layer of the 15 nm hollow
particles exhibiting a higher degree of frustration. Our study sheds light on
the origin of spin-glass-like phenomena and the role of surface spins in
magnetic hollow nanostructures.",1503.00602v2
2018-09-24,Quantum spin-transfer torque induced nonclassical magnetization dynamics and electron-magnetization entanglement,"The standard spin-transfer torque (STT)---where spin-polarized current drives
dynamics of magnetization viewed as a classical vector---requires
noncollinearity between electron spins carried by the current and magnetization
of a ferromagnetic layer. However, recent experiments [A. Zholud et al., Phys.
Rev. Lett. 119, 257201 (2017)] observing magnetization dynamics in spin valves
at cryogenic temperatures, even when electron spin is collinear to
magnetization, point at overlooked quantum effects in STT which can lead to
highly nonclassical magnetization states. Using fully quantum many-body
treatment, where an electron injected as spin-polarized wave packet interacts
with local spins comprising the anisotropic quantum Heisenberg ferromagnetic
chain, we define quantum STT as any time evolution of local spins due to
initial many-body state not being an eigenstate of electron+local-spins system.
For time evolution caused by injected spin-down electron scattering off local
up-spins, entanglement between electron subsystem and local spins subsystem
takes place leading to decoherence and, therefore, shrinking of the total
magnetization but without rotation from its initial orientation which explains
the experiments. Furthermore, the same processes---entanglement and thereby
induced decoherence---are present also in standard noncollinear geometry,
together with the usual magnetization rotation. This is because STT in quantum
many-body picture is caused only by electron spin-down factor state, and the
only difference between collinear and noncollinear geometries is in relative
size of the contribution of initial many-body state containing such factor
state to superpositions of separable many-body quantum states generated during
time evolution.",1809.09090v1
2019-11-28,Manipulating Spin Chirality of Magnetic Skyrmion Bubbles by In-Plane Re-versed Magnetic Fields in (Mn$_{1-x}$Ni$_x$)$_{65}$Ga$_{35}$ ($x = 0.45$) magnet,"Understanding the dynamics of the magnetic skyrmion, a particle-like
topologically stable spin texture, and its response dynamics to external fields
are indis-pensable for the applications in spintronic devices. In this letter,
the Lorentz transmis-sion electron microscopy (LTEM) was used to investigate
the spin chirality of the mag-netic skyrmion bubbles (SKBs) in the
centrosymmetric magnet MnNiGa at room tem-perature. The reversal of SKBs
excited by the in-plane magnetic field has been revealed. Moreover, the
collective behavior of interacting spin chirality can be manipulated by
reversing the directions of the magnetic fields on a wedge-shaped thin plate.
The dy-namic behavior of the bubbles at different position of the thin plate
has been explored with the micromagnetic simulation, indicating a non-uniform
and nontrivial dynamic magnetization on the surfaces and center of the thin
plate during the spin chirality re-versal. The results suggest that the
controllable symmetry breaking of the SKBs arising from thickness variation
provides an ability to manipulate the collective behavior of the spin chirality
with small external fields, leading to a promising application in nonvola-tile
spintronic devices for magnetic skyrmions.",1911.12497v1
2019-12-20,Picosecond spin-orbit torque switching of ferrimagnets,"Spintronics provides an efficient platform for realizing non-volatile memory
and logic devices. In these systems, data is stored in the magnetization of
magnetic materials, and magnetization is switched in the writing process. In
conventional spintronic devices, ferromagnetic materials are used which have a
magnetization dynamics timescale of around the nanoseconds, setting a limit for
the switching speed. Increasing the magnetization switching speed has been one
of the challenges in spintronic research. In this work we take advantage of the
ultrafast magnetization dynamics in ferrimagnetic materials instead of
ferromagnets, and we use femtosecond laser pulses and a photoconductive Auston
switch to create picosecond current pulses for switching the ferrimagnet. By
anomalous Hall and magneto-optic Kerr (MOKE) measurement, we demonstrate the
robust picosecond SOT driven magnetization switching of ferrimagnetic GdFeCo.
The time-resolved MOKE shows more than 50 GHz magnetic resonance frequency of
GdFeCo, indicating faster than 20 ps spin dynamics and tens of picosecond SOT
switching speed. Our work provides a promising route to realize picosecond
operation speed for non-volatile magnetic memory and logic applications.",1912.10129v1
2020-01-21,Multi-scale dynamics of magnetic flux tubes and inverse magnetic energy transfer,"We report on an analytical and numerical study of the dynamics of a
three-dimensional array of identical magnetic flux tubes in the
reduced-magnetohydrodynamic description of the plasma. We propose that the
long-time evolution of this system is dictated by flux-tube mergers and that
such mergers are dynamically constrained by the conservation of the pertinent
(ideal) invariants, {\it viz.} the magnetic potential and axial fluxes of each
tube. We also propose that in the direction perpendicular to the merging plane,
flux tubes evolve in critically-balanced fashion. These notions allow us to
construct an analytical model for how quantities such as the magnetic energy
and the energy-containing scale evolve as functions of time. Of particular
importance is the conclusion that, like its two-dimensional counterpart, this
system exhibits an inverse transfer of magnetic energy that terminates only at
the system scale. We perform direct numerical simulations that confirm these
predictions and reveal other interesting aspects of the evolution of the
system. We find, for example, that the early time evolution is characterized by
a sharp decay of the initial magnetic energy, which we attribute to the
ubiquitous formation of current sheets. We also show that a quantitatively
similar inverse transfer of magnetic energy is observed when the initial
condition is a random, small-scale magnetic seed field.",2001.07291v2
2023-01-17,Three Dimensional Odd Viscosity in Ferrofluids with Vorticity-Magnetization Coupling,"Ferrofluids are a synthetic magnetic colloid consisting of magnetized
nanoparticles surrounded by a repulsive surfactant layer. When subjected to an
external magnetic field the ferrofluid acquires a macroscopic magnetization
density which leads to magnetic behavior that is intricately coupled to the
ambient fluid dynamics. Ferrofluids share several features with the chiral
active fluids composed of unidirectionally spinning hematite cubes, which have
been shown to possess a 2D non-dissipative odd viscosity term (Nature Physics,
15, 1188-1194(2019)). In standard ferrofluid dynamics, 3D versions of parity
breaking terms are not commonly observed, partly because of the small size of
the magnetic particles. In this work, we investigate if there are unique
mechanisms in ferrofluids that can lead to a 3D odd viscosity term. Our results
show that coupling the fluid vorticity ($\vec{\omega}$) to the magnetization
($\vec{M}$) with a term proportional to $\vec{\omega}\cdot\vec{M}$ leads to
parity breaking terms in ferrofluid hydrodynamics, and results in a three
dimensional odd viscosity term when the magnetization is relaxed to the
direction of a uniform and static applied field. Hele-Shaw cells are commonly
used devices to investigate ferrofluids and we demonstrate that this coupling
reproduces the parity odd generalization of Darcy's Law discussed in a recent
work (Phys. Rev. Fluids 7, 114201 (2022)). A potential experimental setup is
discussed which may reveal the presence of this coupling in a ferrofluid
confined to a Hele-Shaw cell.",2301.07096v1
2023-06-02,Phase separation of a magnetic fluid: Asymptotic states and non-equilibrium kinetics,"We study self-assembly in a colloidal suspension of magnetic particles by
performing comprehensive molecular dynamics simulations of the Stockmayer (SM)
model which comprises spherical particles decorated by a magnetic moment. The
SM potential incorporates dipole-dipole interactions along with the usual
Lennard-Jones interaction and exhibits a gas-liquid phase coexistence observed
experimentally in magnetic fluids. When this system is quenched from the
high-temperature homogeneous phase to the coexistence region, the
non-equilibrium evolution to the condensed phase proceeds with the development
of spatial as well as magnetic order. We observe density-dependent coarsening
mechanisms - a diffusive growth law $\ell(t)\sim t^{1/3}$ in the nucleation
regime, and hydrodynamics-driven inertial growth law $\ell(t)\sim t^{2/3}$ in
the spinodal regimes. [$\ell(t)$ is the average size of the condensate at time
$t$ after the quench.] While the spatial growth is governed by the expected
conserved order parameter dynamics, the growth of magnetic order in the
spinodal regime exhibits unexpected non-conserved dynamics. The asymptotic
morphologies have density-dependent shapes which typically include the
isotropic sphere and spherical bubble morphologies in the nucleation region,
and the anisotropic cylinder, planar slab, cylindrical bubble morphologies in
the spinodal region. The structures are robust and nonvolatile and exhibit
characteristic magnetic properties. For example, the oppositely magnetized
hemispheres in the spherical morphology impart the characteristics of a {\it
Janus particle} to it. The observed structures have versatile applications in
catalysis, drug delivery systems, memory devices, and magnetic photonic
crystals, to name a few.",2306.01430v2
2023-07-16,Magnetic Field-Based Reward Shaping for Goal-Conditioned Reinforcement Learning,"Goal-conditioned reinforcement learning (RL) is an interesting extension of
the traditional RL framework, where the dynamic environment and reward sparsity
can cause conventional learning algorithms to fail. Reward shaping is a
practical approach to improving sample efficiency by embedding human domain
knowledge into the learning process. Existing reward shaping methods for
goal-conditioned RL are typically built on distance metrics with a linear and
isotropic distribution, which may fail to provide sufficient information about
the ever-changing environment with high complexity. This paper proposes a novel
magnetic field-based reward shaping (MFRS) method for goal-conditioned RL tasks
with dynamic target and obstacles. Inspired by the physical properties of
magnets, we consider the target and obstacles as permanent magnets and
establish the reward function according to the intensity values of the magnetic
field generated by these magnets. The nonlinear and anisotropic distribution of
the magnetic field intensity can provide more accessible and conducive
information about the optimization landscape, thus introducing a more
sophisticated magnetic reward compared to the distance-based setting. Further,
we transform our magnetic reward to the form of potential-based reward shaping
by learning a secondary potential function concurrently to ensure the optimal
policy invariance of our method. Experiments results in both simulated and
real-world robotic manipulation tasks demonstrate that MFRS outperforms
relevant existing methods and effectively improves the sample efficiency of RL
algorithms in goal-conditioned tasks with various dynamics of the target and
obstacles.",2307.08033v1
2023-09-12,Quantum Diamond Microscope for Dynamic Imaging of Magnetic Fields,"Wide-field imaging of magnetic signals using ensembles of nitrogen-vacancy
(NV) centers in diamond has garnered increasing interest due to its combination
of micron-scale resolution, millimeter-scale field of view, and compatibility
with diverse samples from across the physical and life sciences. Recently,
wide-field NV magnetic imaging based on the Ramsey protocol has achieved
uniform and enhanced sensitivity compared to conventional measurements. Here,
we integrate the Ramsey-based protocol with spin-bath driving to extend the NV
spin dephasing time and improve magnetic sensitivity. We also employ a
high-speed camera to enable dynamic wide-field magnetic imaging. We benchmark
the utility of this quantum diamond microscope (QDM) by imaging magnetic fields
produced from a fabricated wire phantom. Over a $270\times270
\hspace{0.08333em} \mu\mathrm{m}$$^2$ field of view, a median per-pixel
magnetic sensitivity of
$4.1(1)\hspace{0.08333em}\mathrm{nT}$$/\sqrt{\mathrm{Hz}}$ is realized with a
spatial resolution
$\lesssim\hspace{0.08333em}10\hspace{0.08333em}\mu\mathrm{m}$ and
sub-millisecond temporal resolution. Importantly, the spatial magnetic noise
floor can be reduced to the picotesla scale by time-averaging and signal
modulation, which enables imaging of a magnetic-field pattern with a
peak-to-peak amplitude difference of about $300\hspace{0.08333em}\mathrm{pT}$.
Finally, we discuss potential new applications of this dynamic QDM in studying
biomineralization and electrically-active cells.",2309.06587v1
2023-11-07,Chaotic transport in symplectic maps: Applications in plasma,"Chaotic transport is related to the complex dynamical evolution of chaotic
trajectories in Hamiltonian systems, which models various physical processes.
In magnetically confined plasma, it is possible to qualitatively describe the
configuration of the magnetic field via the phase space of suitable symplectic
maps. These phase spaces are of mixed type, where chaos coexists with regular
motion, and the complete understanding of the chaotic transport is a challenge
that, when overcomed, may provide further knowledge into the behaviour of
confined fusion plasma. In this research, we focus our investigation on
properties of chaotic transport in mixed phase spaces of two symplectic maps
that model the magnetic field lines of tokamaks under distinct configurations.
The single-null divertor map, or Boozer map, models the field lines of tokamaks
with poloidal divertors. The ergodic magnetic limiter map, or Ullmann map,
models the magnetic configuration of tokamaks assembled with ergodic magnetic
limiters. We propose two numerical methods developed to study and illustrate
the differences between the transient behaviour of open field lines in both
models while considering induced magnetic configurations that either enhance or
restrain the escaping field lines. The first analysis shows that the spatial
organisation of invariant manifolds creates fitting transport channels for the
open field lines, influencing the average dynamical evolution in the selected
magnetic configurations. The second identifies trajectories that widely differs
from the average chaotic behaviour, specifically detecting the stickiness
phenomenon, which can be related to additional confinement regions in the
nearest surroundings of magnetic islands in the plasma edge. These analyses
may, ultimately, assist in selecting optimal experimental parameters to achieve
specific goals in tokamak discharges.",2311.04028v1
2004-09-22,On the Rotational Dynamics of Magnetically Threaded Disks around Neutron Stars,"We investigate the rotational dynamics of disk accretion around a strongly
magnetized neutron star with an aligned dipole field. The magnetospheric field
is assumed to thread the disk plasma both inside and outside the corotation
radius. As a result of disk-star interaction, the magnetic torque on the disk
affects the structure of accretion flow to yield the observed spin-up or
spin-down rates for a source of given fastness, magnetic field strength, and
mass accretion rate. Within the model we obtain a prescription for the
dynamical viscosity of such magnetically modified solutions for a Keplerian
disk. We then use this prescription to find a model solution for the rotation
rate profile throughout the entire disk including the non-Keplerian inner disk.
We find that the non-Keplerian angular velocity transition region is not
necessarily narrow for a source of given spin state. The boundary layer
approximation, as in the standard magnetically threaded disk model, holds only
in the case of dynamical viscosity decreasing all the way to the innermost edge
of the disk. These results are applied to several observed disk-fed X-ray
pulsars that have exhibited quasi-periodic oscillations (QPOs). The QPO
frequencies provide a constraint on the fastness parameter and enable one to
determine uniquely the width of the angular velocity transition zone for each
source within model assumptions. We discuss the implications of these results
on the value of the critical fastness parameter for a magnetized star in spin
equilibrium. Applications of our model are also made with relevant parameters
from recent numerical simulations of quasi-stationary disk-magnetized star
interactions.",0409545v1
2005-04-12,Complex Topology of the Magnetic Field in Strong Flares,"We report the ""5+1"" dynamical classification of the most frequently observed
topologies of the magnetic field in sunspot groups associated with powerful
flares.",0504259v1
2007-02-19,Dynamics of magnetized relativistic tori oscillating around black holes,"We present a numerical study of the dynamics of magnetized, relativistic,
non-self-gravitating, axisymmetric tori orbiting in the background spacetimes
of Schwarzschild and Kerr black holes. The initial models have a constant
specific angular momentum and are built with a non-zero toroidal magnetic field
component, for which equilibrium configurations have recently been obtained. In
this work we extend our previous investigations which dealt with purely
hydrodynamical thick discs, and study the dynamics of magnetized tori subject
to perturbations which, for the values of the magnetic field strength
considered here, trigger quasi-periodic oscillations lasting for tens of
orbital periods. Overall, we have found that the dynamics of the magnetized
tori analyzed is very similar to that found in the corresponding unmagnetized
models. The spectral distribution of the eigenfrequencies of oscillation shows
the presence of a fundamental p mode and of a series of overtones in a harmonic
ratio 2:3:.... These simulations, therefore, extend the validity of the model
of Rezzolla et al.(2003a) for explaining the high-frequency QPOs observed in
the spectra of LMXBs containing a black-hole candidate also to the case of
magnetized discs with purely toroidal magnetic field distribution. If
sufficiently compact and massive, these oscillations can also lead to the
emission of intense gravitational radiation which is potentially detectable for
sources within the Galaxy.",0702485v1
2007-02-26,Electrical switching of vortex core in a magnetic disk,"A magnetic vortex is a curling magnetic structure realized in a ferromagnetic
disk, which is a promising candidate of a memory cell for future nonvolatile
data storage devices. Thus, understanding of the stability and dynamical
behaviour of the magnetic vortex is a major requirement for developing magnetic
data storage technology. Since the experimental proof of the existence of a
nanometre-scale core with out-of-plane magnetisation in the magnetic vortex,
the dynamics of a vortex has been investigated intensively. However, the way to
electrically control the core magnetisation, which is a key for constructing a
vortex core memory, has been lacking. Here, we demonstrate the electrical
switching of the core magnetisation by utilizing the current-driven resonant
dynamics of the vortex; the core switching is triggered by a strong dynamic
field which is produced locally by a rotational core motion at a high speed of
several hundred m/s. Efficient switching of the vortex core without magnetic
field application is achieved thanks to resonance. This opens up the
potentiality of a simple magnetic disk as a building block for spintronic
devices like a memory cell where the bit data is stored as the direction of the
nanometre-scale core magnetisation.",0702589v1
2010-09-30,The dynamics of magnetization in phase separated manganite around half doping: a case study for Pr$_{0.5}$Sr$_{0.5}$Mn$_{0.925}$Ga$_{0.075}$O$_{3}$,"We investigate the dynamics of magnetization in the phase separated (PS)
state after introducing the quenched disorder at the Mn-site of a manganite
around half doping. The compound,
Pr$_{0.5}$Sr$_{0.5}$Mn$_{0.925}$Ga$_{0.075}$O$_{3}$, exhibits PS with the
coexistence of ferromagnetic (FM) and antiferromagnetic (AFM) clusters where
the size of the FM clusters is substantially reduced due to the disorder
introduced by nonmagnetic Ga substitution. At low temperature, the system
develops a new magnetic anomaly, which is marked by a peak in the zero field
cooled magnetization. Detailed study of linear as well as nonlinear ac
susceptibilities coupled with dc magnetization indicates that this peak arises
due to the thermal blocking of nanometer size FM clusters demonstrating
superparamagnetic behavior. The system, however, exhibits slow magnetic
relaxation, aging effect, memory effect in both field cooled and zero field
cooled magnetization below the blocking temperature. These imply the presence
of collective behavior induced by the interaction between the clusters.
Moreover, the magnetic relaxation measured with positive and negative
temperature excursions exhibits asymmetric response suggesting that the
dynamics in this phase separated system is accounted by the hierarchical model
rather than the droplet model which are commonly used to describe the similar
collective dynamics in glassy system.",1009.6156v1
2016-01-05,Generic properties of magnetic flows,"We obtain Kupka-Smale theorem and Franks lemma for magnetic flows on
manifolds with any dimension. This improves Miranda result on surfaces. However
our methods relies on geometric control theory, like in Rifford and Ruggiero
articles.",1601.00935v1
2016-06-16,Calculating rotating hydrodynamic and magneto-hydrodynamic waves to understand magnetic effects on dynamical tides,"For understanding magnetic effects on dynamical tides, we study the rotating
magneto-hydrodynamic (MHD) flow driven by harmonic forcing. The linear
responses are analytically derived in a periodic box under the local WKB
approximation. Both the kinetic and Ohmic dissipations at the resonant
frequencies are calculated and the various parameters are investigated.
Although magnetic pressure may be negligible compared to thermal pressure,
magnetic field can be important for the first-order perturbation, e.g.
dynamical tides. It is found that magnetic field splits the resonant frequency,
namely the rotating hydrodynamic flow has only one resonant frequency but the
rotating MHD flow has two, one positive and the other negative. In the weak
field regime the dissipations are asymmetric around the two resonant
frequencies and this asymmetry is more striking with a weaker magnetic field.
It is also found that both the kinetic and Ohmic dissipations at the resonant
frequencies are inversely proportional to the Ekman number and the square of
wavenumber. The dissipation at the resonant frequency on small scales is almost
equal to the dissipation at the non-resonant frequencies, namely the resonance
takes its effect on the dissipation at intermediate length scales. Moreover,
the waves with phase propagation perpendicular to magnetic field are much more
damped. It is also interesting to find that the frequency-averaged dissipation
is constant. This result suggests that in compact objects magnetic effects on
tidal dissipation should be considered.",1606.06232v1
2017-05-18,"Comment on ""On the nature of magnetic stripes in cuprate superconductors,"" by H. Jacobsen et al., arXiv:1704.08528v2","Dynamics reduces the orthorhombicity of magnetic stripes in La_2CuO_4+y. The
measured stripe incommensuration can be used to determine the oxygen content of
the sample.",1705.06986v1
2017-12-14,Real Time Visualization of Dynamic Magnetic Fields with a Nanomagnetic FerroLens,"Due to advancements in nanomagnetism and latest nanomagnetic materials and
devices, a new potential field has been opened up for research and applications
which was not possible before. We herein propose a new research field and
application for nanomagnetism for the visualization of dynamic magnetic fields
in real-time. In short, Nano Magnetic Vision. A new methodology, technique and
apparatus were invented and prototyped in order to demonstrate and test this
new application. As an application example the visualization of the dynamic
magnetic field on a transmitting antenna was chosen. Never seen before
high-resolution, photos and real-time color video revealing the actual dynamic
magnetic field inside a transmitting radio antenna rod has been captured for
the first time. The antenna rod is fed with six hundred volts, orthogonal
pulses. This unipolar signal is in the very low frequency (i.e. VLF) range. The
signal combined with an extremely short electrical length of the rod, ensures
the generation of a relatively strong fluctuating magnetic field, analogue to
the signal transmitted, along and inside the antenna. This field is induced
into a ferrolens and becomes visible in real-time within the normal human eyes
frequency spectrum. The name we have given to the new observation apparatus is,
SPIONs Superparamagnetic Ferrolens Microscope (SSFM), a powerful passive
scientific observation tool with many other potential applications in the near
future.",1712.05436v1
2018-07-11,Transport phenomena with chiral fermions in strong magnetic fields,"I discuss the electrical conductivity, bulk viscosity, and heavy-quark
diffusion coefficient in strong magnetic fields putting emphasis on
consequences of the chirality conservation on the perturbative relaxation
dynamics.",1807.04150v1
2023-08-21,Magnetic fields in solar plage regions: insights from high-sensitivity spectropolarimetry,"Plage regions are patches of concentrated magnetic field in the Sun's
atmosphere where hot coronal loops are rooted. While previous studies have shed
light on the properties of plage magnetic fields in the photosphere, there are
still challenges in measuring the overlying chromospheric magnetic fields,
which are crucial to understanding the overall heating and dynamics. Here, we
utilize high-sensitivity, spectropolarimetric data obtained by the four-meter
Daniel K. Inouye Solar Telescope (DKIST) to investigate the dynamic environment
and magnetic field stratification of an extended, decaying plage region. The
data show strong circular polarization signals in both plage cores and
surrounding fibrils. Notably, weak linear polarization signals clearly
differentiate between plage patches and the fibril canopy, where they are
relatively stronger. Inversions of the Ca II 8542 $\mathring{A}$ spectra show
an imprint of the fibrils in the chromospheric magnetic field, with typical
field strength values ranging from $\sim$ 200-300 G in fibrils. We confirm the
weak correlation between field strength and cooling rates in the lower
chromosphere. Additionally, we observe supersonic downflows and strong velocity
gradients in the plage periphery, indicating dynamical processes occurring in
the chromosphere. These findings contribute to our understanding of the
magnetic field and dynamics within plages, emphasizing the need for further
research to explore the expansion of magnetic fields with height and the
three-dimensional distribution of heating rates in the lower chromosphere.",2308.10983v1
2008-04-08,Convective intensification of magnetic fields in the quiet Sun,"Kilogauss-strength magnetic fields are often observed in intergranular lanes
at the photosphere in the quiet Sun. Such fields are stronger than the
equipartition field $B_e$, corresponding to a magnetic energy density that
matches the kinetic energy density of photospheric convection, and comparable
with the field $B_p$ that exerts a magnetic pressure equal to the ambient gas
pressure. We present an idealised numerical model of three-dimensional
compressible magnetoconvection at the photosphere, for a range of values of the
magnetic Reynolds number. In the absence of a magnetic field, the convection is
highly supercritical and is characterised by a pattern of vigorous,
time-dependent, ``granular'' motions. When a weak magnetic field is imposed
upon the convection, magnetic flux is swept into the convective downflows where
it forms localised concentrations. Unless this process is significantly
inhibited by magnetic diffusion, the resulting fields are often much greater
than $B_e$, and the high magnetic pressure in these flux elements leads to
their being partially evacuated. Some of these flux elements contain
ultra-intense magnetic fields that are significantly greater than $B_p$. Such
fields are contained by a combination of the thermal pressure of the gas and
the dynamic pressure of the convective motion, and they are constantly
evolving. These ultra-intense fields develop owing to nonlinear interactions
between magnetic fields and convection; they cannot be explained in terms of
``convective collapse'' within a thin flux tube that remains in overall
pressure equilibrium with its surroundings.",0804.1238v1
2008-04-29,Anisotropic magnetization and sign change of dynamic susceptibility in Na0.85CoO2 single crystal,"The DC and AC magnetic susceptibilities of Na0.85CoO2 single crystals were
measured for the different crystal orientations of H//(ab)- and H//(c)-axis.
The DC-magnetic susceptibility for H//(c)-direction exhibited the
antiferromagnetic transition at TN= 22 K. The thermal hysteresis between the
zero-field-cooled (ZFC) and the field-cooled (FC) magnetization below TN and
the large frustration parameter indicated the spin frustration along the
(c)-axis. For an applied magnetic field in H//(ab)-plane, the DC magnetic
susceptibility exhibited the logarithmic divergent behavior at low temperatures
(T < 6.8 K). This could be understood by the impurity spin effect, dressed by
the spin fluctuation. From the AC magnetic susceptibility measurements, the
real part of the AC-susceptibility for H//(ab) exhibited the spin glass-like
behavior at low temperatures (T < 4 K). Remarkably, for an applied AC magnetic
field with H//(c)-axis, the sign of the AC magnetic susceptibility changed from
a positive to a negative value with increasing AC magnetic field frequency (f >
3 kHz) at low temperatures (T > 7 K). We interpret the sign change of AC
magnetic susceptibility along the (c)-axis in terms of the sudden sign reversal
of the phase difference from in-phase to out-of-phase response with an applied
AC magnetic field in the AC-susceptibility phase space.",0804.4532v1
2011-05-28,Magnetic Flux Expulsion in Star Formation,"Stars form in dense cores of magnetized molecular clouds. If the magnetic
flux threading the cores is dragged into the stars, the stellar field would be
orders of magnitude stronger than observed. This well-known ""magnetic flux
problem"" demands that most of the core magnetic flux be decoupled from the
matter that enters the star. We carry out the first exploration of what happens
to the decoupled magnetic flux in 3D, using an MHD version of the ENZO adaptive
mesh refinement code. The field-matter decoupling is achieved through a sink
particle treatment, which is needed to follow the protostellar accretion phase
of star formation. We find that the accumulation of the decoupled flux near the
accreting protostar leads to a magnetic pressure buildup. The high pressure is
released anisotropically, along the path of least resistance. It drives a
low-density expanding region in which the decoupled magnetic flux is expelled.
This decoupling-enabled magnetic structure has never been seen before in 3D MHD
simulations of star formation. It generates a strong asymmetry in the
protostellar accretion flow, potentially giving a kick to the star. In the
presence of an initial core rotation, the structure presents an obstacle to the
formation of a rotationally supported disk, in addition to magnetic braking, by
acting as a rigid magnetic wall that prevents the rotating gas from completing
a full orbit around the central object. We conclude that the decoupled magnetic
flux from the stellar matter can strongly affect the protostellar collapse
dynamics.",1105.5739v1
2012-01-04,Radiatively Efficient Magnetized Bondi Accretion,"We have carried out a numerical study of the effect of large scale magnetic
fields on the rate of accretion from a uniform, isothermal gas onto a
resistive, stationary point mass. Only mass, not magnetic flux, accretes onto
the point mass. The simulations for this study avoid complications arising from
boundary conditions by keeping the boundaries far from the accreting object.
Our simulations leverage adaptive refinement methodology to attain high spatial
fidelity close to the accreting object. Our results are particularly relevant
to the problem of star formation from a magnetized molecular cloud in which
thermal energy is radiated away on time scales much shorter than the dynamical
time scale. Contrary to the adiabatic case, our simulations show convergence
toward a finite accretion rate in the limit in which the radius of the
accreting object vanishes, regardless of magnetic field strength. For very weak
magnetic fields, the accretion rate first approaches the Bondi value and then
drops by a factor ~ 2 as magnetic flux builds up near the point mass. For
strong magnetic fields, the steady-state accretion rate is reduced by a factor
~ 0.2 \beta^{1/2} compared to the Bondi value, where \beta is the ratio of the
gas pressure to the magnetic pressure. We give a simple expression for the
accretion rate as a function of the magnetic field strength. Approximate
analytic results are given in the Appendixes for both time-dependent accretion
in the limit of weak magnetic fields and steady-state accretion for the case of
strong magnetic fields.",1201.0816v1
2012-06-06,Magnetic-field-dependent photodynamics of single NV defects in diamond: Application to qualitative all-optical magnetic imaging,"Magnetometry and magnetic imaging with nitrogen-vacancy (NV) defects in
diamond rely on the optical detection of electron spin resonance (ESR).
However, this technique is inherently limited to magnetic fields that are weak
enough to avoid electron spin mixing. Here we focus on the high off-axis
magnetic field regime for which spin mixing alters the NV defect spin dynamics.
We first study in a quantitative manner the dependence of the NV defect optical
properties on the magnetic field vector B. Magnetic-field-dependent
time-resolved photoluminescence (PL) measurements are compared to a seven-level
model of the NV defect that accounts for field-induced spin mixing. The model
reproduces the decreases in (i) ESR contrast, (ii) PL intensity and (iii)
excited level lifetime with an increasing off-axis magnetic field. We next
demonstrate that those effects can be used to perform all-optical magnetic
imaging in the high off-axis magnetic field regime. Using a scanning NV defect
microscope, we map the stray field of a magnetic hard disk through both PL and
fluorescence lifetime imaging. This all-optical method for high magnetic field
imaging at the nanoscale might be of interest in the field of nanomagnetism,
where samples producing fields in excess of several tens of milliteslas are
typical.",1206.1201v2
2012-08-23,Solar Force-free Magnetic Fields,"The structure and dynamics of the solar corona is dominated by the magnetic
field. In most areas in the corona magnetic forces are so dominant that all
non-magnetic forces like plasma pressure gradient and gravity can be neglected
in the lowest order. This model assumption is called the force-free field
assumption, as the Lorentz force vanishes. This can be obtained by either
vanishing electric currents (leading to potential fields) or the currents are
co-aligned with the magnetic field lines. First we discuss a mathematically
simpler approach that the magnetic field and currents are proportional with one
global constant, the so-called linear force-free field approximation. In the
generic case, however, the relation between magnetic fields and electric
currents is nonlinear and analytic solutions have been only found for special
cases, like 1D or 2D configurations. For constructing realistic nonlinear
force-free coronal magnetic field models in 3D, sophisticated numerical
computations are required and boundary conditions must be obtained from
measurements of the magnetic field vector in the solar photosphere. This
approach is currently of large interests, as accurate measurements of the
photospheric field become available from ground-based (for example SOLIS) and
space-born (for example Hinode and SDO) instruments. If we can obtain accurate
force-free coronal magnetic field models we can calculate the free magnetic
energy in the corona, a quantity which is important for the prediction of
flares and coronal mass ejections. Knowledge of the 3D structure of magnetic
field lines also help us to interpret other coronal observations, e.g.,
EUV-images of the radiating coronal plasma.",1208.4693v1
2012-11-29,Effects of Resistivity on Magnetized Core-Collapse Supernovae,"We studied roles of a turbulent resistivity in the core-collapse of a
strongly magnetized massive star, carrying out 2D-resistive-MHD simulations.
The three cases with different initial strengths of magnetic field and rotation
are investigated; 1. strongly magnetized rotating core; 2.moderately magnetized
rotating core; 3. very strongly magnetized non-rotating core. In each case,
both an ideal-MHD model and resistive-MHD models are computed. As a result of
computations, each model shows a matter eruption helped by a magnetic
acceleration (and also by a centrifugal acceleration in the rotating cases). We
found that a resistivity attenuates the explosion in case~1 and 2, while it
enhances the explosion in case~3. We also found that in the rotating cases,
main mechanisms for the amplification of a magnetic field in the post-bounce
phase are an outward advection of magnetic field and a winding of poloidal
magnetic field-lines by differential rotation, which are somewhat dampened down
with the presence of a resistivity. Although the magnetorotational instability
seems to occur in the rotating models, it will play only a minor role in a
magnetic field amplification. Another impact of resistivity is that on the
aspect ratio. In the rotating cases, a large aspect ratio of the ejected
matters, $> 2.5$, attained in a ideal-MHD model is reduced to some extent in a
resistive model. These results indicate that a resistivity possibly plays an
important role in the dynamics of strongly magnetized supernovae.",1211.6817v2
2013-07-18,Steady state rheological behaviour of multi-component magnetic suspensions,"In this paper we study the rheological behaviour (in the absence of magnetic
field and upon its application) of multi-component magnetic suspensions that
consist of a mixture of magnetic (iron) and non-magnetic (PMMA) particles
dispersed in a liquid carrier. These suspensions exhibit considerably higher
viscosity and yield stress in the absence of magnetic field than
single-component suspensions of the same solid fraction, as a consequence of
the adsorption of the iron particles on the PMMA ones. The adsorbed layer of
iron particles on the PMMA ones is observed through optical microscopy of
dilute samples and confirmed by attenuated total reflectance. Microscopic
observations also show that the resulting non-magnetic-core-magnetic-shell
composites move upon magnetic field application and aggregate into particle
structures aligned with the applied field. These structures, which consist of
both types of particles, give rise to high values of the static and dynamic
yield stresses upon field application. Actually, both quantities are much
higher than those of a suspension with the same volume fraction of magnetic
particles, and increase when the amount of non-magnetic ones increases. These
trends are adequately predicted by a theoretical model that considers that the
main contribution to the yield stress is the change of the suspension magnetic
permeability when particle chains are deformed by the shear.",1307.4975v1
2015-09-09,The magnetic field of zeta Orionis A,"Zeta Ori A is a hot star claimed to host a weak magnetic field, but no clear
magnetic detection was obtained so far. In addition, it was recently shown to
be a binary system composed of a O9.5I supergiant and a B1IV star. We aim at
verifying the presence of a magnetic field in zeta Ori A, identifying to which
of the two binary components it belongs (or whether both stars are magnetic),
and characterizing the field.Very high signal-to-noise spectropolarimetric data
were obtained with Narval at the Bernard Lyot Telescope (TBL) in France.
Archival HEROS, FEROS and UVES spectroscopic data were also used. The data were
first disentangled to separate the two components. We then analyzed them with
the Least-Squares Deconvolution (LSD) technique to extract the magnetic
information. We confirm that zeta Ori A is magnetic. We find that the
supergiant component zeta Ori Aa is the magnetic component: Zeeman signatures
are observed and rotational modulation of the longitudinal magnetic field is
clearly detected with a period of 6.829 d. This is the only magnetic O
supergiant known as of today. With an oblique dipole field model of the Stokes
V profiles, we show that the polar field strength is ~ 140 G. Because the
magnetic field is weak and the stellar wind is strong, zeta Ori Aa does not
host a centrifugally supported magnetosphere. It may host a dynamical
magnetosphere. Its companion zeta Ori Ab does not show any magnetic signature,
with an upper limit on the undetected field of $\sim$ 300 G.",1509.02773v1
2015-12-30,Bose-Einstein Condensation of Bound Pairs of Relativistic Fermions in a Magnetic Field,"The Bose-Einstein condensation of bound pairs made of equally and oppositely
charged fermions in a magnetic field is investigated using a relativistic
model. The Gaussian fluctuations have been taken into account in order to study
the spectrum of bound pairs in the strong coupling region. We found, in weak
coupling reagion, the condensation temperature increases with an increasing
magnetic field displaying the magnetic catalysis effect. In strong coupling
region, the inverse magnetic catalysis appears when the magnetic field is low
and is replaced by the usual magnetic catalysis effect when magnetic field is
sufficiently high, in contrast to the nonrelativistic case where the inverse
magnetic catalysis prevails in strong coupling region regardless of the
strength of the magnetic field. The resulting response to the magnetic field is
the consequence of the competition between the dimensional reduction by Landau
orbitals in pairing dynamics and the anisotropy of the kinetic spectrum of the
bound pairs. We thus conclude that dimensional reduction dominates in weak
domain and strong coupling one except the small magnetic field region, where
the enhanced fluctuations dominates.",1512.08894v2
2016-11-29,Calculation of Evaluation Variables for High Gradient Magnetic Separation with an Idealized Capture Model,"This paper regards feed mine as a mixture of intergrowths and pure
non-magnetic mineral particles, presents a method to calculate the evaluation
variables such as grade and recovery in high gradient magnetic separation
(HGMS). A idealized capture model is constructed in which the interaction
between particles is not taken into account and only for the initial
aggregation condition that the separator has the highest capture efficiency. In
the model we adopt the functions that use nominal particle radius and magnetic
mineral content as independent variables to describe volume fraction
distribution and capture efficiency of intergrowths respectively. Through
adding multi-wire magnetic fields and setting periodic boundary conditions in
flow field analysis, we modify the computational domain of the single-wire
capture theory to a element domain that periodically appears in the multi-wire
matrix. By means of finite element software, particle trajectories, flow field
and magnetic field are clearly exhibited, and then capture efficiency function
is obtained by interpolation method. The calculated evaluation variables
theoretically represent the best performance of magnetic separator for a given
feed. They can assist mineral engineers to evaluate or compare the effects of
different magnetic separation systems in advance. We use removal of iron
impurity from kaolin as an example to illustrate the presented calculation
method. The results quantitatively compare the evaluation variables of the
separation at different magnetic fields and show that the advantage of higher
magnetic field in separation efficiency decreases with the increase of
saturation magnetization of magnetic mineral.",1611.09481v1
2017-07-03,Distortion of Magnetic Fields in a Starless Core: Near-Infrared Polarimetry of FeSt 1-457,"Magnetic fields are believed to play an important role in controlling the
stability and contraction of dense condensations of gas and dust leading to the
formation of stars and planetary systems. In the present study, the magnetic
field of FeSt 1-457, a cold starless molecular cloud core, was mapped on the
basis of the polarized near-infrared light from 185 background stars after
being dichroically absorbed by dust aligned with the magnetic field in the
core. A distinct ""hourglass-shaped"" magnetic field was identified in the region
of the core, which was interpreted as the first evidence of a magnetic field
structure distorted by mass condensation in a starless core. The steep
curvature of the magnetic field lines obtained in the present study indicates
that the distortion was mainly created during the formation phase of the dense
core. The derived mass-to-magnetic flux ratio indicates that the core is in a
magnetically supercritical state. However, the stability of the core can be
considered to be in a nearly critical state if the additional contributions
from the thermal and turbulent support are included. Further diffusion of the
magnetic field and/or turbulent dissipation would cause the onset of dynamical
collapse of the core. The geometrical relationship between the direction of the
magnetic field lines and the elongation of the core was found to be in good
agreement with the theoretical predictions for the formation of Sun-like stars
under the influence of a magnetic field.",1707.00720v1
2017-09-15,Fragmentation of Filamentary Cloud Permeated by Perpendicular Magnetic Field,"We examine the linear stability of an isothermal filamentary cloud permeated
by a perpendicular magnetic field. Our model cloud is assumed to be supported
by gas pressure against the self-gravity in the unperturbed state. For
simplicity, the density distribution is assumed to be symmetric around the
axis. Also for simplicity, the initial magnetic field is assumed to be uniform
and turbulence is not taken into account. The perturbation equation is
formulated to be an eigenvalue problem. The growth rate is obtained as a
function of the wavenumber for fragmentation along the axis and the magnetic
field strength. The growth rate depends critically on the outer boundary. If
the displacement vanishes in the region very far from the cloud axis (fixed
boundary), cloud fragmentation is suppressed by a moderate magnetic field,
which means the plasma beta is below 1.67 on the cloud axis. If the
displacement is constant along the magnetic field in the region very far from
the cloud, the cloud is unstable even when the magnetic field is infinitely
strong. The cloud is deformed by circulation in the plane perpendicular to the
magnetic field. The unstable mode is not likely to induce dynamical collapse,
since it is excited even when the whole cloud is magnetically subcritical. For
both the boundary conditions the magnetic field increases the wavelength of the
most unstable mode. We find that the magnetic force suppresses compression
perpendicular to the magnetic field especially in the region of low density.",1709.05149v1
2017-11-25,Magnetism and the Invisible Man: The mysteries of coronal cavities,"Magnetism defines the complex and dynamic solar corona. Twists and tangles in
coronal magnetic fields build up energy and ultimately erupt, hurling plasma
into interplanetary space. These coronal mass ejections (CMEs) are transient
riders on the ever-outflowing solar wind, which itself possesses a
three-dimensional morphology shaped by the global coronal magnetic field.
Coronal magnetism is thus at the heart of any understanding of the origins of
space weather at the Earth. However, we have historically been limited by the
difficulty of directly measuring the magnetic fields of the corona, and have
turned to observations of coronal plasma to trace out magnetic structure. This
approach is complicated by the fact that plasma temperatures and densities vary
among coronal magnetic structures, so that looking at any one wavelength of
light only shows part of the picture. In fact, in some regimes it is the lack
of plasma that is a significant indicator of the magnetic field. Such a case is
the coronal cavity: a dark, elliptical region in which strong and twisted
magnetism dwells. I will elucidate these enigmatic features by presenting
observations of coronal cavities in multiple wavelengths and from a variety of
observing vantages, including unprecedented coronal magnetic field measurements
now being obtained by the Coronal Multichannel Polarimeter (CoMP). These
observations demonstrate the presence of twisted magnetic fields within
cavities, and also provide clues to how and why cavities ultimately erupt as
CMEs.",1711.09254v1
2017-12-01,Kinetic simulation of magnetic field generation and collisionless shock formation in expanding laboratory plasmas,"Recent laboratory experiments with laser-produced plasmas have observed and
studied a number of fundamental physical processes relevant to magnetized
astrophysical plasmas, including magnetic reconnection, collisionless shocks,
and magnetic field generation by Weibel instability, opening up new
experimental platforms for laboratory astrophysics. We develop a fully kinetic
simulation model for first-principles simulation of these systems including the
dynamics of magnetic fields---magnetic field generation by the Biermann battery
effect or Weibel instability; advection by the ion flow, Hall effect, and
Nernst effect; and destruction of the field by dissipative mechanisms. Key
dimensionless parameters describing the system are derived for scaling between
kinetic simulation, recent experiments, and astrophysical plasmas. First,
simulations are presented which model Biermann battery magnetic field
generation in plasmas expanding from a thin target. Ablation of two neighboring
plumes leads to the formation of a current sheet as the opposing
Biermann-generated fields collide, modeling recent laser-driven magnetic
reconnection experiments. Second, we simulate recent experiments on
collisionless magnetized shock generation, by expanding a piston plasma into a
pre-magnetized ambient plasma. For parameters considered, the Biermann effect
generates additional magnetic fields in the curved shock front and thereby
increases shock particle reflection. Both cases show the importance of kinetic
processes in the interaction of plasmas with magnetic fields, and open
opportunities to benchmark these important processes through comparison of
theory and experiments.",1712.00152v1
2017-12-13,Magnetically gated accretion in an accreting 'non-magnetic' white dwarf,"White dwarfs are often found in binary systems with orbital periods ranging
from tens of minutes to hours in which they can accrete gas from their
companion stars. In about 15% of these binaries, the magnetic field of the
white dwarf is strong enough ($\geq 10^6$ Gauss) to channel the accreted matter
along field lines onto the magnetic poles. The remaining systems are referred
to as ""non-magnetic"", since to date there has been no evidence that they have a
dynamically significant magnetic field. Here we report an analysis of archival
optical observations of the ""non-magnetic"" accreting white dwarf in the binary
system MV Lyrae (hereafter MV Lyr), whose lightcurve displayed quasi-periodic
bursts of $\approx 30$ minutes duration every $\approx 2$ hours. The
observations indicate the presence of an unstable magnetically-regulated
accretion mode, revealing the existence of magnetically gated accretion, where
disk material builds up around the magnetospheric boundary (at the co-rotation
radius) and then accretes onto the white dwarf, producing bursts powered by the
release of gravitational potential energy. We infer a surface magnetic field
strength for the white dwarf in MV Lyr between $2 \times 10^4 \leq B \leq 10^5$
Gauss, too low to be detectable by other current methods. Our discovery
provides a new way of studying the strength and evolution of magnetic fields in
accreting white dwarfs and extends the connections between accretion onto white
dwarfs, young stellar objects and neutron stars, for which similar magnetically
gated accretion cysles have been identified.",1712.04949v1
2018-07-11,Parametric study and optimization trends for the Von-Kármán-Sodium dynamo experiment,"We present magneto-hydrodynamic simulations of liquid sodium flow with the
PLUTO compressible MHD code. We investigate the influence of the remnant
magnetic field orientation and intensity, impinging velocity field due to Ekman
pumping as well as the impeller dimensions on the magnetic field collimation by
helical flows in between the impeller blades. For a simplified cartesian
geometry we model the flow dynamics of a multi-blades impeller inspired by the
Von-Karman-Sodium (VKS) experiment. The study shows that a remnant magnetic
field oriented in the toroidal direction is the less efficient configuration to
collimate the magnetic field, although if the radial or vertical components are
not negligible the collimation is significantly improved. If the intensity of
the remnant magnetic field increases the system magnetic energy is larger but
the magnetic field collimation efficiency is the same, so the gain of magnetic
energy is smaller as the remnant magnetic field intensity increases. The
magnetic field collimation is modified if the impinging velocity field changes:
the collimation is weaker if the impinging velocity increases from Gamma=0.8 to
0.9, slightly larger if the impinging velocity decreases from Gamma=0.8 to 0.7.
The analysis of the impeller dimensions points out that the most efficient
configuration to collimate the magnetic field requires a ratio between impeller
blade height and base longitude between 0.375 - 0.5. The largest enhancement of
the hypothetical alpha^2 dynamo loop, compared to the hypothetical Omega-alpha
dynamo loop, is observed for the model that mimics TM 73 impeller configuration
rotating in the unscooping direction with a remnant magnetic field of 10^-3 T
orientated in the radial or vertical direction. The optimization trends
obtained in the parametric analysis are also confirmed in simulations with
higher resolution and turbulence degree.",1807.03956v1
2019-08-27,Magnetized 1.5-dimensional advective accretion flows around black holes,"We address the role of large scale magnetic stress in the angular momentum
transport, as well as the formation of different kinds of magnetic barrier in
geometrically thick, optically thin, vertically averaged 1.5-dimensional
advective accretion flows around black holes. The externally generated magnetic
fields are captured by the accretion process from the environment, say,
companion stars or interstellar medium. This field becomes dynamically dominant
near the event horizon of a black hole due to continuous advection of the
magnetic flux. In such magnetically dominated accretion flows, the accreting
matter either decelerates or faces magnetic barrier in vicinity of the black
hole depending on the magnetic field geometry. We find that the accumulated
strong poloidal fields along with certain toroidal field geometry help in the
formation of magnetic barrier which may knock the matter to infinity. When
matter is trying to go back to infinity after getting knocked out by the
barrier, in some cases it is prevented being escaped due to cumulative action
of strong magnetic tension and gravity, and hence another magnetic barrier. We
suggest, this kind of flow may be responsible for the formation of episodic
jets in which magnetic field can lock the matter in between these two barriers.
We also find that for the toroidally dominated disc, the accreting matter
rotates very fast and decelerates towards the central black hole.",1908.10043v1
2020-03-10,Potential Vorticity Mixing in a Tangled Magnetic Field,"A theory of potential vorticity (PV) mixing in a disordered (tangled)
magnetic field is presented. The analysis is in the context of $\beta$-plane
MHD, with a special focus on the physics of momentum transport in the stably
stratified, quasi-2D solar tachocline. A physical picture of mean PV evolution
by vorticity advection and tilting of magnetic fields is proposed. In the case
of weak-field perturbations, quasi-linear theory predicts that the Reynolds and
magnetic stresses balance as turbulence Alfv\'enizes for a larger mean magnetic
field. Jet formation is explored quantitatively in the mean field-resistivity
parameter space. However, since even a modest mean magnetic field leads to
large magnetic perturbations for large magnetic Reynolds number, the physically
relevant case is that of a strong but disordered field. We show that numerical
calculations indicate that the Reynolds stress is modified well before
Alfv\'enization -- i.e. before fluid and magnetic energies balance. To
understand these trends, a double-average model of PV mixing in a stochastic
magnetic field is developed. Calculations indicate that mean-square fields
strongly modify Reynolds stress phase coherence and also induce a magnetic drag
on zonal flows. The physics of transport reduction by tangled fields is
elucidated and linked to the related quench of turbulent resistivity. We
propose a physical picture of the system as a resisto-elastic medium threaded
by a tangled magnetic network. Applications of the theory to momentum transport
in the tachocline and other systems are discussed in detail.",2003.04944v1
2020-04-12,Signatures of Untwisting Magnetic Field in a Small Emerging Bipole in the Solar Photosphere,"We perform a study of fluid motions and its temporal evolution in and around
a small bipolar emerging flux region using observations made by the
Helioseismic and Magnetic Imager (HMI) on-board the Solar Dynamics Observatory
(SDO). We employ local correlation tracking of the Doppler observations to
follow horizontal fluid motions and line-of-sight magnetograms to follow the
flux emergence. Changes in vertical vorticity and horizontal divergence are
used to derive signatures of evolving twists in the magnetic field. Our
analysis reveals that the two polarities of the magnetic flux swirl in opposite
directions in early stages of flux emergence indicating an unwinding of the
pre-emergence twists in the magnetic field. We further find that during the
emergence, there is an increase in swirly motions in the neighbouring
non-magnetic regions. We estimate the magnetic and kinetic energies and find
that magnetic energy is about a factor of ten larger than the kinetic energy.
During the evolution, when the magnetic energy decreases, an increase in the
kinetic energy is observed indicating transfer of energy from the unwinding of
magnetic flux tube to the surrounding fluid motions. Our results thus
demonstrate the presence of pre-emergence twists in emerging magnetic field
that is important in the context of the hemispheric helicity rule warranting a
detailed statistical study in this context. Further, our observations point to
a possible widespread generation of torsional waves in emerging flux regions
due to the untwisting magnetic field with implications for upward energy
transport to the corona.",2004.05615v1
2020-06-23,Hall cascade with fractional magnetic helicity in neutron star crusts,"The ohmic decay of magnetic fields in the crusts of neutron stars is
generally believed to be governed by Hall drift which leads to what is known as
a Hall cascade. Here we show that helical and fractionally helical magnetic
fields undergo strong inverse cascading like in magnetohydrodynamics (MHD), but
the magnetic energy decays more slowly with time $t$: $\propto t^{-2/5}$
instead of $\propto t^{-2/3}$ in MHD. Even for a nonhelical magnetic field
there is a certain degree of inverse cascading for sufficiently strong magnetic
fields. The inertial range scaling with wavenumber $k$ is compatible with
earlier findings for the forced Hall cascade, i.e., proportional to $k^{-7/3}$,
but in the decaying cases, the subinertial range spectrum steepens to a novel
$k^5$ slope instead of the $k^4$ slope in MHD. The energy of the large-scale
magnetic field can increase quadratically in time through inverse cascading.
For helical fields, the energy dissipation is found to be inversely
proportional to the large-scale magnetic field and proportional to the fifth
power of the root-mean square (rms) magnetic field. For neutron star conditions
with an rms magnetic field of a few times $10^{14}\,$G, the large-scale
magnetic field might only be $10^{11}\,$G, while still producing magnetic
dissipation of $10^{33}\,$erg$\,$s$^{-1}$ for thousands of years, which could
manifest itself through X-ray emission. Finally, it is shown that the
conclusions from local unstratified models agree rather well with those from
stratified models with boundaries.",2006.12984v4
2021-03-02,"On the alignment of haloes, filaments and magnetic fields in the simulated cosmic web","The continuous flow of gas and dark matter across scales in the cosmic web
can generate correlated dynamical properties of haloes and filaments (and the
magnetic fields they contain). With this work, we study the halo spin
properties and orientation with respect to filaments, and the morphology of the
magnetic field around these objects, for haloes with masses in the range
1e8-1e14 Msun and filaments up to 8 Mpc long. Furthermore, we study how these
properties vary in presence, or lack thereof, of different (astro)physical
processes and with different magnetic initial conditions. We perform
cosmological magnetohydrodynamical simulations with the Eulerian code Enzo and
we develop a simple and robust algorithm to study the filamentary connectivity
of haloes in three dimensions. We investigate the morphological and magnetic
properties and focus on the alignment of the magnetic field along filaments:
our analysis suggests that the degree of this alignment is partially dependent
on the physical processes involved, as well as on magnetic initial conditions.
We discuss the contribution of this effect on a potential attempt to detect the
magnetic field surrounding these objects: we find that it introduces a bias in
the estimation of the magnetic field from Faraday rotation measure techniques.
Specifically, given the strong tendency we find for extragalactic magnetic
fields to align with the filaments axis, the value of the magnetic field can be
underestimated by a factor 3, because this effect contributes to making the
line-of-sight magnetic field (for filaments in the plane of the sky) much
smaller than the total one.",2103.01943v1
2021-03-05,Relationship between magnetic field properties and statistical flow using numerical simulation and magnetic feature tracking on solar photosphere,"We perform radiative magnetohydrodynamic calculations for the solar quiet
region to investigate the dependence of statistical flow on magnetic properties
and the three-dimensional (3D) structure of magnetic patches in the presence of
large-scale flow that mimics differential rotation. It has been confirmed that
strong magnetic field patches move faster in the longitudinal direction at the
solar surface. Consequently, strong magnetic patches penetrate deeper into the
solar interior. The motion of the deep-rooted magnetic patches is influenced by
the faster differential rotation in the deeper layer. In this study, we perform
realistic radiative magnetohydrodynamic calculations using R2D2 code to
validate that stronger patches have deeper roots. We also add large-scale flow
to mimic the differential rotation. The magnetic patches are automatically
detected and tracked, and we evaluate the depth of 30,000 magnetic patches. The
velocities of 2.9 million magnetic patches are then measured at the
photosphere. We obtain the dependence of these values on the magnetic
properties, such as field strength and flux. Our results confirm that strong
magnetic patches tend to show deeper roots and faster movement, and we compare
our results with observations using the point spread function of instruments at
the Hinode and Solar Dynamics Observatory (SDO). Our result is quantitatively
consistent with previous observational results of the SDO.",2103.03789v1
2021-11-14,Spontaneous Formation of Outflows Powered by Rotating Magnetized Accretion Flows in a Galactic Center,"We investigate how magnetically driven outflows are powered by a rotating,
weakly magnetized accretion flow onto a supermassive black hole using
axisymmetric magnetohydrodynamic simulations. Our proposed model focuses on the
accretion dynamics on an intermediate scale between the Schwarzschild radius
and the galactic scale, which is $\sim$1-100 pc. We demonstrate that a rotating
disk formed on a parsec-scale acquires poloidal magnetic fields via accretion
and this produces an asymmetric bipolar outflow at some point. The formation of
the outflow was found to follow the growth of strongly magnetized regions
around disk surfaces (magnetic bubbles). The bipolar outflow grew continuously
inside the expanding bubbles. We theoretically derived the growth condition of
magnetic bubbles for our model that corresponds to a necessary condition for
outflow growth. We found that the north-south asymmetric structure of the
bipolar outflow originates from the complex motions excited by accreting flows
around the outer edge of the disk. The bipolar outflow comprises multiple
mini-outflows and downflows (failed outflows). The mini-outflows emanate from
the magnetic concentrations (magnetic patches). The magnetic patches exhibit
inward drifting motions, thereby making the outflows unsteady. We demonstrate
that the inward drift can be modeled using a simple magnetic patch model that
considers magnetic angular momentum extraction. This study could be helpful for
understanding how asymmetric and non-steady outflows with complex substructures
are produced around supermassive black holes without the help of strong
radiation from accretion disks or entrainment by radio jets such as molecular
outflows in radio-quiet active galactic nuclei, NGC 1377.",2111.07373v1
2022-03-28,Nonlinear geometrically exact dynamics of fluid-conveying cantilevered hard magnetic soft pipe with uniform and nonuniform magnetizations,"It is generally acknowledged that a hanging cantilevered pipe conveying fluid
becomes unstable by flutter-type instability at a critical flow velocity;
moreover, the pipe undergoes periodic self-excited oscillations in the
post-flutter region. Additionally, the critical flow velocity increases when
the magnetized pipe is exposed to an actuating parallel magnetic field. The
question arises as to whether the actuating magnetic field leads to lessening
the oscillation amplitude of the system in the post-flutter region. To answer
the question, the nonlinear responses of a fluid-conveying cantilevered hard
magnetic soft pipe with uniform and nonuniform magnetizations under an
actuating parallel magnetic field are examined. In the case of the nonuniform
magnetization, the mass density and elastic modulus of the pipe in addition to
its residual magnetic flux density vary along its length. The mathematical
formulation is constructed via a nonlinear geometrically exact model and is
solved by employing the Galerkin technique in conjunction with the Runge-Kutta
finite difference scheme. The numerical results are then analyzed to reveal the
role of magnetization in the magneto-hydro-elastic responses of the system in
the absence and presence of magnetic field.",2203.14618v2
2022-09-30,Tunable Magnets: modeling and validation for dynamic and precision applications,"Actuator self-heating limits the achievable force and can cause unwanted
structural deformations. This is especially apparent in quasi-static actuation
systems that require the actuator to maintain a stable position over an
extended period. As a solution, we use the concept of a Tunable Magnet. Tunable
magnets rely on in-situ magnetization state tuning of AlNico to create an
infinitely adjustable magnetic flux. They consist of an AlNiCo low coercivity
permanent magnet together with a magnetizing coil. After tuning, the AlNiCo
retains its magnetic field without further energy input, which eliminates the
static heat dissipation. To enable implementation in actuation systems, the
AlNiCo needs to be robustly tunable in the presence of a varying system
air-gap. We achieve this by implementing a magnetization state tuning method,
based on a magnetic circuit model of the actuator, measured AlNiCo BH data and
air-gap flux feedback control. The proposed tuning method consists of 2 main
steps. The prediction step, during which the required magnet operating point is
determined, and the demagnetization step, where a feedback controller drives a
demagnetization current to approach this operating point. With this method
implemented for an AlNiCo 5 tunable magnet in a reluctance actuator
configuration, we achieve tuning with a maximum error of 15.86 ""mT"" and a
minimum precision of 0.67 ""mT"" over an air-gap range of 200 ""{\mu}m"". With this
tuning accuracy, actuator heating during static periods is almost eliminated.
Only a small bias current is needed to compensate for the tuning error.",2210.00142v1
2023-05-04,Magnetic properties of Nd6Fe13Cu single crystals,"The understanding of coercivity mechanism in high performance Nd-Fe-B
permanent magnets relies on the analysis of the magnetic properties of all
phases present in the magnets. By adding Cu in such compounds, a new Nd6Fe13Cu
grain boundary phase is formed, however, the magnetic properties of this phase
and its role in the magnetic decoupling of the matrix Nd2Fe14B grains are still
insufficiently studied. In this work, we have grown Nd6Fe13Cu single crystals
by the reactive flux method and studied their magnetic properties in detail. It
is observed that below the N\'eel temperature (TN = 410 K), the Nd6Fe13Cu is
antiferromagnetic in zero magnetic field; whereas when a magnetic field is
applied along the a-axis, a spin-flop transition occurs at approx. 6 T,
indicating a strong competition between antiferromagnetic and ferromagnetic
interactions in two Nd layers below and above the Cu layers. Our atomistic spin
dynamics simulation confirms that an increase in temperature and/or magnetic
field can significantly change the antiferromagnetic coupling between the two
Nd layers below and above the Cu layers, which, in turn, is the reason for the
observed spin-flop transition. These results suggest that the role of
antiferromagnetic Nd6Fe13Cu grain boundary phase in the coercivity enhancement
of Nd-Fe-B-Cu magnets is more complex than previously thought, mainly due to
the competition between its antiferro- and ferro-magnetic exchange
interactions.",2305.02889v1
2023-08-03,Unconventional Metallic Magnetism: Non-analyticity and Sign-changing Behavior of Orbital Magnetization in ABC Trilayer Graphene,"We study an unique form of metallic ferromagnetism in which orbital moments
surpasses the role of spin moments in shaping the overall magnetization. This
system emerges naturally upon doping a topologically non-trivial Chern band in
the recently identified quarter metal phase of rhombohedral trilayer graphene.
Our comprehensive scan of the density-interlayer potential parameter space
reveals an unexpected landscape of orbital magnetization marked by two sign
changes and a line of singularities. The sign change originates from an intense
Berry curvature concentrated close to the band-edge, and the singularity arises
from a topological Lifshitz transition that transform a simply connected Fermi
sea into an annular Fermi sea. Importantly, these variations occur while the
groundstate order-parameter (i.e. valley and spin polarization) remains
unchanged. This unconventional relationship between the order parameter and
magnetization markedly contrasts traditional spin ferromagnets, where spin
magnetization is simply proportional to the groundstate spin polarization via
the gyromagnetic ratio. We compute energy and magnetization curves as functions
of collective valley rotation to shed light on magnetization dynamics and to
expand the Stoner-Wohlfarth magnetization reversal model. We provide
predictions on the magnetic coercive field that can be readily tested in
experiments. Our results challenge established perceptions of magnetism,
emphasising the important role of orbital moments in two-dimensional materials
such as graphene and transition metal dichalcogenides, and in turn, expand our
understanding and potential manipulation of magnetic behaviors in these
systems.",2308.01996v2
2023-12-11,Advancing solar magnetic field extrapolations through multi-height magnetic field measurements,"Non-linear force-free extrapolations are a common approach to estimate the 3D
topology of coronal magnetic fields based on photospheric vector magnetograms.
The force-free assumption is a valid approximation at coronal heights, but for
the dense plasma conditions in the lower atmosphere, this assumption is not
satisfied. In this study, we utilize multi-height magnetic field measurements
in combination with physics-informed neural networks to advance solar magnetic
field extrapolations. We include a flexible height-mapping, which allows us to
account for the different formation heights of the observed magnetic field
measurements. The comparison to analytical and simulated magnetic fields
demonstrates that including chromospheric magnetic field measurements leads to
a significant improvement of our magnetic field extrapolations. We also apply
our method to chromospheric line-of-sight magnetograms, from the Vector
Spectromagnetograph (VSM) on the Synoptic Optical Long-term Investigations of
the Sun (SOLIS) observatory, in combination with photospheric vector
magnetograms, from the Helioseismic Magnetic Imager (HMI) onboard the Solar
Dynamic Observatory (SDO). The comparison to observations in extreme
ultraviolet wavelengths shows that the additional chromospheric information
leads to a better agreement with the observed coronal structures. In addition,
our method intrinsically provides an estimate of the corrugation of the
observed magnetograms. With this new approach, we make efficient use of
multi-height magnetic field measurements and advance the realism of coronal
magnetic field simulations.",2312.06823v1
2017-07-10,One-loop QCD thermodynamics in a strong homogeneous and static magnetic field,"We have studied how the equation of state of thermal QCD with two light
flavours is modified in strong magnetic field by calculating the thermodynamic
observables of hot QCD matter up to one-loop, where the magnetic field affects
mainly the quark contribution and the gluonic part is largely unaffected except
for the softening of the screening mass due to the strong magnetic field. To
begin with the effect of magnetic field on the thermodynamics, we have first
calculated the pressure of a thermal QCD medium in strong magnetic field limit
(SML), where the pressure at fixed temperature increases with the magnetic
field faster than the increase with the temperature at constant magnetic field.
This can be envisaged from the dominant scale of thermal medium in SML, which
is the magnetic field, like the temperature in thermal medium in absence of
strong magnetic field. Thus although the presence of strong magnetic field
makes the pressure of hot QCD medium harder but the increase of pressure with
respect to the temperature becomes less steeper. Corroborated to the above
observations, the entropy density is found to decrease with the temperature in
the ambience of strong magnetic field which resonates with the fact that the
strong magnetic field restricts the dynamics of quarks in two dimensions, hence
the phase space gets squeezed resulting the reduction of number of microstates.
Moreover the energy density is seen to decrease and the speed of sound of
thermal QCD medium is increased in the presence of strong magnetic field. These
crucial findings in strong magnetic field could have phenomenological
implications in heavy ion collisions because the expansion dynamics of the
medium produced in noncentral ultrarelativistic heavy ion collisions is
effectively controlled by both the energy density and the speed of sound.",1707.02890v1
2007-10-31,Spin-Torque Driven Magnetization Dynamics: Micromagnetic Modelling,"In this paper we present an overview of recent progress made in the
understanding of the spin-torque induced magnetization dynamics in nanodevices
using mesoscopic micromagnetic simulations. We first specify how a spin-torque
term may be added to the usual Landau-Lifshitz-Gilbert equation of
magnetization motion and detail its physical meaning. After a brief description
of spin-torque driven dynamics in the macrospin approximation, we discuss the
validity of this approximation for various experimentally relevant geometries.
Next, we perform a detailed comparison between accurate experimental data
obtained from nanopillar devices and corresponding numerical modelling. We show
that, on the one hand, many qualitatively important features of the observed
magnetization dynamics (e.g., non-linear frequency shift and frequency jumps
with increasing current) can be satisfactory explained by sophisticated
micromagnetic models, but on the other hand, understanding of these experiments
is still far from being complete. We proceed with the numerical analysis of
point-contact experiments, where an even more complicated magnetization
dynamics is observed. Simulations reveal that such a rich behaviour is due to
the formation of several strongly non-linear oscillation modes. In the last
part of the paper we emphasize the importance of sample characterization and
conclude with some important remarks concerning the relation between
micromagnetic modelling and real experiments.",0710.5924v1
2011-05-16,Finite-size effects on the dynamic susceptibility of CoPhOMe single-chain molecular magnets in presence of a static magnetic field,"The static and dynamic properties of the single-chain molecular magnet
[Co(hfac)$_2$NITPhOMe] are investigated in the framework of the Ising model
with Glauber dynamics, in order to take into account both the effect of an
applied magnetic field and a finite size of the chains. For static fields of
moderate intensity and short chain lengths, the approximation of a
mono-exponential decay of the magnetization fluctuations is found to be valid
at low temperatures; for strong fields and long chains, a multi-exponential
decay should rather be assumed. The effect of an oscillating magnetic field,
with intensity much smaller than that of the static one, is included in the
theory in order to obtain the dynamic susceptibility $\chi(\omega)$. We find
that, for an open chain with $N$ spins, $\chi(\omega)$ can be written as a
weighted sum of $N$ frequency contributions, with a sum rule relating the
frequency weights to the static susceptibility of the chain. Very good
agreement is found between the theoretical dynamic susceptibility and the ac
susceptibility measured in moderate static fields ($H_{\rm dc}\le 2$ kOe),
where the approximation of a single dominating frequency turns out to be valid.
For static fields in this range, new data for the relaxation time, $\tau$
versus $H_{\rm dc}$, of the magnetization of CoPhOMe at low temperature are
also well reproduced by theory, provided that finite-size effects are included.",1105.3009v1
2015-07-30,Asymmetric driven dynamics of Dzyaloshinskii domain walls in ultrathin ferromagnetic strips with perpendicular magnetic anisotropy,"The dynamics of domain walls in ultrathin ferromagnetic strips with
perpendicular magnetic anisotropy is studied from both numerical and analytical
micromagnetics. The influence of the interfacial Dzyaloshinskii-Moriya
interaction associated to a bi-layer strip arrangement has been considered,
giving rise to the formation of Dzyaloshinskii domain walls. Such walls possess
under equilibrium conditions an inner magnetization structure defined by a
certain orientation angle that make them to be considered as intermediate
configurations between Bloch and N\'eel walls. Two different dynamics are
considered, a field-driven and a current-driven dynamics, in particular, the
one promoted by the spin torque due to the Spin-Hall effect. Results show an
inherent asymmetry associated with the rotation of the domain wall
magnetization orientation before reaching the stationary regime, characterized
by a constant terminal speed. For a certain initial DW magnetization
orientation at rest, the rotation determines whether the reorientation of the
DW magnetization prior to reach stationary motion is smooth or abrupt. This
asymmetry affects the DW motion that can even reverse for a short period of
time. Additionally, it is found that the terminal speed in the case of the
current-driven dynamics may depend on either the initial DW magnetization
orientation at rest or the sign of the longitudinally injected current.",1507.08406v2
2016-11-20,The effect of dynamical compressive and shear strain on magnetic anisotropy in low symmetry ferromagnetic film,"Dynamical strain generated upon excitation of a metallic film by a
femtosecond laser pulse may become a versatile tool enabling control of
magnetic state of thin films and nanostructures via inverse magnetostriction on
a picosecond time scale. Here we explore two alternative approaches to
manipulate magnetocrystalline anisotropy and excite magnetization precession in
a low-symmetry film of a magnetic metallic alloy galfenol (Fe,Ga) either by
injecting picosecond strain pulse into it from a substrate or by generating
dynamical strain of complex temporal profile in the film directly. In the
former case we realize ultrafast excitation of magnetization dynamics solely by
strain pulses. In the latter case optically-generated strain emerged abruptly
in the film modifies its magnetocrystalline anisotropy, competing with
heat-induced change of anisotropy parameters. We demonstrate that the
optically-generated strain remains efficient for launching magnetization
precession, when the heat-induced changes of anisotropy parameters do not
trigger the precession anymore. We emphasize that in both approaches the
ultrafast change of magnetic anisotropy mediating the precession excitation
relies on mixed, compressive and shear, character of the dynamical strain,
which emerges due to low-symmetry of the metallic film under study.",1611.06599v3
2018-03-03,Evidence for a dynamical ground state in a frustrated pyrohafnate Tb2Hf2O7,"We report the physical properties of Tb2Hf2O7 based on ac magnetic
susceptibility \chi_ac(T), dc magnetic susceptibility \chi(T), isothermal
magnetization M(H), and heat capacity C_p(T) measurements combined with muon
spin relaxation (\muSR) and neutron powder diffraction measurements. No
evidence for long-range magnetic order is found down to 0.1 K. However,
\chi_ac(T) data present a frequency-dependent broad peak (near 0.9 K at 16 Hz)
indicating slow spin dynamics. The slow spin dynamics is further evidenced from
the \muSR data (characterized by a stretched exponential behavior) which show
persistent spin fluctuations down to 0.3 K. The neutron powder diffraction data
collected at 0.1 K show a broad peak of magnetic origin (diffuse scattering)
but no magnetic Bragg peaks. The analysis of the diffuse scattering data
reveals a dominant antiferromagnetic interaction in agreement with the negative
Weiss temperature. The absence of long-range magnetic order and the presence of
slow spin dynamics and persistent spin fluctuations together reflect a
dynamical ground state in Tb2Hf2O7.",1803.01193v1
2019-07-03,Dynamic magnetostriction for antiferromagnets,"In this paper we study the switching properties of the dynamics of magnetic
moments, that interact with an elastic medium. To do so we construct a
Hamiltonian framework, that can take into account the dynamics in phase space
of the variables that describe the magnetic moments in a consistent way. It is
convenient to describe the magnetic moments as bilinears of anticommuting
variables that are their own conjugates. However, we show how it is possible to
avoid having to deal directly with the anticommuting variables themselves, only
using them to deduce non-trivial constraints on the magnetoelastic couplings.
We construct the appropriate Poisson bracket and a geometric integration
scheme, that is symplectic in the extended phase space and that allows us to
study the switching properties of the magnetization, that are relevant for
applications, for the case of a toy model for antiferromagnetic NiO, under
external stresses. In the absence of magnetoelastic coupling, we recover the
results reported in the literature and in our previous studies. In the presence
of the magnetoelastic coupling, the characteristic oscillations of the
mechanical system have repercussions on the N\'eel order parameter dynamics.
This is particularly striking for the spin accumulation which is more than
doubled by the coupling to the strain ; here as well, the mechanical
oscillations are reflected on the magnetic dynamics. As a consequence of such a
stress induced strain, the switching time of the magnetization is slightly
faster and the amplitude of the magnetization enhanced.",1907.01857v2
2021-07-13,Dynamical and static spin structure factors of Heisenberg antiferromagnet on honeycomb lattice in the presence of Dzyaloshinskii-Moriya interaction,"We have theoretically studied the spin structure factors of Heisenberg model
on honeycomb lattice in the presence of longitudinal magnetic field, i.e.
magnetic field perpendicular to the honeycomb plane, and Dzyaloshinskii-Moriya
interaction. The possible effects of next nearest neighbor exchange constant
are investigated in terms of anisotropy in the Heisenberg interactions. This
spatial anisotropy is due to the difference between nearest neighbor exchange
coupling constant and next nearest neighbor exchange coupling constant. The
original spin model hamiltonian is mapped to a bosonic model via a hard core
bosonic transformation where an infinite hard core repulsion is imposed to
constrain one boson occupation per site. Using Green's function approach, the
energy spectrum of quasiparticle excitation has been obtained. The spectrum of
the bosonic gas has been implemented in order to obtain two particle propagator
which corresponds to spin structure factor of original Heisenberg chain model
Hamiltonian. The results show the position of peak in the dynamical transverse
spin structure factor at fixed value for Dzyaloshinskii Moriya interaction
moves to higher frequency with magnetic field. Also the intensity of dynamical
transverse spin structure factor is not affected by magnetic field. However the
Dzyaloshinskii Moriya interaction strength causes to decrease the intensity of
dynamical transverse spin structure factor. The increase of magnetic field does
not varied the frequency position of peaks in dynamical longitudinal spin
susceptibility however the intensity reduces with magnetic field. Our results
show static transverse structure factor is found to be monotonically decreasing
with magnetic field and temperature for different vlaues of next nearest
neighbor coupling exchange constant.",2107.05862v1
2021-10-27,Hamilton-Jacobi Equations of Controlled Magnetic Hamiltonian System with Nonholonomic Constraint,"In order to describe the impact of different geometric structures and
constraints for the dynamics of a regular controlled Hamiltonian system, in
this paper, we first define a kind of controlled magnetic Hamiltonian (CMH)
system, and give a good expression of the dynamical vector field of the CMH
system, such that we can describe the magnetic vanishing condition and the
CMH-equivalence, and derive precisely the geometric constraint conditions of
the magnetic symplectic form for the dynamical vector field of the CMH system,
which are called the Type I and Type II of Hamilton-Jacobi equation. Secondly,
we prove that the CMH-equivalence for the CMH systems leaves the solutions of
corresponding to Hamilton-Jacobi equations invariant, if the associated
magnetic Hamiltonian systems are equivalent. Thirdly, we consider the CMH
system with nonholonomic constraint, and derive a distributional CMH system,
which is determined by a non-degenerate distributional two-form induced from
the magnetic symplectic form. Then we drive precisely two types of
Hamilton-Jacobi equation for the distributional CMH system. Moreover, we
generalize the above results for the nonholonomic reducible CMH system with
symmetry, and prove two types of Hamilton-Jacobi theorems for the nonholonomic
reduced distributional CMH system. These research works reveal the deeply
internal relationships of the magnetic symplectic forms, the nonholonomic
constraints, the dynamical vector fields and controls of the CMH systems.",2110.14175v2
2022-05-20,A feasibility analysis towards the simulation of hysteresis with spin-lattice dynamics,"We use spin-lattice dynamics simulations to study the possibility of modeling
the magnetic hysteresis behavior of a ferromagnetic material. The temporal
evolution of the magnetic and mechanical degrees of freedom is obtained through
a set of two coupled Langevin equations. Hysteresis loops are calculated for
different angles between the external field and the magnetocrystalline
anisotropy axes. The influence of several relevant parameters is studied,
including the field frequency, magnetic damping, magnetic anisotropy (magnitude
and type), magnetic exchange, and system size. The role played by a moving
lattice is also discussed. For a perfect bulk ferromagnetic system we find
that, at low temperatures, the exchange and lattice dynamics barely affect the
loops, while the field frequency and magnetic damping have a large effect on
it. The influence of the anisotropy magnitude and symmetry are found to follow
the expected behavior. We show that a careful choice of simulation parameters
allows for an excellent agreement between the spin-lattice dynamics
measurements and the paradigmatic Stoner-Wohlfarth model. Furthermore, we
extend this analysis to intermediate and high temperatures for the perfect bulk
system and for spherical nanoparticles, with and without defects, reaching
values close to the Curie temperature. In this temperature range, we find that
lattice dynamics has a greater role on the magnetic behavior, especially in the
evolution of the defective samples. The present study opens the possibility for
more accurate inclusion of lattice defects and thermal effects in hysteresis
simulations",2205.10418v3
2000-07-13,Dynamical Structure Factors of the S=1/2 Bond-Alternating Spin Chain with a Next-Nearest-Neighbor Interaction in Magnetic Fields,"The dynamical structure factor of the S=1/2 bond-alternating spin chain with
a next-nearest-neighbor interaction in magnetic field is investigated using the
continued fraction method based on the Lanczos algorithm. When the plateau
exists on the magnetization curve, the longitudinal dynamical structure factor
shows a large intensity with a periodic dispersion relation, while the
transverse one shows a large intensity with an almost dispersionless mode. The
periodicity and the amplitude of the dispersion relation in the longitudinal
dynamical structure factor are sensitive to the coupling constants. The
dynamical structure factor of the S=1/2 two-leg ladder in magnetic field is
also calculated in the strong interchain-coupling regime.
  The dynamical structure factor shows gapless or gapful behavior depending on
the wave vector along the rung.",0007219v1
2003-10-13,Multiple dynamic transitions in anisotropic Heisenberg ferromagnet driven by polarised magnetic field,"Uniaxially (Z-axis) anisotropic Heisenberg ferromagnet, in presence of time
dependent (but uniform over space) magnetic field, is studied by Monte Carlo
simulation. The time dependent magnetic field was taken as elliptically
polarised where the resultant field vector rotates in X-Z plane. The system is
cooled (in presence of the elliptically polarised magnetic field) from high
temperature. As the temperature decreases, it was found that in the low
anisotropy limit the system undergoes three successive dynamical phase
transitions. These three dynamic transitions were confirmed by studying the
temperature variation of dynamic 'specific heat'. The temperature variation of
dynamic 'specific heat' shows three peaks indicating three dynamic transition
points.",0310288v2
2010-04-23,Fast-forward of quantum adiabatic dynamics in electro-magnetic field,"We show a method to accelerate quantum adiabatic dynamics of wavefunctions
under electro-magnetic field by developing the previous theory (Masuda &
Nakamura 2008 and 2010). Firstly we investigate the orbital dynamics of a
charged particle. We derive the driving field which accelerates quantum
adiabatic dynamics in order to obtain the final adiabatic states except for the
spatially uniform phase such as the adiabatic phase in any desired short time.
Fast-forward of adiabatic squeezing and transport in the electro-magnetic field
is exhibited. Secondly we investigate spin dynamics under the magnetic field,
showing the fast-forward of adiabatic spin inversion and of adiabatic dynamics
in Landau-Zener model. The connection of the present framework with
Kato-Berry's transitionless quantum driving is elucidated in Appendix.",1004.4108v1
2011-12-12,Theory of laser-induced demagnetization at high temperatures,"Laser-induced demagnetization is theoretically studied by explicitly taking
into account interactions among electrons, spins and lattice. Assuming that the
demagnetization processes take place during the thermalization of the
sub-systems, the temperature dynamics is given by the energy transfer between
the thermalized interacting baths. These energy transfers are accounted for
explicitly through electron-magnons and electron-phonons interaction, which
govern the demagnetization time scale. By properly treating the spin system in
a self-consistent random phase approximation, we derive magnetization dynamic
equations for a broad range of temperature. The dependence of demagnetization
on the temperature and pumping laser intensity is calculated in detail. In
particular, we show several salient features for understanding magnetization
dynamics near the Curie temperature. While the critical slowdown in dynamics
occurs, we find that an external magnetic field can restore the fast dynamics.
We discuss the implication of the fast dynamics in the application of heat
assisted magnetic recording.",1112.2428v1
2014-11-27,The critical dynamics of the models of iron-vanadium magnetic superlattice,"We report the results of a numerical investigation of static and dynamic
critical behavior of the anisotropic easy-plane Heisenberg model which is used
as a model for iron-vanadium magnetic superlattices. Models of iron-vanadium
magnetic superlattice investigated with different values of intralayer and
interlayer exchange interactions ratio. Numerical experiment technique, based
on the dynamic finite-size scaling theory. Basic features of the time evolution
of dynamic parameters for the researched systems are studied. The effect of
interlayer exchange interaction value on a character of the critical dynamics
is determined. It is confirmed the possibility of application of the
Hamiltonian with strong easy-plane anisotropy for investigation both static and
dynamic critical behavior of complex planar magnetic models.",1411.7509v1
2018-10-17,Unified theory of magnetization dynamics with relativistic and nonrelativistic spin torques,"Spin torques play a crucial role in operative properties of modern spintronic
devices. To study current-driven magnetization dynamics, spin-torque terms
providing the action of spin-polarized currents have previously often been
added in a phenomenological way to the Landau-Lifshitz-Gilbert equation
describing the local spin dynamics, yet without derivation from fundamental
principles. Here, starting from the Dirac-Kohn-Sham theory and incorporating
nonlocal spin transport we rigorously derive the various spin-torque terms that
appear in current-driven magnetization dynamics. In particular we obtain an
extended magnetization dynamics equation that precisely contains the
nonrelativistic adiabatic and relativistic nonadiabatic spin-transfer torques
(STTs) of the Berger and Zhang-Li forms as well as relativistic spin-orbit
torques (SOTs). We derive in addition a previously unnoticed relativistic
spin-torque term and moreover show that the various obtained spin-torque terms
do not appear in the same mathematical form in both the Landau-Lifshitz and
Landau-Lifshitz-Gilbert equations of spin dynamics.",1810.07438v1
2020-01-24,MagNet: Discovering Multi-agent Interaction Dynamics using Neural Network,"We present the MagNet, a neural network-based multi-agent interaction model
to discover the governing dynamics and predict evolution of a complex
multi-agent system from observations. We formulate a multi-agent system as a
coupled non-linear network with a generic ordinary differential equation (ODE)
based state evolution, and develop a neural network-based realization of its
time-discretized model. MagNet is trained to discover the core dynamics of a
multi-agent system from observations, and tuned on-line to learn agent-specific
parameters of the dynamics to ensure accurate prediction even when physical or
relational attributes of agents, or number of agents change. We evaluate MagNet
on a point-mass system in two-dimensional space, Kuramoto phase synchronization
dynamics and predator-swarm interaction dynamics demonstrating orders of
magnitude improvement in prediction accuracy over traditional deep learning
models.",2001.09001v2
2021-02-24,udkm1Dsim -- A Python toolbox for simulating 1D ultrafast dynamics in condensed matter,"The udkm1Dsim toolbox is a collection of Python classes and routines to
simulate the thermal, structural, and magnetic dynamics after laser excitation
as well as the according X-ray scattering response in one-dimensional sample
structures. The toolbox provides the capabilities to define arbitrary layered
structures on the atomic level including a rich database of element-specific
physical properties. The excitation of dynamics is represented by an
$N$-temperature-model which is commonly applied in ultrafast physics.
Structural dynamics due to thermal stresses are calculated by a linear-chain
model of masses and springs. The implementation of specific magnetic dynamics
can be easily accomplished by the user employing a generalized magnetization
interface class. The resulting X-ray diffraction response is computed by
kinematical or dynamical X-ray theory which can also include
polarization-dependent magnetic scattering. The udkm1Dsim toolbox is highly
modular and allows for injecting user-defined inputs at any step within the
simulation procedure.",2102.12144v2
2024-03-19,Unraveling the dynamics of magnetization in topological insulator-ferromagnet heterostructures via spin-orbit torque,"Spin-orbit coupling stands as a pivotal determinant in the realm of condensed
matter physics. In recent, its profound influence on spin dynamics opens up a
captivating arena with promising applications. Notably, the topological
insulator-ferromagnet heterostructure has been recognized for inducing spin
dynamics through applied current, driven by spin-orbit torque. Building upon
recent observations revealing spin flip signals within this heterostructure,
our study elucidates the conditions governing spin flips by studying the
magnetization dynamics. We establish that the interplay between spin-anisotropy
and spin-orbit torque plays a crucial role in shaping the physics of
magnetization dynamics within the heterostructure. Furthermore, we categorize
various modes of magnetization dynamics, constructing a comprehensive phase
diagram across distinct energy scales, damping constants, and applied
frequencies. This research not only offers insights into controlling spin
direction but also charts a new pathway to the practical application of
spin-orbit coupled systems.",2403.12701v1
2014-08-22,Collective modes and Kosterlitz-Thouless transition in a magnetic field in the planar Nambu--Jona-Lasino model,"It is known that a constant magnetic field is a strong catalyst of dynamical
chiral symmetry breaking in 2+1 dimensions, leading to generating dynamical
fermion mass even at weakest attraction. In this work we investigate the
collective modes associated with the dynamical chiral symmetry breaking in a
constant magnetic field in the (2+1)-dimensional Nambu--Jona-Lasinio model with
continuous U(1) chiral symmetry. We introduce a self-consistent scheme to
evaluate the propagators of the collective modes at the leading order in 1/N.
The contributions from the vacuum and from the magnetic field are separated
such that we can employ the well-established regularization scheme for the case
of vanishing magnetic field. The same scheme can be applied to the study of the
next-to-leading order correction in 1/N. We show that the sigma mode is always
a lightly bound state with its mass being twice the dynamical fermion mass for
arbitrary strength of the magnetic field. Since the dynamics of the collective
modes is always 2+1 dimensional, the finite temperature transition should be of
the Kosterlitz-Thouless (KT) type. We determine the KT transition temperature
$T_{\rm KT}$ as well as the mass melting temperature $T^*$ as a function of the
magnetic field. It is found that the pseudogap domain $T_{\rm KT}<T<T^*$ is
enlarged with increasing strength of the magnetic field. The influence of a
chiral imbalance or axial chemical potential $\mu_5$ is also studied. We find
that even a constant axial chemical potential $\mu_5$ can lead to inverse
magnetic catalysis of the KT transition temperature in 2+1 dimensions. The
inverse magnetic catalysis behavior is actually the de Haas--van Alphen
oscillation induced by the interplay between the magnetic field and the Fermi
surface.",1408.5364v2
2002-09-06,The Transition State in Magnetization Reversal,"We consider a magnet with uniaxial anisotropy in an external magnetic field
along the anisotropy direction, but with a field magnitude smaller than the
coercive field. There are three representative magnetization configurations
corresponding to three extrema of the free energy. The equilibrium and
metastable configurations, which are magnetized approximately parallel and
antiparallel to the applied field, respectively, both correspond to local
free-energy minima. The third extremum configuration is the saddle point
separating these minima. It is also called the transition state for
magnetization reversal. The free-energy difference between the metastable and
transition-state configurations determines the thermal stability of the magnet.
However, it is difficult to determine the location of the transition state in
both experiments and numerical simulations. Here it is shown that the
computational Projective Dynamics method, applied to the time dependence of the
total magnetization, can be used to determine the transition state. From
large-scale micromagnetic simulations of a simple model of magnetic nanowires
with no crystalline anisotropy, the magnetization associated with the
transition state is found to be linearly dependent on temperature, and the
free-energy barrier is found to be dominated by the entropic contribution at
reasonable temperatures and external fields. The effect of including
crystalline anisotropy is also discussed. Finally, the influence of the spin
precession on the transition state is determined by comparison of the
micromagnetic simulations to kinetic Monte Carlo simulations of precession-free
(overdamped) dynamics.",0209166v1
2007-10-21,New mechanism of generation of large-scale magnetic field in a sheared turbulent plasma,"A review of recent studies on a new mechanism of generation of large-scale
magnetic field in a sheared turbulent plasma is presented. This mechanism is
associated with the shear-current effect which is related to the W x J-term in
the mean electromotive force. This effect causes the generation of the
large-scale magnetic field even in a nonrotating and nonhelical homogeneous
sheared turbulent convection whereby the alpha effect vanishes. It is found
that turbulent convection promotes the shear-current dynamo instability, i.e.,
the heat flux causes positive contribution to the shear-current effect.
However, there is no dynamo action due to the shear-current effect for small
hydrodynamic and magnetic Reynolds numbers even in a turbulent convection, if
the spatial scaling for the turbulent correlation time is k^{-2}, where k is
the small-scale wave number. We discuss here also the nonlinear mean-field
dynamo due to the shear-current effect and take into account the transport of
magnetic helicity as a dynamical nonlinearity. The magnetic helicity flux
strongly affects the magnetic field dynamics in the nonlinear stage of the
dynamo action. When the magnetic helicity flux is not small, the saturated
level of the mean magnetic field is of the order of the equipartition field
determined by the turbulent kinetic energy. The obtained results are important
for elucidation of origin of the large-scale magnetic fields in astrophysical
and cosmic sheared turbulent plasma.",0710.3934v1
2009-09-28,Magnetic order and dynamics of the charge-ordered antiferromagnet La1.5Sr0.5CoO4,"We describe neutron scattering experiments performed to investigate the
magnetic order and dynamics of half-doped La1.5Sr0.5CoO4. This layered
perovskite exhibits a near-ideal checkerboard pattern of Co2+/Co3+ charge order
at temperatures below ~ 800 K. Magnetic correlations are observed at
temperatures below ~ 60 K but static magnetic order only becomes established at
31 K, a temperature at which a kink is observed in the susceptibility. On
warming above 31 K we observed a change in the magnetic correlations which we
attribute either to a spin canting or to a change in the proportion of
inequivalent magnetic domains. The magnetic excitation spectrum is dominated by
an intense band extending above a gap of approximately 3 meV up to a maximum
energy of 16 meV. A weaker band exists in the energy range 20-30 meV. We show
that the excitation spectrum is in excellent quantitative agreement with the
predictions of a spin-wave theory generalized to include the full magnetic
degrees of freedom of high-spin Co2+ ions in an axially distorted crystal
field, coupled by Heisenberg exchange interactions. The magnetic order is found
to be stabilized by dominant antiferromagnetic Co2+ -- Co2+ interactions acting
in a straight line through Co3+. No evidence is found for magnetic scattering
from the Co3+ ions, supporting the view that Co3+ is in the S = 0 state in this
material.",0909.4964v1
2010-04-30,Dynamical Monte Carlo investigation of spin reversals and nonequilibrium magnetization of single-molecule magnets,"In this paper, we combine thermal effects with Landau-Zener (LZ) quantum
tunneling effects in a dynamical Monte Carlo (DMC) framework to produce
satisfactory magnetization curves of single-molecule magnet (SMM) systems. We
use the giant spin approximation for SMM spins and consider regular lattices of
SMMs with magnetic dipolar interactions (MDI). We calculate spin reversal
probabilities from thermal-activated barrier hurdling, direct LZ tunneling, and
thermal-assisted LZ tunnelings in the presence of sweeping magnetic fields. We
do systematical DMC simulations for Mn$_{12}$ systems with various temperatures
and sweeping rates. Our simulations produce clear step structures in
low-temperature magnetization curves, and our results show that the thermally
activated barrier hurdling becomes dominating at high temperature near 3K and
the thermal-assisted tunnelings play important roles at intermediate
temperature. These are consistent with corresponding experimental results on
good Mn$_{12}$ samples (with less disorders) in the presence of little
misalignments between the easy axis and applied magnetic fields, and therefore
our magnetization curves are satisfactory. Furthermore, our DMC results show
that the MDI, with the thermal effects, have important effects on the LZ
tunneling processes, but both the MDI and the LZ tunneling give place to the
thermal-activated barrier hurdling effect in determining the magnetization
curves when the temperature is near 3K. This DMC approach can be applicable to
other SMM systems, and could be used to study other properties of SMM systems.",1004.5461v2
2010-05-28,Magnetic helicity fluxes in interface and flux transport dynamos,"Dynamos in the Sun and other bodies tend to produce magnetic fields that
possess magnetic helicity of opposite sign at large and small scales,
respectively. The build-up of magnetic helicity at small scales provides an
important saturation mechanism. In order to understand the nature of the solar
dynamo we need to understand the details of the saturation mechanism in
spherical geometry. In particular, we want to understand the effects of
magnetic helicity fluxes from turbulence and meridional circulation. We
consider a model with just radial shear confined to a thin layer (tachocline)
at the bottom of the convection zone. The kinetic alpha owing to helical
turbulence is assumed to be localized in a region above the convection zone.
The dynamical quenching formalism is used to describe the build-up of mean
magnetic helicity in the model, which results in a magnetic alpha effect that
feeds back on the kinetic alpha effect. In some cases we compare with results
obtained using a simple algebraic alpha quenching formula. In agreement with
earlier findings, the magnetic alpha effect in the dynamical alpha quenching
formalism has the opposite sign compared with the kinetic alpha effect and
leads to a catastrophic decrease of the saturation field strength with
increasing magnetic Reynolds numbers. However, at high latitudes this quenching
effect can lead to secondary dynamo waves that propagate poleward due to the
opposite sign of alpha. Magnetic helicity fluxes both from turbulent mixing and
from meridional circulation alleviate catastrophic quenching.",1005.5335v1
2016-05-11,"Optical orientation of hole magnetic polarons in (Cd,Mn)Te/(Cd,Mn,Mg)Te quantum wells","The optically induced spin polarization in (Cd,Mn)Te/(Cd,Mn,Mg)Te
diluted-magnetic-semiconductor quantum wells is investigated by means of
picosecond pump-probe Kerr rotation. At 1.8 K temperature, additionally to the
oscillatory signals from photoexcited electrons and Manganese spins precessing
about an external magnetic field, a surprisingly long-lived (up to 60 ns)
nonoscillating spin polarization is detected. This polarization is related to
optical orientation of equilibrium magnetic polarons involving resident holes.
The suggested mechanism for the optical orientation of the equilibrium magnetic
polarons indicates that the detected polaron dynamics originates from unexcited
magnetic polarons. The polaron spin dynamics is controlled by the anisotropic
spin structure of the heavy-hole resulting in a freezing of the polaron
magnetic moment in one of the two stable states oriented along the structure
growth axis. Spin relaxation between these states is prohibited by a potential
barrier, which depends on temperature and magnetic field. The magnetic polaron
relaxation is accelerated with increasing temperature and in magnetic field.",1605.03362v1
2017-03-09,Influence of a large-scale field on energy dissipation in magnetohydrodynamic turbulence,"In magnetohydrodynamic (MHD) turbulence, the large-scale magnetic field sets
a preferred local direction for the small-scale dynamics, altering the
statistics of turbulence from the isotropic case. This happens even in the
absence of a total magnetic flux, since MHD turbulence forms randomly oriented
large-scale domains of strong magnetic field. It is therefore customary to
study small-scale magnetic plasma turbulence by assuming a strong background
magnetic field relative to the turbulent fluctuations. This is done, for
example, in reduced models of plasmas, such as reduced MHD, reduced-dimension
kinetic models, gyrokinetics, etc., which make theoretical calculations easier
and numerical computations cheaper. Recently, however, it has become clear that
the turbulent energy dissipation is concentrated in the regions of strong
magnetic field variations. A significant fraction of the energy dissipation may
be localized in very small volumes corresponding to the boundaries between
strongly magnetized domains. In these regions the reduced models are not
applicable. This has important implications for studies of particle heating and
acceleration in magnetic plasma turbulence. The goal of this work is to
systematically investigate the relationship between local magnetic field
variations and magnetic energy dissipation, and to understand its implications
for modeling energy dissipation in realistic turbulent plasmas.",1703.03433v1
2017-05-31,Lattice and spin dynamics in a low-symmetry antiferromagnet NiWO$_4$,"Lattice and magnetic dynamics of NiWO$_4$ single crystals were studied with
the use of polarized Raman spectroscopy in a wide temperature range of 10-300 K
including the antiferromagnetic ordering temperature $T_N$=62 K. Static
magnetic measurements were used for characterizing the single crystals. All
Raman-active phonons predicted by the group theory were observed and
characterized. Magnetic symmetry analysis was used to determine possible
magnetic space groups for NiWO$_4$ which can be also applied to any other
isostructural crystal with the same magnetic propagation vector k=(1/2,0,0).
Though the magnetic structure of NiWO$_4$ is relatively simple, a rich set of
narrow and broad magnetic excitations with different polarization properties
and temperature behavior in the very broad frequency range of 10-200 cm$^{-1}$
was observed, with some modes surviving at temperatures much higher than $T_N$
up to 220 K. Part of the magnetic excitations was identified as acoustic and
optical spin-wave branches which allow us to construct exchange structure and
estimate exchange and anisotropy constants with the use of linear spin-wave
theory. Since the magnetic structure can be described as exchange-coupled AFM
chains of $S=1$ ions, previously unobserved magnetic excitation at 24 cm$^{-1}$
is tentatively assigned to a Haldane gap mode.",1705.11137v1
2018-05-24,Magnetic suppression of zonal flows on a beta plane,"Zonal flows in rotating systems have been previously shown to be suppressed
by the imposition of a background magnetic field aligned with the direction of
rotation. Understanding the physics behind the suppression may be important in
systems found in astrophysical fluid dynamics, such as stellar interiors.
However, the mechanism of suppression has not yet been explained. In the
idealized setting of a magnetized beta plane, we provide a theoretical
explanation that shows how magnetic fluctuations directly counteract the growth
of weak zonal flows. Two distinct calculations yield consistent conclusions.
The first, which is simpler and more physically transparent, extends the
Kelvin-Orr shearing wave to include magnetic fields and shows that weak,
long-wavelength shear flow organizes magnetic fluctuations to absorb energy
from the mean flow. The second calculation, based on the quasilinear,
statistical CE2 framework, is valid for arbitrary wavelength zonal flow and
predicts a self-consistent growth rate of the zonal flow. We find that a
background magnetic field suppresses zonal flow if the bare Alfven frequency is
comparable to or larger than the bare Rossby frequency. However, suppression
can occur for even smaller magnetic fields if the resistivity is sufficiently
small enough to allow sizable magnetic fluctuations. Our calculations reproduce
the $\eta/B_0^2 = \text{const.}$ scaling that describes the boundary of
zonation, as found in previous work, and we explicitly link this scaling to the
amplitude of magnetic fluctuations.",1805.09847v2
2019-01-05,Investigating the Effect of Magnetic Dipole-Dipole Interaction on Magnetic Particle Spectroscopy (MPS): Implications for Magnetic Nanoparticle-based Bioassays and Magnetic Particle Imaging (MPI),"Superparamagnetic iron oxide nanoparticles (SPIONs), with comparable size to
biomolecules (such as proteins, nucleic acids, etc.) and unique magnetic
properties, good biocompatibility, low toxicity, potent catalytic behavior, are
promising candidates for many biomedical applications. There is one property
present in most SPION systems, yet it has not been fully exploited, which is
the dipole-dipole interaction (also called dipolar interaction) between the
SPIONs. It is known that the magnetic dynamics of an ensemble of SPIONs are
substantially influenced by the dipolar interactions. However, the exact way it
affects the performance of magnetic particle-based bioassays and magnetic
particle imaging (MPI) is still an open question. The purpose of this paper is
to give a partial answer to this question. This is accomplished by numerical
simulations on the dipolar interactions between two nearby SPIONs and
experimental measurements on an ensemble of SPIONs using our lab-based magnetic
particle spectroscopy (MPS) system. Our results show that even moderate changes
in the SPION concentration may have substantial effects on the magnetic
dynamics of the SPION system and the harmonic signal magnitudes can be
increased or decreased by 60%, depending on the values of MPS system
parameters.",1901.01355v1
2020-11-12,Dynamics of test particles around a Bardeen black hole surrounded by perfect fluid dark matter,"We study the dynamics of (i) neutral test particles, (ii) magnetically
charged test particles, and (iii) test magnetic dipole around a regular Bardeen
black hole surrounded by perfect fluid dark matter (PFDM). It has been shown
how the magnetic charge of the black hole and the parameter of the surrounding
PFDM can influence the innermost stable circular orbit (ISCO) radius of a test
particle. We have found that the ISCO radius is strongly affected as a
consequence of the combined effect of the magnetic charge parameter and the
perfect fluid dark matter. The black hole magnetic charge parameter $g$ and the
combined effect of perfect fluid dark matter can mimic the black hole rotation
parameter up to $a/M\approx 0.9$. It has been observed that the ISCO for
magnetic dipole disappears at the values exceeding the calculated upper value
for the magnetic interaction parameter $\beta$. The upper limit decreases with
the increase of both the dark matter and magnetic charge parameters. Finally,
as an astrophysical application, we have analyzed degeneracy effects of spin of
Kerr black holes and magnetic charge of regular Bardeen black holes for the
different values of the dark matter parameter providing exactly the same value
for ISCO radius of a magnetic dipole with the same value of the parameter
$\beta=10.2$ of the magnetar called PSR J1745-2900 orbiting around supermassive
black hole Sagittarius A*.",2011.06148v1
2021-03-08,Optical Magnetic Multipolar Resonances in Large Dynamic Metamolecules,"Dynamic metamolecules (DMMs) are composed of a dielectric core made of
hydrogel surrounded by randomly-packed plasmonic beads that can display
magnetic resonances when excited by light at optical frequencies. Their optical
properties can be controlled by controlling their core diameter through
temperature variations. We have recently shown that DMMs display strong optical
magnetism, including magnetic dipole and magnetic quadrupole resonances,
offering significant potential for novel applications. Here, we use a T-matrix
approach to characterize the magnetic multipole resonance modes of model
metamolecules and explore their presence in experimental data. We show that
high-order multipole resonances become prominent as the bead size and the
overall structure sizes are increased, and when the the inter-bead gap is
decreased. In this limit, mode mixing among high-order magnetic multipole modes
also become significant, particularly in the directional scattering spectra. We
discuss trends in magnetic scattering observed in both experiments and
simulations, and provide suggestions for experimental design and verification
of high-order optical magnetic resonances in the forward or backward scattering
spectra. In addition, angular scattering of higher-order magnetic modes can
display Fano-like interference patterns that should be experimentally
detectable.",2103.04495v2
2021-03-20,Dynamics of charged particles and magnetic dipoles around magnetized quasi-Schwarzschild black holes,"In the present paper, we have investigated the motion of charged particles
together with magnetic dipoles to determine how well the spacetime deviation
parameter $\epsilon$ and external uniform magnetic field can mimic the spin of
a rotating Kerr black hole. Investigation of charged particle motion has shown
that the deviation parameter $\epsilon$ in the absence of external magnetic
fields can mimic the rotation parameter of Kerr spacetime up to $a/M
\approx0.5$. The combination of external magnetic field and deviation parameter
can do even a better job mimicking the rotation parameter up to
$a/M\simeq0.93$, which corresponds to the rapidly rotating case. Study of the
dynamics of magnetic dipoles around quasi-Schwarzschild black holes in the
external magnetic field has shown that there are degeneracy values of ISCO
radius of test particles at $\epsilon_{cr}>\epsilon\geq 0.35$ which may lead to
two different values of the innermost stable circular orbit (ISCO) radius. When
the deviation parameter is in the range of $\epsilon \in (-1,\ 1)$, it can
mimic the spin of a rotating Kerr black hole in the range $a/M \in (0.0537, \
0.3952)$ for magnetic dipoles with values of magnetic coupling parameter $\beta
\in [-0.25,\ 0.25]$ in corotating orbits.",2103.11090v1
2021-07-26,Pseudo-magnetic field-induced ultra-slow carrier dynamics in periodically strained graphene,"The creation of pseudo-magnetic fields in strained graphene has emerged as a
promising route to allow observing intriguing physical phenomena that would be
unattainable with laboratory superconducting magnets. Scanning tunneling
spectroscopy experiments have successfully measured the pseudo-Landau levels
and proved the existence of pseudo-magnetic fields in various strained graphene
systems. These giant pseudo-magnetic fields observed in highly deformed
graphene can substantially alter the optical properties of graphene beyond a
level that can be feasible with an external magnetic field, but the
experimental signatures of the influence of such pseudo-magnetic fields have
yet to be unveiled. Here, using time-resolved infrared pump-probe spectroscopy,
we provide unambiguous evidence for ultra-slow carrier dynamics enabled by
pseudo-magnetic fields in periodically strained graphene. Strong
pseudo-magnetic fields of ~100 T created by non-uniform strain in graphene
nanopillars are found to significantly decelerate the relaxation processes of
hot carriers by more than an order of magnitude. Our finding presents
unforeseen opportunities for harnessing the new physics of graphene enabled by
pseudo-magnetic fields for optoelectronics and condensed matter physics.",2107.11973v1
2021-07-27,Dynamo instabilities in plasmas with inhomogeneous chiral chemical potential,"We study the dynamics of magnetic fields in chiral magnetohydrodynamics,
which takes into account the effects of an additional electric current related
to the chiral magnetic effect in high energy plasmas. We perform direct
numerical simulations, considering weak seed magnetic fields and
inhomogeneities of the chiral chemical potential mu_5 with a zero mean. We
demonstrate that a small-scale chiral dynamo can occur in such plasmas if
fluctuations of mu_5 are correlated on length scales that are much larger than
the scale on which the dynamo growth rate reaches its maximum. Magnetic
fluctuations grow by many orders of magnitude due to the small-scale chiral
dynamo instability. Once the nonlinear backreaction of the generated magnetic
field on fluctuations of mu_5 sets in, the ratio of these scales decreases and
the dynamo saturates. When magnetic fluctuations grow sufficiently to drive
turbulence via the Lorentz force before reaching maximum field strength, an
additional mean-field dynamo phase is identified. The mean magnetic field grows
on a scale that is larger than the integral scale of turbulence after the
amplification of the fluctuating component saturates. The growth rate of the
mean magnetic field is caused by a magnetic alpha effect that is proportional
to the current helicity. With the onset of turbulence, the power spectrum of
mu_5 develops a universal k^(-1) scaling independently of its initial shape,
while the magnetic energy spectrum approaches a k^(-3) scaling.",2107.13028v3
2022-03-04,Reservoir Computing with Spin Waves in Skyrmion Crystal,"Magnetic skyrmions are nanometric spin textures characterized by a quantized
topological invariant in magnets and often emerge in a crystallized form called
skyrmion crystal in an external magnetic field. We propose that magnets hosting
a skyrmion crystal possess high potential for application to reservoir
computing, which is one of the most successful information processing
techniques inspired by functions of human brains. Our skyrmion-based reservoir
exploits precession dynamics of magnetizations, i.e., spin waves, propagating
in the skyrmion crystal. Because of complex interferences and slow relaxations
of the spin-wave dynamics, the skyrmion spin-wave reservoir attains several
important characteristics required for reservoir computing, e.g., the
generalization ability, the nonlinearity, and the short-term memory. We
investigate these properties by imposing three standard tasks to test the
performances of reservoir, i.e., the duration-estimation task, the short-term
memory task, and the parity-check task. Through these investigations, we
demonstrate that magnetic skyrmion crystals are promising materials for
spintronics reservoir devices. Because magnetic skyrmions emerge spontaneously
in magnets via self-organization process under application of a static magnetic
field, the proposed skyrmion reservoir requires neither advanced
nanofabrication nor complicated manufacturing for production in contrast to
other previously proposed magnetic reservoirs constructed with fabricated
spintronics devices. Our proposal is expected to realize a breakthrough in the
research of spintronics reservoirs of high performance.",2203.02160v1
2022-03-15,Some Aspects of Rotation and Magnetic Field Morphology in the Infrared Dark Cloud G34.43+00.24,"The infrared dark clouds (IRDCs) are molecular clouds with relatively greater
values in their magnetic field strengths. For example, the IRDC G34.43+00.24
(G34) has magnetic field strength of the order of a few hundred micro-Gauss. In
this study, we investigate if the dynamic motions of charged particles in an
IRDC such as G34 can produce this magnetic field strength inside it. The
observations show that the line-of-sight velocity of G34 has global gradient.
We assume that the measured global velocity gradient can correspond to the
cloud rotation. We attribute a large-scale current density to this rotating
cloud by considering a constant value for the incompleteness of charge
neutrality and the velocity differences between the positive and negative
particles with very low ionization fractions. We use the numerical package
FISHPACK to obtain the magnetic field strength and its morphology on the
plane-of-sky within G34. The results show that the magnetic field strengths are
of the order of several hundred micro-Gauss, and its morphology in the
plane-of-sky is somewhat consistent with the observational results. We also
obtain the relationship between magnetic field strength and density in G34. The
results show that with increasing density, the magnetic field strength
increases approximately as a power-law function. The amount of power is
approximately equal to 0.45, which is suitable for molecular clouds with strong
magnetic fields. Therefore, we can conclude that the dynamical motion of IRDCs,
and especially their rotations, can amplify the magnetic field strengths within
them.",2203.07689v1
2022-04-12,Magnetization statics and dynamics in (Ir/Co/Pt)$_6$ multilayers with Dzyaloshinskii-Moriya interaction,"Magnetic multilayers of (Ir/Co/Pt)$_6$ with interfacial Dzyaloshinskii-Moriya
interaction (IDMI) were deposited by magnetron sputtering with Co thickness
$d=1.8$ nm. Exploiting magneto-optical Kerr effect in longitudinal mode
microscopy, magnetic force microscopy, and vibrating sample magnetometry, the
magnetic field-driven evolution of domain structures and magnetization
hysteresis loops have been studied. The existence of weak stripe domains
structure was deduced -- tens micrometers size domains with in-plane ""core""
magnetization modulated by hundred of nanometers domains with out-of-plane
magnetization. Micromagnetic simulations interpreted such magnetization
distribution. Quantitative evaluation of IDMI was carried out using Brillouin
light scattering (BLS) spectroscopy as the difference between Stokes and
anti-Stokes peak frequencies $\Delta f$. Due to the additive nature of IDMI,
the asymmetric combination of Ir and Pt covers led to large values of effective
IDMI energy density $D_\mathrm{eff}$. It was found that Stokes and anti-Stokes
frequencies as well as $\Delta f$, measured as a function of in-plane applied
magnetic field, show hysteresis. These results are explained under the
consideration of the influence of IDMI on the dynamics of the in-plane
magnetized ""core"" with weak stripe domains.",2204.05956v1
2022-12-25,Theory of superdiffusive spin transport in noncollinear magnetic multilayers,"Ultrafast demagnetization induced by femtosecond laser pulses in thin
metallic layers is caused by the outflow of spin-polarized hot electron
currents describable by the superdiffusive transport model. These
laser-generated spin currents can cross the interface into another magnetic
layer and give rise to magnetization dynamics in magnetic spin valves with
noncollinear magnetizations. To describe ultrafast transport and spin dynamics
in such nanostructures we develop here the superdiffusive theory for general
noncollinear magnetic multilayers. Specifically, we introduce an Al/Ni/Ru/Fe/Ru
multilayer system with noncollinear Ni and Fe magnetic moments and analyze how
the ultrafast demagnetization and spin-transfer torque depend on the
noncollinearity. We employ ab initio calculations to compute the spin- and
energy-dependent transmissions of hot electrons at the interfaces of the
multilayer. Taking into account multiple electron scattering at interfaces and
spin mixing in the spacer layer we find that the laser-induced demagnetization
of the Ni layer and magnetization change of the Fe layer strongly depend on the
angle between their magnetizations. Similarly, the spin-transfer torques on the
Ni and Fe layers and the total spin momentum absorbed in the Ni and Fe layer
are found to vary markedly with the amount of noncollinearity.
  These results suggest that changing the amount of noncollinearity in magnetic
multilayers one can efficiently control the hot electron spin transport, which
may open a way toward achieving fast, laser-driven spintronic devices.",2212.12961v1
2001-02-16,Finite-temperature magnetism of transition metals: an LDA+DMFT approach,"We present an ab initio quantum theory of the finite temperature magnetism of
iron and nickel. A recently developed technique which combines dynamical
mean-field theory with realistic electronic structure methods, successfully
describes the many-body features of the one electron spectra and the observed
magnetic moments below and above the Curie temperature.",0102297v1
2001-10-30,Nonlinear Hall effect in an AC electric field and related phenomena,"It has been shown that in metals and semiconductors the joint action of
permanent magnetic and AC electric fields leads to arising of DC surface
electric current. This is due to essentially non-linear dynamics of electronic
gas in surface layer.The surface current is linear with respect to external
magnetic field and depends quadratically on an amplitude of AC electric
field.The conditions for spontaneous generation of magnetic field have been
found.",0110616v1
2002-10-31,Superradiant Operation of Spin Masers,"The theory of spin superradiance, developed earlier for nuclear magnets, is
generalized to a wider class of spin systems, such as granular magnets and
molecular magnets. The latter may possess strong single-site magnetocrystalline
anisotropy, whose role in nonlinear spin dynamics is analysed. Transient as
well as pulsing superradiant regimes are described. These coherent regimes may
be employed in the operation of spin masers.",0210698v1
2003-03-02,Long Range Dynamics Related to Magnetic Impurity in the 2D Heisenberg Antiferromagnet,"We consider a magnetic impurity in the two-dimensional Heisenberg
antifferomagnet with long range antiferromagnetic order. At low temperature the
impurity magnetic susceptibility has a Curie term ($\propto 1/T$) and a
logarithmic correction ($\propto \ln(T)$). We calculate the correction and
derive related Ward identity for the impurity-spin-wave vertex.",0303016v1
2003-12-10,Spin Gain Transistor in Ferromagnetic Semiconductors: the Semiconductor Bloch Equations Approach,"Scheme and principle of operation of a spin gain transistor are proposed: a
large unmagnetized current creates density sufficient for the ferromagnetic
transition; a small magnetized current initiates spontaneous magnetization;
large magnetized current is extracted. Therefore spin gain of more than 1000 is
predicted. Collective dynamics of spins under Coulomb exchange interaction is
described via Semiconductor Bloch Equations.",0312243v1
2003-12-22,Spin dynamics of the ferromagnetic superconductor UGe$_{2}$,"Inelastic neutron scattering was used to study the low energy magnetic
excitations of the ferromagnetic superconductor UGe$_{2}$. The ferromagnetic
fluctuations are of Ising nature with a non-conserved magnetization and have an
intermediate behavior between localized and itinerant magnetism.",0312584v1
2007-03-24,Critical exponents of small one-dimensional ising magnetic,"Within the framework of a generalized Ising model, a one-dimensional magnetic
of a finite length with free ends is considered. The correlation length
exponent, dynamic critical exponent z of the magnet is calculated taking into
account the next nearest neighbor interactions and the external field.",0703639v1
2006-02-07,Magnetic Geodesic Flows on Coadjoint Orbits,"We describe a class of completely integrable $G$-invariant magnetic geodesic
flows on (co)adjoint orbits of a compact connected Lie group $G$ with magnetic
field given by the Kirillov-Konstant 2-form.",0602016v2
2006-03-10,Dynamo in Helical MHD Turbulence: Quantum Field Theory Approach,"A quantum field model of helical MHD stochastically forced by gaussian
hydrodynamic, magnetic and mixed noices is investigated. These helical noises
lead to an exponential increase of magnetic fluctuations in the large scale
range. Instabilities, which are produced in this process, are eliminated by
spontaneous symmetry breaking mechanism accompanied by creation of the
homogeneous stationary magnetic field.",0603020v1
2002-08-08,Bayesian analysis of magnetic island dynamics,"We examine a first order differential equation with respect to time coming up
in the description of magnetic islands in magnetically confined plasmas. The
free parameters of this equation are obtained by employing Bayesian probability
theory. Additionally a typical Bayesian change point is solved in the process
of obtaining the data.",0208036v1
2002-09-12,Dipolar relaxation of cold sodium atoms in a magnetic field,"A quantum mechanical close coupling theory of spin relaxation in the
stretched hyperfine level of sodium is presented. We calculate the dipolar
relaxation rate of magnetically trapped cold sodium atoms in the magnetic
field. The influence of shape resonances and the anisotropy of the dipolar
interaction on the collision dynamics are explored. We examine the sensitivity
of the calculated cross sections on the choice of asymptotic atomic state
basis.",0209052v2
2007-04-23,Fluctuation theorems and orbital magnetism in nonequilibrium state,"We study Langevin dynamics of a driven charged particle in the presence as
well as in the absence of magnetic field. We discuss the validity of various
work fluctuation theorems using different model potentials and external drives.
We also show that one can generate an orbital magnetic moment in a
nonequilibrium state which is absent in equilibrium.",0704.2992v1
2007-05-04,Numerical study of liquid metal flow in a rectangular duct under the influence of a heterogenous magnetic field,"The paper presents 3D numerical results for the laminar liquid metal flow in
a rectangular duct compared with experimental results. It is shown that the
magnetic interaction parameter $N$ is the main parameter governing the flow
provided turbulent pulsations are locally suppressed by magnetic field.",0705.0633v1
2007-05-15,Controlled vortex core switching in a magnetic nanodisk by a rotating field,"The switching process of the vortex core in a Permalloy nanodisk affected by
a rotating magnetic field is studied theoretically. A detailed description of
magnetization dynamics is obtained by micromagnetic simulations.",0705.2046v2
2008-02-08,The algorithm for simulating of phase transition in Ising magnetic,"Simple algorithm of dynamics of Ising magnetic is described. The algorithm
can be implemented on conventional digital computer and can be used for
construction of specialized processor for simulation of ferromagnetic systems.
The algorithm gives a simple way to calculate 1D correlation functions for 1D
Ising magnetic.",0802.1133v2
2008-07-03,Supersymmetry of the magnetic vortex,"The N=2 supersymmetry of the Pauli Hamiltonian in any static magnetic field
in the plane combines, for the magnetic vortex, with Jackiw's bosonic
$o(2)\times o(2,1)$ symmetry, into an $o(2)\times osp(1/2)$ dynamical
supersymmetry.",0807.0569v1
2008-08-25,Magnetization of a half-quantum vortex in a spinor Bose-Einstein condensate,"Magnetization dynamics of a half-quantum vortex in a spin-1 Bose-Einstein
condensate with a ferromagnetic interaction are investigated by mean-field and
Bogoliubov analyses. The transverse magnetization is shown to break the
axisymmetry and form threefold domains. This phenomenon originates from the
topological structure of the half-quantum vortex and spin conservation.",0808.3315v1
2008-10-21,"Coherent manipulation of magnetization precession in ferromagnetic semiconductor (Ga,Mn)As with successive optical pumping","We report dynamic control of magnetization precession by light alone. A
ferromagnetic (Ga,Mn)As epilayer was used for experiments. Amplitude of
precession was modulated to a large extent by tuning the time interval between
two successive optical pump pulses which induced torques on magnetization
through a non-thermal process. Nonlinear effect in precession motion was also
discussed.",0810.3728v1
2009-04-14,Poisson-Vlasov in a strong magnetic field: A stochastic solution approach,"Stochastic solutions are obtained for the Maxwell-Vlasov equation in the
approximation where magnetic field fluctuations are neglected and the
electrostatic potential is used to compute the electric field. This is a
reasonable approximation for plasmas in a strong external magnetic field. Both
Fourier and configuration space solutions are constructed.",0904.2214v2
2012-05-09,Asymmetric chiral alignment in magnetized plasma turbulence,"Multi species turbulence in inhomogeneous magnetised plasmas is found to
exhibit symmetry breaking in the dynamical alignment of a third species with
the fluctuating electron density and vorticity with respect to the magnetic
field direction and the species' relative background gradients. The possibility
of truly chiral aggregation of charged molecules in magnetized space plasma
turbulence is discussed.",1205.1891v3
2012-11-16,Measurement of a time-periodic magnetic field by rotating coil,"A novel technique of the measurement data processing is developed which
allows to apply the rotating coil method for measurement of a dynamic magnetic
field, periodic in time. The developed technique allows to obtain
time-dependent multipoles in a single measurement which takes place during one
power cycle of the magnet or the coil rotation period, whichever is longer.",1211.3974v1
2016-07-04,Analogue Magnetism: An Ansatz,"We present an ansatz for the relationship between magnetic flux density and
fluid vorticity evolution equations. We also suggest that the magnetic flux
density evolution equations be compared to the evolution equation for an
effective vorticity ($\omega_{eff}$), which bears a power law relation to the
ordinary vorticity.",1607.00980v2
2016-12-03,Euler potentials and Magnetic fields,"It is known that the cross product of gradients of two scalars, the Euler
potentials (EP), may represent magnetic fields lines. We examine the utility of
such potential in the broader magneto-genesis and dynamo theories, and find
that a reinterpretation of the potentials offer a new understanding of the role
EP may play in the evolution of magnetic fields.",1612.00956v2
2018-05-25,Carleman estimate for the Schrödinger equation and application to magnetic inverse problems,"We prove that the stationary magnetic potential vector and the electrostatic
potential entering the dynamic magnetic Schr\""odinger equation can be Lipschitz
stably retrieved through finitely many local boundary measurements of the
solution. The proof is by means of a specific global Carleman estimate for the
Schr\""odinger equation, established in the first part of the paper.",1805.10076v1
2018-07-05,Mechanisms for magnetic Field generation in precessing cubes,"It is shown that flows in precessing cubes develop at certain parameters
large axisymmetric components in the velocity field which are large enough to
either generate magnetic fields by themselves, or to contribute to the dynamo
effect if inertial modes are already excited and acting as a dynamo. This
effect disappears at small Ekman numbers. The critical magnetic Reynolds number
also increases at low Ekman numbers because of turbulence and small scale
structures.",1807.02047v1
2023-01-24,Spin Dynamics in van der Waals Magnetic Systems,"The discovery of atomic monolayer magnetic materials has stimulated intense
research activities in the two-dimensional (2D) van der Waals (vdW) materials
community. The field is growing rapidly and there has been a large class of 2D
vdW magnetic compounds with unique properties, which provides an ideal platform
to study magnetism in the atomically thin limit. In parallel, based on
tunneling magnetoresistance and magneto-optical effect in 2D vdW magnets and
their heterostructures, emerging concepts of spintronic and optoelectronic
applications such as spin tunnel field-effect transistors and spin-filtering
devices are explored. While the magnetic ground state has been extensively
investigated, reliable characterization and control of spin dynamics play a
crucial role in designing ultrafast spintronic devices. Ferromagnetic resonance
(FMR) allows direct measurements of magnetic excitations, which provides
insight into the key parameters of magnetic properties such as exchange
interaction, magnetic anisotropy, gyromagnetic ratio, spin-orbit coupling,
damping rate, and domain structure. In this review article, we present an
overview of the essential progress in probing spin dynamics of 2D vdW magnets
using FMR techniques. Given the dynamic nature of this field, we focus mainly
on broadband FMR, optical FMR, and spin-torque FMR, and their applications in
studying prototypical 2D vdW magnets. We conclude with the recent advances in
laboratory- and synchrotron-based FMR techniques and their opportunities to
broaden the horizon of research pathways into atomically thin magnets.",2301.09822v2
2006-02-07,Do magnetic fields contribute to the dynamical stability of the secondaries of CVs against mass transfer?,"We show the presence of CVs with orbital periods in the range 3.5-7 h and
mass ratios q=M2/M1 above the critical values for dynamically stable mass
transfer. We explore whether the magnetic fields produced in the secondaries
alter the theoretical mass ratio limits allowing dynamical stability. Since
magnetic fields change the specific heats and the polytropic exponent gamma of
the gas in the convective envelope, the mass-radius adiabatic exponent xi_ad
also changes. We find that turbulent magnetic fields can produce 10% less
restrictive critical q-profiles while large-scale toroidal and poloidal fields
have smaller effects. Thus, magnetic fields alone do not account for the
stability of all the anomalous CVs. However, we found that the small variations
of xi_ad induced by solar-type magnetic cycles explain the amplitudes of the
cyclic accretion luminosity variations shown by several CVs.",0602157v3
1998-12-01,Simulation of Magnetization Switching in Biaxial Single-Domain Ferromagnetic Particles,"The magnetization switching dynamics of biaxial single-domain homogeneous
ferromagnetic particles, in which the two easy axes are perpendicular to each
other, is simulated using a 4-state clock model. A zero-field mapping of the
statics between the symmetric 4-state clock model and two decoupled Ising
models is extended to non-zero field statics and to the dynamics. This
significantly simplifies the analysis of the simulation results. We measure the
magnetization switching time of the model and analyze the results using droplet
theory. The switching dynamics in the asymmetric model is more complicated. If
the easy axis is perpendicular to the stable magnetization direction, the
system can switch its magnetization via two different channels, one very fast
and the other very slow. A maximum value for the switching field as a function
of system size is obtained. The asymmetry affects the switching fields
differently, depending on whether the switching involves one single droplet or
many droplets of spins in the stable magnetization configuration. The angular
dependence of the switching field in symmetric and asymmetric models is also
studied.",9812010v1
2001-07-02,Thermodynamics and Spin Tunneling Dynamics in Ferric Wheels with Excess Spin,"We study theoretically the thermodynamic properties and spin dynamics of a
class of magnetic rings closely related to ferric wheels, antiferromagnetic
ring systems, in which one of the Fe (III) ions has been replaced by a dopant
ion to create an excess spin. Using a coherent-state spin path integral
formalism, we derive an effective action for the system in the presence of a
magnetic field. We calculate the functional dependence of the magnetization and
tunnel splitting on the magnetic field and show that the parameters of the spin
Hamiltonian can be inferred from the magnetization curve. We study the spin
dynamics in these systems and show that quantum tunneling of the Neel vector
also results in tunneling of the total magnetization. Hence, the spin
correlation function shows a signature of Neel vector tunneling, and electron
spin resonance (ESR) techniques or AC susceptibility measurements can be used
to measure both the tunneling and the decoherence rate. We compare our results
with exact diagonalization studies on small ring systems. Our results can be
easily generalized to a wide class of nanomagnets, such as ferritin.",0107025v1
2001-10-09,"Ruthenocuprates RuSr2(Eu,Ce)2Cu2O10: Intrinsic magnetic multilayers","We report ac susceptibility data on RuSr_2(Eu,Ce)_2Cu_2O_(10-y) (Ru-1222, Ce
content x=0.5 and 1.0), RuSr_2GdCu_2O_8 (Ru-1212) and SrRuO_3. Both Ru-1222
(x=0.5, 1.0) sample types exhibit unexpected magnetic dynamics in low magnetic
fields: logarithmic time relaxation, switching behavior, and `inverted'
hysteresis loops. Neither Ru-1212 nor SrRuO_3 exhibit such magnetic dynamics.
The results are interpreted as evidence of the complex magnetic order in
Ru-1222. We propose a specific multilayer model to explain the data, and note
that superconductivity in the ruthenocuprate is compatible with both the
presence and absence of the magnetic dynamics.",0110181v1
2003-12-18,Current induced magnetization dynamics in current perpendicular to the plane spin valves,"We observe magnetization dynamics induced by spin momentum transfer in the
noise spectra of current perpendicular to the plane giant magnetoresistance
spin valves. The dynamics are observable only for those combinations of current
direction and magnetic configuration in which spin transfer acts to reorient
the free layer magnetization away from the direction set by the net magnetic
field. Detailed measurements as a function of magnetic configuration reveal an
evolution of the noise spectra, going from a spectrum with a well-defined noise
peak when the free layer is roughly collinear with the pinned layer to a
spectrum dominated by 1/f noise when the free layer is in an orthogonal
configuration. Finally, the amplitude of the corresponding resistance noise
increases rapidly with increasing current until it saturates at a value that is
a substantial fraction of the magnetoresistance between parallel and
antiparallel states.",0312504v2
2004-08-18,Spin-Fermion Model of Magnetism: Quasi-particle Many-Body Dynamics,"Theoretical foundations and applications of the generalized spin-fermion
(sp-d) exchange lattice model to various magnetic systems, e.g. rare-earth
metals and compounds and magnetic semiconductors are discussed. The
capabilities of the model to describe spin quasi-particle spectra are
investigated. The main emphasis is made on the dynamical behavior of two
interacting subsystems, the localized spins and spin density of itinerant
carriers. A nonperturbative many-body approach, the Irreducible Green Functions
(IGF) method, is used to describe the quasi-particle dynamics.
  Scattering states are investigated and three branches of magnetic excitations
are calculated in the regime characteristic of a magnetic semiconductor. For a
simplified version of the model (Kondo lattice model) we study the spectra of
quasi-particle excitations with special attention given to diluted magnetic
semiconductors in simple approximation, to demonstrate the possibilities of the
IGF approach. For this, to include the effects of disorder, a modified mean
fields are determined self-consistently. The approach permits to investigate
and clarify the role of various interactions and disorder effects in unified
and coherent fashion.",0408406v1
2006-11-01,Self-assembly and dynamics of magnetic holes,"Nonmagnetic particles in magnetized ferrofluids have been denoted magnetic
holes and are in many ways ideal model systems to test various forms of
particle self assembly and dynamics. Some case studies to be reviewed here
include:
  Chaining of magnetic holes
  Braid theory and Zipf relation used in dynamics of magnetic microparticles
  Interactions of magnetic holes in ferrofluid layers
  The objectives of these works have been to find simple characterizations of
complex behavior of particles with dipolar interactions.",0611008v1
2007-02-01,High-energy magnetic excitations from dynamic stripes in LBCO (x=1/8),"We use inelastic neutron scattering to study the temperature dependence of
magnetic excitations (for energies up to 100 meV) in the cuprate
La$_{1.875}$Ba$_{0.125}$CuO$_4$. This compound exhibits stripe order below a
temperature of ~50 K; previous measurements have shown that the magnetic
excitations of the stripe-ordered phase have an hour-glass-like dispersion,
with a saddle point at ~50 meV. Here we compare measurements in the disordered
phase taken at 65 and 300 K. At energies on the scale of $k_{\rm B}T$, there is
substantial momentum-broadening of the signal, and the low-energy
incommensurate features can no longer be resolved at 300 K. In contrast, there
is remarkably little change in the scattered signal for energies greater than
$k_{\rm B}T$. In fact, the momentum-integrated dynamic susceptibility is almost
constant with temperature. We suggest that the continuity of higher-energy
magnetic excitations is strong evidence for dynamic stripes in the
high-temperature, disordered phase. We reconsider the nature of the magnetic
dispersion, and we discuss the correspondences between the thermal evolution of
magnetic stripe correlations with other electronic properties of the cuprates.",0702027v1
2007-08-10,Dynamic Origin of Vortex Core Switching in Soft Magnetic Nanodots,"The magnetic vortex with the in-plane curling magnetization and the
out-of-plane magnetization at the core is a unique ground state in nanoscale
magnetic elements. This kind of magnetic vortex can be used as a memory unit
for information storage, through its downward or upward core-orientation and,
thus, controllable core switching deserves some special attention. Our
analytical and micromagnetic calculations reveal that the origin of the vortex
core reversal is a gyrotropic field. This field is induced by vortex dynamic
motion and is proportional to the velocity of the moving vortex. Our
calculations elucidate the physical origin of the vortex core dynamic reversal
and offer a key to effective manipulation of the vortex-core orientation.",0708.1359v1
2007-10-26,Electron Transport Driven by Nonequilibrium Magnetic Textures,"Spin-polarized electron transport driven by inhomogeneous magnetic dynamics
is discussed in the limit of a large exchange coupling. Electron spins rigidly
following the time-dependent magnetic profile experience spin-dependent
fictitious electric and magnetic fields. We show that the electric field
acquires important corrections due to spin dephasing, when one relaxes the
spin-projection approximation. Furthermore, spin-flip scattering between the
spin bands needs to be taken into account in order to calculate voltages and
spin accumulations induced by the magnetic dynamics. A phenomenological
approach based on the Onsager reciprocity principle is developed, which allows
us to capture the effect of spin dephasing and make a connection to the well
studied problem of current-driven magnetic dynamics. A number of results that
recently appeared in the literature are related and generalized.",0710.5193v2
2008-05-21,Non-equilibrium thermodynamic study of magnetization dynamics in the presence of spin-transfer torque,"The dynamics of magnetization in the presence of spin-transfer torque was
studied. We derived the equation for the motion of magnetization in the
presence of a spin current by using the local equilibrium assumption in
non-equilibrium thermodynamics. We show that, in the resultant equation, the
ratio of the Gilbert damping constant, $\alpha$, and the coefficient, $\beta$,
of the current-induced torque, called non-adiabatic torque, depends on the
relaxation time of the fluctuating field $\tau_{c}$. The equality
$\alpha=\beta$ holds when $\tau_c$ is very short compared to the time scale of
magnetization dynamics. We apply our theory to current-induced magnetization
reversal in magnetic multilayers and show that the switching time is a
decreasing function of $\tau_{c}$.",0805.3306v1
2008-08-19,Magnetization and spin dynamics of a Cr-based magnetic cluster: Cr$_{7}$Ni,"We study the magnetization and the spin dynamics of the Cr$_7$Ni ring-shaped
magnetic cluster. Measurements of the magnetization at high pulsed fields and
low temperature are compared to calculations and show that the spin Hamiltonian
approach provides a good description of Cr$_7$Ni magnetic molecule. In
addition, the phonon-induced relaxation dynamics of molecular observables has
been investigated. By assuming the spin-phonon coupling to take place through
the modulation of the local crystal fields, it is possible to evaluate the
decay of fluctuations of two generic molecular observables. The nuclear
spin-lattice relaxation rate $1/T_1$ directly probes such fluctuations, and
allows to determine the magnetoelastic coupling strength.",0808.2621v1
2010-02-05,Direct observation of Oersted-field-induced magnetization dynamics in magnetic nanostripes,"We have used time-resolved x-ray photoemission electron microscopy to
investigate the magnetization dynamics induced by nanosecond current pulses in
NiFe/Cu/Co nanostripes. A large tilt of the NiFe magnetization in the direction
transverse to the stripe is observed during the pulses. We show that this
effect cannot be quantitatively understood from the amplitude of the Oersted
field and the shape anisotropy. High frequency oscillations observed at the
onset of the pulses are attributed to precessional motion of the NiFe
magnetization about the effective field. We discuss the possible origins of the
large magnetization tilt and the potential implications of the static and
dynamic effects of the Oersted field on current-induced domain wall motion in
such stripes.",1002.1302v3
2011-11-22,Magnetic Dynamics and Spin Freezing in the Ferromagnetic Phase of Ba-Doped Perovskite Cobaltites,"The magnetic properties of polycrystaline La0.5Ba0.5CoO2.5+d (d = 0.44 and
0.485) where investigated using muon spin relaxation and bulk magnetization
experiments. Below a Curie temperature of T_C = 160 K and 180 K, respectively,
a magnetic phase with a macroscopic ferromagnetic moment forms. muSR proves
that the full volume inhomogeneously orders at T_C but that there is
microscopically a phase separation into phase volumes possessing a fully static
and a slow magnetic dynamic behavior. A peak in the dynamic relaxation rate 35
K respectively 55 K below the Curie temperature indicates the freezing of spin
components. The spin fluctuations are thermally activated with a typical
Jahn-Teller like phonon energy of E_a = 340 K. Together with co-operative
Jahn-Teller distortions appearing below the spin freezing temperature, this
points to a strong magneto elastic coupling as the cause for the magnetic
fluctuations and the spin freezing.",1111.5232v1
2011-12-01,Relativistic magnetohydrodynamics in one dimension,"We derive a number of solution for one-dimensional dynamics of relativistic
magnetized plasma that can be used as benchmark estimates in relativistic
hydrodynamic and magnetohydrodynamic numerical codes.
  First, we analyze the properties of simple waves of fast modes propagating
orthogonally to the magnetic field in relativistically hot plasma. The magnetic
and kinetic pressures obey different equations of state, so that the system
behaves as a mixture of gases with different polytropic indices. We find the
self-similar solutions for the expansion of hot strongly magnetized plasma into
vacuum.
  Second, we derive linear hodograph and Darboux equations for the relativistic
Khalatnikov potential, which describe arbitrary one-dimensional isentropic
relativistic motion of cold magnetized plasma and find their general and
particular solutions. The obtained hodograph and Darboux equations are very
powerful: system of highly non-linear, relativistic, time dependent equations
describing arbitrary (not necessarily self-similar) dynamics of highly
magnetized plasma reduces to a single linear differential equation.",1112.0249v1
2012-01-31,Nonreciprocal Directional Dichroism and Toroidalmagnons in Helical Magnets,"We investigate a dynamical magnetoelectric effect due to a magnetic resonance
in helical spin structures through the coupling between magnetization and
electric polarization via a spin current mechanism. We show that the magnon has
both the dynamical magnetic moment $\Delta M^\omega$ and the electric moment
$\Delta P^\omega$ ($\perp \Delta M^\omega$), i.e., a dynamical toroidal moment,
under external magnetic fields, and thus it is named the {\em toroidalmagnon}.
The toroidalmagnon exists in most conical spin structures owing to the
generality of the spin current mechanism. In the absorption of electromagnetic
waves, the toroidalmagnon excitation process generally induces a nonreciprocal
directional dichroism as a consequence of an interference of the magnetic and
electric responses.",1201.6458v1
2012-03-19,Bistability between equatorial and axial dipoles during magnetic field reversals,"Numerical simulations of the geodynamo in presence of an heterogeneous
heating are presented. We study the dynamics and the structure of the magnetic
field when the equatorial symmetry of the flow is broken. If the symmetry
breaking is sufficiently strong, the m = 0 axial dipolar field is replaced by
an hemispherical magnetic field, dominated by an oscillating m = 1 magnetic
field. Moreover, for moderate symmetry breaking, a bistability between the
axial and the equatorial dipole is observed. In this bistable regime, the axial
magnetic field exhibits chaotic switches of its polarity, involving the
equatorial dipole during the transition period. This new scenario for magnetic
field reversals is discussed within the framework of the Earth's dynamo.",1203.4144v1
2012-05-31,Time-Resolved Magnetic Relaxation of a Nanomagnet on Subnanosecond Time Scales,"We present a two-current-pulse temporal correlation experiment to study the
intrinsic subnanosecond nonequilibrium magnetic dynamics of a nanomagnet during
and following a pulse excitation. This method is applied to a model
spin-transfer system, a spin valve nanopillar with perpendicular magnetic
anisotropy. Two-pulses separated by a short delay (< 500 ps) are shown to lead
to the same switching probability as a single pulse with a duration that
depends on the delay. This demonstrates a remarkable symmetry between magnetic
excitation and relaxation and provides a direct measurement of the magnetic
relaxation time. The results are consistent with a simple finite temperature
Fokker-Planck macrospin model of the dynamics, suggesting more coherent
magnetization dynamics in this short time nonequilibrium limit than near
equilibrium.",1205.7034v1
2013-03-28,Spin Torque on Magnetic Textures Coupled to the Surface of a Three-Dimensional Topological Insulator,"We investigate theoretically the spin torque and magnetization dynamic in a
thin ferromagnetic (FM) layer with spatially varying magnetization. The FM
layer is deposited on the surface of a topological insulator (TI). In the limit
of the adiabatic relaxation of electron spin along the magnetization, the
interaction between the exchange interaction and the Rashba-like surface
texture of a TI yields a topological gauge field. Under the gauge field and an
applied current, spin torque is induced according to the direction of the
current. We derived the corresponding effective anisotropy field and hence the
modified Landau-Lifshitz-Gilbert equation, which describes the spin torque and
the magnetization dynamic. In addition, we study the effective field for
exemplary magnetic textures, such as domain wall, skyrmion, and vortex
configurations. The estimated strength of the effective field is comparable to
the switching fields of typical FM materials, and hence can significantly
influence the dynamics of the FM layer.",1303.7031v1
2013-10-24,Towards wafer scale inductive determination of magnetostatic and dynamic parameters of magnetic thin films and multilayers,"We investigate an inductive probe head suitable for non-invasive
characterization of the magnetostatic and dynamic parameters of magnetic thin
films and multilayers on the wafer scale. The probe is based on a planar
waveguide with rearward high frequency connectors that can be brought in close
contact to the wafer surface. Inductive characterization of the magnetic
material is carried out by vector network analyzer ferromagnetic resonance.
Analysis of the field dispersion of the resonance allows the determination of
key material parameters such as the saturation magnetization MS or the
effective damping parameter Meff. Three waveguide designs are tested. The
broadband frequency response is characterized and the suitability for inductive
determination of MS and Meff is compared. Integration of such probes in a wafer
prober could in the future allow wafer scale in-line testing of magnetostatic
and dynamic key material parameters of magnetic thin films and multilayers.",1310.6578v1
2013-12-20,SU(2s+1) symmetry and nonlinear dynamic equations of spin s magnets,"The article is devoted to the description of dynamics of magnets with
arbitrary spin on the basis of the Hamiltonian formalism. The relationship
between the magnetic ordering and Poisson bracket subalgebras of the magnetic
degrees of freedom for spin s=1/2; 1; 3/2 has been established. We have been
obtained non-linear dynamic equations without damping for normal and degenerate
non-equilibrium states of high-spin magnets with the properties of the SO(3),
SU(4), SU(2)$\times$SU(2), SU(3), SO(4), SO(5) symmetry of exchange
interaction. The connection between models of the magnetic exchange energy and
the Casimir invariants has been discussed.",1312.5828v2
2013-12-22,Dynamical Skyrmion State in a Spin Current Nano-Oscillator with Perpendicular Magnetic Anisotropy,"We study the spectral characteristics of spin current nano-oscillators based
on the Pt/[Co/Ni] magnetic multilayer with perpendicular magnetic anisotropy.
By varying the applied magnetic field and current, both localized and
propagating spin wave modes of the oscillation are achieved. At small fields,
we observe an abrupt onset of the modulation sidebands. We use micromagnetic
simulations to identify this state as a dynamical magnetic skyrmion stabilized
in the active device region by spin current injection, whose current-induced
dynamics is accompanied by the gyrotropic motion of the core due to the skew
deflection. Our results demonstrate a practical route for controllable skyrmion
manipulation by spin current in magnetic thin films.",1312.6343v2
2014-01-21,Dynamics of clumps embedded in a hot accretion flow with toroidal magnetic field,"Dynamics of clumps within a magnetized advection dominated accretion flow is
investigated by solving the collisionless Boltzmann equation and considering
the drag force due to the relative velocity between the clumps and the gas.
Toroidal component of the magnetic field is assumed to be dominant. Dynamical
properties of the hot gaseous component such as the radial and the rotational
velocities are affected by the magnetic effects, and so, drag force varies
depending on the strength of magnetic field and the velocity dispersion of the
clumps is then modified significantly. We show that when magnetic pressure is
less than the gas pressure, the root of the averaged radial velocity square of
the clumps decreases at the inner parts of the hot flow and increases slightly
at its outer edge.",1401.5190v1
2014-05-09,Current-induced magnetization dynamics in two magnetic insulators separated by a normal metal,"We study the dynamics of spin valves consisting of two layers of magnetic
insulators separated by a normal metal in the macrospin model. A current
through the spacer generates a spin Hall current that can actuate the
magnetization via the spin-transfer torque. We derive expressions for the
effective Gilbert damping and the critical currents for the onset of
magnetization dynamics including the effects of spin pumping that can be tested
by ferromagnetic resonance experiments. The current generates an amplitude
asymmetry between the in-phase and out-of-phase modes. We briefly discuss
superlattices of metals and magnetic insulators.",1405.2267v1
2014-06-18,Quantum magnetism with ultracold molecules,"This article gives an introduction to the realization of effective quantum
magnetism with ultracold molecules in an optical lattice, reviews experimental
and theoretical progress, and highlights future opportunities opened up by
ongoing experiments. Ultracold molecules offer capabilities that are otherwise
difficult or impossible to achieve in other effective spin systems, such as
long-ranged spin-spin interactions with controllable degrees of spatial and
spin anisotropy and favorable energy scales. Realizing quantum magnetism with
ultracold molecules provides access to rich many-body behaviors, including many
exotic phases of matter and interesting excitations and dynamics.
Far-from-equilibrium dynamics plays a key role in our exposition, just as it
did in recent ultracold molecule experiments realizing effective quantum
magnetism. In particular, we show that dynamical probes allow the observation
of correlated many-body spin physics, even in polar molecule gases that are not
quantum degenerate. After describing how quantum magnetism arises in ultracold
molecules and discussing recent observations of quantum magnetism with polar
molecules, we survey prospects for the future, ranging from immediate goals to
long-term visions.",1406.4758v1
2014-07-28,Eddy current effects in the magnetization dynamics of ferromagnetic metal nanoparticles,"We develop an analytical model for describing the magnetization dynamics in
ferromagnetic metal nanoparticles, which is based on the coupled system of the
Landau-Lifshitz-Gilbert (LLG) and Maxwell equations. By solving Maxwell's
equations in the quasi-static approximation and finding the magnetic field of
eddy currents, we derive the closed LLG equation for the magnetization that
fully accounts for the effects of conductivity. We analyze the difference
between the LLG equations in metallic and dielectric nanoparticles and show
that these effects can strongly influence the magnetization dynamics. As an
example illustrating the importance of eddy currents, the phenomenon of
precessional switching of magnetization is considered.",1407.7466v1
2014-12-08,Magnetization Dynamics driven by Non-equilibrium Spin-Orbit Coupled Electron Gas,"The dynamics of magnetization coupled to an electron gas via s-d exchange
interaction is investigated by using density matrix technique. Our theory shows
that non-equilibrium spin accumulation induces a spin torque and the electron
bath leads to a damping of the magnetization. For the two-dimensional
magnetization thin film coupled to the electron gas with Rashba spin-orbit
coupling, the result for the spin-orbit torques is consistent with the previous
semi-classical theory. Our theory predicts a damping of the magnetization,
which is absent in the semi-classical theory. The magnitude of the damping due
to the electron bath is comparable to the intrinsic Gilbert damping and may be
important in describing the magnetization dynamics of the system.",1412.2479v1
2016-04-02,Exploring the Origin and Dynamics of Solar Magnetic Fields,"The Sun is a magnetically active star and is the source of the solar wind,
electromagnetic radiation and energetic particles which affect the heliosphere
and the Earths atmosphere. The magnetic field of the Sun is responsible for
most of the dynamic activity of the Sun. This thesis research seeks to
understand solar magnetic field generation and the role that magnetic fields
play in the dynamics of the solar atmosphere. Specifically, this thesis focuses
on two themes: in the first part, we study the origin and behaviour of solar
magnetic fields using magnetohydrodynamic dynamo theory and modelling, and in
the second part, utilizing observations and data analysis we study two major
problems in solar physics, namely, the coronal heating problem and initiation
mechanisms of solar flares.",1604.00563v1
2016-05-26,Handling magnetic anisotropy and dynamic magnetic response of ferromagnetic multilayers in flexible substrates,"We investigate the dynamic magnetic response of ferromagnetic flexible
NiFe/Ta and FeCuNbSiB/Ta multilayers under external stress. We explore the
possibility of handling magnetic anisotropy and dynamic magnetic response of
flexible magnetostrictive nanostructures. We quantify the sensitivity of the
multilayers under external stress by calculating the ratio $|\Delta
\textrm{MI}|/|\Delta \bar{\sigma}|$ and show that considerable values can be
reached by tuning the magnetic field, frequency, magnetostriction constant, and
external stress. The results reveal possibilities of application of flexible
magnetostrictive multilayers under external stress and place them as very
attractive candidates as element sensor for the development of sensitive smart
touch sensors.",1605.08193v2
2016-05-27,Edge states and skyrmion dynamics in nanostripes of frustrated magnets,"Magnetic skyrmions are particle-like topological excitations recently
discovered in chiral magnets. Their small size, topological protection and the
ease with which they can be manipulated by electric currents generated much
interest in using skyrmions for information storage and processing. Recently,
it was suggested that skyrmions with additional degrees of freedom can exist in
magnetically frustrated materials. Here, we show that dynamics of skyrmions and
antiskyrmions in nanostripes of frustrated magnets is strongly affected by
complex spin states formed at the stripe edges. These states create multiple
edge channels which guide the skyrmion motion. Non-trivial topology of edge
states gives rise to complex current-induced dynamics, such as emission of
skyrmion-antiskyrmion pairs. The edge state topology can be controlled with an
electric current through the exchange of skyrmions and antiskyrmions between
the edges of a magnetic nanostructure. These results can lead to conceptually
new electronic devices.",1605.08645v1
2016-06-06,Tracking local magnetic dynamics via high-energy charge excitations in a relativistic Mott insulator,"We use time- and energy-resolved optical spectroscopy to investigate the
coupling of electron-hole excitations to the magnetic environment in the
relativistic Mott insulator Na$_2$IrO$_3$. We show that, on the picosecond
timescale, the photoinjected electron-hole pairs delocalize on the hexagons of
the Ir lattice via the formation of quasi-molecular orbital (QMO) excitations
and the exchange of energy with the short-range-ordered zig-zag magnetic
background. The possibility of mapping the magnetic dynamics, which is
characterized by typical frequencies in the THz range, onto high-energy (1-2
eV) charge excitations provides a new platform to investigate, and possibly
control, the dynamics of magnetic interactions in correlated materials with
strong spin-orbit coupling, even in the presence of complex magnetic phases.",1606.01667v2
2016-08-12,Ultrafast inhomogeneous magnetization dynamics analyzed by interface-sensitive nonlinear magneto-optics,"We analyze laser-induced ultrafast, spatially inhomogeneous magnetization
dynamics of epitaxial Co/Cu(001) films in a 0.4-10 nm thickness range with
time-resolved magnetization-induced second harmonic generation, which probes
femtosecond spin dynamics at the vacuum/Co and Co/Cu interfaces. The
interference of these two contributions makes the overall signal particularly
sensitive to differences in the transient magnetization redistribution between
the two interfaces, i.e. ultrafast magnetization profiles in the ferromagnetic
film. We find in films of up to 3 nm thickness a stronger demagnetization at
the surface, because the film thickness is smaller than the effective mean free
path of the spin current mediating the demagnetization, i.e. the difference
between the mean free paths of the majority and minority carriers. For film
thicknesses larger than 3 nm, the magnetization profile reverses, since
majority spins can escape into the conducting substrate only from the
interface-near region.",1608.03842v1
2017-08-02,Strongly exchange-coupled and surface-state-modulated magnetization dynamics in Bi2Se3/YIG heterostructures,"We report strong interfacial exchange coupling in Bi2Se3/yttrium iron garnet
(YIG) bilayers manifested as large in-plane interfacial magnetic anisotropy
(IMA) and enhancement of damping probed by ferromagnetic resonance (FMR). The
IMA and spin mixing conductance reached a maximum when Bi2Se3 was around 6
quintuple-layer (QL) thick. The unconventional Bi2Se3 thickness dependence of
the IMA and spin mixing conductance are correlated with the evolution of
surface band structure of Bi2Se3, indicating that topological surface states
play an important role in the magnetization dynamics of YIG.
Temperature-dependent FMR of Bi2Se3/YIG revealed signatures of magnetic
proximity effect of $T_c$ as high as 180 K, and an effective field parallel to
the YIG magnetization direction at low temperature. Our study sheds light on
the effects of topological insulators on magnetization dynamics, essential for
development of TI-based spintronic devices.",1708.00593v1
2018-07-05,Domain wall dynamics for an in-plane magnetized thin film with large perpendicular hard axis anisotropy including Dzyaloshinskii-Moriya interaction,"We consider a thin ferromagnetic layer to which an external field or a
current are applied along an in plane easy axis. The perpendicular hard axis
anisotropy constant is large so that the out of plane magnetization component
is smaller than the in plane components. A perturbation approach is used to
obtain the profile and velocity of the moving domain wall. The dynamics of the
in plane components of the magnetization is governed by a reaction diffusion
equation which determines the speed of the profile. We find a simple analytic
expression for the out of plane magnetization showing a symmetric distortion
due to the motion in addition to the asymmetric component due to the
Dzyaloshinskii--Moriya interaction. The results obtained complement previous
studies in which either the Dzyalozhinskii vector or the out of plane hard axis
anisotropy were assumed small. In the regime studied the Walker breakdown is
not observed but the reaction diffusion dynamics predicts a slowing down of the
domain wall for sufficiently large magnetic field. The transition point depends
on the applied field, saturation magnetization and easy axis anisotropy.",1807.02032v1
2019-07-04,Resonant Acoustic Wave Assisted Spin-Transfer-Torque Switching of Nanomagnets,"We report the possibility of achieving an order of magnitude reduction in the
energy dissipation needed to write bits in perpendicular magnetic tunnel
junctions (p-MTJs) by simulating the magnetization dynamics under a combination
of resonant surface acoustic waves (r-SAW) and spin-transfer-torque (STT). The
magnetization dynamics were simulated using the Landau-Lifshitz-Gilbert
equation under macrospin assumption with the inclusion of thermal noise. The
resonant magnetization dynamics in the magnetostrictive nanomagnet build over
few 10s of cycles of SAW application that drives the magnetization to precess
in a cone with a deflection of ~45 degrees from the perpendicular direction.
This reduces the STT current density required to switch the magnetization
direction without increasing the STT application time or degrading the
switching probability in the presence of room temperature thermal noise. This
could lead to a pathway to achieve energy efficient switching of spin transfer
torque random access memory (STTRAM) whose lateral dimensions can be scaled
aggressively despite using materials with low magnetostriction by employing
resonant excitation.",1907.02255v1
2020-01-28,Magnetic oscillation modes in square lattice artificial spin ice,"Small amplitude dipolar oscillations are considered in artificial spin ice on
a square lattice in two dimensions. The net magnetic moment of each elongated
magnetic island in the spin ice is assumed to have Heisenberg-like dynamics.
Each island's magnetic moment is assumed to be influenced by shape anisotropies
and by the dipolar interactions with its nearest neighbors. The magnetic
dynamics is linearized around one of the ground states, leading to an $8\times
8$ matrix to be diagonalized for the magnetic spin wave modes. Analytic
solutions are found and classified as antisymmetric and symmetric with regard
to their in-plane dynamic fluctuations. Although only the leading dipolar
interactions are included, modes similar to these may be observable
experimentally.",2001.10409v1
2020-07-23,Nonreciprocal surface magnetoelastic dynamics,"Motivated by recent experiments, we investigate the nonreciprocal
magnetoelastic interaction between the surface acoustic phonons of dielectric
non-magnetic substrates and magnons of proximity nanomagnets. The magnetization
dynamics exerts rotating forces at the edges of the nanomagnet that causes the
nonreciprocal interaction with surface phonons due to its rotation-momentum
locking. This coupling induces the nonreciprocity of the surface phonon
transmission and a nearly complete phonon diode effect by several (tens of)
magnetic nanowires of high (ordinary) magnetic quality. Phase-sensitive
microwave transmission is also nonreciprocal that can pick up clear signals of
the coherent phonons excited by magnetization dynamics. Nonreciprocal pumping
of phonons by precessing magnetization is predicted using Landauer-B\""uttiker
formalism.",2007.12240v3
2020-10-21,A Frustrated Bimeronium: Static Structure and Dynamics,"We show a topological spin texture called ""bimeronium"" in magnets with
in-plane magnetization. It is a topological counterpart of skyrmionium in
perpendicularly magnetized magnets and can be seen as a combination of two
bimerons with opposite topological charges. We report the static structure and
spin-orbit-torque-induced dynamics of an isolated bimeronium in a magnetic
monolayer with frustrated exchange interactions. We study the anisotropy and
magnetic field dependences of a static bimeronium. We also explore the
bimeronium dynamics driven by the damping-like spin-orbit torque. We find that
the bimeronium shows steady rotation when the spin polarization direction is
parallel to the easy axis. Moreover, we demonstrate the annihilation of the
bimeronium when the spin polarization direction is perpendicular to the easy
axis. Our results are useful for understanding fundamental properties of
bimeronium structures and may offer an approach to build bimeronium-based
spintronic devices.",2010.10822v2
2020-11-13,Magnetic dynamics with Weyl fermions,"Transport of charge and valley degrees of freedom coupled to order-parameter
dynamics in magnetic Weyl semimetals is studied in the framework of
nonequilibrium thermodynamics. In addition to the established valley-related
transport anomalies that are rooted in band-structure topology, we construct
dissipative couplings between the three dynamic constituents of the system
driven out of equilibrium by electromagnetic perturbations. We show how the
valley degree of freedom mediates an effective coupling between the charge and
magnetic sectors of the system, through a combination of the chiral anomaly, on
the electric side, and the Onsager-paired valley torque and pumping, on the
magnetic side. This work compliments previous studies of magnetic Weyl
semimetals by a more systematic analysis of collective dissipation. We discuss
several concrete examples of the valley-mediated current-driven magnetic
instabilities and charge pumping, and extend the theory to the
antiferromagnetic case.",2011.07141v1
2021-01-12,"Shielding of external magnetic field by dynamic nuclear polarization in (In,Ga)As quantum dots","The dynamics of the coupled electron-nuclear spin system is studied in an
ensemble of singly-charged (In,Ga)As/GaAs quantum dots (QDs) using periodic
optical excitation at 1 GHz repetition rate. In combination with the
electron-nuclei interaction, the highly repetitive excitation allows us to lock
the electron spins into magnetic resonance in a transverse external magnetic
field. Sweeping the field to higher values, the locking leads to an effective
""diamagnetic"" response of significant strength due to dynamic nuclear
polarization, which shields the QD electrons at least partly from the external
field and can even keep the internal magnetic field constant up to 1.3 T field
variation. We model the effect through a magnetic field-dependent polarization
rate of the nuclei, from which we suggest a strategy for adjusting the nuclear
polarization through the detuning between optical excitation and electronic
transition, in addition to tuning the magnetic field.",2101.04469v2
2021-01-26,Tuning the dynamics of magnetic droplet solitons using dipolar interactions,"Magnetic droplets are dissipative magnetodynamical solitons that can form
under current driven nanocontacts in magnetic layers with large perpendicular
magnetic anisotropy. Here, we extend the original droplet theory by studying
the impact of the dipolar interactions on the dynamics of droplet solitons. By
varying the thickness of the free layer of a spin torque nano-oscillator, we
systematically tune the internal field of the free layer to investigate the
dynamics of droplet solitons. Our numerical results show that increasing the
free layer thickness increases the droplet threshold current, decreases the
droplet frequency and diameter, enlarges the current hysteresis and also
modifies the structure of the droplet. The Oersted field of the current breaks
the phase coherency and deteriorates the stability of the droplet in free
layers with larger thicknesses. Moreover, our findings show a simple relation
to determine the impact of the free layer thickness on the droplet nucleation
boundaries. Our study presents the missing brick on the physics behind magnetic
droplet solitons, and further illustrates that magnetic droplets in thinner
layers possess more promising characteristics for spintronic applications and
enable devices with higher speed of operation.",2101.10716v1
2021-04-12,Strongly anisotropic spin dynamics in magnetic topological insulators,"The recent discovery of magnetic topological insulators has opened new
avenues to explore exotic states of matter that can emerge from the interplay
between topological electronic states and magnetic degrees of freedom, be it
ordered or strongly fluctuating. Motivated by the effects that the dynamics of
the magnetic moments can have on the topological surface states, we investigate
the magnetic fluctuations across the
(MnBi$_{\text{2}}$Te$_{\text{4}}$)(Bi$_{\text{2}}$Te$_{\text{3}}$)$_{\text{n}}$
family. Our paramagnetic electron spin resonance experiments reveal contrasting
Mn spin dynamics in different compounds, which manifests in a strongly
anisotropic Mn spin relaxation in MnBi$_{\text{2}}$Te$_{\text{4}}$ while being
almost isotropic in MnBi$_{\text{4}}$Te$_{\text{7}}$. Our density-functional
calculations explain these striking observations in terms of the sensitivity of
the local electronic structure to the Mn spin-orientation, and indicate that
the anisotropy of the magnetic fluctuations can be controlled by the carrier
density, which may directly affect the electronic topological surface states.",2104.05659v1
2021-06-21,Spin structure factors of doped monolayer Germanene in the presence of spin-orbit coupling,"In this paper, we present a Kane-Mele model in the presence of magnetic field
and next nearest neighbors hopping amplitudes for investigations of the spin
susceptibilities of Germanene layer. Green's function approach has been
implemented to find the behavior of dynamical spin susceptibilities of
Germanene layer within linear response theoryand in the presence of magnetic
field and spin-orbit coupling at finite temperature. Our results show the
magnetic excitation mode for both longitudinal and transverse components of
spin tends to higher frequencies with spin-orbit coupling strength. Moreover
the frequency positions of sharp peaks in longitudinal dynamical spin
susceptibility are not affected by variation of magnetic field while the peaks
in transverse dynamical susceptibility moves to lower frequencies with magnetic
field. The effects of electron doping on frequency behaviors of spin
susceptibilities have been addressed in details. Finally the temperature
dependence of static spin structure factors due to the effects of spin-orbit
coupling, magnetic field and chemical potential has been studied.",2106.11377v1
2021-09-27,Absence of Normal Fluctuations in an Integrable Magnet,"We investigate dynamical fluctuations of transferred magnetization in the
one-dimensional lattice Landau--Lifshitz magnet with uniaxial anisotropy,
representing an emblematic model of interacting spins. We demonstrate that the
structure of fluctuations in thermal equilibrium depends radically on the
characteristic dynamical scale. In the ballistic regime, typical fluctuations
are found to follow a normal distribution and scaled cumulants are finite. In
stark contrast, on the diffusive and superdiffusive timescales, relevant
respectively for the easy-axis and isotropic magnet at vanishing total
magnetization, typical fluctuations are no longer Gaussian and, remarkably,
scaled cumulants are divergent. The observed anomalous features disappear upon
breaking integrability, suggesting that the absence of normal fluctuations is
intimately tied to the presence of soliton modes. In a nonequilibrium setting
of the isotropic magnet with weakly polarized step-profile initial state we
find a slow drift of dynamical exponent from the superdiffusive towards the
diffusive value.",2109.13088v3
2021-12-22,Dynamic structure factor of the magnetized one-component plasma: crossover from weak to strong coupling,"Plasmas in strong magnetic fields have been mainly studied in two distinct
limiting cases--that of weak and strong nonideality with very different
physical properties. While the former is well described by the familiar theory
of Braginskii, the latter regime is closer to the behavior of a Coulomb liquid.
Here we study in detail the transition between both regimes. We focus on the
evolution of the dynamic structure factor of the magnetized one-component
plasma from weak to strong coupling, which is studied with first-principle
molecular dynamics simulations. The simulations show the vanishing of Bernstein
modes and the emergence of higher harmonics of the upper hybrid mode across the
magnetic field, a redistribution of spectral power between the two main
collective modes under oblique angles, and a suppression of plasmon damping
along the magnetic field. Comparison with results from various models,
including the random phase approximation, a Mermin-type dielectric function,
and the Quasi-Localized Charge Approximation show that none of the theories is
capable of reproducing the crossover that occurs when the coupling parameter is
on the order of unity. The findings are relevant to the scattering spectra,
stopping power, and transport coefficients of correlated magnetized plasmas.",2112.11981v1
2022-08-19,Phononic-crystal cavity magnomechanics,"Establishing a way to control magnetic dynamics and elementary excitations
(magnons) is crucial to fundamental physics and the search for novel phenomena
and functions in magnetic solid-state systems. Electromagnetic waves have been
developed as means of driving and sensing in magnonic and spintronics devices
used in magnetic spectroscopy, non-volatile memory, and information processors.
However, their millimeter-scale wavelengths and undesired cross-talk have
limited operation efficiency and made individual control of densely integrated
magnetic systems difficult. Here, we utilize acoustic waves (phonons) to
control magnetic dynamics in a miniaturized phononic crystal micro-cavity and
waveguide architecture. We demonstrate acoustic pumping of localized
ferromagnetic magnons, where their back-action allows dynamic and
mode-dependent modulation of phononic cavity resonances. The phononic crystal
platform enables spatial driving, control and read-out of tiny magnetic states
and provides a means of tuning acoustic vibrations with magnons. This
alternative technology enhances the usefulness of magnons and phonons for
advanced sensing, communications and computation architectures that perform
transduction, processing, and storage of classical and quantum information.",2208.09105v1
2022-09-06,Current-driven magnetization dynamics and their correlation with magnetization configurations in perpendicularly magnetized tunnel junctions,"We study spin-transfer-torque driven magnetization dynamics of a
perpendicular magnetic tunnel junction (MTJ) nanopillar. Based on the
combination of spin-torque ferromagnetic resonance and microwave spectroscopy
techniques, we demonstrate that the free layer (FL) and the weak pinned
reference layer (RL) exhibit distinct dynamic behaviors with opposite frequency
vs. field dispersion relations. The FL can support a single coherent spin-wave
(SW) mode for both parallel and antiparallel configurations, while the RL
exhibits spin-wave excitation only for the antiparallel state. These two SW
modes corresponding to the FL and RL coexist at an antiparallel state and
exhibit a crossover phenomenon of oscillation frequency with increasing the
external magnetic field, which could be helpful in the mutual synchronization
of auto-oscillations for SW-based neuromorphic computing.",2209.02271v1
2022-12-07,Emergent Magneto-multipoles and Nonlinear Responses of a Magnetic Hopfion,"The three-dimensional emergent magnetic field $\textbf{B}^e$ of a magnetic
hopfion gives rise to emergent magneto-multipoles in a similar manner to the
multipoles of classical electromagnetic field. Here, we show that the nonlinear
responses of a hopfion are characterized by its emergent magnetic toroidal
moment ${T}^e_z = \frac{1}{2}\int (\textbf{r}\times \textbf{B}^e)_z dV$ and
emergent magnetic octupole component ${\it \Gamma}^e =\int [(x^2+y^2)B^e_z - xz
B^e_x - y z B^e_y] dV$. The hopfion exhibits nonreciprocal dynamics (nonlinear
hopfion Hall effect) under an ac driving current applied along (perpendicular
to) the direction of ${T}^e_z$. The sign of nonreciprocity and nonlinear Hall
angle is determined by the polarity and chirality of hopfion. The nonlinear
electrical transport induced by a magnetic hopfion is also discussed. This work
reveals the vital roles of emergent magneto-multipoles in nonlinear hopfion
dynamics and could stimulate further investigations on the dynamical responses
of topological spin textures induced by emergent electromagnetic multipoles.",2212.03500v1
2023-01-04,Dynamics of Argon Gas Bubbles Rising in Liquid Steel in the Presence of Transverse Magnetic Field,"Bubbly flows are present in various industrial processes including
metallurgical processes in which gas bubbles are injected at the bottom of bulk
liquid metal to stir the liquid metal and homogenize the metal. Understanding
the motion of such bubbles is essential, as it has been shown that bubble
flotation can remove inclusions. In this work, we have numerically studied
three-dimensional dynamics of a pair of inline Argon bubbles rising in molten
steel under the influence of a transverse magnetic field. We have explored the
effects of two transverse magnetic field strengths (Bx = 0 and 0.2 T). The
bubbles' motion and transient rise velocities are compared under different
magnetic fields. The shape deformations and path of the bubbles are discussed.
The flow structures behind the bubbles are analyzed. We found that structures
are more organized and elongated under a magnetic field, whereas it is complex
and intertwined when the magnetic field is not included. We have used a
geometry construction-based volume of fluid (VOF) method to track interface,
maintain mass balance and estimate the interface curvature. Additionally, we
have incorporated a Sharp Surface Force Method (SSF) for surface tension
forces. The algorithm is able to minimize the spurious velocities.",2301.01797v1
2023-01-09,Dynamic structure factor and excitation spectrum of the one-component plasma: the case of weak to moderate magnetization,"Magnetized plasmas are well known to exhibit a rich spectrum of collective
modes. Here, we focus on the density modes in dense or cold plasmas, where
strong coupling effects alter the mode spectrum known from traditional weakly
coupled plasmas. In particular, we study the dynamic structure factor (DSF) of
the magnetized one-component plasma with molecular dynamics simulations.
Extending our previous results [H.~K\""ahlert and M.~Bonitz, Phys. Rev. Research
\textbf{2022}, 4, 013197], it is shown that Bernstein modes can be observed in
the weakly magnetized regime, where they are found below the upper hybrid
frequency, provided the coupling strength is sufficiently low. We investigate
the DSF for a variety of different wave numbers and plasma parameters and show
that even small magnetization can give rise to a strong zero-frequency mode
perpendicular to the magnetic field and change the dispersion as well as the
damping of the upper hybrid mode.",2301.03425v1
2023-02-10,Photocarrier Transport of Ferroelectric Photovoltaic Thin Films Detected by the Magnetic Dynamics of Adjacent Ferromagnetic Layers,"We have observed photocarrier transport behaviors in
BiFeO$_3$/La$_{1-x}$Sr$_x$MnO$_3$~(BFO/LSMO) heterostructures by using
time-resolved synchrotron x-ray magnetic circular dichroism in reflectivity.
The magnetization of LSMO layers was used as a probe of photo-induced carrier
dynamics in the photovoltaic BFO layers. During the photo-induced
demagnetization process, the decay time of LSMO~($x$=0.2) magnetization
strongly depends on the ferroelectric polarization direction of the BFO layer.
The variation of decay time should be attributed to the different sign of
accumulated photocarriers at the BFO/LSMO interface induced by the photovoltaic
effect of the BFO layer. The photocarriers can reach the BFO/LSMO interface and
influence the magnetization distribution in the LSMO layers within the
timescale of $\sim$100~ps. Our results provide a novel strategy to investigate
carrier dynamics and mechanisms of optical control of magnetization in thin
film heterostructures.",2302.05000v1
2023-03-28,Analysis of ultrafast magnetization switching dynamics in exchange-coupled ferromagnet-ferrimagnet heterostructures,"Magnetization switching in ferromagnets has so far been limited to the
current-induced spin-orbit-torque effects. Recent observation of
helicity-independent all-optical magnetization switching in exchange-coupled
ferromagnet ferrimagnet heterostructures expanded the range and applicability
of such ultrafast heat-driven magnetization switching. Here we report the
element-resolved switching dynamics of such an exchange-coupled system, using a
modified microscopic three-temperature model. We have studied the effect of i)
the Curie temperature of the ferromagnet, ii) ferrimagnet composition, iii) the
long-range RKKY exchange-coupling strength, and iv) the absorbed optical energy
on the element-specific time-resolved magnetization dynamics. The phase-space
of magnetization illustrates how the RKKY coupling strength and the absorbed
optical energy influence the switching time. Our analysis demonstrates that the
threshold switching energy depends on the composition of the ferrimagnet and
the switching time depends on the Curie temperature of the ferromagnet as well
as RKKY coupling strength. This simulation anticipates new insights into
developing faster and more energy-efficient spintronics devices.",2303.16294v1
2023-08-28,Real-time dynamics of axial charge and chiral magnetic current in a non-Abelian expanding plasma,"Understanding axial charge dynamics driven by changes in Chern-Simons number
densities is a key aspect in understanding the chiral magnetic effect (CME) in
heavy-ion collisions. Most phenomenological simulations assume that a large
amount of axial charge is produced in the initial stages and that axial charge
is conserved throughout the simulation. Within an (expanding) homogeneous
holographic plasma, we investigate the real-time axial charge relaxation
dynamics and their impact on the chiral magnetic current. Moreover, we discuss
the real-time interplay of the non-Abelian and the Abelian chiral anomaly in
the presence of a strong magnetic field. In the expanding plasma, the
Chern-Simons diffusion rate and thus the axial charge relaxation rate are time
dependent due to the decaying magnetic field. We quantify the changes in the
late time falloffs and establish a horizon formula for the chiral magnetic
current.",2308.14829v2
2023-10-12,Dynamics of particles with electric charge and magnetic dipole moment near Schwarzschild-MOG black hole,"Investigations of electromagnetic interactions between test-charged and
magnetized particles are important in the dynamics of the particles in strong
gravitational fields around black holes. Here, we study the dynamics of a
particle having an electric charge and a magnetic dipole moment in the
spacetime of a Schwarzschild black hole in modified gravity (MOG), called
Schwarzschild-MOG black hole. First, we provide a solution of Maxwell equations
for the angular component of electromagnetic four potentials in the
Schwarzschild-MOG spacetime. Then, we derive equations of motion and effective
potential for circular motion of such particles using a hybrid form of the
Hamilton-Jacobi equation which includes both interactions of electric charge
and magnetic dipole moment with the external magnetic field assumed as
asymptotically uniform, and interaction between the particles and the MOG
field. Also, we investigate the effects of the three types of interactions on
the radius of innermost stable circular orbits (ISCOs) and the energy \&
angular momentum of the particles at their corresponding ISCOs. Finally, we
provide detailed analyses of the effects of the three interactions mentioned
above on the center of mass energy in the collisions between neutral,
electrically charged, and magnetized particles.",2310.08046v1
2024-01-26,Efficient Control of Magnetization Dynamics Via W/CuO$_\text{x}$ Interface,"Magnetization dynamics, which determine the speed of magnetization switching
and spin information propagation, play a central role in modern spintronics.
Gaining its control will satisfy the different needs of various spintronic
devices. In this work, we demonstrate that the surface oxidized Cu
(CuO$_\text{x}$) can be employed for the tunability of magnetization dynamics
of ferromagnet (FM)/heavy metal (HM) bilayer system. The capping CuO$_\text{x}$
layer in CoFeB/W/CuO$_\text{x}$ trilayer reduces the magnetic damping value in
comparison with the CoFeB/W bilayer. The magnetic damping even becomes lower
than that of the CoFeB/CuO$_\text{x}$ by ~ 16% inferring the stabilization of
anti-damping phenomena. Further, the reduction in damping is accompanied by a
very small reduction in the spin pumping-induced output DC voltage in the
CoFeB/W/CuO$_\text{x}$ trilayer. The simultaneous observation of anti-damping
and spin-to-charge conversion can be attributed to the orbital Rashba effect
observed at the HM/CuO$_\text{x}$ interface. Our experimental findings
illustrate that the cost-effective CuO$_\text{x}$ can be employed as an
integral part of modern spintronics devices owing to its rich underneath
spin-orbital physics.",2401.14708v1
2000-12-10,Helicity in Hydro and MHD Reconnection,"Helicity, a measure of the linkage of flux lines, has subtle and largely
unknown effects upon dynamics. Both magnetic and hydrodynamic helicity are
conserved for ideal systems and could suppress nonlinear dynamics. What
actually happens is not clear because in a fully three-dimensional system there
are additional channels whereby intense, small-scale dynamics can occur. This
contribution shows one magnetic and one hydrodynamic case where for each the
presence of helicity does not suppress small-scale intense dynamics of the type
that might lead to reconnection.",0012210v1
1999-03-03,Dynamics of Ordering of Isotropic Magnets,"We study the dynamics of ordering of the nonconserved and conserved
Heisenberg magnet. The dynamics consists of two parts - an irreversible
dissipation into a heat bath and a reversible precession induced by a torque
due to the local molecular field. For quenches both to T=0 and T=T_c, we show
that the torque is irrelevant when the dynamics is nonconserved but relevant
when the dynamics is conserved and is governed by a new nontrivial fixed point.",9903052v1
2015-04-18,Dynamic phase transitions in a ferromagnetic thin film system: A Monte Carlo simulation study,"Dynamic phase transition properties of ferromagnetic thin film system under
the influence both bias and time dependent magnetic fields have been elucidated
by means of kinetic Monte Carlo simulation with local spin update Metropolis
algorithm. The obtained results after a detailed analysis suggest that bias
field is the conjugate field to dynamic order parameter, and it also appears to
define a phase line between two antiparallel dynamic ordered states depending
on the considered system parameters. Moreover, the data presented in this study
well qualitatively reproduce the recently published experimental findings where
time dependent magnetic behavior of a uniaxial cobalt films is studied in the
neighborhood of dynamic phase transition point.",1504.04749v1
2018-02-08,Anomalous phonon lifetime shortening in paramagnetic CrN caused by magneto-lattice coupling: A combined spin and ab initio molecular dynamics study,"We study the mutual coupling of spin fluctuations and lattice vibrations in
paramagnetic CrN by combining atomistic spin dynamics and ab initio molecular
dynamics. The two degrees of freedom are dynamically coupled leading to
non-adiabatic effects. Those effects suppress the phonon life times at low
temperature compared to an adiabatic approach. The here identified dynamic
coupling provides an explanation for the experimentally observed unexpected
temperature dependence of the thermal conductivity of magnetic semiconductors
above the magnetic ordering temperature.",1802.02934v1
2020-06-04,Analytical model of the inertial dynamics of a magnetic vortex,"We present an analytical model to account for the inertial dynamics of a
magnetic vortex. The model is based on a deformation of the core profile based
on the D\""oring kinetic field, whereby the deformation amplitudes are promoted
to dynamical variables in a collective-coordinate approach that provides a
natural extension to the Thiele model. This extended model accurately describes
complex transients due to inertial effects and the variation of the effective
mass with velocity. The model also provides a quantitative description of the
inertial dynamics leading up to vortex core reversal, which is analogous to the
Walker transition in domain wall dynamics.",2006.02866v1
2001-06-18,Turbulent Dynamo in Asymptotic Giant Branch Stars,"Using recent results on the operation of turbulent dynamos, we show that a
turbulent dynamo can amplify a large scale magnetic field in the envelopes of
asymptotic giant branch (AGB) stars. We propose that a slow rotation of the AGB
envelope can fix the symmetry axis, leading to the formation of an axisymmetric
magnetic field structure. Unlike solar-type alpha-omega dynamos, the rotation
has only a small role in amplifying the toroidal component of the magnetic
field. The large-scale magnetic field is strong enough for the formation of
magnetic cool spots on the AGB stellar surface. The spots can regulate dust
formation, hence mass loss rate, leading to axisymmetric mass loss and the
formation of elliptical planetary nebulae (PNe). Despite its role in forming
cool spots, the large scale magnetic field is too weak to play a dynamic role
and directly influence the wind from the AGB star. We find other problems in
models where the magnetic field plays a dynamic role in shaping the AGB winds,
and argue that they cannot explain the formation of nonspherical PNe.",0106301v1
2002-01-30,Magnetic Aggregation I: Aggregation Dynamics and Numerical Modelling,"Focussing on preplanetary grains growth, we discuss the properties of dust
aggregation driven by magnetic dipole forces. While there is no direct evidence
for the existence of magnetic grains present in the solar nebula, there are
reasons to assume they may have been present. We derive analytical expressions
for the cross-section of two interacting dipoles. The effective cross section
depends upon the strength of the magnetic dipoles and the initial velocities.
For typical conditions the magnetic cross section is between 2 and 3 orders of
magnitude larger than the geometric cross section. We study the growth dynamics
of magnetic grains and find that the mass of the aggregates should increase
with time like t^3.2 whereas Brownian motion growth behaves like t^2. A
numerical tool is introduced which can be used to model dust aggregation in
great detail, including the treatment of contact forces, aggregate
restructuring processes and long-range forces. This tool is used to simulate
collisions between magnetic grains or clusters and to validate the analytical
cross-sections. The numerically derived cross section is in excellent agreement
with the analytical expression. The numerical tool is also used to demonstrate
that structural changes in the aggregates during collisions can be significant.",0201500v1
2002-09-15,The Mean Electromotive Force for MHD Turbulence: the Case of a Weak Mean Magnetic Field and Slow Rotation,"The mean electromotive force in the framework of mean--field
magnetohydrodynamics is found with a particular attention to the effect of a
mean rotation of the fluid on the mean electromotive force. Anisotropy of the
turbulence due to gradients of its intensity or its helicity are admitted. The
mean magnetic field is considered to be weak enough to exclude quenching
effects. Magnetic fluctuations with a zero mean magnetic field are taken into
account. The Omega x J -effect works in the same way with velocity fluctuations
and magnetic fluctuations. A contribution to the electromotive force connected
with the symmetric parts of the gradient tensor of the mean magnetic field was
found in the case of rotation, resulting from velocity or magnetic
fluctuations. The implications of the results for the mean electromotive force
for mean--field dynamo models are discussed with special emphasis to dynamos
working without alpha-effect.",0209287v1
2006-02-06,Magnetic field vector retrieval with HMI,"The Helioseismic and Magnetic Imager (HMI), on board the Solar Dynamics
Observatory (SDO), will begin data acquisition in 2008. It will provide the
first full disk, high temporal cadence observations of the full Stokes vector
with a 0.5 arc sec pixel size. This will allow for a continuous monitoring of
the Solar magnetic field vector. HMI data will advance our understanding of the
small and large-scale magnetic field evolution, its relation to the solar and
global dynamic processes, coronal field extrapolations, flux emergence,
magnetic helicity and the nature of the polar magnetic fields. We summarize
HMI's expected operation modes, focusing on the polarization cross-talk induced
by the solar oscillations and how this affects the magnetic field vector
determinations.",0602130v1
2004-03-10,Role of Correlation and Exchange for Quasi-particle Spectra of Magnetic and Diluted Magnetic Semiconductors,"Theoretical foundation and application of the generalized spin-fermion (sp-d)
exchange lattice model to magnetic and diluted magnetic semiconductors are
discussed. The capabilities of the model to describe spin quasi-particle
spectra are investigated. The main emphasis is made on the dynamic behavior of
two interacting subsystems, the localized spins and spin density of itinerant
carriers. A nonperturbative many-body approach, the Irreducible Green Functions
(IGF) method, is used to describe the quasi-particle dynamics. Scattering
states are investigated and three branches of magnetic excitations are
calculated in the regime, characteristic of a magnetic semiconductor. For a
simplified version of the model (Kondo lattice model) we study the spectra of
quasi-particle excitations with special attention given to diluted magnetic
semiconductors. For this, to include the effects of disorder, modified mean
fields are determined self-consistently. The role of the Coulomb correlation
and exchange is clarified by comparing of both the cases.",0403266v1
2004-08-18,Theory of Magnetic Polaron,"The concept of magnetic polaron is analysed and developed to elucidate the
nature of itinerant charge carrier states in magnetic semiconductors and
similar complex magnetic materials. By contrasting the scattering and bound
states of carriers within the $s-d$ exchange model, the nature of bound states
at finite temperatures is clarified. The free magnetic polaron at certain
conditions is realized as a bound state of the carrier (electron or hole) with
the spin wave. Quite generally, a self-consistent theory of a magnetic polaron
is formulated within a nonperturbative many-body approach, the Irreducible
Green Functions (IGF) method which is used to describe the quasiparticle
many-body dynamics at finite temperatures. Within the above many-body approach
we elaborate a self-consistent picture of dynamic behavior of two interacting
subsystems, the localized spins and the itinerant charge carriers. In
particular, we show that the relevant generalized mean fields emerges naturally
within our formalism. At the same time, the correct separation of elastic
scattering corrections permits one to consider the damping effects (inelastic
scattering corrections) in the unified and coherent fashion. The damping of
magnetic polaron state, which is quite different from the damping of the
scattering states, finds a natural interpretation within the present
self-consistent scheme.",0408404v2
2007-07-03,Strings and D-branes in a supersymmetric magnetic flux background,"We investigate how the presence of RR magnetic $F_{p+2}$ fluxes affects the
energy of classical Dp branes, for specific string theory supersymmetric
backgrounds which are solutions to the leading order in $\alpha'$ including
back-reaction effects. The Dp brane dynamics is found to be similar to the well
known dynamics of particles and strings moving in magnetic fields. We find a
class of BPS solutions which generalize the BPS fundamental strings or BPS
branes with momentum and winding to the case of non-zero magnetic fields.
Remarkably, the interaction with the magnetic fields does not spoil the
supersymmetry of the solution, which turns out to be invariant under four
supersymmetry transformations. We find that magnetic fields can significantly
reduce the energy of some BPS strings and Dp branes, in particular, some
macroscopic Dp branes become light for sufficiently large magnetic fields.",0707.0455v2
2008-02-12,Temperature dependent magnetization dynamics of magnetic nanoparticles,"Recent experimental and theoretical studies show that the switching behavior
of magnetic nanoparticles can be well controlled by external time-dependent
magnetic fields. In this work, we inspect theoretically the influence of the
temperature and the magnetic anisotropy on the spin-dynamics and the switching
properties of single domain magnetic nanoparticles (Stoner-particles). Our
theoretical tools are the Landau-Lifshitz-Gilbert equation extended as to deal
with finite temperatures within a Langevine framework. Physical quantities of
interest are the minimum field amplitudes required for switching and the
corresponding reversal times of the nanoparticle's magnetic moment. In
particular, we contrast the cases of static and time-dependent external fields
and analyze the influence of damping for a uniaxial and a cubic anisotropy.",0802.1740v1
2008-04-29,Magnetic fields in star formation: from galaxies to stars,"Magnetic fields are important at every scale in the star formation process:
from the dynamics of the ISM in galaxies, to the collapse of turbulent
molecular clouds to form stars and in the fragmentation of individual star
forming cores. The recent development of a robust algorithm for MHD in the
Smoothed Particle Hydrodynamics method has enabled us to perform simulations of
star formation including magnetic fields at each of these scales. This paper
focusses on three questions in particular: What is the effect of magnetic
fields on fragmentation in star forming cores? How do magnetic fields affect
the collapse of turbulent molecular clouds to form stars? and: What effect do
magnetic fields have on the dynamics of the interstellar medium?",0804.4647v1
2008-06-28,Theory of spin magnetohydrodynamics,"We develop a phenomenological hydrodynamic theory of coherent magnetic
precession coupled to electric currents. Exchange interaction between electron
spin and collective magnetic texture produces two reciprocal effects:
spin-transfer torque on the magnetic order parameter and the Berry-phase gauge
field experienced by the itinerant electrons. The dissipative processes are
governed by three coefficients: the ohmic resistance, Gilbert damping of the
magnetization, and the ""beta coefficient"" describing viscous coupling between
magnetic dynamics and electric current, which stems from spin mistracking of
the magnetic order. We develop general magnetohydrodynamic equations and
discuss the net dissipation produced by the coupled dynamics. The latter in
particular allows us to determine a lower bound on the magnetic-texture
resistivity.",0806.4656v2
2008-10-08,Self-Similar Solution of Hot Accretion Flows with Ordered Magnetic Field and Outflow,"Observations and numerical magnetohydrodynamic (MHD) simulations indicate the
existence of outflows and ordered large-scale magnetic fields in the inner
region of hot accretion flows. In this paper we present the self-similar
solutions for advection-dominated accretion flows (ADAFs) with outflows and
ordered magnetic fields. Stimulated by numerical simulations, we assume that
the magnetic field has a strong toroidal component and a vertical component in
addition to a stochastic component. We obtain the self-similar solutions to the
equations describing the magnetized ADAFs, taking into account the dynamical
effects of the outflow. We compare the results with the canonical ADAFs and
find that the dynamical properties of ADAFs such as radial velocity, angular
velocity and temperature can be significantly changed in the presence of
ordered magnetic fields and outflows. The stronger the magnetic field is, the
lower the temperature of the accretion flow will be, and the faster the flow
rotates. The relevance to observations is briefly discussed.",0810.1341v1
2009-06-04,Thermoelectric spin transfer in textured magnets,"We study charge and energy transport in a quasi-1D magnetic wire in the
presence of magnetic textures. The energy flows can be expressed in a fashion
similar to charge currents, leading to new energy-current induced spin torques.
In analogy to charge currents, we can identify two reciprocal effects:
spin-transfer torque on the magnetic order parameter induced by energy current
and the Berry-phase gauge field induced energy flow. In addition, we
phenomenologically introduce new $\beta-$like viscous coupling between magnetic
dynamics and energy current into the LLG equation, which originates from spin
mistracking of the magnetic order. We conclude that the new viscous term should
be important for the thermally induced domain wall motion. We study the
interplay between charge and energy currents and find that many of the effects
of texture motion on the charge currents can be replicated with respect to
energy currents. For example, the moving texture can lead to energy flows which
is an analogue of the electromotive force in case of charge currents. We
suggest a realization of cooling effect by magnetic texture dynamics.",0906.1002v2
2009-07-03,Compaction dynamics of a magnetized powder,"We have investigated experimentally the influence of a magnetic interaction
between the grains on the compaction dynamics of a granular pile submitted to a
series of taps. The granular material used to perform this study is a mixture
of metallic and glass grains. The packing is immersed in an homogeneous
external magnetic field. The magnetic field induces an interaction between the
metallic grains that constitutes the tunable cohesion. The compaction
characteristic time and the asymptotic packing fraction have been measured as a
function of the Bond number which is the ratio between the cohesive magnetic
force and the grain weight. These measurements have been performed for
different fractions of metallic beads in the pile. When the pile is only made
of metallic grains, the characteristic compaction time increases as the square
root of the Bond number. While the asymptotic packing fraction decreases as the
inverse of the Bond number. For mixtures, when the fraction of magnetized
grains in the pile is increased, the characteristic time increases while the
asymptotic packing fraction decreases. A simple mesoscopic model based on the
formation of granular chains along the magnetic field direction is proposed to
explain the observed macroscopic properties of the packings.",0907.0646v1
2009-07-28,Global galactic dynamo driven by cosmic-rays and exploding magnetized stars,"We report first results of first global galactic-scale CR-MHD simulations of
cosmic-ray-driven dynamo. We investigate the dynamics of magnetized
interstellar medium (ISM), which is dynamically coupled with the cosmic-ray
(CR) gas. We assume that exploding stars deposit small-scale, randomly
oriented, dipolar magnetic fields into the differentially rotating ISM,
together with a portion of cosmic rays, accelerated in supernova shocks. We
conduct numerical simulations with the aid of a new parallel MHD code PIERNIK.
We find that the initial magnetization of galactic disks by exploding
magnetized stars forms a favourable conditions for the cosmic-ray-driven
dynamo. We demonstrate that dipolar magnetic fields supplied on small
SN-remnant scales, can be amplified exponentially, by the CR-driven dynamo, to
the present equipartition values, and transformed simultaneously to large
galactic-scales. The resulting magnetic field structure in an evolved galaxy
appears spiral in the face-on view and reveals the so called X-shaped structure
in the edge-on view.",0907.4891v2
2009-10-08,Morphology and Dynamics of the Low Solar Chromosphere,"The Interferometric Bidimensional Spectrometer (IBIS) installed at the Dunn
Solar Telescope of the NSO/SP is used to investigate the morphology and
dynamics of the lower chromosphere and the virtually non-magnetic fluctosphere
below. The study addresses in particular the structure of magnetic elements
that extend into these layers. We choose different quiet Sun regions in and
outside coronal holes. In inter-network regions with no significant magnetic
flux contributions above the detection limit of IBIS, we find intensity
structures with the characteristics of a shock wave pattern. The magnetic flux
elements in the network are long lived and seem to resemble the spatially
extended counterparts to the underlying photospheric magnetic elements. We
suggest a modification to common methods to derive the line-of-sight magnetic
field strength and explain some of the difficulties in deriving the magnetic
field vector from observations of the fluctosphere.",0910.1381v1
2009-12-17,Morphology and dynamics of photospheric and chromospheric magnetic fields,"We use joint observations obtained with the Hinode space observatory and the
Interferometric Bidimensional Spectrometer (IBIS) installed at the DST of the
NSO/SP to investigate the morphology and dynamics of (a) non-magnetic and (b)
magnetic regions in the fluctosphere. In inter-network regions with no
significant magnetic flux contributions above the detection limit of IBIS, we
find intensity structures with similar characteristics as those seen in
numerical simulations by Wedemeyer-B\""ohm (2008). The magnetic flux elements in
the network are stable and seem to resemble the spatially extended counterparts
to the underlying photospheric magnetic elements. We will explain some of the
difficulties in deriving the magnetic field vector from observations of the
fluctosphere.",0912.3285v1
2010-03-30,Dynamics of braided coronal loops II: Cascade to multiple small-scale reconnection events,"Aims: Our aim is to investigate the resistive relaxation of a magnetic loop
that contains braided magnetic flux but no net current or helicity. The loop is
subject to line-tied boundary conditions. We investigate the dynamical
processes that occur during this relaxation, in particular the magnetic
reconnection that occurs, and discuss the nature of the final equilibrium.
Methods: The three-dimensional evolution of a braided magnetic field is
followed in a series of resistive MHD simulations. Results: It is found that,
following an instability within the loop, a myriad of thin current layers
forms, via a cascade-like process. This cascade becomes more developed and
continues for a longer period of time for higher magnetic Reynolds number.
During the cascade, magnetic flux is reconnected multiple times, with the level
of this `multiple reconnection' positively correlated with the magnetic
Reynolds number. Eventually the system evolves into a state with no more
small-scale current layers. This final state is found to approximate a
non-linear force-free field consisting of two flux tubes of oppositely-signed
twist embedded in a uniform background field.",1003.5784v2
2010-04-13,Incommensurate magnetic order and dynamics induced by spinless impurities in YBa$_2$Cu$_3$O$_{6.6}$,"We report an inelastic-neutron-scattering and muon-spin-relaxation study of
the effect of 2% spinless (Zn) impurities on the magnetic order and dynamics of
YBa$_2$Cu$_3$O$_{6.6}$, an underdoped high-temperature superconductor that
exhibits a prominent spin-pseudogap in its normal state. Zn substitution
induces static magnetic order at low temperatures and triggers a large-scale
spectral-weight redistribution from the magnetic resonant mode at 38 meV into
uniaxial, incommensurate spin excitations with energies well below the
spin-pseudogap. These observations indicate a competition between
incommensurate magnetic order and superconductivity close to a quantum critical
point. Comparison to prior data on La$_{2-x}$Sr$_x$CuO$_{4}$ suggests that this
behavior is universal for the layered copper oxides and analogous to
impurity-induced magnetic order in one-dimensional quantum magnets.",1004.2139v1
2010-10-22,Thermally activated magnetization reversal in monoatomic magnetic chains on surfaces studied by classical atomistic spin-dynamics simulations,"We analyze the spontaneous magnetization reversal of supported monoatomic
chains of finite length due to thermal fluctuations via atomistic spin-dynamics
simulations. Our approach is based on the integration of the Landau-Lifshitz
equation of motion of a classical spin Hamiltonian at the presence of
stochastic forces. The associated magnetization lifetime is found to obey an
Arrhenius law with an activation barrier equal to the domain wall energy in the
chain. For chains longer than one domain-wall width, the reversal is initiated
by nucleation of a reversed magnetization domain primarily at the chain edge
followed by a subsequent propagation of the domain wall to the other edge in a
random-walk fashion. This results in a linear dependence of the lifetime on the
chain length, if the magnetization correlation length is not exceeded. We
studied chains of uniaxial and tri-axial anisotropy and found that a tri-axial
anisotropy leads to a reduction of the magnetization lifetime due to a higher
reversal attempt rate, even though the activation barrier is not changed.",1010.4730v2
2010-11-04,Probing the magnetic ground state of the molecular Dysprosium triangle,"We present zero field muon spin lattice relaxation measurements of a
Dysprosium triangle molecular magnet. The local magnetic fields sensed by the
implanted muons indicate the coexistence of static and dynamic internal
magnetic fields below $T^* ~35$ K. Bulk magnetization and heat capacity
measurements show no indication of magnetic ordering below this temperature. We
attribute the static fields to the slow relaxation of the magnetization in the
ground state of Dy3. The fluctuation time of the dynamic part of the field is
estimated to be ~0.55 $\mu$s at low temperatures",1011.1164v1
2011-07-27,The effects of plasma beta and anisotropy instabilities on the dynamics of reconnecting magnetic fields in the heliosheath,"The plasma {\beta} (the ratio of the plasma pressure to the magnetic
pressure) of a system can have a large effect on its dynamics as high {\beta}
enhances the effects of pressure anisotropies. We investigate the effects of
{\beta} in a system of stacked current sheets that break up into magnetic
islands due to magnetic reconnection. We find significant differences between
{\beta} < 1 and {\beta} > 1. At low {\beta} growing magnetic islands are
modestly elongated and become round as contraction releases magnetic stress and
reduces magnetic energy. At high {\beta} the increase of the parallel pressure
in contracting islands causes saturation of modestly elongated islands as
island cores approach the marginal firehose condition. Only highly elongated
islands reach finite size. The kinking associated with the Weibel and firehose
instabilities prevents full contraction of these islands, leading to a final
state of highly elongated islands in which further reconnection is suppressed.
The results are directly relevant to reconnection in the sectored region of the
heliosheath and possibly to saturation mechanisms of the magnetorotational
instability in accretion flows.",1107.5558v2
2011-11-14,Efficiency of internal shocks in magnetized relativistic jets,"We study the dynamic and radiative efficiency of conversion of
kinetic-to-thermal/magnetic energy by internal shocks in relativistic
magnetized outflows. A parameter study of a large number of collisions of
cylindrical shells is performed. We explore how, while keeping the total flow
luminosity constant, the variable fluid magnetization influences the efficiency
and find that the interaction of shells in a mildly magnetized jet yields
higher dynamic, but lower radiative efficiency than in a non-magnetized flow. A
multi-wavelength radiative signature of different shell magnetization is
computed assuming that relativistic particles are accelerated at internal
shocks.",1111.3170v2
2011-12-11,Spin-polarized current effect on antiferromagnet magnetization in a ferromagnet - antiferromagnet nanojunction: Theory and simulation,"Spin-polarized current effect is studied on the static and dynamic
magnetization of the antiferromagnet in a ferromagnet - antiferromagnet
nanojunction. The macrospin approximation is generalized to antiferromagnets.
Canted antiferromagnetic configuration and resulting magnetic moment are
induced by an external magnetic field. The resonance frequency and damping are
calculated, as well as the threshold current density corresponding to
instability appearance. A possibility is shown of generating low-damping
magnetization oscillations in terahertz range. The fluctuation effect is
discussed on the canted antiferromagnetic configuration. Numerical simulation
is carried out of the magnetization dynamics of the antiferromagnetic layer in
the nanojunction with spin-polarized current. Outside the instability range,
the simulation results coincide completely with analytical calculations using
linear approximation. In the instability range, undamped oscillations occur of
the longitudinal and transverse magnetization components.",1112.2362v1
2012-02-23,Radiation effects in a muon collider ring and dipole magnet protection,"The requirements and operating conditions for a Muon Collider Storage Ring
(MCSR) pose significant challenges to superconducting magnets. The dipole
magnets should provide a high magnetic field to reduce the ring circumference
and thus maximize the number of muon collisions during their lifetime. One
third of the beam energy is continuously deposited along the lattice by the
decay electrons at the rate of 0.5 kW/m for a 1.5-TeV c.o.m. and a luminosity
of 1034 cm-2s-1. Unlike dipoles in proton machines, the MCSR dipoles should
allow this dynamic heat load to escape the magnet helium volume in the
horizontal plane, predominantly towards the ring center. This paper presents
the analysis and comparison of radiation effects in MCSR based on two dipole
magnets designs. Tungsten masks in the interconnect regions are used in both
cases to mitigate the unprecedented dynamic heat deposition and radiation in
the magnet coils.",1202.5333v1
2012-03-14,Turbulent Magnetic Field Amplification from Spiral SASI Modes: Implications for Core-Collapse Supernovae and Proto-Neutron Star Magnetization,"We extend our investigation of magnetic field evolution in three-dimensional
flows driven by the stationary accretion shock instability (SASI) with a suite
of higher-resolution idealized models of the post-bounce core-collapse
supernova environment. Our magnetohydrodynamic simulations vary in initial
magnetic field strength, rotation rate, and grid resolution. Vigorous
SASI-driven turbulence inside the shock amplifies magnetic fields
exponentially; but while the amplified fields reduce the kinetic energy of
small-scale flows, they do not seem to affect the global shock dynamics. The
growth rate and final magnitude of the magnetic energy are very sensitive to
grid resolution, and both are underestimated by the simulations. Nevertheless
our simulations suggest that neutron star magnetic fields exceeding $10^{14}$ G
can result from dynamics driven by the SASI, \emph{even for non-rotating
progenitors}.",1203.3108v1
2012-09-19,From Weakly to Strongly Magnetized Isotropic MHD Turbulence,"High Reynolds number isotropic magneto-hydro-dynamic turbulence in the
presence of large scale magnetic fields is investigated as a function of the
magnetic field strength. For a variety of flow configurations the energy
dissipation rate \epsilon, follows the Kolmogorov scaling \epsilon ~ U^3/L even
when the large scale magnetic field energy is twenty times larger than the
kinetic. Further increase of the magnetic energy showed a transition to the
\epsilon ~ U^2 B / L scaling implying that magnetic shear becomes more
efficient at this point at cascading the energy than the velocity fluctuations.
Strongly helical configurations form helicity condensates that deviate from
these scalings. Weak turbulence scaling was absent from the investigation.
Finally, the magnetic energy spectra showed support for the Kolmogorov spectrum
k^{-5/3} while kinetic energy spectra are closer to the Iroshnikov-Kraichnan
spectrum k^{-3/2}.",1209.4225v1
2013-06-27,Current Sheets Formation in Tangled Coronal Magnetic Fields,"We investigate the dynamical evolution of magnetic fields in closed regions
of solar and stellar coronae. To understand under which conditions current
sheets form, we examine dissipative and ideal reduced magnetohydrodynamic
models in cartesian geometry, where two magnetic field components are present:
the strong guide field $B_0$, extended along the axial direction, and the
dynamical orthogonal field $\mathbf{b}$. Magnetic field lines thread the system
along the axial direction, that spans the length $L$, and are line-tied at the
top and bottom plates. The magnetic field $b$ initially has only large scales,
with its gradient (current) length-scale of order $\ell_b$. We identify the
magnetic intensity threshold $b/B_0 \sim \ell_b/L$. For values of $b$ below
this threshold, field-line tension inhibits the formation of current sheets,
while above the threshold they form quickly on fast ideal timescales. In the
ideal case, above the magnetic threshold, we show that current sheets thickness
decreases in time until it becomes smaller than the grid resolution, with the
analyticity strip width $\delta$ decreasing at least exponentially, after which
the simulations become under-resolved.",1306.6634v1
2013-08-08,Domain wall tilting in the presence of the Dzyaloshinskii-Moriya interaction in out-of-plane magnetized magnetic nanotracks,"We show that the Dzyaloshinskii-Moriya interaction (DMI) can lead to a
tilting of the domain wall (DW) surface in perpendicularly magnetized magnetic
nanotracks when DW dynamics is driven by an easy axis magnetic field or a spin
polarized current. The DW tilting affects the DW dynamics for large DMI and the
tilting relaxation time can be very large as it scales with the square of the
track width. The results are well explained by an analytical model based on a
Lagrangian approach where the DMI and the DW tilting are included. We propose a
simple way to estimate the DMI in a magnetic multilayers by measuring the
dependence of the DW tilt angle on a transverse static magnetic field. Our
results shed light on the current induced DW tilting observed recently in Co/Ni
multilayers with inversion asymmetry, and further support the presence of DMI
in these systems.",1308.1824v1
2013-10-18,Ultrafast thermally induced magnetic switching in synthetic ferrimagnets,"Synthetic ferrimagnets are composite magnetic structures formed from two or
more anti- ferromagnetically coupled magnetic sublattices with different
magnetic moments. Here we report on atomistic spin simulations of the
laser-induced magnetization dynamics on such synthetic ferrimag- nets, and
demonstrate that the application of ultrashort laser pulses leads to
sub-picoscond magnetization dynamics and all-optical switching in a similar
manner as in ferrimagnetic alloys. Moreover, we present the essential material
properties for successful laser-induced switching, demonstrating the
feasibility of using a synthetic ferrimagnet as a high density magnetic storage
element without the need of a write field.",1310.5170v2
2014-03-24,Magnetic structure and dynamics of a strongly one-dimensional cobalt$^{II}$ metal-organic framework,"We investigate the magnetism of the
Co$^{II}_4$(OH)$_2$(C$_1$$_0$H$_1$$_6$O$_4$)$_3$ metal-organic framework which
displays complex inorganic chains separated from each other by distances of 1
to 2 nm, and which orders at ~5.4 K. The zero-field magnetic structure is
determined using neutron powder diffraction: it is mainly antiferromagnetic but
posseses a ferromagnetic component along the $\textbf{c}$-axis. This magnetic
structure persists in presence of a magnetic field. Ac susceptibility
measurements confirm the existence of a single thermally activated regime over
7 decades in frequency ($E/k_B\approx64 K$) whereas time-dependent relaxation
of the magnetization after saturation in an external field leads to a two times
smaller energy barrier. These experiments probe the slow dynamics of domain
walls within the chains: we propose that the ac measurements are sensitive to
the motion of existing domain walls within the chains, while the magnetization
measurements are governed by the creation of domain walls.",1403.5931v1
2014-12-28,Tunable Transient Decay Times in Nonlinear Systems: Application to Magnetic Precession,"The dynamical motion of the magnetization plays a key role in the properties
of magnetic materials. If the magnetization is initially away from the
equilibrium direction in a magnetic nanoparticle, it will precess at a natural
frequency and, with some damping present, will decay to the equilibrium
position in a short lifetime. Here we investigate a simple but important
situation where a magnetic nanoparticle is driven non-resonantly by an
oscillating magnetic field, not at the natural frequency. We find a surprising
result that the lifetime of the transient motion is strongly tunable, by
factors of over 10,000, by varying the amplitude of the driving field.",1412.8224v3
2015-09-17,Antiferromagnetic spintronics,"Antiferromagnetic materials are magnetic inside, however, the direction of
their ordered microscopic moments alternates between individual atomic sites.
The resulting zero net magnetic moment makes magnetism in antiferromagnets
invisible on the outside. It also implies that if information was stored in
antiferromagnetic moments it would be insensitive to disturbing external
magnetic fields, and the antiferromagnetic element would not affect
magnetically its neighbors no matter how densely the elements were arranged in
a device. The intrinsic high frequencies of antiferromagnetic dynamics
represent another property that makes antiferromagnets distinct from
ferromagnets. The outstanding question is how to efficiently manipulate and
detect the magnetic state of an antiferromagnet. In this article we give an
overview of recent works addressing this question. We also review studies
looking at merits of antiferromagnetic spintronics from a more general
perspective of spin-ransport, magnetization dynamics, and materials research,
and give a brief outlook of future research and applications of
antiferromagnetic spintronics.",1509.05296v1
2015-11-02,A practical approach to extract symplectic transfer maps numerically for arbitrary magnetic elements,"We introduce a practical approach to extract the symplectic transfer maps for
arbitrary magnetic beam-line elements. Beam motion in particle accelerators
depends on linear and nonlinear magnetic fields of the beam-line elements.
These elements are usually modeled as magnetic multipoles with constant field
strengths in the longitudinal direction (i.e., hard-edge model) in accelerator
design and modeling codes. For magnets with complicated structures such as
insertion devices or fields with significant longitudinal variation effects,
the simplified models may not be sufficient to char- acterize beam dynamics
behaviors accurately. A numerical approach has been developed to extract
symplectic transfer maps from particle trajectory tracking simulation that uses
magnetic field data provided by three-dimensional magnetic field modeling codes
or experimental measurements. The extracted transfer maps can be used in linear
optics design and nonlinear dynamics optimization to achieve more realistic
results.",1511.00710v1
2015-12-01,Epitaxial patterning of nanometer-thick Y3Fe5O12 films with low magnetic damping,"Magnetic insulators such as yttrium iron garnet, Y3Fe5O12, with extremely low
magnetic damping have opened the door for low power spin-orbitronics due to
their low energy dissipation and efficient spin current generation and
transmission. We demonstrate reliable and efficient epitaxial growth and
nanopatterning of Y3Fe5O12 thin-film based nanostructures on insulating
Gd3Ga5O12 substrates. In particular, our fabrication process is compatible with
conventional sputtering and liftoff, and does not require aggressive ion
milling which may be detrimental to the oxide thin films. Structural and
magnetic properties indicate good qualities, in particular low magnetic damping
of both films and patterned structures. The dynamic magnetic properties of the
nanostructures are systematically investigated as a function of the lateral
dimension. By comparing to ferromagnetic nanowire structures, a distinct edge
mode in addition to the main mode is identified by both experiments and
simulations, which also exhbits cross-over with the main mode upon varying the
width of the wires. The non-linear evolution of dynamic modes over
nanostructural dimensions highlights the important role of size confinement to
their material properties in magnetic devices where Y3Fe5O12 nanostructures
serve as the key functional component.",1512.00286v1
2016-04-29,Current induced magnetization dynamics and magnetization switching in superconducting ferromagnetic hybrid (F$|$S$|$F) structures,"We investigate the current induced magnetization dynamics and magnetization
switching in an unconventional p-wave superconductor sandwiched between two
misaligned ferromagnetic layers by numerically solving Landau-Lifshitz-Gilbert
equation modified with current induced Slonczewski's spin torque term. A
modified form of Ginzburg-Landau free energy functional has been used for this
purpose. We demonstrated the possibility of current induced magnetization
switching in the spin-triplet ferromagnetic superconducting hybrid structures
with strong easy axis anisotropy and the condition for magnetization reversal.
The switching time for such arrangement is calculated and is found to be highly
dependent on the magnetic configuration along with the biasing current. This
study would be useful in designing practical superconducting-spintronic
devices.",1604.08704v3
2017-02-12,Electric-field-driven domain wall dynamics in perpendicularly magnetized multilayers,"We report on reversible electric-field-driven magnetic domain wall motion in
a Cu/Ni multilayer on a ferroelectric BaTiO$_3$ substrate. In our
heterostructure, strain-coupling to ferroelastic domains with in-plane and
perpendicular polarization in the BaTiO$_3$ substrate causes the formation of
domains with perpendicular and in-plane magnetic anisotropy, respectively, in
the Cu/Ni multilayer. Walls that separate magnetic domains are elastically
pinned onto ferroelectric domain walls. Using magneto-optical Kerr effect
microscopy, we demonstrate that out-of-plane electric field pulses across the
BaTiO$_3$ substrate move the magnetic and ferroelectric domain walls in unison.
Our experiments indicate an exponential increase of domain wall velocity with
electric field strength and opposite domain wall motion for positive and
negative field pulses. Magnetic fields do not affect the velocity of magnetic
domain walls, but independently tailor their internal spin structure, causing a
change in domain wall dynamics at high velocities.",1702.03553v1
2017-05-02,Self-similar structure of ultra-relativistic magnetized termination shocks and the role of reconnection in long-lasting GRB ouflows,"We consider the double-shock structure of ultra-relativistic flows produced
by the interaction of magnetized wind with magnetized external medium. The
contact discontinuity (CD) is a special point in the flow - density, kinetic
pressure and magnetic field experience a jump or are non-analytic on the CD. To
connect dynamically the outside region (the forward shock flow) with the inside
region (the reverse shock flow) requires resolving flow singularities at the
contact discontinuity. On the CD the pressure is communicated exclusively by
the magnetic field on both sides - the CD becomes an Alfven tangential
discontinuity. Thus, the dynamic amplification of the magnetic field leads to a
formation of a narrow magnetosheath. We discussed a possibility that particles
emitting early $X$-ray afterglows, as well as Fermi GeV photons, are
accelerated via magnetic reconnection processes in the post-reverse shock
region, and in the magnetosheath in particular.",1705.01014v1
2017-06-10,Mapping the magnonic landscape in patterned magnetic structures,"We report the development of a hybrid numerical / analytical model capable of
mapping the spatially-varying distributions of the local ferromagnetic
resonance (FMR) frequency and dynamic magnetic susceptibility in a wide class
of patterned and compositionally modulated magnetic structures. Starting from
the numerically simulated static micromagnetic state, the magnetization is
deliberately deflected orthogonally to its equilibrium orientation, and the
magnetic fields generated in response to this deflection are evaluated using
micromagnetic software. This allows us to calculate the elements of the
effective demagnetizing tensor, which are then used within a linear analytical
formalism to map the local FMR frequency and dynamic magnetic susceptibility.
To illustrate the typical results that one can obtain using this model, we
analyze three micromagnetic systems boasting non-uniformity in either one or
two dimensions, and successfully explain the spin-wave emission observed in
each case, demonstrating the ubiquitous nature of the Schl\""omann excitation
mechanism underpinning the observations. Finally, the developed model of local
FMR frequency could be used to explain how spin waves could be confined and
steered using magnetic non-uniformities of various origins, rendering it a
powerful tool for the mapping of the graded magnonic index in magnonics.",1706.03212v1
2017-06-27,How to form a millisecond magnetar? Magnetic field amplification in protoneutron stars,"Extremely strong magnetic fields of the order of $10^{15}\,{\rm G}$ are
required to explain the properties of magnetars, the most magnetic neutron
stars. Such a strong magnetic field is expected to play an important role for
the dynamics of core-collapse supernovae, and in the presence of rapid rotation
may power superluminous supernovae and hypernovae associated to long gamma-ray
bursts. The origin of these strong magnetic fields remains, however, obscure
and most likely requires an amplification over many orders of magnitude in the
protoneutron star. One of the most promising agents is the magnetorotational
instability (MRI), which can in principle amplify exponentially fast a weak
initial magnetic field to a dynamically relevant strength. We describe our
current understanding of the MRI in protoneutron stars and show recent results
on its dependence on physical conditions specific to protoneutron stars such as
neutrino radiation, strong buoyancy effects and large magnetic Prandtl number.",1706.08733v1
2017-10-03,Orbital Effect of the Magnetic Field in Dynamical Mean-Field Theory,"The availability of large magnetic fields at international facilities and of
simulated magnetic fields that can reach the flux-quantum-per-unit-area level
in cold atoms, calls for systematic studies of orbital effects of the magnetic
field on the self-energy of interacting systems. Here we demonstrate
theoretically that orbital effects of magnetic fields can be treated within
single-site dynamical mean-field theory with a translationally invariant
quantum impurity problem. As an example, we study the one-band Hubbard model on
the square lattice using iterated perturbation theory as an impurity solver. We
recover the expected quantum oscillations in the scattering rate and we show
that the magnetic fields allow the interaction-induced effective mass to be
measured through the single-particle density of states accessible in tunneling
experiments. The orbital effect of magnetic fields on scattering becomes
particularly important in the Hofstadter butterfly regime.",1710.01396v1
2018-02-12,Dynamics and morphology of chiral magnetic bubbles in perpendicularly magnetized ultra-thin films,"We study bubble domain wall dynamics using micromagnetic simulations in
perpendicularly magnetized ultra-thin films with disorder and
Dzyaloshinskii-Moriya interaction. Disorder is incorporated into the material
as grains with randomly distributed sizes and varying exchange constant at the
edges. As expected, magnetic bubbles expand asymmetrically along the axis of
the in-plane field under the simultaneous application of out-of-plane and
in-plane fields. Remarkably, the shape of the bubble has a ripple-like part
which causes a kink-like (steep decrease) feature in the velocity versus
in-plane field curve. We show that these ripples originate due to the
nucleation and interaction of vertical Bloch lines. Furthermore, we show that
the Dzyaloshinskii-Moriya interaction field is not constant but rather depends
on the in-plane field. We also extend the collective coordinate model for
domain wall motion to a magnetic bubble and compare it with the results of
micromagnetic simulations.",1802.04215v1
2018-07-16,Magnetic freezing transition in a CoO/Permalloy bilayer revealed by transverse ac susceptibility,"We utilize variable-temperature, variable-frequency magneto-optical
transverse magnetic susceptibility technique to study the static and dynamical
magnetic properties of a thin-film CoO/Permalloy bilayer. Our measurements
demonstrate that in the studied system, the directional asymmetry of the
hysteresis loop is associated mainly with the difference in the reversal
mechanisms between the two reversed states of magnetization stabilized by the
exchange-induced uniaxial anisotropy. The latter is found to be much larger
than the exchange-induced unidirectional anisotropy of the ferromagnet. We also
observe an abrupt variation of the frequency-dependent imaginary part of ac
susceptibility near the exchange bias blocking temperature, consistent with the
magnetic freezing transition inferred from the previous time-domain studies of
magnetic aging in similar systems. The developed measurement approach enables
precise characterization of the dynamical and static characteristics of
thin-film magnetic heterostructures that can find applications in
reconfigurable magnonic and neuromorphic circuits.",1807.05990v2
2018-11-02,Large-scale Model of the Axisymmetric Dynamo with Feedback Effects,"A dynamo model is presented, based on a previously introduced kinematic
model, in which the reaction of the magnetic field on the mass flow through the
Lorentz force is included. Given the base mass flow corresponding to the case
with no magnetic field, and assuming that the modification of this flow due to
the Lorentz force can be treated as a perturbation, a complete model of the
large-scale magnetic field dynamics can be obtained. The input needed consists
in the large-scale meridional and zonal flows, the small-scale magnetic
diffusivity, and a constant parameter entering the expression of the
$\alpha$-effect. When applied to a solar-like star, the model shows a realistic
dynamics of the magnetic field, including cycle duration, consistent field
amplitudes with the correct parity, progression of the zonal magnetic field
towards the equator, and motion toward the poles of the radial field at high
latitudes. Also, the radial and zonal components show a correct phase relation,
and, at the surface level, the magnetic helicity is predominantly negative in
the northern hemisphere and positive in the southern hemisphere.",1811.00933v1
2019-03-26,Nuclear spin assisted quantum tunnelling of magnetic monopoles in spin ice,"Extensive work on single molecule magnets has identified a fundamental mode
of relaxation arising from the nuclear-spin assisted quantum tunnelling of
nearly independent and quasi-classical magnetic dipoles. Here we show that
nuclear-spin assisted quantum tunnelling can also control the dynamics of
purely emergent excitations: magnetic monopoles in spin ice. Our low
temperature experiments were conducted on canonical spin ice materials with a
broad range of nuclear spin values. By measuring the magnetic relaxation, or
monopole current, we demonstrate strong evidence that dynamical coupling with
the hyperfine fields bring the electronic spins associated with magnetic
monopoles to resonance, allowing the monopoles to hop and transport magnetic
charge. Our result shows how the coupling of electronic spins with nuclear
spins may be used to control the monopole current. It broadens the relevance of
the assisted quantum tunnelling mechanism from single molecular spins to
emergent excitations in a strongly correlated system.",1903.11122v1
2019-09-04,Two-axis cavity optomechanical torque characterization of magnetic microstructures,"Significant new functionality is reported for torsion mechanical tools aimed
at full magnetic characterizations of both spin statics and dynamics in micro-
and nanostructures. Specifically, two orthogonal torque directions are
monitored and the results co-analyzed to separate magnetic moment and magnetic
susceptibility contributions to torque, as is desired for characterization of
anisotropic three-dimensional structures. The approach is demonstrated through
application to shape and microstructural disorder-induced magnetic anisotropies
in lithographically patterned permalloy, and will have utility for the
determination of important magnetic thin-film and multilayer properties
including interface anisotropy and exchange bias. The results reflect
remarkable sensitivity of the out-of-plane magnetic torque to the nature of
small edge domains perpendicular to the applied field direction, and also
contain tantalizing indications of direct coupling to spin dynamics at the
frequency of the mechanics.",1909.01949v1
2020-04-28,"Current-Induced Dynamics of Chiral Magnetic Structures: Creation, Motion, and Applications","Magnetic textures can be manipulated by electric currents via the mechanisms
of spin-transfer and spin-orbit-torques. We review how these torques can be
exploited to create chiral magnetic textures in magnets with broken inversion
symmetries, including domain walls and skyrmions. These chiral textures can
also be moved by (electric) currents and obey very rich dynamics. For example,
magnetic domain walls feature the famous Walker breakdown, and magnetic whirls
are subject to the skyrmion Hall effect, which is rooted in their real-space
topology. These properties led to a variety of potential novel applications
which we briefly overview.",2004.13535v2
2020-05-06,Distinct Ultrafast Electronic and Magnetic Response in M-edge Magnetic Circular Dichroism,"Experimental investigations of ultrafast magnetization dynamics increasingly
employ resonant magnetic spectroscopy in the ultraviolet spectral range.
Besides allowing to disentangle the element-specific transient response of
functional magnetic systems, these techniques also promise to access attosecond
to few-femtosecond dynamics of spin excitations. Here, we report on a
systematic study of transient magnetic circular dichroism (MCD) on the
transition metals Fe, Co and Ni as well as on a FeNi and GdFe alloy and reveal
a delayed onset between the electronic and magnetic response. Supported by
\textit{ab-initio} calculations, we attribute our observation to a transient
energy shift of the absorption and MCD spectra at the corresponding elemental
resonances due to non-equilibrium changes of electron occupations.",2005.02872v1
2020-08-19,Field-Induced Magnetic Monopole Plasma in Artificial Spin Ice,"Artificial spin ices (ASIs) are interacting arrays of
lithographically-defined nanomagnets in which novel frustrated magnetic phases
can be intentionally designed. A key emergent description of fundamental
excitations in ASIs is that of magnetic monopoles -- mobile quasiparticles that
carry an effective magnetic charge. Here we demonstrate that the archetypal
square ASI lattice can host, in specific regions of its magnetic phase diagram,
high-density plasma-like regimes of mobile magnetic monopoles. By passively
""listening"" to spontaneous monopole noise in thermal equilibrium, we reveal
their intrinsic dynamics and show that monopole kinetics are minimally
correlated (that is, most diffusive) in the plasma phase. These results open
the door to on-demand monopole regimes having field-tunable densities and
dynamic properties, thereby providing a new paradigm for probing the physics of
effective magnetic charges in synthetic matter.",2008.08635v1
2020-09-17,Peculiar magnetic dynamics across the in-field transition in Ca3Co2O6,"The discovery of multiple coexisting magnetic phases in a
crystallographically homogeneous compound Ca$_3$Co$_2$O$_6$ has stimulated an
ongoing research activity. In recent years the main focus has been on the zero
field state properties, where exceedingly long time scales have been
established. In this study we report a detailed investigation of static and
dynamic properties of Ca$_3$Co$_2$O$_6$ across the magnetic field induced
transition around 3.5 T. This region has so far been practically neglected
while we argue that in some aspects it represents a simpler version of the
transition across the $B = 0$ state. Investigating the frequency dependence of
the ac susceptibility we reveal that on the high field side ($B > 3.5$ T) the
response corresponds to a relatively narrow distribution of magnetic clusters.
The distribution appears very weakly dependent on magnetic field, with an
associated energy barrier of around 200 K. Below 3.5 T a second contribution
arises, with much smaller characteristic frequencies and a strong temperature
and magnetic field dependence. We discuss these findings in the context of
intra-chain and inter-chain clustering of magnetic moments.",2009.08201v1
2020-12-06,Properties and dynamics of meron topological spin textures in the two-dimensional magnet CrCl3,"Merons are nontrivial topological spin textures highly relevant for many
phenomena in solid state physics. Despite their importance, direct observation
of such vortex quasiparticles is scarce and has been limited to a few complex
materials. Here we show the emergence of merons and antimerons in recently
discovered two-dimensional (2D) CrCl3 at zero magnetic field. We show their
entire evolution from pair creation, their diffusion over metastable domain
walls, and collision leading to large magnetic monodomains. Both quasiparticles
are stabilized spontaneously during cooling at regions where in-plane magnetic
frustration takes place. Their dynamics is determined by the interplay between
the strong in-plane dipolar interactions and the weak out-of-plane magnetic
anisotropy stabilising a vortex core within a radius of 8-10 nm. Our results
push the boundary to what is currently known about non-trivial spin structures
in 2D magnets and open exciting opportunities to control magnetic domains via
topological quasiparticles.",2012.03296v2
2021-07-21,Radical pairs can explain magnetic field and lithium effects on the circadian clock,"Drosophila's circadian clock can be perturbed by magnetic fields, as well as
by lithium administration. Cryptochromes are critical for the circadian clock.
Further, the radical pairs in cryptochrome also can explain magnetoreception in
animals. Based on a simple radical pair mechanism model of the animal magnetic
compass, we show that both magnetic fields and lithium can influence the spin
dynamics of the naturally occurring radical pairs and hence modulate the
circadian clock's rhythms. Using a simple chemical oscillator model for the
circadian clock, we show that the spin dynamics influence a rate in the
chemical oscillator model, which translates into a change in the circadian
period. Our model can reproduce the results of two independent experiments,
magnetic fields and lithium effects on the circadian clock. Our model predicts
that stronger magnetic fields would shorten the clock's period. We also predict
that lithium influences the clock in an isotope-dependent manner. Furthermore,
our model also predicts that magnetic fields and hyperfine interactions
modulate oxidative stress. The findings of this work suggest that quantum
nature and entanglement of radical pairs might play roles in the brain, as
another piece of evidence in addition to recent results on xenon anesthesia and
lithium effects on hyperactivity.",2107.10677v1
2021-09-23,Drift of suspended single-domain nanoparticles in a harmonically oscillating gradient magnetic field,"We study the nonlinear dynamics of single-domain ferromagnetic nanoparticles
in a viscous liquid induced by a harmonically oscillating gradient magnetic
field in the absence and presence of a static uniform magnetic field. Under
some physically reasonable assumptions, we derive a coupled set of stiff
ordinary differential equations for the magnetization angle and particle
coordinate describing the rotational and translational motions of
nanoparticles. Analytical solutions of these equations are determined for
nanoparticles near and far from the coordinate origin, and their correctness is
confirmed numerically. We show that if a uniform magnetic field is absent, the
magnetization angle and particle coordinate of each nanoparticle are periodic
functions of time. In contrast, the presence of a uniform magnetic field makes
these functions aperiodic. In this case, we perform a detailed analysis of the
nanoparticle dynamics and predict the appearance of the drift motion (directed
transport) of nanoparticles. We calculate both analytically and numerically the
drift velocity, study its dependence on time and model parameters, analyze the
physical origin of the drift phenomenon and discuss its potential biomedical
applications.",2109.11253v1
2021-11-09,Instability and Turbulent Relaxation in a Stochastic Magnetic Field,"An analysis of instability dynamics in a stochastic magnetic field is
presented for the tractable case of the resistive interchange. Externally
prescribed static magnetic perturbations convert the eigenmode problem to a
stochastic differential equation, which is solved by the method of averaging.
The dynamics are rendered multi-scale, due to the size disparity between the
test mode and magnetic perturbations. Maintaining quasi-neutrality at all
orders requires that small-scale convective cell turbulence be driven by
disparate scale interaction. The cells in turn produce turbulent mixing of
vorticity and pressure, which is calculated by fluctuation-dissipation type
analyses, and are relevant to pump-out phenomena. The development of
correlation between the ambient magnetic perturbations and the cells is
demonstrated, showing that turbulence will `lock on' to ambient stochasticity.
Magnetic perturbations are shown to produce a magnetic braking effect on
vorticity generation at large scale. Detailed testable predictions are
presented. The relations of these findings to the results of available
simulations and recent experiments are discussed.",2111.05396v1
2022-02-03,Entropic force and real-time dynamics of holographic quarkonium in a magnetic field,"We continue the study of a recently constructed holographic QCD model
supplemented with magnetic field. We consider the holographic dual of a quark,
anti-quark pair and investigate its entropy beyond the deconfinement phase
transition in terms of interquark distance, temperature and magnetic field. We
obtain a clear magnetic field dependence in the strongly decreasing entropy
near deconfinement and in the entropy variation for growing interquark
separation. We uncover various supporting evidences for inverse magnetic
catalysis. The emergent entropic force is found to become stronger with
magnetic field, promoting the quarkonium dissociation. We also probe the
dynamical dissociation of the quarkonium state, finding a faster dissociation
with magnetic field. At least the static predictions should become amenable to
a qualitative comparison with future lattice data.",2202.01486v2
2022-05-02,Relativistic magnetic explosions,"Many explosive astrophysical events, like magnetars' bursts and flares, are
magnetically driven. We consider dynamics of such magnetic explosions -
relativistic expansion of highly magnetized and highly magnetically
over-pressurized clouds. The corresponding dynamics is qualitatively different
from fluid explosions due to the topological constraint of the conservation of
the magnetic flux. Using analytical, relativistic MHD as well as force-free
calculations, we find that the creation of a relativistically expanding,
causally disconnected flow obeys a threshold condition: it requires
sufficiently high initial over-pressure and sufficiently quick decrease of the
pressure in the external medium (the pre-explosion wind). In the subcritical
case the magnetic cloud just ""puffs-up"" and quietly expands with the pre-flare
wind. We also find a compact analytical solution to the Prendergast's problem -
expansion of force-free plasma into vacuum.",2205.00695v1
2022-06-21,Magnetism in doped infinite-layer NdNiO2 studied by combined density functional theory and dynamical mean-field theory,"The recent observation of superconductivity in infinite-layer nickelates has
brought intense debate on the established knowledge of unconventional
superconductivity based on the cuprates. Despite many similarities, the
nickelates differ from the cuprates in many characteristics, the most notable
one among which is the magnetism. Instead of a canonical antiferromagnetic Mott
insulator as the undoped cuprates, from which the superconductivity is
generally believed to arise upon doping, the undoped nickelates show no sign of
magnetic ordering in experiments. Through a combined density functional theory,
dynamical mean-field theory, and model study, we show that although the
increased energy splitting between O-$p$ orbital and Cu/Ni-$d$ orbital
($\Delta_{dp}$) results in larger magnetic moment in nickelates, it also leads
to stronger antiferromagnetism/ferromagnetism competition, and weaker magnetic
exchange coupling. Meanwhile, the self-doping effect caused by Nd-$d$ orbital
screens the magnetic moment of Ni. The Janus-faced effect of $\Delta_{dp}$ and
self-doping effect together give a systematic understanding of magnetic
behavior in nickelates and explain recent experimental observations.",2206.10094v2
2023-01-04,Magnetization dynamics with time-dependent spin-density functional theory: significance of exchange-correlation torques,"In spin-density-functional theory (SDFT) for noncollinear magnetic materials,
the Kohn-Sham system features exchange-correlation (xc) scalar potentials and
magnetic fields. The significance of the xc magnetic fields is not very well
explored; in particular, they can give rise to local torques on the
magnetization, which are absent in standard local and semilocal approximations.
Exact benchmark solutions for a five-site extended Hubbard lattice at half
filling and in the presence of spin-orbit coupling are compared with SDFT
results obtained using orbital-dependent exchange-only approximations. The
magnetization dynamics following short-pulse excitations is found to be
reasonably well described in the exchange-only approximation for weak to
moderate interactions. For stronger interactions and near transitions between
magnetically ordered and frustrated phases, exchange and correlation torques
tend to compensate each other and must both be accounted for.",2301.01509v1
2023-04-12,Micromagnetics simulations and phase transitions of ferromagnetics with Dzyaloshinskii-Moriya interaction,"Magnetic skyrmions widely exist in a diverse range of magnetic systems,
including chiral magnets with a non-centrosymmetric structure characterized by
Dzyaloshinkii-Moriya interaction~(DMI). In this study, we propose a generalized
semi-implicit backward differentiation formula projection method, enabling the
simulations of the Landau-Lifshitz~(LL) equation in chiral magnets in a typical
time step-size of $1$ ps, markedly exceeding the limit subjected by existing
numerical methods of typically $0.1$ ps. Using micromagnetics simulations, we
show that the LL equation with DMI reveals an intriguing dynamic instability in
magnetization configurations as the damping varies. Both the isolated
skyrmionium and skyrmionium clusters can be consequently produced using a
simple initialization strategy and a specific damping parameter. Assisted by
the string method, the transition path between skyrmion and skyrmionium, along
with the escape of a skyrmion from the skyrmion clusters, are then thoroughly
examined. The numerical methods developed in this work not only provide a
reliable paradigm to investigate the skyrmion-based textures and their
transition paths, but also facilitate the understandings for magnetization
dynamics in complex magnetic systems.",2304.05789v1
2023-05-25,Energetic perspective on emergent inductance exhibited by magnetic textures in the pinned regime,"Spatially varying magnetic textures can exhibit electric-current-induced
dynamics as a result of the spin-transfer torque effect. When such a magnetic
system is electrically driven, an electric field is generated, which is called
the emergent electric field. In particular, when magnetic-texture dynamics are
induced under the application of an AC electric current, the emergent electric
field also appears in an AC manner, notably, with an out-of-phase time profile,
thus exhibiting inductor behaviour, often called an emergent inductor. Here we
show that the emergent inductance exhibited by magnetic textures in the pinned
regime can be explained in terms of the current-induced energy stored in the
magnetic system. We numerically find that the inductance values defined from
the emergent electric field and the current-induced magnetization-distortion
energy, respectively, are in quantitative agreement in the so-called adiabatic
limit. Our findings indicate that emergent inductors retain the basic concept
of conventional inductors; that is, the energy is stored under the application
of electric current.",2305.16123v2
2023-06-06,Large effective magnetic fields from chiral phonons in rare-earth halides,"Time-reversal symmetry (TRS) is pivotal for materials optical, magnetic,
topological, and transport properties. Chiral phonons, characterized by atoms
rotating unidirectionally around their equilibrium positions, generate dynamic
lattice structures that break TRS. Here we report that coherent chiral phonons,
driven by circularly polarized terahertz light pulses, can polarize the
paramagnetic spins in CeF3 like a quasi-static magnetic field on the order of 1
Tesla. Through time-resolved Faraday rotation and Kerr ellipticity, we found
the transient magnetization is only excited by pulses resonant with phonons,
proportional to the angular momentum of the phonons, and growing with magnetic
susceptibility at cryogenic temperatures, as expected from the spin-phonon
coupling model. The time-dependent effective magnetic field quantitatively
agrees with that calculated from phonon dynamics. Our results may open a new
route to directly investigate mode-specific spin-phonon interaction in
ultrafast magnetism, energy-efficient spintronics, and non-equilibrium phases
of matter with broken TRS.",2306.03852v1
2023-06-12,Correlation of density fluctuation in a magnetized QCD matter near the critical end point,"The dynamical correlation of density fluctuation in quark gluon plasma with a
critical end point has been investigated within the scope of the
M\""uller-Israel-Stewart theory in the presence of static ultra-high external
magnetic field. The dynamic structure factor of the density fluctuation
exhibits three Lorentzian peaks in absence of external magnetic field- a
central Rayleigh peak and two Brillouin peaks situated symmetrically on the
opposite sides of the Rayleigh peak. The spectral structure displays five peaks
in presence of the magnetic field due to the coupling of the magnetic field
with the hydrodynamic fields in second-order hydrodynamics. The emergence of
the extra peaks is due to the asymmetry in the pressure gradient caused by the
external magnetic field in the system. Interestingly, it is observed that near
the critical end point, all the Brillouin peaks disappear irrespective of the
presence or absence of the external magnetic field.",2306.06905v2
2023-09-06,In-plane dominant anisotropy stochastic magnetic tunnel junction for probabilistic computing: A Fokker-Planck study,"Recently there is considerable interest to realize efficient and low-cost
true random number generators (RNGs) for practical applications. One important
way is through the use of bistable magnetic tunnel junctions (MTJs). Here we
study the magnetization dynamics of an MTJ, with a focus to realize efficient
random bit generation under the assumption that the orientation dependence of
the energy of the nanomagnet is described by two perpendicular in-plane
anisotropies. We find that a high rate of random bit generation is achievable
away from the pure easy-axis situation by tuning a single parameter $H_z$ so
that it is either (a) toward a barrierless-like single easy plane situation
when $H_z$ reduces to zero, or (b) toward a stronger easy plane situation when
$H_z$ becomes increasingly negative where transitions between low energy states
are confined in the stronger easy plane that contains the saddle points. We
find that the MTJs maintain their fast magnetization dynamical characteristics
even in the presence of a magnetic field. Our findings provide a valuable guide
to achieving efficient generation of probabilistic bits for applications in
probabilistic computing.",2309.03056v1
2023-10-10,Dynamic Terahertz Beamforming Based on Magnetically Switchable Hyperbolic Materials,"In this work, we introduce a concept to enable dynamic beamforming of
terahertz (THz) wavefronts using applied magnetic fields (B). The proposed
system exploits the magnetically switchable hyperbolic dispersion of the InSb
semiconductor. This phenomenology, combined with diffractive surfaces and
magnetic tilting of scattered fields, allows the design of a metasurface that
works with either circularly or linearly polarized wavefronts. In particular,
we demonstrate numerically that the transmitted beam tilting can be manipulated
with the direction and magnitude of B. Numerical results, obtained through the
finite element method (FEM), are qualitatively supported by semi-analytical
results from the generalized dipole theory. Motivated by potential applications
in future Tera-WiFi active links, a proof of concept is conducted for the
working frequency f = 300 GHz. The results indicate that the transmitted field
can be actively tuned to point in five different directions with beamforming of
45{\deg}, depending on the magnitude and direction of B. In addition to
magnetic beamforming, we also demonstrate that our proposal exhibits magnetic
circular dichroism (MCD), which can also find applications in magnetically
tunable THz isolators for one-way transmission/reflection.",2310.06529v1
2006-06-26,Dynamical PN Evolution with Magnetic Fields,"Hydrodynamical simulations played an important role in understanding the
dynamics and shaping of planetary nebulae in the past century. However,
hydrodynamical simulations were just a first order approach. The new millennium
arrived with the generalized understanding that the effects of magnetic fields
were necessary to study the dynamics of planetary nebulae. Thus, B-fields
introduced a whole new number of physical possibilities for the modeling. In
this paper, we review observational works done in the last 5 years and several
works on magnetohydrodynamics about proto-planetary nebulae, since all the
effort has been focused on that stage, and discuss different scenarios for the
origin of magnetized winds, and the relation binary-bipolararity.",0606632v1
1998-10-21,Large scale instabilities in two-dimensional magnetohydrodynamics,"The stability of a sheared magnetic field is analyzed in two-dimensional
magnetohydrodynamics with resistive and viscous dissipation. Using a
multiple-scale analysis, it is shown that at large enough Reynolds numbers the
basic state describing a motionless fluid and a layered magnetic field, becomes
unstable with respect to large scale perturbations. The exact expressions for
eddy-viscosity and eddy-resistivity are derived in the nearby of the critical
point where the instability sets in. In this marginally unstable case the
nonlinear phase of perturbation growth obeys to a Cahn-Hilliard-like dynamics
characterized by coalescence of magnetic islands leading to a final new
equilibrium state. High resolution numerical simulations confirm quantitatively
the predictions of multiscale analysis.",9810026v1
1996-01-11,Universality and Scaling in Short-time Critical Dynamics,"Numerically we simulate the short-time behaviour of the critical dynamics for
the two dimensional Ising model and Potts model with an initial state of very
high temperature and small magnetization. Critical initial increase of the
magnetization is observed. The new dynamic critical exponent $\theta$ as well
as the exponents $z$ and $2\beta/\nu$ are determined from the power law
behaviour of the magnetization, auto-correlation and the second moment.
Furthermore the calculation has been carried out with both Heat-bath and
Metropolis algorithms. All the results are consistent and therefore
universality and scaling are confirmed.",9601037v1
2001-05-29,Dynamics of a tunneling magnetic impurity: Kondo effect induced incoherence,"We study how the formation of the Kondo compensation cloud influences the
dynamical properties of a magnetic impurity that tunnels between two positions
in a metal. The Kondo effect dynamically generates a strong tunneling
impurity-conduction electron coupling, changes the temperature dependence of
the tunneling rate, and may ultimately result in the destruction of the
coherent motion of the particle at zero temperature. We find an interesting
two-channel Kondo fixed point as well for a vanishing overlap between the
electronic states that screen the magnetic impurity. We propose a number of
systems where the predicted features could be observed.",0105548v1
2002-12-10,Nonlinear Dynamics of a Bose-Einstein Condensate in a Magnetic Waveguide,"We have studied the internal and external dynamics of a Bose-Einstein
condensate in an anharmonic magnetic waveguide. An oscillating condensate
experiences a strong coupling between the center of mass motion and the
internal collective modes. Due to the anharmonicity of the magnetic potential,
not only the center of mass motion shows harmonic frequency generation, but
also the internal dynamics exhibit nonlinear frequency mixing. We describe the
data with a theoretical model to high accuracy. For strong excitations we test
the experimental data for indications of a chaotic behavior.",0212220v2
2003-12-10,Dynamic scaling of fronts in the quantum XX chain,"The dynamics of the transverse magnetization in the zero-temperature XX chain
is studied with emphasis on fronts emerging from steplike initial magnetization
profiles. The fronts move with fixed velocity and display a staircase like
internal structure whose dynamic scaling is explored both analytically and
numerically. The front region is found to spread with time sub-diffusively with
the height and the width of the staircase steps scaling as t^(-1/3) and t^1/3,
respectively. The areas under the steps are independent of time, thus the
magnetization relaxes in quantized ""steps"" of spin-flips.",0312250v1
2005-05-09,Dynamical properties of S=1 bond-alternating Heisenberg chains in transverse magnetic fields,"We calculate dynamical structure factors of the S=1 bond-alternating
Heisenberg chain with a single-ion anisotropy in transverse magnetic fields,
using a continued fraction method based on the Lanczos algorithm. In the
Haldane-gap phase and the dimer phase, dynamical structure factors show
characteristic field dependence. Possible interpretations are discussed. The
numerical results are in qualitative agreement with recent results for
inelastic neutron-scattering experiments on the S=1 bond-alternating
Heisenberg-chain compound $\rm{Ni(C_{9}D_{24}N_{4})(NO_{2})ClO_{4}}$ and the
S=1 Haldane-gap compound $\rm{Ni(C_{5}D_{14}N_{2})_{2}N_{3}(PF_{6})}$ in
transverse magnetic fields.",0505196v1
2006-08-10,Spin Dynamics of a Magnetic Antivortex,"We report on a study of the dynamics of a magnetic antivortex in a submicron,
asteroid shaped, permalloy ferromagnet using micromagnetic simulations. As with
vortex states in disk and square geometries, a gyrotropic mode was found in
which a shifted antivortex core orbits about the center of the asteroid. Pulsed
magnetic fields were used to generate azimuthal or radial spin wave modes,
depending on the field orientation. The degeneracy of low frequency azimuthal
mode frequencies is lifted by gyrotropic motion of the antivortex core, and
restored by inserting a hole in the center of the particle to suppress this
motion. We briefly compare the dynamics of the vortex state of the asteroid to
the antivortex. The size dependence of the antivortex modes is reported.",0608245v1
1994-12-08,Dimensional Reduction and Dynamical Chiral Symmetry Breaking by a Magnetic Field in $3+1$ Dimensions,"It is shown that in $3+1$ dimensions, a constant magnetic field is a catalyst
of dynamical chiral symmetry breaking, leading to generating a fermion mass
even at the weakest attractive interaction between fermions. The essence of
this effect is the dimensional reduction $D \rightarrow D-2$ ($3+1 \rightarrow
1+1$) in the dynamics of fermion pairing in a magnetic field. The effect is
illustrated in the Nambu-Jona-Lasinio model. Possible applications of this
effect are briefly discussed.",9412257v1
1995-11-30,Catalysis of Dynamical Symmetry Breaking by a Magnetic Field,"A constant magnetic field in 3+1 and 2+1 dimensions is a strong catalyst of
dynamical chiral symmetry breaking, leading to the generation of a fermion mass
even at the weakest attractive interaction between fermions. The essence of
this effect is the dimensional reduction $D/rightarrow D-2$ in the dynamics of
fermion pairing in a magnetic field. The effect is illustrated in the
Nambu-Jona-Lasinio model and QED. Possible applications of this effect and its
extension to inhomogeneous field configurations are discussed.",9511224v1
2003-03-31,Charged Grains In Saturn's F-Ring: Interaction With Saturn's Magnetic Field,"Saturn's dynamic F-Ring still presents a challenge for understanding and
explaining the kinematic processes that lead to the changing structure visible
in our observations of this ring. This study examines the effect of Saturn's
magnetic field on the dynamics of micron-sized grains that may become
electrically charged due to interaction with plasma in Saturn's rigidly
corotating magnetosphere. The numerical model calculates the dynamics of
charged dust grains and includes forces due to Saturn's gravitational field,
the plasma polarization electric field, a third order harmonic expansion of
Saturn's magnetic field, and the F Ring's Shepherding moons, Prometheus and
Pandora.",0303123v1
1995-05-05,Topics in nonhamiltonian (magnetic-type) interaction of classical hamiltonian dynamical systems. I,"A convenient algebraic structure to describe some forms of dynamics of two
hamiltonian systems with nonpotential (magnetic--type) interaction is
considered. An algebraic mechanism of generation of such dynamics is explored
on simple ""toy"" examples and models. Nonpotential chains and their continuum
limits are also considered. Examples of hybrid couplings with both potential
and nonpotential (magnetic--type) interactions are discussed.",9505001v2
2007-06-27,Molecular dynamics simulations of the dipolar-induced formation of magnetic nanochains and nanorings,"Iron, cobalt and nickel nanoparticles, grown in the gas phase, are known to
arrange in chains and bracelet-like rings due to the long-range dipolar
interaction between the ferromagnetic (or super-paramagnetic) particles. We
investigate the dynamics and thermodynamics of such magnetic dipolar
nanoparticles for low densities using molecular dynamics simulations and
analyze the influence of temperature and external magnetic fields on two- and
three-dimensional systems. The obtained phase diagrams can be understood by
using simple energetic arguments.",0706.3920v1
2008-02-12,Dynamical Coupling Between Ferromagnets Due to Spin Transfer Torque,"We use a combination of analytic calculations and numerical simulations to
demonstrate that electrical current flowing through a magnetic bilayer induces
dynamical coupling between the layers. The coupling originates from the
dependence of the spin transfer torque exerted on the layers on the relative
orientations of their magnetic moments. We demonstrate that such coupling
modifies the behaviors of both layers, significantly affecting the the
stability of the current-induced dynamical regimes and the efficiency of
current-induced magnetic reversal.",0802.1560v1
2008-03-14,Does there arise a significant enhancement of the dynamical quark mass in external magnetic field?,"Recently, it was found in QED that the generation of a dynamical electron
mass in a strong magnetic field is significantly enhanced by the perturbative
electron mass. In the present paper, the related question of a possible
enhancement of the dynamical quark mass in an external magnetic field and with
a bare mass term is investigated in the Nambu--Jona-Lasinio model.",0803.2191v2
2008-10-05,Current-Induced Dynamics in Almost Symmetric Magnetic Nanopillars,"Magnetic nanodevices usually include a free layer whose configuration can be
changed by spin-polarized current via the spin transfer effect, and a fixed
reference layer. Here, we demonstrate that the roles of the free and the
reference layers interchange over a small range of their relative thicknesses.
Precession of both layers can be induced by spin transfer in symmetric devices,
but the dynamics of one of the layers is rapidly suppressed in asymmetric
devices. We interpret our results in terms of the dynamical coupling between
magnetic layers due to spin transfer.",0810.0863v1
2009-07-08,Magnetic hysteresis in a molecular Ising ferrimagnet: Glauber dynamics approach,"Motivated by recent experimental results reporting giant coercive fields in
Co(II)-based molecular magnets we present a theory of hysteresis phenomena
based on the Glauber stochastic dynamics. Unusual form of hysteresis loops is
similar to those of found in Co-based quasi-one-dimensional ferrimagnet CoPhOMe
at low temperatures. Temperature dependence of the coercive field has a
characteristic form with an inflection that may serve as an indicator of the
Glauber dynamics in real compounds. A relevance of the model for other Co-based
molecular magnets is discussed.",0907.1348v1
2009-10-23,Hamiltonian reduction for the magnetic dynamics in antiferromagnetic crystals,"The nonlinear spin dynamics in antiferromagnetic crystals is studied for the
magnetic structures similar to that of hematite. For the case when only two
magnetization vectors are non-zero and the Hamiltonian has an axial symmetry, a
reduction to a Hamiltonian system with one degree of freedom is performed,
based on the corresponding conservation law. The analysis of the phase
portraits of this system provides tractable analytical and geometric
descriptions of the regimes of nonlinear spin dynamics in the crystal.",0910.4527v3
2009-12-09,Microscopic Theory of Current-Spin Interaction in Ferromagnets,"Interplay between magnetization dynamics and electric current in a conducting
ferromagnet is theoretically studied based on a microscopic model calculation.
First, the effects of the current on magnetization dynamics (spin torques) are
studied with special attention to the ""dissipative"" torques arising from
spin-relaxation processes of conduction electrons. Next, an analysis is given
of the ""spin motive force"", namely, a spin-dependent 'voltage' generation due
to magnetization dynamics, which is the reaction to spin torques. Finally, an
attempt is presented of a unified description of these effects.",0912.1676v1
2010-01-12,Dynamics of random dipoles : chaos {\it vs} ferromagnetism,"The microcanonical dynamics of an ensemble of random magnetic dipoles in a
needle has been investigated. Analyzing magnetic reversal times, a transition
between a chaotic paramagnetic phase and an integrable ferromagnetic phase has
been numerically found. In particular, a simple criterium for transition has
been formulated. Close to the transition point the statistics of average
magnetic reversal times and fluctuations have been studied and critical
exponents numerically given.",1001.1849v2
2010-07-02,Stochastic heating of a molecular nanomagnet,"We study the excitation dynamics of a single molecular nanomagnet by static
and pulsed magnetic fields. Based on a stability analysis of the classical
magnetization dynamics we identify analytically the fields parameters for which
the energy is stochastically pumped into the system in which case the
magnetization undergoes diffusively and irreversibly a large angle deflection.
An approximate analytical expression for the diffusion constant in terms of the
fields parameters is given and assessed by full numerical calculations.",1007.0360v1
2011-07-01,Current induced magnetization reversal on the surface of a topological insulator,"We study dynamics of the magnetization coupled to the surface Dirac fermions
of a three di- mensional topological insulator. By solving the
Landau-Lifshitz-Gilbert equation in the presence of charge current, we find
current induced magnetization dynamics and discuss the possibility of mag-
netization reversal. The torque from the current injection depends on the
transmission probability through the ferromagnet and shows nontrivial
dependence on the exchange coupling. The mag- netization dynamics is a direct
manifestation of the inverse spin-galvanic effect and hence another ferromagnet
is unnecessary to induce spin transfer torque in contrast to the conventional
setup.",1107.0116v1
2011-09-30,Topological charge pumping effect by the magnetization dynamics on the Surface of Three-Dimensional Topological Insulators,"We discuss a current dynamics on the surface of a 3-dimensional topological
insulator induced by magnetization precession of a ferromagnet attached. It is
found that the magnetization dynamics generates a direct charge current when
the precession axis is within the surface plane. This rectification effect is
due to a quantum anomaly and is topologically protected. The robustness of the
rectification effect against first-varying exchange field and impurities is
confirmed by explicit calculation.",1109.6816v2
2011-11-16,Magnetic Sensitivity and Entanglement Dynamics of the Chemical Compass,"We present the quantum limits to the magnetic sensitivity of a new kind of
magnetometer based on biochemical reactions. Radical-ion-pair reactions, the
biochemical system underlying the chemical compass, are shown to offer a new
and unique physical realization of a magnetic field sensor competitive to
modern atomic or condensed matter magnetometers. We elaborate on the quantum
coherence and entanglement dynamics of this sensor, showing that they provide
the physical basis for testing our understanding of the fundamental quantum
dynamics of radical-ion-pair reactions.",1111.3974v2
2012-03-29,Soliton Magnetization Dynamics in Spin-Orbit Coupled Bose-Einstein Condensates,"Ring-trapped Bose-Einstein condensates subject to spin-orbit coupling support
localized dark soliton excitations that show periodic density dynamics in real
space. In addition to the density feature, solitons also carry a localized
pseudo-spin magnetization that exhibits a rich and tunable dynamics. Analytic
results for Rashba-type spin-orbit coupling and spin-invariant interactions
predict a conserved magnitude and precessional motion for the soliton
magnetization that allows for the simulation of spin-related geometric phases
recently seen in electronic transport measurements.",1203.6684v1
2012-06-11,Scaling in Dynamic Susceptibility of Herbertsmithite and Heavy-Fermion Metals,"We present a theory of the dynamic magnetic susceptibility of quantum spin
liquid. The obtained results are in good agreement with experimental facts
collected on herbertsmithite ZnCu3(OH)6Cl2 and on heavy-fermion metals, and
allow us to predict a new scaling in magnetic fields in the dynamic
susceptibility. Under the application of strong magnetic fields quantum spin
liquid becomes completely polarized. We show that this polarization can be
viewed as a manifestation of gapped excitations when investigating the
spin-lattice relaxation rate.",1206.2324v2
2012-12-03,Numerically Exact Long Time Magnetization Dynamics at the Nonequilibrium Kondo Crossover of the Anderson Impurity Model,"We investigate the dynamical and steady-state spin response of the
nonequilibrium Anderson model to magnetic fields, bias voltage, and temperature
using a numerically exact method combining a bold-line quantum Monte Carlo
technique with the memory function formalism. We obtain converged results in a
range of previously inaccessible regimes, in particular the crossover to the
Kondo domain. We provide detailed predictions for novel nonequilibrium
phenomena, including non-monotonic temperature dependence of observables at
high bias voltage and oscillatory quench dynamics at high magnetic fields.",1212.0546v1
2012-12-10,Mechanism of Fast Axially--Symmetric Reversal of Magnetic Vortex Core,"The magnetic vortex core in a nanodot can be switched by an alternating
transversal magnetic field. We propose a simple collective coordinate model
which describes comprehensive vortex core dynamics, including resonant
behavior, weakly nonlinear regimes, and reversal dynamics. A chaotic dynamics
of the vortex polarity is predicted. All analytical results were confirmed by
micromagnetic simulations.",1212.2183v1
2013-06-05,Inertial longitudinal magnetization reversal for non-Heisenberg ferromagnets,"We analyze theoretically the novel pathway of ultrafast spin dynamics for
ferromagnets with high enough single-ion anisotropy (non-Heisenberg
ferromagnets). This longitudinal spin dynamics includes the coupled
oscillations of the modulus of the magnetization together with the quadrupolar
spin variables, which are expressed through quantum expectation values of
operators bilinear on the spin components. Even for a simple single-element
ferromagnet, such a dynamics can lead to an inertial magnetization reversal
under the action of an ultrashort laser pulse.",1306.1108v2
2013-09-10,Spin rectification induced by dynamical Hanle effect,"Dynamic response of spin accumulation to a time-dependent magnetic field has
been investigated in a ferromagnetic/nonmagnetic bilayer under ferromagnetic
resonance. In this system, magnetization precession driven by a microwave
generates direct-current (dc) and alternate-current (ac) spin accumulation in
the nonmagnetic layer by the spin pumping. The ac spin accumulation is coupled
with the microwave magnetic field through a dynamical Hanle spin precession,
giving rise to rectified spin accumulation comparable with the dc spin
accumulation directly generated by the spin pumping.",1309.2365v1
2013-12-19,Cyclotron dynamics of interacting bosons in artificial magnetic fields,"We study theoretically quantum dynamics of interacting bosons in artificial
magnetic fields as engineered in recent ultracold atomic experiments, where
quantum cyclotron orbital motion has been observed. With exact numerical
simulations and perturbative analyses, we find that interactions induce damping
in the cyclotron motion. The damping time is found to be dependent on
interaction and tunneling strengths monotonically, while its dependence on
magnetic flux is non-monotonic. Sufficiently strong interactions would render
bosons dynamically localized inhibiting the cyclotron motion. The damping
predicted by us can be construed as an interaction-induced quantum decoherence
of the cyclotron motion.",1312.5747v2
2018-07-17,Stability of paramagnetic spheroid in precessing field,"Stability analysis of paramagnetic prolate and oblate spheroidal particle in
precessing magnetic field is studied. Bifurcation diagram is calculated
analytically in dependence of magnetic field frequency and precession angle.
The direction of particle in synchronous regime is calculated. Rotational
dynamics and mean rotational frequency in asynchronous regime is found.
Obtained theoretical model gives possibility to calculate analytically dynamics
of the particle in limiting case (when motion is periodic). Theoretically
obtained models were compared with experimental results of rod like particle
dynamics in precessing magnetic field. Experimental results are in good
agreement with proposed theory.",1807.06379v1
2018-08-01,Long range dynamical coupling between magnetic adatoms mediated by a 2D topological insulator,"We study the spin excitation spectra and the dynamical exchange coupling
between iron adatoms on a Bi bilayer nanoribbon. We show that the topological
character of the edge states is preserved in the presence of the magnetic
adatoms. Nevertheless, they couple significantly to the edge spin currents, as
witnessed by the large and long-ranged dynamical coupling we obtain in our
calcula- tions. The large effective magnetocrystalline anisotropy of the
magnetic adatoms combined with the transport properties of the topologically
protected edge states make this system a strong candidate for implementation of
spintronics devices and quantum information and/or computation protocols.",1808.00347v1
2018-12-28,Exponential energy-preserving methods for charged-particle dynamics in a strong and constant magnetic field,"In this paper, exponential energy-preserving methods are formulated and
analysed for solving charged-particle dynamics in a strong and constant
magnetic field. The resulting method can exactly preserve the energy of the
dynamics. Moreover, it is shown that the magnetic moment of the considered
system is nearly conserved over a long time along this exponential
energy-preserving method, which is proved by using modulated Fourier
expansions. Other properties of the method including symmetry and convergence
are also studied. An illustrated numerical experiment is carried out to
demonstrate the long-time behaviour of the method.",1812.11038v1
2018-12-31,Quench dynamics and defects formation in the Ising chain in a transverse magnetic field,"We study analytically and numerically quench dynamics and defects formation
in the quantum Ising model in the presence of a time-dependent transverse
magnetic field. We generalize the Landau-Ziner formula to the case of
non-adiabatic evolution of the quantum system. For a quasi-static magnetic
field, with a slow dependence on time, our outcomes are similar to the results
predicted by the Landau-Zener formula. However, a quench dynamics under a
shock-wave load is more complicated. The final state of the system depends on
the amplitude and pulse velocity, resulting in the mixture of ground and
excited states and significant density of defects.",1812.11887v1
2020-04-04,Energy and momentum conservation in spin transfer,"We utilize simulations of spin-polarized electron scattering by a chain of
localized quantum spins to show that energy and linear momentum conservation
laws impose strong constraints on the properties of magnetic excitations
induced by spin transfer. In turn, electron's orbital and spin dynamics depends
on the dynamical characteristics of the local spins. Our results suggest the
possibility to achieve precise control of spin transfer-driven magnetization
dynamics by tailoring the spectral characteristics of the magnetic systems and
the driving electrons.",2004.01957v1
2020-06-14,Ideal memristor based on viscous magnetization dynamics driven by spin torque,"We show that ideal memristors - devices whose resistance is proportional to
the charge that flows through them - can be realized using spin torque-driven
viscous magnetization dynamics. The latter can be accomplished in the spin
liquid state of thin-film heterostructures with frustrated exchange, where
memristive response is tunable by proximity to the glass transition, while
current-induced Joule heating facilitates nonvolatile operation and
second-order memristive functionality beneficial for neuromorphic applications.
Ideal memristive behaviors can be achieved in other systems characterized by
viscous dynamics of physical, electronic, or magnetic degrees of freedom.",2006.07996v1
2020-09-01,The relativistic dynamics of oppositely charged two fermions interacting with external uniform magnetic field,"We investigated the relativistic dynamics of oppositely charged two fermions
interacting with an external uniform magnetic field. We chose the interaction
of each fermion with the external magnetic field in the symmetric gauge, and
obtained a precise solution of the corresponding fully-covariant two-body Dirac
equation that derived from Quantum Electrodynamics via Action principle. The
dynamic symmetry of the system we deal with allowed us to determine the
relativistic Landau levels of such a spinless composite system, without using
any group theoretical method. As a result, we determined the eigenfunctions and
eigenvalues of the corresponding two-body Dirac Hamiltonian",2009.06380v2
2003-04-06,Large N dynamics in QED in a magnetic field,"The expression for the dynamical mass of fermions in QED in a magnetic field
is obtained for a large number of the fermion flavor N in the framework of 1/N
expansion. The existence of a threshold value N_{thr}, dividing the theories
with essentially different dynamics, is established. For the number of flavors
N << N_{thr}, the dynamical mass is very sensitive to the value of the coupling
constant \alpha_b, related to the magnetic scale \mu = |eB|. For N of order
N_{thr} or larger, a dynamics similar to that in the Nambu-Jona-Lasinio model
with cutoff of order |eB| and the dimensional coupling constant G \sim
1/(N|eB|) takes place. In this case, the value of the dynamical mass is
essentially \alpha_b independent (the dynamics with an infrared stable fixed
point). The value of N_{thr} separates a weak coupling dynamics (with
\tilde{\alpha}_b \equiv N\alpha_b << 1) from a strong coupling one (with
\tilde{\alpha}_b \gtrsim 1) and is of order 1/\alpha_b.",0304059v1
2019-05-11,Guiding center dynamics as motion on a formal slow manifold in loop space,"Since the late 1950's, the dynamics of a charged particle's ``guiding center""
in a strong, inhomogeneous magnetic field have been understood in terms of
near-identity coordinate transformations. The basic idea has been to
approximately transform away the coupling between the fast gyration around
magnetic fields lines and the remaining slow dynamics. This basic understanding
now serves as a foundation for describing the kinetic theory of strongly
magnetized plasmas. I present a new way to understand guiding center dynamics
that does not involve complicated coordinate transformations. Starting from a
dynamical systems formulation of the motion of parameterized loops in a charged
particle's phase space, I identify a formal slow manifold in loop space.
Dynamics on this formal slow manifold are equivalent to guiding center dynamics
to all orders in perturbation theory. After demonstrating that loop space
dynamics comprises an infinite-dimensional noncanonical Hamiltonian system, I
recover the well-known Hamiltonian formulation of guiding center motion by
restricting the (pre-) symplectic structure on loop space to the
finite-dimensional guiding center formal slow manifold.",1905.04410v2
2020-06-25,Temperature-Dependent Anisotropic Magnetoresistance and Spin-Torque-Driven Vortex Dynamics in a Single Microdisk,"Spin-orbit-torque-driven dynamics have recently gained interest in the field
of magnetism due to the reduced requirement of current densities and an
increase in efficiency, as well as the ease of implementation of different
devices and materials. From a practical point of view, the low-frequency
dynamics below 1 GHz is particularly interesting since dynamics associated with
magnetic domains lie in this frequency range. While spin-torque excitation of
high-frequency modes has been extensively studied, the intermediate
low-frequency dynamics have received less attention, although spin torques
could potentially be used for both manipulation of the spin texture, as well as
the excitation of dynamics. In this work, we demonstrate that it is possible to
drive magnetic vortex dynamics in a single microdisk by spin-Hall torque at
varying temperatures, and relate the results to transport properties. We find
that the gyrotropic mode of the core couples to the low-frequency microwave
signal and produces a measurable voltage. The dynamic measurements are in
agreement with magnetic transport measurements and are supported by
micromagnetic simulations. Our results open the door for integrating magnetic
vortex devices in spintronic applications.",2006.14454v1
2010-03-13,The nonisothermal stage of magnetic star formation. II. Results,"In a previous paper we formulated the problem of the formation and evolution
of fragments (or cores) in magnetically-supported, self-gravitating molecular
clouds in axisymmetric geometry, accounting for the effects of ambipolar
diffusion and Ohmic dissipation, grain chemistry and dynamics, and radiative
transfer. Here we present results of star formation simulations that accurately
track the evolution of a protostellar fragment over eleven orders of magnitude
in density (from 300 cm^-3 to \approx 10^14 cm^-3), i.e., from the early
ambipolar-diffusion--initiated fragmentation phase, through the magnetically
supercritical, dynamical-contraction phase and the subsequent magnetic
decoupling stage, to the formation of a protostellar core in near hydrostatic
equilibrium. As found by Fiedler & Mouschovias (1993), gravitationally-driven
ambipolar diffusion leads to the formation and subsequent dynamic contraction
of a magnetically supercritical core. Moreover, we find that ambipolar
diffusion, not Ohmic dissipation, is responsible for decoupling all the species
except the electrons from the magnetic field, by a density \approx 3 x 10^12
cm^-3. Magnetic decoupling precedes the formation of a central stellar object
and ultimately gives rise to a concentration of magnetic flux (a `magnetic
wall') outside the hydrostatic core --- as also found by Tassis & Mouschovias
(2005a,b) through a different approach. At approximately the same density at
which Ohmic dissipation becomes more important than ambipolar diffusion
(\gtrsim 7 x 10^12 cm^-3), the grains carry most of the electric charge as well
as the electric current. The prestellar core remains disclike down to radii ~
10 AU, inside which thermal pressure becomes important. The magnetic flux
problem of star formation is resolved for at least strongly magnetic newborn
stars by this stage of the evolution, i.e., by a central density \approx 10^14
cm^-3. The hydrostatic core has radius \approx 2 AU, density \approx 10^14
cm^-3, temperature \approx 300 K, magnetic field strength \approx 0.2 G,
magnetic flux \approx 5 x 10^18 Wb, luminosity ~ 10^-3 L_\odot, and mass ~
10^-2 M_\odot.",1003.2722v1
2011-07-13,Evolution and Distribution of Magnetic Fields from AGNs in Galaxy Clusters II. The Effects of Cluster Size and Dynamical State,"Theory and simulations suggest that magnetic fields from radio jets and lobes
powered by their central super massive black holes can be an important source
of magnetic fields in the galaxy clusters. This is paper II in a series of
studies where we present self-consistent high-resolution adaptive mesh
refinement cosmological magnetohydrodynamic (MHD) simulations that
simultaneously follow the formation of a galaxy cluster and evolution of
magnetic fields ejected by an active galactic nucleus (AGN). We studied 12
different galaxy clusters with virial masses ranging from 1 $\times$ 10$^{14}$
to 2 $\times$ 10$^{15}$ M$_{\odot}$. In this work we examine the effects of the
mass and merger history on the final magnetic properties. We find that the
evolution of magnetic fields is qualitatively similar to those of previous
studies. In most clusters, the injected magnetic fields can be transported
throughout the cluster and be further amplified by the intra-cluster medium
(ICM) turbulence during the cluster formation process with hierarchical
mergers, while the amplification history and the magnetic field distribution
depend on the cluster formation and magnetism history. This can be very
different for different clusters. The total magnetic energies in these clusters
are between 4 $\times$ 10$^{57}$ and $10^{61}$ erg, which is mainly decided by
the cluster mass, scaling approximately with the square of the total mass.
Dynamically older relaxed clusters usually have more magnetic fields in their
ICM. The dynamically very young clusters may be magnetized weakly since there
is not enough time for magnetic fields to be amplified.",1107.2599v2
2020-11-11,Destruction of long-range magnetic order in an external magnetic field and the associated spin dynamics in Cu2GaBO5 and Cu2AlBO5 ludwigites,"The quantum spin systems Cu$_2$M'BO$_5$ (M' = Al, Ga) with the ludwigite
crystal structure consist of a structurally ordered Cu$^{2+}$ sublattice in the
form of three-leg ladders, interpenetrated by a structurally disordered
sublattice with a statistically random site occupation by magnetic Cu$^{2+}$
and nonmagnetic Ga$^{3+}$ or Al$^{3+}$ ions. A microscopic analysis based on
density-functional-theory calculations for Cu$_2$GaBO$_5$ reveals a frustrated
quasi-two-dimensional spin model featuring five inequivalent antiferromagnetic
exchanges. A broad low-temperature $^{11}$B nuclear magnetic resonance points
to a considerable spin disorder in the system. In zero magnetic field,
antiferromagnetic order sets in below $T_\text{N}$ $\approx$ 4.1 K and ~2.4 K
for the Ga and Al compounds, respectively. From neutron diffraction, we find
that the magnetic propagation vector in Cu$_2$GaBO$_5$ is commensurate and lies
on the Brillouin-zone boundary in the (H0L) plane, $\mathbf{q}_\text{m}$ =
(0.45 0 -0.7), corresponding to a complex noncollinear long-range ordered
structure with a large magnetic unit cell. Muon spin relaxation is monotonic,
consisting of a fast static component typical for complex noncollinear spin
systems and a slow dynamic component originating from the relaxation on
low-energy spin fluctuations. Gapless spin dynamics in the form of a diffuse
quasielastic peak is also evidenced by inelastic neutron scattering. Most
remarkably, application of a magnetic field above 1 T destroys the static
long-range order, which is manifested in the gradual broadening of the magnetic
Bragg peaks. We argue that such a crossover from a magnetically long-range
ordered state to a spin-glass regime may result from orphan spins on the
structurally disordered magnetic sublattice, which are polarized in magnetic
field and thus act as a tuning knob for field-controlled magnetic disorder.",2011.05819v1
1999-01-28,Doped bilayer antiferromagnets: Hole dynamics on both sides of a magnetic ordering transition,"The two-layer square lattice quantum antiferromagnet with spins 1/2 shows a
magnetic order-disorder transition at a critical ratio of the interplane to
intraplane couplings. We investigate the dynamics of a single hole in a bilayer
antiferromagnet described by a t-J Hamiltonian. To model the spin background we
propose a ground-state wave function for the undoped system which covers both
magnetic phases and includes transverse as well as longitudinal spin
fluctuations. The photoemission spectrum is calculated using the spin-polaron
picture for the whole range of the ratio of the magnetic couplings. This allows
for the study of the hole dynamics of both sides of the magnetic order-disorder
transition. For small interplane coupling we find a quasiparticle with
properties known from the single-layer antiferromagnet, e.g., the dispersion
minimum is at (pi/2,pi/2). For large interplane coupling the hole dispersion is
similar to that of a free fermion (with reduced bandwidth). The cross-over
between these two scenarios occurs inside the antiferromagnetic phase which
indicates that the hole dynamics is governed by the local environment of the
hole.",9901326v2
2004-01-28,Muon-spin-relaxation and magnetic-susceptibility studies of effects of the magnetic impurity Ni on the Cu-spin dynamics and superconductivity in La_2-x_Sr_x_Cu_1-y_Ni_y_O_4_ with x = 0.13,"Effects of the magnetic impurity Ni on the Cu-spin dynamics and
superconductivity have been studied in La_2-x_Sr_x_Cu_1-y_Ni_y_O_4_ with x =
0.13 changing y finely up to 0.10. Compared with the case of the nonmagnetic
impurity Zn, it has been found from the muon-spin-relaxation measurements that
a large amount of Ni is required to stabilize a magnetic order of Cu spins.
However, the evolution toward the stabilization of the magnetic order with
increasing impurity concentration is qualitatively similar to each other. The
area of the non-superconducting and slowly fluctuating or static region of Cu
spins around Ni has been found to be smaller than that around Zn, suggesting
that the pinning of rather long-ranged dynamical spin correlation such as the
so-called dynamical stripe by Ni is weaker than that by Zn. This may be the
reason why Zn destroys the superconductivity in the hole-doped high-T_c_
cuprates more markedly than Ni.",0401566v2
2006-01-28,Electric readout of magnetization dynamics in a ferromagnet-semiconductor system,"We apply an analysis of time-dependent spin-polarized current in a
semiconductor channel at room temperature to establish how the magnetization
configuration and dynamics of three ferromagnetic terminals, two of them biased
and third connected to a capacitor, affect the currents and voltages. In a
steady state, the voltage on the capacitor is related to spin accumulation in
the channel. When the magnetization of one of the terminals is rotated, a
transient current is triggered. This effect can be used for electrical
detection of magnetization reversal dynamics of an electrode or for dynamical
readout of the alignment of two magnetic contacts.",0601642v2
2006-09-18,General Form of Magnetization Damping: Magnetization dynamics of a spin system evolving nonadiabatically and out of equilibrium,"Using an effective Hamiltonian including the Zeeman and internal
interactions, we describe the quantum theory of magnetization dynamics when the
spin system evolves non-adiabatically and out of equilibrium. The
Lewis-Riesenfeld dynamical invariant method is employed along with the
Liouville-von Neumann equation for the density matrix. We derive a dynamical
equation for magnetization defined with respect to the density operator with a
general form of magnetization damping that involves the non-equilibrium
contribution in addition to the Landau-Lifshitz-Gilbert equation. Two special
cases of the radiation-spin interaction and the spin-spin exchange interaction
are considered. For the radiation-spin interaction, the damping term is shown
to be of the Gilbert type, while in the spin-spin exchange interaction case the
results depend on a coupled chain of correlation functions.",0609431v2
2006-02-22,Dynamical mass generation in strongly coupled Quantum Electrodynamics with weak magnetic fields,"We study the dynamical generation of masses for fundamental fermions in
quenched quantum electrodynamics in the presence of weak magnetic fields using
Schwinger-Dyson equations. Contrary to the case where the magnetic field is
strong, in the weak field limit the coupling should exceed certain critical
value in order for the generation of masses to take place, just as in the case
where no magnetic field is present. The weak field limit is defined as eB <<
m(0)^2, where m(0) is the value of the dynamically generated mass in the
absence of the field. We carry out a numerical analysis to study the magnetic
field dependence of the mass function above critical coupling and show that in
this regime the dynamically generated mass and the chiral condensate for the
lowest Landau level increase proportionally to (eB)^2.",0602209v2
2002-07-28,Generation of multiscale magnetic field by parity-invariant time-periodic flows,"We study generation of magnetic fields involving large spatial scales by
time- and space-periodic small-scale parity-invariant flows. The anisotropic
magnetic eddy diffusivity tensor is calculated by the standard procedure
involving expansion of magnetic modes and their growth rates in power series in
the scale ratio. Our simulations, conducted for flows with random harmonic
composition and exponentially decaying energy spectra, demonstrate that
enlargement of the spatial scale of magnetic field is beneficial for generation
by time-periodic flows. However, they turn out, in general, to be less
efficient dynamos, than steady flows.",0207112v3
2009-12-22,Billiards in magnetic fields: A molecular dynamics approach,"We present a computational scheme based on classical molecular dynamics to
study chaotic billiards in static external magnetic fields. The method allows
to treat arbitrary geometries and several interacting particles. We test the
scheme for rectangular single-particle billiards in magnetic fields and find a
sequence of regularity islands at integer aspect ratios. In the case of two
Coulomb-interacting particles the dynamics is dominated by chaotic behavior.
However, signatures of quasiperiodicity can be identified at weak interactions,
as well as regular trajectories at strong magnetic fields. Our scheme provides
a promising tool to monitor the classical limit of many-electron semiconductor
nanostructures and transport systems up to high magnetic fields.",0912.4336v1
2010-01-06,Critical properties of Sudden Quench Dynamics in the anisotropic XY Model,"We study the zero temperature quantum dynamical critical behavior of the
anisotropic XY chain under a sudden quench in a transverse field. We
demonstrate theoretically that both quench magnetic susceptibility and
two-particle quench correlation can be used to describe the dynamical quantum
phase transition (QPT) properties. Either the quench magnetic susceptibility or
the derivative of correlation functions as a function of initial magnetic field
$a$ exhibits a divergence at the critical points when final magnetic field $b$
is fixed. A special case that final magnetic field $b$ is just at the critical
point is discussed separately. Some of the critical exponents of the dynamical
QPT are obtained and the long-range correlation of the quench system is
analyzed. We also compare our result with that of the static QPT.",1001.0909v3
2011-11-21,Electron-phonon scattering dynamics in ferromagnets on ultrafast timescales: Influence of the phonon temperature,"The magnetization response of bulk ferromagnets after excitation by an
ultrashort optical pulse is calculated using a dynamical model of the
Elliott-Yafet type that includes the effects of the spin-orbit interaction in
the ab-initio ferromagnetic band structure, the electron-phonon interaction at
the level of Boltzmann scattering integrals, and dynamical changes in the
temperature of the phonon bath. Using realistic parameters for the ultrashort
optical pulse, the computed maximum magnetization quenching achievable with
electron-phonon scattering in a fixed band structure is much smaller than the
quenching observed in experiments. Heating of the phonon bath is found to not
appreciably change the magnetization dynamics on ultrashort timescales.",1111.4783v1
2011-12-26,Thin-Film Magnetization Dynamics on the Surface of a Topological Insulator,"We theoretically study the magnetization dynamics of a thin ferromagnetic
film exchange-coupled with a surface of a strong three-dimensional topological
insulator. We focus on the role of electronic zero modes imprinted by domain
walls (DW's) or other topological textures in the magnetic film.
Thermodynamically reciprocal hydrodynamic equations of motion are derived for
the DW responding to electronic spin torques, on the one hand, and fictitious
electromotive forces in the electronic chiral mode fomented by the DW, on the
other. An experimental realization illustrating this physics is proposed based
on a ferromagnetic strip, which cuts the topological insulator surface into two
gapless regions. In the presence of a ferromagnetic DW, a chiral mode
transverse to the magnetic strip acts as a dissipative interconnect, which is
itself a dynamic object that controls (and, inversely, responds to) the
magnetization dynamics.",1112.5884v2
2012-10-08,Many-body dynamics of p-wave Feshbach molecule production: a mean-field approach,"We study the mean-field dynamics of p-wave Feshbach molecule production in an
ultra cold gas of Fermi atoms in the same internal state. We derive a separable
potential to describe the low-energy scattering properties of such atoms, and
use this potential to solve the mean-field dynamics during a magnetic field
sweep. Initially, on the negative scattering length side of a Feshbach
resonance the gas is described by the BCS theory. We adapt the method by
Szyma\'{n}ska et al. [Phys. Rev. Lett. 94, 170402 (2005)] to p-wave interacting
Fermi gases and model the conversion dynamics of the gas into a Bose-Einstein
condensate of molecules on the other side of the resonance under the influence
of a linearly varying magnetic field. We have analyzed the dependence of the
molecule production efficiency on the density of the gas, temperature, initial
value of the magnetic field, and magnetic field ramp speed. Our results show
that in this approximation molecule production by a linear magnetic field sweep
is highly dependent on the initial state.",1210.2317v2
2012-12-12,Magnetic moment type of lifting from particle dynamics to Vlasov-Maxwell dynamics,"Techniques for coordinate changes that depend on both dependent and
independent variables are developed and applied to the Maxwell-Vlasov
Hamiltonian theory. Particle coordinate changes with a new velocity variable
dependent on the magnetic field, with spatial coordinates unchanged, are lifted
to transform the noncanonical Poisson bracket and, thus, the field Hamiltonian
structure of the Vlasov-Maxwell equation. Several examples are given including
magnetic coordinates, where the velocity is decomposed into components parallel
and perpendicular to the local magnetic field, and the case of spherical
velocity coordinates. An example of the lifting procedure is performed to
obtain a simplified version of gyrokinetics, where the magnetic moment is used
as a coordinate and the dynamics is reduced by elimination of the electric
field energy in the Hamiltonian.",1212.3007v1
2013-05-29,Hydrodynamic theory of coupled current and magnetization dynamics in spin-textured antiferromagnets,"Antiferromagnets with vanishingly small (or zero) magnetization are
interesting candidates for spintronics applications. In the present paper we
propose two models for description of the current-induced phenomena in
antiferromagnetic textures. We show that the magnetization that originates from
rotation or oscillations of antiferromagnetic vector can, via $sd$-exchange
coupling, polarize the current and give rise to adiabatic and nonadiabatic spin
torques. Due to the Lorentz-type dynamics of antiferromagnetic moments (unlike
the Galilenian-like dynamics in ferromagnets), the adiabatic spin torque
affects the characteristic lengthscale of the moving texture. Nonadiabatic spin
torque contributes to the energy pumping and can induce the stable motion of
antiferromagnetic texture, but, in contrast to ferromagnets, has pure dynamic
origin. We also consider the current-induced phenomena in artificial
antiferromagnets where the current maps the staggered magnetization of the
structure. In this case the effect of nonadiabatic spin torque is similar to
that in ferromagnetic constituents of the structure. In particular, the current
can remove degeneracy of the translational antiferromagnetic domains
indistinguishable in the external magnetic field and thus can set into motion
the 180$^\circ$ domain wall.",1305.6734v2
2013-07-17,Role of helicities for the dynamics of turbulent magnetic fields,"Investigations of the inverse cascade of magnetic helicity are conducted with
pseudospectral, three-dimensional direct numerical simulations of forced and
decaying incompressible magnetohydrodynamic turbulence. The high-resolution
simulations which allow for the necessary scale-separation show that the
observed self-similar scaling behavior of magnetic helicity and related
quantities can only be understood by taking the full nonlinear interplay of
velocity and magnetic fluctuations into account. With the help of the
eddy-damped quasi-normal Markovian approximation a probably universal relation
between kinetic and magnetic helicities is derived that closely resembles the
extended definition of the prominent dynamo pseudoscalar $\alpha$. This
unexpected similarity suggests an additional nonlinear quenching mechanism of
the current-helicity contribution to $\alpha$.",1307.4603v2
2013-09-01,Quasistationarity in a long-range interacting model of particles moving on a sphere,"We consider a long-range interacting system of $N$ particles moving on a
spherical surface under an attractive Heisenberg-like interaction of infinite
range, and evolving under deterministic Hamilton dynamics. The system may also
be viewed as one of globally coupled Heisenberg spins. In equilibrium, the
system has a continuous phase transition from a low-energy magnetized phase, in
which the particles are clustered on the spherical surface, to a high-energy
homogeneous phase. The dynamical behavior of the model is studied analytically
by analyzing the Vlasov equation for the evolution of the single-particle
distribution, and numerically by direct simulations. The model is found to
exhibit long lived non-magnetized quasistationary states (QSSs) which in the
thermodynamic limit are dynamically stable within an energy range where the
equilibrium state is magnetized. For finite $N$, these states relax to
equilibrium over a time that increases algebraically with $N$. In the
dynamically unstable regime, non-magnetized states relax fast to equilibrium
over a time that scales as $\log N$. These features are retained in presence of
a global anisotropy in the magnetization.",1309.0194v2
2013-11-28,Conservative effects in spin-transfer-driven magnetization dynamics,"It is shown that under appropriate conditions spin-transfer-driven
magnetization dynamics in a single-domain nanomagnet is conservative in nature
and admits a specific integral of motion, which is reduced to the usual
magnetic energy when the spin current goes to zero. The existence of this
conservation law is connected to the symmetry properties of the dynamics under
simultaneous inversion of magnetisation and time. When one applies an external
magnetic field parallel to the spin polarization, the dynamics is transformed
from conservative into dissipative. More precisely, it is demonstrated that
there exists a state function such that the field induces a monotone relaxation
of this function toward its minima or maxima, depending on the field
orientation. These results hold in the absence of intrinsic damping effects.
When intrinsic damping is included in the description, a competition arises
between field-induced and damping-induced relaxations, which leads to the
appearance of limit cycles, that is, of magnetization self-oscillations.",1311.7344v1
2014-05-25,Spin Hall phenomenology of magnetic dynamics,"We study the role of spin-orbit interactions in the coupled magnetoelectric
dynamics of a ferromagnetic film coated with an electrical conductor. While the
main thrust of this work is phenomenological, several popular simple models are
considered microscopically in some detail, including Rashba and Dirac
two-dimensional electron gases coupled to a magnetic insulator, as well as a
diffusive spin Hall system. We focus on the long-wavelength magnetic dynamics
that experiences current-induced torques and produces fictitious electromotive
forces. Our phenomenology provides a suitable framework for analyzing
experiments on current-induced magnetic dynamics and reciprocal charge pumping,
including the effects of magnetoresistance and Gilbert-damping anisotropies,
without a need to resort to any microscopic considerations or modeling.
Finally, some remarks are made regarding the interplay of spin-orbit
interactions and magnetic textures.",1405.6354v2
2015-01-06,Berry-phase effects and electronic dynamics in noncollinear antiferromagnetic texture,"Antiferromagnets (AFMs), in contrast to ferromagnets, show a nontrivial
magnetic structure with zero net magnetization. However, they share a number of
spintronic effects with ferromagnets, including spin-pumping and spin transfer
torques. Both phenomena stem from the coupled dynamics of free carriers and
localized magnetic moments. In the present paper I study the adiabatic dynamics
of a spin-polarized electrons in a metallic AFM exhibiting a noncollinear
120$^\circ$ magnetic structure. I show that the slowly varying AFM spin texture
produces a non-Abelian gauge potential related to the time/space gradients of
the N\'{e}el vectors. Corresponding emergent electric and magnetic fields
induce rotation of spin and influence the orbital dynamics of free electrons. I
discuss both the possibility of a topological spin Hall effect in the vicinity
of topological AFM solitons with nonzero curvature and rotation of the electron
spin traveling through the AFM domain wall.",1501.01189v2
2015-03-11,Dynamical influence of vortex-antivortex pairs in magnetic vortex oscillators,"We study the magnetization dynamics in a nanocontact magnetic vortex
oscillators as function of temperature. Low temperature experiments reveal that
the dynamics at low and high currents differ qualitatively. At low currents, we
excite a temperature independent standard oscillation mode, consisting in the
gyrotropic motion of a free layer vortex about the nanocontact. Above a
critical current, a sudden jump of the frequency is observed, concomitant with
a substantial increase of the frequency versus current slope factor. Using
micromagnetic simulation and analytical modeling, we associate this new regime
to the creation of a vortex-antivortex pair in the pinned layer of the spin
valve. The vortex-antivortex distance depends on the Oersted field which favors
a separation, and on the exchange bias field, which favors pair merging. The
pair in the pinned layer provides an additional spin torque altering the
dynamics of the free layer vortex, which can be quantitatively accounted for by
an analytical model.",1503.03365v2
2015-04-11,Magnetic response of a disordered binary ferromagnetic alloy to an oscillating magnetic field,"By means of Monte Carlo simulation with local spin update Metropolis
algorithm, we have elucidated non-equilibrium phase transition properties and
stationary-state treatment of a disordered binary ferromagnetic alloy of the
type $A_{p}B_{1-p}$ on a square lattice. After a detailed analysis, we have
found that the system shows many interesting and unusual thermal and magnetic
behaviors, for instance, the locations of dynamic phase transition points
change significantly depending upon amplitude and period of the external
magnetic field as well as upon the active concentration of $A-$ type
components. Much effort has also been dedicated to clarify the hysteresis
tools, such as coercivity, dynamic loop area as well as dynamic correlations
between time dependent magnetizations and external time dependent applied field
as a functions of period and amplitude of field as well as active concentration
of of $A-$ type components, and outstanding physical findings have been
reported in order to better understand the dynamic process underlying present
system.",1504.02874v1
2015-05-09,Simulation of flux expulsion and associated dynamics in a two-dimensional magnetohydrodynamic channel flow,"We consider a plane channel flow of an electrically conducting fluid which is
driven by a mean pressure gradient in the presence of an applied magnetic field
that is streamwise periodic with zero mean. Magnetic flux expulsion and the
associated bifurcation in such a configuration is explored using direct
numerical simulations (DNS). The structure of the flow and magnetic fields in
the Hartmann regime (where the dominant balance is through Lorentz forces) and
the Poiseuille regime (where viscous effects play a significant role) are
studied and detailed comparisons to the existing one-dimensional model of
Kamkar and Moffatt (J. Fluid. Mech., Vol.90, pp 107-122, 1982) are drawn to
evaluate the validity of the model. Comparisons show good agreement of the
model with DNS in the Hartmann regime, but significant diferences arising in
the Poiseuille regime when non-linear effects become important. The effects of
various parameters like the magnetic Reynolds number, imposed field wavenumber
etc. on the bifurcation of the flow are studied. Magnetic field line
reconnections occuring during the dynamic runaway reveal a specific two-step
pattern that leads to the gradual expulsion of flux in the core region.",1505.02313v1
2016-03-25,The influence of the structural transition on magnetic fluctuations in NaFeAs,"NaFeAs belongs to a class of Fe-based superconductors which parent compounds
show separated structural and magnetic transitions. Effects of the structural
transition on spin dynamics therefore can be investigated separately from the
magnetic transition. A plateau in dynamic spin response is observed in a
critical region around the structural transition temperature T_S. It is
interpreted as due to the stiffening of spin fluctuations along the in-plane
magnetic hard axis due to the dxz and dyz orbital ordering. The appearance of
anisotropic spin dynamics in the critical region above the T_S at T* offers a
dynamic magnetic scattering mechanism for anisotropic electronic properties in
the commonly referred ""nematic phase"".",1603.07873v2
2016-08-24,Dynamics of small trapped one-dimensional Fermi gas under oscillating magnetic fields,"Deterministic preparation of an ultracold harmonically trapped
one-dimensional Fermi gas consisting of a few fermions has been realized by the
Heidelberg group. Using Floquet formalism, we study the time dynamics of two-
and three-fermion systems in a harmonic trap under an oscillating magnetic
field. The oscillating magnetic field produces a time-dependent interaction
strength through a Feshbach resonance. We explore the dependence of these
dynamics on the frequency of the oscillating magnetic field for
non-interacting, weakly interacting, and strongly interacting systems. We
identify the regimes where the system can be described by an effective
two-state model and an effective three-state model. We find an unbounded
coupling to all excited states at the infinitely strong interaction limit and
several simple relations that characterize the dynamics. Based on our findings,
we propose a technique for driving transition from the ground state to the
excited states using an oscillating magnetic field.",1608.06966v2
2017-04-15,Cross effect of magnetic field and charge current on antiferromagnetic dynamics,"We theoretically examine a cross effect of magnetic field and charge current
on antiferromagnetic domain wall dynamics. Since antiferromagnetic materials
are largely insensitive to external magnetic fields in general, charge current
has been shown recently as an alternative and efficient way to manipulate
antiferromagnets. We find a new role of the magnetic field in the
antiferromagnetic dynamics that appears when it is combined with charge
current, demonstrating a domain wall motion in the presence of both field and
current. We show that a spatially-varying magnetic field can shift the
current-driven domain-wall velocity, depending on the domain-wall structure and
the direction of the field-gradient. Our result suggests a novel concept of
field-control of current-driven antiferromagnetic dynamics.",1704.04602v1
2017-05-09,Topological spin dynamics in cubic FeGe near room temperature,"Understanding spin-wave dynamics in chiral magnets is a key step for the
development of high-speed, spin-wave based spintronic devices that take
advantage of chiral and topological spin textures for their operation. Here we
present an experimental and theoretical study of spin-wave dynamics in a cubic
B20 FeGe single crystal. Using the combination of waveguide microwave
absorption spectroscopy (MAS), micromagnetic simulations, and analytical
theory, we identify the resonance dynamics in all magnetic phases (field
polarized, conical, helical, and skyrmion phases). Because the resonance
frequencies of specific chiral spin textures are unique, quantitative agreement
between our theoretical predictions and experimental findings for all resonance
frequencies and spin wave modes enables us to unambiguously identify chiral
magnetic phases and to demonstrate that MAS is a powerful tool to efficiently
extract a magnetic phase diagram. These results provide a new tool to
accelerate the integration of chiral magnetic materials into spintronic
devices.",1705.03397v1
2018-01-26,Dynamics of magnetic skyrmion clusters driven by spin-polarized current with a spatially varied polarization,"Magnetic skyrmions are promising candidates for future information
technology. Here, we present a micromagnetic study of isolated skyrmions and
skyrmion clusters in ferromagnetic nanodisks driven by the spin-polarized
current with spatially varied polarization. The current-driven skyrmion
clusters can be either dynamic steady or static, depending on the spatially
varied polarization profile. For the dynamic steady state, the skyrmion cluster
moves in a circle in the nanodisk, while for the static state, the skyrmion
cluster is static. The frequency of the circular motion of skyrmion is also
studied. Furthermore, the dependence of the skyrmion cluster dynamics on the
magnetic anisotropy and Dzyaloshinskii-Moriya interaction is investigated. Our
results may provide a pathway to realize magnetic skyrmion cluster based
devices.",1801.08699v3
2018-04-04,Velocity enhancement by synchronization of magnetic domain walls,"Magnetic domain walls are objects whose dynamics is inseparably connected to
their structure. In this work we investigate magnetic bilayers, which are
engineered such that a coupled pair of domain walls, one in each layer, is
stabilized by a cooperation of Dzyaloshinskii-Moriya interaction and
flux-closing mechanism. The dipolar field mediating the interaction between the
two domain walls, links not only their position but also their structure. We
show that this link has a direct impact on their magnetic field induced
dynamics. We demonstrate that in such a system the coupling leads to an
increased domain wall velocity with respect to single domain walls. Since the
domain wall dynamics is observed in a precessional regime, the dynamics
involves the synchronization between the two walls, to preserve the flux
closure during motion. Properties of these coupled oscillating walls can be
tuned by an additional in-plane magnetic field enabling a rich variety of
states, from perfect synchronization to complete detuning.",1804.01385v1
2018-04-23,Probing condensed matter physics with magnetometry based on nitrogen-vacancy centres in diamond,"The magnetic fields generated by spins and currents provide a unique window
into the physics of correlated-electron materials and devices. Proposed only a
decade ago, magnetometry based on the electron spin of nitrogen-vacancy (NV)
defects in diamond is emerging as a platform that is excellently suited for
probing condensed matter systems: it can be operated from cryogenic
temperatures to above room temperature, has a dynamic range spanning from DC to
GHz, and allows sensor-sample distances as small as a few nanometres. As such,
NV magnetometry provides access to static and dynamic magnetic and electronic
phenomena with nanoscale spatial resolution. Pioneering work focused on
proof-of-principle demonstrations of its nanoscale imaging resolution and
magnetic field sensitivity. Now, experiments are starting to probe the
correlated-electron physics of magnets and superconductors and to explore the
current distributions in low-dimensional materials. In this Review, we discuss
the application of NV magnetometry to the exploration of condensed matter
physics, focusing on its use to study static and dynamic magnetic textures, and
static and dynamic current distributions.",1804.08742v1
2018-12-17,Phonon-induced quantum ratchet in the exciton spin dynamics in diluted magnetic semiconductors in a magnetic field,"Magnetically doped semiconductors are well known for their giant Zeeman
splittings which can reach several meV even in relatively small external
magnetic fields. After preparing a nonequilibrium exciton distribution via
optical excitation, the spin dynamics in diluted magnetic semiconductor quantum
wells is typically governed by spin-flip scattering processes due to the
exciton-impurity exchange interaction. Our theoretical calculations show that
the giant Zeeman splitting in these materials in combination with the influence
of longitudinal acoustic phonons lead to a quantum ratchet-type dynamics,
resulting in an almost complete reversal of the carrier spin polarization at
very low temperatures. Furthermore, we find that the predictions of a much
simpler rate-equation approach qualitatively agree with a more advanced and
numerically demanding quantum kinetic description of the spin dynamics for a
wide range of temperatures, although quantitative difference are noticeable.",1812.06734v2
2019-10-11,Hannay Angles in Magnetic Dynamics,"We consider, within the framework developed by Hannay for classical
integrable systems [Journal of Physics A: Mathematical and General {\bf 18},
221 (1985)], the geometric phases that occur in semi-classical magnetic
dynamics. Such geometric phases are generically referred to as Hannay angles,
and, in the context of magnetic dynamics, may arise as a result of both
adiabatically-varying ellipticity and axis of magnetization precession. We
elucidate both effects and their interplay for single-domain magnetic dynamics
within a simple model with time-dependent anisotropies and external field.
Subsequently, we consider spin waves and rederive, from our classical approach,
some known results on what is commonly referred to as the magnon Berry phase.
As an aside, these results are used to give an interpretation for geometric
phases that occur in superfluids. Finally, we develop a Green's function
formalism for elliptical magnons. Within this formalism, we consider magnon
transport in a mesoscopic ring and show how it is influenced by interference
effects that are tuned by the Hannay angle that results from a varying
ellipticity. Our results may inform the field of magnonics that seeks to
utilize spin waves in applications.",1910.05099v2
2020-06-23,Magnetization dynamics in synthetic antiferromagnets: Role of dynamical energy and mutual spin pumping,"We investigate magnetization dynamics in asymmetric interlayer exchange
coupled Py/Ru/Py trilayers using both vector network analyzer-based and
electrically detected ferromagnetic resonance techniques. Two different
ferromagnetic resonance modes, in-phase and out-of-phase, are observed across
all three regimes of the static magnetization configurations, through
antiparallel alignment at low fields, the spin-flop transition at intermediate
fields, and parallel alignment at high fields. The nonmonotonic behavior of the
modes as a function of the external field is explained in detail by analyzing
the interlayer exchange and Zeeman energies and is found to be solely governed
by the interplay of their dynamical components. In addition, the linewidths of
both modes were determined across the three regimes and the different behaviors
of the linewidths versus external magnetic field are attributed to mutual spin
pumping induced in the samples. Interestingly, the difference between the
linewidths of the out-of-phase and in-phase modes decreases at the spin-flop
transition and is reversed between the antiparallel and parallel aligned
magnetization states.",2006.13031v1
2020-12-24,Dynamical correlation enhanced orbital magnetization in VI$_{3}$,"The effect of electronic correlations on the orbital magnetization in real
materials has not been explored beyond a static mean-field level. Based on the
dynamical mean-field theory, the effect of electronic correlations on the
orbital magnetization in layered ferromagnet VI$_3$ has been studied. A
comparison drawn with the results obtained from density functional theory
calculations robustly establishes the crucial role of dynamical correlations in
this case. In contrast to the density functional theory that leads to
negligible orbital magnetization in VI$_3$, in dynamical mean-field approach
the orbital magnetization is greatly enhanced. Further analysis show that this
enhancement is mainly due to the enhanced local circulations of electrons,
which can be attributed to a better description of the localization behavior of
correlated electrons in VI$_3$. The conclusion drawn in our study could be
applicable to a wide range of layered materials in this class.",2012.13151v1
2021-07-21,Dynamics of two ferromagnetic insulators coupled by superconducting spin current,"A conventional superconductor sandwiched between two ferromagnets can
maintain coherent equilibrium spin current. This spin supercurrent results from
the rotation of odd-frequency spin correlations induced in the superconductor
by the magnetic proximity effect. In the absence of intrinsic magnetization,
the superconductor cannot maintain multiple rotations of the triplet component
but instead provides a Josephson type weak link for the spin supercurrent. We
determine the analogue of the current-phase relation in various circumstances
and show how it can be accessed in experiments on dynamic magnetization. In
particular, concentrating on the magnetic hysteresis and the ferromagnetic
resonance response, we show how the spin supercurrent affects the
nonequilibrium dynamics of magnetization which depends on a competition between
spin supercurrent mediated static exchange contribution and a dynamic spin
pumping contribution. Depending on the outcome of this competition, a mode
crossing in the system can either be an avoided crossing or mode locking.",2107.09959v2
2021-10-07,Ultrafast element- and depth-resolved magnetization dynamics probed by transverse magneto-optical Kerr effect spectroscopy in the soft x-ray range,"We report on time- and angle-resolved transverse magneto-optical Kerr effect
spectroscopy in the soft x-ray range that, by analysis via
polarization-dependent magnetic scattering simulations, allows us to determine
the spatio-temporal and element-specific evolution of femtosecond laser-induced
spin dynamics in nanostructured magnetic materials. In a ferrimagnetic GdFe
thin film system, we correlate a reshaping spectrum of the magneto-optical Kerr
signal to depth-dependent magnetization dynamics and disentangle contributions
due to non-equilibrium electron transport and nanoscale heat diffusion on their
intrinsic time scales. Our work provides a quantitative insight into
light-driven spin dynamics occurring at buried interfaces of complex magnetic
heterostructures, which can be tailored and functionalized for future
opto-spintronic devices.",2110.03599v2
2022-03-15,Standing spin wave excitation in Bi:YIG films via temperature induced anisotropy changes and magnetoacoustic coupling,"Based on micromagnetic simulations and experimental observations of the
magnetization and lattice dynamics following the direct optical excitation of
the magnetic insulator Bi:YIG or indirect excitation via an optically opaque
Pt/Cu double layer, we disentangle the dynamical effects of magnetic anisotropy
and magnetoelastic coupling. The strain and temperature of the lattice are
quantified via modeling ultrafast x-ray diffraction data. Measurements of the
time-resolved magneto-optical Kerr effect agree well with the magnetization
dynamics simulated according to the excitation via two mechanisms: The
magneto-acoustic coupling to the experimentally verified strain dynamics and
the ultrafast temperature-induced transient change in the magnetic anisotropy.
The numerical modeling proves that for direct excitation both mechanisms drive
the fundamental mode with opposite phase. The relative ratio of standing
spin-wave amplitudes of higher order modes indicates that both mechanisms are
substantially active.",2203.07851v1
2022-03-17,A 3D BEM algorithm for simulations of magnetic fluid droplet dynamics,"This paper outlines a numerical algorithm that could be used for simulating
full 3D dynamics of magnetic fluid droplet shapes in external magnetic fields,
by solving boundary integral equations. The algorithm works with arbitrary
droplet and carrier fluid viscosity ratios. It is validated with known
theoretical relationships. Thus it may be used to evaluate various
approximations often used in description of ellipsoidal droplets by comparing
droplet dynamics calculated from them to the results obtained numerically from
first principles here. The algorithm may be used for investigations of droplet
configurations in arbitrary magnetic fields, as well as for indirectly
calculating its physical properties and predicting the magnetic field
thresholds above which droplet shape can develop instabilities in the form of
various spikes.",2203.09499v1
2022-03-26,Nonequilibrium dynamics in a spin valve with noncollinear magnetization,"We utilize a hybrid quantum-classical equation of motion approach to
investigate the spin dynamics and spin-transfer torque in a spin valve under
bias voltage. We show that the interplay between localized classical magnetic
moments and conduction electrons induces a complex effective exchange coupling
between the magnetic layers. This leads to a declination of magnetizations from
layers anisotropy axes even in equilibrium. Introducing a finite bias voltage
triggers spin currents and related spin-transfer torques which further tilt the
magnetizations and govern the relaxation processes of the spin dynamics.
Analyzing different scenarios of the applied bias voltage, we show that
symmetric and asymmetric voltage drops can lead to relaxation times of the spin
dynamics that differ by several orders of magnitude at comparable charge
currents. In both cases we observe resonant features, where the relaxation is
boosted whenever the chemical potential of the leads matches the maxima in the
density of the states of the spin-valve electrons.",2203.14068v2
2023-01-04,Role of ionic surfactant in magnetic dynamics of self-assembled dispersions of nanoplatelets,"In complex colloidal systems, interparticle interactions strongly affect the
dynamics of the constituting particles. A study of the dynamical response also
provides invaluable information on the character of those interactions. Here we
demonstrate how tuning the electrostatic interactions by an ionic surfactant in
dispersions of magnetic nanoplatelets leads to developing new dynamic modes in
magnetic response spectra. The collective modes can be induced or suppressed by
either varying the concentration ratio of the magnetic nanoplatelets (MP) to
the surfactant or increasing the MP concentration reflecting the nanoscale
characteristics of this fluid magnet.",2301.01652v1
2023-07-11,Quantum dynamic response-based NV-diamond magnetometry: Robustness to decoherence and applications in motion detection of magnetic nanoparticles,"We propose a novel quantum sensing protocol that leverages the dynamical
response of physical observables to quenches in quantum systems. Specifically,
we use the nitrogen-vacancy (NV) color center in diamond to realize both scalar
and vector magnetometry via quantum response. Furthermore, we suggest a method
for detecting the motion of magnetic nanoparticles, which is challenging with
conventional interference-based sensors. To achieve this, we derive the closed
exact form of the Berry curvature corresponding to NV centers and design
quenching protocols to extract the Berry curvature via dynamical response. By
constructing and solving non-linear equations, the magnetic field and
instantaneous motion velocity of the magnetic nanoparticle can be deduced. We
investigate the feasibility of our sensing scheme in the presence of
decoherence and show through numerical simulations that it is robust to
decoherence. Intriguingly, we have observed that a vanishing nuclear spin
polarization in diamond actually benefits our dynamic sensing scheme, which
stands in contrast to conventional Ramsey-based schemes. In comparison to
Ramsey-based sensing schemes, our proposed scheme can sense an arbitrary
time-dependent magnetic field, as long as its time dependence is nearly
adiabatic.",2307.05255v1
2023-09-26,Orbital Pumping by Magnetization Dynamics in Ferromagnets,"We show that dynamics of the magnetization in ferromagnets can pump the
orbital angular momentum, which we denote by orbital pumping. This is the
reciprocal phenomenon to the orbital torque that induces magnetization dynamics
by the orbital angular momentum in non-equilibrium. The orbital pumping is
analogous to the spin pumping established in spintronics but requires the
spin-orbit coupling for the orbital angular momentum to interact with the
magnetization. We develop a formalism that describes the generation of the
orbital angular momentum by magnetization dynamics within the adiabatic
perturbation theory. Based on this, we perform first-principles calculation of
the orbital pumping in prototypical $3d$ ferromagnets, Fe, Co, and Ni. The
results show that the ratio between the orbital pumping and the spin pumping
ranges from 5 to 15 percents, being smallest in Fe and largest in Ni. This
implies that ferromagnetic Ni is a good candidate for measuring the orbital
pumping. Implications of our results on experiments are also discussed.",2309.14817v2
2023-10-13,Unified framework of the microscopic Landau-Lifshitz-Gilbert equation and its application to Skyrmion dynamics,"The Landau-Lifshitz-Gilbert (LLG) equation is widely used to describe
magnetization dynamics. We develop a unified framework of the microscopic LLG
equation based on the nonequilibrium Green's function formalism. We present a
unified treatment for expressing the microscopic LLG equation in several
limiting cases, including the adiabatic, inertial, and nonadiabatic limits with
respect to the precession frequency for a magnetization with fixed magnitude,
as well as the spatial adiabatic limit for the magnetization with slow
variation in both its magnitude and direction. The coefficients of those terms
in the microscopic LLG equation are explicitly expressed in terms of
nonequilibrium Green's functions. As a concrete example, this microscopic
theory is applied to simulate the dynamics of a magnetic Skyrmion driven by
quantum parametric pumping. Our work provides a practical formalism of the
microscopic LLG equation for exploring magnetization dynamics.",2310.08807v1
2023-12-14,Passive dynamical decoupling of trapped ion qubits and qudits,"We propose a method to dynamically decouple every magnetically sensitive
hyperfine sublevel of a trapped ion from magnetic field noise, simultaneously,
using integrated circuits to adiabatically rotate its local quantization field.
These integrated circuits allow passive adjustment of the effective
polarization of any external (control or noise) field. By rotating the ion's
quantization direction relative to this field's polarization, we can perform
`passive' dynamical decoupling (PDD), inverting the linear Zeeman sensitivity
of every hyperfine sublevel. This dynamically decouples the entire ion, rather
than just a qubit subspace. Fundamentally, PDD drives the transition
$m_{F}\rightarrow -m_{F}$ for every magnetic quantum number $m_{F}$ in the
system--with only one operation--indicating it applies to qudits with constant
overhead in the dimensionality of the qudit. We show how to perform pulsed and
continuous PDD, weighing each technique's insensitivity to external magnetic
fields versus their sensitivity to diabaticity and control errors. Finally, we
show that we can tune the sinusoidal oscillation of the quantization axis to a
motional mode of the crystal in order to perform a laser-free two qubit gate
that is insensitive to magnetic field noise.",2312.09399v1
2017-05-25,Spin-orbit torque-driven skyrmion dynamics revealed by time-resolved X-ray microscopy,"Magnetic skyrmions are topologically-protected spin textures with attractive
properties suitable for high-density and low-power spintronic device
applications. Much effort has been dedicated to understanding the dynamical
behaviours of the magnetic skyrmions. However, experimental observation of the
ultrafast dynamics of this chiral magnetic texture in real space, which is the
hallmark of its quasiparticle nature, has so far remained elusive. Here, we
report nanosecond-dynamics of a 100 nm-size magnetic skyrmion during a current
pulse application, using a time-resolved pump-probe soft X-ray imaging
technique. We demonstrate that distinct dynamic excitation states of magnetic
skyrmions, triggered by current-induced spin-orbit torques, can be reliably
tuned by changing the magnitude of spin-orbit torques. Our findings show that
the dynamics of magnetic skyrmions can be controlled by the spin-orbit torque
on the nanosecond time scale, which points to exciting opportunities for
ultrafast and novel skyrmionic applications in the future.",1705.09019v1
2019-06-28,Dynamics of groups of magnetically driven artificial microswimmers,"Magnetically driven artificial microswimmers have the potential to
revolutionize many biomedical technologies, such as minimally-invasive
microsurgery, micro-particle manipulation, and localized drug delivery.
However, many of these applications will require the controlled dynamics of
teams of these micro-robots with minimal feedback. In this work, we study the
motion and fluid dynamics produced by groups of artificial microswimmers driven
by a torque induced through a uniform, rotating magnetic field. Through
Stokesian dynamics simulations, we show that the swimmer motion produces a
rotational velocity field in the plane orthogonal to the direction of the
magnetic field's rotation, which causes two interacting swimmers to move in
circular trajectories in this plane around a common center. The resulting over
all motion is on a helical trajectory for the swimmers. We compare the highly
rotational velocity field of the fluid to the velocity field generated by a
rotlet, the point-torque singularity of Stokes flows, showing that this is a
reasonable approximation on the time average. Finally, we study the motion of
larger groups of swimmers and show that these groups tend to move coherently,
especially when swimmer magnetizations are uniform. This coherence is achieved
because the group center remains almost constant in the plane orthogonal to the
net motion of the swimmers. The results in the paper will prove useful for
controlling the ensemble dynamics of small collections of magnetic swimmers.",1906.12049v2
2022-07-02,Robust magnetic order upon ultrafast excitation of an antiferromagnet,"The ultrafast manipulation of magnetic order due to optical excitation is
governed by the intricate flow of energy and momentum between the electron,
lattice and spin subsystems. While various models are commonly employed to
describe these dynamics, a prominent example being the microscopic three
temperature model (M3TM), systematic, quantitative comparisons to both the
dynamics of energy flow and magnetic order are scarce. Here, we apply a M3TM to
the ultrafast magnetic order dynamics of the layered antiferromagnet
GdRh$_2$Si$_2$. The femtosecond dynamics of electronic temperature, surface
ferromagnetic order, and bulk antiferromagnetic order were explored at various
pump fluences employing time- and angle-resolved photoemission spectroscopy and
time-resolved resonant magnetic soft x-ray diffraction, respectively. After
optical excitation, both the surface ferromagnetic order and the bulk
antiferromagnetic order dynamics exhibit two-step demagnetization behaviors
with two similar timescales (<1 ps, ~10 ps), indicating a strong exchange
coupling between localized 4f and itinerant conduction electrons. Despite a
good qualitative agreement, the M3TM predicts larger demagnetization than our
experimental observation, which can be phenomenologically described by a
transient, fluence-dependent increased N\'eel temperature. Our results indicate
that effects beyond a mean-field description have to be considered for a
quantitative description of ultrafast magnetic order dynamics.",2207.00789v1
2024-03-29,Theory of the inverse Faraday effect in dissipative Rashba electron systems: Floquet engineering perspective,"We theoretically study the inverse Faraday effect (IFE), i.e., photo-induced
magnetization, in two-dimensional Rashba spin-orbit coupled electron systems
irradiated by a circularly polarized light. Quantum master (GKSL) equation
enables us to accurately compute the laser driven dynamics, taking inevitable
dissipation effects into account. To find the universal features of
laser-driven magnetization and its dynamics, we investigate (i) the
nonequilibrium steady state (NESS) driven by a continuous wave (CW) and (ii)
ultrafast spin dynamics driven by short laser pulses. In the NESS (i), the
laser-induced magnetization and its dependence of several parameters (laser
frequency, laser field strength, temperature, dissipation strength, etc.) are
shown to be in good agreement with the predictions from Floquet theory for
dissipative systems in the high-frequency regime. In the case (ii), we focus on
ferromagnetic metal states by introducing an effective magnetic field to the
Rashba model as the mean field of electron-electron interaction. We find that a
precession of the magnetic moment occurs due to the pulse-driven instantaneous
magnetic field and the initial phase of the precession is controlled by
changing the sign of light polarization. This is well consistent with the spin
dynamics observed in experiments of laser-pulse-driven IFE. We discuss how the
pulse-driven dynamics are captured by the Floquet theory. Our results provides
a microscopic method to compute ultrafast dynamics in many electron systems
irradiated by intense light.",2403.20110v1
2023-07-16,Emergent electric field from magnetic resonances in a one-dimensional chiral magnet,"The emergent electric field (EEF) is a fictitious electric field acting on
conduction electrons through the Berry phase mechanism. The EEF is generated by
the dynamics of noncollinear spin configurations and becomes nonzero even in
one dimension. Although the EEF has been studied for several one-dimensional
chiral magnets, most of the theoretical studies were limited with respect to
the strength and direction of the magnetic fields. Furthermore, the effect of
edges of the system has not been clarified, whereas it can be crucial in nano-
and micro-scale samples. Here, we perform a theoretical study on the
momentum-frequency profile of the EEF in a one-dimensional chiral magnet while
changing the strength and direction of the magnetic field for both bulk and
finite-size chains with edges. As the bulk contributions, we find that the EEF
is resonantly enhanced at the magnetic resonance frequencies; interestingly,
the higher resonance modes are more clearly visible in the EEF response than in
the magnetic one. Furthermore, we show that the EEF is amplified along with the
solitonic feature of the spin texture introduced by the static magnetic field
perpendicular to the chiral axis. We also show that the static magnetic field
parallel to the chiral axis drives the EEF in the field direction, in addition
to much slower drift motion in the opposite direction associated with the
Archimedean screw dynamics, suggesting a DC electric current generation. As the
edge contributions, we find additional resonance modes localized at the edges
of the system that are also more clearly visible in the EEF response than the
magnetic one. Our results reveal that the emergent electric phenomena in
one-dimensional chiral magnets can be tuned by the magnetic field and the
sample size, and provide not only a good probe of the magnetic resonances but
also a platform for the applications to electronic devices.",2307.08017v1
2022-02-09,Magnetic helicity and energy of emerging solar active regions and their erruptivity,"Aims. We investigate the role of the accumulation of both magnetic helicity
and magnetic energy in the generation of coronal mass ejections (CMEs) from
emerging solar active regions (ARs). Methods. Using vector magnetic field data
obtained by the Helioseismic and Magnetic Imager on board the Solar Dynamics
Observatory, we calculate the magnetic helicity and magnetic energy injection
rates as well as the resulting accumulated budgets in 52 emerging ARs from the
start time of magnetic flux emergence until they reach heliographic longitude
of 45$^{\circ}$ West (W45). Results. Seven of the ARs produced CMEs while 45
did not. In a statistical sense, the eruptive ARs accumulate larger budgets of
both magnetic helicity and energy than the noneruptive ones over intervals that
start from flux emergence start time and end (i) at the end of flux emergence
phase, and (ii) when the AR produces its first CME or crosses W45, whichever
happens first. We found magnetic helicity and energy thresholds of $9 \times
10^{41}$ Mx$^2$ and $2 \times 10^{32}$ erg, respectively, which, if crossed,
ARs are likely to erupt. The segregation, in terms of accumulated magnetic
helicity and energy budgets, of the eruptive ARs from the noneruptive ones is
violated in one case when an AR erupts early in its emergence phase and in six
cases with noneruptive ARs exhibiting large magnetic helicity and energy
budgets. Decay index calculations may indicate that these ARs did not erupt
because the overlying magnetic field provided stronger or more extended
confinement than in eruptive ARs. Conclusions. Our results indicate that
emerging ARs tend to produce CMEs when they accumulate significant budgets of
both magnetic helicity and energy. Any study of their eruptive potential should
place magnetic helicity on equal footing with magnetic energy.",2202.04353v2
2022-04-22,Nuclear spin self compensation system for moving MEG sensing with optical pumped atomic spin co-magnetometer,"Recording the moving MEGs of a person in which a person's head could move
freely as we record the brain's magnetic field is a hot topic in recent years.
Traditionally, atomic magnetometers are utilized for moving MEGs recording and
a large compensation coil system is utilized for background magnetic field
compensation. Here we described a new potential candidate: an optically pumped
atomic co-magnetometer(OPACM) for moving MEGs recording. In the OPACM,
hyper-polarized nuclear spins could produce a magnetic field which will shield
the background fluctuation low frequency magnetic field noise while the the
fast changing MEGs signal could be recorded. The nuclear spins look like an
automatic magnetic field shields and dynamically compensate the fluctuated
background magnetic field noise. In this article, the magnetic field
compensation is studied theoretically and we find that the compensation is
closely related to several parameters such as the electron spin magnetic field,
the nuclear spin magnetic field and the holding magnetic field. Based on the
model, the magnetic field compensation could be optimized. We also
experimentally studied the magnetic field compensation and the responses of the
OPACM to different frequencies of magnetic field are measured. We show that the
OPACM owns a clear suppression of low frequency magnetic field under 1Hz and
response to magnetic field's frequencies around the band of the MEGs. Magnetic
field sensitivity of $3fT/Hz^{1/2}$ has been achieved. Finally, we do a
simulation for the OPACM as it is utilized for moving MEGs recording. For
comparison, the traditional compensation system for moving MEGs recording is
based on a coil which is around 2m in dimension while our compensation system
is only 2mm in dimension. Moreover, our compensation system could work in situ
and will not affect each other.",2204.10481v1
1993-08-13,Escape of trapped electrons from a helium surface: a dynamical theory,"We present a dynamical theory which incorporates the electron-electron
correlations and the effects of external magnetic fields for an electron
escaping from a helium surface. Analytical expressions for the escape rate can
be obtained in various limits. In particular, the tunneling rate with a
parallel magnetic field is presented explicitly.",9308020v1
2003-09-26,Vortex dynamics in dilute two dimensional Josephson junction arrays,"We have investigated the dynamics of vortices in a dilute two dimensional
Josephson junction array where a fraction of the superconducting islands is
missing. We have used the multiple trapping model to calculate the mobility of
vortices and the frequency dependence of the resistance and inductance of the
array.",0309620v1
2005-05-26,Jordan-Wigner Approach to Dynamic Correlations in 2D Spin-1/2 Models,"We discuss the dynamic properties of the square-lattice spin-1/2 XY model
obtained using the two-dimensional Jordan-Wigner fermionization approach. We
argue the relevancy of the fermionic picture for interpreting the neutron
scattering measurements in the two-dimensional frustrated quantum magnet
Cs_2CuCl_4.",0505641v1
2002-06-27,"Comment on ``Electron Mass Operator in a Strong Magnetic Field and Dynamical Chiral Symmetry Breaking""","This is a comment on the paper ``Electron Mass Operator in a Strong Magnetic
Field and Dynamical Chiral Symmetry Breaking"" by A. V. Kuznetsov and N. V.
Mikheev [Phys. Rev. Lett. 89 (2002) 011601]. We show that the main conclusions
of the paper are incorrect.",0206289v1
2007-05-28,The Dynamical Mechanism of the Aharonov-Bohm Effect,"In this paper, it is emphasized that the dynamical cause for the A-B effect
is the superimposed energy between the magnetic field produced by the moving
charges and that in the solenoid, instead of the existence of the vector
potential. If such a superposition between the magnetic fields can be
eliminated, the A-B effect should not be observed any more. To verify this
viewpoint, a new experimental method using a SQUID is suggested in this paper.",0705.4011v1
2008-06-24,Magnetic Susceptibility of Quark Matter,"Magnetic properties of quark matter is discussed by evaluating the spin
susceptibility within Fermi-liquid theory. We take into account the dynamical
and static screening effects. At finite temperature, an anomalous T^2 ln T term
for susceptibity is shown as a non-Fermi-liquid effect due to the dynamical
screening of transverse gluons.",0806.3882v1
2008-11-02,Static and Dynamic irreversible magnetic properties of high temperature superconductors,"This thesis describes experimental and theoretical study of static and
dynamic aspects of the irreversible magnetic behavior of high-Tc
superconductors. Experimentally, conventional magnetometry and novel Hall-probe
array techniques are employed. Using both techniques extends significantly the
experimental possibilities and yields a wealth of new experimental results.",0811.0177v1
2010-09-16,Momentum conservation in dissipationless reduced-fluid dynamics,"The momentum conservation law for general dissipationless reduced-fluid
(e.g., gyrofluid) models is derived by Noether method from a variational
principle. The reduced-fluid momentum density and the reduced-fluid canonical
momentum-stress tensor both exhibit polarization and magnetization effects as
well as an internal torque associated with dynamical reduction. As an
application, we derive an explicit gyrofluid toroidal angular-momentum
conservation law for axisymmetric toroidal magnetized plasmas.",1009.3218v1
2012-08-27,Positive topological entropy for multi-bump magnetic fields,"We study the dynamics of a charged particle in a planar magnetic field which
consists of $n\geq 2$ disjoint localized peaks. We show that, under mild
geometric conditions, this system is semi-conjugated to the full shift on $n$
symbols and, hence, carries positive topological entropy.",1208.5437v1
2014-02-14,Extremely high-resolution measurements of microwave magnetisation dynamics in magnetic thin films and nanostructures,"In this work we discuss the use of interferometric measurement technique to
study microwave magnetization dynamics on ferromagnetic nanostructures. We
demonstrate that in this way one can resolve features which are impossible to
resolve with broadband ferromagnetic resonance and traveling spin wave
spectroscopy otherwise.",1402.3459v1
2015-04-16,Dynamical localization of Dirac particles in electromagnetic fields with dominating magnetic potentials,"We consider two-dimensional massless Dirac operators in a radially symmetric
electromagnetic field. In this case the fields may be described by
one-dimensional electric and magnetic potentials $V$ and $A$. We show dynamical
localization in the regime when $\displaystyle\lim_{r\to\infty}|V|/|A |<1$,
where dense point spectrum occurs.",1504.04077v1
2016-09-22,Manipulation of magnetic Skyrmions with a Scanning Tunneling Microscope,"The dynamics of a single magnetic Skyrmion in an atomic spin system under the
influence of Scanning Tunneling Microscope is investigated by computer
simulations solving the Landau-Lifshitz-Gilbert equation. Two possible
scenarios are described: manipulation with aid of a spin-polarized tunneling
current and by an electric field created by the scanning tunneling microscope.
The dynamics during the creation and annihilation process is studied and the
possibility to move single Skyrmions is showed.",1609.06797v1
2017-08-22,Dynamics of X-state in anisotropic Heisenberg spin with magnetic field,"We study the dynamics of X-state for anisotropic Heisenberg spin system using
quantum fidelity. It is shown that while Bell diagonal state, a special class
of X-state, is stationary, there exists a set of two parametric states which
are stationary in presence of external uniform magnetic field.",1708.06632v1
2019-04-22,About stability of plasma in a magnetic field,"The dynamics of the oscillator system is investigated. The conditions under
which this dynamics becomes unstable are determined. In particular, it is shown
that plasma in constant magnetic field becomes unstable if its density exceeds
a certain critical value. The role of chaotic regimes on confinement of
particles (oscillators) in confining potentials is studied.",1904.09950v1
2002-02-19,Mass loading of pulsar winds,"The dynamics of relativistic magnetized mass loaded outflows carrying
toroidal magnetic field is analyzed in the context of Pulsar Wind Nebulae
(PWNs). Mass loading is very efficient in slowing down super-relativistic
magnetized flows and weakening of relativistic shocks. We suggest that
weakening of relativistic reverse shocks by mass loading in PWNs is responsible
for the low radiative efficiencies of the majority of the PWNs. Mass loading
may also result in a shock transition near the fast magnetosonic point; this is
unlikely to happen in majority of PWNs. The evolution of magnetized mass loaded
flows beyond the reverse shock is complicated: after initial deceleration to
the minimal velocity required to transport the magnetic flux, the mass loaded
flows have to {\it accelerate}. In order to be able to expand to infinity,
magnetized flows should either become time dependent or destroy the toroidal
magnetic flux by developing internal instabilities. Destruction of the magnetic
flux initiated by mass loading may allow for the flow to slow down to
sub-relativistic velocities and resolve the $\sigma$ paradox of the pulsar wind
nebula.",0202344v1
2005-01-28,Why Magnetic Fields Cannot be the Main Agent Shaping Planetary Nebulae,"An increasing amount of literature reports the detection of magnetic fields
in asymptotic giant branch (AGB) stars and in central stars of planetary
nebulae (PNs). These detections lead to claims that the magnetic fields are the
main agent shaping the PNs. In this paper, I examine the energy and angular
momentum carried by magnetic fields expelled from AGB stars, as well as other
physical phenomena that accompany the presence of large scale fields, such as
those claimed in the literature. I show that a single star cannot supply the
energy and angular momentum if the magnetic fields have the large coherent
structure required to shape the circumstellar wind. Therefore, the structure of
non-spherical planetary nebulae cannot be attributed to dynamically important
large scale magnetic fields. I conclude that the observed magnetic fields
around evolved stars can be understood by locally enhanced magnetic loops which
can have a secondary role in the shaping of the PN. The primary role, I argue,
rests with the presence of a companion.",0501647v2
1999-04-22,Deceptive Apparent Nonadiabatic Magnetization Process,"We discuss the effect of the thermal environment on the low-temperature
response of the magnetization of uniaxial magnets to a time-dependent applied
magnetic field. At sufficiently low temperatures the staircase magnetization
curves observed in molecular magnets such as Mn_{12} and Fe_8 display little
temperature dependence. However the changes of the magnetization at each step
do not seem to be directly related to the probability for a quantum mechanical
nonadiabatic transition. In order to explain this deceptive apparent
nonadiabatic behavior, we study the quantum dynamics of the system in a thermal
environment and propose a relation between the observed magnetization steps and
the quantum mechanical transition probability due to the nonadiabatic
transition.",9904323v4
2003-03-15,Carrier States and Ferromagnetism in Diluted Magnetic Semiconductors,"Applying the dynamical coherent potential approximation to a simple model, we
have systematically studied the carrier states in $A_{1-x}$Mn$_xB$-type diluted
magnetic semiconductors (DMS's). The model calculation was performed for three
typical cases of DMS's: The cases with strong and moderate exchange
interactions in the absence of nonmagnetic potentials, and the case with strong
attractive nonmagnetic potentials in addition to moderate exchange interaction.
When the exchange interaction is sufficiently strong, magnetic impurity bands
split from the host band. Carriers in the magnetic impurity band mainly stay at
magnetic sites, and coupling between the carrier spin and the localized spin is
very strong. The hopping of the carriers among the magnetic sites causes
ferromagnetism through a {\it double-exchange (DE)-like} mechanism. We have
investigated the condition for the DE-like mechanism to operate in DMS's. The
result reveals that the nonmagnetic attractive potential at the magnetic site
assists the formation of the magnetic impurity band and makes the DE-like
mechanism operative by substantially enhancing the effect of the exchange
interaction. Using conventional parameters we have studied the carrier states
in Ga$_{1-x}$Mn$_x$As. The result shows that the ferromagnetism is caused
through the DE-like mechanism by the carriers in the bandtail originating from
the impurity states.",0303285v1
2005-06-06,Thin superconductors and SQUIDs in perpendicular magnetic field,"It is shown how the static and dynamic electromagnetic properties can be
calculated for thin flat superconducting films of any shape and size, also
multiply connected as used for SQUIDs, and for any value of the effective
magnetic London penetration depth Lambda. As examples, the distributions of
sheet current and magnetic field are obtained for rectangular and circular
films without and with slits and holes, in response to an applied perpendicular
magnetic field and to magnetic vortices moving in the film. The self energy and
interaction of vortices with each other and with an applied magnetic field
and/or transport current are given. Due to the long ranging magnetic stray
field, these energies depend on the size and shape of the film and on the
vortex position even in large films, in contrast to the situation in large bulk
superconductors. The focussing of magnetic flux into the central hole of square
films without and with a radial slit is compared.",0506144v1
2006-10-06,Influence of topography and Co domain walls on the magnetization reversal of the FeNi layer in FeNi/Al$\_2$O$\_3$/Co magnetic tunnel junctions,"We have studied the magnetization reversal dynamics of FeNi/Al$\_2$O$\_3$/Co
magnetic tunnel junctions deposited on step-bunched Si substrates using
magneto-optical Kerr effect and time-resolved x-ray photoelectron emission
microscopy combined with x-ray magnetic circular dichroism (XMCD-PEEM).
Different reversal mechanisms have been found depending on the substrate miscut
angle. Larger terraces (smaller miscut angles) lead to a higher nucleation
density and stronger domain wall pinning. The width of domain walls with
respect to the size of the terraces seems to play an important role in the
reversal. We used the element selectivity of XMCD-PEEM to reveal the strong
influence of the stray field of domain walls in the hard magnetic layer on the
magnetic switching of the soft magnetic layer.",0610174v1
2001-01-09,Hysteresis in layered spring magnets,"This article addresses a problem of micromagnetics: the reversal of magnetic
moments in layered spring magnets. A one-dimensional model is used of a film
consisting of several atomic layers of a soft material on top of several atomic
layers of a hard material. Each atomic layer is taken to be uniformly
magnetized, and spatial inhomogeneities within an atomic layer are neglected.
The state of such a system is described by a chain of magnetic spin vectors.
Each spin vector behaves like a spinning top driven locally by the effective
magnetic field and subject to damping (Landau-Lifshitz-Gilbert equation). A
numerical integration scheme for the LLG equation is presented that is
unconditionally stable and preserves the magnitude of the magnetization vector
at all times. The results of numerical investigations for a bilayer in a
rotating in-plane magnetic field show hysteresis with a basic period of $2\pi$
at moderate fields and hysteresis with a basic period of $\pi$ at strong
fields.",0101077v1
2006-06-29,Positive Topological Entropy for Magnetic Flows on Surfaces,"We study the topological entropy of the magnetic flow on a closed riemannian
surface. We prove that if the magnetic flow has a non-hyperbolic closed orbit
in some energy set T^cM= E^{-1}(c), then there exists an exact $
C^\infty$-perturbation of the 2-form $ \Omega $ such that the new magnetic flow
has positive topological entropy in T^cM. We also prove that if the magnetic
flow has an infinite number of closed orbits in T^cM, then there exists an
exact C^1-perturbation of $ \Omega $ with positive topological entropy in T^cM.
The proof of the last result is based on an analog of Franks' lemma for
magnetic flows on surfaces, that is proven in this work, and Ma\~n\'e's
techniques on dominated splitting. As a consequence of those results, an exact
magnetic flow on S^2 in high energy levels admits a C^1-perturbation with
positive topological entropy. In the appendices we show that an exact magnetic
flow on the torus in high energy levels admits a $ C^\infty $-perturbation with
positive topological entropy.",0606740v3
2007-09-12,Numerical Simulations of Equatorially-Asymmetric Magnetized Supernovae: Formation of Magnetars and Their Kicks,"A series of numerical simulations on magnetorotational core-collapse
supernovae are carried out. Dipole-like configurations which are offset
northward are assumed for the initially strong magnetic fields together with
rapid differential rotations. Aims of our study are to investigate effects of
the offset magnetic field on magnetar kicks and on supernova dynamics. Note
that we study a regime where the proto-neutron star formed after collapse has a
large magnetic field strength approaching that of a ``magnetar'', a highly
magnetized slowly rotating neutron star. As a result, equatorially-asymmetric
explosions occur with a formation of the bipolar jets. Resultant magnetar's
kick velocities are $\sim 300-1000$ km s$^{-1}$. We find that the acceleration
is mainly due to the magnetic pressure while the somewhat weaker magnetic
tension works toward the opposite direction, which is due to stronger magnetic
field in the northern hemisphere. Noted that observations of magnetar's proper
motions are very scarce, our results supply a prediction for future
observations. Namely, magnetars possibly have large kick velocities, several
hundred km s$^{-1}$, as ordinary neutron stars do, and in an extreme case they
could have those up to 1000 km s$^{-1}$.",0709.1795v1
2008-09-22,Elementary transitions and magnetic correlations in two-dimensional disordered nanoparticle ensembles,"The magnetic relaxation processes in disordered two-dimensional ensembles of
dipole-coupled magnetic nanoparticles are theoretically investigated by
performing numerical simulations. The energy landscape of the system is
explored by determining saddle points, adjacent local minima, energy barriers,
and the associated minimum energy paths (MEPs) as functions of the structural
disorder and particle density. The changes in the magnetic order of the
nanostructure along the MEPs connecting adjacent minima are analyzed from a
local perspective. In particular, we determine the extension of the correlated
region where the directions of the particle magnetic moments vary
significantly. It is shown that with increasing degree of disorder the magnetic
correlation range decreases, i.e., the elementary relaxation processes become
more localized. The distribution of the energy barriers, and their relation to
the changes in the magnetic configurations are quantified. Finally, some
implications for the long-time magnetic relaxation dynamics of nanostructures
are discussed.",0809.3640v1
2009-03-19,Turbulent transport and dynamo in sheared MHD turbulence with a non-uniform magnetic field,"We investigate three-dimensional magnetohydrodynamics turbulence in the
presence of both a large-scale velocity and non-uniform magnetic field. By
assuming a turbulence driven by an external forcing with both helical and
non-helical spectra, we investigate the combined effect of these two shears on
turbulence intensity and turbulent transport represented by turbulent
diffusivities (turbulent viscosity, $\alpha$ and $\beta$ effect) in analogy
with Reynolds. We show that turbulence viscosity and diffusivity are quenched
for strong flow shear and magnetic field in the limits of weak and strong
magnetic field, respectively. For a weak flow shear, we further show that the
magnetic shear increases the turbulence intensity while decreasing the
turbulent transport. In the presence of a strong flow shear, the effect of the
magnetic shear is found to oppose the effect of flow shear (which reduces
turbulence due to shear stabilization) by enhancing turbulence and transport,
thereby weakening the strong quenching by flow shear stabilization. In the case
of a strong magnetic field (compared to flow shear), magnetic shear increases
turbulence intensity and quenches turbulent transport.",0903.3352v2
2009-05-03,Small-scale magnetic helicity losses from a mean-field dynamo,"Using mean-field models with a dynamical quenching formalism we show that in
finite domains magnetic helicity fluxes associated with small-scale magnetic
fields are able to alleviate catastrophic quenching. We consider fluxes that
result either from advection by a mean flow, the turbulent mixing down the
gradient of mean small-scale magnetic helicity concentration, or the explicit
removal which may be associated with the effects of coronal mass ejections in
the Sun. In the absence of shear, all the small-scale magnetic helicity fluxes
are found to be equally strong both for large-scale and small-scale fields. In
the presence of shear there is also an additional magnetic helicity flux
associated with the mean field, but this flux does not alleviate catastrophic
quenching. Outside the dynamo-active region there are neither sources nor sinks
of magnetic helicity, so in a steady state this flux must be constant. It is
shown that unphysical behavior emerges if the small-scale magnetic helicity
flux is forced to vanish within the computational domain.",0905.0242v1
2010-07-02,Bohr quantum theory of the magnetic monopoles and classical electron electromagnetic mass problem,"In the first part of this work we apply Bohr (old or naive quantum atomic)
theory for analysis of the remarkable electro-dynamical problem of magnetic
monopoles. We reproduce formally exactly some basic elements of the Dirac
magnetic monopoles theory, especially Dirac electric/magnetic charge
quantization condition. It follows after application of Bohr theory at the
system, simply called magnetic monopole ""atom"", consisting of the practically
standing, massive magnetic monopole as the ""nucleus"" and electron rotating
stable around magnetic monopole under magnetic and electrostatic interactions.
Also, in the second part of this work we suggest a simple solution of the
classical electron electromagnetic mass problem.",1007.0340v9
2011-02-01,One-dimentional magnonic crystal as a medium with magnetically tunable disorder on a periodical lattice,"We show that periodic magnetic nanostructures (magnonic crystals) represent
an ideal system for studying excitations on disordered periodical lattices
because of the possibility of controlled variation of the degree of disorder by
varying the applied magnetic field. Ferromagnetic resonance (FMR) data
collected inside minor hysteresis loops for a periodic array of Permalloy
nanowires of alternating width and magnetic force microscopy images of the
array taken after running each of these loops were used to establish convincing
evidence that there is a strong correlation between the type of FMR response
and the degree of disorder of the magnetic ground state. We found two types of
dynamic responses: anti-ferromagnetic (AFM) and ferromagnetic (FM), which
represent collective spin wave modes or collective magnonic states. Depending
on the history of sample magnetization either AFM or FM state is either the
fundamental FMR mode or represents a state of a magnetic defect on the
artificial crystal. A fundamental state can be transformed into a defect one
and vice versa by controlled magnetization of the sample.",1102.0069v1
2011-10-04,Negative magnetization of Li2Ni2Mo3O12 having a spin system composed of distorted honeycomb lattices and linear chains,"We study themagnetism of a spin-1 substance Li2Ni2Mo3O12. The spin system
consists of distorted honeycomb lattices and linear chains of Ni2+ spins. Li+
ions enter about 25% and 50% of the honeycomb and chain Ni sites, respectively,
creating disorder in both spin subsystems. A magnetic phase transition occurs
at Tc = 8.0 K in the zero magnetic field. In low magnetic fields, the
magnetization increases rapidly below Tc, decreases below 7 K, and finally
becomes negative at low temperatures. We determine the magnetic structure using
neutron powder diffraction results. The honeycomb lattices and linear chains
show antiferromagnetic and ferromagnetic long-range order, respectively. We
investigate static and dynamic magnetic properties using the local probe
technique of muon spin relaxation. We discuss the origin of the negative
magnetization.",1110.0587v1
2011-10-27,Magnetically Controlling the Explosion of Dirac Fermions during the Oxidation of Graphene,"The different physical properties of multilayered graphene or graphite
relative to single layer graphene result from the Dirac spins symmetry in
graphene and the Pauli spin symmetry in graphite. The Dirac symmetry in
multilayers of graphene (graphite) is hindered by interlayer interactions.
Different magnetizations, electronics and chemistry of graphite and graphene
follow from absence of interlayer interactions in graphene. The distinct
kinetics and dynamics of graphite and graphene by oxidation by the Hummer's
method in weak external magnetic field are observed in this work. Graphite
manifest enhanced non-explosive oxidation of Pauli spins in weak magnetic field
with background paramagnetic oxygen slowing the magnetic acceleration. Graphene
and graphite oxide manifest explosive oxidation and magnetically decelerated
explosive oxidation of Dirac spins in weak magnetic field for temperatures
below 20 oCelsius. The explosive oxidation of graphene and its deceleration in
weak external magnetic field are interpreted resulting from the giant
nonlocality and spin Hall Effect in the chemically reacting graphene. This is
the first identification, analysis and interpretation of the chemistry of the
Dirac spins and the magnetochemistry of relativistic electrons.",1110.6060v1
2012-01-05,Direct measurement of the three dimensional magnetization vector trajectory in GaMnAs by a magneto-optical pump-and-probe method,"We report on a quantitative experimental determination of the
three-dimensional magnetization vector trajectory in GaMnAs by means of the
static and time-resolved pump-and-probe magneto-optical measurements. The
experiments are performed in a normal incidence geometry and the time evolution
of the magnetization vector is obtained without any numerical modeling of
magnetization dynamics. Our experimental method utilizes different polarization
dependences of the polar Kerr effect and magnetic linear dichroism to
disentangle the pump-induced out-of-plane and in-plane motions of
magnetization, respectively. We demonstrate that the method is sensitive enough
to allow for the determination of small angle excitations of the magnetization
in GaMnAs. The method is readily applicable to other magnetic materials with
sufficiently strong circular and linear magneto-optical effects.",1201.1213v1
2012-02-01,Magnet designs for muon collider ring and interactions regions,"Conceptual designs of superconducting magnets for the storage ring of a Muon
Collider with a 1.5 TeV c.o.m. energy and an average luminosity of 10 34
cm-2s-1 are presented. All magnets are based on Nb3Sn superconductor and
designed to provide an adequate operating field/field gradient in the aperture
with the critical current margin required for reliable magnet operation in the
machine. Magnet cross-sections were optimized to achieve the accelerator field
quality in the magnet aperture occupied with beams. The magnets and
corresponding protective measures are designed to handle about 0.5 kW/m of
dynamic heat load from the muon beam decays. Magnet parameters are reported and
compared with the requirements.",1202.0270v1
2012-03-26,Local electric current correlation function in an exponentially decaying magnetic field,"The effect of an exponentially decaying magnetic field on the dynamics of
Dirac fermions in 3+1 dimensions is explored. The spatially decaying magnetic
field is assumed to be aligned in the third direction, and is defined by
{\mathbf{B}}(x)=B(x){\mathbf{e}}_{z}, with B(x)=B_{0}e^{-\xi\ x/\ell_{B}}.
Here, \xi\ is a dimensionless damping factor and \ell_{B}=(eB_{0})^{-1/2} is
the magnetic length. As it turns out, the energy spectrum of fermions in this
inhomogeneous magnetic field can be analytically determined using the Ritus
method. Assuming the magnetic field to be strong, the chiral condensate and the
\textit{local} electric current correlation function are computed in the lowest
Landau level (LLL) approximation and the results are compared with those
arising from a strong homogeneous magnetic field. Although the constant
magnetic field B_{0} can be reproduced by taking the limit of \xi-> 0 and/or
x-> 0 from B(x), these limits turn out to be singular once the quantum
corrections are taken into account.",1203.5634v2
2012-09-26,Fermi Surface Reconstruction by Dynamic Magnetic Fluctuations and Spin-Charge Separation Near an O(3) Quantum Critical Point,"Stimulated by the small/large Fermi surface controversy in the cuprates we
consider a small number of holes injected into the bilayer antiferromagnet. The
system has an O(3) quantum critical point (QCP) separating the magnetically
ordered and the magnetically disordered phases. We demonstrate that nearly
critical quantum magnetic fluctuations can change the Fermi surface topology
and also lead to spin charge separation (SCS) in two dimensions. We demonstrate
that in the physically interesting regime there is a magnetically driven
Lifshitz point (LP) inside the magnetically disordered phase. At the LP the
topology of the hole Fermi surface is changed. The position of the LP, while
being close to the position of the QCP is generally different. Dependent on the
additional hole hopping integrals $t^{\prime}$ and $t^{\prime\prime}$, the LP
can be located either in the magnetically ordered phase and/or in the
magnetically disordered phase. We also demonstrate that in this regime the hole
spin and charge necessarily separate when approaching the QCP. The considered
model sheds light on generic problems concerning the physics of the cuprates.",1209.6051v1
2012-10-06,Magnetic induction maps in a magnetized spherical Couette flow experiment,"The DTS experiment is a spherical Couette flow experiment with an imposed
dipolar magnetic field. Liquid sodium is used as a working fluid. In a series
of measurement campaigns, we have obtained data on the mean axisymmetric
velocity, the mean induced magnetic field and electric potentials. All these
quantities are coupled through the induction equation. In particular, a strong
omega-eff ect is produced by di fferential rotation within the fluid shell,
inducing a significant azimuthal magnetic field. Taking advantage of the simple
spherical geometry of the experiment, I expand the azimuthal and meridional
fields into Legendre polynomials and derive the expressions that relate all
measurements to the radial functions of the velocity field for each harmonic
degree. For small magnetic Reynolds numbers Rm the relations are linear, and
the azimuthal and meridional equations decouple. Selecting a set of
measurements for a given rotation frequency of the inner sphere (Rm = 9.4), I
invert simultaneously the velocity and the magnetic data and thus reconstruct
both the azimuthal and the meridional fields within the fluid shell. The
results demonstrate the good internal consistency of the measurements, and
indicate that turbulent non-axisymmetric fluctuations do not contribute
significantly to the axisymmetric magnetic induction.",1210.1990v2
2012-10-12,Reversal of magnetization of a single-domain magnetic particle by the ac field of time-dependent frequency,"We report numerical and analytical studies of the reversal of the magnetic
moment of a single-domain magnetic particle by a circularly polarized ac field
of time-dependent frequency. For the time-linear frequency sweep, the phase
diagrams are computed that illustrate the dependence of the reversal on the
frequency sweep rate v, the amplitude of the ac field h, the magnetic
anisotropy field d, and the damping parameter alpha. It is shown that the most
efficient magnetization reversal requires a non-linear time dependence of the
frequency, omega(t), for which an exact analytical formula is derived with
account of damping. The necessary condition of the reversal is h > alpha d.
Implementation of a small-scale magnetization reversal is proposed in which a
nanomagnet is electromagnetically coupled to two weak superconducting links
controlled by the voltage. Dynamics of such a system is analyzed with account
of the back effect of the magnet on the superconducting links.",1210.3530v1
2012-12-06,A Superparamagnetic State Induced by a Spin Reorientation Transition in Ultrathin Magnetic Films,"We investigate a spin reorientation transition (SRT) in ultrathin magnetic
films by Monte-Carlo simulations. We assume that the lateral size of the film
is relatively small and it has a single-domain structure. To gain insights into
the SRT, we measure a free-energy as a function of perpendicular and in-plane
magnetizations. As a result, we find that the system is in a superparamagnetic
state at the SRT temperature. The disappearance of magnetization around the SRT
temperature, which is observed in experiments, emerges due to dynamical
fluctuations in magnetization which are inherent in the superparamagnetic
state. This observation is in contrast to that in large ultrathin magnetic
films that the disappearance of magnetization is caused by a static magnetic
structure with many complex domains.",1212.1228v2
2013-04-10,"Reconnection Diffusion, Star Formation and Numerical Simulations","We consider fast magnetic reconnection that takes place within turbulent
magnetic flux and show that the process results in diffusion of magnetic fields
and matter, which we term reconnection diffusion. The process of reconnection
diffusion is based on the model of 3D reconnection of weakly turbulent magnetic
fields and is applicable to both fully ionized and partially ionized gas. The
rate of reconnection diffusion does not depend on the level of ionization and
therefore the usually employed ambipolar diffusion idea gets irrelevant for
magnetic field transport in turbulent fluids. We claim that the reconnection
diffusion process is a manifestation of the violation of flux conservation in
highly conducting turbulent fluids. We discuss the consequences of reconnection
diffusion for star formation and stress. We show that reconnection diffusion on
large scales is independent of small scale magnetic field dynamics of magnetic
fields. We conclude that numerical simulations correctly represents the
diffusion of actual astrophysical magnetic fields in flows with substantially
larger Lundquist numbers if these simulated regions regions are turbulent.",1304.3133v1
2013-04-14,Influence of initial conditions on the large-scale dynamo growth rate,"To investigate the effect of energy and helicity on the growth of magnetic
field, helical kinetic forcing was applied to the magnetohydrodynamic(MHD)
system that had a specific distribution of energy and helicity as initial
conditions. Simulation results show the saturation of a system is not
influenced by the initial conditions, but the growth rate of large scale
magnetic field is proportionally dependent on the initial large scale magnetic
energy and helicity. It is already known that the helical component of small
scale magnetic field(i.e., current helicity $<{\bf j}\cdot {\bf b}>$) quenches
the growth of large scale magnetic field. However, $<{\bf j}\cdot {\bf b}>$ can
also boost the growth of large scale magnetic field by changing its sign and
magnitude. In addition, simulation shows the nonhelical magnetic field can
suppress the velocity field through Lorentz force. Comparison of the profiles
of evolving magnetic and kinetic energy indicates that kinetic energy migrates
backward when the external energy flows into the three dimensional MHD system,
which means the velocity field may play a preceding role in the very early MHD
dynamo stage.",1304.3870v3
2013-05-06,Inverse magnetic catalysis induced by sphalerons,"The recently discovered inverse magnetic catalysis around the critical
temperature indicates that some important information is missing in our current
understanding of conventional chiral dynamics of QCD, which is enhanced by the
magnetic field. In this work, we provide a mechanism to explain that the
inverse magnetic catalysis around the critical temperature is induced by
sphalerons. At high temperatures, sphaleron transitions between distinct
classical vacua cause an asymmetry between the number of right- and left-handed
quarks due to the axial anomaly of QCD. In the presence of a strong magnetic
field, the chiral imbalance is enhanced and destroys the right- and left-handed
pairings, which naturally induces a decreasing critical temperature of the
chiral phase transition for increasing magnetic field. The inverse magnetic
catalysis at finite baryon density, and the critical end point in the presence
of a strong magnetic field is also explored in this work.",1305.1100v1
2013-07-29,Electrocapillary instability of magnetic fluid peak,"The paper presents an experimental study of the capillary electrostatic
instability occurring under effect of a constant electric field on a magnetic
fluid individual peak. The peaks under study occur at disintegration of a
magnetic fluid layer applied on a flat electrode surface under effect of a
perpendicular magnetic field. The electrocapillary instability shows itself as
an emission of charged drops jets from the peak point in direction of the
opposing electrode. The charged drops emission repeats periodically and results
in the peak shape pulsations. It is shown that a magnetic field affects the
electrocapillary instability occurrence regularities and can stimulate its
development. The critical electric and magnetic field strengths at which the
instability occurs have been measured; their dependence on the peak size is
shown. The hysteresis in the system has been studied; it consists in that the
charged drops emission stops at a lesser electric (or magnetic) field strength
than that of the initial occurrence. The peak pulsations frequency depending on
the magnetic and electric field strengths and on the peak size has been
measured.",1307.7734v1
2013-12-05,Transient jet formation and state transitions from large-scale magnetic reconnection in black hole accretion discs,"Magnetically arrested accretion discs (MADs), where the magnetic pressure in
the inner disc is dynamically important, provide an alternative mechanism for
regulating accretion to what is commonly assumed in black hole systems. We show
that a global magnetic field inversion in the MAD state can destroy the jet,
significantly increase the accretion rate, and move the effective inner disc
edge in to the marginally stable orbit. Reconnection of the MAD field in the
inner radii launches a new type of transient outflow containing hot plasma
generated by magnetic dissipation. This transient outflow can be as powerful as
the steady magnetically-dominated Blandford-Znajek jet in the MAD state. The
field inversion qualitatively describes many of the observational features
associated with the high luminosity hard to soft state transition in black hole
X-ray binaries: the jet line, the transient ballistic jet, and the drop in rms
variability. These results demonstrate that the magnetic field configuration
can influence the accretion state directly, and hence the magnetic field
structure is an important second parameter in explaining observations of
accreting black holes across the mass and luminosity scales.",1312.1691v1
2014-01-05,Generation of Magnetic Field on the Accretion Disk around a Proto-First-Star,"The generation process of magnetic field around a proto-first-star is
studied. Utilizing the recent numerical result of proto-first-star formation
based upon the radiation hydrodynamics simulations, we assess the magnetic
field strength generated by the radiative force and the Biermann battery
effect. We find that magnetic field of \sim 10^{-9} G is generated on the
surface of the accretion disk around the proto-first-star. The field strength
on the accretion disk is smaller by two orders of magnitude than the critical
value, above which the gravitational fragmentation of the disk is suppressed.
Thus, the generated seed magnetic field hardly affect the dynamics of on-site
first star formation directly, unless efficient amplification process is taken
into consideration. We also find that the generated magnetic field is
continuously blown out from the disk on the outflows to the poles, that are
driven by the thermal pressure of photoheated gas. The strength of the diffused
magnetic field in low density regions is \sim 10^{-14}-10^{-13}G at n_H=10^3
cm^{-3} which could play important roles on the next generation star formation,
as well as the seeds of magnetic field exist in present-day universe.",1401.0905v1
2014-02-04,Laser-induced magnetization curve,"We propose an all optical ultrafast method to highly magnetize general
quantum magnets using a circularly polarized terahertz laser. The key idea is
to utilize a circularly polarized laser and its chirping. Through this method,
one can obtain magnetization curves of a broad class of quantum magnets as a
function of time even without any static magnetic field. We numerically
demonstrate the laser-induced magnetization process in realistic quantum spin
models and find a condition for the realization. The onset of magnetization can
be described by a many-body version of Landau-Zener mechanism. In a particular
model, we show that a plateau state with topological properties can be realized
dynamically.",1402.0881v2
2014-04-14,Orbital magnetization of correlated electrons with arbitrary band topology,"Spin-orbit coupling introduces chirality into electronic structure. This can
have profound effects on the magnetization induced by orbital motion of
electrons. Here we derive a formula for the orbital magnetization of
interacting electrons in terms of the full Green's function and vertex
functions. The formula is applied within dynamical mean-field theory to the
Kane-Mele-Hubbard model that allows both topological and trivial insulating
phases. We study the insulating and metallic phases in the presence of an
exchange magnetic field. In the presence of interactions, the orbital
magnetization of the quantum spin Hall insulating phase with inversion symmetry
is renormalized by the bulk quasi-particle weight. The orbital magnetization
vanishes for the in-plane antiferromagnetic phase with trivial topology. In the
metallic phase, the enhanced effective spin-orbit coupling due to the
interaction sometimes leads to an enhancement of the orbital magnetization.
However, at low doping, magnetization is suppressed at large interaction
strengths.",1404.3673v3
2014-04-28,Magnetic Prandtl number dependence of kinetic to magnetic dissipation ratio,"Using direct numerical simulations of three-dimensional hydromagnetic
turbulence, either with helical or non-helical forcing, we show that the ratio
of kinetic-to-magnetic energy dissipation always increases with the magnetic
Prandtl number, i.e., the ratio of kinematic viscosity to magnetic diffusivity.
This dependence can always be approximated by a power law, but the exponent is
not the same in all cases. For non-helical turbulence, the exponent is around
1/3, while for helical turbulence it is between 0.6 and 2/3. In the
statistically steady state, the rate of the energy conversion from kinetic into
magnetic by the dynamo must be equal to the Joule dissipation rate. We
emphasize that for both small-scale and large-scale dynamos, the efficiency of
energy conversion depends sensitively on the magnetic Prandtl number, and thus
on the microphysical dissipation process. To understand this behavior, we also
study shell models of turbulence and one-dimensional passive and active scalar
models. We conclude that the magnetic Prandtl number dependence is
qualitatively best reproduced in the one-dimensional model as a result of
dissipation via localized Alfven kinks.",1404.6964v2
2014-04-29,Nanowire Spin Torque Oscillator Driven by Spin Orbit Torques,"Spin torque from spin current applied to a nanoscale region of a ferromagnet
can act as negative magnetic damping and thereby excite self-oscillations of
its magnetization. In contrast, spin torque uniformly applied to the
magnetization of an extended ferromagnetic film does not generate
self-oscillatory magnetic dynamics but leads to reduction of the saturation
magnetization. Here we report studies of the effect of spin torque on a system
of intermediate dimensionality - a ferromagnetic nanowire. We observe coherent
self-oscillations of magnetization in a ferromagnetic nanowire serving as the
active region of a spin torque oscillator driven by spin orbit torques. Our
work demonstrates that magnetization self-oscillations can be excited in a
one-dimensional magnetic system and that dimensions of the active region of
spin torque oscillators can be extended beyond the nanometer length scale.",1404.7262v2
2014-06-08,Perpendicular magnetic anisotropy and magnetization process in CoFeB/Pd multilayer films,"Perpendicular magnetic anisotropy (PMA) and dynamic magnetization reversal
process in [CoFeB $t$ nm/Pd 1.0 nm]$_n$ ($t$ = 0.4, 0.6, 0.8, 1.0, and 1.2 nm;
$n$ = 2 - 20) multilayer films have been studied by means of magnetic
hysteresis and Kerr effect measurements. Strong and controllable PMA with an
effective uniaxial anisotropy up to 7.7$\times$ 10$^6$ J.m$^{-3}$ and a
saturation magnetization as low as 200 emu/cc are achieved. Surface/interfacial
anisotropy of CoFeB/Pd interfaces, the main contribution to the PMA, is
separated from the effective uniaxial anisotropy of the films, and appears to
increase with the number of the CoFeB/Pd bilayers. Observation of the magnetic
domains during a magnetization reversal process using polar magneto-optical
Kerr microscopy shows the detailed behavior of nucleation and displacement of
the domain walls.",1406.2028v1
2014-06-27,On the plasma flow inside magnetic tornadoes on the Sun,"High-resolution observations with the Swedish 1-m Solar Telescope (SST) and
the Solar Dynamics Observatory (SDO) reveal rotating magnetic field structures
that extend from the solar surface into the chromosphere and the corona. These
so-called magnetic tornadoes are primarily detected as rings or spirals of
rotating plasma in the Ca II 854.2 nm line core (also known as chromospheric
swirls). Detailed numerical simulations show that the observed chromospheric
plasma motion is caused by the rotation of magnetic field structures, which
again are driven by photospheric vortex flows at their footpoints. Under the
right conditions, two vortex flow systems are stacked on top of each other. We
refer to the lower vortex, which extends from the low photosphere into the
convection zone, as intergranular vortex flow (IVF). Once a magnetic field
structure is co-located with an IVF, the rotation is mediated into the upper
atmospheric layers and an atmospheric vortex flow (AVF, or magnetic tornado) is
generated. In contrast to the recent work by Shelyag et al., we demonstrate
that particle trajectories in a simulated magnetic tornado indeed follow
spirals and argue that the properties of the trajectories decisively depend on
the location in the atmosphere and the strength of the magnetic field.",1406.7270v1
2014-09-05,Effect of Size of the Computational Domain on Spherical Nonlinear Force-Free Modeling of Coronal Magnetic Field Using SDO/HMI Data,"The solar coronal magnetic field produces solar activity, including extremely
energetic solar flares and coronal mass ejections (CMEs). Knowledge of the
structure and evolution of the magnetic field of the solar corona is important
for investigating and understanding the origins of space weather. Although the
coronal field remains difficult to measure directly, there is considerable
interest in accurate modeling of magnetic fields in and around sunspot regions
on the Sun using photospheric vector magnetograms as boundary data. In this
work, we investigate effects of the size of the domain chosen for coronal
magnetic field modeling on resulting model solution. We apply spherical
Optimization procedure to vector magnetogram data of Helioseismic and Magnetic
Imager (HMI) onboard Solar Dynamics Observatory (SDO) with four Active Region
observed on 09 March 2012 at 20:55UT. The results imply that quantities like
magnetic flux density, electric current density and free magnetic energy
density of ARs of interest are significantly different from the corresponding
quantities obtained in the same region within the wider field of view. The
difference is even more pronounced in the regions where there are connections
to outside the domain.",1409.1775v1
2014-09-15,Nonlinear magneto-optical response to light carrying orbital angular momentum,"We predict a non-thermal magneto-optical effect for magnetic insulators
subject to intense light carrying orbital angular momentum (OAM). Using a
classical approach to second harmonic generation in non-linear media with
specific symmetry properties we predict a significant nonlinear contribution to
the local magnetic field triggered by light with OAM. The resulting magnetic
field originates from the displacement of electrons driven by the electrical
field (with amplitude $E_0$) of the spatially inhomogeneous optical pulse,
modeled here as a Laguerre-Gaussian beam carrying OAM. In particular, the
symmetry properties of the irradiated magnet allow for magnetic field responses
which are second-order ($\sim E_0^2$) and fourth-order ($\sim E_0^4$) in
electric-field strength and have opposite signs. For sufficiently high laser
intensities, terms $\sim E_0^4$ dominate and generate magnetic field strengths
which can be as large as several Tesla. Moreover, changing the OAM of the laser
beam is shown to determine the direction of the total light-induced magnetic
field, which is further utilized to study theoretically the non-thermal
magnetization dynamics.",1409.4385v1
2014-10-08,Flat-band ferromagnetism in the multilayer Lieb optical lattice,"We theoretically study magnetic properties of two-component cold fermions in
half-filled multilayer Lieb optical lattices, i.e., two, three, and several
layers, using the dynamical mean-field theory. We clarify that the magnetic
properties of this system become quite different depending on whether the
number of layers is odd or even. In odd-number-th layers in an odd-number-layer
system, finite magnetization emerges even with an infinitesimal interaction.
This is a striking feature of the flatband ferromagnetic state in multilayer
systems as a consequence of the Lieb theorem. In contrast, in even-number
layers, magnetization develops from zero on a finite interaction. These
different magnetic behaviours are triggered by the flat bands in the local
density of states and become identical in the limit of the infinite-layer
(i.e., three-dimensional) system. We also address how interlayer hopping
affects the magnetization process. Further, we point out that layer
magnetization, which is a population imbalance between up and down atoms on a
layer, can be employed to detect the emergence of the flat-band ferromagnetic
state without addressing sublattice magnetization.",1410.2166v1
2015-07-12,"Non-equilibrium behavior of the magnetization in the helimagnetic phases of the rare earth alloys R_{1-x}Y_{x} (R = Gd, Tb, Dy)","We have performed DC and AC magnetization measurements for the rare-earth
magnetic alloy systems Gd_{0.62}Y_{0.38}, Tb_{0.86}Y_{0.14}, and
Dy_{0.97}Y_{0.03}. These materials commonly exhibit a proper helical magnetic
structure, and ferromagnetic structure at lower temperatures.In all of these
materials, a difference between zero-field-cooled (ZFC) magnetization and
field-cooled (FC) magnetization and a hysteresis loop in the M-H curve have
been observed in the helimagnetic phases. The non-equilibrium behavior is
possibly caused by a common nature, e. g., chiral domain structures. In
addition to the above behavior, strong non-linearity of the magnetization and
slow spin dynamics have been observed around the N'eel temperature only in
Gd$_{0.62}$Y$_{0.38}$. The spin-glass like behavior observed in
Gd_{0.62}Y_{0.38} could be related to a novel glassy state such as a
helical-glass state.",1507.03202v3
2015-08-17,Increased magnetic damping of a single domain wall and adjacent magnetic domains detected by spin torque diode in a nanostripe,"We use spin-torque resonance to probe simultaneously and separately the
dynamics of a magnetic domain wall and of magnetic domains in a nanostripe
magnetic tunnel junction. Thanks to the large associated resistance variations
we are able to analyze quantitatively the resonant properties of these single
nanoscale magnetic objects. In particular, we find that the magnetic damping of
both domains and domain walls is doubled compared to the damping value of their
host magnetic layer. We estimate the contributions to damping arising from
dipolar couplings between the different layers in the junction and from the
intralayer spin pumping effect. We find that they cannot explain the large
damping enhancement that we observe. We conclude that the measured increased
damping is intrinsic to large amplitudes excitations of spatially localized
modes or solitons such as vibrating or propagating domain walls",1508.04043v1
2015-09-25,Self-similar inverse cascade of magnetic helicity driven by the chiral anomaly,"For systems with charged chiral fermions, the imbalance of chirality in the
presence of magnetic field generates an electric current - this is the Chiral
Magnetic Effect (CME). We study the dynamical real-time evolution of
electromagnetic fields coupled by the anomaly to the chiral charge density and
the CME current by solving the Maxwell-Chern-Simons equations. We find that the
CME induces the inverse cascade of magnetic helicity towards the large
distances, and that at late times this cascade becomes self-similar, with
universal exponents. We also find that in terms of gauge field topology the
inverse cascade represents the transition from linked electric and magnetic
fields (Hopfions) to the knotted configuration of magnetic field
(Chandrasekhar-Kendall states). The magnetic reconnections are accompanied by
the pulses of the CME current directed along the magnetic field lines. We
devise an experimental signature of these phenomena in heavy ion collisions,
and speculate about implications for condensed matter systems.",1509.07790v2
2016-01-29,Switching of chiral magnetic skyrmions by picosecond magnetic field pulses via transient topological states,"Magnetic chiral skyrmions are vortex like spin structures that appear as
stable or meta-stable states in magnetic materials due to the interplay between
the symmetric and antisymmetric exchange interactions, applied magnetic field
and/or uniaxial anisotropy. Their small size and internal stability make them
prospective objects for data storage but for this, the controlled switching
between skyrmion states of opposite polarity and topological charge is
essential. Here we present a study of magnetic skyrmion switching by an applied
magnetic field pulse based on a discrete model of classical spins and atomistic
spin dynamics. We found a finite range of coupling parameters corresponding to
the coexistence of two degenerate isolated skyrmions characterized by mutually
inverted spin structures with opposite polarity and topological charge. We
demonstrate how for a wide range of material parameters a short inclined
magnetic field pulse can initiate the reliable switching between these states
at GHz rates. Detailed analysis of the switching mechanism revealed the complex
path of the system accompanied with the excitation of a chiral-achiral meron
pair and the formation of an achiral skyrmion.",1601.08212v1
2016-02-12,Magnetized quark matter with a magnetic-field dependent coupling,"It was recently derived that the QCD running coupling is a function of the
magnetic field strength under the strong magnetic field approximation. Inspired
by this progress and based on the self-consistent solutions of gap equations,
the properties of two-flavor and three-flavor quark matter are studied in the
framework of the Nambu-Jona-Lasinio model with a magnetic-field dependent
running coupling. We find that the dynamical quark masses as functions of the
magnetic field strength are not monotonous in the fully chirally broken phase.
Furthermore, the stability of magnetized quark matter with the running coupling
is enhanced by lowering the free energy per baryon, which is expected to be
more stable than that of the conventional constant coupling case. It is
concluded that the magnetized strange quark matter described by running
coupling can be absolutely stable.",1602.03939v2
2016-03-03,Cloaking the magnons,"We propose two approaches to cloak the spin waves (magnons) by investigating
magnetization dynamics. One approach is based on a spatially inhomogeneous
anisotropic magnetic moment tensor. The other mechanism is using a spatially
inhomogeneous anisotropic gyromagnetic factor tensor and an inhomogeneous
external magnetic field. For both approaches, the damping tensor is also
inhomogeneous and anisotropic. The magnetic characteristic functions of the
magnetic materials have been theoretically derived for both mechanisms. A
non-magnetic core, which prevents magnons from entering and consequently
distorts the spin wave propagation, can be cloaked by a structured magnetic
shell to redirect the spin wave around the core using the above design
mechanisms. We discuss the feasibility of the proposed mechanisms in an
ensemble of quantum dot molecules and magnetic semiconductors. The proposed
approaches shed light on transformation magnonics, and can be utilized for
future spin-wave lenses, concentrators, low back-scattering waveguides, and
ultimately quantum computing.",1603.01000v1
2016-05-11,Algebraic non-integrability of magnetic billiards,"We consider billiard ball motion in a convex domain of the Euclidean plane
bounded by a piece-wise smooth curve influenced by the constant magnetic field.
We show that if there exists a polynomial in velocities integral of the
magnetic billiard flow then every smooth piece $\gamma$ of the boundary must be
algebraic and either is a circle or satisfies very strong restrictions. In
particular in the case of ellipse it follows that magnetic billiard is
algebraically not integrable for all magnitudes of the magnetic field. We
conjecture that circle is the only integrable magnetic billiard not only in the
algebraic sense, but for a broader meaning of integrability. We also introduce
the model of Outer magnetic billiards. As an application of our method we prove
analogous results on algebraically integrable Outer magnetic billiards.",1605.03372v1
2016-05-31,The diamagnetic phase transition in Magnetars,"Neutron stars are ideal astrophysical laboratories for testing theories of
the de Haas-van Alphen (dHvA) effect and diamagnetic phase transition which is
associated with magnetic domain formation. The ""magnetic interaction"" between
delocalized magnetic moments of electrons (the Shoenberg effect), can result in
an effect of the diamagnetic phase transition into domains of alternating
magnetization (Condon's domains). Associated with the domain formation are
prominent magnetic field oscillation and anisotropic magnetic stress which may
be large enough to fracture the crust of magnetar with a super-strong field.
Even if the fracture is impossible as in ""low-field"" magnetar, the depinning
phase transition of domain wall motion driven by low field rate (mainly due to
the Hall effect) in the randomly perturbed crust can result in a
catastrophically variation of magnetic field. This intermittent motion, similar
to the avalanche process, makes the Hall effect be dissipative. These
qualitative consequences about magnetized electron gas are consistent with
observations of magnetar emission, and especially the threshold critical
dynamics of driven domain wall can partially overcome the difficulties of
""low-field"" magnetar bursts and the heating mechanism of transient, or
""outbursting"" magnetar.",1605.09467v1
2016-06-22,Induced magnetization and power loss for a periodically driven system of ferromagnetic nanoparticles with randomly oriented easy axes,"We study the effect of an elliptically polarized magnetic field on a system
of non-interacting, single-domain ferromagnetic nanoparticles characterized by
a uniform distribution of easy axis directions. Our main goal is to determine
the average magnetization of this system and the power loss in it. In order to
calculate these quantities analytically, we develop a general perturbation
theory for the Landau-Lifshitz-Gilbert (LLG) equation and find its steady-state
solution for small magnetic field amplitudes. On this basis, we derive the
second-order expressions for the average magnetization and power loss,
investigate their dependence on the magnetic field frequency, and analyze the
role of subharmonic resonances resulting from the nonlinear nature of the LLG
equation. For arbitrary amplitudes, the frequency dependence of these
quantities is obtained from the numerical solution of this equation. The impact
of transitions between different regimes of regular and chaotic dynamics of
magnetization, which can be induced in nanoparticles by changing the magnetic
field frequency, is examined in detail.",1606.07131v1
2016-07-05,Magnetic moment of inertia within the breathing model,"An essential property of magnetic devices is the relaxation rate in magnetic
switching which strongly depends on the energy dissipation and magnetic inertia
of the magnetization dynamics. Both parameters are commonly taken as a
phenomenological entities. However very recently, a large effort has been
dedicated to obtain Gilbert damping from first principles. In contrast, there
is no ab initio study that so far has reproduced measured data of magnetic
inertia in magnetic materials. In this letter, we present and elaborate on a
theoretical model for calculating the magnetic moment of inertia based on the
torque-torque correlation model. Particularly, the method has been applied to
bulk bcc Fe, fcc Co and fcc Ni in the framework of the tight-binding
approximation and the numerical values are comparable with recent experimental
measurements. The theoretical results elucidate the physical origin of the
moment of inertia based on the electronic structure. Even though the moment of
inertia and damping are produced by the spin-orbit coupling, our analysis shows
that they are caused by undergo different electronic structure mechanisms.",1607.01307v1
2016-09-28,Effective magnetic correlations in hole-doped graphene nanoflakes,"The magnetic properties of zig-zag graphene nanoflakes (ZGNF) are
investigated within the framework of the dynamical mean-field theory. At
half-filling and for realistic values of the local interaction, the ZGNF is in
a fully compensated antiferromagnetic (AF) state, which is found to be robust
against temperature fluctuations. Introducing charge carriers in the AF
background drives the ZGNF metallic and stabilizes a magnetic state with a net
uncompensated moment at low temperature. The change in magnetism is ascribed to
the delocalization of the doped holes in the proximity of the edges, which
mediate ferromagnetic correlations between the localized magnetic moments.
Depending on the hole concentration, the magnetic transition may display a
pronounced hysteresis over a wide range of temperature, indicating the
coexistence of magnetic states with different symmetry. This suggests the
possibility of achieving the electrostatic control of the magnetic state of
ZGNFs to realize a switchable spintronic device.",1609.08812v1
2016-10-28,Spin-Orbit Torque Efficiency in Compensated Ferrimagnetic Cobalt-Terbium Alloys,"Despite the potential advantages of information storage in
antiferromagnetically coupled materials, it remains unclear whether one can
control the magnetic moment orientation efficiently because of the cancelled
magnetic moment. Here, we report spin-orbit torque induced magnetization
switching of ferrimagnetic Co1-xTbx films with perpendicular magnetic
anisotropy. Current induced switching is demonstrated in all of the studied
film compositions, including those near the magnetization compensation point.
The spin-orbit torque induced effective field is further quantified in the
domain wall motion regime. A divergent behavior that scales with the inverse of
magnetic moment is confirmed close to the compensation point, which is
consistent with angular momentum conservation. Moreover, we also quantify the
Dzyaloshinskii-Moriya interaction energy in the Ta/Co1-xTbx system and we find
that the energy density increases as a function of the Tb concentration. The
demonstrated spin-orbit torque switching, in combination with the fast magnetic
dynamics and minimal net magnetization of ferrimagnetic alloys, promises
spintronic devices that are faster and with higher density than traditional
ferromagnetic systems.",1610.09200v1
2016-11-21,Spectropolarimetric evidence for a siphon flow along an emerging magnetic flux tube,"We study the dynamics and topology of an emerging magnetic flux concentration
using high spatial resolution spectropolarimetric data acquired with the
Imaging Magnetograph eXperiment on board the Sunrise balloon-borne solar
observatory. We obtain the full vector magnetic field and the line-of-sight
(LOS) velocity through inversions of the Fe I line at 525.02 nm with the SPINOR
code. The derived vector magnetic field is used to trace magnetic field lines.
Two magnetic flux concentrations with different polarity and LOS velocities are
found to be connected by a group of arch-shaped magnetic field lines. The
positive polarity footpoint is weaker (1100 G) and displays an upflow, while
the negative polarity footpoint is stronger (2200 G) and shows a downflow. This
configuration is naturally interpreted as a siphon flow along an arched
magnetic flux tube.",1611.06732v1
2016-11-23,Dynamics of magnetic nano-flake vortices in Newtonian fluids,"We study the rotational motion of nano-flake ferromagnetic discs suspended in
a Newtonian fluid, as a potential material owing the vortex-like magnetic
configuration. Using analytical expressions for hydrodynamic, magnetic and
Brownian torques, the stochastic angular momentum equation is determined in the
dilute limit conditions under applied magnetic field. Results are compared
against experimental ones and excellent agreement is observed. We also estimate
the uncertainty in the orientation of the discs due to the Brownian torque when
an external magnetic field aligns them. Interestingly, this uncertainty is
roughly proportional to the ratio of thermal energy of fluid to the magnetic
energy stored in the discs. Our approach can be implemented in many practical
applications including biotechnology and multi-functional fluidics.",1611.07680v1
2016-12-07,Effects of a small magnetic field on homoclinic bifurcations in a low-Prandtl-number fluid,"Effects of a uniform magnetic field on homoclinic bifurcations in
Rayleigh-B\'{e}nard convection in a fluid of Prandtl number $Pr = 0.01$ are
investigated using direct numerical simulations (DNS). A uniform magnetic field
is applied either in the vertical or in the horizontal direction. For a weak
vertical magnetic field, the possibilities of both forward and backward
homoclinic bifurcations are observed leading to a spontaneous merging of two
limit cycles into one as well as a spontaneous breaking of a limit cycle into
two for lower values of the Chandrasekhar's number ($Q\leq 5$). A slightly
stronger magnetic field makes the convective flow time independent giving the
possibility of stationary patterns at the secondary instability. For horizontal
magnetic field, the $x\leftrightharpoons y$ symmetry is destroyed and neither a
homoclinic gluing nor a homoclinic breaking is observed. Two low-dimensional
models are also constructed: one for a weak vertical magnetic field and another
for a weak horizontal magnetic field. The models qualitatively capture the
features observed in DNS and help understanding the unfolding of bifurcations
close to the onset of magnetoconvection.",1612.02137v1
2017-03-08,"Interplay of Dirac electrons and magnetism in AMnBi2 (A=Ca, Sr)","Dirac materials exhibit intriguing low-energy carrier dynamics that offer a
fertile ground for novel physics discovery. Of particular interest is the
interplay of Dirac carriers with other quantum phenomena, such as magnetism.
Here we report on a two-magnon Raman scattering study of AMnBi2 (A=Ca, Sr), a
prototypical magnetic Dirac system comprising alternating Dirac-carrier and
magnetic layers. We present the first accurate determination of the exchange
energies in these compounds and, by comparison to the reference compound
BaMn2Bi2, we show that the Dirac-carrier layers in AMnBi2 significantly enhance
the exchange coupling between the magnetic layers, which in turn drives a
charge-gap opening along the Dirac locus. Our findings break new grounds in
unveiling the fundamental physics of magnetic Dirac materials, which offer a
novel platform for probing a distinct type of spin-Fermion interaction. The
outstanding properties of these materials allow a delicate manipulation of the
interaction between the Dirac carriers and magnetic moments, thus holding great
promise for applications in magnetic Dirac devices.",1703.02712v1
2017-09-12,Effect of magnetic field on the phase transition in a dusty plasma,"The formation of self-consistent crystalline structure is a well-known
phenomenon in complex plasmas. In most experiments the pressure and rf power
are the main controlling parameters in determining the phase of the system. We
have studied the effect of externally applied magnetic field on the
configuration of plasma crystals, suspended in the sheath of a radio-frequency
discharge using the Magnetized Dusty Plasma Experiment (MDPX) device.
Experiments are performed at a fixed pressure and rf power where a crystalline
structure is formed within a confining ring. The magnetic field is then
increased from 0 to 1.28 T. We report on the breakdown of the crystalline
structure with increasing magnetic field. The magnetic field affects the
dynamics of the plasma particles and first leads to a rotation of the crystal.
At higher magnetic field, there is a radial variation (shear) in the angular
velocity of the moving particles which we believe leads to the melting of the
crystal. This melting is confirmed by evaluating the variation of the pair
correlation function as a function of magnetic field.",1709.03744v1
2017-10-03,Synchronous magnetic control of water droplets in bulk ferrofluid,"We present a microfluidic platform for two-dimensional manipulation of water
droplets immersed in bulk oil-based ferrofluid. Although non-magnetic, the
droplets are exclusively controlled by magnetic fields, without any
pressure-driven flow. The diphasic fluid layer is trapped in a submillimeter
Hele-Shaw chamber that includes permalloy tracks on its substrate. An in-plane
rotating magnetic field magnetizes the permalloy tracks, thus producing local
magnetic gradients, while an orthogonal magnetic field magnetizes the bulk
ferrofluid. To minimize the magnetostatic energy of the system, droplets are
attracted towards the locations of the tracks where ferrofluid is repelled.
Using this technique, we demonstrate synchronous generation and propagation of
water droplets, analyze PIV data of the bulk ferrofluid flow and study the
kinematics of propagation. In addition, we show controlled break-up of droplets
and droplet-to-droplet interactions. Finally, we discuss future applications
owing to the potential biocompatibility of the droplets.",1710.00961v2
2018-02-13,"From mean-field localized magnetism to itinerant spin fluctuations in the ""Non-metallic metal"" - FeCrAs","FeCrAs displays an unusual electrical response that is neither metallic in
character nor divergent at low temperatures, as expected for an insulating
response, and therefore it has been termed a ""nonmetal-metal"". We carried out
neutron scattering experiments on powder and single crystal samples to study
the magnetic dynamics and critical fluctuations in FeCrAs. Magnetic neutron
diffraction measurements find Cr3+ magnetic order setting in at 115 K with the
mean-field critical exponent. Neutron spectroscopy, however, observes gapless
stiff magnetic fluctuations emanating from magnetic positions with propagation
wave vector q_0=(1/3,1/3), which persists up to at least 80 meV. The magnetism
in FeCrAs therefore displays a response which resembles that of itinerant
magnets at high energy transfers, such as chromium alloys. We suggest that the
presence of stiff high-energy spin fluctuations is the origin of the unusual
temperature dependence of the resistivity.",1802.05271v2
2018-06-20,Magnetic field dependence of antiferromagnetic resonance in NiO,"We report on measurements of magnetic field and temperature dependence of
antiferromagnetic resonances in the prototypical antiferromagnet NiO. The
frequencies of the magnetic resonances in the vicinity of 1 THz have been
determined in the time-domain via time-resolved Faraday measurements after
selective excitation by narrow-band superradiant terahertz (THz) pulses at
temperatures down to 3K and in magnetic fields up to 10 T. The measurements
reveal two antiferromagnetic resonance modes, which can be distinguished by
their characteristic magnetic field dependencies. The nature of the two modes
is discussed by comparison to an eight-sublattice antiferromagnetic model,
which includes superexchange between the next-nearest-neighbor Ni spins,
magnetic dipolar interactions, cubic magneto-crystalline anisotropy, and Zeeman
interaction with the external magnetic field. Our study indicates that a
two-sublattice model is insufficient for the description of spin dynamics in
NiO, while the magnetic-dipolar interactions and magneto-crystalline anisotropy
play important roles.",1806.07968v1
2018-07-10,The dual nature of magnetism in a uranium heavy fermion system,"The duality between localized and itinerant nature of magnetism in
$5\textit{f}$ electron systems has been a longstanding puzzle. Here, we report
inelastic neutron scattering measurements, which reveal both local and
itinerant aspects of magnetism in a single crystalline system of
UPt$_{2}$Si$_{2}$. In the antiferromagnetic state, we observe broad continuum
of diffuse magnetic scattering with a resonance-like gap of $\approx$ 7 meV,
and surprising absence of coherent spin-waves, suggestive of itinerant
magnetism. While the gap closes above the Neel temperature, strong dynamic spin
correlations persist to high temperature. Nevertheless, the size and
temperature dependence of the total magnetic spectral weight can be well
described by local moment with $J=4$. Furthermore, polarized neutron
measurements reveal that the magnetic fluctuations are mostly transverse, with
little or none of the longitudinal component expected for itinerant moments.
These results suggest that a dual description of local and itinerant magnetism
is required to understand UPt$_{2}$Si$_{2}$, and by extension, other 5$f$
systems in general.",1807.03686v1
2019-01-17,"Influences of interfacial oxidization on surface magnetic energy, magnetic damping and spin-orbit-torques in Pt / ferromagnet / capping structures","We investigate the effect of capping layer (CAP) on the interfacial magnetic
anisotropy energy density (K_S), magnetic damping ({\alpha}), and spin-orbit
torques (SOTs) in heavy-metal (Pt) / ferromagnet (Co or Py) / CAP (MgO/Ta,
HfOx, or TaN). At room temperature (RT) the CAP materials influence the
effective magnitude of K_S, which is associated with a formation of interfacial
magnetic oxides. The dynamical dissipation parameters of Co are considerably
influenced by the CAP (especially MgO) while those of Py are not. This is
possibly due to an extra magnetic damping via spin-pumping process across the
Co/CoO interface and incoherent magnon generation (spin fluctuation) in the
interfacial CoO. It is also observed that both anti-damping and field-like SOT
efficiencies vary marginally with the CAP in the thickness ranges we examined.
Our results reveal the crucial role of interfacial oxides on the perpendicular
magnetic anisotropy, magnetic damping, and SOTs.",1901.05777v1
2019-02-06,Measuring magnetic fields with magnetic field insensitive transitions,"Magnetometry is an important tool prevalent in many applications such as
fundamental research, material characterization and biological imaging. Atomic
magnetometry conventionally makes use of two quantum states, the energy
difference of which depends linearly on the magnetic field due to the Zeeman
effect. The magnetic field is evaluated from repeated measurements of the
accumulated dynamic phase between the two Zeeman states in a superposition.
Here we propose a magnetometry method that employs a superposition of clock
states with energies that do not depend, to first-order, on the magnetic field
magnitude. Our method makes use of the geometrical dependence of the
clock-states wavefunctions on the magnetic field orientation. We propose
detailed schemes for measuring both static and time-varying magnetic fields,
and analyze the sensitivity of these methods. We show that, similarly to
Zeeman-based methods, the smallest measurable signal scales inversely with the
system coherence-time, which for clock transitions is typically significantly
longer than for magnetically sensitive transitions. Finally, we experimentally
demonstrate our method on an ensemble of optically trapped 87Rb atoms.",1902.02370v1
2019-02-24,Evidence of a cluster spin-glass state in the B-site disordered perovskite SrTi0.5Mn0.5O3,"SrTi0.5Mn0.5O3 (STMO) is a chemically disordered perovskite having random
distribution of Ti and Mn over 1b site. Striking discrepancies about the
structural and magnetic properties of STMO demands detailed analysis which is
addressed. To explore the magnetic ground state of STMO, static and dynamic
magnetic properties were studied over a broad temperature range (2-300 K). The
dc, ac magnetization show a cusp like peak at Tf ~ 14 K, which exhibits field
and frequency dependence. The thermoremanent magnetization is characterized by
using stretched exponential function and characteristic time suggests the
existence of spin clusters. Also the other features observed in magnetic memory
effect, muon spin resonance/rotation and neutron powder diffraction confirm the
existence of cluster spin glass state in STMO, rather than the long range
ordered ground state. Intriguingly, the observed spin relaxation can be
attributed to the dilute magnetism due to non-magnetic doping at Mn-site and
competing antiferromagnetic and ferromagnetic interactions resulting from the
site disorder.",1902.08961v1
2019-02-27,Emergence of Flat-Band Magnetism and Half-Metallicity in Twisted Bilayer Graphene,"Evidence of flat-band magnetism and half-metallicity in compressed twisted
bilayer graphene is provided with first-principles calculations. We show that
dynamic band-structure engineering in twisted bilayer graphene is possible by
controlling the chemical composition with extrinsic doping, the interlayer
coupling strength with pressure, and the magnetic ordering with external
electric field. By varying the rotational order and reducing the interlayer
separation an unbalanced distribution of charge density resulting in the
spontaneous apparition of localized magnetic moments without disrupting the
structural integrity of the bilayer. Weak exchange correlation between magnetic
moments is estimated in large unit cells. External electric field switches the
local magnetic ordering from ferromagnetic to anti-ferromagnetic.
Substitutional doping shifts the chemical potential of one spin distribution
and leads to half-metallicity. Flakes of compressed twisted bilayer graphene
exhibit spontaneous magnetization, demonstrating that correlation between
magnetic moments is not a necessary condition for their formation.",1902.10287v2
2019-10-22,Controlled nonlinear magnetic damping in spin-Hall nano-devices,"Large-amplitude magnetization dynamics is substantially more complex compared
to the low-amplitude linear regime, due to the inevitable emergence of
nonlinearities. One of the fundamental nonlinear phenomena is the nonlinear
damping enhancement, which imposes strict limitations on the operation and
efficiency of magnetic nanodevices. In particular, nonlinear damping prevents
excitation of coherent magnetization auto-oscillations driven by the injection
of spin current into spatially extended magnetic regions. Here, we propose and
experimentally demonstrate that nonlinear damping can be controlled by the
ellipticity of magnetization precession. By balancing different contributions
to anisotropy, we minimize the ellipticity and achieve coherent magnetization
oscillations driven by spatially extended spin current injection into a
microscopic magnetic disk. Our results provide a novel route for the
implementation of efficient active spintronic and magnonic devices driven by
spin current.",1910.09801v1
2019-10-22,Magnetic properties of clusters of supracolloidal magnetic polymers with central attraction,"Supracolloidal magnetic polymers (SMPs) are structures made by crosslinking
magnetic particles. In this work, using Langevin dynamics simulations, we study
the zero-field magnetic properties of clusters formed in suspensions of SMPs
with different topologies -- chains, rings, X and Y -- that interact via
Stockmayer potential. We find that the presence of central attraction,
resulting in the formation of large compact clusters, leads to a dramatic
decrease of the suspension initial susceptibility, independently from SMP
topology. However, the largest decrease corresponds to chain-like SMPs with
strongly interacting particles. This is due to the higher rotational degrees of
freedom of SMPs with such topology, which allows the particles to reorganise
themselves inside the clusters in such a way that their magnetic moments form
energetically advantageous vortex structures with negligible net magnetic
moments.",1910.10040v1
2019-11-04,Negative Magnetic Diffusivity beta replacing alpha effect in Helical Dynamo,"The alpha effect is known to be an indispensable energy source of the
poloidal magnetic field in the sun or planet. However, the alpha effect is
quenched as the magnetic field grows due to the conservation of magnetic
helicity. With these conventional understanding, what indeed generates and
sustains the observed poloidal magnetic field remains a mystery. To solve this
contradiction between theory and real nature, we derived a semi-analytic
representation of alpha and beta using large scale magnetic helicity and
energy. Applying the simulation data to alpha and beta, we found that the
negative beta effect is a promising substitution of the quenched alpha effect.
However, since the negative beta effect contradicts the conventional dynamo
theory, we derived the new beta expression referring to the field structure
model. This analytic result with the field relation between velocity U and
magnetic field B shows that the beta effect in the helical system is not a
fixed one. Rather, it plays a variable and dynamic role in dynamo depending on
the interaction between the poloidal velocity field and relative strength of
the large scale magnetic field.",1911.01039v1
2019-11-08,GMC Collisions as Triggers of Star Formation. VII. The Effect of Magnetic Field Strength on Star Formation,"We investigate the formation of stars within giant molecular clouds (GMCs)
evolving in environments of different global magnetic field strength and
large-scale dynamics. Building upon a series of magnetohydrodynamic (MHD)
simulations of non-colliding and colliding GMCs, we employ density- and
magnetically-regulated star formation sub-grid models in clouds which range
from moderately magnetically supercritical to near critical. We examine gas and
star cluster morphologies, magnetic field strengths and relative orientations,
pre-stellar core densities, temperatures, mass-to-flux ratios and velocities,
star formation rates and efficiencies over time, spatial clustering of stars,
and kinematics of the stars and natal gas. The large scale magnetic criticality
of the region greatly affects the overall gas evolution and star formation
properties. GMC collisions enhance star formation rates and efficiencies in
magnetically supercritical conditions, but may actually inhibit them in the
magnetically critical case. This may have implications for star formation in
different Galactic environments such as the Galactic Center and the main
Galactic disk.",1911.03141v2
2020-05-09,Bulk and surface topological indices for a skyrmion string,"The magnetic skyrmion is a topological magnetic vortex, and its topological
nature is characterized by an index called skyrmion number which is a mapping
of the magnetic moments defined on a two-dimensional space to a unit sphere. In
three-dimensions, a skyrmion, i.e., a vortex penetrating though the magnet
naturally forms a string, which terminates at the surfaces of the magnet or in
the bulk. For such a string, the topological indices, which control its
topological stability are less trivial. Here, we show theoretically, in terms
of numerical simulation for the current-driven motion of a skyrmion string in a
film sample with the step edges on the surface, that the topological indices
relevant to the stability are the followings; (i) skyrmion number along the
developed surface, and (ii) the monopole charge in the bulk defined as the
integral over the surface enclosing a singular magnetic configuration. As long
as the magnetic configuration is slowly varying, the former is conserved while
its changes is associated with nonzero monopole charge. The skyrmion number and
the monoplole charge offer a coherent understanding of the stability of the
topological magnetic texture and the nontrivial dynamics of skyrmion strings.",2005.04516v1
2020-05-13,Magneto-optics of excitons interacting with magnetic ions in CdSe/CdMnS colloidal nanoplatelets,"Excitons in diluted magnetic semiconductors represent excellent probes for
studying the magnetic properties of these materials. Various magneto-optical
effects, which depend sensitively on the exchange interaction of the excitons
with the localized spins of the magnetic ions can be used for probing. Here, we
study core/shell CdSe/(Cd,Mn)S colloidal nanoplatelets hosting diluted magnetic
semiconductor layers. The inclusion of the magnetic Mn$^{2+}$ ions is evidenced
by three magneto-optical techniques using high magnetic fields up to 15 T:
polarized photoluminescence, optically detected magnetic resonance, and
spin-flip Raman scattering. In particular, information on the Mn$^{2+}$
concentration in the CdS shell layers can be obtained from the spin-lattice
relaxation dynamics of the Mn$^{2+}$ spin system.",2005.06548v1
2020-05-27,Effects of the Zhang-Li Torque on Spin Torque nano Oscillators,"Spin-torque nano-oscillators (STNO) are microwave auto-oscillators based on
magnetic resonances having a nonlinear response with the oscillating amplitude,
which provides them with a large frequency tunability including the possibility
of mutual synchronization. The magnetization dynamics in STNO are induced by
spin transfer torque (STT) from spin currents and can be detected by changes in
electrical resistance due to giant magnetoresistance or tunneling
magnetoresistance. The STT effect is usually treated as a damping-like term
that reduces magnetic dissipation and promotes excitation of magnetic modes.
However, an additional term, known as Zhang-Li term has an effect on
magnetization gradients such as domain walls, and could have an effect on
localized magnetic modes in STNO. Here we study the effect of Zhang-Li torques
in magnetic excitations produced in STNO with a nanocontact geometry. Using
micromagnetic simulations we find that Zhang-Li torque modify threshold
currents of magnetic modes and their effective sizes. Additionally we show that
effects can be controlled by changing the ratio between nanocontact size and
layer thickness.",2005.13364v1
2020-06-15,Magnetic Droplet Solitons,"Magnetic droplet solitons are dynamical magnetic textures that form due to an
attractive interaction between spin waves in thin films with perpendicular
magnetic anisotropy. Spin currents and the spin torques associated with these
currents enable their formation as they provide a means to excite
non-equilibrium spin wave populations and compensate their decay. Recent years
have seen rapid advances in experiments that realize and study magnetic
droplets. Important advances include the first direct x-ray images of droplets,
determination of their threshold and sustaining currents, measurement of their
generation and annihilation time and evidence for drift instabilities, which
can limit their lifetime in spin-transfer nanocontacts. This article reviews
these studies and contrasts these solitons to other types of spin-current
excitations such as spin-wave bullets, and static magnetic textures, including
magnetic vortices and skyrmions. Magnetic droplet solitons can also serve as
current controlled microwave frequency oscillators with potential applications
in neuromorphic chips as nonlinear oscillators with memory.",2006.08808v2
2020-07-16,Viscosity of the magnetized strongly coupled one-component plasma,"The viscosity tensor of the magnetized one-component plasma, consisting of
five independent shear viscosity coefficients, a bulk viscosity coefficient,
and a cross coefficient, is computed using equilibrium molecular dynamics
simulations and the Green-Kubo relations. A broad range of Coulomb coupling and
magnetization strength conditions are studied. Magnetization is found to
strongly influence the shear viscosity coefficients when the gyrofrequency
exceeds the Coulomb collision frequency. Three regimes are identified as the
Coulomb coupling strength and magnetization strength are varied. The Green-Kubo
relations are used to separate kinetic and potential energy contributions to
each viscosity coefficient, showing how each contribution depends upon the
magnetization strength. The shear viscosity coefficient associated with the
component of the stress tensor parallel to the magnetic field, and the two
coefficients associated with the component perpendicular to the magnetic field,
are all found to merge to a common value at strong Coulomb coupling.",2007.08417v1
2020-09-04,Strong enhancement of magnetic order from bulk to stretched monolayer FeSe as Hund's metals,"Despite of the importance of magnetism in possible relation to other key
properties in iron-based superconductors, its understanding is still far from
complete especially for FeSe systems. On one hand, the origin of the absence of
magnetic orders in bulk FeSe is yet to be clarified. On the other hand, it is
still not clear how close monolayer FeSe on SrTiO$_3$, with the highest
transition temperature among iron-based superconductors, is to a magnetic
instability. Here we investigate magnetic properties of bulk and monolayer FeSe
using dynamical mean-field theory combined with density-functional theory. We
find that suppressed magnetic order in bulk FeSe is associated with the
reduction of inter-orbital charge fluctuations, an effect of Hund's coupling,
enhanced by a larger crystal field splitting. Meanwhile, spatial isolation of
Fe atoms in expanded monolayer FeSe leads into a strong magnetic order, which
is completely destroyed by a small electron doping. Our work provides a
comprehensive understanding of the magnetic order in iron-based superconductors
and other general multi-orbital correlated systems as Hund's metals.",2009.02024v1
2020-09-14,Nonlinear high-frequency magnetic response of magnetoferritin metacrystals governed by spin thermodynamics,"A theory is developed of the time-dependent magnetization of the metacrystal
composed of magnetoferritin macromolecules. Such superstructures, comprising up
to several millions of superparamagnetic nanoparticles encapsulated in protein
shells, can be created artificially using biochemical assembling technologies.
They have been also shown to occur naturally in sensitive cells of the inner
ear of birds, which suggests their possible involvement in the detection of the
geomagnetic field for orientation and navigation of migratory animals. The
dynamics of the magnetic system of the magnetoferritin metacrystal, comprising
a very large number of magnetic moments coupled by long-range dipole forces, is
exceedingly complex. In order to find the response of the metacrystal to
high-frequency magnetic fields, we used a thermodynamic approach borrowed from
the theory of nuclear spin systems of solids. The resulted theory yields the
time-dependent superspin temperature and magnetization induced by oscillating
magnetic fields of arbitrary strength. The predicted dependence of the
high-frequency response on the static magnetic field can be used for
experimental detection and characterization of magnetoferritin metacrystals in
biological tissues.",2009.06235v1
2020-11-20,A phenomenological theory of the optical magnetization reversal,"All-optical switching of the magnetization in magnetic nanostructures by
femtosecond circularly polarized laser pulses has been demonstrated in several
systems. We present a Landau-Lifshitz-Lambda (LLL) model which describes the
magnetization dynamics using three density states: two ferromagnetic grounds
states and an excited optical state. One of the ferromagnetic ground states is
optically excited by circularly polarized light to a spin reversed state, which
is then ""Coulomb collapsed"" to the magnetization reversed ground state. The
time evolution of the optically excited states is described by a Lindblad
master equation, in which the optical excitation is introduced via the
Hamiltonian. Dissipation terms are introduced via Lindblad operators. The LLL
model combines the precessional motion of the magnetization described by the
Landau-Lifshitz theory, with the response of the three level Lambda system. The
optical excitation lasts for the duration of the laser pulse and the system
relaxes at a fast rate due to the electron-electron interaction. We study the
solution of the eigenvalues problem of the optical equation of motion for the
magnetization and identify a coherent and incoherent regimes and derive an LLL
model that can be integrated with existing micromagnetic codes to describe
optical excitation of magnetic materials.",2011.10247v1
2021-06-02,Topological Magnons: A Review,"At sufficiently low temperatures magnetic materials often enter a correlated
phase hosting collective, coherent magnetic excitations such as magnons or
triplons. Drawing on the enormous progress on topological materials of the last
few years, recent research has led to new insights into the geometry and
topology of these magnetic excitations. Berry phases associated to magnetic
dynamics can lead to observable consequences in heat and spin transport while
analogues of topological insulators and semimetals can arise within magnon band
structures from natural magnetic couplings. Magnetic excitations offer a
platform to explore the interplay of magnetic symmetries and topology, to drive
topological transitions using magnetic fields. examine the effects of
interactions on topological bands and to generate topologically protected spin
currents at interfaces. In this review, we survey progress on all these topics,
highlighting aspects of topological matter that are unique to magnon systems
and the avenues yet to be fully investigated.",2106.01430v1
2021-09-06,Magnetic loading of magnetars' flares,"Magnetars, the likely sources of Fast Radio Bursts (FRBs), produce both
steady highly relativistic magnetized winds, and occasional ejection events. We
demonstrate that the requirement of conservation of the magnetic flux dominates
the overall dynamics of magnetic explosions. This is missed in conventional
hydrodynamic models of the ejections as expanding shell with parametrically
added magnetic field, as well as one-dimensional models of magnetic
disturbances. Most of the initial free energy of an explosion is actually spent
on stretching its own internal magnetic field, while doing minimal $pdV$ work
against the surrounding. Magnetic explosions from magnetars come into force
balance with the pre-flares wind close to the light cylinder. They are then
advected quietly with the wind, or propagate as electromagnetic disturbances.
No powerful shock waves are generated in the wind.",2109.02524v1
2021-10-22,Magnetization dynamics fingerprints of an excitonic condensate $t_{2g}^{4}$ magnet,"The competition between spin-orbit coupling $\lambda$ and electron-electron
interaction $U$ leads to a plethora of novel states of matter, extensively
studied in the context of $t_{2g}^4$ and $t_{2g}^5$ materials, such as
ruthenates and iridates. Excitonic magnets -- the antiferromagnetic state of
bounded electron-hole pairs -- is a prominent example of phenomena driven by
those competing energy scales. Interestingly, recent theoretical studies
predicted that excitonic magnets can be found in the ground-state of
spin-orbit-coupled $t_{2g}^4$ Hubbard models. Here, we present a detailed
computational study of the magnetic excitations in that excitonic magnet,
employing one-dimensional chains (via density matrix renormalization group) and
small two-dimensional clusters (via Lanczos). Specifically, first we show that
the low-energy spectrum is dominated by a dispersive (acoustic) magnonic mode,
with extra features arising from the $\lambda=0$ state in the phase diagram.
Second, and more importantly, we found a novel magnetic excitation forming a
high-energy optical mode with the highest intensity at wavevector $q\to 0$. In
the excitonic condensation regime at large $U$, we also have found a novel
high-energy $\pi$-mode composed solely of orbital excitations. These unique
fingerprints of the $t_{2g}^4$ excitonic magnet are important in the analysis
of neutron and RIXS experiments.",2110.11828v1
2021-10-31,Thermally induced all-optical ferromagnetic resonance in thin YIG films,"All-optical ferromagnetic resonance (AO-FMR) is a powerful tool for local
detection of micromagnetic parameters, such as magnetic anisotropy, Gilbert
damping or spin stiffness. In this work we demonstrate that the AO-FMR method
can be used in thin films of Yttrium Iron Garnet (YIG) if a metallic capping
layer (Au, Pt) is deposited on top of the film. Magnetization precession is
triggered by heating of the metallic layer with femtosecond laser pulses. The
heating modifies the magneto-crystalline anisotropy of the YIG film and shifts
the quasi-equilibrium orientation of magnetization, which results in
precessional magnetization dynamics. The laser-induced magnetization precession
corresponds to a uniform (Kittel) magnon mode, with the precession frequency
determined by the magnetic anisotropy of the material as well as the external
magnetic field, and the damping time set by a Gilbert damping parameter. The
AO-FMR method thus enables measuring local magnetic properties, with spatial
resolution given only by the laser spot size.",2111.00586v1
2021-11-04,Mechanism of magnetic diode in artificial honeycomb lattice,"Spin diode is important prerequisite to practical manifestation of spin
electronics. Yet, a functioning magnetic diode at room temperature is still
illusive. Here, we reveal diode-type phenomena due to magnetic charge mediated
conduction in artificial honeycomb geometry, made of concave shape single
domain permalloy element. We find that honeycomb lattice defies symmetry by
populating vertices with low and high multiplicity magnetic charges, causing
asymmetric magnetization, in applied current of opposite polarity. High
multiplicity units create highly resistive network, thereby inhibiting magnetic
charge dynamics propelled electrical conduction. However, practical realization
of this effect requires modest demagnetization factor in constituting element.
Concave structure fulfills the condition. Subsequently, magnetic diode behavior
emerges across broad thermal range of $T$ = 40K - 300K. The finding is a
departure from the prevailing notion of spin-charge interaction as the sole
guiding principle behind spintronics. Consequently, a new vista, mediated by
magnetic charge interaction, is envisaged for spintronic research",2111.02929v3
2021-12-02,Observing Magnetic Monopoles in Spin Ice using Electron Holography,"While there is compelling evidence for the existence of magnetic monopoles in
spin ice, the direct observation of a point-like source of magnetic field in
these systems remains an open challenge. One promising approach is electron
holography, which combines atomic-scale resolution with extreme sensitivity to
magnetic vector potentials, through the Aharonov-Bohm effect. Here we explore
what holography can teach us about magnetic monopoles in spin ice, through
experiments on artificial spin ice, and numerical simulations of pyrochlore
spin ice. In the case of artificial spin ice, we show that holograms can be
used to measure local magnetic charge. For pyrochlore spin ice, we demonstrate
that holographic experiments are capable of resolving both magnetic monopoles
and their dynamics, including the emergence of electric fields associated with
fluctuations of closed loops of spins. These results establish that the
observation of both magnetic monopoles and emergent electric fields in
pyrochlore spin ice is a realistic possibility in an electron microscope with
sufficiently--high phase resolution.",2112.01362v2
2022-04-29,Saturation of turbulent helical dynamos,"The presence of large scale magnetic fields in nature is often attributed to
the inverse cascade of magnetic helicity driven by turbulent helical dynamos.
In this work we show that in turbulent helical dynamos, the inverse flux of
magnetic helicity towards the large scales $\Pi_{\mathcal{H}}$ is bounded by
$|\Pi_{\mathcal{H}}|\le c \epsilon k_\eta^{-1}$, where $\epsilon$ is the energy
injection rate, $k_\eta$ is the Kolmogorov magnetic dissipation wavenumber and
$c$ an order one constant. Assuming the classical isotropic turbulence scaling,
the inverse flux of magnetic helicity $\Pi_{\mathcal{H}}$ decreases at least as
a $-3/4$ power-law with the magnetic Reynolds number $Rm$ : $|\Pi_{\mathcal{H}}
| \le c \epsilon \ell_f Rm^{-3/4}\max[Pm,1]^{1/4}$, where $Pm$ the magnetic
Prandtl number and $\ell_f$ the forcing lengthscale. We demonstrate this
scaling with $Rm$ using direct numerical simulations of turbulent dynamos
forced at intermediate scales. The results further indicate that nonlinear
saturation is achieved by a balance between the inverse cascade and dissipation
at domain size scales $L$ for which the saturation value of the magnetic energy
is bounded by ${\mathcal{E}}_\text{m}\leq c L (\epsilon \ell_f)^{2/3}
Rm^{1/4}\max[1,Pm]^{1/4}$. Numerical simulations also demonstrate this bound.",2204.14091v2
2022-09-07,Intrinsic chiral field as vector potential of the magnetic current in the zig-zag lattice of magnetic dipoles,"Chiral magnetic insulators manifest novel phases of matter where the sense of
rotation of the magnetization is associated with exotic transport phenomena.
Effective control of such phases and their dynamical evolution points to the
search and study of chiral fields like the Dzyaloshinskii-Moriya interaction.
Here we combine experiments, numerics, and theory to study a zig-zag dipolar
lattice as a model of an interface between magnetic in-plane layers with
perpendicular magnetization. The zig-zag lattice comprises two parallel
sublattices of dipoles with perpendicular easy plane of rotation. The dipolar
energy of the system is exactly separable into a sum of symmetric and
antisymmetric long-range exchange interactions between dipoles, where the
antisymmetric coupling generates a nonlocal Dzyaloshinskii-Moriya field which
stabilizes winding textures with the form of chiral solitons. The
Dzyaloshinskii-Moriya interaction acts as a vector potential or gauge field of
the magnetic current and gives rise to emergent magnetic and electric fields
that allow the manifestation of the magnetoelectric effect in the system.",2209.03052v2
2022-11-07,Determining Perpendicular Magnetic Anisotropy in Fe/MgO/Fe Magnetic Tunnel Junction: A DFT-Based Spin-Orbit Torque Method,"In our JunPy package, we have combined the first-principles calculated
self-consistent Hamiltonian with divide-and-conquer technique to successfully
determine the magnetic anisotropy (MA) in an Fe/MgO/Fe magnetic tunnel junction
(MTJ). We propose a comprehensive analytical derivation to clarify the crucial
roles of spin-orbit coupling that mediates the exchange and spin-orbit
components of spin torque, and the kinetic and spin-orbit components of spin
current accumulation. The angular dependence of cumulative spin-orbit torque
(SOT) indicates a uniaxial MA corresponding to the out-of-plane rotations of
magnetic moments of the free Fe layers. Different from the conventional MA
energy calculation and the phenomenological theory for a whole MTJ, our results
provide insight into the orbital-resolved SOT for atomistic spin dynamics
simulation in emergency complex magnetic heterojunctions.",2211.03603v3
2023-05-19,Hall effect on the magnetic reconnections during the evolution of a three-dimensional magnetic flux rope,"We present a novel Hall magnetohydrodynamics (HMHD) numerical simulation of a
three-dimensional (3D) magnetic flux rope (MFR) -- generated by magnetic
reconnections from an initial 3D bipolar sheared field. Magnetic reconnections
during the HMHD evolution are compared with the MHD. In both simulations, the
MFRs generate as a consequence of the magnetic reconnection at null points
which has not been realized in contemporary simulations. Interestingly, the
evolution is faster and more intricate in the HMHD simulation. Repetitive
development of the twisted magnetic field lines (MFL) in the vicinity of 3D
nulls (reconnection site) is unique to the HMHD evolution of the MFR. The
dynamical evolution of magnetic field lines around the reconnection site being
affected by the Hall forcing, correspondingly affects the large-scale
structures.",2305.11660v1
2023-07-02,Magnetic braking during direct collapse black hole formation,"Magnetic fields are expected to be efficiently amplified during the formation
of the first massive black holes via the small-scale dynamo and in the presence
of strong accretion shocks occurring during gravitational collapse. Here, we
analyze high-resolution cosmological magneto-hydrodynamical simulations of
gravitational collapse in atomic cooling halos, exploring the dynamical role of
magnetic fields, particularly concerning the effect of magnetic braking and
angular momentum transport. We find that after the initial amplification,
magnetic fields contribute to the transport of angular momentum and reduce it
compared to pure hydrodynamical simulations. However, the magnetic and Reynolds
torques do not fully compensate for the inward advection of angular momentum,
which still accumulates over timescales of $\sim1$~Myr. A Jeans analysis
further shows that magnetic pressure strongly contributes to suppressing
fragmentation on scales of $0.1-10$~pc. Overall, the presence of magnetic
fields thus aids in the transport of angular momentum and favors the formation
of massive objects.",2307.02256v1
2023-07-28,Complex Network View of the Sun's Magnetic Patches: I. Identification,"Solar and stellar magnetic patches (i.e., magnetic fluxes that reach the
surface from the interior) are believed to be the primary sources of a star's
atmospheric conditions. Hence, detecting and identifying these features (also
known as magnetic elements) are among the essential topics in the community.
Here, we apply the complex network approach to recognize the solar magnetic
patches. For this purpose, we use the line-of-sight magnetograms provided by
the Helioseismic and Magnetic Imager on board the Solar Dynamic Observatory. We
construct the magnetic network following a specific visibility graph condition
between pairs of pixels with opposite polarities and search for possible links
between these regions. The complex network approach also provides the ability
to rank the patches based on their connectivity (i.e., degree of nodes) and
importance (i.e., PageRank). The use of the developed algorithm in the
identification of magnetic patches is examined by tracking the features in
consecutive frames, as well as making a comparison with the other approaches to
identification. We find that this method could conveniently identify features
regardless of their sizes. For small-scale (one or two pixels) features, we
estimate the average of 8% false-positive and 1% false-negative errors.",2307.15523v1
2023-08-29,Layer-dependent magnetism and spin fluctuations in atomically thin van der Waals magnet CrPS4,"van der Waals (vdW) magnets, an emerging family of two-dimensional (2D)
materials, have received tremendous attention due to their rich fundamental
physics and significant potential for cutting-edge technological applications.
In contrast to the conventional bulk counterparts, vdW magnets exhibit
significant tunability of local material properties, such as stacking
engineered interlayer coupling and layer-number dependent magnetic and
electronic interactions, which promise to deliver previously unavailable merits
to develop multifunctional microelectronic devices. As a further ingredient of
this emerging topic, here we report nanoscale quantum sensing and imaging of
atomically thin vdW magnet chromium thiophosphate CrPS4, revealing its
characteristic layer-dependent 2D static magnetism and dynamic spin
fluctuations. We also show a large tunneling magnetoresistance in CrPS4-based
spin filter vdW heterostructures. The excellent material stability, robust
strategy against environmental degradation, in combination with tailored
magnetic properties highlight the potential of CrPS4 in developing
state-of-the-art 2D spintronic devices for next-generation information
technologies.",2308.15396v1
2023-09-01,Wall-attached convection under strong inclined magnetic fields,"We employ a linear stability analysis and direct numerical simulations to
study the characteristics of wall-modes in thermal convection in a rectangular
box under strong and inclined magnetic fields. The walls of the convection cell
are electrically insulated. The stability analysis assumes periodicity in the
spanwise direction perpendicular to the plane of the homogeneous magnetic
field. Our study shows that for a fixed vertical magnetic field, the imposition
of horizontal magnetic fields results in an increase of the critical Rayleigh
number along with a decrease in the wavelength of the wall modes. The wall
modes become tilted along the direction of the resulting magnetic fields and
therefore extend further into the bulk as the horizontal magnetic field is
increased. Once the modes localized on the opposite walls interact, the
critical Rayleigh number decreases again and eventually drops below the value
for onset with a purely vertical field. We find that for sufficiently strong
horizontal magnetic fields, the steady wall modes occupy the entire bulk and
therefore convection is no longer restricted to the sidewalls. The above
results are confirmed by direct numerical simulations of the nonlinear
evolution of magnetoconvection.",2309.00745v1
2023-09-23,Magnetic Reconnection and Associated Particle Acceleration in High-energy Astrophysics,"Magnetic reconnection occurs ubiquitously in the universe and is often
invoked to explain fast energy release and particle acceleration in high-energy
astrophysics. The study of relativistic magnetic reconnection in the
magnetically dominated regime has surged over the past two decades, revealing
the physics of fast magnetic reconnection and nonthermal particle acceleration.
Here we review these recent progresses, including the magnetohydrodynamic and
collisionless reconnection dynamics as well as particle energization. The
insights in astrophysical reconnection strongly connect to the development of
magnetic reconnection in other areas, and further communication is greatly
desired. We also provide a summary and discussion of key physics processes and
frontier problems, toward a better understanding to the roles of magnetic
reconnection in high-energy astrophysics.",2309.13382v1
1999-04-09,"Magnetically Driven Warping, Precession and Resonances in Accretion Disks","The inner region of the accretion disk onto a rotating magnetized central
star (neutron star, white dwarf or T Tauri star) is subjected to magnetic
torques which induce warping and precession of the disk. The origin of these
torques lies in the interaction between the (induced) surface current on the
disk and the horizontal magnetic field (parallel to the disk) produced by the
inclined magnetic dipole. Under quite general conditions, there exists a
magnetic warping instability in which the magnetic torque drives the disk plane
away from the equatorial plane of the star toward a state where the disk normal
vector is perpendicular to the spin axis. Viscous stress tends to suppress the
warping instability at large radii, but the magnetic torque always dominates as
the disk approaches the magnetosphere boundary. The magnetic torque also drives
the tilted inner disk into retrograde precession around the stellar spin axis.
Moreover, resonant magnetic forcing on the disk can occur which may affect the
dynamics of the disk. The magnetically driven warping instability and
precession may be related to a number observational puzzles, including: (1)
Spin evolution (torque reversal) of accreting X-ray pulsars; (2) Quasi-periodic
oscillations in low-mass X-ray binaries; (3) Super-orbital periods in X-ray
binaries; (4) Photometric period variations of T Tauri stars.",9904110v3
1999-12-07,The Formation and Structure of a Strongly Magnetized Corona above Weakly Magnetized Accretion Disks,"We use three-dimensional magnetohydrodynamical (MHD) simulations to study the
formation of a corona above an initially weakly magnetized, isothermal
accretion disk. We also describe a modification to time-explicit numerical
algorithms for MHD which enables us to evolve highly stratified disks for many
orbital times. We find that MHD turbulence driven by the magnetorotational
instability (MRI) produces strong amplification of weak fields within two scale
heights of the disk midplane in a few orbital times. About 25 % of the magnetic
energy generated by the MRI within two scale heights escapes due to buoyancy,
producing a strongly magnetized corona above the disk. Most of the buoyantly
rising magnetic energy is dissipated between 3 and 5 scale heights, suggesting
the corona will also be hot. The average vertical disk structure consists of a
weakly magnetized turbulent core below a strongly magnetized corona which is
stable to the MRI. The largescale field structure in both the disk and corona
is toroidal. The functional form of the stress is flat within two scale
heights, but proportional to the density above two scale heights.
  For initially weak uniform vertical fields, we find the exponential growth of
magnetic field via axisymmetric vertical modes of the MRI produces strongly
buoyant sheets of magnetic energy which break the disk apart into horizontal
channels. These channels rise several scale heights vertically before the onset
of the Parker instability distorts the sheets and allows matter to flow back
towards the midplane and reform a disk. We suggest this evolution may be
relevant to the dynamical processes which disrupt the inner regions of a disk
when it interacts with a strongly magnetized central object.",9912135v1
2005-11-25,Magnetar giant flares and afterglows as relativistic magnetized explosions,"(Abridged) We propose that giant flares on Soft Gamma-Ray Repeaters produce
relativistic, strongly magnetized, weakly baryon loaded magnetic clouds,
somewhat analogous to solar coronal mass ejection (CME) events. Flares are
driven by unwinding of internal non-potential magnetic fields which leads to
slow build-up of magnetic energy outside of the neutron star. For large
magnetospheric currents, corresponding to a large twist of external magnetic
field, magnetosphere becomes dynamically unstable on \Alfven crossing times
scale of inner magnetosphere, $t_A \sim R_{NS}/c \sim 30 \mu$sec. Released
magnetic energy results in formation of a strongly magnetized, pair-loaded,
quasi-spherically expanding flux rope, topologically connected by magnetic
field to the neutron star during the prompt flare emission. Magnetic stresses
of the tied flux rope lead to late collimation of the expansion, on time scales
longer than giant flare duration. Relativistic bulk motion of the expanding
magnetic cloud, directed at an angle $\theta \sim 135^\circ$ to the line of
sight (away from the observer), results in a strongly non-spherical forward
shock with observed non-relativistic apparent expansion and bulk motion
velocities $\beta_{app} \sim \cot \theta/2 \sim 0.4 $ at times of first radio
observations approximately one week after the burst. Interaction with a shell
of wind-shocked ISM and then with the unshocked ISM leads to deceleration to
non-relativistic velocities approximately one month after the flare.",0511711v1
2006-08-31,Stability Properties of Magnetic Tower Jets,"Stability properties of ``magnetic tower'' jets propagating in the
gravitationally stratified background have been examined by performing
three-dimensional magnetohydrodynamic simulations. The current-carrying,
Poynting flux-dominated magnetic tower jet, which possesses a highly wound
helical magnetic field, is subject to the current-driven instability (CDI). We
find that, under general physical conditions including small perturbations in
the initial background profiles, the propagating magnetic tower jets develop
the non-axisymmetric, $m=1$ kink mode of the CDI. The kink mode grows on the
local Alfv\'en crossing time scale. In addition, two types of kink modes appear
in the system. At the central region where external thermal pressure
confinement is strong, only the internal kink mode is excited and will grow. A
large distance away from the central region where the external thermal pressure
becomes low, the external kink mode is observed. As a result, the exterior of
magnetic tower jets will be deformed into a large-scale wiggled structure. We
also discuss extensively the different physical processes that contribute to
the overall stability properties of the magnetic tower jets. Specifically, when
the jet propagates in an initially unperturbed background, we find that they
can survive the kink mode beyond the point predicted by the well-known
Kruskal-Shafranov (K-S) criterion. The stabilization in this case comes mainly
from the dynamical relaxation of magnetic twists during the propagation of
magnetic towers; the magnetic pitch is reduced and the corresponding K-S
critical wavelength becomes longer as the tower jet proceeds. Furthermore, we
show that the pressure-driven and Kelvin-Helmholtz instabilities do not occur
in the magnetic tower jets.",0609007v1
2007-05-26,Catastrophic eruption of magnetic flux rope in the corona and solar wind with and without magnetic reconnection,"It is generally believed that the magnetic free energy accumulated in the
corona serves as a main energy source for solar explosions such as coronal mass
ejections (CMEs). In the framework of the flux rope catastrophe model for CMEs,
the energy may be abruptly released either by an ideal magnetohydrodynamic
(MHD) catastrophe, which belongs to a global magnetic topological instability
of the system, or by a fast magnetic reconnection across preexisting or
rapidly-developing electric current sheets. Both ways of magnetic energy
release are thought to be important to CME dynamics. To disentangle their
contributions, we construct a flux rope catastrophe model in the corona and
solar wind and compare different cases in which we either prohibit or allow
magnetic reconnection to take place across rapidly-growing current sheets
during the eruption. It is demonstrated that CMEs, even fast ones, can be
produced taking the ideal MHD catastrophe as the only process of magnetic
energy release. Nevertheless, the eruptive speed can be significantly enhanced
after magnetic reconnection sets in. In addition, a smooth transition from slow
to fast eruptions is observed when increasing the strength of the background
magnetic field, simply because in a stronger field there is more free magnetic
energy at the catastrophic point available to be released during an eruption.
This suggests that fast and slow CMEs may have an identical driving mechanism.",0705.3885v1
2008-04-07,A new probe of magnetic fields during high-mass star formation: Zeeman splitting of 6.7 GHz methanol masers,"Context: The role of magnetic fields during high-mass star formation is a
matter of fierce debate, yet only a few direct probes of magnetic field
strengths are available.
  Aims: The magnetic field is detected in a number of massive star-forming
regions through polarization observations of 6.7 GHz methanol masers. Although
these masers are the most abundant of the maser species occurring during
high-mass star formation, most magnetic field measurements in the high-density
gas currently come from OH and H2 O maser observations.
  Methods: The 100-m Effelsberg telescope was used to measure the Zeeman
splitting of 6.7 GHz methanol masers for the first time. The observations were
performed on a sample of 24 bright northern maser sources.
  Results: Significant Zeeman splitting is detected in 17 of the sources with
an average magnitude of 0.56 m/s . Using the current best estimate of the 6.7
GHz methanol maser Zeeman splitting coefficient and a geometrical correction,
this corresponds to an absolute magnetic field strength of 23 mG in the
methanol maser region.
  Conclusions: The magnetic field is dynamically important in the dense maser
regions. No clear relation is found with the available OH maser magnetic field
measurements. The general sense of direction of the magnetic field is
consistent with other Galactic magnetic field measurements, although a few of
the masers display a change of direction between different maser features. Due
to the abundance of methanol masers, measuring their Zeeman splitting provides
the opportunity to construct a comprehensive sample of magnetic fields in
high-mass star-forming regions.",0804.1141v1
2009-04-13,Magnetic Coupling in the Quiet Solar Atmosphere,"Three kinds of magnetic couplings in the quiet solar atmosphere are
highlighted and discussed, all fundamentally connected to the Lorentz force.
First the coupling of the convecting and overshooting fluid in the surface
layers of the Sun with the magnetic field. Here, the plasma motion provides the
dominant force, which shapes the magnetic field and drives the surface dynamo.
Progress in the understanding of the horizontal magnetic field is summarized
and discussed. Second, the coupling between acoustic waves and the magnetic
field, in particular the phenomenon of wave conversion and wave refraction. It
is described how measurements of wave travel times in the atmosphere can
provide information about the topography of the wave conversion zone, i.e., the
surface of equal Alfv\'en and sound speed. In quiet regions, this surface
separates a highly dynamic magnetic field with fast moving magnetosonic waves
and shocks around and above it from the more slowly evolving field of high-beta
plasma below it. Third, the magnetic field also couples to the radiation field,
which leads to radiative flux channeling and increased anisotropy in the
radiation field. It is shown how faculae can be understood in terms of this
effect. The article starts with an introduction to the magnetic field of the
quiet Sun in the light of new results from the Hinode space observatory and
with a brief survey of measurements of the turbulent magnetic field with the
help of the Hanle effect.",0904.2026v1
2009-07-16,Particle size effect on magnetic properties of interacting La0.67Sr0.33MnO3 nanoparticles,"Magnetic nanoparticles of La0.67Sr0.33MnO3 (LSMO) manganite with mean
particle sizes of 13, 16, 18 and 21 nm were prepared by the sol-gel method. The
crystal structure and mean particle size of the synthesized powders were
estimated by X-ray diffraction (XRD) analysis using rietveld refinement and
transmission electron microscopy (TEM). Fourier transform infrared (FTIR)
transmission spectroscopy revealed that stretching and bending modes are
influenced by calcinations temperature. Dc magnetization versus magnetic field
of the samples was carried out at room temperature. Magnetic dynamics of the
samples was studied by the measurement of ac magnetic susceptibility versus
temperature at different frequencies and ac magnetic fields. A
frequency-dependent peak was observed in ac magnetic susceptibility versus
temperature which is well described by Vogel-Fulcher and critical slowing down
laws, and empirical and parameters. By fitting the experimental data with
Vogel-Fulcher and critical slowing down laws, the relaxation time,
characteristic temperature, magnetic anisotropy energy, effective magnetic
anisotropy constant and critical exponent zv have been estimated. The obtained
values of c1, c2, T0 and t0 from the Vogel-Fulcher law support of the presence
of strong interaction between magnetic nanoparticles. The values of zv and t0
obtained from critical slowing down fit suggest the presence of superspin glass
behavior in LSMO nanoparticles of different sizes.",0907.2815v1
2011-03-10,Special Relativistic Magnetohydrodynamic Simulation of Two-Component Outflow Powered by Magnetic Explosion on Compact Stars,"The nonlinear dynamics of outflows driven by magnetic explosion on the
surface of a compact star is investigated through special relativistic
magnetohydrodynamic simulations. We adopt, as the initial equilibrium state, a
spherical stellar object embedded in hydrostatic plasma which has a density
$\rho(r) \propto r^{- \alpha}$ and is threaded by a dipole magnetic field. The
injection of magnetic energy at the surface of compact star breaks the
equilibrium and triggers a two-component outflow. At the early evolutionary
stage, the magnetic pressure increases rapidly around the stellar surface,
initiating a magnetically driven outflow. A strong forward shock driven outflow
is then excited. The expansion velocity of the magnetically driven outflow is
characterized by the Alfv\'en velocity on the stellar surface, and follows a
simple scaling relation $v_{\rm mag} \propto {v_{\rm A}}^{1/2}$. When the
initial density profile declines steeply with radius, the strong shock is
accelerated self-similarly to relativistic velocity ahead of the magnetically
driven component. We find that it evolves according to a self-similar relation
$\Gamma_{\rm sh} \propto r_{\rm sh}$, where $\Gamma_{\rm sh}$ is the Lorentz
factor of the plasma measured at the shock surface $r_{\rm sh}$. Purely
hydrodynamic process would be responsible for the acceleration mechanism of the
shock driven outflow. Our two-component outflow model, which is the natural
outcome of the magnetic explosion, can provide a better understanding of the
magnetic active phenomena on various magnetized compact stars.",1103.1939v1
2013-07-19,Quantitative magnetic imaging at the nanometer scale by ballistic electron magnetic microscopy,"We demonstrate quantitative ballistic electron magnetic microscopy (BEMM)
imaging of simple model Fe(001) nanostructures. We use in situ nanostencil
shadow mask resistless patterning combined with molecular beam epitaxy
deposition to prepare under ultra-high vacuum conditions nanostructured
epitaxial Fe/Au/Fe/GaAs(001) spin-valves. In this epitaxial system, the
magnetization of the bottom Fe/GaAs(001) electrode is parallel to the [110]
direction, defining accurately the analysis direction for the BEMM experiments.
The large hot-electron magnetoresistance of the Fe/Au/Fe/GaAs(001) epitaxial
spin-valve allows us to image various stable magnetic configurations on the
as-grown Fe(001) microstructures with a high sensitivity, even for small
misalignments of both magnetic electrodes. The angular dependence of the
hot-electron magnetocurrent is used to convert magnetization maps calculated by
micromagnetic simulations into simulated BEMM images. The calculated BEMM
images and magnetization rotation profiles show quantitative agreement with
experiments and allow us to investigate the magnetic phase diagram of these
model Fe(001) microstructures. Finally, magnetic domain reversals are observed
under high current density pulses. This opens the way for further BEMM
investigations of current-induced magnetization dynamics.",1307.5150v1
2013-12-17,Measurement of the magnetic interaction between two electrons,"Electrons have an intrinsic, indivisible, magnetic dipole aligned with their
internal angular momentum (spin). The magnetic interaction between two
electrons can therefore impose a change in their spin orientation. This
process, however, was never observed in experiment. The challenge is two-fold.
At the atomic scale, where the coupling is relatively large, the magnetic
interaction is often overshadowed by the much larger coulomb exchange
counterpart. In typical situations where exchange is negligible, magnetic
interactions are also very weak and well below ambient magnetic noise. Here we
report on the first measurement of the magnetic interaction between two
electronic spins. To this end, we used the ground state valence electrons of
two $^{88}$Sr$^+$ ions, co-trapped in an electric Paul trap and separated by
more than two micrometers. We measured the weak, millihertz scale
(alternatively $10^{-18}$ eV or $10^{-14}$ K), magnetic interaction between
their electronic spins. This, in the presence of magnetic noise that was six
orders of magnitude larger than the respective magnetic fields the electrons
apply on each other. Cooperative spin dynamics was kept coherent for 15 s
during which spin-entanglement was generated. The sensitivity necessary for
this measurement was provided by restricting the spin evolution to a
Decoherence-Free Subspace (DFS) which is immune to collective magnetic field
noise. Finally, by varying the separation between the two ions, we were able to
recover the inverse cubic distance dependence of the interaction. The reported
method suggests an alternative route to the search of long-range anomalous
spin-spin forces and can be generalized to include Quantum Error Correction
codes for other cases of extremely weak signal detection.",1312.4881v1
2014-05-19,Approaching the Ground State of Frustrated A-site Spinels: A Combined Magnetization and Polarized Neutron Scattering Study,"We re-investigate the magnetically frustrated, {\it
diamond-lattice-antiferromagnet} spinels FeAl$_2$O$_4$ and MnAl$_2$O$_4$ using
magnetization measurements and diffuse scattering of polarized neutrons. In
FeAl$_2$O$_4$, macroscopic measurements evidence a ""cusp"" in zero field-cooled
susceptibility around 13~K. Dynamic magnetic susceptibility and {\it memory
effect} experiments provide results that do not conform with a canonical
spin-glass scenario in this material. Through polarized neutron scattering
studies, absence of long-range magnetic order down to 4~K is confirmed in
FeAl$_2$O$_4$. By modeling the powder averaged differential magnetic neutron
scattering cross-section, we estimate that the spin-spin correlations in this
compound extend up to the third nearest-neighbour shell. The estimated value of
the Land\'{e} $g$ factor points towards orbital contributions from Fe$^{2+}$.
This is also supported by a Curie-Weiss analysis of the magnetic
susceptibility. MnAl$_2$O$_4$, on the contrary, undergoes a magnetic phase
transition into a long-range ordered state below $\approx$ 40~K, which is
confirmed by macroscopic measurements and polarized neutron diffraction.
However, the polarized neutron studies reveal the existence of prominent
spin-fluctuations co-existing with long-range antiferromagnetic order. The
magnetic diffuse intensity suggests a similar short range order as in
FeAl$_2$O$_4$. Results of the present work supports the importance of spin-spin
correlations in understanding magnetic response of frustrated magnets like
$A$-site spinels which have predominant short-range spin correlations
reminiscent of the ""spin liquid"" state.",1405.4694v1
2014-08-08,Stable Magnetic Droplet Solitons in Spin Transfer Nanocontacts,"Magnetic thin films with perpendicular magnetic anisotropy (PMA) have
localized excitations that correspond to reversed dynamically precessing
magnetic moments, known as magnetic droplet solitons. Fundamentally, these
excitations are associated with an attractive interaction between elementary
spin-excitations (i.e., magnons) and were predicted to occur in PMA materials
in the absence of damping [1,2]. While damping, present in all magnetic
materials, suppresses these excitations, it is now possible to compensate
damping by spin transfer torques through electrical current flow in nanometer
scale contacts to ferromagnetic thin films [3,4]. A theory predicts the
appearance of magnetic droplet solitons at a threshold current in nanocontacts
[5] and, recently, experimental signatures of droplet nucleation have been
reported [6]. However, thus far, they have been observed to be nearly
reversible excitations, with only partially reversed magnetization and to be
subject to instabilities that cause them to drift away from the nanocontacts
(i.e., drift instabilities) [6]. Here we show that magnetic droplet solitons
can be stabilized in a spin transfer nanocontact. Further, they exhibit a
strong hysteretic response to fields and currents and a nearly fully reversed
magnetization in the contact. These observations, in addition to their
fundamental interest, open up new applications for magnetic droplet solitons as
multi-state high frequency current and field tunable oscillators.",1408.1902v1
2015-08-10,Magneto-optical imaging of thin magnetic films using spins in diamond,"Imaging the fields of magnetic materials provides crucial insight into the
physical and chemical processes surrounding magnetism, and has been a key
ingredient in the spectacular development of magnetic data storage. Existing
approaches using the magneto-optic Kerr effect (MOKE), x-ray and electron
microscopy have limitations that constrain further development, and there is
increasing demand for imaging and characterisation of magnetic phenomena in
real time with high spatial resolution. In this work, we show how the
magneto-optical response of an array of negatively-charged nitrogen-vacancy
spins in diamond can be used to image and map the sub-micron stray magnetic
field patterns from thin ferromagnetic films. Using optically detected magnetic
resonance, we demonstrate wide-field magnetic imaging over 100x100 {\mu}m^2
with a diffraction-limited spatial resolution of 440 nm at video frame rates,
under ambient conditions. We demonstrate a novel all-optical spin relaxation
contrast imaging approach which can image magnetic structures in the absence of
an applied microwave field. Straightforward extensions promise imaging with
sub-{\mu}T sensitivity and sub-optical spatial and millisecond temporal
resolution. This work establishes practical diamond-based wide-field microscopy
for rapid high-sensitivity characterisation and imaging of magnetic samples,
with the capability for investigating magnetic phenomena such as domain wall
and skyrmion dynamics and the spin Hall effect in metals.",1508.02135v1
2015-10-05,A hybrid finite-difference/boundary element procedure for the simulation of turbulent MHD duct flow at finite magnetic Reynolds numbers,"A conservative coupled finite difference-boundary element computational
procedure for the simulation of turbulent magnetohydrodynamic flow in a
straight rectangular duct at finite magnetic Reynolds number is presented. The
flow is assumed to be periodic in the streamwise direction and is driven by a
mean pressure gradient. The duct walls are considered to be electrically
insulating. The co-evolution of the velocity and magnetic fields as described
respectively by the Navier-Stokes and the magnetic induction equations,
together with the coupling of the magnetic field between the conducting domain
and the non-conducting exterior is solved using the magnetic field formulation.
The aim is to simulate localized magnetic fields interacting with turbulent
duct flow. Detailed verification of the implementation of the numerical scheme
is conducted in the limiting case of low magnetic Reynolds number by comparing
with the results obtained using a quasistatic approach that has no coupling
with the exterior. The rigorous procedure with non-local magnetic boundary
conditions is compared versus simplified pseudo-vacuum boundary conditions and
the differences are quantified. Our first direct numerical simulations of
turbulent Hartmann duct flow at moderate magnetic Reynolds numbers and a low
flow Reynolds number show significant differences in the duct flow turbulence,
even at low interaction level between the flow and magnetic field",1510.01205v1
2015-10-15,Time-resolved magnetophotoluminescence studies of magnetic polaron dynamics in type-II quantum dots,"We used continuous wave photoluminescence (cw-PL) and time resolved
photoluminescence (TR-PL) spectroscopy to compare the properties of magnetic
polarons (MP) in two related spatially indirect II-VI epitaxially grown quantum
dot systems. In the ZnTe/(Zn,Mn)Se system the holes are confined in the
non-magnetic ZnTe quantum dots (QDs), and the electrons reside in the magnetic
(Zn,Mn)Se matrix. On the other hand, in the (Zn,Mn)Te/ZnSe system, the holes
are confined in the magnetic (Zn,Mn)Te QDs, while the electrons remain in the
surrounding non-magnetic ZnSe matrix. The magnetic polaron formation energies
in both systems were measured from the temporal red-shift of the band-edge
emission. The magnetic polaron exhibits distinct characteristics depending on
the location of the Mn ions. In the ZnTe/(Zn,Mn)Se system the magnetic polaron
shows conventional behavior with decreasing with increasing temperature T and
increasing magnetic field B. In contrast, in the (Zn,Mn)Te/ZnSe system has
unconventional dependence on temperature T and magnetic field B; is weakly
dependent on T as well as on B. We discuss a possible origin for such a
striking difference in the MP properties in two closely related QD systems.",1510.04376v1
2016-09-15,Bright hot impacts by erupted fragments falling back on the Sun: magnetic channelling,"Dense plasma fragments were observed to fall back on the solar surface by the
Solar Dynamics Observatory after an eruption on 7 June 2011, producing strong
EUV brightenings. Previous studies investigated impacts in regions of weak
magnetic field. Here we model the $\sim~300$ km/s impact of fragments
channelled by the magnetic field close to active regions. In the observations,
the magnetic channel brightens before the fragment impact. We use a 3D-MHD
model of spherical blobs downfalling in a magnetized atmosphere. The blob
parameters are constrained from the observation. We run numerical simulations
with different ambient density and magnetic field intensity. We compare the
model emission in the 171\AA~ channel of the Atmospheric Imaging Assembly with
the observed one. We find that a model of downfall channelled in a $\sim~1$MK
coronal loop confined by a magnetic field of $\sim~10-20$G, best explains
qualitatively and quantitatively the observed evolution. The blobs are highly
deformed, further fragmented, when the ram pressure becomes comparable to the
local magnetic pressure and they are deviated to be channelled by the field,
because of the differential stress applied by the perturbed magnetic field.
Ahead of them, in the relatively dense coronal medium, shock fronts propagate,
heat and brighten the channel between the cold falling plasma and the solar
surface. This study shows a new mechanism which brightens downflows channelled
by the magnetic field, such as in accreting young stars, and also works as a
probe of the ambient atmosphere, providing information about the local plasma
density and magnetic field.",1609.04634v1
2016-10-26,Conversion of Internal Gravity Waves into Magnetic Waves,"Asteroseismology probes the interiors of stars by studying oscillation modes
at a star's surface. Although pulsation spectra are well understood for
solar-like oscillators, a substantial fraction of red giant stars observed by
Kepler exhibit abnormally low-amplitude dipole oscillation modes. Fuller et al.
(2015) suggests this effect is produced by strong core magnetic fields that
scatter dipole internal gravity waves (IGWs) into higher multipole IGWs or
magnetic waves. In this paper, we study the interaction of IGWs with a magnetic
field to test this mechanism. We consider two background stellar structures:
one with a uniform magnetic field, and another with a magnetic field that
varies both horizontally and vertically. We derive analytic solutions to the
wave propagation problem and validate them with numerical simulations. In both
cases, we find perfect conversion from IGWs into magnetic waves when the IGWs
propagate into a region exceeding a critical magnetic field strength. Downward
propagating IGWs cannot reflect into upward propagating IGWs because their
vertical wavenumber never approaches zero. Instead, they are converted into
upward propagating slow (Alfvenic) waves, and we show they will likely
dissipate as they propagate back into weakly magnetized regions. Therefore,
strong internal magnetic fields can produce dipole mode suppression in red
giants, and gravity modes will likely be totally absent from the pulsation
spectra of sufficiently magnetized stars.",1610.08506v2
2019-02-09,Full particle-in-cell simulation of the interaction between two plasmas for laboratory experiments on the generation of magnetized collisionless shocks with high-power lasers,"A preliminary numerical experiment is conducted for laboratory experiments on
the generation of magnetized collisionless shocks with high-power lasers by
using one-dimensional particle-in-cell simulation. The present study deals with
the interaction between a moving Aluminum plasma and a Nitrogen plasma at rest.
In the numerical experiment, the Nitrogen plasma is unmagnetized or magnetized
by a weak external magnetic field. Since the previous study suggested the
generation of spontaneous magnetic field in the piston (Aluminum) plasma due to
the Biermann battery, the effect of the magnetic field is of interest. Sharp
jumps of electron density and magnetic field are observed around the interface
between the two plasmas as long as one of the two plasmas is magnetized, which
indicates the formation of tangential electron-magneto-hydro-dynamic
discontinuity. When the Aluminum plasma is magnetized, strong compression of
both density and magnetic field takes place in the pure Aluminum plasma during
the gyration of Nitrogen ions in the Aluminum plasma region. The formation of a
shock downstream is indicated from the shock jump condition. The result
suggests that the spontaneous magnetic field in the piston (Aluminum) plasma
plays an essential role in the formation of a perpendicular collisionless
shock.",1902.03345v1
2019-02-21,Interpreting magnetic helicity flux in solar flux emergence,"Magnetic helicity flux gives information about the topology of a magnetic
field passing through a boundary. In solar physics applications, this boundary
is the photosphere and magnetic helicity flux has become an important quantity
in analysing magnetic fields emerging into the solar atmosphere. In this work
we investigate the evolution of magnetic helicity flux in magnetohydrodynamic
(MHD) simulations of solar flux emergence. We consider emerging magnetic fields
with different topologies and investigate how the magnetic helicity flux
patterns corresponds to the dynamics of emergence. To investigate how the
helicity input is connected to the emergence process, we consider two forms of
the helicity flux. The first is the standard form giving topological
information weighted by magnetic flux. The second form represents the net
winding and can be interpreted as the standard helicity flux less the magnetic
flux. Both quantities provide important and distinct information about the
structure of the emerging field and these quantities differ significantly for
mixed sign helicity fields. A novel aspect of this study is that we account for
the varying morphology of the photosphere due to the motion of the dense plasma
lifted into the chromosphere. Our results will prove useful for the
interpretation of magnetic helicity flux maps in solar observations.",1902.07997v1
2019-04-15,Improvements of the Longitudinal Magnetic Field Measurement from the Solar Magnetic Field Telescope at Huairou Solar Observing Station,"The weak-field approximation implying linear relationship between Stokes
$V/I$ and longitudinal magnetic field, $B_{\Vert}$, often suffers from
saturation observed in strong magnetic field regions such as sunspot umbra. In
this work, we intend to improve the magnetic field observations carried out by
the \textit{Solar Magnetic Field Telescope} (SMFT) at Huairou Solar Observing
Station, China. We propose using non-linear relationship between Stokes $V/I$
and $B_{\Vert}$ to derive the magnetic field. To determine the form of the
relationship, we perform a cross-calibration of the observed SMFT data and
magnetograms provided by the \textit{Helioseismic and Magnetic Imager} on board
the \textit{Solar Dynamics Observatory}. The algorithm of the magnetic field
derivation is described in details. We show that using non-linear relationship
between Stokes $V/I$ and $B_{\Vert}$ allows us to eliminate magnetic field
saturation inside sunspot umbra. The proposed technique enables one to enhance
the reliability of the SMFT magnetic field data obtained even long before the
space-based instrumentation era, since 1987.",1904.07081v7
2019-05-02,Evolution of Magnetic Helicity in Solar Cycle 24,"We propose a novel approach to reconstruct the surface magnetic helicity
density on the Sun or sun-like stars. The magnetic vector potential is
determined via decomposition of vector magnetic field measurements into
toroidal and poloidal components. The method is verified using data from a
non-axisymmetric dynamo model. We apply the method to vector field synoptic
maps from Helioseismic and Magnetic Imager (HMI) onboard of Solar Dynamics
Observatory (SDO) to study evolution of the magnetic helicity density during
solar cycle 24. It is found that the mean helicity density of the
non-axisymmetric magnetic field of the Sun evolves in a way which is similar to
that reported for the current helicity density of the solar active regions. It
has predominantly the negative sign in the northern hemisphere, and it is
positive in the southern hemisphere. Also, the hemispheric helicity rule for
the non-axisymmetric magnetic field showed the sign inversion at the end of
cycle 24. Evolution of magnetic helicity density of large-scale axisymmetric
magnetic field is different from that expected in dynamo theory. On one hand,
the mean large- and small-scale components of magnetic helicity density display
the hemispheric helicity rule of opposite sign at the beginning of cycle 24.
However, later in the cycle, the two helicities exhibit the same sign in
contrast with the theoretical expectations.",1905.00772v1
2019-08-28,Non-monotonic response of a sheared magnetic liquid crystal to an external field,"Utilizing molecular dynamics simulations, we report a non-monotonic
dependence of the shear stress on the strength of an external magnetic field
($H$) in a liquid-crystalline mixture of magnetic and non-magnetic anisotropic
particles. This non-monotonic behavior is in sharp contrast with the
well-studied monotonic $H$-dependency of the shear stress in conventional
ferrofluids, where the shear stress increases with $H$ until it reaches a
saturation value. We relate the origin of this non-monotonicity to the
competing effects of particle alignment along the shear-induced direction, on
the one hand, and the magnetic field direction, on the other hand. To isolate
the role of these competing effects, we consider a two-component mixture
composed of particles with effectively identical steric interactions, where the
orientations of a small fraction, i.e.\ the magnetic ones, are coupled to the
external magnetic field. By increasing $H$ from zero, the orientations of the
magnetic particles show a Fr\'{e}ederickz-like transition and eventually start
deviating from the shear-induced orientation, leading to an increase in shear
stress. Upon further increase of $H$, a demixing of the magnetic particles from
the non-magnetic ones occurs which leads to a drop in shear stress, hence
creating a non-monotonic response to $H$. Unlike the equilibrium demixing
phenomena reported in previous studies, the demixing observed here is neither
due to size-polydispersity nor due to a wall-induced nematic transition. Based
on a simplified Onsager analysis, we rather argue that it occurs solely due to
packing entropy of particles with different shear- or magnetic-field-induced
orientations.",1908.10815v1
2019-11-11,Gate Tunable Magnetism and Giant Magnetoresistance in ABC-stacked Few-Layer Graphene,"Magnetism is a prototypical phenomenon of quantum collective state, and has
found ubiquitous applications in semiconductor technologies such as dynamic
random access memory (DRAM). In conventional materials, it typically arises
from the strong exchange interaction among the magnetic moments of d- or
f-shell electrons. Magnetism, however, can also emerge in perfect lattices from
non-magnetic elements. For instance, flat band systems with high density of
states (DOS) may develop spontaneous magnetic ordering, as exemplified by the
Stoner criterion. Here we report tunable magnetism in rhombohedral-stacked
few-layer graphene (r-FLG). At small but finite doping (n~10^11 cm-2), we
observe prominent conductance hysteresis and giant magnetoconductance that
exceeds 1000% as a function of magnetic fields. Both phenomena are tunable by
density and temperature, and disappears for n>10^12 cm-2 or T>5K. These results
are confirmed by first principles calculations, which indicate the formation of
a half-metallic state in doped r-FLG, in which the magnetization is tunable by
electric field. Our combined experimental and theoretical work demonstrate that
magnetism and spin polarization, arising from the strong electronic
interactions in flat bands, emerge in a system composed entirely of carbon
atoms. The electric field tunability of magnetism provides promise for
spintronics and low energy device engineering.",1911.04450v1
2020-05-28,Unconventional critical behaviors at the magnetic phase transition of Co3Sn2S2 kagome ferromagnet,"Co3Sn2S2 has generated a growing interest as a rare example of the highly
uniaxial anisotropic kagome ferromagnet showing a combination of
frustrated-lattice magnetism and topology. Recently, via precise measurements
of the magnetization and AC susceptibility we have found a low-field anomalous
magnetic phase (A-phase) with very slow spin dynamics that appears just below
the Curie temperature (T_C). The A-phase hosts high-density domain bubbles
after cooling through T_C as revealed in a previous in-situ Lorentz-TEM study.
Here, we present further signatures of the anomalous magnetic transition (MT)
at T_C revealed by a study of the critical behaviors of the magnetization and
magnetocaloric effect using a high-quality single crystal. Analyses of numerous
magnetization isotherms around T_C (177 K) using different approaches (the
modified Arrot plot, Kouvel-Fisher method and magnetocaloric effect) result in
consistent critical exponents that do not satisfy the theoretical predictions
of standard second-order-MT models. Scaling analyses for the magnetization,
magnetic entropy change and field-exponent of the magnetic entropy change, all
consistently show low-field deviations below TC from the universal curves. Our
results reveal that the MT of Co3Sn2S2 can not be explained as a conventional
second-order type and suggest an anomalous magnetic state below T_C.",2005.13920v2
2020-08-10,Investigation of temperature dependence of magnetic properties of Cr$_2$O$_3$ thin film structure using a magnetic field imaging technique based on Nitrogen-Vacancy centres in diamond crystal,"This work presents a magnetic field imaging method based on color centres in
diamond crystal applied to thin film structure. To demonstrate the capacity of
our device we have used it for characterization of magnetic properties in
microscopic scale of Cr$_2$O$_3$ thin film structure above and below N\'eel
temperature. The obtained measurement results clearly identify the detection of
the magnetic phase transition of Cr$_2$O$_3$ thin film with an unexpected
diamagnetic like behaviour at 19$^{\circ}$C (below the N\'eel temperature of
Cr$_2$O$_3$). To have better insights in the magnetic fields created by the
thin films we present simulations of the magnetic fields near the thin film
surface. We also analysed the optically detected magnetic resonance (ODMR)
profiles to be sure that the measured property is related only to the shift of
the optically detected magnetic resonance. We demonstrate how Nitrogen-Vacancy
centre based magnetometry can deliver a convenient platform for research of
phase transition dynamics and spatial magnetic field distributions of thin
films, that can rival other widely used measurement techniques.",2008.04371v2
2022-02-19,Formation and Immediate Deformation of a Small Filament Through Intermittent Magnetic Interactions,"It is generally believed that filament formation involves a process of the
accumulation of magnetic energy. However, in this paper we discuss the idea
that filaments will not erupt and will only deform when the stored magnetic
energy is released gradually. Combining high-quality observations from Solar
Dynamics Observatory and other instruments, we present the formation and
immediate deformation of a small filament (F1) in the active region (AR) 12760
on 28-30 April 2020. Before the filament formation, three successive dipoles
quickly emerged with separation motions in the center of AR 12760. Due to the
magnetic interaction between magnetic dipoles and pre-existing positive
polarities, coronal brightenings consequently appeared in the overlying
atmosphere. Subsequently, because of the continuous cancellation of magnetic
flux that happened around the adjacent ends of F1 and another nearby filament
(F2), the magnetic reconections occurred intermittently occurred between F1 and
F2. Finally, F1 lessened in the shear, and F2 became shorter. All the results
show that the formation of F1 was closely associated with intermittent
interactions between the sequence of emerging dipoles and pre-existing magnetic
polarities, and the immediate deformation of F1 was intimately related to
intermittent interactions between F1 and F2. We also suggest that the
intermittent magnetic interactions driven by the continuous magnetic activities
(magnetic-flux emergence, cancellation, and convergence) play an important role
in the formation and deformation of filaments.",2202.09633v2
2022-05-16,Pseudostreamer influence on flux rope evolution,"A critical aspect of solar activity is the coupling between eruptions and the
surrounding coronal magnetic field, which determines the trajectory and
morphology of the eruptive event. Pseudostreamers (PSs) are coronal magnetic
structures formed by arcs of twin loops capped by magnetic field lines from
coronal holes of the same polarity that meet at a central spine. They contain a
single magnetic null point in the spine, potentially influencing the evolution
of nearby flux ropes (FRs). To understand the net effect of the PS on FR
eruptions is first necessary to study diverse and isolated FR-PS scenarios,
which are not influenced by other magnetic structures. We performed numerical
simulations in which a FR structure is in the vicinity of a PS magnetic
configuration. The combined magnetic field of the PS and the FR results in the
formation of two magnetic null points. We evolve this scenario by numerically
solving the magnetohydrodynamic equations in 2.5D. The simulations consider a
fully ionised compressible ideal plasma in the presence of a gravitational
field and a stratified atmosphere. We find that the dynamic behaviour of the FR
can be categorised into three different classes based on the FR trajectories
and whether it is eruptive or confined. Our analysis indicates that the
magnetic null points are decisive in the direction and intensity of the FR
deflection and their hierarchy depends on the topological arrangement of the
scenario. Moreover, the PS lobe acts as a magnetic cage enclosing the FR. We
report that the total unsigned magnetic flux of the cage is a key parameter
defining whether the FR is ejected or not.",2205.07936v1
2023-12-19,Data-driven simulations of magnetic field evolution in Active Region 11429: Magneto-friction method using PENCIL code,"Coronal magnetic fields evolve quasi statically over long time scales and
dynamically over short time scales. As of now there exists no regular
measurements of coronal magnetic fields, and therefore generating the coronal
magnetic field evolution using the observations of the magnetic field at the
photosphere is of fundamental requirement to understand the origin of the
transient phenomena from the solar active regions. Using the magnetofriction
(MF) approach, we aim to simulate the coronal field evolution in the solar
active region 11429. The MF method is implemented in open source \PC along with
a driver module to drive the initial field with different boundary conditions
prescribed from observed vector magnetic fields at the photosphere. In order to
work with vector potential and the observations, we prescribe three types of
bottom boundary drivers with varying free-magnetic energy. The MF simulation
reproduces the magnetic structure, which better matches to the sigmoidal
morphology exhibited by AIA images at the pre-eruptive time. We found that the
already sheared field further driven by the sheared magnetic field, will
maintain and further build the highly sheared coronal magnetic configuration,
as seen in AR 11429. Data-driven MF simulation is a viable tool to generate the
coronal magnetic field evolution, capturing the formation of the twisted flux
rope and its eruption.",2312.12124v1
2024-01-10,Quantum magnetism in the frustrated square lattice oxyhalides YbBi2IO4 and YbBi2ClO4,"Square-lattice systems offer a direct route for realizing 2D quantum
magnetism with frustration induced by competing interactions. In this work, the
square-lattice materials YbBi2IO4 and YbBi2ClO4 were investigated using a
combination of magnetization and specific heat measurements on polycrystalline
samples. Specific heat measurements provide evidence for long-range magnetic
order below TN = 0.21 K (0.25 K) for YbBi2IO4 (YbBi2ClO4). On the other hand, a
rather broad maximum is found in the temperature-dependent magnetic
susceptibility, located at Tmax = 0.33 K (0.38 K) in YbBi2IO4 (YbBi2ClO4),
consistent with the quasi-2D magnetism expected for the large separation
between the magnetic layers. Estimation of the magnetic entropy supports the
expected Kramers' doublet ground state for Yb3+ and the observed paramagnetic
behavior is consistent with a well-isolated doublet. Roughly two-thirds of the
entropy is consumed above TN, due to a combination of the quasi-2D behavior and
magnetic frustration. The impact of frustration is examined from the viewpoint
of a simplified J1-J2 square lattice model, which is frustrated for
antiferromagnetic interactions. Specifically, a high-temperature series
expansion analysis of the temperature-dependent specific heat and magnetization
data yields J2/J1 = 0.30 (0.23) for YbBi2IO4 (YbBi2ClO4). This simplified
analysis suggests strong frustration that should promote significant quantum
fluctuations in these compounds, and thus motivates future work on the static
and dynamic magnetic properties of these materials.",2401.05283v1
2024-02-03,Role of time-varying magnetic field on QGP equation of state,"The phase diagram of quantum chromodynamics (QCD) and its associated
thermodynamic properties of quark gluon plasma (QGP) are studied in the
presence of time dependent magnetic field. The study plays a pivotal role in
the field of cosmology, astrophysics, and heavy ion collisions. In order to
explore the structure of quark gluon plasma to deal with the dynamics of quarks
and gluons, we investigate the equation of state (EoS) not only in the
environment of static magnetic field but also in the presence of time-varying
magnetic fields. So, for determining the equation of state of QGP at non zero
magnetic fields, we revisited our earlier model where the effect of time
varying magnetic field was not taken into consideration. Using the
phenomenological model, some appealing features are noticed depending upon the
three different scales; effective mass of quark, temperature, and time
independent as well as time-dependent magnetic field. Earlier the effective
mass of quark was incorporated in our calculations and in the current work, it
is modified for static and time-varying magnetic fields. Thermodynamic
observables including pressure, energy density, entropy, etc. are calculated
for a wide range of temperature and time-dependent as well as time-independent
magnetic fields. Finally, we claim that the EoS are highly affected in the
presence of a magnetic field. Our results are notable compared to other
approaches and found to be advantageous for the measurement of QGP equation of
state. These crucial findings with and without time-varying magnetic field
could have phenomenological implications in various sectors of high energy
physics.",2402.02176v1
2022-11-15,Picosecond Spin-Orbit Torque Induced Coherent Magnetization Switching in a Ferromagnet,"Electrically controllable non-volatile magnetic memories show great potential
for the replacement of semiconductor-based technologies. Recently there has
been strong interest in spin-orbit torque (SOT) induced magnetization reversal
due to the device's increased lifetime and speed of operation. However, recent
SOT switching studies reveal an incubation delay in the ~ns range due to
stochasticity in the nucleation of a magnetic domain during reversal. Here, we
experimentally demonstrate ultrafast SOT-induced magnetization switching
dynamics of a ferromagnet with no incubation delay by avoiding the nucleation
process and driving the magnetization coherently. We employ an ultrafast
photo-conducting switch and a co-planar strip line to generate and guide ~ps
current pulses into the heavy metal/ferromagnet layer stack and induce
ultrafast SOT. We use magneto-optical probing to investigate the magnetization
switching dynamics with sub-picosecond time resolution. Depending on the
relative current pulse and in-plane magnetic field polarities, we observe
either an ultrafast demagnetization and subsequent recovery along with a
SOT-induced precessional oscillation, or ultrafast SOT switching. The
magnetization zero-crossing occurs in ~70 ps, which is approximately an order
of magnitude faster than previous studies. Complete switching needs ~250 ps and
is limited by the heat diffusion to the substrate. We use a macro-magnetic
simulation coupled with an ultrafast heating model to analyze the effect of
ultrafast thermal anisotropy torque and current-induced torque in the observed
dynamics. Good agreement between our experimental results and the macro-spin
model shows that the switching dynamics are coherent and present no noticeable
incubation delay. Our work suggests a potential pathway toward dramatically
increasing the writing speed of SOT magnetic random-access memory devices.",2211.08266v1
2009-01-23,Magnetically-actuated artificial cilia for microfluidic propulsion,"Natural cilia are hair-like microtubule-based structures that are able to
move fluid at low Reynolds number through asymmetric motion. In this paper we
follow a biomimetic approach to design artificial cilia lining the inner
surface of microfluidic channels with the goal to propel fluid. The artificial
cilia consist of polymer films filled with magnetic nanoparticles. The
asymmetric, non-reciprocating motion is generated by tuning an external
magnetic field. To obtain the magnetic field and associated magnetization local
to the cilia we solve the Maxwell equations, from which the magnetic torques
can be deduced. To obtain the ciliary motion we solve the dynamic equations of
motion which are then fully coupled to the fluid dynamic equations that
describe fluid flow around the cilia. By doing so we show that by properly
tuning the applied magnetic field, asymmetric ciliary motion can be generated
that is able to propel fluid in a microchannel. The results are presented in
terms of three dimensionless parameters that fully delineate the asymmetry and
cycle time as a function of the relative contribution of elastic, inertial,
magnetic and viscous fluid forces.",0901.3687v1
2010-07-10,Magnetic Lattice Dynamics of the Oxygen-Free FeAs Pnictides: How Sensitive are Phonons to Magnetic Ordering?,"To shed light on the role of magnetism on the superconducting mechanism of
the oxygen-free FeAs pnictides, we investigate the effect of magnetic ordering
on phonon dynamics in the low-temperature orthorhombic parent compounds, which
present a spin-density wave. The study covers both the 122 (AFe2As2; A=Ca, Sr,
Ba) and 1111 (AFeAsF; A=Ca, Sr) phases. We extend our recent work on the Ca
(122 and 1111) and Ba (122) cases by treating computationally and
experimentally the 122 and 1111 Sr compounds. The effect of magnetic ordering
is investigated through detailed non-magnetic and magnetic lattice dynamical
calculations. The comparison of the experimental and calculated phonon spectra
shows that the magnetic interactions/ordering have to be included in order to
reproduce well the measured density of states. This highlights a
spin-correlated phonon behavior which is more pronounced than the apparently
weak electron-phonon coupling estimated in these materials. Furthermore, there
is no noticeable difference between phonon spectra of the 122 Ba and Sr,
whereas there are substantial differences when comparing these to CaFe2As2
originating from different aspects of structure and bonding.",1007.1711v1
2010-10-18,Investigation of Dynamics of Self-Similarly Evolving Magnetic Clouds,"Magnetic clouds (MCs) are ""magnetized plasma clouds"" moving in the solar
wind. MCs transport magnetic flux and helicity away from the Sun. These
structures are not stationary but feature temporal evolution. Commonly,
simplified MC models are considered. The goal of the present study is to
investigate the dynamics of more general, radially expanding MCs. They are
considered as cylindrically symmetric magnetic structures with low plasma
{\beta}. In order to study MC`evolution the self-similar approach method and a
numerical approach are used. It is shown that the forces are balanced in the
considered self-similarly evolving, cylindrically symmetric magnetic
structures. Explicit analytical expressions for magnetic field, plasma
velocity, density and pressure within MCs are derived. These solutions are
characterized by conserved values of magnetic flux and helicity. We also
investigate the dynamics of self-similarly evolving MCs by means of the
numerical code ""Graale"". In addition, their expansion in a medium with higher
density and higher plasma {\beta} is studied. It is shown that the physical
parameters of the MCs maintain their self-similar character throughout their
evolution. Conclusions. A comparison of the different self-similar and
numerical solutions allows us to conclude that the evolving MCs are quite
adequately described by our self-similar solutions - they retain their
self-similar, coherent nature for quite a long time and over large distances
from the Sun.",1010.3573v1
2011-02-28,High Reynolds number magnetohydrodynamic turbulence using a Lagrangian model,"With the help of a model of magnetohydrodynamic (MHD) turbulence tested
previously, we explore high Reynolds number regimes up to equivalent
resolutions of 6000^3 grid points in the absence of forcing and with no imposed
uniform magnetic field. For the given initial condition chosen here, with equal
kinetic and magnetic energy, the flow ends up being dominated by the magnetic
field, and the dynamics leads to an isotropic Iroshnikov-Kraichnan energy
spectrum. However, the locally anisotropic magnetic field fluctuations
perpendicular to the local mean field follow a Kolmogorov law. We find that the
ratio of the eddy turnover time to the Alfven time increases with wavenumber,
contrary to the so-called critical balance hypothesis. Residual energy and
helicity spectra are also considered; the role played by the conservation of
magnetic helicity is studied, and scaling laws are found for the magnetic
helicity and residual helicity spectra. We put these results in the context of
the dynamics of a globally isotropic MHD flow which is locally anisotropic
because of the influence of the strong large-scale magnetic field, leading to a
partial equilibration between kinetic and magnetic modes for the energy and the
helicity.",1102.5581v2
2015-07-11,"Realization of the thermal equilibrium in inhomogeneous magnetic systems by the Landau-Lifshitz-Gilbert equation with stochastic noise, and its dynamical aspects","It is crucially important to investigate effects of temperature on magnetic
properties such as critical phenomena, nucleation, pinning, domain wall motion,
coercivity, etc. The Landau-Lifshitz-Gilbert (LLG) equation has been applied
extensively to study dynamics of magnetic properties. Approaches of Langevin
noises have been developed to introduce the temperature effect into the LLG
equation. To have the thermal equilibrium state (canonical distribution) as the
steady state, the system parameters must satisfy some condition known as the
fluctuation-dissipation relation. In inhomogeneous magnetic systems in which
spin magnitudes are different at sites, the condition requires that the ratio
between the amplitude of the random noise and the damping parameter depends on
the magnitude of the magnetic moment at each site. Focused on inhomogeneous
magnetic systems, we systematically showed agreement between the stationary
state of the stochastic LLG equation and the corresponding equilibrium state
obtained by Monte Carlo simulations in various magnetic systems including
dipole-dipole interactions. We demonstrated how violations of the condition
result in deviations from the true equilibrium state. We also studied the
characteristic features of the dynamics depending on the choice of the
parameter set. All the parameter sets satisfying the condition realize the same
stationary state (equilibrium state). In contrast, different choices of
parameter set cause seriously different relaxation processes. We show two
relaxation types, i.e., magnetization reversals with uniform rotation and with
nucleation.",1507.03075v1
2016-12-22,Imaging Magnetization Structure and Dynamics in Ultrathin YIG/Pt Bilayers with High Sensitivity Using the Time-Resolved Longitudinal Spin Seebeck Effect,"We demonstrate an instrument for time-resolved magnetic imaging that is
highly sensitive to the in-plane magnetization state and dynamics of thin-film
bilayers of yttrium iron garnet (Y3Fe5O12,YIG)/Pt: the time-resolved
longitudinal spin Seebeck (TRLSSE) effect microscope. We detect the local,
in-plane magnetic orientation within the YIG by focusing a picosecond laser to
generate thermally-driven spin current from the YIG into the Pt by the spin
Seebeck effect, and then use the inverse spin Hall effect in the Pt to
transduce this spin current to an output voltage. To establish the time
resolution of TRLSSE, we show that pulsed optical heating of patterned YIG (20
nm)/Pt(6 nm)/Ru (2 nm) wires generates a magnetization-dependent voltage pulse
of less than 100 ps. We demonstrate TRLSSE microscopy to image both static
magnetic structure and gigahertz-frequency magnetic resonance dynamics with
sub-micron spatial resolution and a sensitivity to magnetic orientation below
0.3$^{\circ}/\sqrt{\text{Hz}}$ in ultrathin YIG.",1612.07610v2
2018-09-19,Magnetic non-contact friction from domain wall dynamics actuated by oscillatory mechanical motion,"Magnetic friction is a form of non-contact friction arising from the
dissipation of energy in a magnet due to spin reorientation in a magnetic
field. In this paper we study magnetic friction in the context of
micromagnetics, using our recent implementation of smooth spring-driven motion
[Phys. Rev. E. 97, 053301 (2018)] to simulate ring-down measurements in two
setups where domain wall dynamics is induced by mechanical motion. These
include a single thin film with a domain wall in an external field and a setup
mimicking a magnetic cantilever tip and substrate, in which the two magnets
interact through dipolar interactions. We investigate how various micromagnetic
parameters influence the domain wall dynamics actuated by the oscillatory
spring-driven mechanical motion and the resulting damping coefficient. Our
simulations show that the magnitude of magnetic friction can be comparable to
other forms of non-contact friction. For oscillation frequencies lower than
those inducing excitations of the internal structure of the domain walls, the
damping coefficient is found to be independent of frequency. Hence, our results
obtained in the frequency range from 8 to 112 MHz are expected to be relevant
also for typical experimental setups operating in the 100 kHz range.",1809.07130v2
2019-01-05,Tunable Magnetic Skyrmions in Ultrathin Magnetic Nanostructures for Cellular-Level Neurostimulation,"In 2016, the Global Burden of Disease reported that neurological disorders
were the principal cause of disability-adjusted life years (DALYs) and the
second leading cause of deaths. Research in the last decade has pushed
neuroscience to design and implement low-cost, efficient, implantable, flexible
electrodes/probes and 3D arrays for neuron stimulation and sensing. Electrical
arrays used in current CMOS-based technologies can be affected by the migration
of cells (such as astrocytes) that attempt to seal off the electronic devices,
causing increased impedance and alternations in the electric field. In this
regard, magnetic nanodevices can be better candidates. A wide assortment of
magnetic skyrmion-based device ideas and models have as of late been proposed
featuring their potential applications. In this paper we propose a highly
tunable skyrmion-based spintronic nanodevice for neuron stimulation. The
effects of tunable material and magnetic properties specifically
Dzyaloshinskii-Moriya interaction (DMI), perpendicular magnetic anisotropy
(PMA) constant, number of skyrmions and device dimensions on stable skyrmion
nucleation and smooth skyrmion dynamics in a magnetic ultra-thin film have been
extensively studied. The aim of this study was to meet the standard therapeutic
specifications of neuron stimulation, which is an electric field of about 10
mV/mm for a duration of 50 {\mu}s. From Faraday's Laws of Induction, skyrmion
dynamics that generates an alternating magnetic flux density induces an
electric field for a certain time duration in the ultra-thin film. The results
of this work show that on tuning the skyrmion dynamics, the induced electric
field can reach the standard value required for neurostimulation, thereby
providing a strong futuristic possibility to exploit skyrmion-based spintronic
nanodevices for neuron stimulation.",1901.01349v1
2020-10-24,Dynamics of test particles around regular black holes in modified gravity,"In the work, we have presented detailed analyses of the event horizon and
curvature properties of spacetime around a regular black hole in modified
gravity so-called regular MOG black hole. The motion of neutral, electrically
charged and magnetized particles with magnetic dipole moment, in the close
environment of the regular MOG black hole immersed in an external
asymptotically uniform magnetic field has been also explored. Through the study
on the motion of the neutral test particle, we have obtained that the positive
(negative) values of the MOG parameter mimic the spin of a rotating Kerr black
hole giving the same values for innermost stable pro-grade (retrograde) orbits
of the particles in the range of the spin parameter $a/M \in(-0.4125, \
0.6946)$. The efficiency of energy release from the accretion disk by the
Novikov-Thorne model has been calculated and shown that the efficiency is
linearly proportional to the increase of the MOG parameter $\alpha$, and
exceeds up to 15 \% at the critical limiting value of the MOG parameter
$\alpha_{\rm cr}$. The study of the charged particles dynamics has shown that
innermost stable circular orbits (ISCOs) of the particle increases with the
increase of the MOG parameter, while the increase of cyclotron frequency being
responsible to the magnetic interaction causes the decrease of the latter.
Moreover, the dynamics of magnetic dipoles has shown that the increase of the
MOG and the magnetic coupling parameters lead to an increase of the inner
radius and the width of the accretion disk consisting of magnetized particles.
Finally, we have focused on showing the range of values of the magnetic
coupling parameter causing the orbits to be stable.",2010.12863v1
2021-06-23,Spin dynamics of itinerant electrons: local magnetic moment formation and Berry phase,"The state-of-the-art theoretical description of magnetic materials relies on
solving effective Heisenberg spin problems or their generalizations to
relativistic or multi-spin-interaction cases that explicitly assume the
presence of local magnetic moments in the system. We start with a general
interacting fermionic model that is often obtained in ab initio electronic
structure calculations and show that the corresponding spin problem can be
introduced even in the paramagnetic regime, which is characterized by a zero
average value of the magnetization. Further, we derive a physical criterion for
the formation of the local magnetic moment and confirm that the latter exists
already at high temperatures well above the transition to the ordered magnetic
state. The use of path-integral techniques allows us to disentangle spin and
electronic degrees of freedom and to carefully separate rotational dynamics of
the local magnetic moment from Higgs fluctuations of its absolute value. It
also allows us to accurately derive the topological Berry phase and relate it
to a physical bosonic variable that describes dynamics of the spin degrees of
freedom. As the result, we demonstrate that the equation of motion in the case
of a large magnetic moment takes a conventional Landau-Lifshitz form that
explicitly accounts for the Gilbert damping due to itinerant nature of the
original electronic model.",2106.12462v3
2021-12-16,Plastic vortex creep and dimensional crossovers in the highly anisotropic superconductor HgBa$_2$CuO$_{4+x}$,"In type-II superconductors exposed to magnetic fields between upper and lower
critical values, $H_{c1}$ and $H_{c2}$, penetrating magnetic flux forms a
lattice of vortices whose motion can induce dissipation. Consequently, the
magnetization $M$ of superconductors is typically progressively weakened with
increasing magnetic field $B \propto n_v$ (for vortex density $n_v$). However,
some materials exhibit a non-monotonic $M(B)$, presenting a maximum in $M$ at
what is known as the second magnetization peak. This phenomenon appears in most
classes of superconductors, including low $T_c$ materials, iron-based, and
cuprates, complicating pinpointing its origin and garnering intense interest.
Here, we report on vortex dynamics in optimally doped and overdoped
HgBa$_2$CuO$_{4+x}$ crystals, with a focus on a regime in which plastic
deformations of the vortex lattice govern magnetic properties. Specifically, we
find that both crystals exhibit conspicuous second magnetization peaks and,
from measurements of the field- and temperature- dependent vortex creep rates,
identify and characterize phase boundaries between elastic and plastic vortex
dynamics, as well as multiple previously unreported transitions within the
plastic flow regime. We find that the second magnetization peak coincides with
the elastic-to-plastic crossover for a very small range of high fields, and a
sharp crossover within the plastic flow regime for a wider range of lower
fields. We find evidence that this transition in the plastic flow regime is due
to a dimensional crossover, specifically a transition from 3D to 2D plastic
dynamics.",2112.08667v1
2022-04-05,Reverse shock forming condition for magnetized relativistic outflows: reconciling theories and simulations,"Reverse shock (RS) emission can be used to probe the properties of the
relativistic ejecta, especially the degree of magnetization $\sigma$, in
gamma-ray burst (GRB) afterglows. However, there has been confusion in the
literature regarding the physical condition for the RS formation, and the role
of magnetic fields in the RS dynamics in the Poynting-flux-dominated regime is
not fully understood. Exploiting the shock jump conditions, we characterize the
properties of a magnetized RS. We compare the RS dynamics and forming
conditions from different theories and numerical simulations, and reconcile the
discrepancies among them. The strict RS forming condition is found to be
$\sigma < \sigma_\mathrm{cr}=(8/3)\gamma_4^2(n_1/n_4)$, where $n_4$ and $n_1$
are the rest-frame number densities of the ejecta and the ambient medium,
respectively, $\gamma_4$ is the bulk Lorentz factor, and $\sigma_\mathrm{cr}$
is the critical magnetization. Contrary to previous claims, we prove that this
condition agrees with other theoretical and simulated results, which can be
further applied to the setup and consistency check of future numerical
simulations. Using this condition, we propose a characteristic radius for RS
formation, and categorize the magnetized shell into three regimes: 'thick
shell' (relativistic RS), 'thin shell' (trans-relativistic RS), and 'no RS'
regimes. The critical magnetization $\sigma_\mathrm{cr}$ is generally below
unity for thin shells, but can potentially reaches $\sim 100-1000$ in the
'thick shell' regime. Our results could be applied to the dynamical evolution
of Poynting-flux-dominated ejecta, with potential applications to
self-consistent lightcurve modelling of magnetized relativistic outflows.",2204.02315v2
2022-06-03,Anisotropic Laser-Pulse-Induced Magnetization Dynamics in van der Waals Magnet Fe$_3$GeTe$_2$,"Femtosecond laser-pulse excitation provides an energy efficient and fast way
to control magnetization at the nanoscale, providing great potential for
ultrafast next-generation data manipulation and nonvolatile storage devices.
Ferromagnetic van der Waals materials have garnered much attention over the
past few years due to their low dimensionality, excellent magnetic properties,
and large response to external stimuli. Nonetheless, their behaviour upon fs
laser-pulse excitation remains largely unexplored. Here, we investigate the
ultrafast magnetization dynamics of a thin flake of Fe$_3$GeTe$_2$ (FGT) and
extract its intrinsic magnetic properties using a microscopic framework. We
find that our data is well described by our modelling, with FGT undergoing a
slow two-step demagnetization, and we experimentally extract the
spin-relaxation timescale as a function of temperature, magnetic field and
excitation fluence. Our observations indicate a large spin-flip probability in
agreement with a theoretically expected large spin-orbit coupling, as well as a
weak interlayer exchange coupling. The spin-flip probability is found to
increase when the magnetization is pulled away from its quantization axis,
opening doors to an external control over the spins in this material. Our
results provide a deeper understanding of the dynamics van der Waals materials
upon fs laser-pulse excitation, paving the way towards two-dimensional
materials-based ultrafast spintronics.",2206.01452v1
2022-09-29,Spin Dynamics in Patterned Magnetic Multilayers with Perpendicular Magnetic Anisotropy,"The magnetization dynamics in nanostructures has been extensively studied in
the last decades, and nanomagnetism has evolved significantly over that time,
discovering new effects, developing numerous applications, and identifying
promising new directions. This includes magnonics, an emerging research field
oriented on the study of spin-wave dynamics and their applications. In this
context, thin ferromagnetic films with perpendicular magnetic anisotropy (PMA)
offer interesting opportunities to study spin waves, in particular, due to
out-of-plane magnetization in remanence or at relatively weak external magnetic
fields. This is the only magnetization configuration offering isotropic
in-plane spin-wave propagation within the sample plane, the forward volume
magnetostatic spin-wave geometry. The isotropic dispersion relation is highly
important in designing signal-processing devices, offering superior prospects
for direct replicating various concepts from photonics into magnonics.
Analogous to photonic or phononic crystals, which are the building blocks of
optoelectronics and phononics, magnonic crystals are considered as key
components in magnonics applications. Arrays of nanodots and structured
ferromagnetic thin films with a periodic array of holes, popularly known as
antidot lattices based on PMA multilayers have been recently studied. Novel
magnonic properties related to propagating spin-wave modes, exploitation of the
band gaps, and confined modes, were demonstrated. Also, the existence of
nontrivial magnonic band topologies has been shown. Moreover, the combination
of PMA and Dzyaloshinskii-Moriya interaction leads to the formation of chiral
magnetization states, including N\'eel domain walls, skyrmions, and skyrmionium
states.",2209.14824v2
2023-03-08,Improving Néel domain walls dynamics and skyrmion stability using He ion irradiation,"Magnetization reversal and domain wall dynamics in Pt/Co/AlOx trilayers have
been tuned by He+ ion irradiation. Fluences up to 1.5x10$^{15}$ ions/cm$^2$
strongly decrease the perpendicular magnetic anisotropy (PMA), without
affecting neither the spontaneous magnetization nor the strength of the
Dzyaloshinskii-Moriya interaction (DMI). This confirms the robustness of the
DMI interaction against interfacial chemical intermixing, already predicted by
theory. In parallel with the decrease of the PMA in the irradiated samples, a
strong decrease of the depinning field is observed. This allows the domain
walls to reach large maximum velocities with lower magnetic fields with respect
to those needed for the pristine films. Decoupling PMA from DMI can therefore
be beneficial for the design of low energy devices based on domain wall
dynamics. When the samples are irradiated with larger He+ fluences, the
magnetization gets close to the out-of-plane/in-plane reorientation transition
where 100nm size magnetic skyrmions are stabilized. We observe that as the He+
fluence increases, the skyrmion size decreases while these magnetic textures
become more stable against the application of an external magnetic field.",2303.04543v1
2024-01-21,Study of Reconnection Dynamics and Plasma Relaxation in MHD simulation of a Solar Flare,"Self-organization in continuous systems is associated with dissipative
processes. In particular, for magnetized plasmas, it is known as magnetic
relaxation, where the magnetic energy is converted into heat and kinetic energy
of flow through the process of magnetic reconnection. An example of such a
system is the solar corona, where reconnection manifests as solar transients
like flares and jets. Consequently, toward investigation of plasma relaxation
in solar transients, we utilize a novel approach of data-constrained MHD
simulation for an observed solar flare. The selected active region NOAA 12253
hosts a GOES M1.3 class flare. The investigation of extrapolated coronal
magnetic field in conjunction with the spatiotemporal evolution of the flare
reveals a hyperbolic flux tube (HFT), overlying the observed brightenings. MHD
simulation is carried out with the EULAG-MHD numerical model to explore the
corresponding reconnection dynamics. The overall simulation shows signatures of
relaxation. For a detailed analysis, we consider three distinct sub-volumes. We
analyze the magnetic field line dynamics along with time evolution of
physically relevant quantities like magnetic energy, current density, twist,
and gradients in magnetic field. In the terminal state, none of the sub-volumes
are seen to reach a force-free state, thus remaining in non-equilibrium,
suggesting the possibility of further relaxation. We conclude that the extent
of relaxation depends on the efficacy and duration of reconnection, and hence,
on the energetics and time span of the flare.",2401.11417v1
2014-06-26,"Spin Supercurrent, Magnetization Dynamics, and Phi-State in Spin-Textured Josephson Junctions","The prospect of combining the dissipationless nature of superconducting
currents with the spin-polarization of magnetic materials is interesting with
respect to exploring superconducting analogues of topics in spintronics. In
order to accomplish this aim, it is pivotal to understand how
spin-supercurrents interact dynamically with magnetization textures. We
investigate the appearance of a spin-supercurrent and the resulting
magnetization dynamics in a textured magnetic Josephson current by using three
experimentally relevant models: i) a S/F/S junction with spin-active
interfaces, ii) a S/F1/F2/F3/S Josephson junction with a ferromagnetic
trilayer, and iii) a Josephson junction containing a domain wall. In all of
these cases, the supercurrent is spin-polarized and exerts a spin-transfer
torque on the ferromagnetic interlayers which causes magnetization dynamics.
Using a scattering matrix formalism in the clean limit, we compute the
Andreev-bound states and free energy of the system which is used to solve the
Landau-Lifshiftz-Gilbert equation. We compute both how the inhomogeneous
magnetism influences the phase-dependence of the charge supercurrent as well as
the magnetization dynamics caused by the spin-supercurrent. Using a realistic
experimental parameter set, we find that the supercurrent can induce
magnetization switching that is controlled by the superconducting phase
difference. Moreover, we demonstrate that the combined effect of chiral spin
symmetry breaking and interface scattering causes the system to act as a phase
battery that may supply any superconducting phase difference phi in the ground
state. Such a phi junction is accompanied by an anomalous supercurrent
appearing even at zero phase difference, and we demonstrate that the flow
direction of this current is controlled by the chirality of the magnetization
configuration.",1406.7016v1
2014-10-26,Ising Metamagnet Driven by Propagating Magnetic Field Wave: Nonequilibrium Phases and Transitions,"The nonequilibrium responses of Ising metamagnet (layered antiferromagnet) to
the propagating magnetic wave are studied by Monte Carlo simulation. Here, the
spatio-temporal variations of magnetic field keeps the system away from
equilibrium. The sublattice magnetisations show dynamical symmetry breaking in
the low temperature ordered phase. The nonequilibrium phase transitions are
studied from the temperature dependences of dynamic staggered order parameter,
its derivative and the dynamic specific heat. The transitions are marked by the
peak position of dynamic specific heat and the position of dip of the
derivative of dynamic staggered order parameter. It is observed that, for lower
values of the amplitudes of the propagating magnetic field, if the system is
cooled from a high temperature, it undergoes a phase transition showing sharp
peak of dynamic specific heat and sharp dip of the derivative of the dynamic
staggered order parameter. However, for higher values of the amplitude of the
propagating magnetic field, system exhibits multiple phase transitions. A
comprehensive phase diagram is also plotted. The transition, for vanishingly
small value of the amplitude of the propagating wave, is very close to that for
equilibrium ferro-para phase transition of cubic Ising ferromagnet.",1410.6983v2
2007-03-21,Modeling the (upper) solar atmosphere including the magnetic field,"The atmosphere of the Sun is highly structured and dynamic in nature. From
the photosphere and chromosphere into the transition region and the corona
plasma-$\beta$ changes from above to below one, i.e. while in the lower
atmosphere the energy density of the plasma dominates, in the upper atmosphere
the magnetic field plays the governing role -- one might speak of a ``magnetic
transition''. Therefore the dynamics of the overshooting convection in the
photosphere, the granulation, is shuffling the magnetic field around in the
photosphere. This leads not only to a (re-)structuring of the magnetic field in
the upper atmosphere, but induces also the dynamic reaction of the coronal
plasma e.g. due to reconnection events. Therefore the (complex) structure and
the interaction of various magnetic patches is crucial to understand the
structure, dynamics and heating of coronal plasma as well as its acceleration
into the solar wind.
  The present article will emphasize the need for three-dimensional modeling
accounting for the complexity of the solar atmosphere to understand these
processes. Some advances on 3D modeling of the upper solar atmosphere in
magnetically closed as well as open regions will be presented together with
diagnostic tools to compare these models to observations. This highlights the
recent success of these models which in many respects closely match the
observations.",0703575v1
2008-06-13,Coherent control of nanomagnet dynamics via ultrafast spin torque pulses,"The magnetization orientation of a nanoscale ferromagnet can be manipulated
using an electric current via the spin transfer effect. Time domain
measurements of nanopillar devices at low temperatures have directly shown that
magnetization dynamics and reversal occur coherently over a timescale of
nanoseconds. By adjusting the shape of a spin torque waveform over a timescale
comparable to the free precession period (100-400 ps), control of the
magnetization dynamics in nanopillar devices should be possible. Here we report
coherent control of the free layer magnetization in nanopillar devices using a
pair of current pulses as narrow as 30 ps with adjustable amplitudes and delay.
We show that the switching probability can be tuned over a broad range by
timing the current pulses with the underlying free-precession orbits, and that
the magnetization evolution remains coherent for more than 1 ns even at room
temperature. Furthermore, we can selectively induce transitions along
free-precession orbits and thereby manipulate the free magnetic moment motion.
We expect this technique will be adopted for further elucidating the dynamics
and dissipation processes in nanomagnets, and will provide an alternative for
spin torque driven spintronic devices, such as resonantly pumping microwave
oscillators, and ultimately, for efficient reversal of memory bits in magnetic
random access memory (MRAM).",0806.2297v1
2010-12-11,Dynamical efficiency of collisionless magnetized shocks in relativistic jets,"The so-called internal shock model aims to explain the light-curves and
spectra produced by non-thermal processes originated in the flow of blazars and
gamma-ray bursts. A long standing question is whether the tenuous collisionless
shocks, driven inside a relativistic flow, are efficient enough to explain the
amount of energy observed as compared with the expected kinetic power of the
outflow. In this work we study the dynamic efficiency of conversion of
kinetic-to- thermal/magnetic energy of internal shocks in relativistic
magnetized outflows. We find that the collision between shells with a non-zero
relative velocity can yield either two oppositely moving shocks (in the frame
where the contact surface is at rest), or a reverse shock and a forward
rarefaction. For moderately magnetized shocks (magnetization {\sigma} ~ 0.1),
the dynamic efficiency in a single two-shell interaction can be as large as
40%. Hence, the dynamic efficiency of moderately magnetized shocks is larger
than in the corresponding unmagnetized two-shell interaction. We find that the
efficiency is only weakly dependent on the Lorentz factor of the shells and,
thus internal shocks in the magnetized flow of blazars and gamma-ray bursts are
approximately equally efficient.",1012.2474v1
2012-09-13,Random field Ising model swept by propagating magnetic field wave: Athermal nonequilibrium phase diagram,"The dynamical steady state behaviour of the random field
  Ising ferromagnet swept by a propagating magnetic field wave is studied at
zero temperature by Monte Carlo simulation in two dimensions. The distribution
of the random field is bimodal type. For a fixed set of values of the frequency
and wavelength of propagating magnetic field wave and the strength of the
random field, four distinct dynamical steady states or nonequilibrium phases
were identified. These four nonequilibrium phases are characterised by
different values of structure factors. State of first kind, where all spins are
parallel (up). The second one is, the propagating type, where the sharp strips
formed by parallel spins are found to move coherently. The third one is also
propagating type, where the boundary of the strips of spins is not very sharp.
The fourth kind, shows no propagation of stripes of magnetic spins, forming a
homogeneous distribution of up and down spins. This is disordered phase The
appearance of these four dynamical phases or modes depends on the value of the
amplitude of propagating magnetic field wave and the strength of random
(static) field. A phase diagram has also been drawn, in the plane formed by the
amplitude of propagating field and the strength of random field. It is checked
that, the existence of these dynamical phases is neither a finite size effect
nor a transient phenomenon.",1209.2841v2
2012-09-27,"Magnetic order, magnetic correlations and spin dynamics in the pyrochlore antiferromagnet Er2Ti2O7","Er2Ti2O7 is believed to be a realization of an XY antiferromagnet on a
frustrated lattice of corner-sharing regular tetrahedra. It is presented as an
example of the order-by-disorder mechanism in which fluctuations lift the
degeneracy of the ground state, leading to an ordered state. Here we report
detailed measurements of the low temperature magnetic properties of Er2Ti2O7,
which displays a second-order phase transition at T_N \simeq 1.2 K with
coexisting short- and long-range orders. Magnetic-susceptibility studies show
that there is no spin-glass-like irreversible effect. Heat-capacity
measurements reveal that the paramagnetic critical exponent is typical of a
3-dimensional XY magnet while the low-temperature specific heat sets an upper
limit on the possible spin-gap value and provides an estimate for the spin-wave
velocity. Muon spin relaxation measurements show the presence of spin dynamics
in the nanosecond time scale down to 21 mK. This time range is intermediate
between the shorter time characterizing the spin dynamics in Tb2Sn2O7, which
also displays long- and short-range magnetic order, and the time scale typical
of conventional magnets. Hence the ground state is characterized by exotic spin
dynamics. We determine the parameters of a symmetry-dictated Hamiltonian
restricted to the spins in a tetrahedron, by fitting the paramagnetic diffuse
neutron scattering intensity for two reciprocal lattice planes. These data are
recorded in a temperature region where the assumption that the correlations are
limited to nearest neighbors is fair.",1209.6153v1
2013-01-02,The Equations of Magnetoquasigeostrophy,"The dynamics contained in magnetized layers of exoplanet atmospheres are
important to understand in order to characterize what observational signatures
they may provide for future observations. It is important to develop a
framework to begin studying and learning the physical processes possible under
those conditions and what, if any, features contained in them may be observed
in future observation missions. The aims of this study is to formally derive,
from scaling arguments, a manageable reduced set of equations for analysis,
i.e. a magnetic formulation of the equations of quasigeostrophy appropriate for
a multi-layer atmosphere. The main goal is to provide a simpler theoretical
platform to explore the dynamics possible within confined magnetized layers of
exoplanet atmospheres. We primarily use scaling arguments to derive the reduced
equations of ""magnetoquasigeostrophy"" which assumes dynamics to take place in
an atmospheric layer which is vertically thin compared to its horizontal
scales. The derived equation set retains features existing in standard
shallow-water magnetohydrodynamic equations but are absent in more classical
derivations of the quasi-geostrophic limit, namely, the non-divergence of the
in-plane components of the magnetic field. We liken this non-divergence of the
in-plane magnetic fields as indicative of a quantity whose behaviour mimics a
two-dimensional ""pseudo""-magnetic monopole source. We also find, using the same
scaling argument procedures, appropriate limits of the fundamental parameters
of the system which yield reduced equations describing the flow dynamics
primarily characterized by magnetostrophic balance. The standard scaling
arguments employed here show how traditional magnetized quasigeostrophic
equations connect to their magnetized shallow water forms. The equations
derived are amenable to analysis using well-known techniques.",1301.0285v1
2015-09-09,Emergence of non-twisted magnetic fields in the Sun: Jets and atmospheric response,"Aims. We study the emergence of a non-twisted flux tube from the solar
interior into the solar atmosphere. We investigate whether the length of the
buoyant part of the flux tube (i.e. {\lambda}) affects the emergence of the
field and the dynamics of the evolving magnetic flux system. Methods. We
perform three-dimensional (3D), time-dependent, resistive, compressible MHD
simulations using the Lare3D code. Results. We find that there are considerable
differences in the dynamics of the emergence of a magnetic flux tube when
{\lambda} is varied. In the solar interior, for larger values of {\lambda}, the
rising magnetic field emerges faster and expands more due to its lower magnetic
tension. As a result, its field strength decreases and its emergence above the
photosphere occurs later than in the smaller {\lambda} case. However, in both
cases, the emerging field at the photosphere becomes unstable in two places,
forming two magnetic bipoles that interact dynamically during the evolution of
the system. Most of the dynamic phenomena occur at the current layer, which is
formed at the interface between the interacting bipoles. We find the formation
and ejection of plasmoids, the onset of successive jets from the interface, and
the impulsive heating of the plasma in the solar atmosphere. We discuss the
triggering mechanism of the jets and the atmospheric response to the emergence
of magnetic flux in the two cases.",1509.02850v1
2016-11-10,Spin-charge coupled dynamics driven by a time-dependent magnetization,"The spin-charge coupled dynamics in a thin, magnetized metallic system are
investigated. The effective driving force acting on the charge carriers is
generated by a dynamical magnetic texture, which can be induced, e.g., by a
magnetic material in contact with a normal-metal system. We consider a general
inversion-asymmetric substrate/normal-metal/magnet structure, which, by
specifying the precise nature of each layer, can mimick various experimentally
employed setups. Inversion symmetry breaking gives rise to an effective Rashba
spin-orbit interaction. We derive general spin-charge kinetic equations which
show that such spin-orbit interaction, together with anisotropic Elliott-Yafet
spin relaxation, yields significant corrections to the magnetization-induced
dynamics. In particular, we present a consistent treatment of the spin density
and spin current contributions to the equations of motion, inter alia
identifying a novel term in the effective force which appears due to a spin
current polarized parallel to the magnetization. This ""inverse spin filter""
contribution depends markedly on the parameter which describes the anisotropy
in spin relaxation. To further highlight the physical meaning of the different
contributions, the spin pumping configuration of typical experimental setups is
analyzed in detail. In the two-dimensional limit the build-up of a DC voltage
is dominated by the spin galvanic (inverse Edelstein) effect. A measuring
scheme that could isolate this contribution is discussed.",1611.03378v3
2019-05-20,Injection locking at fractional frequencies of magnetic tunnel junction (MTJ)-based read sensors' ferromagnetic resonance modes,"Being nonlinear dynamic systems, magnetic read sensors should respond to an
excitation signal of a frequency considerably different from their natural
ferromagnetic resonance (FMR) frequencies. Because of the magnetization
dynamics' inherent nonlinear nature, the sensors' response should be measured
at the DC, excitation frequency, and its multiples (harmonics). In this paper,
we present results of such measurements, accomplished using a one-port
nonlinear vector network analyzer (NVNA), which show distinct resonances at
fractional frequencies of the free layer (FL) FMR mode. Identification of these
resonances, resulting from the nonlinear nature of the spin-torque (ST)-induced
magnetization dynamics, was performed using micromagnetic modeling. In
particular, we show that the measured DC response at the above-mentioned
fractional frequencies can be explained by a low-order nonlinearity and strong
magnetodipolar feedback between magnetic layers adjacent to an MgO barrier.
Additionally, we determined that the simulated harmonic response is strongly
enhanced by the mutual ST effect between these layers. Finally, we demonstrate
that the read sensors' nonlinear magnetization dynamics and, by extension,
their harmonic response are highly sensitive to various magnetic and ST
parameters. Thus, this study shows that using NVNA measurements in conjunction
with micromagnetic modeling can clarify the uncertainty in the definition of
these parameters.",1905.08183v2
2019-09-12,The Hofstadter Butterfly in a Dynamic Cavity-Induced Synthetic Magnetic Field,"Energy bands of electrons in a square lattice potential threaded by a uniform
magnetic field exhibit a fractal structure known as the Hofstadter butterfly.
Here we study a Fermi gas in a 2D optical lattice within a linear cavity with a
tilt along the cavity axis. The hopping along the cavity axis is only induced
by resonant Raman scattering of transverse pump light into a standing wave
cavity mode. Choosing a suitable pump geometry allows to realize the
Hofstadter-Harper model with a cavity-induced dynamical synthetic magnetic
field, which appears at the onset of the superradiant phase transition. The
dynamical nature of this cavity-induced synthetic magnetic field arises from
the delicate interplay between collective superradiant scattering and the
underlying fractal band structure. Using a sixth-order expansion of the free
energy as function of the order parameter and by numerical simulations we show
that at low magnetic fluxes the superradiant ordering phase transition is first
order, while it becomes second order for higher flux. The dynamic nature of the
magnetic field induces a non-trivial deformation of the Hofstadter butterfly in
the superradiant phase. At strong pump far above the self-ordering threshold we
recover the Hofstadter butterfly one would obtain in a static magnetic field.",1909.05975v2
2020-03-01,Topological transitions in superconductor nanomembranes in a magnetic field with submicron inhomogeneity under a strong transport current,"Under a strong transport current, the induced voltage in superconductor
nanomembranes in a magnetic field with submicron inhomogeneity shows a pulse on
a certain interval of the magnetic field. It is a manifestation of a wide
phase-slip domain. The topological transition accompanying the phase-slip
effect consists in the occurrence of two new loops of weak superconducting
currents connecting two regions of superconducting screening currents, which
are disconnected in case of vortex-chain dynamics. In the middle of the
phase-slip domain, rapid dynamics of the superconducting order parameter
consists in decoupling of a spontaneously nucleated vortex-antivortex pair,
subsequent motion of a vortex and an antivortex in the opposite directions
followed by their annihilation with an antivortex and a vortex from the
adjacent pairs. The submicron-scale inhomogeneity of the magnetic field can be
achieved through a direct patterning of the magnetic field applied to a planar
membrane or using advanced nanostructuring, such as roll-up technology, focused
ion-beam deposition or coating carbon nanotubes by superconducting materials.
If the applied magnetic field is orthogonal to the axis of a microtube, which
carries transport current in the azimuthal direction, the phase-slip regime is
characterized by the vortex-antivortex lifetime of 10E-15 s versus 10E-12 s for
disconnected vortex dynamics in the half-tubes. The phase-slip dynamics
determines the voltage-magnetic field and voltage-current characteristics in
nanoarchitectures with multiple disconnected loops of superconducting screening
currents.",2003.00508v1
2021-11-17,Probing the magnetic polaron state in the ferromagnetic semiconductor HgCr$_2$Se$_4$ with resistance fluctuation and muon-spin spectroscopy measurements,"Combined resistance noise and muon-spin relaxation ($\mu$SR) measurements of
the ferromagnetic semiconductor HgCr$_2$Se$_4$ suggest a degree of
magnetoelectric coupling and provide evidence for the existence of isolated
magnetic polarons. These form at elevated temperatures and undergo a
percolation transition with a drastic enhancement of the low-frequency
1/$f$-type charge fluctuations at the insulator-to-metal transition at $\sim 95
- 98$ K in the vicinity of the magnetic ordering temperature $T_C \sim 105 -
107$ K. Upon approaching the percolation threshold from above, the strikingly
unusual dynamics of a distinct two-level fluctuator superimposed on the $1/f$
noise can be described by a slowing down of the dynamics of a nanoscale
magnetic cluster, a magnetic polaron, when taking into account an effective
radius of the polaron depending on the spin correlation length. Coinciding
temperature scales found in $\mu$SR and noise measurements suggest changes in
the magnetic dynamics over a wide range of frequencies and are consistent with
the existence of large polarized and domain-wall-like regions at low
temperatures, that result from the freezing of spin dynamics at the magnetic
polaron percolation transition.",2111.08990v1
2022-06-06,Probing spin dynamics of ultra-thin van der Waals magnets via photon-magnon coupling,"Layered van der Waals (vdW) magnets can maintain a magnetic order even down
to the single-layer regime and hold promise for integrated spintronic devices.
While the magnetic ground state of vdW magnets was extensively studied, key
parameters of spin dynamics, like the Gilbert damping, crucial for designing
ultra-fast spintronic devices, remains largely unexplored. Despite recent
studies by optical excitation and detection, achieving spin wave control with
microwaves is highly desirable, as modern integrated information technologies
predominantly are operated with these. The intrinsically small numbers of
spins, however, poses a major challenge to this.
  Here, we present a hybrid approach to detect spin dynamics mediated by
photon-magnon coupling between high-Q superconducting resonators and ultra-thin
flakes of Cr$_2$Ge$_2$Te$_6$ (CGT) as thin as 11\,nm. We test and benchmark our
technique with 23 individual CGT flakes and extract an upper limit for the
Gilbert damping parameter. These results are crucial in designing on-chip
integrated circuits using vdW magnets and offer prospects for probing spin
dynamics of monolayer vdW magnets.",2206.02460v2
2022-12-30,Commensurate and incommensurate magnetic order in the doped two-dimensional Hubbard model: dynamical mean-field theory analysis,"We develop a dynamical mean-field theory approach for the spiral magnetic
order, changing to a local coordinate frame with preferable spin alignment
along the $z$-axis, which can be considered with the impurity solvers treating
the spin diagonal local Green's function. We furthermore solve the
Bethe-Salpeter equations for nonuniform dynamic magnetic susceptibilities in
the local coordinate frame. We apply this approach to describe the evolution of
magnetic order with doping in the $t-t'$ Hubbard model with $t'=0.15$, which is
appropriate for the description of the doped La$_2$CuO$_4$ high-temperature
superconductor. We find that with doping the antiferromagnetic order changes to
the $(Q,\pi)$ incommensurate one, and then to the paramagnetic phase. The
spectral weight at the Fermi level is suppressed near half filling and
continuously increases with doping. The dispersion of holes in the
antiferromagnetic phase shows qualitative agreement with the results of the
$t$-$J$ model consideration. In the incommensurate phase we find two branches
of hole dispersions, one of which crosses the Fermi level. The resulting Fermi
surface forms hole pockets. We also consider the dispersion of the magnetic
excitations, obtained from the non-local dynamic magnetic susceptibilities. The
transverse spin excitations are gapless, fulfilling the Goldstone theorem; in
contrast to the mean-field approach the obtained magnetic state is found to be
stable. The longitudinal excitations are characterized by a small gap, showing
the rigidity of the spin excitations. For realistic hopping and interaction
parameters we reproduce the experimentally measured spin-wave dispersion of
La$_2$CuO$_4$.",2212.14656v2
2023-06-20,High frequency oscillations in spin-torque nano oscillator due to bilinear coupling,"Exchange coupling in an interfacial context is crucial for spin-torque nano
oscillator (STNO) that consists of a non-magnetic spacer which is alloyed with
a ferromagnetic material. Currently, investigations on the dynamics of the free
layer magnetization and frequency enhancement in the STNO with bilinear
coupling are still being actively pursued. In the present work, we investigate
the dynamics of the STNO in the presence of bilinear coupling but in the
absence of an external magnetic field by analyzing the associated
Landau-Lifshitz-Gilbert-Sloncewski(LLGS) equation, and consequently the impact
of the bilinear coupling on the dynamics of the magnetization of the free layer
is studied. It is observed that the frequency of the oscillations in the
magnetization component along the direction of the pinned layer polarization
can be enhanced above 300 GHz by positive bilinear coupling and up to around 30
GHz by negative bilinear coupling. We further reveal a transition from in-plane
to out-of-plane precession both for positive and negative bi-linear couplings.
We also analyze the switching of the magnetization for different values of
current and bilinear coupling. Our detailed investigations of STNO with
bilinear coupling aim at the possibilities of high-frequency devices by
considering the applied current and bilinear coupling in the absence of a
magnetic field.",2306.11415v1
2011-10-21,Thermodynamics of trajectories of the one-dimensional Ising model,"We present a numerical study of the dynamics of the one-dimensional Ising
model by applying the large-deviation method to describe ensembles of dynamical
trajectories. In this approach trajectories are classified according to a
dynamical order parameter and the structure of ensembles of trajectories can be
understood from the properties of large-deviation functions, which play the
role of dynamical free-energies. We consider both Glauber and Kawasaki
dynamics, and also the presence of a magnetic field. For Glauber dynamics in
the absence of a field we confirm the analytic predictions of Jack and Sollich
about the existence of critical dynamical, or space-time, phase transitions at
critical values of the ""counting"" field $s$. In the presence of a magnetic
field the dynamical phase diagram also displays first order transition
surfaces. We discuss how these non-equilibrium transitions in the 1$d$ Ising
model relate to the equilibrium ones of the 2$d$ Ising model. For Kawasaki
dynamics we find a much simple dynamical phase structure, with transitions
reminiscent of those seen in kinetically constrained models.",1110.4857v1
2015-12-26,An Empirical Relation Between The Large-Scale Magnetic Field And The Dynamical Mass In Galaxies,"The origin and evolution of cosmic magnetic fields as well as the influence
of the magnetic fields on the evolution of galaxies are unknown. Though not
without challenges, the dynamo theory can explain the large-scale coherent
magnetic fields which govern galaxies, but observational evidence for the
theory is so far very scarce. Putting together the available data of
non-interacting, non-cluster galaxies with known large-scale magnetic fields,
we find a tight correlation between the integrated polarized flux density,
S(PI), and the rotation speed, v(rot), of galaxies. This leads to an almost
linear correlation between the large-scale magnetic field B and v(rot),
assuming that the number of cosmic ray electrons is proportional to the star
formation rate, and a super-linear correlation assuming equipartition between
magnetic fields and cosmic rays. This correlation cannot be attributed to an
active linear alpha-Omega dynamo, as no correlation holds with global shear or
angular speed. It indicates instead a coupling between the large-scale magnetic
field and the dynamical mass of the galaxies, B ~ M^(0.25-0.4). Hence, faster
rotating and/or more massive galaxies have stronger large-scale magnetic
fields. The observed B-v(rot) correlation shows that the anisotropic turbulent
magnetic field dominates B in fast rotating galaxies as the turbulent magnetic
field, coupled with gas, is enhanced and ordered due to the strong gas
compression and/or local shear in these systems. This study supports an
stationary condition for the large-scale magnetic field as long as the
dynamical mass of galaxies is constant.",1512.08145v2
2018-02-27,Dynamic magnetism in the disordered hexagonal double perovskite BaTi$_{1/2}$Mn$_{1/2}$O$_{3}$,"Magnetic frustration and disorder are key ingredients to prevent the onset of
magnetic order. In the disordered hexagonal double perovskite
BaTi$_{1/2}$Mn$_{1/2}$O$_{3}$, Mn$^{4+}$ cations, with $S=3/2$ spins, can
either form highly correlated states of magnetic trimers or dimers or remain as
weakly interacting orphan spins. At low temperature ($T$), the dimer response
is negligible, and magnetism is dominated by the trimers and orphans. To
explore the role of magnetic frustration, disorder and possibly of quantum
fluctuations, the low-$T$ magnetic properties of the remaining magnetic degrees
of freedom of BaTi$_{1/2}$Mn$_{1/2}$O$_{3}$ are investigated. Heat-capacity
data and magnetic susceptibility display no evidence for a phase transition to
a magnetically ordered phase but indicate the formation of a correlated spin
state. The low-temperature spin dynamics of this state is then explored by
$\mu$SR experiments. The zero field $\mu^{+}$ relaxation rate data show no
static magnetism down to $T=19$ mK and longitudinal field experiments support
as well that dynamic magnetism persists at low $T$. Our results are interpreted
in terms of a spin glass state which stems from a disordered lattice of orphans
spins and trimers. A spin liquid state in BaTi$_{1/2}$Mn$_{1/2}$O$_{3}$,
however, is not excluded and is also discussed.",1802.09950v2
2022-05-06,Tuning the dynamics of chiral domain walls of ferrimagnetic films with the magneto-ionic effect,"The manipulation of magnetism with a gate voltage is expected to lead the way
towards the realization of energy-efficient spintronics devices and
high-performance magnetic memories. Exploiting magneto-ionic effects under
micro-patterned electrodes in solid-state devices adds the possibility to
modify magnetic properties locally, in a non-volatile and reversible way.
Tuning magnetic anisotropy, magnetization and Dzyaloshinskii-Moriya interaction
allows modifying at will the dynamics of non trivial magnetic textures such as
skyrmions and chiral domain walls in magnetic race tracks. In this work, we
illustrate efficient magneto-ionic effects in a ferrimagnetic Pt/Co/Tb stack
using a ZrO2 thin layer as a solid state ionic conductor. When a thin layer of
terbium is deposited on top of cobalt, it acquires a magnetic moment that
aligns antiparallel to that of cobalt, reducing the effective magnetization.
Below the micro-patterned electrodes, the voltage-driven migration of oxygen
ions in a ZrO2 towards the ferrimagnetic stack partially oxidizes the Tb layer,
leading to the local variation not only of the spontaneous magnetization, but
also of the effective magnetic anisotropy and of the Dzyaloshinskii-Moriya
interaction. This leads to a huge increase of the domain wall velocity, which
varies from 10 m/s in the pristine state to 250 m/s after gating. This
non-volatile and reversible tuning of the domain wall dynamics may lead to
applications to reprogrammable magnetic memories or other spintronic devices.",2205.03158v1
2023-10-11,Modern topics in relativistic spin dynamics and magnetism,"Magnetism is a rich subject touching all aspects of physics. My goal with
this dissertation is to explore spin and magnetic moments in
\emph{relativistic} mechanics from both a quantum and classical perspective. We
emphasize the special case of gyromagnetic ratio $g\!=\!2$ and its relationship
to the algebraic spin structure of wave equations. In relativistic quantum
mechanics, we investigate generalizations of the Dirac equation for arbitrary
magnetic moments for fermions. We analyze the homogeneous magnetic field case
and the Coulomb problem for hydrogen-like atoms with emphasis on the role of
the anomalous magnetic moment (AMM). We explore alternative approaches which
combine mass and the magnetic moment. Classically, we propose a relativistic
covariant model of the Stern-Gerlach force via the introduction of a magnetic
four-potential. This model modifies the covariant torque equations and unites
the Amp\`erian and Gilbertian models for magnetic moments.
  We further study (transition) magnetic dipoles in Majorana neutrinos
specifically analyzing the relationship between flavor mixing and
electromagnetic (EM) fields. We demonstrate EM flavor mixing explicitly in the
2-flavor model and develop a dynamical mass basis with an EM rotation matrix.
EM induced neutrino mass splitting is compared to neutrino mass hierarchy.
  An interesting application of these theoretical developments is to study
primordial magnetization in the early universe during the hot dense
electron-positron plasma epoch. We propose a model of magnetic thermal
matter-antimatter plasmas. We analyze the paramagnetic characteristics of
electron-positron plasma when exposed to an external primordial field.
  Future research outlooks include: Second order equations for anomalous
quantum chromodynamic (QCD) moments, neutrino CP violation in strong EM fields,
and fifth-dimension spin dynamics in Kaluza-Klein theory.",2310.07193v1
1995-09-18,Dimensional Reduction and Catalysis of Dynamical Symmetry Breaking by a Magnetic Field,"It is shown that a constant magnetic field in 3+1 and 2+1 dimensions is a
strong catalyst of dynamical chiral symmetry breaking, leading to the
generation of a fermion dynamical mass even at the weakest attractive
interaction between fermions. The essence of this effect is the dimensional
reduction $D\to D-2$ in the dynamics of fermion pairing in a magnetic field.
The effect is illustrated in the Nambu--Jona--Lasinio (NJL) model and QED. In
the NJL model in a magnetic field, the low--energy effective action and the
spectrum of long wavelength collective excitations are derived. In QED (in
ladder and improved ladder approximations) the dynamical mass of fermions
(energy gap in the fermion spectrum) is determined. Possible applications of
this effect and its extension to inhomogeneous field configurations are
discussed.",9509320v2
2000-09-11,Influence of guiding magnetic field on emission of stimulated photons in generators utilizing periodic slow-wave structures,"Effect of guiding magnetic field on evolution of stimulated emission is
considered. It is shown that the transverse dynamics of electrons contributes
to generation process and that contribution decreases with the magnetic field
grows. The equation of generation for stimulated radiation of electron beam
passing over periodic medium in magnetic field of arbitrary value is derived.
The critical value of guiding field for which the transverse dynamics of
electron don't contribute to emission is determined. It is shown, that
transverse dynamics of electron modifies the boundary conditions. It follows
from the derived generation equation that transverse dynamics yields to ~25%
increase of the increment magnitude in high gain regime. In the limit of small
signal the generation gain twice increases when transverse dynamics evolves.
Obtained results are valid for every FEL system, which use the mechanism of
waves slowing in the slow-wave structures for generation.",0009037v1
2009-10-20,Bifurcation and chaos in spin-valve pillars in a periodic applied magnetic field,"We study the bifurcation and chaos scenario of the macro-magnetization vector
in a homogeneous nanoscale-ferromagnetic thin film of the type used in
spin-valve pillars. The underlying dynamics is described by a generalized
Landau-Lifshitz-Gilbert (LLG) equation. The LLG equation has an especially
appealing form under a complex stereographic projection, wherein the
qualitative equivalence of an applied field and a spin-current induced torque
is transparent. Recently chaotic behavior of such a spin vector has been
identified by Zhang and Li using a spin polarized current passing through the
pillar of constant polarization direction and periodically varying magnitude,
owing to the spin-transfer torque effect. In this paper we show that the same
dynamical behavior can be achieved using a periodically varying applied
magnetic field, in the presence of a constant DC magnetic field and constant
spin current, which is technically much more feasible, and demonstrate
numerically the chaotic dynamics in the system for an infinitely thin film.
Further, it is noted that in the presence of a nonzero crystal anisotropy field
chaotic dynamics occurs at much lower magnitudes of the spin-current and DC
applied field.",0910.3776v1
2011-03-11,Dynamical constraints from field line topology in magnetic flux tubes,"A topological constraint on the dynamics of a magnetic field in a flux tube
arises from the fixed point indices of its field line mapping. This can explain
unexpected behaviour in recent resistive-magnetohydrodynamic simulations of
magnetic relaxation. Here we present the theory for a general periodic flux
tube, representing, for example, a toroidal confinement device or a solar
coronal loop. We show how an ideal dynamics on the side boundary of the tube
implies that the sum of indices over all interior fixed points is invariant.
This constraint applies to any continuous evolution inside the tube, which may
be turbulent and/or dissipative. We also consider the analogous invariants
obtained from periodic points (fixed points of the iterated mapping). Although
there is a countably infinite family of invariants, we show that they lead to
at most two independent dynamical constraints. The second constraint applies
only in certain magnetic configurations. Several examples illustrate the
theory.",1103.2284v2
2011-04-21,Quantitative Aspects of the Dynamical CPA in Harmonic Approximation,"Magnetic and electronic properties of the Hubbard model on the Bethe and fcc
lattices in infinite dimensions have been investigated numerically on the basis
of the dynamical coherent potential approximation (CPA) theory combined with
the harmonic approximation (HA) in order to clarify the quantitative aspects of
the theory. It is shown that the dynamical CPA+HA reproduces well the
sublattice magnetization, the magnetizations, susceptibilities, and the N\'eel
temperatures ($T_{\rm N}$) as well as the Curie temperatures calculated by the
Quantum Monte-Carlo (QMC) method. The critical Coulomb interactions ($U_{\rm
c}$) for the metal-insulator (MI) transition are also shown to agree with the
QMC results above $T_{\rm N}$. Below $T_{\rm N}$, $U_{\rm c}$ deviate from the
QMC values by about 30% at low temperature regime. These results indicate that
the dynamical CPA+HA is applicable to the quantitative description of the
magnetic properties in high dimensional systems, but one needs to take into
account higher-order dynamical corrections in order to describe the MI
transition quantitatively at low temperatures.",1104.4250v1
2013-10-16,Dynamics of skyrmions in chiral magnets: dynamic phase transitions and equation of motion,"We study the dynamics of skyrmions in a metallic chiral magnet. First we show
that skyrmions can be created dynamically by destabilizing the ferromagnetic
background state through a spin polarized current. We then treat skyrmions as
rigid particles and derive the corresponding equation of motion. The dynamics
of skyrmions is dominated by the Magnus force, which accounts for the weak
pinning of skyrmions observed in experiments. Finally we discuss the quantum
motion of skyrmions.",1310.4255v2
2013-10-28,Interaction-Tuned Dynamical Transitions in a Rashba Spin-Orbit Coupled Fermi Gas,"We consider the time evolution of the magnetization in a Rashba
spin-orbit-coupled Fermi gas, starting from a fully-polarized initial state. We
model the dynamics using a Boltzmann equation, which we solve in the
Hartree-Fock approximation. The resulting non-linear system of equations gives
rise to three distinct dynamical regimes with qualitatively different
asymptotic behaviors of the magnetization at long times. The distinct regimes
and the transitions between them are controlled by the interaction strength:
for weakly interacting fermions, the magnetization decays to zero. For
intermediate interactions, it displays undamped oscillations about zero and for
strong interactions, a partially magnetized state is dynamically stabilized.
The dynamics we find is a spin analog of interaction induced self-trapping in
double-well Bose Einstein condensates. The predicted phenomena can be realized
in trapped Fermi gases with synthetic spin-orbit interactions.",1310.7639v1
2014-09-10,Flow dynamics and magnetic induction in the von-Karman plasma experiment,"The von-Karman plasma experiment is a novel versatile experimental device
designed to explore the dynamics of basic magnetic induction processes and the
dynamics of flows driven in weakly magnetized plasmas. A high-density plasma
column (10^16 - 10^19 particles.m^-3) is created by two radio-frequency plasma
sources located at each end of a 1 m long linear device. Flows are driven
through JxB azimuthal torques created from independently controlled emissive
cathodes. The device has been designed such that magnetic induction processes
and turbulent plasma dynamics can be studied from a variety of time-averaged
axisymmetric flows in a cylinder. MHD simulations implementing
volume-penalization support the experimental development to design the most
efficient flow-driving schemes and understand the flow dynamics. Preliminary
experimental results show that a rotating motion of up to nearly 1 km/s is
controlled by the JxB azimuthal torque.",1409.3139v1
2014-11-25,2D Superexchange mediated magnetization dynamics in an optical lattice,"The competition of magnetic exchange interactions and tunneling underlies
many complex quantum phenomena observed in real materials. We study
non-equilibrium magnetization dynamics in an extended 2D system by loading
effective spin-1/2 bosons into a spin-dependent optical lattice, and we use the
lattice to separately control the resonance conditions for tunneling and
superexchange. After preparing a non-equilibrium anti-ferromagnetically ordered
state, we observe relaxation dynamics governed by two well-separated rates,
which scale with the underlying Hamiltonian parameters associated with
superexchange and tunneling. Remarkably, with tunneling off-resonantly
suppressed, we are able to observe superexchange dominated dynamics over two
orders of magnitude in magnetic coupling strength, despite the presence of
vacancies. In this regime, the measured timescales are in agreement with simple
theoretical estimates, but the detailed dynamics of this 2D, strongly
correlated, and far-from-equilibrium quantum system remain out of reach of
current computational techniques.",1411.7036v1
2015-08-04,Evolution and Dynamics of a Solar Active Prominence,"The life of a solar active prominence, one of the most remarkable objects on
the Sun, is full of dynamics; after first appearing on the Sun the prominence
continuously evolves with various internal motions and eventually produces a
global eruption toward the interplane- tary space. Here we report that the
whole life of an active prominence is successfully re- produced by performing
as long-term a magnetohydrodynamic simulation of a magnetized prominence plasma
as was ever done. The simulation reveals underlying dynamic processes that give
rise to observed properties of an active prominence: invisible subsurface flows
self- consistently produce the cancellation of magnetic flux observed at the
photosphere, while observed and somewhat counterintuitive strong upflows are
driven against gravity by en- hanced gas pressure gradient force along a
magnetic field line locally standing vertical. The most highlighted dynamic
event, transition into an eruptive phase, occurs as a natural con- sequence of
the self-consistent evolution of a prominence plasma interacting with a
magnetic field, which is obtained by seamlessly reproducing dynamic processes
involved in the forma- tion and eruption of an active prominence.",1508.00672v1
2015-12-02,Dynamical Axion Field in a Magnetic Topological Insulator Superlattice,"We propose that the dynamical axion field can be realized in a magnetic
topological insulator superlattice or a topological paramagnetic insulator. The
magnetic fluctuations of these systems produce a pseudoscalar field which has
an axionic coupling to the electromagnetic field, and thus it gives a
condensed-matter realization of the axion electrodynamics. Compared to the
previously proposed dynamical axion materials where a long range
antiferromagnetic order is required, the systems proposed here have the
advantage that only a uniform magnetization or a paramagnetic state is needed
for the dynamic axion. We further propose several experiments to detect such a
dynamical axion field.",1512.00534v1
2016-03-27,"A Model of Charge Transfer Excitons: Diffusion, Spin Dynamics, and Magnetic Field Effects","In this letter we explore how the microscopic dynamics of charge transfer
(CT) excitons are influenced by the presence of an external magnetic field in
disordered molecular semiconductors. This influence is driven by the dynamic
interplay between the spin and spatial degrees of freedom of the electron-hole
pair. To account for this interplay we have developed a numerical framework
that combines a traditional model of quantum spin dynamics with a
coarse-grained model of stochastic charge transport. This combination provides
a general and efficient methodology for simulating the effects of magnetic
field on CT state dynamics, therefore providing a basis for revealing the
microscopic origin of experimentally observed magnetic field effects. We
demonstrate that simulations carried out on our model are capable of
reproducing experimental results as well as generating theoretical predictions
related to the efficiency of organic electronic materials.",1603.08160v2
2016-04-15,Direct observation of magnetization dynamics generated by nano-contact spin-torque vortex oscillators,"Time-resolved scanning Kerr microscopy has been used to directly image the
magnetization dynamics of nano-contact (NC) spin-torque vortex oscillators
(STVOs) when phase-locked to an injected microwave (RF) current. The Kerr
images reveal free layer magnetization dynamics that extend outside the NC
footprint, where they cannot be detected electrically, but which are crucial to
phase-lock STVOs that share common magnetic layers. For a single NC, dynamics
were observed not only when the STVO frequency was fully locked to that of the
RF current, but also for a partially locked state characterized by periodic
changes in the core trajectory at the RF frequency. For a pair of NCs, images
reveal the spatial character of dynamics that electrical measurements show to
have enhanced amplitude and reduced linewidth. Insight gained from these images
may improve understanding of the conditions required for mutual phase-locking
of multiple STVOs, and hence enhanced microwave power emission.",1604.04455v1
2016-06-07,A functional calculus for the magnetization dynamics,"A functional calculus approach is applied to the derivation of evolution
equations for the moments of the magnetization dynamics of systems subject to
stochastic fields. It allows us to derive a general framework for obtaining the
master equation for the stochastic magnetization dynamics, that is applied to
both, Markovian and non-Markovian dynamics. The formalism is applied for
studying different kinds of interactions, that are of practical relevance and
hierarchies of evolution equations for the moments of the distribution of the
magnetization are obtained. In each case, assumptions are spelled out, in order
to close the hierarchies. These closure assumptions are tested by extensive
numerical studies, that probe the validity of Gaussian or non--Gaussian closure
Ans\""atze.",1606.02137v1
2017-03-28,Coarsening dynamics of an isotropic ferromagnetic superfluid,"In zero magnetic field the ground state manifold of a ferromagnetic spin-1
condensate is SO(3) and exhibits $\mathbb{Z}_2$ vortices as topological
defects. We investigate the phase ordering dynamics of this system after being
quenched into this ferromagnetic phase from a zero temperature unmagnetized
phase. Following the quench, we observe the ordering of both magnetic and gauge
domains. We find that these domains grow diffusively, i.e. with domain size
$L(t)\sim t^{1/2}$, and exhibit dynamic scale invariance. The coarsening
dynamics progresses as $\mathbb{Z}_2$ vortices annihilate, however we find that
at finite energy a number of these vortices persist in small clumps without
influencing magnetic or gauge order. We consider the influence of a small
non-zero magnetic field, which reduces the ground state symmetry, and show that
this sets a critical length scale such that when the domains reach this size
the system dynamically transitions in order parameter and scaling behaviour
from an isotropic to an anisotropic ferromagnetic superfluid.",1703.09360v2
2018-01-29,Recurrence based quantification of dynamical complexity in the Earth's magnetosphere at geospace storm timescales,"Magnetic storms are the most prominent global manifestations of
out-of-equilibrium magnetospheric dynamics. Investigating the dynamical
complexity exhibited by geomagnetic observables can provide valuable insights
into relevant physical processes as well as temporal scales associated with
this phenomenon. In this work, we utilize several innovative data analysis
techniques enabling a quantitative nonlinear analysis of the nonstationary
behavior of the disturbance storm time (Dst) index together with some of the
main drivers of its temporal variability, the $VB_{South}$ electric field
component, the vertical component of the interplanetary magnetic field, $B_z$,
and the dynamic pressure of the solar wind, $P_{dyn}$. Using recurrence
quantification analysis (RQA) and recurrence network analysis (RNA), we obtain
several complementary complexity measures that serve as markers of different
physical processes underlying quiet and storm time magnetospheric dynamics.
Specifically, our approach discriminates the magnetospheric activity in
response to external (solar wind) forcing from primarily internal variability
and provides a physically meaningful classification of magnetic storm periods
based on observations made at the Earth's surface. In this regard, the proposed
methodology could provide a relevant step towards future improved space weather
and magnetic storm forecasts.",1801.09412v1
2018-07-09,Dynamic phase transition properties of kinetic Ising model in the presence of additive white noise,"Using Monte Carlo simulations based on the Metropolis algorithm, we
investigate the dynamic phase transition properties of kinetic Ising model
driven by a sinusoidally oscillating magnetic field in the presence of additive
white noise. We calculate equilibrium and dynamic properties such as the
temperature dependence of average magnetization and magnetic specific heat, as
well as the period dependence of dynamic order parameter and scaled variance.
After determining the critical period at which order-disorder transition takes
place, we perform finite size scaling analysis to extract the exponent ratios,
and discuss the variation of these properties in the presence of noisy magnetic
field. As a general result, we show that for a noisy system, DPT does not fall
into a universality class of the conventional dynamic (and also equilibrium)
universality class of the Ising model.",1807.03077v1
2021-07-17,Magnetization switching in the inertial regime,"We have numerically solved the Landau-Lifshitz-Gilbert (LLG) equation in its
standard and inertial forms to study the magnetization switching dynamics in a
$3d$ thin film ferromagnet. The dynamics is triggered by ultrashort magnetic
field pulses of varying width and amplitude in the picosecond and Tesla range.
We have compared the solutions of the two equations in terms of switching
characteristic, speed and energy analysis. Both equations return qualitatively
similar switching dynamics, characterized by regions of slower precessional
behavior and faster ballistic motion. In case of inertial dynamics, ballistic
switching is found in a 25 % wider region in the parameter space given by the
magnetic field amplitude and width. The energy analysis of the dynamics is
qualitatively different for the standard and inertial LLG equations. In the
latter case, an extra energy channel, interpreted as the kinetic energy of the
system, is available. Such extra channel is responsible for a resonant energy
absorption at THz frequencies, consistent with the occurence of spin nutation.",2107.08234v1
2022-04-06,Multiparticle Collision Dynamics for Ferrofluids,"Detailed studies of the intriguing field-dependent dynamics and transport
properties of confined flowing ferrofluids require efficient mesoscopic
simulation methods that account for fluctuating ferrohydrodynamics. Here, we
propose such a new mesoscopic model for the dynamics and flow of ferrofluids,
where we couple the multi-particle collision dynamics method as a solver for
the fluctuating hydrodynamics equations to the stochastic magnetization
dynamics of suspended magnetic nanoparticles. This hybrid model is validated by
reproducing the magnetoviscous effect in Poiseuille flow, obtaining the
rotational viscosity in quantitative agreement with theoretical predictions. We
also illustrate the new method for the benchmark problem of flow around a
square cylinder. Interestingly, we observe that the length of the recirculation
region is increased whereas the drag coefficient is decreased in ferrofluids
when an external magnetic field is applied, compared with the field-free case
at the same effective Reynolds number. The presence of thermal fluctuations and
the flexibility of this particle-based mesoscopic method provides a promising
tool to investigate a broad range of flow phenomena of magnetic fluids and
could also serve as an efficient way to simulate solvent effects when colloidal
particles are immersed in ferrofluids.",2204.03060v1
2022-06-29,Strongly enhanced dynamics of a charged Rouse dimer by an external magnetic field,"While the dynamics of dimers and polymer chains in a viscous solvent is well
understood within the celebrated Rouse model, the effect of an external
magnetic field on the dynamics of a charged chain is much less understood. Here
we generalize the Rouse model for a charged dimer to include the effect of an
external magnetic field. Our analytically solvable model allows a fundamental
insight into the magneto-generated dynamics of the dimer in the overdamped
limit as induced by the Lorentz-force. Surprisingly, for a dimer of oppositely
charged particles, we find an enormous enhancement of the dynamics of the dimer
center which exhibits even a transient superballistic behavior. This is highly
unusual in an overdamped system for there is neither inertia nor any internal
or external driving. We attribute this to a significant translation and
rotation coupling due to the Lorentz force. We also find that magnetic field
reduces the mobility of a dimer along its orientation and its effective
rotational diffusion coefficient. In principle, our predictions can be tested
by experiments with colloidal particles and complex plasmas.",2206.14883v1
2022-09-30,Nonlinear dynamics and bifurcations of a planar undulating magnetic microswimmer,"Swimming micro-organisms such as flagellated bacteria and sperm cells have
fascinating locomotion capabilities. Inspired by their natural motion, there is
an ongoing effort to develop artificial robotic nano-swimmers for potential
in-body biomedical applications. A leading method for actuation of
nano-swimmers is by applying a time-varying external magnetic field. Such
systems have rich and nonlinear dynamics that calls for simple fundamental
models. A previous work studied forward motion of a simple two-link model with
passive elastic joint, assuming small-amplitude planar oscillations of the
magnetic field about a constant direction. In this work, we found that there
exists a faster, backward motion of the swimmer with very rich dynamics. By
relaxing the small-amplitude assumption, we analyze the multiplicity of
periodic solutions, as well as their bifurcations, symmetry breaking, and
stability transitions. We have also found that the net displacement and/or mean
swimming speed are maximized for optimal choices of various parameters.
Asymptotic calculations are performed for the bifurcation condition and the
swimmer's mean speed. The results may enable significantly improving the design
aspects of magnetically-actuated robotic microswimmer.",2209.15299v3
2022-10-27,Electron-phonon mediated spin-flip as driving mechanism for ultrafast magnetization dynamics in 3$d$ ferromagnets,"Despite intense experimental effort, theoretical proposals and modeling
approaches, a lack of consensus exists about the intrinsic mechanisms driving
ultrafast magnetization dynamics in 3$d$ ferromagnets. In this work, we find
evidence of electron-phonon mediated spin-flip as the driving mechanism for the
ultrafast magnetization dynamics in all three 3$d$ ferromagnets; nickel, iron
and cobalt. We use a microscopic three temperature model with parameters
calculated from first-principles, which has been validated by direct comparison
to the electron and lattice dynamics extracted from previous experiments. By
direct comparison to the experimentally measured magnetization dynamics for
different laser fluence, we determine the spin-flip probability of each
material. In contrast to previous findings but in agreement to ab-initio
predictions, we find that relatively small values of the spin-flip probability
enable ultrafast demagnetization in all three 3$d$ ferromagnets.",2210.15269v1
2024-03-05,Canonical Hamiltonian Guiding Center Dynamics and Its Intrinsic Magnetic Moment,"The concept of guiding center is potent in astrophysics, space plasmas,
fusion researches, and arc plasmas to solve the multi-scale dynamics of
magnetized plasmas. In this letter, we rigorously prove that the guiding center
dynamics can generally be described as a constrained canonical Hamiltonian
system with two constraints in six dimensional phase space, and that the
solution flow of the guiding center lies on a canonical symplectic
sub-manifold. The guiding center can thus be modeled as a pseudo-particle with
an intrinsic magnetic moment, which properly replaces the charged particle
dynamics on time scales larger than the gyro-period. The complete dynamical
behaviors, such as the velocity and force, of the guiding center
pseudo-particle can be clearly deduced from the model. Furthermore, a series of
related theories, such as symplectic numerical methods, the canonical
gyro-kinetic theory, and canonical particle-in-cell algorithms can be
systematically developed based on the canonical guiding center system. The
canonical guiding center theory also provides an enlightenment for the origin
of the intrinsic magnetic moment.",2403.02883v1
2017-07-14,Nonequilibrium phase transition in spin-S Ising ferromagnet driven by Propagating and Standing magnetic field wave,"The dynamical response of spin-S (S=1, 3/2, 2, 3) Ising ferromagnet to the
plane propagating wave , standing magnetic field wave and uniformly oscillating
field with constant frequency are studied separately in two dimensions by
extensive Monte Carlo simulation. Depending upon the strength of the magnetic
field and the value of the spin state of the Ising spin lattice two different
dynamical phases are observed. For a fixed value of S and the amplitude of the
propagating magnetic field wave the system undergoes a dynamical phase
transition from propagating phase to pinned phase as the temperature of the
system is cooled down. Similarly in case with standing magnetic wave the system
undergoes dynamical phase transition from high temperature phase where spins
oscillates coherently in alternate bands of half wavelength of the standing
magnetic wave to the low temperature pinned or spin frozen phase. For a fixed
value of the amplitude of magnetic field oscillation the transition temperature
is observed to decrease to a limiting value as the value of spin S is
increased. The time averaged magnetisation over a full cycle of the magnetic
field oscillation plays the role of the dynamic order parameter. A
comprehensive phase boundary is drawn in the plane of magnetic field amplitude
and dynamic transition temperature. It is found that the phase boundary shrinks
inwards for high value of spin state S. Also in the low temperature (and high
field) region the phase boundaries are closely spaced.",1707.04404v4
2007-05-03,"On the Dynamical Ferromagnetic, Quantum Hall, and Relativistic Effects on the Carbon Nanotubes Nucleation and Growth Mechanism","The mechanism of carbon nanotube (CNT) nucleation and growth has been a
mystery for over 15 years. Prior models have attempted the extension of older
classical transport mechanisms. In July 2000, a more detailed and accurate
nonclassical, relativistic mechanism was formulated considering the detailed
dynamics of the electronics of spin and orbital rehybridization between the
carbon and catalyst via novel mesoscopic phenomena and quantum dynamics.
Ferromagnetic carbon was demonstrated. Here, quantum (Hall) effects and
relativistic effects of intense many body spin-orbital interactions for novel
orbital rehybridization dynamics (Little Effect) are proposed in this new
dynamical magnetic mechanism. This dynamic ferromagnetic mechanism is proven by
imposing dynamic and static magnetic fields during CNT syntheses and observing
the different influence of these external magnetic environments on the
catalyzing spin currents and spin waves and the resulting CNT formation.",0705.0407v1
2008-12-03,Field theory of bicritical and tetracritical points. III. Relaxational dynamics including conservation of magnetization (Model C),"We calculate the relaxational dynamical critical behavior of systems of
$O(n_\|)\oplus O(n_\perp)$ symmetry including conservation of magnetization by
renormalization group (RG) theory within the minimal subtraction scheme in two
loop order. Within the stability region of the Heisenberg fixed point and the
biconical fixed point strong dynamical scaling holds with the asymptotic
dynamical critical exponent $z=2\phi/\nu-1$ where $\phi$ is the crossover
exponent and $\nu$ the exponent of the correlation length. The critical
dynamics at $n_\|=1$ and $n_\perp=2$ is governed by a small dynamical transient
exponent leading to nonuniversal nonasymptotic dynamical behavior. This may be
seen e.g. in the temperature dependence of the magnetic transport coefficients.",0812.0675v1
2011-05-21,Magnetic and electric contributions to the energy loss in a dynamical QCD medium,"The computation of radiative energy loss in a finite size QCD medium with
dynamical constituents is a key ingredient for obtaining reliable predictions
for jet quenching in ultra-relativistic heavy ion collisions. It was previously
shown that energy loss in dynamical QCD medium is significantly higher compared
to static QCD medium. To understand this difference, we here analyze magnetic
and electric contributions to energy loss in dynamical QCD medium. We find that
the significantly higher energy loss in the dynamical case is entirely due to
appearance of magnetic contribution in the dynamical medium. While for
asymptotically high energies, the energy loss in static and dynamical medium
approach the same value, we find that the physical origin of the energy loss in
these two cases is different.",1105.4288v1
2022-01-22,Dynamics of a Charged Thomas Oscillator in an External Magnetic Field,"In this letter, we provide a detailed numerical examination of the dynamics
of a charged Thomas oscillator in an external magnetic field. We do so by
adopting and then modifying the cyclically symmetric Thomas oscillator to study
the dynamics of a charged particle in an external magnetic field. These
dynamical behaviours for weak and strong field strength parameters fall under
two categories; conservative and dissipative. The system shows a complex
quasi-periodic attractor whose topology depends on initial conditions for high
field strengths in the conservative regime. There is a transition from
adiabatic motion to chaos on decreasing the field strength parameter. In the
dissipative regime, the system is chaotic for weak field strength and weak
damping but shows a limit cycle for high field strengths. Such behaviour is due
to an additional negative feedback loop that comes into action at high field
strengths and forces the system dynamics to be stable in periodic oscillations.
For weak damping and weak field strength, the system dynamics mimic Brownian
motion via chaotic walks.",2202.02383v2
2022-07-21,Probing dynamics of a two-dimensional dipolar spin ensemble using single qubit sensor,"Understanding the thermalization dynamics of quantum many-body systems at the
microscopic level is among the central challenges of modern statistical
physics. Here we experimentally investigate individual spin dynamics in a
two-dimensional ensemble of electron spins on the surface of a diamond crystal.
We use a near-surface NV center as a nanoscale magnetic sensor to probe
correlation dynamics of individual spins in a dipolar interacting surface spin
ensemble. We observe that the relaxation rate for each spin is significantly
slower than the naive expectation based on independently estimated dipolar
interaction strengths with nearest neighbors and is strongly correlated with
the timescale of the local magnetic field fluctuation. We show that this
anomalously slow relaxation rate is due to the presence of strong dynamical
disorder and present a quantitative explanation based on dynamic resonance
counting. Finally, we use resonant spin-lock driving to control the effective
strength of the local magnetic fields and reveal the role of the dynamical
disorder in different regimes. Our work paves the way towards microscopic study
and control of quantum thermalization in strongly interacting disordered spin
ensembles.",2207.10688v2
2023-06-23,Input-driven chaotic dynamics in vortex spin-torque oscillator,"A new research topic in spintronics relating to the operation principles of
brain-inspired computing is input-driven magnetization dynamics in nanomagnet.
In this paper, the magnetization dynamics in a vortex spin-torque oscillator
(STO) driven by a series of random magnetic field are studied through a
numerical simulation of the Thiele equation. It is found that input-driven
synchronization occurs in the weak perturbation limit, as found recently. As
well, chaotic behavior is newly found to occur in the vortex core dynamics for
a wide range of parameters, where synchronized behavior is disrupted by an
intermittency. Ordered and chaotic dynamical phases are examined by evaluating
the Lyapunov exponent. The relation between the dynamical phase and the
computational capability of physical reservoir computing is also studied.",2306.13278v1
1999-09-09,Micromagnetic simulations of thermally activated magnetization reversal of nanoscale magnets,"Numerical integration of a stochastic Landau-Lifshitz-Gilbert equation is
used to study dynamic processes in single-domain nanoscale magnets at nonzero
temperatures. Special attention is given to including thermal fluctuations as a
Langevin term, and the Fast Multipole Method is used to calculate dipole-dipole
interactions. It is feasible to simulate these dynamics on the nanosecond time
scale for spatial discretizations that involve on the order of 10000 nodes
using a desktop workstation. The nanoscale magnets considered here are single
pillars with large aspect ratio. Hysteresis-loop simulations are employed to
study the stable and metastable configurations of the magnetization. Each
pillar has magnetic end caps. In a time-dependent field the magnetization of
the pillars is observed to reverse via nucleation, propagation, and coalescence
of the end caps. In particular, the end caps propagate into the magnet and meet
near the middle. A relatively long-lived defect is formed when end caps with
opposite vorticity meet. Fluctuations are more important in the reversal of the
magnetization for fields weaker than the zero-temperature coercive field, where
the reversal is thermally activated. In this case, the process must be
described by its statistical properties, such as the distribution of switching
times, averaged over a large number of independent thermal histories.",9909136v2
2005-09-10,Magnetic dipolar ordering and relaxation in the high-spin molecular cluster compound Mn6,"Few examples of magnetic systems displaying a transition to pure dipolar
magnetic order are known to date, and single-molecule magnets can provide an
interesting example. The molecular cluster spins and thus their dipolar
interaction energy can be quite high, leading to reasonably accessible ordering
temperatures, provided the crystal field anisotropy is sufficiently small. This
condition can be met for molecular clusters of sufficiently high symmetry, as
for the Mn6 compound studied here. Magnetic specific heat and susceptibility
experiments show a transition to ferromagnetic dipolar order at T_{c} = 0.16 K.
Classical Monte-Carlo calculations indeed predict ferromagnetic ordering and
account for the correct value of T_{c}. In high magnetic fields we detected the
contribution of the ^{55}Mn nuclei to the specific heat, and the characteristic
timescale of nuclear relaxation. This was compared with results obtained
directly from pulse-NMR experiments. The data are in good mutual agreement and
can be well described by the theory for magnetic relaxation in highly polarized
paramagnetic crystals and for dynamic nuclear polarization, which we
extensively review. The experiments provide an interesting comparison with the
recently investigated nuclear spin dynamics in the anisotropic single molecule
magnet Mn12-ac.",0509261v2
2012-04-23,Dynamical mean-field theory of indirect magnetic exchange,"To analyze the physical properties arising from indirect magnetic exchange
between several magnetic adatoms and between complex magnetic nanostructures on
metallic surfaces, the real-space extension of dynamical mean-field theory
(R-DMFT) appears attractive as it can be applied to systems of almost arbitrary
geometry and complexity. While R-DMFT describes the Kondo effect of a single
adatom exactly, indirect magnetic (RKKY) exchange is taken into account on an
approximate level only. Here, we consider a simplified model system consisting
of two magnetic Hubbard sites (""adatoms"") hybridizing with a non-interacting
tight-binding chain (""substrate surface""). This two-impurity Anderson model
incorporates the competition between the Kondo effect and indirect exchange but
is amenable to an exact numerical solution via the density-matrix
renormalization group (DMRG). The particle-hole symmetric model at half-filling
and zero temperature is used to benchmark R-DMFT results for the magnetic
coupling between the two adatoms and for the magnetic properties induced in the
substrate. In particular, the dependence of the local adatom and the nonlocal
adatom-adatom static susceptibilities as well as the magnetic response of the
substrate on the distance between the adatoms and on the strength of their
coupling with the substrate is studied. We find both, excellent agreement with
the DMRG data even on subtle details of the competition between RKKY exchange
and the Kondo effect but also complete failure of the R-DMFT, depending on the
parameter regime considered. R-DMFT calculations are performed using the
Lanczos method as impurity solver. With the real-space extension of the
two-site DMFT, we also benchmark a simplified R-DMFT variant.",1204.5111v2
2012-12-06,Simulations of magnetic fields in isolated disk galaxies,"Magnetic fields are known to be dynamically important in the interstellar
medium of our own Galaxy, and they are ubiquitously observed in diffuse gas in
the halos of galaxies and galaxy clusters. Yet, magnetic fields have typically
been neglected in studies of the formation of galaxies, leaving their global
influence on galaxy formation largely unclear. We extend our MHD implementation
in the moving-mesh code Arepo to cosmological problems which include radiative
cooling and the formation of stars. In particular, we replace our previously
employed divergence cleaning approach with a Powell 8-wave scheme, which turns
out to be significantly more stable, even in very dynamic environments. We
verify the improved accuracy through simulations of the MRI in accretion disks,
that reproduce its correct linear growth rate. Using this new MHD code, we
simulate the formation of isolated disk galaxies similar to the Milky Way using
idealized initial conditions with and without magnetic fields. We find that the
magnetic field is quickly amplified in the initial starburst and the
differential rotation of the forming disk until it eventually saturates when it
becomes comparable to the thermal pressure. The additional pressure component
leads to a lower star formation rate at late times compared to simulations
without magnetic fields, and induces changes in the spiral arm structures of
the gas disk. In addition, we observe highly magnetized fountain-like outflows
from the disk. These results are robust with numerical resolution and are
largely independent of the initial magnetic seed field assumed in the initial
conditions, as the amplification process is rapid and self-regulated. Our
findings suggest an important influence of magnetic fields on galaxy formation
and evolution, cautioning against their neglect in theoretical models of
structure formation.",1212.1452v1
2013-01-04,Dynamics of the Transition corona,"Magnetic reconnection between open and closed magnetic field in the corona is
believed to play a crucial role in the corona/heliosphere coupling. At large
scale, the exchange of open/closed connectivity is expected to occur in
pseudo-streamer structures. However, there is neither clear observational
evidence of how such coupling occurs in pseudo-streamers, nor evidence for how
the magnetic reconnection evolves. Using a newly-developed technique, we
enhance the off-limb magnetic fine structures observed with AIA and identify a
pseudo-streamer-like feature located close to the northern coronal hole. After
extrapolating the magnetic field with the PFSS model, we obtain a
pseudo-streamer magnetic topology, null-point related topology bounded by open
field. We compare the magnetic configuration with the UV observations and
identify the magnetic structures expected to be involved in the event. Using an
3D MHD simulation of interchange reconnection, we showed that the evolution of
the UV structures follows the magnetic field dynamics and the UV emitting
structures have a pattern very similar to the plasma emission derived from the
simulation. Our results highlight that the exchange between open and closed in
the pseudo-streamer topology related to an observed event occurs at least
partially at the null-point, similarly to the interchange reconnection in a
single null-point topology. However, our results also indicate that the
interchange reconnection in pseudo-streamers is a gradual physical process
which opposes to the impulsive reconnection of the solar-jet model.",1301.0740v2
2013-01-28,Magnetization dynamics in Co2$MnGe/Al2$O3$/Co tunnel junctions grown on different substrates,"We study static and dynamic magnetic properties of Co2MnGe (13 nm)/Al2O3 (3
nm)/Co (13 nm) tunnel magnetic junctions (TMJ), deposited on various single
crystalline substrates (a-plane sapphire, MgO(100), Si(111)). The results are
compared to the magnetic properties of Co and of Co$_{2}$MnGe single films
lying on sapphire substrates. X-rays diffraction always shows a (110)
orientation of the Co$_{2}$MnGe films. Structural observations obtained by high
resolution transmission electron microscopy confirmed the high quality of the
TMJ grown on sapphire. Our vibrating sample magnetometry measurements reveal
in-plane anisotropy only in samples grown on a sapphire substrate. Depending on
the substrate, the ferromagnetic resonance spectra of the TMJs, studied by the
microstrip technique, show one or two pseudo-uniform modes. In the case of MgO
and of Si substrates only one mode is observed: it is described by magnetic
parameters (g-factor, effective magnetization, in-plane magnetic anisotropy)
derived in the frame of a simple expression of the magnetic energy density;
these parameters are practically identical to those obtained for the Co single
film. With a sapphire substrate two modes are present: one of them does not
appreciably differ from the observed mode in the Co single film while the other
one is similar to the mode appearing in the Co$_{2}$MnGe single film: their
magnetic parameters can thus be determined independently, using a classical
model for the energy density in the absence of interlayer exchange coupling.",1301.6563v1
2013-11-14,New look at the QCD ground state in a magnetic field,"We explore chiral symmetry breaking in a magnetic field within a
Nambu-Jona-Lasinio model of interacting massless quarks including tensor
channels. We show that the new interaction channels open up via Fierz
identities due to the explicit breaking of the rotational symmetry by the
magnetic field. We demonstrate that the magnetic catalysis of chiral symmetry
breaking leads to the generation of two independent condensates, the
conventional chiral condensate and a spin-one condensate. While the chiral
condensate generates a dynamical fermion mass, the new condensate gives rise to
a dynamical anomalous magnetic moment for the fermions. As a consequence, the
spectrum of the excitations in all Landau levels, except the lowest one,
exhibits Zeeman splitting. Since the pair, formed by a quark and an antiquark
with opposite spins, possesses a resultant magnetic moment, an external
magnetic field can align it giving rise to a net magnetic moment for the ground
state. This is the physical interpretation of the spin-one condensate. Our
results show that the magnetically catalyzed ground state in QCD is actually
richer than previously thought. The two condensates contribute to the effective
mass of the LLL quasiparticles in such a way that the critical temperature for
chiral symmetry restoration becomes enhanced.",1311.3400v3
2015-06-22,The Intrinsic Magnetization of Antiferromagnetic Textures,"Antiferromagnets (AFMs) exhibit intrinsic magnetization when the order
parameter spatially varies. This intrinsic spin is present even at equilibrium
and can be interpreted as a twisting of the homogeneous AFM into a state with a
finite spin. Because magnetic moments couple directly to external magnetic
fields, the intrinsic magnetization can alter the dynamics of antiferromagnetic
textures under such influence. Starting from the discrete Heisenberg model, we
derive the continuum limit of the free energy of AFMs in the exchange
approximation and explicitly rederive that the spatial variation of the
antiferromagnetic order parameter is associated with an intrinsic magnetization
density. We calculate the magnetization profile of a domain wall and discuss
how the intrinsic magnetization reacts to external forces. We show
conclusively, both analytically and numerically, that a spatially inhomogeneous
magnetic field can move and control the position of domain walls in AFMs. By
comparing our model to a commonly used alternative parametrization procedure
for the continuum fields, we show that the physical interpretations of these
fields depend critically on the choice of parametrization procedure for the
discrete-to-continuous transition. This can explain why a significant amount of
recent studies of the dynamics of AFMs, including effective models that
describe the motion of antiferromagnetic domain walls, have neglected the
intrinsic spin of the textured order parameter.",1506.06561v2
2015-10-15,Rashba Torque Driven Domain Wall Motion in Magnetic Helices,"Manipulation of the domain wall propagation in magnetic wires is a key
practical task for a number of devices including racetrack memory and magnetic
logic. Recently, curvilinear effects emerged as an efficient mean to impact
substantially the statics and dynamics of magnetic textures. Here, we
demonstrate that the curvilinear form of the exchange interaction of a magnetic
helix results in an effective anisotropy term and Dzyaloshinskii--Moriya
interaction with a complete set of Lifshitz invariants for a one-dimensional
system. In contrast to their planar counterparts, the geometrically induced
modifications of the static magnetic texture of the domain walls in magnetic
helices offer unconventional means to control the wall dynamics relying on
spin-orbit Rashba torque. The chiral symmetry breaking due to the
Dzyaloshinskii-Moriya interaction leads to the opposite directions of the
domain wall motion in left- or right-handed helices. Furthermore, for the
magnetic helices, the emergent effective anisotropy term and
Dzyaloshinskii-Moriya interaction can be attributed to the clear geometrical
parameters like curvature and torsion offering intuitive understanding of the
complex curvilinear effects in magnetism.",1510.04725v2
2016-04-15,"Relationships between fluid vorticity, kinetic helicity and magnetic field at the small-scale (quiet-network) on the Sun","We derive horizontal fluid motions on the solar surface over large areas
covering the quiet-Sun magnetic network from local correlation tracking of
convective granules imaged in continuum intensity and Doppler velocity by the
Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory
(SDO). From these we calculate horizontal divergence, vertical component of
vorticity, and kinetic helicity of fluid motions. We study the correlations
between fluid divergence and vorticity, and that between vorticity (kinetic
helicity) and magnetic field. We find that the vorticity (kinetic helicity)
around small-scale fields exhibits a hemispherical pattern (in sign) similar to
that followed by the magnetic helicity of large-scale active regions
(containing sunspots). We identify this pattern to be a result of the Coriolis
force acting on supergranular-scale flows (both the outflows and inflows), and
is consistent with earlier studies using local helioseismology. Further, we
show that the magnetic fields cause transfer of vorticity from supergranular
inflow regions to outflow regions, and that they tend to suppress the vortical
motions around them when magnetic flux densities exceed about 300 G (HMI). We
also show that such action of magnetic fields leads to marked changes in the
correlations between fluid divergence and vorticity. These results are
speculated to be of importance to local dynamo action if present, and to the
dynamical evolution of magnetic helicity at the small-scale.",1604.04439v1
2016-09-11,Strongly coupled non-Abelian plasmas in a magnetic field,"In this dissertation we use the gauge/gravity duality approach to study the
dynamics of strongly coupled non-Abelian plasmas. Ultimately, we want to
understand the properties of the quark-gluon plasma (QGP), whose scientifc
interest by the scientific community escalated exponentially after its
discovery in the 2000's through the collision of ultrarelativistic heavy ions.
  One can enrich the dynamics of the QGP by adding an external field, such as
the baryon chemical potential (needed to study the QCD phase diagram), or a
magnetic field. In this dissertation, we choose to investigate the magnetic
effects. Indeed, there are compelling evidences that strong magnetic fields of
the order $eB\sim 10 m_\pi^2$ are created in the early stages of
ultrarelativistic heavy ion collisions.
  The chosen observable to scan possible effects of the magnetic field on the
QGP was the viscosity, due to the famous result $\eta/s=1/4\pi$ obtained via
holography. In a first approach we use a caricature of the QGP, the
$\mathcal{N}=4$ super Yang-Mills plasma to calculate the deviations of the
viscosity as we add a magnetic field. We must emphasize, though, that a
magnetized plasma has a priori seven viscosity coefficients (five shears and
two bulks). In addition, we also study in this same model the anisotropic heavy
quark-antiquark potential in the presence of a magnetic field.
  In the end, we propose a phenomenological holographic QCD-like model, which
is built upon the lattice QCD data, to study the thermodynamics and the
viscosity of the QGP with an external strong magnetic field.",1609.03104v1
2016-09-21,"Magnetic Susceptibility of Dirac Fermions, Bi-Sb Alloys, Interacting Bloch Fermions, Dilute Nonmagnetic Alloys, and Kondo Alloys","Wide ranging interest in Dirac Hamiltomian is due to the emergence of novel
materials, namely, graphene, topological insulators and superconductors, the
newly-discovered Weyl semimetals, and still actively-sought after Majorana
fermions in real materials. We give a brief review of the relativistic Dirac
quantum mechanics and its impact in the developments of modern physics. The
quantum band dynamics of Dirac Hamiltonian is crucial in resolving the giant
diamagnetism of bismuth and Bi-Sb alloys. Quantitative agreement of the theory
with the experiments on Bi-Sb alloys has been achieved, and physically
meaningful contributions to the diamagnetism has been identified. We also treat
relativistic Dirac fermion as an interband dynamics in uniform magnetic fields.
For the interacting Bloch electrons, the role of translation symmetry for
calculating the magnetic susceptibility avoids any approximation to second
order in the field. The magnetic susceptibility of Hubbard model and those of
Fermi liquids are readily obtained as limiting cases. The expressions for
magnetic susceptibility of dilute nonmagnetic alloys give a firm theoretical
foundation of the empirical formulas used in fitting experimental results. For
completeness, the magnetic susceptibility of dilute magnetic or Kondo alloys is
also given for high and low temperature regimes.",1609.06419v1
2016-10-26,On the nature of the magnetic Rayleigh-Taylor instability in Astrophysical Plasma: The case of uniform magnetic field strength,"The magnetic Rayleigh-Taylor instability has been shown to play a key role in
many astrophysical systems. The equation for the growth rate of this
instability in the incompressible limit, and the most-unstable mode that can be
derived from it, are often used to estimate the strength of the magnetic field
that is associated with the observed dynamics. However, there are some issues
with the interpretations given. Here we show that the class of most unstable
modes $k_u$ for a given $\theta$, the class of modes often used to estimate the
strength of the magnetic field from observations, for the system leads to the
instability growing as $\sigma^2=1/2 A g k_u$, a growth rate which is
independent of the strength of the magnetic field and which highlights that
small scales are preferred by the system, but not does not give the fastest
growing mode for that given $k$. We also highlight that outside of the
interchange ($\mathbf{k}\cdot\mathbf{B}=0$) and undular ($\mathbf{k}$ parallel
to $\mathbf{B}$) modes, all the other modes have a perturbation pair of the
same wavenumber and growth rate that when excited in the linear regime can
result in an interference pattern that gives field aligned filamentary
structure often seen in 3D simulations. The analysis was extended to a sheared
magnetic field, where it was found that it was possible to extend the results
for a non-sheared field to this case. We suggest that without magnetic shear it
is too simplistic to be used to infer magnetic field strengths in astrophysical
systems.",1610.08317v1
2017-03-22,Magnetic field effects on dynamics of the ethylammonium nitrate ionic liquid confined between polar glass plates,"Self-diffusion and NMR relaxation of ethylammonium (EA) cations were studied
in the protic ionic liquid ethylammonium nitrate (EAN) confined between
parallel polar glass plates separated by a few um as a function of time after
placement in a magnetic field of 9.40 T. Immediately after sample placement,
the diffusion coefficient of EA (D) increased by a factor of 2, while the
transverse NMR relaxation of NH3 protons decreased by factors of up to 22 in
comparison with bulk EAN, according to previously published data. Further
exposure to the magnetic field leads to gradual changes of D, T1 and T2 with a
time constant of 70 min and a total equilibration time of longer than 4 hours.
This process does not depend on the orientation of the glass plates relative to
the magnetic field. Removing the sample from the magnetic field and repeating
the experiment demonstrated that complete recovery of the sample to the
accelerated D and shortened T2 occurs in approximately 24 hours. Thus, EA
cation dynamics is accelerated relative to bulk when the cation is confined
between polar glass plates and stored in the absence of a strong magnetic
field, but demonstrate a trend to change to bulk values of D and T2 after
exposure to a magnetic field. Because the observed characteristic times of the
change far exceed molecular process times in EAN, we relate this phenomenon to
reversible phase transformations occurring with EAN between polar plates
outside the strong magnetic field, and reverse change of the phases after
placing the sample in the magnetic field. This conclusion was confirmed by data
of Raman spectroscopy.",1703.07648v1
2017-03-30,A novel energy-based phase field model for ferrodroplet deformation and breakup in a uniform magnetic field,"In this paper, we propose a novel, thermodynamically consistent phase field
model to simulate the deformation and breakup of a ferrodroplet that is
immersed in a viscous medium and subject to an applied uniform magnetic field.
Instead of using the magnetic body force in the traditional Rosensweig model,
the key idea of this model is to propose a new magnetic energy that enables
direct effects of the magnetic field on the interface evolution. The model can
thereby be derived from the variational principle via minimizing the free
energy of the total system. This energy based modeling idea can be easily
extended to include more external fields in a convenient and consistent way for
more complicated applications involving multiple external fields. We validate
the model by performing a series of numerical simulations, including the
comparison with analytic solutions, the investigation of the effect of
different types of magnetic fields, the dynamical behaviors of the ferrodroplet
breakup under the strong magnetic field, magnetic/velocity/pressure field
distributions, the magnetic energy density, and the inertial phenomenon, etc.",1704.00645v2
2017-07-27,Common Glass-Forming Spin-Liquid State in the Pyrochlore Magnets Dy$_2$Ti$_2$O$_7$ and Ho$_2$Ti$_2$O$_7$,"Despite a well-ordered pyrochlore crystal structure and strong magnetic
interactions between the Dy$^{3+}$ or Ho$^{3+}$ ions, no long range magnetic
order has been detected in the pyrochlore titanates Ho$_2$Ti$_2$O$_7$ and
Dy$_2$Ti$_2$O$_7$. To explore the actual magnetic phase formed by cooling these
materials, we measure their magnetization dynamics using toroidal,
boundary-free magnetization transport techniques. We find that the dynamical
magnetic susceptibility of both compounds has the same distinctive
phenomenology, that is indistinguishable in form from that of the dielectric
permittivity of dipolar glass-forming liquids. Moreover, Ho$_2$Ti$_2$O$_7$ and
Dy$_2$Ti$_2$O$_7$ both exhibit microscopic magnetic relaxation times that
increase along the super-Arrhenius trajectories analogous to those observed in
glass-forming dipolar liquids. Thus, upon cooling below about 2K,
Dy$_2$Ti$_2$O$_7$ and Ho$_2$Ti$_2$O$_7$ both appear to enter the same magnetic
state exhibiting the characteristics of a glass-forming spin-liquid.",1707.09014v3
2018-07-01,Dynamic modeling of levitation of a superconducting bulk by coupled $H$-magnetic field and Arbitrary Lagrangian-Eulerian formulations,"Intrinsically stable magnetic levitation between superconductors and
permanent magnets can be exploited in a variety of applications of great
technical interest in the field of transportation (rail transportation), energy
(flywheels) and industry. In this contribution, we present a new model for the
calculation of levitation forces between superconducting bulks and permanent
magnet, based on the $H$-formulation of Maxwell's equations coupled with an
Arbitrary Lagrangian-Eulerian formulation. The model uses a moving mesh that
adapts at each time step based on the time-change of the distance between a
superconductor bulk and a permanent magnet. The model is validated against a
fixed mesh model (recently in turn validated against experiments) that uses an
analytical approach for calculating the magnetic field generated by the moving
permanent magnet. Then, it is used to analyze the magnetic field dynamics both
in field-cooled and zero-field-cooled conditions and successively used to test
different configurations of permanent magnets and to compare them in terms of
levitation forces. The easiness of implementation of this model and its
flexibility in handling different geometries, material properties, and
application scenarios make the model an attractive tool for the analysis and
optimization of magnetic levitation-based applications.",1807.00329v2
2019-05-31,Heat transfer and flow regimes in quasi-static magnetoconvection with a vertical magnetic field,"Numerical simulations of quasi-static magnetoconvection with a vertical
magnetic field are carried out up to a Chandrasekhar number of $Q=10^8$ over a
broad range of Rayleigh numbers $Ra$. Three magnetoconvection regimes are
identified: two of the regimes are magnetically-constrained in the sense that a
leading-order balance exists between the Lorentz and buoyancy forces, whereas
the third regime is characterized by unbalanced dynamics that is similar to
non-magnetic convection. Each regime is distinguished by flow morphology,
momentum and heat equation balances, and heat transport behavior. One of the
magnetically-constrained regimes appears to represent an `ultimate'
magnetoconvection regime in the dual limit of asymptotically-large buoyancy
forcing and magnetic field strength; this regime is characterized by an
interconnected network of anisotropic, spatially-localized fluid columns
aligned with the direction of the imposed magnetic field that remain
quasi-laminar despite having large flow speeds. As for non-magnetic convection,
heat transport is controlled primarily by the thermal boundary layer.
Empirically, the scaling of the heat transport and flow speeds with $Ra$ appear
to be independent of the thermal Prandtl number within the
magnetically-constrained, high-$Q$ regimes.",1905.13688v2
2020-03-18,A local view of the laser induced magnetic domain dynamics in CoPd stripe domains at the picosecond time scale,"The dynamic of the magnetic structure in a well ordered ferromagnetic CoPd
stripe domain pattern has been investigated upon excitation by femtosecond
infrared laser pulses. Time-resolved X-ray magnetic circular dichroism in
photoemission electron microscopy (TR-XMCD-PEEM) is used to perform real space
magnetic imaging with 100 ps time resolution in order to show local
transformations of the domains structures. Using the time resolution of the
synchrotron radiation facility of the Helmholtz-Zentrum Berlin, we are able to
image the transient magnetic domains in a repetitive pump and probe experiment.
In this work, we study the reversible and irreversible transformations of the
excited magnetic stripe domains as function of the laser fluence. Our results
can be explained by thermal contributions, reducing the XMCD amplitude in each
stripe domain below a threshold fluence of 12 mJ/cm2. Above this threshold
fluence, irreversible transformations of the magnetic domains are observed.
Static XMCD-PEEM images reveal the new partially ordered stripe domain
structures characterized by a new local magnetic domain distribution showing an
organized pattern at the micrometer scale. This new arrangement is attributed
to the recovery of the magnetic anisotropy during heat dissipation under an
Oersted field.",2003.08121v4
2020-04-23,Momentum broadening of heavy quark in a magnetized thermal QCD medium,"Anisotropic momentum diffusion coefficients of heavy quarks have been
computed in a strongly magnetized quark-gluon plasma beyond the static limit
within the framework of Langevin dynamics. Depending on the orientation of the
motion of the heavy quark with respect to the direction of the magnetic field,
five momentum diffusion coefficients of heavy quark have been estimated in the
magnetized thermal medium. Specifically, we have focussed our attention to
temperature range and strength of magnetic field satisfying the condition,
$\it{i.e.}$ $M\gg\sqrt{eB}\gg T$, $M$ being the mass of heavy quark. The light
quarks/antiquarks follow $1+1-$dimensional lowest Landau level (LLL)
kinematics, and heavy quark dynamics are not directly affected by the magnetic
field in the medium. The thermal gluon contribution to the diffusion
coefficient is proportions to $T^3$, whereas, the contribution of light quarks
in the lowest Landau state to the same is seen to be proportional to $T|eB|$.
Furthermore, it is observed that for the case of heavy quark motion parallel to
the magnetic field, the component of diffusion coefficient transverse both to
the field and the heavy quark velocity $(\kappa^{\parallel}_{TT})$ turns out to
be dominant as compared to the component longitudinal to both the field and
motion $(\kappa^{\parallel}_{LL})$, $i.e.$, $\kappa^{\parallel}_{TT}\gg
\kappa^{\parallel}_{LL}$. Further, for the case of heavy quark moving
perpendicular to the magnetic field, it is seen that the diffusion coefficients
transverse to the magnetic field are dominant, i.e., $\kappa^{\perp}_{LT},
\kappa^{\perp}_{TT}\gg \kappa^{\perp}_{TL}$.",2004.11092v2
2020-07-24,Analysis of solar wind energy and the Akasofu parameter for energy dynamics assessment during Supersubstorm,"Solar activities have significant impact on the member of the solar systems
including earth. As earth is a planet with its own magnetic field, solar
emissions with magnetic fields do interact with the earth's own magnetic field
to create geo-magnetic disturbances. Substorms are geo-magnetic disturbances of
shorter duration (few hours or less) which are mainly concentrated in the
auroral region and originates due to the ionospheric current injection at high
latitudes. Supersubstorm are intense substorms as identified by peak activities
in geo-magnetic indices. In this thesis, we have investigated the energy
dynamics during the supersubstorm and analyzed the relation of the energy
components with different geo-magnetic indices and interplanetary magnetic
field (IMF) components. In particular, the solar wind energy levels and the
energy coupled into the magnetosphere as captured by the Akasofu parameter are
studied. A qualitative analysis of the computed energy and its relationship
with different geo-magnetic indices is presented. Correlation and
cross-correlation analysis have been used to get insights into the nature of
the relationship between the energy components and indices. This for example
uncovered the negative relationship between the solar wind energy and the
Akasofu parameter. A summary analysis of the supersubstorm event in comparison
with the quiet phases are made. This has led to insights for example showing
the higher energy levels during supersubstorm and different energy ratios
(Akasofu parameter divided by the solar wind energy) existing for different
supersubstorm.",2007.12352v2
2020-07-30,MHD accretion-ejection: jets launched by a non-isotropic accretion disk dynamo. I. Validation and application of selected dynamo tensorial components,"Astrophysical jets are launched from strongly magnetized systems that host an
accretion disk surrounding a central object. The origin of the jet launching
magnetic field is one of the open questions for modeling the accretion-ejection
process. Here we address the question how to generate the accretion disk
magnetization and field structure required for jet launching. Applying the
PLUTO code, we present the first resistive MHD simulations of jet launching
including a non-scalar accretion disk mean-field $\alpha^2\Omega$-dynamo in the
context of large scale disk-jet simulations. Essentially, we find the
$\alpha_\phi$-dynamo component determining the amplification of the poloidal
magnetic field, which is strictly related to the disk magnetization (and, as a
consequence, to the jet speed, mass and collimation), while the $\alpha_R$ and
$\alpha_\theta$-dynamo components trigger the formation of multiple,
anti-aligned magnetic loops in the disk, with strong consequences on the
stability and dynamics of the disk-jet system. In particular, such loops
trigger the formation of dynamo inefficient zones, which are characterized by a
weak magnetic field, and therefore a lower value of the magnetic diffusivity.
The jet mass, speed and collimation are strongly affected by the formation of
the dynamo inefficient zones. Moreover, the $\theta$-component of the
$\alpha$-dynamo plays a key role when interacting with a non-radial component
of the seed magnetic field. We also present correlations between the strength
of the disk toy dynamo coefficients and the dynamical parameters of the jet
that is launched.",2007.15701v1
2021-05-01,Charged particle motion around a magnetized Reissner-Nordström black hole,"We investigate the dynamics of neutral and charged test particles around
axially symmetric magnetized black hole spacetime. We consider its
electromagnetic field in the black hole vicinity and study its impact on the
dynamics of test particles. We determine the radius of the innermost stable
circular orbit (ISCO) for neutral and charged test particles and show that the
combined effect of black hole electric charge and magnetic field strongly
affects the ISCO radius, thus shrinking its values. We also show that the ISCO
radius of positively (negatively) charged particle initially gets increased
(decreased) and then gets radically altered with an increase in the value of
both black hole electric charge and test particle charge. It turns out that the
repulsive (attractive) Coulomb force dominates over the Lorentz force arising
from the black hole magnetic field. Typically, black hole rotation causes
axially symmetric spacetime case. Similarly, it turns out that a magnetized
black hole solution also causes axially symmetric spacetime as a consequence of
the presence of magnetic field. We study a degeneracy for the value of the ISCO
between the Kerr and the magnetized Reissner-Nordstr\""{o}m black hole
geometries and show that the combined effects of black hole charge and magnetic
field can be mimicked by Kerr spacetime with the spin parameter up to
$a/M\approx 0.8$. Finally, we consider the center of mass energy of colliding
particles and show that an increase in the values of black hole magnetic field
and electric charge leads to high center of mass energy extracted by collision
of two particles.",2105.00342v2
2021-12-27,"Design, Dynamics, and Dissipation of a Torsional-Magnetic Spring Mechanism","We present an analytical and experimental study of torsional magnetic
mechanism where the restoring torque is due to magnetic field interactions
between rotating and fixed permanent magnets. The oscillator consists of a ball
bearing-supported permanent magnet, called the rotor, placed between two fixed
permanent magnets called the stators. Perturbing the rotor from its equilibrium
angle induces a restoring magnetic torque whose effect is modeled as a
torsional spring. This restoring effect is accompanied by dissipation
mechanisms arising from structural viscoelasticity, air and electromagnetic
damping, as well as friction in the ball bearings. To investigate the system
dynamics, we constructed an experimental setup capable of mechanical,
electrical and magnetic measurements. For various rotor-stator gaps in this
setup, we validated an analytical model that assumes viscous and dry (Coulomb)
damping during the rotor free response. Moreover, we forced the rotor by a
neighboring electromagnetic coil into high amplitude oscillations. We observed
unusual resonator nonlinearity: at large rotor-stator gaps, the oscillations
are softening; at reduced gaps, the oscillations stiffen-then-soften. The
developed reduced-order models capture the nonlinear effects of the
rotor-to-stator and the rotor-to-coil distances. These magnetic oscillators are
promising in low-frequency electromagnetic signal transmission and in designing
magneto-elastic metamaterials with tailorable nonlinearity.",2112.13806v1
2022-01-16,Perspective: magnetic nanoparticles in theranostic applications,"Nanomedicine research started exploring the combination of therapy and
diagnostics, so-called theranostics, to offer a more flexible care with
improved outcome. As magnetic nanoparticles show great potential in many
diagnostic and therapeutic applications, they are prime candidates to be used
in a theranostic setting. This perspective gives an overview of
state-of-the-art magnetic nanoparticle-based imaging techniques and theranostic
applications and discusses their opportunities and challenges. To address these
challenges and exploit these opportunities to the fullest, we provide three
promising research directions. The first considers novel magnetic field
sequences, utilizing the rich magnetic dynamics of the particles, which boost
the capabilities of many nanoparticle-based applications. Secondly ,we
introduce the concept of smart theranostics based on feedback mechanisms
between the particle applications and their supporting imaging procedure to
enhance the performance of both and allow real-time monitoring of treatment
efficiency. Finally, we show how data-driven models could enhance therapy and
diagnostics, and handle the platform's large amount of data and decision
support algorithms. The latter research track also includes hybrid models in
which physics-based and data-driven models are combined to overcome challenges
of applications with limited data, as well as to uncover unknown nanoparticle
dynamics. Contrasting other literature, which mainly focuses on developing
magnetic nanoparticles with the right characteristics, we put forward advances
in magnetic nanoparticle imaging techniques and applications to enable the use
of a broader range of magnetic nanoparticles in theranostics. We encourage
researchers to also investigate these aspects to advance theranostic
applications of magnetic nanoparticles to clinical environments.",2201.06058v1
2022-11-22,Bifurcation analysis of a two-dimensional magnetic Rayleigh-Bénard problem,"We perform bifurcation analysis of a two-dimensional magnetic
Rayleigh-B\'enard problem using a numerical technique called deflated
continuation. Our aim is to study the influence of the magnetic field on the
bifurcation diagram as the Chandrasekhar number $Q$ increases, and compare it
to the standard (non-magnetic) Rayleigh-B\'enard problem. We compute steady
states at a high Chandrasekhar number of $Q=10^3$ over a range of the Rayleigh
number $0\leq \text{Ra}\leq 10^5$. These solutions are obtained by combining
deflation with a continuation of steady states at low Chandrasekhar number,
which allows us to explore the influence of the strength of the magnetic field
as $Q$ increases from low coupling, where the magnetic effect is almost
negligible, to strong coupling at $Q=10^3$. We discover a large profusion of
states with rich dynamics and observe a complex bifurcation structure with
several pitchfork, Hopf and saddle-node bifurcations. Our numerical simulations
show that the onset of bifurcations in the problem is delayed when $Q$
increases, while solutions with fluid velocity patterns aligning with the
background vertical magnetic field are privileged. Additionally, we report a
branch of states that stabilizes at high magnetic coupling, suggesting that one
may take advantage of the magnetic field to discriminate solutions.",2211.12163v1
2023-01-13,Vibronic effects on the quantum tunnelling of magnetisation in Kramers single-molecule magnets,"Single-molecule magnets are among the most promising platforms for achieving
molecular-scale data storage and processing. Their magnetisation dynamics are
determined by the interplay between electronic and vibrational degrees of
freedom, which can couple coherently, leading to complex vibronic dynamics.
Building on an ab initio description of the electronic and vibrational
Hamiltonians, we formulate a non-perturbative vibronic model of the low-energy
magnetic degrees of freedom in monometallic single-molecule magnets. Describing
their low-temperature magnetism in terms of magnetic polarons, we are able to
quantify the vibronic contribution to the quantum tunnelling of the
magnetisation, a process that is commonly assumed to be independent of
spin-phonon coupling. We find that the formation of magnetic polarons lowers
the tunnelling probability in both amorphous and crystalline systems by
stabilising the low-lying spin states. This work, thus, shows that spin-phonon
coupling subtly influences magnetic relaxation in single-molecule magnets even
at extremely low temperatures where no vibrational excitations are present.",2301.05557v2
2024-01-19,Exploration of magnon-magnon coupling in an antidot lattice: The role of non-uniform magnetization texture,"We numerically study the spin wave dynamics in an antidot lattice based on a
Co/Pd multilayer structure with reduced perpendicular magnetic anisotropy at
the edges of the antidots. This structure forms a magnonic crystal with a
periodic antidot pattern and a periodic magnetization configuration consisting
of out-of-plane magnetized bulk and in-plane magnetized rims. Our results show
the different behavior of spin waves in the bulk and in the rims under varying
out-of-plane external magnetic field strength, revealing complex spin-wave
spectra and hybridizations between the modes of these two subsystems. A
particularly strong magnon-magnon coupling, due to exchange interactions, is
found between the fundamental bulk spin-wave mode and the second-order radial
rim modes. However, the dynamical coupling between the spin-wave modes at low
frequencies, involving the first-order radial rim modes, is masked by the
changes in the static magnetization at the bulk-rim interface with magnetic
field changes. The study expands the horizons of magnonic-crystal research by
combining periodic structural patterning and non-collinear magnetization
texture to achieve strong magnon-magnon coupling, highlighting the significant
role of exchange interactions in the coupling.",2401.10540v2
2012-02-07,Relaxation-to-creep transition of domain-wall motion in two- dimensional random-field Ising model with ac driving field,"With Monte Carlo simulations, we investigate the relaxation dynamics with a
domain wall for magnetic systems at the critical temperature. The dynamic
scaling behavior is carefully analyzed, and a dynamic roughening process is
observed. For comparison, similar analysis is applied to the relaxation
dynamics with a free or disordered surface",1202.1343v1
2001-09-11,Dynamic Transitions in Pure Ising Magnets under Pulsed and Oscillating Fields,"Response of pure Ising systems to time-dependent external magnetic fields,
like pulsed and oscillating fields, are discussed and compared here. Because of
the two time scales involved, namely the thermodynamic relaxation time of the
system and the pulse width or the time period of the external field,
dynamically broken symmetric phases appear spontaneously when both become
comparable. A particularly simple case is that of an Ising ferromagnet below
its static critical temperature, when it is perturbed for a short duration by a
pulsed magnetic field competing with the existing order in the system. If the
field strength and duration is more than the threshold (dependent on the
temperature), the system, and consequently the magnetization, switches from one
minimum to the other of the static free energy. This magnetization reversal
transition here shows intriguing dynamic transition behaviour, similar to those
for oscillating fields. Monte Carlo studies for such dynamic transitions are
discussed and compared with the mean field results for the same and the Monte
Carlo results for the oscillating field case. In particular, we discuss about
the Monte Carlo results for the fluctuations and their growth behaviour near
this magnetization reversal (dynamic) transition point.",0109188v1
2013-07-02,Local and Nonlocal Advected Invariants and Helicities in Magnetohydrodynamics and Gas Dynamics I: Lie Dragging Approach,"In this paper we discuss conservation laws in ideal magnetohydrodynamics
(MHD) and gas dynamics associated with advected invariants. The invariants in
some cases, can be related to fluid relabelling symmetries associated with the
Lagrangian map. There are different classes of invariants that are advected or
Lie dragged with the flow. Simple examples are the advection of the entropy S
(a 0-form), and the conservation of magnetic flux (an invariant 2-form advected
with the flow). The magnetic flux conservation law is equivalent to Faraday's
equation. We discuss the gauge condition required for the magnetic helicity to
be advected with the flow. The conditions for the cross helicity to be an
invariant are discussed. We discuss the different variants of helicity in fluid
dynamics and in MHD, including: fluid helicity, cross helicity and magnetic
helicity. The fluid helicity conservation law and the cross helcity
conservation law in MHD are derived for the case of a barotropic gas. If the
magnetic field lies in the constant entropy surface, then the gas pressure can
depend on both the entropy and the density. In these cases the conservation
laws are local conservation laws. We obtain nonlocal conservation laws for
fluid helicity and cross helicity for non-barotropic fluids using the Clebsch
variable formulation of gas dynamics and MHD. Ertel's theorem and potential
vorticity, the Hollman invariant, and the Godbillon Vey invariant for special
flows for which the magnetic helicity is zero are also discussed.",1307.1105v2
2017-01-05,Structural dynamics during laser induced ultrafast demagnetization,"The mechanism underlying femtosecond laser pulse induced ultrafast
magnetization dynamics remains elusive despite two decades of intense research
on this phenomenon. Most experiments focused so far on characterizing
magnetization and charge carrier dynamics, while first direct measurements of
structural dynamics during ultrafast demagnetization were reported only very
recently. We here present our investigation of the infrared laser pulse induced
ultrafast demagnetization process in a thin Ni film, which characterizes
simultaneously magnetization and structural dynamics. This is achieved by
employing femtosecond time resolved X-ray resonant magnetic reflectivity
(tr-XRMR) as probe technique. The experimental results reveal unambiguously
that the sub-picosecond magnetization quenching is accompanied by strong
changes in non-magnetic X-ray reflectivity. These changes vary with reflection
angle and changes up to 30$\%$ have been observed. Modeling the X-ray
reflectivity of the investigated thin film, we can reproduce these changes by a
variation of the apparent Ni layer thickness of up to 1$\%$. Extending these
simulations to larger incidence angles we show that tr-XRMR can be employed to
discriminate experimentally between currently discussed models describing the
ultrafast demagnetization phenomenon.",1701.01375v3
2017-05-07,Magnetohydrodynamic modeling of solar coronal dynamics with an initial non-force-free magnetic field,"The magnetic fields in the solar corona are generally neither force-free nor
axisymmetric and have complex dynamics that are difficult to characterize. Here
we simulate the topological evolution of solar coronal magnetic field lines
(MFLs) using a magnetohydrodynamic model. The simulation is initialized with a
non-axisymmetric non-force-free magnetic field that best correlates with the
observed vector magnetograms of solar active regions (ARs). To focus on these
ideas, simulations are performed for the flaring AR 11283 noted for its
complexity and well-documented dynamics. The simulated dynamics develops as the
initial Lorentz force pushes the plasma and facilitates successive magnetic
reconnections at the two X-type null lines present in the initial field.
Importantly, the simulation allows for the spontaneous development of mass
flow, unique among contemporary works, that preferentially reconnects field
lines at one of the X-type null lines. Consequently, a flux rope consisting of
low-lying twisted MFLs, which approximately traces the major polarity inversion
line, undergoes an asymmetric monotonic rise. The rise is attributed to a
reduction in the magnetic tension force at the region overlying the rope,
resulting from the reconnection. A monotonic rise of the rope is in conformity
with the standard scenario of flares. Importantly, the simulated dynamics leads
to bifurcations of the flux rope, which, being akin to the observed filament
bifurcation in AR 11283, establishes the appropriateness of the initial field
in describing ARs.",1705.02568v1
2018-03-13,Magneto-optic dynamics in a ferromagnetic nematic liquid crystal,"We investigate dynamic magneto-optic effects in a ferromagnetic nematic
liquid crystal experimentally and theoretically. Experimentally we measure the
magnetization and the phase difference of the transmitted light when an
external magnetic field is applied. As a model we study the coupled dynamics of
the magnetization, M, and the director field, n, associated with the liquid
crystalline orientational order. We demonstrate that the experimentally studied
macroscopic dynamic behavior reveals the importance of a dynamic cross-coupling
between M and n. The experimental data are used to extract the value of the
dissipative cross-coupling coefficient. We also make concrete predictions about
how reversible cross-coupling terms between the magnetization and the director
could be detected experimentally by measurements of the transmitted light
intensity as well as by analyzing the azimuthal angle of the magnetization and
the director out of the plane spanned by the anchoring axis and the external
magnetic field. We derive the eigenmodes of the coupled system and study their
relaxation rates. We show that in the usual experimental set-up used for
measuring the relaxation rates of the splay-bend or twist-bend eigenmodes of a
nematic liquid crystal one expects for a ferromagnetic nematic liquid crystal a
mixture of at least two eigenmodes.",1803.04898v1
2019-04-12,The emergence of magnetic flux and its role on the onset of solar dynamic events,"A plethora of solar dynamic events, such as the formation of active regions,
the emission of jets and the occurrence of eruptions is often associated to the
emergence of magnetic flux from the interior of the Sun to the surface and
above. Here, we present a short review on the onset, driving and/or triggering
of such events by magnetic flux emergence. We briefly describe some key
observational examples, theoretical aspects and numerical simulations, towards
revealing the mechanisms that govern solar dynamics and activity related to
flux emergence. We show that the combination of important physical processes
like shearing and reconnection of magnetic fieldlines in emerging flux regions
or at their vicinity, can power some of the most dynamic phenomena in the Sun
on various temporal and spatial scales. Based on previous and recent
observational and numerical studies, we highlight that, in most cases, none of
these processes alone can drive and also trigger explosive phenomena releasing
considerable amount of energy towards the outer solar atmosphere and space,
such as flares, jets and large-scale eruptions (e.g. CMEs). In addition, one
has to take into account the physical properties of the emerging field (e.g.
strength, amount of flux, relative orientation to neighbouring and pre-existing
magnetic fields, etc.) in order to better understand the exact role of magnetic
flux emergence on the onset of solar dynamic events.",1904.06274v1
2022-11-01,Deformation and necking of liquid droplets in a magnetic field,"Pendant droplets of water and paramagnetic solutions are studied in the
presence of uniform and nonuniform magnetic fields produced by small permanent
magnet arrays, both in static conditions and during dynamic pinch-off. Static
measurements of the droplet shape are analysed in terms of an apparent surface
tension {\gamma}app or an effective density \r{ho}eff. The change of surface
tension of deionized water in a uniform field of 450 mT is insignificant, 0.19
- 0.21 mNm-1. Measurements on droplets of compensated zero-susceptibility
solutions of Cu2+, Mn2+ and Dy3+ where the shape is unaffected by any magnetic
body force show changes of surface tension of about -1% in 500 mT. Magnetic
field gradients of up to 100 T2m-1 deform the droplets and lead to changes of
\r{ho}eff that are negative for diamagnetic solutions (buoyancy effect) and
positive for paramagnetic solutions. The droplet profile of
strongly-paramagnetic 0.1 Dy M DyCl3 solution is analysed, treating the
nonuniform vertical field gradient as a spatial variation of gravity. The
influence of Maxwell stress on droplet shape is discussed. In dynamic
measurements, the droplet shape at pinch-off is recorded by high-speed
photography and analysed in terms of a relative change of dynamic surface
tension in the presence of a magnetic field. The surface-tension-dependent
pre-factor of the scaling law that governs the pinch-off dynamics shows no
difference for pure water or 0.11 M DyCl3 solutions in the field. The
nonuniform field has no influence in the pinch-off region because the filament
diameter is much less than the capillary length.",2211.00431v1
2005-03-30,Master Equations for pulsed magnetic fields: Application to magnetic molecules,"We extend spin-lattice relaxation theory to incorporate the use of pulsed
magnetic fields for probing the hysteresis effects and magnetization steps and
plateaus exhibited, at low temperatures, by the dynamical magnetization of
magnetic molecules. The main assumption made is that the lattice degrees of
freedom equilibrate in times much shorter than both the experimental time scale
(determined by the sweep rate) and the typical spin-lattice relaxation time. We
first consider the isotropic case (a magnetic molecule with a ground state of
spin $S$ well separated from the excited levels and also the general isotropic
Heisenberg Hamiltonian where all energy levels are relevant) and then we
include small off-diagonal terms in the spin Hamiltonian to take into account
the Landau-Zener-St\""{u}ckelberg (LZS) effect. In the first case, and for an
$S=1/2$ magnetic molecule we arrive at the generalized Bloch equation recently
used for the magnetic molecule \{V$_6$\} in Phys. Rev. Lett. 94, 147204 (2005).
An analogous equation is derived for the magnetization, at low temperatures, of
antiferromagnetic ring systems. The LZS effect is discussed for magnetic
molecules with a low spin ground state, for which we arrive at a very
convenient set of equations that take into account the combined effects of LZS
and thermal transitions. In particular, these equations explain the deviation
from exact magnetization reversal at $B\approx 0$ observed in \{V$_6$\}. They
also account for the small magnetization plateaus (``magnetic Foehn effect''),
following the LZS steps, that have been observed in several magnetic molecules.
Finally, we discuss the role of the Phonon Bottleneck effect at low
temperatures and specifically we indicate how this can give rise to a
pronounced Foehn effect.",0503695v2
2012-04-24,Magnetic buoyancy instabilities in the presence of magnetic flux pumping at the base of the solar convection zone,"We perform idealised numerical simulations of magnetic buoyancy instabilities
in a model of the solar tachocline. We introduce a simplified model of magnetic
flux pumping in an upper layer (the convection zone), and study the effects of
its inclusion on the evolution of buoyancy instabilities in a lower layer (the
radiative interior). We study its effects on the instability of both a
preconceived magnetic slab and of a shear-generated magnetic layer. In the
former, we find that in the regime in which the downward pumping velocity is
comparable with the Alfven speed of the magnetic layer, flux pumping is able to
hold back the bulk of the magnetic field, with only small pockets of strong
field able to rise into the upper layer. In simulations in which the magnetic
layer is generated by shear, we find that the shear velocity is not necessarily
required to exceed that of the pumping (therefore the kinetic energy of the
shear is not required to exceed that of the overlying convection), for strong
localised pockets of magnetic field to be produced which can rise into the
upper layer. This is because magnetic flux pumping acts to store the field
below the interface, allowing it to be amplified both by the shear, and by
vortical fluid motions, until pockets of field can achieve sufficient strength
to rise into the upper layer. In addition, we find that the interface between
the two layers is a natural location for the production of strong vertical
gradients in the magnetic field. If these gradients are sufficiently strong to
allow the development of magnetic buoyancy instabilities, strong shear is not
necessarily required to drive them (c.f. previous work by Vasil & Brummell). We
find that the addition of magnetic flux pumping appears to be able to assist
shear-driven magnetic buoyancy in producing strong flux concentrations that can
rise into the convection zone from the radiative interior.",1204.5432v1
2012-12-25,The Small-Scale Dynamo at Low Magnetic Prandtl Numbers,"The present-day Universe is highly magnetized, even though the first magnetic
seed fields were most probably extremely weak. To explain the growth of the
magnetic field strength over many orders of magnitude fast amplification
processes need to operate. The most efficient mechanism known today is the
small-scale dynamo, which converts turbulent kinetic energy into magnetic
energy leading to an exponential growth of the magnetic field. The efficiency
of the dynamo depends on the type of turbulence indicated by the slope of the
turbulence spectrum v(l) \propto l^{theta}, where v(l) is the eddy velocity at
a scale l. We explore turbulent spectra ranging from incompressible Kolmogorov
turbulence with theta = 1/3 to highly compressible Burgers turbulence with
theta = 1/2. In this work we analyze the properties of the small-scale dynamo
for low magnetic Prandtl numbers Pm, which denotes the ratio of the magnetic
Reynolds number, Rm, to the hydrodynamical one, Re. We solve the Kazantsev
equation, which describes the evolution of the small-scale magnetic field,
using the WKB approximation. In the limit of low magnetic Prandtl numbers the
growth rate is proportional to Rm^{(1-theta)/(1+theta)}. We furthermore discuss
the critical magnetic Reynolds number Rm_crit, which is required for
small-scale dynamo action. The value of Rm_crit is roughly 100 for Kolmogorov
turbulence and 2700 for Burgers. Furthermore, we discuss that Rm_crit provides
a stronger constraint in the limit of low Pm than it does for large Pm. We
conclude that the small-scale dynamo can operate in the regime of low magnetic
Prandtl numbers, if the magnetic Reynolds number is large enough. Thus, the
magnetic field amplification on small scales can take place in a broad range of
physical environments and amplify week magnetic seed fields on short
timescales.",1212.5979v1
2018-07-23,The Quantum Field Of A Magnet Shown By A Nanomagnetic Ferrolens,"It has been more than two hundred years since the first iron filings
experiment, showing us the 2D macroscopic magnetic imprint of the field of a
permanent magnet. However, latest developments in modern nanomagnetic passive
direct observation devices reveal in real-time and color a more intriguing 3D
dynamic and detailed image of the field of a magnet, with surprising new
findings, that can change our perspective for dipole magnetism forever and lead
to new research. This research is a continuation of our previous work (DOI:
10.1016/j.jmmm.2017.12.023). The magnetostatic fields were under our scope and
examined with the aid of the ferrolens. We are presenting experimental and
photographical evidence, demonstrating the true complex 3D Euclidian geometry
of the quantum field of permanent magnets that have never been seen before and
the classic iron filings experiment, apart of its 2D limitations, fails to
depict. An analysis of why and what these iron filings inherent limitations
are, giving us an incomplete and also in some degree misguiding image of the
magnetic field of a magnet is carried out, whereas, as we prove the ferrolens
is free of these limitations and its far more advanced visualization
capabilities is allowing it to show the quantum image with depth of field
information, of the dipole field of a permanent magnet. For the first time the
domain wall (i.e. Bloch or Neel wall) region of the field of a magnet is
clearly made visible by the ferrolens along with what phenomenon is actually
taking place there, leading to the inescapable conclusion, novel observation
and experimental evidence that the field of any dipole magnet actually consists
of two distinct and separate toroidal shaped 3D magnetic bubbles, each located
at either side of the dipole around the exact spatial regions where the two
poles of the magnet reside.",1807.08751v1
2016-01-27,Dynamic nuclear polarization in a magnetic resonance force microscope experiment,"We report achieving enhanced nuclear magnetization in a magnetic resonance
force microscope experiment at 0.6 tesla and 4.2 kelvin using the dynamic
nuclear polarization (DNP) effect. In our experiments a microwire coplanar
waveguide delivered radiowaves to excite nuclear spins and microwaves to excite
electron spins in a 250 nm thick nitroxide-doped polystyrene sample. Both
electron and proton spin resonance were observed as a change in the mechanical
resonance frequency of a nearby cantilever having a micron-scale nickel tip.
NMR signal, not observable from Curie-law magnetization at 0.6 tesla, became
observable when microwave irradiation was applied to saturate the electron
spins. The resulting NMR signal's size, buildup time, dependence on microwave
power, and dependence on irradiation frequency was consistent with a transfer
of magnetization from electron spins to nuclear spins. Due to the presence of
an inhomogenous magnetic field introduced by the cantilever's magnetic tip, the
electron spins in the sample were saturated in a microwave-resonant slice 10's
of nm thick. The spatial distribution of the nuclear polarization enhancement
factor $\epsilon$ was mapped by varying the frequency of the applied
radiowaves. The observed enhancement factor was zero for spins in the center of
the resonant slice, was $\epsilon = +10$ to $+20$ for spins proximal to the
magnet, and was $\epsilon = -10$ to $-20$ for spins distal to the magnet. We
show that this bipolar nuclear magnetization profile is consistent with
cross-effect DNP in a $\sim \! 10^{5} \: \mathrm{T} \: \mathrm{m}^{-1}$
magnetic field gradient. Potential challenges associated with generating and
using DNP-enhanced nuclear magnetization in a nanometer-resolution magnetic
resonance imaging experiment are elucidated and discussed.",1601.07253v2
2018-10-19,Magnetogenesis at Cosmic Dawn: Tracing the Origins of Cosmic Magnetic Fields,"Despite their ubiquity, the origin of cosmic magnetic fields remains unknown.
Various mechanisms have been proposed for their existence including primordial
fields generated by inflation, or amplification and injection by compact
astrophysical objects. Separating the potential impact of each magnetogenesis
scenario on the magnitude and orientation of the magnetic field and their
impact on gas dynamics may give insight into the physics that magnetised our
Universe. In this work, we demonstrate that because the induction equation and
solenoidal constraint are linear with $B$, the contribution from different
sources of magnetic field can be separated in cosmological magnetohydrodynamics
simulations and their evolution and influence on the gas dynamics can be
tracked. Exploiting this property, we develop a magnetic field tracer algorithm
for cosmological simulations that can track the origin and evolution of
different components of the magnetic field. We present a suite of cosmological
magnetohydrodynamical RAMSES simulations that employ this algorithm where the
primordial field strength is varied to determine the contributions of the
primordial and supernovae-injected magnetic fields to the total magnetic energy
as a function of time and spatial location. We find that, for our specific
model, the supernova-injected fields rarely penetrate far from haloes, despite
often dominating the total magnetic energy in the simulations. The magnetic
energy density from the supernova-injected field scales with density with a
power-law slope steeper than 4/3 and often dominates the total magnetic energy
inside of haloes. However, the star formation rates in our simulations are not
affected by the presence of magnetic fields, for the ranges of primordial field
strengths examined. These simulations represent a first demonstration of the
magnetic field tracer algorithm (abridged).",1810.08619v2
2022-03-22,Dynamic Magnetic Crossover at the Origin of the Hidden-Order in van der Waals Antiferromagnet CrSBr,"The van der Waals material CrSBr stands out as a promising two-dimensional
magnet. Especially, its high magnetic ordering temperature and versatile
magneto-transport properties make CrSBr an important candidate for new devices
in the emergent field of two-dimensional magnetic materials. To date, the
magnetic and structural properties of CrSBr have not been fully elucidated.
Here, we report on the detailed temperature-dependent magnetic and structural
properties of this material, by comprehensively combining neutron scattering,
muon spin relaxation spectroscopy, synchrotron X-ray diffraction, and
magnetization measurements. We evidence that this material undergoes a
transition to an A-type antiferromagnetic state below $T_{\rm N} \approx$ 140
K, with a pronounced two-dimensional character as deduced from the determined
critical exponent of $\beta \approx $ 0.18. In our analysis of the
field-induced metamagnetic transition, we find that the ferromagnetic
correlations within the monolayers persist clearly above the N\'eel temperature
in this material. Furthermore, we unravel the low-temperature (i.e. $T < T_{\rm
N}$) magnetic hidden order within the long-range magnetically ordered state. We
find that it is associated to a slowing down of the magnetic fluctuations,
accompanied by a continuous reorientation of the internal magnetic field. These
take place upon cooling below $T_s$ $\approx$ 100 K, until a spin freezing
process occurs at $T$* $\approx$ 40 K. We argue this complex dynamic behavior
to reflect a magnetic crossover driven by the in-plane uniaxial anisotropy,
which is ultimately caused by the mixed-anion character of the material. Our
findings indicate that the magnetic and structural properties of CrSBr widen
its potential application as a component for spin-based electronic devices.",2203.11785v1
2000-01-20,Short-time dynamics and magnetic critical behavior of two-dimensional random-bond Potts model,"The critical behavior in the short-time dynamics for the random-bond Potts
ferromagnet in two-dimensions is investigated by short-time dynamic Monte Carlo
simulations. The numerical calculations show that this dynamic approach can be
applied efficiently to study the scaling characteristic, which is used to
estimate the critical exponents theta, beta/nu and z for the quenched disorered
systems from the power-law behavior of the kth moments of magnetizations.",0001284v2
2000-09-26,Dynamical process for switching dynamics between metastable ordered magnetic state and nonmagnetic ground state in photoinduced phase transition,"We propose a dynamical mechanism of the two-way switching between the
metastable state and the stable state, which has been found in experiments of
photoinduced reversible magnetization and photoinduced structural phase
transition. We find that the two-way switching with a non-symmetry breaking
perturbation such as illumination is possible only in systems with appropriate
parameters. We make it clear that the existence of two time scales in the
dynamical process is important for the two-way switching.",0009392v2
2001-03-06,Short-time dynamics of random-bond Potts ferromagnet with continuous self-dual quenched disorders,"We present Monte Carlo simulation results of random-bond Potts ferromagnet
with the Olson-Young self-dual distribution of quenched disorders in
two-dimensions. By exploring the short-time scaling dynamics, we find universal
power-law critical behavior of the magnetization and Binder cumulant at the
critical point, and thus obtain estimates of the dynamic exponent $z$ and
magnetic exponent $\eta$, as well as the exponent $\theta$. Our special
attention is paid to the dynamic process for the $q$=8 Potts model.",0103130v1
2003-06-04,Nuclear Spin Dynamics in Parabolic Quantum Wells,"We present a detailed analytical and numerical analysis of the nuclear spin
dynamics in parabolic quantum wells. The shallow potential of parabolic quantum
wells permits substantial modification of the electronic wave function in small
electric fields. The nuclear spin relaxation via the hyperfine interaction
depends on the electronic local density of states, therefore the local nuclear
relaxation time depends sensitively on the electric field. For an inhomogeneous
nuclear magnetization, such as generated by dynamic nuclear polarization, the
total nuclear magnetization dynamics can similarly be altered. We examine this
effect quantitatively and the effect of temperature, field, well thickness, and
nuclear spin diffusion.",0306098v2
2004-06-21,Dynamics and thermodynamics in spinor quantum gases,"We discuss magnetism in spinor quantum gases theoretically and experimentally
with emphasis on temporal dynamics of the spinor order parameter in the
presence of an external magnetic field. In a simple coupled Gross-Pitaevskii
picture we observe a dramatic suppression of spin dynamics due to quadratic
Zeeman ''dephasing''. In view of an inhomogeneous density profile of the
trapped condensate we present evidence of spatial variations of spin dynamics.
In addition we study spinor quantum gases as a model system for thermodynamics
of Bose-Einstein condensation. As a particular example we present measurements
on condensate magnetisation due to the interaction with a thermal bath.",0406485v1
2001-07-26,Dynamics with unitary phase operator:implications for Wigner's problem,"We show that for general deformations of SU(2) algebra, the dynamics in terms
of ladder operators is preserved. This is done for a system of precessing
magnetic dipole in magnetic field, using the unitary phase operator which
arises in the polar decomposition of SU(2) operators. It is pointed out that
there is a single phase operator dynamics underlying the dynamics of usual and
deformed ladder operators.",0107133v2
2010-10-06,Theory of Local Dynamical Magnetic Susceptibilities from the Korringa-Kohn-Rostoker Green Function Method,"Within the framework of time-dependent density functional theory combined
with the Korringa-Kohn-Rostoker Green function formalism, we present a real
space methodology to investigate dynamical magnetic excitations from
first-principles. We set forth a scheme which enables one to deduce the correct
effective Coulomb potential needed to preserve the spin-invariance signature in
the dynamical susceptibilities, i.e. the Goldstone mode. We use our approach to
explore the spin dynamics of 3d adatoms and different dimers deposited on a
Cu(001) with emphasis on their decay to particle-hole pairs.",1010.1293v1
2014-08-14,PT-symmetry in macroscopic magnetic structures,"We introduce the notion of PT-symmetry in magnetic nanostructures and show
that they can support a new type of non-Hermitian dynamics. Using the simplest
possible set-up consisting of two coupled ferromagnetic films, one with loss
and another one with a balanced amount of gain, we demonstrate the existence of
a spontaneous PT-symmetry breaking point where both the eigenfrequencies and
eigenvectors are degenerate. Below this point the frequency spectrum is real
indicating stable dynamics while above this point it is complex signaling
unstable dynamics.",1408.3285v1
2015-11-10,Static and dynamic properties of vortex pairs in asymmetric nanomagnets,"Stacked spin-vortex pairs in magnetic multilayered nanopillars, with vertical
separation between the vortices small compared to the vortex core size, exhibit
spin dynamics absent in individual vortices. This dynamics is nonlinear and is
due to the strong direct core-core coupling in the system, dominating
energetically for small- signal excitation. We observe and explain the
appearance of spin resonance modes, forbidden within linear dynamics, and
discuss how they depend on the magnetic and morphological asymmetries in the
samples.",1511.03092v1
2011-09-15,Dynamic mean-field and cavity methods for diluted Ising systems,"We compare dynamic mean-field and dynamic cavity as methods to describe the
stationary states of dilute kinetic Ising models. We compute dynamic mean-field
theory by expanding in interaction strength to third order, and compare to the
exact dynamic mean-field theory for fully asymmetric networks. We show that in
diluted networks the dynamic cavity method generally predicts magnetizations of
individual spins better than both first order (""naive"") and second order
(""TAP"") dynamic mean field theory.",1109.3399v3
2016-02-02,Dynamics of spinor Bose-Einstein condensate subject to dissipation,"We investigate the internal dynamics of the spinor Bose-Einstein Condensates
subject to dissipation by solving the Lindblad master equation. It is shown
that for the condensates without dissipation its dynamics always evolve along
specific orbital in the phase space of ($n_0$, $\theta$) and display three
kinds of dynamical properties including Josephson-like oscillation,
self-trapping-like oscillation and 'running phase'. In contrast, the
condensates subject to dissipation will not evolve along the specific dynamical
orbital. If component-1 and component-(-1) dissipate in different rates, the
magnetization $m$ will not conserve and the system transits between different
dynamical regions. The dynamical properties can be exhibited in the phase space
of ($n_0$, $\theta$, $m$).",1602.00807v1
1999-12-20,Jet Acceleration by Tangled Magnetic Fields,"We explore the possibility that extragalactic radio jets might be accelerated
by highly disorganized magnetic fields that are strong enough to dominate the
dynamics until the terminal Lorentz factor is reached. Following the
twin-exhaust model by Blandford & Rees (1974), the collimation under this
scenario is provided by the stratified thermal pressure from an external
medium. The acceleration efficiency then depends on the pressure gradient of
that medium. In order for this mechanism to work there must be continuous
tangling of the magnetic field, changing the magnetic equation of state away
from pure flux freezing (otherwise conversion of Poynting flux to kinetic
energy flux is suppressed). This is a complementary approach to models in which
the plasma is accelerated by large scale ordered fields. We include a simple
prescription for magnetic dissipation, which leads to tradeoffs among
conversion of magnetic energy into bulk kinetic energy, random particle energy,
and radiation. We present analytic dynamical solutions of such jets, assess the
effects of radiation drag, and comment on observational issues, such as the
predicted polarization and synchrotron brightness. Finally, we try to make the
connection to observed radio galaxies and gamma-ray bursts.",9912429v1
2001-08-15,Qualitative Properties of Magnetic Fields in Scalar Field Cosmology,"We study the qualitative properties of the class of spatially homogeneous
Bianchi VI_o cosmological models containing a perfect fluid with a linear
equation of state, a scalar field with an exponential potential and a uniform
cosmic magnetic field, using dynamical systems techniques. We find that all
models evolve away from an expanding massless scalar field model in which the
matter and the magnetic field are negligible dynamically. We also find that for
a particular range of parameter values the models evolve towards the usual
power-law inflationary model (with no magnetic field) and, furthermore, we
conclude that inflation is not fundamentally affected by the presence of a
uniform primordial magnetic field. We investigate the physical properties of
the Bianchi I magnetic field models in some detail.",0108268v1
2001-10-30,Cluster Magnetic Fields,"Magnetic fields in galaxy clusters have been measured using a variety of
techniques, including: studies of synchrotron relic and halo radio sources
within clusters, studies of inverse Compton X-ray emission from clusters,
surveys of Faraday rotation measures of polarized radio sources both within and
behind clusters, and studies of Cluster Cold Fronts in X-ray images. These
measurements imply that most cluster atmospheres are substantially magnetized,
with typical field strengths of order 1 microGauss with high areal filling
factors out to Mpc radii. There is likely, however, to be considerable
variation in field strengths and topologies both within and between clusters,
especially when comparing dynamically relaxed clusters to those that have
recently undergone a merger. In some locations, such as the cores of cooling
flow clusters, the magnetic fields reach levels of 10 - 40 microGauss, and may
be dynamically important. In all clusters the magnetic fields have a
significant effect on energy transport in the intracluster medium. We also
review current theories on the origin of cluster magnetic fields.",0110655v1
2002-10-18,Nonlinear Magnetic Diffusion and Magnetic Helicity Transport in Galactic Dynamos,"We have extended our previous mean-field galactic dynamo model which included
algebraic and dynamic alpha nonlinearities (Kleeorin et al., A&A, v. 387, 453,
2002), to include also a quenching of turbulent diffusivity. We readily obtain
equilibrium states for the large-scale magnetic field in the local disc dynamo
model, and these fields have strengths that are comparable to the equipartition
field strength. We find that the algebraic nonlinearity alone (i.e. quenching
of both the alpha effect and turbulent magnetic diffusion) cannot saturate the
growth of the mean magnetic field; only the combined effect of algebraic and
dynamic nonlinearities can limit the growth of the mean magnetic field.
However, in contrast to our earlier work without quenching of the turbulent
diffusivity, we cannot now find satisfactory solutions in the no-z
approximation to the axisymmetric galactic dynamo problem.",0210430v1
2005-08-24,Effects of Magnetic Fields on Proto-Neutron Star Winds,"We discuss effects of magnetic fields on proto-neutron star winds by
performing numerical simulation. We assume that the atmosphere of proto-neutron
star has a homogenous magnetic field (ranging from ~10^{12} G to ~10^{15} G)
perpendicular to the radial direction and examine the dependence of the three
key quantities (dynamical time scale, electron fraction, and entropy per
baryon) for the successful r-process on the magnetic field strength. Our
results show that even with a magneter-class field strength, ~10^{15} G, the
feature of the wind dynamics varies only little from that of non-magnetic
winds, and that the condition for successful r-process is not realized.",0508512v2
2006-11-16,Cosmology with inhomogeneous magnetic fields,"We review spacetime dynamics in the presence of large-scale electromagnetic
fields and then consider the effects of the magnetic component on perturbations
to a spatially homogeneous and isotropic universe. Using covariant techniques,
we refine and extend earlier work and provide the magnetohydrodynamic equations
that describe inhomogeneous magnetic cosmologies in full general relativity.
Specialising this system to perturbed Friedmann-Robertson-Walker models, we
examine the effects of the field on the expansion dynamics and on the growth of
density inhomogeneities, including non-adiabatic modes. We look at scalar
perturbations and obtain analytic solutions for their linear evolution in the
radiation, dust and inflationary eras. In the dust case we also calculate the
magnetic analogue of the Jeans length. We then consider the evolution of vector
perturbations and find that the magnetic presence generally reduces the decay
rate of these distortions. Finally, we examine the implications of magnetic
fields for the evolution of cosmological gravitational waves.",0611537v4
1996-09-02,Tunneling with the Lorentz Force and the Friction,"We present a semiclassical study of a transport process, the tunneling, in
the presence of a magnetic field and a dissipative environment. We have found
that the problem can be mapped onto an effective one-dimensional one, and the
tunneling rate is strongly affected by the magnetic field, such as a complete
suppression by a large parallel magnetic field, an example of the dynamical
localization. In such case a small perpendicular component of the field, or the
dissipation, can enhance the tunneling rate. In the small parallel field and
finite temperatures the tunneling rate is finite. Explicit expressions will be
presented in those cases. If viewing the tunneling in the presence of a
magnetic field as a dissipative tunneling process, by varying the magnetic
field and the potential one can obtain the dissipative spectral function
between the subohmic $s =0$ and the superohmic $s = \infty$. In combination
with a real dissipative spectral function, the effect of the magnetic field can
map the spectral function from $s $ to $2-s$, with $s>2$ mapping to $ s = 0$,
revealing a dual symmetry between the friction and the Lorentz force. Two cases
relevant to experiments, the edge state tunneling in a Hall bar and the
tunneling near the dynamical localization will be discussed in detail.",9609021v1
2002-06-21,Heisenberg Dimer Single Molecule Magnets in a Strong Magnetic Field,"We calculate the static and dynamic properties of single crystal, single
molecule magnets consisting of equal spin $S=1/2$ or 5/2 dimers. The spins in
each dimer interact with each other via the Heisenberg exchange interaction and
with the magnetic induction ${\bf B}$ via the Zeeman interaction, and
interdimer interactions are negligible. For antiferromagnetic couplings, the
static magnetization and specific heat exhibit interesting low temperature $T$
and strong ${\bf B}$ quantum effects. We calculate the frequency spectrum of
the Fourier transform of the real part of the time autocorrelation function
${\cal C}_{11}(t)$ for arbitrary $T, {\bf B}$, and compare our results with
those obtained for classical spins. We also calculate the inelastic neutron
magnetic dynamical structure factor $S({\bf q},\omega)$ at arbitrary $T, {\bf
B}$.",0206406v2
2003-02-17,Magnetization dynamics with a spin-transfer torque,"The magnetization reversal and dynamics of a spin valve pillar, whose lateral
size is 64$\times$64 nm$^2$, are studied by using micromagnetic simulation in
the presence of spin transfer torque. Spin torques display both characteristics
of magnetic damping (or anti-damping) and of an effective magnetic field. For a
steady-state current, both M-I and M-H hysteresis loops show unique features,
including multiple jumps, unusual plateaus and precessional states. These
states originate from the competition between the energy dissipation due to
Gilbert damping and the energy accumulation due to the spin torque supplied by
the spin current. The magnetic energy oscillates as a function of time even for
a steady-state current. For a pulsed current, the minimum width and amplitude
of the spin torque for achieving current-driven magnetization reversal are
quantitatively determined. The spin torque also shows very interesting thermal
activation that is fundamentally different from an ordinary damping effect.",0302337v1
2003-07-25,Dynamics of a magnetic moment induced by a spin-polarized current,"Effects of an incoming spin-polarized current on a magnetic moment are
explored. We found that the spin torque occurs only when the incoming spin
changes as a function of time inside of the magnetic film. This implies that
some modifications are necessary in a phenomenological model where the
coefficient of the spin torque term is a constant, and the coefficient is
determined by dynamics instead of geometrical details. The precession of the
magnetization reversal depends on the incoming energy of electrons in the
spin-polarized current. If the incoming energy is smaller than the interaction
energy, the magnetization does not precess while reversing its direction. We
also found that the relaxation time associated with the reversal depends on the
incoming energy. The coupling between an incoming spin and a magnetic moment
can be estimated by measuring the relaxation time.",0307633v1
2003-09-12,Anisotropy of exchange stiffness and its effect on the properties of magnets,"Using the spin-spiral formulation of the tight-binding linear muffin-tin
orbital method, the principal components of the exchange stiffness tensor are
calculated for typical hard magnets including tetragonal CoPt-type and
hexagonal YCo5 alloys. The exchange stiffness is strongly anisotropic in all
studied alloys. This anisotropy makes the domain wall surface tension
anisotropic. Competition between this anisotropic surface tension and
magnetostatic energy controls the formation and dynamics of nanoscale domain
structures in hard magnets. Anisotropic domain wall bending is described in
detail from the general point of view and with application to cellular Sm-Co
magnets. It is shown that the repulsive cell-boundary pinning mechanism in
these magnets is feasible only due to the anisotropic exchange stiffness if
suitably oriented initial pinning centers are available. In polytwinned
CoPt-type magnets the exchange stiffness anisotropy controls the orientation of
macrodomain wall segments. These segments may reorient both statically during
microstructural coarsening and dynamically during the macrodomain wall
splitting in external field. Reorientation of segments may facilitate their
pinning at antiphase boundaries.",0309321v1
2003-10-18,Dynamics of the superconducting condensate in the presence of a magnetic field. Channelling of vortices in superconducting strips at high currents,"On the basis of the time-dependent Ginzburg-Landau equation we studied the
dynamics of the superconducting condensate in a wide two-dimensional sample in
the presence of a perpendicular magnetic field and applied current. We could
identify two critical currents: the current at which the pure superconducting
state becomes unstable ($J_{c2}$ \cite{self1}) and the current at which the
system transits from the resistive state to the superconducting state
($J_{c1}<J_{c2}$). The current $J_{c2}$ decreases monotonically with external
magnetic field, while $J_{c1}$ exhibits a maximum at $H^*$. For sufficient
large magnetic fields the hysteresis disappears and $J_{c1}=J_{c2}=J_c$. In
this high magnetic field region and for currents close to $J_c$ the voltage
appears as a result of the motion of separate vortices. With increasing current
the moving vortices form 'channels' with suppressed order parameter along which
the vortices can move very fast. This leads to a sharp increase of the voltage.
These 'channels' resemble in some respect the phase slip lines which occur at
zero magnetic field.",0310438v1
2004-01-16,Observation of a Transient Magnetization Plateau in a Quantum Antiferromagnet on the Kagome Lattice,"The magnetization process of an S=1/2 antiferromagnet on the kagome lattice,
[Cu_3(titmb)_2(OCOCH_3)_6]H_2O {titmb= 1,3,5-tris(imidazol-1-ylmethyl)-2,4,6
trimethylbenzene} has been measured at very low temperatures in both pulsed and
steady fields. We have found a new dynamical behavior in the magnetization
process: a plateau at one third of the saturation magnetization appears in the
pulsed field experiments for intermediate sweep rates of the magnetic field and
disappears in the steady field experiments. A theoretical analysis using exact
diagonalization yields J_1=-19K and J_2=6K, for the nearest neighbor and second
nearest neighbor interactions, respectively. This set of exchange parameters
explains the very low saturation field and the absence of the plateau in the
thermodynamic equilibrium as well as the two-peak feature in the magnetic heat
capacity. Supported by numerical results we argue that a dynamical order by
disorder phenomenon could explain the transient appearance of the 1/3 plateau
in pulsed field experiments.",0401288v2
2004-04-26,Glauber dynamics in a single-chain magnet: From theory to real systems,"The Glauber dynamics is studied in a single-chain magnet. As predicted, a
single relaxation mode of the magnetization is found. Above 2.7 K, the
thermally activated relaxation time is mainly governed by the effect of
magnetic correlations and the energy barrier experienced by each magnetic unit.
This result is in perfect agreement with independent thermodynamical
measurements. Below 2.7 K, a crossover towards a relaxation regime is observed
that is interpreted as the manifestation of finite-size effects. The
temperature dependences of the relaxation time and of the magnetic
susceptibility reveal the importance of the boundary conditions.",0404620v1
2005-08-11,Magnetization self-organization in a single-domain ferromagnet subject to a spin current,"The Landau-Lifshitz equation for the magnetization dynamics of a
single-domain magnetic system is solved using the methods of self-organization.
The description takes into account the torque due to spin transfer. The
potential energy of the system includes the uniaxial and easy-plane
anisotropies, and the Zeeman energy due to an external magnetic field. The
equilibrium and stationary states are investigated as a function of the spin
current and external magnetic field. The presented bifurcation diagram allows
to determine the margins of a neutral stability mode of the equilibrium and
stationary states for different values of the easy-plane anisotropy constant.
Using the power spectral density method, the trajectory tracing, Hausdorff
dimension, and maximum Lyapunov exponent, the dynamics of the phase states in
an external magnetic field is demonstrated. The analytical transcendent
equations for switching between different equilibrium states are also obtained,
proving the importance of phase averaging.",0508280v1
2005-10-29,First-principles approach to Non-Collinear Magnetism: towards Spin-dynamics,"A description of non-collinear magnetism in the framework of spin-density
functional theory is presented for the exact exchange energy functional which
depends explicitly on two-component spinor orbitals. The equations for the
effective Kohn-Sham scalar potential and magnetic field are derived within the
optimized effective potential (OEP) framework. With the example of a
magnetically frustrated Cr monolayer it is shown that the resulting
magnetization density exhibits much more non-collinear structure than standard
calculations. Furthermore, a time-dependent generalization of the non-collinear
OEP method is well suited for an ab-initio description of spin dynamics. We
also show that the magnetic moments of solids Fe, Co and Ni are well
reproduced.",0510800v3
2005-11-04,Synchronized Magnetization Oscillations in F/N/F Nanopillars,"Current-induced magnetization dynamics in a trilayer structure composed of
two ferromagnetic free layers and a nonmagnetic spacer is examined. Both free
layers are treated as a monodomain magnetic body with an uniform agnetization.
The dynamics of the two magnetizations is modeled by modified
Landau-Lifshitz-Gilbert equations with spin-transfer torque terms. By solving
the equations simultaneously, we discuss their various solutions in detail. We
show that there exists the synchronous motion of two magnetizations among the
various solutions; the magnetizations are resonantly coupled via spin-transfer
torques and perform precessional motions with the same period. The condition to
excite the synchronous motion depends on the difference between the intrinsic
frequencies of the two ferromagnetic free layers as well as the magnitude of
current.",0511095v1
2006-04-28,Magnetic neutron scattering in hole doped cuprate superconductors,"A review is presented of the static and dynamic magnetic properties of
hole-doped cuprate superconductors measured with neutron scattering. A wide
variety of experiments are described with emphasis on the monolayer
La_{2-x}(Sr,Ba)_{x}CuO_{4} and bilayer YBa_{2}Cu_{3}O_{6+x} cuprates. At zero
hole doping, both classes of materials are antiferromagnetic insulators with
large superexchange constants of J > 100 meV. For increasing hole doping, the
cuprates become superconducting at a critical hole concentration of
x_{c}=0.055. The development of new instrumentation at neutron beam sources
coupled with the improvement in materials has lead to a better understanding of
these materials and the underlying spin dynamics over a broad range of hole
dopings. We will describe how the spin dispersion changes across the insulating
to superconducting boundary as well as the static magnetic properties which are
directly coupled with the superconductivity. Experiments directly probing the
competing magnetic and superconducting order parameters involving magnetic
fields, impurity doping, and structural order will be examined. Correlations
between superconductivity and magnetism will also be discussed.",0604667v1
2004-06-26,Two-fluid magnetic island dynamics in slab geometry: II - Islands interacting with resistive walls or static external resonant magnetic perturbations,"The dynamics of a propagating magnetic island interacting with a resistive
wall or a static external magnetic perturbation is investigated using
two-fluid, drift-MHD theory in slab geometry. In both cases, the island
equation of motion is found to take exactly the same form as that predicted by
single-fluid MHD theory. Three separate ion polarization terms are found in the
Rutherford island width evolution equation. The first is the drift-MHD
polarization term for an isolated island, and is completely unaffected by
interaction with a wall or magnetic perturbation. Next, there is the
polarization term due to interaction with a wall or magnetic perturbation which
is predicted by single-fluid MHD theory. Finally, there is a hybrid of the
other two polarization terms. The sign of this term depends on many factors.
However, under normal conditions, it is stabilizing if the unperturbed island
propagates in the ion diamagnetic direction (in the lab. frame), and
destabilizing if it propagates in the electron diamagnetic direction.",0406133v1
2007-10-29,Buoyancy Instabilities in Weakly Magnetized Low Collisionality Plasmas,"I calculate the linear stability of a stratified low collisionality plasma in
the presence of a weak magnetic field. Heat is assumed to flow only along
magnetic field lines. In the absence of a heat flux in the background plasma,
Balbus (2000) demonstrated that plasmas in which the temperature increases in
the direction of gravity are buoyantly unstable to convective-like motions (the
``magnetothermal instability''). I show that in the presence of a background
heat flux, an analogous instability is present when the temperature decreases
in the direction of gravity. The instability is driven by the background heat
flux and the fastest growing mode has a growth time of order the local
dynamical time. Thus, independent of the sign of the temperature gradient,
weakly magnetized low collisionality plasmas are unstable on a dynamical time
to magnetically-mediated buoyancy instabilities. The instability described in
this paper is predicted to be present in clusters of galaxies at radii from ~
0.1-100 kpc, where the observed temperature increases outwards. Possible
consequences for the origin of cluster magnetic fields, ``cooling flows,'' and
the thermodynamics of the intercluster medium are briefly discussed.",0710.5521v1
2007-11-13,On the existence of a reverse shock in magnetized GRB ejecta,"The role of magnetic fields in gamma-ray burst (GRB) flows remains
controversial. The study of the early afterglow phases and, in particular, of
the reverse shock dynamics and associated emission offers a promising probe of
the magnetization of the ejecta. In this paper, we derive the conditions for
the existence of a reverse shock in arbitrarily magnetized ejecta that
decelerate and interact with the circumburst medium. Both constant and
wind-like density profiles are considered. We show, in contrast to previous
estimates, that ejecta with magnetization larger than unity are not crossed by
a reverse shock for a large fraction of the parameter space relevant to GRB
flows. Allowing for shell spreading, there is always a relativistic or mildly
relativistic reverse shock forming in sigma_o < 0.3 ejecta. From this, we
conclude that the paucity of optical flashes, believed to be a distinctive
signature of a reverse shock, may be explained by the existence of dynamically
important magnetic fields in the ejecta.",0711.1980v2
2007-11-23,Dynamics of magnetic topological solitons in soft magnetic nanostripes,"The motions of domain walls driven by magnetic field in soft magnetic
nanostripes were calculated. The domain walls reveal steady motions in the low
fields and oscillations of their internal structure above a critical field. A
developed soliton model of the walls explains its dynamics by the motions of a
limited number of magnetic topological solitons such as vortex and antivortex.
We predict the reduced wall velocity and critical field in the low-field
regime, and increased wall-oscillation frequency in nanostripes, compared to 1D
Walker solution for bulk magnets. The critical field and velocity are
determined by the nanostripe sizes, whereas the oscillation frequency depends
only on the field strength.",0711.3680v1
2008-08-18,The dynamics of internal working surfaces in MHD jets,"The dynamical effects of magnetic fields in models of radiative, Herbig-Haro
(HH) jets have been studied in a number of papers. For example, magnetized,
radiative jets from variable sources have been studied with axisymmetric and 3D
numerical simulations. In this paper, we present an analytic model describing
the effect of a toroidal magnetic field on the internal working surfaces that
result from a variability in the ejection velocity. We find that for parameters
appropriate for HH jets the forces associated with the magnetic field dominate
over the gas pressure force within the working surfaces. Depending on the ram
pressure radial cross section of the jet, the magnetic field can produce a
strong axial pinch, or, alternatively, a broadening of the internal working
surfaces. We check the validity of the analytic model with axisymmetric
numerical simulations of variable, magnetized jets.",0808.2346v1
2009-01-06,Analytic models of heterogenous magnetic fields for liquid metal flow simulations,"A physically consistent approach is considered for defining an external
magnetic field as needed in computational fluid dynamics problems involving
magnetohydrodynamics (MHD). The approach results in simple analytical formulae
that can be used in numerical studies where an inhomogeneous magnetic field
influences a liquid metal flow. The resulting magnetic field is divergence and
curl-free, and contains two components and parameters to vary. As an
illustration, the following examples are considered: peakwise, stepwise,
shelfwise inhomogeneous magnetic fields, and the field induced by a solenoid.
Finally, the impact of the streamwise magnetic field component is shown
qualitatively to be significant for rapidly changing fields.",0901.0624v2
2009-03-05,Signature of magnetic monopole and Dirac string dynamics in spin ice,"Magnetic monopoles have eluded experimental detection since their prediction
nearly a century ago by Dirac. Recently it has been shown that classical
analogues of these enigmatic particles occur as excitations out of the
topological ground state of a model magnetic system, dipolar spin ice. These
quasi-particle excitations do not require a modification of Maxwell's
equations, but they do interact via Coulombs law and are of magnetic origin. In
this paper we present an experimentally measurable signature of monopole
dynamics and show that magnetic relaxation measurements in the spin ice
material $Dy_{2}Ti_{2}O_{7}$ can be interpreted entirely in terms of the
diffusive motion of monopoles in the grand canonical ensemble, constrained by a
network of ""Dirac strings"" filling the quasi-particle vacuum. In a magnetic
field the topology of the network prevents charge flow in the steady state, but
there is a monopole density gradient near the surface of an open system.",0903.1074v3
2009-07-13,A topological gauge field in nanomagnets: spin wave excitations over a slowly moving magnetization background,"We introduce a topological gauge vector potential which influences spin wave
excitations over arbitrary non-uniform, slowly moving magnetization
distribution. The time-component of the gauge potential plays a principal role
in magnetization dynamics, whereas its spatial components can be often
neglected for typical magnetic nanostructures. As an example, we consider spin
modes excited in the vortex state magnetic dots. It is shown that the vortex/
spin wave interaction can be described as a consequence of the gauge field
arising due to non uniform moving vortex magnetization distribution. The
coupled equations of motion of the vortex and spin waves are solved within
small excitation amplitude approximation. The model yields a giant frequency
splitting of the spin wave modes having non-zero overlapping with the vortex
mode as well as a finite vortex mass of dynamical origin.",0907.2226v1
2009-08-26,Influence of an external magnetic field on forced turbulence in a swirling flow of liquid metal,"We report an experimental investigation on the influence of an external
magnetic field on forced 3D turbulence of liquid gallium in a closed vessel. We
observe an exponential damping of the turbulent velocity fluctuations as a
function of the interaction parameter N (ratio of Lorentz force over inertial
terms of the Navier-Stokes equation). The flow structures develop some
anisotropy but do not become bidimensional. From a dynamical viewpoint, the
damping first occurs homogeneously over the whole spectrum of frequencies. For
larger values of N, a very strong additional damping occurs at the highest
frequencies. However, the injected mechanical power remains independent of the
applied magnetic field. The simultaneous measurement of induced magnetic field
and electrical potential differences shows a very weak correlation between
magnetic field and velocity fluctuations. The observed reduction of the
fluctuations is in agreement with a previously proposed mechanism for the
saturation of turbulent dynamos and with the order of magnitude of the Von
Karman Sodium dynamo magnetic field.",0908.3821v1
2009-12-18,Dynamic correlated Cu(2) magnetic moments in superconducting YBa_2(Cu_0.96Co_0.04)_3O_y (y ~ 7),"We have examined the magnetic properties of superconducting
YBa_2(Cu_0.96Co_0.04)_3O_y (y ~ 7, T_sc = 65 K) using elastic neutron
scattering and muon spin relaxation (muSR) on single crystal samples. The
elastic neutron scattering measurements evidence magnetic reflections which
correspond to a commensurate antiferromagnetic Cu(2) magnetic structure with an
associated Neel temperature T_N ~ 400 K. This magnetically correlated state is
not evidenced by the muSR measurements. We suggest this apparent anomaly arises
because the magnetically correlated state is dynamic in nature. It fluctuates
with rates that are low enough for it to appear static on the time scale of the
elastic neutron scattering measurements, whereas on the time scale of the muSR
measurements, at least down to ~ 50 K, it fluctuates too fast to be detected.
The different results confirm the conclusions reached from work on equivalent
polycrystalline compounds: the evidenced fluctuating, correlated Cu(2) moments
coexist at an atomic level with superconductivity.",0912.3637v1
2010-01-19,Dynamical behaviour of the `beads' along the magnetic field lines near a rotating black hole,"The elements of the cold magnetic driven flows behave like beads on the
magnetic field line. The inclination of the field lines at the surface of the
disc plays a crucial role on the nature of the magnetically driven outflow. For
the non-relativistic case, a centrifugally driven outflow of matter from the
disc is possible, if the poloidal component of the magnetic field makes an
angle of less than a critical 60 degrees with the disc surface. The collimated
flows ejected from active galactic nuclei may probably start from the region
near the black hole. We investigate the dynamical behavior of the 'beads' on
the magnetic field line start from the disc near a black hole. It is found that
the critical angle becomes larger than 60 degrees for the rotating black hole
case (close to 90 degrees for a=1), which may imply that the flows are easy
accelerated in the inner edge of the disk surrounding a rotating black hole.",1001.3201v1
2010-03-09,Effect of induced shape anisotropy on magnetic properties of ferromagnetic cobalt nanocubes,"We report on the synthesis of ferromagnetic cobalt nanocubes of various sizes
using thermal pyrolysis method and the effect of shape anisotropy on the static
and dynamic magnetic properties were studied. Shape anisotropy of approximately
10 % was introduced in nanocubes by making nanodiscs using a linear chain amine
surfactant during synthesis process. It has been observed that, ferromagnetism
persisted above room temperature and a sharp drop in magnetic moment at low
temperatures in zero-field cooled magnetization may be associated with the spin
disorder due to the effective anisotropy present in the system. Dynamic
magnetic properties were studied using RF transverse susceptibility
measurements at different temperatures and the singularities due to anisotropy
fields were probed at low temperatures. Symmetrically located broad peaks are
observed in the frozen state at the effective anisotropy fields and the peak
structure is strongly affected by shape anisotropy and temperature.
Irrespective of size the shape anisotropy gave rise to higher coercive fields
and larger transverse susceptibility ratio at all temperatures. The role of
shape anisotropy and the size of the particles on the observed magnetic
behaviour were discussed.",1003.1855v1
2010-04-01,Strong coupling to magnetic fluctuations in the charge dynamics of Fe-based superconductors,"We present a comprehensive comparison of the infrared charge response of two
systems, characteristic of classes of the 122 pnictide (SrFe2As2) and 11
chalcogenide (Fe_1.087Te) Fe compounds with magnetically-ordered ground states.
In the 122 system, the magnetic phase shows a decreased plasma frequency and
scattering, and associated appearance of strong mid-infrared features. The 11
system, with a different magnetic ordering pattern, also shows decreased
scattering, but an increase in the plasma frequency, while no clear
mid-infrared features appear below the ordering temperature. We suggest how
this marked contrast can be understood in terms of the diverse magnetic
ordering patterns of the ground state, and conclude that while the high
temperature phases of these systems are similar, the magnetic ordering strongly
affects the charge dynamical response. In addition, we propose an optical
absorption mechanism which appears to be consistent with information gained
from several different experiments.",1004.0111v1
2010-06-04,Magnetization dynamics of a CrO$_2$ grain studied by micro-Hall magnetometry,"Micro-Hall magnetometry is employed to study the magnetization dynamics of a
single, micron-size CrO$_2$ grain. With this technique we track the motion of a
single domain wall, which allows us to probe the distribution of imperfections
throughout the material. An external magnetic field along the grain's easy
magnetization direction induces magnetization reversal, giving rise to a series
of sharp jumps in magnetization. Supported by micromagnetic simulations, we
identify the transition to a state with a single cross-tie domain wall, where
pinning/depinning of the wall results in stochastic Barkhausen jumps.",1006.0889v1
2010-06-29,Spin dynamics of current driven single magnetic adatoms and molecules,"A scanning tunneling microscope can probe the inelastic spin excitations of a
single magnetic atom in a surface via spin-flip assisted tunneling in which
transport electrons exchange spin and energy with the atomic spin. If the
inelastic transport time, defined as the average time elapsed between two
inelastic spin flip events, is shorter than the atom spin relaxation time, the
STM current can drive the spin out of equilibrium. Here we model this process
using rate equations and a model Hamiltonian that describes successfully spin
flip assisted tunneling experiments, including a single Mn atom, a Mn dimer and
Fe Phthalocyanine molecules. When the STM current is not spin polarized, the
non-equilibrium spin dynamics of the magnetic atom results in non-monotonic
$dI/dV$ curves. In the case of spin polarized STM current, the spin orientation
of the magnetic atom can be controlled parallel or anti-parallel to the
magnetic moment of the tip. Thus, spin polarized STM tips can be used both to
probe and to control the magnetic moment of a single atom.",1006.5608v1
2010-08-31,Influence of Magnetic Helicity in MHD,"Observations have shown that the Sun's magnetic field has helical structures.
The helicity content in magnetic field configurations is a crucial constraint
on the dynamical evolution of the system. Since helicity is connected with the
number of links we investigate configurations with interlocked magnetic flux
rings and one with unlinked rings. It turns out that it is not the linking of
the tubes which affects the magnetic field decay, but the content of magnetic
helicity.",1008.5235v2
2010-09-17,Time resolved X-ray Resonant Magnetic Scattering in reflection geometry,"We present a new setup to measure element-selective magnetization dynamics
using the ALICE chamber (RSI \textbf{74}, 4048 (2003)) at the BESSY II
synchrotron at the Helmholtz-Zentrum Berlin. A magnetic field pulse serves as
excitation, and the magnetization precession is probed by element selective
X-ray Resonant Magnetic Scattering (XRMS). With the use of single bunch
generated x-rays a temporal resolution well below 100 ps is reached. The setup
is realized in reflection geometry and enables investigations of thin films
described here, multilayers, and laterally structured samples. The combination
of the time resolved setup with a cryostat in the ALICE chamber will allow to
conduct temperature-dependent studies of precessional magnetization dynamics
and of damping constants over a large temperature range and for a large variety
of systems in reflection geometry.",1009.3389v1
2010-11-21,Optical detection of spin transport in non-magnetic metals,"We determine the dynamic magnetization induced in non-magnetic metal wedges
composed of silver, copper and platinum by means of Brillouin light scattering
(BLS) microscopy. The magnetization is transferred from a ferromagnetic
Ni80Fe20 layer to the metal wedge via the spin pumping effect. The spin pumping
efficiency can be controlled by adding an insulating but transparent interlayer
between the magnetic and non-magnetic layer. By comparing the experimental
results to a dynamical macroscopic spin-transport model we determine the
transverse relaxation time of the pumped spin current which is much smaller
than the longitudinal relaxation time.",1011.4656v2
2010-12-14,The Magnetic Field of the Milky Way Galaxy,"Cosmic magnetic fields are an integral component of the interstellar medium
(ISM), having influence on scales ranging from star formation to galactic
dynamics. While observations of external galaxies offer a `birds-eye-view' of
magnetic fields within galaxies, it is equally important to explore the
magnetic field of our own Milky Way Galaxy, which offers a more detailed,
albeit more complicated view. Over the past decade there has been a significant
increase in interest in the Galactic magnetic field, fueled largely by
innovations developed through the Canadian Galactic Plane Survey. In this
paper, I review the current state of understanding of the Galactic magnetic
field, and discuss briefly new and future observations that will provide
exciting new insights about the field.",1012.2932v1
2011-01-26,Domain wall dynamics in a single CrO$_2$ grain,"Recently we have reported on the magnetization dynamics of a single CrO$_2$
grain studied by micro Hall magnetometry (P. Das \textit{et al.}, Appl. Phys.
Lett. \textbf{97} 042507, 2010). For the external magnetic field applied along
the grain's easy magnetization direction, the magnetization reversal takes
place through a series of Barkhausen jumps. Supported by micromagnetic
simulations, the ground state of the grain was found to correspond to a flux
closure configuration with a single cross-tie domain wall. Here, we report an
analysis of the Barkhausen jumps, which were observed in the hysteresis loops
for the external field applied along both the easy and hard magnetization
directions. We find that the magnetization reversal takes place through only a
few configuration paths in the free-energy landscape, pointing to a high purity
of the sample. The distinctly different statistics of the Barkhausen jumps for
the two field directions is discussed.",1101.5101v1
2011-03-31,On the effect of rotation on magnetohydrodynamic turbulence at high magnetic Reynolds number,"This article is focused on the dynamics of a rotating electrically conducting
fluid in a turbulent state. As inside the Earth's core or in various industrial
processes, a flow is altered by the presence of both background rotation and a
large scale magnetic field. In this context, we present a set of 3D direct
numerical simulations of incompressible decaying turbulence. We focus on
parameters similar to the ones encountered in geophysical and astrophysical
flows, so that the Rossby number is small, the interaction parameter is large,
but the Elsasser number, defining the ratio between Coriolis and Lorentz
forces, is about unity. These simulations allow to quantify the effect of
rotation and thus inertial waves on the growth of magnetic fluctuations due to
Alfv\'en waves. Rotation prevents the occurrence of equipartition between
kinetic and magnetic energies, with a reduction of magnetic energy at
decreasing Elsasser number {\Lambda}. It also causes a decrease of energy
transfer mediated by cubic correlations. In terms of flow structure, a decrease
of {\Lambda} corresponds to an increase in the misalignment of velocity and
magnetic field.",1103.6236v1
2011-06-12,Critical Behavior of AC Antiferromagnetic and Ferromagnetic Susceptibilities of a Spin-1/2 Metamagnetic Ising System,"In this study, the temperature variations of the equilibrium and the
non-equilibrium antiferromagnetic and ferromagnetic susceptibilities of a
metamagnetic system are examined near the critical point. The kinetic equations
describing the time dependencies of the total and staggered magnetizations are
derived by utilizing linear response theory. In order to obtain dynamic
magnetic relaxation behavior of the system, the stationary solutions of the
kinetic equations in existence of sinusoidal staggered and physical external
magnetic fields are performed. In addition, the static and dynamical mean field
critical exponents are calculated in order to formulate the critical behavior
of antiferromagnetic and ferromagnetic magnetic response of a metamagnetic
system. Finally, a comparison of the findings of this study with previous
theoretical and experimental studies is represented and it is shown that a good
agreement is found with our results.",1106.2306v1
2011-06-23,"Dynamics of a magnetic dimer with exchange, dipolar and Dzyalozhinski-Moriya interaction","We investigate the dynamics of a magnetic system consisting of two magnetic
moments coupled by either exchange, dipole-dipole, or Dzyalozhinski-Moriya
interaction. We compare the switching mechanisms and switching rates as induced
by the three couplings. For each coupling and each configuration of the two
anisotropy axes, we describe the switching modes and, using the kinetic theory
of Langer, we provide (semi-)analytical expressions for the switching rate. We
then compare the three interactions with regard to their efficiency in the
reversal of the net magnetic moment of the dimer. We also investigate how the
energy barriers vary with the coupling. For the dipole-dipole interaction we
find that the energy barrier may either increase or decrease with the coupling
depending on whether the latter is weak or strong. Finally, upon comparing the
various switching rates, we find that the dipole-dipole coupling leads to the
slowest magnetic dimer, as far as the switching of its net magnetic moment is
concerned.",1106.4713v2
2011-09-15,Waves of magnetic moment and generation of waves by neutron beam in quantum magnetized plasma,"This paper is devoted to studying of dispersion of waves in the magnetized
plasma with the spin and exploring of new methods of the generation wave in the
plasma. We consider the dispersion of waves, existed in the plasma in
consequence of dynamic of the magnetic moments. It is shown there are nine new
waves in the magnetized plasma because of the magnetic moments dynamic. We show
there are instabilities at propagation of the neutron beam through the plasma.
Increments of instabilities caused by neutron beam are calculated. For studying
of this effects we generalize and use the method of the many-particle quantum
hydrodynamics. Described processes can play important role at calculation of
the stability and the safeness of the nuclear reactors and the studying of the
processes in the atmosphere of the neutron stars.",1109.3335v2
2011-10-07,Nonlinear Dynamics in a Magnetic Josephson Junction,"We theoretically consider a Josephson junction formed by a ferromagnetic
spacer with a strong spin-orbit interaction or a magnetic spin valve, i.e., a
bilayer with one static and one free layer. Electron spin transport facilitates
a nonlinear dynamical coupling between the magnetic moment and charge current,
which consists of normal and superfluid components. By phenomenologically
adding reactive and dissipative interactions (guided by structural and Onsager
symmetries), we construct magnetic torques and charge pumping, whose
microscopic origins are also discussed. A stability analysis of our coupled
nonlinear systems generates a rich phase diagram with fixed points, limit
cycles, and quasiperiodic states. Our findings reduce to the known phase
diagrams for current-biased nonmagnetic Josephson junctions, on the one hand,
and spin-torque driven magnetic films, on the other, in the absence of coupling
between the magnetic and superconducting order parameters.",1110.1680v1
2011-10-26,Instability of the origami of a ferrofluid drop in a magnetic field,"Capillary origami is the wrapping of an usual fluid drop by a planar elastic
membrane due to the interplay between capillary and elastic forces. Here, we
use a drop of magnetic fluid whose shape is known to strongly depend on an
applied magnetic field. We study the quasi-static and dynamical behaviors of
such a magnetic capillary origami. We report the observation of an overturning
instability that the origami undergoes at a critical magnetic field. This
instability is triggered by an interplay between magnetic and gravitational
energies in agreement with the theory presented here. Additional effects of
elasticity and capillarity on this instability are also discussed.",1110.5768v1
2012-03-05,A dynamical magnetosphere model for periodic Halpha emission from the slowly rotating magnetic O star HD191612,"The magnetic O-star HD191612 exhibits strongly variable, cyclic Balmer line
emission on a 538-day period. We show here that its variable Halpha emission
can be well reproduced by the rotational phase variation of synthetic spectra
computed directly from full radiation magneto-hydrodynamical simulations of a
magnetically confined wind. In slow rotators such as HD191612, wind material on
closed magnetic field loops falls back to the star, but the transient
suspension of material within the loops leads to a statistically overdense, low
velocity region around the magnetic equator, causing the spectral variations.
We contrast such ""dynamical magnetospheres"" (DMs) with the more steady-state
""centrifugal magnetospheres"" of stars with rapid rotation, and discuss the
prospects of using this DM paradigm to explain periodic line emission from also
other non-rapidly rotating magnetic massive stars.",1203.1050v1
2012-06-22,Ultrasonic triggering of giant magnetocaloric effect in MnAs thin films,"Mechanical control of magnetic properties in magnetostrictive thin films
offers the unexplored opportunity to employ surface wave acoustics in such a
way that acoustic triggers dynamic magnetic effects. The strain-induced
modulation of the magnetic anisotropy can play the role of a high frequency
varying effective magnetic field leading to ultrasonic tuning of electronic and
magnetic properties of nanostructured materials, eventually integrated in
semiconductor technology. Here, we report about the opportunity to employ
surface acoustic waves to trigger magnetocaloric effect in
MnAs(100nm)/GaAs(001) thin films. During the MnAs magnetostructural phase
transition, in an interval range around room temperature (0{\deg}C -
60{\deg}C), ultrasonic waves (170 MHz) are strongly attenuated by the phase
coexistence (up to 150 dB/cm). We show that the giant magnetocaloric effect of
MnAs is responsible of the observed phenomenon. By a simple anelastic model we
describe the temperature and the external magnetic field dependence of such a
huge ultrasound attenuation. Strain-manipulation of the magnetocaloric effect
could be a further interesting route for dynamic and static caloritronics and
spintronics applications in semiconductor technology.",1206.5217v1
2013-10-11,Self-assembling paramagnetic colloids in oscillating magnetic fields,"Many fascinating phenomena such as large-scale collective flows, enhanced
fluid mixing and pattern formation have been observed in so-called active
fluids, which are composed of particles that can absorb energy and dissipate it
into the fluid medium. In order to investigate the role of hydrodynamic
interactions in the collective behavior of an active fluid, we choose a model
system: paramagnetic particles submerged in water and activated by an
oscillating magnetic field. The magnetic field induces magnetic attractions
among the paramagnetic particles, activating the particles, and injecting
energy into the fluid. Over many cycles, the particles aggregate together and
form clusters. In order to form clusters, however, the particles must overcome
viscous drag. We investigate the relative roles of viscosity and magnetism.
When the role of viscosity is important, the particles cannot form large
clusters. But when the role of magnetism is important, the particles rapidly
form organized, large clusters. Our results shown in this fluid dynamics video
suggest that viscous stresses slow the clustering rate and decrease the size of
clusters in a self-assembling colloidal system.",1310.3212v2
2014-09-25,Tuning the Band Structures of a 1D Width-Modulated Magnonic Crystal by a Transverse Magnetic Field,"Theoretical studies, based on three independent techniques, of the band
structure of a one-dimensional width-modulated magnonic crystal under a
transverse magnetic field are reported. The band diagram is found to display
distinct behaviors when the transverse field is either larger or smaller than a
critical value. The widths and center positions of bandgaps exhibit unusual
non-monotonic and large field-tunability through tilting the direction of
magnetization. Some bandgaps can be dynamically switched on and off by simply
tuning the strength of such a static field. Finally, the impact of the lowered
symmetry of the magnetic ground state on the spin-wave excitation efficiency of
an oscillating magnetic field is discussed. Our finding reveals that the
magnetization direction plays an important role in tailoring magnonic band
structures and hence in the design of dynamic spin-wave switches.",1409.7183v1
2014-12-01,All-optical magnetization switching in ferrimagnetic alloys: deterministic vs thermally activated dynamics,"Using photo-emission electron microscopy with X-ray magnetic circular
dichroism as a contrast mechanism, new insights into the all-optical
magnetization switching (AOS) phenomenon in GdFe based rare-earth transition
metal ferrimagnetic alloys are provided. From a sequence of static images taken
after single linearly polarized laser pulse excitation, the repeatability of
AOS can be measured with a correlation coefficient. It is found that low
coercivity enables thermally activated domain wall motion, limiting in turn the
repeatability of the switching. Time-resolved measurement of the magnetization
dynamics reveal that while AOS occurs below and above the magnetization
compensation temperature $T_\text{M}$, it is not observed in GdFe samples where
$T_\text{M}$ is absent. Finally, AOS is experimentally demonstrated against an
applied magnetic field of up to 180 mT.",1412.0396v1
2015-02-23,The nature of Itinerant Ferromagnetism of SrRuO3 : A DFT+DMFT Study,"We have investigated the temperature (T)-dependent evolution of electronic
structures and magnetic properties of an itinerant ferromagnet SrRuO3,
employing the combined scheme of the density functional theory and the
dynamical mean-field theory (DFT+DMFT). The inclusion of finite dynamical
correlation effects beyond the DFT well describes not only the incoherent hump
structure observed in the photoemission experiment but also the T-dependent
magnetic properties in accordance with experiments. We have shown that the
magnetization of SrRuO3 evolves with the Stoner behavior below the Curie
temperature (Tc), reflecting the weak itinerant ferromagnetic behavior, but the
local residual magnetic moment persists even above Tc, indicating the local
magnetic moment behavior. We suggest that the ferromagnetism of SrRuO3 has dual
nature of both weak and local moment limits, even though the magnetism of
SrRuO3 is more itinerant than that of Fe.",1502.06322v1
2015-03-04,"Electromagnetic Torques, Precession and Evolution of Magnetic Inclination of Pulsars","We present analytic calculations of the electromagnetic torques acting on a
magnetic neutron star rotating in vacuum, including near-zone torques
associated with the inertia of dipole and quadrupole magnetic fields. We
incorporate these torques into the rotational dynamics of a rigid-body neutron
star, and show that the effects of the inertial torque can be understood as a
modification of the moment of inertia tensor of the star. We apply our
rotational dynamics equation to the Crab pulsar, including intrinsic
distortions of the star and various electromagnetic torques, to investigate the
possibility that the counter-alignment of the magnetic inclination angle, as
suggested by recent observations, could be explained by pulsar precession. We
find that if the effective principal axis of the pulsar is nearly aligned with
either the magnetic dipole axis or the rotation axis, then precession may
account for the observed counter-alignment over decade timescales. Over the
spindown timescale of the pulsar, the magnetic inclination angle always
decreases.",1503.01486v2
2015-05-29,Time-resolved imaging of pulse-induced magnetization reversal with a microwave assist field,"The reversal of the magnetization under the influence of a field pulse has
been previously predicted to be an incoherent process with several competing
phenomena such as domain wall relaxation, spin wave-mediated instability
regions, and vortex-core mediated reversal dynamics. However, there has been no
study on the direct observation of the switching process with the aid of a
microwave signal input. We report a time-resolved imaging study of
magnetization reversal in patterned magnetic structures under the influence of
a field pulse with microwave assistance. The microwave frequency is varied to
demonstrate the effect of resonant microwave-assisted switching. We observe
that the switching process is dominated by spin wave dynamics generated as a
result of magnetic instabilities in the structures, and identify the
frequencies that are most dominant in magnetization reversal.",1505.07936v1
2015-08-10,Timelike geodesics of a modified gravity black hole immersed in an axially symmetric magnetic field,"We investigate the dynamics of a neutral and a charged particle around a
black hole in modified gravity immersed in magnetic field. Our focus is on the
scalar-tensor-vector theory as modified gravity. We are interested to explore
the conditions on the energy of the particle under which it can escape to
infinity after collision with another neutral particle in the vicinity of the
black hole. We calculate escape velocity of particle orbiting in the innermost
stable circular orbit (ISCO) after the collision. We study the effects of
modified gravity on the dynamics of particles. Further we discuss how the
presence of magnetic field in the vicinity of black hole, effects the motion of
the orbiting particle. We show that the stability of ISCO increases due to
presence of magnetic field. It is observed that a particle can go arbitrary
close to the black hole due to presence of magnetic field. Furthermore ISCO for
black hole is more stable as compared with Schwarzschild black hole. We also
discuss the Lyapunov exponent and the effective force acting on the particle in
the presence of magnetic field.",1508.02123v1
2015-08-26,Non-circular skyrmion and its anisotropic response in thin films of chiral magnets under tilted magnetic field,"We study the equilibrium and dynamical properties of skyrmions in thin films
of chiral magnets with oblique magnetic field. The shape of an individual
skyrmion is non-circular and the skyrmion density decreases with the tilt angle
from the normal of films. As a result, the interaction between two skyrmions
depends on the relative angle between them in addition to their separation. The
triangular lattice of skyrmions under a perpendicular magnetic field is
distorted into a centered rectangular lattice for a tilted magnetic field. For
a low skyrmion density, skyrmions form a chain like structure. The dynamical
response of the non-circular skyrmions depends on the direction of external
currents.",1508.06361v1
2015-11-03,Nd2Sn2O7: an all-in-all-out pyrochlore magnet with no divergence-free field and anomalously slow paramagnetic spin dynamics,"We report measurements performed on a polycrystalline sample of the
pyrochlore compound Nd2Sn2O7. It undergoes a second order magnetic phase
transition at Tc ~ 0.91 K to a noncoplanar all-in-all-out magnetic structure of
the Nd3+ magnetic moments. The thermal behavior of the low temperature specific
heat fingerprints excitations with linear dispersion in a three-dimensional
lattice. The temperature independent spin-lattice relaxation rate measured
below Tc and the anomalously slow paramagnetic spin dynamics detected up to ~
30 Tc are suggested to be due to magnetic short-range correlations in
unidimensional spin clusters, i.e., spin loops. The observation of a
spontaneous field in muon spin relaxation measurements is associated with the
absence of a divergence-free field for the ground state of an all-in-all-out
pyrochlore magnet as predicted recently.",1511.00987v1
2015-12-17,A self-consistent spin-diffusion model for micromagnetics,"We propose a three-dimensional micromagnetic model that dynamically solves
the Landau-Lifshitz-Gilbert equation coupled to the full spin-diffusion
equation. In contrast to previous methods, we solve for the magnetization
dynamics and the electric potential in a self-consistent fashion. This
treatment allows for an accurate description of magnetization dependent
resistance changes. Moreover, the presented algorithm describes both spin
accumulation due to smooth magnetization transitions and due to material
interfaces as in multilayer structures. The model and its finite-element
implementation are validated by current driven motion of a magnetic vortex
structure. In a second experiment, the resistivity of a magnetic multilayer
structure in dependence of the tilting angle of the magnetization in the
different layers is investigated. Both examples show good agreement with
reference simulations and experiments respectively.",1512.05519v4
2016-01-06,Dynamic response of an artificial square spin ice,"Magnetization dynamics in an artificial square spin-ice lattice made of
Ni80Fe20 with magnetic field applied in the lattice plane is investigated by
broadband ferromagnetic resonance spectroscopy. The experimentally observed
dispersion shows a rich spectrum of modes corresponding to different
magnetization states. These magnetization states are determined by exchange and
dipolar interaction between individual islands, as is confirmed by a
semianalytical model. In the low field regime below 400 Oe a hysteretic
behavior in the mode spectrum is found. Micromagnetic simulations reveal that
the origin of the observed spectra is due to the initialization of different
magnetization states of individual nanomagnets. Our results indicate that it
might be possible to determine the spin-ice state by resonance experiments and
are a first step towards the understanding of artificial geometrically
frustrated magnetic systems in the high-frequency regime.",1601.01219v1
2016-03-30,Exchange symmetry and low-frequency asymptotics of Green's functions in spin s=1 magnets,"The paper contains the description of the dynamics of non-equilibrium
processes of spin $s=1$ magnets in an external variable field. We have obtained
nonlinear dynamic equations with sources and calculated low-frequency
asymptotics of two-time Green's functions for ferro- and quadrupole magnetic
states with $SO(3)$ and $SU(3)$ exchange symmetry of the Hamiltonian. It has
been shown that for ferro- and quadrupole magnetic states singularities of
Green's functions in wave vectors $1/k$, $1/k^2$ and frequencies $1/\omega,
1/\omega^2$ have well-known character. We set the exact form of the magnetic
anisotropy of these Green's functions. For states with $SO(3)$ symmetry of the
exchange Hamiltonian, we have found Green's functions with quadrupole degrees
of freedom and compared them with Green's functions of magnets having exchange
$SU(3)$ symmetry.",1603.09204v2
2016-06-15,Driving Skyrmions in a Composite Bilayer,"Magnetic Skyrmions and multiferroics are the most interesting objects in
nanostructure science that have great potential in future spin-electronic
technology. The study of multiferroic Skyrmions has attracted much interest in
recent years. This article reports magnetic Bloch Skyrmions induced by an
electric driving field in a composite bilayer (chiral-magnetic/ferroelectric
bilayer) lattice. By using the spin dynamics method, we use a classical
magnetic spin model and an electric pseudospin model, which are coupled by a
strong magnetoelectric coupling in the dynamical simulations. Interestingly, we
observe some skyrmion-like objects in the electric component either during the
switching process or by applying a magnetic field, which is due to the
connection between the electric and the magnetic structures.",1606.04595v3
2016-06-23,Spin-Hall nano-oscillator with oblique magnetization and Dzyaloshinskii-Moriya interaction as generator of skyrmions and nonreciprocal spin-waves,"Spin-Hall oscillators are promising sources of spin-wave signals for
magnonics applications, and can serve as building blocks for magnonic logic in
ultralow power computation devices. Here, we analytically and micromagnetically
study magnetization dynamics excited in a Spin-Hall oscillator with oblique
magnetization when the spin-Hall effect and interfacial Dzyaloshinskii-Moriya
interaction act simultaneously. Our key results are (i) excitation of
nonreciprocal spin-waves propagating perpendicularly to the in-plane projection
of the static magnetization, (ii) skyrmions generation by pure spin-current,
(iii) excitation of a new spin-wave mode with a spiral spatial profile
originating from a gyrotropic rotation of a dynamical skyrmion. These results
demonstrate that Spin-Hall oscillators can be used as generators of magnetic
skyrmions and different types of propagating spin-waves for magnetic data
storage and signal processing applications.",1606.07280v1
2016-08-11,Parametric study of the solar wind interaction with the Hermean magnetosphere for a weak interplanetary magnetic field,"The aim of this study is to simulate the interaction of the solar wind with
the Hermean magnetosphere when the interplanetary magnetic field is weak,
performing a parametric study for all the range of hydrodynamic values of the
solar wind predicted on Mercury for the ENLIL + GONG WSA + Cone SWRC model:
density from $12$ to $180$ cm$^{-3}$, velocity from $200$ to $500$ km/s and
temperatures from $2 \cdot 10^4$ to $18 \cdot 10^4$ K, and compare the results
with a real MESSENGER orbit as reference case. We use the code PLUTO in
spherical coordinates and an asymmetric multipolar expansion for the Hermean
magnetic field. The study shows for all simulations a stand off distance larger
than the Mercury radius and the presence of close magnetic field lines on the
day side of the planet, so the dynamic pressure of the solar wind is not high
enough to push the magnetopause on the planet surface if the interplanetary
magnetic field is weak. The simulations with large dynamic pressure lead to a
large compression of the Hermean magnetic field modifying its topology in the
inner magnetosphere as well as the plasma flows from the magnetosheath towards
the planet surface.",1608.03606v1
2016-10-14,Nambu mechanics for stochastic magnetization dynamics,"The Landau-Lifshitz-Gilbert (LLG) equation describes the dynamics of a damped
magnetization vector that can be understood as a generalization of Larmor spin
precession. The LLG equation cannot be deduced from the Hamiltonian framework,
by introducing a coupling to a usual bath, but requires the introduction of
additional constraints. It is shown that these constraints can be formulated
elegantly and consistently in the framework of dissipative Nambu mechanics.
This has many consequences for both the variational principle and for
topological aspects of hidden symmetries that control conserved quantities. We
particularly study how the damping terms of dissipative Nambu mechanics affect
the consistent interaction of magnetic systems with stochastic reservoirs and
derive a master equation for the magnetization. The proposals are supported by
numerical studies using symplectic integrators that preserve the topological
structure of Nambu equations. These results are compared to computations
performed by direct sampling of the stochastic equations and by using closure
assumptions for the moment equations, deduced from the master equation.",1610.04598v2
2017-02-13,Voltage-driven charge-mediated fast 180 degree magnetization switching in nanoheterostructure at room temperature,"Voltage-driven 180$^\circ$ magnetization switching without electric current
provides the possibility for revolutionizing the spintronics. We demonstrated
the voltage-driven charge-mediated 180$^\circ$ magnetization switching at room
temperature by combining first-principles calculations and
temperature-dependent magnetization dynamics simulation. The electric field
($E$) induced interface charge is found to allow a giant modulation of the
magnetic anisotropy ($K$) of the nanomagnet. Particularly $K$ is revealed to
vary linearly with respect to $E$ and the epitaxial strain. Magnetization
dynamics simulations using the so-obtained $K$ show that both in-plane and
perpendicular 180$^\circ$ switching can be achieved by $E$ pulses. The
temperature effect renders the 180$^\circ$ switching as probability events.
Statistical analysis indicates a fast (around 4 ns) and low-error-probability
180$^\circ$ switching achievable at room temperature by controlling the
magnitude of $E$ and the pulse width. The study inspires the rational design of
miniaturized nanoscale spintronic devices where thermal fluctuation has a great
impact.",1702.03670v2
2017-04-07,Effect of picosecond magnetic pulse on dynamics of electron's subbands in semiconductor bilayer nanowire,"We report on possibility of charge current generation in nanowire made of two
tunnel coupled one-dimensional electron waveguides by means of single magnetic
pulse lasting up to 20 ps. Existence of interlayer tunnel coupling plays a
crucial role in the effect described here as it allows for hybridization of the
wave functions localized in different layers which can be dynamically modified
by applying a time changeable in-plane magnetic field. Results of
time-dependent DFT calculations performed for a bilayer nanowire confining many
electrons show that the effect of such magnetic hybridization relies on tilting
of electrons' energy subbands, to the left or to the right, depending on a sign
of time derivative of oscillating magnetic field due to the Faraday law.
Consequently, the tilted subbands become a source of charge flow along the
wire. Strength of such magneto-induced current oscillations may achieve even
$0.6\mu\textrm{A}$ but it depends on duration of magnetic pulse as well as on
charge density confined in nanowire which has to be unequally distributed
between both transport layers to observe this effect.",1704.02150v1
2017-06-02,Power Loss for a Periodically Driven Ferromagnetic Nanoparticle in a Viscous Fluid: the Finite Anisotropy Aspects,"The joint magnetic and mechanical motion of a ferromagnetic nanoparticle in a
viscous fluid is considered within the dynamical approach. The equation based
on the total momentum conservation law is used for the description of the
mechanical rotation, while the modified Landau-Lifshitz-Gilbert equation is
utilized for the description of the internal magnetic dynamics. The exact
expressions for the particles trajectories and the power loss are obtained in
the linear approximation. The comparison with the results of other widespread
approaches, such as the model of fixed particle and the model of frozen
magnetic moment, is performed. It is established that in the small oscillations
mode the damping precession of the nanopartile magnetic moment is the main
channel of energy dissipation, but the motion of the nanoparticle easy axis can
significantly influence the value of the resulting power loss.",1706.00777v2
2017-06-14,Broadband conversion of microwaves into propagating spin waves in patterned magnetic structures,"We have used time-resolved scanning Kerr microscopy (TRSKM) and micromagnetic
simulations to demonstrate that, when driven by spatially uniform microwave
field, the edges of patterned magnetic samples represent both efficient and
highly tunable sources of propagating spin waves. The excitation is due to the
local enhancement of the resonance frequency induced by the non-uniform dynamic
demagnetizing field generated by precessing magnetization aligned with the
edges. Our findings represent a crucial step forward in the design of nanoscale
spin-wave sources for magnonic architectures, and are also highly relevant to
the understanding and interpretation of magnetization dynamics driven by
spatially-uniform magnetic fields in patterned magnetic samples.",1706.04409v2
2017-10-30,Dissipated power within a turbulent flow forced homogeneously by magnetic particles,"We report measurements of global dissipated power within a turbulent flow
homogeneously forced at small scale by a new forcing technique. The forcing is
random in both time and space within the fluid by using magnetic particles in
an alternating magnetic field. By measuring the growth rate of the fluid
temperature, we show how the dissipated power is governed by the external
control parameters (magnetic field, and number N of particles). We
experimentally found that the mean dissipated power scales linearly with these
parameters, as expected from the magnetic injected power scalings. These
experimental results are well described by simple scaling arguments showing
that the main origins of the energy dissipation are due to viscous turbulent
friction of particles within the fluid and to the inelasticity of collisions.
Finally, by measuring the particle collision statistics, we also show that the
particle velocity is independent of N, and is only fixed by the magnetic
""thermostat"".",1710.10935v1
2017-12-06,Dynamic quantum kagome ice,"The search for two dimensional quantum spin liquids, exotic magnetic states
with an entangled ground state remaining disordered down to zero temperature,
has been a great challenge in frustrated magnetism during the last decades.
Recently, fractionalized excitations, called spinons, emerging from these
states, have been evidenced in kagome and triangular antiferromagnets. In
contrast, quantum ferromagnetic spin liquids in two dimensions, namely quantum
kagome ices, have been less investigated, yet their classical counterparts
exhibit amazing properties, magnetic monopole crystals as well as magnetic
fragmentation. Here we show that, by applying a magnetic field on the
pyrochlore oxide Nd$_2$Zr$_2$O$_7$, which has been shown to develop three
dimensional quantum magnetic fragmentation in zero field, we are able to reduce
the dimension of the system and to create a dynamic kagome ice state. Our
results open the way to the observation of the quantum kagome ice state which
was recently investigated theoretically.",1712.02418v2
2017-12-10,Magnetic field gradient driven dynamics of isolated skyrmions and antiskyrmions in frustrated magnets,"The study of skyrmion/antiskyrmion motion in magnetic materials is very
important in particular for the spintronics applications. In this work, we
study the dynamics of isolated skyrmions and antiskyrmions in frustrated
magnets driven by magnetic field gradient, using the Landau-Lifshitz-Gilbert
simulations on the frustrated classical Heisenberg model on the triangular
lattice. A Hall-like motion induced by the gradient is revealed in bulk system,
similar to that in the well-studied chiral magnets. More interestingly, our
work suggests that the lateral confinement in nano-stripes of the frustrated
system can completely suppress the Hall motion and significantly speed up the
motion along the gradient direction. The simulated results are well explained
by the Thiele theory. It is demonstrated that the acceleration of the motion is
mainly determined by the Gilbert damping constant, which provides useful
information for finding potential materials for skyrmion-based spintronics.",1712.03550v1
2018-02-12,Easy-Plane Magnetic Strip as a Long Josephson Junction,"Spin-torque-biased magnetic dynamics in an easy-plane ferromagnet (EPF) is
theoretically studied in the presence of a weak in-plane anisotropy. While this
anisotropy spoils U(1) symmetry thereby quenching the conventional spin
superfluidity, we show that the system instead realizes a close analog of a
long Josephson junction (LJJ) model. The traditional magnetic-field and
electric-current controls of the latter map respectively onto the symmetric and
antisymmetric combinations of the out-of-plane spin torques applied at the ends
of the magnetic strip. This suggests an alternative route towards realizations
of superfluid-like transport phenomena in insulating magnetic systems. We study
spin-torque-biased phase diagram, providing an analytical solution for static
multidomain phases in the EPF. We adapt an existing self-consistency method for
the LJJ to develop an approximate solution for the EPF dynamics. The LJJ-EPF
mapping allows us to envision superconducting circuit functionality at elevated
temperatures. The results apply equally to antiferromagnets with suitable
effective free energy in terms of the N\'{e}el order instead of magnetization.",1802.04229v1
2018-05-13,Steady and dynamic magnetic phase transitions in interacting quantum dots arrays coupled with leads,"We apply the Hubbard model, non-equilibrium Green's function (NEGF) theory,
exact diagonalization (ED) and the hierarchical equations of motion (HEOM)
method to investigate abundant magnetic phase transitions in the 1D interacting
quantum dots arrays (QDA) sandwiched by non-interaction leads. The spin
polarization phase transitions are firstly studied with a mean-field
approximation. The many-body calculation of the ED method is then used to
verify such transitions. We find with the weak device-leading couplings, the
anti-ferromagnetic (AF) state only exists in the uniform odd-numbered QDA or
the staggered-hopping QDA systems. With increasing the coupling strength or the
bias potentials, there exists the magnetism-to non-magnetism phase transition.
With the spin-resolved HEOM method we also investigate the detailed dynamic
phase transition process of these lead-QDA-lead systems.",1805.04939v1
2018-05-27,Integer quantum Hall conductivity and longitudinal conductivity in silicene under the electric field and magnetic field,"We investigate the integer Hall conductivity and longitudinal conductivity of
silicene under the magnetic field, electric field, and exchange field in this
letter. We focus not only on the low-temperature and $\delta$-function
impurities (i.e., independent of the scattering momentum) case, which only
exist the intra-Landau level transition, but also on the case of inter-Landau
level transition which also with the non-elastic scattering. The resulting
longitudinal conductivity is very different with the intra-Landau level one at
low-temperature. The exprssions of the Hall conductivity, longitudinal
conductivity, valley contributed Hall conductivity, and the spin or valley Hall
conductivity, are deduced in this letter. We also compute the dynamical
polarization under the magnetic field which is a important quantity and has
many exciting and novel properties, and with the screened scattering due to the
charged impurities. The polarization function here is also related to the
Landau level index under the magnetic field, and shows step-like feature rather
than the logarithmically divergenas which appears for the zero-magnetic field
case, and it's also naturally related to the conductivity in silicene. The
generalized Laguerre polynomial is used in both the longitudinal conductivity
and dynamical polarization function under magnetic field.",1805.10656v1
2019-04-05,Common universal behaviors of magnetic domain walls driven by spin-polarized electrical current and magnetic field,"We explore universal behaviors of magnetic domain wall driven by the
spin-transfer of an electrical current, in a ferromagnetic (Ga,Mn)(As,P) thin
film with perpendicular magnetic anisotropy. For a current direction transverse
to domain wall, the dynamics of the thermally activated creep regime and the
depinning transition are found to be compatible with a self-consistent
universal description of magnetic field induced domain wall dynamics. This
common universal behavior, characteristic of the so-called quenched
Edwards-Wilkinson universality class, is confirmed by a complementary and
independent analysis of domain wall roughness. However, the tilting of domain
walls and the formation of facets is produced by the directionality of
interaction with the current, which acts as a magnetic field only in the
direction transverse to domain wall.",1904.03047v2
2019-06-07,Dynamics of position-phase probability density in magnetic resonance,"We consider the behaviour of precessional angle (phase) carried by molecules
of a diffusing specimen under magnetic fields typical of magnetic resonance
experiments. An evolution equation for the ensemble of particles is
constructed, which treats the phase as well as the position of the molecules as
random variables. This ""position-phase (probability) density"" (PPD) is shown to
encode solutions to a family of Bloch-Torrey equations (BTE) for transverse
magnetization density, which is because the PPD is a more fundamental quantity
than magnetization density; the latter emerges from the former upon averaging.
The present paradigm represents a conceptual advantage, since the PPD is a true
probability density subject to Markovian dynamics, rather than an aggregate
magnetization density whose evolution is less intuitive. We also work out the
analytical solution for suitable special cases.",1906.04046v1
2019-10-13,Modelling the photometric variability of magnetic massive stars with the Analytical Dynamical Magnetosphere model,"In this paper, we investigate the photometric variability of magnetic O-type
stars. Such stars possess oblique, predominantly dipolar magnetic fields that
confine their winds roughly axisymmetrically about the magnetic equator, thus
forming a magnetosphere. We interpret their photometric variability as
phase-dependent magnetospheric occultations. For massive star winds dominated
by electron scattering opacity in the optical and NIR, we can compute synthetic
light curves from simply knowing the magnetosphere's mass density distribution.
We exploit the newly-developed Analytical Dynamical Magnetosphere model (ADM)
in order to obtain the predicted circumstellar density structures of magnetic
O-type stars. The simplicity in our light curve synthesis model allows us to
readily conduct a parameter space study. For validation purposes, we first
apply our algorithm to HD 191612, the prototypical Of?p star. Next, we attempt
to model the photometric variability of the Of?p-type stars identified in the
Magellanic Clouds using OGLE photometry. We evaluate the compatibility of the
ADM predictions with the observed photometric variations, and discuss the
magnetic field properties that are implied by our modelling.",1910.05792v1
2019-10-24,Magnetic Field Effect in the Fine-Structure Constant and Electron Dynamical Mass,"We investigate the effect of an applied constant and uniform magnetic field
in the fine-structure constant of massive and massless QED. In massive QED, it
is shown that a strong magnetic field removes the so called Landau pole and
that the fine-structure constant becomes anisotropic having different values
along and transverse to the field direction. Contrary to other results in the
literature, we find that the anisotropic fine-structure constant always
decreases with the field. We also study the effect of the running of the
coupling constant with the magnetic field on the electron mass. We find that in
both cases of massive and massless QED, the electron dynamical mass always
decreases with the magnetic field, what can be interpreted as an inverse
magnetic catalysis effect.",1910.11245v1
2020-01-23,Charged particle dynamics near an X-point of a non-symmetric magnetic field with closed field lines,"Understanding particle drifts in a non-symmetric magnetic field is of primary
interest in designing optimized stellarators to minimize the neoclassical
radial loss of particles. Quasisymmetry and omnigeneity, two distinct
properties proposed to ensure radial localization of collisionless trapped
particles in stellarators, have been explored almost exclusively for magnetic
fields that generate nested flux surfaces. In this work, we extend these
concepts to the case where all the field lines are closed. We then study
charged particle dynamics in the exact non-symmetric vacuum magnetic field with
closed field lines, obtained recently by Weitzner and Sengupta
(arXiv:1909.01890), which possesses X-points. The magnetic field can be used to
construct magnetohydrodynamic equilibrium in the limit of vanishing plasma
pressure. Expanding in the amplitude of the non-symmetric fields, we explicitly
evaluate the omnigeneity and quasisymmetry constraints. We show that the
magnetic field is omnigeneous in the sense that the drift surfaces coincide
with the pressure surfaces. However, it is not quasisymmetric according to the
standard definitions.",2001.08470v1
2020-02-06,Domain structure dynamics in the ferromagnetic Kagome-lattice Weyl semimetal Co$_3$Sn$_2$S$_2$,"Co$_3$Sn$_2$S$_2$, a Weyl semimetal that consists of layers of Kagome
lattices, transitions from a high-temperature paramagnetic phase to a
low-temperature ferromagnetic phase below 177 K. The phase transition occurs
through an intermediate non-trivial magnetic phase, the so-called ""A""-phase
just below the Curie temperature. The ""A""-phase was earlier linked with a
competing anti-ferromagnetic phase, a spin-glass phase and certain indirect
measurements indicated the possibility of magnetic Skyrmions in this phase. We
have imaged the magnetic domain structure in a single crystal of
Co$_3$Sn$_2$S$_2$ at different temperatures, magnetic fields and field-angles
by magnetic force microscopy. At low temperatures, we observed stripe domains
indicating presence of uniaxial anisotropy. Above 130 K, the domain walls
become mobile and they tend to align relatively easily when the magnetic field
is increased along the $c$-axis than in the $a-b$ plane. Our detailed study of
field-dependent domain dynamics reveal that the anomalous nature of the phase
transition just below $T_c$ is dominantly governed by domain wall motion.",2002.02494v1
2020-02-07,Spin-dependent transport through a Weyl semimetal surface,"We experimentally compare two types of interface structures with magnetic and
non-magnetic Weyl semimetals. They are the junctions between a gold normal
layer and magnetic Weyl semimetal Ti$_2$MnAl, and a ferromagnetic nickel layer
and non-magnetic Weyl semimetal WTe$_2$, respectively. Due to the ferromagnetic
side of the junction, we investigate spin-polarized transport through the Weyl
semimetal surface. For both structures, we demonstrate similar current-voltage
characteristics, with hysteresis at low currents and sharp peaks in
differential resistance at high ones. Despite this behavior resembles the known
current-induced magnetization dynamics in ferromagnetic structures, evolution
of the resistance peaks with magnetic field is unusual. We connect the observed
effects with current-induced spin dynamics in Weyl topological surface states.",2002.02673v2
2020-05-22,Error estimates of some splitting schemes for charged-particle dynamics under strong magnetic field,"In this work, we consider the error estimates of some splitting schemes for
the charged-particle dynamics under a strong magnetic field. We first propose a
novel energy-preserving splitting scheme with computational cost per step
independent from the strength of the magnetic field. Then under the maximal
ordering scaling case, we establish for the scheme and in fact for a class of
Lie-Trotter type splitting schemes, a uniform (in the strength of the magnetic
field) and optimal error bound in the position and in the velocity parallel to
the magnetic field. For the general strong magnetic field case, the modulated
Fourier expansions of the exact and the numerical solutions are constructed to
obtain a convergence result. Numerical experiments are presented to illustrate
the error and energy behaviour of the splitting schemes.",2005.11192v1
2020-05-28,Nonequilibrium quasistationary spin disordered state in the Kitaev-Heisenberg magnet $α$-RuCl$_3$,"Excitation by light pulses enables the manipulation of phases of quantum
condensed matter. Here, we photoexcite high-energy holon-doublon pairs as a way
to alter the magnetic free energy landscape of the Kitaev-Heisenberg magnet
$\alpha$-RuCl$_3$, with the aim to dynamically stabilize a proximate spin
liquid phase. The holon-doublon pair recombination through multimagnon emission
is tracked through the time-evolution of the magnetic linear dichroism
originating from the competing zigzag spin ordered ground state. A small
holon-doublon density suffices to reach a spin disordered state. The phase
transition is described within a dynamic Ginzburg-Landau framework,
corroborating the quasistationary nature of the transient spin disordered
phase. Our work provides insight into the coupling between the electronic and
magnetic degrees of freedom in $\alpha$-RuCl$_3$ and suggests a new route to
reach a proximate spin liquid phase in Kitaev-Heisenberg magnets.",2005.14189v1
2020-06-05,Emergent pattern formation of active magnetic suspensions in an external field,"We study collective self-organization of weakly magnetic active suspensions
in a uniform external field by analyzing a mesoscopic continuum model that we
have recently developed. Our model is based on a Smoluchowski equation for a
particle probability density function in an alignment field coupled to a
mean-field description of the flow arising from the activity and the alignment
torque. Performing linear stability analysis of the Smoluchowski equation and
the resulting orientational moment equations combined with non-linear 3D
simulations, we provide a comprehensive picture of instability patterns as a
function of strengths of activity and magnetic field. For sufficiently high
activity and moderate magnetic field strengths, the competition between the
activity-induced flow and external magnetic torque renders a homogeneous polar
steady state unstable. As a result, four distinct dynamical patterns of
collective motion emerge. The instability patterns for pushers include
traveling bands governed by bend-twist instabilities and dynamical aggregates.
For pullers, finite-sized and system spanning pillar-like concentrated regions
predominated by splay deformations emerge which migrate in the field direction.
Notably, at very strong magnetic fields, we observe a reentrant hydrodynamic
stability of the polar steady state.",2006.03352v1
2020-06-19,Topological transport of deconfined hedgehogs in magnets,"We theoretically investigate the dynamics of magnetic hedgehogs, which are
three-dimensional topological spin textures that exist in common magnets,
focusing on their transport properties and connections to spintronics. We show
that fictitious magnetic monopoles carried by hedgehog textures obey a
topological conservation law, based on which a hydrodynamic theory is
developed. We propose a nonlocal transport measurement in the disordered phase,
where the conservation of the hedgehog flow results in a nonlocal signal
decaying inversely proportional to the distance. The bulk-edge correspondence
between hedgehog number and skyrmion number, the fictitious electric charges
arising from magnetic dynamics, and the analogy between bound states of
hedgehogs in ordered phase and the quark confinement in quantum chromodynamics
are also discussed. Our study points to a practical potential in utilizing
hedgehog flows for long-range neutral signal propagation or manipulation of
skyrmion textures in three-dimensional magnetic materials.",2006.10910v3
2020-09-13,The links between magnetic fields and filamentary clouds III: field regulated mass cumulative functions,"During the past decade the dynamical importance of magnetic fields in
molecular clouds has been increasingly recognized, as observational evidence
has accumulated. However, how a magnetic field affect star formation is still
unclear. Typical star formation models still treat a magnetic fields as an
isotropic pressure, ignoring the fundamental property of dynamically important
magnetic fields: their direction. This study builds on our previous work which
demonstrated how the mean magnetic field orientation relative to the global
cloud elongation can affect cloud fragmentation. After the linear mass
distribution reported earlier, we show here that the mass cumulative function
(MCF) of a cloud is also regulated by the field orientation. A cloud elongated
closer to the field direction tends to have a shallower MCF, in other words, a
higher portion of the gas in high density. The evidence is consistent with our
understanding of bimodal star formation efficiency discovered earlier, which is
also correlated with the field orientations.",2009.05915v1
2020-09-28,Thermal Magnetic Fluctuations of a Ferroelectric Quantum Critical Point,"Entanglement of two different quantum orders is of an interest of the modern
condensed matter physics. One of the examples is the dynamical multiferroicity,
where fluctuations of electric dipoles lead to magnetization. We investigate
this effect at finite temperature and demonstrate an elevated magnetic response
of a ferroelectric near the ferroelectric quantum critical point (FE QCP). We
calculate the magnetic susceptibility of a bulk sample on the paraelectric side
of the FE QCP at finite temperature and find enhanced magnetic susceptibility
near the FE QCP. We propose quantum paraelectric strontium titanate (STO) as a
candidate material to search for dynamic multiferroicity. We estimate the
magnitude of the magnetic susceptibility for this material and find that it is
detectable experimentally.",2009.12995v1
2020-10-02,The relative orientation between the magnetic field and gas density structures in non-gravitating turbulent media,"Magnetic fields are a dynamically important agent for regulating structure
formation in the interstellar medium. The study of the relative orientation
between the local magnetic field and gas (column-) density gradient has become
a powerful tool to analyse the magnetic field's impact on the dense gas
formation in the Galaxy. In this study, we perform numerical simulations of a
non-gravitating, isothermal gas, where the turbulence is driven either
solenoidally or compressively. We find that only simulations with an initially
strong magnetic field (plasma-$\beta<1$) show a change in the preferential
orientation between the magnetic field and isodensity contours, from mostly
parallel at low densities to mostly perpendicular at higher densities. Hence,
compressive turbulence alone is not capable of inducing the transition observed
towards nearby molecular clouds. At the same high initial magnetisation, we
find that solenoidal modes produce a sharper transition in the relative
orientation with increasing density than compressive modes. We further study
the time evolution of the relative orientation and find that it remains
unchanged by the turbulent forcing after one dynamical timescale.",2010.01178v1
2020-10-21,Ambipolar diffusion velocity and magnetic field evolution in magnetar core: Generalised theoretical approach,"The magnetic field associated with neutron stars is generally believed to be
threaded inside the star. In the presence of a magnetic field, the plasma
present in the interior of the star goes through several processes that lead to
magnetic field evolution. It is thought that magnetar activities are mainly due
to field decay. The most important process of field decay inside the core of
the star is the ambipolar diffusion of the charged particles present in the
interior plasma. The decay rate due to ambipolar diffusion is directly
connected to the ambipolar velocity of the charged particles under the
influence of the present magnetic field. The ambipolar velocity of the charged
particles depends on the internal dynamics of the particles. We outline a
general method to solve the particle dynamics in the presence of a magnetic
field using a magnetohydrodynamic equation for ambipolar velocity. The equation
is general and applies to all possible surrounding conditions \eg temperature,
and matter states like normal or superfluid.",2010.10776v4
2020-11-17,Single-shot dynamics of spin-orbit torque and spin transfer torque switching in three-terminal magnetic tunnel junctions,"Current-induced spin-transfer torques (STT) and spin-orbit torques (SOT)
enable the electrical switching of magnetic tunnel junctions (MTJs) in
nonvolatile magnetic random access memories. In order to develop faster memory
devices, an improvement of the timescales underlying the current driven
magnetization dynamics is required. Here we report all-electrical time-resolved
measurements of magnetization reversal driven by SOT in a three-terminal MTJ
device. Single-shot measurements of the MTJ resistance during current injection
reveal that SOT switching involves a stochastic two-step process consisting of
a domain nucleation time and propagation time, which have different genesis,
timescales, and statistical distributions compared to STT switching. We further
show that the combination of SOT, STT, and voltage control of magnetic
anisotropy (VCMA) leads to reproducible sub-ns switching with a spread of the
cumulative switching time smaller than 0.2 ns. Our measurements unravel the
combined impact of SOT, STT, and VCMA in determining the switching speed and
efficiency of MTJ devices.",2011.08709v1
2021-01-18,Topological electric driving of magnetization dynamics in insulators,"Established forms of electromagnetic coupling are usually conservative (in
insulators) or dissipative (in metals and semiconductors). Here we point out
the possibility of nondissipative electric driving of magnetization dynamics,
if the valence electronic states have nontrivial topology in the combined space
of crystal momentum and magnetization configuration. We provide a hybrid
insulator system to demonstrate that the topology-based nonconservative
electrical generalized force is capable of supporting sustained magnetization
motion in the presence of Gilbert damping, with quantized and steady energy
pumping into magnetization motion from the electric field. We also generalize
our results to magnetic textures, and discuss electric field induced
Dzyaloshinskii-Moriya interaction which can be nonconservative.",2101.07164v3
2021-02-05,Symmetry-Driven Spin-Wave Gap Modulation in Nanolayered SrRuO3/SrTiO3 Heterostructures: Implications for Spintronic Applications,"A strong correlation between magnetic interaction and topological symmetries
leads to unconventional magneto-transport behavior. Weyl fermions induce
topologically protected spin-momentum locking, which is closely related to
spin-wave gap formation in magnetic crystals. Ferromagnetic SrRuO3, regarded as
a strong candidate for Weyl semimetal, inherently possesses a nonzero spin-wave
gap owing to its strong magnetic anisotropy. In this paper, we propose a method
to control the spin-wave dynamics by nanolayer designing of the SrRuO3/SrTiO3
superlattices. In particular, the six-unit-cell-thick SrRuO3 layers within the
superlattices undergo a phase transition in crystalline symmetry from
orthorhombic to tetragonal, as the thickness of the SrTiO3 layers is modulated
with atomic-scale precision. Consequently, the magnetic anisotropy, anomalous
Hall conductivity, and spin-wave gap could be systematically manipulated. Such
customization of magnetic anisotropy via nanoscale heterostructuring offers a
novel control knob to tailor the magnon excitation energy for future spintronic
applications, including magnon waveguides and filters. Our nanolayer approach
unveils the important correlation between the tunable lattice degrees of
freedom and spin dynamics in topologically non-trivial magnetic materials.",2102.03008v1
2021-02-22,Robust formation of nanoscale magnetic skyrmions in easy-plane thin film multilayers with low damping,"We experimentally demonstrate the formation of room-temperature skyrmions
with radii of about 25\,nm in easy-plane anisotropy multilayers with
interfacial Dzyaloshinskii-Moriya interaction (DMI). We detect the formation of
individual magnetic skyrmions by magnetic force microscopy and find that the
skyrmions are stable in out-of-plane fields up to about 200 mT. We determine
the interlayer exchange coupling as well as the strength of the interfacial
DMI. Additionally, we investigate the dynamic microwave spin excitations by
broadband magnetic resonance spectroscopy. From the uniform Kittel mode we
determine the magnetic anisotropy and low damping $\alpha_{\mathrm{G}} < 0.04$.
We also find clear magnetic resonance signatures in the non-uniform (skyrmion)
state. Our findings demonstrate that skyrmions in easy-plane multilayers are
promising for spin-dynamical applications.",2102.11117v1
2021-03-24,Large Magnetic Moment in Flexoelectronic Silicon at Room Temperature,"Time-dependent rotational electric polarizations have been proposed to
generate temporally varying magnetic moments, for example, through a
combination of ferroelectric polarization and optical phonons. This phenomenon
has been called dynamical multiferroicity, but explicit experimental
demonstrations have been elusive to date. Here, we report the detection of a
temporal magnetic moment as high as 1.2 mu_B/atom in charge-doped thin film of
silicon under flexural strain. We demonstrate that the magnetic moment is
generated by a combination of electric polarization arising from a
flexoelectronic charge separation along the strain gradient and the deformation
potential of phonons. The effect can be controlled by adjusting the external
strain gradient, doping concentration and dopant, and can be regarded as a
dynamical multiferroic effect involving flexoelectronics polarization instead
of ferroelectricity. The discovery of a large magnetic moment in silicon may
enable the use of non-magnetic and non-ferroelectric semiconductors in various
multiferroic and spintronic applications.",2103.12907v1
2021-04-02,Dynamical evolution of magnetic field in the pre-equilibrium quark-gluon plasma,"High-energy heavy-ion collisions generate extremely strong magnetic field
which plays a key role in a number of novel quantum phenomena in quark-gluon
plasma (QGP), such as the chiral magnetic effect (CME). However, due to the
complexity in theoretical modellings of the coupled electromagnetic fields and
the QGP system, especially in the pre-equilibrium stages, the lifetime of the
magnetic field in the QGP medium remains undetermined. We establish, for the
first time, a kinetic framework to study the dynamical decay of the magnetic
field in the early stages of a weakly coupled QGP by solving the coupled
Boltzmann and Maxwell equations. We find that at late times a
magnetohydrodynamical description of the coupled system emerges. With respect
to realistic collisions at RHIC and the LHC, we estimate the residual strength
of the magnetic field in the QGP when the system start to evolve
hydrodynamically.",2104.00831v1
2021-05-03,Dislocation-driven relaxation processes at the conical to helical phase transition in FeGe,"The formation of topological spin textures at the nanoscale has a significant
impact on the long-range order and dynamical response of magnetic materials. We
study the relaxation mechanisms at the conical-to-helical phase transition in
the chiral magnet FeGe. By combining ac susceptibility, magnetic force
microscopy measurements and micromagnetic simulations, we demonstrate how the
motion of magnetic topological defects, here edge dislocations, impacts the
local formation of a stable helimagnetic spin structure. Although the
simulations show that the edge dislocations move with a velocity of about 100
m/s through the helimagnetic background, their dynamics are observed to disturb
the magnetic order on the timescale of minutes due to pinning by randomly
distributed structural defects. The results corroborate the substantial impact
of dislocation motions on the nanoscale spin structure in chiral magnets,
revealing previously hidden effects on the formation of helimagnetic domains
and domain walls.",2105.00658v1
2021-06-26,Orbital Magnetism of Active Viscoelastic Suspension,"We consider a dilute suspension of active (self-propelling) particles in a
visco-elastic fluid. Particles are charged and constrained to move in a two
dimensional harmonic trap. Their dynamics is coupled to a constant magnetic
field applied perpendicular to their motion via Lorentz force. Due to the
finite activity, the generalised fluctuation-dissipation relation (GFDR) breaks
down, driving the system away from equilibrium. While breaking GFDR, we have
shown that the system can have finite classical orbital magnetism only when the
dynamics of the system contains finite inertia. The orbital magnetic moment has
been calculated exactly. Remarkably, we find that when the elastic dissipation
time scale of the medium is larger (smaller) than the persistence time scale of
the self-propelling particles, the system is diamagnetic (paramagnetic).
Therefore, for a given strength of the magnetic field, the system undergoes a
novel transition from diamagnetic to paramagnetic state (and vice-versa) simply
by tuning the time scales of underlying physical processes, such as, active
fluctuations and visco-elastic dissipation. Interestingly, we also find that
the magnetic moment, which vanishes at equilibrium, behaves non-monotonically
with respect to increasing persistence of self-propulsion, that drives the
system out of equilibrium",2106.13918v1
2021-06-29,Magnetic Bloch-point hopping in multilayer skyrmions and associated emergent electromagnetic signatures,"Magnetic multilayers are promising tuneable systems for hosting magnetic
skyrmions at/above room temperature. Revealing their intriguing switching
mechanisms and associated inherent electrical responses are prerequisites for
developing skyrmionic devices. In this work, we theoretically demonstrate the
annihilation of single skyrmions occurring through a multilayer structure,
which is mediated by hopping dynamics of topological hedgehog singularities
known as Bloch points. The emerging intralayer dynamics of Bloch points are
dominated by the Dzyaloshinskii-Moriya interaction, and their propagation can
give rise to solenoidal emergent electric fields in the vicinity. Moreover, as
the topology of spin textures can dominate their emergent magnetic properties,
we show that the Bloch-point hopping through the multilayer will modulate the
associated topological Hall response, with the magnitude proportional to the
effective topological charge. We also investigate the thermodynamic stability
of these states regarding the layer-dependent magnetic properties. This study
casts light on the emergent electromagnetic signatures of skyrmion-based
spintronics, rooted in magnetic-multilayer systems.",2106.15463v1
2021-07-08,Chiral domain wall dynamics in magnetic heterostructures with bulk Dzyaloshinskii-Moriya interactions,"In this work, dynamics of chiral domain walls in long and narrow magnetic
heterostructures based on non-centrosymmetric chiral magnets with bulk
Dzyaloshinskii-Moriya interactions (DMI) and perpendicular magnetic anisotropy
is systematically investigated. The driving forces can be out-of-plane magnetic
fields and in-plane currents, correspondingly both steady and precessional
flows are considered. Their dividing points (the Walker critical field and
current density) are obtained as functions of bulk DMI strength
($D_{\mathrm{b}}$) and the ratio ($\kappa$) of total (crystalline plus shape)
anisotropy in the hard axis over that in the easy one. When far beyond Walker
breakdown, the dependence curve of wall velocity on external in-plane bias
field takes parabolic shape around the compensation point where the total
in-plane field disappears. The center shift is determined by $D_{\mathrm{b}}$,
$\kappa$, and the wall's topological charge, thus can be used to measure the
bulk DMI strength in chiral magnets.",2107.03586v1
2021-07-27,Evidence of symmetry lowering in antiferromagnetic metal TmB12 with dynamic charge stripes,"Precise angle-resolved magnetoresistance and magnetization measurements have
revealed (i) strong charge transport and magnetic anisotropy and (ii) emergence
of a huge number of magnetic phases in the ground state of TmB12
antiferromagnetic metal with fcc crystal structure and dynamic charge stripes.
By analyzing the angular H-fi magnetic phase diagrams reconstructed from
experimental angle-resolved magnetoresistance and magnetization data we argue
that the symmetry lowering is a consequence of suppression of the indirect
Ruderman- Kittel-Kasuya-Yosida (RKKY) exchange along 110 directions between
nearest neighboring magnetic moments of Tm3+ ions and subsequent redistribution
of conduction electrons to quantum fluctuations of the electron density
(stripes). Magnetoresistance components are discussed in terms of charge
scattering on the spin density wave, itinerant ferromagnetic nano-domains and
on-site Tm3+ spin fluctuations.",2107.12896v1
2021-10-26,"Structure and magnetic studies of geometrically frustrated disordered pyrochlores A$_{2}$Zr$_{2}$O$_{7}$: (A = Eu, Gd, Er)","The spin ice system Dy$_{2}$Ti$_{2}$O$_{7}$ exhibits strong
frequency-dependent spin-freezing at $\sim$ 16 K temperature. Although it has
been a matter of discussion for years, the origin of this unusual spin freezing
is still unknown. The replacement of Ti with isovalent Zr leads to the dynamic
magnetic ground state at low temperatures in Dy$_{2}$Zr$_{2}$O$_{7}$ and
prevents the formation of high-temperature spin freezing. Interestingly the
high-temperature spin freezing re-emerges in the presence of the magnetic
field. In this direction, we have studied a series of disordered pyrochlore
oxides A$_{2}$Zr$_{2}$O$_{7}$ (A = Eu, Gd, Er) and compared their crystal
structure, magnetic, and heat capacity behavior with that of
Dy$_{2}$Zr$_{2}$O$_{7}$ and Ho$_{2}$Zr$_{2}$O$_{7}$ systems. Our study shows
that depending on the disordered parameter, the spin-freezing behavior can be
retained by slowing down the spin dynamic with a suitable choice of the
magnetic field. We observe that unlike titanates, modification at the rare
earth site does not make considerable change in the magnetic ground state of
these zirconates compounds.",2110.13573v1
2022-01-07,On the role of magnetic fields into the dynamics and gravitational wave emission of binary neutron stars,"Modelling as a dipole the magnetic interaction of a binary system of neutron
stars, we are able to include the magnetic effects in the Newtonian and in the
inspiral dynamics of the system using an equivalent one-body description.
Furthermore, in the inspiral stage we determine the role of the magnetic
interaction in the waveforms generated by the system and obtain explicit
formulas for the decrease in the separation of the stars, the time to reach a
minimal radius, the gravitational luminosity and the change of gravitational
wave frequency, all this within the quadrupole approximation. For the magnitude
of the magnetic field that is consider to exist in these binaries $\sim 10^{16}
{\rm G}$ we are able to show that its effect on the observable quantities is of
the order of the 2PN correction, already close to the detection range of the
gravitational waves observatories. %} We also discuss cases in which the
magnetic field could have a more significant influence.",2201.02287v1
2022-03-19,Spin wave excitation and directional propagation in presence of magnetic charges in square artificial spin ice,"Artificial spin ice is a special class of engineered lattice of highly shape
anisotropic single domain magnetic nanostructures which is used as one of the
model systems to study the spin ice behavior observed in pyrochlore oxides. The
nanomagnets interact via dipolar interaction which results in correlated
magnetization dynamics exhibiting macroscopic spin configuration states. Here,
we exploit the interplay of underlying magnetic state and external bias field
orientation to study controlled spin wave propagation in square Artificial Spin
Ice (sASI) by performing detailed micromagnetic simulations. We report that
careful selection of vertices with local magnetic charges can effectively
direct the anisotropic spin wave in presence of an external field. Further, we
explore the influence of local charges due to the excited state in
even-coordinated vertices as well as uncompensated charges due to
odd-coordinated vertices on spin wave behavior. Our studies suggest that there
is no perceptible difference on spin wave dynamical behavior due to the origin
of local magnetic charge in sASI. Our results of controlled and directional
spin wave propagation in sASI system may be useful for low-power consumption
based all magnonic on-chip devices.",2203.10345v1
2022-04-14,Effect of Magnetism on Lattice Vibrations: Mössbauer Spectroscopic Evidence,"Based on the available literature reviewed are results pertinent to the
effect of magnetism on lattice dynamics obtained by M\""ossbauer spectroscopy at
57Fe and 119Sn probe atoms. Presented and discussed are sigma-phase Fe-Cr,
Fe-V, Fe-Cr-Ni and lambda-phase (C14 Laves) NbFe2 compounds that belong to the
Frank-Kasper family of phases, metallic chromium and iron mono-arsenide, FeAs.
The common feature of all these metallic systems is itinerant character of
their magnetism. Two spectral parameters relevant to the lattice dynamics viz.
the center shift, CS, and/or the recoil-free fraction, f, are considered as
figures of merit. Their temperature dependences show anomalies at magnetic
ordering temperatures giving evidence that that vibrations in the magnetic
phase are different than those in the paramagnetic phase. The lower the
magnetic ordering temperature, the stronger the difference.",2204.06881v1
2022-05-03,Mechanism for Sequestering Magnetic Energy at Large Scales in Shear-Flow Turbulence,"Straining of magnetic fields by large-scale shear flow, generally assumed to
lead to intensification and generation of small scales, is re-examined in light
of the persistent observation of large-scale magnetic fields in astrophysics.
It is shown that, in magnetohydrodynamic turbulence, unstable shear flows have
the unexpected effect of sequestering magnetic energy at large scales, due to
counteracting straining motion of nonlinearly excited large-scale stable
eigenmodes. This effect is quantified via dissipation rates, energy transfer
rates, and visualizations of magnetic field evolution by artificially removing
the stable modes. These analyses show that predictions based upon physics of
the linear instability alone miss substantial dynamics, including those of
magnetic fluctuations.",2205.01298v2
2022-06-12,Thermally Activated Transitions Between Micromagnetic States,"We review work by the authors on thermal activation in nanoscopic magnetic
systems. These systems present unique difficulties in analyzing noise-induced
escape over a barrier, including the presence of nonlocal interactions,
nongradient terms in the energy functional, and dynamical textures as initial
or saddle states. We begin with a discussion of magnetic reversal between
single-domain configurations of the magnetization. Here the transition (saddle)
state can be either a single-domain or a spatially varying (instanton-like)
configuration, and depending on the system parameters can exhibit either
Arrhenius or non-Arrhenius reversal rates. We then turn to a discussion of
transitions between magnetic textures, which can be either static and
topologically protected or dynamic and not topologically protected. An example
of the latter case is the droplet soliton, a rotating
nontopologically-protected configuration, which we find can occur either as a
metastable or transition state in a nanoscopic magnetic system. After
discussing various issues in calculating transition rates, we present results
for the activation barriers for creation and annihilation of these magnetic
textures. We conclude with a discussion of activated transitions between
topologically protected skyrmion textures and other configurations, on which
work is ongoing.",2206.05826v1
2022-06-14,Broadband microwave detection using electron spins in a hybrid diamond-magnet sensor chip,"Quantum sensing has developed into a main branch of quantum science and
technology. It aims at measuring physical quantities with high resolution,
sensitivity, and dynamic range. Electron spins in diamond are powerful magnetic
field sensors, but their sensitivity in the microwave regime is limited to a
narrow band around their resonance frequency. Here, we realize broadband
microwave detection using spins in diamond interfaced with a thin-film magnet.
A pump field locally converts target microwave signals to the sensor-spin
frequency via the non-linear spin-wave dynamics of the magnet. Two
complementary conversion protocols enable sensing and high-fidelity spin
control over a gigahertz bandwidth, allowing characterization of the spin-wave
band at multiple gigahertz above the sensor-spin frequency. The pump-tunable,
hybrid diamond-magnet sensor chip opens the way for spin-based sensing in the
100-gigahertz regime at small magnetic bias fields.",2206.07013v1
2022-08-03,Magnetic field driven dynamics in twisted bilayer artificial spin ice at superlattice angles,"Geometrical designs of interacting nanomagnets have been studied extensively
in the form of two dimensional arrays called artificial spin ice. These systems
are usually designed to create geometrical frustration and are of interest for
the unusual and often surprising phenomena that can emerge. Advanced
lithographic and element growth techniques have enabled the realization of
complex designs that can involve elements arranged in three dimensions. Using
numerical simulations employing the dumbbell approximation, we examine possible
magnetic behaviours for bilayer artificial spin ice (BASI) in which the
individual layers are rotated with respect to one another. The goal is to
understand how magnetization dynamics are affected by long-range dipolar
coupling that can be modified by varying the layer separation and layer
alignment through rotation. We consider bilayers where the layers are both
either square or pinwheel arrangements of islands. Magnetic reversal processes
are studied and discussed in terms of domain and domain wall configurations of
the magnetic islands. Unusual magnetic ordering is predicted for special angles
which define lateral spin superlattices for the bilayer systems.",2208.02243v1
2022-08-22,Ultrafast Spin Dynamics and Photoinduced Insulator-to-Metal Transition in $α$-$RuCl_3$,"Laser-induced ultrafast demagnetization is a phenomenon of utmost interest
and attracts significant attention because it enables potential applications in
ultrafast optoelectronics and spintronics. As a spin-orbit coupling assisted
magnetic insulator, $\alpha$-$RuCl_3$ provides an attractive platform to
explore the physics of electronic correlations and related unconventional
magnetism. Using time-dependent density functional theory, we explore the
ultrafast laser-induced dynamics of the electronic and magnetic structures in
$\alpha$-$RuCl_3$. Our study unveils that laser pulses can introduce ultrafast
demagnetizations in $\alpha$-$RuCl_3$, accompanied by an out-of-equilibrium
insulator-to-metal transition in a few tens of femtoseconds. The spin response
significantly depends on the laser wavelength and polarization on account of
the electron correlations, band renormalizations and charge redistributions.
These findings provide physical insights into the coupling between the
electronic and magnetic degrees of freedom in $\alpha$-$RuCl_3$ and shed light
on suppressing the long-range magnetic orders and reaching a proximate spin
liquid phase for two-dimensional magnets on an ultrafast timescale.",2208.10222v1
2022-09-18,Magnetically confined wind shock,"Many stars across all classes possess strong enough magnetic fields to
influence dynamical flow of material off the stellar surface. For the case of
massive stars (O and B types), about 10\% of them harbour strong, globally
ordered (mostly dipolar) magnetic fields. The trapping and channeling of their
stellar winds in closed magnetic loops leads to {\it magnetically confined wind
shocks} (MCWS), with pre-shock flow speeds that are some fraction of the wind
terminal speed that can be a few thousand km s$^{-1}$. These shocks generate
hot plasma, a source of X-rays. In the last decade, several developments took
place, notably the determination of the hot plasma properties for a large
sample of objects using \xmm\ and \ch, as well as fully self-consistent MHD
modelling and the identification of shock retreat effects in weak winds. In
addition, these objects are often sources of H$\alpha$ emission which is
controlled by either sufficiently high mass loss rate or centrifugal breakout.
Here we review the theoretical aspects of such magnetic massive star wind
dynamics.",2209.08540v1
2022-11-16,Spin structure and dynamics of the topological semimetal Co$_{3}$Sn$_{2-x}$In$_{x}$S$_{2}$,"The anomalous Hall effect (AHE), typically observed in ferromagnetic (FM)
metals with broken time-reversal symmetry, depends on electronic and magnetic
properties. In Co$_{3}$Sn$_{2-x}$In$_{x}$S$_{2}$, a giant AHE has been
attributed to Berry curvature associated with the FM Weyl semimetal phase, yet
recent studies report complicated magnetism. We use neutron scattering to
determine the spin dynamics and structures as a function of $x$ and provide a
microscopic understanding of the AHE and magnetism interplay. Spin gap and
stiffness indicate a contribution from Weyl fermions consistent with the AHE.
The magnetic structure evolves from $c$-axis ferromagnetism at $x$ = 0 to a
canted antiferromagnetic (AFM) structure with reduced $c$-axis moment and
in-plane AFM order at $x$ = 0.12 and further reduced $c$-axis FM moment at $x$
= 0.3. Since noncollinear spins can induce non-zero Berry curvature in real
space acting as a fictitious magnetic field, our results revealed another AHE
contribution, establishing the impact of magnetism on transport.",2211.08660v1
2022-11-23,Anatomy of spin wave driven magnetic texture motion via magnonic torques,"The interplay between spin wave and magnetic texture represents the
information exchange between the fast and slow dynamical parts of magnetic
systems. Here we formulate a set of magnonic torques acting on background
magnetic texture, by extracting time-invariant information from the fast
precessing spin waves. Under the frame of magnonic torques, we use theoretical
formulations and micromagnetic simulations to investigate the spin wave driven
domain wall motion in two typical symmetry-breaking situations: the rotational
symmetry broken by the Dzyaloshinkii-Moriya interaction, and the translational
symmetry broken by magnetic damping. The torque-based microscopic analyses
provide compact yet quantitative tools to reinterpret the magnetic texture
dynamics induced by spin wave, beyond the conventional framework of global
momentum conservation.",2211.12958v1
2022-12-16,"Unraveling effects of electron correlation in two-dimensional Fe$_{n}$GeTe$_{2}$ (n=3, 4, 5) by dynamical mean field theory","The Fe$_{n}$GeTe$_{2}$ systems are newly discovered two-dimensional
van-der-Waals materials, exhibiting magnetism at room temperature. The
sub-systems belonging to Fe$_{n}$GeTe$_{2}$ class are special because they show
site-dependent magnetic behavior. We focus on the critical evaluation of
magnetic properties and electron correlation effects in Fe$_{n}$GeTe$_{2}$
($n$= 3, 4, 5) (FGT) systems performing first-principles calculations. Three
different ab-initio approaches have been used, viz., i) standard density
functional theory (DFT), ii) incorporating static electron correlation (DFT+U)
and iii) inclusion of dynamic electron correlation effect (DFT+DMFT). Our
results show that DFT+DMFT is the most accurate technique to correctly
reproduce the magnetic interactions and experimentally observed transition
temperatures. The inaccurate values of structural parameters, magnetic moments
and exchange interactions obtained from DFT+U make this method inapplicable for
the FGT family. Correct determination of magnetic properties for this class of
materials is important since they are promising candidates for spin transport
and spintronic applications at room temperature.",2212.08552v1
2022-12-22,Laser Cooling of Trapped Ions in Strongly Inhomogeneous Magnetic Fields,"Hybrid traps for the simultaneous confinement of neutrals and ions have
recently emerged as versatile tools for studying interactions between these
species at very low temperatures. Such traps rely on the combination of
different types of external fields for the confinement of either species
raising the question of interactions between the individual traps. Here, the
influence of a strongly inhomogeneous magnetic field used for trapping neutrals
on the trapping and laser cooling of a single Ca$^+$ ion in a radiofrequency
ion trap is studied theoretically using molecular-dynamics simulations based on
multilevel rate equations. The inhomogeneous magnetic field couples the
different components of the ion motion and introduces position-dependent Zeeman
splittings. Nonetheless, laser cooling is still found to work efficiently as
the ion samples different magnetic field strengths and directions along its
trajectory. Offsetting the centres of the two traps generates a linear
magnetic-field gradient so that multiple lasers are required to address the
resulting range of Zeeman splittings in order to ensure efficient cooling. The
present study yields detailed insights into the ion cooling dynamics in
combined magnetic and radiofrequency electric fields relevant for the
characterisation and optimisation of hybrid trapping experiments.",2212.11863v1
2023-03-13,Intrinsic spin-orbit torque mechanism for deterministic all-electric switching of noncollinear antiferromagnets,"Using a pure electric current to control kagome noncollinear antiferromagnets
is promising in information storage and processing, but a full description is
still lacking, in particular, on intrinsic (i.e., no external magnetic fields
or external spin currents) spin-orbit torques. In this work, we
self-consistently describe the relations among the electronic structure,
magnetic structure, spin accumulations, and intrinsic spin-orbit torques, in
the magnetic dynamics of a noncollinear antiferromagnet driven by a pure
electric current. Our calculation can yield a critical current density
comparable with those in the experiments, when considering the boost from the
out-of-plane magnetic dynamics induced by the current-driven spin accumulation
on individual magnetic moments. We stress the parity symmetry breaking in
deterministic switching among magnetic structures. This work will be helpful
for future applications of noncollinear antiferromagnets.",2303.06929v2
2023-03-28,Anisotropic and frame dependent chaos of suspended strings from a dynamical holographic QCD model with magnetic field,"We investigate both from a qualitative as well as quantitative perspective
the emergence of chaos in the QCD confining string in a magnetic field from a
holographic viewpoint. We use an earlier developed bottom-up solution of the
Einstein-Maxwell-Dilaton action that mimics QCD and its thermodynamics quite
well. Surprisingly, our predictions depend on the used frame: the magnetic
field tends to suppress the chaos in both perpendicular and parallel directions
relative to the magnetic field in the string frame whilst in the Einstein
frame, the chaos suppression only happens in the perpendicular direction, with
an enhanced chaos along the magnetic field. The amount of
suppression/enhancement in both frames does depend on the relative orientation
of the string and magnetic field.",2303.15716v2
2023-06-17,Coherent two-dimensional THz magnetic resonance spectroscopies for molecular magnets: Analysis of Dzyaloshinskii-Moriya interaction,"To investigate the novel quantum dynamic behaviors of magnetic materials that
arise from complex spin-spin interactions, it is necessary to probe the
magnetic response at a speed greater than the spin-relaxation and dephasing
processes. Recently developed two-dimensional (2D) terahertz magnetic resonance
(THz-MR) spectroscopy techniques use the magnetic components of laser pulses,
and this allows investigation of the details of the ultrafast dynamics of spin
systems. For such investigations, quantum treatment -- not only of the spin
system itself but also of the environment surrounding the spin system -- is
important. In our method, based on the theory of multidimensional optical
spectroscopy, we formulate nonlinear THz-MR spectra using an approach based on
the numerically rigorous hierarchical equations of motion. We conduct numerical
calculations of both linear (1D) and 2D THz-MR spectra for a linear chiral spin
chain. The pitch and direction of chirality (clockwise or anticlockwise) are
determined by the strength and sign of the Dzyaloshinskii-Moriya interaction
(DMI). We show that not only the strength but also the sign of the DMI can be
evaluated through the use of 2D THz-MR spectroscopic measurements, while 1D
measurements allow us to determine only the strength.",2306.10222v1
2023-06-27,Statistics of a 2D immersed granular gas magnetically forced in volume,"We present an experimental study of the dynamics of a set of magnets within a
fluid in which a remote torque applied by a vertical oscillating magnetic field
transfers angular momentum to individual magnets. This system differs from
previous experimental studies of granular gas where the energy is injected by
vibrating the boundaries. Here, we do not observe any cluster formation,
orientational correlation and equipartition of the energy. The magnets' linear
velocity distributions are stretched exponentials, similar to 3D
boundary-forced dry granular gas systems, but the exponent does not depend on
the number of magnets. The value of the exponent of the stretched exponential
distributions is close to the value of 3/2 previously derived theoretically.
Our results also show that the conversion rate of angular momentum into linear
momentum during the collisions controls the dynamics of this
homogenously-forced granular gas. We report the differences between this
homogeneously-forced granular gas, ideal gas, and nonequilibrium
boundary-forced dissipative granular gas.",2306.15456v1
2023-09-18,Tunable Tesla-scale magnetic attosecond pulses through ring-current gating,"Coherent control over electron dynamics in atoms and molecules using
high-intensity circularly-polarized laser pulses gives rise to current loops,
resulting in the emission of magnetic fields. We propose and demonstrate with
ab-initio calculations ``current-gating"" schemes to generate direct or
alternating-current magnetic pulses in the infrared spectral region, with
highly tunable waveform and frequency, and showing femtosecond-to-attosecond
pulse duration. In optimal conditions, the magnetic pulse can be highly
isolated from the driving laser and exhibits a high flux density ($\sim1$ Tesla
at few hundred nanometers from the source, with a pulse duration of 787
attoseconds) for application in forefront experiments of ultrafast
spectroscopy. Our work paves the way toward the generation of attosecond
magnetic fields to probe ultrafast magnetization, chiral responses, and spin
dynamics.",2309.09654v1
2023-10-27,Polarization vs. magnetic field: competing eigenbases in laser-driven atoms,"We present experimental results and a theoretical model that illustrate how
competing eigenbases can determine the dynamics of a fluorescing atom. In the
absence of a magnetic field, the atom can get trapped in a dark state, which
inhibits fluorescence. In general, this will happen when the magnetic
degeneracy of the ground state is greater than the one of the excited state. A
canonical way to avoid optical pumping to dark states is to apply a magnetic
field at an angle with respect to the polarization of the exciting light. This
generates a competition of eigenbases which manifests as a crossover between
two regimes dominated either by the laser or the magnetic field. We illustrate
this crossover with fluorescence measurements on a single laser-cooled calcium
ion in a Paul trap and find that it occurs at a critical laser intensity that
is proportional to the external magnetic field. We contrast our results with
numerical simulations of the atomic levels involved and also present a simple
theoretical model that provides excellent agreement with experimental results
and facilitates the understanding of the dynamics.",2310.18525v1
2023-12-05,Can we distinguish black holes with electric and magnetic charges from quasinormal modes?,"We compute the quasinormal modes of static and spherically symmetric black
holes (BHs) with electric and magnetic charges. For the electrically charged
case, the dynamics of perturbations separates into the odd- and even-parity
sectors with two coupled differential equations in each sector. In the presence
of both electric and magnetic charges, the differential equations of four
dynamical degrees of freedom are coupled with each other between odd- and
even-parity perturbations. Despite this notable modification, we show that, for
a given total charge and mass, a BH with mixed electric and magnetic charges
gives rise to the same quasinormal frequencies for fundamental modes. This
includes the case in which two BHs have equal electric and magnetic charges for
each of them. Thus, the gravitational-wave observations of quasinormal modes
during the ringdown phase alone do not distinguish between electrically and
magnetically charged BHs.",2312.03191v1
2023-12-18,Comparative simulations of Kelvin-Helmholtz induced magnetic reconnection at the Earth's magnetospheric flanks,"This study presents 3D resistive Hall-magnetohydrodynamic numerical
simulations focusing on the Kelvin-Helmholtz instability (KHI) dynamics at
Earth's magnetospheric flanks during northward interplanetary magnetic field
(IMF) periods. Through a comparative analysis of two simulations, we explore
the impact of distinct magnetic field orientations on plasma dynamics and
magnetic reconnection events. In one configuration, a uniform magnetic field
results in (double) Mid-Latitude Reconnection (MLR), while in the second
configuration, a magnetic shear induces both Type I Vortex Induced Reconnection
and MLR. Key findings include the symmetries and asymmetries in the latitudinal
distribution of KHI vortices and current sheets and a quantitative comparison
of reconnection events. Particularly noteworthy is the quantification of newly
closed field lines that experienced double reconnection, ultimately becoming
embedded in solar wind plasma at low latitudes while remaining connected to
magnetospheric plasma at high latitudes. The varying abundance of such lines in
the two simulations holds implications for plasma transport at the
magnetopause. This work significantly advances our understanding of
magnetospheric processes, emphasizing the essential role of a three-dimensional
perspective in accurately simulating magnetospheric plasma phenomena.",2312.11161v1
2024-03-12,Plasmon-driven creation of magnetic topological structures,"In the present research, we demonstrate the usage of plasmonic effects in
thin film structures to control magnetic topological textures, specifically
skyrmions and skyrmioniums. We investigate numerically the generation and
alteration of these topological structures caused by hemisphere gold
nanoparticle placed over a magnetic layer coated with a dielectric material.
The electromagnetic and photothermal models are used to clarify the processes
of producing heat and absorption, and the results were implemented in
micromagnetic formalism to reveal the dynamics of magnetization under various
conditions. Our findings demonstrate the significance of the laser pulse
duration and the contact area between nanoparticles and the underlying magnetic
layer in forming topological textures. In particular, we show how to generate a
single skyrmion, multiple skyrmions, and skyrmioniums, and how to dynamically
transition between these states. These results highlight the possibility of
manipulating magnetic textures by using plasmonic effects, which presents
significant opportunities for spintronics and non-conventional computer
applications.",2403.07382v1
2024-03-15,"Structure, control, and dynamics of altermagnetic textures","We present a phenomenological theory of altermagnets, that captures their
unique magnetization dynamics and allows modelling magnetic textures in this
new magnetic phase. Focusing on the prototypical d-wave altermagnets, e.g.
RuO$_2$, we can explain intuitively the characteristic lifted degeneracy of
their magnon spectra, by the emergence of an effective sublattice-dependent
anisotropic spin stiffness arising naturally from the phenomenological theory.
We show that as a consequence the altermagnetic domain walls, in contrast to
antiferromagnets, have a finite gradient of the magnetization, with its
strength and gradient direction connected to the altermagnetic anisotropy, even
for 180$^\circ$ domain walls. This gradient generates a ponderomotive force in
the domain wall in the presence of a strongly inhomogeneous external magnetic
field, which may be achieved through magnetic force microscopy techniques. The
motion of these altermagentic domain walls is also characterized by an
anisotropic Walker breakdown, with much higher speed limits of propagation than
ferromagnets but lower than antiferromagnets.",2403.10218v2
1997-08-23,Dynamics of fluctuating magnetic fields in turbulent dynamos incorporating ambipolar drifts,"Turbulence with a large magnetic Reyonolds number, generically leads to
rapidly growing magnetic noise over and above any mean field. We revisit the
dynamics of this fluctuating field, in homogeneous, isotropic, helical
turbulence. Assuming the turbulence to be Markovian, we first rederive, in a
fairly transparent manner, the equation for the mean field, and corrected
Fokker-Plank type equations for the magnetic correlations. In these equations,
we also incorporate the effects of ambipolar drift which would obtain if the
turbulent medium has a significant neutral component. We apply these equations
to discuss a number of astrophysically interesting problems: (a) the small
scale dynamo in galactic turbulence with a model Kolmogorov spectrum,
incorporating the effect of ambipolar drift; (b) current helicity dynamics and
the quasilinear corrections to the alpha effect; (c) growth of the current
helicity and large-scale magnetic fields due to nonlinear effects.",9708216v1
2005-03-02,On the Spectrum of Magnetohydrodynamic Turbulence,"We propose a phenomenological model for incompressible magnetohydrodynamic
turbulence. We argue that nonlinear-wave interaction weakens as the energy
cascade proceeds to small scales, however, the anisotropy of fluctuations along
the large-scale magnetic field increases, which makes turbulence strong at all
scales. To explain the weakening of the interaction, we propose that
small-scale fluctuations of the velocity and magnetic fields become
increasingly dynamically aligned as their scale decreases, so that turbulent
eddies become locally anisotropic in the plane perpendicular to the large-scale
magnetic field. In the limit of weak anisotropy, that is, weak large-scale
magnetic field, our model reproduces the Goldreich-Sridhar spectrum, while the
limit of strong anisotropy, that is, strong large-scale magnetic field,
corresponds to the Iroshnikov-Kraichnan scaling of the spectrum. This is in
good agreement with recent numerical results.",0503053v2
1997-09-16,Stability of Relativistic Matter With Magnetic Fields,"Stability of matter with Coulomb forces has been proved for non-relativistic
dynamics, including arbitrarily large magnetic fields, and for relativistic
dynamics without magnetic fields. In both cases stability requires that the
fine structure constant alpha be not too large. It was unclear what would
happen for both relativistic dynamics and magnetic fields, or even how to
formulate the problem clearly. We show that the use of the Dirac operator
allows both effects, provided the filled negative energy `sea' is defined
properly. The use of the free Dirac operator to define the negative levels
leads to catastrophe for any alpha, but the use of the Dirac operator with
magnetic field leads to stability.",9709171v1
1998-11-05,Zero Temperature Phase Transition in Spin-ladders: Phase Diagram and Dynamical studies of Cu(Hp)Cl,"In a magnetic field, spin-ladders undergo two zero-temperature phase
transitions at the critical fields Hc1 and Hc2. An experimental review of
static and dynamical properties of spin-ladders close to these critical points
is presented. The scaling functions, universal to all quantum critical points
in one-dimension, are extracted from (a) the thermodynamic quantities
(magnetization) and (b) the dynamical functions (NMR relaxation). A simple
mapping of strongly coupled spin ladders in a magnetic field on the exactly
solvable XXZ model enables to make detailed fits and gives an overall
understanding of a broad class of quantum magnets in their gapless phase
(between Hc1 and Hc2). In this phase, the low temperature divergence of the NMR
relaxation demonstrates its Luttinger liquid nature as well as the novel
quantum critical regime at higher temperature. The general behaviour close
these quantum critical points can be tied to known models of quantum magnetism.",9811068v2
1999-03-10,Classical and Quantum Dynamics in a Random Magnetic Field,"Using the supersymmetry approach, we study spectral statistical properties of
a two-dimensional quantum particle subject to a non-uniform magnetic field. We
focus mainly on the problem of regularisation of the field theory. Our analysis
begins with an investigation of the spectral properties of the purely classical
evolution operator. We show that, although the kinetic equation is formally
time-reversible, density relaxation is controlled by {\em irreversible}
classical dynamics. In the case of a weak magnetic field, the effective kinetic
operator corresponds to diffusion in the angle space, the diffusion constant
being determined by the spectral resolution of the inhomogeneous magnetic
field. Applying these results to the quantum problem, we demonstrate that the
low-lying modes of the field theory are related to the eigenmodes of the
irreversible classical dynamics, and the higher modes are separated from the
zero mode by a gap associated with the lowest density relaxation rate. As a
consequence, we find that the long-time properties of the system are
characterised by universal Wigner-Dyson statistics. For a weak magnetic field,
we obtain a description in terms of the quasi one-dimensional non-linear
$\sigma$-model.",9903169v1
2000-05-08,Spin dynamics of Mn12-acetate in the thermally-activated tunneling regime: ac-susceptibility and magnetization relaxation,"In this work, we study the spin dynamics of Mn12-acetate molecules in the
regime of thermally assisted tunneling. In particular, we describe the system
in the presence of a strong transverse magnetic field. Similar to recent
experiments, the relaxation time/rate is found to display a series of
resonances; their Lorentzian shape is found to stem from the tunneling. The
dynamic susceptibility $\chi(w)$ is calculated starting from the microscopic
Hamiltonian and the resonant structure manifests itself also in $\chi(w)$.
Similar to recent results reported on another molecular magnet, Fe8, we find
oscillations of the relaxation rate as a function of the transverse magnetic
field when the field is directed along a hard axis of the molecules. This
phenomenon is attributed to the interference of the geometrical or Berry phase.
We propose susceptibility experiments to be carried out for strong transverse
magnetic fields to study of these oscillations and for a better resolution of
the sharp satellite peaks in the relaxation rates.",0005135v3
2000-12-08,Interaction potential between dynamic dipoles: polarized excitons in strong magnetic fields,"The interaction potential of a two-dimensional system of excitons with
spatially separated electron-hole layers is considered in the strong magnetic
field limit. The excitons are assumed to have free dynamics in the $x$-$y$
plane, while being constrained or `polarized' in the $z$ direction. The model
simulates semiconductor double layer systems under strong magnetic field normal
to the layers. The {\em residual} interaction between excitons exhibits
interesting features, arising from the coupling of the center-of-mass and
internal degrees of freedom of the exciton in the magnetic field. This coupling
induces a dynamical dipole moment proportional to the center-of-mass magnetic
moment of the exciton. We show the explicit dependence of the inter-exciton
potential matrix elements, and discuss the underlying physics. The unusual
features of the interaction potential would be reflected in the collective
response and non-equilibrium properties of such system.",0012131v1
2001-04-28,Origin of enhanced dynamic nuclear polarization and all-optical nuclear magnetic resonance in GaAs quantum wells,"Time-resolved optical measurements of electron-spin dynamics in a (110) GaAs
quantum well are used to study the consequences of a strongly anisotropic
electron g-tensor, and the origin of previously discovered all-optical nuclear
magnetic resonance. All components of the g-tensor are measured, and a strong
anisotropy even along the in-plane directions is found. The amplitudes of the
spin signal allow the study of the spatial directions of the injected spin and
its precession axis. Surprisingly efficient dynamic nuclear polarization in a
geometry where the electron spins are injected almost transverse to the applied
magnetic field is attributed to an enhanced non-precessing electron spin
component. The small absolute value of the electron g-factor combined with
efficient nuclear spin polarization leads to large nuclear fields that dominate
electron spin precession at low temperatures. These effects allow for sensitive
detection of all-optical nuclear magnetic resonance induced by periodically
excited quantum-well electrons. The mechanism of previously observed Delta m =
2 transitions is investigated and found to be attributable to electric
quadrupole coupling, whereas Delta m = 1 transitions show signatures of both
quadrupole and electron-spin induced magnetic dipole coupling.",0104564v1
2001-07-03,Polarizable particles aggregation under rotating magnetic fields using scattering dichroism,"We used scattering dichroism to study the combined effects of viscous and
magnetic forces on the dynamics of dipolar chains induced in magnetorheological
suspensions under rotating magnetic fields. We found that the chains adjust
their size to rotate synchronously with the field but with a constant phase
lag. Two different behaviors for the dichroism (proportional to the total
number of aggregated particles) and the phase lag are found below or above a
critical frequency. We obtained a linear dependence of the critical frequency
with the square of the magnetization and with the inverse of the viscosity. The
Mason number (ratio of viscous to magnetic forces) governs the dynamics.
Therefore there is a critical Mason number below which, the dichroism remains
almost constant and above which, the rotation of the field prevents the
particle aggregation process from taken place being this the mechanism
responsible for the decrease of dichroism. Our experimental results have been
corroborated with particle dynamics simulations showing good agreement.",0107070v1
2003-05-22,Detection of the Vortex Dynamic Regimes in MgB2 by Third Harmonic AC Susceptibility Measurements,"In a type-II superconductor the generation of higher harmonics in the
magnetic response to an alternating magnetic field is a consequence of the
non-linearity in the I-V relationship. The shape of the current-voltage (I-V)
curve is determined by the current dependence of the thermal activation energy
U(J) and is thus related to the dynamical regimes governing the vortex motion.
In order to investigate the vortex dynamics in MgB2 bulk superconductors we
have studied the fundamental (chi1) and third (chi3) harmonics of the ac
magnetic susceptibility. Measurements have been performed as a function of the
temperature and the dc magnetic field, up to 9 T, for various frequencies and
amplitudes of the ac field. We show that the analysis of the behaviour in
frequency of chi3(T) and chi3(B) curves can provide clear information about the
non-linearity in different regions of the I-V characteristic. By comparing the
experimental curves with numerical simulations of the non-linear diffusion
equation for the magnetic field we are able to resolve the crossover between a
dissipative regime governed by flux creep and one dominated by flux flow
phenomena.",0305523v1
2004-03-18,Nonadiabatic effects in the dynamics of atoms confined in a cylindric time-orbiting-potential magnetic trap,"In a time-orbiting-potential magnetic trap the neutral atoms are confined by
means of an inhomogeneous magnetic field superimposed to an uniform rotating
one. We perform an analytic study of the atomic motion by taking into account
the nonadiabatic effects arising from the spin dynamics about the local
magnetic field. Geometric-like magnetic-fields determined by the Berry's phase
appear within the quantum description. The application of a variational
procedure on the original quantum equation leads to a set of dynamical
evolution equations for the quantum average value of the position operator and
of the spin variables. Within this approximation we derive the
quantum-mechanical ground state configuration matching the classical adiabatic
solution and perform some numerical simulations.",0403466v1
2005-03-16,Dynamic phase separation in La(5/8-y)Pr(y)Ca(3/8)MnO(3),"Detailed magnetization measurements in La_{5/8-y}Pr_{y}Ca_{3/8} MnO_{3},
including magnetic relaxation properties, demonstrate the dynamic nature of the
phase separated state in manganites. The difference between the
field-cooled-cooling and zero-field-cooled magnetization curves signals the
existence in the latter of blocked metastable states separated by high energy
barriers. Results of the magnetic viscosity show that the system becomes
unblocked in a certain temperature window, where large relaxation rates are
observed. We propose a simple phenomenological model in which the system
evolves through a hierarchy of energy barriers, which separates the coexisting
phases. The calculated magnetization curves using this model reproduce all the
qualitative features of the experimental data. The overall results allowed us
to construct an H-T phase diagram, where frozen and dynamical phase separation
regions are clearly distinguished.",0503406v2
2005-12-14,Measurement of Magnetization Dynamics in Single-Molecule Magnets Induced by Pulsed Millimeter-Wave Radiation,"We describe an experiment aimed at measuring the spin dynamics of the Fe8
single-molecule magnet in the presence of pulsed microwave radiation. In
earlier work, heating was observed after a 0.2-ms pulse of intense radiation,
indicating that the spin system and the lattice were out of thermal equilibrium
at millisecond time scale [Bal et al., Europhys. Lett. 71, 110 (2005)]. In the
current work, an inductive pick-up loop is used to probe the photon-induced
magnetization dynamics between only two levels of the spin system at much
shorter time scales (from ns to us). The relaxation time for the magnetization,
induced by a pulse of radiation, is found to be on the order of 10 us.",0512289v1
2006-05-10,Spin-flip processes and ultrafast magnetization dynamics in Co - unifying the microscopic and macroscopic view of femtosecond magnetism,"The femtosecond magnetization dynamics of a thin cobalt film excited with
ultrashort laser pulses has been studied using two complementary pump-probe
techniques, namely spin-, energy- and time-resolved photoemission and
time-resolved magneto-optical Kerr effect. Combining the two methods it is
possible to identify the microscopic electron spin-flip mechanisms responsible
for the ultrafast macroscopic magnetization dynamics of the cobalt film. In
particular, we show that electron-magnon excitation does not affect the overall
magnetization even though it is an efficient spin-flip channel on the sub-200
fs timescale. Instead we find experimental evidence for the relevance of
Elliott-Yafet type spin-flip processes for the ultrafast demagnetization taking
place on a time scale of 300 fs.",0605272v2
2006-09-18,Critical spin liquid at 1/3 magnetization in a spin-1/2 triangular antiferromagnet,"Although magnetically ordered at low temperatures, the spin-1/2 triangular
antiferromagnet Cs_2CuCl_4 exhibits remarkable spin dynamics that strongly
suggest proximity to a spin liquid phase. Here we address the question of
whether a proximate spin liquid may also occur in an applied magnetic field,
leaving a similar imprint on the dynamical spin correlations of this material.
Specifically, we explore a spatially anisotropic Heisenberg spin-1/2 triangular
antiferromagnet at 1/3 magnetization from a dual vortex perspective, and indeed
find a new ``critical'' spin liquid phase described by QED3 with an emergent
SU(6) symmetry. A number of nontrivial predictions are given for the dynamical
spin structure factor in this ``algebraic vortex liquid'' phase, which can be
tested experimentally via inelastic neutron scattering. We also discuss how the
well-studied ``up-up-down'' magnetization plateaus can be captured within our
approach, and further predict the existence of a stable gapless solid phase in
a weakly ordered up-up-down state. Finally, we predict several anomalous
``roton'' minima in the excitation spectrum in the regime of lattice anisotropy
where the canted Neel state appears.",0609439v2
1992-09-17,Differential cross-sections and escape plots for low energy $SU(2)$ BPS magnetic monopole dynamics,"We compute the low-energy classical differential scattering cross-section for
BPS $SU(2)$ magnetic monopoles using the geodesic approximation to the actual
dynamics and 16K parallel processors on a CM2. Numerical experiments suggest
that the quantum BPS magnetic monopole differential cross-section is
well-approximated by the classical BPS magnetic monopole differential
cross-section. In particular, the expected quantum interference effects for
bosons at scattering angle $\theta=\pi/2$ (CoM frame) are contradicted
numerically. We argue that this is due to the topology of the classical
configuration space for these solitons. We also study the scattering and
bounded classical motions of BPS dyons and their global structure in phase
space by constructing `escape plots'. The escape plots contain a surprising
amount of structure, and suggest that the classical dynamics of two BPS $SU(2)$
magnetic monopoles is chaotic and that there are closed and bounded two dyon
motions with isolated energies.",9209063v1
2008-03-29,Tunnel-barrier-enhanced dc voltage signals induced by magnetization dynamics in magnetic tunnel junctions,"We theoretically study the recently observed tunnel-barrier-enhanced dc
voltage signals generated by magnetization precession in magnetic tunnel
junctions. While the spin pumping is suppressed by the high tunneling
impedance, two complimentary processes are predicted to result in a sizable
voltage generation in ferromagnet (F)|insulator (I)|normal-metal (N) and F|I|F
junctions, with one ferromagnet being resonantly excited. Magnetic dynamics in
F|I|F systems induces a robust charge pumping, translating into voltage in open
circuits. In addition, dynamics in a single ferromagnetic layer develops
longitudinal spin accumulation inside the ferromagnet. A tunnel barrier then
acts as a nonintrusive probe that converts the spin accumulation into a
measurable voltage. Neither of the proposed mechanisms suffers from spin
relaxation, which is typically fast on the scale of the exponentially slow
tunneling rates. The longitudinal spin-accumulation buildup, however, is very
sensitive to the phenomenological ingredients of the spin-relaxation picture.",0803.4228v2
2008-06-18,Hybrid Planar FEM in Magnetoresonance Regime: Control of Dynamical Chaos,"We establish the influence of nonlinear electron dynamics in the
magnetostatic field of a hybrid planar free-electron maser on its gain and
interaction efficiency. Even for the `ideal' undulator magnetic field the
presence of uniform longitudinal (guide) magnetic field potentially leads to
the existence of chaotic zone around certain (magnetoresonant) value of the
guide magnetic field. The width of the chaotic zone is given by the Chirikov
resonance-overlap criterion applied to the normal undulator and cyclotron
frequencies with respect to the coupling induced by the undulator magnetic
field. Using analytical asymptotically exact solutions for trajectories of
individual test electrons, we show that the magnetoresonant multiplier in
electron trajectories is also present in the expression for the gain. The same
Chirikov resonance-overlap criterion allows us to estimate analytically the
maximal magnetoresonant gain of a hybrid planar free-electron maser showing
that, in spite of the well-known drop in the gain for the exact
magnetoresonance, the operation regime in the zone of regular dynamics slightly
above the magnetoresonant value of the guide magnetic field is the preferable
one.",0806.2997v1
2008-08-06,Granularity and vortex dynamics in LaO_0.92F_0.08FeAs as probed by harmonics of the AC magnetic susceptibility,"Fundamental and higher harmonics of the AC magnetic susceptibility have been
measured on a LaO_0.92F_0.08FeAs sample as a function of the temperature, at
various amplitudes and frequencies of the AC magnetic field, with a small
superimposed DC field parallel to the AC field. The granularity of the sample
has been investigated and the inter-grain and intra-grain contributions have
been clearly individuated looking at both the first and third harmonics. The
vortex dynamics has been also analyzed, and a comparison with the magnetic
behavior of both the MgB_2 and the cuprate superconductors has been performed.
Some vortex dissipative phenomena, i.e. the thermally activated flux flow and
the flux creep, have been detected in the presented measurements, similar to
what obtained on YBCO. Nevertheless, although the general behavior is similar,
several differences have been also evidenced between these different classes of
superconductors, mainly in the third harmonics. We infer that different vortex
dynamics have to be included into the analysis of the magnetic response in this
iron-based new material.",0808.0804v1
2009-04-23,Kolmogorov-Sinai entropy in field line diffusion by anisotropic magnetic turbulence,"The Kolmogorov-Sinai (KS) entropy in turbulent diffusion of magnetic field
lines is analyzed on the basis of a numerical simulation model and theoretical
investigations. In the parameter range of strongly anisotropic magnetic
turbulence the KS entropy is shown to deviate considerably from the earlier
predicted scaling relations [Rev. Mod. Phys. {\bf 64}, 961 (1992)]. In
particular, a slowing down logarithmic behavior versus the so-called Kubo
number $R\gg 1$ ($R = (\delta B / B_0) (\xi_\| / \xi_\bot)$, where $\delta B /
B_0$ is the ratio of the rms magnetic fluctuation field to the magnetic field
strength, and $\xi_\bot$ and $\xi_\|$ are the correlation lengths in respective
dimensions) is found instead of a power-law dependence. These discrepancies are
explained from general principles of Hamiltonian dynamics. We discuss the
implication of Hamiltonian properties in governing the paradigmatic
""percolation"" transport, characterized by $R\to\infty$, associating it with the
concept of pseudochaos (random non-chaotic dynamics with zero Lyapunov
exponents). Applications of this study pertain to both fusion and astrophysical
plasma and by mathematical analogy to problems outside the plasma physics.
  This research article is dedicated to the memory of Professor George M.
Zaslavsky",0904.3610v1
2010-11-25,Edge-Soliton-Mediated Vortex-Core Reversal Dynamics,"We report a new reversal mechanism of magnetic vortex cores in nanodot
elements driven by out-of-plane currents, occurring through two coupled
edge-solitons via dynamic transformations between magnetic solitons of
different topological charges. This mechanism differs completely from the well
known switching process mediated by the creation and annihilation of
vortex-antivortex pairs in terms of the associated topological solitons,
energies, and spin-wave emissions. Strongly localized out-of-plane gyrotropic
fields induced by the fast motion of the two coupled edge-solitons enable a
magnetization dip that plays a crucial role in the formation of the reversed
core magnetization. This work provides a new physical insight into the dynamic
transformations of magnetic solitons in nanoelements.",1011.5625v1
2010-12-25,External Fields and the Dynamics of Fundamental Flavours in Holographic Duals of Large N Gauge Theories,"Using the gauge-gravity duality we study strongly coupled dynamics of
fundamental flavours in large N_c gauge theories in a constant external field.
We primarily focus on the effects of an external magnetic field. We use two
holographic models realized in the Type IIB and Type IIA supergravity and
present a comparative case study. In both these models, by studying the
dynamics of probe branes, we explicitly demonstrate and discuss the
magnetically induced chiral symmetry breaking effect (""magnetic catalysis"") in
the flavour sector. We also study the associated thermodynamics and the meson
spectrum and realize e.g. Zeeman splitting, stability enhancement of the mesons
in the presence of an external magnetic field etc. By studying the quasinormal
modes of the probe brane fluctuation in the hydrodynamic limit we also obtain
an analytic dispersion relation in the presence of a magnetic field in the Type
IIA model. This dispersion relation consists of a propagating sound mode in the
otherwise diffusive channel and is sourced by the quantum anomaly of the global
U(1) current. We briefly discuss the effects of an external electric field and
observe that the flavour bound states dissociate for sufficiently high electric
fields and an electric current is induced.",1012.5450v1
2011-03-22,Magnetic Interactions in Ball-Milled Spinel Ferrites,"Spinel Fe3O4 nanoparticles have been produced through ball milling in
methyl-alcohol (CH3OH), aiming to obtain samples with similar average particle
sizes <d> and different interparticle interactions. Three samples having
Fe3O4/CH3(OH) mass ratios R of 3 %, 10 % and 50 % wt. were milled for several
hours until particle size reached a steady value (<d> ~ 7-10 nm). A detailed
study of static and dynamic magnetic properties has been undertaken by
measuring magnetization, ac susceptibility and M\""ossbauer data. As expected
for small particles, the Verwey transition was not observed, but instead
superparamagnetic (SPM) behavior was found with transition to a blocked state
at TB ~ 10-20 K. Spin disorder of the resulting particles, independent of its
concentration, was inferred from the decrease of saturation magnetization MS at
low temperatures. For samples having 3% wt. of magnetic particles, dynamic ac
susceptibility measurements show a thermally activated Arrhenius dependence of
the blocking temperature with applied frequency. This behaviour is found to
change as interparticle interactions begin to rule the dynamics of the system,
yielding a spin-glass-like state at low temperatures for R = 50 wt.% sample.",1103.4390v1
2011-08-15,Effects of Magnetic Turbulence on the Dynamics of Pickup Ions in the Ionosheath of Mars,"We study some of the effects that magnetic turbulent fluctuations have on the
dynamics of pickup O+ ions in the magnetic polar regions of the Mars
ionosheath. In particular we study their effect on the bulk velocity profiles
of ions as a function of altitude over the magnetic poles, in order to compare
them with recent Mars Express data; that indicate that their average velocity
is very low and essentially in the anti-sunward direction. We find that, while
magnetic field fluctuations do give rise to deviations from simple ExB-drift
gyromotion, even fluctuation amplitudes much greater than those of in situ
measurements are {\it not} able to reproduce the vertical velocity profile of
O+ ions. We conclude that other physical mechanisms, different from a pure
charged particle dynamics, are acting on pickup ions at the Martian terminator.
A possibility being a viscous-like interaction between the Solar Wind and the
Martian ionosphere at low altitudes.",1108.2935v1
2011-09-30,"Magnetization Dynamics, Gyromagnetic Relation, and Inertial Effects","The gyromagnetic relation - i.e. the proportionality between the angular
momentum $\vec L$ (defined by an inertial tensor) and the magnetization $\vec
M$ - is evidence of the intimate connections between the magnetic properties
and the inertial properties of ferromagnetic bodies. However, inertia is absent
from the dynamics of a magnetic dipole (the Landau-Lifshitz equation, the
Gilbert equation and the Bloch equation contain only the first derivative of
the magnetization with respect to time). In order to investigate this
paradoxical situation, the lagrangian approach (proposed originally by T. H.
Gilbert) is revisited keeping an arbitrary nonzero inertial tensor. A dynamic
equation generalized to the inertial regime is obtained. It is shown how both
the usual gyromagnetic relation and the well-known Landau-Lifshitz-Gilbert
equation are recovered at the kinetic limit, i.e. for time scales above the
relaxation time $\tau$ of the angular momentum.",1109.6782v1
2012-01-03,The inverse cascade of magnetic helicity in magnetohydrodynamic turbulence,"The nonlinear dynamics of magnetic helicity, $H^M$, which is responsible for
large-scale magnetic structure formation in electrically conducting turbulent
media is investigated in forced and decaying three-dimensional
magnetohydrodynamic turbulence. This is done with the help of high resolution
direct numerical simulations and statistical closure theory. The numerically
observed spectral scaling of $H^M$ is at variance with earlier work using a
statistical closure model [Pouquet et al., J. Fluid Mech. \textbf{77} 321
(1976)]. By revisiting this theory a universal dynamical balance relation is
found that includes effects of kinetic helicity, as well as kinetic and
magnetic energy on the inverse cascade of $H^M$ and explains the
above-mentioned discrepancy. Considering the result in the context of
mean-field dynamo theory suggests a nonlinear modification of the
$\alpha$-dynamo effect important in the context of magnetic field excitation in
turbulent plasmas.",1201.0717v3
2013-01-22,Stripe disorder and dynamics in the hole-doped antiferromagnetic insulator La5/3Sr1/3CoO4,"We report on an investigation into the dynamics of the stripe phase of
La5/3Sr1/3CoO4, a material recently shown to have an hour-glass magnetic
excitation spectrum. A combination of magnetic susceptibility, muon-spin
relaxation and nuclear magnetic resonance measurements strongly suggest that
the physics is determined by a disordered configuration of charge and spin
stripes whose frustrated magnetic degrees of freedom are strongly dynamic at
high temperature and which freeze out in a glassy manner as the temperature is
lowered. Our results broadly confirm a recent theoretical prediction, but show
that the charge quenching remains incomplete well below the charge ordering
temperature and reveal, in detail, the manner in which the magnetic degrees of
freedom are frozen.",1301.5124v2
2014-04-10,Optical gyrotropy as a test for dynamic chiral magnetic effect of Weyl semimetals,"Recent angle resolved photoemission spectroscopy measurements have identified
an inversion symmetry breaking Weyl semimetal phase in TaAs and NbAs. In an
inversion symmetry breaking Weyl semimetal the left and the right handed Weyl
points can occur at different energies and the energy mismatch between the Weyl
points of opposite chirality is known as the chiral chemical potential. In the
presence of the chiral chemical potential, the nontrivial Berry curvature of
the Weyl fermions gives rise to the \emph{dynamic} chiral magnetic effect. This
describes how a time dependent magnetic field leads to an electrical current
along the applied field direction, which is also proportional to the field
strength. We derive a general formula for the dynamic chiral magnetic
conductivity of the inversion symmetry breaking Weyl semimetal. We show that
the measurement of the natural optical activity or rotary power provides a
direct confirmation of the existence of the dynamic chiral magnetic effect in
inversion symmetry breaking Weyl semimetals.",1404.2927v2
2015-01-01,Dynamic rotor mode in antiferromagnetic nanoparticles,"We present experimental, numerical, and theoretical evidence for a new mode
of antiferromagnetic dynamics in nanoparticles. Elastic neutron scattering
experiments on 8 nm particles of hematite display a loss of diffraction
intensity with temperature, the intensity vanishing around 150 K. However, the
signal from inelastic neutron scattering remains above that temperature,
indicating a magnetic system in constant motion. In addition, the precession
frequency of the inelastic magnetic signal shows an increase above 100 K.
Numerical Langevin simulations of spin dynamics reproduce all measured neutron
data and reveal that thermally activated spin canting gives rise to a new type
of coherent magnetic precession mode. This ""rotor"" mode can be seen as a
high-temperature version of superparamagnetism and is driven by exchange
interactions between the two magnetic sublattices. The frequency of the rotor
mode behaves in fair agreement with a simple analytical model, based on a high
temperature approximation of the generally accepted Hamiltonian of the system.
The extracted model parameters, as the magnetic interaction and the axial
anisotropy, are in excellent agreement with results from Mossbauer
spectroscopy.",1501.00313v1
2015-01-09,"Collective magnetization dynamics in ferromagnetic (Ga,Mn)As mediated by photo-excited carriers","We present a study of photo-excited magnetization dynamics in ferromagnetic
(Ga,Mn)As films observed by time-resolved magneto-optical measurements. The
magnetization precession triggered by linearly polarized optical pulses in the
absence of an external field shows a strong dependence on photon frequency when
the photo-excitation energy approaches the band-edge of (Ga,Mn)As. This can be
understood in terms of magnetic anisotropy modulation by both laser heating of
the sample and by hole-induced non-thermal paths. Our findings provide a means
for identifying the transition of laser-triggered magnetization dynamics from
thermal to non-thermal mechanisms, a result that is of importance for ultrafast
optical spin manipulation in ferromagnetic materials via non-thermal paths.",1501.02083v1
2015-03-15,Time-Dependent Meson Melting in External Magnetic Field,"The dynamics of a probe D7-brane in an asymptotically AdS-Vaidya background
has been investigated in the presence of an external magnetic field.
Holographically, this is dual to the dynamical meson melting in the N = 2 super
Yang-Milles theory. If the final temperature of the system is large enough, the
probe D7-brane will dynamically cross the horizon (black hole embedding). By
turning on the external magnetic field and raising it sufficiently, the final
embedding of the corresponding D7-brane changes to Minkowski embedding. In the
field theory side, this means that the mesons which melt due to the raise in
the temperature, will form bound states again by applying an external magnetic
field. We will also show that the evolution of the system to its final
equilibrium state is postponed due to the presence of the magnetic field.",1503.04439v2
2015-03-31,Nonstandard electron dynamics in topological insulators subjected to magnetic field: the Berry phase effects,"The quasiclassical dynamics is studied for charge carriers moving on the
surface of 3D topological insulator of Bi2Te3 type and subjected to static
magnetic field. The effects connected to the symmetry changes of electron
isoenergetic surfaces (contours) and to the nonzero Berry curvature are taken
into account. It is shown that in contrast to the standard dynamics of the
electrons moving in constant and uniform magnetic field along the trajectories
defined by the equations E(k)=const and pz=const, here some new effects are
arising, being related to both the appearance of the anomalous velocity term
proportional to the Berry curvature, and to the trajectory bending related to
the additional term for the energy proportional to the orbital momentum of the
wavepacket. This should lead to the changes in cyclotron resonance conditions
of the surface electrons. Although the time reversal invariance and the
topological order are broken in the magnetic field, the investigation of
cyclotron resonance allows determining whether this insulator was trivial or
nontrivial at zero magnetic field.",1503.08959v1
2015-04-06,Dynamics of molecular superrotors in external magnetic field,"We excite diatomic oxygen and nitrogen to high rotational states with an
optical centrifuge and study their dynamics in external magnetic field. Ion
imaging is employed to directly visualize, and follow in time, the rotation
plane of molecular superrotors. The two different mechanisms of interaction
between the magnetic field and the molecular angular momentum in paramagnetic
oxygen and non-magnetic nitrogen lead to the qualitatively different behaviour.
In nitrogen, we observe the precession of the molecular angular momentum around
the field vector. In oxygen, strong spin-rotation coupling results in faster
and richer dynamics, encompassing the splitting of the rotation plane in three
separate components. As the centrifuged molecules evolve with no significant
dispersion of the molecular wave function, the observed magnetic interaction
presents an efficient mechanism for controlling the plane of molecular
rotation.",1504.01432v1
2015-04-09,Spin-transfer torque effects in the dynamic forced response of the magnetization of nanoscale ferromagnets in superimposed ac and dc bias fields in the presence of thermal agitation,"Spin-transfer torque (STT) effects on the stationary forced response of
nanoscale ferromagnets subject to thermal fluctuations and driven by an ac
magnetic field of arbitrary strength and direction are investigated via a
generic nanopillar model of a spin-torque device comprising two ferromagnetic
strata representing the free and fixed layers and a nonmagnetic conducting
spacer all sandwiched between two ohmic contacts. The STT effects are treated
via the Brown magnetic Langevin equation generalized to include the Slonczewski
STT term thereby extending the statistical moment method [Y. P. Kalmykov et
al., Phys. Rev. B 88, 144406 (2013)] to the forced response of the most general
version of the nanopillar model. The dynamic susceptibility, nonlinear
frequency-dependent dc magnetization, dynamic magnetic hysteresis loops, etc.
are then evaluated highlighting STT effects on both the low-frequency thermal
relaxation processes and the high-frequency ferromagnetic resonance, etc.,
demonstrating a pronounced dependence of these on the spin polarization current
and facilitating interpretation of STT experiments.",1504.02319v2
2015-06-02,Detection of spin torque magnetization dynamics through low frequency noise,"We present a comparative study of high frequency dynamics and low frequency
noise in elliptical magnetic tunnel junctions with lateral dimensions under 100
nm presenting current-switching phenomena. The analysis of the high frequency
oscillation modes with respect to the current reveals the onset of a
steady-state precession regime for negative bias currents above $J=10^7
A/cm^2$, when the magnetic field is applied along the easy axis of
magnetization. By the study of low frequency noise for the same samples, we
demonstrate the direct link between changes in the oscillation modes with the
applied current and the normalised low frequency (1/f) noise as a function of
the bias current. These findings prove that low frequency noise studies could
be a simple and powerful technique to investigate spin-torque based
magnetization dynamics.",1506.00901v1
2015-09-18,Nonlinear ac stationary response and dynamic magnetic hysteresis of quantum uniaxial superparamagnets,"The nonlinear ac stationary response of uniaxial paramagnets and
superparamagnets - nanoscale solids or clasters with spin number S ~ 10^0 -
10^4 - in superimposed uniform ac and dc bias magnetic fields of arbitrary
strength, each applied along the easy axis of magnetization, is determined by
solving the evolution equation for the reduced density matrix represented as a
finite set of three-term differential-recurrence relations for its diagonal
matrix elements. The various harmonic components of the magnetization, dynamic
magnetic hysteresis loops, etc. are then evaluated via matrix continued
fractions indicating a pronounced dependence of the nonlinear response on S
arising from the quantum spin dynamics. In the linear response approximation,
the results concur with existing solutions.",1509.05616v2
2015-11-17,Itinerant and localized magnetization dynamics in antiferromagnetic Ho,"Using femtosecond time-resolved resonant magnetic x-ray diffraction at the Ho
L3 absorption edge, we investigate the demagnetization dynamics in
antiferromagnetically ordered metallic Ho after femtosecond optical excitation.
Tuning the x-ray energy to the electric dipole (E1, 2p -> 5d) or quadrupole
(E2, 2p -> 4f) transition allows us to selectively and independently study the
spin dynamics of the itinerant 5d and localized 4f electronic subsystems via
the suppression of the magnetic (2 1 3-tau ) satellite peak. We find
demagnetization timescales very similar to ferromagnetic 4f systems, suggesting
that the loss of magnetic order occurs via a similar spin-flip process in both
cases. The simultaneous demagnetization of both subsystems demonstrates strong
intra-atomic 4f-5d exchange coupling. In addition, an ultrafast lattice
contraction due to the release of magnetostriction leads to a transient shift
of the magnetic satellite peak.",1511.05315v1
2016-03-02,Electric-Field-Induced Spin Resonance in Antiferromagnetic Insulators: Inverse Process of the Dynamical Chiral Magnetic Effect,"We propose a realization of the electric-field-induced antiferromagnetic
resonance. We consider three-dimensional antiferromagnetic insulators with
spin-orbit coupling characterized by the existence of a topological term called
the $\theta$ term. By solving the Landau-Lifshitz-Gilbert equation in the
presence of the $\theta$ term, we show that, in contrast to conventional
methods using ac magnetic fields, the antiferromagnetic resonance state is
realized by ac electric fields along with static magnetic fields. This
mechanism can be understood as the inverse process of the dynamical chiral
magnetic effect, an alternating current generation by magnetic fields. In other
words, we propose a way to electrically induce the dynamical axion field in
condensed matter. We discuss a possible experiment to observe our proposal,
which utilizes the spin pumping from the antiferromagnetic insulator into a
heavy metal contact.",1603.00614v3
2016-05-05,Large-scale dynamics of magnetic helicity,"In this paper we investigate the dynamics of magnetic helicity in
magnetohydrodynamic (MHD) turbulent flows focusing at scales larger than the
forcing scale. Our results show a nonlocal inverse cascade of magnetic
helicity, which occurs directly from the forcing scale into the largest scales
of the magnetic field. We also observe that no magnetic helicity and no energy
is transferred to an intermediate range of scales sufficiently smaller than the
container size and larger than the forcing scale. Thus, the statistical
properties of this range of scales, which increases with scale separation, is
shown to be described to a large extent by the zero flux solutions of the
absolute statistical equilibrium theory exhibited by the truncated ideal MHD
equations.",1605.01697v3
2017-03-20,Relativistic theory of magnetic inertia in ultrafast spin dynamics,"The influence of possible magnetic inertia effects has recently drawn
attention in ultrafast magnetization dynamics and switching. Here we derive
rigorously a description of inertia in the Landau-Lifshitz-Gilbert equation on
the basis of the Dirac-Kohn-Sham framework. Using the Foldy-Wouthuysen
transformation up to the order of $1/c^4$ gives the intrinsic inertia of a pure
system through the 2$^{\rm nd}$ order time-derivative of magnetization in the
dynamical equation of motion. Thus, the inertial damping $\mathcal{I}$ is a
higher order spin-orbit coupling effect, $\sim 1/c^4$, as compared to the
Gilbert damping $\Gamma$ that is of order $1/c^2$. Inertia is therefore
expected to play a role only on ultrashort timescales (sub-picoseconds). We
also show that the Gilbert damping and inertial damping are related to one
another through the imaginary and real parts of the magnetic susceptibility
tensor respectively.",1704.01559v1
2017-12-25,Dynamics of magnetic flux tubes and IR-variability of Young Stellar Objects,"We simulate the dynamics of slender magnetic flux tubes (MFTs) in the
accretion disks of T Tauri stars. The dynamical equations of our model take
into account the aerodynamic and turbulent drag forces, and the radiative heat
exchange between the MFT and ambient gas. The structure of the disk is
calculated with the help of our MHD model of the accretion disks. We consider
the MFTs formed at the distances $0.027-0.8$ au from the star with various
initial radii and plasma betas $\beta_0$. The simulations show that the MFT
with weak magnetic field ($\beta_0=10$) rise slowly with speeds less than the
sound speed. The MFTs with $\beta_0=1$ form outflowing magnetized corona of the
disk. Strongly magnetized MFTs ($\beta_0=0.1$) can cause the outflows with
velocities $20-50$ km s$^{-1}$. The tubes rise periodically over times from
several days to several months according to our simulations. We propose that
periodically rising MFTs can absorb stellar radiation and contribute to the
IR-variability of Young Stellar Objects.",1712.09094v1
2018-06-29,Optical probing of Rayleigh wave driven magneto-acoustic resonance,"The resonant interaction of electrically excited travelling surface acoustic
waves and magnetization has been hitherto probed through the acoustic
component. In this work it is investigated using time-resolved magneto-optical
detection of magnetization dynamics. To that end, we develop an experimental
scheme where laser pulses are used both to generate the acoustic wave frequency
and to probe magnetization dynamics thus ensuring perfect phase locking. The
light polarization dependence of the signal enables to disentangle elasto-optic
and magneto-optic contributions and to obtain the in-plane and out-of-plane
dynamic magnetization components. Magnetization precession is proved to be
driven solely by the acoustic wave. Its amplitude is shown to resonate at the
same field at which we detect piezo-electrically the resonant attenuation of
the acoustic wave, clearly evidencing the magneto-acoustic resonance with high
sensitivity.",1806.11410v1
2018-07-12,Capture and chaotic scattering of a charged particle by magnetic monopole under uniform electric field,"Motivated by the realization of magnetic monopole of Berry curvature by the
energy crossing point, we theoretically study the effect of magnetic monopole
under a uniform electric field in the semiclassical dynamics, which is relevant
to many physical situations such as relaxation through the diabolic point. We
found that the competition between the backward scattering by the monopole
magnetic field and the acceleration by the electric field leads to the bound
state, i.e., capture of a particle near the monopole. Furthermore, the
nonlinearity induced by the magnetic monopole leads to the chaotic behavior in
the transient dynamics, i.e., the transient chaos. We computed characteristic
quantities of the strange saddle which gives rise to the transient chaos, and
verified that the abrupt bifurcation occurs as we tune the system parameter
toward the parameter region in which the system is solvable.",1807.04451v1
2018-12-20,Laser Controlled Spin Dynamics of Ferromagnetic Thin Film from Femtosecond to Nanosecond Timescale,"Laser induced modulation of the magnetization dynamics occurring over various
time-scales have been unified here for a Ni80Fe20 thin film excited by
amplified femtosecond laser pulses. The weak correlation between
demagnetization time and pump fluence with substantial enhancement in
remagnetization time is demonstrated using three-temperature model considering
the temperatures of electron, spin and lattice. The picosecond magnetization
dynamics is modeled using the Landau-Lifshitz-Gilbert equation. With increasing
pump fluence the Gilbert damping parameter shows significant enhancement from
its intrinsic value due to increment in the ratio of electronic temperature to
Curie temperature within very short time scale. The precessional frequency
experiences noticeable red shift with increasing pump fluence. The changes in
the local magnetic properties due to accumulation and dissipation of thermal
energy within the probed volume are described by the evolution of temporal
chirp parameter in a comprehensive manner. A unification of ultrafast magnetic
processes and its control over broad timescale would enable the integration of
various magnetic processes in a single device and use one effect to control
another.",1812.08404v1
2019-04-12,A dynamical magnetic field accompanying the motion of ferroelectric domain walls,"The recently proposed dynamical multiferroic effect describes the generation
of magnetization from temporally varying electric polarization. Here, we show
that the effect can lead to a magnetic field at moving ferroelectric domain
walls, where the rearrangement of ions corresponds to a rotation of
ferroelectric polarization in time. We develop an expression for the dynamical
magnetic field, and calculate the relevant parameters for the example of
90$^\circ$ and 180$^\circ$ domain walls in BaTiO$_3$ using a combination of
density functional theory and phenomenological modeling. We find that the
magnetic field reaches the order of several $\mu$T at the center of the wall,
and we propose two experiments to measure the effect with nitrogen-vacancy
center magnetometry.",1904.06335v1
2019-05-11,Magnetic field line twisting by photospheric vortices: energy storage and release,"We investigate the dynamics of a closed corona cartesian reduced
magnetohydrodynamic (MHD) model where photospheric vortices twist the coronal
magnetic field lines. We consider two corotating or counter-rotating vortices
localized at the center of the photospheric plate, and additionally more
corotating vortices that fill the plate entirely. Our investigation is
specifically devoted to study the fully nonlinear stage, after the linear stage
during which the vortices create laminar and smoothly twisting flux tubes. Our
main goal is to understand the dynamics of photospheric vortices twisting the
field lines of a coronal magnetic field permeated by finite amplitude broadband
fluctuations. We find that depending on the arrangement and handedness of the
photospheric vortices an inverse cascade storing a significant amount of
magnetic energy may occur or not. In the first case a reservoir of magnetic
energy available to large events such as destabilization of a pre-CME
configuration develops, while in the second case the outcome is a turbulent
heated corona. Although our geometry is simplified our simulations are shown to
have relevant implications for coronal dynamics and CME initiation.",1905.04420v1
2019-05-30,Interface moment dynamics and its contribution to spin-transfer torque switching process in magnetic tunnel junctions,"A practical problem for memory applications involving perpendicularly
magnetized magnetic tunnel junctions is the reliability of switching
characteristics at high-bias voltage. Often it has been observed that at
high-bias, additional error processes are present that cause a decrease in
switching probability upon further increase of bias voltage. We identify the
main cause of such error-rise process through examination of switching
statistics as a function of bias voltage and applied field, and the junction
switching dynamics in real time. These experiments show a coincidental onset of
error-rise and the presence of a new low-frequency microwave emission well
below that dictated by the anisotropy field. We show that in a few-macrospin
coupled numerical model, this is consistent with an interface region with
concentrated perpendicular anisotropy, and where the magnetic moment has
limited exchange coupling to the rest of the layers. These results point to the
important role high-frequency interface magnetic moment dynamics play in
determining the switching characteristics of these tunnel junction devices.",1905.13253v1
2019-06-30,"Non-linear spin torque, pumping and cooling in superconductor/ferromagnet systems","We study the effects of the coupling between magnetization dynamics and the
electronic degrees of freedom in a heterostructure of a metallic nanomagnet
with dynamic magnetization coupled with a superconductor containing a steady
spin-splitting field. We predict how this system exhibits a non-linear spin
torque, which can be driven either with a temperature difference or a voltage
across the interface. We generalize this notion to arbitrary magnetization
precession by deriving a Keldysh action for the interface, describing the
coupled charge, heat and spin transport in the presence of a precessing
magnetization. We characterize the effect of superconductivity on the
precession damping and the anti-damping torques. We also predict the full
non-linear characteristic of the Onsager counterparts of the torque, showing up
via pumped charge and heat currents. For the latter, we predict a spin-pumping
cooling effect, where the magnetization dynamics can cool either the nanomagnet
or the superconductor.",1907.00424v2
2019-07-22,Heavy quark dynamics in a hot magnetized QCD medium,"The heavy quark drag and momentum diffusion have been investigated in a hot
magnetized quark-gluon plasma, along the directions parallel and perpendicular
to the magnetic field. The analysis is done within the framework of
Fokker-Planck dynamics by considering the heavy quark scattering with thermal
quarks and gluons at the leading order in the coupling constant. An extended
quasiparticle model is adopted to encode the thermal QCD medium interactions in
the presence of a magnetic field. Further, the higher Landau level effects on
the temperature behaviour of the parallel and perpendicular components of the
drag force and diffusion coefficients have studied. It has been observed that
both the equation of state and the magnetic field play key roles in the
temperature dependence of the heavy quark dynamics.",1907.09556v1
2019-10-11,Analytical and Numerical Methods and Test Calculations of One-Dimensional Force--Free Magnetodynamics on Arbitrary Magnetic Surfaces across Horizons of Spinning Black Holes,"Numerical simulations of the force-free magnetodynamics (FFMD) of the
electromagnetic field around a spinning black hole are useful to investigate
the dynamic electromagnetic processes around a spinning black hole, such as the
emergence of the Blandford--Znajek mechanism. To reveal the basic physics of
magnetic fields around a black hole through the dynamic process, we use
one-dimensional (1D) FFMD along the axisymmetric magnetic surface, which
provides a relatively simple, sufficiently precise, and powerful tool to
analyze the dynamic process around a spinning black hole. We review the
analytic and numerical aspects of 1D FFMD for an arbitrary magnetic surface
around a black hole. In addition, we also show some numerical simulation test
results for three types of magnetic surfaces at the equatorial plane of the
black hole.",1910.04929v1
2020-04-03,Spin-Phonon Coupling and Thermodynamic Behaviour in YCrO3 and LaCrO3: Inelastic Neutron Scattering and Lattice Dynamics,"We report detailed temperature-dependent inelastic neutron scattering and
ab-initio lattice dynamics investigation of magnetic perovskites YCrO3 and
LaCrO3. The magnetic neutron scattering from the Cr ions exhibits significant
changes with temperature and dominates at low momentum transfer regime.
Ab-inito calculations performed including magnetic interactions show that the
effect of magnetic interaction is very signicant on the low- as well as
high-energy phonon modes. We have also shown that the inelastic neutron
spectrum of YCrO3 mimics the magnon spectrum from a G-type antiferromagnetic
system, which is consistent with previously reported magnetic structure in the
compound. The ab-initio lattice dynamics calculations in both the compounds
exhibit anisotropic thermal expansion behaviour in the orthorhombic structure
and predict negative thermal expansion along the crystallographic a-axis at low
temperatures. We identify the anharmonic phonon modes responsible for this
anamolous behaviour in LaCrO3 involving low-energy La vibrations and
distortions of the CrO6 octahedra.",2004.01731v1
2020-08-13,Dynamics of a magnetic gear with two cogging-free operation modes,"The coupling of two rotating spherical magnets is investigated
experimentally. For two specific angles between the input and output rotation
axes, a cogging-free coupling is observed, where the driven magnet is
phase-locked to the driving one. The striking difference between these two
modes of operation is the reversed sense of rotation of the driven magnet. For
other angles the experiments reveal a more complex dynamical behavior, which is
divided in three different classes. This is done by analysing the deviation
from a periodic motion of the driven magnet, and by measuring the total
harmonic distortion of this rotation. The experimental results can be
understood by a mathematical model based on pure dipole-dipole interaction,
with the addition of adequate friction terms.",2008.05922v1
2020-09-07,Breaking of Coulomb blockade by macrospin-assisted tunneling,"A magnet with precessing magnetization pumps a spin current into adjacent
leads. As a special case of this spin pumping, a precessing macrospin
(magnetization) can assist electrons in tunneling. In small systems, however,
the Coulomb blockade effect can block the transport of electrons. Here, we
investigate the competition between macrospin-assisted tunneling and Coulomb
blockade for the simplest system where both effects meet; namely, for a single
tunnel junction between a normal metal and a metallic ferromagnet with
precessing magnetization. By combining Fermi's golden rule with magnetization
dynamics and charging effects, we show that the macrospin-assisted tunneling
can soften or even break the Coulomb blockade. The details of these effects --
softening and breaking of Coulomb blockade -- depend on the macrospin dynamics.
This allows, for example, to measure the macrospin dynamics via a system's
current-voltage characteristics. It also allows to control a spin current
electrically. From a general perspective, our results provide a platform for
the interplay between spintronics and electronics on the mesoscopic scale. We
expect our work to provide a basis for the study of Coulomb blockade in more
complicated spintronic systems.",2009.03373v3
2020-10-17,Fluctuation relations for dissipative systems in constant external magnetic field: theory and molecular dynamics simulations,"It has recently been pointed out that Hamiltonian particle systems in
constant magnetic fields satisfy generalized time-reversal symmetries that
enable to prove useful statistical relationships based on equilibrium
phase-space probability distributions without the need to invert, as commonly
considered necessary, the magnetic field. Among these relations, that hold
without need of Casimir modifications, one finds the standard linear response
Green-Kubo relations, and consequently the Onsager reciprocal relations. Going
beyond linear response is also possible, for instance in terms of transient and
steady state Fluctuation Relations (FRs). Here we highlight how the generalized
time-reversal symmetries ensure that the (transient) FRs theory directly
applies also for systems in external magnetic fields. Furthermore we show that
transient FR can indeed be verified in nonequilibrium molecular dynamics
simulations, for systems subjected to magnetic and electric fields, which are
thermostatted \`a la Nos\'e-Hoover. The result is nontrivial because, since it
is not immediate within which sizes and time scales the effects can actually be
observable, it is not obvious what one may obtain by real molecular dynamics
simulations.",2010.08831v1
2020-11-02,How correlations change the magnetic structure factor of the kagome Hubbard model,"The kagome Hubbard model (KHM) is a paradigmatic example of a frustrated
two-dimensional model. While its strongly correlated regime, described by a
Heisenberg model, is of topical interest due to its enigmatic prospective
spin-liquid ground state, the weakly and moderately correlated regimes remain
largely unexplored. Motivated by the rapidly growing number of metallic kagome
materials (e.g., Mn$_3$Sn, Fe$_3$Sn$_2$, FeSn, Co$_3$Sn$_2$S$_2$,
Gd$_3$Ru$_4$Al$_{12}$), we study the respective regimes of the KHM by means of
three complementary numerical methods: the dynamical mean-field theory (DMFT),
the dynamical vertex approximation (D$\Gamma$A), and determinant quantum Monte
Carlo (DQMC). In contrast to the archetypal square-lattice, we find no
tendencies towards magnetic ordering, as magnetic correlations remain
short-range. Nevertheless, the magnetic correlations undergo a remarkable
crossover as the system approaches the metal-to-insulator transition. The Mott
transition itself does however not affect the magnetic correlations. Our
equal-time and dynamical structure factors can be used as a reference for
inelastic neutron scattering experiments on the growing family of metallic
kagome materials.",2011.01085v1
2020-11-09,Regimes of motion of magnetocapillary swimmers,"The dynamics of a triangular magnetocapillary swimmer is studied using the
lattice Boltzmann method. Performing extensive numerical simulations taking
into account the coupled dynamics of the fluid-fluid interface and of magnetic
particles floating on it and driven by external magnetic fields we identify
several regimes of the swimmer motion. In the regime of high frequencies the
swimmer's maximum velocity is centered around the particle's inverse coasting
time. Modifying the ratio of surface tension and magnetic forces allows to
study the swimmer propagation in the regime of significantly lower frequencies
mainly defined by the strength of the magnetocapillary potential. Finally,
introducing a constant magnetic contribution in each of the particles in
addition to their magnetic moment induced by external fields leads to another
regime characterised by strong in-plane swimmer reorientations that resemble
experimental observations.",2011.07144v1
2021-01-04,Mechanism of barrier crossing dynamics in the presence of both time dependent and independent magnetic fields,"In this paper we have presented the mechanism of the barrier crossing
dynamics of a Brownian particle which is coupled to a thermal bath in the
presence of both time independent and fluctuating magnetic fields. Here the
following three aspects are important in addition to the role of the thermal
bath on the barrier crossing dynamics. Magnetic field induced coupling may
introduce a resonance like effect. Another role of the field is that
enhancement of its strength reduces the frequency factor of the barrier
crossing rate constant. Finally, the fluctuating magnetic field introduces an
induced electric field which activates the Brownian particle to cross the
energy barrier. As a result of interplay among these aspects versatile
non-monotonic behavior may appear in the variation of the rate constant as a
function of the strength of the time independent magnetic field.",2101.00861v1
2021-02-12,Manipulation of magnetic domain wall by ferroelectric switching: Dynamic magnetoelectricity at the nanoscale,"Controlling magnetism using voltage is highly desired for applications, but
remains challenging due to fundamental contradiction between polarity and
magnetism. Here we propose a mechanism to manipulate magnetic domain walls in
ferrimagnetic or ferromagnetic multiferroics using electric field. Different
from those studies based on static domain-level couplings, here the
magnetoelectric coupling relies on the collaborative spin dynamics around
domain walls. Accompanying the reversal of spin chirality driven by
polarization switching, a ""rolling-downhill""-like motion of domain wall is
achieved at the nanoscale, which tunes the magnetization locally. Our mechanism
opens an alternative route to pursuit practical and fast converse
magnetoelectric functions via spin dynamics.",2102.06471v2
2021-04-06,All-Optical Control of Magnetization in Quantum-Confined Ultrathin Magnetic Metals,"All-optical control dynamics of magnetization in sub-10 nm metallic thin
films are investigated, as these films with quantum confinement undergo unique
interactions with femtosecond laser pulses. Our theoretical derivations based
on the free electron model show that the density of states at Fermi level
(DOS_F) and electron-phonon coupling coefficients (G_ep) in ultrathin metals
have very high sensitivity to film thickness within a few Angstroms. As DOS_F
and G_ep depend on thickness, we show that completely different magnetization
dynamics characteristics emerge compared with bulk metals. Our model suggests
highly-efficient energy transfer from fs laser photons to spin waves due to
minimal energy absorption by phonon. This sensitivity to thickness and
efficient energy transfer offers an opportunity to obtain ultrafast on-chip
magnetization dynamics.",2104.02608v1
2021-07-29,Magnon band structure of skyrmion crystals and stereographic projection approach,"Using semiclassical method combined with stereographic projection approach,
we investigate the magnetic dynamics of the skyrmion crystal (SkX), formed in
planar ferromagnet with both Dzyaloshinskii-Moriya interaction and uniform
magnetic field. The topologically non-trivial ground state of SkX is described
in stereographic projection by the complex valued function with simple poles at
skyrmions' positions. We use the earlier proposed ansatz for this ground state
function in the form of the sum of individual skyrmions. The dynamics follows
from the second variation of the classical action. Numerical analysis yields
the magnon band structure of tight-binding form in accordance with previously
known results. There are two sets of bands, one set with a flat dispersion,
topologically trivial and rapidly evolving with magnetic field. Another set is
robust to magnetic field, characterized by pronounced dispersion and with the
Berry curvature which may be sign-reversal in the Brillouin zone. The developed
theory can be straightforwardly generalized for the analysis of magnetic
dynamics in topological spin structures of other types.",2107.14335v1
2021-09-30,Observation of the orbital Hall effect in a light metal Ti,"The orbital angular momentum is a core ingredient of orbital magnetism, spin
Hall effect, giant Rashba spin splitting, orbital Edelstein effect, and
spin-orbit torque. However, its experimental detection is tricky. In
particular, direct detection of the orbital Hall effect remains elusive despite
its importance for electrical control of magnetic nanodevices. Here we report
the direct observation of the orbital Hall effect in a light metal Ti. The Kerr
rotation by the accumulated orbital magnetic moment is measured at Ti surfaces,
whose result agrees with theoretical calculations semiquantitatively and is
supported by the orbital torque measurement in Ti-based magnetic
heterostructures. The results confirm the electron orbital angular momentum as
an essential dynamic degree of freedom, which may provide a novel mechanism for
the electric control of magnetism. The results may also deepen the
understanding of spin, valley, phonon, and magnon dynamics coupled with orbital
dynamics.",2109.14847v1
2022-03-12,The mass-lumped midpoint scheme for computational micromagnetics: Newton linearization and application to magnetic skyrmion dynamics,"We discuss a mass-lumped midpoint scheme for the numerical approximation of
the Landau-Lifshitz-Gilbert equation, which models the dynamics of the
magnetization in ferromagnetic materials. In addition to the classical
micromagnetic field contributions, our setting covers the non-standard
Dzyaloshinskii-Moriya interaction, which is the essential ingredient for the
enucleation and stabilization of magnetic skyrmions. Our analysis also includes
the inexact solution of the arising nonlinear systems, for which we discuss
both a constraint preserving fixed-point solver from the literature and a novel
approach based on the Newton method. We numerically compare the two
linearization techniques and show that the Newton solver leads to a
considerably lower number of nonlinear iterations. Moreover, in a numerical
study on magnetic skyrmions, we demonstrate that, for magnetization dynamics
that are very sensitive to energy perturbations, the midpoint scheme, due to
its conservation properties, is superior to the dissipative tangent plane
schemes from the literature.",2203.06445v1
2022-02-23,"Biot-Savart type magnetic field quantization via prime number theory, applications to symbolic dynamics","In the present work we propose an algorithm based on the theory of prime
numbers for the estimation of the magnetic field in a device of current
carrying circular rings. Using the proposed algorithm, the magnetic field can
be determined in a very good agreement with that resulting from an algorithm
based on the Biot-Savart law. In addition, the prime-numbers-based algorithm
gives quantized values of the magnetic field and reveals previously unknown
behaviors such as special properties of the distribution of the waiting times
at the quantized magnetic field values. Applications of the proposed
prime-numbers-based algorithm to systems exhibiting symbolic dynamics is
presented, proving its ability to provide a measure of the existence or not of
dynamics in the system.",2204.00420v1
2022-06-02,Anisotropic Gigahertz Antiferromagnetic Resonances of the Easy-Axis van der Waals Antiferromagnet CrSBr,"We report measurements of antiferromagnetic resonances in the van der Waals
easy-axis antiferromagnet CrSBr. The interlayer exchange field and
magnetocrystalline anisotropy fields are comparable to laboratory magnetic
fields, allowing a rich variety of gigahertz-frequency dynamical modes to be
accessed. By mapping the resonance frequencies as a function of the magnitude
and angle of applied magnetic field we identify the different regimes of
antiferromagnetic dynamics. The spectra show good agreement with a
Landau-Lifshitz model for two antiferromagnetically-coupled sublattices,
accounting for inter-layer exchange and triaxial magnetic anisotropy. Fits
allow us to quantify the parameters governing the magnetic dynamics: at 5 K,
the interlayer exchange field is $\mu_0 H_E =$ 0.395(2) T, and the hard and
intermediate-axis anisotropy parameters are $\mu_0 H_c =$ 1.30(2) T and $\mu_0
H_a =$ 0.383(7) T. The existence of within-plane anisotropy makes it possible
to control the degree of hybridization between the antiferromagnetic resonances
using an in-plane magnetic field.",2206.01286v2
2022-11-07,Dynamic self-organisation and pattern formation by magnon-polarons,"Nonlinear dynamics can give rise, via the processes of self-organisation and
pattern formation, to the spontaneous manifestation of order in open and
complex systems far from equilibrium. Self-organising systems, transforming the
inflow of energy into information, are ubiquitously found in current topical
areas of science ranging from brainwave entrainment and neuromorphic computing
to energy-efficient data storage technologies. In the latter, magnetic
materials play a pivotal role combining very fast switching with permanent
retention of information. However, it has been shown that, at very short time
scales, magnetisation dynamics become chaotic due to internal instabilities,
resulting in incoherent spin-wave excitations that ultimately destroy magnetic
ordering. Here, contrary to all expectations, we show that such chaos gives
rise to a periodic pattern of reversed magnetic domains, with a feature size
far smaller than the spatial extent of the excitation. We explain this pattern
as a result of phase-synchronisation of magnon-polaron waves, driven by strong
coupling of magnetic and elastic modes. Our results reveal not only the
peculiar formation and evolution of magnon-polarons at short time-scales, but
also present a novel mechanism of magnetization reversal driven by coherent
packets of short-wavelength quasiparticles.",2211.03723v1
2022-11-21,Inertial dynamics and equilibrium correlation functions of magnetization at short times,"The method of moments is developed and employed to analyze the equilibrium
correlation functions of the magnetization of ferromagnetic nanoparticles in
the case of inertial magnetization dynamics. The method is based on the Taylor
series expansion of the correlation functions and the estimation of the
expansion coefficients. This method significantly reduces the complexity of
analysis of equilibrium correlation functions. Analytical expressions are
derived for the first three coefficients for the longitudinal and transverse
correlation functions for the uniaxial magnetocrystalline anisotropy of
ferromagnetic nanoparticles with a longitudinal magnetic field. The limiting
cases of very strong and negligibly weak external longitudinal fields are
considered. The Gordon sum rule for inertial magnetization dynamics is
discussed. In addition, we show that finite analytic series can be used as a
simple and satisfactory approximation for the numerical calculation of
correlation functions at short times.",2211.11515v2
2022-12-19,A systematic formulation of chiral anomalous magnetohydrodynamics,"We present a new way of deriving effective theories of dynamical
electromagnetic fields in general media. It can be used to give a systematic
formulation of magnetohydrodynamics (MHD) with strong magnetic fields,
including systems with chiral matter and Adler-Bell-Jackiw (ABJ) anomaly. We
work in the regime in which velocity and temperature fluctuations can be
neglected. The resulting chiral anomalous MHD incorporates and generalizes the
chiral magnetic effect, the chiral separation effect, the chiral electric
separation effect, as well as recently derived strong-field MHD, all in a
single coherent framework. At linearized level, the theory predicts that the
chiral magnetic wave does not survive dynamical electromagnetic fields. A
different chiral wave, to which we refer as the chiral magnetic electric
separation wave, emerges as a result of dynamical versions of the chiral
electric separation effect and the chiral magnetic effect. We predict its wave
velocity. We also introduce a simple, but solvable nonlinear model to explore
the fate of the chiral instability.",2212.09757v2
2023-02-06,Observation of Coherently Coupled Cation Spin Dynamics in an Insulating Ferrimagnetic Oxide,"Many technologically useful magnetic oxides are ferrimagnetic insulators,
which consist of chemically distinct cations. Here, we examine the spin
dynamics of different magnetic cations in ferrimagnetic NiZnAl-ferrite
(Ni$_{0.65}$Zn$_{0.35}$Al$_{0.8}$Fe$_{1.2}$O$_4$) under continuous microwave
excitation. Specifically, we employ time-resolved x-ray ferromagnetic resonance
to separately probe Fe$^{2+/3+}$ and Ni$^{2+}$ cations on different sublattice
sites. Our results show that the precessing cation moments retain a rigid,
collinear configuration to within $\approx$2$^\circ$. Moreover, the effective
spin relaxation is identical to within $<$10% for all magnetic cations in the
ferrite. We thus validate the oft-assumed ``ferromagnetic-like'' dynamics in
resonantly driven ferrimagnetic oxides, where the magnetic moments from
different cations precess as a coherent, collective magnetization.",2302.03100v1
2023-07-28,Reciprocal Reservoir Induced Non-Hermitian Skin Effect,"The non-Hermitian skin effect (NHSE), which describes the localization of
macroscopic fraction of eigenstates at a specific boundary, is inherently tied
to nonreciprocity. Here, we show that the NHSE can be engineered in an open
magnetic system interacting with a reciprocal reservoir, through the interplay
between the reservoir-induced coherent and dissipative couplings. Based on a
Lindbladian time evolution, we investigate the transient nonreciprocal dynamics
along a spin chain and its unidirectional limit, which allow us to reveal both
consistency with and limitation of the non-Hermitian Hamiltonian approach. We
comment on the connection to the semiclassical dissipative magnetization
dynamics, and identify the key ingredients underlying the NHSE in magnetic
systems as Dzyaloshinskii-Moriya interaction (DMI) and dissipative coupling,
analogous to those in multi-layered magnet-metal heterostructures. Our work
suggests the generality of nonreciprocal dynamics in magnetic systems and may
inspire new schemes for engineering nonreciprocity in other quantum platforms.",2307.15792v1
2023-12-13,Emergent Fermion Dynamical Symmetry for Monolayer Graphene in a Strong Magnetic Field,"We review the physics of monolayer graphene in a strong magnetic field, with
emphasis on highly collective states that emerge from the weakly interacting
system because of correlations (emergent states). After reviewing the general
properties of graphene and of electrons in a magnetic field, we give a brief
introduction to the integer quantum Hall effect (IQHE) and the fractional
quantum Hall effect (FQHE) in a 2D electron gas as foundation to show that
monolayer graphene in a magnetic field exhibits both effects, but with
properties modified by the influence of the graphene crystal. After giving an
introduction to standard methods of dealing with emergent states for this
system, we show that an SO(8) fermion dynamical symmetry governs the emergent
degrees of freedom and that the algebraic and group properties of the dynamical
symmetry provide a new view of strongly correlated states observed in monolayer
graphene subject to a strong magnetic field.",2312.08475v1
2024-02-14,Davydov's soliton in an external alternating magnetic field,"The influence of an external oscillating in time magnetic field on the
dynamics of the Davydov's soliton is investigated. It is shown that it
essentially depends not only on the amplitude and frequency of the magnetic
field, but also on the field orientation with respect to the molecular chain
axis. The soliton velocity and phase are calculated. They are oscillating in
time functions with the frequency of the main harmonic, given by the external
field frequency, and higher multiple harmonics. It is concluded that such
complex effects of external time-depending magnetic fields on the dynamics of
solitons modify the charge transport in low-dimensional molecular systems,
which can affect functioning of the devices based on such systems. These
results suggest also the physical mechanism of therapeutic effects of
oscillating magnetic fields, based on the field influence on the dynamics of
solitons which provide charge transport through biological macro-molecules in
the redox processes.",2402.09172v1
2024-03-11,Effect of Ir growth pressure on the domain wall dynamics in Ta/Pt/Co/Ir/Ta stacks,"The dynamical response of magnetic domain walls to external magnetic fields
in ultra-thin multilayer magnetic films is determined not only by the
composition and thickness of the layers but also by the growth conditions.
Growth conditions can induce significant structural changes inside the layers
and at the interfaces between them, affecting in particular the dynamics of
domain walls, their mobility, elastic tension, and the pinning forces acting on
them. In this work, we focus specifically on the effect of Ir layer growth
pressure in Ta/Pt/Co/Ir/Ta ultra-thin multilayers films. Measurements of the DC
magnetic properties, domain wall velocity and domain morphology in the creep
regime for both constant and alternating field pulses, were performed for a
batch of samples where the Ir layer was grown at different pressures. We find
that the saturation magnetization, the effective anisotropy constant and the
domain wall surface tension grow with increasing pressure and saturate at a
threshold pressure, while the Dzyaloshinskii-Moriya field and the strength of
the disorder remain practically unaltered over the range of pressures
considered.",2403.07141v1
2024-03-16,Machine Learning Exchange Fields for Ab-initio Spin Dynamics,"We add the magnetic degrees of freedom to the widely used Gaussian
Approximation Potential of machine learning (ML) and present a model that
describes the potential energy surface of a crystal based on the atomic
coordinates as well as their noncollinear magnetic moments. Assuming an
adiabatic approximation for the spin directions and magnitudes, the ML model
depends solely on spin coordinates and orientation, resulting in computational
efffciency and enabling ab initio spin dynamics. Leveraging rotational
symmetries of magnetic interactions, the ML model can incorporate various
magnetic interactions, expanding into two-body, three-body terms, etc.,
following the spirit of cluster expansion. For simplicity, we implement the ML
model with a two-body form for the exchange interaction. Comparing total
energies and local fields predicted by the model for noncollinear spin
arrangements with explicit results of constrained noncollinear density
functional calculations for bcc Fe yields excellent results, within 1 meV/spin
for the total energy. Further optimization, including three-body and other
terms, is expected to encompass diverse magnetic interactions and enhance the
model's accuracy. This will extend the model's applicability to a wide range of
materials and facilitate the machine learning ab initio spin dynamics.",2403.10769v1
2015-11-09,Acceleration of charged particles due to chaotic scattering in the combined black hole gravitational field and asymptotically uniform magnetic field,"To test the role of large-scale magnetic fields in accretion processes, we
study dynamics of charged test particles in vicinity of a black hole immersed
into an asymptotically uniform magnetic field. Using the Hamiltonian formalism
of charged particle dynamics, we examine chaotic scattering in the effective
potential related to the black hole gravitational field combined with the
uniform magnetic field. Energy interchange between the translational and
oscillatory modes od the charged particle dynamics provides mechanism for
charged particle acceleration along the magnetic field lines. This energy
transmutation is an attribute of the chaotic charged particle dynamics in the
combined gravitational and magnetic fields only, the black hole rotation is not
necessary for such charged particle acceleration. The chaotic scatter can cause
transition to the motion along the magnetic field lines with small radius of
the Larmor motion or vanishing Larmor radius, when the speed of the particle
translational motion is largest and can be ultra-relativistic. We discuss
consequences of the model of ionization of test particles forming a neutral
accretion disc, or heavy ions following off-equatorial circular orbits, and we
explore the fate of heavy charged test particles after ionization where no kick
of heavy ions is assumed and only switch-on effect of the magnetic field is
relevant. We demonstrate that acceleration and escape of the ionized particles
can be efficient along the Kerr black hole symmetry axis parallel to the
magnetic field lines. We show that strong acceleration of ionized particles to
ultra-relativistic velocities is preferred in the direction close to the
magnetic field lines. Therefore, the process of ionization of Keplerian discs
around Kerr black holes can serve as a model of relativistic jets.",1511.02936v1
2005-11-03,High-Field Low-Frequency Spin Dynamics,"The theory of exchange symmetry of spin ordered states is extended to the
case of high magnetic field. Low frequency spin dynamics equation for
quasi-goldstone mode is derived for two cases of collinear and noncollinear
antiferromagnets.",0511090v1
2006-10-12,The Curie-Weiss model with dynamical external field,"We study a Curie-Weiss model with a random external field generated by a
dynamical system. Probabilistic limit theorems (weak law of large numbers,
central limit theorems) are proven for the corresponding magnetization.",0610394v1
2000-04-10,Classical and quantum dynamics of a spin-1/2,"We reply to a comment on `Semiclassical dynamics of a spin-1/2 in an
arbitrary magnetic field'.",0004046v1
2012-02-20,Variational description of Gibbs-non-Gibbs dynamical transitions for the Curie-Weiss model,"We perform a detailed study of Gibbs-non-Gibbs transitions for the
Curie-Weiss model subject to independent spin-flip dynamics
(""infinite-temperature"" dynamics). We show that, in this setup, the program
outlined in van Enter, Fern\'andez, den Hollander and Redig can be fully
completed, namely that Gibbs-non-Gibbs transitions are equivalent to
bifurcations in the set of global minima of the large-deviation rate function
for the trajectories of the magnetization conditioned on their endpoint. As a
consequence, we show that the time-evolved model is non-Gibbs if and only if
this set is not a singleton for some value of the final magnetization. A
detailed description of the possible scenarios of bifurcation is given, leading
to a full characterization of passages from Gibbs to non-Gibbs -and vice versa-
with sharp transition times (under the dynamics Gibbsianness can be lost and
can be recovered).
  Our analysis expands the work of Ermolaev and Kulske who considered zero
magnetic field and finite-temperature spin-flip dynamics. We consider both zero
and non-zero magnetic field but restricted to infinite-temperature spin-flip
dynamics. Our results reveal an interesting dependence on the interaction
parameters, including the presence of forbidden regions for the optimal
trajectories and the possible occurrence of overshoots and undershoots in the
optimal trajectories. The numerical plots provided are obtained with the help
of MATHEMATICA.",1202.4205v2
2017-06-13,Dynamical storage of quantum states,"We consider a dynamical method of storage of quantum states based on the
spin-1/2 systems with the dipole-dipole interactions in a strong external
magnetic field { supplemented with the special time-reversion procedure}. The
stored information can be extracted at certain time instants.",1706.03945v1
2020-11-14,"Dynamical Effective Field Model for Interacting Ferrofluids: I. Derivations for homogeneous, inhomogeneous, and polydisperse cases","Quite recently I have proposed a nonperturbative dynamical effective field
model (DEFM) to quantitatively describe the dynamics of interacting
ferrofluids. Its predictions compare very well with the results from
simulations. In this paper I put the DEFM on firm theoretical ground by
deriving it within the framework of dynamical density functional theory (DDFT),
in which the relevant part of correlation-induced free energy is approximated
by a function of the instantaneous magnetization. The DEFM is generalized to
inhomogeneous finite-size samples for which the macroscopic and mesoscopic
scale separation is nontrivial due to the presence of long-range dipole-dipole
interactions. The demagnetizing field naturally emerges from microscopic
considerations and is consistently accounted for. The resulting particle
dynamics on the mesoscopic scale only involves macroscopically local quantities
such as local magnetization and Maxwell field. Nevertheless, the local
demagnetizing field essentially couples to magnetization at distant macroscopic
locations. Thus, a two-scale parallel algorithm, involving information transfer
between different macroscopic locations, can be applied to fully resolve
particle rotational dynamics in an inhomogeneous sample. I also derive the DEFM
for polydisperse ferrofluids, in which the dynamics of particles belonging to
different species can be strongly coupled to each other. I discuss the
underlying assumptions in obtaining a thermodynamically consistent polydisperse
magnetization relaxation equation, which is of the same generic form as that
for monodisperse ferrofluids. The theoretical advances presented in this paper
are important for both qualitative understanding and quantitative modeling of
ferrofluid dynamics.",2011.07287v2
2006-10-06,Axisymmetric oscillations of magnetic neutron stars,"We calculate axisymmetric oscillations of rotating neutron stars composed of
the surface fluid ocean, solid crust, and fluid core, taking account of a
dipole magnetic field as strong as $B_S\sim 10^{15}$G at the surface. The
adiabatic oscillation equations for the solid crust threaded by a dipole
magnetic field are derived in Newtonian dynamics, on the assumption that the
axis of rotation is aligned with the magnetic axis so that perturbations on the
equilibrium can be represented by series expansions in terms of spherical
harmonic functions $Y_l^m(\theta,\phi)$ with different degrees $l$ for a given
azimuthal wave number $m$ around the the magnetic axis. Although the three
component models can support a rich variety of oscillation modes, axisymmetric
($m=0$) toroidal $_{l}t_n$ and spheroidal $_ls_n$ shear waves propagating in
the solid crust are our main concerns, where $l$ and $n$ denote the harmonic
degree and the radial order of the modes, respectively. In the absence of
rotation, axisymmetric spheroidal and toroidal modes are completely decoupled,
and we consider the effects of rotation on the oscillation modes only in the
limit of slow rotation. We find that the oscillation frequencies of the
fundamental toroidal torsional modes $_{l}t_n$ in the crust are hardly affected
by the magnetic field as strong as $B_S\sim 10^{15}$G at the surface. As the
radial order $n$ of the shear modes in the crust becomes higher, however, both
spheroidal and toroidal modes become susceptible to the magnetic field and
their frequencies in general get higher with increasing $B_S$. We also find
that the surface $g$ modes and the crust/ocean interfacial modes are suppressed
by a strong magnetic field, and that there appear magnetic modes in the
presence of a strong magnetic field.",0610182v1
2006-10-18,Magnetic Properties and Metastable States in Spin-Crossover Transition of Co-Fe Prussian Blue Analogues,"The combination of spin transitions and magnetic ordering provides an
interesting structure of phase transitions in Prussian blue analogues (PBAs).
To understand the structure of stable and metastable states of Co-Fe PBA, it is
necessary to clarify free energy as a function of magnetization and the
fraction of the high-temperature component. Including the magnetic interaction
between high-temperature states, we study the magnetic phase transition of
Co-Fe PBA in addition to spin transitions. Here, we take into account the
degeneracy changes due to charge transfer between Co and Fe atoms accompanying
the spin transition. In this study, the charge transfer between Co and Fe atoms
is explicitly taken into account and also the ferrimagnetic structure of Co-Fe
PBAs is expressed in the proper way. First, we found systematic changes in the
structures of stable and metastable states as functions of system parameters
using mean field theory. In particular, the existence of a metastable
magnetic-ordered high-temperature state is confirmed at temperatures lower than
that of the hysteresis region of spin transitions. Second, we found that the
magnetic interaction causes complex ordering processes of a spin transition and
a magnetic phase transition. The effect of a magnetic field on the phase
structure is also investigated and we found metamagnetic magnetization
processes. Finally, the dynamical properties of this metastable state are
studied by Monte Carlo method.",0610500v2
2005-05-26,"Shell to shell energy transfer in MHD, Part II: Kinematic dynamo","We study the transfer of energy between different scales for forced
three-dimensional MHD turbulent flows in the kinematic dynamo regime. Two
different forces are examined: a non-helical Taylor Green flow with magnetic
Prandtl number P_M=0.4, and a helical ABC flow with P_M=1. This analysis allows
us to examine which scales of the velocity flow are responsible for dynamo
action, and identify which scales of the magnetic field receive energy directly
from the velocity field and which scales receive magnetic energy through the
cascade of the magnetic field from large to small scales. Our results show that
the turbulent velocity fluctuations are responsible for the magnetic field
amplification in the small scales (small scale dynamo) while the large scale
field is amplified mostly due to the large scale flow. A direct cascade of the
magnetic field energy from large to small scales is also present and is a
complementary mechanism for the increase of the magnetic field in the small
scales. Input of energy from the velocity field in the small magnetic scales
dominates over the energy that is cascaded down from the large scales until the
large-scale peak of the magnetic energy spectrum is reached. At even smaller
scales, most of the magnetic energy input is from the cascading process.",0505189v1
2005-10-30,Measurements of the magnetic field induced by a turbulent flow of liquid metal,"Initial results from the Madison Dynamo Experiment provide details of the
inductive response of a turbulent flow of liquid sodium to an applied magnetic
field. The magnetic field structure is reconstructed from both internal and
external measurements. A mean toroidal magnetic field is induced by the flow
when an axial field is applied, thereby demonstrating the omega effect.
Poloidal magnetic flux is expelled from the fluid by the poloidal flow.
Small-scale magnetic field structures are generated by turbulence in the flow.
The resulting magnetic power spectrum exhibits a power-law scaling consistent
with the equipartition of the magnetic field with a turbulent velocity field.
The magnetic power spectrum has an apparent knee at the resistive dissipation
scale. Large-scale eddies in the flow cause significant changes to the
instantaneous flow profile resulting in intermittent bursts of non-axisymmetric
magnetic fields, demonstrating that the transition to a dynamo is not smooth
for a turbulent flow.",0510265v3
2006-02-17,"Numerical simulations of current generation and dynamo excitation in a mechanically-forced, turbulent flow","The role of turbulence in current generation and self-excitation of magnetic
fields has been studied in the geometry of a mechanically driven, spherical
dynamo experiment, using a three dimensional numerical computation. A simple
impeller model drives a flow which can generate a growing magnetic field,
depending upon the magnetic Reynolds number, Rm, and the fluid Reynolds number.
When the flow is laminar, the dynamo transition is governed by a simple
threshold in Rm, above which a growing magnetic eigenmode is observed. The
eigenmode is primarily a dipole field tranverse to axis of symmetry of the
flow. In saturation the Lorentz force slows the flow such that the magnetic
eigenmode becomes marginally stable. For turbulent flow, the dynamo eigenmode
is suppressed. The mechanism of suppression is due to a combination of a time
varying large-scale field and the presence of fluctuation driven currents which
effectively enhance the magnetic diffusivity. For higher Rm a dynamo reappears,
however the structure of the magnetic field is often different from the laminar
dynamo; it is dominated by a dipolar magnetic field which is aligned with the
axis of symmetry of the mean-flow, apparently generated by fluctuation-driven
currents. The fluctuation-driven currents have been studied by applying a weak
magnetic field to laminar and turbulent flows. The magnetic fields generated by
the fluctuations are significant: a dipole moment aligned with the symmetry
axis of the mean-flow is generated similar to those observed in the experiment,
and both toroidal and poloidal flux expulsion are observed.",0602126v2
2007-08-07,Spin Pumping of Current in Non-Uniform Conducting Magnets,"Using irreversible thermodynamics we show that current-induced spin transfer
torque within a magnetic domain implies spin pumping of current within that
domain. This has experimental implications for samples both with conducting
leads and that are electrically isolated. These results are obtained by
deriving the dynamical equations for two models of non-uniform conducting
magnets: (1) a generic conducting magnet, with net conduction electron density
n and net magnetization $\vec{M}$; and (2) a two-band magnet, with up and down
spins each providing conduction and magnetism. For both models, in regions
where the equilibrium magnetization is non-uniform, voltage gradients can drive
adiabatic and non-adiabatic bulk spin torques. Onsager relations then ensure
that magnetic torques likewise drive adiabatic and non-adiabatic currents --
what we call bulk spin pumping. For a given amount of adiabatic and
non-adiabatic spin torque, the two models yield similar but distinct results
for the bulk spin pumping, thus distinguishing the two models. As in the recent
spin-Berry phase study by Barnes and Maekawa, we find that within a domain wall
the ratio of the effective emf to the magnetic field is approximately given by
$P(2\mu_{B}/e)$, where P is the spin polarization. The adiabatic spin torque
and spin pumping terms are shown to be dissipative in nature.",0708.0997v1
2007-09-04,Magnetic Braking and Protostellar Disk Formation: The Ideal MHD Limit,"Magnetic fields are usually considered dynamically important in star
formation when the dimensionless mass-to-flux ratio is close to, or less than,
unity (lambda<~1). We show that, in disk formation, the requirement is far less
stringent. This conclusion is drawn from a set of 2D (axisymmetric) simulations
of the collapse of rotating, magnetized, singular isothermal cores. We find
that a weak field corresponding to 1ambda~100 can begin to disrupt the
rotationally supported disk through magnetic braking, by creating regions of
rapid, supersonic collapse in the disk. These regions are separated by one or
more centrifugal barriers, where the rapid infall is temporarily halted. The
number of centrifugal barriers increases with lambda. When lambda>~100, they
merge together to form a more or less contiguous, rotationally supported disk.
Even though the magnetic field in such a case is extremely weak on the scale of
dense cores, it is amplified by collapse and differential rotation, to the
extent that its pressure dominates the thermal pressure in both the disk and
its surrounding region. For relatively strongly magnetized cores with
lambda<~10, the disk formation is suppressed completely, as found previously. A
new feature is that the mass accretion is highly episodic, due to reconnection
of the accumulated magnetic field lines. For rotationally supported disks to
appear during the protostellar mass accretion phase of star formation in dense
cores with realistic field strengths, the powerful magnetic brake must be
weakened, perhaps through nonideal MHD effects and/or protostellar winds. We
discuss the possibility of observing a generic product of the magnetic braking,
an extended circumstellar region that is supported by a combination of toroidal
magnetic field and rotation - a ""magnetogyrosphere"".",0709.0445v1
2009-02-18,Magnetized massive stars as magnetar progenitors,"The origin of ultra-intense magnetic fields on magnetars is a mystery in
modern astrophysics. We model the core collapse dynamics of massive progenitor
stars with high surface magnetic fields in the theoretical framework of a
self-similar general polytropic magnetofluid under the self-gravity with a
quasi-spherical symmetry. With the specification of physical parameters such as
mass density, temperature, magnetic field and wind mass loss rate on the
progenitor stellar surface and the consideration of a rebound shock breaking
through the stellar interior and envelope, we find a remnant compact object
(i.e. neutron star) left behind at the centre with a radius of $\sim 10^6$ cm
and a mass range of $\sim 1-3$ solar masses. Moreover, we find that surface
magnetic fields of such kind of compact objects can be $\sim 10^{14}-10^{15}$
G, consistent with those inferred for magnetars which include soft gamma-ray
repeaters (SGRs) and anomalous X-ray pulsars (AXPs). The magnetic field
enhancement factor critically depends on the self-similar scaling index $n$,
which also determines the initial density distribution of the massive
progenitor. We propose that magnetized massive stars as magnetar progenitors
based on the magnetohydrodynamic evolution of the gravitational core collapse
and rebound shock. Our physical mechanism, which does not necessarily require
ad hoc dynamo amplification within a fast spinning neutron star, favours the
`fossil field' scenario of forming magnetars from the strongly magnetized core
collapse inside massive progenitor stars. With a range of surface magnetic
field strengths over massive progenitor stars, our scenario allows a continuum
of magnetic field strengths from pulsars to magnetars.",0902.3111v1
2009-11-24,Magnetic Rossby waves in the solar tachocline and Rieger-type periodicities,"Apart from the 11-year solar cycle, another periodicity around 155-160 days
was discovered during solar cycle 21 in high energy solar flares, and its
presence in sunspot areas and strong magnetic flux has been also reported. This
periodicity has an elusive and enigmatic character, since it usually appears
only near the maxima of solar cycles, and seems to be related with a periodic
emergence of strong magnetic flux at the solar surface. Therefore, it is
probably connected with the tachocline, a thin layer located near the base of
the solar convection zone, where strong dynamo magnetic field is stored. We
study the dynamics of Rossby waves in the tachocline in the presence of a
toroidal magnetic field and latitudinal differential rotation. Our analysis
shows that the magnetic Rossby waves are generally unstable and that the growth
rates are sensitive to the magnetic field strength and to the latitudinal
differential rotation parameters. Variation of the differential rotation and
the magnetic field strength throughout the solar cycle enhance the growth rate
of a particular harmonic in the upper part of the tachocline around the maximum
of the solar cycle. This harmonic is symmetric with respect to the equator and
has a period of 155-160 days. A rapid increase of the wave amplitude could give
place to a magnetic flux emergence leading to observed periodicities in solar
activity indicators related with magnetic flux.",0911.4591v1
2010-07-02,Core of the Magnetic Obstacle,"Rich recirculation patterns have been recently discovered in the electrically
conducting flow subject to a local external magnetic termed ""the magnetic
obstacle"" [Phys. Rev. Lett. 98 (2007), 144504]. This paper continues the study
of magnetic obstacles and sheds new light on the core of the magnetic obstacle
that develops between magnetic poles when the intensity of the external field
is very large. A series of both 3D and 2D numerical simulations have been
carried out, through which it is shown that the core of the magnetic obstacle
is streamlined both by the upstream flow and by the induced cross stream
electric currents, like a foreign insulated insertion placed inside the
ordinary hydrodynamic flow. The closed streamlines of the mass flow resemble
contour lines of electric potential, while closed streamlines of the electric
current resemble contour lines of pressure. New recirculation patterns not
reported before are found in the series of 2D simulations. These are composed
of many (even number) vortices aligned along the spanwise line crossing the
magnetic gap. The intensities of these vortices are shown to vanish toward to
the center of the magnetic gap, confirming the general conclusion of 3D
simulations that the core of the magnetic obstacle is frozen. The implications
of these findings for the case of turbulent flow are discussed briefly.",1007.0336v1
2010-10-28,Global magnetic cycles in rapidly rotating younger suns,"Observations of sun-like stars rotating faster than our current sun tend to
exhibit increased magnetic activity as well as magnetic cycles spanning
multiple years. Using global simulations in spherical shells to study the
coupling of large-scale convection, rotation, and magnetism in a younger sun,
we have probed effects of rotation on stellar dynamos and the nature of
magnetic cycles. Major 3-D MHD simulations carried out at three times the
current solar rotation rate reveal hydromagnetic dynamo action that yields
wreaths of strong toroidal magnetic field at low latitudes, often with opposite
polarity in the two hemispheres. Our recent simulations have explored behavior
in systems with considerably lower diffusivities, achieved with sub-grid scale
models including a dynamic Smagorinsky treatment of unresolved turbulence. The
lower diffusion promotes the generation of magnetic wreaths that undergo
prominent temporal variations in field strength, exhibiting global magnetic
cycles that involve polarity reversals. In our least diffusive simulation, we
find that magnetic buoyancy coupled with advection by convective giant cells
can lead to the rise of coherent loops of magnetic field toward the top of the
simulated domain.",1010.6073v1
2011-07-18,Rossby waves and polar spots in rapidly rotating stars: Implications for stellar wind evolution,"Rapidly rotating stars show short-period oscillations in magnetic activity
and polar appearance of starspots. The aim of this paper is to study
large-scale shallow water waves in the tachoclines of rapidly rotating stars
and their connection to the periodicity and the formation of starspots at high
latitudes. Shallow-water magnetohydrodynamic equations were used to study the
dynamics of large-scale waves at the rapidly rotating stellar tachoclines in
the presence of toroidal magnetic field. Dispersion relations and latitudinal
distribution of wave modes were derived. We found that low-frequency magnetic
Rossby waves tend to be located at poles, but high-frequency magnetic Poincare
waves are concentrated near the equator in rapidly rotating stars. These
results have important implications for the evolution of the stellar wind in
young Sun-like stars. Unstable magnetic Rossby waves may lead to the local
enhancement of magnetic flux at high latitudes of tachoclines in rapidly
rotating stars. The enhanced magnetic flux may rise upwards owing to the
magnetic buoyancy in the form of tubes and appear as starspots at polar
regions. Magnetic Rossby waves may also cause observed short-term periodicity
in the stellar magnetic activity.",1107.3395v1
2011-09-21,Magnetic Field Amplification by Small-Scale Dynamo Action: Dependence on Turbulence Models and Reynolds and Prandtl Numbers,"The small-scale dynamo is a process by which turbulent kinetic energy is
converted into magnetic energy, and thus is expected to depend crucially on the
nature of turbulence. In this work, we present a model for the small-scale
dynamo that takes into account the slope of the turbulent velocity spectrum
v(l) ~ l^theta, where l and v(l) are the size of a turbulent fluctuation and
the typical velocity on that scale. The time evolution of the fluctuation
component of the magnetic field, i.e., the small-scale field, is described by
the Kazantsev equation. We solve this linear differential equation for its
eigenvalues with the quantum-mechanical WKB-approximation. The validity of this
method is estimated as a function of the magnetic Prandtl number Pm. We
calculate the minimal magnetic Reynolds number for dynamo action, Rm_crit,
using our model of the turbulent velocity correlation function. For Kolmogorov
turbulence (theta=1/3), we find that the critical magnetic Reynolds number is
approximately 110 and for Burgers turbulence (theta=1/2) approximately 2700.
Furthermore, we derive that the growth rate of the small-scale magnetic field
for a general type of turbulence is Gamma ~ Re^((1-theta)/(1+theta)) in the
limit of infinite magnetic Prandtl numbers. For decreasing magnetic Prandtl
number (down to Pm approximately larger than 10), the growth rate of the
small-scale dynamo decreases. The details of this drop depend on the
WKB-approximation, which becomes invalid for a magnetic Prandtl number of about
unity.",1109.4571v2
2012-04-03,Magnetic moment non-conservation in magnetohydrodynamic turbulence models,"The fundamental assumptions of the adiabatic theory do not apply in presence
of sharp field gradients as well as in presence of well developed
magnetohydrodynamic turbulence. For this reason in such conditions the magnetic
moment $\mu$ is no longer expected to be constant. This can influence particle
acceleration and have considerable implications in many astrophysical problems.
  Starting with the resonant interaction between ions and a single parallel
propagating electromagnetic wave, we derive expressions for the magnetic moment
trapping width $\Delta \mu$ (defined as the half peak-to-peak difference in the
particle magnetic moment) and the bounce frequency $\omega_b$. We perform
test-particle simulations to investigate magnetic moment behavior when
resonances overlapping occurs and during the interaction of a ring-beam
particle distribution with a broad-band slab spectrum.
  We find that magnetic moment dynamics is strictly related to pitch angle
$\alpha$ for a low level of magnetic fluctuation, $\delta B/B_0 = (10^{-3}, \,
10^{-2})$, where $B_0$ is the constant and uniform background magnetic field.
Stochasticity arises for intermediate fluctuation values and its effect on
pitch angle is the isotropization of the distribution function $f(\alpha)$.
This is a transient regime during which magnetic moment distribution $f(\mu)$
exhibits a characteristic one-sided long tail and starts to be influenced by
the onset of spatial parallel diffusion, i.e., the variance $<(\Delta z)^2 >$
grows linearly in time as in normal diffusion. With strong fluctuations
$f(\alpha)$ isotropizes completely, spatial diffusion sets in and $f(\mu)$
behavior is closely related to the sampling of the varying magnetic field
associated with that spatial diffusion.",1204.0748v1
2013-09-27,Phonons correlation with magnetic excitations in weak ferromagnet YCrO3,"We report on the temperature dependent Raman spectroscopic studies of
orthorombic distorted perovskite YCrO3 in the temperature range of 20-300K.
Temperature dependence of DC-magnetization measurement under field cooled and
zero field cooled modes confirmed the transition temperature (TN ~142K) and
anomalous characteristic temperature (T* ~60K), above which magnetization tends
to saturate. Magnetization isotherm recorded below TN at 125K shows clear loop
opening without magnetization saturation up to 20kOe, indicating the
coexistence of antiferromagnetic (AFM) interaction with weak ferromagnetic
(WFM) phase. Mean field calculation for exchange constants further confirms the
complex magnetic phase below TN. Temperature evolution of lineshape parameters
of selected modes (associated with the octahedral rotation and A-shift in the
unit cell) revealed anomalous phonon shift near Cr3+ magnetic ordering
temperature (TN ~142K). Additional phonon anomaly was identified at T* ~60K in
agreement with the magnetization results and reflects the change in spin
dynamics, plausibly due to the change in Cr-spin configuration. Moreover, the
positive and negative shift in Raman frequency below TN revel the existence of
competing WFM and AFM exchanges. The phonon shift of B3g (3)-octahedral
rotation mode fairly scaled with the square of sublattice magnetization from TN
to T*, below which it start to depart from theconventional behaviour and need
further attention. This correlation between magnetic and Raman data elucidate
the spin-phonon coupling owing to the multiferroic phenomenon in YCrO3.",1309.7254v1
2013-11-05,Cluster glass magnetism in the phase-separated Nd2/3Ca1/3MnO3 perovskite,"A detailed study of the low-temperature magnetic state and the relaxation in
the phase-separated colossal magnetoresistance Nd2/3Ca1/3MnO3 perovskite has
been carried out. Clear experimental evidence of the cluster-glass magnetic
behavior of this compound has been revealed. Well defined maxima in the
in-phase linear ac susceptibility $\chi ^{/}(T)$ were observed, indicative of
the magnetic glass transition at $T_{g}$ = 60 K. Strongly divergent
zero-field-cooled and field-cooled static magnetizations and frequency
dependent ac susceptibility are evident of the glassy-like magnetic state of
the compound at low temperatures. The frequency dependence of the cusp
temperature Tmax of the $\chi ^{/}(T)$ susceptibility was found to follow the
critical slowing down mechanism. The Cole-Cole analysis of the dynamic
susceptibility at low temperature has shown extremely broad distribution of
relaxation times, indicating that spins are frozen at ""macroscopic"" time scale.
Slow relaxation in the zero-field-cooled magnetization has been experimentally
revealed. The obtained results do not agree with a canonical spin-glass state
and indicate a cluster glass magnetic state of the compound below $T_{g}$,
associated with its antiferromagnetic-ferromagnetic nano-phase segregated
state. It was found that the relaxation mechanisms below the cluster glass
freezing temperature $T_{g}$ and above it are strongly different. Magnetic
field up to about $\mu_{0}$H = 0.4 T suppresses the glassy magnetic state of
the compound.",1311.1123v2
2013-11-18,Kelvin-Helmholtz instability of twisted magnetic flux tubes in the solar wind,"Solar wind plasma is supposed to be structured in magnetic flux tubes carried
from the solar surface. Tangential velocity discontinuity near the boundaries
of individual tubes may result in Kelvin-Helmholtz instability, which may
contribute into the solar wind turbulence. While the axial magnetic field may
stabilize the instability, a small twist in the magnetic field may allow to
sub-Alfvenic motions to be unstable. We aim to study the Kelvin-Helmholtz
instability of twisted magnetic flux tube in the solar wind with different
configurations of external magnetic field. We use magnetohydrodynamic equations
in the cylindrical geometry and derive the dispersion equations governing the
dynamics of twisted magnetic flux tube moving along its axis in the cases of
untwisted and twisted external fields. Then we solve the dispersion equations
analytically and numerically and found thresholds for Kelvin-Helmholtz
instability in both cases of external field. Both analytical and numerical
solutions show that the Kelvin-Helmholtz instability is suppressed in the
twisted tube by external axial magnetic field for sub-Alfvenic motions.
However, even small twist in the external magnetic field allows the
Kelvin-Helmholtz instability to be developed for any sub-Alfvenic motions. The
unstable harmonics correspond to vortices with high azimuthal mode numbers,
which are carried by the flow. Twisted magnetic flux tubes can be unstable to
Kelvin-Helmholtz instability when they move with small speed relative to main
solar wind stream, then the Kelvin-Helmholtz vortices may significantly
contribute into the solar wind turbulence.",1311.4383v1
2014-07-10,Magnetic induction and diffusion mechanisms in a liquid sodium spherical Couette experiment,"We present a reconstruction of the mean axisymmetric azimuthal and meridional
flows in the DTS liquid sodium experiment. The experimental device sets a
spherical Couette flow enclosed between two concentric spherical shells where
the inner sphere holds a strong dipolar magnet, which acts as a magnetic
propeller when rotated. Measurements of the mean velocity, mean induced
magnetic field and mean electric potentials have been acquired inside and
outside the fluid for an inner sphere rotation rate of 9 Hz (Rm 28). Using the
induction equation to relate all measured quantities to the mean flow, we
develop a nonlinear least square inversion procedure to reconstruct a fully
coherent solution of the mean velocity field. We also include in our inversion
the response of the fluid layer to the non-axisymmetric time-dependent magnetic
field that results from deviations of the imposed magnetic field from an axial
dipole. The mean azimuthal velocity field we obtain shows super-rotation in an
inner region close to the inner sphere where the Lorentz force dominates, which
contrasts with an outer geostrophic region governed by the Coriolis force, but
where the magnetic torque remains the driver. The meridional circulation is
strongly hindered by the presence of both the Lorentz and the Coriolis forces.
Nevertheless, it contributes to a significant part of the induced magnetic
energy. Our approach sets the scene for evaluating the contribution of velocity
and magnetic fluctuations to the mean magnetic field, a key question for dynamo
mechanisms.",1407.2703v2
2015-06-06,Saturation of the Turbulent Dynamo,"The origin of strong magnetic fields in the Universe can be explained by
amplifying weak seed fields via turbulent motions on small spatial scales and
subsequently transporting the magnetic energy to larger scales. This process is
known as the turbulent dynamo and depends on the properties of turbulence, i.e.
on the hydrodynamical Reynolds number and the compressibility of the gas, and
on the magnetic diffusivity. While we know the growth rate the magnetic energy
in the linear regime, the saturation level, i.e. the ratio of magnetic energy
to turbulent kinetic energy that can be reached, is not known from analytical
calculations. In this paper we present the first scale-dependent saturation
model based on an effective turbulent resistivity which is determined by the
turnover timescale of turbulent eddies and the magnetic energy density. The
magnetic resistivity increases compared to the Spitzer value and the effective
scale on which the magnetic energy spectrum is at its maximum moves to larger
spatial scales. This process ends when the peak reaches a characteristic
wavenumber k* which is determined by the critical magnetic Reynolds number. The
saturation level of the dynamo also depends on the type of turbulence and
differs for the limits of large and small magnetic Prandtl numbers Pm. With our
model we find saturation levels between 43.8% and 1.3% for Pm>>1 and between
2.43% and 0.135% for Pm<<1, where the higher values refer to incompressible
turbulence and the lower ones to highly compressible turbulence.",1506.02182v2
2016-03-08,Magnetic fields in early protostellar disk formation,"We consider formation of accretion disks from a realistically turbulent
molecular gas using 3D MHD simulations. In particular, we analyze the effect of
the fast turbulent reconnection described by the Lazarian & Vishniac (1999)
model for the removal of magnetic flux from a disk. With our numerical
simulations we demonstrate how the fast reconnection enables protostellar disk
formation resolving the so-called ""magnetic braking catastrophe"". In
particular, we provide a detailed study of the dynamics of a 0.5 M$_\odot$
protostar and the formation of its disk for up to several thousands years. We
measure the evolution of the mass, angular momentum, magnetic field, and
turbulence around the star. We consider effects of two processes that strongly
affect the magnetic transfer of angular momentum, both of which are based on
turbulent reconnection: the first, ""reconnection diffusion"", removes the
magnetic flux from the disk, the other involves the change of the magnetic
field's topology, but does not change the absolute value of the magnetic flux
through the disk. We demonstrate that for the first mechanism, turbulence
causes a magnetic flux transport outward from the inner disk to the ambient
medium, thus decreasing the coupling of the disk to the ambient material. A
similar effect is achieved through the change of the magnetic field's topology
from a split monopole configuration to a dipole configuration. We explore how
both mechanisms prevent the catastrophic loss of disk angular momentum and
compare both above turbulent reconnection mechanisms with alternative
mechanisms from the literature.",1603.02691v1
2016-05-16,"Accretion, Outflows, and Winds of Magnetized Stars","Many types of stars have strong magnetic fields that can dynamically
influence the flow of circumstellar matter. In stars with accretion disks, the
stellar magnetic field can truncate the inner disk and determine the paths that
matter can take to flow onto the star. These paths are different in stars with
different magnetospheres and periods of rotation. External field lines of the
magnetosphere may inflate and produce favorable conditions for outflows from
the disk-magnetosphere boundary. Outflows can be particularly strong in the
propeller regime, wherein a star rotates more rapidly than the inner disk.
Outflows may also form at the disk-magnetosphere boundary of slowly rotating
stars, if the magnetosphere is compressed by the accreting matter. In isolated,
strongly magnetized stars, the magnetic field can influence formation and/or
propagation of stellar wind outflows. Winds from low-mass, solar-type stars may
be either thermally or magnetically driven, while winds from massive, luminous
O and B type stars are radiatively driven. In all of these cases, the magnetic
field influences matter flow from the stars and determines many observational
properties. In this chapter we review recent studies of accretion, outflows,
and winds of magnetized stars with a focus on three main topics: (1) accretion
onto magnetized stars; (2) outflows from the disk-magnetosphere boundary; and
(3) winds from isolated massive magnetized stars. We show results obtained from
global magnetohydrodynamic simulations and, in a number of cases compare global
simulations with observations.",1605.04979v1
2016-05-19,"Magnetic field in atypical prominence structures: Bubble, tornado and eruption","Spectropolarimetric observations of prominences have been obtained with the
THEMIS telescope during four years of coordinated campaigns. Our aim is now to
understand the conditions of the cool plasma and magnetism in `atypical'
prominences, namely when the measured inclination of the magnetic field
departs, to some extent, from the predominantly horizontal field found in
`typical' prominences. What is the role of the magnetic field in these
prominence types? Are plasma dynamics more important in these cases than the
magnetic support? We focus our study on three types of `atypical' prominences
(tornadoes, bubbles and jet-like prominence eruptions) that have all been
observed by THEMIS in the He I D_3 line, from which the Stokes parameters can
be derived. The magnetic field strength, inclination and azimuth in each pixel
are obtained by using the Principal Component Analysis inversion method on a
model of single scattering in the presence of the Hanle effect. The magnetic
field in tornadoes is found to be more or less horizontal, whereas for the
eruptive prominence it is mostly vertical. We estimate a tendency towards
higher values of magnetic field strength inside the bubbles than outside in the
surrounding prominence. In all of the models in our database, only one magnetic
field orientation is considered for each pixel. While sufficient for most of
the main prominence body, this assumption appears to be oversimplified in
atypical prominence structures. We should consider these observations as the
result of superposition of multiple magnetic fields, possibly even with a
turbulent field component.",1605.05964v1
2016-06-22,The global distribution of magnetic helicity in the solar corona,"By defining an appropriate field line helicity, we apply the powerful concept
of magnetic helicity to the problem of global magnetic field evolution in the
Sun's corona. As an ideal-magnetohydrodynamic invariant, the field line
helicity is a meaningful measure of how magnetic helicity is distributed within
the coronal volume. It may be interpreted, for each magnetic field line, as a
magnetic flux linking with that field line. Using magneto-frictional
simulations, we investigate how field line helicity evolves in the
non-potential corona as a result of shearing by large-scale motions on the
solar surface. On open magnetic field lines, the helicity injected by the Sun
is largely output to the solar wind, provided that the coronal relaxation is
sufficiently fast. But on closed magnetic field lines, helicity is able to
build up. We find that the field line helicity is non-uniformly distributed,
and is highly concentrated in twisted magnetic flux ropes. Eruption of these
flux ropes is shown to lead to sudden bursts of helicity output, in contrast to
the steady flux along the open magnetic field lines.",1606.06863v3
2016-12-31,Experimental demonstration of complete 180 degree reversal of magnetization in isolated Co-nanomagnets on a PMN-PT substrate with voltage generated strain,"Rotating the magnetization of a shape anisotropic magnetostrictive nanomagnet
with voltage-generated stress/strain dissipates much less energy than most
other magnetization rotation schemes, but its application to writing bits in
non-volatile magnetic memory has been hindered by the fundamental inability of
stress/strain to rotate magnetization by full 180 degrees. Normally,
stress/strain can rotate the magnetization of a shape anisotropic elliptical
nanomagnet by only up to 90 degrees, resulting in incomplete magnetization
reversal. Recently, we predicted that applying uniaxial stress sequentially
along two different axes that are not collinear with the major or minor axis of
the elliptical nanomagnet will rotate the magnetization by full 180 degrees.
Here, we demonstrate this complete 180 degree rotation in elliptical
Co-nanomagnets (fabricated on a piezoelectric substrate) at room temperature.
The two stresses are generated by sequentially applying voltages to two pairs
of shorted electrodes placed on the substrate such that the line joining the
centers of the electrodes in one pair intersects the major axis of a nanomagnet
at ~+30 degrees and the line joining the centers of the electrodes in the other
pair intersects at ~ -30 degrees. A finite element analysis has been performed
to determine the stress distribution underneath the nanomagnets when one or
both pairs of electrodes are activated, and this has been approximately
incorporated into a micromagnetic simulation of magnetization dynamics to
confirm that the generated stress can produce the observed magnetization
rotations. This result portends an extremely energy-efficient non-volatile
""straintronic"" memory technology predicated on writing bits in nanomagnets with
electrically generated stress.",1701.00042v1
2017-02-28,"Photon-Axion Conversion, Magnetic Field Configuration, and Polarization of Photons","We study the evolution of photon polarization during the photon-axion
conversion process with focusing on the magnetic field configuration
dependence. Most previous studies have been carried out in a conventional model
where a network of magnetic domains is considered and each domain has a
constant magnetic field. We investigate a more general model where a network of
domains is still assumed, but each domain has a helical magnetic field. We find
that the asymptotic behavior does not depend on the configuration of magnetic
fields. Remarkably, we analytically obtain the asymptotic values of the
variance of polarization in the conventional model. When the helicity is small,
we show that there appears the damped oscillating behavior in the early stage
of evolution. Moreover, we see that the constraints on the axion coupling and
the cosmological magnetic fields using polarization observations are affected
by the magnetic field configuration. This is because the different transient
behavior of polarization dynamics is caused by the different magnetic field
configuration. Recently, [C. Wang and D. Lai, J. Cosmol. Astropart. Phys. 06
(2016) 006.] claimed that the photon-axion conversion in helical model behaves
peculiarly. However, our helical model gives much closer predictions to the
conventional discontinuous magnetic field configuration model.",1702.08843v2
2017-05-26,Compression of turbulent magnetized gas in Giant Molecular Clouds,"Interstellar gas clouds are often both highly magnetized and supersonically
turbulent, with velocity dispersions set by a competition between driving and
dissipation. This balance has been studied extensively in the context of gases
with constant mean density. However, many astrophysical systems are contracting
under the influence of external pressure or gravity, and the balance between
driving and dissipation in a contracting, magnetized medium has yet to be
studied. In this paper we present three-dimensional (3D) magnetohydrodynamic
(MHD) simulations of compression in a turbulent, magnetized medium that
resembles the physical conditions inside molecular clouds. We find that in some
circumstances the combination of compression and magnetic fields leads to a
rate of turbulent dissipation far less than that observed in non-magnetized
gas, or in non-compressing magnetized gas. As a result, a compressing,
magnetized gas reaches an equilibrium velocity dispersion much greater than
would be expected for either the hydrodynamic or the non-compressing case. We
use the simulation results to construct an analytic model that gives an
effective equation of state for a coarse-grained parcel of the gas, in the form
of an ideal equation of state with a polytropic index that depends on the
dissipation and energy transfer rates between the magnetic and turbulent
components. We argue that the reduced dissipation rate and larger equilibrium
velocity dispersion produced by compressing, magnetized turbulence has
important implications for the driving and maintenance of turbulence in
molecular clouds, and for the rates of chemical and radiative processes that
are sensitive to shocks and dissipation.",1705.09657v2
2017-08-14,Solar Multiple Eruptions From a Confined Magnetic Structure,"How eruption can recur from a confined magnetic structure is discussed based
on the {\it Solar Dynamics Observatory} (SDO) observations of the NOAA active
region 11444, which produced three eruptions within 1.5 hours on March 27,
2012. The active region had the positive polarity magnetic fields in the center
surrounded by the negative polarity fields around. Since such a distribution of
magnetic polarity tends to form a dome-like magnetic fan structure confined
over the active region, the multiple eruptions was puzzling. Our investigation
reveals that this event exhibits several properties distinct from other
eruptions associated with magnetic fan structures: (i) a long filament
encircling the active region was present before the eruptions; (ii) expansion
of the open-closed boundary of the field lines after each eruption suggestive
of the growing fan-dome structure, and (iii) the ribbons inside the closed
magnetic polarity inversion line evolving in response to the expanding
open-closed boundary. It thus appears that in spite of multiple eruptions the
fan-dome structure remained undamaged, and the closing back field lines after
each eruption rather reinforced the fan-dome structure. We argue that the
multiple eruptions could occur in this active region in spite of its confined
magnetic structure because the filament encircling the active region was
adequate for slipping through the magnetic separatrix to minimize the damage to
its overlying fan-dome structure. The result of this study provides a new
insight into the productivity of eruptions from a confined magnetic structure.",1708.04056v1
2017-10-05,Transport theory for femtosecond laser-induced spin-transfer torques,"Ultrafast demagnetization of magnetic layers pumped by a femtosecond laser
pulse is accompanied by a nonthermal spin-polarized current of hot electrons.
These spin currents are studied here theoretically in a spin valve with
noncollinear magnetizations. To this end, we introduce an extended model of
superdiffusive spin transport that enables to treat noncollinear magnetic
configurations, and apply it to the perpendicular spin valve geometry. We show
how spin-transfer torques arise due to this mechanism and calculate their
action on the magnetization present, as well as how the latter depends on the
thicknesses of the layers and other transport parameters. We demonstrate that
there exists a certain optimum thickness of the out-of-plane magnetized
spin-current polarizer such that the torque acting on the second magnetic layer
is maximal. Moreover, we study the magnetization dynamics excited by the
superdiffusive spin-transfer torque due to the flow of hot electrons employing
the Landau-Lifshitz-Gilbert equation. Thereby we show that a femtosecond laser
pulse applied to one magnetic layer can excite small-angle precessions of the
magnetization in the second magnetic layer. We compare our calculations with
recent experimental results.",1710.02083v2
2017-10-13,The first in-situ observation of torsional Alfven waves during the interaction of large-scale magnetic clouds,"The large-scale magnetic cloud such as coronal mass ejections (CMEs) is the
fundamental driver of the space weather. The interaction of the multiple CMEs
in interplanetary space affects their dynamic evolution and geo-effectiveness.
The complex and merged multiple magnetic clouds appear as the in-situ signature
of the interacting CMEs. The Alfven waves are speculated to be one of the major
possible energy exchange/dissipation mechanism during the interaction. However,
no such observational evidence has been found in the literature. The case
studies of CME-CME collision events suggest that the magnetic and thermal
energy of the CME is converted into the kinetic energy. Moreover, the magnetic
reconnection process is justified to be responsible for the merging of multiple
magnetic clouds. Here, we present unambiguous evidence of sunward torsional
Alfven waves in the interacting region after the super-elastic collision of
multiple CMEs. The Walen relation is used to confirm the presence of Alfven
waves in the interacting region of multiple CMEs/magnetic clouds. We conclude
that Alfven waves and magnetic reconnection are the possible energy
exchange/dissipation mechanisms during large-scale magnetic clouds collisions.
The present study has significant implications not only in CME-magnetosphere
interactions but also in the interstellar medium where interactions of
large-scale magnetic clouds are possible.",1710.05755v2
2018-01-13,"Probing Twisted Magnetic Field Using Microwave Observations in an M Class Solar Flare on 11 February, 2014","This work demonstrates the possibility of magnetic field topology
investigations using microwave polarimetric observations. We study a solar
flare of GOES M1.7 class that occurred on 11 February, 2014. This flare
revealed a clear signature of spatial inversion of the radio emission
polarization sign. We show that the observed polarization pattern can be
explained by nonthermal gyrosynchrotron emission from the twisted magnetic
structure. Using observations of the Reuven Ramaty High Energy Solar
Spectroscopic Imager, Nobeyama Radio Observatory, Radio Solar Telescope
Network, and Solar Dynamics Observatory, we have determined the parameters of
nonthermal electrons and thermal plasma and identified the magnetic structure
where the flare energy release occurred. To reconstruct the coronal magnetic
field, we use nonlinear force-free field (NLFFF) and potential magnetic field
approaches. Radio emission of nonthermal electrons is simulated by the GX
Simulator code using the extrapolated magnetic field and the parameters of
nonthermal electrons and thermal plasma inferred from the observations; the
model radio maps and spectra are compared with observations. We have found that
the potential magnetic field approach fails to explain the observed circular
polarization pattern; on the other hand, the Stokes $V$ map is successfully
explained by assuming nonthermal electrons to be distributed along the twisted
magnetic structure determined by the NLFFF extrapolation approach. Thus, we
show that the radio polarization maps can be used for diagnosing the topology
of the flare magnetic structures where nonthermal electrons are injected.",1801.04400v1
2018-03-23,Observation of hedgehog skyrmions in sub-100 nm soft magnetic nanodots,"Magnetic skyrmions are nanometric spin textures of outstanding potential for
spintronic applications due to unique features governed by their non-trivial
topology. It is well known that skyrmions of definite chirality are stabilized
by the Dzyaloshinskii-Moriya exchange interaction (DMI) in bulk
non-centrosimmetric materials or ultrathin films with strong spin-orbit
coupling in the interface. In this work, we report on the detection of magnetic
hedgehog-skyrmions at room temperature in confined systems with neither DMI nor
perpendicular magnetic anisotropy. We show that soft magnetic (permalloy)
nanodots are able to host non- chiral hedgehog skyrmions that can be further
stabilized by the magnetic field arising from the Magnetic Force Microscopy
probe. Analytical calculations and micromagnetic simulations confirmed the
existence of metastable N\'eel skyrmions in permalloy nanodots even without
external stimuli in a certain size range. Our work implies the existence of a
new degree of freedom to create and manipulate skyrmions in soft nanodots. The
stabilization of skyrmions in soft magnetic materials opens a possibility to
study the skymion magnetization dynamics otherwise limited due to the large
damping constant coming from the high spin-orbit coupling in materials with
high magnetic anisotropy.",1803.08768v1
2018-05-15,"Flare Energy Release in the Magnetic Field Polarity Inversion Line During M1.2 Solar Flare of March 15, 2015. Paper I. Onset of Plasma Heating and Electrons Acceleration","We present the study of SOL2015-03-15 M1.2 flare, revealing acceleration of
electrons and plasma heating in the sheared twisted magnetic structure in the
polarity inversion line (PIL). The scope is to make the analysis of nonthermal
electrons dynamics and plasma heating in the highly stressed magnetic loops
interacting in the PIL by using X-ray, microwave, ultraviolet, and optical
observations. It is found that the most probable scenario for the energy
release in the PIL is the tether-cutting magnetic reconnection between the
low-lying (3 Mm above the photosphere) magnetic loops within a twisted magnetic
flux rope. Energetic electrons with the hardest spectrum were appeared at the
onset of plasma heating up to the super-hot temperature of 40 MK. These
electrons are localized in a thin magnetic channel with width of around 0.5 Mm
with high average magnetic field of about 1200 G. The plasma beta in the
super-hot region is less than 0.01. The estimated density of accelerated
electrons is about 10^9 cm^-3 that is much less than the super-hot plasma
density. The energy density flux of non-thermal electrons is estimated up to
3x10^12 ergs cm^-2s^-1 that is much higher than in the currently available
radiative hydrodynamic models. These results revealed that one need to develop
new self-consisting flare models reproducing 3D magnetic reconnection in the
PIL with strong magnetic field, spatial filamentation of energy release,
formation of high energy density populations of nonthermal electrons and
appearance of the super-hot plasma.",1805.05792v1
2018-05-24,Coronal Hole and Solar Global Magnetic Field Evolution in 1976-2012,"Coronal hole spatial-temporal evolution is studied and comparison made with
that of the solar global magnetic field in cycles 21-23 (1976-2012). The
latitude-longitude distribution dynamics of coronal holes and the regularities
in the global magnetic field associated with the solar polar field reversal are
analyzed. Polar and non-polar coronal hole populations are considered. The
investigation reveals some temporal and spatial regularities in coronal hole
distributions that match well the global magnetic-field cycle evolution. The
results show that the non-polar coronal hole longitudinal distribution follows
all configuration changes in the global magnetic-field structure.
Reorganizations of the global magnetic-field and coronal hole distributions
occur simultaneously during a time interval of a few solar rotations. The cycle
evolution of the non-polar coronal holes reflects the transition of the solar
global magnetic field from the zonal structure to sectorial andvice versa. Two
different type waves of non-polar coronal holes are revealed from their
latitudinal distribution. The first one is short poleward waves. They trace the
poleward motion of the unipolar photospheric magnetic fields from approximately
35 deg to the associated pole in each hemisphere and the redevelopment of a
new-polarity polar CH. Although they start the poleward movement before the
change of the polar magnetic field in the associated hemisphere, they reach the
pole after the polar reversal. The other type of non-polar CH wave forms two
sinusoidal branches associated with the positive- and negative-polarity
magnetic fields. The complete period of the wave was equal to approximately 268
CRs (22 years). These wave CHs arrive at high latitudes during declining phases
when the new polarity polar CHs are already completely formed.",1805.09543v1
2018-05-26,Distinct spin-lattice and spin-phonon interactions in monolayer magnetic CrI$_3$,"We apply the density-functional theory to study various phases (including
non-magnetic (NM), anti-ferromagnetic (AFM), and ferromagnetic (FM)) in
monolayer magnetic chromium triiodide (CrI$_3$), a recently fabricated 2D
magnetic material. It is found that: (1) the introduction of magnetism in
monolayer CrI$_3$ gives rise to metal-to-semiconductor transition; (2) the
electronic band topologies as well as the nature of direct and indirect band
gaps in either AFM or FM phases exhibit delicate dependence on the magnetic
ordering and spin-orbit coupling; and (3)the phonon modes involving Cr atoms
are particularly sensitive to the magnetic ordering, highlighting distinct
spin-lattice and spin-phonon coupling in this magnet. First-principles
simulations of the Raman spectra demonstrate that both frequencies and
intensities of the Raman peaks strongly depend on the magnetic ordering. The
polarization dependent $A_{1g}$ modes at 77 cm$^{-1}$ and 130 cm$^{-1}$ along
with the $E_g$ mode at about 50 cm$^{-1}$ in the FM phase may offer a useful
fingerprint to characterize this material. Our results not only provide a
detailed guiding map for experimental characterization of CrI$_3$, but also
reveal how the evolution of magnetism can be tracked by its lattice dynamics
and Raman response.",1805.10479v2
2018-08-14,Magnetic Nanoparticle Based Molecular Communication in Microfluidic Environments,"The possibility to guide and control magnetic nanoparticles in a non-invasive
manner has spawned various applications in biotechnology such as targeted drug
delivery and sensing of biological substances. These applications are
facilitated by the engineering of the size, selective chemical reactivity, and
general chemical composition of the employed particles. Motivated by their
widespread use and favorable properties, in this paper, we provide a
theoretical study of the potential benefits of magnetic nanoparticles for the
design of molecular communication systems. In particular, we consider magnetic
nanoparticle based communication in a microfluidic channel where an external
magnetic field is employed to attract the information-carrying particles to the
receiver. We show that the particle transport affected by Brownian motion,
fluid flow, and an external magnetic field can be mathematically modeled as
diffusion with drift. Thereby, we reveal that the key parameters determining
the magnetic force are the particle size and the magnetic field gradient.
Moreover, we derive an analytical expression for the channel impulse response,
which is used to evaluate the potential gain in the expected number of observed
nanoparticles due to the magnetic field. Furthermore, adopting the symbol error
rate as performance metric, we show that using magnetic nanoparticles can
enable reliable communication in the presence of disruptive fluid flow.
Numerical results obtained by particle-based simulation validate the accuracy
of the derived analytical expressions.",1808.05147v2
2018-08-30,Growth Pattern of Magnetic Field Treated Bacteria,"A study of the induced effect of different types of weak magnetic field
exposure on bacterial growth is performed, comparing the relative changes after
removal from the magnetic fields. This investigation is relevant to understand
the effect of magnetic field exposure on human beings due to electronic
devices. For this purpose, we use four species of common bacteria in reference
to human health and safety including Escherichia coli, Staphylococcus aureus,
Staphylococcus epidermidis, and Pseudomonas aeruginosa. The choice of these
four bacteria also allows us to check for effects which rely upon the
Gram-staining properties or shapes of bacterial species. These species were
initially exposed to static, non-homogeneous and alternating weak magnetic
fields, and then they were grown in incubators in the same environment at 37
{\deg}C simultaneously. Comparative measurements of optical density are then
used to track the sustained impact on bacterial growth in the experimental
samples. Bacteria was first grown in different weak magnetic fields on a plain
glass surface both in liquid and solid media. Magnetic field treated bacteria
were then transferred into similar test tubes to grow in an incubator
concurrently. Bacterial cultures in liquid nutrient broth on plain glass
proliferated faster in most species. Different magnetic fields affect the
growth pattern of bacteria differently, depending on the bacterial strain. The
weak magnetic field seems to decelerate the growth rate, even after the
magnetic field is removed. With application of this study, we can potentially
investigate the effect of weak field exposures on Eukaryotic cells and gene
dynamics.",1808.10333v2
2018-11-02,Spinning superconductors and ferromagnets,"When a magnetic field is applied to a ferromagnetic body it starts to spin
(Einstein-de Haas effect). This demonstrates the intimate connection between
the electron's magnetic moment $\mu_B=e\hbar/2m_ec$, associated with its spin
angular momentum $S=\hbar/2$, and ferromagnetism. When a magnetic field is
applied to a superconducting body it also starts to spin (gyromagnetic effect),
and when a normal metal in a magnetic field becomes superconducting and expels
the magnetic field (Meissner effect) the body also starts to spin. Yet
according to the conventional theory of superconductivity the electron's spin
only role is to label states, and the electron's magnetic moment plays no role
in superconductivity. Instead, within the unconventional theory of hole
superconductivity, the electron's spin and associated magnetic moment play a
fundamental role in superconductivity. Just like in ferromagnets the
magnetization of superconductors is predicted to result from an aggregation of
magnetic moments with angular momenta $\hbar/2$. This gives rise to a ""Spin
Meissner effect"", the existence of a spin current in the ground state of
superconductors. The theory explains how a superconducting body starts spinning
when it expels magnetic fields, it provides a dynamical explanation for the
Meissner effect, and it explains how supercurrents stop without dissipation,
all of which we argue the conventional theory fails to explain. Essential
elements of the theory of hole superconductivity are that superconductivity is
driven by lowering of kinetic energy, which we have also proposed is true for
ferromagnets], that the normal state charge carriers in superconducting
materials are holes, and that the spin-orbit interaction plays a key role in
superconductivity. The theory is proposed to apply to all superconductors.",1811.03938v1
2019-04-30,Tunable ferromagnetic resonance in coupled trilayers with crossed in-plane and perpendicular magnetic anisotropies,"A novel approach to tune the ferromagnetic resonance frequency of a soft
magnetic Ni$_{80}$Fe$_{20}$ (Permalloy = Py) film with in-plane magnetic
anisotropy (IMA) based on the controlled coupling to a hard magnetic
NdCo$_\text{x}$ film with perpendicular magnetic anisotropy (PMA) through a
non-magnetic Al spacer is studied. Using transverse magneto-optical Kerr effect
(TMOKE), alternating gradient magnetometry (AGM) as well as vector network
analyzer ferromagnetic resonance (VNA-FMR) spectroscopy, the influence of both
Co concentration and Al spacer thickness on the static and dynamic magnetic
properties of the coupled IMA/PMA system is investigated. Compared to a single
Py film, two striking effects of the coupling between IMA and PMA layers can be
observed in their FMR spectra. First, there is a significant increase in the
zero-field resonance frequency from 1.3 GHz up to 6.6 GHz, and second, an
additional frequency hysteresis occurs at low magnetic fields applied along the
hard axis. The maximum frequency difference between the frequency branches for
increasing and decreasing magnetic field is as high as 1 GHz, corresponding to
a tunability of about 20% at external fields of typically less than $\pm$70 mT.
The origin of the observed features in the FMR spectra is discussed by means of
magnetization reversal curves.",1904.13275v1
2019-05-14,Versatile Hall magnetometer with variable sensitivity assembly for characterization of the magnetic properties of nanoparticles,"A Hall magnetometer with variable sensitivity is constructed to measure the
magnetic properties of magnetic nanoparticles manufactured by different
methods. This novel magnetometer can also be used to measure bulk materials and
samples in liquids. The magnetometer is constructed with two commercial
Hall-effect sensors in an acrylic structure, which serves as the support for a
micrometer and the circuit board with the sensors. For operation, the
magnetometer it acquires a complete magnetization curve in a few minutes. If
has a magnetic moment sensitivity of 1.3*10-9 Am2 to sensitivity of 493 mV/mT,
the sensitivity can be adjustable in the range of 10 to 493 mV/mT. Its
performance is tested with magnetic nanoparticles. As an application example,
we estimate the mean diameter of the nanoparticles using the magnetic curves.
The results are compared with those obtained by other techniques, such as
transmission electron microscopy (TEM), X-ray diffraction (XRD) and dynamic
light scattering (DLS). The magnetization results are also compared with those
obtained by independent commercial magnetometers, which reveals errors of
approximately 0.31 Am2/kg (i.e., 0.6%) in the saturation region.",1905.05741v2
2020-02-26,The Trigger Mechanism of Recurrent Solar Active Region Jets Revealed by the Magnetic Properties of a Coronal Geyser Site,"Solar active region jets are small-scale collimated plasma eruptions that are
triggered from magnetic sites embedded in sunspot penumbral regions. Multiple
trigger mechanisms for recurrent jets are under debate. Vector magnetic field
data from SDO-HMI observations are used to analyze a prolific photospheric
configuration, identified in extreme ultraviolet observations as a `Coronal
Geyser', that triggered a set of at least 10 recurrent solar active region
jets. We focus on interpreting the magnetic fields of small-scale flaring sites
aiming to understand the processes that govern recurrent jet eruptions. We
perform a custom reprocessing of the SDO-HMI products, including disambiguation
and uncertainty estimation. We scrutinized the configuration and dynamics of
the photospheric magnetic structures. The magnetic configuration is described
via the analysis of the photospheric magnetic vertical fields, to identify the
process is responsible for driving the jet eruptions. We report that the two
widely debated magnetic trigger processes, namely magnetic flux cancellation
and magnetic flux emergence, appear to be responsible on a case by case basis
for generating each eruption in our set. We find that 4 of 10 jets were due to
flux cancellation while the rest were clearly not, and were more likely due to
flux emergence.",2002.11819v1
2020-05-20,How primordial magnetic fields shrink galaxies,"As one of the prime contributors to the interstellar medium energy budget,
magnetic fields naturally play a part in shaping the evolution of galaxies.
Galactic magnetic fields can originate from strong primordial magnetic fields
provided these latter remain below current observational upper limits. To
understand how such magnetic fields would affect the global morphological and
dynamical properties of galaxies, we use a suite of high-resolution constrained
transport magneto-hydrodynamic cosmological zoom simulations where we vary the
initial magnetic field strength and configuration along with the prescription
for stellar feedback. We find that strong primordial magnetic fields delay the
onset of star formation and drain the rotational support of the galaxy,
diminishing the radial size of the galactic disk and driving a higher amount of
gas towards the centre. This is also reflected in mock UVJ observations by an
increase in the light profile concentration of the galaxy. We explore the
possible mechanisms behind such a reduction in angular momentum, focusing on
magnetic braking. Finally, noticing that the effects of primordial magnetic
fields are amplified in the presence of stellar feedback, we briefly discuss
whether the changes we measure would also be expected for galactic magnetic
fields of non-primordial origin.",2005.10269v1
2020-08-06,Paramagnetic spin Hall magnetoresistance,"Spin Hall magnetoresistance (SMR) refers to a resistance change in a metallic
film reflecting the magnetization direction of a magnet attached to the film.
The mechanism of this phenomenon is spin exchange between conduction-electron
spins and magnetization at the interface. SMR has been used to read out
information written in a small magnet and to detect magnetization dynamics, but
it has been limited to magnets; magnetic ordered phases or instability of
magnetic phase transition has been believed to be indispensable. Here, we
report the observation of SMR in a paramagnetic insulator
Gd$_{3}$Ga$_{5}$O$_{12}$ (GGG) without spontaneous magnetization combined with
a Pt film. The paramagnetic SMR can be attributed to spin-transfer torque
acting on localized spins in GGG. We determine the efficiencies of spin torque
and spin-flip scattering at the Pt/GGG interface, and demonstrate these
quantities can be tuned with external magnetic fields. The results clarify the
mechanism of spin-transport at a metal/paramagnetic insulator interface, which
gives new insight into the spintronic manipulation of spin states in
paramagnetic systems.",2008.02446v2
2020-10-07,Magnetostriction in elastomers with mixtures of magnetically hard and soft microparticles: effects of non-linear magnetization and matrix rigidity,"In this contribution a magnetoactive elastomer (MAE) of mixed content, i.e.,
a polymer matrix filled with a mixture of magnetically soft and magnetically
hard spherical particles, is considered. The object we focus at is an
elementary unit of this composite, for which we take a set consisting of a
permanent spherical micromagnet surrounded by an elastomer layer filled with
magnetically soft microparticles. We present a comparative treatment of this
unit from two essentially different viewpoints. The first one is a
coarse-grained molecular dynamics simulation model, which presents the
composite as a bead-spring assembly and is able to deliver information of all
the microstructural changes of the assembly. The second approach is entirely
based on the continuum magnetomechanical description of the system, whose
direct yield is the macroscopic field-induced response of the MAE to external
field, as this model ignores all the microstructural details of the
magnetization process. We find that, differing in certain details, both
frameworks are coherent in predicting that a unit comprising magnetically soft
and hard particles may display a non-trivial re-entrant
(prolate/oblate/prolate) axial deformation under variation of the applied field
strength.
  The flexibility of the proposed combination of the two complementary
frameworks enables us to look deeper into the manifestation of the magnetic
response: with respect to the magnetically soft particles, we compare the
linear regime of magnetization to that with saturation, which we describe by
the Fr\""{o}hlich-Kennelly approximation; with respect to the polymer matrix, we
analyze the dependence of the re-rentrant deformation on its rigidity.",2010.03684v1
2020-11-09,Evolution of three-dimensional coherent structures in Hall magnetohydrodynamics,"The work extends the computational model EULAG-MHD to include Hall
magnetohydrodynamics (HMHD)---important to explore physical systems undergoing
fast magnetic reconnection at the order of the ion inertial length scale.
Examples include solar transients along with reconnections in magnetosphere,
magnetotail and
  laboratory plasmas. The paper documents the results of two distinct sets of
implicit large-eddy simulations in the presence and absence of the Hall forcing
term, initiated with an unidirectional sinusoidal magnetic field. The HMHD
simulation while benchmarking the code also emphasizes the complexity of three
dimensional (3D) evolution over its two dimensional (2D) counterpart. The
magnetic reconnections onset significantly earlier in HMHD. Importantly, the
magnetic field generated by the Hall term breaks any inherent symmetry,
ultimately making the evolution 3D. The resulting 3D reconnections develop
magnetic flux ropes and magnetic flux tubes. Projected on the reconnection
plane, the ropes and tubes appear as magnetic islands, which later break into
secondary islands, and finally coalesce to generate an X-type neutral point.
These findings are in agreement with the theory and contemporary simulations of
HMHD, and thus verify our extension of the EULAG-MHD model. The second set
explores the influence of the Hall forcing on generation and ascend of a
magnetic flux rope from sheared magnetic arcades---a novel scenario instructive
in understanding the coronal transients. The rope evolves through intermediate
complex structures, ultimately breaking locally because of reconnections.
Interestingly, the breakage occurs earlier in the presence of the Hall term,
signifying faster dynamics leading to magnetic topology favorable for
reconnections.",2011.04223v1
2021-01-28,Universal field dependence of magnetic resonance near zero frequency,"Magnetic resonance is a widely-established phenomenon that probes magnetic
properties such as magnetic damping and anisotropy. Even though the typical
resonance frequency of a magnet ranges from gigahertz to terahertz, experiments
also report the resonance near zero frequency in a large class of magnets. Here
we revisit this phenomenon by analyzing the symmetry of the system and find
that the resonance frequency ($\omega$) follows a universal power law $\omega
\varpropto |H-H_c|^p$, where $H_c$ is the critical field at which the resonance
frequency is zero. When the magnet preserves the rotational symmetry around the
external field ($H$), $p = 1$. Otherwise, $p=1/2$. The magnon excitations are
gapped above $H_c$, gapless at $H_c$ and gapped again below $H_c$. The zero
frequency is often accompanied by a reorientation transition in the
magnetization. For the case that $p=1/2$, this transition is described by a
Landau theory for second-order phase transitions. We further show that the spin
current driven by thermal gradient and spin-orbit effects can be significantly
enhanced when the resonance frequency is close to zero, which can be measured
electrically by converting the spin current into electric signals. This may
provide an experimentally accessible way to characterize the critical field.
Our findings provide a unified understanding of the magnetization dynamics near
the critical field, and may, furthermore, inspire the study of magnon transport
near magnetic transitions.",2101.12292v1
2021-07-27,Exploring extreme magnetization phenomena in directly-driven imploding cylindrical targets,"This paper uses extended-magnetohydrodynamics (MHD) simulations to explore an
extreme magnetized plasma regime realisable by cylindrical implosions on the
OMEGA laser facility. This regime is characterized by highly compressed
magnetic fields (greater than 10~kT across the fuel), which contain a
significant proportion of the implosion energy and induce large electrical
currents in the plasma. Parameters governing the different magnetization
processes such as Ohmic dissipation and suppression of instabilities by
magnetic tension are presented, allowing for optimization of experiments to
study specific phenomena. For instance, a dopant added to the target gas-fill
can enhance magnetic flux compression while enabling spectroscopic diagnosis of
the imploding core. In particular, the use of Ar K-shell spectroscopy is
investigated by performing detailed non-LTE atomic kinetics and radiative
transfer calculations on the MHD data. Direct measurement of the core electron
density and temperature would be possible, allowing for both the impact of
magnetization on the final temperature and thermal pressure to be obtained. By
assuming the magnetic field is frozen into the plasma motion, which is shown to
be a good approximation for highly magnetized implosions, spectroscopic
diagnosis could be used to estimate which magnetization processes are ruling
the implosion dynamics; for example, a relation is given for inferring whether
thermally-driven or current-driven transport is dominating.",2107.13032v1
2021-10-01,Numerical Simulation of Superparamagnetic Nanoparticle Motion in Blood Vessels for Magnetic Drug Delivery,"A numerical model is developed for the motion of superparamagnetic
nanoparticles in a non-Newtonian blood flow under the influence of a magnetic
field. The rheological properties of blood are modeled by the Carreau flow and
viscosity, and the stochastic effects of Brownian motion and red blood cell
collisions are considered. The model is validated with existing data and good
agreement with experimental results is shown. The effectiveness of magnetic
drug delivery in various blood vessels is assessed and found to be most
successful in arterioles and capillaries. A range of magnetic field strengths
are modeled using equations for both a bar magnet and a point dipole: it is
shown that the bar magnet is effective at capturing nanoparticles in limited
cases while the point dipole is highly effective across a range of conditions.
A parameter study is conducted to show the effects of changing the dipole
moment, the distance from the magnet to the blood vessel, and the initial
release point of the nanoparticles. The distance from the magnet to the blood
vessel is shown to play a significant role in determining nanoparticle capture
rate. The optimal initial release position is found to be located within the
tumor radius in capillaries and arterioles to prevent rapid diffusion to the
edges of the blood vessel prior to arriving at the tumor, and near the edge of
the magnet when a bar magnet is used.",2110.01988v2
2021-11-05,Quadrupolar magnetic excitations in an isotropic spin-1 antiferromagnet,"The microscopic origins of emergent behaviours in condensed matter systems
are encoded in their excitations. In ordinary magnetic materials, single
spin-flips give rise to collective dipolar magnetic excitations called magnons.
Likewise, multiple spin-flips can give rise to multipolar magnetic excitations
in magnetic materials with spin $\mathbf{S} \boldsymbol{\ge} \mathbf{1}$.
Unfortunately, since most experimental probes are governed by dipolar selection
rules, collective multipolar excitations have generally remained elusive. For
instance, only dipolar magnetic excitations have been observed in isotropic
$\mathbf{S}\boldsymbol{=}\mathbf{1}$ Haldane spin systems. Here, we unveil a
hidden quadrupolar constituent of the spin dynamics in antiferromagnetic
$\mathbf{S}\boldsymbol{=}\mathbf{1}$ Haldane chain material
Y$_\mathbf{2}$BaNiO$_\mathbf{5}$ using Ni $\mathbf{L_3}$-edge resonant
inelastic x-ray scattering. Our results demonstrate that pure quadrupolar
magnetic excitations can be probed without direct interactions with dipolar
excitations or anisotropic perturbations. Originating from on-site double
spin-flip processes, the quadrupolar magnetic excitations in
Y$_\mathbf{2}$BaNiO$_\mathbf{5}$ show a remarkable dual nature of collective
dispersion. While one component propagates as non-interacting entities, the
other behaves as a bound quadrupolar magnetic wave. This result highlights the
rich and largely unexplored physics of higher-order magnetic excitations.",2111.03625v1
2022-02-16,Another type of anomalous velocity caused by the singularity of the magnetic Bloch function in the magnetic Brillouin zone,"The quantum Hall effect is known to be caused by the anomalous velocity that
is expressed as the Berry curvature of the magnetic Bloch band. It is also
known that the Hall conductivity is expressed by the Chern number that is given
as an integral of the Berry curvature and has a nonzero value because of the
singularity of the magnetic Bloch function in the magnetic Brillouin zone. We
have investigated the dynamics of a wave packet composed of magnetic Bloch
functions. It is shown that another type of the anomalous velocity appears in
addition to the group velocity and the anomalous velocity of the magnetic Bloch
state. This another type of the anomalous velocity is caused by the singularity
of the magnetic Bloch function in the magnetic first Brillouin zone (MBZ) and
is perpendicular to the electric field. Furthermore, we present the
acceleration theorem that describes the motion of the wave packet in the MBZ.
It is found that the expression for the acceleration theorem includes novel
terms that come from the singularity of the magnetic Bloch function in the MBZ.",2202.07810v3
2022-04-28,The Role of Magnetic Fields in Triggered Star Formation of RCW 120,"We report on the near-infrared polarimetric observations of RCW 120 with the
1.4 m IRSF telescope. The starlight polarization of the background stars
reveals for the first time the magnetic field of RCW 120. The global magnetic
field of RCW 120 is along the direction of $20^\circ$, parallel to the Galactic
plane. The field strength on the plane of the sky is $100\pm26\,\mu$G. The
magnetic field around the eastern shell shows evidence of compression by the
HII region. The external pressure (turbulent pressure + magnetic pressure) and
the gas density of the ambient cloud are minimum along the direction where RCW
120 breaks out, which explains the observed elongation of RCW 120. The
dynamical age of RCW 120, depending on the magnetic field strength, is
$\sim\,1.6\,\mathrm{Myr}$ for field strength of $100\,\mu$G, older than the
hydrodynamic estimates. In direction perpendicular to the magnetic field, the
density contrast of the western shell is greatly reduced by the strong magnetic
field. The strong magnetic field in general reduces the efficiency of triggered
star formation, in comparison with the hydrodynamic estimates. Triggered star
formation via the ""collect and collapse"" mechanism could occur in the direction
along the magnetic field. Core formation efficiency (CFE) is found to be higher
in the southern and eastern shells of RCW 120 than in the infrared dark cloud
receiving little influence from the HII region, suggesting increase in the CFE
related to triggering from ionization feedback.",2204.13296v2
2022-07-28,Evolution of short-range magnetic correlations in ferromagnetic Ni-V alloys,"We experimentally study how the magnetic correlations develop in a binary
alloy close to the ferromagnetic quantum critical point with small-angle
neutron scattering (SANS). Upon alloying the itinerant ferromagnet nickel with
vanadium, the ferromagnetic order is continuously suppressed. The critical
temperature Tc vanishes when vanadium concentrations reach the critical value
of xc=0.116 indicating a quantum critical point separating the ferromagnetic
and paramagnetic phases. Earlier magnetization and $\mu$SR data have indicated
the presence of magnetic inhomogeneities in Ni(1-x)V(x) and, in particular,
recognize the magnetic clusters close to xc, on the paramagnetic and on the
ferromagnetic sides with nontrivial dynamical properties [R. Wang et al., Phys.
Rev. Lett. 118, 267202 (2017)]. We present the results of SANS study with full
polarization analysis of polycrystalline Ni(1-x)V(x) samples with x=0.10 and
x=0.11 with low critical temperatures Tc below 50 K. For both Ni-V samples
close to xc we find isotropic magnetic short-range correlations in the
nanometer-scale persisting at low temperatures. They are suppressed gradually
in higher magnetic fields. In addition, signatures of long-range ordered
magnetic domains are present below Tc. The fraction of these magnetic clusters
embedded in the ferromagnetic ordered phase grows towards xc and agrees well
with the cluster fraction estimate from the magnetization and $\mu$SR data. Our
SANS studies provide new insights into the nature of the inhomogeneities in a
ferromagnetic alloy close to a quantum critical point.",2207.14196v2
2023-01-17,Magnetic frustration driven by conduction carrier blocking in Nd$_2$Co$_{0.85}$Si$_{2.88}$,"The intermetallic compound Nd$_2$Co$_{0.85}$Si$_{2.88}$ having a triangular
lattice could be synthesized in single-phase only with defect crystal
structure. Investigation through different experimental techniques indicate the
presence of two magnetic transitions in the system. As verified experimentally
and theoretically, the high-temperature transition T$_H$ ~ 140 K is associated
with the development of ferromagnetic interaction between itinerant Co moments,
whereas the low-temperature transition at T$_L$ ~ 6.5 K is due to the coupling
among Nd-4f and Co-3d moments, which is antiferromagnetic in nature. Detailed
studies of temperature-dependent dc magnetic susceptibility, field dependence
of isothermal magnetization, non-equilibrium dynamical behavior, viz. magnetic
relaxation, aging effect, magnetic-memory effect, and temperature dependence of
heat capacity, along with density functional theory (DFT) calculations, suggest
that the ground state is magnetically frustrated spin-glass in nature, having
competing magnetic interactions of equivalent energies. DFT results further
reveal that the 3d/5d-conduction carriers are blocked in the system and act as
a barrier for the 4f-4f RKKY interactions, resulting in spin-frustration.
Presence of vacancy defects in the crystal are also conducive to the
spin-frustration. This is an unique mechanism of magnetic frustration, not
emphasized so far in any of the ternary R$_2$TX$_3$ (R=rare-earth, T=transition
elements and X=Si, Ge, In) type compounds. Due to the competing character of
the itinerant 3d and localized 4f moments, the compound exhibits anomalous
field dependence of magnetic coercivity.",2301.07140v1
2023-04-19,Thickness-dependent magnetic properties in Pt[CoNi]n multilayers with perpendicular magnetic anisotropy,"We systematically investigated the Ni and Co thickness-dependent
perpendicular magnetic anisotropy (PMA) coefficient, magnetic domain
structures, and magnetization dynamics of Pt(5 nm)/[Co(t_Co nm)/Ni(t_Ni
nm)]5/Pt(1 nm) multilayers by combining the four standard magnetic
characterization techniques. The magnetic-related hysteresis loops obtained
from the field-dependent magnetization M and anomalous Hall resistivity (AHR)
\r{ho}_xy found that the two serial multilayers with t_Co = 0.2 and 0.3 nm have
the optimum PMA coefficient K_U well as the highest coercivity H_C at the Ni
thickness t_Ni = 0.6 nm. Additionally, the magnetic domain structures obtained
by Magneto-optic Kerr effect (MOKE) microscopy also significantly depend on the
thickness and K_U of the films. Furthermore, the thickness-dependent linewidth
of ferromagnetic resonance is inversely proportional to K_U and H_C, indicating
that inhomogeneous magnetic properties dominate the linewidth. However, the
intrinsic Gilbert damping constant determined by a linear fitting of
frequency-dependent linewidth does not depend on Ni thickness and K_U. Our
results could help promote the PMA [Co/Ni] multilayer applications in various
spintronic and spin-orbitronic devices.",2304.09366v1
2023-05-05,Complete replacement of magnetic flux in a flux rope during a coronal mass ejection,"Solar coronal mass ejections are the most energetic events in the Solar
System. In their standard formation model, a magnetic flux rope builds up into
a coronal mass ejection through magnetic reconnection that continually converts
overlying, untwisted magnetic flux into twisted flux enveloping the
pre-existing rope. However, only a minority of coronal mass ejections carry a
coherent magnetic flux rope as their core structure, which casts doubt on the
universality of this orderly wrapping process. Here we provide observational
evidence of a different formation and eruption mechanism of a magnetic flux
rope from an S-shaped thread, where its magnetic flux is fully replaced via
flare reconnections. One of the footpoints of the sigmoidal feature slipped and
expanded during the formation, and then moved to a completely new place,
associated with the highly dynamical evolution of flare ribbons and a twofold
increase in magnetic flux through the footpoint, during the eruption. Such a
configuration is not predicted by standard formation models or numerical
simulations and highlights the three-dimensional nature of magnetic
reconnections between the flux rope and the surrounding magnetic field.",2305.03217v1
2023-07-04,Reconciling the magnetic field in central disc galaxies with the dynamical mass using the cosmological simulations,"The Universe is pervaded by magnetic fields in different scales, although for
simplicity, they are ignored in most cosmological simulations. In this paper,
we use the TNG50, which is a large cosmological galaxy formation simulation
that incorporates magnetic fields with an unprecedented resolution. We study
the correlation of the magnetic field with various galaxy properties such as
the total, stellar and gaseous mass, circular velocity, size and star formation
rate. We find a linear correlation between the average magnetic field pervading
the disc of galaxies in relative isolation and their circular velocities. In
addition we observed that in this sample the average magnetic field in the disc
is correlated with the total mass as $\overline{B}\sim
M_{\mathrm{tot,\,R_{\star}}}^{0.2}$. We also find that the massive galaxies
with active wind-driven black hole feedback, do not follow this trend, as their
magnetic field is substantially affected by this feedback mode in the TNG50
simulation. We show that the correlation of the magnetic field with the star
formation rate is a little weaker than the circular velocity. Moreover, we
compare the magnetic field components of the above sample with a compiled
observational sample of non-cluster non-interacting nearby galaxies. Similar to
the observation, we find a coupling between the ordered magnetic field and the
circular velocity of the flat part of the rotation curve in the simulation,
although contrary to the observation, the ordered component is dominant in the
simulation.",2307.01431v2
2023-07-11,Simulation of magnetohydrodynamic flows of liquid metals with heat transfer or magnetic stirring,"We discuss the effects of nonhomogeneous magnetic fields in liquid metal
flows in two different configurations. In the first configuration, we briefly
report the impact of fringing magnetic fields in a turbulent
Rayleigh-B{\'e}nard convection setup, where it was shown that the global heat
transport decreases with an increase of fringe-width. The convective motion in
regions of strong magnetic fields is confined near the sidewalls. In the second
configuration, we numerically study the effects of an oscillating magnetic
obstacle with different frequencies of oscillation on liquid metal flow in a
duct. The Reynolds number is low such that the wake of the stationary magnetic
obstacle is steady. The transverse oscillation of the magnet creates a
sinusoidal time-dependent wake reminiscent of the vortex shedding behind solid
obstacles. We examine the behavior of the streamwise and spanwise components of
the Lorentz forces as well as the work done by the magnets on the fluid. The
frequency of the oscillation of the streamwise component of Lorentz force is
twice that of the spanwise component as in the case of lift and drag on solid
cylindrical obstacles. The total drag force and the energy transferred from the
magnets to the fluid show a non-monotonic dependence on the frequency of
oscillation of the magnetic obstacle indicative of a resonant excitation of the
sinusoidal vortex shedding.",2307.05465v1
2023-10-24,Evolution of MHD Torus and Mass Outflow Around Spinning AGN,"We perform axisymmetric, two-dimensional magnetohydrodynamic (MHD)
simulations to investigate accretion flows around spinning AGN. To mimic the
space-time geometry of spinning black holes, we consider effective Kerr
potential, and the mass of the black holes is $10^8 M_{\odot}$. We initialize
the accretion disc with a magnetized torus by adopting the toroidal component
of the magnetic vector potential. The initial magnetic field strength is set by
using the plasma beta parameter ($\beta_0$). We observe self-consistent
turbulence generated by magneto rotational instability (MRI) in the disc. The
MRI turbulence transports angular momentum in the disc, resulting in an angular
momentum distribution that approaches a Keplerian distribution. We investigate
the effect of the magnetic field on the dynamics of the torus and associated
mass outflow from the disc around a maximally spinning black hole $(a_k =
0.99)$. For the purpose of our analysis, we investigate the magnetic state of
our simulation model. The model $\beta_0 = 10$ indicates the behaviour similar
to the ""magnetically arrested disk (MAD)'' state, and all the other low
magnetic model remains in the SANE state. We observe that mass outflow rates
are significantly enhanced with the increased magnetic field in the disc. We
find a positive correlation between the magnetic field and mass outflow rates.
We also investigate the effect of black hole spin on the magnetized torus
evolution. However, we have not found any significant effect of black hole spin
on mass outflows in our model. Finally, we discuss the possible astrophysical
applications of our simulation results.",2310.15501v1
2023-10-25,Controllable magnetic states in chains of coupled phi-0 Josephson junctions with ferromagnetic weak links,"A superconductor/ferromagnet/superconductor Josephson junction with anomalous
phase shift (phi_0-S/F/S JJ) is a system, where the anomalous ground state
shift phi_0 provides a direct magnetoelectric coupling between a magnetic
moment and a phase of the superconducting condensate. If a chain of such
phi_0-S/F/S JJs are coupled via superconducting leads, the condensate phase,
being a macroscopic quantity, mediates a long-range interaction between the
magnetic moments of the weak links. We study static and dynamic magnetic
properties of such a system. It is shown that it manifests properties of
n-level system, where the energies of the levels are only determined by
projections of the total magnetic moment onto the easy magnetic axis. It is
similar to a magnetic atom in a Zeeman field, but the role of the field is
played by the magnetoelectric coupling. However, unlike an atom in a magnetic
field, the relative order of energies of different states is controlled by
electrical means. It is also demonstrated that the total magnetic moment can be
fully controlled by a supercurrent and the response of the magnetic system to
local external perturbations is highly nonlocal.",2310.16543v2
2023-10-31,Magnetorotational dynamo can generate large-scale vertical magnetic fields in 3D GRMHD simulations of accreting black holes,"Jetted astrophysical phenomena with black hole (BH) engines, including binary
mergers, jetted tidal disruption events, and X-ray binaries, require a
large-scale vertical magnetic field for efficient jet formation. However, a
dynamo mechanism that could generate these crucial large-scale magnetic fields
has not been identified and characterized. We have employed 3D global general
relativistic magnetohydrodynamical (MHD) simulations of accretion disks to
quantify, for the first time, a dynamo mechanism that generates large-scale
magnetic fields. This dynamo mechanism primarily arises from the nonlinear
evolution of the magnetorotational instability (MRI). In this mechanism, large
non-axisymmetric MRI-amplified shearing wave modes, mediated by the
axisymmetric azimuthal magnetic field, generate and sustain the large-scale
vertical magnetic field through their nonlinear interactions. We identify the
advection of magnetic loops as a crucial feature, transporting the large-scale
vertical magnetic field from the outer regions to the inner regions of the
accretion disk. This leads to a larger characteristic size of the, now
advected, magnetic field when compared to the local disk height. We
characterize the complete dynamo mechanism with two timescales: one for the
local magnetic field generation, $t_{\rm g}$, and one for the large-scale scale
advection, $t_{\rm adv}$. Whereas the dynamo we describe is nonlinear, we
explore the potential of linear mean field models to replicate its core
features. Our findings indicate that traditional $\alpha$-dynamo models, often
computed in stratified shearing box simulations, are inadequate and that the
effective large-scale dynamics is better described by the shear current effects
or stochastic $\alpha$-dynamos.",2311.00034v1
2023-11-22,Twisted Magnetic Knots and Links,"For magnetic knots and links in plasmas we introduce an internal twist and
study their dynamical behavior in numerical simulations. We use a set of
helical and non-helical configurations and add an internal twist that cancels
the helicity of the helical configurations or makes a non-helical system
helical. These fields are then left to relax in a magnetohydrodynamic
environment. In line with previous works we confirm the importance of magnetic
helicity in field relaxation. However, an internal twist, as could be observed
in coronal magnetic loops, does not just add or subtract helicity, but also
introduces an alignment of the magnetic field with the electric current, which
is the source term for helicity. This source term is strong enough to lead to a
significant change of magnetic helicity, which for some cases leads to a loss
of the stabilizing properties expressed in the realizability condition. Even a
relatively weak internal twist in these magnetic fields leads to a strong
enough source term for magnetic helicity that for various cases even in a low
diffusion environment we observe an inversion in sign of magnetic helicity
within time scales much shorter than the diffusion time. We conclude that in
solar and stellar fields an internal twist does not automatically result in a
structurally stable configuration and that the alignment of the magnetic field
with the electric current must be taken into account.",2311.13660v1
2023-12-01,The Magnetic Field Calibration of the Full-Disk Magnetograph onboard the Advanced Space based Solar Observatory (ASO-S/FMG),"The Full-disk magnetograph is a main scientific payload onboard the Advanced
Space based Solar Observatory (ASO-S/FMG) that through Stokes parameter
observation to measures the vector magnetic field. The accuracy of
magnetic-field values is an important aspect of checking the quality of the FMG
magnetic-field measurement. According to the design of the FMG, the linear
calibration method under the weak-field approximation is the preferred scheme
for magnetic-field calibration. However, the spacecraft orbital velocity can
affect the position of observed spectral lines, then result in a change of the
polarization-signal strength. Thus, the magnetic field is modulated by the
orbit velocity of the spacecraft. In this article, through cross calibration
between FMG and HMI (Helioseismic and Magnetic Imager onboard the Solar Dynamic
Observatory), the effects of spacecraft orbital velocity on the coefficient of
magnetic-field calibration are investigated. By comparing the magnetic field of
FMG and HMI with spacecraft orbital velocity as an auxiliary reference, the
revised linear-calibration coefficients that depend on spacecraft orbital
velocity are obtained. Magnetic field of FMG corrected by the revised
calibration coefficients removing the effect of spacecraft orbital velocity
will be more accurate and suitable for scientific research.",2312.00319v1
2024-01-05,Machine learning inspired models for Hall effects in non-collinear magnets,"The anomalous Hall effect has been front and center in solid state research
and material science for over a century now, and the complex transport
phenomena in nontrivial magnetic textures have gained an increasing amount of
attention, both in theoretical and experimental studies. However, a clear path
forward to capturing the influence of magnetization dynamics on anomalous Hall
effect even in smallest frustrated magnets or spatially extended magnetic
textures is still intensively sought after. In this work, we present an
expansion of the anomalous Hall tensor into symmetrically invariant objects,
encoding the magnetic configuration up to arbitrary power of spin. We show that
these symmetric invariants can be utilized in conjunction with advanced
regularization techniques in order to build models for the electric transport
in magnetic textures which are, on one hand, complete with respect to the point
group symmetry of the underlying lattice, and on the other hand, depend on a
minimal number of order parameters only. Here, using a four-band tight-binding
model on a honeycomb lattice, we demonstrate that the developed method can be
used to address the importance and properties of higher-order contributions to
transverse transport. The efficiency and breadth enabled by this method
provides an ideal systematic approach to tackle the inherent complexity of
response properties of noncollinear magnets, paving the way to the exploration
of electric transport in intrinsically frustrated magnets as well as
large-scale magnetic textures.",2401.03044v1
2024-01-21,Nonlinear Model Predictive Detumbling of Small Satellites with a Single-axis Magnetorquer,"Various actuators are used in spacecraft to achieve attitude stabilization,
including thrusters, momentum wheels, and control moment gyros. Small
satellites, however, have stringent size, weight, and cost constraints, which
makes many actuator choices prohibitive. Consequently, magnetic torquers have
commonly been applied to spacecraft to attenuate angular rates. Approaches for
dealing with under-actuation due to magnetic control torque's dependency on the
magnetic field and required high magnetic flux densities have been previously
considered. Generally speaking, control of a satellite that becomes
under-actuated as a result of on-board failures has been a recurrent theme in
the literature. Methods for controlling spacecraft with fewer actuators than
degrees of freedom are increasingly in demand due to the increased number of
small satellite launches. Magnetic torquers have been extensively investigated
for momentum management of spacecraft with momentum wheels and for nutation
damping of spin satellites, momentum-biased, and dual-spin satellites.
Nonetheless, severely under-actuated small spacecraft that carry only a
single-axis magnetic torquer have not been previously treated. This note
considers the detumbling of a small spacecraft using only a single-axis
magnetic torquer. Even with a three-axis magnetic torquer, the spacecraft is
under-actuated, while, in the case of only a single axis magnetic torquer, the
problem is considerably more demanding. Our note examines the feasibility of
spacecraft attitude control with a single-axis magnetic torquer and possible
control methods that can be used.",2401.11536v1
2024-02-02,Emerging topological states in EuMn$_2$Bi$_2$: A first principles prediction,"New materials with magnetic order driven topological phases are hugely sought
after for their immense application potential. In this work, we propose a new
compound EuMn$_2$Bi$_2$ from our first principles density functional theory
calculations to host novel topological phases such as Dirac/Weyl semimetal and
topological insulator in its different magnetic states which are energetically
close to one another. We started with an isostructural compound EuMn$_2$As$_2$
where the magnetic structure has been studied experimentally. From our
calculations we could explain the nature of two magnetic transitions observed
experimentally in this system and could also establish the correct magnetic
ground state. Our electronic structure calculations reveal the insulating
nature of the ground state consistent with the experiments. By replacing all As
by Bi in EuMn$_2$As$_2$ and by optimizing the new structure, we obtained the
new compound EuMn$_2$Bi$_2$. We observe this compound to be dynamically stable
from our phonon calculations supporting its experimental preparation in future.
By comparing the total energies of various possible magnetic structures we
identified the ground state. Though the magnetic ground state is found to be
insulating in nature with tiny band gap which is an order of magnitude less
than the same in EuMn$_2$As$_2$, there were other magnetic states energetically
very close to the ground state which display remarkable non-trivial band
topology such as Dirac/Weyl points close to the Fermi level and topological
insulator state. The energetic proximity of these magnetic order driven
topological phases makes them tunable via external handle which indicates that
the proposed new material EuMn$_2$Bi$_2$ would be a very versatile magnetic
topological material.",2402.01328v1
2024-03-25,Self-similar solutions in cylindrical magneto-hydrodynamic blast waves with energy injection at the centre,"The evolution of shocks induced by massive stars does not depend only on the
ambient magnetic field strength, but also on its orientation. In the present
work, the dynamics of a magnetized blast wave is investigated under the
influence of both azimuthal and axial ambient magnetic fields. The blast wave
is driven by a central source and forms a shell that results from the
accumulation of interstellar matter behind the shock front. A similarity form
of the ambient magnetic field and a cylindrical geometry of the blast wave are
assumed to obtain self-similar solutions. The model is studied separately for
both azimuthal and axial magnetic field and applied to stellar wind bubbles and
supernova remnants respectively, using 1D numerical simulations. We found that
the magnetized blast wave differs from the self-similar case without an ambient
magnetic field. The forward shock front goes slower in the azimuthal case and
faster in the axial one. For both tangential orientations, the thickness of the
shell increases with the magnetic strength. In the azimuthal case, the thermal
energy can be converted to magnetic energy near the inner boundary of the
shell. Thus, the temperature drops and the magnetic field increases at the
tangential discontinuity of the stellar wind bubble. In the axial case of a
supernova remnant, the numerical solution al w ays follows a special curve in
the parameter space given by the self-similar model.",2403.16675v1
2013-02-27,Dynamic phase diagrams of the Blume-Capel model in an oscillating field by the path probability method,"We calculate the dynamic phase transition (DPT) temperatures and present the
dynamic phase diagrams in the Blume-Capel model under the presence of a
time-dependent oscillating external magnetic field by using the path
probability method. We study the time variation of the average order parameters
to obtain the phases in the system and the paramagnetic (P), ferromagnetic (F)
and the F + P mixed phases are found. We also investigate the thermal behavior
of the dynamic order parameters to analyze the nature (continuous and
discontinuous) of transitions and to obtain the DPT points. We present the
dynamic phase diagrams in three planes, namely (T, h), (d, T) and (k2/k1, T),
where T is the reduced temperature, h the reduced magnetic field amplitude, d
the reduced crystal-field interaction and the k2, k1 rate constants. The phase
diagrams exhibit dynamic tricritical and reentrant behaviors as well as a
double critical end point and triple point, strongly depending on the values of
the interaction parameters and the rate constants. We compare and discuss the
dynamic phase diagrams with dynamic phase diagrams that are obtained within the
Glauber-type stochastic dynamics based on the mean-field theory and the
effective field theory.",1302.6769v1
2022-08-04,Dynamics of magnetized accretion disks of young stars,"We investigate the dynamics of the accretion disks of young stars with fossil
large-scale magnetic field. The author's magnetohydrodynamic (MHD) model of the
accretion disks is generalized to consider the dynamical influence of the
magnetic field on gas rotation speed and vertical structure of the disks. With
the help of the developed MHD model, the structure of an accretion disk of a
solar mass T Tauri star is simulated for different accretion rates $\dot{M}$
and dust grain sizes $a_d$. The simulations of the radial structure of the disk
show that the magnetic field in the disk is kinematic, and the electromagnetic
force does not affect the rotation speed of the gas for typical values
$\dot{M}=10^{-8}\,M_\odot\,\mbox{yr}^{-1}$ and $a_d=0.1 \mu$m. In the case of
large dust grains, $a_d\geq 1$ mm, the magnetic field is frozen into the gas
and a dynamically strong magnetic field is generated at radial distances from
the star $r\gtrsim 30$ au, the tensions of which slow down the rotation speed
by $\lesssim 1.5$ % of the Keplerian velocity. This effect is comparable to the
contribution of the radial gradient of gas pressure and can lead to the
increase in the radial drift velocity of dust grains in the accretion disks. In
the case of high accretion rate, $\dot{M}\geq
10^{-7}\,M_\odot\,\mbox{yr}^{-1}$, the magnetic field is also dynamically
strong in the inner region of the disk, $r<0.2$ au. The simulations of the
vertical structure of the disk show that, depending on the conditions on the
surface of the disk, the vertical gradient of magnetic pressure can lead to
both decrease and increase in the characteristic thickness of the disk as
compared to the hydrostatic one by 5-20 %. The change in the thickness of the
disk occurs outside the region of low ionization fraction and effective
magnetic diffusion (`dead' zone), which extends from $r=0.3$ to $20$ au at
typical parameters.",2208.02871v1
2007-11-01,Draping of Cluster Magnetic Fields over Bullets and Bubbles -- Morphology and Dynamic Effects,"High-resolution X-ray observations have revealed cavities and `cold fronts'
with sharp edges in temperature, density, and metallicity within galaxy
clusters. Their presence poses a puzzle since these features are not expected
to be hydrodynamically stable, or to remain sharp in the presence of diffusion.
However, a moving core or bubble in even a very weakly magnetized plasma
necessarily sweeps up enough magnetic field to build up a dynamically important
sheath around the object; the layer's strength is set by a competition between
`plowing up' of field and field lines slipping around the core. We show that a
two-dimensional approach to the problem is quite generally not possible. In
three dimensions, we show with analytic arguments and in numerical experiments,
that this magnetic layer modifies the dynamics of a plunging core, greatly
modifies the effects of hydrodynamic instabilities on the core, modifies the
geometry of stripped material, and even slows the fall of the core through
magnetic tension. We derive an expression for the maximum magnetic field
strength, the thickness of the layer, and the opening angle of the magnetic
wake. The morphology of the magnetic draping layer implies the suppression of
thermal conduction across the layer, thus conserving strong temperature
gradients over the contact surface. The intermittent amplification of the
magnetic field as well as the injection of MHD turbulence in the wake of the
core is identified to be due to vorticity generation within the magnetic
draping layer. These results have important consequences for understanding the
physical properties and the complex gasdynamical processes of the intra-cluster
medium, and apply quite generally to motions through other magnetized
environments, e.g., the ISM.",0711.0213v2
2016-04-08,Ultrafast energy and momentum resolved dynamics of magnetic correlations in photo-doped Mott insulator Sr$_2$IrO$_4$,"Measuring how the magnetic correlations throughout the Brillouin zone evolve
in a Mott insulator as charges are introduced dramatically improved our
understanding of the pseudogap, non-Fermi liquids and high $T_C$
superconductivity. Recently, photoexcitation has been used to induce similarly
exotic states transiently. However, understanding how these states emerge has
been limited because of a lack of available probes of magnetic correlations in
the time domain, which hinders further investigation of how light can be used
to control the properties of solids. Here we implement magnetic resonant
inelastic X-ray scattering at a free electron laser, and directly determine the
magnetization dynamics after photo-doping the Mott insulator Sr$_2$IrO$_4$. We
find that the non-equilibrium state 2~ps after the excitation has strongly
suppressed long-range magnetic order, but hosts photo-carriers that induce
strong, non-thermal magnetic correlations. The magnetism recovers its
two-dimensional (2D) in-plane N\'eel correlations on a timescale of a few ps,
while the three-dimensional (3D) long-range magnetic order restores over a far
longer, fluence-dependent timescale of a few hundred ps. The dramatic
difference in these two timescales, implies that characterizing the
dimensionality of magnetic correlations will be vital in our efforts to
understand ultrafast magnetic dynamics.",1604.02439v2
2016-09-06,Effects of magnetic and kinetic helicities on the growth of magnetic fields in laminar and turbulent flows by helical-Fourier decomposition,"We present a numerical and analytical study of incompressible homogeneous
conducting fluids using a helical Fourier representation. We analytically study
both small- and large-scale dynamo properties, as well as the inverse cascade
of magnetic helicity, in the most general minimal subset of interacting
velocity and magnetic fields on a closed Fourier triad. We mainly focus on the
dependency of magnetic field growth as a function of the distribution of
kinetic and magnetic helicities among the three interacting wavenumbers. By
combining direct numerical simulations of the full magnetohydrodynamics
equations with the helical Fourier decomposition we numerically confirm that in
the kinematic dynamo regime the system develops a large-scale magnetic helicity
with opposite sign compared to the small-scale kinetic helicity, a sort of
triad-by-triad $\alpha$-effect in Fourier space. Concerning the small-scale
perturbations, we predict theoretically and confirm numerically that the
largest instability is achieved for the magnetic component with the same
helicity of the flow, in agreement with the Stretch-Twist-Fold mechanism. Vice
versa, in presence of a Lorentz feedback on the velocity, we find that the
inverse cascade of magnetic helicity is mostly local if magnetic and kinetic
helicities have opposite sign, while it is more nonlocal and more intense if
they have the same sign, as predicted by the analytical approach. Our
analytical and numerical results further demonstrate the potential of the
helical Fourier decomposition to elucidate the entangled dynamics of magnetic
and kinetic helicities both in fully developed turbulence and in laminar flows.",1609.01781v2
1994-11-10,Angular Momemtum and Large-Scale Magnetic Field in Textures Seeded Models,"We start by pedagigically reviewing the problem of large scale magnetic
fields. We explain the acquisition of angular momentum in the general context
of Dark Matter models and we use this result to derive a dynamical mechanism
for magnetic field generation. This mechanism does not produce any magnetic
field for standard CDM, but it does for a large class of other models. We apply
this mechanism in the context of the texture scenario of large-scale structure
formation. Resulting constraints on the texture scenario and other models are
discussed.",9411039v1
1998-12-29,Genesis of Flux Ropes Observed by Pioneer-Venus Orbiter in the Photoequilibrium Region At Venus,"Genesis of flux ropes observed by Pioneer-Venus Orbiter in the
photoequilibrium region at Venus is analysed. Stability analysis of the
large-scale magnetic field pushed deep inside during period of high dynamic
pressure of solar wind had shown that the top-side boundary of the magnetic
belt located within the region of photoequilibrium is unstable. The finite
conductivity of plasma tends to stabilise the instability so that the
bottom-side part of the magnetic belt is regular. Non-numerous flux ropes which
were observed above the predicted instability region are likely moving upward
because of the magnetic buoyancy.",9812461v1
1999-02-18,Magnetized Accretion Inside the Marginally Stable Orbit around a Black Hole,"Qualitative arguments are presented to demonstrate that the energy density of
magnetic fields in matter accreting onto a black hole inside the marginally
stable orbit is automatically comparable to the rest-mass energy density of the
accretion flow. Several consequences follow: magnetic effects must be
dynamically significant, but cannot be so strong as to dominate; outward energy
transport in Alfven waves may alter the effective efficiency of energy
liberation; and vertical magnetic stresses in this region may contribute to
""coronal"" activity.",9902267v1
2000-09-25,Magnetic Fields are not ignorable in the dynamics of disks,"Magnetic fields are considered to be dominant when
$\epsilon_{B}\geq\epsilon_{K}$, being $\epsilon_{B}=B^{2}/8\pi$ the magnetic
energy density and $\epsilon_{K}=1/2 \rho\theta^{2}$ the rotation energy
density, for a conventional moderate B= 1 $\mu$G. They are considered to be
negligible when $\epsilon_{B}<\epsilon_{K}$ for $B\sim 10 \mu$G. With no
assumption and no theoretical calculation, we show that magnetic fields cannot
be ignored in the outer parts of a galaxy like the Milky Way and in the whole
disk of a dwarf galaxy.",0009391v1
2005-07-17,Non-leptonic Weak Interaction in Magnetized Quark matter,"We investigated the non-leptonic weak interaction in magnetic field.
  We discussed an improvement of previous method to analytical work out the
rate for weak field case.Our result easily goes over to field-free limit.Then
we calculated the reaction rate in strong magnetic field where the charged
particles are confined to the lowest Landau level. A strong magnetic field
strongly suppressed the rate,which will be foreseen to affect viscous dynamics
in SQM .We also derived a few approximation formulae under given conditions
that can be conveniently applied.",0507390v1
2005-12-23,Galactic dynamo and helicity losses through fountain flow,"Nonlinear behaviour of galactic dynamos is studied, allowing for magnetic
helicity removal by the galactic fountain flow. A suitable advection speed is
estimated, and a one-dimensional mean-field dynamo model with dynamic
alpha-effect is explored. It is shown that the galactic fountain flow is
efficient in removing magnetic helicity from galactic discs. This alleviates
the constraint on the galactic mean-field dynamo resulting from magnetic
helicity conservation and thereby allows the mean magnetic field to saturate at
a strength comparable to equipartition with the turbulent kinetic energy.",0512592v2
2006-01-30,Magnetized Tori around Kerr Black Holes: Analytic Solutions with a Toroidal Magnetic Field,"The dynamics of accretion discs around galactic and extragalactic black holes
may be influenced by their magnetic field. In this paper we generalise the
fully relativistic theory of stationary axisymmetric tori in Kerr metric of
Abramowicz et al.(1978) by including strong toroidal magnetic field and
construct analytic solutions for barotropic tori with constant angular
momentum. This development is particularly important for the general
relativistic computational magnetohydrodynamics that suffers from the lack of
exact analytic solutions that are needed to test computer codes.",0601678v3
2007-03-20,Traveling waves in magnetized Taylor-Couette flow,"We investigate numerically a traveling wave pattern observed in experimental
magnetized Taylor-Couette flow at low magnetic Reynolds number. By accurately
modeling viscous and magnetic boundaries in all directions, we reproduce the
experimentally measured wave patterns and their amplitudes. Contrary to
previous claims, the waves are shown to be transiently amplified disturbances
launched by viscous boundary layers rather than globally unstable
magnetorotational modes.",0703525v2
1994-08-31,Magnetic von-Neumann lattice for two-dimensional electrons in the magnetic field,"One-particle eigenstates and eigenvalues of two-dimensional electrons in the
strong magnetic field with short range impurity and impurities, cosine
potential, boundary potential, and periodic array of short range potentials are
obtained by magnetic von-Neumann lattice in which Landau level wave functions
have minimum spatial extensions. We find that there is a dual correspondence
between cosine potential and lattice kinetic term and that the representation
based on the von-Neumann lattice is quite useful for solving the system's
dynamics.",9408098v1
1997-02-10,Non magnetic molecular Jahn-Teller Mott insulators,"Narrow-band conductors may turn insulating and magnetic as a consequence of
strong electron-electron correlation. In molecular conductors, the concomitance
of a strong Jahn-Teller coupling may give rise to the alternative possibility
of a non-magnetic insulator, with or without a static cooperative Jahn-Teller
distortion. In the latter case the insulator has Mott-like properties, with an
interesting interplay between electron-electron repulsion and the Jahn-Teller
effect, which is dynamical. We study this kind of non-magnetic insulator in a
very simple $E\otimes e$ Jahn-Teller model and we discuss its general
properties in a more general context, also in connection with the insulating
state of K_{4}C_{60} and Rb_{4}C_{60}.",9702079v1
1997-03-02,On the Self-Consistent Spin-Wave Theory of Two-Dimensional Magnets With Impurities,"The self-consistent spin-wave theory is applied to investigate the
magnetization distribution around the impurity in isotropic and easy-axis
two-dimensional ferro- and antiferromagents. The temperature dependences of
host magnetization disturbance and impurity magnetization are calculated. The
short-range order in the isotropic case is investigated. Importance of dynamic
and kinematic interactions of spin waves is demonstrated.",9703010v1
1997-10-22,Muon spin rotation and relaxation in magnetic materials,"A review of the muon spin rotation and relaxation ($\mu$SR) studies on
magnetic materials published from July 1993 is presented. It covers the
investigation of magnetic phase diagrams, of spin dynamics and the analysis of
the magnetic properties of superconductors. We have chosen to focus on selected
experimental works in these different topics. In addition, a list of published
works is provided.",9710235v1
1998-12-15,Magnetic and Charge Correlations of the 2-dimensional $t-t'-U$ Hubbard model,"Using a spin-rotation invariant six-slave boson representation, we study the
square lattice Hubbard model with nearest-neighbor hopping $t$ and next-nearest
neighbor hopping $t'$. We discuss the influence of $t'$ on the charge and
magnetic properties. In the hole-doped domain, we find that a negative $t'$
strongly favors itinerant ferromagnetism over any incommensurate phase,
especially in the strong coupling regime. For positive $t'$ magnetic
fluctuations are suppressed. A tight connection between frustrated charge
dynamics and large magnetic fluctuations is pointed out. A clear tendency
towards striped charge ordering is found in the regime of large positive $t'$.",9812242v1
1999-05-24,Proton NMR for Measuring Quantum-Level Crossing in the Magnetic Molecular Ring Fe10,"The proton nuclear spin-lattice relaxation rate 1/T1 has been measured as a
function of temperature and magnetic field (up to 15 T) in the molecular
magnetic ring Fe10. Striking enhancement of 1/T1 is observed around magnetic
field values corresponding to a crossing between the ground state and the
excited states of the molecule. We propose that this is due to a
cross-relaxation effect between the nuclear Zeeman reservoir and the reservoir
of the Zeeman levels of the molecule. This effect provides a powerful tool to
investigate quantum dynamical phenomena at level crossing.",9905346v1
1999-12-01,Impurity relaxation mechanism for dynamic magnetization reversal in a single domain grain,"The interaction of coherent magnetization rotation with a system of two-level
impurities is studied. Two different, but not contradictory mechanisms, the
`slow-relaxing ion' and the `fast-relaxing ion' are utilized to derive a system
of integro-differential equations for the magnetization. In the case that the
impurity relaxation rate is much greater than the magnetization precession
frequency, these equations can be written in the form of the Landau-Lifshitz
equation with damping. Thus the damping parameter can be directly calculated
from these microscopic impurity relaxation processes.",9912014v1
2000-01-21,Theory of Diluted Magnetic Semiconductor Ferromagnetism,"We present a theory of carrier-induced ferromagnetism in diluted magnetic
semiconductors (III_{1-x} Mn_x V) which allows for arbitrary itinerant-carrier
spin polarization and dynamic correlations. Both ingredients are essential in
identifying the system's elementary excitations and describing their
properties. We find a branch of collective modes, in addition to the spin waves
and Stoner continuum which occur in metallic ferromagnets, and predict that the
low-temperature spin stiffness is independent of the strength of the exchange
coupling between magnetic ions and itinerant carriers. We discuss the
temperature dependence of the magnetization and the heat capacity.",0001320v2
2000-06-23,Importance of Correlation Effects on Magnetic Anisotropy in Fe and Ni,"We calculate magnetic anisotropy energy of Fe and Ni by taking into account
the effects of strong electronic correlations, spin-orbit coupling, and
non-collinearity of intra-atomic magnetization. The LDA+U method is used and
its equivalence to dynamical mean-field theory in the static limit is
emphasized. Both experimental magnitude of MAE and direction of magnetization
are predicted correctly near U=4 eV for Ni and U=3.5 eV for Fe. Correlations
modify one-electron spectra which are now in better agreement with experiments.",0006385v1
2001-07-03,Importance of Correlation Effects on Magnetic Anisotropy in Fe and Ni,"We calculate magnetic anisotropy energy of Fe and Ni by taking into account
the effects of strong electronic correlations, spin-orbit coupling, and
non-collinearity of intra--atomic magnetization. The LDA+U method is used and
its equivalence to dynamical mean--field theory in the static limit is
emphasized. Both experimental magnitude of MAE and direction of magnetization
are predicted correctly near U=1.9 eV, J=1.2 eV for Ni and U=1.2 eV, J=0.8 eV
for Fe. Correlations modify the one--electron spectra which are now in better
agreement with experiments.",0107063v1
2001-09-27,Electric field induced by magnetic flux motion in superconductor containing fractal clusters of a normal phase,"The influence of the fractal clusters of a normal phase, which act as pinning
centers, on the dynamics of magnetic flux in percolative type-II superconductor
is considered. The voltage-current characteristics of such a superconductor are
obtained taking into account the effect of fractal properties of cluster
boundaries on the magnetic flux trapping. It is revealed that the fractality
reduces the electric field arising from magnetic flux motion and thereby raises
the critical current of a superconductor.",0109523v1
2002-03-05,Scaling behaviour of the relaxation in quantum chains,"We consider the nonequilibrium time evolution of the transverse magnetization
in the critical Ising and $XX$ quantum chains. For some inhomogeneously
magnetized initial states we derive analytically the transverse magnetization
profiles and show that they evolve into scaling forms in the long-time limit.
In particular it is seen that the Ising chain exhibits some similarities with
the conserved dynamics $XX$ chain. That is, after a transient regime, the total
residual magnetization in the transverse direction is also conserved in the
Ising case. A class of general initial states is also considered.",0203078v1
2002-09-02,Quasiperiodic tilings under magnetic field,"We study the electronic properties of a two-dimensional quasiperiodic tiling,
the isometric generalized Rauzy tiling, embedded in a magnetic field. Its
energy spectrum is computed in a tight-binding approach by means of the
recursion method. Then, we study the quantum dynamics of wave packets and
discuss the influence of the magnetic field on the diffusion and spectral
exponents. Finally, we consider a quasiperiodic superconducting wire network
with the same geometry and we determine the critical temperature as a function
of the magnetic field.",0209037v1
2002-09-03,Evidence for weak itinerant long-range magnetic correlations in UGe2,"Positive muon spin relaxation measurements performed on the ferromagnet UGe2
reveal, in addition to the well known localized 5f-electron density responsible
for the bulk magnetic properties, the existence of itinerant quasi-static
magnetic correlations. Their critical dynamics is well described by the
conventional dipolar Heisenberg model. These correlations involve small
magnetic moments.",0209054v1
2002-11-22,Nonlinear microscopic relaxation of uniform magnetization precession,"Dynamic relaxation for nonlinear magnetization excitation is analyzed. For
direct processes, such as magnon-electron scattering and two-magnon scattering,
the relaxation rate is determined from the linear case simply by utilizing the
magnetization oscillation frequency for nonlinear excitation. For an indirect
process, such as slow-relaxing impurities, the analysis gives an additional
relaxation term proportional to the excitation level. In all cases the
effective magnetization damping is increased compared to
Landau-Lifshitz-Gilbert damping.",0211499v1
2002-12-16,Characterization of the Energy Structure and Adiabatic Magnetization Process of V15,"The energy structure of the V$_{15}$ is studied using exact diagonalization
methods, and the adiabatic changes of the magnetization in the system with the
sweeping field are investigated. We confirm that the Dzyaloshinskii-Moriya
interaction leads to an energy gap which allows the adiabatic change of the
magnetization which has been found experimentally by Chiorescu et al., and also
predict novel dynamics of the magnetization due to this energy level structure.",0212354v1
2003-05-22,Coherent Spin Oscillations in a Disordered Magnet,"Most materials freeze when cooled to sufficiently low temperature. We find
that magnetic dipoles randomly distributed in a solid matrix condense into a
spin liquid with spectral properties on cooling that are the diametric opposite
of those for conventional glasses. Measurements of the non-linear magnetic
dynamics in the low temperature liquid reveal the presence of coherent spin
oscillations composed of hundreds of spins with lifetimes up to ten seconds.
These excitations can be labeled by frequency, manipulated by the magnetic
fields from a loop of wire, and permit the encoding of information at multiple
frequencies simultaneously.",0305541v1
2003-06-11,Magnetic energy-level diagrams of high-spin (Mn$_{12}$-acetate) and low-spin (V$_{15}$) molecules,"The magnetic energy-level diagrams for models of the Mn12 and V15 molecule
are calculated using the Lanczos method with full orthogonalization and a
Chebyshev-polynomial-based projector method. The effect of the
Dzyaloshinskii-Moriya interaction on the appearance of energy-level repulsions
and its relevance to the observation of steps in the time-dependent
magnetization data is studied. We assess the usefulness of simplified models
for the description of the zero-temperature magnetization dynamics.",0306275v1
2004-02-10,Effect of uniaxial and biaxial crystal-field potential on magnetic properties of a mixed spin-1/2 and spin-1 Ising model on honeycomb lattice,"Magnetic properties of a mixed spin-1/2 and spin-1 Ising model on honeycomb
lattice are exactly investigated within the framework of generalized
star-triangle mapping transformation. The particular attention is focused on
the effect of uniaxial and biaxial crystal-field anisotropies that basically
influence the magnetic behaviour of the spin-1 atoms. Our results for the basic
thermodynamic quantities, as well as the dynamical time-dependent
autocorrelation function indicate the spin tunneling between the $| +1>$ and $|
- 1>$ states in the magnetically ordered phase.",0402280v2
2004-07-20,Field-induced phase transition in the periodic Anderson model,"We investigate the effect of magnetic fields on a Kondo insulator by using
the periodic Anderson model. The analysis by dynamical mean field theory
combined with quantum Monte Carlo simulations reveals that the magnetic field
drives the Kondo insulator to a transverse antiferromagnetic insulator at low
temperatures. We calculate the staggered spin susceptibility and find its
divergence signaling the antiferromagnetic instability. Further investigation
of the spin correlation functions and the magnetization process clarifies how
the magnetic field suppresses the Kondo singlet formation and induces the
transverse antiferromagnetic ordering.",0407518v1
2004-07-28,Magnetic properties of a mixed spin-1/2 and spin-3/2 Ising model with an uniaxial and biaxial crystal-field potential,"Magnetic properties of a mixed spin-1/2 and spin-3/2 Ising model on honeycomb
lattice are investigated within the framework of an exact star-triangle mapping
transformation. The particular attention is focused on the effect of uniaxial
and biaxial crystal-field potentials that basically influence the magnetic
behaviour of the spin-3/2 atoms. Our results for the basic thermodynamic
quantities, as well as the dynamical time-dependent autocorrelation function
indicate the spin tunneling between the $| \pm \frac32>$ and $| \mp \frac12>$
states in two different magnetically ordered phases OP$_1$ and OP$_2$,
respectively.",0407726v2
2004-11-09,Magnetization reversal in spin patterns with complex geometry,"We study field-driven dynamics of spins with antiferromagnetic interaction
along the links of a complex substrate geometry, which is modeled by graphs of
a controlled connectivity distribution. The magnetization reversal occurs in
avalanches of spin flips, which are pinned by the topological constraints of
the underlying graph. The hysteresis loop and avalanche sizes are analyzed and
classified in terms of graph's connectivity and clustering. The results are
relevant for magnets with a hierarchical spatial inhomogeneity and for design
of nanoscale magnetic devices.",0411223v1
2005-04-13,Current-induced suppression of superconductivity in tantalum thin films at zero magnetic field,"We report our findings on the mechanism of current-induced suppression of
superconductivity in amorphous tantalum films as thin as 5nm under zero applied
magnetic field. Our results indicate that the applied current generates
magnetic vortices threading the films, and the dynamics of these vortices leads
to the suppression of the superconductivity. Our findings also imply that the
motion of current-generated magnetic vortices gives rise to nonlinear transport
of which characteristics closely resemble those expected in Kosterlitz-Thouless
theories with strong finite size effects.",0504340v1
2005-08-30,Effects of spin current on ferromagnets,"When a spin-polarized current flows through a ferromagnet, the local
magnetization receives a spin torque. Two consequences of this spin torque are
studied. First, the uniformly magnetized ferromagnet becomes unstable if a
sufficiently large current is applied. The characteristics of the instability
include spin wave generation and magnetization chaos. Second, the spin torque
has profound effects on the structure and dynamics of the magnetic domain wall.
A detail analysis on the domain wall mass, kinetic energy and wall depinning
threshold is given.",0508735v1
2005-08-31,Quantum Spin Effect and Short-Range Order above the Curie Temperature,"Using quantum Heisenberg model calculations with Green's function technique
generalized for arbitrary spins, we found that for a system of small spins the
quantum spin effects significantly contribute to the magnetic short-range order
and strongly affect physical properties of magnets. The spin dynamics
investigation confirms that these quantum spin effects favor the persistence of
propagating spin-wave excitations above the Curie temperature. Our
investigation suggests a reconsideration of prevailing point of view on finite
temperature magnetism to include quantum effects and the magnetic short-range
order.",0508781v1
2005-11-09,Magnetic properties of artificially prepared ordered two-dimensional shunted and unshunted Nb-AlO_x-Nb Josephson Junction Arrays,"In this Chapter, we present our latest experimental results (along with their
theoretical interpretation) related to some unusual magnetic properties of
artificially prepared ordered two-dimensional Josephson junction arrays (of
both shunted and unshunted Nb-AlO_x-Nb tunnel junctions). Namely, we address
the following topics: (i) influence of non-uniform critical current density
profile on magnetic field behavior of AC susceptibility, (ii) origin of the
so-called dynamic reentrance phenomenon, and (iii) manifestation of novel
geometric quantization effects in temperature behavior of AC magnetic response.",0511218v1
2006-01-11,Relaxing-Precessional Magnetization Switching,"A new way of magnetization switching employing both the spin-transfer torque
and the torque by a magnetic field is proposed. The solution of the
Landau-Lifshitz-Gilbert equation shows that the dynamics of the magnetization
in the initial stage of the switching is similar to that in the precessional
switching, while that in the final stage is rather similar to the relaxing
switching. We call the present method the relaxing-precessional switching. It
offers a faster and lower-power-consuming way of switching than the relaxing
switching and a more controllable way than the precessional switching.",0601227v1
2006-03-10,Stray-field-induced modification of coherent spin dynamics,"Electron spins in an InGaAs semiconductor quantum well are used as a
magnetometer of magnetic stray-fields from patterned Fe stripes. Using
time-resolved Faraday rotation, the coherent precession of quantum-well spins
in the inhomogeneous field below the Fe stripes is measured for varying
magnetic fields. Comparing with reference stripes made of Au, we find an
enhancement of the spin precession frequency proportional to the Fe
magnetization, in line with a decrease of the spin decay time, which is
attributed to the inhomogeneous magnetic stray-field in the quantum well layer.",0603296v1
2006-07-03,On-chip detection of ferromagnetic resonance of a single submicron permalloy strip,"We measured ferromagnetic resonance of a single submicron ferromagnetic
strip, embedded in an on-chip microwave transmission line device. The method
used is based on detection of the oscillating magnetic flux due to the
magnetization dynamics, with an inductive pick-up loop. The dependence of the
resonance frequency on applied static magnetic field agrees very well with the
Kittel formula, demonstrating that the uniform magnetization precession mode is
being driven.",0607036v1
2006-09-05,Out-of-plane spin polarization from in-plane electric and magnetic fields,"We show that the joint effect of spin-orbit and magnetic fields leads to a
spin polarization perpendicular to the plane of a two-dimensional electron
system with Rashba spin-orbit coupling and in-plane parallel dc magnetic and
electric fields, for angle-dependent impurity scattering or nonparabolic energy
spectrum, while only in-plane polarization persists for simplified models. We
derive Bloch equations, describing the main features of recent experiments,
including the magnetic field dependence of static and dynamic responses.",0609078v1
2006-11-09,Chiral scattering in complex magnets,"General properties of the chiral scattering of polarized neutrons are
considered for two possible axial vector interactions: Zeeman energy and
non-alternating Dzyaloshinskii-Moriya interaction. Behavior in magnetic field
of helical magnetic structures is discussed for $Mn Si$ and magneto-electric
materials. The dynamical chiral fluctuations in magnetic field are considered
briefly. The chiral fluctuations in materials with the Dzyaloshinskii-Moriya
interaction are discussed and an assumption is made that above the transition
temperature they have to be incommensurate.",0611244v1
2006-11-27,Microscopic Calculation of Spin Torques and Forces,"Spin torques, that is, effects of conduction electrons on magnetization
dynamics, are calculated microscopically in the first order in spatial gradient
and time derivative of magnetization. Special attention is paid to the
so-called \beta-term and the Gilbert damping, \alpha, in the presence of
electrons' spin-relaxation processes, which are modeled by quenched magnetic
impurities. Two types of forces that the electric/spin current exerts on
magnetization are identified based on a general formula relating the force to
the torque.",0611669v1
2006-12-05,Dynamics of Spontaneous Magnetization Reversal in Exchange Biased Heterostructures,"The dependence of thermally induced spontaneous magnetization reversal on
time-dependent cooling protocols was studied. Slower cooling and longer waiting
close to the N\`{e}el temperature of the antiferromagnet ($T_N$) enhances the
magnetization reversal. Cycling the temperature around $T_N$ leads to a thermal
training effect under which the reversal magnitude increases with each cycle.
These results suggest that spontaneous magnetization reversal is energetically
favored, contrary to our present understanding of positive exchange bias.",0612113v1
1996-08-23,The effect of Silk damping on primordial magnetic fields,"We study the effects of plasma viscosity on the dynamics of primordial
magnetic fields by simulating magnetohydrodynamics in the early universe by
appropriate non-linear cascade models. We find numerically that even in the
presence of large kinetic viscosity, magnetic energy is transferred to large
length scales. There are indications, however, that the inverse cascade stops
at a given time which depends on the magnitude of viscosity. For realistic
viscosities we do not find equipartition between magnetic and kinetic energies.",9608422v1
2002-03-02,Spin waves in the magnetized plasma of a supernova and its excitation by neutrino fluxes,"The spin effects on electromagnetic waves in a strongly magnetized plasma
with rare collisions is considered with the help of relativistic kinetic
equations, which takes into account the electron spin dynamics in the
selfconsistent electric and magnetic fields. It is shown that for
electromagnetic waves propagating almost perpendicular to ambient magnetic
field the spin effects become essential in the vicinity of electron
gyrofrequency and the corresponding wave dispersion and growth rate of the
electromagnetic spin waves in the presence of intense quasi monoenergetic
fluxes of neutrino is determined.",0203020v1
2005-06-22,Primordial magnetic seed fields from extra dimensions,"Dynamical extra dimensions break the conformal invariance of Maxwell's
equations in four dimensions. A higher dimensional background with $n$
contracting extra dimensions and four expanding dimensions is matched to an
effectively four dimensional standard radiation dominated universe. The
resulting spectrum for the magnetic field is calculated taking into account
also the momenta along the extra dimensions. Imposing constraints from
observations an upper limit on the strength of magnetic seed fields is found.
Depending on the number of extra dimensions, cosmologically interesting
magnetic fields can be created.",0506212v1
2004-10-04,"Magnetic condensation, Abelian dominance, and instability of Savvidy vacuum in Yang-Mills theory","We propose a novel type of color magnetic condensation originating from
magnetic monopoles so that it provides the mass of off-diagonal gluons in the
Yang-Mills theory.
  This dynamical mass generation enables us to explain the infrared Abelian
dominance and monopole dominance by way of a non-Abelian Stokes theorem, which
supports the dual superconductivity picture of quark confinement. Moreover, we
show that the instability of Savvidy vacuum disappears by sufficiently large
color magnetic condensation.",0410024v1
2005-10-24,A Stable Magnetic Background in SU(2) QCD,"Motivated by the instability of the Savvidy-Nielsen-Olesen (SNO) vacuum we
make a systematic search for a stable magnetic background in pure SU(2) QCD. It
is shown that Wu-Yang monopole-antimonopole pair is unstable under vacuum
fluctuations. However, it is shown that a pair of axially symmetric
monopole-antimonopole string configuration is stable, provided the distance
between the two strings is small enough (less than a critical value). The
existence of a stable monopole-antimonopole string background strongly supports
that a magnetic condensation of monopole-antimonopole pairs can indeed generate
a dynamical symmetry breaking, and thus a desired magnetic confinement of
color, in QCD.",0510201v1
2006-03-04,Sharp Spectral Asymptotics for two-dimensional Schrödinger operator with a strong degenerating magnetic field,"I consider two-dimensional Schr\""odinger operator with degenerating magnetic
field and in the generic situation I derive spectral asymptotics as $h\to +0$
and $\mu\to +\infty$ where $h$ and $\mu$ are Planck and coupling parameters
respectively. The remainder estimate is $O(\mu^{-{\frac 1 2}}h^{-1})$ which is
between $O(\mu^{-1}h^{-1})$ valid as magnetic field non-degenerates and
$O(h^{-1})$ valid as magnetic field is identically 0. As $\mu$ is close to its
maximal reasonable value $O(h^{-2})$ the principal part contains correction
terms associated with short periodic trajectories of the corresponding
classical dynamics.",0603114v1
2001-05-01,Rectangular microwave resonators with magnetic anisotropy. Mapping onto pseudo integrable rhombus,"The rectangular microwave resonator filed by a ferrite with uniaxial magnetic
anisotropy is considered. It is shown that this task can be reduced to an empty
rhombus resonator with vertex angle defined by external magnetic field provided
the magnetic anisotropy of the ferrite is strong. Therefore statistics of eigen
frequencies for TM modes is described by the Brody or semi-Poisson distribution
with some exceptional cases.",0105001v1
2001-11-23,Shape transformations in rotating ferrofluid drops,"Floating drops of magnetic fluid can be brought into rotation by applying a
rotating magnetic field. We report theoretical and experimental results on the
transition from a spheroid equilibrium shape to non-axissymmetrical three-axes
ellipsoids at certain values of the external field strength. The transitions
are continuous for small values of the magnetic susceptibility and show
hysteresis for larger ones. In the non-axissymmetric shape the rotational
motion of the drop consists of a vortical flow inside the drop combined with a
slow rotation of the shape. Nonlinear magnetization laws are crucial to obtain
quantitative agreement between theory and experiment.",0111054v1
2001-12-09,On Generation of magnetic field in astrophysical bodies,"In this letter we compute energy transfer rates from velocity field to
magnetic field in MHD turbulence using field-theoretic method. The striking
result of our field theoretic calculation is that there is a large energy
transfer rate from the large-scale velocity field to the large-scale magnetic
field. We claim that the growth of large-scale magnetic energy is primarily due
to this transfer. We reached the above conclusion without any linear
approximation like that in $\alpha$-dynamo.",0112013v1
2002-12-03,Exact solutions for magnetic annihilation in curvilinear geometry,"New exact solutions of the steady and incompressible 2D MHD equations in
polar coordinates are presented. The solutions describe the process of
reconnective magnetic annihilation in a curved current layer. They are
particularly interesting for modeling magnetic reconnection in solar flares
caused by the interaction of three photospheric magnetic sources.",0212017v1
2004-03-22,Relativistic Quasilinear Diffusion in Axisymmetric Magnetic Geometry for Arbitrary-Frequency Electromagnetic Fluctuations,"A relativistic bounce-averaged quasilinear diffusion equation is derived to
describe stochastic particle transport associated with arbitrary-frequency
electromagnetic fluctuations in a nonuniform magnetized plasma. Expressions for
the elements of a relativistic quasilinear diffusion tensor are calculated
explicitly for magnetically-trapped particle distributions in axisymmetric
magnetic geometry in terms of gyro-drift-bounce wave-particle resonances. The
resonances can destroy any one of the three invariants of the unperturbed
guiding-center Hamiltonian dynamics.",0403105v2
2004-11-17,From normal state to magnetic storms in terms of fractal dynamics,"We show that distinctive alterations in scaling parameters of Dst index time
series occur as a strong magnetic storm approaches. These alterations reveal a
gradual reduction of complexity as the catastrophic event approaches. The
increase of the susceptibility coupled with the transition from anti-persistent
to persistent behavior may indicate that the onset of a severe magnetic storm
is imminent. The preparation of a major magnetic storm could be studied in
terms of ""Intermittent Criticality"". The analysis also suggests that the
continuous scale invariance is partially broken into a discrete scale
invariance symmetry.",0411157v1
2005-05-26,Low magnetic Prandtl number dynamos with helical forcing,"We present direct numerical simulations of dynamo action in a forced Roberts
flow. The behavior of the dynamo is followed as the mechanical Reynolds number
is increased, starting from the laminar case until a turbulent regime is
reached. The critical magnetic Reynolds for dynamo action is found, and in the
turbulent flow it is observed to be nearly independent on the magnetic Prandtl
number in the range from 0.3 to 0.1. Also the dependence of this threshold with
the amount of mechanical helicity in the flow is studied. For the different
regimes found, the configuration of the magnetic and velocity fields in the
saturated steady state are discussed.",0505192v1
2005-06-29,Dynamic and Stagnating Plasma Flow Leading to Magnetic Flux Tube Collimation,"Highly collimated, plasma-filled magnetic flux tubes are frequently observed
on galactic, stellar and laboratory scales. We propose that a single, universal
magnetohydrodynamic pumping process explains why such collimated, plasma-filled
magnetic flux tubes are ubiquitous. Experimental evidence from carefully
diagnosed laboratory simulations of astrophysical jets confirms this assertion
and is reported here. The magnetohydrodynamic process pumps plasma into a
magnetic flux tube and the stagnation of the resulting flow causes this flux
tube to become collimated.",0506221v1
2006-07-28,Suppressing the Rayleigh-Taylor instability with a rotating magnetic field,"The Rayleigh-Taylor instability of a magnetic fluid superimposed on a
non-magnetic liquid of lower density may be suppressed with the help of a
spatially homogeneous magnetic field rotating in the plane of the undisturbed
interface. Starting from the complete set of Navier-Stokes equations for both
liquids a Floquet analysis is performed which consistently takes into account
the viscosities of the fluids. Using experimentally relevant values of the
parameters we suggest to use this stabilization mechanism to provide controlled
initial conditions for an experimental investigation of the Rayleigh-Taylor
instability.",0607263v1
2006-10-19,Dynamic detection of a single bacterium: nonlinear rotation rate shifts of driven magnetic microsphere stages,"We report on a new technique which was used to detect single Escherichia coli
that is based on the changes in the nonlinear rotation of a magnetic
microsphere driven by an external magnetic field. The presence of one
Escherichia Coli bacterium on the surface of a 2.0 micron magnetic microsphere
caused an easily measurable change in the drag of the system and, therefore, in
the nonlinear rotation rate. The straight-forward measurement uses standard
microscopy techniques and the observed average shift in the nonlinear rotation
rate changed by a factor of ~3.8.",0610144v1
2004-04-20,Wave function collapses in a single spin magnetic resonance force microscopy,"We study the effects of wave function collapses in the oscillating cantilever
driven adiabatic reversals (OSCAR) magnetic resonance force microscopy (MRFM)
technique. The quantum dynamics of the cantilever tip (CT) and the spin is
analyzed and simulated taking into account the magnetic noise on the spin. The
deviation of the spin from the direction of the effective magnetic field causes
a measurable shift of the frequency of the CT oscillations. We show that the
experimental study of this shift can reveal the information about the average
time interval between the consecutive collapses of the wave function",0404105v1
2006-10-20,Three-qubit dynamics of entanglement in magnetic field,"A closed system of the equations for the local Bloch vectors and spin
correlation functions is obtained by decomplexification of the Liouville-von
Neumann equation for three magnetic qubits with the exchange interaction, that
takes place in an arbitrary time-dependent external magnetic field. The
numerical comparative analysis of entanglement is carried out depending on
initial conditions and the magnetic field modulation. The present study may be
useful for analysis of interference experiments and in the field of quantum
computing.",0610176v1
2007-07-11,Experimental application of sum rules for electron energy loss magnetic chiral dichroism,"We present a derivation of the orbital and spin sum rules for magnetic
circular dichroic spectra measured by electron energy loss spectroscopy in a
transmission electron microscope. These sum rules are obtained from the
differential cross section calculated for symmetric positions in the
diffraction pattern. Orbital and spin magnetic moments are expressed explicitly
in terms of experimental spectra and dynamical diffraction coefficients. We
estimate the ratio of spin to orbital magnetic moments and discuss first
experimental results for the Fe L_{2,3} edge.",0707.1585v1
2007-11-15,Potential for measurement of the tensor magnetic polarizability of the deuteron in storage ring experiments,"General formulas describing deuteron spin dynamics in storage rings with
allowance for the tensor electric and magnetic polarizabilities are derived. It
is found that an initially tensor-polarized deuteron beam can acquire a final
horizontal vector polarization of the order of 1%. This effect allows one to
measure the tensor magnetic polarizability of the deuteron in storage ring
experiments. We also confirm an existence of the effect found by Baryshevsky
and Gurinovich, hep-ph/0506135 and Baryshevsky, hep-ph/0510158; hep-ph/0603191
that the tensor magnetic polarizability of the deuteron causes the spin
rotation with two frequencies and experiences beating for polarized deuteron
beams in storage rings.",0711.2390v2
2008-05-15,Dynamical Theory of Artificial Optical Magnetism Produced by Rings of Plasmonic Nanoparticles,"We present a detailed analytical theory for the plasmonic nanoring
configuration first proposed in [A. Alu, A. Salandrino, N. Engheta, Opt. Expr.
14, 1557 (2006)], which is shown to provide negative magnetic permeability and
negative index of refraction at infrared and optical frequencies. We show
analytically how the nanoring configuration may provide superior performance
when compared to some other solutions for optical negative index materials,
offering a more 'pure' magnetic response at these high frequencies, which is
necessary for lowering the effects of radiation losses and absorption.
Sensitivity to losses and the bandwidth of operation of this magnetic inclusion
are also investigated in details and compared with other available setups.",0805.2329v1
2008-08-27,"Coherent control of magnetization precession in ferromagnetic semiconductor (Ga,Mn)As","We report single-color, time resolved magneto-optical measurements in
ferromagnetic semiconductor (Ga,Mn)As. We demonstrate coherent optical control
of the magnetization precession by applying two successive ultrashort laser
pulses. The magnetic field and temperature dependent experiments reveal the
collective Mn-moment nature of the oscillatory part of the time-dependent Kerr
rotation, as well as contributions to the magneto-optical signal that are not
connected with the magnetization dynamics.",0808.3738v2
2008-09-16,Stochastic dynamics of magnetization in a ferromagnetic nanoparticle out of equilibrium,"We consider a small metallic particle (quantum dot) where ferromagnetism
arises as a consequence of Stoner instability. When the particle is connected
to electrodes, exchange of electrons between the particle and the electrodes
leads to a temperature- and bias-driven Brownian motion of the direction of the
particle magnetization. Under certain conditions this Brownian motion is
described by the stochastic Landau-Lifshitz-Gilbert equation. As an example of
its application, we calculate the frequency-dependent magnetic susceptibility
of the particle in a constant external magnetic field, which is relevant for
ferromagnetic resonance measurements.",0809.2611v1
2008-10-23,Magnetic moment manipulation by a Josephson current,"We consider a Josephson junction where the weak-link is formed by a
non-centrosymmetric ferromagnet. In such a junction, the superconducting
current acts as a direct driving force on the magnetic moment. We show that the
a.c. Josephson effect generates a magnetic precession providing then a feedback
to the current. Magnetic dynamics result in several anomalies of current-phase
relations (second harmonic, dissipative current) which are strongly enhanced
near the ferromagnetic resonance frequency.",0810.4286v3
2008-12-10,Magnetic Bianchi I Universe in Loop Quantum Cosmology,"We examine the dynamical consequences of homogeneous cosmological magnetic
fields in the framework of loop quantum cosmology. We show that a big-bounce
occurs in a collapsing magnetized Bianchi I universe, thus extending the known
cases of singularity-avoidance. Previous work has shown that perfect fluid
Bianchi I universes in loop quantum cosmology avoid the singularity via a
bounce. The fluid has zero anisotropic stress, and the shear anisotropy in this
case is conserved through the bounce. By contrast, the magnetic field has
nonzero anisotropic stress, and shear anisotropy is not conserved through the
bounce. After the bounce, the universe enters a classical phase. The addition
of a dust fluid does not change these results qualitatively.",0812.1889v1
2009-02-16,Control of Transport-barrier relaxations by Resonant Magnetic Perturbations,"Transport-barrier relaxation oscillations in the presence of resonant
magnetic perturbations are investigated using three-dimensional global fluid
turbulence simulations from first principles at the edge of a tokamak. It is
shown that resonant magnetic perturbations have a stabilizing effect on these
relaxation oscillations and that this effect is due mainly to a modification of
the pressure profile linked to the presence of both residual residual magnetic
island chains and a stochastic layer.",0902.2710v1
2009-04-11,Planetary dynamos,"The theory of planetary dynamos and its applications to observed phenomena of
planetary magnetism are outlined. It is generally accepted that convection
flows driven by thermal or compositional buoyancy are the most likely source
for the sustenance of global planetary magnetic fields. While the existence of
dynamos in electrically conducting fluid planetary cores provides constraints
on properties of the latter, the lack of knowledge about time dependences of
the magnetic fields and about their toroidal components together with the
restricted parameter regions accessible to theory have prevented so far a full
understanding of the phenomena of planetary magnetism.",0904.1795v1
2009-06-02,Magnetic reversals in a simple model of MHD,"We study a simple magnetohydrodynamical approach in which hydrodynamics and
MHD turbulence are coupled in a shell model, with given dynamo constrains in
the large scales. We consider the case of a low Prandtl number fluid for which
the inertial range of the velocity field is much wider than that of the
magnetic field. Random reversals of the magnetic field are observed and it
shown that the magnetic field has a non trivial evolution linked to the nature
of the hydrodynamics turbulence.",0906.0427v1
2009-07-08,Estimate of the magnetic field strength in heavy-ion collisions,"Magnetic fields created in the noncentral heavy-ion collision are studied
within a microscopic transport model, namely the Ultrarelativistic Quantum
Molecular Dynamics model (UrQMD). Simulations were carried out for different
impact parameters within the SPS energy range ($E_{lab} = 10 - 158 A$ GeV) and
for highest energies accessible for RHIC. We show that the magnetic field
emerging in heavy-ion collisions has the magnitude of the order of $eB_y \sim
10^{-1} m_\pi^2$ for the SPS energy range and $eB_y \sim m_\pi^2$ for the RHIC
energies. The estimated value of the magnetic field strength for the LHC energy
amounts to $eB_y \sim 15 m_\pi^2$.",0907.1396v1
2009-07-15,Quantum Melting of Spin Ice: Emergent Cooperative Quadrupole and Chirality,"A quantum melting of the spin ice is proposed for pyrochlore-lattice magnets
Pr$_2TM_2$O$_7$ ($TM=$Ir, Zr, and Sn). The quantum superexchange Hamiltonian
having a nontrivial magnetic anisotropy is derived in the basis of atomic
non-Kramers magnetic doublets. The ground states exhibit a cooperative
ferroquadrupole and pseudospin chirality, forming a magnetic analog of smectic
liquid crystals. Our theory accounts for dynamic spin-ice behaviors
experimentally observed in Pr$_2TM_2$O$_7$.",0907.2536v3
2009-12-27,Electron structure of the Falicov-Kimball model with a magnetic field,"The two-dimensional Falicov-Kimball model in the presence of a perpendicular
magnetic field is investigated by the dynamical mean-field theory. Within the
model the interplay between electron correlations and the fine electron
structure due to the magnetic field is essentially emerged. Without electron
correlations the magnetic field induces the electron structure to the so-called
Hofstadter butterfly. It is found that when electron correlations drives the
metal-insulator transition, they simultaneously smear out the fine structure of
the Hofstadter butterfly. In a long-range ordered phase, the electron
correlation induced gap preserves the fine structure, but it separates the
Hofstadter butterfly into two wings.",0912.5082v1
2010-01-04,Evolutions of Magnetized and Rotating Neutron Stars,"We study the evolution of magnetized and rigidly rotating neutron stars
within a fully general relativistic implementation of ideal
magnetohydrodynamics with no assumed symmetries in three spatial dimensions.
The stars are modeled as rotating, magnetized polytropic stars and we examine
diverse scenarios to study their dynamics and stability properties. In
particular we concentrate on the stability of the stars and possible critical
behavior. In addition to their intrinsic physical significance, we use these
evolutions as further tests of our implementation which incorporates new
developments to handle magnetized systems.",1001.0575v2
2010-01-15,On the Sensitivity of the H$α$ Scattering Polarization to Chromospheric Magnetism,"A particularly interesting line for exploring the physical conditions of the
quiet solar chromosphere is H$\alpha$, but its intensity profile is
magnetically insensitive and the small circular polarization signatures
produced by the longitudinal Zeeman effect come mainly from the underlying
photosphere. Here we show that the Hanle effect in H$\alpha$ provides
quantitative information on the magnetism of the quiet chromosphere. To this
end, we calculate the response function of the emergent scattering polarization
to perturbations in the magnetic field.",1001.2720v1
2010-02-12,Spectropolarimetric investigations of the magnetization of the quiet Sun chromosphere,"This paper reviews some recent advances in the development and application of
polarized radiation diagnostics to infer the mean magnetization of the quiet
solar atmosphere, from the near equilibrium photosphere to the highly
non-equilibrium upper chromosphere. In particular, I show that interpretations
of the scattering polarization observed in some spectral lines suggest that
while the magnetization of the photosphere and upper chromosphere is very
significant, the lower chromosphere seems to be weakly magnetized.",1002.2615v1
2010-05-14,Local helioseismology and the active Sun,"The goal of local helioseismology is to elicit three-dimensional information
about the sub-surface (or far-side) structure and dynamics of the Sun from
observations of the helioseismic wave field at the surface. The physical
quantities of interest include flows, sound-speed deviations and magnetic
fields. However, strong surface magnetic fields induce large perturbations to
the waves making inversions difficult to interpret. The purpose of this paper
is to outline the methods of analysis used in local helioseismology, review
discoveries associated with the magnetic Sun made using local helioseismology
from the past three years, and highlight the efforts towards imaging the
interior in the presence of strong magnetic fields.",1005.2526v1
2010-05-17,Splitting of the Landau levels by magnetic perturbations and Anderson transition in 2D-random magnetic media,"In this note we consider a Landau Hamiltonian perturbed by a random magnetic
potential of Anderson type. For a given number of bands, we prove the existence
of both strongly localized states at the edges of the spectrum and dynamical
delocalization near the center of the bands in the sense that wave packets
travel at least at a given minimum speed. We provide explicit examples of
magnetic perturbations that split the Landau levels into full intervals of
spectrum.",1005.2789v2
2010-05-25,Dynamics of Bianchi I Universe with Magnetized Anisotropic Dark Energy,"We study Bianchi type $I$ cosmological model in the presence of magnetized
anisotropic dark energy. The energy-momentum tensor consists of anisotropic
fluid with anisotropic EoS $p=\omega{\rho}$ and a uniform magnetic field of
energy density $\rho_B$. We obtain exact solutions to the field equations using
the condition that expansion is proportional to the shear scalar. The physical
behavior of the model is discussed with and without magnetic field. We conclude
that universe model as well as anisotropic fluid do not approach isotropy
through the evolution of the universe.",1005.4480v1
2010-08-06,Spin Resonance and dc Current Generation in a Quantum Wire,"We show that in a quantum wire the spin-orbit interaction leads to a narrow
spin resonance at low temperatures, even in the absence of an external magnetic
field. A relatively weak dc magnetic field of a definite direction strongly
increases the resonance absorption. Linearly polarized resonance radiation
produces dynamic magnetization as well as electric and spin currents. The
effect strongly depends on the external magnetic field.",1008.1225v1
2010-09-24,Relaxation properties in classical diamagnetism,"In the present paper the problem of the relaxation of magnetization to
equilibrium (i.e., with no magnetization) is investigated numerically for a
variant of the well-known model introduced by Bohr to study the diamagnetism of
electrons in metals. Such a model is mathematically equivalent to a billiard
with obstacles in a magnetic field. We show that it is not guaranteed that
equilibrium is attained within the typical time scales of microscopic dynamics.
Indeed, considering an out of equilibrium state produced by an adiabatic
switching on of a magnetic field, we show that, depending on the values of the
parameters, one has a relaxation either to equilibrium or to a diamagnetic
(presumably metastable) state. The analogy with the relaxation properties in
the FPU problem is also pointed out.",1009.4861v1
2010-10-28,Magnetic field correlations in a random flow with strong steady shear,"We analyze magnetic kinematic dynamo in a conducting fluid where the
stationary shear flow is accompanied by relatively weak random velocity
fluctuations. The diffusionless and diffusion regimes are described. The growth
rates of the magnetic field moments are related to the statistical
characteristics of the flow describing divergence of the Lagrangian
trajectories. The magnetic field correlation functions are examined, we
establish their growth rates and scaling behavior. General assertions are
illustrated by explicit solution of the model where the velocity field is
short-correlated in time.",1010.5904v1
2010-11-17,Magnetotransport and spin dynamics in an electron gas formed at oxide interfaces,"We investigate the spin-dependent transport properties of a two-dimensional
electron gas formed at oxides' interface in the presence of a magnetic field.
We consider several scenarios for the oxides' properties, including oxides with
co-linear or spiral magnetic and ferroelectric order. For spiral multiferroic
oxides, the magnetoelectric coupling and the topology of the localized magnetic
moments introduce additional, electric field controlled spin-orbit coupling
that affects the magneto-oscillation of the current. An interplay of this
spin-orbit coupling, the exchange field, and of the applied magnetic field
results in a quantum, gate-controlled spin and charge Hall conductance.",1011.3924v1
2010-12-25,Dark Soliton Interaction of Spinor Bose-Einstein Condensates in an Optical Lattice,"We study the magnetic soliton dynamics of spinor Bose-Einstein condensates in
an optical lattice which results in an effective Hamiltonian of anisotropic
pseudospin chain. An equation of nonlinear Schr\""odinger type is derived and
exact magnetic soliton solutions are obtained analytically by means of Hirota
method. Our results show that the critical external field is needed for
creating the magnetic soliton in spinor Bose-Einstein condensates. The soliton
size, velocity and shape frequency can be controlled in practical experiment by
adjusting the magnetic field. Moreover, the elastic collision of two solitons
is investigated in detail.",1012.5469v1
2011-02-01,Fast vacuum decay into particle pairs in strong electric and magnetic fields,"We discuss fermion pair productions in strong electric and magnetic fields.
We point out that, in the case of massless fermions, the vacuum persistency
probability per unit time and volume is zero in the strong electric and
magnetic fields, while it is finite when the magnetic field is absent. The
contribution from the lowest Landau level (LLL) dominates this phenomenon. We
also discuss dynamics of the vacuum decay, using an effective theory of the LLL
projection, taking into account the back reaction.",1102.0050v1
2011-05-24,Tunable anisotropic magnetism in trapped two-component Bose gases,"We theoretically address magnetic ordering at zero and finite temperature in
both homogeneous and trapped Bose-Bose mixtures in optical lattices. By using
Bosonic Dynamical Mean-Field Theory, we obtain the phase diagram of the
homogeneous two-component Bose-Hubbard model in a three-dimensional (3D) cubic
lattice, which features competing magnetic order of XY-ferromagnetic and
anti-ferromagnetic type in addition to the Mott and superfluid states. We show
that these magnetic phases persist also in the presence of a harmonic trap.",1105.4886v1
2011-07-25,Magnetic control of particle-injection in plasma based accelerators,"The use of an external transverse magnetic field to trigger and to control
electron self-injection in laser- and particle-beam driven wakefield
accelerators is examined analytically and through full-scale particle-in-cell
simulations. A magnetic field can relax the injection threshold and can be used
to control main output beam features such as charge, energy, and transverse
dynamics in the ion channel associated with the plasma blowout. It is shown
that this mechanism could be studied using state-of-the-art magnetic fields in
next generation plasma accelerator experiments.",1107.4912v1
2011-11-14,Subdiffusive transport in intergranular lanes on the Sun. The Leighton model revisited,"In this paper we consider a random motion of magnetic bright points (MBP)
associated with magnetic fields at the solar photosphere. The MBP transport in
the short time range [0-20 minutes] has a subdiffusive character as the
magnetic flux tends to accumulate at sinks of the flow field. Such a behavior
can be rigorously described in the framework of a continuous time random walk
leading to the fractional Fokker-Planck dynamics. This formalism, applied for
the analysis of the solar subdiffusion of magnetic fields, generalizes the
Leighton's model.",1111.3234v1
2012-02-16,Stability of nonstationary states of spin-2 Bose-Einstein condensates,"The dynamical stability of nonstationary states of homogeneous spin-2
rubidium Bose-Einstein condensates is studied. The states considered are such
that the spin vector remains parallel to the magnetic field throughout the time
evolution, making it possible to study the stability analytically. These states
are shown to be stable in the absence of an external magnetic field, but they
become unstable when a finite magnetic field is introduced. It is found that
the growth rate and wavelength of the instabilities can be controlled by tuning
the strength of the magnetic field and the size of the condensate.",1202.3658v2
2012-03-03,Motion of a deformable drop of magnetic fluid on a solid surface in a rotating magnetic field,"The behavior of a magnetic fluid drop lying on a solid horizontal surface and
surrounded by a nonmagnetic liquid under the action of a uniform magnetic field
which is rotating in a vertical plane with low frequency (of the order of 1 Hz)
has been investigated experimentally. Shape deformation and translatory motion
of the drop were observed and studied. The drop translation velocity for
different field amplitudes and field frequencies has been measured.",1203.0680v1
2012-03-05,Thermo-mechanical sensitivity calibration of nanotorsional magnetometers,"We report on the fabrication of sensitive nanotorsional resonators, which can
be utilized as magnetometers for investigating the magnetization dynamics in
small magnetic elements. The thermo-mechanical noise is calibrated with the
resonator displacement in order to determine the ultimate mechanical torque
sensitivity of the magnetometer.",1203.0998v1
2012-05-02,Piezoelectric control of the magnetic anisotropy via interface strain coupling in a composite multiferroic structure,"We investigate theoretically the magnetic dynamics in a
ferroelectric/ferromagnetic heterostructure coupled via strain-mediated
magnetoelectric interaction. We predict an electric field-induced magnetic
switching in the plane perpendicular to the magneto-crystalline easy axis, and
trace this effect back to the piezoelectric control of the magnetoelastic
coupling. We also investigate the magnetic remanence and the electric
coercivity.",1205.0391v1
2012-11-27,Solar magneto-convection,"An overview is given about recent developments and results of comprehensive
simulations of magneto-convective processes in the near-surface layers and
photosphere of the Sun. Simulations now cover a wide range of phenomena, from
whole active regions, over individual sunspots and pores, magnetic flux
concentrations and vortices in intergranular lanes, down to the intricate
mixed-polarity structure of the magnetic field generated by small-scale dynamo
action. The simulations in concert with high-resolution observations have
provided breakthroughs in our understanding of the structure and dynamics of
the magnetic fields in the solar photosphere.",1211.6253v1
2012-12-03,Magnetic resonance spectroscopy and characterization of magnetic phases for spinor Bose-Einstein condensates,"The response of spinor Bose-Einstein condensates to dynamical modulation of
magnetic fields is discussed with linear response theory. As an experimentally
measurable quantity, the energy absorption rate (EAR) is considered, and the
response function is found to access quadratic spin correlations which come
from the perturbation of the quadratic Zeeman term. By applying our formalism
to spin-1 condensates, we demonstrate that the EAR spectrum as a function of
the modulation frequency is able to characterize the different magnetically
ordered phases.",1212.0482v2
2012-12-10,Magnetic properties of the neutron in a uniform background field,"We present calculations of the magnetic moment and magnetic polarisability of
the neutron from the background field method. The calculations are performed on
$32^3\times64$ dynamical lattices generated by the PACS-CS collaboration and
made available via the ILDG. We consider uniform fields quantised by the
periodic spatial volume. We explore different approaches for improving the
quality of the fits used in the results. Also included are initial results for
the magnetic moment of the lowest lying negative parity nucleon states.",1212.1963v1
2012-12-20,Low-Temperature Magnetization Dynamics of Magnetic Molecular Solids in a Swept Field,"The swept-field experiments on magnetic molecular solids such as \Fe8 are
studied using Monte Carlo simulations. A kinetic equation is developed to
understand the phenomenon. It is found that the simulations provide a
quantitatively accurate account of the experiments. The kinetic equation
provides a similarly accurate account except at very low sweep velocities,
where it fails modestly. This failure is due to the neglect of short-range
correlations between the dipolar magnetic fields seen by the molecular spins.
Both the simulations and the kinetic equation provide a good understanding of
the distribution of these dipolar fields.",1212.5292v2
2013-02-04,Beam dynamics performances and applications of a low-energy electron-beam magnetic bunch compressor,"Many front-end applications of electron linear accelerators rely on the
production of temporally-compressed bunches. The shortening of electron bunches
is often realized with magnetic bunch compressors located in high-energy
sections of accelerators. Magnetic compression is subject to collective effects
including space charge and self interaction via coherent synchrotron radiation.
In this paper we explore the application of magnetic compression to low-energy
($\sim 40$ MeV), high-charge (nC) electron bunches with low normalized
transverse emittances ($< 5$ $\mu$m).",1302.0726v1
2013-02-06,Quantum dynamics of vortices in mesoscopic magnetic disks,"Model of quantum depinning of magnetic vortex cores from line defects in a
disk geometry and under the application of an in-plane magnetic field has been
developed within the framework of the Caldeira-Leggett theory. The
corresponding instanton solutions are computed for several values of the
magnetic field. Expressions for the crossover temperature Tc and for the
depinning rate \Gamma(T) are obtained. Fitting of the theory parameters to
experimental data is also presented.",1302.1406v1
2013-05-08,On the existence of three closed magnetic geodesics for subcritical energies,"We consider exact magnetic flows on closed orientable surfaces. We show that
for almost every energy $\kappa$ below Ma\~n\'e's critical value of the
universal covering there are always at least three distinct closed magnetic
geodesics with energy $\kappa$. If in addition the energy level is assumed to
be non-degenerate we prove existence of infinitely many closed magnetic
geodesics.",1305.1871v1
2013-07-29,Theoretical Study of Spin-Torque Oscillator with Perpendicularly Magnetized Free Layer,"The magnetization dynamics of spin torque oscillator (STO) consisting of a
perpendicularly magnetized free layer and an in-plane magnetized pinned layer
was studied by solving the Landau-Lifshitz-Gilbert equation. We derived the
analytical formula of the relation between the current and the oscillation
frequency of the STO by analyzing the energy balance between the work done by
the spin torque and the energy dissipation due to the damping. We also found
that the field-like torque breaks the energy balance, and change the
oscillation frequency.",1307.7427v1
2013-08-06,Broadband ferromagnetic resonance characterization of GaMnAs thin films,"The precessional magnetization dynamics of GaMnAs thin films are
characterized by broadband network analyzer ferromagnetic resonance (FMR) in a
coplanar geometry at cryogenic temperatures. The FMR frequencies are
characterized as function of in-plane field angle and field amplitude. Using an
extended Kittel model of the FMR dispersion the magnetic film parameters such
as saturation magnetization and anisotropies are derived. The modification of
the FMR behavior and of the magnetic parameters of the thin film upon annealing
is analyzed.",1308.1232v1
2013-08-09,A Scaling Theory for ac Magnetic Response in Kagome Ice,"A theory for frequency-dependent magnetic susceptibility \chi(\omega) is
developed for thermally activated magnetic monopoles in kagome ice. By mapping
this system to a two-dimensional (2D) Coulomb gas and then to a sine-Gordon
model, we have shown that the susceptibility has a scaling form
\chi(\omega)/\chi(0)={\cal F}(\omega/\omega_1), where the characteristic
\omega_1 is related to a charge correlation length between diffusively moving
monopoles, and to the sine-Gordon principal breather. The dynamical scaling is
universal among superfluid and superconducting films, and 2D XY magnets above
Kosterlitz-Thouless transitions.",1308.2049v1
2013-11-11,Visualization of Rabi oscillations in a magnetic resonance,"A visualization scheme for dynamics of a qudit polarization vector in a
time-dependent magnetic field is presented by solving equations for a density
matrix in Hermitian basis. This is realized by means of mapping solution for
the polarization vector on the three-dimensional spherical curve (vector
hodograph).
  The obtained results obviously display the interference of precessional and
nutational effects on the polarization vector in a magnetic resonance. The
study can find the practical applications in a magnetic resonance and 3D
visualization of computational data.",1311.2537v2
2013-11-12,Electron and Ion Heating during Magnetic Reconnection in Weakly Collisional Plasmas,"Gyrokinetic simulations of magnetic reconnection are presented to investigate
plasma heating for strongly magnetized, weakly collisional plasmas. For a low
plasma beta case, parallel and perpendicular phase mixing strongly enhance
energy dissipation yielding electron heating. Heating occurs for a long time
period after a dynamical process of magnetic reconnection ended. For a higher
beta case, the ratio of ion to electron dissipation rate increases, suggesting
that ion heating (via phase-mixing) may become an important dissipation channel
in high beta plasmas.",1311.2728v2
2013-12-05,The Study on the Effects of Chromaticity and Magnetic Field Tracking Errors at CSNS/RCS,"The Rapid Cycling Synchrotron (RCS) is a key component of the China
Spallation Neutron Source (CSNS). For this type of high intensity proton
synchrotron, the chromaticity, space charge effects and magnetic field tracking
errors between the quadrupoles and the dipoles can induce beta function
distortion and tune shift, and induce resonances. In this paper the combined
effects of chromaticity, magnetic field tracking errors and space charge on
beam dynamics at CSNS/RCS are studied systemically. 3-D simulations with
different magnetic field tracking errors are performed by using the code ORBIT,
and the simulation results are compared with the case without tracking errors.",1312.1475v1
2013-12-09,Magnetic Fields in Stars: Origin and Impact,"Various types of magnetic fields occur in stars: small scale fields, large
scale fields, and internal toroidal fields. While the latter may be ubiquitous
in stars due to differential rotation, small scale fields (spots) may be
associated with envelop convection in all low and high mass stars. The stable
large scale fields found in only about 10 per cent of intermediate mass and
massive stars may be understood as a consequence of dynamical binary
interaction, e.g., the merging of two stars in a binary. We relate these ideas
to magnetic fields in white dwarfs and neutron stars, and to their role in
core-collapse and thermonuclear supernova explosions.",1312.2373v1
2013-12-16,SQUID Metamaterials: Tuneability and Multistability,"An overview of several dynamic properties of SQUID metamaterials is given in
the presence of both constant and alternating magnetic field. The total current
as a function of the driving frequency exhibits hysteretic effects which are
favored by low levels of disorder. Multistability in the current states leads
to multiple magnetic responses with different value of magnetic permeability.
SQUID metamaterials exhibit wide-band tuneability which is periodic with the
applied constant magnetic field; the numerical calculations reproduce fairly
well recent experimental results. Current work also reveals the possibility for
wave transmission through nonlinear bands, which is briefly discussed.",1312.4320v1
2013-12-18,Electromagnetic way of accelerating the magnetic dipoles,"The article considers an opportunity of electrodynamics accelerating the
magnetic dipoles at initial velocity six hundred meters per second, which is
the magnetic dipole gain after pre-gas-dynamic acceleration to finite velocity
eight and half kilometers per second. The acceleration length is more than two
kilometers. When selecting the drag coefficient and the lift coefficient equal
to one hundredth, the dipoles rise to height ten kilometers during a period of
time fourteen seconds, thus reaching the vertical velocity one kilometer per
second and reducing the forward velocity till seven and a half kilometer per
second. The magnetic dipoles reach flight range twelve thousand three hundred
kilometers.",1312.5046v1
2014-04-09,Current-induced spin torque resonance of magnetic insulators,"We formulate a theory of the AC spin Hall magnetoresistance (SMR) in a
bilayer system consisting of a magnetic insulator such as yttrium iron garnet
(YIG) and a heavy metal such as platinum (Pt). We derive expressions for the DC
voltage generation based on the drift-diffusion spin model and quantum
mechanical boundary condition at the interface that reveal a spin torque
ferromagnetic resonance (ST-FMR). We predict that ST-FMR experiments will
reveal valuable information on the current-induced magnetization dynamics of
magnetic insulators and AC spin Hall effect.",1404.2360v2
2014-04-17,Self Diffusion of Yukawa system in presence of external magnetic field,"Diffusion of dust particles is one of the most significant transport
processes of strongly coupled dusty plasma that reflect the nature of inter
particle interaction and characterize thermodynamics of the system. In this
paper the effect of magnetic field on diffusion of dust particles in strongly
coupled dusty plasma is investigated by using molecular dynamics simulations.
Self- diffusion coefficients of Yukawa systems are obtained for a wide range of
plasma parameters and magnetic field strength using Green- Kubo expression
which is based on integrated velocity autocorrelation function (VACF). It is
assumed that dust particles interact with each other by modified Yukawa (i.e.
screened Coulomb) potential. The study gives interesting results of dust
particle diffusion in magnetized plasma.",1404.4462v1
2014-10-13,Vortex gyration mediated by spin waves driven by an out-of-plane oscillating magnetic field,"In this letter we address the vortex core dynamics involved in gyration
excitation and damping change by out-of-plane oscillating magnetic fields. When
the vortex core is at rest under the effect of in-plane bias magnetic fields,
the spin waves excited by the perpendicular magnetic field can induce obvious
vortex gyration. When simultaneously excite spin waves and vortex gyrotropic
motion, the gyration damping changes. Analysis of the system energy allows us
to explain the origin of the spin-wave-mediated vortex gyration.",1410.3230v1
2014-11-03,Optimum reduction of the dynamo threshold by a ferromagnetic layer located in the flow,"We consider a fluid dynamo model generated by the flow on both sides of a
moving layer. The magnetic permeability of the layer is larger than that of the
flow. We show that there exists an optimum value of magnetic permeability for
which the critical magnetic Reynolds number for dynamo onset is smaller than
for a nonmagnetic material and also smaller than for a layer of infinite
magnetic permeability. We present a mechanism that provides an explanation for
recent experimental results. A similar effect occurs when the electrical
conductivity of the layer is large.",1411.0449v1
2015-01-13,Spin crossover in ferropericlase from first-principles molecular dynamics,"Ferropericlase, (Mg,Fe)O, is the second-most abundant mineral of the Earth's
lower mantle. With increasing pressure, the Fe ions in the material begin to
collapse from a magnetic to non-magnetic spin state. We present a
finite-temperature first-principles phase diagram of this spin crossover,
finding a broad pressure range with coexisting magnetic and non-magnetic ions
due to favorable enthalpy of mixing of the two. Furthermore, we find the
electrical conductivity of the mineral to reach semi-metallic values inside the
Earth.",1501.02902v1
2015-03-30,Generation of magnetic fields by large-scale vortices in rotating convection,"We propose a new self-consistent dynamo mechanism for the generation of
large-scale magnetic fields in natural objects. Recent computational studies
have described the formation of large-scale vortices (LSVs) in rotating
turbulent convection. Here we demonstrate that for magnetic Reynolds numbers
below the threshold for small-scale dynamo action, such turbulent flows can
sustain large-scale magnetic fields --- i.e. fields with a significant
component on the scale of the system.",1503.08599v1
2015-10-18,Two-Fluid Theory for Spin Superfluidity in Magnetic Insulators,"We investigate coupled spin and heat transport in easy-plane magnetic
insulators. These materials display a continuous phase transition between
normal and condensate states that is controlled by an external magnetic field.
Using hydrodynamic equations supplemented by Gross-Pitaevski phenomenology and
magnetoelectric circuit theory, we derive a two-fluid model to describe the
dynamics of thermal and condensed magnons, and the appropriate boundary
conditions in a hybrid normal-metal|magnetic-insulator|normal-metal
heterostructure. We discuss how the emergent spin superfluidity can be
experimentally probed via a spin Seebeck effect measurement.",1510.05316v2
2016-01-06,Wave-induced motion of magnetic spheres,"We report an experimental study of the motion of magnetized beads driven by a
travelling wave magnetic field. For sufficiently large wave speed, we report
the existence of a backward motion, in which the sphere can move in the
direction opposite to the driving wave. We show that the transition to this new
state is strongly subcritical and can lead to chaotic motion of the bead. For
some parameters, this counter-propagation of the sphere can be one order of
magnitude faster than the driving wave speed. These results are understood in
the framework of a model based on the interplay among solid friction, air
resistance and magnetic torque.",1601.01111v1
2016-03-28,Three axis vector atomic magnetometer utilizing polarimetric technique,"The three axis magnetic field measurement based on the interaction of a
single elliptically polarized light beam with an atomic system is described.
The magnetic field direction dependent atomic responses are extracted by the
polarimetric detection in combination with laser frequency modulation and
magnetic field modulation techniques. The magnetometer offers additional
critical requirements like compact size and large dynamic range for space
application. Further, the three axis magnetic field is measured using only
reflected signal from the polarimeter, thus can be easily expanded to make
spatial array of detectors or / and high sensitivity field gradient measurement
as required for biomedical application.",1603.08303v1
2016-04-12,Magnetization distribution and domain wall dynamics in nanotube with surface anisotropy,"The period of magnetization oscillations that occur near the surface of a
nanotube or nanowire under the influence of surface magnetic anisotropy is
determined by means of numerical simulation as a function of nanowire geometry
and material parameters. The hopping mode is observed for stationary movement
of a head-to-head domain wall along nanowire axis in applied magnetic field.
The average speed of the domain wall in the hopping mode is found to be several
times less than the stationary velocity of the wall in the absence of surface
anisotropy.",1604.03523v1
2016-04-15,Electron magnetic resonance in magnetic nanoparticles: dependence on the particle size and applicability of the modified giant spin model,"Superparamagnetic nanoparticles containing hundreds and thousands of coupled
electron spins are on the boundary between classical and quantum behavior, and
demonstrate features which are typical for paramagnetic spins and absent in
macroscopic ferromagnetic systems. In order to better understand the evolution
of magnetization dynamics from quantum to classical behavior with the increase
in the system size, we study the electron magnetic resonance signal in
suspensions of iron oxide nanoparticles as the function of the particle size.
The experimental data are compared with numerical simulations based on the
giant spin approach.",1604.04594v2
2016-05-10,Fermion self-energy in magnetized chirally asymmetric QED matter,"The fermion self-energy is calculated for a cold QED plasma with chiral
chemical potential in a magnetic field. It is found that a momentum shift
parameter dynamically generated in such a plasma leads to a modification of the
chiral magnetic effect current. It is argued that the momentum shift parameter
can be relevant for the evolution of magnetic field in the chirally asymmetric
primordial plasma in the early Universe.",1605.03225v1
2016-06-21,Technical Considerations in Magnetic Analogue Models,"The analogy between vorticity and magnetic fields has been a subject of
interest to researchers for a considerable period of time, mainly because of
the structural similarities between the systems of equations that govern the
evolution of the two fields. We recently presented the analysis of magnetic
fields and hydrodynamics vorticity fields and argued for a formal theory of
analogue magnetism. This article provides in depth technical details of the
relevant considerations for the simulation procedures and extends the analyses
to a range of fluids.",1606.06725v1
2016-07-04,Transversal-rotational and zero group velocity modes in tunable magneto-granular phononic crystals,"We report on the design and operation of a 1D magneto-granular phononic
crystal composed of a chain of steel spherical beads on top of permanent
magnets. The magnetic field of the permanent magnets induces forces in the
granular structure. By changing its strength, we can tune the dynamic response
of the granular structure. We present experimental results with evidence of
coupled transversal-rotational modes, and zero group velocities modes. These
observations are well supported by a proposed model taking into account the
mechanical coupling between the beads and the magnets by linear stiffnesses and
including all degrees of freedom in translations and rotations.",1607.00831v2
2016-07-25,MHD stagnation point flow toward a linearly-stretching thermally-insulated sheet with induced magnetic field,"The equations governing the magnetohydrodynamic stagnation point flow toward
a non-conducting, thermally insulated, nonporous, linearly stretching sheet are
cast in a self similar form. Consistent boundary conditions on the velocity,
magnetic field and temperature are invoked. The flow problem involves three
parameters- the magnetic Prandtl number, the magnetic interaction number, and
the ratio of the stretching rate to the strength of the stagnation point flow.
The energy equation includes viscous dissipation and Joule heating, and
introduces the Prandtl number as a fourth parameter. Numerical solutions are
obtained and sample results are presented.",1607.07374v1
2016-08-06,Combined effect of horizontal magnetic field and vorticity on Rayleigh Taylor instability,"In this research, the height, curvature and velocity of the bubble tip in
Rayleigh-Taylor instability at arbitrary Atwood number with horizontal magnetic
field are investigated. To support the earlier simulation and experimental
results, the vorticity generation inside the bubble is introduced. It is found
that, in early nonlinear stage, the temporal evolution of the bubble tip
parameters depend essentially on the strength and initial perturbation of the
magnetic field, although the asymptotic nature coincides with the non magnetic
case. The model proposed here agrees with the previous linear, nonlinear and
simulation observations.",1608.02072v1
2016-09-18,Detecting Chiral Magnetic Effect by Lattice Dynamics,"In the present paper, we propose that the chiral magnetic effect, the direct
consequence of the presence of Weyl points in the band structure, can be
detected by its coupling to certain phonon modes, which behave like pseudo
scalars under point group transformations. Such coupling can generate resonance
between intrinsic plasmon oscillation and the corresponding phonon modes,
leading to dramatic modification of the optical response by the external
magnetic field, which provides a new way to study chiral magnetic effect by
optical measurements.",1609.05442v1
2016-10-07,Magnonic black holes,"We show that the interaction between spin-polarized current and magnetization
dynamics can be used to implement black-hole and white-hole horizons for
magnons - the quanta of oscillations in the magnetization direction in magnets.
We consider three different systems: easy-plane ferromagnetic metals, isotropic
antiferromagnetic metals, and easy-plane magnetic insulators. Based on
available experimental data, we estimate that the Hawking temperature can be as
large as 1 K. We comment on the implications of magnonic horizons for spin-wave
scattering and transport experiments, and for magnon entanglement.",1610.02313v1
2016-12-13,Magnetic Bloch Skyrmion Transport by Electric Fields in a Composite Bilayer,"We investigate a mechanical method to manipulate magnetic Bloch Skyrmions by
applying an electric field in a composite chiral-magnetic (CM)/ferroelectric
(FE) bilayer. The magnetoelectric coupling at the interface allows the electric
field to stimulate magnetic ordering. Therefore it offers the possibility to
generate Skyrmions [Phys. Rev. B 94, 014311 (2016)]. Here, we design a movable
and localized electric field source to drive skyrmion transport along the
bilayer. A traveling velocity of the electric field source must be carefully
chosen to show the stability and effciency of this process. The effects of high
speed operation will be discussed.",1612.03995v1
2017-01-10,The giant effect of magnetic ordering on a sound velocity in a sigma-Fe55Cr45 alloy,"We studied atomic dynamics of sigma-Fe(100-x)Cr(x) (x=45 and 49.5) alloys
using nuclear inelastic scattering of synchrotron radiation. For the
sigma-Fe55Cr45 alloy, the derived reduced iron-partial density of phonon states
reveal a huge difference in the low-energy region between magnetic and
paramagnetic states. The latter implies a ca.36% increase of the sound velocity
in the magnetic phase, which testifies to a magnetically-induced hardening of
the lattice.",1701.02467v1
2017-04-27,Nucleon Magnetic Properties from Lattice QCD with the Background Field Method,"The magnetic moment and magnetic polarisability of the neutron and proton are
investigated using the uniform background-field method and lattice QCD. The
results are calculated using 32^3 x 64 dynamical QCD lattices provided by the
PACS-CS collaboration through the ILDG. Methods of isolating the energy of the
hadrons accounting for Landau level energies are explored. Effective energy
shifts are formed from combinations of correlation functions and their
efficiency in isolating the magnetic moment and polarisability terms is
determined.",1704.08435v1
2017-09-05,Gate-controlled magnonic-assisted switching of magnetization in ferroelectric/ferromagnetic junctions,"Interfacing a ferromagnet with a polarized ferroelectric gate generates a
non-uniform, interfacial spin density coupled to the ferroelectric polarization
allowing so for an electric field control of effective transversal field to
magnetization. Here we study the dynamic magnetization switching behavior of
such a multilayer system based on the Landau-Lifshitz-Baryakhtar equation,
demonstrating that interfacial magnetoelectric coupling is utilizable as a
highly localized and efficient tool for manipulating magnetism.",1709.01286v1
2017-09-14,Remarks on the derivation of Gross-Pitaevskii equation with magnetic Laplacian,"The effective dynamics for a Bose-Einstein condensate in the regime of high
dilution and subject to an external magnetic field is governed by a magnetic
Gross-Pitaevskii equation. We elucidate the steps needed to adapt to the
magnetic case the proof of the derivation of the Gross-Pitaevskii equation
within the ""projection counting"" scheme.",1709.04841v1
2017-11-27,Shape and fission instabilities of ferrofluids in non-uniform magnetic fields,"We study static distributions of ferrofluid submitted to non-uniform magnetic
fields. We show how the normal-field instability is modified in the presence of
a weak magnetic field gradient. Then we consider a ferrofluid droplet and show
how the gradient affects its shape. A rich phase transitions phenomenology is
found. We also investigate the creation of droplets by successive splits when a
magnet is vertically approached from below and derive theoretical expressions
which are solved numerically to obtain the number of droplets and their aspect
ratio as function of the field configuration. A quantitative comparison is
performed with previous experimental results, as well as with our own
experiments, and yields good agreement with the theoretical modeling.",1711.09942v1
2017-12-31,Magnetic Field Inspired Contact Angle Hysteresis Drives Floating Polyolefin Rafts,"Displacement of floating polymer (polyolefin) rafts by steady magnetic field
is reported. The effect is due to the interplay of gravity deforming the
water/vapor interface, contact angle hysteresis and diamagnetic properties of
the liquid support. Magnetic field (ca. 0.06 T) deformed the water/vapor
interface and impacted the interfacial apparent contact angle. This deformation
gave rise to the propulsion force, displacing the polymer raft. The velocity of
displacement of the center of mass of rafts was registered. The effect is
related to the contact angle hysteresis affected by the magnetic field,
enabling the change in the apparent interfacial contact angle. The
semi-quantitative model of the process is suggested",1801.00294v1
2018-01-11,Fullerene Faraday Cage Keeps Magnetic Properties of Inner Cluster Pristine,"Any single molecular magnets (SMM) perspective for application is as good as
its magnetization stability in ambient conditions. Endohedral metallofullerenes
(EMFs) provide a solid basis for promising SMMs. In this study, we investigated
the behavior of functionalized EMFs on a gold surface (EMF-L-Au). Having
followed the systems molecular dynamics paths, we observed that the chemically
locked inner cluster inside fullerene cage will remain locked even at room
temperature due to the ligand-effect. We have located multiple possible minima
with different charge arrangements between EMF-L-Au fragments. Remarkably, the
charge state of the EMF inner cluster remained virtually constant and so
magnetic properties are expected to be untouched.",1801.03808v2
2018-04-11,Magnetic gravity compensation,"Magnetic gravity compensation in fluids is increasingly popular as a means to
achieve low-gravity forphysical and life sciences studies. We explain the
basics of the magnetic gravity compensation and analyzeits advantages and
drawbacks. The main drawback is the spatial heterogeneity of the residual
gravity field.We discuss its causes. Some new results concerning the
heterogeneity estimation and measurement arepresented. A review of the existing
experimental installations and works involving the magnetic gravitycompensation
is given for both physical and life sciences.",1804.03838v1
2018-06-10,Quantum Collapse of a Magnetic Skyrmion,"Quantum collapse of a small skyrmion in a thin magnetic film with
Dzyalishinskii-Moriya (DMI) interaction has been studied. The energy of the
skyrmion and the stability threshold determined by the DMI, the external
magnetic field, and the underlying atomic lattice are investigated analytically
and numerically. The Lagrangian describing the coupled dynamics of the skyrmion
size and the chirality angle is derived. Equations of motion possess an
instanton solution that corresponds to the skyrmion underbarrier contraction
via quantum tunneling with subsequent collapse and decay of the topological
charge. The tunneling rate is computed and the conditions needed to observe
quantum collapse of a skyrmion in a magnetic film are discussed.",1806.03675v1
2018-09-21,Influence of the particle shape on the equilibrium morphologies of supracolloidal magnetic filaments,"We investigate the equilibrium morphologies of linear and ring-shaped
magnetic filaments made from crosslinked ferromagnetic spherical or ellipsoidal
colloidal particles. Using Langevin dynamics simulations, we calculate the
radius of gyration and total magnetic moment of a single filament at zero field
and different temperatures, analyzing the influence of the particles shape, the
strength of their magnetic moment and the filament length. Our results show
that, among such parameters, the shape of the particles has the strongest
qualitative impact on the equilibrium behavior of the filaments.",1809.08094v1
2018-10-17,Out-of-plane auto-oscillation in spin Hall oscillator with additional polarizer,"The theoretical investigation on magnetization dynamics excited by the spin
Hall effect in metallic multilayers having two ferromagnets is discussed. The
relaxation of the transverse spin in one ferromagnet enables us to manipulate
the direction of the spin-transfer torque excited in another ferromagnet,
although the spin-polarization originally generated by the spin Hall effect is
geometrically fixed. Solving the Landau-Lifshitz-Gilbert-Slonczewski equation,
the possibility to excite an out-of-plane auto-oscillation of an in-plane
magnetized ferromagnet is presented. An application to magnetic recording using
microwave-assisted magnetization reversal is also discussed.",1810.07831v1
2019-02-13,"An extension of the fewest switches surface hopping algorithm to complex Hamiltonians and photophysics in magnetic fields: Berry's phase and ""magnetic"" forces","We present a preliminary extension of the fewest switches surface hopping
(FSSH) algorithm to the case of complex Hamiltonians as appropriate for
modeling the dynamics of photoexcited molecules in magnetic fields. We make
ansatze for the direction of momentum rescaling and we account for Berry's
phase effects through ""magnetic"" forces as applicable in the adiabatic limit.
Because Berry's phase is a nonlocal, topological characteristic of a set of
entangled potential energy surfaces, we find that Tully's local FSSH algorithm
can only partially capture the correct physics.",1902.05115v1
2019-07-04,Helical distributed chaos in magnetic field of solar wind,"Helical distributed chaos in magnetic field has been studied using results of
direct numerical simulations (dominated by magnetic helicity), of a laboratory
experiment with plasma wind tunnel and of solar wind measurements (dominated by
combined magnetic and cross helicity effects). The solar wind measurements,
used for the spectral computations, were produced by Helios-1 and Ulysses
missions for low and high heliolatitudes respectively and for high solar wind
speed at $0.4 < R <4.5$ AU (where $R$ is distance from the Sun).",1907.02425v1
2019-07-06,Angle-resolved broadband ferromagnetic resonance apparatus enabled through a spring-loaded sample mounting manipulator,"Broadband ferromagnetic resonance is a useful technique to determine the
magnetic anisotropy and study the magnetization dynamics of magnetic thin
films. We report a spring-loaded sample loading manipulator for reliable sample
mounting and rotation. The manipulator enables maximum signal, enhances system
stability and is particularly useful for fully automated in-plane-field
angle-resolved measurements. This angle-resolved broadband ferromagnetic
resonance apparatus provides a viable method to study anisotropic damping and
weak magnetic anisotropies, both vital for fundamental research and
applications.",1907.03097v1
2019-08-19,Amorphous Ferrimagnets: an Ideal Host for Ultra-Small Skyrmions at Room Temperature,"Recently, magnetic skyrmion has emerged as an active topic of fundamental
study and applications in magnetic materials research. Magnetic skyrmions are
vortex-like spin excitations with topological protection and therefore are more
robust to pinning compared with magnetic domain walls. We employ atomistic
simulations to create room-temperature ultra-small Neel skyrmions in amorphous
ferrimagnet. The fast propagation and low-dissipation dynamics of ultra-small
ferrimagnetic skyrmions make them attractive for utilization as an alternative
to domain walls in spin-based memory and logic devices.",1908.07003v2
2020-05-25,Multiple Spin-Phonon Relaxation Pathways in a Kramer Single-Ion Magnet,"We present a first-principles investigation of spin-phonon relaxation in a
molecular crystal of Co(II) single-ion magnets. Our study combines electronic
structure calculations with machine-learning force fields and unravels the
nature of both the Orbach and the Raman relaxation channels in terms of
atomistic processes. We find that although both mechanisms are mediated by the
excited spin states, the low temperature spin dynamics is dominated by phonons
in the THz energy range, which partially suppress the benefit of having a large
magnetic anisotropy. This study also determines the importance of
intra-molecular motions for both the relaxation mechanisms and paves the way to
the rational design of a new generation of single-ion magnets with tailored
spin-phonon coupling.",2005.12429v1
2020-07-01,Quantum droplet states of a binary magnetic gas,"Quantum droplets can emerge in bosonic binary magnetic gases (BMGs) from the
interplay of short- and long-ranged interactions, and quantum fluctuations. We
develop an extended meanfield theory for this system and use it to predict
equilibrium and dynamical properties of BMG droplets. We present a phase
diagram and characterize miscible and immiscible droplet states. We also show
that a single component self-bound droplet can be used to bind another magnetic
component which is not in the droplet regime. Our results should be realizable
in experiments with mixtures of highly-magnetic lanthanide atoms.",2007.00366v1
2020-08-01,Magnetic ratchet effect in phosphorene,"The magnetic ratchet effect has been studied in phosphorene by the use of the
Boltzmann kinetic equation that is a semi-classical approach. The Hamiltonian
of phosphorene in a steady parallel magnetic field is derived using the
tight-binding model. We consider the effect of the magnetic field on
non--linear dynamics in the presence of an ac laser field and spatial inversion
asymmetry. We have shown that for anisotropic 2D materials and phosphorene, the
ratchet current has the response to three different light polarizations:
linearly polarized light, circularly polarized light, and unpolarized light.",2008.01561v3
2020-11-12,Magnetically powered metachronal waves induce locomotion in self-assemblies,"When tiny soft ferromagnetic particles are placed along a liquid interface
and exposed to a vertical magnetic field, the balance between capillary
attraction and magnetic repulsion leads to self-organization into well-defined
patterns. Here, we demonstrate experimentally that precessing magnetic fields
induce metachronal waves on the periphery of these assemblies, similar to the
ones observed in ciliates and some arthropods. The outermost layer of particles
behaves like an array of cilia or legs whose sequential movement causes a net
and controllable locomotion. This bioinspired many-particle swimming strategy
is effective even at low Reynolds number, using only spatially uniform fields
to generate the waves.",2011.06442v1
2021-04-13,Dynamic Magnetoelastic Boundary Conditions and the Pumping of Phonons,"We derive boundary conditions at the interfaces of magnetoelastic
heterostructures under ferromagnetic resonance for arbitrary magnetization
directions and interface shapes. We apply our formalism to
magnet$\vert$nonmagnet bilayers and magnetic grains embedded in a nonmagnetic
thin film, revealing a nontrivial magnetization angle dependence of acoustic
phonon pumping.",2104.05992v2
2021-04-27,Field-driven dynamics of magnetic Hopfions,"We present micromagnetic simulations on resonant spin wave modes of magnetic
Hopfions up to 15 GHz driven by external magnetic fields. A sharp transition is
found around 32 mT coinciding with a transition from Hopfions to magnetic
torons. The modes exhibit characteristic amplitudes in frequency space
accompanied by unique localization patterns in real space, and are found to be
robust to damping around topological features, particularly vortex lines in
Hopfions and Bloch points in torons. The marked differences in spin wave
spectra between Hopfions, torons and target skyrmions can serve as fingerprints
in future experimental validation studies of these novel 3d topological spin
textures.",2104.13349v1
2021-08-21,Bursty magnetic reconnection at the Earth's magnetopause triggered by high-speed jets,"The impact of high-speed jets -- dynamic pressure enhancements in the
magnetosheath -- on the Earth's magnetopause has been observed to trigger local
magnetic reconnection. We perform a three-dimensional hybrid simulation to
study the magnetosheath and magnetopause under turbulent conditions using a
quasi-radial southward interplanetary magnetic field (IMF). In contrast to
quasi-steady reconnection with a strong southward IMF, we show that after the
impact of a jet on the magnetopause, the magnetopause moves inwards, the
current sheet is compressed and intensified and signatures of local magnetic
reconnection are observed, showing similarities to spacecraft measurements",2108.09419v1
2021-11-22,Single-layer Skyrmions in a van der Waals Antiferromagnet,"Transition metal halides are quasi-two-dimensional van der Waals magnets that
can potentially host antiferromagnetic (AFM) skyrmions induced by competing
exchange interactions. We study theoretically magnetic states of Fe-doped
NiBr$_2$ using spin interactions obtained by fitting experimental data. We find
that AFM interlayer interactions suppress skyrmion lattices but allow for
skyrmions with topological charges $1,2$ and $3$ confined to a single magnetic
layer. These single-layer skyrmions exist in three different magnetic phases
and their dynamics changes drastically at the transition from a uniform to a
non-collinear modulated state.",2111.11420v2
2021-12-08,Electric dipole in a magnetic field: some aspects of the problem,"In the paper some regimes of motion of an electric dipole placed in a uniform
magnetic field are considered. The motion of both three-dimensional and
two-dimensional dipole in the plane perpendicular to the magnetic field is
studied. In the case of a two-dimensional dipole is discussed, in particular,
the regime of chaotic dipole motion, which arises when, along with the magnetic
field, there is an electric field rotating in the plane of motion. The motion
of a three-dimensional dipole is discussed in cases that allow for analytical
consideration. Finally, the same examples are used to discuss the
quantum-mechanical approach to the problem.",2112.04349v1
2021-12-09,Detection of weak magnetic fields using nitrogen-vacancy centers with maximum confidence,"The problem of detection of magnetic fields using NV centers, that is, to
check whether a weak magnetic field is present or not, can be tackled using
quantum state discrimination theory. In this regard, we find the POVMs that
maximize the confidence of any given measurement, taking the interaction time
as well as decoherence into account. We apply our formalism over a wide range
of scenarios encompassing constant and oscillating magnetic fields, while also
using techniques such as dynamical decoupling to improve the confidence and
extending our treatment to ensembles of NV centers.",2112.04856v1
2022-02-22,Skyrmions in magnetic materials,"Skyrmions are vortex-like textures of magnetic moments found in some
magnetically ordered materials. Here we describe the origin of the skyrmion and
discuss the experimental realization of skyrmions in magnetic materials, the
dynamic properties of the skyrmion and the potentially useful interaction of
skyrmions with electrons. Our discussion is based on the physics of fields and
allows us to touch on notions from field theory and topology that illustrate
how skyrmions can be understood as part of a family of defects in an ordered
field, and hint at other topological objects that await our discovery.",2202.11019v1
2022-07-08,Fast and Furious dynamo action in the anisotropic dynamo,"In the limit of large magnetic Reynolds numbers, it is shown that a smooth
differential rotation can lead to fast dynamo action, provided that the
electrical conductivity or magnetic permeability is anisotropic. If the shear
is infinite, for example between two rotating solid bodies, the anisotropic
dynamo becomes furious, meaning that the magnetic growth rate increases toward
infinity with an increasing magnetic Reynolds number.",2207.03950v1
2023-04-01,All-optical magnetization control in CrI$_3$ monolayers: a microscopic theory,"Bright excitons in ferromagnetic monolayers CrI$_3$ efficiently interact with
lattice magnetization, which makes possible all-optical resonant magnetization
control in this material. Using the combination of ab-initio simulations within
Bethe-Salpeter approach, semiconductor Bloch equations and Landau-Lifshitz
equations, we construct a microscopic theory of this effect. Solving
numerically the resulting system of the coupled equations describing the
dynamics of atomic spins and spins of the excitons, we demonstrate the
possibility of a tunable control of macroscopic magnetization of a sample.",2304.00331v2
2023-08-08,Dispersive decay estimates for the magnetic Schrödinger equations,"In this paper, we present a proof of dispersive decay for both linear and
nonlinear magnetic Schr\""odinger equations. To achieve this, we introduce the
fractional distorted Fourier transforms with magnetic potentials and define the
fractional differential operator $\arrowvert J_{A}(t)\arrowvert^{s}$. By
leveraging the properties of the distorted Fourier transforms and the
Strichartz estimates of $\arrowvert J_{A}\arrowvert^{s}u$, we establish the
dispersive bounds with the decay rate $t^{-\frac{n}{2}}$. This decay rate
provides valuable insights into the spreading properties and long-term dynamics
of the solutions to the magnetic Schr\""odinger equations.",2308.04121v1
2023-09-21,Chaotic studies in Magnetic Dipoles,"The present work investigates the effect of an external rotating magnetic
field on a magnetic needle, and aims to study chaotic behaviour. The equation
of motion is modified to include damping and gravity. The bifurcation behaviour
of such evolving systems is also studied. Furthermore, the coupling in pairs of
magnetic needles is also analyzed. In addition, a 2-dimensional array of
coupled needles is set up, and the synchronization properties of the population
is studied. The study is completed by carrying out an investigation of the
effects of noise on the synchronization of such a system.",2309.12180v1
2023-10-10,Video recognition by physical reservoir computing in magnetic materials,"Nonlinear spin dynamics in magnetic materials offers a promising avenue for
implementing physical reservoir computing, one of the most accomplished
brain-inspired frameworks for information processing. In this study, we
investigate the practical utility of magnetic physical reservoirs by assessing
their performance in a video recognition task. Leveraging a recently developed
spatiotemporal parallelization scheme, our reservoir achieves accurate
classifications of previously provided images. Our findings pave the way for
the development of visual sensors based on the magnetic physical reservoir
computing.",2310.06497v1
2023-10-16,Dynamical magneto-rotational instability,"Magneto-rotational instability (MRI) is an important instability mechanism
for rotating flows with magnetic fields. In particular, when the strength of
the magnetic field tends to zero, the stability criterion for rotating flows is
generally different from the classical Rayleigh criterion for rotating flows
without a magnetic field. MRI has wide applications in astrophysics,
particularly to the turbulence and enhanced angular momentum transport in
accretion disks. For the case of vertical magnetic fields, we give rigorous
proof of linear MRI and a complete description of the spectra and semigroup
growth of the linearized operator. Moreover, we prove nonlinear stability and
instability from the sharp linear stability/instability criteria.",2310.10075v3
2023-12-08,Phase Shift in AC Magnetocaloric Effect Measurements as an Indicator of the Order of Magnetic Phase Transitions,"It is shown that the phase shift between an applied weak alternating magnetic
field and the magnetocaloric response signal of the magnetic material is
drastically sensitive to the order of phase transition. Namely, at the
second-order phase transition, the phase shift does not depend on the magnetic
field magnitude, while in the first-order phase transition this one depends
significantly on the field strength. We have shown that this effect follows
from the general critical dynamics theory.",2312.04979v1
2008-09-26,Ultra-fast spin dynamics: the effect of colored noise,"Recent experimental results have pushed the limits of magnetization dynamics
to pico- and femtosecond timescales. This ultra-fast spin dynamics occurs in
extreme conditions of strong and rapidly varying fields and high temperatures.
This situation requires new description of magnetization dynamics, even on a
phenomenological level of the atomistic Landau-Lifshitz-Gilbert equation,
taking into account that the correlation time for electron system could be of
the order of the inverse characteristic spin frequency. For this case we
introduce the thermodynamically correct phenomenological approach for spin
dynamics based on the Landau-Lifshitz-Miyasaki-Seki equation. The influence of
the noise correlation time on longitudinal and transverse magnetization
relaxation is investigated. We also demonstrate the effect of the noise
correlation time on demagnetisation rate of different materials during
laser-induced dynamics.",0809.4595v1
2010-03-09,Magnetic Field Control of the Quantum Chaotic Dynamics of Hydrogen Analogues in an Anisotropic Crystal Field,"We report magnetic field control of the quantum chaotic dynamics of hydrogen
analogues in an anisotropic solid state environment. The chaoticity of the
system dynamics was quantified by means of energy level statistics. We analyzed
the magnetic field dependence of the statistical distribution of the impurity
energy levels and found a smooth transition between the Poisson limit and the
Wigner limit, i.e. transition between regular Poisson and fully chaotic Wigner
dynamics. Effect of the crystal field anisotropy on the quantum chaotic
dynamics, which manifests itself in characteristic transitions between
regularity and chaos for different field orientations, was demonstrated.",1003.1778v1
2011-12-19,Dynamical phases and intermittency of the dissipative quantum Ising model,"We employ the concept of a dynamical, activity order parameter to study the
Ising model in a transverse magnetic field coupled to a Markovian bath. For a
certain range of values of the spin-spin coupling, magnetic field and
dissipation rate, we identify a first order dynamical phase transition between
active and inactive {\em dynamical phases}. We demonstrate that dynamical
phase-coexistence becomes manifest in an intermittent behavior of the bath
quanta emission. Moreover, we establish the connection between the dynamical
order parameter that quantifies the activity, and the longitudinal
magnetization that serves as static order parameter. The system we consider can
be implemented in current experiments with Rydberg atoms and trapped ions.",1112.4273v2
2014-01-13,Pseudo Slice Energy Spread in Dynamics of Electron Beams Moving through Magnetic Bends,"In the previous canonical formulation of beam dynamics for an electron bunch
moving ultrarelativistically through magnetic bending systems, we have shown
that the transverse dynamics equation for a particle in the bunch has a driving
term which behaves as the centrifugal force caused by the particle's initial
potential energy due to collective particle interactions within the bunch. As a
result, the initial potential energy at the entrance of a bending system, which
we call pseudo (kinetic) energy, is indistinguishable from the usual kinetic
energy offset from the design energy in its perturbation to particle optics
through dispersion and momentum compaction. In this paper, in identifying this
centrifugal force on particles as the remnant of the CSR cancellation effect in
transverse particle dynamics, we show how the dynamics equation in terms of the
canonical momentum for beam motion on a curved orbit is related to the
Panofsky-Wenzel theorem for wakefields for beam motion on a straight path. It
is shown that the effect of pseudo energy spread can be measurable only for a
high-peak-current bunch when the pseudo slice energy spread is appreciable
compared to the slice kinetic energy spread. The implication of the pseudo
slice energy spread for bunch dynamics in magnetic bends is discussed.",1401.2868v1
2014-02-25,Leapfrogging vortex rings in the Landau-Lifshitz equation,"Vortex rings are ubiquitous in fluids, with smoke rings being a familiar
example. The interaction of multiple vortex rings produces complex dynamical
behaviour, such as the leapfrogging motion first analysed by Helmholtz more
than a century and a half ago. Here we report on numerical investigations of
vortex ring dynamics in a different setting from fluids, namely, as solutions
of the Landau-Lifshitz equation that models the evolution of the local
magnetization in a ferromagnetic medium. We present the results of the first
study on the dynamics of interacting magnetic vortex rings and provide a novel
link between fluids and magnetism, by showing that a range of phenomena
familiar in fluids are reproduced in ferromagnets. This includes the
leapfrogging motion of a pair of vortex rings and evidence for the chaotic
dynamics of a trio of rings.",1402.6165v1
2015-02-17,Orientational hysteresis in swarms of active particles in external field,"Structure and ordering in swarms of active particles have much in common with
condensed matter systems like magnets or liquid crystals. A number of important
characteristics of such materials can be obtained via dynamic tests such as
hysteresis. In this work, we show that dynamic hysteresis can be observed also
in swarms of active particles and possesses similar properties to the
counterparts in magnetic materials. To study the swarm dynamics, we use
computer simulations of the active Brownian particle model with dissipative
interactions. The swarm is confined to a narrow linear channel and the
one-dimensional polar order parameter is measured. In an oscillating external
field, the order parameter demonstrates dynamic hysteresis with the shape of
the loop and its area varying with the amplitude and frequency of the applied
field, swarm density and the noise intensity. We measure the scaling exponents
for the hysteresis loop area, which can be associated with the controllability
of the swarm. Although the exponents are non-universal and depend on the
system's parameters, their limiting values can be predicted using a generic
model of dynamic hysteresis. We also discuss similarities and differences
between the swarm ordering dynamics and two-dimensional magnets.",1502.05071v2
2017-06-11,Dynamically order-disorder transition in triangular lattice driven by a time dependent magnetic field,"We have elucidated the dynamic phase transition features and finite-size
scaling analysis of the triangular lattice system under the presence of a
square-wave magnetic field. It has been found that as the value of half-period
of the external field reaches its critical value, whose location is estimated
by means of Binder cumulant, the system presents a dynamic phase transition
between dynamically ordered and disordered phases. Moreover, at the dynamic
phase transition point, finite-size scaling of the Monte Carlo results for the
dynamic order parameter and susceptibility give the critical exponents
$\beta/\nu=0.143\pm0.004$ and $\gamma/\nu=1.766\pm0.036$, respectively. The
obtained critical exponents show that present magnetic system belongs to same
universality class with the two-dimensional equilibrium Ising model.",1706.03351v1
2017-11-30,Dynamic MRI Reconstruction from Undersampled Data with an Anatomical Prescan,"The goal of dynamic magnetic resonance imaging (dynamic MRI) is to visualize
tissue properties and their local changes over time that are traceable in the
MR signal. We propose a new variational approach for the reconstruction of
subsampled dynamic MR data, which combines smooth, temporal regularization with
spatial total variation regularization. In particular, it furthermore uses the
infimal convolution of two total variation Bregman distances to incorporate
structural a-priori information from an anatomical MRI prescan into the
reconstruction of the dynamic image sequence. The method promotes the
reconstructed image sequence to have a high structural similarity to the
anatomical prior, while still allowing for local intensity changes which are
smooth in time. The approach is evaluated using artificial data simulating
functional magnetic resonance imaging (fMRI), and experimental dynamic
contrast-enhanced magnetic resonance data from small animal imaging using
radial golden angle sampling of the k-space.",1712.00099v1
2019-07-24,Approximate dynamical eigenmodes of the Ising model with local spin-exchange moves,"We establish that the Fourier modes of the magnetization serve as the
dynamical eigenmodes for the two-dimensional Ising model at the critical
temperature with local spin-exchange moves, i.e., Kawasaki dynamics. We obtain
the dynamical scaling properties for these modes, and use them to calculate the
time evolution of two dynamical quantities for the system, namely the
autocorrelation function and the mean-square deviation of the line
magnetizations. At intermediate times $1 \lesssim t \lesssim L^{z_c}$, where
$z_c=4-\eta=15/4$ is the dynamical critical exponent of the model, we find that
the line magnetization undergoes anomalous diffusion. Following our recent work
on anomalous diffusion in spin models, we demonstrate that the Generalized
Langevin Equation (GLE) with a memory kernel consistently describes the
anomalous diffusion, verifying the corresponding fluctuation-dissipation
theorem with the calculation of the force autocorrelation function.",1907.10296v1
2020-01-08,Observation of Dynamical Quantum Phase Transition with Correspondence in Excited State Phase Diagram,"Dynamical quantum phase transitions are closely related to equilibrium
quantum phase transitions for ground states. Here, we report an experimental
observation of a dynamical quantum phase transition in a spinor condensate with
correspondence in an excited state phase diagram, instead of the ground state
one. We observe that the quench dynamics exhibits a non-analytical change with
respect to a parameter in the final Hamiltonian in the absence of a
corresponding phase transition for the ground state there. We make a connection
between this singular point and a phase transition point for the highest energy
level in a subspace with zero spin magnetization of a Hamiltonian. We further
show the existence of dynamical phase transitions for finite magnetization
corresponding to the phase transition of the highest energy level in the
subspace with the same magnetization. Our results open a door for using
dynamical phase transitions as a tool to probe physics at higher energy
eigenlevels of many-body Hamiltonians.",2001.02686v2
2020-08-26,Critical Dynamics of Anisotropic Antiferromagnets in an External Field,"We numerically investigate the non-equilibrium critical dynamics in
three-dimensional anisotropic antiferromagnets in the presence of an external
magnetic field. The phase diagram of this system exhibits two critical lines
that meet at a bicritical point. The non-conserved components of the staggered
magnetization order parameter couple dynamically to the conserved component of
the magnetization density along the direction of the external field. Employing
a hybrid computational algorithm that combines reversible spin precession with
relaxational Monte Carlo updates, we study the aging scaling dynamics for the
model C critical line, identifying the critical initial slip, autocorrelation,
and aging exponents for both the order parameter and conserved field, thus also
verifying the dynamic critical exponent. We further probe the model F critical
line by investigating the system size dependence of the characteristic spin
wave frequencies near criticality, and measure the dynamic critical exponents
for the order parameter including its aging scaling at the bicritical point.",2008.11541v2
2021-04-22,Dynamical Backaction Magnomechanics,"Dynamical backaction resulting from radiation pressure forces in
optomechanical systems has proven to be a versatile tool for manipulating
mechanical vibrations. Notably, dynamical backaction has resulted in the
cooling of a mechanical resonator to its ground-state, driving phonon lasing,
the generation of entangled states, and observation of the optical-spring
effect. In certain magnetic materials, mechanical vibrations can interact with
magnetic excitations (magnons) via the magnetostrictive interaction, resulting
in an analogous magnon-induced dynamical backaction. In this article, we
directly observe the impact of magnon-induced dynamical backaction on a
spherical magnetic sample's mechanical vibrations. Moreover, dynamical
backaction effects play a crucial role in many recent theoretical proposals;
thus, our work provides the foundation for future experimental work pursuing
many of these theoretical proposals.",2104.11218v2
2021-10-13,Eigenvalue-based micromagnetic analysis of switching in spin-torque-driven structures,"We present an eigenvalue-based approach for studying the magnetization
dynamics in magnetic nanostructures driven by spintronic excitations, such as
spin transfer torque and spin orbit torque. The approach represents the system
dynamics in terms of normal oscillation modes (eigenstates) with corresponding
complex eigenfrequencies. The dynamics is driven by a small number of active
eigenstates and often considering just a single eigenstate is sufficient. We
develop a perturbation theory that provides semi-analytical dynamic solutions
by using eigenstates for the case in the absence of damping and spintronic
excitations as a basis. The approach provides important insights into dynamics
in such systems and allows solving several difficulties in their modeling, such
as extracting the switching current in magnetic random access memories (MRAM)
and understanding switching mechanisms. We show that the presented approach
directly predicts the critical switching current, i.e., switching current for
an infinite time. The approach also provides solutions for the switching
dynamics allowing obtaining the switching current for a finite switching time,
provided that the system symmetry is broken, e.g., by tilting the polarizer, so
that switching by a finite pulse is possible.",2110.07085v2
1999-12-14,Influence of direct URCA processes in a strong magnetic field on dynamics of collapsing star envelope,"Direct URCA processes in a collapsing star envelope with a strong magnetic
field are investigated. It is shown that in the toroidal magnetic field these
processes can develope a torque which quickly unwinds an envelope. A general
expression for a force density along the field direction is obtained. An
influence of the neutrino unwinding effect on a dynamics of the collapsar
envelope is discussed and numerically estimated.",9912288v1
2003-12-03,Stationary perturbation configurations in a composite system of stellar and coplanarly magnetized gaseous singular isothermal discs,"We construct aligned and unaligned stationary perturbation configurations in
a composite system of stellar and coplanarly magnetized gaseous singular
isothermal discs (SIDs) coupled by gravity. In comparison with SID problems
studied earlier, there exist three possible classes of stationary solutions
allowed by more dynamic freedoms. Our exact global perturbation solutions and
critical points are valuable for testing numerical magnetohydrodynamic codes.
For galactic applications, our model analysis contains more realistic elements
and offer useful insights for structures and dynamics of disc galaxies
consisting of stars and magnetized gas.",0312082v1
1997-10-14,Orbit stability in billiards in magnetic field,"We study the stability properties of orbits in dispersing billiards in a
homogeneous magnetic field by using a modified formalism based on the
Bunimovich-Sinai curvature (horocycle method). We identify simple periodic
orbits that can be stabilized by the magnetic field in the four-disk model and
the square-lattice Lorentz gas. The stable orbits can play a key role in
determining the transport properties of these models.",9710009v1
1993-04-02,Dynamical Correlation Theory for an Escape Process,"A dynamical theory which incorporates the electron-electron correlations and
the effects of external magnetic fields for an electron escaping from a helium
surface is presented. The degrees of freedom in the calculation of the escape
rate is reduced from $3N$ to 3 as compared with other approach. Explicit
expressions for the escape rate in various situations are obtained. In
particular, in the weak parallel magnetic field limit the tunneling rate has an
exponential dependence quadratic with magnetic field strength and an unusual
exponential increase linear with temperature.",9304003v1
1996-08-06,Dynamics of the Two-Site Hubbard Model,"The dynamical evolution of a two-site Hubbard model is derived in the
presence of an uniform external magnetic field for a general initial state. The
time evolution of the half-filled (two-particle) case has a complex behaviour.
Under certain initial conditions, the average number of fermions in
one-particle state, in the half-filled case, is not necessarily periodic, even
though the magnetization remains periodic. The results obtained may be applied
to study the magnetization and the transition dipole moment of the organic
charge-transfer salts in the in-phase mode.",9608025v1
1996-12-17,Dynamical Localization: Hydrogen Atoms in Magnetic and Microwave fields,"We show that dynamical localization for excited hydrogen atoms in magnetic
and microwave fields takes place at quite low microwave frequency much lower
than the Kepler frequency. The estimates of localization length are given for
different parameter regimes, showing that the quantum delocalization border
drops significantly as compared to the case of zero magnetic field. This opens
up broad possibilities for laboratory investigations.",9612155v1
1997-05-19,Nucleation Theory of Magnetization Switching in Nanoscale Ferromagnets,"A nucleation picture of magnetization switching in single-domain
ferromagnetic nanoparticles with high local anisotropy is discussed. Relevant
aspects of nucleation theory are presented, stressing the effects of the
particle size on the switching dynamics. The theory is illustrated by Monte
Carlo simulations and compared with experiments on single particles.",9705189v1
1997-08-27,Dynamical Magnetic Susceptibilities in Cu Benzoate,"Recent experiments on the quasi 1-D antiferromagnet Cu Benzoate revealed a
magentic field induced gap coexisting with (ferro)magnetic order. A theory
explaining these findings has been proposed by Oshikawa and Affleck. In the
present work we discuss consequences of this theory for inelastic neutron
scattering experiments by calculating the dynamical magnetic susceptibilities
close to the antiferromagnetic wave vector by the formfactor method.",9708208v3
1999-03-18,Coherent dynamics of localized spins coupled with a two-dimensional hole gas in diluted-magnetic quantum wells,"Spin dynamics in a quantum well made of diluted-magnetic semiconductor is
studied theoretically. The exchange interaction of the ensemble of localized
spins with two-dimensional heavy-hole gas is shown to affect time evolution of
the spin system in an in-plane magnetic field. The Larmor frequency of
localized spins is reduced under influence of an oscillating effective field of
holes.",9903280v1
1999-10-20,Magnetic-field dependence of dynamical vortex response in two-dimensional Josephson junction arrays and superconducting films,"The dynamical vortex response of a two-dimensional array of the resistively
shunted Josephson junctions in a perpendicular magnetic field is inferred from
simulations. It is found that, as the magnetic field is increased at a fixed
temperature, the response crosses over from normal to anomalous, and that this
crossover can be characterized by a single dimensionless parameter. It is
described how this crossover should be reflected in measurements of the complex
impedance for Josephson junction arrays and superconducting films.",9910300v2
2001-02-12,Nonlinear Dynamics of Quasiclassical Spin Moment in a Swept Field,"Quantum dynamics of anisotropic spin system with large spin moment in a swept
magnetic field is theoretically investigated. Magnetic field of this type
induces vortex static electrical field, that breaks down the axial symmetry and
induces new coherent quantum phenomena: the occurrence of band energy spectrum
with continuous spin states, Bloch-type oscillations and interband Zener
tunnelling effect. These quantum phenomena display themselves, in particular,
in magnetization jumps and susceptibility peaks in the investigated spin
system.",0102198v2
2001-04-09,Spin transport in inhomogeneous magnetic fields: a proposal for Stern-Gerlach-like experiments with conduction electrons,"Spin dynamics in spatially inhomogeneous magnetic fields is studied within
the framework of Boltzmann theory. Stern-Gerlach-like separation of spin up and
spin down electrons occurs in ballistic and diffusive regimes, before spin
relaxation sets in. Transient dynamics and spectral response to time-dependent
inhomogeneous magnetic fields are investigated, and possible experimental
observations of our findings are discussed.",0104146v2
2001-05-14,Thermal and Dynamic Effects in Langevin Simulation of Hysteresis in Nanoscale Pillars,"Dynamic quantities related to hysteresis have been measured in micromagnetic
simulations of single-domain nanoscale magnets at nonzero temperature. The
hysteresis-loop area and magnetization-field correlation display the
characteristics of resonance, and the resonance frequency is found to be
temperature dependent. The period-averaged magnetization displays symmetry
breaking at high frequencies.",0105279v1
2001-06-18,Vortex avalanches and magnetic flux fragmentation in superconductors,"We report results of numerical simulations of non isothermal dendritic flux
penetration in type-II superconductors. We propose a generic mechanism of
dynamic branching of a propagating hotspot of a flux flow/normal state
triggered by a local heat pulse. The branching occurs when the flux hotspot
reflects from inhomogeneities or the boundary on which magnetization currents
either vanish, or change direction. Then the hotspot undergoes a cascade of
successive splittings, giving rise to a dissipative dendritic-type flux
structure. This dynamic state eventually cools down, turning into a frozen
multi-filamentary pattern of magnetization currents.",0106353v1
2001-08-03,Frequency dependence of the intergranular magnetic flux penetration in ceramic $YBa_2Cu_3O_x$ uperconductor,"By means of small magnetic field ac susceptibility measurement at 10 kHz we
found that the real and imaginary parts of ceramic $YBa_2Cu_3O_ x$
susceptibility in presence of the external low frequency field close to 0.1 Hz
exhibit frequency dependence. The wide maximum of hysteresis losses and
{\color{black} the exponential dependence} of the {\color{black} effectiveness
of flux penetration} with increasing of external field frequency were obtained.
We observed a nonlinear dynamic magnetic response in presence dc field and we
suggest that this behavior is due to the dynamic and static Josephson
vortex-vortex interaction.",0108060v1
2001-08-04,Magnetic Field Pinning of a Dynamic Electron-Spin-Resonance Line in a GaAs/AlGaAs Heterostructure,"Electrically detected electron spin resonance (ESR) is used to study the
hyperfine interaction of the two-dimensional electrons and the nuclei of the
host lattice in a GaAs/AlGaAs heterostructure. Under the microwave and radio-
frequency double excitations, we have observed that the ESR line can be pinned
in a very narrow range of magnetic field - in the vicinity of the nuclear
magnetic resonance (NMR) of the nuclei of the GaAs crystal. Our observations
suggest that this pinning effect is the result of a competition process between
the ESR induced dynamic nuclear polarization and the NMR induced
depolarization.",0108080v1
2002-11-01,Exploring dynamical magnetism with time-dependent density-functional theory: from spin fluctuations to Gilbert damping,"We use time-dependent spin-density-functional theory to study dynamical
magnetic phenomena. First, we recall that the local-spin-density approximation
(LSDA) fails to account correctly for magnetic fluctuations in the paramagnetic
state of iron and other itinerant ferromagnets. Next, we construct a
gradient-dependent density functional that does not suffer from this problem of
the LSDA. This functional is then used to derive, for the first time, the
phenomenological Gilbert equation of micromagnetics directly from
time-dependent density-functional theory. Limitations and extensions of Gilbert
damping are discussed on this basis, and some comparisons with phenomenological
theories and experiments are made.",0211021v1
2002-11-27,Hidden symmetries of two-electron quantum dots in a magnetic field,"Using a classical and quantum mechanical analysis, we show that the magnetic
field gives rise to dynamical symmetries of a three-dimensional axially
symmetric two-electron quantum dot with a parabolic confinement. These
symmetries manifest themselves as near-degeneracies in the quantum spectrum at
specific values of the magnetic field and are robust at any strength of the
electron-electron interaction.",0211631v1
2003-03-26,Extended Dynamical Mean Field Theory Study of the Periodic Anderson Model,"We investigate the competition of the Kondo and the RKKY interactions in
heavy fermion systems. We solve a periodic Anderson model using Extended
Dynamical Mean Field Theory (EDMFT) with QMC. We monitor simultaneously the
evolution of the electronic and magnetic properties. As the RKKY coupling
increases the heavy fermion quasiparticle unbinds and a local moment forms. At
a critical RKKY coupling there is an onset of magnetic order. Within EDMFT the
two transitions occur at different points and the disapparence of the magnetism
is not described by a local quantum critical point.",0303539v2
2003-04-22,Optical spin transfer in ferromagnetic semiconductors,"Circularly polarized laser pulses that excite electron-hole pairs across the
band gap of (III,Mn)V ferromagnetic semiconductors can be used to manipulate
and to study collective magnetization dynamics. The initial spin orientation of
a photocarrier in a (III,V) semiconductors is determined by the polarization
state of the laser. We show that the photocarrier spin can be irreversibly
transferred to the collective magnetization, whose dynamics can consequently be
flexibly controlled by suitably chosen laser pulses. As illustrations we
demonstrate the feasibility of all optical ferromagnetic resonance and optical
magnetization reorientation.",0304492v2
2003-08-29,Magnetic Order and Dynamics in Stripe-Ordered La2-xSrxNiO4,"We have studied magnetic correlations in several compositions of
stripe-ordered La2-xSrxNiO4. In this paper we show how polarized-neutron
scattering has helped uncover important features of the magnetic ordering and
spin dynamics. In particular, polarization analysis has enabled us (1) to
characterize a spin reorientation transition, (2) to identify anisotropy gaps
in the spin excitation spectrum, and (3) to investigate an anomalous dip in the
spin-wave intensity suggestive of coupling between collective spin and charge
excitations.",0308631v1
2004-06-22,Josephson vortex dynamic in superconducting $YBa_2Cu_3O_x$ ceramics,"The Josephson vortices dynamic in superconducting $YBa_2Cu_3O_x$ ceramics was
studied in frequency range 0.01-0.5 Hz by ac susceptibility measurements in non
complete magnetic flux penetration regime. We have found that with the increase
of magnetic field frequency starting from 0.01 Hz the ac magnetic response
shows complex frequency dependence: in one half period of a sinusoidal field
appear two asymmetrical peaks of real $\Delta\chi\prime$ and imaginary
$\chi\prime\prime$ parts of the ac susceptibility. The analysis of experimental
results has led to an idea that probably the volume (diameter) of Josephson
vortices increases during its movement. The experimental results are discussed
in terms of Josephson vortex - vortex interactions.",0406523v1
2004-07-27,Dynamical Mean Field Theory of Temperature and Field Dependent Band Shifts in Magnetically Coupled Semimetals: Applocation to EuB6,"A model for semimetals such as $EuB_6$, in which band overlaps are controlled
by magnetic order, is presented and is solved in the dynamical mean field
approximation. First order phase boundaries are computed by evaluating free
energies of different states. The phase diagram is determined. A specific and
physically reasonable choice of parameters is found to approximately reproduce
the available data on $EuB_6$. For this material, predictions are made for the
location of a metamagnetic transition and its associated endpoint, and a change
in the order of the magnetic transition.",0407706v1
2004-07-28,Large quantum nonlinear dynamic susceptibility of single-molecule magnets,"The nonlinear dynamical response of Mn$_{12}$ single-molecule magnets is
experimentally found to be very large, quite insensitive to the spin-lattice
coupling constant, and displaying peaks reversed with respect to classical
superparamagnets. It is shown that these features are caused by the strong
field dependence of the relaxation rate due to the detuning of energy levels
between which tunneling takes place. The nonlinear susceptibility technique,
previously overlooked, is thus proposed as a privileged probe to ascertain the
occurrence of quantum effects in mesoscopic magnetic systems.",0407738v1
2004-10-20,Spin-Transfer Torque and Electron-Magnon Scattering,"According to the spin-torque model, current-driven magnetic dynamics in
ferromagnetic multilayers is determined by the transfer of electron spin
perpendicular to the layers' magnetizations. By separating the largest
contributions to the magnetic dynamics, we demonstrate that the dominant effect
of spin-torque is rather due to the electron spin parallel to the field. We
show that this effect can be equivalently described as stimulated
current-driven excitation of spin-waves, and discuss four specifically
quantum-mechanical aspects of spin-transfer, not described by the spin-torque.",0410520v1
2005-07-26,Butterfly hysteresis curve is a signature of adiabatic Landau-Zener transition,"We stress that the so-called butterfly hysteresis curves observed in
dynamical magnetization measurements on systems of low-spin magnetic molecules
such as V-15 and V-6 are a signature of adiabatic Landau-Zener transitions
rather than that of a phonon bottleneck. We investigate the magnetization
dynamics analytically with the help of a simple relaxation theory in the basis
of the adabatic energy levels of the spin 1/2, to a qualitative accordance with
experimental observations. In particular, reversible behavior is found near
zero field, the corresponding susceptibility being bounded by the equilibrium
and adiabatic susceptibilities from below and above, respectively.",0507618v1
2005-10-21,Magnetic Vortex Core Dynamics in a Ferromagnetic Dot,"We report direct imaging by means of x-ray photoemission electron microscopy
of the dynamics of magnetic vortices confined in micron-size circular Permalloy
dots that are 30 nm thick. The vortex core positions oscillate on a 10-ns
timescale in a self-induced magnetostatic potential well after the in-plane
magnetic field is turned off. The measured oscillation frequencies as a
function of the aspect ratio (thickness/radius) of the dots are in agreement
with theoretical calculations presented for the same geometry.",0510595v1
2006-07-04,Dynamics of Pinned Magnetic Vortices,"We observe the dynamics of a single magnetic vortex in the presence of a
random array of pinning sites. At low excitation amplitudes, the vortex core
gyrates about its equilibrium position with a frequency that is characteristic
of a single pinning site. At high amplitudes, the frequency of gyration is
determined by the magnetostatic energy of the entire vortex, which is confined
in a micron-scale disk. We observe a sharp transition between these two
amplitude regimes that is due to depinning of the vortex core from a local
defect. The distribution of pinning sites is determined by mapping fluctuations
in the frequency as the vortex core is displaced by a static in-plane magnetic
field.",0607095v1
2006-08-25,"Revivals, collapses and magnetic-pulse generation in quantum rings","Using a microscopic theory based on the density matrix formalism we
investigate quantum revivals and collapses of the charge polarization and
charge current dynamics in mesoscopic rings driven by short asymmetric
electromagnetic pulses. The collapsed state is utilized for sub-picosecond
switching of the current and associated magnetization, enabling thus the
generation of pulsed magnetic fields with a tunable time structure and shape
asymmetry which provides a new tool to study ultrafast spin-dynamics and
ratchet-based effects.",0608559v1
2006-10-11,Magnetic Field Dependence of Dephasing Rate due to Diluted Kondo Impurities,"We investigate the dephasing rate, 1/tau_phi, of weakly disordered electrons
due to scattering from diluted dynamical impurities. Our previous result for
the weak-localization dephasing rate is generalized from diluted Kondo
impurities to arbitrary dynamical defects with typical energy transfer larger
than 1/tau_phi. For magnetic impurities, we study the influence of magnetic
fields on the dephasing of Aharonov-Bohm oscillations and universal conductance
fluctuations both analytically and using the numerical renormalization group.
These results are compared to recent experiments.",0610304v3
2006-11-07,Enhanced pinning and proliferation of matching effects in a superconducting film with a Penrose array of magnetic dots,"The vortex dynamics in superconducting films deposited on top of a five-fold
Penrose array of magnetic dots is studied by means of transport measurements.
We show that in the low pinning regime (demagnetized dots) a few periodic and
aperiodic matching features coexist. In the strong pinning regime (magnetized
dots) a richer structure of unforeseen periodic and aperiodic vortex patterns
appear giving rise to a clear enhancement of the critical current in a broader
field range. Possible stable vortex configurations are determined by molecular
dynamics simulations.",0611173v1
2007-03-19,Nonequilibrium charge dynamics of light-driven rings threaded by a magnetic flux,"We study theoretically the charge polarization and the charge current
dynamics of a mesoscopic ring driven by short asymmetric electromagnetic pulses
and threaded by an external static magnetic flux. It is shown that the
pulse-induced charge polarization and the associated light-emission is
controllable by tuning the external magnetic flux. Applying two mutually
perpendicular pulses triggers a charge current in the ring. The interplay
between this nonequilibrium and the persistent currents is investigated and the
conditions under which the pulses stop the persistent current are identified.",0703502v1
1998-09-09,SU(2) vacuum dynamics in applied external magnetic field,"The vacuum dynamics of SU(2) lattice gauge theory is studied by means of a
gauge-invariant effective action, both at zero and finite temperature. Working
with lattices up to 32^4 we check the scaling of the energy density with the
magnetic length. We find that the screening at zero temperature of the applied
external magnetic field weakens by increasing the temperature.",9809042v1
1995-01-16,Dynamical Chiral Symmetry Breaking by a Magnetic Field in QED,"It is shown that the chiral symmetry is spontaneously broken by a constant
magnetic field in QED. The dynamical mass of fermions (energy gap in the
fermion spectrum) is $m_{dyn}\simeq
C\sqrt{eB}\exp\left[-\left(\pi/\alpha\right) ^{1/2}\right]$, where $B$ is the
magnetic field, the constant $C$ is of order one and $\alpha=e^2/4\pi$ is the
renormalized coupling constant. Possible applications of this effect are
discussed.",9501304v3
1997-09-14,Magnetic Catalysis of Chiral Symmetry Breaking in QED at Finite Temperature,"The catalysis of chiral symmetry breaking by a magnetic field in the massless
weak-coupling phase of QED is studied. The dynamical mass of a fermion (energy
gap in the fermion spectrum) is shown to depend essentially nonanalytically on
the renormalized coupling constant $\alpha$ in a strong magnetic field. The
temperature of the symmetry restoration is calculated analytically as
$T_c\approx m_{dyn}$, where $m_{dyn}$ is the dynamical mass of a fermion at
zero temperature.",9709339v1
1998-10-25,Yukawa Coupling Contribution to Magnetic Field Induced Dynamical Mass,"By solving the gap equation in the quenched, ladder approximation for an
Abelian gauge model with Yukawa interaction in the presence of a constant
magnetic field, we show that the Yukawa interactions enhance the dynamical
generation of fermion mass. The theory is then studied at finite temperature,
where we prove that the critical magnetic field, required for the mass
generation to be important at temperatures comparable to the electroweak
critical temperature, can be substantially decreased due to the Yukawa
coupling.",9810473v1
2000-01-11,Magnetic Catalysis of Chiral Symmetry Breaking in Gauge Theories,"Non-perturbative effect of the formation of a chiral symmetry breaking
condensate <\bar\psi\psi> and of a dynamically generated fermion mass in QED in
the presence of an external magnetic field is considered. The dynamical mass of
a fermion (energy gap in the fermion spectrum) is shown to depend essentially
nonanalytically on the renormalized coupling constant \alpha in a strong
magnetic field. Possible applications of this effect are discussed.",0001070v1
2003-07-22,A realistic example of chaotic tunneling: The hydrogen atom in parallel static electric and magnetic fields,"Statistics of tunneling rates in the presence of chaotic classical dynamics
is discussed on a realistic example: a hydrogen atom placed in parallel uniform
static electric and magnetic fields, where tunneling is followed by ionization
along the fields direction. Depending on the magnetic quantum number, one may
observe either a standard Porter-Thomas distribution of tunneling rates or, for
strong scarring by a periodic orbit parallel to the external fields, strong
deviations from it. For the latter case, a simple model based on random matrix
theory gives the correct distribution.",0307038v1
2001-01-30,Intermittency and turbulence in a magnetically confined fusion plasma,"We investigate the intermittency of magnetic turbulence as measured in
Reversed Field Pinch plasmas. We show that the Probability Distribution
Functions of magnetic field differences are not scale invariant, that is the
wings of these functions are more important at the smallest scales, a classical
signature of intermittency. We show that scaling laws appear also in a region
very close to the external wall of the confinement device, and we present
evidences that the observed intermittency increases moving towards the wall.",0101106v1
2001-07-11,On the Dynamical Invariants and the Geometric Phases for a General Spin System in a Changing Magnetic Field,"We consider a class of general spin Hamiltonians of the form
$H_s(t)=H_0(t)+H'(t)$ where $H_0(t)$ and $H'(t)$ describe the dipole
interaction of the spins with an arbitrary time-dependent magnetic field and
the internal interaction of the spins, respectively. We show that if $H'(t)$ is
rotationally invariant, then $H_s(t)$ admits the same dynamical invariant as
$H_0(t)$. A direct application of this observation is a straightforward
rederivation of the results of Yan et al [Phys. Lett. A, Vol: 251 (1999) 289
and Vol: 259 (1999) 207] on the Heisenberg spin system in a changing magnetic
field.",0107063v1
2007-07-13,Multiphoton antiresonance in large-spin systems,"We study nonlinear response of a spin $S>1/2$ with easy-axis anisotropy. The
response displays sharp dips or peaks when the modulation frequency is
adiabatically swept through multiphoton resonance. The effect is a consequence
of a special symmetry of the spin dynamics in a magnetic field for the
anisotropy energy $\propto S_z^2$. The occurrence of the dips or peaks is
determined by the spin state. Their shape strongly depends on the modulation
amplitude. Higher-order anisotropy breaks the symmetry, leading to sharp steps
in the response as function of frequency. The results bear on the dynamics of
molecular magnets in a static magnetic field.",0707.2107v1
2008-05-09,"Spin dynamics in (III,Mn)V ferromagnetic semiconductors: the role of correlations","We address the role of correlations between spin and charge degrees of
freedom on the dynamical properties of ferromagnetic systems governed by the
magnetic exchange interaction between itinerant and localized spins. For this
we introduce a general theory that treats quantum fluctuations beyond the
Random Phase Approximation based on a correlation expansion of the Green's
function equations of motion. We calculate the spin susceptibility, spin--wave
excitation spectrum, and magnetization precession damping. We find that
correlations strongly affect the magnitude and carrier concentration dependence
of the spin stiffness and magnetization Gilbert damping.",0805.1320v2
2008-06-04,Magnetization dynamics in optically excited nanostructured nickel films,"In this work, Laser-induced magnetization dynamics of nanostructured nickel
films is investigated. The influence of the nanosize is discussed considering
the time-scale of hundreds of femtoseconds as well as the GHz regime. While no
nanosize effect is observed on the short time-scale, the excited magnetic mode
in the GHz regime can be identified by comparison with micromagnetic
simulations. The thickness dependence reveals insight on the dipole interaction
between single nickel structures. Also, transient reflectivity changes are
discussed.",0806.0711v1
2008-06-05,Magnetization Relaxation and Collective Vortex Pinning in the Fe-Based Superconductor SmFeAsO_0.9F_0.1,"By measuring the dynamic and traditional magnetization relaxations we
investigate the vortex dynamics of the newly discovered superconductor
SmFeAsO_0.9F_0.1 with Tc = 55K. It is found that the relaxation rate is rather
large reflecting a small characteristic pinning energy. Moreover it shows a
weak temperature dependence in wide temperature region, which resembles the
behavior of the cuprate superconductors. Combining with the resistive data
under different magnetic fields, a vortex phase diagram is obtained. Our
results strongly suggest that the model of collective vortex pinning applies to
this new superconductor very well.",0806.0980v2
2008-06-11,Critical current of spin transfer torque-driven magnetization dynamics in magnetic multilayers,"The critical current of the spin transfer torque-driven magnetization
dynamics was studied by taking into account both spin pumping and the finite
penetration depth of the transverse spin current. We successfully reproduced
the recent experimental results obtained by Chen et al. [Phys. Rev. B {\bf 74},
144408 (2006)] and found that the critical current remains finite even in the
zero-thickness limit of the free layer. We showed that the remaining value of
the critical current is determined mainly by spin pumping.",0806.1822v2
2008-07-18,Current-induced dynamics of spiral magnet,"We study the dynamics of the spiral magnet under the charge current by
solving the Landau-Lifshitz-Gilbert equation numerically. In the steady state,
the current ${\vec j}$ induces (i) the parallel shift of the spiral pattern
with velocity $v=(\beta/\alpha)j$ ($\alpha$, $\beta$: the Gilbert damping
coefficients), (ii) the uniform magnetization $M$ parallel or anti-parallel to
the current depending on the chirality of the spiral and the ratio $\beta /
\alpha $, and (iii) the change in the wavenumber $k$ of the spiral. These are
analyzed by the continuum effective theory using the scaling argument, and the
various nonequilibrium phenomena such as the chaotic behavior and
current-induced annealing are also discussed.",0807.2901v1
2008-07-29,Dynamically Induced Zeeman Effect in Massless QED,"It is shown that in non-perturbative massless QED an anomalous magnetic
moment is dynamically induced by an applied magnetic field. The induced
magnetic moment produces a Zeeman splitting for electrons in Landau levels
higher than $l=0$. The expressions for the non-perturbative Lande g-factor and
Bohr magneton are obtained. Possible applications of this effect are outlined.",0807.4744v2
2008-11-28,Effect of Nonlocal Spin-Transfer Torque on Current-Induced Magnetization Dynamics,"Using the self-consistent model, we present nonlocal spin-transfer effects
caused by the feedback between inhomogeneous magnetization and spin-transfer
torque on the current-induced magnetization dynamics in nanomagnets. The
nonlocal effects can substantially improve the coherence time of precession in
nanomagnets and thus reduce the linewidth of power spectrum. This narrow
linewidth results from the nonlinear damping of spin-waves due to the nonlocal
spin torque which is inherent and thus should be considered in future
experiments.",0811.4649v1
2009-01-23,Magnetic pseudodifferential operators with coefficients in C*-algebras,"In previous articles, a magnetic pseudodifferential calculus and a family of
C*-algebras associated with twisted dynamical systems were introduced and the
connections between them have been established. We extend this formalism to
symbol classes of H\""ormander type with an x-behavior modelized by an abelian
C*-algebra. Some of these classes generate C*-algebras associated with the
twisted dynamical system. We show the relevance of these classes to the
spectral analysis of pseudodifferential operators with anisotropic symbols and
magnetic fields.",0901.3704v1
2009-07-03,Dynamical electron transport through a nanoelectromechanical wire in a magnetic field,"We investigate dynamical transport properties of interacting electrons moving
in a vibrating nanoelectromechanical wire in a magnetic field. We have built an
exactly solvable model in which electric current and mechanical oscillation are
treated fully quantum mechanically on an equal footing. Quantum mechanically
fluctuating Aharonov-Bohm phases obtained by the electrons cause nontrivial
contribution to mechanical vibration and electrical conduction of the wire. We
demonstrate our theory by calculating the admittance of the wire which are
influenced by the multiple interplay between the mechanical and the electrical
energy scales, magnetic field strength, and the electron-electron interaction.",0907.0546v1
2009-11-08,Nonequilibrium dynamical ferromagnetism of interacting Single-Molecule Magnets,"We propose a nonequilibrium Monte Carlo (MC) approach to explore
nonequilibrium dynamical ferromagnetism of interacting single molecule magnets
(SMMs). Both quantum spin tunneling and thermally activated spin reversal are
successfully implemented in the same MC simulation framework. Applied to a
typical example, this simulation method satisfactorily reproduces experimental
magnetization curves with experimental parameters. Our results show that both
quantum and classical effects are essential to determine the hysteresis
behaviors. This method is effective and reliable to gain deep insights into
SMMs.",0911.1539v1
2010-03-01,Vortex dynamics in superconducting channels with periodic constrictions,"Vortices confined to superconducting easy flow channels with periodic
constrictions exhibit reversible oscillations in the critical current at which
vortices begin moving as the external magnetic field is varied. This
commensurability scales with the channel shape and arrangement, although
screening effects play an important role. For large magnetic fields, some of
the vortices become pinned outside of the channels, leading to magnetic
hysteresis in the critical current. Some channel configurations also exhibit a
dynamical hysteresis in the flux-flow regime near the matching fields.",1003.0387v2
2010-05-10,Nonlinearity in a dynamo,"Using a rotating flat layer heated from below as an example, we consider
effects which lead to stabilizing an exponentially growing magnetic field in
magnetostrophic convection in transition from the kinematic dynamo to the full
non-linear dynamo. We present estimates of the energy redistribution over the
spectrum and helicity quenching by the magnetic field. We also study the
alignment of the velocity and magnetic fields. These regimes are similar to
those in planetary dynamo simulations.",1005.1557v1
2010-06-21,Quantum Fluctuations and Dynamic Clustering of Fluctuating Cooper Pairs,"We derive exact expressions for the fluctuation conductivity in two
dimensional superconductors as a function of temperature and magnetic field in
the whole fluctuation region above the upper critical field H_{c2}(T). Focusing
on the vicinity of the quantum phase transition near zero temperature, we
propose that as the magnetic field approaches the line near H_{c2}(0) from
above, a peculiar dynamic state consisting of clusters of coherently rotating
fluctuation Cooper-pairs forms and estimate the characteristic size and
lifetime of such clusters. We find the zero temperature magnetic field
dependence of the the transverse magnetoconductivity above H_{c2}(0) in layered
superconductors.",1006.4124v1
2010-10-12,Coherent spin-current oscillations in transverse magnetic fields,"We address the coherence of the dynamics of spin-currents with components
transverse to an external magnetic field for the spin-1/2 Heisenberg chain. We
study current autocorrelations at finite temperatures and the real-time
dynamics of currents at zero temperature. Besides a coherent Larmor
oscillation, we find an additional collective oscillation at higher
frequencies, emerging as a coherent many-magnon effect at low temperatures.
Using numerical and analytical methods, we analyze the oscillation frequency
and decay time of this coherent current-mode versus temperature and magnetic
field.",1010.2351v2
2010-12-16,Magnetic dynamics driven by the spin-current generated via spin-Seebeck effect,"We consider the spin-current driven dynamics of a magnetic nanostructure in a
conductive magnetic wire under a heat gradient in an open circuit, spin Seebeck
effect geometry. It is shown that the spin-current scattering results in a
spin-current torque acting on the nanostructure and leading to precession and
displacement. The scattering leads also to a redistribution of the spin
electrochemical potential along the wire resulting in a break of the
polarity-reversal symmetry of the inverse spin Hall effect voltage with respect
to the heat gradient inversion.",1012.3552v2
2011-02-11,Dynamics of nonlinear ion-waves in Fermi-Dirac electron-positron-ion magnetoplasmas,"Oblique propagation and head-on collisions of solitary structures is studied
in a dense magnetized plasma comprised of relativistic ultra-cold electrons and
positrons and positive dynamic ions using conventional extended multi-scales
technique, in the ground of quantum hydrodynamics model. The variations of
head-on collision phase-shift as well as the characteristic soliton amplitude
and width is evaluated numerically in terms of other plasma parameters such as
mass-density, normalized magnetic field strength, its angle with respect to the
soliton propagation and the relative positron number-density. The relevance of
current investigations, with appropriate plasma parameters for the
astrophysical dense magnetized objects such as white-dwarfs, is addressed.",1102.2365v1
2011-04-21,Spin Damping Monopole,"We present theoretical evidence that a magnetic monopole emerges in dynamic
magnetic systems in the presence of the spin-orbit interaction. The monopole
field is expressed in terms of spin damping associated with magnetization
dynamics. We demonstrate that the observation of this spin damping monopole is
accomplished electrically using Ampere's law for monopole current. Our
discovery suggests the integration of monopoles into electronics, namely,
monopolotronics.",1104.4215v2
2011-05-30,Nonlogarithmic magnetization relaxation in HTSC,"An experimental study of low density captured magnetic flux dynamics in YBCO
single crystal samples near the superconducting phase transition point has been
performed. Vortex dynamics similar to thermally assisted flux flow regime
(TAFF) has been observed for the first time in such weak fields. It will be
shown that Zeldov's thermoactivated creep model successfully describes the
behavior of the magnetization in the YBCO single crystals in our experiments.
Within the framework of this model the estimation of normalized relaxation rate
was made.",1105.5896v1
2011-11-23,Rashba spin torque in an ultrathin ferromagnetic metal layer,"In a two-dimensional ferromagnetic metal layer lacking inversion symmetry,
the itinerant electrons mediate the interaction between the Rashba spin-orbit
interaction and the ferromagnetic order parameter, leading to a Rashba spin
torque exerted on the magnetization. Using Keldysh technique, in the presence
of both magnetism and a spin-orbit coupling, we derive a spin diffusion
equation that provides a coherent description to the diffusive spin dynamics.
The characteristics of the spin torque and its implication on magnetization
dynamics are discussed in the limits of large and weak spin-orbit coupling.",1111.5466v1
2012-01-27,Dynamics of Successive Minor Hysteresis Loops,"Cumulative growth of successive minor hysteresis loops in Co/Pd multilayers
with perpendicular anisotropy was studied in the context of time dependent
magnetization reversal dynamics. We show that in disordered ferromagnets, where
magnetization reversal involves nucleation, domains' expansion and
annihilation, differences between the time dependencies of these processes are
responsible for accumulation of nuclei for rapid domain expansion, for the
asymmetry of forward and backward magnetization reversals and for the
respective cumulative growth of hysteresis loops. Loops stop changing and
become macroscopically reproducible when populations of upward and downward
nucleation domains balance each other and the respective upward and downward
reversal times stabilize.",1201.5791v1
2012-05-12,Ultrafast Time-Resolved Faraday Rotation in EuO Thin Films,"We have investigated the ultrafast spin dynamics in EuO thin films by
time-resolved Faraday rotation spectroscopy. The photoinduced magnetization is
found to be increased in a transient manner, accompanied with subsequent
demagnetization. The dynamical magnetization enhancement showed a maximum
slightly below the Curie temperature with prolonged tails toward both lower and
higher temperatures and dominates the demagnetization counterpart at 55 K. The
magnetization enhancement component decays in ~1 ns. The realization of the
transient collective ordering is attributable to the enhancement of the f-d
exchange interaction.",1205.2818v2
2012-07-17,Ultrafast dynamical path for the switching of a ferrimagnet after femtosecond heating,"Ultrafast laser-induced magnetic switching in rare earth, transition metal
ferrimagnetic alloys has recently been reported to occur by ultrafast heating
alone. Using atomistic simulations and a ferrimagnetic Landau-Lifshitz-Bloch
formalism, we demonstrate that for switching to occur it is necessary that
angular momentum is transferred from the longitudinal to transverse
magnetization components. This dynamical path leads to magnetization switching
and subsequent ultrafast precession caused by the inter-sublattice exchange
field on the nanoscale.",1207.4092v2
2013-01-11,Exact Relation with Two-point Correlation Functions and Phenomenological Approach for Compressible Magnetohydrodynamic Turbulence,"Compressible isothermal magnetohydrodynamic turbulence is analyzed under the
assumption of statistical homogeneity and in the asymptotic limit of large
kinetic and magnetic Reynolds numbers. Following Kolmogorov we derive an exact
relation for some two-point correlation functions which generalizes the
expression recently found for hydrodynamics. We show that the magnetic field
brings new source and flux terms into the dynamics which may act on the
inertial range similarly as a source or a sink for the mean energy transfer
rate. The introduction of a uniform magnetic field simplifies significantly the
exact relation for which a simple phenomenology may be given. A prediction for
axisymmetric energy spectra is eventually proposed.",1301.2470v1
2013-01-16,Kinematic and dynamic vortices in a thin film driven by an applied current and magnetic field,"Using a Ginzburg-Landau model, we study the vortex behavior of a rectangular
thin film superconductor subjected to an applied current fed into a portion of
the sides and an applied magnetic field directed orthogonal to the film.
  Through a center manifold reduction we develop a rigorous bifurcation theory
for the appearance of periodic solutions in certain parameter regimes near the
normal state. The leading order dynamics yield in particular a motion law for
kinematic vortices moving up and down the center line of the sample. We also
present computations that reveal the co-existence and periodic evolution of
kinematic and magnetic vortices.",1301.3801v1
2013-05-23,Absolute cross-section normalization of magnetic neutron scattering data,"We discuss various methods to obtain the resolution volume for neutron
scattering experiments, in order to perform absolute normalization on inelastic
magnetic neutron scattering data. Examples from previous experiments are given.
We also try to provide clear definitions of a number of physical quantities
which are commonly used to describe neutron magnetic scattering results,
including the dynamic spin correlation function and the imaginary part of the
dynamic susceptibility. Formulas that can be used for general purposes are
provided and the advantages of the different normalization processes are
discussed.",1305.5521v1
2013-07-18,Spin correlated dielectric memory and rejuvenation in relaxor ferroelectric CuCrS2,"CuCrS2, a Heisenberg antiferromagnet with layered edge sharing triangular
lattice, exhibits a spiral magnetic order. Dielectric (E) and polarization
studies show magnetoelectric (ME) coupling at Neel temperature (T_N=38 K) where
simultaneous dielectric and magnetic long range order occur. The compound shows
a diffused ferroelectric (FE) transition and slow relaxation dynamics above
T_N, indicative of relaxor FE behavior. Interestingly, memory effect and
magnetic field induced rejuvenation are also observed in E, establishing
cooperative glassy dynamics and ME coupling even above T_N. We discuss the role
of geometrical frustration and metal ligand hybridization for these unusual
properties.",1307.4859v1
2013-11-06,Competing interaction in magnets: the root of ordered disorder or only frustration?,"What does the equilibrium atomic, molecular or spin configuration of a glass
phase look like? Is there only one unique equilibrium configuration or are
there infinitely many configurations of equal energy? The processes and
mechanisms governing the path towards equilibrium, i.e. the dynamics of glassy
systems, provide insights to these questions. Here we discuss the intrinsic
dynamics of different glassy magnets: of spin-glasses, frustrated ferromagnets,
superspin-glasses and other nanostructured systems with competing ferro- and
antiferromagnetic interactions and randomness in their spatial distribution.
This paper is intended as a brief update on some unsolved problems and the
current empirical status in the field of disordered and frustrated magnetism.",1311.1350v1
2013-12-03,Dynamical properties of three terminal magnetic tunnel junctions: spintronics meets spin-orbitronics,"This letter introduces a micromagnetic model able to characterize the
magnetization dynamics in three terminal magnetic tunnel junctions, where the
effects of spin-transfer torque and spin-orbit torque are taken into account.
Our results predict that the possibility to separate electrically those two
torque sources is very promising from a technological point of view for both
next generation of nanoscale spintronic oscillators and microwave detectors. A
scalable synchronization scheme based on the parallel connection of those three
terminal devices is also proposed.",1312.0806v1
2014-05-15,Larmor precession of the octet baryons in view of the general QCD parametrization,"We apply general QCD parametrization to describe an octet baryon at rest in a
static and uniform magnetic field, in order to demonstrate a dynamical
implementation of the hadron model. The derived evolution has the asymptote of
Larmor precession, governed by an effective magnetic dipole moment coupling.
The parameters appear in the effective magnetic dipole moment has been studied
in the original works in kinematic settings. Here we show that the parameters
have actual dynamical meaning, and are indeed measured by the process.",1405.3779v1
2014-06-18,Axial Current driven by Magnetization Dynamics in Weyl Semimetals,"We theoretically study the axial current $\bm{j}_5$ (defined as the
difference between the charge current with opposite chirality) in doped Weyl
semimetal using a Green's function technique. We show that the axial current is
controlled by the magnetization dynamics in a magnetic insulator attached to a
Weyl semimetal. We find that the induced axial current can be detected by using
ferromagnetic resonance or the inverse spin Hall effect and can be converted
into charge current with no accompanying energy loss. These properties make
Weyl semimetal advantageous for application to low-consumption electronics with
new functionality.",1406.4636v1
2014-07-21,Josephson junction with a magnetic vortex,"We have studied Josephson tunneling through a circularly polarized micron or
submicron-size disk of a soft ferromagnetic material. Such a disk contains a
vortex that exhibits rich classical dynamics and has recently been proposed as
a tool to study quantum dynamics of the nanoscale vortex core. The change in
the Josephson current that is related to a tiny displacement of the vortex core
has been computed analytically and plotted numerically for permalloy disks used
in experiments. It is shown that a Josephson junction with a magnetic disk in
the vortex state can be an interesting physical system that may be used to
measure the nanoscale motion of the magnetic vortex.",1407.5419v1
2014-08-22,The mean-field dynamo model,"The 2D Parker's mean-field dynamo equations with a various distributions of
the $\alpha$- and $\omega$-effects are considered. We show that smooth profiles
of $\alpha$ and $\omega$ can produce dipole configuration of the magnetic field
with the realistic magnetic energy spectrum. We emphasize that fluctuating
$\alpha$-effect leads to increase of the magnetic energy at the small scales,
breaking the dipole configuration of the field. The considered geostrophic
profiles of $\alpha$ and $\omega$ correspond to the small-scale
polarwards/equatorwards travelling waves with the small dipole field
contribution. The same result is observed for the dynamic form of the
$\alpha$-quenching, where two branches of the weak and strong solution coexist.",1408.5202v1
2014-09-29,On heteroclinic separators of magnetic fields in electrically conducting fluids,"In this paper we partly solve the problem of existence of separators of a
magnetic field in plasma. We single out in plasma a 3-body with a boundary in
which the movement of plasma is of special kind which we call an (a-d)-motion.
We prove that if the body is the 3-annulus or the ""fat"" orientable surface with
two holes the magnetic field necessarily have a heteroclinic separator. The
statement of the problem and the suggested method for its solution lead to some
theoretical problems from Dynamical Systems Theory which are of interest of
their own.",1409.8189v2
2014-10-14,Dynamics and thermodynamics of a pair of interacting magnetic dipoles,"We consider the dynamics and thermodynamics of a pair of magnetic dipoles
interacting via their magnetic fields. We consider only the ""spin"" degrees of
freedom; the dipoles are fixed in space. With this restriction it is possible
to provide the general solution of the equations of motion in analytical form.
Thermodynamic quantities, such as the specific heat and the zero field
susceptibility are calculated by combining low temperature asymptotic series
and a complete high temperature expansion. The thermal expectation value of the
autocorrelation function is determined for the low temperature regime including
terms linear in $T$. Furthermore, we compare our analytical results with
numerical calculations based on Monte Carlo simulations.",1410.3700v1
2014-10-21,Magnetic switching dynamics in a ferrimagnetic two sub-lattice model including ultrafast exchange scattering,"We study the heat-induced magnetization dynamics in a toy model of a
ferrimagnetic alloy, which includes localized spins antiferromagnetically
coupled to an itinerant carrier system with a Stoner gap. We determine the
one-particle spin-density matrix including exchange scattering between
localized and itinerant bands as well as scattering with phonons. While a
transient ferromagnetic-like state can always be achieved by a sufficiently
strong excitation, this transient ferromagnetic-like state only leads to
magnetization switching for model parameters that also yield a compensation
point in the equilibrium M(T) curve.",1410.5745v2
2015-01-28,Field induced anisotropic cooperativity in a magnetic colloidal glass,"The translational dynamics in a repulsive colloidal glass-former is probed by
time-resolved X-ray Photon Correlation Spectroscopy. In this dense dispersion
of charge-stabilized and magnetic nanoparticles, the interaction potential can
be tuned, from quasi-isotropic to anisotropic by applying an external magnetic
field. Structural and dynamical anisotropies are reported on interparticle
lengthscales associated with highly anisotropic cooperativity, almost two
orders of magnitude larger in the field direction than in the perpendicular
direction and in zero field.",1501.07208v2
2015-03-07,Collisionless spin dynamics in a magnetic field gradient,"We study the collisionless spin dynamics of a harmonically trapped Fermi gas
in a magnetic field gradient. In the absence of interactions, the system
evolution is periodic: the magnetization develops twists, which evolve into a
longitudinal polarization. Recurrences follow. For weak interaction, the
exchange interactions lead to beats in these oscillations. We present an array
of analytic and numerical techniques for studying this physics.",1503.02131v2
2015-04-02,Magnetic Phenomena in Holographic Superconductivity with Lifshitz Scaling,"We investigate the effects of Lifshitz dynamical critical exponent z on a
family of minimal D=4+1 holographic superconducting models, with a particular
focus on magnetic phenomena. We see that it is possible to have a consistent
Ginzburg-Landau approach to holographic superconductivity in a Lifshitz
background. By following this phenomenological approach we are able to compute
a wide array of physical quantities. We also calculate the Ginzburg-Landau
parameter for different condensates, and conclude that in systems with higher
dynamical critical exponent, vortex formation is more strongly unfavored
energetically and exhibit a stronger Type I behavior. Finally, following the
perturbative approach proposed by Maeda, Natsuume and Okamura, we calculate the
critical magnetic field of our models for different values of z.",1504.00444v1
2015-04-09,Quantum dynamics of a spin-1/2 charged particle in the presence of magnetic field with scalar and vector couplings,"The quantum dynamics of a spin-1/2 charged particle in the presence of
magnetic field is analyzed for the general case where scalar and vector
couplings are considered. The energy spectra are explicitly computed for
different physical situations, as well as their dependencies on the magnetic
field strength, spin projection parameter and vector and scalar coupling
constants.",1504.02316v1
2015-05-08,Thermal vector potential theory of magnon-driven magnetization dynamics,"Thermal vector potential formulation is applied to study thermal dynamics of
magnetic structures in insulating ferromagnets. By separating variables of the
magnetic structure and magnons, the equation of motion for the structure
including spin-transfer effect due to thermal magnons is derived in the case of
a domain wall and a vortex. The magnon current is evaluated based on a linear
response theory with respect to the thermal vector potential representing the
temperature gradient. It is shown that the velocity of a domain wall when
driven by thermal magnon has a strong temperature dependence unlike the case of
an electrically-driven domain wall in metals.",1505.01908v1
2015-08-23,Rotational properties of ferromagnetic nanoparticles driven by a precessing magnetic field in a viscous fluid,"We study the deterministic and stochastic rotational dynamics of
ferromagnetic nanoparticles in a precessing magnetic field. Our approach is
based on the system of effective Langevin equations and on the corresponding
Fokker-Planck equation. Two key characteristics of the rotational dynamics, the
average angular frequency of precession of nanoparticles and their average
magnetization, are of our interest. Using the Langevin and Fokker-Planck
equations, we calculate both analytically and numerically these characteristics
in the deterministic and stochastic cases, determine their dependence on the
model parameters, and analyze in detail the role of thermal fluctuations.",1508.05651v1
2016-03-29,Kinematic dynamo in two-dimensional chaotic flow: the initial and final stages,"The small-scale kinematic dynamo in a two-dimensional chaotic flow is
studied. The analytic approach is developed in framework of the
Kraichnan-Kazantsev model. It is shown that the growth of magnetic field
$\bm{B}$ fluctuations stops at large times in accordance with so-called
anti-dynamo theorems. The value of $\bm{B}^2$ increased therewith in square of
the magnetic Prandtl number times. The spatial structure of the correlation
tensor of the magnetic field is found.",1603.08771v1
2016-08-22,Four dimensional quantum oscillator and magnetic-monopole with U(1) dynamical group,"By using of an appropriate transformation, it was shown that the quantum
system of 4 dimensional simple harmonic oscillator can describe the motion of a
charged particle in the presence of a magnetic monopole field. It was shown
that the Dirac magnetic monopole has the hidden algebra of U(1) symmetry and by
reducing of the dimensions of space, the U(1)*U(1) dynamical group for 4D
harmonic oscillator quantum system was obtained. Using the group representation
and based on explicit solution of the obtained differential equation, the
spectrum of system was calculated.",1608.06085v1
2017-06-06,Search for torsional oscillations in isolated sunspots,"In this work we seek evidence for global torsional oscillations in alpha
sunspots. We have used long time series of continuum intensity and magnetic
field vector maps from the Helioseismic and Magnetic Imager (HMI) instrument on
board the Solar Dynamics Observatory (SDO) spacecraft. The time series analysed
here span the total disk passage of 25 isolated sunspots. We found no evidence
of global long-term periodic oscillations in the azimuthal angle of the sunspot
magnetic field within $\sim$ 1 degree. This study could help us to understand
the sunspot dynamics and its internal structure.",1706.01952v1
2017-10-11,Oscillatory behavior of the domain wall dynamics in a curved cylindrical magnetic nanowire,"Understanding the domain wall dynamics is an important issue in modern
magnetism. Here we present results of domain wall displacement in curved
cylindrical nanowires at a constant magnetic field. We show that the average
velocity of a transverse domain wall increases with curvature. Contrary to what
it is observed in stripes, in a curved wire the transverse domain wall
oscillates along and rotates around the nanowire with the same frequency. These
results open the possibility of new oscillation-based applications.",1710.04316v1
2017-12-11,Testing the Dynamic Time Warping algorithm in the Seafloor Spreading Rate Problem,"We offer to apply the powerful Dynamic Time Warping (DTW) algorithm to find
the spreading rate variation by comparing profiles of marine magnetic anomalies
with the synthetic field constructed by the magnetic polarity reference scale.
For the fixed spreading rate and the given interval of the polarity scale we
generate the synthetic magnetic fields which we compare with the field obtained
for temporal variations of seafloor spreading rate.",1712.03920v1
2018-03-08,Relativistic quantum dynamics of twisted electron beams in arbitrary electric and magnetic fields,"General relativistic quantum dynamics of twisted (vortex) Dirac particles is
constructed. The Hamiltonian and equations of motion in the Foldy-Wouthuysen
representation are derived for a twisted relativistic electron in arbitrary
electric and magnetic fields. A critical experiment for a verification of the
results is proposed. The new important effect of a radiative orbital
polarization of a twisted electron beam in a magnetic field resulting in a
nonzero average projection of the intrinsic orbital angular momentum on the
field direction is predicted.",1803.07410v2
2018-05-13,The contact property for magnetic flows on surfaces,"This is the author's PhD Thesis (University of Cambridge, 2014) in its
original form. In the first part, using an invariance result, we compute the
symplectic homology of contact-type energy levels for magnetic systems on
surfaces, provided the energy is very large or very small. In the second part,
which is partially contained in the later paper (Benedetti, Ergod. Theory
Dynam. Syst., 2016), we discuss some rotationally symmetric examples and
establish dynamical convexity for symplectic magnetic flows on low energy
levels.",1805.04916v1
2018-10-18,Spectral analysis of magnetic fields in simulated galaxy clusters,"We introduce a new sample of galaxy clusters obtained from a cosmological
simulation covering an unprecedented dynamical range. All the clusters in our
sample show a clear signature of small-scale dynamo amplification. We show that
it is possible to use dynamo theory for studying the magnetic spectrum in the
intracluster medium. We study if the intrinsic variations on the spectra depend
on the dynamical history of each cluster or on some host cluster properties.",1810.08009v1
2018-10-26,Pointwise vanishing velocity helicity of a flow does not preclude magnetic field generation,"Pointwise zero velocity helicity density is shown not to prevent steady flows
from acting as kinematic dynamos. We present numerical evidences that such
flows can generate both small-scale magnetic fields as well as, by the magnetic
$\alpha$-effect or negative eddy diffusivity mechanisms, large-scale ones. The
flows are constructed as curls of analytically defined space-periodic steady
solenoidal flows, whose vorticity helicity (i.e., kinetic helicity) density is
everywhere zero.",1811.00859v3
2019-06-14,Rotational tuning of the dipole-dipole interaction in a Bose gas of magnetic atoms,"We investigate the dynamics of a Bose-Einstein condensate of magnetic atoms
in which the dipoles are rotated by an external magnetic field. The
time-averaged dipole-dipole interaction between the atoms is effectively tuned
by this rotation, however recent experimental and theoretical developments show
that dynamic instabilities emerge that may cause heating. We present
simulations of a realistic tuning sequence in this system, and characterize the
system behavior and the emergence of instabilities. Our results indicate that
the instabilities develop more slowly as the rotation frequency increases, and
indicate that experiments with tuned dipole-dipole interactions should be
feasible.",1906.06115v2
2019-06-27,Density functional perturbation theory for lattice dynamics with fully relativistic ultrasoft pseudopotentials: the magnetic case,"We extend density functional perturbation theory for lattice dynamics with
fully relativistic ultrasoft pseudopotentials to magnetic materials. Our
approach is based on the application of the time-reversal operator to the
Sternheimer linear system and to its self-consistent solutions. Moreover, we
discuss how to include in the formalism the symmetry operations of the magnetic
point group which require the time-reversal operator. We validate our
implementation by comparison with the frozen phonon method in fcc Ni and in a
monatomic ferromagnetic Pt wire.",1906.11673v1
2019-09-19,Magnetization dynamics of the compensated ferrimagnet $Mn_{2}Ru_{x}Ga$,"Here we study both static and time-resolved dynamic magnetic properties of
the compensated ferrimagnet from room temperature down to 10K, thus crossing
the magnetic compensation temperature $T_{M}$. The behaviour is analysed with a
model of a simple collinear ferrimagnet with uniaxial anisotropy and
site-specific gyromagnetic ratios. We find a maximum zero-applied-field
resonance frequency of $\sim$160GHz and a low intrinsic Gilbert damping
$\alpha$$\sim$0.02, making it a very attractive candidate for various
spintronic applications.",1909.09085v1
2019-11-04,Sum rules in zone axis STEM-orbital angular momentum resolved electron magnetic chiral dichroism,"In this work we derive sum rules for orbital angular momentum(OAM) resolved
electron magnetic chiral dichroism (EMCD) which enable the evaluation of the
strength of spin and orbital components of the atomic magnetic moments in a
crystalline sample. We also demonstrate through numerical simulations that
these rules appear to be only slightly dependent from the dynamical diffraction
of the electron beam in the sample, making possible their application without
the need of additional dynamical diffraction calculations.",1911.02006v1
2019-11-11,The Rayleigh Bubble in Quark Matter under a Strong Magnetic Field,"The transition of QGP from fluid-dynamical regime to freeze-out is
accompanied by the onset of instabilities. In the present paper we investigate
the impact of the magnetic field on the Rayleigh instability. We show that
extremely strong field generated in peripheral heavy ion collisions has an
insignificant influence on the Rayleigh bubble dynamics. Magnetic ""friction""
turns out to be much weaker than the viscous one.",1911.04148v2
2020-01-29,Relative Equilibria of Magnetic Micro-Swimmers,"We revisit the dynamics of a permanent-magnetic rigid body submitted to a
spatially-uniform steadily-rotating magnetic field in Stokes flow. We propose
an analytical parameterisation of the full set of equilibria depending on two
key experimental parameters, and show how it brings further understanding that
helps to optimise magnetisation and operating parameters. The system is often
bistable when it reaches its optimal swimming velocity. A handling strategy is
proposed that guarantees that the correct equilibrium is reached.",2002.00740v1
2020-08-05,Time scales in the thermal dynamics of magnetic dipolar clusters,"The collective behavior of thermally active structures offers clues on the
emergent degrees of freedom and the physical mechanisms that determine the low
energy state of a variety of systems. Here, the thermally active dynamics of
magnetic dipoles at square plaquettes is modeled in terms of Brownian
oscillators in contact with a heat bath. Solution of the Langevin equation for
a set of interacting x-y dipoles allows the identification of the time scales
and correlation length that reveal how interactions, temperature, damping and
inertia may determine the frequency modes of edge and bulk magnetic mesospins
in artificial dipolar systems.",2008.01891v2
2020-08-27,Nutation Resonance in Ferromagnets,"The inertial dynamics of magnetization in a ferromagnet is investigated
theoretically. The analytically derived dynamic response upon microwave
excitation shows two peaks: ferromagnetic and nutation resonances. The exact
analytical expressions of frequency and linewidth of the magnetic nutation
resonance are deduced from the frequency dependent susceptibility determined by
the inertial Landau-Lifshitz-Gilbert equation. The study shows that the
dependence of nutation linewidth on the Gilbert precession damping has a
minimum, which becomes more expressive with increase of the applied magnetic
field.",2008.12221v3
2021-01-19,Dynamics of quantum discord of two coupled spin-1/2 subjected to time-dependent magnetic fields,"We describe the dynamics of quantum discord of two interacting spin-1/2
subjected to controllable time-dependent magnetic fields. The exact time
evolution of discord is given for various input mixed states consisting of
classical mixtures of two Bell states. The quantum discord manifests a complex
oscillatory behaviour in time and is compared with that of quantum
entanglement, measured by concurrence. The interplay of the action of the
time-dependent magnetic fields and the spin-coupling mechanism in the
occurrence and evolution of quantum correlations is examined in detail.",2101.07697v1
2021-06-03,Ultrafast Electron Dynamics in Magnetic Thin Films,"In past decades, ultrafast spin dynamics in magnetic systems have been
associated with heat deposition from high energy laser pulses, limiting the
selective access to spin order. Here we use a long wavelength terahertz pump
optical probe setup to measure structural features in the ultrafast time scale.
We find that complete demagnetisation is possible with <6 THz pulses. This
occurs concurrently with longitudinal acoustic phonons and an electronic
response, followed by the magnetic response. The required fluence for full
demagnetisation is low, ruling out the necessity of a high power light source.",2106.01828v1
2022-06-01,Dynamic Fingerprints of Synthetic Antiferromagnet Nanostructures with Interfacial Dzyaloshinskii-Moriya Interaction,"Synthetic antiferromagnet (SAF) nanostructures with interfacial
Dzyaloshinskii-Moriya interaction can host topologically distinct spin textures
such as skyrmions and thus are regarded as promising candidates for both
spintronics and magnonics applications. Here, we present comprehensive
micromagnetic simulations of such material systems and discuss the rich phase
diagrams that contain various types of magnetic configurations. Aside from the
static properties, we further discuss the resonant excitations of the
calculated magnetic states which include individual skyrmions and skyrmioniums.
Finally, the internal modes of SAF skyrmion clusters are studied and discussed
in the context of magnetic sensing applications based on the dynamic
fingerprint in broadband ferromagnetic resonance measurements.",2206.00554v2
2023-06-13,Angle dependence of $^{15}$N nuclear spin dynamics in diamond NV centers,"We report on the dynamics of the Rabi oscillation and the Larmor precession
of $^{15}$N nuclear spin using nonselective short microwave pulses for
initialization of $^{15}$N nuclear spins. We observe the Larmor precession of
$^{15}$N nuclear spin depending on the angle between the applied magnetic field
and the axis of the nitrogen vacancy center. We propose to utilize the change
of the Larmor frequency of the nuclear spins to detect static magnetic fields
at high sensitivity. Our results may contribute to enhancing the sensitivity of
dc magnetic fields and devising novel protocols using $^{15}$N nuclear spin in
nitrogen vacancy centers in diamonds.",2306.07556v1
2023-06-14,A magnetic falling-sphere viscometer,"We present a falling-sphere viscometer with a magnetized sphere and fluxgate
magnetometers continuously measuring the magnetic field produced at the sensor
positions by the falling sphere. With a fluid volume of 15 ml and within a few
seconds, we directly measure dynamical viscosities in a range between 200 cP
and 3000 cP with a precision of 3\%.",2306.08725v2
2023-09-06,Optimal Control of the 2D Landau-Lifshitz-Gilbert Equation with Control Energy in Effective Magnetic Field,"The optimal control of magnetization dynamics in a ferromagnetic sample at a
microscopic scale is studied. The dynamics of this model is governed by the
Landau-Lifshitz-Gilbert equation on a two-dimensional bounded domain with the
external magnetic field (the control) applied through the effective field. We
prove the global existence and uniqueness of a regular solution in $\mathbb
S^2$ under a smallness condition on control and initial data. We establish the
existence of optimal control and derive a first-order necessary optimality
condition using the Fr\'echet derivative of the control-to-state operator and
adjoint problem approach.",2309.02786v1
2023-10-19,Crossover from string to cluster dynamics following a field quench in spin ice,"We investigate quench dynamics of spin ice after removal of a strong magnetic
field along the [100] crystal direction, using Monte Carlo simulations and
theoretical arguments. We show how the early-time relaxation of the
magnetization can be understood in terms of nucleation and growth of strings of
flipped spins, in agreement with an effective stochastic model that we
introduce and solve analytically. We demonstrate a crossover at longer times to
a regime dominated by approximately isotropic clusters, which we characterize
in terms of their morphology, and present evidence for a percolation transition
as a function of magnetization.",2310.12410v2
2024-01-17,Dynamic behavior of the antiferromagnetically coupled bilayer Ising model,"Using the path probability and lowest approximation of cluster variation
method, we study the dynamic and equilibrium properties of a bilayer magnetic
system, consisting of two ferromagnetic monolayers antiferromagnetically
coupled for different spins $(\sigma=1/2$ and $S=1)$. Firstly, numerical
results of the monolayer and total magnetizations are presented under the
effect of the diverse physical parameters, and the phase diagrams of bilayer
system are discussed. Then, since it is well established that the path
probability method is an effective method for the existence of metastable
states, the time evolution of monolayer- and total magnetizations is
investigated.",2401.09360v1
2009-10-06,Diffusion of magnetic field and removal of magnetic flux from clouds via turbulent reconnection,"The diffusion of astrophysical magnetic fields in conducting fluids in the
presence of turbulence depends on whether magnetic fields can change their
topology via reconnection in highly conducting media. Recent progress in
understanding fast magnetic reconnection in the presence of turbulence is
reassuring that the magnetic field behavior in computer simulations and
turbulent astrophysical environments is similar, as far as magnetic
reconnection is concerned. Our studies of magnetic field diffusion in turbulent
medium reveal interesting new phenomena. In the presence of gravity and
turbulence, our 3D simulations show the decrease of the magnetic flux-to-mass
ratio as the gaseous density at the center of the gravitational potential
increases. We observe this effect both in the situations when we start with
equilibrium distributions of gas and magnetic field and when we follow the
evolution of collapsing dynamically unstable configurations. Thus the process
of turbulent magnetic field removal should be applicable both to quasi-static
subcritical molecular clouds and cores and violently collapsing supercritical
entities. The increase of the gravitational potential as well as the
magnetization of the gas increases the segregation of the mass and magnetic
flux in the saturated final state of the simulations, supporting the notion
that the reconnection-enabled diffusivity relaxes the magnetic field + gas
system in the gravitational field to its minimal energy state. This effect is
expected to play an important role in star formation, from its initial stages
of concentrating interstellar gas to the final stages of the accretion to the
forming protostar.",0910.1117v3
2016-11-29,Magnetic field evolution in tidal disruption events,"When a star gets tidally disrupted by a supermassive black hole, its magnetic
field is expected to pervade its debris. In this paper, we study this process
via smoothed particle magnetohydrodynamical simulations of the disruption and
early debris evolution including the stellar magnetic field. As the gas
stretches into a stream, we show that the magnetic field evolution is strongly
dependent on its orientation with respect to the stretching direction. In
particular, an alignment of the field lines with the direction of stretching
induces an increase of the magnetic energy. For disruptions happening well
within the tidal radius, the star compression causes the magnetic field
strength to sharply increase by an order of magnitude at the time of pericentre
passage. If the disruption is partial, we find evidence for a dynamo process
occurring inside the surviving core due to the formation of vortices. This
causes an amplification of the magnetic field strength by a factor of $\sim
10$. However, this value represents a lower limit since it increases with
numerical resolution. For an initial field strength of 1 G, the magnetic field
never becomes dynamically important. Instead, the disruption of a star with a
strong 1 MG magnetic field produces a debris stream within which magnetic
pressure becomes similar to gas pressure a few tens of hours after disruption.
If the remnant of one or multiple partial disruptions is eventually fully
disrupted, its magnetic field could be large enough to magnetically power the
relativistic jet detected from Swift J1644+57. Magnetized streams could also be
significantly thickened by magnetic pressure when it overcomes the confining
effect of self-gravity.",1611.09853v2
2020-01-19,Disentangling magnetic and grain contrast in polycrystalline FeGe thin films using four-dimensional Lorentz scanning transmission electron microscopy,"The study of nanoscale chiral magnetic order in polycrystalline materials
with a strong Dzyaloshinkii-Moriya interaction (DMI) is interesting for the
observation of magnetic phenomena at grain boundaries and interfaces. One such
material is sputter-deposited B20 FeGe on Si, which has been actively
investigated as the basis for low-power, high-density magnetic memory
technology in a scalable material platform. Although conventional Lorentz
electron microscopy provides the requisite spatial resolution to probe chiral
magnetic textures in single-crystal FeGe, probing the magnetism of sputtered
B20 FeGe is more challenging because the sub-micron crystal grains add
confounding contrast. We address the challenge of disentangling magnetic and
grain contrast by applying 4-dimensional Lorentz scanning transmission electron
microscopy using an electron microscope pixel array detector. Supported by
analytical and numerical models, we find that the most important parameter for
imaging magnetic materials with polycrystalline grains is the ability for the
detector to sustain large electron doses, where having a high-dynamic range
detector becomes extremely important. Despite the small grain size in sputtered
B20 FeGe on Si, using this approach we are still able to observe helicity
switching of skyrmions and magnetic helices across two adjacent grains as they
thread through neighboring grains. We reproduce this effect using micromagnetic
simulations by assuming that the grains have distinct orientation and magnetic
chirality and find that magnetic helicity couples to crystal chirality. Our
methodology for imaging magnetic textures is applicable to other thin-film
magnets used for spintronics and memory applications, where an understanding of
how magnetic order is accommodated in polycrystalline materials is important.",2001.06900v3
2020-05-07,A global model of the magnetorotational instability in protoneutron stars,"Magnetars are highly magnetized neutron stars whose magnetic dipole ranges
from $10^{14}$ to $10^{15}$ G. The MRI is considered to be a promising
mechanism to amplify the magnetic field in fast-rotating protoneutron stars and
form magnetars. This scenario is supported by many local studies showing that
magnetic fields could be amplified by the MRI on small scales. However, the
efficiency of the MRI at generating a dipole field is still unknown. To answer
this question, we study the MRI dynamo in an idealized global model of a fast
rotating protoneutron star with differential rotation. We perform 3D
incompressible MHD simulations in spherical geometry with explicit
diffusivities where the differential rotation is forced at the outer boundary.
We vary the initial magnetic field and investigated different magnetic boundary
conditions. These simulations were compared to local shearing box simulations.
We obtain a self-sustained turbulent MRI-driven dynamo, whose saturated state
is independent of the initial magnetic field. The MRI generates a strong
turbulent magnetic field of $B \geq 2\times 10^{15}$ G and a non-dominant
magnetic dipole, which represents systematically about $5\%$ of the averaged
magnetic field strength. Interestingly, this dipole is tilted towards the
equatorial plane. We find that local shearing box models can reproduce fairly
well several characteristics of global MRI turbulence such as the kinetic and
magnetic spectra. The turbulence is nonetheless more vigorous in the local
models than in the global ones. Overall, our results support the ability of the
MRI to form magnetar-like large-scale magnetic fields. They furthermore predict
the presence of a stronger small-scale magnetic field. The resulting magnetic
field could be important to power outstanding stellar explosions, such as
superluminous supernovae and GRBs.",2005.03567v2
2020-06-08,Field-induced oscillation of magnetization blocking in holmium metallacrown magnet,"Single-molecule magnets (SMMs) are promising elements for quantum
informatics. In the presence of strong magnetic anisotropy, they exhibit
magnetization blocking - a magnetic memory effect at the level of a single
molecule. Recent studies have shown that the SMM performance scales with the
height of magnetization blocking barrier. By employing molecular engineering
this can be significantly modified, remaining independent from other external
factors such as magnetic field. Taking advantage of hyperfine coupling of
electronic and nuclear spins further enhances their functionality, however, a
poor understanding of relaxation mechanisms in such SMMs limits the
exploitation of nuclear-spin molecular qubits. Here we report the opening
discovery of field-dependent oscillation of the magnetization blocking barrier
in a new holmium metallacrown magnet driven by the switch of relaxation
mechanisms involving hyperfine interaction. Single-crystal magnetic hysteresis
measurements combined with first-principles calculations reveal an activated
temperature dependence of magnetic relaxation dominated either by incoherent
quantum tunneling of magnetization at anti-crossing points of
exchange-hyperfine states or by Orbach-like processes at crossing points. We
demonstrate that these relaxation mechanisms can be consecutively switched on
and off by increasing the external field, which paves a way for manipulating
the magnetization dynamics of SMMs using hyperfine interaction.",2006.04401v1
2021-09-19,Improvement of the Helioseismic and Magnetic Imager (HMI) Vector Magnetic Field Inversion Code,"A spectral line inversion code, Very Fast Inversion of the Stokes Vector
(VFISV), has been used since May 2010 to infer the solar atmospheric parameters
from the spectropolarimetric observations taken by the Helioseismic and
Magnetic Imager (HMI) aboard the Solar Dynamics Observatory (SDO). The magnetic
filling factor, the fraction of the surface with a resolution element occupied
by magnetic field, is set to have a constant value of one in the current
version of VFISV. This report describes an improved inversion strategy for the
spectropolarimetric data observed with HMI for magnetic field strengths of
intermediate values in areas spatially not fully resolved. The VFISV inversion
code has been modified to enable inversion of the Stokes profiles with two
different components: one magnetic and one non-magnetic. In this scheme, both
components share the atmospheric components except for the magnetic field
strength, inclination, and azimuth. In order to determine whether the new
strategy is useful, we evaluate the inferred parameters inverted with one
magnetic component (the original version of the HMI inversion) and with two
components (the improved version) using a Bayesian analysis. In pixels with
intermediate magnetic field strengths (e.g. plages), the new version provides
statistically significant values of filling fraction and magnetic field vector.
Not only does the fitting of the Stokes profile improve, but the inference of
the magnetic parameters and line-of-sight velocity are obtained uniquely. The
new strategy is also proven to be effective for mitigating the anomalous
hemispheric bias in the east-west magnetic field component in moderate field
regions.",2109.09131v2
2023-09-23,Origin of spin-driven ferroelectricity and effect of external pressure on the complex magnetism of 6H-perovskite Ba3HoRu2O9,"The compound Ba3HoRu2O9 magnetically orders at 50 K (TN1) followed by another
complex magnetic ordering at 10 K (TN2). The 2nd magnetic phase transition was
characterized by the co-existence of two competing magnetic ground states
associated with two different magnetic wave vectors (K1=1/2 0 0 and K2=1/4 1/4
0). We have investigated the compound through time-of-flight neutron
diffraction, ac susceptibility, complex dielectric spectroscopy, and
magnetization under external pressure. We demonstrate that the non-colinear
structure involving two different magnetic ions, Ru(4d) and Ho(4f), breaks the
spatial inversion symmetry via inverse Dzyaloshinskii-Moriya (D-M) interaction
through strong4d-4f magnetic correlation, which shifts the oxygen atoms and
results in non-zero polarization. Such an observation of inverse D-M
interaction from two different magnetic spins ions which caused
ferroelectricity is rarely observed. The stronger spin-orbit coupling of
4d-orbital might play a major role in creating D-M interaction of non-collinear
spins. We have systematically studied the spin and dipolar dynamics, which
exhibit intriguing behavior with shorter coherence lengths of 2nd magnetic
phase associated with the K2-wave vector. The results manifest the development
of magnetoelectric domains instead of true long-range ordering which justifies
the experimentally obtained low value of ferroelectric polarization. Further,
we have investigated the effect of external pressure on this complex magnetism.
The result reveals an enhancement of ordering temperature by the application of
external pressure (1.6K/GPa). The external pressure might favor stabilizing the
ground state associated with 2nd magnetic phase. Our study shows an
unconventional mechanism of spin-driven ferroelectricity involving inverse D-M
interaction between Ru(4d) and Ho(4f) magnetic ions due to strong 4d-4f
cross-coupling.",2309.13465v3
1998-06-05,Collapse-Driven Outflow in Star-Forming Molecular Cores,"Dynamical collapses of magnetized molecular cloud cores are studied with
magnetohydrodynamical simulations from the run-away collapse phase to the
accretion phase. In the run-away collapse phase, a disk threaded by magnetic
field lines is contracting due to its self-gravity and its evolution is well
expressed by a self-similar solution. The central density increases greatly in
a finite time scale and reaches a density at which an opaque core is formed at
the center. After that, matter accretes to the newly formed core (accretion
phase). In this stage, a rotationally supported disk is formed in a cloud core
without magnetic fields. In contrast, the disk continues to contract in the
magnetized cloud core, since the magnetic fields transfer angular momentum from
the disk. Its rotation motion winds up the threading magnetic field lines.
Eventually, strong toroidal magnetic fields are formed and begin to drive the
outflow, even if there is no toroidal field component initially. Bipolar
molecular outflows observed in protostar candidates are naturally explained by
this model.",9806085v1
1998-09-16,A magnetic model for acoustic modes in roAp stars,"The mechanism for excitation of p-modes in rapidly oscillating, peculiar A
(roAp, or cool chemically peculiar, CP) stars is unknown. Observations strongly
suggest that acoustic modes in roAp stars are causally linked to the stars'
magnetic field. We propose that small fluctuations in the shape of the mean
magnetic field drive magnetosonic waves, which are observed as p-modes in these
stars.
  The dynamic edge region of roAp stars is a force-free spherical shell. When
strongly coupled to the magnetic field, a force-free plasma can oscillate about
a minimum in its mean magnetic energy. We describe the stable eigenmodes for
this energy minimum in a spherical shell with an open boundary. The
wavenumbers, frequencies, and energies of resulting oscillations are consistent
with observations of p-modes in roAp stars. Our magnetic model for p-mode
oscillations in stars does not invoke convection or opacity mechanisms. We also
suggest the possibility of a nonlinear dynamo for such magnetic stars, which
lack the convection needed for the usual (-( dynamo.",9809215v1
2000-01-16,Solar-Like Cycle in Asymptotic Giant Branch Stars,"I propose that the mechanism behind the formation of concentric semi-periodic
shells found in several planetary nebulae (PNs) and proto-PNs, and around one
asymptotic giant branch (AGB) star, is a solar-like magnetic activity cycle in
the progenitor AGB stars. The time intervals between consecutive ejection
events is about 200-1,000 years, which is assumed to be the cycle period (the
full magnetic cycle can be twice as long, as is the 22-year period in the sun).
The magnetic field has no dynamical effects; it regulates the mass loss rate by
the formation of magnetic cool spots. The enhanced magnetic activity at the
cycle maximum results in more magnetic cool spots, which facilitate the
formation of dust, hence increasing the mass loss rate. The strong magnetic
activity implies that the AGB star is spun up by a companion, via a tidal or
common envelope interaction. The strong interaction with a stellar companion
explains the observations that the concentric semi-periodic shells are found
mainly in bipolar PNs.",0001281v1
2002-06-22,Stability and heating of magnetically driven jets from Keplerian accretion discs,"We have performed 3-D numerical magnetohydrodynamic (MHD) jet experiments to
study the instabilities associated with strongly toroidal magnetic fields and
determine if such magnetic configurations in jets are as unstable as similar
situations are found to be in the laboratory and in analytical estimates. A
perfectly conducting Keplerian disc with fixed density, rotational velocity and
pressure is used as a lower boundary for the jet. The energy equation is
solved, with the inclusion of self-consistently computed heating by viscous and
magnetic dissipation. The resulting jets evolve into time-dependent,
non-axisymmetric configurations, but we find only minor disruption of the jets
by for example the kink instability. We find that magnetic dissipation may have
profound effects on the jet flow as: 1) it turns on in highly wound up magnetic
field regions and helps to prevent critical kink situations; 2) it influences
jet dynamics by re-organizing the magnetic field structure and increasing
thermal pressure in the jet; and 3) it influences mass loading by increasing
temperature and pressure at the base of the jet.",0206394v1
2002-07-10,Magnetic dissipation: Spatial and temporal structure,"A magnetically dominated plasma driven by motions on boundaries at which
magnetic field lines are anchored is forced to dissipate the work being done
upon it, no matter how small the electrical resistivity. Numerical experiments
have clarified that dissipation is achieved through the formation of a
hierarchy of electrical current sheets. The probability distribution function
of the local winding of magnetic field lines is nearly Gaussian, with a width
of the order unity. The dissipation is highly irregular in space and time, but
the average level of dissipation is well described by a scaling law that is
independent of the electrical resistivity. If the boundary driving is suspended
for a period of time the magnetic dissipation rapidly drops to insignificant
levels, leaving the magnetic field in a nearly force-free state. Renewed
boundary driving leads to a quick return to dissipation levels compatible with
the rate of boundary work, with dissipation starting much more rapidly than
when starting from idealized initial conditions with a uniform magnetic field.
Application of these concepts to the solar corona lends credibility to
realistic, three-dimensional numerical models that predict emission measures,
coronal structures, and heating rates compatible with observations.",0207205v1
2002-09-06,Scaling laws in decaying helical hydromagnetic turbulence,"We study the evolution of growth and decay laws for the magnetic field
coherence length xi, energy E_M and magnetic helicity H in freely decaying 3D
MHD turbulence. We show that with certain assumptions, self-similarity of the
magnetic power spectrum alone implies that xi~t^{1/2}. This in turn implies
that magnetic helicity decays as H~t^{-2s}, where s=(xi_{diff}/xi_H)^2, in
terms of xi_{diff}, the diffusion length scale, and xi_H, a length scale
defined from the helicity power spectrum. The relative magnetic helicity
remains constant, implying that the magnetic energy decays as E_M~t^{-1/2-2s}.
The parameter s is inversely proportional to the magnetic Reynolds number Re_M,
which is constant in the self-similar regime.",0209119v2
2004-07-04,Bihelical Magnetic Relaxation and Large Scale Magnetic Field Growth,"A unified, three-scale system of equations accommodating nonlinear velocity
driven helical dynamos, as well as time-dependent relaxation of magnetically
dominated unihelical or bihelical systems is derived and solved herein. When
opposite magnetic helicities of equal magnitude are injected on the
intermediate and small scales, the large scale magnetic helicity grows
kinematically (independent of the magnetic Reynolds number) to equal that on
the intermediate scale. For both free and driven relaxation large scale fields
are rapidly produced. Subsequently, a dissipation-limited dynamo, driven by
growth of small scale kinetic helicity, further amplifies the large scale
field. The results are important for astrophysical coronae fed with bihelical
structures by dynamos in their host rotators. The large scale for the rotator
corresponds to the intermediate scale for the corona. That bihelical magnetic
relaxation can produce global scale fields may help to explain the formation of
astrophysical coronal holes and magnetohydrodynamic outflows.",0407073v2
2006-04-27,Gravitational collapse of magnetized clouds. I. Ideal MHD accretion flow,"We study the self-similar collapse of an isothermal magnetized rotating cloud
in the ideal magnetohydrodynamic (MHD) regime. In the limit of small distance
from the accreting protostar we find an analytic solution that corresponds to
free-fall onto a central mass point. The density distribution is not
spherically symmetric but depends on the mass loading of magnetic field lines,
which can be obtained by matching our inner solution to an outer collapse
solution previously computed by Allen, Shu & Li. The concentration of magnetic
field trapped by the central mass point under field-freezing, independent on
the details of the starting state, creates a split monopole configuration where
the magnetic field strength increases as the inverse square of the distance
from the center. Under such conditions, the inflow eventually becomes
subalfvenic and the outward transfer of angular momentum by magnetic braking
very efficient, thus preventing the formation of a centrifugally supported
disk. Instead, the azimuthal velocity of the infalling gas decreases to zero at
the center, and the gas spirals into the star. Therefore, the dissipation of
dynamically important levels of magnetic field is a fundamental requisite for
the formation of protoplanetary disks around young stars.",0604573v1
2006-12-29,The Role of Ambipolar Diffusion in the Formation Process of Moderately Magnetized Diffuse Clouds,"We investigate the dynamical condensation process in a magnetized thermally
bistable medium. We perform one-dimensional two fluid numerical simulations
that describe the neutral and ionized components in the interstellar medium
with purely transverse magnetic fields. We find that the clouds that are formed
as a consequence of the thermal instability always have a magnetic field
strength on the order of a few microgauss irrespective of the initial strength.
This shows good agreement with the measurements of the magnetic field strength
in diffuse clouds. We show analytically that the final magnetic field strength
in the clouds is determined by the balance between ambipolar diffusion and the
accumulation of the magnetic field due to condensation.",0612770v2
2007-03-06,"Rossby waves in ""shallow water"" magnetohydrodynamics","The influence of a toroidal magnetic field on the dynamics of Rossby waves in
a thin layer of ideal conductive fluid on a rotating sphere is studied in the
""shallow water"" magnetohydrodynamic approximation for the first time.
Dispersion relations for magnetic Rossby waves are derived analytically in
Cartesian and spherical coordinates. It is shown that the magnetic field causes
the splitting of low order (long wavelength) Rossby waves into two different
modes, here denoted fast and slow {\em magnetic Rossby waves}. The high
frequency mode (the fast magnetic Rossby mode) corresponds to an ordinary
hydrodynamic Rossby wave slightly modified by the magnetic field, while the low
frequency mode (the slow magnetic Rossby mode) has new and interesting
properties since its frequency is significantly smaller than that of the same
harmonics of pure Rossby and Alfv{\'e}n waves.",0703105v2
2000-03-21,Ground-State Magnetization in Disordered Systems : Exchange vs. Off-Diagonal Interaction Fluctuations,"In a Hartree-Fock picture, itinerant ferromagnetism results from a
competition between kinetic and exchange energy, with the magnetized state
being favored at large interaction strength. In a recent paper \cite{spings} we
showed that in contrast to this average effect, fluctuations of off-diagonal
interaction matrix elements systematically reduce the ground-state
magnetization. When the interaction dominates, the occurence of a non-zero
ground-state magnetization depends on the ratio $\lambda$ between average
exchange energy and the fluctuation amplitude of the off-diagonal matrix
elements, and a nonzero critical value $\lambda_c$ is necessary to magnetize
the ground-state. We extend these results by numerical studies of $\lambda$ for
standard tight-binding models which indicate a regime of intermediate disorder
where off-diagonal fluctuations should play an important role for ground-state
magnetization. We also emphasize the presence of strong correlations between
minimal eigenvalues of different magnetization blocks which further reduce the
probability of a nonzero ground-state spin.",0003352v1
2000-11-03,Spin-Glass State in $\rm CuGa_2O_4$,"Magnetic susceptibility, magnetization, specific heat and positive muon spin
relaxation (\musr) measurements have been used to characterize the magnetic
ground-state of the spinel compound $\rm CuGa_2O_4$. We observe a spin-glass
transition of the S=1/2 $\rm Cu^{2+}$ spins below $\rm T_f=2.5K$ characterized
by a cusp in the susceptibility curve which suppressed when a magnetic field is
applied. We show that the magnetization of $\rm CuGa_2O_4$ depends on the
magnetic histo Well below $\rm T_f$, the muon signal resembles the dynamical
Kubo-Toyabe expression reflecting that the spin freezing process in $\rm
CuGa_2O_4$ results Gaussian distribution of the magnetic moments. By means of
Monte-Carlo simulati we obtain the relevant exchange integrals between the $\rm
Cu^{2+}$ spins in this compound.",0011055v1
2001-01-09,Classical and quantum magnetization reversal studied in nanometer-sized particles and clusters,"Nanometer-sized magnetic particles have generated continuous interest as the
study of their properties has proved to be scientifically and technologically
very challenging. In this article we reviewed the most important theories and
experimental results concerning the magnetization reversal of single-domain
particles,clusters and molecular clusters. Sect.1 reviews briefly the commonly
used measuring techniques. Among them, electrical transport measurements, Hall
probes and micro-SQUID techniques seem to be the most convenient techniques for
low temperature measurements. Sect.2 discusses the mechanisms of magnetization
reversal in single domain particles at zero Kelvin. The influence of
temperature on the magnetization reversal is reported in Sect.3. Finally,
Sect.4 shows that for very small systems or very low temperature, magnetization
can reverse via quantum tunneling. The boundary between classical and quantum
physics has become a very attractive field of research. This section discusses
detailed measurements which demonstrated that molecular magnets offer an unique
opportunity to explore the quantum dynamics of a large but finite spin. We then
discussed tunneling in nanoparticles and showed how one might give a definite
proof of their quantum character at low temperature.",0101104v1
2001-07-12,Neutron scattering study of dipolar spin ice Ho_2Sn_2O_7: Frustrated pyrochlore magnet,"By means of neutron scattering techniques we have investigated the frustrated
pyrochlore magnet Ho_2Sn_2O_7, which was found to show a ferromagnetic spin-ice
behavior below T \simeq 1.4 K by susceptibility measurements. High-resolution
powder-neutron-diffraction shows no detectable disorder of the lattice, which
implies appearance of a random magnetic state solely by frustrated geometry,
i.e., the corner sharing tetrahedra. Magnetic inelastic-scattering spectra show
that Ho magnetic moments behave as an Ising spin system at low temperatures,
and that the spin fluctuation has static character. The system remains in a
short-range ordered state down to T = 0.4 K. By analyzing the wave-number
dependence of the magnetic scattering using a mean field theory, it is
clarified that the Ising spins interact via the dipolar interaction. Therefore
we conclude that Ho_2Sn_2O_7 belongs to the dipolar-spin-ice family. Slow spin
dynamics is exhibited as thermal hysteresis and time dependence of the magnetic
scattering.",0107278v2
2004-03-15,Superparamagnetic relaxation in Cu_{x}Fe_{3-x}O_{4} (x=0.5 and x=1) nanoparticles,"The scope of this article is to report very detailed results of the
measurements of magnetic relaxation phenomena in the new
Cu$_{0.5}$Fe$_{2.5}$O$_{4}$ nanoparticles and known CuFe$_{2}$O$_{4}$
nanoparticles. The size of synthesized particles is (6.5$\pm $1.5)nm. Both
samples show the superparamagnetic behaviour, with the well-defined phenomena
of blocking of magnetic moment. This includes the splitting of
zero-field-cooled and field-cooled magnetic moment curves, dynamical
hysteresis, slow quasi-logarithmic relaxation of magnetic moment below blocking
temperature. The scaling of the magnetic moment relaxation data at different
temperatures confirms the applicability of the simple thermal relaxation model.
The two copper-ferrites with similar structures show significantly different
magnetic anisotropy density and other magnetic properties. Investigated systems
exhibit the consistency of all obtained results.",0403358v1
2004-04-16,Symmetry of Magnetic Quantum Tunneling in the Single-Molecule Magnet Mn12-Acetate,"The symmetry of magnetic quantum tunneling (MQT) in the single molecule
magnet Mn12-acetate has been determined by sensitive low-temperature magnetic
measurements in the pure quantum tunneling regime and high frequency EPR
spectroscopy in the presence of large transverse magnetic fields. The combined
data set definitely establishes the transverse anisotropy terms responsible for
the low temperature quantum dynamics. MQT is due to a disorder induced locally
varying quadratic transverse anisotropy associated with rhombic distortions in
the molecular environment (2nd order in the spin-operators). This is
superimposed on a 4th order transverse magnetic anisotropy consistent with the
global (average) S4 molecule site symmetry. The hard axes associated with these
forms of the transverse anisotropy are not collinear, leading to a complex
interplay between local and global symmetries, the consequences of which are
analyzed in detail.",0404390v2
2006-11-23,Quantum dissipation theory of slow magnetic relaxation mediated by domain-wall motion in one-dimensional chain compound [Mn(hfac)_{2}BNO_{H}}],"Based on a quantum dissipation theory of open systems, we present a
theoretical study of slow dynamics of magnetization for the ordered state of
the new molecule-based magnetic complex [Mn(hfac)_{2}BNO_{H}] composed from
antiferromagnetically coupled ferrimagnetic (5/2,1) spin chains. Experimental
investigations of the magnetization process in pulsed fields have shown that
this compound exhibits a metamagnetic AF-FI transition at a critical field in
the order of the interchain coupling. A strong frequency dependence for the
ac-susceptibility has been revealed in the vicinity of the AF-FI transition and
was associated with an AF-FI interface kink motion. We model these processes by
a field-driven domain-wall motion along the field-unfavorable chains correlated
with a dissipation effect due to a magnetic system-bath coupling. The
calculated longitudinal magnetization has a two-step relaxation after the field
is switched off and are found in good agreement with the experiment. The
relaxation time determined from the imaginary part of the model
ac-susceptibility agrees qualitatively with that found from the remanent
magnetization data.",0611610v1
2006-12-12,Magnetic and Resonance Properties of the Compound (NH3)2(CH2)3CoCl4 - an Antiferromagnet with the Dzyaloshinskii Interaction,"The static magnetic and dynamic properties of the compound (NH3)2(CH2)3CoCl4
are investigated in the temperature interval 0.5-50 K. It is shown that at TN
=2.05 K this compound undergoes a transition to a magnetically ordered
antiferromagnet state. A distinctive feature of this compound is the presence
of temperature hysteresis at the transition to the ordered state (corresponding
to a first order magnetic phase transition) and of a spontaneous magnetic
moment along the x axis. The paramagnetic Curie temperatures are determined.
The frequency-field curves of the AFMR spectrum in the xy plane are
investigated at a temperature below TN. The main magnetic parameters of the
biaxial AFM - the value of the low-frequency gap and the effective magnetic
anisotropy field - are determined.",0612285v1
2007-01-31,Rapidly driven nanoparticles: Mean first-passage times and relaxation of the magnetic moment,"We present an analytical method of calculating the mean first-passage times
(MFPTs) for the magnetic moment of a uniaxial nanoparticle which is driven by a
rapidly rotating, circularly polarized magnetic field and interacts with a heat
bath. The method is based on the solution of the equation for the MFPT derived
from the two-dimensional backward Fokker-Planck equation in the rotating frame.
We solve these equations in the high-frequency limit and perform precise,
numerical simulations which verify the analytical findings. The results are
used for the description of the rates of escape from the metastable domains
which in turn determine the magnetic relaxation dynamics. A main finding is
that the presence of a rotating field can cause a drastic decrease of the
relaxation time and a strong magnetization of the nanoparticle system. The
resulting stationary magnetization along the direction of the easy axis is
compared with the mean magnetization following from the stationary solution of
the Fokker-Planck equation.",0702001v2
1997-07-30,Electroweak Origin of Cosmological Magnetic Fields,"Magnetic fields may have been generated in the electroweak phase transition
through spontaneous symmetry breaking or through the subsequent dynamical
evolution of semiclassical field configurations. Here I demonstrate explicitly
how magnetic fields emerge spontaneously in the phase transition also when no
gradients of the Higgs field are present. Using a simple model, I show that no
magnetic fields are generated, at least initially, from classical two-bubble
collisions in a first-order phase transition. An improved gauge-invariant
definition of the electromagnetic field is advocated which is more appropriate
in the sense that it never allows electrically neutral fields to serve as
sources for the electromagnetic field. In particular, semiclassical
configurations of the Z field alone do not generate magnetic fields. The
possible generation of magnetic fields in the decay of unstable Z-strings is
discussed.",9707513v2
1998-08-12,Anomalous Global Strings and Primordial Magnetic Fields,"We propose a new mechanism for the generation of primordial magnetic fields,
making use of the magnetic fields which are induced by anomalous global strings
which couple to electromagnetism via Wess-Zumino type interactions. This
mechanism can be realized in QCD by utilizing pion strings, global vortices
which appear in the linear sigma model which describes physics below the QCD
confinement scale. During the chiral symmetry breaking phase transition, pion
strings can be produced, thus leading to primordial magnetic fields. We
calculate the magnitude and coherence length of these fields. They are seen to
depend on the string dynamics. With optimistic assumptions, both the magnitude
and coherence scale of the induced magnetic fields can be large enough to
explain the seed magnetic fields of greater than $10^{-23}$ Gauss necessary to
produce the observed galactic magnetic fields via the galactic dynamo
mechanism.",9808306v2
2006-04-15,Magnetic Fields Boosted by Gluon Vortices in Color Superconductivity,"We investigate the effects of an external magnetic field in the gluon
dynamics of a color superconductor with three massless quark flavors. In the
framework of gluon mean-field theory at asymptotic densities, we show that the
long-range component $\widetilde{H}$ of the external magnetic field that
penetrates the CFL phase produces an instability when its strength becomes
larger than the Meissner mass of the charged gluons. As a consequence, the
magnetic field causes the formation of a vortex state characterized by the
condensation of charged gluons and the creation of magnetic flux tubes. Inside
the flux tubes the magnetic field is stronger than the applied one. This
antiscreening effect is connected to the anomalous magnetic moment of the gluon
field. We suggest how this same mechanism could serve to remove the
chromomagnetic instabilities existing in gapless color superconductivity.",0604136v2
2007-01-24,Evolution of Mixed Dirac Particles Interacting with an External Magnetic Field,"We study in the framework of relativistic quantum mechanics the evolution of
a system of two Dirac neutrinos that mix with each other and have non-vanishing
magnetic moments. The dynamics of this system in an external magnetic field is
determined by solving the Pauli-Dirac equation with a given initial condition.
We consider first neutrino spin-flavor oscillations in a constant magnetic
field and derive an analytical expression for the transition probability of
spin-flavor conversion in the limit of small magnetic interactions. We then
investigate ultrarelativistic neutrinos in an transversal magnetic field and
derive their wave functions and transition probabilities with no limitation for
the size of transition magnetic moments. Although we consider neutrinos, our
formalism is straightforwardly applicable to any spin-1/2 particles.",0701209v4
2004-09-27,Magnetic Rigidity of Horocycle flows,"Let $M$ be a closed oriented surface endowed with a Riemannian metric $g$ and
let $\Omega$ be a 2-form. We show that the magnetic flow of the pair
$(g,\Omega)$ has zero asymptotic Maslov index and zero Liouville action if and
only $g$ has constant Gaussian curvature, $\Omega$ is a constant multiple of
the area form of $g$ and the magnetic flow is a horocycle flow.
  This characterization of horocycle flows implies that if the magnetic flow of
a pair $(g,\Omega)$ is $C^1$-conjugate to the horocycle flow of a hyperbolic
metric $\bar{g}$ then there exists a constant $a>0$, such that $ag$ and
$\bar{g}$ are isometric and $a^{-1}\Omega$ is, up to a sign, the area form of
$g$. The characterization also implies that if a magnetic flow is Ma\~n\'e
critical and uniquely ergodic it must be the horocycle flow.
  As a by-product we also obtain results on existence of closed magnetic
geodesics for almost all energy levels in the case weakly exact magnetic fields
on arbitrary manifolds.",0409528v1
2003-08-03,Energy transfer and locality in magnetohydrodynamic turbulence,"The shell-to-shell energy transfer rates for magnetohydrodynamic (MHD)
turbulence are computed analytically, which shows local energy transfer rates
from velocity to velocity, velocity to magnetic, magnetic to velocity, and
magnetic to magnetic fields for nonhelical MHD in the inertial range. It is
also found that for kinetic-energy dominated MHD fluid, there is a preferential
shell-to-shell energy transfer from kinetic to magnetic energy; the transfer is
reversed for magnetic-energy dominated MHD fluid. This property is the reason
for the asymptotic value of Alfven ratio to be close to 0.5. The analytical
results are in close agreement with recent numerical results. When magnetic and
kinetic helicities are turned on, the helical contributions are opposite to the
corresponding nonhelical contributions. The helical energy transfers have
significant nonlocal components.",0308005v3
1999-11-23,Magnetic Oscillations in the Nambu - Jona-Lasinio model,"The phase structure of a simple Nambu-Jona-Lasinio model has been
investigated at non-zero values of $\mu$ and H, where H is an external magnetic
field and $\mu$ is the chemical potential. On this basis magnetic oscillations
effects were considered. It was shown that there are standard (periodic) van
Alphen-de Haas magnetic oscillations of some thermodynamical quantities,
including magnetization, pressure and particle density in the NJL system.
Besides, we have found non-standard, i.e. non-periodic, magnetic oscillations,
since the frequency of oscillations is a H-dependent quantity. Finally, there
arises an oscillating behaviour not only for thermodynamical quantities, but
also for a dynamical quantity like the quark mass. The possibility for magnetic
oscillations of some physical quantities inside neutron stars is discussed.",9911073v1
2001-07-14,A route to magnetic field reversals: an example of an ABC-forced non-linear dynamo,"We are investigating numerically the non-linear behavior of a space-periodic
MHD system with ABC forcing. Most computations are performed for magnetic
Reynolds numbers increasing from 0 to 60 and a fixed kinematic Reynolds number,
small enough for the trivial solution with a zero magnetic field to be stable
to velocity perturbations. At the critical magnetic Reynolds number for the
onset of instability of the trivial solution the dominant eigenvalue of the
kinematic dynamo problem is real. In agreement with the bifurcation theory new
steady states with non-vanishing magnetic field appear in this bifurcation.
Subsequent bifurcations are investigated. A regime is detected, where chaotic
variations of the magnetic field orientation (analogous to magnetic field
reversals) are observed in the temporal evolution of the system.",0107029v3
2006-07-12,The Stoner-Wohlfarth model of Ferromagnetism: Dynamic and Statistical properties,"The physics of magnetic state change or reversal in single domain magnetic
grains (called Stoner particles) is interesting from the fundamental as well as
the applied points of view. A change in magnetization can be finely tuned with
a specific time variation o f an externally applied magnetic field. It may also
occur naturally (without application of a field) at very low tempe rature with
quantum tunneling and at higher temperature with thermal excitation. The
optimal (usually shortest) time altering the magn etisation along with the
smallest applied magnetic field are sought in technological applications s uch
as high-density reading or writing of information, spintronics, quantum inform
ation and quantum communication systems. This work reviews the magnetization
change with a time dependent field and tem perature and discusses the time it
takes to alter the magnetization as a function of the control parameter chosen,
temperature and material parameters.",0607118v4
2007-07-09,Self-Similar Solutions of Viscous-Resistive ADAFs With Poloidal Magnetic Fields,"We carry out the self-similar solutions of viscous-resistive accretion flows
around a magnetized compact object. We consider an axisymmetric, rotating,
isotheral steady accretion flow which contains a poloidal magnetic field of the
central star. The dominant mechanism of energy dissipation is assumed to be the
turbulence viscosity and magnetic diffusivity due to magnetic field of the
central star. We explore the effect of viscosity on a rotating disk in the
presence of constant magnetic diffusivity. We show that the dynamical
quantities of ADAFs are sensitive to the advection and viscosity parameters.
Increase of the $\alpha$ coefficient in the $\alpha$-prescription model
decreases the radial velocity and increases the density of the flow. It also
affects the poloidal magnetic field considerably.",0707.1253v1
2007-07-19,Map of metastable states for thin circular magnetic nano-cylinders,"Nano-magnetic systems of artificially shaped ferromagnetic islands, recently
became a popular subject due to their current and potential applications in
spintronics, magneto-photonics and superconductivity. When the island size is
close to the exchange length of magnetic material (around 15 nm), its magnetic
structure becomes markedly different. It determines both static and dynamic
magnetic properties of elements, but strongly depends on their shape and size.
Here we map this dependence for circular cylindrical islands of a few exchange
lengths in size. We outline the region of metastability of ""C""-type magnetic
states, proving that they are indeed genuine and not a result of pinning on
particle imperfections. A way to create the smallest particles with guaranteed
magnetic vortex state at zero field becomes evident. It is expected that the
map will help focus the efforts in planning of experiments and devices.",0707.2938v2
2007-11-13,Evidence for weak magnetic fields in early-type emission stars,"We report the results of our study of magnetic fields in a sample of 15 Be
stars using spectropolarimetric data obtained at the European Southern
Observatory with the multi-mode instrument FORS1 installed at the 8m Kueyen
telescope. We detect weak photospheric magnetic fields in four stars, HD56014,
HD148184, HD155806, and HD181615. We note that for HD181615 the evolutionary
status is not obvious due to the fact that it is a binary system currently
observed in the initial rapid phase of mass exchange between the two
components. Further, we notify the possible presence of distinct circular
polarisation features in the circumstellar components of Ca II H&K in three
stars, HD58011, HD117357, and HD181615, hinting at a probable presence of
magnetic fields in the circumstellar mass loss disks of these stars. We
emphasize the need for future spectropolarimetric observations of Be stars with
detected magnetic fields to study the temporal evolution of their magnetic
fields and the correlation of magnetic field properties with dynamical
phenomena taking place in the gaseous circumstellar disks of these stars.",0711.2085v1
2007-11-30,"Magnetic fields, spots and weather in chemically peculiar stars","New observational techniques and sophisticated modelling methods has led to
dramatic breakthroughs in our understanding of the interplay between the
surface magnetism, atomic diffusion and atmospheric dynamics in chemically
peculiar stars. Magnetic Doppler images, constructed using spectropolarimetric
observations of Ap stars in all four Stokes parameters, reveal the presence of
small-scale field topologies. Abundance Doppler mapping has been perfected to
the level where distributions of many different chemical elements can be
deduced self-consistently for one star. The inferred chemical spot structures
are diverse and do not always trace underlying magnetic field geometry.
Moreover, horizontal chemical inhomogeneities are discovered in non-magnetic CP
stars and evolving chemical spots are observed for the first time in the bright
mercury-manganese star alpha And. These results show that in addition to
magnetic fields, another important non-magnetic structure formation mechanism
acts in CP stars.",0711.4905v1
2007-12-12,Reentrant spin glass transition in LuFe2O4,"We have carried out a comprehensive investigation of magnetic properties of
LuFe$_2$O$_4$, using AC susceptibility, DC magnetization and specific heat. A
magnetic phase transition around $\sim$236 K was observed with DC magnetization
and specific heat measurements, which is identified as a paramagnetic to
ferrimagnetic transition based on the nonlinear susceptibility data. Upon
further cooling below this temperature, we also observed highly relaxational
magnetic behavior: the DC magnetization exhibits history and time dependence,
and the real and imaginary part of the AC susceptibility shows large frequency
dependence. Dynamic scaling of the AC susceptibility data suggests that this
low temperature phase can be described as a reentrant spin glass phase. We also
discuss magnetic field dependence of the spin glass transition and aging,
memory and rejuvenation effect below the glass transition temperature around
228 K.",0712.1975v1
2008-01-03,Two-Fluid MHD Simulations of Converging HI Flows in the Interstellar Medium. I: Methodology and Basic Results,"We develop an unconditionally stable numerical method for solving the
coupling between two fluids (frictional forces/heatings, ionization, and
recombination), and investigate the dynamical condensation process of thermally
unstable gas that is provided by the shock waves in a weakly ionized and
magnetized interstellar medium by using two-dimensional two-fluid
magnetohydrodynamical simulations. If we neglect the effect of magnetic field,
it is known that condensation driven by thermal instability can generate high
density clouds whose physical condition corresponds to molecular clouds
(precursor of molecular clouds). In this paper, we study the effect of magnetic
field on the evolution of supersonic converging HI flows and focus on the case
in which the orientation of magnetic field to converging flows is orthogonal.
We show that the magnetic pressure gradient parallel to the flows prevents the
formation of high density and high column density clouds, but instead generates
fragmented, filamentary HI clouds. With this restricted geometry, magnetic
field drastically diminishes the opportunity of fast molecular cloud formation
directly from the warm neutral medium, in contrast to the case without magnetic
field.",0801.0486v2
2008-01-28,Magnetized Neutron Star Mergers and Gravitational Wave Signals,"We investigate the influence of magnetic fields upon the dynamics of and
resulting gravitational waves from a binary neutron star merger in full general
relativity coupled to ideal magnetohydrodynamics (MHD). We consider two merger
scenarios, one where the stars begin with initially aligned poloidal magnetic
fields and one with no magnetic field. Both mergers result in a strongly
differentially rotating object. In comparison to the non-magnetized scenario,
the aligned magnetic fields delay the final merger of the two stars. During and
after merger we observe phenomena driven by the magnetic field, including
Kelvin-Helmholtz instabilities in shear layers, winding of the field lines, and
transition from poloidal to toroidal fields. These effects not only produce
electromagnetic radiation, but also can have a strong influence on the
gravitational waves. Thus, there are promising prospects for studying such
systems with both types of waves.",0801.4387v2
2008-03-14,Magnetoresistance and spin-transfer torque in magnetic tunnel junctions,"We comment on both recent progress and lingering puzzles related to research
on magnetic tunnel junctions (MTJs). MTJs are already being used in
applications such as magnetic-field sensors in the read heads of disk drives,
and they may also be the first device geometry in which spin-torque effects are
applied to manipulate magnetic dynamics, in order to make nonvolatile magnetic
random access memory. However, there remain many unanswered questions about
such basic properties as the magnetoresistance of MTJs, how their properties
change as a function of tunnel-barrier thickness and applied bias, and what are
the magnitude and direction of the spin-transfer-torque vector induced by a
tunnel current.",0803.2254v1
2008-04-24,Oersted fields and current density profiles in spin-torque driven magnetization dynamics -- Finite element modelling of realistic geometries,"The classical impact of electrical currents on magnetic nanostructures is
analyzed with numerical calculations of current-density distributions and
Oersted fields in typical contact geometries. For the Oersted field
calculation, a hybrid finite element / boundary element method (FEM/BEM)
technique is presented which can be applied to samples of arbitrary shape.
Based on the FEM/BEM analysis, it is argued that reliable micromagnetic
simulations on spin-tranfer-torque driven magnetization processes should
include precise calculations of the Oersted field, particularly in the case of
pillar contact geometries. Similarly, finite-element simulations demonstrate
that numerical calculations of current-density distributions are required,
e.g., in the case of magnetic strips with an indentation. Such strips are
frequently used for the design of devices based on current-driven domain-wall
motion. A dramatic increase of the current density is found at the apex of the
notch, which is expected to strongly affect the magnetization processes in such
strips.",0804.4010v1
2008-05-18,Evolution of neutron stars with toroidal magnetic fields: Axisymmetric simulation in full general relativity,"We study the stability of neutron stars with toroidal magnetic fields by
magnetohydrodynamic simulation in full general relativity under assumption of
axial symmetry. Nonrotating and rigidly rotating neutron stars are prepared for
a variety of magnetic field configuration. For modeling the neutron stars, the
polytropic equation of state with the adiabatic index $\Gamma=2$ is used for
simplicity. It is found that nonrotating neutron stars are dynamically unstable
for the case that toroidal magnetic field strength varies $\propto
\varpi^{2k-1}$ with $k\geq 2$ (here $\varpi$ is the cylindrical radius),
whereas for $k=1$ the neutron stars are stable. After the onset of the
instability, unstable modes grow approximately in the Alfv\'en time scale and,
as a result, a convective motion is excited to change the magnetic field
profile until a new state, which is stable against axisymmetric perturbation,
is reached. We also find that rotation plays a role in stabilization, although
the instability still sets in in the Alfv\'en time scale when the ratio of
magnetic energy to rotational kinetic energy is larger than a critical value
$\sim 0.2$. Implication for the evolution of magnetized protoneutron stars is
discussed.",0805.2712v1
2008-07-23,N-body + Magnetohydrodynamical Simulations of Merging Clusters of Galaxies: Characteristic Magnetic Field Structures Generated by Bulk Flow Motion,"We present results from N-body + magnetohydrodynamical simulations of merging
clusters of galaxies. We find that cluster mergers cause various characteristic
magnetic field structures because of the strong bulk flows in the intracluster
medium. The moving substructures result in cool regions surrounded by the
magnetic field. These will be recognized as magnetized cold fronts in the
observational point of view. A relatively ordered magnetic field structure is
generated just behind the moving substructure. Eddy-like field configurations
are also formed by Kelvin-Helmholtz instabilities. These features are similarly
seen even in off-center mergers though the detailed structures change slightly.
The above-mentioned characteristic magnetic field structures are partly
recognized in Faraday rotation measure maps. The higher absolute values of the
rotation measure are expected when observed along the collision axis, because
of the elongated density distribution and relatively ordered field structure
along the axis. The rotation measure maps on the cosmic microwave background
radiation, which covers clusters entirely, could be useful probes of not only
the magnetic field structures but also the internal dynamics of the
intracluster medium.",0807.3765v1
2008-08-08,Magnetic Fields In Astrophysical Objects,"Magnetic fields are known to reside in many astrophysical objects and are now
believed to be crucially important for the creation of phenomena on a wide
variety of scales. However, the role of the magnetic field in the bodies that
we observe has not always been clear. In certain situations, the importance of
a magnetic field has been over looked on the grounds that the large-scale
magnetic field was believed to be too weak to play and important role in the
dynamics.
  In this article I discuss some of the recent developments concerning magnetic
fields in stars, planets and accretion discs. I choose to emphasise some of the
situations where it has been suggested that weak magnetic fields may play a
more significant role than previously thought. At the end of the article I list
some of the questions to be answered in the future.",0808.1118v2
2008-12-01,Two dimensional frustrated magnetic state in superconducting RuSr2Eu1.5Ce0.5Cu2O10 (Ru-1222),"In this paper we investigate the magnetic state and the role of the
crystalline structure in RuSr2Eu1.5Ce0.5Cu2O10 (Ru-1222). Measurements were
made in the isomorphic series (Nb1-xRux)Sr2Eu1.5Ce0.5Cu2O10 [(Nb,Ru)-1222],
with x between 0 and 1. 3D XY fluctuations above the magnetic transition were
not observed in Ru-1222, suggesting a weak inter-plane coupling between the
RuO2 layers. The compositional dependence of the magnetic susceptibility shows
a rapid broadening with increasing Nb content, explained in terms of a
cluster-glass state. The variation of several superconducting parameters as a
function of Ru content is linear in the whole concentration range, with no
jumps at the critical concentration for which percolation of long range order
is expected. 3D Arrhenius and Vogel-Fulcher-type dependencies fail to describe
the dynamic properties. Fitting of a generalized Vogel-Fulcher-type dependence,
with ln(tau/tau0) = A(T-T0)^-B, yield B = 2.0, in excellent agreement with
Monte Carlo simulations for 2D systems. The value deduced for T0 agrees well
with the re-opening of hysteresis in the M(H) curves. The observed
superconducting and magnetic features are explained in terms of a scenario of
2D magnetic islands at the RuO2 layers, with no long range magnetic order.",0812.0137v1
2009-01-16,Double-diffusive instabilities of a shear-generated magnetic layer,"Previous theoretical work has speculated about the existence of
double-diffusive magnetic buoyancy instabilities of a dynamically evolving
horizontal magnetic layer generated by the interaction of forced vertically
sheared velocity and a background vertical magnetic field. Here we confirm
numerically that if the ratio of the magnetic to thermal diffusivities is
sufficiently low then such instabilities can indeed exist, even for high
Richardson number shear flows. Magnetic buoyancy may therefore occur via this
mechanism for parameters that are likely to be relevant to the solar
tachocline, where regular magnetic buoyancy instabilities are unlikely.",0901.2388v2
2009-03-24,Theoretical Models of Sunspot Structure and Dynamics,"Recent progress in theoretical modeling of a sunspot is reviewed. The
observed properties of umbral dots are well reproduced by realistic simulations
of magnetoconvection in a vertical, monolithic magnetic field. To understand
the penumbra, it is useful to distinguish between the inner penumbra, dominated
by bright filaments containing slender dark cores, and the outer penumbra, made
up of dark and bright filaments of comparable width with corresponding magnetic
fields differing in inclination by some 30 degrees and strong Evershed flows in
the dark filaments along nearly horizontal or downward-plunging magnetic
fields. The role of magnetic flux pumping in submerging magnetic flux in the
outer penumbra is examined through numerical experiments, and different
geometric models of the penumbral magnetic field are discussed in the light of
high-resolution observations. Recent, realistic numerical MHD simulations of an
entire sunspot have succeeded in reproducing the salient features of the
convective pattern in the umbra and the inner penumbra. The siphon-flow
mechanism still provides the best explanation of the Evershed flow,
particularly in the outer penumbra where it often consists of cool, supersonic
downflows.",0903.4106v1
2009-06-26,"Magnetic anomalies in nanocrystalline Ca3CoRhO6, a geometrically frustrated spin-chain compound,","We have investigated the magnetic behavior of the nano crystals, synthesized
by high-energy ball-milling, for a well-known geometrically frustrated
spin-chain system, Ca3CoRhO6, and compared its magnetic characteristics with
those of the bulk form by measuring ac and dc magnetization. The features
attributable to the onset of 'partially disordered antiferromagnetism'
(characterizing the bulk form) are not seen in the magnetization data of the
nano particles; the magnetic moment at high fields in the very low temperature
range in the magnetically ordered state gets relatively enhanced in the nano
particles. It appears that the ferromagnetic intrachain interaction, judged by
the sign of the paramagnetic Curie temperature, is preserved in the nano
particles. These trends are opposite to those seen in Ca3Co2O6. However, the
complex spin-dynamics as evidenced by large frequency dependence of ac
susceptibility is retained in the nano particles as well. Thus, there are some
similarities and dissimilarities between the properties of the nano particles
and those of the bulk. We believe that these findings would be useful to
understand correlation lengths deciding various properties of geometrical
frustration and/or spin-chain phenomena.",0906.4886v1
2010-02-14,Turbulent small-scale dynamo action in solar surface simulations,"We demonstrate that a magneto-convection simulation incorporating essential
physical processes governing solar surface convection exhibits turbulent
small-scale dynamo action. By presenting a derivation of the energy balance
equation and transfer functions for compressible magnetohydrodynamics (MHD), we
quantify the source of magnetic energy on a scale-by-scale basis. We rule out
the two alternative mechanisms for the generation of small-scale magnetic field
in the simulations: the tangling of magnetic field lines associated with the
turbulent cascade and Alfvenization of small-scale velocity fluctuations
(""turbulent induction""). Instead, we find the dominant source of small-scale
magnetic energy is stretching by inertial-range fluid motions of small-scale
magnetic field lines against the magnetic tension force to produce (against
Ohmic dissipation) more small-scale magnetic field. The scales involved become
smaller with increasing Reynolds number, which identifies the dynamo as a
small-scale turbulent dynamo.",1002.2750v3
2010-05-03,Dynamics of a stored Zeeman coherence grating in an external magnetic field,"We investigate the evolution of a Zeeman coherence grating induced in a cold
atomic cesium sample in the presence of an external magnetic field. The
gratings are created in a three-beam light storage configuration using two
quasi-collinear writing laser pulses and reading with a counterpropagating
pulse after a variable time delay. The phase conjugated pulse arising from the
atomic sample is monitored. Collapses and revivals of the retrieved pulse are
observed for different polarizations of the laser beams and for different
directions of the applied magnetic field. While magnetic field inhomogeneities
are responsible for the decay of the coherent atomic response, a five-fold
increase in the coherence decay time, with respect to no applied magnetic
field, is obtained for an appropriate choice of the direction of the applied
magnetic field. A simplified theoretical model illustrates the role of the
magnetic field mean and its inhomogeneity on the collective atomic response.",1005.0282v1
2010-06-30,Micromagnetic analysis of magnetic noise in ferromagnetic nanowires,"We investigate the magnetic thermal noise in magnetic nanowires with and
without a domain wall by employing micromagnetic simulations. The magnetic
thermal noise due to random thermal fluctuation fields gives important physical
quantities related with the magnetic susceptibility. We find that the resonance
frequency of a domain wall is distinguishable from one of a magnetic domain
itself. For the single domain without a domain wall, the resonance frequency is
well described by the Kittel's formula considering a ferromagnetic specimen as
a simple ellipsoid with demagnetizing factors for various wire widths and
thicknesses. However, additional resonance frequencies from the magnetic domain
wall show the different dependences of the wire width and thickness. It implies
that the spins inside the domain wall have different effective fields and the
spin dynamics.",1006.5763v1
2010-07-02,External field control of collective spin excitations in an optical lattice of $^2Σ$ molecules,"We show that an ensemble of $^2\Sigma$ molecules in the rotationally ground
state trapped on an optical lattice exhibits collective spin excitations that
can be controlled by applying superimposed electric and magnetic fields. In
particular, we show that the lowest energy excitation of the molecular ensemble
at certain combinations of electric and magnetic fields leads to the formation
of a magnetic Frenkel exciton. The exciton bandwidth can be tuned by varying
the electric or magnetic fields. We show that the exciton states can be
localized by creating vacancies in the optical lattice. The localization
patterns of the magnetic exciton states are sensitive to the number and
distribution of vacancies, which can be exploited for engineering many-body
entangled spin states. We consider the dynamics of magnetic exciton wavepackets
and show that the spin excitation transfer between molecules in an optical
lattice can be accelerated or slowed down by tuning an external magnetic or
electric field.",1007.0458v1
2011-02-07,Cosmic-ray driven dynamo in galaxies,"We present recent developments of global galactic-scale numerical models of
the Cosmic Ray (CR) driven dynamo, which was originally proposed by Parker
(1992). We conduct a series of direct CR+MHD numerical simulations of the
dynamics of the interstellar medium (ISM), composed of gas, magnetic fields and
CR components. We take into account CRs accelerated in randomly distributed
supernova (SN) remnants, and assume that SNe deposit small-scale, randomly
oriented, dipolar magnetic fields into the ISM. The amplification timescale of
the large-scale magnetic field resulting from the CR-driven dynamo is
comparable to the galactic rotation period. The process efficiently converts
small-scale magnetic fields of SN-remnants into galactic-scale magnetic fields.
The resulting magnetic field structure resembles the X-shaped magnetic fields
observed in edge-on galaxies.",1102.1300v1
2011-02-25,Excitation of Slow-Modes in Network Magnetic Elements Through Magnetic Pumping,"From radiation magnetohydrodynamic simulations of the solar atmosphere we
find a new mechanism for the excitation of longitudinal slow modes within
magnetic flux concentrations. We find that the convective downdrafts in the
immediate surroundings of magnetic elements are responsible for the excitation
of slow modes. The coupling between the external downdraft and the plasma
motion internal to the flux concentration is mediated by the inertial forces of
the downdraft that act on the magnetic flux concentration. These forces, in
conjunction with the downward movement, pump the internal atmosphere in the
downward direction, which entails a fast downdraft in the photospheric and
chromospheric layers of the magnetic element. Subsequent to the transient
pumping phase, the atmosphere rebounds, causing a slow mode traveling along the
magnetic flux concentration in the upward direction. It develops into a shock
wave in chromospheric heights, possibly capable of producing some kind of
dynamic fibril. We propose an observational detection of this process.",1102.5164v1
2011-03-08,Injection and extraction magnets: kicker magnets,"Each stage of an accelerator system has a limited dynamic range and therefore
a chain of stages is required to reach high energy. A combination of septa and
kicker magnets is frequently used to inject and extract beam from each stage.
The kicker magnets typically produce rectangular field pulses with fast rise-
and/or fall-times, however, the field strength is relatively low. To compensate
for their relatively low field strength, the kicker magnets are generally
combined with electromagnetic septa. The septa provide relatively strong field
strength but are either DC or slow pulsed. This paper discusses injection and
extraction systems with particular emphasis on the hardware required for the
kicker magnet.",1103.1583v1
2011-04-13,Magnetic fields in nearby galaxies,"Observations of synchrotron radiation and the Faraday rotation of its
polarized component allow us to investigate the magnetic properties of the
diffuse interstellar medium in nearby galaxies, on scales down to roughly one
hundred parsecs. All disc galaxies seem to have a mean, or regular, magnetic
field component that is ordered on length scales comparable to the size of the
galaxy as well as a random magnetic field of comparable or greater strength. I
present an overview of what is currently known observationally about galactic
magnetic fields, focusing on the common features among galaxies that have been
studied rather than the distinctive or unusual properties of individual
galaxies. Of particular interest are the azimuthal patterns formed by regular
magnetic fields and their pitch angles as these quantities can be directly
related to the predictions of the mean field dynamo theory, the most promising
theoretical explanation for the apparent ubiquitous presence of regular
magnetic fields in disc galaxies.",1104.2427v1
2011-07-31,Avoiding power broadening in optically detected magnetic resonance of single NV defects for enhanced DC-magnetic field sensitivity,"We report a systematic study of the magnetic field sensitivity of a magnetic
sensor based on a single Nitrogen-Vacancy (NV) defect in diamond, by using
continuous optically detected electron spin resonance (ESR) spectroscopy. We
first investigate the behavior of the ESR contrast and linewidth as a function
of the microwave and optical pumping power. The experimental results are in
good agreement with a simplified model of the NV defect spin dynamics, yielding
to an optimized sensitivity around 2 \mu T/\sqrt{\rm Hz}. We then demonstrate
an enhancement of the magnetic sensitivity by one order of magnitude by using a
simple pulsed-ESR scheme. This technique is based on repetitive excitation of
the NV defect with a resonant microwave \pi-pulse followed by an optimized
read-out laser pulse, allowing to fully eliminate power broadening of the ESR
linewidth. The achieved sensitivity is similar to the one obtained by using
Ramsey-type sequences, which is the optimal magnetic field sensitivity for the
detection of DC magnetic fields.",1108.0178v2
2012-05-31,Magnetic excitations in underdoped Ba(Fe1-xCox)2As2 with x=0.047,"The magnetic excitations in the paramagnetic-tetragonal phase of underdoped
Ba(Fe0.953Co0.047)2As2, as measured by inelastic neutron scattering, can be
well described by a phenomenological model with purely diffusive spin dynamics.
At low energies, the spectrum around the magnetic ordering vector Q_AFM
consists of a single peak with elliptical shape in momentum space. At high
energies, this inelastic peak is split into two peaks across the direction
perpendicular to Q_AFM. We use our fittings to argue that such a splitting is
not due to incommensurability or propagating spin-wave excitations, but is
rather a consequence of the anisotropies in the Landau damping and in the
magnetic correlation length, both of which are allowed by the tetragonal
symmetry of the system. We also measure the magnetic spectrum deep inside the
magnetically-ordered phase, and find that it is remarkably similar to the
spectrum of the paramagnetic phase, revealing the strongly overdamped character
of the magnetic excitations.",1206.0040v1
2012-06-22,How is the magnetic reconnection derived from magnetohydrodynamics equations?,"We clarify how magnetic reconnection can be derived from magnetohydrodynamics
(MHD) equations in a way that is easily understandable to university students.
The essential mechanism governing the time evolution of the magnetic field is
diffusion dynamics. The magnetic field is represented by two components. It is
clarified that the diffusion of a component causes a generation of another
component that is initially zero and, accordingly, that the magnetic force
lines are reconnected. For this reconnection to occur correctly, the initial
magnetic field must be directed oppositely in the two regions, e.g., $y>0$ and
$y<0$; must be concave (convex) for $y>0$ ($y<0$); and must be saturated for
$y$ far from the x axis, which would indicate the existence of the current
sheet. It will be clear that our comprehension based on diffusion runs parallel
to the common qualitative explanation about the magnetic reconnection.",1206.5161v1
2012-08-10,Unique Topological Characterization of Braided Magnetic Fields,"We introduce a topological flux function to quantify the topology of magnetic
braids: non-zero, line-tied magnetic fields whose field lines all connect
between two boundaries. This scalar function is an ideal invariant defined on a
cross-section of the magnetic field, and measures the average poloidal magnetic
flux around any given field line, or the average pairwise crossing number
between a given field line and all others. Moreover, its integral over the
cross-section yields the relative magnetic helicity. Using the fact that the
flux function is also an action in the Hamiltonian formulation of the field
line equations, we prove that it uniquely characterizes the field line mapping
and hence the magnetic topology.",1208.2286v2
2012-09-07,Optimization of the magnetic dynamo,"In stars and planets, magnetic fields are believed to originate from the
motion of electrically conducting fluids in their interior, through a process
known as the dynamo mechanism. In this Letter, an optimization procedure is
used to simultaneously address two fundamental questions of dynamo theory:
""Which velocity field leads to the most magnetic energy growth?"" and ""How large
does the velocity need to be relative to magnetic diffusion?"" In general, this
requires optimization over the full space of continuous solenoidal velocity
fields possible within the geometry. Here the case of a periodic box is
considered. Measuring the strength of the flow with the root-mean-square
amplitude, an optimal velocity field is shown to exist, but without limitation
on the strain rate, optimization is prone to divergence. Measuring the flow in
terms of its associated dissipation leads to the identification of a single
optimal at the critical magnetic Reynolds number necessary for a dynamo. This
magnetic Reynolds number is found to be only 15% higher than that necessary for
transient growth of the magnetic field.",1209.1559v2
2012-11-27,Boundary and finite-size effects in the competition between indirect magnetic exchange and Kondo screening,"A system of conduction electrons can mediate an indirect magnetic exchange
between magnetic impurites. The nonlocal exchange typically competes with the
local Kondo screening of the impurity magnetic moments. In case of magnetic
adatoms on non-magnetic surfaces this competition is expected to be affected by
different confinement effects. Here we study this situation in the regime of
intermediate hybridization strengths by means of real-space dynamical
mean-field theory for a one-dimensional two-impurity Anderson model. Depending
on the presence of a boundary and depending on the system size, a crossover
between nonlocal and local singlet formation is observed in corresponding
magnetic susceptibilities. The crossover is driven by the strong sensitivity of
the Kondo temperature on the local density of states.",1211.6307v2
2013-03-22,Electro/Magnetically Induced Controllable Rotation In Small-scale Liquid Flow,"We study all the possibilities of producing rotating flow in an
incompressible fluid by electric and magnetic fields. We start with a general
theoretical basis and look for different configurations and set-ups which
electric/magnetic field and an electric current affect the vorticity of fluid
resulting in rotation on liquid flow. We assume steady-state conditions and
time-independent electric and magnetic fields as the external body torque.
Regarding the theoretical basis, we propose three experimental set-ups in which
by applying fields on a fluid, rotational vortices are produced: (a) a uniform
electric field and a uniform electric current, (b) a uniform electric current
and a non-uniform magnetic field, and (c) a non-uniform electric current and a
uniform magnetic field. The first case has been reported in detail named
""Liquid Film Motor"". The two other cases are experimentally investigated here
for a cubic an cylindrical cells. The rotational velocity patterns are obtained
by PIV technique, and the results are discussed and justified by a preliminary
estimation based on the torque exerted by magnetic fields on electric currents.
From the log-log plot of angular velocity versus current, the non-linearity
factors of the rotational flow for cylindrical and rectangular geometries are
obtained.",1303.5657v1
2013-05-09,Low-temperature muon spin rotation studies of the monopole charges and currents in Y doped Ho2Ti2O7,"In the ground state of Ho2Ti2O7 spin ice, the disorder of the magnetic
moments follows the same rules as the proton disorder in water ice. Excitations
take the form of magnetic monopoles that interact via a magnetic Coulomb
interaction. Muon spin rotation has been used to probe the low-temperature
magnetic behaviour in single crystal Ho2-xYxTi2O7 (x=0, 0.1, 1, 1.6 and 2). At
very low temperatures, a linear field dependence for the relaxation rate of the
muon precession lambda(B), that in some previous experiments on Dy2Ti2O7 spin
ice has been associated with monopole currents, is observed in samples with
x=0, and 0.1. A signal from the magnetic fields penetrating into the silver
sample plate due to the magnetization of the crystals is observed for all the
samples containing Ho allowing us to study the unusual magnetic dynamics of Y
doped spin ice.",1305.2048v1
2013-05-13,Backhopping effect in magnetic tunnel junctions: comparison between theory and experiment,"We report on the magnetic switching and backhopping effects due to
spin-transfer-torque in magnetic tunnel junctions. Experimental data on the
current-induced switching in junctions with MgO tunnel barrier reveal a random
back-and-forth switching between the magnetization states, which appears when
the current direction favors the parallel magnetic configuration. The effect
depends on the barrier thickness $t_b$, and is not observed in tunnel junctions
with very thin MgO tunnel barriers, $t_b$ $<$ 0.95 nm. Switching dependence on
the bias voltage and barrier thickness is explained in terms of the macrospin
model, with the magnetization dynamics described by the modified
Landau-Lifshitz-Gilbert equation. Numerical simulations indicate that the
competition between in-plane and out-of-plane torque components can result at
high bias voltages in a non-deterministic switching behavior, in agreement with
experimental observations. When the barrier thickness is reduced, the overall
coupling between the magnetic layers across the barrier becomes ferromagnetic,
which suppresses the backhopping effect.",1305.2711v1
2013-09-17,Dependence of solar plasma flows on magnetic field obliquity,"Interactions between flows and magnetic fields in the Sun's plasma can change
surface waves and flows near active regions, are evident in cyclic changes of
large-scale phenomena such as the meridional circulation, and contribute to
dynamics in the long-term solar magnetic cycle, e.g. during the recent
prolonged solar minimum. We investigate possible relationships between these
phenomena.
  We have observed changes in solar surface flow patterns in active regions,
dependent on magnetic field strength and orientation, consistent with the
theoretically predicted Proctor Effect. Other researchers have observed
relationships between changes in solar magnetic fields and meridional
circulation flows. We explore similarities between the Proctor Effect and the
observed interdependence of larger-scale magnetic fields and flows. This may
contribute to understanding of fundamental solar convection and dynamo
processes, e.g. the prolonged magnetic minimum of the most recent cycle.",1309.4468v2
2013-12-11,Chiral dynamics in a magnetic field from the functional renormalization group,"We investigate the quark-meson model in a magnetic field using the exact
functional renormalization group equation beyond the local-potential
approximation. Our truncation of the effective action involves anisotropic wave
function renormalization for mesons, which allows us to investigate how the
magnetic field distorts the propagation of neutral mesons. Solving the flow
equation numerically, we find that the transverse velocity of mesons decreases
with the magnetic field at all temperatures, which is most prominent at zero
temperature. The meson screening masses and the pion decay constants are also
computed. The constituent quark mass is found to increase with magnetic field
at all temperatures, resulting in the crossover temperature that increases
monotonically with the magnetic field. This tendency is consistent with most
model calculations but not with the lattice simulation performed at the
physical point. Our work suggests that the strong anisotropy of meson
propagation may not be the fundamental origin of the inverse magnetic
catalysis.",1312.3124v2
2014-02-27,Sensitive Magnetic Force Detection with a Carbon Nanotube Resonator,"We propose a technique for sensitive magnetic point force detection using a
suspended carbon nanotube (CNT) mechanical resonator combined with a magnetic
field gradient generated by a ferromagnetic gate electrode. Numerical
calculations of the mechanical resonance frequency show that single Bohr
magneton changes in the magnetic state of an individual magnetic molecule
grafted to the CNT can translate to detectable frequency shifts, on the order
of a few kHz. The dependences of the resonator response to device parameters
such as length, tension, CNT diameter, and gate voltage are explored and
optimal operating conditions are identified. A signal-to-noise analysis shows
that in principle, magnetic switching at the level of a single Bohr magneton
can be read out in a single shot on timescales as short as 10 microseconds.
This force sensor should enable new studies of spin dynamics in isolated single
molecule magnets, free from the crystalline or ensemble settings typically
studied.",1402.6780v1
2014-03-06,Influence of small scale magnetic energy and helicity on the growth of large scale magnetic field,"The influence of initially given small scale magnetic energy($E_M(0)$) and
helicity($H_M(0)$) on the magnetohydrodynamics(MHD) dynamo was investigated.
Equations for $E_M$(t), $H_M$(t), and electromotive force($\langle {\bf
v}\times {\bf b}\rangle$, $EMF$) were derived and solved. The solutions
indicate small scale magnetic field(${\bf b}_i$) caused by $E_M$(0) modifies
$EMF$ and generates additional terms of which effect depends on magnetic
diffusivity $\eta$, position of initial conditions($IC$s) $k_f$, and time
($\sim e^{-\eta k_f^2 t}$). ${\bf b}_i$ increases the inverse cascade of energy
resulting in the enhanced growth of large scale magnetic field($\overline{{\bf
B}}$). Simulation data show that $E_M$(0) in small scale boosts the growth
rate, which also proportionally depends on $H_M(0)$. If $E_M$(0) is the same,
positive $H_M(0)$ is more effective for MHD dynamo than negative $H_M(0)$ is.
It was discussed why large scale magnetic helicity should have the opposite
sign of the injected kinetic helicity.",1403.1328v3
2014-03-17,A reentrant phenomenon in magnetic and dielectric properties of Dy2BaNiO5 and an intriguing influence of external magnetic field,"We report that the spin-chain compound Dy2BaNiO5 recently proven to exhibit
magnetoelectric coupling below its Neel temperature (T_N) of 58 K, exhibits
strong frequency-dependent behavior in ac magnetic susceptibility and complex
dielectric properties at low temperatures (<10K), mimicking 'reentrant'
multiglass phenomenon. Such a behavior is not known among undoped compounds. A
new finding in the field of multiferroics is that the characteristic magnetic
feature at such low temperatures moves towards higher temperatures in the
presence of a magnetic-field (H), whereas the corresponding dielectric feature
shifts towards lower temperatures with H, unlike the situation near T_N. This
observation indicates that the alignment of spins by external magnetic fields
tends to inhibit glassy-like slow electric-dipole dynamics, at least in this
system, possibly arising from peculiarities in the magnetic structure.",1403.4166v1
2014-03-27,Shock Corrugation by Rayleigh-Taylor Instability in GRB Afterglow Jets,"Afterglow jets are Rayleigh-Taylor unstable and therefore turbulent during
the early part of their deceleration. There are also several processes which
actively cool the jet. In this letter, we demonstrate that if cooling
significantly increases the compressibility of the flow, the turbulence
collides with the forward shock, destabilizing and corrugating it. In this
case, the forward shock is turbulent enough to produce the magnetic fields
responsible for synchrotron emission via small scale turbulent dynamo. We
calculate light curves assuming the magnetic field is in energy equipartition
with the turbulent kinetic energy and discover that dynamic magnetic fields are
well-approximated by a constant magnetic-to- thermal energy ratio of 1%, though
there is a sizeable delay in the time of peak flux as the magnetic field turns
on only after the turbulence has activated. The reverse shock is found to be
significantly more magnetized than the forward shock, with a
magnetic-to-thermal energy ratio of order 10%. This work motivates future
Rayleigh-Taylor calculations using more physical cooling models.",1403.6895v2
2014-05-09,Statistical study of free magnetic energy and flare productivity of solar active regions,"Photospheric vector magnetograms from Helioseismic and Magnetic Imager on
board the Solar Dynamic Observatory are utilized as the boundary conditions to
extrapolate both non-linear force-free and potential magnetic fields in solar
corona. Based on the extrapolations, we are able to determine the free magnetic
energy (FME) stored in active regions (ARs). Over 3000 vector magnetograms in
61 ARs were analyzed. We compare FME with ARs' flare index (FI) and find that
there is a weak correlation ($<60\%$) between FME and FI. FME shows slightly
improved flare predictability relative to total unsigned magnetic flux of ARs
in the following two aspects: (1) the flare productivity predicted by FME is
higher than that predicted by magnetic flux and (2) the correlation between FI
and FME is higher than that between FI and magnetic flux. However, this
improvement is not significant enough to make a substantial difference in
time-accumulated FI, rather than individual flare, predictions.",1405.2131v1
2014-07-12,Generalized dyons and magnetic dipoles: the issue of angular momentum,"It is known that a non-Abelian magnetic monopole cannot rotate globally
(although it may possess a nonzero angular momentum density). At the same time,
the total angular momentum of a magnetic dipole equals the electric charge. In
this work we question the generality of these results by considering a number
of generalizations of the Georgi-Glashow model. We study two different types of
finite energy, regular configurations: solutions with net magnetic charge and
monopole-antimonopole pairs with zero net magnetic charge. These configurations
are endowed with an electric charge and carry also a nonvanishing angular
momentum density. However, we argue that the qualitative results found in the
Georgi-Glashow model are generic and thus a magnetic monopole cannot spin as
long as the matter fields feature the usual ""monopole"" asymptotic behaviour
independently of the dynamics of the model. A study of the properties of the
dyons and magnetic dipoles in some generalizations of the Georgi-Glashow model
supplemented with higher order Skyrme-like terms in the gauge curvature and
Higgs fields is given quantitatively.",1407.3378v1
2014-07-25,Energy transfers in dynamos with small magnetic Prandtl numbers,"We perform numerical simulation of dynamo with magnetic Prandtl number
$\mathrm{Pm} =0.2$ on $1024^3$ grid, and compute the energy fluxes and the
shell-to-shell energy transfers. These computations indicate that the magnetic
energy growth takes place mainly due to the energy transfers from large-scale
velocity field to large-scale magnetic field and that the magnetic energy flux
is forward. The steady-state magnetic energy is much smaller than the kinetic
energy, rather than equipartition; this is because the magnetic Reynolds number
is near the dynamo transition regime. We also contrast our results with those
for dynamo with $\mathrm{Pm} =20$ and decaying dynamo.",1407.6855v2
2014-08-12,"Mechano-sysntesis and structural, magnetic and thermal characterization of $α$-Fe nanoparticles embedded in a Wüstite matrix","Magnetic materials for specific applications require an accurate control and
complete comprehension of their magnetic properties. In particular,
nanoparticles embedded in a polycrystalline matrix emerge as good candidates
for applications due to the possibility of tuning the magnetic properties
through interface interaction effects. Here, iron/w\""ustite composite is
prepared using high energy mechanical milling from iron powder and water. The
sample is analyzed by X-ray diffraction, dynamic laser light scattering,
M\""ossbauer spectroscopy, field cooling and zero field cooling curves,
magnetization curves, and magnetic hyperthermia. Based on the results, we
identify that the produced sample is like Fe nanoparticles embedded in a
w\""ustite matrix, with high stability in time, and shows noticeable features
such as exchange bias effect at low temperatures and promising temperatures
reached in a short time interval when considered magnetic hyperthermia, $\sim
46\,^\circ$C, becoming an interesting candidate for biological applications,
such as the one employed for cancer therapy.",1408.2703v1
2014-08-14,Particle Capture Efficiency in a Multi-Wire Model for High Gradient Magnetic Separation,"High gradient magnetic separation (HGMS) is an efficient way to remove
magnetic and paramagnetic particles, such as heavy metals, from waste water. As
the suspension flows through a magnetized filter mesh, high magnetic gradients
around the wires attract and capture the particles, removing them from the
fluid. We model such a system by considering the motion of a paramagnetic
tracer particle through a periodic array of magnetized cylinders. We show that
there is a critical Mason number (ratio of viscous to magnetic forces) below
which the particle is captured irrespective of its initial position in the
array. Above this threshold, particle capture is only partially successful and
depends on the particle's entry position. We determine the relationship between
the critical Mason number and the system geometry using numerical and
asymptotic calculations. If a capture efficiency below 100% is sufficient, our
results demonstrate how operating the HGMS system above the critical Mason
number but with multiple separation cycles may increase efficiency.",1408.3313v1
2014-09-16,"Magnetic field configurations of a magnetar throughout its interior and exterior -- core, crust and magnetosphere","We obtained the magnetic field configurations, including both poloidal and
toroidal components, throughout the interior and exterior of magnetars using a
realistic equation of state. We divided the magnetized star into the
hydromagnetic equilibrium core, Hall equilibrium crust and twisted force-free
magnetosphere. We systematically and simultaneously calculated these regions
under various boundary conditions using the Green function relaxation method,
and noted the following interesting characteristics of these numerical results.
First, the strength and structure of core magnetic fields affect the crustal
magnetic fields. Second, the current sheet on the core-crust interface affects
both internal and external magnetic field configurations. Third, the twisted
magnetosphere makes a cross-point of magnetic field lines, such as X-point
geometry, in the magnetosphere. The X-point geometry appears and disappears
according to the strength of the twisted field in the magnetosphere or the
core-crust boundary conditions. Our results mean that both Hall
magnetohydrodynamics secular evolution and magnetospheric dynamical evolution
are deeply affected by conditions of another region and the core-crust stress
of magnetars.",1409.4547v2
2014-09-25,Quantitative simulation of temperature dependent magnetization dynamics and equilibrium properties of elemental ferromagnets,"Atomistic spin model simulations are immensely useful in determining
temperature dependent magnetic prop- erties, but are known to give the
incorrect dependence of the magnetization on temperature compared to exper-
iment owing to their classical origin. We find a single parameter rescaling of
thermal fluctuations which gives quantitative agreement of the temperature
dependent magnetization between atomistic simulations and experi- ment for the
elemental ferromagnets Ni, Fe, Co and Gd. Simulating the sub-picosecond
magnetization dynam- ics of Ni under the action of a laser pulse we also find
quantitative agreement with experiment in the ultrafast regime. This enables
the quantitative determination of temperature dependent magnetic properties
allowing for accurate simulations of magnetic materials at all temperatures.",1409.7397v2
2014-11-12,Block magnetic excitations in the orbitally selective Mott insulator BaFe2Se3,"Iron pnictides and selenides display a variety of unusual magnetic phases
originating from the interplay between electronic, orbital, and lattice degrees
of freedom. Using powder inelastic neutron scattering on the two-leg ladder
BaFe2Se3, we fully characterize the static and dynamic spin correlations
associated with the Fe4 block state, an exotic magnetic ground state observed
in this low-dimensional magnet and in Rb0.89Fe1.58Se2. All the magnetic
excitations of the Fe4 block state predicted by an effective Heisenberg model
with localized spins are observed below 300 meV and quantitatively reproduced.
However, the data only account for 16 mub^2 per Fe2+, approximatively 2/3 of
the total spectral weight expected for localized S=2 moments. Our results
highlight how orbital degrees of freedom in iron-based magnets can conspire to
stabilize an exotic magnetic state.",1411.3337v2
2014-11-13,Review: Magnetic fields of O stars,"Since 2002, strong, organized magnetic fields have been firmly detected at
the surfaces of about 10 Galactic O-type stars. In this paper I will review the
characteristics of the inferred fields of individual stars, as well as the
overall population. I will discuss the extension of the 'magnetic desert',
first inferred among the A-type stars, to O stars up to 60 solar masses. I will
discuss the interaction of the winds of the magnetic stars with the fields
above their surfaces, generating complex 'dynamical magnetosphere' structures
detected in optical and UV lines, and in X-ray lines and continuum. Finally, I
will discuss the detection of a small number of variable O stars in the LMC and
SMC that exhibit spectral characteristics analogous to the known Galactic
magnetic stars, and that almost certainly represent the first known examples of
extra-Galactic magnetic stars.",1411.3604v1
2014-11-14,Photo-induced magnetization enhancement in two-dimensional weakly anisotropic Heisenberg magnets,"By comparing the photo-induced magnetization dynamics in simple layered
systems we show how light-induced modifications of the magnetic anisotropy
directly enhance the magnetization. It is observed that the spin precession in
(CH3NH3)2CuCl4, initiated by a light pulse, increases in amplitude at the
critical temperature TC. The phenomenon is related to the dependence of the
critical temperature on the axial magnetic anisotropy. The present results
underline the possibility and the importance of the optical modifications of
the anisotropy, opening new paths toward the control of the magnetization state
for ultrafast memories.",1411.3918v1
2015-02-21,Geometrically induced magnetic catalysis and critical dimensions,"We discuss the combined effect of magnetic fields and geometry in interacting
fermionic systems. At leading order in the heat-kernel expansion, the infrared
singularity (that in flat space leads to the magnetic catalysis) is regulated
by the chiral gap effect, and the catalysis is deactivated by the effect of the
scalar curvature. We discover that an infrared singularity is found in
higher-order terms that mix the magnetic field with curvature, and these lead
to a novel form of geometrically induced magnetic catalysis. The dynamical mass
squared is then modified not only due to the chiral gap effect by an amount
proportional to the curvature, but also by a magnetic shift $\propto (4-D)eB$,
where $D$ represents the number of space-time dimensions. We argue that $D=4$
is a critical dimension across which the behavior of the magnetic shift changes
qualitatively.",1502.06090v2
2015-04-16,Ferrogels cross-linked by magnetic particles: Field-driven deformation and elasticity studied using computer simulations,"Ferrogels, i.e. swollen polymer networks into which magnetic particles are
immersed, can be considered as ""smart materials"" since their shape and
elasticity can be controlled by an external magnetic field. Using molecular
dynamics simulations on the coarse-grained level we study a ferrogel in which
the magnetic particles act as the cross-linkers of the polymer network. In a
homogeneous external magnetic field the direct coupling between the orientation
of the magnetic moments and the polymers by means of covalent bonds gives rise
to a deformation of the gel, independent of the interparticle dipole-dipole
interaction. In this paper we report quantitative measurements of this
deformation, the gel's elastic moduli and its magnetic response. Our results
demonstrate that these properties depend significantly on the topology of the
polymer network.",1504.04280v2
2015-05-04,High-topological-number magnetic skyrmions and topologically protected dissipative structure,"The magnetic skyrmion with the topological number of unity ($Q=1$) is a
well-known nanometric swirling spin structure in the nonlinear $\sigma$ model
with the Dzyaloshinskii-Moriya interaction. Here, we show that magnetic
skyrmion with the topological number of two ($Q=2$) can be created and
stabilized by applying vertical spin-polarized current though it cannot exist
as a static stable excitation. Magnetic skyrmion with $Q=2$ is a nonequilibrium
dynamic object, subsisting on a balance between the energy injection from the
current and the energy dissipation by the Gilbert damping. Once it is created,
it becomes a topologically protected object against fluctuations of various
variables including the injected current itself. Hence, we may call it a
topologically protected dissipative structure. We also elucidate the nucleation
and destruction mechanisms of the magnetic skyrmion with $Q=2$ by studying the
evolutions of the magnetization distribution, the topological charge density as
well as the energy density. Our results will be useful for the study of the
nontrivial topology of magnetic skyrmions with higher topological numbers.",1505.00522v2
2015-06-12,Generation of large-scale magnetic fields by small-scale dynamo in shear flows,"We propose a new mechanism for turbulent mean-field dynamo in which the
magnetic fluctuations resulting from a small-scale dynamo drive the generation
of large-scale magnetic fields. This is in stark contrast to the common idea
that small-scale magnetic fields should be harmful to large-scale dynamo
action. These dynamos occur in the presence of large-scale velocity shear and
do not require net helicity, resulting from off-diagonal components of the
turbulent resistivity tensor as the magnetic analogue of the ""shear-current""
effect. Given the inevitable existence of non-helical small-scale magnetic
fields in turbulent plasmas, as well as the generic nature of velocity shear,
the suggested mechanism may help to explain generation of large-scale magnetic
fields across a wide range of astrophysical objects.",1506.04109v2
2015-06-17,Continuous wave approach for simulating Ferromagnetic Resonance in nanosized elements,"We present a numerical approach to simulate the Ferromagnetic Resonance (FMR)
of micron and nanosized magnetic elements by a micromagnetic finite difference
method. In addition to a static magnetic field a linearly polarized oscillating
magnetic field is utilized to excite and analyze the spin wave excitations
observed by Ferromagnetic Resonance in the space- and time-domain. Our
continuous wave approach (CW) provides an alternative to the common simulation
method, which uses a pulsed excitation of the magnetic system. It directly
models conventional FMR-experiments and permits the determination of the real
and imaginary part of the complex dynamic susceptibility without the need of
post-processing. Furthermore not only the resonance fields, but also
linewidths, ellipticity, phase relations and relative intensities of the
excited spin wave modes in a spectrum can be determined and compared to
experimental data. The magnetic responses can be plotted as a function of
spatial dimensions yielding a detailed visualization of the spin wave modes and
their localization as a function of external magnetic field and frequency. This
is illustrated for the case of a magnetic micron sized stripe.",1506.05292v1
2015-06-17,The terahertz frontier for ultrafast coherent magnetic switching: Terahertz-induced demagnetization in ferromagnets,"The transition frequency between nonthermal coherent magnetic precessions and
ultrafast demagnetization is arguably the most sought after answer in magnetism
science and technology nowadays. So far, it is believed to be in the terahertz
(THz) range. Here, using an ultra-intense low frequency THz bullet, and thin
magnetic layers, we report on experimental evidences that fully coherent
nonthermal THz magnetic switching may never be reachable in conventional
ferromagnetic thin films. At high excitation intensities, while the spins still
coherently precess with the THz magnetic field, the deposited THz energy
initiates ultrafast demagnetization and ultimately material damage. These
series of phenomena are found to take place simultaneously. The reported
experiments set fundamental limits and raise questions on the coupling between
electronic and magnetic systems and the associated structural dynamics on the
ultrafast time scale.",1506.05397v1
2015-09-17,Dynamics of magnetic single domain particles embedded in a viscous liquid,"Kinetic equations for magnetic nano particles dispersed in a viscous liquid
are developed and analyzed numerically. Depending on the amplitude of an
applied oscillatory magnetic field the particles orient their time averaged
anisotropy axis perpendicular to the applied field for low magnetic field
amplitudes and nearly parallel to the direction of the field for high
amplitudes. The transition between these regions takes place in a narrow field
interval. In the low field region the magnetic moment is locked to some crystal
axis and the energy absorption in an oscillatory driving field is dominated by
viscous losses associated with particle rotation in the liquid. In the opposite
limit the magnetic moment rotates within the particle while its easy axis being
nearly parallel to the external field direction oscillates. The kinetic
equations are generalized to include thermal fluctuations. This leads to a
significant increase of the power absorption in the low and intermediate field
region with a pronounced absorption peak as function of particle size. In the
high field region, on the other hand, the inclusion of thermal fluctuations
reduces the power absorption. The illustrative numerical calculations presented
are performed for magnetic parameters typical for iron oxide.",1509.05233v1
2015-10-19,Effect of magnetism on lattice dynamics in SrFe$_2$As$_2$ as seen via high resolution inelastic x-ray scattering,"Phonon spectra of detwinned {SrFe$_2$As$_2$} crystals, as measured by
inelastic x-ray scattering, show clear anisotropy accompanying the
magneto-structural transition at 200 K. We model the mode splitting using
magnetic DFT calculations, including a phenomenological reduction in
force-constant anisotropy that can be attributed to magnetic fluctuations. This
serves as a starting point for a general model of phonons in this material
applicable to both magnetic and non-magnetic phase. Using this model, the
measured splitting in the magnetic phase below $\it T_{N}$, and the measured
phonon linewidth, we set a lower bound on the mean magnetic fluctuation
frequency above $\it T_{N}$ at 210 K.",1510.05487v2
2016-02-26,The magnetic monopole and the separation between fast and slow magnetic degrees of freedom,"The Landau-Lifshitz-Gilbert (LLG) equation that describes the dynamics of a
macroscopic magnetic moment finds its limit of validity at very short times.
The reason for this limit is well understood in terms of separation of the
characteristic time scales between slow degrees of freedom (the magnetization)
and fast degrees of freedom. The fast degrees of freedom are introduced as the
variation of the angular momentum responsible for the inertia. In order to
study the effect of the fast degrees of freedom on the precession, we calculate
the geometric phase of the magnetization (i.e. the Hannay angle) and the
corresponding magnetic monopole. In the case of the pure precession (the slow
manifold), a simple expression of the magnetic monopole is given as a function
of the slowness parameter, i.e. as a function of the ratio of the slow over the
fast characteristic times.",1602.08470v1
2016-07-26,Two-phase flow of ferrofluids,"Ferrofluids currently are the only type of magnetic liquid materials with
wide practical use. The theory on ferrofluids is an example of success to apply
statistics to science. Ferrofluids are two-phase liquids consisting of
dispersed nanoscale ferromagnetic particles suspended in a carrier fluid. Due
to their tiny size, individual ferromagnetic particles clearly exhibit Brownian
motions. Only when a large number of randomly-moving particles are subject to
an external magnetic field, can they collectively exhibit magnetization at a
macroscale. Using statistical theory, the magnetization of a ferrofluid can be
characterized by the celebrated Langevin equation. The monograph combines
statistical theory and the method of Dirac function to establish equations for
ferrofluids under several conditions, including magnetization relaxation,
magnetization equilibrium, and magnetization freezing. It thus provides a quite
complete account of the theory of ferrofluid dynamics.",1607.07838v1
2016-09-10,Vortex disruption by magnetohydrodynamic feedback,"In an electrically conducting fluid, vortices stretch out a weak, large-scale
magnetic field to form strong current sheets on their edges. Associated with
these current sheets are magnetic stresses, which are subsequently released
through reconnection, leading to vortex disruption, and possibly even
destruction. This disruption phenomenon is investigated here in the context of
two-dimensional, homogeneous, incompressible magnetohydrodynamics. We derive a
simple order of magnitude estimate for the magnetic stresses --- and thus the
degree of disruption --- that depends on the strength of the background
magnetic field (measured by the parameter $M$, a ratio between the Alfv\'en
speed and a typical flow speed) and on the magnetic diffusivity (measured by
the magnetic Reynolds number $\mbox{Rm}$). The resulting estimate suggests that
significant disruption occurs when $M^{2}\mbox{Rm} = O(1)$. To test our
prediction, we analyse direct numerical simulations of vortices generated by
the breakup of unstable shear flows with an initially weak background magnetic
field. Using the Okubo--Weiss vortex coherence criterion, we introduce a vortex
disruption measure, and show that it is consistent with our predicted scaling,
for vortices generated by instabilities of both a shear layer and a jet.",1609.03069v1
2016-09-29,Jet formation in solar atmosphere due to magnetic reconnection,"Using numerical simulations, we show that jets with features of type II
spicules and cold coronal jets corresponding to temperatures $10^{4}$ K can be
formed due to magnetic reconnection in a scenario in presence of magnetic
resistivity. For this we model the low chromosphere-corona region using the C7
equilibrium solar atmosphere model and assuming Resistive MHD rules the
dynamics of the plasma. The magnetic filed configurations we analyze correspond
to two neighboring loops with opposite polarity. The separation of the loops'
feet determines the thickness of a current sheet that triggers a magnetic
reconnection process, and the further formation of a high speed and sharp
structure. We analyze the cases where the magnetic filed strength of the two
loops is equal and different. In the first case, with a symmetric configuration
the spicules raise vertically whereas in an asymmetric configuration the
structure shows an inclination. With a number of simulations carried out under
a 2.5D approach, we explore various properties of excited jets, namely, the
morphology, inclination and velocity. The parameter space involves magnetic
field strength between 20 and 40 G, and the resistivity is assumed to be
uniform with a constant value of the order $10^{-2}\Omega\cdot m$",1609.09422v1
2016-10-05,"Orbital frustration in the S = 1/2 kagome magnet vesignieite, BaCu3V2O8(OD)2","Here we report crystallographic and magnetic studies on high quality samples
of the magnetically frustrated S = 1/2 kagome antiferromagnet vesignieite,
BaCu3V2O8(OD)2. Powder neutron diffraction data collected from samples obtained
by a new hydrothermal synthetic route reveal a previously unobserved trigonal
P3121 structure, similar to the isoelectronic mineral SrCu3V2O8(OH)2. The
refined structure is consistent with orbital frustration of the eg d-orbitals
in a sublattice of the Cu2+ kagome network due to a dynamic Jahn-Teller effect,
which persists below the magnetic transition at TN = 9K and makes the material
an interesting candidate for exploring concomitant spin and orbital
frustration. A combination of crystallographic strain analysis and
magnetisation measurements indicate strong magnetostructural coupling which may
explain the varied magnetic behaviour between samples of vesignieite in the
literature. The revised orbital structure is similar to that found in
volborthite, rather than the quantum spin liquid herbertsmithite, and provides
a convincing argument for the differing magnetic properties found in these
frustrated magnets.",1610.01436v1
2016-12-22,Tunable Finite-Sized Chains to Control Magnetic Relaxation,"The magnetic dynamics of low-dimensional iron ion chains have been studied
with regards to the tunable finite-sized chain length using iron phthalocyanine
thin films. The deposition temperature varies the diffusion length during thin
film growth by limiting the average crystal size in the range from 40 to 110
nm. Using a method common for single chain magnets, the magnetic relaxation
time for each chain length is determined from temporal remanence data and fit
to a stretched exponential form in the temperature range below 5 K, the onset
for magnetic hysteresis. A temperature-independent master curve is generated by
scaling the remanence by its relaxation time to fit the energy barrier for spin
reversal, and the single spin relaxation time. The energy barrier of 95 K is
found to be independent of the chain length. In contrast, the single spin
relaxation time increases with longer chains from under 1 ps to 800 ps. We show
that thin films provide the nano-architecture to control magnetic relaxation
and a testbed to study finite-size effects in low-dimensional magnetic systems.",1612.07397v1
2017-02-17,Magnetic anisotropy in Permalloy: hidden quantum mechanical features,"By means of relativistic, first principles calculations, we investigate the
microscopic origin of the vanishingly low magnetic anisotropy of Permalloy,
here proposed to be intrinsically related to the local symmetries of the alloy.
It is shown that the local magnetic anisotropy of individual atoms in Permalloy
can be several orders of magnitude larger than that of the bulk sample, and
5-10 times larger than that of elemental Fe or Ni. We, furthermore, show that
locally there are several easy axis directions that are favored, depending on
local composition. The results are discussed in the context of perturbation
theory, applying the relation between magnetic anisotropy and orbital moment.
Permalloy keeps its strong ferromagnetic nature due to the exchange energy to
be larger than the magnetocrystalline anisotropy. Our results shine light on
the magnetic anisotropy of permalloy and of magnetic materials in general, and
in addition enhance the understanding of pump-probe measurements and ultrafast
magnetization dynamics.",1702.05414v1
2017-02-21,Topological staggered field-electric effect with bipartite magnets,"We study the interface physics of bipartite magnetic materials deposited on a
topological insulator. This comprises antiferromagnets as well as ferrimagnets
and ferromagnets with multiple magnetic moments per unit cell. If an energy gap
is induced in the Dirac states on the topological surface, a topological
magnetoelectric effect has been predicted. Here, we show that this effect can
act in opposite directions on the two components of the magnet in certain
parameter regions. Consequently, an electric field will mainly generate a
staggered field rather than a net magnetization in the plane. This is relevant
for the current attempts to detect the magnetoelectric effect experimentally,
as well as for possible applications. We take a field-theoretic approach that
includes the quantum fluctuations of both the Dirac fermions on the topological
surface as well as the fermions in the surface layer of the magnet in an
analytically solvable model. The effective Lagrangian and the Landau-Lifshitz
equation describing the interfacial magnetization dynamics are derived.",1702.06483v2
2017-05-25,Fourier optical processing enables new capabilities in diamond magnetic imaging,"Diamond-based magnetic field sensors have attracted great interest in recent
years. In particular, wide-field magnetic imaging using nitrogen-vacancy (NV)
centers in diamond has been previously demonstrated in condensed matter,
biological, and paleomagnetic applications. Vector magnetic imaging with NV
ensembles typically requires an applied field (>10 G) to separate the
contributions from four crystallographic orientations, hindering studies of
magnetic samples that require measurement in low or independently specified
bias fields. Here we decompose the NV ensemble magnetic resonance spectrum
without such a bias field by modulating the collected light at the microscope's
Fourier plane. In addition to enabling vector magnetic imaging at arbitrarily
low fields, our method can be used to extend the dynamic range at a given bias
field. As demonstrated here, optically-detected diamond magnetometry stands to
benefit from Fourier optical approaches, which have already found widespread
utility in other branches of photonics.",1705.09241v1
2017-07-25,Skyrmion mediated voltage controlled switching of ferromagnets for reliable and energy efficient 2-terminal memory,"We propose a two terminal nanomagnetic memory element based on magnetization
reversal of a perpendicularly magnetized nanomagnet employing a unipolar
voltage pulse that modifies the perpendicular anisotropy of the system. Our
work demonstrates that the presence of Dzyaloshinskii-Moriya Interaction (DMI)
can create alternative route for magnetization reversal that obviates the need
for utilizing precessional magnetization dynamics as well as a bias magnetic
field that are employed in traditional voltage control of magnetic anisotropy
(VCMA) based switching of perpendicular magnetization. We show with extensive
micromagnetic simulation, in the presence of thermal noise, that the proposed
skyrmion mediated VCMA switching mechanism is robust at room temperature
leading to extremely low error switching while also being potentially 1-2
orders of magnitude more energy efficient than state of the art spin transfer
torque (STT) based switching.",1707.07777v1
2017-10-05,Magnetic Skyrmionic Polarons,"We study a two-dimensional electron gas exchanged-coupled to a system of
classical magnetic ions. For large Rashba spin-orbit coupling a single electron
can become self-trapped in a skyrmion spin texture self-induced in the magnetic
ions system. This new quasiparticle carries electrical and topological charge
as well as a large spin, and we named it as magnetic skyrmionic polaron. We
study the range of parameters; temperature, exchange coupling, Rashba coupling
and magnetic field, for which the magnetic skyrmionic polaron is the
fundamental state in the system. The dynamics of this quasiparticle is studied
using the collective coordinate approximation, and we obtain that in presence
of an electric field the new quasiparticle shows, because the chirality of the
skyrmion, a Hall effect. Finally we argue that the magnetic skyrmionic polarons
can be found in large Rashba spin-orbit coupling semiconductors as GeMnTe.",1710.02182v1
2017-10-18,Magnetization plateau as a result of the uniform and gradual electron doping in a coupled spin-electron double-tetrahedral chain,"The double-tetrahedral chain in a longitudinal magnetic field, whose nodal
lattice sites occupied by the localized Ising spins regularly alternate with
triangular plaquettes with the dynamics described by the Hubbard model, is
rigorously investigated. It is demonstrated that the uniform change of electron
concentration controlled by the chemical potential in a combination with the
competition between model parameters and the external magnetic field leads to
the formation of one chiral and seven non-chiral phases at the absolute zero
temperature. Rational plateaux at one-third and one-half of the saturation
magnetization can also be identified in the low-temperature magnetization
curves. On the other hand, the gradual electron doping results in eleven
different ground-state regions which distinguish from each other by the
evolution of the electron distribution during this process. Several
doping-dependent magnetization plateaux are observed in the magnetization
process as a result of the continuous change of electron content in the model.",1710.06822v1
2017-11-03,THz-driven demagnetization with Perpendicular Magnetic Anisotropy: Towards ultrafast ballistic switching,"We study THz-driven spin dynamics in thin CoPt films with perpendicular
magnetic anisotropy. Femtosecond magneto-optical Kerr effect measurements show
that demagnetization amplitude of about $1\%$ can be achieved with a peak THz
electric field of $300$~kV/cm, and a corresponding peak magnetic field of
$0.1$~T. The effect is more than an order of magnitude larger than observed in
samples with easy-plane anisotropy irradiated with the same field strength. We
also utilize finite-element simulations to design a meta-material structure
that can enhance the THz magnetic field by more than an order of magnitude,
over an area of several tens of square micrometers. Magnetic fields exceeding
$1$~Tesla, generated in such meta-materials with the available laser-based THz
sources, are expected to produce full magnetization reversal via ultrafast
ballistic precession driven by the THz radiation. Our results demonstrate the
possibility of table-top ultrafast magnetization reversal induced by THz
radiation.",1711.01234v2
2017-11-13,Guided flows in coronal magnetic flux tubes,"There is evidence for coronal plasma flows to break down into fragments and
to be laminar. We investigate this effect by modeling flows confined along
magnetic channels. We consider a full MHD model of a solar atmosphere box with
a dipole magnetic field. We compare the propagation of a cylindrical flow
perfectly aligned to the field to that of another one with a slight
misalignment. We assume a flow speed of 200 km/s, and an ambient magnetic field
of 30 G. We find that while the aligned flow maintains its cylindrical symmetry
while it travels along the magnetic tube, the misaligned one is rapidly
squashed on one side, becoming laminar and eventually fragmented because of the
interaction and backreaction of the magnetic field. This model could explain an
observation of erupted fragments that fall back as thin and elongated strands
and end up onto the solar surface in a hedge-like configuration, made by the
Atmospheric Imaging Assembly on board the Solar Dynamics Observatory. The
initial alignment of plasma flow plays an important role in determining the
possible laminar structure and fragmentation of flows while they travel along
magnetic channels.",1711.04641v1
2017-11-15,Competition of striped magnetic order and partial Kondo screened state in the Kondo lattice model,"We analyze the magnetically ordered phases in the Kondo lattice model on a
square lattice around quarter filling using the dynamical mean field theory. We
find that close to quarter filling besides the paramagnetic phase, at least
three magnetic phases compete. These phases are a ferromagnetic state, a
partial Kondo screened state, which is a combination of charge order and
magnetic order, and a striped magnetic phase, which is ferromagnetically
ordered in one direction and antiferromagnetically in the other direction. We
analyze the spectral properties of these states and show that the PKS state is
insulating with a flat band above the Fermi energy, while the other states are
metallic. Furthermore, we demonstrate that the energy gain by the Kondo effect
is larger in the PKS state, while the striped magnetic state can gain more
energy from the RKKY interaction. Thus, while the striped magnetic state is
stable at weak coupling, the PKS state becomes stable at intermediate coupling.",1711.05402v1
2018-01-12,Moving magnets in a micromagnetic finite difference framework,"We present a method and an implementation for smooth linear motion in a
finite difference-based micromagnetic simulation code, to be used in simulating
magnetic friction and other phenomena involving moving microscale magnets. Our
aim is to accurately simulate the magnetization dynamics and relative motion of
magnets while retaining high computational speed. To this end, we combine
techniques for fast scalar potential calculation and cubic b-spline
interpolation, parallelizing them on a Graphics Processing Unit (GPU). The
implementation also includes the possibility of explicitly simulating eddy
currents in the case of conducting magnets. We test our implementation by
providing numerical examples of stick-slip motion of thin films pulled by a
spring and the effect of eddy currents on the switching time of magnetic
nanocubes.",1801.04181v2
2018-01-31,How to manipulate magnetic states of antiferromagnets,"Antiferromagnetic materials, which have drawn considerable attention
recently, have fascinating features: they are robust against perturbation,
produce no stray fields, and exhibit ultrafast dynamics. Discerning how to
efficiently manipulate the magnetic state of an antiferromagnet is key to the
development of antiferromagnetic spintronics. In this review, we introduce four
main methods (magnetic, strain, electrical, and optical) to mediate the
magnetic states and elaborate on intrinsic origins of different
antiferromagnetic materials. Magnetic control includes a strong magnetic field,
exchange bias, and field cooling, which are traditional and basic. Strain
control involves the magnetic anisotropy effect or metamagnetic transition.
Electrical control can be divided into two parts, electric field and electric
current, both of which are convenient for practical applications. Optical
control includes thermal and electronic excitation, an inertia-driven
mechanism, and terahertz laser control, with the potential for ultrafast
antiferromagnetic manipulation. This review sheds light on effective usage of
antiferromagnets and provides a new perspective on antiferromagnetic
spintronics.",1801.10332v1
2018-02-07,"Rotation of the photospheric magnetic field through solar cycles 21, 22, 23","Rotation of the large scale solar magnetic field has a great importance for
the understanding of solar dynamic, for the search of longitidinal structure
and for the study of solar-terrestrial relations. 30-year long observations
taken at the Wilcox Solar Observatory (USA) in 21-23 cycles have been analysed
carefully to deduce magnetic field rotation rate at different latitudes in both
hemispheres and its variability in time. The WSO data appear to indicate that
additionally to the differential rotation along the latitudes there are running
waves of fast rotation of the magnetic field. These torsional waves are running
from the poles to the equator with a period of 11 years. The rotation of the
magnetic field (RMF) is almost rigid at latitudes above 55 degrees in both
hemispheres. The rotation rate in the sub-polar regions is slower when the
magnetic field is strong there (during minima of solar activity), and faster
when the magnetic field changes polarity (during maxima of solar activity).",1802.02461v1
2018-04-04,Anomalous nonlinearity of the magnonic edge mode,"Nonlinearity of magneto-dynamics is typically described by a single constant,
$\mathcal{N}$, with positive and negative values indicating repulsion and
attraction of magnons, respectively. In thin magnetic films with easy-plane
magnetic anisotropy, magnon attraction is typically achieved for an in-plane
magnetization. At sufficient stimulus, e.g. via application of spin transfer
torque, the attraction can give rise to self-localized magnetic solitons, such
as spin wave bullets, which shrink as their amplitude increases. In contrast,
for an oblique magnetization above a certain critical angle, the repulsion of
magnons only allows for propagating modes, which expand when pumped more
strongly. Here we demonstrate, both analytically and using micromagnetic
simulations, that such a dichotomic description is inadequate for magnonic edge
modes, which naturally appear in confined magnetic systems. In particular, we
demonstrate that the confinement potential of such modes is nonlinear in nature
and its contribution makes $\mathcal{N}$ non-monotonically dependent on their
amplitude. As a prominent example, edge modes show compression and expansion
for negative and positive $\mathcal{N}$, yet remain localized. In striking
contrast to the extended geometries, edge magnons might also repeal even for an
in-plane magnetization.",1804.01585v1
2018-04-13,Magnetic states in a three-dimensional topological Kondo insulator,"We theoretically study the magnetic phase diagram of a three-dimensional
topological Kondo insulator by means of real-space dynamical mean field theory.
We find that ferromagnetically ordered states become stable upon hole doping.
Besides a wide ferromagnetic phase, we observe surface magnetism close to
half-filling, which corresponds to an A-type antiferromagnetic state. We
further study the impact of the magnetism on the symmetry protected surface
states and find that depending on the surface and the magnetization direction,
surface states are still protected by reflection symmetry present in our model.
The symmetry protected surface states are shifted away by the magnetization
from their original high symmetry momenta in the Brillouin zone. Remarkably,
due to the magnetization, the surface states are deformed, resulting in the
appearance of arcs in the momentum resolved spectrum.",1804.04802v1
2018-06-20,Magnetic Imaging and Microscopy,"The magnetic domain configuration of a system reveals a wealth of information
about the fundamental magnetic properties of that system and can be a critical
factor in the operation of magnetic devices. Not only are the details of the
domain structure strongly governed by materials parameters, but in thin-films
and mesoscopic elements the geometry has an often pivotal effect, providing a
convenient handle to tailor desired domain states. Furthermore a full
understanding of a system requires, in addition, investigation of the dynamic
evolution of the spin-state, which is of particular importance for applications
relying on e.g. the switching of magnetic elements. Here we review some of the
main modern techniques for magnetic imaging, highlighting their respective
advantages and limitations. The methods for imaging domain configurations and
spin structures cover various spatial and temporal resolution scales and
encompass those based on electron and x-ray microscopy as well as scanning
probe techniques. Furthermore, away from the discipline of condensed-matter
physics, magnetic effects are instrumental in a number of techniques for
medical imaging, some key examples of which we also present.",1806.07767v2
2018-07-11,Dynamical coupling of dilute magnetic impurities with quantum spin liquid state in the S = 3/2 dimer compound Ba3ZnRu2O9,"We have investigated the dilute magnetic impurity effect on the magnetic
properties of a quantum spin liquid candidate Ba3ZnRu2O9 and a spin gapped
compound Ba3CaRu2O9. The magnetic ground state of each compound stands against
2% substitution of magnetic impurities for Zn or Ca. We have found that the
magnetic response of these impurities, which behave as paramagnetic spins,
depends on the host materials and the difference of the two manifests itself in
the Weiss temperature, which can hardly be explained by the dilute magnetic
impurities alone in the case of Ba3ZnRu2O9. We consider a contribution from the
Ru5+ ions which would appear only in the substituted Ba3ZnRu2O9 and discuss a
possible physical meaning of the observed Weiss temperature.",1807.04076v1
2018-12-25,Quiet Region Jet by Eruption of Minifilament and Associated Change in Magnetic Flux,"We observe a coronal jet around 22:08 UT on 23 rd March of 2017, in a quiet
region towards the north-east of the solar disk. A minifilament eruption leads
to this jet. We analyze dynamics of the minifilament from its formation until
its eruption. We observe that the minifilament starts forming around 15:50 UT.
Then at a later time, it disappears but again appears and finally erupts as a
blowout jet around 22:08 UT. We study the evolution of photospheric magnetic
field beneath the minifilament. Initially, we observe subsequent increase and
decrease of positive magnetic flux. Prior to the initiation of the eruption,
positive magnetic flux shows a continuous decrease until the minifilament
disappears. The positive magnetic flux increases and decreases due to new
positive flux emergence and cancellation of positive magnetic flux with
negative magnetic flux respectively.",1812.10090v1
2019-02-28,The growth of chiral magnetic instability in a large-scale magnetic field,"The chiral magnetic effect emerges from a miroscopic level, and its
interesting consequences have been discussed in the dynamics of the early
universe, neutron stars and quark-gluon plasma. An instability is caused by
anomalous electric current along magnetic field. We investigate effects of
plasma motion on the instability in terms of linearized perturbation theory. A
magnetic field can inhibit magnetohydrodynamic waves to a remarkable degree and
thereby affects the instability mode. We also found that the unstable mode is
consisted of coupling between Alfven and one of magneto-acoustic waves.
Therefore, the propagation of a mixed Alfven wave driven by magnetic tension is
very important. The direction of unperturbed magnetic field favors the wave
propagation of the instability mode, when Alfven speed exceeds sound speed.",1902.10888v1
2019-03-11,High Field Anomalies of Equilibrium and Ultrafast Magnetism in Rare-Earth-Transition Metal Ferrimagnets,"Magneto-optical spectroscopy in fields up to 30 Tesla reveals anomalies in
the equilibrium and ultrafast magnetic properties of the ferrimagnetic
rare-earth-transition metal alloy TbFeCo. In particular, in the vicinity of the
magnetization compensation temperature, each of the magnetizations of the
antiferromagnetically coupled Tb and FeCo sublattices show triple hysteresis
loops. Contrary to state-of-the-art theory, which explains such loops by sample
inhomogeneities, here we show that they are an intrinsic property of the
rare-earth ferrimagnets. Assuming that the rare-earth ions are paramagnetic and
have a non-zero orbital momentum in the ground state and, therefore, a large
magnetic anisotropy, we are able to reproduce the experimentally observed
behavior in equilibrium. The same theory is also able to describe the
experimentally observed critical slowdown of the spin dynamics in the vicinity
of the magnetization compensation temperature, emphasizing the role played by
the orbital momentum in static and ultrafast magnetism of ferrimagnets.",1903.04293v1
2019-04-02,Controlled self-aggregation of polymer-based nanoparticles employing shear flow and magnetic fields,"Star polymers with magnetically functionalized end groups are presented as a
novel polymeric system whose morphology, self-aggregation, and orientation can
easily be tuned by exposing these macromolecules simultaneously to an external
magnetic field and to shear forces. Our investigations are based on a
specialized simulation technique which faithfully takes into account the
hydrodynamic interactions of the surrounding, Newtonian solvent. We find that
the combination of magnetic field (including both strength and direction) and
shear rate controls the mean number of magnetic clusters, which in turn is
largely responsible for the static and dynamic behavior. While some properties
are similar to comparable non-magnetic star polymers, others exhibit novel
phenomena; examples of the latter include the breakup and reorganization of the
clusters beyond a critical shear rate, and a strong dependence of the
efficiency with which shear rate is translated into whole-body rotations on the
direction of the magnetic field.",1904.01535v1
2019-05-02,Direct Evidence of Torque-mediated Optical Magnetism,"We report experimental evidence of a mechanism that supports and intensifies
induced magnetization at optical frequencies without the intervention of
spin-orbit or spin-spin interactions. Energy-resolved spectra of scattered
light, recorded at moderate intensities (108 W/cm2) and short timescales (<150
fs) in a series of non-magnetic molecular liquids, reveal the signature of
torque dynamics driven jointly by the electric and magnetic field components of
light at the molecular level. While past experiments have recorded radiant
magnetization from magneto-electric interactions of this type, no evidence has
been provided to date of the inelastic librational features expected in
cross-polarized light scattering spectra due to the Lorentz force acting in
combination with optical magnetic torque. Here, torque is shown to account for
inelastic components in the magnetic scattering spectrum under conditions that
produce no such features in electric dipole scattering, in excellent agreement
with quantum theoretical predictions",1905.00552v1
2019-05-10,Sedimentation of a suspension of paramagnetic particles in an external magnetic field,"We investigate the sedimentation of initially packed paramagnetic particles
in presence of a homogeneous external magnetic field, in a Hele-Shaw cell
filled with water. Although the magnetic susceptibility of the particles is
small and the particle-particle induced magnetic interactions are significantly
smaller compared to the gravitational acceleration, we do observe a measurable
reduction of the decompaction rate as the amplitude of the applied magnetic
field is increased. While induced magnetic dipole-dipole interactions between
particles can be either attracting or repulsive depending on the particles
relative alignment, our observations reveal an effective overall enhancement of
the cohesion of the initial pack of particles due to the induced interactions,
very likely promoting internal chain forces in the initial pack of particles.
The influence of the magnetic field on the particles once they disperse after
being decompacted is on the other hand found to remain marginal.",1905.04036v1
2019-06-23,Probing magnetism via spin dynamics in graphene/2D-ferromagnet heterostructures,"The recent discovery of two-dimensional magnetic insulators has generated a
great deal of excitement over their potential for nanoscale manipulation of
spin or magnetism. One intriguing use for these materials is to put them in
contact with graphene, with the goal of making graphene magnetic while
maintaining its unique electronic properties. Such a system could prove useful
in applications such as magnetic memories, or could serve as a host for exotic
states of matter. Proximity to a magnetic insulator will alter the spin
transport properties of graphene, and the strength of this interaction can be
probed with Hanle spin precession experiments. To aid in the analysis of such
experiments, in this work we derive an explicit expression for Hanle spin
precession in graphene interfaced with a ferromagnetic insulator whose
magnetization points perpendicular to the graphene plane. We find that this
interface results in a shifted and asymmetric Hanle response, and we discuss
how this behavior can be used to interpret measurements of spin transport in
these systems.",1906.09568v2
2019-07-12,Extending generalized Debye analysis to long timescale magnetic relaxation,"As the ability to generate magnetic anisotropy in molecular materials
continues to hit new milestones, concerted effort has shifted towards
understanding, and potentially controlling, the mechanisms of magnetic
relaxation across a large time and temperature space. Slow magnetic relaxation
in molecules is highly temperature-, field-, and environment-dependent with the
relevant timescale easily traversing ten orders of magnitude for current
single-molecule magnets (SMM). The prospect of synthetic control over the
nature of (and transition probabilities between) magnetic states make
unraveling the underlying mechanisms an important yet daunting challenge.
Currently, instrumental considerations dictate that the characteristic
relaxation time, $\tau$, is determined by separate methods depending on the
timescale of interest. Static and dynamic probe fields are used for long and
short timescales, respectively. Each method captures a distinct,
non-overlapping time range, and experimental differences lead to the
possibility of fundamentally different meanings for $\tau$ being plotted and
fitted globally as a function of temperature. Herein, we present a method to
generate long-timescale waveforms with standard vibrating sample magnetometry
(VSM) instrumentation, allowing extension of alternating current (AC) magnetic
impedance measurements to SMMs and other superparamagnets with arbitrarily long
relaxation time.",1907.05962v1
2019-07-25,Non-destructive Ultrafast Steering of Magnetic Vortex by Terahertz Pulses,"Electric control of magnetic vortex dynamics in a reproducible way and on an
ultrafast time scale is key element in the quest for efficient spintronic
devices with low-energy consumption. To be useful the control scheme should
ideally be swift, scalable, non-invasive, and resulting in reliable magnetic
switching. Such requirements and in particular the reproducibility of altering
the vortex chirality and/or polarity are not yet met by magnetic vortex
switching via external magnetic fields, spin-polarized currents, spin waves, or
laser pulses. Here we demonstrate a novel packaged-skyrmion mediated vortex
switching process driven by a simple sequence of picosecond electrical field
pulses via magneto-electric interactions. Both the vortex chirality and
polarity show a well-defined reversal behaviour. The unambiguously repeated
switching between four different magnetic vortex states provides an
energy-efficient, highly localized and coherent control method for non-volatile
magnetic vortex-based information storage and handling.",1907.11275v1
2019-08-09,Modeling Spin Dynamics in the Singlet Ground State Garnet Ho3Ga5O12,"Materials containing non-Kramers magnetic ions can show unusual quantum
excitations because of the exact mapping of the two-singlet crystal-field
ground state to a quantum model of Ising spins in a transverse magnetic field.
Here, we model the magnetic excitation spectrum of garnet-structured Ho3Ga5O12,
which has a two-singlet crystal-field ground state. We use a reaction-field
approximation to explain published inelastic neutron-scattering data [Zhou et
al., Phys. Rev. B 78, 140406(R) (2008)] using a three-parameter model
containing the magnetic dipolar interaction, the two-singlet crystal-field
splitting, and the nuclear hyperfine coupling. Our study clarifies the magnetic
Hamiltonian of Ho3Ga5O12, reveals that the nuclear hyperfine interaction drives
magnetic ordering in this system, and provides a framework for quantitative
analysis of magnetic excitation spectra of materials with singlet crystal-field
ground states.",1908.03530v1
2019-08-28,Raman scattering study of lattice and magnetic excitations in CrAs,"We report on the lattice and spin dynamics in the monopnictide CrAs using
polarized Raman scattering. This system exhibits a first-order
magneto-structural phase transition at T_N~ 265 K, below which the magnetic
moments of Cr form an incommensurate double helical magnetic structure while
the unit cell volume abruptly expands. Across the transition, the frequencies
of the fully symmetric Ag phonons strongly renormalize, which along with the
results of first-principle calculations suggest the presence of a sizeable
coupling of the phonons to the magnetic degrees of freedom in this compound. In
addition, we observe two broad modes around 350 and 1700 cm-1 in the magnetic
phase, which we associate with two-magnon Raman scattering. This work provides
one of the few examples of magnetic Raman scattering from a non-collinear
magnet.",1908.10908v2
2019-09-06,Time-dependent Ginzburg-Landau treatment of RF Magnetic Vortices in Superconductors: Vortex-Semiloops in a Spatially Nonuniform Magnetic Field,"We apply time-dependent Ginzburg Landau (TDGL) numerical simulations to study
the finite frequency electrodynamics of superconductors subjected to intense rf
magnetic field. Much recent TDGL work has focused on spatially uniform external
magnetic field and largely ignores the Meissner state screening response of the
superconductor. In this work, we solve the TGDL equations for a spatially
non-uniform magnetic field created by a point magnetic dipole in the vicinity
of a semi-infinite superconductor. A novel two-domain simulation is performed
to accurately capture the effect of the inhomogeneous applied fields and the
resulting screening currents. The creation and dynamics of vortex semiloops
penetrating deep into the superconductor domain is observed and studied, and
the resulting third-harmonic nonlinear response of the sample is calculated.
The effect of point-like defects on vortex semi-loop behaviour is also studied.
This simulation method will assist our understanding of the limits of
superconducting response to intense rf magnetic fields.",1909.02714v2
2019-09-16,Dissipation-induced rotation of suspended ferromagnetic nanoparticles,"We report the precessional rotation of magnetically isotropic ferromagnetic
nanoparticles in a viscous liquid that are subjected to a rotating magnetic
field. In contrast to magnetically anisotropic nanoparticles, the rotation of
which occurs due to coupling between the magnetic and lattice subsystems
through magnetocrystalline anisotropy, the rotation of isotropic nanoparticles
is induced only by magnetic dissipation processes. We propose a theory of this
phenomenon based on a set of equations describing the deterministic magnetic
and rotational dynamics of such particles. Neglecting inertial effects, we
solve these equations analytically, find the magnetization and particle
precessions in the steady state, determine the components of the particle
angular velocity and analyze their dependence on the model parameters. The
possibility of experimental observation of this phenomenon is also discussed.",1909.07450v2
2019-11-26,Magnetic domain wall motion in SrRuO$_3$ thin films,"Influence of substrate miscut on magnetization dynamics in SrRuO$_3$ (SRO)
thin films was studied. Two films were grown on SrTiO$_3$ substrates with high
($\sim1^{\circ}$) and low ($\sim0.1^{\circ}$) miscut angles, respectively. As
expected, high miscut angle leads to suppression of multi-variant growth. By
means of SQUID magnetometry, comparable relaxation effects were observed in
both the multi-variant and the nearly single-variant sample. Differences in the
magnetization reversal process were revealed by magnetic force microscopy. It
showed that the multi-variant growth leads to higher density of defects acting
as pinning or nucleation sites for magnetic domains, which consequently results
in deterioration of magnetic properties. It was demonstrated that the use of
high miscut substrate is important for fabrication of high quality SRO thin
films with low density of crystallographic defects and excellent magnetic
properties.",1911.11655v1
2019-12-08,Magnetic excitations in magnetization plateaux of a frustrated spin ladder,"Magnetization plateaux emerging in quantum spin systems due to spontaneously
breaking of translational symmetry have been reported both theoretically and
experimentally. The broken symmetry can induce reconstruction of elementary
excitations such as Goldstone and Higgs modes, whereas its microscopic
mechanism and reconstructed quasi-particle in magnetization-plateau phases have
remained unclear so far. Here we theoretically study magnetic excitations in
the magnetization-plateau phases of a frustrated spin ladder by using dynamical
density-matrix renormalization-group method. Additionally, analytical
approaches with perturbation theory are performed to obtain intuitive view of
magnetic excitations. Comparison between numerical and analytical results
indicates the presence of a reconstructed quasi-particle originating from
spontaneously broken translational symmetry, which is realized as a collective
mode of spin trimer called trimeron.",1912.03693v2
2020-01-08,Thermal Fluctuations In The Landau-Lifshitz-Bloch Model,"A formulation for thermal noise in the stochastic form of the Landau Lifshitz
Bloch equation used for modeling the magnetization dynamics at elevated
temperatures is presented. The diffusion coefficients for thermal fluctuations
are obtained via the Fokker-Plank equation using the mean field approximation
of the field in defining the free energy. The presented model leads to a mean
magnetization consistent with the equilibrium magnetization for small and large
particles. The distribution of the magnetization magnitude is of a Poisson-like
type rather than the classical Boltzmann distribution. The presented model was
tested by studying the equilibrium magnetization in macrospin particles at high
temperatures. The model is appealing for multi-scale modeling, such as modeling
heat assisted magnetic recording systems and all-optical magnetization reversal",2001.02403v2
2020-01-27,Acoustic ferromagnetic resonance and spin pumping induced by surface acoustic waves,"Voltage induced magnetization dynamics of magnetic thin films is a valuable
tool to study anisotropic fields, exchange couplings, magnetization damping and
spin pumping mechanism. A particularly well established technique is the
ferromagnetic resonance (FMR) generated by the coupling of microwave photons
and magnetization eigenmodes in the GHz range. Here we review the basic
concepts of the so-called acoustic ferromagnetic resonance technique (a-FMR)
induced by the coupling of surface acoustic waves (SAW) and magnetization of
thin films. Interestingly, additional to the benefits of the microwave excited
FMR technique, the coupling between SAW and magnetization also offers fertile
ground to study magnon-phonon and spin rotation couplings. We describe the
in-plane magnetic field angle dependence of the a-FMR by measuring the
absorption / transmission of SAW and the attenuation of SAW in the presence of
rotational motion of the lattice, and show the consequent generation of spin
current by acoustic spin pumping.",2001.09581v1
2020-03-06,Spin-accumulation induced magnetic texture in a metal-insulator bilayer,"We consider the influence of a spin accumulation in a normal metal on the
magnetic statics and dynamics in an adjacent magnetic insulator. In particular,
we focus on arbitary angles between the spin accumulation and the easy-axis of
the magnetic insulator. Based on Landau-Lifshitz-Gilbert phenomenology
supplemented with magnetoelectronic circuit theory, we find that the magnetic
texture twists into a stable configuration that turns out to be described by a
virtual, or image, domain wall configuration, i.e., a domain wall outside the
ferromagnet. We show that even when the spin accumulation is perpendicular to
the anisotropy axis, the magnetic texture develops a component parallel to the
spin accumulation for sufficiently large spin bias. The emergence of this
parallel component gives rise to threshold behavior in the spin Hall
magnetoresistance and nonlocal magnon transport. This threshold can be used to
design novel spintronic and magnonic devices that can be operated without
external magnetic fields.",2003.03226v1
2020-03-09,The impact of magnetic field on the conformations of supracolloidal polymer-like structures with super-paramagnetic monomers,"We investigate the properties of magnetic supracolliodal polymers -- magnetic
filaments (MFs) -- with super-paramagnetic monomers, with and without Van der
Waals (VdW) attraction between them. We employ molecular dynamics (MD)
simulations to elucidate the impact of crosslinking mechanism on the structural
and magnetic response of MFs to an applied external homogeneous magnetic field.
We consider two models: plain crosslinking, which results in a flexible
backbone; and constrained crosslinking, which provides significant stiffens
against bending. We find that for plain crosslinking, even a slight increase of
the central attraction leads to collapsed MF conformations. Structures that
initially exhibit spherical symmetry evolve into cylindrically symmetric ones,
with growing magnetic field strength. Plain crosslinking also allows for
conformational bistability. MFs with constrained crosslinking tend to, instead,
unravel in field. In both crosslinking scenarios, central attraction is able to
hinder low-field magnetic response of MFs, albeit the bistability of plainly
crosslinked MFs manifests itself also in the high-field region.",2003.04200v1
2020-03-25,Two-photon driven magnon-pair resonance as a signature of spin-nematic order,"We theoretically study the nonlinear magnetic resonance driven by intense
laser or electromagnetic wave in a fully polarized frustrated magnet near a
less-visible spin-nematic ordered phase. In general, both magnons and magnon
pairs (two-magnon bound state) appear as the low-energy excitation in the
saturated state of spin-nematic magnets. Their excitation energies are usually
in terahertz (THz) or gigahertz range. Magnon pairs with angular momentum
2$\hbar$ can be excited by the simultaneous absorption of two photons, and such
multi-photon processes occur if the applied THz laser is strong enough. We
compute laser-driven magnetic dynamics of a frustrated four-spin system with
both magnon ($\hbar$) and magnon-pair (2$\hbar$) like excitations which is
analogous to a macroscopic frustrated magnet with a spin nematic phase. We
estimate the required strength of magnetic field of laser for the realization
of two photon absorption, taking into account dissipation effects with the
Lindblad equation. We show that intense THz laser with ac magnetic field of
0.1-1.0 Tesla is enough to observe magnon-pair resonance.",2003.11240v2
2020-03-26,Energy storage in magnetic textures driven by vorticity flow,"An experimentally feasible energy-storage concept is formulated based on
vorticity (hydro)dynamics within an easy-plane insulating magnet. The free
energy, associated with the magnetic winding texture, is built up in a circular
easy-plane magnetic structure by injecting a vorticity flow in the radial
direction. The latter is accomplished by electrically induced spin-transfer
torque, which pumps energy into the magnetic system in proportion to the vortex
flux. The resultant magnetic metastable state with a finite winding number can
be maintained for a long time because the process of its relaxation via phase
slips is exponentially suppressed when the temperature is well below the Curie
temperature. We propose to characterize the vorticity-current interaction
underlying the energy-loading mechanism through its contribution to the
effective electric inductance in the rf response. Our proposal may open an
avenue for naturally powering spintronic circuits and nontraditional
magnet-based neuromorphic networks.",2003.12121v1
2020-03-30,Electronic correlation effects and local magnetic moments in L1$_0$ phase of FeNi,"We study the electronic and magnetic properties of L1$_0$ phase of FeNi, a
perspective rare-earth-free permanent magnet, by using a combination of density
functional and dynamical mean-field theory. Although L1$_0$ FeNi has a slightly
tetragonally distorted fcc lattice, we find that magnetic properties of its
constituent Fe atoms resemble those in pure bcc Fe. In particular, our results
indicate the presence of well-localized magnetic moments on Fe sites, which are
formed due to Hund's exchange. At the same time, magnetism of Ni sites is much
more itinerant. Similarly to pure bcc Fe, the self-energy of Fe $3d$ states is
found to show the non-Fermi-liquid behavior. This can be explained by
peculiarities of density of Fe $3d$ states, which has pronounced peaks near the
Fermi level. Our study of local spin correlation function and momentum
dependence of particle-hole bubble suggests that the magnetic exchange in this
substance is expected to be of RKKY-type, with iron states providing
local-moment contribution, and the states corresponding to nickel sites
(including virtual hopping to iron sites) providing itinerant contribution.",2003.13347v2
2020-07-20,Stacking Domain Wall Magnons in Twisted van der Waals Magnets,"Using bilayer CrI$_3$ as an example, we demonstrate that stacking domain
walls in van der Waals magnets can host one dimensional (1D) magnon channels,
which have lower energies than bulk magnons. Interestingly, some magnon
channels are hidden in magnetically homogeneous background and can only be
inferred with the knowledge of stacking domain walls. Compared to 1D magnons
confined in magnetic domain walls, 1D magnons in stacking domain walls are more
stable against external perturbations. We show that the relaxed moir\'e
superlattices of small-angle twisted bilayer CrI$_3$ is a natural realization
of stacking domain walls and host interconnected moir\'e magnon network. Our
work reveals the importance of stacking domain walls in understanding magnetic
properties of van der Waals magnets, and extends the scope of stacking
engineering to magnetic dynamics.",2007.10398v2
2020-08-05,Magnetism of Nd$_2$O$_3$ single crystals near the Néel temperature,"Single crystals of Nd$_2$O$_3$ were grown and characterized using neutron
scattering and thermodynamic measurements. Nd$_2$O$_3$ has long-range
antiferromagnetic order below $T_{\rm N}$ = 0.55 K and specific heat
measurements have demonstrated that a significant amount of the magnetic
entropy is released above $T_{\rm N}$. Inelastic neutron scattering experiments
reveal a magnetic mode(s) with little dispersion peaked at $\approx$ 0.37 meV
that is of greatest intensity below $T_{\rm N}$ but persists above 2$T_{\rm
N}$. This persistence of dynamic correlations is likely related to frustrated
interactions associated with the nearly-ideal stacked triangular lattice
geometry of $J_{\textrm{eff}}$ = 1/2 spins on Nd$^{3+}$ ions. The magnetization
is observed to be strongly anisotropic at all temperatures due to crystal field
effects, with easy-plane anisotropy observed. A non-compensated magnetic
structure is inferred from the temperature-dependence of the magnetization when
a magnetic field of sufficient strength is applied within the basal plane near
$T_{\rm N}$, and the evolution of the long-range order is summarized in a
temperature-field phase diagram.",2008.02298v1
2020-08-14,Inefficient magnetic-field amplification in supersonic laser-plasma turbulence,"We report a laser-plasma experiment that was carried out at the LMJ-PETAL
facility and realized the first magnetized, turbulent, supersonic plasma with a
large magnetic Reynolds number ($\mathrm{Rm} \approx 45$) in the laboratory.
Initial seed magnetic fields were amplified, but only moderately so, and did
not become dynamically significant. A notable absence of magnetic energy at
scales smaller than the outer scale of the turbulent cascade was also observed.
Our results support the notion that moderately supersonic,
low-magnetic-Prandtl-number plasma turbulence is inefficient at amplifying
magnetic fields.",2008.06594v1
2020-09-07,Magnetic vortex liquid and anomalous thermal Hall conductivity in frustrated magnets with bond-dependent interactions,"Recently, the observation of large thermal Hall conductivities in correlated
insulators with no apparent broken symmetry have generated immense interest and
debates on the underlying ground states. Here, considering frustrated magnets
with bond-dependent interactions, which are realized in the so-called Kitaev
materials, we theoretically demonstrate that a large thermal Hall conductivity
can originate from a classical ground state without any magnetic order. We
discover a novel liquid state of magnetic vortices, which are inhomogeneous
spin textures embedded in the background of polarized spins, under out-of-plane
magnetic fields. In the classical regime, different configurations of vortices
form a degenerate manifold. We study the static and dynamical properties of the
magnetic vortex liquid state at zero and finite temperatures. In particular, we
show that the spin excitation spectrum resembles a continuum of nearly flat
Chern bands, which ultimately leads to a large thermal Hall conductivity.
Possible connections to experiments are discussed.",2009.03332v1
2020-09-24,Magnetic anisotropy and exchange paths for octa- and tetrahedrally coordinated Mn$^{2+}$ ions in the honeycomb multiferroic Mn$_2$Mo$_3$O$_8$,"We investigated the static and dynamic magnetic properties of the polar
ferrimagnet Mn$_2$Mo$_3$O$_8$ in three magnetically ordered phases via
magnetization, magnetic torque, and THz absorption spectroscopy measurements.
The observed magnetic field dependence of the spin-wave resonances, including
Brillouin zone-center and zone-boundary excitations, magnetization, and torque,
are well described by an extended two-sublattice antiferromagnetic classical
mean-field model. In this orbitally quenched system, the competing weak
easy-plane and easy-axis single-ion anisotropies of the two crystallographic
sites are determined from the model and assigned to the tetra- and octahedral
sites, respectively, by ab initio calculations.",2009.11683v1
2020-10-15,Magnetic Manifold Hamiltonian Monte Carlo,"Markov chain Monte Carlo (MCMC) algorithms offer various strategies for
sampling; the Hamiltonian Monte Carlo (HMC) family of samplers are MCMC
algorithms which often exhibit improved mixing properties. The recently
introduced magnetic HMC, a generalization of HMC motivated by the physics of
particles influenced by magnetic field forces, has been demonstrated to improve
the performance of HMC. In many applications, one wishes to sample from a
distribution restricted to a constrained set, often manifested as an embedded
manifold (for example, the surface of a sphere). We introduce magnetic manifold
HMC, an HMC algorithm on embedded manifolds motivated by the physics of
particles constrained to a manifold and moving under magnetic field forces. We
discuss the theoretical properties of magnetic Hamiltonian dynamics on
manifolds, and introduce a reversible and symplectic integrator for the HMC
updates. We demonstrate that magnetic manifold HMC produces favorable sampling
behaviors relative to the canonical variant of manifold-constrained HMC.",2010.07753v1
2020-11-13,Magnetic anisotropy of individual maghemite mesocrystals,"Interest in creating magnetic metamaterials has led to methods for growing
superstructures of magnetic nanoparticles. Mesoscopic crystals of maghemite
($\gamma\text{-Fe}_2\text{O}_3$) nanoparticles can be arranged into highly
ordered body-centered tetragonal lattices of up to a few micrometers. Although
measurements on disordered ensembles have been carried out, determining the
magnetic properties of individual mesoscopic crystals is challenging due to
their small total magnetic moment. Here, we overcome these challenges by
utilizing sensitive dynamic cantilever magnetometry to study individual
micrometer-sized $\gamma\text{-Fe}_2\text{O}_3$ mesocrystals. These
measurements reveal an unambiguous cubic anisotropy, resulting from the
crystalline anisotropy of the constituent maghemite nanoparticles and their
alignment within the mesoscopic lattice. The signatures of anisotropy and its
orgins come to light because we combine the self-assembly of highly ordered
mesocrystals with the ability to resolve their individual magnetism. This
combination is promising for future studies of the magnetic anisotropy of other
nanoparticles, which are too small to investigate individually.",2011.06885v1
2020-11-17,Improving magnetic nanothermometry accuracy through mixing-frequency excitation,"In this study, we proposed a temperature model of magnetic nanoparticle
relaxation and a phase measurement method under a mixing-frequency excitation
field, which can improve the temperature accuracy of magnetic nanothermometry.
According to the Debye-based magnetization model for magnetic nanoparticles,
the phases at the mixing frequencies are used to solve the relaxation phase
delay to the magnetic field with the higher frequency. The method could improve
the signal-to-noise ratio of the magnetic response signal, and also weaken the
phase shift of the detection coils caused by temperature changes. Experimental
results show that the method can achieve static temperature measurement error
less than 0.1K and dynamic temperature measurement error less than 0.2K.",2011.08532v1
2020-12-23,Electric manipulation of domain walls in magnetic Weyl semimetals via the axial anomaly,"We show how the axial (chiral) anomaly induces a spin torque on the
magnetization in magnetic Weyl semimetals. The anomaly produces an imbalance in
left- and right-handed chirality carriers when non-orthogonal electric and
magnetic fields are applied. Such imbalance generates a spin density which
exerts a torque on the magnetization, the strength of which can be controlled
by the intensity of the applied electric field. We show how this results in an
electric control of the chirality of domain walls, as well as in an improvement
of the domain wall dynamics, by delaying the onset of the Walker breakdown. The
measurement of the electric field mediated changes in the domain wall chirality
would constitute a direct proof of the axial anomaly. Additionally, we show how
quantum fluctuations of electronic Fermi arc states bound to the domain wall
naturally induce an effective magnetic anisotropy, allowing for high domain
wall velocities even if the intrinsic anisotropy of the magnetic Weyl semimetal
is small.",2012.12785v2
2021-02-19,Collective modes of polarizable holographic media in magnetic fields,"We consider a neutral holographic plasma with dynamical electromagnetic
interactions in a finite external magnetic field. The Coulomb interactions are
introduced via mixed boundary conditions for the Maxwell gauge field. The
collective modes at finite wave-vector are analyzed in detail and compared to
the magneto-hydrodynamics results valid only at small magnetic fields.
Surprisingly, at large magnetic field, we observe the appearance of two
plasmon-like modes whose corresponding effective plasma frequency grows with
the magnetic field and is not supported by any background charge density.
Finally, we identify a mode collision which allows us to study the radius of
convergence of the linearized hydrodynamics expansion as a function of the
external magnetic field. We find that the radius of convergence in momentum
space, related to the diffusive transverse electromagnetic mode, increases
quadratically with the strength of the magnetic field.",2102.09969v2
2021-05-25,Dynamics of particles with cubic magnetic anisotropy in a viscous liquid,"The specific absorption rate (SAR) of a dilute assembly of spherical iron
nanoparticles with cubic anisotropy distributed in a viscous liquid is
calculated using the solution of stochastic Landau - Lifshitz equation for unit
magnetization vector and stochastic equations for multiple particle directors
that specify the spatial orientation of the nanoparticle in a liquid. The
viscous and magnetic magnetization reversal modes of particles are revealed at
low and sufficiently high amplitudes of alternating magnetic field,
respectively. The SAR of iron nanoparticle assembly is shown to exceed
significantly that of iron oxide nanoparticles with uniaxial anisotropy at the
same amplitudes and frequencies of applied magnetic field. The linear response
theory is shown to be valid only at small magnetic field amplitudes, H0 < 50 -
70 Oe.",2105.11916v1
2021-06-17,Quantum error as an emergent magnetic field,"We investigate the effect of quantum errors on a monitored Brownian
Sachdev-Ye-Kitaev (SYK) model featuring a measurement-induced phase transition
that can be understood as a symmetry-breaking transition of an effective $Z_4$
magnet in the replica space. The errors describe the loss of information about
the measurement outcomes and are applied during the non-unitary evolution or at
the end of the evolution. In the former case, we find that this error can be
mapped to an emergent magnetic field in the $Z_4$ magnet, and as a consequence,
the symmetry is explicitly broken independent of the measurement rate. R\'enyi
entropies computed by twisting boundary conditions now generate domain walls
even in the would-be symmetric phase at a high measurement rate. The entropy is
therefore volume-law irrespective of the measurement rate. In the latter case,
the error-induced magnetic field only exists near the boundary of the magnet.
Varying the magnetic field leads to a pinning transition of domain walls,
corresponding to error threshold of the quantum code prepared by the
non-unitary SYK dynamics.",2106.09635v1
2021-07-14,Magnetic correlations in triangular antiferromagnet FeGa$_2$S$_4$,"The crystal structure and magnetic correlations in triangular antiferromagnet
FeGa$_2$S$_4$ are studied by x-ray diffraction, magnetic susceptibility,
neutron diffraction and neutron inelastic scattering. We report significant
mixing at the cation sites and disentangle magnetic properties dominated by
major and minor magnetic sites. The magnetic short-range correlations at 0.77
\AA$^{-1}$ correspond to the major sites and being static at base temperature
they evolve into dynamic correlations around 30 - 50 K. The minor sites
contribute to the magnetic peak at 0.6 \AA$^{-1}$, which vanishes at 5.5 K. Our
analytical studies of triangular lattice models with bilinear and biquadratic
terms provide the ratios between exchanges for the proposed ordering vectors.
The modelling of the inelastic neutron spectrum within linear spin wave theory
results in the set of exchange couplings $J_1=1.7$\,meV, $J_2=0.9$\,meV,
$J_3=0.8$\,meV for the bilinear Heisenberg Hamiltonian. However, not all
features of the excitation spectrum are explained with this model.",2107.06904v1
2021-07-27,Production of a chiral magnetic anomaly with emerging turbulence and mean-field dynamo action,"In relativistic magnetized plasmas, asymmetry in the number densities of
left- and right-handed fermions, i.e., a non-zero chiral chemical potential
mu_5, leads to an electric current along the magnetic field. This causes a
chiral dynamo instability for a uniform mu_5, but our simulations reveal a
dynamo even for fluctuating mu_5 with zero mean. It produces
magnetically-dominated turbulence and generates mean magnetic fields via the
magnetic alpha effect. Eventually, a universal scale-invariant k^(-1) spectrum
of mu_5 and a k^(-3) magnetic spectrum are formed independently of the initial
condition.",2107.12945v3
2021-09-22,Spin precession of binary neutron stars with magnetic dipole moments,"Spin precession equations including the spin-orbit (SO), spin-spin (SS),
quadrupole-monopole (QM) and magnetic dipole-magnetic dipole (DD) leading-order
interactions are derived for compact binary systems in order to investigate the
DD contribution in the orbit-averaged spin precession equations for binary
neutron star systems neglecting the gravitational radiation-reaction effect. It
is known that the magnitudes of spins are not conserved quantities due to the
DD interaction. We give a simple analytical description for the pure DD
interaction making the magnitudes of spins almost constant by neglecting the
SO, SS and QM contributions. We also demonstrate the evolutions of the relative
angles of spins and magnetic dipoles with the help of numeric simulations
including all contributions (SO, SS, QM and DD) and introduce a dimensionless
magnetic dipole parameter to characterize the strength of magnetic fields for
some realistic neutron star binaries. We find that for realistic configurations
the strong magnetic fields of neutron stars can modify the spin dynamics over
long periods of time.",2109.10722v1
2021-11-29,What Seismic Minimum Reveals About Solar Magnetism Below the Surface?,"The Sun's magnetic field varies in multiple time scales. Observations show
that the minimum between cycles 24 and 25 was the second consecutive minimum
which was deeper and wider than several earlier minima. Since the active
regions observed at the Sun's surface are manifestations of the magnetic field
generated in the interior, it is crucial to investigate/understand the dynamics
below the surface. In this context, we report, by probing the solar interior
with helioseismic techniques applied to long-term oscillations data from the
Global Oscillation Network Group (GONG), that the seismic minima in deeper
layers have been occurring about a year earlier than that at the surface for
the last two consecutive solar cycles. Our findings also demonstrate a decrease
in strong magnetic fields at the base of the convection zone, the primary
driver of the surface magnetic activity. We conclude that the magnetic fields
located in the core and near-surface shear layers, in addition to the
tachocline fields, play an important role in modifying the oscillation
frequencies. This provides evidence that further strengthens the existence of a
relic magnetic field in the Sun's core since its formation.",2111.14323v1
2021-12-02,Stability of plasmas through magnetic helicity,"Magnetic helicity, and more broadly magnetic field line topology, imposes
constraints on the plasma dynamics. Helically interlocked magnetic rings are
harder to bring into a topologically non-trivial state than two rings that are
not linked. This particular restriction has the consequence that helical
plasmas exhibit increased stability in laboratory devices, in the Sun and in
the intergalactic medium. Here we discuss how a magnetic field is stabilizing
the plasma and preventing it from disruption by the presence of magnetic
helicity. We present observational results, numerical experiments and
analytical results that illustrate how helical magnetic fields strongly
contribute to the long-term stability of some plasmas. We discuss several
cases, such as that of solar corona, tokamaks, the galactic and extragalactic
medium, with a special emphasis on extragalactic bubbles.",2112.01193v1
2021-12-07,Gradual charge order melting in Bi0.5Ca0.5MnO3 induced by ultrahigh magnetic field,"We have investigated the magnetic properties of Bi0.5Ca0.5MnO3 under
ultrahigh magnetic fields by magnetization measurements. This compound shows
the metamagnetic transition which indicates the magnetic field induced charge
order melting at the room temperature T = 293 K. We reveled that the
temperature dependence of the critical field, where the highest critical field
is higher than 100 T. At low temperatures, it has been found that the
metamagnetic transitions become broad and vanish in the field ascending process
below 70 K. On the other hand, in the field descending process, a clear
metamagnetic transition is observed even below 70 K. This unusual behavior can
be understood as the gradual charge order melting by magnetic field, and its
simultaneous reformation. A coexistence of multiple order parameters can
account for the observed characteristic dynamics of the first order phase
transition. Finally, we constructed the B-T phase diagram with the unusual
critical feature at ultrahigh magnetic fields.",2112.03507v1
2022-01-31,Anisotropic magnetized neutron star,"As we know, the effect of strong magnetic field causes the anisotropy for the
magnetized compact objects. Therefore, in this paper, we have studied the
structure properties of anisotropic case of magnetized neutron star. We have
derived the equation of state (EoS) of neutron star matter for two forms of
magnetic fields, one uniform and one density dependent. We have solved the
generalized Tolman Oppenheimer Volkoff equations to examine the maximum mass
and corresponding radius, Schwarzschild radius, graviitational redshift,
Kretschmann scalar, and Buchdahl theorem for this system. It was shown that the
maximum mass and radius of neutron star are increasing functions of the
magnetic field. Also redshift, strength of gravity, and Kretschmann scalar
increase as the magnetic field increases. In addition, the dynamical stability
of anisotrop neutron star has been investigated, and finally a comparison with
the empirical results has been made.",2202.01207v1
2022-02-08,"Electronic, Magnetic and Vibrational Properties of Single Layer Aluminum Oxide","The structural, magnetic, vibrational and electronic properties of single
layer aluminum oxide (AlO2) are investigated by performing state-of-the-art
first-principles calculations. Total energy optimization and phonon
calculations reveal that aluminum oxide forms a distorted octahedral structure
(1T'-AlO2) in its single layer limit. It is also shown that surfaces of
1T'-AlO2 display magnetic behavior originating from the O atoms. While the
ferromagnetic (FM) state is the most favorable magnetic order for 1T'-AlO2,
transformation to a dynamically stable antiferromagnetic (AFM) state upon a
slight distortion in the crystal structure is also possible. It is also shown
that Raman activities (350-400 cm^-1) obtained from the vibrational spectrum
can be utilized to distinguish the possible magnetic phases of the crystal
structure. Electronically, both FM and the AFM phases are semiconductors with
an indirect band gap and they can form a type-III vdW heterojunction with
graphene-like ultra-thin materials. Moreover, it is predicted that presence of
oxygen defects that inevitably occur during synthesis and production do not
alter the magnetic state, even at high vacancy density. Apparently, ultra-thin
1T'-AlO2 with its stable crystal structure, semiconducting nature and robust
magnetic state is a quite promising material for nanoscale device applications.",2202.04144v1
2022-02-10,Field-driven skyrmion motion through velocity equipartition between skyrmions and a domain wall,"Magnetic skyrmions, as a whirling spin texture with axisymmetry, cannot be
propelled by a uniform magnetic field. Therefore, reported skyrmion motions
have been induced using other sorts of stimuli; typically, electric currents in
magnetic metals as well as spin waves, thermal gradient, and field gradient
have manifested their ability to drive skyrmion motion. Here, we demonstrate,
through micromagnetic simulations and analytically, that magnetic skyrmions can
be displaced by a uniform perpendicular magnetic field via a domain-wall (DW)
mediator. For a fixed field strength, the velocity of a skyrmion train evolves
in terms of 1 / (1 + Ns) with Ns denoting the number of skyrmions. Based on
Thiele's model, we reproduce the velocity-Ns relation first identified from
numerical results and reveal that the skyrmion-DW and inter-skyrmion repulsions
offer the direct driving force for skyrmion motion. This study underlines the
role of spin textures' interaction in skyrmion dynamics, and opens an
alternative route for skyrmion manipulation especially relevant to insulating
magnets. Given the correspondence between magnetism and electricity, we
anticipate that the scheme should also work for polar skyrmions in
ferroelectrics.",2202.05149v1
2022-02-15,Notes about collapse in magnetohydrodynamics,"We discuss a problem about magnetic collapse as a possible process for
singularity formation of the magnetic field in a finite time within ideal
magneto-hydrodynamics for incompressible fluids. This process is very important
from the point of view of various astrophysical applications, in particular, as
a mechanism of magnetic filaments formation in the convective zone of the Sun.
The collapse possibility is connected with compressibility of continuously
distributed magnetic field lines. A well-known example of the formation of
magnetic filaments in the kinematic dynamo approximation with a given velocity
field, first considered by Parker in 1963, rather indicates that the increase
in the magnetic field is exponential in time. In the case of the kinematic
approximation for the induction equation, the magnetic filaments formation is
shown to occur in areas with a hyperbolic velocity profile.",2202.08121v1
2022-04-09,Spectra of magnetic turbulence in a relativistic plasma,"We present a phenomenological and numerical study of strong Alfv\'enic
turbulence in a magnetically dominated collisionless relativistic plasma with a
strong background magnetic field. In contrast with the non-relativistic case,
the energy in such turbulence is contained in magnetic and electric
fluctuations. We argue that such turbulence is analogous to turbulence in a
strongly magnetized non-relativistic plasma in the regime of broken
quasi-neutrality. Our 2D particle-in-cell numerical simulations of turbulence
in a relativistic pair plasma find that the spectrum of the total energy has
the scaling $k^{-3/2}$, while the difference between the magnetic and electric
energies, the so-called residual energy, has the scaling $k^{-2.4}$. The
electric and magnetic fluctuations at scale $\ell$ exhibit dynamic alignment
with the alignment-angle scaling close to $\cos\phi_\ell\propto \ell^{1/4}$. At
scales smaller than the (relativistic) plasma inertial scale, the energy
spectrum of relativistic inertial Alfv\'en turbulence steepens to $k^{-3.5}$.",2204.04530v3
2022-04-29,Magnetic domains in 2D moiré lattices with square and hexagonal symmetry,"We report the persistence of magnetic domains lying in moir\'e patterns with
square and hexagonal symmetries. Our investigation is based on the dynamical
description of two magnetic domains represented by a two species bosonic
mixture of $^{87}$Rb ultracold atoms, being each specie initially localized in
the left and right halves of a moir\'e lattice defined by a specific angle
$\theta$. To demonstrate the persistence of such initial domains, we follow the
time evolution of the superfluid spin texture, and in particular, the
magnetization on each halve. The two-component superfluid, confined in the
moir\'e pattern plus a harmonic trap, was described through the time dependent
Gross-Pitaevskii coupled equations for moir\'e lattices having $90 \times 90$
sites. Results showed the existence of rotation-angle-dependent structures for
which the initial magnetic domain is preserved for both, square and hexagonal
moir\'e patterns; above $\theta >10^\circ$ the initial magnetic domain is never
destroyed. Stationary magnetic states for a single component Bose condensate
allowed us to identify the lattice parameter associated with moir\'e crystals
that emerge for twisting angles belonging to the intervals $\theta \in
\left(0^\circ ,30^\circ \right)$ and $\theta \in \left(0^\circ ,45^\circ
\right)$ for hexagonal and square geometries respectively.",2204.14200v1
2022-05-17,Restriction on the form of quark anomalous magnetic moment from lattice QCD results,"The quark anomalous magnetic moment (AMM) is dynamically generated through
the spontaneous chiral symmetry breaking. It has been revealed that even though
its exact form is still unknown, the quark AMM is essential to explore quark
matter properties and QCD phase structure under external magnetic fields. In
this study, we take three different forms of the quark AMM and investigate its
influence on the chiral phase transition under magnetic field. In general, a
negative quark AMM plays the role as magnetic catalyzer and a positive quark
AMM plays the role of magnetic inhibition. It is found that a constant quark
AMM drives an unexpected 1st order chiral phase transition; a quark AMM
proportional to the chiral condensate gives a flip of the sign on the chiral
condensate; and a quark AMM proportional to the square of chiral condensate can
produce results of chiral condensate as functions of the temperature and the
magnetic field in good agreement with the lattice result.",2205.08169v1
2022-07-01,Whistler Waves As a Signature of Converging Magnetic Holes in Space Plasmas,"Magnetic holes are plasma structures that trap a large number of particles in
a magnetic field that is weaker than the field in its surroundings. The
unprecedented high time-resolution observations by NASA's Magnetospheric
Multi-Scale (MMS) mission enable us to study the particle dynamics in magnetic
holes in the Earth's magnetosheath in great detail. We reveal the local
generation mechanism of whistler waves by a combination of Landau-resonant and
cyclotron-resonant wave-particle interactions of electrons in response to the
large-scale evolution of a magnetic hole. As the magnetic hole converges, a
pair of counter-streaming electron beams form near the hole's center as a
consequence of the combined action of betatron and Fermi effects. The beams
trigger the generation of slightly-oblique whistler waves. Our conceptual
prediction is supported by a remarkable agreement between our observations and
numerical predictions from the Arbitrary Linear Plasma Solver (ALPS). Our study
shows that wave-particle interactions are fundamental to the evolution of
magnetic holes in space and astrophysical plasmas.",2207.00273v1
2022-09-05,Generation and routing of nanoscale droplet solitons without compensation of magnetic damping,"Magnetic droplet soliton is a localized dynamic spin state which can serve as
a nanoscale information carrier and nonlinear oscillator. The present opinion
is that the formation of droplet solitons requires the compensation of magnetic
damping by a torque created by a spin-polarized electric current or pure spin
current. Here we demonstrate theoretically that nanoscale droplet solitons can
be generated and routed in ferromagnetic nanostructures with voltage-controlled
magnetic anisotropy in the presence of uncompensated magnetic damping.
Performing micromagnetic simulations for the MgO/Fe/MgO trilayer with almost
perpendicular-to-plane magnetization, we reveal the formation of the droplet
soliton under a nanoscale gate electrode subjected to a sub-nanosecond voltage
pulse. The soliton lives up to 50 ns at room temperature and can propagate over
micrometer distances in a ferromagnetic waveguide due to nonzero gradient of
the demagnetizing field. Furthermore, we show that an electrical routing of the
soliton to different outputs of a spintronic device can be realized with the
aid of an additional semiconducting nanostripe electrode creating controllable
gradient of the perpendicular magnetic anisotropy.",2209.01893v1
2022-09-19,Dissipative magnetic structures and scales in small-scale dynamos,"Small-scale dynamos play important roles in modern astrophysics, especially
on Galactic and extragalactic scales. Owing to dynamo action, purely
hydrodynamic Kolmogorov turbulence hardly exists and is often replaced by
hydromagnetic turbulence. Understanding the size of dissipative magnetic
structures is important in estimating the time scale of Galactic scintillation
and other observational and theoretical aspects of interstellar and
intergalactic small-scale dynamos. Here we show that, during the kinematic
phase of the small-scale dynamo, the cutoff wavenumber of the magnetic energy
spectra scales as expected for large magnetic Prandtl numbers, but continues in
the same way also for moderately small values - contrary to what is expected.
For a critical magnetic Prandtl number of about 0.3, the dissipative and
resistive cutoffs are found to occur at the same wavenumber. In the nonlinearly
saturated regime, the critical magnetic Prandtl number becomes unity. The
cutoff scale now has a shallower scaling with magnetic Prandtl number below a
value of about three, and a steeper one otherwise compared to the kinematic
regime.",2209.08717v2
2023-01-01,Magnetized Thick Disks around Boson Stars,"The effects of magnetic fields on accretion disks around compact objects are
of high importance in the study of their general properties and dynamics. Here
we analyze the influence of magnetic fields on thick accretion disks around
rotating boson stars. We assume a uniform constant specific angular momentum
distribution and a polytropic equation of state. The purely hydrodynamical
thick disk solutions are extended to magnetized solutions by adding a toroidal
magnetic field and then analyzed in terms of a magnetization parameter. We
consider one-centered solutions as well as two-centered solutions and focus on
retrograde tori, since they are more distinctive due to their unique
properties. Our computed solutions indicate that strong magnetic fields
influence the characteristics of thick disks around rotating boson stars and
possibly affect their unique features.",2301.00449v2
2023-02-15,Unique Compact Representation of Magnetic Fields using Truncated Solid Harmonic Expansions,"Precise knowledge of magnetic fields is crucial in many medical imaging
applications like magnetic resonance imaging or magnetic particle imaging (MPI)
as they are the foundation of these imaging systems. For the investigation of
the influence of field imperfections on imaging, a compact and unique
representation of the magnetic fields using real solid spherical harmonics,
which can be obtained by measuring a few points of the magnetic field only, is
of great assistance. In this manuscript, we review real solid harmonic
expansions as a general solution of Laplace's equation including an efficient
calculation of their coefficients using spherical t-designs. We also provide a
method to shift the reference point of an expansion by calculating the
coefficients of the shifted expansion from the initial ones. These methods are
used to obtain the magnetic fields of an MPI system. Here, the field-free-point
of the spatial encoding field serves as unique expansion point. Lastly, we
quantify the severity of the distortions of the static and dynamic fields in
MPI by analyzing the expansion coefficients.",2302.07591v1
2023-03-21,Revealing Emergent Magnetic Charge in an Antiferromagnet with Diamond Quantum Magnetometry,"Whirling topological textures play a key role in exotic phases of magnetic
materials and offer promise for logic and memory applications. In
antiferromagnets, these textures exhibit enhanced stability and faster dynamics
with respect to ferromagnetic counterparts, but they are also difficult to
study due to their vanishing net magnetic moment. One technique that meets the
demand of highly sensitive vectorial magnetic field sensing with negligible
backaction is diamond quantum magnetometry. Here, we show that the archetypal
antiferromagnet, hematite, hosts a rich tapestry of monopolar, dipolar and
quadrupolar emergent magnetic charge distributions. The direct readout of the
previously inaccessible vorticity of an antiferromagnetic spin texture provides
the crucial connection to its magnetic charge through a duality relation. Our
work defines a novel paradigmatic class of magnetic systems to explore
two-dimensional monopolar physics, and highlights the transformative role that
diamond quantum magnetometry could play in exploring emergent phenomena in
quantum materials.",2303.12125v1
2023-03-27,Switching the Magnetization in Quantum Antiferromagnets,"The orientation of the order parameter of quantum magnets can be used to
store information in a dense and efficient way. Switching this order parameter
corresponds to writing data. To understand how this can be done, we study a
precessional reorientation of the sublattice magnetization in an (an)isotropic
quantum antiferromagnet induced by an applied magnetic field. We use a
description including the leading quantum and thermal fluctuations, namely
Schwinger boson mean-field theory, because this theory allows us to describe
both ordered phases and the phases in between them, as is crucial for
switching. An activation energy has to be overcome requiring a minimum applied
field $h_\text{t}$ which is given essentially by the spin gap. It can be
reduced significantly for temperatures approaching the N\'eel temperature
facilitating switching. The time required for switching diverges when the field
approaches $h_\text{t}$ which is the signature of an inertia in the
magnetization dynamics. The temporal evolution of the magnetization and of the
energy reveals signs of dephasing. The switched state has lost a part of its
coherence because the magnetic modes do not evolve in phase.",2303.15398v1
2023-04-27,Magnetic Field Line Separation by Random Ballistic Decorrelation in Transverse Magnetic Turbulence,"The statistics of the magnetic field line separation provide insight into how
a bundle of field lines spreads out and the dispersion of non-thermal particles
in a turbulent environment, which underlies various astrophysical phenomena.
Its diffusive character depends on the distance along the field line, the
initial separation, and the characteristics of the magnetic turbulence. This
work considers the separation of two magnetic field lines in general transverse
turbulence in terms of the magnetic power spectrum in three-dimensional
wavenumber space. We apply non-perturbative methods using Corrsin's hypothesis
and assume random ballistic decorrelation to calculate the ensemble average
field line separation for general transverse magnetic turbulence. For 2D+slab
power spectra, our analytic formulae and computer simulations give similar
results, especially at low slab fraction. Our analytical expression also
demonstrates several features of field line separation that are verified by
computer simulations.",2304.14067v1
2023-05-13,Dual Nature of Magnetism Driven by Momentum Dependent f-d Kondo Hybridization,"Intricate nature of magnetism in uranium-based Kondo lattices is a
consequence of correlations between U-5$f$ and conduction electrons. Using
linearized quasiparticle self-consistent GW plus dynamical mean-field theory,
we demonstrate a crossover from incoherent to coherent $f$-$d$ Kondo cloud in
the paramagnetic phase of UTe$_2$ with reduced volumes, USbTe and USbSe. As the
transition occurs, we observe an augmented $f$-$d$ coherence and Pauli-like
magnetic susceptibility, with a substantial frozen magnetic moment of U-5$f$
persisting. We show that momentum dependent $f$-$d$ hybridization is
responsible for the magnetic moments arising from the renormalized $f$
electrons' van Hove singularity. Our findings provide a unique perspective to
explain the dual nature of magnetism and the long-range magnetic ordering
induced by pressure in UTe$_2$.",2305.08003v1
2023-06-14,An N-loop potential energy model for levitated mm-scale magnets in cm-scale superconducting coaxial microwave resonators,"The levitation of a macroscopic object within a superconducting resonator
provides a unique and novel platform to study optomechanics, quantum
information, and gravitational wave detection. Existing mirror-method and
single-loop models for calculating magnet levitation are insufficient for
predicting the position and motion of the levitated magnet. If the
cavity-magnet interaction is modeled using a large number of smaller surface
current loops, one can quantitatively model the dynamics of the levitation of
the magnet within the cavity. The magnet's most-likely position and orientation
can be predicted for non-trivial cavity geometries and cavity orientations.
Knowing the potential energy landscape within the cavity configuration also
provides a means to estimate the resonant mechanical frequencies at which the
levitated magnet vibrates, and enables tailoring the cavity design for specific
outcomes.",2306.08662v1
2023-10-19,Micromagnetic Simulation of Three-dimensional Nanoarchitectures,"The thesis discusses micromagnetic simulation studies on high-frequency
magnetic dynamics in three-dimensional ferromagnetic nanoarchitectures made of
interconnected magnetic nanowire networks. Such artificial magnetic materials
with nanoscale features have recently emerged as a vivid topic of research, as
their geometry has a decisive impact on their magnetic properties. By studying
their static magnetization structure, we find that these systems display a
behavior analogous to that of 3D artificial spin ice lattices, with frustrated
interactions and the emergence of monopole-like defect structures at the wires'
intersection points. Our simulations reveal a high activity of these defect
sites in the magnonic high-frequency spectrum. We study various 3D
nanoarchitectures and show that their geometry and magnetization state results
in characteristic high-frequency signatures. Controlling these features could
open new pathways for magnonics research and reprogrammable magnetic
metamaterials.",2310.12839v1
2023-11-10,Magnetic Field Strength Effects on Nucleosynthesis from Neutron Star Merger Outflows,"Magnetohydrodynamic turbulence drives the central engine of post-merger
remnants, potentially powering both a nucleosynthetically active disk wind and
the relativistic jet behind a short gamma ray burst. We explore the impact of
the magnetic field on this engine by simulating three post-merger black hole
accretion disks using general relativistic magnetohydrodynamics with Monte
Carlo neutrino transport, in each case varying the initial magnetic field
strength. We find increasing ejecta masses associated with increasing magnetic
field strength. We find that a fairly robust main r -process pattern is
produced in all three cases, scaled by the ejected mass. Changing the initial
magnetic field strength has a considerable effect on the geometry of the
outflow and hints at complex central engine dynamics influencing lanthanide
outflows. We find that actinide production is especially sensitive to magnetic
field strength, with overall actinide mass fraction calculated at 1 Gyr
post-merger increasing by more than a factor of six with a tenfold increase in
magnetic field strength. This hints at a possible connection to the variability
in actinide enhancements exhibited by metal poor, r -process-enhanced stars.",2311.05796v1
2023-12-20,Spin wave confinement in hybrid superconductor-ferrimagnet nanostructure,"Eddy currents in a superconductor shield the magnetic field in its interior
and are responsible for the formation of a magnetic stray field outside of the
superconducting structure. The stray field can be controlled by the external
magnetic field and affect the magnetization dynamics in the magnetic system
placed in its range. In the case of a hybrid system consisting of a
superconducting strip placed over a magnetic layer, we predict theoretically
the confinement of spin waves in the well of the static stray field. The number
of bound states and their frequencies can be controlled by an external magnetic
field. We have presented the results of semi-analytical calculations
complemented by numerical modeling.",2312.13029v1
2024-02-02,THz spin-wave excitations in the transverse conical phase of BiFeO$_3$,"Although BiFeO$_3$ is one of the most studied multiferroic materials, recent
magnetization and neutron scattering studies have revealed a new magnetic phase
in this compound - the transverse conical phase. To study the collective spin
excitations of this phase, we performed THz spectroscopy in magnetic fields up
to 17 T at and above room temperature. We observed five spin-wave branches in
the magnetic phase with long wavelength conical modulation. Using a numerical
spin dynamics model we found two kinds of excitations with magnetic moments
oscillating either along or perpendicular to the static fields. Remarkably, we
detected strong directional dichroism, an optical manifestation of the
magnetoelectric effect, for two spin-wave modes of the conical phase. According
to our experiments, the stability of the conical state is sensitive to the
magnetic field history and it can become (meta)stable at or close to zero
magnetic field, which may allow exploiting its magnetoelectric properties at
room temperature.",2402.01417v1
2024-02-09,Capturing the slow relaxation time of superparamagnetic colloids in time-varying fields,"Superparamagnetic colloids present interesting assembly dynamics and
propulsion in time-varying magnetic fields due to their magnetic relaxation.
However, little is known about the mechanisms governing this magnetic
relaxation, which is commonly attributed to the interactions and polydispersity
of the ferromagnetic nanoparticles distributed within the colloid. We measure
this relaxation from the effective potential between colloids subjected to
rotating magnetic fields. Remarkably, our results indicate the presence of
magnetic relaxation times much longer than what has been reported, which
furthers our understanding of the magnetization of colloids in complex magnetic
fields.",2402.06802v1
2024-02-27,Origin of magnetic switching cascades in tetrahedral CoFe nanostructures,"We present a comprehensive study of small-scale three-dimensional (3D)
tetrahedral CoFe nanostructure arrays prepared by focused electron beam-induced
deposition (FEBID) and placed in two distinct orientations with respect to the
direction of an external magnetic field. Using ultra-sensitive micro-Hall
magnetometry we obtain angular-dependent magnetic stray field hysteresis loops
that show characteristic cascading magnetic switching close to zero magnetic
field. By employing micromagnetic simulations we could reproduce the hysteresis
loops and identify characteristic field dependent magnetic configurations
including a vortex-type groundstate. From this we derive a coarse-graining
macrospin model and show that the complex switching behavior can be explained
by the reorientation dynamics of non-interacting uniaxial anisotropic magnetic
grains modeled as a superposition of Stoner-Wohlfarth particles.",2402.17594v1
2024-03-11,$^{27}$Al NMR study of the magnetic Weyl semimetal CeAlGe,"Motivated by the recent observations of electronic correlation effect [M.
Corasaniti \textit{et al}., Phys. Rev. B \textbf{104}, L121112 (2021)] and
topology-stabilized magnetic fluctuations [N. Drucker \textit{et al}., Nat.
Commun. \textbf{14}, 5182 (2023)] in the noncentrosymmetric magnetic Weyl
semimetal candidate CeAlGe, we performed systematic studies on the local static
and dynamic spin susceptibilities by $^{27}$Al nuclear magnetic resonance. Due
to the large spin susceptibility from Ce-$4f$ electrons, the theoretically
predicted responses from Weyl fermions are overwhelmed. A Knight-shift anomaly
is observed below $T^*\sim50$ K, a signature of the onset of coherent Kondo
coupling. In addition, an anomalous peak is found in $1/T_1T$ near 15 K, well
above the magnetic ordering temperature $T_N \approx 5$ K, which probably is a
consequence of topology-stabilized magnetic fluctuations. These results
highlight the interplay among electronic correlation, magnetism and band
topology in this family of Kondo Weyl semimetals.",2403.06476v1
2024-03-29,Local magnetic moment formation and Kondo screening in the presence of Hund exchange: the two-band Hubbard model analysis,"We study formation and screening of local magnetic moments in the two-band
Hubbard model in the presence of Hund exchange interaction using dynamic mean
field theory approach. The characteristic temperatures of the formation,
beginning and full screening of local magnetic moments are obtained from the
analysis of temperature dependencies of the orbital, charge, and spin
susceptibilities, as well as the respective instantaneous correlation
functions. At half filling we find the phase diagram, which is similar to the
single-orbital case with wide region of formation of local magnetic moments
below the orbital Kondo temperature. Similarly to the single-orbital case, with
decreasing temperature the screening of local magnetic moments is preceded by
appearance of fermionic quasiparticles. In the two-band case the quasiparticles
are however present also in some temperature region above the Mott insulating
phase. At finite doping we find broad regime of presence of local magnetic
moments, which coexist with incoherently or coherently moving holes. We also
find, similarly to the half filled case, finite temperature interval in the
doped regime, when the fermionic quasiparticles are formed, but do not yet
screen local magnetic moments.",2403.20036v1
2024-04-04,Direct visualization of local magnetic domain dynamics in a 2D Van der Walls material/ferromagnet interface,"Exploring new strategies for controlling the magnetic domain propagation is
the key to realize ultrafast, high-density domain wall-based memory and logic
devices for next generation computing. These strategies include strain
modulation in multiferroic devices, geometric confinement and area-selective
pinning of domain wall. 2D Van der Waals materials introduce localized
modifications to the interfacial magnetic order, enabling control over the
propagation of magnetic domains. Here, using Lorentz-Transmission Electron
Microscopy (L-TEM) along with the Modified Transport of Intensity equations
(MTIE), we demonstrate controlled domain expansion with in-situ magnetic field
in a ferromagnet (Permalloy, NiFe) interfacing with a 2D Van der Waals material
Graphene (Gr). The Gr/NiFe interface exhibits distinctive domain expansion rate
with magnetic field selectively near the interface which is further analyzed
using micromagnetic simulations. Our findings are crucial for comprehending
direct visualization of interface controlled magnetic domain expansion,
offering insights for developing future domain wall-based technology.",2404.03177v1
2002-06-27,"A turn-over scenario for rotating magnetic white dwarfs: models with several values of mass, angular momentum, and magnetic field","We study a white dwarf with differential rotation and magnetic field, for
which the symmetry axis of the toroidal field, the magnetic axis of the
poloidal field, and the principal axis I_3 coincide permanently. This common
axis, so-called ""magnetic symmetry axis"", inclines at a small angle relative to
the spin axis of the model, the so-called ""turn-over angle"". Such an almost
axisymmetric model undergoes an early evolutionary phase of secular timescale,
with moment of inertia along the spin axis, I_{zz}=I_{33}, greater than the
moments of inertia along the equatorial axes, I_{11}=I_{22}, since rotation and
poloidal field (both deriving oblate configurations) dominate over the toroidal
field (deriving prolate configurations). During this phase, the model suffers
from secular angular momentum loss due to weak magnetic dipole radiation
activated by the poloidal field, leading gradually to ""dynamical asymmetry""
with I_{11}>I_{33}. However, a dynamically asymmetric configuration tends to
turn over spontaneously, i.e., to rotate about axis with moment of inertia
greater than I_{33}, with angular momentum remaining invariant. So, the fate of
a dynamically asymmetric configuration is to become an oblique rotator and,
eventually, a perpendicular rotator. During the so-called ""turn-over phase"",
the turn-over angle increases spontaneously up to 90 degrees on a ""turn-over
timescale"" t_{TOV}. In the present paper, we study numerically the turn-over
phase for models of several masses, angular momenta, and magnetic fields.",0206476v1
2000-07-27,Short range ferromagnetism and spin glass state in $\mathrm{Y_{0.7}Ca_{0.3}MnO_{3}}$,"Dynamic magnetic properties of $\mathrm{Y_{0.7}Ca_{0.3}MnO_{3}}$ are
reported. The system appears to attain local ferromagnetic order at
$T_{\mathrm{SRF}} \approx 70$ K. Below this temperature the low field
magnetization becomes history dependent, i.e. the zero field cooled (ZFC) and
field cooled (FC) magnetization deviate from each other and closely logarithmic
relaxation appears at our experimental time scales (0.3-$10^{4}$ sec). The zero
field cooled magnetization has a maximum at $T_{\mathrm{f}}\approx 30$ K,
whereas the field cooled magnetization continues to increase, although less
sharply, also below this temperature. Surprisingly, the dynamics of the system
shows non-equilibrium spin glass (SG) features not only below the maximum in
the ZFC magnetization, but also in the temperature region between this maximum
and $T_{\mathrm{SRF}}$. The aging and temperature cycling experiments show only
quantitative differences in the dynamic behavior above and below the maximum in
the ZFC-magnetization; similarly, memory effects are observed in both
temperature regions. We attribute the high temperature behavior to the
existence of clusters of short range ferromagnetic order below
$T_{\mathrm{SRF}}$; the configuration evolves into a conventional spin glass
state at temperatures below $T_{\mathrm{f}}$.",0007439v2
2004-09-09,Nonlocal magnetization dynamics in ferromagnetic heterostructures,"Two complementary effects modify the GHz magnetization dynamics of nanoscale
heterostructures of ferromagnetic and normal materials relative to those of the
isolated magnetic constituents: On the one hand, a time-dependent ferromagnetic
magnetization pumps a spin angular-momentum flow into adjacent materials and,
on the other hand, spin angular momentum is transferred between ferromagnets by
an applied bias, causing mutual torques on the magnetizations. These phenomena
are manifestly nonlocal: they are governed by the entire spin-coherent region
that is limited in size by spin-flip relaxation processes. We review recent
progress in understanding the magnetization dynamics in ferromagnetic
heterostructures from first principles, focusing on the role of spin pumping in
layered structures. The main body of the theory is semiclassical and based on a
mean-field Stoner or spin-density--functional picture, but quantum-size effects
and the role of electron-electron correlations are also discussed. A growing
number of experiments support the theoretical predictions. The formalism should
be useful to understand the physics and to engineer the characteristics of
small devices such as magnetic random-access memory elements.",0409242v3
2006-02-24,Magnetization dynamics in dysprosium orthoferrites via inverse Faraday effect,"The ultrafast non-thermal control of magnetization has recently become
feasible in canted antiferromagnets through photomagnetic instantaneous pulses
[A.V. Kimel {\it et al.}, Nature {\bf 435}, 655 (2005)]. In this experiment
circularly polarized femtosecond laser pulses set up a strong magnetic field
along the wave vector of the radiation through the inverse Faraday effect,
thereby exciting non-thermally the spin dynamics of dysprosium orthoferrites. A
theoretical study is performed by using a model for orthoferrites based on a
general form of free energy whose parameters are extracted from experimental
measurements. The magnetization dynamics is described by solving coupled
sublattice Landau-Lifshitz-Gilbert equations whose damping term is associated
with the scattering rate due to magnon-magnon interaction. Due to the inverse
Faraday effect and the non-thermal excitation, the effect of the laser is
simulated by magnetic field Gaussian pulses with temporal width of the order of
hundred femtoseconds. When the field is along the z-axis, a single resonance
mode of the magnetization is excited. The amplitude of the magnetization and
out-of-phase behavior of the oscillations for fields in z and -z directions are
in good agreement with the cited experiment. The analysis of the effect of the
temperature shows that magnon-magnon scattering mechanism affects the decay of
the oscillations on the picosecond scale. Finally, when the field pulse is
along the x-axis, another mode is excited, as observed in experiments. In this
case the comparison between theoretical and experimental results shows some
discrepancies whose origin is related to the role played by anisotropies in
orthoferrites.",0602593v1
2009-01-13,Dynamic percolation of electric conductivity and a trend towards fractal skeletal structuring in a random ensemble of magnetized nanodust,"Numerical modeling of electrodynamic aggregation is carried out for a random
ensemble of magnetized nanodust taken as a many body system of strongly
magnetized thin rods (i.e., one-dimensional static magnetic dipoles), which
possess electric conductivity and static electric charge, screened with its own
static plasma sheath. The self-assembling of quasi-linear filaments from an
ensemble of randomly situated basic blocks and the electric short-circuiting
between biased electrodes are shown to be supported by the alignment of blocks
in an external magnetic field. Statistical analysis of short-circuiting time
allows tracing the dynamic percolation of electric conductivity and shows a
decrease of percolation threshold for volume fraction, as compared with the
observed percolation of carbon nanotubes in liquids and polymer composites.
Modeling of short-circuiting stage of evolution is continued with tracing the
dynamics of pinching of electric current filaments to show the interplay of all
the magnetic and electric mechanisms of filaments networking. A trend towards a
fractal skeletal structuring (namely, repeat of original basic block at a
larger length scale) is illustrated with the evidence for generation of a
bigger magnetic dipole.",0901.1835v1
2010-07-30,Tests of Dynamical Flux Emergence as a Mechanism for CME Initiation,"Current coronal mass ejection (CME) models set their lower boundary to be in
the lower corona. They do not calculate accurately the transfer of free
magnetic energy from the convection zone to the magnetically dominated corona
because they model the effects of flux emergence using kinematic boundary
conditions or simply assume the appearance of flux at these heights. We test
the importance of including dynamical flux emergence in CME modeling by
simulating, in 2.5D, the emergence of sub-surface flux tubes into different
coronal magnetic field configurations. We investigate how much free magnetic
energy, in the form of shear magnetic field, is transported from the convection
zone to the corona, and whether dynamical flux emergence can drive CMEs. We
find that multiple coronal flux ropes can be formed during flux emergence, and
although they carry some shear field into the corona, the majority of shear
field is confined to the lower atmosphere. Less than 10% of the magnetic energy
in the corona is in the shear field, and this, combined with the fact that the
coronal flux ropes bring up significant dense material, means that they do not
erupt. Our results have significant implications for all CME models which rely
on the transfer of free magnetic energy from the lower atmosphere into the
corona but which do not explicitly model this transfer. Such studies of flux
emergence and CMEs are timely, as we have new capabilities to observe this with
Hinode and SDO, and therefore to test the models against observations.",1007.5484v2
2010-12-17,Influence of vortices and phase fluctuations on thermoelectric transport properties of superconductors in a magnetic field,"We study heat transport and thermoelectric effects in two-dimensional
superconductors in a magnetic field. These are modeled as granular
Josephson-junction arrays, forming either regular or random lattices. We employ
two different models for the dynamics, relaxational model-A dynamics or
resistively and capacitively shunted Josephson junction (RCSJ) dynamics. We
derive expressions for the heat current in these models, which are then used in
numerical simulations to calculate the heat conductivity and the Nernst
coefficient for different temperatures and magnetic fields. At low temperatures
and zero magnetic field the heat conductivity in the RCSJ model is calculated
analytically from a spin wave approximation, and is seen to have an anomalous
logarithmic dependence on the system size, and also to diverge in the
completely overdamped limit C -> 0. From our simulations we find at low
magnetic fields that the Nernst signal displays a characteristic ""tilted hill""
profile similar to experiments and a non-monotonic temperature dependence of
the heat conductivity. We also investigate the effects of granularity and
randomness, which become important for higher magnetic fields. In this regime
geometric frustration strongly influences the results in both regular and
random systems and leads to highly non-trivial magnetic field dependencies of
the studied transport coefficients.",1012.3876v2
2012-06-01,"Dynamics of Coronal Bright Points as seen by Sun Watcher using Active Pixel System detector and Image Processing (SWAP), Atmospheric Imaging Assembly AIA), and Helioseismic and Magnetic Imager (HMI)","The \textit{Sun Watcher using Active Pixel system detector and Image
Processing}(SWAP) on board the \textit{PRoject for OnBoard Autonomy\todash 2}
(PROBA\todash 2) spacecraft provides images of the solar corona in EUV channel
centered at 174 \AA. These data, together with \textit{Atmospheric Imaging
Assembly} (AIA) and the \textit{Helioseismic and Magnetic Imager} (HMI) on
board \textit{Solar Dynamics Observatory} (SDO), are used to study the dynamics
of coronal bright points. The evolution of the magnetic polarities and
associated changes in morphology are studied using magnetograms and
multi-wavelength imaging. The morphology of the bright points seen in
low-resolution SWAP images and high-resolution AIA images show different
structures, whereas the intensity variations with time show similar trends in
both SWAP 174 and AIA 171 channels. We observe that bright points are seen in
EUV channels corresponding to a magnetic-flux of the order of $10^{18}$ Mx. We
find that there exists a good correlation between total emission from the
bright point in several UV\todash EUV channels and total unsigned photospheric
magnetic flux above certain thresholds. The bright points also show periodic
brightenings and we have attempted to find the oscillation periods in bright
points and their connection to magnetic flux changes. The observed periods are
generally long (10\todash 25 minutes) and there is an indication that the
intensity oscillations may be generated by repeated magnetic reconnection.",1206.0095v1
2013-12-20,Exciton spin dynamics of colloidal CdTe nanocrystals in magnetic field,"The recombination and spin dynamics of excitons in colloidal CdTe
nanocrystals (NCs) are studied by time-resolved photoluminescence in high
magnetic fields up to 15 T and at cryogenic temperatures. The recombination
decay shows a nonexponential temporal behavior, with the longest component
corresponding to the dark excitons having 260 ns decay time at zero magnetic
field and 4.2 K temperature. This long component shortens to 150 ns at 15 T due
to the magnetic-field-induced mixing of the bright and dark exciton states. The
spin dynamics, assessed through the evolution of the magnetic-field-induced
circular polarization degree of the photoluminescence, has a fast component
shorter than 1 ns related to the bright excitons and a slow component of 5-10
ns associated with the dark excitons. The latter shortens with increasing
magnetic field, which is characteristic for a phonon-assisted spin relaxation
mechanism. The relatively low saturation level of the associated
magnetic-field-induced circular polarization degree of -30 % is explained by a
model that suggests the CdTe NCs to constitute an ensemble of prolate and
oblate NCs, both having a structural quantization axis. The exciton g-factor of
2.4-2.9 evaluated from fitting the experimental data in the frame of the
suggested approach is in good agreement with the expected value for the dark
excitons in CdTe NCs.",1312.5929v1
2016-09-17,Quantum dynamics of a macroscopic magnet operating as environment of a mechanical oscillator,"We study the dynamics of a bipartite quantum system in a way such that its
formal description keeps holding even if one of its parts becomes macroscopic:
the problem is related with the analysis of the quantum-to-classical crossover,
but our approach implies that the whole system stays genuinely quantum. Aim of
the work is to understand 1) if, 2) to what extent, and possibly 3) how, the
evolution of a macroscopic environment testifies to the coupling with its
microscopic quantum companion. To this purpose we consider a magnetic
environment made of a large number of spin-1/2 particles, coupled with a
quantum mechanical oscillator, possibly in the presence of an external magnetic
field. We take the value of the total environmental-spin S constant and large,
which allows us to consider the environment as one single macroscopic system,
and further deal with the hurdles of the spin-algebra via approximations that
are valid in the large-S limit. We find an insightful expression for the
propagator of the whole system, where we identify an effective ""back-action""
term, i.e. an operator acting on the magnetic environment only, and yet missing
in the absence of the quantum principal system. This operator emerges as a
time-dependent magnetic anisotropy whose character, whether uniaxial or planar,
also depends on the detuning between the level-splitting in the spectrum of the
free magnetic system, induced by the possible presence of the external field,
and the frequency of the oscillator. The time-dependence of the anisotropy is
analysed, and its effects on the dynamics of the magnet, as well as its
relation with the entangling evolution of the overall system, are discussed.",1609.05389v1
2017-06-05,How important is non-ideal physics in simulations of sub-Eddington accretion onto spinning black holes?,"Black holes with accretion rates well below the Eddington rate are expected
to be surrounded by low-density, hot, geometrically thick accretion disks. This
includes the two black holes being imaged at sub-horizon resolution by the
Event Horizon Telescope. In these disks, the mean free path for Coulomb
interactions between charged particles is large, and the accreting matter is a
nearly collisionless plasma. Despite this, numerical simulations have so far
modeled these accretion flows using ideal magnetohydrodynamics. Here, we
present the first global, general relativistic, 3D simulations of accretion
flows onto a Kerr black hole including the non-ideal effects most likely to
affect the dynamics of the disk: the anisotropy between the pressure parallel
and perpendicular to the magnetic field, and the heat flux along magnetic field
lines. We show that for both standard and magnetically arrested disks, the
pressure anisotropy is comparable to the magnetic pressure, while the heat flux
remains dynamically unimportant. Despite this large pressure anisotropy,
however, the time-averaged structure of the accretion flow is strikingly
similar to that found in simulations treating the plasma as an ideal fluid. We
argue that these similarities are largely due to the interchangeability of the
viscous and magnetic shear stresses as long as the magnetic pressure is small
compared to the gas pressure, and to the sub-dominant role of pressure/viscous
effects in magnetically arrested disks. We conclude by highlighting outstanding
questions in modeling the dynamics of low collisionality accretion flows.",1706.01533v1
2017-08-08,Spin-orbit-torque driven magnetoimpedance in Pt-layer/magnetic-ribbon heterostructures,"When a flow of electron passes through a paramagnetic layer with strong
spin-orbit-coupling such as platinum (Pt), a net spin current is produced via
spin Hall effect (SHE). This spin current can exert a torque on the
magnetization of an adjacent ferromagnetic layer which can be probed via
magnetization dynamic response, e.g. spin-torque ferromagnetic resonance
(ST-FMR). Nevertheless, that effect in lower frequency magnetization dynamic
regime (MHz) where skin effect occurs in high permeability ferromagnetic
conductors namely the magneto-impedance (MI) effect can be fundamentally
important which has not been studied so far. Here, by utilizing the MI effect
in magnetic-ribbon/Pt heterostructure with high magnetic permeability that
allows the ac current effectively confined at the skin depth of ~100 nm
thickness, the effect of spin-orbit-torque (SOT) induced by the SHE probed via
MI measurement is investigated. We observed a systematic MI frequency shift
that increases by increasing the applied current amplitude and thickness of the
Pt layer (varying from 0 nm to 20 nm). In addition, the role of Pt layer in
ribbon/Pt heterostructure is evaluated with ferromagnetic resonance (FMR)
effect representing standard Gilbert damping increase as the result of presence
of the SHE. Our results unveil the role of SOT in dynamic control of the
transverse magnetic permeability probed with impedance spectroscopy as useful
and valuable technique for detection of future SHE devices.",1708.02402v2
2017-09-10,Dynamical Anisotropic Response of Black Phosphorus under Magnetic Field,"Black phosphorus (BP) has emerged as a promising material candidate for next
generation electronic and optoelectronic devices due to its high mobility,
tunable band gap and highly anisotropic properties. In this work, polarization
resolved ultrafast mid-infrared transient reflection spectroscopy measurements
are performed to study the dynamical anisotropic optical properties of BP under
magnetic fields up to 9 T. The relaxation dynamics of photoexcited carrier is
found to be insensitive to the applied magnetic field due to the broadening of
the Landau levels and large effective mass of carriers. While the anisotropic
optical response of BP decreases with increasing magnetic field, its
enhancement due to the excitation of hot carriers is similar to that without
magnetic field. These experimental results can be well interpreted by the
magneto-optical conductivity of the Landau levels of BP thin film, based on an
effective k*p Hamiltonian and linear response theory. These findings suggest
attractive possibilities of multi-dimensional controls of anisotropic response
(AR) of BP with light, electric and magnetic field, which further introduces BP
to the fantastic magnetic field sensitive applications.",1709.03079v1
2017-12-04,Trend reversal in the magnetic-field dependence of exciton spin-transfer rates in diluted magnetic semiconductors due to non-Markovian dynamics,"We investigate theoretically the influence of an external magnetic field on
the spin dynamics of excitons in diluted magnetic semiconductor quantum wells.
To this end, we apply a quantum kinetic theory beyond the Markov approximation
which reveals that non-Markovian effects can significantly influence the
exciton spin dynamics. If the magnetic field is oriented parallel to the growth
direction of the well, the Markovian spin-transfer rate decreases monotonically
with increasing field as predicted by Fermi's golden rule. The quantum kinetic
theory follows this result qualitatively but predicts pronounced quantitative
differences in the spin-transfer rate as well as in the long-time spin
polarization. However, for an in-plane magnetic field, where the Markovian
spin-transfer rate first drops and then increases again, quantum kinetic
effects become so pronounced that the Markovian trend is completely reversed.
This is made evident by a distinct maximum of the rate followed by a monotonic
decrease. The deviations can be traced back to a redistribution of carriers in
energy space caused by correlations between excitons and magnetic dopants. The
same effect leads to a finite electron-spin polarization at long times in
longitudinal as well as transverse fields which is much larger than the
corresponding Markovian prediction.",1712.00978v2
2018-07-19,Magnetization nutation induced by surface effects in nanomagnets,"We investigate the magnetization dynamics of ferromagnetic nanoparticles in
the atomistic approach taking account of surface anisotropy and the spin
misalignment it causes. We demonstrate that such inhomogeneous spin
configurations induce nutation in the dynamics of the particle's magnetization.
More precisely, in addition to the ordinary precessional motion with frequency
$f_{p}\sim10\,{\rm GHz}$, we find that the dynamics of the net magnetic moment
exhibits two more resonance peaks with frequencies $f_{c}$ and $f_{n}$ which
are higher than the frequency $f_{p} : f_{c}=4\times f_{p}\sim40\,{\rm GHz}$ is
related with the oscillations of the particle's magnetic moment between the
minima of the effective potential induced by weak surface anisotropy. On the
other hand, the much higher frequency $f_{n}\sim1\,{\rm THz}$ is attributed to
the magnetization fluctuations at the atomic level driven by exchange
interaction. We have compared our results on nutation induced by surface
effects with those rendered by the macroscopic approach based on the
Landau-Lifshitz-Gilbert equation augmented by an inertial term (proportional to
the second-order time derivative of the macroscopic moment) with a
phenomenological coefficient. The good agreement between the two models have
allowed us to estimate the latter coefficient in terms of the atomistic
parameters such as the surface anisotropy constant. We have thus proposed a new
origin for the magnetization nutations as being induced by surface effects and
have interpreted the corresponding resonance peaks and their frequencies.",1807.07392v1
2018-11-05,Evidence of long-range ferromagnetic order and spin frustration effects in the double perovskite $La_2CoMnO_6$,"We present a comprehensive study on the magnetic structure, dynamics, and
phase evolution in the single-phase double perovskite $La_2CoMnO_6$. The mixed
valence state due to oxygen deficiency is verified by X-ray photoelectron
spectroscopy, and confirms a double ferromagnetic transition observed in DC
magnetization. Neutron diffraction reveals that the magnetic structure is
dominated by long-range ferromagnetic ordering, which is further corroborated
by a critical exponents analysis of the paramagnetic to ferromagnetic phase
transition. An analysis of the magnetization dynamics by means of linear and
nonlinear ac magnetic susceptibilities marks the presence of two distinct
cluster glass-like states that emerge at low temperatures. The isothermal
entropy change as a function of temperature and magnetic field (H) is exploited
to investigate the mechanism of stabilization of the magnetic phases across the
H-T phase diagram. In the regime of the phase diagram where thermal energy is
sufficiently low, regions of competing interactions due to local disorder
become stabilized and display glass-like dynamics. The freezing mechanism of
clusters is illustrated using a unique probe of transverse susceptibility that
isolates the effects of the local anisotropy of the spin clusters. The results
are summarized in a new H-T phase diagram of $La_2CoMnO_6$ revealed for the
first time from these data.",1811.01835v1
2019-03-13,Nutation spectroscopy of a nanomagnet driven into deeply nonlinear ferromagnetic resonance,"Strongly out-of-equilibrium regimes in magnetic nanostructures exhibit novel
properties, linked to the nonlinear nature of magnetization dynamics, which are
of great fundamental and practical interest. Here, we demonstrate that
field-driven ferromagnetic resonance can occur with substantial spatial
coherency at unprecedented large angle of magnetization precessions, which is
normally prevented by the onset of spin-wave instabilities and magnetization
turbulent dynamics. Our results show that this limitation can be overcome in
nanomagnets, where the geometric confinement drastically reduces the density of
spin-wave modes. The obtained deeply nonlinear ferromagnetic resonance regime
is probed by a new spectroscopic technique based on the application of a second
excitation field. This enables to resonantly drive slow coherent magnetization
nutations around the large angle periodic trajectory. Our experimental findings
are well accounted for by an analytical model derived for systems with uniaxial
symmetry. They also provide new means for controlling highly nonlinear
magnetization dynamics in nanostructures, which open interesting applicative
opportunities in the context of magnetic nanotechnologies.",1903.05411v1
2020-10-22,Magnetic properties and quench dynamics of two interacting ultracold molecules in a trap,"We theoretically investigate the magnetic properties and nonequilibrium
dynamics of two interacting ultracold polar and paramagnetic molecules in a
one-dimensional harmonic trap in external electric and magnetic fields. The
molecules interact via a multichannel two-body contact potential, incorporating
the short-range anisotropy of intermolecular interactions. We show that various
magnetization states arise from the interplay of the molecular interactions,
electronic spins, dipole moments, rotational structures, external fields, and
spin-rotation coupling. The rich magnetization diagrams depend primarily on the
anisotropy of the intermolecular interaction and the spin-rotation coupling.
These specific molecular properties are challenging to calculate or measure.
Therefore, we propose the quench dynamics experiments for extracting them from
observing the time evolution of the analyzed system. Our results indicate the
possibility of controlling the molecular few-body magnetization with the
external electric field and pave the way towards studying the magnetization of
ultracold molecules trapped in optical tweezers or optical lattices and their
application in quantum simulation of molecular multichannel many-body
Hamiltonians and quantum information storing.",2010.11899v3
2021-07-13,Investigation of spin-phonon coupling and local magnetic properties in magnetoelectric Fe2TeO6,"Spin-phonon coupling originated from spin-lattice correlation depends upon
different exchange interactions in transition metal oxides containing 3d
magnetic ions. Spin-lattice coupling can influence the coupling mechanism in
magnetoelectric material. To understand the spin-lattice correlation in inverse
trirutile Fe2TeO6 (FTO), magnetic properties and phonon spectra are studied.
Signature of short-range magnetic correlation induced by 5/2-5/2 dimeric
interaction and magnetic anomaly at 150 K is perceived apart from the familiar
sharp transition (TN~210K) corresponding to long-range order by magnetization
and heat capacity measurement. The magnetic transitions and the spin dynamics
are further locally probed by muon spin resonance ({\mu}SR) measurement in both
zero fields (ZF) and longitudinal field (LF) mode. Three dynamically distinct
temperature regimes; (i) T >TN, (ii) TN>T>150 K, and (iii) T<150 K, are
observed. A swift change in spin dynamics is realized at 150K by {\mu}SR,
though previous studies suggest long-range antiferromagnetic order. The
observation of renormalization of different Raman modes below 210K suggests the
existence of spin-phonon coupling in the material. The coupling strength is
quantified as in the range 0.1-1.2 cm-1 following the two-spin cluster
approximation. We propose that the spin-phonon coupling is mediated by the
Fe-O2-Fe interbilayer exchange play a significant role in ME coupling observed
in the material.",2107.06269v2
2022-12-20,"A Global Simulation of the Dynamo, Zonal Jets, and Vortices on Saturn","The fluid dynamics in planet Saturn gives rise to alternating east-west jet
streams, large cyclonic and anticyclonic vortices, and a dipole-dominant
magnetic field which is highly axisymmetric about the planetary rotation axis.
Modelling these features in a self-consistent manner is crucial for
understanding the dynamics of Saturn's interior and atmosphere. Here we report
a turbulent high-resolution dynamo simulation in a spherical shell which
produces these features simultaneously for the first time. A crucial model
ingredient is a long-hypothesised stably stratified layer (SSL), sandwiched
between a deep metallic hydrogen layer and an outer low-conductivity molecular
layer, born out of limited solubility of Helium inside metallic Hydrogen at
certain depths. The model spontaneously produces polar cyclones and significant
low and mid latitude jet stream activities in the molecular layer. The
off-equatorial low-latitude jet streams partially penetrate into the SSL and
interact with the magnetic field. This helps to axisymmetrize the magnetic
field about the rotation axis and convert some of the poloidal magnetic field
to toroidal field, which appears as two global magnetic energy rings
surrounding the deeper dynamo region. The simulation also mimics a distinctive
dip in the fifth spherical harmonic in Saturn's magnetic energy spectrum as
inferred from the Cassini Grand Finale measurements. Our model highlights the
role of an SSL in shaping the fluid dynamical and magnetic features of giant
planets, as exemplified at Saturn.",2212.10617v1
2023-11-06,A Reduced Ideal MHD System for Nonlinear Magnetic Field Turbulence in Plasmas with Approximate Flux Surfaces,"This paper studies the nonlinear evolution of magnetic field turbulence in
proximity of steady ideal MHD configurations characterized by a small electric
current, a small plasma flow, and approximate flux surfaces, a physical setting
that is relevant for plasma confinement in stellarators. The aim is to gather
insight on magnetic field dynamics, to elucidate accessibility and stability of
three-dimensional MHD equilibria, as well as to formulate practical methods to
compute them. Starting from the ideal MHD equations, a reduced dynamical system
of two coupled nonlinear PDEs for the flux function and the angle variable
associated with the Clebsch representation of the magnetic field is obtained.
It is shown that under suitable boundary and gauge conditions such reduced
system preserves magnetic energy, magnetic helicity, and total magnetic flux.
The noncanonical Hamiltonian structure of the reduced system is identified, and
used to show the nonlinear stability of steady solutions against perturbations
involving only one Clebsch potential. The Hamiltonian structure is also applied
to construct a dissipative dynamical system through the method of double
brackets. This dissipative system enables the computation of MHD equilibria by
minimizing energy until a critical point of the Hamiltonian is reached.
Finally, an iterative scheme based on the alternate solution of the two steady
equations in the reduced system is proposed as a further method to compute MHD
equilibria. A theorem is proven which states that the iterative scheme
converges to a nontrivial MHD equilbrium as long as solutions exist at each
step of the iteration.",2311.03095v1
2012-09-11,Study of the 3D Coronal Magnetic Field of Active Region 11117 Around the Time of a Confined Flare Using a Data-Driven CESE--MHD Model,"We apply a data-driven MHD model to investigate the three-dimensional (3D)
magnetic field of NOAA active region (AR) 11117 around the time of a C-class
confined flare occurred on 2010 October 25. The MHD model, based on the
spacetime conservation-element and solution-element (CESE) scheme, is designed
to focus on the magnetic-field evolution and to consider a simplified solar
atomsphere with finite plasma $\beta$. Magnetic vector-field data derived from
the observations at the photoshpere is inputted directly to constrain the
model. Assuming that the dynamic evolution of the coronal magnetic field can be
approximated by successive equilibria, we solve a time sequence of MHD
equilibria basing on a set of vector magnetograms for AR 11117 taken by the
Helioseismic and Magnetic Imager (HMI) on board the {\it Solar Dynamic
Observatory (SDO)} around the time of flare. The model qualitatively reproduces
the basic structures of the 3D magnetic field, as supported by the visual
similarity between the field lines and the coronal loops observed by the
Atmospheric Imaging Assembly (AIA), which shows that the coronal field can
indeed be well characterized by the MHD equilibrium in most time. The magnetic
configuration changes very limited during the studied time interval of two
hours. A topological analysis reveals that the small flare is correlated with a
bald patch (BP, where the magnetic field is tangent to the photoshpere),
suggesting that the energy release of the flare can be understood by magnetic
reconnection associated with the BP separatrices. The total magnetic flux and
energy keep increasing slightly in spite of the flare, while the computed
magnetic free energy drops during the flare with an amount of $\sim 10^{30}$
erg, which seems to be adequate to provide the energy budget of the minor
C-class confined flare.",1209.2207v1
2014-02-05,Magnetic Helicity and Large Scale Magnetic Fields: A Primer,"Magnetic fields of laboratory, planetary, stellar, and galactic plasmas
commonly exhibit significant order on large temporal or spatial scales compared
to the otherwise random motions within the hosting system. Such ordered fields
can be measured in the case of planets, stars, and galaxies, or inferred
indirectly by the action of their dynamical influence, such as jets. Whether
large scale fields are amplified in situ or a remnant from previous stages of
an object's history is often debated for objects without a definitive magnetic
activity cycle. Magnetic helicity, a measure of twist and linkage of magnetic
field lines, is a unifying tool for understanding large scale field evolution
for both mechanisms of origin. Its importance stems from its two basic
properties: (1) magnetic helicity is typically better conserved than magnetic
energy; and (2) the magnetic energy associated with a fixed amount of magnetic
helicity is minimized when the system relaxes this helical structure to the
largest scale available. Here I discuss how magnetic helicity has come to help
us understand the saturation of and sustenance of large scale dynamos, the need
for either local or global helicity fluxes to avoid dynamo quenching, and the
associated observational consequences. I also discuss how magnetic helicity
acts as a hindrance to turbulent diffusion of large scale fields, and thus a
helper for fossil remnant large scale field origin models in some contexts. I
briefly discuss the connection between large scale fields and accretion disk
theory as well. The goal here is to provide a conceptual primer to help the
reader efficiently penetrate the literature.",1402.0933v2
2014-02-20,Diagnosis of Magnetic and Electric Fields of Chromospheric Jets through Spectropolarimetric Observations of HI Paschen Lines,"Magnetic fields govern the plasma dynamics in the outer layers of the solar
atmosphere, and electric fields acting on neutral atoms that move across the
magnetic field enable us to study the dynamical coupling between neutrals and
ions in the plasma. In order to measure the magnetic and electric fields of
chromospheric jets, the full Stokes spectra of the Paschen series of neutral
hydrogen in a surge and in some active region jets that took place at the solar
limb were observed on May 5, 2012, using the spectropolarimeter of the Domeless
Solar Telescope at Hida observatory, Japan. First, we inverted the Stokes
spectra taking into account only the effect of magnetic fields on the energy
structure and polarization of the hydrogen levels. Having found no definitive
evidence of the effects of electric fields in the observed Stokes profiles, we
then estimated an upper bound for these fields by calculating the polarization
degree under the magnetic field configuration derived in the first step, with
the additional presence of a perpendicular (Lorentz type) electric field of
varying strength. The inferred direction of the magnetic field on the plane of
the sky (POS) approximately aligns to the active region jets and the surge,
with magnetic field strengths in the range 10 G < B < 640 G for the surge.
Using magnetic field strengths of 70, 200, and 600 G, we obtained upper limits
for possible electric fields of 0.04, 0.3, and 0.8 V/cm, respectively. Because
the velocity of neutral atoms of hydrogen moving across the magnetic field
derived from these upper limits of the Lorentz electric field is far below the
bulk velocity of the plasma perpendicular to the magnetic field as measured by
the Doppler shift, we conclude that the neutral atoms must be highly frozen to
the magnetic field in the surge.",1402.4903v1
2018-10-23,"B-site spin-state and anti-site disorder driven multiple-magnetic phases: Griffiths phase, re-entrant cluster glass and exchange bias in double perovskite Pr$_2$CoFeO$_6$","We report the comprehensive experimental results identifying the magnetic
spin ordering and the magnetization dynamics of a double perovskite Pr2CoFeO6
by employing the (dc and ac) magnetization, powder neutron diffraction (NPD)
and X-ray magnetic circular dichroism (XMCD) techniques. X-ray diffraction and
neutron diffraction studies revealed that Pr2CoFeO6 adopts a B-site disordered
orthorhombic structure with space group Pnma. Additionally, ab initio band
structure calculations performed on this system suggested an insulating
anti-ferromagnetic (Fe-Fe) ground state. Magnetometry study showed the system
to possess a spectrum of interesting magnetic phases including long range
antiferromagnetic (canted) spin ordering (TN ~269 K), Griffiths phase,
re-entrant cluster glass (RCG) (TG~ 34 K) and exchange bias. However, the NPD
study divulged the exhibition of a long range G-type (below TN ~269 K) of spin
ordering by Fe spins. Spin dynamics study by ac susceptibility technique
confirmed the system possessing long range ordering at higher
temperatureundergoes a RCG transition at ~34 K. Existence of Griffiths phase
was confirmed by non-analytic field variation of magnetization and Heisenberg
type temporal spin relaxation above long range ordering temperature TN ~269 K.
The anti-site disorder related to the B-sites (Co/Fe) is found to be the main
driving force forthe observed multiple magnetic phases. Furthermore, the
electronic structure probed by the X-ray absorption spectroscopy (XAS) study
suggested a nominal valance state of +3 for both of the B-site ions (Co/Fe)
which in turn triggered the anti-site disorder in the system. Magnetic, XRD,
NPD and XAS analysis yielded a low spin state (LS) for the Co3+ ions. The
random non-magnetic dilution of magnetic Fe3+ (HS) ions by Co3+ (LS) ions
essentially played a crucial role in manifesting the magnetic properties of the
system.",1810.09788v1
2021-01-05,Magnetic Imaging of the Outer Solar Atmosphere (MImOSA): Unlocking the driver of the dynamics in the upper solar atmosphere,"The magnetic activity of the Sun directly impacts the Earth and human life.
Likewise, other stars will have an impact on the habitability of planets
orbiting these host stars. The lack of information on the magnetic field in the
higher atmospheric layers hampers our progress in understanding solar magnetic
activity. Overcoming this limitation would allow us to address four paramount
long-standing questions: (1) How does the magnetic field couple the different
layers of the atmosphere, and how does it transport energy? (2) How does the
magnetic field structure, drive and interact with the plasma in the
chromosphere and upper atmosphere? (3) How does the magnetic field destabilise
the outer solar atmosphere and thus affect the interplanetary environment? (4)
How do magnetic processes accelerate particles to high energies? New
ground-breaking observations are needed to address these science questions. We
suggest a suite of three instruments that far exceed current capabilities in
terms of spatial resolution, light-gathering power, and polarimetric
performance: (a) A large-aperture UV-to-IR telescope of the 1-3 m class aimed
mainly to measure the magnetic field in the chromosphere by combining high
spatial resolution and high sensitivity. (b) An extreme-UV-to-IR coronagraph
that is designed to measure the large-scale magnetic field in the corona with
an aperture of about 40 cm. (c) An extreme-UV imaging polarimeter based on a 30
cm telescope that combines high throughput in the extreme UV with polarimetry
to connect the magnetic measurements of the other two instruments. This mission
to measure the magnetic field will unlock the driver of the dynamics in the
outer solar atmosphere and thereby greatly advance our understanding of the Sun
and the heliosphere.",2101.01566v1
2004-11-17,Pulsar dynamics: magnetic dipole model revisited,Manuscript withdrawn by the authors.,0411507v2
1999-06-14,Universal property of single topological singularity dynamics,"Using the random matrix theory we demonstrate explicitly the insensitivity of
the transverse force on a moving vortex to non-intrinsic and non-magnetic
impurities in a superconductor.",9906189v1
2005-02-18,Dynamics of coupled vortices in layered magnetic nanodots,"The spin dynamics are calculated for a model system consisting of
magnetically soft, layered nanomagnets, in which two ferromagnetic (F)
cylindrical dots, each with a magnetic vortex ground state, are separated by a
non-magnetic spacer (N). This permits a study of the effects of interlayer
magnetostatic interactions on the vortex dynamics. The system was explored by
applying the equations of motion for the vortex core positions. The restoring
force was calculated taking into account the magnetostatic interactions
assuming a realistic surface charge free spin distribution. For tri-layer F/N/F
dots with opposite chiralities and the same core polarizations (lowest energy
state), two eigenmodes are predicted analytically and confirmed via
micromagnetic simulations. One mode is in the sub-GHz range for submicron dot
diameters and corresponds to quasi-circular rotation of the cores about the dot
center. A second mode is in the MHz range corresponding to a small amplitude
rotation of the mean core position. The eigenfrequencies depend strongly on the
geometrical parameters of the system, suggesting that magnetostatic effects
play a dominant role in determining the vortex dynamics.",0502469v1
1994-07-26,Dynamical Flavor Symmetry Breaking by a Magnetic Field in $2+1$ Dimensions,"It is shown that in $2+1$ dimensions, a constant magnetic field is a strong
catalyst of dynamical flavor symmetry breaking, leading to generating a fermion
dynamical mass even at the weakest attractive interaction between fermions. The
essence of this effect is that in a magnetic field, in $2+1$ dimensions, the
dynamics of fermion pairing is essentially one-dimensional. The effect is
illustrated in the Nambu-Jona-Lasinio model in a magnetic field. The low-energy
effective action in this model is derived and the thermodynamic properties of
the model are considered. The relevance of this effect for planar condensed
matter systems and for $3+1$ dimensional theories at high temperature is
pointed out.",9407168v2
2005-11-23,Fluctuation of magnetic induction in von Kármán swirling flows,"Studies of magnetic induction in von K\'arm\'an swirling flows have so far
linked the time-averaged induced magnetic field to the structure of the mean
flow. They have evidenced the Omega and Parker mechanism generated respectively
by the flow differential rotation and helicity, which underly the Duddley and
James \cite{DuddleyJames} dynamos. Using an array of Hall probes we study here
the dynamical regime. In the experimental flow, turbulence is fully developed
and large fluctuations are observed in the magnetic induction processes. We
find that the large scale turbulent fluctuations have different characteristics
when induction results from the differential rotation or from the dynamics of
the stagnation point in the mid plane of the von K\'arm\'an flow. Symmetry
considerations indicate that the dynamical flow spends half of its time away
from the time-averaged structure. The consequences of these observations for
dynamo experiments are discussed.",0511204v1
2006-06-27,Investigation of the Chaotic Dynamics of an Electron Beam with a Virtual Cathode in an External Magnetic Field,"The effect of the strength of the focusing magnetic field on chaotic dynamic
processes occurring inan electron beam with a virtual cathode, as well as on
the processes whereby the structures form in the beamand interact with each
other, is studied by means of two-dimensional numerical simulations based on
solving a self-consistent set of Vlasov-Maxwell equations. It is shown that, as
the focusing magnetic field is decreased,the dynamics of an electron beam with
a virtual cathode becomes more complicated due to the formation andinteraction
of spatio-temporal longitudinal and transverse structures in the interaction
region of a vircator. The optimum efficiency of the interaction of an electron
beam with the electromagnetic field of the vircator isachieved at a
comparatively weak external magnetic field and is determined by the
fundamentally two-dimensional nature of the motion of the beam electrons near
the virtual cathode.",0606235v1
2008-11-09,X-ray imaging of the dynamic magnetic vortex core deformation,"Magnetic platelets with a vortex configuration are attracting considerable
attention. The discovery that excitation with small in-plane magnetic fields or
spin polarised currents can switch the polarisation of the vortex core did not
only open the possibility of using such systems in magnetic memories, but also
initiated the fundamental investigation of the core switching mechanism itself.
Micromagnetic models predict that the switching is mediated by a
vortex-antivortex pair, nucleated in a dynamically induced vortex core
deformation. In the same theoretical framework, a critical core velocity is
predicted, above which switching occurs. Although these models are extensively
studied and generally accepted, experimental support has been lacking until
now. In this work, we have used high-resolution time-resolved X-ray microscopy
to study the detailed dynamics in vortex structures. We could reveal the
dynamic vortex core deformation preceding the core switching. Also, the
threshold velocity could be measured, giving quantitative comparison with
micromagnetic models.",0811.1348v2
2009-03-16,Non-stationary magnetization dynamics driven by spin transfer torque,"This paper shows that the presence of two dynamical regimes, characterized by
different precessional-axis, is the origin of the non-monotonic behavior of the
output integrated power for large-amplitude magnetization precession driven by
spin-polarized current in nanoscale exchange biased spin-valves. In particular,
at the transition current between those two regimes exists an abruptly loss in
the integrated output power. After the introduction of a time-frequency
analysis of magnetization dynamics based on the wavelet transform, we performed
a numerical experiment by means of micromagnetic simulations. Our results
predicted that, together with a modulation of the frequency of the main excited
mode of the magnetization precession, at high non-linear dynamical regime the
instantaneous output power of the spin-torque oscillator can disappear and then
reappear at nanosecond scale.",0903.2751v1
2009-03-27,Dynamical magnetoelectric effects induced by the Dzyaloshinskii-Moriya interaction in multiferroics,"We study the dynamical interplay between ferroelectricity and magnetism in a
multiferroic with a helical magnetic order. We show that the dynamical
exchange-striction induces a biquadratic interaction between the spins and
transverse phonons resulting in quantum fluctuations of the spontaneous
ferroelectric polarization $\mathbf{P}$ in the ferroelectric phase. The
hybridization between the spin wave and the fluctuation of the electric
polarization leads to low-lying transverse phonon modes. Those are
perpendicular to $\mathbf{P}$ and to the helical spins at small wave vector but
then turn parallel to $\mathbf{P}$ at a wave vector close to the magnetic
modulation vector. For helical magnetic structure, the spin chirality which
determines the direction of $\mathbf{P}$, also possesses a long-range order.
Due to the dynamical Dzyaloshiskii-Moriya interaction, the spin-chirality is
strongly coupled to the spin fluctuation which implies an on-site inversion of
the spin-chirality in the ordered spin-1/2 system and results in a finite
scattering intensity of polarized neutrons from a cycloidal helimagnet.",0903.4751v1
2010-06-25,Spin and charge dynamics in [TbPc$_2$]$^0$ and [DyPc$_2$]$^0$ single molecule magnets,"Magnetization, AC susceptibility and $\mu$SR measurements have been performed
in neutral phthalocyaninato lanthanide ([LnPc$_2]^0$) single molecule magnets
in order to determine the low-energy levels structure and to compare the
low-frequency spin excitations probed by means of macroscopic techniques, such
as AC susceptibility, with the ones explored by means of techniques of
microscopic character, such as $\mu$SR. Both techniques show a high temperature
thermally activated regime for the spin dynamics and a low temperature
tunneling one. While in the activated regime the correlation times for the spin
fluctuations estimated by AC susceptibility and $\mu$SR basically agree, clear
discrepancies are found in the tunneling regime. In particular, $\mu$SR probes
a faster dynamics with respect to AC susceptibility. It is argued that the
tunneling dynamics probed by $\mu$SR involves fluctuations which do not yield a
net change in the macroscopic magnetization probed by AC susceptibiliy. Finally
resistivity measurements in [TbPc$_2]^0$ crystals show a high temperature
nearly metallic behaviour and a low temperature activated behaviour.",1006.4919v1
2010-10-05,Magnetic Monopole Dynamics in Spin Ice,"One of the most remarkable examples of emergent quasi-particles, is that of
the ""fractionalization"" of magnetic dipoles in the low energy configurations of
materials known as ""spin ice"", into free and unconfined magnetic monopoles
interacting via Coulomb's 1/r law [Castelnovo et. al., Nature, 451, 42-45
(2008)]. Recent experiments have shown that a Coulomb gas of magnetic charges
really does exist at low temperature in these materials and this discovery
provides a new perspective on otherwise largely inaccessible phenomenology. In
this paper, after a review of the different spin ice models, we present
detailed results describing the diffusive dynamics of monopole particles
starting both from the dipolar spin ice model and directly from a Coulomb gas
within the grand canonical ensemble. The diffusive quasi-particle dynamics of
real spin ice materials within ""quantum tunneling"" regime is modeled with
Metropolis dynamics, with the particles constrained to move along an underlying
network of oriented paths, which are classical analogues of the Dirac strings
connecting pairs of Dirac monopoles.",1010.0970v1
2010-10-15,"Length matters, periodically (the movie)","We perform Rayleigh-Taylor Instability experiments, using magnetic levitation
to force a controlled initial perturbation.",1010.3157v1
2011-06-15,Dissipative hydrodynamic equation of a ferromagnetic Bose-Einstein condensate: Analogy to magnetization dynamics in conducting ferromagnets,"The hydrodynamic equation of a spinor Bose-Einstein condensate (BEC) gives a
simple description of spin dynamics in the condensate. We introduce the
hydrodynamic equation of a ferromagnetic BEC with dissipation originating from
the energy dissipation of the condensate. The dissipative hydrodynamic equation
has the same form as an extended Landau-Lifshitz-Gilbert (LLG) equation, which
describes the magnetization dynamics of ferromagnets interacting with
spin-polarized currents. Employing the dissipative hydrodynamic equation, we
demonstrate the magnetic domain pattern dynamics of a ferromagnetic BEC in the
presence and absence of a current of particles, and discuss the effects of the
current on domain pattern formation. We also discuss the characteristic lengths
of domain patterns that have domain walls with and without finite
magnetization.",1106.2876v4
2012-03-05,Coherent multi-flavour spin dynamics in a fermionic quantum gas,"Microscopic spin interaction processes are fundamental for global static and
dynamical magnetic properties of many-body systems. Quantum gases as pure and
well isolated systems offer intriguing possibilities to study basic magnetic
processes including non-equilibrium dynamics. Here, we report on the
realization of a well-controlled fermionic spinor gas in an optical lattice
with tunable effective spin ranging from 1/2 to 9/2. We observe long-lived
intrinsic spin oscillations and investigate the transition from two-body to
many-body dynamics. The latter results in a spin-interaction driven melting of
a band insulator. Via an external magnetic field we control the system's
dimensionality and tune the spin oscillations in and out of resonance. Our
results open new routes to study quantum magnetism of fermionic particles
beyond conventional spin 1/2 systems.",1203.0948v1
2013-01-25,Driven skyrmions and dynamical transitions in chiral magnets,"We study the dynamics of skyrmions in chiral magnets in the presence of a
spin polarized current. The motion of skyrmions in the ferromagnetic background
excites spin waves and contributes to additional damping. At a large current,
the spin wave spectrum becomes gapless and skyrmions are created dynamically
from the ferromagnetic state. At an even higher current, these skyrmions are
strongly deformed due to the damping and become unstable at a threshold
current, leading to a chiral liquid. We show how skyrmions can be created by
increasing the current in the magnetic spiral state. We then construct a
dynamic phase diagram for a chiral magnet with a current. The instability
transitions between different states can be observed as experimentally clear
signatures in the transport measurements, such as jumps and hysteresis.",1301.5963v2
2013-03-26,A unified model for the dynamics of driven ribbon with strain and magnetic order parameters,"We develop a unified model to explain the dynamics of driven one dimensional
ribbon for materials with strain and magnetic order parameters. We show that
the model equations in their most general form explain several results on
driven magnetostrictive metallic glass ribbons such as the period doubling
route to chaos as a function of a dc magnetic field in the presence of a
sinusoidal field, the quasiperiodic route to chaos as a function of the
sinusoidal field for a fixed dc field, and induced and suppressed chaos in the
presence of an additional low amplitude near resonant sinusoidal field. We also
investigate the influence of a low amplitude near resonant field on the period
doubling route. The model equations also exhibit symmetry restoring crisis with
an exponent close to unity. The model can be adopted to explain certain results
on magnetoelastic beam and martensitic ribbon under sinusoidal driving
conditions. In the latter case, we find interesting dynamics of a periodic one
orbit switching between two equivalent wells as a function of an ac magnetic
field that eventually makes a direct transition to chaos under resonant driving
condition. The model is also applicable to magnetomartensites and materials
with two order parameters.",1303.6621v1
2013-09-10,Vortices and Impurities,"We describe the BPS dynamics of vortices in the presence of impurities. We
argue that a moduli space of solitons survives the addition of both electric
and magnetic impurities. However, dynamics on the moduli space is altered. In
the case of electric impurities, the metric remains unchanged but the dynamics
is accompanied by a connection term, acting as an effective magnetic field over
the moduli space. We give an expression for this connection and compute the
vortex-impurity bound states in simple cases. In contrast, magnetic impurities
distort the metric on the moduli space. We show that magnetic impurities can be
viewed as vortices associated to a second, frozen, gauge group. We provide a
D-brane description of the dynamics of vortices in product gauge groups and
show how one can take the limit such that a subset of the vortices freeze.",1309.2644v2
2014-08-26,Chimeras in SQUID Metamaterials,"Regular lattices comprising superconducting quantum interference devices
(SQUIDs) form magnetic metamaterials exhibiting extraordinary properties,
including tunability, dynamic multistability, and negative magnetic
permeability. The SQUIDs in a metamaterial interact through nonlocal, magnetic
dipole-dipole forces that makes it possible for counter-intuitive dynamic
states referred to as chimera states to appear; the latter feature clusters of
SQUIDs with synchronous dynamics which coexist with clusters exhibiting
asynchronous behavior. The spontaneous appearance of chimera states is
demonstrated numerically for one-dimensional SQUID metamaterials driven by an
alternating magnetic field in which the fluxes threading the SQUID rings are
randomly initialized; then, chimera states appear generically for sufficiently
strong initial excitations, which exhibit relatively long lifetimes. The
synchronization and metastability levels of the chimera states are discussed in
terms of appropriate measures. Given that both one- and two-dimensional SQUID
metamaterials have been already fabricated and investigated in the laboratory,
the presence of a chimera state could in principle be detected with presently
available experimental setups.",1408.6072v3
2015-05-22,Relaxation dynamics of vortex lines in disordered type-II superconductors following magnetic field and temperature quenches,"We study the effects of rapid temperature and magnetic field changes on the
non-equilibrium relaxation dynamics of magnetic vortex lines in disordered
type-II superconductors by employing an elastic line model and performing
Langevin molecular dynamics simulations. In a previously equilibrated system,
either the temperature is suddenly changed, or the magnetic field is
instantaneously altered which is reflected in adding or removing flux lines to
or from the system. The subsequent aging properties are investigated in samples
with either randomly distributed point-like or extended columnar defects, which
allows to distinguish the complex relaxation features that result from either
type of pinning centers. One-time observables such as the radius of gyration
and the fraction of pinned line elements are employed to characterize
steady-state properties, and two-time correlation functions such as the vortex
line height autocorrelations and their mean-square displacement are analyzed to
study the non-linear stochastic relaxation dynamics in the aging regime.",1505.06240v2
2015-08-19,Chiral Magnetic Effect and Anomalous Hall Effect in Antiferromagnetic Insulators with Spin-Orbit Coupling,"We search for dynamical magnetoelectric phenomena in three-dimensional
correlated systems with spin-orbit coupling. We focus on the antiferromagnetic
insulator phases where the dynamical axion field is realized by the fluctuation
of the antiferromagnetic order parameter. It is shown that the dynamical chiral
magnetic effect, an alternating current generation by magnetic fields, emerges
due to the time dependences of the order parameter such as antiferromagnetic
resonance. It is also shown that the anomalous Hall effect arises due to the
spatial variations of the order parameter such as antiferromagnetic domain
walls. Our study indicates that spin excitations in antiferromagnetic
insulators with spin-orbit coupling can result in nontrivial charge responses.
Moreover, observing the chiral magnetic effect and anomalous Hall effect in our
system is equivalent to detecting the dynamical axion field in condensed
matter.",1508.04590v2
2016-06-18,Enhanced diffusion and anomalous transport of magnetic colloids driven above a two-state flashing potential,"We combine experiment and theory to investigate the diffusive and
subdiffusive dynamics of paramagnetic colloids driven above a two-state
flashing potential. The magnetic potential was realized by periodically
modulating the stray field of a magnetic bubble lattice in a uniaxial ferrite
garnet film. At large amplitudes of the driving field, the dynamics of
particles resembles an ordinary random walk with a frequency-dependent
diffusion coefficient. However, subdiffusive and oscillatory dynamics at short
time scales is observed when decreasing the amplitude. We present a persistent
random walk model to elucidate the underlying mechanism of motion, and perform
numerical simulations to demonstrate that the anomalous motion originates from
the dynamic disorder in the structure of the magnetic lattice, induced by
slightly irregular shape of bubbles.",1606.05783v1
2016-07-01,Orientational dynamics of colloidal ribbons self-assembled from microscopic magnetic ellipsoids,"We combine experiments and theory to investigate the orientational dynamics
of dipolar ellipsoids, which self-assemble into elongated ribbon-like
structures due to the presence in each particle of a permanent magnetic moment
perpendicular to the long axis. Monodisperse hematite ellipsoids are
synthesized via sol-gel technique, and arrange into ribbons in presence of
static or time-dependent magnetic fields. We find that under an oscillating
field, the ribbons reorient perpendicular to the field direction, in contrast
with the behaviour observed under a static field. This observation is explained
theoretically by treating a chain of interacting ellipsoids as a single
particle with an orientational and demagnetizing field energy. The model allows
describing the orientational behaviour of the chain and captures well its
dynamics at different strengths of the actuating field. The understanding of
the complex dynamics and assembly of anisotropic magnetic colloids is a
necessary step towards controlling the structure formation which has direct
applications in different fluid-based microscale technologies.",1607.00256v1
2016-10-17,Anisotropic Boltzmann-Gibbs dynamics of strongly magnetized Vlasov-Fokker-Planck equations,"We consider various sets of Vlasov-Fokker-Planck equations modeling the
dynamics of charged particles in a plasma under the effect of a strong magnetic
field. For each of them in a regime where the strength of the magnetic field is
effectively stronger than that of collisions we first formally derive
asymptotically reduced models. In this regime, strong anisotropic phenomena
occur; while equilibrium along magnetic field lines is asymptotically reached
our asymptotic models capture a non trivial dynamics in the perpendicular
directions. We do check that in any case the obtained asymptotic model defines
a well-posed dynamical system and when self consistent electric fields are
neglected we provide a rigorous mathematical justification of the formally
derived systems. In this last step we provide a complete control on solutions
by developing anisotropic hypocoercive estimates.",1610.05138v2
2016-12-07,Non-equilibrium study of the Chiral Magnetic Effect from real-time simulations with dynamical fermions,"We present a real-time lattice approach to study the non-equilibrium dynamics
of vector and axial charges in $SU(N) \times U(1)$ gauge theories. Based on a
classical description of the non-Abelian and Abelian gauge fields, we include
dynamical fermions and develop operator definitions for (improved) Wilson and
overlap fermions that allow us to study real-time manifestations of the axial
anomaly from first principles. We present a first application of this approach
to anomalous transport phenomena such as the Chiral Magnetic Effect (CME) and
Chiral Separation Effect (CSE) by studying the dynamics of fermions during and
after a $SU(N)$ sphaleron transition in the presence of a $U(1)$ magnetic
field. We investigate the fermion mass and magnetic field dependence of the
suggested signatures of the CME and CSE and point out some important aspects
which need to be accounted for in the macroscopic description of anomalous
transport phenomena.",1612.02477v2
2017-02-15,Current Induced Dynamics of Multiple Skyrmions with Domain Wall Pair and Skyrmion-based Majority Gate Design,"As an intriguing ultra-small particle-like magnetic texture, skyrmion has
attracted lots of research interests in next-generation ultra-dense and low
power magnetic memory/logic designs. Previous studies have demonstrated a
single skyrmion-domain wall pair collision in a specially designed magnetic
racetrack junction. In this work, we investigate the dynamics of multiple
skyrmions with domain wall pair in a magnetic racetrack. The numerical
micromagnetic simulation results indicate that the domain wall pair could be
pinned or depinned by the rectangular notch pinning site depending on both the
number of skyrmions in the racetrack and the magnitude of driving current
density. Such emergent dynamical property could be used to implement a
threshold-tunable step function, in which the inputs are skyrmions and
threshold could be tuned by the driving current density. The threshold-tunable
step function is widely used in logic and neural network applications. We also
present a three-input skyrmion-based majority logic gate design to demonstrate
the potential application of such dynamic interaction of multiple skyrmions and
domain wall pair.",1702.04814v2
2017-02-21,Spin texture motion in antiferromagnetic and ferromagnetic nanowires,"We propose a Hamiltonian dynamics formalism for the current and magnetic
field driven dynamics of ferromagnetic and antiferromagnetic domain walls in
one dimensional systems. To demonstrate the power of this formalism, we derive
Hamilton equations of motion via Poisson brackets based on the
Landau-Lifshitz-Gilbert phenomenology, and add dissipative dynamics via the
evolution of the energy. We use this approach to study current induced domain
wall motion and compute the drift velocity. For the antiferromagnetic case, we
show that a nonzero magnetic moment is induced in the domain wall, which
indicates that an additional application of a magnetic field would influence
the antiferromagnetic domain-wall dynamics. We consider both cases of the
magnetic field being parallel and transverse to the N{\'e}el field. Based on
this formalism, we predict an orientation switch mechanism for
antiferromagnetic domain walls which can be tested with the recently discovered
N{\'e}el spin orbit torques.",1702.06274v1
2018-03-14,Dynamics of distorted skyrmions in strained chiral magnets,"In this work, we study the microscopic dynamics of distorted skyrmions in
strained chiral magnets [K. Shibata et al., Nat. Nanotech. 10, 589 (2015)]
under gradient magnetic field or electric current by Landau-Lifshitz-Gilbert
simulations of the anisotropic spin model. It is observed that the dynamical
responses are also anisotropic, and the velocities of the distorted skyrmions
are periodically dependent on the directions of the external stimuli.
Furthermore, in addition to the uniform motion, our work also demonstrates
anti-phase harmonic vibrations of the two skyrmions in nanostripes, and the
frequencies are mainly determined by the exchange anisotropy. The simulated
results are well explained by Thiele theory, which may provide useful
information in understanding the dynamics of the distorted skyrmions in
strained chiral magnets.",1803.05298v1
2018-10-26,Phonon impact on the dynamics of resonantly excited and hot excitons in diluted magnetic semiconductors,"Phonons are well known to be the main mechanism for the coupling between
bright and dark excitons in nonmagnetic semiconductors. Here, we investigate
diluted magnetic semiconductors where this process is in direct competition
with the scattering at localized magnetic impurities. To this end, a recently
developed quantum kinetic description of the exciton spin dynamics in diluted
magnetic semiconductor quantum wells is extended by the scattering with
longitudinal acoustic phonons. A strong phonon impact is found in the
redistribution of exciton momenta on the exciton parabola that becomes
especially prominent for high temperatures and exciton distributions further
away from the exciton resonance which are optically dark. Despite their impact
on the energetic redistribution, acoustic phonons virtually do not affected the
exciton spin dynamics as the exciton-impurity interaction always dominates.
Furthermore, it turns out that the exciton spin lifetime increases by roughly
one order of magnitude for nonequilibrium hot exciton distributions and, in
addition, pronounced quantum kinetic signatures in the exciton spin dynamics
appearing after resonant optical excitation are drastically reduced.",1810.11241v2
2018-10-05,Zero-field dynamics stabilized by in-plane shape anisotropy in MgO-based spin-torque oscillators,"Here we demonstrate numerically that shape anisotropy in MgO-based
spin-torque nano-oscillators consisting of an out-of-plane magnetized free
layer and an in-plane polarizer is necessary to stabilize out-of-plane
magnetization precession without the need of external magnetic fields. As the
in-plane anisotropy is increased, a gradual tilting of the magnetization
towards the in-plane easy direction is introduced, favouring zero-field
dynamics over static in-plane states. Above a critical value, zero-field
dynamics are no longer observed. The optimum ratio of in-plane shape to
out-of-plane uniaxial anisotropy, for which large angle out-of-plane zero-field
dynamics occur within the widest current range, is reported.",1811.00106v1
2019-10-17,A multi-scale approach for magnetisation dynamics: Unraveling exotic magnetic states of matter,"Crystallographic lattice defects strongly influence dynamical properties of
magnetic materials at both microscopic and macroscopic length scales. A
multi-scale approach to magnetisation dynamics, which is presented in this
paper, accurately captures such effects. The method is illustrated using
examples of systems with localized, non-trivial topological properties, e.g. in
the form of skyrmions and chiral domain walls that interact with lattice
dislocations. Technical aspects of the methodology involve multi-scale
magnetisation dynamics that connects atomistic and continuum descriptions. The
technique is capable of solving the Landau-Lifshitz-Gilbert equations
efficiently in two regions of a magnetic material --- the mesoscopic and the
atomistic regions, which are coupled in a seamless way. It is demonstrated that
this methodology allows simulating realistically-sized magnetic skyrmions
interacting with material defects and novel physical effects, uncovered using
this theoretical methodology, are described.",1910.07807v1
2020-02-03,Dynamical phase transitions for the activity biased Ising model in a magnetic field,"We consider large deviations of the dynamical activity -- defined as the
total number of configuration changes within a time interval -- for mean-field
and one-dimensional Ising models, in the presence of a magnetic field. We
identify several dynamical phase transitions that appear as singularities in
the scaled cumulant generating function of the activity. In particular, we find
low-activity ferromagnetic states and a novel high-activity phase, with
associated first- and second-order phase transitions. The high-activity phase
has a negative susceptibility to the magnetic field. In the mean-field case, we
analyse the dynamical phase coexistence that occurs on first-order transition
lines, including the optimal-control forces that reproduce the relevant large
deviations. In the one-dimensional model, we use exact diagonalisation and
cloning methods to perform finite-size scaling of the first-order phase
transition at non-zero magnetic field.",2002.00905v2
2020-10-06,Emergent spin dynamics enabled by lattice interactions in a bicomponent artificial spin ice,"Artificial spin ice (ASI) are arrays on nanoscaled magnets that can serve
both as models for frustration in atomic spin ice as well as for exploring new
spin-wave-based strategies to transmit, process, and store information. Here,
we exploit the intricate interplay of the magnetization dynamics of two
dissimilar ferromagnetic metals arranged on complimentary lattice sites in a
square ASI to effectively modulate the spin-wave properties. We show that the
interaction between the two sublattices results in unique spectra attributed to
each sublattice and we observe inter- and intra-lattice dynamics facilitated by
the distinct magnetization properties of the two materials. The dynamic
properties are systematically studied by angular-dependent broadband
ferromagnetic resonance and confirmed by micromagnetic simulations. We show
that the combination of materials with dissimilar magnetic properties enables
the realization of a wide range of two-dimensional structures potentially
opening the door to new concepts in nanomagnonics.",2010.03008v1
2021-03-22,Universal dynamics of magnetic monopoles in two-dimensional kagomé ice,"A magnetic monopole in spin ice is a novel quasiparticle excitation in
condensed matter physics, and we found that the ac frequency dependent magnetic
susceptibility $\chi(\omega)$ in the two-dimensional (2D) spin ice (so-called
kagom\'{e} ice) of Dy$_2$Ti$_2$O$_7$ shows a single scaling form. This behavior
can be understood in terms of the dynamical scaling law for 2D Coulomb gas (CG)
systems [Phys. Rev. B 90, 144428 (2014)], characterized by the charge
correlation length $\xi (\propto1/\sqrt{\omega_1})$, where $\omega_{1}$ is a
characteristic frequency proportional to the peak position of the imaginary
part of $\chi(\omega)$. It is a generic behavior among a wide variety of models
such as the vortex dynamics of 2D superconductors, 2D superfluids, classical XY
magnets, and dynamics of melting of Wigner crystals.",2103.12101v1
2021-08-28,Intrinsic Spike Timing Dependent Plasticity in Stochastic Magnetic Tunnel Junctions Mediated by Heat Dynamics,"The quest for highly efficient cognitive computing has led to extensive
research interest for the field of neuromorphic computing. Neuromorphic
computing aims to mimic the behavior of biological neurons and synapses using
solid-state devices and circuits. Among various approaches, emerging
non-volatile memory technologies are of special interest for mimicking
neuro-synaptic behavior. These devices allow the mapping of the rich dynamics
of biological neurons and synapses onto their intrinsic device physics. In this
letter, we focus on Spike Timing Dependent Plasticity (STDP) behavior of
biological synapses and propose a method to implement the STDP behavior in
Magnetic Tunnel Junction (MTJ) devices. Specifically, we exploit the
time-dependent heat dynamics and the response of an MTJ to the instantaneous
temperature to imitate the STDP behavior. Our simulations, based on a
macro-spin model for magnetization dynamics, show that, STDP can be imitated in
stochastic magnetic tunnel junctions by applying simple voltage waveforms as
the spiking response of pre- and post-neurons across an MTJ device.",2108.12684v1
2022-10-31,Dynamical fractal and anomalous noise in a clean magnetic crystal,"Fractals -- objects with non-integer dimensions -- occur in manifold settings
and length scales in nature, ranging from snowflakes and lightning strikes to
natural coastlines. Much effort has been expended to generate fractals for use
in many-body physics. Here, we identify an emergent dynamical fractal in a
disorder-free, stoichiometric three-dimensional magnetic crystal in
thermodynamic equilibrium. The phenomenon is born from constraints on the
dynamics of the magnetic monopole excitations in spin ice, which restrict them
to move on the fractal. This observation explains the anomalous exponent found
in magnetic noise experiments in the spin ice compound Dy$_2$Ti$_2$O$_7$, and
it resolves a long standing puzzle about its rapidly diverging relaxation time.
The capacity of spin ice to exhibit such striking phenomena holds promise of
further surprising discoveries in the cooperative dynamics of even simple
topological many-body systems.",2211.00051v2
2022-11-10,Transition to chaos and modal structure of magnetized Taylor-Couette flow,"Taylor-Couette flow is often used as a simplified model for complex rotating
flows in the interior of stars and accretion disks. The flow dynamics in these
objects is influenced by magnetic fields. For example, quasi-Keplerian flows in
Taylor-Couette geometry become unstable to a travelling or standing wave in an
external magnetic field if the fluid is conducting; there is an instability
even when the flow is hydrodynamically stable. This magnetorotational
instability leads to the development of chaotic states and, eventually,
turbulence, when the cylinder rotation is sufficiently fast. The transition to
turbulence in this flow can be complex, with the coexistence of parameter
regions with spatio-temporal chaos and regions with quasi-periodic behaviour,
involving one or two additional modulating frequencies. Although the unstable
modes of a periodic flow can be identified with Floquet analysis, here we adopt
a more flexible equation-free data-driven approach. We analyse the data from
the transition to chaos in the magnetized Taylor-Couette flow and identify the
flow structures related to the modulating frequencies with Dynamic Mode
Decomposition; this method is based on approximating nonlinear dynamics with a
linear infinite-dimensional Koopman operator. With the use of these structures,
one can construct a nonlinear reduced model for the transition.",2211.05545v1
2023-03-27,Inertial effects in ultrafast spin dynamics,"The dynamics of magnetic moments consist of a precession around the magnetic
field direction and a relaxation towards the field to minimize the energy.
While the magnetic moment and the angular momentum are conventionally assumed
to be parallel to each other, at ultrafast time scales their directions become
separated due to inertial effects. The inertial dynamics give rise to
additional high-frequency modes in the excitation spectrum of magnetic
materials. Here, we review the recent theoretical and experimental advances in
this emerging topic and discuss the open challenges and opportunities in the
detection and the potential applications of inertial spin dynamics.",2303.15251v2
2023-05-31,Novel slow dynamics of phase transition in the partially ordered frustrated magnet DyRu2Si2,"DyRu2Si2 is a frustrated magnet to exhibit multiple magnetic phase transition
in zero and finite magnetic fields. We investigated and characterized the phase
transition between the partially-ordered antiferromagnetic phases at zero field
by ac susceptibility measurements. Detailed ac susceptibility measurements
reveal the novel critical dynamics of the phase transition; extremely slow
dynamics with the relaxation time in the order of 10-100 msec, speed-up of the
dynamics on cooling indicating its non-thermally activated origin and growing
of the ferromagnetic correlations towards the phase transition temperature. On
the basis of these findings, we propose the novel phase transition process,
namely, the spontaneous striped-arrangement of the precedently emergent
""belt-like"" ferromagnetic spin texture.",2305.19656v2
2023-11-15,Single- and two-particle observables in the Emery model: a dynamical mean-field perspective,"We compare the dynamical mean-field descriptions of the single-band Hubbard
model and the three-band Emery model at the one- and two-particle level for
parameters relevant to high-Tc superconductors. We show that even within
dynamical mean-field theory, accounting solely for temporal fluctuations, the
intrinsic multi-orbital nature of the Emery model introduces effective
non-local correlations. These lead to a non-Curie-like temperature-dependence
of the magnetic susceptibility, also seen in nuclear magnetic resonance
experiments in the pseudogap regime by M. Avramovska, et al. [Journal of
Superconductivity and Novel Magnetism 33, 2621 (2020)]. We demonstrate the
agreement of our results with these experiments for a large range of dopings
and trace back the effective non-local correlations to an emerging
oxygen-copper singlet by analyzing a minimal finite-size cluster model. Despite
this correct description of the hallmark of the pseudogap at the two-particle
level, i.e., the drop in the Knight shift of nuclear magnetic resonance,
dynamical mean-field theory fails to properly describe the spectral properties
of the pseudogap.",2311.09023v1
2024-02-27,Semiclassical approach to spin dynamics of a ferromagnetic S=1 chain,"Motivated by recent experimental progress in the quasi-one-dimensional
quantum magnet NiNb$_2$O$_6$, we study the spin dynamics of an S=1
ferromagnetic Heisenberg chain with single-ion anisotropy by using a
semiclassical molecular dynamics approach. This system undergoes a quantum
phase transition from a ferromagnetic to a paramagnetic state under a
transverse magnetic field, and the magnetic responses reflecting this
transition is well described by our semiclassical method. We show that at
low-temperature the transverse component of the dynamical structure factor
depicts clearly the magnon dispersion, and the longitudinal component exhibits
two continua associated with single- and two-magnon excitations, respectively.
These spin excitation spectra show interesting temperature dependence as
effects of magnon interactions.Our findings shed light on experimental
detection of spin excitations in a large class of quasi-one-dimensional
magnets.",2402.17416v1
1998-01-30,Global three-dimensional simulations of magnetic field evolution in a galactic disk. II. Gas rich galaxies,"A fully three-dimensional computation of induction processes in a disk is
performed to investigate the evolution of a large-scale magnetic field in
gas-rich barred and spiral galaxies in the presence of field diffusion. As
input parameters we use time dependent velocity fields obtained from
self-consistent 3D N-body calculations of galactic dynamics. Our present work
primarily analyzes the influence of the gas flows in non-axisymmetric
gravitational disturbances (like spiral arms and bars) on the behavior of the
magnetic field. The magnetic field is found to evolve towards structures
resembling dynamical spiral arms and bars, however its distribution is often
much more complicated than the velocity field itself. We show that a random
configuration of seed magnetic fields quickly dissipates, and is therefore
unable to explain the magnetic field strength observed in nearby galaxies. The
detailed comparison between the simulated velocity and magnetic fields allows
us to establish that the reversals of magnetic vectors appear in the vicinity
of the corotation radius of non-axisymmetric disk structures, where the
velocity field shows rather abrupt changes. We argue that this explains the
observed field reversals in our Galaxy.",9801311v1
1998-08-13,Constraints on the Formation and Evolution of Circumstellar Disks in Rotating Magnetized Cloud Cores,"We use magnetic collapse models to place some constraints on the formation
and angular momentum evolution of circumstellar disks which are embedded in
magnetized cloud cores. Previous models have shown that the early evolution of
a magnetized cloud core is governed by ambipolar diffusion and magnetic
braking, and that the core takes the form of a nonequilibrium flattened
envelope which ultimately collapses dynamically to form a protostar. In this
paper, we focus on the inner centrifugally-supported disk, which is formed only
after a central protostar exists, and grows by dynamical accretion from the
flattened envelope. We estimate a centrifugal radius for the collapse of mass
shells within a rotating, magnetized cloud core. The centrifugal radius of the
inner disk is related to its mass through the two important parameters
characterizing the background medium: the background rotation rate $\Omb$ and
the background magnetic field strength $\Bref$. We also revisit the issue of
how rapidly mass is deposited onto the disk (the mass accretion rate) and use
several recent models to comment upon the likely outcome in magnetized cores.
Our model predicts that a significant centrifugal disk (much larger than a
stellar radius) will be present in the very early (Class 0) stage of
protostellar evolution. Additionally, we derive an upper limit for the disk
radius as it evolves due to internal torques, under the assumption that the
star-disk system conserves its mass and angular momentum even while most of the
mass is transferred to a central star.",9808140v1
2002-01-14,"Dynamical Simulations of Magnetically Channeled Line-Driven Stellar Winds: I. Isothermal, Nonrotating, Radially Driven Flow","We present numerical magnetohydrodynamic (MHD) simulations of the effect of
stellar dipole magnetic fields on line-driven wind outflows from hot, luminous
stars. Unlike previous fixed-field analyses, the simulations here take full
account of the dynamical competition between field and flow, and thus apply to
a full range of magnetic field strength, and within both closed and open
magnetic topologies. A key result is that the overall degree to which the wind
is influenced by the field depends largely on a single, dimensionless, `wind
magnetic confinement parameter', $\eta_{\ast}$ ($ = B_{eq}^2 R_\ast^2/{\dot M}
v_\infty$), which characterizes the ratio between magnetic field energy density
and kinetic energy density of the wind. For weak confinement $\eta_{\ast} \le
1$, the field is fully opened by the wind outflow, but nonetheless for
confinements as small as $\eta_{\ast}=1/10$ can have a significant
back-influence in enhancing the density and reducing the flow speed near the
magnetic equator. For stronger confinement $\eta_{\ast} > 1$, the magnetic
field remains closed over a limited range of latitude and height about the
equatorial surface, but eventually is opened into a nearly radial configuration
at large radii.",0201195v1
2009-05-06,Formation of gaseous arms in barred galaxies with dynamically important magnetic field : 3D MHD simulations,"We present results of three-dimensional nonlinear MHD simulations of a
large-scale magnetic field and its evolution inside a barred galaxy with the
back reaction of the magnetic field on the gas. The model does not consider the
dynamo process. To compare our modeling results with observations, we construct
maps of the high-frequency (Faraday-rotation-free) polarized radio emission on
the basis of simulated magnetic fields. The model accounts for the effects of
projection and the limited resolution of real observations. We performed 3D MHD
numerical simulations of barred galaxies and polarization maps. The main result
is that the modeled magnetic field configurations resemble maps of the
polarized intensity observed in barred galaxies. They exhibit polarization
vectors along the bar and arms forming coherent structures similar to the
observed ones. In the paper, we also explain the previously unsolved issue of
discrepancy between the velocity and magnetic field configurations in this type
of galaxies. The dynamical influence of the bar causes gas to form spiral waves
that travel outwards. Each gaseous spiral arm is accompanied by a magnetic
counterpart, which separates and survives in the inter-arm region. Because of a
strong compression, shear of non-axisymmetric bar flows and differential
rotation, the total energy of modeled magnetic field grows constantly, while
the azimuthal flux grows slightly until $0.05\Gyr$ and then saturates.",0905.0845v1
2010-10-19,Numerical Simulations of Dynamos Generated in Spherical Couette Flows,"We numerically investigate the efficiency of a spherical Couette flow at
generating a self-sustained magnetic field. No dynamo action occurs for
axisymmetric flow while we always found a dynamo when non-axisymmetric
hydrodynamical instabilities are excited. Without rotation of the outer sphere,
typical critical magnetic Reynolds numbers $Rm_c$ are of the order of a few
thousands. They increase as the mechanical forcing imposed by the inner core on
the flow increases (Reynolds number $Re$). Namely, no dynamo is found if the
magnetic Prandtl number $Pm=Rm/Re$ is less than a critical value $Pm_c\sim 1$.
Oscillating quadrupolar dynamos are present in the vicinity of the dynamo
onset. Saturated magnetic fields obtained in supercritical regimes (either
$Re>2 Re_c$ or $Pm>2Pm_c$) correspond to the equipartition between magnetic and
kinetic energies. A global rotation of the system (Ekman numbers $E=10^{-3},
10^{-4}$) yields to a slight decrease (factor 2) of the critical magnetic
Prandtl number, but we find a peculiar regime where dynamo action may be
obtained for relatively low magnetic Reynolds numbers ($Rm_c\sim 300$). In this
dynamical regime (Rossby number $Ro\sim -1$, spheres in opposite direction) at
a moderate Ekman number ($E=10^{-3}$), a enhanced shear layer around the inner
core might explain the decrease of the dynamo threshold. For lower $E$
($E=10^{-4}$) this internal shear layer becomes unstable, leading to small
scales fluctuations, and the favorable dynamo regime is lost. We also model the
effect of ferromagnetic boundary conditions. Their presence have only a small
impact on the dynamo onset but clearly enhance the saturated magnetic field in
the ferromagnetic parts. Implications for experimental studies are discussed.",1010.3859v2
2011-04-20,Stokes Diagnostics of Magneto-Acoustic Wave Propagation in the Magnetic Network on the Sun,"The solar atmosphere is magnetically structured and highly dynamic. Owing to
the dynamic nature of the regions in which the magnetic structures exist, waves
can be excited in them. Numerical investigations of wave propagation in
small-scale magnetic flux concentrations in the magnetic network on the Sun
have shown that the nature of the excited modes depends on the value of plasma
beta (the ratio of gas to magnetic pressure) where the driving motion occurs.
Considering that these waves should give rise to observable characteristic
signatures, we have attempted a study of synthesized emergent spectra from
numerical simulations of magneto-acoustic wave propagation. We find that the
signatures of wave propagation in a magnetic element can be detected when the
spatial resolution is sufficiently high to clearly resolve it, enabling
observations in different regions within the flux concentration. The
possibility to probe various lines of sight around the flux concentration bears
the potential to reveal different modes of the magnetohydrodynamic waves and
mode conversion. We highlight the feasibility of using the Stokes-V asymmetries
as a diagnostic tool to study the wave propagation within magnetic flux
concentrations. These quantities can possibly be compared with existing and new
observations in order to place constraints on different wave excitation
mechanisms.",1104.4069v2
2012-01-06,Experimental observation of the optical spin transfer torque,"The spin transfer torque is a phenomenon in which angular momentum of a spin
polarized electrical current entering a ferromagnet is transferred to the
magnetization. The effect has opened a new research field of electrically
driven magnetization dynamics in magnetic nanostructures and plays an important
role in the development of a new generation of memory devices and tunable
oscillators. Optical excitations of magnetic systems by laser pulses have been
a separate research field whose aim is to explore magnetization dynamics at
short time scales and enable ultrafast spintronic devices. We report the
experimental observation of the optical spin transfer torque, predicted
theoretically several years ago building the bridge between these two fields of
spintronics research. In a pump-and-probe optical experiment we measure
coherent spin precession in a (Ga,Mn)As ferromagnetic semiconductor excited by
circularly polarized laser pulses. During the pump pulse, the spin angular
momentum of photo-carriers generated by the absorbed light is transferred to
the collective magnetization of the ferromagnet. We interpret the observed
optical spin transfer torque and the magnetization precession it triggers on a
quantitative microscopic level. Bringing the spin transfer physics into optics
introduces a fundamentally distinct mechanism from the previously reported
thermal and non-thermal laser excitations of magnets. Bringing optics into the
field of spin transfer torques decreases by several orders of magnitude the
timescales at which these phenomena are explored and utilized.",1201.1436v1
2012-05-20,Externally-driven transformations of vortex textures in flat submicrometer magnets,"Two effects of oscillatory transformations of vortex textures in flat
nanomagnets due to the application of an external field or a spin-polarized
electric current are analytically described with relevance to soft-magnetic
structures of submicrometer sizes (whose thickness is significantly bigger than
the magnetostatic exchange length). These are changes of a domain wall (DW)
structure in a long magnetic stripe (oscillations between a transverse DW, a
vortex DW, and an antivortex DW) and periodic vortex-core reversals in a
circular magnetic dot which are accompanied by oscillatory displacements of the
vortex from the dot center. In nanostructures of smaller thicknesses
(comparable to the exchange length), where nonlocal magnetostatic effects are
very strong because of fast spatial variation of the magnetization, similar
phenomena have been widely studied previously. Here, the dynamics is
investigated within a local approach including magnetostatic field via boundary
conditions on solutions to the Landau-Lifshitz-Gilbert equation only. Both the
DWs in stripes and vortex states of the dot are treated as fragments of a
cross-tie DW. Despite similarity of the cyclic transformations of the ordering
to the dynamics of more strongly confined nanomagnets, details of motion
(trajectories) of the vortices and antivortices (Bloch lines) of the textures
under study are different, which is related to prohibition of rapid jumps of
the polarization of Bloch lines. In addition to the magnetization rotation
about the direction of magnetic field or current polarization, the evolution of
textures is shown to relate to oscillatory changes of the direction of a
cross-tie DW with respect to any arbitrary axis in the magnet plane accompanied
by oscillations of the DW width.",1205.5008v1
2012-06-24,Dynamical Relaxation of Coronal Magnetic Fields. III. 3D Spiral Nulls,"Context: The majority of studies on stressed 3D magnetic null points consider
magnetic reconnection driven by an external perturbation, but the formation of
a genuine current sheet equilibrium remains poorly understood. This problem has
been considered more extensively in two-dimensions, but lacks a generalization
into 3D fields.
  Aims: 3D magnetic nulls are more complex than 2D nulls and the field can take
a greater range of magnetic geometries local to the null. Here, we focus on one
type and consider the dynamical non-resistive relaxation of 3D spiral nulls
with initial spine-aligned current. We aim to provide a valid
magnetohydrostatic equilibrium, and describe the electric current accumulations
in various cases, involving a finite plasma pressure.
  Methods: A full MHD code is used, with the resistivity set to zero so that
reconnection is not allowed, to run a series of experiments in which a
perturbed spiral 3D null point is allowed to relax towards an equilibrium, via
real, viscous damping forces. Changes to the initial plasma pressure and other
magnetic parameters are investigated systematically.
  Results: For the axi-symmetric case, the evolution of the field and the
plasma is such that it concentrates the current density in two cone-shaped
regions along the spine, thus concentrating the twist of the magnetic field
around the spine, leaving a radial configuration in the fan plane. The plasma
pressure redistributes in order to maintain the current density accumulations.
However, it is found that changes in the initial plasma pressure do not modify
the final state significantly. In the cases where the initial magnetic field is
not axi-symmetric, a infinite-time singularity of current perpendicular to the
fan is found at the location of the null.",1206.5527v1
2013-10-21,The influence of the magnetic field on the spectral properties of blazars,"We explore the signature imprinted by dynamically relevant magnetic fields on
the spectral energy distribution (SED) of blazars. It is assumed that the
emission from these sources originates from the collision of cold plasma
shells, whose magnetohydrodynamic evolution we compute by numerically solving
Riemann problems. We compute the SEDs including the most relevant radiative
processes and scan a broad parameter space that encompasses a significant
fraction of the commonly accepted values of not directly measurable physical
properties. We reproduce the standard double hump SED found in blazar
observations for unmagnetized shells, but show that the prototype double hump
structure of blazars can also be reproduced if the dynamical source of the
radiation field is very ultrarelativistic both, in a kinematically sense
(namely, if it has Lorentz factors \gtrsim 50) and regarding its magnetization
(e.g., with flow magnetizations \sigma \simeq 0.1). A fair fraction of the {\em
blazar sequence} could be explained as a consequence of shell magnetization:
negligible magnetization in FSRQs, and moderate or large (and uniform)
magnetization in BL Lacs. The predicted photon spectral indices (\gph) in the
\gamma-ray band are above the observed values (\Gamma_{\rm ph, obs} \lesssim
2.6 for sources with redshifts 0.4\le z \le 0.6) if the magnetization of the
sources is moderate (\sigma \simeq 10^{-2}).",1310.5441v2
2014-10-23,First detection of surface magnetic fields in Post-AGB stars : the cases of U Monocerotis and R Scuti,"While several observational investigations have revealed the presence of
magnetic fields in the circumstellar envelopes, jets and outflows of
post-Asymptotic Giant Branch stars (PAGBs) and planetary nebulae (PNe), none
has clearly demonstrated their presence at the stellar surface. The lack of
information on the strength of the surface magnetic fields prevents us from
performing any thorough assessment of their dynamic capability (i.e. material
mixing, envelope shaping, etc). We present new high resolution
spectropolarimetric (Stokes V ) observations of a sample of PAGB stars,
realised with the instruments ESPaDOnS and Narval, where we searched for the
presence of photospheric magnetic fields. Out of the seven targets investigated
the RV Tauri stars U Mon and R Sct display a clear Zeeman signature and return
a definite detection after performing a least squares deconvolution (LSD)
analysis. The remaining five PAGBs show no significant detection. We derived
longitudinal magnetic fields of 10.2 +/- 1.7 G for U Mon and 0.6 +/- 0.6 G for
R Sct. In both cases the Stokes profiles point towards an interaction of the
magnetic field with the atmosphere dynamics. This first discovery of weak
magnetic fields (i.e. ~10 gauss level) at the stellar surface of PAGB stars
opens the door to a better understanding of magnetism in evolved stars.",1410.6224v1
2015-03-16,Dynamics of Current and Field Driven Domain Wall Motion under the Influence of Transverse Magnetic Field,"The dynamics of transverse Neel domain wall in a ferromagnetic nanostrip in
the presence of driving field, current and transverse magnetic field is
investigated by the Landau-Lifshitz-Gilbert(LLG) equation with the adiabatic
and non-adiabatic spin-transfer torques both analytically and numerically. The
analytical expressions for the velocity, width, excitation angle and
displacement for the domain wall are obtained by using small angle
approximation along with Walkers trial function. The results show that the
initial velocity of the domain wall can be controlled by the adiabatic
spin-transfer torque and the saturated velocity can be controlled by the
non-adiabatic spin-transfer torque and driving field. The large increase in the
saturated velocity of the domain wall driven by current and field due to the
transverse magnetic field is identified through the presence of driving field.
There is no impact in the saturated velocity of the domain wall driven by
current from the transverse magnetic field. For the domain wall driven by the
current in the presence of the transverse magnetic field, the saturated
velocity remains constant. The transverse magnetic field along with current and
driving field is more advantageous that the transverse magnetic field along
with current for increasing the saturated velocity of the domain wall. The
numerical results showed that the saturated velocity is increased by the
transverse magnetic field with the irrespective of the directions of the
driving field and current further it is higher and lower when the directions of
driving field and current are antiparallel and parallel respectively. The
obtained analytical solutions are closely coincided with the computed numerical
results.",1503.04560v2
2015-05-06,Anisotropy of the solar network magnetic field around the average supergranule,"Supergranules in the quiet Sun are outlined by a web-like structure of
enhanced magnetic field strength, the so-called magnetic network. We aim to map
the magnetic network field around the average supergranule near disk center. We
use observations of the line-of-sight component of the magnetic field from the
Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory
(SDO). The average supergranule is constructed by coaligning and averaging over
3000 individual supergranules. We determine the positions of the supergranules
with an image segmentation algorithm that we apply on maps of the horizontal
flow divergence measured using time-distance helioseismology. In the center of
the average supergranule the magnetic (intranetwork) field is weaker by about
2.2 Gauss than the background value (3.5 Gauss), whereas it is enhanced in the
surrounding ring of horizontal inflows (by about 0.6 Gauss on average). We find
that this network field is significantly stronger west (prograde) of the
average supergranule than in the east (by about 0.3 Gauss). With time-distance
helioseismology, we find a similar anisotropy. The observed anisotropy of the
magnetic field adds to the mysterious dynamical properties of solar
supergranulation.",1505.01427v2
2015-10-05,Chiral magnetic effect and natural optical activity in (Weyl) metals,"We consider the phenomenon of natural optical activity, and related chiral
magnetic effect in metals with low carrier concentration. To reveal the
correspondence between the two phenomena, we compute the optical conductivity
of a noncentrosymmetric metal to linear order in the wave vector of the light
wave, specializing to the low-frequency regime. We show that it is the orbital
magnetic moment of quasiparticles that is responsible for the natural optical
activity, and thus the chiral magnetic effect. While for purely static magnetic
fields the chiral magnetic effect is known to have a topological origin and to
be related to the presence of Berry curvature monopoles (Weyl points) in the
band structure, we show that the existence of Berry monopoles is not required
for the dynamic chiral magnetic effect to appear; the latter is thus not unique
to Weyl metals. The magnitude of the dynamic chiral magnetic effect in a
material is related to the trace of its gyrotropic tensor. We discuss the
conditions under which this trace is non-zero; in noncentrosymmetric Weyl
metals it is found to be proportional to the energy-space dipole moment of
Berry curvature monopoles. The calculations are done within both the
semiclassical kinetic equation, and Kubo linear response formalisms, with
coincident results.",1510.01304v1
2016-12-19,The Effects of Magnetic Fields on the Dynamics of Radiation Pressure Dominated Massive Star Envelopes,"We use three dimensional radiation magneto-hydrodynamic simulations to study
the effects of magnetic fields on the energy transport and structure of
radiation pressure dominated main sequence massive star envelopes at the region
of the iron opacity peak. We focus on the regime where the local thermal
timescale is shorter than the dynamical timescale, corresponding to inefficient
convective energy transport. We begin with initially weak magnetic fields
relative to the thermal pressure, from 100-1000G in differing geometries. The
unstable density inversion amplifies the magnetic field, increasing the
magnetic energy density to values close to equipartition with the turbulent
kinetic energy density. By providing pressure support, the magnetic field's
presence significantly increases the density fluctuations in the turbulent
envelope, thereby enhancing the radiative energy transport by allowing photons
to diffuse out through low density regions. Magnetic buoyancy brings small
scale magnetic fields to the photosphere and increases the vertical energy
transport with the energy advection velocity proportional to the Alfv\'en
velocity, although in all cases we study photon diffusion still dominates the
energy transport. The increased radiative and advective energy transport causes
the stellar envelope to shrink by several scale heights. We also find larger
turbulent velocity fluctuations compared to the purely hydrodynamic case,
reaching $\approx$ 100 km/s at the stellar photosphere. The photosphere also
shows vertical oscillations with similar averaged velocities and periods of a
few hours. The increased turbulent velocity and oscillations will have strong
impacts on the line broadening and periodic signals in massive stars.",1612.06434v1
2017-06-20,Energy transfer in compressible magnetohydrodynamic turbulence,"Magnetic fields, compressibility and turbulence are important factors in many
terrestrial and astrophysical processes. While energy dynamics, i.e. how energy
is transferred within and between kinetic and magnetic reservoirs, has been
previously studied in the context of incompressible magnetohydrodynamic (MHD)
turbulence, we extend shell-to-shell energy transfer analysis to the
compressible regime. We derive four new transfer functions specifically
capturing compressibility effects in the kinetic and magnetic cascade, and
capturing energy exchange via magnetic pressure. To illustrate their viability,
we perform and analyze four simulations of driven isothermal MHD turbulence in
the sub- and supersonic regime with two different codes. On the one hand, our
analysis reveals robust characteristics across regime and numerical method. For
example, energy transfer between individual scales is local and forward for
both cascades with the magnetic cascade being stronger than the kinetic one.
Magnetic tension and magnetic pressure related transfers are less local and
weaker than the cascades. We find no evidence for significant nonlocal
transfer. On the other hand, we show that certain functions, e.g., the
compressive component of the magnetic energy cascade, exhibit a more complex
behavior that varies both with regime and numerical method. Having established
a basis for the analysis in the compressible regime, the method can now be
applied to study a broader parameter space.",1706.06339v2
2018-02-27,Magnetic field dependence of the nonlinear magnetic response and tricritical point in the monoaxial chiral helimagnet Cr$_{1/3}$NbS$_{2}$,"We present a comprehensive study of the magnetization dynamics and phase
evolution in Cr$_{1/3}$NbS$_{2}$, which realizes a chiral soliton lattice
(CSL). The magnetic field dependence of the ac magnetic response is analyzed
for five harmonic components, $M_{n\omega}(H)$ $(n =1-5)$, using a phase
sensitive measurement over a frequency range, $f = 11 - 10,000$ Hz. At a
critical field, the modulated CSL continuously evolves from a helicity-rich to
a ferromagnetic domain-rich structure, where the crossover is revealed by the
onset of an anomalous nonlinear magnetic response that coincides with extremely
slow dynamics. The behavior is indicative of the formation of a spatially
coherent array of large ferromagnetic domains which relax on macroscopic
time-scales. The frequency dependence of the ac magnetic loss displays an
asymmetric distribution of relaxation times across the highly nonlinear CSL
regime, which shift to shorter time-scales with increasing temperature. We
experimentally resolve the tricritical point at $T_{TCP}$ in a temperature
regime above the ferromagnetic Curie temperature which separates the linear and
nonlinear regimes of the CSL at the phase transition. A comprehensive phase
diagram is constructed which summarized the features of the field and
temperature dependence of the magnetic crossovers and phase transitions in
Cr$_{1/3}$NbS$_{2}$.",1802.09742v2
2018-03-02,Response and phase transition of a Kitaev spin liquid in a local magnetic field,"We study the response of the Kitaev spin liquid (KSL) to a local magnetic
field perpendicular to the Kitaev honeycomb lattice. The local magnetic field
induces a dynamical excitation of a flux pair in the spin liquid and the system
can be described by a generally particle-hole asymmetric interacting resonant
level model. The dynamical excitation of the flux pair closes the flux gap in
the spectrum of the spin correlation function locally for the gapless KSL even
from the perturbative response to a weak magnetic field. Beyond the
perturbative regime, the p-h asymmetry competes with the magnetic field and
results in a rich phase diagram. Moreover, the magnetic field breaks the gauge
equivalence of the ferromagnetic and anti-ferromagnetic Kitaev couplings of the
ground state and leads to very different behaviors for the two cases. The
anti-ferromagnetic case experiences a first order phase transition to the
polarized state during magnetization whereas the ferromagnetic case does not.
This study can be generalized to the Kitaev model in a uniform magnetic field
and may help understand issues in recent experiments on KSL candidates.",1803.01011v4
2018-05-15,Non-linear Galactic Dynamos and the Magnetic Rädler Effect,"We show that the magnetic analogue of the R\""{a}dler effect of mean-field
dynamo theory leads to a non-linear backreaction that quenches a large-scale
galactic dynamo, and can result in saturation of the large-scale magnetic field
at near-equipartition with turbulent kinetic energy density. In a rotating
fluid containing small-scale magnetic fluctuations, anisotropic terms in the
mean electromotive force are induced via the Coriolis effect and these terms
lead to a reduction of the growth rate in a predominantly $\alpha\Omega$-type
galactic dynamo (Chamandy & Singh 2017). By including the generation of
small-scale magnetic fluctuations by turbulent tangling of the large-scale
magnetic field, one obtains a negative feedback effect that quenches the dynamo
and leads to the saturation of the large-scale field. This saturation mechanism
is found to be competitive with the dynamical $\alpha$-quenching mechanism for
realistic galactic parameter values. Furthermore, in the context of the
dynamical $\alpha$-quenching model, a separate non-linear term is obtained
which has the same form as the helicity flux term of Vishniac & Cho (2001), but
which depends on the strength of small-scale magnetic fluctuations. We briefly
discuss the observational implications of the magnetic R\""{a}dler effect for
galaxies.",1805.05548v2
2018-05-21,Confinement of the Crab Nebula with tangled magnetic field by its supernova remnant,"A pulsar wind is a relativistic outflow dominated by Poynting energy at its
base. Based on the standard ideal magnetohydrodynamic (MHD) model of pulsar
wind nebulae (PWNe) with the ordered magnetic field, the observed slow
expansion $v_{\rm PWN} \ll c$ requires the wind to be dominated by kinetic
energy at the upstream of its termination shock, which conflicts with the
pulsar wind theory ($\sigma$-problem). In this paper, we extend the standard
model of PWNe by phenomenologically taking into account conversion of the
ordered to turbulent magnetic field and dissipation of the turbulent magnetic
field. Disordering of the magnetic structure is inferred from the recent
three-dimensional relativistic ideal MHD simulations, while magnetic
dissipation is a non-ideal MHD effect requiring a finite resistivity. We apply
this model to the Crab Nebula and find that the conversion effect is important
for the flow deceleration, while the dissipation effect is not. Even for
Poynting-dominated pulsar wind, we obtain the Crab Nebula's $v_{\rm PWN}$ by
adopting a finite conversion time-scale of $\sim 0.3$ yr. Magnetic dissipation
primarily affects the synchrotron radiation properties. Any values of the
pulsar wind magnetization $\sigma_{\rm w}$ are allowed within the present model
of the PWN dynamics alone, and even a small termination shock radius of $\ll
0.1$ pc reproduces the observed dynamical features of the Crab Nebula. In order
to establish a high-$\sigma_{\rm w}$ model of PWNe, it is important to extend
the present model by taking into account the broadband spectrum and its spacial
profiles.",1805.08325v1
2018-06-22,"Magnetic clustering, half-moons, and shadow pinch points as signals of a proximate Coulomb phase in frustrated Heisenberg magnets","We study the formation of magnetic clusters in frustrated magnets in their
cooperative paramagnetic regime. For this purpose, we consider the
$J_1$-$J_2$-$J_3$ classical Heisenberg model on kagome and pyrochlore lattices
with $J_2 = J_3=J$. In the absence of farther-neighbor couplings, $J=0$, the
system is in the Coulomb phase with magnetic correlations well characterized by
pinch-point singularities. Farther-neighbor couplings lead to the formation of
magnetic clusters, which can be interpreted as a counterpart of
topological-charge clusters in Ising frustrated magnets [T. Mizoguchi, L. D. C.
Jaubert and M. Udagawa, Phys. Rev. Lett. {\bf 119}, 077207 (2017)]. The
concomitant static and dynamical magnetic structure factors, respectively
$\mathcal{S}({\bm{q}})$ and $\mathcal{S}({\bm{q}},\omega)$, develop half-moon
patterns. As $J$ increases, the continuous nature of the Heisenberg spins
enables the half-moons to coalesce into connected `star' structures spreading
across multiple Brillouin zones. These characteristic patterns are a dispersive
complement of the pinch point singularities, and signal the proximity to a
Coulomb phase. Shadows of the pinch points remain visible at finite energy,
$\omega$. This opens the way to observe these clusters through (in)elastic
neutron scattering experiments. The origin of these features are clarified by
complementary methods: large-$N$ calculations, semi-classical dynamics of the
Landau-Lifshitz equation, and Monte Carlo simulations. As promising candidates
to observe the clustering states, we revisit the origin of ""spin molecules""
observed in a family of spinel oxides $AB_2$O$_4$ ($A=$ Zn, Hg, Mg, $B=$ Cr,
Fe).",1806.08534v2
2018-12-13,Dispersion of small magnetic elements inside active regions on the Sun,"A process of diffusion of small-scale magnetic elements inside four active
regions (ARs) was analyzed. Line-of-sight magnetograms acquired by the
Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamic Observatory
(SDO) during a two-day time interval around the AR culmination time were
utilized. Small magnetic elements of size of 3-100 squared HMI pixels with the
field strength above the detection threshold of 30 Mx cm$^{-2}$ were detected
and tracked. The turbulent diffusion coefficient was retrieved using the
pair-separation technique. Comparison with the previously reported quiet-sun
(QS) diffusivity was performed. It was found that: i) dispersion of small-scale
magnetic elements inside the AR area occurs in the regime close to normal
diffusion, whereas well-pronounced super-diffusion is observed in QS; ii) the
diffusivity regime operating in an AR (the magnitude of the spectral index and
the range of the diffusion coefficient) does not seem to depend on the
individual properties of an AR, such as total unsigned magnetic flux, state of
evolution, and flaring activity. We conclude that small-scale magnetic elements
inside an AR do not represent an undisturbed photosphere, but they rather are
intrinsic part of the whole coherent magnetic structure forming an active
region. Moreover, turbulence of small-scale elements in an AR is not closely
related to processes above the photosphere, but it rather carries the footprint
of the sub-photospheric dynamics.",1812.05469v1
2019-02-05,Unconventional magnetism in the 4d$^{4}$ based ($S=1$) honeycomb system Ag$_{3}$LiRu$_{2}$O$_{6}$,"We have investigated the thermodynamic and local magnetic properties of the
Mott insulating system Ag$_{3}$LiRu$_{2}$O$_{6}$ containing Ru$^{4+}$
(4$d$$^{4}$) for novel magnetism. The material crystallizes in a monoclinic
$C2/m$ structure with RuO$_{6}$ octahedra forming an edge-shared
two-dimensional honeycomb lattice with limited stacking order along the
$c$-direction. The large negative Curie-Weiss temperature ($\theta_{CW}$ = -57
K) suggests antiferromagnetic interactions among Ru$^{4+}$ ions though magnetic
susceptibility and heat capacity show no indication of magnetic long-range
order down to 1.8 K and 0.4 K, respectively. $^{7}$Li nuclear magnetic
resonance (NMR) shift follows the bulk susceptibility between 120-300 K and
levels off below 120 K. Together with a power-law behavior in the temperature
dependent spin-lattice relaxation rate between 0.2 and 2 K, it suggest dynamic
spin correlations with gapless excitations. Electronic structure calculations
suggest an $S = 1$ description of the Ru-moments and the possible importance of
further neighbour interactions as also bi-quadratic and ring-exchange terms in
determining the magnetic properties. Analysis of our $\mu$SR data indicates
spin freezing below 5 K but the spins remain on the borderline between static
and dynamic magnetism even at 20 mK.",1902.01653v1
2019-03-07,Spin-orbit Torque and Spin Hall Effect-based Cellular Level Therapeutic Neuromodulators: Modulating Neuron Activities through Spintronic Nanodevices,"Artificial modulation of a neuronal subset through ion channels activation
can initiate firing patterns of an entire neural circuit in vivo. As nanovalves
in the cell membrane, voltage-gated ion channels can be artificially controlled
by the electric field gradient that caused by externally applied time varying
magnetic fields. Herein, we theoretically investigate the feasibility of
modulating neural activities by using magnetic spintronic nanostructures. An
antiferromagnet/ferromagnet (AFM/FM) structure is explored as neuromodulator.
For FM layer with perpendicular magnetization, stable bidirectional
magnetization switching can be achieved by applying in-plane currents through
AFM layer to induce the spin-orbit torque (SOT) due to the spin Hall effect
(SHE). This Spin-orbit Torque Neurostimulator (SOTNS) utilizes in-plane charge
current pulses to switch the magnetization in FM layer. The time changing
magnetic stray field induces electric field that modulates the surrounding
neurons. The Object Oriented Micromagnetic Framework (OOMMF) is used to
calculate space and time dependent magnetic dynamics of SOTNS structure. The
current driven magnetization dynamics in SOTNS has no mechanically moving
parts. Furthermore, the size of SOTNS can be down to tens of nanometers, thus,
arrays of SOTNSs could be fabricated, integrated together and patterned on a
flexible substrate, which gives us much more flexible control of the
neuromodulation with cellular resolution.",1903.02726v1
2019-04-05,Coexistence of magnetic order and persistent spin dynamics in a quantum kagome antiferromagnet with no intersite mixing,"One of the key questions concerning frustrated lattices that has lately
emerged is the role of disorder in inducing spin-liquid-like properties. In
this context, the quantum kagome antiferromagnets YCu$_3$(OH)$_6$Cl$_3$, which
has been recently reported as the first geometrically perfect realization of
the kagome lattice with negligible magnetic/non-magnetic intersite mixing and a
possible quantum-spin-liquid ground state, is of particular interest. However,
contrary to previous conjectures, here we show clear evidence of bulk magnetic
ordering in this compound below $T_N=15$\,K by combining bulk magnetization and
heat capacity measurements, and local-probe muon spin relaxation measurements.
The magnetic ordering in this material is rather unconventional in several
respects. Firstly, a crossover regime where the ordered state coexists with the
paramagnetic state extends down to $T_N/3$ and, secondly, the fluctuation
crossover is shifted far below $T_N$. Moreover, a reduced magnetic-entropy
release at $T_N$ and persistent spin dynamics that is observed at temperatures
as low as $T/T_N=1/300$ could be a sign of emergent excitations of correlated
spin-loops or, alternatively, a sign of fragmentation of each magnetic moment
into an ordered and a fluctuating part.",1904.02878v2
2019-05-28,Magnetic Inhibition of Centrifugal Instability in Astrophysical Jets,"Recently it was shown that the centrifugal instability may be important in
the dynamics of astrophysical jets undergoing reconfinement by external
pressure. However, these studies were limited to the case of unmagnetised
flows. Here we explore the role of the magnetic field within both the Newtonian
and relativistic frameworks. Since the jet problem is rather complicated, we
focus instead on the simpler problem of cylindrical rotation and axial magnetic
field, which shares significant similarity with the jet problem, and consider
only axisymmetric perturbations. The studied equilibrium configurations involve
a cylindrical interface and they are stable to non-magnetic centrifugal and
magneto-rotational instabilities everywhere except this interface. We use a
heuristic approach to derive the local stability criterion for the interface in
the magnetic case and numerical simulations to verify the role of the magnetic
field. The theory and simulations agree quite well for Newtonian models but
indicate a potential discrepancy for the relativistic models in the limit of
high Lorentz factor of the rotational motion at the interface. In general, the
magnetic field sets a critical wavelength below which the centrifugal modes are
stabilised. We discuss the implication of our findings for the astrophysical
jets, which suggest that the centrifugal instability develops only in jets with
relatively low magnetisation. Namely, the magnetic pressure has to be below the
thermal one and for the relativistic case the jets have to be kinetic-energy
dominated.",1905.11650v2
2020-02-19,Softening of breathing elastic mode and trigonal elastic mode in disordered pyrochlore magnet NaCaCo$_2$F$_7$,"Cobalt pyrochlore fluoride NaCaCo$_2$F$_7$ is a disordered frustrated magnet
composed of Co$^{2+}$ ions with an effective spin-$\frac{1}{2}$ magnetic moment
and exhibits spin freezing below $T_f \sim$2.4 K. We perform ultrasound
velocity measurements on a single crystal of the cubic NaCaCo$_2$F$_7$. The
temperature dependence of the bulk modulus (the breathing elastic mode)
exhibits Curie-type softening upon cooling below $\sim$20 K down to $T_f$,
which is suppressed by the magnetic field. This Curie-type softening should be
a precursor to the enhancement of the strength of exchange disorder via the
spin-lattice coupling, which causes the spin freezing. In contrast to the
magnetic-field-suppressed Curie-type softening in the bulk modulus, the
trigonal shear modulus exhibits softening with a characteristic minimum upon
cooling, which is enhanced by the magnetic field at temperatures below $\sim$20
K. This magnetic-field-enhanced elastic anomaly in the trigonal shear modulus
suggests a coupling of the lattice to the dynamical spin-cluster state. For
NaCaCo$_2$F$_7$, the observed elastic anomalies reveal an occurrence of
magnetic-field-induced crossover from an isostructural lattice instability
toward the spin freezing to a trigonal lattice instability arising from the
emergent dynamical spin-cluster state.",2002.07980v2
2020-04-07,Magnetic Order Driven Ultrafast Phase Transition in NdNiO$_3$,"Ultrashort x-ray pulses can be used to disentangle magnetic and structural
dynamics and are accordingly utilized here to study the photoexcitation of
NdNiO$_3$ (NNO), a model nickelate exhibiting structural and magnetic dynamics
that conspire to induce an IMT. During the course of the photoinduced insulator
to metal transition (IMT) with above gap excitation, we observe an ultrafast
($<$ 180 fs) quenching of magnetic order followed by a time delayed collapse of
the insulating phase probed by X-ray absorption and THz transmission (~ 450 fs)
that correlates with the slowest optical phonon mode involved in the structural
transition. A simultaneous order-disorder response at the Ni site and
displacive response at the Nd site coexist in the ultrafast magnetic response.
Crucially, we observe the optical phonon through its coherent coupling with Nd
magnetic order, demonstrating that the magnetic and structural degrees of
freedom both contribute in driving the IMT. Density functional theory (DFT)
calculations reveal a consistent scenario where optically driven inter-site
charge transfer (ICT) drives a collapse of antiferromagnetic order that in turn
destabilizes the charge-ordered phase resulting in an IMT. These experiments
provide new modalities for control of electronic phase transitions in quantum
materials based on ultrafast interplay between structural and magnetic orders
created by femtosecond photoexcitation.",2004.03694v2
2020-08-20,Two-dimensional numerical study for magnetic field dependence of neutrino-driven core-collapse supernova models,"We study the effects of the magnetic field on the dynamics of non-rotating
stellar cores by performing two-dimensional (2D), magnetohydrodynamics (MHD)
simulations. To this end, we have updated our neutrino-radiation-hydrodynamics
supernova code to include MHD employing a divergence cleaning method with both
careful treatments of finite volume and area reconstructions. By changing the
initial strength of the magnetic field, the evolution of $15.0$, $18.4$ and
$27.0$ $M_\odot$ presupernova progenitors is investigated. An intriguing
finding in our study is that the neutrino-driven explosion occurs regardless of
the strength of the initial magnetic field. For the 2D models presented in this
work, the neutrino heating is the main driver for the explosion, whereas the
magnetic field secondary contributes to the pre-explosion dynamics. Our results
show that the strong magnetic field weakens the growth of the neutrino-driven
turbulence in the small scale compared to the weak magnetic field. This results
in the slower increase of the turbulent kinetic energy in the postshock region,
leading to the slightly delayed onset of the shock revival for models with the
stronger initial magnetic field.",2008.08984v2
2020-10-10,Coherent Helicity-Dependent Spin-Phonon Oscillations in the Ferromagnetic van der Waals Crystal CrI3,"The discovery of two-dimensional (2D) systems hosting intrinsic long-range
magnetic order represents a seminal addition to the rich physical landscape of
van der Waals (vdW) materials. CrI3 has emerged as perhaps the most salient
example, as the interdependence of crystalline structure and magnetic order,
along with strong light-matter interactions provides a promising platform to
explore the optical control of magnetic, vibrational, and charge degrees of
freedom at the 2D limit. However, the fundamental question of how this
relationship between structure and magnetism manifests on their intrinsic
timescales has rarely been explored. Here, we use ultrafast optical
spectroscopy to probe magnetic and vibrational dynamics in CrI3, revealing
demagnetization dynamics governed by spin-flip scattering and remarkably, a
strong transient exchange-mediated interaction between lattice vibrations and
spin oscillations. The latter yields a coherent spin-coupled phonon mode that
is highly sensitive to the helicity of the driving pulse in the magnetically
ordered phase. Our results shed light on the nature of spin-lattice coupling in
vdW magnets on ultrafast timescales and highlight their potential for
applications requiring non-thermal, high-speed control of magnetism at the
nanoscale.",2010.04915v1
2020-11-18,Magnetoelastic effect in impact dynamics of nonNewtonian ferrofluid droplets,"In this article, we propose, with the aid of detailed experiments and scaling
analysis, the existence of magneto-elastic effects in the impact hydrodynamics
of non-Newtonian ferrofluid droplets on superhydrophobic (SH) surfaces in
presence of a magnetic field. The effects of magnetic Bond number (Bom), Weber
number (We), polymer concentration and magnetic nanoparticle (Fe3O4)
concentration in the ferrofluids were investigated. In comparison to Newtonian
ferrofluid droplets, addition of polymers caused rebound suppression of the
droplets relatively at lower Bom for a fixed magnetic nanoparticle
concentration and We. We further observed that for a fixed polymer
concentration and We, increasing magnetic nanoparticle concentration also
triggers earlier rebound suppression with increasing Bom. In the absence of the
magnetic nanoparticles, the non-Newtonian droplets do not show rebound
suppression for the range of Bom investigated. Likewise, the Newtonian
ferrofluids show rebound suppression at large Bom. This intriguing interplay of
elastic effects of polymer chains and the magnetic nanoparticles, dubbed as the
magneto-elastic effect is noted to lead to the rebound suppression. We
establish a scaling relationship to show that the rebound suppression is
observed as manifestation of onset of magneto-elastic instability only when the
proposed magnetic Weissenberg number (Wim) exceeds unity. We also put forward a
phase map to identify the various regimes of impact ferrohydrodynamics of such
droplets, and the occurrence of the magneto-elastic effect.",2011.09258v1
2021-06-18,Global Three-Dimensional Simulations of Outer Protoplanetary Disks with Ambipolar Diffusion,"The structure and evolution of protoplanetary disks (PPDs) are largely
governed by disk angular momentum transport, mediated by magnetic fields. In
the most observable outer disk, PPD gas dynamics is primarily controlled by
ambipolar diffusion as the dominant non-ideal magnetohydrodynamic (MHD) effect.
In this work, we study the gas dynamics in outer PPDs by conducting a set of
global 3D non-ideal MHD simulations with ambipolar diffusion and net poloidal
magnetic flux, using the Athena++ MHD code, with resolution comparable to local
simulations. Our simulations demonstrate the co-existence of magnetized disk
wind and turbulence driven by the magneto-rotational instability (MRI). While
MHD wind dominates disk angular momentum transport, the MRI turbulence also
contributes significantly. We observe that magnetic flux spontaneously
concentrate into axisymmetric flux sheets, leading to radial variations in
turbulence levels, stresses, and accretion rates. Annular substructures arise
as a natural consequence of magnetic flux concentration. The flux concentration
phenomena show diverse properties with different levels of disk magnetization
and ambipolar diffusion. The disk generally loses magnetic flux over time,
though flux sheets could prevent the leak of magnetic flux in some cases. Our
results demonstrate the ubiquity of disk annular substructures in weakly MRI
turbulent outer PPDs, and imply a stochastic nature of disk evolution.",2106.10167v2
2021-09-23,Translatory and rotatory motion of Exchange-Bias capped Janus particles controlled by dynamic magnetic field landscapes,"Magnetic Janus particles (MJPs), fabricated by covering a non-magnetic
spherical particle with a hemispherical magnetic in-plane exchange-bias layer
system cap, display an onion magnetization state for comparably large diameters
of a few microns. In this work, the motion characteristics of these MJPs will
be investigated when they are steered by a magnetic field landscape over
prototypical parallel-stripe domains, dynamically varied by superposed external
magnetic field pulse sequences, in an aqueous medium. We demonstrate, that due
to the engineered magnetization state in the hemispherical cap, a comparably
fast, directed particle transport and particle rotation can be induced.
Additionally, by modifying the frequency of the applied pulse sequence and the
strengths of the individual field components, we observe a possible separation
between a combined or an individual occurrence of these two types of motion.
Our findings bear importance for lab-on-a-chip systems, where particle
immobilization on a surface via analyte bridges shall be used for low
concentration analyte detection and a particle rotation over a defined position
of a substrate may dramatically increase the immobilization (and therefore
analyte detection) probability.",2109.11533v1
2021-12-12,Effects of forcing mechanisms on the multiscale properties of magnetohydrodynamics,"We performed numerical simulations to study the response of
magnetohydrodynamics (MHD) to large-scale stochastic forcing mechanisms
parametrized by one parameter, $0 \le a \le1$, going from direct injection on
the velocity field ($a = 1$) to stirring acts on the magnetic field only ($a =
0$). We study the multi-scale properties of the energy transfer, by splitting
the total flux in channels mediated by (i) the kinetic non-linear advection,
(ii) the Lorentz force, (iii) the magnetic advection and (iv) magnetic
stretching term. We further decompose the fluxes in two sub-channels given by
heterochiral and homochiral components in order to distinguish forward, inverse
and flux-loop cascades. We show that there exists a quasi-singular role of the
magnetic forcing mechanism for $a \sim 1$: a small injection on the magnetic
field $a < 1$ can strongly deplete the mean flux of kinetic energy transfer
throughout the kinetic non-linear advection channel. We also show that this
negligible mean flux is the result of a flux-loop balance between heterochiral
(direct) and homochiral (inverse) transfers. Conversely, both homochiral and
heterochiral channels transfer energy forward for the other three channels.
Cross exchange between velocity and the magnetic field is reversed around $a =
0.4$ and except when $a \sim 1$ we always observe that heterochiral mixed
velocity-magnetic energy triads tend to move energy from magnetic to velocity
fields. Our study is an attempt to further characterize the multi-scale nature
of MHD dynamics, by disentangling different properties of the total energy
transfer mechanisms, which can be useful for improving sub-grid-modelling.",2112.06286v1
2021-12-24,Magnetic field amplification and structure formation by the Rayleigh-Taylor instability,"We report on results of high resolution two fluid non-linear simulations of
the Rayleigh Taylor Instability (RTI) at the interface between a solar
prominence and the corona. These follow results reported earlier by Popescu
Braileanu et al. (2021a,b) on linear and early non-linear RTI dynamics in this
environment. The simulations use a two fluid model that includes collisions
between neutrals and charges, including ionization/recombination, energy and
momentum transfer, and frictional heating. High resolution 2.5D magnetized RTI
simulations with the magnetic field dominantly normal to and slightly sheared
with respect to the prominence plane demonstrate that in a fully developed
state of RTI a large fraction of the gravitational energy of a prominence
thread can be converted into quasi-turbulent energy of the magnetic field. RTI
magnetic energy generation is further accompanied by magnetic and plasma
density structure formation, including dynamic formation, break-up, and merging
of current sheets and plasmoid sub-structures. The simulations show the role of
flow decoupling and ionization/recombination reactions between the neutrals and
charges on the structure formation in magnetized RTI. We provide a careful
examination of sources and form of numerical dissipation of the evolving
magnetic field structures.",2112.13043v3
2022-01-05,Controllable nonreciprocal optical response and handedness-switching in magnetized spin orbit coupled graphene,"Starting from a low-energy effective Hamiltonian model, we theoretically
calculate the dynamical optical conductivity and permittivity tensor of a
magnetized graphene layer with Rashba spin orbit coupling (SOC). Our results
reveal a transverse Hall conductivity correlated with the usual nonreciprocal
longitudinal conductivity. Further analysis illustrates that for intermediate
magnetization strengths, the relative magnitudes of the magnetization and SOC
can be identified experimentally by two well-separated peaks in the dynamical
optical response (both the longitudinal and transverse components) as a
function of photon frequency. Moreover, the frequency dependent permittivity
tensor is obtained for a wide range of chemical potentials and magnetization
strengths. Employing experimentally realistic parameter values, we calculate
the circular dichroism of a representative device consisting of magnetized spin
orbit coupled graphene and a dielectric insulator layer, backed by a metallic
plate. The results reveal that this device has different relative
absorptivities for right-handed and left-handed circularly polarized
electromagnetic waves. It is found that the magnetized spin orbit coupled
graphene supports strong handedness-switchings, effectively controlled by
varying the chemical potential and magnetization strength with respect to the
SOC strength.",2201.01779v2
2022-04-30,Constraints on the magnetized Ernst black hole spacetime through quasiperiodic oscillations,"We study the dynamics of test particles around a magnetized Ernst black hole
considering its magnetic field in the environment surrounding the black hole.
We show how its magnetic field can influence the dynamics of particles and
epicyclic motion around the black hole. Based on the analysis, we find that the
radius of the innermost stable circular orbit (ISCO) for both neutral and
charged test particles and epicyclic frequencies are strongly affected by the
influence of the magnetic field. We also show that the ISCO radius of charged
particles decreases rapidly. It turns out that the gravitational and Lorentz
forces of the magnetic field are combined, thus strongly shrinking the values
of the ISCO of charged test particles. Finally, we obtain the generic form for
the epicyclic frequencies and select three microquasars with known
astrophysical quasiperiodic oscillation (QPO) data to constrain the magnetic
field.We show that the magnetic field is of the order of magnitude $B\sim
10^{-7}$ Gauss, taking into account the motion of neutral particles in circular
orbit about the black hole.",2205.00270v3
2022-05-24,Competing magnetic phases in the frustrated spin-1/2 chain compound $β$-TeVO$_4$ probed by NMR,"In frustrated spin-1/2 chains the competition between the nearest- and
next-nearest-neighbor exchange interactions leads to a rich phase diagram that
becomes even richer in the presence of perturbations in their material
realizations. These effects are still largely unexplored, so that new insight
into static and dynamic magnetism, in particular by sensitive local probes, is
highly desired. Here we present a comprehensive $^{17}$O nuclear magnetic
resonance study of $\beta$-TeVO$_4$, where the anisotropy of the main exchange
interactions and additional weak interchain exchange interactions complement
the theoretical phase diagram. Our results confirm the dynamical nature of the
intriguing spin-stripe phase that has been reported in previous studies. In
addition, we find that the magnetic order in the high-field phase, which
develops just below the magnetization saturation, is consistent with an unusual
type of spin-density-wave (SDW) order with different alignments of the magnetic
moments on the neighboring chains. This is reminiscent of the ordering in the
SDW phase, realized in the absence of the magnetic field, and is thus most
likely stabilized by magnetic anisotropy.",2205.11875v1
2023-01-30,Viscoelastic response of an active particle under the action of magnetic field,"We consider the dynamics of a charged inertial active Ornstein-Uhlenbeck
particle in a viscoelastic suspension under the action of an uniform magnetic
field. With the help of both numerical simulation and analytical framework, we
exactly investigate the viscoelastic response of the particle to the magnetic
field by means of particle trajectories, mean displacement, and mean square
displacement (MSD) calculations. The simulated particle trajectories and MSD
calculations reveal that the steady state response of a confined harmonic
particle to the magnetic field show interesting features due to the complex
interplay of underlying physical processes such as elastic dissipation and
active fluctuations in the medium. When the activity time scale of the dynamics
is larger than the elastic dissipation timescale (or vice-versa), the steady
state MSD shows an enhancement (or suppression) with increase in the field
strength. In both the cases, for very strong magnetic field the MSD
interestingly approaches the value at the equilibrium limit where the
generalized fluctuation dissipation relation is satisfied and the particle
behaves like an inertial passive Brownian particle. Thus, for a finite magnetic
field, the system exhibits a re-entrant type transition from active to passive
and then to active behaviour with persistence of elastic dissipation in the
medium. However, for a free particle, the overall displacement covered always
gets suppressed with increase in the strength of the magnetic field and finally
approaches zero for a very strong magnetic field.",2301.13179v1
2023-02-20,"Magnetic support, wind-driven accretion, coronal heating, and fast outflows in a thin magnetically arrested disc","Accretion discs properties should deviate from standard theory when magnetic
pressure exceeds the thermal pressure. To quantify these deviations, we present
a systematic study of the dynamical properties of magnetically arrested discs
(MADs), the most magnetized type of accretion disc. Using an artificial cooling
function to regulate the gas temperature, we study MADs of three different
thermal thicknesses, $h_\mathrm{th}/r=0.3, 0.1$ and $0.03$. We find that the
radial structure of the disc is never mostly supported by the magnetic field.
In fact, thin MADs are very near Keplerian. However, as discs gets colder, they
become more magnetized and the largest deviations from standard theory appear
in our thinnest disc with $h_\mathrm{th}/r=0.03$. In this case, the disc is
much more extended vertically and much less dense than in standard theory
because of vertical support from the turbulent magnetic pressure and
wind-driven angular momentum transport that enhances the inflow speed. The thin
disc also dissipates a lot of thermal energy outside of $z/r = \pm 0.03$ and a
significant fraction of this dissipation happens in mildly relativistic winds.
The enhanced dissipation in low-density regions could possibly feed coronae in
X-ray binaries (XRBs) and active galactic nuclei (AGN). Wind-driven accretion
will also impact the dynamical evolution of accretion discs and could provide a
mechanism to explain the rapid evolution of changing-look AGN and the secular
evolution of XRBs. Finally, our MAD winds have terminal velocities and mass
loss rates in good agreement with the properties of ultra-fast outflows
observed in AGN.",2302.10226v2
2023-05-25,Observation of c-axis Magnetization at Low Temperatures in Weak Ferromagnet FeBO$_3$ Reveals a Spin-Reorientation Transition,"The weak ferromagnet FeBO$_3$ is well known for being a unique system for
modelling and testing magnetic dynamics primarily due to relatively simple and
localized magnetic structure and its interesting spin wave dynamics. At room
temperature, it has slightly canted iron moments lying in the a-b plane that
result in a strong antiferromagnetic moment and a weak ferromagnetic moment,
which results in pronounced ferromagnetic and antiferromagnetic spin modes.
However, some previous studies have shown unusual low-temperature behavior that
suggests a phase transition. By performing low-temperature magnetization
measurements, both in bulk and on the mesoscale, we have observed a low
temperature magnetic texture in this material in which a large c-axis
magnetization occurs. Magnetic fields along the c-axis as high as 1300 Oe were
observed close to the sample surface. This presents evidence for the onset of a
Morin transition or another type of spin-reorientation phase transition wherein
the Fe3+ moments would acquire a c-axis component to their canting below a
critical temperature. The observation of this c-axis magnetization suggests
that there is a different ground state in this material than has been
previously expected and could be due to as yet unexplored intricacies of the
Dzyaloshinskii-Moriya interaction.",2305.16082v1
2023-06-26,Deciphering competing interactions of Kitaev-Heisenberg-$Γ$ system in clusters,"We study Kitaev-Heisenberg-$\Gamma$ Hamiltonian on finite size clusters and
investigate various zero and finite temperature properties in detail and
explore the key differences due to relative sign and magnitude of Kitaev($K$),
Heisenberg $(J)$) and $\Gamma$ interaction in the presence of magnetic field.
Nearest-neighbor spin-spin correlation function shows step like character when
$J$ is positive only irrespective of sign of $K$. Similarly the low energy
spectrum under magnetic field depends on the sign of $K$. Dynamics of Majorana
fermions depend greatly on external magnetic field which can even stabilize the
gauge fields to Kitaev limit in the presence of $J$. Majorana dynamics also
shows that $K-J-\Gamma$ model to be a testbed to manipulate quantum speed
limit. For small magnetic field, magnetization shows a dome like structure for
an intermediate temperature range for positive $K$. However a positive $J$ can
cause this for negative $K$ also. The zero temperature magnetisation and
susceptibility shows large dependence on external magnetic field and relative
sign and magnitude of $K$ and $J$. Either a negative $K$ or $J$ is shown to
give qualitatively similar susceptibility for different magnetic field only.
For the specific heat a negative $K$ or $J$ favors mostly single peak structure
while a two peak structure is favored for positive $K$ or $J$, though the
magnitude of magnetic field can causes re-entrant behavior also. We briefly
discuss effect of $\Gamma$ and compare with recent experiments.",2306.14839v1
2023-07-13,Manifestation of chiral magnetic current in Floquet-Weyl semimetals,"Materials that can host macroscopic persistent current are important because
they are useful for energy storage. However, there are very few examples of
such materials in nature. Superconductors are known as an example in which flow
of supercurrent can persist up to 100,000 years. The chiral magnetic current is
possibly the second example predicted by the chiral magnetic effect. It was
proposed to be realized in recently discovered Weyl semimetals. However, a
no-go theorem negates the chiral magnetic effect and shows that the chiral
magnetic current is generally absent in any equilibrium condensed-matter
system. Here we show how to break the no-go theorem by resorting to dynamical
transitions in time-frequency space. By driving an insulator using a
time-periodic potential and coupling it to a phonon heat bath that provides
suitable dissipation, we show that a Floquet-Weyl semi-metallic phase with
Fermi-Dirac-like distribution emerges. Furthermore, we show that even in the
presence of a static magnetic field, the resulting steady Floquet-Weyl
semimetal supports non-vanishing chiral magnetic current. Our dynamical model
provides a systematic way to fully realize the chiral magnetic effect in
condensed matter systems.",2307.06735v1
2023-09-25,Tunable topological magnetism in superlattices of nonmagnetic B20 systems,"We predict topological magnetic properties of B20 systems, that are organized
in atomically thin multilayers. In particular we focus on FeSi/CoSi and
FeSi/FeGe superlattices with different number of layers and interface
structure. We demonstrate that absence of long range magnetic order, previously
observed in bulk FeSi and CoSi, is broken near the FeSi/CoSi interface, where a
magnetic state with non-trivial topology appears. Using electronic structure
calculations in combination with the magnetic force theorem, we calculate the
Heisenberg and Dzyaloshinskii-Moriya (DM) interactions in these systems. With
this information, we perform atomistic spin dynamics simulations at finite
temperature and applied magnetic field for large supercells with up to
$2\cdot10^6$ spins to capture the complexity of non-collinear textures induced
by the DM interaction. The spin dynamics simulations predict the formation of
antiskyrmions in a [001]-oriented FeSi/CoSi multilayer, intermediate skyrmions
in a [111]-oriented FeSi/CoSi system and Bloch skyrmions in the FeSi/FeGe (001)
system. The size of different types of skyrmions is found to vary between 7 nm
and 37 nm. The varying topological magnetic texture in these systems can be
attributed to the complex asymmetric structure of the DM micromagnetic matrix,
which is different from previously known topological magnets. Furthermore,
through structural engineering, we demonstrate that both FM and AFM skyrmions
can be stabilized, where the latter are especially appealing for applications
due to the zero skyrmion Hall effect. The proposed B20 multilayers show
potential for further exploration and call for experimental confirmation.",2309.14421v1
2023-11-06,A new quantum hydrodynamic description of ferroelectricity in spiral magnets,"The strong coupling between magnetism and ferroelectricity was found in rare
earth manganites, where the electric polarization could be induced by special
magnetic ordering. There is no theoretical model that would allow us to study
the static and dynamic properties of electric polarization in strongly
correlated magnetic dielectrics. In the presented research, we have taken the
main step towards the construction of such a fundamental model, and made a
direct connection between the microscopic Katsura-Nagaosa-Balatsky theory and
Mostovoy's phenomenological model for magnetically induced polarization. A
novel description of the ferroelectricity of spin origin is proposed within the
framework of the many-particle quantum hydrodynamics method. It is applied to
the study of cells of magnetic ions, where the electric dipole moment is
proportional to the vector product of spins. Our approach is based on the
many-particle Pauli equation, where the influence of an external magnetic field
is considered. We define the electric dipole moment operator of the ion cell
and introduce the macroscopic polarization as the quantum mechanical average of
that operator. We formulate a model for the description of nonequilibrium
polarization and derive a new polarization evolution equation. The polarization
switching in ferroelectric magnets with the spiral spin-density-wave state is
considered, and we demonstrate that the proposed model yields known results and
can predict novel effects. The dynamic magnetoelectric effect can be
investigated by employing this novel equation to study the evolution of
polarization.",2311.03610v4
2023-11-10,Kinetic versus Magnetic Chaos in Toroidal Plasmas: A systematic quantitative comparison,"Magnetic field line chaos occurs under the presence of non-axisymmetric
perturbations of an axisymmetric equilibrium and is manifested by the
destruction of smooth flux surfaces formed by the field lines. These
perturbations also render the particle motion, as described by the guiding
center dynamics, non-integrable and, therefore, chaotic. However, the
chaoticities of the magnetic field lines and the particle orbits significantly
differ both in strength and radial location in a toroidal configuration, except
for the case of very low-energy particles whose orbits closely follow the
magnetic field lines. The chaoticity of more energetic particles, undergoing
large drifts with respect to the magnetic field lines, crucially determines the
confinement properties of a toroidal device but cannot be inferred from that of
the underlying magnetic field. In this work, we implement the Smaller ALignment
Index (SALI) method for detecting and quantifying chaos, allowing for a
systematic comparison between magnetic and kinetic chaos. The efficient
quantification of chaos enables the assignment of a value characterizing the
chaoticity of each orbit in the space of the three constants of the motion,
namely energy, magnetic moment and toroidal momentum. The respective diagrams
provide a unique overview of the different effects of a specific set of
perturbations on the entire range of trapped and passing particles, as well as
the radial location of the chaotic regions, offering a valuable tool for the
study of particle energy and momentum transport and confinement properties of a
toroidal fusion device.",2311.06213v2
2023-12-29,Spin-glass states generated in van der Waals magnet Cr$_2$Ge$_2$Te$_6$ by alkali-ion intercalation,"Tuning magnetic properties in layered van der Waals (vdW) materials has
captured a significant attention due to the efficient control of ground-states
by heterostructuring and external stimuli. Electron doping by electrostatic
gating, interfacial charge transfer and intercalation is particularly effective
in manipulating the exchange and spin-orbit properties, resulting in a control
of Curie temperature ($T_{\text{C}}$) and magnetic anisotropy. Here, we
discover an uncharted role of intercalation to generate magnetic frustration.
As a model study, we intercalate Na atoms into the vdW gaps of pristine
Cr$_2$Ge$_2$Te$_6$ (CGT) where generated magnetic frustration leads to emerging
spin-glass states coexisting with a ferromagnetic order. A series of dynamic
magnetic susceptibility measurements/analysis confirms the formation of
magnetic clusters representing slow dynamics with a distribution of relaxation
times. The intercalation also modifies other macroscopic physical parameters
including the significant enhancement of $T_{\text{C}}$ from 66K\, to 240\,K
and the switching of magnetic easy-hard axis direction. Our study identifies
intercalation as a unique route to generate emerging frustrated spin states in
simple vdW crystals.",2312.17554v2
2005-03-18,Relativistic Magnetohydrodynamics In Dynamical Spacetimes: Numerical Methods And Tests,"Many problems at the forefront of theoretical astrophysics require the
treatment of magnetized fluids in dynamical, strongly curved spacetimes. Such
problems include the origin of gamma-ray bursts, magnetic braking of
differential rotation in nascent neutron stars arising from stellar core
collapse or binary neutron star merger, the formation of jets and magnetized
disks around newborn black holes, etc. To model these phenomena, all of which
involve both general relativity (GR) and magnetohydrodynamics (MHD), we have
developed a GRMHD code capable of evolving MHD fluids in dynamical spacetimes.
Our code solves the Einstein-Maxwell-MHD system of coupled equations in
axisymmetry and in full 3+1 dimensions. We evolve the metric by integrating the
BSSN equations, and use a conservative, shock-capturing scheme to evolve the
MHD equations. Our code gives accurate results in standard MHD code-test
problems, including magnetized shocks and magnetized Bondi flow. To test our
code's ability to evolve the MHD equations in a dynamical spacetime, we study
the perturbations of a homogeneous, magnetized fluid excited by a gravitational
plane wave, and we find good agreement between the analytic and numerical
solutions.",0503420v2
2003-06-10,Muon-spin-relaxation and magnetic-susceptibility studies of the effects of nonmagnetic impurities on the Cu-spin dynamics and superconductivity in La_2-x_Sr_x_Cu_1-y_Zn_y_O_4_ around x = 0.115,"Zero-field muon-spin-relaxation (muSR) and magnetic-susceptibility
measurements have been carried out in La_2-x_Sr_x_Cu_1-y_Zn_y_O_4_ with x =
0.10, 0.115 and 0.13 changing the Zn-concentration y finely up to 0.10, with
the aim to clarify effects of the nonmagnetic impurity Zn on the Cu-spin
dynamics and superconductivity. The muSR measurements have revealed that, in
each x, a magnetic order of Cu spins is observed at y ~ 0.0075, while it
disappears and Cu spins turn into a fast fluctuating state for y > 0.03. From
the magnetic-susceptibility measurements, on the other hand, it has been found
that the volume fraction of the superconducting state rapidly decreases through
the slight doping of Zn and that its y dependence corresponds to the y
dependence of the volume fraction of the fast fluctuating region of Cu spins
estimated from the muSR results. Both the rapid decrease of the volume fraction
of the superconducting state and the formation of the magnetic order through
the slight doping of Zn can be interpreted in terms of the development of the
so-called ""Swiss cheese"" model. That is, it is concluded that Cu spins in a
non-superconducting region around Zn exhibit slowing-down of the fluctuations
or form an incoherent or coherent static magnetic order. Moreover, the
formation of a non-superconducting region around Zn is considered to be due to
the pinning of the dynamical spin correlation or dynamical stripe correlations.",0306233v2
2008-07-26,Dynamical Feedback of Self-generated Magnetic Fields in Cosmic Ray Modified Shocks,"We present a semi-analytical kinetic calculation of the process of non-linear
diffusive shock acceleration (NLDSA) which includes the magnetic field
amplification due to cosmic ray induced streaming instability, the dynamical
reaction of the amplified magnetic field and the possible effects of turbulent
heating. The approach is specialized to parallel shock waves and the parameters
we chose are the ones appropriate to forward shocks in Supernova Remnants. Our
calculation allows us to show that the net effect of the amplified magnetic
field is to enhance the maximum momentum of accelerated particles while
reducing the concavity of the spectra, with respect to the standard predictions
of NLDSA. This is mainly due to the dynamical reaction of the amplified field
on the shock, which noticeably reduces the modification of the shock precursor.
The total compression factors which are obtained for parameters typical of
supernova remnants are $R_{tot}\sim 7-10$, in good agreement with the values
inferred from observations. The strength of the magnetic field produced through
excitation of streaming instability is found in good agreement with the values
inferred for several remnants if the thickness of the X-ray rims are
interpreted as due to severe synchrotron losses of high energy electrons. We
also discuss the relative role of turbulent heating and magnetic dynamical
reaction in driving the reduction of the precursor modification.",0807.4261v2
2012-07-09,Dynamic phase transition properties and hysteretic behavior of a ferrimagnetic core-shell nanoparticle in the presence of a time dependent magnetic field,"We have presented dynamic phase transition features and stationary-state
behavior of a ferrimagnetic small nanoparticle system with a core-shell
structure. By means of detailed Monte Carlo simulations, a complete picture of
the phase diagrams and magnetization profiles have been presented and the
conditions for the occurrence of a compensation point $T_{comp}$ in the system
have been investigated. According to N\'{e}el nomenclature, the magnetization
curves of the particle have been found to obey P-type, N-type and Q-type
classification schemes under certain conditions. Much effort has been devoted
to investigation of hysteretic response of the particle and we observed the
existence of triple hysteresis loop behavior which originates from the
existence of a weak ferromagnetic core coupling $J_{c}/J_{sh}$, as well as a
strong antiferromagnetic interface exchange interaction $J_{int}/J_{sh}$. Most
of the calculations have been performed for a particle in the presence of
oscillating fields of very high frequencies and high amplitudes in comparison
with exchange interactions which resembles a magnetic system under the
influence of ultrafast switching fields. Particular attention has also been
paid on the influence of the particle size on the thermal and magnetic
properties, as well as magnetic features such as coercivity, remanence and
compensation temperature of the particle. We have found that in the presence of
ultrafast switching fields, the particle may exhibit a dynamic phase transition
from paramagnetic to a dynamically ordered phase with increasing ferromagnetic
shell thickness.",1207.2023v2
2017-05-03,Linearly polarized GHz magnetization dynamics of spin helix modes in the ferrimagnetic insulator Cu$_{2}$OSeO$_{3}$,"Linear dichroism -- the polarization dependent absorption of electromagnetic
waves -- is routinely exploited in applications as diverse as structure
determination of DNA or polarization filters in optical technologies. Here
filamentary absorbers with a large length-to-width ratio are a prerequisite.
For magnetization dynamics in the few GHz frequency regime strictly linear
dichroism was not observed for more than eight decades. Here, we show that the
bulk chiral magnet Cu$_{2}$OSeO$_{3}$ exhibits linearly polarized magnetization
dynamics at an unexpectedly small frequency of about 2 GHz. Unlike optical
filters that are assembled from filamentary absorbers, the magnet provides
linear polarization as a bulk material for an extremely wide range of
length-to-width ratios. In addition, the polarization plane of a given mode can
be switched by 90$^\circ$ via a tiny variation in width. Our findings shed a
new light on magnetization dynamics in that ferrimagnetic ordering combined
with anisotropic exchange interaction offers strictly linear polarization and
cross-polarized modes for a broad spectrum of sample shapes. The discovery
allows for novel design rules and optimization of microwave-to-magnon
transduction in emerging microwave technologies.",1705.01582v1
2018-07-03,Observations of solar chromospheric heating at sub-arcsec spatial resolution,"A wide variety of phenomena such as gentle but persistent brightening,
dynamic slender features (~100 km), and compact (~1'') ultraviolet (UV) bursts
are associated with the heating of the solar chromosphere. High spatio-temporal
resolution is required to capture the finer details of the likely magnetic
reconnection-driven, rapidly evolving bursts. Such observations are also needed
to reveal their similarities to large-scale flares, which are also thought to
be reconnection driven, and more generally their role in chromospheric heating.
Here we report observations of chromospheric heating in the form of a UV burst
obtained with the balloon-borne observatory, SUNRISE. The observed burst
displayed a spatial morphology similar to that of a large-scale solar flare
with circular ribbon. While the co-temporal UV observations at 1.5'' spatial
resolution and 24s cadence from the Solar Dynamics Observatory showed a compact
brightening, the SUNRISE observations at diffraction-limited spatial resolution
of 0.1'' at 7s cadence revealed a dynamic sub-structure of the burst that it is
composed of extended ribbon-like features and a rapidly evolving arcade of thin
(~0.1'' wide) magnetic loop-like features, similar to post-flare loops. Such a
dynamic sub-structure reveals the small-scale nature of chromospheric heating
in these bursts. Furthermore, based on magnetic field extrapolations, this
heating event is associated with a complex fan-spine magnetic topology. Our
observations strongly hint at a unified picture of magnetic heating in the
solar atmosphere from some large-scale flares to small-scale bursts, all being
associated with such a magnetic topology.",1807.01078v1
2018-07-30,Comparison of dynamical and equilibrium surface spin-reorientations: Inferences on the nature of the phase transitions in the presence of dipole interactions,"The influence of long-range dipole interactions on two dimensional magnetism
has been studied extensively in the spin-reorientation transition of
ferromagnetic ultrathin films. Although there is a great deal of experimental
information on the perpendicular domain phase that is stabilized by dipole
interactions, the transitions to or from the domain phase are subtle and
difficult to characterize experimentally. Magnetic susceptibility measurements
show no divergence in the vicinity of the spin-reorientation transition as a
function of thickness -- a null result that is difficult to interpret with
confidence. This article reports separate dynamical and equilibrium versions of
the reorientation transition in Fe/2ML Ni/W(110) films, using measurements of
the magnetic susceptibility as the films are grown. The dynamical version
occurs when increasing the film thickness causes the domain walls to depin, and
the system moves from a configuration that minimizes local energetics to one
that minimizes global energetics. The dynamical transition \textit{is} marked
by a divergent magnetic susceptibility measured with a field applied along the
in-plane W[001] direction. A comparative study of the two versions of the same
spin-reorientation transition aids in the experimental characterization of the
effects of dipole interactions on the phase transitions. This comparison
confirms the original null result found in magnetic susceptibility measurements
of the equilibrium transition; despite its name, the spin-reorientation
transition in ferromagnetic ultrathin films has no critical phase transition in
either the magnetization or its orientation.",1807.11434v2
2019-02-27,Deterministic field-free skyrmion nucleation at a nano-engineered injector device,"Magnetic skyrmions are topological solitons that exhibit an increased
stability against annihilation, and can be displaced with low current
densities, making them a promising candidate as an information carrier. In
order to demonstrate a viable skyrmion-based memory device, it is necessary to
reliably and reproducibly nucleate, displace, detect, and delete the magnetic
skyrmions. While the skyrmion displacement and detection have both been
investigated in detail, much less attention has been dedicated to the study of
the sub-ns dynamics of the skyrmion nucleation process. Only limited studies on
the statics and above-ns dynamics have been performed, leaving still many open
questions on the dynamics of the nucleation process. Furthermore, the vast
majority of the presently existing studies focus on the nucleation from random
natural pinning sites, or from patterned constrictions in the magnetic material
itself, which limit the functionality of the skyrmion-based device. Those
limitations can be overcome by the fabrication of a dedicated injector device
on top of the magnetic material. In this study, we investigate the nucleation
of magnetic skyrmions from a dedicated nano-engineered injector, demonstrating
the reliable magnetic skyrmion nucleation at the remnant state. The sub-ns
dynamics of the skyrmion nucleation process were also investigated, allowing us
to shine light on the physical processes driving the nucleation.",1902.10435v1
2019-11-16,Characterizing Magnetized Plasmas with Dynamic Mode Decomposition,"Accurate and efficient plasma models are essential to understand and control
experimental devices. Existing magnetohydrodynamic or kinetic models are
nonlinear, computationally intensive, and can be difficult to interpret, while
often only approximating the true dynamics. In this work, data-driven
techniques recently developed in the field of fluid dynamics are leveraged to
develop interpretable reduced-order models of plasmas that strike a balance
between accuracy and efficiency. In particular, dynamic mode decomposition
(DMD) is used to extract spatio-temporal magnetic coherent structures from the
experimental and simulation datasets of the HIT-SI experiment.
Three-dimensional magnetic surface probes from the HIT-SI experiment are
analyzed, along with companion simulations with synthetic internal magnetic
probes. A number of leading variants of the DMD algorithm are compared,
including the sparsity-promoting and optimized DMD. Optimized DMD results in
the highest overall prediction accuracy, while sparsity-promoting DMD yields
physically interpretable models that avoid overfitting. These DMD algorithms
uncover several coherent magnetic modes that provide new physical insights into
the inner plasma structure. These modes were subsequently used to discover a
previously unobserved three-dimensional structure in the simulation, rotating
at the second injector harmonic. Finally, using data from probes at
experimentally accessible locations, DMD identifies a resistive kink mode, a
ubiquitous instability seen in magnetized plasmas.",1911.06938v1
2021-08-20,NMR measurements in dynamically controlled field pulse,"We present the architecture of the versatile NMR spectrometer with
software-defined radio (SDR) technology and its application to the dynamically
controlled pulsed magnetic fields. The pulse-field technology is the only
solution to access magnetic fields greater than 50 T, but the NMR experiment in
the pulsed magnetic field was difficult because of the continuously changing
field strength. The dynamically controlled field pulse allows us to perform NMR
experiment in a quasi-steady field condition by creating a constant magnetic
field for a short time around the peak of the field pulse. We confirmed the
reproducibility of the field pulses using the NMR spectroscopy as a high
precision magnetometer. With the highly reproducible field strength we
succeeded in measuring the nuclear spin-lattice relaxation rate $1/T_1$, which
had never been measured by the pulse-field NMR experiment without dynamic field
control. We also implement the NMR spectrum measurement with both the
frequency-sweep and field-sweep modes and discuss the appropriate choice of
these modes depending on the magnetic properties of sample to be measured. This
development, with further improvement at a long-duration field pulse, will
innovate the microscopic measurement in extremely high magnetic fields.",2108.09163v1
2021-12-11,Optical excitation of electromagnons in hexaferrite,"Understanding ultrafast magnetization dynamics on the microscopic level is of
strong current interest due to the potential for applications in information
storage. In recent years, the spin-lattice coupling has been recognized to be
essential for ultrafast magnetization dynamics. Magnetoelectric multiferroics
of type II possess intrinsic correlations among magnetic sublattices and
electric polarization (P) through spin-lattice coupling, enabling fundamentally
coupled dynamics between spins and lattice. Here we report on ultrafast
magnetization dynamics in a room-temperature multiferroic hexaferrite
possessing ferrimagnetic and antiferromagnetic sublattices, revealed by
time-resolved resonant x-ray diffraction. A femtosecond above-bandgap
excitation triggers a coherent magnon in which the two magnetic sublattices
entangle and give rise to a transient modulation of P. A novel microscopic
mechanism for triggering the coherent magnon in this ferrimagnetic insulator
based on the spin-lattice coupling is proposed. Our finding opens up a novel
but general pathway for ultrafast control of magnetism.",2112.05961v1
2023-07-27,The dynamical magnetic-induced quark spin polarization at the early stage in heavy-ion collisions,"There has been significant interest in recent years in studying the spin
polarization of hyperons in heavy-ion collisions. However, describing the
dynamic spin polarization of quarks in quark-gluon plasma fireballs in detail
presents a significant challenge. In this study, we conducted a
phenomenological investigation of the dynamical spin polarization of quarks
induced by the magnetic field at the pre-thermal stage in heavy-ion collisions
using the recently developed theoretical tool of chiral kinetic theory. This
study presents a comprehensive analysis of the dynamic process of quark spin
polarization induced by magnetic fields. Our findings demonstrate that the spin
polarization of quarks is highly sensitive to the interactions between quarks.
These interactions can delay the decay of the early spin polarization vector
while accelerating the decay of the later spin polarization vector.
Specifically, our simulations model the detailed process of how magnetic fields
polarize quarks within the fireball by the average spin vector and reveal that
quark interactions lead to an acceleration effect on the average spin, causing
it to rapidly increase from zero before being rapidly destroyed. Notably, the
induced magnetic field in the fireball has an incomplete electromagnetic
response effect, and its increase in the opposite direction of the external
magnetic field decaying has a delay effect.",2308.00075v1
2024-03-05,Dynamics of magnetic self-propelled particles in a harmonic trap,"Artificial active particles, exemplified by Hexbugs (HB), serve as valuable
tools for investigating the intricate dynamics of active matter systems.
Leveraging their stochastic motion, Hexbugs provides an excellent experimental
model. In this study, we utilize Hexbugs equipped with disk-like armor and
embedded magnetic dipoles, transforming them into Magnetic Self-Propelled
Particles (MSPP). We explore the emergence of collective and stationary states
numerically and experimentally by confining these MSPPs within a parabolic
domain acting as a harmonic potential. Our findings unveil a diverse range of
metastable configurations intricately linked to the complex dynamics inherent
in the system. We discern that particle number, activity, and the balance
between magnetic and harmonic potential strengths predominantly influence the
metastability of these structures. By employing these parameters as control
factors, we compare and contrast the behavior of MSPPs with disk-like magnetic
Active Brownian Particles (ABPs) in the overdamped limit of vanishing inertia.
Our numerical predictions reproduce most of the experimental observations,
highlighting the crucial role of magnetic dipole interactions in developing
novel configurations for active particles within parabolic domains. These
configurations include chains, clusters, and vortex formations characterized by
a specific pattern in the particle spatial distribution. Notably, we observe
that the influence of inertia is not fundamental in generating metastable
configurations in these confined systems. Instead, the particle's shape,
activity, and orientation are dominant factors. This comparative analysis
provides insights into the distinctive features and dynamics of MSPP within
confined environments, shedding light on the role of short-range polar
interactions.",2403.02569v1
2008-09-30,Dynamics of diluted magnetic semiconductors from atomistic spin dynamics simulations: Mn doped GaAs as a case study,"The dynamical behavior of the magnetism of diluted magnetic semiconductors
(DMS) has been investigated by means of atomistic spin dynamics simulations.
The conclusions drawn from the study are argued to be general for DMS systems
in the low concentration limit, although all simulations are done for 5%
Mn-doped GaAs with various concentrations of As antisite defects. The
magnetization curve, $M(T)$, and the Curie temperature $T_C$ have been
calculated, and are found to be in good correspondence to results from Monte
Carlo simulations and experiments. Furthermore, equilibrium and non-equilibrium
behavior of the magnetic pair correlation function have been extracted. The
dynamics of DMS systems reveals a substantial short ranged magnetic order even
at temperatures at or above the ordering temperature, with a non-vanishing pair
correlation function extending up to several atomic shells. For the high As
antisite concentrations the simulations show a short ranged anti-ferromagnetic
coupling, and a weakened long ranged ferromagnetic coupling. For sufficiently
large concentrations we do not observe any long ranged ferromagnetic
correlation. A typical dynamical response shows that starting from a random
orientation of moments, the spin-correlation develops very fast ($\sim$ 1ps)
extending up to 15 atomic shells. Above $\sim$ 10 ps in the simulations, the
pair correlation is observed to extend over some 40 atomic shells. The
autocorrelation function has been calculated and compared with ferromagnets
like bcc Fe and spin-glass materials. We find no evidence in our simulations
for a spin-glass behaviour, for any concentration of As antisites. Instead the
magnetic response is better described as slow dynamics, at least when compared
to that of a regular ferromagnet like bcc Fe.",0809.5187v1
2004-09-30,A Study of Dynamic Finite Size Scaling Behavior of the Scaling Functions-Calculation of Dynamic Critical Index of Wolff Algorithm,"In this work we have studied the dynamic scaling behavior of two scaling
functions and we have shown that scaling functions obey the dynamic finite size
scaling rules. Dynamic finite size scaling of scaling functions opens
possibilities for a wide range of applications. As an application we have
calculated the dynamic critical exponent ($z$) of Wolff's cluster algorithm for
2-, 3- and 4-dimensional Ising models. Configurations with vanishing initial
magnetization are chosen in order to avoid complications due to initial
magnetization. The observed dynamic finite size scaling behavior during early
stages of the Monte Carlo simulation yields $z$ for Wolff's cluster algorithm
for 2-, 3- and 4-dimensional Ising models with vanishing values which are
consistent with the values obtained from the autocorrelations. Especially, the
vanishing dynamic critical exponent we obtained for $d=3$ implies that the
Wolff algorithm is more efficient in eliminating critical slowing down in Monte
Carlo simulations than previously reported.",0409762v1
2018-03-16,Energetics of turbulence generated by chiral MHD dynamos,"An asymmetry in the number density of left- and right-handed fermions is
known to give rise to a new term in the induction equation that can result in a
dynamo instability. At high temperatures, when a chiral asymmetry can survive
for long enough, this chiral dynamo instability can amplify magnetic fields
efficiently, which in turn drive turbulence via the Lorentz force. While it has
been demonstrated in numerical simulations that this chiral magnetically driven
turbulence exists and strongly affects the dynamics of the magnetic field, the
details of this process remain unclear. The goal of this paper is to analyse
the energetics of chiral magnetically driven turbulence and its effect on the
generation and dynamics of the magnetic field using direct numerical
simulations. We study these effects for different initial conditions, including
a variation of the initial chiral chemical potential and the magnetic Prandtl
number, Pm. In particular, we determine the ratio of kinetic to magnetic
energy, Upsilon, in chiral magnetically driven turbulence. Within the parameter
space explored in this study, Upsilon reaches a value of approximately
0.064-0.074 -independently of the initial chiral asymmetry and for Pm=1. Our
simulations suggest, that Upsilon decreases as a power law when increasing Pm
by decreasing the viscosity. While the exact scaling depends on the details of
the fitting criteria and the Reynolds number regime, an approximate result of
Upsilon(Pm)=0.1 Pm^{-0.4} is reported. Using the findings from our numerical
simulations, we analyse the energetics of chiral magnetically driven turbulence
in the early Universe.",1803.06350v2
2011-11-16,Non-Equilibrium Chemistry of Dynamically Evolving Prestellar Cores: I. Basic Magnetic and Non-Magnetic Models and Parameter Studies,"We combine dynamical and non-equilibrium chemical modeling of evolving
prestellar molecular cloud cores, and explore the evolution of molecular
abundances in the contracting core. We model both magnetic cores, with varying
degrees of initial magnetic support, and non-magnetic cores, with varying
collapse delay times. We explore, through a parameter study, the competing
effects of various model parameters in the evolving molecular abundances,
including the elemental C/O ratio, the temperature, and the cosmic-ray
ionization rate. We find that different models show their largest quantitative
differences at the center of the core, whereas the outer layers, which evolve
slower, have abundances which are severely degenerate among different dynamical
models. There is a large range of possible abundance values for different
models at a fixed evolutionary stage (central density), which demonstrates the
large potential of chemical differentiation in prestellar cores. However,
degeneracies among different models, compounded with uncertainties induced by
other model parameters, make it difficult to discriminate among dynamical
models. To address these difficulties, we identify abundance ratios between
particular molecules, the measurement of which would have maximal potential for
discrimination among the different models examined here. In particular, we find
that the ratios between NH3 and CO; NH2 and CO; NH3 and HCO+ are sensitive to
the evolutionary timescale, and that the ratio between HCN and OH is sensitive
to the C/O ratio. Finally, we demonstrate that measurements of the central
deviation (central depletion or enhancement) of abundances of certain molecules
are good indicators of the dynamics of the core.",1111.3948v2
2016-06-27,Blume-Capel ferromagnet driven by propagating and standing magnetic field wave: Dynamical modes and nonequilibrium phase transition,"The dynamical responses of Blume-Capel (S=1) ferromagnet to the plane
propagating (with fixed frequency and wavelength) and standing magnetic field
waves are studied sepa- rately in two dimensions by extensive Monte Carlo
simulation. Depending on the values of temperature, amplitude of the
propagating magnetic field and the strength of anisotropy, two different
dynamical phases are observed. For a fixed value of anisotropy and the
amplitude of the propagating magnetic field, the system undergoes a dynamical
phase transition from a driven spin wave propagating phase to a pinned or spin
frozen state as the system is cooled down. The time averaged magnetisation over
a full cycle of the propagating magnetic field plays the role of the dynamic
order parameter. A comprehensive phase diagram is plotted in the plane formed
by the amplitude of the propagating wave and the temperature of the system. It
is found that the phase boundary shrinks inward as the anisotropy increases.
The phase boundary, in the plane described by the strength of the anisotropy
and temperature, is also drawn. This phase boundary was observed to shrink
inward as the field amplitude increases.",1606.08155v3
2018-06-14,Magnetohydrodynamics and magneto-solutal transport mediated evaporation dynamics in paramagnetic pendent droplets under field stimulus,"Evaporation kinetics of pendant droplets is an area of immense importance in
several applications in addition to possessing rich fluid and thermal transport
physics. The present article experimentally and analytically sheds insight into
the augmented evaporation dynamics of paramagnetic pendent droplets in the
presence of a magnetic field stimulus. Literature provides information that
solutal advection and solutal Marangoni effect lead to enhancement of
evaporation in droplets with ionic inclusions. The major crux of the present
article remains to modulate the thermosolutal advection with the aid of
magnetic field and comprehend the dynamics of the evaporation process under
such complex multiphysics interactions. Experimental observations reveal that
the evaporation rate enhances as a direct function of the magnetic moment of
the solvated magnetic element ions, thereby pinpointing at the magnetophoretic
and magneto-solutal advection. Additionally, flow visualization by PIV
illustrates that the internal advection currents within the droplet are
strengthened in magnitude as well as distorted in orientation by the magnetic
field. A mathematical formalism based on magnetothermal and magnetosolutal
advection effects has been proposed via scaling analysis of the species and
energy conservation equations. The formalism takes into account all major
governing factors such as the magnetothermal and magnetosolutal Marangoni
numbers, magneto Prandtl and magneto Schmidt numbers and the Hartmann number.
The modeling establishes the magnetosolutal advection component to be the
domineering factor in augmented evaporation dynamics. Accurate validation of
the experimental internal circulation velocity is obtained from the proposed
model. The present study reveals rich insight on the magneto thermosolutal
hydrodynamics aspects in paramagnetic droplets.",1806.05627v1
2018-11-27,Hybrid magneto-dynamical modes in a single magnetostrictive nanomagnet on a piezoelectric substrate arising from magneto-elastic modulation of precessional dynamics,"Magneto-elastic (or ""straintronic"") switching has emerged as an extremely
energy-efficient mechanism for switching the magnetization of magnetostrictive
nanomagnets in magnetic memory, logic and non-Boolean circuits. Here, we
investigate the ultrafast magneto-dynamics associated with straintronic
switching in a single quasi-elliptical magnetostrictive Co nanomagnet deposited
on a piezoelectric PMN-PT substrate using time-resolved magneto-optical Kerr
effect (TR-MOKE) measurements. The pulsed laser pump beam in the TR-MOKE plays
a dual role: it causes precession of the nanomagnet's magnetization about an
applied bias magnetic field and it also generates surface acoustic waves (SAWs)
in the piezoelectric substrate that produce periodic strains in the
magnetostrictive nanomagnet and modulate the precessional dynamics. This
modulation gives rise to intriguing hybrid magneto-dynamical modes in the
nanomagnet, with rich spin wave texture. The characteristic frequencies of
these modes are 5-15 GHz, indicating that strain can affect magnetization in a
magnetostrictive nanomagnet in time scales much smaller than 1 ns (~100 ps).
This can enable ~10 GHz-range magneto-elastic nano-oscillators that are
actuated by strain instead of a spin-polarized current, as well as ultrafast
magneto-electric generation of spin waves for magnonic logic circuits,
holograms, etc.",1811.11297v1
2019-08-29,High-temperature spin dynamics in the Heisenberg chain: Magnon propagation and emerging KPZ-scaling in the zero-magnetization limit,"The large-scale dynamics of quantum integrable systems is often dominated by
ballistic modes due to the existence of stable quasi-particles. We here
consider as an archetypical example for such a system the spin-$\frac{1}{2}$
XXX Heisenberg chain that features magnons and their bound states. An
interesting question, which we here investigate numerically, arises with
respect to the fate of ballistic modes at finite temperatures in the limit of
zero magnetization $m{=}0$. At a finite magnetization density $m$, the spin
autocorrelation function $\Pi(x,t)$ (at high temperatures) typically exhibits a
trimodal behavior with left- and right-moving quasi-particle modes and a broad
center peak with slower dynamics. The broadening of the fastest propagating
modes exhibits a sub-diffusive $t^{1/3}$ scaling at large magnetization
densities, $m {\rightarrow} \frac{1}{2}$, familiar from non-interacting models;
it crosses over into a diffusive scaling $t^{1/2}$ upon decreasing the
magnetization to smaller values. The behavior of the center peak appears to
exhibit a crossover from transient super-diffusion to ballistic relaxation at
long times. In the limit $m{\to}0$, the weight carried by the propagating peaks
tends to zero; the residual dynamics is carried only by the central peak; it is
sub-ballistic and characterized by a dynamical exponent $z$ close to the value
$\frac{3}{2}$ familiar from Kardar-Parisi-Zhang (KPZ) scaling. We confirm,
employing elaborate finite-time extrapolations, that the spatial scaling of the
correlator $\Pi$ is in excellent agreement with KPZ-type behavior and analyze
the corresponding corrections.",1908.11432v2
2019-09-24,Spin-transfer dynamics in MgO-based magnetic tunnel junctions with an out-of-plane magnetized free layer and an in-plane polarizer,"Here, we present an analytical and numerical model describing the
magnetization dynamics in MgO-based spin-torque nano-oscillators with an
in-plane magnetized polarizer and an out-of-plane free layer. We introduce the
spin-transfer torque asymmetry by considering the cosine angular dependence of
the resistance between the two magnetic layers in the stack. For the analytical
solution, dynamics are determined by assuming a circular precession trajectory
around the direction perpendicular to the plane, as set by the effective field,
and calculating the energy integral over a single precession period. In a more
realistic approach, we include the bias dependence of the tunnel
magnetoresistance, which is assumed empirically to be a piecewise linear
function of the applied voltage. The dynamical states are found by solving the
stability condition for the Jacobian matrix for out-of-plane static states. We
find that the bias dependence of the tunnel magnetoresistance, which is an
inseparable effect in every tunnel junction, exhibits drastic impact on the
spin-torque nano-oscillator phase diagram, mainly by increasing the critical
current for dynamics and quenching the oscillations at high currents. The
results are in good agreement with our experimental data published elsewhere.",1909.10983v1
2019-12-16,Spin-current manipulation of photoinduced magnetization dynamics in heavy metal / ferromagnet double layer based nanostructures,"Spin currents offer a way to control static and dynamic magnetic properties,
and therefore they are crucial for next-generation MRAM devices or spin-torque
oscillators. Manipulating the dynamics is especially interesting within the
context of photo-magnonics. In typical $3d$ transition metal ferromagnets like
CoFeB, the lifetime of light-induced magnetization dynamics is restricted to
about 1 ns, which e.g. strongly limits the opportunities to exploit the wave
nature in a magnonic crystal filtering device. Here, we investigate the
potential of spin-currents to increase the spin wave lifetime in a functional
bilayer system, consisting of a heavy metal (8 nm of $\beta$-Tantalum
(Platinum)) and 5 nm CoFeB. Due to the spin Hall effect, the heavy metal layer
generates a transverse spin current when a lateral charge current passes
through the strip. Using time-resolved all-optical pump-probe spectroscopy, we
investigate how this spin current affects the magnetization dynamics in the
adjacent CoFeB layer. We observed a linear spin current manipulation of the
effective Gilbert damping parameter for the Kittel mode from which we were able
to determine the system's spin Hall angles. Furthermore, we measured a strong
influence of the spin current on a high-frequency mode. We interpret this mode
an an exchange dominated higher order spin-wave resonance. Thus we infer a
strong dependence of the exchange constant on the spin current.",1912.07728v1
2020-02-10,Faster chiral versus collinear magnetic order recovery after optical excitation revealed by femtosecond XUV scattering,"While chiral spin structures stabilized by Dzyaloshinskii-Moriya interaction
(DMI) are candidates as novel information carriers, their dynamics on the fs-ps
timescale is little known. Since with the bulk Heisenberg exchange and the
interfacial DMI two distinct exchange mechanisms are at play, the ultra-fast
dynamics of the chiral order needs to be ascertained and compared to the
dynamics of the conventional collinear order. Using an XUV free-electron laser
we determine the fs-ps temporal evolution of the chiral order in domain walls
in a magnetic thin film sample by an IR pump - X-ray magnetic scattering probe
experiment. Upon demagnetisation we observe that the dichroic (CL-CR) signal
connected with the chiral order correlator $m_z m_x$ in the domain walls
recovers significantly faster than the (CL+CR) sum signal representing the
average collinear domain magnetisation $m_z^2 + m_x^2$. We explore possible
explanations based on spin structure dynamics and reduced transversal
magnetisation fluctuations inside the domain walls and find that the latter can
explain the experimental data leading to different dynamics for collinear
magnetic order and chiral magnetic order.",2002.03971v2
2020-05-28,Spintronics meets nonadiabatic molecular dynamics: Geometric spin torque and damping on noncollinear classical magnetism due to electronic open quantum system,"We analyze a quantum-classical hybrid system of steadily precessing slow
classical localized magnetic moments, forming a head-to-head domain wall,
embedded into an open quantum system of fast nonequilibrium electrons. The
electrons reside within a metallic wire connected to macroscopic reservoirs.
The model captures the essence of dynamical noncollinear and noncoplanar
magnetic textures in spintronics, while making it possible to obtain the exact
time-dependent nonequilibrium density matrix of electronic system and split it
into four contributions. The Fermi surface contribution generates dissipative
(or damping-like in spintronics terminology) spin torque on the moments, and
one of the two Fermi sea contributions generates geometric torque dominating in
the adiabatic regime. When the coupling to the reservoirs is reduced, the
geometric torque is the only nonzero contribution. Locally it has both
nondissipative (or field-like in spintronics terminology) and damping-like
components, but with the sum of latter being zero, which act as the
counterparts of geometric magnetism force and electronic friction in
nonadiabatic molecular dynamics. Such current-independent geometric torque is
absent from widely used micromagnetics or atomistic spin dynamics modeling of
magnetization dynamics based on the Landau-Lifshitz-Gilbert equation, where
previous analysis of Fermi surface-type torque has severely underestimated its
magnitude.",2005.14153v2
2022-02-03,Nonequilibrium dynamics of $α$-RuCl$_{3}$ -- a time-resolved magneto-optical spectroscopy study,"We present time-resolved magneto-optical spectroscopy on the magnetic
Mott-Hubbard-insulating Kitaev spin liquid candidate $\alpha$-RuCl$_3$ to
investigate the nonequilibrium dynamics of its antiferromagnetically ordered
zigzag groundstate after photoexcitation. A systematic study of the transient
magnetic linear dichroism under different experimental conditions (temperature,
external magnetic field, photoexcitation density) gives direct access to the
dynamical interplay of charge excitations with the zigzag ordered state on
ultrashort time scales. We observe a rather slow initial demagnetization (few
to 10s of ps) followed by a long-lived non-thermal antiferromagnetic
spin-disordered state (100$-$1000s of ps), which can be understood in terms of
holons and doublons disordering the antiferromagnetic background after
photoexcitation. Varying temperature and fluence in the presence of an external
magnetic field reveals two distinct photoinduced dynamics associated with the
zigzag and quantum paramagnetic disordered phases. The photo-induced
non-thermal spin-disordered state shows universal compressed-exponential
recovery dynamics related to the growth and propagation of zigzag domains on
nanosecond time scales, which is interpreted within the framework of the
Fatuzzo-Labrune model for magnetization reversal. The study of nonequilibrium
states in strongly correlated materials is a relatively unexplored topic, but
our results are expected to be extendable to a large class of Mott-Hubbard
insulator materials with strong spin-orbit coupling.",2202.01648v1
2023-02-28,Magnetic field control of light-induced spin accumulation in monolayer MoSe$_2$,"Semiconductor transition metal dichalcogenides (TMDs) have equivalent
dynamics for their two spin/valley species. This arises from their
energy-degenerated spin states, connected via time-reversal symmetry. When an
out-of-plane magnetic field is applied, time-reversal symmetry is broken and
the energies of the spin-polarized bands shift, resulting in different bandgaps
and dynamics in the K$_+$ and K$_-$ valleys. Here, we use time-resolved Kerr
rotation to study the magnetic field dependence of the spin dynamics in
monolayer MoSe$_2$. We show that the magnetic field can control the
light-induced spin accumulation of the two valley states, with a small effect
on the recombination lifetimes. We unveil that the magnetic field-dependent
spin accumulation is in agreement with hole spin dynamics at the longer
timescales, indicating that the electron spins have faster relaxation rates. We
propose a rate equation model that suggests that lifting the energy-degeneracy
of the valleys induces an ultrafast spin-flip toward the stabilization of the
valley with the higher valence band energy. Our results provide an experimental
insight into the ultrafast charge and spin dynamics in TMDs and a way to
control it, which will be useful for the development of new spintronic and
valleytronic applications.",2302.14206v1
2023-06-20,Convolutional neural networks for large-scale dynamical modeling of itinerant magnets,"Complex spin textures in itinerant electron magnets hold promises for
next-generation memory and information technology. The long-ranged and often
frustrated electron-mediated spin interactions in these materials give rise to
intriguing localized spin structures such as skyrmions. Yet, simulations of
magnetization dynamics for such itinerant magnets are computationally difficult
due to the need for repeated solutions to the electronic structure problems. We
present a convolutional neural network (CNN) model to accurately and
efficiently predict the electron-induced magnetic torques acting on local
spins. Importantly, as the convolutional operations with a fixed kernel
(receptive field) size naturally take advantage of the locality principle for
many-electron systems, CNN offers a scalable machine learning approach to spin
dynamics. We apply our approach to enable large-scale dynamical simulations of
skyrmion phases in itinerant spin systems. By incorporating the CNN model into
Landau-Lifshitz-Gilbert dynamics, our simulations successfully reproduce the
relaxation process of the skyrmion phase and stabilize a skyrmion lattice in
larger systems. The CNN model also allows us to compute the effective receptive
fields, thus providing a systematic and unbiased method for determining the
locality of the original electron models.",2306.11833v1
2023-07-10,Kibble-Zurek Mechanism for Nonequilibrium Generation of Magnetic Monopoles in Spin Ices,"The proliferation of topological defects is a common out-of-equilibrium
phenomenon when a system is driven into a phase of broken symmetry. The
Kibble-Zurek mechanism (KZM) provides a theoretical framework for the critical
dynamics and generation of topological defects in such scenarios. One of the
early applications of KZM is the estimation of heavy magnetic monopoles left
behind by the cosmological phase transitions in the early universe. The
scarcity of such relic monopoles, which contradicts the prediction of KZM, is
one of the main motivations for cosmological inflationary theories. On the
other hand, magnetic monopoles as emergent quasi-particles have been observed
in spin ices, a peculiar class of frustrated magnets that remain disordered at
temperatures well below the energy scale of exchange interaction. Here we study
the annihilation dynamics of magnetic monopoles when spin ice is cooled to zero
temperature in a finite time. Through extensive Glauber dynamics simulations,
we find that the density of residual monopole follows a power law dependence on
the annealing rate. A kinetic reaction theory that precisely captures the
annihilation process from Monte Carlo simulations is developed. We further show
that the KZM can be generalized to describe the critical dynamics of spin ice,
where the exponent of the power-law behavior is determined by the dynamic
critical exponent $z$ and the cooling protocol.",2307.05267v1
1999-09-22,Deterministic Equations of Motion and Dynamic Critical Phenomena,"Taking the two-dimensional $\phi^4$ theory as an example, we numerically
solve the deterministic equations of motion with random initial states.
Short-time behavior of the solutions is systematically investigated. Assuming
that the solutions generate a microcanonical ensemble of the system, we
demonstrate that the second order phase transition point can be determined
already from the short-time dynamic behavior. Initial increase of the
magnetization and critical slowing down are observed. The dynamic critical
exponent z, the new exponent $\theta$ and the static exponents $\beta$ and
$\nu$ are estimated. Interestingly, the deterministic dynamics with random
initial states is in a same dynamic universality class of Monte Carlo dynamics.",9909322v1
2002-06-29,Solvable Kinetic Gaussian Model in External Field,"In this paper, the single-spin transition dynamics is used to investigate the
kinetic Gaussian model in a periodic external field. We first derive the
fundamental dynamic equations, and then treat an isotropic d-dimensional
hypercubic lattice Gaussian spin system with Fourier's transformation method.
We obtain exactly the local magnetization and the equal-time pair correlation
function. The critical characteristics of the dynamical, the complex
susceptibility, and the dynamical response are discussed. The results show that
the time evolution of the dynamical quantities and the dynamical responses of
the system strongly depend on the frequency and the wave vector of the external
field.",0207007v1
2007-10-29,Dynamic Phase Transitions in Superconductivity,"In this Letter, the dynamic phase transitions of the time-dependent
Ginzburg-Landau equations are analyzed using a newly developed dynamic
transition theory and a new classification scheme of dynamics phase
transitions. First, we demonstrate that there are two type of dynamic
transitions, jump and continuous, dictated by the sign of a nondimensional
parameter R. This parameter is computable, and depends on the material
property, the applied field, and the geometry of domain that the sample
occupies. Second, using the parameter R, precise analytical formulas for
critical domain size, and for critical magnetic fields are derived.",0710.5436v1
2008-10-09,Dynamics of strongly correlated ions in a partially ionized quantum plasma,"A scheme which allows to compute the dynamics of strongly correlated
classical ions embedded into a partiallzy ionized quantum plasma by first
principles molecular dynamics is presented. The dynamically screened dust
approach of Joyce and Lampe [Phys. Rev. Lett. {\bf 88}, 095006 (2002] ist
generalized to quantum systems. The electrons are treated fully
quantum-mechanically taking into account their dynamical screening of the
ion-ion interaction in linear response on the basis of an extended Mermin
formula. The scheme allows to include the effect of the electron dynamics,
electron streaming, wake effects and electron magnetization.",0810.1659v1
2016-06-15,Quantum dynamics of trapped ions in a dynamic field gradient using dressed states,"Novel ion traps that provide either a static or a dynamic magnetic gradient
field allow for the use of radio frequency (rf) radiation for coupling internal
and motional states of ions, which is essential for conditional quantum logic.
We show that the coupling mechanism in the presence of a dynamic gradient is
the same, in a dressed state basis, as in the case of a static gradient. Then,
it is shown how demanding experimental requirements arising when using a
dynamic gradient could be overcome. Thus, using dressed states in a dynamic
gradient field could decisively reduce experimental complexity on the route
towards a scalable device for quantum information science based on rf-driven
trapped ions.",1606.04821v1
2007-10-18,Magnetic fields and the dynamics of spiral galaxies,"We investigate the dynamics of magnetic fields in spiral galaxies by
performing 3D MHD simulations of galactic discs subject to a spiral potential.
Recent hydrodynamic simulations have demonstrated the formation of inter-arm
spurs as well as spiral arm molecular clouds provided the ISM model includes a
cold HI phase. We find that the main effect of adding a magnetic field to these
calculations is to inhibit the formation of structure in the disc. However,
provided a cold phase is included, spurs and spiral arm clumps are still
present if $\beta \gtrsim 0.1$ in the cold gas. A caveat to two phase
calculations though is that by assuming a uniform initial distribution, $\beta
\gtrsim 10$ in the warm gas, emphasizing that models with more consistent
initial conditions and thermodynamics are required. Our simulations with only
warm gas do not show such structure, irrespective of the magnetic field
strength. Furthermore, we find that the introduction of a cold HI phase
naturally produces the observed degree of disorder in the magnetic field, which
is again absent from simulations using only warm gas. Whilst the global
magnetic field follows the large scale gas flow, the magnetic field also
contains a substantial random component that is produced by the velocity
dispersion induced in the cold gas during the passage through a spiral shock.
Without any cold gas, the magnetic field in the warm phase remains relatively
well ordered apart from becoming compressed in the spiral shocks. Our results
provide a natural explanation for the observed high proportions of disordered
magnetic field in spiral galaxies and we thus predict that the relative
strengths of the random and ordered components of the magnetic field observed
in spiral galaxies will depend on the dynamics of spiral shocks.",0710.3558v1
2021-09-18,"Time-dependent, long-term hydrodynamic simulations of the inner protoplanetary disk I: The importance of stellar magnetic torques","We conduct simulations of the inner regions of protoplanetary disks (PPDs) to
investigate the effects of protostellar magnetic fields on their long-term
evolution. We use an inner boundary model that incorporates the influence of a
stellar magnetic field. The position of the inner disk is dependent on the mass
accretion rate as well as the magnetic field strength. We use this model to
study the response of a magnetically truncated inner disk to an episodic
accretion event. Additionally, we vary the protostellar magnetic field strength
and investigate the consequences of the magnetic field on the long-term
behavior of PPDs. We use the fully implicit 1+1D TAPIR code which solves the
axisymmetric hydrodynamic equations self-consistently. Our model allows us to
investigate disk dynamics close to the star and to conduct long-term evolution
simulations simultaneously and includes the radial radiation transport in the
stationary diffusion limit. We include stellar magnetic torques, the influence
of a pressure gradient, and a variable inner disk radius in the TAPIR code to
describe the innermost disk region in a more self-consistent manner and can
show that this approach alters the disk dynamics considerably compared to a
simplified diffusive evolution equation, especially during outbursts. The
influences of a prescribed stellar magnetic field, local pressure gradients,
and a variable inner disk radius result in a more consistent description of the
gas dynamics in the innermost regions of PPDs. Combining magnetic torques
acting on the innermost disk regions with the long-term evolution of PPDs
yields previously unseen results, whereby the whole disk structure is affected
over its entire lifetime. Additionally, we want to emphasize that a combination
of our 1+1D model with more sophisticated multi-dimensional codes could improve
the understanding of PPDs even further.",2109.08919v1
2022-01-05,Insensitivity of a turbulent laser-plasma dynamo to initial conditions,"It has recently been demonstrated experimentally that a turbulent plasma
created by the collision of two inhomogeneous, asymmetric, weakly magnetised
laser-produced plasma jets can generate strong stochastic magnetic fields via
the small-scale turbulent dynamo mechanism, provided the magnetic Reynolds
number of the plasma is sufficiently large. In this paper, we compare such a
plasma with one arising from two pre-magnetised plasma jets whose creation is
identical save for the addition of a strong external magnetic field imposed by
a pulsed magnetic field generator (`MIFEDS'). We investigate the differences
between the two turbulent systems using a Thomson-scattering diagnostic, X-ray
self-emission imaging and proton radiography. The Thomson-scattering spectra
and X-ray images suggest that the presence of the external magnetic field has a
limited effect on the plasma dynamics in the experiment. While the presence of
the external magnetic field induces collimation of the flows in the colliding
plasma jets and the initial strengths of the magnetic fields arising from the
interaction between the colliding jets are significantly larger as a result of
the external field, the energy and morphology of the stochastic magnetic fields
post-amplification are indistinguishable. We conclude that, for turbulent
laser-plasmas with super-critical magnetic Reynolds numbers, the
dynamo-amplified magnetic fields are determined by the turbulent dynamics
rather than the seed fields and modest changes in the initial flow dynamics of
the plasma, a finding consistent with theoretical expectations and simulations
of turbulent dynamos.",2201.01705v1
2017-06-06,Driven spin wave modes in XY ferromagnet: Nonequilibrium phase transition,"The dynamical responses of XY ferromagnet driven by linearly polarised
propagating and standing magnetic field wave have been studied by Monte Carlo
simulation in three dimensions. In the case of propagating magnetic field wave
(with specified amplitude, frequency and the wavelength), the low temperature
dynamical mode is a propagating spin wave and the system becomes structureless
(or random) in the high temperature. A dynamical symmetry breaking phase
transition is observed at a finite (nonzero) temperature. This symmetry
breaking is confirmed by studying the statistical distribution of the angle of
the spin vector. The dynamic nonequilibrium transition temperature was found to
decrease as the amplitude of the propagating magnetic field wave increased. A
comprehensive phase boundary is drawn in the plane formed by temperature and
amplitude of propagating field wave. The phase boundary was observed to shrink
(in the low temperature side) for longer wavelength of the propagating magnetic
wave. In the case of standing magnetic field wave, the low temperature
excitation is a standing spin wave which becomes structureless (or random) in
the high temperature. Here also, like the case of propagating magnetic wave, a
dynamical symmetry breaking nonequilibrium phase transition was observed. A
comprehensive phase boundary is drawn. Unlike the case of propagating magnetic
wave, the phase boundary does not show any systematic variation with the
wavelength of the standing magnetic field wave. In the limit of vanishingly
small amplitude of the field, the phase boundaries approach the recent Monte
Carlo estimate of equilibrium transition temperature.",1706.01619v6
2022-06-17,Quantum Dynamics of Magnetic Skyrmions: Consistent Path Integral Formulation,"We present a path integral formalism for the intrinsic quantum dynamics of
magnetic skyrmions coupled to a thermal background of magnetic fluctuations.
Upon promoting the skyrmion's collective coordinate $\boldsymbol{R}$ to a
dynamic variable and integrating out the magnonic heat bath, we derive the
generalized equation of motion for $\boldsymbol{R}$ with a non-local damping
term that describes a steady-state skyrmion dynamics at finite temperatures.
Being essentially temperature dependent, the intrinsic damping is shown to
originate from the coupling of thermally activated magnon modes to the
adiabatic potential driven by a rigid skyrmion motion, which can be regarded as
another manifestation of emergent electrodynamics inherent to topological
magnetic textures. We further argue that the diagonal components of the damping
term act as the source of dissipation and inertia, while its off-diagonal
components modify the gyrotropic motion of a magnetic skyrmion. By means of
numerical calculations for the lattice spin model of chiral ferromagnets, we
study the temperature behavior of the intrinsic damping as a function of
magnetic field in periodic and confined geometries. The intrinsic damping is
demonstrated to be highly non-local, revealing its quantum-mechanical nature,
that becomes more pronounced with increasing temperature. At high temperatures
when the magnon occupation factors are large, the intrinsic damping is shown to
yield a modified Thiele's equation with the additional non-local dissipative
and mass terms that exhibit an almost linear temperature behavior. Our results
provide a microscopic background for semiclassical magnetization dynamics and
establish a framework for understanding spin caloritronics effects in
topological magnetic textures.",2206.08532v2
2022-02-28,"Density Functional description of spin, lattice, and spin-lattice dynamics in antiferromagnetic and paramagnetic phases at finite temperatures","Describing the (a) electronic and magnetic properties (EMP) of
antiferromagnetic or paramagnetic phases of compounds generally requires the
knowledge of (b) the spin configurations and lattice structure (SCLS) of such
phases at a given temperature. Indeed, studying the coupling between (a) and
(b) has been an outstanding challenge in the theory of magnetism. The
traditional approach to electronic phases of matter has generally focused on
solving the problem of EMP regarding the SCLS as a spectator degree of freedom
(DOF). Yet, it has been recognized that EMP of a compound generally respond
self-consistently to changes in SCLS and vice versa. We construct here a
practical, density functional theory (DFT)-based approach that provides the
SCLS as a function of temperature, involving the description of spin, lattice,
and spin-lattice dynamics of different magnetic phases. We distinguish three
Levels of dynamics: (I) dynamics of the spin DOF treated via noncollinear
Heisenberg Monte-Carlo with exchange energies from DFT, (II) dynamics of the
lattice DOF treated by ab initio molecular dynamics (AIMD) employing a fixed
spin configuration from Level I at the simulated temperature, and (III)
coupling of spin and lattice dynamics via Landau-Lifshitz-Gilbert spin dynamics
combined with AIMD. Such SCLS at each of the three levels are used as inputs to
DFT supercell calculations, providing the EMP at each temperature. The results
of this sequence include electronic band structures, band gaps, density of
states, as well as the statistical distribution of local moments and the
short-range order parameters, each as a function of temperature. Using NiO as a
test case, we address the separability of the DOF in magnetic insulators for a
minimal description of electronic and magnetic properties, demonstrating that
inclusion of spin dynamics and, to some level, lattice dynamics is enough to
explain the EMP.",2202.13723v2
2018-03-01,Classical and quantum spin dynamics of the honeycomb $Γ$ model,"Quantum to classical crossover is a fundamental question in dynamics of
quantum many-body systems. In frustrated magnets, for example, it is highly
non-trivial to describe the crossover from the classical spin liquid with a
macroscopically-degenerate ground-state manifold, to the quantum spin liquid
phase with fractionalized excitations. This is an important issue as we often
encounter the demand for a sharp distinction between the classical and quantum
spin liquid behaviors in real materials. Here we take the example of the
classical spin liquid in a frustrated magnet with novel bond-dependent
interactions to investigate the classical dynamics, and critically compare it
with quantum dynamics in the same system. In particular, we focus on signatures
in the dynamical spin structure factor. Combining Landau-Lifshitz dynamics
simulations and the analytical Martin-Siggia-Rose (MSR) approach, we show that
the low energy spectra are described by relaxational dynamics and highly
constrained by the zero mode structure of the underlying degenerate classical
manifold. Further, the higher energy spectra can be explained by precessional
dynamics. Surprisingly, many of these features can also be seen in the
dynamical structure factor in the quantum model studied by finite-temperature
exact diagonalization. We discuss the implications of these results, and their
connection to recent experiments on frustrated magnets with strong spin-orbit
coupling.",1803.00601v2
2021-10-01,Intrinsic energy flow in laser-excited 3$d$ ferromagnets,"Ultrafast magnetization dynamics are governed by energy flow between
electronic, magnetic, and lattice degrees of freedom. A quantitative
understanding of these dynamics must be based on a model that agrees with
experimental results for all three subsystems. However, ultrafast dynamics of
the lattice remain largely unexplored experimentally. Here, we combine
femtosecond electron diffraction experiments of the lattice dynamics with
energy-conserving atomistic spin dynamics (ASD) simulations and ab-initio
calculations to study the intrinsic energy flow in the 3d ferromagnets cobalt
(Co) and iron (Fe). The simulations yield a good description of experimental
data, in particular an excellent description of our experimental results for
the lattice dynamics. We find that the lattice dynamics are influenced
significantly by the magnetization dynamics due to the energy cost of
demagnetization. Our results highlight the role of the spin system as the
dominant heat sink in the first hundreds of femtoseconds. Together with
previous findings for nickel [Zahn et al., Phys. Rev. Research 3, 023032
(2021)], our work demonstrates that energy-conserving ASD simulations provide a
general and consistent description of the laser-induced dynamics in all three
elemental 3d ferromagnets.",2110.00525v2
2003-10-07,Magnetic Spin-Up of Line-Driven Winds,"We summarize recent 2D MHD simulations of line-driven stellar winds from
rotating hot-stars with a dipole magnetic field aligned to the star's rotation
axis. For moderate to strong fields, much wind outflow is initially along
closed magnetic loops that nearly corotate as a solid body with the underlying
star, thus providing a torque that results in an effective angular momentum
spin-up of the outflowing material. But instead of forming the ``magnetically
torqued disk'' (MTD) postulated in previous phenemenological analyses, the
dynamical simulations here show that material trapped near the tops of such
closed loops tends either to fall back or break out, depending on whether it is
below or above the Keplerian corotation radius. Overall the results raise
serious questions about whether magnetic torquing of a wind outflow could
naturally result in a Keplerian circumstellar disk. However, for very strong
fields, it does still seem possible to form a %``magnetically confined,
centrifugally supported, rigid-body disk'', centrifugally supported,
``magnetically rigid disk'' (MRD), in which the field not only forces material
to maintain a rigid-body rotation, but for some extended period also holds it
down against the outward centrifugal force at the loop tops. We argue that such
rigid-body disks seem ill-suited to explain the disk emission from Be stars,
but could provide a quite attractive paradigm for circumstellar emission from
the magnetically strong Bp and Ap stars.",0310179v1
2004-01-30,Neutrino Processes in Strong Magnetic Fields and Implications for Supernova Dynamics,"The processes of neutrino (antineutrino) absorption and electron (positron)
capture on nucleons provide the dominant mechanisms for heating and cooling the
material between the protoneutron star and the stalled shock in a core-collapse
supernova. Observations suggest that some neutron stars are born with magnetic
fields of at least 10^15 G while theoretical considerations give an upper limit
of 10^18 G for the protoneutron star magnetic fields. We calculate the rates
for the above neutrino processes in strong magnetic fields of 10^16 G. We find
that the main effect of such magnetic fields is to change the equations of
state through the phase space of electron and positron, which differs from the
classical case due to quantization of the motion of electron and positron
perpendicular to the magnetic field. As a result, the cooling rate can be
greatly reduced by magnetic fields of 10^16 G for typical conditions below the
stalled shock and a nonuniform protoneutron star magnetic field (e.g., a dipole
field) can introduce a large angular dependence of the cooling rate. In
addition, strong magnetic fields always lead to an angle-dependent heating rate
by polarizing the spin of nucleons. The implications of our results for the
neutrino-driven supernova mechanism are discussed.",0401634v2
2004-10-20,A Genetic Algorithm-Based Exploration of Three Filament Models: A Case for the Magnetic Support of the G11.11-0.12 Infrared-Dark Cloud,"The G11.11-0.12 infrared-dark cloud has a filamentary appearance, both in
extinction against the diffuse infrared emission of the Galactic plane and in
emission at 850 microns. We use a novel computational technique based on an
advanced genetic algorithm to explore thoroughly 3 different models of
self-gravitating, pressure truncated filaments and to constrain their
parameters. Specifically, the models tested are the non-magnetic Ostriker
(1964) model, a generalized version of the magnetic Stodolkiewicz (1963) model,
and the magnetic Fiege & Pudritz (2000) model. Previous results showed that
G11.11-0.12 has a much steeper r^{-4} radial density profile than other
filaments, where the density varies approximately as r^{-2}, and that this
steep density profile is consistent with the Ostriker (1964) model. We present
a more complete analysis that shows that the radial structure of G11.11-0.12 is
consistent with regimes of each of these models. All of the magnetic models
that agree with the data are threaded by a dominant poloidal magnetic field,
and most have dynamically significant fields. Thus, G11.11-0.12 is an excellent
candidate for radial support by a magnetic field that is predominantly
poloidal. We predict the polarization patterns expected for both magnetic
models and show that the two magnetic models produce different polarization
patterns that should be distingished by observations.",0410474v1
2006-08-31,The Cosmic Battery Revisited,"We reinvestigate the generation and accumulation of magnetic flux in
optically thin accretion flows around active gravitating objects. The source of
the magnetic field is the azimuthal electric current associated with the
Poynting-Robertson drag on the electrons of the accreting plasma. This current
generates magnetic field loops which open up because of the differential
rotation of the flow. We show through simple numerical simulations that what
regulates the generation and accumulation of magnetic flux near the center is
the value of the plasma conductivity. Although the conductivity is usually
considered to be effectively infinite for the fully ionized plasmas expected
near the inner edge of accretion disks, the turbulence of those plasmas may
actually render them much less conducting due to the presence of anomalous
resistivity. We have discovered that if the resistivity is sufficiently high
throughout the turbulent disk while it is suppressed interior to its inner
edge, an interesting steady-state process is established: accretion carries and
accumulates magnetic flux of one polarity inside the inner edge of the disk,
whereas magnetic diffusion releases magnetic flux of the opposite polarity to
large distances. In this scenario, magnetic flux of one polarity grows and
accumulates at a steady rate in the region inside the inner edge and up to the
point of equipartition when it becomes dynamically important. We argue that
this inward growth and outward expulsion of oppositely-directed magnetic fields
that we propose may account for the ~30 min cyclic variability observed in the
galactic microquasar GRS1915+105.",0608701v1
2003-01-16,Magnetic Relaxation in the Peak Effect Region of CeRu$_2$,"The different pinning strengths of the flux line lattice in the peak effect
(PE) region of a polycrystalline sample of CeRu$_2$ with a superconducting
transition temperature {$T_c = 6.1$ K} have been probed by means of
magnetization measurements with a SQUID magnetometer as the temperature $T$ and
the magnetic field $H$ are varied. Magnetic relaxation measurements were used
to monitor the flux line dynamics in the PE region. For {$T < 4.5$ K} and $H <
H_P$, where $H_P$ is the field where the magnetization reaches a maximum in the
PE region, the relaxation rate was found to be significantly larger in the
descending-field branch of the PE than it is in other sections of the PE
region. For {$T \geq 4.5$ K}, the relaxation rate in the entire PE region is so
large that the magnetization reached a stable (equilibrium) value within
{$10^4$ s}. This experimentally determined stable state appears as an increase
of the magnetization in the PE region and has a dome shape superimposed on a
linear interpolation through the PE region. It was also found that the PE in
CeRu$_2$ can be suppressed by rapid thermal cycling of the sample between {10
K} and {300 K} four times. The reversible magnetization after the PE has been
suppressed coincides with the linear interpolation through the PE region, in
contrast to the behavior of the equilibrium magnetization when the PE is
present.
  PACS number: 74.25.Qt, 74.70.Ad",0301281v1
2004-07-28,Ab initio study of canted magnetism of finite atomic chains at surfaces,"By using ab initio methods on different levels we study the magnetic ground
state of (finite) atomic wires deposited on metallic surfaces. A
phenomenological model based on symmetry arguments suggests that the
magnetization of a ferromagnetic wire is aligned either normal to the wire and,
generally, tilted with respect to the surface normal or parallel to the wire.
  From a first principles point of view, this simple model can be best related
to the so--called magnetic force theorem calculations being often used to
explore magnetic anisotropy energies of bulk and surface systems. The second
theoretical approach we use to search for the canted magnetic ground state is
first principles adiabatic spin dynamics extended to the case of fully
relativistic electron scattering. First, for the case of two adjacent Fe atoms
an a Cu(111) surface we demonstrate that the reduction of the surface symmetry
can indeed lead to canted magnetism. The anisotropy constants and consequently
the ground state magnetization direction are very sensitive to the position of
the dimer with respect to the surface. We also performed calculations for a
seven--atom Co chain placed along a step edge of a Pt(111) surface. As far as
the ground state spin orientation is concerned we obtain excellent agreement
with experiment. Moreover, the magnetic ground state turns out to be slightly
noncollinear.",0407732v1
2008-02-04,Cosmological MHD simulation of a cooling flow cluster,"Various observations of magnetic fields in the Intra-Cluster Medium (ICM),
most of the time restricted to cluster cores, point towards field strength of
the order of a few microG (synchrotron radiation from radio relics and radio
halos, inverse Compton radiation in X-rays and Faraday rotation measure of
polarised background sources). Both the origin and the spatial structure of
galaxy clusters magnetic fields are still under debate. In particular, the
radial profile of the magnetic field, from the core of clusters to their
outskirts, is of great importance for cosmic rays propagation within the Cosmic
Web. In this letter, we highlight the importance of cooling processes in
amplifying the magnetic field in the core of galaxy clusters up to one order of
magnitude above the typical amplification obtained for a pure adiabatic
evolution. We have performed a ""zoom'' cosmological simulation of a 3 keV
cluster, including dark matter and gas dynamics, atomic cooling, UV heating and
star formation using the newly developed MHD solver in the AMR code RAMSES.
Magnetic field amplification proceeds mainly through gravitational contraction.
Shearing motions due to turbulence provide additional amplification in the
outskirts of the cluster, while magnetic reconnection during mergers causes
magnetic field dissipation in the core. Cooling processes have a strong impact
on the magnetic field structure in the cluster. First, due to the sharp rise of
the gas density in the centre, gravitational amplification is significantly
amplified, when compared to the non--radiative run. Second, due to cooling
processes, shearing motions are much stronger in the core than in the adiabatic
case, leading to additional field amplification and no significant magnetic
reconnection.",0802.0490v3
2008-12-30,Covalent bonding and magnetism in cuprates,"The importance of covalent bonding for the magnetism of 3d metal complexes
was first noted by Pauling in 1931. His point became moot, however, with the
success of the ionic picture of Van Vleck, where ligands influence magnetic
electrons of 3d ions mainly through electrostatic fields. Anderson's theory of
spin superexchange later established that covalency is at the heart of
cooperative magnetism in insulators, but its energy scale was believed to be
small compared to other inter-ionic interactions and therefore it was
considered a small perturbation of the ionic picture. This assertion fails
dramatically in copper oxides, which came to prominence following the discovery
of high critical temperature superconductors (HTSC). Magnetic interactions in
cuprates are remarkably strong and are often considered the origin of the
unusually high superconducting transition temperature, Tc. Here we report a
detailed survey of magnetic excitations in the one-dimensional cuprate Sr2CuO3
using inelastic neutron scattering (INS). We show that although the
experimental dynamical spin structure factor is well described by the model
S=1/2 nearest-neighbour Heisenberg Hamiltonian typically used for cuprates, the
magnetic intensity is modified dramatically by strong hybridization of Cu 3d
states with O p states, showing that the ionic picture of localized 3d
Heisenberg spin magnetism is grossly inadequate. Our findings provide a natural
explanation for the puzzle of the missing INS magnetic intensity in cuprates
and have profound implications for understanding current and future
experimental data on these materials.",0812.5007v1
2009-01-20,A Model of Magnetically Induced Disc-Corona for Black Hole Binaries,"We propose a model of magnetic connection (MC) of a black hole with its
surrounding accretion disc based on large-scale magnetic field. The MC gives
rise to transport of energy and angular momentum between the black hole and the
disc, and the closed field lines pipe the hot matter evaporated from the disc,
and shape it in the corona above the disc to form a magnetically induced
disc-corona system, in which the corona has the same configuration as the
large-scale magnetic field. We numerically solve the dynamic equations in the
context of the Kerr metric, in which the large-scale magnetic field is
determined by dynamo process and equipartition between magnetic pressure and
gas pressure. Thus we can obtain a global solution rather than assuming the
distribution of large-scale magnetic field beforehand. The main MC effects lie
in three aspects. (1) The rotational energy of a fast-spinning black hole can
be extracted, enhancing the dissipation in the accretion disc, (2) the closed
field lines provide a natural channel for corona matter escaping from disc and
finally falling into black hole, and (3) the scope of the corona can be bounded
by the conservation of magnetic flux. We simulate the high-energy spectra of
this system by using Monte-Carlo method, and find that the relative hardness of
the spectra decreases as accretion rate or black hole spin $a_*$ increases. We
fit the typical X-ray spectra of three black-hole binaries (GRO J1655-40, XTE
1118+480 and GX 339-4) in the low/hard or very high state.",0901.3002v1
2009-02-02,Stray-fields based features observed for low and high magnetic fields in Ni$_{80}$Fe$_{20}$-Nb-Ni$_{80}$Fe$_{20}$ trilayers,"We report on the influence of stray fields for both low and high magnetic
fields applied parallel to trilayers consisting of a low-T Nb interlayer and
two outer Ni Fe layers having in-plane anisotropy. At low magnetic fields these
trilayers exhibit a pronounced magnetoresistance effect. Its dynamic transport
behavior is presented through detailed I-V characteristics. More importantly,
the detailed evolution of the longitudinal and transverse magnetic components
of the trilayers is presented from close to well below T . These data clearly
show that below T and for low magnetic fields the transport properties of the
Nb interlayer are influenced by transverse stray-fields that motivate
subsequent transverse magnetic coupling of the outer Ni Fe layers. By
generalizing this experimental finding we propose that the generic prerequisite
for the occurrence of intense magnetoresistance effects in relative trilayers
is that the coercive fields of the outer ferromagnetic layers should almost
coincide since the simultaneous occurrence of magnetic domains all over their
surface will promote a transverse magnetic coupling mediated by the
accompanying transverse stray fields. Finally, the trilayer's upper-critical
field exhibits a pronounced suppression for low magnetic fields indicative of a
behavior, while for high values the conventional behavior is recovered. A
similar process is observed in both Nb-Ni Fe bilayers and Nb single layers.
However, significant qualitative and quantitative differences exist between
these samples. Based on a mechanism that is motivated by longitudinal
stray-fields existing exclusively in the high-field regime we propose a
possible interpretation for this experimental finding.",0902.0314v1
2009-12-16,Protostellar collapse: radiative and magnetic feedbacks on small scale fragmentation,"It is established that both radiative transfer and magnetic field have a
strong impact on the collapse and the fragmentation of prestellar dense cores,
but no consistent calculation exists yet at such scales. We present original
AMR calculations including magnetic field (in the ideal MHD limit) and
radiative transfer, within the Flux Limited Diffusion approximation, of the
collapse of a 1 solar mass dense core. We compare the results with calculations
performed with a barotropic EOS. We show that radiative transfer has an
important impact on the collapse and the fragmentation, through the cooling or
heating of the gas, and is complementary of the magnetic field. A larger field
yields a stronger magnetic braking, increasing the accretion rate and thus the
effect of the radiative feedback. Even for a strongly magnetized core, where
the dynamics of the collapse is dominated by the magnetic field, radiative
transfer is crucial to determine the temperature and optical depth
distributions, two potentially accessible observational diagnostics. A
barotropic EOS cannot account for realistic fragmentation. The diffusivity of
the numerical scheme, however, is found to strongly affect the output of the
collapse, leading eventually to spurious fragmentation. Both radiative transfer
and magnetic field must be included in numerical calculations of star formation
to obtain realistic collapse configurations and observable signatures.
Nevertheless, the numerical resolution and the robustness of the solver are of
prime importance to obtain reliable results. When using an accurate solver, the
fragmentation is found to always remain inhibited by the magnetic field, at
least in the ideal MHD limit, even when radiative transfer is included.",0912.3138v2
2010-04-09,Oscillations of weakly viscous conducting liquid drops in a strong magnetic field,"We analyse small-amplitude oscillations of a weakly viscous electrically
conducting liquid drop in a strong uniform DC magnetic field. An asymptotic
solution is obtained showing that the magnetic field does not affect the shape
eigenmodes, which remain the spherical harmonics as in the non-magnetic case.
Strong magnetic field, however, constrains the liquid flow associated with the
oscillations and, thus, reduces the oscillation frequencies by increasing
effective inertia of the liquid. In such a field, liquid oscillates in a
two-dimensional (2D) way as solid columns aligned with the field. Two types of
oscillations are possible: longitudinal and transversal to the field. Such
oscillations are weakly damped by a strong magnetic field - the stronger the
field, the weaker the damping, except for the axisymmetric transversal and
inherently 2D modes. The former are overdamped because of being incompatible
with the incompressibility constraint, whereas the latter are not affected at
all because of being naturally invariant along the field. Since the magnetic
damping for all other modes decreases inversely with the square of the field
strength, viscous damping may become important in a sufficiently strong
magnetic field. The viscous damping is found analytically by a simple energy
dissipation approach which is shown for the longitudinal modes to be equivalent
to a much more complicated eigenvalue perturbation technique. This study
provides a theoretical basis for the development of new measurement methods of
surface tension, viscosity and the electrical conductivity of liquid metals
using the oscillating drop technique in a strong superimposed DC magnetic
field.",1004.1548v2
2010-04-26,Magnetic helicity fluxes in an alpha-squared dynamo embedded in a halo,"We present the results of simulations of forced turbulence in a slab where
the mean kinetic helicity has a maximum near the mid-plane, generating
gradients of magnetic helicity of both large and small-scale fields. We also
study systems that have poorly conducting buffer zones away from the midplane
in order to assess the effects of boundaries. The dynamical alpha quenching
phenomenology requires that the magnetic helicity in the small-scale fields
approaches a nearly static, gauge independent state. To stress-test this steady
state condition we choose a system with a uniform sign of kinetic helicity, so
that the total magnetic helicity can reach a steady state value only through
fluxes through the boundary, which are themselves suppressed by the velocity
boundary conditions. Even with such a set up, the small-scale magnetic helicity
is found to reach a steady state. In agreement with earlier work, the magnetic
helicity fluxes of small-scale fields are found to be turbulently diffusive. By
comparing results with and without halos, we show that artificial constraints
on magnetic helicity at the boundary do not have a significant impact on the
evolution of the magnetic helicity, except that ""softer"" (halo) boundary
conditions give a lower energy of the saturated mean magnetic field.",1004.4591v1
2010-06-21,Simulation of the Formation of a Solar Active Region,"We present a radiative magnetohydrodynamics simulation of the formation of an
Active Region on the solar surface. The simulation models the rise of a buoyant
magnetic flux bundle from a depth of 7.5 Mm in the convection zone up into the
solar photosphere. The rise of the magnetic plasma in the convection zone is
accompanied by predominantly horizontal expansion. Such an expansion leads to a
scaling relation between the plasma density and the magnetic field strength
such that $B\propto\varrho^{1/2}$. The emergence of magnetic flux into the
photosphere appears as a complex magnetic pattern, which results from the
interaction of the rising magnetic field with the turbulent convective flows.
Small-scale magnetic elements at the surface first appear, followed by their
gradual coalescence into larger magnetic concentrations, which eventually
results in the formation of a pair of opposite polarity spots. Although the
mean flow pattern in the vicinity of the developing spots is directed radially
outward, correlations between the magnetic field and velocity field
fluctuations allow the spots to accumulate flux. Such correlations result from
the Lorentz-force driven, counter-streaming motion of opposite-polarity
fragments. The formation of the simulated Active Region is accompanied by
transient light bridges between umbrae and umbral dots. Together with recent
sunspot modeling, this work highlights the common magnetoconvective origin of
umbral dots, light bridges and penumbral filaments.",1006.4117v2
2011-01-15,Disk Formation in Magnetized Clouds Enabled by the Hall Effect,"Stars form in dense cores of molecular clouds that are observed to be
significantly magnetized. A dynamically important magnetic field presents a
significant obstacle to the formation of protostellar disks. Recent studies
have shown that magnetic braking is strong enough to suppress the formation of
rotationally supported disks in the ideal MHD limit. Whether non-ideal MHD
effects can enable disk formation remains unsettled. We carry out a first study
on how disk formation in magnetic clouds is modified by the Hall effect, the
least explored of the three non-ideal MHD effects in star formation (the other
two being ambipolar diffusion and Ohmic dissipation). For illustrative
purposes, we consider a simplified problem of a non-self-gravitating,
magnetized envelope collapsing onto a central protostar of fixed mass. We find
that the Hall effect can spin up the inner part of the collapsing flow to
Keplerian speed, producing a rotationally supported disk. The disk is generated
through a Hall-induced magnetic torque. Disk formation occurs even when the
envelope is initially non-rotating, provided that the Hall coefficient is large
enough. When the magnetic field orientation is flipped, the direction of disk
rotation is reversed as well. The implication is that the Hall effect can in
principle produce both regularly rotating and counter-rotating disks around
protostars. We conclude that the Hall effect is an important factor to consider
in studying the angular momentum evolution of magnetized star formation in
general and disk formation in particular.",1101.3018v1
2011-03-09,Fast Magnetic Reconnection and Spontaneous Stochasticity,"Magnetic field-lines in astrophysical plasmas are expected to be frozen-in at
scales larger than the ion gyroradius. The rapid reconnection of magnetic flux
structures with dimensions vastly larger than the gyroradius requires a
breakdown in the standard Alfv\'en flux-freezing law. We attribute this
breakdown to ubiquitous MHD plasma turbulence with power-law scaling ranges of
velocity and magnetic energy spectra. Lagrangian particle trajectories in such
environments become ""spontaneously stochastic"", so that infinitely-many
magnetic field-lines are advected to each point and must be averaged to obtain
the resultant magnetic field. The relative distance between initial magnetic
field lines which arrive to the same final point depends upon the properties of
two-particle turbulent dispersion. We develop predictions based on the
phenomenological Goldreich & Sridhar theory of strong MHD turbulence and on
weak MHD turbulence theory. We recover the predictions of the Lazarian &
Vishniac theory for the reconnection rate of large-scale magnetic structures.
Lazarian & Vishniac also invoked ""spontaneous stochasticity"", but of the
field-lines rather than of the Lagrangian trajectories. More recent theories of
fast magnetic reconnection appeal to microscopic plasma processes that lead to
additional terms in the generalized Ohm's law, such as the collisionless Hall
term. We estimate quantitatively the effect of such processes on the
inertial-range turbulence dynamics and find them to be negligible in most
astrophysical environments. For example, the predictions of the
Lazarian-Vishniac theory are unchanged in Hall MHD turbulence with an extended
inertial range, whenever the ion skin depth $\delta_i$ is much smaller than the
turbulent integral length or injection-scale $L_i.$",1103.1882v2
2011-09-15,Suppression of the large-scale Lorentz force by turbulence,"The components of the total stress tensor (Reynolds stress plus Maxwell
stress) are computed within the quasilinear approximation for a driven
turbulence influenced by a large-scale magnetic background field. The
conducting fluid has an arbitrary magnetic Prandtl number and the turbulence
without the background field is assumed as homogeneous and isotropic with a
free Strouhal number St. The total large-scale magnetic tension is always
reduced by the turbulence with the possibility of a `catastrophic quenching'
for large magnetic Reynolds number Rm so that even its sign is reversed. The
total magnetic pressure is enhanced by turbulence with short correlation time
(`white noise') but it is reduced by turbulence with long correlation time.
Also in this case the sign of the total pressure may reverse but only for
special turbulences with sufficiently large St> 1.
  The turbulence-induced terms of the stress tensor are suppressed by strong
magnetic fields. For the tension term this quenching grows with the square of
the Hartmann number of the magnetic field. For microscopic (i.e. small)
diffusivity values the magnetic tension term becomes thus highly quenched even
for field amplitudes much smaller than their equipartition value. In the
opposite case of large-eddy simulations the magnetic quenching is only mild but
then also the turbulence-induced Maxwell tensor components for weak fields
remain rather small.",1109.3345v2
2011-12-02,General relativistic simulations of black hole-neutron star mergers: Effects of magnetic fields,"As a neutron star (NS) is tidally disrupted by a black hole (BH) companion at
the end of a BH-NS binary inspiral, its magnetic fields will be stretched and
amplified. If sufficiently strong, these magnetic fields may impact the
gravitational waveforms, merger evolution and mass of the remnant disk.
Formation of highly-collimated magnetic field lines in the disk+spinning BH
remnant may launch relativistic jets, providing the engine for a short-hard
GRB. We analyze this scenario through fully general relativistic,
magnetohydrodynamic (GRMHD) BHNS simulations from inspiral through merger and
disk formation. Different initial magnetic field configurations and strengths
are chosen for the NS interior for both nonspinning and moderately spinning
(a/M=0.75) BHs aligned with the orbital angular momentum. Only strong interior
(Bmax~10^17 G) initial magnetic fields in the NS significantly influence merger
dynamics, enhancing the remnant disk mass by 100% and 40% in the nonspinning
and spinning BH cases, respectively. However, detecting the imprint of even a
strong magnetic field may be challenging for Advanced LIGO. Though there is no
evidence of mass outflows or magnetic field collimation during the preliminary
simulations we have performed, higher resolution, coupled with longer disk
evolutions and different initial magnetic field configurations, may be required
to definitively assess the possibility of BHNS binaries as short-hard GRB
progenitors.",1112.0568v4
2011-12-16,Response of the Photospheric Magnetic Field to the X2.2 Flare on 2011 February 15,"It is well known that the long-term evolution of the photospheric magnetic
field plays an important role in building up free energy to power solar
eruptions. Observations, despite being controversial, have also revealed a
rapid and permanent variation of the photospheric magnetic field in response to
the coronal magnetic field restructuring during the eruption. The Helioseismic
and Magnetic Imager instrument (HMI) on board the newly launched Solar Dynamics
Observatory (SDO) produces seeing-free full-disk vector magnetograms at
consistently high resolution and high cadence, which finally makes possible an
unambiguous and comprehensive study of this important back-reaction process. In
this study, we present a near disk-center, GOES -class X2.2 flare, which
occurred in NOAA AR 11158 on 2011 February 15. Using the magnetic field
measurements made by HMI, we obtained the first solid evidence of a rapid (in
about 30 minutes) and irreversible enhancement in the horizontal magnetic field
at the flaring magnetic polarity inversion line (PIL) by a magnitude of ~30%.
It is also shown that the photospheric field becomes more sheared and more
inclined. This field evolution is unequivocally associated with the flare
occurrence in this sigmoidal active region, with the enhancement area located
in between the two chromospheric flare ribbons and the initial conjugate hard
X-ray footpoints. These results strongly corroborate our previous conjecture
that the photospheric magnetic field near the PIL must become more horizontal
after eruptions, which could be related to the newly formed low-lying fields
resulted from the tether-cutting reconnection.",1112.3948v1
2012-03-09,Requirements for the Analysis of Quiet Sun Internetwork Magnetic Elements with EST and ATST,"The quiet Sun internetwork is permeated by weak and highly inclined magnetic
fields whose physical properties, dynamics, and magnetic interactions are not
fully understood. High spatial resolution magnetograms show them as discrete
magnetic elements that appear/emerge and disappear/cancel continuously over the
quiet Sun surface. The 4-m European Solar Telescope (EST) and the Advanced
Technology Solar Telescope (ATST) will obtain two-dimensional, high cadence,
high precision polarimetric measurements at the diffraction limit (30 km).
Here, we compile the basic requirements for the observation of internetwork
fields with EST and ATST (field of view, cadence, instrument configuration,
etc). More specifically, we concentrate on the field-of-view requirements. To
set them we analyze the proper motion of internetwork magnetic elements across
the solar surface. We use 13 hours of magnetograms taken with the Hinode
satellite to identify and track thousands of internetwork magnetic element in
an isolated supergranular cell. We calculate the velocity components of each
element and the mean distance they travel. The results show that, on average,
magnetic elements in the interior of supergranular cells move toward the
network. The radial velocity is observed to depend on the distance to the
center of the supergranule. Internetwork magnetic elements travel 400 on
average. These results suggest that ATST and EST should cover, at least, one
supergranular cell to obtain a complete picture of the quiet Sun internetwork.",1203.2185v1
2012-03-20,Linear stability analysis of the Hall magnetorotational instability in a spherical domain,"We investigate the stability of the Hall-MHD system and determine its
importance for neutron stars at their birth, when they still consist of
differentially rotating plasma permeated by extremely strong magnetic fields.
We solve the linearised Hall-MHD equations in a spherical shell threaded by a
homogeneous magnetic field. With the fluid/flow coupling and the Hall effect
included, the magnetorotational instability and the Hall effect are both acting
together. Results differ for magnetic fields aligned with the rotation axis and
anti-parallel magnetic fields. For a positive alignment of the magnetic field
the instability grows on a rotational time-scale for any sufficiently large
magnetic Reynolds number. Even the magnetic fields which are stable against the
MRI due to the magnetic diffusion are now susceptible to the shear-Hall
instability. In contrast, the negative alignment places strong restrictions on
the growth and the magnitude of the fields, hindering the effectiveness of the
Hall-MRI. While non-axisymmetric modes of the MRI can be suppressed by strong
enough rotation, there is no such restriction when the Hall effect is present.
The implications for the magnitude and the topology of the magnetic field of a
young neutron star may be significant.",1203.4348v2
2012-09-27,Flares and Magnetic Non-potentiality of NOAA AR 11158,"The magnetic non-potentiality is important for understanding flares and other
solar activities in active regions (ARs). Five non-potential parameters, i.e.,
electric current, current helicity, source field, photospheric free energy, and
angular shear, are calculated in this work to quantify the non-potentiality of
NOAA AR 11158. Benefited from high spatial resolution, high cadence, and
continuously temporal coverage of vector magnetograms from the Helioseismic and
Magnetic Imager on board the Solar Dynamics Observatory, both the long-term
evolution of the AR and the rapid change during flares have been studied. We
confirmed that, comparing with the magnetic flux, the magnetic non-potentiality
has a closer connection with the flare, and the emerging flux regions are
important for the magnetic non-potentiality and flares. The main results of
this work are as follows. (1) The vortex in the source field directly displays
the deflection of horizontal magnetic field. The deflection is corresponding to
the fast rotated sunspot with a time delay, which suggests that the sunspot
rotation leads to an increase of the non-potentiality. (2) Two areas that have
evident changes of the azimuth of the vector magnetic field are found near the
magnetic polarity inversion line. The change rates of the azimuth are about 1.3
deg/h and 3.6 deg/h, respectively. (3) Rapid and prominent increases are found
in the variation of helicity during four flares in their initial brightening
regions. The recovery of the increases takes 3-4 h for the two biggest flares
(X2.2 and M6.6), while only takes about 2 h for the other two smaller flares
(M2.2 and M1.6).",1209.6192v1
2012-10-14,Self-induced pinning of vortices in the presence of ac driving force in magnetic superconductors,"We derive the response of the magnetic superconductors in the vortex state to
the ac Lorentz force, $F_L(t)=F_{{\rm ac}}\sin(\omega t)$, taking into account
the interaction of vortices with the magnetic moments described by the
relaxation dynamics (polaronic effect). At low amplitudes of the driving force
$F_{{\rm ac}}$ the dissipation in the system is suppressed due to the
enhancement of the effective viscosity at low frequencies and due to formation
of the magnetic pinning at high frequencies $\omega$. In the adiabatic limit
with low frequencies $\omega$ and high amplitude of the driving force $F_{ac}$,
the vortex and magnetic polarization form a vortex polaron when $F_L(t)$ is
small. When $F_L$ increases, the vortex polaron accelerates and at a threshold
driving force, the vortex polaron dissociates and the motion of vortex and the
relaxation of magnetization are decoupled. When $F_L$ decreases, the vortex is
retrapped by the background of remnant magnetization and they again form vortex
polaron. This process repeats when $F_L(t)$ increases in the opposite
direction. Remarkably, after dissociation, decoupled vortices move in the
periodic potential induced by magnetization which remains for some periods of
time due to retardation after the decoupling. At this stage vortices oscillate
with high frequencies determined by the Lorentz force at the moment of
dissociation. We derive also the creep rate of vortices and show that magnetic
moments suppress creep rate.",1210.3820v2
2012-12-05,"High Precision Magnetic Linear Dichroism Measurements in (Ga,Mn)As","Investigation of magnetic materials using the first-order magneto-optical
Kerr effects (MOKE) is well established and is frequently used in the
literature. On the other hand, the utilization of the second-order (or
quadratic) magneto-optical (MO) effects for the material research is rather
rare. This is due to the small magnitude of quadratic MO signals and the fact
that the signals are even in magnetization (i.e., they do not change a sign
when the magnetization orientation is flipped), which makes it difficult to
separate second-order MO signals from various experimental artifacts. In 2005 a
giant quadratic MO effect - magnetic linear dichroism (MLD) - was observed in
the ferromagnetic semiconductor (Ga,Mn)As. This discovery not only provided a
new experimental tool for the investigation of in-plane magnetization dynamics
in (Ga,Mn)As using light at normal incidence, but it also motivated the
development of experimental techniques for the measurement of second-order MO
effects in general. In this paper we compare four different experimental
techniques that can be used to measure MLD and to separate it from experimental
artifacts. We show that the most reliable results are obtained when the
harmonic dependence of MLD on a mutual orientation of magnetization and light
polarization plane is used together with the in-situ rotation of the sample
followed by the magnetic field-induced rotation of magnetization. Using this
technique we measure the MLD spectra of (Ga,Mn)As in a broad spectral range
from 0.1 eV to 2.7 eV and we observe that MLD has a comparable magnitude as
polar MOKE signals in this material.",1212.0956v1
2012-12-21,Magnetic fields in our Milky Way Galaxy and nearby galaxies,"Magnetic fields in our Galaxy and nearby galaxies have been revealed by
starlight polarization, polarized emission from dust grains and clouds at
millimeter and submillimeter wavelength, the Zeeman effect of spectral lines or
maser lines from clouds or clumps, diffuse radio synchrotron emission from
relativistic electrons in interstellar magnetic fields, and the Faraday
rotation of background radio sources as well as pulsars for our Milky Way. It
is easy to get a global structure for magnetic fields in nearby galaxies, while
we have observed many details of magnetic fields in our Milky Way, especially
by using pulsar rotation measure data. In general, magnetic fields in spiral
galaxies probably have a large-scale structure. The fields follow the spiral
arms with or without the field direction reversals. In the halo of spiral
galaxies magnetic fields exist and probably also have a large-scale structure
as toroidal and poloidal fields, but seem to be slightly weaker than those in
the disk. In the central region of some galaxies, poloidal fields have been
detected as vertical components. Magnetic field directions in galaxies seem to
have been preserved during cloud formation and star formation, from large-scale
diffuse interstellar medium to molecular clouds and then to the cloud cores in
star formation regions or clumps for the maser spots. Magnetic fields in
galaxies are passive to dynamics.",1212.5464v2
2013-01-08,Magnetic energy cascade in spherical geometry: I. The stellar convective dynamo case,"We present a method to characterize the spectral transfers of magnetic energy
between scales in simulations of stellar convective dynamos. The full triadic
transfer functions are computed thanks to analytical coupling relations of
spherical harmonics based on the Clebsch-Gordan coefficients. The method is
applied to mean field $\alpha\Omega$ dynamo models as benchmark tests. From the
physical standpoint, the decomposition of the dynamo field into primary and
secondary dynamo families proves very instructive in the $\alpha\Omega$ case.
The same method is then applied to a fully turbulent dynamo in a solar
convection zone, modeled with the 3D MHD ASH code. The initial growth of the
magnetic energy spectrum is shown to be non-local. It mainly reproduces the
kinetic energy spectrum of convection at intermediate scales. During the
saturation phase, two kinds of direct magnetic energy cascades are observed in
regions encompassing the smallest scales involved in the simulation. The first
cascade is obtained through the shearing of magnetic field by the large scale
differential rotation that effectively cascades magnetic energy. The second is
a generalized cascade that involves a range of local magnetic and velocity
scales. Non-local transfers appear to be significant, such that the net
transfers cannot be reduced to the dynamics of a small set of modes. The
saturation of the large scale axisymmetric dipole and quadrupole are detailed.
In particular, the dipole is saturated by a non-local interaction involving the
most energetic scale of the magnetic energy spectrum, which points out the
importance of the magnetic Prandtl number for large-scale dynamos.",1301.1606v1
2014-03-21,Imaging superconducting vortex core and lattice with the scanning tunneling microscope,"The observation of vortices in superconductors was a major breakthrough in
developing the conceptual background for superconducting applications. Each
vortex carries a flux quantum, and the magnetic field radially decreases from
the center. Techniques used to make magnetic field maps, such as magnetic
decoration, give vortex lattice images in a variety of systems. However, strong
type II superconductors allow penetration of the magnetic field over large
distances, of order of the magnetic penetration depth \lambda.
Superconductivity survives up to magnetic fields where, for imaging purposes,
there is nearly no magnetic contrast. Static and dynamic properties of vortices
are largely unknown at such high magnetic fields. Reciprocal space studies
using neutron scattering give insight into the collective behavior. But the
microscopic details of vortex arrangements and their motion remain difficult to
obtain. Direct real space visualization can be made using scanning tunneling
microscopy and spectroscopy (STM/S). Instead of using magnetic contrast, the
electronic density of states describes spatial variations of the quasiparticle
and pair wavefunction properties. These are of order of the superconducting
coherence length \xi, which is much smaller than \lambda. In principle,
individual vortices can be imaged using STM up to the upper critical field
where vortex cores, of size \xi, overlap. In this review, we describe recent
advances in vortex imaging made with scanning tunneling microscopy and
spectroscopy. We introduce the technique and discuss vortex images which reveal
the influence of the Fermi surface distribution of the superconducting gap on
the internal structure of vortices, the collective behavior of the lattice in
different materials and conditions, and the observation of vortex lattice
melting.",1403.5514v1
2014-06-02,Low-metallicity star formation: Relative impact of metals and magnetic fields,"Low-metallicity star formation poses a central problem of cosmology, as it
determines the characteristic mass scale and distribution for the first and
second generations of stars forming in our Universe. Here, we present a
comprehensive investigation assessing the relative impact of metals and
magnetic fields, which may both be present during low-metallicity star
formation. We show that the presence of magnetic fields generated via the
small-scale dynamo stabilises the protostellar disc and provides some degree of
support against fragmentation. In the absence of magnetic fields, the
fragmentation timescale in our model decreases by a factor of ~10 at the
transition from Z=0 to Z>0, with subsequently only a weak dependence on
metallicity. Similarly, the accretion timescale of the cluster is set by the
large-scale dynamics rather than the local thermodynamics. In the presence of
magnetic fields, the primordial disc can become completely stable, therefore
forming only one central fragment. At Z>0, the number of fragments is somewhat
reduced in the presence of magnetic fields, though the shape of the mass
spectrum is not strongly affected in the limits of the statistical
uncertainties. The fragmentation timescale, however, increases by roughly a
factor of 3 in the presence of magnetic fields. Indeed, our results indicate
comparable fragmentation timescales in primordial runs without magnetic fields
and Z>0 runs with magnetic fields.",1406.0346v1
2014-07-11,Modelling MHD accretion-ejection - from the launching area to propagation scales,"We present results of axisymmetric magnetohydrodynamic (MHD) simulations
investigating the launching of jets and outflows from a magnetically diffusive
accretion disk. The time evolution of the disk structure is self-consistently
taken into account. In contrast to previous works we have applied {\em
spherical} coordinates for the numerical grid, implying substantial benefits
concerning the numerical resolution and the stability of the simulation. Thanks
to the new setup we were able to run simulations for more than 150,000
dynamical times on a domain extending 1500 inner disk radii with a resolution
of up to 24 cells per disk height in the inner disk. Depending on the disk
magnetization, jet launching occurs in two different but complementary regimes
- jets driven predominantly by centrifugal or magnetic forces. These regimes
differ in the ejection efficiency concerning mass, energy and angular momentum.
We show that it is the {\em actual} disk magnetization and not so much the
initial magnetization which describes the disk-jet evolution best. Considering
the actual disk magnetization we also find that simulations starting with
different initial magnetization evolve in a similar - typical - way as due to
advection and diffusion the magnetic flux in the disk evolves in time.
Exploring a new, modified diffusivity model we confirm the self-similar
structure of the global jet-launching disk, obtaining power laws for the radial
profiles of the disk physical variables such as density, magnetic field
strength, or accretion velocity.",1407.3136v1
2015-02-06,The magnetic structure of surges in small-scale emerging flux regions,"Aims: To investigate the relationship between surges and magnetic
reconnection during the emergence of small-scale active regions. In particular,
to examine how the large-scale geometry of the magnetic field, shaped by
different phases of reconnection, guides the flowing of surges. Methods: We
present three flux emergence models. The first model, and the simplest,
consists of a region emerging into a horizontal ambient field that is initially
parallel to the top of the emerging region. The second model is the same as the
first but with an extra smaller emerging region which perturbs the main region.
This is added to create a more complex magnetic topology and to test how this
complicates the development of surges compared to the first model. The last
model has a non-uniform ambient magnetic field to model the effects of
emergence near a sunspot field and impose asymmetry on the system through the
ambient magnetic field. At each stage, we trace the magnetic topology to
identify the locations of reconnection. This allows for field lines to be
plotted from different topological regions, highlighting how their geometry
affects the development of surges. Results: In the first model, we identify
distinct phases of reconnection. Each phase is associated with a particular
geometry for the magnetic field and this determines the paths of the surges.
The second model follows a similar pattern to the first but with a more complex
magnetic topology and extra eruptions. The third model highlights how an
asymmetric ambient field can result in preferred locations for reconnection,
subsequently guiding the direction of surges.",1502.01842v1
2015-03-25,Rigorous numerical study of strong microwave photon-magnon coupling in all-dielectric magnetic multilayers,"We demonstrate theoretically a strong local enhancement of the intensity of
the in-plane microwave magnetic field in multilayered structures made from a
magneto-insulating yttrium iron garnet (YIG) layer sandwiched between two
non-magnetic layers with a high dielectric constant matching that of YIG. The
enhancement is predicted for the excitation regime when the microwave magnetic
field is induced inside the multilayer by the transducer of a stripline
Broadband Ferromagnetic Resonance (BFMR) setup. By means of a rigorous
numerical solution of the Landau-Lifshitz-Gilbert equation consistently with
the Maxwell's equations, we investigate the magnetisation dynamics in the
multilayer. We reveal a strong photon-magnon coupling, which manifests itself
as anti-crossing of the ferromagnetic resonance (FMR) magnon mode supported by
the YIG layer and the electromagnetic resonance mode supported by the whole
multilayered structure. The frequency of the magnon mode depends on the
external static magnetic field, which in our case is applied tangentially to
the multilayer in the direction perpendicular to the microwave magnetic field
induced by the stripline of the BFMR setup. The frequency of the
electromagnetic mode is independent of the static magnetic field. Consequently,
the predicted photon-magnon coupling is sensitive to the applied magnetic field
and thus can be used in magnetically tuneable metamaterials based on
simultaneously negative permittivity and permeability achievable thanks to the
YIG layer. We also suggest that the predicted photon-magnon coupling may find
applications in microwave quantum information systems.",1503.07282v1
2015-08-22,3D magnetic field configuration of small-scale reconnection events in the solar plasma atmosphere,"The outer solar atmosphere, i.e., the corona and the chromosphere, is replete
with small energy-release events, which are accompanied by transient
brightening and jet-like ejections. These events are considered to be magnetic
reconnection events in the solar plasma, and their dynamics have been studied
using recent advanced observations from the Hinode spacecraft and other
observatories in space and on the ground. These events occur at different
locations in the solar atmosphere, and vary in their morphology and amount of
the released energy. The magnetic field configurations of these reconnection
events are inferred based on observations of magnetic fields at the
photospheric level. Observations suggest that these magnetic configurations can
be classified into two groups. In the first group, two anti-parallel magnetic
fields reconnect to each other, yielding a 2D emerging flux configuration. In
the second group, helical or twisted magnetic flux tubes are parallel or at a
relative angle to each other. Reconnection can occur only between anti-parallel
components of the magnetic flux tubes and may be referred to as component
reconnection. The latter configuration type may be more important for the
larger class of small-scale reconnection events. The two types of magnetic
configurations can be compared to counter-helicity and co-helicity
configurations, respectively, in laboratory plasma collision experiments.",1508.05481v1
2015-12-14,The magnetic shear-current effect: generation of large-scale magnetic fields by the small-scale dynamo,"A novel large-scale dynamo mechanism, the magnetic shear-current effect, is
discussed and explored. The effect relies on the interaction of magnetic
fluctuations with a mean shear flow, meaning the saturated state of the
small-scale dynamo can drive a large-scale dynamo -- in some sense the inverse
of dynamo quenching. The dynamo is nonhelical, with the mean-field $\alpha$
coefficient zero, and is caused by the interaction between an off-diagonal
component of the turbulent resistivity and the stretching of the large-scale
field by shear flow. Following up on previous numerical and analytic work, this
paper presents further details of the numerical evidence for the effect, as
well as an heuristic description of how magnetic fluctuations can interact with
shear flow to produce the required electromotive force. The pressure response
of the fluid is fundamental to this mechanism, which helps explain why the
magnetic effect is stronger than its kinematic cousin, and the basic idea is
related to the well-known lack of turbulent resistivity quenching by magnetic
fluctuations. As well as being interesting for its applications to general high
Reynolds number astrophysical turbulence, where strong small-scale magnetic
fluctuations are expected to be prevalent, the magnetic shear-current effect is
a likely candidate for large-scale dynamo in the unstratified regions of
ionized accretion disks. Evidence for this is discussed, as well as future
research directions and the challenges involved with understanding details of
the effect in astrophysically relevant regimes.",1512.04511v2
2016-03-14,Lifshitz scaling effects on holographic paramagnetism/ferromagneism phase transition,"In the probe limit, we investigate holographic paramagnetism-ferromagnetism
phase transition in the four-dimensional (4D) and five-dimensional(5D) Lifshitz
black holes by means of numerical and semi-analytical methods, which is
realized by introducing a massive 2-form field coupled to the Maxwell field. We
find that the Lifshitz dynamical exponent $z$ contributes evidently to magnetic
moment and hysteresis loop of single magnetic domain quantitatively not
qualitatively. Concretely, in the case without external magnetic field, the
spontaneous magnetization and ferromagnetic phase transition happen when the
temperature gets low enough, and the critical exponent for the magnetic moment
is always $1/2$, which is in agreement with the result from mean field theory.
And the increasing $z$ enhances the phase transition and increases the DC
resistivity which behaves as the colossal magnetic resistance effect in some
materials. Furthermore, in the presence of the external magnetic field, the
magnetic susceptibility satisfies the Cure-Weiss law with a general $z$. But
the increase of $z$ will result in shortening the period of the external
magnetic field.",1603.04149v1
2016-04-25,Dynamically flavored description of holographic QCD in the presence of a magnetic field,"We construct the gravitational solution of the Witten-Sakai-Sugimoto model by
introducing a magnetic field on the flavor brane. With taking into account
their backreaction, we re-solve the type IIA supergravity in the presence of
the magnetic field. Our calculation shows the gravitational solutions are
magnetic-dependent and analytic both in the bubble (confined) and black brane
(deconfined) case. We study the dual field theory at the leading order in the
ratio of the number of flavors and colors, also in the Veneziano limit. Some
physical properties related to the hadronic physics in an external magnetic
field are discussed by using our confined backreaction solution
holographically. We also investigate the thermodynamics and holographic
renormalization of this model in both phases by our solution. Since the
backreaction of the magnetic field is considered in our gravitational solution,
it allows us to study the Hawking-Page transition with flavors and colors of
this model in the presence of the magnetic field. Finally we therefore obtain
the holographic phase diagram with the contributions from the flavors and the
magnetic field. Our holographic phase diagram is in agreement with lattice QCD
result qualitatively, which thus can be interpreted as the inhibition of
confinement or chirally broken symmetry by the magnetic field.",1604.07197v6
2016-05-12,Temporal and spatial relationship of flare signatures and the force-free coronal magnetic field,"We investigate the plasma and magnetic environment of active region NOAA
11261 on 2 August 2011 around a GOES M1.4 flare/CME (SOL2011-08-02T06:19). We
compare coronal emission at (extreme) ultraviolet and X-ray wavelengths, using
SDO AIA and RHESSI images, in order to identify the relative timing and
locations of reconnection-related sources. We trace flare ribbon signatures at
ultraviolet wavelengths, in order to pin down the intersection of previously
reconnected flaring loops at the lower solar atmosphere. These locations are
used to calculate field lines from 3D nonlinear force-free magnetic field
models, established on the basis of SDO HMI photospheric vector magnetic field
maps. With this procedure, we analyze the quasi-static time evolution of the
coronal model magnetic field previously involved in magnetic reconnection. This
allows us, for the first time, to estimate the elevation speed of the current
sheet's lower tip during an on-disk observed flare, as a few kilometers per
second. Comparison to post-flare loops observed later above the limb in STEREO
EUVI images supports this velocity estimate. Furthermore, we provide evidence
for an implosion of parts of the flaring coronal model magnetic field, and
identify the corresponding coronal sub-volumes associated to the loss of
magnetic energy. Finally, we spatially relate the build up of magnetic energy
in the 3D models to highly sheared fields, established due to dynamic relative
motions of polarity patches within the active region.",1605.03703v1
2016-07-22,Magnetic diffuse scattering in artificial kagome spin ice,"The study of magnetic correlations in dipolar-coupled nanomagnet systems with
synchrotron x-ray scattering provides a means to uncover emergent phenomena and
exotic phases, in particular in systems with thermally active magnetic moments.
From the diffuse signal of soft x-ray resonant magnetic scattering, we have
measured magnetic correlations in a highly dynamic artificial kagome spin ice
with sub-70-nm Permalloy nanomagnets. On comparing experimental scattering
patterns with Monte Carlo simulations based on a needle-dipole model, we
conclude that kagome ice I phase correlations exist in our experimental system
even in the presence of moment fluctuations, which is analogous to bulk spin
ice and spin liquid behavior. In addition, we describe the emergence of
quasi-pinch-points in the magnetic diffuse scattering in the kagome ice I
phase. These quasi-pinch-points bear similarities to the fully developed pinch
points with singularities of a magnetic Coulomb phase, and continually evolve
into the latter on lowering the temperature. The possibility to measure
magnetic diffuse scattering with soft x rays opens the way to study magnetic
correlations in a variety of nanomagnetic systems.",1607.06673v1
2016-07-28,Convection-driven kinematic dynamos at low Rossby and magnetic Prandtl numbers,"Most large-scale planetary magnetic fields are thought to be driven by low
Rossby number convection of a low magnetic Prandtl number fluid. Here kinematic
dynamo action is investigated with an asymptotic, rapidly rotating dynamo model
for the plane layer geometry that is intrinsically low magnetic Prandtl number.
The thermal Prandtl number and Rayleigh number are varied to illustrate
fundamental changes in flow regime, ranging from laminar cellular convection to
geostrophic turbulence in which an inverse energy cascade is present. A
decrease in the efficiency of the convection to generate a dynamo, as
determined by an increase in the critical magnetic Reynolds number, is observed
as the buoyancy forcing is increased. This decreased efficiency may result from
both the loss of correlations associated with the increasingly disordered
states of flow that are generated, and boundary layer behavior that enhances
magnetic diffusion locally. We find that the spatial characteristics of
$\alpha$, and thus the large-scale magnetic field, is dependent only weakly on
changes in flow behavior. However, our results are limited to the linear,
kinematic dynamo regime, and future simulations including the Lorentz force are
therefore necessary to assess the robustness of this result. In contrast to the
large-scale magnetic field, the behavior of the small-scale magnetic field is
directly dependent on, and therefore shows significant variations with, the
small-scale convective flow field.",1607.08346v1
2017-01-16,Noncollinear magnetic ordering in the Shastry-Sutherland Kondo lattice model: Insulating regime and the role of Dzyaloshinskii-Moriya interaction,"We investigate the necessary conditions for the emergence of complex,
noncoplanar magnetic configurations in a Kondo lattice model with classical
local moments on the geometrically frustrated Shastry-Sutherland lattice and
their evolution in an external magnetic field. We demonstrate that
topologically nontrivial spin textures, including a new canted flux state, with
nonzero scalar chirality arise dynamically from realistic short-range
interactions. Our results establish that a finite Dzyaloshinskii-Moriya (DM)
interaction is necessary for the emergence of these novel magnetic states when
the system is at half filling, for which the ground state is insulating. We
identify the minimal set of DM vectors that are necessary for the stabilization
of chiral magnetic phases. The noncoplanarity of such structures can be tuned
continually by applying an external magnetic field. This is the first part in a
series of two papers; in the following paper the effects of frustration,
thermal fluctuations, and magnetic field on the emergence of novel noncollinear
states at metallic filling of itinerant electrons are discussed. Our results
are crucial in understanding the magnetic and electronic properties of the
rare-earth tetraboride family of frustrated magnets with separate spin and
charge degrees of freedom.",1701.04297v2
2017-09-13,Distortion of Magnetic Fields in a Starless Core II: 3D Magnetic Field Structure of FeSt 1-457,"Three dimensional (3D) magnetic field information on molecular clouds and
cores is important for revealing their kinematical stability (magnetic support)
against gravity which is fundamental for studying the initial conditions of
star formation. In the present study, the 3D magnetic field structure of the
dense starless core FeSt 1-457 is determined based on the near-infrared
polarimetric observations of the dichroic polarization of background stars and
simple 3D modeling. With an obtained angle of line-of-sight magnetic
inclination axis $\theta_{\rm inc}$ of $45^{\circ}\pm10^{\circ}$ and previously
determined plane-of-sky magnetic field strength $B_{\rm pol}$ of $23.8\pm12.1$
$\mu{\rm G}$, the total magnetic field strength for FeSt 1-457 is derived to be
$33.7\pm18.0$ $\mu{\rm G}$. The critical mass of FeSt 1-457, evaluated using
both magnetic and thermal/turbulent support is ${M}_{\rm cr} = 3.70\pm0.92$
${\rm M}_{\odot}$, which is identical to the observed core mass, $M_{\rm
core}=3.55\pm0.75$ ${\rm M}_{\odot}$. We thus conclude that the stability of
FeSt 1-457 is in a condition close to the critical state. Without infalling gas
motion and no associated young stars, the core is regarded to be in the
earliest stage of star formation, i.e., the stage just before the onset of
dynamical collapse following the attainment of a supercritical condition. These
properties would make FeSt 1-457 one of the best starless cores for future
studies of the initial conditions of star formation.",1709.04544v2
2017-10-04,The dynamo effect in decaying helical turbulence,"We show that in decaying hydromagnetic turbulence with initial kinetic
helicity, a weak magnetic field eventually becomes fully helical. The sign of
magnetic helicity is opposite to that of the kinetic helicity - regardless of
whether or not the initial magnetic field was helical. The magnetic field
undergoes inverse cascading with the magnetic energy decaying approximately
like t^{-1/2}. This is even slower than in the fully helical case, where it
decays like t^{-2/3}. In this parameter range, the product of magnetic energy
and correlation length raised to a certain power slightly larger than unity, is
approximately constant. This scaling of magnetic energy persists over long time
scales. At very late times and for domain sizes large enough to accommodate the
growing spatial scales, we expect a cross-over to the t^{-2/3} decay law that
is commonly observed for fully helical magnetic fields. Regardless of the
presence or absence of initial kinetic helicity, the magnetic field experiences
exponential growth during the first few turnover times, which is suggestive of
small-scale dynamo action. Our results have applications to a wide range of
experimental dynamos and astrophysical time-dependent plasmas, including
primordial turbulence in the early universe.",1710.01628v4
2017-10-20,Tidal dissipation in rotating fluid bodies: the presence of a magnetic field,"We investigate effects of the presence of a magnetic field on tidal
dissipation in rotating fluid bodies. We consider a simplified model consisting
of a rigid core and a fluid envelope, permeated by a background magnetic field
(either a dipolar field or a uniform axial field). The wavelike tidal responses
in the fluid layer are in the form of magnetic-Coriolis waves, which are
restored by both the Coriolis force and the Lorentz force. Energy dissipation
occurs through viscous damping and Ohmic damping of these waves. Our numerical
results show that the tidal dissipation can be dominated by Ohmic damping even
with a weak magnetic field. The presence of a magnetic field smooths out the
complicated frequency-dependence of the dissipation rate, and broadens the
frequency spectrum of the dissipation rate, depending on the strength of the
background magnetic field. However, the frequency-averaged dissipation is
independent of the strength and structure of the magnetic field, and of the
dissipative parameters, in the approximation that the wave-like response is
driven only by the Coriolis force acting on the non-wavelike tidal flow.
Indeed, the frequency-averaged dissipation quantity is in good agreement with
previous analytical results in the absence of magnetic fields. Our results
suggest that the frequency-averaged tidal dissipation of the wavelike
perturbations is insensitive to detailed damping mechanisms and dissipative
properties.",1710.07690v2
2017-11-17,"Frustrated magnetism in tetragonal CoSe, analogue to superconducting FeSe","Recently synthesized metastable tetragonal CoSe, isostructural to the FeSe
superconductor, offers a new avenue for investigating systems in close
proximity to the iron-based superconductors. We present magnetic and transport
property measurements on powders and single crystals of CoSe. High field
magnetic susceptibility measurements indicate a suppression of the previously
reported 10 K ferromagnetic transition with the magnetic susceptibility
exhibiting time-dependence below the proposed transition. Dynamic scaling
analysis of the time-dependence yields a critical relaxation time of $\tau^{*}
= 0.064 \pm 0.008 $ s which in turn yields an activation energy of $E_{a}^{*}$
= 14.84 $\pm$ 0.59 K and an ideal glass temperature $T_{0}^{*}$ = 8.91 $\pm$
0.09 K from Vogel-Fulcher analysis. No transition is observed in resistivity
and specific heat measurements, but both measurements indicate that CoSe is
metallic. These results are interpreted on the basis of CoSe exhibiting
frustrated magnetic ordering arising from competing magnetic interactions.
Arrott analysis of single crystal magnetic susceptibility has indicated that
the magnetic moments lie in the $ab$-plane so frustration may arise from
intralayer magnetic fluctuations as well as interlayer coupling. The results
have implications for understanding the superconductivity in the iron
chalcogenide systems as well as utilizing CoSe as a host for chemical and
physical manipulation to tune and explore emergent phenomena within an
expanding new class of transition metal chalcogenides.",1711.06725v2
2017-12-02,Magnetic reconnection in strongly magnetized regions of the low solar chromosphere,"Magnetic reconnection in strongly magnetized regions around the temperature
minimum region of the low solar atmosphere is studied by employing MHD-based
simulations of a partially ionized plasma within a reactive 2.5D multi-fluid
model. It is shown that in the absence of magnetic nulls in a low $\beta$
plasma the ionized and neutral fluid flows are well-coupled throughout the
reconnection region. However, non-equilibrium ionization-recombination dynamics
play a critical role in determining the structure of the reconnection region,
lead to much lower temperature increases and a faster magnetic reconnection
rate as compared to simulations that assume plasma to be in
ionization-recombination equilibrium. The rate of ionization of the neutral
component of the plasma is always faster than recombination within the current
sheet region even when the initial plasma $\beta$ is as high as $\beta_0=1.46$.
When the reconnecting magnetic field is in excess of a kilogauss and the plasma
$\beta$ is lower than 0.0145, the initially weakly ionized plasmas can become
fully ionized within the reconnection region and the current sheet can be
strongly heated to above $2.5\times10^4$~K, even as most of the collisionally
dissipated magnetic energy is radiated away. The Hall effect increases the
reconnection rate slightly, but in the absence of magnetic nulls it does not
result in significant asymmetries or change the characteristics of the
reconnection current sheet down to meter scales.",1712.00582v2
2017-12-12,Magnetic Activity in the Galactic Centre Region -- Fast Downflows along Rising Magnetic Loops,"We studied roles of the magnetic field on the gas dynamics in the Galactic
bulge by a three-dimensional global magnetohydrodynamical simulation data,
particularly focusing on vertical flows that are ubiquitously excited by
magnetic activity. In local regions where the magnetic filed is stronger, it is
frequently seen that fast down-flows slide along inclined magnetic field lines
that are associated with buoyantly rising magnetic loops. The vertical velocity
of these down-flows reaches ~ 100 km s$^{-1}$ near the foot-point of the loops
by the gravitational acceleration toward the Galactic plane. The two footpoints
of rising magnetic loops are generally located at different radial locations
and the field lines are deformed by the differential rotation. The angular
momentum is transported along the field lines, and the radial force balance
breaks down. As a result, a fast downflow is often observed only at the one
footpoint located at the inner radial position. The fast downflow compresses
the gas to form a dense region near the footpoint, which will be important in
star formation afterward. Furthermore, the horizontal components of the
velocity are also fast near the foot-point because the down-flow is accelerated
along the magnetic sliding slope. As a result, the high-velocity flow creates
various characteristic features in a simulated position-velocity diagram,
depending on the viewing angle.",1712.04209v2
2018-04-16,Magnetic reconnection in the low solar chromosphere with a more realistic radiative cooling model,"Magnetic reconnection is the most likely mechanism responsible for the high
temperature events that are observed in strongly magnetized locations around
the temperature minimum in the low solar chromosphere. This work improves upon
our previous work [""Magnetic Reconnection in Strongly Magnetized Regions of the
Low Solar Chromosphere"", The Astrophysical Journal 852, 95 (2018) ] by using a
more realistic radiative cooling model computed from the OPACITY project and
the CHIANTI database. We find that the rate of ionization of the neutral
component of the plasma is still faster than recombination within the current
sheet region. For low $\beta$ plasmas, the ionized and neutral fluid flows are
well-coupled throughout the reconnection region resembling the single-fluid
Sweet-Parker model dynamics. Decoupling of the ion and neutral inflows appears
in the higher $\beta$ case with $\beta_0=1.46$, which leads to a reconnection
rate about three times faster than the rate predicted by the Sweet-Parker
model. In all cases, the plasma temperature increases with time inside the
current sheet, and the maximum value is above $2\times10^4$~K when the
reconnection magnetic field strength is greater than $500$~G. While the more
realistic radiative cooling model does not result in qualitative changes of the
characteristics of magnetic reconnection, it is necessary for studying the
variations of the plasma temperature and ionization fraction inside current
sheets in strongly magnetized regions of the low solar atmosphere. It is also
important for studying energy conversion during the magnetic reconnection
process when the hydrogen-dominated plasma approaches full ionization.",1804.05631v2
2018-06-23,The Higgs Factory Muon Collider Superconducting Magnets and Their Protection Against Beam Decay Radiation,"Low-energy medium-luminosity Muon Collider (MC) is being studied as a
possible Higgs Factory (HF). Electrons from muon decays will deposit more than
300 kW in superconducting magnets of the HF collider ring. This imposes
significant challenges to superconducting (SC) magnets used in the MC storage
ring (SR) and interaction regions (IR). The magnet designs are proposed which
provide high operating gradient and magnetic field in a large aperture to
accommodate the large size of muon beams due to low \b{eta}* as well as the
cooling system to intercept the large heat deposition from the showers induced
by decay electrons. Specific distribution of heat deposition in the lattice
elements MC SR requires large aperture magnets to accommodate thick high-Z
absorbers to protect the SC coils. Based on the developed MARS15 model and
intense simulations, a sophisticated radiation protection system was designed
for the collider SR and IR to bring the peak power density in the
superconducting coils below the quench limit and reduce the dynamic heat
deposition in the cold mass by a factor of 100. The system consists of tight
tungsten masks in the magnet interconnect regions and elliptical tungsten
liners in magnet aperture optimized for each magnet. It also reduces the
background particle fluxes in the collider detector.",1806.08883v2
2018-07-04,An orbitally derived single-atom magnetic memory,"A single magnetic atom on a surface epitomizes the scaling limit for magnetic
information storage. Indeed, recent work has shown that individual atomic spins
can exhibit magnetic remanence and be read out with spin-based methods,
demonstrating the fundamental requirements for magnetic memory. However, atomic
spin memory has been only realized on thin insulating surfaces to date,
removing potential tunability via electronic gating or distance-dependent
exchange-driven magnetic coupling. Here, we show a novel mechanism for
single-atom magnetic information storage based on bistability in the orbital
population, or so-called valency, of an individual Co atom on semiconducting
black phosphorus (BP). Distance-dependent screening from the BP surface
stabilizes the two distinct valencies and enables us to electronically
manipulate the relative orbital population, total magnetic moment and spatial
charge density of an individual magnetic atom without a spin-dependent readout
mechanism. Furthermore, we show that the strongly anisotropic wavefunction can
be used to locally tailor the switching dynamics between the two valencies.
This orbital memory derives stability from the energetic barrier to atomic
relaxation and demonstrates the potential for high-temperature single-atom
information storage.",1807.01668v1
2018-10-07,Spin dynamics and field-induced magnetic phase transition in the honeycomb Kitaev magnet $α$-Li$_2$IrO$_3$,"The layered honeycomb iridate $\alpha$-Li$_2$IrO$_3$ displays an
incommensurate magnetic structure with counterrotating moments on
nearest-neighbor sites, proposed to be stabilized by strongly-frustrated
anisotropic Kitaev interactions between spin-orbit entangled Ir$^{4+}$ magnetic
moments. Here we report powder inelastic neutron scattering measurements that
observe sharply dispersive low-energy magnetic excitations centered at the
magnetic ordering wavevector, attributed to Goldstone excitations of the
incommensurate order, as well as an additional intense mode above a gap
$\Delta\simeq2.3$ meV. Zero-field muon-spin relaxation measurements show clear
oscillations in the muon polarization below the N\'{e}el temperature $T_{\rm
N}\simeq15$ K with a time-dependent profile consistent with bulk incommensurate
long-range magnetism. Pulsed field magnetization measurements observe that only
about half the saturation magnetization value is reached at the maximum field
of 64 T. A clear anomaly near 25 T indicates a transition to a phase with
reduced susceptibility. The transition field has a Zeeman energy comparable to
the zero-field gapped mode, suggesting gap suppression as a possible mechanism
for the field-induced transition.",1810.03212v1
2019-01-06,"Quasi-2D magnetism and origin of the Dirac semimetallic behavior in nonstoichiometric Sr$_{1-y}$Mn$_{1-z}$Sb$_{2}$ (y, z~$<$0.1)","Nonstoichiometric Sr$_{1-y}$Mn$_{1-z}$Sb$_{2}$ (y, z~$<$0.1) is known to
exhibit a coexistence of magnetic order and the nontrivial semimetallic
behavior related to Dirac or Weyl fermions. Here, we report inelastic neutron
scattering analyses of the spin dynamics and density functional theory studies
on the electronic properties of Sr$_{1-y}$Mn$_{1-z}$Sb$_{2}$. We observe a
relatively large spin excitation gap $\approx$ 8.5 meV at 5 K, and the
interlayer magnetic exchange constant only 2.8 \% of the dominant intralayer
magnetic interaction, providing evidence that Sr$_{1-y}$Mn$_{1-z}$Sb$_{2}$
exhibits a quasi-2D magnetism. Using density functional theory, we find a
strong influence of magnetic orders on the electronic band structure and the
Dirac dispersions near the Fermi level along the Y-S direction in the presence
of a ferromagnetic ordering. Our study unveils novel interplay between the
magnetic order, magnetic transition, and electronic property in
Sr$_{1-y}$Mn$_{1-z}$Sb$_{2}$, and opens new pathways to control the
relativistic band structure through magnetism in ternary compounds.",1901.01565v1
2019-03-11,Novel dynamics and critical currents in fast superconducting vortices at high pulsed magnetic fields,"Non-linear electrical transport studies at high-pulsed magnetic fields, above
the range accessible by DC magnets, are of direct fundamental relevance to the
physics of superconductors, domain-wall, charge-density waves, and topological
semi-metal. All-superconducting very-high field magnets also make it
technologically relevant to study vortex matter in this regime. However, pulsed
magnetic fields reaching 100 T in milliseconds impose technical and fundamental
challenges that have prevented the realization of these studies. Here, we
present a technique for sub-microsecond, smart, current-voltage measurements,
which enables determining the superconducting critical current in pulsed
magnetic fields, beyond the reach of any DC magnet. We demonstrate the
excellent agreement of this technique with low DC field measurements on
Y$_{0.77}$Gd$_{0.23}$Ba$_2$Cu$_3$O$_7$ coated conductors with and without
BaHfO$_3$ nanoparticles. Exploring the uncharted high magnetic field region, we
discover a characteristic influence of the magnetic field rate of change
($dH/dt$) on the current-voltage curves in a superconductor. We fully capture
this unexplored vortex physics through a theoretical model based on the
asymmetry of the vortex velocity profile produced by the applied current.",1903.04658v2
2019-03-26,"Precision measurement of magnetic field from near to far, from fine to large scales in ISM","Magnetic fields have important or dominant effects in many areas of
astrophysics, but have been very difficult to quantify. Spectropolarimetry from
Ground State Alignment (GSA) has been suggested as a direct tracer of magnetic
field in interstellar diffuse medium. Owing to the long life of the atoms on
ground states the Larmor precession in an external magnetic field imprints the
direction of the field onto the polarization of absorbing species. This
provides a unique tool for studies of sub-gauss magnetic fields using
polarimetry of UV, optical and radio lines. Many spectral lines with strong
signals from GSA are in the UV band. By discerning magnetic fields in gas with
different dynamical properties, high spectral resolution measurement of
spectral polarization will allow the study of 3D magnetic field distribution
and interstellar turbulence. GSA provides also a unique chance to map 3D
direction of magnetic field on small scales, e.g., disks, where grain alignment
is unreliable. The range of objects suitable for studies is extremely wide and
includes magnetic fields in the interplanetary medium, in the interstellar
medium, and in circumstellar regions as well as diffuse media in extragalactic
objects.",1903.11065v1
2019-05-20,Micromagnetic Modelling of the Heat Assisted Switching Process in high Anisotropy FePt Granular Thin Films,"The dynamic process of assisted magnetic switchins has been simulated to
investigate the associated physics. The model uses a Voronoi construction to
determine the physical structure of the nano granular thin film recording
media; and the Landau-Lifshitz-Bloch (LLB) equation is solved to evolve the
magnetic system in time. The reduction of the magnetization is determined over
a range of peak system temperatures and for a number of anisotropy values. The
results show that the HAMR process is not simply magnetization reversal over a
thermally reduced energy barrier. To achieve full magnetization reversal (for
all anisotropies investigated) an applied field strength of at least 6kOe is
required and the peak system temperature must reach at least the Curie point
(Tc). When heated to Tc the magnetization associated with each grain is
destroyed, which invokes the non-precessional linear reversal mode. Reversing
the magnetization through this linear reversal mode is favourable, as the
reversal time is two orders of magnitude smaller than that associated with
precession. Under these conditions, as the temperature decreases to ambient,
the magnetization recovers in the direction of the applied field, completing
the reversal process. Also the model produces results which are consistent with
the concept of thermal writability; when heating the media to Tc, the smaller
grains require a larger field strength to reverse the magnetization.",1905.08134v1
2019-07-15,Nature of the magnetic moment of cobalt in ordered FeCo alloy,"The magnets are typically classified into Stoner and Heisenberg type,
depending on the itinerant or localized nature of the constituent magnetic
moments. In this work, we investigate theoretically the behaviour of the
magnetic moments of iron and cobalt in their B2-ordered alloy. The results
based on local spin density approximation (LSDA) for the density functional
theory (DFT) suggest that the Co magnetic moment strongly depends on the
directions of the surrounding magnetic moments, which usually indicates the
Stoner-type mechanism of magnetism. This is consistent with the disordered
local moment (DLM) picture of the paramagnetic state, where the magnetic moment
of cobalt gets substantially suppressed. We argue that this is due to the lack
of strong on-site electron correlations, which we take into account by
employing a combination of DFT and dynamical mean-field theory (DMFT). Within
LDA+DMFT, we find a substantial quasiparticle mass renormalization and a non
Fermi-liquid behaviour of Fe-$3d$ orbitals. The resulting spectral functions
are in very good agreement with measured spin-resolved photoemission spectra.
Our results suggest that local correlations play an essential role in
stabilizing a robust local moment on Co in the absence of magnetic order at
high temperatures.",1907.06613v5
2019-07-18,Magnetic Power Spectra of Emerging Active Regions,"Magnetic field data provided by the Helioseismic and Magnetic Imager on board
the Solar Dynamics Observatory were utilized to explore the changes in the
magnetic energy of four active regions (ARs) during their emergence. We found
that at the very early stage of the magnetic flux emergence, an abrupt
steepening of the magnetic power spectrum takes place leading to rapid increase
of the absolute value of the negative spectra power index $\alpha$ in $E(k)$
$\sim$ $k^{\alpha}$. As the emergence proceeds, the energy increases at all
scales simultaneously implying that elements of all sizes do appear in the
photosphere. Meanwhile, the energy gain at scales larger than $\approx$10 Mm
prevails as compared to that at smaller scales. Both direct (i.e.,
fragmentation of large structures into smaller ones) and inverse (i.e., merging
of small magnetic features into larger elements) cascades are readily observed
during the emergence. However, in the case of inverse cascade, the total energy
gained at large scales exceeds the energy loss at smaller scales assuming
simultaneous appearance of large-scale magnetic entities from beneath the
photosphere. We conclude that most of the time the energy may grow at all
scales. We also cannot support the point of view regarding the dominant role of
the inverse cascade in the formation of an AR. Although the coalescence of
small magnetic elements into larger pores and sunspots is observed, our
analysis shows that the prevailed energy contribution to an AR comes from
emergence of large-scale structures.",1907.07952v1
2019-07-26,Forecast of Daily Major Flare Probability Using Relationships between Vector Magnetic Properties and Flaring Rates,"We develop forecast models of daily probability of major flares (M- and
X-class) based on empirical relationships between photospheric magnetic
parameters and daily flaring rates from May 2010 to April 2018. In this study,
we consider ten magnetic parameters characterizing size, distribution, and
non-potentiality of vector magnetic fields from Solar Dynamics Observatory
(SDO)/Helioseismic and Magnetic Imager (HMI) and Geostationary Operational
Environmental Satellites (GOES) X-ray flare data. The magnetic parameters are
classified into three types: the total unsigned parameters, the total signed
parameters, and the mean parameters. We divide the data into two sets
chronologically: 70% for training and 30% for testing. The empirical
relationships between the parameters and flaring rates are used to predict
flare occurrence probabilities for a given magnetic parameter value. Major
results of this study are as follows. First, major flare occurrence rates are
well correlated with ten parameters having correlation coefficients above 0.85.
Second, logarithmic values of flaring rates are well approximated by linear
equations. Third, the total unsigned and signed parameters achieved better
performance for predicting flares than the mean parameters in terms of
verification measures of probabilistic and converted binary forecasts. We
conclude that the total quantity of non-potentiality of magnetic fields is
crucial for flare forecasting among the magnetic parameters considered in this
study. When this model is applied for operational use, this model can be used
using the data of 21:00 TAI with a slight underestimation of 2 - 6.3%.",1907.11373v1
2019-10-25,Nature of hyperfine interactions in TbPc$_2$ single-molecule magnets: Multireference ab-initio study,"Lanthanide-based single-ion magnetic molecules can have large magnetic
hyperfine interactions as well as large magnetic anisotropy. Recent
experimental studies reported tunability of these properties by changes of
chemical environments or by application of external stimuli for device
applications. In order to provide insight onto the origin and mechanism of such
tunability, here we investigate the magnetic hyperfine and nuclear quadrupole
interactions for $^{159}$Tb nucleus in TbPc$_2$ (Pc=phthalocyanine)
single-molecule magnets using multireference ab-initio methods including
spin-orbit interaction. Since the electronic ground and first-excited
(quasi)doublets are well separated in energy, the microscopic Hamiltonian can
be mapped onto an effective Hamiltonian with an electronic pseudo-spin $S=1/2$.
From the ab-initio-calculated parameters, we find that the magnetic hyperfine
coupling is dominated by the interaction of the Tb nuclear spin with electronic
orbital angular momentum. The asymmetric $4f$-like electronic charge
distribution leads to a strong nuclear quadrupole interaction with significant
non-axial terms for the molecule with low symmetry. The ab-initio calculated
electronic-nuclear spectrum including the magnetic hyperfine and quadrupole
interactions is in excellent agreement with experiment. We further find that
the non-axial quadrupole interactions significantly influence the avoided level
crossings in magnetization dynamics and that the molecular distortions affect
mostly the Fermi contact terms as well as the non-axial quadrupole
interactions.",1910.11952v1
2020-04-01,Selectively Controlled Magnetic Microrobots with Opposing Helices,"Magnetic microrobots that swim through liquid media are of interest for
minimally invasive medical procedures, bioengineering, and manufacturing. Many
of the envisaged applications, such as micromanipulation and targeted cargo
delivery, necessitate the use and adequate control of multiple microrobots,
which will increase the velocity, robustness, and efficacy of a procedure.
While various methods involving heterogeneous geometries, magnetic properties,
and surface chemistries have been proposed to enhance independent control, the
main challenge has been that the motion between all microwsimmers remains
coupled through the global control signal of the magnetic field. Katsamba and
Lauga proposed transchiral microrobots, a theoretical design with magnetized
spirals of opposite handedness. The competition between the spirals can be
tuned to give an intrinsic nonlinearity that each device can function only
within a given band of frequencies. This allows individual microrobots to be
selectively controlled by varying the frequency of the rotating magnetic field.
Here we present the experimental realization and characterization of
transchiral micromotors composed of independently driven magnetic helices. We
show a swimming micromotor that yields negligible net motion until a critical
frequency is reached and a micromotor that changes its translation direction as
a function of the frequency of the rotating magnetic field. This work
demonstrates a crucial step towards completely decoupled and addressable
swimming magnetic microrobots.",2004.00687v1
2020-05-08,Sublattice magnetizations of ultrathin ferrimagnetic lamellar nanostructures between cobalt leads,"In this work we model the salient magnetic properties of the alloy lamellar
ferrimagnetic nanostructures
$[Co_{1-c}Gd_c]_{\ell^{\prime}}[Co]_\ell[Co_{1-c}Gd_c]_{\ell^{\prime}}$ between
$Co$ semi-infinite leads. We have employed the Ising spin effective field
theory (EFT) to compute the reliable magnetic exchange constants for the pure
cobalt $J_{Co-Co}$ and gadolinium $J_{Gd-Gd}$ materials, in complete agreement
with their experimental data. The sublattice magnetizations of the $Co$ and
$Gd$ sites on the individual hcp atomic (0001) planes of the $Co-Gd$ layered
nanostructures are computed for each plane and corresponding sites, by using
the combined EFT and mean field theory (MFT) spin methods. The sublattice
magnetizations, effective site magnetic moments, and ferrimagnetic compensation
characteristics for the individual hcp atomic planes of the embedded
nanostructures, are computed as a function of temperature, and for various
stable eutectic concentrations in the range $c\leq$ 0.5. The theoretical
results for the sublattice magnetizations and the local magnetic variables of
these ultrathin ferrimagnetic lamellar nanostructured systems, between cobalt
leads, are necessary for the study of their magnonic transport properties, and
eventually their spintronic dynamic computations. The method developed in this
work is general and can be applied to comparable magnetic systems
nanostructured with other materials.",2005.03965v3
2020-05-11,Constraining the primordial magnetic field with dwarf galaxy simulations,"Using a set of cosmological hydro-dynamical simulations, we constrained the
properties of primordial magnetic fields by studying their impact on the
formation and evolution of dwarf galaxies. We performed a large set of
simulations (8 dark matter only and 72 chemo-hydrodynamical) including
primordial magnetic fields through the extra density fluctuations they induce
at small length scales ($k \geq 10\,h\,\rm{Mpc^{-1}}$) in the matter power
spectrum. We explored a large variety of primordial magnetic fields with
strength $B_\lambda$ ranging from $0.05$ to $0.50\,\textrm{nG}$ and magnetic
energy spectrum slopes $n_B$ from $-2.9$ to $-2.1$. Strong magnetic fields
characterized by a high amplitude ($B_\lambda=0.50,\,0.20\,\textrm{nG}$ with
$n_B=-2.9$) or by a steep initial power spectrum slope ($n_B=-2.1,-2.4$, with
$B_\lambda=0.05\,\textrm{nG}$) induce perturbations in the mass scales from
$10^7$ to $10^9\,\rm{M}_{\odot}$. In this context emerging galaxies see their
star formation rate strongly boosted. They become more luminous and metal rich
than their counterparts without primordial magnetic fields. Such strong fields
are ruled out by their inability to reproduce the observed scaling relations of
dwarf galaxies. They predict dwarf galaxies to be at the origin of an
unrealistically early reionization of the Universe and also overproduce
luminous satellites in the Local Group. Weaker magnetic fields impacting the
primordial density field at corresponding masses $\lesssim
10^6\,\rm{M}_{\odot}$, produce a large number of mini dark halos orbiting the
dwarfs, however out of reach for current lensing observations. This study
allows for the first time to constrain the properties of primordial magnetic
fields based on realistic cosmological simulations of dwarf galaxies.",2005.05401v1
2020-09-03,Polarity Free Magnetic Repulsion and Magnetic Bound State,"This is a report on a dynamic autonomous magnetic interaction which does not
depend on polarities resulting in short ranged repulsion involving one or more
inertial bodies and a new class of bound state based on this interaction. Both
effects are new to the literature, found so far. Experimental results are
generalized and reported qualitatively. Working principles of these effects are
provided within classical mechanics and found consistent with observations and
simulations. The effects are based on the interaction of a rigid and finite
inertial body (an object having mass and moment of inertia) endowed with a
magnetic moment with a cyclic inhomogeneous magnetic field which does not
require to have a local minimum. Such a body having some degrees of freedom
involved in driven harmonic motion by this interaction can experience a net
force in the direction of the weak field regardless of its position and
orientation or can find stable equilibrium with the field itself autonomously.
The former is called polarity free magnetic repulsion and the latter magnetic
bound state. Experiments show that a bound state can be obtained between two
free bodies having magnetic dipole moment. Various schemes of trapping bodies
having magnetic moments by rotating fields are realized as well as rotating
bodies trapped by a static dipole field in presence of gravity. Also, a special
case of bound state called bipolar bound state between free dipole bodies is
investigated.",2009.07082v1
2020-10-12,Gravity and rotation drag the magnetic field in high-mass star formation,"The formation of hot stars out of the cold interstellar medium lies at the
heart of astrophysical research. Understanding the importance of magnetic
fields during star formation remains a major challenge. With the advent of the
Atacama Large Millimeter Array, the potential to study magnetic fields by
polarization observations has tremendously progressed. However, the major
question remains how much magnetic fields shape the star formation process or
whether gravity is largely dominating. Here, we show that for the high-mass
star-forming region G327.3 the magnetic field morphology appears to be
dominantly shaped by the gravitational contraction of the central massive gas
core where the star formation proceeds. We find that in the outer parts of the
region, the magnetic field is directed toward the gravitational center of the
region. Filamentary structures feeding the central core exhibit U-shaped
magnetic field morphologies directed toward the gravitational center as well,
again showing the gravitational drag toward the center. The inner part then
shows rotational signatures, potentially associated with an embedded disk, and
there the magnetic field morphology appears to be rotationally dominated.
Hence, our results demonstrate that for this region gravity and rotation are
dominating the dynamics and shaping the magnetic field morphology.",2010.05825v1
2020-12-03,"Polar magneto-optical Kerr effect in antiferromagnetic M$_2$As (M=Cr, Mn, Fe) under an external magnetic field","Antiferromagnetic metals attract tremendous interest for memory applications
due to their expected fast response dynamics in the terahertz frequency regime.
Reading from and writing information into these materials is not easily
achievable using magnetic fields, due to weak high-order magneto-optical
signals and robustness of the magnetic structure against external magnetic
fields. Polarized electromagnetic radiation is a promising alternative for
probing their response, however, when ideal antiferromagnetic symmetry is
present, this response vanishes. Hence, in this work we combine
first-principles simulations with measurements of the polar magneto-optical
Kerr effect under external magnetic fields, to study magneto-optical response
of antiferromagnetic M$_2$As (M=Cr, Mn, and Fe). We devise a computational
scheme to compute the magnetic susceptibility from total-energy changes using
constraints on magnetic-moment tilting. Our predictions of the spectral
dependence of polar magneto-optical Kerr rotation and ellipticity allow us to
attribute these effects to breaking of the magnetic symmetry. We show that
tilting affects the exchange interaction, while the spin-orbit interaction
remains unaffected as the tilting angle changes. Our work provides
understanding of the polar magneto-optical Kerr effect on a band structure
level and underscores the importance of the magnetic susceptibility when
searching for materials with large magneto-optical response.",2012.02090v1
2020-12-18,Untangling the global coronal magnetic field with multiwavelength observations,"Magnetism defines the complex and dynamic solar corona. Coronal mass
ejections (CMEs) are thought to be caused by stresses, twists, and tangles in
coronal magnetic fields that build up energy and ultimately erupt, hurling
plasma into interplanetary space. Even the ever-present solar wind possesses a
three-dimensional morphology shaped by the global coronal magnetic field,
forming geoeffective corotating interaction regions. CME evolution and the
structure of the solar wind depend intimately on the coronal magnetic field, so
comprehensive observations of the global magnetothermal atmosphere are crucial
both for scientific progress and space weather predictions. Although some
advances have been made in measuring coronal magnetic fields locally, synoptic
measurements of the global coronal magnetic field are not yet available.
  We conclude that a key goal for 2050 should be comprehensive, ongoing 3D
synoptic maps of the global coronal magnetic field. This will require the
construction of new telescopes, ground and space-based, to obtain
complementary, multiwavelength observations sensitive to the coronal magnetic
field. It will also require development of inversion frameworks capable of
incorporating multi-wavelength data, and forward analysis tools and simulation
testbeds to prioritize and establish observational requirements on the proposed
telescopes.",2012.09992v1
2021-01-21,Spin liquid behavior of a three-dimensional magnetic system Ba$_3$NiIr$_2$O$_9$ with $S$ = 1,"The quantum spin liquid (QSL) is an exotic phase of magnetic materials where
the spins continue to fluctuate without any symmetry breaking down to zero
temperature. Among the handful reports of QSL with spin $S\ge$1, examples with
magnetic ions on a three-dimensional magnetic lattice are extremely rare since
both larger spin and higher dimension tend to suppress quantum fluctuations. In
this work, we offer a new strategy to achieve 3-D QSL with high spin by
utilizing two types of transition metal ions, both are magnetically active but
located at crystallographically inequivalent positions. We design a 3-D
magnetic system Ba$_3$NiIr$_2$O$_9$ consisting of interconnected corner shared
NiO$_6$ octahedra and face shared Ir$_2$O$_9$ dimer, both having triangular
arrangements in \textit{a-b} plane. X-ray absorption spectroscopy measurements
confirm the presence of Ni$^{2+}$ ($S$=1). Our detailed thermodynamic and
magnetic measurements reveal that this compound is a realization of gapless QSL
state down to at least 100 mK. Ab-initio calculations find a strong magnetic
exchange between Ir and Ni sublattices and in-plane antiferromagnetic coupling
between the dimers, resulting in dynamically fluctuating magnetic moments on
both the Ir and Ni sublattice.",2101.08445v1
2021-02-02,Magnetically Modified Spherical Accretion in GRMHD: Reconnection-Driven Convection and Jet Propagation,"We present 3D general relativistic magnetohydrodynamic(GRMHD) simulations of
zero angular momentum accretion around a rapidly rotating black hole, modified
by the presence of initially uniform magnetic fields. We consider serveral
angles between the magnetic field direction and the black hole spin. In the
resulting flows, the midplane dynamics are governed by magnetic
reconnection-driven turbulence in a magnetically arrested (or a nearly
arrested) state. Electromagnetic jets with outflow efficiencies ~10-200% occupy
the polar regions, reaching several hundred gravitational radii before they
dissipate due to the kink instability. The jet directions fluctuate in time and
can be tilted by as much as ~30 degrees with respect to black hole spin, but
this tilt does not depend strongly on the tilt of the initial magnetic field. A
jet forms even when there is no initial net vertical magnetic flux since
turbulent, horizon-scale fluctuations can generate a net vertical field
locally. Peak jet power is obtained for an initial magnetic field tilted by
40-80 degrees with respect to the black hole spin because this maximizes the
amount of magnetic flux that can reach the black hole. These simulations may be
a reasonable model for low luminosity black hole accretion flows such as Sgr A*
or M87.",2102.01694v1
2021-05-13,In Search of Subsurface Oceans within the Uranian Moons,"The Galileo mission to Jupiter discovered magnetic signatures associated with
hidden sub-surface oceans at the moons Europa and Callisto using the phenomenon
of magnetic induction. These induced magnetic fields originate from
electrically conductive layers within the moons and are driven by Jupiter's
strong time-varying magnetic field. The ice giants and their moons are also
ideal laboratories for magnetic induction studies. Both Uranus and Neptune have
a strongly tilted magnetic axis with respect to their spin axis, creating a
dynamic and strongly variable magnetic field environment at the orbits of their
major moons. Although Voyager 2 visited the ice giants in the 1980s, it did not
pass close enough to any of the moons to detect magnetic induction signatures.
However, Voyager 2 revealed that some of these moons exhibit surface features
that hint at recent geologically activity, possibly associated with sub-surface
oceans. Future missions to the ice giants may therefore be capable of
discovering sub-surface oceans, thereby adding to the family of known ocean
worlds in our solar system. Here, we assess magnetic induction as a technique
for investigating sub-surface oceans within the major moons of Uranus.
Furthermore, we establish the ability to distinguish induction responses
created by different interior characteristics that tie into the induction
response: ocean thickness, conductivity, and depth, and ionospheric
conductance. The results reported here demonstrate the possibility of
single-pass ocean detection and constrained characterization within the moons
of Miranda, Ariel, and Umbriel, and provide guidance for magnetometer selection
and trajectory design for future missions to Uranus.",2105.06087v1
2021-05-29,Magnetic Field and Frequency Dependent AC Susceptibility of High-Tc YBCO Single Crystal,"The temperature dependence of AC susceptibility (ACS) has been measured for a
very high-quality plate-like slightly overdoped YBCO single crystal for
different frequencies and AC magnetic field amplitudes. Frequency dependence of
the ACS is weak irrespective of the magnetic field orientation but significant
effects of field orientation with respect to the CuO2 planes and field
magnitude on real and imaginary components of fundamental ACS were observed.
The height of the loss peak saturates as full penetration of magnetic field is
achieved. The peak temperature, Tp, in \c{hi}"" shifts to lower temperatures
with increasing magnetic field amplitude for both HIIc and HIIab. The value of
Tp depends on the orientation of the magnetic field with respect to the
crystallographic axes, illustrating the anisotropy in the magnetic flux
dynamics. The superconducting transition width increases weakly with increasing
magnetic field. The Cole-Cole plot [\c{hi}""(\c{hi}')] shows qualitatively and
quantitatively identical features for HIIc and HIIab, independent of the
orientation of the magnetic field with respect to the sample geometry and
shielding current paths. The general features of \c{hi}""(\c{hi}') implies that,
there is no flux creep for the range of frequencies and AC fields employed in
this investigation. The maximum value of the loss peak and its position with
respect to \c{hi}' in the Cole-Cole plot are largely consistent with the Bean
critical state model. Slightly increased peak value in comparison to the
predicted peak value within the Bean critical state model is probably due to a
weak field dependence of Jc. The results obtained here are compared with
various theoretical models and experimental findings. Prominent differences are
noted and discussed in details in this study.",2105.14321v1
2022-03-17,"Solving Critical Problems of the Muon Collider Higgs Factory: Optics, Magnets and their Protection, Detector Backgrounds","A low-energy medium-luminosity Muon Collider (MC) is being studied as a
possible Higgs Factory (HF). Electrons from muon decays will deposit more than
300 kW in superconducting magnets of the HF collider ring. This imposes
significant challenges to superconducting (SC) magnets used in the MC storage
ring (SR) and interaction regions (IR). Magnet designs are proposed which
provide high operating gradient and magnetic field in a large aperture to
accommodate the large size of muon beams (due to low beta*), as well as a
cooling system to intercept the large heat deposition from the showers induced
by decay electrons. The distribution of heat deposition in the MC SR lattice
elements requires large-aperture magnets in order to accommodate thick high-Z
absorbers to protect the SC coils. Based on the developed MARS15 model and
intensive simulations, a sophisticated radiation protection system was designed
for the collider SR and IR to bring the peak power density in the
superconducting coils below the quench limit and reduce the dynamic heat
deposition in the cold mass by a factor of 100. The system consists of tight
tungsten masks in the magnet interconnect regions and elliptical tungsten
liners in the magnet aperture optimized individually for each magnet. These
also reduce the background particle fluxes in the collider detector.",2203.09010v1
2022-03-24,"Bipolar Ephemeral Active Regions, Magnetic Flux Cancellation, and Solar Magnetic Explosions","We examine the cradle-to-grave magnetic evolution of 10 bipolar ephemeral
active regions (BEARs) in solar coronal holes, especially aspects of the
magnetic evolution leading to each of 43 obvious microflare events. The data
are from Solar Dynamics Observatory: 211 A coronal EUV images and line-of-sight
photospheric magnetograms. We find evidence that (1) each microflare event is a
magnetic explosion that results in a miniature flare arcade astride the
polarity inversion line (PIL) of the explosive lobe of the BEARs anemone
magnetic field; (2) relative to the BEAR's emerged flux-rope omega loop, the
anemone's explosive lobe can be an inside lobe, an outside lobe, or an inside &
outside lobe; (3) 5 events are confined explosions, 20 events are
mostly-confined explosions, and 18 events are blowout explosions, which are
miniatures of the magnetic explosions that make coronal mass ejections (CMEs);
(4) contrary to the expectation of Moore et al (2010), none of the 18 blowout
events explode from inside the BEARs omega loop during the omega loops
emergence; (5) before and during each of the 43 microflare events there is
magnetic flux cancellation at the PIL of the anemone's explosive lobe. From
finding evident flux cancellation at the underlying PIL before and during all
43 microflare events - together with BEARs evidently being miniatures of all
larger solar bipolar active regions - we expect that in essentially the same
way, flux cancellation in sunspot active regions prepares and triggers the
magnetic explosions for many major flares and CMEs.",2203.13287v1
2022-04-11,Magnetic order and spin liquid behavior in [Mo3+]^{11+} triangular magnets,"Molecular magnets based on [Mo$_3$]$^{11+}$ units with one unpaired electron
per trimer have attracted recent interest due to the identification of quantum
spin liquid candidacy in some family members. Here, we present comprehensive
measurements on polycrystalline samples of ZnScMo$_3$O$_8$, MgScMo$_3$O$_8$,
and Na$_3$Sc$_2$Mo$_5$O$_{16}$ with the same Mo$_3$O$_{13}$ magnetic building
blocks. The crystal structures are characterized with x-ray or neutron powder
diffraction and the magnetic ground states are determined by performing ac and
dc susceptibility, specific heat, neutron powder diffraction, and $\mu$SR
measurements. Our work indicates that ZnScMo$_3$O$_8$ and MgScMo$_3$O$_8$ have
ferromagnetic Curie-Weiss temperatures of 18.5 K and 11.9 K, ordered ground
states with net moments (low-moment ferromagnetism or canted
antiferromagnetism), and zero field ordering temperatures of $T_c =$ 6 K and
$<$ 2 K respectively. On the other hand, Na$_3$Sc$_2$Mo$_5$O$_{16}$ hosts a
dynamical magnetic ground state with no evidence for magnetic ordering or spin
freezing down to 20 mK despite an antiferromagnetic Curie-Weiss temperature of
-36.2 K, and therefore is a candidate for quantum spin liquid behavior. By
comparing the present results to past work on the same family of materials, we
construct a phase diagram which illustrates that the magnetic ground states of
these Mo-based molecular magnets are very sensitive to small changes in the
nearest neighbor Mo-Mo distance.",2204.05377v1
2022-04-19,Magnetic field effects in biology from the perspective of the radical pair mechanism,"A large and growing body of research shows that weak magnetic fields can
significantly influence various biological systems, including plants, animals,
and humans. However, the underlying mechanisms behind these phenomena remain
elusive. It is remarkable that the magnetic energies implicated in these
effects are much smaller than thermal energies. Here we review these
observations, of which there are now hundreds, and we suggest that a viable
explanation is provided by the radical pair mechanism, which involves the
quantum dynamics of the electron and nuclear spins of naturally occurring
transient radical molecules. While the radical pair mechanism has been studied
in detail in the context of avian magnetoreception, the studies reviewed here
show that magnetosensitivity is widespread throughout biology. We review
magnetic field effects on various physiological functions, organizing them
based on the type of the applied magnetic fields, namely static, hypomagnetic,
and oscillating magnetic fields, as well as isotope effects. We then review the
radical pair mechanism as a potential unifying model for the described magnetic
field effects, and we discuss plausible candidate molecules that might
constitute the radical pairs. We review recent studies proposing that the
quantum nature of the radical pairs provides promising explanations for xenon
anesthesia, lithium effects on hyperactivity, magnetic field and lithium
effects on the circadian clock, and hypomagnetic field effects on neurogenesis
and microtubule assembly. We conclude by discussing future lines of
investigation in this exciting new area of quantum biology related to weak
magnetic field effects.",2204.09147v1
2022-09-29,Nonlinearity and Parametric Amplification of Superconducting Nanowire Resonators in Magnetic Field,"Nonlinear superconducting devices, typically based on Josephson Junction (JJ)
nonlinearities, are the basis for superconducting quantum electronics,
enabling, e.g., the formation of isolated two-level superconducting qubits and
amplifiers. While emerging spin, hybrid spin/superconducting (including
Majorana), and nano-magneto-optical quantum systems could benefit tremendously
from superconducting nonlinearities, the presence of strong magnetic fields in
these systems are incompatible with conventional JJ devices, which are highly
sensitive to applied magnetic fields. One potential solution is the use of
kinetic inductance (KI) nonlinearity. To date, only linear kinetic inductance
(KI) devices have been shown to operate in high magnetic fields, while
nonlinear KI device operation in high magnetic fields has received virtually no
attention. Here, we study the nonlinearity of superconducting nanowire (NW) KI
resonators and their performance as parametric amplifiers for in-plane magnetic
fields. We study the Kerr coefficients of NW KI resonators made from 10 nm-thin
NbTiN films with characteristic impedance up to 3 k$\Omega$, and demonstrate
both nondegenerate and degenerate parametric amplification, at magnetic fields
up to 2 T, for the first time. We find that narrow KI resonators of width 0.1
$\mu$m are robust, in terms of gain, dynamic range and noise, to magnetic
fields up to $\sim$2 T, while wider KI resonators of width 1 $\mu$m suffer
significant suppression in the gain around at fields well below 2 T. Around 8
dB deamplification is observed for coherent states for a 0.1 $\mu$m KI
resonator, implying the capability of noise squeezing. These results open a new
pathway to developing nonlinear quantum devices that operate in or generate
high magnetic fields such as spin, hybrid spin/superconducting, and
magneto-opto-mechanical devices.",2209.14523v1
2022-10-19,Three-dimensional numerical simulations of ambipolar diffusion in NS cores in the one-fluid approximation: instability of poloidal magnetic field,"We numerically model evolution of magnetic fields inside a neutron star under
the influence of ambipolar diffusion in the weak-coupling mode in the one-fluid
MHD approximation. Our simulations are three-dimensional and performed in
spherical coordinates. Our model covers the neutron star core and includes
crust where the magnetic field decay is due to Ohmic decay. We discover an
instability of poloidal magnetic field under the influence of ambipolar
diffusion. This instability develops in the neutron star core and grows on a
timescale of 0.2 dimensionless times, reaching saturation by 2 dimensionless
times. The instability leads to formation of azimuthal magnetic field with
azimuthal wavenumber $m=14$ (at the moment of saturation) which keeps merging
and reaches $m=4$ by 16 dimensionless times. Over the course of our simulations
(16 dimensionless times) the surface dipolar magnetic field decays, reaching 20
percent of its original value and keeps decaying. The decay timescale for the
total magnetic energy is six dimensionless times. The ambipolar diffusion
induces electric currents in the crust where these currents dissipate
efficiently. Strong electric currents in the crust lead to heating, which could
correspond to luminosities of $\approx 10^{29}$ erg s$^{-1}$ during hundreds of
Myrs for an initial magnetic field of $10^{14}$ G. Ambipolar diffusion leads to
formation of small-scale magnetic fields at the neutron star surface.",2210.10869v1
2022-11-04,Small-scale dynamo in cool main sequence stars. II. The effect of metallicity,"All cool main sequence stars including our Sun are thought to have magnetic
fields. Observations of the Sun revealed that even in quiet regions small-scale
turbulent magnetic fields are present. Simulations further showed that such
magnetic fields affect the subsurface and photospheric structure, and thus the
radiative transfer and emergent flux. Since small-scale turbulent magnetic
fields on other stars cannot be directly observed, it is imperative to study
their effects on the near surface layers numerically. Until recently
comprehensive three-dimensional simulations capturing the effect of small-scale
turbulent magnetic fields only exists for the solar case. A series of
investigations extending SSD simulations for other stars has been started. Here
we aim to examine small-scale turbulent magnetic fields in stars of solar
effective temperature but different metallicity. We investigate the properties
of three-dimensional simulations of the magneto-convection in boxes covering
the upper convection zone and photosphere carried out with the MURaM code for
metallicity values of $ \rm M/H = \{-1.0, 0.0, 0.5\}$ with and without a
small-scale-dynamo. We find that small-scale turbulent magnetic fields enhanced
by a small-scale turbulent dynamo noticeably affect the subsurface dynamics and
significantly change the flow velocities in the photosphere. Moreover,
significantly stronger magnetic field strengths are present in the convection
zone for low metallicity. Whereas, at the optical surface the averaged vertical
magnetic field ranges from 64G for M/H = 0.5 to 85G for M/H = -1.0.",2211.02722v1
2022-11-15,Dynamical criticality of spin-shear coupling in van der Waals antiferromagnets,"The interplay between a multitude of electronic, spin, and lattice degrees of
freedom underlies the complex phase diagrams of quantum materials. Layer
stacking in van der Waals (vdW) heterostructures is responsible for exotic
electronic and magnetic properties, which inspires stacking control of
two-dimensional magnetism. Beyond the interplay between stacking order and
interlayer magnetism, we discover a spin-shear coupling mechanism in which a
subtle shear of the atomic layers can have a profound effect on the intralayer
magnetic order in a family of vdW antiferromagnets. Using time-resolved x-ray
diffraction and optical linear dichroism measurements, interlayer shear is
identified as the primary structural degree of freedom that couples with
magnetic order. The recovery times of both shear and magnetic order upon
optical excitation diverge at the magnetic ordering temperature with the same
critical exponent. The time-dependent Ginzburg-Landau theory shows that this
concurrent critical slowing down arises from a linear coupling of the
interlayer shear to the magnetic order, which is dictated by the broken mirror
symmetry intrinsic to the monoclinic stacking. Our results highlight the
importance of interlayer shear in ultrafast control of magnetic order via
spin-mechanical coupling.",2211.08189v1
2022-11-17,Shear viscosity coefficient of magnetized QCD medium near chiral phase transition,"We study the properties of the shear viscosity coefficient of quark matter at
finite temperature and chemical potential near chiral phase transition in a
strong background magnetic field. A strong magnetic field induces anisotropic
features, phase-space Landau-level quantization, and if the magnetic field is
sufficiently strong, interferes with prominent QCD phenomena such as dynamical
quark mass generation, likely affecting the quark matter transport
characteristics. The modified Nambu-Jona-Lasinio (NJL) model with inverse
magnetic catalysis effect by fitting the Lattice QCD (LQCD) results is used to
calculate the changes of quasiparticle related thermodynamic quantities, and
the shear viscosity of the system medium, which is analyzed under the
relaxation time approximation. We quantify the influence of the order of chiral
phase transition and the critical endpoint on dissipative phenomena in such a
magnetized medium. When the magnetic field exists, the shear viscosity
coefficient of the dissipative fluid system can be decomposed into five
different components. In strong field limit, we make a detailed study of the
dependencies of $\eta_{2}$ and $\eta_{4}$ on temperature and magnetic field for
the first order phase transition and critical endpoint transition.
respectively. It is found that $\eta_{2}$ and $\eta_{4}$ both decrease with
magnetic field and increase with temperature, and the discontinuities of
$\eta_{2}$ and $\eta_{4}$ occur at the first order phase transition point.",2211.09281v2
2022-12-23,Deformation transients of confined droplets within interacting electric and magnetic field environment,"A theoretical exploration and an analytical model for the
electro-magneto-hydrodynamics (EMHD) of leaky dielectric liquid droplets,
suspended in an immiscible confined fluid domain has been presented. The
analytical solution for the system, under small deformation approximation, in
creeping flow regime, has been put forward. Study of the droplet deformation
suggests that its temporal evolution is exponential, and dependents on the
electric and magnetic field interaction. Further, the direction of the applied
magnetic field with respect to the electric field decides whether the
contribution of magnetic forces opposes or aids the interfacial net electrical
force due to the electric field. Validation of the proposed model at the
asymptotic limits of vanishing magnetic field show that the model accurately
reduces to the case of transient electrohydrodynamic model. We also propose a
magnetic discriminating function to quantify the steady-state droplet
deformation in the presence of interacting electric and magnetic fields. The
change of droplets from spherical shape to prolate, and oblate spheroids,
correspond to magnetic discriminating function >0 and <0 regimes, respectively.
It is shown that with the aid of low magnitude magnetic field, a substantial
augmentation in the deformation parameter, and the associated EMHD circulation
within and around the droplet is achieved. The analysis also reveals the
deformation lag and specific critical parameters that aid or suppressed this
lag behaviour; discussed in terms of relevant non-dimensional parameters.",2212.12135v2
2023-01-13,Exploring magnetic loops and serpentine fields in the quiet Sun with the GRIS-IFU,"Synthetic observations produced from radiative magnetohydronamic simulations
have predicted that higher polarization fractions in the quiet solar
photosphere would be revealed by increasing the total integration time of
observations at GREGOR resolutions. We present recently acquired disk centre
observations of the Fe I $15648.5$ $\mathrm{\AA}$ line obtained with the GREGOR
telescope equipped with the GRIS-IFU during excellent seeing conditions,
showing exceptionally high polarization fractions. Our observation reveal an
internetwork region with a majority ($>60\%$) of magnetised pixels displaying a
clear transverse component of the magnetic field. This result is in stark
contrast to previous disk-centre GRIS-IFU observations in this spectral line,
which had predominantly vertical magnetic fields in the deep photosphere. At
the same time, the median magnetic field strength is weaker than previous
GRIS-IFU observations, indicating that the larger fraction of polarization
signals cannot be explained by a more active target. We use the Stokes
Inversion based on Response functions (SIR) code to analyse the data,
performing over $45$ million inversions, and interrogate the impact of two
conflicting approaches to the treatment of noise on the retrieval of the
magnetic inclination and azimuth. We present several case studies of the zoo of
magnetic features present in these data, including small-scale magnetic loops
that seem to be embedded in a sea of magnetism, and serpentine fields, focusing
on regions where full-vector spectropolarimetry has been achieved. We also
present a new open-source Python 3 analysis tool, SIR Explorer (SIRE), that we
use to examine the dynamics of these small-scale magnetic features.",2301.05591v1
2023-02-01,Temperature-independent ferromagnetic resonance shift in Bi-doped YIG garnets through magnetic anisotropy tuning,"Thin garnet films are becoming central for magnon-spintronics and
spin-orbitronics devices as they show versatile magnetic properties together
with low magnetic losses. These fields would benefit from materials in which
heat does not affect the magnetization dynamics, an effect known as the
non-linear thermal frequency shift. In this study, low damping Bi substituted
Iron garnet (Bi:YIG) ultra-thin films have been grown using Pulsed Laser
Deposition. Through a fine tuning of the growth parameters, the precise control
of the perpendicular magnetic anisotropy allows to achieve a full compensation
of the dipolar magnetic anisotropy. Strikingly, once the growth conditions are
optimized, varying the growth temperature from 405 {\deg}C to 475 {\deg}C as
the only tuning parameter induces the easy-axis to go from out-of-plane to
in-plane. For films that are close to the dipolar compensation, Ferromagnetic
Resonance measurements yield an effective magnetization $\mu _{0}M_{eff} (T)$
that has almost no temperature dependence over a large temperature range (260 K
to 400 K) resulting in an anisotropy temperature exponent of 2. These findings
put Bi:YIG system among the very few materials in which the temperature
dependence of the magnetic anisotropy varies at the same rate than the
saturation magnetization. This interesting behavior is ascribed
phenomenologically to the sizable orbital moment of $Bi^{3+}$.",2302.00585v1
2023-02-23,Revealing magnetic field structure at the surfaces of protoplanetary disks via near-infrared circular polarization,"Context. Magnetic fields play a fundamental role in the dynamical evolution
of protoplanetary disks, in particular via magnetically induced disk winds. The
magnetic field structure at the disk surface is crucial for driving the disk
winds; however, it is still poorly understood observationally. Aims. We explore
a new method to probe the magnetic field structure at the disk surface using
near-infrared (NIR) circular polarization. Near-infrared circular polarization
arises when unpolarized stellar light is scattered by magnetically aligned
grains at the disk surface. In this study, we aim to clarify to what extent the
observed circular polarization pattern can be used to diagnose the magnetic
field structure. Methods. We first calculated light scattering properties of
aligned spheroids, and the results were then used to create expected
observational images of the degree of circular polarization at a NIR
wavelength. Results. Magnetically aligned grains can produce circular
polarization, particularly when the field configuration deviates from a purely
toroidal field. We find that disk azimuthal dependence of the degree of
circular polarization tends to exhibit a double peaked profile when the field
structure is favorable for driving disk winds by centrifugal force. We also
find that even if the disk is spatially unresolved, a net circular polarization
can possibly be nonzero. We also show that the amplitude of circular
polarization is strongly dependent on grain composition and axis ratio.
Conclusions. Our results suggest that circular polarization observations would
be useful to study the magnetic field structure and dust properties at the disk
surface.",2302.12340v1
2023-07-17,"Unravelling the structure of magnetised molecular clouds with SILCC-Zoom: sheets, filaments and fragmentation","To what extent magnetic fields affect how molecular clouds (MCs) fragment and
create dense structures is an open question. We present a numerical study of
cloud fragmentation using the SILCC-Zoom simulations. These simulations follow
the self-consistent formation of MCs in a few hundred parsec sized region of a
stratified galactic disc; and include magnetic fields, self-gravity,
supernova-driven turbulence, as well as a non-equilibrium chemical network. To
discern the role of magnetic fields in the evolution of MCs, we study seven
simulated clouds, five with magnetic fields, and two without, with a maximum
resolution of 0.1 parsec. Using a dendrogram we identify hierarchical
structures which form within the clouds. Overall, the magnetised clouds have
more mass in a diffuse envelope with a number density between 1-100 cm$^{-3}$.
We find that six out of seven clouds are sheet-like on the largest scales, as
also found in recent observations, and with filamentary structures embedded
within, consistent with the bubble-driven MC formation mechanism. Hydrodynamic
simulations tend to produce more sheet-like structures also on smaller scales,
while the presence of magnetic fields promotes filament formation. Analysing
cloud energetics, we find that magnetic fields are dynamically important for
less dense, mostly but not exclusively atomic structures (typically up to $\sim
100 - 1000$~cm$^{-3}$), while the denser, potentially star-forming structures
are energetically dominated by self-gravity and turbulence. In addition, we
compute the magnetic surface term and demonstrate that it is generally
confining, and some atomic structures are even magnetically held together. In
general, magnetic fields delay the cloud evolution and fragmentation by $\sim$
1 Myr.",2307.08746v1
2023-07-23,How Magnetic Erosion Affects the Drag-Based Kinematics of Fast Coronal Mass Ejections,"In order to advance our understanding of the dynamic interactions between
coronal mass ejections (CMEs) and the magnetized solar wind, we investigate the
impact of magnetic erosion on the well-known aerodynamic drag force acting on
CMEs traveling faster than the ambient solar wind. In particular, we start by
generating empirical relationships for the basic physical parameters of CMEs
that conserve their mass and magnetic flux. Furthermore, we examine the impact
of the virtual mass on the equation of motion by studying a variable-mass
system. We next implement magnetic reconnection into CME propagation, which
erodes part of the CME magnetic flux and outer-shell mass, on the drag acting
on CMEs, and we determine its impact on their time and speed of arrival at 1
AU. Depending on the strength of the magnetic erosion, the leading edge of the
magnetic structure can reach near-Earth space up to $\approx$ three hours
later, compared to the non-eroded case. Therefore, magnetic erosion may have a
significant impact on the propagation of fast CMEs and on predictions of their
arrivals at 1 AU. Finally, the modeling indicates that eroded CMEs may
experience a significant mass decrease. Since such a decrease is not observed
in the corona, the initiation distance of erosion may lie beyond the
field-of-view of coronagraphs (i.e. 30 $\mathrm{R_{\odot}}$).",2307.12370v1
2023-07-25,Shear viscosity coefficient of magnetized QCD medium with anomalous magnetic moments near chiral phase transition,"We study the properties of the shear viscosity coefficient of quark matter
near the chiral phase transition at finite temperature and chemical potential,
and the kinds of high temperature, high density and strong magnetic field
background. The strong magnetic field induces anisotropy, that is, the
quantization of Landau energy levels in phase space. If the magnetic field is
strong enough, it will interfere with significant QCD phenomena, such as the
generation of dynamic quark mass, which may affect the transport properties of
quark matter. The inclusion of the anomalous magnetic moments of the quarks at
finite density into the Nambu-Jona-Lasinio model gives rise to additional spin
polarization magnetic effects. It is found that both the ratio $\eta/s$ of
shear viscosity coefficient to entropy and the collision relaxation time $\tau$
show similar trend with temperature, both of which reach minima around the
critical temperature. The shear viscosity coefficient of the dissipative fluid
system can be decomposed into five different components as the strong magnetic
field exists. The influences of the order of chiral phase transition and the
critical end point on dissipative phenomena in such a magnetized medium are
quantitatively investigated. It is found that ${\eta}_{1}$, ${\eta}_{2}$,
${\eta}_{3}$, and ${\eta}_{4}$ all increase with temperature. For first-order
phase transitions, ${\eta}_{1}$, ${\eta}_{2}$, ${\eta}_{3}$, and ${\eta}_{4}$
exhibit discontinuous characteristics.",2307.13193v2
2023-11-21,Magnetic helicity in rotating neutron stars,"We study the magnetic helicity evolution in neutron stars (NSs). First, we
analyze how the surface terms affect the conservation law for the sum of the
chiral imbalance of the charged particle densities and the density of the
magnetic helicity. Our results are applied to a system with a finite volume,
which can be a magnetized NS. We show that the contribution of these surface
terms can potentially lead to the reconnection of magnetic field lines followed
by X-ray or gamma bursts observed in magnetars. Comparing the new quantum
surface term with the classical contribution known in the standard magnetic
hydrodynamics, we obtain that its contribution dominates over the classical
term only for NS with a rigid rotation. Second, we study the dynamics of chiral
electrons which interact electroweakly with a background fermions having the
velocity, arbitrarily depending on spatial coordinates, and the nonuniform
density. We derive the the kinetic equations and the effective action for right
and left particles. The correction to the Adler anomaly and to the anomalous
electric current are obtained in the case of a rotating matter. Then, the
obtained results are applied for the study of the magnetic helicity flow inside
a magnetized rotating NS. We compute the characteristic time of the helicity
change and demonstrate that it coincides with the the magnetic cycle period of
certain pulsars.",2311.12557v1
2024-02-02,Controllable frequency tunability and parabolic-like threshold current behavior in spin Hall nano-oscillators,"We investigate the individual impacts of critical magnetodynamical
parameters-effective magnetization and magnetic damping-on the auto-oscillation
characteristics of nano-constriction-based Spin Hall Nano-Oscillators (SHNOs).
Our micromagnetic simulations unveil a distinctive non-monotonic relationship
between current and auto-oscillation frequency in out-of-plane magnetic fields.
The influence of effective magnetization on frequency tunability varies with
out-of-plane field strengths. At large out-of-plane fields, the frequency
tunability is predominantly governed by effective magnetization, achieving a
current tunability of 1 GHz/mA-four times larger than that observed at the
lowest effective magnetization. Conversely, at low out-of-plane fields,
although a remarkably high-frequency tunability of 4 GHz/mA is observed, the
effective magnetization alters the onset of the transition from a linear-like
mode to a spin-wave bullet mode. Magnetic damping primarily affects the
threshold current with negligible impact on auto-oscillation frequency
tunability. The threshold current scales linearly with increased magnetic
damping at a constant out-of-plane field but exhibits a parabolic behavior with
variations in out-of-plane fields. This behavior is attributed to the
qualitatively distinct evolution of the auto-oscillation mode across different
out-of-plane field values. Our study not only extends the versatility of SHNOs
for oscillator-based neuromorphic computing with controllable frequency
tunability but also unveils the intricate auto-oscillation dynamics in
out-of-plane fields.",2402.01570v1
2024-02-17,Characterizing the dynamical magnetosphere of the extremely slowly rotating magnetic O9.7 V star HD 54879 using rotational modulation of the H $α$ profile,"The magnetic field in the O9.7 V star HD54879 has been monitored for almost a
decade. Spectropolarimetric observations reveal a rather strong mean
longitudinal magnetic field that varies with a period of about 7.41 yr.
Observations in the H$\alpha$ line show a variation with the same period, while
the H$\beta$ line shows only little variation. Assuming the periodic variation
to be caused by a slow rotation and a dipolar magnetic field, we find a
magnetic field strength of $\ge$2 kG at the magnetic poles. With the relatively
low mass loss rate of $10^{-9} M_\odot$ yr$^{-1}$, this star is a case of
extremely strong magnetic confinement. Both theoretical arguments and numerical
simulations indicate the presence of an extended disk of increased gas density
in the equatorial plane of the magnetic field, where gas from the line-driven
stellar wind is trapped. This disk is likely to be the origin of the observed
H$\alpha$ emission, which peaks together with the strongest line-of-sight
magnetic field. The profile of the H$\alpha$ line is resolved in several
components and shows a remarkable variability with the rotation period.",2402.11244v2
2024-02-22,Unexpected versatile electrical transport behaviors of ferromagnetic nickel films,"Perpendicular magnetic anisotropy (PMA) of magnets is paramount for
electrically controlled spintronics due to their intrinsic potentials for
higher memory density, scalability, thermal stability and endurance, surpassing
an in-plane magnetic anisotropy (IMA). Nickel film is a long-lived fundamental
element ferromagnet, yet its electrical transport behavior associated with
magnetism has not been comprehensively studied, hindering corresponding
spintronic applications exploiting nickel-based compounds. Here, we
systematically investigate the highly versatile magnetism and corresponding
transport behavior of nickel films. As the thickness reduces within the general
thickness regime of a magnet layer for a memory device, the hardness of nickel
films' ferromagnetic loop of anomalous Hall effect increases and then
decreases, reflecting the magnetic transitions from IMA to PMA and back to IMA.
Additionally, the square ferromagnetic loop changes from a hard to a soft one
at rising temperatures, indicating a shift from PMA to IMA. Furthermore, we
observe a butterfly magnetoresistance resulting from the anisotropic
magnetoresistance effect, which evolves in conjunction with the thickness and
temperature-dependent magnetic transformations as a complementary support. Our
findings unveil the rich magnetic dynamics and most importantly settle down the
most useful guiding information for current-driven spintronic applications
based on nickel film: The hysteresis loop is squarest for the ~8 nm-thick
nickel film, of highest hardness with Rxyr/Rxys~1 and minimum Hs-Hc, up to 125
K; otherwise, extra care should be taken for a different thickness or at a
higher temperature.",2402.14275v1
2024-03-24,Unveiling the underlying structure of axial-vector bottom-charm tetraquarks in the light of their magnetic moments,"The magnetic moment yields an excellent framework to explore the inner
structure of particles determined by the quark-gluon dynamics of QCD, as it is
the leading-order response of a bound system to a weak external magnetic field.
Motivated by this, in this study, the magnetic moments of possible axial-vector
$T_{bc\bar u \bar u}$, $T_{bc\bar d \bar d}$, and $T_{bc\bar u \bar d}$
tetraquarks are obtained with the help of light-cone QCD sum rules. For this
purpose, we assume that these states are represented as a diquark-antidiquark
picture with different structures and interpolating currents. The magnetic
moment results derived using different diquark-antidiquark configurations
differ substantially from each other. This can be translated into more than one
tetraquark state with the same quantum number and quark content yet possessing
different magnetic moments. From the numerical results obtained, we have
concluded that the magnetic moments of the $T_{bc}$ states can project their
inner structure, which can be used for their quantum numbers and quark-gluon
organization. The contribution of individual quarks to the magnetic moments is
also analyzed for completeness. We hope that our predictions of the magnetic
moments of the $T_{bc}$ tetraquarks, together with the results of other
theoretical investigations of the spectroscopic parameters and decay widths of
these interesting tetraquarks, may be valuable in the search for these states
in future experiments and in unraveling the internal structure of these
tetraquarks.",2403.16191v1
2024-03-27,Complexity of emerging magnetic flux during lifetime of solar ephemeral regions,"As a relatively active region, ephemeral region (ER) exhibits highly complex
pattern of magnetic flux emergence. We aim to study detailed secondary flux
emergences (SFEs) which we define as bipoles that they appear close to ERs and
finally coalesce with ERs after a period. We study the SFEs during the whole
process from emergence to decay of 5 ERs observed by the Helioseismic and
Magnetic Imager (HMI) aboard Solar Dynamics Observatory (SDO) . The maximum
unsigned magnetic flux for each ER is around $10^{20}$ Mx. Each ER has tens of
SFEs with an average emerging magnetic flux of approximately 5$\times10^{18}$
Mx. The frequency of normalized magnetic flux for all the SFEs follows a power
law distribution with an index of -2.08 . The majority of SFEs occur between
the positive and negative polarities of ER , and their growth time is
concentrated within one hour. The magnetic axis of SFE is found to exhibit a
random distribution in the 5 ERs. We suggest that the relationship between SFEs
and ERs can be understood by regarding the photospheric magnetic field
observations as cross-sections of an emerging magnetic structure. Tracking the
ERs' evolution, we propose that these SFEs in ERs may be sequent emergences
from the bundle of flux tube of ERs, and that SFEs are partially emerged
$\Omega$-loops.",2403.18979v1
2013-03-25,"Polaron-like vortices, dissociation transition and self induced pinning in magnetic superconductors","In magnetic superconductors vortices polarize spins nonuniformly and
repolarize them when moving. At a low spin relaxation rate and at low bias
currents vortices carrying magnetic polarization clouds become polaron-like and
their velocities are determined by the effective drag coefficient which is
significantly bigger than the Bardeen-Stephen (BS) one. As current increases,
vortices release polarization clouds and the velocity as well as the voltage in
the I-V characteristics jump to values corresponding to the BS drag coefficient
at a critical current $J_c$. The nonuniform components of the magnetic field
and magnetization drop as velocity increases resulting in weaker polarization
and {\it discontinuous} dynamic dissociation depinning transition.
Experimentally the jump shows up as a depinning transition and the
corresponding current at the jump is the depinning current. As current
decreases, on the way back, vortices are retrapped by polarization clouds at
the current $J_r<J_c$. As a result, polaronic effect suppresses dissipation and
enhances critical current. Borocarbides (RE)Ni$_2$B$_2$C with a short
penetration length and highly polarizable rare earth spins seem to be optimal
systems for a detailed study of vortex polaron formation by measuring I-V
characteristics. We propose also to use superconductor-magnet multilayer
structure to study polaronic mechanism of pinning with the goal to achieve high
critical currents. The magnetic layers should have large magnetic
susceptibility to enhance the coupling between vortices and magnetization in
magnetic layers while the relaxation of the magnetization should be slow. For
Nb and proper magnet multilayer structure, we estimate the critical current
density $J_c\sim 10^{9}\ \rm{A/m^2}$ at magnetic field $B\approx 1$ T.",1303.6184v1
2013-04-19,Magnetic field strength distribution of magnetic bright points inferred from filtergrams and spectro-polarimetric data,"Small scale magnetic fields can be observed on the Sun in G-band filtergrams
as MBPs (magnetic bright points) or identified in spectro-polarimetric
measurements due to enhanced signals of Stokes profiles. These magnetic fields
and their dynamics play a crucial role in understanding the coronal heating
problem and also in surface dynamo models. MBPs can theoretically be described
to evolve out of a patch of a solar photospheric magnetic field with values
below the equipartition field strength by the so-called convective collapse
model. After the collapse, the magnetic field of MBPs reaches a higher stable
magnetic field level. The magnetic field strength distribution of small scale
magnetic fields as seen by MBPs is inferred. Furthermore, we want to test the
model of convective collapse and the theoretically predicted stable value of
about 1300 G. We used four different data sets of high-resolution Hinode/SOT
observations that were recorded simultaneously with the broadband filter device
(G-band, Ca II-H) and the spectro-polarimeter. To derive the magnetic field
strength distribution of these small scale features, the spectropolarimeter
(SP) data sets were treated by the Merlin inversion code. The four data sets
comprise different solar surface types: active regions (a sunspot group and a
region with pores), as well as quiet Sun. In all four cases the obtained
magnetic field strength distribution of MBPs is similar and shows peaks around
1300 G. This agrees well with the theoretical prediction of the convective
collapse model. The resulting magnetic field strength distribution can be
fitted in each case by a model consisting of log-normal components. The
important parameters, such as geometrical mean value and multiplicative
standard deviation, are similar in all data sets, only the relative weighting
of the components is different.",1304.5508v1
2015-01-02,Measuring magnetism in the Milky Way with the Square Kilometre Array,"Magnetic fields in the Milky Way are present on a wide variety of sizes and
strengths, influencing many processes in the Galactic ecosystem such as star
formation, gas dynamics, jets, and evolution of supernova remnants or pulsar
wind nebulae. Observation methods are complex and indirect; the most used of
these are a grid of rotation measures of unresolved polarized extragalactic
sources, and broadband polarimetry of diffuse emission. Current studies of
magnetic fields in the Milky Way reveal a global spiral magnetic field with a
significant turbulent component; the limited sample of magnetic field
measurements in discrete objects such as supernova remnants and HII regions
shows a wide variety in field configurations; a few detections of magnetic
fields in Young Stellar Object jets have been published; and the magnetic field
structure in the Galactic Center is still under debate.
  The SKA will unravel the 3D structure and configurations of magnetic fields
in the Milky Way on sub-parsec to galaxy scales, including field structure in
the Galactic Center. The global configuration of the Milky Way disk magnetic
field, probed through pulsar RMs, will resolve controversy about reversals in
the Galactic plane. Characteristics of interstellar turbulence can be
determined from the grid of background RMs. We expect to learn to understand
magnetic field structures in protostellar jets, supernova remnants, and other
discrete sources, due to the vast increase in sample sizes possible with the
SKA. This knowledge of magnetic fields in the Milky Way will not only be
crucial in understanding of the evolution and interaction of Galactic
structures, but will also help to define and remove Galactic foregrounds for a
multitude of extragalactic and cosmological studies.",1501.00416v1
2015-03-02,Quantum field theory in a magnetic field: From quantum chromodynamics to graphene and Dirac semimetals,"A range of quantum field theoretical phenomena driven by external magnetic
fields and their applications in relativistic systems and quasirelativistic
condensed matter ones, such as graphene and Dirac/Weyl semimetals, are
reviewed. We start by introducing the underlying physics of the magnetic
catalysis. The dimensional reduction of the low-energy dynamics of relativistic
fermions in an external magnetic field is explained and its role in catalyzing
spontaneous symmetry breaking is emphasized. The general theoretical
consideration is supplemented by the analysis of the magnetic catalysis in
quantum electrodynamics, chromodynamics and quasirelativistic models relevant
for condensed matter physics. By generalizing the ideas of the magnetic
catalysis to the case of nonzero density and temperature, we argue that other
interesting phenomena take place. The chiral magnetic and chiral separation
effects are perhaps the most interesting among them. In addition to the general
discussion of the physics underlying chiral magnetic and separation effects, we
also review their possible phenomenological implications in heavy-ion
collisions and compact stars. We also discuss the application of the magnetic
catalysis ideas for the description of the quantum Hall effect in monolayer and
bilayer graphene, and conclude that the generalized magnetic catalysis,
including both the magnetic catalysis condensates and the quantum Hall
ferromagnetic ones, lies at the basis of this phenomenon. We also consider how
an external magnetic field affects the underlying physics in a class of
three-dimensional quasirelativistic condensed matter systems, Dirac semimetals.
While at sufficiently low temperatures and zero density of charge carriers,
such semimetals are expected to reveal the regime of the magnetic catalysis,
the regime of Weyl semimetals with chiral asymmetry is realized at nonzero
density...",1503.00732v2
2018-06-28,Nature of the energy source powering solar coronal loops driven by nanoflares,"Magnetic energy is required to heat the corona, the outer atmosphere of the
Sun, to millions of degrees. We study the nature of the magnetic energy source
that is probably responsible for the brightening of coronal loops driven by
nanoflares in the cores of solar active regions. We consider observations of
two active regions (ARs), 11890 and 12234, in which nanoflares have been
detected. To this end, we use ultraviolet (UV) and extreme ultraviolet (EUV)
images from the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics
Observatory (SDO) for coronal loop diagnostics. These images are combined with
the co-temporal line-of-sight magnetic field maps from the Helioseismic and
Magnetic Imager (HMI) onboard SDO to investigate the connection between coronal
loops and their magnetic roots in the photosphere. The core of these ARs
exhibit loop brightening in multiple EUV channels of AIA, particularly in its
9.4 nm filter. The HMI magnetic field maps reveal the presence of a complex
mixed polarity magnetic field distribution at the base of these loops. We
detect the cancellation of photospheric magnetic flux at these locations at a
rate of about $10^{15}$ Mx s$^{-1}$. The associated compact coronal
brightenings directly above the cancelling magnetic features are indicative of
plasma heating due to chromospheric magnetic reconnection. We suggest that the
complex magnetic topology and the evolution of magnetic field, such as flux
cancellation in the photosphere and the resulting chromospheric reconnection,
can play an important role in energizing active region coronal loops driven by
nanoflares. Our estimate of magnetic energy release during flux cancellation in
the quiet Sun suggests that chromospheric reconnection can also power the quiet
corona.",1806.11045v2
2020-03-19,Realizing an Isotropically Coercive Magnetic Layer for Memristive Applications by Analogy to Dry Friction,"We investigate the possibility of realizing a spintronic memristive device
based on the dependence of the tunnel conductance on the relative angle between
the magnetization of the two magnetic electrodes in in-plane magnetized tunnel
junctions. For this, it is necessary to design a free layer whose magnetization
can be stabilized along several or even any in-plane direction between the
parallel and the antiparallel magnetic configurations. We experimentally show
that this can be achieved by exploiting antiferromagnet-ferromagnet exchange
interactions in a regime where the antiferromagnet is thin enough to induce
enhanced coercivity and no exchange bias. The frustration of exchange
interactions at the interfaces due to competing ferro- and antiferromagnetic
interactions is at the origin of an isotropic dissipation mechanism yielding
isotropic coercivity. From a modeling point of view, it is shown that this
isotropic dissipation can be described by a dry friction term in the
Landau-Lifshitz-Gilbert equation. The influence of this dry friction term on
the magnetization dynamics of an in-plane magnetized layer submitted to a
rotating in-plane field is investigated both analytically and numerically. The
possibility to control the free layer magnetization orientation in an in-plane
magnetized magnetic tunnel junction by using the spin transfer torque from an
additional perpendicular polarizer is also investigated through macrospin
simulation. It is shown that the memristor function can be achieved by the
injection of current pulses through the stack in the presence of an in-plane
static field transverse to the reference layer magnetization, the aim of which
is to limit the magnetization rotation between 0{\deg} and 180{\deg}.",2003.08850v1
2021-06-15,Modeling the Magnetization Dynamics for Large Ensembles of Immobilized Magnetic Nanoparticles in Multi-dimensional Magnetic Particle Imaging,"Magnetic nanoparticles (MNPs) play an important role in biomedical
applications including imaging modalities such as MRI and magnetic particle
imaging (MPI). The latter one exploits the non-linear magnetization response of
a large ensemble of magnetic nanoparticles to magnetic fields which allows
determining the spatial distribution of the MNP concentration from measured
voltage signals. Currently, modeling the voltage signals of large ensembles of
MNPs in an MPI environment is not yet accurately possible, especially for
liquid tracers in multi-dimensional magnetic excitation fields. Thus, the
voltage-to-image mapping is still obtained in a time consuming calibration
procedure. While the ferrofluidic case can be seen as the typical setting, more
recently immobilized and potentially oriented MNPs have received considerable
attention. By aligning the particles during immobilization, one can encode the
angle of the easy axis into the magnetization response providing a
sophisticated benchmark system for model-based approaches. In this work, we
address the modeling problem for immobilized, oriented MNPs in the context of
MPI. We investigate a model-based approach where the magnetization response is
simulated by a N\'eel rotation model for the particle's magnetic moments and
the ensemble magnetization is obtained by solving a Fokker-Planck equation
approach. Since the parameters of the model are a-priori unknown, we
investigate different methods for performing a parameter identification and
discuss two models: One where a single function vector is used from the space
spanned by the model parameters and another where a superposition of function
vectors is considered. We show that our model can much more accurately
reproduce the orientation dependent signal response when compared to the
equilibrium model, which marks the current state-of-the-art for model-based
system matrix simulations in MPI.",2106.08040v1
2014-07-19,Signatures of spin-glass behavior in the induced magnetic moment system PrRuSi3,"We have investigated the magnetic and transport properties of a ternary
intermetallic compound PrRuSi3 using dc magnetization, ac susceptibility,
specific heat, electrical resistivity, neutron diffraction, inelastic neutron
scattering and muSR measurements. The magnetic susceptibility and specific heat
data reveal the signatures of spin-glass behavior in PrRuSi3 with a freezing
temperature of 9.8 K. At low magnetic fields, we observe two sharp anomalies
(at 4.9 and 8.6 K) in magnetic susceptibility data. In contrast, the specific
heat data show only a broad Schottky-type anomaly centered around 10 K.
However, muSR reveals very low frequency coherent oscillations at 1.8 K with an
onset of fast relaxation below 12 K indicating a long range magnetically
ordered ground state with very small moment. On the other hand, no magnetic
Bragg peaks are observed in low temperature neutron diffraction data at 1.8 K.
These two contradictory ground states, spin-glass versus magnetic order, can be
explained if the spin-glass behavior in PrRuSi3 is considered due to the
dynamic fluctuations of the crystal field levels as has been proposed for
spin-glass behavior in PrAu2Si2. Two sharp inelastic excitations near 2.4 meV
and 14.7 meV are observed in the inelastic neutron scattering (INS) spectra
between 4 K and 50 K. Further, exchange coupling J_ex obtained from the
analysis of INS data with CEF model provides evidence for the spontaneously
induced magnetic order with a CEF-split singlet (Gamma_t4) ground state.
However, the exchange coupling seems to be close to the critical value for the
induced moment magnetism, therefore we tend to believe that the dynamic
fluctuations between the ground state singlet and excited doublet CEF levels is
responsible for spin-glass behavior in PrRuSi3.",1407.5209v1
2016-04-25,An extensive numerical survey of the correlation between outflow dynamics and accretion disk magnetization,"We investigate the accretion-ejection process of jets from magnetized
accretion disks. We apply a novel approach to the jet-launching problem in
order to obtain correlations between the physical properties of the jet and the
underlying disk. We extend and confirm the previous works of
\citet{2009MNRAS.400..820T} and \citet{2010A&A...512A..82M} by scanning a large
parameter range for the disk magnetization, $\mu_{\rm D} = 10^{-3.5} ...
10^{-0.7}$. We disentangle the disk magnetization at the foot point of the
outflow as the main parameter that governs the properties of the outflow. We
show how the four jet integrals known from steady-state MHD are correlated to
the disk magnetization at the jet foot point. This agrees with the usual
findings of the steady-state theory, however, here we obtain these correlations
from time-dependent simulations that include the dynamical evolution of the
disk in the treatment. In particular, we obtain robust correlations between the
local disk magnetization and (i)the outflow velocity, (ii) the jet mass
loading, (iii) jet angular momentum, and (iv) the local mass accretion rate.
Essentially we find that strongly magnetized disks launch more energetic and
faster jets, and, due to a larger Alfv\'en lever arm, these jets extract more
angular momentum from the underlying disk. These kind of disk-jet systems have,
however, a smaller mass loading parameter and a lower mass
ejection-to-accretion ratio. The jets are launched at the disk surface where
the magnetization is $\mu(r,z) \simeq 0.1$. The magnetization rapidly increases
vertically providing the energy reservoir for subsequent jet acceleration. We
find indication for a critical disk magnetization $\mu_{\rm D} \simeq 0.01$
that separates the regimes of magnetocentrifugally-driven and magnetic
pressure-driven jets.",1604.07313v1
2019-02-21,EVN observations of 6.7 GHz methanol maser polarization in massive star-forming regions IV. Magnetic field strength limits and structure for 7 additional sources,"MHD simulations show that the magnetic field can drive molecular outflows
during the formation of massive protostars. The best probe to observationally
measure both the morphology and the strength of this magnetic field at scales
of 10-100 au is maser polarization. We measure the direction of magnetic fields
at mas resolution around a sample of massive star-forming regions to determine
whether there is a relation between the orientation of the magnetic field and
of the outflows. In addition, by estimating the magnetic field strength via the
Zeeman splitting measurements, the role of magnetic field in the dynamics of
the massive star-forming region is investigated. We selected a flux-limited
sample of 31 massive star-forming regions to perform a statistical analysis of
the magnetic field properties with respect to the molecular outflows
characteristics. We report the linearly and circularly polarized emission of
6.7 GHz CH3OH masers towards seven massive star-forming regions of the total
sample with the EVN. The sources are: G23.44-0.18, G25.83-0.18, G25.71-0.04,
G28.31-0.39, G28.83-0.25, G29.96-0.02, and G43.80-0.13. We identified a total
of 219 CH3OH maser features, 47 and 2 of which showed linearly and circularly
polarized emission, respectively. We measured well-ordered linear polarization
vectors around all the massive YSOs and Zeeman splitting towards G25.71-0.04
and G28.83-0.25. Thanks to recent theoretical results, we were able to provide
lower limits to the magnetic field strength from our Zeeman splitting
measurements. We further confirm (based on ~80% of the total flux-limited
sample) that the magnetic field on scales of 10-100 au is preferentially
oriented along the outflow axes. The estimated magnetic field strength of
|B_{||}|>61 mG and >21 mG towards G25.71-0.04 and G28.83-0.2, respectively,
indicates that it dominates the dynamics of the gas in both regions.",1902.08210v1
2020-12-02,Reconnection-controlled decay of magnetohydrodynamic turbulence and the role of invariants,"We present a new theoretical picture of magnetically dominated, decaying
turbulence in the absence of a mean magnetic field. We demonstrate that such
turbulence is governed by the reconnection of magnetic structures, and not by
ideal dynamics, as has previously been assumed. We obtain predictions for the
magnetic-energy decay laws by proposing that turbulence decays on reconnection
timescales, while respecting the conservation of certain integral invariants
representing topological constraints satisfied by the reconnecting magnetic
field. As is well known, the magnetic helicity is such an invariant for
initially helical field configurations, but does not constrain non-helical
decay, where the volume-averaged magnetic-helicity density vanishes. For such a
decay, we propose a new integral invariant, analogous to the Loitsyansky and
Saffman invariants of hydrodynamic turbulence, that expresses the conservation
of the random (scaling as $\mathrm{volume}^{1/2}$) magnetic helicity contained
in any sufficiently large volume. Our treatment leads to novel predictions for
the magnetic-energy decay laws: in particular, while we expect the canonical
$t^{-2/3}$ power law for helical turbulence when reconnection is fast (i.e.,
plasmoid-dominated or stochastic), we find a shallower $t^{-4/7}$ decay in the
slow `Sweet-Parker' reconnection regime, in better agreement with existing
numerical simulations. For non-helical fields, for which there currently exists
no definitive theory, we predict power laws of $t^{-10/9}$ and $t^{-20/17}$ in
the fast- and slow-reconnection regimes, respectively. We formulate a general
principle of decay of turbulent systems subject to conservation of Saffman-like
invariants, and propose how it may be applied to MHD turbulence with a strong
mean magnetic field and to isotropic MHD turbulence with initial equipartition
between the magnetic and kinetic energies.",2012.01393v3
2024-02-15,Time evolution of the galactic $B- ρ$ relation: the impact of the magnetic field morphology,"One of the most frequently used indicators to characterize the magnetic
field's influence on star formation is the relation between magnetic field
strength and gas density ($B-\rho$ relation), usually expressed as $B \propto
\rho^{\kappa}$. The value of $\kappa$ is an indication of the dynamical
importance of the magnetic field during gas compression. Investigating the
global magnetic field's impact on this relation and its evolution, we conduct
MHD simulations of Milky-Way-like galaxies including gravity, star formation,
and supernova feedback along with non-equilibrium chemistry up to $H_2$
formation fueling star formation. Two initial magnetic field morphologies are
studied: one completely ordered (toroidal) and the other completely random. In
these models, we study the dynamical importance of the magnetic field through
the plasma $\beta$ and the $B-\rho$ relation. For both magnetic morphologies,
low-density regions are thermally supported, while high-density regions are
magnetically dominated. Equipartition is reached earlier and at lower densities
in the toroidal model. However, the $B-\rho$ relation is not unique even within
the same galaxy, as it consistently includes two different branches for a given
density, with $\kappa$ ranging from about 0.2 to 0.8. The mean value of
$\kappa$ for each model also displays significant variations over time, which
supersede the differences between the two models. While our findings suggest
that the magnetic field morphology does influence the galactic $B-\rho$
relation, its impact is transient, since time-averaged differences between the
models fall within the large temporal scatter. The context and time-dependent
nature of the $B-\rho$ relation underscore the need for comprehensive research
and observations to understand the intricate role of magnetic fields in star
formation processes across diverse galactic environments.",2402.10268v2
2002-10-11,Dynamics of spin-2 Bose condensate driven by external magnetic fields,"Dynamic response of the F=2 spinor Bose-Einstein condensate (BEC) under the
influence of external magnetic fields is studied. A general formula is given
for the oscillation period to describe population transfer from the initial
polar state to other spin states. We show that when the frequency and the
reduced amplitude of the longitudinal magnetic field are related in a specific
manner, the population of the initial spin-0 state will be dynamically
localized during time evolution. The effects of external noise and nonlinear
spin exchange interaction on the dynamics of the spinor BEC are studied. We
show that while the external noise may eventually destroy the Rabi oscillations
and dynamic spin localization, these coherent phenomena are robust against the
nonlinear atomic interaction.",0210252v1
2003-02-27,A new approach to dynamic finite-size scaling,"In this work we have considered the Taylor series expansion of the dynamic
scaling relation of the magnetization with respect to small initial
magnetization values in order to study the dynamic scaling behaviour of 2- and
3-dimensional Ising models. We have used the literature values of the critical
exponents and of the new dynamic exponent $x_0$ to observe the dynamic
finite-size scaling behaviour of the time evolution of the magnetization during
early stages of the Monte Carlo simulation. For 3-dimensional Ising Model we
have also presented that this method opens the possibility of calculating $z$
and $x_0$ separately. Our results show good agreement with the literature
values. Measurements done on lattices with different sizes seem to give very
good scaling.",0302571v1
2007-10-09,NMR Time Reversal as a Probe of Incipient Turbulent Spin Dynamics,"We demonstrate time reversal of nuclear spin dynamics in highly magnetized
dilute liquid 3He-4He mixtures through effective inversion of long-range
dipolar interactions. These experiments, which involve using magic sandwich NMR
pulse sequences to generate spin echoes, probe the spatiotemporal development
of turbulent spin dynamics and promise to serve as a versatile tool for the
study and control of dynamic magnetization instabilities. We also show that a
repeated magic sandwich pulse sequence can be used to dynamically stabilize
modes of nuclear precession that are otherwise intrinsically unstable. To date,
we have extended the effective precession lifetimes of our magnetized samples
by more than three orders of magnitude.",0710.1765v1
2008-08-06,Quantum response to time-dependent external fields,"Recently, explicit real time dynamics has been studied in various systems.
These quantum mechanical dynamics could provide new recipes in information
processing. We study quantum dynamics under time dependent external fields, and
explore how to control the quantum state, and also how to bring the state into
a target state. Here, we investigate a pure quantum mechanical dynamics,
dynamics in quantum Monte Carlo simulation and also in quantum master equation.
For the control magnetic states, operators which do not commute with
magnetization are important. We study case of the transverse Ising model, in
which we compare natures of thermal and quantum fluctuations. We also study the
cases of the Dzyaloshinsky-Moriya interaction, where we find a peculiar energy
level structure. Moreover we study the case of itinerant magnetic state, where
we study the change from the Mott insulator to the Nagaoka ferromagnetic state.
Effects of dissipation are also discussed.",0808.0815v4
2008-10-01,Fractal geometry of Ising magnetic patterns: signatures of criticality and diffusive dynamics,"We investigate the geometric properties displayed by the magnetic patterns
developing on a two-dimensional Ising system, when a diffusive thermal dynamics
is adopted. Such a dynamics is generated by a random walker which diffuses
throughout the sites of the lattice, updating the relevant spins. Since the
walker is biased towards borders between clusters, the border-sites are more
likely to be updated with respect to a non-diffusive dynamics and therefore, we
expect the spin configurations to be affected. In particular, by means of the
box-counting technique, we measure the fractal dimension of magnetic patterns
emerging on the lattice, as the temperature is varied. Interestingly, our
results provide a geometric signature of the phase transition and they also
highlight some non-trivial, quantitative differences between the behaviors
pertaining to the diffusive and non-diffusive dynamics.",0810.0216v1
2009-01-19,Barriers for the reduction of transport due to the EXB drift in magnetized plasmas,"We consider a 1 1/2degrees of freedom Hamiltonian dynamical system, which
models the chaotic dynamics of charged test-particles in a turbulent electric
field, across the confining magnetic field in controlled thermonuclear fusion
devices. The external electric field E = \nabla\bigvee is modeled by a
phenomenological potential V and the magnetic field B is considered uniform. It
is shown that, by introducing a small additive control term to the external
electric field, it is possible to create a transport barrier for this dynamical
system. The robustness of this control method is also investigated. This
theoretical study indicates that alternative transport barriers can be
triggered without requiring a control action on the device scale as in present
Internal Transport Barriers (ITB).",0901.2848v1
2009-08-21,Large-scale horizontal flows in the solar photosphere V: Possible evidence for the disconnection of bi-polar sunspot groups from their magnetic roots,"In a recent paper (Svanda et al., 2008, A&A 477, 285) we pointed out that,
based on the tracking of Doppler features in the full-disc MDI Dopplergrams,
the active regions display two dynamically different regimes. We speculated
that this could be a manifestation of the sudden change in the active regions
dynamics, caused by the dynamic disconnection of sunspots from their magnetic
roots as proposed by Schuessler & Rempel (2005, A&A 441, 337). Here we
investigate the dynamic behaviour of the active regions recorded in the
high-cadence MDI data over the last solar cycle in order to confirm the
predictions in the Schuessler's & Rempel's paper. We find that, after drastic
reduction of the sample, which is done to avoid disturbing effects, a large
fraction of active regions displays a sudden decrease in the rotation speed,
which is compatible with the mechanism of the dynamic disconnection of sunspots
from their parental magnetic structures.",0908.3183v1
2011-10-05,A tunable cancer cell filter using magnetic beads: cellular and fluid dynamic simulations,"In the field of biomedicine magnetic beads are used for drug delivery and to
treat hyperthermia. Here we propose to use self-organized bead structures to
isolate circulating tumor cells using lab-on-chip technologies. Typically blood
flows past microposts functionalized with antibodies for circulating tumor
cells. Creating these microposts with interacting magnetic beads makes it
possible to tune the geometry in size, position and shape. We develop a
simulation tool that combines micromagnetics, discrete particle dynamics and
fluid dynamics, in order to design micropost arrays made of interacting beads.
For the simulation of blood flow we use the Lattice-Boltzmann method with
immersed elastic blood cell models. Parallelization distributes large fluid and
particle dynamic simulations over available resources to reduce overall
calculation time.",1110.0995v1
2014-03-21,Interface boundary conditions for dynamic magnetization and spin wave dynamics in a ferromagnetic layer with the interface Dzyaloshinskii-Moriya interaction,"In this work we derive the interface exchange boundary conditions for the
classical linear dynamics of magnetization in ferromagnetic layers with the
interface Dzyaloshinskii-Moriya interaction (IDMI). We show that IDMI leads to
pinning of dynamic magnetization at the interface. An unusual peculiarity of
the IDMI-based pinning is that its scales as the spin-wave wave number. We
incorporate these boundary conditions into an existing numerical model for the
dynamics of the Damon-Eshbach spin wave in ferromagnetic films. IDMI affects
the dispersion and the frequency non-reciprocity of the traveling Damon-Eshbach
spin wave. For a broad range of film thicknesses L and wave numbers the results
of the numerical simulations of the spin wave dispersion are in a good
agreement with a simple analytical expression which shows that the contribution
of IDMI to the dispersion scales as 1/L, similarly to the effect of other types
of interfacial anisotropy. Suggestions to experimentalists how to detect the
presence of IDMI in a spin wave experiment are given.",1403.5385v4
2014-10-07,Spin dynamics of a confined electron interacting with magnetic or nuclear spins: A semiclassical approach,"A physically transparent and mathematically simple semiclassical model is
employed to examine dynamics in the central-spin problem. The results reproduce
a number of previous findings obtained by various quantum approaches and, at
the same time, provide information on the electron spin dynamics and Berry's
phase effects over a wider range of experimentally relevant parameters than
available previously. This development is relevant to dynamics of bound
magnetic polarons and spin dephasing of an electron trapped by an impurity or a
quantum dot, and coupled by a contact interaction to neighboring localized
magnetic impurities or nuclear spins. Furthermore, it substantiates the
applicability of semiclassical models to simulate dynamic properties of
spintronic nanostructures with a mesoscopic number of spins.",1410.1717v1
2015-11-14,Controlling skyrmion helicity via engineered Dzyaloshinskii-Moriya interactions,"Single magnetic skyrmion dynamics in chiral magnets with a spatially
inhomogeneous Dzyaloshinskii-Moriya interaction (DMI) is considered. Based on
the relation between DMI coupling and skyrmion helicity, it is argued that the
latter must be included as an extra degree of freedom in the dynamics of
skyrmions. An effective description of the skyrmion dynamics for an arbitrary
inhomogeneous DMI coupling is obtained through the collective coordinates
method. The resulting generalized Thiele's equation is a dynamical system for
the center of mass position and helicity of the skyrmion. It is found that the
dissipative tensor and hence the Hall angle become helicity dependent. The
skyrmion position and helicity dynamics are fully characterized by our model in
two particular examples of engineered DMI coupling: half-planes with
opposite-sign DMI and linearly varying DMI. In light of the experiment of
Shibata {\it et al}. [Nature Nanotech. {\bf 8}, 723 (2013)] on the magnitude
and sign of the DMI, our results constitute the first step toward a more
complete understanding of the skyrmion helicity as a new degree of freedom that
could be harnessed in future high-density magnetic storage and logic devices.",1511.04584v1
2016-05-10,Magnetic field induced dynamical instabilities in an anti-ferromagnetic spin-1 Bose-Einstein condensate,"We theoretically investigate four types of dynamical instability, in
particular the periodic and oscillatory type $I_O$, in an anti-ferromagnetic
spin-1 Bose-Einstein condensate in a nonzero magnetic field, by employing the
coupled-mode theory and numerical method. This is in sharp contrast to the
dynamical stability of the same system in zero field. Remarkably, a pattern
transition from a periodic dynamical instability $I_O$ to a uniform one $III_O$
occurs at a critical magnetic field. All the four types of dynamical
instability and the pattern transition are ready to be detected in $^{23}$Na
condensates within the availability of the current experimental techniques.",1605.02864v1
2017-10-26,Spin dynamics in MgO based magnetic tunnel junctions with dynamical exchange coupling,"We study the spin dynamics in Fe|MgO|Fe tunnel junction with the dynamical
exchange coupling by coupled Landau-Lifshitz-Gilbert equations. The effects of
spin pumping on the spin dynamics are investigated in detail. It is observed
that the spin pumping can stabilize a quasi-antiparallel state rather than a
quasi-parallel one. More interestingly, our work suggests that the spin pumping
torque can efficiently modulate the magnetization, similar to the
thermal-bias-driven and electricbias-driven spin torques.",1710.09666v2
2018-03-20,Dynamic magneto-optic coupling in a ferromagnetic nematic liquid crystal,"Hydrodynamics of complex fuids with multiple order parameters is governed by
a set of dynamic equations with many material constants, of which only some are
easily measurable. We present a unique example of a dynamic magneto-optic
coupling in a ferromagnetic nematic liquid, in which long-range orientational
order of liquid crystalline molecules is accompanied by long-range magnetic
order of magnetic nanoplatelets. We investigate the dynamics of the
magneto-optic response experimentally and theoretically and find out that it is
significantly affected by the dissipative dynamic cross-coupling between the
nematic and magnetic order parameters. The cross-coupling coefficient
determined by fitting the experimental results with a macroscopic theory is of
the same order of magnitude as the dissipative coefficient (rotational
viscosity) that governs the reorientation of pure liquid crystals.",1803.07075v1
2018-11-13,Phase locking of a pair of nano ferromagnetic oscillators on a topological insulator,"We investigate the magnetization dynamics of a pair of ferromagnetic
insulators (FMIs) deposited on the surface of a topological insulator (TI). Due
to the nonlinear nature of the underlying physics and intrinsic dynamics, the
FMIs can exhibit oscillatory behaviors even under a constant applied voltage.
The motion of the surface electrons of the TI, which obeys relativistic quantum
mechanics, provides a mechanism of direct coupling between the FMIs. In
particular, the spin polarized current of the TI surface electrons can affect
the magnetization of the two FMIs, which in turn modulates the electron
transport, giving rise to a hybrid relativistic quantum/classical nonlinear
dynamical system. We find robust phase and anti-phase locking between the
magnetization dynamics. As driving the surface electrons of a TI only requires
extremely low power, our finding suggests that nano FMIs coupled by a spin
polarized current on the surface of TI have the potential to serve as the
fundamental building blocks of unconventional, low-power computing paradigms.",1811.05551v1
2019-04-08,Learning time-stepping by nonlinear dimensionality reduction to predict magnetization dynamics,"We establish a time-stepping learning algorithm and apply it to predict the
solution of the partial differential equation of motion in micromagnetism as a
dynamical system depending on the external field as parameter. The data-driven
approach is based on nonlinear model order reduction by use of kernel methods
for unsupervised learning, yielding a predictor for the magnetization dynamics
without any need for field evaluations after a data generation and training
phase as precomputation. Magnetization states from simulated micromagnetic
dynamics associated with different external fields are used as training data to
learn a low-dimensional representation in so-called feature space and a map
that predicts the time-evolution in reduced space. Remarkably, only two degrees
of freedom in feature space were enough to describe the nonlinear dynamics of a
thin-film element. The approach has no restrictions on the spatial
discretization and might be useful for fast determination of the response to an
external field.",1904.04215v3
2019-05-28,The Effects of Solar Wind Dynamic Pressure on the Structure of the Topside Ionosphere of Mars,"We use Mars Atmosphere and Volatile EvolutioN observations of the upstream
solar wind, and Mars Express observations of ionospheric electron densities and
magnetic fields, to study how the topside ionosphere ($>$ 320 km) of Mars is
affected by variations in solar wind dynamic pressure. We find that high solar
wind dynamic pressures result in the topside ionosphere being depleted of
plasma at all solar zenith angles, coincident with increased induced magnetic
field strengths. The depletion of topside plasma in response to high solar wind
dynamic pressures is observed in both weak and strong crustal magnetic field
regions. Taken together, our results suggest that high solar wind dynamic
pressures lead to ionospheric compression, increased ion escape, and reduced
day-to-night plasma transport in the high-altitude nightside ionosphere.",1905.12011v2
2019-08-25,Spin-reorientation critical dynamics in the two-dimensional XY model with a domain wall,"In recent years, static and dynamic properties of non-$180^\circ$ domain
walls in magnetic materials have attracted a great deal of interest. In this
paper, spin-reorientation critical dynamics in the two-dimensional XY model is
investigated with Monte Carlo simulations and theoretical analyses based on the
Langevin equation. At the Kosterlitz-Thouless phase transition, dynamic scaling
behaviors of the magnetization and the two-time correlation function are
carefully analyzed, and critical exponents are accurately determined. When the
initial value of the angle between adjacent domains is slightly lower than
$\pi$, a critical exponent is introduced to characterize the abnormal power-law
increase of the magnetization in the horizontal direction inside the domain
interface, which is measured to be $\psi=0.0568(8)$. Besides, the relation
$\psi=\eta/2z$ is analytically deduced from the Langevin dynamics in the
long-wavelength approximation, well consistent with numerical results.",1908.09247v1
2020-06-08,Time-dependent spintronic anisotropy in magnetic molecules,"We theoretically study the quench dynamics of induced anisotropy of a
large-spin magnetic molecule coupled to spin-polarized ferromagnetic leads. The
real-time evolution is calculated by means of the time-dependent density-matrix
numerical renormalization group method implemented within the matrix product
states framework, which takes into account all correlations in very accurate
manner. We determine the system's response to a quench in the spin-dependent
coupling to ferromagnetic leads. In particular, we focus on the transient
dynamics associated with crossing from the weak to the strong coupling regime,
where the Kondo correlations become important. The dynamics is examined by
calculating the time-dependent expectation values of the spin-quadrupole moment
and the associated spin operators. We identify the relevant time scales
describing the quench dynamics and determine the influence of the molecule's
effective exchange coupling and leads spin-polarization on the dynamical
behavior of the system. Furthermore, the generalization of our predictions for
large values of molecule's spin is considered. Finally, we analyze the effect
of finite temperature and show that it gives rise to a reduction of magnetic
anisotropy by strong suppression of the time-dependent spin-quadrupole moment
due to thermal fluctuations.",2006.04431v1
2021-11-16,The effect of shaping on turbulent dynamics in RFP simulations,"We study the influence of the shape of the plasma container on the dynamics
of the Reversed Field Pinch (RFP). The geometries we consider are periodic
cylinders with elliptical and circular-shaped cross-sections. Numerical
simulations of fully nonlinear visco-resistive magnetohydrodynamics are carried
out to illustrate how the plasma dynamics are affected by shaping. It is shown
that independent of the plasma shape, the quantity $\beta$, comparing the
hydrodynamic pressure to the magnetic pressure, decreases for increasing values
of the Lundquist number, but the pressure gradient fluctuations remain roughly
constant, when compared to the Lorentz force. Different elliptical shapes of
the cross-section of the domain lead to the excitation of different toroidal
(or axial) magnetic and dynamic modes. Furthermore, it is found that in a
geometry with circular cross-section a significant local poloidal angular
momentum is observed, absent in the geometries with elliptical cross-section.
Since the confinement is dominantly determined by plasma movement, and the
dynamics of the velocity and magnetic field are modified by the modification of
the geometry, shaping can thus affect the performance of RFP-devices.",2111.08790v1
2022-03-02,A.C. susceptibility as a probe of low-frequency magnetic dynamics,"The experimental technique of a.c. susceptibility can be used as a probe of
magnetic dynamics in a wide variety of systems. Its use is restricted to the
low-frequency regime and thus is sensitive to relatively slow processes. Rather
than measuring the dynamics of single spins, a.c. susceptibility can be used to
probe the dynamics of collective objects, such as domain walls in ferromagnets
or vortex matter in superconductors. In some frustrated systems, such as spin
glasses, the complex interactions lead to substantial spectral weight of
fluctuations in the low-frequency regime, and thus a.c. susceptibility can play
a unique role. We review the theory underlying the technique and magnetic
dynamics more generally and give applications of a.c. susceptibility to a wide
variety of experimental situations.",2203.00989v1
2023-05-26,Accounting for Quantum Effects in Atomistic Spin Dynamics,"Atomistic spin dynamics (ASD) is a standard tool to model the magnetization
dynamics of a variety of materials. The fundamental dynamical model underlying
ASD is entirely classical. In this letter, we present two approaches to
effectively incorporate quantum effects into ASD simulations, thus enhancing
their low temperature predictions. The first allows to simulate the magnetic
behavior of a quantum spin system by solving the equations of motions of a
classical spin system at an effective temperature. This effective temperature
is determined a priori from the microscopic properties of the system. The
second approach is based on a semi-classical model where classical spins
interact with an environment with a quantum-like power spectrum. The parameters
that characterize this model can be calculated ab initio or extracted from
experiments. This semi-classical model quantitatively reproduces the
low-temperature behavior of a magnetic system, thus accounting for the quantum
mechanical aspects of its dynamics. The methods presented here can be readily
implemented in current ASD simulations with no additional complexity cost.",2305.17082v1
2024-02-27,Expansion dynamics of Bose-Einstein condensates in a synthetic magnetic field,"We investigate the expansion dynamics of spin-orbit-coupled Bose-Einstein
condensates subjected to a synthetic magnetic field, after their release from
an external harmonic trap. Our findings reveal that the condensate experiences
a spin-dependent rotation and separation due to the rigid-like rotational
velocity field, which leads to a spin density deflection. The deflection angle
reaches a peak at a time that is inversely related to the frequency of the
harmonic trap. When the detuning gradient is below a critical value for vortex
nucleation, our analytical results derived from a spinor hydrodynamic theory
align closely with numerical results using the coupled Gross-Pitaevskii
equations. Beyond this critical value, we also numerically simulated the
expansion dynamics of the condensates containing vortices with negative
circulation. Our findings highlight the pivotal role of the rigid-like
rotational velocity field on the dynamics of the condensate and may stimulate
further experimental investigations into the rich superfluid dynamics induced
by synthetic magnetic fields.",2402.17691v1
2024-03-22,Coherent Phonon Control of Ultrafast Magnetization Dynamics in Fe$_\text{3}$GeTe$_\text{2}$ from Time-Dependent Ab Initio Theory,"Exploring ultrafast magnetization control in two-dimensional (2D) magnets
through optically driven coherent phonons has been well-established. Yet, the
microscopic interplay between spin dynamics and lattice degrees of freedom
remains less explored. Employing real-time time-dependent density functional
theory (rt-TDDFT) coupled with Ehrenfest dynamics, we systematically
investigate laser-induced spin-nuclei dynamics with coherent phonon excitation
in the 2D ferromagnet Fe3GeTe2. We found that selectively pre-exciting three
typical coherent phonon modes results in up to a 53% additional spin moment
loss in an out-of-plane A2 1g mode within ~50 fs. Coherent phonon control of
spin dynamics is closely linked to laser pulse parameters. The underlying
microscopic mechanism of this phenomenon is primarily governed by coherent
phonon-induced asymmetric spin-resolved charge transfer following the
disappearance of the laser pulse, thereby enabling effective control of the
spin moment loss. Our findings offer a novel insight into the coupling of
coherent phonons with spin systems in 2D limits on femtosecond timescales.",2403.15204v1
1993-10-18,General Relativistic Dynamics of Irrotational Dust: Cosmological Implications,"The non--linear dynamics of cosmological perturbations of an irrotational
collisionless fluid is analyzed within General Relativity. Relativistic and
Newtonian solutions are compared, stressing the different role of boundary
conditions in the two theories. Cosmological implications of relativistic
effects, already present at second order in perturbation theory, are studied
and the dynamical role of the magnetic part of the Weyl tensor is elucidated.",9310036v1
1993-03-23,Kramers map approach for stabilization of hydrogen atom in a magnetic field,"The phenomenon of stabilization of highly excited states of hydrogen atom in
a strong monochromatic field is discussed. Approximate description of dynamics
by the introduced Kramers map allows to understand the main properties of this
phenomenon on the basis of analogy with the Kepler map. Analogy between the
stabilization and the channeling of particles in a crystal is also discussed.",9303014v1
1994-02-17,Localization and dynamic persistent currents in long cylinders,"A dynamic response to a magnetic field in a long disordered cylinder is
considered. We show that, although at high frequencies conduction is classical
in all directions, the low frequency behavior corresponds to localization in
the longitudinal direction and to a diamagnetic dynamic persistent current in
the transversal one. The current density does not vanish even in the limit of
the infinitely long cylinder.",9402071v1
1997-03-04,Dissipative Dynamics of Solitons in Planar Ferromagnets,"Dynamics of magnetic bubbles in planar ferromagnets described by the
Landau-Lifshitz equation with dissipation is analyzed. The pure O(3) sigma
model has static multisoliton solutions, characterized by a number of
parameters. The parameters describe a finite dimensional manifold. A small
perturbation of energy functional with respect to the sigma model forces
solitons to move. Multisoliton dynamics is effectively reduced to a flow in the
parameter space.",9703045v1
1993-03-23,Relativistic Dynamics of Spin in Strong External Fields,"The dynamics of relativistic spinning particles in strong external
electromagnetic or gravitational fields is discussed. Spin-orbit coupling is
shown to affect such relativistic phenomena as time-dilation and perihelion
shift. Possible applications include muon decay in a magnetic field and the
dynamics of neutron stars in binary systems.",9303124v1
1994-12-29,Dynamical Supersymmetry of a Vortex System with $1/r^{2}$ Potential,"The Pauli Hamiltonian for a spin $\frac{1}{2}$ charged particle interacting
with a point magnetic vortex and $1/r^{2}$ potential exhibits a dynamical
supersymmetry $Osp(1,1)$ on the plane except at the origin. Using this
symmetry, the spectrum and the wave functions have been obtained. And, the
dynamical supersymmetry could be imported to the case when an external harmonic
potential is added.",9412225v1
2003-05-09,Applications of Hofer's geometry to Hamiltonian dynamics,"We prove the following three results in Hamiltonian dynamics.
  1. The Weinstein conjecture holds true for every displaceable hypersurface of
contact type. 2. Every magnetic flow on a closed Riemannian manifold has
contractible closed orbits for a dense set of small energies. 3. Every closed
Lagrangian submanifold of an arbitrary symplectic manifold whose fundamental
group injects and which admits a Riemannian metric without closed geodesics has
the intersection property.",0305146v1
2003-12-24,Effective Dynamics of Magnetic Vortices,"We study solutions of Ginzburg-Landau-type evolution equations (both
dissipative and Hamiltonian) with initial data representing collections of
widely-spaced vortices. We show that for long times, the solutions continue to
describe collections of vortices, and we identify (to leading order in the
vortex separation) the dynamical system describing the motion of the vortex
centres (effective dynamics).",0312438v1
1996-10-31,Relativistic Dynamics and the Deuteron Axial Current,"The deuteron axial current is sensitive both to the form of the
implementation of relativistic dynamics as well as to the details of the
deuteron D state at moderate momentum transfer, making it a natural partner to
the magnetic form factor for exploring details of nucleon-nucleon dynamics and
associated electroweak properties.",9610050v1
2000-08-13,Nonlinear Beam Dynamics and Effects of Wigglers,"We present the applications of variational--wavelet approach for the
analytical/numerical treatment of the effects of insertion devices on beam
dynamics. We investigate the dynamical models which have polynomial
nonlinearities and variable coefficients. We construct the corresponding
wavelet representation for wigglers and undulator magnets.",0008046v1
2007-12-19,Dynamic Phase Transitions for Ferromagnetic Systems,"This article presents a phenomenological dynamic phase transition theory for
ferromagnetism, leading to a precise description of the dynamic transitions,
and to a physical predication on the spontaneous magnetization. The analysis
also suggests asymmetry of fluctuations in both the ferromagnetism and the PVT
systems.",0712.3082v1
2009-09-30,Low energy dynamics of spinor condensates,"We present a derivation of the low energy Lagrangian governing the dynamics
of the spin degrees of freedom in a spinor Bose condensate, for any phase in
which the average magnetization vanishes. This includes all phases found within
mean-field treatments except for the ferromagnet, for which the low energy
dynamics has been discussed previously. The Lagrangian takes the form of a
sigma model for the rotation matrix describing the local orientation of the
spin state of the gas.",0909.5620v1
1996-12-09,Classical Billiards in a Magnetic Field and a Potential,"We study billiards in plane domains, with a perpendicular magnetic field and
a potential. We give some results on periodic orbits, KAM tori and adiabatic
invariants. We also prove the existence of bound states in a related scattering
problem.",9612016v1
1992-04-10,Universal quantum critical dynamics of two-dimensional antiferromagnets,"The universal dynamic and static properties of two dimensional
antiferromagnets in the vicinity of a zero-temperature phase transition from
long-range magnetic order to a quantum disordered phase are studied. Random
antiferromagnets with both N\'{e}el and spin-glass long-range magnetic order
are considered. Explicit quantum-critical dynamic scaling functions are
computed in a 1/N expansion to two-loops for certain non-random, frustrated
square lattice antiferromagnets. Implications for neutron scattering
experiments on the doped cuprates are noted.",9204001v2
1997-03-12,Projected dynamics for metastable decay in Ising models,"The magnetization switching dynamics in the kinetic Ising model is projected
onto a one-dimensional absorbing Markov chain. The resulting projected dynamics
reproduces the direct simulation results with great accuracy. A scheme is
proposed to utilize simulation data for small systems to obtain the metastable
lifetime for large systems and/or for very weak magnetic fields, for which
direct simulation is not feasible.",9703115v1
1997-11-19,Nonequlibrium phase transition in the kinetic Ising model: Dynamical symmetry breaking by randomly varying magnetic field,"The nonequilibrium dynamic phase transition, in the two dimensional kinetic
Ising model in presence of a randomly varying (in time but uniform in space)
magnetic field, has been studied both by Monte Carlo simulation and by solving
the mean field dynamic equation of motion for the average magnetisation. In
both the cases, the time averaged magnetisation vanishes from a nonzero value
depending upon the values of the width of randomly varying field and the
temperature. The phase boundary lines are drawn in the plane formed by the
width of the random field and the temperature.",9711191v2
1997-12-10,Pseudo-gap behavior in dynamical properties of high-Tc cuprates,"Dynamical properties of 2D antiferromagnets with hole doping are investigated
to see the effects of short range local magnetic order on the temperature
dependence of the dynamical magnetic susceptibility. We show the pseudo-gap
like behavior of the temperature dependence of the NMR relaxation rate. We also
discuss implications of the results in relations to the observed spin gap like
behavior of low-doped copper oxide high-$T_c$ superconductors.",9712104v2
1999-11-08,Stability and dynamics of free magnetic polarons,"The stability and dynamics of a free magnetic polaron are studied by Monte
Carlo simulation of a classical two-dimensional Heisenberg model coupled to a
single electron. We compare our results to the earlier mean-field analysis of
the stability of the polaron, finding qualitative similarity but quantitative
differences. The dynamical simulations give estimates of the temperature
dependence of the polaron diffusion, as well as a crossover to a tunnelling
regime.",9911109v1
2000-04-26,Generalized Numerical Renormalization Group for Dynamical Quantities,"In this paper we introduce a new approach for calculating dynamical
properties within the numerical renormalization group. It is demonstrated that
the method previously used fails for the Anderson impurity in a magnetic field
due to the absence of energy scale separation. The problem is solved by
evaluating the Green function with respect to the reduced density matrix of the
full system, leading to accurate spectra in agreement with the static
magnetization. The new procedure (denoted as DM-NRG) provides a unifying
framework for calculating dynamics at any temperature and represents the
correct extension of Wilson's original thermodynamic calculation.",0004458v1
2000-12-05,Algorithm for molecular dynamics simulations of spin liquids,"A new symplectic time-reversible algorithm for numerical integration of the
equations of motion in magnetic liquids is proposed. It is tested and applied
to molecular dynamics simulations of a Heisenberg spin fluid. We show that the
algorithm exactly conserves spin lengths and can be used with much larger time
steps than those inherent in standard predictor-corrector schemes. The results
obtained for time correlation functions demonstrate the evident dynamic
interplay between the liquid and magnetic subsystems.",0012059v1
2001-03-06,Monte Carlo Simulations of Short-time Critical Dynamics with a Conserved Quantity,"With Monte Carlo simulations, we investigate short-time critical dynamics of
the three-dimensional anti-ferromagnetic Ising model with a globally conserved
magnetization $m_s$ (not the order parameter). From the power law behavior of
the staggered magnetization (the order parameter), its second moment and the
auto-correlation, we determine all static and dynamic critical exponents as
well as the critical temperature. The universality class of $m_s=0$ is the same
as that without a conserved quantity, but the universality class of non-zero
$m_s$ is different.",0103136v1
2001-11-18,Length scales coupling for nonlinear dynamical problems in magnetism,"The dynamics of real magnets is often governed by several interacting
processes taking place simultaneously at different length scales. For dynamical
simulations the relevant length scales should be coupled, and the energy
transfer accurately described. We show that in this case the micromagnetic
theory is not always reliable. We present a coarse-graining approach applicable
to nonlinear problems, which provides a unified description of all relevant
length scales, allowing a smooth, seamless coupling. The simulations performed
on model systems show that the coarse-graining approach achieves nearly the
precision of all-atom simulations.",0111324v1
2003-03-03,Dimensional Crossover of Vortex Dynamics Induced by Gd Substitution on Bi2212 Single Crystals,"The vortex dynamics of Bi$_2$Sr$_2$Ca$_{1-x}$Gd$_x$Cu$_2$O$_{8+\delta}$
single crystals is investigated by magnetic relaxation and hysteresis
measurements. By substituting $Ca$ with $Gd$, it is found that the interlayer
Josephson coupling is weakened and the anisotropy is increased, which leads to
the change of vortex dynamics from 3D elastic to 2D plastic vortex creep.
Moreover, the second magnetization peak, which can be observed in samples near
the optimal doping, is absent in the strongly underdoped (with 2D vortex)
region.",0303020v1
2004-11-04,Record dynamics and the observed temperature plateau in the magnetic creep rate of type II superconductors,"We use Monte Carlo simulations of a coarse grained three dimensional model to
demonstrate that the experimentally observed approximate temperature
independence of the magnetic creep rate for a broad range of temperatures may
be explained in terms of record dynamics, {\it viz.} the dynamical properties
of the times at which a stochastic fluctuating signal establishes records.",0411104v2
2005-03-16,Artificial scaling laws of the dynamical magnetic susceptibility in heavy-fermion systems,"We report here how artificial, thus erroneous, scaling laws of the dynamical
magnetic susceptibility can be obtained when data are not treated carefully. We
consider the example of the heavy-fermion system
Ce$_{0.925}$La$_{0.075}$Ru$_{2}$Si$_{2}$ and we explain how different kinds of
artificial scaling laws in $E/T^\beta$ can be plotted in a low temperature
regime where the dynamical susceptibility is nearly temperature independent.",0503410v1
2005-05-25,Relaxation dynamics and colossal magnetocapacitive effect in CdCr2S4,"A thorough investigation of the relaxational dynamics in the recently
discovered multiferroic CdCr2S4 showing a colossal magnetocapacitive effect has
been performed. Broadband dielectric measurements without and with external
magnetic fields up to 10 T provide clear evidence that the observed
magnetocapacitive effect stems from enormous changes of the relaxation dynamics
induced by the development of magnetic order.",0505629v1
2006-03-15,Dynamic hysteresis from zigzag domain walls,"We investigate dynamic hysteresis in ferromagnetic thin films with zigzag
domain walls. We introduce a discrete model describing the motion of a wall in
a disordered ferromagnet with in-plane magnetization, driven by an external
magnetic field, considering the effects of dipolar interactions and anisotropy.
We analyze the effects of external field frequency and temperature on the
coercive field by Monte Carlo simulations, and find a good agreement with the
experimental data reported in literature for Fe/GaAs films. This implies that
dynamic hysteresis in this case can be explained by a single propagating domain
wall model without invoking domain nucleation.",0603406v2
2006-05-25,Time Quantified Monte Carlo Algorithm for Interacting Spin Array Micromagnetic Dynamics,"In this paper, we reexamine the validity of using time quantified Monte Carlo
(TQMC) method [Phys. Rev. Lett. 84, 163 (2000); Phys. Rev. Lett. 96, 067208
(2006)] in simulating the stochastic dynamics of interacting magnetic
nanoparticles. The Fokker-Planck coefficients corresponding to both TQMC and
Langevin dynamical equation (Landau-Lifshitz-Gilbert, LLG) are derived and
compared in the presence of interparticle interactions. The time quantification
factor is obtained and justified. Numerical verification is shown by using TQMC
and Langevin methods in analyzing spin-wave dispersion in a linear array of
magnetic nanoparticles.",0605621v1
2001-01-17,QED$_3$ with Dynamical Fermions in an External Magnetic Field,"In this paper, we present results of numerical lattice simulations of
two-flavor QED in three space-time dimensions. First, we provide evidence that
chiral symmetry is spontaneously broken in the chiral and continuum limit. Next
we discuss the role of an external magnetic field $B$ on the dynamically
generated fermion mass. We investigate the $B$-dependence of the condensate
through calculations with dynamical fermions using the non-compact formulation
of the gauge field, and compare the results with those of a comparable study
using the quenched approximation.",0101011v1
2000-07-15,Magnetogenesis and the dynamics of internal dimensions,"The dynamical evolution of internal space-like dimensions breaks the
invariance of the Maxwell's equations under Weyl rescaling of the (conformally
flat) four-dimensional metric. Depending upon the number and upon the dynamics
of internal dimensions large scale magnetic fields can be created. The
requirements coming from magnetogenesis together with the other cosmological
constraints are examined under the assumption that the internal dimensions
either grow or shrink (in conformal time) prior to a radiation dominated epoch.
If the internal dimensions are growing the magnitude of the generated magnetic
fields can seed the galactic dynamo mechanism.",0007163v1
1997-01-17,"Magnetic Monopoles, Gauge Invariant Dynamical Variables and Georgi Glashow Model","We investigate Georgi-Glashow model in terms of a set of explicitly SO(3)
gauge invariant dynamical variables. In the new description a novel compact
abelian gauge invariance emerges naturally. As a consequence magnetic monopoles
occur as point like ""defects"" in space time. Their non-perturbative
contribution to the partition function is explicitly included. This procedure
corresponds to dynamical ""abelian projection"" without gauge fixing. In the
Higgs phase the above abelian invariance is to be identified with
electromagnetism. We also study the effect of $\theta$ term in the above
abelian theory.",9701083v1
2000-07-13,The averaged dynamics of the hydrogen atom in crossed electric and magnetic fields as a perturbed Kepler problem,"We treat the classical dynamics of the hydrogen atom in perpendicular
electric and magnetic fields as a celestial mechanics problem. By expressing
the Hamiltonian in appropriate action-angle variables, we separate the
different time scales of the motion. The method of averaging then allows us to
reduce the system to two degrees of freedom, and to classify the most important
periodic orbits.",0007018v1
2005-12-05,Dynamically controlled toroidal and ring-shaped magnetic traps,"We present traps with toroidal $(T^{2})$ and ring-shaped topologies, based on
adiabatic potentials for radio-frequency dressed Zeeman states in a ring-shaped
magnetic quadrupole field. Simple adjustment of the radio-frequency fields
provides versatile possibilities for dynamical parameter tuning, topology
change, and controlled potential perturbation. We show how to induce toroidal
and poloidal rotations, and demonstrate the feasibility of preparing degenerate
quantum gases with reduced dimensionality and periodic boundary conditions. The
great level of dynamical and even state dependent control is useful for atom
interferometry.",0512017v2
2007-02-27,Reduction and approximation in gyrokinetics,"The gyrokinetics formulation of plasmas in strong magnetic fields aims at the
elimination of the angle associated with the Larmor rotation of charged
particles around the magnetic field lines. In a perturbative treatment or as a
time-averaging procedure, gyrokinetics is in general an approximation to the
true dynamics. Here we discuss the conditions under which gyrokinetics is
either an approximation or an exact operation in the framework of reduction of
dynamical systems with symmetry",0702234v2
1998-10-22,"Non-Abelian Geometric Phase, Floquet Theory, and Periodic Dynamical Invariants","For a periodic Hamiltonian, periodic dynamical invariants may be used to
obtain non-degenerate cyclic states. This observation is generalized to the
degenerate cyclic states, and the relation between the periodic dynamical
invariants and the Floquet decompositions of the time-evolution operator is
elucidated. In particular, a necessary condition for the occurrence of cyclic
non-adiabatic non-Abelian geometrical phase is derived. Degenerate cyclic
states are obtained for a magnetic dipole interacting with a precessing
magnetic field.",9810064v1
1999-04-29,Semiclassical dynamics of a spin-1/2 in an arbitrary magnetic field,"The spin coherent state path integral describing the dynamics of a
spin-1/2-system in a magnetic field of arbitrary time-dependence is considered.
Defining the path integral as the limit of a Wiener regularized expression, the
semiclassical approximation leads to a continuous minimal action path with
jumps at the endpoints. The resulting semiclassical propagator is shown to
coincide with the exact quantum mechanical propagator. A non-linear
transformation of the angle variables allows for a determination of the
semiclassical path and the jumps without solving a boundary-value problem. The
semiclassical spin dynamics is thus readily amenable to numerical methods.",9904102v1
2003-06-13,Quantum Dynamics of Magnetic and Electric Dipoles and Berry's Phase,"We study the quantum dynamics of neutral particle that posseses a permanent
magnetic and electric dipole moments in the presence of an electromagnetic
field. The analysis of this dynamics demonstrates the appearance of a quantum
phase that combines the Aharonov-Casher effect and the He-Mckellar-Wilkens
effect. We demonstrate that this phase is a special case of the Berry's quantum
phase. A series of field configurations where this phase would be found are
presented. A generalized Casella-type effect is found in one these
configurations. A physical scenario for the quantum phase in an interferometric
experiment is proposed.",0306090v1
2007-04-23,Dynamical stability for finite quantum spin chains against a time-periodic inhomogeneous perturbation,"We investigate dynamical stability of the ground state against a
time-periodic and spatially-inhomogeneous magnetic field for finite quantum XXZ
spin chains. We use the survival probability as a measure of stability and
demonstrate that it decays as $P(t) \propto t^{-1/2}$ under a certain
condition. The dynamical properties should also be related to the level
statistics of the XXZ spin chains with a constant spatially-inhomogeneous
magnetic field. The level statistics depends on the anisotropy parameter and
the field strength. We show how the survival probability depends on the
anisotropy parameter, the strength and frequency of the field.",0704.2922v1
2007-08-15,Three-dimensional MHD simulation of expanding magnetic flux ropes,"Three-dimensional, time-dependent numerical simulations of the dynamics of
magnetic flux ropes are presented. The simulations are targeted towards an
experiment previously conducted at CalTech (Bellan, P. M. and J. F. Hansen,
Phys. Plasmas, 5, 1991 (1998)) which aimed at simulating Solar prominence
eruptions in the laboratory. The plasma dynamics is described by ideal MHD
using different models for the evolution of the mass density. Key features of
the reported experimental observations like pinching of the current loop, its
expansion and distortion into helical shape are reproduced in the numerical
simulations. Details of the final structure depend on the choice of a specific
model for the mass density.",0708.2049v1
2008-02-21,Effects of Quark Interactions on Dynamical Chiral Symmetry Breaking by a Magnetic Field,"It is shown how the strong interaction dynamics of a multi-quark Lagrangian
affects the catalysis of dynamical symmetry breaking by a constant magnetic
field in (3+1) dimensions. Attention is drawn to the local minima structure of
the theory.",0802.3193v2
2008-03-24,Geometric Phase for Neutral Particle in the Presence of a Topological Defect,"In this paper we study the quantum dynamics of a neutral particle in the
presence of a topological defect. We investigate the appearance of a geometric
phase in the relativistic quantum dynamics of neutral particle which possesses
permanent magnetic and electric dipole moments in the presence of an
electromagnetic fields in this curved background. The nonrelativistic quantum
dynamics are investigated using the Foldy-Wouthuysen expansion. The
gravitational Aharonov-Casher and He-Mckellar-Wilkens effects are investigated
for a series of electric and magnetic fields configurations.",0803.3428v1
2008-05-05,The Dynamics of Chern-Simons Vortices,"We study vortex dynamics in three-dimensional theories with Chern-Simons
interactions. The dynamics is governed by motion on the moduli space M in the
presence of a magnetic field. For Abelian vortices, the magnetic field is shown
to be the Ricci form over M; for non-Abelian vortices, it is the first Chern
character of a suitable index bundle. We derive these results by integrating
out massive fermions and following the fate of their zero modes.",0805.0602v2
2008-05-06,Short-time dynamics in the 1D long-range Potts model,"We present numerical investigations of the short-time dynamics at criticality
in the 1D Potts model with power-law decaying interactions of the form
1/r^{1+sigma}. The scaling properties of the magnetization, autocorrelation
function and time correlations of the magnetization are studied. The dynamical
critical exponents theta' and z are derived in the cases q=2 and q=3 for
several values of the parameter $\sigma$ belonging to the nontrivial critical
regime.",0805.0719v2
2009-10-23,First-principles dynamical CPA to finite-temperature magnetism of transition metals,"We present here the first-principles dynamical CPA (coherent potential
approximation) combined with the tight-binding LMTO LDA+U method towards
quantitative calculations of the electronic structure and magnetism at finite
temperatures in transition metals and compounds. The theory takes into account
the single-site dynamical charge and spin fluctuations using the functional
integral technique as well as an effective medium. Numerical results for Fe,
Co, and Ni show that the theory explains quantitatively the high-temperature
properties such as the effective Bohr magneton numbers and the excitation
spectra in the paramagnetic state, and describes the Curie temperatures
semiquantitatively.",0910.4486v1
2010-07-08,Dynamical mobility edge for various random Landau Hamiltonians,"We review recent results obtained within the framework of the integer quantum
Hall effect in the spirit of the work of Germinet, Klein, Schenker in
\cite{GKS}. Landau Hamiltonians perturbed by random electric or magnetic
perturbations are shown to exhibit a dynamical mobility edge, that is a
transition between a regime of dynamical localization and a regime of non
trivial transport at a minimal rate. The focus is put on three situations of
interest: 1) unbounded ergodic electric potentials, for which Landau gaps are
filled; 2) non ergodic electric potentials; 3) random magnetic potentials.",1007.1329v1
2010-10-15,Dynamics of Ferrofluidic Drops Impacting Superhydrophobic Surfaces,"This is a fluid dynamics video illustrating the impact of ferrofluidic
droplets on surfaces of variable wettability. Surfaces studied include mica,
teflon, and superhydrophobic. A magnet is placed beneath each surface, which
modifies the behavior of the ferrofluid by applying additional downward force
apart from gravity resulting in reduced droplet size and increased droplet
velocity. For the superhydrophobic droplet a jetting phenomena is shown which
only occurs in a limited range of impact speeds, higher than observed before,
followed by amplified oscillation due to magnetic field as the drop stabilizes
on the surface.",1010.3297v1
2011-01-29,The Quantum Hall Effect in Graphene,"We investigate the quantum Hall effect in graphene. We argue that in graphene
in presence of an external magnetic field there is dynamical generation of mass
by a rearrangement of the Dirac sea. We show that the mechanism breaks the
lattice valley degeneracy only for the $n=0$ Landau levels and leads to the new
observed $\nu = \pm 1$ quantum Hall plateaus. We suggest that our result can be
tested by means of numerical simulations of planar Quantum Electro Dynamics
with dynamical fermions in an external magnetic fields on the lattice.",1101.5703v2
2011-06-14,Anisotropic Inverse Cascade toward Zonal Flow in Magnetically Confined Plasmas,"We propose a new mechanism for the generation of zonal flows in magnetically
confined plasmas, complementing previous theories based on a modulational
instability. We derive a new conservation law that operates in the regime of
weakly nonlinear dynamics, and show that it serves to focus the inverse cascade
of turbulent drift wave energy into zonal flows. This mechanism continues to
operate in the absence of the separation of dynamical scales typically assumed
in instability calculations.",1106.2746v1
2012-07-13,Magnetic relaxation in bilayers of yttrium iron garnet/platinum due to the dynamic coupling at the interface,"We show that in ferromagnetic (FM)/normal metal (NM) bilayers the dynamic
coupling at the interface transfers an additional magnetic relaxation from the
heavily damped motion of the conduction electron spins in the NM layer to the
FM spins. While the FM relaxation rates due to two-magnon scattering and spin
pumping decrease rapidly with increasing FM film thickness, the damping due to
the dynamic coupling does not depend on the FM film thickness. The proposed
mechanism explains the very large broadening of ferromagnetic resonance lines
in thick films of yttrium iron garnet after deposition of a Pt layer.",1207.3330v1
2012-09-03,Critical domain-wall dynamics of model B,"With Monte Carlo methods, we simulate the critical domain-wall dynamics of
model B, taking the two-dimensional Ising model as an example. In the
macroscopic short-time regime, a dynamic scaling form is revealed. Due to the
existence of the quasi-random walkers, the magnetization shows intrinsic
dependence on the lattice size $L$. A new exponent which governs the
$L$-dependence of the magnetization is measured to be $\sigma=0.243(8)$.",1209.0360v1
2012-11-15,On a regularization of the magnetic gas dynamics system of equations,"A brief derivation of a specific regularization for the magnetic gas dynamic
system of equations is given in the case of general equations of gas state (in
presence of a body force and a heat source). The entropy balance equation in
two forms is also derived for the system. For a constant regularization
parameter and under a standard condition on the heat source, we show that the
entropy production rate is nonnegative.",1211.3539v1
2013-01-21,Dynamics of the One-Dimensional Ising Model without Detailed Balance Condition,"We study an irreversible Markov chain Monte Carlo method based on a skew
detailed balance condition for an one-dimensional Ising model. Dynamical
behavior of the magnetization density is analyzed in order to understand the
properties of this method. As a result, it is found theoretically that the
relaxation time of the magnetization density is reduced by using some
transition probabilities satisfying the skew detailed balance condition, in
comparison to that with the corresponding transition probability with the
detailed balance condition, and that one of the transition probabilities
changes the dynamical critical exponent even with a local spin update.",1301.4801v1
2013-04-21,Dynamic Magnetoelectric Effect in Ferromagnet-Superconductor Tunnel Junctions,"We study the magnetization dynamics in a ferromagnet-insulator-superconductor
tunnel junction and the associated buildup of the electrical polarization. We
show that for an open circuit, the induced voltage varies strongly and
nonmonotonically with the precessional frequency, and can be enhanced
significantly by the superconducting correlations. For frequencies much smaller
or much larger than the superconducting gap, the voltage drops to zero, while
when these two energy scales are comparable, the voltage is peaked at a value
determined by the driving frequency. We comment on the potential utilization of
the effect for the low-temperature spatially-resolved spectroscopy of magnetic
dynamics.",1304.5750v2
2013-09-13,Domain dynamics in nonequilibrium random-field Ising models,"We employ Monte Carlo simulations in order to study dynamics of the
magnetization and domain growth processes in the random-field Ising models with
uniform and Gaussian random field distributions of varying strengths. Domain
sizes are determined directly using the Hoshen-Kopelman algorithm. For either
case, both the magnetization and the largest domain growth dynamics are found
to follow the power law with generally different exponents, which exponentially
decay with the random field strength. Moreover, for relatively small random
fields the relaxation is confirmed to comply with different regimes at early
and later times. No significant differences were found between the results for
the uniform and Gaussian distributions, in accordance with the universality
assumption.",1309.3589v2
2014-05-02,Dynamic nuclear polarization from current-induced electron spin polarization,"Current-induced electron spin polarization is shown to produce nuclear
hyperpolarization through dynamic nuclear polarization. Saturated fields of
several millitesla are generated upon the application of electric field over a
timescale of a hundred seconds in InGaAs epilayers and measured using optical
Larmor magnetometry. The dependence on temperature, external magnetic field,
and applied voltage is investigated. We find an asymmetry in which the
saturation nuclear field depends on the relative alignment of the electrically
generated spin polarization and the external magnetic field, which we attribute
to an interplay between various electron spin dynamical processes.",1405.0342v1
2015-04-22,Dynamical supersymmetry on the XXX spin chain,"We show the XXX model has the N = 2 dynamical supersymmetry. Using the
supercharges defined by the Jordan-Wigner fermions, it was found that the
anti-commutation relation of the supercharges gives the Hamiltonian of the XXX
model with magnetic field. In order to compare the length-change supercharges
with the conventional ones, we interpreted their actions in the spinon basis.
In the last part of this paper, we propose the application of the dynamical
supersymmetry to the models with magnetic impurities through the Bethe-ansatz
analysis.",1504.05676v2
2015-08-23,Event-chain algorithm for the Heisenberg model: Evidence for $z \simeq 1$ dynamic scaling,"We apply the event-chain Monte Carlo algorithm to the three-dimensional
ferromagnetic Heisenberg model. The algorithm is rejection-free and also
realizes an irreversible Markov chain that satisfies global balance. The
autocorrelation functions of the magnetic susceptibility and the energy
indicate a dynamical critical exponent $z \approx 1$ at the critical
temperature, while that of the magnetization does not measure the performance
of the algorithm. This seems to be the first report that the event-chain Monte
Carlo algorithm substantially reduces the dynamical critical exponent from the
conventional value of $z\simeq 2$.",1508.05661v2
2016-02-29,Analogue Magnetism Revisited,"In this article we revisit the significance of the often debated structural
similarity between the equations of electromagnetism and fluid dynamics.
Although the matching of the two sets of equations has successfully been done
for non-dissipative forms of the equations, little has been done for cases
where the dissipative terms are non-negligible. We consider the consequence of
non-negligible viscosity and diffusivity, and how the fine-tuning of these
parameters could allow fluid dynamics to be used to indirectly study certain
properties of magnetic fields.",1602.09105v1
2016-04-15,Exactly solvable time-dependent models of two interacting two-level systems,"Two coupled two-level systems placed under external time-dependent magnetic
fields are modeled by a general Hamiltonian endowed with a symmetry that
enables us to reduce the total dynamics into two independent two-dimensional
sub-dynamics. Each of the sub-dynamics is shown to be brought into an exactly
solvable form by appropriately engineering the magnetic fields and thus we
obtain an exact time evolution of the compound system. Several physically
relevant and interesting quantities are evaluated exactly to disclose
intriguing phenomena in such a system.",1604.04519v1
2016-12-05,Dynamical realization of magnetic states in a strongly interacting Bose mixture,"We describe the dynamical preparation of magnetic states in a strongly
interacting two-component Bose gas in a harmonic trap. By mapping this system
to an effective spin chain model, we obtain the dynamical spin densities and
the fidelities for a few-body system. We show that the spatial profiles transit
between ferromagnetic and antiferromagnetic states as the intraspecies
interaction parameter is slowly increased.",1612.01570v4
2017-02-09,Transient spin dynamics in a single-molecule magnet,"We explore the limitations and validity of semi-classically formulated spin
equations of motion. Using a single-molecule magnet as a test model, we employ
three qualitatively different approximation schemes. From a microscopic model,
we derive a generalized spin equation of motion in which the parameters have a
non-local time-dependence. This dynamical equation is simplified to the
Landau-Lifshitz-Gilbert equation with i) time-dependent, and ii)
time-independent parameters. We show that transient dynamics is essentially
non-existing in the latter approximation, while the former breaks down in the
regime of strong coupling between the spin and the itinerant electrons.",1702.02820v2
2017-05-11,Tunable spin dynamics in chiral soliton lattice,"We study dynamics of a chiral soliton lattice (CSL) in a classical
one-dimensional spin chain coupled to the conduction electrons under an
electric field. The CSL has attracted much interest because its period can be
easily controlled by an external magnetic field. We clarify the dependence of
the CSL dynamics on its period. A collective coordinate and an SU(2) gauge
method are used for the analysis. It turns out that the velocity of the CSL
becomes slower as the period becomes longer. We also mention a relation between
the velocity and the magnetic resistance.",1705.04086v2
2017-05-15,Spin and tunneling dynamics in an asymmetrical double quantum dot with spin - orbit coupling,"In this article we study the spin and tunneling dynamics as a function of
magnetic field in a one-dimensional GaAs double quantum dot with both the
Dresselhaus and Rashba spin-orbit coupling. In particular we consider different
spatial widths for the spin-up and spin-down electronic states. We find that
the spin dynamics is a superposition of slow as well as fast Rabi oscillations.
It is found that the Rashba interaction strength as well as the external
magnetic field strongly modifies the slow Rabi oscillations which is
particularly useful for single qubit manipulation for possible quantum computer
applications.",1705.05395v1
2017-05-29,"Part of a collection of reviews on antiferromagnetic spintronics. Antiferromagnetic dynamics, spin-texures, and nanostructures","Antiferromagnets as active elements of spintronics can be faster than their
ferromagnetic counterparts and more robust to magnetic noise. Owing to the
strongly exchange-coupled magnetic sublattice structure, antiferromagnetic
order parameter dynamics are qualitatively different and thus capable of
engendering novel device functionalities. In this review, we discuss
antiferromagnetic textures -- nanoparticles, domain walls, and skyrmions, --
under the action of different spin torques. We contrast the antiferromagnetic
and ferromagnetic dynamics, with a focus on the features that can be relevant
for applications.",1705.10377v1
2019-01-03,Observing zero-field spin dynamics with spin noise in a pi-pulse modulated field,"Spin noise spectroscopic study of spin dynamics in a zero magnetic field is
commonly frustrated by the dominating 1/f noise. We show that in a pi-pulse
modulated magnetic field, spin noise spectrum centered one-half of the field
modulation frequency reveals spin dynamics in zero-fields, free of any
low-frequency noises.",1901.00714v3
2019-02-26,Dynamic spin-charge coupling: spin Hall magnetoresistance in non-magnetic conductors,"The dynamic coupling between spin and charge currents in non-magnetic
conductors is considered. As a consequence of this coupling, the spin dynamics
is directly reflected in the electrical impedance of the sample, with a
relevant frequency scale defined by spin relaxation and spin diffusion. This
allows the observation of the electron spin resonance by purely electrical
measurements.",1902.09784v1
2020-08-24,Reconstructions of the electron dynamics in magnetic field and the geometry of complex Fermi surfaces,"The paper considers the semiclassical dynamics of electrons on complex Fermi
surfaces in the presence of strong magnetic fields. The reconstructions of the
general topological structure of such dynamics are accompanied by the
appearance of closed extremal trajectories of a special form, closely related
to geometry and topology of the Fermi surface. The study of oscillation
phenomena on such trajectories allows, in particular, to propose a relatively
simple method for refining the parameters of the dispersion relation in metals
with complex Fermi surfaces.",2008.10226v2
2020-09-22,From Chaotic Spin Dynamics to Non-collinear Spin Textures in YIG Nano-films by Spin Current Injection,"In this article I report about a numerical investigation of nonlinear spin
dynamics in a magnetic thin-film, made of Yttrium-Iron-Garnet (YIG). This film
is exposed to a small in-plane oriented magnetic field, and strong spin
currents. The rich variety of findings encompass dynamic regimes hosting
localized, non-propagating solitons, a turbulent chaotic regime, which
condenses into a quasi-static phase featuring a non-collinear spin texture.
Eventually, at largest spin current, a homogeneously switched state is
established.",2009.10628v1
2020-10-02,Dynamic Phase Diagram of an Orthogonal Spin Torque device: Topological Category,"The magnetization evolution of the free layer in an orthogonal spin-torque
device is studied based on a macrospin model. The trajectory of magnetization
vector under various conditions has shown rich nonlinear properties. The phase
diagram is obtained in the parameter spaces of current density and the
polarization distribution (the ratio of in-plane and out-of-plane polarizers).
These dynamic phases can be classified according to their nonlinear behaviors
which are topologically different, namely limit point and/or limit cycle. The
topological classification is meaningful to design the ultra-fast spin-torque
devices under different dynamic conditions towards various applications such as
memory and oscillators.",2010.00749v1
2021-02-22,A review of modelling in ferrimagnetic spintronics,"In this review we introduce computer modelling and simulation techniques
which are used for ferrimagnetic materials. We focus on models where thermal
effects are accounted for, atomistic spin dynamics and finite temperature
macrospin approaches. We survey the literature of two of the most commonly
modelled ferrimagnets in the field of spintronics--the amorphous alloy GdFeCo
and the magnetic insulator yttrium iron garnet. We look at how generic models
and material specific models have been applied to predict and understand
spintronic experiments, focusing on the fields of ultrafast magnetisation
dynamics, spincaloritronics and magnetic textures dynamics and give an outlook
for modelling in ferrimagnetic spintronics.",2102.11004v1
2022-03-31,Long-time dynamical behavior for a piezoelectric system with magnetic effect and nonlinear dampings,"This paper is concerned with the long-time dynamical behavior of a
piezoelectric system with magnetic effect, which has nonlinear damping terms
and external forces with a parameter. At first, we use the nonlinear semigroup
theory to prove the well-posedness of solutions. Then, we investigate the
properties of global attractors and the existence of exponential attractors.
Finally, the upper semicontinuity of global attractors has been investigated.",2203.16736v1
2022-07-31,Dynamics of a particle in the generalised Ellis-Bronnikov wormhole on the rotating Archimede's spiral,"A particle moving on curved magnetic field lines in the wormhole metrics is
considered. The dynamics of a single particle is studied on a co-rotating
parameterized magnetic field line. Being interested in the force-free dynamics
we choose the rotating Archimedes' spiral embedded in the generalized
Ellis-Bronnikov metrics. By analysing the phase space, we explore how the
physical parameters affect the motion of the particle and it is shown that the
particles may reach the force free regime, eventually escaping the wormhole.",2208.00521v1
1995-08-17,Reproducible noise in a macroscopic system: magnetic avalanches in Perminvar,"A study of magnetic avalanches in Perminvar, an Fe-Ni-Co alloy, shows that
some avalanches are almost exactly reproducible from one magnetic field cycle
to the next, while others show significant variability. Averaging over many
cycles produces a fingerprint reflecting the reproducibility of the noise. The
fingerprint is not strongly temperature or driving-rate dependent, indicating
that the variability is a consequence of dynamical effects. We also find that
the slope of the cycle-averaged magnetization, d<M>/dH, is correlated with the
cycle-to-cycle variations in magnetization, suggesting an analogy with the
fluctuation-dissipation relationship from equilibrium thermodynamics.",9508005v1
1998-11-17,Magnetic Fields on the Dynamics of the ICM,"Could the discrepancies found in the determination of mass in clusters of
galaxies, from gravitational lensing data and from X-rays observations, be
consequence of the standard description of the ICM, in which it is assumed
hydrostatic equilibrium maintained by thermal pressure? In analogy to the
interstellar medium of the Galaxy, it is expected a non-thermal term of
pressure, which contains contributions of magnetic fields. We follow the
evolution of the ICM, considering a term of magnetic pressure, aiming at
answering the question whether or not these discrepancies can be explained via
non-thermal terms of pressure. Our results suggest that the magnetic pressure
could only affect the dynamics of the ICM on scales as small as $\la 1 {\rm
kpc}$. These results are compared to the observations of large and small scale
magnetic fields and we are successful at reproducing the available data.",9811264v1
1999-07-26,"A Spherical Model for ""Starless"" Cores of Magnetic Molecular Clouds and Dynamical Effects of Dust Grains","In the standard picture of isolated star formation, dense ``starless'' cores
are formed out of magnetic molecular clouds due to ambipolar diffusion. Under
the simplest spherical geometry, I demonstrate that ``starless'' cores formed
this way naturally exhibit a large scale inward motion, whose size and speed
are comparable to those detected recently by Taffala et al. and Williams et al.
in ``starless'' core L1544. My model clouds have a relatively low mass (of
order 10 $M_\odot$) and low field strength (of order 10 $\mu$G) to begin with.
They evolve into a density profile with a central plateau surrounded by a
power-law envelope, as found previously. The density in the envelope decreases
with radius more steeply than those found by Mouschovias and collaborators for
the more strongly magnetized, disk-like clouds.
  At high enough densities, dust grains become dynamically important by greatly
enhancing the coupling between magnetic field and the neutral cloud matter. The
trapping of magnetic flux associated with the enhanced coupling leads, in the
spherical geometry, to a rapid assemblage of mass by the central protostar,
which exacerbates the so-called ``luminosity problem'' in star formation.",9907358v1
2002-04-29,Dynamic nonlinearity in large scale dynamos with shear,"We supplement the mean field dynamo growth equation with the total magnetic
helicity evolution equation. This provides an explicitly time dependent model
for alpha quenching in dynamo theory. For dynamos without shear, this approach
accounts for the observed large scale field growth and saturation in numerical
simulations. After a significant kinematic phase, the dynamo is resistively
quenched, i.e. the saturation time depends on the microscopic resistivity. This
is independent of whether or not the turbulent diffusivity is resistively
quenched. We find that the approach is also successful for dynamos that include
shear and exhibit migratory waves (cycles). In this case however, whether or
not the cycle period remains of the order of the dynamical time scale at large
magnetic Reynolds numbers does depend how on how the turbulent magnetic
diffusivity quenches. Since this is unconstrained by magnetic helicity
conservation, the diffusivity is presently an input parameter. Comparison to
current numerical experiments suggests a turbulent diffusivity that depends
only weakly on the magnetic Reynolds number, but higher resolution simulations
are needed.",0204497v2
2006-06-02,Dynamics of the fast solar tachocline: II. Migrating field,"We present detailed numerical calculations of the fast solar tachocline based
on the assumption that the dynamo field dominates over the dynamics of the
tachocline. In the present paper of the series, we focus on three shortfalls of
the earlier models. First, instead of the simple oscillating dipole poloidal
field we study the more general magnetic field structures reminiscent of the
butterfly diagram. The migrating field is prescribed as the observed
axisymmetric radial magnetic field Stenflo (1988, 1994). Our results are in
good agreement with our analitical estimate and our previous works in
Forgacs-Dajka & Petrovay (2001,2002), but the polar ""dip"" in isorotational
surfaces is strongly reduced in this case. On the other hand, a more realistic
model should have a magnetic diffusivity decreasing significantly inside the
radiative interior, so we also explore the effect of diffusivity and magnetic
Prandtl number varying with depth. We found that the downwards decreasing
magnetic diffusivity and Prandtl number have no significant effect on the
solution, although the temporal variation of the tachocline thickness has
decreased.",0606055v1
1995-05-19,MAGNETISATION REVERSAL AND DOMAIN STRUCTURE IN THIN MAGNETIC FILMS: THEORY AND COMPUTER SIMULATION,"A model is introduced for the theoretical description of nanoscale magnetic
films with high perpendicular anisotropy. In the model the magnetic film is
described in terms of single domain magnetic grains, interacting via exchange
as well as via dipolar forces. Additionally, the model contains anisotropy
energy and a coupling to an external magnetic field. Disorder is taken into
account in order to describe realistic domain and domain wall structures.
Within this framework the dependence of the energy on the film thickness can be
discussed. The influence of a finite temperature as well as the dynamics can be
modeled by a Monte Carlo simulation. The results on the hysteresis loops, the
domain configurations, and the dynamics during the reversal process are in good
agreement with experimental findings.",9505084v1
1996-11-30,Slow antiferromagnetic dynamics in the low temperature tetragonal phase of La_{2-x}Sr_xCuO_4 as revealed by ESR of Gd spin probes,"Measuring the ESR of Gd spin probes we have studied the magnetic properties
of the copper oxide planes in the low temperature tetragonal (LTT) phase of Eu
doped La_{2-x}Sr_xCuO_4. The data give evidence that at particular levels of Sr
and Eu doping the frequency of the antiferromagnetic fluctuations in the LTT
phase dramatically decreases at low temperatures by almost three orders of
magnitude. However, no static magnetic order has been found for T>8K in
contrast to the observation by neutron scattering of stripe ordering of spins
below 50K in a Nd doped La_{2-x}Sr_xCuO_4 single crystal. To our opinion static
order in the Nd doped compound is induced due to the interaction between the Cu
spins with the rare earth magnetic moments. Therefore, a really characteristic
property of the magnetism in the LTT structural phase may be not static
magnetic order at elevated temperatures but rather extremely slow
antiferromagnetic dynamics.",9612011v1
1999-05-05,Vortex dynamics in bulk HTS with levitation techniques,"We have developed a set of experimental techniques with levitation to
investigate the macroscopic magnetic properties of bulk HTS: (i) resonance
oscillation technique; (ii) high speed magnetic rotor technique; (iii) the
method of viscous motion of a permanent magnet through a HTS aperture under a
steady force. Early, we used these techniques to investigate the magnetic flux
dynamics in granular HTS. Now, we have extended and modified these techniques
to study the large grain melt-textured HTS. With resonance oscillations
technique we obtain the energy losses vs ac magnetic field amplitude at HTS
surface and can derive the values of critical current density with high
accuracy. We extract information about distribution of critical current in a
thin undersurface layer and, in this way, check the HTS degradation processes.
The treatment of the HTS surface that has influence on the surface pinning was
investigated with this technique.",9905051v1
2002-03-21,A molecular dynamics study on the equilibrium magnetization properties and structure of ferrofluids,"We investigate in detail the initial susceptibility, magnetization curves,
and microstructure of ferrofluids in various concentration and particle dipole
moment ranges by means of molecular dynamics simulations. We use the Ewald
summation for the long-range dipolar interactions, take explicitly into account
the translational and rotational degrees of freedom, coupled to a Langevin
thermostat. When the dipolar interaction energy is comparable with the thermal
energy, the simulation results on the magnetization properties agree with the
theoretical predictions very well. For stronger dipolar couplings, however, we
find systematic deviations from the theoretical curves. We analyze in detail
the observed microstructure of the fluids under different conditions. The
formation of clusters is found to enhance the magnetization at weak fields and
thus leads to a larger initial susceptibility. The influence of the particle
aggregation is isolated by studying ferro-solids, which consist of magnetic
dipoles frozen in at random locations but which are free to rotate. Due to the
artificial suppression of clusters in ferro-solids the observed susceptibility
is considerably lowered when compared to ferrofluids.",0203445v1
2004-03-08,Mean-field magnetization relaxation in conducting ferromagnets,"Collective ferromagnetic motion in a conducting medium is damped by the
transfer of the magnetic moment and energy to the itinerant carriers. We
present a calculation of the corresponding magnetization relaxation as a
linear-response problem for the carrier dynamics in the effective exchange
field of the ferromagnet. In electron systems with little intrinsic spin-orbit
interaction, a uniform magnetization motion can be formally eliminated by going
into the rotating frame of reference for the spin dynamics. The ferromagnetic
damping in this case grows linearly with the spin-flip rate when the latter is
smaller than the exchange field and is inversely proportional to the spin-flip
rate in the opposite limit. These two regimes are analogous to the
""spin-pumping"" and the ""breathing Fermi-surface"" damping mechanisms,
respectively. In diluted ferromagnetic semiconductors, the hole-mediated
magnetization can be efficiently relaxed to the itinerant-carrier degrees of
freedom due to the strong spin-orbit interaction in the valence bands.",0403224v2
2004-06-24,Models for the magnetic ac susceptibility of granular superferromagnetic CoFe/Al$_2$O$_3$,"The magnetization and magnetic ac susceptibility, $\chi = \chi' - i \chi''$,
of superferromagnetic systems are studied by numerical simulations. The
Cole-Cole plot, $\chi''$ vs. $\chi'$, is used as a tool for classifying
magnetic systems by their dynamical behavior. The simulations of the
magnetization hysteresis and the ac susceptibility are performed with two
approaches for a driven domain wall in random media. The studies are motivated
by recent experimental results on the interacting nanoparticle system
Co$_{80}$Fe$_{20}$/Al$_{2}$O$_{3}$ showing superferromagnetic behavior. Its
Cole-Cole plot indicates domain wall motion dynamics similarly to a disordered
ferromagnet, including pinning and sliding motion. With our models we can
successfully reproduce the features found in the experimental Cole-Cole plots.",0406590v2
2004-12-07,Solving the Master Equation for Extremely Long Time Scale Calculations,"The dynamics of magnetic reversal process plays an important role in the
design of the magnetic recording devices in the long time scale limit. In
addition to long time scale, microscopic effects such as the entropic effect
become important in magnetic nano-scale systems. Many advanced simulation
methods have been developed, but few have the ability to simulate the long time
scale limit and to accurately model the microscopic effects of nano-scale
systems at the same time. We develop a new Monte Carlo method for calculating
the dynamics of magnetic reversal at arbitrary long time. For example, actual
calculations were performed up to 1e50 Monte Carlo steps. This method is based
on microscopic interactions of many constituents and the master equation for
magnetic probability distribution function is solved symbolically.",0412151v1
2005-03-30,Dynamics of Magnetic Defects in Heavy Fermion LiV2O4 from Stretched Exponential 7Li NMR Relaxation,"7Li NMR measurements on LiV2O4 from 0.5 to 4.2 K are reported. A small
concentration of magnetic defects within the structure drastically changes the
7Li nuclear magnetization relaxation versus time from a pure exponential as in
pure LiV2O4 to a stretched exponential, indicating glassy behavior of the
magnetic defects. The stretched exponential function is described as arising
from a distribution of 7Li nuclear spin-lattice relaxation rates and we present
a model for the distribution in terms of the dynamics of the magnetic defects.
Our results explain the origin of recent puzzling 7Li NMR literature data on
LiV2O4 and our model is likely applicable to other glassy systems.",0503723v1
2005-07-22,Dynamical Response of Fermi Condensate to Varying Magnetic Fields,"We investigate the dynamical response of strongly interacting ultra-cold
fermionic atoms near Feshbach resonance to varying magnetic fields. Following
the experimental practices, we calculate the response of the atoms to
oscillating and to linearly ramped magnetic fields respectively. For
oscillating magnetic fields, depending on the frequencies and the amplitudes of
the oscillations, the response of the pair excitation gap shows either linear
or rich non-linear behaviour. In addition, both the spectral studies through
the linear response theory and the time-domain simulations suggest the
existence of a resonant frequency corresponding to the pair dissociation
threshold. For linearly ramped magnetic fields, the response of the excitation
gap shows damped oscillations. The final value of the excitation gap depends on
the rate of the field sweep.",0507540v1
2005-08-30,Large angle magnetization dynamics measured by time-resolved ferromagnetic resonance,"A time-resolved ferromagnetic resonance technique was used to investigate the
magnetization dynamics of a 10 nm thin Permalloy film. The experiment consisted
of a sequence of magnetic field pulses at a repetition rate equal to the
magnetic systems resonance frequency. We compared data obtained by this
technique with conventional pulsed inductive microwave magnetometry. The
results for damping and frequency response obtained by these two different
methods coincide in the limit of a small angle excitation. However, when
applying large amplitude field pulses, the magnetization had a non-linear
response. We speculate that one possible cause of the nonlinearity is related
to self-amplification of incoherence, known as the Suhl instabilities.",0508708v1
2005-11-09,Current driven magnetization dynamics in helical spin density waves,"A mechanism is proposed for manipulating the magnetic state of a helical spin
density wave using a current. In this paper, we show that a current through a
bulk system with a helical spin density wave induces a spin transfer torque,
giving rise to a rotation of the order parameter.The use of spin transfer
torque to manipulate the magnetization in bulk systems does not suffer from the
obstacles seen for magnetization reversal using interface spin transfer torque
in multilayered systems. We demonstrate the effect by a quantitative
calculation of the current induced magnetization dynamics of Erbium. Finally we
propose a setup for experimental verification.",0511224v2
2006-06-29,Dynamics of molecular nanomagnets in time-dependent external magnetic fields: Beyond the Landau-Zener-Stückelberg model,"The time evolution of the magnetization of a magnetic molecular crystal is
obtained in an external time-dependent magnetic field, with sweep rates in the
kT/s range. We present the 'exact numerical' solution of the time dependent
Schr\""{o}dinger equation, and show that the steps in the hysteresis curve can
be described as a sequence of two-level transitions between adiabatic states.
The multilevel nature of the problem causes the transition probabilities to
deviate significantly from the predictions of the Landau-Zener-St\""{u}ckelberg
model. These calculations allow the introduction of an efficient approximation
method that accurately reproduces the exact results. When including phase
relaxation by means of an appropriate master equation, we observe an interplay
between coherent dynamics and decoherence. This decreases the size of the
magnetization steps at the transitions, but does not modify qualitatively the
physical picture obtained without relaxation.",0606763v1
2006-11-30,Edge singularities in high-energy spectra of gapped one-dimensional magnets in strong magnetic fields,"We use the dynamical density matrix renormalization group technique to show
that the high-energy part of the spectrum of a S=1 Haldane chain, placed in a
strong external magnetic field $H$ exceeding the Haldane gap $\Delta$, contains
edge singularities, similar to those known to exist in the low-energy spectral
response. It is demonstrated that in the frequency range $\omega\gtrsim \Delta$
the longitudinal (with respect to the applied field) dynamical structure factor
is dominated by the power-law singularity
$S^{\parallel}(q=\pi,\omega)\propto(\omega-\omega_{0})^{-\alpha'}$. We study
the behavior of the high-energy edge exponent $\alpha'$ and the edge
$\omega_{0}$ as functions of the magnetic field. The existence of edge
singularities at high energies is directly related to the Tomonaga-Luttinger
liquid character of the ground state at $H>\Delta$ and is expected to be a
general feature of one-dimensional gapped spin systems in high magnetic fields.",0611772v2
2007-02-09,Magnetic vortex oscillator driven by dc spin-polarized current,"Transfer of angular momentum from a spin-polarized current to a ferromagnet
provides an efficient means to control the dynamics of nanomagnets. A peculiar
consequence of this spin-torque, the ability to induce persistent oscillations
of a nanomagnet by applying a dc current, has previously been reported only for
spatially uniform nanomagnets. Here we demonstrate that a quintessentially
nonuniform magnetic structure, a magnetic vortex, isolated within a nanoscale
spin valve structure, can be excited into persistent microwave-frequency
oscillations by a spin-polarized dc current. Comparison to micromagnetic
simulations leads to identification of the oscillations with a precession of
the vortex core. The oscillations, which can be obtained in essentially zero
magnetic field, exhibit linewidths that can be narrower than 300 kHz, making
these highly compact spin-torque vortex oscillator devices potential candidates
for microwave signal-processing applications, and a powerful new tool for
fundamental studies of vortex dynamics in magnetic nanostructures.",0702253v1
1998-02-17,Induced Magnetic moments in three-dimensional gauge theories with external magnetic fields,"We study the appearance of induced parity-violating magnetic moment, in the
presence of external magnetic fields, for even-number of fermion species
coupled to dynamical fields in three dimensions. Specifically, we use a
SU(2)xU(1) gauge model for dynamical gauge symmetry breaking, which is also
proposed recently as a field theoretical model for high-temperature
superconductors. By decomposing the fermionic degrees of freedom in terms of
Landau levels, we show that, in the effective theory with the lowest Landau
levels, a parity-violating magnetic moment interaction is induced by the higher
Landau levels when the fermions are massive. The possible relevance of this
result for a recently observed phenomenon in high-temperature superconductors
is also discussed.",9802119v2
2004-03-11,Twisted Crossed Products and Magnetic Pseudodifferential Operators,"There is a connection between the Weyl pseudodifferential calculus and
crossed product C*-algebras associated with certain dynamical systems. And in
fact both topics are involved in the quantization of a non-relativistic
particle moving in R^n. Our paper studies the situation in which a variable
magnetic field is also present. The Weyl calculus has to be modified, giving a
functional calculus for a family of operators (positions and magnetic momenta)
with highly non-trivial commutation relations. On the algebraic side, the
dynamical system is twisted by a cocyle defined by the flux of the magnetic
field, leading thus to twisted crossed products. We outline the interplay
between the modified pseudodifferential setting and the C*-algebraic formalism
at an abstract level as well as in connection with magnetic field.",0403016v1
1999-01-26,"Transverse wavevector dependent and frequency dependent dielectric function, magnetic permittivity and generalized conductivity of interaction site fluids. MD calculations for the TIP4P water","It is shown that the dielectric properties of interaction site models of
polar fluids can be investigated in computer experiment using not only the
charge fluctuations but also correlations corresponding to a current of moving
charges. This current can be associated with a generalized dynamical
polarization or separated into electric and magnetic components. The first
approach deals with the dielectric permittivity related to a generalized
conductivity, whereas the second one leads to the functions describing
polarization and magnetization fluctuations separately. The last way is only
the source to calculate the magnetic susceptibility for a system of interaction
sites. The transverse wavevector- and frequency-dependent dielectric functions
and magnetic susceptibility are evaluated for the TIP4P water model in a very
wide scale of wavelengths and frequencies using molecular dynamics simulations.
We demonstrate that the transverse part of the dielectric functions may differ
drastically from their longitudinal component. A relationship between the two
approaches is discussed and the limiting transition to the static dielectric
constant in the infinite-wavelength regime is analyzed. The propagation of
transverse electromagnetic waves in the TIP4P water is also considered.",9901048v1
2004-11-08,"On the jets, kinks, and spheromaks formed by a planar magnetized coaxial gun","Measurements of the various plasma configurations produced by a planar
magnetized coaxial gun provide insight into the magnetic topology evolution
resulting from magnetic helicity injection. Important features of the
experiments are a very simple coaxial gun design so that all observed
geometrical complexity is due to the intrinsic physical dynamics rather than
the source shape and use of a fast multiple-frame digital camera which provides
direct imaging of topologically complex shapes and dynamics. Three key
experimental findings were obtained: (1) formation of an axial collimated jet
[Hsu and Bellan, Mon. Not. R. Astron. Soc. 334, 257 (2002)] that is consistent
with a magnetohydrodynamic description of astrophysical jets, (2)
identification of the kink instability when this jet satisfies the
Kruskal-Shafranov limit, and (3) the nonlinear properties of the kink
instability providing a conversion of toroidal to poloidal flux as required for
spheromak formation by a coaxial magnetized source [Hsu and Bellan, Phys. Rev.
Lett. 90, 215002 (2003)]. A new interpretation is proposed for how the n=1
central column instability provides flux amplification during spheromak
formation and sustainment, and it is shown that jet collimation can occur
within one rotation of the background poloidal field.",0411089v1
2005-05-13,Dynamics of Entanglement for One-Dimensional Spin Systems in an External Time-Dependent Magnetic Field,"We study the dynamics of entanglement for the XY-model, one-dimensional spin
systems coupled through nearest neighbor exchange interaction and subject to an
external time-dependent magnetic field. Using the two-site density matrix, we
calculate the time-dependent entanglement of formation between nearest neighbor
qubits. We investigate the effect of varying the temperature, the anisotropy
parameter and the external time-dependent magnetic field on the entanglement.
We have found that the entanglement can be localized between nearest neighbor
qubits for certain values of the external time-dependent magnetic field.
Moreover, as known for the magnetization of this model, the entanglement shows
nonergodic behavior, it does not approach its equilibrium value at the infinite
time limit.",0505101v1
2007-09-05,Time-Domain Studies of Very-Large-Angle Magnetization Dynamics Excited by Spin Transfer Torques,"We describe time-domain measurements which provide new information about the
large-angle nonlinear dynamics of nanomagnets excited by spin-transfer torque
from a spin-polarized current. Sampling-oscilloscope measurements, which
average over thousands of experimental time traces, show that the mean reversal
time for spin-transfer-driven magnetic switching has a step-like dependence on
magnetic field, because an integer number of precession cycles is required for
reversal. Storage-oscilloscope measurements of individual experimental traces
reveal non-periodic large-amplitude resistance variations at values of magnetic
field and current in a crossover region between the regimes of
spin-transfer-driven switching and steady-state precession. We also observe
directly the existence of time-dependent switching, on the nanosecond scale,
between different precessional modes and between a precessional mode and a
static state, at particular values of magnetic field and current bias.",0709.0560v1
2007-09-08,Transition rates for a S \ge 1 model coupled to a phonon bath,"We investigate transition rates between different spin configurations for $S
\ge 1$ spins weakly coupled to a $d$-dimensional phonon bath. This study is
motivated by understanding observed magnetization relaxation as a function of
temperature in diverse magnetic systems such as arrays of magnetic
nanoparticles and magnetic molecules. We assume that the magnetization of the
spin system relaxes through consecutive emission or absorption of a single
phonon. From a weak, linear spin-phonon coupling Hamiltonian, we derive
transition rates that would be used to examine dynamic properties of the system
in kinetic Monte Carlo simulations. Although the derived phonon-assisted
transition rates satisfy detailed balance, in the case of two and three
dimensional phonon baths, transitions between degenerate states are not
allowed. Thus, if there are no alternative paths along which the spin system
can relax, the relaxation time diverges. Otherwise, the system finds other
paths, which leads to an increase in the relaxation time and energy barrier.
However, when higher-order phonon processes are included in the transition
rates, it is found that the system can reach the states which were inaccessible
due to the forbidden transitions. As a result, the system recovers some of the
dynamic properties obtained using the Glauber transition rate.",0709.1237v1
2007-10-22,Dynamo quenching due to shear flow,"We provide a theory of dynamo ($\alpha$ effect) and momentum transport in
three-dimensional magnetohydrodynamics. For the first time, we show that the
$\alpha$ effect is severely reduced by the shear even in the absence of
magnetic field. The $\alpha$ effect is further suppressed by magnetic fields
well below equipartition (with the large-scale flow) with different scalings
depending on the relative strength of shear and magnetic field. The turbulent
viscosity is also found to be significantly reduced by shear and magnetic
fields, with positive value. These results suggest a crucial effect of shear
and magnetic field on dynamo quenching and momentum transport reduction, with
important implications for laboratory and astrophysical plasmas, in particular
for the dynamics of the Sun.",0710.4005v2
2008-02-26,The Influence of Helical Magnetic Fields in the Dynamics and Emission of Relativistic Jets,"We present numerical relativistic magnetohydrodynamic and emission
simulations aimed to study the role played by the magnetic field in the
dynamics and emission of relativistic jets in Active Galactic Nuclei. We focus
our analysis on the study of the emission from recollimation shocks since they
may provide an interpretation for the stationary components seen at
parsec-scales in multiple sources. We show that the relative brightness of the
knots associated with the recollimation shocks decreases with increasing jet
magnetization, suggesting that jets presenting stationary components may have a
relatively weak magnetization, with magnetic fields of the order of
equipartition or below.",0802.3826v1
2008-03-01,External Fields and Chiral Symmetry Breaking in the Sakai-Sugimoto Model,"Using the Sakai-Sugimoto model we study the effect of an external magnetic
field on the dynamics of fundamental flavours in both the confined and
deconfined phases of a large N_c gauge theory. We find that an external
magnetic field promotes chiral symmetry breaking, consistent with the ""magnetic
catalysis"" observed in the field theory literature, and seen in other studies
using holographic duals. The external field increases the separation between
the deconfinement temperature and the chiral symmetry restoring temperature. In
the deconfined phase we investigate the temperature-magnetic field phase
diagram and observe, for example, there exists a maximum critical temperature
(at which symmetry is restored) for very large magnetic field. We find that
this and certain other phenomena persist for the Sakai-Sugimoto type models
with probe branes of diverse dimensions. We comment briefly on the dynamics in
the presence of an external electric field.",0803.0038v3
2008-04-18,Polarisation selective magnetic vortex dynamics and core reversal in rotating magnetic fields,"A magnetic vortex occurs as an equilibrium configuration in thin
ferromagnetic platelets of micron and sub-micron size and is characterised by
an in-plane curling magnetisation. At the centre, a magnetic singularity is
avoided by an out-of-plane magnetisation core. This core has a gyrotropic
excitation mode, which corresponds to a circular motion of the vortex around
its equilibrium position, where the rotation sense is determined by the
direction of the vortex core magnetisation, its polarisation. Unlike linear
fields or spin polarised currents, which excite both polarisation states, an
in-plane rotating field can selectively excite one of the polarisation states.
Here we report the observation of vortex dynamics in response to rotating
magnetic fields, imaged with time-resolved scanning X-ray microscopy. We
demonstrate that the rotating field only excites the gyrotropic mode if the
rotation sense of the field coincides with the vortex gyration sense and that
such a field can selectively reverse the vortex polarisation.",0804.2944v2
2008-06-17,Dynamics of magnetization in frustrated spin-chain systems: Ca$_3$Co$_2$O$_6$,"The magnetization dynamics of the triangular lattice of Ising spin chains is
investigated in the framework of a two-dimensional model. The rigid chains are
assumed to interact with the nearest neighboring chains, an external magnetic
field, and a heat reservoir that causes the chains to change their states
randomly with time. A probability of a single spin-flip process is assumed in a
Glauber-like form. This technique allows describing properly the steps in the
magnetization curves observed in Ca$_3$Co$_2$O$_6$ and their dependence on a
magnetic field sweep rate and temperature. A transition from a low-temperature
to high-temperature phase is also observed.",0806.2692v1
2008-10-08,Anti-vortex dynamics in magnetic nanostripes,"In a thin magnetic nanostripe, an anti-vortex nucleates inside a moving
domain wall when driven by an in-plane magnetic field greater than the
so-called Walker field. The nucleated anti-vortex must cross the width of the
nanostripe before the domain wall can propagate again, leading to low average
domain wall speeds. A large out-of-plane magnetic field, applied
perpendicularly to the plane of the nanostripe, inhibits the nucleation of the
anti-vortex leading to fast domain wall speeds for all in-plane driving fields.
We present micromagnetic simulation results relating the anti-vortex dynamics
to the strength of the out-of-plane field. An asymmetry in the motion is
observed which depends on the alignment of the anti-vortex core magnetic
moments to the direction of the out-of-plane field. The size of the core is
directly related to its crossing speed, both depending on the strength of the
perpendicular field and the alignment of the core moments and direction of the
out-of-plane field.",0810.1538v1
2008-10-17,"Magnetization dynamics down to zero field in dilute (Cd,Mn)Te quantum wells","The evolution of the magnetization in (Cd,Mn)Te quantum wells after a short
pulse of magnetic field was determined from the giant Zeeman shift of
spectroscopic lines. The dynamics in absence of magnetic field was found to be
up to three orders of magnitude faster than that at 1 T. Hyperfine interaction
and strain are mainly responsible for the fast decay. The influence of a hole
gas is clearly visible: at zero field anisotropic holes stabilize the system of
Mn ions, while in a magnetic field of 1 T they are known to speed up the decay
by opening an additional relaxation channel.",0810.3100v1
2008-11-12,Wave turbulence on the surface of a ferrofluid submitted to a magnetic field,"We report the observation of wave turbulence on the surface of a ferrofluid
mechanically forced and submitted to a static normal magnetic field. We show
that magnetic surface waves arise only above a critical field. The power
spectrum of their amplitudes displays a frequency-power law leading to the
observation of a magnetic wave turbulence regime which is experimentally shown
to involve a 4-wave interaction process. The existence of the regimes of
gravity, magnetic and capillary wave turbulence is reported in the phase space
parameters as well as a triple point of coexistence of these three regimes.
Most of these features are understood using dimensional analysis or the
dispersion relation of the ferrohydrodynamics surface waves.",0811.1943v1
2009-03-30,Universal Holographic Chiral Dynamics in an External Magnetic Field,"In this work we further extend the investigation of holographic gauge
theories in external magnetic fields, continuing earlier work. We study the
phenomenon of magnetic catalysis of mass generation in 1+3 and 1+2 dimensions,
using D3/D7- and D3/D5-brane systems, respectively. We obtain the low energy
effective actions of the corresponding pseudo Goldstone bosons and study their
dispersion relations. The D3/D7 system exhibits the usual
Gell-Mann--Oakes--Renner (GMOR) relation and a relativistic dispersion
relation, while the D3/D5 system exhibits a quadratic non-relativistic
dispersion relation and a modified linear GMOR relation. The low energy
effective action of the D3/D5 system is related to that describing magnon
excitations in a ferromagnet. We also study properties of general Dp/Dq systems
in an external magnetic field and verify the universality of the magnetic
catalysis of dynamical symmetry breaking.",0903.5345v2
2009-06-29,Three-dimensional evolution of magnetic and velocity shear driven instabilities in a compressible magnetized jet,"The problem of three-dimensional combined magnetic and velocity shear driven
instabilities of a compressible magnetized jet modeled with a plane
neutral/current double vortex sheet in the framework of the resistive
magnetohydrodynamics is addressed. The resulting dynamics given by the
stream+current sheet interaction is analyzed and the effects of a variable
geometry of the basic fields are considered. Depending on the basic asymptotic
magnetic field configuration, a selection rule of the linear instability modes
can be obtained. Hence, the system follows a two-stage path developing either
through a fully three-dimensional dynamics with a rapid evolution of kink modes
leading to a final turbulent state, or rather through a driving two-dimensional
instability pattern that develops on parallel planes on which a
reconnection+coalescence process takes place.",0906.5383v1
2009-09-25,Modeling domain wall dynamics in thin magnetic strips with disorder,"We present a line-based model of transverse domain walls in thin magnetic
strips, to study the effect of bulk disorder on the domain wall dynamics within
the thermally activated creep regime. The creep velocity is found to exhibit a
non-linear dependence on both applied magnetic fields and electric currents,
characterized by similar creep exponents for both forms of the external drive.
We discuss briefly the significance of the inherently stochastic thermally
activated domain wall motion from the point of view of spintronics
applications, where it generally is essential to be able to control the domain
wall displacement in a deterministic manner.",0909.4659v1
2009-10-06,Dynamics of reflection of ultracold atoms from a periodic 1D magnetic lattice potential,"We report on an experimental study of the dynamics of the reflection of
ultracold atoms from a periodic one-dimensional magnetic lattice potential. The
magnetic lattice potential of period 10 \textmu m is generated by applying a
uniform bias magnetic field to a microfabricated periodic structure on a
silicon wafer coated with a multilayered TbGdFeCo/Cr magneto-optical film. The
effective thickness of the magnetic film is about 960 nm. A detailed study of
the profile of the reflected atoms as a function of externally induced periodic
corrugation in the potential is described. The effect of angle of incidence is
investigated in detail. The experimental observations are supported by
numerical simulations.",0910.0953v1
2010-04-21,Effects of Spin Fluctuations and Anomalous Thermal Expansion of delta-Plutonium,"We suggest a model for the magnetic dynamics of - plutonium and its alloys in
order to show that the dynamical fluctuations of the magnetization density, or
spin fluctuations, may be responsible for the anomalies of their observed
thermal expansion. We show that due to strong magneto-elastic coupling, spin
fluctuations may essentially contribute to the volume strain by giving a
negative magneto-volume contribution that is proportional to the squared local
magnetic moment and the magnetic Gruneisen constant which is negative in -
plutonium. In the presented model, the local magnetic moment increases as the
temperature rises, resulting in the interplay between the positive
contributions to the volume strain from the lattice and the negative
contribution from spin fluctuations, and finally leads to the Invar anomaly or
to the negative coefficient of thermal expansion. Our results agree closely
with the measured thermal expansion data for Pu-Ga alloys.",1004.3658v1
2010-07-06,The dynamics of magnetic vortex states in a single permalloy nanoparticle,"We demonstrate a novel method allowing the study of the magnetic state
dynamics of a single nanoparticle by means of electron transport measurements.
Elliptical 550 nm x 240 nm permalloy nanoparticles are wired with non-magnetic
leads for magnetotransport measurements in the presence of a radio-frequency
(RF) field. Their resistance exhibits sharp jumps due to the anisotropic
magnetoresistance even at room temperature. An RF field induces DC voltage
across the nanoparticle which can be partially depleted at a certain RF
frequency when a magnetic vortex core resonance is present. An application of
an additional DC magnetic field eliminates the vortex and reinstates the
unperturbed DC voltage level. The vortex core resonance frequencies are found
and the smallest resonance widths are estimated to be less than 6 MHz.",1007.0958v1
2010-07-29,Effect of a high-frequency magnetic field on the resonant behavior displayed by a spin-$1/2$ particle under the influence of a rotating magnetic field,"In this paper, we investigate the role of a high-frequency magnetic field in
the resonant behavior displayed by a spin-$1/2$ particle under the influence of
a rotating magnetic field. We propose two alternative methods for analyzing the
system dynamics, namely, the averaging method and the multiple scale method.
The analytical results achieved by applying these two methods are compared with
those obtained from the numerical solution of the Schr\""odinger equation. This
comparison leads to the conclusion that the multiple scale method provides a
better understanding of the system dynamics than the averaging method. In
particular, the averaging method predicts the complete destruction of the
resonant behavior by an appropriate choice of the parameter values of the
high-frequency magnetic field. This conclusion is disproved both by the
numerical results, and also by the results obtained from the multiple scale
method.",1007.5194v1
2010-08-12,Magnetization dynamics in the inertial regime: nutation predicted at short time scales,"The dynamical equation of the magnetization has been reconsidered with
enlarging the phase space of the ferromagnetic degrees of freedom to the
angular momentum. The generalized Landau-Lifshitz-Gilbert equation that
includes inertial terms, and the corresponding Fokker-Planck equation, are then
derived in the framework of mesoscopic non-equilibrium thermodynamics theory. A
typical relaxation time $\tau$ is introduced describing the relaxation of the
magnetization acceleration from the inertial regime towards the precession
regime defined by a constant Larmor frequency. For time scales larger than
$\tau$, the usual Gilbert equation is recovered. For time scales below $\tau$,
nutation and related inertial effects are predicted. The inertial regime offers
new opportunities for the implementation of ultrafast magnetization switching
in magnetic devices.",1008.2177v1
2010-10-20,Electron spin decoherence in diluted magnetic quantum wells,"We study electron spin dynamics in diluted magnetic quantum wells. The
electrons are coupled by exchange interaction with randomly distributed
magnetic ions polarized by magnetic field. This coupling leads to both spin
relaxation and spin decoherence. We demonstrate that even very small spatial
fluctuations of quantum well width dramatically increase rate of decoherence.
Depending on the strength of exchange interaction and the amplitude of the
fluctuations the decoherence can be homogeneous or inhomogeneous. In the
homogeneous regime, the transverse (with respect to magnetic field) component
of the electron spin decays on the short time scale exponentialy, while the
long-time spin dynamics is non-exponential demonstrating long-lived power law
tail. In the inhomogeneous case, the transverse spin component decays
exponentially with the exponent quadratic in time.",1010.4284v1
2010-12-27,Phenomenology of Current-Induced Dynamics in Antiferromagnets,"We derive a phenomenological theory of current-induced staggered
magnetization dynamics in antiferromagnets. The theory captures the reactive
and dissipative current-induced torques and the conventional effects of
magnetic fields and damping. A Walker ansatz describes the dc current-induced
domain-wall motion when there is no dissipation. If magnetic damping and
dissipative torques are included, the Walker ansatz remains robust when the
domain-wall moves slowly. As in ferromagnets, the domain-wall velocity is
proportional to the ratio between the dissipative-torque and the magnetization
damping. In addition, a current-driven antiferromagnetic domain-wall acquires a
net magnetic moment.",1012.5655v2
2011-01-18,Statistical Mechanics and the Physics of the Many-Particle Model Systems,"The development of methods of quantum statistical mechanics is considered in
light of their applications to quantum solid-state theory. We discuss
fundamental problems of the physics of magnetic materials and the methods of
the quantum theory of magnetism, including the method of two-time temperature
Green's functions, which is widely used in various physical problems of
many-particle systems with interaction. Quantum cooperative effects and
quasiparticle dynamics in the basic microscopic models of quantum theory of
magnetism: the Heisenberg model, the Hubbard model, the Anderson Model, and the
spin-fermion model are considered in the framework of novel
self-consistent-field approximation. We present a comparative analysis of these
models; in particular, we compare their applicability for description of
complex magnetic materials. The concepts of broken symmetry, quantum
protectorate, and quasiaverages are analyzed in the context of quantum theory
of magnetism and theory of superconductivity. The notion of broken symmetry is
presented within the nonequilibrium statistical operator approach developed by
D.N. Zubarev. In the framework of the latter approach we discuss the derivation
of kinetic equations for a system in a thermal bath. Finally, the results of
investigation of the dynamic behavior of a particle in an environment, taking
into account dissipative effects, are presented.",1101.3423v1
2011-01-26,Magnetic dynamo action in random flows with zero and finite correlation times,"Hydromagnetic dynamo theory provides the prevailing theoretical description
for the origin of magnetic fields in the universe. Here we consider the problem
of kinematic, small-scale dynamo action driven by a random, incompressible,
non-helical, homogeneous and isotropic flow. In the Kazantsev dynamo model the
statistics of the driving flow are assumed to be instantaneously correlated in
time. Here we compare the results of the model with the dynamo properties of a
simulated flow that has equivalent spatial characteristics as the Kazantsev
flow but different temporal statistics. In particular, the simulated flow is a
solution of the forced Navier-Stokes equations and hence has a finite
correlation time. We find that the Kazantsev model typically predicts a larger
magnetic growth rate and a magnetic spectrum that peaks at smaller scales.
However, we show that by filtering the diffusivity spectrum at small scales it
is possible to bring the growth rates into agreement and simultaneously align
the magnetic spectra.",1101.5181v2
2011-06-17,Current-induced synchronized magnetization reversal of two-body Stoner particles with dipolar interaction,"We investigate magnetization reversal of two-body uniaxial Stoner particles,
by injecting spin-polarized current through a spin-valve structure. The
two-body Stoner particles perform synchronized dynamics and can act as an
information bit in computer technology. In the presence of magnetic
dipole-dipole interaction (DDI) between the two particles, the critical
switching current $I_c$ for reversing the two dipoles is analytically obtained
and numerically verified in two typical geometric configurations. $I_c$
bifurcates at a critical DDI strength, where $I_c$ can be decreased to about
70% of the usual value without DDI. Moreover, we also numerically investigate
the magnetic hysteresis loop, magnetization self-precession, reversal time and
the synchronization stability phase diagram for the two-body system in the
synchronized dynamics regime.",1106.3454v1
2011-07-01,Catastrophic quenching in alpha-Omega dynamos revisited,"At large magnetic Reynolds numbers, magnetic helicity evolution plays an
important role in astrophysical large-scale dynamos. The recognition of this
fact led to the development of the dynamical alpha quenching formalism, which
predicts catastrophically low mean fields in open systems. Here we show that in
oscillatory alpha-Omega dynamos this formalism predicts an unphysical magnetic
helicity transfer between scales. An alternative technique is proposed where
this artifact is removed by using the evolution equation for the magnetic
helicity of the total field in the shearing--advective gauge. In the
traditional dynamical alpha quenching formalism, this can be described as an
additional magnetic helicity flux of small-scale fields that does not appear in
homogeneous alpha-squared dynamos. In alpha-Omega dynamos, the alternative
formalism is shown to lead to larger saturation fields than previously obtained
with the traditional formalism.",1107.0238v2
2011-07-28,Optimal control of vortex core polarity by resonant microwave pulses,"In a vortex-state magnetic nano-disk, the static magnetization is curling in
the plane, except in the core region where it is pointing out-of-plane, either
up or down leading to two possible stable states of opposite core polarity p.
Dynamical reversal of p by large amplitude motion of the vortex core has
recently been demonstrated experimentally,raising fundamental interest for
potential application in magnetic storage devices. Here we demonstrate coherent
control of p by single and double microwave pulse sequences, taking advantage
of the resonant vortex dynamics in a perpendicular bias magnetic field.
Optimization of the microwave pulse duration required to switch p allows to
experimentally infer the characteristic decay time of the vortex core in the
large oscillation regime. It is found to be more than twice shorter than in the
small oscillation regime, raising the fundamental question of the non-linear
behaviour of magnetic dissipation.",1107.5700v1
2011-08-01,Spin Pumping and Spin Transfer,"Spin pumping is the emission of a spin current by a magnetization dynamics
while spin transfer stands for the excitation of magnetization by spin
currents. Using Onsager's reciprocity relations we prove that spin pumping and
spin-transfer torques are two fundamentally equivalent dynamic processes in
magnetic structures with itinerant electrons. We review the theory of the
coupled motion of the magnetization order parameter and electron for textured
bulk ferromagnets (e.g. containing domain walls) and heterostructures (such as
spin valves). We present first-principles calculations for the
material-dependent damping parameters of magnetic alloys. Theoretical and
experimental results agree in general well.",1108.0385v3
2011-09-04,Numerical simulations of relativistic magnetic reconnection with Galerkin methods,"We present the results of two-dimensional magnetohydrodynamical numerical
simulations of relativistic magnetic reconnection, with particular emphasis on
the dynamics of Petschek-type configurations with high Lundquist numbers, S ~
10^5-10^8. The numerical scheme adopted, allowing for unprecedented accuracy
for this type of calculations, is based on high order finite volume and
discontinuous Galerkin methods as recently proposed by Dumbser & Zanotti
(2009). The possibility of producing high Lorentz factors is discussed, by
studying the effects produced on the dynamics by different magnetization and
resistivity regimes. We show that Lorentz factors close to ~4 can be produced
for a plasma magnetization parameter sigma=20. Moreover, we find that the
Sweet-Parker layers are unstable, generating secondary magnetic islands, but
only for S>S_c~10^8, much larger than what is reported in the Newtonian regime.",1109.0746v1
2011-09-30,Phase separation dynamics in a two-dimensional magnetic mixture,"Based on classical density functional theory (DFT), we investigate the
demixing phase transition of a two-dimensional, binary Heisenberg fluid
mixture. The particles in the mixture are modeled as Gaussian soft spheres,
where one component is characterized by an additional classical spin-spin
interaction of Heisenberg type. Within the DFT we treat the particle
interactions using a mean-field approximation. For certain magnetic coupling
strengths we calculate phase diagrams in the density-concentration plane. For
sufficiently large coupling strengths and densities, we find a demixing phase
transition driven by the ferromagnetic interactions of the magnetic species. We
also provide a microscopic description (i.e., density profiles) of the
resulting non-magnetic/magnetic fluid-fluid interface. Finally, we investigate
the phase separation using dynamical density functional theory (DDFT),
considering both nucleation processes and spinodal demixing.",1109.6756v1
2011-10-10,Star Formation and the Hall Effect,"Magnetic fields play an important role in star formation by regulating the
removal of angular momentum from collapsing molecular cloud cores. Hall
diffusion is known to be important to the magnetic field behaviour at many of
the intermediate densities and field strengths encountered during the
gravitational collapse of molecular cloud cores into protostars, and yet its
role in the star formation process is not well-studied. This thesis describes a
semianalytic self-similar model of the collapse of rotating isothermal
molecular cloud cores with both Hall and ambipolar diffusion, presenting
similarity solutions that demonstrate that the Hall effect has a profound
influence on the dynamics of collapse.
  ...
  Hall diffusion also determines the strength of the magnetic diffusion and
centrifugal shocks that bound the pseudo and rotationally-supported discs, and
can introduce subshocks that further slow accretion onto the protostar. In
cores that are not initially rotating Hall diffusion can even induce rotation,
which could give rise to disc formation and resolve the magnetic braking
catastrophe. The Hall effect clearly influences the dynamics of gravitational
collapse and its role in controlling the magnetic braking and radial diffusion
of the field would be worth exploring in future numerical simulations of star
formation.",1110.2168v1
2011-11-26,Quantum depinning of the magnetic vortex core in micron-size permalloy disks,"The vortex state, characterized by an in-plane closed flux domain structure
and an out-of-plane magnetization at its centre (the vortex core), is one of
the magnetic equilibria of thin soft ferromagnetic micron-size dots. In the
last two decades many groups have been working on the dynamics of the magnetic
moment in nanomagnetic materials at low temperatures, it giving rise to the
observation of quantum relaxations and quantum hysteresis cycles. For the first
time, we report experimental evidence of quantum dynamics of the vortex core of
micron-size permalloy (Fe$_{19}$Ni$_{81}$) disks induced by the application of
an in-plane magnetic field. It is attributed to the quantum tunneling of the
vortex core through pinning barriers, which are associated to structural
defects in the dots, towards its equilibrium position.",1111.6171v1
2012-04-12,On the dynamics of mechanical failures in magnetized neutron-star crusts,"We consider the dynamics of a mechanical failure induced by a shear stress in
a strongly magnetized neutron-star crust. We show that even if the elastic
properties of the crust allow the creation of a shear crack, the strongly
sheared magnetic field around the crack leads to a back-reaction from the
Lorentz force which does not allow large relative displacement of the crack
surfaces. Instead, the global evolution of the crack proceeds on a slow
resistive time scale, and is unable to release any substantial mechanical
energy. Our calculations demostrate that for {\it some} magnetic-field
configurations, the magnetic forces cause, effectively, a plastic deformation
of the crust when the resulting elastic shear stress exceeds the critical value
for mechanical failure.",1204.2605v1
2012-06-07,Magnetic particle hyperthermia: Power losses under circularly polarized field in anisotropic nanoparticles,"The deterministic Landau-Lifshitz-Gilbert equation has been used to
investigate the nonlinear dynamics of magnetization and the specific loss power
in magnetic nanoparticles with uniaxial anisotropy driven by a rotating
magnetic field, generalizing the results obtained for the isotropic case found
in [P. F. de Chatel, I. Nandori, J. Hakl, S. Meszaros and K. Vad, J. Phys.:
Condens. Matter 21, 124202 (2009)]. As opposed to many applications of
magnetization reversal in single-domain ferromagnetic particles where losses
must be minimized, in this paper, we study the mechanisms of dissipation used
in cancer therapy by hyperthermia which requires the enhancement of energy
losses. We show that for circularly polarized field, the loss energy per cycle
is decreased by the anisotropy compared to the isotropic case when only
dynamical effects are taken into account. Thus, in this case, in the low
frequency limit, a better heating efficiency can be achieved for isotropic
nanoparticles. The possible role of thermal fluctuations is also discussed.
Results obtained are compared to experimental data.",1206.1544v2
2012-07-20,Current induced domain wall dynamics in the presence of a transverse magnetic field in out-of-plane magnetized materials,"An analytical model was developped to describe the current induced DW
dynamics of a Bloch DW in the presence of an external transverse magnetic
field. The model takes into account the DW deformation and the magnetization
tilting in the domain. The model is compared to the results of micromagnetic
simulation and an excellent agreement is obtained. In the steady state regime,
the model shows that the domain tilting does not change the DW mobility. An
external or current induced transverse magnetic field such as the Oersted or
Rashba field can prevent the Walker breakdown leading to a higher domain wall
velocity.",1207.4970v1
2012-09-02,Heavy flavor suppression in a dynamical QCD medium with finite magnetic mass,"Reliable predictions for jet quenching in ultra-relativistic heavy ion
collisions require accurate computation of radiative energy loss. While all
available energy loss calculations assume zero magnetic mass, in accordance
with the one-loop perturbative calculations, different non-perturbative
approaches report a non-zero magnetic mass at RHIC and LHC. We generalized the
dynamical energy loss formalism, to consistently include a possibility for
existence of non-zero magnetic screening. We show that this generalization
indicates a fundamental constraint on electric to magnetic mass ratio, which
appears to be supported by lattice QCD simulations. Jet suppression patterns,
obtained from this newly developed generalization, show reasonable agreement
with the available RHIC and LHC measurements.",1209.0198v1
2012-09-26,Nonequilibrium dynamics of a spin-3/2 Blume Capel model with quenched random crystal field,"The relaxation and complex magnetic susceptibility treatments of a spin-3/2
Blume-Capel model with quenched random crystal field on a two dimensional
square lattice are investigated by a method combining the statistical
equilibrium theory and the thermodynamics of linear irreversible processes.
Generalized force and flux are defined in irreversible thermodynamics limit.
The kinetic equation for the magnetization is obtained by using linear response
theory. Temperature and also crystal field dependencies of the relaxation time
are obtained in the vicinity of phase transition points. We found that the
relaxation time exhibits divergent treatment near the order-disorder phase
transition point as well as near the isolated critical point whereas it
displays cusp behavior near the first order phase transition point. In
addition, much effort has been devoted to investigation of complex magnetic
susceptibility response of the system to changing applied field frequencies and
it is observed that the considered disordered magnetic system exhibits unusual
and interesting behaviors. Furthermore, dynamical mean field critical exponents
for the relaxation time and complex magnetic susceptibility are calculated in
order to formulate the critical behavior of the system. Finally, a comparison
of our observations with those of recently published studies is represented and
it is shown that there exists a qualitatively good agreement.",1209.5880v1
2012-09-28,Tunable diffusion of magnetic particles in a quasi-one-dimensional channel,"The diffusion of a system of ferromagnetic dipoles confined in a
quasi-one-dimensional parabolic trap is studied using Brownian dynamics
simulations. We show that the dynamics of the system is tunable by an in-plane
external homogeneous magnetic field. For a strong applied magnetic field, we
find that the mobility of the system, the exponent of diffusion and the
crossover time among different diffusion regimes can be tuned by the
orientation of the magnetic field. For weak magnetic fields, the exponent of
diffusion in the subdiffusive regime is independent of the orientation of the
external field.",1209.6472v2
2013-03-19,Dynamics of Chromospheric Upflows and Underlying Magnetic Fields,"We used \ha-0.1 nm and magnetic field (at 1.56$\mu$) data obtained with the
New Solar Telescope to study the origin of the disk counterparts to type II
spicules, so-called rapid blueshifted excursions (RBEs). The high time cadence
of our chromospheric (10 s) and magnetic field (45 s) data allowed us to
generate x-t plots using slits parallel to the spines of the RBEs. These plots,
along with potential field extrapolation, led us to suggest that the occurrence
of RBEs is generally correlated with the appearance of new, mixed or unipolar
fields in close proximity to network fields. RBEs show a tendency to occur at
the interface between large-scale fields and small-scale dynamic magnetic loops
and thus are likely to be associated with existence of a magnetic canopy.
Detection of kinked and/or inverse ""Y"" shaped RBEs further confirm this
conclusion.",1303.4766v1
2013-05-01,Gravitational collapse of a magnetized fermion gas with finite temperature,"We examine the dynamics of a self--gravitating magnetized electron gas at
finite temperature near the collapsing singularity of a Bianchi-I spacetime.
Considering a general and appropriate and physically motivated initial
conditions, we transform Einstein--Maxwell field equations into a complete and
self--consistent dynamical system amenable for numerical work. The resulting
numerical solutions reveal the gas collapsing into both, isotropic
(""point-like"") and anisotropic (""cigar-like"") singularities, depending on the
initial intensity of the magnetic field. We provide a thorough study of the
near collapse behavior and interplay of all relevant state and kinematic
variables: temperature, expansion scalar, shear scalar, magnetic field,
magnetization and energy density. A significant qualitative difference in the
behavior of the gas emerges in the temperature range $\hbox{T}
sim10^{4}\hbox{K}$ and $\hbox{T}\sim 10^{7}\hbox{K}$.",1305.0308v1
2013-05-30,Simulation of magnetic active polymers for versatile microfluidic devices,"We propose to use a compound of magnetic nanoparticles (20-100 nm) embedded
in a flexible polymer (Polydimethylsiloxane PDMS) to filter circulating tumor
cells (CTCs). The analysis of CTCs is an emerging tool for cancer biology
research and clinical cancer management including the detection, diagnosis and
monitoring of cancer. The combination of experiments and simulations lead to a
versatile microfluidic lab-on-chip device. Simulations are essential to
understand the influence of the embedded nanoparticles in the elastic PDMS when
applying a magnetic gradient field. It combines finite element calculations of
the polymer, magnetic simulations of the embedded nanoparticles and the fluid
dynamic calculations of blood plasma and blood cells. With the use of magnetic
active polymers a wide range of tunable microfluidic structures can be created.
The method can help to increase the yield of needed isolated CTCs.",1305.7071v1
2013-07-02,Field-induced magnetic structures in Tb2Ti2O7 spin liquid under field H || [111],"We have studied the field-induced magnetic structures of Tb2Ti2O7 pyrochlore
by single-crystal neutron diffraction under a field applied along the [111]
local anisotropy axis, up to H=12T and down to T=40mK. We collected a hundred
magnetic Bragg peaks for each field and temperature value and refined the
magnetic structures with k=0 propagation vector by performing a symmetry
analysis in the space group R-3m, reducing the number of free parameters to
three only. We observe a gradual reorientation of the Tb magnetic moments
towards the field direction, close to a ""3-in, 1-out / 1-in, 3-out"" spin
structure in the whole field range 0.05-12T. We perform a quantitative
comparison with mean-field calculations and we propose the presence of a
low-temperature dynamic symmetry breaking of the local trigonal symmetry, akin
to a dynamic Jahn-Teller effect, i.e. preserving the overall cubic symmetry. We
discuss the possible origin of this off-diagonal mixing term in the crystal
field hamiltonian in terms of quadrupole-quadrupole interaction or
magneto-elastic effects.",1307.0806v1
2013-10-05,Estimation of Plasma Properties and Magnetic Field in a Prominence-like Structure as Observed by SDO/AIA,"We analyze a prominence-like cool plasma structure as observed by Atmospheric
Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO). We perform
the Differential Emission Measure (DEM) analysis using various filters of AIA,
and also deduce the temperature and density structure in and around the
observed flux-tube. In addition to deducing plasma parameters, we also find an
evidence of multiple harmonics of fast magnetoacoustic kink waves in the
observed prominence-like magnetic structure. Making use of estimated plasma
parameters and observed wave parameters, under the baseline of MHD seismology,
we deduce magnetic field in the flux-tube. The wave period ratio P1/P2 = 2.18
is also observed in the flux-tube, which carries the signature of magnetic
field divergence where we estimate the tube expansion factor as 1.27. We
discuss constraints in the estimation of plasma and magnetic field properties
in such a structure in the current observational perspective, which may shed
new light on the localized plasma dynamics and heating scenario in the solar
atmosphere.",1310.2135v1
2013-11-02,Resonantly exited precession motion of three-dimensional vortex core in magnetic nanospheres,"We found resonantly excited precession motions of a three-dimensional vortex
core in soft magnetic nanospheres and controllable precession frequency with
the sphere diameter 2R, as studied by micromagnetic numerical and analytical
calculations. The precession angular frequency for an applied static field
$H_{DC}$ is given as $\omega_{MV}= \gamma_{eff} H_{DC}$, where $\gamma_{eff} =
\gamma <m_{\Gamma}>$ is the effective gyromagnetic ratio in collective vortex
dynamics, with the gyromagnetic ratio $\gamma$ and the average magnetization
component $<m_{\Gamma}>$ of the ground-state vortex in the core direction.
Fitting to the micromagnetic simulation data for $<m_{\Gamma}>$ yields a simple
explicit form of $<m_{\Gamma}> = (73.6 \pm 3.4)(l_{ex}/2R)^{2.20 \pm 0.14}$,
where $l_{ex}$ is the exchange length of a given material. This dynamic
behavior might serve as a foundation for potential bio-applications of
size-specific resonant excitation of magnetic vortex-state nanoparticles, for
example, magnetic particle resonance imaging.",1311.0346v3
2013-11-13,Thermally activated in-plane magnetization rotation induced by spin torque,"We study the role of thermal fluctuations on the spin dynamics of a thin
permalloy film with a focus on the behavior of spin torque and find that the
thermally assisted spin torque results in new aspects of the magnetization
dynamics. In particular, we uncover the formation of a finite, spin
torque-induced, in-plane magnetization component. The orientation of the
in-plane magnetization vector depends on the temperature and the spin-torque
coupling. We investigate and illustrate that the variation of the temperature
leads to a thermally-induced rotation of the in-plane magnetization.",1311.3120v1
2014-02-03,Influence of Magnetorotational Instability on Neutrino Heating: A New Mechanism for Weakly Magnetized Core-Collapse Supernovae,"We investigated the impacts of magnetorotational instability (MRI) on the
dynamics of weakly magnetized, rapidly rotating core-collapse by conducting
high resolution MHD simulations in axisymmetry with simplified neutrino
transfer. We found that an initially sub-magnetar class magnetic field is
drastically amplified by MRI and substantially affects the dynamics thereafter.
Although the magnetic pressure is not strong enough to eject matter, the
amplified magnetic field efficiently transfers angular momentum from higher to
lower latitudes, which causes the expansion of the heating region at low
latitudes due to the extra centrifugal force. This then enhance the efficiency
of neutrino heating and eventually leads to neutrino-driven explosion. This is
a new scenario of core-collapse supernovae that has never been demonstrated by
numerical simulations so far.",1402.0513v2
2014-02-05,Re-entrant superspin glass phase in La$_{0.82}$Ca$_{0.18}$MnO$_3$ ferromagnetic insulator,"We report results of magnetization and ac susceptibility measurements down to
very low fields on a single crystal of the perovskite manganite,
La$_{0.82}$Ca$_{0.18}$MnO$_3$. This composition falls in the intriguing
ferromagnetic insulator region of the manganite phase diagram. In contrast to
earlier beliefs, our investigations reveal that the system is magnetically (and
in every other sense) single-phase with a ferromagnetic ordering temperature of
$\sim$ 170 K. However, this ferromagnetic state is magnetically frustrated, and
the system exhibits pronounced glassy dynamics below 90 K. Based on measured
dynamical properties, we propose that this quasi-long-ranged ferromagnetic
phase, and associated superspin glass behavior, is the true magnetic state of
the system, rather than being a macroscopic mixture of ferromagnetic and
antiferromagnetic phases as often suggested. Our results provide an
understanding of the quantum phase transition from an antiferromagnetic
insulator to a ferromagnetic metal via this ferromagnetic insulating state as a
function of $x$ in La$_{1-x}$Ca$_x$MnO$_3$, in terms of the possible formation
of magnetic polarons.",1402.1148v1
2014-02-07,Dynamics and energetics of emergent magnetic monopoles in chiral magnets,"The formation and destruction of topologically quantized magnetic whirls,
so-called skyrmions, in chiral magnets is driven by the creation and motion of
singular hedgehog defects. These can be identified with emergent magnetic
monopoles and antimonopoles. We investigate how the energetics of and forces
between monopoles and antimonopoles influence their creation rate and dynamics.
We study a single skyrmion line defect in the helical phase using both
micromagnetic simulations and a Ginzburg-Landau analysis. Monopole-antimonople
pairs are created in a thermally activated process, largely controlled by the
(core) energy of the monopole. The force between monopoles and antimonopoles is
linear in distance and described by a string tension. The sign and size of the
string tension determines the stability of the phases and the velocity of the
monopoles.",1402.1721v1
2014-03-20,Spin Dynamics Simulation of the magnetization reversal process in FM/AFM bilayer structures by Anisotropic Heisenberg Model,"We have studied the magnetization reversal process in FM/AFM bilayer
structures through of spin dynamics simulation. It has been observed that the
magnetization behavior is different at each branch of the hysteresis loop as
well as the exchange-bias behavior. On the descending branch a sudden change of
the magnetization is observed while on the ascending branch is observed a bland
change of the magnetization. The occurrence of the asymmetry in the hysteresis
loop and the variation in the exchange-bias is due to anisotropy which is
introduced only in the coupling between ferromagnetic (FM) and
antiferromagnetic (AFM) layers.",1403.5279v1
2014-03-24,Modeling an elastic beam with piezoelectric patches by including magnetic effects,"Models for piezoelectric beams using Euler-Bernoulli small displacement
theory predict the dynamics of slender beams at the low frequency accurately
but are insufficient for beams vibrating at high frequencies or beams with low
length-to-width aspect ratios. A more thorough model that includes the effects
of rotational inertia and shear strain, Mindlin-Timoshenko small displacement
theory, is needed to predict the dynamics more accurately for these cases.
Moreover, existing models ignore the magnetic effects since the magnetic
effects are relatively small. However, it was shown recently \cite{O-M1} that
these effects can substantially change the controllability and stabilizability
properties of even a single piezoelectric beam. In this paper, we use a
variational approach to derive models that include magnetic effects for an
elastic beam with two piezoelectric patches actuated by different voltage
sources. Both Euler-Bernoulli and Mindlin-Timoshenko small displacement
theories are considered. Due to the magnetic effects, the equations are quite
different from the standard equations.",1403.6144v1
2014-04-28,Dynamics of Particles Around a Schwarzschild-like Black Hole in the Presence of Quintessence and Magnetic Field,"We investigate the dynamics of a neutral and a charged particle around a
static and spherically symmetric black hole in the presence of quintessence
matter and external magnetic field. We explore the conditions under which the
particle moving around the black hole could escape to infinity after colliding
with another particle. The innermost stable circular orbit (ISCO) for the
particles are studied in detail. Mainly the dependence of ISCO on dark energy
and on the presence of external magnetic field in the vicinity of black hole is
discussed. By using the Lyapunov exponent, we compare the stabilities of the
orbits of the particles in the presence and absence of dark energy and magnetic
field. The expressions for the center of mass energies of the colliding
particles near the horizon of the black hole are derived. The effective force
on the particles due to dark energy and magnetic field in the vicinity of black
hole is also discussed.",1404.7123v3
2014-07-22,Explaining Jupiter's magnetic field and equatorial jet dynamics,"Spacecraft data reveal a very Earth-like Jovian magnetic field. This is
surprising since numerical simulations have shown that the vastly different
interiors of terrestrial and gas planets can strongly affect the internal
dynamo process. Here we present the first numerical dynamo that manages to
match the structure and strength of the observed magnetic field by embracing
the newest models for Jupiter's interior. Simulated dynamo action primarily
occurs in the deep high electrical conductivity region while zonal flows are
dynamically constrained to a strong equatorial jet in the outer envelope of low
conductivity. Our model reproduces the structure and strength of the observed
global magnetic field and predicts that secondary dynamo action associated to
the equatorial jet produces banded magnetic features likely observable by the
Juno mission. Secular variation in our model scales to about 2000 nT per year
and should also be observable during the one year nominal mission duration.",1407.5940v2
2014-09-09,Magnetization pumping and dynamics in a Dzyaloshinskii-Moriya magnet,"We formulate a phenomenological description of thin ferromagnetic layers with
inversion asymmetry where the single-domain magnetic dynamics experiences
magnon current-induced torques and leads to magnon-motive forces. We first
construct a phenomenological theory based on irreversible thermodynamics,
taking into account the symmetries of the system. Furthermore, we confirm that
these effects originate from Dzyaloshinskii-Moriya interactions from the
analysis based on the stochastic Landau-Lifshitz-Gilbert equation. Our
phenomenological results generalize to a general form of Dzyaloshinskii-Moriya
interactions and to other systems, such as pyrochlore crystals and chiral
magnets. Possible applications include spin current generation, magnetization
reversal and magnonic cooling.",1409.2846v3
2014-11-18,Particle Dynamics Around Weakly Magnetized Reissner-Nordström Black Hole,"Considering the geometry of Reissner-Nordstr\""{o}m (RN) black hole immersed
in magnetic field we have studied the dynamics of neutral and charged
particles. A collision of particles in the inner stable circular orbit is
considered and the conditions for the escape of colliding particles from the
vicinity of black hole are given. The trajectories of the escaping particle are
discussed. Also the velocity required for this escape is calculated. It is
observed that there are more than one stable regions if magnetic field is
present in the accretion disk of black hole so the stability of ISCO increases
in the presence of magnetic field. Effect of magnetic field on the angular
motion of neutral and charged particles is observed graphically.",1411.4811v2
2014-12-23,Morphology and dynamics of solar prominences from 3D MHD simulations,"In this paper we present a numerical study of the time evolution of solar
prominences embedded in sheared magnetic arcades. The prominence is represented
by a density enhancement in a background stratified atmosphere and is connected
to the photosphere through the magnetic field. By solving the ideal
magnetohydrodynamic (MHD) equations in three dimensions we study the dynamics
for a range of parameters representative of real prominences. Depending on the
parameters considered, we find prominences that are suspended above the
photosphere, i.e., detached prominences, but also configurations resembling
curtain or hedgerow prominences whose material continuously connects to the
photosphere. The plasma$-\beta$ is an important parameter that determines the
shape of the structure. In many cases magnetic Rayleigh-Taylor (MRT)
instabilities and oscillatory phenomena develop. Fingers and plumes are
generated, affecting the whole prominence body and producing vertical
structures in an essentially horizontal magnetic field. However, magnetic shear
is able to reduce or even to suppress this instability.",1412.7438v1
2015-01-21,Perturbation Theory for Propagating Magnetic Droplet Solitons,"Droplet solitons are a strongly nonlinear, inherently dynamic structure in
the magnetization of ferromagnets, balancing dispersion (exchange energy) with
focusing nonlinearity (strong perpendicular anisotropy). Large droplet solitons
have the approximate form of a circular domain wall sustained by precession
and, in contrast to single magnetic vortices, are predicted to propagate in an
extended, homogeneous magnetic medium. In this work, multiscale perturbation
theory is utilized to develop an analytical framework for investigating the
impact of additional physical effects on the behavior of a propagating droplet.
After first developing soliton perturbation theory in the general context of
Hamiltonian systems, a number of physical phenomena of current interest are
investigated. These include droplet-droplet and droplet-boundary interactions,
spatial magnetic field inhomogeneities, spin transfer torque induced forcing in
a nanocontact device, and damping. Their combined effects demonstrate the
fundamental mechanisms for a previously observed droplet drift instability and
under what conditions a slowly propagating droplet can be supported by the
nanocontact, important considerations for applications. This framework
emphasizes the particle-like dynamics of the droplet, providing analytically
tractable and practical predictions for modern experimental configurations.",1501.05276v1
2015-02-05,Improved efficiency of heat generation in nonlinear dynamics of magnetic nanoparticles,"The deterministic Landau-Lifshitz-Gilbert equation has been used to
investigate the nonlinear dynamics of magnetization and the specific loss power
in magnetic nanoparticles with uniaxial anisotropy driven by a rotating
magnetic field. We propose a new type of applied field, which is
""simultaneously rotating and alternating"", i.e. the direction of the rotating
external field changes periodically. We show that a more efficient heat
generation by magnetic nanoparticles is possible with this new type of applied
field and we suggest its possible experimental realization in cancer therapy
which requires the enhancement of loss energies.",1502.01619v2
2015-03-09,Magnetization reversal in exchange-spring bilayer system under circularly polarized microwave field,"Microwave assisted magnetization reversal are studied in the bulk bilayer
exchange coupled system. We investigate the nonlinear magnetization reversal
dynamics in a perpendicular exchange spring media using Landau-Lifshitz
equation. In the limit of the infinite thickness of the system, the propagation
field leads the reversal of the system. The reduction of the switching field
and the magnetization profile in the extended system are studied numerically.
The possibility to study the dynamics analytically is discussed and an
approximation where two P-modes are coupled by an interaction field is
presented. The ansatz used for the interaction field is validated by comparison
with the numerical results. This approach is shown to be equivalent to two
exchange coupled macrospins.",1503.02572v1
2015-04-13,A theoretical study of the dynamics of paramagnetic superrotors in external magnetic fields,"We present a detailed theoretical study of oxygen molecules in high
rotational states (molecular superrotors) interacting with an external magnetic
field. The system shows rich dynamics, ranging from a spin-selective splitting
of the angular distribution over molecular alignment to an inversion of the
rotational direction. We find that the observed magneto-rotational effects are
due to a spin-mediated precession of the orbital angular momentum around the
magnetic field. Analytical expressions for the precession frequency in the
limits of weak and strong magnetic fields are derived and used to support the
proposed mechanism. In addition, we provide the procedure for a numerical
treatment of oxygen superrotors in an external magnetic field.",1504.03278v2
2015-06-08,Dynamical Generation of Topological Magnetic Lattices for Ultracold Atoms,"We propose a scheme to dynamically synthesize a space-periodic effective
magnetic field for neutral atoms by time-periodic magnetic field pulses. When
atomic spin adiabatically follows the direction of the effective magnetic
field, an adiabatic scalar potential together with a geometric vector potential
emerges for the atomic center-of-mass motion, due to the Berry phase effect.
While atoms hop between honeycomb lattice sites formed by the minima of the
adiabatic potential, complex Peierls phase factors in the hopping coefficients
are induced by the vector potential, which facilitate a topological Chern
insulator. With further tuning of external parameters, both a topological phase
transition and topological flat bands can be achieved, highlighting realistic
prospects for studying strongly correlated phenomena in this system. Our Letter
presents an alternative pathway towards creating and manipulating topological
states of ultracold atoms by magnetic fields.",1506.02418v2
2015-10-14,Non-local topological magnetoelectric effect by Coulomb interaction at a topological insulator-ferromagnet interface,"The interface between a topological insulator and a ferromagnetic insulator
exhibits an interesting interplay of topological Dirac electrons and magnetism.
As has been shown recently, the breaking of time-reversal invariance by
magnetic order generates a Chern-Simons term in the action, that in turn leads
to a Berry phase and a magnetoelectric effect of topological origin. Here, we
consider the system in the presence of long-range Coulomb interaction between
the Dirac electrons, and find that the magnetoelectric effect of the
fluctuating electric field becomes non-local. We derive a Landau-Lifshitz
equation for the fluctuation-induced magnetization dynamics and the
Euler-Lagrange equation of the Coulomb field by explicit one-loop calculations.
Via the Coulomb interaction, divergences in the in-plane magnetization affect
the magnetization dynamics over large distances in a topologically protected
way.",1510.04285v2
2015-11-11,Photoinduced demagnetization and insulator-to-metal transition in ferromagnetic insulating BaFeO$_3$ thin films,"We studied the electronic and magnetic dynamics of ferromagnetic insulating
BaFeO3 thin films by using pump-probe time-resolved resonant x-ray reflectivity
at the Fe 2p edge. By changing the excitation density, we found two distinctly
different types of demagnetization with a clear threshold behavior. We assigned
the demagnetization change from slow (~ 150 ps) to fast (< 70 ps) to a
transition into a metallic state induced by laser excitation. These results
provide a novel approach for locally tuning magnetic dynamics. In analogy to
heat assisted magnetic recording, metallization can locally tune the
susceptibility for magnetic manipulation, allowing to spatially encode magnetic
information.",1511.03365v1
2015-11-24,Electronic and Magnetic Properties of single Fe atoms on a CuN Surface; Effects of Electron Correlations,"The electronic structure and magnetic properties of a single Fe adatom on a
CuN surface have been studied using density functional theory in the local spin
density approximation (LSDA), the LSDA+U approach and the local density
approximation plus dynamical mean-field theory (LDA+DMFT). The impurity problem
in LDA+DMFT is solved through exact diagonalization and in the Hubbard-I
approximation. The comparison of the one-particle spectral functions obtained
from LSDA, LSDA+U and LDA+DMFT show the importance of dynamical correlations
for the electronic structure of this system. Most importantly, we focused on
the magnetic anisotropy and found that neither LSDA, nor LSDA+U can explain the
measured, high values of the axial and transverse anisotropy parameters.
Instead, the spin excitation energies obtained from our LDA+DMFT approach with
exact diagonalization agree significantly better with experimental data. This
affirms the importance of treating fluctuating magnetic moments through a
realistic many-body treatment when describing this class of nano-magnetic
systems. Moreover, it facilitates insight to the role of the hybridization with
surrounding orbitals.",1511.07909v1
2015-12-14,Formation of a Topological Monopole Lattice and its Dynamics in Three-dimensional Chiral Magnets,"Topologically protected swirl of the magnetic texture known as the Skyrmion
has become ubiqui- tous in both metallic and insulating chiral magnets.
Meanwhile the existence of its three-dimensional analogue, known as the
magnetic monopole, has been suggested by various indirect experimental sig-
natures in MnGe compound. Theoretically, Ginzburg-Landau arguments in favor of
the formation of a three-dimensional crystal of monopoles and anti-monopoles
have been put forward, however no microscopic model Hamiltonian was shown to
support such a phase. Here we present strong numerical evidence from Monte
Carlo simulations for the formation of a rock-salt crystal structure of
monopoles and anti-monopoles in short-period chiral magnets. Real-time
simulation of the spin dynamics suggests there is only one collective mode in
the monopole crystal state in the frequency range of several GHz for the
material parameters of MnGe.",1512.04153v1
2016-01-19,Chimera states and synchronization in magnetically driven SQUID metamaterials,"Superconducting QUantum Interference Device (SQUID) metamaterials are
superconducting artificial media whose function relies both on their geometry
and the extraordinary properties of superconductivity and the Josephson effect.
Recent experiments on one- and two-dimensional radio-frequency (rf) SQUID
metamaterials have revealed their wide-band tuneability, significantly reduced
losses, dynamic multistability, and tunable broadband transparency. The
simplest version of an rf SQUID involves a superconducting ring interrupted by
a Josephson junction; this device is a highly nonlinear resonator with a strong
response to applied magnetic fields. SQUID metamaterials exhibit peculiar
magnetic properties such as negative diamagnetic permeability, predicted both
for the quantum and the classical regime. The applied alternating fields induce
(super)currents in the SQUID rings, which are therefore coupled through
dipole-dipole magnetic forces. This interaction is weak due to its magnetic
nature. However, it couples the SQUIDs non-locally since it falls-off as the
inverse cube of their center-to-center distance.",1601.04843v1
2016-03-15,Temporal upscaling in micro magnetism via heterogeneous multiscale methods,"We consider a multiscale strategy addressing the disparate scales in the
Landau-Lifschitz equations in micro-magnetism. At the microscopic scale, the
dynamics of magnetic moments are driven by a high frequency field. On the
macroscopic scale we are interested in simulating the dynamics of the
magnetisation without fully resolving the microscopic scales.
  The method follows the framework of heterogeneous multiscale methods and it
has two main ingredients: a micro- and a macroscale model. The microscopic
model is assumed to be known exactly whereas the macro model is incomplete as
it lacks effective quantities. The two models use different temporal and
spatial scales and effective parameter values for the macro model are computed
on the fly, allowing for improved efficiency over traditional one-scale
schemes.
  For the analysis, we consider a single spin under a high frequency field and
show that effective quantities can be obtained accurately with step-sizes much
larger than the size of the microscopic scales required to resolve the
microscopic features. Numerical results both for a single magnetic particle as
well as a chain of interacting magnetic particles are given to validate the
theory.",1603.04920v2
2016-07-11,A Method for stability analysis of magnetic bearings : Basic stability criteria,"In this work I outline a general procedure for dynamic modeling and stability
analysis of a magnetic bearing, which is a rotating shaft confined inside a
chamber through electromagnetic forces alone. I consider the simplest type of
self-propelled bearing, namely a permanent magnet synchronous motor and an
induction motor rotor freely suspended inside the corresponding stator, and
having no eccentricity-fedback control algorithm. Writing Euler's equations for
the rotor mechanics and Maxwell's equations for the electromagnetic field leads
to a systematic technique for analysing the dynamics of the complete system.
Physical arguments indicate that that two essential components for rotor
confinement are a spatial gradient in the stator magnetic field and a torque
angle lying in the second quadrant. These predictions are confirmed through the
linear stability analysis. The direct practical utility of the results is
mitigated by the presence of a repeated eigenvalue in the linearized equations.
Despite this limitation, the analysis presented can act as a good starting
point for more accurate treatments of other magnetic bearing configurations.",1607.07889v1
2016-09-29,Proposal for a Leaky-Integrate-Fire Spiking Neuron based on Magneto-Electric Switching of Ferro-magnets,"The efficiency of the human brain in performing classification tasks has
attracted considerable research interest in brain-inspired neuromorphic
computing. Hardware implementations of a neuromorphic system aims to mimic the
computations in the brain through interconnection of neurons and synaptic
weights. A leaky-integrate-fire (LIF) spiking model is widely used to emulate
the dynamics of neuronal action potentials. In this work, we propose a spin
based LIF spiking neuron using the magneto-electric (ME) switching of
ferro-magnets. The voltage across the ME oxide exhibits a typical
leaky-integrate behavior, which in turn switches an underlying ferro-magnet.
Due to the effect of thermal noise, the ferro-magnet exhibits probabilistic
switching dynamics, which is reminiscent of the stochasticity exhibited by
biological neurons. The energy-efficiency of the ME switching mechanism coupled
with the intrinsic non-volatility of ferro-magnets result in lower energy
consumption, when compared to a CMOS LIF neuron. A device to system-level
simulation framework has been developed to investigate the feasibility of the
proposed LIF neuron for a hand-written digit recognition problem",1609.09158v1
2016-11-07,Dynamics of the photosphere along the solar cycle from SDO/HMI,"As the global magnetic field of the Sun has an activity cycle, one expects to
observe some variation of the dynamical properties of the flows visible in the
photosphere. We investigate the flow field during the solar cycle by analysing
SDO/HMI observations of continuum intensity, Doppler velocity and longitudinal
magnetic field. We first picked data at disk center during 6 years along the
solar cycle with a 48-hour time step in order to study the overall evolution of
the continuum intensity and magnetic field. Then we focused on thirty 6-hour
sequences of quiet regions without any remnant of magnetic activity separated
by 6 months, in summer and winter, when disk center latitude B0 is close to
zero. The horizontal velocity was derived from the local correlation tracking
technique over a field of view of 216.4Mm x 216.4Mm located at disk center. Our
measurements at disk center show the stability of the flow properties between
meso- and supergranular scales along the solar cycle. The network magnetic
field, produced locally at disk center independently from large scale dynamo,
together with continuum contrast, vertical and horizontal flows, seem to remain
constant during the solar cycle.",1611.02152v1
2016-12-22,C-field smoothing by trajectory dynamically tracing compensation in rubidium fountain clock,"We present a trajectory dynamically tracing compensation method to smooth the
spatial fluctuation of the static magnetic field (C-field) that provides a
quantization axis in the fountain clock. The C-field coil current is
point-to-point adjusted in accordance to the atoms experienced magnetic field
along the flight trajectory. A homogeneous field with a 0.2 nT uncertainty is
realized compared to 5 nT under the static magnetic field with constant current
during the Ramsey interrogation. The corresponding uncertainty associated with
the second-order Zeeman shift that we calculate is improved by one order of
magnitude. The technique provides an alternative method to improve the magnetic
field uniformity particularly for large-scale equipment that is difficult to
machine with magnetic shielding. Our method is simple, robust, and essentially
important in frequency evaluations concerning the dominant uncertainty
contribution due to the quadratic Zeeman shift.",1612.07469v2
2016-12-29,Theory of electron transport and magnetization dynamics in metallic ferromagnets,"Magnetic electric effects in ferromagnetic metals are discussed from the
viewpoint of effective spin electromagnetic field that couples to conduction
electron spin. The effective field in the adiabatic limit is the spin Berry's
phase in space and time, and it leads to spin motive force (voltage generated
by magnetization dynamics) and topological Hall effect due to spin chirality.
Its gauge coupling to spin current describes the spin transfer effect, where
magnetization structure is driven by an applied spin current. The idea of
effective gauge field can be extended to include spin relaxation and Rashba
spin-orbit interaction. Voltage generation by the inverse Edelstein effect in
junctions is interpreted as due to the electric component of Rashba-induced
spin gauge field. The spin gauge field arising from the Rashba interaction
turns out to coincides with troidal moment, and causes asymmetric light
propagation (directional dichroism) as a result of the Doppler shift. Rashba
conductor without magnetization is shown to be natural metamaterial exhibiting
negative refraction.",1612.09019v1
2017-01-09,"Boundary-driven Lindblad dynamics of random quantum spin chains : strong disorder approach for the relaxation, the steady state and the current","The Lindblad dynamics of the XX quantum chain with large random fields $h_j$
(the couplings $J_j$ can be either uniform or random) is considered for
boundary-magnetization-drivings acting on the two end-spins. Since each
boundary-reservoir tends to impose its own magnetization, we first study the
relaxation spectrum in the presence of a single reservoir as a function of the
system size via some boundary-strong-disorder renormalization approach. The
non-equilibrium-steady-state in the presence of two reservoirs can be then
analyzed from the effective renormalized Linbladians associated to the two
reservoirs. The magnetization is found to follow a step profile, as found
previously in other localized chains. The strong disorder approach allows to
compute explicitly the location of the step of the magnetization profile and
the corresponding magnetization-current for each disordered sample in terms of
the random fields and couplings.",1701.02102v2
2017-02-16,"Magnetization dynamics in magnonic structures with different geometries: interfaces, notches and waveguides","The discovery of the ultrafast magnetization dynamics twenty years ago has
led to a broad variety of experimental techniques to explore phenomena in
magnetic materials with high temporal resolution. In the present article we
present a study dealing with broadband excitation of spin-wave packets at
different magnonic crystal continuous magnetic film interfaces. Similar to
protected conducting states on the surfaces of topological band insulators,
these interfaces exhibit surface spin-wave modes that propagate out of the
crystal into the continuous film. The propagation distance depends on the
direction of the applied magnetic field as well as the surface geometry of the
crystal.",1702.05041v1
2017-04-01,Thermally nucleated magnetic reversal in CoFeB/MgO nanodots,"Power consumption is the main limitation in the development of new high
performance random access memory for portable electronic devices. Magnetic RAM
(MRAM) with CoFeB/MgO based magnetic tunnel junctions (MTJs) is a promising
candidate for reducing the power consumption given its non-volatile nature
while achieveing high performance. The dynamic properties and switching
mechanisms of MTJs are critical to understanding device operation and to enable
scaling of devices below 30 nm in diameter. Here we show that the magnetic
reversal mechanism is incoherent and that the switching is thermally nucleated
at device operating temperatures. Moreover, we find an intrinsic thermal
switching field distribution arising on the sub-nanosecond timescale even in
the absence of size and anisotropy distributions or material defects. These
features represent the characteristic signature of the dynamic properties in
MTJs and give an intrinsic limit to reversal reliability in small magnetic
nanodevices.",1704.00106v1
2017-04-26,Ferrofluid Capillary Rise in Porous Medium under the Action of Non-Uniform Magnetic Field,"The capillary flow of a ferrofluid in a single cylindrical capillary tube and
through a sandy porous medium under the action of a non-uniform magnetic field
is studied experimentally. The dynamics of the capillary rise and the static
case have been considered. It has been shown that the non-uniform magnetic
field with upward directed gradient accelerates the capillary rise; contrary,
the non-uniform magnetic field with downward directed gradient decelerates the
capillary rise. Time dependences of the ferrofluid height and maximum reachable
height of ferrofluid have been analyzed. The method of the study of ferrofluid
capillary rise based on the use of magnetic measurements has been proposed. It
has been demonstrated that porous material parameters can be extracted from the
results of measurements of the inductances of the solenoid with porous medium
inside and the small sensing coil within a single experiment.",1704.08310v1
2017-05-10,Focusing and sorting of ellipsoidal magnetic particles in microchannels,"We present a simple method to control the position of ellipsoidal magnetic
particles in microchannel Poiseuille flow using a static uniform magnetic
field. The magnetic field is utilized to pin the particle orientation, and the
hydrodynamic interactions between ellipsoids and channel walls allow control of
the transverse position of the particles. We employ a far-field hydrodynamic
theory and simulations using the boundary element method and Brownian dynamics
to show how magnetic particles can be focussed and segregated by size and
shape. This is of importance for particle manipulation in lab-on-a-chip
devices.",1705.03665v2
2017-05-26,Static and Dynamic Magnetic Properties of FeMn/Pt Multilayers,"Recently we have demonstrated the presence of spin-orbit toque in FeMn/Pt
multilayers which, in combination with the anisotropy field, is able to rotate
its magnetization consecutively from 0o to 360o without any external field.
Here, we report on an investigation of static and dynamic magnetic properties
of FeMn/Pt multilayers using combined techniques of magnetometry, ferromagnetic
resonance, inverse spin Hall effect and spin Hall magnetoresistance
measurements. The FeMn/Pt multilayer was found to exhibit ferromagnetic
properties, and its temperature dependence of saturation magnetization can be
fitted well using a phenomenological model by including a finite distribution
in Curie temperature due to subtle thickness variations across the multilayer
samples. The non-uniformity in static magnetic properties is also manifested in
the ferromagnetic resonance spectra, which typically exhibit a broad resonance
peak. A damping parameter of around 0.106 is derived from the frequency
dependence of ferromagnetic resonance linewidth, which is comparable to the
reported values for other types of Pt-based multilayers. Clear inverse spin
Hall signals and spin Hall magnetoresistance have been observed in all samples
below the Curie temperature, which corroborate the strong spin-orbit torque
effect observed previously.",1705.09423v1
2017-07-18,Magnetocapillary self-assemblies: locomotion and micromanipulation along a liquid interface,"This paper presents an overview and discussion of magnetocapillary
self-assemblies. New results are presented, in particular concerning the
possible development of future applications. These self-organizing structures
possess the notable ability to move along an interface when powered by an
oscillatory, uniform magnetic field. The system is constructed as follows. Soft
magnetic particles are placed on a liquid interface, and submitted to a
magnetic induction field. An attractive force due to the curvature of the
interface around the particles competes with an interaction between magnetic
dipoles. Ordered structures can spontaneously emerge from these conditions.
Furthermore, time-dependent magnetic fields can produce a wide range of dynamic
behaviours, including non-time-reversible deformation sequences that produce
translational motion at low Reynolds number. In other words, due to a
spontaneous breaking of time-reversal symmetry, the assembly can turn into a
surface microswimmer. Trajectories have been shown to be precisely
controllable. As a consequence, this system offers a way to produce microrobots
able to perform different tasks. This is illustrated in this paper by the
capture, transport and release of a floating cargo, and the controlled mixing
of fluids at low Reynolds number.",1707.05515v1
2017-07-25,Probing $α$-RuCl$_3$ Beyond Magnetic Order: Effects of Temperature and Magnetic Field,"Recent studies have brought $\alpha$-RuCl$_3$ to the forefront of
experimental searches for materials realizing Kitaev spin-liquid physics. This
material exhibits strongly anisotropic exchange interactions afforded by the
spin-orbit coupling of the 4d Ru centers. We investigate the dynamical response
at finite temperature and magnetic field for a realistic model of the magnetic
interactions in $\alpha$-RuCl$_3$. These regimes are thought to host
unconventional paramagnetic states that emerge from the suppression of magnetic
order. Using exact diagonalization calculations of the quantum model
complemented by semi-classical analysis, we find a very rich evolution of the
spin dynamics as the applied field suppresses the zigzag order and stabilizes a
quantum paramagnetic state that is adiabatically connected to the fully
polarized state at high fields. At finite temperature, we observe large
redistributions of spectral weight that can be attributed to the anisotropic
frustration of the model. These results are compared to recent experiments, and
provide a roadmap for further studies of these regimes.",1707.08144v2
2017-09-22,Effects of the temperature and magnetic-field dependent coupling on the properties of QCD matter,"To reflect the asymptotic freedom in the thermal direction, a
temperature-dependent coupling was proposed in the literature. We investigate
its effect on QCD matter with and without strong magnetic fields. Compared with
the fixed coupling constant, the running coupling leads to a drastic change in
the dynamical quark mass, entropy density, sound velocity, and specific heat.
The crossover transition of QCD matter at finite temperature is characterized
by the pseudocritical temperature $T_\mathrm{pc}$, which is generally
determined by the peak of the derivative of the quark condensate with respect
to the temperature $d\phi/dT$, or equivalently, by the derivative of the quark
dynamical mass $d M/dT$. In a strong magnetic field, the temperature- and
magnetic-field-dependent coupling $G(eB,T)$ was recently introduced to account
for inverse magnetic catalysis. We propose an analytical relation between the
two criteria $d\phi/dT$ and $dM/dT$ and show a discrepancy between them in
finding the pseudocritical temperature. The magnitude of the discrepancy
depends on the behavior of $dG/dT$.",1709.07773v1
2017-10-24,Magnetic field in expanding quark-gluon plasma,"Intense electromagnetic fields are created in the quark-gluon plasma by the
external ultra-relativistic valence charges. The time-evolution and the
strength of this field are strongly affected by the electrical conductivity of
the plasma. Yet, it has recently been observed that the effect of the magnetic
field on the plasma flow is small. We compute the effect of plasma flow on
magnetic field and demonstrate that it is less than 10\%. These observations
indicate that the plasma hydrodynamics and the dynamics of electromagnetic
field decouple. Thus, it is a very good approximation, on the one hand, to
study QGP in the background electromagnetic field generated by external sources
and, on the other hand, to investigate the dynamics of magnetic field in the
background plasma. We also argue that the wake induced by the magnetic field in
plasma is negligible.",1710.08793v1
2017-11-15,Oscillatory spin-orbit torque switching induced by field-like torques,"Deterministic magnetization switching using spin-orbit torque (SOT) has
recently emerged as an efficient means to electrically control the magnetic
state of ultrathin magnets. The SOT switching still lacks in oscillatory
switching characteristics over time, therefore, it is limited to bipolar
operation where a change in polarity of the applied current or field is
required for bistable switching. The coherent rotation based oscillatory
switching schemes cannot be applied to SOT because the SOT switching occurs
through expansion of magnetic domains. Here, we experimentally achieve
oscillatory switching in incoherent SOT process by controlling domain wall
dynamics. We find that a large field-like component can dynamically influence
the domain wall chirality which determines the direction of SOT switching.
Consequently, under nanosecond current pulses, the magnetization switches
alternatively between the two stable states. By utilizing this oscillatory
switching behavior we demonstrate a unipolar deterministic SOT switching scheme
by controlling the current pulse duration.",1711.05369v1
2017-11-21,Coupling magneto-elastic Lagrangians to spin transfer torque sources,"The consequences of coupling magnetic and elastic degrees of freedom, where
spins and deformations are carried by point-like objects subject to local
interactions, are studied, theoretically and by detailed numerical simulations.
From the constrained Lagrangians we derive consistent equations of motion for
the coupled dynamical variables. In order to probe the dynamics of such a
system, we consider external perturbations, such as spin transfer torques for
the magnetic part, and homogeneous stresses for the elastic part, associated to
their corresponding damping. This approach is applied to the study of ultrafast
switching processes in anti-ferromagnetic systems, which have recently
attracted attention as candidates for anti-ferromagnetic spintronic devices.
Our strategy is then checked in simple, but instructive, situations. We carried
out numerical experiments to study, in particular, how the magnetostrictive
coupling and external stresses affect the nature of the switching processes in
a prototype anti-ferromagnetic material.",1711.08062v2
2017-12-20,Strong fields and neutral particle magnetic moment dynamics,"Interaction of magnetic moment of point particles with external
electromagnetic fields experiences unresolved theoretical and experimental
discrepancies. In this work we point out several issues within the relativistic
quantum mechanics and the QED and we describe effects related to a new
covariant classical model of magnetic moment dynamics. Using this framework we
explore the invariant acceleration experienced by neutral particles coupled to
an external plane wave field through the magnetic moment: we study the case of
ultra relativistic Dirac neutrinos with magnetic moment in the range of
$10^{-11}$ to $10^{-20}$ $\mu_\mathrm{B}$; and we address the case of slowly
moving neutrons. We explore how critical accelerations for neutrinos can be
experimentally achieved in laser-pulse interactions. The radiation of
accelerated neutrinos can serve as an important test distinguishing between
Majorana and Dirac nature of neutrinos.",1712.07698v2
2018-01-22,Magnetic domain wall creep and depinning: a scalar field model approach,"Magnetic domain wall motion is at the heart of new magneto-electronic
technologies and hence the need for a deeper understanding of domain wall
dynamics in magnetic systems. In this context, numerical simulations using
simple models can capture the main ingredients responsible for the complex
observed domain wall behavior. We present a scalar-field model for the
magnetization dynamics of quasi-two-dimensional systems with a perpendicular
easy axis of magnetization which allows a direct comparison with typical
experimental protocols, used in polar magneto-optical Kerr effect microscopy
experiments. We show that the thermally activated creep and depinning regimes
of domain wall motion can be reached, and the effect of different quenched
disorder implementations can be assessed with the model. In particular, we show
that the depinning field increases with the mean grain size of a Voronoi
tessellation model for the disorder.",1801.07324v1
2018-03-20,All-optical nonequilibrium pathway to stabilizing magnetic Weyl semimetals in pyrochlore iridates,"Nonequilibrium many-body dynamics is becoming one of the central topics of
modern condensed matter physics. Floquet topological states were suggested to
emerge in photodressed band structures in the presence of periodic laser
driving. Here we propose a viable nonequilibrium route without requiring
coherent Floquet states to reach the elusive magnetic Weyl semimetallic phase
in pyrochlore iridates by ultrafast modification of the effective
electron-electron interaction with short laser pulses. Combining \textit{ab
initio} calculations for a time-dependent self-consistent reduced Hubbard $U$
controlled by laser intensity and nonequilibrium magnetism simulations for
quantum quenches, we find dynamically modified magnetic order giving rise to
transiently emerging Weyl cones that are probed by time- and angle-resolved
photoemission spectroscopy. Our work offers a unique and realistic pathway for
nonequilibrium materials engineering beyond Floquet physics to create and
sustain Weyl semimetals. This may lead to ultrafast, tens-of-femtoseconds
switching protocols for light-engineered Berry curvature in combination with
ultrafast magnetism.",1803.07447v2
2018-04-06,Quantum-impurity relaxometry of magnetization dynamics,"Prototypes of quantum impurities (QI), such as NV and SiV centers in diamond,
have been recently growing in popularity due to their minimally invasive and
high-resolution magnetic field sensing. Here, we focus on quantum-impurity
relaxometry as a method to probe collective excitations in magnetic insulators.
We develop a general framework that relates the experimentally-measurable
quantum-impurity relaxation rates to the properties of a magnetic system via
the noise emitted by the latter. We suggest that, when the quantum-impurity
frequency lies within the spin-wave gap, quantum-impurity relaxometry can be
effectively deployed to detect signatures of the coherent spin dynamics, such
as magnon condensation, both in ferromagnetic and antiferromagnetic systems, as
well as open prospects to nonintrusively probe spin-wave transport regimes in
magnetic insulators.",1804.02417v1
2018-04-30,Modular Compact Modeling of Magnetic Tunnel Junction Devices,"This paper describes a robust, modular, and physics- based circuit framework
to model conventional and emerging Magnetic Tunnel Junction (MTJ) devices.
Magnetization dynamics are described by the stochastic Landau-Lifshitz-Gilbert
(sLLG) equation whose results are rigorously benchmarked with a Fokker-Planck
Equation (FPE) description of magnet dynamics. We then show how sLLG is coupled
to transport equations of MTJ-based devices in a unified circuit platform. Step
by step, we illustrate how the physics-based MTJ model can be extended to
include different spintronics phenomena, including spin-transfer-torque (STT),
voltage-control of magnetic anisotropy (VCMA) and spin-orbit torque (SOT)
phenomena by experimentally benchmarked examples. To demonstrate how our
approach can be used in the exploration of novel MTJ-based devices, we also
present a recently proposed MEMS resonator- driven spin-torque nano oscillator
(STNO) that can reduce the phase noise of STNOs. We briefly elaborate on the
use of our framework beyond conventional devices.",1805.00066v2
2018-06-11,Spin dynamics of coupled spin ladders near quantum criticality in Ba2CuTeO6,"We report inelastic neutron scattering measurements of the magnetic
excitations in Ba2CuTeO6, proposed by ab initio calculations to magnetically
realize weakly coupled antiferromagnetic two-leg spin-1/2 ladders. Isolated
ladders are expected to have a singlet ground state protected by a spin gap.
Ba2CuTeO6 orders magnetically, but with a small Neel temperature relative to
the exchange strength, suggesting that the interladder couplings are relatively
small and only just able to stabilize magnetic order, placing Ba2CuTeO6 close
in parameter space to the critical point separating the gapped phase and Neel
order. Through comparison of the observed spin dynamics with linear spin wave
theory and quantum Monte Carlo calculations, we propose values for all relevant
intra- and interladder exchange parameters, which place the system on the
ordered side of the phase diagram in proximity to the critical point. We also
compare high field magnetization data with quantum Monte Carlo predictions for
the proposed model of coupled ladders.",1806.04052v2
2018-06-27,Dynamic interface formation in magnetic thin film heterostructures,"Magnetic thin film heterostructures have been widely studied for fundamental
interests in the emergence of novel phenomena accompanied by the
heterointerface formation as well as their promising practical potential.
Combining x-ray magnetic circular dichroism with scanning tunneling microscopy,
we show for Mn/Fe thin film heterostructures that the interfacial factors
dominating electronic and magnetic properties of the entire system dynamically
change with the amount of the Mn overlayer. Element specific magnetization
curves of the Fe layer exhibit a two-step spin reorientation transition from
out-of-plane to in-plane direction with increasing the Mn coverage.
Atomic-scale characterizations of structural and electronic properties in
combination with the first-principles calculations successfully unravel the
roles of the entangled interfacial factors, and clarify the driving forces of
the transition. The first step of the transition at a low Mn coverage is
dominantly induced by the formation of FeMn disordered alloy at the
heterointerface, and the electronic hybridization with interfacial FeMn ordered
alloy is dominant as the origin of the second step of the transition at a high
Mn coverage.",1806.10543v1
2018-07-02,Picosecond acoustic excitation driven ultrafast magnetization dynamics in dielectric Bi-substituted yttrium iron garnet,"Using femtosecond optical pulses, we have investigated the ultrafast
magnetization dynamics induced in a dielectric film of bismuth-substituted
yttrium iron garnet (Bi-YIG) buried below a thick Cu/Pt metallic bilayer. We
show that exciting the sample from Pt surface launches an acoustic strain pulse
propagating into the garnet film. We discovered that this strain pulse induces
a coherent magnetization precession in the Bi-YIG at the frequency of the
ferromagnetic resonance. The observed phenomena can be explain by
strain-induced changes of magnetocristalline anisotropy via the inverse
magnetostriction effect. These findings open new perspectives toward the
control of the magnetization in magnetic garnets embedded in complex
heterostructure devices.",1807.00610v1
2018-08-16,Dynamic multiferroicity of a ferroelectric quantum critical point,"Quantum matter hosts a large variety of phases, some coexisting, some
competing; when two or more orders occur together, they are often entangled and
cannot be separated. Dynamical multiferroicity, where fluctuations of electric
dipoles lead to magnetisation, is an example where the two orders are
impossible to disentangle. Here we demonstrate elevated magnetic response of a
ferroelectric near the ferroelectric quantum critical point (FE QCP) since
magnetic fluctuations are entangled with ferroelectric fluctuations. We thus
suggest that any ferroelectric quantum critical point is an \textit{inherent}
multiferroic quantum critical point. We calculate the magnetic susceptibility
near the FE QCP and find a region with enhanced magnetic signatures that
appears near the FE QCP, and controlled by the tuning parameter of the
ferroelectric phase. The effect is small but observable - we propose quantum
paraelectric strontium titanate as a candidate material where the magnitude of
the induced magnetic moments can be $\sim 5 \times 10^{-7} \mu_{B}$ per unit
cell near the FE QCP.",1808.05509v2
2018-08-25,Optically driven collective spin excitations and magnetization dynamics in the Néel-type skyrmion host GaV$_4$S$_8$,"GaV$_4$S$_8$ is a multiferroic semiconductor hosting magnetic cycloid (Cyc)
and N\'eel-type skyrmion lattice (SkL) phases with a broad region of thermal
and magnetic stability. Here, we use time-resolved magneto-optical Kerr
spectroscopy and micro-magnetic simulations to demonstrate the coherent
generation of collective spin excitations in the Cyc and SkL phases driven by
an optically-induced modulation of uniaxial anisotropy. Our results shed light
on spin-dynamics in anisotropic materials hosting skyrmions and pave a new
pathway for the optical control of their magnetic order.",1808.08476v1
2018-10-12,High resolution dynamic imaging of the delay- and tilt-free motion of Néel domain walls in perpendicularly magnetized superlattices,"We report on the time-resolved investigation of current- and field-induced
domain wall motion in perpendicularly magnetized microwires exhibiting
asymmetric exchange interaction by means of scanning transmission x-ray
microscopy using a time step of 200 ps. Dynamical domain wall velocities on the
order of 50-100 m s$^{-1}$ were observed. The improvement in the temporal
resolution allowed us to observe the absence of incubation times for the motion
of the domain wall, together with indications for a negligible inertia.
Furthermore, we observed that, for short current and magnetic field pulses, the
magnetic domain walls do not exhibit a tilting during its motion, providing a
mechanism for the fast, tilt-free, motion of magnetic domain walls.",1810.05384v1
2018-10-26,Dynamics of a high-density plasma in a magnetic field,"The paper considers the dynamics of the plasma generated by the
explosive-emission cells of the cathode spot of a vacuum arc in a magnetic
field. It is shown that the expansion of the (high-density) plasma in a
transverse magnetic field may cause asymmetry in the plasma density
distribution at the cathode spot boundary. The asymmetry, in turn, increases
the probability of the ignition of new explosive-emission cells in the region
of a stronger magnetic field in the 'anti-amperian' direction of BxI. The
disturbed plasma density distribution estimated in the MHD approximation is
presented. In addition, the velocity of the 'retrograde' spot motion (ignition
of new explosive-emission cells) in the stronger field region is estimated as a
function of the external magnetic field strength. The velocity estimates (a few
to tens of m/(s T)) are shown to agree with experimental data",1810.11384v2
2018-12-04,Effect of (Cu/Fe)O5 bipyramid size and separation on magnetic and dielectric properties of rare earth layered perovskite LaBaCuFeO5 and LuBaCuFeO5,"We report structural, magnetic and dielectric properties of layered
perovskite materials LnBaCuFeO5 (Ln = La and Lu). LaBaCuFeO5 shows magnetic
cluster glass behavior below 60 K owing to the competing ferromagnetic and
antiferromagnetic exchange interactions. Glassy dynamics of electric dipoles
has also been observed in the vicinity of the magnetic glass transition
temperature. The presence of significant coupling between spin and polar
degrees of freedom results in the multiglass feature in LaBaCuFeO5. The
LuBaCuFeO5 compound undergoes YBaCuFeO5 like commensurate to incommensurate
antiferromagnetic transition at 175 K. Large magnetic irreversibility below 17
K in this compound suggests the presence of strong spin anisotropy. In
addition, in this compound the interaction between the dipoles is not strong
enough, which results in the absence of glassy dynamics of electric dipoles.
The contrasting behavior of two compounds is possibly due to variation in the
ferromagnetic and antiferromagnetic interactions along c-axis, which is the
manifestation of structural modification arising out of the difference in the
ionic radii of La and Lu.",1812.01239v2
2018-12-13,Current-induced nucleation and dynamics of skyrmions in a Co-based Heusler alloy,"We demonstrate room-temperature stabilization of dipolar magnetic skyrmions
with diameters in the range of $100$ nm in a single ultrathin layer of the
Heusler alloy Co$_2$FeAl (CFA) under moderate magnetic fields. Current-induced
skyrmion dynamics in microwires is studied with a scanning Nitrogen-Vacancy
magnetometer operating in the photoluminescence quenching mode. We first
demonstrate skyrmion nucleation by spin-orbit torque and show that its
efficiency can be significantly improved using tilted magnetic fields, an
effect which is not specific to Heusler alloys and could be advantageous for
future skyrmion-based devices. We then show that current-induced skyrmion
motion remains limited by strong pinning effects, even though CFA is a magnetic
material with a low magnetic damping parameter.",1812.05345v1
2019-01-17,Trace formula for the magnetic Laplacian,"The Guillemin-Uribe trace formula is a semiclassical version of the Selberg
trace formula and more general Duistermaat-Guillemin formula for elliptic
operators on compact manifolds, which reflects the dynamics of magnetic
geodesic flows in terms of eigenvalues of a natural differential operator (the
magnetic Laplacian) associated with the magnetic field. In this paper, we give
a survey of basic notions and results related with the Guillemin-Uribe trace
formula and provide concrete examples of its computation for two-dimensional
constant curvature surfaces with constant magnetic fields and for the Katok
example.",1901.05699v2
2019-02-24,Magnetic Nanoparticle Relaxation Dynamics-based Magnetic Particle Spectroscopy (MPS) for Rapid and Wash-free Molecular Sensing,"Magnetic nanoparticles (MNPs) have been extensively used as contrasts and
tracers for bioimaging, heating sources for tumor therapy, carriers for
controlled drug delivery, and labels for magnetic immunoassays. Here, we
describe a MNP relaxation dynamics-based magnetic particle spectroscopy (MPS)
method for the quantitative detection of molecular biomarkers. In MPS
measurements, the harmonics of oscillating MNPs are recorded and used as a
metric for the freedom of rotational motion, which indicates the bound states
of the MNPs. These harmonics can be collected from microgram quantities of iron
oxide nanoparticles within 10 seconds. Using a streptavidin-biotin binding
system, we demonstrate the feasibility of using MPS to sense these molecular
interactions, showing this method is able to achieve rapid, wash-free
bioassays, and is suitable for future point-of-care (POC), sensitive, and
versatile diagnosis.",1902.08867v2
2019-07-03,Anisotropic string tensions and inversely magnetic catalyzed deconfinement from a dynamical AdS/QCD model,"We extend previous work on dynamical AdS/QCD models by introducing an extra
ingredient under the form of a background magnetic field, this to gain insight
into the influence such field can have on crucial QCD observables. Therefore,
we construct a closed form analytic solution to an Einstein-Maxwell-dilaton
system with a magnetic field. We specifically focus on the deconfinement
transition, reporting inverse magnetic catalysis, and on the string tension,
reporting a weaker/stronger confinement along/perpendicular to the magnetic
field. The latter, being of importance to potential modelling of heavy
quarkonia, is in qualitative agreement with lattice findings.",1907.01852v2
2019-09-02,Spontaneous order in ensembles of rotating magnetic droplets,"Ensembles of elongated magnetic droplets in a rotating field are studied
experimentally. In a given range of field strength and frequency the droplets
form rotating structures with a triangular order - rotating crystals. A model
is developed to describe ensembles of several droplets, taking into account the
hydrodynamic interactions between the rotating droplets in the presence of a
solid wall below the rotating ensemble. A good agreement with the
experimentally observed periodic dynamics for an ensemble of four droplets is
obtained. During the rotation, the tips of the elongated magnetic droplets
approach close to one another. An expression is derived that gives the magnetic
interaction between such droplets by taking into account the coulombian forces
between magnetic charges on the droplet tips.",1909.00661v1
2019-10-09,Multi-accretion events from corotating and counterrotating SMBHs tori,"Ringed accretion disks (RADs) are aggregates of corotating and
counterrotating toroidal accretion disks orbiting a central Kerr super-massive
Black Hole (SMBH) in AGNs. The dimensionless spin of the central BH and the
fluids relative rotation are proved to strongly affect the RAD dynamics. There
is evidence of a strict correlation between SMBH spin, fluid rotation and
magnetic fields in RADs formation and evolution. Recently, the model was
extended to consider RADs constituted by several magnetized accretion tori and
the effects of a toroidal magnetic field in RAD dynamics have been
investigated. The analysis poses constraints on tori formation and emergence of
RADs instabilities in the phases of accretion onto the central attractor and
tori collision emergence. Magnetic fields and fluids rotation are proved to be
strongly constrained and influence tori formation and evolution in RADs, in
dependence on the toroidal magnetic fields parameters. Eventually, the RAD
frame investigation constraints specific classes of tori that could be observed
around some specific SMBHs identified by their dimensionless spin",1910.03925v1
2019-12-15,A semi-analytical approach to calculating the dynamic modes of magnetic vortices with Dzyaloshinskii-Moriya interactions,"Here we introduce a Landau-Lifshitz based diagonalization (LLD) method, and
use this approach to calculate the effects of the interfacial Dzyaloshinskii
Moriya interactions (DMI) on the radial-type spin wave modes of magnetic
vortices in circular disks. The LLD method is a semi-analytical approach that
involves the diagonalization of the magnetostatic kernel, exchange, and DMI
contributions to extract the system eigenfrequencies and eigenmodes. The
magnetic vortex state provides a convenient model system in which to
investigate the effects of the DMI on the dynamics of a magnetic structures
with confined geometries. Our calculations show that the DMI leads to shifts of
the mode frequencies that are similar in magnitude to what is observed for spin
waves of a comparable wavelength in extended films. However, unlike what is
found in thin films, only the down-shifted modes are observed in the disks, and
these corresponds to modes that propagate either radially outward or inward,
depending on the vortex circulation. The semi-analytical calculations agree
well with full micromagnetic simulations. This technique also applies to other
systems with cylindrical symmetry, for example, magnetic skyrmions.",1912.06963v1
2020-03-03,Optical and magnetic excitations in the underscreened quasi-quartet Kondo lattice,"The underscreened Kondo lattice consisting of a single twofold degenerate
conduction band and a CEF split 4f-electron quasi-quartet has non-conventional
quasiparticle dispersions obtained from the constrained mean-field theory. An
additional genuinely heavy band is found in the main hybridization band gap of
the upper and lower hybridzed bands whose heavy effective mass is controlled by
the CEF splitting. Its presence should profoundly influence the dynamical
optical and magnetic response functions. In the former, the onset of the
optical conductivity is not the main hybridisation energy but the much lower
Kondo energy scale which appears in the direct transitions to the additional
heavy band. The dynamical magnetic response is also strongly modified by the
in-gap heavy band which can lead to unconventional resonant excitations that
may be interpreted as coherent CEF-Kondo lattice magnetic exciton bands. Their
instability at low temperature signifies the onset of induced excitonic
magnetism in the underscreened Kondo lattice.",2003.01535v2
2020-03-10,Wide-bandwidth atomic magnetometry via instantaneous-phase retrieval,"We develop and demonstrate a new protocol that allows sensing of magnetic
fields in an extra-ordinary regime for atomic magnetometry. Until now, the
demonstrated bandwidth for atomic magnetometry has been constrained to be
slower than the natural precession of atomic spins in a magnetic field---the
Larmor frequency. We demonstrate a new approach that tracks the instantaneous
phase of atomic spins to measure arbitrarily modulated magnetic fields with
frequencies up to fifty times higher than the Larmor frequency. By accessing
this regime, we demonstrate magnetic-field measurements across four decades in
frequency up to 400 kHz, over three orders of magnitude wider than conventional
atomic magnetometers. Furthermore, we demonstrate that our protocol can
linearly detect transient fields 100--fold higher in amplitude than
conventional methods. We highlight the bandwidth and dynamic range of the
technique by measuring a magnetic field with a broad and dynamical spectrum.",2003.04526v1
2020-03-20,Normal forms for strong magnetic systems on surfaces: Trapping regions and rigidity of Zoll systems,"We prove a normal form for strong magnetic fields on a closed, oriented
surface and use it to derive two dynamical results for the associated flow.
First, we show the existence of KAM tori and trapping regions provided a
natural non-resonance condition holds. Second, we prove that the flow cannot be
Zoll unless (i) the Riemannian metric has constant curvature and the magnetic
function is constant, or (ii) the magnetic function vanishes and the metric is
Zoll. We complement the second result by exhibiting an exotic magnetic field on
a flat two-torus yielding a Zoll flow for arbitrarily small rescalings.",2003.09141v4
2020-03-25,Magnon Trap by Chiral Spin Pumping,"Chiral spin pumping is the generation of a unidirectional spin current in
half of ferromagnetic films or conductors by dynamic dipolar stray fields from
close-by nanomagnets. We formulate a general theory of long-range chiral
interactions between magnets mediated by unidirectional traveling waves, e.g.,
spin waves in a magnetic film or microwaves in a waveguide. The traveling waves
emitted by an excited magnet can be perfectly trapped by a second, initially
passive, magnet by a dynamical interference effect. When both magnets are
excited by a uniform microwave, the chiral interaction between them creates a
large imbalance in their magnon numbers.",2003.11581v2
2020-05-10,Charged particle motion around non-singular black holes in conformal gravity in the presence of external magnetic field,"We consider electromagnetic fields and charged particle dynamics around
non-singular black holes in conformal gravity immersed in an external,
asymptotically uniform magnetic field. First, we obtain analytic solutions of
the electromagnetic field equation around rotating non-singular black holes in
conformal gravity. We show that the radial components of the electric and
magnetic fields increase with the increase of the parameters $L$ and $N$ of the
black hole solution. Second, we study the dynamics of charged particles. We
show that the increase of the values of the parameters $L$ and $N$ and of
magnetic field causes a decrease in the radius of the innermost stable circular
orbits (ISCO) and the magnetic coupling parameter can mimic the effect of
conformal gravity giving the same ISCO radius up to $\omega_{\rm B}\leq 0.07$
when $N=3$.",2005.04752v2
2020-05-25,Complex magnetic ordering and associated topological Hall effect in a two-dimensional metallic chiral magnet,"Motivated by recent experiments on the observation of room temperature
skyrmions in a layered heterostructure and subsequent demonstration of
topological Hall effect in the same system, we have studied a minimal model of
itinerant electrons coupled to local moments with competing interactions in an
external magnetic field. Working in the limit of strong magneto-electric
coupling where the fast dynamics of the electrons can be decoupled from the
slow dynamics of the local moments (treated as classical spins), we analyze the
multiple field induced magnetic phases and the associated electronic transport
properties in these regimes. Our results help understand the microscopic origin
of the observed phenomena and further provide crucial insight into
unconventional magneto-transport in metallic chiral magnets.",2005.12307v1
2020-06-03,Generation of Solar Spicules and Subsequent Atmospheric Heating,"Spicules are rapidly evolving fine-scale jets of magnetized plasma in the
solar chromosphere. It remains unclear how these prevalent jets originate from
the solar surface and what role they play in heating the solar atmosphere.
Using the Goode Solar Telescope at the Big Bear Solar Observatory, we observed
spicules emerging within minutes of the appearance of opposite-polarity
magnetic flux around dominant-polarity magnetic field concentrations. Data from
the Solar Dynamics Observatory showed subsequent heating of the adjacent
corona. The dynamic interaction of magnetic fields (likely due to magnetic
reconnection) in the partially ionized lower solar atmosphere appears to
generate these spicules and heat the upper solar atmosphere.",2006.02571v1
2020-09-03,Measurements and modelling of stray magnetic fields and the simulation of their impact on the Compact Linear Collider at 380 GeV,"The Compact Linear Collider (CLIC) targets a nanometre beam size at the
collision point. Realising this beam size requires the generation and transport
of ultra-low emittance beams. Dynamic imperfections can deflect the colliding
beams, leading to a collision with a relative offset. They can also degrade the
emittance of each beam. Both of these effects can significantly impact the
luminosity of CLIC. In this paper, we examine a newly considered dynamic
imperfection: stray magnetic fields. Measurements of stray magnetic fields in
the Large Hadron Collider tunnel are presented and used to develop a
statistical model that can be used to realistically generate stray magnetic
fields in simulations. The model is used in integrated simulations of CLIC at
380GeV including mitigation systems for stray magnetic fields to evaluate their
impact on luminosity.",2009.01524v2
2020-09-27,Local negative magnetic permeability and possibility of observation of breather excitations in magnetic metamaterials,"It is shown that the long-wave dynamics and magnetic properties of
one-dimensional systems constructed of the inductively and capacitively coupled
split-ring resonators are described by the regularized nonlinear dispersive
Klein-Gordon equations. It is found that in such systems a highfrequency
magnetic field excites dynamic solitons on a \glqq pedestal\grqq-stable
breathers, oscillating in anti-phase with respect to the background of uniform
oscillations, which means the existence of regions with a negative magnetic
permeability in the system. If supplemented by a medium with negative
permittivity, such a system forms a \glqq left-handed\grqq metamaterial in
which the regions with the breather excitations are transparent to
electromagnetic radiation. This makes it possible to observe them
experimentally.",2009.12792v1
2020-12-21,Nonlinear dynamics of topological ferromagnetic textures for frequency multiplication,"We propose that the non-linear radio-frequency dynamics and nanoscale size of
topological magnetic structures associated to their well-defined internal modes
advocate for their use as in-materio scalable frequency multipliers for
spintronic systems. Frequency multipliers allow for frequency conversion
between input and output frequencies, and thereby significantly increase the
range of controllably accessible frequencies. In particular, we explore the
excitation of eigenmodes of topological magnetic textures by fractions of the
corresponding eigenfrequencies. We show via micromagnetic simulations that
low-frequency perturbations to the system can efficiently excite bounded modes
with a higher amplitude. For example, we excited the eigenmodes of isolated
ferromagnetic skyrmions by applying half, a third and a quarter of the
corresponding eigenfrequency. We predict that the frequency multiplication via
magnetic structures is a general phenomenon which is independent of the
particular properties of the magnetic texture, and works also for magnetic
vortices, droplets and other topological textures.",2012.11481v2
2021-02-17,Temperature induced collapse of spin dimensionality in magnetic metamaterials,"Spin and spatial dimensionalities are universal concepts, essential for
describing both phase transitions and dynamics in magnetic materials. Lately,
these ideas have been adopted to describe magnetic properties of metamaterials,
as well as to replicate and explore ensembles of mesospins belonging to
different universality classes. Here, we take the next step by investigating
magnetic metamaterials not conforming to the conventional framework of
continuous phase transitions. Instead of a continuous decrease in the moment
with temperature, discrete steps are possible, resulting in a binary transition
in the interactions of the elements. The transition is enabled by nucleation
and annihilation of vortex cores, shifting topological charges between the
interior and the edges of the elements. Consequently, the mesospins can be
viewed as shifting their spin dimensionality, from 2 (XY-like) to 0, (vortices)
at the transition. The results provide insight into how dynamics at different
length-scales couple, which can lead to thermally driven topological
transitions in magnetic metamaterials.",2102.08731v2
2021-02-11,Magnonics in collinear magnetic insulating systems,"In the last decades, collinear magnetic insulating systems have emerged as
promising energy-saving information carriers. Their elementary collective spin
excitations, i.e., magnons, can propagate for long distances bypassing the
Joule heating effects that arise from electron scattering in metal-based
devices. This tutorial article provides an introduction to theoretical and
experimental advances in the study of magnonics in collinear magnetic
insulating systems. We start by outlining the quantum theory of spin waves in
ferromagnetic and antiferromagnetic systems and we discuss their quantum
statistics. We review the phenomenology of spin and heat transport of the
coupled coherent and incoherent spin dynamics and the interplay between
magnetic excitations and lattice degrees of freedom. Finally, we introduce the
reader to the key ingredients of two experimental probes of magnetization
dynamics, spin transport and NV-center relaxometry setups, and discuss
experimental findings relevant to the outlined theory.",2102.08829v1
2021-04-08,Probing the interplay between lattice dynamics and short-range magnetic correlations in CuGeO3 with femtosecond RIXS,"Investigations of magnetically ordered phases on the femtosecond timescale
have provided significant insights into the influence of charge and lattice
degrees of freedom on the magnetic sub-system. However, short-range magnetic
correlations occurring in the absence of long-range order, for example in
spin-frustrated systems, are inaccessible to many ultrafast techniques. Here,
we show how time-resolved resonant inelastic X-ray scattering (trRIXS) is
capable of probing such short-ranged magnetic dynamics in a charge-transfer
insulator through the detection of a Zhang-Rice singlet exciton. Utilizing
trRIXS measurements at the O K-edge, and in combination with model
calculations, we probe the short-range spin-correlations in the frustrated spin
chain material CuGeO3 following photo-excitation, revealing a strong coupling
between the local lattice and spin sub-systems.",2104.03557v1
2021-04-14,Persisting correlation between electrical transport and magnetic dynamics in M-type hexaferrites,"In this work we present frequency-dependent magnetic susceptibility and dc
electric transport properties of three different compositions of hexaferrite
Ba$_{1-x}$Pb$_{x}$Fe$_{12-y}$Al$_{y}$O$_{19}$. We find a correlation between
activation energies of dc electric transport and ac magnetic susceptibility
which persists in the whole researched range of aluminium substitution $x=0$ to
$3.3$. This result is discussed in the context of charged magnetic domain
walls, the pinning of which is determined by charge carriers activated over the
transport gap. Our work points toward a general relaxational mechanism in
ferrimagnetic semiconductors which directly affects dynamic magnetic properties
via electric transport.",2104.06964v2
2021-04-15,Magnetic connections across the chromosphere-corona transition region,"The plasma contributing to emission from the Sun between the cool
chromosphere ($\le 10^4$K) and hot corona ($\ge 10^6$K) has been subjected to
many different interpretations. Here we look at the magnetic structure of this
transition region (TR) plasma, based upon the implications of CLASP2 data of an
active region recently published by Ishikawa et al., and earlier IRIS and SDO
data of quiet regions. Ishikawa et al. found that large areas of sunspot plages
are magnetically unipolar as measured in the cores of \ion{Mg}{2} resonance
lines, formed in the lower transition region under low plasma-$\beta$
conditions. Here we show that IRIS images in the line cores have fibrils which
well aligned with the overlying coronal loop segments seen in the 171 \AA{}
channel of SDO. When the TR emission in active regions arise from plasma
magnetically and thermally connected to the corona, then the line cores can
provide the first credible magnetic boundary conditions for force-free
calculations extended to the corona. We also re-examine IRIS images of dynamic
TR cool loops previously reported as a major contributor to transition region
emission from the quiet Sun. Dynamic cool loops contribute only a small
fraction of the total TR emission from the quiet Sun.",2104.07753v1
2021-05-28,Characterization of the stability and dynamics of a laser-produced plasma expanding across strong magnetic field,"Magnetized laser-produced plasmas are central to many new studies in
laboratory astrophysics, inertial confinement fusion, and industrial
applications. Here we present the results of large-scale, three-dimensional
magneto-hydrodynamic simulations of the dynamics of a laser-produced plasma
expanding into a transverse magnetic field with a strength of tens of Tesla.
The simulations show the plasma is confined by the strong magnetic field into a
slender slab structured by the magnetized Rayleigh-Taylor instability that
develops at the plasma-vacuum interface. We find that by perturbing the initial
velocity of the plume the slab can develop kink-like motion which disrupts its
propagation.",2105.13800v1
2021-07-21,Pinning and gyration dynamics of magnetic vortices revealed by correlative Lorentz and bright-field imaging,"Topological magnetic textures are of great interest in various scientific and
technological fields. To allow for precise control of nanoscale magnetism, it
is of great importance to understand the role of intrinsic defects in the host
material. Here, we use conventional and time-resolved Lorentz microscopy to
study the effect of grain size in polycrystalline permalloy films on the
pinning and gyration orbits of vortex cores inside magnetic nanoislands. To
assess static pinning, we use in-plane magnetic fields to shift the core across
the island while recording its position. This enables us to produce highly
accurate two-dimensional maps of pinning sites. Based on this technique, we can
generate a quantitative map of the pinning potential for the core, which we
identify as being governed by grain boundaries. Furthermore, we investigate the
effects of pinning on the dynamic behavior of the vortex core using
stroboscopic Lorentz microscopy, harnessing a new photoemission source that
accelerates image acquisition by about two orders of magnitude. We find
characteristic changes to the vortex gyration in the form of increased
dissipation and enhanced bistability in samples with larger grains.",2107.10208v1
2021-08-19,Saturation of large-scale dynamo in anisotropically forced turbulence,"Turbulent dynamo theories have faced difficulties in obtaining evolution of
large-scale magnetic fields on short dynamical time-scales due to the
constraint imposed by magnetic helicity balance. This has critical implications
for understanding the large-scale magnetic field evolution in astrophysical
systems like the Sun, stars and galaxies. Direct numerical simulations (DNS) in
the past with isotropically forced helical turbulence have shown that
large-scale dynamo saturation time-scales are dependent on the magnetic
Reynolds number (Rm). In this work, we have carried out periodic box DNS of
helically forced turbulence leading to a large-scale dynamo with two kinds of
forcing function, an isotropic one based on that used in PENCIL-CODE and an
anisotropic one based on Galloway-Proctor flows. We show that when the
turbulence is forced anisotropically, the nonlinear (saturation) behaviour of
the large-scale dynamo is only weakly dependent on Rm. In fact the magnetic
helicity evolution on small and large scales in the anisotropic case is
distinctly different from that in the isotropic case. This result possibly
holds promise for the alleviation of important issues like catastrophic
quenching.",2108.08740v2
2021-09-07,Inertial spin dynamics in epitaxial cobalt films,"We investigate the spin dynamics driven by terahertz magnetic fields in
epitaxial thin films of cobalt in its three crystalline phases. The terahertz
magnetic field generates a torque on the magnetization which causes it to
precess for about 1 ps, with a sub-picosecond temporal lag from the driving
force. Then, the magnetization undergoes natural damped THz oscillations at a
frequency characteristic of the crystalline phase. We describe the experimental
observations solving the inertial Landau-Lifshitz-Gilbert equation. Using the
results from the relativistic theory of magnetic inertia, we find that the
angular momentum relaxation time $\eta$ is the only material parameter needed
to describe all the experimental evidence. Our experiments suggest a
proportionality between $\eta$ and the strength of the magneto-crystalline
anisotropy.",2109.03076v2
2021-09-26,Transition state dynamics of a driven magnetic free layer,"Magnetization switching in ferromagnetic structures is an important process
for technical applications such as data storage in spintronics, and therefore
the determination of the corresponding switching rates becomes essential. We
investigate a free-layer system in an oscillating external magnetic field
resulting in an additional torque on the spin. The magnetization dynamics
including inertial damping can be described by the phenomenological Gilbert
equation. The magnetization switching between the two stable orientations on
the sphere then requires the crossing of a potential region characterized by a
moving rank-1 saddle. We adopt and apply recent extensions of transition state
theory for driven systems to compute both the time-dependent and average
switching rates of the activated spin system in the saddle region.",2109.12605v1
2021-10-09,Single-shot imaging of ultrafast all-optical magnetization dynamics with a spatio-temporal resolution,"We present a laboratory system for single-shot magneto-optical (MO) imaging
of ultrafast magnetization dynamics with high-sensitivity of MO rotation. We
create a stack of MO images repeatedly employing a single pair of a pump and
defocused probe pulses to induce and visualize MO changes in the sample. Both
laser beams are independently wavelength-tunable allowing for a flexible,
resonant adjustable two-color pump and probe scheme. To increase the MO
contrast the probe beam is spatially filtered. We performed the all-optical
switching experiment in Co-doped yttrium iron garnet films (YIG:Co) to
demonstrate the capability of the presented method. We determine the
spatial-temporal distribution of the effective field of photo-induced
anisotropy driving the all-optical switching of the magnetization in YIG:Co
film without an external magnetic field. Moreover, using this imaging method,
we tracked the process of the laser-induced magnetization precession.",2110.04506v1
2021-10-21,Dynamics of the multiferroic LiCuVO4 influenced by electric field,"We present the electron spin resonance study of the influence of an electric
field on the low-field multiferroic magnetic state in LiCuVO4. The shift of the
magnetic resonance spectra in the electric field has been observed
experimentally. Symmetry analysis has been conducted in order to describe the
static properties of the magnetic system. The low-frequency dynamics of LiCuVO4
in magnetic and electric fields was considered in the framework of hydrodynamic
approach. It was shown that the application of the external electric field
leads to the change of the configuration of the magnetic system before and
after spin-flop reorientation. Satisfactory agreement was obtained between the
results of experimental studies and theoretical consideration.",2110.11095v1
2021-10-25,Topological defects in superconducting open nanotubes under gradual and abrupt switch-on of the transport current and magnetic field,"We analyze the dynamics of the order parameter in superconducting open
nanotubes under a strong transport current in an external homogeneous magnetic
field using the time-dependent Ginzburg-Landau equation. Near the critical
transport current, the dissipation processes are driven by vortex and phase
slip dynamics. The transition between the vortex and phase-slip regimes is
found to depend on the external magnetic field only weakly if the magnetic
field and/or the transport current are switched on gradually. In the case of an
abrupt switch-on of the magnetic field or transport current, the system can be
triggered to the stable phase-slip regime, within a certain window of
parameters. Finally, a hysteresis effect in the current-voltage characteristics
is predicted in superconducting open nanotubes.",2110.12745v2
2021-12-23,Electric-field-induced parametric excitation of exchange magnons in a CoFeB/MgO junction,"Electric-field controlled magnetization dynamics is an important integrant in
low power spintronic devices. In this letter, we demonstrate electric-field
induced parametric excitation for CoFeB/MgO junctions by using interfacial
in-plane magnetic anisotropy. When the in-plane magnetic anisotropy and the
external magnetic field are parallel to each other, magnons are efficiently
excited by using electric-field induced parametric excitation. Its wavelength
and wavenumber are tuned by changing input power and frequency of the applied
voltage. A generalized phenomenological model is developed to explain the
underlying role of the electric-field torque. Electrical excitation with no
Joule heating offers a good opportunity for developing magnonic devices and
exploring various nonlinear dynamics in magnetic systems.",2112.12308v2
2021-12-23,Nutation spin waves in ferromagnets,"Magnetization dynamics and spin waves in ferromagnets are investigated using
the inertial Landau-Lifshitz-Gilbert equation. Taking inertial magnetization
dynamics into account, dispersion relations describing the propagation of
nutation spin waves in an arbitrary direction relative to the applied magnetic
field are derived via Maxwell's equations. It is found that the inertia of
magnetization causes the hybridization of electromagnetic waves and nutation
spin waves in ferromagnets, hybrid nutation spin waves emerge, and the redshift
of frequencies of precession spin waves is initiated, which transforms to
precession-nutation spin waves. These effects depend sharply on the direction
of wave propagation relative to the applied magnetic field. Moreover, the waves
propagating parallel to the applied field are circularly polarized, while the
waves propagating perpendicular to that field are elliptically polarized. The
characteristics of these spin nutation waves are also analyzed.",2112.12503v3
2022-01-25,Complex vortex-antivortex dynamics in the magnetic superconductor EuFe$_{2}$(As$_{0.7}$P$_{0.3}$)$_{2}$,"We report on the investigation of the magnetic superconductor
EuFe$_{2}$(As$_{0.7}$P$_{0.3}$)$_{2}$ based on muon-spin spectroscopy and ac
magnetic susceptibility ($\chi$) measurements. The dependence of the internal
field at the muon site on temperature is indicative of a ferromagnetic ordering
of Eu$^{2+}$ magnetic moments and only the conventional magnon scattering
governs the longitudinal relaxation rate at low temperatures. At the same time,
we observe a rich phenomenology for the imaginary component of the
susceptibility $\chi^{\prime\prime}$ by means of both standard ac
susceptibility and a novel technique based on a microwave coplanar waveguide
resonator. In particular, we detect activated trends for several features in
$\chi^{\prime\prime}$ over frequencies spanning ten orders of magnitude. We
interpret our results in terms of the complex dynamics of vortices and
antivortices influenced by the underlying structure of magnetic domains.",2201.10216v1
2022-02-04,Vector vorticity of skyrmionic texture: an internal degree of freedom tunable by magnetic field,"Different from the skyrmion driven by the Dzyaloshinskii-Moriya interaction
in non-centrosymmetric materials, the skyrmionic texture in centrosymmetric
magnet may possess the extra internal degrees of freedom, which greatly enrich
its morphologies, imply the continuous deformation allowed by topological
protection, and enable flexible tunability and potential functionality. To
describe the internal degree of freedom to full extent, the conventional
integer-valued scalar vorticity is extended into a vector vorticity with
continuous rotation allowed. The further simplified vorticity angle, together
with helicity angle, represents the whole rotation freedom of spin space. The
centrosymmetric magnet with frustration provides a perfect platform for
realizing a controllable manipulation on these internal degrees of freedom,
where the magnetic field can be applied to tune vorticity continuously in both
crystal and isolated forms of skyrmionic texture, and the helicity can be
further controlled by electric field. Moreover, the simulation reveals the
distinctive dynamic effects related to the vorticity modulation, namely, a
straight motion can be generated by rotating magnetic field to tune vorticity,
and the vorticity can also be controlled to modulate the dynamics induced by
the spin polarized current.",2202.02091v1
2022-03-10,Ultrafast optically induced magnetic state transition in 2D antiferromagnets,"Manipulating spin in antiferromagnetic (AFM) materials has great potential in
AFM opto-spintronics. Laser pulses can induce a transient ferromagnetic (FM)
state in AFM metallic systems, but have never been proven in two-dimensional
(2D) AFM semiconductors and related van der Waals (vdW) heterostructures. Here,
using 2D vdW heterostructures of FM MnS2 and AFM MXenes as prototypes, we
investigated optically induced interlayer spin transfer dynamics based on the
real-time time-dependent density functional theory (rt-TDDFT). We observed that
laser pulses induce significant spin injection and the interfacial
atom-mediated spin transfer from MnS2 to Cr2CCl2. In particular, we first
demonstrated the transient FM state in semiconducting AFM/FM heterostructures
during photoexcited processes. Because the proximity magnetism breaks the
magnetic symmetry of Cr2CCl2 in heterostructures. Our results provide the
microscopic understanding for optically controlled interlayer spin dynamics in
2D magnetic heterostructures and open a new way to manipulate magnetic orders
in ultrafast opto-spintronics.",2203.05124v1
2022-04-21,Temperature-dependent magnetism in Fe foams via spin-lattice dynamics,"Spin-lattice dynamics is used to study the magnetic properties of Fe foams.
The temperature dependence of the magnetization in foams is determined as a
function of the fraction of surface atoms in foams, nsurf. The Curie
temperature of foams decreases approximately linearly with nsurf, while the
critical exponent of the magnetization increases considerably more strongly. If
the data are plotted as a function of the fraction of surface atoms, reasonable
agreement with recent data on vacancy-filled Fe crystals and novel data on
void-filled crystals is observed for the critical . Critical temperature and
critical exponent also depend on the coordination of surface atoms. Although
the decrease we find is relatively small, it hints to the possibility of
improved usage of topology to taylor magnetic properties.",2204.10123v1
2022-07-03,Continuous-stage symplectic adapted exponential methods for charged-particle dynamics with arbitrary electromagnetic fields,"This paper is devoted to the numerical symplectic approximation of the
charged-particle dynamics (CPD) with arbitrary electromagnetic fields. By
utilizing continuous-stage methods and exponential integrators, a general class
of symplectic methods is formulated for CPD under a homogeneous magnetic field.
Based on the derived symplectic conditions, two practical symplectic methods up
to order four are constructed where the error estimates show that the proposed
second order scheme has a uniform accuracy in the position w.r.t. the strength
of the magnetic field. Moreover, the symplectic methods are extended to CPD
under non-homogeneous magnetic fields and three algorithms are formulated.
Rigorous error estimates are investigated for the proposed methods and one
method is proved to have a uniform accuracy in the position w.r.t. the strength
of the magnetic field. Numerical experiments are provided for CPD under
homogeneous and non-homogeneous magnetic fields, and the numerical results
support the theoretical analysis and demonstrate the remarkable numerical
behavior of our methods.",2207.00970v1
2022-07-18,Magnetoelectric effects in Josephson junctions,"The review is devoted to the fundamental aspects and characteristic features
of the magnetoelectric effects, reported in the literature on Josephson
junctions (JJs). The main focus of the review is on the manifestations of the
direct and inverse magnetoelectric effects in various types of Josephson
systems. They provide a coupling of the magnetization in
superconductor/ferromagnet/superconductor JJs to the Josephson current. The
direct magnetoelectric effect is a driving force of spin torques acting on the
ferromagnet inside the JJ. Therefore it is of key importance for the electrical
control of the magnetization. The inverse magnetoelectric effect accounts for
the back action of the magnetization dynamics on the Josephson subsystem, in
particular, making the JJ to be in the resistive state in the presence of the
magnetization dynamics of any origin. The perspectives of the coupling of the
magnetization in JJs with ferromagnetic interlayers to the Josephson current
via the magnetoelectric effects are discussed.",2207.08876v1
2022-07-21,Modeling the Photometric Variability of Alpha$^2$ CVn with a Dynamical Magnetosphere,"Alpha$^2$ Canum Venaticorum (AM CVn) is a strongly magnetic star with
peculiar chemical signatures and periodic variability that have been long
attributed to the diffusion of magnetic elements through the photosphere,
leading to chemical spots across the stellar surface. However, recent studies
of other magnetic hot stars are consistent with magnetospheric clouds above the
surface. Here we take a renewed approach to modeling AM CVn with a simplified
dynamical magnetosphere (DM) and a tilted, offset magnetic dipole to reproduce
its Transiting Exoplanet Survey Satellite (TESS) variability. Our dipole model
also reproduces well the magnetic surface map of AM CVn from Silvester,
Kochukhov, & Wade (2014). Its ultraviolet variability, from IUE archival
spectra, is also consistent with traditional reddening models. We further
discuss the implications of a magnetosphere on other observable quantities from
the system to conclude that it is unlikely to be present in AM CVn.",2207.11151v1
2022-08-20,Unravelling the contributions to spin-lattice relaxation in Kramers single-molecule magnets,"The study of how spin interacts with lattice vibrations and relaxes to
equilibrium provides unique insights on its chemical environment and the
relation between electronic structure and molecular composition. Despite its
importance for several disciplines, ranging from magnetic resonance to quantum
technologies, a convincing interpretation of spin dynamics in crystals of
magnetic molecules is still lacking due to the challenging experimental
determination of the correct spin relaxation mechanism. We apply ab initio spin
dynamics to a series of twelve coordination complexes of Co(II) and Dy(III)
ions selected among $\sim$240 compounds that largely cover the literature on
single-molecule magnets and well represent different regimes of spin
relaxation. Simulations reveal that the Orbach spin relaxation rate of known
compounds mostly depends on the ions' zero-field splitting and little on the
details of molecular vibrations. Raman relaxation is instead found to be also
significantly affected by the features of low-energy phonons. These results
provide a complete understanding of the factors limiting spin lifetime in
single-molecule magnets and revisit years of experimental investigations by
making it possible to transparently distinguish Orbach and Raman relaxation
mechanisms.",2208.09674v1
2022-09-07,Convergence analysis of an implicit finite difference method for the inertial Landau-Lifshitz-Gilbert equation,"The Landau-Lifshitz-Gilbert (LLG) equation is a widely used model for fast
magnetization dynamics in ferromagnetic materials. Recently, the inertial LLG
equation, which contains an inertial term, has been proposed to capture the
ultra-fast magnetization dynamics at the sub-picosecond timescale.
Mathematically, this generalized model contains the first temporal derivative
and a newly introduced second temporal derivative of magnetization.
Consequently, it produces extra difficulties in numerical analysis due to the
mixed hyperbolic-parabolic type of this equation with degeneracy. In this work,
we propose an implicit finite difference scheme based on the central difference
in both time and space. A fixed point iteration method is applied to solve the
implicit nonlinear system. With the help of a second order accurate constructed
solution, we provide a convergence analysis in $H^1$ for this numerical scheme,
in the $\ell^\infty (0, T; H_h^1)$ norm. It is shown that the proposed method
is second order accurate in both time and space, with unconditional stability
and a natural preservation of the magnetization length. In the hyperbolic
regime, significant damping wave behaviors of magnetization at a shorter
timescale are observed through numerical simulations.",2209.02914v2
2022-09-13,Inter-Correlation Between Sunspot Oscillations and Their Internal Structures,"Three- and five-minute oscillations are commonly found in any sunspot. As
they are modulated by the internal thermal and magnetic structures of a
sunspot, therehence, they could be used as an effective tool for sunspot
seismology. In this paper, we investigate the properties of oscillations in
sunspot groups with varying size and magnetic field, and aim to establish the
relationships between sunspot oscillations and its internal structure
comparatively. We selected three groups of unipolar sunspot with approximately
axial-symmetric magnetic field and calculated their Fourier spectra based on
the Ultraviolet(UV)/Extreme ultraviolet(EUV) emission intensity variations
recorded by the Solar Dynamics Observatory/Atmospheric Imaging Assembly
(SDO/AIA). We found that the distribution of three minute oscillation is
defined by the joint effect of diverging magnetic field and the stratification
of sunspot atmosphere. Its distribution could be modified by any invading
magnetic structures in the umbra. Whereas the five minute oscillations are more
prominent in small spots, it implies that five minute oscillation is very
closely connected with umbral dynamics.",2209.05982v1
2022-09-28,Analytic characterization of sub-Alfvénic turbulence energetics,"Magnetohydrodynamic (MHD) turbulence is a cross-field process relevant to
many systems. A prerequisite for understanding these systems is to constrain
the role of MHD turbulence, and in particular the energy exchange between
kinetic and magnetic forms. The energetics of strongly magnetized and
compressible turbulence has so far resisted attempts to understand them.
Numerical simulations reveal that kinetic energy can be orders of magnitude
larger than fluctuating magnetic energy. We solve this lack-of-balance puzzle
by calculating the energetics of compressible and sub-Alfv\'enic turbulence
based on the dynamics of coherent cylindrical fluid parcels. Using the MHD
Lagrangian, we prove analytically that the bulk of the magnetic energy
transferred to kinetic is the energy stored in the coupling between the ordered
and fluctuating magnetic field. The analytical relations are in striking
agreement with numerical data, up to second order terms.",2209.14143v5
2023-02-22,Reversal of coupled vortices in advanced spintronics: A mechanistic study,"This study conducts a comprehensive investigation into the reversal mechanism
of magnetic vortex cores in a nanopillar system composed of two coupled
ferromagnetic dots under zero magnetic field conditions. The research employs a
combination of experimental and simulation methods to gain a deeper
understanding of the dynamics of magnetic vortex cores. The findings reveal
that by applying a constant direct current, the orientation of the vortex cores
can be manipulated, resulting in a switch in one of the dots at a specific
current value. The micromagnetic simulations provide evidence that this switch
is a consequence of a deformation in the vortex profile caused by the
increasing velocity of the vortex cores resulting from the constant amplitude
of the trajectory as frequency increases. These findings offer valuable new
insights into the coupled dynamics of magnetic vortex cores and demonstrate the
feasibility of manipulating their orientation using direct currents under zero
magnetic field conditions. The results of this study have potential
implications for the development of vortex-based non-volatile memory
technologies. \end{abstract}",2302.11616v1
2023-04-18,Guidance of the resonance energy flow in the mechanism of coupled magnetic pendulums,"This paper presents a methodology of controlling the resonance energy
exchange in mechanical system consisting of two weakly coupled magnetic
pendulums interacting with the magnetic field generated by coils placed
underneath. It is shown that properly guided magnetic fields can effectively
change mechanical potentials in a way that the energy flow between the
oscillators takes the desired direction. Studies were considered by using a
specific set of descriptive functions characterizing the total excitation
level, its distribution between the pendulums, and the phase shift. The
developed control strategies are based on the observation that, in the case of
antiphase oscillation, the energy is moving from the pendulum subjected to the
repelling magnetic field, to the oscillator under the attracting field. In
contrast, during the inphase oscillations, the energy flow is reversed.
Therefore, closed-loop controller requires only the information about phase
shift, which is easily estimated from dynamic state signals through the
coherency index. Advantage of suggested control strategy is that the temporal
rate of inputs is dictated by the speed of beating, which is relatively slow
compared to the carrying oscillations.",2304.09755v1
2023-05-23,Non-equilibrium dynamics of dipole-charged fields in the Proca theory,"We discuss the dynamics of field configurations encoded in the certain class
of electric (magnetic) dipole-charged states in the Proca theory of the real
massive vector field. We construct such states so as to ensure that the long
distance structure of the mean electromagnetic field in them is initially set
by the formula describing the electromagnetic field of the electric (magnetic)
dipole. We analyze then how such a mean electromagnetic field evolves in time.
We find that far away from the center of the initial field configuration, the
long range component of the mean electromagnetic field harmonically oscillates,
which leads to the phenomenon of the periodic oscillations of the electric
(magnetic) dipole moment. We also find that near the center of the initial
field configuration, the mean electromagnetic field escapes from its initial
arrangement and a spherical shock wave propagating with the speed of light
appears in the studied system. A curious configuration of the axisymmetric mean
electric field is found to accompany the mean magnetic field in magnetic
dipole-charged states.",2305.13908v1
2023-06-14,Propagators for molecular dynamics in a magnetic field,"Ab initio molecular dynamics in a magnetic field requires solving equations
of motion with velocity-dependent forces -- namely, the Lorentz force arising
from the nuclear charges moving in a magnetic field and the Berry force arising
from the shielding of these charges from the magnetic field by the surrounding
electrons. In this work, we revisit two existing propagators for these
equations of motion, the auxiliary-coordinates-and-momenta (ACM) propagator and
the Tajima propagator (TAJ), and compare them with a new exponential (EXP)
propagator based on the Magnus expansion. Additionally, we explore limits (for
example, the zero-shielding limit), the implementation of higher-order
integration schemes, and series truncation to reduce computational cost by
carrying out simulations of a HeH$^+$ model system for a wide range of field
strengths. While being as efficient as the TAJ propagator, the EXP propagator
is the only propagator that converges to both the schemes of Spreiter and
Walter (derived for systems without shielding of the Lorentz force) and to the
exact cyclotronic motion of a charged particle. Since it also performs best in
our model simulations, we conclude that the EXP propagator is the recommended
propagator for molecules in magnetic fields.",2306.08332v1
2023-06-26,Repulsively bound magnon excitations of a spin-1/2 XXZ chain in a staggered transverse field,"We study the excitation spectrum of the one-dimensional spin-1/2 XXZ chain
with antiferromagnetic Ising anisotropy across a magnetic quantum phase
transition induced by the application of a site-dependent transverse magnetic
field. Motivated by the chain antiferromagnet BaCo$_2$V$_2$O$_8$, we consider a
situation where the transverse magnetic field has a strong uniform component
and a weaker staggered part. To determine the nature of the excitations giving
rise to the spin dynamical structure factor, we use a combination of analytical
approaches and the numerically exact time-dependent matrix product state
method. We identify below the quantum phase transition high-energy many-body
two-magnon and three-magnon repulsively bound states which are clearly visible
due to the staggered component of the magnetic field. At high magnetic fields
and low temperature, single magnons dominate the dynamics. Our theory results
are in very good agreement with terahertz spectroscopy experimental results
presented in [Wang et al., in preparation].",2306.14742v2
2023-07-10,Angular dependence of spin-orbit torque in monolayer $Fe_3GeTe_2$,"In ferromagnetic systems lacking inversion symmetry, an applied electric
field can control the ferromagnetic order parameters through the spin-orbit
torque. The prototypical example is a bilayer heterostructure composed of a
ferromagnet and a heavy metal that acts as a spin current source. In addition
to such bilayers, spin-orbit coupling can mediate spin-orbit torques in
ferromagnets that lack bulk inversion symmetry. A recently discovered example
is the two-dimensional monolayer ferromagnet $Fe_3GeTe_2$. In this work, we use
first-principles calculations to study the spin-orbit torque and ensuing
magnetic dynamics in this material. By expanding the torque versus
magnetization direction as a series of vector spherical harmonics, we find that
higher order terms (up to $\ell=4$) are significant and play important roles in
the magnetic dynamics. They give rise to deterministic, magnetic field-free
electrical switching of perpendicular magnetization.",2307.04900v2
2023-08-28,Spin-wave spectral analysis in crescent-shaped ferromagnetic nanorods,"The research on the properties of spin waves (SWs) in three-dimensional
nanosystems is an innovative idea in the field of magnonics. Mastering and
understanding the nature of magnetization dynamics and binding of SWs at
surfaces, edges, and in-volume parts of three-dimensional magnetic systems
enables the discovery of new phenomena and suggests new possibilities for their
use in magnonic and spintronic devices. In this work, we use numerical methods
to study the effect of geometry and external magnetic field manipulations on
the localization and dynamics of SWs in crescent-shaped (CS) waveguides. It is
shown that changing the magnetic field direction in these waveguides breaks the
symmetry and affects the localization of eigenmodes with respect to the static
demagnetizing field. This in turn has a direct effect on their frequency.
Furthermore, CS structures were found to be characterized by significant
saturation at certain field orientations, resulting in a cylindrical
magnetization distribution. Thus, we present chirality-based nonreciprocal
dispersion relations for high-frequency SWs, which can be controlled by the
field direction (shape symmetry) and its amplitude (saturation).",2308.14678v1
2023-09-06,Creation and dynamics of spin fluctuations in a noisy magnetic field,"We theoretically and numerically investigate the spin fluctuations induced in
a thermal atomic ensemble by an external fluctuating uniaxial magnetic field,
in the context of a standard spin noise spectroscopy (SNS) experiment. We show
that additional spin noise is excited, which dramatically depends on the
magnetic noise variance and bandwidth, as well as on the power of the probe
light and its polarization direction. We develop an analytical perturbative
model proving that this spin noise first emerges from the residual optical
pumping in the medium, which is then converted into spin fluctuations by the
magnetic noise and eventually detected using SNS. The system studied is a
spin-1 system, which thus shows both Faraday rotation and ellipticity noises
induced by the random magnetic fluctuations. The analytical model gives results
in perfect agreement with the numerical simulations, with potential
applications in future experimental characterization of stray field properties
and their influence on spin dynamics.",2309.02916v1
2023-09-14,Spiking Dynamics in Dual Free Layer Perpendicular Magnetic Tunnel Junctions,"Spintronic devices have recently attracted a lot of attention in the field of
unconventional computing due to their non-volatility for short and long term
memory, non-linear fast response and relatively small footprint. Here we report
how voltage driven magnetization dynamics of dual free layer perpendicular
magnetic tunnel junctions enable to emulate spiking neurons in hardware. The
output spiking rate was controlled by varying the dc bias voltage across the
device. The field-free operation of this two terminal device and its robustness
against an externally applied magnetic field make it a suitable candidate to
mimic neuron response in a dense Neural Network (NN). The small energy
consumption of the device (4-16 pJ/spike) and its scalability are important
benefits for embedded applications. This compact perpendicular magnetic tunnel
junction structure could finally bring spiking neural networks (SNN) to
sub-100nm size elements.",2309.07535v1
2023-10-23,How gravity stabilises instability: the case of magnetic micro-convection,"Finding solutions for better mixing in microfluidics remains an important
challenge, including understanding fundamental aspects of these processes. Here
we investigate the magnetic micro-convection on water and miscible magnetic
fluid interface in a vertical microfluidic chip to understand what is the role
of gravity, as fluids have different densities. Our model is reduced to two
dimensionless quantities - magnetic and gravitational Rayleigh numbers.
Numerical simulation results show that static magnetic field generate rich
dynamics. This is confirmed quantitatively with careful experiments in
initially stagnant fluids. We also show that the length of resulting mixing is
limited by gravity. For this we construct a master curve, exploiting the
measurements of critical field. A three-fluid layer model and linear stability
analysis on its interfaces allows us to explain the limitation mechanism. Our
results can help in the development of instability based micromixers.",2310.15323v1
2024-01-11,Room-temperature Magnetic Thermal Switching by Suppressing Phonon-Magnon Scattering,"Thermal switching materials, whose thermal conductivity can be controlled
externally, show great potential in contemporary thermal management.
Manipulating thermal transport properties through magnetic fields has been
accomplished in materials that exhibit a high magnetoresistance. However, it is
generally understood that the lattice thermal conductivity attributed to
phonons is not significantly impacted by the magnetic fields. In this study, we
experimentally demonstrate the significant impact of phonon-magnon scattering
on the thermal conductivity of the rare-earth metal gadolinium near room
temperature, which can be controlled by a magnetic field to realize thermal
switching. Using first-principles lattice dynamics and spin-lattice dynamics
simulations, we attribute the observed change in phononic thermal conductivity
to field-suppressed phonon-magnon scattering. This research suggests that
phonon-magnon scattering in ferromagnetic materials is crucial for determining
their thermal conductivity, opening the door to innovative
magnetic-field-controlled thermal switching materials.",2401.05607v1
2004-07-27,Relativistic field theories in a magnetic background as noncommutative field theories,"We study the connection of the dynamics in relativistic field theories in a
strong magnetic field with the dynamics of noncommutative field theories
(NCFT). As an example, the Nambu-Jona-Lasinio models in spatial dimensions $d
\geq 2$ are considered. We show that this connection is rather sophisticated.
In fact, the corresponding NCFT are different from the conventional ones
considered in the literature. In particular, the UV/IR mixing is absent in
these theories. The reason of that is an inner structure (i.e., dynamical
form-factors) of neutral composites which plays an important role in providing
consistency of the NCFT. An especially interesting case is that for a magnetic
field configuration with the maximal number of independent nonzero tensor
components. In that case, we show that the NCFT are finite for even $d$ and
their dynamics is quasi-(1+1)-dimensional for odd $d$. For even $d$, the NCFT
describe a confinement dynamics of charged particles. The difference between
the dynamics in strong magnetic backgrounds in field theories and that in
string theories is briefly discussed.",0407219v3
2019-11-07,Charged dust close to outer mean-motion resonances in the heliosphere,"We investigate the dynamics of charged dust close to outer mean-motion
resonances with planet Jupiter. The importance of the interplanetary magnetic
field on the orbital evolution of dust is clearly demonstrated. New dynamical
phenomena are found that do not exist in the classical problem of uncharged
dust. We find changes in the orientation of the orbital planes of dust
particles, an increased amount of chaotic orbital motions, sudden 'jumps' in
the resonant argument, and a decrease in time of temporary capture due to the
Lorentz force. Variations in the orbital planes of dust grain orbits are found
to be related to the angle between the orbital angular momentum and magnetic
axes of the heliospheric field and the rotation rate of the Sun. These
variations are bound using a simplified model derived from the full dynamical
problem using first order averaging theory. It is found that the interplanetary
magnetic field does not affect the capture process, that is still dominated by
the other non-gravitational forces. Our study is based on a dynamical model in
the framework of the inclined circular restricted three-body problem.
Additional forces include solar radiation pressure, solar wind drag, the
Poynting-Robertson effect, and the influence of a Parker spiral type
interplanetary magnetic field model. The analytical estimates are derived on
the basis of Gauss' form of planetary equations of motion. Numerical results
are obtained by simulations of dust grain orbits together with the system of
variational equations. Chaotic regions in phase space are revealed by means of
Fast Lyapunov Chaos Indicators.",1911.02778v1
2020-08-11,"Lattice dynamics and ultrafast energy flow between electrons, spins, and phonons in a 3d ferromagnet","The ultrafast dynamics of magnetic order in a ferromagnet are governed by the
interplay between electronic, magnetic and lattice degrees of freedom. In order
to obtain a microscopic understanding of ultrafast demagnetization, information
on the response of all three subsystems is required. A consistent description
of demagnetization and microscopic energy flow, however, is still missing.
Here, we combine a femtosecond electron diffraction study of the ultrafast
lattice response of nickel to laser excitation with ab initio calculations of
the electron-phonon interaction and energy-conserving atomistic spin dynamics
simulations. Our model is in agreement with the observed lattice dynamics and
previously reported electron and magnetization dynamics. Our approach reveals
that the spin system is the dominating heat sink in the initial few hundreds of
femtoseconds and implies a transient non-thermal state of the spins. Our
results provide a clear picture of the microscopic energy flow between
electronic, magnetic and lattice degrees of freedom on ultrafast timescales and
constitute a foundation for theoretical descriptions of demagnetization that
are consistent with the dynamics of all three subsystems.",2008.04611v2
2021-03-06,Exceptional spectrum and dynamic magnetization,"A macroscopic effect can be induced by a local non-Hermitian term in a
many-body system, when it manifests simultaneously level coalescence of a full
real degeneracy spectrum, leading to exceptional spectrum. In this paper, we
propose a family of systems that support such an intriguing property. It is
generally consisted of two arbitrary identical Hermitian sub-lattices in
association with unidirectional couplings between them. We show exactly that
all single-particle eigenstates coalesce in pairs even only single
unidirectional coupling appears. It means that all possible initial states obey
the exceptional dynamics, resulting in some macroscopic phenomena, which never
appears in a Hermitian system. As an application, we study the dynamic
magnetization induced by complex fields in an itinerant electron system. It
shows that an initial saturated ferromagnetic state at half-filling can be
driven into its opposite state according to the dynamics of high-order
exceptional point. Any Hermitian quench term cannot realize a steady opposite
saturated ferromagnetic state. Numerical simulations for the dynamical
processes of magnetization are performed for several representative situations,
including lattice dimensions, global random and local impurity distributions.
It shows that the dynamic magnetization processes exhibit universal behavior.",2103.04109v2
2021-11-25,The role of q-spin singlet pairs of physical spins in the dynamical properties of the spin-1/2 Heisenberg-Ising XXZ chain,"Dynamical correlation functions contain important physical information on
correlated spin models. Here a dynamical theory suitable suitable to the
isotropic spin-1/2 Heisenberg chain in a longitudinal magnetic field is
extended to anisotropy larger than one . The aim of this paper is the study of
the line shape of the spin dynamical structure factor components +-, -+ and zz
of the spin-1/2 Heisenberg-Ising chain in a longitudinal magnetic field near
their sharp peaks. To reach that goal, the nature of the specific type of
elementary magnetic configurations both unbound and associated with real
Bethe-ansatz rapidities and bound and described by complex Bethe strings is
clarified: They are singlet q-spin pairs of physical spins 1/2. We derive
analytical expressions for the line shapes of dynamical structure factor
components +-, -+ and zz valid in the vicinity of lines of sharp peaks. Those
are mostly located at and just above lower thresholds of continua associated
with both real-rapidity states with only unbound q-spin singlet pairs and
states with real and complex rapidities populated by such unbound pairs and a
single 2-string or a single 3-string. The latter contain 2 and 3 bound q-spin
singlet pairs, respectively. Our results provide physically interesting and
important information on the microscopic processes that determine the dynamical
properties of the non-perturbative spin-1/2 Heisenberg-Ising chain in a
longitudinal magnetic field.",2111.13118v1
2022-03-22,Adiabatic spin dynamics and effective exchange interactions from constrained tight-binding electronic structure theory: Beyond the Heisenberg regime,"We consider an implementation of the adiabatic spin dynamics approach in a
tight-binding description of the electronic structure. The adiabatic
approximation for spin-degrees of freedom assumes that the faster electronic
degrees of freedom are always in a quasi-equilibrium state, which significantly
reduces the numerical complexity in comparison to the full electron dynamics.
Non-collinear magnetic configurations are stabilized by a constraining field,
which allows to directly obtain the effective magnetic field from the negative
of the constraining field. While the dynamics are shown to conserve energy, we
demonstrate that adiabatic spin dynamics does not conserve the total spin
angular momentum when the lengths of the magnetic moments are allowed to
change, which is confirmed by numerical simulations. Furthermore, we develop a
method to extract an effective two-spin exchange interaction from the energy
curvature tensor of non-collinear states, which we calculate at each time step
of the numerical simulations. We demonstrate the effect of non-collinearity on
this effective exchange and limitations due to multi-spin interactions in
strongly non-collinear configurations beyond the regime where the Heisenberg
model is valid. The relevance of the results are discussed with respect to
experimental pump-probe experiments that follow the ultra-fast dynamics of
magnetism.",2203.11759v3
2022-09-20,Terahertz spin dynamics in rare-earth orthoferrites,"Recent interest in developing fast spintronic devices and laser-controllable
magnetic solids has sparked tremendous experimental and theoretical efforts to
understand and manipulate ultrafast dynamics in materials. Studies of spin
dynamics in the terahertz (THz) frequency range are particularly important for
elucidating microscopic pathways toward novel device functionalities. Here, we
review THz phenomena related to spin dynamics in rare-earth orthoferrites, a
class of materials promising for antiferromagnetic spintronics. We expand this
topic into a description of four key elements. (1) We start by describing THz
spectroscopy of spin excitations for probing magnetic phase transitions in
thermal equilibrium. While acoustic magnons are useful indicators of spin
reorientation transitions, electromagnons that arise from dynamic
magnetoelectric couplings serve as a signature of inversion-symmetry-breaking
phases at low temperatures. (2) We then review the strong laser driving
scenario, where the system is excited far from equilibrium and thereby subject
to modifications to the free energy landscape. Microscopic pathways for
ultrafast laser manipulation of magnetic order are discussed. (3) Furthermore,
we review a variety of protocols to manipulate coherent THz magnons in time and
space, which are useful capabilities for antiferromagnetic spintronic
applications. (4) Finally, new insights on the connection between dynamic
magnetic coupling in condensed matter and the Dicke superradiant phase
transition in quantum optics are provided. By presenting a review on an array
of THz spin phenomena occurring in a single class of materials, we hope to
trigger interdisciplinary efforts that actively seek connections between
subfields of spintronics, which will facilitate the invention of new protocols
of active spin control and quantum phase engineering.",2209.09468v1
2022-12-30,Selection rules for ultrafast laser excitation and detection of spin correlations dynamics in a cubic antiferromagnet,"Exchange interactions determine the correlations between microscopic spins in
magnetic materials. Probing the dynamics of these spin correlations on
ultrashort length and time scales is, however rather challenging, since it
requires simultaneously high spatial and high temporal resolution. Recent
experimental demonstrations of laser-driven two-magnon modes - zone-edge
excitations in antiferromagnets governed by exchange coupling - posed questions
about the microscopic nature of the observed spin dynamics, the mechanism
underlying its excitation, and their macroscopic manifestation enabling
detection. Here, on the basis of a simple microscopic model, we derive the
selection rules for cubic systems that describe the polarization of pump and
probe pulses required to excite and detect dynamics of nearest-neighbor spin
correlations, and can be employed to isolate such dynamics from other magnetic
excitations and magneto-optical effects. We show that laser-driven spin
correlations contribute to optical anisotropy of the antiferromagnet even in
the absence of spin-orbit coupling. In addition, we highlight the role of
subleading anisotropy in the spin system and demonstrate that the dynamics of
the antiferromagnetic order parameter occurs only in next-to-leading order,
determined by the smallness of the magnetic anisotropy as compared to the
isotropic exchange interactions in the system. We expect that our results will
stimulate and support further studies of magnetic correlations on the shortest
length and time scale.",2212.14698v2
2023-05-23,Laminar and transiently disordered dynamics of a magnetic skyrmion pipe flow,"The world is full of fluids that flow. The fluid nature of flowing skyrmionic
quasiparticles is of fundamental physical interest and plays an essential role
in the transport of many skyrmions. Here, we report the laminar and transiently
disordered dynamic behaviors of many magnetic skyrmions flowing in a pipe
channel. The skyrmion flow driven by a uniform current may show a lattice
structural transition. The skyrmion flow driven by a non-uniform current shows
a dynamically varying lattice structure. A large uniform current could result
in the compression of skyrmions toward the channel edge, leading to the
transition of the skyrmion pipe flow into an open-channel flow with a free
surface. Namely, the width of the skyrmion flow could be adjusted by the
driving current. Skyrmions on the free surface may form a single shear layer
adjacent to the main skyrmion flow. In addition, although we focus on the
skyrmion flow dynamics in a clean pipe channel without any pinning or defect
effect, we also show that a variation of magnetic anisotropy in the pipe
channel could lead to more complicated skyrmion flow dynamics and pathlines.
Our results reveal the fluid nature of skyrmionic quasiparticles that may
motivate future research on the complex flow physics of magnetic textures.",2305.13590v2
2002-11-14,The magnetic state of Yb in Kondo-lattice YbNi2B2C,"We report neutron scattering experiments performed to investigate the dynamic
magnetic properties of the Kondo-lattice compound YbNi2B2C. The spectrum of
magnetic excitations is found to be broad, extending up to at least 150 meV,
and contains inelastic peaks centred near 18 meV and 43 meV. At low energies we
observe quasielastic scattering with a width Gamma = 2.1 meV. The results
suggest a Yb3+ ground state with predominantly localized 4f electrons subject
to (i) a crystalline electric field (CEF) potential, and (ii) a Kondo
interaction, which at low temperatures is about an order of magnitude smaller
than the CEF interaction. From an analysis of the dynamic magnetic response we
conclude that the crystalline electric field acting on the Yb ions has a
similar anisotropy to that in other RNi2B2C compounds, but is uniformly
enhanced by almost a factor of 2. The static and dynamic magnetic properties of
YbNi2B2C are found to be reconciled quite well by means of an approximation
scheme to the Anderson impurity model, and this procedure also indicates that
the effective Kondo interaction varies with temperature due to the crystal
field splitting. We discuss the nature of the correlated-electron ground state
of YbNi2B2C based on these and other experimental results, and suggest that
this compound might be close to a quantum critical point on the non-magnetic
side.",0211277v3
2003-03-11,Magnetic artificial atoms based on thin-film ferrite disk particles,"Semiconductor quantum wells can be considered as an example of artificial
atoms. Following the ideas used in the effective-mass theory, one can describe
electron states in the quantum-well structure based on the Schrodinger-like
equation for the envelope function. In recent years, there has been a renewed
interest in high frequency dynamic properties of finite size magnetic
structures. In a series of new publications, confinement phenomena of
high-frequency magnetization dynamics in magnetic particles have been the
subject of much experimental and theoretical attention. Till now, however,
there are no phenomenological models of a ferrite particle with high-frequency
magnetization dynamics that use the effective-mass approximation and the
Schrodinger-like equation to analyze energy eigenstates of a whole
ferrite-particle system, similarly to semiconductor quantum wells.
Magnetostatic (MS) oscillations in ferrite samples have the wavelength much
smaller than the electromagnetic wavelength at the same frequency and, at the
same time, much larger than the exchange-interaction spin wavelength. This
intermediate position between the pure electromagnetic and spin-wave
(exchange-interaction) processes reveals very special behaviors of the
geometrical effects. The confined effects for MS oscillations in normally
magnetized thin-film ferrite disks demonstrate very unique properties of
artificial atomic structures.",0303196v1
2007-11-21,How to fix a broken symmetry: Quantum dynamics of symmetry restoration in a ferromagnetic Bose-Einstein condensate,"We discuss the dynamics of a quantum phase transition in a spin-1
  Bose-Einstein condensate when it is driven from the magnetized
broken-symmetry phase to the unmagnetized ``symmetric'' polar phase. We
determine where the condensate goes out of equilibrium as it approaches the
critical point, and compute the condensate magnetization at the critical point.
This is done within a quantum Kibble-Zurek scheme traditionally employed in the
context of symmetry-breaking quantum phase transitions. Then we study the
influence of the nonequilibrium dynamics near a critical point on the
condensate magnetization. In particular, when the quench stops at the critical
point, nonlinear oscillations of magnetization occur. They are characterized by
a period and an amplitude that are inversely proportional. If we keep driving
the condensate far away from the critical point through the unmagnetized
``symmetric'' polar phase, the amplitude of magnetization oscillations slowly
decreases reaching a non-zero asymptotic value. That process is described by
the equation that can be mapped onto the classical mechanical problem of a
particle moving under the influence of harmonic and ``anti-friction'' forces
whose interplay leads to surprisingly simple fixed-amplitude oscillations. We
obtain several scaling results relating the condensate magnetization to the
quench rate, and verify numerically all analytical predictions.",0711.3431v3
2010-10-22,Spin-Transfer Torque Induced Vortex Dynamics in Fe/Ag/Fe Nanopillars,"We report experimental and analytical work on spin-transfer torque induced
vortex dynamics in metallic nanopillars with in-plane magnetized layers. We
study nanopillars with a diameter of 150 nm, containing two Fe layers with a
thickness of 15 nm and 30 nm respectively, separated by a 6 nm Ag spacer. The
sample geometry is such that it allows for the formation of magnetic vortices
in the Fe disks. As confirmed by micromagnetic simulations, we are able to
prepare states where one magnetic layer is homogeneously magnetized while the
other contains a vortex. We experimentally show that in this configuration
spin-transfer torque can excite vortex dynamics and analyze their dependence on
a magnetic field applied in the sample plane. The center of gyration is
continuously dislocated from the disk center, and the potential changes its
shape with field strength. The latter is reflected in the field dependence of
the excitation frequency. In the second part we propose a novel mechanism for
the excitation of the gyrotropic mode in nanopillars with a perfectly
homogeneously magnetized in-plane polarizing layer. We analytically show that
in this configuration the vortex can absorb energy from the spin-polarized
electric current if the angular spin-transfer efficiency function is
asymmetric. This effect is supported by micromagnetic simulations.",1010.4791v2
2010-12-07,Effects of Magnetic Fields on Photoionised Pillars and Globules,"The effects of initially uniform magnetic fields on the formation and
evolution of dense pillars and cometary globules at the boundaries of H II
regions are investigated using 3D radiation-magnetohydrodynamics simulations.
It is shown, in agreement with previous work, that a strong initial magnetic
field is required to significantly alter the non-magnetised dynamics because
the energy input from photoionisation is so large that it remains the dominant
driver of the dynamics in most situations. Additionally it is found that for
weak and medium field strengths an initially perpendicular field is swept into
alignment with the pillar during its dynamical evolution, matching magnetic
field observations of the `Pillars of Creation' in M16 and also some cometary
globules. A strong perpendicular magnetic field remains in its initial
configuration and also confines the photoevaporation flow into a bar-shaped
dense ionised ribbon which partially shields the ionisation front and would be
readily observable in recombination lines. A simple analytic model is presented
to explain the properties of this bright linear structure. These results show
that magnetic field strengths in star-forming regions can in principle be
significantly constrained by the morphology of structures which form at the
borders of H II regions.",1012.1500v2
2011-03-22,Dipolar Interactions between Iron-Oxide Nanoparticles in Frozen Ferrofluids and Ferronematics,"We present a detailed study of the magnetic behavior of iron-oxide
(gamma-Fe2O3 and Fe3O4) nanoparticles constituents of ferrofluids (FF's) with
average particle sizes <d> = 2.5 and 10 nm. The particles were dispersed in the
frozen liquid carrier (pure FF) and in a frozen lyotropic liquid crystalline
matrix in the nematic phase or ferronematic (FN) (ferrolyomesophase). Both FF
and FN phases displayed superparamagnetic (SPM) behaviour at room temperature,
with blocking temperatures T_B ~ 10 and 100 K for <d> = 2.5 and 10 nm,
respectively. Dynamic ac susceptibility measurements showed a thermally
activated N\'eel-Brown dependence of the blocking temperature with applied
frequency. Our results show that dipolar interactions are small, but
non-negligible, as compared to the single-particle energy barriers from
magnetic anisotropy. From the fit of ac susceptibility we calculated the
effective magnetic anisotropy constant K_{eff} for 2.5 nm maghemite particles.
Although interparticle interactions present in highly diluted samples do not
appreciably modify the dynamic magnetic behavior of isolated particles, the
calculated magnetic anisotropy were abut one order of magnitude larger that the
bulk materials, suggesting the existence of large surface anisotropy. Using the
thermally activated model to fit the dynamic data yielded effective energy
barriers Ea = 3.5x10^{-21} J. From these data, we obtained K_{eff} = 422 kJ/m^3
for the single-particle effective magnetic anisotropy.",1103.4403v1
2011-03-30,Quantum simulation of frustrated magnetism in triangular optical lattices,"Magnetism plays a key role in modern technology as essential building block
of many devices used in daily life. Rich future prospects connected to
spintronics, next generation storage devices or superconductivity make it a
highly dynamical field of research. Despite those ongoing efforts, the
many-body dynamics of complex magnetism is far from being well understood on a
fundamental level. Especially the study of geometrically frustrated
configurations is challenging both theoretically and experimentally. Here we
present the first realization of a large scale quantum simulator for magnetism
including frustration. We use the motional degrees of freedom of atoms to
comprehensively simulate a magnetic system in a triangular lattice. Via a
specific modulation of the optical lattice, we can tune the couplings in
different directions independently, even from ferromagnetic to
antiferromagnetic. A major advantage of our approach is that standard
Bose-Einstein-condensate temperatures are sufficient to observe magnetic
phenomena like N\'eel order and spin frustration. We are able to study a very
rich phase diagram and even to observe spontaneous symmetry breaking caused by
frustration. In addition, the quantum states realized in our spin simulator are
yet unobserved superfluid phases with non-trivial long-range order and
staggered circulating plaquette currents, which break time reversal symmetry.
These findings open the route towards highly debated phases like spin-liquids
and the study of the dynamics of quantum phase transitions.",1103.5944v1
2012-04-16,Non-perturbative Euler-Heisenberg Lagrangian and Paraelectricity in Magnetized Massless QED,"In this paper we calculate the non-perturbative Euler-Heisenberg Lagrangian
for massless QED in a strong magnetic field $H$, where the breaking of the
chiral symmetry is dynamically catalyzed by the external magnetic field via the
formation of an electro-positron condensate. This chiral condensate leads to
the generation of dynamical parameters that have to be found as solutions of
non-perturbative Schwinger-Dyson equations. Since the electron-positron pairing
mechanism leading to the breaking of the chiral symmetry is mainly dominated by
the contributions from the infrared region of momenta much smaller than
$\sqrt{eH}$, the magnetic field introduces a dynamical ultraviolet cutoff in
the theory that also enters in the non-perturbative Euler-Heisenberg action.
Using this action, we show that the system exhibits a significant
paraelectricity in the direction parallel to the magnetic field. The
nonperturbative nature of this effect is reflected in the non-analytic
dependence of the obtained electric susceptibility on the fine-structure
constant. The strong paraelectricity in the field direction is linked to the
orientation of the electric dipole moments of the pairs that form the chiral
condensate. The large electric susceptibility can be used to detect the
realization of the magnetic catalysis of chiral symmetry breaking in physical
systems.",1204.3660v2
2012-09-29,Emergence and Frustration of Magnetic Order with Variable-Range Interactions in a Trapped Ion Quantum Simulator,"Frustration, or the competition between interacting components of a network,
is often responsible for the complexity of many body systems, from social and
neural networks to protein folding and magnetism. In quantum magnetic systems,
frustration arises naturally from competing spin-spin interactions given by the
geometry of the spin lattice or by the presence of long-range antiferromagnetic
couplings. Frustrated magnetism is a hallmark of poorly understood systems such
as quantum spin liquids, spin glasses and spin ices, whose ground states are
massively degenerate and can carry high degrees of quantum entanglement. The
controlled study of frustrated magnetism in materials is hampered by short
dynamical time scales and the presence of impurities, while numerical modeling
is generally intractable when dealing with dynamics beyond N~30 particles.
Alternatively, a quantum simulator can be exploited to directly engineer
prescribed frustrated interactions between controlled quantum systems, and
several small-scale experiments have moved in this direction. In this article,
we perform a quantum simulation of a long-range antiferromagnetic quantum Ising
model with a transverse field, on a crystal of up to N = 16 trapped Yb+ atoms.
We directly control the amount of frustration by continuously tuning the range
of interaction and directly measure spin correlation functions and their
dynamics through spatially-resolved spin detection. We find a pronounced
dependence of the magnetic order on the amount of frustration, and extract
signatures of quantum coherence in the resulting phases.",1210.0142v1
2016-02-29,Universal Dynamic Magnetism in Yb-Pyrochlores with Disparate Ground States,"The ytterbium pyrochlore magnets, Yb2B2O7 (B = Sn, Ti, Ge) are well described
by S_eff = 1/2 quantum spins decorating a network of corner-sharing tetrahedra
and interacting via anisotropic exchange. Structurally, only the non-magnetic
B-site cation, and hence, primarily the lattice parameter, is changing across
the series. Nonetheless, a range of magnetic behaviors are observed: the low
temperature magnetism in Yb2Ti2O7 and Yb2Sn2O7 has ferromagnetic character,
while Yb2Ge2O7 displays an antiferromagnetically ordered Neel state at low
temperatures. While the static properties of the ytterbium pyrochlores are
distinct, inelastic neutron scattering measurements reveal a common character
to their exotic spin dynamics. All three ytterbium pyrochlores show a gapless
continuum of spin excitations, resembling over-damped ferromagnetic spin waves
at low Q. Furthermore, the specific heat of the series also follows a common
form, with a broad, high-temperature anomaly followed by a sharp
low-temperature anomaly at T_C or T_N. The novel spin dynamics we report
correlate strongly with the broad specific heat anomaly only, remaining
unchanged across the sharp anomaly. This result suggests that the primary order
parameter in the ytterbium pyrochlores associated with the sharp anomaly is
""hidden"" and not simple magnetic dipole order.",1602.09011v1
2016-07-29,"A rock-salt type Li-based oxide, Li3Ni2RuO6, exhibiting a chaotic ferrimagnetism with cluster spin-glass dynamics and thermally frozen charge carriers","The area of research to discover new Li containing materials and to
understand their physical properties has been of constant interest due to
applications potential for rechargeable batteries. Here, we present the results
of magnetic investigations on a Li compound, Li3Ni2RuO6, which was believed to
be a ferrimagnet below 80K. While our neutron diffraction (ND) and isothermal
magnetization (M) data support ferrimagnetism, more detailed magnetic studies
establish that this ferrimagnetic phase exhibits some features similar to
spin-glasses. In addition, we find another broad magnetic anomaly around 40-55
K in magnetic susceptibility, attributable to cluster spin-glass phenomenon.
Gradual dominance of cluster spin-glass dynamics with a decrease of temperature
(T) and the apparent spread in freezing temperature suggest that the
ferrimagnetism of this compound is a chaotic one. The absence of a unique
freezing temperature for a crystalline material is interesting. In addition,
pyroelectric current data reveals a feature in the range 40-50 K, attributable
to thermally stimulated depolarization current. We hope this finding motivates
future work to explore whether there is any intriguing correlation of such a
feature with spin-glass dynamics. We attribute these magnetic and electric
dipole anomalies to the crystallographic disorder intrinsic to this compound.",1607.08689v1
2016-08-16,Incommensurate spiral magnetic order on anisotropic triangular lattice: Dynamical mean field study in a spin-rotating frame,"We study the ground-state magnetism of the half-filled Hubbard model on the
anisotropic triangular lattice, where two out of three bonds have hopping $t$
and the third one has $t^\prime$ in a unit triangle. Working in a spin-rotating
frame and using the density matrix renormalization group method as an impurity
solver, we provide a proper description of incommensurate magnetizations at
zero temperature in the framework of the dynamical mean-field theory (DMFT). It
is shown that the incommensurate spiral magnetic order for $t^\prime/t\gtrsim
0.7$ survives the dynamical fluctuations of itinerant electrons in the Hubbard
interaction range from the strong-coupling (localized-spin) limit down to the
insulator-to-metal transition. We also find that the magnetic moment reduction
from the localized-spin limit is pronounced in the vicinity of the transition
between the commensurate N\'eel and incommensurate spiral phases at
$t^\prime/t\sim 0.7$. When the anisotropy parameter $t^\prime/t$ increases from
the N\'eel-to-spiral transition, the magnitude of the magnetic moment
immediately reaches a maximum and then rapidly decreases in the range of larger
$t^\prime/t$ including the isotropic triangular lattice point $t^\prime/t=1$.
This work gives a solid foundation for further extension of the study including
nonlocal correlation effects neglected at the standard DMFT level.",1608.04490v1
2018-08-10,Interactions between Gas Dynamics and Magnetic Fields in the Massive Dense Cores of the DR21 Filament,"We report Submillimeter Array molecular line observations in the 345 GHz band
of five massive dense cores, Cyg-N38, Cyg-N43, Cyg-N48, Cyg-N51, and Cyg-N53 in
the DR21 filament. The molecular line data reveal several dynamical features of
the cores: (1) prominent outflows in all cores seen in the CO and SiO lines,
(2) significant velocity gradients in Cyg-N43 and Cyg-N48 seen in the H13CN and
H13CO+ lines suggesting 0.1-pc-scale rotational motions, and (3) possible
infalls in Cyg-N48 found in the SiO and SO lines. Comparing the molecular line
data and our dust polarization data in Ching et al. (2017), we find that the
gradients of line-of-sight velocities appear to be randomly oriented relative
to the plane-of-sky magnetic fields. Our simulations suggest that this random
alignment implies parallel or random alignment between the velocity gradients
and magnetic fields in the three dimensional space. The linewidths of H13CN
emission are consistently wider than those of H13CO+ emission in the 3''-10''
detectable scales, which can be explained by the existence of ambipolar
diffusion with maximum plane-of-the-sky magnetic field strengths of 1.9 mG and
5.1 mG in Cyg-N38 and Cyg-N48, respectively. Our results suggest that the gas
dynamics may distort the magnetic fields of the cores of into complex
structures and ambipolar diffusion could be important in dissipating the
magnetic energies of the cores.",1808.03459v1
2019-03-07,Maltese Cross anisotropy in Ho0.8Lu0.2B12 antiferromagnetic metal with dynamic charge stripes,"The model strongly correlated electron system Ho0.8Lu0.2B12 which
demonstrates a cooperative Jahn-Teller instability of the boron sub-lattice in
combination with rattling modes of Ho(Lu) ions, dynamic charge stripes and
unusual antiferromagnetic (AF) ground state has been studied in detail at low
temperatures by magnetoresistance, magnetization and heat capacity
measurements. Based on received results it turns out that the angular H-fi-T
magnetic phase diagrams of this non-equilibrium AF metal can be reconstructed
in the form of a Maltese cross. The dramatic AF ground state symmetry lowering
of this dodecaboride with fcc crystal structure can be attributed to the
redistribution of conduction electrons which leave the RKKY oscillations of the
electron spin density to participate in the dynamic charge stripes providing
with extraordinary changes in the indirect exchange interaction between
magnetic moments of Ho3+ ions and resulting in the emergence of a number of
various magnetic phases. It is also shown that the two main contributions to
magnetoresistance in the complex AF phase, the (i) positive linear on magnetic
field and the (ii) negative quadratic component can be separated and analyzed
quantitatively, correspondingly, in terms of charge carrier scattering on spin
density wave (5d) component of the magnetic structure and on local 4f-5d spin
fluctuations of holmium sites.",1903.03000v1
2020-04-06,Effect of Nd and Rh substitution on the spin dynamics of Kondo insulator CeFe2Al10,"The dynamic magnetic properties of the Kondo-insulator state in CeFe2Al10
(spin gap, resonance mode) have been investigated using polarized neutrons on a
single crystal of pure CeFe2Al10. The results indicate that the magnetic
excitations are polarized mainly along the orthorhombic a axis and their
dispersion along the orthorhombic c direction could be determined.
Polycrystalline samples of Nd- and Rh-doped CeFe2Al0 were also studied by the
time-of-flight technique, with the aim of finding out how the low-energy
magnetic excitation spectra change upon isoelectronic substitution of the
rare-earth (Nd) on the magnetic Ce site or electron doping (Rh) on the
transition-element Fe sublattice. The introduction of magnetic Nd impurities
strongly modifies the spin gap in the Ce dynamic magnetic response and causes
the appearance of a quasielastic signal. The crystal-field excitations of Nd,
studied in both LaFe2Al10 and CeFe2Al10, also reveal a significant influence of
f-electron hybridization (largest in the case of Ce) on the crystal-field
potential. As a function of the Rh concentration, a gradual change is observed
from a Kondo-insulator to a metallic Kondo-lattice response, likely reflecting
the decrease in the hybridization energy.",2004.02572v2
2020-05-08,$s$-$d$ model for local and nonlocal spin dynamics in laser-excited magnetic heterostructures,"We discuss a joint microscopic theory for the laser-induced magnetization
dynamics and spin transport in magnetic heterostructures based on the $s$-$d$
interaction. Angular momentum transfer is mediated by scattering of itinerant
$s$ electrons with the localized ($d$ electron) spins. We use the corresponding
rate equations and focus on a spin one-half $d$ electron system, leading to a
simplified analytical expression for the dynamics of the local magnetization
that is coupled to an equation for the non-equilibrium spin accumulation of the
$s$ electrons. We show that this description converges to the microscopic
three-temperature model in the limit of a strong $s$-$d$ coupling. The equation
for the spin accumulation is used to introduce diffusive spin transport. The
presented numerical solutions show that during the laser-induced
demagnetization in a ferromagnetic metal a short-lived spin accumulation is
created that counteracts the demagnetization process. Moreover, the spin
accumulation leads to the generation of a spin current at the interface of a
ferromagnetic and non-magnetic metal. Depending on the specific magnetic
system, both local spin dissipation and interfacial spin transport are able to
enhance the demagnetization rate by providing relaxation channels for the spin
accumulation that is build up during demagnetization in the ferromagnetic
material.",2005.03905v2
2020-05-26,"From magnetic order to quantum disorder: a $μ$SR study of the Zn-barlowite series of $S={\frac{1}{2}}$ kagomé antiferromagnets, Zn$_{x}$Cu$_{4-x}$(OH)$_{6}$FBr","We report a comprehensive muon spectroscopy study of the Zn-barlowite series
of $S={\frac{1}{2}}$ kagom\'e antiferromagnets, Zn$_x$Cu$_{4-x}$(OH)$_{6}$FBr,
for $x=0.00$ to $0.99(1)$. By combining muon spin relaxation and rotation
measurements with state-of-the-art density-functional theory muon-site
calculations, we observe the formation of both $\mu$--F and $\mu$--OH complexes
in Zn-barlowite. From these stopping sites, implanted muon spins reveal the
suppression of long-range magnetic order into a possible quantum spin liquid
state upon increasing concentration of Zn-substitution. In the parent compound
($x=0$), static long-range magnetic order below $T_{\mathsf{N}}=15$ K manifests
itself in the form of spontaneous oscillations in the time-dependent muon
asymmetry signal consistent with the dipolar fields expected from the
calculated muon stopping sites and the previously determined magnetic structure
of barlowite. Meanwhile, in the $x=1.0$ end-member of the series---in which
antiferromagnetic kagom\'e layers of Cu$^{2+}$ $S={\frac{1}{2}}$ moments are
decoupled by diamagnetic Zn$^{2+}$ ions---we observe that dynamic magnetic
moment fluctuations persist down to at least 50 mK, indicative of a quantum
disordered ground state. We demonstrate that this crossover from a static to
dynamic magnetic ground state occurs for compositions of Zn-barlowite with
$x>0.5$, which bears resemblance to dynamical behaviour of the widely studied
Zn-paratacamite series that contains the quantum spin liquid candidate
herbertsmithite.",2005.12615v1
2020-07-08,Solar Vortex Tubes: Vortex Dynamics in the Solar Atmosphere,"In this work, a state-of-the-art vortex detection method, Instantaneous
Vorticity Deviation, is applied to locate three-dimensional vortex tube
boundaries in numerical simulations of solar photospheric magnetoconvection
performed by the MURaM code. We detected three-dimensional vortices distributed
along intergranular regions and displaying coned shapes that extend from the
photosphere to the low chromosphere. Based on a well-defined vortex center and
boundary, we were able to determine averaged radial profiles and thereby
investigate the dynamics across the vortical flows at different height levels.
The solar vortex tubes present nonuniform angular rotational velocity, and, at
all height levels, there are eddy viscosity effects within the vortices, which
slow down the plasma as it moves toward the center. The vortices impact the
magnetic field as they help to intensify the magnetic field at the sinking
points, and in turn, the magnetic field ends up playing an essential role in
the vortex dynamics. The magnetic field was found to be especially important to
the vorticity evolution. On the other hand, it is shown that, in general,
kinematic vortices do not give rise to magnetic vortices unless their
tangential velocities at different height levels are high enough to overcome
the magnetic tension.",2007.04371v1
2020-08-28,Equation of motion and the constraining field in ab initio spin dynamics,"It is generally accepted that the effective magnetic field acting on a
magnetic moment is given by the gradient of the energy with respect to the
magnetization. However, in ab initio spin dynamics within the adiabatic
approximation, the effective field is also known to be exactly the negative of
the constraining field, which acts as a Lagrange multiplier to stabilize an
out-of-equilibrium, non-collinear magnetic configuration. We show that for
Hamiltonians without mean-field parameters both of these fields are exactly
equivalent, while there can be a finite difference for mean-field Hamiltonians.
For density-functional theory (DFT) calculations the constraining field
obtained from the auxiliary Kohn-Sham Hamiltonian is not exactly equivalent to
the DFT energy gradient. This inequality is highly relevant for both ab initio
spin dynamics and the ab initio calculation of exchange constants and effective
magnetic Hamiltonians. We argue that the effective magnetic field and exchange
constants have the highest accuracy in DFT when calculated from the energy
gradient and not from the constraining field.",2008.12547v2
2020-11-13,Cell dynamics simulations of coupled charge and magnetic phase transformation in correlated oxides,"We present a comprehensive numerical study on the kinetics of phase
transition that is characterized by two non-conserved scalar order parameters
coupled by a special linear-quadratic interaction. This particular
Ginzburg-Landau theory has been proposed to describe the coupled charge- and
magnetic transition in nickelates and the collinear stripe phase in cuprates.
The inhomogeneous state of such systems at low temperatures consists of
magnetic domains separated by quasi-metallic domain-walls where the
charge-order is reduced. By performing large-scale cell dynamics simulations,
we find a two-stage phase-ordering process in which a short period of
independent evolution of the two order parameters is followed by a correlated
coarsening process. The long-time growth and coarsening of magnetic domains is
shown to follow the Allen-Cahn power law. We further show that the
nucleation-and-growth dynamics during phase transformation to the ordered
states is well described by the Kolmogorov-Johnson-Mehl-Avrami theory in two
dimensions. On the other hand, the presence of quasi-metallic magnetic domain
walls in the ordered states gives rise to a very different kinetics for phase
transition to the high temperature paramagnetic phase. In this new scenario,
the phase transformation is initiated by the decay of magnetic domain walls
into two insulator-metal boundaries, which subsequently move away from each
other. Implications of our findings to recent nano-imaging experiments on
nickelates are also discussed.",2011.07045v2
2021-03-07,Exploring self-consistency of the equations of axion electrodynamics in Weyl semimetals,"Recent works have provided evidence that an axial anomaly can arise in Weyl
semimetals. If this is the case, then the electromagnetic response of Weyl
semimetals should be governed by the equations of axion electrodynamics. These
equations capture both the chiral magnetic and anomalous Hall effects in the
limit of linear response, while at higher orders their solutions can provide
detectable electromagnetic signatures of the anomaly. In this work, we consider
three versions of axion electrodynamics that have been proposed in the Weyl
semimetal literature. These versions differ in the form of the chiral magnetic
term and in whether or not the axion is treated as a dynamical field. In each
case, we look for solutions to these equations for simple sample geometries
subject to applied external fields. We find that in the case of a linear chiral
magnetic term generated by a non-dynamical axion, self-consistent solutions can
generally be obtained. In this case, the magnetic field inside of the Weyl
semimetal can be magnified significantly, providing a testable signature for
experiments. Self-consistent solutions can also be obtained for dynamical
axions, but only in cases where the chiral magnetic term vanishes identically.
Finally, for a nonlinear form of the chiral magnetic term frequently considered
in the literature, we find that there are no self-consistent solutions aside
from a few special cases.",2103.04232v2
2021-05-08,Comparative study of electric currents and energetic particle fluxes in a solar flare and Earth magnetospheric substorm,"Magnetic field-line reconnection is a universal plasma process responsible
for the conversion of magnetic field energy to the plasma heating and charged
particle acceleration. Solar flares and Earth's magnetospheric substorms are
two most investigated dynamical systems where magnetic reconnection is believed
to be responsible for global magnetic field reconfiguration and energization of
plasma populations. Such a reconfiguration includes formation of a long-living
current systems connecting the primary energy release region and cold dense
conductive plasma of photosphere/ionosphere. In both flares and substorms the
evolution of this current system correlates with formation and dynamics of
energetic particle fluxes. Our study is focused on this similarity between
flares and substorms. Using a wide range of datasets available for flare and
substorm investigations, we compare qualitatively dynamics of currents and
energetic particle fluxes for one flare and one substorm. We showed that there
is a clear correlation between energetic particle bursts (associated with
energy release due to magnetic reconnection) and magnetic field
reconfiguration/formation of current system. We then discuss how datasets of
in-situ measurements in the magnetospheric substorm can help in interpretation
of datasets gathered for the solar flare.",2105.03772v1
2021-10-27,Quasi-static magnetoconvection with a tilted magnetic field,"A numerical study of convection with stress-free boundary conditions in the
presence of an imposed magnetic field that is tilted with respect to the
direction of gravity is carried out in the limit of small magnetic Reynolds
number. The dynamics are investigated over a range of Rayleigh number $Ra$ and
Chandrasekhar numbers up to $Q = 2\times10^6$, with the tilt angle between the
gravity vector and imposed magnetic field vector fixed at $45^{\circ}$. For a
fixed value of $Q$ and increasing $Ra$, the convection dynamics can be broadly
characterized by three primary flow regimes: (1) quasi-two-dimensional
convection rolls near the onset of convection; (2) isolated convection columns
aligned with the imposed magnetic field; and (3) unconstrained convection
reminiscent of non-magnetic convection. The influence of varying $Q$ and $Ra$
on the various fields is analyzed. Heat and momentum transport, as
characterized by the Nusselt and Reynolds numbers, are quantified and compared
with the vertical field case. Ohmic dissipation dominates over viscous
dissipation in all cases investigated. Various mean fields are investigated and
their scaling behavior is analyzed. Provided $Ra$ is sufficiently large, all
investigated values of $Q$ exhibit an inverse kinetic energy cascade that
yields strong `zonal' flows. Relaxation oscillations, as characterized by a
quasi-periodic shift in the predominance of either the zonal or non-zonal
component of the mean flow, appear for sufficiently large $Ra$ and $Q$.",2110.14721v2
2022-02-07,Effects of kinematic and magnetic boundary conditions on the dynamics of convection-driven plane layer dynamos,"Rapidly rotating convection-driven dynamos are investigated under different
kinematic and magnetic boundary conditions using DNS. At a fixed rotation rate,
represented by the Ekman number $E=5\times10^{-7}$, the thermal forcing is
varied from 2 to 20 times its value at the onset of convection
($\mathcal{R}=Ra/Ra_c=2-20$), keeping the fluid properties constant
($Pr=Pr_m=1$). The statistical behavior, force balance and heat transport
characteristics of the dynamos depend on boundary conditions that dictate both
boundary layer and the interior dynamics. At a fixed thermal forcing
($\mathcal{R}=3$), the Ekman plumes in the presence of viscous boundary layers
lead to energetic vortices that result in higher enstrophy and kinetic helicity
with no-slip boundaries compared to free-slip boundaries. The structure and
strength of the magnetic field are also dictated by the boundary conditions.
Though the leading order force balance remains geostrophic, Lorentz force
dominates inside the thermal boundary layer with no-slip, electrically
conducting walls. Here, the Lorentz work term in the turbulent kinetic energy
budget is found to have components that exchange energy from the velocity field
to the magnetic field, and vice-versa. However, with no-slip, insulated walls,
all Lorentz work components perform unidirectional energy transfer to produce
magnetic energy from the kinetic energy of the fluid. The heat transfer
enhancement in dynamos, compared to non-magnetic rotating convection, exhibits
a peak in the range $\mathcal{R}=3-5$. For free-slip conditions, dynamo action
may alter the heat transport by suppressing the formation of large-scale
vortices. However, the highest heat transfer enhancement occurs when the
boundaries are no-slip, electrically conducting walls.",2202.03235v1
2022-04-22,Controlling magnetism with light in a zero orbital angular momentum antiferromagnet,"Antiferromagnetic materials feature intrinsic ultrafast spin dynamics, making
them ideal candidates for future magnonic devices operating at THz frequencies.
A major focus of current research is the investigation of optical methods for
the efficient generation of coherent magnons in antiferromagnetic insulators.
In magnetic lattices endowed with orbital angular momentum, spin-orbit coupling
enables spin dynamics through the resonant excitation of low-energy electric
dipoles such as phonons and orbital resonances which interact with spins.
However, in magnetic systems with zero orbital angular momentum, microscopic
pathways for the resonant and low-energy optical excitation of coherent spin
dynamics are lacking. Here, we consider experimentally the relative merits of
electronic and vibrational excitations for the optical control of zero orbital
angular momentum magnets, focusing on a limit case: the antiferromagnet
manganese thiophoshate (MnPS3), constituted by orbital singlet Mn2+ ions. We
study the correlation of spins with two types of excitations within its band
gap: a bound electron orbital excitation from the singlet orbital ground state
of Mn2+ into an orbital triplet state, which causes coherent spin precession,
and a vibrational excitation of the crystal field that causes thermal spin
disorder. Our findings cast orbital transitions as key targets for magnetic
control in insulators constituted by magnetic centers of zero orbital angular
momentum.",2204.10574v1
2022-06-17,Multiscale Modelling of the Antiferromagnet Mn2Au: From ab-initio to Micromagnetics,"Antiferromagnets (AFMs) are strong candidates for the future spintronic and
memory applications largely because of their inherently fast dynamics and lack
of stray fields, with Mn2Au being one of the most promising. For the numerical
modelling of magnetic material properties, it is common to use ab-initio
methods, atomistic models and micromagnetics. However, each method alone
describes the physics within certain limits. Multiscale methods bridging the
gap between these three approaches have been already proposed for ferromagnetic
materials. Here, we present a complete multiscale model of the AFM Mn2Au as an
exemplar material, starting with results from ab-initio methods going via
atomistic spin dynamics (ASD) to an AFM Landau-Lifshitz-Bloch (AFM-LLB) model.
Firstly, bulk is modelled using a classical spin Hamiltonian constructed based
on earlier first-principles calculations. Secondly, this spin model is used in
the stochastic Landau-Lifshitz-Gilbert (LLG) to calculate temperature-dependent
equilibrium properties, such as magnetization and magnetic susceptibilities.
Thirdly, the temperature dependent micromagnetic parameters are used in the
AFM-LLB. We validate our approach by comparing the ASD and AFM-LLB models for
three paradigmatic cases; (i) Damped magnetic oscillations, (ii) magnetization
dynamics following a heat pulse resembling pump-probe experiments, (iii)
magnetic domain wall motion under thermal gradients.",2206.08625v1
2022-10-28,Field-free-switching state diagram of perpendicular magnetization subjected to conventional and unconventional spin-orbit torques,"The lack of certain crystalline symmetries in strong spin-orbit-coupled
non-magnetic materials allows for the existence of uncoventional spin Hall
responses, with electrically generated transverse spin currents possessing
collinear flow and spin directions. The injection of such spin currents into an
adjacent ferromagnetic layer can excite magnetization dynamics via
unconventional spin-orbit torques, leading to deterministic switching in
ferromagnets with perpendicular magnetic anisotropy. We study the interplay
between conventional and unconventional spin-orbit torques on the magnetization
dynamics of a perpendicular ferromagnet in the small intrinsic damping limit,
and identify a rich set of dynamical regimes that includes deterministic and
probabilistic switching, precessional and pinning states. Contrary to common
belief, we found that there exists a critical conventional spin Hall angle,
beyond which deterministic magnetization switching transitions to a
precessional or pinned state. Conversely, we showed that larger unconventional
spin Hall angle is generally beneficial for deterministic switching. We derive
an approximate expression that qualitatively describes the state diagram
boundary between the full deterministic switching and precessional states and
discuss a criterion for searching symmetry-broken spin Hall materials in order
to maximize switching efficiency. Our work offers a roadmap towards energy
efficient spintronic devices, which might opens doors for applications in
advanced in-memory computing technologies.",2210.16344v1
2022-11-22,Spatially Nonuniform Oscillations in Ferrimagnets Based on an Atomistic Model,"The ferrimagnets, such as GdxFeCo(1-x), can produce ultrafast magnetic
switching and oscillation due to the strong exchange field. The two-sublattices
macrospin model has been widely used to explain the experimental results.
However, it fails in describing the spatial nonuniform magnetic dynamics which
gives rises to many important phenomenons such as the domain walls and
skyrmions. Here we develop the two-dimensional atomistic model and provide a
torque analysis method to study the ferrimagnetic oscillation. Under the
spin-transfer torque, the magnetization oscillates in the exchange mode or the
flipped exchange mode. When the Gd composition is increased, the exchange mode
firstly disappears, and then appears again as the magnetization compensation
point is reached. We show that these results can only be explained by analyzing
the spatial distribution of magnetization and effective fields. In particular,
when the sample is small, a spatial nonuniform oscillation is also observed in
the square film. Our work reveals the importance of spatial magnetic
distributions in understanding the ferrimagnetic dynamics. The method developed
in this paper provides an important tool to gain a deeper understanding of
ferrimagnets and antiferromagnets. The observed ultrafast dynamics can also
stimulate the development of THz oscillators.",2211.12247v1
2022-11-30,Unconventional spin dynamics in the non-collinear phase of a ferrimagnet,"Ferrimagnets containing several partially compensated magnetic sublattices
are considered the most promising materials for all-optical data storage and
for ultrafast communications based on spin waves. There are two magnetic phases
of the ferrimagnets: collinear and non-collinear ones. Up to now spin dynamics
in ferrimagnets has been studied mostly in the collinear state without paying
much attention to the kind of the magnetic phase. Here we investigate laser
induced ultrafast spin dynamics in a rare-earth iron garnet film in the
noncollinear phase as well. We identify a crucial influence of the magnetic
phase on the excited spin modes which allowed us to discover several prominent
effects previously overlooked. In particular, the non-collinearity makes the
quasi-antiferromagnetic mode sensitive to the external magnetic field and
brings its frequency close to the frequency of the quasiferromagnetic mode. The
latter maximizes near the magnetization compensation point and vanishes towards
the collinear phase. Spectacularly, at the phase transition the
quasiferromagnetic mode becomes soft and its amplitude significantly increases
reaching 7{\deg}. This opens new opportunities for the ultrafast control of
spins in ferrimagnets for nonthermal data storage and data processing.",2212.00085v2
2023-01-22,Easy-plane spin Hall oscillator,"Spin Hall oscillators (SHOs) based on bilayers of a ferromagnet (FM) and a
non-magnetic heavy metal (HM) are electrically tunable nanoscale microwave
signal generators. Achieving high output power in SHOs requires driving
large-amplitude magnetization dynamics by a direct spin Hall current. The
maximum possible amplitude of such oscillations with the precession cone angle
nearing $90^\circ$ is predicted for FM layers with easy-plane magnetic
anisotropy and spin Hall current polarization perpendicular to the easy plane.
While many FMs exhibit natural easy-plane anisotropy in the FM film plane, the
spin Hall current in a HM|FM bilayer is polarized in this plane and thus cannot
drive large-amplitude magneto-dynamics. Here we present a new type of SHO
engineered to have the easy-plane anisotropy oriented normal to the film plane,
enabling large-amplitude easy-plane dynamics driven by spin Hall current. Our
experiments and micromagnetic simulations demonstrate that the desired
easy-plane anisotropy can be achieved by tuning the magnetic shape anisotropy
and perpendicular magnetic anisotropy in a nanowire SHO, leading to a
significant enhancement of the generated microwave power. The easy-plane SHO
experimentally demonstrated here is an ideal candidate for realization of a
spintronic spiking neuron. Our results provide a new approach to design of
high-power SHOs for wireless communications, neuromorphic computing, and
microwave assisted magnetic recording.",2301.09228v1
2023-02-08,Phase transitions and spin dynamics of the quasi-one dimensional Ising-like antiferromagnet BaCo$_{2}$V$_{2}$O$_{8}$ in a longitudinal magnetic field,"By combining inelastic neutron scattering and numerical simulations, we study
the quasi-one dimensional Ising-like quantum antiferromagnet
BaCo$_{2}$V$_{2}$O$_{8}$ in a longitudinal magnetic field applied along the
magnetic anisotropy axis, which is also the chain direction. The external field
closes the excitation gap due to the magnetic anisotropy, inducing a transition
from the N\'eel ordered state to an incommensurate longitudinal spin density
wave phase. If the field is increased further, another transition into a
transverse antiferromagnetic phase takes place at 9 T due to the competition
between longitudinal and transverse correlations. We numerically and
experimentally show that the model of XXZ chains connected by a weak interchain
interaction well reproduces this transition. We also calculate the dynamical
susceptibility and demonstrate that it agrees quantitatively with inelastic
neutron scattering measurements. In contrast to the abrupt change of magnetic
ordering, the spectra do not change much at the transition at 9 T, and the spin
dynamics can be described as a Tomonaga-Luttinger liquid. We also refine the
modeling of BaCo$_{2}$V$_{2}$O$_{8}$ by including a four-site periodic term
arising from the crystal structure which enables to account for an anomaly of
the magnetic susceptibility appearing at 19 T as well as for the anticrossing
observed in the inelastic neutron scattering spectra.",2302.03833v1
2023-02-09,Latitudinal Dynamics and Sectoral Structure of the Solar Magnetic Field,"The study of the global structure of the large-scale magnetic field of the
Sun is extremely important for creating a theoretical model of the dynamics of
the Sun and predictions of the real situation in the helio- and
geomagnetosphere. The purpose of the present study was to calculate the
differential rotation period of a large-scale photospheric magnetic field, to
study its behavior over time and to find out whether there is a sectoral
structure of this field along the longitude. However, the choice of the
coordinate system in which to search for it is far from unambiguous. This is
closely related to the fact that the rotation of the Sun is differential in
latitude and varies with depth and over time. Based on the observational data
of the J. Wilcox Solar Observatory for three complete cycles of solar activity
21, 22 and 23, the period of rotation of the magnetic field at various
latitudes and its change in time were calculated. A uniquely stable over 30
years longitude structure was found. It was determined that its speed of
rotation coincides with the one with which the base of the convective shell
rotates, that is, the structuring of the magnetic field of the Sun occurs in
tachocline. This result clearly demonstrates the close connection of solar
activity processes with the topology of magnetic fields, with their dynamics
and depth stratification.",2302.04943v1
2023-02-20,(Nonequilibrium) dynamics of diffusion processes with non-conservative drifts,"The nonequilibrium Fokker-Planck dynamics with a non-conservative drift
field, in dimension $N\geq 2$, can be related with the non-Hermitian quantum
mechanics in a real scalar potential $V$ and in a purely imaginary vector
potential -$iA$ of real amplitude $A$. Since Fokker-Planck probability density
functions may be obtained by means of Feynman's path integrals, the previous
observation points towards a general issue of ""magnetically affine""
propagators, possibly of quantum origin, in real and Euclidean time. In below
we shall follow the $N=3$ ""magnetic thread"", within which one may keep under a
computational control formally and conceptually different implementations of
magnetism (or surrogate magnetism) in the dynamics of diffusion processes. We
shall focus on interrelations (with due precaution to varied, not evidently
compatible, notational conventions) of: (i) the pertinent non-conservatively
drifted diffusions, (ii) the classic Brownian motion of charged particles in
the (electro)magnetic field, (iii) diffusion processes arising within so-called
Euclidean quantum mechanics (which from the outset employs non-Hermitian
""magnetic"" Hamiltonians), (iv) limitations of the usefulness of the Euclidean
map $\exp(-itH_{quant}) \rightarrow \exp(-tH_{Eucl})$, regarding the
probabilistic significance of inferred (path) integral kernels in the
description of diffusion processes.",2302.10154v5
2023-04-29,Dynamics and reversible control of the vortex Bloch-point vortex domain wall in short cylindrical magnetic nanowires,"Fast and efficient switching of nanomagnets is one of the main challenges in
the development of future magnetic memories. We numerically investigate the
evolution of the static and dynamic spin wave (SW) magnetization in short
(50-400 nm length and 120 nm diameter) cylindrical ferromagnetic nanowires,
where competing single vortex (SV) and vortex domain wall with a Bloch point
(BP-DW) magnetization configurations could be formed. For a limited nanowire
length range (between 150 and 300 nm) we demonstrate a reversible microwave
field induced (forward) and opposite spin currents (backwards) transitions
between the topologically different SV and BP-DW states. By tuning the nanowire
length, excitation frequency, the microwave pulse duration and the spin current
values we show that the optimum (low power) manipulation of the BP-DW could be
reached by a microwave excitation tuned to the main SW mode and for nanowire
lengths around 230-250 nm, where single vortex domain wall magnetization
reversal via nucleation and propagation of SV-DW takes place. An analytical
model for dynamics of the Bloch point provides an estimation of the gyrotropic
mode frequency close the one obtained via micromagnetic simulations. A
practical implementation of the method on a device has been proposed involving
microwave excitation and the generation of the opposite spin currents via spin
orbit torque. Our findings open a new pathway for the creation of unforeseen
topological magnetic memories.",2305.00346v1
2023-08-03,A general flux-Based Circuit Theory for Superconducting Josephson Junction Circuits,"Superconducting quantum interference devices (SQUIDs), single flux-quantum
(SFQ) logic circuits, and quantum Josephson junction circuits have been
developed into a family of superconductor integrated circuit, and are widely
applied for subtle magnetic-field measurements, energy-efficient computing, and
quantum computing, respectively. They are Josephson junction networks composed
of Josephson junctions and normal resistor-inductor-capacitor (RLC) components,
working with the fluxoid-quantization principle and Josephson effects to
achieve unique flux-modulated dynamics and characteristics; they react to the
vector potential of magnetic fields rather than the electric potential.
However, the conventional circuit diagrams and nodal analysis methods focus on
the electric charges flowing though branches and nodes, ignoring dynamics of
the magnetic fluxes flowing from loop to loop. This article introduces a
general flux-based circuit theory to unify the analyses of Josephson junction
circuits and normal RLC circuits. This theory presents a
magnetic-flux-generator (MFG) concept to unify Josephson junctions and normal
circuit elements, and abstract both Josephson junction circuits and normal RLC
circuits as MFG network; it derives a general network equation to describe
dynamics of Josephson junction circuits, and invents a kind of magnetic-flux
flow (MFF) diagram to depict the working principles of magnetic-flux flows
inside Josephson junction circuits. The flux-based theory is complementary to
the conventional circuit theories in the design and analysis of superconductor
integrated circuits.",2308.01693v1
2023-10-14,"Chiral magnetism, lattice dynamics, and anomalous Hall conductivity in the novel V$_3$AuN antiferromagnetic antiperovskite","Antiferromagnetic antiperovskites, where magnetically active 3$d$ metal
cations are placed in the octahedral corners of a perovskite structure, are in
the spotlight due to their intertwined magnetic structure and topological
properties. Especially their anomalous Hall conductivity, which can be
controlled by applied strain and/or electric field, makes them highly
attractive in different electronic applications. Here, we present the study and
theoretical understanding of a new antiperovskite compound that can offer
enormous opportunities in a broad set of applications. Using first-principles
calculations, we investigated the structure, lattice dynamics, noncollinear
magnetic ordering, and electronic behavior in the Vanadium-based antiperovskite
V$_3$AuN. We found an antiperovskite structure centered on N similar to the
Mn$_3A$N family as the structural ground state. In such a phase, a
\emph{Pm$\bar{3}$m} ground state was found in contrast to the \emph{Cmcm}
post-antiperovskite layered structure, as in the V$_3A$N, $A$ = Ga, Ge, As, and
P. We studied the lattice dynamics and electronic properties, demonstrating its
vibrational stability in the cubic structure and a chiral antiferromagnetic
noncollinear ordering as a magnetic ground state. Finally, we found that the
anomalous Hall conductivity, associated with the topological features induced
by the magnetic symmetry, is $\sigma_{xy}$ = $-$291 S$\cdot$cm$^{-1}$
($\sigma_{111}$ = $-$504 S$\cdot$cm$^{-1}$). The latter is the largest reported
in the antiferromagnetic antiperovskite family of compounds.",2310.09616v1
2024-02-01,Quantum phase transitions and composite excitations of antiferromagnetic quantum spin trimer chains in a magnetic field,"Motivated by recent advancements in theoretical and experimental studies on
the high-energy excitations, we theoretically explore the quantum phase
transitions and composite excitations of the antiferromagnetic trimer chains in
a magnetic field using the exact diagonalization, density matrix
renormalization group, time-dependent variational principle and cluster
perturbation theory. We utilize the entanglement entropy to uncover the phase
diagram, encompassing the XY-I, $1/3$ magnetization plateau, XY-II and
ferromagnetic phases. The critical XY-I and XY-II phases are both described by
the conformal field theory with the central charge $c \simeq 1$. We reveal the
diverse features of spin dynamics in various phases by using the dynamical
structure factor. In the weak intertrimer interaction regime, we identify the
intermediate-energy and high-energy modes in the XY-I and $1/3$ magnetization
plateau phases as the internal trimer excitations, corresponding to the
propagation of doublon and quarton, respectively. Notably, the magnetic field
splits the high-energy spectra into two branches labeled as the upper quarton
and lower quarton. Furthermore, we also explore the spin dynamics of a
trimerized model closely related to the quantum magnet \ce{Na_2Cu_3Ge_4O_12},
and discuss the possibility of the quarton Bose-Einstein condensation. Our
results can be verified in the inelastic neutron scattering experiments and
provide deep insights for exploring the high-energy exotic excitations.",2402.00272v1
2002-05-01,Recent Developments in Magnetic Dynamo Theory,"Some recent results and open issues in magnetic dynamo theory are addressed.
The distinction between small-scale and mean-field dynamo (MFD) action in
forced turbulent flows is emphasized. Though useful, the MFD has been
controversial. This is partly due to concerns about its need for helically
forced turbulence, but mainly because simple ""textbook"" treatments are
kinematic and linear. The non-linear backreaction of the growing magnetic field
has been suspected to prematurely quench MFD action. To resolve the
controversy, we must first understand those non-linear MFDs which can be
numerically simulated. Recently, there has been progress on this front. For
simple MFDs in closed systems, dynamical quenching models that incorporate a
transfer of magnetic helicity between small and large scales agree reasonably
well with fully 3-D numerical periodic box simulations. Unresolved issues such
as the quenching of turbulent diffusion and the additional physics needed to
apply these results to real systems are also discussed herein. The following
summarizes conceptual progress in describing mean-field magnetic energy growth
in the simplest MFDs:
  For a closed turbulent flow, the non-linear mean-field dynamo, is first fast
and kinematic, then slow and dynamic, and magnetic helicity transfer makes it
so.",0205002v2
2002-07-22,A Cosmic Battery Reconsidered,"We revisit the problem of magnetic field generation in accretion flows onto
black holes owing to the excess radiation force on electrons. This excess force
may arise from the Poynting-Robertson effect.
  Instead of a recent claim of the generation of dynamically important magnetic
fields, we establish the validity of earlier results from 1977 which show only
small magnetic fields are generated.
  The radiative force causes the magnetic field to initially grow linearly with
time. However, this linear growth holds for only a {\it restricted} time
interval which is of the order of the accretion time of the matter.
  The large magnetic fields recently found result from the fact that the linear
growth is unrestricted.
  A model of the Poynting-Robertson magnetic field generation close to the
horizon of a Schwarzschild black hole is solved exactly using General
Relativity, and the field is also found to be dynamically insignificant.
  These weak magnetic fields may however be important as seed fields for
dynamos.",0207476v1
2002-07-23,A model of nonlinear evolution and saturation of the turbulent MHD dynamo,"The growth and saturation of magnetic field in conducting turbulent media
with large magnetic Prandtl numbers are investigated. This regime is very
common in low-density hot astrophysical plasmas. During the early (kinematic)
stage, weak magnetic fluctuations grow exponentially and concentrate at the
resistive scale, which lies far below the hydrodynamic viscous scale. The
evolution becomes nonlinear when the magnetic energy is comparable to the
kinetic energy of the viscous-scale eddies. A physical picture of the ensuing
nonlinear evolution of the MHD dynamo is proposed. Phenomenological
considerations are supplemented with a simple Fokker--Planck model of the
nonlinear evolution of the magnetic-energy spectrum. It is found that, while
the shift of the bulk of the magnetic energy from the subviscous scales to the
velocity scales may be possible, it occurs very slowly -- at the resistive,
rather than dynamical, time scale (for galaxies, this means that generation of
large-scale magnetic fields cannot be explained by this mechanism). The role of
Alfvenic motions and the implications for the fully developed isotropic MHD
turbulence are discussed.",0207503v2
2005-01-18,Plasma instabilities and magnetic-field growth in clusters of galaxies,"We show that under very general conditions, cluster plasmas threaded by weak
magnetic fields are subject to very fast growing plasma instabilities driven by
the anisotropy of the plasma pressure (viscous stress) with respect to the
local direction of the magnetic field. Such an anisotropy will naturally arise
in any weakly magnetized plasma that has low collisionality and is subject to
stirring. The magnetic field must be sufficiently weak for the instabilities to
occur, viz., beta>Re^{1/2}. The instabilities are captured by the extended MHD
model with Braginskii viscosity. However, their growth rates are proportional
to the wavenumber down to the ion gyroscale, so MHD equations with Braginskii
viscosity are not well posed and a fully kinetic treatment is necessary. The
instabilities can lead to magnetic fields in clusters being amplified from seed
strength of ~10^{-18} G to dynamically important strengths of ~10 microG on
cosmologically trivial time scales (~10^8 yr). The fields produced during the
amplification stage are at scales much smaller than observed. Predicting the
saturated field scale and structure will require a kinetic theory of magnetized
cluster turbulence.",0501362v3
2005-10-14,The Magnetic Field in the Star-forming Region Cepheus A from Water Maser Polarization Observations,"We present linear and circular polarization observations of the water masers
in 4 distinct regions spread over 1x2 arcseconds around the HW2 high-mass young
stellar object in the Cepheus A star-forming region. We find magnetic fields
between 100-500 mG in the central maser region, which has been argued to trace
a circumstellar disk. The masers further from HW2 have field strengths between
30-100 mG. In all cases the magnetic field pressure is found to be similar to
the dynamic pressure, indicating that the magnetic field is capable of
controlling the outflow dynamics around HW2. In addition to several water maser
complexes observed before, we also detect a new maser filament, ~1 arcsec (~690
AU) East of HW2, which we interpret as a shocked region between the HW2 outflow
and the surrounding medium. We detect a linear polarization gradient along the
filament as well as a reversal of the magnetic field direction. This is thought
to mark the transition between the magnetic field associated with the outflow
and that found in the surrounding molecular cloud. In addition to the magnetic
field we determine several other physical properties of the maser region,
including density and temperatures as well as the maser beaming angles.",0510452v1
2007-02-06,Magnetic Fields in Stellar Jets,"Although several lines of evidence suggest that jets from young stars are
driven magnetically from accretion disks, existing observations of field
strengths in the bow shocks of these flows imply that magnetic fields play only
a minor role in the dynamics at these locations. To investigate this apparent
discrepancy we performed numerical simulations of expanding magnetized jets
with stochastically variable input velocities with the AstroBEAR MHD code.
Because the magnetic field B is proportional to the density n within
compression and rarefaction regions, the magnetic signal speed drops in
rarefactions and increases in the compressed areas of velocity-variable flows.
In contrast, B ~ n^0.5 for a steady-state conical flow with a toroidal field,
so the Alfven speed in that case is constant along the entire jet. The
simulations show that the combined effects of shocks, rarefactions, and
divergent flow cause magnetic fields to scale with density as an intermediate
power 1 > p > 0.5. Because p > 0.5, the Alfven speed in rarefactions decreases
on average as the jet propagates away from the star. This behavior is extremely
important to the flow dynamics because it means that a typical Alfven velocity
in the jet close to the star is significantly larger than it is in the
rarefactions ahead of bow shocks at larger distances, the one place where the
field is a measurable quantity. We find that the observed values of weak fields
at large distances are consistent with strong fields required to drive the
observed mass loss close to the star. For a typical stellar jet the crossover
point inside which velocity perturbations of 30 - 40 km/s no longer produce
shocks is ~ 300 AU from the source.",0702174v1
1995-12-14,Analytical and computational study of magnetization switching in kinetic Ising systems with demagnetizing fields,"An important aspect of real ferromagnetic particles is the demagnetizing
field resulting from magnetostatic dipole-dipole interaction, which causes
large particles to break up into domains. Sufficiently small particles,
however, remain single-domain in equilibrium. This makes such small particles
of particular interest as materials for high-density magnetic recording media.
In this paper we use analytic arguments and Monte Carlo simulations to study
the effect of the demagnetizing field on the dynamics of magnetization
switching in two-dimensional, single-domain, kinetic Ising systems. For systems
in the ``Stochastic Region,'' where magnetization switching is on average
effected by the nucleation and growth of fewer than two well-defined critical
droplets, the simulation results can be explained by the dynamics of a simple
model in which the free energy is a function only of magnetization. In the
``Multi-Droplet Region,'' a generalization of Avrami's Law involving a
magnetization-dependent effective magnetic field gives good agreement with our
simulations.",9512110v1
2005-03-24,Fast magnetization switching of Stoner particles: A nonlinear dynamics picture,"The magnetization reversal of Stoner particles is investigated from the point
of view of nonlinear dynamics within the Landau-Lifshitz-Gilbert formulation.
The following results are obtained. 1) We clarify that the so-called
Stoner-Wohlfarth (SW) limit becomes exact when damping constant is infinitely
large. Under the limit, the magnetization moves along the steepest energy
descent path. The minimal switching field is the one at which there is only one
stable fixed point in the system. 2) For a given magnetic anisotropy, there is
a critical value for the damping constant, above which the minimal switching
field is the same as that of the SW-limit. 3) We illustrate how fixed points
and their basins change under a field along different directions. This change
explains well why a non-parallel field gives a smaller minimal switching field
and a short switching time. 4) The field of a ballistic magnetization reversal
should be along certain direction window in the presence of energy dissipation.
The width of the window depends on both of the damping constant and the
magnetic anisotropy. The upper and lower bounds of the direction window
increase with the damping constant. The window width oscillates with the
damping constant for a given magnetic anisotropy. It is zero for both zero and
infinite damping. Thus, the perpendicular field configuration widely employed
in the current experiments is not the best one since the damping constant in a
real system is far from zero.",0503594v1
2005-06-09,Magnetic Quantum Phase Transitions in Kondo Lattices,"The identification of magnetic quantum critical points in heavy fermion
metals has provided an ideal setting for experimentally studying quantum
criticality. Motivated by these experiments, considerable theoretical efforts
have recently been devoted to reexamine the interplay between Kondo screening
and magnetic interactions in Kondo lattice systems. A local quantum critical
picture has emerged, in which magnetic interactions suppress Kondo screening
precisely at the magnetic quantum critical point (QCP). The Fermi surface
undergoes a large reconstruction across the QCP and the coherence scale of the
Kondo lattice vanishes at the QCP. The dynamical spin susceptibility exhibits
$\omega/T$ scaling and non-trivial exponents describe the temperature and
frequency dependence of various physical quantities. These properties are to be
contrasted with the conventional spin-density-wave (SDW) picture, in which the
Kondo screening is not suppressed at the QCP and the Fermi surface evolves
smoothly across the phase transition. In this article we discuss recent
microscopic studies of Kondo lattices within an extended dynamical mean field
theory (EDMFT). We summarize the earlier work based on an analytical
$\epsilon$-expansion renormalization group method, and expand on the more
recent numerical results. We also discuss the issues that have been raised
concerning the magnetic phase diagram. We show that the zero-temperature
magnetic transition is second order when double counting of the RKKY
interactions is avoided in EDMFT.",0506207v2
2008-02-29,Indirect exchange coupling between localized magnetic moments in carbon nanotubes: a dynamic approach,"Magnetic moments dilutely dispersed in a metallic host tend to be coupled
through the conduction electrons of the metal. This indirect exchange coupling,
known to occur for a variety of magnetic materials embedded in several
different metallic structures, is of rather long range, especially for
low-dimensional structures like carbon nanotubes. Motivated by recent claims
that the indirect coupling between magnetic moments in precessional motion has
a much longer range than its static counterpart, here we consider how magnetic
atoms adsorbed to the walls of a metallic nanotube respond to a time-dependent
perturbation that induces their magnetic moments to precess. By calculating the
frequency-dependent spin susceptibility we are able to identify resonant peaks
whose respective widths provide information about the dynamic aspect of the
indirect exchange coupling. We show that by departing from a purely static
representation to another in which the moments are allowed to precess, we
change from what is already considered a long range interaction to another
whose range is far superior. In other words, localized magnetic moments
embedded in a metallic structure can feel each other's presence more easily
when they are set in precessional motion. We argue that such an effect can have
useful applications leading to large-scale spintronics devices.",0803.0028v1
2008-03-24,Effect of static and dynamic disorder on electronic transport of $RCo_2$ compounds: a study of $Ho(Al_xCo_{1-x})_2$ alloys,"We present experimental results on thermoelectric power ({\em S}) and
electrical resistivity ($\rho $) of pseudobinary alloys
Ho(Al$_x$Co$_{1-x}$)$_2$ ($0 \leq x \leq 0.1 $), in the temperature range 4.2 K
to 300 K. The work focuses on the effects of static (induced by alloying) and
dynamic (induced by temperature) disorder on the magnetic state and electronic
transport in a metallic system with itinerant metamagnetic instability. Spatial
fluctuations of the local magnetic susceptibility in the alloys lead to a
development of a partially ordered magnetic ground state of the itinerant 3d
electron system. This results in a strong increase of the residual resistivity
and a suppression of the temperature-dependent resistivity. Thermopower
exhibits a complex temperature variation in both the magnetically ordered and
in the paramagnetic state. This complex temperature variation is referred to
the electronic density of states features in vicinity of Fermi energy and to
the interplay of magnetic and impurity scattering. Our results indicate that
the magnetic enhancement of the Co 3d-band in RCo$_{\rm 2}$--based alloys upon
a substitution of Co by non-magnetic elements is mainly related to a
progressive localization of the Co -- 3d electrons caused by disorder. We show
that the magnitude of the resistivity jump at the Curie temperature for
RCo$_{\rm 2}$ compounds exhibiting a first order phase transition is a
non-monotonic function of the Curie temperature due to a saturation of the
3d--band spin fluctuation magnitude at high temperatures.",0803.3380v3
2008-04-29,A new general relativistic magnetohydrodynamics code for dynamical spacetimes,"We present a new numerical code which solves the general relativistic
magneto-hydrodynamics (GRMHD) equations coupled to the Einstein equations for
the evolution of a dynamical spacetime within the conformally-flat
approximation. This code has been developed with the main objective of studying
astrophysical scenarios in which both, high magnetic fields and strong
gravitational fields appear, such as the magneto-rotational collapse of stellar
cores, the collapsar model of GRBs, and the evolution of neutron stars. The
code is based on an existing and thoroughly tested purely hydrodynamics code
and on its extension to accommodate weakly magnetized fluids (passive magnetic
field approximation). The numerical code we present here is based on
high-resolution shock-capturing schemes to solve the GRMHD equations together
with the flux constraint transport method to ensure the solenoidal condition of
the magnetic field. Since the astrophysical applications envisaged do not
deviate much from spherical symmetry, the conformal flatness condition
approximation is used for the formulation of the Einstein equations. In
addition, the code can handle several equations of state, from simple
analytical expressions to microphysical tabulated ones. In this paper we
present stringent tests of our new GRMHD numerical code, which show its ability
to handle all aspects appearing in the astrophysical scenarios for which the
code is intended, namely relativistic shocks, highly magnetized fluids, and
equilibrium configurations of magnetized neutron stars. As an application,
magneto-rotational core collapse simulations of a realistic progenitor are
presented, comparing the results with our previous finding in the passive
magnetic field approximation.",0804.4572v2
2008-07-23,A key role for unusual spin dynamics in ferropnictides,"The 2008 discovery of superconducting ferropnictides with Tc~26K-56K
introduced a new family of materials into the category of high Tc
superconductors. The ongoing project of understanding the superconducting
mechanism and pairing symmetry has already revealed a complicated and often
contradictory underlying picture of the structural and magnetic properties.
There is an almost unprecedented sensitivity of the calculated magnetism and
Fermi surface to structural details that prohibits correspondence with
experiment. Furthermore, experimental probes of the order parameter symmetry
are in surprisingly strong disagreement, even considering the relative
immaturity of the field. Here we outline all of the various and seemingly
contradictory evidences, both theoretical and experimental, and show that they
can be rectified if the system is assumed to be highly magnetic with a spin
density wave that is well-defined but with magnetic twin and anti-phase
boundaries that are dynamic on the time-scale of experiments. Under this
assumption, we find that our calculations can accurately reproduce even very
fine details of the structure, and a natural explanation for the temperature
separation of structural and magnetic transitions is provided. Thus, our theory
restores agreement between experiment and theory in crucial areas, making
further cooperative progress possible on both fronts. We believe that
fluctuating magnetic domains will be an essential component of unravelling the
interplay between magnetic interactions and superconductivity in these newest
high Tc superconductors.",0807.3737v1
2008-11-07,Electrically driven magnetization of diluted magnetic semiconductors actuated by Overhauser effect,"It is well-known that the Curie temperature, and hence the magnetization, in
diluted magnetic semiconductor (DMS) like Ga$_{1-x}$Mn$_x$As can be controlled
by changing the equilibrium density of holes in the material. Here, we propose
that even with a constant hole density, large changes in the magnetization can
be obtained with a relatively small imbalance in the quasi-Fermi levels for
up-spin and down-spin electrons. We show, by coupling mean field theory of
diluted magnetic semiconductor ferromagnetism with master equations governing
the Mn spin-dynamics, that a mere splitting of the up-spin and down-spin
quasi-Fermi levels by 0.1 meV will produce the effect of an external magnetic
field as large as 1 T as long as the alternative relaxation paths for Mn spins
(i.e. spin-lattice relaxation) can be neglected. The physics is similar to the
classic Overhauser effect, also called the dynamic nuclear polarization, with
the Mn impurities playing the role of the nucleus. We propose that a lateral
spin-valve structure in anti-parallel configuration with a DMS as the channel
can be used to demonstrate this effect as quasi-Fermi level splitting of such
magnitude, inside the channel of similar systems, have already been
experimentally demonstrated to produce polarization of paramagnetic impurity
spins.",0811.1225v2
2010-06-15,Applications of Magnetic PsiDO Techniques to Space-adiabatic Perturbation Theory,"In this review, we show how advances in the theory of magnetic
pseudodifferential operators (magnetic $\Psi$DO) can be put to good use in
space-adiabatic perturbation theory (SAPT). As a particular example, we extend
results of [PST03] to a more general class of magnetic fields: we consider a
single particle moving in a periodic potential which is subjectd to a weak and
slowly-varying electromagnetic field. In addition to the semiclassical
parameter $\eps \ll 1$ which quantifies the separation of spatial scales, we
explore the influence of additional parameters that allow us to selectively
switch off the magnetic field.
  We find that even in the case of magnetic fields with components in
$C_b^{\infty}(\R^d)$, e. g. for constant magnetic fields, the results of
Panati, Spohn and Teufel hold, i.e. to each isolated family of Bloch bands,
there exists an associated almost invariant subspace of $L^2(\R^d)$ and an
effective hamiltonian which generates the dynamics within this almost invariant
subspace. In case of an isolated non-degenerate Bloch band, the full quantum
dynamics can be approximated by the hamiltonian flow associated to the
semiclassical equations of motion found in [PST03].",1006.3103v4
2010-10-25,Study on the large scale dynamo transition,"Using the magnetohydrodynamic (MHD) description, we develop a nonlinear
dynamo model that couples the evolution of the large scale magnetic field with
turbulent dynamics of the plasma at small scale by electromotive force (e.m.f.)
in the induction equation at large scale. The nonlinear behavior of the plasma
at small scale is described by using a MHD shell model for velocity field and
magnetic field fluctuations.The shell model allow to study this problem in a
large parameter regime which characterizes the dynamo phenomenon in many
natural systems and which is beyond the power of supercomputers at today. Under
specific conditions of the plasma turbulent state, the field fluctuations at
small scales are able to trigger the dynamo instability. We study this
transition considering the stability curve which shows a strong decrease in the
critical magnetic Reynolds number for increasing inverse magnetic Prandlt
number $\textrm{Pm}^{-1}$ in the range $[10^{-6},1]$ and slows an increase in
the range $[1,10^{8}]$. We also obtain hysteretic behavior across the dynamo
boundary reveling the subcritical nature of this transition. The system,
undergoing this transition, can reach different dynamo regimes, depending on
Reynolds numbers of the plasma flow. This shows the critical role that the
turbulence plays in the dynamo phenomenon. In particular the model is able to
reproduce the dynamical situation in which the large-scale magnetic field jumps
between two states which represent the opposite polarities of the magnetic
field, reproducing the magnetic reversals as observed in geomagnetic dynamo and
in the VKS experiments.",1010.5213v1
2011-02-25,Dynamo in the Intra-Cluster Medium: Simulation of CGL-MHD Turbulent Dynamo,"The standard magnetohydrodynamic (MHD) description of the plasma in the hot,
magnetized gas of the intra-cluster (ICM) medium is not adequate because it is
weakly collisional. In such collisionless magnetized gas, the microscopic
velocity distribution of the particles is not isotropic, giving rise to kinetic
effects on the dynamical scales. These kinetic effects could be important in
understanding the turbulence, as so as the amplification and maintenance of the
magnetic fields in the ICM. It is possible to formulate fluid models for
collisonless or weakly collisional gas by introducing modifications in the MHD
equations. These models are often referred as kinetic MHD (KMHD). Using a KMHD
model based on the CGL-closure, which allows the adiabatic evolution of the two
components of the pressure tensor (the parallel and perpendicular components
with respect to the local magnetic field), we performed 3D numerical
simulations of forced turbulence in order to study the amplification of an
initially weak seed magnetic field. We found that the growth rate of the
magnetic energy is comparable to that of the ordinary MHD turbulent dynamo, but
the magnetic energy saturates in a level smaller than of the MHD case. We also
found that a necessary condition for the dynamo works is to impose limits to
the anisotropy of the pressure.",1102.5139v1
2012-08-07,Magnetar Giant Flares --- Flux Rope Eruptions in Multipolar Magnetospheric Magnetic Fields,"We address a primary question regarding the physical mechanism that triggers
the energy release and initiates the onset of eruptions in the magnetar
magnetosphere. A self-consistent stationary, axisymmetric model of the magnetar
magnetosphere is constructed based on a force-free magnetic field configuration
which contains a helically twisted force-free flux rope. Given the complex
multipolar magnetic fields at the magnetar surface, we also develop a
convenient numerical scheme to solve the GS equation. Depending on the surface
magnetic field polarity, there exist two kinds of magnetic field
configurations, inverse and normal. For these two kinds of configurations,
variations of the flux rope equilibrium height in response to gradual surface
physical processes, such as flux injections and crust motions, are carefully
examined. We find that equilibrium curves contain two branches, one represents
a stable equilibrium branch, the other an unstable equilibrium branch. As a
result, the evolution of the system shows a catastrophic behavior: when the
magnetar surface magnetic field evolves slowly, the height of flux rope would
gradually reach a critical value beyond which stable equilibriums can no longer
be maintained. Subsequently the flux rope would lose equilibrium and the
gradual quasi-static evolution of the magnetar magnetosphere will be replaced
by a fast dynamical evolution. In addition to flux injections, the relative
motion of active regions would give rise to the catastrophic behavior and lead
to magnetic eruptions as well. We propose that a gradual process could lead to
a sudden release of magnetosphere energy on a very short dynamical timescale,
without being initiated by a sudden fracture in the crust of the magnetar. Some
implications of our model are also discussed.",1208.1459v1
2012-10-19,Effects of Strong Magnetic Fields on Photoionised Clouds,"Simulations are presented of the photoionisation of three dense gas clouds
threaded by magnetic fields, showing the dynamical effects of different initial
magnetic field orientations and strengths. For moderate magnetic field
strengths the initial radiation-driven implosion phase is not strongly affected
by the field geometry, and the photoevaporation flows are also similar. Over
longer timescales, the simulation with an initial field parallel to the
radiation propagation direction (parallel field) remains basically
axisymmetric, whereas in the simulation with a perpendicular initial field the
pillar of neutral gas fragments in a direction aligned with the magnetic field.
For stronger initial magnetic fields, the dynamics in all gas phases are
affected at all evolutionary times. In a simulation with a strong initially
perpendicular field, photoevaporated gas forms filaments of dense ionised gas
as it flows away from the ionisation front along field lines. These filaments
are potentially a useful diagnostic of magnetic field strengths in H II regions
because they are very bright in recombination line emission. In the strong
parallel field simulation the ionised gas is constrained to flow back towards
the radiation source, shielding the dense clouds and weakening the ionisation
front, eventually transforming it to a recombination front.",1210.5385v1
2013-11-07,Spin-Orbit Torques and Anisotropic Magnetization Damping in Skyrmion Crystals,"The length scale of the magnetization gradients in chiral magnets is
determined by the relativistic Dzyaloshinskii-Moriya interaction. Thus, even
conventional spin-transfer torques are controlled by the relativistic
spin-orbit coupling in these systems, and additional relativistic corrections
to the current-induced torques and magnetization damping become important for a
complete understanding of the current-driven magnetization dynamics. We
theoretically study the effects of reactive and dissipative homogeneous
spin-orbit torques and anisotropic damping on the current-driven skyrmion
dynamics in cubic chiral magnets. Our results demonstrate that spin-orbit
torques play a significant role in the current-induced skyrmion velocity. The
dissipative spin-orbit torque generates a relativistic Magnus force on the
skyrmions, whereas the reactive spin-orbit torque yields a correction to both
the drift velocity along the current direction and the transverse velocity
associated with the Magnus force. The spin-orbit torque corrections to the
velocity scale linearly with the skyrmion size, which is inversely proportional
to the spin-orbit coupling. Consequently, the reactive spin-orbit torque
correction can be the same order of magnitude as the non-relativistic
contribution. More importantly, the dissipative spin-orbit torque can be the
dominant force that causes a deflected motion of the skyrmions if the torque
exhibits a linear or quadratic relationship with the spin-orbit coupling. In
addition, we demonstrate that the skyrmion velocity is determined by
anisotropic magnetization damping parameters governed by the skyrmion size.",1311.1778v1
2014-03-11,Non-linear galactic dynamos: A toolbox,"We compare various models and approximations for non-linear mean-field
dynamos in disc galaxies to assess their applicability and accuracy, and thus
to suggest a set of simple solutions suitable to model the large-scale galactic
magnetic fields in various contexts. The dynamo saturation mechanisms
considered are the magnetic helicity balance involving helicity fluxes (the
dynamical $\alpha$-quenching) and an algebraic $\alpha$-quenching. The
non-linear solutions are then compared with the marginal kinematic and
asymptotic solutions. We also discuss the accuracy of the no-$z$ approximation.
Although these tools are very different in the degree of approximation and
hence complexity, they all lead to remarkably similar solutions for the mean
magnetic field. In particular, we show that the algebraic $\alpha$-quenching
non-linearity can be obtained from a more physical dynamical $\alpha$-quenching
model in the limit of nearly azimuthal magnetic field. This suggests, for
instance, that earlier results on galactic disc dynamos based on the simple
algebraic non-linearity are likely to be reliable, and that estimates based on
simple, even linear models are often a good starting point. We suggest improved
no-$z$ and algebraic $\alpha$-quenching models, and also incorporate galactic
outflows into a simple analytical dynamo model to show that the outflow can
produce leading magnetic spirals near the disc surface. The simple dynamo
models developed are applied to estimate the magnetic pitch angle and the
arm-interarm contrast in the saturated magnetic field strength for realistic
parameter values.",1403.2562v2
2014-08-03,Nonlinear Force-Free Field Modeling of the Solar Magnetic Carpet and Comparison with SDO/HMI and Sunrise/IMaX Observations,"In the quiet solar photosphere, the mixed polarity fields form a magnetic
carpet, which continuously evolves due to dynamical interaction between the
convective motions and magnetic field. This interplay is a viable source to
heat the solar atmosphere. In this work, we used the line-of-sight (LOS)
magnetograms obtained from the Helioseismic and Magnetic Imager (HMI) on the
\textit{Solar Dynamics Observatory} (\textit{SDO}), and the Imaging
Magnetograph eXperiment (IMaX) instrument on the \textit{Sunrise} balloon-borne
observatory, as time dependent lower boundary conditions, to study the
evolution of the coronal magnetic field. We use a magneto-frictional relaxation
method, including hyperdiffusion, to produce time series of three-dimensional
(3D) nonlinear force-free fields from a sequence of photospheric LOS
magnetograms. Vertical flows are added up to a height of 0.7 Mm in the modeling
to simulate the non-force-freeness at the photosphere-chromosphere layers.
Among the derived quantities, we study the spatial and temporal variations of
the energy dissipation rate, and energy flux. Our results show that the energy
deposited in the solar atmosphere is concentrated within 2 Mm of the
photosphere and there is not sufficient energy flux at the base of the corona
to cover radiative and conductive losses. Possible reasons and implications are
discussed. Better observational constraints of the magnetic field in the
chromosphere are crucial to understand the role of the magnetic carpet in
coronal heating.",1408.0497v1
2014-12-01,Experimental and Numerical Understanding of Localized Spin Wave Mode Behavior in Broadly Tunable Spatially Complex Magnetic Configurations,"Spin wave modes confined in a ferromagnetic film by the spatially
inhomogeneous magnetic field generated by a scanned micromagnetic tip of a
ferromagnetic resonance force microscope (FMRFM) enable microscopic imaging of
the internal fields and spin dynamics in nanoscale magnetic devices. Here we
report a detailed study of spin wave modes in a thin ferromagnetic film
localized by magnetic field configurations frequently encountered in FMRFM
experiments, including geometries in which the probe magnetic moment is both
parallel and antiparallel to the applied uniform magnetic field. We demonstrate
that characteristics of the localized modes, such as resonance field and
confinement radius, can be broadly tuned by controlling the orientation of the
applied field relative to the film plane. Micromagnetic simulations accurately
reproduce our FMRFM spectra allowing quantitative understanding of the
localized modes. Our results reveal a general method of generating tightly
confined spin wave modes in various geometries with excellent spatial
resolution that significantly facilitates the broad application of FMRFM. This
paves the way to imaging of magnetic properties and spin wave dynamics in a
variety of contexts for uncovering new physics of nanoscale spin excitations.",1412.0726v1
2015-03-24,Spin dynamics and frequency dependence of magnetic damping study in soft ferromagnetic FeTaC film with a stripe domain structure,"Perpendicular magnetic anisotropy (PMA) and low magnetic damping are the key
factors for the free layer magnetization switching by spin transfer torque
technique in magnetic tunnel junction devices. The magnetization precessional
dynamics in soft ferromagnetic FeTaC thin film with a stripe domain structure
was explored in broad band frequency range by employing micro-strip
ferromagnetic resonance technique. The polar angular variation of resonance
field and linewidth at different frequencies have been analyzed numerically
using Landau-Lifshitz-Gilbert equation by taking into account the total free
energy density of the film. The numerically estimated parameters Land\'{e}
$g$-factor, PMA constant, and effective magnetization are found to be 2.1,
2$\times10^{5}$ erg/cm$^{3}$ and 7145 Oe, respectively. The frequency
dependence of Gilbert damping parameter ($\alpha$) is evaluated by considering
both intrinsic and extrinsic effects into the total linewidth analysis. The
value of $\alpha$ is found to be 0.006 at 10 GHz and it increases with
decreasing precessional frequency.",1503.07043v5
2015-05-07,"Magnetization relaxation, critical current density and vortex dynamics in a Ba$_{0.66}$K$_{0.32}$BiO$_{3+δ}$ single crystal","We have conducted extensive investigations on the magnetization and its
dynamical relaxation on a Ba$_{0.66}$K$_{0.32}$BiO$_{3+\delta}$ single crystal.
It is found that the magnetization relaxation rate is rather weak compared with
that in the cuprate superconductors, indicating a higher collective vortex
pinning potential (or activation energy), although the intrinsic pinning
potential $U_\mathrm{c}$ is weaker. Detailed analysis leads to the following
discoveries: (1) A second-peak effect on the magnetization-hysteresis-loop was
observed in a very wide temperature region, ranging from 2K to 24K. Its general
behavior looks like that in YBa$_2$Cu$_3$O$_7$; (2) Associated with the second
peak effect, the magnetization relaxation rate is inversely related to the
transient superconducting current density $J_\mathrm{s}$ revealing a quite
general and similar mechanism for the second peak effect in many high
temperature superconductors; (3) A detailed analysis based on the collective
creep model reveals a large glassy exponent $\mu$ and a small intrinsic pinning
potential $U_\mathrm{c}$; (4) Investigation on the volume pinning force density
shows that the data can be scaled to the formula $F_{p}\propto b^p(1-b)^q$ with
$p=2.79$ and $q=3.14$, here $b$ is the reduced magnetic field to the
irreversible magnetic field. The maximum normalized pinning force density
appears near $b\approx0.47$. Finally, a vortex phase diagram is drawn for
showing the phase transitions or crossovers between different vortex phases.",1505.01717v1
2015-11-23,Femtosecond X-ray magnetic circular dichroism absorption spectroscopy at an X-ray free electron laser,"X-ray magnetic circular dichroism spectroscopy using an X-ray free electron
laser is demonstrated with spectra over the Fe L$_{3,2}$-edges. This new
ultrafast time-resolved capability is then applied to a fluence-dependent study
of all-optical magnetic switching dynamics of Fe and Gd magnetic sublattices in
a GdFeCo thin film above its magnetization compensation temperature. At the
magnetic switching fuence, we corroborate the existence of a transient
ferromagnetic-like state. The timescales of the dynamics, however, are longer
than previously observed below the magnetization compensation temperature.
Above and below the switching fluence range, we observe secondary
demagnetization with about 5 ps timescales. This indicates that the spin
thermalization takes longer than 5 ps.",1511.07372v1
2016-07-14,Spin dynamics of diamond nitrogen-vacancy centres at the ground state level anti-crossing and all-optical low frequency magnetic field sensing,"We investigate the photo-induced spin dynamics of single nitrogen-vacancy
(NV) centres in diamond near the electronic ground state level anti-crossing
(GSLAC), which occurs at an axial magnetic field around 1024 G. Using optically
detected magnetic resonance spectroscopy, we first find that the electron spin
transition frequency can be tuned down to 100 kHz for the \NV{14} centre, while
for the \NV{15} centre the transition strength vanishes for frequencies below
about 2 MHz owing to the GSLAC level structure. Using optical pulses to prepare
and readout the spin state, we observe coherent spin oscillations at 1024 G for
the \NV{14}, which originate from spin mixing induced by residual transverse
magnetic fields. This effect is responsible for limiting the smallest
observable transition frequency, which can span two orders of magnitude from
100 kHz to tens of MHz depending on the local magnetic noise. A similar feature
is observed for the \NV{15} centre at 1024 G. As an application of these
findings, we demonstrate all-optical detection and spectroscopy of
externally-generated fluctuating magnetic fields at frequencies from 8 MHz down
to 500 kHz, using a \NV{14} centre. Since the Larmor frequency of most nuclear
spin species lies within this frequency range near the GSLAC, these results
pave the way towards all-optical, nanoscale nuclear magnetic resonance
spectroscopy, using longitudinal spin cross-relaxation.",1607.04006v1
2016-07-20,Magnetic Field Amplification During the Common Envelope Phase,"During the common envelope (CE) phase, a giant star in a binary system
overflows its Roche lobe and unstable mass transfer leads to a spiral-in of the
companion, resulting in a close binary system or in a merger of the stellar
cores. Dynamo processes during the CE phase have been proposed as a mechanism
to generate magnetic fields that are important for forming magnetic white
dwarfs (MWDs) and for shaping planetary nebulae. Here, we present the first
magnetohydrodynamics simulations of the dynamical spiral-in during a CE phase.
We find that magnetic fields are strongly amplified in the accretion stream
around the $1M_\odot$ companion as it spirals into the envelope of a $2M_\odot$
RG. This leads to field strengths of 10 to 100 kG throughout the envelope after
120 d. The magnetic field amplification is consistent with being driven by the
magnetorotational instability. The field strengths reached in our simulation
make the magnetic field interesting for diagnostic purposes, but they are
dynamically irrelevant. They are also too small to explain the formation of the
highest fields found in MWDs, but may be relevant for luminous red novae, and
detecting magnetic fields in these events would support the scenario as
proposed here.",1607.05996v1
2016-08-05,Optical determination of the Neel vector in a CuMnAs thin-film antiferromagnet,"Recent breakthroughs in electrical detection and manipulation of
antiferromagnets have opened a new avenue in the research of non-volatile
spintronic devices. Antiparallel spin sublattices in antiferromagnets,
producing zero dipolar fields, lead to the insensitivity to magnetic field
perturbations, multi-level stability, ultra-fast spin dynamics and other
favorable characteristics which may find utility in fields ranging from
magnetic memories to optical signal processing. However, the absence of a net
magnetic moment and the ultra-short magnetization dynamics timescales make
antiferromagnets notoriously difficult to study by common magnetometers or
magnetic resonance techniques. In this paper we demonstrate the experimental
determination of the Neel vector in a thin film of antiferromagnetic CuMnAs
which is the prominent material used in the first realization of
antiferromagnetic memory chips. We employ a femtosecond pump-probe
magneto-optical experiment based on magnetic linear dichroism. This table-top
optical method is considerably more accessible than the traditionally employed
large scale facility techniques like neutron diffraction and X-ray magnetic
dichroism measurements. This optical technique allows an unambiguous direct
determination of the Neel vector orientation in thin antiferromagnetic films
utilized in devices directly from measured data without fitting to a
theoretical model.",1608.01941v1
2016-08-25,"Charged dust grain dynamics subject to solar wind, Poynting-Robertson drag, and the interplanetary magnetic field","We investigate the combined effect of solar wind, Poynting-Robertson drag,
and the frozen-in interplanetary magnetic field on the motion of charged dust
grains in our solar system. For this reason we derive a secular theory of
motion by the means of averaging method and validate it with numerical
simulations of the un-averaged equations of motions. The theory predicts that
the secular motion of charged particles is mainly affected by the z-component
of the solar magnetic axis, or the normal component of the interplanetary
magnetic field. The normal component of the interplanetary magnetic field leads
to an increase or decrease of semi-major axis depending on its functional form
and sign of charge of the dust grain. It is generally accepted that the
combined effects of solar wind and photon absorption and re-emmision
(Poynting-Robertson drag) lead to a decrease in semi-major axis on secular time
scales. On the contrary, we demonstrate that the interplanetary magnetic field
may counteract these drag forces under certain circumstances. We derive a
simple relation between the parameters of the magnetic field, the physical
properties of the dust grain as well as the shape and orientation of the
orbital ellipse of the particle, which is a necessary conditions for the
stabilization in semi-major axis.",1608.07040v1
2017-01-27,The nature of spin excitations in the one-third magnetization plateau phase of Ba$_3$CoSb$_2$O$_9$,"Magnetization plateaus in quantum magnets---where bosonic quasiparticles
crystallize into emergent spin superlattices---are spectacular yet simple
examples of collective quantum phenomena escaping classical description. While
magnetization plateaus have been observed in a number of spin-1/2
antiferromagnets, the description of their magnetic excitations remains an open
theoretical and experimental challenge. Here, we investigate the dynamical
properties of the triangular-lattice spin-1/2 antiferromagnet
Ba$_3$CoSb$_2$O$_9$ in its one-third magnetization plateau phase using a
combination of nonlinear spin-wave theory and neutron scattering measurements.
The agreement between our theoretical treatment and the experimental data
demonstrates that magnons behave semiclassically in the plateau in spite of the
purely quantum origin of the underlying magnetic structure. This allows for a
quantitative determination of Ba$_3$CoSb$_2$O$_9$ exchange parameters. We
discuss the implication of our results to the deviations from semiclassical
behavior observed in zero-field spin dynamics of the same material and conclude
they must have an intrinsic origin.",1701.07971v4
2017-02-13,Strain-mediated magnetoelectric effect for the electric-field control of magnetic states in nanomagnets,"Electric-field control of magnetism without electric currents potentially
revolutionizes spintronics towards ultralow power. Here by using mechanically
coupled phase field simulations, we computationally demonstrate the application
of the strain-mediated magnetoelectric effect for the electric-field control of
magnetic states in heterostructure. In the model heterostructure constituted of
the soft nanomagnet Co and the piezoelectric substrate PMN-PT, both the
volatility of magnetic states and the magnetization switching dynamics excited
by the electric field are explored. It is found that an electric field can
drive the single-domain nanomagnet into an equilibrium vortex state. The
nanomagnet remains in the vortex state even after removing the electric field
or applying a reverse electric field, i.e. the vortex state is extremely stable
and nonvolatile. Only by utilizing the precessional magnetization dynamics, the
180$^\circ$ magnetization switching is possible in small-sized nanomagnets
which are free of the stable vortex state. Electric-field pulses can realize
the deterministic 180$^\circ$ switching if the electric-field magnitude, pulse
width, and ramp time are carefully designed. The minimum switching time is
found to be less than 10 ns. These results provide useful information for the
design of low-power, reliable, and fast electric-field-controlled spintronics.",1702.03672v2
2017-04-02,Effect of strong magnetic fields on the crust-core transition and inner crust of neutron stars,"The Vlasov equation is used to determine the dispersion relation for the
eigenmodes of magnetized nuclear and neutral stellar matter, taking into
account the anomalous magnetic moment of nucleons. The formalism is applied to
the determination of the dynamical spinodal section, a quantity that gives a
good estimation of the crust-core transition in neutron stars. We study the
effect of strong magnetic fields, of the order of $10^{15}-10^{17}$ G, on the
extension of the crust of magnetized neutron stars. The dynamical instability
region of neutron-proton-electron ($npe$) matter at subsaturation densities is
determined within a relativistic mean field model. It is shown that a strong
magnetic field has a large effect on the instability region, defining the
crust-core transition as a succession of stable and unstable regions due to the
opening of new Landau levels. The effect of the anomalous magnetic moment is
non-negligible for fields larger than 10$^{15}$ G. The complexity of the crust
at the transition to the core and the increase of the crust thickness may have
direct impact on the properties of neutrons stars related with the crust.",1704.00375v1
2017-04-21,Time- and spatially-resolved magnetization dynamics driven by spin-orbit torques,"Current-induced spin-orbit torques (SOTs) represent one of the most effective
ways to manipulate the magnetization in spintronic devices. The orthogonal
torque-magnetization geometry, the strong damping, and the large domain wall
velocities inherent to materials with strong spin-orbit coupling make SOTs
especially appealing for fast switching applications in nonvolatile memory and
logic units. So far, however, the timescale and evolution of the magnetization
during the switching process have remained undetected. Here, we report the
direct observation of SOT-driven magnetization dynamics in Pt/Co/AlO$_x$ dots
during current pulse injection. Time-resolved x-ray images with 25 nm spatial
and 100 ps temporal resolution reveal that switching is achieved within the
duration of a sub-ns current pulse by the fast nucleation of an inverted domain
at the edge of the dot and propagation of a tilted domain wall across the dot.
The nucleation point is deterministic and alternates between the four dot
quadrants depending on the sign of the magnetization, current, and external
field. Our measurements reveal how the magnetic symmetry is broken by the
concerted action of both damping-like and field-like SOT and show that
reproducible switching events can be obtained for over $10^{12}$ reversal
cycles.",1704.06402v1
2017-05-04,EUV and Magnetic Activities Associated with Type-I Solar Radio Bursts,"Type-I bursts (i.e. noise storms) are the earliest-known type of solar radio
emission at the metre wavelength. They are believed to be excited by
non-thermal energetic electrons accelerated in the corona. The underlying
dynamic process and exact emission mechanism still remain unresolved. Here,
with a combined analysis of extreme ultraviolet (EUV), radio and photospheric
magnetic field data of unprecedented quality recorded during a type-I storm on
30 July 2011, we identify a good correlation between the radio bursts and the
co-spatial EUV and magnetic activities. The EUV activities manifest themselves
as three major brightening stripes above a region adjacent to a compact
sunspot, while the magnetic field there presents multiple moving magnetic
features (MMFs) with persistent coalescence or cancelation and a
morphologically similar three-part distribution. We find that the type-I
intensities are correlated with those of the EUV emissions at various
wavelengths with a correlation coefficient of 0.7-0.8. In addition, in the
region between the brightening EUV stripes and the radio sources there appear
consistent dynamic motions with a series of bi-directional flows, suggesting
ongoing small-scale reconnection there. Mainly based on the induced connection
between the magnetic motion at the photosphere and the EUV and radio activities
in the corona, we suggest that the observed type-I noise storms and the EUV
brightening activities are the consequence of small-scale magnetic reconnection
driven by MMFs. This is in support of the original proposal made by Bentely et
al. (Solar Phys. 193, 227, 2000).",1705.01666v1
2017-05-12,Switching dynamics of the spin density wave in superconducting CeCoIn5,"The ordering wave vector $\mathbf{Q}$ of a spin density wave (SDW),
stabilized within the superconducting state of $\mathrm{CeCoIn_5}$ in a high
magnetic field, has been shown to be hypersensitive to the direction of the
field. $\mathbf{Q}$ can be switched from a nodal direction of the $d$-wave
superconducting order parameter to a perpendicular node by rotating the
in-plane magnetic field through the antinodal direction within a fraction of a
degree. Here, we address the dynamics of the switching of $\mathbf{Q}$. We use
a free energy functional based on the magnetization density, which describes
the condensation of magnetic fluctuations of nodal quasiparticles, and show
that the switching process includes closing of the SDW gap at one $\mathbf{Q}$
and then reopening the SDW gap at another $\mathbf{Q}$ perpendicular to the
first one. The magnetic field couples to $\mathbf{Q}$ through the spin-orbit
interaction. Our calculations show that the width of the hysteretic region of
switching depends linearly on the deviation of magnetic field from the critical
field associated with the SDW transition, consistent with our thermal
conductivity measurements. The agreement between theory and experiment supports
our scenario of the hypersensitivity of the $Q$ phase on the direction of
magnetic field, as well as the magnon condensation as the origin of the SDW
phase in $\mathrm{CeCoIn_5}$.",1705.04655v1
2017-08-04,Magnetic field-dependent dynamics and field-driven metal-to-insulator transition of the half-filled Hubbard model: A DMFT+DMRG study,"We study the magnetic field driven metal-to-insulator transition in
half-filled Hubbard model on the Bethe lattice, using the dynamical mean-field
theory by solving the quantum impurity problem with density-matrix
renormalization group algorithm. The method enables us to obtain a
high-resolution spectral densities in the presence of a magnetic field. It is
found that the Kondo resonance at the Fermi level splits at relatively high
magnetic field: the spin-up and spin-down components move away from the Fermi
level and finally form a spin polarized band insulator. By calculating the
magnetization and spin susceptibility, we clarify that an applied magnetic
field drives a transition from a paramagnetic metallic phase to a band
insulting phase. In the weak interaction regime, the nature of the transition
is continuous and captured by the Stoner's description, while in the strong
interaction regime the transition is very likely to be metamagnetic, evidenced
by the hysteresis curve. Furthermore, we determine the phase boundary by
tracking the kink in the magnetic susceptibility, and the step-like change of
the entanglement entropy and the entanglement gap closing. Interestingly, the
phase boundary determined from these two different ways are largely consistent
with each other.",1708.01687v1
2017-09-29,Half-metallicity and magnetism in the Co$_2$MnAl/CoMnVAl heterostructure,"We present a study of the electronic structure and magnetism of Co$_2$MnAl,
CoMnVAl and their heterostructure. We employ a combination of
density-functional theory and dynamical mean-field theory (DFT+DMFT). We find
that Co$_2$MnAl is a half-metallic ferromagnet, whose electronic and magnetic
properties are not drastically changed by strong electronic correlations,
static or dynamic. Non-quasiparticle states are shown to appear in the minority
spin gap without affecting the spin-polarization at the Fermi level predicted
by standard DFT. We find that CoMnVAl is a semiconductor or a semi-metal,
depending on the employed computational approach. We then focus on the
electronic and magnetic properties of the Co$_2$MnAl/CoMnVAl heterostructure,
predicted by previous first principle calculations as a possible candidate for
spin-injecting devices. We find that two interfaces, Co-Co/V-Al and
Co-Mn/Mn-Al, preserve the half-metallic character, with and without including
electronic correlations. We also analyse the magnetic exchange interactions in
the bulk and at the interfaces. At the Co-Mn/Mn-Al interface, competing
magnetic interactions are likely to favor the formation of a non-collinear
magnetic order, which is detrimental for the spin-polarization.",1709.10464v2
2018-05-31,Resonate and Fire Neuron with Fixed Magnetic Skyrmions,"In the brain, the membrane potential of many neurons oscillates in a
subthreshold damped fashion and fire when excited by an input frequency that
nearly equals their eigen frequency. In this work, we investigate theoretically
the artificial implementation of such ""resonate-and-fire"" neurons by utilizing
the magnetization dynamics of a fixed magnetic skyrmion in the free layer of a
magnetic tunnel junction (MTJ). To realize firing of this nanomagnetic
implementation of an artificial neuron, we propose to employ voltage control of
magnetic anisotropy or voltage generated strain as an input (spike or
sinusoidal) signal, which modulates the perpendicular magnetic anisotropy
(PMA). This results in continual expansion and shrinking (i.e. breathing) of a
skyrmion core that mimics the subthreshold oscillation. Any subsequent input
pulse having an interval close to the breathing period or a sinusoidal input
close to the eigen frequency drives the magnetization dynamics of the fixed
skyrmion in a resonant manner. The time varying electrical resistance of the
MTJ layer due to this resonant oscillation of the skyrmion core is used to
drive a Complementary Metal Oxide Semiconductor (CMOS) buffer circuit, which
produces spike outputs. By rigorous micromagnetic simulation, we investigate
the interspike timing dependence and response to different excitatory and
inhibitory incoming input pulses. Finally, we show that such resonate and fire
neurons have potential application in coupled nanomagnetic oscillator based
associative memory arrays.",1806.00076v1
2018-06-29,Post-adiabatic supernova remnants in an interstellar magnetic field: oblique shocks and non-uniform environment,"We present very-high-resolution 1D MHD simulations of the late-stage
supernova remnants (SNR). In the post-adiabatic stage the magnetic field has an
important and significant dynamical effect on the shock dynamics, the flow
structure, and hence the acceleration and emission of cosmic rays. We find that
the tangential component of the magnetic field provides pressure support that
to a fair degree prevents the collapse of the radiative shell and thus limits
the total compression ratio of the partially or fully radiative forward shock.
A consequence is that the spectra of cosmic rays would not be as hard as in
hydrodynamic simulations. We also investigated the effect on the flow profiles
of the magnetic-field inclination and a large-scale gradient in the gas density
and/or the magnetic field. A positive density gradient shortens the
evolutionary stages whereas a shock obliquity lowers the shock compression. The
compression of the tangential component of the magnetic field leads to its
dominance in the downstream region of post-adiabatic shocks for a wide range of
orientation of the upstream field, which may explain why one preferentially
observes tangential radio polarization in old SNRs. As most cosmic rays are
produced at late stages of SNR evolution, the post-adiabatic phase and the
influence of the magnetic field during it are most important for modeling the
cosmic-ray acceleration at old SNRs and the gamma-ray emission from late-stage
SNRs interacting with clouds.",1806.11396v1
2018-07-09,Spiral-arm instability - II: magnetic destabilisation,"Fragmentation of spiral arms can drive the formation of giant clumps and
induce intense star formation in disc galaxies. Based on the spiral-arm
instability analysis of our Paper I, we present linear perturbation theory of
dynamical instability of self-gravitating spiral arms of magnetised gas,
focusing on the effect of toroidal magnetic fields. Spiral arms can be
destabilised by the toroidal fields which cancel Coriolis force, i.e.
magneto-Jeans instability. Our analysis can be applied to multi-component
systems that consist of gas and stars. To test our analysis, we perform ideal
magneto-hydrodynamics simulations of isolated disc galaxies and examine the
simulation results. We find that our analysis can characterise dynamical
instability leading arms to fragment and form clumps if magnetic fields are
nearly toroidal. We propose that dimensionless growth rate of the most unstable
perturbation, which is computed from our analysis, can be used to predict
fragmentation of spiral arms within an orbital time-scale. Our analysis is
applicable as long as magnetic fields are nearly toroidal. Using our analytic
model, we estimate a typical mass of clumps forming from spiral-arm
fragmentation to be consistent with observed giant clumps $\sim10^{7-8}~{\rm
M_\odot}$. Furthermore, we find that, although the magnetic destabilisation can
cause low-density spiral arms to fragment, the estimated mass of resultant
clumps is almost independent from strength of magnetic fields since marginal
instability occurs at long wavelengths which compensate the low densities of
magnetically destabilised arms.",1807.02988v2
2018-09-11,Time variations of the non-potential and volume-threading magnetic helicities,"Relative magnetic helicity is a gauge invariant quantity suitable for the
study of the magnetic helicity content of heliospheric plasmas. Relative
magnetic helicity can be decomposed uniquely into two gauge invariant
quantities, the magnetic helicity of the non-potential component of the field,
and a complementary volume-threading helicity. Recent analysis of numerical
experiments simulating the generation of solar eruptions have shown that the
ratio of the non-potential helicity to the total relative helicity is a clear
marker of the eruptivity of the magnetic system, and that the high value of
that quantity could be a sufficient condition for the onset of the instability
generating the eruptions. The present study introduces the first analytical
examination of the time variations of these non-potential and volume-threading
helicities. The validity of the analytical formulas derived are confirmed with
analysis of three-dimensional (3D) magnetohydrodynamics (MHD) simulations of
solar coronal dynamics. Both the analytical investigation, and the numerical
application show that, unlike magnetic helicity, the non-potential and the
volume-threading helicities are not conserved quantities, even in the ideal MHD
regime. A term corresponding to the transformation between the non-potential
and volume-threading helicities frequently dominates their dynamics. This
finding has an important consequence for their estimation in the solar corona:
unlike with relative helicity, their volume coronal evolution cannot be
ascertained by the flux of these quantities through the volume's boundaries.
Only techniques extrapolating the 3D coronal field will enable both the proper
study of the non-potential and volume-threading helicities, and the
observational analysis of helicity-based solar-eruptivity proxies.",1809.03765v1
2018-12-25,Solar Magnetic Flux Rope Eruption Simulated by a Data-Driven Magnetohydrodynamic Model,"The combination of magnetohydrodynamic (MHD) simulation and multi-wavelength
observations is an effective way to study mechanisms of magnetic flux rope
eruption. We develop a data-driven MHD model using the zero-$\beta$
approximation. The initial condition is provided by nonlinear force-free field
derived by the magneto-frictional method based on vector magnetic field
observed by the Helioseismic and Magnetic Imager (HMI) aboard the Solar
Dynamics Observatory (SDO). The bottom boundary uses observed time series of
the vector magnetic field and the vector velocity derived by the Differential
Affine Velocity Estimator for Vector Magnetograms (DAVE4VM). We apply the
data-driven model to active region 11123 observed from 06:00 UT on 2011
November 11 to about 2 hours later. The evolution of the magnetic field
topology coincides with the flare ribbons observed in the 304 and 1600 \AA $ $
wavebands by the Atmospheric Imaging Assembly. The morphology, propagation
path, and propagation range of the flux rope are comparable with the
observations in 304 \AA . We also find that a data-constrained boundary
condition, where the bottom boundary is fixed to the initial values, reproduces
a similar simulation result. This model can reproduce the evolution of a
magnetic flux rope in its dynamic eruptive phase.",1812.10030v1
2019-05-31,Fingerprints of Kitaev physics in the magnetic excitations of honeycomb iridates,"In the quest for realizations of quantum spin liquids, the exploration of
Kitaev materials - spin-orbit entangled Mott insulators with strong
bond-directional exchanges - has taken center stage. However, in these
materials the local spin-orbital j=1/2 moments typically show long-range
magnetic order at low temperature, thus defying the formation of a spin-liquid
ground state. Using resonant inelastic x-ray scattering (RIXS), we here report
on a proximate spin liquid regime with clear fingerprints of Kitaev physics in
the magnetic excitations of the honeycomb iridates alpha-Li2IrO3 and Na2IrO3.
We observe a broad continuum of magnetic excitations that persists up to at
least 300K, more than an order of magnitude larger than the magnetic ordering
temperatures. We prove the magnetic character of this continuum by an analysis
of the resonance behavior. RIXS measurements of the dynamical structure factor
for energies within the continuum show that dynamical spin-spin correlations
are restricted to nearest neighbors. Notably, these spectroscopic observations
are also present in the magnetically ordered state for excitation energies
above the conventional magnon excitations. Phenomenologically, our data agree
with inelastic neutron scattering results on the related honeycomb compound
RuCl3, establishing a common ground for a proximate Kitaev spin-liquid regime
in these materials.",1905.13590v1
2019-06-18,Dynamical and thermal magnetic properties of the Kitaev spin liquid candidate $α$-RuCl$_3$,"What is the correct low-energy spin Hamiltonian description of
$\alpha$-RuCl$_3$? The material is a promising Kitaev spin liquid candidate,
but is also known to order magnetically, the description of which necessitates
additional interaction terms. The nature of these interactions, their
magnitudes and even signs, remain an open question. In this work we
systematically investigate dynamical and thermodynamic magnetic properties of
proposed effective Hamiltonians. We calculate zero-temperature inelastic
neutron scattering (INS) intensities using exact diagonalization, and magnetic
specific heat using a thermal pure quantum states method. We find that no
single current model satisfactorily explains all observed phenomena of
$\alpha$-RuCl$_3$. In particular, we find that Hamiltonians derived from first
principles can capture the experimentally observed high-temperature peak in the
magnetic specific heat, while overestimating the magnon energy at the zone
center. In contrast, other models reproduce important features of the INS data,
but do not adequately describe the magnetic specific heat. To address this
discrepancy we propose a modified ab initio model that is consistent with both
magnetic specific heat and low-energy features of INS data.",1906.07579v3
2019-09-16,Magnetic topology in fluids,"We study the evolution of turbulent magnetic fields from a topological point
of view, invoking commonplace mathematical tools from general topology and
dynamical systems theory which connect magnetic field evolution to time
reversal invariance, entropy increase and the second law of thermodynamics. We
show that in fact magnetic topology is well-defined only in the phase space
corresponding to a dynamical system governed by the induction equation. Hence
the field's topology and stochasticity can be studied in terms of the
corresponding phase space trajectories rather than the field lines in real
Euclidean space. In fact, our results suggest that magnetic field lines should
not be taken too literally because their existence and uniqueness and more
importantly continuity in time require strong mathematical conditions, hardly
satisfied in astrophysical systems. As for magnetic topology change, it is
shown that the phase space topology is preserved in time for a magnetic field
which, besides satisfying few continuity conditions, solves a time reversal
invariant induction equation. What breaks the time symmetry in the induction
equation is the presence of non-ideal plasma effects at small scales such as
resistivity, which results from random collisions between diffusing electrons
and other particles. The small scale, stochastic disturbances produced thereby
are super-linearly amplified by Richardson diffusion in the turbulent cascade,
which are eventually manifested as large scale reconnection events, somehow
similar to stretching quantum fluctuations during inflation to seed large scale
cosmological structures. This suggests that reconnection is rooted in the
second law of thermodynamics that dictates entropy increase which in turn
breaks the time symmetry.",1909.07325v4
2019-12-05,Collective in-plane magnetization in a 2D XY macrospin system within the framework of generalized Ott-Antonsen theory,"The problem of magnetic transitions between the low-temperature (macrospin
ordered) phases in 2D XY arrays is addressed. The system is modeled as a plane
structure of identical single-domain particles arranged in a square lattice and
coupled by the magnetic dipole-dipole interaction; all the particles possess a
strong easy-plane magnetic anisotropy. The basic state of the system in the
considered temperature range is an antiferromagnetic (AF) stripe structure,
where the macrospins (particle magnetic moments) are still involved in
thermofluctuational motion: the superparamagnetic blocking $T_b$ temperature is
lower than that ($T_\text{af}$) of the AF transition. The description is based
on the stochastic equations governing the dynamics of individual magnetic
moments, where the interparticle interaction is added in the mean field
approximation. With the technique of a generalized Ott-Antonsen theory, the
dynamics equations for the order parameters (including the macroscopic
magnetization and the antiferromagnetic order parameter) and the partition
function of the system are rigorously obtained and analysed. We show that
inside the temperature interval of existence of the AF phase, a static external
field tilted to the plane of the array is able to induce first order phase
transitions from AF to ferromagnetic state; the phase diagrams displaying
stable and metastable regions of the system are presented.",1912.02575v1
2020-07-05,Size and temperature dependent magnetization of iron nanoclusters,"The magnetic behavior of bcc iron nanoclusters, with diameters between 2 and
8 nm, is investigated by means of spin dynamics (SD) simulations coupled to
molecular dynamics (MD-SD), using a distance-dependent exchange interaction.
Finite-size effects in the total magnetization as well as the influence of the
free surface and the surface/core proportion of the nanoclusters are analyzed
in detail for a wide temperature range, going beyond the cluster and bulk Curie
temperatures. Comparison is made with experimental data and with theoretical
models based on the mean-field Ising model adapted to small clusters, and
taking into account the influence of low coordinated spins at free surfaces.
Our results for the temperature dependence of the average magnetization per
atom M(T), including the thermalization of the transnational lattice degrees of
freedom, are in very good agreement with available experimental measurements on
small Fe nanoclusters. In contrast, significant discrepancies with experiment
are observed if the translational degrees of freedom are artificially frozen.
The finite-size effects on M(T) are found to be particularly important near the
cluster Curie temperature. Simulated magnetization above the Curie temperature
scales with cluster size as predicted by models assuming short-range magnetic
ordering (SRMO). Analytical approximations to the magnetization as a function
of temperature and size are proposed.",2007.02230v2
2020-08-10,"Color, Flavor, Temperature and Magnetic Field Dependence of QCD Phase Diagram: Magnetic Catalysis and its Inverse","We study dynamical chiral symmetry breaking for quarks in the fundamental
representation of $SU(N_c)$ for $N_f$ number of light quark flavors. We also
investigate the phase diagram of quantum chromodynamics at finite temperature
$T$ and/or in the presence of a constant external magnetic field $eB$. The
unified formalism for this analysis is provided by a symmetry-preserving
Schwinger-Dyson equations treatment of a vector$\times$vector contact
interaction model which encodes several well-established features of quantum
chromodynamics to mimic the latter as closely as possible. Deconfinement and
chiral symmetry restoration are triggered above a critical value of $N_f$ at
$T=0=eB$. On the other hand, increasing temperature itself screens strong
interactions, thus ensuring that a smaller value of $N_f$ is sufficient to
restore chiral symmetry at higher temperatures. We also observe the well-known
phenomenon of magnetic catalysis for a strong enough magnetic field. However,
we note that if the effective coupling strength of the model decreases as a
function of magnetic field, it can trigger inverse magnetic catalysis in a
certain window of this functional dependence. Our model allows for the
simultaneous onset of dynamical chiral symmetry breaking and confinement for
each case. Qualitative as well as quantitative predictions of our simple but
effective model are in reasonably satisfactory agreement with lattice results
and other reliable and refined predictions based upon intricate continuum
studies of quantum chromodynamics.",2008.03847v1
2020-09-09,Dynamics of magnetized particles around Einstein-Æther black hole with uniform magnetic field,"This work is devoted to study the effects of Einstein-\AE ther gravity on the
dynamics of magnetized particles orbiting a static, spherically symmetric and
uncharged black hole immersed in an external asymptotically uniform magnetic
field in both comoving and proper observers frames. The analysis is carried out
by varying the free parameters $c_{13}$ and $c_{14}$ of the Einstein-\AE ther
theory and noticing their impacts on the particle trajectories, radii of the
innermost stable circular orbits (ISCOs), and the amount of center-of-mass
energy produced as a result of the collision. The strength of the magnetic
field and the location of circular orbits is significantly affected by varying
the above free parameters. We have also made detailed comparisons between the
effects of parameters of Einstein-\AE ther and spin of rotating Kerr black
holes on ISCO followed by magnetized particles and noticed that both black
holes depict similar behaviour for suitable values of $c_{13}$, $c_{14}$, spin
and the magnetic coupling parameters which provide exactly the same values for
the ISCO. Finally, we have analysed the cases when a static \AE ther black hole
can be described as Schwarzschild black hole in modified gravity (MOG) with the
corresponding values of the parameters of the black holes.",2009.04898v2
2020-09-21,Magneto-Elastic Coupling to Coherent Acoustic Phonon Modes in Ferrimagnetic Insulator GdTiO$_3$,"In this work we investigate single crystal GdTiO$_{3}$, a promising candidate
material for Floquet engineering and magnetic control, using ultrafast optical
pump-probe reflectivity and magneto-optical Kerr spectroscopy. GdTiO${}_{3}$ is
a Mott-Hubbard insulator with a ferrimagnetic and orbitally ordered ground
state (\textit{T${}_{C}$} = 32 K). We observe multiple signatures of the
magnetic phase transition in the photoinduced reflectivity signal, in response
to above band-gap 660 nm excitation. Magnetic dynamics measured via Kerr
spectroscopy reveal optical perturbation of the ferrimagnetic order on
spin-lattice coupling timescales, highlighting the competition between the
Gd${}^{3+}$ and Ti${}^{3+}$ magnetic sub-lattices. Furthermore, a strong
coherent oscillation is present in the reflection and Kerr dynamics,
attributable to an acoustic strain wave launched by the pump pulse. The
amplitude of this acoustic mode is highly dependent on the magnetic order of
the system, growing sharply in magnitude at \textit{T${}_{C}$}, indicative of
strong magneto-elastic coupling. The driving mechanism, involving
strain-induced modification of the magnetic exchange interaction, implies an
indirect method of coupling light to the magnetic degrees of freedom and
emphasizes the potential of GdTiO${}_{3}$ as a tunable quantum material.",2009.10222v1
2020-11-19,Topological insulators and semimetals in classical magnetic systems,"Pursuing topological phases in natural and artificial materials is one of the
central topics in modern physical science and engineering. In classical
magnetic systems, spin waves (or magnons) and magnetic solitons (such as domain
wall, vortex, skyrmion, etc) represent two important excitations. Recently, the
topological insulator and semimetal states in magnon- and soliton-based
crystals (or metamaterials) have attracted growing attention owing to their
interesting dynamics and promising applications for designing robust spintronic
devices. Here, we give an overview of current progress of topological phases in
structured classical magnetism. We first provide a brief introduction to spin
wave, and discuss its topological properties including magnon Hall effects,
topological magnon insulators, and Dirac (Weyl) magnon semimetals. Appealing
proposal of topological magnonic devices is also highlighted. We then review
the collective-coordinate approach for describing the dynamics of magnetic
soliton lattice. Pedagogical topological models such as the
Su-Schrieffer-Heeger model and the Haldane model and their manifestation in
magnetic soliton crystals are elaborated. Then we focus on the topological
properties of magnetic solitons, by theoretically analyzing the first-order
topological insulating phases in low dimensional systems and higher-order
topological states in breathing crystals. Finally, we discuss the experimental
realization and detection of the edge states in both the magnonic and solitonic
crystals. We remark the challenges and future prospects before concluding this
article.",2011.09751v1
2020-12-06,Ultrafast ferromagnetic fluctuations preceding magnetoelastic first-order transitions,"First-order magnetic transitions are of both fundamental and technological
interest given that a number of emergent phases and functionalities are thereby
created. Of particular interest are giant magnetocaloric effects, which are
attributed to first-order magnetic transitions and have attracted broad
attention for solid-state refrigeration applications. While the conventional
wisdom is that atomic lattices play an important role in first-order magnetic
transitions, a coherent microscopic description of the lattice and spin degrees
of freedom is still lacking. Here, we study the magnetic phase transition
dynamics on the intermetallic LaFe13-xSix, which is one of the most classical
giant magnetocaloric systems, in both frequency and time domains utilizing
neutron scattering and ultrafast X-ray diffraction. We have observed a strong
magnetic diffuse scattering in the paramagnetic state preceding the first-order
magnetic transition, corresponding to picosecond ferromagnetic fluctuations.
Upon photon-excitation, the ferromagnetic state is completely suppressed in 0.9
ps and recovered in 20 ps. The ultrafast dynamics suggest that the magnetic
degree of freedom dominates this magnetoelastic transition and ferromagnetic
fluctuations might be universally relevant for this kind of compounds.",2012.03186v1
2021-04-01,An energy-conserving dynamical model of GRB afterglows from magnetized forward and reverse shocks,"In the dynamical models of gamma-ray burst (GRB) afterglows, the uniform
assumption of the shocked region is known as provoking total energy
conservation problem. In this work we consider shocks originating from
magnetized ejecta, extend the energy-conserving hydrodynamical model of Yan et
al. (2007) to the MHD limit by applying the magnetized jump conditions from
Zhang & Kobayashi (2005). Compared with the non-conservative models, our
Lorentz factor of the whole shocked region is larger by a factor
$\lesssim\sqrt{2}$. The total pressure of the forward shocked region is higher
than the reversed shocked region, in the relativistic regime with a factor of
about 3 in our interstellar medium (ISM) cases while ejecta magnetization
degree $\sigma<1$, and a factor of about 2.4 in the wind cases. For $\sigma\le
1$, the non-conservative model loses $32-42$% of its total energy for ISM
cases, and for wind cases $25-38$%, which happens specifically in the forward
shocked region, making the shock synchrotron emission from the forward shock
less luminous than expected. Once the energy conservation problem is fixed, the
late time light curves from the forward shock become nearly independent of the
ejecta magnetization. The reverse shocked region doesn't suffer from the energy
conservation problem since the changes of the Lorentz factor are recompensed by
the changes of the shocked particle number density. The early light curves from
the reverse shock are sensitive to the magnetization of the ejecta, thus are an
important probe of the magnetization degree.",2104.00509v2
2021-06-04,Cavitation and charge separation in laser-produced copper and carbon plasma in transverse magnetic field,"In the present work, we report the dynamics and geometrical features of the
plasma plume formed by the laser ablation of copper and graphite (carbon)
targets in the presence of different transverse magnetic field. This work
emphasizes on the effect of atomic mass of the plume species on the diamagnetic
behaviour and geometrical aspect of the expanding plasma plume in the magnetic
field. The time-resolved analysis of the simultaneously captured two
directional images in orthogonal to the expansion axis is carried out for the
comparative study of projected three-dimensional structure of copper and carbon
plasma plume. In the presence of magnetic field, sharp differences are observed
between the copper and carbon plasma plumes in terms of formation of
diamagnetic cavity and structure formation. An elliptical cavity-like structure
is observed in case of copper plasma plume which attains the sharp conical
shape with increasing the time delay or magnetic field strength. On the other
hand, splitted carbon plasma plume appears as a Y-shape structure in the
presence of magnetic field where the cavity-like structure is not observed for
the considered time and magnetic field. Based on the modified energy balance
relation for the elliptic cylindrical geometry, we have also simulated the
dynamics of the plume which is in close agreement with observed plasma
expansion in diamagnetic and non-diamagnetic regions.",2106.02339v1
2021-06-24,Light Bridges Can Suppress the Formation of Coronal Loops,"A light bridge is a magnetic intrusion into a sunspot, it interacts with the
main magnetic field and excites a variety of dynamical processes. In the
letter, we studied magnetic connectivity between a light bridge and coronal
loops rooted at the sunspot. We used the data of the Atmospheric Imaging
Assembly onboard the Solar Dynamics Observatory (SDO) to study the features of
sunspots with light bridges. It is found that if a light bridge anchors at the
umbra-penumbra boundary, the coronal loops could not be formed around the
anchoring point. If the a light bridge become detached from the penumbra, the
coronal loop starts to form again. The vector magnetogram provided by the
Helioseismic Magnetic Imager onboard SDO shows that the anchoring region of a
light bridge usually have an accompanying opposite minor-polarities. We
conjugate that the magnetic field line could connect to these opposite
polarities and form short-range magnetic loops, and therefore, coronal loops
that extend to long-range could not be formed. A model of light bridge is
proposed to explain the magnetic connectivity between a light bridge and the
coronal loops. This model could explain many physical processes associated with
light bridges.",2106.12833v1
2021-08-26,Helical turbulent nonlinear dynamo at large magnetic Reynolds numbers,"The excitation and further sustenance of large-scale magnetic fields in
rotating astrophysical systems, including planets, stars and galaxies, is
generally thought to involve a fluid magnetic dynamo effect driven by helical
magnetohydrodynamic turbulence. While this scenario is appealing on general
grounds, it however currently remains largely unconstrained, notably because a
fundamental understanding of the nonlinear asymptotic behaviour of large-scale
fluid magnetism in the astrophysically-relevant but treacherous regime of large
magnetic Reynolds number $Rm$ is still lacking. We explore this problem using
local high-resolution simulations of turbulent magnetohydrodynamics driven by
an inhomogeneous helical forcing generating a sinusoidal profile of kinetic
helicity, mimicking the hemispheric distribution of kinetic helicity in
rotating turbulent fluid bodies. We identify a transition at large $Rm$ to a
nonlinear state, followed up to $Rm\simeq 3\times 10^3$, consisting of strong,
saturated small-scale magnetohydrodynamic turbulence and a weaker, travelling
coherent large-scale field oscillation. This state is characterized by an
asymptotically small resistive dissipation of magnetic helicity, by its spatial
redistribution across the equator through turbulent fluxes driven by the
hemispheric distribution of kinetic helicity, and by the tentative presence in
the tangled dynamical magnetic field of plasmoids typical of reconnection at
large $Rm$.",2108.12037v2
2021-09-02,Giant effective damping of octupole oscillation in an antiferromagnetic Weyl semimetal,"A magnetic Weyl semimetal is a recent focus of extensive research as it may
exhibit large and robust transport phenomena associated with topologically
protected Weyl points in momentum space. Since a magnetic texture provides a
handle for the configuration of the Weyl points and its transport response,
understanding of magnetic dynamics should form a basis of future control of a
topological magnet. Mn3Sn is an example of an antiferromagnetic Weyl semimetal
that exhibits a large response comparable to the one observed in ferromagnets
despite a vanishingly small magnetization. The non-collinear spin order in
Mn3Sn can be viewed as a ferroic order of cluster magnetic octupole and breaks
the time-reversal symmetry, stabilizing Weyl points and the significantly
enhanced Berry curvature near the Fermi energy. Here we report our first
observation of time-resolved octupole oscillation in Mn3Sn. In particular, we
find the giant effective damping of the octupole dynamics, and it is feasible
to conduct an ultrafast switching at < 10 ps, a hundred times faster than the
case of spin-magnetization in a ferromagnet. Moreover, high domain wall
velocity over 10 km/s is theoretically predicted. Our work paves the path
towards realizing ultrafast electronic devices using the topological
antiferromagnet.",2109.01223v1
2021-12-02,Reaching the equilibrium state of frustrated triangular Ising magnet Ca3Co2O6,"Ca3Co2O6 is a frustrated magnet consisting of a triangular arrangement of
chains of Ising spins. It shows regular magnetization steps vs magnetic field
every 1.2 T that are metastable with very slow dynamics. This has puzzled the
community for many years and given rise to numerous potential theories. Here we
approach the problem by seeking the elusive magnetic equilibrium state at T = 2
K. To this end, we explore two approaches: (1) bypassing the slow dynamics
produced by changing fields by instead field-cooling directly to the target
temperature, and (2) quantum annealing in transverse magnetic fields. While we
observe no measurable effect of the quantum annealing in fields up to 7 T,
which is likely due to the large Ising anisotropy of Co spins in this material,
we find that for the field cooling in longitudinal fields we achieve the
predicted equilibrium 1/3 magnetization. We perform Monte Carlo simulations of
the ground state phase diagram and we also simulate the quantum annealing
process and find good agreement between experiment and theory. Thus we present
an investigation of the elusive ground state properties of the canonical
frustrated triangular system Ca3Co2O6.",2112.01356v1
2021-12-28,"Phonon, Electron, and Magnon Excitations in Antiferromagnetic L1$_{0}$-type MnPt","Antiferromagnetic L1$_{0}$-type MnPt is a material with relatively simple
crystal and magnetic structure, recently attracting interest due to its high
N{\'{e}}el temperature and wide usage as a pinning layer in magnetic devices.
While it is experimentally well characterized, the theoretical understanding is
much less developed, in part due to the challenging accuracy requirements
dictated by the small underlying energy scales that govern magnetic ordering in
antiferromagnetic metals. In this work, we use density functional theory, the
Korringa-Kohn-Rostoker formalism, and a Heisenberg model to establish a
comprehensive theoretical description of antiferromagnetic L1$_{0}$-type MnPt,
along with accuracy limits, by thoroughly comparing to available literature
data. Our simulations show that the contribution of the magnetic dipole
interaction to the magnetocrystalline anisotropy energy of $K_{1}$=1.07$\times
10^{6}$\,J/m$^3$ is comparable in magnitude to the spin-orbit contribution.
Using our result for the magnetic susceptibility of $5.25\times10^{-4}$, a
lowest magnon frequency of about 2.02\,THz is predicted, confirming THz spin
dynamics in this material. From our data for electron, phonon, and magnon
dispersion we compute the individual contributions to the total heat capacity
and show that the dominant term at or above 2\,K arises from phonons. From the
Landau-Lifshitz-Gilbert equation, we compute a N\'{e}el temperature of
990--1070 K. Finally, we quantify the magnitude of the magneto-optical Kerr
effect generated by applying an external magnetic field. Our results provide
insight into the underlying physics, which is critical for a deep understanding
of fundamental limits of the time scale of spin dynamics, stability of the
magnetic ordering, and the possibility of magneto-optical detection of
collective spin motion.",2112.13954v1
2022-02-09,Dynamics of magnetic flux tubes in accretion disks of Herbig Ae/Be stars,"The dynamics of magnetic flux tubes (MFTs) in the accretion disk of typical
Herbig Ae/Be star with fossil large-scale magnetic field is modeled taking into
account the buoyant and drag forces, radiative heat exchange with the
surrounding gas, and the magnetic field of the disk. The structure of the disk
is simulated using our magnetohydrodynamic (MHD) model, taking into account the
heating of the surface layers of the disk with the stellar radiation. The
simulations show that MFTs periodically rise from the innermost region of the
disk with speeds up to $10-12$ km s$^{-1}$. MFTs experience decaying magnetic
oscillations under the action of the external magnetic field near the disk's
surface. The oscillation period increases with distance from the star and
initial plasma beta of the MFT, ranging from several hours at $r=0.012$ au up
to several months at $r=1$ au. The oscillations are characterized by pulsations
of the MFT's characteristics including its temperature. We argue that the
oscillations can produce observed IR-variability of Herbig Ae/Be stars, which
would be more intense than in the case of T Tauri stars, since the disks of
Herbig Ae/Be stars are hotter, denser and have stronger magnetic field.",2202.04412v1
2022-02-22,Small moments without long-range magnetic ordering in the zero-temperature ground state of the double-perovskite iridate Ba$_2$YIrO$_6$,"The spin-orbit coupled double perovskite iridate Ba$_2$YIrO$_6$ with $d^4$
occupancy of Ir is considered as a candidate material for a non-magnetic $J=0$
ground state. The issue of existence of such a state in Ba$_2$YIrO$_6$ however
has opened up intense debates both in experimental and theoretical studies. In
this study, we revisit the issue using \textit{ab-initio} density functional
combined with dynamical mean-field theory to investigate the magnetic
properties of Ba$_2$YIrO$_6$ down to zero temperature. To reach the ground
state, a recently developed impurity solver based on tensor-product states
working directly at zero temperature is employed. We find that Ba$_2$YIrO$_6$
has a small instantaneous non-zero magnetic moment, both at $T=0$ K as well as
at room temperature. We did not observe any evidence of magnetic ordering, not
even at $T=0$ K. From the calculated local magnetic susceptibility we see that
the quantum fluctuations are very strong and effective in screening the
instantaneous moments. This dynamical screening, together with frustration
effects in the fcc lattice that can lead to almost degenerate magnetic ground
states, prevents any long-range ordering.",2202.10794v2
2022-05-04,Cosmic rays and random magnetic traps,"The spatial distribution of cosmic ray (CR) particles in the interstellar
medium (ISM) is of major importance in radio astronomy, where its knowledge is
essential for the interpretation of observations, and in theoretical
astrophysics, where CR contribute to the structure and dynamics of the ISM.
Local inhomogeneities in interstellar magnetic field strength and structure can
affect the local diffusivity and ensemble dynamics of the cosmic ray particles.
Magnetic traps (regions between magnetic mirrors located on the same magnetic
line) can lead to especially strong and persistent features in the CR spatial
distribution. Using test particle simulations, we study the spatial
distribution of an ensemble of CR particles (both protons and electrons) in
various magnetic field configurations, from an idealized axisymmetric trap to
those that emerge in intermittent (dynamo-generated) random magnetic fields. We
demonstrate that both the inhomogeneity in the CR sources and the energy losses
by the CR particles can lead to persistent local inhomogeneities in the CR
distribution and that the protons and electrons have different spatial
distributions. Our results can have profound implications for the
interpretation of the synchrotron emission from astronomical objects, and in
particular its random fluctuations.",2205.01986v2
2022-06-20,Effect of internal magnetic flux on a relativistic spin-1 oscillator in the spinning point source-generated spacetime,"We consider a charged relativistic spin-1 oscillator under the influence of
an internal magnetic flux in a 2+1 dimensional spacetime induced by a spinning
point source. In order to analyze the effects of the internal magnetic flux and
spin of the point source on the relativistic dynamics of such a vector field,
we seek a non-perturbative solution of the associated spin-1 equation derived
as an excited state of Zitterbewegung. By performing an analytical solution of
the resulting equation, we determine exact results for the system in question.
Accordingly, we analyze the effects of spin of the point source and internal
magnetic flux on the relativistic dynamics of the considered test field. We see
that the spin of such a field can be altered by the magnetic flux and this
means that the considered system may behave as a fermion or boson according to
the varying values of the magnetic flux, in principle. We observe that the
internal magnetic flux and the spin of the point source impact on the
relativistic energy levels and probability density functions. Also, our results
indicate that the spin of the point source breaks the symmetry of the energy
levels corresponding to particle-antiparticle states.",2206.09898v3
2022-06-29,Inertial active Ornstein-Uhlenbeck particle in the presence of magnetic field,"We consider an inertial active Ornstein-Uhlenbeck particle in an athermal
bath. The particle is charged, constrained to move in a two-dimensional
harmonic trap, and a magnetic field is applied perpendicular to the plane of
motion. The steady state correlations and the mean square displacement are
studied when the particle is confined as well as when it is set free from the
trap. With the help of both numerical simulation and analytical calculations,
we observe that inertia plays a crucial role in the dynamics in the presence of
a magnetic field. In a highly viscous medium where the inertial effects are
negligible, the magnetic field has no influence on the correlated behaviour of
position as well as velocity. In the time asymptotic limit, the overall
displacement of the confined harmonic particle gets enhanced by the presence of
magnetic field and saturates for a stronger magnetic field. On the other hand,
when the particle is set free, the overall displacement gets suppressed and
approaches zero when strength of the field is very high. Interestingly, it is
seen that in the time asymptotic limit, the confined harmonic particle behaves
like a passive particle and becomes independent of the activity, especially in
the presence of a very strong magnetic field. Similarly, for a free particle
the mean square displacement in the long time limit becomes independent of
activity even for a longer persistence of noise correlation in the dynamics.",2206.14715v1
2022-07-07,Thermal spin dynamics of Kitaev magnets $-$ scattering continua and magnetic field induced phases within a stochastic semiclassical approach,"The honeycomb magnet $\alpha-$RuCl$_3$ is a prime candidate material for
realizing the Kitaev quantum spin liquid (QSL), but it shows long-range
magnetic order at low temperature. Nevertheless, its broad inelastic neutron
scattering (INS) response at finite frequency has been interpreted as that of a
'proximate QSL'. A moderate in-plane magnetic field indeed melts the residual
zigzag order, giving rise to peculiar intermediate field phases before the
high-field polarized state. In INS measurements the low-frequency spin waves
disappear, leading to a broad scattering continuum in the field-induced
intermediate regime, whose nature is currently under debate. Here, we study the
magnetic field dependent spin dynamics of the $K-\Gamma-\Gamma'-$model within a
stochastic semiclassical treatment, which incorporates the effect of
finite-temperature fluctuations. At temperatures relevant for INS experiments,
we show how the excitations of the zigzag phase broaden and that the different
intermediate phases all show a similar continuum response. We discuss the
implications of our results for experiments and highlight the importance of
distinguishing finite temperature fluctuations from genuine quantum
fractionalization signatures in frustrated magnets.",2207.03515v2
2022-09-12,Holes and magnetic polarons in a triangular lattice antiferromagnet,"The intricate interplay between charge motion and magnetic order in
geometrically frustrated lattices is central for the properties of many
two-dimensional quantum materials. The triangular lattice antiferromagnet is a
canonical example of a frustrated system, and here we analyse the dynamics of a
hole in such a lattice focusing on observables that have become accessible in a
new generation of experiments. Using the $t$-$J$ model, we solve the problem
exactly within linear spin wave theory in the limit of strong magnetic
interactions, showing that the ground state is described by a coherent state of
spin waves. The derivation highlights the crucial role played by the
interaction between a static hole and the neighboring spins, which originates
in the geometric frustration and has often been omitted in earlier works.
Furthermore, we show that the non-equilibrium dynamics after a hole has
abruptly been inserted at a lattice site is given exactly by a coherent state
with time-dependent oscillatory coefficients. Physically, this describes a
burst of magnetic frustration propagating through only two-thirds of the
lattice sites, since a destructive interference of spin waves leaves spins
parallel to that removed by the hole unperturbed. After the wave has propagated
through the lattice, the magnetization relaxes to that of the ground state. We
then use our analytical solution to benchmark the widely used self-consistent
Born approximation (SCBA), showing that it is very accurate also for a
triangular lattice. The magnetic polaron spectrum is analysed for general
magnetic interactions using the SCBA, and we compare our results with those for
a square lattice.",2209.05131v1
2022-10-11,The characteristics of flare- and CME-productive solar active regions,"Solar flares and coronal mass ejections (CMEs) cause immediate and adverse
effects on the interplanetary space and geospace. The deeper understanding of
the mechanisms that produce them and the construction of efficient prediction
schemes are of paramount importance. The source regions of flares and CMEs
exhibit some common morphological characteristics associated with strongly
sheared magnetic polarity inversion lines, indicative of the complex magnetic
configurations that store huge amounts of free magnetic energy and helicity.
This knowledge is transformed into parameters that can help us distinguish
efficiently between quiet, flare-, and CME-productive active regions.
Nonetheless, flare and CME prediction still faces a number of challenges. The
magnetic field information is constrained at the photosphere and accessed only
from one vantage point of observation; the dynamic behavior of active regions
is still not fully incorporated into predictions; the stochasticity of flares
and CMEs renders their prediction probabilistic. To meet these challenges, new
properties have been put forward to describe different aspects of magnetic
energy storage mechanisms in active regions and offer the opportunity of
parametric studies for over an entire solar cycle. This inventory of predictors
now includes information from flow fields, transition region/coronal
spectroscopy, data-driven modeling of the coronal magnetic field, as well as
parameterizations of dynamic effects from time series. Further work towards
these directions may help alleviate the current limitations in observing the
magnetic field of higher atmospheric layers. This paper reviews these efforts
as well as the importance of transforming new knowledge into more efficient
predictors and including new types of data.",2210.05453v1
2022-11-10,Static and dynamic magnetic properties of the spin-5/2 triangle lattice antiferromagnet Na3Fe(PO4)2 studied by 31P NMR,"$^{31}$P nuclear magnetic resonance (NMR) measurements have been carried out
to investigate the magnetic properties and spin dynamics of Fe$^{3+}$ ($S$ =
5/2) spins in the two-dimensional triangular lattice (TL) compound
Na$_3$Fe(PO$_4$)$_2$. The temperature ($T$) dependence of nuclear spin-lattice
relaxation rates ($1/T_1$) shows a clear peak around N\'eel temperature,
$T_{\rm N} = 10.9$~K, corresponding to an antiferromagnetic (AFM) transition.
From the temparature dependence of NMR shift ($K$) above $T_{\rm N}$, an
exchange coupling between Fe$^{3+}$ spins was estimated to be $J/k_{\rm
B}\simeq 1.9$~K using the spin-5/2 Heisenberg isotropic-TL model. The
temperature dependence of $1/T_1T$ divided by the magnetic susceptibility
($\chi$), $1/T_1T\chi$, above $T_{\rm N}$ proves the AFM nature of spin
fluctuations below $\sim$ 50 K in the paramagnetic state. In the magnetically
ordered state below $T_{\rm N}$, the characteristic rectangular shape of the
NMR spectra is observed, indicative of a commensurate AFM state in its ground
state. The strong temperature dependence of 1/$T_1$ in the AFM state is well
explained by the two-magnon (Raman) process of the spin waves in a 3D
antiferromagnet with a spin-anisotropy energy gap of 5.7 K. The temperature
dependence of sublattice magnetization is also well reproduced by the spin
waves. Those results indicate that the magnetically ordered state of
Na$_3$Fe(PO$_4$)$_2$ is a conventional 3D AFM state, and no obvious spin
frustration effects were detected in its ground state.",2211.05921v1
2023-03-13,Experimental investigation of the effect of topological insulator on the magnetization dynamics of ferromagnetic metal: $BiSbTe_{1.5}Se_{1.5}$ and $Ni_{80}Fe_{20}$ heterostructure,"We have studied ferromagnetic metal/topological insulator bilayer system to
understand magnetization dynamics of ferromagnetic metal (FM) in contact with a
topological insulator (TI). At magnetic resonance condition, the precessing
magnetization in the metallic ferromagnet ($Ni_{80}Fe_{20}$) injects spin
current into the topological insulator ($BiSbTe_{1.5}Se_{1.5}$), a phenomenon
known as spin-pumping. Due to the spin pumping effect, fast relaxation in the
ferromagnet results in the broadening of ferromagnetic resonance linewidth
($\Delta H$). We evaluated the parameters like effective Gilbert damping
coefficient ($\alpha_{eff}$), spin-mixing conductance ($g_{eff}^{\uparrow
\downarrow}$) and spin current density ($j_S^0$) to confirm a successful spin
injection due to spin-pumping into the $BiSbTe_{1.5}Se_{1.5}$ layer. TIs embody
a spin-momentum locked surface state that span the bulk band-gap. It can act
differently to the FM magnetization than the other normal metals. To probe the
effect of topological surface state, a systematic low temperature study is
crucial as surface state of TI dominates at lower temperatures. The exponential
growth of $\Delta H$ for all different thickness combination of FM/TI bilayers
and effective Gilbert damping coefficient ($\alpha_{eff}$) with lowering
temperature confirms the prediction that spin chemical bias generated from
spin-pumping induces surface current in TI due to spin-momentum locking. The
hump-like feature of magnetic anisotropy field ($H_K$)of the bilayer around 60K
suggests that the decrease of interfacial in-plane magnetic anisotropy can
result from exchange coupling between the TI surface state and the local
moments of FM layer.",2303.07025v2
2023-05-03,Catastrophic magnetic flux avalanches in NbTiN superconducting resonators,"Macroscopic superconducting components are an important building block of
various quantum circuits. Since several of the envisioned applications require
exposure to magnetic fields, it is of utmost importance to explore the impact
of magnetic fields on their performance. Here we explore the complex pattern of
magnetic field penetration and identify its impact on the resonance frequency
of NbTiN superconducting resonators by combining magneto-optical imaging and
high-frequency measurements. At temperatures below approximately half of the
superconducting critical temperature, the development of magnetic flux
avalanches manifests itself as a noisy response in the field-dependent
resonance frequency. Magneto-optical imaging reveals different regimes and
distinguishes the impact of avalanches in the ground plane and resonator. Our
findings demonstrate that superconducting resonators represent a valuable tool
to investigate magnetic flux dynamics. Moreover, the current blooming of
niobium-based superconducting radio-frequency devices makes this report timely
by unveiling the severe implications of magnetic flux dynamics.",2305.02418v2
2023-05-24,Manipulation of magnetic solitons under the influence of DMI gradients,"Magnetic solitons are promising for applications due to their intrinsic
properties such as small size, topological stability, ultralow power
manipulation and potentially ultrafast operations. To date, research has
focused on the manipulation of skyrmions, domain walls, and vortices by applied
currents. The discovery of new methods to control magnetic parameters, such as
the interfacial Dzyaloshinskii-Moriya interaction (DMI) by strain, geometry
design, temperature gradients, and applied voltages promises new avenues for
energetically efficient manipulation of magnetic structures. The latter has
shown significant progress in 2d material-based technology. In this work, we
present a comprehensive study using numerical and analytical methods of the
stability and motion of different magnetic textures under the influence of DMI
gradients. Our results show that under the influence of linear DMI gradients,
N\'eel and Bloch-type skyrmions and radial vortex exhibit motion with finite
skyrmion Hall angle, while the circular vortex undergoes expulsion dynamics.
This work provides a deeper and crucial understanding of the stability and
gradient-driven dynamics of magnetic solitons, and paves the way for the design
of alternative low-power sources of magnetization manipulation in the emerging
field of 2d materials.",2305.15052v1
2023-06-28,2.5-MHD models of circumstellar discs around FS~CMa post-mergers : I. Non-stationary accretion stage,"We investigate the dynamic evolution of gaseous region around FS~CMa
post-mergers. Due to the slow rotation of a central B-type star, the dynamics
is driven mainly by the magnetic field of the central star. Recent observations
have allowed us to set a realistic initial conditions such as, the magnetic
field value ($B_\star\approx6\times10^{3}G$), the mass of the central star
($M_\star=6M_\odot$), and the initial disc density
$\rho_{d0}\in[10^{-13}\mathrm{g\,cm^{-3}},10^{-11}\mathrm{g \, cm^{-3}}] $. We
use the PLUTO code to perform 2.5D-MHD simulations of thin and thick discs
models. Especially relevant for the interpretation of the observed properties
of FS~CMa post-mergers are the results for low-density discs, in which we find
formation of a jet emerging from inner edge of the disc, as well as the
formation of the so called ""hot plasmoid"" in the corona region. Jets are
probably detected as discrete absorption components in the resonance lines of
FS~CMa stars. Moreover, the magnetic field configuration in the low-density
plasma region, favors the appearance of magnetocentrifugal winds from the disc.
The currents toward the star created by the magnetic field may explain
accidentally observed material infall. The disc structure is significantly
changed due to the presence of the magnetic field. The magnetic field is also
responsible for the formation of a hot corona as observed in several FS~CMa
stars through the Raman lines. Our results are valid for all magnetic stars
surrounded by a low density plasma, i.e., some of stars showing the B[e]
phenomenon.",2306.16073v1
2023-07-31,Bioinspired multi-asymmetric magnetized surfaces for tailoring energy-free liquid manipulation and 3-DOF solid transportation,"Through the utilization of smart materials and well-designed structures,
functional surfaces have been developed to enable small-scale liquid/solid
manipulation tasks, thereby facilitating crucial applications in the fields of
microfluidics, soft robotics, and biomedical engineering. However, the design
of functional systems with flexible, tunable, and multimodal liquid/solid
manipulation capabilities remains a challenging endeavor. Here, inspired by
asymmetric structural features in natural plants and metachrony in cross-scale
biological systems, I report a magnetic-responsive functional surface that can
achieve rich liquid operations under static magnetic fields, while also
enabling the transportation of solids with multiple degrees of freedom (DOFs)
under dynamic magnetic fields. The presence of curvature pillars on the
surface, combined with their magnetic-driven tilt/gradient arrangement, imparts
liquids with multi-directional spreading modes based on the asymmetry of
Laplace pressure. I elucidate the mechanisms governing these liquid spreading
modes and subsequently develop compelling liquid operations, such as adjustable
anti-gravity climbing, spontaneous modal shifts in liquid transport, and liquid
mixing. Furthermore, the dynamic metachronal motion of the magnetic pillars can
be harnessed for solid object transportation. I illustrate the
synchronous/asynchronous transport modes of the surface and propose a novel
strategy for achieving 3-DOF solid transportation by coordinating the
arrangement of objects and employing magnetic actuation strategies. This study
presents a new design concept for application-oriented manipulation surfaces,
which hold significant potential for extensive engineering applications.",2307.16740v2
2023-08-02,Magnetohydrodynamical torsional oscillations from thermo-resistive instability in hot jupiters,"Hot jupiter atmospheres may be subject to a thermo-resistive instability
where an increase in the electrical conductivity due to ohmic heating results
in runaway of the atmospheric temperature. We introduce a simplified
one-dimensional model of the equatorial sub-stellar region of a hot jupiter
which includes the temperature-dependence and time-dependence of the electrical
conductivity, as well as the dynamical back-reaction of the magnetic field on
the flow. This model extends our previous one-zone model to include the radial
structure of the atmosphere. Spatial gradients of electrical conductivity
strongly modify the radial profile of Alfv\'en oscillations, leading to
steepening and downwards transport of magnetic field, enhancing dissipation at
depth. We find unstable solutions that lead to self-sustained oscillations for
equilibrium temperatures in the range $T_\mathrm{eq}\approx 1000$--$1200$~K,
and magnetic field in the range $\approx 10$--$100$~G. For a given set of
parameters, self-sustained oscillations occur in a narrow range of equilibrium
temperatures which allow the magnetic Reynolds number to alternate between
large and small values during an oscillation cycle. Outside of this temperature
window, the system reaches a steady state in which the effect of the magnetic
field can be approximated as a magnetic drag term. Our results show that
thermo-resistive instability is a possible source of variability in magnetized
hot jupiters at colder temperatures, and emphasize the importance of including
the temperature-dependence of electrical conductivity in models of atmospheric
dynamics.",2308.00892v1
2023-08-21,Fleeting Small-scale Surface Magnetic Fields Build the Quiet-Sun Corona,"Arch-like loop structures filled with million Kelvin hot plasma form the
building blocks of the quiet-Sun corona. Both high-resolution observations and
magnetoconvection simulations show the ubiquitous presence of magnetic fields
on the solar surface on small spatial scales of $\sim$100\,km. However, the
question of how exactly these quiet-Sun coronal loops originate from the
photosphere and how the magnetic energy from the surface is channeled to heat
the overlying atmosphere is a long-standing puzzle. Here we report
high-resolution photospheric magnetic field and coronal data acquired during
the second science perihelion of Solar Orbiter that reveal a highly dynamic
magnetic landscape underlying the observed quiet-Sun corona. We found that
coronal loops often connect to surface regions that harbor fleeting weaker,
mixed-polarity magnetic field patches structured on small spatial scales, and
that coronal disturbances could emerge from these areas. We suggest that weaker
magnetic fields with fluxes as low as $10^{15}$\,Mx and/or those that evolve on
timescales less than 5\,minutes, are crucial to understand the coronal
structuring and dynamics.",2308.10982v2
2023-09-05,Ultrafast optical polarimetry in magnetic phases of Kondo semi metal CeSb,"We investigated photoinduced ultrafast transient dynamics in different
magnetic phases in CeSb by means of the time resolved magneto-optical
spectroscopy. We observe a distinctive coherent oscillations in the
ground-state antiferromagnetic (AF) phase and the high-magnetic field
ferromagnetic (F) phase. While the AF-phase oscillations frequencies match the
recent Raman scattering findings the F-phase oscillation frequency does not
correspond to the previously observed magnetic excitation. The large
spectroscopic factor, g=3.94, and optical polarization properties suggest that
it corresponds to a previously undetected Ce^{3+} coherent crystal-field state
excitation. The AF-phase oscillations show no magnetic field dependence so
their lattice origin cannot be entirely excluded. The non-oscillatory part of
the transients is qualitatively similar in all investigated magnetic phases
with a faster sub-picosecond dynamics in the ferromagnetic and
ferro-para-magnetic phases and is attributed to differences in the electronic
structure, which affect the photo-excited quasiparticle energy relaxation
kinetics.",2309.02331v2
2024-01-16,Anomalous Proximitized Transport in Metal/Quantum Magnet Heterostructure $\rm{Bi_{2}Ir_{2}O_{7}/Yb_{2}Ti_{2}O_{7}}$,"Fluctuations of quantum spins play a crucial role in the emergence of exotic
magnetic phases and excitations. The lack of the charge degree of freedom in
insulating quantum magnets, however, precludes such fluctuations from mediating
electronic transport. Here we show that the quantum fluctuations of a localized
frustrated magnet induce strong proximitized charge transport of the conduction
electrons in a synthetic heterostructure comprising an epitaxial
$\rm{Bi_{2}Ir_{2}O_{7}}$ ultrathin film on the single crystal of
$\rm{Yb_{2}Ti_{2}O_{7}}$. The proximity effects are evidenced by the scaling
behavior of the $\rm{Bi_{2}Ir_{2}O_{7}}$ resistance in correspondance with the
dynamic scaling of the dynamic spin correlation function of
$\rm{Yb_{2}Ti_{2}O_{7}}$, which is a result of quantum fluctuations near a
multi-phase quantum critical point. The proximitized transport in
$\rm{Bi_{2}Ir_{2}O_{7}}$ can be effectively tuned by magnetic field through
suppressing the quantum spin fluctuations as well as inducing transitions via
magnetic anisotropy in $\rm{Yb_{2}Ti_{2}O_{7}}$. Our work establishes a new
pathway for harnessing quantum spin fluctuations in magnetic insulators with
electric transport, offering exciting prospects for potential applications in
the realm of quantum spintronics.",2401.08125v1
2024-01-24,Simplified Shielded MEG-MRI Multimodal System with Scalar-mode Optically Pumped Magnetometers as MEG Sensors,"Magnetoencephalography (MEG) conventionally operates within high-performance
magnetic shields due to the extremely weak magnetic field signals from the
measured objects and the narrow dynamic range of the magnetic sensors employed
for detection. This limitation results in elevated equipment costs and
restricted usage. Additionally, the information obtained from MEG is functional
images, and to analyze from which part of the brain the signals are coming, it
is necessary to capture morphological images separately. When MEG and
morphological imaging devices are separate, despite their individual high
measurement accuracies, discrepancies in positional information may arise. In
response, we have developed a low-field magnetic resonance imaging system that
incorporates scalar-mode optically pumped magnetometers with a wide dynamic
range and exceptionally high measurement sensitivity as sensors for MEG.
Operating at low magnetic fields eliminates the need for superconducting coils
in magnetic resonance imaging and the high-performance magnetic shields
essential for MEG, promising a substantial cost reduction compared to
traditional approaches. We achieved a noise level of about
$16\,\rm{pT/Hz}^{1/2}$ with a single channel magnetometer, and reached a noise
level of $367\,\rm{fT/cm/Hz}^{1/2}$ through differential measurements. The
system successfully conducted MR imaging on a phantom, demonstrating the
potential of MEG and MRI fusion.",2401.13287v2
2024-02-15,Emergent topological quasiparticle kinetics in constricted nanomagnets,"The ubiquitous domain wall kinetics under magnetic field or current
application describes the dynamic properties in nanostructured magnets.
However, when the geometrical size of a nanomagnetic system is constricted to
the limiting domain wall length scale, the competing energetics between
anisotropy, exchange and dipolar interactions can cause emergent kinetics due
to quasiparticle relaxation, similar to bulk magnets of atomic origin. Here, we
present a joint experimental and theoretical study to support this argument --
constricted nanomagnets, made of antiferromagnetic and paramagnetic neodymium
thin film with honeycomb motif, reveal fast kinetic events at ps time scales
due to the relaxation of chiral vortex loop-shaped topological quasiparticles
that persist to low temperature in the absence of any external stimuli. Such
phenomena are typically found in macroscopic magnetic materials. Our discovery
is especially important considering the fact that paramagnets or
antiferromagnets have no net magnetization. Yet, the kinetics in neodymium
nanostructures is quantitatively similar to that found in ferromagnetic
counterparts and only varies with the thickness of the specimen. This suggests
that a universal, topological quasiparticle mediated dynamical behavior can be
prevalent in nanoscopic magnets, irrespective of the nature of underlying
magnetic material.",2402.10143v2
2024-03-14,Large Eddy Simulations of Magnetized Mergers of Black Holes and Neutron Stars,"The LIGO-Virgo-Kagra collaboration has observed gravitational waves
consistent with the mergers of a black hole and a neutron star, namely GW200105
and GW200115, providing evidence for such cataclysmic events. Although no
electromagnetic counterpart was reported for either of these two events, under
certain conditions black hole--neutron star mergers are expected to form a
significant accretion disk and to produce both a short gamma ray burst and a
kilonova, much as observed in the binary neutron star merger GW170817. Here, we
extend our publicly available code \mhduet to study numerically the merger of a
magnetized neutron star with a black hole. \mhduet employs Large Eddy
Simulation techniques to help capture the magnetic field amplification
resulting from turbulence and other sub-grid scale dynamics in the post-merger
stage. In particular, we simulate a merger with parameters favorable to
producing an accretion disk, focusing on the formation and dynamics of the
turbulent disk and the resulting magnetic field amplification. Following the
tidal disruption and during the formation of the accretion disk, the magnetic
field undergoes significant amplification driven by the Kelvin-Helmholtz
instability, reaching strengths of more than $10^{14}\,\rm{G}$ from a realistic
initial strength of $10^{11}\,\rm{G}$ in short timescales of approximately
$20\,\rm{ms}$. At later times in our simulations the magnetic field growth
occurs at larger scales and is dominated mainly by magnetic winding.",2403.09770v1
2000-06-22,Solar flare radio pulsations as a signature of dynamic magnetic reconnection,"Decimetric radio observations of the impulsive solar flare on October 5,
1992, 09:25 UT show a long series of quasi-periodic pulsations deeply
modulating a continuum in the 0.6-2 GHz range that is slowly drifting toward
lower frequencies. We propose a model in which the pulsations of the radio flux
are caused by quasi-periodic particle acceleration episodes that result from a
dynamic phase of magnetic reconnection in a large-scale current sheet. The
reconnection is dominated by repeated formation and subsequent coalescence of
magnetic islands (known as ``secondary tearing'' or ``impulsive bursty'' regime
of reconnection), while a continuously growing plasmoid is fed by newly
coalescing islands. Such a model, involving a current sheet and a growing
plasmoid, is consistent with the Yohkoh observations of the same flare (Ohyama
& Shibata 1998). We present two-dimensional MHD simulations of dynamic magnetic
reconnection that support the model. Within the framework of the proposed
interpretation, the radio observations reveal details of plasmoid formation in
flares.",0006324v1
2002-05-22,Dynamics and Structure of Three-Dimensional Trans-Alfvenic Jets. II. The Effect of Density and Winds,"Two three-dimensional magnetohydrodynamical simulations of strongly
magnetized conical jets, one with a poloidal and one with a helical magnetic
field, have been performed. In the poloidal simulation a significant sheath
(wind) of magnetized moving material developed and partially stabilized the jet
to helical twisting. The fundamental pinch mode was not similarly affected and
emission knots developed in the poloidal simulation. Thus, astrophysical jets
surrounded by outflowing winds could develop knotty structures along a straight
jet triggered by pinching. Where helical twisting dominated the dynamics,
magnetic field orientation along the line-of-sight could be organized by the
toroidal flow field accompanying helical twisting. On astrophysical jets such
structure could lead to a reversal of the direction of Faraday rotation in
adjacent zones along a jet. Theoretical analysis showed that the different
dynamical behavior of the two simulations could be entirely understood as a
result of dependence on the velocity shear between jet and wind which must
exceed a surface Alfven speed before the jet becomes unstable to helical and
higher order modes of jet distortion.",0205377v1
2006-12-07,Dynamics of a magnetized Bianchi I universe with vacuum energy,"We make use of a flat, axisymmetric Bianchi I metric to investigate the
effects of a magnetic field upon the dynamics of the universe for the case in
which the accompanying fluid is a cosmological constant and derive two exact
solutions to the dynamical equations for this situation. We examine the
behaviour of the scale factor perpendicular and parallel to the field lines,
A(t) and W(t) respectively, and find the expected behaviour. The field has the
strongest effect when A(t) is small, decelerating collapse perpendicular to the
field lines, due to magnetic pressure, and accelerating collapse along the
field lines, due to magnetic tension, while the vacuum energy dominates at late
time, driving accelerated expansion.",0612168v2
1999-04-08,Dynamics of vortex and magnetic lines in ideal hydrodynamics and MHD,"Vortex line and magnetic line representations are introduced for description
of flows in ideal hydrodynamics and MHD, respectively. For incompressible
fluids it is shown that the equations of motion for vorticity ${\bf \Omega}$
and magnetic field with the help of this transformation follow from the
variational principle. By means of this representation it is possible to
integrate the system of hydrodynamic type with the Hamiltonian ${\cal{H}}=\int
|{\bf \Omega}| d{\bf r}$. It is also demonstrated that these representations
allow to remove from the noncanonical Poisson brackets, defined on the space of
divergence-free vector fields, degeneracy connected with the vorticity
frozenness for the Euler equation and with magnetic field frozenness for ideal
MHD. For MHD a new Weber type transformation is found. It is shown how this
transformation can be obtained from the two-fluid model when electrons and ions
can be considered as two independent fluids. The Weber type transformation for
ideal MHD gives the whole Lagrangian vector invariant. When this invariant is
absent this transformation coincides with the Clebsch representation analog
introduced by Zakharov and Kuznetov.",9904015v1
2000-05-30,Dynamical Properties of Spin-Orbital Chains in a Magnetic Field,"The excitation spectrum of the one-dimensional spin-orbital model in a
magnetic field is studied, using a recently developed dynamical density matrix
renormalization group technique. The method is employed on chains with up to 80
sites, and examined for test cases such as the spin-1/2 antiferromagnetic
Heisenberg chain, where the excitation spectrum is known exactly from the Bethe
Ansatz. In the spin-orbital chain, the characteristic dynamical response
depends strongly on the model parameters and the applied magnetic field. The
coupling between the spin and orbital degrees of freedom is found to influence
the incommensuration at finite magnetizations. In the regions of the phase
diagram with only massive spin and orbital excitations, a finite field is
required to overcome the spin gap. An incommensurate orbital mode is found to
become massless in this partially spin-polarized regime, indicating a strong
coupling between the two degrees of freedom. In the critical region with three
elementary gapless excitations, a prominent particle-hole excitation is
observed at higher energies, promoted by the biquadratic term in the model
Hamiltonian of the spin-orbital chain.",0005526v2
2001-09-21,"Dynamic spin-glass behavior in a disorder-free, two-component model of quantum frustrated magnets","Motivated by the observation of a spin-glass transition in almost
disorder-free Kagome antiferromagnets, and by the specific form of the
effective low-energy model of the S=1/2, trimerized Kagome antiferromagnet, we
investigate the possibility to obtain a spin-glass behavior in two-component,
disorder-free models. We concentrate on a toy-model, a modified Ashkin-Teller
model in a magnetic field that couples only to one species of spins, for which
we prove that a dynamic spin-glass behavior occurs. The dynamics of the
magnetization is closely related to that of the underlying Ising model in zero
field in which spins and pseudo-spins are intimately coupled. The spin-glass
like history dependence of the magnetization is a consequence of the ageing of
the underlying Ising model.",0109406v2
2002-08-06,Spin pumping and magnetization dynamics in metallic multilayers,"We study the magnetization dynamics in thin ferromagnetic films and small
ferromagnetic particles in contact with paramagnetic conductors. A moving
magnetization vector causes \textquotedblleft pumping\textquotedblright of
spins into adjacent nonmagnetic layers. This spin transfer affects the
magnetization dynamics similar to the Landau-Lifshitz-Gilbert phenomenology.
The additional Gilbert damping is significant for small ferromagnets, when the
nonmagnetic layers efficiently relax the injected spins, but the effect is
reduced when a spin accumulation build-up in the normal metal opposes the spin
pumping. The damping enhancement is governed by (and, in turn, can be used to
measure) the mixing conductance or spin-torque parameter of the
ferromagnet--normal-metal interface. Our theoretical findings are confirmed by
agreement with recent experiments in a variety of multilayer systems.",0208091v2
2002-09-03,Terahertz magneto-spectroscopy of transient plasmas in semiconductors,"Using synchronized near-infrared (NIR) and terahertz (THz) lasers, we have
performed picosecond time-resolved THz spectroscopy of transient carriers in
semiconductors. Specifically, we measured the temporal evolution of THz
transmission and reflectivity after NIR excitation. We systematically
investigated transient carrier relaxation in GaAs and InSb with varying NIR
intensities and magnetic fields. Using this information, we were able to
determine the evolution of the THz absorption to study the dynamics of
photocreated carriers. We developed a theory based on a Drude conductivity with
time-dependent density and density-dependent scattering lifetime, which
successfully reproduced the observed plasma dynamics. Detailed comparison
between experimental and theoretical results revealed a linear dependence of
the scattering frequency on density, which suggests that electron-electron
scattering is the dominant scattering mechanism for determining the scattering
time. In InSb, plasma dynamics was dramatically modified by the application of
a magnetic field, showing rich magneto-reflection spectra, while GaAs did not
show any significant magnetic field dependence. We attribute this to the small
effective masses of the carriers in InSb compared to GaAs, which made the
plasma, cyclotron, and photon energies all comparable in the density, magnetic
field, and wavelength ranges of the current study.",0209047v1
2003-06-12,Nonequilibrium dynamics and magnetoviscosity of moderately concentrated magnetic liquids: A dynamic mean--field study,"A mean-field Fokker-Planck equation approach to the dynamics of ferrofluids
in the presence of a magnetic field and velocity gradients is proposed that
incorporates magnetic dipole-dipole interactions of the colloidal particles.
The model allows to study the combined effect of a magnetic field and dipolar
interactions on the viscosity of the ferrofluid. It is found that dipolar
interactions lead to additional non-Newtonian contributions to the stress
tensor, which modify the behavior of the non-interacting system. The
predictions of the present model are in qualitative agreement with experimental
results, such as presence of normal stress differences, enhancement and
different anisotropy of magnetoviscous effect and the dependence of the
viscosity on the hydrodynamic volume fraction. A quantitative comparison of the
concentration dependence of the magnetoviscosity shows good agreement with
experimental results for low concentrations.",0306313v1
2005-03-19,"Correlations, spin dynamics, defects: the highly-frustrated Kagomé bilayer","The SrCr$\_{9p}$Ga$\_{12-9p}$O$\_{19}$ and
Ba$\_{2}$Sn$\_{2}$ZnGa$\_{10-7p}$Cr$\_{7p}$O$\_{22}$ compounds are two
highly-frustrated magnets possessing a quasi-two-dimensional Kagom\'{e} bilayer
of spin 3/2 chromium ions with antiferromagnetic interactions. Their magnetic
susceptibility was measured by local Nuclear Magnetic Resonance and non-local
(SQUID) techniques, and their low-temperature spin dynamics by Muon Spin
Resonance. Consistent with the theoretical picture drawn for geometrically
frustrated systems, the Kagom\'{e} bilayer is shown here to exhibit: (i) short
range spin-spin correlations down to a temperature much lower than the
Curie-Weiss temperature, no conventional long-range transition occurring; (ii)
a Curie contribution to the susceptibility from paramagnetic defects generated
by spin vacancies; (iii) low-temperature spin fluctuations, at least down to 30
mK, which are a trademark of a dynamical ground state. These properties point
to a spin-liquid ground state, possibly built on Resonating Valence Bonds with
unconfined spinons as the magnetic excitations.",0503496v1
2005-07-20,All-optical probe of precessional magnetization dynamics in exchange biased NiFe/FeMn bilayers,"An internal anisotropy pulse field is launched by an 8.3 ps short laser
excitation, which triggers precessional magnetization dynamics of a
polycrystalline NiFe/FeMn exchange bias system on the picosecond timescale. Due
to the excitation the unidirectional anisotropy and, thus, the exchange
coupling across the interface between the ferromagnetic and the
antiferromagnetic layer is reduced, leading to a fast reduction of the exchange
bias field and to a dramatic increase of the zero-field susceptibility. The
fast optical unpinning is followed by a slower recovery of the interfacial
exchange coupling dominated by spin-lattice and heat flow relaxation with a
time constant of the order of 160 ps. The measured picosecond time evolution of
the exchange decoupling and restoration is interpreted as an anisotropy pulse
field giving rise to fast precessional magnetization dynamics of the
ferromagnetic layer. The strength of the internal pulse field and even the
initial magnetization deflection direction from the equilibrium orientation can
be controlled by the absorbed photons. The dependence of the effective Gilbert
damping on both small and large angle precessional motion was studied, yielding
that both cases can be modeled with reasonable accuracy within the
Landau-Lifshitz and Gilbert framework.",0507475v1
2006-01-10,Quantum beat phenomenon presence in coherent spin dynamics of spin-2 $^{87}$Rb atoms in a deep optical lattice,"Motivated by the recent experimental work (A. Widera, \textit{et al}, Phys.
Rev. Lett. 95, 19045), we study the collisional spin dynamics of two spin-2 $%
^{87}$Rb atoms confined in a deep optical lattice. When the system is
initialized as $|0,0>$, three different two-particle Zeeman states are involved
in the time evolution due to the conservation of magnetization. For a large
magnetic field $B>0.8$ Guass, the spin coherent dynamics reduces to a Rabi-like
oscillation between the states $|0,0>$ and $% |1,-1>$. However, under a small
magnetic field, a general three-level coherent oscillation displays. In
particular, around a critical magnetic field $B_{c}\simeq 0.48$ Guass, the
probability in the Zeeman states $% |2,-2> $ exhibits a novel quantum beat
phenomenon, ready to be confirmed in future experiments.",0601188v1
2006-04-21,Dynamic approach for micromagnetics close to the Curie temperature,"In conventional micromagnetism magnetic domain configurations are calculated
based on a continuum theory for the magnetization which is assumed to be of
constant length in time and space. Dynamics is usually described with the
Landau-Lifshitz-Gilbert (LLG) equation the stochastic variant of which includes
finite temperatures. Using simulation techniques with atomistic resolution we
show that this conventional micromagnetic approach fails for higher
temperatures since we find two effects which cannot be described in terms of
the LLG equation: i) an enhanced damping when approaching the Curie temperature
and, ii) a magnetization magnitude that is not constant in time. We show,
however, that both of these effects are naturally described by the
Landau-Lifshitz-Bloch equation which links the LLG equation with the theory of
critical phenomena and turns out to be a more realistic equation for
magnetization dynamics at elevated temperatures.",0604508v1
2006-07-03,Slow dynamics at the smeared phase transition of randomly layered magnets,"We investigate a model for randomly layered magnets, viz. a three-dimensional
Ising model with planar defects. The magnetic phase transition in this system
is smeared because static long-range order can develop on isolated rare spatial
regions. Here, we report large-scale kinetic Monte Carlo simulations of the
dynamical behavior close to the smeared phase transition which we characterize
by the spin (time) autocorrelation function. In the paramagnetic phase, its
behavior is dominated by Griffiths effects similar to those in magnets with
point defects. In the tail region of the smeared transition the dynamics is
even slower: the autocorrelation function decays like a stretched exponential
at intermediate times before approaching the exponentially small asymptotic
value following a power law at late times. Our Monte-Carlo results are in good
agreement with recent theoretical predictions based on optimal fluctuation
theory.",0607025v2
2007-01-11,Dynamical chiral symmetry breaking by a magnetic field and multi-quark interactions,"Catalysis of dynamical symmetry breaking by a constant magnetic field in
(3+1) dimensions is considered. We use the three flavour Nambu -- Jona-Lasinio
type model with 't Hooft and eight-quark interaction terms. It is shown that
the multi-quark interactions introduce new additional features to this
phenomenon: (a) the local minimum of the effective potential catalyzed by the
constant magnetic field is smoothed out with increasing strength of the field
at the characteristic scale H~10^{19} G, (b) the multi-quark forces generate
independently another local minimum associated with a larger dynamical fermion
mass. This state may exist even for multi-quark interactions with a subcritical
set of couplings, and is globally stable with respect to a further increase of
the magnetic field.",0701090v2
1996-05-05,The Gross-Neveu Model and the Supersymmetric and Non-Supersymmetric Nambu-Jona-Lasinio Model in a Magnetic Field,"The infrared dynamics in the (3+1)-dimensional supersymmetric and
non-supersymmetric Nambu-Jona-Lasinio model in a constant magnetic field is
studied. While at strong coupling the dynamics in these two models is
essentially different, it is shown that the models become equivalent at weak
coupling. In particular, at weak coupling, as the strength of the magnetic
field goes to infinity, both the supersymmetric and non-supersymmetric
Nambu-Jona-Lasinio models are reduced to a continuum set of independent
(1+1)-dimensional Gross-Neveu models, labeled by the coordinates in the plane
perpendicular to the magnetic field. The relevance of these results for
cosmological models based on supersymmetric dynamics is pointed out.",9605027v4
1998-02-13,Dynamical Generation of Fermion Mass and Magnetic Field in Three-Dimensional QED with Chern-Simons Term,"We study dynamical symmetry breaking in three-dimensional QED with a
Chern-Simons (CS) term, considering the screening effect of $N$ flavor
fermions. We find a new phase of the vacuum, in which both the fermion mass and
a magnetic field are dynamically generated, when the coefficient of the CS term
$\kappa$ equals $N e^2/4 \pi$. The resultant vacuum becomes the finite-density
state half-filled by fermions. For $\kappa=N e^2/2 \pi$, we find the fermion
remains massless and only the magnetic field is induced. For $\kappa=0$,
spontaneous magnetization does not occur and should be regarded as an external
field.",9802101v2
1998-05-17,Yukawa Interactions and Dynamical Generation of Mass in an External Magnetic Field,"In this work we study the dynamical generation of a fermion mass induced by a
constant and uniform external magnetic field in an Abelian gauge model with a
Yukawa term. We show that the Yukawa coupling not only enhances the dynamical
generation of the mass, but it substantially decreases the magnetic field
required for the mass to be generated at temperatures comparable to the
electroweak critical temperature. These results indicate that if large enough
primordial magnetic fields were present during the early universe evolution,
the field-induced generation of fermion masses, which in turn corresponds to
the generation of fermion bound states, may play an important role in the
electroweak phase transition.",9805103v2
2001-08-30,Effect of Dynamical SU(2) Gluons to the Gap Equation of Nambu--Jona-Lasinio Model in Constant Background Magnetic Field,"In order to estimate the effect of dynamical gluons to chiral condensate, the
gap equation of SU(2) gauged Nambu--Jona-Lasinio model, under a constant
background magnetic field, is investigated up to the two-loop order in 2+1 and
3+1 dimensions. We set up a general formulation allowing both cases of electric
as well as magnetic background field. We rely on the proper time method to
maintain gauge invariance. In 3+1 dimensions chiral symmetry breaking
($\chi$SB) is enhanced by gluons even in zero background magnetic field and
becomes much striking as the background field grows larger. In 2+1 dimensions
gluons also enhance $\chi$SB but whose dependence on the background field is
not simple: dynamical mass is not a monotone function of background field for a
fixed four-fermi coupling.",0108225v1
2000-12-04,Dynamo effect in parity-invariant flow with large and moderate separation of scales,"It is shown that non-helical (more precisely, parity-invariant) flows capable
of sustaining a large-scale dynamo by the negative magnetic eddy diffusivity
effect are quite common. This conclusion is based on numerical examination of a
large number of randomly selected flows. Few outliers with strongly negative
eddy diffusivities are also found, and they are interpreted in terms of the
closeness of the control parameter to a critical value for generation of a
small-scale magnetic field. Furthermore, it is shown that, for parity-invariant
flows, a moderate separation of scales between the basic flow and the magnetic
field often significantly reduces the critical magnetic Reynolds number for the
onset of dynamo action.",0012005v2
2007-11-04,On connecting the dynamics of the chromosphere and transition region with Hinode SOT and EIS,"We use coordinated Hinode SOT/EIS observations that include high-resolution
magnetograms, chromospheric and TR imaging and TR/coronal spectra in a first
test to study how the dynamics of the TR are driven by the highly dynamic
photospheric magnetic fields and the ubiquitous chromospheric waves. Initial
analysis shows that these connections are quite subtle and require a
combination of techniques including magnetic field extrapolations,
frequency-filtered time-series and comparisons with synthetic chromospheric and
TR images from advanced 3D numerical simulations. As a first result, we find
signatures of magnetic flux emergence as well as 3 and 5 mHz wave power above
regions of enhanced photospheric magnetic field in both chromospheric,
transition region and coronal emission.",0711.0487v1
2008-02-01,Ultrafast dynamics of a magnetic antivortex - Micromagnetic simulations,"The antivortex is a fundamental magnetization structure which is the
topological counterpart of the well-known magnetic vortex. We study here the
ultrafast dynamic behavior of an isolated antivortex in a patterned Permalloy
thin-film element. Using micromagnetic simulations we predict that the
antivortex response to an ultrashort external field pulse is characterized by
the production of a new antivortex as well as of a temporary vortex, followed
by an annihilation process. These processes are complementary to the recently
reported response of a vortex and, like for the vortex, lead to the reversal of
the orientation of the antivortex core region. In addition to its fundamental
interest, this dynamic magnetization process could be used for the generation
and propagation of spin waves for novel logical circuits.",0802.0093v1
2008-02-11,Dynamic localization of lattice electrons under time dependent electric and magnetic fields,"Applying the method of characteristics leads to wavefunctions and dynamic
localization conditions for electrons on the one dimensional lattice under
perpendicular time dependent electric and magnetic fields. Such conditions
proceed again in terms of sums of products of Bessel functions of the first
kind. However, this time one deals with both the number of magnetic flux quanta
times $\pi $ and the quotients between the Bloch frequency and the ones
characterizing competing fields. Tuning the phases of time dependent
modulations leads to interesting frequency mixing effects providing an
appreciable simplification of dynamic localization conditions one looks for.
The understanding is that proceeding in this manner, the time dependent
superposition mentioned above gets reduced effectively to the influence of
individual ac-fields exhibiting mixed frequency quotients. Besides pure field
limits and superpositions between uniform electric and time dependent magnetic
fields, parity and periodicity effects have also been discussed.",0802.1449v1
2008-05-14,Persistent Spin Dynamics in the $S=1/2$ V$_{15}$ Molecular Nano-Magnet,"We present muon spin lattice relaxation measurements in the V15 spin 1/2
molecular nano-magnet. We find that the relaxation rate in low magnetic fields
(<5 kG) is temperature independent below ~10 K, implying that the molecular
spin is dynamically fluctuating down to 12 mK. These measurements show that the
fluctuation time increases as the temperature is decreased and saturates at a
value of ~6 nsec at low temperatures. The fluctuations are attributed to V15
molecular spin dynamics perpendicular to the applied magnetic field direction,
induced by coupling between the molecular spin and nuclear spin bath in the
system.",0805.2164v1
2008-10-06,Dynamics of a self-gravitating magnetized neutron source,"The dynamics of a self-gravitating neutron gas in presence of a magnetic
field is being studied taking the equation of state of a magnetized neutron gas
obtained in a previous study [2]. We work in a Bianchi I spacetime
characterized by a Kasner metric, this metric allow us to take into account the
anisotropy that introduces the magnetic field. The set of Einstein-Maxwell
field equations for this gas becomes a dynamical system in a 4-dimensional
phase space. We get numerical solutions of the system. In particular there is a
unique point like solution for different initial conditions. Physically this
singular solution may be associated with the collapse of a local volume of
neutron material within a neutron star.",0810.1071v2
2009-01-01,Exact results for the criticality of quench dynamics in quantum Ising models,"Based on the obtained exact results we systematically study the quench
dynamics of a one-dimensional spin-1/2 transverse field Ising model with zero-
and finite-temperature initial states. We focus on the magnetization of the
system after a sudden change of the external field and a coherent
time-evolution process. With a zero-temperature initial state, the quench
magnetic susceptibility as a function of the initial field strength exhibits
strongly similar scaling behaviors to those of the static magnetic
susceptibility, and the quench magnetic susceptibility as a function of the
final field strength shows a discontinuity at the quantum critical point. This
discontinuity remains robust and always occurs at the quantum critical point
even for the case of finite-temperature initial systems, which indicates a
great advantage of employing quench dynamics to study quantum phase
transitions.",0901.0158v2
2009-11-20,Spin Transfer Dynamics in Spin Valves with Out-of-plane Magnetized CoNi Free Layers,"We have measured spin transfer-induced dynamics in magnetic nanocontact
devices having a perpendicularly magnetized Co/Ni free layer and an in-plane
magnetized CoFe fixed layer. The frequencies and powers of the excitations
agree well with the predictions of the single-domain model and indicate that
the excited dynamics correspond to precessional orbits with angles ranging from
zero to 90 degrees as the applied current is increased at a fixed field. From
measurements of the onset current as a function of applied field strength we
estimate the magnitude of the spin torque asymmetry parameter lambda ~ 1.5. By
combining these with spin torque ferromagnetic resonance measurements, we also
estimate the spin wave radiation loss in these devices.",0911.4077v1
2010-09-22,Optimal control of magnetization dynamics in ferromagnetic heterostructures by spin--polarized currents,"We study the switching-process of the magnetization in a
ferromagnetic-normal-metal multilayer system by a spin polarized electrical
current via the spin transfer torque. We use a spin drift-diffusion equation
(SDDE) and the Landau-Lifshitz-Gilbert equation (LLGE) to capture the coupled
dynamics of the spin density and the magnetization dynamic of the
heterostructure. Deriving a fully analytic solution of the stationary SDDE we
obtain an accurate, robust, and fast self-consistent model for the
spin-distribution and spin transfer torque inside general ferromagnetic/normal
metal heterostructures. Using optimal control theory we explore the switching
and back-switching process of the analyzer magnetization in a seven-layer
system. Starting from a Gaussian, we identify a unified current pulse profile
which accomplishes both processes within a specified switching time.",1009.4296v2
2011-09-07,The dynamics of azurite Cu$_3$(CO$_3$)$_2$(OH)$_2$ in a magnetic field as determined by neutron scattering,"Azurite, a natural mineral made up of CuO chains, is also an intriguing
spin-1/2 quantum magnet. There has been much debate as to the 1-dimensional
(1D) nature of this material by theorists and experimentalists alike. The focus
of this debate lies in the interactions between Cu-ions within the
antiferromagnetically ordered state below 1.9 K. We present high-resolution
inelastic neutron scattering data which highlight the complexity of the
magnetic ground state of azurite. The application of magnetic fields and
temperatures were used to probe the excitations revealing important information
about the dynamics in this system. From this we are able to conclude that the
1D Heisenberg antiferromagnetic spin chain model is not sufficient to describe
the dynamics in azurite. Instead additional coupling including interchain
interactions and an anisotropic staggered field are necessary to fully model
the observed excitations.",1109.1369v1
2011-09-30,Charge localization and dynamical spin locking in double quantum dots driven by ac magnetic fields,"We investigate electron localization and dynamical spin locking induced by ac
magnetic fields in double quantum dots. We demonstrate that by tuning the ac
magnetic fields parameters, i.e., the field intensity, frequency, and the phase
difference between the fields within each dot, coherent destruction of
tunneling (and thus charge localization) can be achieved. We show that in
contrast with ac electric fields, ac magnetic fields are able to induce spin
locking, i.e., to freeze the electronic spin, at certain field parameters. We
show how the symmetry of the Hamiltonian determines the quasienergy spectrum
which presents degeneracies at certain field parameters, and how it is
reflected in the charge and spin dynamics.",1109.6934v1
2011-12-13,Quantum dynamical screening of the local magnetic moment in Fe-based superconductors,"We have calculated the local magnetic susceptibility of one of the
prototypical Fe-based superconductors (LaFeAsO) by means of the local density
approximation + dynamical mean field theory as a function of both (imaginary)
time and real frequencies with and without vertex corrections. Vertex
corrections are essential for obtaining the correct $\omega$-dependence, in
particular a pronounced low-energy peak at $\omega \sim 0.2 $eV, which
constitutes the hallmark of the dynamical screening of a large instantaneous
magnetic moment on the Fe atoms. In experiments, however, except for the case
of x-ray absorption spectroscopy (XAS), the magnetic moment or the
susceptibility represent typically the average over long time scales. In this
respect, the frequency range of typical neutron experiments would be too
limited to directly estimate the magnitude of the short-time moment.",1112.3002v2
2012-02-07,Prediction of Giant Spin Motive Force due to Rashba Spin-Orbit Coupling,"Magnetization dynamics in a ferromagnet can induce a spin-dependent electric
field through spin motive force. Spin current generated by the spin-dependent
electric field can in turn modify the magnetization dynamics through
spin-transfer torque. While this feedback effect is usually weak and thus
ignored, we predict that in Rashba spin-orbit coupling systems with large
Rashba parameter $\alpha_{\rm R}$, the coupling generates the spin-dependent
electric field [$\pm(\alpha_{\rm R}m_e/e\hbar) (\vhat{z}\times \partial
\vec{m}/\partial t)]$, which can be large enough to modify the magnetization
dynamics significantly. This effect should be relevant for device applications
based on ultrathin magnetic layers with strong Rashba spin-orbit coupling.",1202.1406v2
2013-04-15,Neutron Scattering and Its Application to Strongly Correlated Systems,"Neutron scattering is a powerful probe of strongly correlated systems. It can
directly detect common phenomena such as magnetic order, and can be used to
determine the coupling between magnetic moments through measurements of the
spin-wave dispersions. In the absence of magnetic order, one can detect diffuse
scattering and dynamic correlations. Neutrons are also sensitive to the
arrangement of atoms in a solid (crystal structure) and lattice dynamics
(phonons). In this chapter, we provide an introduction to neutrons and neutron
sources. The neutron scattering cross section is described and formulas are
given for nuclear diffraction, phonon scattering, magnetic diffraction, and
magnon scattering. As an experimental example, we describe measurements of
antiferromagnetic order, spin dynamics, and their evolution in the
La(2-x)Ba(x)CuO(4) family of high-temperature superconductors.",1304.4214v1
2013-09-18,Self-consistent modeling of the dynamic evolution of magnetic island growth in the presence of stabilizing ECCD,"The most promising technique for the control of neoclassical tearing modes in
tokamak experiments is the compensation of the missing bootstrap current with
electron-cyclotron current drive. In this frame, the dynamics of magnetic
islands has been studied extensively in terms of the modified Rutherford
equation, including the presence of current drive, either analytically
described or computed by numerical methods. In this article, a self-consistent
model for the dynamic evolution of the magnetic island and the driven current
is derived, which takes into account the island's magnetic topology and its
effect on the current drive. The model combines the modified Rutherford
equation with a ray-tracing approach to electron-cyclotron wave propagation and
absorption. Numerical results exhibit a decrease in the time required for
complete stabilization with respect to the conventional computation (not taking
into account the island geometry), which increases with increasing initial
island size and radial misalignment of the deposition.",1309.4571v1
2013-11-08,Curvature effects in statics and dynamics of a thin magnetic shell,"Equations of the magnetization dynamics are derived for an arbitrary curved
2D surface. General static solutions are obtained in the limit of a strong
anisotropy of both signs (easy-surface and easy-normal cases). It is shown that
the effect of the curvature can be treated as appearance of an effective
magnetic field which is aligned along the surface normal for the case of
easy-surface anisotropy and it is tangential to the surface for the case of
easy-normal anisotropy. In general, the existence of such a field denies the
solutions strictly tangential as well as strictly normal to the surface. As an
example we consider static equilibrium solutions and linear dynamics for a cone
surface magnetization.",1311.2026v2
2014-01-08,Transport enhancement and suppression in turbulent magnetic reconnection: A self-consistent turbulence model,"Through the enhancement of transport, turbulence is expected to contribute to
the fast reconnection. However the effects of turbulence are not so
straightforward. In addition to the enhancement of transport, turbulence under
some environment shows effects that suppress the transport. In the presence of
turbulent cross helicity, such a dynamic balance between the transport
enhancement and suppression occurs. As this result of dynamic balance, the
region of effective enhanced magnetic diffusivity is confined to a narrow
region, leading to the fast reconnection. In order to confirm this idea, a
self-consistent turbulence model for the magnetic reconnection is proposed.
With the aid of numerical simulations where turbulence effects are incorporated
in a consistent manner through the turbulence model, the dynamic balance in the
turbulence magnetic reconnection is confirmed.",1401.1498v1
2014-01-15,Landau-Stark states and edge-induced Bloch oscillations in topological lattices,"We consider dynamics of a charged particle in a finite along the $x$
direction square lattice in the presence of normal to the lattice plane
magnetic field and in-plane electric field aligned with the $y$ axis. For
vanishing magnetic field this dynamics would be common Bloch oscillations where
the particle oscillates in the $y$ direction with amplitude inverse
proportional to the electric field. We show that a non-zero magnetic field
crucially modifies this dynamics. Namely, the new Bloch oscillations consist of
time intervals where the particle moves with constant velocity in the $x$
direction intermitted by intervals where it is accelerated or decelerated along
the lattice edges. The analysis is done in terms of the Landau-Stark states
which are eigenstates of a quantum particle in a two-dimensional lattice
subject to (real or synthetic) electric and magnetic fields.",1401.3411v1
2014-03-06,Momentum space anisotropy of electronic correlations in Fe and Ni - an analysis of magnetic Compton profiles,"The total and magnetically resolved Compton profiles are analyzed within the
combined density functional and dynamical mean field theory for the transition
metal elements Fe and Ni. A rather good agreement between the measured and
computed magnetic Compton profiles (MCPs) of Fe and Ni is obtained with the
standard Local Spin Density Approximation (LSDA). By including local but
dynamic many-body correlations captured by Dynamical Mean Field Theory (DMFT),
the calculated magnetic Compton profile is further improved when compared with
experiment. The second moment of the difference of the total Compton profiles
between LSDA and DMFT, along the same momentum direction, has been used to
discuss the strength of electronic correlations in Fe and Ni.",1403.1503v1
2014-03-24,Hyperchaotic Intermittent Convection in a Magnetized Viscous Fluid,"We consider a low-dimensional model of convection in a horizontally
magnetized layer of a viscous fluid heated from below. We analyze in detail the
stability of hydromagnetic convection for a wide range of two control
parameters. Namely, when changing the initially applied temperature difference
or magnetic field strength, one can see transitions from regular to irregular
long-term behavior of the system, switching between chaotic, periodic, and
equilibrium asymptotic solutions. It is worth noting that owing to the induced
magnetic field a transition to hyperchaotic dynamics is possible for some
parameters of the model. We also reveal new features of the generalized Lorenz
model, including both type I and III intermittency.",1403.6169v1
2014-07-17,Nonlinear mirror instability,"Slow dynamical changes in magnetic-field strength and invariance of the
particles' magnetic moments generate ubiquitous pressure anisotropies in weakly
collisional, magnetized astrophysical plasmas. This renders them unstable to
fast, small-scale mirror and firehose instabilities, which are capable of
exerting feedback on the macroscale dynamics of the system. By way of a new
asymptotic theory of the early nonlinear evolution of the mirror instability in
a plasma subject to slow shearing or compression, we show that the instability
does not saturate quasilinearly at a steady, low-amplitude level. Instead, the
trapping of particles in small-scale mirrors leads to nonlinear secular growth
of magnetic perturbations, $\delta B/B \propto t^{2/3}$. Our theory explains
recent collisionless simulation results, provides a prediction of the mirror
evolution in weakly collisional plasmas and establishes a foundation for a
theory of nonlinear mirror dynamics with trapping, valid up to $\delta B/B
=O(1)$.",1407.4707v1
2014-07-23,Modulating spin transfer torque switching dynamics with two orthogonal spin-polarizers by varying the cell aspect ratio,"We study in-plane magnetic tunnel junctions with additional perpendicular
polarizer for subnanosecond-current-induced switching memories. The
spin-transfer-torque switching dynamics was studied as a function of the cell
aspect ratio both experimentally and by numerical simulations using the
macrospin model. We show that the anisotropy field plays a significant role in
the dynamics, along with the relative amplitude of the two spin-torque
contributions. This was confirmed by micromagnetic simulations. Real-time
measurements of the reversal were performed with samples of low and high aspect
ratio. For low aspect ratios, a precessional motion of the magnetization was
observed and the effect of temperature on the precession coherence was studied.
For high aspect ratios, we observed magnetization reversals in less than 1 ns
for high enough current densities, the final state being controlled by the
current direction in the magnetic tunnel junction cell.",1407.6240v2
2014-08-30,Geometric Quantum Noise of Spin,"The presence of geometric phases is known to affect the dynamics of the
systems involved. Here we consider a quantum degree of freedom, moving in a
dissipative environment, whose dynamics is described by a Langevin equation
with quantum noise. We show that geometric phases enter the stochastic noise
terms. Specifically, we consider small ferromagnetic particles (nano-magnets)
or quantum dots close to Stoner instability, and investigate the dynamics of
the total magnetization in the presence of tunneling coupling to the metallic
leads. We generalize the Ambegaokar-Eckern-Sch\""on (AES) effective action and
the corresponding semiclassical equations of motion from the U(1) case of the
charge degree of freedom to the SU(2) case of the magnetization. The Langevin
forces (torques) in these equations are strongly influenced by the geometric
phase. As a first but nontrivial application we predict low temperature quantum
diffusion of the magnetization on the Bloch sphere, which is governed by the
geometric phase. We propose a protocol for experimental observation of this
phenomenon.",1409.0150v2
2014-09-08,"Dynamics of magnetization and carriers at the onset of the photo-excited precession of magnetization in (Ga,Mn)As","Photo-excited precession of magnetization in (Ga,Mn)As is investigated by
measuring time-resolved magneto-optical response and transient differential
reflectivity with pump-and-probe technique. In the time region less than 1 ps,
rapidly oscillating and spike-like signals are observed, respectively, with
excitation of below and above the GaAs band gap. Analysis with gyromagnetic
model and autocorrelation function concludes that those signals are not
attributed to ultrafast demagnetization but due to interference between pump
and probe pulses incorporating sub-ps carrier dynamics characteristic of
low-temperature grown semiconductors. Photo-ionization of Mn ions (Mn2+ ->
Mn3+) is proposed as a mechanism which dynamically induces orbital angular
momentum and affects hole-mediated magnetic anisotropy in (Ga,Mn)As.",1409.2209v1
2014-10-14,Spin precession mapping at ferromagnetic resonance via nuclear resonant scattering,"We probe the spin dynamics in a thin magnetic film at ferromagnetic resonance
by nuclear resonant scattering of synchrotron radiation at the 14.4 keV
resonance of $^{57}$Fe. The precession of the magnetization leads to an
apparent reduction of the magnetic hyperfine field acting at the $^{57}$Fe
nuclei. The spin dynamics is described in a stochastic relaxation model adapted
to the ferromagnetic resonance theory by Smit and Beljers to model the decay of
the excited nuclear state. From the fits of the measured data the shape of the
precession cone of the spins is determined. Our results open a new perspective
to determine magnetization dynamics in layered structures with very high depth
resolution by employing ultrathin isotopic probe layers.",1410.3689v1
2014-11-13,Transverse dynamical magnetic susceptibilities from regular static density functional theory: Evaluation of damping and g-shifts of spin-excitations,"The dynamical transverse magnetic Kohn-Sham susceptibility calculated within
time-dependent density functional theory shows a fairly linear behavior for a
finite energy window. This observation is used to propose a scheme where the
computation of this quantity is greatly simplified. Regular simulations based
on static density functional theory can be used to extract the dynamical
behavior of the magnetic response function. Besides the ability to calculate
elegantly damping of magnetic excitations, we derive along the way useful
equations giving the main characteristics of these excitations: effective
$g$-factors and the resonance frequencies that can be accessed experimentally
using inelastic scanning tunneling spectroscopy or spin-polarized electron
energy loss spectroscopy.",1411.3630v1
2015-02-14,Barkhausen noise in the Random Field Ising Magnet Nd$_2$Fe$_{14}$B,"With sintered needles aligned and a magnetic field applied transverse to its
easy axis, the rare-earth ferromagnet Nd$_2$Fe$_{14}$B becomes a
room-temperature realization of the Random Field Ising Model. The transverse
field tunes the pinning potential of the magnetic domains in a continuous
fashion. We study the magnetic domain reversal and avalanche dynamics between
liquid helium and room temperatures at a series of transverse fields using a
Barkhausen noise technique. The avalanche size and energy distributions follow
power-law behavior with a cutoff dependent on the pinning strength dialed in by
the transverse field, consistent with theoretical predictions for Barkhausen
avalanches in disordered materials. A scaling analysis reveals two regimes of
behavior: one at low temperature and high transverse field, where the dynamics
are governed by the randomness, and the second at high temperature and low
transverse field where thermal fluctuations dominate the dynamics.",1502.04234v1
2015-02-18,"Dynamics of Electric Currents, Magnetic Field Topology and Helioseismic Response of a Solar Flare","The solar flare on July 30, 2011 was of a modest X-ray class (M9.3), but it
made a strong photospheric impact and produced a ""sunquake,"" observed with the
Helioseismic and Magnetic Imager (HMI) on NASA's Solar Dynamics Observatory
(SDO). In addition to the helioseismic waves (also observed with the SDO/AIA
instrument), the flare caused a large expanding area of white-light emission
and was accompanied by substantial restructuring of magnetic fields, leading to
the rapid formation of a sunspot structure in the flare region. The flare
produced no significant hard X-ray emission and no coronal mass ejection. This
indicates that the flare energy release was mostly confined to the lower
atmosphere. The absence of significant coronal mass ejection rules out magnetic
rope eruption as a mechanism of helioseismic waves. We discuss the connectivity
of the flare energy release with the electric currents dynamics and show the
potential importance of high-speed plasma flows in the lower solar atmosphere
during the flare energy release.",1502.05190v1
2015-02-24,Coarsening dynamics driven by vortex-antivortex annihilation in ferromagnetic Bose-Einstein condensates,"In ferromagnetic Bose-Einstein condensates (BECs), the quadratic Zeeman
effect controls magnetic anisotropy, which affects on magnetic domain pattern
formation. While the longitudinal magnetization is dominant (similar to the
Ising model) for a negative quadratic Zeeman energy, the transverse
magnetization is dominant (similar to the XY model) for a positive one. When
the quadratic Zeeman energy is positive, the coarsening dynamics is driven by
vortex-antivortex annihilation in the same way as the XY model. However, due to
superfluid flow of atoms, there exist several combinations of vortex-antivortex
pairs in ferromagnetic BECs, which makes the coarsening dynamics more
complicated than that of the XY model. We propose a revised domain growth law,
which is based on the growth law of the two-dimensional XY model, for a
two-dimensional ferromagnetic BEC with a positive quadratic Zeeman energy.",1502.06673v3
2015-03-24,Far-from-equilibrium dynamics of a strongly coupled non-Abelian plasma with non-zero charge density or external magnetic field,"Using holography, we study the evolution of a spatially homogeneous, far from
equilibrium, strongly coupled N=4 supersymmetric Yang-Mills plasma with a
non-zero charge density or a background magnetic field. This gauge theory
problem corresponds, in the dual gravity description, to an initial value
problem in Einstein-Maxwell theory with homogeneous but anisotropic initial
conditions. We explore the dependence of the equilibration process on different
aspects of the initial departure from equilibrium and, while controlling for
these dependencies, examine how the equilibration dynamics are affected by the
presence of a non-vanishing charge density or an external magnetic field. The
equilibration dynamics are remarkably insensitive to the addition of even large
chemical potentials or magnetic fields; the equilibration time is set primarily
by the form of the initial departure from equilibrium. For initial deviations
from equilibrium which are well localized in scale, we formulate a simple model
for equilibration times which agrees quite well with our results.",1503.07148v2
2015-05-01,Spin-charge transport driven by magnetization dynamics on disordered surface of doped topological insulators,"We theoretically study the spin and charge generation along with the electron
transport on a disordered surface of a doped three-dimensional topological
insulator/magnetic insulator junction by using Green's function techniques. We
find that the spin and charge current are induced by not only local but also
nonlocal magnetization dynamics through nonmagnetic impurity scattering on the
disordered surface of the doped topological insulator. We also clarify that the
spin current as well as charge density are induced by spatially inhomogeneous
magnetization dynamics, and the spin current diffusively propagates on the
disordered surface. Using these results, we discuss both local and nonlocal
spin torques before and after the spin and spin current generation on the
surface, and provide a procedure to detect the spin current.",1505.00106v1
2015-09-07,Quantum speed limit for a relativistic electron in a uniform magnetic field,"We analyze the influence of relativistic effects on the minimum evolution
time between two orthogonal states of a quantum system. Defining the initial
state as an homogeneous superposition between two Hamiltonian eigenstates of an
electron in a uniform magnetic field, we obtain a relation between the minimum
evolution time and the displacement of the mean radial position of the electron
wavepacket. The quantum speed limit time is calculated for an electron dynamics
described by Dirac and Schroedinger-Pauli equations considering different
parameters, such as the strength of magnetic field and the linear momentum of
the electron in the axial direction. We highlight that when the electron
undergoes a region with extremely strong magnetic field the relativistic and
non-relativistic dynamics differ substantially, so that the description given
by Schroedinger-Pauli equation enables the electron traveling faster than c,
which is prohibited by Einstein's theory of relativity. This approach allows a
connection between the abstract Hilbert space and the space-time coordinates,
besides the identification of the most appropriate quantum dynamics used to
describe the electron motion.",1509.02057v1
2016-01-21,Magneto--Coriolis waves in a spherical Couette flow experiment,"The dynamics of fluctuations in a fast rotating spherical Couette flow
experiment in the presence of a strong dipolar magnetic field is investigated
in detail, through a thorough analysis of the experimental data as well as a
numerical study. Fluctuations within the conducting fluid (liquid sodium) are
characterized by the presence of several oscillation modes, identified as
magneto-Coriolis (MC) modes, with definite symmetry and azimuthal number. A
numerical simulation provides eigensolutions which exhibit oscillation
frequencies and magnetic signature comparable to the observation. The main
characteristics of these hydromagnetic modes is that the magnetic contribution
has a fundamental influence on the dynamical properties through the Lorentz
forces, although its importance remains weak in an energetical point of view.
Another specificity is that the Lorentz forces are confined near the inner
sphere where the dipolar magnetic field is the strongest, while the Coriolis
forces are concentrated in the outer fluid volume close to the outer sphere.",1601.05684v1
2016-06-21,Torsion Effects and LLG Equation,"Based on the non-relativistic regime of the Dirac equation coupled to a
torsion pseudo-vector, we study the dynamics of magnetization and how it is
affected by the presence of torsion. We consider that torsion interacting terms
in Dirac equation appear in two ways one of these is thhrough the covariant
derivative considering the spin connection and gauge magnetic field and the
other is through a non-minimal spin torsion coupling. We show within this
framework, that it is possible to obtain the most general Landau, Lifshitz and
Gilbert (LLG) equation including the torsion effects, where we refer to torsion
as a geometric field playing an important role in the spin coupling process. We
show that the torsion terms can give us two important landscapes in the
magnetization dynamics: one of them related with damping and the other related
with the screw dislocation that give us a global effect like a helix damping
sharped. These terms are responsible for changes in the magnetization
precession dynamics.",1606.06610v1
2016-08-12,Dynamic response functions and helical gaps in interacting Rashba nanowires with and without magnetic fields,"A partially gapped spectrum due to the application of a magnetic field is one
of the main probes of Rashba spin-orbit coupling in nanowires. Such a ""helical
gap"" manifests itself in the linear conductance, as well as in dynamic response
functions such as the spectral function, the structure factor, or the
tunnelling density of states. In this paper, we investigate theoretically the
signature of the helical gap in these observables with a particular focus on
the interplay between Rashba spin-orbit coupling and electron-electron
interactions. We show that in a quasi-one-dimensional wire, interactions can
open a helical gap even without magnetic field. We calculate the dynamic
response functions using bosonization, a renormalization group analysis, and
the exact form factors of the emerging sine-Gordon model. For special
interaction strengths, we verify our results by refermionization. We show how
the two types of helical gaps, caused by magnetic fields or interactions, can
be distinguished in experiments.",1608.03756v3
2016-08-08,Simulations of Energetic Particles Interacting with Nonlinear Anisotropic Dynamical Turbulence,"We investigate test-particle diffusion in dynamical turbulence based on a
numerical approach presented before. For the turbulence we employ the nonlinear
anisotropic dynamical turbulence model which takes into account wave
propagation effects as well as damping effects. We compute numerically
diffusion coefficients of energetic particles along and across the mean
magnetic field. We focus on turbulence and particle parameters which should be
relevant for the solar system and compare our findings with different
interplanetary observations. We vary different parameters such as the
dissipation range spectral index, the ratio of the turbulence bendover scales,
and the magnetic field strength in order to explore the relevance of the
different parameters. We show that the bendover scales as well as the magnetic
field ratio have a strong influence on diffusion coefficients whereas the
influence of the dissipation range spectral index is weak. The best agreement
with solar wind observations can be found for equal bendover scales and a
magnetic field ratio of $\delta$B/B0 = 0.75.",1609.05226v1
2016-09-27,Steady State and Dynamics of Joule Heating in Magnetic Tunnel Junctions Observed via the Temperature Dependence of RKKY Coupling,"Understanding quantitatively the heating dynamics in magnetic tunnel
junctions (MTJ) submitted to current pulses is very important in the context of
spin-transfer-torque magnetic random access memory development. Here we provide
a method to probe the heating of MTJ using the RKKY coupling of a synthetic
ferrimagnetic storage layer as a thermal sensor. The temperature increase
versus applied bias voltage is measured thanks to the decrease of the spin-flop
field with temperature. This method allows distinguishing spin transfer torque
(STT) effects from the influence of temperature on the switching field. The
heating dynamics is then studied in real-time by probing the conductance
variation due to spin-flop rotation during heating. This approach provides a
new method for measuring fast heating in spintronic devices, particularly
magnetic random access memory (MRAM) using thermally assisted or spin transfer
torque writing.",1609.08385v1
2016-11-15,Dynamical and Reversible Control of Topological Spin Textures,"Recent observations of topological spin textures brought spintronics one step
closer to new magnetic memories. Nevertheless, the existence of Skyrmions, as
well as their stabilization, require very specific intrinsic magnetic
properties which are usually fixed in magnets. Here we address the possibility
to dynamically control their intrinsic magnetic interactions by varying the
strength of a high-frequency laser field. It is shown that drastic changes can
be induced in the antiferromagnetic exchange interactions and the latter can
even be reversed to become ferromagnetic, provided the direct exchange is
already non-negligible in equilibrium as predicted, for example, in Si doped
with C, Sn, or Pb adatoms. In the presence of Dzyaloshinskii-Moriya
interactions, this enables us to tune features of ferromagnetic Skyrmions such
as their radius, making them easier to stabilize. Alternatively, such
topological spin textures can occur in frustrated triangular lattices. Then, we
demonstrate that a high-frequency laser field can induce dynamical frustration
in antiferromagnets, where the degree of frustration can subsequently be tuned
suitably to drive the material toward a Skyrmionic phase.",1611.05075v2
2017-02-27,Dynamics of Bound Magnon Pairs in the Quasi-One-Dimensional Frustrated Magnet LiCuVO_4,"We report on the dynamics of the spin-1/2 quasi-one-dimensional frustrated
magnet LiCuVO$\mathrm{_4}$ measured by nuclear spin relaxation in high magnetic
fields 10--34 T, in which the ground state has spin-density-wave order. The
spin fluctuations in the paramagnetic phase exhibit striking anisotropy with
respect to the magnetic field. The transverse excitation spectrum probed by
$^{51}$V nuclei has an excitation gap, which increases with field. On the other
hand, the gapless longitudinal fluctuations sensed by $^7$Li nuclei grow with
lowering temperature, but tend to be suppressed with increasing field. Such
anisotropic spin dynamics and its field dependence agree with the theoretical
predictions and are ascribed to the formation of bound magnon pairs, a
remarkable consequence of the frustration between ferromagnetic nearest
neighbor and antiferromagnetic next-nearest-neighbor interactions.",1702.08573v2
2017-03-17,Dynamic nuclear spin polarization in self-assembled quantum dots under zero magnetic field,"We studied the zero-field dynamic nuclear spin polarization in a single
In$_{0.75}$Al$_{0.25}$As/Al$_{0.3}$Ga$_{0.7}$As quantum dot. Even without any
external magnetic field, the positive trion excited by the circularly-polarized
light generated an Overhauser field of up to $\sim$0.8 T. From the excitation
power dependences of the Overhauser field and degree of circular polarization
of the photoluminescence spectra, the relation between the Overhauser field and
Knight field under zero external magnetic field was revealed clearly. In
addition, we found that the nuclear depolarization rate decreased as the
magnitude of the longitudinal magnetic field increased, which seemed to be
caused by the influence of the quadrupolar interaction of nuclear spins.
Further, the key parameters describing the dynamics of a coupled
electron-nuclear spin system, the electron g-factor and the fluctuation of the
Overhauser field, were evaluated in a typical single InAlAs quantum dot.",1703.06046v1
2017-03-28,Resonant photoluminescence and dynamics of a hybrid Mn-hole spin in a positively charged magnetic quantum dot,"We analyze, through resonant photoluminescence, the spin dynamics of an
individual magnetic atom (Mn) coupled to a hole in a semiconductor quantum dot.
The hybrid Mn-hole spin and the positively charged exciton in a CdTe/ZnTe
quantum dot forms an ensemble of $\Lambda$ systems which can be addressed
optically. Auto-correlation of the resonant photoluminescence and resonant
optical pumping experiments are used to study the spin relaxation channels in
this multilevel spin system. We identified for the hybrid Mn-hole spin an
efficient relaxation channel driven by the interplay of the Mn-hole exchange
interaction and the coupling to acoustic phonons. We also show that the optical
$\Lambda$ systems are connected through inefficient spin-flips than can be
enhanced under weak transverse magnetic field. The dynamics of the resonant
photoluminescence in a p-doped magnetic quantum dot is well described by a
complete rate equation model. Our results suggest that long lived hybrid
Mn-hole spin could be obtained in quantum dot systems with large
heavy-hole/light-hole splitting.",1703.09441v2
2017-10-02,Multi-scale modelling of current-induced switching in magnetic tunnel junctions using ab initio spin transfer torques,"There exists a significant challenge in developing efficient magnetic tunnel
junctions with low write currents for non-volatile memory devices. With the aim
of analysing potential materials for efficient current-operated magnetic
junctions we have developed a multi-scale methodology combining the ab initio
calculations of spin-transfer torque with large-scale time-dependent
simulations using atomistic spin dynamics. In this work we introduce our
multi-scale approach including a discussion on a number of possible mapping
schemes the ab initio spin torques into the spin dynamics. We demonstrate this
methodology on a prototype Co/MgO/Co/Cu tunnel junction showing that the spin
torques are primarily acting at the interface between the Co free layer and
MgO. Using spin dynamics we then calculate the reversal switching times for the
free layer and the critical voltages and currents required for such switching.
Our work provides an efficient, accurate and versatile framework for designing
novel current-operated magnetic devices, where all the materials details are
take into account.",1710.00574v1
2017-10-10,Electron conduction in solid state via time varying wavevectors,"In this paper, we study electron wavepacket dynamics in electric and magnetic
fields. We rigorously derive the semiclassical equations of electron dynamics
in electric and magnetic fields. We do it both for free electron and electron
in a periodic potential. We do this by introducing time varying wavevectors
$k(t)$. In the presence of magnetic field, our wavepacket reproduces the
classical cyclotron orbits once the origin of the Schr\""oedinger equation is
correctly chosen to be center of cyclotron orbit. In the presence of both
electric and magnetic fields, our equations for wavepacket dynamics differ from
classical Lorentz force equations. We show that in a periodic potential, on
application of electric field, the electron wave function adiabatically follows
the wavefunction of a time varying Bloch wavevector $k(t)$, with its energies
suitably shifted with time. We derive the effective mass equation and discuss
conduction in conductors and insulators.",1710.03480v2
2017-10-18,Magnetic Dynamics of Phase Separation Domains in GdMn2O5 and Gd0.8Ce0.2Mn2O5 Multiferroics,"Specific features of the magnetic properties and magnetic dynamics of
isolated phase separation domains in GdMn2O5 and Gd0.8Ce0.2Mn2O5 have been
investigated. These domains represent 1D superlattices consisting of dielectric
and conducting layers with the ferromagnetic orientation of their spins. A set
of ferromagnetic resonances of separate superlattice layers has been studied.
The properties of the 1D superlattices in GdMn2O5 and Gd0.8Ce0.2Mn2O5 are
compared with the properties of the previously investigated RMn2O5 (R = Eu, Tb,
Er, and Bi) series. The similarity of the properties for all the RMn2O5
compounds with different R ion types is established. Based on the concepts of
the magnetic dynamics of ferromagnetic multilayers and properties of
semiconductor superlattices, a 1D model of the superlattices in RMn2O5 is
built.",1710.06695v1
2017-11-13,Phases of QCD$_3$ from Non-SUSY Seiberg Duality and Brane Dynamics,"We consider a non-supersymmetric USp Yang-Mills Chern-Simons gauge theory
coupled to fundamental flavours. The theory is realised in type IIB string
theory via an embedding in a Hanany-Witten brane configuration which includes
an orientifold and anti-branes. We argue that the theory admits a magnetic
Seiberg dual. Using the magnetic dual we identify dynamics in field theory and
brane physics that correspond to various phases, obtaining a better
understanding of 3d bosonization and dynamical breaking of flavour symmetry in
USp QCD$_3$ theory. In field theory both phases correspond to magnetic 'squark'
condensation. In string theory they correspond to open string tachyon
condensation and brane reconnection. We also discuss other phases where the
magnetic 'squark' is massive. Finally, we briefly comment on the case of
unitary gauge groups.",1711.04832v2
2018-02-07,Latitudinal structure and dynamic of the photospheric magnetic field,"Analysis of the structure and dynamics of the magnetic field of the Sun is
fundamental for understanding of the origin of solar activity and variability
as well as for the study of solar-terrestrial relations. Observations of the
large scale magnetic field in the photosphere taken at the Wilcox Solar
Observatory from 1976 up to 2007 have been analysed to deduce its latitudinal
and longitudinal structures, its differential rotation, and their variability
in time. This paper is dedicated to the analysis and dynamics of the
latitudinal structure of the solar magnetic field over three solar cycles 21,
22, 23. The main results discussed in this paper are the following: the large
scale latitudinal structure is antisymmetric and composed of four zones with
boundaries located at the equator, -25 and + 25 degrees, stable over 10-11
years with a time delay of about 5-6 years in near-equatorial zones. The
variability and North-South asymmetry of polarity waves running from the
equator to the poles with 2-3 - year period was studied in detail.",1802.02450v2
2018-04-22,Magnetic Droplet Soliton Nucleation in Oblique Fields,"We study the auto-oscillating magnetodynamics in orthogonal spin torque
nano-oscillators (STNOs) as a function of the out-of-plane (OOP) magnetic field
angle. In perpendicular fields and at OOP field angles down to approximately 50
degrees we observe the nucleation of a droplet. However, for field angles below
50 degrees, experiments indicate that the droplet gives way to propagating spin
waves, in agreement with our micromagnetic simulations. Theoretical
calculations show that the physical mechanism behind these observations is the
sign changing of spin-wave nonlinearity (SWN) by angle. In addition, we show
that the presence of a strong perpendicular magnetic anisotropy (PMA) free
layer in the system reverses the angular dependence of the SWN and dynamics in
STNOs with respect to the known behavior determined for the in-plane magnetic
anisotropy free layer. Our results are of fundamental interest in understanding
the rich dynamics of nanoscale solitons and spin-wave dynamics in STNOs.",1804.08047v2
2018-06-01,Dirac-Surface-State Modulated Spin Dynamics in a Ferrimagnetic Insulator at Room Temperature,"This work demonstrates dramatically modified spin dynamics of magnetic
insulator (MI) by the spin-momentum locked Dirac surface states of the adjacent
topological insulator (TI) which can be harnessed for spintronic applications.
As the Bi-concentration x is systematically tuned in 5 nm thick (BixSb1-x)2Te3
TI film, the weight of the surface relative to bulk states peaks at x = 0.32
when the chemical potential approaches the Dirac point. At this concentration,
the Gilbert damping constant of the precessing magnetization in 10 nm thick
Y3Fe5O12 MI film in the MI/TI heterostructures is enhanced by an order of
magnitude, the largest among all concentrations. In addition, the MI acquires
additional strong magnetic anisotropy that favors the in-plane orientation with
similar Bi-concentration dependence. These extraordinary effects of the Dirac
surface states distinguish TI from other materials such as heavy metals in
modulating spin dynamics of the neighboring magnetic layer.",1806.00151v1
2018-06-18,Stellar dynamics of low mass stars from the surface to the interior measured by CoRoT and Kepler,"Continuous high-precision photometry of stars, provided by space missions
such as CoRoT, Kepler, and K2, represents a unique way to study stellar
rotation and magnetism. The coupling of these studies of the surface dynamics
with asteroseismology is changing our view to surface and internal dynamics. In
this proceedings I will provide the latest developments in the understanding of
surface and internal rotation and magnetic fields. I will also discuss the
possible discovery of strong internal magnetic fields of dynamo origin in the
convective cores of stars above 1.2-1.4 solar masses. I will finish by
providing constraints on gyrochronology laws for low-mass stars and put the Sun
into context of its magnetism when compared to other solar-analog stars.",1806.06642v1
2018-11-26,Radiative dynamical mass of planar charged fermion in a constant homogeneous magnetic field,"The effective Lagrangian and mass operator are calculated for planar charged
massive and massless fermions in a constant external homogeneous magnetic field
in the one-loop approximation of the 2+1 dimensional quantum electrodynamics
(QED$_{2+1}$). We obtain the renormalizable effective Lagrangian and the
fermion mass operator for a charged fermion of mass $m$ and then calculate
these quantities for the massless case. The radiative corrections to the mass
of charged massless fermion when it occupies the lowest Landau level are found
for the cases of the pure QED$_{2+1}$ as well as the so-called reduced
QED$_{3+1}$ on a 2-brane. The fermion masses were found can be generated
dynamically by an external magnetic field in the pure QED$_{2+1}$ if the
charged fermion has small bare mass $m_0$ and in the reduced QED$_{3+1}$ on a
2-brane even at $m_0=0$. The dynamical mass seems to be likely to be revealed
in monolayer graphene in the presence of constant homogeneous magnetic field
(normal to the graphene sample).",1811.10414v1
2019-02-06,Accelerating spin-space sampling by auxiliary spin-dynamics and temperature-dependent spin-cluster expansion,"Atomistic simulations of thermodynamic properties of magnetic materials rely
on an accurate modelling of magnetic interactions and an efficient sampling of
the high-dimensional spin space. Recent years have seen significant progress
with a clear trend from model systems to material specific simulations that are
usually based on electronic-structure methods. Here we develop a Hamiltonian
Monte Carlo framework that makes use of auxiliary spin-dynamics and an
auxiliary effective model, the temperature-dependent spin-cluster expansion, in
order to efficiently sample the spin space. Our method does not require a
specific form of the model and is suitable for simulations based on
electronic-structure methods. We demonstrate fast warm-up and a reasonably
small dynamical critical exponent of our sampler for the classical Heisenberg
model. We further present an application of our method to the magnetic phase
transition in bcc iron using magnetic bond-order potentials.",1902.02116v1
2019-02-21,Stochastic ejection of nanocontact droplet solitons via drift instability,"The magnetic droplet soliton is a large amplitude, coherently precessing wave
state that exists in ferromagnetic thin films with perpendicular magnetic
anisotropy. To effectively sustain a droplet, magnetic damping can be locally
compensated in a nanocontact region that imparts spin-transfer torque; this has
been successfully deployed in experiment to directly image the droplet and
probe its dynamics electrically. However, theory predicts and experiments
indicate the existence of a drift instability whereby the droplet is ejected
from the spin-transfer-torque-active region and subsequently decays, an effect
that may be enhanced or possibly induced by thermal fluctuations. Using soliton
perturbation theory and large deviation theory, this work determines the
soliton ejection rate and the most likely path an ejected soliton tracks in the
presence of thermal fields. These results lead to an effective lower bound on
the stability of magnetic droplet solitons in spin-transfer torque nanocontact
devices operating at finite temperature and point to ways in which droplets can
be made more robust.",1902.09310v1
2019-03-03,Dynamical mode coupling and coherence in spin Hall nano-oscillator with perpendicular magnetic anisotropy,"We experimentally study the dynamical modes excited by spin current in Spin
Hall nano-oscillators based on the Pt/[Co/Ni] multilayers with perpendicular
magnetic anisotropy. Both propagating spin wave and localized solitonic modes
of the oscillation are achieved and controlled by varying the applied magnetic
field and current. At room temperature, the generation linewidth broadening
associated with mode hopping was observed at currents close to the transition
between different modes and in the mode coexistence regimes. The mode hopping
was suppressed at cryogenic temperatures, confirming that the coupling between
modes is mediated by thermal magnons. We also demonstrate that coherent
single-mode oscillations with linewidth of 5 MHz can be achieved without
applying external magnetic field. Our results provide insight into the
mechanisms controlling the dynamical coherence in nanomagnetic oscillators, and
guidance for optimizing their applications in spin wave-based electronics.",1903.00822v1
2019-03-07,Interplay between intra- and inter-nanowires dynamic dipolar interactions in the spin wave band structure of Py/Cu/Py nanowires,"We have studied both experimentally and theoretically the reprogrammable spin
wave band structure in Permalloy(10nm)/Cu(5nm)/Permalloy(30nm) nanowire arrays
of width w=280 nm and inter-wire separation in the range from 80 to 280 nm. We
found that, depending on the inter-wire separation, the anti-parallel
configuration, where the magnetizations of the two Permalloy layers point in
opposite directions, is stabilized over specific magnetic field ranges thus
enabling us to directly compare the band structure with that of the parallel
alignment. We show that collective spin waves of the Bloch type propagate
through the arrays with different magnonic bandwidths as a consequence of the
interplay between the intra- and inter-nanowire dynamic dipolar interactions. A
detailed understanding, e.g. whether they have a stationary or propagating
character, is achieved by considering the phase relation (in-phase or
out-of-phase) between the dynamic magnetizations in the two ferromagnetic
layers and their average value. This work opens the path to magnetic
field-controlled reconfigurable layered magnonic crystals that can be used for
future nanoscale magnon spintronic devices.",1903.02935v1
2019-03-15,Effect of a magnetic field on the thermodynamic uncertainty relation,"The thermodynamic uncertainty relation provides a universal lower bound on
the product of entropy production and the fluctuations of any current. While
proven for Markov dynamics on a discrete set of states and for overdamped
Langevin dynamics, its status for underdamped dynamics is still open. We
consider a two-dimensional harmonically confined charged particle in a magnetic
field under the action of an external torque. We show analytically that,
depending on the sign of the magnetic field, the thermodynamic uncertainty
relation does not hold for the currents associated with work and heat. A strong
magnetic field can effectively localize the particle with concomitant bounded
fluctuations and low dissipation. Numerical results for a three-dimensional
variant and for further currents suggest that the existence of such a bound
depends crucially on the specific current.",1903.06480v2
2019-03-18,Role of excited states in the dynamics of excitons and their spins in diluted magnetic semiconductors,"We theoretically investigate the impact of excited states on the dynamics of
the exciton ground state in diluted magnetic semiconductor quantum wells.
Exploiting the giant Zeeman shift in these materials, an external magnetic
field is used to bring transitions between the exciton ground state and excited
states close to resonance. It turns out that, when treating the exciton
dynamics in terms of a quantum kinetic theory beyond the Markov approximation,
higher exciton states are populated already well below the critical magnetic
field required to bring the exciton ground state in resonance to an excited
state. This behavior is explained by exciton-impurity correlations that can
bridge energy differences on the order of a few meV and require a quantum
kinetic description beyond the independent-particle picture. Of particular
interest is the significant spin transfer toward states on the optically dark
$2p$ exciton parabola which are protected against radiative decay.",1903.07564v2
2019-04-09,Manipulation of Magnetic Skyrmions by Superconducting Vortices in Ferromagnet-Superconductor Heterostructures,"Dynamics of magnetic skyrmions in hybrid ferromagnetic films harbors novel
physical phenomena and holds promise for technological applications. In this
work, we discuss the behavior of magnetic skyrmions when coupled to
superconducting vortices in a ferromagnet-superconductor heterostructure. We
use numerical simulations and analytic arguments to reveal broader
possibilities for manipulating the skyrmion-vortex dynamic correlations in the
hybrid system, that are not possible in its separated constituents. We explore
the thresholds of particular dynamic phases, and quantify the phase diagram as
a function of the relevant material parameters, applied current and induced
magnetic torques. Finally, we demonstrate the broad and precise tunability of
the skyrmion Hall-angle in presence of vortices, with respect to currents
applied to either or both the superconductor and the ferromagnet within the
heterostructure.",1904.04537v3
2019-04-28,Effect of magnetic flux advection on the dynamics of shock in accretion flow around a rotating black hole,"We investigate the dynamical behaviour of a magnetized, dissipative,
accretion flow around a rapidly rotating black hole. We solve the
magnetohydrodynamic equations and calculate the transonic accretion solutions
which may contain discontinuous shock transitions. We investigate the effect of
$\zeta-$ parameter (parametrizing the radial variation of the toroidal magnetic
flux advection rate) on the dynamical behaviour of shocks. For a rapidly
rotating black hole and for fixed injection parameters at the outer edge, we
show that stationary shocks are sustained in the global magnetized accretion
solutions for a wide range of $\zeta$ and accretion rate ($\dot{m}$). To
investigate the observational implications, we consider dissipative shocks and
estimate the maximum accessible energy from the post-shock corona (PSC) for
nine stellar mass black hole candidates. We compare this with the observed
radio jet kinetic power reported in the literature, whenever available. We find
close agreement between the estimated values from our model with those reported
in the literature.",1904.12296v2
2019-05-03,Chiral excitations of magnetic droplet solitons driven by their own inertia,"The inertial effects of magnetic solitons play a crucial role in their
dynamics and stability. Yet governing their inertial effects is a challenge for
their use in real devices. Here, we show how to control the inertial effects of
magnetic droplet solitons. Magnetic droplets are strongly nonlinear and
localized autosolitons than can form in current-driven nanocontacts. Droplets
can be considered as dynamical particles with an effective mass. We show that
the dynamical droplet bears a second excitation under its own inertia. These
excitations comprise a chiral profile, and appear when the droplet resists the
force induced by the Oersted field of the current injected into the
nanocontact. We reveal the role of the spin torque on the excitation of these
chiral modes and we show how to control these modes using the current and the
field.",1905.01085v2
2019-05-24,Faraday Rotation Measure Dependence on Galaxy Clusters Dynamics,"We study the magnetic fields in galaxy clusters through Faraday rotation
measurements crossing systems in different dynamical states. We confirm that
magnetic fields are present in those systems and analyze the difference between
relaxed and unrelaxed samples with respect to the dispersion between their
inherent Faraday Rotation measurements. We found an increase of this RM
dispersion and a higher RM overlapping frequency for unrelaxed clusters. This
fact suggests that a large scale physical process is involved in the nature of
unrelaxed systems and possible depolarization effects are present in the
relaxed ones. We show that dynamically unrelaxed systems can enhance magnetic
fields to large coherence lengths. In contrast, the results for relaxed systems
suggests that small-scale dynamo can be a dominant mechanism for sustaining
magnetic fields, leading to intrinsic depolarization.",1905.10303v1
2019-06-11,Effects of the magnetic field direction on the Tsallis statistic,"We extend the use of the Tsallis statistic to measure the differences in gas
dynamics relative to the mean magnetic field present from natural eddy-type
motions existing in magnetohydrodynamical (MHD) turbulence. The variation in
gas dynamics was estimated using the Tsallis parameters on the incremental
probability distribution function of the observables (intensity and velocity
centroid) obtained from compressible MHD simulations. We find that the Tsallis
statistic is susceptible to the anisotropy produce by the magnetic field, even
when anisotropy is percent the Tsallis statistic can be use to determine MHD
parameters such as the Sonic Mach number. We quantize the goodness of the
Tsallis parameters using the coefficient of determination to measure the
differences in the gas dynamics. These parameters also determine the level of
magnetization and compressibility of the medium. To further simulate realistic
spectroscopic observational data we introduced smoothing, noise, and cloud
boundaries to the MHD simulations.",1906.04821v1
2019-07-03,Recurrent network classifier for ultrafast skyrmion dynamics,"By using the supervised learning we train a recurrent neural network to
recognize and classify ultrafast magnetization processes realized in
two-dimensional nanosystems with Dzyaloshinskii-Moriya interaction. Our focus
is on the different types of skyrmion dynamics driven by ultrafast magnetic
pulses. Each process is represented as a sequence of the sorted magnetization
vectors inputted into the network. The trained network can perform an accurate
classification of the skyrmionic processes at zero temperature in wide ranges
of parameters that are the magnetic pulse width and damping factor. The network
performance is also demonstrated on different types of unseen data including
finite temperature processes. Our approach can be easily adapted for creating
an autonomous control system on skyrmion dynamics for experiments or device
operations.",1907.01814v1
2019-07-03,Spin wave excitations of magnetic metalorganic materials,"The Organic Materials Database (OMDB) is an open database hosting about
22,000 electronic band structures, density of states and other properties for
stable and previously synthesized 3-dimensional organic crystals. The web
interface of the OMDB offers various search tools for the identification of
novel functional materials such as band structure pattern matching and density
of states similarity search. In this work the OMDB is extended to include
magnetic excitation properties. For inelastic neutron scattering we focus on
the dynamical structure factor $S(\mathbf{q},\omega)$ which contains
information on the excitation modes of the material. We introduce a new dataset
containing atomic magnetic moments and Heisenberg exchange parameters for which
we calculate the spin wave spectra and dynamic structure factor with linear
spin wave theory and atomistic spin dynamics. We thus develop the materials
informatics tools to identify novel functional organic and metalorganic
magnets.",1907.01817v2
2019-07-11,Image of dynamic local exchange interactions in the dc magnetoresistance of spin-polarized current through a dopant,"We predict strong, dynamical effects in the dc magnetoresistance of current
flowing from a spin-polarized electrical contact through a magnetic dopant in a
nonmagnetic host. Using the stochastic Liouville formalism we calculate
clearly-defined resonances in the dc magnetoresistance when the applied
magnetic field matches the exchange interaction with a nearby spin. At these
resonances spin precession in the applied magnetic field is canceled by spin
evolution in the exchange field, preserving a dynamic bottleneck for spin
transport through the dopant. Similar features emerge when the dopant spin is
coupled to nearby nuclei through the hyperfine interaction. These features
provide a precise means of measuring exchange or hyperfine couplings between
localized spins near a surface using spin-polarized scanning tunneling
microscopy, without any ac electric or magnetic fields, even when the exchange
or hyperfine energy is orders of magnitude smaller than the thermal energy.",1907.05509v1
2019-11-07,Electric field effect on spin waves and magnetization dynamics: role of magnetic moment current,"We show that a static electric field $E_x$ gives rise to a shift of the spin
wave dispersion relation $\omega(q_y-q_E)$ in the direction of the wavenumber
$q_y$ of the quantity $q_E=-\gamma_LE_x/c^2$. This effect is caused by the
magnetic moment current carried by the spin wave itself that generates an
additional phase proportional to the electric field, as in the Aharonov-Casher
effect. This effect is independent from the possibly present magneto-electric
effects of insulating ferromagnets and superimposes to them. By extending this
picture to arbitrary magnetization dynamics, we find that the electric field
gives rise to a dynamic interaction term which has the same chiral from of the
Dzyaloshinskii-Moriya interaction but is fully tunable with the applied
electric field.",1911.02902v1
2019-12-02,"Quaternionic approach on the Dirac-Maxwell, Bernoulli and Navier-Stokes equations for dyonic fluid plasma","Applying the Hamilton's quaternion algebra, we propose the generalized
electromagnetic-fluid dynamics of dyons governed by the combination of the
Dirac-Maxwell, Bernoulli and Navier-Stokes equations. The generalized
quaternionic hydro-electromagnetic field of dyonic cold plasma consist the
electrons and the magnetic monopoles in which there exist dual-mass and
dual-charge species in presence of dyons. We construct the conservation of
energy and conservation of momentum equations by equating the quaternionic
scalar and vector parts for generalized hydro-electromagnetic field of dyonic
cold plasma. We propose the quaternionic form of conservation of energy is
related to the Bernoulli's like equation while the conservation of momentum is
related to Navier-Stokes like equation for dynamics of dyonic plasma fluid.
Further, the continuity equation i.e. the conservation of electric and magnetic
charges with the dynamics of hydro-electric and hydro-magnetic flow of
conducting cold plasma fluid is also analyzed. The quaternionic formalism for
dyonic plasma wave emphasizes that there are two types of waves propagation
namely the Langmuir like wave propagation due to electrons, and the
t-Hooft-Polyakov like wave propagation due to magnetic monopoles.",1912.08046v1
2020-04-20,The role of the quadrupolar interaction in the tunneling dynamics of lanthanide molecular magnets,"Quantum tunneling dominates the low temperature magnetization dynamics in
molecular magnets and presents features that are strongly system dependent. The
current discussion is focused on the terbium(III) bis(phtalocyanine)
([TbPc$_2$]$^{-1}$) complex, that should serve as a prototypical case for
lanthanide molecular magnets. We analyze numerically the effect of non-axial
interactions on the magnitude of the intrinsic tunnel splitting and show that
usual suspects like the transverse ligand field and Zeeman interaction fail to
explain the experimentally observed dynamics. We then propose through the
nuclear quadrupolar interaction a viable mechanism that mixes, otherwise
\textit{almost} degenerate hyperfine states.",2004.09001v1
2020-05-04,Spin-orbit-torque magnonics,"The field of magnonics, which utilizes propagating spin waves for nano-scale
transmission and processing of information, has been significantly advanced by
the advent of the spin-orbit torque. The latter phenomenon can allow one to
overcome two main drawbacks of magnonic devices - low energy efficiency of
conversion of electrical signals into spin wave signals, and fast spatial decay
of spin waves in thin-film waveguiding structures. At first glance, the
excitation and amplification of spin waves by spin-orbit torques can seem to be
straightforward. Recent research indicates, however, that the lack of the
mode-selectivity in the interaction of spin currents with dynamic magnetic
modes and the onset of dynamic nonlinear phenomena represent significant
obstacles. Here, we discuss the possible route to overcoming these limitations,
based on the suppression of nonlinear spin-wave interactions in magnetic
systems with perpendicular magnetic anisotropy. We show that this approach
enables efficient excitation of coherent magnetization dynamics and propagating
spin waves in extended spatial regions, and is expected to enable practical
implementation of complete compensation of spin-wave propagation losses.",2005.01391v1
2020-05-15,Multiple magnon modes in spin-1/2 Heisenberg antiferromagnet on simple square lattice in strong magnetic field,"We discuss spin-$\frac12$ Heisenberg antiferromagnet on simple square lattice
in magnetic field $H$ using recently proposed bond-operator technique. It is
well known that magnetically ordered phases of quantum magnets are well
described at least qualitatively by the conventional spin-wave theory that only
introduces quantum corrections into the classical solution of the problem. We
observe that quantum fluctuations change drastically dynamical properties of
the considered model at $H$ close to its saturation value: the dynamical
structure factor shows anomalies corresponding to Green's function poles which
have no counterparts in the spin-wave theory. That is, quantum fluctuations
produce multiple short-wavelength magnon modes not changing qualitatively the
long-wavelength spin dynamics. Our results are in agreement with previous
quantum Monte-Carlo simulations and exact diagonalization of finite clusters.",2005.07537v3
2020-07-20,Dynamical magnetoelectric coupling in axion insulator thin films,"Axion insulator is an exotic magnetic topological insulator with zero Chern
number but a nonzero quantized Chern-Simons magnetoelectric coupling. A
conclusive experimental evidence for axion insulators is still lacking due to
the small signal of topological magnetoelectric effect (TME). Here we show that
the dynamical magnetoelectric coupling can be induced by the
\emph{out-of-plane} surface magnetization dynamics in axion insulator thin
films, which further generates a polarization current in the presence of an
external magnetic field in the same direction. Such a current is finite in the
bulk and increases as the film thickness $d$ decreases, in opposite to TME
current which decreases as $d$ decreases. Remarkably, the current in thin films
at magnetic resonance is at least ten times larger than that of TME, and thus
may serve as a smoking gun signature for axion insulators.",2007.09869v1
2020-08-18,"Survey of 360$^{\circ}$ domain walls in magnetic heterostructures: topology, chirality and current-driven dynamics","Chirality and current-driven dynamics of topologically nontrivial
360$^{\circ}$ domain walls (360DWs) in magnetic heterostructures (MHs) are
systematically investigated. For MHs with normal substrates, the static 360DWs
are N\'{e}el-type with no chirality. While for those with heavy-metal
substrates, the interfacial Dzyaloshinskii-Moriya interaction (iDMI) therein
makes 360DWs prefer specific chirality. Under in-plane driving charge currents,
as the direct result of ""full-circle"" topology a certain 360DW does not undergo
the ""Walker breakdown""-type process like a well-studied 180$^{\circ}$ domain
wall as the current density increases. Alternatively, it keeps a fixed
propagating mode (either steady-flow or precessional-flow, depending on the
effective damping constant of the MH) until it collapses or changes to other
types of solition when the current density becomes too high. Similarly, the
field-like spin-orbit torque (SOT) has no effects on the dynamics of 360DWs,
while the anti-damping SOT has. For both modes, modifications to the mobility
of 360DWs by iDMI and anti-damping SOT are provided.",2008.08196v1
2020-09-16,Dynamical mass generation for ferromagnetic skyrmions in two dimensions,"Magnetic skyrmions are topological magnetization textures that are
characterized by the homotopy group of two dimensional spheres. Despite years
of intensive research on skyrmions, the fundamental problem of the inertia of a
skyrmion in driven motion remains unresolved. By properly taking into account a
direct coupling between skyrmion motion and the correspondingly excited
magnons, we identify a dynamical mass for the skyrmion in motion. The direct
coupling between skyrmion motion and magnons can be employed to engineer
skyrmion dynamics with magnons through ingenious material and geometry design.",2009.07486v4
2020-10-27,Vertex Dependent Dynamic Response of a Connected Kagome Artificial Spin Ice,"We present the dynamic response of a connected Kagome artificial spin ice
with emphasis on the effect of the vertex magnetization configuration on the
mode characteristics. We use broadband ferromagnetic resonance (FMR)
spectroscopy and micromagnetic simulations to identify and characterize
resonant modes. We find the mode frequencies of elongated, single-domain film
segments not only depend on the orientation of their easy-axis with respect to
the applied magnetic field, but also depend on the vertex magnetization
configuration, which suggests control over the FMR mode can be accomplished by
altering the vertex magnetization. Moreover, we study differences between the
vertex center mode (VCM) and the localized domain wall (LDW) mode. We show that
the LDW mode acts as a signature of the domain wall (DW) nucleation process and
the DW dynamics active during segment reversal events. The results show the VCM
and LDW modes can be controlled using a field protocol, which has important
implications for applications in magnonic and spintronic devices.",2010.13965v1
2020-11-09,Spin-wave dynamics and symmetry breaking in an artificial spin ice,"Artificial spin ices are periodic arrangements of interacting nanomagnets
successfully used to investigate emergent phenomena in the presence of
geometric frustration. Recently, it has been shown that artificial spin ices
can be used as building blocks for creating functional materials, such as
magnonic crystals, and support a large number of programmable magnetic states.
We investigate the magnetization dynamics in a system exhibiting anisotropic
magnetostatic interactions owing to locally broken structural inversion
symmetry. We find a rich spin-wave spectrum and investigate its evolution in an
external magnetic field. We determine the evolution of individual modes, from
building blocks up to larger arrays, highlighting the role of symmetry breaking
in defining the mode profiles. Moreover, we demonstrate that the mode spectra
exhibit signatures of long-range interactions in the system. These results
contribute to the understanding of magnetization dynamics in spin ices beyond
the kagome and square ice geometries and are relevant for the realization of
reconfigurable magnonic crystals based on spin ices.",2011.04505v2
2020-12-11,From one- to two-magnon excitations in the S=3/2 magnet $β$-CaCr$_2$O$_4$,"We apply neutron spectroscopy to measure the magnetic dynamics in the S=3/2
magnet $\beta$-CaCr$_2$O$_4$ (T$_N$=21 K). The low-energy fluctuations, in the
ordered state, resemble large-S linear spin-waves from the incommensurate
ground state. However, at higher energy transfers, these semi-classical and
harmonic dynamics are replaced by an energy and momentum broadened continuum of
excitations. Applying kinematic constraints required for energy and momentum
conservation, sum rules of neutron scattering, and comparison against exact
diagonalization calculations, we show that the dynamics at high-energy
transfers resemble low-S one-dimensional quantum fluctuations.
$\beta$-CaCr$_2$O$_4$ represents an example of a magnet at the border between
classical N\'eel and quantum phases, displaying dual characteristics.",2012.06314v2
2021-01-07,Quantum annealing simulation of out-of-equilibrium magnetization in a spin-chain compound,"Geometrically frustrated spin-chain compounds such as Ca3Co2O6 exhibit
extremely slow relaxation under a changing magnetic field. Consequently, both
low-temperature laboratory experiments and Monte Carlo simulations have shown
peculiar out-of-equilibrium magnetization curves, which arise from trapping in
metastable configurations. In this work we simulate this phenomenon in a
superconducting quantum annealing processor, allowing us to probe the impact of
quantum fluctuations on both equilibrium and dynamics of the system. Increasing
the quantum fluctuations with a transverse field reduces the impact of
metastable traps in out-of-equilibrium samples, and aids the development of
three-sublattice ferrimagnetic (up-up-down) long-range order. At equilibrium we
identify a finite-temperature shoulder in the 1/3-to-saturated phase
transition, promoted by quantum fluctuations but with entropic origin. This
work demonstrates the viability of dynamical as well as equilibrium studies of
frustrated magnetism using large-scale programmable quantum systems, and is
therefore an important step toward programmable simulation of dynamics in
materials using quantum hardware.",2101.02769v1
2021-01-15,Magnetic properties of transition metal dimers probed by inelastic neutron scattering,"The physical characterisation and understanding of molecular magnetic
materials is one of the most important steps towards the integration of such
systems in hybrid spintronic devices. Amongst the many characterisation
techniques employed in such a task, Inelastic Neutron Scattering (INS) stands
as one of the most powerful and sensitive tools to investigate their spin
dynamics. Herein, the magnetic properties and spin dynamics of two dinuclear
complexes, namely [(M(hfacac)$_2$)$_2$(bpym)] (where M = Ni$^{2+}$, Co$^{2+}$,
abbreviated in the following as Ni$_2$, Co$_2$) are reported. These are model
systems that could constitute fundamental units of future spintronic devices.
By exploiting the highly sensitive IN5 Cold INS spectrometer, we are able to
gain a deep insight into the spin dynamics of Ni$_2$ and to fully obtain the
microscopic spin Hamiltonian parameters; while for Co$_2$, a multitude of INS
transitions are observed demonstrating the complexity of the magnetic
properties of octahedral cobalt-based systems.",2101.06225v1
2021-09-08,Emergent inductance by dynamical Aharonov-Casher phases,"We propose a mechanism of inductance operation originating from a dynamical
Aharonov-Casher (AC) phase of an electron in ferromagnets. By taking into
account spin-orbit coupling effects, we extend the theory of emergent
inductance, which has recently been discovered in spiral magnets, to arbitrary
magnetic textures. The inductance of dynamical AC phase origin universally
arises in the coexistence of magnetism and a spin-orbit coupling, even with
spatially-uniform magnetization, allowing its stable operation in wide ranges
of temperature and frequency. Revisiting the widely studied systems, such as
ferromagnets with spatial inversion asymmetry, with the new perspective offered
by our work will lead to opening a new paradigm in the study of AC phase
physics and the spintronics-based power management in ultra-wideband frequency
range.",2109.03558v1
2021-09-22,Hopfion Dynamics in Chiral Magnets,"Resonant spin dynamics of topological spin textures are correlated with their
topological nature, which can be employed to understand this nature. In this
study, we present resonant spin dynamics of three-dimensional topological spin
texture, i.e., Neel and Bloch hopfions. Using micromagnetic simulations, we
stabilize Bloch and Neel hopfions with bulk and interfacial
Dzyaloshinskii-Moriya interaction (DMI), respectively. We identify the ground
state spin configuration of both hopfions, effects of anisotropies, geometric
confinements, and demagnetizing fields. To confirm topological nature, Hopf
number is calculated for each spin texture. Then, we calculate the resonance
frequencies and spin-wave modes of spin precessions under multiple magnetic
fields. Unique resonance frequencies and specific magnetic field dependence can
help to guide experimental studies to identify the three-dimensional
topological spin texture of hopfions in functioning chiral magnets when imaging
is not possible.",2109.10758v1
2021-12-14,Magnetic properties and pseudogap formation in infinite-layer nickelates: insights from the single-band Hubbard model,"We study the magnetic and spectral properties of a single-band Hubbard model
for the infinite-layer nickelate compound LaNiO$_2$. As spatial correlations
turn out to be the key ingredient for understanding its physics, we use two
complementary extensions of the dynamical mean-field theory to take them into
account: the cellular dynamical mean-field theory and the dynamical vertex
approximation. Additionally to the systematic analysis of the doping dependence
of the non-Curie-Weiss behavior of the uniform magnetic susceptibility, we
provide insight into its relation to the formation of a pseudogap regime by the
calculation of the one-particle spectral function and the magnetic correlation
length. The latter is of the order of a few lattice spacings when the pseudogap
opens, indicating a strong-coupling pseudogap formation in analogy to cuprates.",2112.07531v1
2022-03-14,Noncollinear antiferromagnetic textures driven high harmonic generation from magnetic dynamics in the absence of spin-orbit coupling,"We demonstrate the generation of high order harmonics in carrier pumping from
precessing ferromagnetic or antiferromagnetic orders, excited via magnetic
resonance, in the presence of topological antiferromagnetic textures. This
results in an enhancement of the carrier dynamics by orders of magnitude.
Interestingly, the generation process occurs in an intrinsic manner, and is
solely governed by the interplay between the s-d exchange coupling underlying
the noncollinear antiferromagnetic order and the dynamical s-d exchange
parameter of the magnetic drive. Therefore, the relativistic spin-orbit
interaction is not required for the emergence of high harmonics in the pumped
currents. Accordingly, the noncollinear topological antiferromagnetic order is
presented as an alternative to spin-orbit interaction for the purpose of
harnessing high harmonic emission in carrier pumping. Our proposal initiates a
tantalizing perspective for the exploitation of topological magnetic textures
in the context of the highly active domain of ultrafast spintronics.",2203.07310v2
2022-03-27,Making a case for femto- phono- magnetism with FePt,"In the field of femtomagnetism magnetic matter is controlled by ultrafast
laser pulses; here we show that coupling phonon excitations of the nuclei to
spin and charge leads to femto-phono-magnetism, a powerful route to control
magnetic order at ultrafast times. With state-of-the-art theoretical
simulations of coupled spin-, charge-, and lattice-dynamics we identify strong
non-adiabatic spin-phonon coupled modes that dominate early time spin dynamics.
Activating these phonon modes we show leads to an additional (up to 40\% extra)
loss of moment in FePt occurring within 40 femtoseconds of the pump laser
pulse. Underpinning this enhanced ultrafast loss of spin moment we identify a
physical mechanism in which minority spin-current drives an enhanced inter-site
minority charge transfer, in turn promoting increased on-site spin flips. Our
finding demonstrates that the nuclear system, often assumed to play only the
role of an energy sink aiding long time re-magnetisation of the spin system,
can play a profound role in controlling femtosecond spin-dynamics in materials.",2203.14234v1
2022-06-23,Kramers-Moyal analysis of interplanetary magnetic field fluctuations at sub-ion scales,"In the framework of statistical time series analysis of complex dynamics we
present a multiscale characterization of solar wind turbulence in the
near-Earth environment. The data analysis, based on the Markov-process theory,
is meant to estimate the Kramers-Moyal coefficients associated with the
measured magnetic field fluctuations. In fact, when the scale-to-scale dynamics
can be successfully described as a Markov process, first- and second-order
Kramers-Moyal coefficients provide a complete description of the dynamics in
terms of Langevin stochastic process. The analysis is carried out by using
high-resolution magnetic field measurements gathered by Cluster during a fast
solar wind period on January 20, 2007. This analysis extends recent findings in
the near-Sun environment with the aim of testing the universality of the
Markovian nature of the magnetic field fluctuations in the sub-ion/kinetic
domain.",2206.11498v2
2022-07-14,Magnetization dynamics in proximity-coupled superconductor-ferromagnet-superconductor multilayers. Part II,"In this work, we study magnetization dynamics in
superconductor-ferromagnet-superconductor thin-film structures. Results of the
broad-band ferromagnetic resonance spectroscopy are reported for a large set of
samples with varied thickness of both superconducting and ferromagnetic layers
in a wide frequency, field, and temperature ranges. Experimentally the
one-dimensional anisotropic action of superconducting torque on magnetization
dynamics is established; its dependence on thickness of layers is revealed. It
is demonstrated that experimental findings support the recently-proposed
mechanism of the superconducting torque formation via the interplay between the
superconducting kinetic inductance and magnetization precession at
superconductor-ferromagnet interfaces.",2207.06751v2
2022-09-30,Hybrid magnetization dynamics in $\text{Cu}_\text{2}\text{OSeO}_\text{3}$/$\mathrm{NiFe}$ heterostructures,"We investigate the coupled magnetization dynamics in heterostructures of a
single crystal of the chiral magnet $\mathrm{Cu_2OSeO_3}$ (CSO) and a
polycrystalline ferromagnet $\mathrm{NiFe}$ (Py) thin film using broadband
ferromagnetic resonance (FMR) at cryogenic temperatures. We observe the
excitation of a hybrid mode (HM) below the helimagnetic transition temperature
of CSO. This HM is attributed to the spin dynamics at the CSO/Py interface. We
study the HM by measuring its resonance frequencies for in plane rotations of
the external magnetic field. We find that the HM exhibits dominantly four-fold
anisotropy, in contrast to the FMR of CSO and Py.",2210.00897v1
2022-11-01,Strongly nonlinear antiferromagnetic dynamics in high magnetic fields,"Antiferromagnetic (AFM) materials possess a well-recognized potential for
ultrafast data processing thanks to their intrinsic ultrafast spin dynamics,
absence of stray fields, and large spin transport effects. The very same
properties, however, make their manipulation difficult, requiring frequencies
in THz range and magnetic fields of tens of Teslas. Switching of AFM order
implies going into the nonlinear regime, a largely unexplored territory. Here
we use THz light from a free electron laser to drive antiferromagnetic NiO into
a highly nonlinear regime and steer it out of nonlinearity with magnetic field
from a 33-Tesla Bitter magnet. This demonstration of large-amplitude dynamics
represents a crucial step towards ultrafast resonant switching of AFM order.",2211.00353v1
2022-12-06,Dynamics of hybrid magnetic skyrmion driven by spin-orbit torque in ferrimagnets,"Magnetic skyrmions are magnetic textures with topological protection, which
are expected to be information carriers in future spintronic devices. In this
work, we propose a scheme to implement hybrid magnetic skyrmions (HMS) in
ferrimagnets, and we study theoretically and numerically the dynamics of the
HMS driven by spin-orbit torque. It is revealed that the skyrmion Hall effect
depends on the skyrmion helicity and the net angular momentum ({\delta}s),
allowing the effective modulation of the HMS motion through tuning
Dzyaloshinskii-Moriya interaction and {\delta}s. Thus, the Hall effect can be
suppressed through selecting suitable materials to better control the HMS
motion. Moreover, Magnus force for finite {\delta}s suppresses the transverse
motion and enhances the longitudinal propagation, resulting in the HMS dynamics
in ferrimagnets faster than that in antiferromagnets.",2212.02807v1
2023-03-29,Universality in relaxation of spin helices under the $XXZ$- spin chain dynamics,"We describe dynamics of transverse spin-helix state (SHS) -- a product state
with spatially rotating magnetization -- under anisotropic Heisenberg XXZ spin
chain evolution. Due to experimental relevance we especially focus on
magnetization dynamics. At long times the $U(1)$ symmetry of the Hamiltonian is
restored, leading to the decay of transverse magnetization, which can be
described as an exponential decay of a spatially harmonic profile. We show that
the dependence of the short and intermediate-time decay timescale, which in
principle depends on all different parameters, like the wavevector of the
initial helix, the anisotropy, etc., can be described well by a single scaling
function. We also briefly discuss the evolution of magnetization current.",2303.16632v2
2023-05-10,Investigation of Spin-Wave Dynamics in Gyroid Nanostructures,"A new concept in magnonics studies the dynamics of spin waves (SWs) in
three-dimensional nanosystems. It is a natural evolution from conventionally
used planar systems to explore magnetization configurations and dynamics in 3D
nanostructures with lengths near intrinsic magnetic scales. In this work, we
perform broadband ferromagnetic resonance (BBFMR) measurements and
micromagnetic simulations of nanoscale magnetic gyroids - a periodic chiral
structure consisting entirely of chiral triple junctions. Our results show
unique properties of the network, such as the localization of the SW modes,
evoking their topological properties, and the substantial sensitivity to the
direction of the static magnetic field. The presented results open a wide range
of applications in the emerging field of 3D magnonic crystals and spintronics.",2305.06319v2
2023-05-23,Current-driven motion of magnetic topological defects in ferromagnetic superconductors,"Recent years have seen a number of instances where magnetism and
superconductivity intrinsically coexist. Our focus is on the case where
spin-triplet superconductivity arises out of ferromagnetism, and we make a
hydrodynamic analysis of the effect of a charge supercurrent on magnetic
topological defects like domain walls and merons. We find that the emergent
electromagnetic field that arises out of the superconducting order parameter
provides a description for not only the physical quantities such as the local
energy flux density and the interaction between current and defects but also
the energy dissipation through magnetic dynamics of the Gilbert damping, which
becomes more prominent compared to the normal state as superconductivity
attenuates the energy dissipation through the charge sector. In particular, we
reveal that the current-induced dynamics of domain walls and merons in the
presence of the Gilbert damping give rise to the nonsingular $4\pi$ and $2\pi$
phase slips, respectively, revealing the intertwined dynamics of spin and
charge degrees of freedom in ferromagnetic superconductors.",2305.13564v1
2023-05-24,Beyond Walker Breakdown through the Resonant Dissipation: Dramatic Enhancement of Magnetic Domain Wall Velocity via Resonant Excitation of Standing Wave Modes of Domain Wall Structure,"The dynamic behaviors of magnetic domain walls have significant implications
for developing advanced spintronic devices. In this study, we investigate the
intriguing resonance phenomenon within the magnetic domain wall structure and
its profound influence on dynamic motion, focusing on the dissipation
mechanism. By applying a static external magnetic field, we observe a
remarkable amplification of domain wall velocity, surpassing the limitations of
the conventional one-dimensional model. To quantify this enhancement, we
introduce a novel parameter, the distortion variation rate, which captures the
rapid and pronounced changes occurring within the domain wall structure.
Through comprehensive micromagnetic simulations, we establish a robust
relationship between speed and distortion variation rate, thereby validating
our theoretical framework. Our findings provide crucial insights into the
underlying mechanisms governing domain wall dynamics while paving the way for
developing and optimizing next-generation spintronic devices boasting
unparalleled speed and efficiency.",2305.14868v3
2023-06-19,Field-free Line Magnetic Particle Imaging: Radon-based Artifact Reduction with Motion Models,"Magnetic particle imaging is a promising medical imaging technique. Applying
changing magnetic fields to tracer material injected into the object under
investigation results in a change in magnetization. Measurement of related
induced voltage signals enables reconstruction of the particle distribution.
For the field-free line scanner the scanning geometry is similar to the one in
computerized tomography. We make use of these similarities to derive a forward
model for dynamic particle concentrations. We validate our theoretical findings
for synthetic data. By utilizing information about the object's dynamics in
terms of a diffeomorphic motion model, we are able to jointly reconstruct the
particle concentration and the corresponding dynamic Radon data without or with
reduced motion artifacts. Thereby, we apply total variation regularization for
the concentration and an optional sparsity constraint on the Radon data.",2306.10722v1
2023-07-25,Lattice structure dependence of laser-induced ultrafast magnetization switching in ferrimagnets,"The experimental discovery of single-pulse ultrafast magnetization switching
in ferrimagnetic alloys, such as GdFeCo and MnRuGa, opened the door to a
promising route toward faster and more energy efficient data storage. A recent
semi-phenomenological theory has proposed that a fast, laser-induced
demagnetization below a threshold value puts the system into a dynamical regime
where angular momentum transfer between sublattices dominates. Notably, this
threshold scales inversely proportional to the number of exchange-coupled
nearest neighbours considered in the model, which in the simplest case is
directly linked to the underlying lattice structure. In this work, we study the
role of the lattice structure on the laser-induced ultrafast magnetization
switching in ferrimagnets by complementing the phenomenological theory with
atomistic spin dynamics computer simulations. We consider a spin model of the
ferrimagnetic GdFeCo alloy with increasing number of exchange-coupled
neighbours. Within this model, we demonstrate that the laser-induced
magnetization dynamics and switching depends on the lattice structure. Further,
we determine that the critical laser energy for switching reduces for
decreasing number of exchange-coupled neighbours.",2307.13522v1
2023-08-01,Field and Polarization Dependent Quantum Spin Dynamics in Honeycomb Magnet Na$_2$Co$_2$TeO$_6$: Magnetic Excitations and Continuum,"We report terahertz spectroscopic measurements of quantum spin dynamics in
the spin-1/2 honeycomb magnet Na$_2$Co$_2$TeO$_6$ as a function of applied
magnetic field with different terahertz polarizations. Distinct field
dependencies of the resolved spin dynamics are identified in three regimes,
which are separated by two critical fields at $B_{c1}\approx 7$ and
$B_{c2}\approx 10$ T. A polarization selective continuum is observed in the
intermediate phase, featuring spin fluctuations of a proximate quantum spin
liquid.",2308.00631v2
2023-12-13,Kernel Polynomial Method for Linear Spin Wave Theory,"Calculating dynamical spin correlations is essential for matching model
magnetic exchange Hamiltonians to momentum-resolved spectroscopic measurements.
A major numerical bottleneck is the diagonalization of the dynamical matrix,
especially in systems with large magnetic unit cells, such as those with
incommensurate magnetic structures or quenched disorder. In this paper, we
demonstrate an efficient scheme based on the kernel polynomial method for
calculating dynamical correlations of relevance to inelastic neutron scattering
experiments. This method reduces the scaling of numerical cost from cubic to
linear in the magnetic unit cell size.",2312.08349v2
1999-10-26,Spin dynamics simulations of the magnetic dynamics of RbMnF$_3$ and direct comparison with experiment,"Spin-dynamics techniques have been used to perform large-scale simulations of
the dynamic behavior of the classical Heisenberg antiferromagnet in simple
cubic lattices with linear sizes $L\leq 60$. This system is widely recognized
as an appropriate model for the magnetic properties of RbMnF$_3$.
Time-evolutions of spin configurations were determined numerically from coupled
equations of motion for individual spins using a new algorithm implemented by
Krech {\it etal}, which is based on fourth-order Suzuki-Trotter decompositions
of exponential operators. The dynamic structure factor was calculated from the
space- and time-displaced spin-spin correlation function. The crossover from
hydrodynamic to critical behavior of the dispersion curve and spin-wave
half-width was studied as the temperature was increased towards the critical
temperature. The dynamic critical exponent was estimated to be $z=(1.43\pm
0.03)$, which is slightly lower than the dynamic scaling prediction, but in
good agreement with a recent experimental value. Direct, quantitative
comparisons of both the dispersion curve and the lineshapes obtained from our
simulations with very recent experimental results for RbMnF$_3$ are presented.",9910429v1
2000-12-11,Hysteresis and the dynamic phase transition in thin ferromagnetic films,"Hysteresis and the non-equilibrium dynamic phase transition in thin magnetic
films subject to an oscillatory external field have been studied by Monte Carlo
simulation. The model under investigation is a classical Heisenberg spin system
with a bilinear exchange anisotropy in a planar thin film geometry with
competing surface fields. The film exhibits a non-equilibrium phase transition
between dynamically ordered and dynamically disordered phases characterized by
a critical temperature Tcd, whose location of is determined by the amplitude H0
and frequency w of the applied oscillatory field. In the presence of competing
surface fields the critical temperature of the ferromagnetic-paramagnetic
transition for the film is suppressed from the bulk system value, Tc, to the
interface localization-delocalization temperature Tci. The simulations show
that in general Tcd < Tci for the model film. The profile of the time-dependent
layer magnetization across the film shows that the dynamically ordered and
dynamically disordered phases coexist within the film for T < Tcd. In the
presence of competing surface fields, the dynamically ordered phase is
localized at one surface of the film.",0012190v2
2004-09-27,A New Measurement of Dynamic Critical exponent of Wolff Algorithm by Dynamic Finite Size Scaling,"In this work we have calculated the dynamic critical exponent $z$ for 2-, 3-
and 4-dimensional Ising models using the Wolff's algorithm through dynamic
finite size scaling. We have studied time evolution of the average cluster
size, the magnetization and higher moments of the magnetization. It is observed
that dynamic scaling is independent of the algorithm. In this sense,
universality is established for a wide range of algorithms with their own
dynamic critical exponents. For scaling, we have used the literature values of
critical exponents to observe the dynamic finite size scaling and to obtain the
value of $z$. From the simulation data a very good scaling is observed leading
to vanishingly small $z$ values for all three dimensions.",0409696v1
2005-07-11,Critical slowing down in random anisotropy magnets,"We study the purely relaxational critical dynamics with non-conserved order
parameter (model A critical dynamics) for three-dimensional magnets with
disorder in a form of the random anisotropy axis. For the random axis
anisotropic distribution, the static asymptotic critical behaviour coincides
with that of random site Ising systems. Therefore the asymptotic critical
dynamics is governed by the dynamical exponent of the random Ising model.
However, the disorder influences considerably the dynamical behaviour in the
non-asymptotic regime. We perform a field-theoretical renormalization group
analysis within the minimal subtraction scheme in two-loop approximation to
investigate asymptotic and effective critical dynamics of random anisotropy
systems. The results demonstrate the non-monotonic behaviour of the dynamical
effective critical exponent $z_{\rm eff}$.",0507241v1
2012-01-31,Dynamic relaxation of topological defect at Kosterlitz-Thouless phase transition,"With Monte Carlo methods we study the dynamic relaxation of a vortex state at
the Kosterlitz-Thouless phase transition of the two-dimensional XY model. A
local pseudo-magnetization is introduced to characterize the symmetric
structure of the dynamic systems. The dynamic scaling behavior of the
pseudo-magnetization and Binder cumulant is carefully analyzed, and the
critical exponents are determined. To illustrate the dynamic effect of the
topological defect, similar analysis for the the dynamic relaxation with a
spin-wave initial state is also performed for comparison. We demonstrate that a
limited amount of quenched disorder in the core of the vortex state may alter
the dynamic universality class. Further, theoretical calculations based on the
long-wave approximation are presented.",1201.6423v1
2021-11-15,Finite-time dynamical phase transition in non-equilibrium relaxation,"We uncover a finite-time dynamical phase transition in the thermal relaxation
of a mean-field magnetic model. The phase transition manifests itself as a cusp
singularity in the probability distribution of the magnetisation that forms at
a critical time. The transition is due to a sudden switch in the dynamics,
characterised by a dynamical order parameter. We derive a dynamical Landau
theory for the transition that applies to a range of systems with scalar,
parity-invariant order parameters. Close to criticalilty, our theory reveals an
exact mapping between the dynamical and equilibrium phase transitions of the
magnetic model, and implies critical exponents of mean-field type. We argue
that interactions between nearby saddle points, neglected at the mean-field
level, may lead to critical, spatiotemporal fluctuations of the order
parameter, and thus give rise to novel, dynamical critical phenomena.",2111.07681v2
2015-09-23,Anomalously slow spin dynamics and short-range correlations in the quantum spin ice systems Yb2Ti2O7 and Yb2Sn2O7,"We report a positive muon spin relaxation and rotation (\muSR) study of the
quantum spin ice materials Yb2Ti2O7 and Yb2Sn2O7 focusing on the low field
response. In agreement with earlier reports, data recorded in small
longitudinal fields evidence anomalously slow spin dynamics in the microsecond
range below the temperature T_c at which the specific heat displays an intense
peak, namely T_c = 0.24 K and 0.15 K, respectively, for the two systems. We
found that slow dynamics extends above T_c up to at least 0.7 K for both
compounds. The conventional dynamical Gaussian Kubo-Toyabe model describes the
\muSR spectra recorded above T_c. At lower temperatures a published analytical
extension of the Gaussian Kubo-Toyabe model provides a good description,
consistent with the existence of short-range magnetic correlations. While the
physical response of the two systems is qualitatively the same, Yb2Ti2O7
exhibits a much larger local magnetic susceptibility than Yb2Sn2O7 below T_c.
Considering previously reported ac susceptibility, neutron scattering and \muSR
results, we suggest the existence of anomalously slow spin dynamics to be a
common physical property of pyrochlore magnetic materials. The possibility of
molecular spin substructures to be associated to the slow dynamics and
therefore the short-range correlations is mentioned. The slow spin dynamics
observed under field does not exclude the presence of much faster dynamics
detected in extremely low or zero field.",1509.06908v1
2018-01-29,Temporal organization of magnetospheric fluctuations unveiled by recurrence patterns in the Dst index,"Magnetic storms constitute the most remarkable large-scale phenomena of
nonlinear magnetospheric dynamics. Studying the dynamical organization of
macroscopic variability in terms of geomagnetic activity index data by means of
complexity measures provides a promising approach for identifying the
underlying processes and associated time-scales. Here, we apply a suite of
characteristics from recurrence quantification analysis (RQA) and recurrence
network analysis (RNA) in order to unveil some key nonlinear features of the
hourly Disturbance storm-time (Dst) index during periods with magnetic storms
and such of normal variability. Our results demonstrate that recurrence-based
measures can serve as excellent tracers for changes in the dynamical complexity
along non-stationary records of geomagnetic activity. In particular, trapping
time (characterizing the typical length of ""laminar phases"" in the observed
dynamics) and recurrence network transitivity (associated with the number of
the system's effective dynamical degrees of freedom) allow for a very good
discrimination between magnetic storm and quiescence phases. In general, some
RQA and RNA characteristics distinguish between storm and non-storm times
equally well or even better than other previously considered nonlinear
characteristics like Hurst exponent or symbolic dynamics based entropy
concepts. Our results point to future potentials of recurrence characteristics
for unveiling temporal changes in the dynamical complexity of the
magnetosphere.",1802.01426v1
1993-11-02,Magnetic Activity in Thick Accretion Disks and Associated Observable Phenomena I. Flux Expulsion,"We study the dynamics of toroidal magnetic flux tubes, symmetric about the
rotation axis, inside non-magnetic thick accretion disks around black holes. We
present model equations which include effects of gravity, centrifugal force,
pressure gradient force, Coriolis force, drag, magnetic tension and magnetic
buoyancy. We solve them assuming the disk to be adiabatic. We show that under a
wide range of parameters describing the size and the field strength, as well as
angular momentum distribution inside the disk, buoyant flux tubes, either
released on the equatorial plane or at the outer edge of the disk, can gather
in the chimney-like openings near the axis. This behavior makes the chimneys
magnetically most active and could shed light on the origin and acceleration of
cosmic jets, as well as the variabilities observed in Blazars.",9311010v1
2000-02-14,On the Role of Dust Particles in Decoupling of Plasma from Magnetic Field in Laboratory and Space,"The objective of this paper is to apply the recent achievements in
understanding of the non-MHD effects in plasma (acquired both in laboratory
experiments, as well as in theory), to the interstellar phenomena. Applied to
the space plasma, these effects can significantly change the picture of plasma
dynamics. Charged dust particles are always present in the interstellar medium
and can easily remain not magnetized, even when the plasma electrons and ions
are strongly magnetized. In such a medium, the magnetic field can propagate
with the super-Alfven velocity. This breaks the local ""frozen-in"" law for the
magnetic field, which can significantly affect the gravitational collapse
phenomena, the space plasma turbulence spectrum, magnetic dynamo and the rate
of the Fermi acceleration of cosmic rays.",0002293v1
2002-01-29,MHD inverse cascade in the early Universe,"We have carried out numerical simulations of freely decaying
magnetohydrodynamic (MHD) turbulence in three dimensions, which can be applied
to the evolution of stochastic magnetic fields in the early Universe. For
helical magnetic fields an inverse cascade effect is observed in which magnetic
helicity and energy is transfered from smaller scales to larger scales,
accompanied by power law growth in the characteristic length scale of the
magnetic field. The magnetic field quickly reaches a scaling regime with
self-similar evolution, and power law behaviour at high wavenumbers. We also
find power law decay in the magnetic and kinematic energies.",0201466v1
2002-02-05,R-modes in the ocean of a magnetic neutron star,"We study the dynamics of r-modes in the ocean of a magnetic neutron star. We
modeled the star's ocean with a spherical rotating thin shell and assumed that
the magnetic field symmetry axis is not aligned to the shell's spin axis. In
the magnetohydrodynamic approximation, we calculate the frequency of $\ell=m$
r-modes in the shell of an incompressible fluid. Different r-modes with $\ell$
and $\ell\pm2$ are coupled by the {\it inclined} magnetic field. Kinematical
secular effects for the motion of a fluid element in the shell undergoing
$\ell=m=2$ r-mode are studied. The magnetic corrected drift velocity of a given
fluid element undergoing the $\ell=m$ r-mode oscillations is obtained. The
magnetic field increases the magnitude of the fluid drift produced by the
r-mode drift velocity, the high-$\ell$ modes in the ocean fluid will damp
faster than the low-$\ell$ ones.",0202105v3
2005-07-16,Magnetohydrodynamics in full general relativity: Formulation and tests,"A new implementation for magnetohydrodynamics (MHD) simulations in full
general relativity (involving dynamical spacetimes) is presented. In our
implementation, Einstein's evolution equations are evolved by a BSSN formalism,
MHD equations by a high-resolution central scheme, and induction equation by a
constraint transport method. We perform numerical simulations for standard test
problems in relativistic MHD, including special relativistic magnetized shocks,
general relativistic magnetized Bondi flow in stationary spacetime, and a
longterm evolution for self-gravitating system composed of a neutron star and a
magnetized disk in full general relativity. In the final test, we illustrate
that our implementation can follow winding-up of the magnetic field lines of
magnetized and differentially rotating accretion disks around a compact object
until saturation, after which magnetically driven wind and angular momentum
transport inside the disk turn on.",0507383v1
2005-11-09,Solar differential rotation and properties of magnetic clouds,"The most geoeffective solar drivers are magnetic clouds - a subclass of
coronal mass ejections (CME's) distinguished by the smooth rotation of the
magnetic field inside the structure. The portion of CME's that are magnetic
clouds is maximum at sunspot minimum and mimimum at sunspot maximum. This
portion is determined by the amount of helicity carried away by CME's which in
turn depends on the amount of helicity transferred from the solar interior to
the surface, and on the surface differential rotation. The latter can increase
or reduce, or even reverse the twist of emerging magnetic flux tubes, thus
increasing or reducing the helicity in the corona, or leading to the violation
of the hemispheric helicity rule, respectively. We investigate the CME's
associated with the major geomagnetic storms in the last solar cycle whose
solar sources have been identified, and find that in 10 out of 12 cases of
violation of the hemispheric helicity rule or of highly geoeffective CME's with
no magnetic field rotation, they originate from regions with ""anti-solar"" type
of surface differential rotation.",0511257v1
2006-04-21,Modeling the Large Scale Structures of Astrophysical Jets in the Magnetically Dominated Limit,"We suggest a new approach that could be used for modeling both the large
scale behavior of astrophysical jets and the magnetically dominated explosions
in astrophysics. We describe a method for modeling the injection of magnetic
fields and their subsequent evolution in a regime where the free energy is
magnetically dominated. The injected magnetic fields, along with their
associated currents, have both poloidal and toroidal components, and they are
not force free. The dynamic expansion driven by the Lorentz force of the
injected fields is studied using 3-dimensional ideal magnetohydrodynamic
simulations. The generic behavior of magnetic field expansion, the interactions
with the background medium, and the dependence on various parameters are
investigated.",0604469v1
2006-07-24,Effect of Rossby and Alfvén waves on the dynamics of the tachocline,"To understand magnetic diffusion, momentum transport, and mixing in the
interior of the sun, we consider an idealized model of the tachocline, namely
magnetohydrodynamics (MHD) turbulence on a $\beta$ plane subject to a large
scale shear (provided by the latitudinal differential rotation). This model
enables us to self-consistently derive the influence of shear, Rossby and
Alfv\'{e}n waves on the transport properties of turbulence. In the strong
magnetic field regime, we find that the turbulent viscosity and diffusivity are
reduced by magnetic fields only, similarly to the two-dimensional MHD case
(without Rossby waves). In the weak magnetic field regime, we find a crossover
scale ($L\_R$) from a Alfv\'{e}n dominated regime (on small scales) to a Rossby
dominated regime (on large scales). For parameter values typical of the
tachocline, $L\_R$ is larger that the solar radius so that Rossby waves are
unlikely to play an important role in the transport of magnetic field and
angular momentum. This is mainly due to the enhancement of magnetic
back-reaction by shearing which efficiently generates small scales, thus strong
currents.",0607545v1
2006-07-26,Evolution of Collapse Nonuniformity for Rotating Magnetic Interstellar Clouds,"We investigate the formation and evolution of isothermal collapse
nonuniformity for rotating magnetic interstellar clouds. The initial and
boundary conditions correspond to the statement of the problem of homogeneous
cloud contraction from a pressure equilibrium with the external medium. The
initial uniform magnetic field is collinear with the angular velocity. Fast and
slow magnetosonic rarefaction waves are shown to be formed and propagate from
the boundary of the cloud toward its center in the early collapse stages. The
front of the fast rarefaction wave divides the gas mass into two parts. The
density, angular velocity, and magnetic field remain uniform in the inner
region and have nonuniform profiles in the outer region. The rarefaction wave
front surface can take both prolate and oblate shapes along the rotation axis,
depending on the relationship between the initial angular velocity and magnetic
field. We derive a criterion that separates the two regimes of rarefaction wave
dynamics with the dominant role of electromagnetic and centrifugal forces.
Based on analytical estimations and numerical calculations, we discuss possible
scenarios for the evolution of collapse nonuniformity for rotating magnetic
interstellar clouds.",0607588v1
2007-01-08,Collapse and Fragmentation of Molecular Cloud Cores. IX. Magnetic Braking of Initially Filamentary Clouds,"The collapse and fragmentation of initially filamentary, magnetic molecular
clouds is calculated in three dimensions with a gravitational, radiative
hydrodynamics code. The code includes magnetic field effects in an approximate
manner: magnetic pressure, tension, braking, and ambipolar diffusion are all
modelled. Three types of outcomes are observed: direct collapse and
fragmentation into a multiple protostar system, periodic contraction and
expansion without collapse, or periodic contraction and expansion leading
eventually to collapse. While the models begin their evolution at rest except
for the assumed solid-body rotation, they develop weakly supersonic velocity
fields as a result of the rebounding prior to collapse. The models show that
magnetically-supported clouds subject to magnetic braking can undergo dynamic
collapse leading to protostellar fragmentation on scales of 10 AU to 100 AU,
consistent with typical binary star separations.",0701210v1
2007-01-28,"Twist, writhe and energy from the helicity of magnetic perturbed vortex filaments","The twist and writhe numbers and magnetic energy of an orthogonally perturbed
vortex filaments are obtained from the computation of the magnetic helicity of
geodesic and abnormal magnetohydrodynamical (MHD) vortex filament solutions.
Twist is computed from a formula recently derived by Berger and Prior [J. Phys.
A 39 (2006) 8321] and finally writhe is computed from the theorem that the
helicity is proportional to the sum of twist and writhe. The writhe number is
proportional to the total torsion and to integrals of the vector potential. The
magnetic energy is computed in terms of the integral of the torsion squared
which allows us to place a bound to energy in in the case of helical filaments
due to a theorem by Fukumoto [J. Phys. Soc. Japan,(1987)] in fluid vortex
filaments. It is also shown that filament torsion coincides with the magnetic
twist in the case under consideration, where a small orthogonal magnetic field
exist along the thin filament. A new Aharonov-Bohm (AB) phase term is obtained
in the writhe number expression which is not present in the Moffatt-Ricca (Proc
Roy Soc London A,1992) computation.",0701796v1
1996-12-09,Hall Conductivity for Two Dimensional Magnetic Systems,"A Kubo inspired formalism is proposed to compute the longitudinal and
transverse dynamical conductivities of an electron in a plane (or a gas of
electrons at zero temperature) coupled to the potential vector of an external
local magnetic field, with the additional coupling of the spin degree of
freedom of the electron to the local magnetic field (Pauli Hamiltonian). As an
example, the homogeneous magnetic field Hall conductivity is rederived. The
case of the vortex at the origin is worked out in detail. This system happens
to display a transverse Hall conductivity ($P$ breaking effect) which is
subleading in volume compared to the homogeneous field case, but diverging at
small frequency like $1/\omega^2$. A perturbative analysis is proposed for the
conductivity in the random magnetic impurity problem (Poissonian vortices in
the plane). At first order in perturbation theory, the Hall conductivity
displays oscillations close to the classical straight line conductivity of the
mean magnetic field.",9612080v1
1997-04-25,Correlated-electron theory of strongly anisotropic metamagnets,"The microscopic origin of metamagnetism and metamagnetic transitions in
strongly anisotropic antiferromagnets is investigated within a quantum
mechanical theory of correlated electrons. To this end the Hubbard model with
staggered magnetization m_st along an easy axis e in a magnetic field H || e is
studied both analytically and numerically within the dynamical mean field
theory (DMFT). At intermediate couplings the self-consistent DMFT equations,
which become exact in the limit of large coordination number, are solved by
finite temperature Quantum Monte Carlo techniques. The temperature and magnetic
field dependence of the homogeneous and staggered magnetization are calculated
and the magnetic phase diagram is constructed. At half filling the metamagnetic
transitions are found to change from first order at low temperatures to second
order near the N'eel temperature, implying the existence of a multicritical
point. Doping with holes or electrons has a strong effect: the system becomes
metallic, the electronic compressibility increases and the critical
temperatures and fields decrease. These results are related to known properties
of insulating metamagnets such as FeBr_2, metallic metamagnets such as UPdGe,
and the giant and colossal magnetoresistance found in a number of magnetic bulk
systems.",9704209v1
2000-10-19,Magnets with strong geometric frustration,"A non-technical introduction to the theory of magnets with strong geometric
frustration is given, concentrating on magnets on corner-sharing (kagome,
pyrochlore, SCGO and GGG) lattices. Their rich behaviour is traced back to a
large ground-state degeneracy in model systems, which renders them highly
unstable towards perturbations. A systematic classification according to
properties of their ground states is discussed. Other topics addressed in this
overview article include a general theoretical framework for thermal order by
disorder; the dynamics of how the vast regions of phase space accessible at low
temperature are explored; the origin of the featureless magnetic susceptibility
fingerprint of geometric frustration; the role of perturbations; and spin ice.
The rich field of quantum frustrated magnets is also touched on.",0010301v1
2001-05-24,Electric-Field-Induced Phase Separation and Labyrinthine Patterns in Nanocolloids,"In this paper we report universal labyrinthine patterns observed in
nanocolloids originated from electric-field-induced phase separation. For
nanocolloids consisting of magnetic particles (magnetic fluids), the
labyrinthine pattern as the result of the co-existence of two phases is found
to be stable against perturbation by an additional magnetic field. The
relaxation back to the globally disordered pattern after the magnetic field is
removed clearly shows evolution of ""undulation"" thus demonstrates that
superimposing magnetic and electric fields can easily manipulate the patterns
resulted from the phase separation, indicating that a magnetic fluid has an
advantage in exploring the dynamics of defects that has applications in many
fields.",0105484v1
2002-08-26,Tunable Lyapunov exponent in inverse magnetic billiards,"The stability properties of the classical trajectories of charged particles
are investigated in a two dimensional stadium-shaped inverse magnetic domain,
where the magnetic field is zero inside the stadium domain and constant
outside. In the case of infinite magnetic field the dynamics of the system is
the same as in the Bunimovich billiard, i.e., ergodic and mixing. However, for
weaker magnetic fields the phase space becomes mixed and the chaotic part
gradually shrinks. The numerical measurements of the Lyapunov exponent
(performed with a novel method) and the integrable/chaotic phase space volume
ratio show that both quantities can be smoothly tuned by varying the external
magnetic field. A possible experimental realization of the arrangement is also
discussed.",0208491v1
2002-09-25,Spin Glass Behavior in RuSr2Gd1.5Ce0.5Cu2O10,"The dynamics of the magnetic properties of polycrystalline
RuSr2Gd1.5Ce0.5Cu2O10 (Ru-1222) have been studied by ac susceptibility and dc
magnetization measurements, including relaxation and ageing studies. Ru-1222 is
a reported magneto-superconductor with Ru spins magnetic ordering at
temperatures near 100 K and superconductivity in Cu-O2 planes below Tc ~ 40 K.
The exact nature of Ru spins magnetic ordering is still debated and no
conclusion has been reached yet. In this work, a frequency-dependent cusp was
observed in ac susceptibility vs. T measurements, which is interpreted as a
spin glass transition. The change in the cusp position with frequency follows
the Vogel-Fulcher law, which is commonly accepted to describe a spin glass with
magnetically interacting clusters. Such interpretation is supported by
themoremanaent magnetization (TRM) measurements at T = 60 K. TRM relaxations
are well described by a stretched exponential relation, and present significant
ageing effects.",0209593v1
2003-12-15,The role of dynamical polarization of the ligand to metal charge transfer excitations in {\em ab initio} determination of effective exchange parameters,"The role of the bridging ligand on the effective Heisenberg coupling
parameters is analyzed in detail. This analysis strongly suggests that the
ligand-to-metal charge transfer excitations are responsible for a large part of
the final value of the magnetic coupling constant. This permits to suggest a
new variant of the Difference Dedicated Configuration Interaction (DDCI)
method, presently one of the most accurate and reliable for the evaluation of
magnetic effective interactions. This new method treats the bridging ligand
orbitals mediating the interaction at the same level than the magnetic orbitals
and preserves the high quality of the DDCI results while being much less
computationally demanding. The numerical accuracy of the new approach is
illustrated on various systems with one or two magnetic electrons per magnetic
center. The fact that accurate results can be obtained using a rather reduced
configuration interaction space opens the possibility to study more complex
systems with many magnetic centers and/or many electrons per center.",0312370v1
2003-12-30,NMR and LDA evidence for spiral magnetic order in the chain cuprate LiCu2O2,"We report on {6,7}Li nuclear magnetic resonance measurements of the
spin-chain compound LiCu2O2 in the paramagnetic and magnetically ordered
states. Below T about 24 K the NMR lineshape presents a clear signature of
incommensurate (IC) static modulation of the local magnetic field consistent
with an IC spiral modulation of the magnetic moments. {7}Li NMR reveals strong
phason-like dynamical fluctuations extending well below 24 K. We hypothesize
that a series of phase transitions at 24.2, 22.5, and 9 K reflects a ""Devil's
staircase"" type behavior generic for IC systems. LDA based calculations of
exchange integrals reveal a large in-chain frustration leading to a magnetical
spiral.",0312706v1
2004-05-19,Dynamics of the magnetic and structural a -> e phase transition in Iron,"We have studied the high-pressure iron bcc to hcp phase transition by
simultaneous X-ray Magnetic Circular Dichroism (XMCD) and X-ray Absorption
Spectroscopy (XAS) with an X-ray dispersive spectrometer. The combination of
the two techniques allows us to obtain simultaneously information on both the
structure and the magnetic state of Iron under pressure. The magnetic and
structural transitions simultaneously observed are sharp. Both are of first
order in agreement with theoretical prediction. The pressure domain of the
transition observed (2.4 $\pm$ 0.2 GPa) is narrower than that usually cited in
the literature (8 GPa). Our data indicate that the magnetic transition slightly
precedes the structural one, suggesting that the origin of the instability of
the bcc phase in iron with increasing pressure is to be attributed to the
effect of pressure on magnetism as predicted by spin-polarized full potential
total energy calculations.",0405439v4
2004-06-18,Spin pumping and magnetization dynamics in ferromagnet-Luttinger liquid junctions,"We study spin transport between a ferromagnet with time-dependent
magnetization and a conducting carbon nanotube or quantum wire, modeled as a
Luttinger liquid. The precession of the magnetization vector of the ferromagnet
due for instance to an outside applied magnetic field causes spin pumping into
an adjacent conductor. Conversely, the spin injection causes increased
magnetization damping in the ferromagnet. We find that, if the conductor
adjacent to the ferromagnet is a Luttinger liquid, spin pumping/damping is
suppressed by interactions, and the suppression has clear Luttinger liquid
power law temperature dependence. We apply our result to a few particular
setups. First we study the effective Landau-Lifshitz-Gilbert (LLG) coupled
equations for the magnetization vectors of the two ferromagnets in a FM-LL-FM
junction. Also, we compute the Gilbert damping for a FM-LL and a FM-LL-metal
junction.",0406437v1
2004-06-24,Thermal Effects on the Magnetic Field Dependence of Spin Transfer Induced Magnetization Reversal,"We have developed a self-aligned, high-yield process to fabricate CPP
(current perpendicular to the plane) magnetic sensors of sub 100 nm dimensions.
A pinned synthetic antiferromagnet (SAF) is used as the reference layer which
minimizes dipole coupling to the free layer and field induced rotation of the
reference layer. We find that the critical currents for spin transfer induced
magnetization reversal of the free layer vary dramatically with relatively
small changes the in-plane magnetic field, in contrast to theoretical
predictions based on stability analysis of the Gilbert equations of
magnetization dynamics including Slonczewski-type spin-torque terms. The
discrepancy is believed due to thermal fluctuations over the time scale of the
measurements. Once thermal fluctuations are taken into account, we find good
quantitative agreement between our experimental results and numerical
simulations.",0406574v1
2004-08-17,Vortex state oscillations in soft magnetic cylindrical dots,"We have studied magnetic vortex oscillations in soft sub-micron cylindrical
dots with variable thickness and diameter by an analytical approach and
micromagnetic simulations. We have considered two kinds of modes of the vortex
magnetization oscillations: 1) low-frequency translation mode, corresponding to
the movement of the vortex as a whole near its equilibrium position; 2)
high-frequency vortex modes, which correspond to radially symmetric
oscillations of the vortex magnetization, mainly outside the vortex core. The
vortex translational eigenmode was calculated numerically in frequency and time
domains for different dot aspect ratios. To describe the discrete set of vortex
high-frequency modes we applied the linearized equation of motion of dynamic
magnetization over the vortex ground state. We considered only radially
symmetric magnetization oscillations modes. The eigenfrequencies of both kinds
of excitation modes are determined by magnetostatic interactions. They are
proportional to the thickness/diameter ratio and lie in the GHz range for
typical dot sizes.",0408388v1
2004-10-25,Absence of magnetic moments in plutonium,"Many theories published in the last decade propose that either ordered or
disordered local moments are present in elemental plutonium at low
temperatures. We present new experimental data and review previous experimental
results. None of the experiments provide any evidence for ordered or disordered
magnetic moments (either static or dynamic) in plutonium at low temperatures,
in either the alpha- or delta-phases. The experiments presented and discussed
are magnetic susceptibility,electrical resistivity, NMR, specific heat, and
both elastic and inelastic neutronscattering. Many recent calculations
correctly predict experimentally observed atomic volumes, including that of
delta-Pu. These calculations achieve observed densities by the localization of
electrons, which then give rise to magnetic moments. However, localized
magnetic moments have never been observed experimentally in Pu. A theory is
needed that is in agreement with all the experimental observations. Two
theories are discussed that might provide understanding of the ensemble of
unusual properties of Pu, including the absence of experimental evidence for
localized magnetic moments; an issue that has persisted for over 50 years.",0410634v1
2005-01-28,Finite-temperature Simulations for Magnetic Nanostructures,"We examine different models and methods for studying finite-temperature
magnetic hysteresis in nanoparticles and ultrathin films. This includes
micromagnetic results for the hysteresis of a single magnetic nanoparticle
which is misaligned with respect to the magnetic field. We present results from
both a representation of the particle as a one-dimensional array of magnetic
rotors, and from full micromagnetic simulations. The results are compared with
the Stoner-Wohlfarth model. Results of kinetic Monte Carlo simulations of
ultrathin films are also presented. In addition, we discuss other topics of
current interest in the modeling of magnetic hysteresis in nanostructures,
including kinetic Monte Carlo simulations of dynamic phase transitions and
First-Order Reversal Curves.",0501692v2
2005-02-10,Ultra fast bit addressing in a magnetic memory matrix with crossed wire write line geometry,"An ultra fast bit addressing scheme for magnetic random access memories
(MRAM) in a crossed wire geometry is proposed. In the addressing scheme a word
of cells is programmed simultaneously by sub nanosecond field pulses making use
of the magnetization precession of the free layer. Single spin simulations of
the free layer dynamics show that the pulse parameters for programming an
arbitrary word of the array can be chosen such that the magnetization of the
cells to be written performs either a half or a full precessional turn during
application of the programming pulse depending on the initial and final
magnetization orientation of the addressed cells. Such bit addressing scheme
leads to a suppression of the magnetization ringing in all cells of the memory
array thereby allowing ultra high MRAM write clock rates above 1 GHz.",0502263v1
2005-02-28,Magnetization dynamics of two interacting spins in an external magnetic field,"The longitudinal relaxation time of the magnetization of a system of two
exchange coupled spins subjected to a strong magnetic field is calculated
exactly by averaging the stochastic Gilbert-Landau-Lifshitz equation for the
magnetization, i.e., the Langevin equation of the process, over its
realizations so reducing the problem to a system of linear
differential-recurrence relations for the statistical moments (averaged
spherical harmonics). The system is solved in the frequency domain by matrix
continued fractions yielding the complete solution of the two-spin problem in
external fields for all values of the damping and barrier height parameters.
The magnetization relaxation time extracted from the exact solution is compared
with the inverse relaxation rate from Langer's theory of the decay of
metastable states, which yields in the high barrier and intermediate-to-high
damping limits the asymptotic behaviour of the greatest relaxation time.",0502661v2
2005-08-31,Photon-Induced Magnetization Changes in Single-Molecule Magnets,"Microwave radiation applied to single-molecule magnets can induce large
magnetization changes when the radiation is resonant with transitions between
spin levels. These changes are interpreted as due to resonant heating of the
sample by the microwaves. Pulsed-radiation studies show that the magnetization
continues to decrease after the radiation has been turned off with a rate that
is consistent with the spin's characteristic relaxation rate. The measured rate
increases with pulse duration and microwave power, indicating that greater
absorbed radiation energy results in a higher sample temperature. We also
performed numerical simulations that qualitatively reproduce many of the
experimental results. Our results indicate that experiments aimed at measuring
the magnetization dynamics between two levels resonant with the radiation must
be done much faster than the >20-microsecond time scales probed in these
experiments.",0508774v3
2005-11-16,Giant magnetoimpedance in composite wires with insulator layer between non-magnetic core and soft magnetic shell,"A method for calculation of the magnetoimpedance in composite wires having an
insulator layer between non-magnetic core and soft magnetic shell is described.
It is assumed that the magnetic shell has a helical anisotropy and the driving
current flows through the core only. The distribution of eddy currents and
expressions for the impedance are found by means of a solution of Maxwell
equations taking into account the magnetization dynamics within the shell
governed by the Landau-Lifshitz equation. The effect of the insulator layer on
the magnetoimpedance is analyzed.",0511386v1
2005-11-22,Magnetomechanical Torques in Small Magnetic Cantilevers,"We study the dnamics of small magnetic cantilevers, either made from Si
covered by a magnetic film or entirely ferromagnetic ones. The
magnetomechanical torques are found to cause line splittings in ferromagnetic
resonance spectra and magnetization reversal facilitated by mechanical degrees
of freedom. We show that the magnetomechanical torques can extend the limits of
detecting and exciting motion at the nanoscale. A ""nanomotor"" described here
effectively transforms rf magnetic fields into mechanical oscillations. We
furthermore propose to integrate mechanical oscillators into magnetoelectronic
devices that make use of current-induced spin-transfer torques. This opens new
possibilities for electric transducers of nanomechanical motion.",0511548v1
2006-01-30,Multiple transfer of angular momentum quanta from a spin-polarized hole to magnetic ions in ZnMnSe/ZnBeSe quantum wells,"The magnetization kinetics in (Zn,Mn)Se/(Zn,Be)Se quantum wells has been
studied on a ps-time scale after pulsed laser excitation. The magnetization
induced by an external magnetic field is reduced by up to 30% during ~100 ps
due to spin and energy transfer from photocarriers to Mn spin system. The giant
Zeeman splitting leads to a complete spin polarization of the carriers,
resulting in a strong suppression of flip-flop processes between carriers and
magnetic ions. Therefore a multiple angular momentum transfer from each
spin-polarized hole to the Mn ions becomes the dominant mechanism in the
magnetization dynamics. A model based on spin-momentum coupling in the valence
band is suggested for explaining this transfer.",0601661v1
2003-09-17,Asymptotic Regimes of Magnetic Bianchi Cosmologies,"We consider the asymptotic dynamics of the Einstein-Maxwell field equations
for the class of non-tilted Bianchi cosmologies with a barotropic perfect fluid
and a pure homogeneous source-free magnetic field, with emphasis on models of
Bianchi type VII_{0}, which have not been previously studied. Using the
orthonormal frame formalism and Hubble-normalized variables, we show that, as
is the case for the previously studied class A magnetic Bianchi models, the
magnetic Bianchi VII_{0} cosmologies also exhibit an oscillatory approach to
the initial singularity. However, in contrast to the other magnetic Bianchi
models, we rigorously establish that typical magnetic Bianchi VII_{0}
cosmologies exhibit the phenomena of asymptotic self-similarity breaking and
Weyl curvature dominance in the late-time regime.",0309083v1
2004-10-03,"Octet, decuplet and antidecuplet magnetic moments in the chiral quark soliton model revisited","We reanalyse the magnetic moments of the baryon octet, decuplet, and
antidecuplet within the framework of the chiral quark-soliton model, with SU(3)
symmetry breaking taken into account. We consider the contributions of the
mixing of higher representations to the magnetic moment operator arising from
the SU(3) symmetry breaking. Dynamical parameters of the model are fixed by
experimental data for the magnetic moments of the baryon octet and from the
masses of the octet, decuplet and of $\Theta^{+}$. The magnetic moment of
$\Theta^{+}$ depends rather strongly on the pion-nucleon sigma term and reads
$-1.19 {\rm n.m.}$ to $-0.33 {\rm n.m.}$ for $\Sigma_{\pi N} = 45$ and 75 MeV
respectively. The recently reported mass of $\Xi^{--}_{\bar{10}}(1862)$ is
compatible with $\Sigma_{\pi N} = 73$ MeV. As a byproduct the strange magnetic
moment of the nucleon is obtained with a value of $\mu^{(s)}_N =+0.39$ n.m.",0410042v2
2004-12-18,Magnetic moments of the pentaquarks,"We present in this talk a recent analysis for the magnetic moments of the
baryon antidecuplet within the framework of the chiral quark-soliton model with
linear $m_s$ corrections considered. We take into account the mixing of higher
representations to the collective magnetic moment operator, which comes from
the SU(3) symmetry breaking. Dynamical parameters of the model are fixed by
experimental data for the magnetic moments of the baryon octet as well as by
the masses of the octet, decuplet and of $\Theta^{+}$. The magnetic moment of
$\Theta^{+}$ is rather sensitive to the pion-nucleon sigma term and ranges from
$-1.19 {\rm n.m.}$ to $-0.33 {\rm n.m.}$ as the sigma term is varied from
$\Sigma_{\pi N} = 45$ to 75 MeV, respectively. On top of them, we obtained that
the strange magnetic moment of the nucleon has the value of $\mu^{(s)}_N
=+0.39$ n.m. within this scheme and turns out to be almost independent of the
sigma term.",0412270v1
1996-06-10,The Gross-Neveu model on a sphere with a magnetic monopole,"We study, for the first time, the phase structure of the Gross--Neveu model
with a combination of a (constant) gravitational and a magnetic field. This has
been made possible by our finding of an exact solution to the problem, namely
the effective potential for the composite fermions. Then, from the
corresponding implicit equation the phase diagram for the dynamical fermion
mass is calculated numerically for some values of the magnetic field. %(what
can be done with arbitrary precision). For a small magnetic field the phase
diagram hints to the possibility of a second order phase transition at some
critical curvature. With growing magnetic field only the phase with broken
chiral symmetry survives, because the magnetic field prevents the decay of the
chiral condensate. This result is bound to have important consequences in early
universe cosmology.",9606051v1
1996-09-05,Quantum Charged Spinning Particles in a Strong Magnetic Field (a Quantal Guiding Center Theory),"A quantal guiding center theory allowing to systematically study the
separation of the different time scale behaviours of a quantum charged spinning
particle moving in an external inhomogeneous magnetic filed is presented. A
suitable set of operators adapting to the canonical structure of the problem
and generalizing the kinematical momenta and guiding center operators of a
particle coupled to a homogenous magnetic filed is constructed. The Pauli
Hamiltonian rewrites in this way as a power series in the magnetic length $l_B=
\sqrt{\hbar c/eB}$ making the problem amenable to a perturbative analysis. The
first two terms of the series are explicitly constructed. The effective
adiabatic dynamics turns to be in coupling with a gauge filed and a scalar
potential. The mechanism producing such magnetic-induced geometric-magnetism is
investigated in some detail.",9609042v1
2001-07-30,Energy fluxes in helical magnetohydrodynamics and dynamo action,"Renormalized viscosity, renormalized resistivity, and various energy fluxes
are calculated for helical magnetohydrodynamics using perturbative field
theory. The calculation is to first-order in perturbation. Kinetic and magnetic
helicities do not affect the renormalized parameters, but they induce an
inverse cascade of magnetic energy. The sources for the the large-scale
magnetic field have been shown to be (1) energy flux from large-scale velocity
field to large-scale magnetic field arising due to nonhelical interactions, and
(2) inverse energy flux of magnetic energy caused by helical interactions.
Based on our flux results, a premitive model for galactic dynamo has been
constructed. Our calculations yields dynamo time-scale for a typical galaxy to
be of the order of $10^8$ years. Our field-theoretic calculations also reveal
that the flux of magnetic helicity is backward, consistent with the earlier
observations based on absolute equilibrium theory.",0107069v4
2003-11-26,Simulation of induction at low magnetic Prandtl number,"We consider the induction of magnetic field in flows of electrically
conducting fluid at low magnetic Prandtl number and large kinetic Reynolds
number. Using the separation between the magnetic and kinetic diffusive
lengthscales, we propose a new numerical approach. The coupled magnetic and
fluid equations are solved using a mixed scheme, where the magnetic field
fluctuations are fully resolved and the velocity fluctuations at small scale
are modelled using a Large Eddy Simulation (LES) scheme. We study the response
of a forced Taylor-Green flow to an externally applied field: tology of the
mean induction and time fluctuations at fixed locations. The results are in
remarkable agreement with existing experimental data; a global $1/f$ behavior
at long times is also evidenced.",0311130v1
2005-05-03,Magnetization of rotating ferrofluids: predictions of different theoretical models,"We consider a ferrofluid cylinder, that is rotating with constant rotation
frequency \Omega e_z as a rigid body. A homogeneous magnetic field H_0 e_x is
applied perpendicular to the cylinder axis e_z. This causes a nonequilibrium
situation. Therein the magnetization M and the internal magnetic field H are
constant in time and homogeneous within the ferrofluid. According to the
Maxwell equations they are related to each other via H = H_0 - M/2. However, H
and M are not parallel to each other and their directions differ from that of
the applied field H_0. We have analyzed several different theoretical models
that provide equations for the magnetization in such a situation. The
magnetization M is determined for each model as a function of \Omega and H_0 in
a wide range of frequencies and fields. Comparisons are made of the different
model results and the differences in particular of the predictions for the
perpendicular components H_y =-M_y/2 of the fields are analyzed.",0505022v2
2007-01-12,Fully developed turbulent dynamo at low magnetic Prandtl numbers,"We investigate the dynamo problem in the limit of small magnetic Prandtl
number ($\Pm$) using a shell model of magnetohydrodynamic turbulence. The model
is designed to satisfy conservation laws of total energy, cross helicity and
magnetic helicity in the limit of inviscid fluid and null magnetic diffusivity.
The forcing is chosen to have a constant injection rate of energy and no
injection of kinetic helicity nor cross helicity. We find that the value of the
critical magnetic Reynolds number ($\Rm$) saturates in the limit of small
$\Pm$. Above the dynamo threshold we study the saturated regime versus $\Rm$
and $\Pm$. In the case of equipartition, we find Kolmogorov spectra for both
kinetic and magnetic energy except for wave numbers just below the resistive
scale. Finally the ratio of both dissipation scales (viscous to resistive)
evolves as $\Pm^{-3/4}$ for $\Pm < 1$",0701152v1
2007-05-23,A ferrofluid based neural network: design of an analogue associative memory,"We analyse an associative memory based on a ferrofluid, consisting of a
system of magnetic nano-particles suspended in a carrier fluid of variable
viscosity subject to patterns of magnetic fields from an array of input and
output magnetic pads. The association relies on forming patterns in the
ferrofluid during a trainingdphase, in which the magnetic dipoles are free to
move and rotate to minimize the total energy of the system. Once equilibrated
in energy for a given input-output magnetic field pattern-pair the particles
are fully or partially immobilized by cooling the carrier liquid. Thus produced
particle distributions control the memory states, which are read out
magnetically using spin-valve sensors incorporated in the output pads. The
actual memory consists of spin distributions that is dynamic in nature,
realized only in response to the input patterns that the system has been
trained for. Two training algorithms for storing multiple patterns are
investigated. Using Monte Carlo simulations of the physical system we
demonstrate that the device is capable of storing and recalling two sets of
images, each with an accuracy approaching 100%.",0705.3402v1
2007-10-24,Criterion of multi-switching stability for magnetic nanoparticles,"We present a procedure to study the switching and the stability of an array
of magnetic nanoparticles in the dynamical regime. The procedure leads to the
criterion of multi-switching stability to be satisfied in order to have stable
switching. The criterion is used to compare various magnetic-field-induced
switching schemes, either present in the literature or suggested in the present
work. In particular, we perform micromagnetic simulations to study the
magnetization trajectories and the stability of the magnetization after
switching for nanoparticles of elliptical shape. We evaluate the stability of
the switching as a function of the thickness of the particles and the rise and
fall times of the magnetic pulses, both at zero and room temperature.
Furthermore, we investigate the role of the dipolar interaction and its
influence on the various switching schemes. We find that the criterion of
multi-switching stability can be satisfied at room temperature and in the
presence of dipolar interactions for pulses shaped according to CMOS
specifications, for switching rates in the GHz regime.",0710.4480v1
2007-10-26,Transport and magnetic properties in YBaCo2O5.45: Focus on the high-temperature transition,"The electronic transport properties and the magnetic susceptibility were
measured in detail in $YBaCo_2O_{5.45}$. Close to the so-called metal-insulator
transition, strong effects of resistance relaxation, a clear thermal hysteresis
and a sudden increase of the resistance noise are observed. This is likely due
to the first order character of the transition and to the underlying phases
coexistence. Despite these out of equilibrium features, a positive and linear
magneto-resistance is also observed, possibly linked to the heterogeneity of
the state. From a magnetic point of view, the paramagnetic to ordered magnetic
state transition is observed using non linear susceptibilty. This transition
shows the characteristics of a continuous transition, and time dependent
effects can be linked with the dynamics of magnetic domains in presence of
disorder. Thus, when focusing on the order of the transitions, the electronic
one and the magnetic one can not be directly associated.",0710.5008v1
2007-10-30,The Ekman-Hartmann layer in MHD Taylor-Couette flow,"We study magnetic effects induced by rigidly rotating plates enclosing a
cylindrical MHD Taylor-Couette flow at the finite aspect ratio $H/D=10$. The
fluid confined between the cylinders is assumed to be liquid metal
characterized by small magnetic Prandtl number, the cylinders are perfectly
conducting, an axial magnetic field is imposed $\Ha \approx 10$, the rotation
rates correspond to $\Rey$ of order $10^2-10^3$. We show that the end-plates
introduce, besides the well known Ekman circulation, similar magnetic effects
which arise for infinite, rotating plates, horizontally unbounded by any walls.
In particular there exists the Hartmann current which penetrates the fluid,
turns into the radial direction and together with the applied magnetic field
gives rise to a force. Consequently the flow can be compared with a Taylor-Dean
flow driven by an azimuthal pressure gradient. We analyze stability of such
flows and show that the currents induced by the plates can give rise to
instability for the considered parameters. When designing an MHD Taylor-Couette
experiment, a special care must be taken concerning the vertical magnetic
boundaries so they do not significantly alter the rotational profile.",0710.5600v1
2007-11-12,Enhancing Domain Wall Speed in Nanowires with Transverse Magnetic Fields,"Dynamic micromagnetic simulation studies have been completed to observe the
motion of a domain wall in a magnetic nanowire in an effort to increase the
field-driven domain wall speed. Previous studies have shown that the wire
dimensions place a cap on the maximum speed attainable by a domain wall when
driven by a magnetic field placed along the direction of the nanowire. Here we
present data showing a significant increase in the maximum speed of a domain
wall due to the addition of a magnetic field placed perpendicular to the
longitudinal driving field. The results are expressed in terms of the relative
alignment of the transverse field direction with respect to the direction of
the magnetic moments within the domain wall. In particular, when the transverse
field is parallel to the magnetic moments within the domain wall, the velocity
of the wall varies linearly with the strength of the transverse field
increasing by up to 20%. Further examination of the domain wall structure shows
that the length of the domain wall also depends linearly on the strength of the
transverse field. We present a simple model to correlate the effects.",0711.1864v1
2008-01-25,Effect of External Magnetic Field on Critical Current for the Onset of Virtual Cathode Oscillations in Relativistic Electron Beams,"In this Letter we research the space charge limiting current value at which
the oscillating virtual cathode is formed in the relativistic electron beam as
a function of the external magnetic field guiding the beam electrons. It is
shown that the space charge limiting (critical) current decreases with growth
of the external magnetic field, and that there is an optimal induction value of
the magnetic field at which the critical current for the onset of virtual
cathode oscillations in the electron beam is minimum. For the strong external
magnetic field the space charge limiting current corresponds to the analytical
relation derived under the assumption that the motion of the electron beam is
one-dimensional [High Power Microwave Sources. Artech House Microwave Library,
1987. Chapter~13]. Such behavior is explained by the characteristic features of
the dynamics of electron space charge in the longitudinal and radial directions
in the drift space at the different external magnetic fields.",0801.3960v1
2008-12-08,Spontaneous magnetization in QCD and non-Fermi-liquid effects,"Magnetic properties of quark matter at finite temperature are discussed by
evaluating the magnetic susceptibility. Combining the microscopic calculation
of the self-energy for quarks as well as the screening effects for gluons with
Fermi-liquid theory in a consistent way, we study the temperature dependence of
the magnetic susceptibility. The longitudinal gluons have the static screening
given by the Debye mass, and have a standard temperature dependence of
$O(T^2)$. An anomalous $T^2\ln T$ term arises in the magnetic susceptibility as
a novel non-Fermi-liquid effect due to the anomalous self-energy for quarks
given by the dynamic screening for transverse gluons. We then extract the
critical(Curie) temperature and present the magnetic phase diagram on the
density-temperature plane.",0812.1347v2
2009-01-14,Dynamically dominant magnetic fields in the diffuse interstellar medium,"Observations show that magnetic fields in the interstellar medium (ISM) often
do not respond to increases in gas density as would be naively expected for a
frozen-in field. This may suggest that the magnetic field in the diffuse gas
becomes detached from dense clouds as they form. We have investigated this
possibility using theoretical estimates, a simple magneto-hydrodynamic model of
a flow without mass conservation and numerical simulations of a thermally
unstable flow. Our results show that significant magnetic flux can be shed from
dense clouds as they form in the diffuse ISM, leaving behind a magnetically
dominated diffuse gas.",0901.1975v1
2009-02-08,Magnetic and magnetoelectric studies in pure and cation doped BiFeO3,"We report magnetic and magnetoelectric studies on BiFeO3 and divalent cation
(A) suvtitute Bi0.7A0.3FeO3 (A = Sr,Ba, and Sr0.5Ba0.5). It is shown that the
rapid increase of magnetization at the Neel temperature (TN = 642 K) is
suppressed in the co-doped compound A = Sr0.5Ba0.5. All the divalent subtituted
compounds show enhanced magnetization and hysteresis loop. Both longitudinal
and transverse magnetoelectric coefficients were measured using the dynamical
lock-in technique. The co-doped compound shows the highest magnetoelectric
coefficient at room temperature although it is not the compound with the
highest saturation magnetization. It is found that as the size of the A-site
cation increses, the transverse magnetoelectric coeffient increases and exceeds
the longitudinal magnetoelectric coefficient. It is suggested that changes in
magnetic domain structure and magnetostriction are possible reasons for the
observed changes in the magnetoelectric coefficients.",0902.1283v1
2009-02-19,Magnetic-field-enhanced incommensurate magnetic order in the underdoped high-temperature superconductor YBa(2)Cu(3)O(6.45),"We present a neutron-scattering study of the static and dynamic spin
correlations in the underdoped high-temperature superconductor
YBa(2)Cu(3)O(6.45) in magnetic fields up to 15 T. The field strongly enhances
static incommensurate magnetic order at low temperatures and induces a
spectral-weight shift in the magnetic-excitation spectrum. A reconstruction of
the Fermi surface driven by the field-enhanced magnetic superstructure may thus
be responsible for the unusual Fermi surface topology revealed by recent
quantum-oscillation experiments.",0902.3335v2
2009-02-27,The Magnetic Field of Solar Spicules,"Determining the magnetic field of solar spicules is vital for developing
adequate models of these plasma jets, which are thought to play a key role in
the thermal, dynamic, and magnetic structure of the chromosphere. Here we
report on magnetic spicule properties in a very quiet region of the off-limb
solar atmosphere, as inferred from new spectropolarimetric observations in the
HeI 10830 A triplet. We have used a novel inversion code for Stokes profiles
caused by the joint action of atomic level polarization and the Hanle and
Zeeman effects (HAZEL) to interpret the observations. Magnetic fields as strong
as 40G were unambiguously detected in a very localized area of the slit, which
may represent a possible lower value of the field strength of organized network
spicules.",0903.0002v1
2009-05-22,Memory and aging effects in antiferromagnetic nanoparticles,"We investigate slow dynamics of collection of a few antiferromagnetic NiO
nanoparticles. The zero-field cooled magnetization exhibits size dependent
fluctuations. We find memory effects in field cooled magnetization, as well as
aging effects in thermoremenant magnetization of antiferromagnetic
nanoparticles. The antiferromagnetic nanoparticles show a stronger memory
effect than the corresponding effect in the ferromagnetic particles, when the
distribution of particles include very small sizes. The situation reverses for
bigger sizes. The relaxation of the magnetization after a sudden cooling,
heating and removal of fields reiterate the memory effects. We also see a weak
signature of size-dependent magnetization fluctuations in aging effect of
antiferromagnetic nanoparticles. We find a two-step relaxation of
thermoremenant magnetization in antiferromagnetic case, which differs
qualitatively from relaxation of ferromagnetic nanoparticles.",0905.3617v2
2009-06-12,Magnetic dynamics with spin transfer torques near the Curie temperature,"We use atomistic stochastic Landau-Lifshitz-Slonczewski simulations to study
the interaction between large thermal fluctuations and spin transfer torques in
the magnetic layers of spin valves. At temperatures near the Curie temperature
$T_{\rm C}$, spin currents measurably change the size of the magnetization
(i.e. there is a {\it longitudinal} spin transfer effect). The change in
magnetization of the free magnetic layer in a spin valve modifies the
temperature dependence of the applied field-applied current phase diagram for
temperatures near $T_{\rm C}$. These atomistic simulations can be accurately
described by a Landau-Lifshitz-Bloch + Slonczewski equation, which is a
thermally averaged mean field theory. Both the simulation and the mean field
theory show that a longitudinal spin transfer effect can be a substantial
fraction of the magnetization close to $T_{\rm C}$.",0906.2423v1
2009-08-20,MnSi$_{1.7}$ nanoparticles embedded in Si: Superparamagnetism with a collective behavior,"The doping of Mn in Si is attracting research attentions due to the
possibility to fabricate Si-based diluted magnetic semiconductors. However, the
low solubility of Mn in Si favors the precipitation of Mn ions even at
non-equilibrium growth conditions. MnSi$_{1.7}$ nanoparticles are the common
precipitates, which show exotic magnetic properties in comparison with the
MnSi$_{1.7}$ bulk phase. In this paper we present the static and dynamic
magnetic properties of MnSi$_{1.7}$ nanoparticles. Using the Preisach model, we
derive the magnetic parameters, such as the magnetization of individual
particles, the distribution of coercive fields and the inter-particle
interaction field. Time-dependent magnetization measurements reveal a
spin-glass behavior of the system.",0908.2906v3
2009-10-16,Heavy fermion material: Ce versus Yb case,"Heavy fermion compounds are complex systems but excellent materials to study
quantum criticality with the switch of different ground states. Here a special
attention is given on the interplay between magnetic and valence instabilities
which can be crossed or approached by tuning the system by pressure or magnetic
field. By contrast to conventional rare earth magnetism or classical s wave
superconductivity, strong couplings may occur with drastic changes in spin or
charge dynamics. Measurements on Ce materials give already a sound basis with
clear key factors; they have pointed out that close to a magnetic or a valence
criticality unexpected phenomena such as unconventional superconductivity, non
Fermi liquid behaviour and the possibility of re-entrance phenomena under
magnetic field. Recent progresses in the growth of Yb heavy fermion compounds
give the perspectives of clear interplays between valence and magnetic
fluctuations and also the possibility to enter in new situations such as
valence transitions inside a sole doublet crystal field ground state.",0910.3110v2
2010-01-13,On the 3D Structure of the Magnetic Field in Regions of Emerging Flux,"We explore the photospheric and chromospheric magnetic field in an emerging
flux region. An image of the equivalent width of the He I 10830 A red blended
component shows the presence of filamentary structures that might be
interpreted as magnetic loops. We point out that the magnetic field strength in
the chromosphere resembles a smoothed version of that found in the photosphere
and that it is not correlated at all with the above-mentioned equivalent width
map. Lacking other diagnostics, this suggests that one cannot discard the
possibility that the chromospheric field we infer from the observations is
tracing the lower chromosphere of the active region instead of tracing the
magnetic field along loops. If the He I line is formed within magnetic loops,
we point out a potential problem that appears when interpreting observations
using only one component along the line-of-sight.",1001.2108v1
2010-03-01,Switchable Hardening of a Ferromagnet at Fixed Temperature,"The intended use of a magnetic material, from information storage to power
conversion, depends crucially on its domain structure, traditionally crafted
during materials synthesis. By contrast, we show that an external magnetic
field applied transverse to the preferred magnetization of a model disordered
uniaxial ferromagnet is an isothermal regulator of domain pinning. At elevated
temperatures, near the transition into the paramagnet, modest transverse fields
increase the pinning, stabilize the domain structure, and harden the magnet,
until a point where the field induces quantum tunneling of the domain walls and
softens the magnet. At low temperatures, tunneling completely dominates the
domain dynamics and provides an interpretation of the quantum phase transition
in highly disordered magnets as a localization/delocalization transition for
domain walls. While the energy scales of the rare earth ferromagnet studied
here restrict the effects to cryogenic temperatures, the principles discovered
are general and should be applicable to existing classes of highly anisotropic
ferromagnets with ordering at room temperature or above.",1003.0369v1
2010-03-04,Local strong parity violation and new possibilities in experimental study of non-perturbative QCD,"Quark interaction with topologically non-trivial gluonic fields, instantons
and sphalerons, violates \P and \CP symmetry. In the strong magnetic field of a
non-central nuclear collision such interactions lead to the charge separation
along the magnetic field, the so called chiral magnetic effect, which manifests
local parity violations. An experimental observation of the chiral magnetic
effect would be a direct proof for the existence of such physics. Recent STAR
results on charge and the reaction plane dependent correlations are consistent
with theoretical expectations for the chiral magnetic effect. IIn this paper I
discuss different approaches to experimental study of the local parity
violation, and propose future measurements which can clarify the picture. In
particular I propose to use central body-body U+U collisions to disentangle
correlations due to chiral magnetic effect from possible background
correlations due to elliptic flow. Further more quantitative studies can be
performed with collision of isobaric beams.",1003.1127v1
2010-03-05,Magnetoresistance in nanostructures: the role of nonuniform current,"We developed a method to calculate the magnetoresistance of magnetic
nanostructures. We discretize a magnetic disk in small cells and numerically
solve the Landau-Lifshitz-Gilbert (LLG) equation in order to obtain its
magnetization profile. We consider a anisotropic magnetoresistance (AMR) that
depends on the local magnetization as the main source of the magnetoresistance.
We then use it as an input to calculate the resistance and current distribution
numerically, using a relaxation method. We show how magnetoresistance
measurements can be useful to obtain information on the magnetic structure.
Additionally, we obtain non-homogeneous current distributions for different
magnetic configurations in static and dynamical regimes.",1003.1323v2
2010-03-17,Chiral Magnetic Spiral,"We argue that the presence of a very strong magnetic field in the chirally
broken phase induces inhomogeneous expectation values, of a spiral nature along
the magnetic field axis, for the currents of charge and chirality, when there
is finite baryon density or an imbalance between left and right chiralities.
This ""chiral magnetic spiral"" is a gapless excitation transporting the currents
of (i) charge (at finite chirality), and (ii) chirality (at finite baryon
density) along the direction of the magnetic field. In both cases it also
induces in the transverse directions oscillating currents of charge and
chirality. In heavy ion collisions, the chiral magnetic spiral possibly
provides contributions both to the out-of-plane and the in-plane dynamical
charge fluctuations recently observed at RHIC.",1003.3464v1
2010-04-21,"Effect of pressure on the magnetic, transport, and thermal-transport properties of the electron-doped manganite CaMn$_{1-x}$Sb$_{x}$O$_{3}$","We have demonstrated the effect of hydrostatic pressure on magnetic and
transport properties, and thermal transport properties in electron-doped
manganites CaMn$_{1-x}$Sb$_{x}$O$_{3}$. The substitution of Sb$^{5+}$ ion for
Mn $^{4+}$site of the parent matrix causes one-electron doping with the
chemical formula CaMn$^{4+}_{1-2x}$Mn$^{3+}_{x}$Sb$^{5+}_{x}$O$_{3}$
accompanied by a monotonous increase in unit cell volume as a function of $x$.
Upon increasing the doping level of Sb, the magnitudes of both electrical
resistivity and negative Seebeck coefficient are suppressed at high
temperatures, indicating the electron doping. Anomalous diamagnetic behaviors
at $x=0.05$ and 0.08 are clearly observed in field cooled dc magnetization. The
effect of hydrostatic pressure on dc magnetization is in contrast to the
chemical pressure effect due to Sb doping. The dynamical effect of ac magnetic
susceptibility measurement points to the formation of the magnetically
frustrated clusters such as FM clusters embedded in canted AFM matrix.",1004.3669v3
2010-04-20,Wide-range wavevector selectivity of magnon gases in Brillouin light scattering spectroscopy,"Brillouin light scattering spectroscopy is a powerful technique for the study
of fast magnetization dynamics with both frequency- and wavevector resolution.
Here, we report on a distinct improvement of this spectroscopic technique
towards two-dimensional wide-range wavevector selectivity in a backward
scattering geometry. Spin-wave wavevectors oriented perpendicular to the bias
magnetic field are investigated by tilting the sample within the magnet gap.
Wavevectors which are oriented parallel to the applied magnetic field are
analyzed by turning the entire setup, including the magnet system. The setup
features a wide selectivity of wavevectors up to 2.04\cdot 10E5 rad/cm for both
orientations, and allows selecting and measuring wavevectors of dipole- and
exchange-dominated spin waves of any orientation to the magnetization
simultaneously.",1005.5084v1
2010-07-06,Effect of induced magnetic field on peristaltic flow of a micropolar fluid in an asymmetric channel,"Of concern in this paper is an investigation of peristaltic transport of a
physiological fluid in an asymmetric channel under long wave length and
low-Reynolds number assumptions. The flow is assumed to be incompressible,
viscous, electrically conducting micropolar fluid and the effect of induced
magnetic field is taken into account. Exact analytical solutions obtained for
the axial velocity, microrotation component, stream line pattern, magnetic
force function, axial-induced magnetic field as well as the current density
distribution across the channel. The flow phenomena for the pumping
characteristics, trapping and reflux are also investigated. The results
presented reveal that the velocity decreases with the increase of magnetic
field as well as the coupling parameter. Moreover, the trapping fluid can be
eliminated by the application of an external magnetic field. Thus, the study
bears the promise of important applications in physiological systems.",1007.0923v1
2010-08-14,Scaling the energy conversion rate from magnetic field reconnection to different bodies,"Magnetic field reconnection is often invoked to explain electromagnetic
energy conversion in planetary magnetospheres, stellar coronae, and other
astrophysical objects. Because of the huge dynamic range of magnetic fields in
these bodies, it is important to understand energy conversion as a function of
magnetic field strength and related parameters. It is conjectured theoretically
and shown experimentally that the energy conversion rate per unit area in
reconnection scales as the cube of an appropriately weighted magnetic field
strength divided by the square root of an appropriately weighted density. With
this functional dependence, the energy release in flares on the Sun, the large
and rapid variation of the magnetic flux in the tail of Mercury, and the
apparent absence of reconnection on Jupiter and Saturn, may be understood.
Electric fields at the perihelion of the Solar Probe Plus mission may be tens
of volts/meter.",1008.2454v3
2010-09-29,Magnetic spectrum of the two-dimensional antiferromagnet La2CoO4 studied by inelastic neutron scattering,"We report measurements of the magnetic excitation spectrum of the layered
antiferromagnet La2CoO4 by time-of-flight neutron inelastic scattering. In the
energy range probed in our experiments (0-250 meV) the magnetic spectrum
consists of spin-wave modes with strong in-plane dispersion extending up to 60
meV, and a nearly dispersionless peak at 190 meV. The spin-wave modes exhibit a
small (~1 meV) dispersion along the magnetic zone boundary. We show that the
magnetic spectrum can be described very well by a model of a Heisenberg
antiferromagnet that includes the full spin and orbital degrees of freedom of
Co2+ in an axially-distorted crystal field. The collective magnetic dynamics
are found to be controlled by dominant nearest-neighbour exchange interactions,
strong XY-like single-ion anisotropy and a substantial unquenched orbital
angular momentum.",1009.5913v2
2010-09-29,Sensitivity of a Babcock-Leighton Flux-Transport Dynamo to Magnetic Diffusivity Profiles,"We study the influence of various magnetic diffusivity profiles on the
evolution of the poloidal and toroidal magnetic fields in a kinematic flux
transport dynamo model for the Sun. The diffusivity is a poorly understood
ingredient in solar dynamo models. We mathematically construct various
theoretical profiles of the depth-dependent diffusivity, based on constraints
from mixing length theory and turbulence, and on comparisons of poloidal field
evolution on the Sun with that from the flux-transport dynamo model.
  We then study the effect of each diffusivity profile in the cyclic evolution
of the magnetic fields in the Sun, by solving the mean-field dynamo equations.
We investigate effects on the solar cycle periods, the maximum tachocline field
strengths, and the evolution of the toroidal and poloidal field structures
inside the convection zone, due to different diffusivity profiles.
  We conduct three experiments: (I) comparing very different magnetic
diffusivity profiles; (II) comparing different locations of diffusivity
gradient near the tachocline for the optimal profile; and (III) comparing
different slopes of diffusivity gradient for an optimal profile.
  Based on these simulations, we discuss which aspects of depth-dependent
diffusivity profiles may be most relevant for magnetic flux evolution in the
Sun, and how certain observations could help improve knowledge of this dynamo
ingredient.",1009.5965v1
2010-10-11,"Magnetic fields, winds and X-rays of the massive stars in the Orion Nebula Cluster","In some massive stars, magnetic fields are thought to confine the outflowing
radiatively-driven wind. Although theoretical models and MHD simulations are
able to illustrate the dynamics of such a magnetized wind, the impact of this
wind-field interaction on the observable properties of a magnetic star - X-ray
emission, photometric and spectral variability - is still unclear. The aim of
this study is to examine the relationship between magnetism, stellar winds and
X-ray emission of OB stars, by providing empirical observations and confronting
theory. In conjunction with the COUP survey of the Orion Nebula Cluster, we
carried out spectropolarimatric ESPaDOnS observations to determine the magnetic
properties of massive OB stars of this cluster.",1010.2244v1
2010-10-29,Magnetic helicity transport in the advective gauge family,"Magnetic helicity fluxes are investigated in a family of gauges in which the
contribution from ideal magnetohydrodynamics takes the form of a purely
advective flux. Numerical simulations of magnetohydrodynamic turbulence in this
advective gauge family exhibit instabilities triggered by the build-up of
unphysical irrotational contributions to the magnetic vector potential. As a
remedy, the vector potential is evolved in a numerically well behaved gauge,
from which the advective vector potential is obtained by a gauge
transformation. In the kinematic regime, the magnetic helicity density evolves
similarly to a passive scalar when resistivity is small and turbulent mixing is
mild, i.e. when the fluid Reynolds number is not too large. In the dynamical
regime, resistive contributions to the magnetic helicity flux in the advective
gauge are found to be significant owing to the development of small length
scales in the irrotational part of the magnetic vector potential.",1010.6177v2
2010-10-31,Magnetic dynamo action at low magnetic Prandtl numbers,"Amplification of magnetic field due to kinematic turbulent dynamo action is
studied in the regime of small magnetic Prandtl numbers. Such a regime is
relevant for planets and stars interiors, as well as for liquid metal
laboratory experiments. A comprehensive analysis based on the
Kazantsev-Kraichnan model is reported, which establishes the dynamo threshold
and the dynamo growth rates for varying kinetic helicity of turbulent
fluctuations. It is proposed that in contrast with the case of large magnetic
Prandtl numbers, the kinematic dynamo action at small magnetic Prandtl numbers
is significantly affected by kinetic helicity, and it can be made quite
efficient with an appropriate choice of the helicity spectrum.",1011.0202v1
2010-12-07,"The interplay between magnetism, structure, and strong electron-phonon coupling in binary FeAs under pressure","Unlike the ferropnictide superconductors, which crystallize in a tetragonal
crystal structure, binary FeAs forms in an orthorhombic crystal structure,
where the local atomic environment resembles a highly distorted variant of the
FeAs4 tetrahdedral building block of the ferropnictide superconductors.
However, like the parent compounds of the ferropnictide superconductors, FeAs
undergoes magnetic ordering at low temperatures, with no evidence favoring a
superconducting ground state at ambient pressure. We employ pressure-dependent
electrical transport and x-ray diffraction measurements using diamond anvil
cells to characterize the magnetic state and the structure as a function of
pressure. While the MnP-type structure of FeAs persists up to 25 GPa,
compressing continuously with no evidence of structural transformations under
pressure, features in the magnetotransport measurements associated with
magnetism are not observed for pressures in excess of 11 GPa. Where observable,
the features associated with magnetic order at ambient pressure show remarkably
little pressure dependence, and transport measurements suggest that a dynamical
structural instability coupled to the Fermi surface via a strong
electron-phonon interaction may play an important role in enabling magnetism in
FeAs.",1012.1391v1
2010-12-17,Structural and magnetic properties of Co2MnSi thin films,"Co2MnSi (CMS) films of different thicknesses (20, 50 and 100 nm) were grown
by radio frequency (RF) sputtering on a-plane sapphire substrates. Our X-rays
diffraction study shows that, in all the samples, the cubic <110> CSM axis is
normal to the substrate and that there exist well defined preferential in-plane
orientations. Static and dynamic magnetic properties were investigated using
vibrating sample magnetometry (VSM) and micro-strip line ferromagnetic
resonance (MS-FMR), respectively. From the resonance measurements versus the
direction and the amplitude of an applied magnetic field we derive most of the
magnetic parameters: magnetization, gyromagnetic factor, exchange stiffness
coefficient and magnetic anisotropy terms. The in-plane anisotropy can be
described as resulting from the superposition of two terms showing a two-fold
and a four-fold symmetry without necessarily identical principal axes. The
observed behavior of the hysteresis loops is in agreement with this complex
form of the in-plane anisotropy",1012.3837v2
2011-02-13,Magnetization and Lyapunov exponents on a kagome chain with multi-site exchange interaction,"The Ising approximation of the Heisenberg model in a strong magnetic field,
with two, three and six spin exchange interactions is studied on a kagome
chain. The kagome chain can be considered as an approximation of the third
layer of 3He absorbed on the surface of graphite (kagome lattice). By using
dynamical approach we have found one and multi-dimensional mappings (recursion
relations) for the partition function. The magnetization diagrams are plotted
and they show that the kagome chain is separating into four sublattices with
different magnetizations. Magnetization curves of two sublattices exhibit
plateaus at zero and 2/3 of the saturation field. The maximal Lyapunov exponent
for multi-dimensional mapping is considered and it is shown that near the
magnetization plateaus the maximal Lyapunov exponent also exhibits plateaus.",1102.2589v1
2011-03-10,Chiral Properties of Strong Interactions in a Magnetic Background,"We investigate the chiral properties of QCD in presence of a magnetic
background field and in the low temperature regime, by lattice numerical
simulations of N_f = 2 QCD. We adopt a standard staggered discretization, with
a pion mass around 200 MeV, and explore a range of magnetic fields (180 MeV)^2
\leq|e|B \leq (700 MeV)^2, in which we study magnetic catalysis, i.e. the
increase of chiral symmetry breaking induced by the background field. We
determine the dependence of the chiral condensate on the external field,
compare our results with existing model predictions and show that a substantial
contribution to magnetic catalysis comes from the modified distribution of
non-Abelian gauge fields, induced by the magnetic field via dynamical quark
loop effects.",1103.2080v2
2011-03-11,Interaction of a magnetic dipole with a slowly moving electrically conducting plate,"We report an analytical investigation of the force and torque acting upon a
magnetic dipole placed in the vicinity of a moving electrically conducting
nonmagnetic plate. This problem is relevant to contactless electromagnetic flow
measurement in metallurgy and extends previous theoretical works (Thess et al.
Phys. Rev. Lett. 96(2006), 164501; New J. Phys. 9(2007), 299) to the case where
the orientation of the magnetic dipole relative to the plate is arbitrary. It
is demonstrated that for the case of low magnetic Reynolds number the
three-dimensional distributions of the induced electric potential, of the eddy
currents and of the induced magnetic field can be rigorously derived. It is
also shown that all components of the force and torque can be computed without
any further approximation. The results of the present work serve as a benchmark
problem that can be used to verify numerical simulations of more complex
magnetic field distributions.",1103.2245v1
2011-04-04,Evolution of magnetic fields through cosmological perturbation theory,"The origin of galactic and extra-galactic magnetic fields is an unsolved
problem in modern cosmology. A possible scenario comes from the idea of these
fields emerged from a small field, a seed, which was produced in the early
universe (phase transitions, inflation, ...) and it evolves in time.
Cosmological perturbation theory offers a natural way to study the evolution of
primordial magnetic fields. The dynamics for this field in the cosmological
context is described by a cosmic dynamo like equation, through the dynamo term.
In this paper we get the perturbed Maxwell's equations and compute the energy
momentum tensor to second order in perturbation theory in terms of gauge
invariant quantities. Two possible scenarios are discussed, first we consider a
FLRW background without magnetic field and we study the perturbation theory
introducing the magnetic field as a perturbation. The second scenario, we
consider a magnetized FLRW and build up the perturbation theory from this
background. We compare the cosmological dynamo like equation in both scenarios.",1104.0701v4
2011-04-07,Magnetization hysteresis and time decay measurements in FeSe$_{0.50}$Te$_{0.50}$ : Evidence for fluctuation in mean free path induced pinning,"We present results of magnetic measurements relating to vortex phase diagram
in a single crystal of FeSe$_{0.5}$Te$_{0.5}$ which displays second
magnetization peak anomaly for $H \parallel c$. The possible role of the
crystalline anisotropy on vortex pinning is explored via magnetic torque
magnetometry. We present evidence in favor of pinning related to spatial
variations of the charge carrier mean free path leading to small bundle vortex
pinning by randomly distributed (weak) pinning centers for both $H \parallel c$
and $H \perp c$. This is further corroborated using magnetization data for $H
\parallel c$ in a single crystal of FeSe$_{0.35}$Te$_{0.65}$. Dynamical
response across second magnetization peak (SMP) anomaly in
FeSe$_{0.5}$Te$_{0.5}$ has been compared with that across the well researched
phenomenon of peak effect (PE) in a single crystal of CeRu$_2$.",1104.1332v4
2011-05-05,Wave packet dynamics and valley filter in strained graphene,"The time evolution of a wavepacket in strained graphene is studied within the
tight-binding model and continuum model. The effect of an external magnetic
field, as well as a strain-induced pseudo-magnetic field, on the wave packet
trajectories and zitterbewegung are analyzed. Combining the effects of strain
with those of an external magnetic field produces an effective magnetic field
which is large in one of the Dirac cones, but can be practically zero in the
other. We construct an efficient valley filter, where for a propagating
incoming wave packet consisting of momenta around the K and K' Dirac points,
the outgoing wave packet exhibits momenta in only one of these Dirac points,
while the components of the packet that belong to the other Dirac point are
reflected due to the Lorentz force. We also found that the zitterbewegung is
permanent in time in the presence of either external or strain-induced magnetic
fields, but when both the external and strain-induced magnetic fields are
present, the zitterbewegung is transient in one of the Dirac cones, whereas in
the other cone the wave packet exhibits permanent spatial oscillations.",1105.1125v1
2011-05-06,Magnetism in superconducting EuFe$_2$As$_{1.4}$P$_{0.6}$ single crystals studied by local probes,"We have studied the magnetism in superconducting single crystals of EuFe2
As1.4 P0.6 by using the local probe techniques of zero-field muon spin
rotation/relaxation and 151 Eu/57 Fe M\""ossbauer spec- troscopy. All of these
measurements reveal magnetic hyperfine fields below the magnetic ordering
temperature TM = 18 K of the Eu2+ moments. The analysis of the data shows that
there is a coexistence of ferromagnetism, resulting from Eu2+ moments ordered
along the crystallographic c-axis, and superconductivity below TSC \approx 15
K. We find indications for a change in the dynamics of the small Fe magnetic
moments (\sim 0.07 \mu B) at the onset of superconductivity: below TSC the Fe
magnetic moments seem to be ""frozen"" within the ab-plane.",1105.1201v1
2011-05-31,Flow-turbulence interaction in magnetic reconnection,"Roles of turbulence in the context of magnetic reconnection are investigated
with special emphasis on the mutual interaction between flow (large-scale
inhomogeneous structure) and turbulence. In order to evaluate the effective
transport due to turbulence, in addition to the {\it intensity} information of
turbulence represented by the turbulent energy, the {\it structure} information
represented by pseudoscalar statistical quantities (helicities) is important.
On the basis of the evolution equation, mechanisms that provide turbulence with
cross helicity are presented. Magnetic-flux freezing in highly turbulent media
is considered with special emphasis on the spatial distribution of the
turbulent cross helicity. The cross-helicity effects in the context of magnetic
reconnection are also investigated. It is shown that the large-scale flow and
magnetic-field configurations favorable for the cross-helicity generation is
compatible with the fast reconnection. In this sense, turbulence and
large-scale structures promote magnetic reconnection mediated by the turbulent
cross helicity.",1105.6343v2
2011-06-14,A dynamo driven by zonal jets at the upper surface: Applications to giant planets,"We present a dynamo mechanism arising from the presence of barotropically
unstable zonal jet currents in a rotating spherical shell. The shear
instability of the zonal flow develops in the form of a global Rossby mode,
whose azimuthal wavenumber depends on the width of the zonal jets. We obtain
self-sustained magnetic fields at magnetic Reynolds numbers greater than 1000.
We show that the propagation of the Rossby waves is crucial for dynamo action.
The amplitude of the axisymmetric poloidal magnetic field depends on the
wavenumber of the Rossby mode, and hence on the width of the zonal jets. We
discuss the plausibility of this dynamo mechanism for generating the magnetic
field of the giant planets. Our results suggest a possible link between the
topology of the magnetic field and the profile of the zonal winds observed at
the surface of the giant planets. For narrow Jupiter-like jets, the poloidal
magnetic field is dominated by an axial dipole whereas for wide Neptune-like
jets, the axisymmetric poloidal field is weak.",1106.2701v3
2011-06-16,Synchronous Spatial Oscillation of Electron- and Mn-Spin Polarizations in Dilute-Magnetic-Semiconductor Quantum Wells under Spin-Orbit Effective Magnetic Fields,"In semiconductors, spin-orbit effective magnetic fields, i.e., the Rashba and
Dresselhaus fields, are used to control electron-spin polarization. This
operation, however, destroys the electron-spin coherence, and the spin
polarization is limited to the vicinity of a ferromagnetic source electrode. In
this paper, we propose the use of dilute magnetic semiconductors to improve the
coherence of spatially oscillating electron-spin polarization. In dilute
magnetic semiconductors, the electron-spin polarization near the source
electrode dynamically induces the local spin polarization of magnetic
impurities through s-d spin-flip scattering. This impurity-spin polarization
improves, in turn, the coherence of the electron-spin polarization, and this
improved electron-spin polarization induces impurity-spin polarization farther
in the adjacent region. Because of this positive feedback, the coherent and
synchronized spatial oscillations of electron- and impurity-spin polarizations
grow cooperatively. A numerical calculation for a CdMnTe quantum well
demonstrates the validity of this mechanism.",1106.3124v2
2011-07-04,A Generalized Flux Function for Three-dimensional Magnetic Reconnection,"The definition and measurement of magnetic reconnection in three-dimensional
magnetic fields with multiple reconnection sites is a challenging problem,
particularly in fields lacking null points. We propose a generalization of the
familiar two-dimensional concept of a magnetic flux function to the case of a
three-dimensional field connecting two planar boundaries. Using hyperbolic
fixed points of the field line mapping, and their global stable and unstable
manifolds, we define a unique flux partition of the magnetic field. This
partition is more complicated than the corresponding (well-known) construction
in a two-dimensional field, owing to the possibility of heteroclinic points and
chaotic magnetic regions. Nevertheless, we show how the partition reconnection
rate is readily measured with the generalized flux function. We relate our
partition reconnection rate to the common definition of three-dimensional
reconnection in terms of integrated parallel electric field. An analytical
example demonstrates the theory, and shows how the flux partition responds to
an isolated reconnection event.",1107.0594v1
2011-11-08,Magnetic helicity fluxes and their effect on stellar dynamos,"Magnetic helicity fluxes in turbulently driven alpha^2 dynamos are studied to
demonstrate their ability to alleviate catastrophic quenching. A
one-dimensional mean-field formalism is used to achieve magnetic Reynolds
numbers of the order of 10^5. We study both diffusive magnetic helicity fluxes
through the mid-plane as well as those resulting from the recently proposed
alternate dynamic quenching formalism. By adding shear we make a parameter scan
for the critical values of the shear and forcing parameters for which dynamo
action occurs. For this $\alpha\Omega$ dynamo we find that the preferred mode
is antisymmetric about the mid-plane. This is also verified in 3-D direct
numerical simulations.",1111.2023v2
2011-11-16,On Simplified Numerical Turbulence Models in Test-particle Simulations,"Using the conventional approach of superposing plane waves, it is not
possible to create a strictly isotropic turbulent magnetic field structure that
obeys all physical constraints, which are (i) equal mean of all magnetic field
components; (ii) isotropy of the wave vectors; and (iii) vanishing divergence
of the magnetic field. Such magnetic fields are widely implemented in
test-particle Monte-Carlo simulations, which are used to obtain (i) scattering
mean free paths of charged particles; (ii) field line diffusion coefficients.
It is shown that, while the turbulent magnetic field strength plays an
important role for the results, such does not seem to be the case for a
non-zero magnetic field divergence and/or the isotropy of the wave vectors.",1111.3780v1
2012-01-13,Intermediate state switching dynamics in magnetic double layer nanopillars grown by molecular beam epitaxy,"We observe a stable intermediate resistance switching state in the current
perpendicular to plane geometry for all Co/Cu/Co double layer nanopillar
junctions grown by molecular beam epitaxy. This novel state has a resistance
between the resistances of the parallel and antiparallel alignment of both
Co-layer magnetizations. The state, which originates from an additional
in-plane magnetic easy axis, can be reached by spin transfer torque switching
or by an external magnetic field. In addition to spin torque-induced coherent
small-angle spin wave modes we observe a broad microwave emission spectrum. The
latter is attributed to incoherent magnetic excitations that lead to a
switching between the intermediate state and the parallel or antiparallel
alignment of both ferromagnetic layers. We conclude that the additional
magnetic easy axis suppresses a stable trajectory of coherent large-angle
precession, which is not observed in our samples.",1201.2752v1
2012-03-22,Observation of An Evolving Magnetic Flux Rope Prior To and During A Solar Eruption,"Explosive energy release is a common phenomenon occurring in magnetized
plasma systems ranging from laboratories, Earth's magnetosphere, the solar
corona and astrophysical environments. Its physical explanation is usually
attributed to magnetic reconnection in a thin current sheet. Here we report the
important role of magnetic flux rope structure, a volumetric current channel,
in producing explosive events. The flux rope is observed as a hot channel prior
to and during a solar eruption from the Atmospheric Imaging Assembly (AIA)
telescope on board the Solar Dynamic Observatory (SDO). It initially appears as
a twisted and writhed sigmoidal structure with a temperature as high as 10 MK
and then transforms toward a semi-circular shape during a slow rise phase,
which is followed by fast acceleration and onset of a flare. The observations
suggest that the instability of the magnetic flux rope trigger the eruption,
thus making a major addition to the traditional magnetic-reconnection paradigm.",1203.4859v1
2012-06-24,Low-Temperature Low-Field Phases of the Pyrochlore Quantum Magnet Tb$_2$Ti$_2$O$_7$,"By means of ac magnetic-susceptibility measurements, we find evidence for a
new magnetic phase of Tb$_2$Ti$_2$O$_7$ below about 140 mK in zero magnetic
field. In magnetic fields parallel to [111], this phase---exhibiting frequency-
and amplitude-dependent susceptibility and an extremely slow spin
dynamics---extends to about 70 mT, at which it gives way to another phase. The
field dependence of the susceptibility of this second phase, which extends to
about 0.6 T, indicates the presence of a weak magnetization plateau below 50
mK, as has been predicted by a single-tetrahedron four-spin model, giving
support to the underlying proposal that the disordered low-field ground state
of Tb$_2$Ti$_2$O$_7$ is a quantum spin ice.",1206.5541v3
2012-06-24,Chiral Effects and Cosmic Magnetic Fields,"In the presence of cosmic chiral asymmetry, chiral-vorticity and
chiral-magnetic effects can play an important role in the generation and
evolution of magnetic fields in the early universe. We include these chiral
effects in the magnetic field equations and find solutions under simplifying
assumptions. Our numerical and analytical results show the presence of an
attractor solution in which chiral effects produce a strong, narrow, Gaussian
peak in the magnetic spectrum and the magnetic field becomes maximally helical.
The peak in the spectrum shifts to longer length scales and becomes sharper
with evolution. We also find that the dynamics may become non-linear for
certain parameters, pointing to the necessity of a more complete analysis.",1206.5549v1
2012-08-07,Observation of Coherent Helimagnons and Gilbert damping in an Itinerant Magnet,"We study the magnetic excitations of itinerant helimagnets by applying
time-resolved optical spectroscopy to Fe0.8Co0.2Si. Optically excited
oscillations of the magnetization in the helical state are found to disperse to
lower frequency as the applied magnetic field is increased; the fingerprint of
collective modes unique to helimagnets, known as helimagnons. The use of
time-resolved spectroscopy allows us to address the fundamental magnetic
relaxation processes by directly measuring the Gilbert damping, revealing the
versatility of spin dynamics in chiral magnets. (*These authors contributed
equally to this work)",1208.1462v1
2012-10-25,Strong Competition of Superconducting and Magnetic Order Parameters in Ba1-xNaxFe2As2,"We study the interplay of magnetic and superconducting order in single
crystalline hole doped Ba1-xNaxFe2As2 using muon spin relaxation. We find
microscopic coexistence of magnetic order and superconductivity. In a strongly
underdoped specimen the two forms of order coexist without any measurable
reduction of the ordered magnetic moment by superconductivity, while in a
nearly optimally doped sample the ordered magnetic moment is strongly
suppressed below the superconducting transition temperature. This coupling can
be well described within the framework of an effective two-band model
incorporating inter- and intra-band interactions. In optimally doped
Ba1-xNaxFe2As2 we observe no traces of static or dynamic magnetism and the
temperature dependence of the superfluid density is consistent with two s-wave
gaps without nodes.",1210.6881v1
2012-11-09,An Arecibo Survey for Zeeman Splitting in OH Megamaser Galaxies,"We present the results of a comprehensive survey using the Arecibo
Observatory for Zeeman splitting of OH lines in OH megamasers (OHMs). A total
of seventy-seven sources were observed with the Arecibo telescope. Of these,
maser emission could not be detected for eight sources, and two sources were
only ambiguously detected. Another twenty-seven sources were detected at low
signal-to-noise ratios or with interference that prevented placing any useful
limits on the presence of magnetic fields. In twenty-six sources, it was
possible to place upper limits on the magnitude of magnetic fields, typically
between 10-30 mG. For fourteen sources, the Stokes V spectra exhibit features
consistent with Zeeman splitting. Eleven of these fourteen are new detections,
and the remaining three are re-detections of Stokes V detections in Robishaw et
al. (2008). Among confident new detections, we derive magnetic fields
associated with maser regions with magnitudes ranging from 6.1-27.6 mG. The
distribution of magnetic field strengths suggests the magnetic fields in OH
masing clouds in OHMs are larger than those in Galactic OH masers. The results
are consistent with magnetic fields playing a dynamically important role in OH
masing clouds in OHMs.",1211.2023v1
2012-11-13,Magnetization reversal in isolated and interacting single-domain nanoparticles,"Computational and experimental results on the thermally-induced magnetization
reversal in single-domain magnetic nanoparticles are reported. The simulations
are based on the direct integration of the Fokker-Planck equation that governs
the dynamics of the magnetic moment associated with the nanoparticles. A mean
field approximation is used to account for the influence of the dipolar
interaction between nanoparticles. It is shown that the interactions can either
speed up or slow down the reversal process, depending on the angle between the
external magnetic field and the axis of easy magnetization. The numerical
results are in good agreement with experimental measurements on cobalt-platinum
nanoparticles.",1211.3007v1
2012-11-15,Spin transport and tunable Gilbert damping in a single-molecule magnet junction,"We study time-dependent electronic and spin transport through an electronic
level connected to two leads and coupled with a single-molecule magnet via
exchange interaction. The molecular spin is treated as a classical variable and
precesses around an external magnetic field. We derive expressions for charge
and spin currents by means of the Keldysh non-equilibrium Green's functions
technique in linear order with respect to the time-dependent magnetic field
created by this precession. The coupling between the electronic spins and the
magnetization dynamics of the molecule creates inelastic tunneling processes
which contribute to the spin currents. The inelastic spin currents, in turn,
generate a spin-transfer torque acting on the molecular spin. This back-action
includes a contribution to the Gilbert damping and a modification of the
precession frequency. The Gilbert damping coefficient can be controlled by the
bias and gate voltages or via the external magnetic field and has a
non-monotonic dependence on the tunneling rates.",1211.3611v2
2012-11-27,"""Strongly interacting matter in magnetic fields"": an overview","This is an introduction to the volume of Lecture Notes in Physics on
""Strongly interacting matter in magnetic fields"". The volume combines
contributions written by a number of experts on different aspects of the
problem. The response of QCD matter to intense magnetic fields has attracted a
lot of interest recently. On the theoretical side, this interest stems from the
possibility to explore the plethora of novel phenomena arising from the
interplay of magnetic field with QCD dynamics. On the experimental side, the
interest is motivated by the recent results on the behavior of quark-gluon
plasma in a strong magnetic field created in relativistic heavy ion collisions
at RHIC and LHC. The purpose of this introduction is to provide a brief
overview and a guide to the individual contributions where these topics are
covered in detail.",1211.6245v2
2012-12-03,Magnetic effects on nonlinear mechanical properties of a suspended carbon nanotube,"We propose a microscopic model for a nanoelectromechanical system made by a
radio-frequency driven suspended carbon nanotube (CNT) in the presence of an
external magnetic field perpendicular to the current. As a main result, we show
that, when the device is driven far from equilibrium, one can tune the CNT
mechanical properties by varying the external magnetic field. Indeed, the
magnetic field affects the CNT bending mode dynamics inducing an enhanced
damping as well as a noise term due to the electronic phase fluctuations. The
quality factor, as observed experimentally, exhibits a quadratic dependence on
external magnetic field strength. Finally, CNT resonance frequencies as a
function of gate voltage acquire, increasing the magnetic field strength, a
peculiar dip-peak structure that should be experimentally observed.",1212.0267v3
2013-01-10,"Magnetization precession induced by quasi-transverse picosecond strain pulses in (311) ferromagnetic (Ga,Mn)As","Quasi-longitudinal and quasi-transverse picosecond strain pulses injected
into a ferromagnetic (311) (Ga,Mn)As film induce dynamical shear strain in the
film, thereby modulating the magnetic anisotropy and inducing resonant
precession of the magnetization at a frequency ~10 GHz. The modulation of the
out-of-plane magnetization component by the quasitransverse strain reaches
amplitudes as large as 10% of the equilibrium magnetization. Our theoretical
analysis is in good agreement with the observed results, thus providing a
strategy for ultrafast magnetization control in ferromagnetic films by strain
pulses.",1301.2188v1
2013-02-04,"Two-flavor color superconductivity at finite temperature, chemical potential and in the presence of strong magnetic fields","Utilizing an extended two-flavor Nambu-Jona Lasinio (NJL) model, we review
some of the effects of external magnetic fields on two-flavor color
superconducting phase (2SC) at moderate baryon densities in the QCD phase
diagram. The effective action of the extended NJL model consists of two mass
gaps as functions of three intensive quantities, the temperature, the quark
chemical potential and the external magnetic field. The nonzero values of the
mass gaps, chiral and diquark condensates, induce spontaneous chiral and color
symmetry breaking, respectively, and as a result two different phases of quark
matter appear. We find the transition curves between these phases as well as
the critical points in the QCD phase diagram in terms of the intensive
quantities. Imposing a constant strong magnetic field on these two phases, we
show that the mass gaps increase with the magnetic field and the symmetry
breaking region in the QCD phase diagram expands even to the larger values of
temperature and quark chemical potential. This phenomenon is a consequence of
the magnetic catalysis of dynamical symmetry breaking, which is proven before.",1302.0622v1
2013-03-03,The inviscid instability in an electrically conducting fluid affected by a parallel magnetic field,"We investigate inviscid instability in an electrically conducting fluid
affected by a parallel magnetic field. The case of low magnetic
  Reynolds number in Poiseuille flow is considered. When the magnetic field is
sufficiently strong, for a flow with low hydrodynamic Reynolds number, it is
already known that the neutral disturbances are three-dimensional. Our
investigation shows that at high hydrodynamic Reynolds number(inviscid flow),
the effect of the strength of the magnetic field on the fastest growing
perturbations is limited to a decrease of their oblique angle i.e. angle
between the direction of the wave propagation and the basic flow. The waveform
remains unchanged. The detailed analysis of the linear instability provided by
the eigenvalue problem shows that the magnetic field has a stabilizing effect
on the electrically conducting fluid flow. We find also that at least, the
unstability appears if the main flow possesses an inflexion point with a
suitable condition between the velocity of the basic flow and the complex
stability parameter according to Rayleigh's inflexion point theorem.",1303.0534v1
2013-04-22,Magnetic anisotropic energy gap and low energy spin wave excitation in antiferromagnetic block phase of K2Fe4Se5,"Neutron scattering experiments were performed to investigate magnetic order
and magnetic excitations in ternary iron chalcogenide K2Fe4Se5. The formation
of a superlattice structure below 580 K together with the decoupling between
the Fe-vacancy order-disorder transition and the antiferromagnetic order
transition appears to be a common feature in the A2Fe4Se5 family. The study of
spin dynamics of K2Fe4Se5 reveals two distinct energy gaps at the magnetic
Brillouin zone center, which indicates the presence of magnetic anisotropy and
the decrease of local symmetry due to electronic and orbital anisotropy. The
low-energy spin wave excitations of K2Fe4Se5 can be properly described by
linear spin wave theory within a Heisenberg model. Compared to iron pnictides,
K2Fe4Se5 exhibits a more two-dimensional magnetism as characterized by large
differences not only between out-of-plane and in-plane spin wave velocities,
but also between out-of-plane and in-plane exchange interactions.",1304.5950v1
2013-05-22,Fractal multi-scale nature of solar/stellar magnetic field,"An abstract mathematical concept of fractal organization of certain complex
objects received significant attention in astrophysics during last decades. The
concept evolved into a broad field including multi-fractality and
intermittency, percolation theory, self-organized criticality, theory of
catastrophes, etc. Such a strong mathematical and physical approach provide new
possibilities for exploring various aspects of astrophysics. In particular, in
the solar and stellar magnetism, multi-fractal properties of magnetized plasma
turned to be useful for understanding burst-like dynamics of energy release
events, conditions for turbulent dynamo action, nature of turbulent magnetic
diffusivity, and even the dual nature of solar dynamo. In this review, I will
briefly outline how the ideas of multi-fractality are used to explore the above
mentioned aspects of solar magnetism.",1305.5282v1
2013-07-23,Thermodynamic properties of strongly coupled plasma in presence of external magnetic field,"Thermodynamic properties of a Yukawa system consisting of dust particles in
plasma are studied in presence of an external magnetic field. It is assumed
that dust particles interact with each other by modified potential in presence
of magnetic field. Accordingly, a modified expression for internal energy has
been obtained. A molecular dynamics code is developed to calculate this
internal energy for the entire system. Based on the values of internal energy
given by the code Helmholtz free energy and pressure are calculated for the
system. Our study shows novel kind of behaviour for internal energy in presence
of magnetic field. Thermodynamic properties are affected significantly by
magnetic field. The study helps to express internal energy as a function of
Coulomb coupling parameter and magnetic field.",1307.6088v1
2013-08-01,Vortex precession frequency and its amplitude-dependent shift in cylindrical nanomagnets,"Frequency of free magnetic vortex precession in circular soft ferromagnetic
nano-cylinders (magnetic dots) of various sizes is an important parameter, used
in design of spintronic devices (such as spin-torque microwave
nano-oscillators) and characterization of magnetic nanostructures. Here, using
a recently developed collective-variable approach to non-linear dynamics of
magnetic textures in planar nano-magnets, this frequency and its
amplitude-dependent shift are computed analytically and plotted for the full
range of cylinder geometries. The frequency shift is positive in large planar
dots, but becomes negative in smaller and more elongated ones. At certain dot
dimensions a zero frequency shift is realized, which can be important for
enhancing frequency stability of magnetic nano-oscillators.",1308.0240v2
2013-08-23,Experimental evidence of a collinear antiferromagnetic ordering in the frustrated CoAl2O4 spinel,"Nuclear magnetic resonance (NMR), neutron diffaction (ND), x-ray diffraction,
magnetic susceptibility and specific heat measurements on the frustrated A-site
spinel CoAl2O4 compound reveal a collinear antiferromagnetic ordering below Tn
= 9.8(2) K. A high quality powder sample characterized by x-ray diffraction
that indicates a relatively low Co-Al inversion parameter x = 0.057(20) in
(Co1-xAlx)[Al2-xCox]O4, shows a broad maximum around 15 K in magnetic
susceptibility and a sharp peak at Tn in heat capacity. The average ordered
magnetic moment of Co^2+ (S = 3/2) ions at the A-site is estimated to be 2.4(1)
Bohr magneton from NMR and 1.9(5) Bohr magneton from ND which are smaller than
the expected value of 3 Bohr magneton for S = 3/2 and g = 2. Antiferromagnetic
spin uctuations and correlations in the paramagnetic state are revealed from
the magnetic susceptibility, NMR and ND measurements, which are due to spin
frustration and site inversion effects in the system. The ND data also show
short-range dynamic magnetic ordering that persists to a temperature that is
almost twice Tn.",1308.5238v1
2013-09-05,Nuclear magnetization in gallium arsenide quantum dots at zero magnetic field,"Optical and electrical control of the nuclear spin system allows enhancing
the sensitivity of NMR applications and spin-based information storage and
processing. Dynamic nuclear polarization in semiconductors is commonly achieved
in the presence of a stabilizing external magnetic field. Here we report
efficient optical pumping of nuclear spins at zero magnetic field in strain
free GaAs quantum dots. The strong interaction of a single, optically injected
electron spin with the nuclear spins acts as a stabilizing, effective magnetic
field (Knight field) on the nuclei. We optically tune the Knight field
amplitude and direction. In combination with a small transverse magnetic field,
we are able to control the longitudinal and transverse component of the nuclear
spin polarization in the absence of lattice strain i.e. nuclear quadrupole
effects, as reproduced by our model calculations.",1309.1288v1
2013-09-19,Energy Fluxes during Dynamo Reversals,"Using direct numerical simulations of the equations of magnetohydrodynamics,
we study reversals of the magnetic field generated by the flow of an
electrically conducting fluid in a sphere. We show that at low magnetic Prandtl
numbers, Pm=0.5, the decrease of magnetic energy, ohmic dissipation and power
of the Lorentz force during a reversal is followed by an increase of the power
injected by the force driving the flow and an increase of viscous dissipation.
Cross correlations show that the Lorentz energy flux is in advance with respect
to the other energy fluxes. We also observe that during a reversal, the maximum
of the magnetic energy density migrates from one hemisphere to the other and
comes back to its initial position, in agreement with recent experimental
observations. For larger magnetic Prandtl numbers (Pm= 1, 2), the magnetic
field reversals do not display these trends and strongly differ one from
another.",1309.5062v1
2013-09-27,Structures and Lagrangian statistics of the Taylor-Green Dynamo,"The evolution of a Taylor-Green forced magnetohydrodynamic (MHD) system
showing dynamo activity is analyzed via direct numerical simulations. The
statistical properties of the velocity and magnetic field in Eulerian
coordinates and along trajectories of fluid elements (Lagrangian coordinates)
are found to change between the kinematic, non-linear and saturated regime.
Fluid element (tracer) trajectories change from chaotic quasi-isotropic
(kinematic phase) to mean magnetic field aligned (saturated phase). The
probability density functions (PDFs) of the magnetic field change from strongly
non-Gaussian in the kinematic to quasi-Gaussian PDFs in the saturated regime so
that their flatness give a precise handle on the definition of the limiting
points of the three regimes. Also the statistics of the fluctuations of the
kinetic and magnetic energy along fluid trajectories change. All this goes
along with a dramatic increase of the correlation time of velocity and magnetic
field fluctuations experienced by tracers significantly exceeding one turbulent
large-eddy turn-over time. A remarkable consequence is an intermittent scaling
regime of the Lagrangian magnetic field structure functions at unusually long
time scales.",1309.7975v2
2013-11-15,Electric-field-induced strain-mediated magnetoelectric effect in CoFeB-MgO magnetic tunnel junctions,"Magnetoelectric coupling between magnetic and electric dipoles is one of the
cornerstones of modern physics towards developing the most energy-efficient
magnetic data storage. Conventionally, magnetoelectric coupling is achieved in
single-phase multiferroics or in magnetoelectric composite nanostructures
consisting of ferromagnetic and ferroelectric/piezoelectric materials. Here, we
demonstrate an electric-field-induced strain-mediated magnetoelectric effect in
ultrathin CoFeB/MgO magnetic tunnel junction employing non-piezoelectric
material, which is a vitally important structure for spintronic devices, by
using dynamical magnetoelectric and piezoresponse force microscopy measurement
techniques. We show that the applied electric-field induces strain in a few
atomic layers of dielectric MgO which is transferred to magnetostrictive CoFeB
layer, resulting in a magnetoelectric effect of magnitude up to 80.8 V cm-1
Oe-1 under -0.5 V. The demonstrated strain-mediated magnetoelectric effect with
an electric field in magnetic tunnel junctions is a significant step towards
exploring magnetoelectrically controlled spintronic devices for low-power and
high density magnetic data storage applications.",1311.3794v1
2013-12-01,Modeling and stabilizability of voltage-actuated piezoelectric beams with magnetic effects,"Models for piezoelectric beams and structures with piezoelectric patches
generally ignore magnetic effects. This is because the magnetic energy has a
relatively small effect on the overall dynamics. Piezoelectric beam models are
known to be exactly observable, and can be exponentially stabilized in the
energy space by using a mechanical feedback controller. In this paper, a
variational approach is used to derive a model for a piezoelectric beam that
includes magnetic effects. It is proven that the partial differential equation
model is well-posed. Magnetic effects have a strong effect on the
stabilizability of the control system. For almost all system parameters the
piezoelectric beam can be strongly stabilized, but is not exponentially
stabilizable in the energy space. Strong stabilization is achieved using only
electrical feedback. Furthermore, using the same electrical feedback, an
exponentially stable closed-loop system can be obtained for a set of system
parameters of zero Lebesgue measure. These results are compared to those of a
beam without magnetic effects.",1312.0260v1
2013-12-30,Transient coherence of an EIT media under strong phase modulation,"A strong phase modulated coupling field leads to an amplitude modulation of a
probe field in an electromagnetically induced transparency process. Time vs.
detuning plots for different modulation frequencies reveals a transition
between an adiabatic regime where a series of smooth pulses are created with a
global phase dependent upon the detuning, and a non-adiabatic regime where a
strong transient oscillating response is added to these pulses. In the extreme
non-adiabatic regime, where the modulation frequency is higher than the
transient decay time, a coherent interference pattern is revealed. Adding a
magnetic field lifts the hyperfine level degeneracy, resulting in a separation
of the original pulse to three pulses. Every pulse has now a different phase
dependent upon the magnetic field causing an interference effect between the
different magnetic level transients. We explore the dynamics of the magnetic
and non magnetic cases and show its resemblance to the Landau-Zener theory. We
also show that combining the global phase of the pulses with the transient
interference allows for a wide magnetic sensing range without loosing the
sensitivity of a single EIT line.",1312.7708v1
2014-02-13,Unusual spin fluctuations and magnetic frustration in olivine and non-olivine LiCoPO$_4$ detected by $^{31}$P and $^7$Li nuclear magnetic resonance,"We report $^{31}$P and $^{7}$Li nuclear magnetic resonance (NMR) studies in
new non-olivine LiZnPO$_4$-type LiCoPO$_4^\text{tetra}$ microcrystals, where
the Co$^{2+}$ ions are tetrahedrally coordinated. Olivine LiCoPO$_4$, which was
directly transformed from LiCoPO$_4^\text{tetra}$ by an annealing process, was
also studied and compared. The uniform bulk magnetic susceptibility and the
$^{31}$P Knight shift obey the Curie-Weiss law for both materials with a high
spin Co$^{2+}$ ($3d^7$, $S=3/2$), but the Weiss temperature $\Theta$ and the
effective magnetic moment $\mu_\text{eff}$ are considerably smaller in
LiCoPO$_4^\text{tetra}$. The spin-lattice relaxation rate $T_1^{-1}$ reveals a
quite different nature of the spin dynamics in the paramagnetic state of both
materials. Our NMR results imply that strong geometrical spin frustration
occurs in tetrahedrally coordinated LiCoPO$_4$, which may lead to the
incommensurate magnetic ordering.",1402.3082v2
2014-02-22,LDA+DMFT Approach to Magnetocrystalline Anisotropy of Strong Magnets,"The new challenges posed by the need of finding strong rare-earth free
magnets demand methods that can predict magnetization and magnetocrystalline
anisotropy energy (MAE). We argue that correlated electron effects, which are
normally underestimated in band structure calculations, play a crucial role in
the development of the orbital component of the magnetic moments. Because
magnetic anisotropy arises from this orbital component, the ability to include
correlation effects has profound consequences on our predictive power of the
MAE of strong magnets. Here we show that incorporating the local effects of
electronic correlations with dynamical mean-field theory provides reliable
estimates of the orbital moment, the mass enhancement and the MAE of YCo5.",1402.5543v2
2014-03-26,Statistics of the inverse-cascade regime in two-dimensional magnetohydrodynamic turbulence,"We present a detailed direct numerical simulation of statistically steady,
homogeneous, isotropic, two-dimensional magnetohydrodynamic (2D MHD)
turbulence. Our study concentrates on the inverse cascade of the magnetic
vector potential. We examine the dependence of the statistical properties of
such turbulence on dissipation and friction coefficients. We extend earlier
work sig- nificantly by calculating fluid and magnetic spectra, probability
distribution functions (PDFs) of the velocity, magnetic, vorticity, current,
stream-function, and magnetic-vector-potential fields and their increments. We
quantify the deviations of these PDFs from Gaussian ones by computing their
flatnesses and hyperflatnesses. We also present PDFs of the Okubo-Weiss
parameter, which distin- guishes between vortical and extensional flow regions,
and its magnetic analog. We show that the hyperflatnesses of PDFs of the
increments of the stream-function and the magnetic vector potential exhibit
significant scale dependence and we examine the implication of this for the
multiscaling of structure functions. We compare our results with those of
earlier studies.",1403.6607v1
2014-04-15,On the Helicity of Open Magnetic Fields,"We reconsider the topological interpretation of magnetic helicity for
magnetic fields in open domains, and relate this to the relative helicity.
Specifically, our domains stretch between two parallel planes, and each of
these ends may be magnetically open. It is demonstrated that, while the
magnetic helicity is gauge-dependent, its value in any gauge may be physically
interpreted as the average winding number among all pairs of field lines with
respect to some orthonormal frame field. In fact, the choice of gauge is
equivalent to the choice of reference field in the relative helicity, meaning
that the magnetic helicity is no less physically meaningful. We prove that a
particular gauge always measures the winding with respect to a fixed frame, and
propose that this is normally the best choice. For periodic fields, this choice
is equivalent to measuring relative helicity with respect to a potential
reference field. But for aperiodic fields, we show that the potential field can
be twisted. We prove by construction that there always exists a possible
untwisted reference field.",1404.3897v1
2014-05-04,Diffusion of magnetic elements in a supergranular cell,"Small scale magnetic fields (magnetic elements) are ubiquitous in the solar
photosphere. Their interaction can provide energy to the upper atmospheric
layers, and contribute to heat the solar corona. In this work, the dynamic
properties of magnetic elements in the quiet Sun are investigated. The high
number of magnetic elements detected in a supegranular cell allowed us to
compute their displacement spectrum $\langle(\Delta
r)^2\rangle\propto\tau^\gamma$ (being $\gamma>0$, and $\tau$ the time since the
first detection), separating the contribution of the network (NW) and the
internetwork (IN) regions. In particular, we found $\gamma=1.27\pm0.05$ and
$\gamma=1.08\pm0.11$ in NW (at smaller and larger scales, respectively), and
$\gamma=1.44\pm0.08$ in IN. These results are discussed in light of the
literature on the topic, as well as the implications for the build up of the
magnetic network.",1405.0677v1
2014-07-14,Thermal noise induced stochastic resonance in self organizing Fe nanoparticle system,"The natural world is replete with examples of multistable systems, known to
respond to periodic modulations and produce a signal, which exhibits resonance
with noise amplitude. This is a concept not demonstrated in pure materials,
which involve a measured physical property. In a thermoremanent magnetization
experiment with a common magnetic material, Fe, in the nanoparticulate form, we
establish how magnetization in a system of dilute spins during dissipation of
stored magnetic energy, breaks up into spontaneous oscillatory behavior.
Starting at 175 K and aided by temperature (stochastic noise) the oscillation
amplitude goes through a maximum, reminiscent of stochastic resonance. Our
observation of thermal noise induced coherent resonance is due to intrinsic
self-organizing magnetic dynamics of the Fe nanoperticle system without
applying any external periodic force. These results yield new possibilities in
design of magnetic materials and a platform to understand stochastic
interference and phase synchronization in neural activity, as models for neural
communication.",1407.3579v1
2014-09-18,Storage Ring And Interaction Region Magnets For A μ+μ- Higgs Factory,"A low-energy Muon Collider (MC) offers unique opportunities to study the
recently found Higgs boson. However, due to a relatively large beam emittance
with moderate cooling in this machine, large-aperture high- field
superconducting (SC) magnets are required. The magnets need also an adequate
margin to operate at a large radiation load from the muon decay showers.
General specifications of the SC dipoles and quadrupoles for the 125 GeV c.o.m.
Higgs Factory with an average luminosity of ~2x10**31 cm-2s-1 are formulated.
Magnet conceptual designs and parameters are reported. The impact of the magnet
fringe fields on the beam dynamics as well as the IR and lattice magnet
protection from radiation are also reported and discussed.",1409.5461v1
2014-10-07,Metamorphosis of helical magnetorotational instability in the presence axial electric current,"This paper presents numerical linear stability analysis of a cylindrical
Taylor-Couette flow of liquid metal carrying axial electric current in a
generally helical external magnetic field. Axially symmetric disturbances are
considered in the inductionless approximation corresponding to zero magnetic
Prandtl number. Axial symmetry allows us to reveal an entirely new
electromagnetic instability. First, we show that the electric current passing
through the liquid can extend the range of helical magnetorotational
instability (HMRI) indefinitely by transforming it into a purely
electromagnetic instability. Two different electromagnetic instability
mechanisms are identified. The first is an internal pinch-type instability,
which is due to the interaction of the electric current with its own magnetic
field. Axisymmetric mode of this instability requires a free-space component of
the azimuthal magnetic field. When the azimuthal component of the magnetic
field is purely rotational and the axial component is nonzero, a new kind of
electromagnetic instability emerges. The latter driven by the interaction of
electric current with a weak collinear magnetic field in a quiescent fluid
gives rise to a steady meridional circulation coupled with azimuthal rotation.",1410.1750v3
2014-11-04,Magnetoimpedance effect at the high frequency range for the thin film geometry: Numerical calculation and experiment,"The magnetoimpedance effect is a versatile tool to investigate ferromagnetic
materials, revealing aspects on the fundamental physics associated to
magnetization dynamics, broadband magnetic properties, important issues for
current and emerging technological applications for magnetic sensors, as well
as insights on ferromagnetic resonance effect at non-saturated magnetic states.
Here, we perform a theoretical and experimental investigation of the
magnetoimpedance effect for the thin film geometry in a wide frequency range.
We calculate the longitudinal magnetoimpedance for single layered, multilayered
or exchange biased systems from an approach that considers a magnetic
permeability model for planar geometry and the appropriate magnetic free energy
density for each structure. From numerical calculations and experimental
results found in literature, we analyze the magnetoimpedance behavior, and
discuss the main features and advantages of each structure. To test the
robustness of the approach, we directly compare theoretical results with
experimental magnetoimpedance measurements obtained in a wide range of
frequencies for an exchange biased multilayered film. Thus, we provide
experimental evidence to confirm the validity of the theoretical approach
employed to describe the magnetoimpedance in ferromagnetic films, revealed by
the good agreement between numerical calculations and experimental results.",1411.0971v1
2014-11-24,Turbulence Reduces Magnetic Diffusivity in a Liquid Sodium Experiment,"The contribution of small scale turbulent fluctuations to the induction of
mean magnetic field is investigated in our liquid sodium spherical Couette
experiment with an imposed magnetic field.An inversion technique is applied to
a large number of measurements at $Rm \approx 100$ to obtain radial profiles of
the $\alpha$ and $\beta$ effects and maps of the mean flow.It appears that the
small scale turbulent fluctuations can be modeled as a strong contribution to
the magnetic diffusivity that is negative in the interior region and positive
close to the outer shell.Direct numerical simulations of our experiment support
these results.The lowering of the effective magnetic diffusivity by small scale
fluctuations implies that turbulence can actually help to achieve
self-generation of large scale magnetic fields.",1411.6366v2
2014-11-12,Biological effects of pulsating magnetic fields: role of solitons,"In this paper, we analyze biological effects produced by magnetic fields in
order to elucidate the physical mechanisms, which can produce them. We show
that there is a chierarchy of such mechanisms and that the mutual interplay
between them can result in the synergetic outcome. In particular, we analyze
the biological effects of magnetic fields on soliton mediated charge transport
in the redox processes in living organisms. Such solitons are described by
nonlinear systems of equations and represent electrons that are self-trapped in
alpha-helical polypeptides due to the moderately strong electron-lattice
interaction. They represent a particular type of disssipativeless large
polarons in low-dimensional systems. We show that the effective mass of
solitons in the is different from the mass of free electrons, and that there is
a resonant effect of the magnetic fields on the dynamics of solitons, and,
hence, on charge transport that accompanies photosynthesis and respiration.
These effects can result in non-thermal resonant effects of magnetic fields on
redox processes in particular, and on the metabolism of the organism in
general. This can explain physical mechanisms of therapies based on applying
magnetic fields.",1411.6576v1
2014-12-05,Guiding-center transformation of the radiation-reaction force in a nonuniform magnetic field,"In this paper, we present the guiding-center transformation of the
radiation-reaction force of a classical point charge traveling in a nonuniform
magnetic field. The transformation is valid as long as the gyroradius of the
charged particles is much smaller than the magnetic field nonuniformity length
scale, so that the guiding-center Lie-transform method is applicable.
Elimination of the gyromotion time scale from the radiation-reaction force is
obtained with the Poisson bracket formalism originally introduced by [A. J.
Brizard, Phys. Plasmas 11 4429 (2004)], where it was used to eliminate the fast
gyromotion from the Fokker-Planck collision operator. The formalism presented
here is applicable to the motion of charged particles in planetary magnetic
fields as well as in magnetic confinement fusion plasmas, where the
corresponding so-called synchrotron radiation can be detected. Applications of
the guiding-center radiation-reaction force include tracing of charged particle
orbits in complex magnetic fields as well as kinetic description of plasma when
the loss of energy and momentum due to radiation plays an important role, e.g.,
for runaway electron dynamics in tokamaks.",1412.1966v2
2014-12-26,"Potts models with magnetic field: arithmetic, geometry, and computation","We give a sheaf theoretic interpretation of Potts models with external
magnetic field, in terms of constructible sheaves and their Euler
characteristics. We show that the polynomial countability question for the
hypersurfaces defined by the vanishing of the partition function is affected by
changes in the magnetic field: elementary examples suffice to see
non-polynomially countable cases that become polynomially countable after a
perturbation of the magnetic field. The same recursive formula for the
Grothendieck classes, under edge-doubling operations, holds as in the case
without magnetic field, but the closed formulae for specific examples like
banana graphs differ in the presence of magnetic field. We give examples of
computation of the Euler characteristic with compact support, for the set of
real zeros, and find a similar exponential growth with the size of the graph.
This can be viewed as a measure of topological and algorithmic complexity. We
also consider the computational complexity question for evaluations of the
polynomial, and show both tractable and NP-hard examples, using dynamic
programming.",1412.7925v2
2015-01-30,Phase diagram and optimal switching induced by spin Hall effect in a perpendicular magnetic layer,"In a ferromagnet/heavy-metal bilayer device with strong spin Hall effect an
in-plane current excites magnetic dynamics through spin torque. We analyze
bilayers with perpendicular magnetization and calculate three-dimensional phase
diagrams describing switching by external magnetic field at a fixed current. We
then concentrate on the case of a field applied in the plane formed by the film
normal and the current direction. Here we analytically study the evolution of
both the conventional ""up""/""down"" magnetic equilibria and the additional
equilibria created by the spin torque. Expressions for the stability regions of
all equilibria are derived, and the nature of switching at each critical
boundary is discussed. The qualitative picture obtained this way predicts
complex hysteresis patterns that should occur in bilayers. By analyzing the
phase portraits of the system we show that when the spin torque induced
equilibrium exists, switching between ""up"" and ""down"" states proceeds through
it as an intermediate state. Using numeric simulations we analyze the switching
time and compare it to that of a conventional spin torque device with collinear
magnetizations of the polarizer and the free layer.",1501.07787v1
2015-03-16,Persistence of magnetic field driven by relativistic electrons in a plasma,"The onset and evolution of magnetic fields in laboratory and astrophysical
plasmas is determined by several mechanisms, including instabilities, dynamo
effects and ultra-high energy particle flows through gas, plasma and
interstellar-media. These processes are relevant over a wide range of
conditions, from cosmic ray acceleration and gamma ray bursts to nuclear fusion
in stars. The disparate temporal and spatial scales where each operates can be
reconciled by scaling parameters that enable to recreate astrophysical
conditions in the laboratory. Here we unveil a new mechanism by which the flow
of ultra-energetic particles can strongly magnetize the boundary between the
plasma and the non-ionized gas to magnetic fields up to 10-100 Tesla (micro
Tesla in astrophysical conditions). The physics is observed from the first
time-resolved large scale magnetic field measurements obtained in a laser
wakefield accelerator. Particle-in-cell simulations capturing the global plasma
and field dynamics over the full plasma length confirm the experimental
measurements. These results open new paths for the exploration and modelling of
ultra high energy particle driven magnetic field generation in the laboratory.",1503.04696v1
2015-05-19,Ultrafast Magnetic Light,"We propose a novel concept for efficient dynamic tuning of optical properties
of a high refractive index subwavelength nanoparticle with a magnetic Mie-type
resonance by means of femtosecond laser radiation. This concept is based on
ultrafast generation of electron-hole plasma within such nanoparticle,
drastically changing its transient dielectric permittivity. This allows to
manipulate by both electric and magnetic nanoparticle responses, resulting in
dramatic changes of its extinction cross section and scattering diagram.
Specifically, we demonstrate the effect of ultrafast switching-on a Huygens
source in the vicinity of the magnetic dipole resonance. This approach enables
to design ultrafast and compact optical switchers and modulators based on the
""ultrafast magnetic light"" concept.",1505.05185v1
2015-05-25,Magnetism in the three-dimensional layered Lieb lattice: Enhanced transition temperature via flat-band and Van Hove singularities,"We describe the enhanced magnetic transition temperatures $T_c$ of
two-component fermions in three-dimensional layered Lieb lattices, which are
created in cold atom experiments. We determine the phase diagram at
half-filling using the dynamical mean-field theory. The dominant mechanism of
enhanced $T_c$ gradually changes from the (delta-functional) flat-band to the
(logarithmic) Van Hove singularity as the interlayer hopping increases. We
elucidate that the interaction induces an effective flat-band singularity from
a dispersive flat (or narrow) band. We offer a general analytical framework for
investigating the singularity effects, where a singularity is treated as one
parameter in the density of states. This framework provides a unified
description of the singularity-induced phase transitions, such as magnetism and
superconductivity, where the weight of the singularity characterizes physical
quantities. This treatment of the flat-band provides the transition temperature
and magnetization as a universal form (i.e., including the Lambert function).
We also elucidate a specific feature of the magnetic crossover in magnetization
at finite temperatures.",1505.06591v1
2015-06-09,Spin dynamics in a Curie-switch,"Ferromagnetic resonance properties of F$_1$/f/F$_2$/AF multilayers, where
weakly ferromagnetic spacer f is sandwiched between strongly ferromagnetic
layers F$_1$ and F$_2$, with F$_1$ being magnetically soft and F$_2$ -
magnetically hard due to exchange pinning to antiferromagnetic layer AF, are
investigated. Spacer-mediated exchange coupling is shown to strongly affect the
resonance fields of both F$_1$ and F$_2$ layers. Our theoretical calculations
as well as measurements show that the key magnetic parameters of the spacer,
which govern the ferromagnetic resonance in F$_1$/f/F$_2$/AF, are the magnetic
exchange length ($\Lambda$), effective saturation magnetization at $T=0$
$(m_0)$, and effective Curie temperature ($T_{\text{C}}^{\text{eff}}$). The
values of these key parameters are deduced from the experimental data for
multilayers with f = Ni$_x$Cu$_{100-x}$, for the key ranges in Ni-concentration
($x=54\div70$ at. %) and spacer thickness ($d=3\div 6$ nm). The results
obtained provide a deeper insight into thermally-controlled spin precession and
switching in magnetic nanostructures, with potential applications in spin-based
oscillators and memory devices.",1506.02928v1
2015-06-15,Neutrino electromagnetic properties: new approach to oscillations in magnetic fields,"Several new and interesting aspects of neutrino oscillations in a magnetic
field are considered: 1) We develop a standard usually used approach to the
neutrino spin oscillations in the neutrino mass basis and obtain the effective
neutrino spin (and ""spin-mass"") oscillation Hamiltonian that can be used for
description of the neutrino oscillations between different pairs of neutrino
states with different masses and helicities; 2) We derive the exact solution of
the Dirac equation for a massive neutrino with nonzero magnetic moment in the
presence of a constant transversal magnetic field that is rotating along the
direction of the neutrino propagation (the twisting magnetic field) and on the
basis of the obtained energy spectrum the neutrino spin oscillation effective
Hamiltonian is derive; 3) We develop a new approach to neutrino spin
oscillations that is based on the description of the neutrino spin states with
the corresponding spin operator that commutes with the neutrino dynamics
Hamiltonian in the magnetic field. The obtained new results can have important
phenomenological applications.",1506.05311v1
2015-07-09,Temperature evolution of superparamagnetic clusters in single-crystal La0.85Sr0.15CoO3 from nonlinear magnetic ac response and neutron depolarization,"The representative measurements of the second harmonic in ac magnetization
complemented by neutron depolarization have been performed for single-crystal
La0.85Sr0.15CoO3 in the temperature range 97 K < T < 230 K, where occurrence of
a small fraction (~ 0.001) of nanoscale ferromagnetic clusters (FMC) has been
found. Magnetic, geometrical and dynamical parameters of the FMC system have
been evaluated in the temperature range T < 140 K, where superparamagnetic
regime installs, by means of the formalism involving the Fokker-Planck equation
(FPE). With lowering the temperature, the amount of clusters fraction, the
cluster size and magnetic moment along with its diffusion relaxation time
strongly increase, each in its own temperature interval. Below 130 K, FMC
contribute essentially to the total linear magnetic susceptibility. The damping
factor of the order 0.1 proves the importance of precession in thermal
relaxation of the cluster magnetic moment. The FMC are a precursor of
long-range ferromagnetic correlations seen below 100 K with neutron-scattering
techniques. The employed technique supplemented with FPE-based data-treatment
formalism is a novel method for studying superparamagnetic systems.",1507.02593v2
2015-08-17,Magnetization dynamics driven by angle-dependent spin-orbit spin transfer torque,"Spin-orbit spin transfer torque allows an efficient control of magnetization
by an in-plane current. Recent experiments found that the spin-orbit torque has
strong dependence on the magnetization angle [Garello et al., Nature
Nanotechnol. 8, 587 (2013); Qiu et al., Sci. Rep. 4, 4491 (2014)]. We
theoretically investigate magnetization switching and domain wall motion in a
perpendicularly magnetized layer, induced by angle-dependent spin-orbit torque.
We obtain analytic expressions of the switching current and domain wall
velocity, in agreement with numerical results. Based on the expressions, we
find that the spin-orbit torque increasing with the polar angle of
magnetization is beneficial for both switching and domain wall motion. Our
result will serve as a guideline to design and interpret switching and domain
wall experiments based on spin-orbit torque.",1508.04067v2
2015-08-28,Nernst Effect in Magnetized Plasmas,"We present nanosecond timescale Vlasov-Fokker-Planck-Maxwell modeling of
magnetized plasma transport and dynamics in a hohlraum with an applied external
magnetic field, under conditions similar to recent experiments. Self-consistent
modeling of the kinetic electron momentum equation allows for a complete
treatment of the heat flow equation and Ohm's Law, including Nernst advection
of magnetic fields. In addition to showing the prevalence of non-local
behavior, we demonstrate that effects such as anomalous heat flow are induced
by inverse bremsstrahlung heating. We show magnetic field amplification up to a
factor of 3 from Nernst compression into the hohlraum wall. The magnetic field
is also expelled towards the hohlraum axis due to Nernst advection faster than
frozen-in-flux would suggest. Non-locality contributes to the heat flow towards
the hohlraum axis and results in an augmented Nernst advection mechanism that
is included self-consistently through kinetic modeling.",1508.07260v1
2015-09-06,Proposal for a Domain Wall Nano-Oscillator driven by Non-uniform Spin Currents,"We propose a new mechanism and a related device concept for a robust,
magnetic field tunable radio-frequency (rf) oscillator using the self
oscillation of a magnetic domain wall subject to a uniform static magnetic
field and a spatially non-uniform vertical dc spin current. The self
oscillation of the domain wall is created as it translates periodically between
two unstable positions, one being in the region where both the dc spin current
and the magnetic field are present, and the other, being where only the
magnetic field is present. The vertical dc spin current pushes it away from one
unstable position while the magnetic field pushes it away from the other. We
show that such oscillations are stable under noise and can exhibit a quality
factor of over 1000. A domain wall under dynamic translation, not only being a
source for rich physics, is also a promising candidate for advancements in
nanoelectronics with the actively researched racetrack memory architecture,
digital and analog switching paradigms as candidate examples. Devising a stable
rf oscillator using a domain wall is hence another step towards the realization
of an all domain wall logic scheme.",1509.01791v1
2015-11-05,Non-Markovian magnetization dynamics for uniaxial nanomagnets,"A stochastic approach for the description of the time evolution of the
magnetization of nanomagnets is proposed, that interpolates between the
Landau-Lifshitz-Gilbert and the Landau-Lifshitz-Bloch approximations, by
varying the strength of the noise. Its finite autocorrelation time, i.e. when
it may be described as colored, rather than white, is, also, taken into account
and the consequences, on the scale of the response of the magnetization are
investigated. It is shown that the hierarchy for the moments of the
magnetization can be closed, by introducing a suitable truncation scheme, whose
validity is tested by direct numerical solution of the moment equations and
compared to the averages obtained from a numerical solution of the
corresponding colored stochastic Langevin equation. This comparison is
performed on magnetic systems subject to both an external uniform magnetic
field and an internal one-site uniaxial anisotropy.",1511.01693v1
2016-01-06,Primordial magnetic fields from the string network,"Cosmic strings are a type of cosmic defect formed by a symmetry-breaking
phase transition in the early universe. Individual strings would have gathered
to build a network, and their dynamical motion would induce scalar--, vector--
and tensor--type perturbations. In this paper, we focus on the vector mode
perturbations arising from the string network based on the one scale model and
calculate the time evolution and the power spectrum of the associated magnetic
fields. We show that the relative velocity between photon and baryon fluids
induced by the string network can generate magnetic fields over a wide range of
scales based on standard cosmology. We obtain the magnetic field spectrum
before recombination as
$a^2B(k,z)\sim4\times10^{-16}G\mu/((1+z)/1000)^{4.25}(k/{\rm Mpc}^{-1})^{3.5}$
Gauss on super-horizon scales, and
$a^2B(k,z)\sim2.4\times10^{-17}G\mu/((1+z)/1000)^{3.5}(k/{\rm Mpc}^{-1})^{2.5}$
Gauss on sub-horizon scales in co-moving coordinates. This magnetic field grows
up to the end of recombination, and has a final amplitude of approximately
$B\sim10^{-17\sim -18} G\mu$ Gauss at the $k\sim1\ {\rm Mpc}^{-1}$ scale today.
This field might serve as a seed for cosmological magnetic fields.",1601.01059v1
2016-02-03,Topological magnetic dipolar interaction and non-local electric magnetization control in topological insulator heterostructures,"The magnetoelectric effect predicted in topological insulators makes
heterostructures that combine magnetic materials and such insulators promising
candidates for spintronics applications. Here, we theoretically consider a
setup that exhibits two well-separated interfaces between a topological
insulator and a ferromagnetic insulator. We show that there is a topological
magnetic dipole-dipole interaction stemming from long-range Coulomb
interactions. We analytically derive the magnetization dynamics at the two
interfaces and discuss how the long-range coupling can be applied to
non-locally induce the formation of a magnetic texture at one interface by
suitably gating the other interface.",1602.01469v4
2016-02-15,Ferromagnetic Cluster Glass Phase Embedded in a Paramagnetic and Metallic Host in Non-uniform Magnetic System CaRu_{1-x}Sc_{x}O_{3},"We have investigated both static and dynamic magnetic properties of
polycrystalline CaRu$_{1-x}$Sc$_{x}$O$_{3}$ system in order to clarify the role
of Sc ions as a disorder for magnetic ordering. We have observed typical
features of a ferromagnetic cluster glass state below around 40 K: (i) a broad,
frequency-dependent peak in the ac magnetic susceptibility, (ii) a slow
relaxation of the magnetization, and (iii) a continuous increase in the dc
magnetic susceptibility in field cooling process. The composition dependence of
characteristic parameters for the cluster glass state suggests that chemical
segregation can hardly explain the clustering mechanism. We propose a possible
picture that the ferromagnetic clusters are distributed uniformly and form the
glassy state embedded in the paramagnetic and metallic host of CaRuO$_{3}$.",1602.04647v1
2016-03-01,Ferromagnetic resonance and magnetic damping in C-doped Mn5Ge3,"Ferromagnetic resonance (FMR) was used to investigate the static and dynamic
magnetic properties of carbon-doped Mn5Ge3 (C$_{0.1}$ and C$_{0.2}$) thin films
grown on Ge(111). The temperature dependence of magnetic anisotropy shows an
increased perpendicular magneto-crystalline contribution at 80K with an
in-plane easy axis due to the large shape contribution. We find that our
samples show a small FMR linewidth (corresponding to an intrinsic magnetic
damping parameter $\alpha$=0.005), which is a measure of the spin relaxation
and directly related with the magnetic and structural quality of the material.
In the perpendicular-to-plane geometry, the FMR linewidth shows a minimum at
around 200K for all the samples, which seems to be not correlated to the
C-doping. The magnetic relaxation parameters have been determined and indicate
the two-magnon scattering as the main extrinsic contribution. We observe a
change in the main contribution from scattering centres in Mn5Ge3C0.2 at low
temperatures, which could be related to the minimum in linewidth.",1603.00190v1
2016-03-18,"Structural and Magnetic Characterization of Large Area, Free-Standing Thin Films of Magnetic Ion Intercalated Dichalcogenides Mn0.25TaS2 and Fe0.25TaS2","Free-standing thin films of magnetic ion intercalated transition metal
dichalcogenides are produced using ultramicrotoming techniques. Films of
thicknesses ranging from 30nm to 250nm were achieved and characterized using
transmission electron diffraction and X-ray magnetic circular dichroism.
Diffraction measurements visualize the long range crystallographic ordering of
the intercalated ions, while the dichroism measurements directly assess the
orbital contributions to the total magnetic moment. We thus verify the
unquenched orbital moment in Fe0.25TaS2 and measure the fully quenched orbital
contribution in Mn0.25TaS2. Such films can be used in a wide variety of
ultrafast X-ray and electron techniques that benefit from transmission
geometries, and allow measurements of ultrafast structural, electronic, and
magnetization dynamics in space and time.",1603.05921v1
2016-03-23,Magnetic Flux Cancellation in Ellerman Bombs,"Ellerman Bombs (EBs) are often found co-spatial with bipolar photospheric
magnetic fields. We use H$\alpha$ imaging spectroscopy along with Fe I 6302.5
\AA\ spectro-polarimetry from the Swedish 1-m Solar Telescope (SST), combined
with data from the Solar Dynamic Observatory (SDO) to study EBs and the
evolution of the local magnetic fields at EB locations. The EBs are found via
an EB detection and tracking algorithm. We find, using NICOLE inversions of the
spectro-polarimetric data, that on average (3.43 $\pm$ 0.49) x 10$^{24}$ ergs
of stored magnetic energy disappears from the bipolar region during the EBs
burning. The inversions also show flux cancellation rates of 10$^{14}$ -
10$^{15}$ Mx s$^{-1}$, and temperature enhancements of 200 K at the detection
footpoints. We investigate near-simultaneous flaring of EBs due to co-temporal
flux emergence from a sunspot, which shows a decrease in transverse velocity
when interacting with an existing, stationary area of opposite polarity
magnetic flux and the EBs are formed. We also show that these EBs can get
fueled further by additional, faster moving, negative magnetic flux regions.",1603.07100v1
2016-04-18,Spontaneous Formation of Surface Magnetic Structure from Large-scale Dynamo in Strongly-stratified Convection,"We report the first successful simulation of spontaneous formation of surface
magnetic structures from a large-scale dynamo by strongly-stratified thermal
convection in Cartesian geometry. The large-scale dynamo observed in our
strongly-stratified model has physical properties similar to those in earlier
weakly-stratified convective dynamo simulations, indicating that the
$\alpha^2$-type mechanism is responsible for it. Additionally to the
large-scale dynamo, we find that large-scale structures of the vertical
magnetic field are spontaneously formed in the convection zone surface only for
the case of strongly-stratified atmosphere. The organization of the vertical
magnetic field proceeds in the upper convection zone within tens of convective
turn-over time and band-like bipolar structures are recurrently-appeared in the
dynamo-saturated stage. We examine possibilities of several candidates as the
origin of the surface magnetic structure formation, and then suggest the
existence of an as-yet-unknown mechanism for the self-organization of the
large-scale magnetic structure, which should be inherent in the
strongly-stratified convective atmosphere.",1604.05374v1
2016-05-13,Integrable magnetic geodesic flows on 2-torus: new example via quasi-linear system of PDEs,"The only one example has been known of magnetic geodesic flow on the 2-torus
which has a polynomial in momenta integral independent of the Hamiltonian. In
this example the integral is linear in momenta and corresponds to a one
parametric group preserving the Lagrangian function of the magnetic flow. In
this paper the problem of integrability on one energy level is considered. This
problem can be reduced to a remarkable Semi-hamiltonian system of quasi-linear
PDEs and to the question of existence of smooth periodic solutions for this
system. Our main result states that the pair of Liouville metric with zero
magnetic field on the 2-torus can be analytically deformed to a Riemannian
metric with small magnetic field so that the magnetic geodesic flow on an
energy level is integrable by means of a quadratic in momenta integral. Thus
our construction gives a new example of smooth periodic solution to the
Semi-hamiltonian (Rich) quasi-linear system of PDEs.",1605.04234v1
2016-05-31,Correlation effects and orbital magnetism of Co clusters,"Recent experiments on isolated Co clusters have shown huge orbital magnetic
moments in comparison with their bulk and surface counterparts. These clusters
hence provide the unique possibility to study the evolution of the orbital
magnetic moment with respect to the cluster size and how competing interactions
contribute to the quenching of orbital magnetism. We investigate here different
theoretical methods to calculate the spin and orbital moments of Co clusters,
and assess the performances of the methods in comparison with experiments. It
is shown that density functional theory in conventional local density or
generalized gradient approximations, or even with a hybrid functional, severely
underestimates the orbital moment. As natural extensions/corrections we
considered the orbital polarization correction, the LDA+U approximation as well
as the LDA+DMFT method. Our theory shows that of the considered methods, only
the LDA+DMFT method provides orbital moments in agreement with experiment, thus
emphasizing the importance of dynamic correlations effects for determining
fundamental magnetic properties of magnets in the nano-size regime.",1605.09547v1
2016-06-28,Magnetization switching of FePt nanoparticle recording medium by femtosecond laser pulses,"Manipulation of magnetization with ultrashort laser pulses is promising for
information storage device applications. The dynamic of the magnetization
response depends on the energy transfer from the photons to the spins during
the initial laser excitation. A material of special interest for magnetic
storage is FePt nanoparticles , on which optical writing with optical angular
momentum was demonstrated recently by Lambert et al., although the mechanism
remained unclear. Here we investigate experimentally and theoretically the
all-optical switching of FePt nanoparticles. We show that the magnetization
switching is a stochastic process. We develop a complete multiscale model which
allows us to optimize the number of laser shots needed to write the
magnetization of high anisotropy FePt nanoparticles in our experiments. We
conclude that only angular momentum induced optically by the inverse Faraday
effect will provide switching with one single femtosecond laser pulse.",1606.08723v1
2016-07-22,Constraints on dynamo action in plasmas,"Upper bounds are derived on the amount of magnetic energy that can be
generated by dynamo action in collisional and collisionless plasmas with and
without external forcing. A hierarchy of mathematical descriptions is
considered for the plasma dynamics: ideal MHD, visco-resistive MHD, the
double-adiabatic theory of Chew, Goldberger and Low (CGL), kinetic MHD, and
other kinetic models. It is found that dynamo action is greatly constrained in
models where the magnetic moment of any particle species is conserved. In the
absence of external forcing, the magnetic energy then remains small at all
times if it is small in the initial state. In other words, a small ""seed""
magnetic field cannot be amplified significantly, regardless of the nature of
flow, as long as the collision frequency and gyroradius are small enough to be
negligible. A similar conclusion also holds if the system is subject to
external forcing as long as this forcing conserves the magnetic moment of at
least one plasma species and does not greatly increase the total energy of the
plasma (i.e., in practice, is subsonic). Dynamo action therefore always
requires collisions or some small-scale kinetic mechanism for breaking the
adiabatic invariance of the magnetic moment.",1607.06637v1
2016-08-01,Magnetic relaxation phenomena in the Chiral Magnet Fe$_{1-x}$Co$_x$Si: An ac susceptibility study,"We present a systematic study of the ac susceptibility of the chiral magnet
Fe$_{1-x}$Co$_x$Si with $x$ = 0.30 covering four orders of magnitude in
frequencies from 0.1 Hz to 1 kHz, with particular emphasis to the pronounced
history dependence. Characteristic relaxation times ranging from a few
milliseconds to tens of seconds are observed around the skyrmion lattice
A-phase, the helical-to-conical transition and in a region above $T_C$. The
distribution of relaxation frequencies around the A-phase is broad, asymmetric
and originates from multiple coexisting relaxation processes. The pronounced
dependence of the magnetic phase diagram on the magnetic history and cooling
rates as well as the asymmetric frequency dependence and slow dynamics suggest
more complicated physical phenomena in Fe$_{0.7}$Co$_{0.3}$Si than in other
chiral magnets.",1608.00404v2
2016-08-02,Spin dynamics and magnetic interactions of Mn dopants in the topological insulator Bi$_2$Te$_3$,"The magnetic and electronic properties of the magnetically doped topological
insulator Bi$_{\rm 2-x}$Mn$_{\rm x}$Te$_3$ were studied using electron spin
resonance (ESR) and measurements of static magnetization and electrical
transport. The investigated high quality single crystals of Bi$_{\rm
2-x}$Mn$_{\rm x}$Te$_3$ show a ferromagnetic phase transition for $x\geq 0.04$
at $T_{C}\approx 12$ K. The Hall measurements reveal a p-type finite
charge-carrier density. Measurements of the temperature dependence of the ESR
signal of Mn dopants for different orientations of the external magnetic field
give evidence that the localized Mn moments interact with the mobile charge
carriers leading to a Ruderman-Kittel-Kasuya-Yosida-type ferromagnetic coupling
between the Mn spins of order 2-3 meV. Furthermore, ESR reveals a
low-dimensional character of magnetic correlations that persist far above the
ferromagnetic ordering temperature.",1608.00827v2
2016-10-05,Magnetic reconnection: from MHD to QED,"The paper examines the prospects of using laser plasmas for studying novel
regimes of the magnetic field line reconnection and charged particle
acceleration. Basic features of plasma dynamics in the three-dimensional
configurations relevant to the formation of current sheets in a plasma are
addressed by analyzing exact self-similar solutions of the
magneto-hydrodynamics and electron magneto-hydrodynamics equations. Then the
magnetic field annihilation in the ultrarelativistic limit is considered, when
the opposite polarity magnetic field is generated in collisionless plasma by
multiple laser pulses, in the regime with a dominant contribution of the
displacement current exciting a strong large-scale electric field. This field
leads to the conversion of the magnetic energy into the kinetic energy of
accelerated particles inside a thin current sheet. Charged particle
acceleration during magnetic field reconnection is discussed when radiation
friction and quantum electrodynamics effects become dominant.",1610.01873v4
2016-11-19,Inverse Magnetic Catalysis from improved Holographic QCD in the Veneziano limit,"We study the dependence of the chiral condensate on external magnetic field
in the context of holographic QCD at large number of flavors. We consider a
holographic QCD model where the flavor degrees of freedom fully backreact on
the color dynamics. Perturbative QCD calculations have shown that $B$ acts
constructively on the chiral condensate, a phenomenon called ""magnetic
catalysis"". In contrast, recent lattice calculations show that, depending on
the number of flavors and temperature, the magnetic field may also act
destructively, which is called ""inverse magnetic catalysis"". Here we show that
the holographic theory is capable of both behaviors depending on the choice of
parameters. For reasonable choice of the potentials entering the model we find
qualitative agreement with the lattice expectations. Our results provide
insight for the physical reasons behind the inverse magnetic catalysis. In
particular, we argue that the backreaction of the flavors to the background
geometry decatalyzes the condensate.",1611.06339v1
2016-12-01,Magnetic field sensitivity and decoherence spectroscopy of an ensemble of narrow-linewidth nitrogen-vacancy centers close to a diamond surface,"We perform pulsed optically detected electron spin resonance to measure the
DC magnetic field sensitivity and electronic spin coherence time T_2 of an
ensemble of near-surface, high-density nitrogen-vacancy (NV) centers engineered
to have a narrow magnetic resonance linewidth. Combining pulsed spectroscopy
with dynamic nuclear polarization, we obtain the photon-shot-noise-limited DC
magnetic sensitivity of 35 nT Hz^{-0.5}. We find that T_2 is controlled by
instantaneous diffusion, enabling decoherence spectroscopy on residual nitrogen
impurity spins in the diamond lattice and a quantitative determination of their
density. The demonstrated high DC magnetic sensitivity and decoherence
spectroscopy are expected to broaden the application range for two-dimensional
magnetic imaging.",1612.00088v1
2016-12-01,"A fast spinning magnetic white dwarf in the double-degenerate, super-Chandrasekhar system NLTT 12758","We present an analysis of the close double degenerate NLTT 12758, which is
comprised of a magnetic white dwarf with a field of about 3.1 MG and an
apparently non-magnetic white dwarf. We measured an orbital period of 1.154
days and found that the magnetic white dwarf is spinning around its axis with a
period of 23 minutes. An analysis of the atmospheric parameters has revealed
that the cooling ages of the two white dwarfs are comparable, suggesting that
they formed within a short period of time from each other. Our modelling
indicates that the non-magnetic white dwarf is more massive (M=0.83 M_sun) than
its magnetic companion (M=0.69 M_sun) and that the total mass of the system is
higher than the Chandrasekhar mass. Although the stars will not come into
contact over a Hubble time, when they do come into contact, dynamically
unstable mass transfer will take place leading to either an accretion induced
collapse into a rapidly spinning neutron star or a Type Ia supernova.",1612.00325v1
2016-12-16,Origin of doping-induced suppression and reemergence of magnetism in LaFeAsO$_{1-x}$H$_x$,"We investigate the evolution of magnetic properties as a function of hydrogen
doping in iron based superconductor LaFeAsO$_{1-x}$H$_x$ using the dynamical
mean-field theory combined with the density-functional theory. We find that two
independent consequences of the doping, the increase of the electron occupation
and the structural modification, have the opposite effects on the strength of
electron correlation and magnetism, resulting in the minimum of the calculated
magnetic moment around the intermediate doping level as a function of $x$. Our
result provides a natural explanation for the puzzling recent experimental
discovery of the two separated antiferromagnetic phases at low and high doping
limits. Furthermore, the increase of orbital occupation and correlation
strength with the doping results in reduced orbital polarization of $d_{xz/yz}$
orbitals and the enhanced role of $d_{xy}$ orbital in the magnetism at high
doping levels, and their possible implications to the superconductivity are
discussed in line with the essential role of the magnetism.",1612.05520v1
2016-12-19,Carrier localization due to local magnetic order induced by magnetic impurities in Ba(Fe$_{1-x}$TM$_x$)$_2$As$_2$ (TM = Mn and Cr) as seen via optical spectra,"The charge dynamics of Ba(Fe$_{1-x}$TM$_x$)$_2$As$_2$ (TM = Mn and Cr) has
been investigated by optical spectroscopy. It was found that in addition to the
strong suppression of the coherent charge transport, the magnetic impurity
induces a novel spectral feature related to the carrier localization in the
far-infrared region above the antiferromagnetic transition temperature. We
attribute it to the cooperative effect between conduction electrons and local
magnetic order induced by magnetic impurities. The present results demonstrate
that Mn and Cr are not conventional magnetic pair-breakers in iron pnictides.",1612.06234v1
2017-01-09,"Static and Dynamic Magnetic Properties of Spin-1/2 Inequilateral Diamond-Chain Compounds $A_3$Cu$_3$AlO$_2$(SO$_4$)$_4$ ($A$=K, Rb, and Cs)","Spin-1/2 compounds A3Cu3AlO2(SO4)4 (A=K, Rb, and Cs) have one-dimensional
(1D) inequilateral diamond chains. We analyze the temperature dependence of the
magnetic susceptibility and determine the magnetic exchange interactions. In
contrast to the azurite, a dimer is formed on one of the sides of the diamond.
From numerical analyses of the proposed model, we find that the dimer together
with a nearly isolated 1D Heisenberg chain characterizes magnetic properties
including magnetization curve and magnetic excitations. This implies that a
dimer-monomer composite chain without frustration is a good starting point for
describing these compounds.",1701.02198v3
2017-02-09,Magnetic charge injection in spin ice: a new way to fragmentation,"The complexity embedded in condensed matter fertilizes the discovery of new
states of matter, enriched by ingredients like frustration. Illustrating
examples in magnetic systems are Kitaev spin liquids, skyrmions phases, or spin
ices. These unconventional ground states support exotic excitations, for
example the magnetic charges in spin ices, also called monopoles. Beyond their
discovery, an important challenge is to be able to control and manipulate them.
Here, we propose a new mechanism to inject monopoles in a spin ice through a
staggered magnetic field. We show theoretically, and demonstrate experimentally
in the Ho$_2$Ir$_2$O$_7$ pyrochlore iridate, that it results in the
stabilization of a monopole crystal, which exhibits magnetic fragmentation. In
this new state of matter, the magnetic moment fragments into an ordered part
and a persistently fluctuating one. Compared to conventional spin ices, the
different nature of the excitations in this fragmented state opens the way to
novel tunable field-induced and dynamical behaviors.",1702.02864v1
2017-02-13,"Coexistence of localized and itinerant magnetism in intercalated iron-selenide (Li,Fe)OHFeSe","The electronic structure and magnetism of a new magnetic intercalation
compound (Li0.8Fe0.2)OHFeSe are investigated theoretically. The electronic
structure calculations predict that the Fe in the (Li,Fe)OH intercalated layer
is in +2 valence state, i.e. there is electron doping to the FeSe layer,
resulting in the shrinking of the Fermi surface (FS) pocket around Gamma and a
strong suppression of dynamical spin susceptibility at M in comparison with the
bulk FeSe compound. The ground state of the FeSe layer is a striped
antiferromagnetic (SAFM) metal, while the (Li,Fe)OH layer displays a very weak
localized magnetism, with an interlayer ferromagnetic (FM) coupling between the
FeSe and intercalated (Li,Fe)OH layers. Moreover, the (Li,Fe)OH is more than a
block layer; it is responsible for enhancing the antiferromagnetic (AFM)
correlation in the FeSe layer through interlayer magnetic coupling. We propose
that the magnetic spacer layer introduces a tuning mechanism for spin
fluctuations associated with superconductivity in iron-based superconductors.",1702.03845v1
2017-02-13,Atomic-Scale Magnetometry of Dynamic Magnetization,"The spatial resolution of imaging magnetometers has benefited from scanning
probe techniques. The requirement that the sample perturbs the scanning probe
through a magnetic field external to its volume limits magnetometry to samples
with pre-existing magnetization. We propose a magnetometer in which the
perturbation is reversed: the probe's magnetic field generates a response of
the sample, which acts back on the probe and changes its energy. For an NV$^-$
spin center in diamond this perturbation changes the fine-structure splitting
of the spin ground state. Sensitive measurement techniques using coherent
detection schemes then permit detection of the magnetic response of
paramagnetic and diamagnetic materials. This technique can measure the
thickness of magnetically dead layers with better than $0.1$ \AA$~$accuracy.",1702.03863v1
2017-03-27,Spin transfer due to quantum fluctuations of magnetization,"Spin transfer - the transfer of angular momentum from spin-polarized
electrical current to magnetic materials - has been extensively researched as
an efficient mechanism for the electronic manipulation of the static and
dynamic states in nanomagnetic systems, advancing our understanding of
nanomagnetism and electronic transport, and enabling the development of
energy-efficient magnetic nanodevices. Our present understanding of spin
transfer is based on the classical approximation for the magnetization, even
though the spin-polarized electrons mediating spin transfer are treated
quantum-mechanically. Here, we utilize a nanoscale magnetic spin-valve
structure to demonstrate that quantum zero-point fluctuations of magnetization,
neglected in the existing theories of spin transfer, provide the dominant
contribution to this effect at cryogenic temperatures, and remain
non-negligible even at room temperature. The demonstrated quantum spin transfer
(QST) is distinguished by a non-smooth piecewise-linear dependence of the
fluctuation intensity on current, and can be driven not only by the directional
flows of electrons, but also by their thermal motion. This effect can enhance
current-induced phenomena, overcoming the efficiency limitations that are
presently perceived as fundamental to the spin transfer mechanism.",1703.09335v2
2017-04-21,Distribution of eigenstate populations and dissipative beating dynamics in uniaxial single-spin magnets,"Numerical simulations of magnetization reversal of a quantum uniaxial magnet
under a swept magnetic field [Hatomura, \textit{et al}., \textit{Quantum
Stoner-Wohlfarth Model}, Phys. Rev. Lett. \textbf{116}, 037203 (2016)] are
extended. In particular, how the ""wave packet"" describing the time-evolution of
the system is scattered in the successive avoided level crossings is
investigated from the viewpoint of the distribution of the eigenstate
populations. It is found that the peak of the distribution as a function of the
magnetic field does not depend on spin-size $S$, which indicates that the delay
of magnetization reversal due to the finite sweeping rate is the same in both
the quantum and classical cases. The peculiar synchronized oscillations of all
the spin components result in the beating of the spin-length. Here, dissipative
effects on this beating are studied by making use of the generalized
Lindblad-type master equation. The corresponding experimental situations are
also discussed in order to find conditions for experimental observations.",1704.06466v1
2017-04-21,"Interplay between particle microstructure, network topology and sample shape in magnetic gels -- A molecular dynamics simulation study","Ferrogels, i.e., hydrogels loaded with magnetic nanoparticles, have the
ability to deform in external magnetic fields. The precise shape of deformation
and the alignment of the gel in the field, however, depend on the interplay of
several factors. In this paper, we introduce a coarse-grained simulation model,
which takes into account the configuration of magnetic particles in the gel,
the sample shape, and aspects of the polymer network topology. We use this
model to show that in gels with an isotropic microstructure, an external
magnetic field reduces clustering, while this is not the case for uniaxial
gels, in which the particle configuration is anisotropic due to the presence of
a magnetic field during cross-linking. For ellipsoidal gels, we find that a
uniaxial microstructure additionally can override the deformation and alignment
expected as indicated the demagnetization energy. Finally, we examine to what
degree gels with different network topologies maintain the particle
microstructure ``frozen in`` during the cross-linking process.",1704.06578v1
2017-04-26,Sunspots rotation and magnetic transients associated with flares in NOAA AR 11429,"We analyze sunspots rotation and magnetic transients in NOAA AR 11429 during
two X-class (X5.4 and X1.3) flares using the data from the Helioseismic and
Magnetic Imager on board the \emph{Solar Dynamics Observatory}. A large leading
sunspot with positive magnetic polarity rotated counterclockwise. As expected,
the rotation was significantly affected by the two flares. The magnetic
transients induced by the flares were clearly evident in the sunspots with
negative polarity. They were moving across the sunspots with speed of order
$3-7\ \rm km \ s^{-1}$. Furthermore, the trend of magnetic flux evolution of
these sunspots exhibited changes associated with the flares. These results may
shed light on the understanding of the evolution of sunspots.",1704.08018v2
2017-07-03,Heavy-mass magnetic modes in pyrochlore iridates due to dominant Dzyaloshinskii-Moriya interaction,"Materials with strong spin-orbit interactions are presently a main target in
the search for systems with novel magnetic properties. Magnetic anisotropies
can be very large in such compounds, ranging from strongly frustrated Kitaev
exchange in honeycomb iridates and the associated spin-liquid states to robust
antisymmetric couplings in square-lattice Sr_2IrO_4 . Here we predict from ab
initio quantum chemistry calculations that another highly unusual regime is
realized in pyrochlore iridium oxides: the nearest-neighbor Heisenberg
interaction can vanish whilst the antisymmetric Dzyaloshinskii- Moriya exchange
reaches values as large as 5 meV, a result which challenges common notions and
existing phenomenological models of magnetic superexchange. The resulting spin
excitation spectra reveal a very flat magnon dispersion in the Nd- and Tb-based
pyrochlore iridates, suggesting the possibility of using these modes to store
magnetic information. Indeed the magnetization dynamics indicates that these
modes are unable to propagate out of the excitation region.",1707.00500v1
2017-07-11,Spin switching via quantum dot spin valves,"We develop a theory for spin transport and magnetization dynamics in a
quantum-dot spin valve, i.e., two magnetic reservoirs coupled to a quantum dot.
Our theory is able to take into account effects of strong correlations. We
demonstrate that, as a result of these strong correlations, the dot gate
voltage enables control over the current-induced torques on the magnets, and,
in particular, enables voltage-controlled magnetic switching. The electrical
resistance of the structure can be used to read out the magnetic state. Our
model may be realized by a number of experimental systems, including magnetic
scanning-tunneling microscope tips and artificial quantum dot systems.",1707.03373v4
2017-07-21,Decay Rate of Magnetic Dipoles near Non-magnetic Nanostructures,"In this article, we propose a concise theoretical framework based on mixed
field-susceptibilities to describe the decay of magnetic dipoles induced by
non--magnetic nanostructures. This approach is first illustrated in simple
cases in which analytical expressions of the decay rate can be obtained. We
then show that a more refined numerical implementation of this formalism
involving a volume discretization and the computation of a generalized
propagator can predict the dynamics of magnetic dipoles in the vicinity of
nanostructures of arbitrary geometries. We finally demonstrate the versatility
of this numerical method by coupling it to an evolutionary optimization
algorithm. In this way we predict a structure geometry which maximally promotes
the decay of magnetic transitions with respect to electric emitters.",1707.07006v3
2017-09-18,Skyrmion morphology in ultrathin magnetic films,"Nitrogen-vacancy magnetic microscopy is employed in quenching mode as a
non-invasive, high resolution tool to investigate the morphology of isolated
skyrmions in ultrathin magnetic films. The skyrmion size and shape are found to
be strongly affected by local pinning effects and magnetic field history.
Micromagnetic simulations including static disorder, based on a physical model
of grain-to-grain thickness variations, reproduce all experimental observations
and reveal the key role of disorder and magnetic history in the stabilization
of skyrmions in ultrathin magnetic films. This work opens the way to an
in-depth understanding of skyrmion dynamics in real, disordered media.",1709.06027v2
2017-09-22,A Co(II)-based Molecular Magnet with a 6 T Coercive Field,"Hard magnets with high coercivity, such as Nd2Fe14B and SmCo5 alloys, can
maintain magnetisation under a high reverse external magnetic field and have
therefore become irreplaceable parts in many practical applications. Molecular
magnets are promising alternatives, owing to their precise and designable
chemical structures, tuneable functionalities and controllable synthetic
methods. Here, we demonstrate that an unusually large coercive field can be
achieved in a single-chain magnet. Systematic characterisations, including
magnetic susceptibility, heat capacity and neutron diffraction measurements,
show that the observed giant coercive field originates from the spin dynamics
along the one-dimensional chain of the compound because of the strong exchange
coupling between Co(II) centres and radicals.",1709.07697v2
2017-10-05,Actuated rheology of magnetic micro-swimmers suspensions : emergence of motor and brake states,"We study the effect of magnetic field on the rheology of magnetic
micro-swimmers suspensions. We use a model of a dilute suspension under simple
shear and subjected to a constant magnetic field. Particle shear stress is
obtained for both pusher and puller types of micro-swimmers. In the limit of
low shear rate, the rheology exhibits a constant shear stress, called actuated
stress, which only depends on the swimming activity of the particles. This
stress is induced by the magnetic field and can be positive (brake state) or
negative (motor state). In the limit of low magnetic fields, a scaling relation
of the motor-brake effect is derived as a function of the dimensionless
parameters of the model. In this case, the shear stress is an affine function
of the shear rate. The possibilities offered by such an active system to
control the rheological response of a uid are finally discussed.",1710.01954v2
2017-10-17,"Fast, low-current spin-orbit torque switching of magnetic tunnel junctions through atomic modifications of the free layer interfaces","Future applications of spin-orbit torque will require new mechanisms to
improve the efficiency for switching nanoscale magnetic tunnel junctions
(MTJs), while also controlling the magnetic dynamics to achieve fast,
nanosecond scale performance with low write error rates. Here we demonstrate a
strategy to simultaneously enhance the interfacial magnetic anisotropy energy
and suppress interfacial spin memory loss by introducing sub-atomic and
monatomic layers of Hf at the top and bottom interfaces of the ferromagnetic
free layer of an in-plane magnetized three-terminal MTJ device. When combined
with a beta-W spin Hall channel that generates spin-orbit torque, the
cumulative effect is a switching current density of 5.4 x 106 A/cm2, more than
a factor of 3 lower than demonstrated in any other spin-orbit-torque magnetic
memory device at room temperature, and highly reliable switching with current
pulses only 2 ns long.",1710.06391v1
2017-12-11,Transition to turbulent dynamo saturation,"While the saturated magnetic energy is independent of viscosity in dynamo
experiments, it remains viscosity-dependent in state-of-the-art 3D direct
numerical simulations (DNS). Extrapolating such viscous scaling-laws to
realistic parameter values leads to an underestimation of the magnetic energy
by several orders of magnitude. The origin of this discrepancy is that fully 3D
DNS cannot reach low enough values of the magnetic Prandtl number $Pm$. To
bypass this limitation and investigate dynamo saturation at very low $Pm$, we
focus on the vicinity of the dynamo threshold in a rapidly rotating flow: the
velocity field then depends on two spatial coordinates only, while the magnetic
field consists of a single Fourier mode in the third direction. We perform
numerical simulations of the resulting set of reduced equations for $Pm$ down
to $2\cdot 10^{-5}$. This parameter regime is currently out of reach to fully
3D DNS. We show that the magnetic energy transitions from a high-$Pm$ viscous
scaling regime to a low-$Pm$ turbulent scaling regime, the latter being
independent of viscosity. The transition to the turbulent saturation regime
occurs at a low value of the magnetic Prandtl number, $Pm \simeq 10^{-3}$,
which explains why it has been overlooked by numerical studies so far.",1712.03787v1
2018-01-09,Influence of image forces on the interlayer exchange interaction in magnetic tunnel junctions with ferroelectric barrier,"We study interlayer exchange interaction in magnetic tunnel junctions with
ferroelectric barrier. We focus on the influence of image forces on the voltage
dependence of the interlayer magnetic interaction (magneto-electric effect).
The influence of the image forces is twofold: 1) they significantly enforce
magneto-electric effect occurring due to the surface charges at the interface
between ferroelectric and ferromagnets; 2) in combination with voltage
dependent dielectric constant of the ferroelectric barrier image forces cause
an additional contribution to the magneto-electric effect in magnetic tunnel
junctions. This contribution can exceed the one coming from surface charges. We
compare the interlayer exchange coupling voltage variation with spin transfer
torque effect and show that for half-metallic electrodes the interlayer
exchange coupling variation is dominant and defines the magnetic state and
dynamics of magnetization in the tunnel junction.",1801.03113v1
2018-01-23,Correlation between Magnetic Properties and Depinning Field in Field-Driven Domain Wall Dynamics in GdFeCo Ferrimagnets,"The influence of temperature on the magnetic-field-driven domain wall (DW)
motion is investigated in GdFeCo ferrimagnets with perpendicular magnetic
anisotropy (PMA). We find that the depinning field strongly depends on
temperature. Moreover, it is also found that the saturation magnetization
exhibits a similar dependence on temperature to that of depinning field. From
the creep-scaling criticality, a simple relation between the depinning field
and the properties of PMA is clearly identified theoretically as well as
experimentally. Our findings open a way for a better understanding how the
magnetic properties influence on the depinning field in magnetic system and
would be valuably extended to depinning studies in other system.",1801.07436v1
2018-02-26,Small-scale dynamos in simulations of stratified turbulent convection,"Small-scale dynamo action is often held responsible for the generation of
quiet-Sun magnetic fields. We aim to determine the excitation conditions and
saturation level of small-scale dynamos in non-rotating turbulent convection at
low magnetic Prandtl numbers. We use high resolution direct numerical
simulations of weakly stratified turbulent convection. We find that the
critical magnetic Reynolds number for dynamo excitation increases as the
magnetic Prandtl number is decreased, which might suggest that small-scale
dynamo action is not automatically evident in bodies with small magnetic
Prandtl numbers as the Sun. As a function of the magnetic Reynolds number
(${\rm Rm}$), the growth rate of the dynamo is consistent with an ${\rm
Rm}^{1/2}$ scaling. No evidence for a logarithmic increase of the growth rate
with ${\rm Rm}$ is found.",1802.09607v1
2018-03-04,Optimization of Time-Resolved Magneto-optical Kerr Effect Signals for Magnetization Dynamics Measurements,"Recently magnetic storage and magnetic memory have shifted towards the use of
magnetic thin films with perpendicular magnetic anisotropy (PMA). Understanding
the magnetic damping in these materials is crucial, but normal Ferromagnetic
Resonance (FMR) measurements face some limitations. The desire to quantify the
damping in materials with PMA has resulted in the adoption of Time-Resolved
Magneto-optical Kerr Effect (TR-MOKE) measurements. In this paper, we discuss
the angle and field dependent signals in TR-MOKE, and utilize a numerical
algorithm based on the Landau-Lifshitz-Gilbert (LLG) equation to provide
information on the optimal conditions to run TR-MOKE measurements.",1803.01280v2
2018-03-09,Laboratory experiments on dynamo action and magnetically triggered flow instabilities,"Magnetic fields of planets, stars and galaxies are generated by
self-excitation in moving electrically conducting fluids. Once produced,
magnetic fields can play an active role in cosmic structure formation by
destabilizing rotational flows that would be otherwise hydrodynamically stable.
For a long time, both hydromagnetic dynamo action as well as magnetically
triggered flow instabilities had been the subject of purely theoretical
research. Meanwhile, however, the dynamo effect has been observed in
large-scale liquid sodium experiments in Riga, Karlsruhe and Cadarache. In this
paper, we summarize the results of liquid metal experiments devoted to the
dynamo effect and various magnetic instabilities such as the helical and the
azimuthal magnetorotational instability and the Tayler instability. We discuss
in detail our plans for a precession-driven dynamo experiment and a large-scale
Tayler-Couette experiment using liquid sodium, and on the prospects to observe
magnetically triggered instabilities of flows with positive shear.",1803.03421v1
2018-03-19,Vegas: Software package for the atomistic simulation of magnetic materials,"We present an open-source software package, Vegas, for the atomistic
simulation of magnetic materials. Using the classical Heisenberg model and the
Monte Carlo Metropolis algorithm, Vegas provides the required tools to simulate
and analyze magnetic phenomena of a great variety of systems. Vegas stores the
history of the simulation, i.e. the magnetization and energy of the system at
every time step, allowing to analyze static and dynamic magnetic phenomena from
results obtained in a single simulation. Also, standardized input and output
file formats are employed to facilitate the simulation process and the exchange
and archiving of data. We include results from simulations performed using
Vegas, showing its applicability to study different magnetic phenomena.",1803.07030v1
2018-04-10,Magnetometric Mapping of Superconducting RF Cavities,"A scalable mapping system for superconducting RF cavities is presented.
Currently, it combines local temperature measurement with 3D magnetic field
mapping along the outer surface of the resonator. This allows for the
observation of dynamic effects that have an impact on the superconducting
properties of a cavity, such as the normal to superconducting phase transition
or a quench. The system was developed for a single cell 1.3 GHz TESLA-type
cavity, but can be easily adopted to arbitrary other cavity types. A data
acquisition rate of 500 Hz for all channels simultaneously (i.e.2ms)
acquisition time for a complete map) and a magnetic field resolution of
currently up to 14 mA/m/mu0 = 17 nT has been implemented. While temperature
mapping is a well known technique in SRF research, the integration of magnetic
field mapping opens the possibility of detailed studies of trapped magnetic
flux and its impact on the surface resistance. It is shown that magnetic field
sensors based on the anisotropic magnetoresistance (AMR) effect can be used in
the cryogenic environment with improved sensitivity compared to room
temperature. Furthermore, examples of first successful combined temperature and
magnetic-field maps are presented.",1804.03445v1
2018-05-07,Phase dynamics of oscillating magnetizations coupled via spin pumping,"A theoretical formalism is developed to simultaneously solve equation of
motion of the magnetizations in two ferromagnets and the spin-pumping induced
spin transport equation. Based on the formalism, a coupled motion of the
magnetizations in a self-oscillation state is studied. The spin pumping is
found to induce an in-phase synchronization of the magnetizations for the
oscillation around the easy axis. For an out-of-plane self-oscillation around
the hard axis, on the other hand, the spin pumping leads to an in-phase
synchronization in a small current region, whereas an antiphase synchronization
is excited in a large current region. An analytical theory based on the phase
equation reveals that the phase difference between the magnetizations in a
steady state depends on the oscillation direction, clockwise or
counterclockwise, of the magnetizations.",1805.02766v1
2018-05-12,"Computational modeling of magnetoconvection: effects of discretization method, grid refinement and grid stretching","The problem of natural convection in a laterally heated three-dimensional
cubic cavity under the action of an externally imposed magnetic field is
revisited. Flows at the Rayleigh number Ra=10^6 and the Hartmann number Ha=100,
and three different orientations of the magnetic field are considered. The
problem is solved using two independent numerical methods based on the second
order finite-volume discretization schemes on structured Cartesian grids.
Convergence toward grid-independent results is examined versus the grid
refinement and near-wall grid stretching. Converged benchmark-quality results
are obtained. It is shown that for convection flows with a strong magnetic
field a steep, sometimes extremely steep, stretching near some of the
boundaries is needed. Three-dimensional patterns and integral properties of the
converged flow fields are reported and discussed. It is shown that the
strongest magnetic suppression is yielded by the field directed along the
imposed temperature gradient. The horizontal magnetic field perpendicular to
the imposed temperature gradient stabilizes the main convection roll and leads
to a flow with higher kinetic energy and heat transfer rate than in the
non-magnetic case. Applicability of the quasi-two-dimensional model to natural
convection flows in a box is discussed.",1805.04710v1
2018-05-15,Control of phase of the magnetization precession excited by circularly polarized fs-laser pulses,"The inverse Faraday effect induced in magnetic films by ultrashort laser
pulses allows excitation and control of spins at GHz and sub-THz frequencies.
Frequency of the excited magnetization precession is easily tunable by the
external magnetic field. On the other hand, phase of the precession hardly
depends on magnetic field. Here we demonstrate an approach for the control of
the precession phase by variation of the incidence angle of the laser pulses.
In particular, theoretical consideration states that the phase increases with
increase of the incidence angle and for small angles this relation is a direct
proportionality. Experimental studies confirm this conclusion and provide shift
of phase by about 4 deg. for the declination of the incidence angle by 15 deg.
from the normal. This study provides a simple way for additional manipulation
with optically excited magnetization dynamics, which is of importance for
different spintronic applications.",1805.05936v1
2018-06-07,Magnetic domain walls as broadband spin wave and elastic magnetisation wave emitters,"We report on the direct observation of spin wave and elastic wave emission
from magnetic domain walls in ferromagnetic thin films. Driven by alternating
homogeneous magnetic fields the magnetic domain walls act as coherent
magnetisation wave sources. Directional and low damped elastic waves below and
above the ferromagnetic resonance are excited. The wave vector of the
magnetoelastically induced acoustic shear waves is linearly tuned by varying
the excitation frequency. Domain wall emitted magnetostatic surface spin waves
occur at higher frequencies, which characteristics are confirmed by
micromagnetic simulations. The distinct modes of magnetisation wave excitation
from micromagnetic objects are a general physical phenomenon relevant for
dynamic magnetisation processes in structured magnetic films. Magnetic domain
walls can act as reconfigurable antennas for spin wave and elastic wave
generation with control of the wave orientation.",1806.02646v1
2018-06-08,Metachronal motion of artificial magnetic cilia,"Organisms use hair-like cilia that beat in a metachronal fashion to actively
transport fluid and suspended particles. Metachronal motion emerges due to a
phase difference between beating cycles of neighboring cilia and appears as
traveling waves propagating along ciliary carpet. In this work, we demonstrate
biomimetic artificial cilia capable of metachronal motion. The cilia are
micromachined magnetic thin filaments attached at one end to a substrate and
actuated by a uniform rotating magnetic field. We show that the difference in
magnetic cilium length controls the phase of the beating motion. We use this
property to induce metachronal waves within a ciliary array and explore the
effect of operation parameters on the wave motion. The metachronal motion in
our artificial system is shown to depend on the magnetic and elastic properties
of the filaments, unlike natural cilia, where metachronal motion arises due to
fluid coupling. Our approach enables an easy integration of metachronal
magnetic cilia in lab-on-a-chip devices for enhanced fluid and particle
manipulations.",1806.03363v1
2018-06-14,Magnetic inhomogeneity in the copper pseudochalcogenide CuNCN,"Copper carbodiimide, CuNCN, is a geometrically frustrated nitrogen-based
analogue of cupric oxide, whose magnetism remains ambiguous. Here, we employ a
combination of local-probe techniques, including $^{63,\, 65}$Cu nuclear
quadrupole resonance, $^{13}$C nuclear magnetic resonance and muon spin
rotation to show that the magnetic ground state of the Cu$^{2+}$ ($S=1/2$)
spins is frozen and disordered. Moreover, these complementary experiments
unequivocally establish an onset of intrinsically inhomogeneous magnetic state
at $T_h=80$ K. Below $T_h$, the low-temperature frozen component coexist with
the remnant high-temperature dynamical component down to $T_l = 20$ K, where
the latter finally ceases to exist. Based on a scaling of internal magnetic
fields of both components we conclude that the two components coexist on a
microscopic level.",1806.05486v2
2018-06-27,Site-selective Probe of Magnetic Excitations in Rare-earth Nickelates using Resonant Inelastic X-ray Scattering,"We have used high-resolution resonant inelastic x-ray scattering (RIXS) to
study a thin film of NdNiO$_3$, a compound whose unusual spin- and bond-ordered
electronic ground state has been of long-standing interest. Below the magnetic
ordering temperature, we observe well-defined collective magnon excitations
along different high-symmetry directions in momentum space. The magnetic
spectra depend strongly on the incident photon energy, which we attribute to
RIXS coupling to different local electronic configurations of the expanded and
compressed NiO$_6$ octahedra in the bond-ordered state. Both the noncollinear
magnetic ground state and the observed site-dependent magnon excitations are
well described by a model that assumes strong competition between the
antiferromagnetic superexchange and ferromagnetic double-exchange interactions.
Our study provides direct insight into the magnetic dynamics and exchange
interactions of the rare-earth nickelates, and demonstrates that RIXS can serve
as a site-selective probe of magnetism in these and other materials.",1806.10499v1
2018-07-04,Effects of a strong magnetic field on the QCD flux tube,"In this work we investigate the effect of an external magnetic field B on the
shape of flux tubes in QCD by means of lattice simulations, performed with
N_f=2+1 flavors of stout improved dynamical staggered quarks with physical
masses. After having discussed some difficulties in the practical definition of
the flux tube at B=0, we show that these ambiguities do not affect the
determination of the flux tube modifications induced by the magnetic field.
Different results are obtained depending on the relative orientations of the
flux tube and of the magnetic field: they confirm that the magnetic field acts
as transverse confinement catalyser and longitudinal confinement inhibitor;
moreover, the flux tube itself loses its axial symmetry when it is not directed
along the magnetic background.",1807.01673v2
2018-09-26,Cavity Optomechanics of Topological Spin Textures in Magnetic Insulators,"Collective dynamics of topological magnetic textures can be thought of as a
massive particle moving in a magnetic pinning potential. We demonstrate that
inside a cavity resonator this effective mechanical system can feel the
electromagnetic radiation pressure from cavity photons through the
magneto-optical inverse Faraday and Cotton-Mouton effects. We estimate values
for the effective parameters of the optomechanical coupling for two spin
textures -- a Bloch domain wall and a chiral magnetic soliton lattice. The
soliton lattice has magnetic chirality, so that in circularly polarized light
it behaves like a chiral particle with the sign of the optomechanical coupling
determined by the helicity of the light and chirality of the lattice. Most
interestingly, we find a level attraction regime for the soliton lattice, which
is tunable through an applied magnetic field.",1809.10091v2
2018-10-05,Ultrafast demagnetization of Pt magnetic moment in L1${}_0$-FePt probed by hard x-ray free electron laser,"We demonstrate ultrafast magnetization dynamics in a 5d transition metal
using circularly-polarized x-ray free electron laser in the hard x-ray region.
A decay time of light-induced demagnetization of L1${}_0$-FePt was determined
to be $\tau_\textrm{Pt} = 0.6\ \textrm{ps}$ using time-resolved x-ray magnetic
circular dichroism at the Pt L${}_3$ edge, whereas magneto-optical Kerr
measurements indicated the decay time for total magnetization as
$\tau_\textrm{total} = 0.1\ \textrm{ps}$. A transient magnetic state with the
photo-modulated magnetic coupling between the 3d and 5d elements is firstly
demonstrated.",1810.02551v1
2018-10-30,Magnetic field sensing with the kinetic inductance of a high-$T_\mathrm{c}$ superconductor,"We carry out an experimental feasibility study of a magnetic field sensor
based on the kinetic inductance of the high-$T_\mathrm{c}$ superconductor
yttrium barium copper oxide. We pattern thin superconducting films into
radio-frequency resonators that feature a magnetic field pick-up loop. At 77 K
and for film thicknesses down to 75 nm, we observe the persistence of screening
currents that modulate the loop kinetic inductance. According to the
experimental results the device concept appears attractive for sensing
applications in ambient magnetic field environments. We report on a device with
a magnetic field sensitivity of 4 pT/Hz${}^{1/2}$, an instantaneous dynamic
range of 11 $\mu$T, and operability in magnetic fields up to 28 $\mu$T.",1810.12725v1
2018-11-29,Magneto-immutable turbulence in weakly collisional plasmas,"We propose that pressure anisotropy causes weakly collisional turbulent
plasmas to self-organize so as to resist changes in magnetic-field strength. We
term this effect ""magneto-immutability"" by analogy with incompressibility
(resistance to changes in pressure). The effect is important when the pressure
anisotropy becomes comparable to the magnetic pressure, suggesting that in
collisionless, weakly magnetized (high-$\beta$) plasmas its dynamical relevance
is similar to that of incompressibility. Simulations of magnetized turbulence
using the weakly collisional Braginskii model show that magneto-immutable
turbulence is surprisingly similar, in most statistical measures, to critically
balanced MHD turbulence. However, in order to minimize magnetic-field
variation, the flow direction becomes more constrained than in MHD, and the
turbulence is more strongly dominated by magnetic energy (a nonzero ""residual
energy""). These effects represent key differences between pressure-anisotropic
and fluid turbulence, and should be observable in the $\beta\gtrsim1$ turbulent
solar wind.",1811.12421v2
2018-12-26,Time evolution of rotating and magnetized white dwarf stars,"We investigate the evolution of isolated, zero and finite temperature,
massive, uniformly rotating and highly magnetized white dwarf stars under
angular momentum loss driven by magnetic dipole braking. We consider the
structure and thermal evolution of white dwarf isothermal cores taking also
into account the nuclear burning and neutrino emission processes. We estimate
the white dwarf lifetime before it reaches the condition either for a type Ia
supernova explosion or for the gravitational collapse to a neutron star. We
study white dwarfs with surface magnetic fields from $10^6$ to $10^{9}$~G and
masses from $1.39$ to $1.46~M_\odot$ and analyze the behavior of the white
dwarf parameters such as moment of inertia, angular momentum, central
temperature and magnetic field intensity as a function of lifetime. The
magnetic field is involved only to slow down white dwarfs, without affecting
their equation of state and structure. In addition, we compute the
characteristic time of nuclear reactions and dynamical time scale. The
astrophysical consequences of the results are discussed.",1812.10543v3
2019-01-22,Magnetic helicity and fluxes in an inhomogeneous alpha squared dynamo,"Much work on turbulent three-dimensional dynamos has been done using triply
periodic domains, in which there are no magnetic helicity fluxes. Here we
present simulations where the turbulent intensity is still nearly homogeneous,
but now there is a perfect conductor boundary condition on one end and a
vertical field or pseudo-vacuum condition on the other. This leads to migratory
dynamo waves. Good agreement with a corresponding analytically solvable alpha^2
dynamo is found. Magnetic helicity fluxes are studied in both types of models.
It is found that at moderate magnetic Reynolds numbers, most of the magnetic
helicity losses occur at large scales. Whether this changes at even larger
magnetic Reynolds numbers, as required for alleviating the catastrophic dynamo
quenching problem, remains still unclear.",1901.07552v2
2019-02-19,Spin-wave mediated interactions for Majority Computation using Skyrmions and Spin-torque Nano-oscillators,"Recent progress in all-electrical nucleation, detection and manipulation of
magnetic skyrmions has unlocked the tremendous potential of skyrmion-based
spintronic devices. Here, we show via micromagnetic simulations that the stable
magnetic oscillations of STNO radiate spin waves (SWs) that can be scattered in
the presence of skyrmions in the near vicinity. Interference between SWs
emitted by the STNO and SWs scattered by the skyrmion gives rise to interesting
dynamics that leads to amplification or attenuation of STNO's magnetic
oscillations. In the presence of strong Dzyaloshinskii-Moriya interaction
(DMI), the amplified magnetic oscillations evolve into a new skyrmion. These
interactions between skyrmions and STNOs are found to be identical for both
Neel-type and Bloch-type skyrmions, and are not observed between domain walls
and STNOs. These findings offer a novel perspective in processing information
using single skyrmions and we propose a 3-bit majority gate for logic
applications.",1902.06925v2
2019-02-24,Decoupling of the Kinetic and Magnetic Energy Cascades in MHD Turbulence,"Magnetic and kinetic energy in ideal incompressible MHD are not global
invariants and, therefore, it had been justified to discuss only the cascade of
their sum, total energy. We provide a physical argument based on scale-locality
of the cascade, along with compelling evidence that at high Reynolds numbers,
magnetic and kinetic energy budgets statistically decouple beyond a
transitional ""conversion"" range. This arises because magnetic field-line
stretching is a large-scale process which vanishes on average at intermediate
and small scales within the inertial-inductive range, thereby allowing each of
mean kinetic and magnetic energy to cascade conservatively and at an equal
rate. One consequence is that the turbulent magnetic Prandtl number is unity
over the ""decoupled range"" of scales.",1902.08983v1
2019-02-25,Driving magnetization dynamics in an on-demand magnonic crystal by magneto-elastic interaction,"Using spatial light interference of ultrafast laser pulses, we generate a
lateral modulation in the magnetization profile of an otherwise uniformly
magnetized film, whose magnetic excitation spectrum is monitored via the
coherent and resonant interaction with elastic waves. We find an unusual
dependence of the magnetoelastic coupling as the externally applied magnetic
field is angle- and field-tuned relative to the wave vector of the
magnetization modulation, which can be explained by the emergence of spatially
inhomogeneous spin-wave modes. In this regard, the spatial light interference
methodology can be seen as a user-configurable, temporally windowed, on-demand
magnonic crystal, potentially of arbitrary two-dimensional shape, which allows
control and selectivity of the spatial distribution of spin waves. Calculations
of spin waves using a variety of methods, demonstrated here using the
plane-wave method and micromagnetic simulation, can identify the spatial
distribution and associated energy scales of each excitation, which opens the
door to a number of excitation methodologies beyond our chosen elastic wave
excitation.",1902.09186v1
2019-02-27,Nuclear magnetic resonance spectroscopy as a dynamical structural probe of high pressure hydrogen,"An unambiguous crystallographic structure solution for the observed phases
II-VI of high pressure hydrogen does not exist due to the failure of standard
structural probes at extreme pressure. In this work we propose that nuclear
magnetic resonance spectroscopy provides a complementary structural probe for
high pressure hydrogen. We show that the best structural models available for
phases II, III, and IV of high pressure hydrogen exhibit markedly distinct
nuclear magnetic resonance spectra which could therefore be used to
discriminate amongst them. As an example, we demonstrate how nuclear magnetic
resonance spectroscopy could be used to establish whether phase III exhibits
polymorphism. Our calculations also reveal a strong renormalisation of the
nuclear magnetic resonance response in hydrogen arising from quantum
fluctuations, as well as a strong isotope effect. As the experimental
techniques develop, nuclear magnetic resonance spectroscopy can be expected to
become a useful complementary structural probe in high pressure experiments.",1902.10721v2
2019-03-18,Astro2020 Science White Paper: Magnetic Fields and Polarization in the Diffuse Interstellar Medium,"Magnetism is one of the most important forces on the interstellar medium
(ISM), anisotropically regulating the structure and star formation that drive
galactic evolution. Recent high dynamic range observations of diffuse gas and
molecular clouds have revealed new links between interstellar structures and
the ambient magnetic field. ISM morphology encodes rich physical information,
but deciphering it requires high-resolution measurements of the magnetic field:
linear polarization of starlight and dust emission, and Zeeman splitting. These
measure different components of the magnetic field, and crucially, Zeeman
splitting is the only way to directly measure the field strength in the ISM. We
advocate a statistically meaningful survey of magnetic field strengths using
the 21-cm line in absorption, as well as an observational test of the link
between structure formation and field strength using the 21-cm line in
emission. Finally, we report on the serendipitous discovery of linear
polarization of the 21-cm line, which demands both theoretical and
observational follow-up.",1903.07671v1
2019-05-20,Magnetized Gas Collimation of Interstellar Outflow Scaled by Laser-produced Plasma,"Young stellar objects/planetary nebula outflow anisotropies usually involve
wind-wind interactions and magnetic collimation, but detailed structures of
wind and magnetic fields inside collimation region remain undetermined. We
numerically investigated its laboratory counterpart, based on poloidal field
collimation in magnetocentrifugal launching model. Our analog consist of fast
wind: Aluminum plasma generated by laser and magnetized ambient: molecular
Helium and B=5-60 Tesla embedded field. Elevating magnetic field strength or
decreasing gas density can alter expansion morphology, from sphere to prolonged
cavity and ultimately to collimated jet. Outflow patterns can be quantitatively
predicted based on the knowledge of its surroundings through a set of external
Mach numbers. We conclude that such mixed gas and magnetic field dynamics are
consistent with astronomical observations of protostars and planetary nebulae
in certain evolution stages, and here provide a scalable framework allowing
fitting of flow-field structures in astronomical unresolved regions by assuming
their possible geometries on a repeatable laboratory platform.",1905.07935v1
2019-06-19,Stability of a force-free Hall equilibrium and release of magnetic energy,"Conservation of magnetic helicity by the Hall drift does not prevent Hall
instability of helical fields. This conclusion follows from stability analysis
of a force-free spatially-periodic Hall equilibrium. The growth rates of the
instability scale as $\sigma \propto B^{3/4}\eta^{1/4}$ with the field strength
$B$ and magnetic diffusivity $\eta$ and can be large compared to the rate of
resistive decay of the background field. The instability deviates the magnetic
field from the force-free configuration. The unstable eigenmodes include a fine
spatial structure which evolves into current sheets at the nonlinear stage of
the instability. The instability catalyses the resistive release of magnetic
energy. The energy is released in a sequence of spikes, every spike emits
several percent of the total energy. A numerically defined scaling for the
energy released in a single spike permits an extrapolation to astrophysically
relevant values of the Hall number. The instability can be relevant to magnetic
energy release in a neutron star crust and, possibly, in stellar coronae.",1906.07936v1
2019-07-01,Spin currents driven by the Higgs mode in magnetic superconductors,"Higgs mode in superconducting materials describes slowly-decaying
oscillations of the order parameter amplitude. We demonstrate that in magnetic
superconductors with built-in spin-splitting field Higgs mode is strongly
coupled to the spin degrees of freedom allowing for the generation of
time-dependent spin currents.
  Converting such spin currents to electric signals by spin-filtering elements
provides a tool for the second-harmonic generation and the electrical detection
of the Higgs mode generated by the external irradiation. The non-adiabatic spin
torques generated by these spin currents allow for the magnetic detection of
the Higgs mode by measuring the precession of magnetic moment in the adjacent
ferromagnet. We discuss also the reciprocal effect which is the generation of
the Higgs mode by the magnetic precession. Coupling the collective modes in
superconductors to light and magnetic dynamics opens the new direction of
superconducting optospintronics.",1907.00539v1
2019-07-01,Electrically driven spin torque and dynamical Dzyaloshinskii-Moriya interaction in magnetic bilayer systems,"Efficient control of magnetism with electric means is a central issue of
current spintronics research, which opens an opportunity to design integrated
spintronic devices. However, recent well-studied methods are mostly based on
electric-current injection, and they are inevitably accompanied by considerable
energy losses through Joule heating. Here we theoretically propose a way to
exert spin torques into magnetic bilayer systems by application of electric
voltages through taking advantage of the Rashba spin-orbit interaction. The
torques resemble the well-known electric-current-induced torques, providing
similar controllability of magnetism but without Joule-heating energy losses.
The torques also turn out to work as an interfacial Dzyaloshinskii-Moriya
interaction which enables us to activate and create noncollinear magnetism like
skyrmions by electric-voltage application. Our proposal offers an efficient
technique to manipulate magnetizations in spintronics devices without
Joule-heating energy losses.",1907.00601v2
2019-07-02,Prediction of topological Hall effect in a driven magnetic domain wall,"We investigate the possible emergence of topological Hall effect (THE) in a
driven magnetic DW. Numerical simulation based on the
Landau-Lifshitz-Gilbert-Slonczewski (LLGS) equation shows that the emergent
magnetic flux appears when the DW is in a non-equilibrium state. The magnitude
of magnetic flux is modulated by Dzyaloshinskii-Moriya interaction (DMI) or
in-plane longitudinal magnetic field, providing an experimental test of the
predicted THE. These results indicate that the THE can be observed even in a
topologically trivial magnetic DW, and therefore open up new possibility to
electrically detect the dynamical spin structure.",1907.01648v2
2019-07-19,A cryogenic memory element based on an anomalous Josephson junction,"We propose a non-volatile memory element based on a lateral ferromagnetic
Josephson junction with spin-orbit coupling and out-of-plane magnetization. The
interplay between the latter and the intrinsic exchange field of the
ferromagnet leads to a magnetoelectric effect that couples the charge current
through the junction and its magnetization, such that by applying a current
pulse the direction of the magnetic moment in F can be switched. The two memory
states are encoded in the direction of the out-of-plane magnetization. With the
aim to determine the optimal working temperature for the memory element, we
explore the noise-induced effects on the averaged stationary magnetization by
taking into account thermal fluctuations affecting both the Josephson phase and
the magnetic moment dynamics. We investigate the switching process as a
function of intrinsic parameters of the ferromagnet, such as the Gilbert
damping and strength of the spin-orbit coupling, and proposed a non-destructive
readout scheme based on a dc-SQUID. Additionally, we analyze a way to protect
the memory state from external perturbations by voltage gating in systems with
a both linear-in-momentum Rashba and Dresselhaus spin-orbit coupling.",1907.08454v2
2019-09-06,Transverse Force Induced by a Magnetized Wake,"The force on a test charge moving through a strongly magnetized plasma is
calculated using linear response theory. Strong magnetization is found to
generate a component of the force perpendicular to the velocity of the particle
in the plane formed by the velocity and magnetic field vectors. This transverse
force is generated by an asymmetry with respect to the velocity vector in the
induced electrostatic wake potential that is associated with the action of the
Lorentz force on the background plasma. The direction depends on the speed of
the test charge. If it is faster than a critical speed characteristic of the
sound speed, it acts to reduce the component of velocity parallel to the
magnetic field and to increase the gyroradius. In contrast, if the speed is
below this critical speed, it acts to increase the velocity parallel to the
magnetic field and to decrease the gyroradius. Because the transverse force is
perpendicular to the velocity, it does not directly influence the total energy
of the test charge. Nevertheless, it significantly alters the trajectory on a
timescale associated with the Coulomb collision time.",1909.03121v1
2019-10-10,Analytical criteria for magnetization reversal in $\varphi_0$ Josephson junction,"The $\varphi_0$ Josephson junctions formed by ordinary superconductors and a
magnetic non-centrosymmetric interlayer are studied. We derive an analytical
solution for the magnetization dynamics induced by an arbitrary current pulse
and formulate the criteria for magnetization reversal. Using the obtained
results, the form and duration of the current pulse are optimized. The
agreement between analytical and numerical investigations is reached in the
case of a large product of the ratio Josephson to magnetic energy, strength of
spin-orbit interaction and a minimal value of the current pulse. The obtained
results allow one to predict magnetization reversal at the chosen system
parameters.",1910.04419v2
2019-10-10,Formation of Néel Type Skyrmions in an Antidot Lattice with Perpendicular Magnetic Anisotropy,"Magnetic skyrmions are particle-like chiral spin textures found in a magnetic
film with out-of-planeanisotropy and are considered to be potential candidates
as information carriers in next generationdata storage devices. Despite intense
research into the nature of skyrmions and their dynamic prop-erties, there are
several key challenges that still need to be addressed. In particular, the
outstandingissues are the reproducible generation, stabilization and
confinement of skyrmions at room tempera-ture. Here, we present a method for
the capture of nanometer sized magnetic skyrmions in an arrayof magnetic
topological defects in the form of an antidot lattice. With micromagnetic
simulations,we elucidate the skyrmion formation in the antidot lattice and show
that the capture is dependenton the antidot lattice parameters. This behavior
is confirmed with scanning transmission x-ray mi-croscopy measurements. This
demonstration that a magnetic antidot lattice can be implemented asa host to
capture skyrmions provides a new platform for experimental investigations of
skyrmionsand skyrmion based devices.",1910.04515v1
2019-10-30,Laser-produced magnetic-Rayleigh-Taylor unstable plasma slabs in a 20 T magnetic field,"Magnetized laser-produced plasmas are central to many novel laboratory
astrophysics and inertial confinement fusion studies, as well as in industrial
applications. Here we provide the first complete description of the
three-dimensional dynamics of a laser-driven plasma plume expanding in a 20 T
transverse magnetic field. The plasma is collimated by the magnetic field into
a slender, rapidly elongating slab, whose plasma-vacuum interface is unstable
to the growth of the ""classical"", fluid-like magnetized Rayleigh-Taylor
instability.",1910.13778v1
2019-10-31,Ultrafast magnetic dynamics in insulating YBa$_2$Cu$_3$O$_{6+x}$ revealed by time resolved two-magnon Raman Scattering,"Measurement and control of magnetic order and correlations in real time is a
rapidly developing scientific area relevant for magnetic memory and spintronics
[1,15]. In these experiments an ultra-short laser pulse (pump) is first
absorbed by excitations carrying electric dipole moment. These then give their
energy to the magnetic subsystem monitored by a time-resolved probe. A lot of
progress has been made in investigations of ferromagnets but antiferromagnets
are more challenging. Here we introduce time-resolved two-magnon Raman
scattering as a novel real time probe of magnetic correlations especially
well-suited for antiferromagnets. Its application to the antiferromagnetic
charge transfer insulator YBa$_2$Cu$_3$O$_{6+x}$ revealed rapid demagnetization
within 90fs of photoexcitation. The relaxation back to thermal equilibrium is
characterized by much slower timescales. We interpret these results in terms of
slow relaxation of the charge sector and rapid equilibration of the magnetic
sector to a prethermal state characterized by parameters that change slowly as
the charge sector relaxes.",1910.14585v3
2019-11-03,Magnetic damping modulation in $IrMn_{3}/Ni_{80}Fe_{20}$ via the magnetic spin Hall effect,"Non-collinear antiferromagnets can have additional spin Hall effects due to
the net chirality of their magnetic spin structure, which provides for more
complex spin-transport phenomena compared to ordinary non-magnetic materials.
Here we investigated how ferromagnetic resonance of permalloy
($Ni_{80}Fe_{20}$) is modulated by spin Hall effects in adjacent epitaxial
$IrMn_{3}$ films. We observe a large dc modulation of the ferromagnetic
resonance linewidth for currents applied along the [001] $IrMn_{3}$ direction.
This very strong angular dependence of spin-orbit torques from dc currents
through the bilayers can be explained by the magnetic spin Hall effect where
$IrMn_{3}$ provides novel pathways for modulating magnetization dynamics
electrically.",1911.00943v1
2019-11-15,Radiative Properties of Magnetically-Arrested Disks,"Magnetically-arrested disks (MADs) appear when accretion flows are supplied
with a sufficient amount of magnetic flux. In this work, we use results of
magnetohydrodynamic simulations to set the configuration of the magnetic field
and investigate the dynamics and radiative properties of the resulting
accretion flow (i.e., without that of the jet) of MAD. The method developed
here is applied to both the MAD and the standard and normal evolution (SANE)
accretion flow with or without large scale magnetic fields. For the radiative
processes, we include synchrotron, bremsstrahlung and Compton scattering. We
find that, in general, MAD accretion flows have similar spectra to those of the
SANE, which complicates the task of distinguishing MADs from SANEs. At the same
accretion rates, MADs are systematically brighter than SANEs. However, the
critical accretion rate above which the hot solution ceases to exist is lower
in MAD. Consequently, the maximum luminosity the MAD can reach is comparable
but slightly lower than that of SANE, and the dependence on the magnetic flux
is weak. We then discuss implications of our results for active galactic nuclei
and accreting black-hole binaries.",1911.06439v1
2019-11-18,Particle size controlled magnetic loss in magnetite nanoparticles in RF-microwave region,"Frequency dependant complex magnetic permeability is used to understand
RF-microwave behaviour of magnetic nanoparticles in the frequency range 250 MHz
to 3 GHz. The stable dispersions of Fe3O4 nanoparticles with mean size varying
between 11 to 16 nm are prepared for this purpose. The effect of mean particle
size and external static magnetic field over microwave absorption properties of
magnetic fluid is studied. It is observed that frequency of ferrimagnetic
resonance, frequency of maximum absorption, loss tangent and reflection loss
(RL) can be controlled by modifying mean particle size and strength of applied
external static magnetic field. This kind of study can be useful for
radio-microwave devices like tunable attenuator, EM sheilder, and other
applications like Hyperthermia.",1911.07467v1
2020-01-10,Unidirectional tilt of domain walls in equilibrium in biaxial stripes with Dzyaloshinskii-Moriya interaction,"The orientation of a chiral magnetic domain wall in a racetrack determines
its dynamical properties. In equilibrium, magnetic domain walls are expected to
be oriented perpendicular to the stripe axis. We demonstrate the appearance of
a unidirectional domain wall tilt in out-of-plane magnetized stripes with
biaxial anisotropy and Dzyaloshinskii--Moriya interaction (DMI). The tilt is a
result of the interplay between the in-plane easy-axis anisotropy and DMI. We
show that the additional anisotropy and DMI prefer different domain wall
structure: anisotropy links the magnetization azimuthal angle inside the domain
wall with the anisotropy direction in contrast to DMI, which prefers the
magnetization perpendicular to the domain wall plane. Their balance with the
energy gain due to domain wall extension defines the equilibrium magnetization
the domain wall tilting. We demonstrate that the Walker field and the
corresponding Walker velocity of the domain wall can be enhanced in the system
supporting tilted walls.",2001.03408v1
2020-02-09,Impact of longitudinal bulk viscous effects to heavy quark transport in a strongly magnetized hot QCD medium,"The effects of longitudinal bulk viscous pressure on the heavy quark dynamics
have been estimated in a strongly magnetized quark-gluon plasma within the
Fokker-Planck approach. The bulk viscous modification to the momentum
distribution of bulk degrees of freedom has been obtained in the presence of a
magnetic field while incorporating the realistic equation of state of the hot
magnetized QCD medium. As the magnetic field breaks the isotropy of the medium,
the analysis is done along the directions longitudinal and transverse to the
field. The longitudinal bulk viscous contribution is seen to have sizable
effects in the heavy quark momentum diffusion in the magnetized medium. The
dependence of higher Landau levels and the equation of state on the viscous
correction to the heavy quark transport has been explored in the analysis.",2002.03325v2
2020-03-05,Reexaming the ground state and magnetic properties of curium dioxide,"The ground state electronic structure and magnetic behaviors of curium
dioxide (CmO$_{2}$) are controversial. In general, the formal valence of Cm
ions in CmO$_{2}$ should be tetravalent. It implies a $5f^{6.0}$ electronic
configuration and a non-magnetic ground state. However, it is in sharp contrast
with the large magnetic moment measured by painstaking experiments. In order to
clarify this contradiction, we tried to study the ground state electronic
structure of CmO$_{2}$ by means of a combination of density functional theory
and dynamical mean-field theory. We find that CmO$_{2}$ is a wide-gap charge
transfer insulator with strong 5$f$ valence state fluctuation. It belongs to a
mixed-valence compound indeed. The predominant electronic configurations for Cm
ions are $5f^{6.0}$ and $5f^{7.0}$. The resulting magnetic moment agrees quite
well with the experimental value. Therefore, the magnetic puzzle in CmO$_{2}$
can be appropriately explained by the mixed-valence scenario.",2003.02716v2
2020-03-31,Spin-torque switching mechanisms of perpendicular magnetic tunnel junctions nanopillars,"Understanding the magnetization dynamics induced by spin transfer torques in
perpendicularly magnetized magnetic tunnel junction nanopillars and its
dependence on material parameters is critical to optimizing device performance.
Here we present a micromagnetic study of spin-torque switching in a disk-shaped
element as a function of the free layer's exchange constant and disk diameter.
The switching is shown to generally occur by 1) growth of the magnetization
precession amplitude in the element center; 2) an instability in which the
reversing region moves to the disk edge, forming a magnetic domain wall; and 3)
the motion of the domain wall across the element. For large diameters and small
exchange, step 1 leads to a droplet with a fully reversed core that experiences
a drift instability (step 2). While in the opposite case (small diameters and
large exchange), the central region of the disk is not fully reversed before
step 2 occurs. The origin of the micromagnetic structure is shown to be the
disk's non-uniform demagnetization field. Faster, more coherence and energy
efficient switching occur with larger exchange and smaller disk diameters,
showing routes to increase device performance.",2003.13875v1
2020-04-13,Density of states and Delocalization for discrete magnetic random Schrödinger operators,"We study discrete magnetic random Schr\""odinger operators on the square and
honeycomb lattice. For the non-random magnetic operator on the hexagonal
lattice with any rational magnetic flux, we show that the middle two dispersion
surfaces exhibit Dirac cones. We then derive an asymptotic expansion for the
density of states on the honeycomb lattice for oscillations of arbitrary
rational magnetic flux. This allows us, as a corollary, to rigorously study the
quantum Hall effect and conclude dynamical delocalization close to the conical
point under disorder. We obtain similar results for the discrete random
Schr\""odinger operator on the $\mathbb Z^2$-lattice with weak magnetic fields,
close to the bottom and top of its spectrum.",2004.06189v2
2020-05-04,Statistical properties of magnetic structures and energy dissipation during turbulent reconnection in the Earth's magnetotail,"We present the first statistical study of magnetic structures and associated
energy dissipation observed during a single period of turbulent magnetic
reconnection, by using the \textit{in-situ} measurements of the Magnetospheric
Multiscale mission in the Earth's magnetotail on July 26, 2017. The structures
are selected by identifying a bipolar signature in the magnetic field and
categorized as plasmoids or current sheets via an automated algorithm which
examines current density and plasma flow. The size of the plasmoids forms a
decaying exponential distribution ranging from sub-electron up to ion scales.
The presence of substantial number of current sheets is consistent with a
physical picture of dynamic production and merging of plasmoids during
turbulent reconnection. The magnetic structures are locations of significant
energy dissipation via electric field parallel to the local magnetic field,
while dissipation via perpendicular electric field dominates outside of the
structures. Significant energy also returns from particles to fields.",2005.01226v2
2020-06-01,Magnetic field effects in peripheral heavy-ion collisions around 1 GeV/nucleon,"Magnetic field effects on free nucleons are studied in peripheral collisions
of $^{197}$Au + $^{197}$Au at energies ranging from 600 to 1500 MeV/nucleon by
utilizing an isospin-dependent quantum molecular dynamics (IQMD) model. With
the help of angular distributions and two-particle angular correlators, the
magnetic field effect at an impact parameter of 11 fm is found to be more
obvious than at an impact parameter of 8 fm. Moreover, the results suggest that
with an increase in the number of peripheral collisions, protons are more
easily condensed with the magnetic field. Magnetic field effects are further
investigated by the ratio of free neutrons to free protons as functions of a
two-particle correlator $C_{2}$, four-particle correlator $C_{4}$ and
six-particle correlator $C_{6}$ of angle $\phi$, rapidity $Y$ and transverse
momentum $p_{T}$. The results show that weak magnetic field effects could be
revealed more clearly by these multiple-particle correlators, with the larger
number of particle correlators demonstrating a clear signal. The results
highlight a new method to search for weak signals using multi-particle
correlators.",2006.00718v1
2020-06-22,Enhanced x-ray emission arising from laser-plasma confinement by a strong transverse magnetic field,"We analyze, using experiments and 3D MHD numerical simulations, the dynamics
and radiative properties of a plasma ablated by a laser (1 ns,
10$^{12}$-10$^{13}$ W/cm$^2$) from a solid target, as it expands into a
homogeneous, strong magnetic field (up to 30 T) transverse to its main
expansion axis. We find that as soon as 2 ns after the start of the expansion,
the plasma becomes constrained by the magnetic field. As the magnetic field
strength is increased, more plasma is confined close to the target and is
heated by magnetic compression. We also observe a dense slab that rapidly
expands into vacuum after ~ 8 ns; however, this slab contains only ~ 2 % of the
total plasma. As a result of the higher density and increased heating of the
confined plasma, there is a net enhancement of the total x-ray emissivity
induced by the magnetization.",2006.12424v1
2020-07-08,Magnetization around mix jets entering inertial confinement fusion fuel,"Engineering features are known to cause jets of ablator material to enter the
fuel hot-spot in inertial confinement fusion implosions. The Biermann battery
mechanism wraps them in self-generated magnetic field. We show that higher-Z
jets have an additional thermoelectric magnetic source term that is not present
for hydrogen jets, verified here through a kinetic simulation. It has similar
magnitude to the Biermann term. We then include this in an extended
magneto-hydrodynamics approach to post-process an xRAGE radiation-hydrodynamic
implosion simulation. The simulation includes an accurate model for the capsule
fill tube, producing a dense carbon jet that becomes wrapped in a 4000T
magnetic field. A simple spherical carbon mix model shows that this insulates
the electron heat conduction enough to cause contraction of the jet to an
optically thick equilibrium. The denser magnetized jet hydrodynamics could
change its core penetration and therefore the final mix mass, which is known to
be well correlated with fusion yield degradation. Fully exploring this will
require self-consistent magneto-hydrodynamic simulations. Experimental
signatures of this self-magnetization may emerge in the high energy neutron
spectrum.",2007.04293v1
2020-07-15,Janus Monolayers of Magnetic Transition Metal Dichalcogenides as an All-in-One Platform for Spin-Orbit Torque,"We theoretically predict that vanadium-based Janus dichalcogenide monolayers
constitute an ideal platform for spin-orbit-torque memories. Using first
principles calculations, we demonstrate that magnetic exchange and magnetic
anisotropy energies are higher for heavier chalcogen atoms, while the broken
inversion symmetry in the Janus form leads to the emergence of Rashba-like
spin-orbit coupling. The spin-orbit torque efficiency is evaluated using
optimized quantum transport methodology and found to be comparable to heavy
nonmagnetic metals. The coexistence of magnetism and spin-orbit coupling in
such materials with tunable Fermi-level opens new possibilities for monitoring
magnetization dynamics in the perspective of non-volatile magnetic random
access memories.",2007.07579v1
2020-07-16,Strong surface magnetic field generation in relativistic short pulse laser-plasma interaction with an applied seed magnetic field,"While plasma often behaves diamagnetically, we demonstrate that the laser
irradiation of a thin opaque target with an embedded target-transverse seed
magnetic field $B_\mathrm{seed}$ can trigger the generation of an
order-of-magnitude stronger magnetic field with opposite sign at the target
surface. Strong surface field generation occurs when the laser pulse is
relativistically intense and results from the currents associated with the
cyclotron rotation of laser-heated electrons transiting through the target and
the compensating current of cold electrons. We derive a predictive scaling for
this surface field generation, $B_\mathrm{gen} \sim - 2 \pi B_\mathrm{seed}
\Delta x/\lambda_0$, where $\Delta x$ is the target thickness and $\lambda_0$
is the laser wavelength, and conduct 1D and 2D particle-in-cell simulations to
confirm its applicability over a wide range of conditions. We additionally
demonstrate that both the seed and surface-generated magnetic fields can have a
strong impact on application-relevant plasma dynamics, for example
substantially altering the overall expansion and ion acceleration from a
$\mu$m-thick laser-irradiated target with a kilotesla-level seed magnetic
field.",2007.08677v1
2020-07-24,Does elasticity stabilise a magnetic neutron star?,"The configuration of the magnetic field in the interior of a neutron star is
mostly unknown from observations. Theoretical models of the interior magnetic
field geometry tend to be oversimplified to avoid mathematical complexity and
tend to be based on axisymmetric barotropic fluid systems. These static
magnetic equilibrium configurations have been shown to be unstable on a short
time scale against an infinitesimal perturbation. Given this instability, it is
relevant to consider how more realistic neutron star physics affects the
outcome. In particular, it makes sense to ask if elasticity, which provides an
additional restoring force on the perturbations, may stabilise the system. It
is well-known that the matter in the neutron star crust forms an ionic crystal.
The interactions between the crystallized nuclei can generate shear stress
against any applied strain. To incorporate the effect of the crust on the
dynamical evolution of the perturbed equilibrium structure, we study the effect
of elasticity on the instability of an axisymmetric magnetic star. In
particular we determine the critical shear modulus required to prevent magnetic
instability and consider the corresponding astrophysical consequences.",2007.12400v2
2020-07-28,Quantifying the thermal stability in perpendicularly magnetized ferromagnetic nanodisks with forward flux sampling,"The thermal stability in nanostructured magnetic systems is an important
issue for applications in information storage. From a theoretical and
simulation perspective, an accurate prediction of thermally-activated
transitions is a challenging problem because desired retention times are on the
order of 10 years, while the characteristic time scale for precessional
magnetization dynamics is of the order of nanoseconds. Here, we present a
theoretical study of the thermal stability of magnetic elements in the form of
perpendicularly-magnetized ferromagnetic disks using the forward flux sampling
method, which is useful for simulating rare events. We demonstrate how rates of
thermally-activated switching between the two uniformly-magnetized ``up'' and
``down'' states, which occurs through domain wall nucleation and propagation,
vary with the interfacial Dzyaloshinskii-Moriya interaction, which affect the
energy barrier separating these states. Moreover, we find that the average
lifetimes differ by several orders of magnitude from estimates based on the
commonly assumed value of 1 GHz for the attempt frequency.",2007.14382v1
2020-09-30,Designing of Magnetic MAB Phases for Energy Applications,"Based on high-throughput density functional theory calculations, we performed
screening for stable magnetic MAB compounds and predicted potential strong
magnets for permanent magnet and magnetocaloric applications. The
thermodynamical, mechanical, and dynamical stabilities are systematically
evaluated, resulting in 21 unreported compounds on the convex hull, and 434
materials being metastable considering convex hull tolerance to be 100
meV/atom. Analysis based on the Hume-Rothery rules revealed that the valence
electron concentration and size factor difference are of significant importance
in determining the stability, with good correspondence with the local atomic
bonding. We found 71 compounds with the absolute value of magneto-crystalline
anisotropy energy above 1.0 MJ/m$^3$ and 23 compounds with a uniaxial
anisotropy greater than 0.4 MJ/m$^3$, which are potential gap magnets. Based on
the magnetic deformation proxy, 99 compounds were identified as potential
materials with interesting magnetocaloric performance.",2009.14543v1
2020-11-27,Entropic signatures of the skyrmion lattice phase in MnSi1-xAlx and Fe1-yCoySi,"The entropic signatures of magnetic phase transitions in the skyrmion lattice
host compounds MnSi0.962Al0.038 and Fe0.7Co0.3Si were investigated through low
field magnetization and ac susceptibility measurements. These data indicate
that the conical to skyrmion transition that occurs with the application of
magnetic field in MnSi0.962Al0.038 is characterized by clear discontinuity in
the magnetic entropy as expected for first order topological phase transition.
These same magnetoentropic features are negligibly small in isostructural
Fe0.7Co0.3Si due to the level of chemical substitution related disorder and
differences in the spin dynamics (range and timescales). Despite the obvious
similarities in the magnetic structures of these two compounds, the transitions
between these phases is substantially different indicating a surprising
non-universality to the magnetic phase transitions in this class of materials.",2011.14025v1
2020-12-11,Slow Spin Relaxation in Single Endohedral Fullerene Molecules,"Well-protected magnetization, tunable quantum states and long coherence time
are desired for the use of magnetic molecules in spintronics and quantum
information technologies. In this work, endohedral fullerene molecules M@C28
with different transition metal cores were explored through systematic
first-principles calculations and spin dynamics analyses. Many of them have
stable magnetization, giant magnetic anisotropy energy and bias-tunable
structure. In particular, some of them may have coherence time up to several
milliseconds for their quantum spin states at high temperature (~10K) after
full consideration of spin-vibration couplings. These results suggest that
these M@C28 provide a rich pool for the development of single-molecule magnets,
magnetic molecular junctions, and molecular qubits.",2012.06141v2
2020-12-20,Probing the internal magnetism of stars using asymptotic magneto-asteroseismology,"Our knowledge of the dynamics of stars has undergone a revolution thanks to
the simultaneous large amount of high-quality photometric observations
collected by space-based asteroseismology and ground-based high-precision
spectropolarimetry. They allowed us to probe the internal rotation of stars and
their surface magnetism in the whole Hertzsprung-Russell diagram. However, new
methods should still be developed to probe the deep magnetic fields in those
stars. Our goal is to provide seismic diagnoses that allow us to sound the
internal magnetism of stars. Here, we focus on asymptotic low-frequency gravity
modes and high-frequency acoustic modes. Using a first-order perturbative
theory, we derive magnetic splittings of their frequencies as explicit
functions of stellar parameters. As in the case of rotation, we show how
asymptotic gravity and acoustic modes can allow us to probe the different
components of the magnetic field in the cavities where they propagate. This
demonstrates again the high potential of using mixed-modes when this is
possible.",2012.11050v1
2020-12-28,Edge magnetic properties of black phosphorene nanoribbons,"The magnetic properties of black phosphorene nanoribbons are investigated
using static and dynamical mean-field theory. Besides confirming the existence
of ferromagnetic/antiferromagnetic edge magnetism, our detailed calculations
using large unit-cells find a phase-transition at weak interaction strength to
an incommensurate (IC) magnetic phase. A detailed Fourier analysis of the
magnetization patterns in the IC phase shows the existence of a second critical
interaction strength, where the incommensurate phase changes to an
antiferromagnetic (AFM) or ferromagnetic (FM) phase. We demonstrate that the
difference of the ground state energies of the AFM and FM phase is
exponentially small, making it possible to switch between both states by a
small external field. Finally, we analyze the influence of strain and disorder
on the magnetic properties and show that while the IC phase is robust to
Anderson type disorder, it is fragile against strain.",2012.14052v2
2021-01-06,Magnet design optimization of a 100 MeV separated sector medical cyclotron and its injection line,"This paper presents the magnetic design of a 100MeV Separated Sector Medical
Cyclotron (SSMC) as well as the optimization of the injection line magnets
where will be used to transport the 14MeV proton beam from the Azimuthally
Varying Field (AVF) cyclotron to the SSMC. The study demonstrates that the
isochronous magnetic field with a tolerance around 10-4 T can be obtained all
along prescribed path by placing three sets of correction trim coils in the
pole tips. The result of Betatron oscillations is sufficient to verify that the
focusing forces of the magnetic field can hold particles close to the median
plane of the magnet. In order to inject the beam by minimum loss from a 14MeV
Cyclotron to SSMC, it is essential for the beam to fit into the acceptance
ellipse of the separate sector cyclotron. The characteristics of the beam
injection system is calculated and optimized by Trace-3D beam dynamic code.",2101.02294v1
2021-03-07,Phonon Hall Viscosity in Magnetic Insulators,"The Phonon Hall Viscosity is the leading term evincing time-reversal symmetry
breaking in the low energy description of lattice phonons. It may generate
phonon Berry curvature, and can be observed experimentally through the acoustic
Faraday effect and thermal Hall transport. We present a systematic procedure to
obtain the phonon Hall viscosity induced by phonon-magnon interactions in
magnetic insulators under an external magnetic field. We obtain a general
symmetry criterion that leads to non-zero Faraday rotation and Hall
conductivity, and clarify the interplay between lattice symmetry,
spin-orbit-coupling, external magnetic field and magnetic ordering. The
symmetry analysis is verified through a microscopic calculation. By
constructing the general symmetry-allowed effective action that describes the
spin dynamics and spin-lattice coupling, and then integrating out the spin
fluctuations, the leading order time-reversal breaking term in the phonon
effective action, i.e. the phonon Hall viscosity, can be obtained. The analysis
of the square lattice antiferromagnet for a cuprate Mott insulator,
Sr$_2$CuO$_2$Cl$_2$, is presented explicitly, and the procedure described here
can be readily generalized to other magnetic insulators.",2103.04223v1
2021-03-25,Observation of magnetic helicoidal dichroism with extreme ultraviolet light vortices,"We report on the experimental evidence of magnetic helicoidal dichroism,
observed in the interaction of an extreme ultraviolet vortex beam carrying
orbital angular momentum with a magnetic vortex. Numerical simulations based on
classical electromagnetic theory show that this dichroism is based on the
interference of light modes with different orbital angular momenta, which are
populated after the interaction between the light phase chirality and the
magnetic topology. This observation gives insight into the interplay between
orbital angular momentum and magnetism, and sets the framework for the
development of new analytical tools to investigate ultrafast magnetization
dynamics.",2103.13697v1
2021-04-02,Surface plasmon-enhanced photo-magnetic excitation of spin dynamics in Au/YIG:Co magneto-plasmonic crystals,"We report strong amplification of photo-magnetic spin precession in Co-doped
YIG employing a surface plasmon excitation in a metal-dielectric
magneto-plasmonic crystal. Plasmonic enhancement is accompanied by the
localization of the excitation within the 300~nm-thick layer inside the
transparent dielectric garnet. Experimental results are nicely reproduced by
numerical simulations of the photo-magnetic excitation. Our findings
demonstrate the magneto-plasmonic concept of subwavelength localization and
amplification of the photo-magnetic excitation in dielectric YIG:Co and open up
a path to all-optical magnetization switching below diffraction limit with
energy efficiency approaching the fundamental limit for magnetic memories.",2104.01250v1
2021-04-14,An MHD Modeling of Successive X2.2 and X9.3 Solar Flares of 2017 September 6,"Solar active region 12673 produced two successive X-class flares (X2.2 and
X9.3) approximately 3 hours apart in September 2017. The X9.3 was the recorded
largest solar flare in Solar Cycle 24. In this study we perform a
data-constrained magnetohydrodynamic simulation taking into account the
observed photospheric magnetic field to reveal the initiation and dynamics of
the X2.2 and X9.3 flares. According to our simulation, the X2.2 flare is first
triggered by magnetic reconnection at a local site where at the photosphere the
negative polarity intrudes into the opposite-polarity region. This magnetic
reconnection expels the innermost field lines upward beneath which the magnetic
flux rope is formed through continuous reconnection with external twisted field
lines. Continuous magnetic reconnection after the X2.2 flare enhances the
magnetic flux rope, which is lifted up and eventually erupts via the torus
instability. This gives rise to the X9.3 flare.",2104.06639v1
2021-05-03,Light-Induced Control of Magnetic Phases in Kitaev Quantum Magnets,"Leveraging coherent light-matter interaction in solids is a promising new
direction towards control and functionalization of quantum materials, to
potentially realize regimes inaccessible in equilibrium and stabilize new or
useful states of matter. We show how driving the strongly spin-orbit coupled
proximal Kitaev magnet $\alpha$-RuCl$_3$ with circularly-polarized light can
give rise to a novel ligand-mediated magneto-electric effect that both
photo-induces a large dynamical effective magnetic field and dramatically
alters the interplay of competing isotropic and anisotropic exchange
interactions. We propose that tailored light pulses can nudge the material
towards the elusive Kitaev quantum spin liquid as well as probe competing
magnetic instabilities far from equilibrium, and predict that the transient
competition of magnetic exchange processes can be readily observed via
pump-probe spectroscopy.",2105.01062v2
2021-05-20,A hierarchy of multi-moment skyrmion phases in twisted magnetic bilayers,"The recent discovery of two-dimensional (2D) Van der Waals (VdW) magnets is a
crucial turning point in the quantum magnet research field, since quantum
fluctuations and experimental difficulties often elude stable magnetic orders
in 2D. This opens new doors to delve for novel quantum and topological spin
configurations, which may or may not have direct analogs in bulk counterparts.
Here we study a twisted bilayer geometry of 2D magnets in which long-range
spin-spin interactions naturally commence along the inter-layer Heisenberg
($J_{\perp}$) and dipole-dipole ($J_{\rm D}$) channels. The $J_{\perp}-J_{\rm
D}$ parameter space unveils a hierarchy of distinct skyrmions phases, ranging
from point-, rod-, and ring-shaped topological charge distributions.
Furthermore, we predict a novel topological antiferroelectric phase, where
oppositely-charged antiskyrmion pairs are formed, and the corresponding
topological dipole moments become ordered in a N\'eel-like state $-$ hence
dubbed topological antiferroelectric state. The results indicate that twisted
magnetic layer provides a new setting to engineer and tune a plethora of novel
and exotic skyrmion phases and their dynamics.",2105.09521v1
2021-06-01,Magnetic flux expulsion in a superconducting wire,"An electric current generates a magnetic field, and magnetic fields cannot
exist in the interior of type I superconductors. As a consequence of these two
facts, electric currents can only flow near the surface of a type I
superconducting wire so that the self-field vanishes in the interior. Here we
examine how an electric current flowing through the entire cross-section of a
normal conducting wire becomes a surface current when it enters a
superconducting portion of the wire. This geometry provides insight into the
dynamics of magnetic flux expulsion that is not apparent in the Meissner effect
involving expulsion of an externally applied magnetic field. It provides clear
evidence that the motion of magnetic field lines in superconductors is
intimately tied to the motion of charge carriers, as occurs in classical
plasmas (Alfven's theorem) and as proposed in the theory of hole
superconductivity \cite{holesc}, in contradiction with the conventional
London-BCS theory of superconductivity.",2106.00262v1
2021-06-05,Radio afterglow of magnetars' giant flares,"We develop a model for the radio afterglow of the giant flare of SGR 1806-20
arising due to the interaction of magnetically-dominated cloud, an analogue of
Solar Coronal Mass Ejections (CMEs), with the interstellar medium (ISM). The
CME is modeled as a spheromak-like configuration. The CME is first advected
with the magnetar's wind and later interacts with the ISM, creating a strong
forward shock and complicated backwards exhaust flow. Using three-dimensional
magnetohydrodynamic simulations, we study various relative configurations of
the magnetic field of the CME with respect to the ISM's magnetic field. We show
that the dynamics of the forward shock mostly follows the Sedov-Taylor
blastwave, while the internal structure of the shocked medium is considerably
modified by the back flow, creating a multiple shock configuration. We
calculate synthetic synchrotron emissivity maps and light curves using two
assumptions: (i) magnetic field compression; (ii) amplification of the magnetic
field at the shock.We find that models with magnetic field amplification
account better for the observed radio emission.",2106.02918v3
2021-07-01,Van der Waals Magnet based Spin-Valve Devices at Room Temperature,"The discovery of van der Waals (vdW) magnets opened up a new paradigm for
condensed matter physics and spintronic technologies. However, the operations
of active spintronic devices with vdW magnets are so far limited to cryogenic
temperatures, inhibiting its broader practical applications. Here, for the
first time, we demonstrate room temperature spin-valve devices using vdW
itinerant ferromagnet Fe5GeTe2 in heterostructures with graphene. The tunnel
spin polarization of the Fe5GeTe2/graphene vdW interface is detected to be
significantly large ~ 45 % and negative at room temperature. Lateral spin-valve
device design enables electrical control of spin signal and realization of
basic building blocks for device application such as efficient spin injection,
transport, precession, and detection functionalities. Furthermore, measurements
with different magnetic orientations provide unique insights into the magnetic
anisotropy of Fe5GeTe2 and its relation with spin polarization and dynamics in
the heterostructure. These findings open opportunities for the applications of
vdW magnet-based all-2D spintronic devices and integrated spin circuits at
ambient temperatures.",2107.00310v1
2021-07-09,Spontaneous orbital magnetization of mesoscopic dipole dimers,"Ensembles of gold nanoparticles present a magnetic behavior which is at odds
with the weakly diamagnetic response of bulk gold. In particular, an unusual
ferromagnetic order has been unveiled by several experiments. Here we
investigate if the combined effect of orbital magnetism of conduction electrons
and interparticle dipolar interaction can lead to magnetic ordering. Using
different model systems of interacting mesoscopic magnetic dipoles, together
with a microscopic description of the electron dynamics within the
nanoparticles, we find that a spontaneous magnetic moment may arise in dimers
of metallic nanoparticles when the latter are characterized by a large orbital
paramagnetic susceptibility.",2107.04508v3
2021-07-15,The Magnetic Mechanism for Hotspot Reversals in Hot Jupiter Atmospheres,"Magnetically-driven hotspot variations (which are tied to atmospheric wind
variations) in hot Jupiters are studied using non-linear numerical simulations
of a shallow-water magnetohydrodynamic (SWMHD) system and a linear analysis of
equatorial SWMHD waves. In hydrodynamic models, mid-to-high latitude
geostrophic circulations are known to cause a net west-to-east equatorial
thermal energy transfer, which drives hotspot offsets eastward. We find that a
strong toroidal magnetic field can obstruct these energy transporting
circulations. This results in winds aligning with the magnetic field and
generates westward Lorentz force accelerations in hotspot regions, ultimately
causing westward hotspot offsets. In the subsequent linear analysis we find
that this reversal mechanism has an equatorial wave analogy in terms of the
planetary scale equatorial magneto-Rossby waves. We compare our findings to
three-dimensional MHD simulations, both quantitively and qualitatively,
identifying the link between the mechanics of magnetically-driven hotspot and
wind reversals. We use the developed theory to identify physically-motivated
reversal criteria, which can be used to place constraints on the magnetic
fields of ultra-hot Jupiters with observed westward hotspots.",2107.07515v1
2021-07-28,Transport theory and spin-transfer physics for frustrated magnets,"We study the electron dynamics in magnetic conductors with frustrated
interactions dominated by isotropic exchange. We present a transport theory for
itinerant carriers built upon the (single-band) doped Hubbard model and the
slave-boson formalism, which incorporates the spin-exchange with the
magnetically frustrated background into the representation of electron
operators in a clear and controllable way. We also formulate hydrodynamic
equations for the itinerant charge and spin degrees of freedom, whose currents
contain new contributions that depend on the spatiotemporal variations of the
order parameter of the frustrated magnet, which are described by Yang-Mills
fields. Furthermore, we elucidate the transfer of angular momentum from the
itinerant charge fluid to the magnet (i.e., the spin-transfer torque) via
reciprocity arguments. A detailed microscopic derivation of our effective
theory is also provided for one of the simplest models of frustrated magnetism,
namely the Heisenberg antiferromagnet on a triangular lattice. Our findings
point towards the possibility of previously unanticipated Hall physics in these
frustrated platforms.",2107.13330v2
2021-08-31,Kinematic dynamos in triaxial ellipsoids,"Planetary magnetic fields are generated by motions of electrically conducting
fluids in their interiors. The dynamo problem has thus received much attention
in spherical geometries, even though planetary bodies are non-spherical. To go
beyond the spherical assumption, we develop an algorithm that exploits a fully
spectral description of the magnetic field in triaxial ellipsoids to solve the
induction equation with local boundary conditions (i.e. pseudo-vacuum or
perfectly conducting boundaries). We use the method to compute the free-decay
magnetic modes and to solve the kinematic dynamo problem for prescribed flows.
The new method is thoroughly compared with analytical solutions and standard
finite-element computations, which are also used to model an insulating
exterior. We obtain dynamo magnetic fields at low magnetic Reynolds numbers in
ellipsoids, which could be used as simple benchmarks for future dynamo studies
in such geometries. We finally discuss how the magnetic boundary conditions can
modify the dynamo onset, showing that a perfectly conducting boundary can
strongly weaken dynamo action, whereas pseudo-vacuum and insulating boundaries
often give similar results.",2109.03232v1
2021-10-04,Magnetized Ablative Rayleigh-Taylor Instability in 3-D,"3-D extended-MHD simulations of the magnetized ablative Rayleigh-Taylor
instability are presented for the first time. Previous 2-D simulations claiming
perturbation suppression by magnetic tension are shown to be misleading, as
they do not include the most unstable dimension. For perturbation modes along
the applied field direction, the magnetic field simultaneously reduces ablative
stabilization and adds magnetic tension stabilization; the stabilizing term is
found to dominate for applied fields >5T, with both effects increasing in
importance at short wavelengths. For modes perpendicular to the applied field,
magnetic tension does not directly stabilize the perturbation, but can result
in moderately slower growth due to the perturbation appearing 2-D (albeit in a
different orientation to 2-D ICF simulations). In cases where thermal ablative
stabilization is dominant the applied field increases the peak bubble-spike
height. Resistive diffusion is shown to be important for short wavelengths and
long timescales, reducing the effectiveness of tension stabilization.",2110.01666v1
2021-10-06,Strong large scale magnetic fields in rotating convection-driven dynamos: the important role of magnetic diffusion,"Natural dynamos such as planets and stars generate global scale magnetic
field despite the inferred presence of small scale turbulence. Such systems are
known as large scale dynamos and are typically driven by convection and
influenced by rotation. Previous numerical studies of rotating dynamos
generally find that the large scale magnetic field becomes weaker as the flow
becomes more turbulent. The underlying physical processes necessary for
sustaining so-called large scale dynamos is therefore still debated. Here we
use a suite of numerical simulations to show that strong large scale magnetic
fields can be generated in rotating convective turbulence provided that two
conditions are satisfied: (1) the flow remains rotationally constrained; and
(2) magnetic diffusion is important on the small convective length scale. These
findings are in agreement with previous asymptotic predictions and suggest that
natural dynamos might satisfy these two conditions.",2110.03086v2
2021-10-27,Magnetoconductivity of a metal with closed Fermi surface reconstructed by a biaxial density wave,"We investigate quantum dynamics and kinetics of a 2D conductor with closed
Fermi surface reconstructed by a biaxial density wave, in which electrons move
along a two-dimensional periodic net of semiclassical trajectories coupled by
the magnetic breakdown tunnelling under a strong magnetic field. We derive a
quasi-particle dispersion law and magnetoconductivity tensor. The
quasi-particle spectrum is found to be the alternating series of
two-dimensional magnetic energy bands with gaps between them. The longitudinal
magnetoconductivity shows giant oscillations with change of magnetic field,
while the Hall coefficient changes sign and is absent in a wide range of the
magnetic fields in between. Preliminary estimations show that the suggested
magnetoconductivity mechanism may be the origin of such behaviour of the Hall
coefficient vs. magnetic field, as observed in experiments in materials with
analogous topology of the Fermi surface, such as the high-Tc superconducting
cuprates.",2110.14265v1
2021-11-23,Pitch Angle Scattering of Fast Particles by Low Frequency Magnetic Fluctuations,"The adiabatic invariance of the magnetic moment during particle motion is of
fundamental importance to the dynamics of magnetized plasma. The related rate
of pitch angle scattering is investigated here for fast particles that
thermally stream through static magnetic perturbations. For a uniform magnetic
field with a localized perturbation it is found that the curvature parameter
$\kappa^2=\min(R_c/\rho_L)$ does not predict the level of pitch angle
scattering. Instead, based on numerical integration of particle orbits in
prescribed magnetic fields, we derive predictions for the particle scattering
rates, which can be characterized by the relative perturbation amplitude
$A\simeq \delta B/B$, and the particle Larmor radius normalized by the
field-aligned wavelength of the perturbations, $\rho_L/\lambda_{\|}$. Particles
with $\rho_L/\lambda_{\|}\simeq1$, are subject to strong pitch angle
scattering, while the level of scattering vanishes for both the limits of
$\rho_L/\lambda_{\|} \ll 1$ and $\rho_L/\lambda_{\|} \gg 1$. The results are
summarized in terms of a scattering operator, suitable for including the
described scattering in basic kinetic models.",2111.11993v1
2021-12-03,Magnetic field-induced deformation of the spin-density wave microphases in Ca$_3$Co$_2$O$_6$,"The frustrated triangular Ising magnet Ca$_3$Co$_2$O$_6$ has long been known
for an intriguing combination of extremely slow spin dynamics and peculiar
magnetic orders, such as the evenly-spaced non-equilibrium metamagnetic
magnetization steps and the long-wavelength spin density wave (SDW) order, the
latter of which is essentially an emergent crystal of solitons. Recently, an
elaborate field-cooling protocol to bypass the low-field SDW phase was proposed
to overcome the extraordinarily long timescale of spin relaxation that impeded
previous experimental studies in equilibrium, which may point to a deep
connection between the low-temperature slow relaxation and the cooling process
passing through the low-field SDW phase. As the first step to elucidate the
conjectured connection, we investigate the magnetic field-induced deformation
of the SDW state and incommensurate-commensurate transitions, thereby mapping
out the equilibrium in-field phase diagram for a realistic three-dimensional
lattice spin model by using Monte Carlo simulations. We also discuss
Ginzburg-Landau theory that includes several Umklapp terms as well as an
effective sine-Gordon model, which can qualitatively explain the observed
magnetic field-induced deformation of the SDW microphases.",2112.01701v2
2021-12-17,Macro-scale fast flow and magnetic field generation in 2-temperature relativistic electron-ion plasmas of astrophysical objects,"We have shown the simultaneous generation of macro-scale fast flows and
strong magnetic fields in the 2-temperature relativistic electron-ion plasmas
of astrophysical objects due to Unified Reverse Dynamo/Dynamo mechanism. The
resulting dynamical magnetic field amplification and/or flow acceleration is
directly proportional to the initial turbulent kinetic/magnetic (magnetic)
energy; the process is very sensitive to relativistically hot electron-ion
fraction temperature and magneto-fluid coupling. It is shown, that for
realistic physical parameters of White Dwarfs accreting hot astrophysical flow
/ Binary systems there always exists such a real solution of dispersion
relation for which the formation of dispersive strong super-Alfv\'enic
macro-scale flow/outflow with Alfv\'en Mach number $> 10^6$ and/or generation
of super-strong magnetic fields is guaranteed.",2112.09673v1
2022-02-01,Magnetic structures and electronic properties of cubic-pyrochlore ruthenates from first principles,"The magnetic ground states of $R_2$Ru$_2$O$_7$ and $A_2$Ru$_2$O$_7$ with $R=$
Pr, Gd, Ho, and Er, as well as $A=$ Ca, Cd are predicted devising a combination
of the cluster-multipole (CMP) theory and spin-density-functional theory
(SDFT). The strong electronic correlation effects are estimated by the
constrained-random-phase approximation (cRPA) and taken into account within the
dynamical-mean-field theory (DMFT). The target compounds feature $d$-orbital
magnetism on Ru$^{4+}$ and Ru$^{5+}$ ions for $R$ and $A$, respectively, as
well as $f$-orbital magnetism on the $R$ site, which leads to an intriguing
interplay of magnetic interactions in a strongly correlated system. We find
CMP+SDFT is capable of describing the magnetic ground states in these
compounds. The cRPA captures a difference in the screening strength between
$R_2$Ru$_2$O$_7$ and $A_2$Ru$_2$O$_7$ compounds, which leads to a qualitative
and quantitative understanding of the electronic properties within DMFT.",2202.00277v1
2022-02-08,"Spin-Phonon Relaxation in Magnetic Molecules: Theory, Predictions and Insights","Magnetic molecules have played a central role in the development of magnetism
and coordination chemistry and their study keeps leading innovation in
cutting-edge scientific fields such as magnetic resonance, magnetism,
spintronics, and quantum technologies. Crucially, a long spin lifetime well
above cryogenic temperature is a stringent requirement for all these
applications. In this chapter we review the foundations of spin relaxation
theory and provide a detailed overview of first-principles strategies applied
to the problem of spin-phonon relaxation in magnetic molecules. Firstly, we
present a rigorous formalism of spin-phonon relaxation based on open quantum
systems theory. These results are then used to derive classical
phenomenological relations based on the Debye model. Finally, we provide a
prescription of how to map the relaxation formalism onto existing electronic
structure methods to obtain a quantitative picture of spin-phonon relaxation.
Examples from the literature, including both transition metals and lanthanides
compounds, will be discussed in order to illustrate how Direct, Orbach and
Raman relaxation mechanisms can affect spin dynamics for this class of
compounds.",2202.03776v1
2022-02-13,Critical-layer instability of shallow water magnetohydrodynamic shear flows,"In this paper, the instability of shallow water shear flow with a sheared
parallel magnetic field is studied. Waves propagating in such magnetic shear
flows encounter critical levels where the phase velocity relative to the basic
flow $c-U(y)$ matches the Alfv\'en wave velocities $\pm B(y)/\sqrt{\mu\rho}$,
based on the local magnetic field $B(y)$, the magnetic permeability $\mu$ and
the mass density of the fluid $\rho$. It is shown that when the two critical
levels are close to each other, the critical layer can generate an instability.
The instability problem is solved, combining asymptotic solutions at large
wavenumbers and numerical solutions, and the mechanism of instability explained
using the conservation of momentum. For the shallow water MHD system, the paper
gives the general form of the local differential equation governing such
coalescing critical layers for any generic field and flow profiles, and
determines precisely how the magnetic field modifies the purely hydrodynamic
stability criterion based on the potential vorticity gradient in the critical
layer. The curvature of the magnetic field profile, or equivalently the
electric current gradient, $J' = - B''/\mu$ in the critical layer is found to
play a complementary role in the instability.",2202.06272v1
2022-04-09,Magnetized granular particles running and tumbling on $S^{1}$,"It has been shown that a nonvibrated magnetic granular system, when it is
feeded by means an altenating magnetic field, behaves with most of the
distinctive physical features of active matter systems. In this work we focus
our attention on the simplest granular system composed by a single magnetized
spherical particle allocated in a quasi one-dimensional circular channel that
receives energy from a magnetic field reservoir and transduces it into a
running and tumbling motion. The theoretical analysis based on the run and
tumble model on a circle of radius R forecasts the existence of a dynamical
phase transition between an erratic motion (disordered phase) when the
characteristic persistence length of the run and tumble motion, $\ell_{c} <
R/2$, to a persistent motion (ordered phase) when $\ell_{c}> R/2$. It is found
that the limiting behaviours of these phases correspond to a Brownian motion on
the circle and a simple uniform circular motion, respectively. It is
qualitatively shown that the lower magnetization of a particle, the larger
persistence lenght is. It is so at least within the experimental limit of
validity of our experiments. Our results show a very good agreement between
theory and experiment.",2204.04552v1
2022-04-12,Ferrofluid drop impacts and Rosensweig peak formation in a non-uniform magnetic field,"Vertical drop impacts of ferrofluids onto glass slides in a non-uniform
magnetic field have been studied using high-speed photography. Outcomes have
been classified based on the motion of the fluid-surface contact lines, and
formation of peaks (Rosensweig instabilities) which affect the height of the
spreading drop. The largest peaks are nucleated at the edge of a spreading
drop, similarly to crown-rim instabilities in drop impacts with conventional
fluids, and remain there for an extended time. Impact Weber numbers ranged from
18.0 to 489, and the vertical component of the B-field was varied between 0 and
0.37 T at the surface by changing the vertical position of a simple disc magnet
placed below the surface. The falling drop was aligned with the vertical
cylindrical axis of the 25 mm diameter magnet, and the impacts produced
Rosensweig instabilities without splashing. At high magnetic field strengths a
stationary ring of ferrofluid forms approximately above the outer edge of the
magnet.",2204.05523v1
2022-04-22,Resistive instabilities in sinusoidal shear flows with a streamwise magnetic field,"We investigate the linear stability of a sinusoidal shear flow with an
initially uniform streamwise magnetic field in the framework of incompressible
magnetohydrodynamics (MHD) with finite resistivity and viscosity. This flow is
known to be unstable to the Kelvin-Helmholtz instability in the hydrodynamic
case. The same is true in ideal MHD, where dissipation is neglected, provided
the magnetic field strength does not exceed a critical threshold beyond which
magnetic tension stabilizes the flow. Here, we demonstrate that including
viscosity and resistivity introduces two new modes of instability. One of these
modes, which we call a resistively-unstable Alfv\'en wave due to its connection
to shear Alfv\'en waves, exists for any nonzero magnetic field strength as long
as the magnetic Prandtl number $Pm < 1$. We present a reduced model for this
instability that reveals its excitation mechanism to be the negative eddy
viscosity of periodic shear flows described by Dubrulle & Frisch (1991).
Finally, we demonstrate numerically that this mode saturates in a
quasi-stationary state dominated by counter-propagating solitons.",2204.10875v1
2022-05-05,$Λ/{\bar Λ}$ Polarization and Splitting Induced by Rotation and Magnetic Field,"The global polarization of $\Lambda/{\bar \Lambda}$ and the splitting of
${\bar \Lambda}-\Lambda$ polarization induced by rotation and magnetic field
has been investigated in a dynamical quark model by taking into account the
axial vector interaction and the anomalous magnetic moment of quarks. It is
found that the rotation leads to the spin polarization of quarks and
anti-quarks with the same sign, while the magnetic field to opposite sign,
which corresponds to the ${\bar \Lambda}-\Lambda$ polarization splitting. The
combination of the two effects leads to perfect agreement with experiment data.
Quantitatively, the axial vector spin polarization contributes 30$\%$ of the
global polarization and the anomalous magnetic moment of quarks contributes
40$\%$ to the splitting of ${\bar \Lambda}-\Lambda$ polarization. However, at
$\sqrt{s_{NN}} \leq 7.7 \text{GeV}$, it still remains a challenge to reach
enough magnitude of the magnetic field at freeze-out.",2205.02420v3
2022-05-16,Twisting the near-field radiative heat switch in hyperbolic antiferromagnets,"We study the twisted control of the near-field radiative heat transfer
between two hyperbolic antiferromagnetic insulators under external magnetic
fields. We show that the near-field heat flux can be affected by both the twist
angle $\theta$ and the magnitude of the applied magnetic field with different
broken symmetries. Irrespective of twist angle, the external magnetic field
causes the radiative heat flux to change nonmonotonically, and the minimum heat
flux can be found with the magnetic fields of about 1.5 T. Such nonmonotonic
behavior is due to the fact that the magnetic field can radically change the
nature of the magnon polaritons with time reversal symmetry breaking. The field
not only affects the topological structure of surface magnon polaritons, but
also induces the volume magnon polaritons that progressively dominate the heat
transfer as the field increases. We further propose a twist-induced thermal
switch device with inversion symmetry breaking, which can severely regulate
radiative heat flux through different magnetic fields. Our findings account for
a characteristic modulation of radiative heat transfer with implications for
applications in dynamic thermal management.",2205.07648v1
2022-06-14,Scaling of the Hosking integral in decaying magnetically-dominated turbulence,"The Saffman helicity invariant of Hosking and Schekochihin (2021, PRX 11,
041005), which we here call the Hosking integral, has emerged as an important
quantity that may govern the decay properties of magnetically dominated
nonhelical turbulence. Using a range of different computational methods, we
confirm that this quantity is indeed gauge-invariant and nearly perfectly
conserved in the limit of large Lundquist numbers. For direct numerical
simulations with ordinary viscosity and magnetic diffusivity operators, we find
that the solution develops in a nearly self-similar fashion. In a diagram
quantifying the instantaneous decay coefficients of magnetic energy and
integral scale, we find that the solution evolves along a line that is indeed
suggestive of the governing role of the Hosking integral. The solution settles
near a line in this diagram that is expected for a self-similar evolution of
the magnetic energy spectrum. The solution will settle in a slightly different
position when the magnetic diffusivity decreases with time, which would be
compatible with the decay being governed by the reconnection time scale rather
than the Alfv\'en time.",2206.07513v2
2022-06-19,Equation of state of fermions in neutron stars,"Employing both the number density and energy density, the equation of state
of fermions for each microscopic region of a neutron star is derived in the
presence of various statistical effects. It is computed as functions of
temperature (T), chemical potential (\mu) and magnetic field (B) based on the
statistical properties and corresponding phases of neutron stars. It is
revealed that the measurable properties describing the dynamic behavior of a
neutron star are determined by the unusual statistical conditions. It is
demonstrated that, in a strong magnetic field (B larger than\mu), the pressure
(P) shows a logarithmic dependence on B (P is proportional to logarithm
of(\mu/B). It is found that the regions with a higher (lower) magnetic field
(density) have a shorter (longer) life time. Moreover, the relationship between
the magnetic field and chemical potential confirms that the large magnetic
field and high density are short lived as compared to low magnetic field and
low density at the same temperature.",2206.09486v1
2022-06-28,Impact of magnetic field on giant dipole resonance of $^{40}$Ca using the EQMD model,"By taking into account the magnetic field in the extended quantum molecular
dynamics model (EQMD), we analyzed its effects on giant dipole resonance (GDR)
by studying the responses and strengths of the dipole moments. The selected
system is the $^{40}$Ca nucleus which is excited through the Coulomb
interaction by $^{16}$O. Particle acceleration term in Li\'enard-Wiechert
potential is discussed which, however, has small impact on magnetic field. The
peak energy, strength and width of GDR, temperature, and angular momentum of
$^{40}$Ca as a function of beam energy are investigated. It is found that the
magnetic field enhances the peak energy, strength and width of GDR which is not
only due to the temperature effects but also due to the enhancement of the
angular momentum of nucleus. At beam energy {E} $>$ 200 MeV/nucleon, magnetic
field maintains a constant value for the strength of GDR. The work sheds light
on examining important roles of the magnetic field on nuclear structure in
low-intermediate energy heavy-ion collisions.",2206.13921v1
2022-07-05,Noncollinear magnetism in two-dimensional CrTe$_2$,"The discovery of two-dimensional (2D) van der Waals magnets opened
unprecedented opportunities for the fundamental exploration of magnetism in
quantum materials and the realization of next generation spintronic devices.
Here, based on a multiscale modelling approach that combines first-principles
calculations and a Heisenberg model supplied with ab-initio parameters, we
report a strong magnetoelastic coupling in a free-standing monolayer of
CrTe$_2$. We demonstrate that different crystal structures of a single CrTe$_2$
give rise to non-collinear magnetism through magnetic frustration and emergence
of the Dzyaloshinskii-Moriya interaction (DMI). Utilizing atomistic spin
dynamics, we perform a detailed investigation of the complex magnetic
properties pertaining to this 2D material impacted by the presence of various
types of structural distortions akin to charge density waves.",2207.01926v2
2022-07-08,Experimental Evidence of Plasmoids in High-$β$ Magnetic Reconnection,"Magnetic reconnection is a ubiquitous and fundamental process in plasmas by
which magnetic fields change their topology and release magnetic energy.
Despite decades of research, the physics governing the reconnection process in
many parameter regimes remains controversial. Contemporary reconnection
theories predict that long, narrow current sheets are susceptible to the
tearing instability and split into isolated magnetic islands (or plasmoids),
resulting in an enhanced reconnection rate. While several experimental
observations of plasmoids in the regime of low- to intermediate-$\beta$ (where
$\beta$ is the ratio of plasma thermal pressure to magnetic pressure) have been
made, there is a relative lack of experimental evidence for plasmoids in the
high-$\beta$ reconnection environments which are typical in many space and
astrophysical contexts. Here, we report the observation of strong experimental
evidence for plasmoid formation and dynamics in laser-driven high-$\beta$
reconnection experiments.",2207.04119v1
2022-07-16,Magnetization Switching of Single Magnetite Nanoparticles Monitored Optically,"Magnetic nanomaterials record information as fast as picoseconds in computer
memories but retain it for millions of years in ancient rocks. This exceedingly
broad range of times is covered by hopping over a potential energy barrier
through temperature, ultrafast optical excitation for demagnetization or
magnetization manipulation, mechanical stress, or microwaves. As switching
depends on nanoparticle size, shape, orientation, and material properties, only
single-nanoparticle studies can eliminate ensemble heterogeneity. Here, we push
the sensitivity of photothermal magnetic circular dichroism down to individual
20-nm magnetite nanoparticles. Single-particle magnetization curves display
superparamagnetic to ferromagnetic behaviors, depending on size, shape, and
orientation. Some nanoparticles undergo thermally activated switching on time
scales of milliseconds to minutes. Surprisingly, the switching barrier appears
to vary in time, leading to dynamical heterogeneity. Our observations will help
to identify and eventually control the nanoscale parameters influencing the
switching of magnetic nanoparticles, an important step for applications in many
fields.",2207.07866v2
2022-07-18,Nutational switching in ferromagnets and antiferromagnets,"It was demonstrated recently that on ultrashort time scales magnetization
dynamics does not only exhibit precession but also nutation. Here, we
investigate how nutation can contribute to spin switching leading towards
ultrafast data writing. We use analytic theory and atomistic spin simulations
to discuss the behavior of ferromagnets and antiferromagnets in high-frequency
magnetic fields. In ferromagnets, linearly polarized fields align the
magnetization perpendicular to the external field, enabling $90^{\circ}$
switching. For circularly polarized fields in the $xy$ plane, the magnetization
tilts to the $z$ direction. During this tilting, it rotates around the $z$
axis, allowing $180^{\circ}$ switching. In antiferromagnets, external fields
with frequencies higher than the nutation frequency align the order parameter
parallel to the field direction, while for lower frequencies it is oriented
perpendicular to the field. The switching frequency increases with the magnetic
field strength, and it deviates from the Larmor frequency, making it possible
to outpace precessional switching in high magnetic fields.",2207.08566v1
2022-08-15,Tunable Strong Magnetic Anisotropy in Two-Dimensional van der Waals Antiferromagnets,"We show that anisotropic energy of a 2D antiferromagnet is greatly enhanced
via stacking on a magnetic substrate layer, arising from the
sublattice-dependent interlayer magnetic interaction that defines an effective
anisotropic energy. Interestingly, this effective energy couples strongly with
the interlayer stacking order and the magnetic order of the substrate layer,
providing unique mechanical and magnetic means to control the antiferromagnetic
order. These two types of control methods affect distinctly the sublattice
magnetization dynamics, with a change of the ratio of sublattice precession
amplitudes in the former and its chirality in the later. In moir\'{e}
superlattices formed by a relative twist or strain between the layers, the
coupling with stacking order introduces a landscape of effective anisotropic
energy across the moir\'{e}, which can be utilized to create nonuniform
antiferromagnetic textures featuring periodically localized low-energy magnons.",2208.06958v1
2022-09-18,Observation of axisymmetric standard magnetorotational instability in the laboratory,"We report the first direct evidence for the axisymmetric standard
magnetorotational instability (SMRI) from a combined experimental and numerical
study of a magnetized liquid-metal shear flow in a Taylor-Couette cell with
independently rotating and electrically conducting end caps. When a uniform
vertical magnetic field $B_i$ is applied along the rotation axis, the measured
radial magnetic field $B_r$ on the inner cylinder increases linearly with a
small magnetic Reynolds number $Rm$ due to the magnetization of the residue
Ekman circulation. Onset of the axisymmetric SMRI is identified from the
nonlinear increase of $B_r$ beyond a critical $Rm$ in both experiments and
nonlinear numerical simulations. The axisymmetric SMRI exists only at
sufficiently large $Rm$ and intermediate $B_i$, a feature consistent with
theoretical predictions. Our simulations further show that the axisymmetric
SMRI causes the velocity and magnetic fields to contribute an outward flux of
axial angular momentum in the bulk region, just as it should in accretion
disks.",2209.08457v1
2022-10-24,Reduction of energy cost of magnetization switching in a biaxial nanoparticle by use of internal dynamics,"A solution to energy-efficient magnetization switching in a nanoparticle with
biaxial anisotropy is presented. Optimal control paths minimizing the energy
cost of magnetization reversal are calculated numerically as functions of the
switching time and materials properties, and used to derive energy-efficient
switching pulses of external magnetic field. Hard-axis anisotropy reduces the
minimum energy cost of magnetization switching due to the internal torque in
the desired switching direction. Analytical estimates quantifying this effect
are obtained based on the perturbation theory. The optimal switching time
providing a tradeoff between fast switching and energy efficiency is obtained.
The energy cost of switching and the energy barrier between the stable states
can be controlled independently in a biaxial nanomagnet. This provides a
solution to the dilemma between energy-efficient writability and good thermal
stability of magnetic memory elements.",2210.13514v2
2022-10-27,Magnetic Solitons due to interfacial chiral interactions,"We study solitons in a zig-zag lattice of magnetic dipoles. The lattice
comprises two sublattices of parallel chains with magnetic dipoles at their
vertices. Due to orthogonal easy planes of rotation for dipoles belonging to
different sublattices, the total dipolar energy of this system is separable
into a sum of symmetric and chiral long-ranged interactions between the magnets
where the last takes the form of Dzyaloshinskii-Moriya coupling. For a specific
range of values of the offset between sublattices, the dipoles realize an
equilibrium magnetic state in the lattice plane, consisting of one chain
settled in an antiferromagnetic parallel configuration and the other in a
collinear ferromagnetic fashion. If the offset grows beyond this value, the
internal Dzyaloshinskii-Moriya field stabilizes two Bloch domain walls at the
edges of the antiferromagnetic chain. The dynamics of these solitons is studied
by deriving the long-wavelength lagrangian density for the easy axis
antiferromagnet. We find that the chiral couplings between sublattices give
rise to an effective magnetic field that stabilizes the solitons in the
antiferromagnet. When the chains displace respect to each other, an emergent
Lorentz force accelerates the domain walls along the lattice.",2210.15716v2
2022-11-20,Double Multiple-Relaxation-Time model of Lattice-Boltzmann Magnetohydrodynamics at Low Magnetic Reynolds Numbers,"We develop an improved lattice-Boltzmann numerical scheme to solve
magnetohydrodynamic (MHD) equations in the regime of low magnetic Reynolds
numbers, grounded on a manifestly Galilean covariant modeling of the
Navier-Stokes equations. The simulation of the magnetic induction equation
within the lattice-Boltzmann approach to MHD has been usually devised along the
lines of the simplest phenomenological description, the single relaxation time
(SRT) model. In order to deal with well-known stability difficulties of the SRT
framework, we introduce, alternatively, a multi-relaxation-time technique for
the solution of the magnetic induction equation, combined with a novel boundary
condition method to cope with the subtleties of magnetic Boltzmann-like
distributions on curved boundaries. As an application, we investigate open
issues related to the description of transient flow regimes in MHD pipe flows,
subject to non-uniform magnetic fields.",2211.11005v1
2022-11-23,Pressure-Strain Interaction: II. Decomposition in Magnetic Field-Aligned Coordinates,"In weakly collisional and collisionless magnetized plasmas, the
pressure-strain interaction describes the rate of conversion between bulk flow
and thermal energy density. In this study, we derive an analytical expression
for the pressure-strain interaction in a coordinate system with an axis aligned
with the local magnetic field. The result is eight groups of terms
corresponding to different physical mechanisms that can contribute to the
pressure-strain interaction. We provide a physical description of each term.
The results are immediately of interest to weakly collisional and collisionless
magnetized plasmas and the fundamental processes that happen therein, including
magnetic reconnection, magnetized plasma turbulence, and collisionless shocks.
The terms in the field-aligned coordinate decomposition are likely accessible
to measurement with satellite observations.",2211.13161v1
2023-01-19,Maple Leaf Antiferromagnet in a Magnetic Field,"We analyze the quantum antiferromagnet on the maple leaf lattice in the
presence of a magnetic field. Starting from its exact dimer ground state and
for a magnetic field strength of the order of the local dimer spin exchange
coupling, we perform a strong coupling expansion and extract an effective
hardcore boson model. The interplay of effective many-body interactions,
suppressed single-particle dynamics, and correlated hopping gives way to an
intriguing series of superfluid to insulator transitions which correspond to
magnetization plateaux in terms of the maple leaf spin degrees of freedom.
While we find plateaux at intermediate magnetization to be dominated by bosonic
density wave order, we conjecture plateau formation from multi-boson bound
states due to correlated hopping for lower magnetization.",2301.08264v2
2023-03-20,Free-induction-decay magnetic field imaging with a microfabricated Cs vapor cell,"Magnetic field imaging is a valuable resource for signal source localization
and characterization. This work reports an optically pumped magnetometer (OPM)
based on the free-induction-decay (FID) protocol, that implements
microfabricated cesium (Cs) vapor cell technology to visualize the magnetic
field distributions resulting from various magnetic sources placed close to the
cell. The slow diffusion of Cs atoms in the presence of a nitrogen (N$_{2}$)
buffer gas enables spatially independent measurements to be made within the
same vapor cell by translating a $175\,\mu$m probe beam over the sensing area.
For example, the OPM was used to record temporal and spatial information to
reconstruct magnetic field distributions in one and two dimensions. The optimal
magnetometer sensitivity was estimated to be 0.43$\,\mathrm{pT/\sqrt{Hz}}$
within a Nyquist limited bandwidth of $500\,$Hz. Furthermore, the sensor's
dynamic range exceeds the Earth's field of approximately $50\,\mu$T, which
provides a framework for magnetic field imaging in unshielded environments.",2303.10915v2
2023-05-12,Spacetime magnetic hopfions: from internal excitations and braiding of skyrmions,"Spatial topology endows topological solitons, such as skyrmions and hopfions,
with fascinating dynamics. However, the temporal dimension has so far provided
a passive stage on which topological solitons evolve. Here we construct
spacetime magnetic hopfions: magnetic textures in two spatial dimensions that
when excited by a time-periodic drive develop spacetime topology. We uncover
two complementary construction routes using skyrmions by braiding their center
of mass position and by controlling their internal low-energy excitations.
Spacetime magnetic hopfions can be realized in nanopatterned grids to braid
skyrmions and in frustrated magnets under an applied AC electric field. Their
topological invariant, the spacetime Hopf index, can be tuned by the applied
electric field as demonstrated by our collective coordinate modeling and
micromagnetic simulations. The principles we have introduced to actively
control spacetime topology are not limited to magnetic solitons, opening
avenues to explore spacetime topology of general order parameters and fields.",2305.07589v1
2023-07-10,Increasing Flips per Second and Speed of p-Computers by Using Dilute Magnetic Semiconductors to Implement Binary Stochastic Neurons,"Probabilistic computing with binary stochastic neurons (BSN) implemented with
low- or zero-energy barrier nanoscale ferromagnets (LBMs) possessing in-plane
magnetic anisotropy has emerged as an efficient paradigm for solving
computationally hard problems. The fluctuating magnetization of an LBM at room
temperature encodes a p-bit which is the building block of a BSN. Its only
drawback is that the dynamics of common (transition metal) ferromagnets are
relatively slow and hence the number of uncorrelated p-bits that can be
generated per second - the so-called ""flips per second"" (fps) - is
insufficient, leading to slow computational speed in autonomous co-processing
with p-computers. Here, we show that a simple way to increase fps is to replace
commonly used ferromagnets (e.g. Co, Fe, Ni), which have large saturation
magnetization Ms, with a dilute magnetic semiconductor like GaMnAs with much
smaller saturation magnetization. The smaller Ms reduces the energy barrier
within the LBM and increases the fps significantly. It also offers other
benefits such as increased packing density for increased parallelization and
reduced device to device variation. This provides a way to realize the hardware
acceleration and energy efficiency promise of p-computers.",2307.04663v1
2023-07-26,Symmetry of the emergent inductance tensor exhibited by magnetic textures,"Metals hosting gradually varying spatial magnetic textures are attracting
attention as a new class of inductor. Under the application of an alternating
current, the spin-transfer-torque effect induces oscillating dynamics of the
magnetic texture, which subsequently yields the spin-motive force as a back
action, resulting in an inductive voltage response. In general, a second-order
tensor representing a material's response can have an off-diagonal component.
However, it is unclear what symmetries the emergent inductance tensor has and
also which magnetic textures can exhibit a transverse inductance response. Here
we reveal both analytically and numerically that the emergent inductance tensor
should be a symmetric tensor in the so-called adiabatic limit. By considering
this symmetric tensor in terms of symmetry operations that a magnetic texture
has, we further characterize the magnetic textures in which the transverse
inductance response can appear. This finding provides a basis for exploring the
transverse response of emergent inductors, which has yet to be discovered.",2307.14542v2
2023-08-04,Probing the magnetic field strength dependence of the Chiral Magnetic Effect,"The article presents a study aimed at probing the dependence of the Chiral
Magnetic Effect (CME) on the magnetic field strength using the Anomalous
Viscous Fluid Dynamics (AVFD) model in Pb--Pb at LHC energies. The results
demonstrate the quadratic dependence of the correlators used for the study of
the CME in heavy ion collisions on the number of spectators, a proxy of the
magnitude of the magnetic field. The article also presents the extension of
this approach to a two dimensional space, formed by both the aforementioned
proxy of the magnetic field strength but also a proxy of the final state
ellipticity, a key ingredient of the background in these measurements, for each
centrality interval. This provides an exciting possibility to experiments to
isolate the background contributions from the potential CME signal.",2308.02361v3
2023-08-16,Quench Detection in a Superconducting Radio Frequency Cavity with Combine Temperature and Magnetic Field Mapping,"Local dissipation of RF power in superconducting radio frequency cavities
create so called hot spots, primary precursors of cavity quench driven by
either thermal or magnetic instability. These hot spots are detected by a
temperature mapping system, and a large increase in temperature on the outer
surface is detected during cavity quench events. Here, we have used combined
magnetic and temperature mapping systems using anisotropic magnetoresistance
(AMR) sensors and carbon resisters to locate the hot spots and areas with high
trapped flux on a 3.0 GHz single-cell Nb cavity during the RF tests at 2.0 K.
The quench location and hot spots were detected near the equator when the
residual magnetic field in the Dewar is kept < 1 mG. The hot spots and quench
locations moved when the magnetic field is trapped locally, as detected by
T-mapping system. No significant dynamics of trapped flux is detected by AMR
sensors, however, change in magnetic flux during cavity quench is detected by a
flux gate magnetometer, close to the quench location. The result provides the
direct evidence of hot spots and quench events due to localized trapped
vortices.",2308.08686v1
2023-11-06,2D Magnetic Heterostructures: Spintronics and Quantum Future,"The discovery of two-dimensional (2D) magnetism within atomically thin
structures derived from layered crystals has opened up a new realm for
exploring magnetic heterostructures. This emerging field provides a
foundational platform for investigating unique physical properties and
exquisite phenomena at the nanometer and molecular/atomic scales. By
engineering 2D interfaces using physical methods and selecting interlayer
interactions, we unlock the potential for extraordinary exchange dynamics. This
potential extends to high-performance and high-density magnetic memory
applications, as well as future advancements in neuromorphic and quantum
computing. This review delves into recent advances in 2D magnets, elucidates
the mechanisms behind 2D interfaces, and highlights the development of 2D
devices for spintronics and quantum information. Particular focus is placed on
2D magnetic heterostructures with topological properties, promising for a
resilient and low-error information system. Finally, we discuss the trends of
2D heterostructures for future electronics, considering the challenges and
opportunities from physics, material synthesis, and technological prospective.",2311.03505v1
2023-12-15,Laser Thomson Scattering Measurements around Magnetized Model in Rarefied Argon Arcjet Plume,"To elucidate the role of the Hall effect in magnetohydrodynamic (MHD)
aerobraking in rarefied flows,we measured the radial distributions of electron
temperature and density in front of a magnetized model in a rarefied argon
arcjet wind tunnel using the laser Thomson scattering method. We also developed
a water-cooled magnetized model to prevent thermal demagnetization during the
measurement. The measured electron density distributions were in excellent
agreement with computational fluid dynamics (CFD) predictions. It was also
found that the magnetic field had little effect on the electron density
distribution around the model. In the case without the magnetic field, the
measured electron temperature almost agreed with the CFD prediction. However,
the measured electron temperature increase caused by applying the magnetic
field was about 1,000 K less than that of the CFD prediction. This discrepancy
indicates that the location of an insulating boundary in the plasma is far from
the model.",2312.09593v2
2023-12-31,Spinterface Mediated Magnetic Properties of Co20Fe60B20/Alq3 Heterostructures,"Organic semiconductors (OSCs) are suitable materials for spintronics
applications as they form a spinterface when placed next to a ferromagnet,
which in turn leads to novel functionalities. The evolution of spinterface can
tune the global magnetic anisotropy, magnetization reversal, magnetization
dynamics, etc. Planar tris-(8-hydroxyquinoline)aluminum (Alq3) OSC has shown
tremendous potential for spintronics applications, thanks to its efficient
spin-polarized current transport ability. Here, we establish the spinterface
when the Alq3 molecules are deposited on amorphous ferromagnet
Co20Fe60B20(CFB). The $\pi$-d hybridization in CFB/Alq3 enhances the coercive
field and significantly modifies the shape and size of the magnetic domains. A
$\sim$100% increase in uniaxial anisotropic energies and a reduction in
magnetic damping are also evident owing to the strong interfacial
hybridization.",2401.00487v1
2024-02-12,Continuum of magnetic excitations in the Kitaev honeycomb iridate D$_3$LiIr$_2$O$_6$,"Inelastic neutron scattering (INS) measurements of powder
D$_3(^{7}$Li)($^{193}$Ir)$_2$O$_6$ reveal low energy magnetic excitations with
a scattering cross section that is broad in $|Q|$ and consistent with a Kitaev
spin-liquid (KSL) state. The magnetic nature of the excitation spectrum is
demonstrated by longitudinally polarized neutron studies. The total magnetic
moment of 1.7(2)$\mu_B$/Ir inferred from the total magnetic scattering cross
section is consistent with the effective moment inferred from magnetic
susceptibility data and expectations for the $J_{\rm eff}=1/2$ single ion
state. The rise in the dynamic correlation function ${\cal S}(Q,\omega)$ for
$\hbar\omega<5~$meV can be described by a nearest-neighbor Kitaev model with
interaction strength $K\approx-13(5)$~meV. Exchange disorder associated with
the mixed D-Li site could play an important role in stabilizing the low $T$
quantum fluctuating state.",2402.08083v1
2024-02-16,Tuning properties of phase-separated magnetic fluid with temperature,"Phase-separated magnetic fluids provide a very strong magnetic response
($\mu>25$) in a liquid state material. Even small fields can cause a notable
material response, but this depends on its properties, which are often
difficult to control. Here, we investigate how temperature affects the
properties of the system, where phase separation is induced by an increase in
ionic strength. Following the deformations of individual microscopic droplets,
we can extract surface tension, magnetic permeability, and viscosity. We find
that the temperature increase does not affect the surface tension, while the
magnetic permeability and viscosity increase. Applying this knowledge to induce
a shape instability for a drop only using temperature shows the potential of
applicability of our findings.",2402.11117v1
2024-02-21,Magnetic field and radial velocity fluxes at the initial stages of the evolution of solar active regions based on measurements at the photospheric level,"In this article, the physical processes occurring in the convective layer and
the photosphere of the Sun and their connection to the formation of active
regions (ARs) and the development of the corresponding magnetic field are
explored. Specifically, we test the magnetic flux emergence hypothesis and
based on the line-of-sight magnetic field and Doppler shift data obtained from
the Global Oscillation Network Group (GONG) observations. The study encompasses
the analysis of 24 ARs observed during the period from 2011 to 2022. We find a
strong correlation between the magnetic flux and the imbalance of radial
velocity fluxes. The results indicate that the magnetic flux emergence
hypothesis cannot fully explain the evolution of ARs during their early stages
of development.",2402.13806v1
2024-02-26,Oscillatory Hall effect from magnetoelectronic coupling in flexoelectronic silicon,"The magnetoelectronic coupling can be defined as cross-domain coupling
between electronic and magnetic properties, where modulation in magnetic
properties changes the electronic properties. In this letter, an explicit
experimental evidence of magnetoelectronic coupling is presented, which is
uncovered from oscillatory Hall effect response in Hall measurement. The strain
gradient in a MgO (1.8 nm)/p-Si (~400 nm) freestanding sample leads to transfer
of electrons (~5X10^18 cm^-3) from valence to conduction band due to
flexoelectronic charge separation in the p-Si layer. The resulting
flexoelectronic polarization gives rise to temporal magnetic moment from
dynamical multiferroicity. The external magnetic field changes the net temporal
magnetic moment, which causes modulations in charge carrier concentration and
oscillatory Hall effect. The period of oscillatory Hall response is 1.12 T,
which is attributed to the magnitude of temporal magnetic moment. The discovery
of oscillatory Hall effect adds a new member to the family of Hall effects.",2402.16330v1
2024-02-26,Discovery of magnetically guided metal accretion onto a polluted white dwarf,"Dynamically active planetary systems orbit a significant fraction of white
dwarf stars. These stars often exhibit surface metals accreted from debris
disks, which are detected through infrared excess or transiting structures.
However, the full journey of a planetesimal from star-grazing orbit to final
dissolution in the host star is poorly understood. Here, we report the
discovery that the cool metal polluted star WD0816-310 has cannibalized heavy
elements from a planetary body similar in size to Vesta, and where accretion
and horizontal mixing processes have clearly been controlled by the stellar
magnetic field. Our observations unveil periodic and synchronized variations in
metal line strength and magnetic field intensity, implying a correlation
between the local surface density of metals and the magnetic field structure.
Specifically, the data point to a likely persistent concentration of metals
near a magnetic pole. These findings demonstrate that magnetic fields may play
a fundamental role in the final stages of exoplanetary bodies that are recycled
into their white dwarf hosts.",2402.16526v1
2024-02-28,Plasma-induced magnetic phase in 3D $\mathrm{Mn^{II}-Nb^{IV}}$ octacyanidometalate with magnetic sponge behavior,"A new magnetic phase with $T_C = 72 \ \mathrm K$ was obtained by exposing the
three-dimensional $\mathrm{\{ [Mn^{II}(H_2O)_2]_2[Nb^{IV}(CN)_8] \cdot 4H_2O \}
_n}$ coordination ferrimagnet ($T_C = 49 \ \mathrm K$) to air, oxygen,
nitrogen, and argon-based plasma. The X-ray powder diffraction pattern revealed
that the unit cell shrank after plasma treatment, leading to a 20% enhancement
of the superexchange couplings, as estimated from the mean-field approximation
(MFA) model. Although no stable dehydrated form was found in the
thermogravimetric analysis, the observed changes are attributed to the removal
of crystallization water molecules. The plasma-induced magnetic phase could not
be obtained by exposing the studied material to 0% relative humidity during
dynamic vapor sorption. Instead, the material underwent a major structural
reorganization after dehydration, necessitating an extended MFA model to
reproduce the magnetic susceptibility. These findings demonstrate that
plasma-induced changes can create unique magnetic phases in molecule-based
systems that are otherwise unobtainable.",2402.18195v1
2024-03-19,Polarization Dynamics in Paramagnet of Charged Quark-Gluon Plasma,"It is commonly understood that the strong magnetic field produced in heavy
ion collisions is short-lived. The electric conductivity of the quark-gluon
plasma is unable to significantly extend the life time of magnetic field. We
propose an alternative scenario to achieve this: with finite baryon density and
spin polarization by the initial magnetic field, the quark-gluon plasma behaves
as a paramagnet, which may continue to polarize quark after fading of initial
magnetic field. We confirm this picture by calculations in both quantum
electrodynamics and quantum chromodynamics. In the former case, we find a
splitting in the damping rates of probe fermion with opposite spin component
along the magnetic field with the splitting parametrically small than the
average damping rate. In the latter case, we find a similar splitting in the
damping rates of probe quark with opposite spin components along the magnetic
field. The splitting is parametrically comparable to the average damping rate,
providing an efficient way of polarizing strange quarks by the quark-gluon
plasma paramagnet consisting of light quarks.",2403.12615v1
2024-04-03,Enhancing Phase Stability and Coercivity of MnAl $τ$-L$1_0$ with Fe and Ni Substitutions: \textit{ab initio} and Micromagnetic Modeling,"The binary manganese aluminium (MnAl) alloy with L$1_0$ crystal structure,
which exhibits exceptional magnetic properties, is a promising rare earth
element free permanent magnet material. However, according to experimental
reports, synthesizing it in a stable bulk form is extremely challenging. Here,
we propose and theoretically verify an alternative method of stabilizing the
material by partially substituting Mn and Al sites by Fe and Ni and identify
its stability, electronic structure, and magnetic properties from density
functional theory (DFT) calculations. When considering a fixed ($50\%$)-Ni, the
magnetic anisotropy increases with the increasing Fe content, but the trend is
opposite in terms of formation energy. The calculated formation energies
demonstrate that all the binary and quaternary compositions are stable. Through
the analysis of calculated elastic constants and phonon frequencies, we confirm
that all compositions are mechanically and dynamically stable. Most
importantly, both magnetic moment and magnetic anisotropy constant in $50\%$-Fe
substituted composition (equiatomic phase) increases significantly as compared
to the MnAl. Predicted coercivity of equiatomic phase is larger than parent
compound obtained by combining DFT computed parameters with micromagnetic
simulations.",2404.03051v1
2020-11-14,Dynamical Effective Field Model for Interacting Ferrofluids: II. The proper relaxation time and effects of dynamic correlations,"The recently proposed dynamical effective field model (DEFM) is
quantitatively accurate for describing dynamical magnetic response of
ferrofluids. In paper I it is derived under the framework of dynamical density
functional theory, via which the original ensemble of bare Brownian particles
is mapped to an ensemble of dressed particles. However, it remains to clarify
how the characteristic rotational relaxation time of a dressed particle,
denoted by $\tau_r$, is quantitatively related to that of a bare particle,
denoted by $\tau^0_r$. By building macro-micro connections via two different
routes, I reveal that under some gentle assumptions $\tau_r$ can be identified
with the long-time rotational self-diffusion time. I further introduce two
simple but useful integrated correlation factors, describing the effects of
quasi-static (adiabatic) and dynamic (nonadiabatic) inter-particle
correlations, respectively. In terms of both correlation factors I reformulate
the dynamic magnetic susceptibility in an illuminating and elegant form.
Remarkably, it shows that the macro-micro connection is established via two
successive steps: a dynamical coarse-graining with nonadiabatic effects
accounted for by the dynamic factor, followed by equilibrium statistical
mechanical averaging captured by the static factor. Surprisingly,
$\tau_r/\tau^0_r$ is found insensitive to changes of particle volume fraction.
I provide a physical picture to explain it. Furthermore, an empirical formula
is proposed to characterize the dependence of $\tau_r/\tau^0_r$ on
dipole-dipole interaction strength. The DEFM supplemented with this formula
leads to parameter-free predictions in good agreement with results from
Brownian dynamics simulations. The theoretical developments presented in this
paper may have important consequences to studies of ferrofluid dynamics in
particular and other systems modelled by DDFTs in general.",2011.07293v1
2016-12-07,Effects of uniaxial pressure on the quantum tunneling of magnetization in a high-symmetry Mn12 single-molecule magnet,"The symmetry of single-molecule magnets dictates their spin quantum dynamics,
influencing how such systems relax via quantum tunneling of magnetization
(QTM). By reducing a system's symmetry, through the application of a magnetic
field or uniaxial pressure, these dynamics can be modified. We report
measurements of the magnetization dynamics of a crystalline sample of the
high-symmetry [Mn12O12(O2CMe)16(MeOH)4]MeOH single-molecule magnet as a
function of uniaxial pressure applied either parallel or perpendicular to the
sample's ""easy"" magnetization axis. At temperatures between 1.8 and 3.3 K,
magnetic hysteresis loops exhibit the characteristic steplike features that
signal the occurrence of QTM. After applying uniaxial pressure to the sample in
situ, both the magnitude and field position of the QTM steps changed. The step
magnitudes were observed to grow as a function of pressure in both arrangements
of pressure, while pressure applied along (perpendicular to) the sample's easy
axis caused the resonant-tunneling fields to increase (decrease). These
observations were compared with simulations in which the system's Hamiltonian
parameters were changed. From these comparisons, we determined that parallel
pressure induces changes to the second-order axial anisotropy parameter as well
as either the fourth-order axial or fourth-order transverse parameter, or to
both. In addition, we find that pressure applied perpendicular to the easy axis
induces a rhombic anisotropy E ~ D/2000 per kbar that can be understood as
deriving from a symmetry-breaking distortion of the molecule.",1612.02396v3
2017-11-11,Tunable dynamic moduli of magnetic elastomers: from X-$μ$CT characterization to mesoscopic modeling,"Ferrogels and magnetoelastomers are composite materials obtained by embedding
magnetic particles of mesoscopic size in a crosslinked polymeric matrix. They
combine the reversible elastic deformability of polymeric materials with the
high responsivity of ferrofluids to external magnetic fields. These materials
stand out, for example, for large magnetostriction as well as significant
increase of the elastic moduli in the presence of external magnetic fields. By
means of X-ray micro-computed tomography, position and size of each magnetic
particle can be measured with a high degree of accuracy. We here use data
extracted from real magnetoelastic samples as input for coarse-grained
dipole-spring modeling and calculations to investigate magnetostriction,
stiffening, and changes in the normal modes spectrum. More precisely, we assign
to each particle a dipole moment proportional to its volume and set a
randomized network of springs between them that mimics the behavior of the
polymeric elastic matrix. Extending our previously developed methods, we
compute the resulting structural changes in the systems, their overall
distortions, as well as the frequency-dependent elastic moduli when magnetic
interactions are turned on. Particularly, with increasing magnetization, we
observe the formation of chain-like aggregates, resulting in significant
overall deformations. Interestingly, the static elastic moduli can first show a
slight decrease with growing amplitude of the magnetic interactions, before a
pronounced increase appears upon the chain formation. The change of the dynamic
moduli with increasing magnetization depends on the frequency and can even show
nonmonotonic behavior. Overall, we demonstrate how theory and experiments can
complement each other to learn more about the dynamic behavior of this
interesting class of materials.",1711.04165v1
2020-08-06,"Magnetic Energy Release, Plasma Dynamics, and Particle Acceleration during Relativistic Turbulent Magnetic Reconnection","In strongly magnetized astrophysical plasma systems, magnetic reconnection is
believed to be a primary process during which explosive energy release and
particle acceleration occur, leading to significant high-energy emission. Past
years have witnessed active development of kinetic modeling of relativistic
magnetic reconnection, supporting this magnetically dominated scenario. A much
less explored issue is the consequence of 3D dynamics, where turbulent
structures are naturally generated as various types of instabilities develop.
This paper presents a series of 3D, fully-kinetic simulations of relativistic
turbulent magnetic reconnection (RTMR) in positron-electron plasmas with system
domains much larger than kinetic scales. Our simulations start from a
force-free current sheet with several different modes of long wavelength
magnetic field perturbations, which drive additional turbulence in the
reconnection region. Because of this, the current layer breaks up and the
reconnection region quickly evolves into a turbulent layer filled with coherent
structures such as flux ropes and current sheets. We find that plasma dynamics
in RTMR is vastly different from their 2D counterparts in many aspects. The
flux ropes evolve rapidly after their generation, and can be completely
disrupted due to the secondary kink instability. This turbulent evolution leads
to superdiffusion behavior of magnetic field lines as seen in MHD studies of
turbulent reconnection. Meanwhile, nonthermal particle acceleration and
energy-release time scale can be very fast and do not strongly depend on the
turbulence amplitude. The main acceleration mechanism is a Fermi-like
acceleration process supported by the motional electric field, whereas the
non-ideal electric field acceleration plays a subdominant role. We discuss
possible observational implications of 3D RTMR in high-energy astrophysics.",2008.02743v2
2021-07-21,Magnetic field tuning of the spin dynamics in the magnetic topological insulators (MnBi$_{2}$Te$_{4}$)(Bi$_{2}$Te$_{3}$)$_{n}$,"We report a high frequency/high magnetic field electron spin resonance
(HF-ESR) spectroscopy study in the sub-THz frequency domain of the two
representatives of the family of magnetic topological insulators
(MnBi$_{2}$Te$_{4}$)(Bi$_{2}$Te$_{3}$)$_{n}$ with $n = 0$ and 1. The HF-ESR
measurements in the magnetically ordered state at a low temperature of $T = 4$
K combined with the calculations of the resonance modes showed that the spin
dynamics in MnBi$_{\text{4}}$Te$_{\text{7}}$ is typical for an anisotropic
easy-axis type ferromagnet (FM) whereas MnBi$_{\text{2}}$Te$_{\text{4}}$
demonstrates excitations of an anisotropic easy-axis type antiferromagnet
(AFM). However, by applying the field stronger than a threshold value $\sim 6$
T we observed in MnBi$_{\text{2}}$Te$_{\text{4}}$ a crossover from the AFM
resonance modes to the FM modes which properties are very similar to the
ferromagnetic response of MnBi$_{\text{4}}$Te$_{\text{7}}$. We attribute this
remarkably unusual effect unexpected for a canonical easy-axis AFM, which,
additionally, can be accurately reproduced by numerical calculations of the
excitation modes, to the closeness of the strength of the AFM exchange and
magnetic anisotropy energies which appears to be a very specific feature of
this compound. Our experimental data evidences that the spin dynamics of the
magnetic building blocks of these compounds, the Mn-based septuple layers
(SLs), is inherently ferromagnetic featuring persisting short-range FM
correlations far above the magnetic ordering temperature as soon as the SLs get
decoupled either by introducing a nonmagnetic quintuple interlayer, as in
MnBi$_{\text{4}}$Te$_{\text{7}}$, or by applying a moderate magnetic field, as
in MnBi$_{\text{2}}$Te$_{\text{4}}$, which may have an effect on the surface
topological band structure of these compounds.",2107.09929v1
2021-09-09,Numerical reverse engineering of general spin-wave dispersions: Bridge between numerics and analytics using a dynamic-matrix approach,"Modern problems in magnetization dynamics require more and more the numerical
determination of the spin-wave spectra and -dispersion in magnetic systems
where analytic theories are not yet available. Micromagnetic simulations can be
used to compute the spatial profiles and oscillation frequencies of spin-waves
in magnetic system with almost arbitrary geometry and different magnetic
interactions. Although numerical approaches are very versatile, they often do
not give the same insight and physical understanding as analytical theories.
For example, it is not always possible to decide whether a certain feature
(such as dispersion asymmetry, for example) is governed by one magnetic
interaction or the other. Moreover, since numerical approaches typically yield
the normal modes of the system, it is not always feasible to disentangle
hybridized modes. In this manuscript, we build a bridge between numerics and
analytics by presenting a methodology to calculate the individual contributions
to general spin-wave dispersions in a fully numerical manner. We discuss the
general form of any spin-wave dispersion in terms of the effective (stiffness)
fields produced by the modes. Based on a special type of micromagnetic
simulations, the numerical dynamic-matrix approach, we show how to calculate
each stiffness field in the respective dispersion law, separately for each
magnetic interaction. In particular, it becomes possible to disentangle
contributions of different magnetic interactions to the dispersion asymmetry in
systems where non-reciprocity is present. At the same time, dipolar-hybridized
modes can be easily disentangled. Since this method is independent of the
geometry or the involved magnetic interactions at hand, we believe it is
attractive for experimental and theoretical studies of magnetic systems where
there are no analytics yet, but also to aid the development of new analytical
theories.",2109.04521v2
2023-05-19,Secular dipole-dipole stability of magnetic binaries,"The presence of strong large-scale stable magnetic fields in a significant
portion of early-type stars, white dwarfs, and neutron stars is well
established. Despite this, the origins of these fields remain unresolved, with
leading propositions advocating fossil fields, mergers, and shear-driven
dynamos as the main mechanism. A potential key for further insight could lie in
the connection with binarity: notably, magnetism can play a role in the
long-term orbital and rotational dynamics of binaries. In gravitational wave
astronomy, the advanced sensitivity of upcoming detectors such as LISA and the
Einstein Telescope will enable the characterisation of the orbital inspirals of
compact systems, including their magnetic properties. A comprehensive
understanding of the dynamics of magnetism in these systems is required for the
interpretation of the gravitational wave signals and to avoid calibration
biases. Furthermore, this knowledge can be used to create new magnetic
population models and to provide insight into the nature of their internal
fields. The aim of this study is to investigate the secular spin precession
dynamics of binary systems under pure magnetic dipole interactions, focusing on
stars with strong, stable, dipolar fields. We employ an orbit-averaging
procedure for the spin equations and obtain an effective secular description.
By minimising the magnetic energy, we derive the configurations of equilibrium.
We show that among the four states, only one is stable, consisting of the spin
and magnetic axes of one star reversed with respect to the companions', and
orthogonal to the orbital plane. Our long-term stability results disagree with
usual methods, which tend to neglect orbital motion. Finally, we provide
analytical solutions for the system out of equilibrium, which can be used to
derive secular orbital evolution in the context of gravitational wave
astronomy.",2305.15429v1
1999-06-29,Magnetic Fields in Clusters: Theory vs. Observations,"It is now well established that the plasma in galaxy clusters is magnetized.
In some cases, at least, the field is strong enough to be dynamically
important. Perhaps this is the time to move past simple detection experiments,
and to work towards a general understanding of the strength, structure and
maintenance of the cluster field.",9906485v1
1996-09-24,"Comment on ``Generalized Dynamic Scaling for Critical Relaxation"" [Phys. Rev. Lett. 77, 679 (1996), cond-mat/9607013]","We point out that in nonequilibrium relaxational processes the magnetic field
of the initial state (instead of the initial magnetization) has to be
considered as the additional scaling field (related to the initial state).
Taking this into account the interpretation of the scaling behavior observed in
the paper by Zheng becomes more straightforward.",9609232v1
1996-12-27,The magnetic susceptibility of the t-J model at low hole doping,"We compute the dynamical magnetic susceptibility of the t-J model in its
commensurate flux phase at low hole doping. We compare the calculations with
experiments and exact diagonalization studies.",9612231v1
2002-08-21,Re-entrant spin glass and magnetoresistance in Co_{0.2}Zn_{0.8}Fe_{1.6}Ti_{0.4}O_4 spinel oxide,"We have investigated the static and dynamic response of magnetic clusters in
Co_{0.2}Zn_{0.8}Fe_{1.6}Ti_{0.4}O_4 spinel oxide, where a sequence of magnetic
phase transitions, i.e., paramagnetic (PM) to ferromagnetic at T_{C} $\leq$
270K and ferromagnetic to canted spin glass state at T_f$ $\leq$ 125K is
observed.",0208416v1
2002-10-31,Dipolar corrections to the static magnetic susceptibility of condensed $^3$He,"We examine the consequences of a recent model describing correlated
zero-point polarization of the electronic cloud in solid $^3$He. This
polarization arises from the highly anisotropic and correlated dynamic mixing
of the $s$ and $p$ electronic levels ($\sim1$%). The magnetic polarization
introduces a small paramagnetic correction, of $1-0.1$%, to the static
susceptibility of condensed $^3$He. This correction could explain recent
measurements in liquid $^3$He.",0210705v1
2004-03-29,Correlation effects in Co/Cu and Fe/Cr magnetic multilayers,"The electronic structure of Co/Cu(001) and Fe/Cr(001) magnetic multilayers
has been investigated within the local density approximation combined with
dynamical mean field theory. Our calculation shows enhanced density of states
at the Fermi level, suggesting that electronic correlations might play an
important role in the transport properties of multilayers.",0403685v1
2005-04-28,To the theory of interaction between electron and nuclear systems,"Coupled electron-nuclear oscillations in antiferromagnetics with anisotropy
of ""light plane"" type in the strong external magnetic field are under
consideration. The new mode \epsilon_{2k} of these coupled oscillations is
obtained for antiferromagnetic systems by using ""u-v""-Bogolubov?s unitary
transformations. The dynamic shift of the frequency of nuclear magnetic
resonance concerned with this mode is obtained.",0504759v1
2005-05-10,Fluctuation-dissipation considerations and damping models for ferromagnetic materials,"The role of fluctuation-dissipation relations (theorems) for the
magnetization dynamics with Landau-Lifshitz-Gilbert and Bloch-Bloembergen
damping terms are discussed. We demonstrate that the use of the Callen-Welton
fluctuation-dissipation theorem that was proven for Hamiltonian systems can
give an inconsistent result for magnetic systems with dissipation.",0505259v1
2007-02-05,Rabi oscillations from ultrasound in spin systems,"It is shown that ultrasound in the GHz range can generate space-time Rabi
oscillations between spin states of molecular magnets. We compute dynamics of
the magnetization generated by surface acoustic waves and discuss conditions
under which this novel quantum effect can be observed.",0702116v1
2002-09-26,Precession of a particle with anomalous magnetic moment in electromagnetic and gravitational pp-wave fields,"We present an exact solution of the Bargmann-Michel-Telegdi (BMT) equations
for the dynamics of a spin particle in external electromagnetic and
gravitational pp-wave fields. We demonstrate that an anomalous magnetic moment
gives rise to an additional spin rotation which is modulated both by the
electromagnetic and by the gravitational wave periodicities.",0209099v1
2003-08-26,Precision Measurement of the Anomalous Magnetic Moment of the Muon,"A precision measurement of the anomalous magnetic moment (g-2) of the muon
has been made by the E821 collaboration at BNL. This paper is a written version
of a ``hot topics'' talk at ICAP2002 (Cambridge, MA). The principles of the
experiment are discussed, especially the beam dynamics aspects.",0308064v1
1997-04-24,Chiral Symmetry Breaking in QED in a Magnetic Field at Finite Temperature,"The catalysis of chiral symmetry breaking in the massless weakly coupled QED
in a magnetic field at finite temperature is studied. The temperature of the
symmetry restoration is estimated analytically as $T_c\simeq m_{dyn}$, where
$m_{dyn}$ is the dynamical mass of a fermion at zero temperature.",9704394v1
2007-02-01,Progress on chiral symmetry breaking in a strong magnetic field,"The problem of chiral symmetry breaking in QED in a strong magnetic field is
briefly reviewed. Recent progress on issues regarding the gauge fixing
independence of the dynamically generated fermion mass is discussed.",0702010v1
1995-12-15,Chiral Symmetry Breaking by a Magnetic Field in Weak-coupling QED,"Using the nonperturbative Schwinger-Dyson equation, we show that chiral
symmetry in weak-coupling massless QED is dynamically broken by a constant but
arbitrarily strong external magnetic field.",9512114v1
1997-05-26,Magnetic Vortices in a Gauged O(3) Sigma Model with Symmetry Breaking Self-Interaction,"We consider a (2+1) dimensional nonlinear O(3) sigma model with its U(1)
subgroup gauged along with the inclusion of a self-interaction having symmetry
breaking minima.The gauge field dynamics is governed by the Maxwell term.The
model is shown to support topologically stable purely magnetic self-dual
vortices.",9705196v1
2004-12-22,Further Investigation on Chiral Symmetry Breaking in a Uniform External Magnetic Field,"We study chiral symmetry breaking in QED when a uniform external magnetic
field is present. We calculate higher order corrections to the dynamically
generated fermion mass and find them to be small. In so doing we correct an
error in the literature regarding the matrix structure of the fermion
self-energy.",0412278v1
2004-05-31,The longitudinal KAM-cocycle of a magnetic flow,"Let $M$ be a closed oriented surface of negative Gaussian curvature and let
$\Omega$ be a non-exact 2-form. Let $\lambda$ be a small positive real number.
We show that the longitudinal KAM-cocycle of the magnetic flow given by $\la
\Omega$ is a coboundary if and only if the Gaussian curvature is constant and
$\Omega$ is a constant multiple of the area form.",0405598v1
2002-01-08,Spectral cross correlations of magnetic edge states,"We observe strong, non-trivial cross-correlations between the edge states
found in the interior and the exterior of magnetic quantum billiards. Our
analysis is based on a novel definition of the edge state spectral density
which is rigorous, practical and semiclassically accessible.",0201010v1
2002-11-05,Form Factor of a Quantum Graph in a Weak Magnetic Field,"Using periodic orbit theory, we evaluate the form factor of a quantum graph
to which a very weak magnetic field is applied. The first correction to the
diagonal approximation describing the transition between the universality
classes is shown to be in agreement with Pandey and Mehta's formula of
parametric random matrix theory.",0211006v1
2003-06-24,On triple correlations in isotropic electron magnetohydrodynamic turbulence,"The evolution of the correlation characteristics in three-dimensional
isotropic electron magnetohydrodynamic turbulence is investigated. Universal
exact relations between the longitudinal and transverse two-point triple
correlations of the components of the fluctuational magnetic fields and the
rates of dissipation of the magnetic helicity and energy are obtained in the
inertial range.",0306045v2
2004-01-29,Intermittency and the passive nature of the magnitude of the magnetic field,"It is shown that the statistical properties of the magnitude of the magnetic
field in turbulent electrically conducting media resemble, in the inertial
range, those of passive scalars in fully developed three-dimensional fluid
turbulence. This conclusion, suggested by the data from Advanced Composition
Explorer, is supported by a brief analysis of the appropriate
magnetohydrodynamic equations.",0401043v2
2004-09-23,Variational formulations of exact and reduced Vlasov-Maxwell equations,"The foundations of gyrokinetic theory are reviewed with an emphasis on the
applications of Lagrangian and Hamiltonian methods used in the derivation of
nonlinear gyrokinetic Vlasov-Maxwell equations. These reduced dynamical
equations describe the turbulent evolution of low-frequency electromagnetic
fluctuations in nonuniform magnetized plasmas with arbitrary magnetic geometry.",0409119v1
2005-03-02,Quantum Magnetohydrodynamics,"The quantum hydrodynamic model for charged particle systems is extended to
the cases of non zero magnetic fields. In this way, quantum corrections to
magnetohydrodynamics are obtained starting from the quantum hydrodynamical
model with magnetic fields. The quantum magnetohydrodynamics model is analyzed
in the infinite conductivity limit. The conditions for equilibrium in ideal
quantum magnetohydrodynamics are established. Translationally invariant exact
equilibrium solutions are obtained in the case of the ideal quantum
magnetohydrodynamic model.",0503021v1
2005-06-08,Variational principle for linear stability of moving magnetized plasma,"The variational principle for linear stability of three-dimensional,
inhomogenious, compressible, moving magnetized plasma is suggested. The
principle is ``softer'' (easier to be satisfied) than all previously known
variational stability conditions. The key point of the analysis is a
conservation in variations of new integrals inherent in the linearized equation
of the motion that was not earlier discussed in the literature.",0506073v1
1995-08-22,"Superconducting, magnetic, and charge correlations in the doped two-chain Hubbard model","Superconducting, magnetic, and charge correlation functions and dynamic spin
correlation functions of the doped two-chain Hubbard model is studied with the
projector Quantum Monte carlo method and Lanczos recursion method. Of the three
correlation functions, the interchain singlet superconducting correlation
function is the most long range. Our data is not consistent with the
Luther-Emery picture.",9508003v1
2007-05-11,Measurement of electro-magnetic radiation at PHENIX,"Recent results on direct photons and dileptons from the PHENIX xperiment
opened up a possibility of landscaping electro-magnetic radiation over various
kinetic energies in heavy ion collisions. A detail discussion is given based on
a review of the results.",0705.1711v1
2008-05-28,Vanishing beta function for Grosse-Wulkenhaar model in a magnetic field,"We prove that the beta function of the Grosse-Wulkenhaar model including a
magnetic field vanishes at all order of perturbations. We compute the
renormalization group flow of the relevant dynamic parameters and find a
non-Gaussian infrared fixed point. Some consequences of these results are
discussed.",0805.4362v1
2008-08-29,Closed Magnetic Geodesics on $S^2$,"We give existence results for simple closed curves with prescribed geodesic
curvature on $S^{2}$, which correspond to periodic orbits of a charge in a
magnetic field.",0808.4038v4
2008-11-04,Angular Momentum Loss by Magnetic Braking and Gravitational Radiation in Relativistic Binary Stars,"Angular momentum loss (AML) mechanisms and dynamical evolution owing to
magnetic braking and gravitational radiation in relativistic binary stars (RBS)
are studied with use of physical parameters collected from the literature. We
have calculated and compared AML time scales for the RBS with non-degenerate
components and double degenerate (DD) systems.",0811.0455v1
2009-02-17,Chaos at Nonlinear NMR,"The hodographs of magnetization of nonlinear nuclear magnetic resonance are
investigated in the conditions of resonance on the unshifted frequency. It is
shown that, depending on the value of amplitude of the variable field and value
of frequency shift, topologically different hodographs separated from each
other by separatrix are obtained.",0902.2909v1
2009-04-28,Meson Spectra and Magnetic Fields in the Sakai-Sugimoto Model,"We continue our study of the dynamics of the flavour sector of the
Sakai-Sugimoto model in the presence of an external magnetic field, uncovering
several features of the meson spectrum at high and low temperatures. We employ
both analytical and numerical methods to study the coupled non-linear equations
that result from the gravity dual.",0904.4320v1
2009-09-25,Vorticity Flow Stabilization of Magnetized Plasmas,"Exact solutions of a magnetized plasma in a vorticity containing shear flow
for constant temperature are presented. This is followed by the modification of
these solutions by thermomagnetic currents in the presence of temperature
gradients. It is shown that solutions which are unstable for a subsonic flow,
are stable if the flow is supersonic.
  The results are applied to the problem of vorticity shear flow stabilization
of a linear z-pinch discharge.",0909.4750v2
2010-03-13,Nonlinear magnetoplasmons in strongly coupled Yukawa plasmas,"The existence of plasma oscillations at multiples of the magnetoplasmon
frequency in a strongly coupled two-dimensional magnetized Yukawa plasma is
reported, based on extensive molecular dynamics simulations. These modes are
the analogues of Bernstein modes which are renormalized by strong interparticle
correlations. Their properties are theoretically explained by a dielectric
function incorporating the combined effect of a magnetic field, strong
correlations and finite temperature.",1003.2721v1
2010-12-10,Spin relaxation and spin dynamics in semiconductors,"A chapter contribution to book: ""Handbook of Spin Transport and Magnetism"",
ed. by Evegeny Y. Tsymbal and Igor Zutic (Chapman & Hall/CRC, 2011)
http://www.crcpress.com/product/isbn/9781439803776 .",1012.2205v1
2011-08-23,Theory of inverse Faraday effect in disordered metal in THz regime,"We calculate the magnetization dynamics induced by the inverse Faraday effect
in disordered metals in THz regime by using the diagrammatic method. We find
that the induced magnetization is proportional to the frequency of circularly
polarized light.",1108.4522v1
2011-09-07,Planar approximation for spin transfer systems with application to tilted polarizer devices,"Planar spin-transfer devices with dominating easy-plane anisotropy can be
described by an effective one-dimensional equation for the in-plane angle. Such
a description provides an intuitive qualitative understanding of the magnetic
dynamics. We give a detailed derivation of the effective planar equation and
use it to describe magnetic switching in devices with tilted polarizer.",1109.1331v1
2011-09-30,Parker's dynamo and geomagnetic reversals,"Fluctuations of the alpha-effect which break equatorial symmetry of the flow
in the kinematic Parker's dynamo are considered. We show, that even small (a
few percents) fluctuation can leed to the substantial assymmetry of the
magnetic field in the hemispheres as well as the propagation of the dynamo wave
through the equator plane. We also consider how change of the dynamo number can
be used to explain different regimes of magnetic field generation in geodynamo.",1109.6889v1
2012-08-12,On Totally integrable magnetic billiards on constant curvature surface,"We consider billiard ball motion in a convex domain of a constant curvature
surface influenced by the constant magnetic field. We prove that if the
billiard map is totally integrable then the boundary curve is necessarily a
circle. This result is a manifestation of the so-called Hopf rigidity
phenomenon which was recently obtained for classical billiards on constant
curvature surfaces.",1208.2455v1
2012-08-19,Overcoming the Child-Langmuir law via the magnetic mirror effect,"The maximum current in a vacuum tube prescribed by the classical
Child-Langmuir law can be overcome, when the space-charge effect of the induced
potential is mitigated by the mirror effect in a spatially varying magnetic
field. The current could exceed the Child-Langmuir value by as much as a few
factors. The regime of practical interest is examined.",1208.3898v1
2013-01-08,Horizon area-angular momentum-charge-magnetic fluxes inequalities in 5D Einstein-Maxwell-dilaton gravity,"In the present paper we consider 5D spacetimes satisfying the
Einstein-Maxwell-dilaton gravity equations which are $U(1)^2$ axisymmetric but
otherwise highly dynamical. We derive inequalities between the area, the
angular momenta, the electric charge and the magnetic fluxes for any smooth
stably outer marginally trapped surface.",1301.1548v1
2014-12-30,Stability of the electron cyclotron resonance,"We consider the magnetic AC Stark effect for the quantum dynamics of a single
particle in the plane under the influence of an oscillating homogeneous
electric and a constant perpendicular magnetic field. We prove that the
electron cyclotron resonance is insensitive to impurity potentials.",1501.00038v2
2015-12-31,Metastability of the two-dimensional Blume-Capel model with zero chemical potential and small magnetic field,"We consider the two-dimensional Blume-Capel model with zero chemical
potential and small magnetic field evolving on a large but finite torus. We
obtain sharp estimates for the transition time, we characterize the set of
critical configurations, and we prove the metastable behavior of the dynamics
as the temperature vanishes.",1512.09286v1
2017-03-15,(De)Constructing magnetized dimensions,"We provide an origin of family replications in the standard model of particle
physics by constructing renormalizable, asymptotically free, four dimensional
local gauge theories that dynamically generate the fifth and sixth dimensions
with magnetic fluxes.",1703.05263v1
2017-11-09,Emergent electromagnetism induced by topological defects created during magnetization reversal in nanowires,"We report that the irreversible magnetization switching process in
ferromagnetic nanoparticles is governed by the formation and dynamics of
topological point-defects in the form of hedgehog-antihedgehog pairs. After
nucleation, these pairs rapidly separate with speeds exceeding domain wall
velocities, and they generate an emergent electric field of solenoidal
character and substantial magnitude.",1711.03511v1
2019-07-23,Dissipative invariants in MHD turbulence,"Role of the dissipative invariants in isotropic homogeneous MHD turbulence
has been studied using the distributed chaos approach and results of direct
numerical simulations. Effects of weak imposed mean magnetic field and magnetic
fields in undisturbed solar photosphere are also briefly discussed in this
context.",1907.09964v3
2021-05-18,Computational Micromagnetics based on Normal Modes: bridging the gap between macrospin and full spatial discretization,"The Landau-Lifshitz equation governing magnetization dynamics is written in
terms of the amplitudes of normal modes associated with the micromagnetic
system's appropriate ground state. This results in a system of nonlinear
ordinary differential equations (ODEs), the right-hand side of which can be
expressed as the sum of a linear term and nonlinear terms with increasing order
of nonlinearity (quadratic, cubic, etc.). The application of the method to
nanostructured magnetic systems demonstrates that the accurate description of
magnetization dynamics requires a limited number of normal modes, which results
in a considerable improvement in computational speed. The proposed method can
be used to obtain a reduced-order dynamical description of magnetic
nanostructures which allows to adjust the accuracy between low-dimensional
models, such as macrospin, and micromagnetic models with full spatial
discretization. This new paradigm for micromagnetic simulations is tested for
three problems relevant to the areas of spintronics and magnonics: directional
spin-wave coupling in magnonic waveguides, high power ferromagnetic resonance
in a magnetic nanodot, and injection-locking in spin-torque nano-oscillators.
The case studies considered demonstrate the validity of the proposed approach
to systematically obtain an intermediate order dynamical model based on normal
modes for the analysis of magnetic nanosystems. The time-consuming calculation
of the normal modes has to be done only one time for the system. These modes
can be used to optimize and predict the system response for all possible
time-varying external excitations (magnetic fields, spin currents). This is of
utmost importance for applications where fast and accurate system simulations
are required, such as in electronic circuits including magnetic devices.",2105.08829v2
2021-08-26,Dynamo and the Adiabatic Invariant,"The paper considers dynamo generated by a shallow fluid layer in a celestial
body (planet or star). This dynamo is based on the extra invariant for
interacting magnetic Rossby waves. The magnetohydrodynamics (MHD) is linearized
on the background of strong toroidal magnetic field. The extra invariant is
used to show that the background field is maintained.",2108.11548v1
2021-12-31,Demystifying magnetic resonance measurements of the true diffusion propagator,"In a recent work, a method for the magnetic resonance (MR) measurement of the
true diffusion propagator was introduced, which was subsequently implemented
and validated for free diffusion on a benchtop MR scanner. Here, we provide a
brief theoretical description of the method and discuss various experimental
regimes.",2112.15584v1
2022-02-22,3D RMHD simulations of jet-wind interactions in High Mass X-ray Binaries,"The interaction of jets in High-Mass X-ray Binaries (HMXBs) with the strong
winds driven by the hot companion star in the vicinity of the compact object is
fundamental to understand the jet dynamics, non-thermal emission and long-term
stability. However, the role of the jet magnetic field in this process is
unclear. We study the dynamical role of weak and moderate-to-strong toroidal
magnetic fields during the first hundreds of seconds of jet propagation,
focusing on the magnetized flow dynamics and the mechanisms of energy
conversion. We have developed the code L\'ostrego v1.0, a new 3D RMHD code to
simulate astrophysical plasmas in Cartesian coordinates. Using this tool, we
performed the first 3D RMHD numerical simulations of relativistic magnetized
jets propagating through the clumpy stellar wind in a HMXB. The overall
morphology and dynamics of weakly magnetized jet models is similar to previous
hydrodynamical simulations, where the jet head generates a strong shock in the
ambient medium and the initial over-pressure with respect to the stellar wind
drives one or more recollimation shocks. In the time scales of our simulations,
these jets are ballistic and seem to be more stable against internal
instabilities than jets with the same power in the absence of fields. However,
moderate-to-strong toroidal magnetic fields favour the development of
current-driven instabilities and the disruption of the jet within the binary. A
detailed analysis of the energy distribution in the relativistic outflow and
the ambient medium reveals that both magnetic and internal energies can
contribute to the effective acceleration of the jet. We certify that the jet
feedback into the ambient medium is highly dependent on the jet energy
distribution at injection, where hotter, more dilute and/or more magnetized
jets are more efficient, as anticipated by feedback studies in the case of jets
in active galaxies.",2202.11119v1
2023-10-15,Asymptotic Stability of 3D relativistic Collisionless plasma states in a constant magnetic field with a boundary,"Motivated by solar wind models in the low altitude, we explore a boundary
problem of the nonlinear relativistic Vlasov-Poisson systems in the 3D half
space in the presence of a constant vertical magnetic field and strong
background gravity with the inflow boundary condition. As the main result, we
construct stationary solutions and establish their nonlinear dynamical
asymptotic stability.",2310.09865v1
2024-02-02,The chimera revisited: Wall- and magnetically-bounded turbulent flows,"This paper is a first attempt at bringing together various concepts from
research on wall- and magnetically-bounded turbulent flows. Brief reviews of
both fields are provided: The main similarities identified are coherent
(turbulent) structures, flow generation and transport barriers. Examples are
provided and discussed.",2402.07918v1
2009-06-15,Transport of Large Scale Poloidal Flux in Black Hole Accretion,"We report on a global, three-dimensional GRMHD simulation of an accretion
torus embedded in a large scale vertical magnetic field orbiting a
Schwarzschild black hole. This simulation investigates how a large scale
vertical field evolves within a turbulent accretion disk and whether global
magnetic field configurations suitable for launching jets and winds can
develop. We find that a ""coronal mechanism"" of magnetic flux motion, which
operates largely outside the disk body, dominates global flux evolution. In
this mechanism, magnetic stresses driven by orbital shear create large-scale
half-loops of magnetic field that stretch radially inward and then reconnect,
leading to discontinuous jumps in the location of magnetic flux. In contrast,
little or no flux is brought in directly by accretion within the disk itself.
The coronal mechanism establishes a dipole magnetic field in the evacuated
funnel around the orbital axis with a field intensity regulated by a
combination of the magnetic and gas pressures in the inner disk. These results
prompt a reevaluation of previous descriptions of magnetic flux motion
associated with accretion. Local pictures are undercut by the intrinsically
global character of magnetic flux. Formulations in terms of an ""effective
viscosity"" competing with an ""effective resistivity"" are undermined by the
nonlinearity of of the magnetic dynamics and the fact that the same turbulence
driving mass motion (traditionally identified as ""viscosity"") can alter
magnetic topology.",0906.2784v2
2012-09-24,Droplet microfluidics to prepare magnetic polymer vesicles and to confine the heat in magnetic hyperthermia,"In this work, we present two types of microfluidic chips involving magnetic
nanoparticles dispersed in cyclohexane with oleic acid. In the first case, the
hydrophobically coated nanoparticles are self-assembled with an amphiphilic
diblock copolymer by a double-emulsion process in order to prepare giant
magnetic vesicles (polymersomes) in one step and at a high throughput. It was
shown in literature that such diblock copolymer W/O/W emulsion droplets can
evolve into polymersomes made of a thin (nanometric) magnetic membrane through
a dewetting transition of the oil phase from the aqueous internal cores usually
leading to ""acorn-like"" structures (polymer excess) sticking to the membranes.
To address this issue and greatly speed up the process, the solvent removal by
evaporation was replaced by a ""shearing-off"" of the vesicles in a simple PDMS
chip designed to exert a balance between a magnetic gradient and viscous shear.
In the second example, a simple oil-in-oil emulsion chip is used to obtain
regular trains of magnetic droplets that circulate inside an inductor coil
producing a radio-frequency magnetic field. We evidence that the heat produced
by magnetic hyperthermia can be converted into a temperature rise even at the
scale of nL droplets. The results are compared to heat transfer models in two
limiting cases: adiabatic vs. dissipative. The aim is to decipher the delicate
puzzle about the minimum size required for a tumor ""phantom"" to be heated by
radio-frequency hyperthermia in a general scope of anticancer therapy.",1209.5249v5
2013-10-02,Magnetic Field Amplification in Young Galaxies,"The Universe at present is highly magnetized, with fields of the order of a
few 10^-5 G and coherence lengths larger than 10 kpc in typical galaxies like
the Milky Way. We propose that the magnetic field was amplified to this values
already during the formation and the early evolution of the galaxies.
Turbulence in young galaxies is driven by accretion as well as by supernova
(SN) explosions of the first generation of stars. The small-scale dynamo can
convert the turbulent kinetic energy into magnetic energy and amplify very weak
primordial magnetic seed fields on short timescales. The amplification takes
place in two phases: in the kinematic phase the magnetic field grows
exponentially, with the largest growth on the smallest non-resistive scale. In
the following non-linear phase the magnetic energy is shifted towards larger
scales until the dynamo saturates on the turbulent forcing scale. To describe
the amplification of the magnetic field quantitatively we model the
microphysics in the interstellar medium (ISM) of young galaxies and determine
the growth rate of the small-scale dynamo. We estimate the resulting saturation
field strengths and dynamo timescales for two turbulent forcing mechanisms:
accretion-driven turbulence and SN-driven turbulence. We compare them to the
field strength that is reached, when only stellar magnetic fields are
distributed by SN explosions. We find that the small-scale dynamo is much more
efficient in magnetizing the ISM of young galaxies. In the case of
accretion-driven turbulence a magnetic field strength of the order of 10^-6 G
is reached after a time of 24-270 Myr, while in SN-driven turbulence the dynamo
saturates at field strengths of typically 10^-5 G after only 4-15 Myr. This is
considerably shorter than the Hubble time. Our work can help to understand why
present-day galaxies are highly magnetized.",1310.0853v2
2016-07-06,"Binary Neutron Star Mergers and Short Gamma-Ray Bursts: Effects of Magnetic Field Orientation, Equation of State, and Mass Ratio","We present fully GRMHD simulations of the merger of binary neutron star (BNS)
systems. We consider BNSs producing a hypermassive neutron star (HMNS) that
collapses to a spinning black hole (BH) surrounded by a magnetized accretion
disk in a few tens of ms. We investigate whether such systems may launch
relativistic jets and power short gamma-ray bursts. We study the effects of
different equations of state (EOSs), different mass ratios, and different
magnetic field orientations. For all cases, we present a detailed investigation
of the matter dynamics and of the magnetic field evolution, with particular
attention to its global structure and possible emission of relativistic jets.
The main result of this work is that we found the formation of an organized
magnetic field structure. This happens independently of EOS, mass ratio, and
initial magnetic field orientation. We also show that those models that produce
a longer-lived HMNS lead to a stronger magnetic field before collapse to BH.
Such larger fields make it possible, for at least one of our models, to resolve
the MRI and hence further amplify the magnetic field. However, by the end of
our simulations, we do not observe a magnetically dominated funnel and hence
neither a relativistic outflow. With respect to the recent simulations of Ruiz
et al 2016, we evolve models with lower and more realistic initial magnetic
field strengths and, because of computational reasons, we do not evolve the
accretion disk for the long timescales that seem to be required in order to see
a relativistic outflow. Since all our models produce a similar ordered magnetic
field structure, we expect that the results found in Ruiz et al 2016, where
they only considered an equal-mass system with an ideal fluid EOS, should be
general and, at least from a qualitative point of view, independent from
mass-ratio, magnetic field orientation, and EOS.",1607.01791v2
2016-09-22,Ambipolar diffusion regulated collapse of filaments threaded by perpendicular magnetic fields,"We numerically reproduce the density profiles for filaments that are in
magnetohydrostatic and pressure equilibrium with their surroundings obtained in
Tomisaka (2014) and show that these equilibria are dynamically stable. If the
effect of ambipolar diffusion is considered, these filaments lose magnetic
support initiating cloud collapse. The filaments do not lose magnetic flux.
Rather the magnetic flux is redistributed within the filament from the centre
towards the envelope.
  The rate of the collapse is inversely proportional to the fractional
ionisation and two gravitationally-driven ambipolar diffusion regimes for the
collapse are observed as predicted in Mouschovias & Morton (1991). For high
values of the ionisation coefficient, that is $X \geq 10^{-7}$, the gas is
strongly coupled to the magnetic field and the Jeans length is larger than the
ambipolar diffusion length scale. Then the collapse is governed by
magnetically-regulated ambipolar diffusion. For $X \lesssim 10^{-8}$, the gas
is weakly coupled to the magnetic field and the magnetic support is removed by
gravitationally-dominated ambipolar diffusion. Here, neutrals and ions only
collide sporadically, that is the ambipolar diffusion length scale is larger
than the Jeans length.
  When decaying turbulence is included, additional support is provided to the
filament. This slows down the collapse of the filament even in the absence of a
magnetic field. When a magnetic field is present, the collapse rate increases
by a ratio smaller than for the non-magnetic case. This is because of a
speed-up of the ambipolar diffusion due to larger magnetic field gradients
generated by the turbulence and because the ambipolar diffusion aids the
dissipation of turbulence below the ambipolar diffusion length scale. The
highest increase in the rate is observed for the lowest ionisation coefficient
and the highest turbulent intensity.",1609.06879v1
2018-02-13,Estimating the coronal and chromospheric magnetic fields of solar active regions as observed with the Nobeyama Radioheliograph Compared with the Extrapolated Linear Force-Free Field,"Adopting the thermal free-free emission mechanism, the coronal and
chromospheric magnetic fields are derived from the polarization and spectral
observations with the Nobeyama Radioheliograph at 1.76 cm. The solar active
regions (AR) located near the disk center observed on January 8, 2015 (AR
12257) and December 4, 2016 (AR 12615) are used for the estimate of the
chromospheric and coronal magnetic fields with the microwave radio
observations. We compare solar radio maps of active regions for both intensity
and circularly polarized component with the optical maps from observations with
the Helioseismic and Magnetic Imager and the chromosphere and corona transition
region images obtained with the Atmospheric Imaging Assembly instrument, on
board the Solar Dynamic Observatory. We notice from the comparison between
radio maps of both AR that the circular polarization degree in the AR 12257 is
about 2 percent, but the AR 12615 has a higher existent value by 3 percent.
Radio observations provide us for direct measurements of magnetic fields in the
chromospheric and coronal layers. We estimate the coronal magnetic fields using
the Atmospheric Imaging Assembly observations by adopting magnetic loops in the
corona over some patches with weak photospheric magnetic fields. The coronal
magnetic field derived from the Atmospheric Imaging Assembly data was from 90
to 240 Gauss. We also study the coronal magnetic fields based on the structure
of the extrapolated field, where the result of the magnetic fields was in the
range from 35 to 145 Gauss, showing that the difference in the coronal magnetic
fields between both results is attributed to the assumption of the force-free
approximation.",1802.04598v1
2018-06-28,"Fluctuation Dynamo in a Collisionless, Weakly Magnetized Plasma","Results from a numerical study of fluctuation dynamo in a collisionless,
weakly magnetized plasma are presented. The key difference between this dynamo
and its magnetohydrodynamic (MHD) counterpart is the adiabatic production of
magnetic-field-aligned pressure anisotropy by the amplification of a weak seed
field. This in turn drives kinetic instabilities on the ion-Larmor
scale---namely, firehose and mirror---which sever the adiabatic link between
the thermal and magnetic pressures, thereby allowing the dynamo to proceed.
After an initial phase of rapid growth driven by these instabilities, the
magnetic energy grows exponentially and exhibits a $k^{3/2}$ spectrum that
peaks near the resistive scale, similar to the large-magnetic-Prandtl-number
($\mathrm{Pm}\gg{1}$) MHD dynamo. The magnetic field self-organizes into a
folded-sheet topology, with direction reversals at the resistive scale and
field lines curved at the parallel scale of the flow. The effective
$\mathrm{Pm}$ is determined by whether the ion-Larmor scale is above or below
the field-reversing scale: in the former case, particles undergo Bohm-like
diffusion; in the latter case, particles scatter primarily off firehose
fluctuations residing at the ends of the magnetic folds, and the viscosity
becomes anisotropic. The magnetic field ultimately saturates at dynamical
strengths, with its spectral peak migrating towards larger scales. This
feature, along with an anti-correlation of magnetic-field strength and
field-line curvature and a gradual thinning of magnetic sheets into ribbons,
resemble the saturated state of the large-$\mathrm{Pm}$ dynamo, the primary
differences manifesting in firehose/mirror-unstable regions. These results have
implications for magnetic-field growth in the weakly collisional intracluster
medium of galaxy clusters.",1806.11162v3
2019-08-20,Magnetic fields in heavy ion collisions: flow and charge transport,"At the earliest times after a heavy-ion collision, the magnetic field created
by the spectator nucleons will generate an extremely strong, albeit rapidly
decreasing in time, magnetic field. The impact of this magnetic field may have
detectable consequences, and is believed to drive anomalous transport effects
like the Chiral Magnetic Effect (CME). We detail an exploratory study on the
effects of a dynamical magnetic field on the hydrodynamic medium created in the
collisions of two ultrarelativistic heavy-ions, using the framework of
numerical ideal MagnetoHydroDynamics (MHD) with the ECHO-QGP code. In this
study, we consider a magnetic field captured in a conducting medium, where the
conductivity can receive contributions from the electromagnetic conductivity
$\sigma$ and the chiral magnetic conductivity $\sigma_{\chi}$. We first study
the elliptic flow of pions, which we show is relatively unchanged by the
introduction of a magnetic field. However, by increasing the magnitude of the
magnetic field, we find evidence for an enhancement of the elliptic flow in
peripheral collisions. Next, we explore the impact of the chiral magnetic
conductivity on electric charges produced at the edges of the fireball. This
initial $\sigma_\chi$ can be understood as a long-wavelength effective
description of chiral fermion production. We then demonstrate that this chiral
charge, when transported by the MHD medium, produces a charge dipole
perpendicular to the reaction plane which extends a few units in rapidity.
Assuming charge conservation at the freeze-out surface, we show that the
produced charge imbalance can have measurable effects on some experimental
observables, like $v_1$ or $\langle \sin \phi \rangle$. This demonstrates the
ability of a MHD fluid to transport the signature of the initial chiral
magnetic fields to late times.",1908.07605v2
2020-02-25,Investigation of the magnetic ground state of the ordered double perovskite Sr2YbRuO6: a tale of two transitions,"Comprehensive muon spin rotation/relaxation (muSR) and neutron powder
diffraction (NPD) studies supported via bulk measurements have been performed
on the ordered double perovskite Sr2YbRuO6 to investigate the nature of the
magnetic ground state. Two sharp transitions at TN1 ~ 42 K and TN2 ~ 36 K have
been observed in the static and dynamic magnetization measurements, coinciding
with the heat capacity data. In order to confirm the origin of the observed
phase transitions and the magnetic ground state, microscopic evidences are
presented here. An initial indication of long-range magnetic ordering comes
from a sharp drop in the muon initial asymmetry and a peak in the relaxation
rate near TN1. NPD confirms that the magnetic ground state of Sr2YbRuO6
consists of an antiferromagnetic (AFM) structure with interpenetrating lattices
of parallel Yb3+ and Ru5+ moments lying in the ab-plane and adopting a A-type
AFM structure. Intriguingly, a small but remarkable change is observed in the
long-range ordering parameters at TN2 confirming the presence of a weak spin
reorientation (i.e. change in spin configuration) transition of Ru and Yb
moments, as well as a change in the magnetic moment evolution of the Yb3+ spins
at TN2. The temperature dependent behaviour of the Yb3+ and Ru5+ moments
suggests that the 4d-electrons of Ru5+ play a dominating role in stabilizing
the long range ordered magnetic ground state in the double perovskite Sr2YbRuO6
whereas only the Yb3+ moments show an arrest at TN2. The observed magnetic
structure and the presence of a ferromagnetic interaction between Ru- and Yb-
ions are explained with use of the Goodenough-Kanamori-Anderson (GKA) rules.
Possible reasons for the presence of the second magnetic phase transition and
of a compensation point in the magnetization data are linked to competing
mechanisms of magnetic anisotropy.",2002.10985v3
2020-03-09,Distinguishing between Flaring and Non-Flaring Active Regions: A Machine Learning Perspective,"Large scale solar eruptions significantly impact space weather and damages
space-based human infrastructures. It is necessary to predict large scale solar
eruptions, which will enable us to protect our vulnerable infrastructures of
modern society. We aim to investigate the difference between flaring and
non-flaring active regions. We use photospheric vector magnetogram data from
Solar Dynamic Observatory's Helioseismic Magnetic Imager to study the time
evolution of photospheric magnetic parameters on the solar surface. We build a
database of flaring and non-flaring active region observed on the solar surface
from the years 2010 to 2017. We train the machine learning algorithm by the
time evolution of these active region parameters. Finally, we estimate the
performance obtained from this machine learning algorithm. We find the strength
of some magnetic parameters namely total unsigned magnetic flux, total unsigned
magnetic helicity, total unsigned vertical current and total photospheric
magnetic energy density in flaring active regions are much higher compared to
the non-flaring ones. These magnetic parameters in the flaring active region
are highly evolving and complex. We are able to obtain good forecasting
capability with a relatively high value of true skill statistic (TSS). We also
find that time evolution of total unsigned magnetic helicity and total unsigned
magnetic flux have very high ability to distinguish flaring and non-flaring
active regions. It is possible to distinguish flaring active region from the
non-flaring one with good accuracy. We confirm that there is no single common
parameter which can distinguish all flaring active regions from the non flaring
one. However, time evolution of top few magnetic parameters namely total
unsigned magnetic flux and total unsigned magnetic helicity have very high
distinguishing capability.",2003.03878v1
2023-04-26,Exploring magnetic field properties at the boundary of solar pores: A comparative study based on SDO-HMI observations,"The Sun's magnetic fields play an important role in various solar phenomena.
Solar pores are regions of intensified magnetic field strength compared to the
surrounding photospheric environment, and their study can help us better
understand the properties and behaviour of magnetic fields in the Sun. Up to
now, there exists only a single study on magnetic field properties at the
boundary region of a pore. Therefore, the main goal of this work is to increase
the statistics of magnetic properties determining the pore boundary region. We
analyse six solar pores using data from the Helioseismic and Magnetic Imager
instrument on board the Solar Dynamics Observatory. We apply image processing
techniques to extract the relevant features of the solar pores and determine
the boundary conditions of the magnetic fields. We find the maximal vertical
magnetic field values on the boundaries of the studied solar pores to range
from 1400~G to 1600~G, with a standard deviation between 7.8\% and 14.8\%.
These values are lower than those reported in the mentioned preceding study.
However, this can be explained by differences in spatial resolution as well as
the type of data we used. The vertical magnetic field is an important factor in
determining the boundary of solar pores, and it plays a more relevant role than
the intensity gradient. The obtained information will be useful for future
studies on the formation and evolution of magnetic structures of the Sun.
Additionally, this study highlights the importance of high spatial resolution
data for the purpose of accurately characterising the magnetic properties of
solar pores.",2304.13212v1
2023-07-18,The effect of pressure-anisotropy-driven kinetic instabilities on magnetic field amplification in galaxy clusters,"The intracluster medium (ICM) is the low-density diffuse magnetized plasma in
galaxy clusters, which reaches virial temperatures of up to 10^8 K. Under these
conditions, the plasma is weakly collisional and therefore has an anisotropic
pressure tensor with respect to the local direction of the magnetic field. This
triggers very fast, Larmor-scale, pressure-anisotropy-driven kinetic
instabilities that alter magnetic field amplification. We study magnetic field
amplification through a turbulent small-scale dynamo, including the effects of
the kinetic instabilities, during the evolution of a typical massive galaxy
cluster. A specific aim of this work is to establish a redshift limit from
which a dynamo has to start to amplify the magnetic field up to equipartition
with the turbulent velocity field at redshift z=0. We implemented 1D radial
profiles for various plasma quantities for merger trees generated with the
Modified GALFORM algorithm. We assume that turbulence is driven by successive
mergers of dark matter halos and construct effective models for the Reynolds
number Re_eff dependence on the magnetic field in three different magnetization
regimes, including the effects of kinetic instabilities. The magnetic field
growth rate is calculated for the different Re_eff models. The model results in
a higher magnetic field growth rate at higher redshift. For all scenarios
considered, to reach equipartition at z=0, the amplification of the magnetic
field has to start at redshift z_start=1.5 and above. The time to reach
equipartition can be significantly shorter, in cases with systematically
smaller turbulent forcing scales, and for the highest Re_eff models. Merger
trees are useful tools for studying the evolution of magnetic fields in weakly
collisional plasmas. They could also be used to constrain the different stages
of the dynamo that could be observed by future radio telescopes.",2307.09451v2
2023-10-02,Control of the magnetic anisotropy in multi-repeat Pt/Co/Al heterostructures using magneto-ionic gating,"Controlling magnetic properties through the application of an electric field
is a significant challenge in modern nanomagnetism. In this study, we
investigate the magneto-ionic control of magnetic anisotropy in the topmost Co
layer in Ta/Pt/[Co/Al/Pt]$_n$/Co/Al/AlO$_\text{x}$ multilayer stacks comprising
$n +1$ Co layers and its impact on the magnetic properties of the multilayers.
We demonstrate that the perpendicular magnetic anisotropy can be reversibly
quenched through gate-driven oxidation of the intermediary Al layer between Co
and AlO$_\text{x}$, enabling dynamic control of the magnetic layers
contributing to the out-of-plane remanence - varying between $n$ and $n +1$.
For multilayer configurations with $n = 2$ and $n = 4$, we observe reversible
and non-volatile additions of 1/3 and 1/5, respectively, to the anomalous Hall
effect amplitude based on the applied gate voltage. Magnetic imaging reveals
that the gate-induced spin-reorientation transition occurs through the
propagation of a single 90$^{\circ}$ magnetic domain wall separating the
perpendicular and in-plane anisotropy states. In the 5-repetition multilayer,
the modification leads to a doubling of the period of the magnetic domains at
remanence. These results demonstrate that the magneto-ionic control of the
anisotropy of a single magnetic layer can be used to control the magnetic
properties of coupled multilayer systems, extending beyond the gating effects
on a single magnetic layer.",2310.01623v1
2024-01-11,The Decay Process of an α-configuration Sunspot,"The decay of sunspot plays a key role in magnetic flux transportation in
solar active regions (ARs). To better understand the physical mechanism of the
entire decay process of a sunspot, an {\alpha}-configuration sunspot in AR NOAA
12411 was studied. Based on the continuum intensity images and vector magnetic
field data with stray light correction from Solar Dynamics
Observatory/Helioseismic and Magnetic Imager, the area, vector magnetic field
and magnetic flux in the umbra and penumbra are calculated with time,
respectively. Our main results are as follows: (1) The decay curves of the
sunspot area in its umbra, penumbra, and whole sunspot take the appearance of
Gaussian profiles. The area decay rates of the umbra, penumbra and whole
sunspot are -1.56 MSH/day, -12.61 MSH/day and -14.04 MSH/day, respectively; (2)
With the decay of the sunspot, the total magnetic field strength and the
vertical component of the penumbra increase, and the magnetic field of the
penumbra becomes more vertical. Meanwhile, the total magnetic field strength
and vertical magnetic field strength for the umbra decrease, and the
inclination angle changes slightly with an average value of about 20{\deg}; (3)
The magnetic flux decay curves of the sunspot in its umbra, penumbra, and whole
sunspot exhibit quadratic patterns, their magnetic flux decay rates of the
umbra, penumbra and whole sunspot are -9.84 * 10^19 Mx/day, -1.59 * 10^20
Mx/day and -2.60 * 10^20 Mx/day , respectively. The observation suggests that
the penumbra may be transformed into the umbra, resulting in the increase of
the average vertical magnetic field strength and the reduction of the
inclination angle in the penumbra during the decay of the sunspot.",2401.05688v1
2012-10-29,Spin-Transfer Torque Magnetization Reversal in Uniaxial Nanomagnets with Thermal Noise,"We consider the general Landau-Lifshitz-Gilbert (LLG) dynamical theory
underlying the magnetization switching rates of a thin film uniaxial magnet
subject to spin-torque effects and thermal fluctuations (thermal noise). After
discussing the various dynamical regimes governing the switching phenomena, we
present analytical results for the mean switching time behavior. Our approach,
based on explicitly solving the first passage time problem, allows for a
straightforward analysis of the thermally assisted, low spin-torque, switching
asymptotics of thin film magnets. To verify our theory, we have developed an
efficient GPU-based micromagnetic code to simulate the stochastic LLG dynamics
out to millisecond timescales. We explore the effects of geometrical tilts
between the spin-current and uniaxial anisotropy axes on the thermally assisted
dynamics. We find that even in the absence of axial symmetry, the switching
times can be functionally described in a form virtually identical to the
collinear case. We further verify that asymptotic behavior is reached fairly
slowly, thus quantifying the role of thermal noise in the crossover regime
linking deterministic to thermally assisted magnetization reversal.",1210.7675v3
2013-08-09,The contact property for nowhere vanishing magnetic fields on the two-sphere,"In this paper we give some positive and negative results about the contact
property for the energy levels $\Sigma_c$ of a symplectic magnetic field on
$S^2$. In the first part we focus on the case of the area form on a surface of
revolution. We state a sufficient condition for an energy level to be of
contact type and give an example where the contact property fails. If the
magnetic curvature is positive, the dynamics and the action of invariant
measures can be numerically computed. This hints at the conjecture that an
energy level of a symplectic magnetic field with positive magnetic curvature
should be of contact type. In the second part we show that, for small energies,
there exists a convex hypersurface $N_c$ in $\mathbb C^2$ and a covering map
$p_c:N_c \rightarrow \Sigma_c$ such that the pull-back via $p_c$ of the
characteristic distribution on $\Sigma_c$ is the standard characteristic
distribution on $N_c$. As a corollary we prove that there are either two or
infinitely many periodic orbits on $\Sigma_c$. The second alternative holds if
there exists a contractible prime periodic orbit.",1308.2128v2
2015-09-21,Isotropic and Anisotropic Regimes of the Field-Dependent Spin Dynamics in Sr2IrO4: Raman Scattering Studies,"A major focus of experimental interest in Sr2IrO4 has been to clarify how the
magnetic excitations of this strongly spin-orbit coupled system differ from the
predictions of anisotropic 2D spin-1/2 Heisenberg model and to explore the
extent to which strong spin-orbit coupling affects the magnetic properties of
iridates. Here, we present a high-resolution inelastic light (Raman) scattering
study of the low energy magnetic excitation spectrum of Sr2IrO4 and doped
Eu-doped Sr2IrO4 as functions of both temperature and applied magnetic field.
We show that the high-field (H>1.5 T) in-plane spin dynamics of Sr2IrO4 are
isotropic and governed by the interplay between the applied field and the small
in-plane ferromagnetic spin components induced by the Dzyaloshinskii-Moriya
interaction. However, the spin dynamics of Sr2IrO4 at lower fields (H<1.5 T)
exhibit important effects associated with interlayer coupling and in-plane
anisotropy, including a spin-flop transition at Hc in Sr2IrO4 that occurs
either discontinuously or via a continuous rotation of the spins, depending
upon the in-plane orientation of the applied field. These results show that
in-plane anisotropy and interlayer coupling effects play important roles in the
low-field magnetic and dynamical properties of Sr2IrO4.",1509.06409v1
2016-07-11,Magnetorotational dynamo chimeras. The missing link to turbulent accretion disk dynamo models?,"In Keplerian accretion disks, turbulence and magnetic fields may be jointly
excited through a subcritical dynamo process involving the magnetorotational
instability (MRI). High-resolution simulations exhibit a tendency towards
statistical self-organization of MRI dynamo turbulence into large-scale cyclic
dynamics. Understanding the physical origin of these structures, and whether
they can be sustained and transport angular momentum efficiently in
astrophysical conditions, represents a significant theoretical challenge. The
discovery of simple periodic nonlinear MRI dynamo solutions has recently proven
useful in this respect, and has notably served to highlight the role of
turbulent magnetic diffusion in the seeming decay of the dynamics at low
magnetic Prandtl number Pm (magnetic diffusivity larger than viscosity), a
common regime in accretion disks. The connection between these simple
structures and the statistical organization reported in turbulent simulations
remained elusive, though. Here, we report the numerical discovery in moderate
aspect ratio Keplerian shearing boxes of new periodic, incompressible,
three-dimensional nonlinear MRI dynamo solutions with a larger dynamical
complexity reminiscent of such simulations. These ""chimera"" cycles are
characterized by multiple MRI-unstable dynamical stages, but their basic
physical principles of self-sustainment are nevertheless identical to those of
simpler cycles found in azimuthally elongated boxes. In particular, we find
that they are not sustained at low Pm either due to subcritical turbulent
magnetic diffusion. These solutions offer a new perspective into the transition
from laminar to turbulent instability-driven dynamos, and may prove useful to
devise improved statistical models of turbulent accretion disk dynamos.",1607.02903v1
2016-10-13,Dynamic Domains of DTS: Simulations of a Spherical Magnetized Couette Flow,"The Derviche Tourneur Sodium experiment, a spherical Couette
magnetohydrodynamics experiment with liquid sodium as the medium and a dipole
magnetic field imposed from the inner sphere, recently underwent upgrades to
its diagnostics to better characterize the flow and induced magnetic fields
with global rotation. In tandem with the upgrades, a set of direct numerical
simulations were run with the xshells code to give a more complete view of the
fluid and magnetic dynamics at various rotation rates of the inner and outer
spheres. These simulations reveal several dynamic regimes, determined by the
Rossby number. At positive differential rotation there is a regime of
quasigeostrophic flow, with low levels of fluctuations near the outer sphere.
Negative differential rotation shows a regime of what appear to be saturated
hydrodynamic instabilities at low negative differential rotation, followed by a
regime where filamentary structures develop at low latitudes and persist over
five to ten differential rotation periods as they drift poleward. We emphasize
that all these coherent structures emerge from turbulent flows. At least some
of them seem to be related to linear instabilities of the mean flow. The
simulated flows can produce the same measurements as those that the physical
experiment can take, with signatures akin to those found in the experiment.
This paper discusses the relation between the internal velocity structures of
the flow and their magnetic signatures at the surface.",1610.03964v4
2016-11-15,Dynamics of Coronal-Hole Boundaries,"Remote and in-situ observations strongly imply that the slow solar wind
consists of plasma from the hot, closed-field corona that is released onto open
magnetic field lines. The Separatrix Web (S-Web) theory for the slow wind
proposes that photospheric motions, at the scale of supergranules, are
responsible for generating dynamics at coronal-hole boundaries, which result in
the closed plasma release. We use three-dimensional magnetohydrodynamic (3D
MHD) simulations to determine the effect of photospheric flows on the open and
closed magnetic flux of a model corona with a dipole magnetic field and an
isothermal solar wind. A rotational surface motion is used to approximate
photospheric supergranular driving and is applied at the boundary between the
coronal hole and helmet streamer. The resulting dynamics consist primarily of
prolific and efficient interchange reconnection between open and closed flux.
Magnetic flux near the coronal-hole boundary experiences multiple interchange
events, with some flux interchanging over fifty times in one day. Additionally,
we find that the interchange reconnection occurs all along the coronal-hole
boundary, even producing a lasting change in magnetic-field connectivity in
regions that were not driven by the applied motions. Our results show that
these dynamics should be ubiquitous in the Sun and heliosphere. We discuss the
implications of our simulations for understanding the observed properties of
the slow solar wind, with particular focus on the global-scale consequences of
interchange reconnection.",1611.04968v2
2017-06-07,Cosmological dynamics of magnetic Bianchi I in viable $f(R)$ models of gravity,"Standard dynamical system analysis of Einstein-Maxwell equation in $f(R)$
theories is considered in this work. We investigate cosmological dynamics of a
uniform magnetic field in the Orthogonal Spatially Homogeneous (OSH) Bianchi
type I universe with viable $f(R)$ models of gravity. In this work, the $f(R) =
R -\alpha R^n$ and $f(R) = \left( R^b - \Lambda\right)^c$ models are examined
by using our dynamical system analysis. Our results show that both of two
$f(R)$ models have a viable cosmological consequence identical to the analysis
present in Ref.\cite{Amendola:2007nt} for the FLRW background. Contrary to
Ref.\cite{Amendola:2007nt}, we discover in our models that there is an
additional anisotropic and non-zero cosmological magnetic fields fixed point
emerging before the present of the standard matter epoch. This means that the
universe has initially isotropic stage with the intermediated epoch as the
anisotropic background and it ends up with the isotropic late-time
acceleration. The primordial magnetic fields play a crucial role of the shear
evolutions obtained from these two models which have the same scaling of the
cosmic time as $\sigma\sim t^{-\frac13}$, instead of $\sigma\sim t^{-1}$ for
the absence of the primordial magnetic cases.",1706.02279v2
2018-07-10,Dynamic Response of Tunable Phononic Crystals and New Homogenization Approaches in Magnetoactive Composites,"This research investigates dynamic response of tunable periodic structures
and homogenization methods in magnetoelastic composites (MECs). The research on
tunable periodic structures is focused on the design, modeling and
understanding of wave propagation phenomena and the dynamic response of smart
phononic crystals. High amplitude wrinkle formation is employed to study a
one-dimensional phononic crystal slab consists of a thin film bonded to a thick
compliant substrate. Buckling induced surface instability generates a wrinkly
structure triggered by a compressive strain. It is demonstrated that surface
periodic pattern and the corresponding large deformation can control elastic
wave propagation in the low thickness composite slab. Simulation results show
that the periodic wrinkly structure can be used as a smart phononic crystal
which can switch band diagrams of the structure in a transformative manner. A
magnetoactive phononic crystal is proposed which its dynamic properties are
controlled by combined effects of large deformations and an applied magnetic
field. Finite deformations and magnetic induction influence phononic
characteristics of the periodic structure through geometrical pattern
transformation and material properties. A magnetoelastic energy function is
proposed to develop constitutive laws considering large deformations and
magnetic induction in the periodic structure. Analytical and finite element
methods are utilized to compute dispersion relation and band structure of the
phononic crystal for different cases of deformation and magnetic loadings. It
is demonstrated that magnetic induction not only controls the band diagram of
the structure but also has a strong effect on preferential directions of wave
propagation. Moreover, a thermally controlled phononic crystal is designed
using ligaments of bi-materials in the structure.",1810.05678v1
2019-02-18,Dynamics of frequency-swept nuclear spin optical pumping in powdered diamond at low magnetic fields,"A broad effort is underway to improve the sensitivity of nuclear magnetic
resonance through the use of dynamic nuclear polarization. Nitrogen-vacancy
(NV) centers in diamond offer an appealing platform because these paramagnetic
defects can be optically polarized efficiently at room temperature. However,
work thus far has been mainly limited to single crystals because most
polarization transfer protocols are sensitive to misalignment between the NV
and magnetic field axes. Here we study the spin dynamics of NV-13C pairs in the
simultaneous presence of optical excitation and microwave frequency sweeps at
low magnetic fields. We show that a subtle interplay between illumination
intensity, frequency sweep rate, and hyperfine coupling strength leads to
efficient, sweep-direction-dependent 13C spin polarization over a broad range
of orientations of the magnetic field. In particular, our results strongly
suggest that finely-tuned, moderately coupled nuclear spins are key to the
hyperpolarization process, which makes this mechanism distinct from other known
dynamic polarization channels. These findings pave the route to applications
where powders are intrinsically advantageous, including the hyper-polarization
of target fluids in contact with the diamond surface or the use of
hyperpolarized particles as contrast agents for in-vivo imaging.",1902.06805v1
2019-07-25,Magnetic memory effect in ensembles of interacting anisotropic magnetic nanoparticles,"We explore the influence of demagnetization interaction on magnetic memory
effect by varying organization geometry of anisotropic ZnFe$_2$O$_4$
nanoparticles in an ensemble. The static and dynamic behaviour of two
differently organized ensembles, compact ensemble (CE) and hollow core ensemble
(HCE), are extensively studied by both dc and ac susceptibility, magnetic
memory effect and spin relaxation. The frequency-dependence peak shifting of
freezing temperature in both the systems is analyzed properly with the help of
two dynamic scaling models: Vogel-Fulcher law and power law. Presence of
cluster spin-glass phase is reflected from Vogel-Fulcher temperature $T_0$
$\simeq$ 142.58 K for CE, $\simeq$ 97 K for HCE and characteristic time
constant $\tau_0$ $\simeq$ $8.85\times10^{-9}$ s for CE, $\simeq$
$3.8\times10^{-10}$ s for HCE; along with $\delta$T$_{Th}$ $\sim$ 0.1 for CE
and 0.2 for HCE. The power law fitting with dynamic exponent $zv'$ = 6.2 $\pm$
1.1 for CE, 6.3 $\pm$ 0.5 for HCE and single spin flip $\tau^*$ $\simeq$
$7.7\times10^{-11}$ s for CE, $\simeq$ $1.3\times10^{-10}$ s for HCE provide
firm confirmation of cluster spin-glass phase. The progressive spin freezing
across multiple metastable states with prominent memory effects is reflected in
both the systems via nonequilibrium dynamics study. The hollow core geometry
with anisotropic nanoparticles on surface with closer proximity leads to
complex anisotropy energy landscape with enhanced demagnetizing field resulting
highly frustrated surface spins. As a consequence, more prominent magnetic
memory effect is observed in HCE with higher activation energy, reduced
blocking temperature and enhanced coercivity than that of CE.",1907.11116v2
2020-07-18,The dynamics of 3-minute wavefronts and their relation to sunspot magnetic fields,"We present a study of wave processes occurring in solar active region NOAA
11131 on 2010 December 10, captured by the Solar Dynamics Observatory in the
1600A, 304A, and 171A channels. For spectral analysis we employed pixelised
wavelet filtering together with a developed digital technique based on
empirical mode decomposition. We studied the 3-minute wave dynamics to obtain
relationships with the magnetic structuring of the underlying sunspot. We found
that during development of wave trains the motion path occurred along a
preferential direction, and that the broadband wavefronts can be represented as
a set of separate narrowband oscillation sources. These sources become visible
as the waves pass through the umbral inhomogeneities caused by the differing
magnetic field inclination angles. We found the spatial and frequency
fragmentation of wavefronts, and deduced that the combination of narrowband
spherical and linear parts of the wavefronts provide the observed spirality.
Maps of the magnetic field inclination angles confirm this assumption. We
detect the activation of umbral structures as the increasing of oscillations in
the sources along the front ridge. Their temporal dynamics are associated with
the occurrence of umbral flashes. Spatial localisation of the sources is stable
over time and depends on the oscillation period. We propose that these sources
are the result of wave paths along the loops extending outwards from the
magnetic bundles of the umbra.",2007.09369v1
2020-11-20,An ac-susceptibility study of magnetic relaxation phenomena in the antiskyrmion hosting tetragonal Mn-Pt(Pd)-Sn system,"Here, we report an exhaustive study of the frequency-dependent ac-magnetic
susceptibility of the $D_{2d}$ symmetric Heusler system Mn-Pt(Pd)-Sn that hosts
antiskyrmions over a wide temperature range. Magnetic relaxation studies using
Cole-Cole formalism reveal a Debye-type relaxation with a nearly negligible
distribution in relaxation times. In contrast to the archetypical skyrmion
hosts, the high Curie temperature ($ T_C $) of the present system ensures
shorter switching times, and, correspondingly, higher frequencies are required
to probe the relaxation dynamics. We find a non-monotonic variation in the
characteristic relaxation time with distinct maxima at the phase boundaries
\textit{via} helical $\longrightarrow$ antiskyrmion $\longrightarrow$
field-polarized states, indicating slower magnetization dynamics over the
region of phase coexistence. The temperature-dependent relaxation time across
different phases is of the order of $ 10^{-5} - 10^{-4} $ s and follows the
well-known Arrhenius law with reasonable values of the energy barriers. The
present study concerning the magnetization dynamics in the antiskyrmion host
tetragonal Heusler system is an important contribution towards the basic
understanding of the dynamical aspects of antiskyrmions for their potential
applications.",2011.10229v1
2021-05-28,Influence of magnetic fields on ultrafast laser-induced switching dynamics in Co/Gd bilayers,"Recently it has been shown that not only GdFeCo alloys exhibit single-pulse
helicity-independent all-optical switching (HI-AOS), but that this effect is
also seen in Co/Gd bilayers. However, there have been no reports on the
explicit time dynamics of the switching process in these bilayers as of yet.
Furthermore, time-resolved measurements of switching of other materials are
typically done with a constant applied field to reset the magnetization between
consecutive pulses and thus ensure repeatable behavior. In this paper we
experimentally resolve the explicit dynamics of the switching process in Co/Gd,
and the influence of applied magnetic fields on the switching process. We
observe that after a switch within several picoseconds, the magnetization
switches back at a timescale of hundreds of picoseconds. This backswitch
includes a strong dependence on the magnetic field strength even at sub-tesla
fields, significantly smaller than the exchange fields that govern the
switching dynamics. This surprising behaviour is explained by a combination of
longitudinal switching (on a picosecond timescale), precessional switching (on
a nanosecond timescale) and domain-wall motion (on a timescale of 10 ns and
beyond). We discuss these different switching regimes and their relative
importance using simple model calculations.",2105.13862v1
2021-11-05,Ultrafast optical excitation of magnetic dynamics in van der Waals magnets: Coherent magnons and BKT dynamics in NiPS$_3$,"Optical pump-probe experiments carried out in the time domain reveal both the
intrinsic low energy dynamics and its connections to higher energy excitations
in correlated electron systems. In this work, we propose two microscopic
mechanisms for the optical generation of coherent magnetic modes in van der
Waals magnets, and derive the corresponding effective light-spin interactions:
either through pumping atomic orbital excitations resonantly or via a
light-induced Floquet spin Hamiltonian, the ground state of the system is
driven out of equilibrium. The subsequent long-time relaxational dynamics can
then be probed using, e.g. the magneto-optical Kerr effect or transient grating
spectroscopy. As an example, we apply our framework to NiPS$_3$, which is
magnetically ordered in the bulk, and is conjectured to realize the XY model in
the monolayer limit. Our theory makes explicit how the material's low-energy
response depends sensitively on the microscopic details of the light-spin
coupling as well as pump fluence, frequency and polarization. For the case of
bulk NiPS$_3$, we find quantitative agreement with recent experiments by
Afanasiev et al. [Science Advances 7, abf3096 (2021)]. We further propose
pump-probe experiments for monolayer NiPS$_3$ and detail how anomalous
relaxational behaviour may reveal excitations of a (proximate) BKT phase in a
proposed effective XY model.",2111.03674v2
2021-12-31,Spin-injection-generated shock waves and solitons in a ferromagnetic thin film: the spin piston problem,"The unsteady, nonlinear magnetization dynamics induced by spin injection in
an easy-plane ferromagnetic channel subject to an external magnetic field are
studied analytically. Leveraging a dispersive hydrodynamic description, the
Landau-Lifshitz equation is recast in terms of hydrodynamic-like variables for
the magnetization's perpendicular component (spin density) and azimuthal phase
gradient (fluid velocity). Spin injection acts as a moving piston that
generates nonlinear, dynamical spin textures in the ferromagnetic channel with
downstream quiescent spin density set by the external field. In contrast to the
classical problem of a piston accelerating a compressible gas, here, variable
spin injection and field lead to a rich variety of nonlinear wave phenomena
from oscillatory spin shocks to solitons and rarefaction waves. A full
classification of solutions is provided using nonlinear wave modulation theory
by identifying two key aspects of the fluid-like dynamics: subsonic/supersonic
conditions and convex/nonconvex hydrodynamic flux. Familiar waveforms from the
classical piston problem such as rarefaction (expansion) waves and shocks
manifest in their spin-based counterparts as smooth and highly oscillatory
transitions, respectively. The spin shock is an example of a dispersive shock
wave, which arises in many physical systems. New features without a gas
dynamics counterpart include composite wave complexes with ""contact"" spin
shocks and rarefactions. Magnetic supersonic conditions lead to two pronounced
piston edge behaviors including a stationary soliton and an oscillatory
wavetrain. These coherent wave structures have physical implications for the
generation of high frequency spin waves from pulsed injection and persistent,
stable stationary and/or propagating solitons in the presence of magnetic
damping. The analytical results are favorably compared with numerical
simulations.",2112.15301v2
2022-02-17,Escape Dynamics in an Anisotropically Driven Brownian Magneto-System,"Thermally activated escape of a Brownian particle over a potential barrier is
well understood within Kramers theory. When subjected to an external magnetic
field, the Lorentz force slows down the escape dynamics via a rescaling of the
diffusion coefficient without affecting the exponential dependence on the
barrier height. Here, we study the escape dynamics of a charged Brownian
particle from a two-dimensional truncated harmonic potential under the
influence of Lorentz force due to an external magnetic field. The particle is
driven anisotropically by subjecting it to noises with different strengths
along different spatial directions. We show that the escape time can largely be
tuned by the anisotropic driving. While the escape process becomes anisotropic
due to the two different noises, the spatial symmetry is restored in the limit
of large magnetic fields. This is attributed to the Lorentz force induced
coupling between the spatial degrees of freedom which makes the difference
between two noises irrelevant at high magnetic fields. The theoretical
predictions are verified by Brownian dynamics simulations. In principle, our
predictions can be tested by experiments with a Brownian gyrator in the
presence of a magnetic field.",2202.08559v3
2022-11-09,Electron dynamics in planar radio frequency magnetron plasmas: I. The mechanism of Hall heating and the μ-mode,"The electron dynamics and the mechanisms of power absorption in
radio-frequency (RF) driven, magnetically enhanced capacitively coupled plasmas
(MECCPs) at low pressure are investigated. The device in focus is a
geometrically asymmetric cylindrical magnetron with a radially nonuniform
magnetic field in axial direction and an electric field in radial direction.
The dynamics is studied analytically using the cold plasma model and a
single-particle formalism, and numerically with the inhouse energy and charge
conserving particle-in-cell/Monte Carlo collisions code ECCOPIC1S-M. It is
found that the dynamics differs significantly from that of an unmagnetized
reference discharge. In the magnetized region in front of the powered
electrode, an enhanced electric field arises during sheath expansion and a
reversed electric field during sheath collapse. Both fields are needed to
ensure discharge sustaining electron transport against the confining effect of
the magnetic field. The corresponding azimuthal ExB-drift can accelerate
electrons into the inelastic energy range which gives rise to a new mechanism
of RF power dissipation. It is related to the Hall current and is different in
nature from Ohmic heating, as which it has been classified in previous
literature. The new heating is expected to be dominant in many magnetized
capacitively coupled discharges. It is proposed to term it the ""{\mu}-mode"" to
separate it from other heating modes.",2211.04805v1
2023-03-10,Field-driven collapsing dynamics of skyrmions in magnetic multilayers,"Magnetic skyrmions are fascinating topological particle-like textures
promoted by a trade-off among interfacial properties (perpendicular anisotropy
and Dzyaloshinskii-Moriya interaction (DMI)) and dipolar interactions.
Depending on the dominant interaction, complex spin textures, including pure
N\'eel and hybrid skyrmions have been observed in multilayer heterostructures.
A quantification of these different spin textures typically requires a
depth-reoslved magnetic imaging or scattering techniques. In the present work,
we will show qualitatively different collapsing dynamics for pure N\'eel and
hybrid skyrmions induced by a perpendicular magnetic field in two
representative systems, [Pt/Co/Ir]15 and [Ta/CoFeB/MgO]15 multilayers.
Skyrmions in the former stack undergo two morphological transitions, upon
reversing the perpendicular field direction. Skyrmions in [Ta/CoFeB/MgO]15
multilayers exhibit a continuous transition, which is mainly linked to a
reversible change of the skyrmion size. A full micromagnetic phase diagram is
presented to identify these two collapsing mechanisms as a function of material
parameters. Since the two distinct collapsing dynamics rely on the detailed
layer-dependent spin structures of skyrmions, they could be used as potential
fingerprints for identifying the skyrmion type in magnetic multilayers. Our
work suggests the employment of pure and hybrid skyrmions for specific
applications, due to the strong correlation between the skyrmion dynamics and
3-dimentional spin profiles.",2303.05988v1
2024-03-11,Topological solitons stabilized by a background gauge field and soliton-anti-soliton asymmetry,"We study topological lumps supported by the second homotopy group $\pi_2(S^2)
\simeq {\mathbb Z}$ in a gauged $O(3)$ model without any potential term coupled
with a (non)dynamical $U(1)$ gauge field. It is known that gauged-lumps are
stable with an easy-plane potential term but are unstable to expand if the
model has no potential term. In this paper, we find that these gauged lumps
without a potential term can be made stable by putting them in a uniform
magnetic field, irrespective of whether the gauge field is dynamical or not. In
the case of the non-dynamical gauge field, only either of lumps or anti-lumps
stably exists depending on the sign of the background magnetic field, and the
other is unstable to shrink to be singular. We also construct coaxial multiple
lumps whose size and mass exhibit a behaviour of droplets. In the case of the
dynamical gauge field, both the lumps and anti-lumps stably exist with
different masses; the lighter (heavier) one corresponds to the (un)stable one
in the case of the nondynamical gauge field. We find that a lump behaves as a
superconducting ring and traps magnetic field in its inside, with the total
magnetic field reduced from the background magnetic field.",2403.06778v1
2015-01-21,Observations of an Energetically Isolated Quiet Sun Transient: Evidence of Quasi-Steady Coronal Heating,"Increasing evidence for coronal heating contributions from cooler solar
atmospheric layers, notably quiet Sun (QS) conditions, challenges standard
solar atmospheric descriptions of bright transition region (TR) emission. As
such, questions to the role of dynamic QS transients in contributing to the
total coronal energy budget are elevated. Using observations from the {\it
Atmospheric Imaging Assembly} and {\it Heliosemic Magnetic Imager} on board the
{\it Solar Dynamics Observatory}, and numerical model extrapolations of coronal
magnetic fields, we investigate a dynamic QS transient energetically isolated
to the TR and extruding from a common footpoint shared with two heated loop
arcades. A non-casual relationship is established between episodic heating of
the QS transient and wide-spread magnetic field re-organization events, while
evidence is found favoring a magnetic topology typical of eruptive processes.
Quasi-steady interchange reconnection events are implicated as a source of the
transient's visibly bright radiative signature. We consider the QS transient's
temporally stable ($\approx$\,35\,min) radiative nature occurs as a result of
the large-scale magnetic field geometries of the QS and/or relatively quiet
nature of the magnetic photosphere, which possibly act to inhibit energetic
buildup processes required to initiate a catastrophic eruption phase. This work
provides insight to the QS's thermodynamic and magnetic relation to eruptive
processes quasi-steadily heating a small-scale dynamic and TR transient. This
work elevates arguments of non-negligible coronal heating contributions from
cool atmospheric layers in QS conditions, and increases evidence for solar wind
mass feeding of dynamic transients therein.",1501.05211v4
2018-06-24,Dynamics of Insufflated Abdominal Wall Tissue for Magnetically Anchored Surgical Instruments,"Magnetically-anchored surgical devices have recently gained attention in
abdominal surgery, with the use of magnets to anchor surgical devices onto the
insufflated abdominal wall. These anchors have been used to secure passive and
active devices, where active device such as robotic manipulators produce
motions that would excite the dynamics of the non-rigid abdominal wall. Hence,
there is a need to investigate the mechanical dynamics of the abdominal wall
tissue in insufflated state, combined with magnetic anchoring, specifically its
response to mechanical excitations and the expected disturbances to the
operation of the anchored devices. In this paper, loading and unloading tests
are performed on a corresponding porcine specimen for dynamics identification.
The experiment setup was constructed to emulate the insufflated state of the
abdomen with the magnetically anchored mechanism. The tissue responses during
unloading are captured and approximated with a general numerical model, which
is in turn used for the dynamic analysis of the tissue using Bode plot. The
results showed that in such stretched and compressed state, the steady state
displacement of the tissue is approximately zero. The maximum transient error
was found to be 1mm in displacement using a high magnetic anchoring force.
Significant attenuation of the disturbances due to the high stiffness and
damping of the abdominal wall, was observed from 100rad/s in the frequency
response. If a robotic manipulator was attached to the anchoring device, the
typical operating frequency of movements would still produce unattenuated
disturbances. It is expected that some error compensation through suitable
control strategies is required. These outcomes establish the basis for the
controller design and the design specification of the active
magnetically-anchored surgical devices for minimal disturbance impact onto the
abdominal wall tissue.",1806.09067v1
2021-11-26,Magnetic correlations in infinite-layer nickelates: an experimental and theoretical multi-method study,"We report a comprehensive study of magnetic correlations in LaNiO$_{2}$, a
parent compound of the recently discovered family of infinite-layer (IL)
nickelate superconductors, using multiple experimental and theoretical methods.
Our specific heat, muon-spin rotation ($\mu$SR), and magnetic susceptibility
measurements on polycrystalline LaNiO$_{2}$ show that long-range magnetic order
remains absent down to 2 K. Nevertheless, we detect residual entropy in the
low-temperature specific heat, which is compatible with a model fit that
includes paramagnon excitations. The $\mu$SR and low-field static and dynamic
magnetic susceptibility measurements indicate the presence of short-range
magnetic correlations and glassy spin dynamics, which we attribute to local
oxygen non-stoichiometry in the average infinite-layer crystal structure. This
glassy behavior can be suppressed in strong external fields, allowing us to
extract the intrinsic paramagnetic susceptibility. Remarkably, we find that the
intrinsic susceptibility shows non-Curie-Weiss behavior at high temperatures,
in analogy to doped cuprates that possess robust non-local spin fluctuations.
The distinct temperature dependence of the intrinsic susceptibility of
LaNiO$_{2}$ can be theoretically understood by a multi-method study of the
single-band Hubbard model in which we apply complementary cutting-edge quantum
many-body techniques (dynamical mean-field theory, cellular dynamical
mean-field theory and the dynamical vertex approximation) to investigate the
influence of both short- and long-ranged correlations. Our results suggest a
profound analogy between the magnetic correlations in parent (undoped) IL
nickelates and doped cuprates.",2111.13668v1
2017-08-01,Influence of a dispersion of magnetic and nonmagnetic nanoparticles on the magnetic Fredericksz transition of the liquid crystal 5CB,"A long time ago, Brochard and de Gennes predicted the possibility of
significantly decreasing the critical magnetic field of the Fredericksz
transition (the magnetic Fredericksz threshold) in a mixture of nematic liquid
crystals and ferromagnetic particles, the so-called ferronematics. This
phenomenon has rarely been measured, usually due to soft homeotropic anchoring
induced at the nanoparticle surface. Here we present an optical study of the
magnetic Fredericksz transition combined with a light scattering study of the
classical nematic liquid crystal, 5CB, doped with 6 nm diameter magnetic and
non-magnetic nanoparticles. Surprisingly, for both nanoparticles, we observe at
room temperature a net decrease of the threshold field of the Fredericksz
transition at low nanoparticle concentrations, which appears associated with a
coating of the nanoparticles by a brush of polydimethylsiloxane copolymer
chains inducing planar anchoring of the director on the nanoparticle surface.
Moreover the magnetic Fredericksz threshold exhibits non-monotonic behaviour as
a function of the nanoparticle concentration for both types of nanoparticles,
first decreasing down to a value from 23\% to 31\% below that of pure 5CB, then
increasing with a further increase of nanoparticle concentration. This is
interpreted as an aggregation starting at around 0.02 weight fraction that
consumes more isolated nanoparticles than those introduced when the
concentration is increased above $c = 0.05$ weight fraction (volume fraction
$3.5 \times 10^{-2}$). This shows the larger effect of isolated nanoparticles
on the threshold with respect to aggregates. From dynamic light scattering
measurements we deduced that, if the decrease of the magnetic threshold when
the nanoparticle concentration increases is similar for both kinds of
nanoparticles, the origin of this decrease is different for magnetic and
non-magnetic nanoparticles. For non-magnetic nanoparticles, the behavior may be
associated with a decrease of the elastic constant due to weak planar
anchoring. For magnetic nanoparticles there are non-negligible local magnetic
interactions between liquid crystal molecules and magnetic nanoparticles,
leading to an increase of the average order parameter. This magnetic
interaction thus favors an easier liquid crystal director rotation in the
presence of external magnetic field, able to reorient the magnetic moments of
the nanoparticles along with the molecules.",1708.00286v1
2006-11-24,Internal shocks in relativistic outflows: collisions of magnetized shells,"(Abridged): We study the collision of magnetized irregularities (shells) in
relativistic outflows in order to explain the origin of the generic
phenomenology observed in the non-thermal emission of both blazars and
gamma-ray bursts. We focus on the influence of the magnetic field on the
collision dynamics, and we further investigate how the properties of the
observed radiation depend on the strength of the initial magnetic field and on
the initial internal energy density of the flow. The collisions of magnetized
shells and the radiation resulting from these collisions are calculated using
the 1D relativistic magnetohydrodynamics code MRGENESIS. The interaction of the
shells with the external medium prior to their collision is also determined
using an exact solver for the corresponding 1D relativistic magnetohydrodynamic
Riemann problem. Our simulations show that two magnetization parameters - the
ratio of magnetic energy density and thermal energy density, \alpha_B, and the
ratio of magnetic energy density and mass-energy density, \sigma - play an
important role in the pre-collision phase, while the dynamics of the collision
and the properties of the light curves depend mostly on the magnetization
parameter \sigma. The interaction of the shells with the external medium
changes the flow properties at their edges prior to the collision. For
sufficiently dense shells moving at large Lorentz factors (\simgt 25) these
properties depend only on the magnetization parameter \sigma. Internal shocks
in GRBs may reach maximum efficiencies of conversion of kinetic into thermal
energy between 6% and 10%, while in case of blazars, the maximum efficiencies
are \sim 2%.",0611765v3
2005-10-07,Magnetic and Charge Correlations in La{2-x-y}Nd_ySr_xCuO_4: Raman Scattering Study,"Two aspects in connection with the magnetic properties of
La_{2-x-y}Nd_ySr_xCuO_4 single crystals are discussed. The first is related to
long wavelength magnetic excitations in x = 0, 0.01, and 0.03 La_{2-x}Sr_xCuO_4
detwinned crystals as a function of doping, temperature and magnetic field. Two
magnetic modes were observed within the AF region of the phase diagram. The one
at lower energies was identified with the spin-wave gap induced by the
antisymmetric DM interaction and its anisotropic properties in magnetic field
could be well explained using a canonical form of the spin Hamiltonian. A new
finding was a magnetic field induced mode whose dynamics allowed us to discover
a spin ordered state outside the AF order which was shown to persist in a 9 T
field as high as 100 K above the N\'eel temperature T_N for x = 0.01. For these
single magnon excitations we map out the Raman selection rules in magnetic
fields and demonstrate that their temperature dependent spectral weight is
peaked at the N\'eel temperature. The second aspect is related to phononic and
magnetic Raman scattering in La_{2-x-y}Nd_ySr_xCuO_4 with three doping
concentrations: x = 1/8, y = 0; x = 1/8, y = 0.4; and x = 0.01, y = 0. We
observed that around 1/8 Sr doping and independent of Nd concentration there
exists substantial disorder in the tilt pattern of the CuO_6 octahedra in both
the orthorhombic and tetragonal phases which persist down to 10 K and are
coupled to bond disorder in the cation layers. The weak magnitude of existing
charge/spin modulations in the Nd doped structure did not allow us to detect
specific Raman signatures on lattice dynamics or two-magnon scattering around
2200 cm-1.",0510191v1
2009-08-25,Methanol masers probing the ordered magnetic field of W75N,"The role of magnetic fields during the protostellar phase of high-mass
star-formation is a debated topic. In particular, it is still unclear how
magnetic fields influence the formation and dynamic of disks and outflows. Most
current information on magnetic fields close to high-mass protostars comes from
H2O and OH maser observations. Recently, the first 6.7 GHz methanol maser
polarization observations were made, and they reveal strong and ordered
magnetic fields. The morphology of the magnetic field during high-mass
star-formation needs to be investigated on small scales, which can only be done
using very long baseline interferometry observations. The massive star-forming
regionW75N contains three radio sources and associated masers, while a
large-scale molecular bipolar outflow is also present. Polarization
observations of the 6.7 GHz methanol masers at high angular resolution probe
the strength and structure of the magnetic field and determine its relation to
the outflow. Eight of the European VLBI network antennas were used to measure
the linear polarization and Zeeman-splitting of the 6.7 GHz methanol masers in
the star-forming region W75N. We detected 10 methanol maser features, 4 of
which were undetected in previous work. All arise near the source VLA1 of W75N.
The linear polarization of the masers reveals a tightly ordered magnetic field
over more than 2000 AU around VLA1 that is exactly aligned with the large-scale
molecular outflow. This is consistent with the twisted magnetic field model
proposed for explaining dust polarization observations. The Zeeman-splitting
measured on 3 of the maser features indicates a dynamically important magnetic
field in the maser region of the order of 50mG. We suggest VLA1 is the powering
sources of the bipolar outflow.",0908.3585v1
2011-01-10,The structure of the magnetic field in the massive star-forming region W75N,"A debated topic in star formation theory is the role of magnetic fields
during the protostellar phase of high-mass stars. It is still unclear how
magnetic fields influence the formation and dynamics of massive disks and
outflows. Most current information on magnetic fields close to high-mass
protostars comes from polarized maser emissions, which allows us to investigate
the magnetic field on small scales by using very long-baseline interferometry.
The massive star-forming region W75N contains three radio continuum sources
(VLA1, VLA2, and VLA3), at three different evolutionary stages, and associated
masers, while a large-scale molecular bipolar outflow is also present. Very
recently, polarization observations of the 6.7 GHz methanol masers at
milliarsecond resolution have been able to probe the strength and structure of
the magnetic field over more than 2000 AU around VLA1. The magnetic field is
parallel to the outflow, suggesting that VLA1 is its powering source. The
observations of water masers at 22 GHz can give more information about the gas
dynamics and the magnetic fields around VLA1 and VLA2. The NRAO Very Long
Baseline Array was used to measure the linear polarization and the
Zeeman-splitting of the 22 GHz water masers in the star-forming region W75N. We
detected 124 water masers, 36 around VLA1 and 88 around VLA2 of W75N, which
indicate two different physical environments around the two sources, where VLA1
is in a more evolved state. The linear polarization of the masers confirms the
tightly ordered magnetic field around VLA1, which is aligned with the
large-scale molecular outflow, and also reveals an ordered magnetic field
around VLA2, which is not parallel to the outflow. [abridged]",1101.1956v1
2011-11-22,Observation of spin glass state in weakly ferromagnetic Sr$_2$FeCoO$_6$ double perovskite,"We report the observation of spin glass state in the double perovskite oxide
Sr$_{2}$FeCoO$_{6}$ prepared through sol-gel technique. Initial structural
studies using x rays reveal that the compound crystallizes in tetragonal $I
4/m$ structure with lattice parameters, $a$ = 5.4609(2) \AA and $c$ = 7.7113(7)
\AA. The temperature dependent powder x ray studies reveal no structural phase
transition in the temperature range 10 -- 300 K. However, the unit cell volume
shows an anomaly coinciding with the magnetic transition temperature thereby
suggesting a close connection between lattice and magnetism. Neutron
diffraction studies and subsequent bond valence sums analysis show that in
Sr$_{2}$FeCoO$_{6}$, the $B$ site is randomly occupied by Fe and Co in the
mixed valence states of Fe$^{3+}$/Fe$^{4+}$ and Co$^{3+}$/Co$^{4+}$. The random
occupancy and mixed valence sets the stage for inhomogeneous magnetic exchange
interactions and in turn, for the spin glass like state in this double
perovskite which is observed as an irreversibility in temperature dependent dc
magnetization at $T_f\sim$ 75 K. Thermal hysteresis observed in the
magnetization profile of Sr$_{2}$FeCoO$_{6}$ is indicative of the mixed
magnetic phases present. The dynamic magnetic susceptibility displays
characteristic frequency dependence and confirms the spin glass nature of this
material. Dynamical scaling analysis of $\chi'(T)$ yields a critical
temperature $T_{ct}$ = 75.14(8) K and an exponent $z\nu$ = 6.2(2) typical for
spin glasses. The signature of presence of mixed magnetic interactions is
obtained from the thermal hysteresis in magnetization of Sr$_{2}$FeCoO$_{6}$.
Combining the neutron and magnetization results of Sr$_2$FeCoO$_6$, we deduce
the spin states of Fe to be in low spin while that of Co to be in low spin and
intermediate spin.",1111.5136v1
2013-02-22,The Post-Merger Magnetized Evolution of White Dwarf Binaries: The Double-Degenerate Channel of Sub-Chandrasekhar Type Ia Supernovae and the Formation of Magnetized White Dwarfs,"Type Ia supernovae (SNe Ia) play a crucial role as standardizable
cosmological candles, though the nature of their progenitors is a subject of
active investigation. Recent observational and theoretical work has pointed to
merging white dwarf binaries, referred to as the double-degenerate channel, as
the possible progenitor systems for some SNe Ia. Additionally, recent
theoretical work suggests that mergers which fail to detonate may produce
magnetized, rapidly-rotating white dwarfs. In this paper, we present the first
multidimensional simulations of the post-merger evolution of white dwarf
binaries to include the effect of the magnetic field. In these systems, the two
white dwarfs complete a final merger on a dynamical timescale, and are tidally
disrupted, producing a rapidly-rotating white dwarf merger surrounded by a hot
corona and a thick, differentially-rotating disk. The disk is strongly
susceptible to the magnetorotational instability (MRI), and we demonstrate that
this leads to the rapid growth of an initially dynamically weak magnetic field
in the disk, the spin-down of the white dwarf merger, and to the subsequent
central ignition of the white dwarf merger. Additionally, these magnetized
models exhibit new features not present in prior hydrodynamic studies of white
dwarf mergers, including the development of MRI turbulence in the hot disk,
magnetized outflows carrying a significant fraction of the disk mass, and the
magnetization of the white dwarf merger to field strengths $\sim 2 \times 10^8$
G. We discuss the impact of our findings on the origins, circumstellar media,
and observed properties of SNe Ia and magnetized white dwarfs.",1302.5700v2
2014-05-03,3D MHD simulation of linearly polarised Alfven wave dynamics in Arnold-Beltrami-Childress magnetic field,"Previous studies [Malara et al ApJ, 533, 523 (2000)] considered
small-amplitude Alfven wave (AW) packets in Arnold-Beltrami-Childress (ABC)
magnetic field using WKB approximation. In this work linearly polarised Alfven
wave dynamics in ABC magnetic field via direct 3D MHD numerical simulation is
studied for the first time. Gaussian AW pulse with length-scale much shorter
than ABC domain length and harmonic AW with wavelength equal to ABC domain
length are studied for four different resistivities. While it is found that AWs
dissipate quickly in the ABC field, surprisingly, AW perturbation energy
increases in time. In the case of the harmonic AW perturbation energy growth is
transient in time, attaining peaks in both velocity and magnetic perturbation
energies within timescales much smaller than resistive time. In the case of the
Gaussian AW pulse velocity perturbation energy growth is still transient in
time, attaining a peak within few resistive times, while magnetic perturbation
energy continues to grow. It is also shown that the total magnetic energy
decreases in time and this is governed by the resistive evolution of the
background ABC magnetic field rather than AW damping. On contrary, when
background magnetic field is uniform, the total magnetic energy decrease is
prescribed by AW damping, because there is no resistive evolution of the
background. By considering runs with different amplitudes and by analysing
perturbation spectra, possible dynamo action by AW perturbation-induced
peristaltic flow and inverse cascade of magnetic energy have been excluded.
Therefore, the perturbation energy growth is attributed to a new instability.
The growth rate appears to be dependent on the value of the resistivity and
spatial scale of the AW disturbance. Thus, when going beyond WKB approximation,
AW damping, described by full MHD equations, does not guarantee decrease of
perturbation energy.",1405.0587v1
2015-04-24,Small-scale magnetic islands in the solar wind and their role in particle acceleration. Part 1: Dynamics of magnetic islands near the heliospheric current sheet,"Increases of ion fluxes in the keV-MeV range are sometimes observed near the
heliospheric current sheet (HCS) during periods when other sources are absent.
These resemble solar energetic particle (SEP) events, but the events are weaker
and apparently local. Conventional explanations based on either shock
acceleration of charged particles or particle acceleration due to magnetic
reconnection at interplanetary current sheets are not persuasive. We suggest
instead that recurrent magnetic reconnection occurs at the HCS and smaller
current sheets in the solar wind (Zharkova & Khabarova 2012), of which a
consequence is particle energization by the dynamically evolving secondary
current sheets and magnetic islands (Zank et al. 2014; Drake et al. 2006a). The
effectiveness of the trapping and acceleration process associated with magnetic
islands depends in part on the topology of the HCS. We show that the HCS
possesses ripples superimposed on the large-scale flat or wavy structure. We
conjecture that the ripples can efficiently confine plasma and provide
tokamak-like conditions that are favorable for the appearance of small-scale
magnetic islands that merge and/or contract. Particles trapped in the vicinity
of merging islands and experiencing multiple small-scale reconnection events
are accelerated by the induced electric field, and experience first-order Fermi
acceleration in contracting magnetic islands (Zank et al. 2014). We present
multi-spacecraft observations of magnetic island merging and particle
energization in the absence of other sources, providing support for theory and
simulations that show particle energization by reconnection related processes
of magnetic island merging and contraction.",1504.06616v2
2016-04-18,Full particle orbit effects in regular and stochastic magnetic fields,"We present a numerical study of charged particle motion in a time-independent
magnetic field in cylindrical geometry. The magnetic field model consists of an
unperturbed reversed-shear helical part and a perturbation consisting of a
superposition of modes. Contrary to most of the previous studies, the particle
trajectories are computed by directly solving the full Lorentz force equations
of motion in a six-dimensional phase space using a sixth-order, implicit,
symplectic Gauss-Legendre method. The level of stochasticity in the particle
orbits is diagnosed using averaged, effective Poincare sections. It is shown
that when only one mode is present the particle orbits can be stochastic even
though the magnetic field line orbits are not stochastic. The lack of
integrability of the particle orbits in this case is related to separatrix
crossing and the breakdown of the global conservation of the magnetic moment.
Some perturbation consisting of two modes creates resonance overlapping,
leading to Hamiltonian chaos in magnetic field lines. Then, the particle orbits
exhibit a nontrivial dynamics depending on their energy and pitch angle. It is
shown that the regions where the particle motion is stochastic decrease as the
energy increases. The non-monotonicity of the $q$-profile implies the existence
of magnetic ITBs which correspond to shearless flux surfaces located in the
vicinity of the $q$-profile minimum. It is shown that depending on the energy,
these magnetic ITBs might or might not confine particles. That is, magnetic
ITBs act as an energy-dependent particle confinement filter. Magnetic field
lines in reversed-shear configurations exhibit topological bifurcations due to
separatrix reconnection. We show that a similar but more complex scenario
appears in the case of particle orbits that depends in a non-trivial way on the
energy and pitch angle of the particles.",1604.05112v2
2016-08-16,"Europium mixed-valence, long-range magnetic order, and dynamic magnetic response in EuCu$_{2}$(Si$_{x}$Ge$_{1-x}$)$_{2}$","In mixed-valence or heavy-fermion systems, the hybridization between local
$f$ orbitals and conduction band states can cause the suppression of long-range
magnetic order, which competes with strong spin fluctuations. Ce- and Yb-based
systems have been found to exhibit fascinating physical properties
(heavy-fermion superconductivity, non-Fermi-liquid states, etc.) when tuned to
the vicinity of magnetic quantum critical points by use of various external
control parameters (temperature, magnetic field, chemical composition).
Recently, similar effects (mixed-valence, Kondo fluctuations, heavy Fermi
liquid) have been reported to exist in some Eu-based compounds. Unlike Ce (Yb),
Eu has a multiple electron (hole) occupancy of its $4f$ shell, and the magnetic
Eu$^{2+}$ state ($4f^7$) has no orbital component in the usual $LS$ coupling
scheme, which can lead to a quite different and interesting physics. In the
EuCu$_{2}$(Si$_{x}$Ge$_{1-x}$)$_{2}$ series, where the valence can be tuned by
varying the Si/Ge ratio, it has been reported that a significant valence
fluctuation can exist even in the magnetic order regime. This paper presents a
detailed study of the latter material using different microscopic probes
(XANES, M\""ossbauer spectroscopy, elastic and inelastic neutron scattering), in
which the composition dependence of the magnetic order and dynamics across the
series is traced back to the change in the Eu valence state. In particular, the
results support the persistence of valence fluctuations into the
antiferromagnetic state over a sizable composition range below the critical Si
concentration $x_c \approx 0.65$. The sequence of magnetic ground states in the
series is shown to reflect the evolution of the magnetic spectral response.",1608.04687v2
2017-06-20,"Filament formation in wind-cloud interactions. II. Clouds with turbulent density, velocity, and magnetic fields","We present a set of numerical experiments designed to systematically
investigate how turbulence and magnetic fields influence the morphology,
energetics, and dynamics of filaments produced in wind-cloud interactions. We
cover 3D magnetohydrodynamic systems of supersonic winds impacting clouds with
turbulent density, velocity, and magnetic fields. We find that log-normal
density distributions aid shock propagation through clouds, increasing their
velocity dispersion and producing filaments with expanded cross sections and
highly-magnetised knots and sub-filaments. In self-consistently turbulent
scenarios the ratio of filament to initial cloud magnetic energy densities is
~1. The effect of Gaussian velocity fields is bound to the turbulence Mach
number: Supersonic velocities trigger a rapid cloud expansion; subsonic
velocities only have a minor impact. The role of turbulent magnetic fields
depends on their tension and is similar to the effect of radiative losses: the
stronger the magnetic field or the softer the gas equation of state, the
greater the magnetic shielding at wind-filament interfaces and the suppression
of Kelvin-Helmholtz instabilities. Overall, we show that including turbulence
and magnetic fields is crucial to understanding cold gas entrainment in
multi-phase winds. While cloud porosity and supersonic turbulence enhance the
acceleration of clouds, magnetic shielding protects them from ablation and
causes Rayleigh-Taylor-driven sub-filamentation. Wind-swept clouds in turbulent
models reach distances ~15-20 times their core radius and acquire bulk speeds
~0.3-0.4 of the wind speed in one cloud-crushing time, which are three times
larger than in non-turbulent models. In all simulations the ratio of turbulent
magnetic to kinetic energy densities asymptotes at ~0.1-0.4, and convergence of
all relevant dynamical properties requires at least 64 cells per cloud radius.",1706.06607v1
2018-01-19,Polarized-neutron investigation of magnetic ordering and spin dynamics in BaCo$_2$(AsO$_4$)$_2$ frustrated honeycomb-lattice magnet,"The magnetic properties of the cobaltite {\BCAO}, a good realization of the
quasi two-dimensional frustrated honeycomb-lattice system with strong planar
anisotropy, have been reinvestigated by means of spherical neutron polarimetry
with CRYOPAD. From accurate measurements of polarization matrices both on
elastic and inelastic contributions as a function of the scattering vector
{\bf{Q}}, we have been able to determine the low-temperature magnetic structure
of {\BCAO} and reveal its puzzling in-plane spin dynamics. Surprisingly, the
ground-state structure (described by an incommensurate propagation vector
${\bf{k}}_{1}=(k_{x}, 0, k_{z}$), with $k_{x}=0.270{\pm}0.005$ and $k_{z}
\approx -1.31$) appears to be a quasi-collinear structure, and not a simple
helix, as previously determined. In addition, our results have revealed the
existence of a non-negligible out-of-plane moment component $ \approx
0.25{\mu}_{B}$/Co$^{2+}$, representing about 10\% of the in-plane component, as
demonstrated by the presence of finite off-diagonal elements $P_{yz}$ and
$P_{zy}$ of the polarization matrix, both on elastic and inelastic magnetic
contributions. Despite a clear evidence of the existence of a slightly
inelastic contribution of structural origin superimposed to the magnetic
excitations at the scattering vectors ${\bf{Q}}=(0.27, 0, 3.1)$ and
${\bf{Q}}=(0.73, 0, 0.8)$ (energy transfer ${\Delta}E \approx 2.3$ meV), no
strong inelastic nuclear-magnetic interference terms could be detected so far,
meaning that the nuclear and magnetic degrees of freedom have very weak
cross-correlations. The strong inelastic $P_{yz}$ and $P_{zy}$ matrix elements
can be understood by assuming that the magnetic excitations in {\BCAO} are spin
waves associated with trivial anisotropic precessions of the magnetic moments
involved in the canted incommensurate structure.",1801.06588v1
2018-10-25,Time-retarded damping and magnetic inertia in the Landau-Lifshitz-Gilbert equation self-consistently coupled to electronic time-dependent nonequilibrium Green functions,"The conventional Landau-Lifshitz-Gilbert (LLG) equation is a widely used tool
to describe dynamics of local magnetic moments, viewed as classical vectors of
fixed length, with their change assumed to take place simultaneously with the
cause. Here we demonstrate that recently developed [M. D. Petrovi\'{c} {\em et
al.}, {\tt arXiv:1802.05682}] self-consistent coupling of the LLG equation to
time-dependent quantum-mechanical description of electrons microscopically
generates time-retarded damping in the LLG equation described by a memory
kernel which is also spatially dependent. For sufficiently slow dynamics of
local magnetic moments, the memory kernel can be expanded to extract the
Gilbert damping (proportional to first time derivative of magnetization) and
magnetic inertia (proportional to second time derivative of magnetization)
terms whose parameters, however, are time-dependent in contrast to
time-independent parameters used in the conventional LLG equation. We use
examples of single or multiple magnetic moments precessing in an external
magnetic field, as well as field-driven motion of a magnetic domain wall (DW),
to quantify the difference in their time evolution computed from conventional
LLG equation vs. TDNEGF+LLG quantum-classical hybrid approach. The faster DW
motion predicted by TDNEGF+LLG approach reveals that important quantum effects,
stemming from finite amount of time which it takes for conduction electron spin
to react to the motion of classical local magnetic moments, are missing from
conventional classical micromagnetics simulations. We also demonstrate large
discrepancy between TDNEGF+LLG-computed numerically exact and, therefore,
nonperturbative result for charge current pumped by a moving DW and the same
quantity computed by perturbative spin motive force formula combined with the
conventional LLG equation.",1810.11016v2
2019-11-22,Anisotropic electrical conductivity of magnetized hot quark matter,"We studied the effect of a strong magnetic field ($B$) on the electrical
conductivity of hot quark matter. The electrical conductivity is a key
transport coefficient determining the time dependence and strength of magnetic
fields generated in a relativistic heavy-ion collision. A~magnetic field
induces Hall anisotropic conduction, phase-space Landau-level quantization and,
if sufficiently strong, interferes with prominent QCD phenomena such as
dynamical quark mass generation, likely affecting the quark matter electrical
conductivity, which depends strongly on the quark masses. To address these
issues, we used a quasi-particle description of quark matter in which the
electric charge carriers are constituent quarks with temperature- and
magnetic-field-dependent masses predicted by a Nambu--Jona-Lasinio model. The
model accurately describes recent lattice QCD results showing magnetic
catalysis at low temperatures and inverse magnetic catalysis at temperatures
close to the pseudo-critical temperature ($T_{\rm pc}$) of the QCD phase
transition. We found that the magnetic field increases the conductivity
component parallel to it and decreases the transverse component, in qualitative
agreement with recent lattice QCD results. In addition, we found that: (1)~the
space anisotropy of the conductivity increases with~$B$, (2)~the longitudinal
conductivity increases due to phase-space Landau-level quantization, (3)~a
lowest Landau level approximation behaves poorly for temperatures close to
$T_{\rm pc}$, and (5)~inverse magnetic catalysis leaves a distinctive signal in
all components of the conductivity, a prominent peak at $T_{\rm pc}$. Our study
adds to the existing body of work on the hot quark matter electrical
conductivity by incorporating nontrivial temperature and magnetic field effects
on dynamical mass generation.",1911.10005v2
2020-06-22,A dynamo amplifies the magnetic field of a Milky-Way-like galaxy,"The magnetic fields of spiral galaxies are so strong that they cannot be
primordial. Their typical values are over one billion times higher than any
value predicted for the early Universe. Explaining this immense growth and
incorporating it in galaxy evolution theories is one of the long-standing
challenges in astrophysics. So far, the most successful theory for the
sustained growth of the galactic magnetic field is the alpha-omega dynamo. This
theory predicts a characteristic dipolar or quadrupolar morphology for the
galactic magnetic field, which has been observed in external galaxies. However,
so far, there has been no direct demonstration of a mean-field dynamo operating
in direct, multi-physics simulations of spiral galaxies. We do so in this work.
We employ numerical models of isolated, star-forming spiral galaxies that
include a magnetized gaseous disk, a dark matter halo, stars, and stellar
feedback. Naturally, the resulting magnetic field has a complex morphology that
includes a strong random component. Using a smoothing of the magnetic field on
small scales, we are able to separate the mean from the turbulent component and
analyze them individually. We find that a mean-field dynamo naturally occurs as
a result of the dynamical evolution of the galaxy and amplifies the magnetic
field by an order of magnitude over half a Gyr. Despite the highly dynamical
nature of these models, the morphology of the mean component of the field is
identical to analytical predictions. This result underlines the importance of
the mean-field dynamo in galactic evolution. Moreover, by demonstrating the
natural growth of the magnetic field in a complex galactic environment, it
brings us a step closer to understanding the cosmic origin of magnetic fields.",2006.12574v1
2021-02-01,Temporal evolution of small-scale internetwork magnetic fields in the solar photosphere,"While the longitudinal field that dominates photospheric network regions has
been studied extensively, small scale transverse fields have recently been
found to be ubiquitous in the quiet internetwork photosphere. Few observations
have captured how this field evolves. We aim to statistically characterise the
magnetic properties and observe the temporal evolution of small scale magnetic
features. We present two high spatial/temporal resolution observations that
reveal the dynamics of two disk centre internetwork regions taken by the new
GRIS/IFU (GREGOR Infrared Spectrograph Integral Field Unit) with the highly
magnetically sensitive Fe I line pair at 15648.52 {\AA} and 15652.87 {\AA}.
With the SIR code, we consider two inversion schemes: scheme 1 (S1), where a
magnetic atmosphere is embedded in a field free medium, and scheme 2 (S2), with
two magnetic models and a fixed stray light component. S1 inversions returned a
median magnetic field strength of 200 and 240 G for the two datasets,
respectively. We consider the median transverse (horizontal) component, among
pixels with Stokes Q or U, and the median unsigned longitudinal (vertical)
component, among pixels with Stokes V, above a noise threshold. We determined
the former to be 263 G and 267 G, and the latter to be 131 G and 145 G, for the
two datasets, respectively. We present three regions of interest (ROIs),
tracking the dynamics of small scale magnetic features. We apply S1 and S2
inversions to specific profiles, and find S2 produces better approximations
when there is evidence of mixed polarities. We find patches of linear
polarization with magnetic flux density between 130 and 150 G, appearing
preferentially at granule/intergranular lane (IGL) boundaries. The weak hG
magnetic field appears to be organised in terms of complex loop structures,
with transverse fields often flanked by opposite polarity longitudinal fields.",2102.00942v4
2021-11-10,Influence of the Magnetic Sub-Lattices in the Double Perovskite Compound LaCaNiReO$_6$,"The magnetism of double perovskites is a complex phenomenon, determined from
intra- or interatomic magnetic moment interactions, and strongly influenced by
geometry. We take advantage of the complementary length and time scales of the
muon spin rotation, relaxation and resonance ($\mu^+$SR) microscopic technique
and bulk AC/DC magnetic susceptibility measurements to study the magnetic
phases of the LaCaNiReO$_6$ double perovskite. As a result we are able to
discern and report a newly found dynamic phase transition and the formation of
magnetic domains below and above the known magnetic transition of this compound
at T$_N$ = 103 K. $\mu^+$SR, serving as a local probe at crystallographic
interstitial sites, reveals a transition from a metastable ferrimagnetic
ordering below T = 103 K to a stable one below T = 30 K. The fast and slow
collective dynamic state of this system are investigated. Between 103 K < T <
230 K, the following two magnetic environments appear, a dense spin region and
a static-dilute spin region. The paramagnetic state is obtained only above T >
270 K. An evolution of the interaction between Ni and Re magnetic sublattices
in this geometrically frustrated fcc perovskite structure, is revealed as a
function of temperature and magnetic field, through the critical behaviour and
thermal evolution of microscopic and macroscopic physical quantities.",2111.05920v2
2022-08-31,Magnetic signatures on mixed-mode frequencies. II. Period spacings as a probe of the internal magnetism of red giants,"Theoretical works have looked into the various topologies and amplitudes, as
well as the stability of the magnetic field that is expected to be present in
the radiative interior of stars evolving after the main sequence. Such internal
magnetic fields have never been observed in evolved stars. As a result, there
is a major piece missing from our global picture of stars as dynamical bodies.
Asteroseismology opened a window onto stellar internal dynamics, as oscillation
frequencies, amplitudes, and lifetimes are affected by processes that are
taking place inside the star. In this scope, magnetic signatures on mixed-mode
frequencies have recently been characterized, but the task of detection remains
challenging as the mixed-mode frequency pattern is highly complex and affected
by rotational effects, while modes of different radial orders are often
intertwined. In this work, we aim to build a bridge between theoretical
prescriptions and complex asteroseismic data analysis to facilitate a future
search and characterization of internal magnetism with asteroseismology. We
investigate the effect of magnetic fields inside evolved stars with solar-like
oscillations on the estimation of the period spacing of gravity-mode components
of simulated mixed gravito-acoustic modes. We derived a new corrected
stretching function of the power spectrum density to account for the presence
of magnetic signatures on their frequencies. We demonstrate that the strong
dependency of the amplitude of the magnetic signature with mixed-mode
frequencies leads to biased estimates of period spacings towards lower values.
We also show that a careful analysis of the oscillation frequency pattern
through various period spacing estimates and across a broad frequency range
might lead to the first detection of magnetic fields inside red giants and at
the same time, we adjust the measured value of gravity-mode period spacing.",2208.14954v1
1994-05-09,Fragmentation of a Magnetized Filamentary Molecular Cloud Rotating around its Axis,"The dynamical instability of a self-gravitating magnetized filamentary cloud
was investigated while taking account of rotation around its axis. The
filamentary cloud of our model is supported against self-gravity in part by
both a magnetic field and rotation. The density distribution in equilibrium was
assumed to be a function of the radial distance from the axis, $ \rho _0 (r) \,
= \, \rho _{\rm c} \, ( 1 \, + \, r ^2 / 8 H ^2 ) ^{-2} $, where $ \rho _{\rm
c} $ and $ H $ are model parameters specifying the density on the axis and the
length scale, respectively; the magnetic filed was assumed to have both
longitudinal ($z$-) and azimuthal ($\varphi$-) components with a strength of $
B _0 (r) \, \propto \, \sqrt{ \rho _0 (r) } $. The rotation velocity was
assumed to be $ v _{0\varphi} \, = \, \Omega _{\rm c} \, r \, (1 \, + \, r ^2 /
8 H ^2 ) ^{-1/2} $. We obtained the growth rate and eigenfunction numerically
for (1) axisymmetric $ ( m \, = \, 0 ) $ perturbations imposed on a rotating
cloud with a longitudinal magnetic field, (2) non-axisymmetric $ ( m \, = \, 1
) $ perturbations imposed on a rotating cloud with a longitudinal magnetic
filed, and (3) axisymmetric perturbations imposed on a rotating cloud with a
helical magnetic field. The fastest growing perturbation is an axisymmetric one
for all of the model clouds studied. Its wavelength is $ \lambda _{\rm max} \,
\le \, 11.14 \, H $ for a non-rotating cloud without a magnetic field, and is
shorter",9405019v1
1995-10-23,The MHD Kelvin-Helmholtz Instability: A Two-Dimensional Numerical Study,"Using a new numerical code we have carried out two-dimensional simulations of
the nonlinear evolution of unstable sheared magnetohydrodynamic flows. We
considered two cases: a strong magnetic field (Alfven Mach number, M_a = 2.5)
and a weak field (M_a =5). Each flow rapidly evolves until it reaches a nearly
steady condition, which is fundamentally different from the analogous
gasdynamic state. Both MHD flows relax to a stable, laminar flow on timescales
less than or of the order of 15 linear growth times, measured from saturation
of the instability. That timescale is several orders of magnitude less than the
nominal dissipation time for these simulated flows, so this condition
represents an quasi-steady relaxed state. The strong magnetic field case
reaches saturation as magnetic tension in the displaced flow boundary becomes
sufficient to stabilize it. That flow then relaxes in a straightforward way to
the steady, laminar flow condition. The weak magnetic field case, on the other
hand, begins development of the vortex expected for gasdynamics, but that
vortex is destroyed by magnetic stresses that locally become strong. Magnetic
topologies lead to reconnection and dynamical alignment between magnetic and
velocity fields. Together these processes produce a sequence of intermittent
vortices and subsequent relaxation to a nearly laminar flow condition in which
the magnetic cross helicity is nearly maximized. Remaining irregularities
consist of a pair of flux tubes straddling the shear layer. Fluctuations within
those features are closely aligned, representing Alfv\'en waves propagating
locally downstream.",9510115v1
1996-10-22,The galactic dynamo effect due to Parker-shearing instability of magnetic flux tubes. I. General formalism and the linear approximation,"In this paper we investigate the idea of Hanasz & Lesch 1993 that the
galactic dynamo effect is due to the Parker instability of magnetic flux tubes.
In addition to the former approach, we take into account more general physical
conditions in this paper, by incorporating cosmic rays and differential forces
due to the axisymmetric differential rotation and the density waves as well. We
present the theory of slender magnetic flux tube dynamics in the thin flux tube
approximation and the Lagrange description. This is the application of the
formalism obtained for solar magnetic flux tubes by Spruit (1981), to the
galactic conditions. We perform a linear stability analysis for the
Parker-shearing instability of magnetic flux tubes in galactic discs and then
calculate the dynamo coefficients. We present a number of new effects which are
very essential for cosmological and contemporary evolution of galactic magnetic
fields. First of all we demonstrate that a very strong dynamo $\alpha$-effect
is possible in the limit of weak magnetic fields in presence of cosmic rays.
Second, we show that the differential force resulting from axisymmetric
differential rotation and the linear density waves causes that the
$\alpha$-effect is essentially magnified in galactic arms and switched off in
the interarm regions. Moreover, we predict a non-uniform magnetic field in
spiral arms and well aligned one in interarm regions. These properties are well
confirmed by recent observational results by Beck & Hoernes (1996)",9610167v2
2000-02-03,Fast Turbulent Reconnection,"Reconnection is the process by which magnetic fields in a conducting fluid
change their topology. This process is essential for understanding a wide
variety of astrophysical processes, including stellar and galactic dynamos and
astrophysical turbulence. To account for solar flares, solar cycles and the
structure of the galactic magnetic field reconnection must be fast, propagating
with a speed close to the Alfven speed. We show that the presence of a random
magnetic field component substantially enhances the reconnection rate and
enables fast reconnection, i.e. reconnection that does not depend on fluid
resistivity. The enhancement of the reconnection rate is achieved via a
combination of two effects. First of all, only small segments of magnetic field
lines are subject to direct Ohmic annihilation. Thus the fraction of magnetic
energy that goes directly into fluid heating goes to zero as fluid resistivity
vanishes. However, the most important enhancement comes from the fact that
unlike the laminar fluid case where reconnection is constrained to proceed line
by line, the presence of turbulence enables many magnetic field lines to enter
the reconnection zone simultaneously. A significant fraction of magnetic energy
goes into MHD turbulence and this enhances reconnection rates through an
increase in the field stochasticity. In this way magnetic reconnection becomes
fast when field stochasticity is accounted for. As a consequence solar and
galactic dynamos are also fast, i.e. do not depend on fluid resistivity.",0002067v1
2002-01-07,Magnetic fields and radio polarization of barred galaxies - 3D dynamo simulations,"A three-dimensional (3D) MHD model is applied to simulate the evolution of a
large-scale magnetic field in a barred galaxy possessing a gaseous halo
extending to about 2.8 kpc above the galactic plane. As the model input we use
a time-dependent velocity field of molecular gas resulting from self-consistent
3D N-body simulations of a galactic disk. We assume that the gaseous halo
rotates differentially co-rotating with the disk or decreasing its velocity in
the Z direction. The dynamo process included in the model yields the
amplification of the magnetic field as well as the formation of field
structures high above the galactic disk. The simulated magnetic fields are used
to construct the models of a high-frequency (Faraday rotation-free) polarized
radio emission that accounts for effects of projection and limited resolution,
and is thus suitable for direct comparison with observations. We found that the
resultant magnetic field correctly reproduces the observed structures of
polarization B-vectors, forming coherent patterns well aligned with spiral arms
and with the bar. The process initializing a wave-like behavior of the magnetic
field, which efficiently forms magnetic maxima between the spiral arms, is
demonstrated. The inclusion of the galactic halo constitutes a step towards a
realistic model of galactic magnetic fields that includes as many dynamical
components as needed for a realistic description.",0201083v2
2003-06-23,High resolution simulations of nonhelical MHD turbulence,"According to the kinematic theory of nonhelical dynamo action the magnetic
energy spectrum increases with wavenumber and peaks at the resistive cutoff
wavenumber. It has previously been argued that even in the dynamical case the
magnetic energy peaks at the resistive scale. Using high resolution simulations
(up to 1024^3 meshpoints) with no large scale imposed field we show that the
magnetic energy peaks at a wavenumber that is independent of the magnetic
Reynolds number and about 5 times larger than the forcing wavenumber.
Throughout the inertial range the spectral magnetic energy exceeds the kinetic
energy by a factor of 2 to 3. Both spectra are approximately parallel. The
total energy spectrum seems to be close to k^{-3/2}, but there is a strong
bottleneck effect and it is suggested that the asymptotic spectrum is instead
k^{-5/3}. This is supported by the value of the second order structure function
exponent that is found to be zeta_2=0.70, suggesting a k^{-1.70} spectrum. The
third order structure function scaling exponent is very close to unity, in
agreement with Goldreich-Sridhar theory.
  Adding an imposed field tends to suppress the small scale magnetic field. We
find that at large scales the magnetic power spectrum follows then a k^{-1}
slope. When the strength of the imposed field is of the same order as the
dynamo generated field, we find almost equipartition between the magnetic and
kinetic power spectra.",0306453v1
2003-10-07,The Effects of Magnetic Fields on Line-Driven Hot-Star Winds,"This talk summarizes results from recent MHD simulations of the role of a
dipole magnetic field in inducing large-scale structure in the line-driven
stellar winds of hot, luminous stars. Unlike previous fixed-field analyses, the
MHD simulations here take full account of the dynamical competition between the
field and the flow. A key result is that the overall degree to which the wind
is influenced by the field depends largely on a single, dimensionless `wind
magnetic confinement parameter', $\eta_\ast (= B_{eq}^2 R_{\ast}^2/\dot{M}
v_\infty$), which characterizes the ratio between magnetic field energy density
and kinetic energy density of the wind. For weak confinement, $\eta_\ast \le
1$, the field is fully opened by wind outflow, but nonetheless, for confinement
as small as $\eta_\ast=1/10$ it can have significant back-influence in
enhancing the density and reducing the flow speed near the magnetic equator.
For stronger confinement, $\eta_\ast > 1$, the magnetic field remains closed
over limited range of latitude and height above the equatorial surface, but
eventually is opened into nearly radial configuration at large radii. Within
the closed loops, the flow is channeled toward loop tops into shock collisions
that are strong enough to produce hard X-rays. Within the open field region,
the equatorial channeling leads to oblique shocks that are again strong enough
to produce X-rays and also lead to a thin, dense, slowly outflowing ``disk'' at
the magnetic equator.",0310180v1
2003-10-10,Magnetic Braking and Viscous Damping of Differential Rotation in Cylindrical Stars,"Differential rotation in stars generates toroidal magnetic fields whenever an
initial seed poloidal field is present. The resulting magnetic stresses, along
with viscosity, drive the star toward uniform rotation. This magnetic braking
has important dynamical consequences in many astrophysical contexts. For
example, merging binary neutron stars can form ""hypermassive"" remnants
supported against collapse by differential rotation. The removal of this
support by magnetic braking induces radial fluid motion, which can lead to
delayed collapse of the remnant to a black hole. We explore the effects of
magnetic braking and viscosity on the structure of a differentially rotating,
compressible star, generalizing our earlier calculations for incompressible
configurations. The star is idealized as a differentially rotating, infinite
cylinder supported initially by a polytropic equation of state. The gas is
assumed to be infinitely conducting and our calculations are performed in
Newtonian gravitation. Though highly idealized, our model allows for the
incorporation of magnetic fields, viscosity, compressibility, and shocks with
minimal computational resources in a 1+1 dimensional Lagrangian MHD code. Our
evolution calculations show that magnetic braking can lead to significant
structural changes in a star, including quasistatic contraction of the core and
ejection of matter in the outermost regions to form a wind or an ambient disk.
These calculations serve as a prelude and a guide to more realistic MHD
simulations in full 3+1 general relativity.",0310304v1
2004-02-24,Distribution of Faraday Rotation Measure in Jets from Active Galactic Nuclei II. Prediction from our Sweeping Magnetic Twist Model for the Wiggled Parts of AGN Jets and Tails,"Distributions of Faraday rotation measure (FRM) and the projected magnetic
field derived by a 3-dimensional simulation of MHD jets are investigated based
on our ""sweeping magnetic twist model"". FRM and Stokes parameters were
calculated to be compared with radio observations of large scale wiggled AGN
jets on kpc scales. We propose that the FRM distribution can be used to discuss
the 3-dimensional structure of magnetic field around jets and the validity of
existing theoretical models, together with the projected magnetic field derived
from Stokes parameters. In the previous paper, we investigated the basic
straight part of AGN jets by using the result of a 2-dimensional axisymmetric
simulation. The derived FRM distribution has a general tendency to have a
gradient across the jet axis, which is due to the toroidal component of the
magnetic field generated by the rotation of the accretion disk. In this paper,
we consider the wiggled structure of the AGN jets by using the result of a
3-dimensional simulation. Our numerical results show that the distributions of
FRM and the projected magnetic field have a clear correlation with the large
scale structure of the jet itself, namely, 3-dimensional helix. Distributions,
seeing the jet from a certain direction, show a good matching with those in a
part of 3C449 jet. This suggests that the jet has a helical structure and that
the magnetic field (especially the toroidal component) plays an important role
in the dynamics of the wiggle formation because it is due to a current-driven
helical kink instability in our model.",0402545v1
2004-08-10,Alignment of Dust in Molecular Clouds,"Polarimetry is one of the most informative techniques of studying magnetic
fields in molecular clouds. How reliable the interpretation of the polarization
maps in terms of magnetic fields is the issue that the grain alignment theory
addresses.
  We show that grain alignment involves several processes acting
simultaneously, but on different time-scales. We explain that rotating dust
grains get substantial magnetic moment that allows them precess fast about
magnetic field lines. As the result, grains preserve their orientation to
magnetic field when the magnetic field direction fluctuates. We point out to
the importance of internal alignment, i.e. the process forces grain axes to be
aligned in respect to the grain angular momentum. We show that subtle quantum
effects, in particular relaxation related to nuclear magnetic moments of atoms
composing the grain, brings to live complex grain motions, e.g. flips. These
flips substantially alter the dynamics of grain and limit the applicability of
earlier theories that did not account for them. We also briefly review basic
physical processes involved in the alignment of grain angular momentum in
respect to interstellar magnetic field. We claim that the bulk of existing
observational data is consistent with the radiative torque alignment mechanism.
In particular, we show that large grains that are known to exist in the cores
of molecular clouds may be aligned by the attenuated external interstellar
radiation field.",0408174v1
2006-01-26,The magnetic field and confined wind of the O star $θ^1$~Orionis~C,"In this paper we confirm the presence of a globally-ordered, kG-strength
magnetic field in the photosphere of the young O star $\theta^1$~Orionis~C, and
examine the properties of its optical line profile variations. A new series of
high-resolution MuSiCoS Stokes $V$ and $I$ spectra has been acquired which
samples approximately uniformly the rotational cycle of $\theta^1$~Orionis~C.
Using the Least-Squares Deconvolution (LSD) multiline technique, we have
succeeded in detecting variable Stokes $V$ Zeeman signatures associated with
the LSD mean line profile. These signatures have been modeled to determine the
magnetic field geometry. We have furthermore examined the profi le variations
of lines formed in both the wind and photosphere using dynamic spectra. Based
on spectrum synthesis fitting of the LSD profiles, we determine that the polar
strength of the magnetic dipole component is $1150 \la B_{\rm d}\la 1800$~G and
that the magnetic obliquity is $27\degr \la \beta \la 68\degr$, assuming
$i=45\pm 20\degr$. The best-fit values for $i=45\degr$ are $B_{\rm d} = 1300
\pm 150 (1\sigma)$~G and $\beta = 50\degr \pm 6\degr (1\sigma)$. Our data
confirm the previous detection of a magnetic field in this star, and
furthermore demonstrate the sinusoidal variability of the longitudinal field
and accurately determine the phases and intensities of the magnetic extrema.
The analysis of ``photospheric'' and ``wind'' line profile variations supports
previous reports of the optical spectroscopic characteristics, and provides
evidence for infall of material within the magnetic equatorial plane.",0601623v1
2007-02-21,Polarimetry toward the IRAS Vela Shell. II. Extinction and Magnetic Fields,"We explore correlations between visual extinction and polarization along the
western side of the IRAS Vela Shell using a published polarimetric catalog of
several hundreds of objects. Our extinction maps along this ionization front
(I-front) find evidence of clumpy structure with typical masses between 1.5 and
6 solar masses and a mean length scale L ~ 0.47pc. The polarimetric data
allowed us to investigate the distribution of the local magnetic field in small
(~pc) scales across the I-front. Using the dispersion of polarization position
angles, we find variations in the kinetic-to-magnetic energy density ratio of,
at least, one order of magnitude along the I-front, with the magnetic pressure
generally dominating over the turbulent motions. These findings suggest that
the magnetic component has a significant contribution to the dynamical balance
of this region. Along the I-front, the mean magnetic field projected on the sky
is [0.018 +/- 0.013]mG. The polarization efficiency seems to change along the
I-front. We attribute high polarization efficiencies in regions of relatively
low extinction to an optimum degree of grain alignment. Analysis of the
mass-to-magnetic flux ratio shows that this quantity is consistent with the
subcritical regime (lambda < 1), showing that magnetic support is indeed
important in the region. Our data extend the overall lambda-N(H2) relation
toward lower density values and show that such trend continues smoothly toward
low N(H2) values. This provides general support for the evolution of initially
subcritical clouds to an eventual supercritical stage.",0702550v1
2005-11-16,"Dynamic relaxation of magnetic clusters in a ferromagnetic (Ga,Mn)As epilayer","A new scenario of the mechanism of intriguing ferromagnetic properties in
Mn-doped magnetic semiconductor (Ga,Mn)As is examined in detail. We find that
magnetic features seen in zero-field cooled and field cooled magnetizations are
not interpreted with a single domain model [Phys. Rev. Lett. 95, 217204
(2005)], and the magnetic relaxation, which is similar to that seen in magnetic
particles and granular systems, is becoming significant at temperatures above
the lower-temperature peak in the temperature dependence of ac susceptibility,
supporting the cluster/matrix model reported in our previous work [Phys. Rev.
Lett. 94, 147203 (2005)]. Cole-Cole analysis reveals that magnetic interactions
between such (Ga,Mn)As clusters are significant at temperatures below the
higher-temperature peak in the temperature dependent ac susceptibility. The
magnetizations of these films disappear above the temperature showing the
higher-temperature peak, which is generally referred to as the Curie
temperature. However, we suggest that these combined results are evidence that
the temperature is actually the blocking temperature of (Ga,Mn)As clusters with
a relatively high hole concentration compared to the (Ga,Mn)As matrix.",0511392v2
2007-04-25,Orbital magnetization and its effects in spin-chiral ferromagnetic Kagome lattice,"Recently, Berry phase in the semiclassical dynamical of Bloch electrons has
been found to make a correction to the phase-space density of states and a
general multi-band formula for finite-temperature orbital magnetization has
been given [Phys. Rev. Lett. \textbf{97}, 026603 (2006)], where the orbital
magnetization $\mathcal{M}$ consists of two parts, i.e., the conventional part
$M_{c}$ and the Berry-phase correction part $M_{\Omega}$. Using this general
formula, we theoretically investigate the orbital magnetization and its effects
on thermoelectric transport and magnetic susceptibility properties of the
two-dimensional \textit{kagom\'{e}} lattice with spin anisotropies included.
The study in this paper is highly interesting by the occurrence of nonzero
Chern number in the lattice. The spin chirality parameter $\phi$ (see text)
results in profound effects on the orbital magnetization properties. It is
found that the two parts in orbital magnetization opposite each other. In
particular, we show that $M_{c}$ and $M_{\Omega}$ yield the paramagnetic and
diamagnetic responses, respectively. It is further shown that the orbital
magnetization displays fully different behavior in the metallic and insulating
regions, which is due to the different roles $M_{c}$ and $M_{\Omega}$ play in
these two regions. The anomalous Nernst conductivity is also calculated, which
displays a peak-valley structure as a function of the electron Fermi energy.",0704.3305v1
2007-05-13,Recent progresses in the simulation of small-scale magnetic fields,"New high-resolution observations reveal that small-scale magnetic flux
concentrations have a delicate substructure on a spatial scale of 0.1''. Its
basic structure can be interpreted in terms of a magnetic flux sheet or tube
that vertically extends through the ambient weak-field or field-free atmosphere
with which it is in mechanical equilibrium. A more refined interpretation comes
from new three-dimensional magnetohydrodynamic simulations that are capable of
reproducing the corrugated shape of magnetic flux concentrations and their
signature in the visible continuum. Furthermore it is shown that the
characteristic asymmetric shape of the contrast profile of facular granules is
an effect of radiative transfer across the rarefied atmosphere of the magnetic
flux concentration. I also discuss three-dimensional radiation
magnetohydrodynamic simulations of the integral layers from the top of the
convection zone to the mid-chromosphere. They show a highly dynamic
chromospheric magnetic field, marked by rapidly moving filaments of stronger
than average magnetic field that form in the compression zone downstream and
along propagating shock fronts. The simulations confirm the picture of flux
concentrations that strongly expand through the photosphere into a more
homogeneous, space filling chromospheric field. Future directions in the
simulation of small-scale magnetic fields are indicated by a few examples of
very recent work.",0705.1848v1
2008-01-09,Fluctuation-dissipation relations in critical coarsening: crossover from unmagnetized to magnetized initial states,"We study the non-equilibrium dynamics of the spherical ferromagnet quenched
to its critical temperature, as a function of the magnetization of the initial
state. The two limits of unmagnetized and fully magnetized initial conditions
can be understood as corresponding to times that are respectively much shorter
and much longer than a magnetization timescale, as in a recent field
theoretical analysis of the $n$-vector model. We calculate exactly the
crossover functions interpolating between these two limits, for the
magnetization correlator and response and the resulting fluctuation-dissipation
ratio (FDR). For $d>4$ our results match those obtained recently from a
Gaussian field theory. For $d<4$, non-Gaussian fuctuations arising from the
spherical constraint need to be accounted for. We extend our framework from the
fully magnetized case to achieve this, providing an exact solution for the
relevant integral kernel. The resulting crossover behaviour is very rich, with
the asymptotic FDR $X^\infty$ depending non-monotonically on the scaled age of
the system. This is traced back to non-monotonicities of the two-time
correlator, themselves the consequence of large magnetization fluctuations on
the crossover timescale. We correct a trivial error in our earlier calculation
for fully magnetized initial states; the corrected FDR is {\em consistent} with
renormalization group expansions to first order in $4-d$ for the longitudinal
fluctuations of the O(n) model in the limit $n\to\infty$.",0801.1381v1
2008-01-29,"Viscous, Resistive Magnetorotational Modes","We carry out a comprehensive analysis of the behavior of the
magnetorotational instability (MRI) in viscous, resistive plasmas. We find
exact, non-linear solutions of the non-ideal magnetohydrodynamic (MHD)
equations describing the local dynamics of an incompressible, differentially
rotating background threaded by a vertical magnetic field when disturbances
with wavenumbers perpendicular to the shear are considered. We provide a
geometrical description of these viscous, resistive MRI modes and show how
their physical structure is modified as a function of the Reynolds and magnetic
Reynolds numbers. We demonstrate that when finite dissipative effects are
considered, velocity and magnetic field disturbances are no longer orthogonal
(as it is the case in the ideal MHD limit) unless the magnetic Prandtl number
is unity. We generalize previous results found in the ideal limit and show that
a series of key properties of the mean Reynolds and Maxwell stresses also hold
for the viscous, resistive MRI. In particular, we show that the Reynolds stress
is always positive and the Maxwell stress is always negative. Therefore, even
in the presence of viscosity and resistivity, the total mean angular momentum
transport is always directed outwards. We also find that, for any combination
of the Reynolds and magnetic Reynolds numbers, magnetic disturbances dominate
both the energetics and the transport of angular momentum and that the total
mean energy density is an upper bound for the total mean stress responsible for
angular momentum transport. The ratios between the Maxwell and Reynolds
stresses and between magnetic and kinetic energy densities increase with
decreasing Reynolds numbers for any magnetic Reynolds number; the lowest limit
of both ratios is reached in the ideal MHD regime.",0801.4570v1
2008-01-31,"Glassy behavior in the ferromagnetic and the non-magnetic insulating states of the rare earth manganates, Ln0.7Ba0.3MnO3 (Ln = Nd or Gd)","While La0.7Ba0.3MnO3 is a ferromagnetic metal (TC = 340 K) with longrange
ordering, Nd0.7Ba0.3MnO3 shows a transition around 150 K with a small increase
in magnetization, but remains an insulator at all temperatures. Gd0.7Ba0.3MnO3
is non-magnetic and insulating at all temperatures. Low field dc magnetization
and ac susceptibility measurements reveal the presence of a transition at
around 150 K in Nd0.7Ba0.3MnO3, and a complex behavior with different
ordering/freezing transitions at 62, 46 and 36 K in the case of Gd0.7Ba0.3MnO3,
the last one being more prominent. The nature of the field dependence of the
magnetization, combined with the slow magnetic relaxation, ageing and memory
effects, suggests that Nd0.7Ba0.3MnO3 is a cluster glass below 150 K, a
situation similar to that found for La_{1-x}SrxCoO3. Gd0.7Ba0.3MnO3, however,
shows non-equilibrium dynamics characteristic of spin glasses, below 36 K. The
difference in nature of the glassy behavior between Gd0.7Ba0.3MnO3 and
Nd0.7Ba0.3MnO3 probably arises because of the larger disorder arising from the
mismatch between the sizes of the A-site cations in the former. Our results on
Nd0.7Ba0.3MnO3 and Gd0.7Ba0.3MnO3 suggest that the magnetic insulating states
often reported for rare earth manganates of the type Ln1-xAxMnO3 (Ln = rare
earth, A = alkaline earth) are likely to be associated with glassy magnetic
behavior.",0802.0018v1
2008-06-13,Testing magnetically confined wind shock models for Beta Cep using XMM-Newton and Chandra phase-resolved X-ray observations,"(abridged) We have performed a set of phase-resolved X-ray observations of
the magnetic B star Beta Cep, for which theoretical models predict the presence
of a confined wind emitting X-rays from stationary shocks.
  We obtained four observations spaced in rotational phase with XMM-Newton and
with Chandra. A detailed analysis of the data was performed to derive both
photometric and spectral parameters from the EPIC data, searching for
rotational modulation, and to derive the location of the X-ray plasma from the
line ratios in the He-like triplets of N, O and Ne from the RGS data. The LETG
data were used to constrain the presence of bulk motions in the plasma.
  The strong rotational modulation predicted by the early, static magnetically
confined wind model for the X-ray emission is not observed in Beta Cep. The
small modulation present goes in the opposite direction, pointing to the
absence of any optically thick disk of neutral material, and showing a
modulation consistent with the later, dynamic models of magnetically confined
wind models in B stars. The lack of observed bulk motion points to the plasma
being confined by a magnetic field, but the low plasma temperature and lack of
any flaring show that the plasma is not heated by magnetic reconnection.
Therefore, the observations point to X-ray emission from shocks in a
magnetically confined wind, with no evidence of an optically thick, dense disk
at the magnetic equator",0806.2275v1
2008-10-16,Deceleration of arbitrarily magnetized GRB ejecta: the complete evolution,"(Abridged) We aim to quantitatively understand the dynamical effect and
observational signatures of magnetization of the GRB ejecta on the onset of the
afterglow. We perform ultrahigh-resolution one-dimensional relativistic MHD
simulations of the interaction of a radially expanding, magnetized ejecta with
the interstellar medium. The need of ultrahigh numerical resolution derives
from the extreme jump conditions in the region of interaction between the
ejecta and the circumburst medium. We study the evolution of an
ultrarelativistic shell all the way to a the self-similar asymptotic phase. Our
simulations show that the complete evolution can be characterized in terms of
two parameters, namely, the \xi parameter introduced by Sari & Piran (1995) and
the magnetization \sigma_0. We exploit this property by producing numerical
models where the shell Lorentz factor is \gamma_0 ~ tens and rescaling the
results to arbitrarily large \gamma_0. We find that the reverse shock is
typically very weak or absent for ejecta characterized by \sigma_0 >~ 1. The
onset of the forward shock emission is strongly affected by the magnetization.
On the other hand, the magnetic energy of the shell is transfered to the
external medium on a short timescale (~several times the duration of the
burst). The later forward shock emission does not contain information for the
initial magnetization of the flow. The asymptotic evolution of strongly
magnetized shells, after they have suffred a substantial deceleration,
resembles that of hydrodynamic shells, i.e., they fully enter in the
Blandford-McKee self-similar regime.",0810.2961v2
2008-11-14,An 84 microGauss Magnetic Field in a Galaxy at Redshift z=0.692,"The magnetic field pervading our Galaxy is a crucial constituent of the
interstellar medium: it mediates the dynamics of interstellar clouds, the
energy density of cosmic rays, and the formation of stars. The field associated
with ionized interstellar gas has been determined through observations of
pulsars in our Galaxy. Radio-frequency measurements of pulse dispersion and the
rotation of the plane of linear polarization, i.e., Faraday rotation, yield an
average value B ~ 3 microGauss. The possible detection of Faraday rotation of
linearly polarized photons emitted by high-redshift quasars suggests similar
magnetic fields are present in foreground galaxies with redshifts z > 1. As
Faraday rotation alone, however, determines neither the magnitude nor the
redshift of the magnetic field, the strength of galactic magnetic fields at
redshifts z > 0 remains uncertain. Here we report a measurement of a magnetic
field of B ~ 84 microGauss in a galaxy at z =0.692, using the same
Zeeman-splitting technique that revealed an average value of B = 6 microGauss
in the neutral interstellar gas of our Galaxy. This is unexpected, as the
leading theory of magnetic field generation, the mean-field dynamo model,
predicts large-scale magnetic fields to be weaker in the past rather than
stronger.",0811.2408v1
2008-12-03,Growth of Magnetic Fields Induced by Turbulent Motions,"We present numerical simulations of driven magnetohydrodynamic (MHD)
turbulence with weak/moderate imposed magnetic fields. The main goal is to
clarify dynamics of magnetic field growth. We also investigate the effects of
the imposed magnetic fields on the MHD turbulence, including, as a limit, the
case of zero external field. Our findings are as follows. First, when we start
off simulations with weak mean magnetic field only (or with small scale random
field with zero imposed field), we observe that there is a stage at which
magnetic energy density grows linearly with time. Runs with different numerical
resolutions and/or different simulation parameters show consistent results for
the growth rate at the linear stage. Second, we find that, when the strength of
the external field increases, the equilibrium kinetic energy density drops by
roughly the product of the rms velocity and the strength of the external field.
The equilibrium magnetic energy density rises by roughly the same amount.
Third, when the external magnetic field is not very strong (say, less than ~0.2
times the rms velocity when measured in the units of Alfven speed), the
turbulence at large scales remains statistically isotropic, i.e. there is no
apparent global anisotropy of order B_0/v. We discuss implications of our
results on astrophysical fluids.",0812.0817v1
2009-01-06,On the analogy between streamlined magnetic and solid obstacles,"Analogies are elaborated in the qualitative description of two systems: the
magnetohydrodynamic (MHD) flow moving through a region where an external local
magnetic field (magnetic obstacle) is applied, and the ordinary hydrodynamic
flow around a solid obstacle. The former problem is of interest both
practically and theoretically, and the latter one is a classical problem being
well understood in ordinary hydrodynamics. The first analogy is the formation
in the MHD flow of an impenetrable region -- core of the magnetic obstacle --
as the interaction parameter $N$, i.e. strength of the applied magnetic field,
increases significantly. The core of the magnetic obstacle is streamlined both
by the upstream flow and by the induced cross stream electric currents, like a
foreign insulated insertion placed inside the ordinary hydrodynamic flow. In
the core, closed streamlines of the mass flow resemble contour lines of
electric potential, while closed streamlines of the electric current resemble
contour lines of pressure. The second analogy is the breaking away of attached
vortices from the recirculation pattern produced by the magnetic obstacle when
the Reynolds number $Re$, i.e. velocity of the upstream flow, is larger than a
critical value. This breaking away of vortices from the magnetic obstacle is
similar to that occurring past a real solid obstacle. Depending on the inlet
and/or initial conditions, the observed vortex shedding can be either symmetric
or asymmetric.",0901.0685v3
2009-01-28,Spin canting in a Dy-based Single-Chain Magnet with dominant next-nearest neighbor antiferromagnetic interactions,"We investigate theoretically and experimentally the static magnetic
properties of single crystals of the molecular-based Single-Chain Magnet (SCM)
of formula [Dy(hfac)$_{3}$NIT(C$_{6}$H$_{4}$OPh)]$_{\infty}$ comprising
alternating Dy$^{3+}$ and organic radicals. A peculiar inversion between maxima
and minima in the angular dependence of the magnetic molar susceptibility
$\chi_{M}$ occurs on increasing temperature. Using information regarding the
monomeric building block as well as an {\it ab initio} estimation of the
magnetic anisotropy of the Dy$^{3+}$ ion, this anisotropy-inversion phenomenon
can be assigned to weak one-dimensional ferromagnetism along the chain axis.
This indicates that antiferromagnetic next-nearest-neighbor interactions
between Dy$^{3+}$ ions dominate, despite the large Dy-Dy separation, over the
nearest-neighbor interactions between the radicals and the Dy$^{3+}$ ions.
Measurements of the field dependence of the magnetization, both along and
perpendicularly to the chain, and of the angular dependence of $\chi_{M}$ in a
strong magnetic field confirm such an interpretation. Transfer matrix
simulations of the experimental measurements are performed using a classical
one-dimensional spin model with antiferromagnetic Heisenberg exchange
interaction and non-collinear uniaxial single-ion anisotropies favoring a
canted antiferromagnetic spin arrangement, with a net magnetic moment along the
chain axis. The fine agreement obtained with experimental data provides
estimates of the Hamiltonian parameters, essential for further study of the
dynamics of rare-earths based molecular chains.",0901.4409v1
2009-09-27,Space-time symmetry violation of the fields in quasi-2D ferrite particles with magnetic-dipolar-mode oscillations,"In magnetic systems with reduced dimensionality, the effects of dipolar
interactions allow the existence of long-range ordered phases. Long-range
magnetic-dipolar interactions are at the heart of the explanation of many
peculiar phenomena observed in nuclear magnetic resonance, ferromagnetic
resonance, and Bose-Einstein-condensate structures. In this paper we show that
magnetic-dipolar-modes (MDMs) in quasi-2D ferrite disks are characterized by
symmetry breaking effects. Our analysis is based on postulates about a physical
meaning of the magnetostatic-potential function as a complex scalar wave
function, which presumes the long-range phase correlations. An important
feature of the MDM oscillations in a ferrite disk concerns the fact that a
structure with symmetric parameters and symmetric basic equations goes into
eigenstates that are not space-time symmetric. The proper solutions are found
based on an analysis of magnetostatic-wave propagation in a helical coordinate
system. For a ferrite disk, we show that while a composition of two helical
waves may acquire a geometrical phase over-running of during a period, every
separate helical wave has a dynamical phase over-running of and so behaves as a
double-valued function. We demonstrate that unique topological structures of
the fields in a ferrite disk are intimately related to the symmetry breaking
properties of MDM oscillations. The solutions give the MDM power-flow-density
vortices with cores at the disk center and azimuthally running waves of
magnetization. One can expect that the proposed models of long-range ordered
phases and space-time violation properties of magnetic-dipolar interactions can
be used in other magnetic structures, different from the
ferromagnetic-resonance system with reduced dimensionality.",0909.4920v1
2010-04-08,CMB temperature anisotropy at large scales induced by a causal primordial magnetic field,"We present an analytical derivation of the Sachs Wolfe effect sourced by a
primordial magnetic field. In order to consistently specify the initial
conditions, we assume that the magnetic field is generated by a causal process,
namely a first order phase transition in the early universe. As for the
topological defects case, we apply the general relativistic junction conditions
to match the perturbation variables before and after the phase transition which
generates the magnetic field, in such a way that the total energy momentum
tensor is conserved across the transition and Einstein's equations are
satisfied. We further solve the evolution equations for the metric and fluid
perturbations at large scales analytically including neutrinos, and derive the
magnetic Sachs Wolfe effect. We find that the relevant contribution to the
magnetic Sachs Wolfe effect comes from the metric perturbations at
next-to-leading order in the large scale limit. The leading order term is in
fact strongly suppressed due to the presence of free-streaming neutrinos. We
derive the neutrino compensation effect dynamically and confirm that the
magnetic Sachs Wolfe spectrum from a causal magnetic field behaves as
l(l+1)C_l^B \propto l^2 as found in the latest numerical analyses.",1004.1405v2
2010-06-18,Probing the magnetic state by linear and non linear ac magnetic susceptibility measurements in under doped manganite Nd0.8Sr0.2MnO3,"We have thoroughly investigated the entire magnetic states of under doped
ferromagnetic insulating manganite Nd0.8Sr0.2MnO3 through temperature dependent
linear and non linear complex ac magnetic susceptibility measurements. This
ferromagnetic insulating manganite is found to have frequency independent
ferromagnetic to paramagnetic transition temperature at around 140 K. At around
90 K (\approx T_f) the sample shows a second frequency dependent re - entrant
magnetic transition as explored through complex ac susceptibility measurements.
Non linear ac susceptibility measurements (higher harmonics of ac
susceptibility) have also been performed (with and without the superposition of
a dc magnetic field) to further investigate the origin of this frequency
dependence (dynamic behavior at this re-entrant magnetic transition).
Divergence of 3rd order susceptibility in the limit of zero exciting field
indicates a spin glass like freezing phenomena. However, large value of spin
relaxation time (?0= 10-8 s) and small value of coercivity (~22 Oe) obtained at
low temperature (below T_f) from critical slowing down model and dc magnetic
measurements, respectively, are in contrast with what generally observed in a
canonical spin glass (?0 = 10-12 - 10-14 s and very large value of coercivity
below freezing temperature). We have attributed our observation to the
formation of finite size ferromagnetic clusters which are formed as consequence
of intrinsic separation and undergo cluster glass like freezing below certain
temperature in this under doped manganite. The results are supported by the
electronic - and magneto - transport data.",1006.3733v1
2010-06-29,"Systematics of the magnetic-Prandtl-number dependence of homogeneous, isotropic magnetohydrodynamic turbulence","We present the results of our detailed pseudospectral direct numerical
simulation (DNS) studies, with up to $1024^3$ collocation points, of
incompressible, magnetohydrodynamic (MHD) turbulence in three dimensions,
without a mean magnetic field. Our study concentrates on the dependence of
various statistical properties of both decaying and statistically steady MHD
turbulence on the magnetic Prandtl number ${\rm Pr_M}$ over a large range,
namely, $0.01 \leq {\rm Pr_M} \leq 10$. We obtain data for a wide variety of
statistical measures such as probability distribution functions (PDFs) of
moduli of the vorticity and current density, the energy dissipation rates, and
velocity and magnetic-field increments, energy and other spectra, velocity and
magnetic-field structure functions, which we use to characterise intermittency,
isosurfaces of quantities such as the moduli of the vorticity and current, and
joint PDFs such as those of fluid and magnetic dissipation rates. Our
systematic study uncovers interesting results that have not been noted
hitherto. In particular, we find a crossover from larger intermittency in the
magnetic field than in the velocity field, at large ${\rm Pr_M}$, to smaller
intermittency in the magnetic field than in the velocity field, at low ${\rm
Pr_M}$. Furthermore, a comparison of our results for decaying MHD turbulence
and its forced, statistically steady analogue suggests that we have strong
universality in the sense that, for a fixed value of ${\rm Pr_M}$, multiscaling
exponent ratios agree, at least within our errorbars, for both decaying and
statistically steady homogeneous, isotropic MHD turbulence.",1006.5585v1
2011-04-22,Can nothing be a superconductor and a superfluid?,"A superconductor is a material that conducts electric current with no
resistance. Superconductivity and magnetism are known to be antagonistic
phenomena: superconductors expel weak external magnetic field (the Meissner
effect) while a sufficiently strong magnetic field, in general, destroys
superconductivity. In a seemingly contradictory statement, we show that a very
strong magnetic field can turn an empty space into a superconductor. The
external magnetic field required for this effect should be about 10^{16} Tesla
(eB ~ 1 GeV^2). The physical mechanism of the exotic vacuum superconductivity
is as follows: in strong magnetic field the dynamics of virtual quarks and
antiquarks is effectively one-dimensional because these electrically charged
particles tend to move along the lines of the magnetic field. In one spatial
dimension a gluon-mediated attraction between a quark and an antiquark of
different flavors inevitably leads to formation of a colorless spin-triplet
bound state (a vector analogue of the Cooper pair) with quantum numbers of an
electrically charged rho meson. Such quark-antiquark pairs condense to form an
anisotropic inhomogeneous superconducting state similar to the Abrikosov vortex
lattice in a type-II superconductor. The onset of the superconductivity of the
charged rho mesons should also induce an inhomogeneous superfluidity of the
neutral rho mesons. The vacuum superconductivity should survive at very high
temperatures of typical Quantum Chromodynamics (QCD) scale of 10^{12} K (T ~
100 MeV). We propose the phase diagram of QCD in the plane ""magnetic field -
temperature"".",1104.4404v1
2011-09-25,Study of the mixed state of La_{1.83}Sr_{0.17}CuO_{4} by means of muon-spin rotation and magnetization experiments in a low magnetic field,"Muon-spin rotation (muSR) experiments are often used to study the magnetic
field distribution in type-II superconductors in the vortex state. Based on the
determination of the magnetic penetration depth it is frequently
speculated---also controversially---about the order-parameter symmetry of the
studied superconductors. This article reports on a combined muSR and
magnetization study of the mixed state in the cuprate high-temperature
superconductor La_{1.83}Sr_{0.17}CuO_{4} in a low magnetic field of 20 mT
applied along the c axis of a single crystal. The macroscopic magnetization
measurements reveal substantial differences for various cooling procedures.
Yet, indicated changes in the vortex dynamics between different temperature
regions as well as the results of the microscopic muSR experiments are
virtually independent of the employed cooling cycles. Additionally, it is found
that the mean magnetic flux density, locally probed by the muons, strongly
increases at low temperatures. This can possibly be explained by a non-random
sampling of the spatial field distribution of the vortex lattice in this
cuprate superconductor caused by intensified vortex pinning.",1109.5367v2
2011-10-18,Lagrangian coherent structures in photospheric flows and their implications for coronal magnetic structure,"Aims. We show how the build-up of magnetic gradients in the Sun's corona may
be inferred directly from photospheric velocity data. This enables computation
of magnetic connectivity measures such as the squashing factor without recourse
to magnetic field extrapolation.
  Methods.Assuming an ideal evolution in the corona, and an initially uniform
magnetic field, the subsequent field line mapping is computed by integrating
trajectories of the (time-dependent) horizontal photospheric velocity field.
The method is applied to a 12 hour high-resolution sequence of photospheric
flows derived from Hinode/SOT magnetograms.
  Results. We find the generation of a network of quasi-separatrix layers in
the magnetic field, which correspond to Lagrangian coherent structures in the
photospheric velocity. The visual pattern of these structures arises primarily
from the diverging part of the photospheric flow, hiding the effect of the
rotational flow component: this is demonstrated by a simple analytical model of
photospheric convection. We separate the diverging and rotational components
from the observed flow and show qualitative agreement with purely diverging and
rotational models respectively. Increasing the flow speeds in the model
suggests that our observational results are likely to give a lower bound for
the rate at which magnetic gradients are built up by real photospheric flows.
Finally, we construct a hypothetical magnetic field with the inferred topology,
that can be used for future investigations of reconnection and energy release.",1110.3957v2
2011-11-03,Scattering of Scalar Waves by Schwarzschild Black Hole Immersed in Magnetic Field,"The magnetic field is one of the most important constituents of the cosmic
space and one of the main sources of the dynamics of interacting matter in the
universe. The astronomical observations imply the existence of a strong
magnetic fields of up to $10^4-10^8G$ near supermassive black holes in the
active galactic nuclei and even around stellar mass black holes. In this paper,
with the quantum scattering theory, we analysis the Schr\""{o}edinger-type
scalar wave equation of black hole immersed in magnetic field and numerically
investigate its absorption cross section and scattering cross section. We find
that the absorption cross sections oscillate about the geometric optical value
in the high frequency regime. Furthermore in low frequency regime, the magnetic
field makes the absorption cross section weaker and this effect is more
obviously on lower frequency brand. On the other hand, for the effects of
scattering cross sections for the black hole immersed in magnetic field, we
find that the magnetic field makes the scattering flux weaker and its width
narrower in the forward direction. We find that there also exists the glory
phenomenon along the backforward direction. At fixed frequency, the glory peak
is higher and the glory width becomes narrower due to the black hole immersed
in magnetic field.",1111.0825v1
2011-12-16,Growth rate of small-scale dynamo at low magnetic Prandtl numbers,"In this study we discuss two key issues related to a small-scale dynamo
instability at low magnetic Prandtl numbers and large magnetic Reynolds
numbers, namely: (i) the scaling for the growth rate of small-scale dynamo
instability in the vicinity of the dynamo threshold; (ii) the existence of the
Golitsyn spectrum of magnetic fluctuations in small-scale dynamos. There are
two different asymptotics for the small-scale dynamo growth rate: in the
vicinity of the threshold of the excitation of the small-scale dynamo
instability, $\lambda \propto \ln({\rm Rm}/ {\rm Rm}^{\rm cr})$, and when the
magnetic Reynolds number is much larger than the threshold of the excitation of
the small-scale dynamo instability, $\lambda \propto {\rm Rm}^{1/2}$, where
${\rm Rm}^{\rm cr}$ is the small-scale dynamo instability threshold in the
magnetic Reynolds number ${\rm Rm}$. We demonstrated that the existence of the
Golitsyn spectrum of magnetic fluctuations requires a finite correlation time
of the random velocity field. On the other hand, the influence of the Golitsyn
spectrum on the small-scale dynamo instability is minor. This is the reason why
it is so difficult to observe this spectrum in direct numerical simulations for
the small-scale dynamo with low magnetic Prandtl numbers.",1112.3926v2
2012-03-27,General Relativistic Simulations of Magnetized Plasmas around Merging Supermassive Black Holes,"Coalescing supermassive black hole binaries are produced by the mergers of
galaxies and are the most powerful sources of gravitational waves accessible to
space-based gravitational observatories. Some such mergers may occur in the
presence of matter and magnetic fields and hence generate an electromagnetic
counterpart. In this Letter, we present the first general relativistic
simulations of magnetized plasma around merging supermassive black holes using
the general relativistic magnetohydrodynamic code Whisky. By considering
different magnetic field strengths, going from non-magnetically dominated to
magnetically dominated regimes, we explore how magnetic fields affect the
dynamics of the plasma and the possible emission of electromagnetic signals. In
particular we observe a total amplification of the magnetic field of ~2 orders
of magnitude which is driven by the accretion onto the binary and that leads to
much stronger electromagnetic signals, more than a factor of 10^4 larger than
comparable calculations done in the force-free regime where such amplifications
are not possible.",1203.6108v2
2012-05-29,Magnetic Interactions in Coalescing Neutron Star Binaries,"It is expected on both evolutionary and empirical grounds that many merging
neutron star (NS) binaries are composed of a highly magnetized NS in orbit with
a relatively low magnetic field NS. I study the magnetic interactions of these
binaries using the framework of a unipolar inductor model. The e.m.f. generated
across the non-magnetic NS as it moves through the magnetosphere sets up a
circuit connecting the two stars. The exact features of this circuit depend on
the uncertain resistance in the space between the stars R_space. Nevertheless,
I show that there are interesting observational and/or dynamical effects
irrespective of its exact value. When R_space is large, electric dissipation as
great as ~10^{46} erg/s (for magnetar-strength fields) occurs in the
magnetosphere, which would exhibit itself as a hard X-ray precursor in the
seconds leading up to merger. With less certainty, there may also be an
associated radio transient, but this would be observed well past merger (~hrs)
because of interstellar dispersion. When R_space is small, electric dissipation
largely occurs in the surface layers of the magnetic NS. This can reach
~10^{49} erg/s during the final ~1 sec before merger, similar to the energetics
and timescales of short gamma-ray bursts. In addition, for dipole fields
greater than ~10^{12} G and a small R_space, magnetic torques spin up the
magnetized NS. This drains angular momentum from the binary and accelerates the
inspiral. A faster coalescence results in less orbits occurring before merger,
which would impact matched-filtering gravitational-wave searches by
ground-based laser interferometers and could create difficulties for studying
alternative theories of gravity with compact inspirals.",1205.6482v1
2012-06-18,Impact of Dynamic Orbital Correlations on Magnetic Excitations in the Normal State of Iron-Based Superconductors,"We show here that orbital degrees of freedom produce a distinct signature in
the magnetic excitation spectrum of iron-based superconductors above the
magnetic ordering temperature. Because $d_{xz}$ and $d_{yz}$ orbitals are
strongly connected with the Fermi surface topology, the nature of magnetic
excitations can be modified significantly due to the presence of either static
or fluctuating orbital correlations. Within a five-orbital itinerant model, we
show that static orbital order generally leads to an enhancement of
commensurate magnetic excitations even when the original Fermi surface lacks
nesting at $(\pi,0)$ or $(0,\pi)$. When long-range orbital order is absent,
Gaussian fluctuations beyond the standard random-phase approximation (RPA)
capture the effects of fluctuating orbital correlations on the magnetic
excitations. We find that commensurate magnetic excitations can also be
enhanced if the orbital correlations are strong. Our results offer a natural
explanation for the incommensurate-to-commensurate transformation observed in a
recent neutron scattering measurement (Z. Xu, et. al., arXiv:1201.4404), and we
propose that this unusual transformation is an important signature to
distinguish orbital from spin physics in the normal state of the pnictides.
Implications for the magnetic and superconducting states are discussed.",1206.4095v2
2012-06-29,Laser spectroscopy of finite size and covering effects in magnetite nanoparticles,"The experiments on the impact of the size of magnetite clusters on various
magnetic properties (magnetic moment, Curie temperature, blocking temperature
etc.) have been carried out. The methods of magnetic separation, centrifuging
of water suspensions of biocompatible iron oxide nanoparticles (NPs) allow
producing fractions with diameter of nanoparticles in the range of 4{\div}22
nm. The size of NPs are controlled by the methods of dynamic light scattering
(DLS), transmission electron microscopy (TEM) and atomic force microscopy
(AFM). For the first time the DLS method is applied in real time to control the
size during the process of the separation of the NPs in aqueous suspensions.
The changes of the size of NPs cause a shift in the Curie temperature and in
the changes in the specific magnetic properties of the iron NPs. The
experimental data is interpreted on the basis of Monte Carlo simulations for
the classical Heisenberg model with different bulk and surface magnetic
moments. It is demonstrated experimentally and by theoretical modeling that
magnetic properties of magnetite NPs are determined not only by their sizes,
but also by the their surface spin states, while both growing and falling
dependences of the magnetic moment (per Fe3O4 formula unit) being possible,
depending on the number of magnetic atoms in the nanoparticle. Both NPs clean
and covered with a bioresorbable layer clusters have been investigated.",1206.6985v5
2012-09-02,Nanoscale magnetic imaging of a single electron spin under ambient conditions,"The detection of ensembles of spins under ambient conditions has
revolutionized the biological, chemical, and physical sciences through magnetic
resonance imaging and nuclear magnetic resonance. Pushing sensing capabilities
to the individual-spin level would enable unprecedented applications such as
single molecule structural imaging; however, the weak magnetic fields from
single spins are undetectable by conventional far-field resonance techniques.
In recent years, there has been a considerable effort to develop nanoscale
scanning magnetometers, which are able to measure fewer spins by bringing the
sensor in close proximity to its target. The most sensitive of these
magnetometers generally require low temperatures for operation, but measuring
under ambient conditions (standard temperature and pressure) is critical for
many imaging applications, particularly in biological systems. Here we
demonstrate detection and nanoscale imaging of the magnetic field from a single
electron spin under ambient conditions using a scanning nitrogen-vacancy (NV)
magnetometer. Real-space, quantitative magnetic-field images are obtained by
deterministically scanning our NV magnetometer 50 nanometers above a target
electron spin, while measuring the local magnetic field using dynamically
decoupled magnetometry protocols. This single-spin detection capability could
enable single-spin magnetic resonance imaging of electron spins on the nano-
and atomic scales and opens the door for unique applications such as mechanical
quantum state transfer.",1209.0203v1
2012-10-01,Magnetic Field Structures Triggering Solar Flares and Coronal Mass Ejections,"Solar flares and coronal mass ejections (CMEs), the most catastrophic
eruptions in our solar system, have been known to affect terrestrial
environments and infrastructure. However, because their triggering mechanism is
still not sufficiently understood, our capacity to predict the occurrence of
solar eruptions and to forecast space weather is substantially hindered. Even
though various models have been proposed to determine the onset of solar
eruptions, the types of magnetic structures capable of triggering these
eruptions are still unclear. In this study, we solved this problem by
systematically surveying the nonlinear dynamics caused by a wide variety of
magnetic structures in terms of three-dimensional magnetohydrodynamic
simulations. As a result, we determined that two different types of small
magnetic structures favor the onset of solar eruptions. These structures, which
should appear near the magnetic polarity inversion line (PIL), include magnetic
fluxes reversed to the potential component or the nonpotential component of
major field on the PIL. In addition, we analyzed two large flares, the X-class
flare on December 13, 2006 and the M-class flare on February 13, 2011, using
imaging data provided by the Hinode satellite, and we demonstrated that they
conform to the simulation predictions. These results suggest that forecasting
of solar eruptions is possible with sophisticated observation of a solar
magnetic field, although the lead time must be limited by the time scale of
changes in the small magnetic structures.",1210.0598v1
2012-10-08,Chiral Magnetic Effect in Hydrodynamic Approximation,"We review derivations of the chiral magnetic effect (ChME) in hydrodynamic
approximation. The reader is assumed to be familiar with the basics of the
effect. The main challenge now is to account for the strong interactions
between the constituents of the fluid. The main result is that the ChME is not
renormalized: in the hydrodynamic approximation it remains the same as for
non-interacting chiral fermions moving in an external magnetic field. The key
ingredients in the proof are general laws of thermodynamics and the
Adler-Bardeen theorem for the chiral anomaly in external electromagnetic
fields. The chiral magnetic effect in hydrodynamics represents a macroscopic
manifestation of a quantum phenomenon (chiral anomaly). Moreover, one can argue
that the current induced by the magnetic field is dissipation free and talk
about a kind of ""chiral superconductivity"". More precise description is a
ballistic transport along magnetic field taking place in equilibrium and in
absence of a driving force. The basic limitation is exact chiral limit while
the temperature--excitingly enough- does not seemingly matter. What is still
lacking, is a detailed quantum microscopic picture for the ChME in
hydrodynamics. Probably, the chiral currents propagate through
lower-dimensional defects, like vortices in superfluid. In case of superfluid,
the prediction for the chiral magnetic effect remains unmodified although the
emerging dynamical picture differs from the standard one.",1210.2186v1
2012-12-11,A mean field dynamo from negative eddy diffusivity,"Using direct numerical simulations, we verify that ""flow IV"" of Roberts
(1972) exhibits dynamo action dominated by horizontally averaged large-scale
magnetic field. With the test-field method we compute the turbulent magnetic
diffusivity and find that it is negative and overcomes the molecular
diffusivity, thus explaining quantitatively the large-scale dynamo for magnetic
Reynolds numbers above $\approx8$. As expected for a dynamo of this type, but
contrary to $\alpha$-effect dynamos, the two horizontal field components grow
independently of each other and have arbitrary amplitude ratios and phase
differences. Small length scales of the mean magnetic field are shown to be
stabilized by the turbulent magnetic diffusivity becoming positive at larger
wavenumbers. Oscillatory decaying or growing solutions have also been found in
certain wavenumber intervals and sufficiently large values of the magnetic
Reynolds number. For magnetic Reynolds numbers below $\approx0.5$ the turbulent
magnetic diffusivity is confirmed to be positive, as expected for all
incompressible flows. Earlier claims of a dynamo driven by a modified
Taylor-Green flow through negative eddy diffusivity could not be confirmed.",1212.2626v3
2014-03-04,Oscillations in solar Jets Observed with the SOT of Hinode: Viscous Effects during Reconnection,"Transverse oscillatory motions and recurrence behavior in the chromospheric
jets observed by Hinode/SOT are studied. A comparison is considered with the
behavior that was noticed in coronal X-ray jets observed by Hinode/XRT. A jet
like bundle observed at the limb in Ca II H line appears to show a magnetic
topology that is similar to X-ray jets (i.e., the Eiffel tower shape). The
appearance of such magnetic topology is usually assumed to be caused by
magnetic reconnection near a null point. Transverse motions of the jet axis are
recorded but no clear evidence of twist is appearing from the highly processed
movie. The aim is to investigate the dynamical behavior of an incompressible
magnetic X-point occurring during the magnetic reconnection in the jet
formation region. The viscous effect is specially considered in the closed
line-tied magnetic X-shape nulls. We perform the MHD numerical simulation in
2-D by solving the visco-resistive MHD equations with the tracing of velocity
and magnetic field. A qualitative agreement with Hinode observations is found
for the oscillatory and non-oscillatory behaviors of the observed solar jets in
both the chromosphere and the corona. Our results suggest that the viscous
effect contributes to the excitation of the magnetic reconnection by generating
oscillations that we observed at least inside this Ca II H line cool solar jet
bundle.",1403.0814v2
2014-03-13,Ribbons characterize magnetohydrodynamic magnetic fields better than lines: a lesson from dynamo theory,"Blackman & Brandenburg argued that magnetic helicity conservation in dynamo
theory can in principle be captured by diagrams of mean field dynamos when the
magnetic fields are represented by ribbons or tubes, but not by lines. Here we
present such a schematic ribbon diagram for the $\alpha^2$ dynamo that tracks
magnetic helicity and provides distinct scales of large scale magnetic
helicity, small scale magnetic helicity, and kinetic helicity involved in the
process. This also motivates our construction of a new ""2.5 scale"" minimalist
generalization of the helicity-evolving equations for the \alpha^2 dynamo that
separately allows for these three distinct length scales while keeping only two
dynamical equations. We solve these equations and, as in previous studies, find
that the large scale field first grows at a rate independent of the magnetic
Reynolds number R_M before quenching to an R_M dependent regime. But we also
show that the larger the ratio of the wavenumber where the small scale current
helicity resides to that of the forcing scale, the earlier the non-linear
dynamo quenching occurs, and the weaker the large scale field is at the turnoff
from linear growth. The harmony between the theory and the schematic diagram
exemplifies a general lesson that magnetic fields in MHD are better visualized
as two-dimensional ribbons (or pairs of lines) rather than single lines.",1403.3445v2
2014-04-07,The Helioseismic and Magnetic Imager (HMI) Vector Magnetic Field Pipeline: SHARPs -- Space-weather HMI Active Region Patches,"A new data product from the Helioseismic and Magnetic Imager (HMI) onboard
the Solar Dynamics Observatory (SDO) called Space-weather HMI Active Region
Patches (SHARPs) is now available. SDO/HMI is the first space-based instrument
to map the full-disk photospheric vector magnetic field with high cadence and
continuity. The SHARP data series provide maps in patches that encompass
automatically tracked magnetic concentrations for their entire lifetime; map
quantities include the photospheric vector magnetic field and its uncertainty,
along with Doppler velocity, continuum intensity, and line-of-sight magnetic
field. Furthermore, keywords in the SHARP data series provide several
parameters that concisely characterize the magnetic-field distribution and its
deviation from a potential-field configuration. These indices may be useful for
active-region event forecasting and for identifying regions of interest. The
indices are calculated per patch and are available on a twelve-minute cadence.
Quick-look data are available within approximately three hours of observation;
definitive science products are produced approximately five weeks later. SHARP
data are available at http://jsoc.stanford.edu and maps are available in either
of two different coordinate systems. This article describes the SHARP data
products and presents examples of SHARP data and parameters.",1404.1879v1
2014-06-11,Dynamic behavior of magnetic avalanches in the spin-ice compound Dy$_2$Ti$_2$O$_7$,"Avalanches of the magnetization, that is to say an abrupt reversal of the
magnetization at a given field, have been previously reported in the spin-ice
compound Dy$_{2}$Ti$_{2}$O$_{7}$. This out-of-equilibrium process, induced by
magneto-thermal heating, is quite usual in low temperature magnetization
studies. A key point is to determine the physical origin of the avalanche
process. In particular, in spin-ice compounds, the origin of the avalanches
might be related to the monopole physics inherent to the system. We have
performed a detailed study of the avalanche phenomena in three single crystals,
with the field oriented along the [111] direction, perpendicular to [111] and
along the [100] directions. We have measured the changing magnetization during
the avalanches and conclude that avalanches in spin ice are quite slow compared
to the avalanches reported in other systems such as molecular magnets. Our
measurements show that the avalanches trigger after a delay of about 500 ms and
that the reversal of the magnetization then occurs in a few hundreds of
milliseconds. These features suggest an unusual propagation of the reversal,
which might be due to the monopole motion. The avalanche fields seem to be
reproducible in a given direction for different samples, but they strongly
depend on the initial state of magnetization and on how the initial state was
achieved.",1406.2954v1
2014-08-09,Direct observations of magnetic flux rope formation during a solar coronal mass ejection,"Coronal mass ejections (CMEs) are the most spectacular eruptive phenomena in
the solar atmosphere. It is generally accepted that CMEs are results of
eruptions of magnetic flux ropes (MFRs). However, a heated debate is on whether
MFRs pre-exist before the eruptions or they are formed during the eruptions.
Several coronal signatures, \textit{e.g.}, filaments, coronal cavities, sigmoid
structures and hot channels (or hot blobs), are proposed as MFRs and observed
before the eruption, which support the pre-existing MFR scenario. There is
almost no reported observation about MFR formation during the eruption. In this
letter, we present an intriguing observation of a solar eruptive event occurred
on 2013 November 21 with the Atmospheric Imaging Assembly on board the
\textit{Solar Dynamic Observatory}, which shows a detailed formation process of
the MFR during the eruption. The process started with the expansion of a
low-lying coronal arcade, possibly caused by the flare magnetic reconnection
underneath. The newly-formed ascending loops from below further pushed the
arcade upward, stretching the surrounding magnetic field. The arcade and
stretched magnetic field lines then curved-in just below the arcade vertex,
forming an X-point. The field lines near the X-point continued to approach each
other and a second magnetic reconnection was induced. It is this high-lying
magnetic reconnection that led to the formation and eruption of a hot blob
($\sim$ 10 MK), presumably a MFR, producing a CME. We suggest that two
spatially-separated magnetic reconnections occurred in this event, responsible
for producing the flare and the hot blob (CME), respectively.",1408.2000v1
2015-01-20,Evolution of field line helicity during magnetic reconnection,"We investigate the evolution of field line helicity for magnetic fields that
connect two boundaries without null points, with emphasis on localized finite-B
magnetic reconnection. Total (relative) magnetic helicity is already recognized
as an important topological constraint on magnetohydrodynamic processes. Field
line helicity offers further advantages because it preserves all topological
information and can distinguish between different magnetic fields with the same
total helicity. Magnetic reconnection changes field connectivity and field line
helicity reflects these changes; the goal of this paper is to characterize that
evolution. We start by deriving the evolution equation for field line helicity
and examining its terms, also obtaining a simplified form for cases where
dynamics are localized within the domain. The main result, which we support
using kinematic examples, is that during localized reconnection in a complex
magnetic field, the evolution of field line helicity is dominated by a
work-like term that is evaluated at the field line endpoints, namely the scalar
product of the generalized field line velocity and the vector potential.
Furthermore, the flux integral of this term over certain areas is very small
compared to the integral of the unsigned quantity, which indicates that changes
of field line helicity happen in a well-organized pairwise manner. It follows
that reconnection is very efficient at redistributing helicity in complex
magnetic fields despite having little effect on the total helicity.",1501.04856v2
2015-06-22,"The structure of steady, relativistic, magnetised jets with rotation","We present equilibrium models of relativistic magnetised, infinite,
axisymmetric jets with rotation propagating through an homogeneous,
unmagnetised ambient medium at rest. The jet models are characterised by six
functions defining the radial profiles of density, pressure, and the toroidal
and axial components of velocity and magnetic field. Fixing the ambient
pressure and the jet rest-mass density and axial components of the flow
velocity and magnetic field, we analyze the influence of the toroidal magnetic
field and several rotation laws on the structure of the equilibrium models. Our
approach excludes by construction the analysis of the self-consistently
magnetically launched jet models or the force-free equilibrium solutions.
Several forbidden regions in the magnetic pitch angle/magnetization plane are
found where models of the class considered in our study could not be settled.
These forbidden regions are associated with the existence of maximum axial and
toroidal magnetic field components compatible with the prescribed equilibrium
condition at the jet surface, and/or an excess of centrifugal force producing
gaps with negative pressures in the jet. The present study can be easily
extended to jet models with different transversal profiles and magnetic field
configurations.
  In the last part of the paper, we test the ability of our RMHD code to
maintain steady equilibrium models of axisymmetric RMHD jets in one and two
spatial dimensions. The one dimensional numerical simulations serve also as a
consistency proof of the fidelity of the analytical steady solutions discussed
in the first part of the paper. The present study allows us to build initial
equilibrium jet models with selected properties for dynamical (and emission)
simulations of magnetised relativistic jets with rotation.",1506.06519v1
2015-08-10,On the distribution of particle acceleration sites in plasmoid-dominated relativistic magnetic reconnection,"We investigate the distribution of particle acceleration sites, independently
of the actual acceleration mechanism, during plasmoid-dominated, relativistic
collisionless magnetic reconnection by analyzing the results of a
particle-in-cell numerical simulation. The simulation is initiated with
Harris-type current layers in pair plasma with no guide magnetic field,
negligible radiative losses, no initial perturbation, and using periodic
boundary conditions. We find that the plasmoids develop a robust internal
structure, with colder dense cores and hotter outer shells, that is recovered
after each plasmoid merger on a dynamical time scale. We use spacetime diagrams
of the reconnection layers to probe the evolution of plasmoids, and in this
context we investigate the individual particle histories for a representative
sample of energetic electrons. We distinguish three classes of particle
acceleration sites associated with (1) magnetic X-points, (2) regions between
merging plasmoids, and (3) the trailing edges of accelerating plasmoids. We
evaluate the contribution of each class of acceleration sites to the final
energy distribution of energetic electrons -- magnetic X-points dominate at
moderate energies, and the regions between merging plasmoids dominate at higher
energies. We also identify the dominant acceleration scenarios, in order of
decreasing importance -- (1) single acceleration between merging plasmoids, (2)
single acceleration at a magnetic X-point, and (3) acceleration at a magnetic
X-point followed by acceleration in a plasmoid. Particle acceleration is absent
only in the vicinity of stationary plasmoids. The effect of magnetic mirrors
due to plasmoid contraction does not appear to be significant in relativistic
reconnection.",1508.02392v2
2015-09-18,Active region upflows: 2. Data driven MHD modeling,"Context. Observations of many active regions show a slow systematic
outflow/upflow from their edges lasting from hours to days. At present no
physical explanation has been proven, while several suggestions have been put
forward. Aims. This paper investigates one possible method for maintaining
these upflows assuming that convective motions drive the magnetic field to
initiate them through magnetic reconnection. Methods. We use Helioseismic and
Magnetic Imager (HMI) data to provide an initial potential three dimensional
magnetic field of the active region NOAA 11123 on 2010 November 13 where the
characteristic upflow velocities are observed. A simple one-dimensional
hydrostatic atmospheric model covering the region from the photosphere to the
corona is derived. Local Correlation Tracking of the magnetic features in the
HMI data is used to derive a proxy for the time dependent velocity field. The
time dependent evolution of the system is solved using a resistive
three-dimensional MagnetoHydro-Dynamic code. Results. The magnetic field
contains several null points located well above the photosphere, with their fan
planes dividing the magnetic field into independent open and closed flux
domains. The stressing of the interfaces between the different flux domains is
expected to provide locations where magnetic reconnection can take place and
drive systematic flows. In this case, the region between the closed and open
flux is identified as the region where observations find the systematic
upflows. Conclusions. In the present experiment, the driving only initiates
magneto-acoustic waves, without driving any systematic upflows at any of the
flux interfaces.",1509.05639v1
2015-11-11,A global galactic dynamo with a corona constrained by relative helicity,"We present a model for a global axisymmetric turbulent dynamo operating in a
galaxy with a corona which treats the supernovae (SNe) and magneto-rotational
instability (MRI) driven turbulence parameters under a common formalism. The
nonlinear quenching of the dynamo is alleviated by inclusion of small-scale
advective and diffusive magnetic helicity fluxes, which allow the gauge
invariant magnetic helicity to be transferred outside the disk and consequently
build up a corona during the course of dynamo action. The time-dependent dynamo
equations are expressed in a separable form and solved through an eigenvector
expansion constructed using the steady-state solutions of the dynamo equation.
The parametric evolution of the dynamo solution allows us to estimate the final
structure of the global magnetic field and the saturated value of the
turbulence parameter $\alpha_m$, even before solving the dynamical equations
for evolution of magnetic fields in the disk and the corona, along with
$\alpha$-quenching. We then solve these equations simultaneously to study the
saturation of large-scale magnetic field, its dependence on the small-scale
magnetic helicity fluxes and corresponding evolution of the force-free field in
the corona. The quadrupolar large-scale magnetic field in the disk is found to
reach equipartition strength within a timescale of 1 Gyr. The large-scale
magnetic field in the corona obtained is much weaker in strength compared to
the field inside the disk and has only a weak impact on the dynamo operation.",1511.03392v2
2016-01-12,Solar Atmospheric Magnetic Energy Coupling: Broad Plasma Conditions and Spectrum Regimes,"Solar variability investigations that include magnetic energy coupling are
paramount to solving many key solar/stellar physics problems, particularly for
understanding the temporal variability of magnetic energy redistribution and
heating processes. Using three years of observations from the {\it Solar
Dynamics Observatory's} Atmospheric Imaging Assembly and Heliosemic Magnetic
Imager; radiative and magnetic fluxes were measured from gross features and at
full-disk scales, respectively. Magnetic energy coupling analyses support
radiative flux descriptions via a plasma heating connectivity of dominant
(magnetic) and diffuse components, specifically of the predominantly closed
field corona. Our work shows that this relationship favors an energetic
redistribution efficiency across large temperature gradients, and potentially
sheds light on the long withstanding issue of diffuse unresolved low corona
emission. The intimacy of magnetic energy redistribution and plasma conditions
revealed by this work holds significant insight for the field of stellar
physics, as we have provided possible means for probing distant sources in
currently limited and/or undetectable radiation distributions.",1601.02986v6
2016-01-26,N-body model of magnetic flux tubes reconnecting in the solar atmosphere,"The investigation of dynamics of the small scale magnetic field on the Sun
photosphere is necessary to understand the physical processes occurring in the
higher layers of solar atmosphere due to the magnetic coupling between the
photosphere and the corona. We present a simulation able to address these
phenomena investigating the statistics of magnetic loops reconnections. The
simulation is based on N-body model approach and is divided in two
computational layers. We simplify the convection problem, interpreting the
larger convective scale, mesogranulation, as the result of the collective
interaction of convective downflow of granular scale. The N-body advection
model is the base to generate a synthetic time series of nanoflares produced by
interacting magnetic loops. The reconnection of magnetic field lines is the
result of the advection of the magnetic footpoints following the velocity field
generated by the interacting downflows. The model gives a quantitative idea of
how much energy is expected to be released by the reconfiguration of magnetic
loops in the quiet Sun.",1601.07105v1
2016-02-24,Nonaxisymmetric MHD instabilities of Chandrasekhar states in Taylor-Couette geometry,"We consider axially periodic Taylor-Couette geometry with insulating boundary
conditions. The imposed basic states are so-called Chandrasekhar states, where
the azimuthal flow $U_\phi$ and magnetic field $B_\phi$ have the same radial
profiles. Mainly three particular profiles are considered: the Rayleigh limit,
quasi-Keplerian, and solid-body rotation. In each case we begin by computing
linear instability curves and their dependence on the magnetic Prandtl number
Pm. For the azimuthal wavenumber m=1 modes, the instability curves always scale
with the Reynolds number and the Hartmann number. For sufficiently small Pm
these modes therefore only become unstable for magnetic Mach numbers less than
unity, and are thus not relevant for most astrophysical applications. However,
modes with m>10 can behave very differently. For sufficiently flat profiles,
they scale with the magnetic Reynolds number and the Lundquist number, thereby
allowing instability also for the large magnetic Mach numbers of astrophysical
objects. We further compute fully nonlinear, three-dimensional equilibration of
these instabilities, and investigate how the energy is distributed among the
azimuthal (m) and axial (k) wavenumbers. In comparison spectra become steeper
for large m, reflecting the smoothing action of shear. On the other hand
kinetic and magnetic energy spectra exhibit similar behavior: if several
azimuthal modes are already linearly unstable they are relatively flat, but for
the rigidly rotating case where m=1 is the only unstable mode they are so steep
that neither Kolmogorov nor Iroshnikov-Kraichnan spectra fit the results. The
total magnetic energy exceeds the kinetic energy only for large magnetic
Reynolds numbers Rm>100.",1602.07436v1
2016-04-26,Quantifying the Topology and Evolution of a Magnetic Flux Rope Associated with Multi-flare Activities,"Magnetic flux rope (MFR) plays an important role in solar activities. A
quantitative assessment of the topology of an MFR and its evolution is crucial
for a better understanding of the relationship between the MFR and the
associated activities. In this paper, we investigate the magnetic field of
active region 12017 from 2014 March 28 to 29, where 12 flares were triggered by
the intermittent eruptions of a filament (either successful or confined). Using
the vector magnetic field data from the Helioseismic and Magnetic Imager on
board the \textit{Solar Dynamics Observatory}, we calculate the magnetic energy
and helicity injection in the active region, and extrapolate the 3D magnetic
field with a nonlinear force-free field model. From the extrapolations, we find
an MFR that is cospatial with the filament. We further determine the
configuration of this MFR by a closed quasi-separatrix layer (QSL) around it.
Then, we calculate the twist number and the magnetic helicity for the field
lines composing the MFR. The results show that the closed QSL structure
surrounding the MFR gets smaller as a consequence of the flare occurrence. We
also find that the flares in our sample are mainly triggered by kink
instability. Moreover, the twist number varies more sensitively than other
parameters to the occurrence of flares.",1604.07502v1
2016-04-26,"Quark-hadron phase structure, thermodynamics and magnetization of QCD matter","SU($3$) Polyakov linear-sigma model (PLSM) is systematically implemented to
characterize the quark-hadron phase structure and to determine various
thermodynamic quantities and magnetization of the QCD matter. In mean-field
approximation, the dependence of the chiral order-parameter on finite magnetic
field is also calculated. In a wide range of temperatures and magnetic field
strengths, various thermodynamic quantities including trace anomaly, speed of
sound squared, entropy density, specific heat are presented and some magnetic
properties are described, as well. Wherever available these results are
confronted to recent lattice QCD calculations. The temperature dependence of
these quantities confirms our previous result that the transition temperature
is reduced with the increase in the magnetic field strength, i.e. QCD matter is
to be characterized by an inverse magnetic catalysis. Furthermore, the
temperature dependence of the magnetization shows that the conclusion that the
QCD matter has paramagnetic properties slightly below and far above the
pseudo-critical temperature, is confirmed, as well. The excellent agreement
with recent lattice calculations proves that our QCD-like approach (PLSM) seems
to possess the correct degrees-of-freedom in both hadronic and partonic phases
and describes well the dynamics deriving confined hadrons to deconfined
quark-gluon plasma.",1604.08174v2
2016-05-04,"Insight into the magnetism of a distorted Kagome lattice, Dy3Ru4Al12, based on polycrystalline studies","The layered compound with distorted Kagome nets, Dy3Ru4Al12, was previously
reported to undergo antiferromagnetic ordering below (TN=) 7 K, based on
investigations on single crystals. Here, we report the results of our
investigation of ac and dc magnetic susceptibility (\c{hi}), isothermal remnant
magnetization (MIRM), heat-capacity, magnetocaloric effect and
magnetoresistance measurements on polycrystals. The present results reveal that
there is an additional magnetic anomaly around 20 K, as though the N\'eel order
is preceded by the formation of ferromagnetic clusters. We attribute this
feature to geometric frustration of magnetism. In view of the existence of this
phase, the interpretation of the linear-term in the heat-capacity in terms of
spin-fluctuations from the Ru 4d band needs to be revisited. Additionally, in
the vicinity of TN, AC \c{hi} shows a prominent frequency dependence and, below
TN, MIRM exhibits a slow decay with time. This raises a question whether the
antiferromagnetic structure in this compound is characterized by
spin-glass-like dynamics. In contrast to what was reported earlier, there is a
change in the sign of the magnetoresistance (MR) at the metamagnetic
transition. A butter-fly-shaped (isothermal) MR loop (interestingly spanning
over all the four quadrants) is observed at 2 K with distinct evidence for the
magnetic phase co-existence phenomenon in zero field after travelling through
metamagnetic transition field. The results on polycrystals thus provide
additional information about the magnetism of this compound, revealing that the
magnetism of this compound is more complex than what is believed, due to
geometric frustration intrinsic to Kagome net.",1605.01147v1
2016-07-13,Comparison of magnetic properties in a magnetic cloud and its solar source on April 11-14 2013,"In the context of Sun-Earth connection of coronal mass ejections and magnetic
flux ropes (MFRs), we studied the solar active region (AR) and the magnetic
properties of magnetic cloud (MC) event during April 14-15, 2013. We use
in-situ observations from the Advanced Composition Explorer and source AR
measurements from the Solar Dynamic Observatory. The MCs magnetic structure is
reconstructed from the Grad-Shafranov method which reveals a northern component
of the axial field with left-handed helicity. The MC invariant axis is highly
inclined to the ecliptic plane pointing northward and is rotated by $117^o$
with respect to the source region PIL. The net axial flux and current in the MC
are comparatively higher than from the source region. Linear force-free alpha
distribution ($10^{-7}-10^{-6}$ m$^{-1}$) at the sigmoid leg matches the range
of twist number in the MC of 1-2 AU MFR. The MFR is non-linear force-free with
decreasing twist from the axis (9 turns/AU) towards the edge. Therefore
Gold-Hoyle (GH) configuration, assuming a constant twist, is more consistent
with the MC structure than the Lundquist configuration of increasing twist from
the axis to boundary. As an indication to that, the GH configuration yields
better fitting to the global trend of in-situ magnetic field components, in
terms of rms, than the Lundquist model. These cylindrical configurations
improved the MC fitting results when considered the effect of self-similar
expansion of MFR. For such twisting behaviour, this study suggests an
alternative fitting procedure to better characterise the MC magnetic structure
and its source region links.",1607.03811v1
2016-09-01,Magnetic Field Induced Vortex Lattice Transition in HgBa$_{2}$CuO$_{4+δ}$,"Measurements of the $^{17}$O nuclear magnetic resonance (NMR) quadrupolar
spectrum of apical oxygen in HgBa$_{2}$CuO$_{4+\delta}$ were performed over a
range of magnetic fields from 6.4 to 30\,T in the superconducting state. Oxygen
isotope exchanged single crystals were investigated with doping corresponding
to superconducting transition temperatures from 74\,K underdoped, to 78\,K
overdoped. The apical oxygen site was chosen since its NMR spectrum has narrow
quadrupolar satellites that are well separated from any other resonance.
Non-vortex contributions to the spectra can be deconvolved in the time domain
to determine the local magnetic field distribution from the vortices. Numerical
analysis using Brandt's Ginzburg-Landau theory was used to find structural
parameters of the vortex lattice, penetration depth, and coherence length as a
function of magnetic field in the vortex solid phase. From this analysis we
report a vortex structural transition near 15\,T from an oblique lattice with
an opening angle of $73^{\circ}$ at low magnetic fields to a triangular lattice
with $60^{\circ}$ stabilized at high field. The temperature for onset of vortex
dynamics has been identified with vortex lattice melting. This is independent
of the magnetic field at sufficiently high magnetic field similar to that
reported for YBa$_2$Cu$_3$O$_7$ and Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$
and is correlated with mass anisotropy of the material. This behavior is
accounted for theoretically only in the limit of very high anisotropy.",1609.00414v1
2016-09-22,Magnetization oscillations and waves driven by pure spin currents,"Recent advances in the studies of pure spin currents - flows of angular
momentum (spin) not accompanied by the electric currents - have opened new
horizons for the emerging technologies based on the electron's spin degree of
freedom, such as spintronics and magnonics. The main advantage of pure spin
current, as compared to the spin-polarized electric current, is the possibility
to exert spin transfer torque on the magnetization in thin magnetic films
without electrical current flow through the material. In addition to minimizing
Joule heating and electromigration effects, this characteristic enables the
implementation of spin torque devices based on the low-loss insulating magnetic
materials, and offers an unprecedented geometric flexibility. Here we review
the recent experimental achievements in investigations of magnetization
oscillations excited by pure spin currents in different magnetic nanosystems
based on metallic and insulating magnetic materials. We discuss the spectral
properties of spin-current nano-oscillators, and relate them to the spatial
characteristics of the excited dynamic magnetic modes determined by the
spatially-resolved measurements. We also show that these systems support
locking of the oscillations to external microwave signals, as well as their
mutual synchronization, and can be used as efficient nanoscale sources of
propagating spin waves.",1609.06899v1
2016-10-10,Study on creation and destruction of transport barriers via effective safety factors for energetic particles,"Charged particles with low kinetic energy move along magnetic field lines,
but so do not energetic particles. We investigate the topological structure
changes in the phase space of energetic particles with respect to the magnetic
one. For this study cylindrical magnetic fields with non-monotonic safety
factors that induce the magnetic internal transport barrier are considered. We
show that the topological structure of the magnetic field line and of the
particle trajectories can be quite different. We explain this difference using
the concept of effective particle $q$-profile. Using this notion we can
investigate the location and existence of resonances for particle orbits that
are different from the magnetic ones. These are examined both numerically by
integrating an equation of motion and theoretically by use of Alfv\'en's
guiding center theory and by use of the effective reduced Hamiltonian for the
integrable unperturbed system. It is clarified that, for the energetic
particles, the grad $B$ drift effect shifts the resonances and the drift
induced by curvature of the magnetic field line leads to the vanishing of the
resonances. As a result, we give two different mechanisms that lead to the
creation of transport barriers for energetic particles in the region where the
magnetic field line is chaotic.",1610.02867v1
2016-10-26,Magnetic field amplification in turbulent astrophysical plasmas,"Magnetic fields play an important role in astrophysical accretion discs, and
in the interstellar and intergalactic medium. They drive jets, suppress
fragmentation in star-forming clouds and can have a significant impact on the
accretion rate of stars. However, the exact amplification mechanisms of cosmic
magnetic fields remain relatively poorly understood. Here I start by reviewing
recent advances in the numerical and theoretical modelling of the 'turbulent
dynamo', which may explain the origin of galactic and inter-galactic magnetic
fields. While dynamo action was previously investigated in great detail for
incompressible plasmas, I here place particular emphasis on highly compressible
astrophysical plasmas, which are characterised by strong density fluctuations
and shocks, such as the interstellar medium. I find that dynamo action works
not only in subsonic plasmas, but also in highly supersonic, compressible
plasmas, as well as for low and high magnetic Prandtl numbers. I further
present new numerical simulations from which I determine the growth of the
turbulent (un-ordered) magnetic field component ($B_\mathrm{turb}$) in the
presence of weak and strong guide fields ($B_0$). I vary $B_0$ over 5 orders of
magnitude and find that the dependence of $B_\mathrm{turb}$ on $B_0$ is
relatively weak, and can be explained with a simple theoretical model in which
the turbulence provides the energy to amplify $B_\mathrm{turb}$. Finally, I
discuss some important implications of magnetic fields for the structure of
accretion discs, the launching of jets, and the star formation rate of
interstellar clouds.",1610.08132v2
2016-10-26,Skyrmion production on demand by homogeneous DC currents,"Topological magnetic textures - like skyrmions - have become a major player
in the design of next-generation magnetic storage technology due to their
stability and the control of their motion by ultra-low current densities. A
major challenge to develop this new skyrmion-based technology is to achieve the
controlled and deterministic creation of magnetic skyrmions without the need of
complex setups. We demonstrate a solution to this challenge by showing how to
create skyrmions and other magnetic textures in ferromagnetic thin films by
means of a homogeneous DC current and without requiring Dzyaloshinskii-Moriya
interactions. This is possible by exploiting a static loss of stability arising
from the interplay of current-induced spin-transfer torque and a spatially
inhomogeneous magnetization, which can be achieved, e.g., by locally
engineering the anisotropy, the magnetic field, or other magnetic interactions.
The magnetic textures are created controllably, efficiently, and periodically
with a period that can be tuned by the applied current strength. We propose
specific experimental setups realizable with simple materials, such as cobalt
based materials, to observe the periodic formation of skyrmions. We show that
adding chiral interactions will not influence the basics of the generations but
then influence the consequent dynamics with respect to the stabilization of
topological textures. Our findings allow for the production of skyrmions on
demand in simple ferromagnetic thin films by homogeneous DC currents.",1610.08313v2
2017-01-13,The role of magnetic fields in the structure and interaction of supershells,"Large-scale shocks formed by clustered feedback of young OB stars are
considered an important source of mechanical energy for the ISM and a trigger
of molecular cloud formation. Their interaction sites are locations where
kinetic energy and magnetic fields are redistributed between ISM phases. In
this work we study the effect of the magnetic field on the expansion and
fragmentation of supershells and look for the signatures of supershell
collisions on dense structures and on the kinetic and magnetic energy
distribution of the ISM. We performed a series of high-resolution,
three-dimensional simulations of colliding supershells. The shocks are created
by time-dependent feedback and evolve in a diffuse turbulent environment that
is either unmagnetized or has different initial magnetic field configurations.
In the hydrodynamical situation, the expansion law of the superbubbles is
consistent with the radius-time relation that is theoretically predicted for
wind-blown bubbles. The supershells fragment over their entire surface into
small dense clumps that carry more than half of the total kinetic energy in the
volume. However, this is not the case when a magnetic field is introduced,
either in the direction of the collision or perpendicular to the collision. In
magnetized situations, the shell surfaces are more stable to dynamical
instabilities. When the magnetic field opposes the collision, the expansion law
of the supershells also becomes significantly flatter than in the
hydrodynamical case. Although a two-phase medium arises in all cases, in the
MHD simulations the cold phase is limited to lower densities.",1701.03696v1
2017-01-29,Observational Evidence of Magnetic Reconnection for Brightenings and Transition Region Arcades in IRIS observations,"By using a new method of forced-field extrapolation, we study the emerging
flux region AR 11850 observed by the Interface Region Imaging Spectrograph
(IRIS) and Solar Dynamical Observatory (SDO). Our results suggest that the
bright points (BPs) in this emerging region have responses in lines formed from
the upper photosphere to the transition region, with a relatively similar
morphology. They have an oscillation of several minutes according to the
Atmospheric Imaging Assembly (AIA) data at 1600 and 1700 A . The ratio between
the BP intensities measured in 1600 A and 1700 A filtergrams reveals that these
BPs are heated differently. Our analysis of the Helioseismic and Magnetic
Imager (HMI) vector magnetic field and the corresponding topology in AR11850
indicates that the BPs are located at the polarity inversion line (PIL) and
most of them related with magnetic reconnection or cancelation. The heating of
the BPs might be different due to different magnetic topology. We find that the
heating due to the magnetic cancelation would be stronger than the case of bald
patch reconnection. The plasma density rather than the magnetic field strength
could play a dominant role in this process. Based on physical conditions in the
lower atmosphere, our forced-field extrapolation shows consistent results
between the bright arcades visible in slit-jaw image (SJI) 1400 A and the
extrapolated field lines that pass through the bald patches. It provides a
reliable observational evidence for testing the mechanism of magnetic
reconnection for the BPs and arcades in emerging flux region, as proposed in
simulation works.",1701.08356v1
2017-02-16,Plasma Brightenings in a Failed Solar Filament Eruption,"Failed filament eruptions are solar eruptions that are not associated with
coronal mass ejections. In a failed filament eruption, the filament materials
usually show some ascending and falling motions as well as generate bright EUV
emissions. Here we report a failed filament eruption that occurred in a
quiet-Sun region observed by the Atmospheric Imaging Assembly on board the
Solar Dynamics Observatory. In this event, the filament spreads out but gets
confined by the surrounding magnetic field. When interacting with the ambient
magnetic field, the filament material brightens up and flows along the magnetic
field lines through the corona to the chromosphere. We find that some materials
slide down along the lifting magnetic structure containing the filament and
impact the chromosphere to cause two ribbon-like brightenings in a wide
temperature range through kinetic energy dissipation. There is evidence
suggesting that magnetic reconnection occurs between the filament magnetic
structure and the surrounding magnetic fields where filament plasma is heated
to coronal temperatures. In addition, thread-like brightenings show up on top
of the erupting magnetic fields at low temperatures, which might be produced by
an energy imbalance from a fast drop of radiative cooling due to plasma
rarefaction. Thus, this single event of failed filament eruption shows
existence of a variety of plasma brightenings that may be caused by completely
different heating mechanisms.",1702.05136v1
2017-04-27,Magnetorotational Dynamo Action in the Shearing Box,"Magnetic dynamo action caused by the magnetorotational instability is studied
in the shearing-box approximation with no imposed net magnetic flux. Consistent
with recent studies, the dynamo action is found to be sensitive to the aspect
ratio of the box: it is much easier to obtain in tall boxes (stretched in the
direction normal to the disk plane) than in long boxes (stretched in the radial
direction). Our direct numerical simulations indicate that the dynamo is
possible in both cases, given a large enough magnetic Reynolds number. To
explain the relatively larger effort required to obtain the dynamo action in a
long box, we propose that the turbulent eddies caused by the instability most
efficiently fold and mix the magnetic field lines in the radial direction. As a
result, in the long box the scale of the generated strong azimuthal
(stream-wise directed) magnetic field is always comparable to the scale of the
turbulent eddies. In contrast, in the tall box the azimuthal magnetic flux
spreads in the vertical direction over a distance exceeding the scale of the
turbulent eddies. As a result, different vertical sections of the tall box are
permeated by large-scale nonzero azimuthal magnetic fluxes, facilitating the
instability. In agreement with this picture, the cases when the dynamo is
efficient are characterized by a strong intermittency of the local azimuthal
magnetic fluxes.",1704.08636v1
2017-07-26,Stabilizing isolated skyrmions at low magnetic fields exploiting vanishing magnetic anisotropy,"Skyrmions are topologically protected non-collinear magnetic structures.
Their stability and dynamics, arising from their topological character, have
made them ideal information carriers e.g. in racetrack memories. The success of
such a memory critically depends on the ability to stabilize and manipulate
skyrmions at low magnetic fields. The driving force for skyrmion formation is
the non-collinear Dzyaloshinskii-Moriya exchange interaction (DMI) originating
from spin-orbit coupling (SOC). It competes with both the nearest neighbour
Heisenberg exchange interaction and the magnetic anisotropy, which favour
collinear states. While skyrmion lattices might evolve at vanishing magnetic
fields, the formation of isolated skyrmions in ultra-thin films so far required
the application of an external field which can be as high as several T. Here,
we show that isolated skyrmions in a monolayer (ML) of Co epitaxially grown on
a Ru(0001) substrate can be stabilized at magnetic fields as low as 100 mT.
Even though SOC is weak in the 4d element Ru, a homochiral spin spiral ground
state and isolated skyrmions could be detected and laterally resolved using a
combination of tunneling and anisotropic tunneling magnetoresistance effect in
spin-sensitive scanning tunneling microscopy (STM). Density functional theory
(DFT) calculations confirm these chiral magnetic textures, even though the
stabilizing DMI interaction is weak. We find that the key factor is the absence
of magnetocristalline anisotropy in this system which enables non-collinear
states to evolve in spite of weak SOC, opening up a wide choice of materials
beyond 5d elements.",1707.08519v1
2017-09-25,On magnetic field amplification and particle acceleration near non-relativistic astrophysical shocks: Particles in MHD Cells simulations,"We present simulations of magnetized astrophysical shocks taking into account
the interplay between the thermal plasma of the shock and supra-thermal
particles. Such interaction is depicted by combining a grid-based
magneto-hydrodynamics description of the thermal fluid with particle in cell
techniques devoted to the dynamics of supra-thermal particles. This approach,
which incorporates the use of adaptive mesh refinement features, is potentially
a key to simulate astrophysical systems on spatial scales that are beyond the
reach of pure particle-in-cell simulations. We consider in this study
non-relativistic shocks with various Alfvenic Mach numbers and magnetic field
obliquity. We recover all the features of both magnetic field amplification and
particle acceleration from previous studies when the magnetic field is parallel
to the normal to the shock. In contrast with previous particle-in-cell-hybrid
simulations, we find that particle acceleration and magnetic field
amplification also occur when the magnetic field is oblique to the normal to
the shock but on larger timescales than in the parallel case. We show that in
our simulations, the supra-thermal particles are experiencing acceleration
thanks to a pre-heating process of the particle similar to a shock drift
acceleration leading to the corrugation of the shock front. Such oscillations
of the shock front and the magnetic field locally help the particles to enter
the upstream region and to initiate a non-resonant streaming instability and
finally to induce diffuse particle acceleration.",1709.08482v2
2017-11-22,Dynamical onset of superconductivity and retention of magnetic fields in cooling neutron stars,"A superconductor of paired protons is thought to form in the core of neutron
stars soon after their birth. Minimum energy conditions suggest magnetic flux
is expelled from the superconducting region due to the Meissner effect, such
that the neutron star core is largely devoid of magnetic fields for some
nuclear equation of state and proton pairing models. We show via neutron star
cooling simulations that the superconducting region expands faster than flux is
expected to be expelled because cooling timescales are much shorter than
timescales of magnetic field diffusion. Thus magnetic fields remain in the bulk
of the neutron star core for at least 10^6-10^7 yr. We estimate the size of
flux free regions at 10^7 yr to be <~ 100 m for a magnetic field of 10^11 G and
possibly smaller for stronger field strengths. For proton pairing models that
are narrow, magnetic flux may be completely expelled from a thin shell of
approximately the above size after 10^5 yr. This shell may insulate lower
conductivity outer layers, where magnetic fields can diffuse and decay faster,
from fields maintained in the highly conducting deep core.",1711.08480v1
2017-11-23,A Statistical Study of Photospheric Magnetic Field Changes During 75 Solar Flares,"Abrupt and permanent changes of photospheric magnetic fields have been
observed during solar flares. The changes seem to be linked to the
reconfiguration of magnetic fields, but their origin is still unclear. We
carried out a statistical analysis of permanent line-of-sight magnetic field
($B_{\rm LOS}$) changes during 18 X-, 37 M-, 19 C- and 1 B-class flares using
data from Solar Dynamics Observatory/Helioseismic and Magnetic Imager. We
investigated the properties of permanent changes, such as frequency, areas, and
locations. We detected changes of $B_{\rm LOS}$ in 59/75 flares. We find that
strong flares are more likely to show changes, with all flares $\ge$ M1.6
exhibiting them. For weaker flares, permanent changes are observed in 6/17
C-flares. 34.3\% of the permanent changes occurred in the penumbra and 18.9\%
in the umbra. Parts of the penumbra appeared or disappeared in 23/75 flares.
The area where permanent changes occur is larger for stronger flares. Strong
flares also show a larger change of flux, but there is no dependence of the
magnetic flux change on the heliocentric angle. The mean rate of change of
flare-related magnetic field changes is 20.7 Mx cm$^{-2}$ min$^{-1}$. The
number of permanent changes decays exponentially with distance from the
polarity inversion line. The frequency of the strength of permanent changes
decreases exponentially, and permanent changes up to 750 Mx cm$^{-2}$ were
observed. We conclude that permanent magnetic field changes are a common
phenomenon during flares, and future studies will clarify their relation to
accelerated electrons, white light emission, and sunquakes to further
investigate their origin.",1711.08631v2
2017-12-17,"Low temperature ferromagnetic properties, magnetic field induced spin order and random spin freezing effect in Ni1.5Fe1.5O4 ferrite; prepared at different pH values and annealing temperatures","We present the low temperature magnetic properties in Ni1.5Fe1.5O4 ferrite as
the function of pH at which the material was prepared by chemical route and
post annealing temperature. The material is a ferri or ferromagnet, but showed
magnetic blocking and random spin freezing process on lowering the measurement
temperature down to 5 K. The sample prepared at pH =12 and annealed at 800 ^C
showed a sharp magnetization peak at 105 K, the superparamagnetic blocking
temperature of the particles. The magnetization peak remained incomplete within
measurement temperature up to 350 K for rest of the samples, although peak
temperature was brought down by increasing applied dc field. The fitting of
temperature dependence of coercivity data according to Kneller law suggested
random orientation of ferromagnetic particles. The fitting of saturation
magnetization according to Bloch law provided the exponent that largely
deviated from 1.5, a typical value for long ranged ferromagnet. An abrupt
increase of saturation magnetization below 50 K suggested the active role of
frozen surface spins in low temperature magnetic properties. AC susceptibility
data elucidated the low temperature spin freezing dynamics and exhibited the
characters of cluster spin glass in the samples depending on pH value and
annealing temperature.",1712.06114v1
2018-02-28,Magnetically regulated collapse in the B335 protostar ? I. ALMA observations of the polarized dust emission,"The role of the magnetic field during protostellar collapse is poorly
constrained from an observational point of view, although it could be
significant if we believe state-of-the-art models of protostellar formation. We
present polarimetric observations of the 233 GHz thermal dust continuum
emission obtained with ALMA in the B335 Class 0 protostar. Linearly polarized
dust emission arising from the circumstellar material in the envelope of B335
is detected at all scales probed by our observations, from radii of 50 to 1000
au. The magnetic field structure producing the dust polarization has a very
ordered topology in the inner envelope, with a transition from a large-scale
poloidal magnetic field, in the outflow direction, to strongly pinched in the
equatorial direction. This is probably due to magnetic field lines being
dragged along the dominating infall direction since B335 does not exhibit
prominent rotation. Our data and their qualitative comparison to a family of
magnetized protostellar collapse models show that, during the magnetized
collapse in B335, the magnetic field is maintaining a high level of
organization from scales 1000 au to 50 au: this suggests the field is
dynamically relevant and capable of influencing the typical outcome of
protostellar collapse, such as regulating the disk size in B335.",1803.00028v1
2018-03-27,Generation and annihilation time of magnetic droplet solitons,"Magnetic droplet solitons were first predicted to occur in materials with
uniaxial magnetic anisotropy due to a long-range attractive interaction between
elementary magnetic excitations, magnons. A non-equilibrium magnon population
provided by a spin-polarized current in nanocontacts enables their creation and
there is now clear experimental evidence for their formation, including direct
images obtained with scanning x-ray transmission microscopy. Interest in
magnetic droplets is associated with their unique magnetic dynamics that can
lead to new types of high frequency nanometer scale oscillators of interest for
information processing, including in neuromorphic computing. However, there are
no direct measurements of the time required to nucleate droplet solitons or
their lifetime---experiments to date only probe their steady-state
characteristics, their response to dc spin currents. Here we determine the
timescales for droplet annihilation and generation using current pulses.
Annihilation occurs in a few nanoseconds while generation can take several
nanoseconds to a microsecond depending on the pulse amplitude. Micromagnetic
simulations show that there is an incubation time for droplet generation that
depends sensitively on the initial magnetic state of the nanocontact. An
understanding of these processes is essential to utilizing the unique
characteristics of magnetic droplet solitons oscillators, including their high
frequency, tunable and hysteretic response.",1803.10334v1
2018-04-26,Particle acceleration in explosive relativistic reconnection events and Crab Nebula gamma-ray flares,"We develop a model of gamma-ray flares of the Crab Nebula resulting from the
magnetic reconnection events in highly-magnetized relativistic plasma. We first
discuss physical parameters of the Crab nebula and review the theory of pulsar
winds and termination shocks. We also review the principle points of particle
acceleration in explosive reconnection events (Lyutikov et al. 2017a,b).
  It is required that particles producing flares are accelerated in highly
magnetized regions of the nebula. Flares originate from the poleward regions at
the base of Crab's polar outflow, where both the magnetization and the magnetic
field strength are sufficiently high. The post-termination shock flow develops
macroscopic (not related to the plasma properties on the skin-depth scale)
kink-type instabilities. The resulting large-scales magnetic stresses drive
explosive reconnection events on the light-crossing time of the reconnection
region. Flares are produced at the initial stage of the current sheet
development, during the X-point collapse. The model has all the ingredients
needed for Crab flares: natural formation of highly magnetized regions,
explosive dynamics on light travel time, development of high electric fields on
macroscopic scales and acceleration of particles to energies well exceeding the
average magnetic energy per particle.",1804.10291v2
2018-05-14,Optically Pumped Floquet States of Magnetization in Ferromagnets,"Floquet states have been subject of great research interest since
Zel'dovich's pioneering work on the quasienergy of a quantum system subject to
a temporally periodic action. Nowadays periodic modulation of the system
Hamiltonian is mostly achieved by microwaves leading to novel exciting
phenomena in condensed matter physics: Floquet topological insulators, chiral
edge states etc. On the other hand, nonthermal optical control of magnetization
at picosecond time scales is currently a highly appealing research topic for
potential applications in magnetic data storage. Here we combine these two
concepts to investigate Floquet states in the system of exchange-coupled spins
in a ferromagnet. We periodically perturb the magnetization of an iron-garnet
film by a train of circularly-polarized femtosecond laser pulses hitting the
sample at 1 GHz repetition rate and monitor the magnetization dynamics behaving
like a Floquet state. An external magnetic field allows tuning of the Floquet
states leading to a pronounced increase of the precession amplitude by one
order of magnitude at the center of the Brillouin zone, i.e. when the
precession frequency is a multiple of the laser pulse repetition rate. Floquet
states might potentially allow for parametric generation of magnetic
oscillations. The observed phenomena expand the capabilities of coherent
ultrafast optical control of magnetization and pave a way for their application
in quantum computation or data processing.",1805.05822v1
2018-05-18,Transport coefficients of hot magnetized QCD matter beyond the lowest Landau level approximation,"In this article, shear viscosity, bulk viscosity, and thermal conductivity of
a hot QCD medium have been studied in the presence of a strong magnetic field.
To model the hot magnetized QCD matter, an extended quasi-particle description
of the hot QCD equation of state in the presence of the magnetic field has been
adopted. The effects of higher Landau levels on the temperature dependence of
viscous coefficients (bulk and shear viscosities) and thermal conductivity have
been obtained by considering the $1\rightarrow 2$ processes in the presence of
the strong magnetic field. An effective covariant kinetic theory has been set
up in (1+1)-dimensional that includes mean field contributions in terms of
quasi-particle dispersions and magnetic field to describe the Landau level
dynamics of quarks. The sensitivity of these parameters to the magnitude of the
magnetic field has also been explored. Both the magnetic field and mean field
contributions have seen to play a significant role in obtaining the temperature
behaviour of the transport coefficients of hot QCD medium.",1805.07313v2
2018-06-25,On the sensitivity of magnetic cycles in global simulations of solar-like stars,"The periods of magnetic activity cycles in the Sun and solar-type stars do
not exhibit a simple or even single trend with respect to rotation rate or
luminosity. Dynamo models can be used to interpret this diversity, and can
ultimately help us understand why some solar-like stars do not exhibit a
magnetic cycle, whereas some do, and for the latter what physical mechanisms
set their magnetic cycle period. Three-dimensional non-linear
magnetohydrodynamical simulations present the advantage of having only a small
number of tunable parameters, and produce in a dynamically self-consistent
manner the flows and the dynamo magnetic fields pervading stellar interiors. We
conducted a series of such simulations within the EULAG-MHD framework, varying
the rotation rate and luminosity of the modeled solar-like convective
envelopes. We find decadal magnetic cycles when the Rossby number near the base
of the convection zone is moderate (typically between 0.25 and 1). Secondary,
shorter cycles located at the top of the convective envelope close to the
equator are also observed in our numerical experiments, when the local Rossby
number is lower than 1. The deep-seated dynamo sustained in these numerical
experiments is fundamentally non-linear, in that it is the feedback of the
large-scale magnetic field on the large-scale differential rotation that sets
the magnetic cycle period. The cycle period is found to decrease with the
Rossby number, which offers an alternative theoretical explanation to the
variety of activity cycles observed in solar-like stars.",1806.09484v1
2018-07-04,A Study of Magnetic Field Characteristics of Flaring Active Region Based on Nonlinear Force-free Field Extrapolation,"Coronal magnetic fields are responsible for the onset of solar flares and
solar eruptions. However, the type of magnetic field parameters that can be
used to measure the critical condition for a solar eruption is still unclear.
As an effort to understand the possible condition for a solar flare, we have
examined the non-dimensional parameter $\kappa$ introduced by Ishiguro & Kusano
(2017), which contains information about magnetic twist distribution and
magnetic flux in an active region (AR). We introduce a new parameter
$\kappa^\ast$, as a proxy for $\kappa$, and we have analyzed the evolution of
$\kappa^\ast$ during the flaring period of an AR using the nonlinear force-free
field (NLFFF) extrapolated from the photospheric vector magnetic field data.
Using data from the Solar Dynamics Observatory (SDO)/Helioseismic and Magnetic
Imager (HMI), we have calculated $\kappa^\ast$ for the AR NOAA 11158 during its
three-day flaring period. We found that $\kappa^\ast$ increased to a certain
level before two large flares and decreased significantly after their onset.
The results suggest that $\kappa^\ast$ may be used as an indicator of the
necessary condition for the onset of a solar eruption in the AR. Based on this
result, we propose a new method to assess the possibility of a large solar
eruption from an AR by combining the parameter $\kappa^\ast$ and information
about the magnetic energy of the AR.",1807.01436v1
2018-07-12,Effect of the exoplanet magnetic field topology on its magnetospheric radio emission,"The magnetized wind from stars that impact exoplanets should lead to radio
emissions. According to the scaling laws derived in the solar system, the radio
emission should depend on the stellar wind, interplanetary magnetic field, and
topology of the exoplanet magnetosphere. The aim of this study is to calculate
the dissipated power and subsequent radio emission from exoplanet
magnetospheres with different topologies perturbed by the interplanetary
magnetic field and stellar wind, to refine the predictions from scaling laws,
and to prepare the interpretation of future radio detections. We use the
magnetohydrodynamic (MHD) code PLUTO in spherical coordinates to analyze the
total radio emission level resulting from the dissipation of the kinetic and
magnetic (Poynting flux) energies inside the exoplanet's magnetospheres. We
apply a formalism to infer the detailed contribution in the exoplanet radio
emission on the exoplanet's day side and magnetotail. The model is based on
Mercury-like conditions, although the study results are extrapolated to
exoplanets with stronger magnetic fields, providing the lower bound of the
radio emission. The predicted dissipated powers and resulting radio emissions
depends critically on the exoplanet magnetosphere topology and interplanetary
magnetic field (IMF) orientation. The radio emission on the exoplanet's night
and day sides should thus contain information on the exoplanet magnetic field
topology. In addition, if the topology of an exoplanet magnetosphere is known,
the radio emission measurements can be used as a proxy of the instantaneous
dynamic pressure of the stellar wind, IMF orientation, and intensity.",1807.04417v1
2018-07-12,First-Principles-Based Strain and Temperature Dependent Ferroic Phase Diagram of SrMnO$_3$,"Perovskite structure SrMnO$_3$ is a rare example of a multiferroic material
where strain-tuning and/or cation substitution could lead to coinciding
magnetic and ferroelectric ordering temperatures, which would then promise
strong magnetoelectric coupling effects. Here, we establish the temperature and
strain dependent ferroic phase diagram of SrMnO$_3$ using
first-principles-based effective Hamiltonians. All parameters of these
Hamiltonians are calculated using density functional theory, i.e., no fitting
to experimental data is required. Temperature dependent properties are then
obtained from Monte Carlo and molecular dynamics simulations. We observe a
sequence of several magnetic transitions under increasing tensile strain, with
a moderate variation of the corresponding critical temperatures. In contrast,
the ferroelectric Curie temperature increases strongly after its onset around
2.5\,\% strain, and indeed crosses the magnetic transition temperature just
above 3\,\% strain. Our results indicate pronounced magnetoelectric coupling,
manifested in dramatic changes of the magnetic ordering temperatures and
different magnetic ground states as function of the ferroelectric distortion.
In addition, coexisting ferroelectric and ferromagnetic order is obtained for
strains above 4\,\%. Our calculated phase diagram suggests the possibility to
control the magnetic properties of SrMnO$_3$ through an applied electric field,
significantly altering the magnetic transition temperatures, or even inducing
transitions between different magnetic states.",1807.04777v1
2018-08-14,Magnetic nanowires and nanotubes,"We propose a review of the current knowledge about the synthesis, magnetic
properties and applications of magnetic cylindrical nanowires and nanotubes. By
""nano"" we consider diameters reasonably smaller than a micrometer. At this
scale, comparable to micromagnetic and transport length scales, novel
properties appear. At the same time, this makes the underlying physics easier
to understand due to the limiter number of degrees of freedom involved. The
three-dimensional nature and the curvature of these objects contribute also to
their specific properties, compared to patterns flat elements. While the topic
of nanowires and later nanotubes started now decades ago, it is nevertheless
flourishing, thanks to the progress of synthesis, theory and characterization
tools. These give access to ever more complex and thus functional structures,
and also shifting the focus from material-type measurements of large
assemblies, to single-object investigations. We first provide an overview of
common fabrication methods yielding nanowires, nanotubes and structures
engineered in geometry~(change in diameter, shape) or material (segments,
core-shell structures), shape or core-shell. We then review their magnetic
properties: global measurements, magnetization states and switching, single
domain wall statics and dynamics, and spin waves. For each aspect, both theory
and experiments are surveyed. We also mention standard characterization
techniques useful for these. We finally mention emerging applications of
magnetic nanowires and nanotubes, along with the foreseen perspectives in the
topic.",1808.04656v1
2018-09-04,Chiral magnetic interlayer coupling in synthetic antiferromagnets,"The exchange coupling underlies ferroic magnetic coupling and is thus the key
element that governs statics and dynamics of magnetic systems. This fundamental
interaction comes in two flavors - symmetric and antisymmetric coupling. While
symmetric coupling leads to ferro- and antiferromagnetism, antisymmetric
coupling has attracted significant interest owing to its major role in
promoting topologically non-trivial spin textures that promise high-speed and
energy-efficient devices. So far, the antisymmetric exchange coupling rather
short-ranged and limited to a single magnetic layer has been demonstrated,
while the symmetric coupling also leads to long-range interlayer exchange
coupling. Here, we report the missing component of the long-range antisymmetric
interlayer exchange coupling in perpendicularly magnetized synthetic
antiferromagnets with parallel and antiparallel magnetization alignments.
Asymmetric hysteresis loops under an in-plane field unambiguously reveal a
unidirectional and chiral nature of this novel interaction, which cannot be
accounted for by existing coupling mechanisms, resulting in canted
magnetization alignments. This can be explained by spin-orbit coupling combined
with reduced symmetry in multilayers. This new class of chiral interaction
provides an additional degree of freedom for engineering magnetic structures
and promises to enable a new class of three-dimensional topological structures.",1809.01080v2
2019-01-21,Paramagnetic squeezing of a uniformly expanding quark-gluon plasma in and out of equilibrium,"The plasma of quarks and gluons created in ultrarelativistic heavy-ion
collisions turns out to be paramagnetic. In the presence of a background
magnetic field, this paramagnetism thus leads to a pressure anisotropy, similar
to anisotropies appearing in a viscous fluid. In the present paper, we use this
analogy, and develop a framework similar to anisotropic hydrodynamics, to take
the pressure anisotropy caused, in particular, by the nonvanishing
magnetization of a plasma of quarks and gluons into account. We consider the
first two moments of the classical Boltzmann equation in the presence of an
electromagnetic source in the relaxation-time approximation, and derive a set
of coupled differential equations for the anisotropy parameter $\xi_0$ and the
effective temperature $\lambda_0$ of an ideal fluid with nonvanishing
magnetization. We also extend this method to a dissipative fluid with finite
magnetization in the presence of a strong and dynamical magnetic field. We
present a systematic method leading to the one-particle distribution function
of this magnetized dissipative medium in a first-order derivative expansion,
and arrive at analytical expressions for the shear and bulk viscosities in
terms of the anisotropy parameter $\xi$ and effective temperature $\lambda$. We
then solve the corresponding differential equations for $(\xi_0,\lambda_0)$ and
$(\xi,\lambda)$ numerically, and determine, in this way, the proper time and
temperature dependence of the energy density, directional pressures, speed of
sound, and the magnetic susceptibility of a longitudinally expanding magnetized
quark-gluon plasma in and out of equilibrium.",1901.06928v2
2019-02-14,High-speed domain wall racetracks in a magnetic insulator,"Recent reports of current-induced switching of ferrimagnetic oxides coupled
to a heavy metal layer have opened realistic prospects for implementing
magnetic insulators into electrically addressable spintronic devices. However,
key aspects such as the configuration and dynamics of magnetic domain walls
driven by electrical currents in insulating oxides remain unexplored. Here, we
investigate the internal structure of the domain walls in Tm3Fe5O12 (TmIG) and
TmIG/Pt bilayers and demonstrate their efficient manipulation by spin-orbit
torques with velocities of up to 400 m s$^{-1}$ and minimal current threshold
for domain wall flow of 5 x 10$^{6}$ A cm$^{-2}$. Domain wall racetracks
embedded in TmIG are defined by the deposition of Pt current lines, which allow
us to control the domain propagation and magnetization switching in selected
regions of an extended magnetic layer. Scanning nitrogen-vacancy magnetometry
reveals that the domain walls of thin TmIG films are N\'eel walls with
left-handed chirality, with the domain wall magnetization rotating towards an
intermediate N\'eel-Bloch configuration upon deposition of Pt. These results
indicate the presence of a sizable interfacial Dzyaloshinskii-Moriya
interaction in TmIG, which leads to novel possibilities to control the
formation of chiral spin textures in magnetic insulators. Ultimately, domain
wall racetracks provide an efficient scheme to pattern the magnetic landscape
of TmIG in a fast and reversible way",1902.05639v1
2019-02-25,Strong deformation of ferrofluid-filled elastic alginate capsules in inhomogenous magnetic fields,"We present a new system based on alginate gels for the encapsulation of a
ferrofluid drop, which allows us to create millimeter-sized elastic capsules
that are highly deformable by inhomogeneous magnetic fields. We use a
combination of experimental and theoretical work in order to characterize and
quantify the deformation behavior of these ferrofluid-filled capsules. We
introduce a novel method for the direct encapsulation of unpolar liquids by
sodium alginate. The addition of polar alcohol molecules allows us to
encapsulate a ferrofluid as a single phase. This encapsulation method increases
the amount of encapsulated magnetic nanoparticles resulting in high
deformations and offers possible applications of capsules as actuators,
switches, or valves in confined spaces like microfluidic devices. We determine
both elastic moduli of the capsule shell, Young's modulus and Poisson's ratio,
by employing two independent mechanical methods, spinning capsule measurements
and capsule compression between parallel plates. We then show that the observed
magnetic deformation can be fully understood from magnetic forces exerted by
the ferrofluid on the capsule shell if the magnetic field distribution and
magnetization properties of the ferrofluid are known. Using an iterative
solution scheme that couples a finite element / boundary element method for the
magnetic field calculation to the solution of the elastic shape equations, we
achieve quantitative agreement between theory and experiment for deformed
capsule shapes using the Young modulus from mechanical characterization and the
surface Poisson ratio as a fit parameter. This detailed analysis confirms the
results from mechanical characterization that the surface Poisson ratio of the
alginate shell is close to unity, that is, deformations of the alginate shell
are almost area conserving.",1902.09389v1
2019-04-04,Topological-chiral magnetic interactions driven by emergent orbital magnetism,"Two hundred years ago, Andr\'e-Marie Amp\`ere discovered that electric loops
in which currents of electrons are generated by a penetrating magnetic field
can interact with each other. Here we show that Amp\`ere's observation can be
transferred to the quantum realm of interactions between triangular plaquettes
of spins on a lattice, where the electrical currents at the atomic scale are
associated with a peculiar type of the orbital motion of electrons in response
to the non-coplanarity of neighbouring spins playing the role of a magnetic
field. The resulting topological orbital moment underlies the relation of the
orbital dynamics with the topology of the spin structure. We demonstrate that
the interactions of the topological orbital moments with each other and with
the spins of the underlying lattice give rise to a new class of magnetic
interactions $-$ topological chiral interactions $-$ which can dominate over
the celebrated Dzyaloshinskii-Moriya interaction, thus opening a path for the
realization of new classes of chiral magnetic materials with three-dimensional
magnetization textures such as magnetic hopfions.",1904.02369v3
2019-04-12,Data-Optimized Coronal Field Model: I. Proof of Concept,"Deriving the strength and direction of the three-dimensional (3D) magnetic
field in the solar atmosphere is fundamental for understanding its dynamics.
Volume information on the magnetic field mostly relies on coupling 3D
reconstruction methods with photospheric and/or chromospheric surface vector
magnetic fields. Infrared coronal polarimetry could provide additional
information to better constrain magnetic field reconstructions. However,
combining such data with reconstruction methods is challenging, e.g., because
of the optical-thinness of the solar corona and the lack and limitations of
stereoscopic polarimetry. To address these issues, we introduce the
Data-Optimized Coronal Field Model (DOCFM) framework, a model-data fitting
approach that combines a parametrized 3D generative model, e.g., a magnetic
field extrapolation or a magnetohydrodynamic model, with forward modeling of
coronal data. We test it with a parametrized flux rope insertion method and
infrared coronal polarimetry where synthetic observations are created from a
known ""ground truth"" physical state. We show that this framework allows us to
accurately retrieve the ground truth 3D magnetic field of a set of force-free
field solutions from the flux rope insertion method. In observational studies,
the DOCFM will provide a means to force the solutions derived with different
reconstruction methods to satisfy additional, common, coronal constraints. The
DOCFM framework therefore opens new perspectives for the exploitation of
coronal polarimetry in magnetic field reconstructions and for developing new
techniques to more reliably infer the 3D magnetic fields that trigger solar
flares and coronal mass ejections.",1904.06308v1
2019-09-07,A Coupled A-H Formulation for Magneto-Thermal Transients in High-Temperature Superconducting Magnets,"The application of high-temperature superconductors to accelerator magnets
for future particle colliders is under study. Numerical methods are crucial for
an accurate evaluation of the complex dynamical behavior of the magnets,
especially concerning the magnetic field quality and thermal behavior. We
present a coupled $\textbf{A}$-$\textbf{H}$ field formulation for the analysis
of magneto-thermal transients in accelerator magnets. The magnetic field
strength $\textbf{H}$ accounts for the eddy current problem in the source
regions containing the superconducting domains, while the magnetic vector
potential $\textbf{A}$ represents the magnetoquasistatic problem in the normal
and non-conducting domains. Furthermore, we include a slab approximation for
the source regions, making the formulation suitable for large scale models
composed of thousands of tapes. In this work, the relevant equations are
derived and discussed, with emphasis on the coupling conditions. The weak
formulation is derived, and numerical results are provided in order to both,
verify the formulation and scale it to the size of an accelerator magnet.",1909.03312v3
2019-12-10,Dynamical resilience to disorder: the dilute Hubbard model on the Lieb lattice,"In itinerant systems, electron-electron interactions may lead to the
formation of local magnetic moments and their effective exchange coupling,
which in turn gives rise to long-range magnetic order. Therefore, when moment
formation is weakened, such as in the single-band Hubbard model on a square
lattice with the on-site repulsion being randomly switched off on a fraction
$x$ of sites, magnetic order is suppressed beyond some critical $x_c$, which
was found to lie below the classical percolation threshold,
$x_c^\text{(perc,sq)}$. Here we study dilute magnetism in flat band systems,
namely in the Hubbard model on a `Lieb' lattice. Interestingly, we show that
magnetic order persists to $x$ almost twice as large as the classical
percolation threshold for the lattice, thus emphasizing the central role of
electron itinerancy to the magnetic response. The analysis of the
orbital-resolved order parameters reveals that the contribution of the
four-fold coordinated `d' sites to magnetism is dramatically affected by
dilution, while the localized `p' states of the flat band provide the dominant
contribution to long-range correlations. We also examine the transport
properties, which suggest the existence of an insulator-to-metal transition in
the same range of the critical magnetic dilution.",1912.04918v3
2020-01-08,Effect of collisions on the plasma sheath in the presence of an inhomogeneous magnetic field,"A low-pressure magnetized plasma is studied to find the dependency of sheath
properties on ion-neutral collisions in presence of an inhomogeneous magnetic
field. A self-consistent one-dimensional two-fluid hydrodynamic model is
considered, and the system of equations is solved numerically. The study
reveals that the width of the plasma sheath expands and space charge increases
with collisions. The ion-neutral collisions and the inhomogeneous magnetic
field restrict the ions to move towards the surface. The movement of the ions
towards the wall can be controlled by choosing a suitable configuration of the
magnetic field and ion-neutral collision frequency. A comparison between two
different magnetic field configurations has been presented alongside to
differentiate the commonly found scenarios in the field. The outcome of the
study is supposed to help in understanding the complex dynamics of ions in
plasma confinement and plasma processing of materials. Furthermore, the present
work seeks to create a framework for two-fluid modeling of magnetized plasmas
with any arbitrary magnetic field profiles. The analysis provided here is
supposed to act as a basis for any future work in the respective field.",2001.02389v3
2020-02-12,Hyperfine and quadrupole interactions for Dy isotopes in DyPc$_2$ molecules,"Nuclear spin levels play an important role in understanding magnetization
dynamics and implementation and control of quantum bits in lanthanide-based
single-molecule magnets. We investigate the hyperfine and nuclear quadrupole
interactions for $^{161}$Dy and $^{163}$Dy nucleus in anionic DyPc$_2$
(Pc=phthalocyanine) single-molecule magnets, using multiconfigurational
ab-initio methods (beyond density-functional theory) including spin-orbit
interaction. The two isotopes of Dy are chosen because the others have zero
nuclear spin. Both isotopes have the nuclear spin $I=5/2$, although the
magnitude and sign of the nuclear magnetic moment differ from each other. The
large energy gap between the electronic ground and first-excited Kramers
doublets, allows us to map the microscopic hyperfine and quadrupole interaction
Hamiltonian onto an effective Hamiltonian with an electronic pseudo-spin
$S_{\rm eff}=1/2$ that corresponds to the ground Kramers doublet. Our ab-initio
calculations show that the coupling between the nuclear spin and electronic
orbital angular momentum contributes the most to the hyperfine interaction and
that both the hyperfine and nuclear quadrupole interactions for $^{161}$Dy and
$^{163}$Dy nucleus are much smaller than those for $^{159}$Tb nucleus in
TbPc$_2$ single-molecule magnets. The calculated separations of the
electronic-nuclear levels are comparable to experimental data reported for
$^{163}$DyPc$_2$. We demonstrate that hyperfine interaction for Dy Kramers ion
leads to tunnel splitting (or quantum tunneling of magnetization) at zero
field. This effect does not occur for TbPc$_2$ single-molecule magnets. The
magnetic field values of the avoided level crossings for $^{161}$DyPc$_2$ and
$^{163}$DyPc$_2$ are found to be noticeably different, which can be observed
from experiment.",2002.05134v1
2020-03-31,Skyrmions in antiferromagnets: thermal stability and the effect of external field and impurities,"Calculations of skyrmions in antiferromagnets (AFMs) are presented, and their
properties compared with skyrmions in corresponding ferromagnets (FMs). The
rates of skyrmion collapse and escape through the boundary of a track, as well
as the binding to and collapse at a non-magnetic impurity, are calculated as a
function of applied magnetic field. The activation energy for skyrmion
annihilation is the same in AFMs and corresponding FMs in the absence of an
applied magnetic field. The pre-exponential factor in the Arrhenius rate law
is, however, different because skyrmion dynamics is different in the two
systems. An applied magnetic field has opposite effects on skyrmions in the two
types of materials. In AFMs the rate of collapse of skyrmions as well as the
rate of escape through the edge of a magnetic strip decreases slightly with
increasing field, while these rates increase strongly for a skyrmion in the
corresponding FMs when the field is directed antiparallel to the magnetization
in the center of the skyrmion. A non-magnetic impurity is less likely to trap a
skyrmion in AFMs especially in the presence of a magnetic field. This, together
with the established fact that a spin polarized current moves skyrmions in AFMs
in the direction of the current, while in FMs skyrmions move at an angle to the
current, demonstrates that skyrmions in AFMs have several advantageous
properties over skyrmions in FMs for memory and spintronic devices.",2004.00095v2
2020-04-07,Laser-induced magnetization precession in individual magnetoelastic domains of a multiferroic CoFeB/BaTiO$_3$ composite,"Using a magneto-optical pump-probe technique with micrometer spatial
resolution we show that magnetization precession can be launched in individual
magnetic domains imprinted in a Co$_{40}$Fe$_{40}$B$_{20}$ (CoFeB) layer by
elastic coupling to ferroelectric domains in a BaTiO$_{3}$ substrate. The
dependence of the precession parameters on external magnetic field strength and
orientation reveal that by laser-induced ultrafast partial quenching of the
magnetoelastic coupling parameter of CoFeB by $\approx$27% along with 10%
ultrafast demagnetization trigger the magnetization precession. The relation
between the laser-induced reduction of the magnetoelastic coupling and the
demagnetization is approximated by the $n(n+1)/2$-law with n$\approx$2. This
correspondence confirms the thermal origin of the laser-induced anisotropy
change. Based on the analysis and modeling of the excited precession we find
signatures of laser-induced precessional switching, which occurs when the
magnetic field is applied along the hard magnetization axis and its value is
close to the effective magnetoelastic anisotropy field. The precession
excitation process in an individual magnetoelastic domain is found to be
unaffected by neighboring domains. This makes laser-induced changes of
magnetoelastic anisotropy a promising tool for driving magnetization dynamics
and switching in composite multiferroics with spatial selectivity.",2004.03566v5
2020-05-07,Spin caloritronics in a CrBr$_3$-based magnetic van der Waals heterostructure,"The recently reported magnetic ordering in insulating two-dimensional (2D)
materials, such as chromium triiodide (CrI$_3$) and chromium tribromide
(CrBr$_3$), opens new possibilities for the fabrication of magneto-electronic
devices based on 2D systems. Inevitably, the magnetization and spin dynamics in
2D magnets are strongly linked to Joule heating. Therefore, understanding the
coupling between spin, charge and heat, i.e. spin caloritronic effects, is
crucial. However, spin caloritronics in 2D ferromagnets remains mostly
unexplored, due to their instability in air. Here we develop a fabrication
method that integrates spin-active contacts with 2D magnets through hBN
encapsulation, allowing us to explore the spin caloritronic effects in these
materials. The angular dependence of the thermal spin signal of the CrBr$_3$/Pt
system is studied, for different conditions of magnetic field and heating
current. We highlight the presence of a significant magnetic proximity effect
from CrBr$_3$ on Pt revealed by an anomalous Nernst effect in Pt, and suggest
the contribution of the spin Seebeck effect from CrBr$_3$. These results pave
the way for future magnonic devices using air-sensitive 2D magnetic insulators.",2005.03561v1
2020-08-03,Magnetofluidic mixing of a ferrofluid droplet under the influence of time-dependent external field,"We report the experimental investigations on the mixing of a ferrofluid
droplet with a non-magnetic fluid in the presence of a time-dependent magnetic
field on an open surface microfluidic platform. The bright field visualization
technique, in combination with the micro-PIV analysis, is carried out to
explore the internal hydrodynamics of the ferrofluid droplet. Also, using the
Laser-induced fluorescence (micro-LIF) technique, we quantify the mass transfer
occurring between the two droplets, which in effect, determines the underlying
mixing performance under the modulation of the frequency of the applied
magnetic field. We show that the magnetic nanoparticles exhibit complex
spatio-temporal movement inside the ferrofluid droplet domain under the
influence of a time-dependent magnetic field, which, in turn, promotes the
mixing efficiency in the convective mixing regime. Our analysis establishes
that the movement of magnetic nanoparticles in presence of the time-periodic
field strengthens the interfacial instability, which acts like a sparking agent
to initiate an augmented mixing in the present scenario. By performing
numerical simulations, we also review the onset of interfacial instability,
mainly stemming from the susceptibility mismatch between the magnetic and
non-magnetic fluids. Inferences of the present analysis, which focuses on the
simple, wireless, robust, and low-cost open surface micromixing mechanism, will
provide a potential solution for rapid droplets mixing without requiring pH
level or ion concentration dependency of the fluids.",2008.00678v1
2020-08-17,Multiphase Circumnuclear Gas in a Low-$β$ Disk: Turbulence and Magnetic Field Reversals,"We studied the magnetic field structures and dynamics of magnetized
multiphase gas on parsec scales around supermassive black holes by using global
3D magnetohydrodynamics (MHD) simulations. We considered the effect of
radiative cooling and X-ray heating due to active galactic nuclei (AGNs). The
gas disk consists of a multiphase gas with (1) cold ($\leq 10^3$ K) and thin,
and (2) warm ($\sim 10^4$ K) and thick components with a wide range of number
densities. The turbulent magnetic energy at maximum is comparable to the
thermal and turbulent kinetic energies in the turbulent motion. We confirmed
that the turbulent velocity of the warm gas in the ambient cold gas is caused
by magnetoconvective instability. The turbulent magnetic field due to
magnetorotational instability (MRI) is developed in the disk, but the mean
toroidal magnetic field dominates and supports in a quasi-steady state, where
the plasma-$\beta$, the ratio between gas pressure and magnetic pressure, is
low ($\beta < 1$). As often seen in adiabatic MHD simulations of rotating
disks, the direction of the mean toroidal field periodically reverses with time
even in multiphase gas structures. The direction reversal is caused by magnetic
flux vertically escaping from the disk and by the combination of the MRI and
the Parker instability.",2008.07050v4
2020-09-22,Magnon-mediated spin currents in Tm3Fe5O12/Pt with perpendicular magnetic anisotropy,"The control of pure spin currents carried by magnons in magnetic insulator
(MI) garnet films with a robust perpendicular magnetic anisotropy (PMA) is of
great interest to spintronic technology as they can be used to carry, transport
and process information. Garnet films with PMA present labyrinth domain
magnetic structures that enrich the magnetization dynamics, and could be
employed in more efficient wave-based logic and memory computing devices. In
MI/NM bilayers, where NM being a normal metal providing a strong spin-orbit
coupling, the PMA benefits the spin-orbit torque (SOT) driven magnetization's
switching by lowering the needed current and rendering the process faster,
crucial for developing magnetic random-access memories (SOT-MRAM). In this
work, we investigated the magnetic anisotropies in thulium iron garnet (TIG)
films with PMA via ferromagnetic resonance measurements, followed by the
excitation and detection of magnon-mediated pure spin currents in TIG/Pt driven
by microwaves and heat currents. TIG films presented a Gilbert damping constant
{\alpha}~0.01, with resonance fields above 3.5 kOe and half linewidths broader
than 60 Oe, at 300 K and 9.5 GHz. The spin-to-charge current conversion through
TIG/Pt was observed as a micro-voltage generated at the edges of the Pt film.
The obtained spin Seebeck coefficient was 0.54 {\mu}V/K, confirming also the
high interfacial spin transparency.",2009.10299v1
2021-03-04,Fitting an Analytic Magnetic Field to a Prestellar Core,"We deploy and demonstrate the capabilities of the magnetic field model
developed by Ewertowski & Basu (2013) by fitting observed polarimetry data of
the prestellar core FeSt 1-457. The analytic hourglass magnetic field function
derived directly from Maxwell's equations yields a central-to-surface magnetic
field strength ratio in the equatorial plane, as well as magnetic field
directions with relative magnitudes throughout the core. This fit emerges from
a comparison of a single plane of the model with the polarization map that
results from the integrated properties of the magnetic field and dust
throughout the core. Importantly, our fit is independent of any assumed density
profile of the core. We check the robustness of the fit by using the POLARIS
code to create synthetic polarization maps that result from the integrated
scattering and emission properties of the dust grains and their radiative
transfer, employing an observationally-motivated density profile. We find that
the synthetic polarization maps obtained from the model also provides a good
fit to the observed polarimetry. Our model fits the striking feature of
significant curvature of magnetic field lines in the outer part of FeSt 1-457.
Combined with independent column density estimates, we infer that the core of
size $R_{\rm gas}$ has a mildly supercritical mass-to-flux ratio and may have
formed through dynamical motions starting from a significantly larger radius
$R$. A breakdown of flux-freezing through neutral-ion slip (ambipolar
diffusion) could be responsible for effecting such a transition from a
large-scale magnetic field structure to a more compact gas structure.",2103.03324v1
2021-04-28,Statistical description of coalescing magnetic islands via magnetic reconnection,"The physical picture of interacting magnetic islands provides a useful
paradigm for certain plasma dynamics in a variety of physical environments,
such as the solar corona, the heliosheath, and the Earth's magnetosphere. In
this work, we derive an island kinetic equation to describe the evolution of
the island distribution function (in area and in flux of islands) subject to a
collisional integral designed to account for the role of magnetic reconnection
during island mergers. This equation is used to study the inverse transfer of
magnetic energy through the coalescence of magnetic islands in 2D. We solve our
island kinetic equation numerically for three different types of initial
distribution: delta-distribution, Gaussian and power-law distribution. The time
evolution of several key quantities is found to agree well with our analytical
predictions: magnetic energy decays as $\tilde t^{-1}$, the number of islands
decreases as $\tilde t^{-1}$, and the averaged area of islands grows as $\tilde
t$, where $\tilde t$ is the time normalized to the characteristic reconnection
time scale of islands. General properties of the distribution function and the
magnetic energy spectrum are also studied.",2104.13757v2
2021-10-16,Annihilation of Magnetic Islands at the Top of Solar Flare Loops,"The dynamics of magnetic reconnection in the solar current sheet (CS) is
studied by high-resolution 2.5-dimensional MHD simulation. With the commence of
magnetic reconnection, a number of magnetic islands are formed intermittently
and move quickly upward and downward along the CS. When colliding with the
semi-closed flux of flare loops, the downflow islands cause a second
reconnection with a rate even comparable with that in the main CS. Though the
time-integrated magnetic energy release is still dominated by the reconnection
in main CS, the second reconnection can release substantial magnetic energy,
annihilating the main islands and generating secondary islands with various
scales at the flare loop top. The distribution function of the flux of the
second islands is found to follow a power-law varying from
$f\left(\psi\right)\sim\psi^{-1}$ (small scale) to $\psi^{-2}$ (large scale),
which seems to be independent with background plasma $\beta$ and if including
thermal conduction. However, the spatial scale and the strength of the
termination shocks driven by main reconnection outflows or islands decrease if
$\beta$ increases or thermal conduction is included. We suggest that the
annihilation of magnetic islands at the flare loop top, which is not included
in the standard flare model, plays a non-negligible role in releasing magnetic
energy to heat flare plasma and accelerate particles.",2110.08526v3
2022-01-06,Magnetic fields in the formation of the first stars.--II Results,"Beginning with cosmological initial conditions at z=100, we simulate the
effects of magnetic fields on the formation of Population III stars and compare
our results with the predictions of Paper I. We use Gadget-2 to follow the
evolution of the system while the field is weak. We introduce a new method for
treating kinematic fields by tracking the evolution of the deformation tensor.
The growth rate in this stage of the simulation is lower than expected for
diffuse astrophysical plasmas, which have a very low resistivity (high magnetic
Prandtl number); we attribute this to the large numerical resistivity in
simulations, corresponding to a magnetic Prandtl number of order unity. When
the magnetic field begins to be dynamically significant in the core of the
minihalo at z=27, we map it onto a uniform grid and follow the evolution in an
adaptive mesh refinement, MHD simulation in Orion2. The nonlinear evolution of
the field in the Orion2 simulation violates flux-freezing and is consistent
with the theory proposed by Xu & Lazarian. The fields approach equipartition
with kinetic energy at densities ~ 10^10 - 10^12 cm^-3. When the same
calculation is carried out in Orion2 with no magnetic fields, several
protostars form, ranging in mass from ~ 1 to 30 M_sol with magnetic fields,
only a single ~ 30 M_sol protostar forms by the end of the simulation. Magnetic
fields thus suppress the formation of low-mass Pop III stars, yielding a
top-heavy Pop III IMF and contributing to the absence of observed Pop III
stars.",2201.02225v2
2022-01-27,Jet Formation in 3D GRMHD Simulations of Bondi-Hoyle-Lyttleton Accretion,"A black hole (BH) travelling through a uniform, gaseous medium is described
by Bondi-Hoyle-Lyttleton (BHL) accretion. If the medium is magnetized, then the
black hole can produce relativistic outflows. We performed the first 3D,
general-relativistic magnetohydrodynamics simulations of BHL accretion onto
rapidly rotating black holes using the code H-AMR, where we mainly varied the
strength of a background magnetic field that threads the medium. We found that
the ensuing accretion continuously drags to the BH the magnetic flux, which
accumulates near the event horizon until it becomes dynamically important.
Depending on the strength of the background magnetic field, the BHs can
sometimes launch relativistic jets with high enough power to drill out of the
inner accretion flow, become bent by the headwind, and escape to large
distances. While for stronger background magnetic fields the jets are
continuously powered, at weaker field strengths they are intermittent, turning
on and off depending on the fluctuating gas and magnetic flux distributions
near the event horizon. We find that our jets reach extremely high efficiencies
of $\sim100-300\%$, even in the absence of an accretion disk. We also
calculated the drag forces exerted by the gas onto to the BH, finding that the
presence of magnetic fields causes drag forces to be much less efficient than
in unmagnetized BHL accretion, and sometimes become negative, accelerating the
BH rather than slowing it down. Our results extend classical BHL accretion to
rotating BHs moving through magnetized media and demonstrate that accretion and
drag are significantly altered in this environment.",2201.11753v2
2022-02-07,Coulomb correlations and magnetic properties of L1$_0$ FeCo: a DFT+DMFT study,"We consider electronic correlation effects and their impact on magnetic
properties of tetragonally distorted chemically ordered FeCo alloys (L1$_0$
structure) being a promising candidate for rare-earth-free permanent magnets.
We employ a state-of-the-art method combining density functional and dynamical
mean-field theory. According to our results, the predicted Curie temperature
reduces with increase of lattice parameters ratio $c/a$ and reaches nearly 850
K at ${c/a=1.22}$. For all considered $c/a$ from 1 to $\sqrt{2}$, we find
well-localized magnetic moments on Fe sites, which are formed due to strong
correlations originating from Hund's coupling. At the same time, magnetism of
Co sites is more itinerant with a much less lifetime of local magnetic moments.
However, these short-lived local moments are also formed due to Hund's
exchange. Electronic states at Fe sites are characterized by a
non-quasiparticle form of self-energies, while the ones for Co sites are found
to have a Fermi-liquid-like shape with quasiparticle mass enhancement factor
${m^*/m\sim 1.4}$, corresponding to moderately correlated metal. The strong
electron correlations on Fe sites leading to Hund's metal behaviour can be
explained by peculiarities of the density of states, which has pronounced peaks
near the Fermi level, while weaker many-body effects on Co sites can be caused
by stronger deviation from half-filling of their $3d$ states. The obtained
momentum dependence of magnetic susceptibility suggests that the ferromagnetic
ordering is the most favourable one except for the near vicinity of the fcc
structure and the magnetic exchange is expected to be of RKKY type.",2202.03205v2
2022-02-10,Magnetic phase diagram and cluster glass-like properties of stage-1 graphite intercalated FeCl$_3$,"We present a comprehensive investigation of the magnetic properties of
stage-1 graphite intercalated FeCl$_3$ using a combination of DC and AC
magnetic susceptibility, thermoremanent magnetization and field dependent
magnetization measurements. This van der Waals system, with a centrosymmetric
honeycomb lattice, combines frustration and disorder, due to intercalation, and
may be hosting topologically non-trivial magnetic phases. Our study identifies
two magnetic phase transitions at $T_{f1} \approx $ 4.2 K and at $T_{f2}
\approx $ 2.7 K. We find that the paramagnetic state, for $T > T_{f1}$, is
dominated by short-range ferromagnetic correlations. These build up well above
$T_{f1}$ and lead to a significant change in magnetic entropy, which reaches
$\Delta S_M^{Pk} = $ -5.52 J kg$^{-1}$ K$^{-1}$ at 7 T. Between $T_{f1}$ and
$T_{f2}$ we observe slow spin dynamics characteristic of a cluster glass-like
state, whereas for $T$ $ < $ $T_{f2}$ our results indicate the onset of a low
temperature long range ordered state. The analysis of the experimental results
leads to a complex phase diagram, which may serve as a reference for future
investigations searching for topological non-trivial phases in this system.",2202.04983v1
2022-02-16,Magnetic support for neutrino-driven explosion of 3D non-rotating core-collapse supernova models,"The impact of the magnetic field on postbounce supernova dynamics of
non-rotating stellar cores is studied by performing three-dimensional
magnetohydrodynamics simulations with spectral neutrino transport. The
explodability of strongly and weakly magnetized models of $20$ and $27$
$M_{\odot}$ pre-supernova progenitors are compared. We find that although the
efficiency for the conversion of the neutrino heating into turbulent energy
including magnetic fields in the gain region is not significantly different
between the strong and weak field models, the amplified magnetic field due to
the neutrino-driven convection on large hot bubbles just behind stalled shock
results in a faster and more energetic explosion in the strongly magnetized
models. In addition, by comparing the difference between the 2nd- and 5th-order
spatial accuracy of the simulation in the strong field model for $27$
$M_{\odot}$ progenitor, we also find that the higher order accuracy in space is
beneficial to the explosion because it enhances the growth of neutrino-driven
convection in the gain region. Based on our results of core-collapse supernova
simulations for the non-rotating model, a new possibility for the origin of the
magnetic field of the protoneutron star (PNS) is proposed. The magnetic field
is accumulated and amplified to magnetar level, that is, $\mathcal{O}(10^{14})$
G, in the convectively stable shell near the PNS surface.",2202.07967v2
2022-03-21,Magnetic fields in star formation: from clouds to cores,"In this chapter we review recent advances in understanding the roles that
magnetic fields play throughout the star formation process, gained through
observations and simulations of molecular clouds, the dense, star-forming phase
of the magnetised, turbulent interstellar medium (ISM). Recent results broadly
support a picture in which the magnetic fields of molecular clouds transition
from being gravitationally sub-critical and near equipartition with turbulence
in low-density cloud envelopes, to being energetically sub-dominant in dense,
gravitationally unstable star-forming cores. Magnetic fields appear to play an
important role in the formation of cloud substructure by setting preferred
directions for large-scale gas flows in molecular clouds, and can direct the
accretion of material onto star-forming filaments and hubs. Low-mass star
formation may proceed in environments close to magnetic criticality; high-mass
star formation remains less well-understood, but may proceed in more
supercritical environments. The interaction between magnetic fields and
(proto)stellar feedback may be particularly important in setting star formation
efficiency. We also review a range of widely-used techniques for quantifying
the dynamic importance of magnetic fields, concluding that better-calibrated
diagnostics are required in order to use the spectacular range of forthcoming
observations and simulations to quantify our emerging understanding of how
magnetic fields influence the outcome of the star formation process.",2203.11179v1
2022-04-27,The Rise of Buoyant Magnetic Structures through Convection with a Background Magnetic Field,"Inspired by observations of sunspots embedded in active regions, it is often
assumed that large-scale, strong magnetic flux emerges from the Sun's deep
interior in the form of arched, cylindrical structures, colloquially known as
flux tubes. Here, we continue to examine the different dynamics encountered
when these structures are considered as concentrations in a volume-filling
magnetic field rather than as isolated entities in a field-free background. Via
2.5D numerical simulations, we consider the buoyant rise of magnetic flux
concentrations from a radiative zone through an overshooting convection zone
that self-consistently (via magnetic pumping) arranges a volume-filling
large-scale background field. This work extends earlier papers that considered
the evolution of such structures in a purely adiabatic stratification with an
assumed form of the background field. This earlier work established the
existence of a bias that created an increased likelihood of successful rise for
magnetic structures with one (relative) orientation of twist and a decreased
likelihood for the other. When applied to the solar context, this bias is
commensurate with the solar hemispherical helicity rules (SHHR). This paper
establishes the robustness of this selection mechanism in a model incorporating
a more realistic background state, consisting of overshooting convection and a
turbulently-pumped mean magnetic field. Ultimately, convection only weakly
influences the selection mechanism, since it is enacted at the initiation of
the rise, at the edge of the overshoot zone. Convection does however add
another layer of statistical fluctuations to the bias, which we investigate in
order to explain variations in the SHHR.",2204.13078v1
2022-04-29,Novel magnetic ordering in LiYbO2 probed by muon spin relaxation,"The stretched diamond lattice material LiYbO2 has recently been reported to
exhibit two magnetic transitions ($T_{N1} = 1.1 K$, $T_{N2} = 0.45 K$) via
specific heat, magnetization, and neutron scattering measurements [Bordelon et
al., Phys. Rev. B 103, 014420 (2021)]. Here we report complementary magnetic
measurements down to T = 0.28 K via the local probe technique of muon spin
relaxation. While we observe a rapid increase in the zero-field muon
depolarization rate at $T_{N1}$, we do not observe any spontaneous muon
precession for $T < T_{N1}$, which is typically associated with long-range
magnetic ordering. The depolarization rate in the ordered state shows a
surprising sensitivity to magnetic fields applied along the initial spin
polarization direction. Using a simple one-dimensional model, we show that
these results are consistent with the unusual random-phase bipartite
incommensurate magnetic structure proposed by Bordelon et al. for the
intermediate temperature range $T_{N2} < T < T_{N1}$. We also find evidence for
temperature-independent magnetic fluctuations persisting to our lowest
temperatures, but no obvious signature of the transition or spontaneous muon
precession at and below TN2, respectively. This result is suggestive of quantum
dynamics within a highly degenerate ground state.",2205.00052v2
2022-05-24,Electronic structure of the frustrated diamond lattice magnet NiRh$_2$O$_4$,"The $A$-site spinel NiRh$_2$O$_4$ is the only known realization of a spin-1
diamond lattice magnet and is predicted to host unconventional magnetic
phenomena driven by frustrated nearest and next-nearest neighbor exchange as
well as orbital degeneracy. Previous works found no sign of magnetic order but
found a gapped dispersive magnetic excitation indicating a possible valence
bond magnetic ground state. However, the presence of many competing low energy
degrees of freedom and limited empirical microscopic constraints complicates
further analysis. Here, we carry out resonant inelastic x-ray scattering (RIXS)
and x-ray absorption spectroscopy (XAS) to characterize the local electronic
structure of NiRh$_2$O$_4$. The RIXS data can be partly described by a
single-ion model for tetrahedrally coordinated Ni$^{2+}$ and indicates a
tetragonal distortion $\Delta t_2\!=\!70$ meV that splits the $t_2$ orbitals
into a high energy orbital singlet and lower energy orbital doublet. We
identify features of the RIXS spectra that are consistent with a Rh-Ni two-site
excitation indicating strong metal-metal hybridization mediated by oxygen in
NiRh$_2$O$_4$. We also identify signatures of electron-phonon coupling through
the appearance of phonon sidebands that dress crystal field excitations. These
results establish the key energy scales relevant to the magnetism in
NiRh$_2$O$_4$ and further demonstrate that covalency and lattice dynamics play
essential roles in controlling the magnetic ground states of $A$-site spinels.",2205.12296v1
2022-05-29,"Quantum Scattering Theory of Spin Transfer Torque, Spin Pumping and Fluctuations","Spin transfer torque and spin pumping are central reciprocal phenomena in
spintronics. These phenomena occur in hybrid systems of normal metals and
magnets. Spin transfer is the conversion of spin currents in metals to a torque
on the magnetization of magnets. Spin pumping is the emission of spin currents
from precessing magnets. Here, we demonstrate a general way to understand these
effects within a quantum out-of-equilibrium path-integral model. Our results
agree with known expressions for spin transfer and spin pumping in terms of
transverse (mixing) conductances when there are no fluctuations. However, at a
finite temperature, frequency or spin accumulation, the magnet also experiences
fluctuating torques. In the classical regime, when the thermal energy is larger
than the bias voltage and precession frequency, we reproduce the classical
Brownian-Langevin forces associated with spin transfer and spin pumping. At low
temperatures, in the quantum regime, we demonstrate that magnetization
fluctuations differ in the elastic and inelastic electron transport regimes.
Furthermore, we show how additional transport coefficients beyond the mixing
conductance govern the fluctuations. Some of these coefficients are related to
electron shot noise because of the discrete spin angular momentum of electrons.
We estimate the fluctuation coefficients of clean, tunnel, and disordered
junctions and in the case of an insulating magnet. Our results open a path for
exploring low-temperature magnetization dynamics and spin caloritronics.",2205.14672v2
2022-07-02,Spin excitations in the quantum dipolar magnet Yb(BaBO$_3$)$_3$,"We report results of magnetization, specific-heat and muon-spin relaxation
measurements on single crystals of disorder-free Yb$^{3+}$ triangular lattice
Yb(BaBO$_3$)$_3$. The magnetization experiments show anisotropic magnetic
properties with Curie-Weiss temperatures $\theta_{\perp}=-1.40$~K ($H \perp c$)
and $\theta_{\parallel}=-1.16$~K ($H \parallel c$) determined from low
temperature data. The absence of both long-range antiferromagnetic order and
spin freezing is confirmed down to 0.27 K at zero field. A two-level Schottky
anomaly due to the opening of the ground-state Kramers doublet is observed from
the low-temperature specific-heat measurements when the applied magnetic fields
$\mu_0H >0.7$~T. At zero field, the increase of both $C_{\rm mag}/T$ and the
muon spin relaxation rate $\lambda$ below 1~K is due to the electronic spin
excitations, which often exist in quantum magnets where dipole-dipole
interaction creates an anisotropy of magnetic properties. The spin excitation
is also supported by the unusual maximum of field dependence of $\lambda$ due
to the field-induced increase of the density of excitations. We argue that
dipolar interaction is dominant and induces the spin dynamics in the quantum
magnet Yb(BaBO$_3$)$_3$.",2207.00702v1
2022-08-11,"Trion magnetic polarons in (Cd,Mn)Te/(Cd,Mn,Mg)Te quantum wells","A trion magnetic polaron formed by the exchange interaction of a positively
charged exciton (trion) with localized spins of Mn$^{2+}$ ions is found
experimentally in a 4\,nm wide
Cd$_{0.98}$Mn$_{0.02}$Te/Cd$_{0.78}$Mn$_{0.02}$Mg$_{0.2}$Te quantum well
containing resident holes. The experiment is performed at a temperature of 1.6
K using resonant excitation of the trion with circularly polarized light. The
trion is formed from a resident hole, which is in a hole magnetic polaron
state, and a photogenerated electron-hole pair. The dynamical evolution from
the hole magnetic polaron to the trion magnetic polaron is accompanied by a
spin-flip of the electron, which results in negative circular polarization of
the photoluminescence. The degree of circular polarization reaches $-8\%$ at
zero magnetic field and strongly decreases in transverse magnetic fields
exceeding 0.2 T. Our model considerations show that different localization
sizes of the resident and photogenerated holes and the resulting difference in
their exchange interaction with the Mn$^{2+}$ spins maintains Mn spin
polarization. The resulting exchange field of Mn acting on the electron
provides a robust spin polarization of the trion magnetic polaron. We evaluate
the electron exchange energy in the T$^+$MP to be 0.19~meV, and the T$^+$MP
binding energy to be about 0.5 - 1 meV.",2208.05943v1
2022-10-18,Spin re-orientation induced anisotropic magnetoresistance switching in LaCo$_{0.5}$Ni$_{0.5}$O$_{3-δ}$ thin films,"Realization of novel functionalities by tuning magnetic interactions in rare
earth perovskite oxide thin films opens up exciting technological prospects.
Strain-induced tuning of magnetic interactions in rare earth cobaltates and
nickelates is of central importance due to their versatility in electronic
transport properties. Here we reported the spin re-orientation induced
switching of anisotropic magnetoresistance (AMR) and its tunability with strain
in epitaxial LaCo$_{0.5}$Ni$_{0.5}$O$_{3-\delta}$ thin films across the
ferromagnetic transition. Moreover, with strain tuning, we could observe a
two-fold to four-fold symmetry crossover in AMR across the magnetic transition
temperature. The magnetization measurements revealed an onset of ferromagnetic
transition around 50 K, and a further reduction in temperature showed a subtle
change in the magnetization dynamics, which reduced the ferromagnetic
long-range ordering and introduced glassiness in the system. X-ray absorption
and X-ray magnetic circular dichroism spectroscopy measurements over Co and Ni
L edges revealed the Co spin state transition below the magnetic transition
temperature leading to the AMR switching and also the presence of Ni$^{2+}$ and
Co$^{4+}$ ions evidencing the charge transfer from Ni to Co ions. Our work
demonstrated the tunability of magnetic interactions mediated electronic
transport in cobaltate-nickelate thin films, which is relevant in understanding
Ni-Co interactions in oxides for their technological applications such as in
AMR sensors.",2210.09608v3
2022-10-27,Dynamics of a relativistic jet through magnetized media,"The merger of two neutron stars (NSs) produces the emission of gravitational
waves, the formation of a compact object surrounded by a dense and magnetized
environment, and the launching of a collimated and relativistic jet, which will
eventually produce a short gamma-ray burst (SGRB). The interaction of the jet
with the environment has been shown to play a major role in shaping the
structure of the outflow that eventually powers the gamma-ray emission. In this
paper, we present a set of 2.5 dimensional RMHD simulations that follow the
evolution of a relativistic non-magnetized jet through a medium with different
magnetization levels, as produced after the merger of two NSs. We find that the
predominant consequence of a magnetized ambient medium is that of suppressing
instabilities within the jet, and preventing the formation of a series of
collimation shocks. One implication of this is that internal shocks lose
efficiency, causing bursts with low-luminosity prompt emission. On the other
hand, the jet-head velocity and the induced magnetization within the jet are
fairly independent from the magnetization of the ambient medium. Future
numerical studies with a larger domain are necessary to obtain light curves and
spectra in order to better understand the role of magnetized media.",2210.15698v1
2023-01-12,Shear-driven magnetic buoyancy in the solar tachocline: The mean electromotive force due to rotation,"The leading theoretical paradigm for the Sun's magnetic cycle is an
$\alpha\omega$-dynamo process, in which a combination of differential rotation
and turbulent, helical flows produces a large-scale magnetic field that
reverses every 11 years. Most $\alpha\omega$ solar dynamo models rely on
differential rotation in the solar tachocline to generate a strong toroidal
field. The most problematic part of such models is then the production of the
large-scale poloidal field, via a process known as the $\alpha$-effect. Whilst
this is usually attributed to small-scale convective motions under the
influence of rotation, the efficiency of this regenerative process has been
called into question by some numerical simulations. Motivated by likely
conditions within the tachocline, the aim of this paper is to investigate an
alternative mechanism for the poloidal field regeneration, namely the magnetic
buoyancy instability in a shear-generated, rotating magnetic layer. We use a
local, fully compressible model in which an imposed vertical shear winds up an
initially vertical magnetic field. The field ultimately becomes buoyantly
unstable, and we measure the resulting mean electromotive force (EMF). For
sufficiently rapid rotation, we find that a significant component of the mean
EMF is aligned with the direction of the mean magnetic field, which is the
characteristic feature of the classical $\alpha\omega$-dynamo model. Our
results therefore suggest that magnetic buoyancy could contribute directly to
the generation of large-scale poloidal field in the Sun.",2301.05067v1
2023-01-19,Stereoscopic Observation of Simultaneous Longitudinal and Transverse Oscillations in a Single Filament Driven by Two-sided-loop Jet,"We report the first observations of simultaneous large-amplitude longitudinal
and transverse oscillations of a quiescent filament trigged by a two-sided-loop
jet formed by the magnetic reconnection between the filament and an emerging
loop in the filament channel, recorded by the Solar Dynamics Observatory and
the Solar TErrestrial RElations Observatory. The north arm of the jet firstly
pushed the filament mass moving northwardly along the magnetic field lines
consisting of the coronal cavity, then some elevated filament mass fell back
and started to oscillate longitudinally at the bottom of the cavity (i.e., the
magnetic dip). The northernmost part of the filament also showed transverse
oscillation simultaneously. The amplitude and period of the longitudinal
(transverse) oscillation are 12.96 (2.99) Mm and 1.18 (0.33) hours,
respectively. By using the method of filament seismology, the radius of
curvature of the magnetic dip is about 151 Mm, consistent with that obtained by
the 3D reconstruction (166 Mm). Using different physical parameters of the
observed longitudinal and transverse oscillations, the magnetic field strength
of the filament is estimated to be about 23 and 21 Gauss, respectively. By
calculating the energy of the moving filament mass, the minimum energy of the
jet is estimated to be about 1.96 x 10^28 erg. We conclude that the newly
formed jet can not only trigger simultaneous longitudinal and transverse
oscillations in a single filament, but also can be used as a seismology tool
for diagnosing filament information, such as the magnetic structure, magnetic
field strength, and magnetic twists.",2301.07989v1
2023-02-14,Structural and spin-glass properties of single crystal $J_{eff}$ = 1/2 pyrochlore antiferromagnet NaCdCo$_2$F$_7$ : correlating $T_f$ with magnetic-bond-disorder,"Weak bond disorder disrupts the expected spin-liquid ground-state of the
ideal $S$ = 1/2 Heisenberg pyrochlore antiferromagnet. Here we introduce a
single crystal study of the structural and magnetic properties of the
bond-disordered pyrochlore NaCdCo$_2$F$_7$. The magnetic susceptibility appears
isotropic, with a large negative Curie-Weiss temperature ($\theta_{CW}$ =
-108(1) K), however no magnetic order is observed on cooling until a spin-glass
transition at $T_f$ = 4.0 K. AC-susceptibility measurements show a
frequency-dependent shift of the associated cusp in $\chi$ at $T_f$, that can
be fitted well by the empirical Vogel-Fulcher law. The magnetic moment of
$\mu_{eff}$ = 5.4(1) $\mu_{B}$/Co$^{2+}$ indicates a significant orbital
contribution and heat capacity measurements show that down to 1.8 K, well below
$T_f$, only $S_{mag}$ ~2/3 Rln(2) of the magnetic entropy is recovered,
suggestive of residual continued dynamics. Structural and magnetism comparisons
are made with the other known members of the Na$A$Co$_2$F$_7$ family ($A$ =
Ca$^{2+}$, Sr$^{2+}$), confirming the expected relationship between spin-glass
freezing temperature, and extent of magnetic bond disorder brought about by the
size mismatch between A-site ions.",2302.06919v2
2023-02-21,Coronal Magnetic Fields derived with Images acquired during the 21 August 2017 Total Solar Eclipse,"The coronal magnetic field, despite its overwhelming importance to the
physics and dynamics of the corona, has only rarely been measured. Here, the
electron density maps derived from images acquired during the total solar
eclipse of August 21st, 2017 are employed to demonstrate a new technique to
measure the coronal magnetic fields. The strength of the coronal magnetic
fields is derived with a semiempirical formula relating the plasma magnetic
energy density to the gravitational potential energy. The resulting values are
compared with those provided by more advanced coronal field reconstruction
methods based on MHD simulations of the whole corona starting from photospheric
field measurements, finding a very good agreement. Other parameters such as the
plasma-$\beta$ and Alfv\'en velocity are also derived and compared with those
of MHD simulations. Moreover, the plane-of-sky (POS) orientation of the coronal
magnetic fields is derived from the observed inclination of the coronal
features in the filtered images, also finding a close agreement with magnetic
field reconstructions. Hence, this work demonstrates for the first time that
the 2D distribution of coronal electron densities measured during total solar
eclipses is sufficient to provide the coronal magnetic field strengths and
inclinations projected on the POS. These are among the main missing pieces of
information that limited so far our understanding of physical phenomena going
on in the solar corona.",2302.10647v1
2023-05-03,Correlation Induced Magnetic Topological Phases in Mixed-Valence Compound SmB6,"SmB6 is a mixed-valence compound with flat f-electron bands that have a
propensity to magnetism. Here, using a realistic Gamma8 quartet model, we
investigate the dynamical spin susceptibility and describe the in-gap
collective mode observed in neutron scattering experiments. We show that as the
Sm valence increases with pressure, the magnetic correlations enhance and SmB6
undergoes a first-order phase transition into a metallic antiferromagnetic
state, whose symmetry depends on the model parameters. The magnetic orderings
give rise to distinct band topologies: while the A-type order leads to an
overlap between valence and conduction bands in the form of Dirac nodal lines,
the G-type order has a negative indirect gap with weak Z2 indices. We also
consider the spin polarized phase under a strong magnetic field, and find that
it exhibits Weyl points as well as nodal lines close to the Fermi level. The
magnetic phases show markedly different surface states and tunable bulk
transport properties, with important implications for experiments. Our theory
predicts that a magnetic order can be stabilized also by lifting the Gamma8
cubic symmetry, thus explaining the surface magnetism reported in SmB6.",2305.02454v2
2023-06-05,Stockmayer supracolloidal magnetic polymers under the influence of an applied magnetic field and a shear flow,"The idea of creating magnetically controllable colloids whose rheological
properties can be finely tuned on the nano- or micro-scale has caused a lot of
experimental and theoretical effort. The latter resulted in systems whose
building blocks are ranging between single magnetic nanoparticles to complexes
of such nanoparticles bound together by various mechanisms. The binding can be
either chemical or physical, reversible or not. One way to create a system that
is physically bound is to let the precrosslinked supracolloidal magnetic
polymers (SMPs) to cluster due to both magnetic and Van-der-Waals-type forces.
The topology of the SMPs in this case can be used to tune both magnetic and
rheological properties of the resulting clusters as we show in this work. We
employ Molecular Dynamics computer simulations coupled with explicit solvent
modelled by Lattice-Boltzmann method in order to model the behaviour of the
clusters formed by chains, rings, X- and Y-shaped SMPs in a shear flow with
externally applied magnetic field. We find that the shear stabilises the shape
of the clusters not letting them extend in the direction of the field and
disintegrate. The clusters that show the highest response to an applied field
and higher shape stability are those made of Y- and X-like SMPs.",2306.03005v1
2023-06-16,Synchrotron Intensity Gradient Revealing Magnetic Fields in Galaxy Clusters,"Magnetic fields and their dynamical interplay with matter in galaxy clusters
contribute to the physical properties and evolution of the intracluster medium.
However, the current understanding of the origin and properties of cluster
magnetic fields is still limited by observational challenges. In this article,
we map the magnetic fields at hundreds-kpc scales of five clusters RXC J1314.4
-2515, Abell 2345, Abell 3376, MCXC J0352.4 -7401, and El Gordo using the
innovative synchrotron intensity gradient technique in conjunction with
high-resolution radio observations from JVLA and MeerKAT. We demonstrate that
magnetic field orientation of radio relics derived from synchrotron intensity
gradients is in very good agreement with that obtained with synchrotron
polarization. Most important, synchrotron intensity gradients is not limited by
Faraday depolarization in the cluster central regions and allows us to map
magnetic fields in the radio halos of RXC J1314.4 -2515 and El Gordo. We find
that magnetic fields in radio halos exihibit a preferential direction along the
major merger axis and show turbulent structures at higher angular resolution.
Results are consistent with expectations from numerical simulations which
predict turbulent magnetic fields in cluster mergers that are stirred and
amplified by matter motions.",2306.10011v2
2023-07-11,"Magnetic and quadrupole moments of the $Z_{c}(4020)^+$, $Z_{c}(4050)^+$, and $Z_{c}(4600)^{+}$ states in the diquark-antidiquark picture","The magnetic and quadrupole moments of the $Z_{c}(4020)^+$, $Z_{c}(4050)^+$
and $Z_{c}(4600)^{+}$ states are calculated within the QCD light-cone sum
rules. The compact diquark-antidiquark interpolating currents and the
distribution amplitudes of the on-shell photon are used to extract the magnetic
and quadrupole moments of these states. The magnetic moments are acquired as
$\mu_{Z_{c}} = 0.50 ^{+0.22}_{-0.22}~\mu_N$, $\mu_{Z^{1}_{c}}=1.22
^{+0.34}_{-0.32}~\mu_N$, and $\mu_{Z^2_{c}}=2.40 ^{+0.53}_{-0.48}~\mu_N$ for
the $Z_{c}(4020)^+$, $Z_{c}(4050)^+$ and $Z_{c}(4600)^{+}$ states,
respectively. The magnetic moments evaluated for the $Z_{c}4020)^+$,
$Z_{c}(4050)^+$ and $Z_{c}(4600)^{+}$ states are sufficiently large to be
experimentally measurable. The magnetic moment is an excellent platform for
studying the internal structure of hadrons governed by the quark-gluon dynamics
of QCD because it is the leading-order response of a bound system to a weak
external magnetic field. The quadrupole moment results are
$\mathcal{D}_{Z_c}=(0.20 ^{+0.05}_{-0.04}) \times 10^{-3}~\mbox{fm}^2 $,
$\mathcal{D}_{Z_c^1}=(0.57 ^{+0.07}_{-0.08}) \times 10^{-3}~\mbox{fm}^2 $, and
$\mathcal{D}_{Z_c^2}=(0.30 ^{+0.05}_{-0.04}) \times 10^{-3}~\mbox{fm}^2 $ for
the $Z_{c}(4020)^+$, $Z_{c}(4050)^+$ and $Z_{c}(4600)^{+}$ states,
respectively. We obtain a non-zero, but small, value for the quadrupole moments
of the $Z_c$ states, which indicates a non-spherical charge distribution. The
nature and internal structure of these states can be elucidated by comparing
future experimental data on the magnetic and quadrupole moments of the
$Z_{c}(4020)^+$, $Z_{c}(4050)^+$, and $Z_{c}(4600)^{+}$ states with the results
of the present study.",2307.05028v3
2023-10-20,X-Type Antiferromagnets,"Magnetically ordered materials reveal various types of magnetic moment
alignment that affects their functional properties. This makes the exploration
of unconventional magnetic orderings promising for the discovery of new
physical phenomena and spintronic applications. Here, we introduce cross-chain
antiferromagnets, dubbed X-type antiferromagnets, as an uncharted class of
magnetically ordered crystals, where the stacking of two magnetic sublattices
form an orthogonal pattern of intersecting atomic chains. These largely
unexplored X-type antiferromagnets reveal unique spin-dependent transport
properties that are not present in conventional magnets. Using $\beta$-Fe2PO5
as a representative example of such X-type antiferromagnets, we predict the
emergence of sublattice-selective spin-polarized transport, where one magnetic
sublattice is conducting, while the other is not. As a result, spin torque can
be exerted solely on a single sublattice, leading to unconventional ultrafast
dynamics of the N\`eel vector capable of deterministic switching of the
antiferromagnetic domains. Our work uncovers a previously overlooked type of
magnetic moment alignment in antiferromagnets and reveals sublattice-selective
physical properties promising for high-performance spintronic applications.",2310.13271v1
2023-11-01,Generation of Strong Fields to Study the Phase Transitions of Magnetized Warm Dense Matter,"Warm dense matter is a regime where Fermi degenerate electrons take an
important role in the macroscopic properties of a material. While recent
experiments hinged us closer to better understanding the unmagnetized processes
in such dense materials, their magnetized counterpart has been more difficult
to study since kilo-Tesla order magnetic fields are required. Although there
are examples of field compression generating such fields by compressing
pre-magnetized targets, this approach uses a closed configuration, where a
converging liner compresses the magnetic flux dynamically. The plasma produced
to compress the field is so dense that the fast-heating laser beam required to
form warm dense matter cannot penetrate where the field is the highest. In this
paper, numerical simulations are used to show that magnetic field compression
is also possible by shining the laser beams onto the inner surface of the
target, rather than on the outer surface. This approach relies on field
compression done by a low-density high-temperature plasma, rather than a
high-density low-temperature plasma, used in the more conventional approach.
With this novel configuration, the region of the large magnetic field is now
mostly free of plasma so the heating beam can reach the sample, located where
the magnetic field is the strongest.",2311.00879v2
2023-11-06,Controlling effective field contributions to laser-induced magnetization precession by heterostructure design,"Nanoscale heterostructure design can control laser-induced heat dissipation
and strain propagation as well as their efficiency for driving magnetization
precession. We use insulating MgO layers incorporated into metallic Pt-Cu-Ni
heterostructures to block the propagation of hot electrons. Ultrafast x-ray
diffraction (UXRD) experiments quantify how this enables controlling the
spatio-temporal shape of the transient heat and strain, which drive the
magnetization dynamics in the Ni layer. The frequency of the magnetization
precession observed by the time-resolved magneto-optical Kerr effect (MOKE) in
polar geometry is systematically tuned by the magnetic field orientation. The
combined experimental analysis (UXRD and MOKE) and modeling of transient
strain, heat and magnetization uniquely highlights the importance of
quasi-static strain as a driver of precession, when the magnetic material is
rapidly heated via electrons. The concomitant effective field change
originating from demagnetization partially compensates the change induced by
quasi-static strain. Tailored strain pulses shaped via the nanoscale
heterostructure design provide an equally efficient, phase-matched driver of
precession, paving the way for opto-magneto-acoustic devices with low heat
energy deposited in the magnetic layer.",2311.03158v2
2023-11-11,Double-Free-Layer Stochastic Magnetic Tunnel Junctions with Synthetic Antiferromagnets,"Stochastic magnetic tunnel junctions (sMTJ) using low-barrier nanomagnets
have shown promise as fast, energy-efficient, and scalable building blocks for
probabilistic computing. Despite recent experimental and theoretical progress,
sMTJs exhibiting the ideal characteristics necessary for probabilistic bits
(p-bit) are still lacking. Ideally, the sMTJs should have (a) voltage bias
independence preventing read disturbance (b) uniform randomness in the
magnetization angle between the free layers, and (c) fast fluctuations without
requiring external magnetic fields while being robust to magnetic field
perturbations. Here, we propose a new design satisfying all of these
requirements, using double-free-layer sMTJs with synthetic antiferromagnets
(SAF). We evaluate the proposed sMTJ design with experimentally benchmarked
spin-circuit models accounting for transport physics, coupled with the
stochastic Landau-Lifshitz-Gilbert equation for magnetization dynamics. We find
that the use of low-barrier SAF layers reduces dipolar coupling, achieving
uncorrelated fluctuations at zero-magnetic field surviving up to diameters
exceeding ($D\approx 100$ nm) if the nanomagnets can be made thin enough
($\approx 1$-$2$ nm). The double-free-layer structure retains bias-independence
and the circular nature of the nanomagnets provides near-uniform randomness
with fast fluctuations. Combining our full sMTJ model with advanced transistor
models, we estimate the energy to generate a random bit as $\approx$ 3.6 fJ,
with fluctuation rates of $\approx$ 3.3 GHz per p-bit. Our results will guide
the experimental development of superior stochastic magnetic tunnel junctions
for large-scale and energy-efficient probabilistic computation for problems
relevant to machine learning and artificial intelligence.",2311.06642v2
2023-12-07,Evolution of the magnetic field and flows of solar active regions with persistent magnetic bipoles before emergence,"Magnetic active regions on the Sun are harbingers of space weather events.
Understanding the physics of how they form and evolve will improve space
weather forecasting. Our aim is to characterise the surface magnetic field and
flows for a sample of active regions with persistent magnetic bipoles prior to
emergence. We identified 42 emerging active regions (EARs), in the Solar
Dynamics Observatory Helioseismic Emerging Active Region survey (Schunker et
al. 2016), associated with small magnetic bipoles at least one day before the
time of emergence. We then identified a contrasting sample of 42 EARs that
emerge more abruptly without bipoles before emergence. We computed the
supergranulation scale surface flows using helioseismic holography. We averaged
the flow maps and magnetic field maps over all active regions in each sample at
each time interval from 2 days before emergence to 1 day after. We found that
EARs associated with a persistent pre-emergence bipole evolve to be, on
average, lower flux active regions than EARs that emerge more abruptly.
Further, we found that the EARs that emerge more abruptly do so with a
diverging flow of $(3\pm 0.6) \times 10^{-6}$ s$^{-1}$ on the order of 50-100
ms$^{-1}$. Our results suggest that there is a statistical dependence of the
surface flow signature throughout the emergence process on the maximum magnetic
flux of the active region.",2312.04157v1
2023-12-08,"Sub-Alfvenic Turbulence: Magnetic to Kinetic Energy Ratio, Modification of Weak Cascade and Implications for Magnetic Field Strength Measurement","We study the properties of sub-Alfvenic magnetohydrodynamic (MHD) turbulence,
i.e., turbulence with Alfven Mach number $M_A=V_L/V_A<1$, where $V_L$ is the
velocity at the injection scale and $V_A$ is the Alfven velocity. We
demonstrate that weak turbulence can have different regimes depending on
whether it is driven by velocity or magnetic fluctuations. If the turbulence is
driven by isotropic bulk forces, i.e. velocity-driven, in an incompressible
conducting fluid, we predict that the kinetic energy is $M_A^{-2}$ times larger
than the energy of magnetic fluctuations. This effect arises from the long
parallel wavelength tail of the forcing, which excites modes with $k_\|/k_\perp
< M_A$. We also predict that as the turbulent cascade reaches the strong regime
the energy of slow modes exceeds the energy of Alfven modes by a factor
$M_A^{-1}$. These effects are absent if the turbulence is magnetically driven
at the injection scale. We confirm these predictions with numerical
simulations. As the assumption of magnetic and kinetic energy equipartition is
at the core of the Davis-Chandrasekhar-Fermi (DCF) approach to measuring
magnetic field strength in sub-Alfvenic turbulence, we conclude that the DCF
technique is not universally applicable. In particular, we suggest that the
dynamical excitation of long azimuthal wavelength modes in the galactic disk
may compromise the use of the DCF technique. We discuss alternative expressions
that can be used to obtain magnetic field strength from observations.",2312.05399v1
2023-12-14,On statistical zonostrophic instability and the effect of magnetic fields,"Zonal flows are mean flows in the east-west direction, which are ubiquitous
on planets, and can be formed through 'zonostrophic instability': within
turbulence or random waves, a weak large-scale zonal flow can grow
exponentially to become prominent. In this paper, we study the statistical
behaviour of the zonostrophic instability and the effect of magnetic fields. We
use a stochastic white noise forcing to drive random waves, and study the
growth of a mean flow in this random system. The dispersion relation for the
growth rate of the expectation of the mean flow is derived, and properties of
the instability are discussed. In the limits of weak and strong magnetic
diffusivity, the dispersion relation reduces to manageable expressions, which
provide clear insights into the effect of the magnetic field and scaling laws
for the threshold of instability. The magnetic field mainly plays a stabilising
role and thus impedes the formation of the zonal flow, but under certain
conditions it can also have destabilising effects. Numerical simulation of the
stochastic flow is performed to confirm the theory. Results indicate that the
magnetic field can significantly increase the randomness of the zonal flow. It
is found that the zonal flow of an individual realisation may behave very
differently from the expectation. For weak magnetic diffusivity and moderate
magnetic field strengths, this leads to considerable variation of the outcome,
that is whether zonostrophic instability takes place or not in individual
realisations.",2312.08905v1
2023-12-15,Topological magnon gap engineering in van der Waals CrI$_3$ ferromagnets,"The microscopic origin of the topological magnon band gap in CrI$_3$
ferromagnets has been a subject of controversy for years since two main models
with distinct characteristics, i.e., Dzyaloshinskii-Moriya (DM) and Kitaev,
provided possible explanations with different outcome implications. Here we
investigate the angular magnetic field dependence of the magnon gap of CrI$_3$
by elucidating what main contributions play a major role in its generation. We
implement stochastic atomistic spin dynamics simulations to compare the impact
of these two spin interactions on the magnon spectra. We observe three distinct
magnetic field dependencies between these two gap opening mechanisms. First, we
demonstrate that the Kitaev-induced magnon gap is influenced by both the
direction and amplitude of the applied magnetic field, while the DM-induced gap
is solely affected by the magnetic field direction. Second, the position of the
Dirac cones within the Kitaev-induced magnon gap shifts in response to changes
in the magnetic field direction, whereas they remain unaffected by the magnetic
field direction in the DM-induced gap scenario. Third, we find a
direct-indirect magnon band-gap transition in the Kitaev model by varying the
applied magnetic field direction. These differences may distinguish the origin
of topological magnon gaps in CrI$_3$ and other van der Waals magnetic layers.
Our findings pave the way for exploration and engineering topological gaps in
van der Waals materials.",2312.09903v1
2024-02-21,Switchback Patches Evolve into Microstreams via Magnetic Relaxation,"Magnetic switchbacks are distinct magnetic structures characterized by their
abrupt reversal in the radial component of the magnetic field within the
pristine solar wind. Switchbacks are believed to lose magnetic energy with
heliocentric distance. To investigate this switchbacks originating from similar
solar source regions are identified during a radial alignment of the Parker
Solar Probe (PSP; 25.8 solar radii) and Solar Orbiter (SolO; 152 solar radii).
We found that 1) the dynamic and thermal pressures decrease at the switchback
boundaries by up to 20% at PSP and relatively unchanged at SolO and magnetic
pressure jump across the boundary remains negligible at both distances, and 2)
bundles of switchbacks are often observed in switchback patches near the Sun,
and in microstreams farther away. Background proton velocity (vp) is 10%
greater than the pristine solar wind (vsw) in microstreams, whereas vp ~ vsw in
switchback patches. Microstreams contain an average of 30% fewer switchbacks
than switchback patches. It is concluded that switchbacks likely relax
magnetically and equilibrate their plasma with the surrounding environment with
heliocentric distance. Switchback relaxation can, in turn, accelerate the
surrounding plasma. Therefore, it is hypothesized that magnetic relaxation of
switchbacks may cause switchback patches to evolve into microstreams with
heliocentric distance. Statistical analysis of PSP and SolO switchbacks is
underway to further test our hypothesis.",2402.13964v1
2024-03-07,Relative alignment between gas structures and magnetic field in Orion A at different scales using different molecular gas tracers,"Context: Magnetic fields can play crucial roles in high-mass star formation.
Nonetheless, the significance of magnetic fields at various scales and their
relationship with gas structures is largely overlooked. Aims: Our goal is to
examine the relationship between the magnetic field and molecular gas
structures within the Orion A giant molecular cloud at different scales and
density regimes. Methods: We assess the gas intensity structures and column
densities in Orion A by utilizing $^{12}$CO, $^{13}$CO, and C$^{18}$O from
Nobeyama observations. Through comparing Nobeyama observations with
{\it{Planck}} polarization observations on large scales ($\sim0.6$ pc) and JCMT
polarization observations on small scales ($\sim0.04$ pc), we investigate how
the role of magnetic fields change with scale and density. Results: We find a
similar trend from parallel to perpendicular alignment with increasing column
densities in Orion A at both large and small spatial scales. Besides, when
changing from low-density to high-density tracers, the relative orientation
preference changes from random to perpendicular. The self-similar results at
different scales indicate that magnetic fields are dynamically important in
both cloud formation and filament formation. However, magnetic fields
properties at small scales are relative complicated, and the interplay between
magnetic field and star-forming activities needs to be discussed case-by-case.",2403.04274v1
1995-05-30,On the dynamics of a spherical spin-glass,"We carry out an analysis of the effect of a quenched magnetic field on the
dynamics of the spherical Sherrington-Kirkpatrick spin-glass model. We show
that there is a characteristic time introduced by the presence of the field.
Firstly, for times sufficiently small the dynamic scenario of the zero field
case - with aging effects - is reproduced. Secondly, for times larger than the
characteristic time one sees equilibrium dynamics. This dynamical behaviour is
reconciled with the geometry of the energy landscape of the model. We compare
this behaviour with experimental observations at a finite applied field.",9505149v1
2000-03-29,Ultrafast spin dynamics and critical behavior in half-metallic ferromagnet : Sr_2FeMoO_6,"Ultrafast spin dynamics in ferromagnetic half-metallic compound Sr_2FeMoO_6
is investigated by pump-probe measurements of magneto-optical Kerr effect.
Half-metallic nature of this material gives rise to anomalous thermal
insulation between spins and electrons, and allows us to pursue the spin
dynamics from a few to several hundred picoseconds after the optical
excitation. The optically detected magnetization dynamics clearly shows the
crossover from microscopic photoinduced demagnetization to macroscopic critical
behavior with universal power law divergence of relaxation time for wide
dynamical critical region.",0003467v3
2002-06-29,"Dynamical Decimation Renormalization-Group Technique: Kinetic Gaussian Model on Non-Branching, Braching and Multi-branching Koch Curve","A generalizing formulation of dynamical real-space renormalization that suits
for arbitrary spin systems is suggested. The new version replaces the
single-spin flipping Glauber dynamics with the single-spin transition dynamics.
As an application, in this paper we mainly investigate the critical slowing
down of the Gaussian spin model on three fractal lattices, including
nonbranching, branching and multibranching Koch Curve. The dynamical critical
exponent $z$ is calculated for these lattices using an exact decimation
renormalization transformation in the assumption of the magnetic-like
perturbation, and a universal result $z=1/\nu $ is found.",0207008v1
2001-12-19,Pseudochaos,"A family of the billiard-type systems with zero Lyapunov exponent is
considered as an example of dynamics which is between the regular one and
chaotic mixing. This type of dynamics is called ``pseudochaos''. We demonstrate
how the fractional kinetic equation can be introduced for the pseudochaos and
how the main critical exponents of the fractional kinetics can be evaluated
from the dynamics. Problems related to pseudochaos are discussed: Poincare
recurrences, continued fractions, log-periodicity, rhombic billiards, and
others. Pseudochaotic dynamics and fractional kinetics can be applied to
streamlines or magnetic field lines behavior.",0112033v2
1999-04-21,Nonlinear Accelerator Problems via Wavelets: 2. Orbital Dynamics in General Multipolar Field,"In this series of eight papers we present the applications of methods from
wavelet analysis to polynomial approximations for a number of accelerator
physics problems. In this part we consider orbital motion in transverse plane
for a single particle in a circular magnetic lattice in case when we take into
account multipolar expansion up to an arbitrary finite number. We reduce
initial dynamical problem to the finite number (equal to the number of n-poles)
of standard algebraical problem and represent all dynamical variables via an
expansion in the base of periodical wavelets.",9904040v1
1999-04-21,Nonlinear Accelerator Problems via Wavelets: 3. Effects of Insertion Devices on Beam Dynamics,"In this series of eight papers we present the applications of methods from
wavelet analysis to polynomial approximations for a number of accelerator
physics problems. In this part, assuming a sinusoidal field variation, we
consider the analytical treatment of the effects of insertion devices on beam
dynamics. We investigate via wavelet approach a dynamical model which has
polynomial nonlinearities and variable coefficients. We construct the
corresponding wavelet representation. As examples we consider wigglers and
undulator magnets. We consider the further modification of our variational
approach which may be applied in each scale.",9904041v1
2007-09-25,Chaotic Dynamics of Spin-Valve Oscillators,"Recent experimental and theoretical studies on the magnetization dynamics
driven by an electric current have uncovered a number of unprecedented rich
dynamic phenomena. We predict an intrinsic chaotic dynamics that has not been
previously anticipated. We explicitly show that the transition to chaotic
dynamics occurs through a series of period doubling bifurcations. In chaotic
regime, two dramatically different power spectra, one with a well-defined peak
and the other with a broadly distributed noise, are identified and explained.",0709.4045v1
2011-07-04,Moderate Deviations for a Curie-Weiss model with dynamical external field,"In the present paper we prove moderate deviations for a Curie-Weiss model
with external magnetic field generated by a dynamical system, as introduced by
Dombry and Guillotin-Plantard. The results extend those already obtained in the
case of a constant external field by Eichelsbacher and L\""owe. The Curie-Weiss
model with dynamic external field is related to the so called dynamic Z-random
walks. We also prove a moderate deviation result for the dynamic Z-random walk,
completing the list of limit theorems for this object.",1107.0671v1
2015-10-13,Taming the dynamical sign problem in real-time evolution of quantum many-body problems,"Current nonequilibrium Monte Carlo methods suffer from a dynamical sign
problem that makes simulating real-time dynamics for long times exponentially
hard. We propose a new `Inchworm Algorithm', based on iteratively reusing
information obtained in previous steps to extend the propagation to longer
times. The algorithm largely overcomes the dynamical sign problem, changing the
scaling from exponential to quadratic. We use the method to solve the Anderson
impurity model in the Kondo and mixed valence regimes, obtaining results both
for quenches and for spin dynamics in the presence of an oscillatory magnetic
field.",1510.03534v1
2013-08-05,Revisiting linear dynamics of non-axisymmetric perturbations in weakly magnetized accretion discs,"We investigate linear dynamics of non-axisymmetric perturbations in
incompressible, vertically stratified Keplerian discs with a weak vertical
magnetic field in the shearing box approximation. Perturbations are decomposed
into shearing waves whose evolution is followed via numerical integration of
the linearized ideal MHD equations. There are two basic modes in the system --
inertia-gravity waves and magnetic mode, which displays the magnetorotational
instability (MRI). As distinct from previous studies, we introduce
`eigenvariables' characterizing each (counter-propagating) component of the
inertia-gravity and magnetic modes, which are governed by a set of four first
order coupled ordinary differential equations. This allowed us to identify a
new process of linear coupling of the two above non-axisymmetric modes due to
the disc's differential rotation. We did a comparative analysis of the dynamics
of non-axisymmetric and axisymmetric magnetic mode perturbations. It is shown
that the growth of optimal and close-to-optimal non-axisymmetric harmonics of
this mode, having transient nature, can prevail over the exponential growth of
axisymmetric ones (i.e., over the axisymmetric MRI) during dynamical time. A
possible implication of this result for axisymmetric channel solutions is
discussed. Specifically, the formation of the channel may be affected/impeded
by non-axisymmetric modes already at the early linear stage leading to its
untimely disruption -- the outcome strongly depends on the amplitude and
spectrum of initial perturbation. So, this competition may result in an
uncertainty in the magnetic mode's non-linear dynamics. It is also shown that a
maximum non-axisymmetric growth is at vertical wavelengths close to the
scale-height for which compressibility effects are important. This indirectly
suggests that compressibility plays a role in the dynamics of the
non-axisymmetric MRI.",1308.1058v1
2017-02-08,Meridional Circulation Dynamics in a Cyclic Convective Dynamo,"Surface observations indicate that the speed of the solar meridional
circulation in the photosphere varies in anti-phase with the solar cycle. The
current explanation for the source of this variation is that inflows into
active regions alter the global surface pattern of the meridional circulation.
When these localized inflows are integrated over a full hemisphere, they
contribute to the slow down of the axisymmetric poleward horizontal component.
The behavior of this large scale flow deep inside the convection zone remains
largely unknown. Present helioseismic techniques are not sensitive enough to
capture the dynamics of this weak large scale flow. Moreover, the large time of
integration needed to map the meridional circulation inside the convection
zone, also masks some of the possible dynamics on shorter timescales. In this
work we examine the dynamics of the meridional circulation that emerges from a
3D MHD global simulation of the solar convection zone. Our aim is to assess and
quantify the behavior of meridional circulation deep inside the convection
zone, where the cyclic large-scale magnetic field can reach considerable
strength. Our analyses indicate that the meridional circulation morphology and
amplitude are both highly influenced by the magnetic field, via the impact of
magnetic torques on the global angular momentum distribution. A dynamic feature
induced by these magnetic torques is the development of a prominent upward flow
at mid latitudes in the lower convection zone that occurs near the equatorward
edge of the toroidal bands and that peaks during cycle maximum. Globally, the
dynamo-generated large-scale magnetic field drives variations in the meridional
flow, in stark contrast to the conventional kinematic flux transport view of
the magnetic field being advected passively by the flow.",1702.02421v2
2006-08-31,Using the Galactic Dynamics of M7 Dwarfs to Infer the Evolution of Their Magnetic Activity,"We present a spectroscopic study and dynamical analysis of ~2600 M7 dwarfs.
We confirm our previous finding that the fraction of magnetically active stars
decreases with vertical distance from the Galactic plane. We also show that the
mean luminosity of the H-alpha emission has a small but statistically
significant decrease with distance. Using space motions for ~1300 stars and a
simple one-dimensional dynamical simulation, we demonstrate that the drop in
the activity fraction of M7 dwarfs can be explained by thin disk dynamical
heating and a rapid decrease of magnetic activity at a mean stellar age of ~6-7
Gyr.",0609001v1
1995-11-27,Topology and dynamics in ferromagnetic media,"A direct link between the topological complexity of ferromagnetic media and
their dynamics has recently been established through the construction of
unambiguous conservation laws as moments of a topological vorticity. In the
present paper we carry out this program under completely realistic conditions,
with due account of the long-range magnetostatic field and related boundary
effects. In particular, we derive unambiguous expressions for the linear and
angular momentum in a ferromagnetic film which are then used to study the
dynamics of magnetic bubbles under the influence of an applied magnetic-field
gradient. The semi-empirical golden rule of bubble dynamics is verified in its
gross features but not in its finer details. A byproduct of our analysis is a
set of virial theorems generalizing Derrick's scaling relation as well as a
detailed recalculation of the fundamental magnetic bubble.",9511126v2
1996-04-16,Dynamics of Domains in Diluted Antiferromagnets,"We investigate the dynamics of two-dimensional site-diluted Ising
antiferromagnets. In an external magnetic field these highly disordered
magnetic systems have a domain structure which consists of fractal domains with
sizes on a broad range of length scales. We focus on the dynamics of these
systems during the relaxation from a long-range ordered initial state to the
disordered fractal-domain state after applying an external magnetic field. The
equilibrium state with applied field consists of fractal domains with a size
distribution which follows a power law with an exponential cut-off. The
dynamics of the system can be understood as a growth process of this
fractal-domain state in such a way that the equilibrium distribution of domains
develops during time. Following these ideas quantitatively we derive a simple
description of the time dependence of the order parameter. The agreement with
simulations is excellent.",9604094v1
1997-05-22,Monte Carlo Simulation of the Short-time Behaviour of the Dynamic XY Model,"Dynamic relaxation of the XY model quenched from a high temperature state to
the critical temperature or below is investigated with Monte Carlo methods.
When a non-zero initial magnetization is given, in the short-time regime of the
dynamic evolution the critical initial increase of the magnetization is
observed. The dynamic exponent $\theta$ is directly determined. The results
show that the exponent $\theta$ varies with respect to the temperature.
Furthermore, it is demonstrated that this initial increase of the magnetization
is universal, i.e. independent of the microscopic details of the initial
configurations and the algorithms.",9705236v1
2001-02-07,Finite-temperature dynamical magnetic susceptibility of quasi-one-dimensional frustrated spin-1/2 Heisenberg antiferromagnets,"We study the dynamical response of frustrated, quasi-one-dimensional spin-1/2
Heisenberg antiferromagnets at finite temperatures. We allow for the presence
of a Dzyaloshinskii-Moriya interaction. We concentrate on a model of weakly
coupled planes of anisotropic triangular lattices. Combining exact results for
the dynamical response of one dimensional Heisenberg chains with a Random Phase
Approximation (RPA) in the frustrated interchain couplings, we calculate the
dynamical susceptibility in the disordered phase. We investigate the
instability of the disordered phase to the formation of collective modes. We
find a very weak instability to the formation of incommensurate magnetic order
and determine the ordering temperature and wave vector. We also determine the
effects of uniform magnetic fields on the ordering transition.",0102138v2
2001-09-20,Spin dynamics of a one-dimensional spin-1/2 fully anisotropic Ising-like antiferromagnet in a transverse magnetic field,"We consider the one-dimensional Ising-like fully anisotropic S=1/2 Heisenberg
antiferromagnetic Hamiltonian and study the dynamics of domain wall excitations
in the presence of transverse magnetic field $h_x$. We obtain dynamical spin
correlation functions along the magnetic field $S^{xx}(q,\omega)$ and
perpendicular to it $S^{yy}(q,\omega)$. It is shown that the line shapes of
$S^{xx}(q,\omega)$ and $S^{yy}(q,\omega)$ are purely symmetric at the
zone-boundary. It is observed in $S^{yy}(q,\omega)$ for $\pi/2<q<\pi$ that the
spectral weight moves toward low energy side with the increase of $h_x$. This
model is applicable to study the spin dynamics of CsCoCl$_3$ in the presence of
weak interchain interactions.",0109364v1
2004-04-02,Quantum Spin Dynamics in Single-Molecule Magnets,"This thesis contains a thorough investigation of the quantum spin dynamics in
Mn12-ac and Mn6 Single-molecule magnets. In particular, we have investigated
the interplay between quantum tunneling of magnetization and nuclear spin
dynamics in Mn12-ac by ultra-low temperature NMR experiments. We discuss the
effect of quantum tunneling on the nuclear spin-lattice relaxation, the nuclear
spin diffusion, the thermalization of the nuclear spins, the isotope effect,
the influence of strong perpendicular fields, and we propose a model to unify
all our observations. For the isotropic Mn6 molecule, we report the observation
of long-range ferromagentic ordering of the cluster spins, due to dipolar
interactions only. The nuclear spin dynamics provides an interesting comparison
with the case of Mn12-ac. Two large introductory chapters describe the design
of our ultra-low temperature setups, and the theoretical background necessary
to understand the interplay between nuclear spins and quantum tunneling, with
emphasis on the Prokof'ev-Stamp theory of the ""spin-bath"".",0404049v1
2004-11-18,Optoelectronic control of spin dynamics at near-THz frequencies in magnetically doped quantum wells,"We use time-resolved Kerr rotation to demonstrate the optical and electronic
tuning of both the electronic and local moment (Mn) spin dynamics in
electrically gated parabolic quantum wells derived from II-VI diluted magnetic
semiconductors. By changing either the electrical bias or the laser energy, the
electron spin precession frequency is varied from 0.1 to 0.8 THz at a magnetic
field of 3 T and at a temperature of 5 K. The corresponding range of the
electrically-tuned effective electron g-factor is an order of magnitude larger
compared with similar nonmagnetic III-V parabolic quantum wells. Additionally,
we demonstrate that such structures allow electrical modulation of local moment
dynamics in the solid state, which is manifested as changes in the amplitude
and lifetime of the Mn spin precession signal under electrical bias. The large
variation of electron and Mn-ion spin dynamics is explained by changes in
magnitude of the sp&#8722;d exchange overlap.",0411492v1
2006-02-22,Soliton pair dynamics in patterned ferromagnetic ellipses,"Confinement alters the energy landscape of nanoscale magnets, leading to the
appearance of unusual magnetic states, such as vortices, for example. Many
basic questions concerning dynamical and interaction effects remain unanswered,
and nanomagnets are convenient model systems for studying these fundamental
physical phenomena. A single vortex in restricted geometry, also known as a
non-localized soliton, possesses a characteristic translational excitation mode
that corresponds to spiral-like motion of the vortex core around its
equilibrium position. Here, we investigate, by a microwave reflection
technique, the dynamics of magnetic soliton pairs confined in lithographically
defined, ferromagnetic Permalloy ellipses. Through a comparison with
micromagnetic simulations, the observed strong resonances in the subgigahertz
frequency range can be assigned to the translational modes of vortex pairs with
parallel or antiparallel core polarizations. Vortex polarizations play a
negligible role in the static interaction between two vortices, but their
effect dominates the dynamics.",0602509v1
2000-09-12,"Confinement, magnetic Z(N) symmetry and low energy effective theory of gluodynamics","In these notes I explain the idea how one could understand confinement by
studying the low energy effective dynamics of non Abelian gauge theories. I
argue that under some mild assumptions, the low energy dynamics is determined
universally by the spontaneous breaking of the magnetic symmetry introduced by
't Hooft more than 20 years ago. The degrees of freedom in the effective theory
are magnetic vortices. They play a similar role in confining dynamics to the
role played by pions and sigma in the chiral symmetry breaking dynamics.
  I give explicit derivation of the effective theory in 2+1 dimensional weekly
coupled confining models and give arguments that it remains qualitatively the
same in strongly coupled 2+1 dimensional gluodynamics. Confinement in this
effective theory is a very simple classical statement about long range
interaction between topological solitons, which follows by a simple direct
classical calculation from the structure of the effective Lagrangian. I discuss
the elements of this picture which generalize to 3+1 dimensions and point to
the open questions still remaining.",0009138v2
2002-04-17,Electron Mass Operator in a Strong Magnetic Field and Dynamical Chiral Symmetry Breaking,"The electron mass operator in a strong magnetic field is calculated. The
contribution of higher Landau levels of virtual electrons, along with the
ground Landau level, is shown to be essential in the leading log approximation.
The effect of the electron dynamical mass generation by a magnetic field is
investigated. In a model with N charged fermions, it is shown that some
critical number N_{cr} exists for any value of the electromagnetic coupling
constant alpha, such that the fermion dynamical mass is generated with a
doublet splitting for N < N_{cr}, and the dynamical mass does not arise at all
for N > N_{cr}, thus leaving the chiral symmetry unbroken.",0204201v1
2002-05-31,Magnetic catalysis and anisotropic confinement in QCD,"The expressions for dynamical masses of quarks in the chiral limit in QCD in
a strong magnetic field are obtained. A low energy effective action for the
corresponding Nambu-Goldstone bosons is derived and the values of their decay
constants as well as the velocities are calculated. The existence of a
threshold value of the number of colors $N^{thr}_c$, dividing the theories with
essentially different dynamics, is established. For the number of colors $N_c
\ll N^{thr}_c$, an anisotropic dynamics of confinement with the confinement
scale much less than $\Lambda_{QCD}$ and a rich spectrum of light glueballs is
realized. For $N_c$ of order $N^{thr}_c$ or larger, a conventional confinement
dynamics takes place. It is found that the threshold value $N^{thr}_c$ grows
rapidly with the magnetic field [$N^{thr}_c \gtrsim 100$ for $|eB| \gtrsim
(1{GeV})^2$]. In contrast to QCD with a nonzero baryon density, there are no
principal obstacles for checking these results and predictions in lattice
computer simulations.",0205348v3
1996-03-23,RG analysis of magnetic catalysis in Dynamical symmetry breaking,"We perform the renormalization group analysis on the dynamical symmetry
breaking under strong external magnetic field, studied recently by Gusynin,
Miransky and Shovkovy. We find that any attractive four-Fermi interaction
becomes strong in the low energy, thus leading to dynamical symmetry breaking.
When the four-Fermi interaction is absent, the $\beta$-function for the
electromagnetic coupling vanishes in the leading order in $1/N$. By solving the
Schwinger-Dyson equation for the fermion propagator, we show that in $1/N$
expansion, for any electromagnetic coupling, dynamical symmetry breaking occurs
due to the presence of Landau energy gap by the external magnetic field.",9603157v1
2000-09-15,Exact time-correlation functions of quantum Ising chain in a kicking transversal magnetic field,"Spectral analysis of the {\em adjoint} propagator in a suitable Hilbert space
(and Lie algebra) of quantum observables in Heisenberg picture is discussed as
an alternative approach to characterize infinite temperature dynamics of
non-linear quantum many-body systems or quantum fields, and to provide a bridge
between ergodic properties of such systems and the results of classical ergodic
theory. We begin by reviewing some recent analytic and numerical results along
this lines. In some cases the Heisenberg dynamics inside the subalgebra of the
relevant quantum observables can be mapped explicitly into the (conceptually
much simpler) Schr\"" odinger dynamics of a single one-(or few)-dimensional
quantum particle. The main body of the paper is concerned with an application
of the proposed method in order to work out explicitly the general spectral
measures and the time correlation functions in {\em a quantum Ising spin 1/2
chain in a periodically kicking transversal magnetic field}, including the
results for the simpler autonomous case of a static magnetic field in the
appropriate limit. The main result, being a consequence of a purely continuous
non-trivial part of the spectrum, is that the general time-correlation
functions decay to their saturation values as $t^{-3/2}$.",0009031v1
2007-04-06,Model C critical dynamics of random anisotropy magnets,"We study the relaxational critical dynamics of the three-dimensional random
anisotropy magnets with the non-conserved n-component order parameter coupled
to a conserved scalar density. In the random anisotropy magnets the structural
disorder is present in a form of local quenched anisotropy axes of random
orientation. When the anisotropy axes are randomly distributed along the edges
of the n-dimensional hypercube, asymptotical dynamical critical properties
coincide with those of the random-site Ising model. However structural disorder
gives rise to considerable effects for non-asymptotic critical dynamics. We
investigate this phenomenon by a field-theoretical renormalization group
analysis in the two-loop order. We study critical slowing down and obtain
quantitative estimates for the effective and asymptotic critical exponents of
the order parameter and scalar density. The results predict complex scenarios
for the effective critical exponent approaching an asymptotic regime.",0704.0896v1
2007-08-03,Local dynamics and gravitational collapse of a self-gravitating magnetized Fermi gas,"We use the Bianchi-I spacetime to study the local dynamics of a magnetized
self-gravitating Fermi gas. The set of Einstein-Maxwell field equations for
this gas becomes a dynamical system in a 4-dimensional phase space. We consider
a qualitative study and examine numeric solutions for the degenerate zero
temperature case. All dynamic quantities exhibit similar qualitative behavior
in the 3-dimensional sections of the phase space, with all trajectories
reaching a stable attractor whenever the initial expansion scalar H_{0} is
negative. If H_{0} is positive, and depending on initial conditions, the
trajectories end up in a curvature singularity that could be isotropic(singular
""point"") or anisotropic (singular ""line""). In particular, for a sufficiently
large initial value of the magnetic field it is always possible to obtain an
anisotropic type of singularity in which the ""line"" points in the same
direction of the field.",0708.0593v2
2008-08-18,Dynamics of driven vortex-antivortex matter in superconducting films with a magnetic dipole array,"We investigate theoretically vortex-antivortex (v-av) matter moving in thin
superconducting films with a regular array of in-plane magnetic dipoles. Our
model considers v-av pair creation induced by the local current density
generated by the magnetic texture and the transport current and simulates the
dynamics of vortices and antivortices by numerical integration of the Langevin
equation of motion. Calculations of the transport properties at zero applied
field show a strong dependence of the v-av dynamics on the current intensity
and direction. The dynamics of the v-av matter is characterized by a series of
creation and annihilation processes, which reflect on the time dependence of
the electrical field, and by guided motion, resulting in a zero-field
transverse resistance.",0808.2421v1
2008-10-20,"Gas-dynamic shock heating of post-flare loops due to retraction following localized, impulsive reconnection","We present a novel model in which shortening of a magnetic flux tube
following localized, three-dimensional reconnection generates strong
gas-dynamic shocks around its apex. The shortening releases magnetic energy by
progressing away from the reconnection site at the Alfven speed. This launches
inward flows along the field lines whose collision creates a pair of
gas-dynamic shocks. The shocks raise both the mass density and temperature
inside the newly shortened flux tube. Reconnecting field lines whose initial
directions differ by more that 100 degrees can produce a concentrated knot of
plasma hotter that 20 MK, consistent with observations. In spite of these high
temperatures, the shocks convert less than 10% of the liberated magnetic energy
into heat - the rest remains as kinetic energy of bulk motion. These
gas-dynamic shocks arise only when the reconnection is impulsive and localized
in all three dimensions; they are distinct from the slow magnetosonic shocks of
the Petschek steady-state reconnection model.",0810.3661v2
2008-11-12,Spin-Dynamics of the antiferromagnetic S=1/2-Chain at finite magnetic Fields and intermediate Temperatures,"We present a study of the dynamic structure factor of the antiferromagnetic
spin-1/2 Heisenberg chain at finite temperatures and finite magnetic fields.
Using Quantum-Monte-Carlo based on the stochastic series expansion and
Maximum-Entropy methods we evaluate the longitudinal and the transverse dynamic
structure factor from vanishing magnetic fields up to and above the threshold
$B_c$ for ferromagnetic saturation, as well as for high and for intermediate
temperatures. We study the field-induced redistribution of spectral weight
contrasting longitudinal versus transverse excitations. At finite fields below
saturation incommensurate low-energy modes are found consistent with zero
temperature Bethe-Ansatz. The crossover between the field induced ferromagnet
above $B_c$ and the Luttinger liquid below $B_c$ is analyzed in terms of the
transverse spin-dynamics. Evaluating sum-rules we assess the quality of the
analytic continuation and demonstrate excellent consistency of the
Maximum-Entropy results.",0811.1956v2
2009-04-07,Spin dynamics and structure formation in a spin-1 condensate in a magnetic field,"We study the dynamics of a trapped spin-1 condensate in a magnetic field.
First, we analyze the homogeneous system, for which the dynamics can be
understood in terms of orbits in phase space. We analytically solve for the
dynamical evolution of the populations of the various Zeeman components of the
homogeneous system. This result is then applied via a local density
approximation to trapped quasi-1D condensates. Our analysis of the trapped
system in a magnetic field shows that both the mean-field and Zeeman regimes
are simultaneously realized, and we argue that the border between these two
regions is where spin domains and phase defects are generated. We propose a
method to experimentally tune the position of this border.",0904.1160v2
2009-11-30,Small-scale structure and dynamics of the chromospheric magnetic field,"Recent advances in observational performance and numerical simulations have
revolutionised our understanding of the solar chromosphere. This concerns in
particular the structure and dynamics on small spatial and temporal scales. As
a result, the picture of the solar chromosphere changed from an idealised
static and plane-parallel stratification to a complex compound of intermittent
domains, which are dynamically coupled to the layers below and above. In this
picture, the chromosphere in a stricter sense is associated with the typical
fibrillar structure shaped by magnetic fields like it is known from images
taken in the H alpha line core. In internetwork regions below this layer, there
exists a domain with propagating shock waves and weak magnetic fields, which
both probably interact with the overlying large scale field. The existence of
such a sub-canopy domain certainly depends on the properties of the overlying
field. Details of the structure of the lower atmosphere can therefore be
expected to vary significantly from location to location. Here, high-resolution
observations, which were obtained with the CRISP filter at the Swedish Solar
Telescope, are used to derive qualitative constraints for the atmospheric
structure of quiet Sun regions.",0911.5639v1
2010-07-05,Laser-induced Spin Dynamics in Metallic Multilayers,"Electronic excitations in a ferromagnet can trigger ultrafast spin dynamics
with potential applications in a speed increase in magnetic recording. The
project investigates ultrafast magnetization dynamics, which is driven in
metallic layers by ballistic hot electrons. In a ferromagnet these electrons
induce a change in the absolute value of the magnetization M through
spin-dependent scattering. If the electrons are spin-polarized, scattering at
the interface of a noble metal and a ferromagnet results in spin-transfer
torque and hence modifies the direction of M. To reveal the underlying
mechanisms, we study model systems, which are realized by layers of Au with its
large ballistic mean free path and Fe as an itinerant ferromagnet. We aim at
understanding ultrafast demagnetization and femtosecond all-optical generation
of spin transfer torque effects (excitation of hot electrons with a femtosecond
laser). The launched dynamics are probed by magneto-optics in a time-resolved
experiment.",1007.0726v1
2012-04-20,Magnetic order and spin dynamics in the proximity of a ferromagnetic quantum critical point: A μSR study of YbNi4P2,"The local 4f-electronic spin dynamics and magnetic order in YbNi4P2 were
studied by means of muon-spin relaxation measurements. Zero-field muon-spin
relaxation proves static magnetic order with a strongly reduced ordered Yb3+
moment of (2.5-4.6) \times 10-2{\mu}B, below TC = 140 mK. Above TC, the
muon-spin polarization P(t,B) is dominated by quasihomogeneous spin
fluctuations and exhibits a time-field scaling relation P(t,B) =
P(t/B{\gamma}), indicating cooperative critical spin dynamics in the system. At
T = 190 mK, slightly above TC, {\gamma} = 0.81(5), suggesting time-scale
invariant power-law behavior for the dynamic electronic spin-spin
autocorrelation function.",1204.4632v1
2012-07-20,Measurement of the dynamical dipolar coupling in a pair of magnetic nano-disks using a Ferromagnetic Resonance Force Microscope,"We perform an extensive experimental spectroscopic study of the collective
spin-wave dynamics occurring in a pair of magnetic nano-disks coupled by the
magneto-dipolar interaction. For this, we take advantage of the stray field
gradient produced by the magnetic tip of a ferromagnetic resonance force
microscope (f-MRFM) to continuously tune and detune the relative resonance
frequencies between two adjacent nano-objects. This reveals the anti-crossing
and hybridization of the spin-wave modes in the pair of disks. At the exact
tuning, the measured frequency splitting between the binding and anti-binding
modes precisely corresponds to the strength of the dynamical dipolar coupling
$\Omega$. This accurate f-MRFM determination of $\Omega$ is measured as a
function of the separation between the nano-disks. It agrees quantitatively
with calculations of the expected dynamical magneto-dipolar interaction in our
sample.",1207.4919v1
2013-02-07,Three-dimensional artificial neural network model of the dayside magnetopause,"Artificial Neural Networks (ANN) from package NeuroShell 2 are applied for
modeling of dayside magnetopause. The model data set is based on the
magnetopause crossings and on the corresponding hour-averaged measurements of
solar wind plasma and interplanetary magnetic field. ANN model represents 3D
shape and size of the dayside magnetopause in dependence on solar wind dynamic
pressure, Bz and By components of interplanetary magnetic field. The model
permits firstly to describe dynamics of the magnetic cusp and global asymmetry
of the dayside magnetopause under wide range of the external conditions.
Slightly change of the magnetopause size with change of By absolute value is
presented quantitatively in the form of analytical expression. ANN model shows
three regimes of the magnetopause dynamics that controlled by Bz component:
pressure balance regime (Bz>0 nT), reconnection regime (0>Bz>-10 nT) and regime
of reconnection saturation (Bz<-10 nT). Three different regimes of magnetopause
dynamics at the subsolar point are described by modified pressure balance
equation obtained from ANN model.",1302.1704v1
2013-04-02,Structures and dynamics of small scales in decaying magnetohydrodynamic turbulence,"The topological and dynamical features of small scales are studied in the
context of decaying magnetohydrodynamic turbulent flows using direct numerical
simulations. Joint probability density functions (PDFs) of the invariants of
gradient quantities related to the velocity and the magnetic fields demonstrate
that structures and dynamics at the time of maximum dissipation depend on the
large scale initial conditions. This is evident in particular from the fact
that each flow has a different shape for the joint PDF of the invariants of the
velocity gradient in contrast to the universal teardrop shape of hydrodynamic
turbulence. The general picture that emerges from the analysis of the
invariants is that regions of high vorticity are correlated with regions of
high strain rate $\bm S$ also in contrast to hydrodynamic turbulent flows.
Magnetic strain dominated regions are also well correlated with region of high
current density $\bm j$. Viscous dissipation ($\propto \bm S^2$) as well as
Ohmic dissipation ($\propto \bm j^2$) reside in regions where strain and
rotation are locally almost in balance. The structures related to the velocity
gradient possess different characteristics than those associated with the
magnetic field gradient with the latter being locally more quasi-two
dimensional.",1304.0695v1
2013-07-05,Instabilities of rotational flows in azimuthal magnetic fields of arbitrary radial dependence,"Using the WKB approximation we perform a linear stability analysis for a
rotational flow of a viscous and electrically conducting fluid in an external
azimuthal magnetic field that has an arbitrary radial profile B_{phi}(R). In
the inductionless approximation, we find the growth rate of the
three-dimensional perturbation in a closed form and demonstrate in particular
that it can be positive when the velocity profile is Keplerian and the magnetic
field profile is slightly shallower than R^{-1}.",1307.1576v2
2013-10-19,Stochastic Resonance and Dynamic First-Order Pseudo-Phase Transitions in the Irreversible Growth of Thin Films under Spatially Periodic Magnetic Fields,"We study the irreversible growth of magnetic thin films under the influence
of spatially periodic fields by means of extensive Monte Carlo simulations. We
find first-order pseudo-phase transitions that separate a dynamically
disordered phase from a dynamically ordered phase. By analogy with
time-dependent oscillating fields applied to Ising-type models, we
qualitatively associate this dynamic transition with the
localization/delocalization transition of ""spatial hysteresis"" loops. Depending
on the relative width of the magnetic film, $L$, compared to the wavelength of
the external field, $\lambda$, different transition regimes are observed. For
small systems ($L<\lambda$), the transition is associated with the Standard
Stochastic Resonance regime, while, for large systems ($L>\lambda$), the
transition is driven by Anomalous Stochastic Resonance. The origin of the
latter is identified as due to the emergence of an additional relevant
lengthscale, namely the roughness of the spin domain switching interface. The
distinction between different stochastic resonance regimes is discussed at
length, both qualitatively by means of snapshot configurations, as well as
quantitatively via residence-length and order-parameter probability
distributions.",1310.5259v1
2014-03-19,Vibrational free energy and phase stability of paramagnetic and antiferromagnetic CrN from ab-initio molecular dynamics,"We present a theoretical first-principles method to calculate the free energy
of a magnetic system in its high-temperature paramagnetic phase, including
vibrational, electronic, and magnetic contributions. The method for calculating
free energies is based on ab-initio molecular dynamics and combines a treatment
of disordered magnetism using disordered local moments molecular dynamics
(DLM-MD) with the temperature dependent effective potential (TDEP) method to
obtain the vibrational contribution to the free energy. We illustrate the
applicability of the method by obtaining the anharmonic free energy for the
paramagnetic cubic and the antiferromagnetic orthorhombic phases of chromium
nitride. The influence of lattice dynamics on the transition between the two
phases is demonstrated by constructing the temperature-pressure phase diagram.",1403.4766v1
2015-01-22,Topologically Protected Dynamics of Spin Textures,"We study current-induced dynamics of spin textures in thin magnetic
nanowires. We derive effective equations of motion describing the dynamics of
the domain-wall soft modes associated with topological defects. Because the
magnetic domain walls are topological objects, these equations are universal
and depend only on a few parameters. We obtain spin spiral domain-wall
structure in ferromagnetic wires with Dzyaloshinskii-Moriya interaction and
critical current dependence on this interaction. We also find the most
efficient way to move the domain walls by resonant current pulses and propose a
procedure to determine their dynamics by measuring the voltage induced by a
moving domain wall. Based on translationally non-invariant nanowires, we show
how to make prospective magnetic memory nanodevices much more energy efficient.",1501.05366v1
2015-03-08,Antiferromagnetic order and spin dynamics in iron-based superconductors,"High-transition temperature (high-$T_c$) superconductivity in the iron
pnictides/chalcogenides emerges from the suppression of the static
antiferromagnetic order in their parent compounds, similar to copper oxides
superconductors. This raises a fundamental question concerning the role of
magnetism in the superconductivity of these materials. Neutron scattering, a
powerful probe to study the magnetic order and spin dynamics, plays an
essential role in determining the relationship between magnetism and
superconductivity in high-$T_c$ superconductors. The rapid development of
modern neutron time-of-flight spectrometers allows a direct determination of
the spin dynamical properties of iron-based superconductors throughout the
entire Brillouin zone. In this review, we present an overview of the neutron
scattering results on iron-based superconductors, focusing on the evolution of
spin excitation spectra as a function of electron/hole-doping and isoelectronic
substitution. We compare spin dynamical properties of iron-based
superconductors with those of copper oxide and heavy fermion superconductors,
and discuss the common features of spin excitations in these three families of
unconventional superconductors and their relationship with superconductivity.",1503.02340v2
2015-04-22,Chromatic and Dispersive Effects in Nonlinear Integrable Optics,"Proton accumulator rings and other circular hadron accelerators are
susceptible to intensity-driven parametric instabilities because the
zero-current charged particle dynamics are characterized by a single tune.
Landau damping can suppress these instabilities, which requires energy spread
in the beam or introducing nonlinear magnets such as octupoles. However, this
approach reduces dynamic aperture. Nonlinear integrable optics can suppress
parametric instabilities independent of energy spread in the distribution,
while preserving the dynamic aperture. This novel approach promises to reduce
particle losses and enable order-of-magnitude increases in beam intensity. In
this paper we present results, obtained using the Lie operator formalism, on
how chromaticity and dispersion affect particle orbits in integrable optics. We
conclude that chromaticity in general breaks the integrability, unless the
vertical and horizontal chromaticities are equal. Because of this, the
chromaticity correcting magnets can be weaker and fewer correcting magnet
families are required, thus minimizing the impact on dynamic aperture.",1504.05981v2
2015-06-16,Stochastic approximation of dynamical exponent at quantum critical point,"We have developed a unified finite-size scaling method for quantum phase
transitions that requires no prior knowledge of the dynamical exponent $z$.
During a quantum Monte Carlo simulation, the temperature is automatically tuned
by the Robbins-Monro stochastic approximation method, being proportional to the
lowest gap of the finite-size system. The dynamical exponent is estimated in a
straightforward way from the system-size dependence of the temperature. As a
demonstration of our novel method, the two-dimensional $S=1/2$ quantum $XY$
model in uniform and staggered magnetic fields is investigated in the
combination of the world-line quantum Monte Carlo worm algorithm. In the
absence of the uniform magnetic field, we obtain the fully consistent result
with the Lorentz invariance at the quantum critical point, $z=1$, i.e., the
three-dimensional classical $XY$ universality class. Under a finite uniform
magnetic field, on the other hand, the dynamical exponent becomes two, and the
mean-field universality with effective dimension $(2+2)$ governs the quantum
phase transition.",1506.04837v1
2015-07-29,Taub-NUT Dynamics with a Magnetic Field,"We study classical and quantum dynamics on the Euclidean Taub-NUT geometry
coupled to an abelian gauge field with self-dual curvature and show that, even
though Taub-NUT has neither bounded orbits nor quantum bound states, the
magnetic binding via the gauge field produces both. The conserved Runge-Lenz
vector of Taub-NUT dynamics survives, in a modified form, in the gauged model
and allows for an essentially algebraic computation of classical trajectories
and energies of quantum bound states. We also compute scattering cross sections
and find a surprising electric-magnetic duality. Finally, we exhibit the
dynamical symmetry behind the conserved Runge-Lenz and angular momentum vectors
in terms of a twistorial formulation of phase space.",1507.08165v1
2015-08-07,Spin Dynamics in Driven Composite Multiferroics,"A spin dynamics approach has been used to study the behavior of the magnetic
spins and the electric pseudo-spins in a 1-D composite multiferroic chain with
a linear magneto-electric coupling at the interface. The response is
investigated with either external magnetic or electric fields driving the
system. The spin dynamics is based on the Landau-Lifshitz-Gilbert equation. A
Gaussian white noise is later added into the dynamic process to include the
thermal effects. The interface requires a closer inspection of the
magneto-electric effects. Thus, we construct a 2-D ladder model to describe the
behavior of the magnetic spins and the electric pseudo-spins with different
magneto-electric couplings.",1508.01584v2
2015-11-19,Novel considerations about the non-equilibrium regime of the tricritical point in a metamagnetic model: localization and tricritical exponents,"We have investigated the time-dependent regime of a two-dimensional
metamagnetic model at its tricritical point via Monte Carlo simulations. First
of all, we obtained the temperature and magnetic field corresponding to the
tricritical point of the model by using a refinement process based on
optimization of the coefficient of determination in the log-log fit of
magnetization decay as function of time. With these estimates in hand, we
obtained the dynamic tricritical exponents $\theta $ and $z$ and the static
tricritical exponents $\nu $ and $\beta $ by using the universal power-law
scaling relations for the staggered magnetization and its moments at early
stage of the dynamic evolution. Our results at tricritical point confirm that
this model belongs to the two-dimensional Blume-Capel model universality class
for both static and dynamic behaviors, and also they corroborate the conjecture
of Janssen and Oerding for the dynamics of tricritical points.",1511.06377v1
2016-01-06,All-electrical detection of spin dynamics in magnetic antidot lattices by the inverse spin Hall effect,"The understanding of spin dynamics in laterally confined structures on
sub-micron length scales has become a significant aspect of the development of
novel magnetic storage technologies. Numerous ferromagnetic resonance
measurements, optical characterization by Kerr microscopy and Brillouin light
scattering spectroscopy and x-ray studies were carried out to detect the
dynamics in patterned magnetic antidot lattices. Here, we investigate
Oersted-field driven spin dynamics in rectangular Ni80Fe20/Pt antidot lattices
with different lattice parameters by electrical means and compare them to
micromagnetic simulations. When the system is driven to resonance, a dc voltage
across the length of the sample is detected that changes its sign upon field
reversal, which is in agreement with a rectification mechanism based on the
inverse spin Hall effect. Furthermore, we show that the voltage output scales
linearly with the applied microwave drive in the investigated range of powers.
Our findings have direct implications on the development of engineered
magnonics applications and devices.",1601.01221v1
2017-02-16,Visualization of the global flow structure in a modified Rayleigh-Benard setup using contactless inductive flow tomography,"Rayleigh-Benard convection is not only a classical problem in fluid dynamics
but plays also an important role in many metallurgical and crystal growth
applications. The measurement of the flow field and of the dynamics of the
emerging large-scale circulation in liquid metals is a challenging task due to
the opaqueness and the high temperature of the melts. Contactless inductive
flow tomography is a technique to visualize the mean three-dimensional flow
structure in liquid metals by measuring the flow induced magnetic field
perturbations under the influence of one or several applied magnetic fields. In
this paper, we present first measurements of the flow induced magnetic field in
a Rayleigh-Benard setup, which are also used to investigate the dynamics of the
large-scale circulation. Additionally, we investigate numerically the quality
of the reconstruction of the three-dimensional flow field for different sensor
configurations.",1702.04880v1
2017-04-24,Magnetic field fluctuations analysis for the ion trap implementation of the quantum Rabi model in the the deep strong coupling regime,"The dynamics of the quantum Rabi model in the deep strong coupling regime is
theoretically analyzed in a trapped-ion setup. Recognizably, the main hallmark
of this regime is the emergence of collapses and revivals, whose faithful
observation is hindered under realistic magnetic dephasing noise. Here we
discuss how to attain a faithful implementation of the quantum Rabi model in
the deep strong coupling regime which is robust against magnetic field
fluctuations and at the same time provides a large tunability of the simulated
parameters. This is achieved by combining standing wave laser configuration
with continuous dynamical decoupling. In addition, we study the role that
amplitude fluctuations play to correctly attain the quantum Rabi model using
the proposed method. In this manner the present work further supports the
suitability of continuous dynamical decoupling techniques in trapped-ion
settings to faithfully realize different interacting dynamics.",1704.07303v2
2017-07-07,Magnetoelectric antiferromagnets as platforms for the manipulation of solitons,"We study the magnetic dynamics of magnetoelectric antiferromagnetic thin
films, where an unconventional boundary ferromagnetism coexists with the bulk
N\'{e}el phase below the N\'{e}el temperature. The spin exchange between the
two order parameters yields an effective low-energy theory that is formally
equivalent to that of a ferrimagnet. Dynamics of domain walls and skyrmions are
analyzed within the collective variable approach, from which we conclude that
they behave as massive particles moving in a viscous medium subjected to a
gyrotropic force. We find that the film thickness can be used as a control
parameter for the motion of these solitons. In this regard, it is shown that an
external magnetic field can drive the dynamics of domain walls, whose terminal
velocity is tunable with the sample thickness. Furthermore, the classification
of the skyrmion dynamics is sensitive to the spatial modulation of the sample
thickness, which can be easily engineered with the present (thin-film)
deposition techniques. Current-driven spin transfer can trigger drifting orbits
of skyrmions, which can be utilized as racetracks for these magnetic textures.",1707.02374v1
2017-08-21,Relaxation of Compressible Euler Flow in a Toroidal Domain,"It is shown that the universal steady Euler flow field, independent of
boundary shape or symmetry, in a toroidal domain with fixed boundary obeys a
nonlinear Beltrami equation, with the nonlinearity arising from a
Boltzmann-like, velocity-dependent factor. Moreover, this is a relaxed velocity
field, in the sense that it extremizes the total kinetic energy in the domain
under free variations of the velocity field, constrained only by tangential
velocity and vorticity boundary conditions and conservation of total fluid
helicity and entropy. This is analogous to Woltjer-Taylor relaxation of plasma
magnetic field to a stationary state. However, unlike the magnetic field case,
attempting to derive slow, quasi-relaxed dynamics from Hamilton's action
principle, with constant total fluid helicity as a constraint, fails to agree,
in the static limit, with the nonlinear Beltrami solution of the Euler
equations. Nevertheless, an action principle that gives a quasi-relaxed
dynamics that does agree can be formulated, by introducing a potential
representation of the velocity field and defining an analogue of the magnetic
helicity as a new constraint. A Hamiltonian form of quasi-relaxed fluid
dynamics is also given.",1708.06193v1
2017-10-10,Dynamical regimes of vortex flow in type-II superconductors with parallel twin boundaries,"We explore the dynamics of driven magnetic flux lines in disordered type-II
superconductors in the presence of twin boundaries oriented parallel to the
direction of the applied magnetic field, using a three-dimensional elastic line
model simulated with Langevin molecular dynamics. The lines are driven
perpendicular to the planes to model the effect of an electric current applied
parallel to the planes and perpendicular to the magnetic field. A study of the
long-time non-equilibrium steady states for varying sample orientation and
thickness reveals a rich collection of dynamical regimes spanning the depinning
crossover region that separates the pinned and moving-lattice states of vortex
matter. We observe the emergence of a preferred direction for the ordering of
the Abrikosov lattice in the free-flowing vortex regime due to asymmetric
pinning by the planar defects. We have performed novel direct measurements of
flux line excitations such as half-loops and double kinks to aid the
characterization of the topologically rich flux flow profile.",1710.03679v2
2018-03-01,Aging dynamics in quenched noisy long-range quantum Ising models,"We consider the $d$-dimensional transverse-field Ising model with power-law
interactions $J/r^{d+\sigma}$ in the presence of a noisy longitudinal field
with zero average. We study the longitudinal-magnetization dynamics of an
initial paramagnetic state after a sudden switch-on of both the interactions
and the noisy field. While the system eventually relaxes to an
infinite-temperature state with vanishing magnetization correlations, we find
that two-time correlation functions show aging at intermediate times. Moreover,
for times shorter than the inverse noise strength $\kappa$ and distances longer
than $a(J/\kappa)^{2/\sigma}$ with $a$ being the lattice spacing, we find a
critical scaling regime of correlation and response functions consistent with
the model A dynamical universality class with an initial-slip exponent
$\theta=1$ and dynamical critical exponent $z=\sigma/2$. We obtain our results
analytically by deriving an effective action for the magnetization field
including the noise in a non-perturbative way. The above scaling regime is
governed by a non-equilibrium fixed point dominated by the noise fluctuations.",1803.00280v2
2018-03-05,Off-resonant all-optical switching dynamics in a ferromagnetic model system,"We present a theoretical study of the the effects of off-resonant polarized
optical fields on a ferromagnetic model system. We determine the light-induced
dynamics of itinerant carriers in a system that includes magnetism at the
mean-field level and spin-orbit coupling. We investigate an all-optical
switching process for ferromagnets, which is close to the one proposed by
Qaiumzadeh et al. [Phys. Rev. B 88, 064416] for the inverse Faraday effect. By
computing the optically driven coherent dynamics together with incoherent
scattering mechanisms we go beyond a perturbation expansion in powers of the
optical field. We find an important contribution of a dynamic Stark effect
coupling of the Raman type between the magnetic bands, which leads to a
polarization-dependent effect on the magnetization that may support or oppose
switching, but also contributes to demagnetization via an increase in
electronic energy.",1803.01924v1
2018-04-24,Coupling between dynamic magnetic and charge-order correlations in the cuprate superconductor Nd$_{2-x}$Ce$_{x}$CuO$_4$,"Charge order has now been observed in several cuprate high-temperature
superconductors. We report a resonant inelastic x-ray scattering experiment on
the electron-doped cuprate Nd$_{2-x}$Ce$_{x}$CuO$_4$ that demonstrates the
existence of dynamic correlations at the charge order wave vector. Upon cooling
we observe a softening in the electronic response, which has been predicted to
occur for a d-wave charge order in electron-doped cuprates. At low
temperatures, the energy range of these excitations coincides with that of the
dispersive magnetic modes known as paramagnons. Furthermore, measurements where
the polarization of the scattered photon is resolved indicate that the dynamic
response at the charge order wave vector primarily involves spin-flip
excitations. Overall, our findings indicate a coupling between dynamic magnetic
and charge-order correlations in the cuprates.",1804.09185v1
2018-07-26,Reversal Time of Jump-Noise Dynamics for Large Nucleation,"The jump-noise is a phenomenological stochastic process used to model the
thermal fluctuation of magnetization in nanomagnets. In this work, the large
nucleation regime of jump-noise dynamics is studied, and its reversal time is
characterized from Monte Carlo simulations and analysis. Results show that the
reversal time of jump-noise dynamics for large nucleation is asymptotically
equal to the time constant associated with a single jump-noise scattering event
from the energy minimum in the energy landscape of the magnetization. The
reversal time for large nucleation depends linearly on the height of the energy
barrier for large barriers. The significance of the large nucleation regime of
jump-noise dynamics to phenomenologically explain the magnetoelectric switching
of antiferromagnetic order parameter is also prospected.",1807.09922v1
2018-08-17,Director switching dynamics of ferromagnetic nematic liquid crystals,"Successful realization of ferromagnetic nematic liquid crystals has opened up
the possibility to experimentally study a completely new set of fundamental
physical phenomena. In this contribution we present a detailed investigation of
some aspects of the static response and the complex dynamics of ferromagnetic
liquid crystals under the application of an external magnetic field.
Experimental results are then compared with a macroscopic model. Dynamics of
the director were measured by optical methods and analyzed in terms of a
theoretical macroscopic model. A dissipative cross-coupling coefficient
describing the dynamic coupling between the two system order parameters, the
magnetization and the nematic director, is needed to explain the results. In
this contribution we examine the dependency of this coefficient on material
parameters and the saturation magnetization and the liquid crystal host.
Despite the complexity of the system, the theoretical description allows for a
proper interpretation of the results and is connected to several microscopic
aspects of the colloidal suspension.",1808.05843v1
2018-11-10,Spin dynamics in lattices of spinor atoms with quadratic Zeeman effect,"A lattice system of spinor atoms or molecules experiencing quadratic Zeeman
effect is considered. This can be an optical lattice with sufficiently deep
wells at lattice sites, so that the system is in an isolating state, where
atoms are well localized. But their effective spins can move in the presence of
external magnetic fields. The dynamics of spins, starting from an initial
nonequilibrium state, is investigated. The system is immersed into a magnetic
coil of an electric circuit, creating a magnetic feedback field. Two types of
quadratic Zeeman effect are treated, a nonresonant, so-called static-current
quadratic Zeeman effect and a quasi-resonant alternating-current quadratic
Zeeman effect. Spin dynamics in these conditions is highly nonlinear. Different
regimes of spin dynamics, starting from a strongly nonequilibrium state, are
studied. Conditions for realizing fast spin reversal are found, which can be
used in quantum information processing and spintronics.",1811.04271v1
2018-11-21,Metastable hard-axis polar state of a spinor Bose-Einstein condensate under a magnetic field gradient,"We investigate the stability of a hard-axis polar state in a spin-1
antiferromagnetic Bose-Einstein condensate under a magnetic field gradient,
where the easy-plane spin anisotropy is controlled by a negative quadratic
Zeeman energy $q<0$. In a uniform magnetic field, the axial polar state is
dynamically unstable and relaxes into the planar polar ground state. However,
under a field gradient $B'$, the excited spin state becomes metastable down to
a certain threshold $q_{th}$ and as $q$ decreases below $q_{th}$, its intrinsic
dynamical instability is rapidly recalled. The incipient spin excitations in
the relaxation dynamics appear with stripe structures, indicating the
rotational symmetry breaking by the field gradient. We measure the dependences
of $q_{th}$ on $B'$ and the sample size, and we find that $q_{th}$ is highly
sensitive to the field gradient in the vicinity of $B'=0$, exhibiting power-law
behavior of $|q_{th}|\propto B'^{\alpha}$ with $\alpha \sim 0.5$. Our results
demonstrate the significance of the field gradient effect in the quantum
critical dynamics of spinor condensates.",1811.08544v1
2019-01-23,The near and far of a pair of magnetic capillary disks,"Control on microscopic scales depends critically on our ability to manipulate
interactions with different physical fields. The creation of micro-machines
therefore requires us to understand how multiple fields, such as surface
capillary or electro-magnetic, can be used to produce predictable behaviour.
Recently, a spinning micro-raft system was developed that exhibited both static
and dynamic self-assembly [Wang et al. (2017) Sci. Adv. 3, e1602522]. These
rafts employed both capillary and magnetic interactions and, at a critical
driving frequency, would suddenly change from stable orbital patterns to static
assembled structures. In this paper, we explain the dynamics of two interacting
micro-rafts through a combination of theoretical models and experiments. This
is first achieved by identifying the governing physics of the orbital patterns,
the assembled structures, and the collapse separately. We find that the orbital
patterns are determined by the short range capillary interactions between the
disks, while the explanations of the other two behaviours only require the
capillary far field. Finally we combine the three models to explain the
dynamics of a new micro-raft experiment.",1901.09703v1
2019-02-08,Simulation of the Magnetization Dynamics of a Single Domain BiFeO$_3$ Thin Film,"The switching dynamics of a single-domain BiFeO$_3$ thin films is
investigated through combining the dynamics of polarization and Neel vector.
The evolution of the ferroelectric polarization is described by the
Landau-Khalatnikov (LK) equation, and the Landau-Lifshitz-Gilbert (LLG)
equations for spins in two sublattices to model the time evolution of the
antiferromagnetic order (Neel vector) in a G-type antiferromagnet. This work
theoretically demonstrates that due to the rotation of the magnetic hard axis
following the polarization reversal, the Neel vector can be switched by 180
degrees, while the weak magnetization can remain unchanged. The simulation
results are consistent with the ab initio calculation, where the Neel vector
rotates during polarization rotation, and also match our calculation of the
dynamics of order parameter using Landau-Ginzburg theory. We also find that the
switching time of the Neel vector is determined by the speed polarization
switching and is predicted to be as short as 30 ps.",1902.03330v1
2019-08-01,Theory of current-driven dynamics of spin textures on a surface of topological insulators,"Spin-transfer torque is one of the important physical quantities to
understand for successful application of topological insulators to spintronics.
In this paper, we present analytical expressions of the spin-transfer torques
on a surface of a magnetic topological insulator by including the higher-order
contributions of momentum, $k^2$-term and the hexagonal warping. We obtain six
different types of the spin-transfer torque including both the field-like and
the damping-like torques; the four of them appear only when the higher-order
momentum contributions are included. In addition, we discuss the dynamics of
magnetic skyrmions driven by the spin-transfer torques on the surface of the
topological insulator. Unlike the skyrmion dynamics in conventional metals, we
find that the dynamics significantly depends on the internal structure of
magnetic textures.",1908.00152v1
2019-08-13,Ultrafast spin dynamics in the proximate quantum spin liquid α-RuCl3,"{\alpha}-RuCl3 is a Kitaev material suggested to be a proximate quantum spin
liquid in a certain temperature and magnetic field range. Nonequilibrium
measurements of transient dynamics have been proposed to detect fractionalized
particles that emerge in the spin liquid and to possibly drive the system into
novel photoinduced magnetic states that cannot be accessed by conventional
equilibrium probes. Here we study ultrafast spin dynamics of photoinduced
excitations in {\alpha}-RuCl3 using pump-probe transient grating spectroscopy.
In the real part of the complex transient reflectance change {\Delta}R/R, we
observe the long-range antiferromagnetic correlation near the N\'eel
temperature. Most intriguingly, above the N\'eel temperature in the Kitaev
paramagnetic phase, we reveal a photoexcitation component sensitive to the
in-plane magnetic field in the imaginary part of {\Delta}R/R. This component
exhibits two distinct lifetimes of about tens of picoseconds. This
photoexcitation component may be connected to novel photoexcited states in the
Kitaev quantum spin liquid, and its lifetimes likely reflect the dynamics of
unconventional spin excitations in the Kitaev model.",1908.04807v2
2019-11-08,Magnetic Field Effect on Dynamics of Entanglement for Time-dependent Harmonic Oscillator,"We investigate the dynamics of entanglement, uncertainty and mixedness by
solving time dependent Schr\""{o}dinger equation for two-dimensional harmonic
oscillator with time dependent frequency and coupling parameter subject to a
static magnetic field. We compute the purities (global/marginal) and then
calculate explicitly the linear entropy $S_{L}$ as well as logarithmic
negativity $\mathcal{N}$ using the symplectic parametrization of vacuum state.
We introduce the spectral decomposition to diagonalize the marginal state and
get the expression of von Neumann entropy $S_{von}$ and establish its link with
$S_{L}$. We use the Wigner formalism to derive the Heisenberg uncertainties and
{show their dependencies on both $S_{L}$ and the coupling parameters
$\gamma_{i}$ $ (i=1,2)$ of the quadrature term $x_{i}p_{i}$.} We graphically
study the dynamics of the three features (entanglement, uncertainty, mixedness)
and present the similar topology with respect to time. We show the effects of
the magnetic field and quenched values of $J(t)$ and $\omega_{2}(t)$ on these
three dynamics, which lead eventually to control and handle them.",1911.03153v2
2020-02-27,Ultrafast magnetization dynamics in half-metallic Co$_2$FeAl Heusler alloy,"We report on optically induced, ultrafast magnetization dynamics in the
Heusler alloy $\mathrm{Co_{2}FeAl}$, probed by time-resolved magneto-optical
Kerr effect. Experimental results are compared to results from electronic
structure theory and atomistic spin-dynamics simulations. Experimentally, we
find that the demagnetization time ($\tau_{M}$) in films of
$\mathrm{Co_{2}FeAl}$ is almost independent of varying structural order, and
that it is similar to that in elemental 3d ferromagnets. In contrast, the
slower process of magnetization recovery, specified by $\tau_{R}$, is found to
occur on picosecond time scales, and is demonstrated to correlate strongly with
the Gilbert damping parameter ($\alpha$). Our results show that
$\mathrm{Co_{2}FeAl}$ is unique, in that it is the first material that clearly
demonstrates the importance of the damping parameter in the remagnetization
process. Based on these results we argue that for $\mathrm{Co_{2}FeAl}$ the
remagnetization process is dominated by magnon dynamics, something which might
have general applicability.",2002.12255v1
2020-06-19,Spin-glass thermo-remanent magnetization revisited: a numerical and analytical study,"Thermoremanent magnetization data for the 3D Edwards-Anderson spin glass are
generated using the Waiting Time Method as simulational tool and interpreted
using Record Dynamics. We verify that clusters of contiguous spins are
overturned by quakes, non-equilibrium events linked to record sized energy
fluctuations and show that quaking is a log-Poisson process, i.e. a Poisson
process whose average depends on the logarithm of the system age, counted from
the initial quench. Our results compare favorably with experimental
thermoremanent magnetization findings and with the spontaneous fluctuation
dynamics of the E-A model.
  The logarithm growth of the size of overturned clusters is related to similar
experimental results and to the growing length scale of the spin-spin spatial
correlation function. The analysis buttresses the applicability of the Waiting
Time Method as a simulational tool and of Record Dynamics as coarse-graining
method for aging dynamics.",2006.11214v2
2020-06-23,Experimental Observation of Quantum Many-Body Excitations of $E_8$ Symmetry in the Ising Chain Ferromagnet CoNb$_2$O$_6$,"Close to the quantum critical point of the transverse-field Ising spin-chain
model, an exotic dynamic spectrum was predicted to emerge upon a perturbative
longitudinal field. The dynamic spectrum consists of eight particles and is
governed by the symmetry of the $E_8$ Lie algebra. Here we report on
high-resolution terahertz spectroscopy of quantum spin dynamics in the
ferromagnetic Ising-chain material CoNb$_2$O$_6$. At 0.25 K in the magnetically
ordered phase we identify characteristics of the first six $E_8$ particles,
$\mathbf{m}_1$ to $\mathbf{m}_6$, and the two-particle
($\mathbf{m}_1+\mathbf{m}_2$) continuum in an applied transverse magnetic field
of $B_c^{1D}=4.75$ T, before the three-dimensional magnetic order is suppressed
above $B_c^{3D}\approx 5.3$ T. The observation of the higher-energy particles
($\mathbf{m}_3$ to $\mathbf{m}_6$) above the low-energy two-particle continua
features quantum many-body effects in the exotic dynamic spectrum.",2006.12956v2
2020-07-30,Dynamical torques from Shiba states in $s$-wave superconductors,"Magnetic impurities inserted in a $s$-wave superconductor give rise to
spin-polarized in-gap states called Shiba states. We study the back-action of
these induced states on the dynamics of the classical moments. We show that the
Shiba state pertains to both reactive and dissipative torques acting on the
precessing classical spin that can be detected through ferromagnetic resonance
measurements. Moreover, we highlight the influence of the bulk states as well
as the effect of the finite linewidth of the Shiba state on the magnetization
dynamics. Finally, we demonstrate that the torques are a direct measure of the
even and odd frequency triplet pairings generated by the dynamics of the
magnetic impurity. Our approach offers non-invasive alternative to the STM
techniques used to probe the Shiba states.",2007.15392v2
2020-08-29,Frequency analysis and dynamic aperture studies in ELENA with realistic 3D magnetic fields,"We briefly present recent progress with our algorithm and its implementation
called SIMPA described in a previous paper. The algorithm has a new and unique
approach to long-term 4D tracking of charged particles in arbitrary static
electromagnetic fields. Using the improvements described in this paper, we made
frequency analysis and dynamic aperture studies in ELENA. The effect of the end
fields and the perturbation introduced by the magnetic system of the electron
cooler on dynamic aperture is shown. A special feature of this study is that we
have not introduced any multipole errors into the model. The dynamic aperture
calculated in this paper is the direct consequence of the geometry of the
magnetic elements. Based on the results, we make a few suggestions to reduce
the losses during the deceleration of the beam.",2008.12945v1
2021-04-13,Spin dynamics of the director state in frustrated hyperkagome systems,"We present an experimental study of the magnetic structure and dynamics of
two frustrated hyperkagome compounds, Gd3Ga5O12 and Gd3Al5O12. It has
previously been shown that Gd3Ga5O12 exhibits long-range correlations of
multipolar directors, that are formed from antiferromagnetic spins on loops of
ten ions. Using neutron diffraction and Reverse Monte Carlo simulations we
prove the existence of similar magnetic correlations in Gd3Al5O12, showing the
ubiquity of these complex structures in frustrated hyperkagome materials.
  Using inelastic neutron scattering we shed further light on the director
state and the associated low lying magnetic excitations. In addition we have
measured quasielastic dynamics that show evidence of spin diffusion. Finally,
we present AC susceptibility measurements on both Gd3Ga5O12 and Gd3Al5O12,
revealing a large difference in the low frequency dynamics between the two
otherwise similar compounds.",2104.05987v2
2021-06-21,Resonant Measurement of Non-Reorientable Spin-Orbit Torque from a Ferromagnetic Source Layer Accounting for Dynamic Spin Pumping,"Using a multilayer structure containing (cobalt detector layer)/(copper
spacer)/(Permalloy source layer), we show experimentally how the
non-reorientable spin-orbit torque generated by the Permalloy source layer (the
component of spin-orbit torque that does not change when the Permalloy
magnetization is rotated) can be measured using spin-torque ferromagnetic
resonance (ST-FMR) with lineshape analysis. We find that dynamic spin pumping
between the magnetic layers exerts torques on the magnetic layers as large or
larger than the spin-orbit torques, so that if dynamic spin pumping is
neglected the result would be a large overestimate of the spin-orbit torque.
Nevertheless, the two effects can be separated by performing ST-FMR as a
function of frequency. We measure a non-reorientable spin torque ratio
$\xi_{\text{Py}} = 0.04 \pm 0.01$ for spin current flow from Permalloy through
an 8 nm Cu spacer to the Co, and a strength of dynamic spin pumping that is
consistent with previous measurements by conventional ferromagnetic resonance.",2106.11127v1
2021-09-06,Applying explicit symplectic integrator to study chaos of charged particles around magnetized Kerr black hole,"In a recent work of Wu, Wang, Sun and Liu, a second-order explicit symplectic
integrator was proposed for the integrable Kerr spacetime geometry. It is still
suited for simulating the nonintegrable dynamics of charged particles moving
around the Kerr black hole embedded in an external magnetic field. Its
successful construction is due to the contribution of a time transformation.
The algorithm exhibits a good long-term numerical performance in stable
Hamiltonian errors and computational efficiency. As its application, the
dynamics of order and chaos of charged particles is surveyed. In some
circumstances, an increase of the dragging effects of the spacetime seems to
weaken the extent of chaos from the global phase-space structure on Poincare
sections. However, an increase of the magnetic parameter strengthens the
chaotic properties. On the other hand, fast Lyapunov indicators show that there
is no universal rule for the dependence of the transition between different
dynamical regimes on the black hole spin. The dragging effects of the spacetime
do not always weaken the extent of chaos from a local point of view.",2109.02295v1
2021-11-10,Periodicities in a multiply connected geometry from quenched dynamics,"Exploring the lowest energy configurations of a quantum system is consistent
with the counting statistics of the frequently appeared states from quenching
dynamics. By studying the Little-Parks periodicities in a multiply-connected
ring-shaped geometry from the holographic technique, it is found that the
frequently appeared states from dynamics incline to have lower free energies.
In particular, the resulting winding numbers from quenched dynamics are
constrained in a normal distribution for a fixed magnetic flux threading the
ring. Varying the magnetic fluxes, Little-Parks periodicities will take place
with periods identical to the flux quantum $\Phi_0$. Favorable solutions with
lowest free energies perform first order phase transitions which transform
between distinct winding numbers as the magnetic flux equals half-integers
multiplying $\Phi_0$.",2111.05568v3
2021-12-08,Skyrmion Dynamics in the Presence of Deformation,"Magnetic skyrmions are topological spin textures promising for future
high-density and nonvolatile memory. It is crucial to understand the
current-driven skyrmion dynamics in the presence of deformation, of which an
analytical model, however, remains elusive. Here we extend Thiele's model by
considering both the radial and tangential forces. Our model attributes the
skyrmion deformation to the current-induced rotational symmetry breaking, which
includes magnetization canting and domain wall width variation. Our predictions
of skyrmion radius and nonlinear dynamics are consistent with micromagnetic
simulation results. Besides, we show that by applying an in-plane magnetic
field, the deformation of a skyrmion can be suppressed, and even the
compression of a skyrmion can be achieved. Our model provides a generic way to
analyze the skyrmion deformation and may inspire applications based on
nonlinear skyrmion dynamics.",2112.04073v1
2021-12-13,Selective quantum Zeno effect of ultracold atom-molecule scattering in dynamic magnetic fields,"We demonstrated that final states of ultracold scattering between atom and
molecule can be selectively produced using dynamic magnetic fields of multiple
frequencies. The mechanism of the dynamic magnetic field control is based on a
generalized quantum Zeno effect for the selected scattering channels. In
particular, we use an atom-molecule spin flip scattering to show that the
transition to the selected final spin projection of the molecule in the
inelastic scattering can be suppressed by dynamic modulation of coupling
between the Floquet engineered initial and final states.",2112.06481v2
2022-03-09,Nonequilibrium Hole Dynamics in Antiferromagnets: Damped Strings and Polarons,"We develop a nonperturbative theory for hole dynamics in antiferromagnetic
spin lattices, as described by the $t$-$J$ model. This is achieved by
generalizing the selfconsistent Born approximation to nonequilibrium systems,
making it possible to calculate the full time-dependent many-body wave
function. Our approach reveals three distinct dynamical regimes, ultimately
leading to the formation of magnetic polarons. Following the initial ballistic
stage of the hole dynamics, coherent formation of string excitations gives rise
to characteristic oscillations in the hole density. Their damping eventually
leaves behind magnetic polarons that undergo ballistic motion with a greatly
reduced velocity. The developed theory provides a rigorous framework for
understanding nonequilibrium physics of defects in quantum magnets and
quantitatively explains recent observations from cold-atom quantum simulations
in the strong coupling regime.",2203.04789v2
2022-05-18,Dynamical magnetic response in superconductors with finite momentum pairs,"We derive the dynamical magnetic response functions in the Fulde-Ferrell (FF)
state of a superconductor with inversion symmetry. The pair momentum 2q is
obtained by minimization of the condensation energy and the resulting
quasiparticle states and spectral functions exhibit the segmentation into
paired and unpaired regions due to the finite q. The dynamical magnetic
susceptibility is then calculated in linear response formalism in the FF state
with finite-q condensate resulting from s-wave or d-wave pairing. We show that
quasiparticle excitations inside as well as between paired and unpaired
segments contribute to the dynamical response. We discuss its dependence on
frequency and momentum transfer which develops a characteristic
symmetry-breaking parallel to q. Furthermore, we investigate the possible
influence on Knight shift and in the case of d-wave pairing on the spin
resonance formation in the FF state.",2205.09207v1
2022-06-22,Direct optical probing of ultrafast spin dynamics in a magnetic semiconductor,"We uncovered for the first time the spin dynamics involved in the birth and
growth of giant spin polarons in a magnetic semiconductor. For this purpose, we
developed a new measurement technique, which for the first time provides direct
access to the spin dynamics, irrespective of phonons and carriers involved in
the process. Moreover, we solved for the first time the Landau-Liftshitz
equation in the specific scenario of spin polarons, which fits our data
excellently, and demonstrates for the first time that the spin polaron growth
slows down dramatically when the sample is cooled in the paramagnetic phase.
Finally, temperature dependent Monte Carlo simulations were performed, which
are in excellent agreement with the observed slowdown, which demonstrates for
the first time that fluctuations in the Weiss field play a decisive role in
spin coherence generation induced by light in magnetic materials. These results
offer a new tool and new insight for spin dynamics investigations.",2206.10933v1
2022-09-21,Manipulating THz Spin Current Dynamics by the Dzyaloshinskii-Moriya Interaction in Antiferromagnetic Hematite,"An important vision of modern magnetic research is to use antiferromagnets as
controllable and active ultrafast components in spintronic devices. Hematite is
a promising model material in this respect because its pronounced
Dzyaloshinskii-Moriya interaction leads to the coexistence of
antiferromagnetism and weak ferromagnetism. Here, we use femtosecond laser
pulses to drive terahertz spin currents from hematite into an adjacent Pt
layer. We find two contributions to the generation of the spin current with
distinctly different dynamics: the impulsive stimulated Raman scatting that
relies on the antiferromagnetic order and the ultrafast spin Seebeck effect
that relies on the net magnetization. The total THz spin current dynamics can
thus be manipulated by a medium-strength magnetic field. The controllability of
the THz spin current achieved in hematite opens the pathway toward controlling
the exact spin current dynamics from ultrafast antiferromagnetic spin sources.",2209.10175v1
2023-01-26,"Optical orientation of excitons in a longitudinal magnetic field in indirect band gap (In,Al)As/AlAs quantum dots with type-I band alignment","The exciton recombination and spin dynamics in (In,Al)As/AlAs quantum dots
(QDs) with indirect band gap and type-I band alignment are studied. The
negligible (less than $0.2~\mu$eV) value of the anisotropic exchange
interaction in these QDs prevents a mixing of the excitonic basis states with
pure spin and allows for the formation of spin polarized bright excitons for
quasi-resonant circularly polarized excitation. In a longitudinal magnetic
field, the recombination and spin dynamics of the excitons are controlled by
the hyperfine interaction between the electron and nuclear spins. A QD blockade
by dark excitons is observed in magnetic field eliminating the impact of the
nuclear spin fluctuations. A kinetic equation model, which accounts for the
population dynamics of the bright and dark exciton states as well as for the
spin dynamics, has been developed, which allows for a quantitative description
of the experimental data.",2301.11138v1
2023-03-05,Electrical detection of antiferromagnetic dynamics in Gd-Co thin films by using a 154-GHz gyrotron irradiation,"THz magnetization dynamics is a key property of antiferromagnets as well as
ferrimagnets that could harness the THz forefront and spintronics. While most
of the present THz measurement techniques are for bulk materials whose
sensitivities rely on the volume of the material, measurement techniques
suitable for thin films are quite limited. In this study, we explored and
demonstrated electrical detection of the antiferromagnetic dynamics in
ferrimagnetic Gd-Co thin films by using a 154 GHz gyrotron, a high-power
electromagnetic wave source. Captured resonant modes allow us to characterize
the peculiar magnetization dynamics of the Gd-Co around the net angular
momentum compensation. As the gyrotron frequency is scalable up to THz, our
demonstration can be an important milestone toward the THz measurements for
antiferro- and ferri- magnetic thin films.",2303.02639v1
2023-04-25,Vortex dynamics induced by scanning SQUID susceptometry,"We measured the local magnetic response of a niobium thin film by applying a
millitesla-scale AC magnetic field using a micron-scale field coil and
detecting the response with a micron-scale pickup loop in a scanning
superconducting quantum interference device (SQUID) susceptometry measurement.
Near the film's critical temperature, we observed a step-like nonlinear and
dissipative magnetic response due to the dynamics of a small number of
vortex-antivortex pairs induced in the film by the local applied AC field. We
modeled the dynamics of the measurement using a combined two-dimensional
London-Maxwell and time-dependent Ginzburg-Landau approach, allowing us to
construct a detailed real-space picture of the vortex motion causing the
observed dissipative response. This work pushes scanning SQUID susceptometry of
two-dimensional superconductors beyond the regime of linear response and lays
the foundation for microscopic studies of vortex dynamics and pinning in
superconducting devices and more exotic materials systems.",2304.13093v2
2023-06-20,Dynamic image reconstruction with motion priors in application to 3D magnetic particle imaging,"Various imaging modalities allow for time-dependent image reconstructions
from measurements where its acquisition also has a time-dependent nature.
Magnetic particle imaging (MPI) falls into this class of imaging modalities and
it thus also provides a dynamic inverse problem. Without proper consideration
of the dynamic behavior, motion artifacts in the reconstruction become an
issue. More sophisticated methods need to be developed and applied to the
reconstruction of the time-dependent sequences of images. In this context, we
investigate the incorporation of motion priors in terms of certain
flow-parameter-dependent PDEs in the reconstruction process of time-dependent
3D images in magnetic particle imaging. The present work comprises the method
development for a general 3D+time setting for time-dependent linear forward
operators, analytical investigation of necessary properties in the MPI forward
operator, modeling aspects in dynamic MPI, and extensive numerical experiments
on 3D+time imaging including simulated data as well as measurements from a
rotation phantom and in-vivo data from a mouse.",2306.11625v1
2020-04-26,Dynamical crossover in the transient quench dynamics of short-range transverse field Ising models,"Dynamical detection of quantum phases and phase transitions (QPT) in quenched
systems with experimentally convenient initial states is a topic of interest
from both theoretical and experimental perspectives. Quenched from polarized
states, longitudinal magnetization decays exponentially to zero in time for the
short-range transverse-field Ising model (TFIM) and hence, has a featureless
steady state regime, which prevents it from exhibiting dynamical phase
transitions of type-I. In this paper, we ask whether the transient regimes of
such non-equilibrium processes probed by single-site observables, that is
magnetization per site, could encode information about the underlying QPT. The
decay rates of time-dependent and single-site observables exhibit a dynamical
crossover that separates two dynamical regions, ordered and disordered, both of
which have distinct nonequilibrium responses. We construct a dynamical order
parameterlike quantity that exhibits a scaling law at the vicinity of the
crossover. Our results reveal that scaling law exponent in short times at the
close vicinity of the dynamical crossover is significantly different than the
one predicted by analytical theory for long times. When integrability is
strongly broken, the crossover boundary turns into a region that separates two
other dynamical regions that act like dynamically-ordered and -disordered
regimes.",2004.12287v5
2024-01-05,Solutions to the Landau-Lifshitz-Gilbert equation in the frequency space: Discretization schemes for the dynamic-matrix approach,"The dynamic matrix method addresses the Landau-Lifshitz-Gilbert (LLG)
equation in the frequency domain by transforming it into an eigenproblem.
Subsequent numerical solutions are derived from the eigenvalues and
eigenvectors of the dynamic matrix. In this work we explore discretization
methods needed to obtain a matrix representation of the dynamic operator, a
fundamental counterpart of the dynamic matrix. Our approach opens a new set of
linear algebra tools for the dynamic matrix method and expose the
approximations and limitations intrinsic to it. Moreover, our discretization
algorithms can be applied to various discretization schemes, extending beyond
micromagnetism problems. We present some application examples, including a
technique to obtain the dynamic matrix directly from the magnetic free energy
function of an ensemble of macrospins, and an algorithmic method to calculate
numerical micromagnetic kernels, including plane wave kernels. We also show how
to exploit symmetries and reduce the numerical size of micromagnetic
dynamic-matrix problems by a change of basis. This procedure significantly
reduces the size of the dynamic matrix by several orders of magnitude while
maintaining high numerical precision. Additionally, we calculate analytical
approximations for the dispersion relations in magnonic crystals. This work
contributes to the understanding of the current magnetization dynamics methods,
and could help the development and formulations of novel analytical and
numerical methods for solving the LLG equation within the frequency domain.",2401.02933v2
2005-06-27,The dynamical disconnection of sunspots from their magnetic roots,"After a dynamically active emergence phase, magnetic flux at the solar
surface soon ceases to show strong signs of the subsurface dynamics of its
parent magnetic structure. This indicates that some kind of disconnection of
the emerged flux from its roots in the deep convection zone should take place.
We propose a mechanism for the dynamical disconnection of the surface flux
based upon the buoyant upflow of plasma along the field lines. Such flows arise
in the upper part of a rising flux loop during the final phases of its buoyant
ascent towards the surface. The combination of the pressure buildup by the
upflow and the cooling of the upper layers of an emerged flux tube by radiative
losses at the surface lead to a progressive weakening of the magnetic field in
several Mm depth. When the field strength has become sufficiently low,
convective motions and the fluting instability disrupt the flux tube into thin,
passively advected flux fragments, thus providing a dynamical disconnection of
the emerged part from its roots. We substantiate this scenario by considering
the quasi-static evolution of a sunspot model under the effects of radiative
cooling, convective energy transport, and pressure buildup by a prescribed
inflow at the bottom of the model. For inflow speeds in the range shown by
simulations of thin flux tubes, we find that the disconnection takes place in a
depth between 2 and 6 Mm for disconnection times up to 3 days.",0506654v1
2002-05-08,Quasiparticles governing the zero-temperature dynamics of the 1D spin-1/2 Heisenberg antiferromagnet in a magnetic field,"The T=0 dynamical properties of the one-dimensional (1D) $s=1/2$
  Heisenberg antiferromagnet in a uniform magnetic field are studied via Bethe
ansatz for cyclic chains of $N$ sites. The ground state at magnetization
$0<M_z<N/2$, which can be interpreted as a state with $2M_z$ spinons or as a
state of $M_z$ magnons, is reconfigured here as the vacuum for a different
species of quasiparticles, the {\em psinons} and {\em antipsinons}. We
investigate three kinds of quantum fluctuations, namely the spin fluctuations
parallel and perpendicular to the direction of the applied magnetic field and
the dimer fluctuations. The dynamically dominant excitation spectra are found
to be sets of collective excitations composed of two quasiparticles excited
from the psinon vacuum in different configurations. The Bethe ansatz provides a
framework for (i) the characterization of the new quasiparticles in relation to
the more familiar spinons and magnons, (ii) the calculation of spectral
boundaries and densities of states for each continuum, (iii) the calculation of
transition rates between the ground state and the dynamically dominant
collective excitations, (iv) the prediction of lineshapes for dynamic structure
factors relevant for experiments performed on a variety of quasi-1D
antiferromagnetic compounds, including KCuF$_3$,
  Cu(C$_4$H$_4$N$_2)(NO_3)_2$, and CuGeO$_3$.",0205142v1
2004-02-16,Low energy excitations and dynamic Dzyaloshinskii-Moriya interaction in $α'$-NaV$_2$O$_5$ studied by far infrared spectroscopy,"We have studied far infrared transmission spectra of alpha'-NaV2O5 between 3
and 200cm-1 in polarizations of incident light parallel to a, b, and c
crystallographic axes in magnetic fields up to 33T. The triplet origin of an
excitation at 65.4cm-1 is revealed by splitting in the magnetic field. The
magnitude of the spin gap at low temperatures is found to be magnetic field
independent at least up to 33T. All other infrared-active transitions appearing
below Tc are ascribed to zone-folded phonons. Two different dynamic
Dzyaloshinskii-Moriya (DM) mechanisms have been discovered that contribute to
the oscillator strength of the otherwise forbidden singlet to triplet
transition. 1. The strongest singlet to triplet transition is an electric
dipole transition where the polarization of the incident light's electric field
is parallel to the ladder rungs, and is allowed by the dynamic DM interaction
created by a high frequency optical a-axis phonon. 2. In the incident light
polarization perpendicular to the ladder planes an enhancement of the singlet
to triplet transition is observed when the applied magnetic field shifts the
singlet to triplet resonance frequency to match the 68cm-1 c-axis phonon
energy. The origin of this mechanism is the dynamic DM interaction created by
the 68cm-1 c-axis optical phonon. The strength of the dynamic DM is calculated
for both mechanisms using the presented theory.",0402419v2
2004-06-23,Aging and Memory Effects in Superparamagnets and Superspin Glasses,"Many dense magnetic nanoparticle systems exhibit slow dynamics which is
qualitatively indistinguishable from that observed in atomic spin glasses and
its origin is attributed to dipole interactions among particle moments (or
superspins). However, even in dilute nanoparticle systems where the dipole
interactions are vanishingly small, slow dynamics is observed and is attributed
solely to a broad distribution of relaxation times which in turn comes from
that of the anisotropy energy barriers. To clarify characteristic differences
between the two types of slow dynamics, we study a simple model of a
non-interacting nanoparticle system (a superparamagnet) analytically as well as
ferritin (a superparamagnet) and a dense Fe-N nanoparticle system (a superspin
glass) experimentally. It is found that superparamagnets in fact show aging (a
waiting time dependence) of the thermoremanent-magnetization as well as various
memory effects. We also find some dynamical phenomena peculiar only to
superspin glasses such as the flatness of the field-cooled magnetization below
the critical temperature and memory effects in the zero-field-cooled
magnetization. These dynamical phenomena are qualitatively reproduced by the
random energy model, and are well interpreted by the so-called droplet theory
in the field of the spin-glass study.",0406546v2
2005-08-03,Nonequilibrium multicritical behavior in anisotropic Heisenberg ferromagnet driven by oscillating magnetic field,"The Heisenberg ferromagnet (uniaxially anisotropic along z-direction), in the
presence of time dependent (but uniform over space) magnetic field, is studied
by Monte Carlo simulation. The time dependent magnetic field was taken as
elliptically polarised in such a way that the resulting field vector rotates in
the XZ plane. In the limit of low anisotropy, the dynamical responses of the
system are studied as functions of temperature and the amplitudes of the
magnetic field. As the temperature decreases, it aws found that the system
undergoes multiple dynamical phase transitions. In this limit, the multiple
transitions were studied in details and the phase diagram for this observed
multicritical behaviour was drawn in the field amplitude and temperature
palne.The natures (continuous/discontinuous) of the transitions are determined
by the temperature variations of fourth order Binder cumulant ratio and the
distributions of the order parameter near the transition points. The
transitions are supported by finite size study. The temperature variations of
the variances of dynamic order parameter components (for different system
sizes) indicate the existence of diverging length scale near the dynamic
transition points. The frequency dependences of the transition temperatures of
the multiple dynamic transition are also studied briefly.",0508103v2
2009-11-20,Magnetisation dynamics in exchange coupled spring systems with perpendicular anisotropy,"Magnetisation dynamics in exchange spring magnets have been studied using
simulations of the FePt/Fe bilayer system. The FePt hard layer exhibits strong
perpendicular magnetocrystalline anisotropy, while the soft (Fe) layer has
negligible magnetocrystalline anisotropy. The variation of the local spin
orientation in the Fe layer is determined by the competition of the exchange
coupling interaction with the hard layer and the magnetostatic energy which
favours in-plane magnetisation. Dynamics were studied by monitoring the
response of the Fe layer magnetisation after the abrupt application of a
magnetic field which causes the systems to realign via precessional motion.
This precessional motion allows us to obtain the frequency spectrum and hence
examine the dynamical magnetisation motion. Since the rotation of the spins in
the soft layer does not have a well defined magnetic anisotropy, the system
does not present the usual frequency field characteristics for a thin film.
Additionally we obtain multi-peaked resonance spectra for the application of
magnetic fields perpendicular to the film plane, though we discount the
existence of spin wave modes and propose that this arises due to variations in
the local effective field across the Fe layer. The dynamic response is only
considered in the Fe layer, with the FePt layer held fixed in the perpendicular
orientation.",0911.4137v2
2014-05-25,Proton spin dynamics in polymer melts: new perspectives for experimental investigations of polymer dynamics,"Significant progress was made in recent years in the understanding of the
proton spin kinetics in polymer melts. Generally, the proton spin kinetics is
determined by intramolecular and intermolecular magnetic dipole-dipole
contributions of proton spins. During many decades it was postulated that the
main contribution is a result of intramolecular magnetic dipole-dipole
interactions of protons belonging to the same polymer segment. It appears that
this postulate is far from reality. The relative weights of intra- and
intermolecular contributions are time dependent and sensitive to details of
polymer chain dynamics. It is shown that for isotropic models of polymer
dynamics the influence of the intermolecular magnetic dipole-dipole
interactions increases faster with increasing evolution time (i.e. decreasing
frequency) than the corresponding influence of the intramolecular counterpart.
On the other hand, an inverted situation is predicted by the tube-reptation
model: here the influence of the intramolecular magnetic dipole-dipole
interactions increases faster with time than the contribution from
intermolecular interactions. The intermolecular contribution in the proton
relaxation of polymer melts can experimentally be isolated using the isotope
dilution technique and this opens a new perspective for experimental
investigations of polymer dynamics by proton NMR.",1405.6366v1
2017-04-26,Anomalous lattice dynamics in a sigma-Fe60V40 alloy: Mossbauer spectroscopic study,"Lattice dynamics in a sigma-Fe60V40 compound, which shows a re-entrant
magnetism and orders ferromagnetic ally at ca. 170K, was investigated with the
Mossbauer spectroscopy in the temperature interval of 5-300 K. Two relevant
spectral parameters viz. the average center shift, <CS>, and the relative
recoil-free fraction, f/fo, were explored. The former yielded the Debye
temperature, T_D1, and the mean-square velocity of vibration, <v^2>, while the
latter T_D2 and the mean-square amplitude of vibrations, <x^2>. Significant
differences in the lattice-dynamical behaviors in the magnetic and paramagnetic
phases were revealed. In particular, the values of T_D were notably lower and
those of f/fo greatly higher in the former. This anomalous result has likely
its origin in a remarkably high inharmonic contribution to the vibrations found
for the ground magnetic state (spin-glass). Especially anomalous behavior vs.
temperature exhibits <x^2> where four well defined ranges could have been
identified and ascribed to the paramagnetic, ferromagnetic and two spin-glass
phases. Linear correlations between <v^2>-<x^2> were found within each of the
four ranges. They enabled determination of force constants, hence a change of
the potential energy, Ep, in each of the ranges. The total change of Ep=30 meV
while the corresponding one of the kinetic energy, determined from the
knowledge of <v^2>, was Ek=21 meV. The lack of balance between Ep and Ek
follows from the anharmonic lattice-dynamical behavior observed in the
spin-glass state. The results give a strong evidence that magnetism can
significantly affect the lattice dynamics.",1704.08054v1
2020-11-25,Spin dynamics in bulk MnNiGa and Mn$_{1.4}$Pt$_{0.9}$Pd$_{0.1}$Sn investigated by muon spin relaxation,"We report muon spin relaxation and magnetometry studies of bulk
Mn$_{1.4}$Pt$_{0.9}$Pd$_{0.1}$Sn and MnNiGa, two materials which have recently
been proposed to host topological magnetic states in thin lamella
(antiskyrmions for Mn$_{1.4}$Pt$_{0.9}$Pd$_{0.1}$Sn and biskyrmions for
MnNiGa), and show spin reorientation transitions in bulk. These measurements
shed light on the magnetic dynamics surounding the two magnetic phase
transitions in each material. In particular, we demonstrate that the behaviour
approaching the higher temperature transition in both samples is best
understood by considering a slow decrease in the frequency of dynamics with
temperature, rather than the sharp critical slowing down typical of second
order transitions. Furthermore, at low temperatures the two samples both show
spin dynamics over a broad range of frequencies that persist below the spin
reorienation transition. The dynamic behavior we identify gives new insight
into the bulk magnetism of these materials that may help underpin the
stabilization of the topologically non-trivial phases that are seen in thin
lamellae.",2011.12994v2
2021-04-30,Ab-Initio Molecular Dynamics with Screened Lorentz Forces. Part I. Calculation and Atomic Charge Interpretation of Berry Curvature,"The dynamics of a molecule in a magnetic field is significantly different
form its zero-field counterpart. One important difference in the presence of a
field is the Lorentz force acting on the nuclei, which can be decomposed as the
sum of the bare nuclear Lorentz force and a screening force due to the
electrons. This screening force is calculated from the Berry curvature and can
change the dynamics qualitatively. It is therefore important to include the
contributions from the Berry curvature in molecular dynamics simulations in a
magnetic field. In this work, we present a scheme for calculating the Berry
curvature numerically, by a finite-difference technique, addressing challenges
related to the arbitrary global phase of the wave function. The Berry curvature
is calculated as a function of bond distance for H$_2$ at the restricted and
unrestricted Hartree--Fock levels of theory and for CH$^{+}$ as a function of
the magnetic field strength at the restricted Hartree--Fock level of theory.
The calculations are carried out using basis sets of contracted Gaussian
functions equipped with London phase factors (London orbitals) to ensure
gauge-origin invariance. In the paper, we also interpret the Berry curvature in
terms of atomic charges and discuss its convergence in basis sets with and
without London phase factors. Calculation of the Berry curvature allows for its
inclusion in \textit{ab initio} molecular dynamics simulations in a magnetic
field.",2104.14861v1
2021-05-10,Ab-Initio Molecular Dynamics with Screened Lorentz Forces. Part II. Efficient Propagators and Rovibrational Spectra in Strong Magnetic Fields,"Strong magnetic fields have a large impact on the dynamics of molecules. In
addition to the changes of the electronic structure, the nuclei are exposed to
the Lorentz force with the magnetic field being screened by the electrons. In
this work, we explore these effects using ab-initio molecular dynamics
simulations based on an effective Hamiltonian calculated at the Hartree-Fock
level of theory. To correctly include these non-conservative forces in the
dynamics, we have designed a series of novel propagators that show both good
efficiency and stability in test cases. As a first application, we analyze
simulations of He and H$_2$ at two field strengths characteristic of magnetic
white dwarfs (0.1 $B_0 = 2.35 \times 10^4$ T and $B_0 = 2.35 \times 10^5$ T).
While the He simulations clearly demonstrate the importance of electron
screening of the Lorentz force in the dynamics, the extracted rovibrational
spectra of H$_2$ reveal a number of fascinating features not observed in the
field-free case: couplings of rotations/vibrations with the cyclotron rotation,
overtones with unusual selection rules, and hindered rotations that transmute
into librations with increasing field strength. We conclude that our presented
framework is a powerful tool to investigate molecules in these extreme
environments.",2105.04225v1
2021-11-01,Dynamics of position disordered Ising spins with a soft-core potential,"We theoretically study magnetization relaxation of Ising spins distributed
randomly in a $d$-dimension homogeneous and Gaussian profile under a soft-core
two-body interaction potential $\propto1/[1+(r/R_c)^\alpha]$ ($\alpha\ge d$),
where $r$ is the inter-spin distance and $R_c$ is the soft-core radius. The
dynamics starts with all spins polarized in the transverse direction. In the
homogeneous case, an analytic expression is derived at the thermodynamic limit,
which starts as $\propto\exp(-t^2)$ and follows a stretched-exponential law
asymptotically at long time with an exponent $\beta=d/\alpha$. In between an
oscillating behaviour is observed with a damping amplitude. For Gaussian
samples, the degree of disorder in the system can be controlled by the ratio
$l_\rho/R_c$ with $l_\rho$ the mean inter-spin distance and the magnetization
dynamics is investigated numerically. In the limit of $l_\rho/R_c\ll1$, a
coherent many-body dynamics is recovered for the total magnetization despite of
the position disorder of spins. In the opposite limit of $l_\rho/R_c\gg1$, a
similar dynamics as that in the homogeneous case emerges at later time after a
initial fast decay of the magnetization. We obtain a stretched exponent of
$\beta\approx0.18$ for the asymptotic evolution with $d=3, \alpha=6$, which is
different from that in the homogeneous case ($\beta=0.5$).",2111.00779v1
2021-11-09,Dynamics of charged particles moving around Kerr black hole with inductive charge and external magnetic field,"We mainly focus on the effects of small changes of parameters on the dynamics
of charged particles around the Kerr black hole surrounded by an external
magnetic field, which can be considered as a tidal environment. The radial
motions of charged particles on the equatorial plane are studied via an
effective potential. It is found that the particle energies at the local maxima
values of the effective potentials increase with an increase of the black hole
spin and the particle angular momenta, but decrease with an increase of one of
the inductive charge parameter and magnetic field parameter. The radii of
stable circular orbits on the equatorial plane also increase, whereas those of
the innermost stable circular orbits decrease. On the other hand, the effects
of small variations of the parameters on the orbital regular and chaotic
dynamics of charged particles on the non-equatorial plane are traced by means
of a time-transformed explicit symplectic integrator, Poincar\'{e} sections and
fast Lyapunov indicators. It is shown that the dynamics sensitively depends on
small variations of the inductive charge parameter, magnetic field parameter,
energy and angular momentum. Chaos occurs easily as each of the dynamical
parameters increases. When the dragging effects of the spacetime increase, the
chaotic properties are not always weakened under some circumstances.",2111.04900v1
2022-04-30,Electron dynamics in small magnetospheres: insights from global fully-kinetic plasma simulations of planet Mercury,"The planet Mercury possesses a small but highly dynamic magnetosphere in
which the role and dynamics of electrons are still largely unknown. We aim at
modeling the global dynamics of solar wind electrons impinging on Mercury's
magnetosphere. Particular relevance is given to local acceleration processes
and the global circulation patterns. The goals of this work are pursued by
means of three-dimensional, fully kinetic particle-in-cell simulations modeling
the interaction of the solar wind with the Hermean magnetosphere. This method
allows a self-consistent representation of the plasma dynamics from the large
planetary scale down to the electron kinetic scale. Numerical simulations are
carried out using two different solar wind conditions: purely northward or
purely southward interplanetary magnetic field direction. We find a high plasma
current (of the order of few $\mu$A/m2) flowing at the magnetospheric
boundaries (bow shock and magnetopause) dominated by electrons. This current is
driven by the small-scale electron physics resolved in our model. Furthermore,
we observe strong electron acceleration up to tens of keV as a consequence of
magnetic reconnection when the interplanetary magnetic field is directed
southward. Such energetic electrons are partially trapped in the dipolar
magnetic field of the planet mainly at nightside. Finally, by studying the
distribution of electrons in our simulations along Mariner10 and BepiColombo
first-Mercury-flyby trajectories, we propose that both spacecraft observed this
energetic quasi-trapped electron population around closest approach.",2205.00229v1
2022-08-12,Dynamic Sensor Matching based on Geomagnetic Inertial Navigation,"Optical sensors can capture dynamic environments and derive depth information
in near real-time. The quality of these digital reconstructions is determined
by factors like illumination, surface and texture conditions, sensing speed and
other sensor characteristics as well as the sensor-object relations.
Improvements can be obtained by using dynamically collected data from multiple
sensors. However, matching the data from multiple sensors requires a shared
world coordinate system. We present a concept for transferring multi-sensor
data into a commonly referenced world coordinate system: the earth's magnetic
field. The steady presence of our planetary magnetic field provides a reliable
world coordinate system, which can serve as a reference for a position-defined
reconstruction of dynamic environments. Our approach is evaluated using
magnetic field sensors of the ZED 2 stereo camera from Stereolabs, which
provides orientation relative to the North Pole similar to a compass. With the
help of inertial measurement unit informations, each camera's position data can
be transferred into the unified world coordinate system. Our evaluation reveals
the level of quality possible using the earth magnetic field and allows a basis
for dynamic and real-time-based applications of optical multi-sensors for
environment detection.",2208.06233v2
2022-09-08,Effects of the rf current and bias field direction on the transition from linear to non-linear gyrotropic dynamics in magnetic vortex structures,"We present a frequency-domain study of the dynamic behavior of a magnetic
vortex core within a single Permalloy disk by means of electrical detection and
micromagnetic simulations. When exciting the vortex core dynamics in a
non-linear regime, the lineshape of the rectified dc signal reveals a resonance
peak splitting which depends on the excitation amplitude. Using micromagnetic
simulations, we show that at high excitation power the peak splitting
originates from the nanosecond time scale quasi-periodic switching of the
vortex core polarity. Using lock-in detection, the rectified voltage is
integrated over a ms time scale, so that the net signal detected between the
two resonant peaks for a given range of parameters cancels out. The results are
in agreement with the reported effects of the in-plane static field magnitude
on the gyration dynamics, and complement them by detailed analysis of the
effects of the rf current amplitude and the azimuthal angle of the in-plane
bias magnetic field. Systematic characterization shows that a transition from
linear to nonlinear dynamical regime can be controlled by rf current as well as
by varying the magnitude and the direction of the bias magnetic field.",2209.03701v2
2023-07-30,Spin dynamics in ordered phases of anisotropic triangular-lattice antiferromagnet Cs2CoBr4,"We study spin dynamics of ordered phases of Cs2CoBr4 in a magnetic field
using electron spin resonance (ESR) technique and theoretical analysis. This
material hosts weakly interacting distorted-triangular-lattice planes of
spin-3/2 Co(2+) ions which can be viewed as spin chains coupled by frustrating
interactions. Strong single-ion anisotropy allows to describe the low-energy
spin dynamics of this system by an effective strongly anisotropic
pseudospin-1/2 model. Our ESR data show up to seven branches of magnetic
resonance in four magnetic phases arising due to subtle interplay of
frustration, low dimensionality and strong anisotropy. In particular, in the
low-field collinear stripe phase, the field evolution of modes lying below 200
GHz is described reasonably good by spectra of spin-1 and spin-0 quasiparticles
which we obtain using the bond-operator technique. These well-defined
excitations can be treated as conventional magnons and bound states of two
magnons, respectively. In contrast, numerous excitations lying above 200 GHz
are not captured by our theory due to pronounced one-dimensional correlations
inside spin chains which govern the spin dynamics at high enough energies. As
it was shown before, these modes can be most naturally interpreted as bound
states of domain walls in individual chains and their sequence resembles the
so-called ""Zeeman ladder"" in anisotropic Ising-like spin chains. Thus, Cs2CoBr4
is a system showing spin-dynamics in ordered state characteristic of both
two-dimensional and one-dimensional magnets.",2307.16251v1
2023-08-29,Wall mode dynamics and transition to chaos in magnetoconvection with a vertical magnetic field,"Quasistatic magnetoconvection of a low Prandtl number fluid
($\textrm{Pr}=0.025)$ with a vertical magnetic field is considered in a unit
aspect ratio box with no-slip boundaries. At high relative magnetic field
strengths, given by the Hartmann number $\textrm{Ha}$, the onset of convection
is known to result from a sidewall instability giving rise to the wall mode
regime. Here, we carry out 3D direct numerical simulations of unprecedented
length to map out the parameter space at $\textrm{Ha} = 200, 500, 1000$,
varying the Rayleigh number ($\textrm{Ra}$) between $6\times10^5 \lesssim
\textrm{Ra} \lesssim 5\times 10^8$. We track the development of stable
equilibria produced by this primary instability, identify bifurcations leading
to limit cycles, and eventually to chaotic dynamics. At {$\textrm{Ha}=200$},
the steady wall mode solution undergoes a symmetry-breaking bifurcation
producing a state featuring a coexistence between wall modes and a large-scale
roll in the centre of the domain which persists to higher $\textrm{Ra}$.
However, under a stronger magnetic field at $\textrm{Ha}=1000$, the steady wall
mode solution undergoes a Hopf bifurcation producing a limit cycle which
further develops to solutions that shadow an orbit homoclinic to a saddle
point. Upon a further increase in $\textrm{Ra}$, the system undergoes a
subsequent symmetry break producing a coexistence between wall modes and a
large-scale roll, although the large-scale roll exists only for a small range
of $\textrm{Ra}$, and chaotic dynamics primarily arise due to a mixture of
chaotic wall mode dynamics and arrays of cellular structures.",2308.15165v2
2024-01-22,Time-Resolved Imaging Reveals Transiently Chaotic Spin-Orbit-Torque-Driven Dynamics Under Controlled Conditions,"Spin-orbit torques (SOTs) act as efficient drivers for nanoscale magnetic
systems, such as in magnetic tunnel junctions, nano-oscillators and racetrack
geometries. In particular, in combination with materials exhibiting high
Dzyaloshinskii--Moriya interaction, SOTs are considered to result in
well-controlled deterministic magnetisation dynamics and are, therefore, used
as robust drives to move and create magnetic skyrmions. In contrast to these
expectations, we here find unpredictable, transiently chaotic dynamics induced
by SOT at an artificial anisotropy-engineered defect in a magnetic racetrack.
Based on these controlled conditions, we directly observe the nanoscale
dynamics with holography-based, time-resolved x-ray imaging. In concert with
micromagnetic simulations, we disclose a regime of violent picosecond
fluctuations, including topological instabilities that, remarkably, result in
deterministic final configurations. In addition, our images expose previously
unseen skyrmion shedding and highlight the potential of transiently chaotic
pathways for topological switching. Our approach offers new perspectives for
the investigation and application of highly non-linear SOT dynamics in
spintronics materials.",2401.12130v1
2024-02-16,Live magnetic observation of parahydrogen hyperpolarization dynamics,"Nuclear spin hyperpolarization is used in physics, chemistry, and medicine to
produce strong magnetization unachievable by equilibrium polarization
techniques. Hyperpolarization enables magnetic resonance spectroscopy and
imaging with minute samples, and is used to produce MRI spin-tracers and
polarized physics targets. Although widely used, the dynamics of the
hyperpolarization process have never been studied `live' due to the extremely
low (Hz-band) frequencies involved, and/or detector saturation by the driving
fields used. Here, we use an atomic magnetometer with sub-pT sensitivity to
observe, in real time, the complex dynamics of hyperpolarization, without
disturbing or disrupting the process. We start by examining
parahydrogen-induced $^1$H and $^{13}$C magnetization build-up during adiabatic
eigenbasis transformations in the $\mu$T-field avoided state crossings at the
heart of the process; we see live hyperpolarization dynamics including coherent
oscillations, leakage mechanisms and dipolar shifts that would be challenging
or impossible to observe by post hoc measurement. We then extend the methods to
observe the chemical-exchange-driven $^{13}$C hyperpolarization of
[1-$^{13}$C]-pyruvate -- the most important spin tracer for clinical metabolic
imaging. Beyond the interests of hyperpolarization, the observation of
adiabatic transitions in real-time is a fundamentally new approach to NMR,
reveals previously hidden nuclear spin dynamics and enables quantum control and
live process optimization in a variety of chemical scenarios.",2402.10766v1
2012-05-14,Tuning the phase transition dynamics by variation of cooling field and metastable phase fraction in Al doped Pr$_{0.5}$Ca$_{0.5}$MnO$_3$,"We report the effect of field, temperature and thermal history on the time
dependence in resistivity and magnetization in the phase separated state of Al
doped Pr$_{0.5}$Ca$_{0.5}$MnO$_3$. The rate of time dependence in resistivity
is much higher than that of magnetization and it exhibits a different cooling
field dependence due to percolation effects. Our analysis show that the time
dependence in physical properties depends on the phase transition dynamics
which can be effectively tuned by variation of temperature, cooling field and
metastable phase fraction. The phase transition dynamics can be broadly divided
into the arrested and un-arrested regimes, and in the arrested regime, this
dynamics is mainly determined by time taken in the growth of critical nuclei.
An increase in cooling field and/or temperature shifts this dynamics from
arrested to un-arrested regime, and in this regime, this dynamics is determined
by thermodynamically allowed rate of formation of critical nuclei which in turn
depends on the cooling field and available metastable phase fraction. At a
given temperature, a decrease in metastable phase fraction shifts the crossover
from arrested to un-arrested regimes towards lower cooling field. It is rather
significant that inspite of the metastable phase fraction calculated from
resistivity being somewhat off from that of magnetization, their cooling field
dependence exhibits a striking similarity which indicate that the dynamics in
arrested and un-arrested regimes are so different that it comes out vividly
provided that the measurements are done around percolation threshold.",1205.3040v1
2019-05-24,"Fluid Dynamics of Earth's core: geodynamo, inner core dynamics, core formation","This chapter is build from three 1.5 hours lectures given in Udine in april
2018 on various aspects of Earth's core dynamics. The chapter starts with a
short historical note on the discovery of Earth's magnetic field and core
(section 1). We then turn to an introduction of magnetohydrodynamics (section
2), introducing and discussing the induction equation and the form and effects
of the Lorentz force. Section 3 is devoted to the description of Earth's
magnetic field, introducing its spherical harmonics description and showing how
it can be used to demonstrate the internal origin of the geomagnetic origin. We
then move to an introduction of the convection-driven model of the geodynamo
(section 4), discussing our current understanding of the dynamics of Earth's
core, obtaining heuristically the Ekman dependency of the critical Rayleigh
number for natural rotating convection, and introducing the equations and
non-dimensional parameters used to model a convectively driven dynamo. The
following section deals with the energetics of the geodynamo (section 5). The
final two section deal with the dynamics of the inner core, focusing on the
effect of the magnetic field (section 6), and with the formation of the core
(section 7). Given the wide scope of this chapter and the limited time
available, this introduction to Earth's core dynamics is by no means intended
to be comprehensive. For more informations, the interested reader may refer to
Jones (2011), Olson (2013), or Christensen and Wicht (2015) on the geomagnetic
field and the geodynamo, to Sumita and Bergman (2015), Deguen (2012) and
Lasbleis and Deguen (2015) on the dynamics of the inner core, and to Rubie et
al. (2015) on core formation.",1905.10416v1
2022-04-23,Critical Dynamics of the Antiferromagnetic $O(3)$ Nonlinear Sigma Model with Conserved Magnetization,"We study the near-equilibrium critical dynamics of the $O(3)$ nonlinear sigma
model describing isotropic antiferromagnets with non-conserved order parameter
reversibly coupled to the conserved total magnetization. To calculate response
and correlation functions, we set up a description in terms of Langevin
stochastic equations of motion, and their corresponding Janssen--De~Dominicis
response functional. We find that in equilibrium, the dynamics is
well-separated from the statics, at least to one-loop order in a perturbative
treatment with respect to the static and dynamical nonlinearities. Since the
static nonlinear sigma model must be analyzed in a dimensional $d = 2 +
\varepsilon$ expansion about its lower critical dimension $d_\textrm{lc} = 2$,
whereas the dynamical mode-coupling terms are governed by the upper critical
dimension $d_c = 4$, a simultaneous perturbative dimensional expansion is not
feasible, and the reversible critical dynamics for this model cannot be
accessed at the static critical renormalization group fixed point. However, in
the coexistence limit addressing the long-wavelength properties of the
low-temperature ordered phase, we can perform an $\epsilon = 4 - d$ expansion
near $d_c$. This yields anomalous scaling features induced by the massless
Goldstone modes, namely sub-diffusive relaxation for the conserved
magnetization density with asymptotic scaling exponent $z_\Gamma = d - 2$ which
may be observable in neutron scattering experiments. Intriguingly, if
initialized near the critical point, the renormalization group flow for the
effective dynamical exponents recovers their universal critical values $z_c = d
/ 2$ in an intermediate crossover region.",2204.11145v2
2022-11-06,Dynamical singularity of the rate function for quench dynamics in finite-size quantum systems,"The dynamical quantum phase transition is characterized by the emergence of
nonanalytic behaviors in the rate function, corresponding to the occurrence of
exact zero points of the Loschmidt echo in the thermodynamical limit. In
general, exact zeros of the Loschmidt echo are not accessible in a finite-size
quantum system except for some fine-tuned quench parameters. In this work, we
study the realization of the dynamical singularity of the rate function for
finite-size systems under the twist boundary condition, which can be introduced
by applying a magnetic flux. By tuning the magnetic flux, we illustrate that
exact zeros of the Loschmidt echo can be always achieved when the postquench
parameter is across the underlying equilibrium phase transition point, and thus
the rate function of a finite-size system is divergent at a series of critical
times. We demonstrate our theoretical scheme by calculating the
Su-Schrieffer-Heeger model and the Creutz model in detail and exhibit its
applicability to more general cases. Our result unveils that the emergence of
dynamical singularity in the rate function can be viewed as a signature for
detecting dynamical quantum phase transition in finite-size systems. We also
unveil that the critical times in our theoretical scheme are independent on the
systems size, and thus it provides a convenient way to determine the critical
times by tuning the magnetic flux to achieve the dynamical singularity of the
rate function.",2211.03135v2
2022-11-17,As a matter of dynamical range -- scale dependent energy dynamics in MHD turbulence,"Magnetized turbulence is ubiquitous in many astrophysical and terrestrial
plasmas but no universal theory exists. Even the detailed energy dynamics in
magnetohydrodynamic (MHD) turbulence are still not well understood. We present
a suite of subsonic, super-Alfv\'enic, high plasma-beta MHD turbulence
simulations that only vary in their dynamical range, i.e., in their separation
between the large-scale forcing and dissipation scales, and their dissipation
mechanism (implicit large eddy simulation, ILES, versus and direct numerical
simulation, DNS). Using an energy transfer analysis framework we calculate the
effective, numerical viscosities and resistivities and demonstrate and that all
ILES calculations of MHD turbulence are resolved and correspond to an
equivalent visco-resistive MHD turbulence calculation. Increasing the number of
grid points used in an ILES corresponds to lowering the dissipation
coefficients, i.e., larger (kinetic and magnetic) Reynolds numbers for a
constant forcing scale. Independently, we use this same framework to
demonstrate that -- contrary to hydrodynamic turbulence -- the cross-scale
energy fluxes are not constant in MHD turbulence. This applies both to
different mediators (such as cascade processes or magnetic tension) for a given
dynamical range as well as to a dependence on the dynamical range itself, which
determines the physical properties of the flow. We do not observe any
indication of convergence even at the highest resolution (largest Reynolds
numbers) simulation at $2{,}048^3$ cells, calling into question whether an
asymptotic regime in MHD turbulence exists, and, if so, what it looks like.",2211.09750v1
1998-12-09,Nonlinear dynamic susceptibilities of interacting and noninteracting nanoparticle systems,"The linear and cubic dynamic susceptibilities of solid dispersions of
nano-sized maghemite particles have been measured for three samples with a
volume concentration of magnetic particles ranging from 0.3 % to 17 %, in order
to study the effect of dipole-dipole interactions. Significant differences
between the dynamic response of the three samples are observed. The dynamic
susceptibilities of the most dilute sample compare reasonably well with an
existing theory for the linear and cubic dynamic susceptibilities of an
assembly of noninteracting, uniaxial magnetic particles. The nonlinear dynamic
response of the most concentrated sample exhibits at low temperatures some of
the features observed in a Ag(11 at.% Mn) spin glass.",9812143v2
2003-08-23,On the universality of the fluctuation-dissipation ratio in non-equilibrium critical dynamics,"The two-time nonequilibrium correlation and response functions in 1D kinetic
classical spin systems with non-conserved dynamics and quenched to their
zero-temperature critical point are studied. The exact solution of the kinetic
Ising model with Glauber dynamics for a wide class of initial states allows for
an explicit test of the universality of the non-equilibrium limit
fluctuation-dissipation ratio X_{\infty}. It is shown that the value of
X_{\infty} depends on whether the initial state has finitely many domain walls
or not and thus two distinct dynamic universality classes can be identified in
this model. Generic 1D kinetic spin systems with non-conserved dynamics fall
into the same universality classes as the kinetic Glauber-Ising model provided
the dynamics is invariant under the C-symmetry of simultaneous spin and
magnetic-field reversal. While C-symmetry is satisfied for magnetic systems, it
need not be for lattice gases which may therefore display hitherto unexplored
types of non-universal kinetics.",0308466v2
2003-09-26,Spin Glasses: Model systems for non-equilibrium dynamics,"Spin glasses are frustrated magnetic systems due to a random distribution of
ferro- and antiferromagnetic interactions. An experimental three dimensional
(3d) spin glass exhibits a second order phase transition to a low temperature
spin glass phase regardless of the spin dimensionality. In addition, the low
temperature phase of Ising and Heisenberg spin glasses exhibits similar
non-equilibrium dynamics and an infinitely slow approach towards a
thermodynamic equilibrium state. There are however significant differences in
the detailed character of the dynamics as to memory and rejuvenation phenomena
and the influence of critical dynamics on the behaviour. In this article, some
aspects of the non-equilibrium dynamics of an Ising and a Heisenberg spin glass
are briefly reviewed and some comparisons are made to other glassy systems that
exhibit magnetic non-equilibrium dynamics.",0309602v1
2004-09-04,Stochastic dynamic simulations of fast remagnetization processes: recent advances and applications,"Numerical simulations of fast remagnetization processes using the stochastic
dynamics are widely used to study various magnetic systems. In this paper we
first address several crucial methodological problems of such simulations: (i)
the influence of the finite-element discretization on the simulated dynamics,
(ii) choice between Ito and Stratonovich stochastic calculi by the solution of
micromagnetic stochastic equations of motion and (iii) non-trivial correlation
properties of the random (thermal) field. Next we discuss several examples to
demonstrate the great potential of the Langevin dynamics for studying fast
remagnetization processes in technically relevant applications: we present
numerical analysis of equilibrium magnon spectra in patterned structures, study
thermal noise effects on the magnetization dynamics of nanoelements in pulsed
fields and show some results for a remagnetization dynamics induced by a
spin-polarized current.",0409098v1
2006-10-05,Dynamic dipole and quadrupole phase transitions in the kinetic spin-1 model,"The dynamic phase transitions have been studied, within a mean-field
approach, in the kinetic spin-1 Ising model Hamiltonian with arbitrary bilinear
and biquadratic pair interactions in the presence of a time varying
(sinusoidal) magnetic field by using the Glauber-type stochastic dynamics. The
nature (first- or second-order) of the transition is characterized by
investigating the behavior of the thermal variation of the dynamic order
parameters. The dynamic phase transitions (DPTs) are obtained and the phase
diagrams are constructed in the temperature and magnetic field amplitude plane
and found six fundamental types of phase diagrams. Phase diagrams exhibit one
or two dynamic tricritical points depending on the biquadratic interaction (K).
Besides the disordered (D) and ferromagnetic (F) phases, the FQ + D, F + FQ and
F + D coexistence phase regions also exist in the system and the F and F + D
phases disappear for high values of K.",0610149v1
2010-05-04,Critical Slowing Down Along the Dynamic Phase Boundary in Ising Meanfield Dynamics,"We studied the dynamical phase transition in kinetic Ising ferromagnets
driven by oscillating magnetic field in meanfield approximation. The meanfield
differential equation was solved by sixth order Runge-Kutta-Felberg method. The
time averaged magnetisation plays the role of the dynamic order parameter. We
studied the relaxation behaviour of the dynamic order parameter close to the
transition temperature, which depends on the amplitude of the applied magnetic
field. We observed the critical slowing down along the dynamic phase boundary.
We proposed a power law divergence of the relaxation time and estimated the
exponent. We also found its dependence on the field amplitude and compared the
result with the exact value in limiting case.",1005.0565v1
2010-12-11,Monte Carlo study of dynamic phase transition in Ising metamagnet driven by oscillating magnetic field,"The dynamical responses of Ising metamagnet (layered antiferromagnet) in the
presence of a sinusoidally oscillating magnetic field are studied by Monte
Carlo simulation. The time average staggered magnetisation plays the role of
dynamic order parameter. A dynamical phase transition was observed and a phase
diagram was plotted in the plane formed by field amplitude and temperature. The
dynamical phase boundary is observed to shrink inward as the relative
antiferromagnetic strength decreases. The results are compared with that
obtained from pure ferromagnetic system. The shape of dynamic phase boundary
observed to be qualitatively similar to that obtained from previous meanfield
calculations.",1012.2457v2
2012-01-30,Universal dynamical phase diagram of lattice spin models and strongly correlated ultracold atoms in optical lattices,"We study semiclassical dynamics of anisotropic Heisenberg models in two and
three dimensions. Such models describe lattice spin systems and hard core
bosons in optical lattices. We solve numerically Landau-Lifshitz type equations
on a lattice and show that in the phase diagram of magnetization and
interaction anisotropy, one can identify several distinct regimes of dynamics.
These regions can be distinguished based on the character of one dimensional
solitonic excitations, and stability of such solitons to transverse modulation.
Small amplitude and long wavelength perturbations can be analyzed analytically
using mapping of non-linear hydrodynamic equations to KdV type equations.
Numerically we find that properties of solitons and dynamics in general remain
similar to our analytical results even for large amplitude and short distance
inhomogeneities, which allows us to obtain a universal dynamical phase diagram.
As a concrete example we study dynamical evolution of the system starting from
a state with magnetization step and show that formation of oscillatory regions
and their stability to transverse modulation can be understood from the
properties of solitons. In regimes unstable to transverse modulation we observe
formation of lump type solutions with modulation in all directions. We discuss
implications of our results for experiments with ultracold atoms.",1201.6400v1
2014-08-05,Dynamics of a Mn spin coupled to a single hole confined in a quantum dot,"Using the emission of the positively charged exciton as a probe, we analyze
the dynamics of the optical pumping and the dynamics of the relaxation of a Mn
spin exchange-coupled with a confined hole spin in a II-VI semiconductor
quantum dot. The hole-Mn spin can be efficiently initialized in a few tens of
$ns$ under optical injection of spin polarized carriers. We show that this
optical pumping process and its dynamics are controlled by electron-Mn
flip-flops within the positively charged exciton-Mn complex. The pumping
mechanism and its magnetic field dependence are theoretically described by a
model including the dynamics of the electron-Mn complex in the excited state
and the dynamics of the hole-Mn complex in the ground state of the positively
charged quantum dot. We measure at zero magnetic field a spin relaxation time
of the hole-Mn spin in the $\mu s$ range or shorter. This hole-Mn spin
relaxation is induced by the presence of valence band mixing in self-assembled
quantum dots.",1408.0930v2
2014-11-14,Dynamical instabilities and transient short-range order in the fermionic Hubbard model,"We study the dynamics of magnetic correlations in the half-filled fermionic
Hubbard model following a fast ramp of the repulsive interaction. We use
Schwinger-Keldysh self-consistent second-order perturbation theory to
investigate the evolution of single-particle Green's functions and solve the
non-equilibrium Bethe-Salpeter equation to study the dynamics of magnetic
correlations. This approach gives us new insights into the interplay between
single-particle relaxation dynamics and the growth of antiferromagnetic
correlations. Depending on the ramping time and the final value of the
interaction, we find different dynamical behavior which we illustrate using a
dynamical phase diagram. Of particular interest is the emergence of a transient
short-range ordered regime characterized by the strong initial growth of
antiferromagnetic correlations followed by a decay of correlations upon
thermalization. The discussed phenomena can be probed in experiments with
ultracold atoms in optical lattices.",1411.4049v1
2015-07-06,Bose-Hubbard ladder subject to effective magnetic field: quench dynamics in a harmonic trap,"Motivated by a recent experiment with optical lattices that has realized a
ladder geometry with an effective magnetic field (Atala et al., Nature Physics
\textbf{10}, 588 (2014)), we study the dynamics of bosons on a tight-binding
two-leg ladder with complex hopping amplitudes. This system displays a quantum
phase transition even without interactions. We study the non-equilibrium
dynamics without and with interactions, in the presence of a harmonic trapping
potential. In particular we consider dynamics induced by quenches of the
trapping potential and of the magnitude of the rung hopping. We present a
striking ""slowing down"" effect in the collective mode dynamics near the phase
transition. This manifestation of a slowing down phenomenon near a quantum
phase transition can be visualized unusually directly: the collective mode
dynamics can be followed experimentally in real time and real space by imaging
the atomic cloud.",1507.01402v1
2016-02-09,Dynamics of cholesteric liquid crystals in the presence of random magnetic fields,"Based on dynamic renormalization group techniques, this letter analyzes the
effects of external stochastic perturbations on the dynamical properties of
cholesteric liquid crystals, studied in presence of a random magnetic field.
Our analysis quantifies the nature of the temperature dependence of the
dynamics; the results also highlight a hitherto unexplored regime in
cholesteric liquid crystal dynamics. We show that stochastic fluctuations drive
the system to a second-ordered Kosterlitz-Thouless phase transition point,
eventually leading to a Kardar-Parisi-Zhang (KPZ) universality class. The
results go beyond quasi-first order mean-field theories, and provides the first
theoretical understanding of a KPZ phase in distorted nematic liquid crystal
dynamics.",1602.03219v1
2016-11-03,Spatio-temporal Dynamics of Intrinsic Networks in Functional Magnetic Imaging Data Using Recurrent Neural Networks,"We introduce a novel recurrent neural network (RNN) approach to account for
temporal dynamics and dependencies in brain networks observed via functional
magnetic resonance imaging (fMRI). Our approach directly parameterizes temporal
dynamics through recurrent connections, which can be used to formulate blind
source separation with a conditional (rather than marginal) independence
assumption, which we call RNN-ICA. This formulation enables us to visualize the
temporal dynamics of both first order (activity) and second order (directed
connectivity) information in brain networks that are widely studied in a static
sense, but not well-characterized dynamically. RNN-ICA predicts dynamics
directly from the recurrent states of the RNN in both task and resting state
fMRI. Our results show both task-related and group-differentiating directed
connectivity.",1611.00864v2
2017-01-27,Collective dynamics in atomistic models with coupled translational and spin degrees of freedom,"Using an atomistic model that simultaneously treats the dynamics of
translational and spin degrees of freedom, we perform combined molecular and
spin dynamics simulations to investigate the mutual influence of the phonons
and magnons on their respective frequency spectra and lifetimes in
ferromagnetic bcc iron. By calculating the Fourier transforms of the space- and
time-displaced correlation functions, the characteristic frequencies and the
linewidths of the vibrational and magnetic excitation modes were determined.
Comparison of the results with that of the stand-alone molecular dynamics and
spin dynamics simulations reveals that the dynamic interplay between the
phonons and magnons leads to a shift in the respective frequency spectra and a
decrease in the lifetimes. Moreover, in the presence of lattice vibrations,
additional longitudinal magnetic excitations were observed with the same
frequencies as the longitudinal phonons.",1701.07906v1
2019-03-24,"Synchronized, periodic, and chaotic dynamics in spin torque oscillator with two free layers","A phase diagram of the magnetization dynamics is studied by numerically
solving the Landau-Lifshitz-Gilbert (LLG) equation in a spin torque oscillator
consisting of asymmetric two free layers that are magnetized in in-plane
direction. We calculated the dynamics for a wide range of current density for
both low and high field cases, and found many dynamical phases such as
synchronization, auto-oscillation with different frequencies, and chaotic
dynamics. The observation of the synchronization indicates the presence of a
dynamical phase which has not been found experimentally by using the
conventional electrical detection method. The auto-oscillations with different
frequencies lead to an oscillation of magnetoresistance with a high frequency,
which can be measured experimentally. The chaotic and/or periodic behavior of
magnetoresistance in a high current region, on the other hand, leads to a
discontinuous change of the peak frequency in Fourier spectrum.",1903.09938v2
2019-07-18,Dynamical formation of a magnetic polaron in a two-dimensional quantum antiferromagnet,"We numerically study the real-time dynamics of a single hole created in the
$t-J$ model on a square lattice. Initially, the hole spreads ballistically with
a velocity proportional to the hopping matrix element. At intermediate to long
times, the dimensionality as well as the spin background determine the hole
dynamics. A hole created in the ground state of a two dimensional quantum
antiferromagnet propagates again ballistically at long times but with a
velocity proportional to the spin exchange coupling, showing the formation of a
magnetic polaron. We provide an intuitive explanation of this dynamics in terms
of a parton construction, which leads to a good quantitative agreement with the
numerical simulations. In the limit of infinite temperature and no spin
exchange couplings, the dynamics can be approximated by a quantum random walk
on the Bethe lattice. Adding Ising interactions corresponds to an effective
disordered potential, which can dramatically slow down the hole propagation,
consistent with subdiffusive dynamics.",1907.08214v1
2020-05-13,Manipulation of heteronuclear spin dynamics with microwave and vector light shift,"We report the observation and manipulation of heteronuclear spin dynamics in
a spin-1 mixture of ultracold $^{87}$Rb and $^{23}$Na atoms. The dynamics is
driven by the interspecies spin-dependent interaction and shows a pronounced
dependence on magnetic fields with influences from both linear and quadratic
Zeeman shifts. Similar to the well-studied homonuclear cases, the interspecies
spin dynamics can be controlled by tuning the quadratic Zeeman shift with
far-detuned microwave fields. In addition, we successfully realize spin
dynamics control with vector light shifts which act as a species-selective
effective magnetic field on $^{87}$Rb atoms. Both methods show negligible loss
of atoms thus will be powerful techniques for investigating spin dynamics with
fast temporal and high spatial resolutions.",2005.06098v1
2021-03-02,Open Quantum-System Simulation of Faraday's Induction Law via Dynamical Instabilities,"We propose a novel type of a Bose-Hubbard ladder model based on an open
quantum-gas--cavity-QED setup to study the physics of dynamical gauge
potentials. Atomic tunneling along opposite directions in the two legs of the
ladder is mediated by photon scattering from transverse pump lasers to two
distinct cavity modes. The resulting interplay between cavity photon
dissipation and the optomechanical atomic back-action then induces an
average-density-dependent dynamical gauge field. The dissipation-stabilized
steady-state atomic motion along the legs of the ladder leads either to a pure
chiral current, screening the induced dynamical magnetic field as in the
Meissner effect, or generates simultaneously chiral and particle currents. For
sufficiently strong pump the system enters into a dynamically unstable regime
exhibiting limit-cycle and period-doubled oscillations. Intriguingly, an
electromotive force is induced in this dynamical regime as expected from an
interpretation based on Faraday's law of induction for the time-dependent
synthetic magnetic flux.",2103.01979v2
2021-07-02,Vortex Dynamics in Amorphous MoSi Superconducting Thin Films,"Vortex dynamics in superconductors have received a great deal of attention
from both fundamental and applied researchers over the past few decades.
Because of its critical role in the energy relaxation process of type II
superconductors, vortex dynamics have been deemed a key factor for the emerging
superconducting devices, but the effect of irradiation on vortex dynamics
remains unclear. With the support of electrical transport measurements under
external magnetic fields and irradiation, photon effect on vortex dynamics in
amorphous MoSi (a MoSi) superconducting thin films are investigated in this
work. The magnetic field dependent critical vortex velocity v* derived from the
Larkin Ovchinnikov model is not significantly affected by irradiation. However,
vortex depinning is found to be enhanced by photon-induced reduction in
potential barrier, which mitigates the adverse effect of film inhomogeneity on
superconductivity in the a MoSi thin films. The thorough understanding of the
vortex dynamics in a MoSi thin films under the effect of external stimuli is of
paramount importance for both further fundamental research in this area and
optimization of future superconducting devices.",2107.01046v2
2022-01-04,Quantum dynamics of Gaudin magnets,"Quantum dynamics of many-body systems is a fascinating and significant
subject for both theory and experiment. The question of how an isolated
many-body system evolves to its steady state after a sudden perturbation or
quench still remains challenging. In this paper, using the Bethe ansatz wave
function, we study the quantum dynamics of an inhomogeneous Gaudin magnet. We
derive explicit analytical expressions for various local dynamic quantities
with an arbitrary number of flipped bath spins, such as: the spin distribution
function, the spin-spin correlation function, and the Loschmidt echo. We also
numerically study the relaxation behavior of these dynamic properties, gaining
considerable insight into coherence and entanglement between the central spin
and the bath. In particular, we find that the spin-spin correlations relax to
their steady value via a nearly logarithmic scaling, whereas the Loschmidt echo
shows an exponential relaxation to its steady value. Our results advance the
understanding of relaxation dynamics and quantum correlations of long-range
interacting models of Gaudin type.",2201.01025v1
2022-11-07,Simulation of the Einstein-de Haas effect combining molecular and spin dynamics,"The spin and lattice dynamics of a ferromagnetic nanoparticle are studied via
molecular dynamics and with semi-classical spin dynamics simulations where spin
and lattice degrees of freedom are coupled via a dynamic uniaxial anisotropy
term. We show that this model conserves total angular momentum, whereas spin
and lattice angular momentum are not conserved. We carry out simulations of the
the Einstein-de Haas effect for a Fe nanocluster with more than 500 atoms that
is free to rotate, using a modified version of the open-source spinlattice
dynamics code (SPILADY). We show that the rate of angular momentum transfer
between spin and lattice is proportional to the strength of the magnetic
anisotropy interaction. The addition of the anisotropy allows full spin-lattice
relaxation to be achieved on previously reported timescales of \sim 100 ps and
for tight-binding magnetic anisotropy energies comparable to those of small Fe
nanoclusters.",2211.03706v1
2024-02-08,Absence of breakdown of ferrodark solitons exhibiting snake instability,"We investigate the dynamical stability and real time dynamics of the
two-types of ferrodark solitons (FDSs) which occur as topological magnetic
domain walls in the easy-plane phase of a quasi-two-dimensional (2D)
ferromagnetic spin-1 Bose-Einstein condensate. The type-I FDS has positive
inertial mass and exhibits a single dynamical instability that generates in
plane spin winding, causing polar-core spin vortex dipoles. The positive
inertial mass leads to the elastic oscillations of the soliton under transverse
perturbations. The type-II FDS has negative inertial mass and exhibits a snake
instability and a spin-twist instability, with the latter involving the
generation of out of plane spin winding. Distinct from the normal dynamics of
negative mass solitons under long wave length transverse perturbations, the
snake instability does not lead to the type-II FDS breaking down. Instead,
segments of the type-II FDS convert to type-I and mass vortex dipoles are
produced. The resulting hybridized-chain of the two soliton types and vortices
exhibits complex 2D soliton dynamics at long times while the vortices remain
confined and the topological structure of a magnetic domain wall is preserved.",2402.05351v1
2010-04-20,"Near-Infrared Imaging Polarimetry of the Serpens Cloud Core: Magnetic Field Structure, Outflows, and Inflows in A Cluster Forming Clump","We made deep NIR imaging polarimetry toward the Serpens cloud core. The
polarization vector maps enable us to newly detect 24 small IR reflection
nebulae with YSOs. Polarization measurements of NIR point sources indicate an
hourglass-shaped magnetic field, of which symmetry axis is nearly perpendicular
to the elongation of the C18O (J=1-0) or submillimeter continuum emission. The
bright part of C18O (J=1-0), submillimeter continuum cores as well as many
class 0/I objects are located just toward the constriction region of the
hourglass-shaped magnetic field. Applying the CF method, the magnetic field
strength was estimated to be ~100 muG, suggesting that the ambient region of
the Serpens cloud core is moderately magnetically supercritical. These suggest
that the Serpens cloud core first contracted along the magnetic field to be an
elongated cloud, which is perpendicular to the magnetic field, and that then
the central part contracted cross the magnetic field due to the high density in
the central region of the cloud core, where star formation is actively
continuing. Comparison of this magnetic field with the previous observations of
molecular gas and large-scale outflows suggests a possibility that the cloud
dynamics is controlled by the magnetic field, protostellar outflows and
gravitational inflows. This appears to be in good agreement with the
outflow-driven turbulence model and implies the importance of the magnetic
field to continuous star formation in the center region of the cluster forming
region.",1004.3409v1
2011-04-20,Magnetic fields in supernova remnants and pulsar-wind nebulae,"We review the observations of supernova remnants (SNRs) and pulsar-wind
nebulae (PWNe) that give information on the strength and orientation of
magnetic fields. Radio polarimetry gives the degree of order of magnetic
fields, and the orientation of the ordered component. Many young shell
supernova remnants show evidence for synchrotron X-ray emission. The spatial
analysis of this emission suggests that magnetic fields are amplified by one to
two orders of magnitude in strong shocks. Detection of several remnants in TeV
gamma rays implies a lower limit on the magnetic-field strength (or a
measurement, if the emission process is inverse-Compton upscattering of cosmic
microwave background photons). Upper limits to GeV emission similarly provide
lower limits on magnetic-field strengths. In the historical shell remnants,
lower limits on B range from 25 to 1000 microGauss. Two remnants show
variability of synchrotron X-ray emission with a timescale of years. If this
timescale is the electron-acceleration or radiative loss timescale, magnetic
fields of order 1 mG are also implied. In pulsar-wind nebulae, equipartition
arguments and dynamical modeling can be used to infer magnetic-field strengths
anywhere from about 5 microGauss to 1 mG. Polarized fractions are considerably
higher than in SNRs, ranging to 50 or 60% in some cases; magnetic-field
geometries often suggest a toroidal structure around the pulsar, but this is
not universal. Viewing-angle effects undoubtedly play a role. MHD models of
radio emission in shell SNRs show that different orientations of upstream
magnetic field, and different assumptions about electron acceleration, predict
different radio morphology. In the remnant of SN 1006, such comparisons imply a
magnetic-field orientation connecting the bright limbs, with a non-negligible
gradient of its strength across the remnant.",1104.4047v2
2011-07-25,Modelling interaction of relativistic and non-relativistic winds in binary system PSR B1259-63/SS2883 - II. Impact of magnetization and anisotropy of the pulsar wind,"In this paper, we present a numerical study of the properties of the flow
produced by the collision of a magnetized anisotropic pulsar wind with its
environment in binary system. We compare the impact of both the magnetic field
and the wind anisotropy to the benchmark case of a purely hydrodynamical (HD)
interaction of isotropic winds, which has been studied in detail by Bogovalov
et al. (2008). We consider the interaction in axisymmetric approximation, i.e.
the pulsar rotation axis is assumed to be oriented along the line between the
pulsar and the optical star and the effects related to the pulsar orbiting are
neglected. The impact of the magnetic field is studied for the case of weak
magnetization (with magnetization parameter $\sigma<0.1$), which is consistent
with conventional models of pulsar winds. The effects related to anisotropy in
pulsar winds are modeled assuming that the kinetic energy flux in a
non-magnetized pulsar wind is strongly anisotropic, with the minimum at the
pulsar rotation axis and the maximum in the perpendicular direction. We show
that, although both considered effects change the shape of the region occupied
by the terminated pulsar wind, their impact appears to be small. In particular,
for the magnetization of the pulsar wind below 0.1, the magnetic field pressure
remains well below the plasma pressure in the post-shock region. Thus, in the
case of interaction of a pulsar with the stellar wind environment (opposite to
the case of plerions, i.e. the pulsar interaction with the interstellar medium,
when the magnetic field becomes dynamically important independently on the wind
magnetization) the HD approach represents a feasible approximation for
numerical modelling.",1107.4831v1
2011-10-31,Linear stability of magnetohydrodynamic flow in a perfectly conducting rectangular duct,"We analyse numerically the linear stability of a liquid metal flow in a
rectangular duct with perfectly electrically conducting walls subject to a
uniform transverse magnetic field. A non-standard three dimensional vector
stream function/vorticity formulation is used with Chebyshev collocation method
to solve the eigenvalue problem for small-amplitude perturbations. A relatively
weak magnetic field is found to render the flow linearly unstable as two weak
jets appear close to the centre of the duct at the Hartmann number Ha \approx
9.6. In a sufficiently strong magnetic field, the instability following the
jets becomes confined in the layers of characteristic thickness \delta \sim
Ha^{-1/2} located at the walls parallel to the magnetic field. In this case the
instability is determined by \delta, which results in both the critical
Reynolds and wavenumbers numbers scaling as \sim \delta^{-1}. Instability modes
can have one of the four different symmetry combinations along and across the
magnetic field. The most unstable is a pair of modes with an even distribution
of vorticity along the magnetic field. These two modes represent strongly
non-uniform vortices aligned with the magnetic field, which rotate either in
the same or opposite senses across the magnetic field. The former enhance while
the latter weaken one another provided that the magnetic field is not too
strong or the walls parallel to the field are not too far apart. In a strong
magnetic field, when the vortices at the opposite walls are well separated by
the core flow, the critical Reynolds and wavenumbers for both of these
instability modes are the same: Re_c \approx 642Ha^{1/2}+8.9x10^3Ha^{-1/2} and
k_c \approx 0.477Ha^{1/2}. The other pair of modes, which differs from the
previous one by an odd distribution of vorticity along the magnetic field, is
more stable with approximately four times higher critical Reynolds number.",1111.0036v2
2011-11-29,Magnetic properties and spin dynamics of 3d-4f molecular complexes,"We present the magnetic properties of three recently synthesized binuclear
molecular complexes [NiNd], [NiGd] and [ZnGd] investigated by dc magnetization
and proton nuclear magnetic resonance (NMR) measurements. The high-temperature
magnetic properties are related to the independent paramagnetic behavior of the
two magnetic metal ions within the binuclear entities both in [NiNd] and
[NiGd]. On lowering the temperature, the formation of a magnetic dimer, with a
low-spin ground state due to antiferromagnetic interaction (J/kB = -25 K)
between Ni2+ and Nd3+, is found in the case of [NiNd], while in [NiGd] a
ferromagnetic interaction (J/kB = 3.31 K) between the magnetic ions leads to a
high-spin (S = 9/2) ground state. The temperature dependence of the proton
nuclear spin lattice relaxation rate 1/T1 in [NiNd] is driven by the
fluctuation of the hyperfine field at the nuclear site due to relaxation of the
magnetization. At high temperature the independent Ni2+ and Nd3+ spins
fluctuate fast while at low temperature we observe a slowing down of the
fluctuation of the total magnetization of the dimer because of the insurgence
of antiferromagnetic spin correlations. The relaxation mechanism in [NiNd] at
low temperature is interpreted by a single, temperature dependent, correlation
frequency wc\simT^3.5, which reflects the life time broadening of the exchange
coupled spins via spin-phonon interaction. The proton NMR signal in [NiGd]
could be detected just at room temperature, due to the shortening of relaxation
times when T is decreased. The magnetic properties of [ZnGd] are the ones
expected from a weakly interacting assembly of isolated moments except for
anomalies in the susceptibility and NMR results below 15 K which currently
cannot be explained.",1111.7008v1
2012-08-03,"Spin dynamics, short-range order and superparamagnetism in superconducting ferromagnet RuSr2Gd1.4Ce0.6Cu2O10-δ","We report structural, detailed DC and linear/non-linear AC, isothermal and
thermoremanent magnetization study of the rutheno-cuprate superconducting
ferromagnet RuSr2Gd1.4Ce0.6Cu2O10-{\delta} (GdRu-1222). Structural analysis, by
employing Rietveld refinement of X-ray diffraction pattern, reveals that
GdRu-1222 crystallizes in tetragonal phase with I4/mmm space group. GdRu-1222
is a reported superconducting ferromagnet with Ru spins magnetic ordering at
around 110 K and superconductivity below 40 K in Cu-O2 planes. Detailed
linear/non-linear first and higher order harmonic of AC susceptibility studies
unveiled the complex magnetism of GdRu-1222. A frequency dependent cusp is
observed in AC susceptibility ({\chi}ac) vs. T measurements. The change in cusp
position with applied frequency followed the well known Vogel-Fulcher law,
which is a feature to describe a spin-glass (SG) system with possibility of
embedded homogeneous/non-homogeneous magnetically interacting/non-interacting
ferromagnetic clusters. Such an interpretation is also supported by
thermoremanent magnetization (TRM) study at T = 60 K. Detailed interpretation
of AC magnetization results revealed the formation of magnetic (ferromagnetic)
homogenous/non-homogenous clusters of different sizes embedded in spin-glass
(SG) matrix. The magnetization vs. applied field loops do not saturate, even at
high applied fields (50 kOe), resulting in the short-range magnetic ordering in
the system, which causes the formation of clusters that freeze at low
temperatures. Temperature variation of first- and third-order susceptibility
harmonics show good agreement with Wohlfarth's model (WM), leading to the
superparamagnetism (SPM) state. Detailed magnetization (DC and AC both) results
and their analysis helped in explaining the temperature dependent magnetism of
the GdRu-1222 system.",1208.0760v2
2013-05-15,The plasmoid instability during asymmetric inflow magnetic reconnection,"Theoretical studies of the plasmoid instability generally assume that the
reconnecting magnetic fields are symmetric. We relax this assumption by
performing two-dimensional resistive magnetohydrodynamic simulations of the
plasmoid instability during asymmetric inflow magnetic reconnection. Magnetic
asymmetry modifies the onset, scaling, and dynamics of this instability.
Magnetic islands develop preferentially into the weak magnetic field upstream
region. Outflow jets from individual X-points impact plasmoids obliquely rather
than directly as in the symmetric case. Consequently, deposition of momentum by
the outflow jets into the plasmoids is less efficient, the plasmoids develop
net vorticity, and shear flow slows down secondary merging between islands.
Secondary merging events have asymmetry along both the inflow and outflow
directions. Downstream plasma is more turbulent in cases with magnetic
asymmetry because islands are able to roll around each other after exiting the
current sheet. As in the symmetric case, plasmoid formation facilitates faster
reconnection for at least small and moderate magnetic asymmetries. However,
when the upstream magnetic field strengths differ by a factor of four, the
reconnection rate plateaus at a lower value than expected from scaling the
symmetric results. We perform a parameter study to investigate the onset of the
plasmoid instability as a function of magnetic asymmetry and domain size. There
exist domain sizes for which symmetric simulations are stable but asymmetric
simulations are unstable, suggesting that moderate magnetic asymmetry is
somewhat destabilizing. We discuss the implications for plasmoid and flux rope
formation in solar eruptions, laboratory reconnection experiments, and space
plasmas. The differences between symmetric and asymmetric simulations provide
some hints regarding the nature of the three-dimensional plasmoid instability.",1305.3646v1
2014-10-23,The 2014 Magnetism Roadmap,"Magnetism is a very fascinating and dynamic field. Especially in the last 30
years it has experienced many major advances in the full range from novel
fundamental phenomena to new products. Applications such as hard disk drives
and magnetic sensors are part of our daily life, and new applications, such as
in non-volatile computer random access memory, are expected to surface shortly.
Thus it is timely for describing the current status, and current and future
challenges in the form of a Roadmap article. This 2014 Magnetism Roadmap
provides a view on several selected, currently very active innovative
developments. It consists of 12 sections, each written by an expert in the
field and addressing a specific subject, with strong emphasize on future
potential. This Roadmap cannot cover the entire field. We have selected several
highly relevant areas without attempting to provide a full review - a future
update will have room for more topics. The scope covers mostly nano-magnetic
phenomena and applications, where surfaces and interfaces provide additional
functionality. New developments in fundamental topics such as interacting
nano-elements, novel magnon-based spintronics concepts, spin-orbit torques and
spin-caloric phenomena are addressed. New materials, such as organic magnetic
materials and permanent magnets are covered. New applications are presented
such as nano-magnetic logic, non-local and domain-wall based devices,
heat-assisted magnetic recording, magnetic random access memory, and
applications in biotechnology. May the Roadmap serve as a guideline for future
emerging research directions in modern magnetism.",1410.6404v1
2014-11-24,B fields in OB stars (BOB): on the detection of weak magnetic fields in the two early B-type stars beta CMa and epsilon CMa,"Within the context of the ""B fields in OB stars (BOB)"" collaboration, we used
the HARPSpol spectropolarimeter to observe the early B-type stars beta CMa
(HD44743; B1 II/III) and epsilon CMa (HD52089; B1.5 II). For both stars, we
consistently detected the signature of a weak (<30 G in absolute value)
longitudinal magnetic field. We determined the physical parameters of both
stars and characterise their X-ray spectrum. For beta CMa, our mode
identification analysis led to determining a rotation period of 13.6+/-1.2 days
and of an inclination angle of the rotation axis of 57.6+/-1.7 degrees, with
respect to the line of sight. On the basis of these measurements and assuming a
dipolar field geometry, we derived a best fitting obliquity of ~22 degrees and
a dipolar magnetic field strength (Bd) of ~100 G (60<Bd<230 G within 1 sigma),
below what is typically found for other magnetic massive stars. For epsilon CMa
we could only determine a lower limit on the dipolar magnetic field strength of
13 G. For this star, we determine that the rotation period ranges between 1.3
and 24 days. Both stars are expected to have a dynamical magnetosphere. We also
conclude that both stars are most likely core hydrogen burning and that they
have spent more than 2/3 of their main sequence lifetime. A histogram of the
distribution of the dipolar magnetic field strength for the magnetic massive
stars known to date does not show the magnetic field ""desert"" observed instead
for intermediate-mass stars. The biases involved in the detection of (weak)
magnetic fields in massive stars with the currently available instrumentation
and techniques imply that weak fields might be more common than currently
observed. Our results show that, if present, even relatively weak magnetic
fields are detectable in massive stars and that more observational effort is
probably still needed to properly access the magnetic field incidence.",1411.6490v1
2016-02-15,Magnetic field and early evolution of circumstellar disks,"The magnetic field plays a central role in the formation and evolution of
circumstellar disks. The magnetic field connects the rapidly rotating central
region with the outer envelope and extracts angular momentum from the central
region during gravitational collapse of the cloud core. This process is known
as magnetic braking. Both analytical and multidimensional simulations have
shown that disk formation is strongly suppressed by magnetic braking in
moderately magnetized cloud cores in the ideal magnetohydrodynamic limit. On
the other hand, recent observations have provided growing evidence of a
relatively large disk several tens of astronomical units in size existing in
some Class 0 young stellar objects. This introduces a serious discrepancy
between the theoretical study and observations. Various physical mechanisms
have been proposed to solve the problem of catastrophic magnetic braking, such
as misalignment between the magnetic field and the rotation axis, turbulence,
and non-ideal effect. In this paper, we review the mechanism of magnetic
braking, its effect on disk formation and early evolution, and the mechanisms
that resolve the magnetic braking problem. In particular, we emphasize the
importance of non-ideal effects. The combination of magnetic diffusion and
thermal evolution during gravitational collapse provides a robust formation
process for the circumstellar disk at the very early phase of protostar
formation. The rotation induced by the Hall effect can supply a sufficient
amount of angular momentum for typical circumstellar disks around T Tauri
stars. By examining the combination of the suggested mechanisms, we conclude
that the circumstellar disks commonly form in the very early phase of protostar
formation.",1602.04538v2
2017-04-25,Measuring surface magnetic fields of red supergiant stars,"RSG stars are very massive cool evolved stars. Recently, a weak magnetic
field was measured at the surface of $\alpha$ Ori and this is so far the only
M-type supergiant for which a direct detection of a surface magnetic field has
been reported. By extending the search for surface magnetic field in a sample
of late-type supergiants, we want to determine whether the surface magnetic
field detected on $\alpha$ Ori is a common feature among the M-type
supergiants. With the spectropolarimeter Narval at TBL we undertook a search
for surface magnetic fields in a sample of cool supergiant stars, and we
analysed circular polarisation spectra using the least-squares deconvolution
technique. We detect weak Zeeman signatures of stellar origin in the targets CE
Tau, $\alpha^1$ Her and $\mu$ Cep. For the latter star, we also show that
cross-talk from the strong linear polarisation signals detected on this star
must be taken into account. For CE Tau and $\mu$ Cep, the longitudinal
component of the detected surface fields is at the Gauss-level, such as in
$\alpha$~Ori. We measured a longitudinal field almost an order of magnitude
stronger for $\alpha^1$ Her. We also report variability of the longitudinal
magnetic field of CE Tau and $\alpha^1$ Her, with changes in good agreement
with the typical atmospheric dynamics time-scales. We also report a
non-detection of magnetic field at the surface of the yellow supergiant star
$\rho$ Cas. The two RSG stars of our sample, CE Tau and $\mu$ Cep, display
magnetic fields very similar to that of $\alpha$ Ori. The non-detection of a
magnetic field on the post-RSG star $\rho$ Cas suggests that the magnetic field
disappears, or at least becomes undetectable with present methods, at later
evolutionary stages. Our analysis of $\alpha^1$ Her supports the proposed
reclassification of the star as an AGB star.",1704.07761v2
2017-08-05,Proton imaging of stochastic magnetic fields,"Recent laser-plasma experiments report the existence of dynamically
significant magnetic fields, whose statistical characterisation is essential
for understanding the physical processes these experiments are attempting to
investigate. In this paper, we show how a proton imaging diagnostic can be used
to determine a range of relevant magnetic field statistics, including the
magnetic-energy spectrum. To achieve this goal, we explore the properties of an
analytic relation between a stochastic magnetic field and the image-flux
distribution created upon imaging that field. We conclude that features of the
beam's final image-flux distribution often display a universal character
determined by a single, field-scale dependent parameter - the contrast
parameter - which quantifies the relative size of the correlation length of the
stochastic field, proton displacements due to magnetic deflections, and the
image magnification. For stochastic magnetic fields, we establish the existence
of four contrast regimes - linear, nonlinear injective, caustic and diffusive -
under which proton-flux images relate to their parent fields in a qualitatively
distinct manner. As a consequence, it is demonstrated that in the linear or
nonlinear injective regimes, the path-integrated magnetic field experienced by
the beam can be extracted uniquely, as can the magnetic-energy spectrum under a
further statistical assumption of isotropy. This is no longer the case in the
caustic or diffusive regimes. We also discuss complications to the
contrast-regime characterisation arising for inhomogeneous, multi-scale
stochastic fields, as well as limitations currently placed by experimental
capabilities on extracting magnetic field statistics. The results presented in
this paper provide a comprehensive description of proton images of stochastic
magnetic fields, with applications for improved analysis of given proton-flux
images.",1708.01738v1
2018-07-26,Is there a left-handed magnetic field in the solar neighborhood? Exploring helical magnetic fields in the interstellar medium through dust polarization power spectra,"The full-sky Planck polarization data at 850um revealed unexpected properties
of the E and B mode power spectra of dust emission in the interstellar medium
(ISM). The positive cross-correlation between the total dust intensity, T, with
the B modes has raised new questions about the physical mechanisms that affect
dust polarization, such as the Galactic magnetic-field structure. This is key
both to better understanding ISM dynamics and to accurately describing Galactic
foregrounds to the polarization of the Cosmic Microwave Background (CMB). In
this theoretical paper we investigate the possibility that the observed
cross-correlations in the dust polarization power spectra, and specifically
between T and B, can be related to a parity-odd quantity in the ISM such as the
magnetic helicity. We produce synthetic dust polarization data, derived from 3D
analytical toy models of density structures and helical magnetic fields, to
compare with the E and B modes of observations. Focusing on the observed T-B
correlation, we propose a new line of interpretation of the Planck observations
based on a large-scale helical component of the Galactic magnetic field in the
solar neighborhood. Our analysis shows that: I) the sign of magnetic helicity
does not affect E and B modes for isotropic magnetic-field configurations; II)
helical magnetic fields threading interstellar filaments cannot reproduce the
Planck results; III) a weak helical left-handed magnetic field structure in the
solar neighborhood may explain the T-B correlation seen in the Planck data.
This work suggests a new perspective for the interpretation of the dust
polarization power spectra, which strongly supports the imprint of a
large-scale structure of the Galactic magnetic field in the solar neighborhood.",1807.10188v3
2018-10-25,"Magnetic Properties of Dust Grains, Effect of Precession and Radiative Torque Alignment","Alignment of dust grains in astrophysical environments results in the
polarization of starlight as well as the polarization of radiation emitted by
dust. We demonstrate the advances in grain alignment theory allow the use of
linear and circular polarization to probe not only the magnetic field, but also
dust composition, the dust environment, etc. We revisit the process of grain
alignment by Radiative Torques (RATs) and focus on constraining magnetic
susceptibility of grains via observations. We discuss the possibility of
observational testing of the magnetic properties of grains as the alignment
changes from being in respect to the magnetic field to being in respect to the
radiation direction. This opens both a possibility of constraining the
uncertain parameters of the RATs theory and provides a new way of measuring
magnetic fields in interstellar medium and circumstellar regions. We provide a
detailed discussion of the precession induced both by the magnetic field and
the anisotropic radiation and revisit a number of key processes related to
magnetic response of the grains. We consider various effects that increase the
rate of magnetic relaxation both in silicate and carbonaceous grains. In
particular, we find a new relaxation process related to the change of the
amplitude of internal magnetization within a wobbling triaxial grain and
identify a range of grain sizes in which this effect can dominate the internal
alignment of angular momentum within grain axes. We show that these relaxation
processes significantly change the dynamics of grains in the presence of RATs.
We apply our analysis for observed grain alignment in special environments to
put constraints on the enhanced magnetic properties of dust grains in the cloud
near supernovae, in cometary coma, and protoplanetary disks.",1810.10686v2
2019-01-14,Magneto-structural and dynamic susceptibility studies on rare-earth rich intermetallic compound,"Complex magneto structural behaviour of rare rich intermetallic Tb$_3$Co is
reported in this study. Below the 84K (T$_N$), it undergoes a first order
magnetic transition 72K confirmed from specific heat and magnetization
measurements. Detailed study using magnetization, specific heat, neuron
diffraction and ac-susceptibility measurements suggests that the compound in
the question possesses canted magnetic structure. Neutron diffraction reveals
that Tb moments are aligned antiferromagnetically (AFM) in ab plane and
ferromagnetically (FM) along c direction. This study focusses on the
temperature evolution of magnetic order at low temperatures. Neutron
Diffraction clearly shows that magnetic structure remains more or less same up
to 70K although low field magnetization exhibits a transition like feature
around 30K. Field dependent neutron diffraction suggests that a strong
spin-lattice coupling is present at lower temperature region (below 20K)
compared to that for T above 40K. This is responsible for the observed drop in
ZFC magnetization around 30K and is not due to any change in magnetic
structure. Another important but surprising result that observed is the
signature of magnetic glass below 72K in frequency dependent ac susceptibility
measurements. Real part of ac-susceptibility data shows dispersion with
frequency which on detailed analysis provides evidence for spin glass behaviour
in the compound riding on top of non-collinear AFM order. Higher order
harmonics in ac non-linear) susceptibilities, which are expected to show a
well-defined behaviour for pure FM or AFM or spin glass or cluster glass
systems, were also measured. However, the non-linear susceptibilities do not
exhibit any well-known variation but show complex behaviour thus indicating
Tb$_3$Co is neither a pure AFM or FM or spin glass system but provides another
evidence for non-collinear magnetic structure.",1901.04191v1
2019-08-25,"Electric and magnetic axion quark nuggets, their stability and their detection","The present work studies the dynamics of axion quark nuggets introduced in
\cite{zhitnitsky} and exploited in the works
\cite{zhitnitsky2}-\cite{zhitnitsky13}. The new feature considered here is the
possibility that these nuggets become ferromagnetic. This possibility was
pointed out in \cite{tatsumi}, although ferromagnetism may also take place due
some anomaly terms found in \cite{son}-\cite{son2}. The purpose of the present
letter however, is not to give evidence in favor or against these statements.
Instead, it is focused in some direct consequences of this ferromagnetic
behavior, if it exists. The first is that the nugget magnetic field induces an
electric field due to the axion wall, which may induce pair production by
Schwinger effect. Depending on the value of the magnetic field, the pair
production can be quite large. A critical value for such magnetic field at the
surface of the nugget is obtained, and it is argued that the value of the
magnetic field of \cite{tatsumi} is at the verge of stability and may induce
large pair production. The consequences of this enhanced pair production may be
unclear. It may indicate that the the nugget evaporates, but on the other hand
it may be just an indication that the intrinsic magnetic field disappears and
the nuggets evolves to a non magnetized state such as
\cite{zhitnitsky}-\cite{zhitnitsky13}. The interaction of such magnetic and
electric nugget with the troposphere of the earth is also analyzed. However, if
the magnetic field does not decay before the actual universe, then this would
lead to high energy electron flux due to its interaction with the electron
gases of the Milky Way. This suggests that these magnetized quarks may be a
considerably part of dark matter, but only if their hypothetical magnetic and
electric fields are evaporated.",1908.09409v5
2019-11-05,The nature of mean-field generation in three classes of optimal dynamos,"In recent years, several optimal dynamos have been discovered. They minimize
the magnetic energy dissipation or, equivalently, maximize the growth rate at a
fixed magnetic Reynolds number. In the optimal dynamo of Willis (2012, Phys.
Rev. Lett. 109, 251101), we find mean-field dynamo action for planar averages.
One component of the magnetic field grows exponentially while the other decays
in an oscillatory fashion near onset. This behavior is different from that of
an alpha^2 dynamo, where the two non-vanishing components of the planar
averages are coupled and have the same growth rate. For the Willis dynamo, we
find that the mean field is excited by a negative turbulent magnetic
diffusivity, which has a non-uniform spatial profile near onset. The temporal
oscillations in the decaying component are caused by the corresponding
component of the diffusivity tensor being complex when the mean field is
decaying and, in this way, time-dependent. The growing mean field can be
modeled by a negative magnetic diffusivity combined with a positive magnetic
hyperdiffusivity. In two other classes of optimal dynamos of Chen et al. (2015,
J. Fluid Mech. 783, 23), we find, to some extent, similar mean-field dynamo
actions. When the magnetic boundary conditions are mixed, the two components of
the planar averaged field grow at different rates when the dynamo is 15%
supercritical. When the mean magnetic field satisfies homogeneous boundary
conditions (where the magnetic field is tangential to the boundary), mean-field
dynamo action is found for one-dimensional averages, but not for planar
averages. Despite having different spatial profiles, both dynamos show negative
turbulent magnetic diffusivities. Our finding suggests negative turbulent
magnetic diffusivities may support a broader class of dynamos than previously
thought, including these three optimal dynamos.",1911.01712v2
2020-01-03,"3D Turbulent Reconnection: Theory, Tests and Astrophysical Implications","Magnetic reconnection, topological change in magnetic fields, is a
fundamental process in magnetized plasmas. It is associated with energy release
in regions of magnetic field annihilation, but this is only one facet of this
process. Astrophysical flows normally have very large Reynolds numbers and are
expected to be turbulent, in agreement with observations. In strong turbulence
magnetic lines constantly reconnect everywhere at all scales, making magnetic
reconnection an intrinsic part of turbulent cascade. We note that this is
inconsistent with the usual practice of regarding magnetic lines as persistent
dynamical elements. A number of theoretical, numerical, and observational
studies, starting with Lazarian & Vishniac (1999), demonstrated that 3D
turbulence makes magnetic reconnection fast and that these two processes are
intrinsically connected. We discuss the dramatic violation of the textbook
concept of magnetic flux-freezing in the presence of turbulence and demonstrate
that in the presence of turbulence the plasma effects are subdominant to
turbulence as far as the magnetic reconnection is concerned. This justifies an
MHD-like treatment of magnetic reconnection at scales much larger than the
relevant plasma scales. We discuss numerical and observational evidences
supporting the turbulent reconnection model. In particular, we show that
tearing reconnection is suppressed in 3D and, unlike the 2D case, the 3D
reconnection induces turbulence that makes reconnection independent of
resistivity. We show that turbulent reconnection dramatically affects the key
astrophysical processes, e.g., star formation, turbulent dynamo, acceleration
of cosmic rays. We provide criticism of the concept of ""reconnection-mediated
turbulence"" and explain why turbulent reconnection is very different from
enhanced turbulent resistivity and hyper-resistivity, and why the latter has
fatal conceptual flaws.",2001.00868v1
2020-01-29,Helicity proxies from linear polarisation of solar active regions,"The alpha effect is believed to play a key role in the generation of the
solar magnetic field. A fundamental test for its significance in the solar
dynamo is to look for magnetic helicity of opposite signs in the two
hemispheres, and at small and large scales. However, measuring magnetic
helicity is compromised by the inability to fully infer the magnetic field
vector from observations of solar spectra, caused by what is known as the ""pi
ambiguity"" of spectropolarimetric observations. We decompose linear
polarisation into parity-even and parity-odd E and B polarisations, which are
not affected by the ""pi ambiguity"". Furthermore, we study whether the
correlations of spatial Fourier spectra of B and parity-even quantities such as
E or temperature T are a robust proxy for magnetic helicity of solar magnetic
fields. We analyse polarisation measurements of active regions observed by the
Helioseismic and Magnetic Imager on board the Solar Dynamics observatory.
Theory predicts the magnetic helicity of active regions to have, statistically,
opposite signs in the two hemispheres. We then compute the parity-odd E B and T
B correlations, and test for systematic preference of their sign based on the
hemisphere of the active regions. We find that: (i) E B and T B correlations
are a reliable proxy for magnetic helicity, when computed from linear
polarisation measurements away from spectral line cores, and (ii) E
polarisation reverses its sign close to the line core. Our analysis reveals
Faraday rotation to not have a significant influence on the computed parity-odd
correlations. The EB decomposition of linear polarisation appears to be a good
proxy for magnetic helicity independent of the ""pi ambiguity"". This allows us
to routinely infer magnetic helicity directly from polarisation measurements.",2001.10884v2
2020-12-07,Predicting the Evolution of Photospheric Magnetic Field in Solar Active Regions Using Deep Learning,"The continuous observation of the magnetic field by Solar Dynamics
Observatory (SDO)/Helioseismic and Magnetic Imager (HMI) produces numerous
image sequences in time and space. These sequences provide data support for
predicting the evolution of photospheric magnetic field. Based on the
spatiotemporal long short-term memory(LSTM) network, we use the preprocessed
data of photospheric magnetic field in active regions to build a prediction
model for magnetic field evolution. Because of the elaborate learning and
memory mechanism, the trained model can characterize the inherent relationships
contained in spatiotemporal features. The testing results of the prediction
model indicate that (1) the prediction pattern learned by the model can be
applied to predict the evolution of new magnetic field in the next 6 hour that
have not been trained, and predicted results are roughly consistent with real
observed magnetic field evolution in terms of large-scale structure and
movement speed; (2) the performance of the model is related to the prediction
time; the shorter the prediction time, the higher the accuracy of the predicted
results; (3) the performance of the model is stable not only for active regions
in the north and south but also for data in positive and negative regions.
Detailed experimental results and discussions on magnetic flux emergence and
magnetic neutral lines finally show that the proposed model could effectively
predict the large-scale and short-term evolution of the photospheric magnetic
field in active regions. Moreover, our study may provide a reference for the
spatiotemporal prediction of other solar activities.",2012.03584v1
2021-01-15,Direct evidence for magnetic reconnection at the boundaries of magnetic switchbacks with Parker Solar Probe,"Parker Solar Probe's first encounters with the Sun revealed the presence of
ubiquitous localised magnetic deflections in the inner heliosphere; these
structures, often called switchbacks, are particularly striking in solar wind
streams originating from coronal holes. We report the direct evidence for
magnetic reconnection occuring at the boundaries of three switchbacks crossed
by Parker Solar Probe (PSP) at a distance of 45 to 48 solar radii of the Sun
during its first encounter. We analyse the magnetic field and plasma parameters
from the FIELDS and SWEAP instruments. The three structures analysed all show
typical signatures of magnetic reconnection. The ion velocity and magnetic
field are first correlated and then anti-correlated at the inbound and outbound
edges of the bifurcated current sheets with a central ion flow jet. Most of the
reconnection events have a strong guide field and moderate magnetic shear but
one current sheet shows indications of quasi anti-parallel reconnection in
conjunction with a magnetic field magnitude decrease by $90\%$. Given the
wealth of intense current sheets observed by PSP, reconnection at switchbacks
boundaries appears to be rare. However, as the switchback boundaries accomodate
currents one can conjecture that the geometry of these boundaries offers
favourable conditions for magnetic reconnection to occur. Such a mechanism
would thus contribute in reconfiguring the magnetic field of the switchbacks,
affecting the dynamics of the solar wind and eventually contributing to the
blending of the structures with the regular wind as they propagate away from
the Sun.",2101.06279v2
2021-01-28,Phase diagram for strongly interacting matter in the presence of a magnetic field using the Polyakov-Nambu-Jona-Lasinio model with magnetic field dependent coupling strengths,"We study the phase diagram for strongly interacting matter using the 't Hooft
determinant extended Nambu--Jona-Lasinio model with a Polyakov loop in the
light and strange quark sectors (\emph{up}, \emph{down} and \emph{strange})
focusing on the effect of a magnetic field dependence of the coupling strengths
of these interactions. This dependence was obtained so as to reproduce recent
lattice QCD results for the magnetic field dependence of the quarks dynamical
masses.
  A finite magnetic field is known to induce several additional first-order
phase transition lines with the respective Critical End Points (CEP) in the
temperature-quark chemical potential phase diagram when compared to the zero
magnetic field case. A study of the magnetic field dependence in the range
$eB=0-0.6~\mathrm{GeV}^2$ of the location of these CEPs reveals that the
initial one as well as several of the new ones only survive up to a critical
magnetic field. Only two remain in the upper limit of the studied magnetic
field strength. A comparison of the results obtained with versions of the model
with and without Polyakov loop is also done.
  We also found that the inclusion of the magnetic field dependence on the
coupling strengths, while not changing the qualitative features of the phase
diagram, affects the location of these CEPs. The comparison of results with and
without a regularization cutoff in the medium part of the integrals does not
show a significant change.",2101.12004v2
2024-02-26,Black hole in a combined magnetic field: ionized accretion disks in the jetlike and looplike configurations,"Magnetic fields surrounding black holes are responsible for various
astrophysical phenomena related to accretion processes and relativistic jets.
Depending on the source, the configuration of the field lines may differ
significantly, affecting the trajectories of charged particles and the
corresponding observables. Usually, the magnetic fields around black holes are
modeled within a single source or current generating the field. However,
magnetic fields can have more than a single origin, being a combination of
different fields, such as, e.g., that of an accretion disk and external
large-scale or Galactic ones. In this paper, we propose a combined magnetic
field solution given by the superposition of the uniform and Blandford-Znajek
split-monopole magnetic fields in a strong gravity regime of the Schwarzschild
black hole. We show that when the combined magnetic field components are
aligned, the resulting field is of a paraboloidal jetlike shape. Such a
configuration is supported by relativistic jet observations and is often
utilized in general relativistic magnetohydrodynamical simulations. In the
opposite orientation of the two field components, we observe looplike field
structures magnetically connecting the black hole with an accretion disk and
the magnetic null points, which can be related to the regions of magnetic
reconnection. In the combined magnetic field configurations, we analyze the
dynamics of charged particles, study their stability conditions, and find the
locations of stable off-equatorial structures close to the symmetry axis. We
consider an ionization of Keplerian accretion disk as a particular scenario of
particle scattering. From the numerical experiments, we conclude that charged
particles in the jetlike combination show a strong tendency to escape from the
black hole. In contrast, the looplike combination supports accretion of charged
particles into the black hole.",2402.16529v1
2000-12-01,Two-dimensional disk dynamos with vertical outflows into a halo,"We study the effects of vertical outflows on mean-field dynamos in disks.
These outflows could be due to thermal winds or magnetic buoyancy. We analyse
numerical solutions of the nonlinear mean-field dynamo equations using a
two-dimensional finite-difference model. Contrary to expectations, a modest
vertical velocity can enhance dynamo action. This can lead to super-exponential
growth of the magnetic field and to higher magnetic energies at saturation in
the nonlinear regime.",0012013v1
2000-12-05,The inverse cascade in turbulent dynamos,"The emergence of a large scale magnetic field from randomly forced isotropic
strongly helical flows is discussed in terms of the inverse cascade of magnetic
helicity and the alpha-effect. In simulations of such flows the maximum field
strength exceeds the equipartition field strength for large scale separation.
However, helicity conservation controls the speed at which this final state is
reached. In the presence of open boundaries magnetic helicity fluxes out of the
domain are possible. This reduces the timescales of the field growth, but it
also tends to reduce the maximum attainable field strength.",0012112v1
2001-05-21,A Solvable Model for Nonlinear Mean Field Dynamo,"We formulate a solvable model that describes generation and saturation of
mean magnetic field in a dynamo with kinetic helicity, in the limit of large
magnetic Prandtl number. This model is based on the assumption that the
stochastic part of the velocity field is Gaussian and white in time (the
Kazantsev-Kraichnan ensemble), while the regular part describing the back
reaction of the magnetic field is chosen from balancing the viscous and Lorentz
stresses in the MHD Navier-Stokes equation. The model provides an analytical
explanation for previously obtained numerical results.",0105354v1
2001-12-15,Grain Dynamics In Magnetized Interstellar Gas,"Interstellar medium is turbulent and this induces relative motions of dust
grains. We calculate relative velocities of charged grains in a partially
ionized magnetized gas. We account for anisotropy of magnetohydrodynamic (MHD)
turbulence, grain coupling with magnetic field, and the turbulence cutoff
arising from the ambipolar drag. We obtain grain velocities for turbulence with
parameters consistent with those in HI and dark clouds. Those velocities are
smaller than those in earlier papers, where MHD effects were disregarded.
Finally, we compare grain velocities arising from photoelectric emission,
radiation pressure and thrusts from molecular hydrogen formation. We conclude
that turbulence should prevent segregation of grains of different sizes.",0112365v2
2003-12-09,Early Disk Evolution,"A variety of processes play a role in the evolution of protostellar disks.
Here I focus on the uncertain issue of magnetic field-disk coupling and its
implications for magnetically-driven turbulence and disk-driven winds. At
present it is clear that the magnetic field plays a crucial role in disk
evolution, but detailed conclusions cannot be drawn because the complicated
interplay between dynamics and the evolution of the grain population remains to
be explored.",0312220v1
2004-03-14,Nearly Minimal Magnetogenesis,"We propose a new mechanism for magnetic field generation from inflation, by
which strong magnetic fields can be generated on cosmological scales. These
fields may be observable by cosmic microwave background radition measurements,
and may have a dynamical impact on structure formation. The mechanism is based
on the observation that a light nearly minimally coupled charged scalar may be
responsible for the creation of a negative photon mass-squared (provided the
scalar field coupling to the curvature scalar is small but negative), which in
turn results in abundant photon production - and thus in growing magnetic
fields - during inflation.",0403335v1
2004-09-06,Chiral Symmetry Breaking in Presence of Strong Quantizing Magnetic Fields- A Nambu-Jona-Lasino Model with Semi-Classical Approximation,"The breaking of chiral symmetry of light quarks at zero temperature in
presence of strong quantizing magnetic field is studied using
Nambu-Jona-Lasinio (NJL) model with Thomas-Fermi type semi-classical formalism.
It is found that the dynamically generated light quark mass can never become
zero if the Landau levels are populated and increases with the increase of
magnetic field strength.",0409109v2
2005-03-23,"Comment on ""Gamma-ray burst early afterglows: reverse shock emission from an arbitrarily magnetized ejecta"" by Zhang and Kobayashi (2004)","Zhang and Kobayashi (2004) attempted to calculate early afterglow emission
from a system of forward and reverse shocks in GRB outflows for the case of
magnetized ejecta. We point out a fundamental error in the underlying dynamical
model. According to the authors, energy and momentum carried by the magnetic
field of the ejecta are not transfered to the forward shock. This is an
incorrect assumption that invalidates the results.",0503505v1
2006-08-02,"Grain Alignment, Polarization and Magnetic Fields","Aligned non-spherical dust particles polarize starlight passing through the
dust cloud. They also emit polarized far infrared and sub-mm radiation.
Substantial progress in understanding of grain alignment theory makes the
interpretation of the polarized radiation in terms of underlying magnetic
fields much more reliable. I discuss a number of fundamental processes that
affect grain alignment. In particular, I shall discuss how subtle effects
related to nuclear spins of the atoms alter the dynamics of dust grains. I
shall discuss how the theory explains the existing observational data and
demonstrate when the polarization can and cannot be interpreted in terms of the
underlying magnetic fields.",0608049v1
2007-02-08,Sunspot models with bright rings,"A theoretical sunspot model is provided including magnetic suppression of the
diffusivities and also a strong stratification of density and temperature. Heat
diffusion alone with given magnetic field and zero mean flow only produces
(after a very long relaxation time) dark spots without any bright ring. Models
with full dynamics of both field and flow, however, provide rings and also the
observed correlation of ring temperature excess and the spot size. The rings
are formed as the result of heat transport by the resulting flow system and
increased thermal diffusivity due to reduced magnetic quenching around spots.",0702232v2
1992-09-05,Aspects of Spin Dynamics in the Cuprate Superconductors,"One of the interesting aspects of the CuO superconductors is that
superconductivity is happening so close to the antiferromagnetic state. The
nuclear magnetic resonance and the recent neutron scattering experiments
clearly indicate that magnetic correlations persist in to the heavily doped
regime. In this paper we will discuss some of the details of the coupling of
the nuclear magnetic spin to the conduction electron spins. Furthermore we will
show that a simple band structure can explain the recent neutron scattering
data in the \LaSrCuO material for the optimal concentration of $x\approx 0.15$
if the lifetime effects are included.",9209005v1
1996-10-07,Self similar Barkhausen noise in magnetic domain wall motion,"A model for domain wall motion in ferromagnets is analyzed. Long-range
magnetic dipolar interactions are shown to give rise to self-similar dynamics
when the external magnetic field is increased adiabatically. The power spectrum
of the resultant Barkhausen noise is of the form $1/\omega^\alpha$, where
$\alpha\approx 1.5$ can be estimated from the critical exponents for interface
depinning in random media.",9610053v2
1996-11-13,Dramatic Switching of Magnetic Exchange in a Classic Transition Metal Oxide: Evidence for Orbital Ordering,"Spin correlations in metallic and insulating phases of $V_2O_3$ and its
derivatives are investigated using magnetic neutron scattering. Metallic
samples have incommensurate spin correlations varying little with hole doping.
Paramagnetic insulating samples have spin correlations only among near
neighbors. The transition from either of these phases into the low temperature
insulating antiferromagnetic phase is accompanied by an abrupt change of
dynamic magnetic short range order. Our results support the idea that the
transition into the antiferromagnetic insulator is also an orbital ordering
transition.",9611109v1
1997-07-10,Incommensurate Magnetic Fluctuations in YBa2Cu3O6.6,"We use inelastic neutron scattering to demonstrate that at low temperatures,
the low frequency magnetic fluctuations in YBa_2Cu_3O_{6.6} ($T_c=62.7$ K) are
incommensurate, being found at positions displaced by $\pm\delta$ ($0.057\pm
0.006$ r.l.u.) along the $[\pi,\pi]$ direction from the wave vector $(\pi,\pi)$
associated with the antiferromagnetic order of the parent insulator,
YBa_2Cu_3O_{6}. The dynamical susceptibility $\chi''(q,\omega)$ at the
incommensurate positions increases on cooling below $T_c$, accompanied by a
suppression of magnetic fluctuations at the commensurate points.",9707112v1
1997-10-01,Semiclassical Approach to Orbital Magnetism of Interacting Diffusive Quantum Systems,"We study interaction effects on the orbital magnetism of diffusive mesoscopic
quantum systems. By combining many-body perturbation theory with semiclassical
techniques, we show that the interaction contribution to the ensemble averaged
quantum thermodynamic potential can be reduced to an essentially classical
operator. We compute the magnetic response of disordered rings and dots for
diffusive classical dynamics. Our semiclassical approach reproduces the results
of previous diagrammatic quantum calculations.",9710014v1
1997-11-13,On the Dynamics of the 4d Spin Glass in a magnetic field,"We study the four dimensional Gaussian spin glass in presence of a magnetic
field. Using off-equilibrium numerical simulations we have found that the
probability distribution of the overlaps is built in the same way as that of
the Mean Field approximation with replica symmetry breaking. Finally we have
studied the violation of the fluctuation-dissipation theorem in presence of
magnetic field.",9711122v1
1998-04-22,A magnetic model for the incommensurate I phase of spin-Peierls systems,"A magnetic model is proposed for describing the incommensurate I phase of
spin-Peierls systems. Based on the harmonicity of the lattice distortion, its
main ingredient is that the distortion of the lattice adjusts to the average
magnetization such that the system is always gapful. The presence of dynamical
incommensurabilities in the fluctuation spectra is also predicted. Recent
experimental results for CuGeO_3 obtained by NMR, ESR and light scattering
absorption are well understood within this model.",9804242v1
1998-04-23,Response in kinetic Ising model to oscillating magnetic fields,"Ising models obeying Glauber dynamics in a temporally oscillating magnetic
field are analyzed. In the context of stochastic resonance, the response in the
magnetization is calculated by means of both a mean-field theory with
linear-response approximation, and the time-dependent Ginzburg-Landau equation.
Analytic results for the temperature and frequency dependent response,
including the resonance temperature, compare favorably with simulation data.",9804251v1
1998-07-14,Non-adiabatic scattering of a classical particle in an inhomogeneous magnetic field,"We study the violation of the adiabaticity of the electron dynamics in a
slowly varying magnetic field. We formulate and solve exactly a non-adiabatic
scattering problem. In particular, we consider scattering on a magnetic field
inhomogeneity which models scatterers in the composite-fermion theory of the
half-filled Landau level. The calculated non-adiabatic shift of the guiding
center is exponentially small and exhibits an oscillatory behavior related to
the ""self-commensurability"" of the drifting cyclotron orbit. The analytical
results are complemented with a numerical simulation.",9807213v1
1998-12-28,The Kronig-Penney-Ising picture of the colossal magnetoresistance,"From general arguments, it is shown that a magnetic Kronig-Penney model based
on the thermodynamics of an Ising model can be used for describing the Colossal
Magnetoresistance (CMR) phenomenon. The model considers a tunneling-like
transmission process of hopping electrons through a dynamic lattice
characterized by evolving magnetic clusters. In this model, correlations
between the magnetic states are considered to be more relevant than the lattice
strain effects for obtaining the CMR features. Physical arguments lead to the
theoretical description of the intrinsic temperature and field dependences of
the CMR observed in typical manganite materials.",9812406v1
1999-01-26,Antiferromagnetism in doped anisotropic two-dimensional spin-Peierls systems,"We study the formation of antiferromagnetic correlations induced by impurity
doping in anisotropic two-dimensional spin-Peierls systems. Using a mean-field
approximation to deal with the inter-chain magnetic coupling, the intra-chain
correlations are treated exactly by numerical techniques. The magnetic coupling
between impurities is computed for both adiabatic and dynamical lattices and is
shown to have an alternating sign as a function of the impurity-impurity
distance, hence suppressing magnetic frustration. An effective model based on
our numerical results supports the coexistence of antiferromagnetism and
dimerization in this system.",9901286v2
2000-01-10,Irreversibility temperature from magnetic relaxation,"Simulation of the magnetic relaxation in a model of hard superconductor
reveals a new time scale below the irreversibility temperature. The relaxation
in this new time scale, which appears in an intermediate time window, is a
power law and is related to self organization of the magnetic flux around the
critical current density J_c during the relaxation. Emergence of a new time
scale which along with the time scale for long time relaxation due to thermally
activated process dynamically identifies the irreversibility temperature.",0001096v1
2000-02-14,Magnetization Switching in Nanowires: Monte Carlo Study with Fast Fourier Transformation for Dipolar Fields,"For the investigations of thermally activated magnetization reversal in
systems of classical magnetic moments numerical methods are desirable. We
present numerical studies which base on time quantified Monte Carlo methods
where the long-range dipole-dipole interaction is calculated with the aid of
fast Fourier transformation. As an example, we study models for ferromagnetic
nanowires comparing our numerical results for the characteristic time of the
reversal process also with numerical data from Langevin dynamics simulations
where the fast Fourier transformation method is well established. Depending on
the system geometry different reversal mechanism occur like coherent rotation,
nucleation, and curling.",0002201v1
2000-04-17,Formation of Directed Beams from Atom Lasers,"The problem of creating well-collimated beams of atoms escaping from a trap
is studied. This problem is of high importance for the realization of atom
lasers. Nonadiabatic dynamics of neutral atoms in nonuniform magnetic fields,
typical of quadrupole magnetic traps, has been considered. The main result of
this report is that an unusual semiconfining regime of motion is found, when
atoms are confined from one side of an axis but are not confined from another
side. This regime can be achieved by means of only magnetic fields. The formed
atomic beam can be forwarded in arbitrary direction.",0004267v1
2001-05-31,Quantum transport in parallel magnetic fields: A realization of the Berry-Robnik symmetry phenomenon,"We analyze the magnetoconductance of two-dimensional electron and hole gases
(2DEGs) subject to a parallel magnetic field. It is shown that, for confining
potential wells which are symmetric with respect to spatial inversion, a
temperature-dependent weak localization signal exists even in the presence of a
magnetic field. Deviations from this symmetry lead to magnetoconductance
profiles that contain information on both, the geometry of the confining
potential and characteristics of the disorder.",0105623v1
2001-06-08,Droplets in Disordered Metallic Quantum Critical Systems,"We present a field theory for a structurally disordered magnetic system
coupled to a metallic environment near a quantum critical point. We show that
close to the magnetic quantum critical point droplets are formed due to the
disorder and undergo dissipative quantum dynamics. We show that the problem has
a characteristic energy scale, the droplet Kondo temperature, that determines
the crossover energy scale from weak to strong coupling. Our results have
direct significance for the Griffiths-McCoy singularities of itinerant magnets.",0106176v1
2001-06-14,Magnetic field-induced insulating behavior in highly oriented pyrolitic graphite,"We propose an explanation for the apparent semimetal-insulator transition
observed in highly oriented pyrolitic graphite in the presence of magnetic
field perpendicular to the layers. We show that the magnetic field opens an
excitonic gap in the linear spectrum of the Coulomb interacting quasiparticles,
in a close analogy with the phenomenon of dynamical chiral symmetry breaking in
the relativistic theories of the 2+1-dimensional Dirac fermions. Our
strong-coupling appoach allows for a non-perturbative description of the
corresponding critical behavior.",0106261v3
2001-06-19,Observation of Convergent Oscillations of the Flux Line Lattice as a Result of Magnetic Flux Jumping in Hard Superconductors,"We have monitored new peculiarities of the dynamics of catastrophic
avalanches of the magnetic flux in superconducting Nb, Nb-Ti, and YBaCuO
samples: i) convergent oscillations of the magnetic flux; ii) a threshold for
entering the huge flux avalanches in the shielding experiments; iii) a
threshold for the exit of a residual flux in the trapping experiments. The
observed phenomena are interpreted in terms of a theoretical model which takes
into account the inertial properties of the vortex matter.",0106379v2
2001-06-26,Fragility of the spin-glass-like collective state to a magnetic field in an interacting Fe-C nanoparticle system,"The effect of applied magnetic fields on the collective nonequilibrium
dynamics of a strongly interacting Fe-C nanoparticle system has been
investigated. It is experimentally shown that the magnetic aging diminishes to
finally disappear for fields of moderate strength. The field needed to remove
the observable aging behavior increases with decreasing temperature. The same
qualitative behavior is observed in an amorphous metallic spin glass
(Fe_{0.15}Ni_{0.85})_{75}P_{16}B_6Al_3.",0106530v1
2002-03-18,Relaxation of Shallow Donor Electron Spin due to Interaction with Nuclear Spin Bath,"We study the low-temperature dynamics of a shallow donor, e.g., $^{31}$P,
impurity electron spin in silicon, interacting with the bath of nuclear spins
of the $^{29}$Si isotope. For small applied magnetic fields, the electron spin
relaxation is controlled by the steady state distribution of the nuclear spins.
We calculate the relaxation times $T_1$ and $T_2$ as functions of the external
magnetic field, and conclude that nuclear spins play an important role in the
donor electron spin decoherence in Si:P at low magnetic fields.",0203362v1
2002-06-19,Magnetic-field induced triplet superconductivity in the Hubbard model on a triangular lattice,"We propose theoretically that a magnetic field can realize spin-triplet
superconductivity in repulsively interacting electron systems having strong
ferromagnetic spin fluctuations. We confirm the general idea for the
low-density Hubbard model on a triangular lattice, whose Fermi surface consists
of disconnected pieces, by calculating the pairing susceptibility in a moderate
magnetic field with the quantum Monte-Carlo method combined with the dynamical
cluster approximation.",0206358v1
2002-08-25,Spin dynamics of a tetrahedral cluster magnet,"We study the magnetism of a lattice of coupled tetrahedral spin-1/2 clusters
which might be of relevance to the tellurate compounds Cu2Te2O5X2, with X=Cl,
Br. Using the flow equation method we perform a series expansion in terms of
the inter-tetrahedral exchange couplings starting from the quadrumer limit.
Results will be given for the magnetic instabilities of the quadrumer phase and
the dispersion of elementary triplet excitations. In limiting cases of our
model of one- or two dimensional character we show our results to be consistent
with findings on previously investigated decoupled tetrahedral chains and the
Heisenberg model on the 1/5-depleted square lattice.",0208472v1
2002-10-16,Quantum dynamics of crystals of molecular nanomagnets inside a resonant cavity,"It is shown that crystals of molecular nanomagnets exhibit enhanced magnetic
relaxation when placed inside a resonant cavity. Strong dependence of the
magnetization curve on the geometry of the cavity has been observed, providing
evidence of the coherent microwave radiation by the crystals. A similar
dependence has been found for a crystal placed between Fabry-Perot
superconducting mirrors. These observations open the possibility of building a
nanomagnetic microwave laser pumped by the magnetic field.",0210340v1
2002-10-17,Magnetization reversal times in the 2D Ising model,"We present a theoretical framework which is generally applicable to the study
of time scales of activated processes in systems with Brownian type dynamics.
This framework is applied to a prototype system: magnetization reversal times
in the 2D Ising model. Direct simulation results for the magnetization reversal
times, spanning more than five orders of magnitude, are compared with
theoretical predictions; the two agree in most cases within 20%.",0210370v1
2002-11-12,Putting hydrodynamic interactions to work: tagged particle separation,"Separation of magnetically tagged cells is performed by attaching markers to
a subset of cells in suspension and applying fields to pull from them in a
variety of ways. The magnetic force is proportional to the field gradient, and
the hydrodynamic interactions play only a passive, adverse role. Here we
propose using a homogeneous rotating magnetic field only to make tagged
particles rotate, and then performing the actual separation by means of
hydrodynamic interactions, which thus play an active role. The method, which we
explore here theoretically and by means of numerical simulations, lends itself
naturally to sorting on large scales.",0211243v2
2002-12-03,Thermal stability of coupled ferromagnetic and superparamagnetic particles,"We consider a single-domain ferromagnetic particle with uniaxial anisotropy
coupled to a single-domain soft ferromagnetic particle (superparamagnetic
particle). The problem of thermally agitated magnetization reversal in this
case can be reduced to the random magnetization dynamics of the first particle
with an effectively larger anisotropy field. The magnetic external field is
also altered in a manner that depends on the sign of the coupling and can be
either enhanced or suppressed.",0212041v1
2003-03-04,Field-tuned quantum tunneling in a supramolecule dimer $[Mn_4]_2$,"Field-tuned quantum tunneling in two single-molecule magnets coupled
antiferromagnetically and formed a supramolecule dimer is studied. We obtain
step-like magnetization curves by means of the numerically exact solution of
the time-dependent Schr\H{o}dinger equation. The steps in magnetization curves
show the phenomenon of quantum resonant tunneling quantitatively. The effects
of the sweeping rate of applied field is discussed. These results obtained from
quantum dynamical evolution well agree with the recent experiment[W.Wernsdorfer
et al. Nature 416(2002)406].",0303048v1
2003-04-11,Quantum dynamics of exchange biased Single Molecule Magnets,"We present a new family of exchange biased Single Molecule Magnets in which
antiferromagnetic coupling between the two components results in quantum
behaviour different from that of the individual SMMs. Our experimental
observations and theoretical analysis suggest a means of tuning the quantum
tunnelling of magnetization in SMMs. See also: W. Wernsdorfer, N.
Aliaga-Alcalde, D. Hendrickson, G. Christou, Nature 416 (2002) 406.",0304279v1
2003-07-21,Nuclear Magnetic Resonance in a Ferromagnet-Semiconductor Heterostructure,"We report the observation of nuclear magnetic resonance (NMR) in a
ferromagnet-semiconductor heterostructure in the presence of a spin-polarized
current. Spin-polarized electrons injected from a metallic ferromagnet generate
a large nuclear spin population in a GaAs quantum well by dynamic polarization.
The characteristic time for the polarization process is approximately 20 sec,
and the nuclear polarization can persist for several minutes after the current
is turned off. Resonant depolarization is observed in the presence of an AC
magnetic field or when the injection current is modulated at the NMR frequency.",0307512v1
2004-04-01,Temperature dependence of spin-transfer-induced switching of nanomagnets,"We measure the temperature, magnetic-field, and current dependence for the
switching of nanomagnets by a spin-polarized current. Depending on current
bias, switching can occur between either two static magnetic states or a static
state and a current-driven precessional mode. In both cases, the switching is
thermally activated and governed by the sample temperature, not a higher
effective magnetic temperature. The activation barriers for switching between
static states depend linearly on current, with a weaker dependence for dynamic
to static switching.",0404003v1
2004-05-26,Nonequilibrium Extension of the Landau-Lifshitz-Gilbert Equation for Magnetic Systems,"Using the invariant operator method for an effective Hamiltonian including
the radiation-spin interaction, we describe the quantum theory for
magnetization dynamics when the spin system evolves nonadiabatically and out of
equilibrium, $d \hat{\rho}/dt \neq 0$. It is shown that the vector parameter of
the invariant operator and the magnetization defined with respect to the
density operator, both satisfying the quantum Liouville equation, still obey
the Landau-Lifshitz-Gilbert equation.",0405599v1
2004-06-21,"Magnetic susceptibility, exchange interactions and spin-wave spectra in the local spin density approximation","Starting from exact expression for the dynamical spin susceptibility in the
time-dependent density functional theory a controversial issue about exchange
interaction parameters and spin-wave excitation spectra of itinerant electron
ferromagnets is reconsidered. It is shown that the original expressions for
exchange integrals based on the magnetic force theorem (J. Phys. F14 L125
(1984)) are optimal for the calculations of the magnon spectrum whereas static
response function is better described by the ``renormalized'' magnetic force
theorem by P. Bruno (Phys. Rev. Lett. 90, 087205 (2003)). This conclusion is
confirmed by the {\it ab initio} calculations for Fe and Ni.",0406488v1
2004-07-23,Low-Temperature Glassy Response of Ultrathin Manganite Films to Electric and Magnetic Fields,"The glassy response of thin films of La0.8Ca0.2MnO3 to external magnetic and
gated electrostatic fields in a field-effect geometry has been studied at low
temperatures. A hierarchical response with irreversible memory effects,
non-ergodic time evolution, aging and annealing behavior of the resistance
suggest that the dynamics are governed by strain relaxation for both electronic
and magnetic perturbations. Cross-coupling of charge, spin, and strain have
been exploited to tune the coercivity of an ultrathin manganite film by
electrostatic gating.",0407607v1
2005-06-04,Magnetic Soliton and Soliton Collisions of Spinor Bose-Einstein Condensates in an Optical Lattice,"We study the magnetic soliton dynamics of spinor Bose-Einstein condensates in
an optical lattice which results in an effective Hamiltonian of anisotropic
pseudospin chain. A modified Landau-Lifshitz equation is derived and exact
magnetic soliton solutions are obtained analytically. Our results show that the
time-oscillation of the soliton size can be controlled in practical experiment
by adjusting of the light-induced dipole-dipole interaction. Moreover, the
elastic collision of two solitons is investigated.",0506101v1
2005-09-22,Current-induced motion of a domain wall in magnetic nanowires,"The dynamics of current-induced motion of a magnetic domain wall in a
quasi-one-dimensional ferromagnet with both easy-axis and easy-plane
anisotropy, is studied. We pay a special attention to the case of a sharp
domain wall, and calculate the spin torque created by the electric current. The
torque has two components, one of which is acting as a driving force for the
motion of the domain wall while the other distorts its shape, forcing thus the
magnetic moments to deviate from the easy plane.",0509574v1
2006-01-15,Self-Induced Quasistationary Magnetic Fields,"The interaction of electromagnetic radiation with temporally dispersive
magnetic solids of small dimensions may show very special resonant behaviors.
The internal fields of such samples are characterized by
magnetostatic-potential scalar wave functions. The oscillating modes have the
energy orthogonality properties and unusual pseudo-electric (gauge) fields.
Because of a phase factor, that makes the states single valued, a persistent
magnetic current exists. This leads to appearance of an eigen-electric moment
of a small disk sample. One of the intriguing features of the mode fields is
dynamical symmetry breaking.",0601319v1
2006-02-04,Spin-transfer-driven ferromagnetic resonance of individual nanomagnets,"We demonstrate a technique that enables ferromagnetic resonance (FMR)
measurements of the normal modes for magnetic excitations in individual
nanoscale ferromagnets, smaller in volume by a factor of 1000 than can be
probed by other methods. The measured peak shapes indicate two regimes of
response: simple FMR and phase locking. Studies of the resonance frequencies,
amplitudes, and linewidths as a function of microwave power, DC current, and
magnetic field provide detailed new information about the exchange, damping,
and spin-transfer torques that govern the dynamics in magnetic nanostructures.",0602105v1
2006-02-17,Andreev drag effect via magnetic quasiparticle focusing in normal-superconductor nanojunctions,"We study a new hybrid normal-superconductor (NS) pi-junction in which the
non-local current can be orders of magnitude larger than that in earlier
proposed systems. We calculate the electronic transport of this NS hybrid when
an external magnetic field is applied. It is shown that the non-local current
exhibits oscillations as a function of the magnetic field, making the effect
tunable with the field. The underlying classical dynamics is qualitatively
discussed.",0602423v2
2006-05-08,Microscopic Calculation of Spin Torques in Disordered Ferromagnets,"Effects of conduction electrons on magnetization dynamics, represented by
spin torques, are calculated microscopically in the first order in spatial
gradient and time derivative of magnetization. Special attention is paid to the
so-called $\beta$-term and the Gilbert damping, $\alpha$, in the presence of
electrons' spin-relaxation processes, which are modeled by quenched magnetic
(and spin-orbit) impurities. The obtained results such as $\alpha \ne \beta$
hold for localized as well as itinerant ferromagnetism.",0605186v1
2006-06-21,Quantum Dynamics of Spin Wave Propagation Through Domain Walls,"Through numerical solution of the time-dependent Schrodinger equation, we
demonstrate that magnetic chains with uniaxial anisotropy support stable
structures, separating ferromagnetic domains of opposite magnetization. These
structures, domain walls in a quantum system, are shown to remain stable if
they interact with a spin wave. We find that a domain wall transmits the
longitudinal component of the spin excitations only. Our results suggests that
continuous, classical spin models described by LLG equation cannot be used to
describe spin wave-domain wall interaction in microscopic magnetic systems.",0606549v1
2006-06-25,Quantum chaos and fluctuations in isolated nuclear spin systems,"Using numerical simulations we investigate dynamical quantum chaos in
isolated nuclear spin systems. We determine the structure of quantum states,
investigate the validity of the Curie law for magnetic susceptibility and find
the spectrum of magnetic noise. The spectrum is the same for positive and
negative temperatures. The study is motivated by recent interest in
condensed-matter experiments for searches of fundamental parity- and
time-reversal-invariance violations. In these experiments nuclear spins are
cooled down to microkelvin temperatures and are completely decoupled from their
surroundings. A limitation on statistical sensitivity of the experiments arises
from the magnetic noise.",0606635v1
2006-06-26,Self Consistent NEGF-LLG Model for Spin-Torque Based Devices,"We present here a self consistent solution of quantum transport, using the
Non Equilibrium Green's Function (NEGF) method, and magnetization dynamics,
using the Landau-Lifshitz-Gilbert (LLG) formulation. We have applied this model
to study current induced magnetic switching due to `spin torque' in a device
where the electronic transport is ballistic and the free magnetic layer is
sandwiched between two anti-parallel ferromagnetic contacts. The device shows
clear hysteretic current-voltage characteristics, at room temperature, with a
sharp transition between the bistable states and hence can be used as a
non-volatile memory. We show that the proposed design may allow reducing the
switching current by an order of magnitude.",0606648v2
2006-06-27,Magnetic Susceptibility of the Double-Exchange Model in the RKKY Limit,"The magnetic susceptibility and Edwards-Anderson order parameter q of the
spin-glass-like (SGL) phase of the double-exchange model are evaluated in the
weak-coupling or RKKY limit. Dynamical mean-field theory is used to show that
q=M(T/Tsg)^2, where M is the classical Brillouin function and Tsg is the SGL
transition temperature. The correlation length of the SGL phase is determined
by a correlation parameter Q that maximizes Tsg and minimizes the free energy.
The magnetic susceptibility has a cusp at Tsg and reaches a nonzero value as T
goes to zero.",0606699v1
2006-08-19,Spin dynamics in molecular ring nanomagnets: Significant effect of acoustic phonons and magnetic anisotropies,"The nuclear spin-lattice relaxation rate 1/T_1_ is calculated for magnetic
ring clusters by fully diagonalizing their microscopic spin Hamiltonians.
Whether the nearest-neighbor exchange interaction J is ferromagnetic or
antiferromagnetic, 1/T_1_ versus temperature T in ring nanomagnets may be
peaked at around k_B_T=|J| provided the lifetime broadening of discrete energy
levels is in proportion to T^3^. Experimental findings for ferromagnetic and
antiferromagnetic Cu^II^ rings are reproduced with crucial contributions of
magnetic anisotropies as well as acoustic phonons.",0608438v1
2006-08-22,Clustering induced suppression of ferromagnetism in diluted magnets,"Ferromagnetism in diluted magnets in the compensated regime p << x is shown
to be suppressed by the formation of impurity spin clusters. The majority bulk
spin couplings are shown to be considerably weakened by the preferential
accumulation of holes in spin clusters, resulting in low-energy magnon
softening and enhanced low-temperature decay of magnetic order. A locally
self-consistent magnon renormalization analysis of spin dynamics shows that
although strong intra-cluster correlations tend to prolong global order, T_c is
still reduced compared to the ordered case.",0608474v2
2006-08-31,Magnetic correlations around the Mott transition in the Kagomé lattice Hubbard model,"We study the magnetic properties around the Mott transition in the Kagom\'e
lattice Hubbard model by the cellular dynamical mean field theory combined with
quantum Monte Carlo simulations. By investigating the q-dependence of the
susceptibility, we find a dramatic change of the dominant spin fluctuations
around the Mott transition. The spin fluctuations in the insulating phase favor
down to the lowest temperature a spatial spin configuration in which
antiferromagnetic correlations are strong only in one chain direction but
almost vanishing in the others.",0608699v1
2006-09-19,Spin echo without an external permanent magnetic field,"The spin echo techniques aim at the elimination of the effect of a random
magnetic field on the spin evolution. These techniques conventionally utlize
the application of a permanent field which is much stronger than the random
one. The strong field, however, may also modify the magnetic response of the
medium containing the spins, thus altering their ``natural'' dynamics. We
suggest an iterative scheme for generating a sequence of pulses which create an
echo without an external permanent field. The approximation to the ideal echo
improves with the sequence length.",0609490v1
2006-09-20,Coherent spin dynamics in quantum wells in quantizing magnetic field,"We investigate theoretically the coherent longitudinal and transversal spin
relaxation of photoexcited electrons in quantum wells in quantized magnetic
fields. We find the relaxation time for typical quantum well parameters between
100 and 1000 ps. For a realistic random potential the relaxation process
depends on the electron energy and g-factor, demonstrating oscillations in the
spin polarization accompanying the spin relaxation. The dependence of spin
relaxation on applied field, and thus on the corresponding ""magnetic"" length,
can be used to characterize the spatial scale of disorder in quantum wells.",0609522v1
2007-01-17,Flux-biased mesoscopic rings,"Kinetics of magnetic flux in a thin mesoscopic ring biased by a strong
external magnetic field is described equivalently by dynamics of a Brownian
particle in a tilted washboard potential. The 'flux velocity', i.e. the
averaged time derivative of the total magnetic flux in the ring, is a candidate
for a novel characteristics of mesoscopic rings. Its global properties reflect
the possibility of accommodating persistent currents in the ring.",0701403v2
2007-02-02,Current-induced magnetic vortex core switching in a Permalloy nanodisk,"We report on the switching of a magnetic vortex core in a sub-micron
Permalloy disk, induced by a short current pulse applied in the film plane.
Micromagnetic simulations including the adiabatic and non-adiabatic spin-torque
terms are used to investigate the current-driven magnetization dynamics. We
predict that a core reversal can be triggered by current bursts a tenth of a
nanosecond long. The vortex core reversal process is found to be the same as
when an external field pulse is applied. The control of a vortex core's
orientation using current pulses introduces the technologically relevant
possibility to address individual nanomagnets within dense arrays.",0702048v1
2001-09-25,Magnetic Brane-worlds,"We investigate brane-worlds with a pure magnetic field and a perfect fluid.
We extend earlier work to brane-worlds, and find new properties of the Bianchi
type I brane-world. We find new asymptotic behaviours on approach to the
singularity and classify the critical points of the dynamical phase space. It
is known that the Einstein equations for the magnetic Bianchi type I models are
in general oscillatory and are believed to be chaotic, but in the brane-world
model this chaotic behaviour does not seem to be possible.",0109084v2
2002-10-22,Bianchi type I universes with dilaton and magnetic fields,"We consider the dynamics of a Bianchi type I spacetime in the presence of
dilaton and magnetic fields. The general solution of the Einstein-Maxwell
dilaton field equations can be obtained in an exact parametric form. Depending
on the numerical values of the parameters of the model there are three distinct
classes of solutions. The time evolution of the mean anisotropy, shear and
deceleration parameter is considered in detail and it is shown that a
magnetic-dilaton anisotropic Bianchi type I geometry does not isotropize, the
initial anisotropy being present in the universe for all times.",0210073v1
1997-04-18,"Comment on ""Cherenkov Radiation by Neutrinos in a Supernova Core""","Mohanty and Samal have shown that the magnetic-moment interaction with
nucleons contributes significantly to the photon dispersion relation in a
supernova core, and with an opposite sign relative to the usual plasma effect.
Because of a numerical error they overestimated the magnetic-moment term by two
orders of magnitude, but it is still of the same order as the plasma effect. It
appears that the Cherenkov processes gamma+nu -> nu and nu -> nu+gamma remain
forbidden, but a final verdict depends on a more detailed investigation of the
dynamical magnetic susceptibility of a hot nuclear medium.",9704352v1
2000-09-26,External Magnetic Fields in QFT: A Non-Perturbative Approach,"A discussion of the influence of boundaries and scalar field interactions in
the non-perturbative dynamics of fermions in an external magnetic field, along
with their possible applications to condensed matter and cosmology, is briefly
presented. The significance of the results for electroweak baryogenesis in the
presence of an external magnetic field is indicated.",0009303v1
2002-03-26,Magnetic moments of the Lambda(1405) and Lambda(1670) resonances,"By using techniques of unitarized chiral perturbation theory, where the
$\Lambda(1405)$ and $\Lambda(1670)$ resonances are dynamically generated, we
evaluate the magnetic moments of these resonances and their transition magnetic
moment. The results obtained here differ appreciably from those obtained with
existing quark models. The width for the $\Lambda(1670) \to \Lambda(1405)
\gamma$ transition is also evaluated leading to a branching ratio of the order
of $2 \times 10^{-6}$.",0203248v1
1997-01-29,Chiral Symmetry Breaking in a Uniform External Magnetic Field,"Using the nonperturbative Schwinger-Dyson equation, we show that chiral
symmetry is dynamically broken in QED at weak couplings when an external
magnetic field is present, and that chiral symmetry is restored at temperatures
above $T_c \simeq \alpha\pi^2/\sqrt{2 \pi |eH|}$, where $\alpha$ is the fine
structure constant and $H$ is the magnetic field strength.",9701172v1
1998-10-19,The effective potential of composite fields in weakly coupled QED in a uniform external magnetic field,"The effective potential for the composite fields responsible for chiral
symmetry breaking in weakly coupled QED in a magnetic field is derived. The
global minimum of the effective potential is found to acquire a non-vanishing
expectation value of the composite fields that leads to generating the
dynamical fermion mass by an external magnetic field. The results are compared
with those for the Nambu-Jona-Lasinio model.",9810144v1
1998-10-22,The Aharonov-Bohm Problem Revisited,"The properties of a nonrelativistic charged particle in two dimensions in the
presence of an arbitrary number of nonquantized magnetic fluxes are
investigated in free space as well as in a uniform magnetic field. The fluxes
are represented mathematically as branch points in one of the complex
coordinates. It is found that in order to construct solutions, the fluxes have
to be treated in general as dynamical objects dual to the charges. A medium
made up of fluxes acts like an anti-magnetic field and tends to expel the
charges.",9810182v2
1999-09-06,Dynamical Symmetry Breaking in Planar QED,"We investigate (2+1)-dimensional QED coupled with Dirac fermions both at zero
and finite temperature. We discuss in details two-components (P-odd) and
four-components (P-even) fermion fields. We focus on P-odd and P-even Dirac
fermions in presence of an external constant magnetic field. In the spontaneous
generation of the magnetic condensate survives even at infinite temperature. We
also discuss the spontaneous generation of fermion mass in presence of an
external magnetic field.",9909029v2
2002-07-17,Non Abelian Magnetic Monopoles and Dynamics of Confinement,"Magnetic monopoles having non-Abelian charges have been found recently to
play a crucial role in the infrared in a class of supersymmetric gauge
theories. We argue that these ""dual quarks"" can naturally be identified with
the non-Abelian magnetic monopoles of the type first discussed by Goddard,
Nuyts and Olive. Our argument is based on a few simple observations as regards
to their charge structure, flavor quantum numbers, and some general properties
of electromagnetic duality.",0207161v1
2005-10-17,Stable Monopole-Antimonopole String Background in SU(2) QCD,"Motivated by the instability of the Savvidy-Nielsen-Olesen vacuum we make a
systematic search for a stable magnetic background in pure SU(2) QCD. It is
shown that a pair of axially symmetric monopole and antimonopole strings is
stable, provided that the distance between the two strings is less than a
critical value. The existence of a stable monopole-antimonopole string
background strongly supports that a magnetic condensation of
monopole-antimonopole pairs can generate a dynamical symmetry breaking, and
thus the magnetic confinement of color in QCD.",0510140v1
1999-02-02,Rigidity for periodic magnetic fields,"We study the motion of a charge on a conformally flat Riemannian torus in the
presence of magnetic field. We prove that for any non-zero magnetic field there
always exist orbits of this motion which have conjugate points. We conjecture
that the restriction of conformal flatness of the metric is not essential for
this result. This would provide a ``twisted'' version of the recent
generalisation of Hopf's rigidity result obtained by Burago and Ivanov.",9902013v1
2002-04-15,Thresholds to Chaos and Ionization for the Hydrogen Atom in Rotating Fields,"We analyze the classical phase space of the hydrogen atom in crossed magnetic
and circularly polarized microwave fields in the high frequency regime, using
the Chirikov resonance overlap criterion and the renormalization map. These
methods are used to compute thresholds to large scale chaos and to ionization.
The effect of the magnetic field is a strong stabilization of a set of
invariant tori which bound the trajectories and prevent stochastic ionization.
In order to ionize, larger amplitudes of the microwave field are necessary in
the presence of a magnetic field.",0204033v1
2003-01-30,Local amplitude equation from non-local dynamics,"We derive a closed equation for the shape of the free surface of a magnetic
fluid subject to an external magnetic field. The equation is strongly non-local
due to the long range character of the magnetic interaction. We develop a
systematic multiple scale perturbation expansion in which the non-locality is
reduced to the occurrence of the Hilbert transform of the surface profile. The
resulting third order amplitude equation describing the slow modulation of the
basic pattern is shown to be purely local.",0301046v1
2000-06-17,Time-odd mean fields in the rotating frame: microscopic nature of nuclear magnetism,"The microscopic role of nuclear magnetism in rotating frame is investigated
for the first time in the framework of the cranked relativistic mean field
theory. It is shown that nuclear magnetism modifies the expectation values of
single-particle spin, orbital and total angular momenta along the rotational
axis effectively creating additional angular momentum. This effect leads to the
increase of kinematic and dynamic moments of inertia at given rotational
frequency and has an impact on effective alignments.",0006034v1
2001-06-27,The Kraichnan-Kazantsev dynamo,"The problem of the dynamo effect for a Kraichnan incompressible helicity-free
velocity field is considered. Exploiting a quantum formalism first introduced
by Kazantsev (A.P. Kazantsev, Sov. Phys. JETP 26, 1031-1034 (1968)), we show
that a critical magnetic Reynolds number exists for the presence of dynamo. The
value of the Prandtl number influences the spatial distribution of the magnetic
field and its growth in time. The magnetic field correlation length is always
the largest between the diffusive scale and the viscous scale of the flow. In
the same way the field growth is characterized by a time scale that corresponds
to the largest between the diffusive and the viscous characteristic time.",0106090v1
2001-06-28,Preferential Acceleration of Coherent Magnetic Structures and Bursty Bulk Flows in Earth's Magnetotail,"Observations indicate that the magnetotail convection is turbulent and
bi-modal, consisting of fast bursty bulk flows (BBF) and a nearly stagnant
background. We demonstrate that this observed phenomenon may be understood in
terms of the intermittent interactions, dynamic mergings and preferential
accelerations of coherent magnetic structures under the influence of a
background magnetic field geometry that is consistent with the development of
an X-point mean-field structure.",0106098v1
2003-06-05,Statistical anisotropy of magnetohydrodynamic turbulence,"Direct numerical simulations of decaying and forced magnetohydrodynamic (MHD)
turbulence without and with mean magnetic field are analyzed by higher-order
two-point statistics. The turbulence exhibits statistical anisotropy with
respect to the direction of the local magnetic field even in the case of global
isotropy. A mean magnetic field reduces the parallel-field dynamics while in
the perpendicular direction a gradual transition towards two-dimensional MHD
turbulence is observed with $k^{-3/2}$ inertial-range scaling of the
perpendicular energy spectrum. An intermittency model based on the Log-Poisson
approach, $\zeta_p=p/g^2 +1 -(1/g)^{p/g}$, is able to describe the observed
structure function scalings.",0306045v1
2004-05-06,Fluid pumped by magnetic stress,"A magnetic field rotating on the free surface of a ferrofluid layer is shown
to induce considerable fluid motion toward the direction the field is rolling.
The measured flow velocity i) increases with the square of the magnetic field
amplitude, ii) is proportional to the thickness of the fluid layer, and iii)
has a maximum at a driving frequency of about 3 kHz. The pumping speed can be
estimated with a two-dimensional flow model.",0405025v2
2004-11-21,Plateau Instability of Liquid Crystalline Cylinder in Magnetic Field,"The capillary instability of a LC cylinder in magnetic field is considered
using an energy approach. The boundary problem is solved in the linear
approximation of the anisotropy $\chi_a$ of the magnetic susceptibility $\chi$.
The effect of anisotropy, in the region $1\gg|\chi|>|\chi_a|\gg \chi^2$, can be
strong enough to counteract and even reverse the tendency of the field to
enhance stabilization by enlarging the cut--off wave number $k_s$ beyond the
conventional one set by Rayleigh.",0411200v1
2005-01-07,Anharmonic mixing in a magnetic trap,"We have experimentally observed re-equilibration of a magnetically trapped
cloud of metastable neon atoms after it was put in a non-equilibrium state.
Using numerical simulations we show that anharmonic mixing, equilibration due
to the collisionless dynamics of atoms in a magnetic trap, is the dominant
process in this equilibration. We determine the dependence of its time on trap
parameters and atom temperature. Furthermore we observe in the simulations a
resonant energy exchange between the radial and axial trap dimensions at a
ratio of trap frequencies \omega_r / \omega_z = 3/2. This resonance is
explained by a simple oscillator model.",0501033v1
2005-06-01,Multi-electron giant dipole resonances of atoms in crossed electric and magnetic fields,"Multi-electron giant dipole resonances of atoms in crossed electric and
magnetic fields are investigated. Stationary configurations corresponding to a
highly symmetric arrangement of the electrons on a decentered circle are
derived, and a normal-mode stability analysis is performed. A classification of
the various modes, which are dominated either by the magnetic or Coulomb
interactions, is provided. A six-dimensional wave-packet dynamical study, based
on the MCTDH approach, is accomplished for the two-electron resonances,
yielding in particular lifetimes of more than 0.1 $\mu$s for strong electric
fields.",0506007v1
2005-08-02,N-Electron Giant Dipole States in Crossed Electric and Magnetic Fields,"Multi-electron giant dipole resonances of atoms in crossed electric and
magnetic fields are investigated. Stationary configurations corresponding to a
highly symmetric arrangement of the electrons on a decentered circle are
derived, and a normal-mode and stability analysis are performed. A
classification of the various modes, which are dominated by the magnetic field
or the Coulomb interactions, is provided. Based on the MCTDH approach, we carry
out a six-dimensional wave-packet dynamical study for the two-electron
resonances, yielding in particular lifetimes of more than 0.1 $\mu$s for strong
electric fields.",0508021v1
2001-06-28,Trapped-Atom-Interferometer in a Magnetic Microtrap,"We propose a configuration of a magnetic microtrap which can be used as an
interferometer for three-dimensionally trapped atoms. The interferometer is
realized via a dynamic splitting potential that transforms from a single well
into two separate wells and back. The ports of the interferometer are
neighboring vibrational states in the single well potential. We present a
one-dimensional model of this interferometer and compute the probability of
unwanted vibrational excitations for a realistic magnetic potential. We
optimize the speed of the splitting process in order suppress these excitations
and conclude that such interferometer device should be feasible with currently
available microtrap technique.",0106162v1
2002-10-07,Single-Spin Measurement and Decoherence in Magnetic Resonance Force Microscopy,"We consider a simple version of a cyclic adiabatic inversion (CAI) technique
in magnetic resonance force microscopy (MRFM). We study the problem: What
component of the spin is measured in the CAI MRFM? We show that the
non-destructive detection of the cantilever vibrations provides a measurement
of the spin component along the effective magnetic field. This result is based
on numerical simulations of the Hamiltonian dynamics (the Schrodinger equation)
and the numerical solution of the master equation.",0210043v1
2005-03-18,On decoherence in noncommutative plane with perpendicular magnetic field,"In the last years noncommutative quantum mechanics has been investigated
intensively. We consider the influence of magnetic field on decoherence of a
system in the noncommutative quantum plane. Particularly, we point out a model
in which the magnetic field allows {\it in situ} dynamical control of
decoherence as well as, in principle, observation of noncommutativity.",0503163v1
2007-04-27,Constrained flow around a magnetic obstacle,"Many practical applications exploit an external local magnetic field --
magnetic obstacle -- as an essential part of their constructions. Recently, it
has been demonstrated that the flow of an electrically conducting fluid
influenced by an external field can show several kinds of recirculation. The
present paper reports a 3D numerical study whose some results are compared with
an experiment about such a flow in a rectangular duct.",0704.3700v2
2007-05-14,On the Enhanced Reverse Beta Processes in Graphene-Iron Composite Nanostructures at High Temperatures in Strong Magnetic Field,"Strong dense many-spin interactions have been proposed to organize novel
orbital dynamics (the Little Effect) for novel chemical and catalytic
phenomena. The recent determinations of the relativistic and quantum Hall
effects of carriers in graphene under strong magnetic confinement have
substantiated the Little Effect. Moreover such nonclassical phenomena under the
stronger magnetic confinement of ferro-nanocatalysts are here shown to organize
reverse beta processes and possibly pycnonuclear reactions under high
temperature and high-pressure conditions. Such processes have implications for
reverse beta reactions and nuclear reactions within the interior of the earth
and new technologies for carbon nanotube-ferrometal and nanographene-ferrometal
composites.",0705.1909v1
2007-06-12,Theory of nonlinear particle acceleration at shocks and self-generation of the magnetic field,"We present some recent developments in the theory of particle acceleration at
shock fronts in the presence of dynamical reaction of the accelerated particles
and self-generation of magnetic field due to streaming instability. The spectra
of accelerated particles, the velocity, magnetic field and temperature profiles
can be calculated in this approach anywhere in the precursor and in the
downstream region. The implications for the origin of cosmic rays and for the
phenomenology of supernova remnants will be discussed.",0706.1714v1
2007-07-14,Nonequilibrium Work distributions for a trapped Brownian particle in a time dependent magnetic field,"We study the dynamics of a trapped, charged Brownian particle in presence of
a time dependent magnetic field. We calculate work distributions for different
time dependent protocols. In our problem thermodynamic work is related to
variation of vector potential with time as opposed to the earlier studies where
the work is related to time variation of the potentials which depends only on
the coordinates of the particle. Using Jarzynski identity and Crook's equality
we show that the free energy of the particle is independent of the magnetic
field, thus complementing the Bohr-van Leeuwen theorem. We also show that our
system exhibits a parametric resonance in certain parameter space.",0707.2131v1
2007-08-14,Circumstellar Magnetic Field Diagnostics from Line Polarization,"Given that dynamically significant magnetic fields in at least some massive
stars have now been measured, our contribution addresses the question, to what
extent can fields be directly detected in circumstellar gas? The question
speaks directly to the very interesting topic of line-driving physics coupled
with magnetized plasmas, and how this coupling produces structure in the wind
flow. We focus our attention on weak-field diagnostics. These come in two main
types: the Hanle effect, which pertains to coherence effects for linear
polarization from line scattering, and the weak longitudinal Zeeman effect,
which pertains to circular polarization in lines.",0708.1942v2
2007-08-31,Increasing the Efficiency of Energy Scavengers,"In this paper, a methodology for designing efficient energy scavengers is
proposed. The scavenger consists of a cantilever beam on which piezoelectric
films and a mass are mounted. The mass at the tip of the beam is known as the
proof mass and the device is called either an energy scavenger or a beam-mass
system. The proof mass is a permanent magnet, where in its vicinity attracting
permanent magnets are placed. It is shown that when the magnets have
appropriate strengths and are placed appropriately, the vibration of the
beam-mass system can be amplified, thereby the scavenged energy is increased.
Examples are given throughout the paper.",0708.4412v2
2007-09-06,Magnetogenesis from Rotating Cosmic String Loops,"We present a mechanism to create vortices in a plasma via gravitational
dragging behind rotating cosmic string loops. The vortical motions create
magnetic fields by means of the Harrison-Rees mechanism; the fields are further
enhanced through galactic collapse and dynamo amplification. Employing the
Velocity dependent One Scale model (VOS) for the string network and
incorporating loop dynamics, we compute the magnetic fields generated around
the time of decoupling: these are just strong and coherent enough to account
for presently observed magnetic fields in spiral galaxies if efficient dynamos
with $\Gamma_{dy}^{-1}\approx 0.3$ Gyr are present.",0709.0735v1
2007-09-07,Theory of Electromotive Force Induced by Domain Wall Motion,"We formulate a theory on the dynamics of conduction electrons in the presence
of moving magnetic textures in ferromagnetic materials. We show that the
variation of local magnetization in both space and time gives rise to
topological fields, which induce electromotive forces on the electrons.
Universal results are obtained for the emf induced by both transverse and
vortex domain walls traveling in a magnetic film strip, and their measurement
may provide clear characterization on the motion of such walls.",0709.1117v3
2007-10-02,Harmonic oscillator model for current- and field-driven magnetic vortices,"In experiments the distinction between spin-torque and Oersted-field driven
magnetization dynamics is still an open problem. Here, the gyroscopic motion of
current- and field-driven magnetic vortices in small thin-film elements is
investigated by analytical calculations and by numerical simulations. It is
found that for small harmonic excitations the vortex core performs an
elliptical rotation around its equilibrium position. The global phase of the
rotation and the ratio between the semi-axes are determined by the frequency
and the amplitude of the Oersted field and the spin torque.",0710.0532v1
2007-11-07,"Comment on Effects of transverse magnetic field on mixed convection in wall plume of power-law fluids, by Gorla, Lee, Nakamura and Pop, IJES, 1993","Comment on Effects of transverse magnetic field on mixed convection in wall
plume of power-law fluids, by Rama Subba Reddy Gorla, Jin Kook Lee, Shoichiro
Nakamura and Ioan Pop [International Journal of Engineering Science, 31 (1993)
1035-1045]. In the above paper the authors treat the boundary layer mixed
convection flow of a power-law fluid along a vertical adiabatic surface in a
transverse magnetic field with a steady thermal source at the leading edge. The
governing non-similar equations are solved by means of a novel finite
difference scheme. However, there are two fundamental errors in this paper and
the presented results do not have any practical value.",0711.1015v1
2007-11-19,Nonlinear mode conversion in monodomain magnetic squares,"Modifications of spatial distributions of dynamic magnetization corresponding
to spinwave eigenmodes of magnetic squares subjected to a strong microwave
excitation field have been studied experimentally and theoretically. We show
that an increase of the excitation power leads to a nonlinear generation of
long-wavelength spatial harmonics caused by the nonlinear cross coupling
between the eigenmodes. The analysis of the experimental data shows that this
process is mainly governed by the action of the nonlinear spin-wave damping.
This conclusion is further supported by the numerical calculations based on the
complex Ginzburg-Landau equation phenomenologically taking into account the
nonlinear damping.",0711.2872v1
2008-01-15,Broadband ferromagnetic resonance of Ni81Fe19 wires using a rectifying effect,"The broadband ferromagnetic resonance measurement using the rectifying effect
of Ni81Fe19 wire has been investigated. One wire is deposited on the center
strip line of the coplanar waveguide (CPW) and the other one deposited between
two strip lines of CPW. The method is based on the detection of the
magnetoresistance oscillation due to the magnetization dynamics induced by the
radio frequency field. The magnetic field dependences of the resonance
frequency and the rectification spectrum are presented and analytically
interpreted on the standpoint of a uniform magnetization precession model.",0801.2203v1
2008-02-06,Self-organized plane arrays of metallic magnetic elements,"We studied the dynamic magnetic properties of plane periodical arrays of
circular permalloy nano-dots fabricated using a self-organized mask formed by
polysterene nanospheres on the surface of a Permalloy film. Conventional
(microwave cavity) and broadband coplanar-line ferromagnetic resonance setups
were used for the measurements. We found several well resolved resonance peaks.
This result shows that the self-organized mask fabrication technique is able to
produce high-quality samples with small dispersion of geometrical and magnetic
parameters.",0802.0731v1
2008-02-25,Tunnel barrier enhanced voltage signals generated by magnetization precession of a single ferromagnetic layer,"We report the electrical detection of magnetization dynamics in an
Al/AlOx/Ni80Fe20/Cu tunnel junction, where a Ni80Fe20 ferromagnetic layer is
brought into precession under the ferromagnetic resonance (FMR) conditions. The
dc voltage generated across the junction by the precessing ferromagnet is
enhanced about an order of magnitude compared to the voltage signal observed
when the contacts in this type of multilayered structure are ohmic. We discuss
the relation of this phenomenon to magnetic spin pumping and speculate on other
possible underlying mechanisms responsible for the enhanced electrical signal.",0802.3646v1
2008-03-18,Quantum mechanics on curved 2D systems with electric and magnetic fields,"We derive the Schroedinger equation for a spinless charged particle
constrained to a curved surface with electric and magnetics fields applied. The
particle is confined on the surface using a thin-layer procedure, giving rise
to the well-known geometric potential. The electric and magnetic fields are
included via the four-potential. We find that there is no coupling between the
fields and the surface curvature and that, with a proper choice of the gauge,
the surface and transverse dynamics are exactly separable. Finally, the
Hamiltonian for the cylinder, sphere and torus are analytically derived.",0803.2670v1
2008-04-18,Magnetic Component of Quark-Gluon Plasma,"We describe recent developments of the ""magnetic scenario"" of sQGP. We show
that at $T=(0.8-1.3)T_c$ there is a dense plasma of monopoles, capable of
supporting metastable flux tubes. Their existence allows to quantitatively
explained the non-trivial $T$-dependence of the static $\bar Q Q$ potential
energy calculated on the lattice. By molecular dynamics simulation we derived
transport properties (shear viscosity and diffusion constant) and showed that
the best liquid is given by most symmetric plasma, with 50%-50% of electric and
magnetic charges. The results are close to those of the ``perfect liquid''
observed at RHIC.",0804.3102v1
2008-05-15,Spin polarizations and spin Hall currents in a two-dimensional electron gas with magnetic impurities,"We consider a two-dimensional electron gas in the presence of Rashba
spin-orbit coupling, and study the effects of magnetic s-wave impurities and
long-range non-magnetic disorder on the spin-charge dynamics of the system. We
focus on voltage induced spin polarizations and their relation to spin Hall
currents. Our results are obtained using the quasiclassical Green function
technique, and hold in the full range of the disorder parameter $\alpha
p_F\tau$.",0805.2346v2
2008-05-27,Self-sustained magnetoelectric oscillations in magnetic resonant tunneling structures,"The dynamic interplay of transport, electrostatic, and magnetic effects in
the resonant tunneling through ferromagnetic quantum wells is theoretically
investigated. It is shown that the carrier-mediated magnetic order in the
ferromagnetic region not only induces, but also takes part in intrinsic,
robust, and sustainable high-frequency current oscillations over a large window
of nominally steady bias voltages. This phenomenon could spawn a new class of
quantum electronic devices based on ferromagnetic semiconductors.",0805.4068v1
2008-06-04,External magnetic field effects on a distorted kagome antiferromagnet,"We report bulk magnetization, and elastic and inelastic neutron scattering
measurements under an external magnetic field, $H$, on the weakly coupled
distorted kagome system, Cu_{2}(OD)_3Cl. Our results show that the ordered
state below 6.7 K is a canted antiferromagnet and consists of large
antiferromagnetic $ac$-components and smaller ferromagnetic $b$-components. By
first-principle calculations and linear spin wave analysis, we present a simple
spin hamiltonian with non-uniform nearest neighbor exchange interactions
resulting in a system of coupled spin trimers with a single-ion anisotropy that
can qualitatively reproduce the spin dynamics of Cu_{2}(OD)_3Cl.",0806.0767v1
2008-06-04,Electric Field Induced Transformation of Magnetic Domain Structure in Magnetoelectric Iron Garnet Films,"The room temperature magnetoelectric effect was observed in epitaxial iron
garnet films that appeared as magnetic domain wall motion induced by electric
field. The films grown on gadolinium-gallium garnet substrates with various
crystallographic orientations were examined. The effect was observed in (210)
and (110) films and was not observed in (111) films. Dynamic observation of the
domain wall motion in 400 V voltage pulses gave the value of domain wall
velocity in the range 30-50 m/s. The same velocity was achieved in magnetic
field pulse about 50 Oe.",0806.0811v1
2008-07-27,Magnetic-dipolar and electromagnetic vortices in quasi-2D ferrite disks,"Magnetic-dipolar-mode (MDM) oscillations in a quasi-2D ferrite disk show
unique dynamical symmetry properties resulting in appearance of topologically
distinct structures. Based on the magnetostatic (MS) spectral problem
solutions, in this paper we give an evidence for eigen MS power-flow-density
vortices in a ferrite disk. Due to these circular eigen power flows, the MDMs
are characterized by MS energy eigen states. It becomes evident that the reason
of stability of the vortex configurations in saturated ferrite samples is
completely different from the nature of stability in magnetically soft
cylindrical dots. We found a clear correspondence between analytically derived
MDM vortex states and numerically modeled electromagnetic vortices in quasi-2D
ferrite disks.",0807.4281v1
2008-07-31,Scattering Theory of Gilbert Damping,"The magnetization dynamics of a single domain ferromagnet in contact with a
thermal bath is studied by scattering theory. We recover the
Landau-Liftshitz-Gilbert equation and express the effective fields and Gilbert
damping tensor in terms of the scattering matrix. Dissipation of magnetic
energy equals energy current pumped out of the system by the time-dependent
magnetization, with separable spin-relaxation induced bulk and spin-pumping
generated interface contributions. In linear response, our scattering theory
for the Gilbert damping tensor is equivalent with the Kubo formalism.",0807.5009v1
2008-09-09,Finite Volume Study of the Delta Magnetic Moments Using Dynamical Clover Fermions,"We calculate the magnetic dipole moment of the Delta baryon using a
background magnetic field on 2+1-flavors of clover fermions on anisotropic
lattices. We focus on the finite volume effects that can be significant in
background field studies, and thus we use two different spatial volumes in
addition to several quark masses.",0809.1629v1
2008-10-05,Electric Field Driven Magnetic Domain Wall Motion in Iron Garnet Film,"The dynamic observation of domain wall motion induced by electric field in
magnetoelectric iron garnet film is reported. Measurements in 800 kV/cm
electric field pulses gave the domain wall velocity ~45 m/s. Similar velocity
was achieved in magnetic field pulse about 50 Oe. Reversible and irreversible
micromagnetic structure transformation is demonstrated. These effects are
promising for applications in spintronics and magnetic memory.",0810.0850v1
2008-10-24,Dynamical view of pair creation in uniform electric and magnetic fields,"Pair creation in a uniform classical electromagnetic field (Schwinger
mechanism) is studied focusing on the time evolution of the distribution of
created particles. The time evolution of the distribution in time-dependent
fields is also presented as well as effects of back reaction. Motivated by the
Glasma flux tube, which may be formed at the initial stage of heavy-ion
collisions, we investigate effects of a magnetic field parallel to an electric
field, and find that the magnetic field makes the evolution of a fermion system
faster.",0810.4429v2
2008-10-28,Ferroplasmas: Magnetic Dust Dynamics in a Conducting Fluid,"We consider a dusty plasma, in which the dust particles have a magnetic
dipole moment. A Hall-MHD type of model, generalized to account for the
intrinsic magnetization, is derived. The model is shown to be energy
conserving, and the energy density and flux is derived. The general dispersion
relation is then derived, and we show that kinetic Alfv\'en waves exhibit an
instability for a low temperature and high density plasma. We discuss the
implication of our results.",0810.5005v2
2008-11-06,Large-scale intermittency of liquid-metal channel flow in a magnetic field,"We predict a novel flow regime in liquid metals under the influence of a
magnetic field. It is characterised by long periods of nearly steady,
two-dimensional flow interrupted by violent three-dimensional bursts. Our
prediction has been obtained from direct numerical simulations in a channel
geometry at low magnetic Reynolds number and translates into physical
parameters which are amenable to experimental verification under laboratory
conditions. The new regime occurs in a wide range of parameters and may have
implications for metallurgical applications.",0811.0912v1
2008-11-16,Soliton dynamics for the nonlinear Schrödinger equation with magnetic field,"The semiclassical limit of a nonlinear focusing Schr\""odinger equation in
presence of nonconstant electric and magnetic potentials V,A is studied by
taking as initial datum the ground state solution of an associated autonomous
elliptic equation. The concentration curve of the solutions is a
parameterization of the solutions of a Newton ODE involving the electric force
as well as the magnetic force via the Lorenz law of electrodynamics.",0811.2584v4
2008-11-26,Coherent Activation of Zero-Field Fiske Modes in Arrays of Josephson Junctions,"Series arrays of Josephson junctions show evidence of a mode in which all the
junctions oscillate in synchronism on voltage resonances appearing, in zero
external magnetic field, at multiples of the fundamental Fiske step spacing.
The measurements show that the current amplitude of the resonances increases
linearly as their voltages are summed. Investigation of the nature of the
coherent mode by magnetic field responses of arrays and isolated juctions
reveals that the oscillations take place in a parameter plane region where dc
magnetic fields only activate boundary current and flux-quanta dynamics can
take place.",0811.4317v1
2008-12-14,"Magnetic resonance in the cuprates -- exciton, plasmon, or π-mode","We re-analyzed the issue whether the resonance peak observed in neutron
scattering experiments on the cuprates is an exciton, a \pi-resonance, or a
magnetic plasmon. We considered a toy model with on-cite Hubbard U and
nearest-neighbor interactions in both charge and spin channels. We found that
the resonance is predominantly an exciton, even if magnetic interaction is
absent and d-wave pairing originates from attractive density-density
interaction. Our results indicate that one cannot distinguish between spin and
charge-mediated pairing interactions by just looking at the resonance peak in
the dynamic spin susceptibility.",0812.2697v1
2009-02-03,Electromagnon Resonances in the Optical Response of Spiral Magnets,"The optical response of spiral magnets is studied, with special attention to
its electromagnon features. We show that these features trace back to the
resonant magnetoelectric response resulting from the spiral ordering
(irrespective of any concomitant ferroelectricity). This response, being
magnetoelectric in nature, not always can be reduced to an effective electric
permittivity. We argue that electromagnons in spiral magnets can produce, in
addition to the observed peaks in the optical absorption of multiferroics, a
(dynamically enhanced) optical rotation and a negative refractive index
behavior.",0902.0495v2
2009-02-23,Magnetic field variations and the seismicity of solar active regions,"Dynamical changes in the solar corona have proven to be very important in
inducing seismic waves into the photosphere. Different mechanisms for their
generation have been proposed. In this work, we explore the magnetic field
forces as plausible mechanisms to generate sunquakes as proposed by Hudson,
Fisher and Welsch. We present a spatial and temporal analysis of the
line-of-sight magnetic field variations induced by the seismically active 2003
October 29 and 2005 January 15 solar flares and compare these results with
other supporting observations.",0902.3856v2
2009-03-21,Analytical results on the magnetization of the Hamiltonian Mean Field model,"The violent relaxation and the metastable states of the Hamiltonian
Mean-Field model, a paradigmatic system of long-range interactions, is studied
using a Hamiltonian formalism. Rigorous results are derived algebraically for
the time evolution of selected macroscopic observables, e.g., the global
magnetization. The high and low energy limits are investigated and the
analytical predictions are compared with direct $N$-body simulations. The
method we use enables us to re-interpret the out-of-equilibrium phase
transition separating magnetized and (almost) unmagnetized regimes.",0903.3641v1
2009-04-01,Current-induced microwave excitation of a domain wall confined in a magnetic wire with bi-axial anisotropy,"We studied the current-induced magnetization dynamics of a domain wall
confined in a magnetic wire with bi-axial anisotropy. We showed that above the
threshold current density, breathing-mode excitation, where the thickness of
the domain wall oscillates, is induced by spin-transfer torque. We found that
the breathing-mode can be applied as a source of microwave oscillation because
the resistance of the domain wall is a function of the domain wall thickness.
In a current sweep simulation, the frequency of the breathing-mode exhibits
hysteresis because of the confinement.",0904.0044v1
2009-05-27,NMR and muSR in Highly Frustrated Magnets,"We present a brief overview on some of the most significant achievements
obtained by means of NMR and muSR techniques in highly frustrated magnets.
First the basic quantities measured by the two techniques will be presented and
their connection to the microscopic static and dynamical spin susceptibility
recalled. Then the main findings will be outlined, starting from the most
simple frustrated units, the molecular nanomagnets, to artificially built
frustrated systems as $^{3}$He on graphite, to magnets with a macroscopically
degenerate ground-state as the ones on a pyrochlore or kagom\'e lattices.",0905.4414v1
2009-06-14,Magneto-optical imaging of magnetic deflagration in Mn12-Acetate,"For the first time, the morphology and dynamics of spin avalanches in
Mn12-Acetate crystals using magneto-optical imaging has been explored. We
observe an inhomogeneous relaxation of the magnetization, the spins reversing
first at one edge of the crystal and a few milliseconds later at the other end.
Our data fit well with the theory of magnetic deflagration, demonstrating that
very slow deflagration rates can be obtained, which makes new types of
experiments possible.",0906.2562v1
2009-06-20,A numerical model of the VKS experiment,"We present numerical simulations of the magnetic field generated by the flow
of liquid sodium driven by two counter-rotating impellers (VKS experiment).
Using a dynamo kinematic code in cylindrical geometry, it is shown that
different magnetic modes can be generated depending on the flow configuration.
While the time averaged axisymmetric mean flow generates an equatorial dipole,
our simulations show that an axial field of either dipolar or quadrupolar
symmetry can be generated by taking into account non-axisymmetric components of
the flow. Moreover, we show that by breaking a symmetry of the flow, the
magnetic field becomes oscillatory. This leads to reversals of the axial dipole
polarity, involving a competition with the quadrupolar component.",0906.3792v1
2009-06-29,Performance of Magnetic Quantum Cellular Automata and Limitations due to Thermal Noise,"Operation parameters of magnetic quantum cellular automata are evaluated for
the purposes of reliable logic operation. The dynamics of the nanomagnets is
simulated via the Landau-Lifshitz-Gilbert equations with a stochastic magnetic
field corresponding to thermal fluctuations. It is found that in the macrospin
approximation the switching speed does not change under scaling of both size
and distances between nanomagnets. Thermal fluctuations put a limitation on the
size of nanomagnets, since the gate error rate becomes excessive for
nanomagnets smaller than 200nm at room temperature.",0906.5172v1
2009-07-05,Magnetic quantum tunnelling in Fe8 with excited nuclei,"We investigate the effect of dynamic nuclear spin fluctuation on quantum
tunneling of the magnetization (QTM) in the molecular magnet Fe8 by increasing
the nuclei temperature using radio frequency (RF) pulses before the hysteresis
loop measurements. The RF pulses do not change the electrons spin temperature.
Independently we show that the nuclear spin-spin relaxation time T2 has strong
temperature dependence. Nevertheless, we found no effect of the nuclear spin
temperature on the tunneling probability. This suggests that in our
experimental conditions only the hyperfine field strength is relevant for QTM.
We demonstrate theoretically how this can occur.",0907.0844v1
2009-07-16,Long-lived spin memory in Mn-doped GaAs: Time resolved study,"We study the electron spin dynamics in p-type GaAs doped with magnetic Mn
acceptors by means of time-resolved pump-probe and photoluminescence
techniques. Measurements in transverse magnetic fields show a long spin
relaxation time of 20 ns that can be uniquely related to electrons. Application
of weak longitudinal magnetic fields above 100 mT extends the spin relaxation
times up to microseconds which is explained by suppression of the
Bir-Aronov-Pikus spin relaxation for the electron on the Mn acceptor.",0907.2851v1
2009-08-20,Theory of anisotropic exchange in laterally coupled quantum dots,"The effects of spin-orbit coupling on the two-electron spectra in lateral
coupled quantum dots are investigated analytically and numerically. It is
demonstrated that in the absence of magnetic field the exchange interaction is
practically unaffected by spin-orbit coupling, for any interdot coupling,
boosting prospects for spin-based quantum computing. The anisotropic exchange
appears at finite magnetic fields. A numerically accurate effective spin
Hamiltonian for modeling spin-orbit-induced two-electron spin dynamics in the
presence of magnetic field is proposed.",0908.2961v2
2009-10-14,Multiple eruptions from magnetic flux emergence,"In this paper we study the effects of a toroidal magnetic flux tube emerging
into a magnetized corona, with an emphasis on large-scale eruptions. The
orientation of the fields is such that the two flux systems are almost
antiparallel when they meet. We follow the dynamic evolution of the system by
solving the 3D MHD equations using a Lagrangian remap scheme. Multiple
eruptions are found to occur. The physics of the trigger mechanisms are
discussed and related to well-known eruption models.",0910.2616v1
2009-10-14,Note on a non-critical holographic model with a magnetic field,"We consider a noncritical holographic model constructed from an intersecting
brane configuration D4/$\bar{\rm{D4}}$-D4 with an external magnetic field. We
investigate the influences of this magnetic field on strongly coupled dynamics
by the gauge/gravity correspondence.",0910.2661v5
2009-10-23,Chiral symmetry restoration and strong CP violation in a strong magnetic background,"Motivated by the phenomenological scenario of the chiral magnetic effect that
can be possibly found in high-energy heavy ion collisions, we study the role of
very intense magnetic fields and strong CP violation in the phase structure of
strong interactions and, more specifically, their influence on the nature of
the chiral transition. Direct implications for the dynamics of phase conversion
and its time scales are briefly discussed. Our results can also be relevant in
the case of the early universe.",0910.4525v1
2009-12-09,Single magnetic molecule between conducting leads: Effect of mechanical rotations,"We study spin-rotation effects in a magnetic molecule bridged between two
conducting leads. Dynamics of the total angular momentum couples spin tunneling
to the mechanical rotations. Landau-Zener spin transition produced by the
time-dependent magnetic field generates a unique pattern of mechanical
oscillations that can be detected by measuring the electronic tunneling current
through the molecule.",0912.1882v1
2009-12-12,Induction in a von Karman flow driven by ferromagnetic impellers,"We study magnetohydrodynamics in a von K\'arm\'an flow driven by the rotation
of impellers made of material with varying electrical conductivity and magnetic
permeability. Gallium is the working fluid and magnetic Reynolds numbers of
order unity are achieved. We find that specific induction effects arise when
the impeller's electric and magnetic characteristics differ from that of the
fluid. Implications in regards to the VKS dynamo are discussed.",0912.2429v1
2010-01-24,Stability problem in dynamo,"It is shown, that the saturated $\alpha$-effect taken from the nonlinear
dynamo equations for the thin disk can still produce exponentially growing
magnetic field in the case, when this field does not feed back on the $\alpha$.
For negative dynamo number (stationary regime) stability is defined by the
structure of the spectra of the linear problem for the positive dynamo numbers.
Stability condition for the oscillatory solution (positive dynamo number) is
also obtained and related to the phase shift of the original magnetic field,
which produced saturated $\alpha$ and magnetic field in the kinematic regime.
Results can be used for explanation of the similar effect observed in the shell
models simulations as well in the 3D dynamo models in the plane layer and
sphere.",1001.4234v1
2010-04-23,Magnetic traveling-stripe-forcing: enhanced transport in the advent of the Rosensweig instability,"A new kind of contactless pumping mechanism is realized in a layer of
ferrofluid via a spatio-temporally modulated magnetic field. The resulting
pressure gradient leads to a liquid ramp, which is measured by means of X-rays.
The transport mechanism works best if a resonance of the surface waves with the
driving is achieved. The behavior can be understood semi-quantitatively by
considering the magnetically influenced dispersion relation of the fluid.",1004.4151v1
2010-05-10,Direct and inverse cascades in the geodynamo,"The rapid rotation of planets causes cyclonic thermal turbulence in their
cores which may generate the large-scale magnetic fields observed outside the
planets. We consider the model which enables us reproduce the typical features
of small-scale geostrophic flows in physical and wave spaces. We present
estimates of kinetic and magnetic energy fluxes as a function of the wave
number. The joint existence of forward and inverse cascades are demonstrated.
We also consider the mechanism of magnetic field saturation at the end of the
kinematic dynamo regime.",1005.1546v1
2010-05-31,Atomic levels in superstrong magnetic fields and D=2 QED of massive electrons: screening,"The photon polarization operator in superstrong magnetic fields induces the
dynamical photon ""mass"" which leads to screening of Coulomb potential at small
distances $z\ll 1/m$, $m$ is the mass of an electron. We demonstrate that this
behaviour is qualitatively different from the case of D=2 QED, where the same
formula for a polarization operator leads to screening at large distances as
well. Because of screening the ground state energy of the hydrogen atom at the
magnetic fields $B \gg m^2/e^3$ has the finite value $E_0 = -me^4/2
\ln^2(1/e^6)$.",1005.5695v1
2010-06-29,Supertransient magnetohydrodynamic turbulence in Keplerian shear flows,"A subcritical transition to turbulence in magnetized Keplerian shear flows is
investigated using a statistical approach. Three-dimensional numerical
simulations of the shearing box equations with zero net magnetic flux are
employed to determine the transition from decaying to sustained turbulence as a
function of the magnetic Reynolds number Rm. The results reveal no clear
transition to sustained turbulence as the average lifetime of the transients
grows as an exponential function of Rm, in accordance with a type-II
supertransient law.",1006.5669v1
2010-07-22,Neutron magnetic form factor in strongly correlated materials,"We introduce a formalism to compute the neutron magnetic form factor Fm(q)
within a first-principles Density Functional Theory (DFT) + Dynamical Mean
Field Theory (DMFT). The approach treats spin and orbital interactions on the
same footing and reduces to earlier methods in the fully localized or the fully
itinerant limit. We test the method on various actinides of current interest
NpCoGa5, PuSb and PuCoGa5; we show that PuCoGa5 is in mixed valent state, which
naturally explains the measured magnetic form factor.",1007.3997v2
2010-08-02,Chern number spins of Mn acceptor magnets in GaAs,"We determine the effective total spin $J$ of local moments formed from
acceptor states bound to Mn ions in GaAs by evaluating their magnetic Chern
numbers. We find that when individual Mn atoms are close to the sample surface,
the total spin changes from $J = 1$ to $J = 2$, due to quenching of the
acceptor orbital moment. For Mn pairs in bulk, the total $J$ depends on the
pair orientation in the GaAs lattice and on the separation between the Mn
atoms. We point out that Berry curvature variation as a function of local
moment orientation can profoundly influence the quantum spin dynamics of these
magnetic entities.",1008.0275v1
2010-08-17,Anisotropic magnetic fluctuations in the ferromagnetic superconductor UCoGe studied by angle-resolved ^{59}Co NMR,"We have carried out direction-dependent ^{59}Co NMR experiments on a single
crystal sample of the ferromagnetic superconductor UCoGe in order to study the
magnetic properties in the normal state. The Knight shift and nuclear
spin-lattice relaxation rate measurements provide microscopic evidence that
both static and dynamic susceptibilities are ferromagnetic with strong Ising
anisotropy. We discuss that superconductivity induced by these magnetic
fluctuations prefers spin-triplet pairing state.",1008.2837v1
2010-09-24,Parametric excitation of a magnetic nanocontact by a microwave field,"We demonstrate that magnetic oscillations of a current-biased magnetic
nanocontact can be parametrically excited by a microwave field applied at twice
the resonant frequency of the oscillation. The threshold microwave amplitude
for the onset of the oscillation decreases with increasing bias current, and
vanishes at the transition to the auto-oscillation regime. The parametrically
excited oscillation mode is the same as the one in the auto-oscillation regime,
enabling studies of both the passive and the active dynamics of the oscillator.
Theoretical analysis shows that measurements of parametric excitation provide
quantitative information about the relaxation rate, the spin transfer
efficiency, and the nonlinearity of the nanomagnetic system.",1009.4837v1
2010-11-01,Optical control of magnetization of micron-size domains in antiferromagnetic NiO single crystals,"We propose Raman-induced collinear difference-frequency generation (DFG) as a
method to manipulate dynamical magnetization. When a fundamental beam
propagates along a threefold rotational axis, this coherent second-order
optical process is permitted by angular momentum conservation through the
rotational analogue of the Umklapp process. As a demonstration, we
experimentally obtained polarization properties of collinear magnetic DFG along
a [111] axis of a single crystal of antiferromagnetic NiO with micro
multidomain structure, which excellently agreed with the theoretical
prediction.",1011.0327v1
2010-11-10,Macrospin Tunneling and Magnetopolaritons with Nanomechanical Interference,"We theoretically address the quantum dynamics of a nanomechanical resonator
coupled to the macrospin of a magnetic nanoparticle by both instanton and
perturbative approaches. We demonstrate suppression of the tunneling between
opposite magnetizations by nanomechanical interference. By approximating the
macrospin as a two-level system, we describe magnetopolaritons and their
destruction by interference. The predictions can be verified experimentally by
a molecular magnet attached to a nanomechanical bridge.",1011.2242v2
2010-11-17,Generalized Split-Octonion Electrodynamics,"Starting with the usual definitions of octonions and split octonions in terms
of Zorn vector matrix realization, we have made an attempt to write the
consistent form of generalized Maxwell's equations in presence of electric and
magnetic charges (dyons). We have thus written the generalized potential,
generalized field, and generalized current of dyons in terms of split octonions
and accordingly the split octonion forms of generalized Dirac Maxwell's
equations are obtained in compact and consistent manner. This theory reproduces
the dynamic of electric (magnetic) in the absence of magnetic (electric)
charges.",1011.3922v1
2011-01-05,Effects of La non-magnetic impurities on the spin resonance of CeCoIn$_{5}$,"The influence of La non magnetic impurities on the spin dynamics of
CeCoIn$_{5}$ was studied by inelastic neutron scattering. In La-substituted
systems, the spin resonance peak (observed at $\Omega_{res}=0.55 meV$ in the
pure system) is shifted to lower energies but the ratio
$\Omega_{res}/k_{B}T_{c}$ remains unchanged. The excitation broadens till it
reaches 0.3 meV equal to the value of the quasi-elastic signal in the normal
state. The evolution of La substitution is compared with the evolution of the
magnetic resonance in Ni and Zn substituted YBa$_{2}$Cu$_{3}$O$_{7}$.",1101.1018v1
2011-01-17,Alleviation of catastrophic quenching in solar dynamo model with nonlocal alpha-effect,"The nonlocal alpha-effect of Babcock-Leighton type is not prone to the
catastrophic quenching due to conservation of magnetic helicity. This is shown
with a dynamo model, which jointly applies the nonlocal alpha-effect, the
diamagnetic pumping, and dynamical equation for the magnetic alpha-effect. The
same model shows catastrophic quenching when the alpha-effect is changed to its
local formulation. The nonlocal model shows the preferred excitation of
magnetic fields of dipolar symmetry, which oscillate with a period of about ten
years and have a toroidal-to-polar fields ratio of about a thousand.",1101.3115v1
2011-01-21,Global fluctuations in magnetohydrodynamic dynamos,"The spectrum of temporal fluctuations of total magnetic energy for several
dynamo models is different from white noise at frequencies smaller than the
inverse of the turnover time of the underlying turbulent velocity field.
Examples for this phenomenon are known from previous work and we add in this
paper simulations of the G.O. Roberts dynamo and of convectively driven dynamos
in rotating spherical shells. The appearance of colored noise in the magnetic
energy is explained by simple phenomenological models. The Kolmogorov theory of
turbulence is used to predict the spectrum of kinetic and magnetic energy
fluctuations in the inertial range.",1101.4141v1
2011-01-25,Onset and saturation of ion heating by odd-parity rotating-magnetic-fields in a field-reversed configuration,"Heating of figure-8 ions by odd-parity rotating magnetic fields ($RMF_o$)
applied to an elongated field-reversed configuration (FRC) is investigated. The
largest energy gain occurs at resonances ($s \equiv \omega_R/ \omega$) of the
$RMF_o$ frequency, $\omega_R$, with the figure-8 orbital frequency, $\omega$,
and is proportional to $s^2$ for $s-even$ resonances and to $s$ for $s-odd$
resonances. The threshold for the transition from regular to stochastic orbits
explains both the onset and saturation of heating. The FRC magnetic geometry
lowers the threshold for heating below that in the tokamak by an order of
magnitude.",1101.4774v1
2011-03-24,Effect of nonadiabatic spin transfer torque on domain wall resonance frequency and mass,"The dynamics of a magnetic domain wall in a semi circular nanowire loop is
studied by an analytical model and micromagnetic simulations. We find a damped
sinusoidal oscillation of the domain wall for small displacement angles around
its equilibrium position under an external magnetic field in the absence of
currents. By studying the effect of current induced nonadiabatic spin transfer
torque on the magnetic domain wall resonance frequency and mass, a red shift is
found in the resonance frequency and domain wall mass increases by increasing
the ratio of nonadiabatic spin torque to adiabatic contribution above 1.",1103.4670v1
2011-04-22,Stochastic Resonance in a simple model of magnetic reversals,"We discuss the effect of stochastic resonance in a simple model of magnetic
reversals. The model exhibits statistically stationary solutions and bimodal
distribution of the large scale magnetic field. We observe a non trivial
amplification of stochastic resonance induced by turbulent fluctuations, i.e.
the amplitude of the external periodic perturbation needed for stochastic
resonance to occur is much smaller than the one estimated by the equilibrium
probability distribution of the unperturbed system. We argue that similar
amplifications can be observed in many physical systems where turbulent
fluctuations are needed to maintain large scale equilibria.",1104.4417v1
2011-06-03,Spectral scaling laws in MHD turbulence simulations and in the solar wind,"The question is addressed to what extent incompressible magnetohydrodynamics
(MHD) can describe random magnetic and velocity fluctuations measured in the
solar wind. It is demonstrated that distributions of spectral indices for the
velocity, magnetic field, and total energy obtained from high resolution
numerical simulations are qualitatively and quantitatively similar to solar
wind observations at 1 AU. Both simulations and observations show that in the
inertial range the magnetic field spectrum E_b is steeper than the velocity
spectrum E_v with E_b >~ E_v and that the residual energy E_R = E_b-E_v
decreases nearly following a k_perp^-2 scaling.",1106.0700v1
2011-06-20,A note on the magnetic spatial forcing of a ferrofluid layer,"We report on the response of a thin layer of ferrofluid to a spatially
modulated magnetic field. This field is generated by means of a constant
current in a special arrangement of aluminum wires. The full surface profile of
the liquid layer is recorded by means of the absorption of X-rays. The outcome
is analyzed particularly with regard to the magnetic self focusing effect under
a deformable fluid layer.",1106.3857v1
2011-06-22,Electric spectroscopy of vortex states and dynamics in magnetic disks,"Spin-polarized radio frequency (RF) currents and RF-Oersted fields resonantly
excite a magnetic vortex core confined in a micron-scale soft magnetic disk. In
this study, we measured the rectifying voltage spectra caused by the
anisotropic magnetoresistance oscillation due to the gyration of the vortex
with different polarity and chirality. The measured spectra are presented such
that we can determine the vortex properties and strength of the spin torques
and Oersted field accurately and directly through analytical calculation.",1106.4516v2
2011-06-23,Landau quantization effects in ultracold atom-ion collisions,"We study ultracold atom-ion collisions in the presence of an external
magnetic field. At low collision energy the field can drastically modify the
translational motion of the ion, which follows quantized cyclotron orbits. We
present a rigorous theoretical approach for the calculation of quantum
scattering amplitudes in these conditions. Collisions in different magnetic
field regimes, identified by the size of the cyclotron radius with respect to
the range of the interaction potential, are investigated. Our results are
important in cases where use of a magnetic field to control the atom-ion
collision dynamics is envisioned.",1106.4672v1
2011-09-20,Hysteresis for ferromagnetism: asymptotics of some 2-scale Landau-Lifshitz model,"We study a 2-scale version of the Landau-Lifshitz system of ferromagnetism,
introduced by Starynkevitch to modelize hysteresis: the response of the
magnetization is fast compared to a slowly varying applied magnetic fi eld.
Taking the exchange term into account, in space dimension 3, we prove that,
under some natural stability assumption on the equilibria of the system, the
strong solutions follow the dynamics of these equilibria. We also give explicit
examples of relevant equilibria and exterior magnetic fields, when the
ferromagnetic medium occupies some ellipsoidal domain.",1109.4352v1
2011-09-22,Bistability and chaos in Taylor-Green dynamo,"Using direct numerical simulations we study dynamo action under the
Taylor-Green forcing with Prandtl number less than one. We observe bistability
with a weak magnetic field branch and a strong magnetic field branch. Both the
dynamo branches undergo subcritical dynamo transition. We also observe host of
dynamo states including constant, periodic, quasiperiodic, and chaotic magnetic
fields. One of the chaotic state originates through a quasiperiodic route with
phase locking, while another chaotic attractor appears to follow
Newhouse-Ruelle-Takens route to chaos. We also observe intermittent transitions
among quasiperiodic and chaotic states for a given Taylor-Green forcing.",1109.4894v1
2011-09-27,Ferroelectric charge order stabilized by antiferromagnetism in multiferroic LuFe2O4,"Neutron diffraction measurements on multiferroic LuFe2O4 show changes in the
antiferromagnetic (AFM) structure characterized by wavevector q = (1/3 1/3 1/2)
as a function of electric field cooling procedures. The increase of intensity
from all magnetic domains and the decrease in the 2D magnetic order observed
below the Neel temperature are indicative of increased ferroelectric charge
order. The AFM order changes the dynamics of the CO state, and stabilizes it.
It is determined that the increase in electric polarization observed at the
magnetic ordering temperature is due to a transition from paramagnetic 2D
charge order to AFM 3D charge order.",1109.5762v1
2011-09-29,Lattice Planar QED in external magnetic field,"We investigate planar Quantum ElectroDynamics (QED) with two degenerate
staggered fermions in an external magnetic field on the lattice. Our
preliminary results indicate that in external magnetic fields there is
dynamical generation of mass for two-dimensional massless Dirac fermions in the
weak coupling region. We comment on possible implications to the quantum Hall
effect in graphene.",1109.6549v1
2011-10-13,Vertically Self-Gravitating ADAFs in the Presence of Toroidal Magnetic Field,"Force due to the self-gravity of the disc in the vertical direction is
considered to study its possible effects on the structure of a magnetized
advection-dominated accretion disc. We present steady-sate self similar
solutions for the dynamical structure of such a type of the accretion flows.
Our solutions imply reduced thickness of the disc because of the self-gravity.
It also imply that the thickness of the disc will increase by adding the
magnetic field strength.",1110.2941v1
2011-12-06,Three-magnetic fields,"A completely new mechanism to generate the observed amount of large-scale
cosmological magnetic fields is introduced in the context of three-form
inflation. The amplification of the fields occurs via fourth order dynamics of
the vector perturbations and avoids the backreaction problem that plagues most
previously introduced mechanisms.",1112.1356v2
2011-12-28,Spectral signatures of magnetic Bloch oscillations in one-dimensional easy-axis ferromagnets,"Domain walls in a one-dimensional gapped easy-axis ferromagnet can exhibit
Bloch oscillations in an applied magnetic field. We investigate how exchange
couplings modify this behavior within an approximation based on noninteracting
domain-wall bound states. In particular, we obtain analytical results for the
spectrum and the dynamic structure factor, and show where in momentum space to
expect equidistant energy levels, the Wannier-Zeeman ladder, which is the
spectral signature of magnetic Bloch oscillations. We compare our results to
previous calculations employing a single domain-wall approximation, and make
predictions relevant for the material $CoCl2\cdot2H2O$.",1112.6195v2
2012-01-17,Spin wave excitation in magnetic insulators by spin-transfer torque,"We study the excitation of spin waves in magnetic insulators by the
current-induced spin-transfer torque. We predict preferential excitation of
surface spin waves induced by an easy-axis surface anisotropy with critical
current inversely proportional to the penetration depth and surface anisotropy.
The surface modes strongly reduce the critical current and enhance the
excitation power of the current-induced magnetization dynamics.",1201.3442v1
2012-02-06,Mean-field quantum dynamics with magnetic fields,"We consider a system of $N$ bosons in three dimensions interacting through a
mean-field Coulomb potential in an external magnetic field. For initially
factorized states we show that the one-particle density matrix associated with
the solution of the $N$-body Schr\""odinger equation converges to the projection
onto the solution of the magnetic Hartree equation in trace norm and in energy
as $N \rightarrow \infty$. Estimates on the rate of convergence are provided.",1202.1065v2
2012-03-07,3D Simulations of Magnetic Massive Star Winds,"Due to computational requirements and numerical difficulties associated with
coordinate singularity in spherical geometry, fully dynamic 3D
magnetohydrodynamic (MHD) simulations of massive star winds are not readily
available. Here we report preliminary results of the first such a 3D simulation
using $\theta^1$ Ori C (O5.5 V) as a model. The oblique magnetic rotator
$\theta^1$ Ori C is a source of hard X-ray emitting plasma in its circumstellar
environment. Our numerical model can explain both the hardness and the location
of the X-ray emission from this star confirming that magnetically confined wind
shock (MCWS) is the dominating mechanism for hard Xrays in some massive stars.",1203.1523v1
2012-03-29,The role of large-scale magnetic field in the morphology and evolution of extragalactic radio sources,"We discuss a model of formation of extragalactic radio sources when the
parent optical galaxy has a large-scale dipolar magnetic field. The study of
dynamics of ejected from the central part of optical galaxy clouds of
relativistic particles in dipolar magnetic field gives a possibility to explain
main morphological features and physical properties of formed extragalactic
radio sources. We bring some results of statistical analyses and correlations
between physical parameters for more than 500 radio sources. In appendix we
present the data of all used extragalactic radio sources with the references
for them.",1203.6549v1
2012-04-24,"Magnetostatic Spin Waves and Magnetic-Wave Chaos in Ferromagnetic Films. III. Numeric Simulations of Microwave-Band Magnetic Chaos, Its Synchronization and Application to Secure Communication","Selected results of original numeric simulations of non-linear magnetostatic
spin waves and microwave-frequency magnetic chaos in ferrite films are
expounded, as third part of the work whose first two parts are recent arXive
preprints 1204.0200 and 1204.2423 . Especially we consider crucial role of
parametric processes in creating the chaos and simultaneously obstacles to its
synchronization, and examine some possibilities of good enough synchronization
(to an extent allowing its use for direct secure communication in microwave
band).",1204.5410v1
2012-04-27,Chiral Symmetry Breaking in Planar QED in External Magnetic Fields,"We investigate planar quantum electrodynamics (QED) with two degenerate
staggered fermions in an external magnetic field on the lattice. We argue that
in external magnetic fields there is dynamical generation of mass for
two-dimensional massless Dirac fermions in the weak-coupling region. We
extrapolate our lattice results to the quantum Hall effect in graphene.",1204.6112v1
2012-05-16,The magnetic fields and magnetospheres of hot stars,"Strong advances in direct evidence of magnetic fields in hot massive stars
have been possible thanks to the new generation of high-resolution
spectropolarimeters such as ESPaDOnS (on the Canada-France-Hawaii Telescope) or
HARPSpol (on the 3.6m ESO telescope). UV and optical high-resolution
spectroscopy has also been very useful to study the magnetospheres of massive
stars. In this contribution I review the observing tools and our current
knowledge concerning the detection and characterisation of the magnetic fields
and magnetospheres in hot stars.",1205.3603v1
2012-06-20,Magnetoelectric nature of skyrmions in a chiral magnetic insulator Cu2OSeO3,"Dielectric properties were investigated under various magnitudes and
directions of magnetic field (H) for a chiral magnetic insulator Cu2OSeO3. We
found that the skyrmion crystal induces electric polarization (P) along either
in-plane or out-of-plane direction of the spin vortices depending on the
applied H-direction. The observed H-dependence of P in ferrimagnetic,
helimagnetic, and skyrmion crystal state can be consistently described by the
d-p hybridization model, highlighting an important role of relativistic
spin-orbit interaction in the magnetoelectric coupling in Cu2OSeO3. Our
analysis suggests that each skyrmion particle can locally carry electric dipole
or quadrupole, which implies that the dynamics of skyrmions are controllable by
the external electric field.",1206.4404v1
2012-06-26,Entanglement and Sources of Magnetic Anisotropy in Radical Pair-Based Avian Magnetoreceptors,"One of the principal models of magnetic sensing in migratory birds rests on
the quantum spin-dynamics of transient radical pairs created photochemically in
ocular cryptochrome proteins. We consider here the role of electron spin
entanglement and coherence in determining the sensitivity of a radical
pair-based geomagnetic compass and the origins of the directional response. It
emerges that the anisotropy of radical pairs formed from spin-polarized
molecular triplets could form the basis of a more sensitive compass sensor than
one founded on the conventional hyperfine-anisotropy model. This property
offers new and more flexible opportunities for the design of biologically
inspired magnetic compass sensors.",1206.5946v1
2012-07-18,"Comment on ""Geometric phase of the gyromotion for charged particles in a time-dependent magnetic field"" [Phys. Plasmas 18, 072505 (2011)]","The geometric analysis of the gyromotion for charged particles in a
time-dependent magnetic field by J. Liu and H. Qin [Phys. Plasmas 18, 072505
(2011)] is reformulated in terms of the spatial angles that represent the
instantaneous orientation of the magnetic field. This new formulation, which
includes the equation of motion for the pitch angle, clarifies the
decomposition of the gyroangle-averaged equation of motion for the gyrophase
into its dynamic and geometric contributions.",1207.4356v1
2012-08-02,"Magnetic moments of the low-lying $J^P=\,1/2^-$, $3/2^-$ $Λ$ resonances within the framework of the chiral quark model","The magnetic moments of the low-lying spin-parity $J^P=$ $1/2^-$, $3/2^-$
$\Lambda$ resonances, like, for example, $\Lambda(1405)$ $1/2^-$,
$\Lambda(1520)$ $3/2^-$, as well as their transition magnetic moments, are
calculated using the chiral quark model. The results found are compared with
those obtained from the nonrelativistic quark model and those of unitary chiral
theories, where some of these states are generated through the dynamics of two
hadron coupled channels and their unitarization.",1208.0477v1
2012-09-03,Pressure induced ferromagnetism in antiferromagnetic Fe_1.03Te,"The magnetic properties of Fe1.03Te under hydrostatic pressure up to p ~ 5.7
GPa were investigated by means of muon spin rotation, dc magnetization, and
neutron depolarization measurements. With increasing pressure the
antiferromagnetic ordering temperature TN decreases continuously from 70 K at
ambient pressure towards higher pressures. Surprisingly, the commensurate
antiferromagnetic order of FeTe enters a region of incommensurate and dynamical
magnetic order before at p ~ 1.7 GPa the system turns ferromagnetic. The
ferromagnetic ordering temperature TC increases with increasing pressure.",1209.0383v1
2012-09-17,Electrical excitation and detection of magnetic dynamics with impedance matching,"Motivated by the prospects of increased measurement bandwidth, improved
signal to noise ratio and access to the full complex magnetic susceptibility we
develop a technique to extract microwave voltages from our high resistance (10
k{\Omega}) (Ga,Mn)As microbars. We drive magnetization precession with
microwave frequency current, using a mechanism that relies on the spin orbit
interaction. A capacitively coupled lambda/2 microstrip resonator is employed
as an impedance matching network, enabling us to measure the microwave voltage
generated during magnetisation precession.",1209.3643v2
2012-10-04,All Spin Nano-magnetic State Elements,"We propose an all spin state element to enable all spin state machines using
spin currents and nanomagnets. We demonstrate via numerical simulations the
operation of a state element a critical building block for synchronous,
sequential logic computation. The numerical models encompass
Landau-Lifshitz-Gilbert (LLG) nanomagnet dynamics with stochastic models and
vector spin-transport in metallic magnetic and non-magnetic channels. Combined
with all spin combinatorial logic, the state elements can enable synchronous
and asynchronous computing elements.",1210.1613v1
2012-11-18,Controlling the Spin Torque Efficiency with Ferroelectric Barriers,"Non-equilibrium spin-dependent transport in magnetic tunnel junctions
comprising a ferroelectric barrier is theoretically investigated. The exact
solutions of the free electron Schr\""odinger equation for electron tunneling in
the presence of interfacial screening are obtained by combining Bessel and Airy
functions. We demonstrate that the spin transfer torque efficiency, and more
generally the bias-dependence of tunneling magneto- and electroresistance, can
be controlled by switching the ferroelectric polarization of the barrier. This
effect provides a supplementary way to electrically control the current-driven
dynamic states of the magnetization and related magnetic noise in spin transfer
devices.",1211.4196v1
2012-11-29,Pumping $ac$ Josephson current in the Single Molecular Magnets by spin nutation,"We demonstrate that an {\it ac} Josephson current is pumped through the
Single Molecular Magnets (SMM) by the spin nutation. The spin nutation is
generated by applying a time dependent magnetic field to the SMM. We obtain the
flowing charge current through the junction by working in the tunneling limit
and employing Green's function technique. At the resonance conditions some
discontinuities and divergencies are appeared in the normal and Josephson
currents, respectively. Such discontinuities and divergencies reveal themselves
when the absorbed/emitted energy, owing to the interaction of the
quasiparticles with the spin dynamics are in the range of the superconducting
gap.",1211.6879v1
2012-12-19,Nanomagnetic Logic and Magnetization Switching Dynamics in Spin Torque Majority Gates,"Spin torque majority gates are modeled and several regimes of magnetization
switching (some leading to failure) are discovered. The switching speed and
noise margins are determined for STMGs and an adder based on it. With switching
time of 3ns at current of 80uA, the adder computational throughput is
comparable to that of a CMOS adder.",1212.4547v1
2013-01-08,Magnetic chirality as probed by neutron scattering,"We review the concept of chirality, at first briefly in a general context
then in the specific framework of the spin networks. We next discuss to what
extent neutron scattering appears as an unconvertible tool to probe magnetic
chirality in the static and dynamical regimes of the spins. The remarkable
chiral ground state and excitations of the Fe-langasite compound finally serves
to illustrate the use of neutron polarimetry in the experimental studies of the
magnetic chirality.",1301.1616v1
2013-01-10,Application of Harmonic Maps $CP^{(N-1)}$ on SU(N) Bogomolny Equation for BPS Magnetic Monopoles,"In this thesis we study dynamic of magnetic monopoles from Lagrangian density
in Yang-Mills-Higgs field theory. In particular, we discuss BPS (Bogomolny
Prasad Sommerfield) magnetic monopoles, described by SU(N) Bogomolny equations,
which has field equations in form of non-linear coupled matrix field equations.
One of the methods to simplify SU(N) Bogomolny equations is by using harmonic
maps $CP^{(N-1)}$. This method has relation with $Gr(n,N)} \sigma$-model and
can transform SU(N) Bogomolny equation into more simple scalar field equations
that depends only on one variable. As an example, we consider the case of SU(2)
Bogomolny equation.",1301.2017v1
2013-03-21,Magneto-optics of monolayer and bilayer graphene,"The optical conductivity of graphene and bilayer graphene in quantizing
magnetic fields is studied. Both dynamical conductivities, longitudinal and
Hall's, are analytically evaluated. The conductivity peaks are explained in
terms of electron transitions. Correspondences between the transition
frequencies and the magneto-optical features are established using the
theoretical results. The main optical transitions obey the selection rule with
$\Delta n=1$ for the Landau number $n$. The Faraday rotation and light
transmission in the quantizing magnetic fields are calculated. The effects of
temperatures and magnetic fields on the chemical potential are considered.
\pacs{71.20.Nr, 78.20.Ci, 78.20.Bh}",1303.5214v1
2013-04-23,Intermittency in spherical Couette dynamos,"We investigate dynamo action in three-dimensional numerical simulations of
turbulent spherical Couette flows. Close to the onset of dynamo action, the
magnetic field exhibits an intermittent behavior, characterized by a series of
short bursts of the magnetic energy separated by low-energy phases. We show
that this behavior corresponds to the so-called on-off intermittency. This
behavior is here reported for dynamo action with realistic boundary conditions.
We investigate the role of magnetic boundary conditions in this phenomenon.",1304.6212v1
2013-06-06,The origins of $k^{-2}$ spectrum in the decaying Taylor-Green magnetohydrodynamic turbulent flows,"We investigate the origins of $k^{-2}$ spectrum in a decaying Taylor-Green
magnetohydrodynamic flow with zero large scale magnetic flux that was reported
in Lee et al. (2010). A possible candidate for this scaling exponent has been
the weak turbulence phenomenology. From our numerical simulations, we observe
that current sheets in the magnetic Taylor-Green flow are formed in regions of
magnetic discontinuities. Based on this observation and by studying the
influence of the current sheets on the energy spectrum, using a filtering
technique, we argue that the discontinuities are responsible for the $-2$ power
law scaling of the energy spectra of this flow.",1306.1380v1
2013-07-11,Ultra-Low-Intensity Magneto-Optical and Mechanical Effects in Metal Nanocolloids,"We theoretically and numerically investigate the linear and nonlinear
magneto-optical contributions to the refractive index of metal nanospheres. The
analysis is in good agreement with the experimental extinction spectra of gold
nanocolloid solutions, with threshold magnetic fields less than 1 mT when
illuminated with light intensities less than 1 W/cm^2. Plasmonic current loops
and vortex power flows provide a new framework for the dynamical interaction
between material magnetization, light polarization and nano-surfaces. The
photo-induced magneto-optical interaction of non-magnetic metal nanocolloids
imparts significant, non-negligible torque forces in nanofluids.",1307.3188v1
2013-07-22,Analytical expression for the harmonic Hall voltages in evaluating spin orbit torques,"Solid understanding of current induced torques is key to the development of
current and voltage controlled magnetization dynamics in ultrathin magnetic
heterostructures. A versatile technique is needed to evaluate such torques in
various systems. Here we examine the adiabatic (low frequency) harmonic Hall
voltage measurement that has been recently developed to study current induced
effective field that originate from the spin orbit effects. We analytically
derive a form that can be used to evaluate the harmonic Hall voltages and
extract relevant parameters in two representative systems, i.e. out of plane
and in-plane magnetized systems. Contributions from the anomalous Hall and
planar Hall effects are considered.",1307.5603v1
2013-08-21,Thermal transport due to quantum interference in magnetic tunnel junctions,"We study the thermal transport in magnetic tunnel junctions. Thermal
gradients across the tunneling barrier appear around the Fowler-Nordheim
tunneling regime, due to the current-induced heat caused by quantum
interference. Both thermovoltage and thermal temperature follow a linear
response with the applied current, which is an evidence for a thermoelectric
effect. By increasing the barrier transparency, the dynamics of thermoelectric
properties is observed with the current. Accordingly, a large range of the
Seebeck coefficient, 10 - 1000 {\mu}V/K, has been obtained in magnetic tunnel
junctions.",1308.4483v1
2013-09-16,Fundamental solutions of MHD Stokes flow,"A simple analytical solution is obtained for the MHD stokeslet in a
homogeneous magnetic field. This solution represents the flow past a small
particle and can also be interpreted as the flow sufficiently far away from a
body of finite size. Fundamental solutions are found in terms of velocity,
pressure and scalar potential distributions for the flows due to either a
concentrated force or a current source. The former consists of two basic
solutions for the force parallel and transverse to the magnetic field,
respectively. All fundamental solutions have the characteristic length scale of
the Hartmann boundary layer and two parabolic wakes developing along the
magnetic field.",1309.3886v1
2013-09-19,Neutrino oscillations in the presence of the crust magnetization,"It is noted that the crustal magnetic spectrum exhibits the signal from the
partly correlated domain dipoles on the space-scale up to approximately 500 km.
This suggests the nonzero correlation among the dynamical variables of the
ferromagnetic magnetization phenomenon on the small domain scale inside the
earth's crust also. Therefore the influence of the mean of the zero component
of the polarization on the CP matter-induced violation indexes is discussed.",1309.5115v1
2013-09-25,Magnetoplasmons in rotating dusty plasmas,"A rotating dusty plasma apparatus was constructed to provide the possibility
of experimental emulation of extremely high magnetic fields by means of the
Coriolis force, observable in a co-rotating measurement frame. We present
collective excitation spectra for different rotation rates with a magnetic
induction equivalent of up to 3200 Tesla. We identify the onset of
magnetoplasmon-equivalent mode dispersion in the rotating macroscopic
two-dimensional single-layer dusty plasma. The experimental results are
supported by molecular dynamics simulations of 2D magnetized Yukawa systems.",1309.6416v1
2013-10-08,Single-file and normal diffusion of magnetic colloids in modulated channels,"Diffusive properties of interacting magnetic dipoles confined in a parabolic
narrow channel and in the presence of a periodic modulated (corrugated)
potential along the unconfined direction are studied using Brownian dynamics
simulations. We compare our simulation results with the analytical result for
the effective diffusion coefficient of a single-particle by Festa and d'Agliano
[Physica A \textbf{90}, 229 (1978)] and show the importance of inter-particle
interaction on the diffusion process. We present results for the diffusion of
magnetic dipoles as a function of linear density, strength of the periodic
modulation and commensurability factor.",1310.2153v1
2013-10-09,Constants of motion for the magnetic force: the angular momentum and the Laplace-Runge-Lenz vector,"It is well-known that an electric charge under a uniform magnetic field has a
bidimensional motion if its initial position and velocity are perpendicular to
this magnetic field. Although some constants of motion, as the energy and
angular momentum, have been identified for this system, its features hide
others. In this work, we build generalizations of the angular momentum and the
Laplace-Runge-Lenz vector and show that these vectors are constants of motion.
Moreover, from them, we find four dynamically independent conserved qualities.",1310.2591v3
2013-10-10,Phenomenology of current-skyrmion interactions in thin films with perpendicular magnetic anisotropy,"We study skyrmions in magnetic thin films with structural inversion asymmetry
perpendicular to the film plane. We determine the magnetization texture of a
single skyrmion and its dependence on the strength of the Dzyaloshinskii-Moriya
interaction relative to the magnetostatic energy. Furthermore, we construct a
phenomenological model that describes the interaction between the motion of
skyrmions and electric currents to lowest order in spin-orbit coupling. We
estimate the experimental verifiable velocities for current-driven motion of
skyrmion textures based on available results obtained from domain walls
dynamics.",1310.2850v2
2013-11-12,Photonic de Haas-van Alphen effect,"Based on the recently proposed concept of effective gauge potential and
magnetic field for photons, we numerically demonstrate a photonic de Haas-van
Alphen effect. We show that in a dynamically modulated photonic resonator
lattice exhibiting an effect magnetic field, the trajectories of the light beam
at a given frequency have the same shape as the constant energy contour for the
photonic band structure of the lattice in the absence of the effective magnetic
field.",1311.2860v1
2013-11-18,On the range of the attenuated magnetic ray transform for connections and Higgs fields,"For a two-dimensional simple magnetic system, we study the attenuated
magnetic ray transform $I_{A,\Phi}$, with attenuation given by a unitary
connection $A$ and a skew-Hermitian Higgs field $\Phi$. We give a description
for the range of $I_{A,\Phi}$ acting on $\mathbb C^n$-valued tensor fields.",1311.4582v2
2013-12-18,Interplay between chromoelectric and chromomagnetic gluons in Yang-Mills thermodynamics,"We propose an effective theory of SU(3) gluonic matter where interactions
between color-electric and color-magnetic gluons are constrained by the center
and scale symmetries. Through matching to the dimensionally-reduced magnetic
theories, the magnetic gluon condensate qualitatively changes its thermal
behavior above the critical temperature. We argue its phenomenological
consequences for the thermodynamics, in particular the dynamical breaking of
scale invariance.",1312.5245v1
2013-12-23,Acoustic parametric pumping of spin waves,"Recent experiments demonstrated generation of spin currents by ultrasound. We
can understand this acoustically induced spin pumping in terms of the coupling
between magnetization and lattice waves. Here we study the parametric
excitation of magnetization by longitudinal acoustic waves and calculate the
acoustic threshold power. The induced magnetization dynamics can be detected by
the spin pumping into an adjacent normal metal that displays the inverse spin
Hall effect.",1312.6499v1
2014-02-14,Thermodynamically self-consistent non-stochastic micromagnetic model for the ferromagnetic state,"In this work, a self-consistent thermodynamic approach to micromagnetism is
presented. The magnetic degrees of freedom are modeled using the
Landau-Lifshitz-Baryakhtar theory, that separates the different contributions
to the magnetic damping, and thereby allows them to be coupled to the electron
and phonon systems in a self-consistent way. We show that this model can
quantitatively reproduce ultrafast magnetization dynamics in Nickel.",1402.3487v3
2014-02-17,Photon Polarization Precession Spectroscopy for High-Resolution Studies of Spinwaves,"A new type of spectroscopy for high-resolution studies of spin waves that
relies on resonant scattering of hard x-rays is introduced. The energy transfer
in the scattering process is encoded in the precession of the polarization
vector of the scattered photons. Thus, the energy resolution of such a
spectroscopy is independent of the bandwidth of the probing radiation. The
measured quantity resembles the intermediate scattering function of the
magnetic excitations in the sample. At pulsed x-ray sources, especially x-ray
lasers, the proposed technique allows to take single-shot spectra of the
magnetic dynamics. The method opens new avenues to study low-energy
non-equilibrium magnetic processes in a pump-probe setup.",1402.4795v1
2014-03-04,Ultrafast switching of the electric polarization and magnetic chirality in BiFeO3 by an electric field,"Using a first-principles-based effective Hamiltonians within molecular
dynamics simulations, we discover that applying an electric field that is
opposite to the initial direction of the polarization results in a switching of
both the polarization and the magnetic chirality vector of multiferroic BiFeO3
at an ultrafast pace (namely of the order of picoseconds). We discuss the
origin of such a double ultrafast switching, which is found to involve original
intermediate magnetic states and may hold promise for designing various
devices.",1403.0671v1
2014-03-24,Skyrmionic spin Seebeck effect via dissipative thermomagnonic torques,"We derive thermomagnonic torque and its ""$\beta$-type"" dissipative correction
from the stochastic Landau-Lifshitz-Gilbert equation. The ""$\beta$-type""
dissipative correction describes viscous coupling between magnetic dynamics and
magnonic current and it stems from spin mistracking of the magnetic order. We
show that thermomagnonic torque is important for describing temperature
gradient induced motion of skyrmions in helical magnets while dissipative
correction plays an essential role in generating transverse Magnus force. We
propose to detect such skyrmionic motion by employing the transverse spin
Seebeck effect geometry.",1403.6160v2
2014-04-11,Utilizing Nitrogen Vacancy Centers to measure oscillating magnetic fields,"We show how nitrogen vacancy (NV) centers can be used to determine the
amplitude, phase and frequency of unknown weak monochromatic and multichromatic
oscillating magnetic fields using only the periodic dynamical decoupling (PDD)
and Carr-Purcell-Meiboom-Gill (CPMG) sequences. The effect of decoherence on
the measurement of the magnetic field parameters is explicitly analyzed, and we
take into account the fact that different pulse sequences suppress decoherence
to different extents. Since the sensitivity increases with increasing sensing
time while it decreases due to decoherence, we use the Fisher information
matrix in order to optimize the number of pulses that should be used.",1404.3133v1
2014-08-05,Vinamax: a macrospin simulation tool for magnetic nanoparticles,"We present Vinamax, a simulation tool for nanoparticles that aims at
simulating magnetization dynamics on very large timescales. To this end, each
in dividual nanoparticle is approximated by a macrospin. Vinamax numerically
solves the Landau-Lifshitz equation by adopting a dipole approximation method,
while temperature effects can be taken into account with two stochastic
methods. It describes the influence of demagnetizing and anisotropy fields on
magnetic nanoparticles at finite temperatures in a space and time-dependent
externally applied field. Vinamax can be used in biomedical research where
nanoparticle imaging techniques are under developement.",1408.0908v1
2014-09-15,Enhanced synchronization in an array of spin torque nano oscillators in the presence of oscillating external magnetic field,"We demonstrate that the synchronization of an array of electrically coupled
spin torque nano-oscillators (STNO) modelled by
Landau-Lifshitz-Gilbert-Slonczewski (LLGS) equation can be enhanced appreciably
in the presence of a common external microwave magnetic field. The applied
microwave magnetic field stabilizes and enhances the regions of synchronization
in the parameter space of our analysis, where the oscillators are exhibiting
synchronized oscillations thereby emitting improved microwave power. To
characterize the synchronized oscillations we have calculated the locking range
in the domain of external source frequency.",1409.4384v2
2014-09-21,Effective potential energy for relativistic particles in the field of inclined rotating magnetized sphere,"The dynamics of a charged relativistic particle in electromagnetic field of a
rotating magnetized celestial body with the magnetic axis inclined to the axis
of rotation is studied. The covariant Lagrangian function in the rotating
reference frame is found. Effective potential energy is defined on the base of
the first integral of motion. The structure of the equipotential surfaces for a
relativistic charged particle is studied and depicted for different values of
the dipole moment. It is shown that there are trapping regions for the
particles of definite energies.",1409.5976v1
2014-12-14,Landau levels for discrete-time quantum walks in artificial magnetic fields,"A new family of 2D discrete-time quantum walks (DTQWs) is presented and shown
to coincide, in the continuous limit, with the Dirac dynamics of a spin 1/2
fermion coupled to a constant and uniform magnetic field. Landau levels are
constructed, not only in the continuous limit, but also for the DTQWs i.e. for
finite non-vanishing values of the time- and position-step, by a perturbative
approach in the step. Numerical simulations support the above results and
suggest that the magnetic interpretation is valid beyond the scope of the
continuous limit. The possibility of quantum simulation of condensed-matter
systems by DTQWs is also discussed.",1412.4337v2
2014-12-15,Nonequilibrium spin current through interacting quantum dots,"We develop a theory for charge and spin current between two canted magnetic
leads flowing through a quantum dot with an arbitrary local interaction. For a
noncollinear magnetic configuration, we calculate equilibrium and
nonequilibrium current biased by voltage or temperature difference or pumped by
magnetic dynamics. We are able to explicitly separate the equilibrium and
nonequilibrium contributions to the current, both of which can be written in
terms of the full retarded Green's function on the dot. Taking the specific
example of a single-level quantum dot with a large on-site Coulomb interaction,
we calculate the total spin current near the Kondo regime, which we find to be
generally enhanced in magnitude as compared to the noninteracting case.",1412.4663v1
2014-12-18,Coherent elastic excitation of spin waves,"We model the injection of elastic waves into a ferromagnetic film (F) by a
non-magnetic transducer (N). We compare the configurations in which the
magnetization is normal and parallel to the wave propagation. The lack of axial
symmetry in the former results in the emergence of evanescent interface states.
We compute the energy-flux transmission across the N$|$F interface and
sound-induced magnetization dynamics in the ferromagnet. We predict efficient
acoustically induced pumping of spin current into a metal contact attached to
F.",1412.5820v2
2015-01-30,Memory functions of magnetic skyrmions,"Magnetic skyrmion, a swirling spin texture, in chiral magnets is
characterized by (i) nano-scale size ($\sim$1nm -- 100nm), (ii) topological
stability, and (iii) gyro-dynamics. These features are shown to be advantageous
for (a) high-density data-storage, (b) nonvolatile memory, and (c) ultra-low
current and energy cost manipulation, respectively. By the numerical
simulations of Landau-Lifshitz-Gilbert equation, the elementary functions of
skyrmions are demonstrated aiming at the design principles of skyrmionic memory
devices.",1501.07650v1
2015-02-14,Energy dissipation in single-domain ferromagnetic nanoparticles: Dynamical approach,"We study, both analytically and numerically, the phenomenon of energy
dissipation in single-domain ferromagnetic nanoparticles driven by an
alternating magnetic field. Our interest is focused on the power loss resulting
from the Landau-Lifshitz-Gilbert equation, which describes the precessional
motion of the nanoparticle magnetic moment. We determine the power loss as a
function of the field amplitude and frequency and analyze its dependence on
different regimes of forced precession induced by circularly and linearly
polarized magnetic fields. The conditions to maximize the nanoparticle heating
are also analyzed.",1502.04222v1
2015-03-16,Detection of spin waves in permalloy using planar Hall effect,"Rectification of microwave oscillations of magnetization in a permalloy film
is realized using planar Hall effect. Two different rectified signals are
obtained: a signal from the linearly excited uniform magnetization precession
at the frequency of the external pumping and a signal from the pairs of
contra-propagating short-wavelength spin waves parametrically generated at a
half of the pumping frequency. The second, most unusual, rectified signal is
caused by the uniform component of the dynamic magnetization created due to the
interference of the phase correlated pairs of parametric spin waves.",1503.04638v1
2015-03-23,Quantum Stoner-Wohlfarth model,"The quantum mechanical counterpart of the famous Stoner-Wohlfarth model -- an
easy-axis magnet in a tilted magnetic field -- is studied theoretically and
through simulations, as a function of the spin-size $S$ in a sweeping
longitudinal field. Beyond the classical Stoner-Wohlfarth transition, the
sweeping field-induced adiabatic change of states slows down as $S$ increases,
leading to a dynamical quantum phase transition. This result is described as a
critical phenomenon associated with Landau-Zener tunneling gaps at metastable
quasi-avoided crossings. Furthermore, a beating of the magnetization is
discovered after the Stoner-Wohlfarth transition. The period of the beating,
obtained analytically, arises from a new type of quantum phase factor.",1503.06658v1
2015-03-31,On the existence of periodic orbits for magnetic systems on the two-sphere,"We prove that there exist periodic orbits on almost all compact regular
energy levels of a Hamiltonian function defined on a twisted cotangent bundle
over the two-sphere. As a corollary, given any Riemannian two-sphere and a
magnetic field on it, there exists a closed magnetic geodesic for almost all
kinetic energy levels.",1503.09035v2
2015-04-17,Critical analysis and remedy of switching failures in straintronic logic using Bennett clocking in the presence of thermal fluctuations,"Straintronic logic is a promising platform for beyond Moore's law computing.
Using Bennett clocking mechanism, information can propagate through an array of
strain-mediated multiferroic nanomagnets exploiting the dipolar coupling
between the magnets without having to physically interconnect them. Here we
perform a critical analysis of switching failures, i.e., error in information
propagation due to thermal fluctuations through a chain of such straintronic
devices. We solved stochastic Landau-Lifshitz-Gilbert equation considering
room-temperature thermal perturbations and show that magnetization switching
may fail due to inherent magnetization dynamics accompanied by thermally
broadened switching delay distribution. Avenues available to circumvent such
issue are proposed.",1504.04618v1
2015-06-05,Spin dynamics in pressure-induced magnetically-ordered phases in PHCC,"We present inelastic neutron scattering experiments on the $S=1/2$ frustrated
gapped quantum magnet piperazinium hexachlorodicuprate under applied
hydrostatic pressure. These results show that at 9 kbar the magnetic triplet
excitations in the system are gapless, contrary to what was previously
reported. We show that the changes in the excitation spectrum can be primarily
attributed to the change in a single exchange pathway.",1506.01876v1
2015-06-11,The Effect of Correlations on the Heat Transport in a Magnetized Plasma,"In a classical ideal plasma, a magnetic field is known to reduce the heat
conductivity perpendicular to the field whereas it does not alter the one along
the field. Here we show that, in strongly correlated plasmas that are observed
at high pressure or/and low temperature, a magnetic field reduces the
perpendicular heat transport much less and even {\it enhances} the parallel
transport. These surprising observations are explained by the competition of
kinetic, potential and collisional contributions to the heat conductivity. Our
results are based on first principle molecular dynamics simulations of a
one-component plasma.",1506.03605v1
2015-08-11,"Disks and Jets - Gravity, Rotation and Magnetic Fields","Magnetic fields are fundamental to the dynamics of both accretion disks and
the jets that they often drive. We review the basic physics of these phenomena,
the past and current efforts to model them numerically with an emphasis on the
jet-disk connection, and the observational constraints on the role of magnetic
fields in the jets of active galaxies on all scales.",1508.02546v1
2015-09-14,Spin Transport in Antiferromagnetic Insulators Mediated by Magnetic Correlations,"We report a systematic study of spin transport in antiferromagnetic (AF)
insulators having a wide range of ordering temperatures. Spin current is
dynamically injected from Y3Fe5O12 (YIG) into various AF insulators in
Pt/insulator/YIG trilayers. Robust, long-distance spin transport in the AF
insulators is observed, which shows strong correlation with the AF ordering
temperatures. We find a striking linear relationship between the spin decay
length in the AFs and the damping enhancement in YIG, suggesting the critical
role of magnetic correlations in the AF insulators as well as at the AF/YIG
interfaces for spin transport in magnetic insulators.",1509.04336v1
2015-11-05,Force law in material media and quantum phases,"We show that the known expressions for the force on a point-like dipole are
incompatible with the relativistic transformation of force, and in this respect
we apply the Lagrangian approach to the derivation of the correct equation for
force on a small electric/magnetic dipole. The obtained expression for the
generalized momentum of a moving dipole predicts two novel quantum effects with
non-topological and non-dynamic phases, when an electric dipole is moving in an
electric field, and when a magnetic dipole is moving in a magnetic field,
correspondingly. The implications of the obtained results are discussed.",1511.04341v1
2015-11-14,Parametric resonance induced chaos in magnetic damped driven pendulum,"A damped driven pendulum with a magnetic driving force, appearing from a
solenoid, where ac current flows is considered. The solenoid acts on the
magnet, which is located at the free end of the pendulum. In this system, the
existence and interrelation of chaos and parametric resonance is theoretically
examined. Derived analytical results are supported by numerical simulations and
conducted experiments.",1511.04593v2
2016-01-16,On the integrable magnetic geodesic flow on a 2-torus,"In this paper the magnetic geodesic flow on a 2-torus is considered. We study
a semi-hamiltonian quasi-linear PDEs which is equivalent to the existence of
polynomial in momenta first integral of magnetic geodesic flow on fixed energy
level. It is known that diagonal metric associated with this system is Egorov
one if degree of the first integral is equal to 2 or 3. In this paper we prove
this fact in the case of existence of the first integral of any degree.",1601.04160v1
2016-03-16,Magnetic properties of a two-dimensional electron gas strongly coupled to light,"Considering the quantum dynamics of 2DEG exposed to both a stationary
magnetic field and an intense high-frequency electromagnetic wave, we found
that the wave decreases the scattering-induced broadening of Landau levels.
Therefore, various magnetoelectronic properties of two-dimensional
nanostructures (density of electronic states at Landau levels,
magnetotransport, etc) are sensitive to the irradiation by light. Thus, the
elaborated theory paves a way to optical controlling magnetic properties of
2DEG.",1603.04965v2
2016-04-02,Fluctuations of electrical conductivity: a new source for astrophysical magnetic fields,"We consider the generation of magnetic field by the flow of a fluid for which
the electrical conductivity is nonuniform. A new amplification mechanism is
found which leads to dynamo action for flows much simpler than those considered
so far. In particular, the fluctuations of the electrical conductivity provide
a way to bypass anti-dynamo theorems. For astrophysical objects, we show
through three-dimensional global numerical simulations that the
temperature-driven fluctuations of the electrical conductivity can amplify an
otherwise decaying large scale equatorial dipolar field. This effect could play
a role for the generation of the unusually tilted magnetic field of the iced
giants Neptune and Uranus.",1604.00469v1
2016-04-13,Dipolar Rings of Microscopic Ellipsoids: Magnetic Manipulation and Cell Entrapment,"We study the formation and dynamics of dipolar rings composed by microscopic
ferromagnetic ellipsoids, which self-assemble in water by switching the
direction of the applied field. We show how to manipulate these fragile
structures and control their shape via application of external static and
oscillating magnetic fields. We introduce a theoretical framework which
describes the ring deformation under an applied field, allowing to understand
the underlying physical mechanism. Our microscopic rings are finally used to
capture, entrap and later release a biological cell via magnetic command, i.e.
performing a simple operation which can be implemented in other microfluidic
devices which make use of ferromagnetic particles.",1604.03895v2
2016-04-20,Magnetic monopoles in noncommutative quantum mechanics,"We discuss certain generalization of the Hilbert space of states in
noncommutaive quantum mechanics that, as we show, introduces magnetic monopoles
into the theory. Such generalization arises very naturally in the considered
model, but can be easily reproduced in ordinary quantum mechanics as well. This
approach offers a different viewpoint on the Dirac quantization condition and
other important relations for magnetic monopoles. We focus mostly on the
kinematic structure of the theory, but investigate also a dynamical problem
(with the Coulomb potential).",1604.05968v3
2016-05-04,Parker's model in geodynamo,"We consider how information on geostrophic flows in the planetary cores,
taken from 3D simulations in the sphere, can be used in 2D Parker's geodynamo
model with the simple forms of the $\alpha$-quenching. Using cluster computer
systems dependence of dynamo equations solution on the magnitudes of $\alpha$-
and $\omega$-effects is studied. We show that geostrophical flow can produce
the well-known Z-structure of the poloidal magnetic field without the feed-back
of the magnetic field onto the large-scale velocity field. The influence of
fluctuations of $\alpha$-effect on magnetic field generation, its spectral
properties, in respect to the geodynamo applications, is also discussed.",1605.01321v1
2016-06-11,Chiral magnetic effect by synthetic gauge fields,"We study the dynamical generation of the chiral chemical potential in a Weyl
metal constructed from a three-dimensional optical lattice and subject to
synthetic gauge fields. By numerically solving the Boltzmann equation with the
Berry curvature in the presence of parallel synthetic electric and magnetic
fields, we find that the spectral flow and the ensuing chiral magnetic current
emerge. We show that the spectral flow and the chiral chemical potential can be
probed by time-of-flight imaging.",1606.03589v1
2016-07-18,Heat transfer in MHD flow due to a linearly stretching sheet with induced magnetic field,"The full MHD problem of the flow and heat transfer due to a linearly
stretching sheet in the presence of a transverse magnetic field is put in a
self-similar form. Traditionally ignored physical processes such as induced
magnetic field, viscous dissipation, Joule heating, and work shear are included
and their importance is established. Cases of prescribed surface temperature,
prescribed heat flux, surface feed (injection or suction), velocity slip, and
thermal slip are also considered. The problem is shown to admit self
similarity. Sample numerical solutions are obtained for chosen combinations of
the flow parameters.",1607.05720v2
2016-10-26,On the motion of free interface in two-dimensional ideal incompressible MHD,"For the free boundary problem of the plasma-vacuum interface to ideal
incompressible magnetohydrodynamics (MHD) in two-dimensional space, the a
priori estimates of solutions are proved in Sobolev norms by adopting a
geometrical point of view. In the vacuum region, the magnetic field is
described by the div-curl system of pre-Maxwell dynamics, while at the
interface the total pressure is continuous and the magnetic field is tangent to
the boundary. We prove that the $L^2$ norms of any order covariant derivatives
of the magnetic field in vacuum and on the boundaries are bounded in terms of
initial data and the second fundamental forms of the free interface and the
rigid wall.",1610.08285v2
2016-11-05,Averaging of nonlinear Schrödinger equations with strong magnetic confinement,"We consider the dynamics of nonlinear Schr\""odinger equations with strong
constant magnetic fields. In an asymptotic scaling limit the system exhibits a
purely magnetic confinement, based on the spectral properties of the Landau
Hamiltonian. Using an averaging technique we derive an associated effective
description via an averaged model of nonlinear Schr\""odinger type. In a special
case this also yields a derivation of the LLL equation.",1611.01574v2
2016-11-07,Purcell magneto-elastic swimmer controlled by an external magnetic field,"This paper focuses on the mechanism of propulsion of a Purcell swimmer whose
segments are magnetized and react to an external magnetic field applied into
the fluid. By an asymptotic analysis, we prove that it is possible to steer the
swimmer along a chosen direction when the control functions are prescribed as
an oscillating field. Moreover, we discuss what are the main obstructions to
overcome in order to get classical controllability result for this system.",1611.02020v1
2016-11-26,Growth of the inner core in the mean-field dynamo model,"Application of Parker's dynamo model to the geodynamo with the growing inner
core is considered. It is shown that decrease of the inner core size, where
intensive magnetic field generation takes place, leads to the multi-polar
magnetic field in the past. This effect reflects the decrease of the region of
the effective magnetic field generation. The process is accompanied by increase
of the reversals number and decrease of intensity of the geomagnetic field. The
constraints on the mechanisms of convection in the liquid core are discussed.",1611.08652v1
2016-11-29,Investigation of thermalization in giant-spin models by different Lindblad schemes,"The theoretical understanding of time-dependence in magnetic quantum systems
is of great importance in particular for cases where a unitary time evolution
is accompanied by relaxation processes. A key example is given by the dynamics
of single-molecule magnets where quantum tunneling of the magnetization
competes with thermal relaxation over the anisotropy barrier. In this article
we investigate how good a Lindblad approach describes the relaxation in giant
spin models and how the result depends on the employed operator that transmits
the action of the thermal bath.",1611.09695v1
2016-12-27,Reinventing atomistic magnetic simulations with spin-orbit coupling,"We propose a powerful extension to combined molecular and spin dynamics that
fully captures the coupling between the atomic and spin subsystems via
spin-orbit interactions. Its foundation is the inclusion of the local magnetic
anisotropies that arise as a consequence of the lattice symmetry breaking due
to phonons or defects. We demonstrate that our extension enables the exchange
of angular momentum between the atomic and spin subsystems, which is critical
to the challenges arising in the study of fluctuations and non-equilibrium
processes in complex, natural, and engineered magnetic materials.",1612.08503v1
2017-01-09,Quantum Ring in Gapped Graphene Layer with Wedge Disclination in the Presence of an Uniform Magnetic Field,"In this paper we investigate the relativistic quantum dynamics of a massive
excitation in a graphene layer with a wedge disclination in the presence of an
uniform magnetic field. We use a Dirac oscillator type coupling to introduce
the confining potential for massive fermions in this system. We obtain the
energy spectrum and eigenfunctions for the quantum ring pierced by
Aharonov-Bohm flux resulting in appearance of persistent current and
spontaneous magnetization.",1701.02051v2
2017-01-11,Theory of Quantum Oscillations of Magnetization in Kondo Insulators,"The Kondo lattice model of spin-1/2 local moments coupled to the conduction
electrons at half-filling is studied for its orbital response to magnetic field
on bipartite lattices. Through an effective charge dynamics, in a canonical
representation of electrons that appropriately describes the Kondo insulating
ground state, the magnetization is found to show de Haas-van Alphen
oscillations from intermediate to weak Kondo coupling. These oscillations are
ascribed to Lifshitz-like inversion of a dispersion of the gapped
quasiparticles, whose chemical potential surface is measured by the oscillation
frequency. Such oscillations are also predicted to occur in spin-density wave
insulators.",1701.02825v2
2017-01-24,Accelerated Magnetic Resonance Spectroscopy with Vandermonde Factorization,"Multi-dimensional magnetic resonance spectroscopy is an important tool for
studying molecular structures, interactions and dynamics in bio-engineering.
The data acquisition time, however, is relatively long and non-uniform sampling
can be applied to reduce this time. To obtain the full spectrum,a
reconstruction method with Vandermonde factorization is proposed.This method
explores the general signal property in magnetic resonance spectroscopy: Its
time domain signal is approximated by a sum of a few exponentials. Results on
synthetic and realistic data show that the new approach can achieve faithful
spectrum reconstruction and outperforms state-of-the-art low rank Hankel matrix
method.",1701.07017v1
2017-04-13,The hottest hot Jupiters may host Atmospheric Dynamos,"Hot Jupiters have proven themselves to be a rich class of exoplanets which
test our theories of planetary evolution and atmospheric dynamics under extreme
conditions. Here, we present three-dimensional magnetohydrodynamic simulations
and analytic results which demonstrate that a dynamo can be maintained in the
thin, stably-stratified atmosphere of a hot Jupiter, independent of the
presumed deep-seated dynamo. This dynamo is maintained by conductivity
variations arising from strong asymmetric heating from the planets' host star.
The presence of a dynamo significantly increases the surface magnetic field
strength and alters the overall planetary magnetic field geometry, possibly
affecting star-planet magnetic interactions.",1704.04197v1
2017-05-31,Topological lattice using multi-frequency radiation,"We describe a novel technique for creating an artificial magnetic field for
ultra-cold atoms using a periodically pulsed pair of counter propagating Raman
lasers that drive transitions between a pair of internal atomic spin states: a
multi-frequency coupling term. In conjunction with a magnetic field gradient,
this dynamically generates a rectangular lattice with a non-staggered magnetic
flux. For a wide range of parameters, the resulting Bloch bands have
non-trivial topology, reminiscent of Landau levels, as quantified by their
Chern numbers.",1705.11101v2
2017-07-26,Interfacial exchange interactions and magnetism of Ni2MnAl/Fe bilayers,"Based on a multi-scale calculations, combining ab-initio methods with spin
dynamics simulations, we perform a detailed study of the magnetic behavior of
Ni2MnAl/Fe bilayers. Our simulations show that such a bilayer exhibits a small
exchange bias effect when the Ni2MnAl Heusler alloy is in a disordered B2
phase. Additionally, we present an effective way to control the magnetic
structure of the Ni2MnAl antiferromagnet, in the pseudo-ordered B2-I as well as
the disordered B2 phases, via a spin-flop coupling to the Fe layer.",1707.08651v1
2017-08-03,Interaction of ultracold non-ideal ion-electron plasma with a uniform magnetic field,"The method of molecular dynamics is used to study behavior of a ultracold
non-ideal ion-electron Be plasma in a uniform magnetic field. Our simulations
yield an estimate for the rate of electron-ion collisions which is
non-monotonicallydependent on the magnetic field magnitude. Also they
explicitly show that there are two types of diffusion: classical one,
corresponding to Brownian motion of particles, and Bohm diffusion when the
trajectory of particles (guiding centers) includes substantial lengths of drift
motion.",1708.00990v4
2017-08-21,Dynamo transition in a five-mode helical model,"We construct a five-mode helical dynamo model containing three velocity and
two magnetic modes and solve it analytically. This model exhibits dynamo
transition via supercritical pitchfork bifurcation. We show that the critical
magnetic Reynolds number for dynamo transition ($\mathrm{Rm}_c$) asymptotes to
constant values for very low and very high magnetic Prandtl numbers
($\mathrm{Pm}$). Beyond dynamo transition, secondary bifurcations lead to
periodic, quasi-periodic, and chaotic dynamo states as the forcing amplitude is
increased and chaos appears through a quasi-periodic route.",1708.06124v1
2017-10-19,Optically Pumped Magnetometry in Arbitrarily Oriented Magnetic Fields,"Optically pumped atomic magnetometers (OPMs) offer highly sensitive magnetic
measurements using compact hardware, offering new possibilities for practical
precision sensors. Double-resonance OPM operation is well suited to unshielded
magnetometry, due to high sensor dynamic range. However, sensor response is
highly anisotropic with variation in the orientation of the magnetic field. We
present data quantifying these effects and discuss implications for the design
of practical sensors.",1710.07024v1
2017-10-24,Equilibrium properties of magnetic filament suspensions,"Langevin dynamics is used to study equilibrium properties of the suspension
of magnetic filaments (chains of nanoparticles permanently crosslinked with
polymers). It is shown that the filament suspension generally has a larger
magnetic susceptibility than the system of unlinked nanoparticles with the same
average particle concentration. However, actual susceptibility gain strongly
depends on length and flexibility of filaments. It is also shown that in a
strong gravitational (centrifugal) field sedimentation profiles of filaments
are less homogeneous than that of unlinked particles. The spatial distribution
of filaments weakly depends on the intensity of interparticle dipole-dipole
interactions.",1710.08956v2
2017-11-17,Electron heating in the laser and static electric and magnetic fields,"A 2D slab approximation of the interactions of electrons with intense
linearly polarized laser radiation and static electric and magnetic fields is
widely used for both numerical simulations and simplified semi-analytical
models. It is shown that in this case electron dynamics can be conveniently
described in the framework of 3/2 dimensional Hamiltonian approach. The
electron acceleration beyond a standard ponderomotive scaling, caused by the
synergistic effects of the laser and static electro-magnetic fields, is due to
an onset of stochastic electron motion.",1711.06714v4
2018-02-02,Nonequilibrium Kondo-vs-RKKY Scenarios in Nanoclusters,"Ultrafast manipulations of magnetic phases are eliciting increasing attention
from the scientific community, because potentially relevant to the
understanding of nonequilibrium phase transitions and to novel technologies.
Here, we focus on manipulations applied to magnetic impurities in metallic
hosts. By considering small nanoring geometries, we show how currents can
induce a dynamical switching between different types of exchange interactions
in these systems. Our work thus opens a study window on nonequilibrium
Doniach's magnetic phase diagrams, and time-dependent Kondo-vs-RKKY scenarios.",1802.00838v2
2018-02-19,Algebraic non-integrability of magnetic billiards on the Sphere and Hyperbolic plane,"We consider billiard ball motion in a convex domain on a constant curvature
surface influenced by the constant magnetic field. We examine the existence of
integral of motion which is polynomial in velocities. We prove that if such an
integral exists then the boundary curve of the domain determines an algebraic
curve in $\mathbf{C}^3$ which must be nonsingular. Using this fact we deduce
that for any domain different from round disc for all but finitely many values
of the magnitude of the magnetic field billiard motion does not have Polynomial
in velocities integral of motion.",1802.06582v1
2018-04-27,Angular Dependent Magnetization Dynamics with Mirror-symmetric Excitations in Artificial Quasicrystalline Nanomagnet Lattices,"We report angle-dependent spin-wave spectroscopy on aperiodic
quasicrystalline magnetic lattices, i.e., Ammann, Penrose P2 and P3 lattices
made of large arrays of interconnected Ni$_{80}$Fe$_{20}$ nanobars. Spin-wave
spectra obtained in the nearly saturated state contain distinct sets of
resonances with characteristic angular dependencies for applied in-plane
magnetic fields. Micromagnetic simulations allow us to attribute detected
resonances to mode profiles with specific mirror symmetries. Spectra in the
reversal regime show systematic emergence and disappearance of spin wave modes
indicating reprogrammable magnonic characteristics.",1804.10630v1
2018-07-18,Concentration phenomena for a fractional Choquard equation with magnetic field,"We consider the following nonlinear fractional Choquard equation $$
\varepsilon^{2s}(-\Delta)^{s}_{A/\varepsilon} u + V(x)u =
\varepsilon^{\mu-N}\left(\frac{1}{|x|^{\mu}}*F(|u|^{2})\right)f(|u|^{2})u
\mbox{ in } \mathbb{R}^{N}, $$ where $\varepsilon>0$ is a parameter, $s\in (0,
1)$, $0<\mu<2s$, $N\geq 3$, $(-\Delta)^{s}_{A}$ is the fractional magnetic
Laplacian, $A:\mathbb{R}^{N}\rightarrow \mathbb{R}^{N}$ is a smooth magnetic
potential, $V:\mathbb{R}^{N}\rightarrow \mathbb{R}$ is a positive potential
with a local minimum and $f$ is a continuous nonlinearity with subcritical
growth. By using variational methods we prove the existence and concentration
of nontrivial solutions for $\varepsilon>0$ small enough.",1807.07442v1
2018-08-27,Hamiltonian Nature of Monopole Dynamics,"Classical electromagnetism with magnetic monopoles is not a Hamiltonian field
theory because the Jacobi identity for the Poisson bracket fails. The Jacobi
identity is recovered only if all of the species have the same ratio of
electric to magnetic charge or if an electron and a monopole can never collide.
Without the Jacobi identity, there are no local canonical coordinates or
Lagrangian action principle. To build a quantum theory of magnetic monopoles,
we either must explain why the positions of electrons and monopoles can never
coincide or we must resort to new quantization techniques.",1808.08689v3
2018-10-11,Observation of magnetic structural universality using transverse NMR relaxation,"Transverse NMR relaxation from spins diffusing through a random magnetic
medium is sensitive to its structure on a mesoscopic scale. In particular, this
results in the time-dependent relaxation rate. We show analytically and
numerically that this rate approaches the long-time limit in a power-law
fashion, with the exponent reflecting the disorder class of mesoscopic magnetic
structure. The spectral line shape acquires a corresponding non-analytic power
law singularity at zero frequency. We experimentally detect a change in the
dynamical exponent as a result of the transition into a maximally random jammed
state characterized by hyperuniform correlations.",1810.04847v1
2018-11-05,Surface acoustic wave coupled to magnetic resonance on a multiferroic CuB$_2$O$_4$,"We observed surface acoustic wave (SAW) propagation on a multiferroic
material CuB$_2$O$_4$ with use of two interdigital transducers (IDTs). The
period of IDT fingers is as short as 1.6 $\mu$m so that the frequency of SAW is
3 GHz, which is comparable with that of magnetic resonance. In
antiferromagnetic phase, the SAW excitation intensity varied with the magnitude
and direction of the magnetic field, owing to the dynamical coupling between
SAWs and antiferromagnetic resonance of CuB$_2$O$_4$. The microscopic mechanism
is discussed based on the symmetrically allowed magentoelastic coupling.",1811.01527v1
2018-11-29,Extended statistical analysis of emerging solar active regions,"We use observations of line-of-sight magnetograms from Helioseismic and
Magnetic Imager (HMI) on board of Solar Dynamics Observatory (SDO) to
investigate polarity separation, magnetic flux, flux emergence rate, twist and
tilt of solar emerging active regions. Functional dependence of polarity
separation and maximum magnetic flux of an active region is in agreement with a
simple model of flux emergence as the result of buoyancy forces. Our
investigation did not reveal any strong dependence of emergence rate on twist
properties of active regions.",1811.12089v1
2018-12-04,Polynomial integrals of magnetic geodesic flows on the 2-torus on several energy levels,"In this paper the geodesic flow on a 2-torus in a non-zero magnetic field is
considered. Suppose that this flow admits an additional first integral $F$ on
$N+2$ different energy levels which is polynomial in momenta of arbitrary
degree $N$ with analytic periodic coefficients. It is proved that in this case
the magnetic field and metrics are functions of one variable and there exists a
linear in momenta first integral on all energy levels.",1812.01290v1
2018-12-05,Magnetic control of polariton spin transport,"We show the full control of the polarization dynamics of a propagating
exciton-polariton condensate in a planar microcavity by using a magnetic field
applied in the Voigt geometry. The change of the spin-beat frequency, the
suppression of the optical spin Hall effect and the rotation of the
polarization pattern by the magnetic field are theoretically reproduced by
accounting for the magneto-induced mixing of exciton-polariton and dark, spin
forbidden, exciton states.",1812.01919v1
2019-01-04,Hydrodynamics of three-dimensional skyrmions in frustrated magnets,"We study the nucleation and collective dynamics of Shankar skyrmions [R.
Shankar, Journal de Physique 38, 1405 (1977)] in the class of frustrated
magnetic systems described by an SO(3) order parameter, including multi-lattice
antiferromagnets and amorphous magnets. We infer the expression for the
spin-transfer torque that injects skyrmion charge into the system and the
Onsager-reciprocal pumping force that enables its detection by electrical
means. The thermally-assisted flow of topological charge gives rise to an
algebraically decaying drag signal in nonlocal transport measurements. We
contrast our findings to analogous effects mediated by spin supercurrents.",1901.01208v1
2019-01-10,An upper limit on primordial magnetic fields from ultra-faint dwarf galaxies,"The presence of primordial magnetic fields increases the minimum halo mass in
which star formation is possible at high redshifts. Estimates of the dynamical
mass of ultra-faint dwarf galaxies (UFDs) within their half-light radius
constrain their virialized halo mass before their infall into the Milky Way.
The inferred halo mass and formation redshift of the UFDs place upper bounds on
the primordial comoving magnetic field, $B_0$. We derive an upper limit of
$0.50\pm 0.086$ ($0.31\pm 0.04$) nG on $B_0~$ assuming the average formation
redshift of the UFD host halos is $z_{\rm form}=$ 10 (20), respectively.",1901.03341v2
2019-01-28,Hölder Stable Recovery of Time-Dependent Electromagnetic Potentials Appearing in a Dynamical Anisotropic Schrödinger Equation,"We consider the inverse problem of H\""oldder-stably determining the time- and
space-dependent coefficients of the Schr\""odinger equation on a simple
Riemannian manifold with boundary of dimension $n\geq2$ from knowledge of the
Dirichlet-to-Neumann map. Assuming the divergence of the magnetic potential is
known, we show that the electric and magnetic potentials can be H\""older-stably
recovered from these data. Here we also remove the smallness assumption for the
solenoidal part of the magnetic potential present in previous results.",1901.09728v1
2019-03-21,Magnetic Skyrmions in Atomic Thin CrI$_3$ Monolayer,"In this letter, we report the visualization of topologically protected spin
textures, in the form of magnetic skyrmions, in recently discovered
monoatomic-thin two-dimensional CrI$_3$. By combining density functional theory
and atomistic spin dynamic simulation, we demonstrate that an application of
out-of-plane electric field to CrI$_3$ lattice favors the formation of sub-10
nm skyrmions at 0 K temperature. The spin texture arises due to a strong
correlation between magneto-crystalline anisotropy, Dzyaloshinskii-Moriya
interaction and the vertical electric field, whose shape and size could be
tuned with the magnetic field. Such finding will open new avenues for
atomic-scale quantum engineering and precision sensing.",1903.08797v1
2019-04-25,Scale-invariant Helical Magnetic Fields from Inflation,"We discuss a model which can generate scale-invariant helical magnetic fields
on large scales ($\lesssim 1$Mpc) in the primordial universe. It is also shown
that the electric conductivity becomes significant and terminates
magnetogenesis even before reheating is completed. By solving the
electromagnetic dynamics taking conductivity into account, we find that
magnetic fields with amplitude $B\simeq 10^{-15}{\rm G}$ at present can be
generated without encountering a backreaction or strong coupling problem.",1904.11428v1
2019-06-17,Electronic structure and Magneto-transport in MoS$_2$/Phosphorene van der Waals heterostructure,"The time-dependent spin current mediated spin transfer torque behaviour has
been investigated via scattering formalism within density functional theory
framework supported by Green's function. Quantum magnetotransport
characteristics have been revealed in a model semiconducting
MoS$_2$/phosphorene van der Waals heterostructure. The dynamics of spin current
channelized heterolayer transport has been studied with rotational variation in
magnetization angle. It is observed that the time-dependent spin transport
torque remains invariant irrespective of magnetization angle direction. The
polarized spin-current is persistent with the external magnetic field for
potential applicability towards spintronics.",1906.09187v1
2019-06-22,Effects of a quadrupolar magnetic term in a Generalized Störmer problem,"A new generalized St\""ormer problem is proposed. The charged particles motion
around a rotating axisymmetric magnetic planet is studied using various
conditions mainly in planetary magnetospheres. It is shown that the existence
of specific trajectories is due to the quadrupolar and dipolar magnetic terms.
Moreover, the equilibrium and stability behaviors and the corresponding orbital
frequencies for various kinds of charged dust grains are treated giving rise to
a new model describing the dynamical system and leading to more interesting
results.",1906.09481v1
2019-07-01,Determination of energy flux rate in homogeneous ferrohydrodynamic turbulence using two-point statistics,"In ferrofluids the suspended ferromagnetic particles agglomerate due to the
interaction between the particle magnetic moment and the external magnetic
field, which in turn, influences the turbulence and relaxation time. The
relaxation time becomes large when we are considering turbulence drag force
instead of viscous drag force in Brownian motion. We investigate that the total
energy conservation in ferrofluids taking into interaction between the external
magnetic field and the ferrofluid.",1907.00604v1
2019-10-09,Scattering of exchange spin waves from regions of modulated magnetization,"We investigate the reflection coefficient of spin waves propagating in an
ultra-thin ferromagnetic film with regions where saturation magnetization is
modulated. We find analytically and using micromagnetic simulations that there
are transmission resonances that depend on the width of the regions and on the
energy of excitation. Our results resemble the quantum mechanical
Ramsauer-Townsend effect in which an electron with certain energies can
propagate above a potential field without scattering. Our findings are useful
for reconfigurable magnonic devices where the saturation magnetization can be
dynamically controlled via a thermal landscape.",1910.04303v1
2019-10-28,Taylor state merging at SSX: experiment and simulation,"We describe experiments and simulations of dynamical merging with two Taylor
state plasmas in the SSX device. Taylor states are formed by magnetized plasma
guns at opposite ends of the device. We have performed experiments with Taylor
states of either sense of magnetic helicity (right-handed twist or left-handed
twist). We present results of both counter-helicity merging (one side
left-handed, the other right-handed) and co-helicity merging (both sides
left-handed). Experiments show significant ion heating, consistent with
magnetic reconnection. Magnetohydrodynamic simulations of these experiments
reveal the structure of the final relaxed, merged state.",1910.13861v1
2019-11-07,Optimal Actuation of Flagellar Magnetic Micro-Swimmers,"We present an automated procedure for the design of optimal actuation for
flagellar magnetic microswimmers based on numerical optimization. Using this
method, a new magnetic actuation method is provided which allows these devices
to swim significantly faster compared to the usual sinusoidal actuation. This
leads to a novel swimming strategy which makes the swimmer perform a 3D
figure-8 trajectory. This shows that a faster propulsion is obtained when the
swimmer is allowed to go out-of-plane. This approach is experimentally
validated on a scaled-up flexible swimmer.",1911.02805v1
2019-11-07,On the Complexity of a Charged Quantum Oscillator,"In this paper, we study the effect of both the electric and the magnetic
fields on the rate of complexity growth. Our system is a charged quantum
oscillator and over a period of time, we study the maximum dynamic evolution of
quantum states which might lead to a strong bound on the rate of computation.
We show that by turning on the electric field, the rate of complexity decreases
whereas this rate has increasing behavior when the magnetic field is turned on.
In this regard, we also find a critical value of the magnetic field, beyond
which this rate changes its behavior drastically.",1911.08886v2
2019-12-19,On the effect of time-dependent inhomogeneous magnetic fields on the particle momentum spectrum in electron-positron pair production,"Electron-positron pair production in spatially and temporally inhomogeneous
electric and magnetic fields is studied within the Dirac-Heisenberg-Wigner
formalism (quantum kinetic theory) through computing the corresponding Wigner
functions. The focus is on discussing the particle momentum spectrum regarding
signatures of Schwinger and multiphoton pair production. Special emphasis is
put on studying the impact of a strong dynamical magnetic field on the particle
distribution functions. As the equal-time Wigner approach is formulated in
terms of partial integro-differential equations an entire section of the
manuscript is dedicated to present numerical solution techniques applicable to
Wigner function approaches in general.",1912.09359v1
2020-02-21,Rayleigh-Bénard Convection in Strong Vertical Magnetic Field: Flow Structure and Verification of Numerical Method,"Direct numerical simulations are performed to study turbulent
Rayleigh-B\'{e}nard convection in a vertical cylindrical cavity with uniform
axial magnetic field. Flows at high Hartmann and Rayleigh numbers are
considered. The calculations reveal that, similarly to the behavior observed in
Rayleigh-B\'{e}nard convection with strong rotation, flows at strong magnetic
field develop a central vortex, while the heat transfer is suppressed.",2002.09404v2
2020-03-02,Chirality transfer & chiral turbulence in gauge theories,"Chirality transfer between fermions and gauge fields plays a crucial role for
understanding the dynamics of anomalous transport phenomena such as the Chiral
Magnetic Effect. In this proceeding we present a first principles study of
these processes based on classical-statistical real-time lattice simulations of
strongly coupled QED $(e^2N_f=64)$. Our simulations demonstrate that a
chirality imbalance in the fermion sector triggers chiral plasma instabilities
in the gauge field sector, which ultimately lead to the generation of long
range helical magnetic fields via a self-similar turbulent cascade of the
magnetic helicity.",2003.00763v1
2020-04-13,Reduction of divisors for classical superintegrable $GL(3)$ magnetic chain,"Variables of separation for classical $GL(3)$ magnetic chain obtained by
Sklyanin form a generic positive divisor $D$ of degree $n$ on a genus $g$
non-hyperelliptic algebraic curve. Because $n>g$ this divisor $D$ has unique
representative $\rho(D)$ in the Jacobian which can be constructed by using
dim$|D|=n-g$ steps of Abel's algorithm. We study properties of the
corresponding chain of divisors and prove that the classical $GL(3)$ magnetic
chain is a superintegrable system with dim$|D|=2$ superintegrable Hamiltonians.",2004.05775v2
2020-04-20,Decoherence measurements in crystals of molecular magnets,"Decoherence processes in crystals of molecular magnets are prototypical for
interacting electronic spin systems. We analyze the Landau-Zener dynamics of
the archetypical TbPc$_2$ complex diluted in a diamagnetic monocrystal. The
dependence of the tunneling probability on the field sweep rate is evaluated in
the framework of the recently proposed master equation in which the decoherence
processes are described through a phenomenological Lindblad operator. Thus, we
showcase low temperature magnetic measurements that complement resonant
techniques in determining small tunnel splittings and dephasing times.",2004.09011v1
2020-05-30,A phenomenological model for the spontaneous exchange bias effect,"In this work we propose an alternative model to explain the spontaneous
exchange bias (SEB) effect observed in spin glass (SG)-like systems. As in a
previously proposed model (Ref. 1), it is based on the unconventional dynamics
of the SG-like moments at the magnetic hysteresis cycle. However, using a
reliable estimate of the amount of SG-spins that are relaxing during the cycle,
the new model can correctly describe the changes in the SEB observed for
measurements performed at different temperatures and different maximum applied
fields.",2006.00230v1
2020-05-29,Investigation of spin-$0$ massive charged particle subject to a homogeneous magnetic field with potentials in a topologically trivial flat class of Gödel-type space-time,"In this paper, we investigate relativistic quantum dynamics of spin- $0$
massive charged particle subject to a homogeneous magnetic field in the
G\""{o}del-type space-time with potentials. We solve the Klein-Gordon equation
subject to a homogeneous magnetic field in a topologically trivial flat class
of G\""{o}del-type space-time in the presence of a Cornell-type scalar and
Coulomb-type vector potentials and analyze the effects on the energy
eigenvalues and eigenfunctions.",2006.00907v1
2020-06-15,Low MHD turbulence: the role of boundaries,"In this short review, we present the main known features of MHD Turbulence at
Low Magnetic Reynolds number, for which the flow isn't intense nor electrically
conductive enough to disturb an externally applied magnetic field. The emphasis
is deliberately placed on the very specific physical mechanisms of these flows,
rather than their numerical modelling. We also focus on homogeneous magnetic
fields which have received most attention. Since the basic properties of these
flows have been thoroughly reviewed a number of times, this review is
deliberately biased towards flows in bounded domains, in which the tendency to
two-dimensionality observed in MHD flows casts the boundaries of the domain
into a leading role.",2006.08187v1
2020-08-25,Friction Force in Strongly Magnetized Plasmas,"A charged particle moving through a plasma experiences a friction force that
commonly acts antiparallel to its velocity. It was recently predicted that in
strongly magnetized plasmas, in which the plasma particle gyro-frequency
exceeds the plasma frequency, the friction also includes a transverse component
that is perpendicular to both the velocity and Lorentz force. Here, this
prediction is confirmed using molecular dynamics simulations, and it is shown
that the relative magnitude of the transverse component increases with plasma
coupling strength. This result influences single particle motion and
macroscopic transport in strongly magnetized plasmas found in a broad range of
applications.",2008.11126v1
2020-09-07,Anomalous In-plane Magnetic Anisotropy in Strain-mediated Converse Magnetoelectric Coupling,"Magnetic axis rotation (MAR) in ferromagnetic (FM) layers is crucial for
strain-mediated converse magnetoelectric coupling. Employing the density
functional theory (DFT), we computationally study the magnetic anisotropy of
selected deformed FM materials such as body-centered iron. The results show
that the short axis is more energy-favorable at high in-plane strain difference
than previously predicted phenomenologically. This anomalous trend and the
complex energy behaviors at different strain conditions explain why
spin-lattice dynamics (SLD) simulation does not produce in-plane MAR and imply
couplings between different energy terms together with high order coefficient
contributions.",2009.03045v2
2020-09-07,Aharonov-Bohm effect on spin-$0$ scalar massive charged particle with a uniform magnetic field in Som-Raychaudhuri space-time with a cosmic string,"We study the relativistic quantum dynamics of spin-$0$ massive charged
particle in a G\""{o}del-type space-time with electromagnetic interactions. We
solve the Klein-Gordon equation subject to a uniform magnetic field in the
Som-Raychaudhuri space-time with a cosmic string. In addition, we include a
magnetic quantum flux into the relativistic quantum system obtain the energy
eigenvalues and analyze an analogue of the Aharonov-Bohm (AB) effect.",2009.07080v2
2020-11-11,Reduction of back switching by large damping ferromagnetic material,"Recent studies on magnetization dynamics induced by spin-orbit torque have
revealed a weak dependence of the critical current for magnetization switching
on the damping constant of a ferromagnetic free layer. This study, however,
reveals that the damping constant nevertheless plays a key role in
magnetization switching induced by spin-orbit torque. An undesirable switching,
returning to an initial state, named as back switching, occurs in a ferromagnet
with an easy axis parallel to the current direction. Numerical and theoretical
analyses reveal that back switching is strongly suppressed when the damping
constant of the ferromagnet is large.",2011.05566v1
2021-04-11,Orbital dynamics in 2D topological and Chern insulators,"Within a relativistic quantum formalism we examine the role of second-order
corrections caused by the application of magnetic fields in two-dimensional
topological and Chern insulators. This allows to reach analytical expressions
for the change of the Berry curvature, orbital magnetic moment, density of
states and energy determining their canonical grand potential and transport
properties. The present corrections, which become relevant at relatively low
fields due to the small gap characterizing these systems, unveil a zero-field
diamagnetic susceptibility which can be tuned by the external magnetic field.",2104.05126v1
2021-04-30,Micromagnetic modeling of magnon coherent states in a nonuniform magnetic field,"The study of the dynamics of magnetically ordered states in strong excitation
through micromagnetic modeling has become relevant due to the observation of
magnon Bose condensation. In particular, the question has arisen about the
possibility of describing the coherent quantum state by the quasi-classical
Landau-Lifshitz-Gilbert equations. We performed micromagnetic simulations of
magnetization precession with a high angle of deviation in an out-of-plane
nonuniform dc field. Our results confirm the formation of coherent magnon state
under conditions of high excitation. This coherent state extends over long
distances and described by a spatially inhomogeneous amplitude and a
homogeneous precession phase.",2104.14804v1
2021-06-03,Magnon spin transport around the compensation magnetic field in easy-plane antiferromagnetic insulators,"In this work, we theoretically study the magnon spin transport in easy-plane
antiferromagnetic insulators in the presence of an in-plane magnetic field. By
exactly calculating the magnon spectrum, we find the band splitting due to the
magnetic anisotropy can be fully compensated by the external field at a
particular strength, which makes its dynamics nearly equivalent to an easy-axis
antiferromagnet. As a result, the intrinsic magnon spin Hall effect due to the
dipole-dipole interaction, previously predicted in easy-axis antiferromagnets
is activated in easy-plane antiferromagnets. The compensation feature also
allows the field control of magnon spin lifetime and hence the spin diffusion
length. The compensation feature is robust against the biaxial anisotropy.",2106.02178v1
2021-07-14,Transitionless quantum driving in spin echo,"Spin echo can be used to refocus random dynamical phases caused by
inhomogeneities in control fields and thereby retain the purity of a spatial
distribution of quantum spins. This technique for accurate spin control is an
essential ingredient in many applications, such as nuclear magnetic resonance,
magnetic resonance imaging, and quantum information processing. Here, we show
how all the elements of a spin echo sequence can be performed at high speed by
means of transitionsless quantum driving. This technique promises accurate
control of rapid quantum spin evolution. We apply the scheme to universal
nonadiabatic geometric single- and two-qubit gates in a nuclear magnetic
resonance setting.",2107.06598v2
2021-11-30,Global weak solutions for the Landau-Lifshitz-Gilbert-Vlasov-Maxwell system coupled via emergent electromagnetic fields,"Motivated by recent models of current driven magnetization dynamics, we
examine the coupling of the Landau-Lifshitz-Gilbert equation and classical
electron transport governed by the Vlasov-Maxwell system. The interaction is
based on space-time gyro-coupling in the form of emergent electromagnetic
fields of quantized helicity that add up to the conventional Maxwell fields. We
construct global weak solutions of the coupled system in the framework of
frustrated magnets with competing first and second order gradient interactions
known to host topological solitons such as magnetic skyrmions and hopfions.",2111.15482v1
2021-12-20,Thermal Coherence of Heisenberg Model With Dipole Interaction and Magnetic External Field,"The dynamics of coherence in a two-qubit Heisenberg XXX model with magnetic
field in the presence of the intrinsic decoherence is investigated. We discuss
the influence of dipole parameter, spin distance, magnetic field and initial
state parameter on coherence of the long time limit situation. Then, we discuss
how decoherence changes with time with the initial state parameter and the
decoherence parameter.",2112.10341v1
2021-12-30,Estimations and scaling laws for stellar magnetic fields,"In rapidly rotating turbulence (Rossby number much less than unity), the
standard mixing length theory for turbulent convection breaks and Coriolis
force enters the force balance such that magnetic field eventually depends on
rotation. By simplifying the self-sustained magnetohydrodynamics dynamo
equations of electrically conducting fluid motion, with the aid of theory of
isotropic non-rotating or anisotropic rotating turbulence driven by thermal
convection, we make estimations and derive scaling laws for stellar magnetic
fields with slow and fast rotation. Our scaling laws are in good agreement with
the observations.",2112.15103v3
2022-03-10,Study of the Kramers-Fokker-Planck quadratic operator with a constant magnetic field,"We study the quadratic Kramers-Fokker-Planck operator with a constant
magnetic field and with a quadratic potential. We describe the exact expression
of the norm of the semi-group associated to the operator near the equilibrium.
At this level, explicit and accurate estimates of this norm are shown in small
and long times as well as uniform-in-time estimates when the magnetic parameter
$b$ tends to infinity.",2203.05273v1
2022-05-28,Magnetoclinicity Instability,"In strongly compressible magnetohydrodynamic turbulence, obliqueness between
the large-scale density gradient and magnetic field gives an electromotive
force mediated by density variance (intensity of density fluctuation). This
effect is named ``magnetoclinicity'', and is expected to play an important role
in large-scale magnetic-field generation in astrophysical compressible
turbulent flows. Analysis of large-scale instability due to the
magnetoclinicity effect shows that the mean magnetic-field perturbation is
destabilised at large scales in the vicinity of strong mean density gradient in
the presence of density variance.",2205.14453v1
2022-06-15,Twisted light affects ultrafast demagnetization,"Irradiation with an ultrashort laser pulse can completely destroy the
magnetic order of ferromagnetic thin films on the femtosecond timescale. This
phenomenon holds great potential for ultrafast spintronics and information
processing and is an active field of research. It is still an open question if
the angular momentum of light can support this effect. While it has been shown
that the spin of light only has a negligible influence, we experimentally
demonstrate the influence of ultrashort laser pulses with orbital angular
momentum (OAM) on the magnetization dynamics of a thin nickel film. Our results
reveal that the photonic OAM affects the demagnetization behavior within the
first hundreds of femtoseconds depending on the handedness of the OAM with
respect to the direction of the sample magnetization.",2206.07502v1
2022-06-28,Meaning of the splitting process for the transition to self-sustained turbulence in a magnetized cylindrical plasma,"When turbulent structures split more frequently before they decay, persistent
turbulence forms in neutral fluid shear flows. Whether this concept can be
extended to linear magnetized plasmas is investigated here and compared to the
behavior of the pipe flow. With increasing control parameter the dynamics in
the magnetized plasmas is known to undergo several changes from a quasiperiodic
to a phase locked to a weakly turbulent regime. When the phase-locked regime
breaks down, the splitting time approaches the decreasing lifetime reflecting
self-sustained turbulence, as known from the pipe flow.",2206.14038v1
2022-07-26,Rectification of the spin Seebeck current in noncollinear antiferromagnets,"In the absence of an external magnetic field and a spin-polarized charge
current, an antiferromagnetic system supports two degenerate magnon modes. An
applied thermal bias activates the magnetic dynamics, leading to a magnon flow
from the hot to the cold edge (magnonic spin Seebeck current). Both degenerate
bands contribute to the magnon current but the orientations of the magnetic
moments underlying the magnons are opposite in different bands. Therefore,
while the magnon current is nonzero, the net spin current is zero.",2207.12890v1
2022-10-05,Nonlinear effective dynamics of Brownian particle in magnetized plasma,"An effective description is presented for a Brownian particle in a magnetized
plasma. In order to systematically capture various corrections to linear
Langevin equation, we construct effective action for the Brownian particle, to
quartic order in its position. The effective action is first derived within
non-equilibrium effective field theory formalism, and then confirmed via a
microscopic holographic model consisting of an open string probing magnetic
AdS$_5$ black brane. For practical usage, the non-Gaussian effective action is
converted into Fokker-Planck type equation, which is an Euclidean analog of
Schr$\ddot{\rm o}$dinger equation and describes time evolution of probability
distribution for particle's position and velocity.",2210.02274v1
2022-10-28,Magnetorheological Axisymmetric Actuator with Permanent Magnet,"This study examines the concept of axisymmetric actuator based on the
magnetorheological membrane, electromagnet and permanent magnet. The
construction of the actuator enables its application in wide range of practical
devices like pumping, loudspeaker or varying-stiffness button. This work will
highlight its working principle especially the influence of permanent magnet.
Furthermore, the model of devices will be defined relaying on the Hammerstein
model. To show the properties of the actuator and to perform the model
identification, the set of experiments was run taking into account static and
dynamic working conditions.",2210.15968v1
2022-12-02,Energy pathways in large- and small-scale convection-driven dynamos,"We investigate the energy pathways between the velocity and the magnetic
fields in a rotating plane layer dynamo driven by Rayleigh-B\'enard convection
using direct numerical simulations. The kinetic and magnetic energies are
divided into mean and turbulent components to study the production, transport,
and dissipation associated with large and small-scale dynamos. This energy
balance-based characterization reveals distinct mechanisms for large- and
small-scale magnetic field generation in dynamos, depending on the nature of
the velocity field and the conditions imposed at the boundaries.",2212.00969v2
2023-02-14,Particle and guiding-center orbits in crossed electric and magnetic fields,"The problem of the charged-particle motion in crossed electric and magnetic
fields is investigated, and the validity of the guiding-center representation
is assessed in comparison with the exact particle dynamics. While the magnetic
field is considered to be straight and uniform, the (perpendicular) radial
electric field is nonuniform. The Hamiltonian guiding-center theory of
charged-particle motion is presented for arbitrary radial electric fields, and
explicit examples are provided for the case of a linear radial electric field.",2302.07119v2
2023-03-06,On the topology of the magnetic lines of solutions of the MHD equations,"We construct examples of smooth periodic solutions to the Magnetohydrodynamic
equations in dimension 2 with positive resistivity for which the topology of
the magnetic lines changes under the flow. By Alfv\'en's theorem this is known
to be impossible in the ideal case (resistivity = 0). In the resistive case the
reconnection of the magnetic lines is known to occur and has deep physical
implications, being responsible for many dynamic phenomena in astrophysics. The
construction is a simplified proof of [3] and in addition we consider the case
of the forced system.",2303.02999v1
2023-03-07,Four-thirds law of energy and magnetic helicity in electron and Hall magnetohydrodynamic fluids,"In this paper, by exploiting the feature of the Hall term, we establish some
local version four-thirds laws for the dissipation rates of energy and magnetic
helicity in both electron and Hall magnetohydrodynamic equations in the sense
of Duchon-Robert type. New 4/3 laws for the dissipation rates of magnetic
helicity in these systems are first observed and four-thirds law involving the
dissipation rates of energy for the Hall magnetohydrodynamic equations
generalizes the work of Galtier.",2303.04290v1
2023-04-19,Regions without flux surfaces of given class for magnetic fields in toroidal geometry,"A Converse KAM method for 3D vector fields, establishing regions through
which pass no invariant 2-tori transverse to a given direction field, is tested
on some helical perturbations of an axisymmetric magnetic field in toroidal
geometry. It finds regions corresponding to magnetic islands and chaos for the
fieldline flow. Minimization of these regions is proposed as a tool to help in
the design of plasma confinement devices of tokamak and stellarator type.",2304.09613v3
2023-07-10,Coexistence of self-similar and anomalous scalings in turbulent small-scale solar magnetic fields,"We report an evidence that self-similar and anomalous scalings coexist in a
turbulent medium, particularly in fluctuations of the magnetic field flux
density in magnetized plasma of the solar photosphere. The structure function
scaling exponents in the inertial range have been analyzed for fluctuations
grouped according to the sign of the path-dependent stochastic entropy
production. It is found that the scaling exponents for fluctuations with the
positive entropy production follow the phenological linear dependence for the
magnetohydrodynamic turbulence. For fluctuations with the negative entropy
production, the scaling is anomalous.",2307.04703v1
2023-07-13,Chiral magnetovortical instability,"We demonstrate that in a chiral plasma subject to an external magnetic field,
the chiral vortical effect can induce a new type of magnetohydrodynamic
instability which we refer to as the {\it chiral magnetovortical instability}.
This instability arises from the mutual evolution of the magnetic and vortical
fields. It can cause a rapid amplification of the magnetic fields by
transferring the chirality of the constituent particles to the cross helicity
of the plasma.",2307.06746v1
2023-07-18,"Propagation of Coupled Acoustic, Electromagnetic and Spin Waves in Saturated Ferromagnetoelastic Solids","We study the propagation of plane waves in an unbounded body of a saturated
ferromagnetoelastic solid. The equations by Tiersten for small fields
superposed on finite initial fields in a saturated ferromagnetoelastic material
are employed, with their quasistatic magnetic field extended to dynamic
electric and magnetic fields for electromagnetic waves. Dispersion relations of
the plane waves are obtained. The cutoff frequencies and long wave
approximation of the dispersion curves are determined. Results show that
acoustic, electromagnetic and magnetic spin waves are coupled in such a
material. For YIG which is a cubic crystal without piezoelectric coupling, the
acoustic and electromagnetic waves are not directly coupled but they can still
interact indirectly through spin waves.",2307.09171v1
2023-07-27,Newton's Off-Center Circular Orbits and the Magnetic Monopole,"Introducing a radially dependent magnetic field into Newton's off-center
circular orbits potential so as to preserve the $E=0$ dynamical symmetry leads
to a unique choice of field that can be identified as the inclusion of a
magnetic monopole in the inverse stereographically projected problem. One finds
also a phenomenological correspondence with that of the linearly damped Kepler
model. The presence of the monopole field deforms the symmetry algebra by a
central extension, and the quantum mechanical version of this algebra reveals a
number of zero modes equal to that counted using the index theorem of elliptic
operators.",2307.15222v2
2023-09-01,Multichannel quantum defect theory with a frame transformation for ultracold molecular collisions in magnetic fields,"We extend the powerful formalism of multichannel quantum defect theory
combined with a frame transformation (MQDT-FT) to ultracold molecular
collisions in magnetic fields. By solving the coupled-channel equations with
hyperfine and Zeeman interactions omitted at short range, MQDT-FT enables a
drastically simplified description of the intricate quantum dynamics of
ultracold molecular collisions in terms of a small number of short-range
parameters. We apply the formalism to ultracold Mg + NH collisions in a
magnetic field, achieving a 10$^4$-fold reduction in computational effort.",2309.00263v1
2023-09-18,Tuning domain wall oscillation frequency in bent nanowires through a mechanical analogy,"In this work, we present a theoretical model for domain wall (DW)
oscillations in a curved magnetic nanowire with a constant curvature under the
action of a uniaxial magnetic field. Our results show that the DW dynamics can
be described as that of the mechanical pendulum, and both the NW curvature and
the external magnetic field influence its oscillatory frequency. A comparison
between our theoretical approach and experimental data in the literature shows
an excellent agreement. The results presented here can be used to design
devices demanding the proper control of the DW oscillatory motion in NWs.",2309.10051v1
2023-10-22,Suppressing Rayleigh-Plateau Instability with a Magnetic Force Field for Deformable Interfaces Engineering,"The Rayleigh-Plateau instability (RPI) is a classical hydrodynamics
phenomenon that prevents a jet of liquid to flow indefinitely within air or
another liquid. Here, we show how adding a magnetic force field makes possible
its suppression. Enclosing the jet in a ferrofluid held by magnetic forces
allows flow focusing without sheath flow, which completely avoids dripping
failure at small flow rates and provides conditional stability for a continuous
fluid jet. Highly deformable liquid interfaces withstanding spatial and time
varying flow conditions within a large parameter space can be realized.",2310.14280v1
2023-10-25,Variational Gaussian approximation for the magnetic Schrödinger equation,"In the present paper we consider the semiclassical magnetic Schr\""odinger
equation, which describes the dynamics of particles under the influence of a
magnetic field. The solution of the time-dependent Schr\""odinger equation is
approximated by a single Gaussian wave packet via the time-dependent
Dirac--Frenkel variational principle. For the approximation we derive ordinary
differential equations of motion for the parameters of the variational
solution. Moreover, we prove $L^2$-error bounds and observable error bounds for
the approximating Gaussian wave packet.",2310.16734v1
2023-10-31,Annihilation of electroweak dumbbells,"We study the annihilation of electroweak dumbbells and the dependence of
their dynamics on initial dumbbell length and twist. Untwisted dumbbells decay
rapidly while maximally twisted dumbbells collapse to form a compact
sphaleron-like object, before decaying into radiation. The decay products of a
dumbbell include electromagnetic magnetic fields with energy that is a few
percent of the initial energy. The magnetic field from the decay of twisted
dumbbells carries magnetic helicity with magnitude that depends on the twist,
and handedness that depends on the decay pathway.",2311.00026v1
2023-12-03,Thermally Averaged Magnetic Anisotropy Tensors via Machine Learning Based on Gaussian Moments,"We propose a machine learning method to model molecular tensorial quantities,
namely the magnetic anisotropy tensor, based on the Gaussian-moment
neural-network approach. We demonstrate that the proposed methodology can
achieve an accuracy of 0.3--0.4 cm$^{-1}$ and has excellent generalization
capability for out-of-sample configurations. Moreover, in combination with
machine-learned interatomic potential energies based on Gaussian moments, our
approach can be applied to study the dynamic behavior of magnetic anisotropy
tensors and provide a unique insight into spin-phonon relaxation.",2312.01415v1
2023-12-11,A local study of dynamo action driven by precession,"We demonstrate an efficient magnetic dynamo due to precession-driven
hydrodynamic turbulence in the local model. Dynamo growth rate increases with
Poincar\'{e} and magnetic Prandtl numbers. Spectral analysis shows that the
dynamo acts over a broad range of scales: at large (system size) and
intermediate scales it is driven by 2D vortices and shear of the background
precessional flow, while at smaller scales it is mainly driven by 3D inertial
waves. These results are important for understanding magnetic field generation
and amplification in precessing planets and stars.",2312.06835v1
2023-12-14,Magnetic Taylor-Proudman constraint explains the flow into Tangent Cylinders,"Our understanding of planetary dynamos and liquid cores relies on the
existence of a Tangent Cylinder (TC). This imaginary surface that physically
separates polar and equatorial regions stems from Taylor-Proudman's Constraint
(TPC) due to planetary rotation only. Here we establish mathematically and
verify experimentally that a magnetic field aligned with the rotation incurs a
flow \emph{into} the TC, linked to the flow along it by a \emph{magnetic} TPC.
This constraint sheds new light on flows in planetary cores and flows subject
to both magnetic fields and rotation in general.",2312.08840v1
2023-12-25,Fermion-antifermion pairs in Bonnor-Melvin magnetic space-time with non-zero cosmological constant,"We study the relativistic dynamics of fermion-antifermion pairs in the
Bonnor-Melvin magnetic (BMM) spacetime in non-zero cosmology. We focus on the
$(1+2)$-dimensional cylindrically symmetric BMM-spacetime background. Within
the context of such a magnetized universe, we rigorously investigate the
fully-covariant two-body Dirac equation. We derive the corresponding radial
part of the equation and obtain an exact closed form analytical solution for
the problem at hand. Moreover, we report some intriguing parallels with
relativistic and non-relativistic quantum oscillators in flat spaces. Notably,
our findings suggest a compelling correlation between the cosmological constant
and the energy spectrum of fermion-antifermion systems, hinting at profound
connections between quantum realms and cosmology.",2401.02441v1
2024-01-16,Singular vortex pairs follow magnetic geodesics,"We consider pairs of point vortices having circulations $\Gamma_1$ and
$\Gamma_2$ and confined to a two-dimensional surface $S$. In the limit of zero
initial separation $\varepsilon$, we prove that they follow a magnetic geodesic
in unison, if properly renormalized. Specifically, the ``singular vortex pair""
moves as a single charged particle on the surface with a charge of order
$1/\varepsilon^2$ in a magnetic field $B$ which is everywhere normal to the
surface and of strength $|B|=\Gamma_1 +\Gamma_2$. In the case
$\Gamma_1=-\Gamma_2$, this gives another proof of Kimura's conjecture (Kimura
1999) that singular dipoles follow geodesics.",2401.08512v1
2024-01-30,Coherent and dissipative coupling in a magneto-mechanical system,"Hybrid elastic and spin waves hold promises for energy-efficient and
versatile generation and detection of magnetic signals, with potentially long
coherence times. Here we report on the combined elastic and magnetic dynamics
in a one-dimensional magneto-mechanical crystal composed of an array of
magnetic nanowires. Phononic and magnonic modes are impulsively excited by an
optical ultrafast trigger and their decay is monitored by time resolved Magneto
Optical Kerr Effect, with complementary Brillouin Light Scattering measurements
and micromagnetic simulations. The strength and degree of mixing of coherent
and dissipative coupling of the quasi-particles is determined quantitatively.",2401.16994v1
2024-02-01,Investigating the magnetism of Ni from a momentum space perspective,"For more than three decades, clear discrepancies have existed between spin
densities in momentum space revealed by Magnetic Compton scattering experiments
and theoretical calculations based on density functional theory (DFT). Here by
making a wide comparison between different theoretical methods, including DFT,
DFT combined with dynamical mean field theory, and Hedin's $GW$ approximation,
we discover how the magnetic Compton profiles of Ni can be predicted remarkably
well. We find that the essential ingredients missing in DFT are (i) local spin
fluctuations and (ii) a non-local treatment of electron correlations.",2402.00799v1
2024-03-04,Carroll Strings with an Extended Symmetry Algebra,"Starting from the Polyakov action we consider two distinct Carroll limits in
target space, keeping the string worldsheet relativistic. The resulting
magnetic and chiral Carroll string models exhibit different symmetries and
dynamics. Both models have an infinite dimensional symmetry algebra with
Carroll symmetry included in a finite dimensional subalgebra. For the magnetic
model, this is the so-called string Carroll algebra. The chiral model realises
an extended version of the string Carroll algebra. The magnetic model does not
have any transverse string excitations. The chiral model is less restrictive
and includes arbitrary left-moving modes that carry transverse momentum but do
not contribute to the energy in target space.",2403.01984v1
2024-03-04,All-optical control of skyrmion configuration in CrI$_3$ monolayer,"The potential for manipulating characteristics of skyrmions in a CrI$_3$
monolayer using circularly polarised light is explored. The effective
skyrmion-light interaction is mediated by bright excitons whose magnetization
is selectively influenced by the polarization of photons. The light-induced
skyrmion dynamics is illustrated by the dependencies of the skyrmion size and
the skyrmion lifetime on the intensity and polarization of the incident light
pulse. Two-dimensional magnets hosting excitons thus represent a promising
platform for the control of topological magnetic structures by light.",2403.02223v1
2024-03-18,Nonlinear Pulse Dynamics in a Magnetized and Nonextensive Quark-Gluon Plasma,"We investigate the propagation of nonlinear energy density waves in a
nonextensive quark-gluon plasma under the influence of a magnetic field using
the reductive perturbation technique. For a nonextensive MIT bag equation of
state, we obtain the governing equation for the first order perturbation of the
energy density. We observe that an increase in the strength of the magnetic
field results in the localization of the waves.",2403.11630v1
2024-03-28,Gyrokinetic limit of the 2D Hartree equation in a large magnetic field,"We study the dynamics of two-dimensional interacting fermions submitted to a
homogeneous transverse magnetic field. We consider a large magnetic field
regime, with the gap between Landau levels set to the same order as that of
potential energy contributions. Within the mean-field approximation, i.e.
starting from Hartree's equation for the first reduced density matrix, we
derive a drift equation for the particle density. We use vortex coherent states
and the associated Husimi function to define a semi-classical density almost
satisfying the limiting equation. We then deduce convergence of the density of
the true Hartree solution by a Dobrushin-type stability estimate for the
limiting equation.",2403.19226v1
2013-04-01,Two Dimensional Incommensurate and Three Dimensional Commensurate Magnetic Order and Fluctuations in $La_{2-x}Ba_{x}CuO_{4}$,"We present neutron scattering measurements on single crystals of lightly
doped $La_{2-x}Ba_{x}CuO_{4}$, with $0 \leq x \leq? 0.035$. These reveal the
evolution of the magnetism in this prototypical doped Mott insulator from a
three dimensional (3D) commensurate (C) antiferromagnetic ground state, which
orders at a relatively high TN, to a two dimensional (2D) incommensurate (IC)
ground state with finite ranged static correlations, which appear below a
relatively low effective TN. At low temperatures, the 2D IC magnetism co-exists
with the 3D C magnetism for doping concentrations as low as ? 0.0125. We find
no signal of a 3D C magnetic ground state by x $\sim$? 0.025, consistent with
the upper limit of x $\sim$? 0.02 observed in the sister family of doped Mott
insulators, $La_{2-x}Sr_{x}CuO_{4}$. The 2D IC ground states observed for
$0.0125 \leq x \leq 0.035$ are diagonal, and are rotated by 45 degrees within
the orthorhombic basal plane compared with those previously reported for
samples with superconducting ground states: $La_{2-x}Ba_{x}CuO_{4}$, with $0.05
\leq? x \leq? 0.095. We construct a phase diagram based solely on magnetic
order parameter measurements, which displays much of the complexity of standard
high temperature superconductivity phase diagrams discussed in the literature.
Analysis of high energy-resolution inelastic neutron scattering at moderately
low temperatures shows a progressive depletion of the very low energy dynamic
magnetic susceptibility as x increases from 0.0125 to 0.035. This low energy,
dynamic susceptibility falls off? with increasing temperature on a scale much
higher than the effective 2D IC TN appropriate to these materials. Appreciable
dynamic 2D IC magnetic fluctuations inhabit much of the ""pseudogap"" regime of
the phase diagram.",1304.0362v1
2013-04-02,Modeling Multi-Magnet Networks Interacting Via Spin Currents,"The significant experimental advances of the last few decades in dealing with
the interaction of spin currents and nanomagnets, at the device level, has
allowed envisioning a broad class of devices that propose to implement
information processing using spin currents and nanomagnets. To analyze such
spin-magnet logic circuits, in general, we have developed a coupled
spin-transport/ magnetization-dynamics simulation framework that could be
broadly applicable to various classes of spin-valve/ spin-torque devices.
Indeed, the primary purpose of this chapter is to describe in detail, the
overall approach we have developed to include a description of spin transport
coupled with magnetization dynamics and to show how it was benchmarked against
available data on experiments.
  We address non-collinear spin-transport in Section-2 using a lumped
""4-component spin-circuit formalism"" that describes the interaction of
non-collinear magnets (required for modeling spin torque), by computing
4-component currents and voltages at every node of a ""circuit"". For modeling
the magnetization dynamics, we use the standard Landau-Lifshitz-Gilbert (LLG)
equation with the Slonczewski and the field-like terms included for spin
torque. Section-3 describes how this LLG model is coupled with the spin
transport model to analyze spin-torque experiments and spin-magnet circuits in
general.
  We include MATLAB codes in the Appendix to facilitate a ""hands-on""
understanding of our model and hope it will enable interested readers to
conveniently analyze their own experiments, develop a deeper insight into
spin-magnet circuits or come up with their own creative designs.",1304.0742v3
2013-05-09,Flux emergence in a magnetized convection zone,"We study the influence of a dynamo magnetic field on the buoyant rise and
emergence of twisted magnetic flux-ropes, and their influence on the global
external magnetic field. We ran 3D MHD numerical simulations using the ASH code
and analysed the dynamical evolution of such buoyant flux-ropes from the bottom
of the convection zone until the post-emergence phases. The global nature of
this model represents very crudely and inaccurately the local dynamics of the
buoyant rise, but allows to study the influence of global effects such as
self-consistently generated differential rotation, meridional circulation and
Coriolis forces. Although motivated by the solar context, this model cannot be
thought of as a realistic model of the rise of magnetic structures and their
emergence in the Sun where the local dynamics are completely different. The
properties of initial phases of the buoyant rise in good agreement with
previous studies. However, the effects of the interaction of the background
dynamo field become increase as the flux-ropes evolve. During the buoyant rise
across the CZ, the flux-rope's magnetic field strength and scales as $rho^a$,
with $a\lesssim 1$. An increase of velocity, density and current precedes flux
emergence at all longitudes. The geometry, latitude and relative orientation of
the flux-ropes with respect to the background magnetic field influences the
rise speeds, zonal flow amplitudes (which develop within the flux-ropes) and
the corresponding surface signatures. This influences the morphology, duration
and amplitude of the associated surface shearing and Poynting flux. The emerged
flux influences the system's global polarity, leading in some cases to a
polarity reversal while inhibiting background dynamo from doing so in some
others. The emerged magnetic flux is slowly advected poleward, while being
diffused and assimilated by the background dynamo field.",1305.2159v1
2014-07-08,Effect of divalent Ba cation substitution with Sr on coupled multiglass state in the magnetoelectric multiferroic compound Ba3NbFe3Si2O14,"(Ba/Sr)3NbFe3Si2O14 is a magnetoelectric multiferroic with an incommensurate
antiferromagnetic spiral magnetic structure which induces electric polarization
at 26 K. The structure, as revealed by x-rays and neutrons, as well as static
and dynamic magnetic and dielectric properties of these compounds have been
studied down to 6 K under different conditions. Both the compounds have similar
crystal structure but with different lattice constants down to 6 K. The Ba-and
Sr-compounds exhibit a transition at 26 K and 25 K respectively, as indicated
by the specific heat capacity and dc specific magnetization, into an
antiferromagnetic state. Although Ba and Sr are isovalent, they exhibit very
different static and dynamic magnetic behavior. The Ba-compound exhibits both
thermal and magnetic field hysteresis with the thermal hysteresis decreasing
with increasing magnetic field, a behavior typical of glasses. The glassy
behavior is also clearly seen in the ac susceptibility studies which show a
dispersive peak in the range 40 K to 90 K in the frequency range 10^1 Hz to
10^4 Hz. The dispersive behavior follows a cluster glass critical slowing
dynamics with a freezing temperature of 35 K and a critical exponent of 3.9, a
value close to the 3-D Ising model. The Sr-compound however does not exhibit
any dispersive behavior except for the invariant transition at 25 K in ac
susceptibility with no magnetic field hysteresis at all temperatures. The
dielectric constant studied in the frequency range 10^1 Hz to 10^6 Hz also
reflects the magnetic behavior of the two compounds. The Ba-compound has two
distinct dispersive peaks near TN and in the range 40 K to 125 K while the
Sr-compound has a single dispersive peak in the range 40 K to 80 K. The
activation energy of the high temperature dispersive peak in both compounds
however is found to be similar, 71 meV and 65 meV respectively for Ba- and
Sr-compounds.",1407.2049v1
2018-10-09,Cluster spin-glass behaviour and memory effect in Cr$_{0.5}$Fe$_{0.5}$Ga,"We report the structural, static, and dynamic properties of
Cr$_{0.5}$Fe$_{0.5}$Ga by means of powder x-ray diffraction, magnetization,
heat capacity, magnetic relaxation, and magnetic memory effect measurements. DC
magnetization and AC susceptibility studies reveal a spin-glass transition at
around $T_{\rm f} \simeq 22$~K. An intermediate value of the relative shift in
freezing temperature $\delta T_{\rm f} \simeq 0.017$, obtained from the AC
susceptibility data reflects the formation of cluster spin-glass states. The
frequency dependence of $T_{\rm f}$ is also analyzed within the framework of
dynamic scaling laws. The analysis using power law yields a time constant for a
single spin flip $\tau* \simeq 1.1\times10^{-10}$~s and critical exponent
$z\nu^{\prime}=4.2\pm0.2$. On the other hand, the Vogel-Fulcher (VF) law yields
the time constant for a single spin flip $\tau_0 \simeq 6.6\times10^{-9}$~s, VF
temperature $T_{\rm 0}=21.1\pm0.1$~K, and an activation energy $E_{\rm
a}/k_{\rm B} \simeq 16$~K. The value of $\tau*$ and $\tau_0$ along with a
non-zero value of $T_{\rm 0}$ provide further evidence for the cluster
spin-glass behaviour. The magnetic field dependent $T_{\rm f}$ follows the de
Almeida-Thouless line with a non-mean-field type instability, reflecting either
a different universality class or strong anisotropy in the spin system. A
detailed non-equilibrium dynamics study via relaxation and memory effect
experiments demonstrates striking memory effects. All the above observations
render a cluster spin-glass behaviour which is triggered by magnetic
frustration due to competing antiferromagnetic and ferromagnetic interactions
and magnetic site disorder. Moreover, the asymmetric response of magnetic
relaxation with respect to the change in temperature, below $T_{\rm f}$ can be
explained by the hierarchical model.",1810.03890v1
2020-07-06,Magnetic field amplification in accretion discs around the first stars: implications for the primordial IMF,"Magnetic fields play an important role in the dynamics of present-day
molecular clouds. Recent work has shown that magnetic fields are equally
important for primordial clouds, which form the first stars in the Universe.
While the primordial magnetic field strength on cosmic scales is largely
unconstrained, theoretical models strongly suggest that a weak seed field
existed in the early Universe. We study how the amplification of such a weak
field can influence the evolution of accretion discs around first stars, and
thus affect the primordial initial mass function (IMF). We perform a suite of
3D ideal magneto-hydrodynamic (MHD) simulations with different initial field
strengths and numerical resolutions. We find that, in simulations with
sufficient spatial resolution to resolve the Jeans scale during the collapse,
even initially weak magnetic fields grow exponentially to become dynamically
important due to both the so-called 'small-scale turbulent dynamo' and the
'large-scale mean-field dynamo'. Capturing the small-scale dynamo action
depends primarily on how well we resolve the Jeans length, while capturing the
large-scale dynamo depends on the Jeans resolution as well as the maximum
absolute resolution. Provided enough resolution, we find that fragmentation
does not depend strongly on the initial field strength, because even weak
fields grow to become strong. However, fragmentation in runs with magnetic
fields differs significantly from those without magnetic fields. We conclude
that the development of dynamically strong magnetic fields during the formation
of the first stars is likely inevitable, and that these fields had a
significant impact on the primordial IMF.",2007.02678v2
2021-01-22,Spin dynamics in NaFeAs and NaFe$_{0.53}$Cu$_{0.47}$As probed by resonant inelastic X-ray scattering,"The parent compounds of iron-based superconductors are magnetically-ordered
bad metals, with superconductivity appearing near a putative magnetic quantum
critical point. The presence of both Hubbard repulsion and Hund's coupling
leads to rich physics in these multiorbital systems, and motivated descriptions
of magnetism in terms of itinerant electrons or localized spins. The
NaFe$_{1-x}$Cu$_x$As series consists of magnetically-ordered bad metal ($x=0$),
superconducting ($x\approx0.02$) and magnetically-ordered
semiconducing/insulating ($x\approx0.5$) phases, providing a platform to
investigate the connection between superconductivity, magnetism and electronic
correlations. Here we use X-ray absorption spectroscopy and resonant inelastic
X-ray scattering to study the valence state of Fe and spin dynamics in two
NaFe$_{1-x}$Cu$_x$As compounds ($x=0$ and 0.47). We find that magnetism in both
compounds arises from Fe$^{2+}$ atoms, and exhibits underdamped dispersive spin
waves in their respective ordered states. The dispersion of spin excitations in
NaFe$_{0.53}$Cu$_{0.47}$As is consistent with being quasi-one-dimensional.
Compared to NaFeAs, the band top of spin waves in NaFe$_{0.53}$Cu$_{0.47}$As is
slightly softened with significantly more spectral weight of the spin
excitations. Our results indicate the spin dynamics in
NaFe$_{0.53}$Cu$_{0.47}$As arise from localized magnetic moments and suggest
the iron-based superconductors are proximate to a correlated insulating state
with localized iron moments.",2101.08948v1
2022-11-07,Quantum-classical approach to spin and charge pumping and the ensuing radiation in THz spintronics: Example of ultrafast-light-driven Weyl antiferromagnet Mn$_3$Sn,"The interaction of fs light pulses with magnetic materials has been intensely
studied for more than two decades in order to understand ultrafast
demagnetization in single magnetic layers or THz emission from their bilayers
with nonmagnetic spin-orbit (SO) materials. Here we develop a multiscale
quantum-classical formalism -- where conduction electrons are described by
quantum master equation of the Lindblad type; classical dynamics of local
magnetization is described by the Landau-Lifshitz-Gilbert (LLG) equation; and
incoming light is described by classical vector potential while outgoing
electromagnetic radiation is computed using Jefimenko equations for retarded
electric and magnetic fields -- and apply it a bilayer of antiferromagnetic
Weyl semimetal Mn$_3$Sn with noncollinear local magnetization in contact with
SO-coupled nonmagnetic material. Our QME+LLG+Jefimenko scheme makes it possible
to understand how fs light pulse generates directly spin and charge pumping and
electromagnetic radiation by the latter, including both odd and even high
harmonics (of the pulse center frequency) up to order $n \le 7$. The directly
pumped spin current then exert spin torque on local magnetization whose
dynamics, in turn, pumps additional spin and charge currents radiating in the
THz range. By switching on and off LLG dynamics and SO couplings, we unravel
which microscopic mechanism contribute the most to emitted THz radiation --
charge pumping by local magnetization of Mn$_3$Sn in the presence of its
intrinsic SO coupling is far more important than standardly assumed (for other
types of magnetic layers) spin pumping and subsequent spin-to-charge conversion
within the neighboring nonmagnetic SO-coupled material.",2211.03645v3
2019-05-14,Dynamics of an itinerant spin-3 atomic dipolar gas in an optical lattice,"Arrays of ultra-cold dipolar gases loaded in optical lattices are emerging as
powerful quantum simulators of the many-body physics associated with the rich
interplay between long-range dipolar interactions, contact interactions,
motion, and quantum statistics. In this work we report on our investigation of
the quantum many-body dynamics of a large ensemble of bosonic magnetic chromium
atoms with spin S = 3 in a three-dimensional lattice as a function of lattice
depth. Using extensive theory and experimental comparisons we study the
dynamics of the population of the different Zeeman levels and the total
magnetization of the gas across the superfluid to the Mott insulator
transition. We are able to identify two distinct regimes: At low lattice
depths, where atoms are in the superfluid regime, we observe that the spin
dynamics is strongly determined by the competition between particle motion,
onsite interactions and external magnetic field gradients. Contact spin
dependent interactions help to stabilize the collective spin length, which sets
the total magnetization of the gas. On the contrary, at high lattice depths,
transport is largely frozen out. In this regime, while the spin populations are
mainly driven by long range dipolar interactions, magnetic field gradients also
play a major role in the total spin demagnetization. We find that dynamics at
low lattice depth is qualitatively reproduced by mean-field calculations based
on the Gutzwiller ansatz; on the contrary, only a beyond mean-field theory can
account for the dynamics at large lattice depths. While the cross-over between
these two regimes does not correspond to sharp features in the observed
dynamical evolution of the spin components, our simulations indicate that it
would be better revealed by measurements of the collective spin length.",1905.06123v1
2020-09-22,Bethe strings in the dynamical structure factor of the spin-1/2 Heisenberg XXX chain,"Recently there has been a renewed interest in the spectra and role in
dynamical properties of excited states of the spin-1/2 Heisenberg
antiferromagnetic chain in longitudinal magnetic fields associated with Bethe
strings. The latter are bound states of elementary magnetic excitations
described by Bethe-ansatz complex non-real rapidities. Previous studies on this
problem referred to finite-size systems. Here we consider the thermodynamic
limit and study it for the isotropic spin-1/2 Heisenberg XXX chain in a
longitudinal magnetic field. We confirm that also in that limit the most
significant spectral weight contribution from Bethe strings leads to gapped
continua in the spectra of the spin +- and xx dynamical structure factors. The
contribution of Bethe strings to the zz dynamical structure factor is found to
be small at low spin densities and to become negligible upon increasing that
density above 0.317. For the -+ dynamical structure factor, that contribution
is found to be negligible at finite magnetic field. We derive analytical
expressions for the line shapes of the +-, xx, and zz dynamical structure
factors valid in the vicinity of singularities located at and just above the
gapped lower thresholds of the Bethe-string states's spectra. As a side result
and in order to provide an overall physical picture that includes the relative
location of all spectra with a significant amount of spectral weight, we
revisit the general problem of the line-shape of the transverse and
longitudinal spin dynamical structure factors at finite magnetic field and
excitation energies in the vicinity of other singularities. This includes those
located at and just above the lower thresholds of the spectra that stem from
excited states described by only real Bethe-ansatz rapidities.",2009.10499v1
2016-02-02,Carrier-impurity spin transfer dynamics in paramagnetic II-VI diluted magnetic semiconductors in the presence of a wave-vector-dependent magnetic field,"Quantum kinetic equations of motion for carrier and impurity spins in
paramagnetic II-VI diluted magnetic semiconductors in a $\mathbf{k}$-dependent
effective magnetic field are derived, where the carrier-impurity correlations
are taken into account. In the Markov limit, rates for the electron-impurity
spin transfer can be derived for electron spins parallel and perpendicular to
the impurity spins corresponding to measurable decay rates in Kerr experiments
in Faraday and Voigt geometry. Our rigorous microscopic quantum kinetic
treatment automatically accounts for the fact that, in an individual spin
flip-flop scattering process, a spin flip of an electron is necessarily
accompanied by a flop of an impurity spin in the opposite direction and the
corresponding change of the impurity Zeeman energy influences the final energy
of the electron after the scattering event. This shift in the electron energies
after a spin flip-flop scattering processes, which usually has been overlooked
in the literature, turns out to be especially important in the case of
extremely diluted magnetic semiconductors in an external magnetic field. As a
specific example for a $\mathbf{k}$-dependent effective magnetic field the
effects of a Rashba field on the dynamics of the carrier-impurity correlations
in a Hg$_{1-x-y}$Cd$_y$Mn$_x$Te quantum well are described. It is found that,
although accounting for the Rashba interaction in the dynamics of the
correlations leads to a modified $\mathbf{k}$-space dynamics, the time
evolution of the total carrier spin is not significantly influenced.
Furthermore, a connection between the present theory and the description of
collective carrier-impurity precession modes is presented.",1602.00911v2
2019-09-06,Magnetically actuated artificial microswimmers as mobile microparticle manipulators,"Micro-scale swimming robots have been envisaged for many medical applications
such as targeted drug delivery, where the microrobot will be expected to
navigate in a fluid through channels carrying a payload. Alternatively, in many
cases, such a payload does not have to be physically bound to the swimmer, but
may be instead manipulated and steered through the channel by the microrobot.
We investigate this problem of contactless manipulation of a microparticle by
mobile microswimmer in a fluid at low Reynolds number. We consider a model of a
magnetically actuated artificial microswimmer, whose locomotion through a fluid
induces a disturbance velocity field in the fluid, that then acts to propel a
cargo particle in its vicinity. The problem investigated in this paper is
therefore one of coupled locomotion-manipulation of two bodies in a fluid. The
magnetic swimmer's motion is actuated by an externally applied magnetic field
of constant strength but whose direction rotates at a constant rate in a plane.
The swimmer propels itself in the direction perpendicular to this plane if the
frequency associated with the periodic magnetic field is above a critical
frequency. Below this critical frequency, the swimmer tumbles in place without
net locomotion. The coupled fluid-swimmer-cargo particle dynamics are solved
numerically using the method of Stokesian dynamics. The induced motion of the
cargo particle is shown to be controllable. This is achieved by switching the
planes of rotation of the magnetic field and switching frequency of the
magnetic field above and below the critical frequency. While a swimmer with a
specific geometry has been used in the model, the results of this paper are
applicable to swimmers with other geometries and means of propulsion. The
results of this paper show that microswimmers can be utilized as mobile
manipulators of microparticles in a fluid.",1909.05646v1
2019-12-11,Robust cycloid crossover driven by anisotropy in the skyrmion host GaV$_\mathbf{4}$S$_\mathbf{8}$,"We report on the anomalous magnetization dynamics of the
cycloidally-modulated spin textures under the influence of uniaxial anisotropy
in multiferroic $\mathrm{GaV_4S_8}$. The temperature and field dependence of
the linear ac susceptibility [$\chi_{1\omega}^{\prime}(T,H)$], ac magnetic loss
[$\chi_{1\omega}^{\prime\prime}(T,H)$], and nonlinear ac magnetic response
[$M_{3\omega}(T,H)$] are examined across the magnetic phase diagram in the
frequency range $f = 10-10000$ Hz. According to recent theory, skyrmion
vortices under axial crystal symmetry are confined along specific orientations,
resulting in enhanced robustness against oblique magnetic fields and altered
spin dynamics. We characterize the magnetic response of each spin texture and
find that the dynamic rigidity of the N\'eel skyrmion lattice appears enhanced
compared to Bloch-type skyrmions in cubic systems, even in the multidomain
state. Anomalous $M_{3\omega}$ and strong dissipation emerge over the same
phase regime where strong variations in the cycloid pitch were observed on
lowering temperature in recent small-angle neutron scattering experiments
[White et al., Phys. Rev. B 97, 020401(R) (2018)]. Here, we show that strong
anisotropy also drives an extended crossover of the zero-field cycloid texture
in $\mathrm{GaV_4S_8}$. The frequency dependence of these dynamic signatures is
consistent with that of a robust anharmonic spin texture exhibiting a
correlated domain arrangement. The results underpin the essential role of
magnetic anisotropy in enhancing the rigidity of topological spin textures for
diverse applications.",1912.05367v2
2020-11-15,Adiabaticity of spin dynamics in diamond nitrogen vacancy centers in time-dependent magnetic fields,"We study the spin dynamics of diamond nitrogen vacancy (NV) centers in an
oscillating magnetic field along the symmetry axis of the NV in the presence of
transverse magnetic fields. It is well-known that the coupling between the
otherwise degenerate Zeeman levels $|M_S=\pm1\rangle$ due to strain and
electric fields is responsible for a Landau-Zener process near the
pseudo-crossing of the adiabatic energy levels when the axial component of the
oscillating magnetic field changes sign. We derive an effective two-level
Hamiltonian for the NV system that includes coupling between the two levels via
virtual transitions into the third far-detuned level $|M_S=0\rangle$ induced by
transverse magnetic fields. This coupling adds to the coupling due to strain
and electric fields, with a phase that depends on the direction of the
transverse field in the plane perpendicular to the NV axis. Hence, the {\em
total coupling} of the Zeeman levels can be tuned to control the adiabaticity
of spin dynamics by fully or partially compensating the effect of the strain
and electric fields, or by enhancing it. Moreover, by varying the strength and
direction of the transverse magnetic fields, one can determine the strength and
direction of the local strain and electric fields at the position of the NV
center, and even the {\em external} stress and electric field. The nuclear spin
hyperfine interaction is shown to introduce a nuclear spin dependent offset of
the axial magnetic field for which the pseudo-crossing occurs, while the
adiabaticity remains unaffected by the nuclear spin. If the NV center is
coupled to the environment, modeled by a bath with a Gaussian white noise
spectrum, as appropriate for NVs near the diamond surface, then the spin
dynamics is accompanied by relaxation of the Zeeman level populations and
decoherence with a non-monotonic decrease of the purity of the system.",2011.07609v3
2022-10-26,Phase stability of Fe from first-principles: atomistic spin dynamics coupled with ab initio molecular dynamics simulations and thermodynamic integration,"The calculation of free energies from first principles in materials is a
formidable task which enables the prediction of phase stability with high
accuracy; these calculations are complicated in magnetic materials by the
interplay of electronic, magnetic, and vibrational degrees of freedom. In this
work, we show the feasibility and accuracy of the calculation of phase
stability in magnetic systems with ab initio methods and thermodynamic
integration by sampling the magnetic and vibrational phase space with coupled
atomistic spin dynamics-ab initio molecular dynamics (ASD-AIMD) simulations
[Stockem et al., PRL 121, 125902 (2018)], where energies and interatomic forces
are calculated with density functional theory (DFT). We employ the method to
calculate the phase stability of Fe at ambient pressure from 800 K up to 1800
K. The Gibbs free energy difference between fcc and bcc Fe at zero pressure as
a function of temperature is calculated carrying out thermodynamic integration
over temperature on the energies at the DFT level from ASD-AIMD, using a
reference free energy difference calculated in the paramagnetic state at
temperatures much higher than the magnetic transition temperatures with
thermodynamic integration over stress-strain variables with disordered local
moment (DLM)-AIMD simulations. We show the importance of the magnetic ordering
temperature of bcc Fe on the $\alpha$ to $\gamma$ structural transition
temperature, whereas the $\gamma$ to $\delta$ transition is well reproduced
independently of the exchange interactions. The Gibbs free energy difference
between the two structures is within 5 meV/atom from the CALPHAD estimate, and
both transition temperatures are reproduced within 150 K. The present work
paves the way to free energy calculations in magnetic materials from first
principles with accuracy in the order of 1 meV/atom.",2210.14718v2
2024-03-01,Dichotomous Dynamics of Magnetic Monopole Fluids,"A recent advance in the study of emergent magnetic monopoles was the
discovery that monopole motion is restricted to dynamical fractal trajectories
(J. Hall\'en et al, Science 378, 1218 (2022)) thus explaining the
characteristics of magnetic monopole noise spectra (Dusad, R. et al. Nature
571, 234 (2019); Samarakoon, A. M. et al. Proc. Natl. Acad. Sci. 119,
e2117453119 (2022)). Here we apply this new theory to explore the dynamics of
field-driven monopole currents, finding them comprised of two quite distinct
transport processes: initially swift fractal rearrangements of local monopole
configurations followed by conventional monopole diffusion. This theory also
predicts a characteristic frequency dependence of the dissipative loss-angle
for AC-field-driven currents. To explore these novel perspectives on monopole
transport, we introduce simultaneous monopole current control and measurement
techniques using SQUID-based monopole current sensors. For the canonical
material Dy2Ti2O7, we measure ${\Phi}(t)$, the time-dependence of magnetic flux
threading the sample when a net monopole current $J(t) = \dot{\Phi}(t)/\mu_0$
is generated by applying an external magnetic field $B_0(t)$. These experiments
find a sharp dichotomy of monopole currents, separated by their distinct
relaxation time-constants before and after $t \approx 600 {\mu}s$ from monopole
current initiation. Application of sinusoidal magnetic fields $B_0(t) =
Bcos({\omega}t)$ generates oscillating monopole currents whose loss angle
${\theta}(f)$ exhibits a characteristic transition at frequency $f \approx 1.8$
kHz over the same temperature range. Finally, the magnetic noise power is also
dichotomic, diminishing sharply after $t \approx 600 {\mu}s$. This complex
phenomenology represents a new form of heterogeneous dynamics generated by the
interplay of fractionalization and local spin configurational symmetry.",2403.00726v1
1999-06-28,Does magnetic pressure affect the ICM dynamics?,"A possible discrepancy found in the determination of mass from gravitational
lensing data, and from X-rays observations, has been largely discussed in the
latest years (for instance, Miralda-Escude & Babul (1995)). Another important
discrepancy related to these data is that the dark matter is more centrally
condensed than the X-ray-emitting gas, and also with respect to the galaxy
distribution (Eyles et al. 1991). Could these discrepancies be consequence of
the standard description of the ICM, in which it is assumed hydrostatic
equilibrium maintained by thermal pressure? We follow the evolution of the ICM,
considering a term of magnetic pressure, aiming at answering the question
whether or not these discrepancies can be explained via non-thermal terms of
pressure. Our results suggest that the magnetic pressure could only affect the
dynamics of the ICM on scales as small as < 1kpc. Our models are constrained by
the observations of large and small scale fields and we are successful at
reproducing available data, for both Faraday rotation limits and inverse
Compton limits for the magnetic fields. In our calculations the radius (from
the cluster center) in which magnetic pressure reaches equipartition is smaller
than radii derived in previous works, as a consequence of the more realistic
treatment of the magnetic field geometry and the consideration of a sink term
in the cooling flow.",9906453v2
2001-05-18,Structure of Small-Scale Magnetic Fields in the Kinematic Dynamo Theory,"A weak fluctuating magnetic field embedded into a turbulent conducting medium
grows exponentially while its characteristic scale decays. In the ISM and
protogalactic plasmas, the magnetic Pr is very large, so a broad spectrum of
growing magnetic fluctuations is excited at subviscous scales. We study the
statistical correlations that are set up in the field pattern and show that the
magnetic-field lines possess a folding structure, where most of the scale
decrease is due to rapid transverse field direction reversals, while the scale
of the field variation along itself stays approximately constant. Specifically,
we find that the field strength and the field-line curvature are
anticorrelated, and the curvature possesses a stationary limiting distribution
with the bulk located at the values of curvature comparable to the
characteristic wave number of the velocity field and a power tail extending to
large values of curvature. The regions of large curvature, therefore, occupy
only a small fraction of the total volume of the system. Our theoretical
results are corroborated by direct numerical simulations. The implication of
the folding effect is that the advent of the Lorentz back reaction occurs when
the magnetic energy approaches that of the smallest turbulent eddies. Our
results also directly apply to the problem of statistical geometry of the
material lines in a random flow.",0105322v3
2002-06-30,Self-Gravitating Magnetically-Supported Protostellar Disks and the Formation of Substellar Companions,"Isolated low-mass stars are formed, in the standard picture, from the
collapse of dense cores condensed out of strongly magnetized molecular clouds.
The dynamically collapsing inflow traps nearly half of the critical magnetic
flux needed for the core support and deposits it in a small region surrounding
the protostar. It has been argued previously that the deposited flux can slow
down the inflow, allowing matter to pile up and settle along field lines into a
magnetically supported, circumstellar disk. Here we show that the disk
typically contains $\sim 10%$ of the stellar mass, and that it could become
self-gravitating under plausible conditions during the rapidly accreting,
``Class 0'' phase of star formation. Subsequent fragmentation of the
self-gravitating, magnetically subcritical disk, driven by magnetic diffusion,
could produce fragments of substellar masses, which collapse to form brown
dwarfs and possibly massive planets. This scenario predicts substellar object
formation at distances of order 100 AU from the central star, although orbital
evolution is possible after formation. It may provide an explanation for the
small, but growing, number of brown dwarf companions found around nearby stars
by direct imaging. The relatively large formation distances make the substellar
companions vulnerable to dynamic ejection, particularly in binary (multiple)
systems and dense clusters. Those ejected may account for, at least in part,
the isolated brown dwarfs and perhaps free-floating planetary mass objects.",0207014v1
2003-04-28,Dynamics of magnetic flux tubes in close binary stars II. Nonlinear evolution and surface distributions,"Observations of magnetically active close binaries with orbital periods of a
few days reveal the existence of starspots at preferred longitudes (with
respect to the direction of the companion star). We numerically investigate the
non-linear dynamics and evolution of magnetic flux tubes in the convection
zoneof a fast-rotating component of a close binary system and explore whether
the tidal effects are able to generate non-uniformities in the surface
distribution of erupting flux tubes. Assuming a synchronised system with a
rotation period of two days and consisting of two solar-type components, both
the tidal force and the deviation of the stellar structure from spherical shape
are considered in lowest-order perturbation theory. The magnetic field is
initially stored in the form of toroidal magnetic flux rings within the stably
stratified overshoot region beneath the convection zone. Once the field has
grown sufficiently strong, instabilities initiate the formation of rising flux
loops, which rise through the convection zone and emerge at the stellar
surface. We find that although the magnitude of tidal effects is rather small,
they nevertheless lead to the formation of clusters of flux tube eruptions at
preferred longitudes on opposite sides of the star, which result from the
cumulative and resonant character of the action of tidal effects on rising flux
tubes. The longitude distribution of the clusters depends on the initial
parameters of flux tubes in the overshoot region like magnetic field strength
and latitude, implying that there is no globally unique preferred longitude
along a fixed direction.",0304498v1
2003-12-03,Ultra-Relativistic Magneto-Hydro-Dynamic Jets in the context of Gamma Ray Bursts,"We present a detailed numerical study of the dynamics and evolution of
ultrarelativistic magnetohydrodynamic jets in the black hole-disk system under
extreme magnetization conditions. We find that Lorentz factors of up to 3000
are achieved and derived a modifiedMichel scaling (Gamma ~ sigma) which allows
for a wide variation in the flow Lorentz factor. Pending contamination induced
by mass-entrainment, the linear Michel scaling links modulations in the
ultrarelativistic wind to variations in mass accretion in the disk for a given
magnetization. The jet is asymptotically dominated by the toroidal magnetic
field allowing for efficient collimation. We discuss our solutions (jets) in
the context of Gamma ray bursts and describe the relevant features such as the
high variability in the Lorentz factor and how high collimation angles (~ 0-5
degrees), or cylindrical jets, can be achieved. We isolate a jet instability
mechanism we refer to as the ""bottle-neck"" instability which essentially relies
on a high magnetization and a recollimation of the magnetic flux surfaces. The
instability occurs at large radii where any dissipation of the magnetic energy
into radiation would in principle result in an optically thin emission.",0312090v1
2004-08-24,Magnetorotational instability in protoplanetary discs,"We investigate the linear growth and vertical structure of the
magnetorotational instability (MRI) in weakly ionised, stratified accretion
discs. The magnetic field is initially vertical and dust grains are assumed to
have settled towards the midplane, so charges are carried by electrons and ions
only. Solutions are obtained at representative radial locations from the
central protostar for different choices of the initial magnetic field strength,
sources of ionisation, and disc surface density.
  The MRI is active over a wide range of magnetic field strengths and fluid
conditions in low conductivity discs. For the minimum-mass solar nebula model,
incorporating cosmic ray ionisation, perturbations grow at 1 AU for B < 8 G.
For a significant subset of these strengths (0.2 - 5 G), the growth rate is of
order the ideal MHD rate (0.75 Omega). Similarly, when cosmic rays are assumed
to be excluded from the disc by the winds emitted by the magnetically active
protostar, unstable modes grow at this radius for B less than about 2 G.
  This study shows that, despite the low magnetic coupling, the magnetic field
is dynamically important for a large range of fluid conditions and field
strengths in protostellar discs. Hall diffusion largely determines the
structure and growth rate of these perturbations for all studied radii. At
radii of order 1 AU, in particular, it is crucial to incorporate the full
conductivity tensor in studies of the dynamics of astrophysical discs.
(Abridged)",0408427v1
1998-04-29,"Magnetic correlations and quantum criticality in the insulating antiferromagnetic, insulating spin liquid, renormalized Fermi liquid, and metallic antiferromagnetic phases of the Mott system V_2O_3","Magnetic correlations in all four phases of pure and doped vanadium
sesquioxide V_2O_3 have been examined by magnetic thermal neutron scattering.
While the antiferromagnetic insulator can be accounted for by a Heisenberg
localized spin model, the long range order in the antiferromagnetic metal is an
incommensurate spin-density-wave, resulting from a Fermi surface nesting
instability. Spin dynamics in the strongly correlated metal are dominated by
spin fluctuations in the Stoner electron-hole continuum. Furthermore, our
results in metallic V_2O_3 represent an unprecedentedly complete
characterization of the spin fluctuations near a metallic quantum critical
point, and provide quantitative support for the SCR theory for itinerant
antiferromagnets in the small moment limit. Dynamic magnetic correlations for
energy smaller than k_BT in the paramagnetic insulator carry substantial
magnetic spectral weight. However, the correlation length extends only to the
nearest neighbor distance. The phase transition to the antiferromagnetic
insulator introduces a sudden switching of magnetic correlations to a different
spatial periodicity which indicates a sudden change in the underlying spin
Hamiltonian. To describe this phase transition and also the unusual short range
order in the paramagnetic state, it seems necessary to take into account the
orbital degrees of freedom associated with the degenerate d-orbitals at the
Fermi level in V_2O_3.",9804320v3
2001-11-24,Static Correlation and Dynamical Properties of Tb3+-moments in Tb2Ti2O7 -Neutron Scattering Study-,"Static and dynamical properties of the magnetic moment system of pyrochlore
compound Tb2Ti2O7 with strong magnetic frustration, have been investigated down
to the temperature T=0.4 K by neutron scattering on a single crystal sample.
The scattering vector (Q)-dependence of the magnetic scattering intensity
becomes appreciable with decreasing T at around 30 K, indicating the
development of the magnetic correlation. From the observed energy profiles, the
elastic, quasi elastic and inelastic components have been separately obtained.
The quasi elastic component corresponds to the diffusive motion of the magnetic
moments within the lowest states, which are formed of the lowest energy levels
of Tb3+ ions. Magnetic correlation pattern which can roughly reproduce the
Q-dependence of the scattering intensities of the elastic and quasi elastic
component is discussed based on the trial calculations for clusters of 7
moments belonging to two corner-sharing tetrahedra. A possible origin of the
glassy state, which develops at around 1.5 K with decreasing T is discussed.",0111466v3
2005-08-18,Probing spin dynamics and quantum relaxation in LiY0.998Ho0.002F4 via 19F NMR,"We report measurements of 19F nuclear spin-lattice relaxation 1/T1 as a
function of temperature and external magnetic field in LiY0.998Ho0.002F4 single
crystal, a single-ion magnet exhibiting interesting quantum effects. The 19F
1/T1 is found to depend on the coupling with the diluted rare-earth (RE)
moments. Depending on the temperature range, a fast spin diffusion regime or a
diffusion limited regime is encountered. In both cases we find it possible to
use the 19F nucleus as a probe of the rare-earth spin dynamics. The results for
1/T1 show a behavior similar to that observed in molecular nanomagnets, a
result which we attribute to the discreteness of the energy levels in both
cases. At intermediate temperatures the lifetime broadening of the crystal
field split RE magnetic levels follows a T3 power law. At low temperature the
field dependence of 1/T1 shows peaks in correspondence to the critical magnetic
fields for energy level crossings (LC). The results can be explained by
inelastic scattering between the fluorine nuclear spins and the RE magnetic
levels. A key result of this study is that the broadening of the levels at LC
is found to be become extremely small at low temperatures, about 1.7 mT, a
value which is comparable to the weak dipolar fields at the RE lattice
positions. Thus, unlike the molecular magnets, decoherence effects are strongly
suppressed, and it may be possible to measure directly the level repulsions at
avoided level crossings.",0508406v1
2005-11-30,"A Novel ""Magnetic"" Field And Its Dual Non-Commutative Phase Space","In this paper we have studied a new form of Non-Commutative (NC) phase space
with an operatorial form of noncommutativity. A point particle in this space
feels the effect of an interaction with an ""{\it{internal}}"" magnetic field,
that is singular at a specific position $\theta^{-1}$. By ""internal"" we mean
that the effective magnetic fields depends essentially on the particle
properties and modifies the symplectic structure. Here $\theta $ is the NC
parameter and induces the coupling between the particle and the ""internal""
magnetic field. The magnetic moment of the particle is computed. Interaction
with an {\it{external}} physical magnetic field reveals interesting features
induced by the inherent fuzziness of the NC phase space: introduction of
non-trivial structures into the charge and mass of the particle and possibility
of the particle dynamics collapsing to a Hall type of motion. The dynamics is
studied both from Lagrangian and symplectic (Hamiltonian) points of view. The
canonical (Darboux) variables are also identified. We briefly comment, that the
model presented here, can play interesting role in the context of (recently
observed) {\it{real}} space Berry curvature in material systems.",0511302v4
2007-05-30,Magnetic fluctuations in n-type high-$T_c$ superconductors reveal breakdown of fermiology,"By combining experimental measurements of the quasiparticle and dynamical
magnetic properties of optimally electron-doped Pr$_{0.88}$LaCe$_{0.12}$CuO$_4$
with theoretical calculations we demonstrate that the conventional fermiology
approach cannot possibly account for the magnetic fluctuations in these
materials. In particular, we perform tunneling experiments on the very same
sample for which a dynamical magnetic resonance has been reported recently and
use photoemission data by others on a similar sample to characterize the
fermionic quasiparticle excitations in great detail. We subsequently use this
information to calculate the magnetic response within the conventional
fermiology framework as applied in a large body of work for the hole-doped
superconductors to find a profound disagreement between the theoretical
expectations and the measurements: this approach predicts a step-like feature
rather than a sharp resonance peak, it underestimates the intensity of the
resonance by an order of magnitude, it suggests an unreasonable temperature
dependence of the resonance, and most severely, it predicts that most of the
spectral weight resides in incommensurate wings which are a key feature of the
hole-doped cuprates but have never been observed in the electron-doped
counterparts. Our findings strongly suggest that the magnetic fluctuations
reflect the quantum-mechanical competition between antiferromagnetic and
superconducting orders.",0705.4424v1
2008-02-14,Extragalactic jets with helical magnetic fields: relativistic MHD simulations,"Extragalactic jets are inferred to harbor dynamically important, organized
magnetic fields which presumably aid in the collimation of the relativistic jet
flows. We here explore by means of grid-adaptive, high resolution numerical
simulations the morphology of AGN jets pervaded by helical field and flow
topologies. We concentrate on morphological features of the bow shock and the
jet beam behind the Mach disk, for various jet Lorentz factors and magnetic
field helicities. We investigate the influence of helical magnetic fields on
jet beam propagation in overdense external medium. We use the AMRVAC code,
employing a novel hybrid block-based AMR strategy, to compute ideal plasma
dynamics in special relativity. The helicity of the beam magnetic field is
effectively transported down the beam, with compression zones in between
diagonal internal cross-shocks showing stronger toroidal field regions. In
comparison with equivalent low-relativistic jets which get surrounded by
cocoons with vortical backflows filled by mainly toroidal field, the high speed
jets demonstrate only localized, strong toroidal field zones within the
backflow vortical structures. We find evidence for a more poloidal, straight
field layer, compressed between jet beam and backflows. This layer decreases
the destabilizing influence of the backflow on the jet beam. In all cases, the
jet beam contains rich cross-shock patterns, across which part of the kinetic
energy gets transferred. For the high speed reference jet considered here,
significant jet deceleration only occurs beyond distances exceeding ${\cal
O}(100 R_j)$, as the axial flow can reaccelerate downstream to the internal
cross-shocks. This reacceleration is magnetically aided, due to field
compression across the internal shocks which pinch the flow.",0802.2034v1
2008-03-03,Slipping friction of an optically and magnetically manipulated microsphere rolling at a glass-water interface,"The motion of submerged magnetic microspheres rolling at a glass-water
interface has been studied using magnetic rotation and optical tweezers
combined with bright-field microscopy particle tracking techniques. Individual
microspheres of varying surface roughness were magnetically rotated both in and
out of an optical trap to induce rolling, along either plain glass cover slides
or glass cover slides functionalized with polyethylene glycol. It has been
observed that the manipulated microspheres exhibited nonlinear dynamic
rolling-while-slipping motion characterized by two motional regimes: At low
rotational frequencies, the speed of microspheres free-rolling along the
surface increased proportionately with magnetic rotation rate; however, a
further increase in the rotation frequency beyond a certain threshold revealed
a sharp transition to a motion in which the microspheres slipped with respect
to the external magnetic field resulting in decreased rolling speeds. The
effects of surface-microsphere interactions on the position of this threshold
frequency are posed and investigated. Similar experiments with microspheres
rolling while slipping in an optical trap showed congruent results.",0803.0328v1
2008-03-27,Spherically Symmetric Accretion Flows: Minimal Model with MHD Turbulence,"The first spherical accretion model was developed 55 years ago, but the
theory is yet far from being complete. The real accretion flow was found to be
time-dependent and turbulent. This paper presents the minimal MHD spherical
accretion model that separately deals with turbulence. Treatment of turbulence
is based on simulations of several regimes of collisional MHD. The effects of
freezing-in amplification, dissipation, dynamo action, isotropization, and
constant magnetic helicity are self-consistently included. The assumptions of
equipartition and magnetic field isotropy are released. Correct dynamics of
magnetized flow is calculated. Diffusion, convection, and radiation are not
accounted for. Two different types of Radiatively Inefficient accretion flows
are found: a transonic non-rotating flow (I), a flow with effective transport
of angular momentum outward (II). Non-rotating flow has an accretion rate
several times smaller than Bondi rate, because turbulence inhibits accretion.
Flow with angular momentum transport has accretion rate about 10-100 times
smaller than Bondi rate. The effects of highly helical turbulence, states of
outer magnetization, and different equations of state are discussed. The flows
were found to be convectively stable on average, despite gas entropy increases
inward. The proposed model has a small number of free parameters and the
following attractive property. Inner density in the non-rotating magnetized
flow was found to be several times lower than density in a non-magnetized
accretion. Still several times lower density is required to explain the
observed low IR luminosity and low Faraday rotation measure of accretion onto
Sgr A*.",0803.3909v1
2008-07-12,Time-Dependent of Accretion Flow with Toroidal Magnetic Field,"In the present study time evolution of quasi-spherical polytropic accretion
flow with toroidal magnetic field is investigated. The study especially focused
the astrophysically important case in which the adiabatic exponent
$\gamma=5/3$. In this scenario, it was assumed that the angular momentum
transport is due to viscous turbulence and used $\alpha$-prescription for
kinematic coefficient of viscosity. The equations of accretion flow are solved
in a simplified one-dimensional model that neglects the latitudinal dependence
of the flow. In order to solve the integrated equations which govern the
dynamical behavior of the accretion flow, self-similar solution was used. The
solution provides some insight into the dynamics of quasi-spherical accretion
flow and avoids many of the strictures of the steady self-similar solution. The
effect of the toroidal magnetic field is considered with additional variable
$\beta[=p_{mag}/p_{gas}]$, where $p_{mag}$ and $p_{gas}$ are the magnetic and
gas pressure, respectively. The solution indicates a transonic point in the
accretion flow, that this point approaches to central object by adding strength
of the magnetic field. Also, by adding strength of the magnetic field, the
radial-thickness of the disk decreases and the disk compresses. It was
analytically indicated that the radial velocity is only a function of Alfv'en
velocity. The model implies that the flow has differential rotation and is
sub-Keplerian at all radii.",0807.1983v2
2009-02-13,Mesoscale dynamics on the Sun's surface from HINODE observations,"Aims: The interactions of velocity scales on the Sun's surface, from
granulation to supergranulation are still not understood, nor are their
interaction with magnetic fields. We thus aim at giving a better description of
dynamics in the mesoscale range which lies between the two scales mentioned
above. Method: We analyse a 48h high-resolution time sequence of the quiet Sun
photosphere at the disk center obtained with the Solar Optical Telescope
onboard Hinode. The observations, which have a field of view of 100
\arcsec$\times$ 100 \arcsec, typically contain four supergranules. We monitor
in detail the motion and evolution of granules as well as those of the radial
magnetic field. Results: This analysis allows us to better characterize Trees
of Fragmenting Granules issued from repeated fragmentation of granules,
especially their lifetime statistics. Using floating corks advected by measured
velocity fields, we show their crucial role in the advection of the magnetic
field and in the build up of the network. Finally, thanks to the long duration
of the time series, we estimate that the turbulent diffusion coefficient
induced by horizontal motion is approximately $430 \mathrm{km}^2
\mathrm{s}^{-1}$. Conclusions: These results demonstrate that the long living
families contribute to the formation of the magnetic network and suggest that
supergranulation could be an emergent length scale building up as small
magnetic elements are advected and concentrated by TFG flows. Our estimate for
the magnetic diffusion associated with this horizontal motion might provide a
useful input for mean-field dynamo models.",0902.2299v1
2009-06-29,Generation of a symmetric magnetic field by thermal convection in a plane rotating layer,"We investigate numerically magnetic field generation by thermal convection
with square periodicity cells in a rotating horizontal layer of
electrically-conducting fluid with stress-free electrically perfectly
conducting boundaries for Rayleigh numbers in the interval 5100\le R\le 5800.
Dynamos of three kinds, apparently not encountered before, are presented: 1)
Steady and time-periodic regimes, where the flow and magnetic field are
symmetric about a vertical axis. In regimes with this symmetry, the global
alpha-effect is insignificant, and the complex structure of the system of
amplitude equations controlling weakly nonlinear stability of the system to
perturbations with large spatial and temporal scales suggests that the
perturbations are likely to exhibit uncommon complex patterns of behaviour, to
be studied in the future work. 2) Periodic in time regimes, where magnetic
field is always concentrated in the interior of the convective layer, in
contrast to the behaviour first observed by St Pierre (1993) and often
perceived as generic for electrically infinitely conducting boundaries. 3) A
dynamo exhibiting chaotic behaviour of heteroclinic nature, where a sample
trajectory enjoys excursions between a periodic magnetohydrodynamic regime and
rolls. The rolls are amagnetic, but generate magnetic field kinematically. As a
result, magnetic energy falls off almost to zero, while the rolls are
approached.",0906.5380v2
2010-05-12,Using Faraday Rotation to Probe MHD Instabilities in Intracluster Media,"It has recently been suggested that conduction-driven magnetohydrodynamic
(MHD) instabilities may operate at all radii within an intracluster medium
(ICM), and profoundly affect the structure of a cluster's magnetic field. Where
MHD instabilities dominate the dynamics of an ICM, they will re-orient magnetic
field lines perpendicular to the temperature gradient inside a cooling core, or
parallel to the temperature gradient outside it. This characteristic structure
of magnetic field could be probed by measurements of polarized radio emission
from background sources. Motivated by this possibility we have constructed 3-d
models of a magnetized cooling core cluster and calculated Faraday rotation
measure (RM) maps in the plane of the sky under realistic observing conditions.
We compare a scenario in which magnetic field geometry is characterized by
conduction driven MHD instabilities to that where it is determined by isotropic
turbulent motions. We find that future high-sensitivity spectro-polarimetric
measurements of RM, such as will be enabled by the Expanded Very Large Array
and Square Kilometer Array can distinguish between these two cases with
plausible exposure times. Such observations will test the existence of
conduction-driven MHD instabilities in dynamically relaxed cooling core
clusters. More generally, our findings imply that observations of Faraday RM
should be able to discern physical mechanisms that result in qualitatively
different magnetic field topologies, without a priori knowledge about the
nature of the processes.",1005.2193v2
2010-06-11,Power-Law NLED-Based Magnetic Universe Can Mimic Phantom Behavior,"We study the cosmic dynamics of a magnetic universe supported by non-linear
electrodynamics (NLED) Lagrangeans that are proportional to powers of the
electromagnetic invariant $\propto F^{1/(1-m)}$ ($m$ is an overall constant).
For simplicity we focus in the case when $F$ depends on the magnetic field
alone, a case dubbed in the bibliography as ''magnetic universe''. Our results
demonstrate that, depending on the values of the free parameter $m$, the
magnetic field can mimic phantom field behavior, an effect previously found in
other contexts. It is demonstrated that, since there are found equilibrium
points in the phase space of these models that can be associated with
magnetic-dominated past and future attractors, a combination of positive and
negative powers of $F$ may lead to interesting cosmological behavior. In
particular, a cosmological scenario where the universe might evolve from a past
NLED-driven (non-inflationary) state into a future (late-time) -- also
NLED-driven -- inflationary stage, transiting through a matter-dominated
solution, is envisioned. The impact of braneworld gravity on the dynamics
driven by such NLED Lagrangeans is also investigated. It is demonstrated that,
due to phantom property at late times, the non-linear electromagnetic effects
may play an important role in deciding the fate of the cosmic evolution.
Randall-Sundrum brane effects, in particular, modify the nature of the starting
point of the cosmic evolution, as well as the fate of the expansion: both, the
big bang singularity and the big rip event -- inherent in general relativity
with phantom matter source --, may be avoided.",1006.2274v2
2010-07-07,Magnetic Quenching of Turbulent Diffusivity: Reconciling Mixing-length Theory Estimates with Kinematic Dynamo Models of the Solar Cycle,"The turbulent magnetic diffusivity in the solar convection zone is one of the
most poorly constrained ingredients of mean-field dynamo models. This lack of
constraint has previously led to controversy regarding the most appropriate set
of parameters, as different assumptions on the value of turbulent diffusivity
lead to radically different solar cycle predictions. Typically, the dynamo
community uses double step diffusivity profiles characterized by low values of
diffusivity in the bulk of the convection zone. However, these low diffusivity
values are not consistent with theoretical estimates based on mixing-length
theory -- which suggest much higher values for turbulent diffusivity. To make
matters worse, kinematic dynamo simulations cannot yield sustainable magnetic
cycles using these theoretical estimates. In this work we show that magnetic
cycles become viable if we combine the theoretically estimated diffusivity
profile with magnetic quenching of the diffusivity. Furthermore, we find that
the main features of this solution can be reproduced by a dynamo simulation
using a prescribed (kinematic) diffusivity profile that is based on the
spatiotemporal geometric-average of the dynamically quenched diffusivity. Here,
we provide an analytic fit to the dynamically quenched diffusivity profile,
which can be used in kinematic dynamo simulations. Having successfully
reconciled the mixing-length theory estimated diffusivity profile with
kinematic dynamo models, we argue that they remain a viable tool for
understanding the solar magnetic cycle.",1007.1262v1
2010-10-12,Differential Rotation and Magnetism in Simulations of Fully Convective Stars,"Stars of sufficiently low mass are convective throughout their interiors, and
so do not possess an internal boundary layer akin to the solar tachocline.
Because that interface figures so prominently in many theories of the solar
magnetic dynamo, a widespread expectation had been that fully convective stars
would exhibit surface magnetic behavior very different from that realized in
more massive stars. Here I describe how recent observations and theoretical
models of dynamo action in low-mass stars are partly confirming, and partly
confounding, this basic expectation. In particular, I present the results of
3--D MHD simulations of dynamo action by convection in rotating spherical
shells that approximate the interiors of 0.3 solar-mass stars at a range of
rotation rates. The simulated stars can establish latitudinal differential
rotation at their surfaces which is solar-like at ``rapid'' rotation rates
(defined within) and anti-solar at slower rotation rates; the differential
rotation is greatly reduced by feedback from strong dynamo-generated magnetic
fields in some parameter regimes. I argue that this ``flip'' in the sense of
differential rotation may be observable in the near future. I also briefly
describe how the strength and morphology of the magnetic fields varies with the
rotation rate of the simulated star, and show that the maximum magnetic
energies attained are compatible with simple scaling arguments.",1010.2509v1
2010-11-12,"Magnetization Dynamics, Bennett Clocking and Associated Energy Dissipation in Multiferroic Logic","It has been recently shown that multiferroic logic - where logic bits are
encoded in the magnetization orientation of a nanoscale magnetostrictive layer
elastically coupled to a piezoelectric layer - can be Bennett clocked with
small electrostatic potentials of few tens of mV applied to the piezoelectric
layer. The potential generates stress in the magnetostrictive layer and rotates
its magnetization by a large angle to carry out Bennett clocking. This method
of clocking is far more energy-efficient than using spin transfer torque. In
order to assess if such a clocking scheme can be also reasonably fast, we have
studied the magnetization dynamics of a multiferroic logic array with nearest
neighbor dipole coupling using the Landau-Lifshitz-Gilbert (LLG) equation. We
find that switching delays of ~ 3 ns (clock rates of 0.33 GHz) can be achieved
with proper design provided we clock non-adiabatically and dissipate ~48,000 kT
(at room temperature) of energy per clock cycle per bit flip in the clocking
circuit. This dissipation far exceeds the energy barrier separating the two
logic states, which we assumed to be 32 kT to yield a bit error probability of
. Had we used spin transfer torque to switch with the same ~ 3 ns delay, the
energy dissipation would have been much larger (~ $6 \times 106$ kT). This
shows that spin transfer torque, widely used in magnetic random access memory,
is an inefficient way to switch a magnet, and multiferroic logic clocked with
voltage-induced stress is a superior nanomagnetic logic scheme.",1011.2914v2
2011-10-10,Magnetar Oscillations II: spectral method,"The seismological dynamics of magnetars is largely determined by a strong
hydro-magnetic coupling between the solid crust and the fluid core. In this
paper we set up a ""spectral"" computational framework in which the magnetar's
motion is decomposed into a series of basis functions which are associated with
the crust and core vibrational eigenmodes. A general-relativistic formalism is
presented for evaluation of the core Alfven modes in the magnetic-flux
coordinates, as well for eigenmode computation of a strongly magnetized crust
of finite thickness. By considering coupling of the crustal modes to the
continuum of Alfven modes in the core, we construct a fully relativistic
dynamical model of the magnetar which allows: i) Fast and long simulations
without numerical dissipation. ii) Very fine sampling of the stellar structure.
We find that the presence of strong magnetic field in the crust results in
localizing of some high-frequency crustal elasto-magnetic modes with the radial
number n>1 to the regions of the crust where the field is nearly horizontal.
While the hydro-magnetic coupling of these localized modes to the Alfven
continuum in the core is reduced, their energy is drained on a time-scale much
less than 1 second. Therefore the puzzle of the observed QPOs with frequencies
larger than 600 Hz still stands.",1110.2107v1
2011-11-17,Non-Equilibrium Chemistry of Dynamically Evolving Prestellar Cores: II. Ionization and Magnetic Field,"We study the effect that non-equilibrium chemistry in dynamical models of
collapsing molecular cloud cores has on measurements of the magnetic field in
these cores, the degree of ionization, and the mean molecular weight of ions.
We find that OH and CN, usually used in Zeeman observations of the
line-of-sight magnetic field, have an abundance that decreases toward the
center of the core much faster than the density increases. As a result, Zeeman
observations tend to sample the outer layers of the core and consistently
underestimate the core magnetic field. The degree of ionization follows a
complicated dependence on the number density at central densities up to 10^5
cm^{-3} for magnetic models and 10^6 cm^{-3} in non-magnetic models. At higher
central densities the scaling approaches a power-law with a slope of -0.6 and a
normalization which depends on the cosmic-ray ionization rate {\zeta} and the
temperature T as ({\zeta}T)^1/2. The mean molecular weight of ions is
systematically lower than the usually assumed value of 20 - 30, and, at high
densities, approaches a value of 3 due to the asymptotic dominance of the H3+
ion. This significantly lower value implies that ambipolar diffusion operates
faster.",1111.4218v2
2012-01-18,Statics and dynamics of weakly coupled antiferromagnetic spin-1/2 ladders in a magnetic field (PhD thesis),"We investigate weakly coupled spin-1/2 ladders in a magnetic field. The work
is motivated by recent experiments on the compound
$\mathrm{(C_5H_{12}N)_2CuBr_4}$ (BPCB). We use a combination of numerical and
analytical methods, in particular the density matrix renormalization group
(DMRG) technique, to explore the phase diagram and the excitation spectra of
such a system. We give detailed results on the temperature dependence of the
magnetization and the specific heat, and the magnetic field dependence of the
nuclear magnetic resonance (NMR) relaxation rate of single ladders. For coupled
ladders, treating the weak interladder coupling within a mean field approach,
we compute the transition temperature of triplet condensation and its
corresponding antiferromagnetic order parameter. Existing experimental
measurements are discussed and compared to our theoretical results. Furthermore
we compute, using time dependent DMRG, the dynamical correlations of a single
spin ladder. Our results allow to directly describe the inelastic neutron
scattering cross section up to high energies. We focus on the evolution of the
spectra with the magnetic field and compare their behavior for different
couplings. The characteristic features of the spectra are interpreted using
different analytical approaches such as the mapping onto a spin chain, a
Luttinger liquid (LL) or onto a t-J model. For values of parameters for which
such measurements exist, we compare our results to inelastic neutron scattering
experiments on the compound BPCB and find excellent agreement. We make
additional predictions for the high energy part of the spectrum that are
potentially testable in future experiments.",1201.5276v1
2012-04-03,The Small-Scale Dynamo and Non-Ideal MHD in Primordial Star Formation,"We study the amplification of magnetic fields during the formation of
primordial halos. The turbulence generated by gravitational infall motions
during the formation of the first stars and galaxies can amplify magnetic
fields very efficiently and on short timescales up to dynamically significant
values. Using the Kazantsev theory, which describes the so-called small-scale
dynamo - a magnetohydrodynamical process converting kinetic energy from
turbulence into magnetic energy - we can then calculate the growth rate of the
small-scale magnetic field. Our calculations are based on a detailed chemical
network and we include non-ideal magnetohydrodynamical effects such as
ambipolar diffusion and Ohmic dissipation. We follow the evolution of the
magnetic field up to larger scales until saturation occurs on the Jeans scale.
Assuming a weak magnetic seed field generated by the Biermann battery process,
both Burgers and Kolmogorov turbulence lead to saturation within a rather small
density range. Such fields are likely to become relevant after the formation of
a protostellar disk and, thus, could influence the formation of the first stars
and galaxies in the Universe.",1204.0658v2
2012-10-12,Distribution of Magnetic Discontinuities in the Solar Wind and in MHD Turbulence,"The statistical properties of magnetic discontinuities in the solar wind are
investigated by measuring fluctuations in the magnetic field direction, given
by the rotation Delta theta that the magnetic field vector undergoes during
time interval Delta t. We show that the probability density function for
rotations, P(Delta theta), can be described by a simple model in which the
magnetic field vector rotates with a relative increment (Delta B)/B that is
lognormally distributed. We find that the probability density function of
increments, P((Delta B)/B), has a remarkable scaling property: the normalized
variable x=[(Delta B)/B]*[(Delta t)/(Delta t_0)]^-a has a universal lognormal
distribution for all time intervals Delta t. We then compare measurements from
the solar wind with those from direct numerical simulations of
magnetohydrodynamic (MHD) turbulence. We find good agreement for P(Delta theta)
obtained in the two cases when the magnetic guide-field to fluctuations ratio
B_0/b_rms is chosen accordingly. However, the scale invariance of P((Delta
B)/B) is broken in the MHD simulations with relatively limited inertial
interval, which causes P(Delta theta) to scale with measurement interval
differently than in the solar wind.",1210.3613v1
2012-11-13,Tracing Electron Beams in the Sun's Corona with Radio Dynamic Imaging Spectroscopy,"We report observations of type III radio bursts at decimeter wavelengths
(type IIIdm bursts) -- signatures of suprathermal electron beams propagating in
the low corona -- using the new technique of radio dynamic imaging spectroscopy
provided by the recently upgraded Karl G. Jansky Very Large Array (VLA). For
the first time, type IIIdm bursts were imaged with high time and frequency
resolution over a broad frequency band, allowing electron beam trajectories in
the corona to be deduced. Together with simultaneous hard X-ray (HXR) and
extreme ultraviolet (EUV) observations, we show these beams emanate from an
energy release site located in the low corona at a height below ~15 Mm, and
propagate along a bundle of discrete magnetic loops upward into the corona. Our
observations enable direct measurements of the plasma density along the
magnetic loops, and allow us to constrain the diameter of these loops to be
less than 100 km. These over-dense and ultra-thin loops reveal the
fundamentally fibrous structure of the Sun's corona. The impulsive nature of
the electron beams, their accessibility to different magnetic field lines, and
the detailed structure of the magnetic release site revealed by the radio
observations indicate that the localized energy release is highly fragmentary
in time and space, supporting a bursty reconnection model that involves
secondary magnetic structures for magnetic energy release and particle
acceleration.",1211.3058v2
2013-01-18,Collapse and Fragmentation of Magnetic Molecular Cloud Cores with the Enzo AMR MHD Code. I. Uniform Density Sphere,"Magnetic fields are important contributers to the dynamics of collapsing
molecular cloud cores, and can have a major effect on whether collapse results
in a single protostar or fragmentation into a binary or multiple protostar
system. New models are presented of the collapse of magnetic cloud cores using
the adaptive mesh refinement (AMR) code Enzo2.0. The code was used to calculate
the ideal magnetohydrodynamics (MHD) of initially spherical, uniform density
and rotation clouds with density perturbations, i.e., the Boss and Bodenheimer
(1979) standard isothermal test case for three dimensional (3D) hydrodynamics
(HD) codes. After first verifying that Enzo reproduces the binary fragmentation
expected for the non-magnetic test case, a large set of models was computed
with varied initial magnetic field strengths and directions with respect to the
cloud core axis of rotation (parallel or perpendicular), density perturbation
amplitudes, and equations of state. Three significantly different outcomes
resulted: (1) contraction without sustained collapse, forming a denser cloud
core, (2) collapse to form a single protostar with significant spiral arms, and
(3) collapse and fragmentation into binary or multiple protostar systems, with
multiple spiral arms. Comparisons are also made with previous MHD calculations
of similar clouds with barotropic equations of state. These results for the
collapse of initially uniform density spheres illustrate the central importance
of both magnetic field direction and field strength for determining the outcome
of dynamic protostellar collapse.",1301.4495v1
2014-01-08,Dynamical quark mass generation in a strong external magnetic field,"We investigate the effect of a strong magnetic field on dynamical chiral
symmetry breaking in quenched and unquenched QCD. To this end we apply the
Ritus formalism to the coupled set of (truncated) Dyson-Schwinger equations for
the quark and gluon propagator under the presence of an external constant
Abelian magnetic field. We work with an approximation that is trustworthy for
large fields eH > \Lambda_{QCD}^2 but is not restricted to the lowest Landau
level. We confirm the linear rise of the quark condensate with large external
field previously found in other studies and observe the transition to the
asymptotic power law at extremely large fields. We furthermore quantify the
validity of the lowest Landau level approximation and find substantial
quantitative differences to the full calculation even at very large fields. We
discuss unquenching effects in the strong field propagators, condensate and the
magnetic polarization of the vacuum. We find a significant weakening of
magnetic catalysis caused by the back reaction of quarks on the Yang-Mills
sector. Our results support explanations of the inverse magnetic catalysis
found in recent lattice studies due to unquenching effects.",1401.1647v2
2014-12-04,Cross-correlation Aided Transport in Stochastically Driven Accretion Flows,"Origin of linear instability resulting in rotating sheared accretion flows
has remained a controversial subject for long. While some explanations of such
non-normal transient growth of disturbances in the Rayleigh stable limit were
available for magnetized accretion flows, similar instabilities in absence of
magnetic perturbations remained unexplained. This dichotomy was resolved in two
recent publications by Chattopadhyay, {\it et al} where it was shown that such
instabilities, especially for non-magnetized accretion flows, were introduced
through interaction of the inherent stochastic noise in the system (even a
\enquote{cold} accretion flow at 3000K is too \enquote{hot} in the statistical
parlance and is capable of inducing strong thermal modes) with the underlying
Taylor-Couette flow profiles. Both studies, however, excluded the additional
energy influx (or efflux) that could result from nonzero cross-correlation of a
noise perturbing the velocity flow, say, with the noise that is driving the
vorticity flow (or equivalently the magnetic field and magnetic vorticity flow
dynamics). In this article we show that nonzero noise cross-correlations
essentially renormalize the strength of temporal correlations. Apart from an
overall boost in the energy rate (both for spatial and temporal correlations,
and hence in the ensemble averaged energy spectra), this results in mutual
competition in growth rates of affected variables often resulting in
suppression of oscillating Alfven waves at small times while leading to faster
saturations at relatively longer time scales. The effects are seen to be more
pronounced with magnetic field fluxes where the noise cross-correlation
magnifies the strength of the field concerned.",1412.1752v1
2015-01-18,Dynamics of Dirac strings and monopole-like excitations in chiral magnets under a current drive,"Skyrmion lines in metallic chiral magnets carry an emergent magnetic field
experienced by the conduction electrons. The inflow and outflow of this field
across a closed surface is not necessarily equal, thus it allows for the
existence of emergent monopoles. One example is a segment of skyrmion line
inside a crystal, where a monopole and antimonopole pair is connected by the
emergent magnetic flux line. This is a realization of Dirac string-like
excitations. Here we study the dynamics of monopoles in chiral magnets under an
electric current. We show that in the process of creation of skyrmion lines,
skyrmion line segments are first created via the proliferation of monopoles and
antimonopoles. Then these line segments join and span the whole system through
the annihilation of monopoles. The skyrmion lines are destroyed via the
proliferation of monopoles and antimonopoles at high currents, resulting in a
chiral liquid phase. We also propose to create the monopoles in a controlled
way by applying an inhomogenous current to a crystal. Remarkably, an electric
field component in the magnetic field direction proportional to the current
squared in the low current region is induced by the motion of distorted
skyrmion lines, in addition to the Hall and longitudinal voltage. The existence
of monopoles can be inferred from transport or imaging measurements.",1501.04356v2
2015-02-10,Magnetic-field and doping dependence of low-energy spin fluctuations in the antiferroquadrupolar compound Ce(1-x)La(x)B(6),"CeB(6) is a model compound exhibiting antiferroquadrupolar (AFQ) order, its
magnetic properties being typically interpreted within localized models. More
recently, the observation of strong and sharp magnetic exciton modes forming in
its antiferromagnetic (AFM) state at both ferromagnetic and AFQ wave vectors
suggested a significant contribution of itinerant electrons to the spin
dynamics. Here we investigate the evolution of the AFQ excitation upon the
application of an external magnetic field and the substitution of Ce with
non-magnetic La, both parameters known to suppress the AFM phase. We find that
the exciton energy decreases proportionally to T_N upon doping. In field, its
intensity is suppressed, while its energy remains constant. Its disappearance
above the critical field of the AFM phase is preceded by the formation of two
modes, whose energies grow linearly with magnetic field upon entering the AFQ
phase. These findings suggest a crossover from itinerant to localized spin
dynamics between the two phases, the coupling to heavy-fermion quasiparticles
being crucial for a comprehensive description of the magnon spectrum.",1502.03139v2
2015-04-13,The Evolution and Impacts of Magnetorotational Instability in Magnetized Core-Collapse Supernovae,"We carried out 2D-axisymmetric MHD simulations of core-collapse supernovae
for rapidly-rotating magnetized progenitors. By changing both the strength of
the magnetic field and the spatial resolution, the evolution of the
magnetorotational instability (MRI) and its impacts upon the dynamics are
investigated. We found that the MRI greatly amplifies the seed magnetic fields
in the regime where not the Alfv\'en mode but the buoyant mode plays a primary
role in the exponential growth phase. The MRI indeed has a powerful impact on
the supernova dynamics. It makes the shock expansion faster and the explosion
more energetic, with some models being accompanied by the collimated-jet
formations. These effects, however, are not made by the magnetic pressure
except for the collimated-jet formations. The angular momentum transfer induced
by the MRI causes the expansion of the heating region, by which the accreting
matter gain an additional time to be heated by neutrinos. The MRI also drifts
low-$Y_p$ matter from the deep inside of the core to the heating region, which
makes the net neutrino heating rate larger by the reduction of the cooling due
to the electron capture. These two effects enhance the efficiency of the
neutrino heating, which is found to be the key to boost the explosion. Indeed
we found that our models explode far more weakly when the net neutrino heating
is switched off. The contribution of the neutrino heating to the explosion
energy could reach 60\% even in the case of strongest magnetic field in the
current simulations.",1504.03035v2
2015-05-21,Exact two-dimensionalization of low-magnetic-Reynolds-number flows subject to a strong magnetic field,"We investigate the behavior of flows, including turbulent flows, driven by a
horizontal body-force and subject to a vertical magnetic field, with the
following question in mind: for very strong applied magnetic field, is the flow
mostly two-dimensional, with remaining weak three-dimensional fluctuations, or
does it become exactly 2D, with no dependence along the vertical?
  We first focus on the quasi-static approximation, i.e. the asymptotic limit
of vanishing magnetic Reynolds number Rm << 1: we prove that the flow becomes
exactly 2D asymptotically in time, regardless of the initial condition and
provided the interaction parameter N is larger than a threshold value. We call
this property ""absolute two-dimensionalization"": the attractor of the system is
necessarily a (possibly turbulent) 2D flow.
  We then consider the full-magnetohydrodynamic equations and we prove that,
for low enough Rm and large enough N, the flow becomes exactly two-dimensional
in the long-time limit provided the initial vertically-dependent perturbations
are infinitesimal. We call this phenomenon ""linear two-dimensionalization"": the
(possibly turbulent) 2D flow is an attractor of the dynamics, but it is not
necessarily the only attractor of the system. Some 3D attractors may also exist
and be attained for strong enough initial 3D perturbations.
  These results shed some light on the existence of a dissipation anomaly for
magnetohydrodynamic flows subject to a strong external magnetic field.",1505.05662v1
2015-06-02,Respective influence of in-plane and out-of-plane spin-transfer torques in magnetization switching of perpendicular magnetic tunnel junctions,"The relative contributions of in-plane (damping-like) and out-of-plane
(field-like) spin-transfer-torques in the magnetization switching of
out-of-plane magnetized magnetic tunnel junctions (pMTJ) has been theoretically
analyzed using the transformed Landau-Lifshitz (LL) equation with the STT
terms. It is demonstrated that in a pMTJ structure obeying macrospin dynamics,
the out-of-plane torque influences the precession frequency but it does not
contribute significantly to the STT switching process (in particular to the
switching time and switching current density), which is mostly determined by
the in-plane STT contribution. This conclusion is confirmed by finite
temperature and finite writing pulse macrospin simulations of the current-field
switching diagrams. It contrasts with the case of STT-switching in in-plane
magnetized MTJ in which the field-like term also influences the switching
critical current. This theoretical analysis was successfully applied to the
interpretation of voltage-field STT switching diagrams experimentally measured
on perpendicular MTJ pillars 36 nm in diameter, which exhibit macrospin-like
behavior. The physical nonequivalence of Landau and Gilbert dissipation terms
in presence of STT-induced dynamics is also discussed.",1506.00780v2
2015-06-18,Dynamical mean-field study of partial Kondo screening in the periodic Anderson model on the triangular lattice,"The competition between Kondo screening and indirect magnetic exchange is
studied for a system with geometrical frustration using dynamical mean-field
theory (DMFT). We systematically scan the weak- to strong-coupling regime of
the periodic Anderson model on the triangular lattice for a wide range of
fillings n. The magnetic phase diagram is derived using a site-dependent DMFT
approach by self-consistent mapping onto three independent single-impurity
models corresponding to the three correlated f orbitals in the unit cell. At
half-filling, the system is a non-magnetic Kondo insulator for all considered
interaction strengths U>0 which immediately develops into a non-magnetic
metallic Kondo-singlet phase for fillings slightly below half-filling. On the
other hand, indirect magnetic exchange between the f moments results in
antiferromagnetic order at lower fillings. The antiferromagnetic and the
Kondo-singlet phases are separated in the U-n phase diagram by an extended
region of partial Kondo screening, i.e., a phase where the magnetic moment at
one site in the unit cell is Kondo screened while the remaining two are coupled
antiferromagnetically. At even lower fillings the system crosses over from a
local-moment to a mixed-valence regime where the minimization of the kinetic
energy in a strongly correlated system gives rise to a metallic and partially
polarized ferromagnetic state.",1506.05755v2
2015-06-22,Analytic Bjorken flow in one-dimensional relativistic magnetohydrodynamics,"In the initial stage of relativistic heavy-ion collisions, strong magnetic
fields appear due to the large velocity of the colliding charges. The evolution
of these fields appears as a novel and intriguing feature in the
fluid-dynamical description of heavy-ion collisions. In this work, we study
analytically the one-dimensional, longitudinally boost-invariant motion of an
ideal fluid in the presence of a transverse magnetic field. Interestingly, we
find that, in the limit of ideal magnetohydrodynamics, i.e., for infinite
conductivity, and irrespective of the strength of the initial magnetization,
the decay of the fluid energy density $e$ with proper time $\tau$ is the same
as for the time-honored ""Bjorken flow"" without magnetic field. Furthermore,
when the magnetic field is assumed to decay $\sim \tau^{-a}$, where $a$ is an
arbitrary number, two classes of analytic solutions can be found depending on
whether $a$ is larger or smaller than one. In summary, the analytic solutions
presented here highlight that the Bjorken flow is far more general than
formerly thought. These solutions can serve both to gain insight on the
dynamics of heavy-ion collisions in the presence of strong magnetic fields and
as testbeds for numerical codes.",1506.06620v2
2015-08-11,Anisotropic Formation of Magnetized Cores in Turbulent Clouds,"In giant molecular clouds (GMCs), shocks driven by converging turbulent flows
create high-density, strongly-magnetized regions that are locally sheetlike. In
previous work, we showed that within these layers, dense filaments and embedded
self-gravitating cores form by gathering material along the magnetic field
lines. Here, we extend the parameter space of our three-dimensional, turbulent
MHD core formation simulations. We confirm the anisotropic core formation model
we previously proposed, and quantify the dependence of median core properties
on the pre-shock inflow velocity and upstream magnetic field strength. Our
results suggest that bound core properties are set by the total dynamic
pressure (dominated by large-scale turbulence) and thermal sound speed c_s in
GMCs, independent of magnetic field strength. For models with Mach number
between 5 and 20, the median core masses and radii are comparable to the
critical Bonnor-Ebert mass and radius defined using the dynamic pressure for
P_ext. Our results correspond to M_core = 1.2 c_s^4/sqrt(G^3 rho_0 v_0^2) and
R_core = 0.34 c_s^2/sqrt(G rho_0 v_0^2) for rho_0 and v_0 the large-scale mean
density and velocity. For our parameter range, the median M_core ~ 0.1-1 M_sun,
but a very high pressure cloud could have lower characteristic core mass. We
find cores and filaments form simultaneously, and filament column densities are
a factor ~2 greater than the surrounding cloud when cores first collapse. We
also show that cores identified in our simulations have physical properties
comparable to those observed in the Perseus cloud. Superthermal cores in our
models are generally also magnetically supercritical, suggesting that the same
may be true in observed clouds.",1508.02710v1
2015-08-14,Numerical MHD Simulation of the Coupled Evolution of Collisional Plasma and Magnetic Field in the Solar Chromosphere. I. Gradual and Impulsive Energisation,"The dynamical coupling between the solar chromospheric plasma and magnetic
field is investigated by numerically solving a fully self-consistent,
two-dimensional initial-value problem for the nonlinear collisional MHD
equations including electric resistivity, thermal conduction, and, in some
cases, gas-dynamic viscosity. The processes in the contact zone between two
horizontal magnetic fields of opposite polarities are considered. The plasma is
assumed to be initially motionless and having a temperature of 50,000 K uniform
throughout the plasma volume; the characteristic magnetic field corresponds to
a plasma $\beta\gtrsim 1$. In a physical-time interval of 17~seconds typically
covered by a computational run, the plasma temperature gradually increases by a
factor of two to three. Against this background, an impulsive (in 0.1 seconds
or less) increase in the current-aligned plasma velocity occurs at the site of
the current-layer thinning (sausage-type deformation, or $m=0$ pinch
instability). Such a ""velocity burst"" can be interpreted physically as an event
of suprathemal-proton generation. Further development of the sausage
instability results in an increase in the kinetic temperature of the protons to
high values, even to those observed in flares. The form of our system of MHD
equations indicates that such increases are a property of the exact solution of
the system at an appropriate choice of the parameters. Magnetic reconnection
does not manifest itself in this solution: it would generate flows forbidden by
the chosen geometry. Therefore, the pinch-sausage effect can act as an
energiser of the upper chromosphere and be an alternative to the
magnetic-reconnection process as the producer of flares.",1508.03626v1
2015-12-18,Electronic Correlation and Magnetism in the Ferromagnetic Metal Fe3GeTe2,"Motivated by the search for design principles of rare-earth-free strong
magnets, we present a study of electronic structure and magnetic properties of
the ferromagnetic metal Fe3GeTe2 within local density approximation (LDA) of
the density functional theory, and its combination with dynamical mean-field
theory (DMFT). For comparison to these calculations, we have measured magnetic
and thermodynamic properties as well as X-ray magnetic circular dichroism and
the photoemission spectrum of single crystal Fe3GeTe2. We find that the
experimentally determined Sommerfeld coefficient is enhanced by an order of
magnitude with respect to the LDA value. This enhancement can be partially
explained by LDA+DMFT. In addition, the inclusion of dynamical electronic
correlation effects provides the experimentally observed magnetic moments, and
the spectral density is in better agreement with photoemission data. These
results establish the importance of electronic correlations in this
ferromagnet.",1512.05847v2
2016-04-25,Frequency-based nanoparticle sensing over large field ranges using the ferromagnetic resonances of a magnetic nanodisc,"Using finite element micromagnetic simulations, we study how resonant
magnetisation dynamics in thin magnetic discs with perpendicular anisotropy are
influenced by magnetostatic coupling to a magnetic nanoparticle. We identify
resonant modes within the disc using direct magnetic eigenmode calculations and
study how their frequencies and profiles are changed by the nanoparticle's
stray magnetic field. We demonstrate that particles can generate shifts in the
resonant frequency of the disc's fundamental mode which exceed resonance
linewidths in recently studied spin torque oscillator devices. Importantly, it
is shown that the simulated shifts can be maintained over large field ranges
(here up to 1T). This is because the resonant dynamics (the basis of
nanoparticle detection here) respond directly to the nanoparticle stray field,
i.e. detection does not rely on nanoparticle-induced changes to the magnetic
ground state of the disk. A consequence of this is that in the case of small
disc-particle separations, sensitivities to the particle are highly mode- and
particle-position-dependent, with frequency shifts being maximised when the
intense stray field localised directly beneath the particle can act on a large
proportion of the disc's spins that are undergoing high amplitude precession.",1604.07277v2
2016-05-06,Nanosecond-timescale low error switching of in-plane magnetic tunnel junctions through dynamic Oersted-field assisted spin-Hall effect,"We investigate fast-pulse switching of in-plane-magnetized magnetic tunnel
junctions (MTJs) within 3-terminal devices in which spin-transfer torque is
applied to the MTJ by the giant spin Hall effect. We measure reliable
switching, with write error rates down to $10^{-5}$, using current pulses as
short as just 2 ns in duration. This represents the fastest reliable switching
reported to date for any spin-torque-driven magnetic memory geometry, and
corresponds to a characteristic time scale that is significantly shorter than
predicted possible within a macrospin model for in-plane MTJs subject to
thermal fluctuations at room temperature. Using micromagnetic simulations, we
show that in the 3-terminal spin-Hall devices the Oersted magnetic field
generated by the pulse current strongly modifies the magnetic dynamics excited
by the spin-Hall torque, enabling this unanticipated performance improvement.
Our results suggest that in-plane MTJs controlled by Oersted-field-assisted
spin-Hall torque are a promising candidate for both cache memory applications
requiring high speed and for cryogenic memories requiring low write energies.",1605.02104v2
2017-01-05,Collective vortex pinning and crossover between second order to first order transition in optimally doped Ba1-xKxBiO3 single crystals,"Measurements on magnetization and relaxation have been carried out on an
optimally doped Ba$_{0.59}$K$_{0.47}$BiO$_{3+\delta}$ single crystal with $T_c$
= 31.3 K. Detailed analysis is undertaken on the data. Both the dynamical
relaxation and conventional relaxation have been measured leading to the
self-consistent determination of the magnetization relaxation rate. It is found
that the data are well described by the collective pinning model leading to the
glassy exponent of about $\mu\approx$ 1.64 to 1.68 with the magnetic fields of
1 and 3 T. The analysis based on Maley's method combining with the conventional
relaxation data allows us to determine the current dependent activation energy
$U$ which yields a $\mu$ value of about 1.23 to 1.29 for the magnetic fields of
1 and 3 T. The second magnetization peaks appear in wide temperature region
from 2 K to 24 K. The separation between the second peak field and the
irreversibility field becomes narrow when temperature is increased. When the
two fields are close to each other, we find that the second peak evolves into a
step like transition of magnetization, suggesting a crossover from the second
order to first order transition. Finally, we present a vortex phase diagram and
demonstrate that the vortex dynamics in Ba$_{1-x}$K$_x$BiO$_3$ can be used as a
model system for studying the collective vortex pining.",1701.01346v1
2017-07-09,Signatures of pairing in the magnetic excitation spectrum of strongly correlated ladders,"Magnetic interactions are widely believed to play a crucial role in the
microscopic mechanism leading to high critical temperature superconductivity.
It is therefore important to study the signatures of pairing in the magnetic
excitation spectrum of simple models known to show unconventional
superconducting tendencies. Using the Density Matrix Renormalization Group
technique, we calculate the dynamical spin structure factor $S({\bf k},\omega)$
of a generalized $t-U-J$ Hubbard model away from half-filling in a two-leg
ladder geometry. The addition of $J$ enhances pairing tendencies. We analyze
quantitatively the signatures of pairing in the magnetic excitation spectra. We
found that the superconducting pair-correlation strength, that can be estimated
independently from ground state properties, is closely correlated with the
integrated low-energy magnetic spectral weight in the vicinity of $(\pi,\pi)$.
In this wave-vector region, robust spin incommensurate features develop with
increasing doping. The branch of the spectrum with rung direction wave-vector
$k_{rung}=0$ does not change substantially with doping where pairing dominates,
and thus plays a minor role. We discuss the implications of our results for
neutron scattering experiments, where the spin excitation dynamics of
hole-doped quasi-one dimensional magnetic materials can be measured, and also
address implications for recent resonant inelastic X-ray scattering
experiments.",1707.02626v1
2017-10-17,Observation of superparamagnetism in coexistence with quantum anomalous Hall C=$\pm$1 and C=0 Chern states,"Simultaneous transport and scanning nanoSQUID-on-tip magnetic imaging studies
in Cr-(Bi,Sb)$_2$Te$_3$ modulation-doped films reveal the presence of
superparamagnetic order within the quantum anomalous Hall regime. In contrast
to the expectation that a long-range ferromagnetic order is required for
establishing the quantum anomalous Hall state, superparamagnetic dynamics of
weakly interacting nanoscale magnetic islands is observed both in the plateau
transition regions as well as within the fully quantized C=$\pm$1 Chern
plateaus. Modulation doping of the topological insulator films is found to give
rise to significantly larger superparamagnetic islands as compared to uniform
magnetic doping, evidently leading to enhanced robustness of the quantum
anomalous Hall effect. Nonetheless, even in this more robust quantum state,
attaining full quantization of transport coefficients requires magnetic
alignment of at least 95% of the superparamagnetic islands. The
superparamagnetic order is also found within the incipient C=0 zero Hall
plateau, which may host an axion state if the top and bottom magnetic layers
are magnetized in opposite directions. In this regime, however, a significantly
lower level of island alignment is found in our samples, hindering the
formation of the axion state. Comprehension and control of superparamagnetic
dynamics is thus a key factor in apprehending the fragility of the quantum
anomalous Hall state and in enhancing the endurance of the different quantized
states to higher temperatures for utilization of robust topological protection
in novel devices.",1710.06446v1
2017-10-20,Transverse instabilities of stripe domains in magnetic thin films with perpendicular magnetic anisotropy,"Stripe domains are narrow, elongated, reversed regions that exist in magnetic
materials with perpendicular magnetic anisotropy. Stripe domains appear as a
pair of domain walls that can exhibit topology with a nonzero chirality. Recent
experimental and numerical investigations identify an instability of stripe
domains in the long direction as a means of nucleating isolated magnetic
skyrmions. Here, the onset and nonlinear evolution of transverse instabilities
for a dynamic stripe domain known as the bion stripe are investigated. Both
non-topological and topological variants of the bion stripe are shown to
exhibit a long-wavelength transverse instability with different characteristic
features. In the former, small transverse variations in the stripe's width lead
to a neck instability that eventually pinches the non-topological stripe into a
chain of two-dimensional breathers composed of droplet soliton pairs. In the
latter case, small variations in the stripe's center results in a snake
instability whose topological structure leads to the nucleation of dynamic
magnetic skyrmions and antiskyrmions as well as perimeter-modulated droplets.
Quantitative, analytical predictions for both the early, linear evolution and
the long-time, nonlinear evolution are achieved using an averaged Lagrangian
approach that incorporates both exchange (dispersion) and anisotropy
(nonlinearity). The method of analysis is general and can be applied to other
filamentary structures.",1710.07718v1
2017-11-03,Geometric theory of the natural optical activity in noncentrosymmetric metals,"This is a PhD dissertation. Ignited by the chiral anomaly of recently
discovered Weyl (semi-)metals, we study the chiral magnetic effect and the
natural optical activity of noncentrosymmetric metals. Both phenomena are
related to the linear-in-$\mathbf{q}$ spatial dispersion of the optical
conductivity tensor, and can be calculated within the formalism of the
semiclassical kinetic equation. Therefore, we calculate the dispersion of
optical conductivity up to the linear order of the wave vector, in the low
frequency regime, with both the semiclassical Boltzmann equation and the Kubo
formula. The two different methods of calculation provide us the same result.
In this result, the static and dynamic chiral magnetic effects are revealed to
have different origin: one comes from topology, related to Berry monopoles, and
the other has a geometric origin, which is determined by the orbital magnetic
moment. The Faraday rotation of the polarization of light transmitted through a
slab of the sample provides us the most direct way to measure the magnitude of
dynamic chiral magnetic effect. We develop an effective medium theory for
electromagnetic wave propagating through gapless nonuniform systems, to
calculate macroscopic sample inhomogeneities induced corrections to the chiral
magnetic conductivity. We show that, in metals with low carrier density, the
way in which macroscopic fluctuations of the local conductivity affect the
frequency dependent of the measured optical polarization rotation angle: by
creating a sharp feature near the plasma edge, which can be detected by
experiments. Then we narrow down our research a bit further to the current
induced magnetization.",1711.01289v2
2018-01-11,Occurrence and persistence of magnetic elements in the quiet Sun,"Turbulent convection efficiently transports energy up to the solar
photosphere, but its multi-scale nature and dynamic properties are still not
fully understood. Several works in the literature have investigated the
emergence of patterns of convective and magnetic nature in the quiet Sun at
spatial and temporal scales from granular to global. Aims. To shed light on the
scales of organisation at which turbulent convection operates, and its
relationship with the magnetic flux therein, we studied characteristic spatial
and temporal scales of magnetic features in the quiet Sun. Methods. Thanks to
an unprecedented data set entirely enclosing a supergranule, occurrence and
persistence analysis of magnetogram time series were used to detect spatial and
long-lived temporal correlations in the quiet Sun and to investigate their
nature. Results. A relation between occurrence and persistence representative
for the quiet Sun was found. In particular, highly recurrent and persistent
patterns were detected especially in the boundary of the supergranular cell.
These are due to moving magnetic elements undergoing motion that behaves like a
random walk together with longer decorrelations ($\sim2$ h) with respect to
regions inside the supergranule. In the vertices of the supegranular cell the
maximum observed occurrence is not associated with the maximum persistence,
suggesting that there are different dynamic regimes affecting the magnetic
elements.",1801.03871v1
2018-05-04,Selective measurements of intertwined multipolar orders: non-Kramers doublets on a triangular lattice,"Motivated by the rapid experimental progress on the spin-orbit-coupled Mott
insulators, we propose and study a generic spin model that describes the
interaction between the non-Kramers doublets on a triangular lattice and is
relevant for triangular lattice rare-earth magnets. We predict that the system
supports both pure quadrupolar orders and intertwined multipolar orders in the
phase diagram. Besides the multipolar orders, we explore the magnetic
excitations to reveal the dynamic properties of the systems. Due to the
peculiar properties of the non-Kramers doublets and the selective coupling to
the magnetic field, we further study the magnetization process of the system in
the magnetic field. We point out the selective measurements of the static and
dynamic properties of the intertwined multipolarness in the neutron scattering,
NMR and $\mu$SR probes and predict the experimental consequences. The relevance
to the existing materials such as TmMgGaO$_{4}$, Pr-based and Tb-based magnets,
and many ternary chalcogenides is discussed. Our results illustrate the rich
physics and the promising direction in the interplay between strong
spin-orbit-entangled multipole moments and the geometrical frustration.",1805.01865v3
2018-06-01,"Magnetic phases of skyrmion-hosting GaV$_4$S$_{8-y}$Se$_{y}$ ($y = 0, 2, 4, 8$) probed with muon spectroscopy","We present the results of a muon-spin spectroscopy investigation of
GaV$_4$S$_{8-y}$Se$_{y}$ with $y=0, 2, 4$ and 8. Zero-field measurements
suggest that GaV$_{4}$Se$_{8}$ and GaV$_{4}$S$_{8}$ have distinct magnetic
ground states, with the latter material showing an anomalous
temperature-dependence of the local magnetic field. It is not possible to
evolve the magnetic state continuously between these two systems, with the
intermediate $y=2$ and $4$ materials showing glassy magnetic behaviour at low
temperature. The skyrmion lattice (SkL) phase is evident in the $y=0$ and 8
materials through an enhanced response of the muon-spin relaxation to the
emergent dynamics that accompany the SkL. For our polycrystalline samples of
GaV$_4$Se$_{8}$, this enhanced dynamic response is confined to a smaller region
of the magnetic field-temperature phase diagram than the previous reports of
the SkL in single crystals.",1806.00412v3
2018-06-14,Selection of axial dipole from a seed magnetic field in rapidly rotating dynamo models,"In this study, we investigate preferences of dipolar magnetic structure from
a seed magnetic field in the rapidly rotating spherical shell dynamo models. In
this study, we set up a realistic model to show the effect of the Lorentz force
in the polarity selection. The important results that has come out from our
study is that the magnetic field acts on the flow much before the saturation.
Our study suggests that the growth of the magnetic field is not a kinematic
effect as one might think off, rather a dynamic effect. This dynamic effect
grows as the field generated with time and finally brings the saturation to the
dynamo action. Previous studies show that Lorentz force effect the flow when
Elsasser number more or less 1 and the studies were focused on the saturation
by looking at the time-averaged quantities. However, in this study, we show a
clear effect of the Lorentz force even at Elsasser number of $0.3-0.4$. To show
the effect of the Lorentz force, we did two different simulations, one is a
nonlinear model and another is kinematic model and shows that how a magnetic
field can change the flow structure and by doing that the generated field
changes, while this kind of behavior is not observed in kinematic dynamo
models. This study shows a scale dependent behaviour of the kinetic helicity at
two different spectral range.",1806.05368v1
2018-08-21,Pseudospin-lattice coupling in the spin-orbit Mott insulator Sr2IrO4,"Spin-orbit entangled magnetic dipoles, often referred to as pseudospins,
provide a new avenue to explore novel magnetism inconceivable in the weak
spin-orbit coupling limit, but the nature of their low-energy interactions
remains to be understood. We present a comprehensive study of the static
magnetism and low-energy pseudospin dynamics in the archetypal spin-orbit Mott
insulator Sr2IrO4. We find that in order to understand even basic magnetization
measurements, a formerly overlooked in-plane anisotropy is fundamental. In
addition to magnetometry, we use neutron diffraction, inelastic neutron
scattering and resonant elastic and inelastic x-ray scattering to identify and
quantify the interactions that determine the global symmetry of the system and
govern the linear responses of pseudospins to external magnetic felds and their
low-energy dynamics. We find that a pseudospin-only Hamiltonian is insufficient
for an accurate description of the magnetism in Sr2IrO4 and that
pseudospin-lattice coupling is essential. This finding should be generally
applicable to other pseudospin systems with sizable orbital moments sensitive
to anisotropic crystalline environments.",1808.06920v2
2018-09-12,Low-Temperature and Dynamic Magnetism of Highly Frustrated 5d2 Li4MgOsO6 Polymorphs in Comparison with 5d3 Li3Mg2OsO6,"Geometric magnetic frustration (GMF) has attracted substantial interest due
to the exotic physics and rich phase diagrams revealed by the cancellation of
normally-dominant magnetic interactions, giving impetus for the search for
novel frustrated systems, most often based on antiferromagnetic correlations
between magnetic ions decorating triangular or tetrahedral lattices. We report
here low-temperature magnetic susceptibility and muon spin relaxation results
on Li4MgOsO6 and Li3Mg2OsO6, members of the A5BO6 ""rock salt ordered"" family of
frustrated materials. In Li3Mg2OsO6 we find spin freezing below 12K. In
Li4MgOsO6, which can crystallize into either orthorhombic Fddd or monoclinic
C2/m crystal symmetries depending on synthesis conditions, we find magnetism
consistent with glassy-like behavior dominating below 2K, with partial ordering
and evidence for dynamics at somewhat higher temperatures.",1809.04687v3
2018-10-19,Effect of Meissner screening and trapped magnetic flux on magnetization dynamics in thick Nb/Ni80Fe20/Nb trilayers,"We investigate the influence of Meissner screening and trapped magnetic flux
on magnetization dynamics for a Ni80Fe20 film sandwiched between two thick Nb
layers (100 nm) using broadband (5-20 GHz) ferromagnetic resonance (FMR)
spectroscopy. Below the superconducting transition Tc of Nb, significant
zero-frequency line broadening (5-6 mT) and DC resonance field shift (50 mT) to
a low field are both observed if the Nb thickness is comparable to the London
penetration depth of Nb films (>= 100 nm). We attribute the observed peculiar
behaviors to the increased incoherent precession near the Ni80Fe20/Nb interface
and the effectively focused magnetic flux in the middle Ni80Fe20 caused by
strong Meissner screening and (defect-)trapped flux of the thick adjacent Nb
layers. This explanation is supported by static magnetic properties of the
samples and comparison with FMR data on thick Nb/Ni80Fe20 bilayers. Great care
should therefore be taken in the analysis of FMR response in ferromagnetic
Josephson structures with thick superconductors, a fundamental property for
high-frequency device applications of spin-polarized supercurrents.",1810.08443v2
2018-11-21,Does magnetic field modify tidal dynamics in the convective envelope of Solar mass stars?,"The energy dissipation of wave-like tidal flows in the convective envelope of
low-mass stars is one of the key physical mechanisms that shape the orbital and
rotational dynamics of short-period planetary systems. Tidal flows, and the
excitation, propagation, and dissipation of tidally-induced inertial waves can
be modified by stellar magnetic fields (e.g., Wei 2016, 2018, Lin and Ogilvie
2018). It is thus important to assess for which stars, at which location of
their internal structure, and at which phase of their evolution, one needs to
take into account the effects of magnetic fields on tidal waves. Using scaling
laws that provide the amplitude of dynamo-generated magnetic fields along the
rotational evolution of these stars (e.g., Christensen et al. 2009, Brun et al.
2015), combined with detailed grids of stellar rotation models (e.g., Amard et
al. 2016), we examine the influence of a magnetic field on tidal forcing and
dissipation near the tachocline of solar-like stars. We show that full
consideration of magnetic fields is required to compute tidal dissipation, but
not necessarily for tidal forcing.",1811.08649v1
2018-12-06,Finite Dissipation in Anisotropic Magnetohydrodynamic Turbulence,"In presence of an externally supported, mean magnetic field a turbulent,
conducting medium, such as plasma, becomes anisotropic. This mean magnetic
field, which is separate from the fluctuating, turbulent part of the magnetic
field, has considerable effects on the dynamics of the system. In this paper,
we examine the dissipation rates for decaying incompressible
magnetohydrodynamic (MHD) turbulence with increasing Reynolds number, and in
the presence of a mean magnetic field of varying strength. Proceeding
numerically, we find that as the Reynolds number increases, the dissipation
rate asymptotes to a finite value for each magnetic field strength, confirming
the K\'arm\'an-Howarth hypothesis as applied to MHD. The asymptotic value of
the dimensionless dissipation rate is initially suppressed from the
zero-mean-field value by the mean magnetic field but then approaches a constant
value for higher values of the mean field strength. Additionally, for
comparison, we perform a set of two-dimensional (2DMHD) and a set of reduced
MHD (RMHD) simulations. We find that the RMHD results lie very close to the
values corresponding to the high mean-field limit of the three-dimensional runs
while the 2DMHD results admit distinct values far from both the zero mean field
cases and the high mean field limit of the three-dimensional cases. These
findings provide firm underpinnings for numerous applications in space and
astrophysics wherein von K\'arm\'an decay of turbulence is assumed.",1812.02379v1
2018-12-28,High-order two-fluid plasma solver for direct numerical simulations of plasma flows with full transport phenomena,"The two-fluid plasma equations for a single ion species, with full transport
terms, including temperature and magnetic field dependent ion and electron
viscous stresses and heat fluxes, frictional drag force, and ohmic heating term
have been implemented in the CFDNS code and solved by using sixth-order
non-dissipative compact finite differences for plasma flows in several
different regimes. In order to be able to fully resolve all the dynamically
relevant time and length scales, while maintaining computational feasibility,
the assumptions of infinite speed of light and negligible electron inertia have
been made. Non-dimensional analysis of the two-fluid plasma equations shows
that, by varying the characteristic/background number density, length scale,
temperature, and magnetic strength, the corresponding Hall, resistive, and
ideal magnetohydrodynamics (MHD) equations can be recovered as limiting cases.
The accuracy and robustness of this two-fluid plasma solver in handling plasma
flows in different regimes have been validated against four canonical problems:
Alfven and whistler dispersion relations, electromagnetic plasma shock, and
magnetic reconnection. For all test cases, by using physical dissipation and
diffusion, with negligible numerical dissipation/diffusion, fully converged
Direct Numerical Simulations (DNS)-like solutions are obtained when the ion
Reynolds number based on grid size is smaller than a threshold value which is
about 2.3 in this study. For the magnetic reconnection problem, the results
show that the magnetic flux saturation time and value converge when the ion and
magnetic Reynolds numbers are large enough. Thus, the DNS-like results become
relevant to practical problems with much larger Reynolds numbers.",1812.11237v1
2019-01-05,Generation and transport of magnetic flux in accretion-ejection flows,"Astrophysical accretion flows are associated with energetic emission of
radiation and outflows (winds and jets). Extensive observations of these two
processes in X-ray binary outbursts are available. A convincing understanding
of their dynamics remains, however, elusive. The main agent that controls the
dynamics is believed to be a large scale magnetic field that threads the
system. We propose that during the quiescent state, the field is held in place
by a delicate balance between inward advection and outward diffusion through
the accreting matter. We also propose that the source of the field is a growing
toroidal electric current generated by the aberrated radiation pressure on the
innermost plasma electrons in orbit around the central black hole. This is the
astrophysical mechanism of the Cosmic Battery. When the return magnetic field
outside the toroidal electric current diffuses through the surrounding disk,
the disk magnetic field and its associated accretion rate gradually increase,
thus leading the system to an outburst. After the central accretion flow
approaches equipartition with radiation, it is disrupted, and the Cosmic
Battery ceases to operate. The outward field diffusion is then reversed,
magnetic flux reconnects with the flux accumulated around the central black
hole and disappears. The magnetic field and the associated accretion rate
slowly decrease, and the system is gradualy driven back to quiescence. We
conclude that the action (or inaction) of the Cosmic Battery may be the missing
key that will allow us to understand the long-term evolution of astrophysical
accretion-ejection flows.",1901.01404v3
2019-03-26,Astro2020 Science White Paper: Probing Magnetic Reconnection in Solar Flares - New Perspectives from Radio Dynamic Imaging Spectroscopy,"Magnetic reconnection is a fundamental physical process in many laboratory,
space, and astrophysical plasma contexts. Solar flares serve as an outstanding
laboratory to study the magnetic reconnection and the associated energy release
and conversion processes under plasma conditions difficult to reproduce in the
laboratory, and with considerable spatiotemporal details not possible elsewhere
in astrophysics. Here we emphasize the unique power of remote-sensing
observations of solar flares at radio wavelengths. In particular, we discuss
the transformative technique of broadband radio dynamic imaging spectroscopy in
making significant contributions to addressing several outstanding challenges
in magnetic reconnection, including the capability of pinpointing magnetic
reconnection sites, measuring the time-evolving reconnecting magnetic fields,
and deriving the spatially and temporally resolved distribution function of
flare-accelerated electrons.",1903.11192v1
2019-03-28,Magnetic filed dynamics and varying plasma emission in large coronal loops,"In this study we report detailed observations of magnetic environment at four
footpoints of two warm coronal loops observed on 5 May 2016 in NOAA AR 12542
(Loop I) and 17 Dec 2015 in NOAA AR 12470 (Loop II). These loops were
connecting a plage region with sunspot periphery (Loop I) and a sunspot umbra
(Loop II). We used Solar Dynamics Observatory (SDO) and Goode Solar Telescope
(GST) data to describe the phenomenon and understand its causes. The study
indicates loop brightening episodes were associated with magnetic flux
emergence and cancellation processes observed in SDO's Helioseismic and
Magnetic Imager (HMI) and GST's Near InfraRed Imaging Spectrapolarimeter
(NIRIS) data. The observed activity was driven by magnetic reconnection between
small-scale emerging dipoles and large-scale pre-existing fields, suggesting
that the reconnection occurred in the lower chromosphere at the edge of an
extended plage region, where the loops were rooted. We suggest that plasma,
evaporated during these reconnection events, gradually filled the loops and as
it cooled the visible density front propagated from one footpoint of the loop
to another at a rate of 90-110 km s$^{-1}$. This study also indicates that at
least some of the bright loops seen in SDO Atmospheric Imaging Assembly images
rooted in sunspot umbra may be heated due to magnetic activity taking place at
the remote (non-sunspot) footpoint.",1903.12148v1
2019-04-11,FSRQ/BL Lac dichotomy as the magnetized advective accretion process around black holes: a unified classification of blazars,"The $Fermi$ blazar observations show a strong correlation between
$\gamma$-ray luminosities and spectral indices. BL Lac objects are less
luminous with harder spectra than flat-spectrum radio quasars (FSRQs).
Interestingly FSRQs are evident to exhibit a Keplerian disc component along
with a powerful jet. We compute the jet intrinsic luminosities by beaming
corrections determined by different cooling mechanisms. Observed $\gamma$-ray
luminosities and spectroscopic measurements of broad emission lines suggest a
correlation of the accretion disc luminosity with jet intrinsic luminosity.
Also, theoretical and observational inferences for these jetted sources
indicate a signature of hot advective accretion flow and a dynamically dominant
magnetic field at jet-footprint. Indeed it is difficult to imagine the powerful
jet launching from a geometrically thin Keplerian disc. We propose a
magnetized, advective disc-outflow symbiosis with explicit cooling to address a
unified classification of blazars by controlling both the mass accretion rate
and magnetic field strength. The large scale strong magnetic fields influence
the accretion dynamics, remove angular momentum from the infalling matter, help
in the formation of strong outflows/jets, and lead to synchrotron emissions
simultaneously. We suggest that the BL Lacs are more optically thin and
magnetically dominated than FSRQs at the jet-footprint to explain their
intrinsic $\gamma$-ray luminosities.",1904.05898v1
2019-04-21,Spectropolarimetric analysis of an active region filament. I. Magnetic and dynamical properties from single component inversions,"The determination of the magnetic filed vector in solar filaments is possible
by interpreting the Hanle and Zeeman effects in suitable chromospheric spectral
lines like those of the He I multiplet at 10830 A. We study the vector magnetic
field of an active region filament (NOAA 12087). Spectropolarimetric data of
this active region was acquired with the GRIS instrument at the GREGOR
telescope and studied simultaneously in the chromosphere with the He I 10830 A
multiplet and in the photosphere with the Si I 10827 A line. As it is usual
from previous studies, only a single component model is used to infer the
magnetic properties of the filament. The results are put into a solar context
with the help of the Solar Dynamic Observatory images. Some results clearly
point out that a more complex inversion had to be done. Firstly, the Stokes $V$
map of He I does not show any clear signature of the presence of the filament.
Secondly, the local azimuth map follows the same pattern than Stokes $V$ as if
the polarity of Stokes $V$ were conditioning the inference to very different
magnetic field even with similar linear polarization signals. This indication
suggests that the Stokes $V$ could be dominated by the below magnetic field
coming from the active region, and not, from the filament itself. Those and
more evidences will be analyzed in depth and a more complex inversion will be
attempted in the second part of this series.",1904.09593v1
2019-06-18,The structure of magnetic turbulence in the heliosheath region observed by Voyager 2 at 106 AU,"It is currently believed that the turbulent fluctuations pervade the
outermost heliosphere. Turbulence, magnetic reconnection, and their link may be
responsible for magnetic energy conversion in these regions. The governing
mechanisms of such anisotropic and compressible magnetic turbulence in the
inner heliosheath (IHS) and in the local interstellar medium (LISM) still lack
a thorough description. The present literature mainly concerns large scales
which are not representative of the inertial-cascade dynamics of turbulence.
Moreover, lack of broadband spectral analysis makes the IHS dynamics critically
understudied. Our recent study shows that 48 s magnetic-field data from the
Voyager mission are appropriate for a spectral analysis over a frequency range
of six decades, from 5 x 10-8 Hz to 10-2 Hz. Here, focusing on the Voyager 2
observation interval from 2013.824 to 2016.0, we describe the structure of
turbulence in a sector zone of the IHS. A spectral break around 7 x 10-7 Hz
(magnetic structures with size l~1.3 Astronomical Units) separates the
energy-injection regime from the inertial-cascade regime of turbulence. A
second scale is observed around 6 x 10-5 Hz (l~ 0.017 AU) and corresponds to a
peak of compressibility and intermittency of fluctuations.",1906.07520v1
2019-07-18,Kosterlitz-Thouless Melting of Magnetic Order in the Triangular Quantum Ising Material TmMgGaO$_4$,"Frustrated magnets host the promises of material realizations of new paradigm
of quantum matter, while direct comparison of unbiased model calculations with
experimental measurements is still very challenging. Here, we design and
implement a protocol of employing many-body computation methodologies for
accurate model calculation -- both equilibrium and dynamical properties -- of a
frustrated rare-earth magnet TmMgGaO$_4$ (TMGO), which perfectly explains the
corresponding experimental findings. Our results confirm TMGO is an ideal
realization of triangular-lattice Ising model with an intrinsic transverse
field. The magnetic order of TMGO is predicted to melt through two successive
Kosterlitz-Thouless (KT) phase transitions, with a floating KT phase in
between. The dynamical spectra calculated suggest remnant images of a vanishing
magnetic stripe order that represent vortex-antivortex pairs, resembling rotons
in a superfluid helium film. TMGO therefore constitutes a rare quantum magnet
for realizing KT physics and we further propose experimental detections of its
intriguing properties.",1907.08173v3
2019-10-17,3-Dimensional Dusty Plasma in a Strong Magnetic Field: Observation of Rotating Dust Tori,"The paper reports on the dynamics of a 3-dimensional dusty plasma in a strong
magnetic field. An electrostatic potential well created by a conducting or
non-conducting ring in the rf discharge confines the charged dust particles. In
the absence of the magnetic field, dust grains exhibit a thermal motion about
their equilibrium position. As the magnetic field crosses a threshold value (B
$>$ 0.02 T), the edge particles start to rotate and form a vortex in the
vertical plane. At the same time, the central region particles either exhibit
thermal motion or $\vec{E} \times \vec{B}$ motion in the horizontal plane. At B
$>$ 0.15 T, the central region dust grains start to rotate in the opposite
direction resulting in a pair of counter-rotating vortices in the vertical
plane. The characteristics of the vortex pair change with increasing the
strength of the magnetic field (B $\sim$ 0.8 T). At B $>$ 0.8 T, dust grains
exhibit very complex motion in the rotating torus. The angular frequency
variation of rotating particles indicates a differential or sheared dust
rotation in a vortex. The angular frequency increases with increasing the
magnetic field from 0.05 T to 0.8 T. The ion drag force and dust charge
gradient along with the E-field are considered as possible energy sources for
driving the edge vortex flow and central region vortex motion, respectively.
The directions of rotation also confirm the different energy sources
responsible for the vortex motion.",1910.07846v2
2019-11-25,Quantum field theory of topological spin dynamics,"We develop a field theory of quantum magnets and magnetic (semi)metals, which
is suitable for the analysis of their universal and topological properties. The
systems of interest include collinear, coplanar and general non-coplanar
magnets. At the basic level, we describe the dynamics of magnetic moments using
smooth vector fields in the continuum limit. Dzyaloshinskii-Moriya interaction
is captured by a non-Abelian vector gauge field, and chiral spin couplings
related to topological defects appear as higher-rank antisymmetric tensor gauge
fields. We distinguish type-I and type-II magnets by their equilibrium response
to the non-Abelian gauge flux, and characterize the resulting lattices of
skyrmions and hedgehogs, the spectra of spin waves, and the chiral response to
external perturbations. The general spin-orbit coupling of electrons is
similarly described by non-Abelian gauge fields, including higher-rank tensors
related to the electronic Berry flux. Itinerant electrons and local moments
exchange their gauge fluxes through Kondo and Hund interactions. Hence, by
utilizing gauge fields, this theory provides a unifying physical picture of
``intrinsic'' and ``topological'' anomalous Hall effects, spin-Hall effects,
and other correlations between the topological properties of electrons and
moments. We predict ``topological'' magnetoelectric effect in materials prone
to hosting hedgehogs. Links to experiments and model calculations are provided
by deriving the couplings and gauge fields from generic microscopic models,
including the Hubbard model with spin-orbit interactions. We establish the
possible existence of novel quantum spin liquids that exhibit a fractional
magnetoelectric effect, and discuss a few applications to topological magnetic
conductors like Mn$_3$Sn and Pr$_2$Ir$_2$O$_7$.",1911.11155v2
2020-01-13,The Surprisingly Small Impact of Magnetic Fields On The Inner Accretion Flow of Sagittarius A* Fueled By Stellar Winds,"We study the flow structure in 3D magnetohydrodynamic (MHD) simulations of
accretion onto Sagittarius A* via the magnetized winds of the orbiting
Wolf-Rayet stars. These simulations cover over 3 orders of magnitude in radius
to reach $\approx$ 300 gravitational radii, with only one poorly constrained
parameter (the magnetic field in the stellar winds). Even for winds with
relatively weak magnetic fields (e.g., plasma $\beta$ $\sim$ $10^6$), flux
freezing/compression in the inflowing gas amplifies the field to $\beta$ $\sim$
few well before it reaches the event horizon. Overall, the dynamics, accretion
rate, and spherically averaged flow profiles (e.g., density, velocity) in our
MHD simulations are remarkably similar to analogous hydrodynamic simulations.
We attribute this to the broad distribution of angular momentum provided by the
stellar winds, which sources accretion even absent much angular momentum
transport. We find that the magneto-rotational instability is not important
because of i) strong magnetic fields that are amplified by flux
freezing/compression, and ii) the rapid inflow/outflow times of the gas and
inefficient radiative cooling preclude circularization. The primary effect of
magnetic fields is that they drive a polar outflow that is absent in
hydrodynamics. The dynamical state of the accretion flow found in our
simulations is unlike the rotationally supported tori used as initial
conditions in horizon scale simulations, which could have implications for
models being used to interpret Event Horizon Telescope and GRAVITY observations
of Sgr A*.",2001.04469v1
2020-02-19,Linear and nonlinear hydromagnetic stability in laminar and turbulent flows,"We consider the evolution of arbitrarily large perturbations of a prescribed
pure hydrodynamical flow of an electrically conducting fluid. We study whether
the flow perturbations as well as the generated magnetic fields decay or grow
with time and constitute a dynamo process. For that purpose we derive a
generalized Reynolds-Orr equation for the sum of the kinetic energy of the
hydrodynamic perturbation and the magnetic energy. The flow is confined in a
finite volume so the normal component of the velocity at the boundary is zero.
The tangential component is left arbitrary in contrast with previous works. For
the magnetic field we mostly employ the classical boundary conditions where the
field extends in the whole space. We establish critical values of hydrodynamic
and magnetic Reynolds numbers below which arbitrarily large initial
perturbations of the hydrodynamic flow decay. This involves generalization of
the Rayleigh-Faber-Krahn inequality for the smallest eigenvalue of an elliptic
operator. For high Reynolds number turbulence we provide an estimate of
critical magnetic Reynolds number below which arbitrarily large fluctuations of
the magnetic field decay.",2002.08275v2
2020-05-15,Diffusion of large-scale magnetic fields by reconnection in MHD turbulence,"The rate of magnetic field diffusion plays an essential role in several
astrophysical plasma processes. It has been demonstrated that the omnipresent
turbulence in astrophysical media induces fast magnetic reconnection, which
consequently leads to large-scale magnetic flux diffusion at a rate independent
of the plasma microphysics. This process is called ``reconnection diffusion''
(RD) and allows for the diffusion of fields which are dynamically important.
The current theory describing RD is based on incompressible magnetohydrodynamic
(MHD) turbulence. In this work, we have tested quantitatively the predictions
of the RD theory when magnetic forces are dominant in the turbulence dynamics
(Alfv\'{e}nic Mach number $M_A < 1$). We employed the \textsc{Pencil Code} to
perform numerical simulations of forced MHD turbulence, extracting the values
of the diffusion coefficient $\eta_{RD}$ using the Test-Field method. Our
results are consistent with the RD theory ($\eta_{RD} \sim M_A^{3}$ for $M_A <
1$) when turbulence approaches the incompressible limit (sonic Mach number $M_S
\lesssim 0.02$), while for larger $M_S$ the diffusion is faster ($\eta_{RD}
\sim M_A^{2}$). This work shows for the first time simulations of compressible
MHD turbulence with the suppression of the cascade in the direction parallel to
the mean magnetic field, which is consistent with incompressible weak
turbulence theory. We also verified that in our simulations the energy
cascading time does not follow the scaling with $M_A$ predicted for the weak
regime, in contradiction with the RD theory assumption. Our results generally
support and expand the RD theory predictions.",2005.07775v2
2020-06-19,X-ray detection of ultrashort spin current pulses in synthetic antiferromagnets,"We explore the ultrafast generation of spin currents in magnetic multilayer
samples by applying fs laser pulses to one layer and measuring the magnetic
response in the other layer by element-resolved x-ray spectroscopy. In
Ni(5~nm)/Ru(2~nm)/Fe(4~nm), the Ni and Fe magnetization directions couple
antiferromagnetically due to the RKKY interaction, but may be oriented parallel
through an applied magnetic field. After exciting the top Ni layer with a fs
laser pulse, we find that also the Fe layer underneath demagnetizes, with a
$4.1 \pm 1.9$\% amplitude difference between parallel and antiparallel
orientation of the Ni and Fe magnetizations. We attribute this difference to
the influence of a spin current generated by the fs laser pulse that transfers
angular momentum from the Ni into the Fe layer. Our results confirm that
superdiffusive spin transport plays a role in determining the sub-ps
demagnetization dynamics of synthetic antiferromagnetic layers, but also
evidence large depolarization effects due to hot electron dynamics, which are
independent of the relative alignment of the magnetization in Ni and Fe.",2006.11030v1
2020-08-07,Spin dynamics under the influence of elliptically rotating fields: Extracting the field topology from time-averaged quantities,"We focus on quantum systems that can be effectively described as a localized
spin-$s$ particle subject to a static magnetic field coplanar to a coexisting
elliptically rotating time-periodic field. Depending on the values taken on by
the static and rotating components, the total magnetic field shows two regimes
with different topological properties. Along the boundary that separates these
two regimes, the total magnetic field vanishes periodically in time and the
system dynamics becomes highly nonadiabatic. We derive a relation between two
time-averaged quantities of the system which is linked to the topology of the
applied magnetic field. Based on this finding, we propose a measurable quantity
that has the ability to indicate the topology of the total magnetic field
without knowing a priori the value of the static component. We also propose a
possible implementation of our approach by a trapped-ion quantum system. The
results presented here are independent of the initial state of the system. In
particular, when the system is initialized in a Floquet state, we find some
interesting properties of the quasienergy spectrum which are linked to the
topological change of the total magnetic field. Throughout the paper, the
theoretical results are illustrated with numerical simulations for the case of
a two-level quantum system.",2008.03078v2
2020-08-11,Near-horizon structure of escape zones of electrically charged particles around weakly magnetized rotating black hole. II. Acceleration and escape in the oblique magnetosphere,"Strong gravity and magnetic fields are key ingredients that power processes
of accretion and ejection near compact objects. While the particular mechanisms
that operate here are still discussed, it seems that the presence of an ordered
magnetic field is crucial for the acceleration and collimation of relativistic
jets of electrically charged particles on superhorizon length scales. In this
context, we further study the effect of a large-scale magnetic field on the
dynamics of charged particles near a rotating black hole. We consider a
scenario in which the initially neutral particles on regular geodesic orbits in
the equatorial plane are destabilized by a charging process (e.g., by
photoionization). Some charged particles are accelerated out of the equatorial
plane, and they follow jetlike trajectories with relativistic velocities. In
our previous paper, we investigated this scenario for the case of perfect
alignment of the magnetic field with the axis of rotation; i.e., the system was
considered axisymmetric. Here we relax this assumption and investigate
nonaxisymmetric systems in which the magnetic field is arbitrarily inclined
with respect to the black hole spin. We study the system numerically in order
to locate the zones of escaping trajectories and compute the maximum (terminal)
escape velocity. It appears that breaking the axial symmetry (even by small
inclination angles) substantially increases the fraction of escaping orbits and
allows the acceleration to ultrarelativistic velocities that were excluded in
the axisymmetric setup. The presence of transient chaotic dynamics in the
launching region of the relativistic outflow is confirmed with chaotic
indicators.",2008.04630v2
2020-08-19,Dynamics of Composite Domain Walls in Multiferroics in Magnetic Field and Their Instability,"We study theoretically the dynamics of composite domain walls (DW) in
multiferroic material GdFeO${}_3$ driven by magnetic field $H$. Two
antiferromagnetic orders of Fe and Gd spins interact Gd-ion displacement in
this system with coupling $c$ at low temperatures, and we have numerically
simulated the corresponding time-dependent Ginzburg-Landau equations in which
magnetic field $H$ couples to Fe-spin order parameter. We vary $H$ and $c$
systematically and calculate velocity $v$ and inner structure in a stationary
state for magnetic DW and magneto-electric DW. DW mobility $v/H$ increases with
$H$ and decreases with $c$ for both DWs, but their characteristics differ
between the two. We have also studied analytically the smooth characteristics
of magneto-electric DW by perturbation theory. Another finding is a splitting
instability at large H. A magneto-electric DW splits into a pair of magnetic DW
and electric DW when $c$ is large, while a magnetic DW splits when $c$ is
small. Internal structure of composite DW deforms with increasing $c$ and $H$,
and modulations in different order parameters separate in space. Their relative
distances show a noticeable enhancement with approaching the splitting
instability.",2008.08600v1
2020-09-22,Dynamic $^{14}\rm N$ nuclear spin polarization in nitrogen-vacancy centers in diamond,"We studied the dynamic nuclear spin polarization of nitrogen in negatively
charged nitrogen-vacancy (NV) centers in diamond both experimentally and
theoretically over a wide range of magnetic fields from 0 to 1100 G covering
both the excited-state level anti-crossing and the ground-state level
anti-crossing magnetic field regions. Special attention was paid to the less
studied ground-state level anti-crossing region. The nuclear spin polarization
was inferred from measurements of the optically detected magnetic resonance
signal. These measurements show that a very large (up to $96 \pm 2\%$) nuclear
spin polarization of nitrogen can be achieved over a very broad range of
magnetic field starting from around 400 G up to magnetic field values
substantially exceeding the ground-state level anti-crossing at 1024 G. We
measured the influence of angular deviations of the magnetic field from the NV
axis on the nuclear spin polarization efficiency and found that, in the
vicinity of the ground-state level anti-crossing, the nuclear spin polarization
is more sensitive to this angle than in the vicinity of the excited-state level
anti-crossing. Indeed, an angle as small as a tenth of a degree of arc can
destroy almost completely the spin polarization of a nitrogen nucleus. In
addition, we investigated theoretically the influence of strain and optical
excitation power on the nuclear spin polarization.",2009.10516v1
2020-12-21,Electronic correlations and magnetic interactions in infinite-layer NdNiO$_2$,"The large antiferromagnetic exchange coupling in the parent high-$T_{\rm c}$
cuprate superconductors is believed to play a crucial role in pairing the
superconducting carriers. The recent observation of superconductivity in
hole-doped infinite-layer (IL-) NdNiO$_2$ brings to the fore the relevance of
magnetic coupling in high-$T_{\rm c}$ superconductors, particularly because no
magnetic ordering is observed in the undoped IL-NdNiO$_2$ unlike in parent
copper oxides. Here, we investigate the electronic structure and the nature of
magnetic exchange in IL-NdNiO$_2$ using state-of-the-art many-body quantum
chemistry methods. From a systematic comparison of the electronic and magnetic
properties with isostructural cuprate IL-CaCuO$_2$, we find that the on-site
dynamical correlations are significantly stronger in IL-NdNiO$_2$ compared to
the cuprate analog. These dynamical correlations play a critical role in the
magnetic exchange resulting in an unexpectedly large antiferromagnetic nearest
neighbor isotropic $J$ of 77 meV between the Ni$^{1+}$ ions within the
$ab$-plane. While we find many similarities in the electronic structure between
the nickelate and the cuprate, the role of electronic correlations is
profoundly different in the two. We further discuss the implications of our
findings in understanding the origin of superconductivity in nickelates.",2012.11179v1
2021-02-16,Lifshitz scaling effects on the holographic paramagnetic-ferromagnetic phase transition,"We disclose the effects of Lifshitz dynamical exponent $z$ on the properties
of holographic paramagnetic-ferromagnetic phase transition in the background of
Lifshitz spacetime. To preserve the conformal invariance in higher dimensions,
we consider the Power-Maxwell (PM) electrodynamics as our gauge field. We
introduce a massive $2$-form coupled to the PM field and perform the numerical
shooting method in the probe limit by assuming the PM and the $2$-form fields
do not back-react on the background geometry. The obtained results indicate
that the critical temperature decreases with increasing the strength of the
power parameter $q$ and dynamical exponent $z$. Besides, the formation of the
magnetic moment in the black hole background is harder in the absence of an
external magnetic field. At low temperatures, and in the absence of an external
magnetic field, our result show the spontaneous magnetization and the
ferromagnetic phase transition. We find that the critical exponent takes the
universal value $\beta= 1/2$ regardless of the parameters $q, z, d$, which is
in agreement with the mean field theory. In the presence of an external
magnetic field, the magnetic susceptibility satisfies the Curie-Weiss law.",2102.08047v3
2021-03-03,Emergence of internetwork magnetic fields through the solar atmosphere,"Internetwork (IN) magnetic fields are highly dynamic, short-lived magnetic
structures that populate the interior of supergranular cells. Since they emerge
all over the Sun, these small-scale fields bring a substantial amount of flux,
and therefore energy, to the solar surface. Because of this, IN fields are
crucial for understanding the quiet Sun (QS) magnetism. However, they are weak
and produce very small polarization signals, which is the reason why their
properties and impact on the energetics and dynamics of the solar atmosphere
are poorly known. Here we use coordinated, high-resolution, multiwavelength
observations obtained with the Swedish 1-m Solar Telescope (SST) and the
\textit{Interface Region Imaging Spectrograph} (IRIS) to follow the evolution
of IN magnetic loops as they emerge into the photosphere and reach the
chromosphere and transition region. We studied in this paper three flux
emergence events having total unsigned magnetic fluxes of $1.9\times10^{18}$,
$2.5\times10^{18}$, and $5.3\times10^{18}$~Mx. The footpoints of the emerging
IN bipoles are clearly seen to appear in the photosphere and to rise up through
the solar atmosphere, as observed in \ion{Fe}{1} 6173 \AA\/ and \ion{Mg}{1}
b$_2$ 5173 \AA\/ magnetograms, respectively. For the first time, our
polarimetric measurements taken in the chromospheric \ion{Ca}{2} 8542 \AA\/
line provide direct observational evidence that IN fields are capable of
reaching the chromosphere. Moreover, using IRIS data, we study the effects of
these weak fields on the heating of the chromosphere and transition region.",2103.02213v1
2021-04-23,A Kinetic Model of Friction in Strongly Coupled Strongly Magnetized Plasmas,"Plasmas that are strongly magnetized in the sense that the gyrofrequency
exceeds the plasma frequency exhibit novel transport properties that are not
well understood. As a representative example, we compute the friction force
acting on a massive test charge moving through a strongly coupled and strongly
magnetized one-component plasma using a generalized Boltzmann kinetic theory.
Recent works studying the weakly coupled regime have shown that strong
magnetization leads to a transverse component of the friction force that is
perpendicular to both the Lorentz force and velocity of the test charge; in
addition to the stopping power component aligned antiparallel to the velocity.
Recent molecular dynamics simulations have also shown that strong Coulomb
coupling in addition to strong magnetization gives rise to a third component of
the friction force in the direction of the Lorentz force. Here, we show that
the generalized Boltzmann kinetic theory captures these effects, and generally
agrees well with the molecular dynamics simulations over a broad range of
Coulomb coupling and magnetization strength regimes. The theory is also used to
show that a ""gyro"" component of the friction in the direction of the Lorentz
force arises due to asymmetries associated with gyromotion during short-range
collisions. Computing the average motion of the test charge through the
background plasma, the transverse force is found to strongly influence the
trajectory by changing the gyroradius and the gyro friction force is found to
slightly change the gyrofrequency of the test charge resulting in a phase
shift.",2104.11718v1
2021-06-27,Hadron gas in the presence of a magnetic field using non-extensive statistics: A transition from diamagnetic to paramagnetic system,"Non-central heavy-ion collisions at ultra-relativistic energies are unique in
producing magnetic fields of the largest strength in the laboratory. Such
fields being produced at the early stages of the collision could affect the
properties of Quantum Chromodynamics (QCD) matter formed in the relativistic
heavy-ion collisions. The transient magnetic field leaves its reminiscence,
which in principle, can affect the thermodynamic and transport properties of
the final state dynamics of the system. In this work, we study the
thermodynamic properties of a hadron gas in the presence of an external static
magnetic field using a thermodynamically consistent non-extensive Tsallis
distribution function. Various thermodynamical observables such as energy
density ($\epsilon$), entropy density ($s$), pressure ($P$) and speed of sound
($c_{\rm s}$) are studied. Investigation of magnetization ($M$) is also
performed and this analysis reveals an interplay of diamagnetic and
paramagnetic nature of the system in the presence of a magnetic field of
varying strength. Further, to understand the system dynamics under equilibrium
and non-equilibrium conditions, the effect of the non-extensive parameter ($q$)
on the above observables is also studied.",2106.14297v2
2021-10-14,Long-Term Evolution of Magnetic Fields in Flaring Active Region NOAA 12673,"During the lifetime of AR 12673, its magnetic field evolved drastically and
produced numerous large flares. In this study, using full maps of the Sun
observed by the Solar Dynamics Observatory and the Solar Terrestrial Relations
Observatory, we identified that AR 12673 emerged in decayed AR 12665, which had
survived for two solar rotations. Although both ARs emerged at the same
location, they possessed different characteristics and different flare
productivities. Therefore, it is important to study the long-term magnetic
evolution of both ARs to identify the distinguishing characteristics of an AR
that can produce large solar flares. We used the Spaceweather Helioseismic and
Magnetic Imager Active Region Patch data to investigate the evolution of the
photospheric magnetic field and other physical properties of the recurring ARs
during five Carrington rotations. All these investigated parameters dynamically
evolved through a series of solar rotations. We compared the long-term
evolution of AR 12665 and AR 12673 to understand the differences in their
flare-producing properties. We also studied the relation of the long-term
evolution of these ARs with the presence of active longitude. We found that the
magnetic flux and complexity of AR 12673 developed much faster than those of AR
12665. Our results confirmed that a strong emerging flux that emerged in the
pre-existing AR near the active longitude created a very strong and complex AR
that produced large flares.",2110.07369v1
2021-11-30,The dynamical structure of the outflows driven by a large-scale magnetic field,"Large-scale magnetic field is crucial in launching and collimating the
jets/outflows. It is found that the magnetic flux can be efficiently
transported inward by the fast moving corona above a thin disk. In this work we
investigate the dynamical structure of the outflows driven by the large-scale
magnetic field advected by the hot corona. With derived large-scale magnetic
field, the outflow solution along every field line is obtained by solving a set
of magneto-hydrodynamical (MHD) Equations self-consistently with boundary
conditions at the upper surface of the corona. We find that the terminal speeds
of the outflows driven from the inner region of the disk are $\sim 0.01c-0.1c$.
The temperatures of the outflows at the large distance from the black hole are
still as high as several ten keV. The properties of the magnetic outflows
derived in this work are roughly consistent with the fast outflows detected in
some luminous quasars and X-ray binaries. The total mass loss rate in the
outflows from the corona is about $7\%-12\%$ of the mass accretion rate of the
disk. The three dimension field geometry, the velocity, temperature and density
of the outflows derived in this work can be used for calculating the emergent
spectra and their polarization of the accretion disk/corona/outflow systems.
Our results may help understand the features of the observed spectra of X-ray
binaries and active galactic nuclei.",2111.15115v2
2022-02-08,Ultrafast Emergence of Ferromagnetism in Antiferromagnetic FeRh in High Magnetic Fields,"Ultrafast heating of FeRh by a femtosecond laser pulse launches a
magneto-structural phase transition from an antiferromagnetic to a
ferromagnetic state. Aiming to reveal the ultrafast kinetics of this
transition, we studied magnetization dynamics with the help of the
magneto-optical Kerr effect in a broad range of temperatures (from 4 K to 400
K) and magnetic fields (up to 25 T). Three different types of ultrafast
magnetization dynamics were observed and, using a numerically calculated H-T
phase diagram, the differences were explained by different initial states of
FeRh corresponding to a (i) collinear antiferromagnetic, (ii) canted
antiferromagnetic and (iii) ferromagnetic alignment of spins. We argue that
ultrafast heating of FeRh in the canted antiferromagnetic phase launches
practically the fastest possible emergence of magnetization in this material.
The magnetization emerges on a time scale of 2 ps, which corresponds to the
earlier reported time-scale of the structural changes during the phase
transition.",2202.03931v4
2022-02-16,Turbulent dynamo in the two-phase interstellar medium,"Magnetic fields are a dynamically important component of the turbulent
interstellar medium (ISM) of star-forming galaxies. These magnetic fields are
due to a dynamo action, which is a process of converting turbulent kinetic
energy to magnetic energy. A dynamo that acts at scales less than the turbulent
driving scale is known as the turbulent dynamo. The ISM is a multiphase medium
and observations suggest that the properties of magnetic fields differ with the
phase. Here, we aim to study how the properties of the turbulent dynamo depend
on the phase. We simulate the non-isothermal turbulent dynamo in a two-phase
medium (most previous work assumes an isothermal gas). We show that the warm
phase ($T\ge10^3~{\rm K}$) is transsonic and the cold phase ($T<10^3~{\rm K}$)
is supersonic. We find that the growth rate of magnetic fields in the
exponentially growing stage is similar in both phases. We compute the terms
responsible for amplification and destruction of vorticity and show that in
both phases vorticity is amplified due to turbulent motions, further amplified
by the baroclinic term in the warm phase, and destroyed by the term for viscous
interactions in the presence of logarithmic density gradients in the cold
phase. We find that the final ratio of magnetic to turbulent kinetic energy is
lower in the cold phase due to a stronger Lorentz force. We show that the
non-isothermal turbulent dynamo is significantly different from its isothermal
counterpart and this demonstrates the need for studying the turbulent dynamo in
a multiphase medium.",2202.08324v2
2022-03-06,Spin-Dependent Graph Neural Network Potential for Magnetic Materials,"The development of machine learning interatomic potentials has immensely
contributed to the accuracy of simulations of molecules and crystals. However,
creating interatomic potentials for magnetic systems that account for both
magnetic moments and structural degrees of freedom remains a challenge. This
work introduces SpinGNN, a spin-dependent interatomic potential approach that
employs the graph neural network (GNN) to describe magnetic systems. SpinGNN
consists of two types of edge GNNs: Heisenberg edge GNN (HEGNN) and
spin-distance edge GNN (SEGNN). HEGNN is tailored to capture Heisenberg-type
spin-lattice interactions, while SEGNN accurately models multi-body and
high-order spin-lattice coupling. The effectiveness of SpinGNN is demonstrated
by its exceptional precision in fitting a high-order spin Hamiltonian and two
complex spin-lattice Hamiltonians with great precision. Furthermore, it
successfully models the subtle spin-lattice coupling in BiFeO3 and performs
large-scale spin-lattice dynamics simulations, predicting its antiferromagnetic
ground state, magnetic phase transition, and domain wall energy landscape with
high accuracy. Our study broadens the scope of graph neural network potentials
to magnetic systems, serving as a foundation for carrying out large-scale
spin-lattice dynamic simulations of such systems.",2203.02853v2
2022-04-12,Three-dimensional close-to-substrate trajectories of magnetic microparticles in dynamically changing magnetic field landscapes,"The transport of magnetic nano- or microparticles in microfluidic devices
using artificially designed magnetic field landscapes (MFL) is promising for
the implementation of key functionalities in Lab-on-a-chip (LOC) systems. A
close-to-substrate transport is hereby instrumental to use changing
particle-substrate interactions upon analyte binding for analytics and
diagnostics. Here, we present an essential prerequisite for such an
application, namely the label-free quantitative experimental determination of
the three-dimensional trajectories of superparamagnetic particles (SPP)
transported by a dynamically changing MFL above a topographically flat
substrate. The evaluation of the SPP sharpness within defocused video-recorded
images, acquired by an optical bright-field microscope, was employed to obtain
a vertical z-coordinate. This method applied to a prototypical transport
scheme, using the static MFL of parallel-stripe domains superposed by a
particular magnetic field pulse sequence, revealed a hopping-like motion of the
magnetic particles, previously predicted by theory. Maximum vertical particle
jumps of several micrometers have been observed experimentally, corroborating
theoretical estimates for the particle-substrate distance. As our findings pave
the way towards precise quantification of particle-substrate separations in the
discussed transport system, they bear deep implications for future LOC
detection schemes using only optical microscopy.",2204.05958v1
2022-05-30,Skyrmion-Magnetic Tunnel Junction Synapse with Mixed Synaptic Plasticity for Neuromorphic Computing,"Magnetic skyrmion-based data storage and unconventional computing devices
have gained increasing attention due to their topological protection, small
size, and low driving current. However, skyrmion creation, deletion, and motion
are still being studied. In this study, we propose a skyrmion-based
neuromorphic magnetic tunnel junction (MTJ) device with both long- and
short-term plasticity (LTP and STP) (mixed synaptic plasticity). We showed that
plasticity could be controlled by magnetic field, spin-orbit torque (SOT), and
the voltage-controlled magnetic anisotropy (VCMA) switching mechanism. LTP
depends on the skyrmion density and is manipulated by the SOT and magnetic
field while STP is controlled by the VCMA. The LTP property of the device was
utilized for static image recognition. By incorporating the STP feature, the
device gained additional temporal filtering ability and could adapt to a
dynamic environment. The skyrmions were conserved and confined to a nanotrack
to minimize the skyrmion nucleation energy. The synapse device was trained and
tested for emulating a deep neural network. We observed that when the skyrmion
density was increased, the inference accuracy improved: 90% accuracy was
achieved by the system at the highest density. We further demonstrated the
dynamic environment learning and inference capabilities of the proposed device.",2205.14915v1
2022-06-16,Compressible two-dimensional turbulence: cascade reversal and sensitivity to imposed magnetic field,"We study the impact of compressibility on two-dimensional turbulent flows,
such as those modeling astrophysical disks. We demonstrate that the direction
of cascade undergoes continuous transition as the Mach number Ma increases,
from inverse at zero Ma, to direct at infinite Ma. Thus, at Ma of order one
comparable amounts of energy flow from the scale of the pumping to large and
small scales, in accord with previous data. For supersonic turbulence with
large Ma, the cascade is direct, as in three dimensions, which results in
multifractal density field. For compressible flows of conducting fluids,
imposing external magnetic field allows to manipulate the flow producing
possibly large changes even at small Mach number. Thus Zeldovich antidynamo
theorem, by which at zero Ma the magnetic field is zero in the steady state,
must be used with caution. Real flows have finite Ma and, however small it is,
for large magnetic flux through the disk, the magnetic field changes the flow
appreciably, or rearranges it completely. For large Ma, relevant for
astrophysical disks, the magnetic field energy is enhanced by a large factor as
compared to estimates based on the mean field. We propose to use
two-dimensional Burgers turbulence, whose three-dimensional counterpart is used
for studies of the large-scale structure of the Universe, as a model for
supersonic thin accretion disks.",2206.10367v2
2022-08-08,Van der Waals engineering of ultrafast carrier dynamics in magnetic heterostructures,"Heterostructures composed of the intrinsic magnetic topological insulator
MnBi$_2$Te$_4$ and its non-magnetic counterpart Bi$_2$Te$_3$ host distinct
surface band structures depending on the stacking order and exposed
termination, allowing fine control of their magnetic, electronic and optical
properties. Here, we probe the ultrafast dynamical response of MnBi$_2$Te$_4$
and MnBi$_4$Te$_7$ following near-infrared optical excitation using time- and
angle-resolved photoemission spectroscopy. We gain access to the
out-of-equilibrium surface electronic structure of both MnBi$_2$Te$_4$ and
Bi$_2$Te$_3$ surface terminations of MnBi$_4$Te$_7$, revealing an instantaneous
occupation of states that are resonant with the optical excitation in the
Bi$_2$Te$_3$ layer followed by carrier extraction into the adjacent
MnBi$_2$Te$_4$ layers with a laser fluence-tunable delay of up to 350 fs. The
ensuing thermal relaxation processes are driven by in-plane phonon scattering
with significantly slower relaxation times in the magnetic MnBi$_2$Te$_4$
septuple layers. The competition of interlayer charge transfer and intralayer
phonon scattering establishes MnBi$_2$Te$_4$-based compounds as a platform for
controlling ultrafast charge transfer processes in combination with magnetism
and topology in van der Waals heterostructures.",2208.04098v1
2022-12-21,Charge transfer and disorder-induced spin relaxation in La2NiMnO6 crystallites,"Investigation of the electronic and spin structure in double perovskites is
recently attracting significant attention, mainly driven by their unique
multifunctional properties and other underlying charge and spin dynamics.
Herein, using X-ray photoelectron spectroscopy (XPS), we explore the influence
of variable fractions of Mn3+/Mn4+ cation in different crystallite sizes of
La2NiMnO6 that control the various completing exchange interactions of Ni/Mn
cations responsible for multiple magnetic transitions. The enhanced itinerant
electron due to Mn4+ + Ni2+ to Mn3+ + Ni3+ charge transfer emerged as a
shoulder like characteristics at the low binding energy in the Mn-2P core-level
spectrum. The various approaches such as difference in saturation
magnetization, presence of multiple charge valance, and magnetic entropy
calculations confirm the presence of antisites disorder and it varies as a
function of milling. As milling provides excess energy that helps with
nucleation or cation ordering. Competing magnetic interactions driven by mixed
valences and disorder were established across a cluster glassy phase in the
crystallites. Electron spin resonance spectroscopy (ESR) was utilized to probe
the temperature-driven ferromagnetic-cluster spin-glass transition with
modified g-factor ranging from 2.050 to 2.037. The line width of the ESR
signals increases across the ferromagnetic to cluster-glass phase transition
due to spin freezing. This phase transition is further characterized by
temperature-dependent ac-magnetic susceptibility measurements. Argand diagram
for the ac-susceptibility of the interacting crystallites suggests a collective
magnetization relaxation dynamic in the proximity of spin-glass freezing
temperature of La2NiMnO6.",2212.10845v2
2023-03-28,Generation of third-harmonic spin oscillation from strong spin precession induced by terahertz magnetic near fields,"The ability to drive a spin system to state far from the equilibrium is
indispensable for investigating spin structures of antiferromagnets and their
functional nonlinearities for spintronics. While optical methods have been
considered for spin excitation, terahertz (THz) pulses appear to be a more
convenient means of direct spin excitation without requiring coupling between
spins and orbitals or phonons. However, room-temperature responses are usually
limited to small deviations from the equilibrium state because of the
relatively weak THz magnetic fields in common approaches. Here, we studied the
magnetization dynamics in a HoFeO3 crystal at room temperature. A custom-made
spiral-shaped microstructure was used to locally generate a strong multicycle
THz magnetic near field perpendicular to the crystal surface; the maximum
magnetic field amplitude of about 2 T was achieved. The observed time-resolved
change in the Faraday ellipticity clearly showed second- and third-order
harmonics of the magnetization oscillation and an asymmetric oscillation
behaviour. Not only the ferromagnetic vector M but also the antiferromagnetic
vector L plays an important role in the nonlinear dynamics of spin systems far
from equilibrium.",2303.16016v1
2023-05-16,Impact of radiative cooling on the magnetised geometrically thin accretion disk around Kerr black hole,"It is believed that the spectral state transitions of the outbursts in X-ray
binaries (XRBs) are triggered by the rise of the mass accretion rate due to
underlying disc instabilities. Recent observations found that characteristics
of disc winds are probably connected with the different spectral states, but
the theoretical underpinnings of it are highly ambiguous. To understand the
correlation between disc winds and the dynamics of the accretion flow, we have
performed General Relativistic Magneto-hydrodynamic (GRMHD) simulations of an
axisymmetric thin accretion disc with different accretion rates and magnetic
field strengths. Our simulations have shown that the dynamics and the
temperature properties depend on both accretion rates and magnetic field
strengths. We later found that these properties greatly influence spectral
properties. We calculated the average coronal temperature for different
simulation models, which is correlated with high-energy Compton emission. Our
simulation models reveal that the average coronal temperature is
anti-correlated with the accretion rates, which is correlated with the magnetic
field strengths. We also found that the structured component of the disc winds
(Blandford-Payne disc wind) predominates as the accretion rates and magnetic
field strengths increase. In contrast, the turbulent component of the disc
winds ($B_{\rm tor}$ disc wind) predominates as the accretion rates and
magnetic field strengths decrease. Our results suggest that the disc winds
during an outburst in XRBs can only be understood if the magnetic field
contribution varies over time (e.g., MAXI J1820+070).",2305.09698v1
2023-05-24,Charged spinning and magnetized test particles orbiting quantum improved charged black holes,"In the present work, we aimed to investigate the dynamics of spinning charged
and magnetized test particles around both electrically and magnetically charged
quantum-improved black holes. We derive the equations of motion for charged
spinning test particles using the Mathisson-Papapetrou-Dixon equations with the
Lorentz coupling term. The radius of innermost stable circular orbits (ISCOs),
specific angular momentum, and energy for charged spinless, uncharged spinning,
and charged spinning test particles around the charged and non-charged
quantum-improved black holes are analyzed separately. We found that the quantum
parameter increases the maximum spin value, $s_\text{max}$, which leads to the
nonphysical motion (superluminal motion) of the charged spinning test particle,
whereas the black hole charge decreases its value. We also found that, in
contrast to the Reissner Nordstr\""om black hole, spinning charged test
particles in the quantum-improved charged black hole have higher
$s_\text{max}$; moreover, positively charged spinning particles can have higher
values of $s_\text{max}$ near the extreme black hole cases when compared with
uncharged spinning particles. Finally, we investigate the magnetized test
particle's dynamics around a quantum-improved magnetically charged black hole
in Quantum Einstein Gravity using the Hamilton-Jacobi equation. We show that
the presence of $\omega$ increases the maximum value of the effective potential
and decreases the minimum energy and angular momentum of magnetized particles
at their circular orbits. We found an upper constraint in the black hole charge
at the ISCO.",2305.15350v1
2023-06-27,MHD Simulation in Galactic Center Region with Radiative Cooling and Heating,"We investigate the role of magnetic field on the gas dynamics in a galactic
bulge region by three dimensional simulations with radiative cooling and
heating. While high-temperature corona with $T>10^6\ {\rm K}$ is formed in the
halo regions, the temperature near the mid-plane is $\lesssim 10^4\ {\rm K}$
following the thermal equilibrium curve determined by the radiative cooling and
heating. Although the thermal energy of the interstellar gas is lost by
radiative cooling, the saturation level of the magnetic field strength does not
significantly depend on the radiative cooling and heating. The magnetic field
strength is amplified to $10\ {\rm \mu G}$ on average, and reaches several
hundred ${\rm \mu G}$ locally. We find the formation of magnetically dominated
regions at mid-latitudes in the case with the radiative cooling and heating,
which is not seen in the case without radiative effect. The vertical thickness
of the mid-latitude regions is $50-150\ {\rm pc}$ at the radial location of
$0.4-0.8\ {\rm kpc}$ from the galactic center, which is comparable to the
observed vertical distribution of neutral atomic gas. When we take the average
of different components of energy density integrated over the galactic bulge
region, the magnetic energy is comparable to the thermal energy. We conclude
that the magnetic field plays a substantial role in controlling the dynamical
and thermal properties of the galactic bulge region.",2306.15761v2
2023-07-05,Dynamical Effects of Magnetic Opacity in Neutron Star Accretion Columns,"We present relativistic, radiation magnetohydrodynamic simulations of
supercritical neutron star accretion columns in Cartesian geometry, including
temperature-dependent, polarization-averaged Rosseland mean opacities
accounting for classical electron scattering in a magnetic field. Just as in
our previous pure Thomson scattering simulations, vertical oscillations of the
accretion shock and horizontally propagating entropy waves (photon bubbles) are
present in all our simulations. However, at high magnetic fields
$\gtrsim10^{12}$~G, the magnetic opacities produce significant differences in
the overall structure and dynamics of the column. At fixed accretion rate,
increasing the magnetic field strength results in a shorter accretion column,
despite the fact that the overall opacity within the column is larger.
Moreover, the vertical oscillation amplitude of the column is reduced.
Increasing the accretion rate at high magnetic fields restores the height of
the column. However, a new, slower instability takes place at these field
strengths because they are in a regime where the opacity increases with
temperature. This instability causes both the average height of the column and
the oscillation amplitude to substantially increase on a time scale of
$\sim10$~ms. We provide physical explanations for these results, and discuss
their implications for the observed properties of these columns, including
mixed fan-beam/pencil-beam emission patterns caused by the oscillations.",2307.02410v1
2023-08-16,A New Magneto-Micropolar Boundary Layer Model for Liquid Flows -- Effect of Micromagnetorotation (MMR),"In this paper, we present a micropolar continuum model based on the theory of
magnetohydrodynamics. In particular, the effect of micromagnetorotation (MMR)
is taken into account in the derivation of an initial-boundary value problem
(i-bvp) within magneto-micorpolar flows. MMR is a phenomenon that is related to
the micromotions of the magnetic liquid particles in the presence of externally
applied magnetic field. In all previous investigations magnetization was
supposed to be parallel to applied magnetic field therefore its effect in the
lateral direction is neglected. This assumption is not correct in
magnetic-micropolar flows. Since, magnetic-micropolar flows are anisotropic in
nature. Here, we present a model accounting for this MMR effect. The
constitutive equation for the MMR is described and the governing system of flow
dynamics is described in the form of PDEs. Boundary layer flow assumptions are
used to derive an initial-boundary value problem in ODEs. As a consequence, two
newly defined parameters arises that are related to the MMR. The effects of
these parameters on the flow characteristics are investigated. The developed
i-bvp is solved through the shooting method using MATLAB routines. Effects of
MMR are analyzed on the miro-rotational and hydrodynamic velocities profiles.
Some interesting features of the flow are observed. Results are presented
through graphs and discussed in detail. It is worth mentioning that the model
presented is first of its kind in the literature and has a great potential in
investigating boundary layer flows within micropolar continuum with other
physical aspects of the flow pertinent to engineering and biomedical
applications.",2308.08457v1
2023-11-12,Emergence of magnetic structure in supersonic isothermal magnetohydrodynamic turbulence,"The inverse transfer of magnetic helicity is a fundamental process which may
explain large scale magnetic structure formation and sustainement. Until very
recently, direct numerical simulations (DNS) of the inverse transfer in
magnetohydrodynamics (MHD) turbulence have been done in incompressible MHD or
at low Mach numbers only. We review first results obtained through DNS of the
isothermal MHD equations at Mach numbers ranging from subsonic to about 10. The
spectral exponent of the magnetic helicity spectrum becomes flatter with
increasing compressibility. When considering the Alfv\'en velocity in place of
the magnetic field however, results found in incompressible MHD, including a
dynamic balance between shear and twist, can be extended to supersonic MHD. In
the global picture of an inverse transfer of magnetic helicity, three phenomena
are at work: a local direct transfer mediated by the large scale velocity
field, a local inverse transfer mediated by the intermediate scale velocity
field and a nonlocal inverse transfer mediated by the small scale velocity
field. The compressive part of the velocity field is geometrically favored in
the local direct transfer and contributes to the nonlocal inverse transfer, but
plays no role in the local inverse transfer.",2311.06803v1
2023-11-27,Quantum magnetometry using discrete-time quantum walk,"Quantum magnetometry uses quantum resources to measure magnetic fields with
precision and accuracy that cannot be achieved by its classical counterparts.
In this paper, we propose a scheme for quantum magnetometry using discrete-time
quantum walk (DTQW) where multi-path interference plays a central role. The
dynamics of a spin-half particle implementing DTQW on a one-dimensional lattice
gets affected by magnetic fields, and the controlled dynamics of DTQW help in
estimating the fields' strength. To gauge the effects of the field, we study
the variance of the particle's position probability distribution (PD) and use
it to determine the direction of the magnetic field maximally affecting the
quantum walk. We then employ statistical tools like quantum Fisher information
(QFI) and Fisher information (FI) of the particle's position and spin
measurements to assess the system's sensitivity to the magnetic fields. We find
that one can use the position and spin measurements to estimate the strengths
of the magnetic fields. Calculations for an electron implementing quantum walk
of fifty time steps show that the estimate had a root-mean-square error of the
order of 0.1 picoTesla. Moreover, the sensitivity of our system can be tuned to
measure any desired magnetic field. Our results indicate that the system can be
used as a tool for optimal quantum magnetometry.",2311.15801v2
2024-01-03,Spin-Transfer-Torque Induced Spatially Nonuniform Switching in Ferrimagnets,"Ferrimagnet (FiM), (FeCo)1-xGdx, attracts research attention due to its
ultrafast magnetic dynamics and finite net magnetization. Incorporating FiM
into the magnetic tunnel junction will be beneficial to further improve the
writing speed of magnetic random access memory (MRAM). It is commonly assumed
that the FeCo and Gd atoms are switched together due to the strong exchange
coupling, which remains valid even if one performs the two-sublattice macrospin
simulation. Interestingly, using the atomistic model developed by our group, it
is clearly seen that different atoms are not switched together. In addition,
our study reveals that the nature of switching is spatially nonuniform even in
the small sample with the dimension of 20 nm-20 nm. Furthermore, the
characteristics of nonuniformity are completely different for samples with
different Gd composition (x). When x is close to the magnetization compensation
point, successful switching cannot be obtained, but is accompanied by the
stable oscillation. The atom type that dominates the oscillation is different
from that predicted by the two-sublattice macrospin model. In addition, the
size of singular region is a non-monotonic function of current density. All
these results can only be understood by considering the spatial nonuniform
magnetization dynamics.",2401.01741v1
2024-03-18,Coarsening of chiral domains in itinerant electron magnets: A machine learning force field approach,"Frustrated itinerant magnets often exhibit complex noncollinear or
noncoplanar magnetic orders which support topological electronic structures. A
canonical example is the anomalous quantum Hall state with a chiral spin order
stabilized by electron-spin interactions on a triangular lattice. While a
long-range magnetic order cannot survive thermal fluctuations in two
dimensions, the chiral order which results from the breaking of a discrete
Ising symmetry persists even at finite temperatures. We present a scalable
machine learning (ML) framework to model the complex electron-mediated
spin-spin interactions that stabilize the chiral magnetic domains in a
triangular lattice. Large-scale dynamical simulations, enabled by the ML
force-field models, are performed to investigate the coarsening of chiral
domains after a thermal quench. While the chiral phase is described by a broken
$Z_2$ Ising-type symmetry, we find that the characteristic size of chiral
domains increases linearly with time, in stark contrast to the expected
Allen-Cahn domain growth law for a non-conserved Ising order parameter field.
The linear growth of the chiral domains is attributed to the orientational
anisotropy of domain boundaries. Our work also demonstrates the promising
potential of ML models for large-scale spin dynamics of itinerant magnets.",2403.11705v1
2008-10-22,Phonon Control of Magnetic Relaxation in the Pyrochlore Slab Compounds SCGO and BSZCGO,"We are interested in the phonon response in the frustrated magnets
SrCr$_{9x}$Ga$_{12-9x}$O$_{19}$ (SCGO) and
Ba$_{2}$Sn$_{2}$ZnCr$_{7x}$Ga$_{10-7x}$O$_{22}$ (BSZCGO). The motivation of the
study is the recently discovered, phonon-driven, magnetic relaxation in the
SCGO compound [Mutka et al. PRL {\bf 97} 047203 (2006)] pointing out the
importance of a low-energy ($\hbar\omega\sim$7 meV) phonon mode. In neutron
scattering experiments on these compounds the phonon signal is partly masked by
the magnetic signal from the Cr moments and we have therefore examined in
detail the non-magnetic isostructural counterparts SrGa$_{12}$O$_{19}$ (SGO)
and Ba$_{2}$Sn$_{2}$ZnGa$_{10}$O$_{22}$ (BSZGO). Our {\it ab-initio} lattice
dynamics calculations on SGO reveal a peak in the vibrational density of states
matching with the neutron observations on SGO and SCGO. A strong contribution
in the vibrational density of states comes from the partial contribution of the
Ga atoms on the 2b and 12k sites, involving modes at the M--point of the
hexagonal system. These modes comprise dynamics of the kagom\'e planes of the
pyrochlore slab magnetic sub-lattice, 12k sites, and therefore can drive
magnetic relaxation via spin-phonon coupling. Both BSZCGO and BSZGO show a
similar low-energy Raman peak but no corresponding peak in the neutron
determined density of states of BSZGO is seen. However, a strong non-Debye
enhancement of low-energy phonon response is observed. We attribute this
particular feature to the Zn/Ga disorder on the 2$d$ -site, already evoked
earlier to affect the magnetic properties of BSZCGO. We propose that this
disorder-induced phonon response explains the absence of a characteristic
energy scale and the much faster magnetic relaxation observed in BSZCGO.",0810.3941v2
2012-03-30,The magnetic field of IRAS 16293-2422 as traced by shock-induced H2O masers,"Shock-induced H2O masers are important magnetic field tracers at very high
density gas. Water masers are found in both high- and low-mass star-forming
regions, acting as a powerful tool to compare magnetic field morphologies in
both mass regimes. In this paper, we show one of the first magnetic field
determinations in the low-mass protostellar core IRAS 16293-2422 at volume
densities as high as 10^(8-10) cm^-3. Our goal is to discern if the collapsing
regime of this source is controlled by magnetic fields or other factors like
turbulence. We used the Very Large Array (VLA) to carry out
spectro-polarimetric observations in the 22 GHz Zeeman emission of H2O masers.
From the Stokes V line profile, we can estimate the magnetic field strength in
the dense regions around the protostar. A blend of at least three maser
features can be inferred from our relatively high spatial resolution data set
(~ 0.1""), which is reproduced in a clear non-Gaussian line profile. The
emission is very stable in polarization fraction and position angle across the
channels. The maser spots are aligned with some components of the complex
outflow configuration of IRAS 16293-2422, and they are excited in zones of
compressed gas produced by shocks. The post-shock particle density is in the
range of 1-3 x 10^9 cm^-3, consistent with typical water masers pumping
densities. Zeeman emission is produced by a very strong line-of-sight magnetic
field (B ~ 113 mG). The magnetic field pressure derived from our data is
comparable to the ram pressure of the outflow dynamics. This indicates that the
magnetic field is energetically important in the dynamical evolution of IRAS
16293-2422.",1204.0004v1
2014-01-29,Analytical Models of Exoplanetary Atmospheres. I. Atmospheric Dynamics via the Shallow Water System,"Within the context of exoplanetary atmospheres, we present a comprehensive
linear analysis of forced, damped, magnetized shallow water systems, exploring
the effects of dimensionality, geometry (Cartesian, pseudo-spherical and
spherical), rotation, magnetic tension and hydrodynamic and magnetic sources of
friction. Across a broad range of conditions, we find that the key governing
equation for atmospheres and quantum harmonic oscillators are identical, even
when forcing (stellar irradiation), sources of friction (molecular viscosity,
Rayleigh drag and magnetic drag) and magnetic tension are included. The global
atmospheric structure is largely controlled by a single, key parameter that
involves the Rossby and Prandtl numbers. This near-universality breaks down
when either molecular viscosity or magnetic drag acts non-uniformly across
latitude or a poloidal magnetic field is present, suggesting that these effects
will introduce qualitative changes to the familiar chevron-shaped feature
witnessed in simulations of atmospheric circulation. We also find that
hydrodynamic and magnetic sources of friction have dissimilar phase signatures
and affect the flow in fundamentally different ways, implying that using
Rayleigh drag to mimic magnetic drag is inaccurate. We exhaustively lay down
the theoretical formalism (dispersion relations, governing equations and
time-dependent wave solutions) for a broad suite of models. In all situations,
we derive the steady state of an atmosphere, which is relevant to interpreting
infrared phase and eclipse maps of exoplanetary atmospheres. We elucidate a
pinching effect that confines the atmospheric structure to be near the equator.
Our suite of analytical models may be used to decisively develop physical
intuition and as a reference point for three-dimensional, magnetohydrodynamic
(MHD) simulations of atmospheric circulation.",1401.7571v2
2014-01-30,Mixing in Magnetized Turbulent Media,"Turbulent motions are essential to the mixing of entrained fluids and are
also capable of amplifying weak initial magnetic fields by small-scale dynamo
action. Here we perform a systematic study of turbulent mixing in magnetized
media, using three-dimensional magnetohydrodynamic simulations that include a
scalar concentration field. We focus on how mixing depends on the magnetic
Prandtl number, Pm, from 1 to 4 and the Mach number, M}, from 0.3 to 2.4. For
all subsonic flows, we find that the velocity power spectrum has a k^-5/3 slope
in the early, kinematic phase, but steepens due to magnetic back reactions as
the field saturates. The scalar power spectrum, on the other hand, flattens
compared to k^-5/3 at late times, consistent with the Obukohov-Corrsin picture
of mixing as a cascade process. At higher Mach numbers, the velocity power
spectrum also steepens due to the presence of shocks, and the scalar power
spectrum again flattens accordingly. Scalar structures are more intermittent
than velocity structures in subsonic turbulence while for supersonic
turbulence, velocity structures appear more intermittent than the scalars only
in the kinematic phase. Independent of the Mach number of the flow, scalar
structures are arranged in sheets in both the kinematic and saturated phases of
the magnetic field evolution. For subsonic turbulence, scalar dissipation is
hindered in the strong magnetic field regions, probably due to Lorentz forces
suppressing the buildup of scalar gradients, while for supersonic turbulence,
scalar dissipation increases monotonically with increasing magnetic field
strength. At all Mach numbers, mixing is significantly slowed by the presence
of dynamically-important small-scale magnetic fields, implying that mixing in
the interstellar medium and in galaxy clusters is less efficient than modeled
in hydrodynamic simulations.",1401.8001v2
2015-11-16,Determination of intrinsic damping of perpendicularly magnetized ultrathin films from time resolved precessional magnetization measurements,"Magnetization dynamics are strongly influenced by damping. An effective
damping constant {\alpha}eff is often determined experimentally from the
spectral linewidth of the free induction decay of the magnetization after the
system is excited to its non-equilibrium state. Such an {\alpha}eff, however,
reflects both intrinsic damping as well as inhomogeneous broadening. In this
paper we compare measurements of the magnetization dynamics in ultrathin
non-epitaxial films having perpendicular magnetic anisotropy using two
different techniques, time-resolved magneto optical Kerr effect (TRMOKE) and
hybrid optical-electrical ferromagnetic resonance (OFMR). By using an external
magnetic field that is applied at very small angles to the film plane in the
TRMOKE studies, we develop an explicit closed-form analytical expression for
the TRMOKE spectral linewidth and show how this can be used to reliably extract
the intrinsic Gilbert damping constant. The damping constant determined in this
way is in excellent agreement with that determined from the OFMR method on the
same samples. Our studies indicate that the asymptotic high-field approach that
is often used in the TRMOKE method to distinguish the intrinsic damping from
the effective damping may result in significant error, because such high
external magnetic fields are required to make this approach valid that they are
out of reach. The error becomes larger the lower is the intrinsic damping
constant, and thus may account for the anomalously high damping constants that
are often reported in TRMOKE studies. In conventional ferromagnetic resonance
(FMR) studies, inhomogeneous contributions can be readily distinguished from
intrinsic damping contributions from the magnetic field dependence of the FMR
linewidth. Using the analogous approach, we show how reliable values of the
intrinsic damping can be extracted from TRMOKE.",1511.04802v1
2016-06-07,A moving mesh unstaggered constrained transport scheme for magnetohydrodynamics,"We present a constrained transport (CT) algorithm for solving the 3D ideal
magnetohydrodynamic (MHD) equations on a moving mesh, which maintains the
divergence-free condition on the magnetic field to machine-precision. Our CT
scheme uses an unstructured representation of the magnetic vector potential,
making the numerical method simple and computationally efficient. The scheme is
implemented in the moving mesh code Arepo. We demonstrate the performance of
the approach with simulations of driven MHD turbulence, a magnetized disc
galaxy, and a cosmological volume with primordial magnetic field. We compare
the outcomes of these experiments to those obtained with a previously
implemented Powell divergence-cleaning scheme. While CT and the Powell
technique yield similar results in idealized test problems, some differences
are seen in situations more representative of astrophysical flows. In the
turbulence simulations, the Powell cleaning scheme artificially grows the mean
magnetic field, while CT maintains this conserved quantity of ideal MHD. In the
disc simulation, CT gives slower magnetic field growth rate and saturates to
equipartition between the turbulent kinetic energy and magnetic energy, whereas
Powell cleaning produces a dynamically dominant magnetic field. Such difference
has been observed in adaptive-mesh refinement codes with CT and
smoothed-particle hydrodynamics codes with divergence-cleaning. In the
cosmological simulation, both approaches give similar magnetic amplification,
but Powell exhibits more cell-level noise. CT methods in general are more
accurate than divergence-cleaning techniques, and, when coupled to a moving
mesh can exploit the advantages of automatic spatial/temporal adaptivity and
reduced advection errors, allowing for improved astrophysical MHD simulations.",1606.02310v2
2016-07-02,Interface-Induced Phenomena in Magnetism,"This article reviews static and dynamic interfacial effects in magnetism,
focusing on interfacially-driven magnetic effects and phenomena associated with
spin-orbit coupling and intrinsic symmetry breaking at interfaces. It provides
a historical background and literature survey, but focuses on recent progress,
identifying the most exciting new scientific results and pointing to promising
future research directions. It starts with an introduction and overview of how
basic magnetic properties are affected by interfaces, then turns to a
discussion of charge and spin transport through and near interfaces and how
these can be used to control the properties of the magnetic layer. Important
concepts include spin accumulation, spin currents, spin transfer torque, and
spin pumping. An overview is provided to the current state of knowledge and
existing review literature on interfacial effects such as exchange bias,
exchange spring magnets, spin Hall effect, oxide heterostructures, and
topological insulators. The article highlights recent discoveries of
interface-induced magnetism and non-collinear spin textures, non-linear
dynamics including spin torque transfer and magnetization reversal induced by
interfaces, and interfacial effects in ultrafast magnetization processes.",1607.00439v4
2016-11-02,Topological insulator in a helicoidal magnetization field,"A key feature of topological insulators is the robustness of the electron
energy spectrum. At a surface of a topological insulator, Dirac point is
protected by the characteristic symmetry of the system. The breaking of the
symmetry opens a gap in the energy spectrum. Therefore, topological insulators
are very sensitive to magnetic fields, which can open a gap in the electronic
spectrum. Concerning ""internal"" magnetic effects, for example the situation
with doped magnetic impurities, is not trivial. A single magnetic impurity is
not enough to open the band gap, while in the case of a ferromagnetic chain of
deposited magnetic impurities the Dirac point is lifted. However, a much more
interesting case is when localized magnetic impurities form a chiral spin
order. Our first principle density functional theory calculations have shown
that this is the case for Fe deposited on the surface of Bi2Se3 topological
insulator. But not only magnetic impurities can form a chiral helicoidal spin
texture. An alternative way is to use chiral multiferroics (prototype material
is LiCu2O2) that induce a proximity effect. The theoretical approach we present
here is valid for both cases. We observed that opposite to a ferromagnetically
ordered case, a chiral spin order does not destroy the Dirac point. We also
observed that the energy gap appears at the edges of the new Brillouin zone.
Another interesting result concerns the spin dynamics. We derived an equation
for the spin density dynamics with a spin current and relaxation terms. We have
shown that the motion of the conductance electron generates a magnetic torque
and exerts a certain force on the helicoidal texture.",1611.00642v1
2017-01-05,Magnetic switching in granular FePt layers promoted by near-field laser enhancement,"Light-matter interaction at the nanoscale in magnetic materials is a topic of
intense research in view of potential applications in next-generation
high-density magnetic recording. Laser-assisted switching provides a pathway
for overcoming the material constraints of high-anisotropy and high-packing
density media, though much about the dynamics of the switching process remains
unexplored. We use ultrafast small-angle x-ray scattering at an x-ray
free-electron laser to probe the magnetic switching dynamics of FePt
nanoparticles embedded in a carbon matrix following excitation by an optical
femtosecond laser pulse. We observe that the combination of laser excitation
and applied static magnetic field, one order of magnitude smaller than the
coercive field, can overcome the magnetic anisotropy barrier between ""up"" and
""down"" magnetization, enabling magnetization switching. This magnetic switching
is found to be inhomogeneous throughout the material, with some individual FePt
nanoparticles neither switching nor demagnetizing. The origin of this behavior
is identified as the near-field modification of the incident laser radiation
around FePt nanoparticles. The fraction of not-switching nanoparticles is
influenced by the heat flow between FePt and a heat-sink layer.",1701.01237v1
2017-04-13,Low energy magnon dynamics and magneto-optics of the skyrmionic Mott insulator Cu$_2$OSeO$_3$,"In this work, we present a comprehensive study of the low energy optical
magnetic response of the skyrmionic Mott insulator Cu$_2$OSeO$_3$ via high
resolution time-domain THz spectroscopy. In zero field, a new magnetic
excitation not predicted by spin-wave theory with frequency $f$ = 2.03 THz is
observed and shown, with accompanying time-of-flight neutron scattering
experiments, to be a zone folded magnon from the $\mathrm{R}$ to
$\mathrm{\Gamma}$ points of the Brillouin zone. Highly sensitive polarimetry
experiments performed in weak magnetic fields, $\mu_0$H $<$ 200 mT, observe
Faraday and Kerr rotations which are proportional to the sample magnetization,
allowing for optical detection of the skyrmion phase and construction of a
magnetic phase diagram. From these measurements, we extract a critical exponent
of $\beta$ = 0.35 $\pm$ 0.04, in good agreement with the expected value for the
3D Heisenberg universality class of $\beta$ = 0.367. In large magnetic fields,
$\mu_0$H $>$ 5 T, we observe the magnetically active uniform mode of the
ferrimagnetic field polarized phase whose dynamics as a function of field and
temperature are studied. In addition to extracting a $g_\text{eff}$ = 2.08
$\pm$ 0.03, we observe the uniform mode to decay through a non-Gilbert damping
mechanism and to possesses a finite spontaneous decay rate, $\Gamma_0$
$\approx$ 25 GHz, in the zero temperature limit. Our observations are
attributed to Dzyaloshinkii-Moriya interactions, which have been proposed to be
exceptionally strong in Cu$_2$OSeO$_3$ and are expected to impact the low
energy magnetic response of such chiral magnets.",1704.04228v1
2017-06-27,The pulsating magnetosphere of the extremely slowly rotating magnetic $β$ Cep star $ξ^1$ CMa,"$\xi^1$ CMa is a monoperiodically pulsating, magnetic $\beta$ Cep star with
magnetospheric X-ray emission which, uniquely amongst magnetic stars, is
clearly modulated with the star's pulsation period. The rotational period
$P_{\rm rot}$ has yet to be identified, with multiple competing claims in the
literature. We present an analysis of a large ESPaDOnS dataset with a 9-year
baseline. The longitudinal magnetic field $\langle B_{\rm Z}\rangle$ shows a
significant annual variation, suggesting that $P_{\rm rot}$ is at least on the
order of decades. The possibility that the star's H$\alpha$ emission originates
around a classical Be companion star is explored and rejected based upon VLTI
AMBER and PIONIER interferometry, indicating that the emission must instead
originate in the star's magnetosphere and should therefore also be modulated
with $P_{\rm rot}$. Period analysis of H$\alpha$ equivalent widths measured
from ESPaDOnS and CORALIE spectra indicates $P_{\rm rot} > 30$ yr. All evidence
thus supports that $\xi^1$ CMa is a very slowly rotating magnetic star hosting
a dynamical magnetosphere. H$\alpha$ also shows evidence for modulation with
the pulsation period, a phenomenon which we show cannot be explained by
variability of the underlying photospheric line profile, i.e. it may reflect
changes in the quantity and distribution of magnetically confined plasma in the
circumstellar environment. In comparison to other magnetic stars with similar
stellar properties, $\xi^1$ CMa is by far the most slowly rotating magnetic
B-type star, is the only slowly rotating B-type star with a magnetosphere
detectable in H$\alpha$ (and thus, the coolest star with an optically
detectable dynamical magnetosphere), and is the only known early-type magnetic
star with H$\alpha$ emission modulated by both pulsation and rotation.",1706.08820v1
2017-09-14,Origin and evolution of magnetic fields in PMS stars : influence of rotation and structural changes,"During stellar evolution, especially in the PMS, stellar structure and
rotation evolve significantly causing major changes in the dynamics and global
flows of the star. We wish to assess the consequences of these changes on
stellar dynamo, internal magnetic field topology and activity level. To do so,
we have performed a series of 3D HD and MHD simulations with the ASH code. We
choose five different models characterized by the radius of their radiative
zone following an evolutionary track computed by a 1D stellar evolution code.
These models characterized stellar evolution from 1 Myr to 50 Myr. By
introducing a seed magnetic field in the fully convective model and spreading
its evolved state through all four remaining cases, we observe systematic
variations in the dynamical properties and magnetic field amplitude and
topology of the models. The five MHD simulations develop strong dynamo field
that can reach equipartition state between the kinetic and magnetic energy and
even super- equipartition levels in the faster rotating cases. We find that the
magnetic field amplitude increases as it evolves toward the ZAMS. Moreover the
magnetic field topology becomes more complex, with a decreasing axisymmetric
component and a non-axisymmetric one becoming predominant. The dipolar
components decrease as the rotation rate and the size of the radiative core
increase. The magnetic fields possess a mixed poloidal-toroidal topology with
no obvious dominant component. Moreover the relaxation of the vestige dynamo
magnetic field within the radiative core is found to satisfy MHD stability
criteria. Hence it does not experience a global reconfiguration but slowly
relaxes by retaining its mixed stable poloidal-toroidal topology.",1709.04667v1
2019-06-09,Spin dynamics and unconventional magnetism in insulating La$_{(1-2x)}$Sr$_{2x}$Co$_{(1-x)}$Nb$_{x}$O$_3$,"We study the structural, magnetic, transport and electronic properties of
LaCoO$_3$ with Sr/Nb co-substitution, i.e.,
La$_{(1-2x)}$Sr$_{2x}$Co$_{(1-x)}$Nb$_{x}$O$_3$ using x-ray and neutron
diffraction, dc and ac-magnetization, neutron depolarization, dc-resistivity
and photoemission measurements. The powder x-ray and neutron diffraction data
were fitted well with the rhombohedral crystal symmetry (space group
\textit{R$\bar{3}$c}) in Rietveld refinement analysis. The calculated effective
magnetic moment ($\approx$3.85~$\mu_B$) and average spin ($\approx$1.5) of Co
ions from the analysis of magnetic susceptibility data are consistent with 3+
state of Co ions in intermediate-spin (IS) and high-spin (HS) states in the
ratio of $\approx$50:50, i.e., spin-state of Co$^{3+}$ is preserved at least up
to $x=$ 0.1 sample. Interestingly, the magnetization values were significantly
increased with respect to the $x=$ 0 sample, and the M-H curves show
non-saturated behavior up to an applied maximum magnetic field of $\pm$70 kOe.
The ac-susceptibility data show a shift in the freezing temperature with
excitation frequency and the detailed analysis confirm the slower dynamics and
a non-zero value of the Vogel-Fulcher temperature T$_0$, which suggests for the
cluster spin glass. The unusual magnetic behavior indicates the presence of
complex magnetic interactions at low temperatures. The dc-resistivity
measurements show the insulating nature in all the samples. However, relatively
large density of states $\approx$10$^{22}$ eV$^{-1}$cm$^{-3}$ and low
activation energy $\approx$130~meV are found in $x=$ 0.05 sample. Using x-ray
photoemission spectroscopy, we study the core-level spectra of La 3$d$, Co
2$p$, Sr 3$d$, and Nb 3$d$ to confirm the valence state.",1906.03659v1
2022-06-11,Field evolution of magnetic phases and spin dynamics in the honeycomb lattice magnet Na2Co2TeO6: 23Na NMR study,"We report on the results of 23Na NMR in the honeycomb lattice magnet
Na2Co2TeO6 which has been nominated as a Kitaev material. Measurements of
magnetic shift and width of the NMR line as functions of temperature and
magnetic field show that a spin-disordered phase does not appear up to a field
of 9 T. In the antiferromagnetic phase just below the Neel temperature TN, we
find a temperature region extending down to ~TN/2 where the nuclear
spin-lattice relaxation rate 1/T1 remains enhanced and is further increased by
a magnetic field. This region crosses over to a low temperature region
characterized by the rapidly decreasing 1/T1 which is less field-sensitive.
These observations suggest incoherent spin excitations with a large spectral
weight at low energies in the intermediate temperature region transforming to
more conventional spin-wave excitations at low temperatures. The drastic change
of the low-energy spin dynamics is likely caused by strong damping of spin
waves activated only in the intermediate temperature region, which may be
realized for triple-q magnetic order possessing partially-disordered moments as
scattering centers of spin waves. In the paramagnetic phase near TN, dramatic
field suppression of 1/T1 is observed. From analysis of the temperature
dependence of 1/T1 based on the renormalized-classical description of a
two-dimensional quantum antiferromagnet, we find the field-dependent spin
stiffness constant that scales with TN as a function of magnetic field. This
implies field suppression of the energy scale characterizing both
two-dimensional spin correlations and three-dimensional long-range order, which
may be associated with an increasing effect of frustration in magnetic fields.",2206.05409v3
2023-04-14,Magnetic properties of a spin-orbit entangled Jeff=1/2 three-dimensional frustrated rare-earth hyperkagome,"The interplay between competing degrees of freedom can stabilize non-trivial
magnetic states in correlated electron materials. Frustration-induced strong
quantum fluctuations can evade long-range magnetic ordering leading to exotic
quantum states such as spin liquids in two-dimensional spin-lattices such as
triangular and kagome structures. However, the experimental realization of
dynamic and correlated quantum states is rare in three-dimensional (3D)
frustrated magnets wherein quantum fluctuations are less prominent. Herein, we
report the crystal structure, magnetic susceptibility, electron spin resonance
(ESR) and specific heat studies accompanied by crystal electric field (CEF)
calculations on a 3D frustrated magnet Yb3Sc2Ga3O12. In this material, Yb3+
ions form a three-dimensional network of corner-sharing triangles known as
hyperkagome lattice without any detectable anti-site disorder. Our results
reveal a low energy state with Jeff = 1/2 degrees of freedom in the Kramers
doublet state. The zero field-cooled and field cooled magnetic susceptibility
taken in 0.001 T rules out the presence of spin-freezing down to 1.8K. The
Curie-Weiss (CW) fit to low-T susceptibility data yields a small and negative
CW temperature indicating the presence of a weak antiferromagnetic interaction
between Jeff = 1/2 (Yb3+) moments. The Yb-ESR displays a broad line of
non-Lorentzian shape that suggests considerable magnetic anisotropy in
Yb3Sc2Ga3O12. The CEF calculations suggest that the ground state is well
separated from the excited states, which are in good agreement with
experimental results. The absence of long-range magnetic ordering indicates a
dynamic liquid-like ground state at least down to 130 mK. Furthermore, zero
field specific heat shows a broad maximum around 200 mK suggesting the presence
of short-range spin correlations in this 3D frustrated antiferromagnet.",2304.07350v2
2024-01-16,Critical magnetic Reynolds number of the turbulent dynamo in collisionless plasmas,"The intracluster medium of galaxy clusters is an extremely hot and diffuse,
nearly collisionless plasma, which hosts dynamically important magnetic fields
of $\sim \mu {\rm G}$ strength. Seed magnetic fields of much weaker strength of
astrophysical or primordial origin can be present in the intracluster medium.
In collisional plasmas, which can be approximated in the magneto-hydrodynamical
(MHD) limit, the turbulent dynamo mechanism can amplify weak seed fields to
strong dynamical levels efficiently by converting turbulent kinetic energy into
magnetic energy. However, the viability of this mechanism in weakly collisional
or completely collisionless plasma is much less understood. In this study, we
explore the properties of the collisionless turbulent dynamo by using
three-dimensional hybrid-kinetic particle-in-cell simulations. We explore the
properties of the collisionless turbulent dynamo in the kinematic regime for
different values of the magnetic Reynolds number, ${\rm Rm}$, initial
magnetic-to-kinetic energy ratio, $(E_{\rm{mag}}/E_{\rm{kin}})_{\rm{i}}$, and
initial Larmor ratio, $(r_{\rm Larmor}/L_{\rm box})_{\rm i}$, i.e., the ratio
of the Larmor radius to the size of the turbulent system. We find that in the
`un-magnetised' regime, $(r_{\rm Larmor}/L_{\rm box})_{\rm i} > 1$, the
critical magnetic Reynolds number for the dynamo action ${\rm Rm}_{\rm crit}
\approx 107 \pm 3$. In the `magnetised' regime, $(r_{\rm Larmor}/L_{\rm
box})_{\rm i} \lesssim 1$, we find a marginally higher ${\rm Rm}_{\rm crit} =
124 \pm 8$. We find that the growth rate of the magnetic energy does not depend
on the strength of the seed magnetic field when the initial magnetisation is
fixed. We also study the distribution and evolution of the pressure anisotropy
in the collisionless plasma and compare our results with the MHD turbulent
dynamo.",2401.08499v1
2024-03-06,Dark Dragon Breaks Magnetic Chain: Dynamical Substructures of IRDC G28.34 Form in Supported Environments,"We have comprehensively studied the multi-scale physical properties of the
infrared dark cloud (IRDC) G28.34 (the Dragon cloud) with dust polarization and
molecular line data from Planck, FCRAO-14m, JCMT, and ALMA. We find that the
averaged magnetic fields of clumps tend to be either parallel with or
perpendicular to the cloud-scale magnetic fields, while the cores in clump MM4
tend to have magnetic fields aligned with the clump fields. Implementing the
relative orientation analysis (for magnetic fields, column density gradients,
and local gravity), Velocity Gradient Technique (VGT), and modified
Davis-Chandrasekhar-Fermi (DCF) analysis, we find that: G28.34 is located in a
trans-to-sub-Alfv\'{e}nic environment ($\mathcal{M}_{A}=0.74$ within $r=15$
pc); the magnetic field is effectively resisting gravitational collapse in
large-scale diffuse gas, but is distorted by gravity within the cloud and
affected by star formation activities in high-density regions; and the
normalized mass-to-flux ratio tends to increase with increasing density and
decreasing radius. Considering the thermal, turbulent, and magnetic supports,
we find that the environmental gas of G28.34 is in a super-virial (supported)
state, the infrared dark clumps may be in a near-equilibrium state, and core
MM4-core4 is in a sub-virial (gravity-dominant) state. In summary, we suggest
that magnetic fields dominate gravity and turbulence in the cloud environment
at large scales, resulting in relatively slow cloud formation and evolution
processes. Within the cloud, gravity could overwhelm magnetic fields and
turbulence, allowing local dynamical star formation to happen.",2403.03437v2
1997-03-21,"Star Formation in Cold, Spherical, Magnetized Molecular Clouds","We present an idealized, spherical model of the evolution of a magnetized
molecular cloud due to ambipolar diffusion. This model allows us to follow the
quasi-static evolution of the cloud's core prior to collapse and the subsequent
evolution of the remaining envelope. By neglecting the thermal pressure
gradients in comparison with magnetic stresses and by assuming that the ion
velocity is small compared with the neutral velocity, we are able to find exact
analytic solutions to the MHD equations. We show that, in the case of a
centrally condensed cloud, a core of finite mass collapses into the origin
leaving behind a quasi-static envelope, whereas initially homogeneous clouds
never develop any structure in the absence of thermal stresses, and collapse as
a whole. Prior to the collapse of the core, the cloud's evolution is
characterized by two phases: a long, quasi-static phase where the relevant
timescale is the ambipolar diffusion time (treated in this paper), and a short,
dynamical phase where the characteristic timescale is the free-fall time. The
collapse of the core is an ""outside-in"" collapse. The quasi-static evolution
terminates when the cloud becomes magnetically supercritical; thereafter its
evolution is dynamical, and a singularity develops at the origin-a protostar.
After the initial formation of the protostar, the outer envelope continues to
evolve quasi-statically, while the region of dynamical infall grows with
time-an ""inside-out"" collapse. We use our solution to estimate the magnetic
flux trapped in the collapsing core and the mass accretion rate onto the newly
formed protostar. Our results agree, within factors of order unity, with the
numerical results of Fiedler & Mouschovias (1992) for the physical quantities
in the midplane of",9703144v1
2002-11-14,Collapse of a Magnetized Star to a Black Hole,"We study of the collapse of a magnetized spherical star to a black hole in
general relativity theory. The matter and gravitational fields are described by
the exact Oppenheimer-Snyder solution for the collapse of a spherical,
homogeneous dust ball. We adopt a ``dynamical Cowling approximation'' whereby
the matter and the geometry (metric), while highly dynamical, are unaffected by
the electromagnetic fields. The matter is assumed to be perfectly conducting
and threaded by a dipole magnetic field at the onset of collapse. We determine
the subsequent evolution of the magnetic and electric fields without
approximation; the fields are determined analytically in the matter interior
and numerically in the vacuum exterior. We apply junction conditions to match
the electromagnetic fields across the stellar surface. We use this model to
experiment with several coordinate gauge choices for handling spacetime
evolution characterized by the formation of a black hole and the associated
appearance of singularities. These choices range from ``singularity avoiding''
time coordinates to ``horizon penetrating'' time coordinates accompanied by
black hole excision. The later choice enables us to integrate the
electromagnetic fields arbitrarily far into the future. At late times the
longitudinal magnetic field in the exterior has been transformed into a
transverse electromagnetic wave; part of the electromagnetic radiation is
captured by the hole and the rest propagates outward to large distances. The
solution we present for our simple scenario can be used to test codes designed
to treat more general evolutions of relativistic MHD fluids flowing in strong
gravitational fields in dynamical spacetimes.",0211339v1
1999-04-01,Pauli principle and chaos in a magnetized disk,"We present results of a detailed quantum mechanical study of a gas of $N$
noninteracting electrons confined to a circular boundary and subject to
homogeneous dc plus ac magnetic fields $(B=B_{dc}+B_{ac}f(t)$, with
$f(t+2\pi/\omega_0)=f(t)$). We earlier found a one-particle {\it classical}
phase diagram of the (scaled) Larmor frequency
$\tilde\omega_c=omega_c/\omega_0$ {\rm vs} $\epsilon=B_{ac}/B_{dc}$ that
separates regular from chaotic regimes. We also showed that the quantum
spectrum statistics changed from Poisson to Gaussian orthogonal ensembles in
the transition from classically integrable to chaotic dynamics. Here we find
that, as a function of $N$ and $(\epsilon,\tilde\omega_c)$, there are clear
quantum signatures in the magnetic response, when going from the
single-particle classically regular to chaotic regimes. In the quasi-integrable
regime the magnetization non-monotonically oscillates between diamagnetic and
paramagnetic as a function of $N$. We quantitatively understand this behavior
from a perturbation theory analysis. In the chaotic regime, however, we find
that the magnetization oscillates as a function of $N$ but it is {\it always}
diamagnetic. Equivalent results are also presented for the orbital currents. We
also find that the time-averaged energy grows like $N^2$ in the
quasi-integrable regime but changes to a linear $N$ dependence in the chaotic
regime. In contrast, the results with Bose statistics are akin to the
single-particle case and thus different from the fermionic case. We also give
an estimate of possible experimental parameters were our results may be seen in
semiconductor quantum dot billiards.",9904024v1
2006-07-25,Spin dynamics in the diluted ferromagnetic Kondo lattice model,"The interplay of disorder and competing interactions is investigated in the
carrier-induced ferromagnetic state of the Kondo lattice model within a
numerical finite-size study in which disorder is treated exactly. Competition
between impurity spin couplings, stability of the ferromagnetic state, and
magnetic transition temperature are quantitatively investigated in terms of
magnon properties for different models including dilution, disorder, and
weakly-coupled spins. A strong optimization is obtained for T_c at hole doping
p << x, highlighting the importance of compensation in diluted magnetic
semiconductors. The estimated T_c is in good agreement with experimental
results for Ga_{1-x}Mn_x As for corresponding impurity concentration, hole
bandwidth, and compensation. Finite-temperature spin dynamics is quantitatively
studied within a locally self-consistent magnon renormalization scheme, which
yields a substantial enhancement in T_c due to spin clustering, and highlights
the nearly-paramagnetic spin dynamics of weakly-coupled spins. The large
enhancement in density of low-energy magnetic excitations due to disorder and
competing interactions results in a strong thermal decay of magnetization,
which fits well with the Bloch form M_0(1-BT^{3/2}) at low temperature, with B
of same order of magnitude as obtained in recent squid magnetization
measurements on Ga_{1-x}Mn_x As samples.",0607633v1
2006-12-17,Phase separation and suppression of critical dynamics at quantum transitions of itinerant magnets: MnSi and (Sr$_{1-x}$Ca$_{x}$)RuO$_{3}$,"Quantum phase transitions (QPTs) have been studied extensively in correlated
electron systems. Characterization of magnetism at QPTs has, however, been
limited by the volume-integrated feature of neutron and magnetization
measurements and by pressure uncertainties in NMR studies using powderized
specimens. Overcoming these limitations, we performed muon spin relaxation
($\mu$SR) measurements which have a unique sensitivity to volume fractions of
magnetically ordered and paramagnetic regions, and studied QPTs from itinerant
heli/ferro magnet to paramagnet in MnSi (single-crystal; varying pressure) and
(Sr$_{1-x}$Ca$_{x}$)RuO$_{3}$ (ceramic specimens; varying $x$). Our results
provide the first clear evidence that both cases are associated with
spontaneous phase separation and suppression of dynamic critical behavior,
revealed a slow but dynamic character of the ``partial order'' diffuse spin
correlations in MnSi above the critical pressure, and, combined with other
known results in heavy-fermion and cuprate systems, suggest a possibility that
a majority of QPTs involve first-order transitions and/or phase separation.",0612437v1
2009-10-19,Flux dynamics and vortex phase diagram in $Ba(Fe_{1-x}Co_x)_2As_2$ single crystals revealed by magnetization and its relaxation,"Magnetization and its relaxation have been measured in
$Ba(Fe_{1-x}Co_x)_2As_2$ single crystals at various doping levels ranging
fromvery underdoped to very overdoped regimes. Sizable magnetization relaxation
rate has been observed in all samples, indicating a moderate vortex motion and
relatively small characteristic pinning energy. Detailed analysis leads to the
following conclusions: (1) A prominent second-peak (SP) effect was observed in
the samples around the optimal doping level (x $\approx$ 0.08), but it becomes
invisible or very weak in the very underdoped and overdoped samples; (2) The
magnetization relaxation rate is inversely related to the transient
superconducting current density revealing the non-monotonic field and
temperature dependence through the SP region; (3) A very sharp magnetization
peak was observed near zero field which corresponds to a much reduced
relaxation rate; (4) A weak temperature dependence of relaxation rate or a
plateau was found in the intermediate temperature region. Together with the
treatment of the Generalized-Inversion-Scheme, we suggest that the vortex
dynamics is describable by the collective pinning model. Finally, vortex phase
diagrams were drawn for all the samples showing a systematic evolution of
vortex dynamics.",0910.3600v2
2011-02-16,Experimental characterization of coherent magnetization transport in a one-dimensional spin system,"We experimentally characterize the non-equilibrium, room-temperature
magnetization dynamics of a spin chain evolving under an effective
double-quantum Hamiltonian. We show that the Liouville space operators
corresponding to the magnetization and the two-spin correlations evolve 90
degrees out of phase with each other, and drive the transport dynamics. For a
nearest-neighbor-coupled N-spin chain, the dynamics are found to be restricted
to a Liouville operator space whose dimension scales only as N^2, leading to a
slow growth of multi-spin correlations. Even though long-range couplings are
present in the real system, we find excellent agreement between the analytical
predictions and our experimental results, confirming that leakage out of the
restricted Liouville space is slow on the timescales investigated. Our results
indicate that the group velocity of the magnetization is 6.04 +/- 0.38 um/s,
corresponding to a coherent transport over N ~ 26 spins on the experimental
timescale. As the double-quantum Hamiltonian is related to the standard
one-dimensional XX Hamiltonian by a similarity transform, our results can be
directly extended to XX quantum spin chains, which have been extensively
studied in the context of both quantum magnetism and quantum information
processing.",1102.3400v2
2013-08-01,Simulation of the dynamics in the magnetotail current sheet,"The dynamics in the magnetosphere current sheet was simulated following
transformations to the rectangular NxM array of cells (cellular automaton)
originally proposed by Koselov and Koselova (2002). The magnetosphere part of
the modeling system was organized as a rectangular arrangement of cells with a
stored energy, a local redistribution of the energy will exist when a value
threshold is exceeded in one of the cells. We assume that the threshold value
in each cell depends on external control parameter which influences the long
boundaries of the rectangular array (40x80). The model dynamics controlled by
the z-component of the interplanetary magnetic field (Bz) as well as analogies
between the model transient processes and the observed substorm auroral
activations are discussed. The Bz correspond to temporary windows of the solar
wind for a group of magnetic clouds and plasmoids. The model simulates
organized patterns in the energy distribution. The function of distribution of
probability (or PDF) of the size of the avalanches have a power-law form,
showing evidences of a system with Self-Organized Criticality (SOC). The
occurrence of big avalanches in the model is related with the activation of
some intense magnetic substorm. The arrival to the terrestrial magnetopause of
a solar wind with Bz negative is a fundamental condition of occurrence of
magnetic substorms. While more negative it is the Bz IMF component bigger it is
the value of the AE index and trigger a strong magnetic substorm.",1308.0350v1
2013-12-10,On the alignment of PNe and local magnetic field at the galactic centre: MHD numerical simulations,"For the past decade observations of the alignement of PNe symmetries with
respect to the galactic disk have led to conflicting results. Recently
observational evidence for alignment between PNe and local interstellar
magnetic fields in the central part of the Galaxy ($b < 5^\circ$) has been
found. We studied the role of the interstellar magnetic field on the dynamical
evolution of a PN by means of an analytical model and from 3D MHD numerical
simulations. We test under what conditions typical ejecta would have their
dynamics severely modified by an interstellar magnetic field. We found that
uniform fields of $> 100\mu$G are required in order to be dynamically dominant.
This is found to occur only at later evolutionary stages, therefore being
unable to change the general morphology of the nebula. However, the symmetry
axis of bipolar and elliptical nebulae end up aligned to the external field.
This result can explain why different samples of PNe result in different
conclusions regarding the alignment of PNe. Objects located at high galactic
latitudes, or at large radii, should present no preferential alignment with
respect to the galactic plane. PNe located at the galactic centre and low
latitudes would, on the other hand, be preferentiably aligned to the disk.
Finally, we present synthetic polarization maps of the nebulae to show that the
polarization vectors, as well as the field lines at the expanding shell, are
not uniform even in the strongly magnetized case, indicating that polarization
maps of nebulae are not adequate in probing the orientation, or intensity, of
the dominant external field.",1312.2910v1
2014-01-23,Electron-spin dynamics induced by photon spins,"Strong rotating magnetic fields may cause a precession of the electron's spin
around the rotation axis of the magnetic field. The superposition of two
counterpropagating laser beams with circular polarization and opposite helicity
features such a rotating magnetic field component but also carries spin. The
laser's spin density, that can be expressed in terms of the lase's
electromagnetic fields and potentials, couples to the electron's spin via a
relativistic correction to the Pauli equation. We show that the quantum
mechanical interaction of the electron's spin with the laser's rotating
magnetic field and with the laser's spin density counteract each other in such
a way that a net spin rotation remains with a precession frequency that is much
smaller than the frequency one would expect from the rotating magnetic field
alone. In particular, the frequency scales differently with the laser's
electric field strength depending on if relativistic corrections are taken into
account or not. Thus, the relativistic coupling of the electron's spin to the
laser's spin density changes the dynamics not only quantitatively but also
qualitatively as compared to the nonrelativistic theory. The electron's spin
dynamics is a genuine quantum mechanical relativistic effect.",1401.5976v2
2014-02-05,Magnetization dynamics: path-integral formalism for the stochastic Landau-Lifshitz-Gilbert equation,"We construct a path-integral representation of the generating functional for
the dissipative dynamics of a classical magnetic moment as described by the
stochastic generalization of the Landau-Lifshitz-Gilbert equation proposed by
Brown, with the possible addition of spin-torque terms. In the process of
constructing this functional in the Cartesian coordinate system, we critically
revisit this stochastic equation. We present it in a form that accommodates for
any discretization scheme thanks to the inclusion of a drift term. The
generalized equation ensures the conservation of the magnetization modulus and
the approach to the Gibbs-Boltzmann equilibrium in the absence of non-potential
and time-dependent forces. The drift term vanishes only if the mid-point
Stratonovich prescription is used. We next reset the problem in the more
natural spherical coordinate system. We show that the noise transforms
non-trivially to spherical coordinates acquiring a non-vanishing mean value in
this coordinate system, a fact that has been often overlooked in the
literature. We next construct the generating functional formalism in this
system of coordinates for any discretization prescription. The functional
formalism in Cartesian or spherical coordinates should serve as a starting
point to study different aspects of the out-of-equilibrium dynamics of magnets.
Extensions to colored noise, micro-magnetism and disordered problems are
straightforward.",1402.1200v2
2014-06-19,"Neutrino Magnetic Moment, CP Violation and Flavor Oscillations in Matter","We consider collective oscillations of neutrinos, which are emergent
nonlinear flavor evolution phenomena instigated by neutrino-neutrino
interactions in astrophysical environments with sufficiently high neutrino
densities. We investigate the symmetries of the problem in the full three
flavor mixing scheme and in the exact many-body formulation by including the
effects of CP violation and neutrino magnetic moment. We show that, similar to
the two flavor scheme, several dynamical symmetries exist for three flavors in
the single-angle approximation if the net electron background in the
environment and the effects of the neutrino magnetic moment are negligible.
Moreover, we show that these dynamical symmetries are present even when the CP
symmetry is violated in neutrino oscillations. We explicitly write down the
constants of motion through which these dynamical symmetries manifest
themselves in terms of the generators of the SU(3) flavor transformations. We
also show that the effects due to the CP-violating Dirac phase factor out of
the many-body evolution operator and evolve independently of nonlinear flavor
transformations if neutrino electromagnetic interactions are ignored. In the
presence of a strong magnetic field, CP-violating effects can still be
considered independently provided that an effective definition for neutrino
magnetic moment is used.",1406.5489v1
2014-06-24,Dynamic Topology and Flux Rope Evolution During Non-linear Tearing of 3D Null Point Current Sheets,"In this work the dynamic magnetic field within a tearing-unstable
three-dimensional (3D) current sheet about a magnetic null point is described
in detail. We focus on the evolution of the magnetic null points and flux ropes
that are formed during the tearing process. Generally, we find that both
magnetic structures are created prolifically within the layer and are
non-trivially related. We examine how nulls are created and annihilated during
bifurcation processes, and describe how they evolve within the current layer.
The type of null bifurcation first observed is associated with the formation of
pairs of flux ropes within the current layer. We also find that new nulls form
within these flux ropes, both following internal reconnection and as adjacent
flux ropes interact. The flux ropes exhibit a complex evolution, driven by a
combination of ideal kinking and their interaction with the outflow jets from
the main layer. The finite size of the unstable layer also allows us to
consider the wider effects of flux rope generation. We find that the unstable
current layer acts as a source of torsional MHD waves and dynamic braiding of
magnetic fields. The implications of these results to several areas of
heliophysics are discussed.",1406.6120v1
2014-08-14,Structural stability and thermodynamics of CrN magnetic phases from ab initio and experiment,"The dynamical and thermodynamic phase stabilities of the stoichiometric
compound CrN including different structural and magnetic configurations are
comprehensively investigated using a first-principles density-functional-theory
(DFT) plus U approach in conjunction with experimental measurements of the
thermal expansion. Comparing DFT and DFT+U results with experimental data
reveals that the treatment of electron correlations using methods beyond
standard DFT is crucial. The non-magnetic face-centered cubic B1-CrN phase is
both, elastically and dynamically unstable, even under high pressure, while CrN
phases with non-zero local magnetic moments are predicted to be dynamically
stable within the framework of the DFT+U scheme. Furthermore, the impact of
different treatments for the exchange-correlation (xc)-functional is
investigated by carrying out all computations employing the local density
approximation and generalized gradient approximation. To address
finite-temperature properties, both, magnetic and vibrational contributions to
the free energy have been computed employing our recently developed spin-space
averaging method. The calculated phase transition temperature between
low-temperature antiferromagnetic and high-temperature paramagnetic (PM) CrN
variants is in excellent agreement with experimental values and reveals the
strong impact of the choice of the xc-functional. The temperature-dependent
linear thermal expansion coefficient of CrN is experimentally determined by the
wafer curvature method from a reactive magnetron sputter deposited single-phase
B1-CrN thin film with dense film morphology. A good agreement is found between
experimental and ab initio calculated linear thermal expansion coefficients of
PM B1-CrN. Other thermodynamic properties, such as the specific heat capacity,
have been computed as well and compared to previous experimental data.",1408.3201v1
2015-01-06,Exciton spin dynamics and photoluminescence polarization of CdSe/CdS dot-in-rod nanocrystals in high magnetic fields,"The exciton spin dynamics and polarization properties of the related emission
are investigated in colloidal CdSe/CdS dot-in-rod (DiR) and spherical
core/shell nanocrystal (NC) ensembles by magneto-optical photoluminescence (PL)
spectroscopy in magnetic fields up to 15 T. It is shown that the degree of
circular polarization (DCP) of the exciton emission induced by the magnetic
field is affected by the NC geometry as well as the exciton fine structure and
can provide information on nanorod orientation. A theory to describe the
circular and linear polarization properties of the NC emission in magnetic
field is developed. It takes into account phonon mediated coupling between the
exciton fine structure states as well as the dielectric enhancement effect
resulting from the anisotropic shell of DiR NCs. This theoretical approach is
used to model the experimental results and allows us to explain most of the
measured features. The spin dynamics of the dark excitons is investigated in
magnetic fields by time-resolved photoluminescence. The results highlight the
importance of confined acoustic phonons in the spin relaxation of dark
excitons. The bare core surface as well as the core/shell interface give rise
to an efficient spin relaxation channel, while the surface of core/shell NCs
seems to play only a minor role.",1501.01169v1
2015-10-08,Elliptic Annular Josephson Tunnel Junctions in an external magnetic field: The dynamics,"We analyze the dynamics of a magnetic flux quantum (current vortex) trapped
in a current-biased long planar elliptic annular Josephson tunnel junction. The
system is modeled by a perturbed sine-Gordon equation that determines the
spatial and temporal behavior of the phase difference across the tunnel barrier
separating the two superconducting electrodes. In the absence of an external
magnetic field the fluxon dynamics in an elliptic annulus does not differ from
that of a circular annulus where the stationary fluxon speed merely is
determined by the system losses. The interaction between the vortex magnetic
moment and a spatially homogeneous in-plane magnetic field gives rise to a
tunable periodic non-sinusoidal potential which is strongly dependent on the
annulus aspect ratio. We study the escape of the vortex from a well in the
tilted potential when the bias current exceeds the depinning current. The
smallest depinning current as well as the lowest sensitivity of the annulus to
the external field is achieved when the eccentricity is equal to -1. The
presented extensive numerical results are in good agreement with the findings
of the perturbative approach. We also probe the rectifying properties of an
asymmetric potential implemented with an egg-shaped annulus formed by two
semi-elliptic arcs.",1510.02254v1
2015-12-02,Investigation of intergranular bright points from the New Vacuum Solar Telescope,"Six high-resolution TiO-band image sequences from the New Vacuum Solar
Telescope (NVST) are used to investigate the properties of intergranular bright
points (igBPs). We detect the igBPs using a Laplacian and morphological
dilation algorithm (LMD) and track them using a three-dimensional segmentation
algorithm automatically, and then investigate the morphologic, photometric and
dynamic properties of igBPs, in terms of equivalent diameter, the intensity
contrast, lifetime, horizontal velocity, diffusion index, motion range and
motion type. The statistical results confirm the previous studies based on
G-band or TiO-band igBPs from the other telescopes. It illustrates that the TiO
data from the NVST have a stable and reliable quality, which are suitable for
studying the igBPs. In addition, our method is feasible to detect and track the
igBPs in the TiO data from the NVST. With the aid of the vector magnetograms
obtained from the Solar Dynamics Observatory /Helioseismic and Magnetic Imager,
the properties of igBPs are found to be influenced by their embedded magnetic
environments strongly. The area coverage, the size and the intensity contrast
values of igBPs are generally larger in the regions with higher magnetic flux.
However, the dynamics of igBPs, including the horizontal velocity, the
diffusion index, the ratio of motion range and the index of motion type are
generally larger in the regions with lower magnetic flux. It suggests that the
absence of strong magnetic fields in the medium makes it possible for the igBPs
to look smaller and weaker, diffuse faster, move faster and further in a
straighter path.",1512.00737v1
2016-05-08,Decay of isotropic flow and anisotropic flow with rotation or magnetic field or both in a weakly nonlinear regime,"We investigate numerically the decay of isotropic, rotating,
magnetohydrodynamic (MHD), and rotating MHD flows in a periodic box. The
Reynolds number $Re$ defined with the box size and the initial velocity is
$100$ at which the flows are in a weakly nonlinear regime, i.e. not laminar but
far away from the fully turbulent state. The decay of isotropic flow has two
stages, the first stage for the development of small scales and the second
stage for the viscous dissipation. In the rapidly rotating flow, fast rotation
induces the inertial wave and causes the large-scale structure to inhibit the
development of the first stage and retard the flow decay. In the MHD flow, the
imposed field also causes the large-scale structure but facilitates the flow
decay in the first stage because of the energy conversion from flow to magnetic
field. Magnetic Reynolds number $Rm$ is important for the dynamics of the MHD
flow, namely a high $Rm$ induces the Alfv\'en wave but a low $Rm$ cannot. In
the rotating MHD flow, slower rotation tends to convert more kinetic energy to
magnetic energy. The orientation between the rotational and magnetic axes is
important for the dynamics of the rotating MHD flow, namely the energy
conversion is more efficient and the stronger wave is induced when the two axes
are not parallel than when they are parallel.",1605.02267v1
2017-01-31,Flux Accretion and Coronal Mass Ejection Dynamics,"Coronal mass ejections (CMEs) are the primary drivers of severe space weather
disturbances in the heliosphere. Models of CME dynamics have been proposed that
do not fully include the effects of magnetic reconnection on the forces driving
the ejection. Both observations and numerical modeling, however, suggest that
reconnection likely plays a major role in most, if not all, fast CMEs. Here, we
theoretically investigate the accretion of magnetic flux onto a rising ejection
by reconnection involving the ejection's background field. This reconnection
alters the magnetic structure of the ejection and its environment, thereby
modifying the forces acting upon the ejection, generically increasing its
upward acceleration. The modified forces, in turn, can more strongly drive the
reconnection. This feedback process acts, effectively, as an instability, which
we refer to as a reconnective instability. Our analysis implies that CME models
that neglect the effects of reconnection cannot accurately describe observed
CME dynamics. Our ultimate aim is to understand changes in CME acceleration in
terms of observable properties of magnetic reconnection, such as the amount of
reconnected flux. This flux can be estimated from observations of flare ribbons
and photospheric magnetic fields.",1701.09082v5
2017-02-27,Current Induced Damping of Nanosized Quantum Moments in the Presence of Spin-Orbit Interaction,"Motivated by the need to understand current-induced magnetization dynamics at
the nanoscale, we have developed a formalism, within the framework of Keldysh
Green function approach, to study the current-induced dynamics of a
ferromagnetic (FM) nanoisland overlayer on a spin-orbit-coupling (SOC) Rashba
plane. In contrast to the commonly employed classical micromagnetic LLG
simulations the magnetic moments of the FM are treated {\it quantum
mechanically}. We obtain the density matrix of the whole system consisting of
conduction electrons entangled with the local magnetic moments and calculate
the effective damping rate of the FM. We investigate two opposite limiting
regimes of FM dynamics: (1) The precessional regime where the magnetic
anisotropy energy (MAE) and precessional frequency are smaller than the
exchange interactions, and (2) The local spin-flip regime where the MAE and
precessional frequency are comparable to the exchange interactions. In the
former case, we show that due to the finite size of the FM domain, the
\textquotedblleft Gilbert damping\textquotedblright does not diverge in the
ballistic electron transport regime, in sharp contrast to Kambersky's breathing
Fermi surface theory for damping in metallic FMs. In the latter case, we show
that above a critical bias the excited conduction electrons can switch the
local spin moments resulting in demagnetization and reversal of the
magnetization. Furthermore, our calculations show that the bias-induced
antidamping efficiency in the local spin-flip regime is much higher than that
in the rotational excitation regime.",1702.08408v2
2017-03-30,Structure distribution and turbulence in self-consistently supernova-driven ISM of multiphase magnetized galactic discs,"Galaxy evolution and star formation are two multi-scale problems tightly
linked to each other. To understand the interstellar cycle, which triggers
galaxy evolution, it is necessary to describe simultaneously the large-scale
evolution widely induced by the feedback processes and the details of the gas
dynamics that controls the star formation process through gravitational
collapse. We perform a set of three-dimensional high-resolution numerical
simulations of a turbulent, self-gravitating and magnetized interstellar medium
within a $1\ \mathrm{kpc}$ stratified box with supernova feedback correlated
with star-forming regions. In particular, we focus on the role played by the
magnetic field and the feedback on the galactic vertical structure, the star
formation rate (SFR) and the flow dynamics. For this purpose we vary their
respective intensities. We extract properties of the dense clouds arising from
the turbulent motions and compute power spectra of various quantities. Using a
distribution of supernovae sufficiently correlated with the dense gas, we find
that supernova explosions can reproduce the observed SFR, particularly if the
magnetic field is on the order of a few $\mu G$. The vertical structure, which
results from a dynamical and an energy equilibrium is well reproduced by a
simple analytical model, which allows us to estimate the coupling between the
gas and the supernovae. We found the coupling to be rather low and on the order
of 1.5$\%$. Strong magnetic fields may help to increase this coupling by a
factor of about 2-3. To characterize the flow we compute the power spectra of
various quantities in 3D but also in 2D in order to account for the
stratification of the galactic disc.",1703.10421v1
2017-12-04,Effect of FePd alloy composition on the dynamics of artificial spin ice,"Artificial spin ices (ASI) are arrays of single domain nano-magnetic islands,
arranged in geometries that give rise to frustrated magnetostatic interactions.
It is possible to reach their ground state via thermal annealing. We have made
square ASI using different FePd alloys to vary the magnetization via
co-sputtering. From a polarized state the samples were incrementally heated and
we measured the vertex population as a function of temperature using magnetic
force microscopy. For the higher magnetization FePd sample, we report an onset
of dynamics at $T = 493$ K, with a rapid collapse into $>90\%$ ground state
vertices. In contrast, the low magnetization sample started to fluctuate at
lower temperatures, $T = 393$ K and over a wider temperature range but only
reached a maximum of $25\%$ of ground state vertices. These results indicate
that the interaction strength, dynamic temperature range and pathways can be
finely tuned using a simple co-sputtering process. In addition we have compared
our experimental values of the blocking temperature to those predicted using
the simple N\'{e}el-Brown two-state model and find a large discrepancy which we
attribute to activation volumes much smaller than the island volume.",1712.02404v1
2018-03-01,Quantum Interference Controls the Electron Spin Dynamics in n-GaAs,"Manifestations of quantum interference effects in macroscopic objects are
rare. Weak localization is one of the few examples of such effects showing up
in the electron transport through solid state. Here we show that weak
localization becomes prominent also in optical spectroscopy via detection of
the electron spin dynamics. In particular, we find that weak localization
controls the free electron spin relaxation in semiconductors at low
temperatures and weak magnetic fields by slowing it down by almost a factor of
two in $n$-doped GaAs in the metallic phase. The weak localization effect on
the spin relaxation is suppressed by moderate magnetic fields of about 1 T,
which destroy the interference of electron trajectories, and by increasing the
temperature. The weak localization suppression causes an anomalous decrease of
the longitudinal electron spin relaxation time $T_1$ with magnetic field, in
stark contrast with well-known magnetic field induced increase in $T_1$. This
is consistent with transport measurements which show the same variation of
resistivity with magnetic field. Our discovery opens a vast playground to
explore quantum magneto-transport effects optically in the spin dynamics.",1803.00352v2
2018-04-18,"Onset of Photospheric Impacts and Helioseismic Waves in X9.3 Solar Flare of September 6, 2017","The X9.3 flare of September 6, 2017, was the most powerful flare of Solar
Cycle 24. It generated strong white-light emission and multiple helioseismic
waves (sunquakes). By using data from Helioseismic and Magnetic Imager (HMI)
onboard the Solar Dynamics Observatory (SDO) as well as hard X-ray data from
KONUS instrument onboard WIND spacecraft, and Anti-Coincidence System (ACS)
onboard the INTERGRAL space observatory, we investigate spatio-temporal
dynamics of photospheric emission sources, identify sources of helioseismic
waves and compare the flare photospheric dynamics with the hard X-ray (HXR)
temporal profiles. The results show that the photospheric flare impacts started
to develop in compact regions in close vicinity of the magnetic polarity
inversion line (PIL) in the pre-impulsive phase before detection of the HXR
emission. The initial photospheric disturbances were localized in the region of
strong horizontal magnetic field of the PIL, and, thus, are likely associated
with a compact sheared magnetic structure elongated along the PIL. The acoustic
egression power maps revealed two primary sources of generation of sunquakes,
which were associated with places of the strongest photospheric impacts in the
pre-impulsive phase and the early impulsive phase. This can explain the two
types of helioseismic waves observed in this flare. Analysis of the
high-cadence HMI filtergrams suggests that the flare energy release developed
in the form of sequential involvement of compact low-lying magnetic loops that
were sheared along the PIL.",1804.06565v1
2018-09-07,Magnetohydrodynamic Modeling of a Solar Eruption Associated with X9.3 Flare Observed in Active Region 12673,"On SOL2017-09-06 solar active region 12673 produced an X9.3 flare which is
regarded as largest to occur in solar cycle 24. In this work we have preformed
a magnetohydrodynamic (MHD) simulation in order to reveal the three-dimensional
(3D) dynamics of the magnetic fields associated with the X9.3 solar flare. We
first performed an extrapolation of the 3D magnetic field based on the observed
photospheric magnetic field prior to the flare and then used it as the initial
condition for an MHD simulation. Consequently, the simulation showed a dramatic
eruption. In particular, we found that a large coherent flux rope composed of
highly twisted magnetic field lines is formed during the eruption. A series of
small flux ropes are found to lie along a magnetic polarity inversion line
prior to the flare. Reconnection occurring between each small flux rope during
the early stages of the eruption forms the large and highly twisted flux
rope.Furthermore, we found a writhing motion of the erupting flux rope. The
understanding of these dynamics is important in increasing the accuracy of
space weather forecasting. We report on the detailed dynamics of the 3D
eruptive flux rope and discuss the possible mechanisms of the writhing motion.",1809.02309v1
2018-12-18,Petahertz Spintronics,"The enigmatic coupling between electronic and magnetic phenomena was one of
the riddles propelling the development of modern electromagnetism. Today, the
fully controlled electric field evolution of ultrashort laser pulses permits
the direct and ultrafast control of electronic properties of matter and is the
cornerstone of light-wave electronics. In sharp contrast, because there is no
first order interaction between light and spins, the magnetic properties of
matter can only be affected indirectly on the much slower tens-of-femtosecond
timescale in a sequence of optical excitation followed by the rearrangement of
the spin structure. Here we record an orders of magnitude faster magnetic
switching with sub-femtosecond response time by initiating optical excitations
with near-single-cycle laser pulses in a ferromagnetic layer stack. The
unfolding dynamics are tracked in real-time by a novel attosecond time-resolved
magnetic circular dichroism (atto-MCD) detection scheme revealing optically
induced spin and orbital momentum transfer (OISTR) in synchrony with light
field driven charge relocation. In tandem with ab-initio quantum dynamical
modelling, we show how this mechanism provides simultaneous control over
electronic and magnetic properties that are at the heart of spintronic
functionality. This first incarnation of attomagnetism observes light field
coherent control of spin-dynamics in the initial non-dissipative temporal
regime and paves the way towards coherent spintronic applications with
Petahertz clock rates.",1812.07420v1
2019-04-26,Non perturbative and thermal dynamics of confined fields in dual QCD,"In order to study the detailed dynamics and associated non-perturbative
features of QCD, a dual version of the color gauge theory based on the
topologically viable homogeneous fiber bundle approach has been analysed taking
into account its magnetic symmetry structure. In the dynamically broken phase
of magnetic symmetry, the associated flux tube structure on a S 2 -sphere in
the magnetically condensed state of the dual QCD vacuum has been analyzed for
the profiles of the color electric field using flux quantization and stability
conditions. The color electric field has its intimate association with the
vector mode of the magnetically condensed QCD vacuum and such field
configurations have been analyzed to show that the color electric flux gets
localized towards the poles for a large sphere case while it gets uniformly
distributed for the small sphere case in the infrared sector of QCD. The
critical flux tube densities have been computed for various couplings and are
shown to be in agreement with that for lead-ion central collisions in the near
infrared sector of QCD. The possible annihilation/unification of flux tubes
under some typical flux tube density and temperature conditions in the magnetic
symmetry broken phase of QCD has also been analyzed and shown to play an
important role in the process of QGP formation. The thermal variation of the
profiles of the color electic field is further investigated which indicates the
survival of flux tubes even in the thermal domain that leads the possibility of
the formation of some exotic states like QGP in the intermedate regime during
the quark-hadron phase transition.",1904.11714v1
2019-06-05,The formation and dissipation of current sheets and shocks due to compressive waves in a stratified atmosphere containing a magnetic null,"We study the propagation and dissipation of magnetohydrodynamic waves in a
set of numerical models that each include a solar--like stratified atmosphere
and a magnetic field with a null point. All simulations have the same magnetic
field configuration but different transition region heights. Compressive wave
packets introduced in the photospheric portion of the simulations refract
towards the null and collapse it into a current sheet, which then undergoes
reconnection. The collapsed null forms a current sheet due to a strong magnetic
pressure gradient caused by the inability of magnetic perturbations to cross
the null. Although the null current sheet undergoes multiple reconnection
episodes due to repeated reflections off the lower boundary, we find no
evidence of oscillatory reconnection arising from the dynamics of the null
itself. Wave mode conversion around the null generates a series of slow mode
shocks localized near each separatrix. The shock strength is asymmetric across
each separatrix, and subsequent shock damping therefore creates a tangential
discontinuity across each separatrix, with long--lived current densities. A
parameter study of the injected wave energy to reach the null confirms our
previous WKB estimates. Finally, using current estimates of the photospheric
acoustic power, we estimate that the shock and Ohmic heating we describe may
account for $\approx1-10\%$ of the radiative losses from coronal bright points
with similar topologies, and are similarly insufficient to account for losses
from larger structures such as ephemeral regions. At the same time, the
dynamics are comparable to proposed mechanisms for generating type--II
spicules.",1906.02317v1
2019-09-20,Impact of disorder on dynamics and ordering in the honeycomb lattice iridate Na$_2$IrO$_3$,"Kitaev's honeycomb spin-liquid model and its proposed realization in
materials such as $\alpha$-RuCl$_3$, Li$_2$IrO$_3$ and Na$_2$IrO$_3$ continue
to present open questions about how the dynamics of a spin-liquid are modified
in the presence of non-Kitaev interactions as well as the presence of
inhomogeneities. Here we use $^{23}$Na nuclear magnetic resonance to probe both
static and dynamical magnetic properties in single crystal Na$_2$IrO$_3$. We
find that the NMR shift follows the bulk susceptibility above 30 K but deviates
from it below; moreover below $T_N$ the spectra show a broad distribution of
internal magnetic fields. Both of these results provide evidence for
inequivalent magnetic sites at low temperature, suggesting inhomogeneities are
important for the magnetism. The spin-lattice relaxation rate is isotropic and
diverges at $T_N$, suggesting that the Kitaev cubic axes may control the
critical quantum spin fluctuations. In the ordered state, we observe gapless
excitations, which may arise from site substitution, emergent defects from
milder disorder, or possibly be associated with nearby quantum paramagnetic
states distinct from the Kitaev spin liquid.",1909.09356v2
2020-01-31,Coupling large eddies and waves in turbulence: Case study of magnetic helicity at the ion inertial scale,"In turbulence, for neutral or conducting fluids, a large ratio of scales is
excited because of the possible occurrence of inverse cascades to large, global
scales together with direct cascades to small, dissipative scales, as observed
in the atmosphere and oceans, or in the solar environment. In this context,
using direct numerical simulations with forcing, we analyze scale dynamics in
the presence of magnetic fields with a generalized Ohm's law including a Hall
current. The ion inertial length epsilon_H serves as the control parameter at
fixed Reynolds number. Both the magnetic and generalized helicity -- invariants
in the ideal case -- grow linearly with time, as expected from classical
arguments. The cross-correlation between the velocity and magnetic field grows
as well, more so in relative terms for a stronger Hall current. We find that
the helical growth rates vary exponentially with epsilon_H, provided the ion
inertial scale resides within the inverse cascade range. These exponential
variations are recovered phenomenologically using simple scaling arguments.
They are directly linked to the wavenumber power-law dependence of generalized
and magnetic helicity, k^(-2), in their inverse ranges. This illustrates and
confirms the important role of the interplay between large and small scales in
the dynamics of turbulent flows.",2001.11625v1
2020-03-09,Magnetic connectivity between the light bridge and penumbra in a sunspot,"A light bridge is a prominent structure commonly observed within a sunspot.
Its presence usually triggers a wealth of dynamics in a sunspot, and has a
lasting impact on sunspot evolution. However, the fundamental structure of
light bridges is still not well understood. In this study, we used the
high-resolution spectropolarimetry data obtained by the Solar Optical Telescope
onboard the Hinode satellite to analyze the magnetic and thermal structure of a
light bridge at \AR. We also combined the high-cadence $1700\unit{\AA}$ channel
data provided by the Atmospheric Imaging Assembly onboard the Solar Dynamic
Observatory to study the dynamics on this bridge. We found that a pair of blue
and red Doppler shift patches at two ends of this bridge, this pattern appears
to be the convective motion directed by the horizontal component of the
magnetic field aligned with the spine of this bridge. Paired upward and
downward motions implies that the light bridge could have a two-legged or
undulate magnetic field. Significant four minute oscillations in the emission
intensity of the $1700\unit{\AA}$ bandpass were detected at two ends, which had
overlap with the paired blue and red shift patches. The oscillatory signals at
the light bridge and the penumbra were highly correlated with each other.
Although they are separated in space at the photosphere, the periodicity seems
to have a common origin from the underneath. Therefore, we infer that the light
bridge and penumbra could share a common magnetic source and become fragmented
at the photosphere by magneto-convection.",2003.03976v1
2020-06-29,Long term time dependent frequency analysis of chaotic waves in the weakly magnetized spherical Couette system,"The long therm behavior of chaotic flows is investigated by means of time
dependent frequency analysis. The system under test consists of an electrically
conducting fluid, confined between two differentially rotating spheres. The
spherical setup is exposed to an axial magnetic field. The classical Fourier
Transform method provides a first estimation of the time dependence of the
frequencies associated to the flow, as well as its volume-averaged properties.
It is however unable to detect strange attractors close to regular solutions in
the Feigenbaum as well as Newhouse-Ruelle-Takens bifurcation scenarios. It is
shown that Laskar's frequency algorithm is sufficiently accurate to identify
these strange attractors and thus is an efficient tool for classification of
chaotic flows in high dimensional dynamical systems. Our analysis of several
chaotic solutions, obtained at different magnetic field strengths, reveals a
strong robustness of the main frequency of the flow. This frequency is
associated to an azimuthal drift and it is very close to the frequency of the
underlying unstable rotating wave. In contrast, the main frequency of
volume-averaged properties can vary almost one order of magnitude as the
magnetic forcing is decreased. We conclude that, at the moderate differential
rotation considered, unstable rotating waves provide a good description of the
variation of the main time scale of any flow with respective variations in the
magnetic field.",2006.15145v2
2020-10-29,Domain wall dynamics in antiferromagnetically-coupled double-lattice systems,"In ferromagnetic materials, the rich dynamics of magnetic domain walls (DWs)
under magnetic field or current have been successfully described using the
well-known q-{\phi} analytical model. We demonstrate here that this simple
unidimensional model holds for multiple-sublattice materials such as
ferrimagnetic alloys or synthetic antiferromagnets (SAF) by using effective
parameters, and is in excellent agreement with double-lattice micromagnetic
simulations. We obtain analytical laws for the DW velocity and internal
precession angle as a function of net magnetisation for different driving
forces (magnetic field, spin transfer and spin-orbit torques) and different
propagation regimes in ferrimagnetic alloys and SAFs. The model predicts that
several distinctive dynamical features occur near or at the magnetic and the
angular compensation points when the net magnetization or the net angular
momentum of the system vanishes, and we discuss the experimental observations
that have been reported for some of them. Using a higher degree-of-freedom
analytical model that accounts for inter-sublattice distortions, we give
analytical expressions for these distortions that agree with the micromagnetic
simulations. This model shows that the DW velocity and precession rate are
independent of the strength of the inter-sublattice exchange coupling, and
justifies the use of the simpler effective parameters model.",2010.15443v1
2021-02-16,Dynamic symmetry breaking in chiral magnetic systems,"The Dzyaloshinskii-Moriya interaction (DMI) in magnetic systems stabilizes
spin textures with preferred chirality, applicable to next-generation memory
and computing architectures. In perpendicularly magnetized
heavy-metal/ferromagnet films, the interfacial DMI originating from structural
inversion asymmetry and strong spin-orbit coupling favors chiral N\'eel-type
domain walls (DWs) whose energetics and mobility remain at issue. Here, we
characterize a new effect in which domains expand unidirectionally in response
to a combination of out-of-plane and in-plane magnetic fields, with the growth
direction controlled by the in-plane field strength. These growth
directionalities and symmetries with applied fields cannot be understood from
static treatments alone. We theoretically demonstrate that perpendicular field
torques stabilize steady-state magnetization profiles highly asymmetric in
elastic energy, resulting in a dynamic symmetry breaking consistent with the
experimental findings. This phenomenon sheds light on the mechanisms governing
the dynamics of N\'eel-type DWs and expands the utility of field-driven DW
motion to probe and control chiral DWs.",2102.07909v4
2021-10-24,$^{139}$La-NMR study of spin-dynamics coupled with hole mobility in $T$*-type La$_{0.86}$Eu$_{0.86}$Sr$_{0.28}$CuO$_{4-δ}$,"In $T$*-type cuprate oxides with five oxygen coordination, little is known
about the relationship between the spin correlations and dope carriers. We
performed $^{139}$La-nuclear magnetic resonance (NMR) and electrical
resistivity measurements on an as-sintered (AS) and oxidation annealed (OA)
polycrystalline $T$*-type La$_{0.86}$Eu$_{0.86}$S$_{0.28}$CuO$_4$ (LESCO) to
investigate its magnetic and superconducting (SC) properties. Upon cooling, the
NMR spectrum of AS LESCO broadened below 3 K, at which the nuclear spin-lattice
relaxation rate $1/T_1$ against the temperature exhibited a maximum, thereby
indicating the appearance of static magnetism. The temperature dependence of
$1/T_1$ between 3 K and 20 K was similar to that of the resistivity displaying
the semiconducting behavior. Furthermore, the energy scale of the transport gap
and spin-dynamics estimated was found to be comparable. These results suggest a
close connection between the mobility of the doped carriers and low-energy
spin-dynamics, as reported for lightly doped $T$-type La$_{2-x}$Sr$_x$CuO$_4$.
In the OA SC sample, we confirmed the absence of a magnetic order and the
Korringa relation above 10 K. Therefore, in the $T$*-type LESCO with $x$ =
0.28, the magnetic state coupled with holes drastically turns to the weakly
correlated metallic state by oxidation annealing.",2110.12333v1
2021-11-29,Quantifying the Computational Capability of a Nanomagnetic Reservoir Computing Platform with Emergent Magnetization Dynamics,"Devices based on arrays of interconnected magnetic nano-rings with emergent
magnetization dynamics have recently been proposed for use in reservoir
computing applications, but for them to be computationally useful it must be
possible to optimise their dynamical responses. Here, we use a phenomenological
model to demonstrate that such reservoirs can be optimised for classification
tasks by tuning hyperparameters that control the scaling and input rate of data
into the system using rotating magnetic fields. We use task-independent metrics
to assess the rings' computational capabilities at each set of these
hyperparameters and show how these metrics correlate directly to performance in
spoken and written digit recognition tasks. We then show that these metrics,
and performance in tasks, can be further improved by expanding the reservoir's
output to include multiple, concurrent measures of the ring arrays magnetic
states.",2111.14603v2
2022-03-18,Semi-classical simulation of spin-1 magnets,"Theoretical studies of magnets have traditionally concentrated on either
classical spins, or the extreme quantum limit of spin-1/2. However, magnets
built of spin-1 moments are also intrinsically interesting, not least because
they can support quadrupole, as well as dipole moments, on a single site. For
this reason, spin-1 models have been extensively studied as prototypes for
quadrupolar (spin-nematic) order in magnetic insulators, and Fe-based
superconductors. At the same time, because of the presence of quadrupoles, the
classical limit of a spin-1 moment is not an $O(3)$ vector, a fact which must
be taken into account in describing their properties. In this Article we
develop a method to simulate spin-1 magnets based on a $u(3)$ algebra which
treats both dipole and quadrupole moments on equal footing. This approach is
amenable to both classical and quantum calculations, and we develop the
techniques needed to calculate thermodynamic properties through Monte Carlo
simulations and classical low-temperature expansion, and dynamical properties,
through ""molecular dynamics"" simulations and a multiple-boson expansion. As a
case study, we present detailed analytic and numerical results for the
thermodynamic properties of ferroquadrupolar order on the triangular lattice,
and its associated dynamics. At low temperatures, we show that it is possible
to ""correct"" for the effects of classical statistics in simulations, and
extrapolate to the zero-temperature quantum results found in flavour-wave
theory.",2203.09819v3
2022-03-22,Quantum sensing and imaging of spin-orbit-torque-driven spin dynamics in noncollinear antiferromagnet Mn3Sn,"Novel noncollinear antiferromagnets with spontaneous time-reversal symmetry
breaking, nontrivial band topology, and unconventional transport properties
have received immense research interest over the past decade due to their rich
physics and enormous promise in technological applications. One of the central
focuses in this emerging field is exploring the relationship between the
microscopic magnetic structure and exotic material properties. Here, the
nanoscale imaging of both spin-orbit-torque-induced deterministic magnetic
switching and chiral spin rotation in noncollinear antiferromagnet Mn3Sn films
using nitrogen-vacancy (NV) centers is reported. Direct evidence of the
off-resonance dipole-dipole coupling between the spin dynamics in Mn3Sn and
proximate NV centers is also demonstrated with NV relaxometry measurements.
These results demonstrate the unique capabilities of NV centers in accessing
the local information of the magnetic order and dynamics in these emergent
quantum materials and suggest new opportunities for investigating the interplay
between topology and magnetism in a broad range of topological magnets.",2203.11465v1
2022-04-04,Study of general relativistic magnetohydrodynamic accretion flow around black holes,"We present a novel approach to study the global structure of steady,
axisymmetric, advective, geometrically thin, magnetohydrodynamic (MHD)
accretion flow around black holes in full general relativity (GR). Considering
ideal MHD conditions and relativistic equation of state (REoS), we solve the
governing equations to obtain all possible smooth global accretion solutions.
We examine the dynamical and thermodynamical properties of accreting matter in
terms of the flow parameters, namely energy (${\cal E}$), angular momentum
(${\cal L}$), and local magnetic fields. For a thin GRMHD flow, we observe that
toroidal component ($b^\phi$) of the magnetic fields generally dominates over
radial component ($b^r$) at the disk equatorial plane. This evidently suggests
that toroidal magnetic field indeed plays important role in regulating the disk
dynamics. We further notice that the disk remains mostly gas pressure ($p_{\rm
gas}$) dominated ($\beta = p_{\rm gas}/p_{\rm mag} > 1$, $p_{\rm mag}$ refers
magnetic pressure) except at the near horizon region, where magnetic fields
become dynamically important ($\beta \sim 1$). We observe that Maxwell stress
is developed that eventually yields angular momentum transport inside the disk.
Towards this, we calculate the viscosity parameter ($\alpha$) that appears to
be radially varying. In addition, we examine the underlying scaling relation
between $\alpha$ and $\beta$, which clearly distinguishes two domains coexisted
along the radial extent of the disk. Finally, we discuss the utility of the
present formalism in the realm of GRMHD simulation studies.",2204.01412v2
2022-04-13,"Chaos, complexity, and intermittent turbulence in space plasmas","Intermittent turbulence is key for understanding the stochastic nonlinear
dynamics of space, astrophysical, and laboratory plasmas. We review the
observation and theory of chaos and complexity in plasmas, and elucidate their
links to intermittent plasma turbulence. First, we present evidence of magnetic
reconnection and intermittent magnetic turbulence in coronal mass ejections in
the solar corona and solar wind via remote and in situ observations. The
signatures of turbulent magnetic reconnection, i.e., bifurcated current sheet,
reconnecting jet, parallel/anti-parallel Alfv\'en waves, and spiky dynamical
pressure pulse, as well as fully-developed Kolmogorov intermittent turbulence,
are detected at the leading edge of an interplanetary coronal mass ejection and
the interface region of two interplanetary magnetic flux ropes. Methods for
quantifying the degree of coherence, amplitude-phase synchronization, and
multifractality of nonlinear multiscale fluctuations are discussed. The
stochastic chaotic nature of Alfv\'en intermittent structures driven by
magnetic reconnection is determined by a complexity-entropy analysis. Next, we
discuss the theory of chaos, intermittency, and complexity for nonlinear
Alfv\'en waves, and parametric decay and modulational wave-wave interactions,
in the absence/presence of noise. The transition from order to chaos is studied
using the bifurcation diagram. Two types of plasma chaos are considered: type-I
Pomeau-Manneville intermittency and crisis-induced intermittency. The role of
transient structures known as chaotic saddles in deterministic and stochastic
chaos in plasmas is investigated. Alfv\'en complexity associated with
noise-induced intermittency, in the presence of multistability, is studied.",2204.06133v1
2022-06-30,Origin and dynamics of umbrella states in rare-earth iron garnets,"Rare-earth iron garnets $R_{3}$Fe$_{5}$O$_{12}$ are fascinating insulators
with very diverse magnetic phases. Their strong potential in spintronic devices
has encouraged a renewal of interest in the study of their low temperature spin
structures and spin wave dynamics. A striking feature of rare-earth garnets
with $R$-ions exhibiting strong crystal-field effects like Tb-, Dy-, Ho-, and
Er-ions is the observation of low-temperature non-collinear magnetic structures
featuring ""umbrella-like"" arrangements of the rare-earth magnetic moments. In
this study, we demonstrate that such umbrella magnetic states are naturally
emerging from the crystal-field anisotropies of the rare-earth ions. By means
of a general model endowed with only the necessary elements from the crystal
structure, we show how umbrella-like spin structures can take place and
calculate the canting-angle as a result of the competition between the
exchange-interaction of the rare-earth and the iron ions as well as the
crystal-field anisotropy. Our results are compared to experimental data, and a
study of the polarised spin wave dynamics is presented. Our study improves the
understanding of umbrella-like spin structures and paves the way for more
complex spin wave calculations in rare-earth iron garnets with non-collinear
magnetic phases.",2207.00017v2
2022-11-14,Heavily Damped Precessional Switching with Very Low Write-error Rate in Elliptical-cylinder Magnetic Tunnel Junction,"Voltage-induced dynamic switching in magnetic tunnel junctions (MTJs) is a
writing technique for voltage-controlled magnetoresistive random access memory
(VCMRAM), which is expected to be an ultimate non-volatile memory with
ultra-low power consumption. In conventional dynamic switching, the width of
sub-nanosecond write voltage pulses must be precisely controlled to achieve a
sufficiently low write-error rate (WER). This very narrow tolerance of pulse
width is the biggest technical difficulty in developing VCMRAM. Heavily damped
precessional switching is a writing scheme for VCMRAM with a substantially high
tolerance of pulse width although the minimum WER has been much higher than
that of conventional dynamic switching with an optimum pulse width. In this
study, we theoretically investigate the effect of MTJ shape and the direction
of the applied magnetic field on the WER of heavily damped precessional
switching. The results show that the WER in elliptical-cylinder MTJ can be
several orders of magnitude smaller than that in usual circular-cylinder MTJ
when the external magnetic field is applied parallel to the minor axis of the
ellipse. The reduction in WER is due to the fact that the demagnetization field
narrows the component of the magnetization distribution perpendicular to the
plane direction immediately before the voltage is applied.",2211.07148v1
2023-01-18,Emergence of field-induced memory effect in spin ices,"Out-of-equilibrium investigation of strongly correlated materials deciphers
the hidden equilibrium properties. Herein, we have investigated the
out-of-equilibrium magnetic properties of polycrystalline Dy2Ti2O7 and Ho2Ti2O7
spin ices. The experimental results show the emergence of magnetic
field-induced anomalous hysteresis observed only in temperature/magnetic
field-dependent ac susceptibility measurements. The observed memory effect
(anomalous thermomagnetic hysteresis) strongly depends on the driving thermal
and non-thermal variables. Contrary, in the absence of the magnetic field,
dipolar interaction induced Ising paramagnetic to spin ice crossover develops a
liquid-gas transition type hysteresis below 4 K. Unlike field-induced
hysteresis, it shows weak dependency on thermal and non-thermal variables. Due
to the non-colinear spin structure, the applied dc bias magnetic field produces
quench disorder sites in the cooperative Ising spin matrix and suppresses the
spin-phonon coupling. These quench disorders create dynamic spin correlations
governed by quantum fluctuations, having slow spin relaxation and quick decay
times, which additionally contribute to ac susceptibility. The initial
conditions and measurement protocol decide the magnitude and sign of this
dynamical term contributing to ac susceptibility. It has been suggested that
such kind of out-of-equilibrium properties emerge by the cumulative effect of
geometric frustration, disorder, quantum fluctuations, and the cooperative
nature of spin dynamics of these materials.",2301.07741v1
2023-05-04,Impact of electric charges on chaos in magnetized $Reissner-Nordström$ spacetimes,"We consider the motion of test particles around a $Reissner-Nordstr\""{o}m$
black hole immersed into a strong external magnetic field modifying the
spacetime structure. When the particles are neutral, their dynamics are
nonintegrable because the magnetic field acts as a gravitational effect, which
destroys the existence of a fourth motion constant in the
$Reissner-Nordstr\""{o}m$ spacetime. A time-transformed explicit symplectic
integrator is used to show that the motion of neutral particles can be chaotic
under some circumstances. When test particles have electric charges, their
motions are subject to an electromagnetic field surrounding the black hole as
well as the gravitational forces from the black hole and the magnetic field. It
is found that increasing both the magnetic field and the particle energy or
decreasing the particle angular momentum can strengthen the degree of chaos
regardless of whether the particles are neutral or charged. The effect of
varying the black hole positive charge on the dynamical transition from order
to chaos is associated with the electric charges of particles. The dynamical
transition of neutral particles has no sensitive dependence on a change of the
black hole charge. An increase of the black hole charge weakens the chaoticity
of positive charged particles, whereas enhances the chaoticity of negative
charged particles. With the magnitude of particle charge increasing, chaos
always gets stronger.",2305.02702v1
2023-08-22,Dynamics of K$_2$Ni$_2$(SO$_4$)$_3$ governed by proximity to a 3D spin liquid model,"Quantum spin liquids (QSLs) have become a key area of research in magnetism
due to their remarkable properties, such as long-range entanglement, fractional
excitations, pinch-point singularities, and topologically protected phenomena.
In recent years, the search for QSLs has expanded into the three-dimensional
world, where promising features have been found in materials that form
pyrochlore and hyper-kagome lattices, despite the suppression of quantum
fluctuations due to high dimensionality. One such material is the $S = 1$
K$_2$Ni$_2$(SO$_4$)$_3$ compound, which belongs to the langbeinite family
consisting of two interconnected trillium lattices. Although magnetically
ordered, K$_2$Ni$_2$(SO$_4$)$_3$ has been found to exhibit a highly dynamical
and correlated state which can be driven into a pure quantum spin liquid under
magnetic fields of only $B \simeq 4$~T. In this article, we combine inelastic
neutron scattering measurements with pseudo-fermion functional renormalization
group (PFFRG) and classical Monte Carlo (cMC) calculations to study the
magnetic properties of K$_2$Ni$_2$(SO$_4$)$_3$, revealing a high level of
agreement between the experiment and theory. We further reveal the origin of
the dynamical state in K$_2$Ni$_2$(SO$_4$)$_3$ by studying a larger set of
exchange parameters, uncovering an `island of liquidity' around a focal point
given by a magnetic network composed of tetrahedra on a trillium lattice.",2308.11746v1
2023-10-10,A numerical investigation of quasi-static magnetoconvection with an imposed horizontal magnetic field,"Quasi-static Rayleigh-B\'enard convection with an imposed horizontal magnetic
field is investigated numerically for Chandrasekhar numbers up to $Q=10^6$ with
stress free boundary conditions. Both $Q$ and the Rayleigh number ($Ra$) are
varied to identify the various dynamical regimes that are present in this
system. We find three primary regimes: (I) a two-dimensional (2D) regime in
which the axes of the convection rolls are oriented parallel to the imposed
magnetic field; (II) an anisotropic three-dimensional (3D) regime; and (III) a
mean flow regime characterized by a large scale horizontal flow directed
transverse to the imposed magnetic field. The transition to 3D dynamics is
preceded by a series of 2D transitions in which the number of convective rolls
decreases as $Ra$ is increased. For sufficiently large $Q$, there is an
eventual transition to two rolls just prior to the 2D/3D transition. The 2D/3D
transition occurs when inertial forces become comparable to the Lorentz force,
i.e. when $\sqrt{Q}/Re = O(1)$; 2D, magnetically constrained states persist
when $\sqrt{Q}/Re \gtrsim O(1)$. Within the 2D regime we find heat and momentum
transport scalings that are consistent with the hydrodynamic asymptotic
predictions of Chini and Cox [Phys. Fluids \textbf{21}, 083603 (2009)]: the
Nusselt number ($Nu$) and Reynolds number ($Re$) scale as $Nu \sim Ra^{1/3}$
and $Re \sim Ra^{2/3}$, respectively. For $Q=10^6$, we find that the scaling
behavior of $Nu$ and $Re$ breaks down at large values of $Ra$ due to a sequence
of bifurcations and the eventual manifestation of mean flows.",2310.06683v1
2024-02-05,Inferring Fundamental Properties of the Flare Current Sheet Using Flare Ribbons: Oscillations in the Reconnection Flux Rates,"Magnetic reconnection is understood to be the main physical process that
facilitates the transformation of magnetic energy into heat, motion, and
particle acceleration during solar eruptions. Yet, observational constraints on
reconnection region properties and dynamics are limited due to lack of
high-cadence and high-spatial-resolution observations. By studying the
evolution and morphology of post-reconnected field-lines footpoints, or flare
ribbons and vector photospheric magnetic field, we estimate the magnetic
reconnection flux and its rate of change with time to study the flare
reconnection process and dynamics of the current sheet above. We compare
high-resolution imaging data to study the evolution of the fine structure in
flare ribbons as ribbons spread away from the polarity inversion line. Using
data from two illustrative events (one M- and X-class flare), we explore the
relationship between the ribbon-front fine structure and the temporal
development of bursts in the reconnection region. Additionally, we use the
RibbonDB database to perform statistical analysis of 73 (C- to X-class) flares
and identify QPP's properties using the Wavelet Transform. Our main finding is
the discovery of quasi-periodic pulsations (QPP) signatures in the derived
magnetic reconnection rates in both example events and the large flare sample.
We find that the oscillations' periods range from one to four minutes.
Furthermore, we find nearly co-temporal bursts in Hard X-ray (HXR) emission
profiles. We discuss how dynamical processes in the current sheet involving
plasmoids can explain the nearly-co-temporal signatures of quasi periodicity in
the reconnection rates and HXR emission.",2402.03567v1
2024-03-17,Merons and magnetoelectric switching in centrosymmetric spiral magnets,"Spiral multiferroics exhibit a strong coupling between magnetic and
ferroelectric orders, allowing cross-control. Since the seminal work of Kimura
et al. in 2003, these materials have attracted great interest galvanized by the
prospect of new high-efficiency memory devices, where magnetic (electric) bits
are switched via an external electric (magnetic) field. Nevertheless, the
mechanism underlying such a switching process - the electric field-driven
dynamics of domain walls (DWs) - is still poorly understood. We address this
problem for meron DWs, which represent one of the main DW types in spiral
multiferroics and consist of an array of meron (half-skyrmion) strings. Minimum
energy walls feature merons with alternating topological charges and move as
relativistic massive particles, with the limiting velocity set by magnon speed.
Low-energy defects in this alternating charge sequence, which appear during
domain nucleation, can lead to DWs with net topological charge. This induces a
peculiar non-local dynamics where all the spins in the system rotate, merons
translate within the wall, and the DW mobility is suppressed. The topological
charge of the wall and the meron helicity can be easily modified via an
external magnetic and electric field, respectively, offering fine control over
DW dynamics. Defects within the meron strings, analogous to Bloch points, have
hedgehog-like spin texture and are strongly pinned to the lattice. The
fascinating interplay between domain wall motion, translation of merons within
the wall, and precession of spins in the entire domains opens a new playground
for the electric manipulation of topological spin textures.",2403.11195v1
2016-03-04,Novel Dynamical Phenomena in Magnetic systems,"Dynamics of Ising models is a much studied phenomenon and has emerged as a
rich field of present-day research. An important dynamical feature commonly
studied is the quenching phenomenon below the critical temperature. In this
thesis we have studied the zero temperature quenching dynamics of different
Ising spin systems. First we have studied the zero temperature quenching
dynamics of two dimensional Ising spin system with competating interactions.
Then we have studied the effect of randomness or disorder on the quenching
dynamics of Ising spin system. We have studied the effect of the nature of
randomness on zero temperature quenching dynamics of one dimensional Ising
model on two type of complex networks. A model for opinion dynamics also has
been proposed in this thesis, in which the binary opinions of the individuals
are determined according to the size of their neighboring domains. This model
can be equivalently defined in terms of Ising spin variables and the various
quantities studied have one to one correspondence with magnetic systems.
Introducing disorder in this model through a parameter called rigidity
parameter $\rho$ (probability that people are completely rigid and never change
their opinion), the transition to a heterogeneous society at $\rho = 0^{+}$ is
obtained. The Model (Model I) has been generalized introducing a parameter
named as size sensitivity parameter to modify the dynamics of the proposed
model and a macroscopic crossover in time is observed for the intermediate
values of this parameter.",1603.01646v1
2017-02-07,One-dimensional Quantum Spin Dynamics of Bethe String States,"Quantum dynamics of strongly correlated systems is a challenging problem.
Although the low energy fractional excitations of one dimensional integrable
models are often well-understood, exploring quantum dynamics in these systems
remains challenging in the gapless regime, especially at intermediate and high
energies. Based on the algebraic Bethe ansatz formalism, we study spin dynamics
in a representative one dimensional strongly correlated model, {\it i.e. }, the
antiferromagnetic spin-$\frac{1}{2}$ XXZ chain with the Ising anisotropy, via
the form-factor formulae. Various excitations at different energy scales are
identified crucial to the dynamic spin structure factors under the guidance of
sum rules. At small magnetic polarizations, gapless excitations dominate the
low energy spin dynamics arising from the magnetic-field-induced
incommensurability. In contrast, spin dynamics at intermediate and high
energies is characterized by the two- and three-string states, which are
multi-particle excitations based on the commensurate N\'eel ordered background.
Our work is helpful for experimental studies on spin dynamics in both condensed
matter and cold atom systems beyond the low energy effective Luttinger liquid
theory. Based on an intuitive physical picture, we speculate that the dynamic
feature at high energies due to the multi-particle anti-bound state excitations
can be generalized to non-integrable spin systems.",1702.01854v2
2019-03-21,Emergent topology and symmetry-breaking order in correlated quench dynamics,"Quenching a quantum system involves three basic ingredients: the initial
phase, the post-quench target phase, and the non-equilibrium dynamics which
carries the information of the former two. Here we propose a dynamical theory
to characterize both the topology and symmetry-breaking order in correlated
quantum system, through quenching the Haldane-Hubbard model from an initial
magnetic phase to topologically nontrivial regime. The equation of motion for
the complex pseudospin dynamics is obtained with the flow equation method, with
the pseudospin evolution shown to obey a microscopic
Landau-Lifshitz-Gilbert-Bloch equation. We find that the correlated quench
dynamics exhibit robust universal behaviors on the so-called band-inversion
surfaces (BISs), from which the nontrivial topology and magnetic order can be
extracted. In particular, the topology of the post-quench regime can be
characterized by an emergent dynamical topological pattern of quench dynamics
on BISs, which is robust against dephasing and heating induced by interactions;
the pre-quench symmetry-breaking orders is read out from a universal scaling
behavior of the quench dynamics emerging on the BIS, which is valid beyond the
mean-field regime. This work opens a way to characterize both the topology and
symmetry-breaking orders by correlated quench dynamics.",1903.09144v3
2001-07-12,Magnetic fields in barred galaxies. II. Dynamo models,"We study the generation and maintenance of large-scale magnetic fields in
barred galaxies. We take a velocity field (with strong noncircular components)
from a published gas dynamical simulation of Athanassoula (1992), and use this
as input to a galactic dynamo calculation. Our work is largely motivated by
recent high quality VLA radio observations of the barred galaxy NGC 1097, and
we compare our results in detail with the regular magnetic fields deduced from
these observations. We are able to reproduce most of the conspicuous
large-scale features of the observed regular field, including the field
structure in the central regions, by using a simple mean-field dynamo model in
which the intensity of interstellar turbulence (more precisely, the turbulent
diffusivity) is enhanced by a factor of 2-6 in the dust lanes and near the
circumnuclear ring. We argue that magnetic fields can be dynamically important,
and therefore should be included in models of gas flow in barred galaxies.",0107214v2
2001-10-08,The effect of hyperdiffusivity on turbulent dynamos with helicity,"In numerical studies of turbulence, hyperviscosity is often used as a tool to
extend the inertial subrange and to reduce the dissipative subrange. By
analogy, hyperdiffusivity (or hyperresistivity) is sometimes used in
magnetohydrodynamics. The underlying assumption is that only the small scales
are affected by this manipulation. In the present paper, possible side effects
on the evolution of the large scale magnetic field are investigated. It is
found that for turbulent flows with helicity, hyperdiffusivity causes the
dynamo-generated magnetic field to saturate at a higher level than normal
diffusivity. This result is successfully interpreted in terms of magnetic
helicity conservation, which also predicts that full saturation is only reached
after a time comparable to the large scale magnetic (hyper)diffusion time.",0110171v1
2001-11-16,Dynamics of Walls: Fermion Scattering in Magnetic Field,"We investigate the scattering of fermions off walls in the presence of a
magnetic field. We consider both the bubble wall and the kink domain wall. By
solving the Dirac equation for fermions in the presence of a domain wall in an
external magnetic field, we investigate the dependence on the magnetic field of
the transmission and reflection coefficients. In the case of kink domain wall,
we also consider the solutions localized on the wall. The possibile role of the
ferromagnetic domain walls in the dynamics of the early Universe is also
discussed.",0111327v1
2002-10-09,Dynamics of relativistic reconnection,"The dynamics of the steady-state Sweet--Parker-type reconnection is analyzed
in relativistic regime when energy density in the inflowing region is dominated
by magnetic field. The structure of reconnection layer (its thickness, inflow
and outflow velocities) depends on the ratio of two large dimensionless
parameters of the problem - magnetization parameter $\sigma \gg 1$ (the ratio
of the magnetic to particle energy-densities in the inflowing region) and the
Lundquist number $S$. The inflow velocity may be relativistic (for $ S<
\sigma$) or non-relativistic (for $ S > \sigma$), while the outflowing plasma
is moving always relativisticly. For extremely magnetized plasmas with $\sigma
\geq S^2$, the inflow four-velocity becomes of the order of the \Alfven
four-velocity.",0210206v1
2003-02-06,Magnetic Protoneutron Star Winds and r-Process Nucleosynthesis,"Because of their neutron-richness and association with supernovae,
post-explosion protoneutron star winds are thought to be a likely astrophysical
site for rapid neutron capture nucleosynthesis (the r-process). However, the
most recent models of spherical neutrino-driven protoneutron star winds do not
produce robust r-process nucleosynthesis for `canonical' neutron stars with a
gravitational mass of 1.4M_sun and coordinate radius of 10km. These models fail
variously; either the flow entropy is too low, the electron fraction is too
high, or the dynamical expansion timescale is too long. To date, no models have
included the effects of an ordered dipole magnetic field. We show that a strong
magnetic field can trap the outflow in the neutrino heating region, thus
leading to much higher matter entropy. We estimate both the trapping timescale
and the resulting entropy amplification. For sufficiently large energy
deposition rates, the trapped matter emerges dynamically from the region of
closed magnetic field lines and escapes to infinity. We find that ordered
dipoles with surface fields of greater than 6 times 10^14 Gauss increase the
asymptotic entropy sufficiently for robust r-process nucleosynthesis.",0302132v1
2004-01-09,Two Dimensional Poynting Flux Dominated Flow onto a Schwarzschild Black Hole,"We discuss the dynamics of the accretion flow onto a black hole driven by
Poynting flux in a simplified model of a two-dimensional accretion disk. In
this simplified model, the condition of the stationary accretion flow is found
to impose a nontrivial constraint on the magnetic field configuration. The
effect of the magnetic field on the accretion flow is discussed in detail using
the paraboloidal and hyperboloidal type configuration for the poloidal
structure suggested by Blandford in 1976. It is demonstrated explicitly that
the angular velocity of the disk, $\Omega_D$, deviates from the Keplerian
angular velocity. The angular velocity of the rigidly-rotating magnetic
surface, $\Omega_F$, does not have to be the same as the angular velocity of
the disk for the paraboloidal type configuration. But for the hyperboloidal
type configuration, it is found that we can set $\Omega_F = \Omega_D$, which
corresponds to an accretion disk of perfect conductor. We discuss the numerical
solutions of the stream equation for stationary accretion flow in the
Schwarzschild background using a paraboloidal type configuration. The dynamics
of the accretion disk is found to depend strongly on the ratio of the accretion
rate to the magnetic field strength.",0401143v1
2004-06-07,Magnetic Effects Change Our View of the Heliosheath,"There is currently a controversy as to whether Voyager 1 has already crossed
the Termination Shock, the first boundary of the Heliosphere. The region
between the Termination Shock and the Heliopause, the Helisheath, is one of the
most unknown regions theoretically. In the Heliosheath magnetic effects are
crucial, as the solar magnetic field is compressed at the Termination Shock by
the slowing flow. Recently, our simulations showed that the Heliosheath
presents remarkable dynamics, with turbulent flows and the presence of a jet
flow at the current sheet that is unstable due to magnetohydrodynamic
instabilities \cite{opher,opher1}. In this paper we review these recent
results, and present an additional simulation with constant neutral atom
background. In this case the jet is still present but with reduced intensity.
Further study, e.g., including neutrals and the tilt of the solar rotation from
the magnetic axis, is required before we can definitively address how the
Heliosheath behaves. Already we can say that this region presents remarkable
dynamics, with turbulent flows, indicating that the Heliosheath might be very
different from what we previously thought.",0406184v1
2005-04-19,Reply to Lyutikov's comments on Zhang & Kobayashi (2005),"Lyutikov (astro-ph/0503505) raised a valid point that for shock deceleration
of a highly magnetized outflow, the fate of the magnetic fields after shock
crossing should be considered. However, his comment that the deceleration
radius should be defined by the total energy rather than by the baryonic
kinetic energy is incorrect. As strictly derived from the shock jump conditions
in Zhang & Kobayashi (2005), during the reverse shock crossing process the
magnetic energy is not tapped. As a result, the fireball deceleration radius is
defined by the baryonic energy only. The magnetic energy is expected to be
transferred to the circumburst medium after the reverse shock disappears. The
evolution of the system then mimicks a continuously-fed fireball. As a result,
Lyutikov's naive conclusion that the forward shock dynamics is independent on
the ejecta content is also incorrect. The shock deceleration dynamics and the
reverse shock calculation presented in Zhang & Kobayashi (2005) are robust and
correct.",0504425v1
1999-03-19,Anisotropic non-perturbative zero modes for passively advected magnetic fields,"A first analytic assessment of the role of anisotropic corrections to the
isotropic anomalous scaling exponents is given for the $d$-dimensional
kinematic magneto-hydrodynamics problem in the presence of a mean magnetic
field. The velocity advecting the magnetic field changes very rapidly in time
and scales with a positive exponent $\xi$. Inertial-range anisotropic
contributions to the scaling exponents, $\zeta_j$, of second-order magnetic
correlations are associated to zero modes and have been calculated
non-perturbatively. For $d=3$, the limit $\xi\mapsto 0$ yields
$\protect{\zeta_j=j-2+ \xi (2j^3 +j^2 -5 j - 4)/[2(4 j^2 - 1)]} $ where $j$
($j\geq 2$) is the order in the Legendre polynomial decomposition applied to
correlation functions. Conjectures on the fact that anisotropic components
cannot change the isotropic threshold to the dynamo effect are also made.",9903026v2
1999-09-14,Nonperturbative Spectrum of Anomalous Scaling Exponents in the Anisotropic Sectors of Passively Advected Magnetic Fields,"We address the scaling behavior of the covariance of the magnetic field in
the three-dimensional kinematic dynamo problem when the boundary conditions
and/or the external forcing are not isotropic. The velocity field is gaussian
and $\delta$-correlated in time, and its structure function scales with a
positive exponent $\xi$. The covariance of the magnetic field is naturally
computed as a sum of contributions proportional to the irreducible
representations of the SO(3) symmetry group. The amplitudes are non-universal,
determined by boundary conditions. The scaling exponents are universal, forming
a discrete, strictly increasing spectrum indexed by the sectors of the symmetry
group. When the initial mean magnetic field is zero, no dynamo effect is found,
irrespective of the anisotropy of the forcing. The rate of isotropization with
decreasing scales is fully understood from these results.",9909020v1
1997-02-02,Effect of spin-glass order on magnetic polarons in semimagnetic semiconductors,"A theory accounting for the specific features of magnetic polarons (MP) in
the presence of spin glass order is presented. We derive and solve
selfconsistent equations for i) the polaron magnetisation, ii) the
thermodynamically averaged carrier--spin, and iii) for the spin glass order
parameter. The temperature dependence of these quantities is analysed in
detail. The modification of the spin glass phase due to the presence of the
exchange field of the carrier inside the magnetic polaron volume is
investigated. The onset of spin glass order leads to a plateau--like flattening
in the temperature dependence of the MP energy at low temperatures. It is found
that solutions of spin glass equations are needed to optimally fit the
experimental data of the temperature dependence of the exciton magnetic polaron
(EMP) energy in (Cd,Mn)Te. Moreover, the dynamical aspects of the MP formation
are discussed. Our model predicts qualitatively different temperature
dependences of the MP formation time in different dynamical scenarios.",9702013v1
1998-05-05,Thermally Activated Resonant Magnetization Tunneling in Molecular Magnets: Mn_12Ac and others,"The dynamical theory of thermally activated resonant magnetization tunneling
in uniaxially anisotropic magnetic molecules such as Mn_12Ac (S=10) is
developed.The observed slow dynamics of the system is described by master
equations for the populations of spin levels.The latter are obtained by the
adiabatic elimination of fast degrees of freedom from the density matrix
equation with the help of the perturbation theory developed earlier for the
tunneling level splitting [D. A. Garanin, J. Phys. A, 24, L61 (1991)]. There
exists a temperature range (thermally activated tunneling) where the escape
rate follows the Arrhenius law, but has a nonmonotonic dependence on the bias
field due to tunneling at the top of the barrier. At lower temperatures this
regime crosses over to the non-Arrhenius law (thermally assisted tunneling).
The transition between the two regimes can be first or second order, depending
on the transverse field, which can be tested in experiments. In both regimes
the resonant maxima of the rate occur when spin levels in the two potential
wells match at certain field values. In the thermally activated regime at low
dissipation each resonance has a multitower self-similar structure with
progressively narrowing peaks mounting on top of each other.",9805057v1
1999-12-09,Dynamics of the Local Moment Induced by Nonmagnetic Defects in Cuprates,"We present a study of the spin dynamics of magnetic defects induced by Li
substitution of the plane Cu in the normal state of YBa$_2$Cu$_3$O$_{6+x}$. The
fluctuations of the coupled Cu magnetic moments in the vicinity of Li are
probed by near-neighbour $^{89}$Y {\it and} $^7$Li NMR spin lattice relaxation.
The data indicates that the magnetic perturbation fluctuates as a single entity
with a correlation time $\tau$ which scales with the local static
susceptibility. This behaviour is reminiscent of the low $T$ Kondo state of
magnetic impurities in conventional metals. Surprisingly it extends well above
the ``Kondo'' temperature for the underdoped pseudogapped case.",9912165v2
2000-11-22,Fractal Structure of the Harper Map Phase Diagram from Topological Hierarchical Classification,"It is suggested a topological hierarchical classification of the infinite
many Localized phases figuring in the phase diagram of the Harper equation for
anisotropy parameter $\epsilon$ versus Energy $E$ with irrational magnetic flux
$\omega$. It is also proposed a rule that explain the fractal structure of the
phase diagram. Among many other applications, this system is equivalent to the
Semi-classical problem of Bloch electrons in a uniform magnetic field, the
Azbel-Hofstadter model, where the discrete magnetic translations operators
constitute the quantum algebra $U_q(sl_2)$ with $q^2=e^{i2\pi\omega}$. The
magnetic flux is taken to be the golden mean $\omega^*=(\sqrt{5}-1)/2$ and is
obtained by successive rational approximants $\omega_m=F_{m-1}/F_m$ with $F_m$
given by the Fibonacci sequence $F_m$.[OUTP-00-08S, \texttt{cond-mat/0011396}]",0011396v2
2001-03-25,Dynamical turbulent flow on the Galton board with friction and magnetic field,"We study numerically and analytically the dynamics of charged particles on
the Galton board, a regular lattice of disc scatters, in the presence of
constant external force, magnetic field and friction. It is shown that under
certain conditions friction leads to the appearance of a strange chaotic
attractor. In this regime the average velocity and direction of particle flow
can be effectively affected by electric and magnetic fields. We discuss the
applications of these results to the charge transport in antidot superlattices
and stream of suspended particles in a viscous flow through scatters.",0103515v1
2001-11-14,Ultrafast Spin Dynamics in GaAs/GaSb/InAs Heterostructures Probed by Second Harmonic Generation,"We report the first application of pump-probe second harmonic generation
(SHG) measurements to characterize optically-induced magnetization in
non-magnetic multilayer semiconductors. In the experiment, coherent spins are
selectively excited by a pump beam in the GaAs layer of GaAs/GaSb/InAs
structures. However, the resulting net magnetization manifests itself through
the induced SHG probe signal from the GaSb/InAs interface, thus indicating a
coherent spin transport across the heterostructure. We find that the
magnetization dynamics is governed by an interplay between the spin density
evolution at the interfaces and the spin dephasing.",0111266v1
2002-07-30,Microscopic relaxation mechanisms and linear magnetization dynamics,"Linear magnetization dynamics in the presense of a thermal bath is analyzed
for two general classes of microscopic damping mechanisms. The resulting
stochastic differential equations are always in the form of a damped harmonic
oscillator driven by a thermal field. The damping term contains both the
interaction mechanisms and the symmetry of the magnetic system. Back
transformation from the oscillator coordinates to the magnetization variables
results in a macroscopic tensor form of damping that reflects the system
anisotropy. Scalar Landau-Lifshitz-Gilbert damping term is valid only for
systems with axial symmetry. Analysis of FMR linewith measurements versus
frequency, temperature, and film thickness in NiFe films shows good agreement
with a combination of slow-relaxing impurity and magnon-electron confluence
processes.",0207721v1
2002-08-13,Electron Spin Dynamics in Semiconductors without Inversion Symmetry,"We present a microscopic analysis of electron spin dynamics in the presence
of an external magnetic field for non-centrosymmetric semiconductors in which
the D'yakonov-Perel' spin-orbit interaction is the dominant spin relaxation
mechanism. We implement a fully microscopic two-step calculation, in which the
relaxation of orbital motion due to electron-bath coupling is the first step
and spin relaxation due to spin-orbit coupling is the second step. On this
basis, we derive a set of Bloch equations for spin with the relaxation times
T_1 and T_2 obtained microscopically. We show that in bulk semiconductors
without magnetic field, T_1 = T_2, whereas for a quantum well with a magnetic
field applied along the growth direction T_1 = T_2/2 for any magnetic field
strength.",0208261v1
2002-09-07,Neural network analysis of the magnetization reversal in magnetic dot arrays,"We simulated the remagnetization dynamics of the ultra-dense and ultra-thin
magnetic dot array system with dipole-dipole and exchange coupling
interactions. Within the proposed 2D XY superlattice model, the square dots are
modeled by the spatially modulated exchange-couplings. The dipole-dipole
interactions were approximated by the hierarchical sums and dynamics was
reduced to damping term of the Landau-Lifshitz-Gilbert equation. The simulation
of 40 000 spin system leads to nonequilibrium nonuniform configurations with
soliton-antisoliton pairs detected at intra-dot and inter-dot scales. The
classification of intra-dot magnetic configurations was performed using the
self-adaptive neural networks with varying number of neurons.",0209186v1
2003-01-17,Current-Induced Magnetization Reversal in High Magnetic Fields in Co/Cu/Co Nanopillars,"Current-induced magnetization dynamics in Co/Cu/Co trilayer nanopillars
(~100nm in diameter) has been studied experimentally for large applied fields
perpendicular to the layers. An abrupt and hysteretic increase in dynamic
resistance is observed at high current densities for one polarity of the
current, comparable to the giant magnetoresistance effect observed at low
fields. A micromagnetic model, that includes a spin-transfer torque, suggests
that the current induces a complete reversal of the thin Co layer to alignment
antiparallel to the applied field-that is, to a state of maximum magnetic
energy.",0301324v4
2003-09-24,Magnetic susceptibilities of diluted magnetic semiconductors and anomalous Hall-voltage noise,"The carrier spin and impurity spin densities in diluted magnetic
semiconductors are considered using a semiclassical approach. Equations of
motions for the spin densities and the carrier spin current density in the
paramagnetic phase are derived, exhibiting their coupled diffusive dynamics.
The dynamical spin susceptibilities are obtained from these equations. The
theory holds for p-type and n-type semiconductors doped with magnetic ions of
arbitrary spin quantum number. Spin-orbit coupling in the valence band is shown
to lead to anisotropic spin diffusion and to a suppression of the Curie
temperature in p-type materials. As an application we derive the Hall-voltage
noise in the paramagnetic phase. This quantity is critically enhanced close to
the Curie temperature due to the contribution from the anomalous Hall effect.",0309547v1
2004-02-10,Magnetism in semiconductors: A dynamical mean field study of ferromagnetism in Ga_{1-x}Mn_xAs,"We employ the dynamical mean field approximation to perform a systematic
study of magnetism in Ga_{1-x}Mn_xAs. Our model incorporates the effects of the
strong spin-orbit coupling on the J=3/2 GaAs valence bands and of the exchange
interaction between the randomly distributed magnetic ions and the itinerant
holes. The ferromagnetic phase transition temperature T_c is obtained for
different values of the impurity-hole coupling J_c and of the hole
concentration n_h at the Mn doping of x=0.05. We also investigate the
temperature dependence of the local magnetization and spin polarization of the
holes. By comparing our results with those for a single band Hamiltonian in
which the spin-orbit coupling is switched off, we conclude that the spin-orbit
coupling in Ga_{1-x}Mn_xAs gives rise to frustration in the ferromagnetic
order, strengthening recent findings by Zarand and Janko (Phys. Rev. Lett. 89,
047201 (2002)).",0402289v1
2004-03-02,Vortex configurations and metastability in mesoscopic superconductors,"The vortex dynamics in mesoscopic superconducting cylinders with rectangular
cross section under an axially applied magnetic field is investigated in the
multivortex London regime. The rectangles considered range from a square up to
an infinite slab. The flux distribution and total flux carried by a vortex
placed in an arbitrary position of the sample is calculated analytically by
assuming Clem's solution for the vortex core. The Bean-Livingston energy
barrier is also analytically calculated in this framework. A Langevin algorithm
simulates the flux penetration and dynamical evolution of the vortices as the
external field is slowly cycled. The simulated magnetization process is
governed by metastable states. The magnetization curves are hysteretic, with
paramagnetic response in part of the downward branch, and present a series of
peaks corresponding to the entry or expulsion of a single vortex. For elongated
rectangles, the vortices arrange themselves into parallel vortex chains and an
additional modulation of the magnetization, corresponding to creation or
destruction of a vortex chain, comes out.",0403077v1
2004-03-25,Ab-initio spin dynamics applied to nanoparticles: canted magnetism of a finite Co chain along a Pt(111) surface step edge,"In order to search for the magnetic ground state of surface nanostructures we
extended first principles adiabatic spin dynamics to the case of fully
relativistic electron scattering. Our method relies on a constrained density
functional theory whereby the evolution of the orientations of the spin-moments
results from a semi-classical Landau-Lifshitz equation. This approach is
applied to a study of the ground state of a finite Co chain placed along a step
edge of a Pt(111) surface. As far as the ground state spin orientation is
concerned we obtain excellent agreement with the experiment. Furthermore we
observe noncollinearity of the atom-resolved spin and orbital moments. In terms
of magnetic force theorem calculations we also demonstrate how a reduction of
symmetry leads to the existence of canted magnetic states.",0403637v1
2004-04-05,"Large-angle, gigahertz-rate random telegraph switching induced by spin-momentum transfer","We show that spin-polarized dc current passing through a small magnetic
element induces two-state, random telegraph switching of the magnetization via
the spin-momentum transfer effect. The resistances of the states differ by up
to 50% of the change due to complete magnetization reversal. Fluctuations are
seen for a wide range of currents and magnetic fields, with rates that can
exceed 2 GHz, and involve collective motion of a large volume (10^4 nm^3) of
spins. Switching rate trends with field and current indicate that increasing
temperature alone cannot explain the dynamics. The rates approach a stochastic
regime wherein dynamics are governed by both precessional motion and thermal
perturbations.",0404109v1
2004-04-07,The Effect of Columnar Disorder on the Superconducting Transition of a Type-II Superconductor in Zero Applied Magnetic Field,"We investigate the effect of random columnar disorder on the superconducting
phase transition of a type-II superconductor in zero applied magnetic field
using numerical simulations of three dimensional XY and vortex loop models. We
consider both an unscreened model, in which the bare magnetic penetration
length is approximated as infinite, and a strongly screened model, in which the
magnetic penetration length is of order the vortex core radius. We consider
both equilibrium and dynamic critical exponents. We show that, as in the
disorder free case, the equilibrium transitions of the unscreened and strongly
screened models lie in the same universality class, however scaling is now
anisotropic. We find for the correlation length exponent $\nu=1.2\pm 0.1$, and
for the anisotropy exponent $\zeta=1.3\pm 0.1$. We find different dynamic
critical exponents for the unscreened and strongly screened models.",0404179v1
2004-05-31,Electron Dynamics in a Coupled Quantum Point Contact Structure with a Local Magnetic Moment,"We develop a theoretical model for the description of electron dynamics in
coupled quantum wires when the local magnetic moment is formed in one of the
wires. We employ a single-particle Hamiltonian that takes account of the
specific geometry of potentials defining the structure as well as electron
scattering on the local magnetic moment. The equations for the wave functions
in both wires are derived and the approach for their solution is discussed. We
determine the transmission coefficient and conductance of the wire having the
local magnetic moment and show that our description reproduces the
experimentally observed features.",0405705v1
2004-06-21,Basic considerations for magnetization dynamics in the combined presence of spin-transfer torques and thermal fluctuations,"This article reviews basic theoretical features of Gilbert magnetization
dynamics of a single domain magnetic film in the presence of Slonczewski
spin-transfer torques, with and without thermal fluctuations taken into
account. Rather than showing results of detailed numerical calculations, the
discussion here is restricted to basic analytical results and conclusions which
can mostly be derived from simply the form of the equations of motion, as well
as elementary considerations based on classical stability analysis and the
fluctuation-dissipation theorem. The presents work describes how interesting
features of spin-transfer may be viewed as arising from non-equilibrium
thermodynamics that are a direct consequence of the nonreciprocal nature of
spin-transfer torques. The present article discusses fairly general results for
spin-torque induced instability without thermal fluctuations, as well as the
case of thermally activated magnetization reversal in uniaxial devices in the
combined presence of external fields, thermal fluctuations, and spin-transfer
torques. The results will be discussed and briefly compared and contrasted with
that of prior work.",0406486v1
2004-07-27,Study of Gas-Fluidization Dynamics with Laser-Polarized 129Xe,"We report initial NMR studies of gas dynamics in a particle bed fluidized by
laser-polarized xenon (129Xe) gas. We have made preliminary measurements of two
important characteristics: gas exchange between the bubble and emulsion phases;
and the gas velocity distribution in the bed. We used T2* contrast to
differentiate the bubble and emulsion phases by choosing solid particles with
large magnetic susceptibility, in order. Experimental tests demonstrated that
this method was successful in eliminating 129Xe magnetization in the emulsion
phase, which enabled us to observe the time-dependence of the bubble
magnetization. By employing the pulsed field gradient method, we also measured
the gas velocity distribution within the bed. These results clearly show the
onset of bubbling and can be used to deduce information about gas and particle
motion in the fluidized bed.",0407708v1
2004-11-10,Dynamical nuclear polarization and nuclear magnetic fields in semiconductor nanostructures,"We investigate the dynamic nuclear polarization from the hyperfine
interaction between nonequilibrium electronic spins and nuclear spins coupled
to them in semiconductor nanostructures. We derive the time and position
dependence of the induced nuclear spin polarization and dipolar magnetic
fields. In GaAs/AlGaAs parabolic quantum wells the nuclear spin polarization
can be as high as 80% and the induced nuclear magnetic fields can approach a
few gauss with an associated nuclear resonance shift of the order of kHz when
the electronic system is 100% spin polarized. These fields and shifts can be
tuned using small electric fields. We discuss the implications of such control
for optical nuclear magnetic resonance experiments in low-dimensional
semiconductor nanostructures.",0411277v1
2004-11-11,Dynamical breakdown of the Ising spin-glass order under a magnetic field,"The dynamical magnetic properties of an Ising spin glass
Fe$_{0.55}$Mn$_{0.45}$TiO$_3$ are studied under various magnetic fields. Having
determined the temperature and static field dependent relaxation time
$\tau(T;H)$ from ac magnetization measurements under a dc bias field by a
general method, we first demonstrate that these data provide evidence for a
spin-glass (SG) phase transition only in zero field. We next argue that the
data $\tau(T;H)$ of finite $H$ can be well interpreted by the droplet theory
which predicts the absence of a SG phase transition in finite fields.",0411291v2
2004-12-22,Topology and spin dynamics in magnetic molecules,"We investigate the role of topology and distortions in the quantum dynamics
of magnetic molecules, using a cyclic spin system as reference. We consider
three variants of antiferromagnetic molecular ring, i.e. Cr$_8$, Cr$_7$Zn and
Cr$_7$Ni, characterized by low lying states with different total spin $S$. We
theoretically and experimentally study the low-temperature behavior of the
magnetic torque as a function of the applied magnetic field. Near level
crossings, this observable selectively probes quantum fluctuations of the total
spin (''$S$ mixing"") induced by lowering of the ideal ring symmetry. We show
that while a typical distortion of a model molecular structure is very
ineffective in opening new $S$-mixing channels, the spin topology is a major
ingredient to control the degree of $S$ mixing. This conclusion is further
substantiated by low-temperature heat capacity measurements.",0412628v1
2005-07-19,Energy-resolved inelastic electron scattering off a magnetic impurity,"We study inelastic scattering of energetic electrons off a Kondo impurity. If
the energy E of the incoming electron (measured from the Fermi level) exceeds
significantly the Kondo temperature T_K, then the differential inelastic
cross-section \sigma (E,w), i.e., the cross-section characterizing scattering
of an electron with a given energy transfer w, is well-defined. We show that
\sigma (E,w) factorizes into two parts. The E-dependence of \sigma (E,w) is
logarithmically weak and is due to the Kondo renormalization of the effective
coupling. We are able to relate the w-dependence to the spin-spin correlation
function of the magnetic impurity. Using this relation, we demonstrate that in
the absence of magnetic field the dynamics of the impurity spin causes the
electron scattering to be inelastic at any temperature. Quenching of the spin
dynamics by an applied magnetic field results in a finite elastic component of
the electron scattering cross-section. The differential scattering
cross-section may be extracted from the measurements of relaxation of hot
electrons injected in conductors containing localized spins.",0507431v2
2006-01-27,Dynamics of thin-film spin-flip transistors with perpendicular source-drain magnetizations,"A ""spin-flip transistor"" is a lateral spin valve consisting of ferromagnetic
source drain contacts to a thin-film normal-metal island with an electrically
floating ferromagnetic base contact on top. We analyze the
\emph{dc}-current-driven magnetization dynamics of spin-flip transistors in
which the source-drain contacts are magnetized perpendicularly to the device
plane by magnetoelectronic circuit theory and the macrospin
Landau-Lifshitz-Gilbert equation. Spin flip scattering and spin pumping effects
are taken into account. We find a steady-state rotation of the base
magnetization at GHz frequencies that is tuneable by the source-drain bias. We
discuss the advantages of the lateral structure for high-frequency generation
and actuation of nanomechanical systems over recently proposed nanopillar
structures.",0601630v1
2006-02-07,Dynamics of magnetic domain wall motion after nucleation: Dependence on the wall energy,"The dynamics of magnetic domain wall motion in the FeNi layer of a
FeNi/Al2O3/Co trilayer has been investigated by a combination of x-ray magnetic
circular dichroism, photoelectron emission microscopy, and a stroboscopic
pump-probe technique. The nucleation of domains and subsequent expansion by
domain wall motion in the FeNi layer during nanosecond-long magnetic field
pulses was observed in the viscous regime up to the Walker limit field. We
attribute an observed delay of domain expansion to the influence of the domain
wall energy that acts against the domain expansion and that plays an important
role when domains are small.",0602174v1
2006-07-25,The magnetization-driven random field Ising model at T=0,"We study the hysteretic evolution of the random field Ising model (RFIM) at
T=0 when the magnetization M is controlled externally and the magnetic field H
becomes the output variable. The dynamics is a simple modification of the
single-spin-flip dynamics used in the H-driven situation and consists in
flipping successively the spins with the largest local field. This allows to
perform a detailed comparison between the microscopic trajectories followed by
the system with the two protocols. Simulations are performed on random graphs
with connectivity z=4 (Bethe lattice) and on the 3-D cubic lattice. The same
internal energy U(M)is found with the two protocols when there is no
macroscopic avalanche and it does not depend on whether the microscopic states
are stable or not. On the Bethe lattice, the energy inside the macroscopic
avalanche also coincides with the one that is computed analytically with the
H-driven algorithm along the unstable branch of the hysteresis loop. The output
field, defined here as dU/dM, exhibits very large fluctuations with the
magnetization and is not self-averaging. Relation to the experimental situation
is discussed.",0607644v1
2006-11-29,Irregular Spin Tunnelling for Spin-1 Bose-Einstein Condensates in a Sweeping Magnetic Field,"We investigate the spin tunnelling of spin-1 Bose-Einstein condensates in a
linearly sweeping magnetic field with a mean-field treatment. We focus on the
two typical alkali Bose atoms $^{87}Rb$ and $^{23}Na$ condensates and study
their tunnelling dynamics according to different sweeping rates of external
magnetic fields. In the adiabatic (i.e., slowly sweeping) and sudden (i.e.,
fast sweeping) limits, no tunnelling is observed. For the case of moderate
sweeping rates, the tunnelling dynamics is found to be very sensitive on the
sweeping rates with showing a chaotic-like tunnelling regime. With magnifying
the regime, however, we find interestedly that the plottings become resolvable
under a resolution of $10^{-4}$ G/s where the tunnelling probability with
respect to the sweeping rate shows a regular periodic-like pattern. Moreover, a
conserved quantity standing for the magnetization in experiments is found can
dramatically affect the above picture of the spin tunnelling. Theoretically we
have given a reasonable interpretation to the above findings and hope our
studies would bring more attention to spin tunnelling experimentally.",0611734v1
1999-09-14,Dynamics of magnetic Bianchi VI_0 cosmologies,"Methods of dynamical systems analysis are used to show rigorously that the
presence of a magnetic field orthogonal to the two commuting Killing vector
fields in any spatially homogeneous Bianchi type VI_0 vacuum solution to
Einstein's equation changes the evolution toward the singularity from
convergent to oscillatory. In particular, it is shown that the alpha-limit set
(for time direction that puts the singularity in the past) of any of these
magnetic solutions contains at least two sequential Kasner points of the BKL
sequence and the orbit of the transition solution between them. One of the
Kasner points in the alpha-limit set is non-flat, which leads to the result
that each of these magnetic solutions has a curvature singularity.",9909043v4
1998-03-21,Magnetohydrodynamics of the Early Universe and the Evolution of Primordial Magnetic Fields,"We show that the decaying magnetohydrodynamic turbulence leads to a more
rapid growth of the correlation length of a primordial magnetic field than that
caused by the expansion of the Universe. As an example, we consider the
magnetic fields created during the electroweak phase transition. The expansion
of the universe alone would yield a correlation length at the present epoch of
1 AU, whereas we find that the correlation length is likely of order 100 AU,
and cannot possibly be longer than $10^4$ AU for non-helical fields. If the
primordial field is strongly helical, the correlation length can be much
larger, but we show that even in this case it cannot exceed 100 pc. All these
estimates make it hard to believe that the observed galactic magnetic fields
can result from the amplification of seed fields generated at the electroweak
phase transition by the standard galactic dynamo.",9803412v2
1998-02-08,Abelianization of SU(N) Gauge Theory with Gauge Invariant Dynamical Variables and Magnetic Monopoles,"It is shown that SU(N) gauge theory coupled to adjoint Higgs can be
explicitly re-written in terms of SU(N) gauge invariant dynamical variables
with local physical interactions. The resultant theory has a novel compact
abelian $U(1)^{(N - 1)}$ gauge invariance. The above abelian gauge invariance
is related to the adjoint Higgs field and not to the gauge group SU(N). In this
abelianized version the magnetic monopoles carrying the magnetic charges of
$(N-1)$ types have a natural origin and therefore appear explicitly in the
partition function as Dirac monopoles along with their strings. The gauge
invariant electric and magnetic charges with respect to $U(1)^{(N-1)}$ gauge
groups are shown to be vectors in root and co-root lattices of SU(N)
respectively. Therefore, the Dirac quantization condition corresponds to SU(N)
Cartan matrix elements being integers. We also study the effect of the $\theta$
term in the abelian version of the theory.",9802050v1
2000-06-13,"Microscopic Pictures of Dynamical Symmetry Breaking in Supersymmetric $SU(n_c)$, $USp(2n_c)$ and $SO(n_c)$ Theories","Several distinct mechanisms of confinement and dynamical symmetry breaking
(DSB) are identified, in a class of supersymmetric $SU(n_c)$, $USp(2n_c)$ and
$SO(n_c)$ gauge theories. In some of the vacua, the magnetic monopoles carrying
nontrivial flavor quantum numbers condense, causing confinement and symmetry
breaking simultaneously. In more general classes of vacua, however, the
effective low-energy degrees of freedom are found to be constituents of the
monopoles - dual (magnetic) quarks. These magnetic quarks condense and give
rise to confinement and DSB. We find two more important classes of vacua, one
is in various universality classes of nontrivial superconformal theories
(SCFT), another in free-magnetic phase.",0006086v1
2007-02-17,Magneto-electrostatic trapping of ground state OH molecules,"We report the magnetic confinement of neutral, ground state hydroxyl radicals
(OH) at a density of $\sim3\times10^{3}$ cm$^{-3}$ and temperature of $\sim$30
mK. An adjustable electric field of sufficient magnitude to polarize the OH is
superimposed on the trap in either a quadrupole or homogenous field geometry.
The OH is confined by an overall potential established via molecular state
mixing induced by the combined electric and magnetic fields acting on the
molecule's electric dipole and magnetic dipole moments, respectively. An
effective molecular Hamiltonian including Stark and Zeeman terms has been
constructed to describe single molecule dynamics inside the trap. Monte Carlo
simulation using this Hamiltonian accurately models the observed trap dynamics
in various trap configurations. Confinement of cold polar molecules in a
magnetic trap, leaving large, adjustable electric fields for control, is an
important step towards the study of low energy dipole-dipole collisions.",0702146v1
2005-01-27,Tunnelling of condensate magnetization in a double-well potential,"We study quantum dynamical properties of a spin-1 atomic Bose-Einstein
condensate in a double-well potential. Adopting a mean field theory and single
spatial mode approximation, we characterize our model system as two coupled
spins. For certain initial states, we find full magnetization oscillations
between wells not accompanied by mass (or atom numbers) exchange. We identify
dynamic regimes of collective spin variables arising from nonlinear
self-interactions that are different from the usual Josephson oscillations. We
also discuss magnetization beats and incomplete oscillations of collective spin
variables other than the magnetization. Our study points to an alternative
approach to observe coherent tunnelling of a condensate through a (spatial)
potential barrier.",0501165v1
2007-03-31,Formation of quasi-solitons in transverse confined ferromagnetic film media,"The formation of quasi-2D spin-wave waveforms in longitudinally magnetized
stripes of ferrimagnetic film was observed by using time- and space-resolved
Brillouin light scattering technique. In the linear regime it was found that
the confinement decreases the amplitude of dynamic magnetization near the
lateral stripe edges. Thus, the so-called effective dipolar pinning of dynamic
magnetization takes place at the edges.
  In the nonlinear regime a new stable spin wave packet propagating along a
waveguide structure, for which both transversal instability and interaction
with the side walls of the waveguide are important was observed. The
experiments and a numerical simulation of the pulse evolution show that the
shape of the formed waveforms and their behavior are strongly influenced by the
confinement.",0704.0024v1
2007-04-24,Element-resolved x-ray ferrimagnetic and ferromagnetic resonance spectroscopy,"We report on the measurement of element-specific magnetic resonance spectra
at gigahertz frequencies using x-ray magnetic circular dichroism (XMCD). We
investigate the ferrimagnetic precession of Gd and Fe ions in Gd-substituted
Yttrium Iron Garnet, showing that the resonant field and linewidth of Gd
precisely coincide with Fe up to the nonlinear regime of parametric
excitations. The opposite sign of the Gd x-ray magnetic resonance signal with
respect to Fe is consistent with dynamic antiferromagnetic alignment of the two
ionic species. Further, we investigate a bilayer metal film,
Ni$_{80}$Fe$_{20}$(5 nm)/Ni(50 nm), where the coupled resonance modes of Ni and
Ni$_{80}$Fe$_{20}$ are separately resolved, revealing shifts in the resonance
fields of individual layers but no mutual driving effects. Energy-dependent
dynamic XMCD measurements are introduced, combining x-ray absorption and
magnetic resonance spectroscopies.",0704.3139v2
2007-05-24,Local spin dynamic arising from the non-perturbative SU(2) gauge field of the spin orbit effect,"We use the non-perturbative gauge field approach to study the effects of spin
orbit coupling on the dynamic of magnetic moment. We present a general equation
of motion (EOM) which unifies i) the spin orbit coupling effect derived from
the SU(2) spin gauge field, and ii) the moment chirality effect previously
derived from the topological U(1)xU(1) rotation gauge under the adiabatic
condition. We present a modified Landau-Liftshitz-Gilbert equation and discuss
the implication of the modified EOM in various technological applications, such
as current-induced switching and trajectory of magnetic moments in spin-valve
multilayers, magnetic memory and diluted magnetic semiconductor.",0705.3502v1
2007-05-30,Two Modes of Magnetization Switching in a Simulated Iron Nanopillar in an Obliquely Oriented Field,"Finite-temperature micromagnetics simulations are employed to study the
magnetization-switching dynamics driven by a field applied at an angle to the
long axis of an iron nanopillar. A bi-modal distribution in the switching times
is observed, and evidence for two competing modes of magnetization-switching
dynamics is presented. For the conditions studied here, temperature $T = 20$ K
and the reversal field 3160 Oe at an angle of 75$^\circ$ to the long axis,
approximately 70% of the switches involve unstable decay (no free-energy
barrier) and 30% involve metastable decay (a free-energy barrier is crossed).
The latter are indistinguishable from switches which are constrained to start
at a metastable free-energy minimum. Competition between unstable and
metastable decay could greatly complicate applications involving magnetization
switches near the coercive field.",0705.4454v1
2007-05-30,Weather in stellar atmosphere: the dynamics of mercury clouds in alpha Andromedae,"The formation of long-lasting structures at the surfaces of stars is commonly
ascribed to the action of strong magnetic fields. This paradigm is supported by
observations of evolving cool spots in the Sun and active late-type stars, and
stationary chemical spots in the early-type magnetic stars. However, results of
our seven-year monitoring of mercury spots in non-magnetic early-type star
alpha Andromedae show that the picture of magnetically-driven structure
formation is fundamentally incomplete. Using an indirect stellar surface
mapping technique, we construct a series of 2-D images of starspots and
discover a secular evolution of the mercury cloud cover in this star. This
remarkable structure formation process, observed for the first time in any
star, is plausibly attributed to a non-equilibrium, dynamical evolution of the
heavy-element clouds created by atomic diffusion and may have the same
underlying physics as the weather patterns on terrestrial and giant planets.",0705.4469v1
2007-06-04,Sum-rules for electron energy-loss near-edge spectra,"We derive four sum-rule expressions for spectra measured in electron
energy-loss near edge structure experiments. These sum-rules permit the
determination spin and orbital magnetic moments, spin-orbit interaction and
number of states, analogously to the sum rules of x-ray magnetic circular
dichroism. The derivation of the sum-rules is based on dynamical electron
diffraction theory and the properties of the mixed dynamic form-factor. The
accuracy of the sum-rules is tested by a complete evaluation of the thickness
dependent electron energy-loss spectra for iron, cobalt, and nickel crystals.
We find that the sum-rules reproduce both spin and orbital moments with very
good accuracy. Our results provide a foundation for the use of the energy loss
magnetic chiral dichroism technique as a quantitative probe of element specific
magnetic properties.",0706.0402v2
2007-09-18,Theory of current-driven magnetization dynamics in inhomogeneous ferromagnets,"We give a brief account of recent developments in the theoretical
understanding of the interaction between electric currents and inhomogeneous
ferromagnetic order parameters. We start by discussing the physical origin of
the spin torques responsible for this interaction and construct a
phenomenological description. We then consider the electric current-induced
ferromagnetic instability and domain-wall motion. Finally, we present a
microscopic justification of the phenomenological description of current-driven
magnetization dynamics, with particular emphasis on the dissipative terms, the
so-called Gilbert damping $\alpha$ and the $\beta$ component of the adiabatic
current-driven torque.",0709.2937v2
2007-12-12,Anisotropic turbulent model for solar coronal heating,"Context : We present a self-consistent model of solar coronal heating,
originally developed by Heyvaert & Priest (1992), in which we include the
dynamical effect of the background magnetic field along a coronal structure by
using exact results from wave MHD turbulence (Galtier et al. 2000). Aims : We
evaluate the heating rate and the microturbulent velocity for comparison with
observations in the quiet corona, active regions and also coronal holes.
Methods :The coronal structures are assumed to be in a turbulent state
maintained by the slow erratic motions of the magnetic footpoints. A
description for the large-scale and the unresolved small-scale dynamics are
given separately. From the latter, we compute exactly (or numerically for
coronal holes) turbulent viscosites that are finally used in the former to
close self-consistently the system and derive the heating flux expression.
Results : We show that the heating rate and the turbulent velocity compare
favorably with coronal observations. Conclusions : Although the Alfven wave
turbulence regime is strongly anisotropic, and could reduce a priori the
heating efficiency, it provides an unexpected satisfactory model of coronal
heating for both magnetic loops and open magnetic field lines.",0712.2029v2
2008-01-28,Switching phenomena in magnetic vortex dynamics,"A magnetic nanoparticle in a vortex state is a promising candidate for the
information storage. One bit of information corresponds to the upward or
downward magnetization of the vortex core (vortex polarity). Generic properties
of the vortex polarity switching are insensitive of the way how the vortex
dynamics was excited: by an AC magnetic field, or by an electrical current. We
study theoretically the switching process and describe in detail its mechanism,
which involves the creation and annihilation of an intermediate
vortex-antivortex pair.",0801.4045v2
2008-05-14,Bypassing Cowling's theorem in axisymmetric fluid dynamos,"We present a numerical study of the magnetic field generated by an
axisymmetrically forced flow in a spherical domain. At small enough Reynolds
number, Re, the flow is axisymmetric and generates an equatorial dipole above a
critical magnetic Reynolds number Rmc . The magnetic field thus breaks
axisymmetry, in agreement with Cowling's theorem. This structure of the
magnetic field is however replaced by a dominant axial dipole when Re is larger
and allows non axisymmetric fluctuations in the flow. We show here that even in
the absence of such fluctuations, an axial dipole can also be generated, at low
Re, through a secondary bifurcation, when Rm is increased above the dynamo
threshold. The system therefore always find a way to bypass the constraint
imposed by Cowling's theorem. We understand the dynamical behaviors that result
from the interaction of equatorial and axial dipolar modes using simple model
equations for their amplitudes derived from symmetry arguments.",0805.2123v1
2008-09-05,MHD turbulence in a channel with spanwise magnetic field,"The effect of a uniform spanwise magnetic field on a turbulent channel flow
is investigated for the case of low magnetic Reynolds number. DNS and LES
computations are performed for two values of the hydrodynamic Reynolds number
(10^4 and 2\times 10^4) and the Hartmann number varying in a wide range. It is
shown that the main effect of the magnetic field is the suppression of
turbulent velocity fluctuations and momentum transfer in the wall-normal
direction. This leads to drag reduction and transformation of the mean flow
profile. The centerline velocity grows, the mean velocity gradients near the
wall decrease, and the typical horizontal dimensions of the coherent structures
enlarge upon increasing the Hartmann number. Comparison between LES and DNS
results shows that the dynamic Smagorinsky model accurately reproduces the flow
transformation.",0809.0964v1
2008-10-03,Dilution effects in Ho$_{2-x}$Y$_x$Sn$_2$O$_7$: from the Spin Ice to the single-ion magnet,"A study of the modifications of the magnetic properties of
Ho$_{2-x}$Y$_x$Sn$_2$O$_7$ upon varying the concentration of diamagnetic
Y$^{3+}$ ions is presented. Magnetization and specific heat measurements show
that the Spin Ice ground-state is only weakly affected by doping for $x\leq
0.3$, even if non-negligible changes in the crystal field at Ho$^{3+}$ occur.
In this low doping range $\mu$SR relaxation measurements evidence a
modification in the low-temperature dynamics with respect to the one observed
in the pure Spin Ice. For $x\to 2$, or at high temperature, the dynamics
involve fluctuations among Ho$^{3+}$ crystal field levels which give rise to a
characteristic peak in $^{119}$Sn nuclear spin-lattice relaxation rate. In this
doping limit also the changes in Ho$^{3+}$ magnetic moment suggest a variation
of the crystal field parameters.",0810.0608v1
2008-11-17,Magnetic Moments of Delta and Omega^- Baryons with Dynamical Clover Fermions,"We calculate the magnetic dipole moment of the Delta(1232) and Omega^-
baryons with 2+1-flavors of clover fermions on anisotropic lattices using a
background magnetic field. This is the first dynamical calculation of these
magnetic moments using a background field technique. The calculation for
Omega^- is done at the physical strange quark mass, with the result in units of
the physical nuclear magneton mu_Omega^-= -1.93(8)(12) (where the first error
is statistical and the second is systematic) compared to the experimental
number: -2.02(5). The Delta has been studied at three unphysical quark masses,
corresponding to pion mass m_pi = 366, 438, and 548 MeV. The pion mass
dependence is compared with the behavior obtained from chiral effective field
theory.",0811.2440v1
2008-11-25,Electronic theory of ultrafast spin dynamics,"NiO is a good candidate for ultrafast magnetic switching because of its large
spin density, antiferromagnetic order, and clearly separated intragap states.
In order to detect and monitor the switching dynamics, we develop a systematic
approach to study optical second harmonic generation (SHG) in NiO, both at the
(001) surface and in the bulk. In our calculations NiO is modeled as a doubly
embedded cluster. All intragap \emph{d}-states of the bulk and the (001)
surface are obtained with highly-correlational quantum chemistry and propagated
in time under the influence of a static magnetic field and a laser pulse. We
find that demagnetization and switching can be best achieved in a subpicosecond
regime with linearly rather than circularly polarized light. We also show the
importance of including an external magnetic field in order to distinguish
spin-up and spin-down states and the necessity of including magnetic-dipole
transitions in order to realize the $\Lambda$-process in the centrosymmetric
bulk. Having already shown the effects of phonons in the SHG for the bulk NiO
within the frozen-phonon approximation, and following the same trail of
thoughts, we discuss the role of phonons in a fully quantized picture as a
symmetry-lowering mechanism in the switching scenario and investigate the
electronic and lattice temperature effects.",0811.4042v1
2009-01-13,Spinor condensate of $^{87}$Rb as a dipolar gas,"We consider a spinor condensate of $^{87}$Rb atoms in F=1 hyperfine state
confined in an optical dipole trap. Putting initially all atoms in $m_F=0$
component we find that the system evolves towards a state of thermal
equilibrium with kinetic energy equally distributed among all magnetic
components. We show that this process is dominated by the dipolar interaction
of magnetic spins rather than spin mixing contact potential. Our results show
that because of a dynamical separation of magnetic components the spin mixing
dynamics in $^{87}$Rb condensate is governed by dipolar interaction which plays
no role in a single component rubidium system in a magnetic trap.",0901.1763v2
2009-02-16,Nature-inspired microfluidic propulsion using magnetic actuation,"In this work we mimic the efficient propulsion mechanism of natural cilia by
magnetically actuating thin films in a cyclic but non-reciprocating manner. By
simultaneously solving the elasto-dynamic, magnetostatic and fluid mechanics
equations, we show that the amount of fluid propelled is proportional to the
area swept by the cilia. By using the intricate interplay between film
magnetization and applied field we are able to generate a pronounced asymmetry
and associated flow. We delineate the functional response of the system in
terms of three dimensionless parameters that capture the relative contribution
of elastic, inertial, viscous and magnetic forces.",0902.2632v1
2009-03-06,Structure and Dynamics of Sunspots,"The physics of Sunspots is a fascinating and demanding field of research in
solar astronomy. Interaction of magnetic fields and plasma flows takes place in
a tangled magnetic geometry and occurs on spatial scales that pose a continuous
challenge for existing instrumentation and for the unambiguous interpretation
of spectropolarimetric observations. Thus, the main properties of sunspots are
well established but its fine structure is not yet fully understood.
  In this contribution we summarize the current knowledge of the magnetic and
dynamic properties of sunspots at the photospheric level based on selected
observations featuring the highest possible spatial and spectral resolution. We
concentrate on light bridges, umbral dots, penumbral filaments and the
notorious dark cores in penumbral filaments. We report on the morphology of the
fine structure elements but mostly focus on observations of their line-of-sight
velocities and magnetic field parameters. We briefly comment on results from
recent radiative MHD simulations and more schematic model ideas that attempt to
rationalize observations of the penumbra.",0903.1300v1
2009-04-05,Static and Dynamic Magnetism in Underdoped Superconductor BaFe$_{1.92}$Co$_{0.08}$As$_2$,"We report neutron scattering measurements on single crystals of
BaFe$_{1.92}$Co$_{0.08}$As$_2$. The magnetic Bragg peak intensity is reduced by
6 % upon cooling through T$_C$. The spin dynamics exhibit a gap of 8 meV with
anisotropic three-dimensional (3d) interactions. Below T$_C$ additional
intensity appears at an energy of $\sim$4.5(0.5) meV similar to previous
observations of a spin resonance in other Fe-based superconductors. No further
gapping of the spin excitations is observed below T$_C$ for energies down to 2
meV. These observations suggest the redistribution of spectral weight from the
magnetic Bragg position to a spin resonance demonstrating the direct
competition between static magnetic order and superconductivity.",0904.0767v1
2009-06-05,One-dimensional stability of parallel shock layers in isentropic magnetohydrodynamic,"Extending investigations of Barker, Humpherys, Lafitte, Rudd, and Zumbrun for
compressible gas dynamics and Freist\""uhler and Trakhinin for compressible
magnetohydrodynamics, we study by a combination of asymptotic ODE estimates and
numerical Evans function computations the one-dimensional stability of parallel
isentropic magnetohydrodynamic shock layers over the full range of physical
parameters (shock amplitude, strength of imposed magnetic field, viscosity,
magnetic permeability, and electrical resistivity) for a $\gamma$-law gas with
$\gamma\in [1,3]$. Other $\gamma$-values may be treated similarly, but were not
checked numerically. Depending on magnetic field strength, these shocks may be
of fast Lax, intermediate (overcompressive), or slow Lax type; however, the
shock layer is independent of magnetic field, consisting of a purely
gas-dynamical profile. In each case, our results indicate stability.
Interesting features of the analysis are the need to renormalize the Evans
function in order to pass continuously across parameter values where the shock
changes type or toward the large-amplitude limit at frequency $\lambda=0$ and
the systematic use of winding number computations on Riemann surfaces.",0906.1208v1
2009-06-13,On the lack of universality in decaying magnetohydrodynamic turbulence,"Using computations of three-dimensional magnetohydrodynamic (MHD) turbulence
with a Taylor-Green flow, whose inherent time-independent symmetries are
implemented numerically, and in the absence of either a forcing function or an
imposed uniform magnetic field, we show that three different inertial ranges
for the energy spectrum may emerge for three different initial magnetic fields,
the selecting parameter being the ratio of the Alfven to the eddy turnover
time. Equivalent computational grids range from 128^3 to 2048^3 points with a
unit magnetic Prandtl number and a Taylor Reynolds number of up to 1500 at the
peak of dissipation. We also show convergence of our results with Reynolds
number. Our study is consistent with previous findings of a variety of energy
spectra in MHD turbulence by studies performed in the presence of both a
forcing term with a given correlation time and a strong, uniform magnetic
field. In contrast to the previous studies, however, the ratio of
characteristic time scales here can only be ascribed to the intrinsic nonlinear
dynamics of the flows under study.",0906.2506v1
2009-09-03,Spontaneous Magnetic Ordering in a Ferromagnetic Spinor Dipolar Bose-Einstein Condensate,"We study the spin dynamics in a spin-1 ferromagnetic Bose-Einstein condensate
with magnetic dipole-dipole interaction (MDDI) based on the Gross-Pitaevskii
and Bogoliubov theories. We find that various magnetic structures such as
checkerboards and stripes emerge in the course of the dynamics due to the
combined effects of spin-exchange interaction, MDDI, quadratic Zeeman and
finite-size effects, and non-stationary initial conditions. However, the
short-range magnetic order observed by the Berkeley group [Phys. Rev. Lett.
{\bf 100}, 170403 (2008)] is not fully reproduced in our calculations; the
periodicity of the order differs by a factor of three and the checkerboard
pattern eventually dissolves in our numerical simulations. Possible reasons for
the discrepancy are discussed.",0909.0565v2
2009-09-03,Simulation of the current dynamics in superconductors: Application to magnetometry measurements,"A simple model for simulating the current dynamics and the magnetic
properties of superconductors is presented. Short simulation times are achieved
by solving the differential form of Maxwell's equations inside the sample,
whereas integration is only required at the surface to meet the exact boundary
conditions. The procedure reveals the time and position dependence of the
current density and the magnetic induction (B) making it very convenient to
apply field dependent material parameters for the simulation of magnetization
loops, relaxation measurements, etc. Two examples, which are important for
standard magnetometry experiments, are discussed. Firstly, we prove that
evaluating the critical current density (J_c) from experiment by applying
Bean's model reveals almost the exact J_c(B) behavior, if the evaluation is
corrected by a simple numerical expression. Secondly, we show that the
superconducting volume fraction of a sample can be directly determined from
magnetization loops by carefully comparing experiment and simulation in the
field range, where the current loops are differently oriented within the
sample.",0909.0597v1
2009-09-30,Two-Dimensional Spin Dynamics in the Itinerant Ferromagnet LaCoPO Revealed by Magnetization and $^{31}$P-NMR Measurements,"We have performed magnetization and $^{31}$P-NMR measurements on the
itinerant ferromagnet LaCoPO (Curie temperature $T_{\rm Curie}\sim 44$ K) with
a layered structure in order to investigate spin dynamics in the paramagnetic
state. The linear scaling between the Knight shift $K$ at the P site and the
bulk susceptibility $\chi$ above $T_{\rm Curie}$ indicates that the P nucleus
is suitable for investigating magnetic properties. The temperature and magnetic
field dependences of the nuclear spin-lattice relaxation rate divided by the
temperature $1/T_1T$ at the P site show characteristic features of itinerant
ferromagnets, such as ZrZn$_2$ and Y(Co$_{1-x}$Al$_x$)$_2$. In addition, the
relationship between $1/T_1T$ and $\chi$ above $T_{\rm Curie}$ suggests that
ferromagnetic fluctuations possess a two-dimensional (2D) characteristic. The
present data show that LaCoPO is a unique ferromagnet, where the 2D
fluctuations anticipated from the crystal structure are predominant down to
almost $T_{\rm Curie}$.",0909.5641v1
2009-10-06,Effect of dipolar interactions for domain wall dynamics in magnetic thin films,"We study the effect of long range dipolar forces on the dynamics and
morphology of domain walls in magnetic thin films by numerical simulations of
the spin-1 random field Ising model. By studying the size distribution of
avalanches of domain wall motion arising as a response to quasistatic external
driving, we observe a cross-over from the case dominated by short range
interactions to another universality class where the long range dipolar forces
become important. This crossover is accompanied with a change of the domain
wall morphology from a rough wall to walls with zigzag structure.",0910.1040v1
2009-11-12,An ac field probe for the magnetic ordering of magnets with random anisotropy,"A Monte Carlo simulation is carried out to investigate the magnetic ordering
in magnets with random anisotropy (RA). Our results show peculiar similarities
to recent experiments that the real part of ac susceptibility presents two
peaks for weak RA and only one for strong RA regardless of glassy critical
dynamics manifested for them. We demonstrate that the thermodynamic nature of
the low-temperature peak is a ferromagnetic-like dynamic phase transition to
quasi-long range order (QLRO) for the former. Our simulation, therefore, is
able to be incorporated with the experiments to help clarify the existence of
the QLRO theoretically predicted so far.",0911.2352v1
2009-11-20,On Chern-Simons corrections to magnetohydrodynamics equations,"We study the effect of a (3+1)-dimensional Chern-Simons electrodynamics on
the equations governing the dynamics of magnetized plasma and fields. In this
model, the Chern-Simons (CS) part consists of a dynamical pseudo-scalar field
whose space-time derivatives couple with the electromagnetic field. We explore
the CS corrections to the evolution equation for the magnetic field in a plasma
with non-zero electrical resistivity. We revisit Cowling's theorem in this
context and observe that the CS corrections lead to possibly small but non-zero
source terms for axisymmetric magnetic field. The scalar product of electric
and magnetic fields play the role of source of the pseudo-scalar field, and
therefore, pulsars and magnetars are likely astrophysical candidates to
generate propagating pseudo-scalar waves. Although aligned electric field gets
shorted out by flowing charges in large parts of the magnetosphere, there are
vacuum gaps in the vicinity of pulsars where strong $\vec E. \vec B$ is
expected to be present. We derive a wave solution for the pseudo-scalar field
generated by the time-varying $\vec E. \vec B$ associated with a pulsar.",0911.3967v1
2009-11-27,Multiple vortex-antivortex pair generation in magnetic nanodots,"The interaction of a magnetic vortex with a rotating magnetic field causes
the nucleation of a vortex--antivortex pair leading to a vortex polarity
switching. The key point of this process is the creation of a dip, which can be
interpreted as a nonlinear resonance in the system of certain magnon modes with
nonlinear coupling. The usually observed single-dip structure is a particular
case of a multidip structure. The dynamics of the structure with $n$ dips is
described as the dynamics of nonlinearly coupled modes with azimuthal numbers
$m=0,\pm n,\pm 2n$. The multidip structure with arbitrary number of vortex
antivortex pairs can be obtained in vortex-state nanodisk using a space- and
time-varying magnetic field. A scheme of a possible experimental setup for
multidip structure generation is proposed.",0911.5353v2
2009-12-01,Trapping ultracold dysprosium: a highly magnetic gas for dipolar physics,"Ultracold dysprosium gases, with a magnetic moment ten times that of alkali
atoms and equal only to terbium as the most magnetic atom, are expected to
exhibit a multitude of fascinating collisional dynamics and quantum dipolar
phases, including quantum liquid crystal physics. We report the first laser
cooling and trapping of half a billion Dy atoms using a repumper-free
magneto-optical trap (MOT) and continuously loaded magnetic confinement, and we
characterize the trap recycling dynamics for bosonic and fermionic isotopes.
The first inelastic collision measurements in the few partial wave, 100 uK to 1
mK, regime are made in a system possessing a submerged open electronic f-shell.
In addition, we observe unusual stripes of intra-MOT <10 uK sub-Doppler cooled
atoms.",0912.0050v3
2010-05-12,Parametric amplification of matter waves in dipolar spinor Bose-Einstein condensates,"Spin-changing collisions may lead under proper conditions to the parametric
amplification of matter waves in spinor Bose-Einstein condensates. Magnetic
dipole-dipole interactions, although typically very weak in alkaline atoms, are
shown to play a very relevant role in the amplification process. We show that
these interactions may lead to a strong dependence of the amplification
dynamics on the angle between the trap axis and the magnetic-field orientation.
We analyze as well the important role played by magnetic-field gradients, which
modify also strongly the amplification process. Magnetic-field gradients must
be hence carefully controlled in future experiments, in order to observe
clearly the effects of the dipolar interactions in the amplification dynamics.",1005.2011v1
2010-05-12,Ca II K Spectral Study of an Emerging Flux Region using Domeless Solar Telescope in Hida Observatory,"A cooperative observation with Hida observatory and Hinode satellite was
performed on an emerging flux region. The successive Ca II K spectro-heliograms
of the emerging flux region were taken by the Domeless Solar Telescope of Hida
observatory. Hinode observed the emerging flux region with Ca II H and Fe I
Stokes IQUV filtergrams. In this study, detailed dynamics and temporal
evolution of the magnetic flux emergence was studied observationally. The event
was first detected in the photospheric magnetic field signals. 3 minutes later,
the horizontal expansion of the dark area was detected. And then, 7 minutes
later than the horizontal expansion, the emerging loops were detected with the
maximal rise speed of 2.1 km/s at chromospheric heights. The observed dynamics
of emerging magnetic flux from the photosphere to the upper chromosphere is
well consistent with the results of previous simulation works. The gradual
rising phase of flux tubes with a weak magnetic strength was confirmed by our
observation.",1005.2025v1
2010-06-08,Linear-Temperature Dependence of Static Magnetic Susceptibility in LaFeAsO from Dynamical Mean-Field Theory,"In this Letter we report the LDA+DMFT (method combining Local Density
Approximation with Dynamical Mean-Field Theory) results for magnetic properties
of parent superconductor LaFeAsO in paramagnetic phase. Calculated uniform
magnetic susceptibility shows linear dependence at intermediate temperatures in
agreement with experimental data. For high temperatures ($>$1000 K)
calculations show saturation and then susceptibility decreases with
temperature. Contributions to temperature dependence of the uniform
susceptibility are strongly orbitally dependent. It is related to the form of
the orbitally-resolved spectral functions near the Fermi energy with strong
temperature dependent narrow peaks for some of the orbitals. Our results
demonstrate that linear-temperature dependence of static magnetic
susceptibility in pnictide superconductors can be reproduced without invoking
antiferromagnetic fluctuations.",1006.1509v2
2010-07-02,Interaction of a Nanomagnet with a Weak Superconducting Link,"We study electromagnetic interaction of a nanomagnet with a weak
superconducting link. Equations that govern coupled dynamics of the two systems
are derived and investigated numerically. We show that the presence of a small
magnet in the proximity of a weak link may be detected through Shapiro-like
steps caused by the precession of the magnetic moment. Despite very weak
magnetic field generated by the weak link, a time-dependent bias voltage
applied to the link can initiate a non-linear dynamics of the nanomagnet that
leads to the reversal of its magnetic moment. We also consider quantum problem
in which a nanomagnet interacting with a weak link is treated as a two-state
spin system due to quantum tunneling between spin-up and spin-down states.",1007.0416v2
2010-07-20,Evidence for Dynamically Important Magnetic Fields in Molecular Clouds,"Recent observational evidence that magnetic fields are dynamically important
in molecular clouds, compared to self-gravity and turbulence, is reviewed and
illustrated with data from the NGC 2024 region. One piece of evidence,
turbulence anisotropy, was found in the diffuse envelope of a cloud (Av~1;
Heyer et al. 2008); our data further suggests turbulence anisotropy in the
cloud (Av >7) and even near the cloud core (Av~100). The data also shows that
magnetic fields can channel gravitational contraction even for a region with
super-critical N(H2)/2Blos ratio (the ratio between the observed column density
and two times the line-of-sight observed field strength), a parameter which has
been widely used by observers to estimate core mass-to-flux ratios. Although
the mass-to-flux ratio is constant under the flux-freezing condition, we show
that N(H2)/2Blos grows with time if gravitational contraction is anisotropic
due to magnetic fields.",1007.3312v2
2010-07-22,Theory of the power spectrum of spin-torque nanocontact vortex oscillators,"Spin-transfer torques in magnetic nanocontacts can lead to self-sustained
magnetization oscillations that involve large-amplitude gyrotropic vortex
motion. This dynamics consists of a steady state orbit around the nanocontact,
which is made possible because the intrinsic magnetic damping is compensated by
spin torques. In this article, we present an analytical theory of the power
spectrum of these oscillations based on a rigid-vortex model. The appearance of
vortex oscillations in nanocontacts is not associated with a Hopf bifurcation:
there is no critical current and the only precondition for steady-state
oscillations at finite currents is the existence of a vortex in the system, in
contrast with conventional spin-torque oscillators that involve large-angle
magnetization precession. The oscillation frequency is found to depend linearly
on the applied current and inversely proportional to the orbital radius. By
solving the associated Langevin problem for the vortex dynamics, the lineshape
and linewidth for the power spectrum are also obtained. Under typical
experimental conditions, a Lorentzian lineshape with a current-independent
linewidth is predicted. Good quantitative agreement between the theory and
recent experiments is shown.",1007.3859v1
2010-09-22,Temperature- and Magnetic-Field-Dependent Optical Properties of Heavy Quasiparticles in YbIr2Si2,"We report the temperature- and magnetic-field-dependent optical conductivity
spectra of the heavy electron metal YbIr$_2$Si$_2$. Upon cooling below the
Kondo temperature ($T_{\rm K}$), we observed a typical charge dynamics that is
expected for a formation of a coherent heavy quasiparticle state. We obtained a
good fitting of the Drude weight of the heavy quasiparticles by applying a
modified Drude formula with a photon energy dependence of the quasiparticle
scattering rate that shows a similar power-law behavior as the temperature
dependence of the electrical resistivity. By applying a magnetic field of 6T
below $T_{\rm K}$, we found a weakening of the effective dynamical mass
enhancement by about 12% in agreement with the expected decrease of the
$4f$-conduction electron hybridization on magnetic field.",1009.4290v2
2010-11-01,Decay of trefoil and other magnetic knots,"Two setups with interlocked magnetic flux tubes are used to study the
evolution of magnetic energy and helicity on magnetohydrodynamical (MHD)
systems like plasmas. In one setup the initial helicity is zero while in the
other it is finite. To see if it is the actual linking or merely the helicity
content that influences the dynamics of the system we also consider a setup
with unlinked field lines as well as a field configuration in the shape of a
trefoil knot. For helical systems the decay of magnetic energy is slowed down
by the helicity which decays slowly. It turns out that it is the helicity
content, rather than the actual linking, that is significant for the dynamics.",1011.0417v3
2010-12-13,"Comment on paper by L. M. Malyshkin and S. Boldyrev, ""Magnetic dynamo action at low magnetic Prandtl numbers"", PRL 105, 215002 (2010)","Is the scaling, \lambda \sim Rm^{1/2}, for the growth rate of small-scale
dynamo instability at low magnetic Prandtl numbers and large magnetic Reynolds
numbers, Rm, valid in the vicinity of the threshold? Our analysis and even
numerical solution (Malyshkin and Boldyrev, 2010) of the dynamo equations for a
Gaussian white-noise velocity field (the Kazantsev-Kraichnan model) imply that
the answer is negative. Contrary to the claim by Malyshkin and Boldyrev (2010),
there are two different asymptotics for the dynamo growth rate: in the vicinity
of the threshold and far from the threshold.",1012.2773v1
2010-12-14,Constraints on the Galactic Magnetic Field from the Canadian Galactic Plane Survey,"The Galactic magnetic field is important in the dynamics of our Galaxy. It is
believed to play a role in star formation and influence the structure of the
Galaxy. In order to understand how the Galactic magnetic field originally
formed or how it is evolving, we must first determine its present topology. To
this end, we have used observations from the Canadian Galactic Plane Survey
(CGPS) to calculate the highest source density of rotation measures (RM) to
date in the disk of the Galaxy. Using these data, we estimate the Galactic
longitude of the RM null point in the outer Galaxy (where the RMs of
extragalactic sources are observed to pass through zero, on average, with
increasing Galactic longitude). We have also examined the RM scale height using
the CGPS latitude extension. The values of these parameters offer critical
constraints for modeling the large-scale magnetic field in the Galactic disk.",1012.2934v1
2011-01-10,Dynamical Signatures of Edge-State Magnetism on Graphene Nanoribbons,"We investigate the edge-state magnetism of graphene nanoribbons using
projective quantum Monte Carlo simulations and a self-consistent mean-field
approximation of the Hubbard model. The static magnetic correlations are found
to be short ranged. Nevertheless, the correlation length increases with the
width of the ribbon such that already for ribbons of moderate widths we observe
a strong trend towards mean-field-type ferromagnetic correlations at a zigzag
edge. These correlations are accompanied by a dominant low-energy peak in the
local spectral function and we propose that this can be used to detect
edge-state magnetism by scanning tunneling microscopy. The dynamic spin
structure factor at the edge of a ribbon exhibits an approximately linearly
dispersing collective magnonlike mode at low energies that decays into Stoner
modes beyond the energy scale where it merges into the particle-hole continuum.",1101.1882v2
2011-03-08,Slow dynamics of interacting antiferromagnetic nanoparticles,"We study magnetic relaxation dynamics, memory and aging effects in
interacting polydisperse antiferromagnetic NiO nanoparticles by solving a
master equation using a two-state model. We investigate the effects of
interactions using dipolar, Nearest-Neighbour Short-Range (NNSR) and Long-Range
Mean-Field (LRMF) interactions. The magnetic relaxation of the nanoparticles in
a time-dependent magnetic field has been studied using LRMF interaction. The
size-dependent effects are suppressed in the ac-susceptibility, as the
frequency is increased. We find that the memory dip, that quantifies the memory
effect is about the same as that of non-interacting nanoparticles for the NNSR
case. There is a stronger memory-dip for LRMF, and a weaker memory-dip for the
dipolar interactions. We have also shown a memory effect in the
Zero-field-cooled magnetization for the dipolar case, a signature of glassy
behaviour, from Monte-Carlo studies.",1103.1667v1
2011-03-19,Magnetic dynamics of single domain Ni nanoparticles,"The dynamic magnetic properties of Ni nanoparticles diluted in an amorphous
SiO2 matrix prepared from a modified sol-gel method have been studied by the
frequency f dependence of the ac magnetic susceptibility \c{hi}(T). For samples
with similar average radii ~ 3-4 nm, an increase of the blocking temperature
from TB ~ 20 to ~ 40 K was observed for Ni concentrations of ~ 1.5 and 5 wt.%,
respectively, assigned to the effects of dipolar interactions. Both the
in-phase \c{hi}'(T) and the out-of-phase \c{hi}""(T) maxima follow the
predictions of the thermally activated N\'eel-Arrhenius model. The effective
magnetic anisotropy constant Keff inferred from \c{hi}""(T) versus f data for
the 1.5 wt.% Ni sample is close to the value of the magnetocrystalline
anisotropy of bulk Ni, suggesting that surface effects are negligible in the
present samples. In addition, the contribution from dipolar interactions to the
total anisotropy energy Ea in specimens with 5 wt.% Ni was found to be
comparable to the intrinsic magnetocrystalline anisotropy barrier.",1103.3783v1
2011-04-18,Entanglement dynamics of one-dimensional driven spin systems in time-varying magnetic fields,"We study the dynamics of entanglement for a one-dimensional spin chain with a
nearest neighbor time dependent Heisenberg coupling J(t) between the spins in
presence of a time dependent external magnetic field h(t) at zero and finite
temperatures. We consider different forms of time dependence for the coupling
and magnetic field; exponential, hyperbolic and periodic. We examined the
system size effect on the entanglement asymptotic value. It was found that for
a small system size the entanglement starts to fluctuate within a short period
of time after applying the time dependent coupling. The period of time
increases as the system size increases and disappears completely as the size
goes to infinity. We also found that when J(t) is periodic the entanglement
shows a periodic behavior with the same period, which disappears upon applying
periodic magnetic field with the same frequency. Solving the particular case
where J(t) and h(t) are proportional exactly, we showed that the asymptotic
value of entanglement depends only on the initial conditions regardless of the
form of J(t) and h(t) applied at t > 0.",1104.3521v1
2011-05-06,Chiral transition in a magnetic field and at finite baryon density,"We consider the quark-meson model with two quark flavors in a constant
external magnetic field $B$ at finite temperature $T$ and finite baryon
chemical potential $\mu_B$. We calculate the full renormalized effective
potential to one-loop order in perturbation theory. We study the system in the
large-$N_c$ limit, where we treat the bosonic modes at tree level. It is shown
that the system exhibits dynamical chiral symmetry breaking, i. e. that an
arbitrarily weak magnetic field breaks chiral symmetry dynamically, in
agreement with earlier calculations using the NJL model. We study the influence
on the phase transition of the fermionic vacuum fluctuations. For strong
magnetic fields, $|qB|\sim5m_{\pi}^2$ and in the chiral limit, the transition
is first order in the entire $\mu_B-T$ plane if vacuum fluctuations are not
included and second order if they are included. At the physical point, the
transition is a crossover for $\mu_B=0$ with and without vacuum fluctuations.",1105.1290v3
2011-08-30,Residual energy in magnetohydrodynamic turbulence and in the solar wind,"Recent observations indicate that kinetic and magnetic energies are not in
equipartition in the solar wind turbulence. Rather, magnetic fluctuations are
more energetic and have somewhat steeper energy spectrum compared to the
velocity fluctuations. This leads to the presence of the so-called residual
energy E_r=E_v-E_b in the inertial interval of turbulence. This puzzling effect
is addressed in the present paper in the framework of weak turbulence theory.
Using a simple model of weakly colliding Alfv\'en waves, we demonstrate that
the kinetic-magnetic equipartition indeed gets broken as a result of nonlinear
interaction of Alfv\'en waves. We establish that magnetic energy is indeed
generated more efficiently as a result of these interactions, which proposes an
explanation for the solar wind observations.",1108.6072v1
2011-09-02,Angle-resolved NMR: quantitative theory of 75As T1 relaxation rate in BaFe2As2,"While NMR measurements of nuclear energy spectra are routinely used to
characterize the static properties of quantum magnets, the dynamical
information locked in NMR 1/T1 relaxation rates remains notoriously difficult
to interpret. The difficulty arises from the fact that information about all
possible low-energy spin excitations of the electrons, and their coupling to
the nuclear moments, is folded into a single number, 1/T1. Here we develop a
quantitative theory of the NMR 1/T1 relaxation rate in a collinear
antiferromagnet, focusing on the specific example of BaFe2As2. One of the most
striking features of magnetism in BaFe2As2 is a strong dependence of 1/T1 on
the orientation of the applied magnetic field. By careful analysis of the
coupling between the nuclear and electronic moments, we show how this
anisotropy arises from the ""filtering"" of spin fluctuations by the form-factor
for transferred hyperfine interactions. This allows us to make convincing,
quantitative, fits to experimental 1/T1 data for BaFe2As2, for different field
orientations. We go on to show how a quantitative, angle-dependent theory for
the relaxation rate leads to new ways of measuring the dynamical parameters of
magnetic systems, in particular the spin wave velocities.",1109.0384v2
2011-10-24,Pressure balance at the magnetopause: Experimental studies,"The pressure balance at the magnetopause is formed by magnetic field and
plasma in the magnetosheath, on one side, and inside the magnetosphere, on the
other side. In the approach of dipole earth's magnetic field configuration and
gas-dynamics solar wind flowing around the magnetosphere, the pressure balance
predicts that the magnetopause distance R depends on solar wind dynamic
pressure Pd as a power low R ~ Pd^alpha, where the exponent alpha=-1/6. In the
real magnetosphere the magnetic filed is contributed by additional sources:
Chapman-Ferraro current system, field-aligned currents, tail current, and
storm-time ring current. Net contribution of those sources depends on
particular magnetospheric region and varies with solar wind conditions and
geomagnetic activity. As a result, the parameters of pressure balance,
including power index alpha, depend on both the local position at the
magnetopause and geomagnetic activity. In addition, the pressure balance can be
affected by a non-linear transfer of the solar wind energy to the
magnetosheath, especially for quasi-radial regime of the subsolar bow shock
formation proper for the interplanetary magnetic field vector aligned with the
solar wind plasma flow.",1110.5209v1
2011-10-24,Dynamically configurable and optimizable Zeeman slower using permanent magnets and servomotors,"We report on the implementation of a dynamically configurable, servomotor-
controlled, permanent magnet Zeeman slower for quantum optics experiments with
ultracold atoms and molecules. This atom slower allows for switching between
magnetic field profiles that are designed for different atomic species.
Additionally, through feedback on the atom trapping rate, we demonstrate that
computer-controlled genetic optimization algorithms applied to the magnet
positions can be used in situ to obtain field profiles that maximize the
trapping rate for any given experimental conditions. The device is lightweight,
remotely controlled, and consumes no power in steady state; it is a step toward
automated control of quantum optics experiments.",1110.5351v2
2011-11-28,Structural and magnetic dynamics of a laser induced phase transition in FeRh,"We use time-resolved x-ray diffraction and magnetic optical Kerr effect to
study the laser induced antiferromagnetic to ferromagnetic phase transition in
FeRh. The structural response is given by the nucleation of independent
ferromagnetic domains (\tau_1 ~ 30ps). This is significantly faster than the
magnetic response (\tau_2 ~ 60ps) given by the subsequent domain realignment.
X-ray diffraction shows that the two phases co-exist on short time-scales and
that the phase transition is limited by the speed of sound. A nucleation model
describing both the structural and magnetic dynamics is presented.",1111.6435v2
2011-12-05,Phase transitions of quasistationary states in the Hamiltonian Mean Field model,"The out-of-equilibrium dynamics of the Hamiltonian Mean Field (HMF) model is
studied in presence of an externally imposed magnetic field h. Lynden-Bell's
theory of violent relaxation is revisited and shown to adequately capture the
system dynamics, as revealed by direct Vlasov based numerical simulations in
the limit of vanishing field. This includes the existence of an
out-of-equilibrium phase transition separating magnetized and non magnetized
phases. We also monitor the fluctuations in time of the magnetization, which
allows us to elaborate on the choice of the correct order parameter when
challenging the performance of Lynden-Bell's theory. The presence of the field
h removes the phase transition, as it happens at equilibrium. Moreover, regions
with negative susceptibility are numerically found to occur, in agreement with
the predictions of the theory.",1112.1101v1
2011-12-20,Magnetic catalysis with massive dynamical flavours,"Within gauge/gravity duality, we construct a backreacted supergravity
background dual to SU(Nc) N=4 SYM coupled to Nf massive fundamental flavours in
the presence of an external magnetic field. Our solution is perturbative in a
parameter that counts the number of the internal flavour loops. The background
has a hollow cavity in the bulk of the geometry, where it is similar to the
supergravity dual of a non-commutative SYM. The radius of this cavity is
related to the dynamically generated mass of the fundamental fields. We apply
our construction to study the effect of magnetic catalysis and develop an
appropriate renormalization scheme to compute the free energy and the
fundamental condensate of the dual gauge theory as a function of the bare mass.
While at leading order in the expansion of the perturbative parameter, the free
energy and the fundamental condensate agree with the results obtained in the
quenched approximation, at next order we show that the effect of magnetic
catalysis is enhanced and the contribution to the condensate of the theory from
internal fundamental loops runs logarithmically with the finite cutoff
Lambda_UV.",1112.4807v1
2012-01-04,Quasiparticles of string solutions in the spin-1/2 antiferromagnetic Heisenberg chain in a magnetic field,"Spectral properties of the spin-1/2 antiferromagnetic Heisenberg chain in a
magnetic field are investigated by using exact Bethe-ansatz solutions. We argue
that not only quasiparticles called psinon and antipsinon but also a
quasiparticle representing a 2-string in the Bethe ansatz plays an important
role for dynamical properties in a magnetic field. Combined with psinon and
antipsinon, the quasiparticle for a 2-string forms a continuum in the
high-energy regime for transverse dynamical structure factor
$S^{+-}(k,\omega)$. In the zero-field limit, the continuum is located on the
mode of the lowest excited states in zero field called the des
Cloizeaux-Pearson mode. In a magnetic field, the continuum separates from other
low-energy continua, and reduces to the mode of bound states of overturned
spins from the fully polarized state near the saturation field. We confirm the
relevance through comparisons with available experimental results on the
quasi-one-dimensional antiferromagnet CuCl$_2\cdot$2N(C$_5$D$_5$).",1201.0844v1
2012-01-04,Low-temperature magnetization in geometrically frustrated Tb2Ti2O7,"The nature of the low temperature ground state of the pyrochlore compound
Tb2Ti2O7 remains a puzzling issue. Dynamic fluctuations and short-range
correlations persist down to 50 mK, as evidenced by microscopic probes. In
parallel, magnetization measurements show irreversibilities and glassy behavior
below 200 mK. We have performed magnetization and AC susceptibility
measurements on four single crystals down to 57 mK. We did not observe a clear
plateau in the magnetization as a function of field along the [111] direction,
as suggested by the quantum spin ice model. In addition to a freezing around
200 mK, slow dynamics are observed in the AC susceptibility up to 4 K. The
overall frequency dependence cannot be described by a canonical spin-glass
behavior.",1201.0859v2
2012-01-20,Lagrangian chaos in an ABC--forced nonlinear dynamo,"The Lagrangian properties of the velocity field in a magnetized fluid are
studied using three-dimensional simulations of a helical magnetohydrodynamic
dynamo. We compute the attracting and repelling Lagrangian coherent structures,
which are dynamic lines and surfaces in the velocity field that delineate
particle transport in flows with chaotic streamlines and act as transport
barriers. Two dynamo regimes are explored, one with a robust coherent mean
magnetic field and one with intermittent bursts of magnetic energy. The
Lagrangian coherent structures and the statistics of finite--time Lyapunov
exponents indicate that the stirring/mixing properties of the velocity field
decay as a linear function of the magnetic energy. The relevance of this study
for the solar dynamo problem is discussed.",1201.4324v1
2012-02-10,The Chern-Simons diffusion rate in strongly coupled N=4 SYM plasma in an external magnetic field,"We calculate the Chern-Simons diffusion rate in a strongly coupled N=4 SUSY
Yang-Mills plasma in the presence of a constant external $U(1)_R$ magnetic flux
via the holographic correspondence. Due to the strong interactions between the
charged fields and non-Abelian gauge fields, the external Abelian magnetic
field affects the thermal Yang-Mills dynamics and increases the diffusion rate,
regardless of its strength. We obtain the analytic results for the Chern-Simons
diffusion rate both in the weak and strong magnetic field limits. In the latter
limit, we show that the diffusion rate scales as $B\times T^2$ and this can be
understood as a result of a dynamical dimensional reduction.",1202.2161v2
2012-03-05,Frequency generation by a magnetic vortex-antivortex dipole in spin-polarized current,"A vortex-antivortex (VA) dipole may be generated due to a spin-polarized
current flowing through a nano-aperture in a magnetic element. We study the
vortex dipole dynamics using the Landau-Lifshitz equation in the presence of an
in-plane applied magnetic field and a Slonczewski spin-torque term with
in-plane polarization. We establish that the vortex dipole is set in steady
state rotational motion. The frequency of rotation is due to two independent
forces: the interaction between the two vortices and the external magnetic
field. The nonzero skyrmion number of the dipole is responsible for both forces
giving rise to rotational dynamics. The spin-torque acts to stabilize the
vortex dipole motion at a definite vortex-antivortex separation distance. We
give analytical and numerical results for the angular frequency of rotation and
VA dipole features as functions of the parameters.",1203.0880v1
2012-05-29,Thermally-Assisted Spin-Transfer Torque Magnetization Reversal in Uniaxial Nanomagnets,"We simulate the stochastic Landau-Lifshitz-Gilbert (LLG) dynamics of a
uniaxial nanomagnet out to sub-millisecond timescales using a graphical
processing unit based micromagnetic code and determine the effect of
geometrical tilts between the spin-current and uniaxial anisotropy axes on the
thermally assisted reversal dynamics. The asymptotic behavior of the switching
time ($I\rightarrow 0$, $<\tau>\propto\exp(-\xi(1-I)^2)$) is approached
gradually, indicating a broad crossover regime between ballistic and thermally
assisted spin transfer reversal. Interestingly, the mean switching time is
shown to be nearly independent of the angle between the spin current and
magnet's uniaxial axes. These results have important implications for modeling
the energetics of thermally assisted magnetization reversal of spin transfer
magnetic random access memory bit cells.",1205.6509v1
2012-08-31,Cosmological Ohm's law and dynamics of non-minimal electromagnetism,"The origin of large-scale magnetic fields in cosmic structures and the
intergalactic medium is still poorly understood. We explore the effects of
non-minimal couplings of electromagnetism on the cosmological evolution of
currents and magnetic fields. In this context, we revisit the mildly non-linear
plasma dynamics around recombination that are known to generate weak magnetic
fields. We use the covariant approach to obtain a fully general and non-linear
evolution equation for the plasma currents and derive a generalised Ohm law
valid on large scales as well as in the presence of non-minimal couplings to
cosmological (pseudo-)scalar fields. Due to the sizeable conductivity of the
plasma and the stringent observational bounds on such couplings, we conclude
that modifications of the standard (adiabatic) evolution of magnetic fields are
severely limited in these scenarios. Even at scales well beyond a Mpc, any
departure from flux freezing behaviour is inhibited.",1208.6547v2
2012-09-21,Broadband probing magnetization dynamics of the coupled vortex state permalloy layers in nanopillars,"Broadband magnetization response of coupled vortex state magnetic dots in
layered nanopillars was explored as a function of in-plane magnetic field and
interlayer separation. For dipolarly coupled circular Py(25 nm)/Cu(20 nm)/Py(25
nm) nanopillars of 600 nm diameter, a small in-plane field splits the
eigenfrequencies of azimuthal spin wave modes inducing an abrupt transition
between in-phase and out-of-phase kinds of the low-lying coupled spin wave
modes. The critical field for this splitting is determined by antiparallel
chiralities of the vortices in the layers. Qualitatively similar (although more
gradual) changes occur also in the exchange coupled Py(25 nm)/Cu(1 nm)/Py(25
nm) tri-layer nanopillars. These findings are in qualitative agreement with
micromagnetic dynamic simulations.",1209.4822v1
2012-10-23,Averaged equation for energy diffusion on a graph reveals bifurcation diagram and thermally assisted reversal times in spin-torque driven nanomagnets,"Driving nanomagnets by spin-polarized currents offers exciting prospects in
magnetoelectronics, but the response of the magnets to such currents remains
poorly understood. We show that an averaged equation describing the diffusion
of energy on a graph captures the low-damping dynamics of these systems. From
this equation we obtain the bifurcation diagram of the magnets, including the
critical currents to induce stable precessional states and magnetization
switching, as well as the mean times of thermally assisted magnetization
reversal in situations where the standard reaction rate theory of Kramers is no
longer valid. These results match experimental observations and give a
theoretical basis for a N\'eel-Brown-type formula with an effective energy
barrier for the reversal times.",1210.6253v3
2012-11-26,Low temperature dynamic freezing and the fragility of ordering in Tb2Sn2O7,"We have probed the low temperature magnetic behavior of the ordered spin ice
material Tb2Sn2O7 through ac magnetic susceptibility measurements of both the
pure material and samples with small percentages of Ti substituted on the Sn
sublattice. We observe a clear signature for the previously reported ordering
transition at TC = 850 mK, as well as evidence for dynamic freezing at
temperatures well below TC, confirming the persistence of significant magnetic
fluctuations deep in the spin-ordered regime. The long range ordering
transition was completely suppressed with as little as 5% Ti for Sn
substitution, and 10% Ti substitution resulted in a spin-glass-like spin
freezing transition near 250 mK. The results demonstrate that the long range
magnetic ordering is surprisingly fragile in this system.",1211.6047v1
2012-12-13,Real-time MOKE microscopy made simple,"We present a simple and effective instrument for simultaneous real-time
imaging and hysteresis of the anisotropic magnetic domain dynamics in thin
films using the Magneto-Optical Kerr Effect (MOKE). We furthermore illustrate
that magnetic imaging allows a more accurate interpretation of the
magnetization reversal processes than the conventional hysteresis
characterization. In particular, we present a case where the onset of a
double-step reversal observed in imaging remains invisible in the spatially
integrated hysteresis loops. When complemented by precise tuning of the
external magnetic field orientation, our system reveals the singular
anisotropic variations of the domain dynamics near the hard-axis in epitaxial
thin films, thus shedding light on the reported, but as yet unexplained, hard
axis coercivity behavior.",1212.3259v1
2012-12-31,Forced MHD turbulence in three dimensions using Taylor-Green symmetries,"We examine the scaling laws of MHD turbulence for three different types of
forcing functions and imposing at all times the four-fold symmetries of the
Taylor-Green (TG) vortex generalized to MHD; no uniform magnetic field is
present and the magnetic Prandtl number is equal to unity. We also include a
forcing in the induction equation, and we take the three configurations studied
in the decaying case in [E. Lee et al. Phys. Rev.E {\bf 81}, 016318 (2010)]. To
that effect, we employ direct numerical simulations up to an equivalent
resolution of $2048^3$ grid points. We find that, similarly to the case when
the forcing is absent, different spectral indices for the total energy spectrum
emerge, corresponding to either a Kolmogorov law, an Iroshnikov-Kraichnan law
that arises from the interactions of turbulent eddies and Alfv\'en waves, or to
weak turbulence when the large-scale magnetic field is strong. We also examine
the inertial range dynamics in terms of the ratios of kinetic to magnetic
energy, and of the turn-over time to the Alfv\'en time, and analyze the
temporal variations of these quasi-equilibria.",1212.6902v2
2013-02-01,Vortex mechanics in planar nano-magnets,"A collective-variable approach for the study of non-linear dynamics of
magnetic textures in planar nano-magnets is proposed. The variables are just
arbitrary parameters (complex or real) in the specified analytical function of
a complex variable, describing the texture in motion. Starting with such a
function, a formal procedure is outlined, allowing a (non-linear) system of
differential equations of motion to be obtained for the variables. The
resulting equations are equivalent to Landau-Lifshitz-Gilbert dynamics as far
as the definition of collective variables allows it. Apart from the
collective-variable specification, the procedure does not involve any
additional assumptions (such as translational invariance or steady-state
motion). As an example, the equations of weakly non-linear motion of a magnetic
vortex are derived and solved analytically. A simple formula for the dependence
of the vortex precession frequency on its amplitude is derived. The results are
verified against special cases from the literature and agree quantitatively
with experiments and simulations.",1302.0222v3
2013-03-02,Two-band effect on the vortex dynamics and critical current density in an anisotropic MgB$_2$ thin film,"We report the influence of intrinsic superconducting parameters on the vortex
dynamics and the critical current densities of a MgB$_2$ thin film. The small
magnetic penetration depth of \lambda = 50 nm at T=4 K is related to a clean
\pi-band, and transport and magnetization data show an upper critical field
similar to those reported in clean single crystals. We find a high self-field
critical current density Jc, which is strongly reduced with applied magnetic
field, and attribute this to suppression of the superconductivity in the
\pi-band. The temperature dependence of the creep rate S(T) at low magnetic
field can be explained by a simple Anderson-Kim mechanism. The system shows
high pinning energies at low field that are strongly suppressed by high field,
which is consistent with a two-band contribution.",1303.0352v1
2013-05-16,Topics in microphysics of relativistic plasmas,"Astrophysical plasmas can have parameters vastly different from the more
studied laboratory and space plasmas. In particular, the magnetic fields can be
the dominant component of the plasma, with energy-density exceeding the
particle rest-mass energy density. Magnetic fields then determine the plasma
dynamical evolution, energy dissipation and acceleration of non-thermal
particles. Recent data coming from astrophysical high energy missions, like
magnetar bursts and Crab nebula flares, point to the importance of magnetic
reconnection in these objects.
  In this review we outline a broad spectrum of problems related to the
astrophysical relevant processes in magnetically dominated relativistic
plasmas. We discuss the problems of large scale dynamics of relativistic
plasmas, relativistic reconnection and particle acceleration at reconnecting
layers, turbulent cascade in force-fee plasmas. A number of astrophysical
applications are also discussed.",1305.3838v1
2013-06-12,Non-equilibrium quantum magnetism in a dipolar lattice gas,"Research on quantum magnetism with ultra-cold gases in optical lattices is
expected to open fascinating perspectives for the understanding of fundamental
problems in condensed-matter physics. Here we report on the first realization
of quantum magnetism using a degenerate dipolar gas in an optical lattice. In
contrast to their non-dipolar counterparts, dipolar lattice gases allow for
inter-site spin-spin interactions without relying on super-exchange energies,
which constitutes a great advantage for the study of spin lattice models. In
this paper we show that a chromium gas in a 3D lattice realizes a lattice model
resembling the celebrated t-J model, which is characterized by a
non-equilibrium spinor dynamics resulting from inter-site Heisenberg-like
spin-spin interactions provided by non-local dipole-dipole interactions.
Moreover, due to its large spin, chromium lattice gases constitute an excellent
environment for the study of quantum magnetism of high-spin systems, as
illustrated by the complex spin dynamics observed for doubly-occupied sites.",1306.2754v2
2013-07-23,Engineering Weyl nodes in Dirac semimetals by a magnetic field,"We study the phase diagram of a Dirac semimetal in a magnetic field at a
nonzero charge density. It is shown that there exists a critical value of the
chemical potential at which a first-order phase transition takes place. At
subcritical values of the chemical potential the ground state is a gapped state
with a dynamically generated Dirac mass and a broken chiral symmetry. The
supercritical phase is the normal (gapless) phase with a nontrivial chiral
structure: it is a Weyl semimetal with a pair of Weyl nodes for each of the
original Dirac points. The nodes are separated by a dynamically induced chiral
shift. The direction of the chiral shift coincides with that of the magnetic
field and its magnitude is determined by the quasiparticle charge density, the
strength of the magnetic field, and the strength of the interaction. The
rearrangement of the Fermi surface accompanying this phase transition is
described.",1307.6230v2
2013-07-25,Geometrical crossover in two-body systems in a magnetic field,"An algebraic approach is formulated in the harmonic approximation to describe
a dynamics of two-fermion systems, confined in three-dimensional axially
symmetric parabolic potential, in an external magnetic field. The fermion
interaction is considered in the form U_(M)(r)= alpha_(M)r^{-M} (alpha_(M)>0,
M>0). The formalism of a semi-simple Lie group is applied to analyse symmetries
of the considered system. Explicit algebraic expressions are derived in terms
of system's parameters and the magnetic field strength to trace the evolution
of the equilibrium shape. It is predicted that the interplay of classical and
quantum correlations may lead to a quantum shape transition from a lateral to a
vertical localization of fermions in the confined system. The analytical
results demonstrate a good agreement with numerical results for two-electron
quantum dots in the magnetic field, when classical correlations dominate in the
dynamics.",1307.6833v2
2013-09-14,Electric-field induced domain-wall dynamics: depinning and chirality switching,"We theoretically study the equilibrium and dynamic properties of nanoscale
magnetic tunnel junctions (MTJs) and magnetic wires, in which an electric field
controls the magnetic anisotropy through spin-orbit coupling. By performing
micromagnetic simulations, we construct a rich phase diagram and find that, in
particular, the equilibrium magnetic textures can be tuned between Neel and
Bloch domain walls in an elliptical MTJ. Furthermore, we develop a
phenomenological model of a quasi-one-dimensional domain wall confined by a
parabolic potential and show that, near the Neel-to-Bloch-wall transition, a
pulsed electric field induces precessional domain-wall motion which can be used
to reverse the chirality of a Neel wall and even depin it. This domain-wall
motion controlled by electric fields, in lieu of applied current, may provide a
model for ultra-low-power domain-wall memory and logic devices.",1309.3693v1
2013-10-09,Chargeless spin current for switching and coupling of domain walls in magnetic nanowires,"The demonstration of the generation and control of a pure spin current
(without net charge flow) by electric fields and/or temperature gradient has
been an essential leap in the quest for low-power consumption electronics. The
key issue of whether and how such a current can be utilized to drive and
control information stored in magnetic domain walls (DWs) is still outstanding
and is addressed here. We demonstrate that pure spin current acts on DWs in a
magnetic stripe with an effective spin-transfer torque resulting in a mutual
DWs separation dynamics and picosecond magnetization reversal. In addition,
long-range ($\sim$ mm) antiferromagnetic DWs coupling emerges. If one DW is
pinned by geometric constriction, the spin current induces a dynamical spin
orbital interaction that triggers an internal electric field determined by
$\vec{E} \sim \hat{e}_{x} \cdot (\vec{n}_{1} \times \vec{n}_{2})$ where
$\vec{n}_{1/2}$ are the effective DWs orientations and $\hat{e}_{x} $ is their
spatial separation vector. This leads to charge accumulation or persistent
electric current in the wire. As DWs are routinely realizable and tuneable, the
predicted effects bear genuine potential for power-saving spintronics devices.",1310.2331v1
2013-12-05,Zero modes in magnetic systems: general theory and an efficient computational scheme,"The presence of topological defects in magnetic media often leads to normal
modes with zero frequency (zero modes). Such modes are crucial for long-time
behavior, describing, for example, the motion of a domain wall as a whole.
Conventional numerical methods to calculate the spin-wave spectrum in magnetic
media are either inefficient or they fail for systems with zero modes. We
present a new efficient computational scheme that reduces the magnetic
normal-mode problem to a generalized Hermitian eigenvalue problem also in the
presence of zero modes. We apply our scheme to several examples, including
two-dimensional domain walls and Skyrmions, and show how the effective masses
that determine the dynamics can be calculated directly. These systems highlight
the fundamental distinction between the two types of zero modes that can occur
in spin systems, which we call special and inertial zero modes. Our method is
suitable for both conservative and dissipative systems. For the latter case, we
present a perturbative scheme to take into account damping, which can also be
used to calculate dynamical susceptibilities.",1312.1503v1
2013-12-19,Deconfinement transition in two-flavour lattice QCD with dynamical overlap fermions in an external magnetic field,"We study the influence of an external magnetic field on the deconfinement
transition in two-flavour lattice QCD with physical quark charges. We use
dynamical overlap fermions without any approximation such as fixed topology and
perform simulations on a $16^3 \times 6$ lattice and at a pion mass around
$500MeV$. The pion mass (as well as the lattice spacing) was determined in
independent runs on $12^3 \times 24$ lattices. We consider two temperatures,
one of which is close to the deconfinement transition and one which is above.
Within our limited statistics the dependence of the Polyakov loop and chiral
condensate on the magnetic field supports the ""inverse magnetic catalysis""
scenario in which the transition temperature decreases as the field strength
grows for temperature not to far above the critical temperature.",1312.5628v2
2013-12-24,Two driven coupled qubits in a time varying magnetic field: exact and approximate solutions,"We study the dynamics of a two-qubit system coupled through time dependent
anisotropic $XYZ$ Heisenberg interaction in presence of a time varying
non-uniform external magnetic field. Exact results are presented for the time
evolution of the system under certain integrability conditions. Furthermore,
the corresponding entanglement of the system is studied for different values of
the involved parameters. We found that the time evolution and different
properties of entanglement such as amplitude, frequency and profile can be
finely tuned by the interplay of the characteristics of the time-dependent
magnetic field and exchange coupling. Also we show how the discrete symmetries
of the system Hamiltonian, which splits its Hilbert space into two distinct
subspaces, can be utilized to deduce the dynamics in one of its two distinct
subspaces from the other one. Moreover, approximate results for the time
evolution are provided utilizing the rotating wave as well as the perturbation
approximations for the special case of either static magnetic field or exchange
coupling. We compare the range of validity of the two approximation methods and
their effectiveness in treating the considered system and determine their
critical parameters.",1312.6883v1
2014-01-17,Exact scattering matrix of graphs in magnetic field and quantum noise,"We consider arbitrary quantum wire networks modelled by finite, noncompact,
connected quantum graphs in the presence of an external magnetic field.
  We find a general formula for the total scattering matrix of the network in
terms of its local scattering properties and its metric structure. This is
applied to a quantum ring with $N$ external edges. Connecting the external
edges of the ring to heat reservoirs, we study the quantum transport on the
graph in ambient magnetic field. We consider two types of dynamics on the ring:
the free Schr\""odinger and the free massless Dirac equations. For each case, a
detailed study of the thermal noise is performed analytically.
  Interestingly enough, in presence of a magnetic field, the standard linear
Johnson-Nyquist law for the low temperature behaviour of the thermal noise
becomes nonlinear. The precise regime of validity of this effect is discussed
and a typical signature of the underlying dynamics is observed.",1401.4967v1
2014-04-17,Wave packet dynamics in monolayer MoS$_2$ with and without a magnetic field,"We study the dynamics of electrons in monolayer Molybdenum Disulfide
(MoS$_2$), in the absence as well as presence of a transverse magnetic field.
Considering the initial electronic wave function to be a Gaussian wave packet,
we calculate the time dependent expectation value of position and velocity
operators. In the absence of the magnetic field, the time dependent average
values of position and velocity show damped oscillations dependent on the width
of the wave packet. In the presence of a transverse magnetic field, the wave
packet amplitude shows oscillatory behaviour over short timescales associated
with classical cyclotron orbit, followed by the phenomena of spontaneous
collapse and revival over larger timescales. We relate the timescales of these
effects and our results can be useful for the interpretation of experiments
with trapped ions.",1404.4534v2
2014-05-22,Giant magnetoelectric effect in magnetic tunnel junctions coupled to an electromagnetic environment,"We study the magnetization dynamics in
ferromagnet$\mid$insulator$\mid$ferromagnet and
ferromagnet$\mid$insulator$\mid$normal metal ultra-small tunnel junctions, and
the associated voltage drop in the presence of an electromagnetic environment
assisting the tunneling processes. We show that the environment strongly
affects the resulting voltage, which becomes a highly non-linear function of
the precession cone angle $\theta$. We find that voltages comparable to the
driving frequency $\omega$ can be reached even for small precession cone angles
$\theta$, in stark contrast to the case where the environment is absent. Such
an effect could be useful for the detection local magnetization precessions in
textured ferromagnets or, conversely, for probing the environment via the
magnetization dynamics.",1405.5744v2
2014-05-26,Structural and Magnetic Dynamics in the Magnetic Shape Memory Alloy Ni$_2$MnGa,"Magnetic shape memory Heusler alloys are multiferroics stabilized by the
correlations between electronic, magnetic and structural order. To study these
correlations we use time resolved x-ray diffraction and magneto-optical Kerr
effect experiments to measure the laser induced dynamics in a Heusler alloy
Ni$_2$MnGa film and reveal a set of timescales intrinsic to the system. We
observe a coherent phonon which we identify as the amplitudon of the modulated
structure and an ultrafast phase transition leading to a quenching of the
incommensurate modulation within 300~fs with a recovery time of a few ps. The
thermally driven martensitic transition to the high temperature cubic phase
proceeds via nucleation within a few ps and domain growth limited by the speed
of sound. The demagnetization time is 320~fs, which is comparable to the
quenching of the structural modulation.",1405.6534v1
2014-10-16,Nonlinear dynamics of Josephson vortices in a film screen under dc and ac magnetic fields,"We present detailed numerical simulations of Josephson vortices in a long
Josephson junction perpendicular to a thin film screen under strong dc and ac
magnetic fields. By solving the sine-Gordon equation, we calculated the
threshold magnetic field for penetration of fluxons as a function of frequency,
and the power dissipated by oscillating fluxons as functions of the ac field
amplitude and frequency. We considered the effects of superimposed ac and dc
fields, and a bi-harmonic magnetic field resulting in a vortex ratchet
dynamics. The results were used to evaluate the contribution of weak-linked
grain boundaries to the nonlinear surface resistance of polycrystalline
superconductors under strong electromagnetic fields, particularly thin film
screens and resonator cavities.",1410.4606v1
2014-11-03,Curvature effects in statics and dynamics of low dimensional magnets,"We develop an approach to treat magnetic energy of a ferromagnet for
arbitrary curved wires and shells on the assumption that the anisotropy
contribution much exceeds the dipolar and other weak interactions. We show that
the curvature induces two effective magnetic interactions: effective magnetic
anisotropy and effective Dzyaloshinskii-like interaction. We derive an equation
of magnetisation dynamics and propose a general static solution for the limit
case of strong anisotropy. To illustrate our approach we consider the
magnetisation structure in a ring wire and a cone surface: ground states in
both systems essentially depend on the curvature excluding strictly tangential
solutions even in the case of strong anisotropy. We derive also the spectrum of
spin waves in such systems.",1411.0646v2
2014-11-12,Quantum magnetism and topological ordering via enhanced Rydberg-dressing near Förster-resonances,"We devise a cold-atom approach to realizing a broad range of bi-linear
quantum magnets. Our scheme is based on off-resonant single-photon excitation
of Rydberg $P$-states (Rydberg-dressing), whose strong interactions are shown
to yield controllable XYZ-interactions between effective spins, represented by
different atomic ground states. The distinctive features of F\""orster-resonant
Rydberg atom interactions are exploited to enhance the effectiveness of
Rydberg-dressing and, thereby, yield large spin-interactions that greatly
exceed corresponding decoherence rates. We illustrate the concept on a spin-1
chain implemented with cold Rubidium atoms, and demonstrate that this permits
the dynamical preparation of topological magnetic phases. Generally, the
described approach provides a viable route to exploring quantum magnetism with
dynamically tuneable (an)isotropic interactions as well as variable space- and
spin-dimensions in cold-atom experiments.",1411.3118v2
2014-11-19,Thermally driven classical Heisenberg chain with a spatially varying magnetic field: Thermal rectification and Negative differential thermal resistance,"Thermal rectification and negative differential thermal resistance are two
important features that have direct technological relevance. In this paper, we
study the classical one dimensional Heisenberg model, thermally driven by heat
baths attached at the two ends of the system, and in presence of an external
magnetic field that varies monotonically in space. Heat conduction in this
system is studied using a local energy conserving dynamics. It is found that,
by suitably tuning the spatially varying magnetic field, the homogeneous
symmetric system exhibits both thermal rectification and negative differential
thermal resistance. Thermal rectification, in some parameter ranges, shows
interesting dependences on the average temperature T and the system size N -
rectification improves as T and N is increased. Using the microscopic dynamics
of the spins we present a physical picture to explain the features observed in
rectification as exhibited by this system and provide supporting numerical
evidences. Emergence of NDTR in this system can be controlled by tuning the
external magnetic field alone which can have possible applications in the
fabrication of thermal devices.",1411.5200v1
2015-02-20,Magneto-Plasmonics and Resonant Interaction of Light with Dynamic Magnetisation in Metallic and All-Dielectric Nanostructures (Review),"A significant interest in combining plasmonics and magnetism at the nanoscale
gains momentum in both photonics and magnetism sectors that are concerned with
the resonant enhancement of light-magnetic-matter interaction in
nanostructures. These efforts result in a considerable amount of literature,
which is difficult to collect and digest in limited time. Furthermore, there is
insufficient exchange of results between the two research sectors.
Consequently, the goal of this review paper is to bridge this gap by presenting
an overview of recent progress in the field of magneto-plasmonics from two
different points of view: magneto-plasmonics, and magnonics and magnetisation
dynamics. It is expected that this presentation style will make this review
paper of particular interest to both general physical audience and specialists
conducting research on photonics, plasmonics, Brillouin light scattering
spectroscopy of magnetic nanostructures and magneto-optical Kerr effect
magnetometry, as well as ultrafast all-optical and THz-wave excitation of spin
waves. Moreover, readers interested in a new, rapidly emerging field of
all-dielectric nanophotonics will find a section about all-magneto-dielectric
nanostructures.",1502.05783v1
2015-02-25,Magnetic Flux Dynamics in Horizontally Cooled Superconducting Cavities,"Previous studies on magnetic flux expulsion as a function of cooling details
have been performed for superconducting niobium cavities with the cavity beam
axis placed parallel respect to the helium cooling flow, and findings showed
that for sufficient cooling thermogradients all magnetic flux could be expelled
and very low residual resistance could be achieved. In this paper we
investigate the flux trapping and its impact on radio frequency surface
resistance when the resonators are positioned perpendicularly to the helium
cooling flow, which is representative of how superconducting radio-frequency
(SRF) cavities are cooled in an accelerator. We also extend the studies to
different directions of applied magnetic field surrounding the resonator.
Results show that in the cavity horizontal configuration there is a different
impact of the various field components on the final surface resistance, and
that several parameters have to be considered to understand flux dynamics. A
newly discovered phenomenon of concentration of flux lines at the cavity top
leading to cavity equator temperature rise is presented.",1502.07291v3
2015-03-10,Microwave-induced dynamic switching of magnetic skyrmion cores in nanodots,"The nonlinear dynamic behavior of a magnetic skyrmion in circular nanodots
was studied numerically by solving the Landau-Lifshitz-Gilbert equation with a
classical spin model. We show that a skyrmion core reversal can be achieved
within nanoseconds using a perpendicular oscillating magnetic field. Two
symmetric switching processes that correspond to excitations of the breathing
mode and the mixed mode (combination of the breathing mode and a radial
spin-wave mode) are identified. For excitation of the breathing mode, the
skyrmion core switches through nucleation of a new core from a transient
uniform state. In the mixed mode, the skyrmion core reverses with the help of
spins excited both at the edge and core regions. Unlike the magnetic vortex
core reversal, the excitation of radial spin waves does not dominate the
skyrmion core reversal process.",1503.02869v1
2015-04-30,Optical diode effect in the room-temperature multiferroic BiFeO$_3,"Multiferroics permit the magnetic control of the electric polarization and
electric control of the magnetization. These static magnetoelectric (ME)
effects are of enormous interest: The ability to read and write a magnetic
state current-free by an electric voltage would provide a huge technological
advantage. Dynamic or optical ME effects are equally interesting because they
give rise to unidirectional light propagation as recently observed in
low-temperature multiferroics. This phenomenon, if realized at room
temperature, would allow the development of optical diodes which transmit
unpolarized light in one, but not in the opposite direction. Here, we report
strong unidirectional transmission in the room-temperature multiferroic
BiFeO$_3$ over the gigahertz--terahertz frequency range. Supporting theory
attributes the observed unidirectional transmission to the spin-current driven
dynamic ME effect. These findings are an important step toward the realization
of optical diodes, supplemented by the ability to switch the transmission
direction with a magnetic or electric field.",1505.00033v1
2015-06-08,Demagnetization dynamics of non-interacting trapped fermions,"Motivated by several experimental efforts to understand spin diffusion and
transport in ultracold fermionic gases, we study the spin dynamics of initially
spin-polarized ensembles of harmonically trapped non-interacting spin-1/2
fermionic atoms, subjected to a magnetic field gradient. We obtain simple
analytic expressions for spin observables in the presence of both constant and
linear magnetic field gradients, with and without a spin-echo pulse, and at
zero and finite temperatures. The analysis shows the relevance of spin-motional
coupling in the non-interacting regime where the demagnetization decay rate at
short times can be faster than the experimentally measured rates in the
strongly interacting regime under similar trapping conditions. Our calculations
also show that particle motion limits the ability of a spin-echo pulse to
remove the effect of magnetic field inhomogeneity, and that a spin-echo pulse
can instead lead to an increased decay of magnetization at times comparable to
the trapping period.",1506.02353v1
2015-06-15,Detection of topological states in two-dimensional Dirac systems by the dynamic spin susceptibility,"We discuss dynamic spin susceptibility (DSS) in two-dimensional (2D) Dirac
electrons with spin-orbit interactions to characterize topological insulators.
The imaginary part of the DSS appears as an absorption rate in response to a
transverse ac magnetic field, just as in an electron spin resonance experiment
for localized spin systems. We found that when the system is in a static
magnetic field, the topological state can be identified by an anomalous
resonant peak of the imaginary part of the DSS as a function of the frequency
of the transverse magnetic field $\omega$. In the absence of a static magnetic
field, the imaginary part of the DSS becomes a continuous function of $\omega$
with a threshold frequency $\omega_{\rm c}$. In this case, the topological and
the trivial phases can also be distinguished by the values of $\omega_{\rm c}$
and by the line shapes. Thus the DSS is an experimentally observable physical
quantity to characterize a topological insulator directly from bulk properties,
without observing a topological transition.",1506.04691v2
2015-09-11,Comparison between a quantum kinetic theory of spin transfer dynamics in Mn doped bulk semiconductors and its Markov limit for non-zero Mn magnetization,"We investigate the transfer between carrier and Mn spins due to the
s-d-exchange interaction in a Mn doped bulk semiconductor within a microscopic
quantum kinetic theory. We demonstrate that the spin transfer dynamics is
qualitatively different for components of the carrier spin parallel and
perpendicular to the Mn magnetization. From our quantum kinetic equations we
have worked out the corresponding Markov limit which is equivalent to rate
equations based on Fermi's golden rule. The resulting equations resemble the
widely used Landau-Lifshitz-Gilbert-equations, but also describe genuine spin
transfer due to quantum corrections. Although it is known that the Markovian
rate description works well for bulk systems when the initial Mn magnetization
is zero, we find large qualitative deviations from the full quantum kinetic
theory for finite initial Mn magnetizations. These deviations mainly reflect
corrections of higher than leading order in the interaction which are not
accounted for in golden rule-type rates.",1509.03479v1
2015-10-02,Evidence for unidimensional low-energy excitations as the origin of persistent spin dynamics in geometrically frustrated magnets,"We report specific heat, magnetic, and muon spin relaxation measurements
performed on a polycrystalline sample of the normal spinel CdHo2S4. The
rare-earth ions sit on a lattice of corner-sharing regular tetrahedra as in
pyrochlore compounds. Magnetic ordering is detected at Tc ~ 0.87 K. From
spin-lattice relaxation rate measurements on both sides of Tc we uncover
similar magnetic excitation modes driving the so-called persistent spin
dynamics at T < Tc. Unidimensional excitations are argued to be at its origin.
Often observed spin loop structures are suggested to support these excitations.
The possibility of a generic mechanism for their existence is discussed.",1510.00544v1
2015-10-20,Nonaxisymmetric linear instability of cylindrical magnetohydrodynamic Taylor-Couette flow,"We consider the nonaxisymmetric modes of instability present in
Taylor-Couette flow under the application of helical magnetic fields, mainly
for magnetic Prandtl numbers close to the inductionless limit, and conduct a
full examination of marginal stability in the resulting parameter space. We
allow for the azimuthal magnetic field to be generated by a combination of
currents in the inner cylinder and fluid itself, and introduce a parameter
governing the relation between the strength of these currents. A set of
governing eigenvalue equations for the nonaxisymmetric modes of instability are
derived and solved by spectral collocation with Chebyshev polynomials over the
relevant parameter space, with the resulting instabilities examined in detail.
We find that by altering the azimuthal magnetic field profiles the azimuthal
magnetorotational instability, nonaxisymmetric helical magnetorotational
instability, and Tayler instability yield interesting dynamics, such as
different preferred mode types, and modes with azimuthal wave number $m>1$.
Finally, a comparison is given to the recent WKB analysis performed by Kirillov
$et$ $al$. [Kirillov, Stefani, and Fukumoto, J. Fluid Mech. 760, 591 (2014)]
and its validity in the linear regime.",1510.05932v1
2015-11-25,Three-dimensional magnetic cloak working from DC to 250 kHz,"Invisible cloaking is one of major outcomes of the metamaterial research, but
the practical potential, in particular for high frequencies (e.g., microwave to
visible light), is fatally challenged by the complex material properties they
usually demand. On the other hand, it will be advantageous and also
technologically instrumental to design cloaking devices for applications at low
frequencies where electromagnetic components are favorably uncoupled. In this
work, we vastly develop the bilayer approach to create a three-dimensional
magnetic cloak able to work in both static and dynamic fields. Under the
quasi-static approximation, we demonstrate a perfect magnetic cloaking device
with a large frequency band from zero to 250 kHz. The practical potential of
our device is experimentally verified by using a commercial metal detector,
which may lead us to having a real cloaking application where the dynamic
magnetic field can be manipulated in desired ways.",1511.08005v1
2015-12-01,Competition of static magnetic and dynamic photon forces in electronic transport through a quantum dot,"We investigate theoretically the balance of the static magnetic and the
dynamical photon forces in the electron transport through a quantum dot in a
photon cavity with a single photon mode. The quantum dot system is connected to
external leads and the total system is exposed to a static perpendicular
magnetic field. We explore the transport characteristics through the system by
tuning the ratio, $\hbar\omega_{\gamma} / \hbar\omega_c$, between the photon
energy, $\hbar\omega_{\gamma}$, and the cyclotron energy, $\hbar\omega_c$.
Enhancement in the electron transport with increasing electron-photon coupling
is observed when $\hbar\omega_{\gamma} / \hbar\omega_c > 1$. In this case the
photon field dominates and stretches the electron charge distribution in the
quantum dot, extending it towards the contacts area for the leads. Suppression
in the electron transport is found when $\hbar\omega_{\gamma} / \hbar\omega_c <
1$, as the external magnetic field causes circular confinement of the charge
density around the dot.",1512.00392v1
2016-01-07,Driving Skyrmions on a Composite Multiferroic Lattice,"Magnetic skyrmions and multiferroics are the most interesting objects in
nanostructure science that have great potential in future spin-electronic
technology. The study of the multiferroic skyrmions has attracted much interest
in recent years. This paper reports on the magnetic skyrmions induced by an
electric driving field on a composite multiferroic lattice. By using the spin
dynamics method, we use a classical magnetic spin model and an electric
pseudospin model, which are coupled by a strong magnetoelectric coupling in the
dynamical simulations. Interestingly, we observe some skyrmion-like objects on
the electric component during the switching process, which is due to the
connection between the electric and the magnetic structures.",1601.01383v1
2016-02-03,Dynamic Error in Strain Induced Magnetization Reversal of Nanomagnets due to Incoherent Switching and Formation of Metastable States: A Size-dependent Study,"Modulation of stress anisotropy of magnetostrictive nanomagnets with strain
offers an extremely energy-efficient method of magnetization reversal. The
reversal process, however, is often incoherent and hence error-prone in the
presence of thermal noise at room temperature. Occurrence of incoherent
metastable states in the potential landscape of the nanomagnet can further
exacerbate the error. Stochastic micromagnetic simulations at room temperature
are used to understand and calculate energy dissipations and switching error
probabilities in this important magnetization switching methodology. We find
that these quantities have an intriguing dependence on nanomagnet size: small
nanomagnets perform better owing to the fact that they are more resilient to
the formation of metastable states and magnetization dynamics in them is more
coherent. However, for a fixed stress anisotropy energy density, smaller
nanomagnets will also have poorer resilience against thermal instability. Thus,
the challenge in straintronics is to maximize the stress anisotropy energy
density by developing materials and processes that yield the largest
magnetostriction.",1602.01491v2
2016-02-12,Mass Effect on Axial Charge Dynamics,"We studied effect of finite quark mass on the dynamics of axial charge using
the D3/D7 model in holography. The mass term in axial anomaly equation affects
both the fluctuation (generation) and dissipation of axial charge. We studied
the dependence of the effect on quark mass and external magnetic field. For
axial charge generation, we calculated the mass diffusion rate, which
characterizes the helicity flipping rate. The rate is a non-monotonous function
of mass and can be significantly enhanced by the magnetic field. The diffusive
behavior is also related to a divergent susceptibility of axial charge. For
axial charge dissipation, we found that in the long time limit, the mass term
dissipates all the charge effectively generated by parallel electric and
magnetic fields. The result is consistent with a relaxation time approximation.
The rate of dissipation through mass term is a monotonous increasing function
of both quark mass and magnetic field.",1602.03952v2
2016-02-25,Orbital magnetism of ultracold fermionic gases in a lattice: dynamical mean-field approach,"We study finite-temperature properties of ultracold four-component mixtures
of alkaline-earth-like atoms in optical lattices that can be effectively
described by the two-band spin-$1/2$ Hubbard model including the Hund's
exchange coupling term. Our main goal is to investigate the effect of exchange
interactions on finite-temperature magnetic phases for a wide range of lattice
fillings. We use the dynamical mean-field theory approach and its real-space
generalization to obtain finite-temperature phase diagrams including
transitions to magnetically-ordered phases. It allows to determine optimal
experimental regimes for approaching long-range ferromagnetic ordering in
ultracold gases. We also calculate the entropy in the vicinity of
magnetically-ordered phases, which provides quantitative predictions for
on-going and future experiments aiming at approaching and studying long-range
ordered states in optical lattices.",1602.08031v2
2016-04-05,Domain wall-gated skyrmion logic,"Skyrmions and domain walls are typical spin textures of significant
technological relevance to magnetic memory and logic applications, where they
are used as carriers of information. The unique topology of skyrmions makes
them to display distinct dynamical properties compared to domain walls. Some
studies have demonstrated that the two topologically inequivalent magnetic
objects could be interconverted by cleverly designed geometric structures.
Here, we numerically address the skyrmion-domain wall collision in a magnetic
racetrack by introducing relative motion between the two objects based on a
specially designed junction. An electric current serves as the driving force
that moves a skyrmion toward a trapped domain-wall pair. We observe different
types of collision dynamics by changing the driving parameters. Most
importantly, the domain wall modulation of skyrmion transport is realized in
this system, allowing a set of domain wall-gated logical NOT, NAND, and NOR
gates to be constructed. By providing a promising logic architecture that is
fully compatible with racetrack memories, this work is expected to speed up the
development of skyrmion-based magnetic computation.",1604.01310v1
2016-04-22,First evidence for a Sm$^{3+}$-type contribution to the magnetic form factor in the quasielastic spectral response of intermediate valence SmB$_{6}$,"The momentum transfer dependence of the magnetic form factor associated with
the quasielastic spectral component in the dynamic magnetic response of
intermediate valence SmB$_{6}$ has been measured using inelastic neutron
scattering on a double-isotope ($^{154}$Sm, $^{11}$B) single crystal. The
experimental dependence differs qualitatively from those obtained earlier for
the inelastic signals, as well as from the field-induced magnetic form factor
of SmB$_{6}$ obtained by polarized neutron diffraction. This observation is
interpreted by specifically considering the Curie-type contributions to the
dynamic susceptibility, which arise from the mixing of 4f$^{5}$ and 4f$^{6}$
J-multiplets into the intermediate valence state wave function.",1604.06717v1
2016-05-07,Persistence of slow dynamics in Tb(OETAP)$_2$ single molecule magnets embedded in conducting polymers,"The spin dynamics of Tb(OETAP)$_2$ single ion magnets was investigated by
means of muon spin resonance ($\mu$SR) both in the bulk material as well as
when the system is embedded into PEDOT:PSS polymer conductor. The
characteristic spin fluctuation time is characterized by a high temperature
activated trend, with an energy barrier around 320 K, and by a low temperature
tunneling regime. When the single ion magnet is embedded into the polymer the
energy barrier only slightly decreases and the fluctuation time remains of the
same order of magnitude even at low temperature. This finding shows that these
single molecule magnets preserve their characteristics which, if combined with
those of the conducting polymer, result in a hybrid material of potential
interest for organic spintronics.",1605.02148v1
2016-05-27,A reduced model for precessional switching of thin-film nanomagnets under the influence of spin-torque,"We study the magnetization dynamics of thin-film magnetic elements with
in-plane magnetization subject to a spin-current flowing perpendicular to the
film plane. We derive a reduced partial differential equation for the in-plane
magnetization angle in a weakly damped regime. We then apply this model to
study the experimentally relevant problem of switching of an elliptical element
when the spin-polarization has a component perpendicular to the film plane,
restricting the reduced model to a macrospin approximation. The macrospin
ordinary differential equation is treated analytically as a weakly damped
Hamiltonian system, and an orbit-averaging method is used to understand
transitions in solution behaviors in terms of a discrete dynamical system. The
predictions of our reduced model are compared to those of the full
Landau--Lifshitz--Gilbert--Slonczewski equation for a macrospin.",1605.08698v1
2016-05-31,Electric field control of magnon-induced magnetization dynamics in multiferroics,"We consider theoretically the effect of an inhomogeneous magnetoelectric
coupling on the magnon-induced dynamics of a ferromagnet. The magnon-mediated
magnetoelectric torque affects both the homogeneous magnetization and
magnon-driven domain wall motion. In the domains, we predict a reorientation of
the magnetization, controllable by the applied electric field, which is almost
an order of magnitude larger than that observed in other physical systems via
the same mechanism. The applied electric field can also be used to tune the
domain wall speed and direction of motion in a linear fashion, producing domain
wall velocities several times the zero field velocity. These results show that
multiferroic systems offer a promising arena to achieve low-dissipation
magnetization rotation and domain wall motion by exciting spin-waves.",1606.00007v2
2016-06-13,Dynamo generated by the centrifugal instability,"We present a new scenario for magnetic field amplification where an
electrically conducting fluid is confined in a differentially rotating,
spherical shell with thin aspect-ratio. When the angular momentum sufficiently
decreases outwards, an hydrodynamic instability develops in the equatorial
region, characterised by pairs of counter-rotating toroidal vortices similar to
those observed in cylindrical Couette flow. These spherical Taylor-Couette
vortices generate a subcritical dynamo magnetic field dominated by
non-axisymmetric components. We show that the critical magnetic Reynolds number
seems to reach a constant value at large Reynolds number and that the global
rotation can strongly decrease the dynamo onset. Our numerical results are
understood within the framework of a simple dynamical system, and we propose a
low-dimensional model for subcritical dynamo bifurcations. Implications for
both laboratory dynamos and astrophysical magnetic fields are finally
discussed.",1606.04078v1
2016-06-29,"Kinetics of local $""$magnetic$""$ moment and non-stationary spin-polarized current in the single impurity Anderson-model","We perform theoretical investigation of the localized state dynamics in the
presence of interaction with the reservoir and Coulomb correlations. We analyze
kinetic equations for electron occupation numbers with different spins taking
into account high order correlation functions for the localized electrons. We
reveal that in the stationary state electron occupation numbers with the
opposite spins always have the same value - the stationary state is a
$""$paramagnetic$""$ one. $""$Magnetic$""$ properties can appear only in the
non-stationary characteristics of the single-impurity Anderson model and in the
dynamics of the localized electrons second order correlation functions. We
found, that for deep energy levels and strong Coulomb correlations, relaxation
time for initial $""$magnetic$""$ state can be several orders larger than for
$""$paramagnetic$""$ one. So, long-living $""$magnetic$""$ moment can exist in the
system. We also found non-stationary spin polarized currents flowing in
opposite directions for the different spins in the particular time interval.",1606.08975v1
2016-07-29,Exchange interaction-driven dynamic nuclear polarization in Mn-doped InGaAs/GaAs quantum dots,"We investigated optical spin orientation and dynamic nuclear polarization
(DNP) in individual self-assembled InGaAs/GaAs quantum dots (QDs) doped by a
single Mn atom, a magnetic impurity providing a neutral acceptor A$^0$ with an
effective spin $J=1$. We find that the spin of an electron photo-created in
such a quantum dot can be efficiently oriented by a quasi-resonant
circularly-polarized excitation. For the electron spin levels which are made
quasi-degenerate by a magnetic field compensating the exchange interaction
$\Delta_e$ with A$^0$, there is however a full depolarization due the
anisotropic part of the exchange. Still, in most studied QDs, the spin
polarized photo-electrons give rise to a pronounced DNP which grows with a
longitudinal magnetic field until a critical field where it abruptly vanishes.
For some QDs, several replica of such DNP sequence are observed at different
magnetic fields. This striking behavior is qualitatively discussed as a
consequence of different exchange interactions experienced by the electron,
driving the DNP rate via the energy cost of electron-nucleus spin flip-flops.",1607.08870v2
2016-08-29,Impurity effects on spin dynamics in magnetic and superconducting iron pnictides and chalcogenides,"In this paper we summarize the effects of magnetic and nonmagnetic impurities
on the spin dynamics in Febased superconductors and their parent compounds. The
effects of chemical substitution, vacancies, and disorder on the suppression or
stabilization of superconductivity and spin-density-wave phases are reviewed in
the context of recent neutron-spectroscopy measurements of spin excitations. We
also present new results on the structure of magnetic fluctuations in BaFe2As2
single crystals doped with Mn local moments and discuss them in relationship to
the previously reported (pi, pi) branch of checkerboard magnetic excitations.",1608.08123v4
2016-08-30,Quantum speed limit time in a magnetic resonance,"A visualization for dynamics of a qudit spin vector in a time-dependent
magnetic field is realized by means of mapping a solution for a spin vector on
the three-dimensional spherical curve (vector hodograph). The obtained results
obviously display the quantum interference of precessional and nutational
effects on the spin vector in the magnetic resonance. For any spin the bottom
bounds of the quantum speed limit time (QSL) are found. It is shown that the
bottom bound goes down when using multilevel spin systems. Under certain
conditions the non-nil minimal time, which is necessary to achieve the
orthogonal state from the initial one, is attained at spin S=2. An estimation
of the product of two and three standard deviations of the spin components are
presented. We discuss the dynamics of the mutual uncertainty, conditional
uncertainty and conditional variance in terms of spin standard deviations. The
study can find practical applications in the magnetic resonance, 3D
visualization of computational data and in designing of optimized information
processing devices for quantum computation and communication.",1608.08537v3
2016-09-07,Cu nuclear magnetic resonance study of charge and spin stripe order in La$_{1.875}$Ba$_{0.125}$CuO$_4$,"We present a Cu nuclear magnetic/quadrupole resonance study of the charge
stripe ordered phase of LBCO, with detection of previously unobserved
('wiped-out') signal. We show that spin-spin and spin-lattice relaxation rates
are strongly enhanced in the charge ordered phase, explaining the apparent
signal decrease in earlier investigations. The enhancement is caused by
magnetic, rather than charge fluctuations, conclusively confirming the
long-suspected assumption that spin fluctuations are responsible for the
wipeout effect. Observation of the full Cu signal enables insight into the spin
and charge dynamics of the stripe-ordered phase, and measurements in external
magnetic fields provide information on the nature and suppression of spin
fluctuations associated with charge order. We find glassy spin dynamics, in
agreement with previous work, and incommensurate static charge order with
charge modulation amplitude similar to other cuprate compounds, suggesting that
the amplitude of charge stripes is universal in the cuprates.",1609.02162v1
2016-09-12,Currentless reversal of Néel vector in antiferromagnets,"The bias driven perpendicular magnetic anisotropy is a magneto-electric
effect that can realize 90$^\circ$ magnetization rotation and even 180$% ^\circ
$ flip along the easy axis in the ferromagnets with a minimal energy
consumption. This study theoretically demonstrates a similar phenomenon of the
N\'{e}el vector reversal via a short electrical pulse that can mediate
perpendicular magnetic anisotropy in the antiferromagnets. The analysis based
on the dynamical equations as well as the micro-magnetic simulations reveals
the important role of the inertial behavior in the antiferromagnets that
facilitates the N\'{e}el vector to overcome the barrier between two free-energy
minima of the bistable states along the easy axis. In contrast to the
ferromagnets, this N\'{e}el vector reversal does not accompany angular moment
transfer to the environment, leading to acceleration in the dynamical response
by a few orders of magnitude. Further, a small switching energy requirement of
a few attojoules illustrates an added advantage of the phenomenon in low-power
spintronic applications.",1609.03454v2
2016-09-26,Relativistic theory of spin relaxation mechanisms in the Landau-Lifshitz-Gilbert equation of spin dynamics,"Starting from the Dirac-Kohn-Sham equation we derive the relativistic
equation of motion of spin angular momentum in a magnetic solid under an
external electromagnetic field. This equation of motion can be written in the
form of the well-known Landau-Lifshitz-Gilbert equation for a harmonic external
magnetic field, and leads to a more general magnetization dynamics equation for
a general time-dependent magnetic field. In both cases with an electronic
spin-relaxation term which stems from the spin-orbit interaction. We thus
rigorously derive, from fundamental principles, a general expression for the
anisotropic damping tensor which is shown to contain an isotropic Gilbert
contribution as well as an anisotropic Ising-like and a chiral,
Dzyaloshinskii-Moriya-like contribution. The expression for the spin relaxation
tensor comprises furthermore both electronic interband and intraband
transitions. We also show that when the externally applied electromagnetic
field possesses spin angular momentum, this will lead to an optical spin torque
exerted on the spin moment.",1609.07901v1
2016-11-03,Magnetic avalanches in granular ferromagnets: Thermal activated collective behavior,"We present a numerical study on the thermal activated avalanche dynamics in
granular materials composed of ferromagnetic clusters embedded in a
non-magnetic matrix. A microscopic dynamical simulation based on the
reaction-diffusion process is developed to modeling the magnetization process
of such systems. The large-scale simulations presented here explicitly
demonstrate inter-granular collective behavior induced by thermal activation of
spin tunneling. In particular, we observe an intriguing criticality controlled
by the rate of energy dissipation. We show that thermal activated avalanches
can be understood in the framework of continuum percolation and the emergent
dissipation induced criticality is in the universality class of
three-dimensional percolation transition. Implications of these results to the
phase-separated states of colossal magnetoresistance materials and other
artificial granular magnetic systems are also~discussed.",1611.01098v1
2016-12-06,Quantum Dynamics of Skyrmions in Chiral Magnets,"We study the quantum propagation of a Skyrmion in chiral magnetic insulators
by generalizing the micromagnetic equations of motion to a finite-temperature
path integral formalism, using field theoretic tools. Promoting the center of
the Skyrmion to a dynamic quantity, the fluctuations around the Skyrmionic
configuration give rise to a time-dependent damping of the Skyrmion motion.
From the frequency dependence of the damping kernel, we are able to identify
the Skyrmion mass, thus providing a microscopic description of the kinematic
properties of Skyrmions. When defects are present or a magnetic trap is
applied, the Skyrmion mass acquires a finite value proportional to the
effective spin, even at vanishingly small temperature. We demonstrate that a
Skyrmion in a confined geometry provided by a magnetic trap behaves as a
massive particle owing to its quasi-one-dimensional confinement. An additional
quantum mass term is predicted, independent of the effective spin, with an
explicit temperature dependence which remains finite even at zero temperature.",1612.01885v2
2017-01-10,Capturing ultrafast magnetic dynamics by time-resolved soft x-ray magnetic circular dichroism,"Experiments of time-resolved x-ray magnetic circular dichroism (Tr-XMCD) and
resonant x-ray scattering at a beamline BL07LSU in SPring-8 with a
time-resolution of under 50 ps are presented. A micro-channel plate is utilized
for the Tr-XMCD measurements at nearly normal incidence both in the partial
electron and total fluorescence yield (PEY and TFY) modes at the L2,3
absorption edges of the 3d transition-metals in the soft x-ray region. The
ultrafast photo-induced demagnetization within 50 ps is observed on the
dynamics of a magnetic material of FePt thin film, having a distinct threshold
of the photon density. The spectrum in the PEY mode is less-distorted both at
the L2,3 edges compared with that in the TFY mode and has the potential to
apply the sum rule analysis for XMCD spectra in pump-probed experiments.",1701.03156v2
2017-01-19,Universality of the helimagnetic transition in cubic chiral magnets: Small angle neutron scattering and neutron spin echo spectroscopy studies of Fe$_{1-x}$Co$_x$Si,"We present a comprehensive Small Angle Neutron Scattering (SANS) and Neutron
Spin Echo Spectroscopy (NSE) study of the structural and dynamical aspects of
the helimagnetic transition in Fe$_{1-x}$Co$_x$Si with $x$ = 0.30. In contrast
to the sharp transition observed in the archetype chiral magnet MnSi, the
transition in Fe$_{1-x}$Co$_x$Si is gradual and long-range helimagnetic
ordering coexists with short-range correlations over a wide temperature range.
The dynamics are more complex than in MnSi and involve long relaxation times
with a stretched exponential relaxation which persists even under magnetic
field. These results in conjunction with an analysis of the hierarchy of the
relevant length scales show that the helimagnetic transition in
Fe$_{1-x}$Co$_x$Si differs substantially from the transition in MnSi and
question the validity of a universal approach to the helimagnetic transition in
chiral magnets.",1701.05448v2
2017-02-03,A dynamic analysis of magnetized plasma sheath in a collisionless scenario with ion sources,"In this paper, the central concern is to analyze the influence of the forces
that controls the ion dynamics inside a magnetized plasma sheath under
collisionless conditions. The ionization effects are considered within the
sheath. The magnetic field is tilted in x-z plane and makes an angle with the
x-axis. The motivation of the paper is to see the effect of both field strength
variation and the variation of the inclination angle on the force fields inside
the sheath. The pitch length and pitch angle for the particle velocity field is
also calculated and has been found to vary widely with the inclination angle
and the strength of the magnetic field. The role of the Lorentz force and
energy acquired by the ions while moving towards the wall is highlighted. A
comparison between two different ion sources has also been foregrounded.",1702.00944v2
2017-03-22,Skyrmion dynamics in a frustrated ferromagnetic film and current-induced helicity locking-unlocking transition,"The helicity-orbital coupling is an intriguing feature of magnetic skyrmions
in frustrated magnets. Here, we explore the skyrmion dynamics in a frustrated
magnet based on the $J_{1}$-$J_{2}$-$J_{3}$ classical Heisenberg model
explicitly by including the dipole-dipole interaction. The skyrmion energy
acquires a helicity dependence due to the dipole-dipole interaction, resulting
in the current-induced translational motion with a fixed helicity. The lowest
energy states are the degenerate Bloch-type states, which can be used for
building the binary memory. By increasing the driving current, the helicity
locking-unlocking transition occurs, where the translational motion changes to
the rotational motion. Furthermore, we demonstrate that two skyrmions can
spontaneously form a bound state. The separation of the bound state forced by a
driving current is also studied. In addition, we show the annihilation of a
pair of skyrmion and antiskyrmion. Our results reveal the distinctive
frustrated skyrmions may enable viable applications in topological magnetism.",1703.07501v3
2017-04-13,Simulating chiral magnetic effect and anomalous transport phenomena in the pre-equilibrium stages of heavy-ion collisions,"We present a first principles approach to study the Chiral Magnetic Effect
during the pre-equilibrium stage of a heavy-ion collision. We discuss the
dynamics of the Chiral Magnetic Effect and Chiral Magnetic Wave based on
real-time lattice simulations with dynamical (Wilson and Overlap) fermions
simultaneously coupled to color and electromagnetic fields. While for light
quarks we observe a dissipation-less transport of charges as in anomalous
hydrodynamics, we demonstrate that for heavier quarks the effects of explicit
chiral symmetry breaking lead to a significant reduction of the associated
currents.",1704.05887v2
2017-04-30,Modelling the Lagrangian of magnetized plasmas with low-frequency magnetic perturbations by a gyrokinetic Ampere-Poisson model,"Following the method in Ref.(1), this paper introduces a fundamental
Lagrangian 1-form on the particle's coordinates, which determines the dynamics
of all ions and electrons of the magnetized plasma with low-frequency magnetic
perturbations. An Ampere-Vlasov model is utilized to model this fundamental
Lagrangian 1-form based on a kind of coarse-grained scheme. With the
Cary-Littlejohn single-parameter Lie transform method, a new fundamental
Lagrangian 1-form on the gyrocenter coordinates is derived through transforming
the one on particle's coordinates. This new 1-form determines the dynamics of
all ions and electrons on the gyrocenter coordinates. A new Ampere-Vlasov model
totally defined on the gyrocenter coordinates is developed to model the new
Lagrangian 1-form. By incorporating the electrostatic perturbation into the
Lagrangian 1-form, we eventually derived an Ampere-Poisson-Vlasov model defined
on the gyrocenter coordinates.",1705.00282v1
2017-05-11,Spin Dynamics and Magnetoelectric Coupling Mechanism of Co4Nb2O9,"Neutron powder diffraction experiments reveal that Co4Nb2O9 forms a
noncollinear in-plane magnetic structure with Co2+ moments lying in the ab
plane. The spin-wave excitations of this magnet were measured by using
inelastic neutron scattering and soundly simulated by a dynamic model involving
nearest and next-nearest neighbour exchange interactions, in-plane anisotropy
and the Dzyaloshinskii-Moriya interaction. The in-plane magnetic structure of
Co4Nb2O9 is attributed to the large in-plane anisotropy while the
noncollinearity of the spin configuration is attributed to the
Dzyaloshinskii-Moriya interaction. The high magnetoelectric coupling effect of
Co4Nb2O9 in fields can be explained by its special in-plane magnetic structure.",1705.04017v3
2017-05-16,Characterizing the ionospheric current pattern response to southward and northward IMF turnings with dynamical SuperMAG correlation networks,"We characterize the response of the quiet time (no substorms or storms)
large-scale ionospheric transient equivalent currents to north-south and
south-north IMF turnings by using a dynamical network of ground-based
magnetometers. Canonical correlation between all pairs of SuperMAG magnetometer
stations in the Northern Hemisphere (magnetic latitude (MLAT) 50-82$^{\circ}$)
is used to establish the extent of near-simultaneous magnetic response between
regions of magnetic local time-MLAT. Parameters and maps that describe
spatial-temporal correlation are used to characterize the system and its
response to the turnings aggregated over several hundred events. We find that
regions that experience large increases in correlation post turning coincide
with typical locations of a two-cell convection system and are influenced by
the interplanetary magnetic field $\mathit{B}_{y}$. The time between the
turnings reaching the magnetopause and a network response is found to be
$\sim$8-10 min and correlation in the dayside occurs 2-8 min before that in the
nightside.",1705.05698v1
2017-05-16,How does an asymmetric magnetic field change the vertical structure of a hot accretion flow?,"This paper explores the effects of large-scale magnetic fields in hot
accretion flows for asymmetric configurations with respect to the equatorial
plane. The solutions that we have found show that the large-scale asymmetric
magnetic field can significantly affect the dynamics of the flow and also cause
notable outflows in the outer parts. Previously, we treated a viscous resistive
accreting disc in the presence of an odd symmetric $\textbf{B-}$ field about
the equatorial plane. Now we extend our earlier work by taking into account
another configuration of large-scale magnetic field which is no longer
symmetric. We provide asymmetric field structures with small deviations from
even and odd symmetric $\textbf{B-}$field. Our results show that the disc's
dynamics and appearance become different above and below the equatorial plane.
The set of solutions also predicts that even a small deviation in a symmetric
field causes the disc to compress on one side and expand on the other. In some
cases, our solution represents a very strong outflow from just one side of the
disc. Therefore, the solution may potentially explain the origin of one-sided
jets in radio galaxies.",1705.07190v2
2017-06-05,Dynamic anisotropy in MHD turbulence induced by mean magnetic field,"In this paper, we study the development of anisotropy in strong MHD
turbulence in the presence of a large scale magnetic field B 0 by analyzing the
results of direct numerical simulations. Our results show that the developed
anisotropy among the different components of the velocity and magnetic field is
a direct outcome of the inverse cascade of energy of the perpendicular velocity
components u? and a forward cascade of the energy of the parallel component u k
. The inverse cascade develops for a strong B0, where the flow exhibits a
strong vortical structure by the suppression of fluctuations along the magnetic
field. Both the inverse and the forward cascade are examined in detail by
investigating the anisotropic energy spectra, the energy fluxes, and the shell
to shell energy transfers among different scales.",1706.01250v2
2017-06-09,Localization of a magnetic moment using a two-qubit probe,"A nanomagnet precessing in an external magnetic field can be treated as a
source of narrow-bandwidth magnetic noise, that leaves characteristic
fingerprints in decoherence of a nearby spin qubit undergoing dynamical
decoupling. We show how, by measurements of two-qubit coherence, a noise sensor
composed of qubit pair can be used to reconstruct the position of the
nanomagnet. Such localization of noise source is possible with only two qubit
probes, because the course of coherence decay under appropriately designed
dynamical decoupling sequences contain information not only about noises
experienced by each qubit, but also about their cross-correlations. We test the
applicability of the proposed protocol on an example of two qubits coupled to
the nanomagnet via dipolar interaction. We also show how, using a two-qubit
sensor possessing a particular symmetry, one can localize the nanomagnet even
when the sensor-magnet coupling law is unknown.",1706.02948v2
2017-07-11,Putative spin liquid in the triangle-based iridate Ba$_3$IrTi$_2$O$_9$,"We report on thermodynamic, magnetization, and muon spin relaxation
measurements of the strong spin-orbit coupled iridate Ba$_3$IrTi$_2$O$_9$,
which constitutes a new frustration motif made up a mixture of edge- and
corner-sharing triangles. In spite of strong antiferromagnetic exchange
interaction of the order of 100~K, we find no hint for long-range magnetic
order down to 23 mK. The magnetic specific heat data unveil the $T$-linear and
-squared dependences at low temperatures below 1~K. At the respective
temperatures, the zero-field muon spin relaxation features a persistent spin
dynamics, indicative of unconventional low-energy excitations. A comparison to
the $4d$ isostructural compound Ba$_3$RuTi$_2$O$_9$ suggests that a concerted
interplay of compass-like magnetic interactions and frustrated geometry
promotes a dynamically fluctuating state in a triangle-based iridate.",1707.03140v1
2017-08-17,Front dynamics and entanglement in the XXZ chain with a gradient,"We consider the XXZ spin chain with a magnetic field gradient and study the
profiles of the magnetization as well as the entanglement entropy. For a slowly
varying field it is shown that, by means of a local density approximation, the
ground-state magnetization profile can be obtained with standard Bethe ansatz
techniques. Furthermore, it is argued that the low-energy description of the
theory is given by a Luttinger liquid with slowly varying parameters. This
allows us to obtain a very good approximation of the entanglement profile using
a recently introduced technique of conformal field theory in curved spacetime.
Finally, the front dynamics is also studied after the gradient field has been
switched off, following arguments of generalized hydrodynamics for integrable
systems. While for the XX chain the hydrodynamic solution can be found
analytically, the XXZ case appears to be more complicated and the magnetization
profiles are recovered only around the edge of the front via an approximate
numerical solution.",1708.05187v2
2017-08-29,Magnetic field generation by pointwise zero-helicity three-dimensional steady flow of incompressible electrically conducting fluid,"We introduce six families of three-dimensional space-periodic steady
solenoidal flows, whose kinetic helicity density is zero at any point. Four
families are analytically defined. Flows in four families have zero helicity
spectrum. Sample flows from five families are used to demonstrate numerically
that neither zero kinetic helicity density, nor zero helicity spectrum prohibit
generation of large-scale magnetic field by the two most prominent dynamo
mechanisms: the magnetic $\alpha$-effect and negative eddy diffusivity. Our
computations also attest that such flows often generate small-scale field for
sufficiently small magnetic molecular diffusivity. These findings indicate that
kinetic helicity and helicity spectrum are not the quantities controlling the
dynamo properties of a flow regardless of whether scale separation is present
or not.",1708.08770v4
2017-08-29,Element-resolved ultrafast demagnetization rates in ferrimagnetic CoDy,"Femtosecond laser induced ultrafast magnetization dynamics have been studied
in multisublattice CoxDy1-x alloys. By performing element and time-resolved
X-ray spectroscopy, we distinguish the ultrafast quenching of Co3d and Dy4f
magnetic order when the initial temperatures are below (T=150K) or above
(T=270K) the temperature of magnetic compensation (Tcomp). In accordance with
former element-resolved investigations and theoretical calculations, we observe
different dynamics for Co3d and Dy4f spins. In addition we observe that, for a
given laser fluence, the demagnetization amplitudes and demagnetization times
are not affected by the existence of a temperature of magnetic compensation.
However, our experiment reveals a twofold increase of the ultrafast
demagnetization rates for the Dy sublattice at low temperature. In parallel, we
measure a constant demagnetization rate of the Co3d sublattice above and below
Tcomp. This intriguing difference between the Dy4f and Co3d sublattices calls
for further theoretical and experimental investigations.",1708.08918v2
2017-09-29,Designing Magnetic Droplet Soliton Nucleation Employing Spin Polarizer,"We show by means of micromagnetic simulations that spin polarizer in
nano-contact spin torque oscillators (NC-STOs) as the representative of the
fixed layer in an orthogonal pseudo-spin valve (P-SV) can be employed to design
and to control magnetic droplet soliton nucleation and dynamics. We found that
using a tilted spin polarizer layer decreases the droplet nucleation time which
is more suitable for high speed applications. However, a tilted spin polarizer
increases the nucleation current and decreases the frequency stability of the
droplet. Additionally, by driving the magnetization inhomogenously at the
nano-contact region, it is found that a tilted spin polarizer reduces the
precession angle of the droplet and through an interplay with the Oersted field
of the DC current, it breaks the spatial symmetry of the droplet profile. Our
findings explore fundamental insight into nano-scale magnetic droplet soliton
dynamics with potential tunability parameters for future microwave electronics.",1709.10325v3
2017-10-12,Magnetic droplet solitons generated by pure spin currents,"Magnetic droplets are dynamical solitons that can be generated by locally
suppressing the dynamical damping in magnetic films with perpendicular
anisotropy. To date, droplets have been observed only in nanocontact
spin-torque oscillators operated by spin-polarized electrical currents. Here,
we experimentally demonstrate that magnetic droplets can be nucleated and
sustained by pure spin currents in nanoconstriction-based spin Hall devices.
Micromagnetic simulations support our interpretation of the data, and indicate
that in addition to the stationary droplets, propagating solitons can be also
generated in the studied system, which can be utilized for the information
transmission in spintronic applications.",1710.04430v1
2017-10-13,Mode-Dependent Damping in Metallic Antiferromagnets Due to Inter-Sublattice Spin Pumping,"Damping in magnetization dynamics characterizes the dissipation of magnetic
energy and is essential for improving the performance of spintronics-based
devices. While the damping of ferromagnets has been well studied and can be
artificially controlled in practice, the damping parameters of
antiferromagnetic materials are nevertheless little known for their physical
mechanisms or numerical values. Here we calculate the damping parameters in
antiferromagnetic dynamics using the generalized scattering theory of
magnetization dissipation combined with the first-principles transport
computation. For the PtMn, IrMn, PdMn and FeMn metallic antiferromagnets, the
damping coefficient associated with the motion of magnetization ($\alpha_m$) is
one to three orders of magnitude larger than the other damping coefficient
associated with the variation of the N\'eel order ($\alpha_n$), in sharp
contrast to the assumptions made in the literature.",1710.04766v1
2017-10-27,Semiconducting double-dot exchange-only qubit dynamics in presence of magnetic and charge noises,"The effects of magnetic and charge noises on the dynamical evolution of the
double-dot exchange-only qubit (DEOQ) is theoretically investigated. The DEOQ
consisting of three electrons arranged in an electrostatically defined double
quantum dot deserves special interest in quantum computation applications due
to its advantages in terms of fabrication, control and manipulation in view of
implementation of fast single and two qubit operations through only electrical
tuning. The presence of the environmental noise due to nuclear spins and charge
traps, in addition to fluctuations in the applied magnetic field and charge
fluctuations on the electrostatic gates adopted to confine the electrons, is
taken into account including random magnetic field and random coupling terms in
the Hamiltonian. The behavior of the return probability as a function of time
for initial conditions of interest is presented. Moreover, through an
envelope-fitting procedure on the return probabilities, coherence times are
extracted when model parameters take values achievable experimentally in
semiconducting devices.",1710.10032v2
2017-10-28,High frequency dynamics modulated by collective magnetization reversal in artificial spin ice,"Spin-torque ferromagnetic resonance (ST-FMR) arises in heavy
metal/ferromagnet heterostructures when an alternating charge current is passed
through the bilayer stack. The methodology to detect the resonance is based on
the anisotropic magnetoresistance, which is the change in the electrical
resistance due to different orientations of the magnetization. In connected
networks of ferromagnetic nanowires, known as artificial spin ice, the
magnetoresistance is rather complex owing to the underlying collective behavior
of the geometrically frustrated magnetic domain structure. Here, we demonstrate
ST-FMR investigations in a square artificial spin-ice system and correlate our
observations to magnetotransport measurements. The experimental findings are
described using a simulation approach that highlights the importance of the
correlated dynamics response of the magnetic system. Our results open the
possibility of designing reconfigurable microwave oscillators and
magnetoresistive devices based on connected networks of nanomagnets.",1710.10534v1
2017-11-08,Magnetic skyrmion bubble motion driven by surface acoustic waves,"We study the dynamical control of a magnetic skyrmion bubble by using
counter-propagating surface acoustic waves (SAWs) in a ferromagnet. First, we
determine the bubble mass and derive the force due to SAWs acting on a magnetic
bubble using Thiele's method. The force that pushes the bubble is proportional
to the strain gradient for the major strain component. We then study the
dynamical pinning and motion of magnetic bubbles by SAWs in a nanowire. In a
disk geometry, we propose a SAWs-driven skyrmion bubble oscillator with two
resonant frequencies.",1711.03041v3
2017-11-14,Temperature effects on drift of suspended single-domain particles induced by the Magnus force,"We study the temperature dependence of the drift velocity of single-domain
ferromagnetic particles induced by the Magnus force in a dilute suspension. A
set of stochastic equations describing the translational and rotational
dynamics of particles is derived, and the particle drift velocity that depends
on components of the average particle magnetization is introduced. The
Fokker-Planck equation for the probability density of magnetization
orientations is solved analytically in the limit of strong thermal fluctuations
for both the planar rotor and general models. Using these solutions, we
calculate the drift velocity and show that the out-of-plane fluctuations of
magnetization, which are not accounted for in the planar rotor model, play an
important role. In the general case of arbitrary fluctuations, we investigate
the temperature dependence of the drift velocity by numerically simulating a
set of effective stochastic differential equations for the magnetization
dynamics.",1711.05338v1
2017-11-16,Transient dynamics of nonlinear magneto-optical rotation,"We analyze nonlinear magneto-optical rotation (NMOR) in rubidium vapor
subjected to continuously-scanned magnetic field. By varying magnetic-field
sweep rate, a transition from traditionally-observed dispersive-like NMOR
signals (low sweep rate) to oscillating signals (higher sweep rates) is
demonstrated. The transient oscillatory behavior is studied versus light and
magnetic-field parameters, revealing a strong dependence of the signals on
magnetic-sweep rate and light intensity. The experimental results are supported
with density-matrix calculations, which enable quantitative analysis of the
effect. Fitting of the signals simulated versus different parameters with a
theoretically-motivated curve reveals presence of oscillatory and static
components in the signals. The components depend differently on the system
parameters, which suggests their distinct nature. The investigations provide
insight into dynamics of ground-state coherence generation and enable
application of NMOR in detection of transient spin couplings.",1711.06028v1
2017-12-28,Theory of magnetic skyrmion glass,"Skyrmions and skyrmion crystal (SkX) discovered in chiral magnets show unique
physical properties due to their nontrivial topology such as the stability
against the annihilation and the motion driven by the ultralow current density,
which can be advantageous for the device applications such as magnetic
memories. Especially, the chiral dynamics, i.e., the velocity perpendicular to
the force acting on a skyrmion, is a key to avoid the impurity potential and
enhances its mobility. However, the collective pinning of SkX occurs by the
disorder, which is crucial for its low energy properties. Here, we study
theoretically the low energy dynamics of SkX in the presence of disorder
effects in terms of replica field theory, and reveal nonreciprocal collective
modes and their electromagnetic responses along the direction of external
magnetic field. The physical quantities such as the relaxation rate of
$\mu$SR/NMR and the pinning frequency show a dramatic change associated with
the topological phase transition from the helical state to SkX. These results
provide a firm basis to explore the glassy state of SkX.",1712.09782v1
2018-01-10,Spin and motion dynamics with zigzag ion crystals in transverse magnetic field gradients,"We investigate the dynamics of ion crystals in zigzag configuration in
transverse magnetic field gradients. A surface-electrode Paul trap is employed
to trap $^{40}$Ca$^+$ ions and features submerged wires to generate magnetic
field gradients of up to 16.3(9) T/m at the ions position. With the gradient
aligned in the direction perpendicular to the axis of weakest confinement,
along which linear ion crystals are formed, we demonstrate magnetic field
gradient induced coupling between the spin and ion motion. For crystals of
three ions across their linear-to-zigzag structural transition we perform
sideband spectroscopy upon directly driving the spins with a radiofrequency
field. Furthermore, we observe the rich excitation spectrum of vibrational
modes in a planar crystal comprised of four ions.",1801.03391v2
2018-01-17,Mimicking inflation with 2-fluid systems in a strong gradient magnetic field,"In the standard cosmological picture the Universe underwent a brief period of
near-exponential expansion, known as Inflation. This provides an explanation
for structure formation through the amplification of perturbations by the rapid
expansion of the fabric of space. Although this mech- anism is theoretically
well understood, it cannot be directly observed in nature. We propose a novel
experiment combining fluid dynamics and strong magnetic field physics to
simulate cosmo- logical inflation. Our proposed system consists of two
immiscible, weakly magnetised fluids moving through a strong magnetic field in
the bore of a superconducting magnet. By precisely controlling the propagation
speed of the interface waves, we can capture the essential dynamics of
inflation- ary fluctuations: interface perturbations experience a shrinking
effective horizon and are shown to transition from oscillatory to squeezed and
frozen regimes at horizon crossing.",1801.05843v1
2018-01-18,Axion-plasmon polaritons in strongly magnetized plasmas,"Axions are hypothetical particles related to the violation of the
charge-parity symmetry, being the most prone candidates for dark matter.
Multiple attempts to prove their existence are currently performed in different
physical systems. Here, we anticipate the possibility of the axions coupling to
the electrostatic (Langmuir) modes of a strongly magnetized plasma, by showing
that a new quasi-particle can be defined, the axion-plasmon polariton. The
excitation of axions can be inferred from the pronounced modification of the
dispersion relation of the Langmuir waves, a feature that we estimate to be
accessible in state-of-the-art plasma-based experiments. We further show that,
under extreme density and magnetic field conditions (e.g. at the interior of
dense neutron stars), the axion-plasmon polariton becomes dynamically unstable,
similarly to the case of the Jeans instability occurring in self-gravitating
fluids. This latter result anticipates a plausible mechanism to the creation of
axion-like particles in the universe.",1801.06254v1
2018-02-12,Dynamics of a magnetic skyrmionium driven by spin waves,"The magnetic skyrmionium is a skyrmion-like structure but carries a zero net
skyrmion number, which can be used as a building block for non-volatile
information processing devices. Here, we study the dynamics of a magnetic
skyrmionium driven by propagating spin waves. It is found that the skyrmionium
can be effectively driven into motion by spin waves showing tiny skyrmion Hall
effect, of which the mobility is much better than that of the skyrmion at the
same condition. We also show that the skyrmionium mobility depends on the
nanotrack width and damping coefficient, and can be controlled by an external
out-of-plane magnetic field. Besides, we demonstrate the skyrmionium motion
driven by spin waves is inertial. Our results indicate that the skyrmionium is
a promising building block for building spin-wave spintronic devices.",1802.03868v2
2018-02-20,Magnetic exchange and susceptibilities in fcc iron: A supercell dynamical mean-field theory study,"We study the momentum- and temperature dependencies of magnetic
susceptibilities and magnetic exchange in paramagnetic fcc iron by a
combination of density functional theory and supercell dynamical mean-field
theory (DFT+DMFT). We find that in agreement with experimental results the
antiferromagnetic correlations with the wave vector close to $(0,0,2\pi)$
dominate at low temperatures (as was also obtained previously theoretically),
while the antiferromagnetic and ferromagnetic correlations closely compete at
the temperatures $T\sim 1000$ K, where $\gamma$-iron exists in nature. Inverse
staggered susceptibility has linear temperature dependence at low temperatures,
with negative Weiss temperature $\theta_{\rm stagg} \approx -340$ K; the
inverse local susceptibility is also linear at not too low temperatures,
showing well formed local moments. Analysis of magnetic exchange shows that the
dominant contribution comes from first two coordination spheres. In agreement
with the analysis of the susceptibility, the nearest-neighbor exchange is found
to be antiferromagnetic at low temperatures, while at temperature of the
$\alpha$-$\gamma$ structural phase transition its absolute value becomes small,
and the system appears on the boundary between the regimes with strongest
antiferro- and ferromagnetic correlations.",1802.07061v2
2018-02-26,Devil's Staircases in SFS Josephson Junctions,"We study the effect of coupling between the superconducting current and
magnetization in the superconductor/ferromagnet/superconductor Josephson
junction under an applied circularly polarized magnetic field. Manifestation of
ferromagnetic resonance in the frequency dependence of the amplitude of the
magnetization and the average critical current density is demonstrated
numerically. The IV-characteristics show subharmonic steps that form devil's
staircases, following a continued fraction algorithm. The origin of the found
steps is related to the effect of the magnetization dynamics on the phase
difference in the Josephson junction. The dynamics of our system is described
by a generalized RCSJ model coupled to the Landau-Lifshitz-Gilbert equation. In
the suplement we justify analytically the appearance of the fractional steps in
IV-characteristics of the superconductor/ferromagnet/superconductor Josephson
junction.",1802.09212v2
2018-03-01,Coupling of phonons with orbital dynamics and magnetism in CuSb$_2$O$_6$,"Strongly interacting phonons and orbital excitations are observed in the same
energy range for CuSb$_2$O$_6$, unlocking a so-far unexplored type of
electron-phonon interaction. An orbital wave at $\sim 550$ cm$^{-1}$ softens on
warming and strongly interferes with a phonon at $\sim 500$ cm$^{-1}$, giving
rise to a merged excitation of mixed character. An electronic continuum grows
on warming to the orbital ordering temperature $T_{OO}$=400 K, generating an
important phonon decay channel. This direct and simultaneous observation of
orbital and vibrational excitations reveals details of their combined dynamics.
In addition, phonon frequency anomalies due to magnetic correlations are
observed below $\sim 150$ K, much above the three-dimensional magnetic ordering
temperature $T_N^{3D}=8.5$ K, confirming one-dimensional magnetic correlations
along Cu-O-O-Cu linear chains in the paramagnetic state.",1803.00559v3
2018-04-05,Stochastic ferrimagnetic Landau-Lifshitz-Bloch equation for finite magnetic structures,"Precise modeling of the magnetization dynamics of nanoparticles with finite
size effects at fast varying temperatures is a computationally challenging
task. Based on the Landau-Lifshitz-Bloch (LLB) equation we derive a coarse
grained model for disordered ferrimagnets, which is both fast and accurate.
First, we incorporate stochastic fluctuations to the existing ferrimagnetic LLB
equation. Further, we derive a thermodynamic expression for the temperature
dependent susceptibilities, which is essential to model finite size effects.
Together with the zero field equilibrium magnetization the susceptibilities are
used in the stochastic ferrimagnetic LLB to simulate a $5\times10$ nm$^2$
ferrimagnetic GdFeCo particle with 70 % FeCo and 30 % Gd under various external
applied fields and heat pulses. The obtained trajectories agree well with those
of an atomistic model, which solves the stochastic Landau-Lifshitz-Gilbert
equation for each atom. Additionally, we derive an expression for the
intergrain exchange field which couple the ferromagnetic sublattices of a
ferrimagnet. A comparison of the magnetization dynamics obtained from this
simpler model with those of the ferrimagnetic LLB equation shows a perfect
agreement.",1804.01724v1
2018-04-27,Two-Dimensional Computation of Pulsed Magnetic Field Diffusion Dynamics in Gold Cone with Consideration of Inductive Heating and Temperature Dependence of Electrical Conductivity,"Application of an external kilo-tesla-level magnetic field, which can be
generated using high-intensity laser, to a target is a promising scheme to
reduce spray angle of a laser-driven relativistic electron beam (REB) for
enhancing the isochoric heating of a dense plasma with the laser-driven REB.
Here we have developed a two-dimensional electro-magnetic dynamics (2D-EMD)
simulation code to solve Maxwell equations with considerations of the inductive
heating and temperature-dependence of electrical conductivity of a material for
calculating temporally and spatially resolved two-dimensional profile of the
externally applied magnetic field in a gold-cone-attached target.",1804.10410v1
2018-05-19,Optically driven spin pumping mediating collective magnetization dynamics in a spin valve structure,"We demonstrate spin pumping, i.e. the generation of a pure spin current by
precessing magnetization, without application of microwave radiation commonly
used in spin pumping experiments. We use femtosecond laser pulses to
simultaneously launch the magnetization precession in each of two ferromagnetic
layers of a Galfenol-based spin valve and monitor the temporal evolution of the
magnetizations. The spin currents generated by the precession cause a dynamic
coupling of the two layers. This coupling has dissipative character and is
especially efficient when the precession frequencies in the two layers are in
resonance, where coupled modes with strongly different decay rates are formed.",1805.07669v1
2018-05-22,Solar coronal loop dynamics near the null point above active region NOAA 2666,"We analyse observations of a saddle-like structure in the corona above the
western limb of the Sun on 2017 July 18. The structure was clearly outlined by
coronal loops with typical coronal temperature no more than 1 MK. The dynamics
of loops showed convergence toward the centre of the saddle in the vertical
direction and divergence in the horizontal direction. The event is a clear
example of smooth coronal magnetic field reconnection. No heating
manifestations in the reconnection region or magnetically connected areas were
observed. Potential magnetic field calculations, which use as the boundary
condition the SDO/HMI magnetogram taken on July 14, showed the presence of a
null point at the height of 122"" above the photosphere just at the centre of
the saddle structure. The shape of field lines fits the fan-spine magnetic
configuration above NOAA 2666.",1805.08540v1
2018-05-25,Macromagnetic simulation for reservoir computing utilizing spin dynamics in magnetic tunnel junctions,"The figures-of-merit for reservoir computing (RC), using spintronics devices
called magnetic tunnel junctions (MTJs), are evaluated. RC is a type of
recurrent neural network. The input information is stored in certain parts of
the reservoir, and computation can be performed by optimizing a linear
transform matrix for the output. While all the network characteristics should
be controlled in a general recurrent neural network, such optimization is not
necessary for RC. The reservoir only has to possess a non-linear response with
memory effect. In this paper, macromagnetic simulation is conducted for the
spin-dynamics in MTJs, for reservoir computing. It is determined that the
MTJ-system possesses the memory effect and non-linearity required for RC. With
RC using 5-7 MTJs, high performance can be obtained, similar to an echo-state
network with 20-30 nodes, even if there are no magnetic and/or electrical
interactions between the magnetizations.",1805.09977v3
2018-05-30,Magnonic band gap and mode hybridization in continuous Permalloy film induced by vertical coupling with an array of Permalloy ellipses,"We investigate magnonic band structure in thin homogeneous permalloy film
decorated with periodic array of elliptically shaped permalloy dots and
separated by non-magnetic Pt spacer. We demonstrated experimentally formation
of the magnonic band structure for Damon-Eshbach wave propagating in permalloy
film with the band gap opened at the Brillouin zone border and band splitting
at smaller wavenumbers, due to the Bragg interference and interaction of
propagating wave of the continuous film with a standing resonant mode of the
nano-ellipses, respectively. The shape anisotropy of the permalloy nanodots
allows to control the spin wave dynamics through the switch between two states
of the magnetization with respect to the underneath film magnetization, thus
enabling magnonic band structure reprogrammability. With numerical analysis we
show, that predominant role in formation of the magnonic band structure is
played by a vertical dynamic coupling between propagating wave in the film and
magnetization oscillations in the nanodots.",1805.12178v2
2018-06-30,Scalable Dissolution-Dynamic Nuclear Polarization with Rapid Transfer of a Polarized Solid,"In dissolution-dynamic nuclear polarization, nuclear spins are hyperpolarized
at cryogenic temperatures using radicals and microwave irradiation. The
hyperpolarized solid is dissolved with hot solvent and the solution is
transferred to a secondary magnet where strongly enhanced magnetic resonance
signals are observed. Here we present a method for transferring the
hyperpolarized solid. A bullet containing the frozen, hyperpolarized sample is
ejected using pressurized helium gas, and shot into a receiving structure in
the secondary magnet, where the bullet is retained and the polarized solid is
dissolved rapidly. The transfer takes approximately 70 ms. A solenoid, wound
along the entire transfer path ensures adiabatic transfer and limits
radical-induced low-field relaxation. The method is fast and scalable towards
small volumes suitable for high-resolution nuclear magnetic resonance
spectroscopy while maintaining high concentrations of the target molecule.
Polarization levels of approximately 30% have been observed for 1-$^{\sf
13}$C-labelled pyruvic acid in solution.",1807.00223v2
2018-08-03,Magnetic reconnection detonation in supernova remnants,"As a key process that refreshes the interstellar medium, the dynamics and
radiative properties of the supernova remnant (SNR) expansion front not only
reflect the physical environment of the old interstellar medium (ISM)
surrounding the supernova, but they also provide information about the
refreshed ISM. However the expansion dynamics of SNRs cannot be simply
explained by the conventional law of spherical shock wave propagation; on the
other hand, the high energy radiation requires an additional electron
acceleration mechanism in the shock front beyond thermal collision. We consider
herein the detonation wave description of the SNR expansion, in which magnetic
reconnection follows the shock front and transfers the SNR magnetic field
energy to both fluid thermal energy and particle kinetic energy. The structure
of the magnetic reconnection detonation (MRD) is identified based on scaling
analysis in this paper. By applying the MRD description of the SNR expansion
shock to the example of the Crab Nebula, this paper shows that the MRD
description can explain both the accelerative expansion of the nebula as well
as the origin of the luminous expanding shell.",1808.01089v1
2018-08-17,Laser-induced antiferromagnetic-like resonance in amorphous ferrimagnets,"The magnetization dynamics for ferrimagnets at the angular momentum
compensation temperature T_A is believed to be analogous to that for
antiferromagnets. We investigated the pulsed-laser-induced magnetization
dynamics in amorphous rare-earth transition-metal ferrimagnet films with a T_A
just above room temperature. For a low pulse fluence, the magnetization
precession frequency decreases as the applied magnetic field increases, whereas
for a higher pulse fluence, it increases as the applied field increases. The
result was well explained by the left-handed and right-handed precession modes
of the antiferromagnetic-like resonance at temperatures below and above T_A,
respectively, and the data were in agreement with the theoretical simulation.
The study demonstrated the experimental route to achieving antiferromagnetic
resonance in ferrimagnets using a pulsed laser.",1808.05707v1
2018-10-04,Magnetic moment generation in small gold nanoparticles via the plasmonic inverse Faraday effect,"We theoretically investigate the creation of a magnetic moment in gold
nanoparticles by circularly polarized laser light. To this end, we describe the
collective electron dynamics in gold nanoparticles using a semiclassical
approach based on a quantum hydrodynamic model that incorporates the prin-
cipal quantum many-body and nonlocal effects, such as the electron spill-out,
the Hartree potential, and the exchange and correlation effects. We use a
variational approach to investigate the breathing and the dipole dynamics
induced by an external electric field. We show that gold nanoparticles can
build up a static magnetic moment through the interaction with a circularly
polarized laser field. We analyze that the responsible physical mechanism is a
plasmonic, orbital inverse Faraday effect, which can be understood from the
time-averaged electron current that contains currents rotating on the
nanoparticles surface. The computed laser-induced magnetic moments are
sizeable, of about 0.35 muB/atom for a laser intensity of 450 GW/cm2 at plasmon
resonance.",1810.02097v1
2018-11-02,A two-step symmetric method for charged-particle dynamics in a normal or strong magnetic feld,"The study of the long time conservation for numerical methods poses
interesting and challenging questions from the point of view of geometric
integration. In this paper, we analyze the long time energy and magnetic moment
conservations of two-step symmetric methods for charged-particle dynamics. A
two-step symmetric method is proposed and its long time behaviour is shown not
only in a normal magnetic feld but also in a strong magnetic feld. The
approaches to dealing with these two cases are based on the backward error
analysis and modulated Fourier expansion, respectively. It is obtained from the
analysis that the method has better long time conservations than the
variational method which was researched recently in the literature.",1811.01060v2
2018-11-30,Influence of Landau levels in the phonon dispersion of Weyl semimetals,"Weyl semimetals display unusual electronic transport properties when placed
under magnetic fields. Here, we investigate how magnetic fields alter the
dynamics of long wavelength lattice vibrations in these materials. To that end,
we develop a theory for the phonon dispersion, which incorporates contributions
from chiral and nonchiral Landau levels, electron-phonon interactions,
electron-electron interactions, and disorder. We predict (i) a
magnetic-field-induced hybridization between optical phonons and plasmons, (ii)
avoided crossings between pseudoscalar optical phonons and electronic
excitations originating from nonchiral Landau levels, (iii) a sharp dependence
of the sound velocity on the relative angle between the sound propagation and
the magnetic field. We compare our results to recent theoretical studies on the
signatures of the chiral anomaly in phonon dynamics.",1811.12899v2
2018-12-06,Multistep Bloch-line-mediated Walker breakdown in ferromagnetic strips,"A well-known feature of magnetic field driven dynamics of domain walls in
ferromagnets is the existence of a threshold driving force at which the
internal magnetization of the domain wall starts to precess -- a phenomenon
known as the Walker breakdown -- resulting in an abrupt drop of the domain wall
propagation velocity. Here, we report on micromagnetic simulations of magnetic
field driven domain wall dynamics in thin ferromagnetic strips with
perpendicular magnetic anisotropy which demonstrate that in wide enough strips
Walker breakdown is a multistep process: It consists of several distinct
velocity drops separated by short linear parts of the velocity vs field curve.
These features originate from the repeated nucleation, propagation and
annihilation of an increasing number of Bloch lines within the domain wall as
the driving field magnitude is increased. This mechanism arises due to
magnetostatic effects breaking the symmetry between the two ends of the domain
wall.",1812.02545v2
2019-01-14,Inducing out-of-plane precession of magnetization for microwave assisted magnetic recording using an oscillating polarizer in spin torque oscillator,"We investigated the dynamics of a novel design of spin torque oscillator
(STO) for microwave assisted magnetic recording. Using Ni$_{80}$Fe$_{20}$
(NiFe) as the polarizer and Fe$_{67}$Co$_{33}$ (FeCo) as the field generating
layer, we experimentally observed the magnetization reversal of NiFe, followed
by multiple signals in the power spectra as the bias voltage increased. The
signals reflected the out-of-plane precession (OPP) mode oscillation of both
FeCo and NiFe, as well as the magnetoresistance effect of the STO device, which
had the frequency equal to the difference between the oscillation frequency of
NiFe and FeCo. Such dynamics were reproduced by micromagnetic simulation. In
addition to the merit of realizing the OPP mode oscillation with a simple and
thin structure suitable for a narrow gap recording head, the experimental
results using this design suggested that a large cone angle of $\sim$
70$^{\circ}$ for the OPP mode oscillation of FeCo was achieved, which was
estimated based on the macrospin model.",1901.04208v1
2019-03-19,Spin Dynamics of CPMG sequence in time-dependent magnetic fields,"We analyze the effects of time dependent magnetic and RF fields on the spin
dynamics of the Carr-Purcell-Meiboom-Gill (CPMG) sequence. The analysis is
based on the decomposition of the magnetization into the eigenmodes of the
propagator of a single refocusing cycle. For sufficiently slow changes in the
external fields, the magnetization follows the changing eigenmodes
adiabatically. This results in echo amplitudes that show regular modulations
with time. Faster field changes can induce transitions between the eigenmodes.
Such non-adiabatic behavior occurs preferentially at particular offsets of the
Larmor frequency from the RF frequency where the eigenmodes become nearly
degenerate. We introduce the instantaneous adiabaticity parameter ${\cal A}(t)$
that accurately predicts the crossover from the adiabatic to the non-adiabatic
regime and allows the classification of field fluctuations. ${\cal A}(t)$ is
determined solely by the properties of a single refocusing cycle under static
conditions and the instantaneous value of the field offset and its temporal
derivative. The analytical results are compared with numerical simulations.",1903.08006v1
2019-04-19,Sub-terahertz ferrimagnetic spin-transfer torque oscillator,"A theory of magnetization dynamics in ferrimagnetic materials with
antiparallel aligned spin sub-lattices under action of spin-transfer torques
(STT) is developed. We consider magnetization dynamics in GdFeCo layers in two
cases of magnetic anisotropy: easy plane and easy axis. We demonstrate that,
(i) for the easy plane anisotropy the precession of the N\'{e}el vector is
conical and the cone angle depends on the STT strength and the value of spin
non-compensation, while the frequency of precession can reach sub-THz
frequencies; (ii) for the easy axis anisotropy two regimes are possible:
deterministic switching of the net magnetization and a conical sub-THz
precession depending on the STT strength.",1904.09341v1
2019-05-03,Generalized quenching of large-scale magnetic dynamos in anisotropic flows,"The buildup of small-scale magnetic helicity which accompanies the oppositely
signed growth on large scales is central to conventional dynamical quenching
theories of mean-field dynamos. However, the conventional formalism presumes
isotropic turbulence and thereby excludes part of the magnetic Lorentz
back-reaction. This renders it insufficient to predict the full quenching for
general anisotropic flows. To overcome this deficiency, we derive a new
generalized quenching formalism that includes the full back-reacting Lorentz
force, and a new ""selective-damping-$\tau$"" closure which conserves magnetic
helicity. We apply the formalism to examples of $\bm\alpha^2$ dynamos and show
its predicted quenching for different cases of turbulence---isotropic helical,
anisotropic helical, and anisotropic non-helical. It predicts
stronger-than-conventional quenching in general, but reduces to the
conventional case in the helical isotropic limit.",1905.01256v4
2019-05-24,Inverse cascade of hybrid helicity in $B Ω$-MHD turbulence,"We investigate the impact of a solid-body rotation $\Omega_0$ on the
large-scale dynamics of an incompressible magnetohydrodynamic turbulent flow in
presence of a background magnetic field $\bf B_0$ and at low Rossby number.
Three-dimensional direct numerical simulations are performed in a periodic box,
at unit magnetic Prandtl number and with a forcing at intermediate wavenumber
$k_f=20$. When $\Omega_0$ is aligned with $\bf B_0$ (i.e. $\theta \equiv
\widehat{\left(\Omega_{0}, \bf B_0 \right)} = 0$), inverse transfer is found
for the magnetic spectrum at $k<k_f$. This transfer is stronger when the
forcing excites preferentially right-handed (rather than left-handed)
fluctuations; it is smaller when $\theta>0$ and becomes weak when $\theta \ge
35^o$. These properties are understood as the consequence of an inverse cascade
of hybrid helicity which is an inviscid/ideal invariant of this system when
$\theta=0$. Hybrid helicity emerges, therefore, as a key element for
understanding rotating dynamos. Implication of these findings on the origin of
the alignment of the magnetic dipole with the rotation axis in planets and
stars is discussed.",1905.10250v1
2019-06-11,"Vortex dynamic, pinning and irreversibility field investigation in EuRbFe4As4 superconductor","We performed systematic AC susceptibility and magnetic moment measurements to
investigate the vortex dynamics and pinning in the $EuRbFe_4As_4$ single
crystal as a function of temperature, frequency, and DC magnetic field. The
vortex solid-liquid line was determined and it fits well with
$H(T_p)=H_0(1-t_p)^\beta$ using $\beta$=1.74-1.91, for $H\parallel ab$. It
indicates a rather high pinning strength of the vortex system. The activation
energy $U_0$ was determined from thermally activated flux creep theory and
reached 6700 K at low fields, suggesting strong vortex pinning. A field
dependence of $U_0(H\parallel ab)\sim H^a$ with $a=0.47$ suggests thermally
activated plastic pinning or caused by planar defects. Magnetic moment
measurements also confirmed strong pinning in a $EuRbFe_4As_4$ superconductor
and the superconducting response gives the main contribution to the $M(H)$
hysteresis. Additionally, we found evidence of long-range magnetic interactions
in $Eu^{2+}$ sublattice and the FM-like nature of $Eu^{2+}$ atoms ordering.",1906.04597v2
2019-06-25,Melting of Spin Ice state through structural disorder in Dy2Zr2O7,"Neutron scattering, a.c. magnetic susceptibility and specific heat studies
have been carried out on polycrystalline Dy2Zr2O7. Unlike the pyrochlore spin
ice Dy2Ti2O7, Dy2Zr2O7 crystallizes into the fluorite structure and the
magnetic Dy3+ moments randomly reside on the corner-sharing tetrahedral
sublattice with non-magnetic Zr ions. Antiferromagnetic spin correlations
develop below 10 K but remain dynamic down to 40 mK. These correlations extend
over the length of two tetrahedra edges and grow to 6 nearest neighbors with
the application of a 20 kOe magnetic field. No Pauling's residual entropy was
observed and by 8 K the full entropy expected for a two level system is
released. We propose that the disorder melts the spin ice state seen in the
chemically ordered Dy2Ti2O7 compound, but the spins remain dynamic in a
disordered, liquid-like state and do not freeze into a glass-like state that
one might intuitively expect.",1906.10738v1
2019-07-04,Domain wall dynamics due to femtosecond laser-induced superdiffusive spin transport,"Manipulation of magnetic domain walls via a helicity-independent laser pulse
has recently been experimentally demonstrated and various physical mechanisms
leading to domain wall dynamics have been discussed. Spin-dependent
superdiffusive transport of hot electrons has been identified as one of the
possible ways how to affect a magnetic domain wall. Here, we develop a model
based on superdiffusive spin-dependent transport to study the laser-induced
transport of hot electrons through a smooth magnetic domain wall. We show that
the spin transfer between neighboring domains can enhance ultrafast
demagnetization in the domain wall. More importantly, our calculations reveal
that when the laser pulse is properly focused on to the vicinity of the domain
wall, it can excite sufficiently strong spin currents to generate a
spin-transfer torque that can rapidly move the magnetic domain wall by several
nanometers in several hundreds of femtoseconds, leading to a huge
nonequilibrium domain wall velocity.",1907.02419v1
2019-08-31,Dispersions of Many-Body Bethe strings,"Complex bound states of magnetic excitations, known as Bethe string, were
predicted almost a century ago to exist in one-dimensional quantum magnets 1.
The dispersions of the string states have so far remained the subject of
intensive theoretical studies 2-7. By performing neutron scattering experiments
on the one-dimensional Heisenberg-Ising antiferromagnet SrCo2V2O8 in high
longitudinal magnetic fields, we reveal in detail the dispersion relations of
the string states over the full Brillouin zone, as well as their magnetic field
dependences. Furthermore the characteristic energy, the scattering intensity
and linewidth of the observed string states exhibit excellent agreement with
our precise Bethe Ansatz calculations. Our results establish the important role
of string states in the quantum spin dynamics of one-dimensional systems, and
will invoke studies of their dynamical properties in more general many-body
systems.",1909.00146v1
2019-09-12,Perspectives on spin hydrodynamics in ferromagnetic materials,"The field of spin hydrodynamics aims to describe magnetization dynamics from
a fluid perspective. For ferromagnetic materials, there is an exact mapping
between the Landau-Lifshitz equation and a set of dispersive hydrodynamic
equations. This analogy provides ample opportunities to explore novel
magnetization dynamics and magnetization states that can lead to applications
relying entirely upon magnetic materials, for example, long-distance transport
of information. This article provides an overview of the theoretical
foundations of spin hydrodynamics and their physical interpretation in the
context of spin transport. We discuss other proposed applications for spin
hydrodynamics as well as our view on challenges and future research directions.",1909.05756v1
2019-09-27,Dynamics of longitudinal magnetization in transverse-field quantum Ising model: from symmetry-breaking gap to Kibble-Zurek mechanism,"We show that the symmetry-breaking gap of the quantum Ising model in the
transverse field can be extracted from free evolution of the longitudinal
magnetization taking place after a gradual quench of the magnetic field. We
perform for this purpose numerical simulations of the Ising chains with either
periodic or open boundaries. We also study the condition for adiabaticity of
evolution of the longitudinal magnetization finding excellent agreement between
our simulations and the prediction based on the Kibble-Zurek theory of
non-equilibrium phase transitions. Our results should be relevant for ongoing
cold atom and ion experiments targeting either equilibrium or dynamical aspects
of quantum phase transitions. They could be also useful for benchmarking D-Wave
machines.",1909.12853v3
2019-10-11,Investigation of spin orbit torque driven dynamics in ferromagnetic heterostructures,"We use time-resolved (TR) measurements based on the polar magneto-optical
Kerr effect (MOKE) to study the magnetization dynamics excited by spin orbit
torques in Py (Permalloy)/Pt and Ta/CoFeB bilayers. The analysis reveals that
the field-like (FL) spin orbit torque (SOT) dominates the amplitude of the
first oscillation cycle of the magnetization precession and the damping-like
(DL) torque determines the final steady-state magnetization. In our bilayer
samples, we have extracted the effective fields, hFL and hDL, of the two SOTs
from the time-resolved magnetization oscillation spectrum. The extracted values
are in good agreement with those extracted from time-integrated DCMOKE
measurements, suggesting that the SOTs do not change at high frequencies. We
also find that the amplitude ratio of the first oscillation to steady state is
linearly proportional to the ratio hFL/hDL. The first oscillation amplitude is
inversely proportional to, whereas the steady state value is independent of,
the applied external field along the current direction.",1910.04945v1
2019-12-01,On the Chaplygin sphere in a magnetic field,"We consider the possibility of using Dirac's ideas of the deformation of
Poisson brackets in nonholonomic mechanics. As an example, we analyze the
composition of external forces that do no work and reaction forces of
nonintegrable constraints in the model of a nonholonomic Chaplygin sphere on a
plane. We prove that, when a solenoidal field is applied, the general
mechanical energy, the invariant measure and the conformally Hamiltonian
representation of the equations of motion are preserved. In addition, we
consider the case of motion of the nonholonomic Chaplygin sphere in a constant
magnetic field taking dielectric and ferromagnetic (superconducting) properties
of the sphere into account. As a by-product we also obtain two new integrable
cases of the Hamiltonian rigid body dynamics in a constant magnetic field
taking the magnetization by rotation effect into account.",1912.00373v1
2019-12-11,Lyapunov exponents and entanglement entropy transition on the noncommutative hyperbolic plane,"We study quantum dynamics on noncommutative spaces of negative curvature,
focusing on the hyperbolic plane with spatial noncommutativity in the presence
of a constant magnetic field. We show that the synergy of noncommutativity and
the magnetic field tames the exponential divergence of operator growth caused
by the negative curvature of the hyperbolic space. Their combined effect
results in a first-order transition at a critical value of the magnetic field
in which strong quantum effects subdue the exponential divergence for {\it all}
energies, in stark contrast to the commutative case, where for high enough
energies operator growth always diverge exponentially. This transition
manifests in the entanglement entropy between the `left' and `right' Hilbert
spaces of spatial degrees of freedom. In particular, the entanglement entropy
in the lowest Landau level vanishes beyond the critical point. We further
present a non-linear solvable bosonic model that realizes the underlying
algebraic structure of the noncommutative hyperbolic plane with a magnetic
field.",1912.10805v3
2020-03-10,Collective Transport of Magnetic Microparticles at a Fluid Interface through Dynamic Self-Assembled Lattices,"The transport of motile entities across modulated energy landscapes plays an
important role in a range of phenomena in biology, colloidal science and
solid-state physics. Here, an easily implementable strategy that allows for the
collective and monitored transport of microparticles at fluid-fluid interfaces
is introduced. Adsorbed magnetic microparticles are carried on time-dependent
magnetic potentials, generated by a dynamic self-assembled lattice of
different-sized magnetic particles. In such binary systems, the sudden
reorientation of the applied field triggers the rapid exchange between
attractive and repulsive configurations, enabling for the ballistic transfer of
the carriers through the lattice. As the number of motile entities increases,
the induced current increases, before reaching a maximum, while the loaded
interface gradually displays bidirectional transport. The described methodology
can be tuned through the applied field and exploited for the monitored guidance
of adsorbed molecules on liquid surfaces, the segregation of colloidal
mixtures, the induced motion of defects in photonic crystals or the design of
new self-assembled microrobots.",2003.04594v1
2020-05-31,Ultrafast dynamics of light-induced spin crossovers under high pressure,"Within the multielectron model of magnetic insulator with spin crossover
under high pressure we have studied the mean field phase diagrams in pressure
temperature plane and dynamics of a sudden excited non equilibrium spin state.
We obtain the different relaxation of the magnetization, high spin/low spin
occupation numbers, and the metal oxygen bond length for different values of
the external pressure. We found the long living oscillations of magnetization
without pressure and at small external pressure. Close to crossover pressure
the smooth relaxation is accompanied with a set of sharp strongly non linear
oscillations of magnetization and HS/LS occupation numbers that are accompanied
by the Franck Condon resonances.",2006.00539v1
2020-08-03,"Energy Transfer by Nonlinear Alfvén Waves in the Solar Chromosphere, and Its Effect on Spicule Dynamics, Coronal Heating, and Solar Wind Acceleration","Alfv\'en waves are responsible for the transfer of magnetic energy in the
magnetized plasma. They are involved in heating solar atmosphere and driving
solar wind through various nonlinear processes. Since the magnetic field
configurations directly affect the nonlinearity of Alfv\'en waves, it is
important to investigate how they relate to the solar atmosphere and wind
structure through the nonlinear propagation of Alfv\'en waves. In this study,
we carried out the one-dimensional magnetohydrodynamic simulations to realize
the above relation. The results show that when the nonlinearity of Alfv\'en
waves in the chromosphere exceeds a critical value, the dynamics of the solar
chromosphere (e.g., spicule) and the mass loss rate of solar wind tend to be
independent of the energy input from the photosphere. In a situation where the
Alfv\'en waves are highly nonlinear, the strong shear torsional flow generated
in the chromosphere ``fractures'' the magnetic flux tube. This corresponds to
the formation of chromospheric intermediate shocks, which limit the
transmission of the Poynting flux into the corona by Alfv\'en waves and also
inhibits the propagation of chromospheric slow shock.",2008.00643v1
2020-08-03,Small-Scale Dynamo in Stably Stratified Turbulence,"We present numerical investigations into three principal properties of the
small-scale dynamo in stably stratified turbulence: the onset criterion, the
growth rate, and the nature of the magnetic field anisotropy in the kinematic
regime. The results suggest that all three dynamo properties are controlled by
the scale separation between the Ozmidov scale and the viscous or resistive
scale. In addition to the critical magnetic Reynolds number, this allows for
the definition of critical buoyancy and magnetic buoyancy Reynolds numbers for
stratified small-scale dynamo onset in the high and low magnetic Prandtl number
regimes, respectively. The presence of a small-scale dynamo in stellar
radiative zones could affect dynamics through resulting Maxwell stresses and/or
influence large-scale dynamo mechanisms in regions of differential rotation.
Taking the solar radiative zone as a representative example and applying the
onset criterion, we find that the stratification is strong enough to make the
small-scale dynamo marginally active in the stably stratified turbulence of the
solar tachocline.",2008.01025v3
2020-09-07,As a matter of tension -- kinetic energy spectra in MHD turbulence,"Magnetized turbulence is ubiquitous in many astrophysical and terrestrial
systems but no complete, uncontested theory even in the simplest form,
magnetohydrodynamics (MHD), exists. Many theories and phenomenologies focus on
the joint (kinetic and magnetic) energy fluxes and spectra. We highlight the
importance of treating kinetic and magnetic energies separately to shed light
on MHD turbulence dynamics. We conduct an implicit large eddy simulation of
subsonic, super-Alfv\'enic MHD turbulence and analyze the scale-wise energy
transfer over time. Our key finding is that the kinetic energy spectrum
develops a scaling of approximately $k^{-4/3}$ in the stationary regime as the
kinetic energy cascade is suppressed by magnetic tension. This motivates a
reevaluation of existing MHD turbulence theories with respect to a more
differentiated modeling of the energy fluxes.",2009.03342v1
2020-09-17,Magnetic Gradient Fluctuations from Quadrupolar $^{73}$Ge in Si/SiGe Exchange-Only Qubits,"We study the time-fluctuating magnetic gradient noise mechanisms in pairs of
Si/SiGe quantum dots using exchange echo noise spectroscopy. We find through a
combination of spectral inversion and correspondence to theoretical modeling
that quadrupolar precession of the $^{73}$Ge nuclei play a key role in the
spin-echo decay time $T_2$, with a characteristic dependence on magnetic field
and the width of the Si quantum well. The $^{73}$Ge noise peaks appear at the
fundamental and first harmonic of the $^{73}$Ge Larmor resonance, superimposed
over $1/f$ noise due to $^{29}$Si dipole-dipole dynamics, and are dependent on
material epitaxy and applied magnetic field. These results may inform the needs
of dynamical decoupling when using Si/SiGe quantum dots as qubits in quantum
information processing devices.",2009.08079v1
2020-09-20,Non-classical spin transfer effects in an antiferromagnet,"We simulate scattering of electrons by a chain of antiferromagnetically
coupled quantum Heisenberg spins, to analyze spin-transfer effects not
described by the classical models of magnetism. Our simulations demonstrate
efficient excitation of dynamical states that would be forbidden by the
semiclassical symmetries, such as generation of multiple magnetic excitation
quanta by a single electron. Furthermore, quantum interference of spin
wavefunctions enables generation of magnetization dynamics with amplitudes
exceeding the transferred magnetic moment. The efficiency of excitation is
almost independent of the electron spin polarization, and is governed mainly by
the transfer of energy. Non-classical spin transfer may thus enable efficient
electronic control of antiferromagnets not limited by the classical
constraints.",2009.09540v2
2020-09-30,Electronic structure and finite temperature magnetism of yttrium iron garnet,"Yttrium iron garnet is a complex ferrimagnetic insulator with 20 magnon modes
which is used extensively in fundamental experimental studies of magnetisation
dynamics. As a transition metal oxide with moderate gap (2.8 eV), yttrium iron
garnet requires a careful treatment of electronic correlation. We have applied
quasiparticle self-consistent GW to provide a fully ab initio description of
the electronic structure and resulting magnetic properties, including the
parameterisation of a Heisenberg model for magnetic exchange interactions.
Subsequent spin dynamical modelling with quantum statistics extends our
description to the magnon spectrum and thermodynamic properties such as the
Curie temperature, finding favourable agreement with experimental measurements.
This work provides a snapshot of the state-of-the art in modelling of complex
magnetic insulators.",2009.14601v1
2020-10-09,"A Magnetized, Moon-Forming Giant Impact","The Moon is believed to have formed in the aftermath of a giant impact
between a planetary mass body and the proto-Earth. In a typical giant impact
scenario, a disk of vapor, liquid, and solid debris forms around the
proto-Earth and--after possibly decades of evolution--condenses to form the
Moon. Using state-of-the-art numerical simulations, we investigate the
dynamical effects of magnetic fields on the Moon-forming giant impact. We show
that turbulence generated by the collision itself, shear in the boundary layer
between the post-impact debris field and the proto-Earth, and turbulence in the
vapor component of the disk amplify the field to dynamically significant
strengths. Magnetically driven turbulence promotes angular momentum transport
in the protolunar disk. Debris material is accreted onto the proto-Earth,
making Moon formation less efficient, while the disk is forced to spread to
larger radii, cooling at its outer edge. Magnetic fields speed the evolution of
the vapor component of the protolunar disk and hasten the formation of the
Moon.",2010.04798v1
2020-11-03,Driving spin chirality by electron dynamics in laser-excited antiferromagnets,"Optical generation of complex spin textures is one of the most exciting
challenges of modern spintronics. Here, we uncover a distinct physical
mechanism for imprinting spin chirality into collinear magnets with short laser
pulses. By simultaneously treating the laser-ignited evolution of electronic
structure and magnetic order, we show that their intertwined dynamics can
result in an emergence of quasi-stable chiral states. We find that laser-driven
chirality does not require any auxiliary external fields or intrinsic
spin-orbit interaction to exist, and it can survive on the time scale of
nanoseconds even in the presence of thermal fluctuations, which makes the
uncovered mechanism relevant for understanding various optical experiments on
magnetic materials. Our findings open a new perspective at the interaction of
complex chiral magnetism with light.",2011.01670v3
2020-11-23,New approach to describe two coupled spins in a variable magnetic field,"We propose a method to describe the evolution of two spins coupled by
hyperfine interaction in an external time-dependent magnetic field. We apply
the approach to the case of hyperfine interaction with axial symmetry, which
can be solved exactly in a constant, appropriately oriented magnetic field. In
order to treat the nonstationary dynamical problem, we modify the
time-dependent Schr\""odinger equation through a change of representation that,
by exploiting an instantaneous (adiabatic) basis makes the time-dependent
Hamiltonian diagonal at any time instant. The solution of the transformed
time-dependent Schr\""odinger in the form of chronologically ordered exponents
with transparent pre-exponential coefficients is reported. This solution is
highly simplified when an adiabatically varying magnetic field perturbs the
system. The approach here proposed may be used for the perturbative treatment
of other dynamical problems with no exact solution.",2011.11569v1
2020-12-16,Magnetic Compton profiles of Ni beyond the one-particle picture: numerically exact and perturbative solvers of dynamical mean-field theory,"We calculated the magnetic Compton profiles (MCPs) of Ni using density
functional theory supplemented by electronic correlations treated within
dynamical mean-field theory (DMFT). We present comparisons between the
theoretical and experimental MCPs. The theoretical MCPs were calculated using
the KKR method with the perturbative spin-polarized T-matrix fluctuation
exchange approximation DMFT solver, as well as with the full potential linear
augmented planewave method with the numerically exact continuous-time quantum
Monte Carlo DMFT solver. We show that the total magnetic moment decreases with
the intra-atomic Coulomb repulsion $U$, which is also reflected in the
corresponding MCPs. The total magnetic moment obtained in experimental
measurements can be reproduced by intermediate values of $U$. The spectral
function reveals that the minority X$_2$ Fermi surface pocket shrinks and gets
shallower with respect to the density functional theory calculations.",2012.08812v2
2021-01-04,Importance of magnetic fields in highly eccentric discs with applications to tidal disruption events,"Whether tidal disruption events (TDEs) circularise or accrete directly as a
highly eccentric disc is the subject of current research and appears to depend
sensitively on the disc thermodynamics. In a previous paper we applied the
theory of eccentric discs to TDE discs using an $\alpha-$prescription for the
disc stress, which leads to solutions that exhibit extreme, potentially
unphysical, behaviour. In this paper we further explore the dynamical vertical
structure of highly eccentric discs using alternative stress models that are
better motivated by the behaviour of magnetic fields in eccentric discs. We
find that the presence of a coherent magnetic field has a stabilising effect on
the dynamics and can significantly alter the behaviour of highly eccentric
radiation dominated discs. We conclude that magnetic fields are important for
the evolution of TDE discs.",2101.01221v1
2021-01-20,Dynamic mitigation of filamentation instability and magnetic reconnection in sheet-current sustained plasma,"Dynamic mitigation is presented for filamentation instability and magnetic
reconnection in a plasm driven by a wobbling electron sheet current. The
wobbling current introduces an oscillating perturbation and smooths the
perturbation. The sheet current creates an anti-parallel magnetic field in
plasma. The initial small perturbation induces the electron beam filamentation
and the magnetic reconnection. When the wobbling or oscillation motion is added
to the sheet electron beam along the sheet current surface, the perturbation
phase is mixed and consequently the instability growth is delayed remarkably.
Normally plasma instabilities are discussed by the growth rate, because it
would be difficult to measure or detect the phase of the perturbations in
plasmas. However, the phase of perturbation can be controlled externally, for
example, by the driver wobbling motion. The superimposition of perturbations
introduced actively results in the perturbation smoothing, and the instability
growth can be reduced, like feed-forward control.",2101.07939v1
2021-01-26,Hydrodynamical study of Terahertz emission in magnetized graphene field-effect transistors,"Several hydrodynamic descriptions of charge transport in graphene have been
presented in the late years. We discuss a general hydrodynamic model governing
the dynamics of a two-dimensional electron gas in a magnetized field-effect
transistor in the slow drift regime. The Dyakonov--Shur instability is
investigated including the effect of weak magnetic fields (i.e. away from
Landau levels). We show that the gap on the dispersion relation prevents the
instability to reach the lower frequencies thus imposing a limit on the Mach
number of the electronic flow. Furthermore, we discuss that the presence of the
external magnetic field decreases the growth rate of the instability, as well
as the saturation amplitude. The numerical results from our simulations and the
presented higher order dynamic mode decomposition support such reasoning.",2101.10786v2
2021-05-03,Three-dimensional core-collapse supernovae with complex magnetic structures: I. Explosion dynamics,"Magnetic fields can play a major role in the dynamics of outstanding
explosions associated to violent events such as GRBs and hypernovae, since they
provide a natural mechanism to harness the rotational energy of the central
proto-neutron star and power relativistic jets through the stellar progenitor.
As the structure of such fields is quite uncertain, most numerical models of
MHD-driven core-collapse supernovae consider an aligned dipole as initial
magnetic field, while the field's morphology can actually be much more complex.
We present three-dimensional simulations of core-collapse supernovae with more
realistic magnetic structures, such as quadrupolar fields and, for the first
time, an equatorial dipolar field. Configurations other than an aligned dipole
produce weaker explosions and less collimated outflows, but can at the same
time be more efficient in extracting the rotational energy from the PNS. This
energy is then stored in the surroundings of the PNS, rather than powering the
polar jets. A significant axial dipolar component is also produced by models
starting with a quadrupolar field, pointing to an effective dynamo mechanism
operating in proximity of the PNS surface.",2105.00665v2
2021-07-22,Are GRMHD Mean-Field Dynamo Models of Thick Accretion Disks SANE?,"The remarkable results by the Event Horizon Telescope collaboration
concerning the emission from M87* and, more recently, its polarization
properties, require an increasingly accurate modeling of the plasma flows
around the accreting black hole. Radiatively inefficient sources such as M87*
and Sgr A* are typically modeled with the SANE (standard and normal evolution)
paradigm, if the accretion dynamics is smooth, or with the MAD (magnetically
arrested disk) paradigm, if the black hole's magnetosphere reacts by halting
the accretion sporadically, resulting in a highly dynamical process. While the
recent polarization studies seem to favor MAD models, this may not be true for
all sources, and SANE accretion surely still deserves attention. In this work,
we investigate the possibility of reaching the typical degree of magnetization
and other accretion properties expected for SANE disks by resorting to the
mean-field dynamo process in axisymmetric GRMHD simulations, which are supposed
to mimic the amplifying action of an unresolved magnetorotational
instability-driven turbulence. We show that it is possible to reproduce the
main diagnostics present in the literature by starting from very unfavorable
initial configurations, such as a purely toroidal magnetic field with
negligible magnetization.",2107.10630v1
2021-07-27,"Dipolar Ising Model: Phases, Growth Laws and Universality","The behavior of many magnetic and dielectric solids, and the more
contemporary magnetic super-lattices, is governed by dipolar interactions. They
are anisotropic and long-ranged, having varied consequences ranging from ground
states with complicated magnetic order to the presence of glassy dynamics
characterised by a plethora of relaxation times.These systems are well-captured
by the dipolar Ising model (DIM) with nearest-neighbor exchange interactions
(J) and long-range dipolar interactions (D). Depending on the relative
interaction strength {\Gamma} =J/D, there are four phases of distinct magnetic
order and symmetry. Using Monte Carlo simulations, we perform deep quenches to
study domain growth or coarsening in the d= 3 DIM. This important
non-equilibrium phenomenon has not been addressed as dipolar interactions are
notoriously difficult to handle theoretically. Our study reveals that, in spite
of the anisotropy in interactions and diversity in ground state configurations,
we observe universality in the ordering dynamics of all phases.",2107.12803v1
2021-08-11,Optical excitation and probing of the antiferromagnetic modes with non-uniform in-depth distribution in birefringent antiferromagnetic crystals,"Optical pump-probe setups are commonly used for excitation and investigation
of the spin dynamics in various types of magnetic materials. However, usually
the spatially homogeneous excitation is considered. In the present study we
describe an approach for optical excitation of the nonuniform THz spin dynamics
and for probing its spatial distribution inside a magnetic crystal. We propose
to illuminate a crystal with laser pulses of properly adjusted polarization to
benefit from a strong optical birefringence inherent to the crystal. It results
in an unusual behavior of the effective magnetic field generated by the pulses
due to the inverse Faraday effect and the peculiar sign-changing dependence of
the direct Faraday effect inside the crystal. The study is performed exemplary
for yttrium orthoferrite crystal although the proposed approach is applicable
for various magnetic materials with optical anisotropy.",2108.05443v2
2021-08-16,Bell-state generation for spin qubits via dissipative coupling,"We theoretically investigate the dynamics of two spin qubits interacting with
a magnetic medium. A systematic formal framework for this qubit-magnet hybrid
system is developed in terms of the steady-state properties of the magnetic
medium. Focusing on the induced dissipative coupling between the spin qubits,
we show how a sizable long-lived entanglement can be established via the
magnetic environment, in the absence of any coherent coupling. Moreover, we
demonstrate that maximally-entangled two-qubit states (Bell states) can be
achieved in this scheme when complemented by proper postselection. In this
situation, the time evolution of the entanglement is governed by a
non-Hermitian Hamiltonian, where dynamical phases are separated by an
exceptional point. The resultant Bell state is robust against weak random
perturbations and does not require the preparation of a particular initial
state. Our study may find applications in quantum information science, quantum
spintronics, and for sensing of nonlocal quantum correlations.",2108.07365v2
2021-09-01,Collective hydrodynamic transport of magnetic microrollers,"We investigate the collective transport properties of microscopic magnetic
rollers that propel close to a surface due to a circularly polarized, rotating
magnetic field. The applied field exerts a torque to the particles, which
induces a net rolling motion close to a surface. The collective dynamics of the
particles result from the balance between magnetic dipolar interactions and
hydrodynamic ones. We show that, when hydrodynamics dominate, i.e. for high
particle spinning, the collective mean velocity linearly increases with the
particle density. In this regime we analyse the clustering kinetics, and find
that hydrodynamic interactions between the anisotropic, elongated particles,
induce preferential cluster growth along a direction perpendicular to the
driving one, leading to dynamic clusters that easily break and reform during
propulsion.",2109.00289v2
2021-09-28,Radical pairs may play a role in microtubule reorganization,"The exact mechanism behind general anesthesia remains an open question in
neuroscience. It has been proposed that anesthetics selectively prevent
consciousness and memory via acting on microtubules (MTs). It is known that the
magnetic field modulates MT organization. A recent study shows that a radical
pair model can explain the isotope effect in xenon-induced anesthesia and
predicts magnetic field effects on anesthetic potency. Further, reactive oxygen
species are also implicated in MT stability and anesthesia. Based on a simple
radical pair mechanism model and a simple mathematical model of MT
organization, we show that magnetic fields can modulate spin dynamics of
naturally occurring radical pairs in MT. We show that the spin dynamics
influence a rate in the reaction cycle, which translates into a change in the
MT density. We can reproduce magnetic field effects on the MT concentration
that have been observed. Our model also predicts additional effects at slightly
higher fields. Our model further predicts that the effect of zinc on the MT
density exhibits isotopic dependence. The findings of this work make a
connection between microtubule-based and radical pair-based quantum theories of
consciousness.",2109.14055v1
2021-10-28,The dynamic chirality flips of Skyrmion bubbles,"Magnetic skyrmion, a topological magnetic domain with complex non-coplanar
spin texture, appears a disk-like structure in two dimensions. Exploring
three-dimensional spin texture and related chirality switching has drawn
enormous interests from the perspective of fundamental research. Here, the
three-dimensional magnetic moment of the skyrmion bubbles in centrosymmetric
Mn-Ni-Ga were reconstructed with the vector field tomography approach via
Lorentz transmission electron microscopy. The type of the bubbles was
determined from investigating the magnetic vectors in entire space. We found
that the bubbles switched their chirality easily but still keep the polarity to
remain the singularity of the bubbles within the material. Our results offer
valuable insights into the fundamental mechanisms underlying the spin chirality
flips dynamics of skyrmion bubbles.",2110.14898v1
2021-11-24,A generalized model of magnon kinetics and subgap magnetic noise,"Magnetic noise spectroscopy provides a noninvasive probe of spin dynamics in
magnetic materials. We consider two-dimensional magnetically ordered insulators
with magnon excitations, especially those supporting long-distance magnon
transport, where nitrogen-vacancy (NV) centers enable the access to (nearly)
ballistic transport regime of magnons. We develop a generalized theory to
describe the magnon transport across a wide range of length scales. The
longitudinal dynamic spin susceptibility is derived from the Boltzmann equation
and extended to a Lindhard form, which is modified by both the spin-conserving
magnon collisions and spin relaxation. Our result is consistent with the
diffusive (ballistic) model for the length scale much larger (smaller) than the
magnon mean free path, and provides a description for the intermediate regime.
We also give a prediction for the NV transition rate in different magnon
transport regimes.",2111.12613v2
2021-12-16,Geometric continuous-stage exponential energy-preserving integrators for charged-particle dynamics in a magnetic field from normal to strong regimes,"This paper is concerned with geometric exponential energy-preserving
integrators for solving charged-particle dynamics in a magnetic field from
normal to strong regimes. We firstly formulate the scheme of the methods for
the system in a uniform magnetic field by using the idea of continuous-stage
methods, and then discuss its energy-preserving property. Moreover, symmetric
conditions and order conditions are analysed. Based on those conditions, we
propose two practical symmetric continuous-stage exponential energy-preserving
integrators of order up to four. Then we extend the obtained methods to the
system in a nonuniform magnetic field and derive their properties including the
symmetry, convergence and energy conservation. Numerical experiments
demonstrate the efficiency of the proposed methods in comparison with some
existing schemes in the literature.",2112.08576v1
2021-12-29,Harnessing re-programmable phase transitions to control the propagation of sound waves,"Metamaterials can enable peculiar static and dynamic behavior (such as
negative effective mass density, dynamical stiffness, and Poisson's ratio) due
to their geometry rather than their chemical composition. The geometry of these
metamaterials can be thought of as the phase of the material, which is usually
fixed once the material is fabricated. While there exist many theoretical and
numerical studies of metamaterials that can change phase, or re-program,
experimental realizations remain limited due to challenges in
manufacturability, the destructive nature of the re-programming and inherent
non-linearities. Through a combination of analytical, numerical and
experimental analyses, we utilize tunable, self-assembled, nonlinear magnetic
lattices to realize metamaterials with reversible phase transitions. Our
metamaterials are composed of free-floating disks, with embedded permanent
magnets, confined within magnetic boundaries. We exploit the non-destructive
nature of the adjustable magnetic boundaries to create a set of re-programmable
metamaterials to control the propagation of sound waves. Furthermore, we
demonstrate a robust, real-time tunable wave filter at ultra-low frequencies.
Our findings can expand the metamaterials horizon into functional and tunable
devices.",2112.14787v1
2021-12-30,Spin Torque Oscillations Triggered by In-plane Field,"We study the dynamics of a spin torque nano oscillator that consists of
parallelly magnetized free and pinned layers by numerically solving the
associated Landau-Lifshitz-Gilbert-Slonczewski equation in the presence of a
field-like torque. We observe that an in-plane magnetic field which is applied
for a short interval of time ($<$1ns) triggers the magnetization to exhibit
self-oscillations from low energy initial magnetization state. Also, we confirm
that the frequency of oscillations can be tuned over the range $\sim$25 GHz to
$\sim$72 GHz by current, even in the absence of field-like torque. We find the
frequency enhancement up to 10 GHz by the presence of field-like torque. We
determine the Q-factor for different frequencies and show that it increases
with frequency. Our analysis with thermal noise confirms that the system is
stable against thermal noise and the dynamics is not altered appreciably by it.",2112.15004v1
2022-01-05,Topological characterization of dynamic chiral magnetic textures using machine learning,"Recently proposed spintronic devices use magnetic skyrmions as bits of
information. The reliable detection of those chiral magnetic objects is an
indispensable requirement. Yet, the high mobility of magnetic skyrmions leads
to their stochastic motion at finite temperatures, which hinders the precise
measurement of the topological numbers. Here, we demonstrate the successful
training of artificial neural networks to reconstruct the skyrmion number in
confined geometries from time-integrated, dimensionally reduced data. Our
results prove the possibility to recover the topological charge from a
time-averaged measurement and hence smeared dynamic skyrmion ensemble, which is
of immediate relevance to the interpretation of experimental results,
skyrmion-based computing, and memory concepts",2201.01629v2
2022-02-07,Magnetization dynamics affected by phonon pumping,"""Pumping"" of phonons by a dynamic magnetization promises to extend the range
and functionality of magnonic devices. We explore the impact of phonon pumping
on room-temperature ferromagnetic resonance (FMR) spectra of bilayers of thin
yttrium iron garnet (YIG) films on thick gadolinium gallium garnet substrates
over a wide frequency range. At low frequencies the Kittel mode hybridizes
coherently with standing ultrasound waves of a bulk acoustic resonator to form
magnon polarons that induce rapid oscillations of the magnetic susceptibility,
as reported before. At higher frequencies, the phonon resonances overlap,
merging into a conventional FMR line, but with an increased line width. The
frequency dependence of the increased line broadening follows the predictions
from phonon pumping theory in the thick substrate limit. In addition, we find
substantial magnon-phonon coupling of a perpendicular standing spin wave (PSSW)
mode. This evidences the importance of the mode overlap between the acoustic
and magnetic modes, and provides a route towards engineering the magnetoelastic
mode coupling.",2202.03331v1
2022-02-10,Dimensional reduction and the generalized pion in a magnetic field within the NJL model,"In this work, the mass of the neutral pion is investigated in the presence of
background magnetic fields in the framework of the Nambu--Jona-Lasinio model.
Taking into account the anisotropic four-fermion interactions, a tensor current
arises in the magnetized QCD system, which forms an anomalous magnetic moment
(AMM) coupling in the Dirac equation for the quarks. By solving the gap
equations, we find that the sign of the dynamically generated AMM is opposite
to the sign of the quark's charge and its magnitude is definitely smaller than
the constituent mass. We construct two generalized Nambu-Goldstone pions, which
emerge as combinations of the quantum fluctuations around the conventional
scalar and the emergent tensor chiral condensates. We analytically demonstrate
that the Goldstone nature has been spoiled by the dimensional reduction in the
two-particle state and the corresponding decreasing mass of the lighter
generalized pionic mode is a remnant of the infrared dynamics.",2202.05090v2
2022-03-03,Beam dynamics corrections to the Run-1 measurement of the muon anomaly $a_μ$ at Fermilab,"The Fermi National Accelerator Laboratory (FNAL) Muon $g-2$ Experiment has
measured the positive muon magnetic anomaly $a_{\mu} \equiv (g_{\mu}-2)/2$ with
a precision of 0.46 parts per million, with data collected during its first
physics run in 2018. The experimental result combined with the measurement from
the former experiment at Brookhaven National Laboratory increases the tension
with the Standard Model expectation to $4.2\sigma$, thus strengthening evidence
for new physics. The magnetic anomaly is determined from the precision
measurements of the muon spin precession frequency, relative to the muon
momentum, and the average magnetic field seen by the beam. In an ideal case
with muons orbiting in a horizontal plane with a uniform vertical magnetic
field, the anomalous precession frequency $\omega_a$ is given by the difference
between the spin~($s$) and cyclotron~($c$) frequencies, $\omega_a = \omega_s -
\omega_c$. The observable $\omega_a$ is proportional to $a_{\mu}$. This
proceeding presents the beam dynamics systematic corrections that are required
to adjust the measured muon precession frequency $\omega^{m}_{a}$ to its true
physical value $\omega_{a}$.",2203.02021v1
2022-03-22,Intrinsic ferromagnetism and restrictive thermodynamic stability in MA$_2$N$_4$ and Janus VSiGeN$_4$ monolayers,"The seminal experimental discovery of the remarkably stable MoSi$_2$N$_4$
monolayer has led to a handful of predicted magnetic two-dimensional (2D)
materials in the MA$_2$Z$_4$ family (M = transition metals, A = Si, Ge, and Z =
N, P, As). These magnetic monolayers were predicted to be dynamically stable,
but none of them has been synthesized to date. In this Research Letter, from
first-principles thermodynamic stability analysis, we demonstrate that only the
nitrides are thermodynamically stable and this occurs under N-rich conditions.
Based on this finding, we propose two ferromagnetic, semiconducting Janus
monolayers in the family: VSiGeN$_4$ and VSiSnN$_4$. They are both dynamically
and thermally stable, but only the former is thermodynamically stable.
Intriguingly, Janus VSiGeN$_4$ and VSiSnN$_4$ monolayers show weak in-plane
anisotropy compared with the VSi$_2$N$_4$ monolayer. These two emerging Janus
magnetic semiconductors offer opportunities for studying 2D magnetism and spin
control for spintronics applications.",2203.11605v3
2022-04-03,Zero absolute vorticity plane Couette flow as an hydrodynamic representation of quantum energy states under perpendicular magnetic field,"Here we extend the Madelung transformation of the Schr\""{o}dinger equation
into a fluid-like form to include the influence of an external electromagnetic
field on a charged particle. The vorticity of the Madelung fluid is then in the
opposite direction to the imposed magnetic field and equal in magnitude to the
cyclotron angular frequency. When the particle motion is confined to a plane,
perpendicular to an imposed magnetic field, the equivalent flow dynamics is
that of zero absolute vorticity obtained in a quasi 2D rotating frame, where
the cyclotron frequency plays a role equivalent to that of the Coriolis
frequency in a rotating frame. We show how the Landau levels and the extended
modes in the integer quantum Hall effect are all mapped into such zero absolute
vorticity-like plane Couette flows, where the latter exhibit a geostrophic-like
balance between the magnetic force and the gradients of the quantum (Bohm)
potential and the electric force.",2204.01013v1
2022-05-23,Three-dimensional direct numerical simulation of free-surface magnetohydrodynamic wave turbulence,"We report on three-dimensional direct numerical simulation of wave turbulence
on the free surface of a magnetic fluid subjected to an external horizontal
magnetic field. A transition from capillarywave turbulence to anisotropic
magneto-capillary wave turbulence is observed for an increasing field. At high
enough field, wave turbulence becomes highly anisotropic, cascading mainly
perpendicularly to the field direction, in good agreement with the prediction
of a phenomenological model, and with anisotropic Alfv{\'e}n wave turbulence.
Although surface waves on a magnetic fluid are different from Alfv{\'e}n waves
in plasma, a strong analogy is found with similar wave spectrum scalings and
similar magnetic-field dependent dispersionless wave velocities.",2205.11516v1
2022-07-25,Local fluxes in MHD Turbulence,"Using highly resolved direct numerical simulations we examine the statistical
properties of the local energy flux rate $\Pi_\ell(x)$ towards small scales for
three isotropic turbulent magnetohydrodynamic flows, which differ in strength
and structure of the magnetic field. We analyse the cascade process both in the
kinetic and magnetic energy, disentangling the different flux contributions to
the overall energy dynamics. The results show that the probability distribution
of the local energy flux develops long tails related to extreme events, similar
to the hydrodynamic case. The different terms of the energy flux display
different properties and show sensitivity on the type of the flow examined. We
further examine the joint pdf between the local energy flux and the gradients
of the involved fields. The results point out a correlation with the magnetic
field gradients, showing however a dispersion much stronger than what is
observed in hydrodynamic flows. Finally, it is also shown that the local energy
flux shows some dependence on the local amplitude of the magnetic field. The
present results have implications for subgrid scale models that we discuss.",2207.12335v1
2022-07-28,Dynamical chiral magnetic current and instability in Weyl semimetals,"Weyl semimetals realize massless relativistic fermions with two Weyl nodes
separated in energy and momentum space, whose low-energy physics is described
by Dirac fermions with an axial gauge constant. Here, we study their
electromagnetic linear responses based on the effective field theory and on the
chiral kinetic theory. Although the static chiral magnetic effect is canceled
by the Chern-Simons current under the Pauli-Villars regularization, a dynamical
magnetic field is found capable of driving an electric current along its
direction, with the total transported charge being independent of temperature
and chemical potential for a uniform field. We also incorporate dissipation in
the relaxation-time approximation and study collective excitations coupled with
Maxwell electromagnetic fields when Weyl node populations deviate from
equilibrium. Their dispersion relations at low frequency and long wavelength
are determined only by electric, chiral magnetic, and anomalous Hall
conductivities, which predict unstable modes leading to anisotropic generation
of electromagnetic waves oriented to the direction of Weyl node separation.",2207.14272v3
2022-09-06,Magnetic-induced Spontaneous Scalarization in Dynamcial Chern-Simons Gravity,"In the framework of the dynamical Chern-Simons gravity, we study the scalar
field perturbations of the Reissner-Nordstr\""{o}m-Melvin spacetime, which
describes a charged black hole permeated by a uniform magnetic field. In the
presence of the magnetic field, the scalar field acquires an effective mass
whose square takes negative value in the half domain of the angular direction.
This inevitably introduces the tachyonic instability and associated spontaneous
scalarization as long as the coupling constant between the scalar field and the
Chern-Simons invariant exceeds a threshold value. We study the object pictures
of the time evolutions of the scalar field perturbations at the linear level,
and find that the presence of the magnetic field will dramatically change the
waveforms and associated ringdown modes. Nonlinear evolutions for the unstable
perturbations are also performed in the decoupling limit, which demonstrate the
scalar cloud as the final fate. Influences of the coupling constant and the
black hole charge on the wave dynamics are also studied.",2209.02268v2
2022-12-19,Multi-Meron Interactions and Statistics in Two-Dimensional Materials,"As a fundamental type of topological spin textures in two-dimensional (2D)
magnets, a magnetic meron carries half-integer topological charge and forms a
pair with its antithesis to keep the stability in materials. However, it is
challenging to quantitatively calculate merons and their dynamics by using the
widely used continuum model because of the characteristic highly inhomogeneous
spin textures. In this work, we develop a discrete method to address the
concentrated spin structures around the core of merons. We reveal a
logarithmic-scale interaction between merons when their distance is larger than
twice their core size and obtain subsequent statistics of meron gas. The model
also predicts how these properties of single and paired merons evolve with
magnetic exchange interactions, and the results are in excellent agreement with
the Monte Carlo simulations using the parameters of real 2D van der Waals
magnetic materials. This discrete approach not only shows equilibrium static
statistics of meron systems but also is useful to further explore the dynamic
properties of merons through the quantified pairing interactions.",2212.09698v1
2022-12-29,Non-volatile Electric Control of Magnetic and Topological Properties of MnBi2Te4 Thin Films,"In this letter, we propose a mechanism to control the magnetic properties of
topological quantum material (TQM) by using magnetoelectric coupling: this
mechanism uses a heterostructure of TQM with two-dimensional (2D) ferroelectric
material, which can dynamically control the magnetic order by changing the
polarization of the ferroelectric material and induce possible topological
phase transitions. This concept is demonstrated using the example of the
bilayer MnBi2Te4 on ferroelectric In2Se3 or In2Te3, where the polarization
direction of the 2D ferroelectrics determines the interfacial band alignment
and consequently the direction of the charge transfer. This charge transfer, in
turn, enhances the stability of the ferromagnetic state of MnBi2Te4 and leads
to a possible topological phase transition between the quantum anomalous Hall
(QAH) effect and the zero plateau QAH. Our work provides a route to dynamically
alter the magnetic ordering of TQMs and could lead to the discovery of new
multifunctional topological heterostructures.",2212.14331v1
2022-12-30,Unique determination of localized basis in molecular spin,"Localized basis plays an important role in comprehending the magnetic
dynamics in molecular spins from a physics perspective. Nonetheless, the
uniqueness and rigor of its determination have received limited attention. In
this study, we propose a new determination of the localized basis applicable to
both non-Kramers and Kramers molecular spin systems, leveraging the
time-reversal symmetry of the spin Hamiltonian and the molecular spin's main
magnetic axis. By introducing this, we establish a distinct and practical means
of determining the localized basis, enabling the association of a molecular
spin wave function with either an ""up"" or ""down"" magnetic moment orientation in
molecular spins. This finding facilitates a comprehensive interpretation of
magnetic dynamics and simplifies the construction of theoretical models for
materials analysis.",2212.14639v3
2023-01-12,Identification of Magnetic Field Errors in Synchrotrons based on Deep Lie Map Networks,"Magnetic field errors pose a limitation in the performance of synchrotrons,
as they excite non-systematic resonances, reduce dynamic aperture and may
result in beam loss. Their effect can be compensated assuming knowledge of
their location and strength. Established identification procedures are based on
orbit response matrices or resonance driving terms. While they sequentially
build a field error model for subsequent accelerator sections, a method
detecting field errors in parallel could save valuable beam time. We introduce
deep Lie map networks, which enable construction of an accelerator model
including multipole components for the magnetic field errors by linking charged
particle dynamics with machine learning methodology in a data-driven approach.
Based on simulated beam-position-monitor readings for the example case of SIS18
at GSI, we demonstrate inference of location and strengths of gradient and
sextupole errors for all accelerator sections in parallel. The obtained refined
accelerator model may support setup of corrector magnets in operation to allow
more precise control over tunes, chromaticities and resonance compensation.",2301.04914v2
2023-03-24,Magnetic and antimagnetic rotational bands data tables,"The experimental results of 252 magnetic rotational bands reported in 123
nuclei and 38 antimagnetic rotational bands reported in 27 nuclei are collected
and listed in the present work, including energy, spin, parity, magnetic dipole
reduced transition probability B(M1), electric quadrupole reduced transition
probability B(E2), B(M1)/B(E2) ratio, and the ratio of the dynamic moment of
inertia to the electric quadrupole reduced transition probability J(2)/B(E2).
Following the presentation of the kinematic moment of inertia J(1), dynamic
moment of inertia J(2), and I versus rotational frequency, as well as energy
staggering parameter S(I), B(M1), B(E2), B(M1)/B(E2), and J(2)/B(E2) versus I
in A 60, 80, 110, 140, and 190 mass regions, a brief discussion is provided.
Based on the systematic studies, some nuclei are predicted to be candidates for
magnetic or antimagnetic rotation.",2303.13849v1
2023-04-26,Confining and escaping magnetic field lines in Tokamaks: Analysis via symplectic map,"In magnetically confined plasma, it is possible to qualitatively describe the
magnetic field configuration via phase spaces of suitable symplectic maps.
These phase spaces are of mixed type, where chaos coexists with regular motion,
and the complete understanding of the chaotic transport is a challenge that,
when overcome, may provide further knowledge into the behaviour of confined
fusion plasma. This work presents two numerical investigations into
characteristics of mixed phase spaces which model distinct magnetic
configurations in tokamaks under different perturbation regimes. The first
approach relies on a recurrence-based analysis of ensembles of chaotic
trajectories to detect open field lines that widely differ from the average.
The second focuses on the transient dynamical behaviour of field lines before
they escape the systems. These two methods provide insights into the influence
of stickiness and invariant manifolds on the evolution of chaotic trajectories,
improving our understanding of how these features affect the chaotic transport
and diffusion properties in mixed phase spaces. These theoretical and numerical
approaches may enhance our comprehension of confined plasma behaviour and
plasma-wall interactions.",2304.13810v1
2023-05-03,"Impacts of the half-skyrmion spin topology, spin-orbit torque, and dynamic symmetry breaking on the growth of magnetic stripe domains","We have performed an experimental and modeling-based study of the spin-orbit
torque-induced growth of magnetic stripe domains in heavy metal/ferromagnet
thin-film heterostructures that possess chiral N\'eel-type domain walls due to
an interfacial Dzyaloshinskii-Moriya interaction. In agreement with previous
reports, the stripe domains stabilized in these systems exhibit a significant
transverse growth velocity relative to the applied current axis. This behavior
has previously been attributed to the Magnus force-like skyrmion Hall effect of
the stripe domain spin topology, which is analogous to that of a half-skyrmion.
However, through analytic modeling of the in-plane torques generated by
spin-orbit torque, we find that a dynamical reconfiguration of the domain wall
magnetization profile is expected to occur - promoting motion with similar
directionality and symmetry as the skyrmion Hall effect. These results further
highlight the sensitivity of spin-orbit torque to the local orientation of the
domain wall magnetization profile and its contribution to domain growth
directionality.",2305.02181v1
2023-05-29,Phase transition of three-dimensional finite-sized charged dust clusters in a plasma environment,"The dynamics of a harmonically trapped three-dimensional Yukawa ball of
charged dust particles immersed in plasma is investigated as function of
external magnetic field and Coulomb coupling parameter using molecular dynamics
simulation. It is shown that the harmonically trapped dust particles organize
themselves into nested spherical shells. The particles start rotating in a
coherent order as the magnetic field reaches a critical value corresponding to
the coupling parameter of the system of dust particles. The magnetically
controlled charged dust cluster of finite size undergoes a first-order phase
transition from disordered to ordered phase. At sufficiently high coupling and
strong magnetic field, the vibrational mode of this finite-sized charged dust
cluster freezes, and the system retains only rotational motion.",2305.17902v1
2023-07-13,Magnon-magnon coupling in synthetic ferrimagnets,"Magnetic multilayers with interlayer exchange coupling have been widely
studied for both static and dynamic regimes. Their dynamical responses depend
on the exchange coupling strength and magnetic properties of individual layers.
Magnetic resonance spectra in such systems are conveniently discussed in terms
of coupling of acoustic and optical modes. At a certain value of applied
magnetic field, the two modes come close to being degenerate and the spectral
gap indicates the strength of mode hybridisation. In this work, we
theoretically and experimentally study the mode hybridisation of
interlayer-exchange-coupled moments with dissimilar magnetisation and thickness
of two ferromagnetic layers. In agreement with symmetry analysis for
eigenmodes, our low-symmetry multilayers exhibit sizable spectral gaps for all
experimental conditions. The spectra agree well with the predictions from the
Landau-Lifshitz-Gilbert equation at the macrospin limit whose parameters are
independently fixed by static measurements.",2307.06888v2
2023-07-20,Dynamical Energy Dissipation of Relativistic Magnetic Bullets,"To demonstrate the magnetic energy dissipation via relativistic shocks, we
carry out spherically symmetrical one-dimensional special relativistic
magneto-hydrodynamic simulations of highly magnetised outflows with an adaptive
mesh refinement method. We first investigate the detail of the dynamical energy
dissipation via interaction between a single ejecta and an external medium. The
energy dissipation timescales, which affect the early behaviour of the
afterglow emission in gamma-ray bursts, are estimated for a wide range of
magnetisation. In addition, we demonstrate the internal shock dissipation in
multiple interactions between magnetically dominated relativistic ejecta and
kinetically dominated non-relativistic winds. Our numerical results show that
almost 10% of the magnetic energy in the ejecta can be converted into the
thermal energy of the relativistic and low-magnetised outflows via shocks in
the rarefaction waves or the winds. Such hot and less magnetised outflows are
relevant for observed non-thermal emissions in blazars or gamma-ray bursts.",2307.10576v1
2023-08-24,Stability threshold of the 2D Couette flow in a homogeneous magnetic field using symmetric variables,"We consider a 2D incompressible and electrically conducting fluid in the
domain $\mathbb{T}\times\mathbb{R}$. The aim is to quantify stability
properties of the Couette flow $(y,0)$ with a constant homogenous magnetic
field $(\beta,0)$ when $|\beta|>1/2$. The focus lies on the regime with small
fluid viscosity $\nu$, magnetic resistivity $\mu$ and we assume that the
magnetic Prandtl number satisfies
$\mu^2\lesssim\mathrm{Pr}_{\mathrm{m}}=\nu/\mu\leq 1$. We establish that small
perturbations around this steady state remain close to it, provided their size
is of order $\varepsilon\ll\nu^{2/3}$ in $H^N$ with $N$ large enough.
Additionally, the vorticity and current density experience a transient growth
of order $\nu^{-1/3}$ while converging exponentially fast to an $x$-independent
state after a time-scale of order $\nu^{-1/3}$. The growth is driven by an
inviscid mechanism, while the subsequent exponential decay results from the
interplay between transport and diffusion, leading to the dissipation
enhancement. A key argument to prove these results is to reformulate the system
in terms of symmetric variables, inspired by the study of inhomogeneous fluid,
to effectively characterize the system's dynamic behavior.",2308.12589v1
2023-08-24,Effect of magnetic field on the long-time dynamics of two charged coupled harmonic oscillators in the presence of a common heat bath,"In this paper, the moderately long-time behaviours of the position
autocorrelation, position velocity correlation and velocity autocorrelation
functions for two charged coupled harmonic oscillators connected to a heat bath
are derived in the presence of a magnetic field via the Quantum Langevin
equation. It is seen that at long times the correlation functions at T->0
exhibit a power law decay with the coefficients of the power laws being
completely different for the two masses that affect the overall trends of the
decays of the correlation functions. The effect of the bath-induced force on
mass m1 mediated by the interaction of m2 with the common heat bath is studied
and the results are highlighted in the presence of an external magnetic field.
This brings out the distinctness of the coupled oscillator scenario from the
independent single oscillator model studied earlier in the literature. The
results in the absence of a magnetic field are also presented, which are
extremely important for analyzing the dynamics of atoms in protein molecules at
low temperatures.",2308.12668v1
2023-09-14,Paramagnon Heat Capacity and Anomalous Thermopower in Anisotropic Magnetic Systems: Understanding Inter-Layer Spin Correlations in a Magnetically Disordered Phase,"The interplay between entropy transport and charge carriers-paramagnon
interaction in the Onsager linear system has been a subject of debate due to
the limited theoretical and experimental understanding of paramagnon heat
capacity. In this study, we investigate this interplay in an anisotropic
layered magnetic system using cluster mean-field theory with spin quantum
correlations. By examining spin correlation functions between different spins
with various types of clustering, we derive the spin correlation function as a
function of distance and temperature for the inter-layer clusters both below
and above the magnetic order phase transition. Our analysis reveals that
paramagnons characterized by pronounced spin correlations among inter-layer
nearest-neighbor spins exhibit a non-zero heat capacity, providing valuable
insights into the dynamics of entropy transport. The findings align with
experimental observations, lending strong support to the validity of the
paramagnon drag thermopower concept. This study sheds light on the intricate
dynamics and thermodynamic properties of paramagnons, advancing our
understanding of entropy transport in complex systems.",2309.07646v1
2023-11-24,Light--absorbed orbital angular momentum in the linear response regime,"In exploring the light-induced dynamics within the linear response regime,
this study investigates the induced orbital angular momentum on a wide variety
of electronic structures. We derive a general expression for the torque induced
by light on different electronic systems based on their characteristic
dielectric tensor. We demonstrate that this phenomenon diverges from the
inverse Faraday effect as it produces an orbital magnetization persistent
post-illumination. Indeed, our results reveal that, while isotropic
non-dissipative materials do not absorb orbital angular momentum from
circularly polarized light, any symmetry-breaking arrangement of matter, be it
spatial or temporal, introduces novel channels for the absorption of orbital
angular momentum, or magnetization. Most notably, in dissipative materials,
circularly polarized light imparts a torque corresponding to a change in
orbital angular momentum of $\hbar$ per absorbed photon. The potential of these
mechanisms to drive helicity-dependent magnetic phenomena paves the way for a
deeper understanding of light-matter interactions. Notably, the application of
pump-probe techniques in tandem with our findings allows experimentalists to
quantitatively assess the amount of orbital angular momentum transferred to
electrons in matter, thus hopefully enhancing our ability to steer ultrafast
light-induced magnetization dynamics.",2311.14774v1
2023-11-30,Two-scale exponential integrators with uniform accuracy for three-dimensional charged-particle dynamics under strong magnetic field,"The numerical simulation of three-dimensional charged-particle dynamics (CPD)
under strong magnetic field is challenging. In this paper, we introduce a new
methodology to design two-scale exponential integrators for three-dimensional
CPD whose magnetic field's strength is inversely proportional to a
dimensionless parameter $0<\varepsilon \ll 1$. By dealing with the transformed
form of three-dimensional CPD, we linearize the magnetic field and put the rest
part in a nonlinear function which can be shown to be small. Based on which and
the proposed two-scale exponential integrators, a class of novel integrators is
formulated. The corresponding uniform accuracy over
$\mathcal{O}(1/\varepsilon^{\beta})$ time interval is
$\mathcal{O}(\varepsilon^{r\beta} h^r)$ for the $r$-th order integrator with
the time stepsize $h$, $r=1,2,3,4$ and $0<\beta<1$. A rigorous proof of this
error bound is presented and a numerical test is performed to illustrate the
error behaviour of the proposed integrators.",2311.18615v1
2023-12-15,Dynamic behavior of a magnetic system driven by an oscillatory external temperature,"The dynamic effects on a magnetic system exposed to a time-oscillating
external temperature are studied using Monte Carlo simulations on the classic
2D Ising Model.
  The time dependence of temperature is defined as $T(t)=T_0 + A \cdot
\sin(2\pi t/\tau)$. Magnetization $M(t)$ and period-averaged magnetization
$\langle Q\rangle$ are analyzed to characterize out-of-equilibrium phenomena.
Hysteresis-like loops in $M(t)$ are observed as a function of $T(t)$. The area
of the loops is well-defined outside the critical Ising temperature ($T_c$) but
takes more time to close it when the system crosses the critical curve. Results
show a power-law dependence of $\langle Q\rangle$ (the averaged area of loops)
on both $L$ and $\tau$, with exponents $\alpha=1.0(1)$ and $\beta=0.70(1)$,
respectively.
  Furthermore, the impact of shifting the initial temperature $T_0$ on $\langle
Q\rangle$ is analyzed, suggesting the existence of an effective
$\tau$-dependent critical temperature $T_c(\tau)$. A scaling law behavior for
$\langle Q\rangle$ is found on the base of this $\tau$-dependent critical
temperature.",2312.10164v1
2023-12-20,Modelling of impurity heating during reconnections in a Reverse Field Pinch device as due to parallel electric field acceleration and chaos-induced thermalization,"The ion temperature during magnetic reconnections measured along the
direction of the magnetic field at the MST Reverse Field Pinch has not yet
received a satisfactory theoretical explanation. In this work we argue that it
is consistent with a picture of ion energization by the parallel electric
fields generated by the plasma during reconnection, and thermalization due to
the chaotic ion dynamics. Three possible sources of randomness are pointed out:
particle motion along static stochastic magnetic field lines, breakup of the
adiabatic ion dynamics caused by the strong gradient nonlinearity of the
magnetic field, parallel acceleration along stochastic time-dependent electric
fields. The three mechanisms are likely operative simultaneously; regardless
the specific mechanism active, the correct scaling laws with ion parameters are
recovered. Furthermore, we argue that quantitative agreement can be obtained by
feeding the model with realistic values for the plasma conditions.",2312.13067v1
2023-12-21,Pseudo-spectral Landau-Lifshitz description of magnetization dynamics,"Magnetic materials host a wealth of nonlinear dynamics, textures, and
topological defects. This is possible due to the competition between strong
nonlinearity and dispersion that act at the atomic scale as well as long-range
interactions. However, these features are difficult to analytically and
numerically study because of the vastly different temporal and spatial scales
involved. Here, we present a pseudo-spectral approach for the Landau-Lifshitz
equation that invokes energy and momentum conservation embodied in the magnon
dispersion relation to accurately describe both atomic and continuum limits.
Furthermore, this approach enables analytical study at every scale. We show the
applicability of this model in both the continuum and atomic limit by
investigating modulational instability and ultrafast evolution of magnetization
due to transient grating, respectively, in a 1D ferromagnetic chain with
perpendicular magnetic anisotropy. This model provides the possibility of
grid-independent multiscale numerical approaches that will enable the
description of singularities within a single framework.",2312.14068v1
2023-12-23,Pulse-driven depinning of magnetic gap modes in ferromagnetic films,"Manipulation of magnons in artificial magnonic crystals (MCs) leads to
fascinating nonlinear wave phenomena such as the generation of gap solitons,
which has been mostly limited to one-dimensional systems. Here, we propose a
model system for the magnetization in two-dimensional MCs subjected to a
periodic external magnetic field, describing the dynamics of magnetic gap
solitons (MGSs) formed by nonlinear self-trapping. We show the formation,
stability, and dynamics for various two-dimensional gap modes, including gap
solitons and vortical ones. Their existence regions depend on the anisotropic
axis orientation of the ferromagnetic film. The Bloch oscillation and depinning
propagation of MGSs under constant spin-current injections are discovered and
characterized. We design a scheme of pulse current injection to achieve
distortionless propagation of MGSs. These findings show that the 2D magnonic
crystals can be viewed as a building block for MGSs-based storage and
transmission, where the propagation and localization are variously controlled
and reconfigurable.",2312.15200v1
2024-02-03,Numerical MHD simulations of solar flares and their associated small-scale structures,"Using numerical simulations, we study the formation and dynamics of
post-flare loops in a local region of the solar atmosphere. The MHD equations
rule the post-flare structures' dynamic evolution, including space-dependent
magnetic resistivity and highly anisotropic thermal conduction on a 2.5 D
slice. We use an initial magnetic configuration consisting of a vertical
current sheet, which helps trigger the magnetic reconnection process.
Specifically, we study two scenarios, one with only resistivity and the second
with resistivity plus thermal conduction. Numerical simulations show
differences in the global morphology of the post-flare substructures in both
cases. In particular, localized resistivity produces more substructure on the
loops related to a Ritchmyer-Meshkov Instability (RMI). On the other hand, in
the scenario with resistivity plus thermal conduction, the post-flare loops are
smooth, and no apparent substructures develop. Besides, in the $z-$component of
the current density for the Res+TC scenario, we observe the development of
multiple small magnetic islands along the current sheet.",2402.02161v1
2024-02-05,Transition to chaos and magnetic field generation in rotating Rayleigh-Bénard convection,"Hydrodynamic and magnetohydrodynamic convective attractors in
three-dimensional rotating Rayleigh-B\'enard convection are studied numerically
by varying the Taylor and Rayleigh numbers as control parameters. First, an
analysis of hydrodynamic attractors and their bifurcations is conducted, where
routes to chaos via quasiperiodicity are identified. Second, the behaviour of
the magnetohydrodynamic system is investigated by introducing a seed magnetic
field and measuring its growth or decay as a function of the Taylor number,
while keeping the Rayleigh number fixed. Analysis of the attractors shows that
rotation has a significant impact on magnetic field generation in
Rayleigh-B\'enard convection, with the critical magnetic Prandtl number
changing nonmonotonically with the rotation rate. It is argued that a
nonhysteretic blowout bifurcation with on-off intermittency is responsible for
the transitions to dynamo.",2402.02908v1
2024-02-07,Macroscopic Magnetic Dynamics,"Ferromagnetic metals and spin-polarized $^{3}$He are spin 1/2 systems with
the same macroscopic symmetry, and thus should have macroscopic magnetic
dynamics with the same structure. Using Onsager's irreversible thermodynamics,
we develop a theory for these systems that contains two relaxation times (one
for the magnetization $\vec{M}$ and the other for the spin current
$\vec{J}_{i}$), a magnetic compressibility, and a mean-field parameter.
Currently spintronics data on metallic ferromagnets are analyzed using a
complex decay length from a theory employing a diffusion constant, a lifetime,
and a mean-field parameter. The present theory leads to a complex decay length
with the same structure. On neglecting decay of $\vec{M}$, the present theory
applies to liquids and gases. For macroscopic equations the particle statistics
is not relevant, so the theory also applies to bosons. The theory predicts a
longitudinal spin wave whose velocity we estimate for liquid $^{3}$He and for
paramagnetic metals; but such a wave should also occur for ferromagnets and for
gases.",2402.04639v1
2024-02-16,Spin projection noise and the magnetic sensitivity of optically pumped magnetometers,"Present protocols for obtaining the ultimate magnetic sensitivity of
optically pumped magnetometers (OPMs) utilizing alkali-metal ensembles rely on
uncorrelated atoms in stretched states. A new approach for calculating the spin
projection noise (SPN)-limited signal to noise ratio (SNR) and the magnetic
sensitivity of OPMs is proposed. Our model is based solely on the mean-field
density matrix dynamics and in contrast to previous models, it applies to both
low and high field regimes, it takes into account the degree of spin
polarization, the intra- and interhyperfine correlations, the decoherence
processes, the atom-light coupling and the effects of the spin dynamics on the
spin-noise spectra. Fine tuning of the probe frequency allow us to explore
different hyperfine states and ground-state correlations. Especially in the
spin-exchange-relaxation-free (SERF) regime, alongside the magnetic resonance
narrowing and the increased number density, hallmarks of SERF magnetometers, we
report on a new SERF feature; the reduction of spin-projection noise at the
spin precession frequency as a consequence of strongly-correlated hyperfine
spins that attenuate and redistribute SPN when properly probed.",2402.10746v2
2024-03-06,"Buzdin, Shapiro and Chimera Steps in $\varphi_0$ Josephson Junctions. II. Bifurcation, Switching, and Hysteresis","The dynamics of magnetization and current-voltage characteristics of the
superconductor-ferromagnet-superconductor $\varphi_0$ Josephson junction in the
presence of external electromagnetic radiation have been studied. Effects of
radiation magnetic component are investigated in the frameworks of one- and
two-signal models. The implementation of two types of dynamical states of
magnetization is demonstrated. These states have a phase shift of $\pi$ in the
synchronization region of magnetic precession and Josephson oscillations and
differ in the nature of their time dependence. Transitions between these states
with increasing and decreasing bias current show hysteresis, which is reflected
in the bifurcation diagram and the current-voltage characteristics. We also
provide an experimental way to test the obtained results by measuring the phase
shift in voltage temporal dependence at fixed current value for both sweeping
directions. The results obtained can find application in various fields of
superconducting spintronics and quantum computing.",2403.03621v1
1997-11-01,The dissipative effect of thermal radiation loss in high-temperature dense plasmas,"A dynamical model based on the two-fluid dynamical equations with energy
generation and loss is obtained and used to investigate the self-generated
magnetic fields in high-temperature dense plasmas such as the solar core. The
self-generation of magnetic fields might be looked at as a
self-organization-type behavior of stochastic thermal radiation fields, as
expected for an open dissipative system according to Prigogine's theory of
dissipative structures.",9711002v1
2005-03-26,On Quasilinear Perpendicular Diffusion,"Quasilinear perpendicular diffusion of charged particles in fluctuating
electromagnetic fields is the focus of this paper. A general transport
parameter for perpendicular diffusion is presented being valid for an arbitrary
turbulence geometry and a plasma wave dispersion relation varying arbitrarily
in wavevector. The new diffusion coefficient is evaluated in detail for slab
turbulence geometry for two special cases: (1) Alfv\'enic turbulence and (2)
dynamical magnetic turbulence. Furthermore, perpendicular diffusion in 2D
geometry is considered for a purely dynamical magnetic turbulence. The
derivations and numerical calculations presented here cast serious doubts on
the applicability of quasilinear theory for perpendicular diffusion.
Furthermore, they emphasize that nonlinear effects play a crucial role in the
context of perpendicular diffusion.",0503576v1
1995-09-05,Computation of Kolmogorov's Constant in Magnetohydrodynamic Turbulence,"In this paper we calculate Kolmogorov's constant for magnetohydrodynamic
turbulence to one loop order in perturbation theory using the direct
interaction approximation technique of Kraichnan. We have computed the
constants for various $E^u(k)/E^b(k)$, i.e., fluid to magnetic energy ratios
when the normalized cross helicity is zero. We find that $K$ increases from
1.47 to 4.12 as we go from fully fluid case $(E^b=0)$ to a situation when $%
E^u/E^b=0.5$, then it decreases to 3.55 in a fully magnetic limit $(E^u=0)$.
When $E^u/E^b=1$, we find that $K=3.43$.",9509001v1
1996-07-29,On developed turbulence in a weak compressibile conductive fluid,"A method of construction of decomposition of correlation functions of
developed turbulence in a compressible fluid on Mach number {\em Ma} is
generalized now for a model of stochastic magnetic hydrodynamics. With the help
of the field theory renormalization group method the composite operators of
transversal fields are studied. It is shown, that in the case of hydrodynamical
interaction of fluid flows is prevalent the compressible effects are essential
in inertial range. On the contrary the corrections to developed turbulent
spectra due to compressibility are insignificant if the magnetic interaction of
flows is more important one.",9607015v1
1999-01-20,The Effect of Radiation on the Stochastic Web,"A charged particle circling in a uniform magnetic field and kicked by an
electric field is considered. Under the assumption of small magnetic field, an
iterative map is developed. Comparison between the (relativistic) non-radiative
case and the (relativistic) radiative case shows that in both cases one can
observe a stochastic web structure, and that both cases are qualitatively
similar.",9901015v1
1999-10-11,Decaying magnetohydrodynamics: effects of initial conditions,"We study the effects of homogenous and isotropic initial conditions on
decaying Magnetohydrodynamics (MHD). We show that for an initial distribution
of velocity and magnetic field fluctuations, appropriately defined structure
functions decay as power law in time. We also show that for a suitable choice
of initial cross-correlations between velocity and magnetic fields even order
structure functions acquire anomalous scaling in time where as scaling
exponents of the odd order structure functions remain unchanged. We discuss our
results in the context of fully developed MHD turbulence.",9910017v1
1993-03-26,Nonequilibrium Dynamics and Aging in the Three--Dimensional Ising Spin Glass Model,"The low temperature dynamics of the three dimensional Ising spin glass in
zero field with a discrete bond distribution is investigated via MC
simulations. The thermoremanent magnetization is found to decay algebraically
and the temperature dependent exponents agree very well with the experimentally
determined values. The nonequilibrium autocorrelation function $C(t,t_w)$ shows
a crossover at the waiting (or {\em aging}) time $t_w$ from algebraic {\em
quasi-equilibrium} decay for times $t$$\ll$$t_w$ to another, faster algebraic
decay for $t$$\gg$$t_w$ with an exponent similar to one for the remanent
magnetization.",9303048v1
1993-08-27,Phase Transition Study of Superconducting Microstructures,"The presented results are part of a feasibility study of superheated
superconducting microstructure detectors. The microstructures (dots) were
fabricated using thin film patterning techniques with diameters ranging from
$50\mu$m up to $500\mu$m and thickness of $1\mu$m. We used arrays and single
dots to study the dynamics of the superheating and supercooling phase
transitions in a magnetic field parallel to the dot surface. The phase transi-
tions were produced by either varying the applied magnetic field strength at a
constant temperature or changing the bath temperature at a constant field.
Preliminary results on the dynamics of the phase transitions of arrays and
single indium dots will be reported.",9308033v1
1995-04-26,Chaos in Andreev Billiards,"A new type of classical billiard - the Andreev billiard - is investigated
using the tangent map technique. Andreev billiards consist of a normal region
surrounded by a superconducting region. In contrast with previously studied
billiards, Andreev billiards are integrable in zero magnetic field, {\it
regardless of their shape}. A magnetic field renders chaotic motion in a
generically shaped billiard, which is demonstrated for the Bunimovich stadium
by examination of both Poincar\'e sections and Lyapunov exponents. The issue of
the feasibility of certain experimental realizations is addressed.",9504110v1
1996-07-24,Long Wavelength Anomalous Diffusion Mode in the 2D XY Dipole Magnet,"In 2D XY ferromagnet the dipole force induces a strong interaction between
spin-waves in the long-wavelength limit. The major effect of this interaction
is the transformation of a propagating spin-wave into a diffusion mode. We
study the anomalous dynamics of such diffusion modes. We find that the
Janssen-De Dominics functional, which governs this dynamics, approaches the
non-Gaussian fixed-point. A spin-wave propagates by an anomalous anisotropic
diffusion with the dispersion relation: $i\omega{\sim}k_{y}^{\Delta_y}$ and
$i\omega{\sim}k_{x}^{\Delta_x}$, where ${\Delta_y}=47/27$ and
${\Delta_x}=47/36$. The low-frequency response to the external magnetic field
is found.",9607168v1
1997-01-15,Spin dynamics of the spin-Peierls compound CuGeO_3 under magnetic field,"The magnetic field--driven transition in the spin-Peierls system CuGeO_3
associated with the closing of the spin gap is investigated numerically. The
field dependence of the spin dynamical structure factor (seen by inelastic
neutron scattering) and of the momentum dependent static susceptibility are
calculated. In the dimerized phase (H<H_c), we suggest that the strong field
dependence of the transverse susceptibility could be experimentally seen from
the low temperature spin-echo relaxation rate 1/T_{2G} or the second moment of
the NMR spectrum. Above H_c low energy spin excitations appear at
incommensurate wave vectors where the longitudinal susceptibility chi_{zz}(q)
peaks.",9701107v1
1997-02-12,Quantum internal modes of solitons in 1d easy-plane antiferromagnet in strong magnetic field,"In presence of a strong external magnetic field the dynamics of solitons in a
one-dimensional easy-plane Heisenberg antiferromagnet exhibits a number of
peculiarities. Dynamics of internal soliton degrees of freedom is essentially
quantum, and they are strongly coupled to the ""translational"" mode of soliton
movement. These peculiarities lead to considerable changes in the response
functions of the system which can be detected experimentally.",9702114v1
1997-05-16,Origin of strong scarring of wavefunctions in quantum wells in a tilted magnetic field,"The anomalously strong scarring of wavefunctions found in numerical studies
of quantum wells in a tilted magnetic field is shown to be due to special
properties of the classical dynamics of this system. A certain subset of
periodic orbits are identified which are nearly stable over a very large
interval of variation of the classical dynamics; only this subset are found to
exhibit strong scarring. Semiclassical arguments shed further light on why
these orbits dominate the experimentally observed tunneling spectra.",9705167v1
1997-06-13,Dynamic instabilities in resonant tunneling induced by a magnetic field,"We show that the addition of a magnetic field parallel to the current induces
self sustained intrinsic current oscillations in an asymmetric double barrier
structure. The oscillations are attributed to the nonlinear dynamic coupling of
the current to the charge trapped in the well, and the effect of the external
field over the local density of states across the system. Our results show that
the system bifurcates as the field is increased, and may transit to chaos at
large enough fields.",9706144v1
1997-08-05,Dynamical properties of the pinned Wigner crystal,"We study various dynamical properties of the weakly pinned Wigner crystal in
a high magnetic field. Using a Gaussian variational method we can compute the
full frequency and field dependence of the real and imaginary parts of the
diagonal and Hall conductivities. The zero temperature Hall resistivity is
independent of frequency and remains unaffected by disorder at its classical
value. We show that, depending on the inherent length scales of the system, the
pinning peak and the threshold electric field exhibit strikingly different
magnetic field dependences.",9708034v2
1998-10-23,Bloch Electrons in a Magnetic Field - Why Does Chaos Send Electrons the Hard Way?,"We find that a 2D periodic potential with different modulation amplitudes in
x- and y-direction and a perpendicular magnetic field may lead to a transition
to electron transport along the direction of stronger modulation and to
localization in the direction of weaker modulation. In the experimentally
accessible regime we relate this new quantum transport phenomenon to avoided
band crossing due to classical chaos.",9810313v3
1998-12-06,The dynamo effect - A dynamic renormalisation group approach,"The Dynamo effect is used to describe the generation of magnetic fields in
astrophysical objects. However, no rigorous derivation of the dynamo equation
is available. We justify the form of the equation using an Operator Product
Expansion (OPE) of the relevant fields. We also calculate the coefficients of
the OPE series using a dynamic renormalisation group approach and discuss the
time evolution of the initial conditions on the initial seed magnetic field.",9812090v1
1998-12-17,Magnetic Field Induced Phase Transitions in YBa2Cu4O8,"The $c$-axis resistivity measurements in YBa_2Cu_4O_8 from Hussey et al. for
magnetic field orientations along the c-axis as well as within the ab-plane are
analyzed and interpreted using the scaling theory for static and dynamic
classical critical phenomena. We identify a superconductor to normal conductor
transition for both field orientations as well as a normal conductor to
insulator transition at a critical field H_c||a with dynamical critical
exponent z=1, leading to a multicritical point where superconducting, normal
conducting and insulating phases coexist.",9812301v1
1999-01-05,A low-temperature dynamic mode scanning force microscope operating in high magnetic fields,"A scanning force microscope was implemented operating at temperatures below
4.2K and in magnetic fields up to 8T. Piezoelectric quartz tuning forks were
employed for non optical tip-sample distance control in the dynamic operation
mode. Fast response was achieved by using a phase-locked loop for driving the
mechanical oscillator. Possible applications of this setup for various scanning
probe techniques are discussed.",9901023v1
2000-02-09,Vortex Pinning and Dynamics in Layerd Superconductors with Periodic Pinning Arrays,"We examine vortex dynamics and pinning in layered superconductors using
three-dimensional molecular dyanmics simulations of magnetically interacting
pancake vortices. Our model treats the magnetic interactions of the pancakes
exactly, with longe-range logarithmic interactions both within and between
planes. At the matching field the vortices are aligned with the pinning array.
As a function of tilt angle for the pinning arrays a series of commensuration
effects occur, seen as peaks in the critical current, due to pancakes finding a
favorable alignment.",0002123v1
2000-06-06,A critical examination of the spin dynamics in high-$T_C$ cuprates,"A critical examination of the spin dynamics in high-$T_C$ cuprates is made on
the light of inelastic neutron scattering results obtained by different groups.
Recent neutron data show that incommensurate magnetic peaks in YBCO belong to
the same excitation as the resonance peak observed at $(\pi/a,\pi/a)$. Being
only observed in the superconducting state, the incommensurability is then
rather difficult to reconcile with a stripe picture. We also discuss the link
between the resonance peak spectral weight and the superconducting condensation
energy.",0006085v2
2000-07-24,rf susceptibility of La_{1-x}Sr_{x}MnO_{3} single crystals : magnetic signatures of structural changes,"A sensitive tunnel diode oscillator (TDO) operating at 4MHz is used to probe
the dynamic response of La_{1-x}Sr_{x}MnO_{3} single crystals for
x=0.125,0.175,0.28 and 0.33 doping. Systematics of the measured change in
reactance as a function of temperature (30K<T<320K) and DC magnetic field
(0<H<6kOe) reveal distinct temperature and field scales associated with the
dynamic response of spin. It is notable that these features are far more
striking than the corresponding features in static measurements. The results
are discussed in the context of structural changes leading to polaron ordering.",0007377v1
2000-09-21,Dynamics of spin correlations in the spin-1/2 isotropic XY chain in a transverse field,"Dynamic xx spin pair correlation functions for the isotropic spin-1/2 XY
chain are calculated numerically for long open chains in the presence of a
transverse magnetic field at finite temperature. As an application we discuss
the temperature dependence of the spin-spin relaxation time in PrCl_3.",0009334v1
2000-10-05,Critical dynamics of an interacting magnetic nanoparticle system,"Effects of dipole-dipole interactions on the magnetic relaxation have been
investigated for three Fe-C nanoparticle samples with volume concentrations of
0.06, 5 and 17 vol%. While both the 5 and 17 vol% samples exhibit collective
behavior due to dipolar interactions, only the 17 vol% sample displays critical
behavior close to its transition temperature. The behaviour of the 5 vol%
sample can be attributed to a mixture of collective and single particle
dynamics.",0010090v2
2000-12-06,A macroscopically frustrated Ising model,"A disordered spin glass model where both static and dynamical properties
depend on macroscopic magnetizations is presented. These magnetizations
interact via random couplings and, therefore, the typical quenched realization
of the system exhibit a macroscopic frustration. The model is solved by using a
revisited replica approach, and the broken symmetry solution turns out to
coincide with the symmetric solution. Some dynamical aspects of the model are
also discussed, showing how it could be a useful tool for describing some
properties of real systems as, for example, natural ecosystems or human social
systems.",0012089v1
2001-04-29,"Driven Heisenberg Magnets: Nonequilibrium Criticality, Spatiotemporal Chaos and Control","We drive a $d$-dimensional Heisenberg magnet using an anisotropic current.
The continuum Langevin equation is analysed using a dynamical renormalization
group and numerical simulations. We discover a rich steady-state phase diagram,
including a critical point in a new nonequilibrium universality class, and a
spatiotemporally chaotic phase. The latter may be `controlled' in a robust
manner to target spatially periodic steady states with helical order.",0104566v2
2001-05-08,Glassy dynamics of the inhomogeneous metallic phase in La1-xCaxMnO3,"The anelastic spectra (elastic energy loss and dynamic modulus versus
temperature) of La1-xCaxMnO3 have been measured for 0.33< x < 0.5. A peak in
the imaginary part of the elastic susceptibility is found slightly below the
Curie temperature, whose temperature and frequency dependences are typical of
the dielectric and magnetic susceptibilities of relaxor ferroelectrics, spin
glasses and other inhomogeneous frustrated systems. The fluctuations can be
associated with the magnetic clusters which are observed by several techniques
or with charge-ordered insulating domains in the low temperature inhomogeneous
phase. The temperature dependent distribution function of the fluctuation
energy barriers has been extracted by the analysis of the experimental curves.",0105159v1
2001-08-11,Spin dynamics from time-dependent spin density-functional theory,"We derive the spin-wave dynamics of a magnetic material from the
time-dependent spin density functional theory in the linear response regime.
The equation of motion for the magnetization includes, besides the static spin
stiffness, a ""Berry curvature"" correction and a damping term. A gradient
expansion scheme based on the homogeneous spin-polarized electron gas is
proposed for the latter two quantities, and the first few coefficients of the
expansion are calculated to second order in the Coulomb interaction.",0108193v1
2001-11-26,Ferromagnetic transition in a double-exchange system containing impurities in the Dynamical Mean Field Approximation,"We formulate the Dynamical Mean Field Approximation equations for the
double-exchange system with quenched disorder for arbitrary relation between
Hund exchange coupling and electron band width. Close to the
ferromagnetic-paramagnetic transition point the DMFA equations can be reduced
to the ordinary mean field equation of Curie-Weiss type. We solve the equation
to find the transition temperature and present the magnetic phase diagram of
the system.",0111491v4
2002-04-28,Dynamical density correlation function of 1D Mott insulators in a magnetic field,"We consider the one dimensional (1D) extended Hubbard model at half filling
in the presence of a magnetic field. Using field theory techniques we calculate
the dynamical density-density correlation function $\chi_{nn}(\omega,q)$ in the
low-energy limit. When excitons are formed, a singularity appears in
$\chi_{nn}(\omega,q)$ at a particular energy and momentum transfer.",0204594v1
2002-08-21,The spin angular gradient approximation in the density functional theory,"A spin angular gradient approximation for the exchange correlation magnetic
field in the density functional formalism is proposed. The usage of such
corrections leads to a consistent spin dynamical approach beyond the local
approximation. The proposed technique does not contain any approximations for
the form of potential and can be used in modern full potential band structure
methods. The obtained results indicate that the direct 'potential' exchange in
3d magnets is rather small compared to the indirect 'kinetic' exchange, thus
justifies the dynamical aspect of the local density approximation in 3d metals.",0208420v1
2002-10-26,Dynamic exchange coupling in magnetic bilayers,"A long-ranged dynamic interaction between ferromagnetic films separated by
normal-metal spacers is reported, which is communicated by nonequilibrium spin
currents. It is measured by ferromagnetic resonance (FMR) and explained by an
adiabatic spin-pump theory. In FMR the spin-pump mechanism of spatially
separated magnetic moments leads to an appreciable increase in the FMR line
width when the resonance fields are well apart, and results in a dramatic
line-width narrowing when the FMR fields approach each other.",0210588v2
2003-04-22,"Dynamical Systems, Topology and Conductivity in Normal Metals","New observable integer-valued numbers of the topological origin were revealed
by the present authors studying the conductivity theory of single crystal 3D
normal metals in the reasonably strong magnetic field ($B \leq 10^{3} Tl$). Our
investigation is based on the study of dynamical systems on Fermi surfaces for
the motion of semi-classical electron in magnetic field. All possible
asymptotic regimes are also found for $B \to \infty$ based on the topological
classification of trajectories.",0304471v2
2003-05-15,Inhomogeneous Ferromagnetism and Unconventional Charge Dynamics in Disordered Double Exchange Magnets,"We solve the double exchange model in the presence of arbitrary
substitutional disorder by using a self consistently generated effective
Hamiltonian for the spin degrees of freedom. The magnetic properties are
studied through classical Monte Carlo while the effective exchange, $D_{ij}$,
are calculated by solving the disordered fermion problem, and renormalised
self-consistently with increasing temperature. We present exact results on the
conductivity, magnetoresistance, optical response and `real space' structure of
the inhomogeneous ferromagnetic state, and compare our results with charge
dynamics in disordered La_{1-x}Sr_xMnO_3. The large sizes, ${\cal O} (10^3)$,
accessible within our method allows a complete, controlled calculation on the
disordered strongly interacting problem.",0305352v1
2003-06-13,Scaling of the magnetic response in doped antiferromagnets,"A theory of the anomalous $\omega/T$ scaling of the dynamic magnetic response
in cuprates at low doping is presented. It is based on the memory function
representation of the dynamical spin suceptibility in a doped antiferromagnet
where the damping of the collective mode is constant and large, whereas the
equal-time spin correlations saturate at low $T$. Exact diagonalization results
within the t-J model are shown to support assumptions. Consequences, both for
the scaling function and the normalization amplitude, are well in agreement
with neutron scattering results.",0306366v2
2003-06-19,Griffiths phase in diluted magnetic semiconductors,"We study the effects of disorder in the vicinity of the ferromagnetic
transition in a diluted magnetic semiconductor in the strongly localized
regime. We derive an effective polaron Hamiltonian, which leads to the
Griffiths phase above the ferromagnetic transition point. The Griffiths-McCoy
effects yield non-perturbative contributions to the dynamic susceptibility. We
explicitly derive the long-time susceptibility, which has a pseudo-scaling
form, with the dynamic critical exponent being expressed through the
percolation indices.",0306488v2
2003-07-16,"The quantum dynamics of atomic magnets, co-tunneling and dipolar-biased tunneling","Multi-spins tunneling cross-relaxations in an ensemble of weakly-coupled
Ho$^{3+}$ ions, mediated by weak anisotropic dipolar interactions, can be
evidenced by ac-susceptibility measurements in a high temperature regime. Based
on a four-body representation, including the rare-earth nuclear spin, two-ions
tunneling mechanisms can be attributed to both dipolar-biased tunneling and
co-tunneling processes. The co-reversal involving entangled pairs of magnetic
moments is discussed with a particular emphasis, giving new evidences to
elucidate the many-body quantum dynamics.",0307405v1
2003-11-14,Magnetization reversal and spin dynamics exchange in biased F/AF bilayers probed with complex permeability spectra,"The spin dynamics of the ferromagnetic pinned layer of
ferro-antiferromagnetic coupled NiFe/MnNi bilayers is investigated in a broad
frequency range (30 MHz-6 GHz). A phenomenological model based on the
Landau-Lifshitz equation for the complex permeability of the F/AF bilayer is
proposed. The experimental results are compared to theoretical predictions. We
show that the resonance frequencies, measured during the magnetization, are
likewise hysteretic.",0311338v1
2003-11-14,Dynamic roughening and fluctuations of dipolar chains,"Nonmagnetic particles in a carrier ferrofluid acquire an effective dipolar
moment when placed in an external magnetic field. This fact leads them to form
chains that will roughen due to Brownian motion when the magnetic field is
decreased. We study this process through experiments, theory and simulations,
three methods that agree on the scaling behavior over 5 orders of magnitude.
The RMS width goes initially as $t^{1/2}$, then as $t^{1/4}$ before it
saturates. We show how these results complement existing results on polymer
chains, and how the chain dynamics may be described by a recent non-Markovian
formulation of anomalous diffusion.",0311340v1
2004-02-12,Spin Dynamics of Double-Exchange Manganites with Magnetic Frustration,"This work examines the effects of magnetic frustration due to competing
ferromagnetic and antiferromagnetic Heisenberg interactions on the spin
dynamics of the double-exchange model. When the local moments are non-colinear,
a charge-density wave forms because the electrons prefer to sit on lines of
sites that are coupled ferromagnetically. With increasing hopping energy, the
local spins become aligned and the average spin-wave stiffness increases. Phase
separation is found only within a narrow range of hopping energies. Results of
this work are applied to the field-induced jump in the spin-wave stiffness
observed in the manganite Pr$_{1-x}$Ca$_x$MnO$_3$ with $0.3 \le x \le 0.4$.",0402344v1
2004-02-25,Spin Dynamics of a Canted Antiferromagnet in a Magnetic Field,"The spin dynamics of a canted antiferromagnet with a quadratic spin-wave
dispersion near $\vq =0$ is shown to possess a unique signature. When the
anisotropy gap is negligible, the spin-wave stiffness $\dsw (\vq, B) =
(\omega_{\vq}-B)/q^2$ depends on whether the limit of zero field or zero
wavevector is taken first. Consequently, $\dsw $ is a strong function of
magnetic field at a fixed wavevector. Even in the presence of a sizeable
anisotropy gap, the field dependence of both $\dsw $ and the gap energy
distinguishes a canted antiferromagnet from a phase-separated mixture
containing both ferromagnetic and antiferromagnetic regions.",0402633v2
2004-04-01,Semiclassical dynamics of domain walls in the one-dimensional Ising ferromagnet in a transverse field,"We investigate analytically and numerically the dynamics of domain walls in a
spin chain with ferromagnetic Ising interaction and subject to an external
magnetic field perpendicular to the easy magnetization axis (transverse field
Ising model). The analytical results obtained within the continuum
approximation and numerical simulations performed for discrete classical model
are used to analyze the quantum properties of domain walls using the
semiclassical approximation. We show that the domain wall spectrum shows a band
structure consisting of 2$S$ non-intersecting zones.",0404014v1
2004-12-24,"""Glassy Dynamics"" in Ising Spin Glasses -- Experiment and Simulation","The field-cooled magnetization (FCM) processes of Ising spin glasses under
relatively small fields are investigated by experiment on
Fe_{0.55}Mn_{0.45}TiO_3 and by numerical simulation on the three-dimensional
Edwards-Anderson model. Both results are explained in a unified manner by means
of the droplet picture. In particular, the cusp-like behavior of the FCM is
interpreted as evidence, not for an equilibrium phase transition under a finite
magnetic field, but for a dynamical (`blocking') transition frequently observed
in glassy systems.",0412674v2
2005-02-24,Lifetime Widths of Surface States on Magnetic Lanthanide Metals,"Low-temperature scanning tunneling spectroscopy is used to study electronic
structure and dynamics of d-like surface states of trivalent lanthanide metals
from La to Lu. The magnetic exchange splitting of these states is found to
scale with the 4f spin multiplied by an effective exchange-coupling constant
that increases with 4f occupancy in an approximately linear way. The dynamics
of the surfaces states, as revealed by the lifetime width, is dominated by
electron-phonon scattering in the occupied region and by electron-magnon
scattering in the unoccupied region, respectively.",0502581v1
2005-06-24,Dynamics of Dipolar Spinor Condensates,"We study the semiclassical dynamics of a spinor condensate with the magnetic
dipole-dipole interaction included. The time evolution of the population
imbalance and the relative phase among different spin components depends
greatly on the relative strength of interactions as well as on the initial
conditions. The interplay of spin exchange and dipole-dipole interaction makes
it possible to manipulate the atomic population on different components,
leading to the phenomena of spontaneous magnetization and Macroscopic Quantum
Self Trapping. Simple estimate demonstrates that these effects are accessible
and controllable by modifying the geometry of the trapping potential.",0506630v1
2005-08-30,Current induced transverse spin-wave instability in thin ferromagnets: beyond linear stability analysis,"A sufficiently large unpolarized current can cause a spin-wave instability in
thin nanomagnets with asymmetric contacts. The dynamics beyond the instability
is understood in the perturbative regime of small spin-wave amplitudes, as well
as by numerically solving a discretized model. In the absence of an applied
magnetic field, our numerical simulations reveal a hierarchy of instabilities,
leading to chaotic magnetization dynamics for the largest current densities we
consider.",0508732v1
2005-12-02,Exchange enhanced anisotropy in ferromagnetic/antiferromagnetic multilayers: dynamic consequences,"The phenomena of exchange anisotropy is well known in terms of static
magnetization properties such as enhanced coercivity and magnetization loop
shifts. These effects are primarily associated with effective anisotropies
introduced into the ferromagnet by exchange coupling with a strongly
anisotropic antiferromagnet. These effective anisotropies can be understood as
manifestations of a more fundamental exchange induced susceptibility. We show
that a consequence of this view is that a class of unusual dynamic effects
associated with the exchange susceptibility should also exist. The effects
become apparent near the ordering temperature of the antiferromagnet and affect
domain wall velocities, domain wall resonances, and precessional switching of
the ferromagnet.",0512034v1
2005-12-08,Nature of non-magnetic strongly-correlated state in delta-plutonium,"Ab-initio relativistic dynamical mean-field theory is applied to resolve the
long-standing controversy between theory and experiment in the ""simple""
face-centered cubic phase of plutonium called delta-Pu. In agreement with
experiment, neither static nor dynamical magnetic moments are predicted. In
addition, the quasiparticle density of states reproduces not only the peak
close to the Fermi level, which explains the large coefficient of electronic
specific heat, but also main 5f features observed in photoelectron
spectroscopy.",0512185v1
2006-04-10,Transport of magnetic vortices by surface acoustic waves,"In a thin film of superconducting YBCO the impact of surface acoustic waves
(SAWs) traveling on the piezoelectric substrate is investigated. A pronounced
interaction between the ultrasonic waves and the vortex system in the type II
superconductor is observed. The occurrence of a SAW-induced dc voltage
perpendicular to the sound path is interpreted as {\em dynamic pinning} of
vortices by the piezoacoustic SAW, which acts as a conveyor for the fluxquanta.
Its antisymmetry with respect to the magnetic field directly evidences the
induced, directed flux motion.This dynamic manipulation of vortices can be seen
as an important step towards flux-based electronic devices.",0604254v1
2006-07-21,Magnetic quantum phase transition in an anisotropic Kondo lattice,"The quantum phase transition between paramagnetic and antiferromagnetic
phases of the Kondo lattice model with Ising anisotropy in the intersite
exchange is studied within the framework of extended dynamical mean-field
theory. Nonperturbative numerical solutions at zero temperature point to a
continuous transition for both two- and three-dimensional magnetism. In the
former case, the transition is associated with critical local physics,
characterized by a vanishing Kondo scale and by an anomalous exponent in the
dynamics close in value to that measured in heavy-fermion CeCu_{5.9}Au_{0.1}.",0607566v3
2006-09-08,Quantum Dynamics of Molecular Nanomagnets in a Resonant Cavity and the Maser Effect,"We study the dynamics of molecular nanomagnets through a fully quantum
mechanical model describing high-spin and high-anisotropy magnetic molecules
subjected to a time-dependent magnetic field along the quantization axis, which
continuously inverts the population of spin states. Crystals of molecular
nanomagnets placed inside a resonant cavity interact with a quantized
electromagnetic field. Relaxation of excited states takes place by means of
spin-photon interaction, allowing stimulated emission of radiation and creating
a maser effect.",0609205v1
2006-09-12,Nonlinear dynamics of quantum dot nuclear spins,"We report manifestly nonlinear dependence of quantum dot nuclear spin
polarization on applied magnetic fields. Resonant absorption and emission of
circularly polarized radiation pumps the resident quantum dot electron spin,
which in turn leads to nuclear spin polarization due to hyperfine interaction.
We observe that the resulting Overhauser field exhibits hysteresis as a
function of the external magnetic field. This hysteresis is a consequence of
the feedback of the Overhauser field on the nuclear spin cooling rate. A
semi-classical model describing the coupled nuclear and electron spin dynamics
successfully explains the observed hysteresis but leaves open questions for the
low field behaviour of the nuclear spin polarization.",0609291v1
2006-11-25,Dynamics of Vortex Core Switching in Ferromagnetic Nanodisks,"Dynamics of magnetic vortex core switching in nanometer-scale permalloy disk,
having a single vortex ground state, was investigated by micromagnetic
modeling. When an in-plane magnetic field pulse with an appropriate strength
and duration is applied to the vortex structure, additional two vortices, i.e.,
a circular- and an anti-vortex, are created near the original vortex core.
Sequentially, the vortex-antivortex pair annihilates. A spin wave is created at
the annihilation point and propagated through the entire element; the relaxed
state for the system is the single vortex state with a switched vortex core.",0611640v1
2007-03-27,Gauge Field Formulation of Adiabatic Spin Torques,"Previous calculation of spin torques for small-amplitude magnetization
dynamics around a uniformly magnetized state [J. Phys. Soc. Jpn. {\bf 75}
(2006) 113706] is extended here to the case of finite-amplitude dynamics. This
is achieved by introducing an `` adiabatic'' spin frame for conduction
electrons, and the associated SU(2) gauge field. In particular, the Gilbert
damping is shown to arise from the time variation of the spin-relaxation source
terms in this new frame, giving a new physical picture of the damping. The
present method will allow a `` first-principle'' derivation of spin torques
without any assumptions such as rotational symmetry in spin space.",0703705v1
1993-03-26,Nonequilibrium Dynamics and Aging in the Three--Dimensional Ising Spin Glass Model,"The low temperature dynamics of the three dimensional Ising spin glass in
zero field with a discrete bond distribution is investigated via MC
simulations. The thermoremanent magnetization is found to decay algebraically
and the temperature dependent exponents agree very well with the experimentally
determined values. The nonequilibrium autocorrelation function $C(t,t_w)$ shows
a crossover at the waiting (or {\em aging}) time $t_w$ from algebraic {\em
quasi-equilibrium} decay for times $t$$\ll$$t_w$ to another, faster algebraic
decay for $t$$\gg$$t_w$ with an exponent similar to one for the remanent
magnetization.",9303013v1
1997-04-11,Electroweak properties of light mesons in relativistic hamiltonian dynamics,"The calculation of lepton decay constants and electromagnetic form factors of
pion and kaon is performed in the framework of relativistic hamiltonian
dynamics. The different model wave function is used. Wave function parameters
are fixed from fit of mean square radius of meson. The internal quark structure
is taken into account by electromagnetic quark form factor and quark anomalous
magnetic moment. The pion and kaon form factors depend only weakly on model
wave function. Strong dependence of these values on quark anomalous magnetic
moment is obtained in the transfer momentum region of CEBAF experiments.",9704293v1
2000-06-20,Magnetic string contribution to hadron dynamics in QCD,"Dynamics of a light quark in the field of static source (heavy-light meson)
is studied using the nonlinear Dirac equation, derived recently. Special
attention is paid to the contribution of magnetic correlators and it is found
that it yields a significant increase of string tension at intermediate
distances. The spectrum of heavy-light mesons is computed with account of this
contribution and compared to experimental and lattice data.",0006237v1
2006-05-02,"Comment on ""Dynamical mass generation in strongly coupled quantum electrodynamics with weak magnetic fields""","In a recent article, Ayala et al. [Phys. Rev. D 73, 105009 (2006)] have
studied the dynamical generation of masses for fundamental fermions in strongly
coupled QED in the presence of weak magnetic fields. We argue that the results
and conclusions of the article are not reliable as they are subject to gauge
dependent artifacts resulting from an inconsistent truncation of the
Schwinger-Dyson equations.",0605027v2
2007-03-29,Nonperturbative dynamics in the colour-magnetic QCD vacuum,"In the deconfinement phase of QCD quarks and gluons interact with the dense
stochastic colour-magnetic vacuum. We consider the dynamics of quarks in this
deconfinement phase using the Field Correlators Method and derive an effective
nonperturbative inter-quark potential, in addition to the usual perturbative
short-ranged interaction. We find the resulting angular-momentum-dependent
interaction to be attractive enough to maintain bound states and, for light
quarks (and gluons), to cause emission of quark and gluon pairs. Possible
consequences for the strong interacting quark--gluon plasma are briefly
discussed.",0703306v1
1998-11-09,Dynamical Chiral Symmetry Breaking in QED in a Magnetic Field: Toward Exact Results,"We describe a (first, to the best of our knowledge) essentially soluble
example of dynamical symmetry breaking phenomenon in a 3+1 dimensional gauge
theory without fundamental scalar fields: QED in a constant magnetic field.",9811079v3
2001-12-14,Supersymmetry-based Approach to Quantum Particle Dynamics,"We present the N=2 supersymmetric formulation for the classical and quantum
dynamics of a nonrelativistic charged particle on a curved surface in the
presence of a perpendicular magnetic field. For a particle moving on a
constant-curvature surface in a constant magnetic field, our Hamiltonian
possesses the shape-invariance property in addition. On the surface of a sphere
and also on the hyperbolic plane, we exploit the supersymmetry and
shape-invariance properties to obtain complete solutions to the corresponding
quantum mechanical problems.",0112120v1
2004-07-20,Noncommuting spherical coordinates,"Restricting the states of a charged particle to the lowest Landau level
introduces a noncommutativity between Cartesian coordinate operators. This idea
is extended to the motion of a charged particle on a sphere in the presence of
a magnetic monopole. Restricting the dynamics to the lowest energy level
results in noncommutativity for angular variables and to a definition of a
noncommuting spherical product. The values of the commutators of various
angular variables are not arbitrary but are restricted by the discrete
magnitude of the magnetic monopole charge. An algebra, isomorphic to angular
momentum, appears. This algebra is used to define a spherical star product.
Solutions are obtained for dynamics in the presence of additional angular
dependent potentials.",0407177v1
2003-12-23,An example of a jump from chaos to integrability for magnetic geodesic flows,"It is proved that the motion of a charge particle on a hyperbolic oriented
two-dimensional surface in a magnetic field given by the volume form of the
hyperbolic metric is completely integrable on the energy levels E < 1/2 in
terms of real-analytic integrals. However it was known that on the level E=1/2
every trajectory is transitive and the restrictions of this flow onto the
levels E>1/2 are Anosov flows",0312430v2
2000-06-07,Breaking conjugate pairing in thermostatted billiards by magnetic field,"We demonstrate that in the thermostatted three-dimensional Lorentz gas the
symmetry of the Lyapunov spectrum can be broken by adding to the system an
external magnetic field not perpendicular to the electric field. For
perpendicular field vectors, there is a Hamiltonian reformulation of the
dynamics and the conjugate pairing rule still holds. This indicates that
symmetric Lyapunov spectra has nothing to do with time reversal symmetry or
reversibility; instead, it seems to be related to the existence of a
Hamiltonian connection.",0006010v1
2001-06-05,Fluctuation formula for nonreversible dynamics in the thermostated Lorentz gas,"We investigate numerically the validity of the Gallavotti-Cohen fluctuation
formula in the two and three dimensional periodic Lorentz gas subjected to
constant electric and magnetic fields and thermostated by the Gaussian
isokinetic thermostat. The magnetic field breaks the time reversal symmetry,
and by choosing its orientation with respect to the lattice one can have either
a generalized reversing symmetry or no reversibility at all. Our results
indicate that the scaling property described by the fluctuation formula may be
approximately valid for large fluctuations even in the absence of
reversibility.",0106005v2
2004-06-24,Semi-classical analysis of real atomic spectra beyond Gutzwiller's approximation,"Real atomic systems, like the hydrogen atom in a magnetic field or the helium
atom, whose classical dynamics are chaotic, generally present both discrete and
continuous symmetries. In this letter, we explain how these properties must be
taken into account in order to obtain the proper (i.e. symmetry projected)
$\hbar$ expansion of semiclassical expressions like the Gutzwiller trace
formula. In the case of the hydrogen atom in a magnetic field, we shed light on
the excellent agreement between present theory and exact quantum results.",0406055v2
2005-05-26,Spectral Form Factor for Chaotic Dynamics in a Weak Magnetic Field,"Using semiclassical periodic orbit theory for a chaotic system in a weak
magnetic field, we obtain the form factor predicted by Pandey and Mehta's two
matrix model up to the third order. The third order contribution has a peculiar
term which exists only in the intermediate crossover domain between the GOE
(Gaussian Orthogonal Ensemble) and the GUE (Gaussian Unitary Ensemble)
universality classes. The exact expression is obtained by taking account of the
contribution from encounter regions where orbit loops are connected.",0505055v3
1999-03-13,Neutron Star Vortex Dynamics and Magnetic Field Decay: Implications for High Density Nuclear Matter,"We investigate the effect of the density-dependent proton and neutron gaps on
vortex dynamics in neutron stars. We argue that the persistence of neutron star
magnetic fields on timescales of $10^9$ y suggests a superconducting gap curve
with local maximum at intermediate density. We discuss the implications for
exotic core phenomena such as pion/kaon condensation or a transition to quark
matter.",9903039v1
1999-10-19,Deuteron Magnetic and Quadrupole Moments with a Poincaré Covariant Current Operator in the Front-Form Dynamics,"The deuteron magnetic and quadrupole moments are unambiguosly determined
within the front-form Hamiltonian dynamics, by using a new current operator
which fulfills Poincar\'e, parity and time reversal covariance, together with
hermiticity and the continuity equation. For both quantities the usual
disagreement between theoretical and experimental results is largely removed.",9910049v1
2000-03-15,Dynamical Screening and Radiative Parton Energy Loss in a Quark-gluon Plasma,"Dynamical screening in the magnetic part of the one-gluon exchange
interaction is included in the study of radiative energy loss of a fast parton
propagating inside a quark-gluon plasma. As a result the final radiative energy
loss is about twice as large as when only the electric part of one-gluon
exchange interaction is considered. A non-perturbative magnetic screening mass
is also used in the estimate of the mean-free-path of parton scattering in a
hot QCD matter.",0003033v2
2000-11-29,Finite element approach for simulating quantum electron dynamics in a magnetic field,"A fast and stable numerical method is formulated to compute the time
evolution of a wave function in a magnetic field by solving the time-dependent
Schroedinger equation. This computational method is based on the finite element
method in real space to improved accuracy without any increase of computational
cost. This method is also based on Suzuki's exponential product theory to
afford an efficient way to manage the TD-Schroedinger equation with a vector
potential. Applying this method to some simple electron dynamics, we have
confirmed its efficiency and accuracy.",0011069v1
2005-05-06,Microfluidic capturing-dynamics of paramagnetic bead suspensions,"We study theoretically the capturing of paramagnetic beads by a magnetic
field gradient in a microfluidic channel treating the beads as a continuum.
Bead motion is affected by both fluidic and magnetic forces. The transfer of
momentum from beads to the fluid creates an effective bead-bead interaction
that greatly aids capturing. We demonstrate that for a given inlet flow speed a
critical density of beads exists above which complete capturing takes place.",0505052v1
1997-04-10,On the Torus Quantization of Two Anyons with Coulomb Interaction in a Magnetic Field,"We study two anyons with Coulomb interaction in a uniform magnetic field $B$.
By using the torus quantization we obtain the modified Landau and Zeeman
formulas for the two anyons. Then we derive a simple algebraic equation for the
full spectral problem up to the second order in $B$.",9704022v1
2003-06-16,Quantum Dynamics of the Oscillating Cantilever-Driven Adiabatic Reversals in Magnetic Resonance Force Microscopy,"We simulated the quantum dynamics for magnetic resonance force microscopy
(MRFM) in the oscillating cantilever-driven adiabatic reversals (OSCAR)
technique.
  We estimated the frequency shift of the cantilever vibrations and
demonstrated that this shift causes the formation of a Schrodinger cat state
which has some similarities and differences from the conventional MRFM
technique which uses cyclic adiabatic reversals of spins. The interaction of
the cantilever with the environment is shown to quickly destroy the coherence
between the two possible cantilever trajectories. We have shown that using
partial adiabatic reversals, one can produce a significant increase in the
OSCAR signal.",0306107v1
2003-12-17,Quantum dynamics in single spin measurement,"We study the quantum dynamics of a model for the single-spin measurement in
magnetic-resonance force microscopy. We consider an oscillating driven
cantilever coupled with the magnetic moment of the sample. Then, the cantilever
is damped through an external bath and its readout is provided by a radiation
field. Conditions for reliable measurements will be discussed.",0312147v2
2006-02-13,Entanglement and dynamics of spin-chains in periodically-pulsed magnetic fields: accelerator modes,"We study the dynamics of a single excitation in a Heisenberg spin-chain
subjected to a sequence of periodic pulses from an external, parabolic,
magnetic field. We show that, for experimentally reasonable parameters, a pair
of counter-propagating coherent states are ejected from the centre of the
chain. We find an illuminating correspondence with the quantum time evolution
of the well-known paradigm of quantum chaos, the Quantum Kicked Rotor (QKR).
  From this we can analyse the entanglement production and interpret the
ejected coherent states as a manifestation of so-called `accelerator modes' of
a classically chaotic system.",0602106v2
2006-11-28,Efficient Dynamic Nuclear Polarization at High Magnetic Fields,"By applying a new technique for dynamic nuclear polarization involving
simultaneous excitation of electronic and nuclear transitions, we have enhanced
the nuclear polarization of the nitrogen nuclei in 15N@C60 by a factor of 1000
at a fixed temperature of 3 K and a magnetic field of 8.6 T, more than twice
the maximum enhancement reported to date. This methodology will allow the
initialization of the nuclear qubit in schemes exploiting N@C60 molecules as
components of a quantum information processing device.",0611276v1
2007-04-03,Exactly solvable spin dynamics of an electron coupled to large number of nuclei and the electron-nuclear spin echo in a quantum dot,"The model considered in the paper is used nowadays to describe spin dynamics
of quantum dots after optical excitation. Based on the exact diagonalization of
a model Hamiltonian, we solve the problems of the electron spin polarization
decay and magnetic field dependence of the steady state polarization. The
important role of the nuclear state is shown and methods of its calculation for
different regimes of optical excitation are proposed. The effect of spin echo
observed after application of the magnetic field $\pi$-pulse is predicted.",0704.0391v3
2007-07-27,Excitation of spin dynamics by spin-polarized current in vortex state disks,"A spin-polarized current with the polarization perpendicular to the plane of
a vortex-state disk results in renormalization of the effective damping for a
given magnetization mode, and the effective damping becomes zero if the current
exceeds a threshold value. The lowest threshold current corresponds to the
lowest frequency vortex gyroscopic mode. For larger values of the current the
dynamic magnetization state is characterized by precession of the vortex around
the dot center with non-small amplitude and higher frequency.",0707.4128v1
2007-10-04,Activation of additional energy dissipation processes in the magnetization dynamics of epitaxial chromium dioxide films,"The precessional magnetization dynamics of a chromium dioxide$(100)$ film is
examined in an all-optical pump-probe setup. The frequency dependence on the
external field is used to extract the uniaxial in-plane anisotropy constant.
The damping shows a strong dependence on the frequency, but also on the laser
pump fluency, which is revealed as an important experiment parameter in this
work: above a certain threshold further channels of energy dissipation open and
the damping increases discontinuously. This behavior might stem from spin-wave
instabilities.",0710.0986v2
2007-10-29,Cyclotron motion and magnetic focusing in semiconductor quantum wells with spin-orbit coupling,"We investigate the ballistic motion of electrons in III-V semiconductor
quantum wells with Rashba spin-orbit coupling in a perpendicular magnetic
field. Taking into account the full quantum dynamics of the problem, we explore
the modifications of classical cyclotron orbits due to spin-orbit interaction.
As a result, for electron energies comparable with the cyclotron energy the
dynamics are particularly rich and not adequately described by semiclassical
approximations. Our study is complementary to previous semiclassical approaches
concentrating on the regime of weaker fields.",0710.5395v2
2007-12-01,Solitons in isotropic antiferromagnets: beyond a sigma model,"Isotropic antiferromagnets shows a reach variety of magnetic solitons with
non-trivial static and dynamic properties. One-dimensional soliton elementary
excitations have a periodic dispersion law. For two-dimensional case, planar
antiferromagnetic vortices having non-singular macroscopic core with the
saturated magnetic moment are present. The dynamic properties of these planar
antiferromagnetic vortex are characterized by presence of a gyroforce",0712.0073v1
2008-01-11,Giant dynamical Zeeman split in inverse spin valves,"The inversion of a spin valve device is proposed. Opposite to a conventional
spin valve of a non-magnetic spacer sandwiched between two ferromagnetic
metals, an inverse spin valve is a ferromagnet sandwiched between two
non-magnetic metals. It is predicted that, under a bias, the chemical
potentials of spin-up and spin-down electrons in the metals split at
metal-ferromagnet interfaces, a dynamical Zeeman effect. This split is of the
order of an applied bias. Thus, there should be no problem of generating an
$eV$ split that is not possible to be realized on the earth by the usual Zeeman
effect.",0801.1775v1
2008-03-07,Orbital magnetism in transition-metal systems: The role of local correlation effects,"The influence of correlation effects on the orbital moments for transition
metals and their alloys is studied by first-principle relativistic Density
Functional Theory in combination with the Dynamical Mean-Field Theory. In
contrast to the previous studies based on the orbital polarization corrections
we obtain an improved description of the orbital moments for wide range of
studied systems as bulk Fe, Co and Ni, Fe-Co disordered alloys and 3$d$
impurities in Au. The proposed scheme can give simultaneously a correct
dynamical description of the spectral function as well as static magnetic
properties of correlated disordered metals.",0803.1000v1
2008-06-29,The Magnetic Eden Model,"In the magnetic Eden model (MEM), particles have a spin and grow in contact
with a thermal bath. Although Ising-like interactions affect the growth
dynamics, deposited spins are frozen and not allowed to flip. This review
article focuses on recent developments and future prospects, such as
spontaneous switching phenomena, critical behavior associated with fractal,
wetting, and order-disorder phase transitions, the equilibrium/nonequilibrium
correspondence conjecture, as well as dynamical and critical features of the
MEM defined on complex network substrates.",0806.4767v1
2008-07-11,Probing the spin polarization of current by soft X-ray imaging of current-induced magnetic vortex dynamics,"Time-resolved soft X-ray transmission microscopy is applied to image the
current-induced resonant dynamics of the magnetic vortex core realized in a
micronsized Permalloy disk. The high spatial resolution better than 25 nm
enables us to observe the resonant motion of the vortex core. The result also
provides the spin polarization of the current to be 0.67 +/-0.16 for Permalloy
by fitting the experimental results with an analytical model in the framework
of the spin-transfer torque.",0807.1782v1
2008-09-02,Dynamical Correlations of the Spin-1/2 Heisenberg XXZ Chain in a Staggered Field,"We consider the easy-plane anisotropic spin-1/2 Heisenberg chain in combined
uniform longitudinal and transverse staggered magnetic fields. The low-energy
limit of his model is described by the sine-Gordon quantum field theory. Using
methods of integrable quantum field theory we determine the various components
of the dynamical structure factor. To do so, we derive explicit expressions for
all matrix elements of the low-energy projections of the spin operators
involving at most two particles. We discuss applications of our results to
experiments on one-dimensional quantum magnets.",0809.0398v2
2008-11-18,Dynamics of Einstein - de Haas Effect: Application to Magnetic Cantilever,"Local time-dependent theory of Einstein - de Haas effect is developed. We
begin with microscopicinteractions and derive dynamical equations that couple
elastic deformations with internal twists due to spins. The theory is applied
to the description of the motion of a magnetic cantilever caused by the
oscillation of the domain wall. Theoretical results are compared with a recent
experiment on Einstein - de Haas effect in a microcantilever.",0811.3013v1
2008-11-25,Dynamics of OH(2Pi)-He collisions in combined electric and magnetic fields,"We use accurate quantum mechanical calculations to analyze the effects of
parallel electric and magnetic fields on collision dynamics of OH(2Pi)
molecules. It is demonstrated that spin relaxation in 3He-OH collisions at
temperatures below 0.01 K can be effectively suppressed by moderate electric
fields of order 10 kV/cm. We show that electric fields can be used to
manipulate Feshbach resonances in collisions of cold molecules. Our results can
be verified in experiments with OH molecules in Stark decelerated molecular
beams and electromagnetic traps.",0811.4018v1
2008-12-15,Dynamically dominant excitations of string solutions in the spin-1/2 antiferromagnetic Heisenberg chain in magnetic fields,"Using Bethe-ansatz solutions, we uncover a well-defined continuum in
dynamical structure factor $S^{+-}(k,\omega)$ of the spin-1/2 antiferromagnetic
Heisenberg chain in magnetic fields. It comes from string solutions which
continuously connect the mode of the lowest-energy excitations in the
zero-field limit and that of bound states of overturned spins from the
ferromagnetic state near the saturation field. We confirm the relevance to real
materials through comparisons with experimental results.",0812.2707v1
2009-01-22,Geometrically frustrated antiferromagnets: statistical mechanics and dynamics,"These lecture notes are intended to provide a simple overview of the physics
of geometrically frustrated magnets. The emphasis is on classical and
semiclassical treatments of the statistical mechanics and dynamics of
frustrated Heisenberg models, and on the ways in which the results provide an
understanding of some of the main observed properties of these systems.",0901.3492v1
2009-04-22,Magnetic field-induced soft mode in spin-gapped high-Tc superconductors,"We present an explanation of the dynamical in-gap spin mode in LSCO induced
by an applied magnetic field H as recently observed by J. Chang et al. Our
model consists of a phenomenological spin-only Hamiltonian, and the softening
of the spin mode is caused by vortex pinning of dynamical stripe fluctuations
which we model by a local ordering of the exchange interactions. The spin gap
vanishes experimentally around H=7T which in our scenario corresponds to the
field required for overlapping vortex regions.",0904.3404v1
2009-07-16,Influence of magnetic viscosity on domain wall dynamics under spin-polarized currents,"We present a theoretical study of the influence of magnetic viscosity on
current-driven domain wall dynamics. In particular we examine how domain wall
depinning transitions, driven by thermal activation, are influenced by the
adiabatic and nonadiabatic spin-torques. We find the Arrhenius law that
describes the transition rate for activation over a single energy barrier
remains applicable under currents but with a current-dependent barrier height.
We show that the effective energy barrier is dominated by a linear current
dependence under usual experimental conditions, with a variation that depends
only on the nonadiabatic spin torque coefficient beta.",0907.2867v1
2009-08-11,Dynamical Axion Field in Topological Magnetic Insulators,"Axions are very light, very weakly interacting particles postulated more than
30 years ago in the context of the Standard Model of particle physics. Their
existence could explain the missing dark matter of the universe. However,
despite intensive searches, they have yet to be detected. In this work, we show
that magnetic fluctuations of topological insulators couple to the
electromagnetic fields exactly like the axions, and propose several experiments
to detect this dynamical axion field. In particular, we show that the axion
coupling enables a nonlinear modulation of the electromagnetic field, leading
to attenuated total reflection. We propose a novel optical modulators device
based on this principle.",0908.1537v1
2009-11-16,Anomalous Spin Dynamics of Hubbard Model on Honeycomb Lattices,"In this paper, the honeycomb Hubbard model in optical lattices is
investigated using O(3) non-linear sigma model. A possible quantum non-magnetic
insulator in a narrow parameter region is found near the metal-insulator
transition. We study the corresponding dynamics of magnetic properties, and
find that the narrow region could be widened by hole doping.",0911.3002v1
2010-01-11,Physics of Solar Prominences: II - Magnetic Structure and Dynamics,"Observations and models of solar prominences are reviewed. We focus on
non-eruptive prominences, and describe recent progress in four areas of
prominence research: (1) magnetic structure deduced from observations and
models, (2) the dynamics of prominence plasmas (formation and flows), (3)
Magneto-hydrodynamic (MHD) waves in prominences and (4) the formation and
large-scale patterns of the filament channels in which prominences are located.
Finally, several outstanding issues in prominence research are discussed, along
with observations and models required to resolve them.",1001.1635v1
2010-03-10,Dichotomy between large local and small ordered magnetic moment in Iron-based superconductors,"We study a four band model for iron-based superconductors within local
density approximation + dynamical mean field theory (LDA+DMFT). This
successfully reproduces the results of models which take As p degrees of
freedom explicitly into account and has several physical advantages over the
standard five d-band model. Our findings reveal that the new superconductors
are more strongly correlated than their single-particle properties suggest.
Two-particle correlation functions unveil the dichotomy between local and
ordered magnetic moments in these systems, calling for further experiments to
better resolve the short time scale spin dynamics.",1003.2162v1
2010-03-19,Dynamics of magnetization on the topological surface,"We investigate theoretically the dynamics of magnetization coupled to the
surface Dirac fermions of a three dimensional topological insulator, by
deriving the Landau-Lifshitz-Gilbert (LLG) equation in the presence of charge
current. Both the inverse spin-Galvanic effect and the Gilbert damping
coefficient $\alpha$ are related to the two-dimensional diagonal conductivity
$\sigma_{xx}$ of the Dirac fermion, while the Berry phase of the ferromagnetic
moment to the Hall conductivity $\sigma_{xy}$. The spin transfer torque and the
so-called $\beta$-terms are shown to be negligibly small. Anomalous behaviors
in various phenomena including the ferromagnetic resonance are predicted in
terms of this LLG equation.",1003.3769v1
2010-05-25,Riemann-Lagrange Geometric Dynamics for the Multi-Time Magnetized Non-Viscous Plasma,"In this paper, using Riemann-Lagrange geometrical methods, we construct a
geometrical model on 1-jet spaces for the study of multi-time relativistic
magnetized non-viscous plasma, characterized by a given energy-stress-momentum
distinguished (d-) tensor. In that arena, we give the conservation laws and the
continuity equations for multi-time plasma. The partial differential equations
of the stream sheets (the equivalent of stream lines in the classical
semi-Riemannian geometrical approach of plasma) for multi-time plasma are also
written.",1005.4567v1
2010-06-09,Analytical expression of the magneto-optical Kerr effect and Brillouin light scattering intensity arising from dynamic magnetization,"Time-resolved magneto-optical Kerr effect (MOKE) and Brillouin light
scattering (BLS) spectroscopy are important techniques for the investigation of
magnetization dynamics. Within this article, we calculate analytically the MOKE
and BLS signals from prototypical spin-wave modes in the ferromagnetic layer.
The reliability of the analytical expressions is confirmed by optically exact
numerical calculations. Finally, we discuss the dependence of the MOKE and BLS
signals on the ferromagnetic layer thickness.",1006.1906v1
2010-07-08,Dysprosium magneto-optical traps,"Magneto-optical traps (MOTs) of highly magnetic lanthanides open the door to
explorations of novel phases of strongly correlated matter such as lattice
supersolids and quantum liquid crystals. We recently reported the first MOTs of
the five high abundance isotopes of the most magnetic atom, dysprosium.
Described here are details of the experimental technique employed for
repumper-free Dy MOTs containing up to half a billion atoms. Extensive
characterization of the MOTs' properties---population, temperature, loading,
metastable decay dynamics, trap dynamics---is provided.",1007.1480v1
2010-09-11,Nonlinear optical response of hole-trion systems in quantum dots in tilted magnetic fields,"We discuss, from a theoretical point of view, the four wave mixing
spectroscopy on an ensemble of p-doped quantum dots in a magnetic field
slightly tilted from the in-plane configuration. We describe the system
evolution in the density matrix formalism. In the limit of coherent ultrafast
optical driving, we obtain analytical formulas for the single system dynamics
and for the response of an inhomogeneously broadened ensemble. The results are
compared to the previously studied time-resolved Kerr rotation spectroscopy on
the same system. We show that the Kerr rotation and four wave mixing spectra
yield complementary information on the spin dynamics (precession and damping).",1009.2202v1
2010-10-11,Intrinsic coherence dynamics and phase localization in Aharonov-Bohm Interferometers,"The nonequilibrium real-time dynamics of electron coherence is explored in
the quantum transport through the double-dot Aharonov-Bohm interferometers. We
solve the exact master equation to find the exact quantum state of the device,
from which the changes of the electron coherence through the magnetic flux in
the nonequilibrium transport processes is obtained explicitly. We find that the
relative phase between the two charge states of the double dot localizes to
$\frac{\pi}{2}$ or $-\frac{\pi}{2}$ for all different magnetic flux. This
nontrivial phase localization process can be manifested in the measurable
occupation numbers.",1010.1977v1
2010-11-16,Dynamic susceptibility of a spin ice near the critical point,"We consider spin ice magnets (primarily, $\mathrm{Dy_2Ti_2O_7}$) in the
vicinity of their critical point on the $(H,T)$ plane. We find that the
longitudinal susceptibility diverges at the critical point, leading to the
behaviour qualitatively similar to the one which would result from non-zero
conductance of magnetic charges. We show that dynamics of critical fluctuations
belongs to the universality class of easy-axis ferroelectric and calculate
logarithmic corrections (within two-loop approximation) to the mean-field
critical behavior.",1011.3659v2
2010-11-25,Dissipative dynamics of a two-qubit system: Four-level lasing,"The dissipative dynamics of a two-qubit system is studied theoretically. We
make use of the Bloch-Redfield formalism which explicitly includes the
parameter-dependent relaxation rates. We consider the case of two flux qubits,
when the controlling parameters are the partial magnetic fluxes through the
qubits' loops. The strong dependence of the inter-level relaxation rates on the
controlling magnetic fluxes is demonstrated for the realistic system. This
allows us to propose several mechanisms for lasing in this four-level system.",1011.5598v2
2011-07-28,Random Hysteresis Loops,"Dynamical hysteresis is a phenomenon which arises in ferromagnetic systems
below the critical temperature as a response to adiabatic variations of the
external magnetic field. We study the problem in the context of the mean-field
Ising model with Glauber dynamics, proving that for frequencies of the magnetic
field oscillations of order $N^{2/3}$, with $N$ the size of the system, the
""critical"" hysteresis loop becomes random.",1107.5662v2
2011-11-17,Comment on 'Spin Ice: Magnetic Excitations without Monopole Signature Using $μ$SR' (arXiv:1110.0877),"Dunsiger and co-workers have criticised our paper on magnetic monopole
dynamics spin ice [S. T. Bramwell et al., Nature, 461 956 (2009)]. We consider
their criticisms and results but show that these do not warrant any revision of
our earlier conclusions. In contrast, we point out that their conclusion of a
`new' type of spin dynamics in spin ice is inconsistent with established facts.",1111.4168v1
2011-11-21,Magnetic Compton profiles of Fe and Ni corrected by dynamical electron correlations,"Magnetic Compton profiles (MCPs) of Ni and Fe along [111] direction have been
calculated using a combined Density Functional and many-body theory approach.
At the level of the local spin density approximation the theoretical MCPs does
not describe correctly the experimental results around the zero momentum
transfer. In this work we demonstrate that inclusion of electronic correlations
as captured by Dynamical Mean Field Theory (DMFT) improves significantly the
agreement between the theoretical and the experimental MCPs. In particular, an
energy decomposition of Ni MCPs gives indication of spin polarization and
intrinsic nature of Ni 6 eV satellite, a genuine many-body feature.",1111.4848v1
2011-11-22,Molecular nanomagnets as quantum simulators,"Quantum simulators are controllable systems that can be used to simulate
other quantum systems. Here we focus on the dynamics of a chain of molecular
qubits with interposed antiferromagnetic dimers. We theoretically show that its
dynamics can be controlled by means of uniform magnetic pulses and used to
mimic the evolution of other quantum systems, including fermionic ones. We
propose two proof-of-principle experiments, based on the simulation of the
Ising model in transverse field and of the quantum tunneling of the
magnetization in a spin-1 system.",1111.5201v1
2011-12-14,Dynamics of Thermal Effects in the Spin-Wave Theory of Quantum Antiferromagnets,"We derive a master equation that allows us to study non-equilibrium dynamics
of a quantum antiferromagnet. By resorting to spin-wave theory, we obtain a
closed analytic form for the magnon decay rates. These turn out to be closely
related to form factors, which are experimentally accessible by means of
neutron and Raman scattering. Furthermore, we compute the time evolution of the
staggered magnetization showing that, for moderate temperatures, the magnetic
order is not spoiled even if the coupling is fully isotropic.",1112.3158v3
2011-12-14,Quenched Slonczewski-Windmill in Spin-Torque Vortex-Oscillators,"We present a combined analytical and numerical study on double-vortex
spin-torque nano-oscillators and describe a mechanism that suppresses the
windmill modes. The magnetization dynamics is dominated by the gyrotropic
precession of the vortex in one of the ferromagnetic layers. In the other layer
the vortex gyration is strongly damped. The dominating layer for the
magnetization dynamics is determined by the current polarity. Measurements on
Fe/Ag/Fe nano-pillars support these findings. The results open up a new
perspective for building high quality-factor spin-torque oscillators operating
at selectable, well-separated frequency bands.",1112.3163v1
2012-03-09,"Relaxation, pre-thermalization and diffusion in a noisy Quantum Ising Chain","We study the dynamics of thermalization resulting from a time-dependent noise
in a Quantum Ising Chain subject to a sudden quench of the transverse magnetic
field. For weak noise the dynamics shows a pre-thermalized state at
intermediate time scales, eventually drifting towards an asymptotic infinite
temperature steady state characterized by diffusive behavior. By computing
analytically the density of kinks, as well as the transverse and longitudinal
magnetic field correlators, we characterize these two regimes, their
observability and their signatures in the various physical quantities.",1203.2108v3
2012-05-04,Cyclotron-Bloch dynamics of a quantum particle in a two-dimensional lattice II,"We study the quantum dynamics of a charged particle in a two-dimensional
lattice, subject to constant and homogeneous electric and magnetic fields. We
find that different regimes characterize these motions, depending on a
combination of conditions, corresponding to weak and strong electric field
intensities, rational or irrational directions of the electric field with
respect to the lattice, and small or large values of the magnetic (Peierls)
phase.",1205.0862v1
2012-09-04,Hamiltonian equation of motion and depinning phase transition in two-dimensional magnets,"Based on the Hamiltonian equation of motion of the $\phi^4$ theory with
quenched disorder, we investigate the depinning phase transition of the
domain-wall motion in two-dimensional magnets. With the short-time dynamic
approach, we numerically determine the transition field, and the static and
dynamic critical exponents. The results show that the fundamental Hamiltonian
equation of motion belongs to a universality class very different from those
effective equations of motion.",1209.0513v1
2012-10-30,NMR relaxation in spin ice due to diffusing emergent monopoles,"At low temperatures, spin dynamics in ideal spin ice is due mainly to dilute,
thermally excited magnetic monopole excitations. I consider how these will
affect the dynamics of a nuclear spin (the same theory applies to muon spin
resonance if implanted muons do not diffuse). Up to the time scale for nearby
monopoles to be rearranged, a stretched-exponential form of the relaxation
functions is expected. I work out the expected exponent in that exponential and
the formulas for the $T_1$ (longitudinal) and $T_2$ (dephasing) relaxations, as
a function of the monopole density. Experimental NMR is incompatible with the
predictions and I suggest is due to magnetic impurities.",1210.8137v1
2013-01-18,Field-Free Synthetic-Ferromagnet Spin Torque Oscillator,"We study the magnetization dynamics of spin valve structures with a free
composite synthetic ferromagnet (SyF) that consists of two ferromagnetic layers
coupled through a normal metal spacer. A ferromagnetically coupled SyF can be
excited into dynamical precessional states by an applied current without
external magnetic fields. We analytically determine the stability of these
states in the space spanned by the current density and SyF interlayer exchange
coupling. Numerical simulations confirm our analytical results.",1301.4294v1
2013-10-11,Magnetocapillary Swimmers,"We present an experiment where three mesoscopic soft ferromagnetic beads are
placed onto a liquid surface and submitted to the influence of magnetic fields.
A vertical magnetic field creates a repulsion which counterbalances the
capillary attraction. We show that the competition with a second, oscillating
field, deforms the structure in a non reciprocal way. As a consequence, the
structure is able to swim. This experiment is fully described in a fluid
dynamics video attached to this submission.",1310.3094v1
2013-10-11,Degenerate Rayleigh-Plateau instability in a magnetically annealed colloidal dispersion,"This fluid dynamics video depicts the evolution of a suspension of
paramagnetic colloids under the influence of a uniform, pulsed magnetic field.
At low pulse frequencies, the suspension condenses into columns which decompose
via a Rayleigh-Plateau instability. At high pulse frequencies, the suspension
forms a kinetically arrested, system spanning network. We demonstrate the
degeneration of the Rayleigh-Plateau instability with increasing pulse
frequency.",1310.3226v2
2014-02-11,Sliding window and compressive sensing for low-field dynamic magnetic resonance imaging,"We describe an acquisition/processing procedure for image reconstruction in
dynamic Magnetic Resonance Imaging (MRI). The approach requires sliding window
to record a set of trajectories in the k-space, standard regularization to
reconstruct an estimate of the object and compressed sensing to recover image
residuals. We validated this approach in the case of specific simulated
experiments and, in the case of real measurements, we showed that the procedure
is reliable even in the case of data acquired by means of a low-field scanner.",1402.2453v1
2014-07-25,Escape of Charged Particles Moving around a Weakly Magnetized Kerr Black Hole,"We study the dynamics and escape of charged particles initially orbiting a
weakly magnetized Kerr black hole after they get kicked in the direction normal
to the orbit. The case of neutral particles is analysed first and the escape
conditions are given analytically. A general analysis of charged particles
innermost stable circular orbits (ISCO)s is performed numerically. We then
study the charged particles three-dimensional motion and give an effective
condition for their escape. We also discuss how the black hole rotation affects
the escape of charged particles and the chaoticness in their dynamics.",1407.7069v1
2014-10-03,Dynamics of a vortex dipole across a magnetic phase boundary in a spinor Bose-Einstein condensate,"Dynamics of a vortex dipole in a spin-1 Bose-Einstein condensate in which
magnetic phases are spatially distributed is investigated. When a vortex dipole
travels from the ferromagnetic phase to the polar phase, or vice versa, it
penetrates the phase boundary and transforms into one of the various spin
vortex dipoles, such as a leapfrogging ferromagnetic-core vortex dipole and a
half-quantum vortex dipole. Topological connections of spin wave functions
across the phase boundary are discussed.",1410.0743v1
2014-11-03,Decay rates of magnetic modes below the threshold of a turbulent dynamo,"We measure the decay rates of magnetic field modes in a turbulent flow of
liquid sodium below the dynamo threshold.We observe that turbulent fluctuations
induce energy transfers between modes with different symmetries (dipolar and
quadrupolar). Using symmetry properties, we show how to measure the decay rate
of each mode without being restricted to the one with the smallest damping
rate.We observe that the respective values of the decay rates of these modes
depend on the shape of the propellers driving the flow. Dynamical regimes,
including field reversals, are observed onlywhen the modes are both nearly
marginal. This is in linewith a recently proposed model.",1411.0517v1
2014-11-21,Ballistic dynamics of Dirac particles in electro-magnetic fields,"Investigating properties of two-dimensional Dirac operators coupled to an
electric and a magnetic field (perpendicular to the plane) requires in general
unbounded (vector-) potentials. If the system has a certain symmetry, the
fields can be described by one-dimensional potentials $V$ and $A$. Assuming
that $|A|<|V|$ outside some arbitrary large ball, we show that absolutely
continuous states of the effective Dirac operators spread ballistically. These
results are based on well-known methods in spectral dynamics together with
certain new Hilbert-Schmidt bounds. We use Lorentz boosts to derive these new
estimates.",1411.5998v1
2014-11-30,Spin dynamics of the anisotropic spin-1 antiferromagnetic chain at finite magnetic fields,"We present results of a study of the antiferromagnetic spin-1 chain, subject
to the simultaneous presence of single-ion anisotropy and external magnetic
fields. Using quantum Monte-Carlo based on the stochastic series expansion
method we first uncover a rich quantum phase diagram comprising N\'eel,
Haldane, Luttinger liquid, and large anisotropy phases. Second, we scan across
this phase diagram over a wide range of parameters, evaluating the transverse
dynamic structure factor, which we show to exhibit sharp massive modes, as well
as multi particle continua. For vanishing anisotropy and fields, comparison
with existing results from other analytic and numerical approaches shows
convincing consistency.",1412.0208v2
2015-03-06,Localizing spin dynamics in a spin-1 Bose-Einstein condensate via magnetic pulses,"Spin exchange interaction between atoms in a spin-1 Bose-Einstein condensate
causes atomic spin evolving periodically under the single spatial mode
approximation in the mean field theory. By applying fast magnetic pulses
according to a two-step or a four-step control protocol, we find analytically
that the spin dynamics is significantly suppressed for an arbitrary initial
state. Numerical calculations under single mode approximation are carried out
to confirm the validity and robustness of these protocols. This localization
method can be readily utilized to improve the sensitivity of a magnetometer
based on spin-1 Bose-Einstein condensates.",1503.01828v1
2015-06-18,Dispersive read-out of ferromagnetic resonance for strongly coupled magnons and microwave photons,"We demonstrate the dispersive measurement of ferromagnetic resonance in a
yttrium iron garnet sphere embedded within a microwave cavity. The reduction in
the longitudinal magnetization at resonance is measured as a frequency shift in
the cavity mode coupled to the sphere. This measurement is a result of the
intrinsic non-linearity in magnetization dynamics, indicating a promising route
towards experiments in magnon cavity quantum electro-dynamics.",1506.05631v1
2015-06-20,Spin and orbital exchange interactions from Dynamical Mean Field Theory,"We derive a set of equations expressing the parameters of the magnetic
interactions characterizing a strongly correlated electronic system in terms of
single-electron Green's functions and self-energies. This allows to establish a
mapping between the initial electronic system and a spin model including up to
quadratic interactions between the effective spins, with a general interaction
(exchange) tensor that accounts for anisotropic exchange, Dzyaloshinskii-Moriya
interaction and other symmetric terms such as dipole-dipole interaction. We
present the formulas in a format that can be used for computations via
Dynamical Mean Field Theory algorithms.",1506.06209v1
2015-06-23,Effects of magnetic anisotropy on spin dynamics of ferromagnetic frustrated chain,"By exploiting density-matrix renormalization group techniques, we investigate
the spin dynamics of a spin-1/2 one-dimensional J1-J2 XXZ model with competing
ferromagnetic J1 and antiferromagnetic J2 exchange couplings under applied
magnetic fields. Numerical results of spin excitation spectra show that in the
field-induced spin quadrupole regime, the longitudinal component has a gapless
mode and the transverse component has a gapped mode irrespective of the
exchange anisotropy. The excitation gap of the transverse spin excitation
increases as the exchange anisotropy increases over the XY-like and Ising-like
regions, demonstrating that two-magnon bound states are stabilized due to the
easy-axis anisotropy.",1506.06883v1
2015-09-11,"Formation of star-forming clouds from the magnetised, diffuse interstellar medium","Molecular clouds, the birthplaces of stars in galaxies, form dynamically from
the diffuse atomic gas of the interstellar medium (ISM). The ISM is also
threaded by magnetic fields which have a large impact on its dynamics. In
particular, star forming regions must be magnetically supercrit- ical in order
to accomodate gas clumps which can collapse under their own weight. Based on a
parameter study of three dimensional magneto-hydrodyamical (MHD) simulations,
we show that the long-standing problem of how such supercritical regions are
generated is still an open issue.",1509.03436v1
2015-09-11,Non-Markovian Effects in the Spin Transfer Dynamics in Diluted Magnetic Semiconductors due to Excitation in Proximity to the Band Edge,"The non-Markovian effects in the spin dynamics in diluted magnetic
semiconductors found in quantum kinetic calculations can be reproduced very
well by a much simpler effective single electron theory, if a finite memory is
accounted for. The resulting integro-differential equation can be solved by a
differential transform method, yielding the Taylor series of the solution. From
the comparison of both theories it can be concluded that the non-Markovian
effects are due to the spectral proximity of the excited electrons to the band
edge.",1509.03487v1
2015-12-03,Dynamics of vortices with magnetic impurities,"We investigate the dynamics of BPS vortices in the presence of magnetic
impurities taking the form of axially-symmetric localised lumps and
delta-functions. We present numerical results for vortices on flat space, as
well as exact results for vortices on hyperbolic space in the presence of
delta-function impurities. In fact, delta-function impurities of appropriate
strength can be captured within the moduli space approximation by keeping one
or more of the vortices fixed. We also show that previous work on vortices on
the 2-sphere extends naturally to the inclusion of delta-function impurities.",1512.01054v1
2015-12-20,Shapiro like steps reveals molecular nanomagnets' spin dynamics,"We present an accurate way to detect spin dynamics of a nutating molecular
nanomagnet by inserting it in a tunnel Josephson junction and studying the
current voltage (I-V) characteristic. The spin nutation of the molecular
nanomagnet is generated by applying two circularly polarized magnetic fields.
We demonstrate that modulation of the Josephson current by the nutation of the
molecular nanomagnet's spin appears as a stepwise structure like Shapiro steps
in the I-V characteristic of the junction. Width and heights of these
Shapiro-like steps are determined by two parameters of the spin nutation,
frequency and amplitude of the nutation, which are simply tuned by the applied
magnetic fields.",1512.06406v1
2015-12-30,Distributed chaos and isotropic turbulence,"Power spectrum of the distributed chaos can be represented by a weighted
superposition of the exponential functions which is converged to a stretched
exponential $\propto \exp-(k/k_{\beta})^{\beta }$. An asymptotic theory has
been developed in order to estimate the value of $\beta$ for the isotropic
turbulence. This value has been found to be $\beta =3/4$. Excellent agreement
has been established between this theory and the data of direct numerical
simulations not only for the velocity field but also for the passive scalar,
energy dissipation rate, and magnetic fields. One can conclude that the
isotropic turbulence emerges from the distributed chaos.",1512.08837v3
2016-06-30,Skyrmion dynamics in a chiral magnet driven by periodically varying spin currents,"In this work, we investigated the spin dynamics in a slab of chiral magnets
induced by an alternating (ac) spin current. Periodic trajectories of the
skyrmion in real space are discovered under the ac current as a result of the
Magnus and viscous forces, which originate from the Gilbert damping, the spin
transfer torque, and the $ \beta $-nonadiabatic torque effects. The results are
obtained by numerically solving the Landau-Lifshitz-Gilbert equation and can be
explained by the Thiele equation characterizing the skyrmion core motion.",1606.09326v2
2016-08-23,Dynamic dependence to domain wall propagation through artificial spin ice,"Domain wall propagation dynamics have been studied in nanostructured
artificial kagome spin ice structures. A stripline circuit has been used to
provide localised pulsed magnetic fields within the artificial spin ice
structure. This provides control of the system through electrically assisted
domain wall nucleation events. Synchronisation of the pulsed fields with
additional global magnetic fields and the use of a focussed magneto-optical
Kerr effect magnetometer allows our experiments to probe the domain wall
transit through an extended ASI structure. We find that the propagation
distance depends on the driving field revealing field driven properties of
domain walls below their intrinsic nucleation field.",1608.06436v1
2016-11-14,Dynamic Polarization and Relaxation of 75As Nuclei in Silicon at High Magnetic Field and Low Temperature,"We present the results of experiments on dynamic nuclear polarization and
relaxation of 75As in silicon crystals. Experiments are performed in strong
magnetic fields of 4.6 T and temperatures below 1 K. At these conditions donor
electron spins are fully polarized, and the allowed and forbidden ESR
transitions are well resolved. We demonstrate effective nuclear polarization of
75As nuclei via the Overhauser effect on the time scale of several hundred
seconds. Excitation of the forbidden transitions leads to a polarization
through the solid effect. The relaxation rate of donor nuclei has strong
temperature dependence characteristic of Orbach process.",1611.04384v1
2016-12-23,"Defect Dynamics in Artificial Colloidal Ice: Real-Time Observation, Manipulation and Logic Gate","We study the defect dynamics in a colloidal spin ice system realized by
filling a square lattice of topographic double well islands with repulsively
interacting magnetic colloids. We focus on the contraction of defects in the
ground state, and contraction/expansion in a metastable biased state. Combining
real-time experiments with simulations, we prove that these defects behave like
emergent topological monopoles obeying a Coulomb law with an additional line
tension. We further show how to realize a completely resettable ""NOR"" gate,
which provides guidelines for fabrication of nanoscale logic devices based on
the motion of topological magnetic monopoles.",1612.08003v1
2017-05-09,Comparison of CPMG and Uhrig Dynamic Decoupling (UDD) for tissue refocusing in MRI,"Two different dynamic decoupling strategies, the classic CPMG and the more
recently proposed UDD protocol are compared in the context of magnetic
resonance imaging (MRI). Both sequences were implemented on a 3T human MRI
system and relaxometry was performed for a variety of tissue-mimicking agar and
agarose gels. We find that CPMG provides moderately better decoupling than
\UDD. The results are consistent with experimental noise spectra obtained from
\textit{in vivo} spectroscopy results, which suggest that the type of noise
spectra obtained would favor \CPMG. Theoretical coherence pathway analysis
further suggests that \UDD\ is more susceptible to systematic errors due to
static magnetic and radio-frequency field inhomogeneity or imperfect excitation
and refocusing pulse profiles.",1705.03292v1
2017-05-16,On free Lie algebras and particles in electro-magnetic fields,"The Poincar\'e algebra can be extended (non-centrally) to the Maxwell algebra
and beyond. These extensions are relevant for describing particle dynamics in
electro-magnetic backgrounds and possibly including the backreaction due the
presence of multipoles. We point out a relation of this construction to free
Lie algebras that gives a unified description of all possible kinematic
extensions, leading to a symmetry algebra that we call Maxwell${}_\infty$. A
specific dynamical system with this infinite symmetry is constructed and
analysed.",1705.05854v2
2017-08-10,Control of the gyration dynamics of magnetic vortices by the magnetoelastic effect,"The influence of a strain-induced uniaxial magnetoelastic anisotropy on the
magnetic vortex core dynamics in microstructured magnetostrictive
Co$_{40}$Fe$_{40}$B$_{20}$ elements was investigated with time-resolved
scanning transmission x-ray microscopy. The measurements revealed a
monotonically decreasing eigenfrequency of the vortex core gyration with the
increasing magnetoelastic anisotropy, which follows closely the predictions
from micromagnetic modeling.",1708.03123v1
2017-08-22,A dynamical mean-field theory study of stripe order and d-wave superconductivity in the two-dimensional Hubbard model,"We use cellular dynamical mean-field theory with extended unit cells to study
the ground state of the two-dimensional repulsive Hubbard model at finite
doping. We calculate the energy of states with d-wave superconductivity
coexisting with spatially uniform magnetic order and find that they are
energetically favoured in a large doping region as compared to the uniform
solution. We study the spatial form of the superconducting and magnetic order
parameters at different doping values.",1708.06749v2
2017-09-13,Ultrafast spin-lattice relaxation in ferromagnets including effective spin-orbit fields,"We investigate ultrafast demagnetization due to electron-phonon interaction
in a model band-ferromagnet. We show that the microscopic mechanism behind the
spin dynamics due to electron-phonon interaction is the interplay of scattering
and the precession around momentum-dependent effective internal spin-orbit
magnetic fields. The resulting magnetization dynamics can only be mimicked by
spin-flip transitions if the spin precession around the internal fields is
sufficiently fast (compared to the scattering time) so that it averages out the
transverse spin components.",1709.04253v1
2017-11-23,Effective Resistivity in Collisionless Magnetic Reconnection,"Magnetic reconnection (MR) in collisionless plasma is often attributed to the
off-diagonal electron Reynolds stress, which can give rise to a large induction
electric field in the reconnection region. However, in magneto-hydro-dynamics
(MHD) simulations of MR, it is difficult to implement the full Reynolds stress,
which is kinetic in nature. In this paper, we propose a theoretical model of
effective resistivity from the first principle of particle dynamics. The
derived theoretical formulation of the effective resistivity is verified by
full particle-in-cell (PIC) simulations, and the corresponding physics is
discussed.",1711.08702v1
2018-01-15,Uncertainty Quantification For A Permanent Magnet Synchronous Machine With Dynamic Rotor Eccentricity,"The influence of dynamic eccentricity on the harmonic spectrum of the torque
of a permanent magnet synchronous machine is studied. The spectrum is
calculated by an energy balance method. Uncertainty quantification is applied
by using generalized Polynomial Chaos and Monte Carlo. It is found that the
displacement of the rotor impacts the spectrum of the torque the most.",1801.04828v1
2018-02-26,Dynamics and Instabilities of the Shastry-Sutherland Model,"We study the excitation spectrum in the dimer phase of the Shastry-Sutherland
model by using an unbiased variational method that works in the thermodynamic
limit. The method outputs dynamical correlation functions in all possible
channels. This output is exploited to identify the order parameters with the
highest susceptibility (single or multitriplon condensation in a specific
channel) upon approaching a quantum phase transition in the magnetic field
versus the $J'/J$ phase diagram. We find four different instabilities:
antiferro spin nematic, plaquette spin nematic, stripe magnetic order, and
plaquette order, two of which have been reported in previous studies.",1802.09428v2
2018-03-22,Transient dynamics of strongly coupled spin vortex pairs: effects of anharmonicity and resonant excitation on inertial switching,"Spin vortices in magnetic nanopillars are used as GHz oscillators, with
frequency however essentially fixed in fabrication. We demonstrate a model
system of a two-vortex nanopillar, in which the resonance frequency can be
changed by an order of magnitude, without using high dc magnetic fields. The
effect is due to switching between the two stable states of the vortex pair,
which we show can be done with low-amplitude fields of sub-ns duration. We
detail the relevant vortex-core dynamics and explain how field anharmonicity
and phase control can be used to enhance the performance.",1803.08413v1
2018-06-04,Dynamical quadrupole structure factor of frustrated ferromagnetic chain,"We investigate the dynamical quadrupole structure factor of a spin-1/2
$J_{1}$-$J_{2}$ Heisenberg chain with competing ferromagnetic $J_{1}$ and
antiferromagnetic $J_{2}$ in a magnetic field by exploiting density-matrix
renormalization group techniques. In a field-induced spin nematic regime, we
observe gapless excitations at $q=\pi$ according to quasi-long-range
antiferro-quadrupole correlations. The gapless excitation mode has a quadratic
form at the saturation, while it changes into a linear dispersion as the
magnetization decreases.",1806.01434v1
2018-08-30,Axion instability and non-linear electromagnetic effect,"We investigate the instability due to dynamical axion field near the
topological phase transition of insulators. We first point out that the
amplitude of dynamical axion field is bounded for magnetic insulators in
general, which suppresses the axion instability. Near the topological phase
transition, however, the axion field may have a large fluctuation, which
decreases the critical electric field for the instability and increases the
axion induced magnetic flux density. Using two different model Hamiltonians, we
report the electromagnetic response of the axion field in details.",1808.10466v1
2018-10-14,Explicit symplectic adapted exponential integrators for charged-particle dynamics in a strong and constant magnetic field,"This paper studies explicit symplectic adapted exponential integrators for
solving charged-particle dynamics in a strong and constant magnetic field. We
first formulate the scheme of adapted exponential integrators and then derive
its symplecticity conditions. Based on the symplecticity conditions, we propose
five practical explicit symplectic adapted exponential integrators. Two
numerical experiments are carried out and the numerical results demonstrate the
remarkable numerical behavior of the new methods.",1810.06038v2
2018-11-06,Nonlinear dynamic interpretation of quantum spin,"In an effort to provide an alternative method to represent a quantum spin, a
precise nonlinear dynamics semi-classical model is used to show that standard
quantum spin analysis can be obtained. The model includes a multi-body,
anti-ferromagnetic ordering, highly coupled quantum spin and a semi-classical
interpretation of the torque on a spin magnetic moment in the presence of a
magnetic field. The deterministic nonlinear differential coupling equation is
used to introduce chaos, which is necessary to reproduce the correct
statistical quantum results.",1811.02624v1
2018-11-06,Nonlinear Dynamics Semi-classical Model of Quantum Spin,"A nonlinear dynamics semi-classical model is used to show that standard
quantum spin analysis can be obtained. The model includes a classically driven
nonlinear differential equation with dissipation and a semi-classical
interpretation of the torque on a spin magnetic moment in the presence of a
realistic magnetic field, which will represent two equilibrium positions. The
highly complicated driven nonlinear dissipative semi-classical model is used to
introduce chaos, which is necessary to produce the correct statistical quantum
results. The resemblance between this semi-classical spin model and the
thoroughly studied classical driven-damped nonlinear pendulum are shown and
discussed.",1811.02645v1
2018-11-14,Fractional calculus approach for the phase dynamics of Josephson junction under the influence of magnetic field,"This article presents the phase dynamics of an inline long Josephson junction
in voltage state under the influence of constant external magnetic field.
Fractional calculus approach is used to model the evolution of the phase
difference between the macroscopic wave functions of the two superconductors
across the junction. The governing non-linear partial differential equation is
then solved using finite difference-finite element schemes. Other quantities of
interest like Josephson current density and voltage across the junction are
also computed. The effects of various parameters in the model on phase
difference, Josephson current density and voltage are analyzed graphically with
the help of numerical simulations.",1811.05691v1
2019-01-24,Dynamic axion field in the magnetoelectric antiferromagnet chromia,"Chromia is a magnetoelectric insulator whose bulk magnetoelectric
susceptibility contains a non-zero pseudoscalar component, like that present in
magnetized topological insulators. We reveal the dynamic axion field of chromia
by measuring the temperature dependence of its non-linear magnetoelectric
response using a lock-in technique with an AC electric field stimulus. The
electric field dependence, magnetic field dependence, and temperature behavior
of the second harmonic lock-in signal are all in agreement with theoretical
expectations. Our results demonstrate that chromia magnetoelectric response
couples strongly to its low-energy quantum and thermal spin fluctuations.",1901.08536v1
2019-03-08,Dynamical phase transition in the 1D-transverse field Ising chain characterized by the transverse magnetization spectral function,"We study a 1D-Quantum Ising Model in transverse field driven out of
equilibrium by performing a composite quantum quench to deduce the asymptotic
properties of the transverse magnetization stationary state via the analysis of
the spectral function. What emerges is that, in correspondence of the dynamical
phase transition transition predicted for this model, the spectral function
vanishes giving a hint of the criticality. This result suggests also that
linear response experiment can be used to detect this kind of phase transition.",1903.03439v1
2019-04-01,Synchrony breakdown and noise-induced oscillation death in ensembles of serially connected spin-torque oscillators,"We consider collective dynamics in the ensemble of serially connected
spin-torque oscillators governed by the Landau-Lifshitz-Gilbert-Slonczewski
magnetization equation. Proximity to homoclinicity hampers synchronization of
spin-torque oscillators: when the synchronous ensemble experiences the
homoclinic bifurcation, the Floquet multiplier, responsible for the temporal
evolution of small deviations from the ensemble mean, diverges. Depending on
the configuration of the contour, sufficiently strong common noise, exemplified
by stochastic oscillations of the current through the circuit, may suppress
precession of the magnetic field for all oscillators. We derive the explicit
expression for the threshold amplitude of noise, enabling this suppression.",1904.00897v1
2019-05-21,Collective spin dynamics of Z2 vortex crystals in triangular Kitaev-Heisenberg antiferromagnets,"We show that the mesoscopic incommensurate $\mathbb{Z}_2$ vortex crystals
proposed for layered triangular anisotropic magnets can be most saliently
identified by two distinctive signatures in dynamical spin response
experiments: The presence of pseudo-Goldstone `phonon' modes at low frequencies
$\omega$, associated with the collective vibrations of the vortex cores, and a
characteristic multi-scattered intensity profile at higher $\omega$, arising
from a large number of Bragg reflections and magnon bandgaps. These are direct
fingerprints of the large vortex sizes and magnetic unit cells and the
solitonic spin profile around the vortex cores.",1905.08694v2
2019-10-18,Kardar-Parisi-Zhang universality from soft gauge modes,"The emergence of superdiffusive spin dynamics in integrable classical and
quantum magnets is well established by now, but there is no generally valid
theoretical explanation for this phenomenon. A fundamental difficulty is that
the hydrodynamic fluctuations of conserved quasiparticle modes are purely
diffusive. We argue that in isotropic integrable magnets, a complete
hydrodynamic description must include soft ""gauge"" degrees of freedom, that
arise from spontaneous breaking of the Bethe pseudovacuum symmetry. We show
that the coarse-grained time evolution of these modes lies in the
Kardar-Parisi-Zhang universality class of dynamics.",1910.08266v3
2019-11-25,Spin liquids in geometrically perfect triangular antiferromagnets,"The cradle of quantum spin liquids, triangular antiferromagnets show strong
proclivity to magnetic order and require deliberate tuning to stabilize a
spin-liquid state. In this brief review, we juxtapose recent theoretical
developments that trace the parameter regime of the spin-liquid phase, with
experimental results for Co-based and Yb-based triangular antiferromagnets.
Unconventional spin dynamics arising from both ordered and disordered ground
states is discussed, and the notion of a geometrically perfect triangular
system is scrutinized to demonstrate non-trivial imperfections that may assist
magnetic frustration in stabilizing dynamic spin states with peculiar
excitations.",1911.11157v2
2020-01-25,Field-induced freezing in the unfrustrated Ising antiferromagnet,"We study instantaneous quenches from infinite temperature to well below $T_c$
in the two-dimensional square lattice Ising antiferromagnet in the presence of
a longitudinal external magnetic field. Under single-spin-flip Metropolis
algorithm Monte Carlo dynamics, this protocol produces a pair of magnetization
plateaus that prevent the system from reaching the equilibrium ground state
except for some special values of the field. We explain the plateaus in terms
of local spin configurations that are stable under the dynamics.",2001.09268v2
2019-11-04,THz graphene W-shaped three-port circulator with dynamical control,"We propose a new graphene-based THz circulator. It consists of a circular
graphene resonator and three graphene nanoribbon waveguides of W-geometry
placed on a dilectric substrate. Surface plasmon-polariton waves propagate in
the waveguides. The nanoribbon excites in the resonator dipole resonance.
Nonreciprocity of the device is defined by nonsymmetry of the conductivity
tensor of the magnetized graphene. The circulator with the central frequency
7.5 THz has the bandwidth 4.25\% for isolation -15 dB, insertion loss -2.5 dB.
Applied DC magnetic field is 0.56T and Fermi energy of graphene
$\epsilon_F=0.15$eV. The Fermi energy allows one to control dynamically the
circulator responses.",2001.10078v1
2020-05-29,On the asymptotic dynamics of 2-D magnetic quantum systems,"In this work we provide results on the long time localisation in space
(dynamical localisation) of certain two-dimensional magnetic quantum systems.
The underlying Hamiltonian may have the form $H=H_0+W$, where $H_0$ is
rotationally symmetric, has dense point spectrum, and $W$ is a perturbation
that breaks the rotational symmetry.
  In the latter case, we also give estimates for the growth of the angular
momentum operator in time.",2006.00134v1
2020-07-15,"Comment on: ""Relativistic quantum dynamics of a charged particle in cosmic string spacetime in the presence of magnetic field and scalar potential''. Eur. Phys. J. C (2012) 72:2051","We analyze the results of a paper on ``Relativistic quantum dynamics of a
charged particle in cosmic string spacetime in the presence of magnetic field
and scalar potential''. We show that the authors did not obtain the spectrum of
the eigenvalue equation but only one eigenvalue for a specific relationship
between model parameters. In particular, the existence of allowed cyclotron
frequencies conjectured by the authors is a mere artifact of the truncation
condition used to obtain exact solutions to the radial eigenvalue equation.",2008.03376v1
2020-11-09,Topological charge analysis of dynamic process of transition to Néel-type skyrmion: role of domain wall skyrmions,"Magnetic skyrmions are intriguing topological spin textures that promise
future high-density spintronic devices. The creation of magnetic skyrmions has
been understood based on the energetics of skyrmions, but the detailed dynamic
process of the skyrmion creation remains unclear. Here topological evolution in
conversion from uneven domains to N\'eel-skyrmions is investigated using
micromagnetic simulations. We find that, rather than the overall topological
charge, annihilation of novel topological defects, i.e., recently suggested
domain wall skyrmions, dominantly govern the skyrmion creation process. Also,
the topological charge evolution is interpreted in terms of the number and the
combination of such topological defects.",2011.05299v1
2021-11-05,A First-Order Approach to Model Simultaneous Control of Multiple Microrobots,"The control of swarm systems is relatively well understood for simple robotic
platforms at the macro scale. However, there are still several unanswered
questions about how similar results can be achieved for microrobots. In this
paper, we propose a modeling framework based on a dynamic model of magnetized
self-propelling Janus microrobots under a global magnetic field. We verify our
model experimentally and provide methods that can aim at accurately describing
the behavior of microrobots while modeling their simultaneous control. The
model can be generalized to other microrobotic platforms in low Reynolds number
environments.",2111.03232v1
2021-12-06,Analysis of finite-size effect of infinite-range Ising model under Glauber dynamics,"We consider an infinite-range Ising model under the Glauber dynamics and
determine the finite-size effect on the distribution of two spin variables as a
perturbation of $O \left( 1/N \right)$. Based on several considerations,
ordinary differential equations are derived for describing the time development
of both a two-body correlation and the autocorrelation function of
magnetization. The results of the calculation fit the simulation results,
unless the perturbation theory breaks down because of critical phenomena or
magnetization reversal.",2112.02730v1
2021-12-18,Stability analysis of the Hindmarsh-Rose neuron under electromagnetic induction,"We consider the Hindmarsh-Rose neuron model modified by taking into account
the effect of electromagnetic induction on membrane potential. We study the
impact of the magnetic flux on the neuron dynamics, through the analysis of the
stability of fixed points. Increasing magnetic flux reduces the number of
equilibrium points and favors their stability. Therefore, electromagnetic
induction tends to regularize chaotic regimes and to affect regular and
quasi-regular ones by reducing the number of spikes or even destroying the
oscillations.",2112.09987v1
2022-02-25,Regions without invariant tori of given class for the planar circular restricted three-body problem,"A method to establish regions of phase space through which pass no invariant
tori transverse to a given direction field is applied to the planar circular
restricted three-body problem. Implications for the location of stable orbits
for planets around a binary star are deduced. It is expected that lessons
learnt from this problem will be useful for applications of the method to other
contexts such as flux surfaces for magnetic fields, guiding centre motion in
magnetic fields, and classical models of chemical reaction dynamics.",2202.12691v1
2022-04-29,Long term analysis of splitting methods for charged-particle dynamics,"In this paper, we rigorously analyze the energy, momentum and magnetic moment
behaviours of two splitting methods for solving charged-particle dynamics. The
near-conservations of these invariants are given for the system under constant
magnetic field or quadratic electric potential. By the approach named as
backward error analysis, we derive the modified equations and modified
invariants of the splitting methods and based on which, the near-conservations
over long times are proved. Some numerical experiments are presented to
demonstrate these long time behaviours.",2205.06659v1
2022-09-13,Small-scale chaotic dynamo and spontaneous breaking of local reflectional symmetry in magnetohydrodynamics,"It is shown, using results of direct numerical simulations, that the
spontaneous breaking of local reflectional symmetry (and corresponding
localized kinetic and magnetic helicities) can dominate chaotic dynamics of the
small-scale MHD dynamo in the Rayleigh-B{\'e}nard convection, in kinetically
forced flows under strong stratification, in the near-surface solar convection,
and in kinetically forced flows at large magnetic Reynolds numbers. The notions
of deterministic and helical distributed chaos have been used for this purpose.
The coexistence of large- and small-scale dynamo mechanisms and applications of
the obtained results to quiet and active regions of the solar photosphere have
been briefly discussed.",2209.06089v1
2022-11-28,Dynamical susceptibility of Skyrmion crystal,"Using stereographic projection approach we develop a theory for calculation
of dynamical susceptibility tensor of Skyrmion crystals (SkX), formed in thin
ferromagnetic films with Dzyaloshinskii-Moriya interaction and in the external
magnetic field. Staying whenever possible within analytical framework, we
employ the model anzats for static SkX configuration and discuss small
fluctuations around it. The obtained formulas are numerically analyzed in the
important case of uniform susceptibility, accessible in magnetic resonance (MR)
experiments. We show that, in addition to three characteristic MR frequencies
discussed earlier both theoretically and experimentally, one should also expect
several resonances of smaller amplitude at somewhat higher frequencies.",2211.15740v1
2023-01-17,Dynamics of self-gravitating systems in non-linearly magnetized chameleonic Brans-Dicke gravity,"We study the effects of magnetic fields of non-linear electrodynamics in
chameleonic Brans-Dicke theory under the existence of anisotropic spherical
fluid. In particular, we explore dissipative and non-dissipative
self-gravitating systems in the quasi-homologous regime with the minimal
complexity constraint. As a result, under the aforementioned circumstances,
several analytic solutions are found. Furthermore, by analyzing the dynamics of
a dissipative fluid, it is demonstrated that a void covering the center can
satisfy the Darmois criteria. The temperature of the self gravitating systems
is also investigated.",2301.07105v1
2023-09-25,Dunkl-Pauli Equation in the Presence of a Magnetic Field,"The Pauli equation, an important equation of quantum mechanics, allows us to
study the dynamics of spin-$1/2$ particles. The Dunkl derivative, when used
instead of the ordinary derivative, leads to obtaining parity-dependent
solutions. Motivated by these facts, in this work, we consider a
two-dimensional nonrelativistic spin-$1/2$ particle system in the presence of
an external magnetic field, and we investigate its parity-dependent dynamics by
solving the Pauli equation analytically. Next, we assume the system to be in
thermal equilibrium, and we examine various thermal quantities of the system.",2309.14081v2
2023-12-07,Thermomagnetic Anomalies by Magnonic Criticality in Ultracold Atomic Transport,"We investigate thermomagnetic transport in an ultracold atomic system with
two ferromagnets linked via a magnetic quantum point contact. Using
nonequilibrium Green's function approach, we show a divergence in spin
conductance and a slowing down of spin relaxation that manifest in the weak
effective-Zeeman-field limit. These anomalous spin dynamics result from the
magnonic critical point at which magnons become gapless due to spontaneous
magnetization. Our findings unveil untapped dynamics in ultracold atomic
systems, opening new avenues in thermomagnetism.",2312.04280v1
2024-01-31,Magnetic Bloch oscillations in a non-Hermitian quantum Ising chain,"We investigate the impacts of an imaginary transverse field on the dynamics
of magnetic domain walls in a quantum Ising chain. We show that an imaginary
field plays a similar role as a real transverse field in forming a low-lying
Wannier-Stark ladder. However, analytical and numerical calculations of the
time evolutions in both systems show that the corresponding Bloch oscillations
exhibit totally different patterns for the same initial states. These findings
reveal the nontrivial effect of non-Hermiticity on quantum spin dynamics.",2401.17586v2
2024-03-27,Specificity of $τ$ -- approximation for chaotic electron trajectories on complex Fermi surfaces,"The work examines a special behavior of the magnetic conductivity of metals
that arises when chaotic electron trajectories appear on the Fermi surface.
This behavior is due to the scattering of electrons at singular points of the
dynamic system describing the dynamics of electrons in $\, {\bf p}$-space, and
caused by small-angle scattering of electrons on phonons. In this situation,
the electronic system is described by a ""non-standard"" relaxation time, which
plays the main role in a certain range of temperature and magnetic field
values.",2403.18457v1
1999-01-27,Femtosecond electron and spin dynamics probed by nonlinear optics,"A theoretical calculation is performed for the ultrafast spin dynamics in
nickel using an exact diagonalization method. The present theory mainly focuses
on a situation where the intrinsic charge and spin dynamics is probed by the
nonlinear (magneto-)optical responses on the femtosecond time scale, i.e.
optical second harmonic generation (SHG) and the nonlinear magneto-optical Kerr
effect (NOLIMOKE). It is found that the ultrafast charge and spin dynamics are
observable on the time scale of 10 fs. The charge dynamics proceeds ahead of
the spin dynamics, which indicates the existence of a spin memory time. The
fast decay results from the loss of coherence in the initial excited state.
Both the material specific and experimental parameters affect the dynamics. We
find that the increase of exchange interaction mainly accelerates the spin
dynamics rather than the charge dynamics. A reduction of the hopping integrals,
such as present at interfaces, slows down the spin dynamics significantly.
Besides, it is found that a spectrally broad excitation yields the intrinsic
speed limit of the charge (SHG) and spin dynamics (NOLIMOKE) while a narrower
width prolongs the dynamics. This magnetic interface dynamics then should
become accessible to state of art time resolved nonlinear-optical experiments.",9901303v1
2003-10-29,Dynamical obstruction in a constrained system and its realization in lattices of superconducting devices,"Hard constraints imposed in statistical mechanics models can lead to
interesting thermodynamical behaviors, but may at the same time raise
obstructions in the thoroughfare to thermal equilibration. Here we study a
variant of Baxter's 3-color model in which local interactions and defects are
included, and discuss its connection to triangular arrays of Josephson
junctions of superconductors and \textit{kagom\'e} networks of superconducting
wires. The model is equivalent to an Ising model in a hexagonal lattice with
the constraint that the magnetization of each hexagon is $\pm 6$ or 0. For
ferromagnetic interactions, we find that the system is critical for a range of
temperatures (critical line) that terminates when it undergoes an exotic first
order phase transition with a jump from a zero magnetization state into the
fully magnetized state at finite temperature. Dynamically, however, we find
that the system becomes frozen into domains. The domain walls are made of
perfectly straight segments, and domain growth appears frozen within the time
scales studied with Monte Carlo simulations. This dynamical obstruction has its
origin in the topology of the allowed reconfigurations in phase space, which
consist of updates of closed loops of spins. As a consequence of the dynamical
obstruction, there exists a dynamical temperature, lower than the (avoided)
static critical temperature, at which the system is seen to jump from a
``supercooled liquid'' to a ``polycrystalline'' phase. In contrast, for
antiferromagnetic interactions, we argue that the system orders for
infinitesimal coupling because of the constraint, and we observe no interesting
dynamical effects.",0310710v1
2018-03-08,Probing the collective dynamics of nuclear spin bath in a rare-earth ion doped crystal,"Probing collective spin dynamics is a current challenge in the field of
magnetic resonance spectroscopy and has important applications in material
analysis and quantum information protocols. Recently, the rare-earth ion doped
crystals are an attractive candidate for making long-lived quantum memory.
Further enhancement of its performance would benefit from the direct knowledge
on the dynamics of nuclear-spin bath in the crystal. Here we detect the
collective dynamics of nuclear-spin bath located around the rare-earth ions in
a crystal using dynamical decoupling spectroscopy method. From the measured
spectrum, we analyze the configuration of the spin bath and characterize the
flip-flop time between two correlated nuclear spins in a long time scale ($\sim
$1s). Furthermore, we experimentally demonstrate that the rare-earth ions can
serve as a magnetic quantum sensor for external magnetic field. These results
suggest that the rare-earth ion is a useful probe for complex spin dynamics in
solids and enable quantum sensing in the low-frequency regime, revealing
promising possibilities for applications in diverse fields.",1803.03075v2
2020-03-20,Probing the Possibilities of Ergodicity in the 1D Spin-1/2 XY Chain with Quench Dynamics,"Ergodicity sits at the heart of the connection between statistical mechanics
and dynamics of a physical system. By fixing the initial state of the system
into the ground state of the Hamiltonian at zero temperature and tuning a
control parameter, we consider the occurrence of the ergodicity with quench
dynamics in the one-dimensional (1D) spin-1/2 XY model in a transverse magnetic
field. The ground-state phase diagram consists of two ferromagnetic and
paramagnetic phases. It is known the magnetization in this spin system is
non-ergodic. We set up two different experiments as we call them single and
double quenches and test the dynamics of the magnetization along the $Z$-axis
and the spin-spin correlation function along the $X$-axis which are the order
parameters of the zero-temperature phases . Our exact results reveal that for
single quenches at zero-temperature, the ergodicity depends on the initial
state and the order parameter. Interestingly on the other setup, a double
quench on a cyclic path, ergodicity is completely broken for starting from the
phase corresponding to the order parameter. Otherwise, it depends on the first
quenched point, and the quench time $T$ when the model spent before a second
quench in the way back which gives an ability to controlling the ergodicity in
the system. Therefore, and contrary to expectations, in the mentioned model the
ergodicity can be observed with probing quench dynamics at zero-temperature.
Our results provide further insight into the zero-temperature dynamical
behavior of quantum systems and their connections to the ergodicity phenomenon.",2003.09462v2
2021-01-15,Bethe strings in the spin dynamical structure factor of the Mott-Hubbard phase in one-dimensional fermionic Hubbard model,"The spectra and role in the spin dynamical properties of bound states of
elementary magnetic excitations named Bethe strings that occur in some
integrable spin and electronic one-dimensional models have recently been
identified and realized in several materials by experiments. Corresponding
theoretical studies have usually relied on the one-dimensional spin-1/2
Heisenberg antiferromagnet in a magnetic field. At the isotropic point, it
describes the large onsite repulsion U limit of the spin degrees of freedom of
the one-dimensional fermionic Hubbard model with one electron per site in a
magnetic field h. In this paper we consider the thermodynamic limit and study
the effects of lowering the latter quantum problem ratio u=U/4t, where t is the
first-neighbor transfer integral, on the line-shape singularities in regions at
and just above the lower thresholds of the transverse and longitudinal spin
dynamical structure factors. The most significant spectral weight contribution
from Bethe strings leads to a gapped continuum in the spectrum of the spin
dynamical structure factor +-. Our study focuses on the line shape
singularities at and just above the gapped lower threshold of that continuum,
which have been identified in experiments. Our results are consistent with the
contribution of Bethe strings to the spin dynamical structure factor zz being
small at low spin densities and becoming negligible upon increasing that
density. Our results provide physically important information about how
electron itinerancy affects the spin dynamics.",2101.06198v1
2022-12-19,Nonequilibrium dynamics of the Ising model on heterogeneous networks with an arbitrary distribution of threshold noise,"The Ising model on networks plays a fundamental role as a testing ground for
understanding cooperative phenomena in complex systems. Here we solve the
synchronous dynamics of the Ising model on random graphs with an arbitrary
degree distribution in the high-connectivity limit. Depending on the
distribution of the threshold noise that governs the microscopic dynamics, the
model evolves to nonequilibrium stationary states. We obtain an exact dynamical
equation for the distribution of local magnetizations, from which we find the
critical line that separates the paramagnetic from the ferromagnetic phase. For
random graphs with a negative binomial degree distribution, we demonstrate that
the stationary critical behavior as well as the long-time critical dynamics of
the first two moments of the local magnetizations depend on the distribution of
the threshold noise. In particular, for an algebraic threshold noise, these
critical properties are determined by the power-law tails of the distribution
of thresholds. We further show that the relaxation time of the average
magnetization inside each phase exhibits the standard mean-field critical
scaling. The values of all critical exponents considered here are independent
of the variance of the negative binomial degree distribution. Our work
highlights the importance of certain details of the microscopic dynamics for
the critical behavior of nonequilibrium spin systems.",2212.09424v2
2023-08-17,Heat-conserving three-temperature model for ultrafast demagnetization of 3d ferromagnets,"We study the ultrafast magnetization dynamics of bcc Fe and fcc Co using the
recently suggested heat-conserving three-temperature model (HC3TM), together
with atomistic spin- and lattice dynamics simulations. It is shown that this
type of Langevin-based simulation is able to reproduce observed trends of the
ultrafast magnetization dynamics of fcc Co and bcc Fe, in agreement with
previous findings for fcc Ni. The simulations are performed by using parameters
that to as large extent as possible are obtained from electronic structure
theory. The one parameter that was not calculated in this way, was the damping
term used for the lattice dynamics simulations, and here a range of parameters
were investigated. It is found that this term has a large influence on the
details of the magnetization dynamics. The dynamics of iron and cobalt is
compared with previous results for nickel and similarities and differences in
the materials' behavior are analysed following the absorption of a femtosecond
laser pulse. Importantly, for all elements investigated so far with this model,
we obtain a linear relationship between the value of the maximally demagnetized
state and the fluence of the laser pulse, which is in agreement with
experiments.",2308.08996v1
1995-08-28,Evolution of a Magnetic Flux Tube in a Sunspot Penumbra,"The motion of an individual magnetic flux tube inside the penumbra of a
sunspot is studied numerically. Here, we present preliminary results. The thin
flux tube approximation together with a simplified radiative heat exchange with
the surroundings is used to study the evolution of a flux tube embedded into a
background given by a global magneto-static sunspot model. The investigation is
undertaken in order to verify the conjecture that convection in sunspot
penumbrae occurs by an interchange of magnetic flux tubes. The code being
developed can be used to study dynamic aspects of filamentary structure in the
penumbra: the temporal and spatial fluctuations of the temperature and the
magnetic field, the motion of bright penumbral grains, or the Evershed effect.
Here we present the evolution of a wave formed by the tube whose fragment
emerges in the penumbral photosphere and migrates towards the umbra. The
properties of this wave show qualitative features of the observed bright
penumbral grains with corresponding upward velocity and its correlation with
brightness and the inclination of the magnetic field, and also of the Evershed
effect.",9508128v1
1996-10-24,Primordial Magnetic Fields from Cosmological First Order Phase Transitions,"We give an improved estimate of primordial magnetic fields generated during
cosmological first order phase transitions. We examine the charge distribution
at the nucleated bubble wall and its dynamics. We consider instabilities on the
bubble walls developing during the phase transition. It is found that damping
of these instabilities due to viscosity and heat conductivity caused by
particle diffusion can be important in the QCD phase transition, but is
probably negligible in the electroweak transition. We show how such
instabilities together with the surface charge densities on bubble walls excite
magnetic fields within a certain range of wavelengths. We discuss how these
magnetic seed fields may be amplified by MHD effects in the turbulent fluid.
The strength and spectrum of the primordial magnetic field at the present time
for the cases where this mechanism was operative during the electroweak or the
QCD phase transition are estimated. On a 10 Mpc comoving scale, field strengths
of the order 10**(-29) G for electroweak and 10**(-20) G for QCD, could be
attained for reasonable phase transition parameters.",9610201v1
1997-07-28,A Possible Dynamical Effect of a Primordial Magnetic Field,"The possible existence of a primordial magnetic field in the universe has
been previously investigated in many articles. Studies involving the influence
of a magnetic field in the nucleosyntesis era, studies considering the effects
in the formation of structures during the radiation era and the matter era have
been considered. We here assume the existence of a primordial magnetic field
and study its effect, in particular, in the formation of voids. The study is
twofold: to put constraints on the strength of the magnetic field during the
recombination era and to preview its effects on the formation of voids.",9707303v1
1997-08-27,The Magnetic Field Spectrum in a Plasma in Thermal Equilibrium in the Epoch of Primordial Nucleosynthesis,"The low frequency magnetic field spectrum in the primordial plasma is of
particular interest as a possible origin of magnetic fields in the universe
(e.g., Tajima et al. 1992 and Cable and Tajima 1992). We derive the magnetic
field spectrum in the primordial plasma, in particular, at the epoch of
primordial nucleosynthesis. The pioneering study of Cable and Tajima (1992) of
the electromagnetic fluctuations, based on the Fluctuation-Dissipation Theorem,
is extended. Our model describes both the thermal and collisional effects in a
plasma. It is based on a kinetic description with the BGK collision term. It is
shown that the zero-frequency peak found by Cable and Tajima (1992) decreases.
At high frequencies, the blackbody spectrum is obtained naturally without the
necessity of the link procedure used by them. At low frequencies ($\omega \leq
4\omega_{pe}$, where $\omega_{pe}$ is the electron plasma frequency) it is
shown that the magnetic field spectrum has more energy than the blackbody
spectrum in vacuum.",9708251v1
1999-06-18,Solar Nebula Magnetohydrodynamics,"The dynamical state of the solar nebula depends critically upon whether or
not the gas is magnetically coupled. The presence of a subthermal field will
cause laminar flow to break down into turbulence. Magnetic coupling, in turn,
depends upon the ionization fraction of the gas. The inner most region of the
nebula ($\lesssim 0.1$ AU) is magnetically well-coupled, as is the outermost
region ($\gtrsim 10$ AU). The magnetic status of intermediate scales ($\sim 1$
AU) is less certain. It is plausible that there is a zone adjacent to the inner
disk in which turbulent heating self-consistently maintains the requisite
ionization levels. But the region adjacent to the active outer disk is likely
to be magnetically ``dead.'' Hall currents play a significant role in nebular
magnetohydrodynamics.
  Though still occasionally argued in the literature, there is simply no
evidence to support the once standard claim that differential rotation in a
Keplerian disk is prone to break down into shear turbulence by nonlinear
instabilities. There is abundant evidence---numerical, experimental, and
analytic---in support of the stabilizing role of Coriolis forces.
Hydrodynamical turbulence is almost certainly not a source of enhanced
turbulence in the solar nebula, or in any other astrophysical accretion disk.",9906317v1
2000-06-17,3-D MHD Numerical Simulations of Cloud-Wind Interactions,"We present results from three-dimensional (3-D) numerical simulations
investigating the magnetohydrodynamics of cloud-wind interactions. The initial
cloud is spherical while the magnetic field is uniform and transverse to the
cloud motion. A simplified analytical model that describes the magnetic energy
evolution in front of the cloud is developed and compared with simulation
results. In addition, it is found the interaction of the cloud with a
magnetized interstellar medium (ISM) results in the formation of a highly
structured magnetotail. The magnetic flux in the wake of the cloud organizes
into flux ropes and a reconnection, current sheet is developed, as field lines
of opposite polarity are brought close together near the symmetry axis. At the
same time, magnetic pressure is strongly enhanced at the leading edge of the
cloud from the stretching of the field lines that occurs there. This has an
important dynamical effect on the subsequent evolution of the cloud, since some
unstable modes tend to be strongly enhanced.",0006240v1
2000-08-08,Disk Formation by AGB Winds in Dipole Magnetic Fields,"We present a simple, robust mechanism by which an isolated star can produce
an equatorial disk. The mechanism requires that the star have a simple dipole
magnetic field on the surface and an isotropic wind acceleration mechanism. The
wind couples to the field, stretching it until the field lines become mostly
radial and oppositely directed above and below the magnetic equator, as occurs
in the solar wind. The interaction between the wind plasma and magnetic field
near the star produces a steady outflow in which magnetic forces direct plasma
toward the equator, constructing a disk. In the context of a slow (10 km/s)
outflow (10^{-5} M_sun/yr) from an AGB star, MHD simulations demonstrate that a
dense equatorial disk will be produced for dipole field strengths of only a few
Gauss on the surface of the star. A disk formed by this model can be
dynamically important for the shaping of Planetary Nebulae.",0008129v1
2000-11-29,Conservation of Magnetic Helicity and Its Constraint on $α$-Effect of Dynamo Theory,"Dynamical studies of MHD turbulence on the one hand, and arguments based upon
magnetic helicity on the other, have yielded seemingly contradictory estimates
for the $\alpha$ parameter in turbulent dynamo theory. Here we show, with
direct numerical simulation of three-dimensional magnetohydrodynamic turbulence
with a mean magnetic field, $\OB$, that the constraint on the dynamo
$\alpha$-effect set by the magnetic helicity is time-dependent. A time-scale
$t_c$ is introduced such that for $t<t_c$, the $\alpha$-coefficient calculated
from the simulation is close to the result of Pouquet et al and Field et al,
$-\frac{\tau_{cor}}{3}(< \v \cdot \nabla \times \v > - < \b \cdot \nabla \times
\b >)$; for $t>t_c$, the classical result of the $\alpha$-coefficient given by
the Mean-Field Electrodynamics is reduced by a factor of $1/({R_m
|\OB|^2/v_{rms}^2})$, as argued by Gruzinov & Diamond, Seehafer and Cattaneo &
Hughes. Here, $R_m$ is the magnetic Reynolds number, $v_{rms}$ the rms velocity
of the turbulence, $\tau_{cor}$ the correlation time of the turbulence, and
$\overline B$ is in velocity unit. The applicability of and connection between
different models of dynamo theory are also discussed.",0011548v2
2001-01-10,On Turbulent Reconnection,"We examine the dynamics of turbulent reconnection in 2D and 3D reduced MHD by
calculating the effective dissipation due to coupling between small-scale
fluctuations and large-scale magnetic fields. Sweet-Parker type balance
relations are then used to calculate the global reconnection rate. Two
approaches are employed -- quasi-linear closure and an eddy-damped fluid model.
Results indicate that despite the presence of turbulence, the reconnection rate
remains inversely proportional to $\sqrt{R_m}$, as in the Sweet-Parker
analysis. In 2D, the global reconnection rate is shown to be enhanced over the
Sweet-Parker result by a factor of magnetic Mach number. These results are the
consequences of the constraint imposed on the global reconnection rate by the
requirement of mean square magnetic potential balance. The incompatibility of
turbulent fluid-magnetic energy equipartition and stationarity of mean square
magnetic potential is demonstrated.",0101161v1
2001-05-02,Ring Formation in Magnetically Subcritical Clouds and Multiple Star Formation,"We study numerically the ambipolar diffusion-driven evolution of
non-rotating, magnetically subcritical, disk-like molecular clouds, assuming
axisymmetry. Previous similar studies have concentrated on the formation of
single magnetically supercritical cores at the cloud center, which collapse to
form isolated stars. We show that, for a cloud with many Jeans masses and a
relatively flat mass distribution near the center, a magnetically supercritical
ring is produced instead. The supercritical ring contains a mass well above the
Jeans limit. It is expected to break up, through both gravitational and
possibly magnetic interchange instabilities, into a number of supercritical
dense cores, whose dynamic collapse may give rise to a burst of star formation.
Non-axisymmetric calculations are needed to follow in detail the expected ring
fragmentation into multiple cores and the subsequent core evolution.
Implications of our results on multiple star formation in general and the
northwestern cluster of protostars in the Serpens molecular cloud core in
particular are discussed.",0105028v1
2002-01-30,The Magnetic Field Structure in W51A,"We present 850 micron imaging polarimetry of the W51A massive star forming
region performed with SCUBA on the JCMT. From the polarimetry we infer the
column-averaged magnetic field direction, projected onto the plane of the sky.
We find that the magnetic field geometry in the region is complicated. We
compare the field geometry with 6 cm and CS J=7-6 emission and determine that
the magnetic field must be relatively weak and plays a passive role, allowing
itself to be shaped by pressure forces and dynamics in the ionised and neutral
gases. Comparisons are drawn between our data and 1.3 mm BIMA interferometric
polarimetry data, from which we conclude that the magnetic field must increase
in importance as we move to smaller scales and closer to sites of active star
formation.",0201499v1
2004-12-14,Dynamics of the Flows Accreting onto a Magnetized Neutron Star,"Non-stationary column accretion onto a surface of a magnetized neutron star
is studied with a numerical code based on modified first-order Godunov method
with splitting. Formation and evolution of shocks in the column is modeled for
accretion rates ranging from 10^{15} g/s to 10^{16} g/s and surface magnetic
fields ranging from 5*10^{11} G to 10^{13} G. Non-stationary solutions with
plasma deceleration at collisionless oscillating shocks are found. The kinetic
energy of the accreting flow efficiently transforms into a cyclotron radiation
field. Collisionless stopping of the flow allows a substantial part of
accreting CNO nuclei to avoid spallation and reach the surface. The nuclei
survival fraction depends on the surface magnetic field, being higher at lower
magnetic fields.",0412319v1
2005-01-13,Quasilinear Drift Of Cosmic Rays In Weak Turbulent Electromagnetic Fields,"A general quasilinear transport parameter for particle drift in arbitrary
turbulence geometry is presented. The new drift coefficient is solely
characterized by a nonresonant term and is evaluated for slab and
two-dimensional turbulence geometry. The calculations presented here
demonstrate that fluctuating electric fields are a key quantity for
understanding quasilinear particle drift in slab geometry. It is shown that
particle drift does not exist in unpolarized and purely magnetic slab
fluctuations. This is in stark contrast to previous models, which are
restricted to slab geometry and the field line random walk limit. The
evaluation of the general transport parameter for two-dimensional turbulence
geometry, presented here for the first time for dynamical magnetic turbulence,
results in a drift coefficient valid for a magnetic power spectrum and
turbulence decay rate varying arbitrarily in wavenumber. For a two-component,
slab/two-dimensional turbulence model, numerical calculations are presented.
The new quasilinear drift, induced by the magnetic perturbations, is compared
with a standard drift expression related to the curvature and gradient of an
unperturbed heliospheric background magnetic field. The considerations
presented here offer a solid ground and natural explanation for the hitherto
puzzling observation that drift models often describe observations much better
when drift effects are reduced.",0501243v1
2006-09-06,Dynamo Action in the Solar Convection Zone and Tachocline: Pumping and Organization of Toroidal Fields,"We present the first results from three-dimensional spherical shell
simulations of magnetic dynamo action realized by turbulent convection
penetrating downward into a tachocline of rotational shear. This permits us to
assess several dynamical elements believed to be crucial to the operation of
the solar global dynamo, variously involving differential rotation resulting
from convection, magnetic pumping, and amplification of fields by stretching
within the tachocline. The simulations reveal that strong axisymmetric toroidal
magnetic fields (about 3000 G in strength) are realized within the lower stable
layer, unlike in the convection zone where fluctuating fields are predominant.
The toroidal fields in the stable layer possess a striking persistent
antisymmetric parity, with fields in the northern hemisphere largely of
opposite polarity to those in the southern hemisphere. The associated mean
poloidal magnetic fields there have a clear dipolar geometry, but we have not
yet observed any distinctive reversals or latitudinal propagation. The presence
of these deep magnetic fields appears to stabilize the sense of mean fields
produced by vigorous dynamo action in the bulk of the convection zone.",0609153v1
2007-02-02,Magnetized Hypermassive Neutron Star Collapse: a candidate central engine for short-hard GRBs,"Hypermassive neutron stars (HMNSs) are equilibrium configurations supported
against collapse by rapid differential rotation and likely form as transient
remnants of binary neutron star mergers. Though HMNSs are dynamically stable,
secular effects such as viscosity or magnetic fields tend to bring HMNSs into
uniform rotation and thus lead to collapse. We simulate the evolution of
magnetized HMNSs in axisymmetry using codes which solve the
Einstein-Maxwell-MHD system of equations. We find that magnetic braking and the
magnetorotational instability (MRI) both contribute to the eventual collapse of
HMNSs to rotating black holes surrounded by massive, hot accretion tori and
collimated magnetic fields. Such hot tori radiate strongly in neutrinos, and
the resulting neutrino-antineutrino annihilation could power short-hard GRBs.",0702080v1
1996-04-16,Composite Fermions with Orbital Magnetization,"For quantum Hall systems, in the limit of large magnetic field (or
equivalently small electron band mass $m_b$), the static response of electrons
to a spatially varying magnetic field is largely determined by kinetic energy
considerations. This response is not correctly given in existing approximations
based on the Fermion Chern-Simons theory of the partially filled Landau level.
We remedy this problem by attaching an orbital magnetization to each fermion to
separate the current into magnetization and transport contributions, associated
with the cyclotron and guiding center motions respectively. This leads to a
Chern-Simons Fermi liquid description of the $\nu=\frac{1}{2m}$ state which
correctly predicts the $m_b$ dependence of the static and dynamic response in
the limit $m_b \rightarrow 0$.",9604103v1
1996-10-28,Stability and Instability of Relativistic Electrons in Classical Electro magnetic Fields,"The stability of matter composed of electrons and static nuclei is
investigated for a relativistic dynamics for the electrons given by a suitably
projected Dirac operator and with Coulomb interactions. In addition there is an
arbitrary classical magnetic field of finite energy. Despite the previously
known facts that ordinary nonrelativistic matter with magnetic fields, or
relativistic matter without magnetic fields is already unstable when the fine
structure constant, is too large it is noteworthy that the combination of the
two is still stable provided the projection onto the positive energy states of
the Dirac operator, which defines the electron, is chosen properly. A good
choice is to include the magnetic field in the definition. A bad choice, which
always leads to instability, is the usual one in which the positive energy
states are defined by the free Dirac operator. Both assertions are proved here.",9610195v4
1998-03-25,A Mesoscopic Approach to the ``Negative'' Viscosity Effect in Ferrofluids,"We present a mesoscopic approach to analyze the dynamics of a single magnetic
dipole under the influence of an oscillating magnetic field, based on the
formulation of a Fokker-Planck equation. The dissipated power and the viscosity
of a suspension of such magnetic dipoles are calculated from non-equilibrium
thermodynamics of magnetized systems. By means of this method we have found a
non-monotonous behaviour of the viscosity as a function of the frequency of the
field which has been referred to as the ``negative'' viscosity effect.
Moreover, we have shown that the viscosity depends on the vorticity field thus
exhibiting non-Newtonian behaviour. Our analysis is complemented with numerical
simulations which reproduce the behaviour of the viscosity we have found and
extend the scope of our analytical approach to higher values of the magnetic
field.",9803305v2
2000-02-09,Structure and Magnetism of well-defined cobalt nanoparticles embedded in a niobium matrix,"Our recent studies on Co-clusters embedded in various matrices reveal that
the co-deposition technique (simultaneous deposition of two beams : one for the
pre-formed clusters and one for the matrix atoms) is a powerful tool to prepare
magnetic nanostructures with any couple of materials even though they are
miscible. We study, both sharply related, structure and magnetism of the Co/Nb
system. Because such a heterogeneous system needs to be described at different
scales, we used microscopic and macroscopic techniques but also local selective
absorption ones. We conclude that our clusters are 3 nm diameter f.c.c
truncated octahedrons with a pure cobalt core and a solid solution between Co
and Nb located at the interface which could be responsible for the magnetically
inactive monolayers we found. The use of a very diluted Co/Nb film, further
lithographed, would allow us to achieve a pattern of microsquid devices in view
to study the magnetic dynamics of a single-Co cluster.",0002126v2
2000-03-24,Magnetic Behavior of a Mixed Ising Ferrimagnetic Model in an Oscillating Magnetic Field,"The magnetic behavior of a mixed Ising ferrimagnetic system on a square
lattice, in which the two interpenetrating square sublattices have spins +- 1/2
and spins +-1,0, in the presence of an oscillating magnetic field has been
studied with Monte Carlo techniques. The model includes nearest and
next-nearest neighbor interactions, a crystal field and the oscillating
external field. By studying the hysteretic response of this model to an
oscillating field we found that it qualitatively reproduces the increasing of
the coercive field at the compensation temperature observed in real
ferrimagnets, a crucial feature for magneto-optical applications. This behavior
is basically independent of the frequency of the field and the size of the
system. The magnetic response of the system is related to a dynamical
transition from a paramagnetic to a ferromagnetic phase and to the different
temperature dependence of the relaxation times of both sublattices.",0003403v1
2000-10-25,"Susceptibility, Magnetization Process and ESR Studies on the Helical Spin System RbCuCl$_{3}$","The static and dynamic magnetic properties of an $S=1/2$ stacked triangular
antiferromagnet RbCuCl$_{3}$ with a helical spin structure due to lattice
distortion were investigated by susceptibility, high-field magnetization
process and ESR measurements. The susceptibilities were analyzed by
high-temperature expansion approximation in terms of ferromagnetic intrachain
coupling and the antiferromagnetic interchain coupling. The magnetization
saturates at $H_{\rm s}\approx 66.8$ T at 1.5 K, the value of which is twice
that of CsCuCl$_3$. A small magnetization jump indicative of a phase transition
was observed at $H_{\rm c}=21.2$ T $\approx (1/3)H_{\rm s}$ for $H{\perp}c$.
ESR modes observed for $H\parallel c$ are well described by the calculation
based on the helical spin structure. From the present measurements, the
ferromagnetic intrachain and two kinds of antiferromagnetic interchain exchange
interactions, and the planar anisotropy energy were determined as $J_0/k_{\rm
B}=25.7$ K, $J_1/k_{\rm B}=-10.6$ K, $J_1'/k_{\rm B}=-17.4$ K, and
${\Delta}J_0/k_{\rm B}=-0.45$ K, respectively.",0010388v2
2001-03-07,Tunneling from a correlated 2D electron system transverse to a magnetic field,"We show that, in a magnetic field parallel to the 2D electron layer, strong
electron correlations change the rate of tunneling from the layer
exponentially. It results in a specific density dependence of the escape rate.
The mechanism is a dynamical Mossbauer-type recoil, in which the Hall momentum
of the tunneling electron is partly transferred to the whole electron system,
depending on the interrelation between the rate of interelectron momentum
exchange and the tunneling duration. We also show that, in a certain
temperature range, magnetic field can enhance rather than suppress the
tunneling rate. The effect is due to the magnetic field induced energy exchange
between the in-plane and out-of-plane motion. Magnetic field can also induce
switching between intra-well states from which the system tunnels, and a
transition from tunneling to thermal activation. Explicit results are obtained
for a Wigner crystal. They are in qualitative and quantitative agreement with
the relevant experimental data, with no adjustable parameters.",0103151v1
2001-05-04,Magnetic properties of nanoparticles in the Bethe-Peierls approximation,"In this work we present a new method to calculate the classical properties of
magnetic nanoparticles. Based on the Bethe-Peierls (pair) approximation, we
developed a simple system of equations for the classical magnetization of spins
at any position within the nanoparticle. The nearest neightbor pair
correlations are treated exactly for Ising spins, and the method can be
generalized for various lattice symmetries. The master equation is solved for
the Glauber dynamics (single-spin-flip) in order to obtain the time evolution
of the magnetization. The capabilities of the model are demonstrated through
the calculation of hysteresis loops as well as field cooling (FC) and zero
field cooling (ZFC) magnetization curves of heterogeneous nanoparticles. The
present method can be an alternative to the usually complex and time consuming
methods employed in micromagnetism.",0105106v2
2001-06-04,Haldane-Gapped Spin Chains as Luttinger Liquids: Correlation Functions at Finite Field,"We study the behavior of Heisenberg, antiferromagnetic, integer-spin chains
in the presence of a magnetic field exceeding the attendant spin gap. For
temperatures much smaller than the gap, the spin chains exhibit Luttinger
liquid behavior. We compute exactly both the corresponding Luttinger parameter
and the Fermi velocity as a function of magnetic field. This enables the
computation of a number of correlators from which we derive the spin
conductance, the expected form of the dynamic structure factor relevant to
inelastic neutron scattering experiments, and NMR relaxation rates. We also
comment upon the robustness of the magnetically induced gapless phase both to
finite temperature and finite couplings between neighbouring chains.",0106037v2
2002-01-14,Magnetic Field Scaling in Spin Glasses and the Mean-Field Theory,"The scaling of the magnetic field dependence of the remanent magnetization
for different temperatures and different spin-glass samples is studied.
Particular attention is paid to the effect of the de Almeida-Thouless (AT)
critical line on spin-glass dynamics. It is shown that results of the
mean-field theory of aging phenomena, with two additional experimentally
justified assumptions, predict $H/H_{AT}(T)$ scaling for remanent magnetization
curves. Experiments on a single crystal Cu:Mn 1.5 at % sample in the
temperature interval from $0.7T_{g}$ to $0.85T_{g}$ give results consistent
with this scaling. Magnetization vs. field curves for different Cu:Mn and
thiospinel samples also scale together. These experimental results support the
predictions of the mean-field theory of aging phenomena.",0201226v1
2002-11-19,NMR and Mossbauer study of spin dynamics and electronic structure of Fe{2+x}V{1-x}Al and Fe2VGa,"In order to assess the magnetic ordering process in Fe2VAl and the related
material Fe2VGa, we have carried out nuclear magnetic resonance (NMR) and
Mossbauer studies. 27Al NMR relaxation measurements covered the temperature
range 4 -- 500 K in Fe(2+x)V(1-x)Al samples. We found a peak in the NMR
spin-lattice relaxation rate, 27T1^-1, corresponding to the magnetic
transitions in each of these samples. These peaks appear at 125 K, 17 K, and
165 K for x = 0.10, 0, and - 0.05 respectively, and we connect these features
with critical slowing down of the localized antisite defects. Mossbauer
measurements for Fe2VAl and Fe2VGa showed lines with no hyperfine splitting,
and isomer shifts nearly identical to those of the corresponding sites in Fe3Al
and Fe3Ga, respectively. We show that a model in which local band filling leads
to magnetic regions in the samples, in addition to the localized antisite
defects, can account for the observed magnetic ordering behavior.",0211416v2
2003-01-20,Magnetic-Field-Induced Localization of Quasiparticles in Underdoped La$_{2-x}$Sr$_x$CuO$_4$ Single Crystals,"Magnetic-field-induced ordering of electrons around vortices is a striking
phenomenon recently found in high-$T_c$ cuprates. To identify its consequence
in the quasiparticle dynamics, the magnetic-field ($H$) dependence of the
low-temperature thermal conductivity $\kappa$ of La$_{2-x}$Sr$_x$CuO$_4$
crystals is studied for a wide doping range. It is found that the behavior of
$\kappa(H)$ in the sub-Kelvin region changes drastically across optimum doping,
and the data for underdoped samples are indicative of unusual
magnetic-field-induced localization of quasiparticles; this localization
phenomenon is probably responsible for the unusual ""insulating normal state""
under high magnetic fields.",0301339v2
2003-02-17,Thermally assisted magnetization reversal in the presence of a spin-transfer torque,"We propose a generalized stochastic Landau-Lifshitz equation and its
corresponding Fokker-Planck equation for the magnetization dynamics in the
presence of spin transfer torques. Since the spin transfer torque can pump a
magnetic energy into the magnetic system, the equilibrium temperature of the
magnetic system is ill-defined. We introduce an effective temperature based on
a stationary solution of the Fokker-Planck equation. In the limit of high
energy barriers, the law of thermal agitation is derived. We find that the
N\'{e}el-Brown relaxation formula remains valid as long as we replace the
temperature by an effective one that is linearly dependent of the spin torque.
We carry out the numerical integration of the stochastic Landau-Lifshitz
equation to support our theory. Our results agree with existing experimental
data.",0302339v2
2003-02-26,Persistence of Manifolds in Nonequilibrium Critical Dynamics,"We study the persistence P(t) of the magnetization of a d' dimensional
manifold (i.e., the probability that the manifold magnetization does not flip
up to time t, starting from a random initial condition) in a d-dimensional spin
system at its critical point. We show analytically that there are three
distinct late time decay forms for P(t) : exponential, stretched exponential
and power law, depending on a single parameter \zeta=(D-2+\eta)/z where D=d-d'
and \eta, z are standard critical exponents. In particular, our theory predicts
that the persistence of a line magnetization decays as a power law in the d=2
Ising model at its critical point. For the d=3 critical Ising model, the
persistence of the plane magnetization decays as a power law, while that of a
line magnetization decays as a stretched exponential. Numerical results are
consistent with these analytical predictions.",0302537v1
2003-03-08,Current-Driven Magnetic Excitations in Permalloy-Based Multilayer Nanopillars,"We study current-driven magnetization switching in nanofabricated
Ni84Fe16/Cu/Ni84Fe16 trilayers at 295 K and 4.2 K. The shape of the hysteretic
switching diagram at low magnetic field changes from 295 K to 4.2 K. The
reversible behavior at higher field involves two phenomena, a threshold current
for magnetic excitations closely correlated with the switching current, and a
peak in differential resistance characterized by telegraph noise, with average
period that decreases exponentially with current and shifts with temperature.
We interpret both static and dynamic results at 295 K and 4.2 K in terms of
thermal activation over a potential barrier, with a current dependent effective
magnetic temperature.",0303149v2
2003-06-05,Quasiparticle vanishing driven by geometrical frustration,"We investigate the single hole dynamics in the triangular t-J model. We study
the structure of the hole spectral function, assuming the existence of a 120
magnetic Neel order. Within the self-consistent Born approximation (SCBA) there
is a strong momentum and t sign dependence of the spectra, related to the
underlying magnetic structure and the particle-hole asymmetry of the model. For
positive t, and in the strong coupling regime, we find that the low energy
quasiparticle excitations vanish outside the neighbourhood of the magnetic
Goldstone modes; while for negative t the quasiparticle excitations are always
well defined. In the latter, we also find resonances of magnetic origin whose
energies scale as (J/t)^2/3 and can be identified with string excitations. We
argue that this complex structure of the spectra is due to the subtle interplay
between magnon-assisted and free hopping mechanisms. Our predictions are
supported by an excellent agreement between the SCBA and the exact results on
finite size clusters. We conclude that the conventional quasiparticle picture
can be broken by the effect of geometrical magnetic frustration.",0306147v3
2003-09-08,Absence of magnetic order in Yb3Ga5O12: relation between phase transition and entropy in geometrically frustrated materials,"From muon spin relaxation spectroscopy experiments, we show that the sharp
peak (lambda type anomaly) detected by specific heat measurements at 54 mK for
the ytterbium gallium garnet compound, Yb3Ga5O12, does not correspond to the
onset of a magnetic phase transition, but to a pronounced building up of
dynamical magnetic pair-correlations. Beside the lambda anomaly, a broad hump
is observed at higher temperature in the specific heat of this garnet and other
geometrically frustrated compounds. Comparing with other frustrated magnetic
systems we infer that a ground state with long-range order is reached only when
at least 1/4-1/3 of the magnetic entropy is released at the lambda transition.",0309185v1
2003-12-13,Pathways of structural and magnetic transitions in ferromagnetic shape memory alloys,"A fundamental question in the study of ferromagnetic shap ememory alloys is
the nature of magnetoelastic coupling and the extent it drives the structural
transformation. This question also holds the key to developing new and
optimized alloys that combine high strains at low switching field. In the
present study it is shown that the reconfiguration of the micromagnetic
structure is enslaved to and follows the martensitic transformation in these
alloys using the NiMnGa and FePd alloy systems. This is determined by
developing a new, high speed electronic method to study the temperature
dependence of domain dynamics, called magnetic transition spectra. The sequence
of magnetic and structural transition was found to be as follows. For cooling,
structural transition followed by magnetic transition and for heating, magnetic
transition followed by structural transition.",0312343v1
2004-09-22,Hofstadter spectrum in electric and magnetic fields,"The problem of Bloch electrons in two dimensions subject to magnetic and
intense electric fields is investigated. Magnetic translations, electric
evolution and energy translation operators are used to specify the solutions of
the Schr\""odinger equation. For rational values of the magnetic flux quanta per
unit cell and commensurate orientations of the electric field relative to the
original lattice, an extended superlattice can be defined and a complete set of
mutually commuting space-time symmetry operators is obtained. Dynamics of the
system is governed by a finite difference equation that exactly includes the
effects of: an arbitrary periodic potential, an electric field orientated in a
commensurable direction of the lattice, and coupling between Landau levels. A
weak periodic potential broadens each Landau level in a series of minibands,
separated by the corresponding minigaps. The addition of the electric field
induces a series of avoided and exact crossing of the quasienergies, for
sufficiently strong electric field the spectrum evolves into equally spaced
discreet levels, in this ""magnetic Stark ladder"" the energy separation is an
integer multiple of $ h E / a B $, with $a$ the lattice parameter.",0409579v1
2004-10-05,The Non--Ergodicity Threshold: Time Scale for Magnetic Reversal,"We prove the existence of a non-ergodicity threshold for an anisotropic
classical Heisenberg model with all-to-all couplings. Below the threshold, the
energy surface is disconnected in two components with positive and negative
magnetizations respectively. Above, in a fully chaotic regime, magnetization
changes sign in a stochastic way and its behavior can be fully characterized by
an average magnetization reversal time. We show that statistical mechanics
predicts a phase--transition at an energy higher than the non-ergodicity
threshold. We assess the dynamical relevance of the latter for finite systems
through numerical simulations and analytical calculations. In particular, the
time scale for magnetic reversal diverges as a power law at the ergodicity
threshold with a size-dependent exponent, which could be a signature of the
phenomenon.",0410119v3
2005-01-13,Fast Switching of Bistable Magnetic Nanowires Through Collective Spin Reversal,"The use of magnetic nanowires as memory units is made possible by the
exponential divergence of the characteristic time for magnetization reversal at
low temperature, but the slow relaxation makes the manipulation of the frozen
magnetic states difficult. We suggest that finite-size segments can show a fast
switching if collective reversal of the spins is taken into account. This
mechanism gives rise at low temperatures to a scaling law for the dynamic
susceptibility that has been experimentally observed for the dilute molecular
chain Co(hfac)2NitPhOMe. These results suggest a possible way of engineering
nanowires for fast switching of the magnetization.",0501316v1
2005-03-02,A two dimensional model for ferromagnetic martensites,"We consider a recently introduced 2-D square-to-rectangle martensite model
that explains several unusual features of martensites to study ferromagnetic
martensites. The strain order parameter is coupled to the magnetic order
parameter through a 4-state clock model. Studies are carried out for several
combinations of the ordering of the Curie temperatures of the austenite and
martensite phases and, the martensite transformation temperature. We find that
the orientation of the magnetic order which generally points along the short
axis of the rectangular variant, changes as one crosses the twin or the
martensite-austenite interface. The model shows the possibility of a subtle
interplay between the growth of strain and magnetic order parameters as the
temperature is decreased. In some cases, this leads to qualitatively different
magnetization curves from those predicted by earlier mean field models.
Further, we find that strain morphology can be substantially altered by the
magnetic order. We have also studied the dynamic hysteresis behavior.
  The corresponding dissipation during the forward and reverse cycles has
features similar to the Barkhausen's noise.",0503054v1
2005-03-07,Neutron scattering studies of the spin ices Ho2Ti2O7 and Dy2Ti2O7 in applied magnetic field,"Neutron diffraction has been used to investigate the magnetic correlations in
single crystals of the spin ice materials Ho2Ti2O7 and Dy2Ti2O7 in an external
magnetic field applied along either the [001] or [1-10] crystallographic
directions. With the field applied along [001] a long range ordered groundstate
is selected from the spin ice manifold. With the field applied along [1-10] the
experiments show that the spin system is separated into parallel (alpha) and
perpendicular (beta) chains with respect to the field. This leads to partial
ordering and the appearance of quasi-one-dimensional magnetic structures. In
both field orientations this frustrated spin system is defined by the
appearance of metastable states, magnetization plateaux and unusually slow,
field regulated dynamics.",0503160v1
2005-05-02,Absence of spin superradiance in resonatorless magnets,"A spin system is considered with a Hamiltonian typical of molecular magnets,
having dipole-dipole interactions and a single-site magnetic anisotropy. In
addition, spin interactions through the common radiation field are included. A
fully quantum-mechanical derivation of the collective radiation rate is
presented. An effective narrowing of the dipole-dipole attenuation, due to high
spin polarization is taken into account. The influence of the radiation rate on
spin dynamics is carefully analysed. It is shown that this influence is
completely negligible. No noticeable collective effects, such as superradiance,
can appear in molecular magnets, being caused by electromagnetic spin
radiation. Spin superradiance can arise in molecular magnets only when these
are coupled to a resonant electric circuit, as has been suggested earlier by
one of the authors in Laser Phys. {\bf 12}, 1089 (2002).",0505024v1
2005-08-10,Stripes and superconductivity in cuprate superconductors,"One type of order that has been observed to compete with superconductivity in
cuprates involves alternating charge and antiferromagnetic stripes. Recent
neutron scattering studies indicate that the magnetic excitation spectrum of a
stripe-ordered sample is very similar to that observed in superconducting
samples. In fact, it now appears that there may be a universal magnetic
spectrum for the cuprates. One likely implication of this universal spectrum is
that stripes of a dynamic form are present in the superconducting samples. On
cooling through the superconducting transition temperature, a gap opens in the
magnetic spectrum, and the weight lost at low energy piles up above the gap;
the transition temperature is correlated with the size of the spin gap.
Depending on the magnitude of the spin gap with respect to the magnetic
spectrum, the enhanced magnetic scattering at low temperature can be either
commensurate or incommensurate. Connections between stripe correlations and
superconductivity are discussed.",0508272v1
2005-12-22,Semiclassical degeneracies and ordering for highly frustrated magnets in a field,"We discuss ground state selection by quantum fluctuations in frustrated
magnets in a strong magnetic field. We show that there exist dynamical
symmetries -- one a generalisation of Henley's gauge-like symmetry for
collinear spins, the other the quantum relict of non-collinear weathervane
modes -- which ensure a partial survival of the classical degeneracies. We
illustrate these for the case of the kagome magnet, where we find zero-point
energy differences to be rather small everywhere except near the collinear
`up-up-down` configurations, where there is rotational but not translational
symmetry breaking. In the effective Hamiltonian, we demonstrate the presence of
a term sensitive to a topological `flux'. We discuss the connection of such
problems to gauge theories by casting the frustrated lattices as medial
lattices of appropriately chosen simplex lattices, and in particular we show
how the magnetic field can be used to tune the physical sector of the resulting
gauge theories.",0512594v1
2006-01-10,Magneto-mechanical interplay in spin-polarized point contacts,"We investigate the interplay between magnetic and structural dynamics in
ferromagnetic atomic point contacts. In particular, we look at the effect of
the atomic relaxation on the energy barrier for magnetic domain wall migration
and, reversely, at the effect of the magnetic state on the mechanical forces
and structural relaxation. We observe changes of the barrier height due to the
atomic relaxation up to 200%, suggesting a very strong coupling between the
structural and the magnetic degrees of freedom. The reverse interplay is weak,
i.e. the magnetic state has little effect on the structural relaxation at
equilibrium or under non-equilibrium, current-carrying conditions.",0601200v1
2006-01-26,Spin dynamics characterization in magnetic dots,"The spin structure in a magnetic dot, which is an example of a quantum
few-body system, is studied as a function of exchange coupling strength and dot
size with in the semiclassical approximation on a discrete lattice. As the
exchange coupli ng is decreased or the size is increased, the ground state
undergoes a phase cha nge from a single domain ferromagnet to a spin vortex.
The line separating these two phases has been calculated numerically for small
system sizes. %, and analytically for larger dots. The dipolar interaction has
been fully included in our calculations. Magnon frequencies in such a dot have
also been calculated in both phases by the linearized equation of motion
method. These results have also been reproduced f rom the Fourier transform of
the spin autocorrelation function. From the magnon Density Of States (DOS), it
is possible to identify the magnetic phase of the dot. Furthermore, the magnon
modes have been characterized for both the ferromagnetic and the vortex phase,
and the magnon instability mechanism leading to the vortex-ferro transition has
also been identified. The results can also be used to compute finite
temperature magnetization or vort icity of magnetic dots.",0601624v1
2006-05-10,Magnetic excitations in a new anisotropic Kagomé antiferromagnet,"The Nd-langasite compound contains planes of magnetic Nd3+ ions on a lattice
topologically equivalent to a kagom\'{e} net. The magnetic susceptibility does
not reveal any signature of long-range ordering down to 2 K but rather a
correlated paramagnetism with significant antiferromagnetic interactions
between the Nd and a single-ion anisotropy due to crystal field effect.
Inelastic neutron scattering on Nd-langasite powder and single-crystal allowed
to probe its very peculiar low temperature dynamical magnetic correlations.
They present unusual dispersive features and are broadly localized in
wave-vector Q revealing a structure factor associated to characteristics short
range-correlations between the magnetic atoms. From comparison with theoretical
calculations, these results are interpreted as a possible experimental
observation of a spin liquid state in an anisotropic kagom\'{e}
antiferromagnet.",0605282v1
2006-10-20,Probing Anomalous Longitudinal Fluctuations of the Interacting Bose Gas via Bose-Einstein Condensation of Magnons,"The emergence of a finite staggered magnetization in quantum Heisenberg
antiferromagnets subject to a uniform magnetic field can be viewed as
Bose-Einstein condensation of magnons. Using non-perturbative results for the
infrared behavior of the interacting Bose gas, we present exact results for the
staggered spin-spin correlation functions of quantum antiferromagnets in a
magnetic field at zero temperature. In particular, we show that in dimensions 1
< D <= 3 the longitudinal dynamic structure factor S_parallel(q,omega)
describing staggered spin fluctuations in the direction of the staggered
magnetization exhibits a critical continuum whose weight can be controlled
experimentally by varying the magnetic field.",0610575v3
2007-03-23,Transition-metal dimers and physical limits on magnetic anisotropy,"Recent advances in nanoscience have raised interest in the minimum bit size
required for classical information storage, i.e. for bistability with
suppressed quantum tunnelling and energy barriers that exceed ambient
temperatures. In the case of magnetic information storage much attention has
centred on molecular magnets[1] with bits consisting of ~ 100 atoms, magnetic
uniaxial anisotropy energy barriers ~ 50 K, and very slow relaxation at low
temperatures. In this article we draw attention to the remarkable magnetic
properties of some transition metal dimers which have energy barriers
approaching ~ 500 K with only two atoms. The spin dynamics of these ultra small
nanomagnets is strongly affected by a Berry phase which arises from
quasi-degeneracies at the electronic Highest Occupied Molecular Orbital (HOMO)
energy. In a giant spin-approximation, this Berry phase makes the effective
reversal barrier thicker. [1] Gatteschi, D., Sessoli, R. & Villain, J.
Molecular Nanomagnets. (Oxford, New York 2006).",0703607v2
2004-10-08,Rotation Effects and The Gravito-Magnetic Approach,"Gravito-electromagnetism is somewhat ubiquitous in relativity. In fact, there
are many situations where the effects of gravitation can be described by
formally introducing ""gravito-electric"" and ""gravito-magnetic"" fields, starting
from the corresponding potentials, in analogy with the electromagnetic theory
(see also A. Tartaglia's contribution to these proceedings). The ""many faces of
gravito-electromagnetism"" are related to rotation effects in both approximated
and full theory approaches. Here we show that, by using a 1+3 splitting,
relativistic dynamics can be described in terms of gravito-electromagnetic
(GEM) fields in full theory. On the basis of this formalism, we introduce a
""gravito-magnetic Aharonov-Bohm effect"", which allows to interpret some
rotation effects as gravito-magnetic effects. Finally, we suggest a way for
measuring the angular momentum of celestial bodies by studying the
gravito-magnetic effects on the propagation of electromagnetic signals.",0410039v2
1996-09-05,Electric-Magnetic Duality and the Heavy Quark Potential,"We use the assumption of electric-magnetic duality to express the heavy quark
potential in QCD in terms of a Wilson Loop $W_{eff}(\Gamma)$ determined by the
dynamics of a dual theory which is weakly coupled at long distances. The
classical approximation gives the leading contribution to $W_{eff}(\Gamma)$ and
yields a velocity dependent heavy quark potential which for large $R$ becomes
linear in $R$, and which for small $R$ approaches lowest order perturbative
QCD. The corresponding long distance interaction between color magnetic
monopoles is governed by a Yukawa potential. As a consequence the magnetic
interaction between the color magnetic moments of the quarks is exponentially
damped. The semi-classical corrections to $W_{eff}(\Gamma)$ due to fluctuations
of the classical flux tube should lead to an effective string theory free from
the conformal anomaly.",9609269v1
1999-03-02,The Sphaleron in a Magnetic Field and Electroweak Baryogenesis,"The presence of a primordial magnetic field in the early universe affects the
dynamic of the electroweak phase transition enhancing its strength. This effect
may enlarge the window for electroweak baryogenesis in the minimal
supersymmetric extension of the standard model or even resurrect the
electroweak baryogenesis scenario in the standard model. We compute the
sphaleron energy in the background of the magnetic field and show that, due to
the sphaleron dipole moment, the barrier between topologically inequivalent
vacua is lowered. Therefore, the preservation of the baryon asymmetry calls for
a much stronger phase transition than required in the absence of a magnetic
field. We show that this effect overwhelms the gain in the phase transition
strength, and conclude that magnetic fields do not help electroweak
baryogenesis.",9903227v1
1999-05-05,Magnetic Oscillations in Dense Cold Quark Matter with Four-Fermion Interactions,"The phase structures of Nambu-Jona-Lasinio models with one or two flavours
have been investigated at non-zero values of $\mu$ and $H$, where $H$ is an
external magnetic field and $\mu$ is the chemical potential. In the phase
portraits of both models there arise infinitely many massless chirally
symmetric phases, as well as massive ones with spontaneously broken chiral
invariance, reflecting the existence of infinitely many Landau levels. Phase
transitions of first and second orders and a lot of tricritical points have
been shown to exist in phase diagrams. In the massless case, such a phase
structure leads unavoidably to the standard van Alphen-de Haas magnetic
oscillations of some thermodynamical quantities, including magnetization,
pressure and particle density. In the massive case we have found an oscillating
behaviour not only for thermodynamical quantities, but also for a dynamical
quantity as the quark mass. Besides, in this case we have non-standard, i.e.
non-periodic, magnetic oscillations, since the frequency of oscillations is an
$H$-dependent quantity.",9905253v1
2003-06-13,Magnetised Neutron Stars : An Overview,"In the presence of strong magnetic field reported to have been observed on
the surface of some neutron stars and on what are called Magnetars, a host of
physical phenomenon from the birth of a neutron star to free streaming neutrino
cooling phase will be modified. In this review I will discuss the effect of
magnetic field on the equation of state of high density nuclear matter by
including the anomalous magnetic moment of the nucleons into consideration. I
would then go over to discuss the neutrino interaction processes in strong as
well as in weak magnetic fields. The neutrino processes are important in
studying the propagation of neutrinos and in studying the energy loss, Their
study is a prerequisite for the understanding of actual dynamics of supernova
explosion and on the stabilization of radial pulsation modes through the effect
on bulk viscosity. The anisotropy introduced in the neutrino emission and
through the modification of the shape of the neutrino sphere may explain the
observed pulsar kicks.",0306116v1
2004-05-18,Magnetic moments of the exotic pentaquark baryons within the chiral quark-soliton model,"We present in this talk recent results of the magnetic moments of the baryon
antidecuplet within the framework of the chiral quark-soliton model in the
chiral limit. The dynamic parameters of the model are fixed by using the
experimental data for those of the baryon octet. Sum rules for the magnetic
moments are derived. We found that the magnetic moments of the baryon
antidecuplet have opposite signs to their charges. The magnetic moments of the
neutral baryon antidecuplet turn out to be compatible with zero.",0405171v1
2004-05-24,Condensation of Tubular D2-branes in Magnetic Field Background,"It is known that in the Minkowski vacuum a bunch of IIA superstrings with
D0-branes can be blown-up to a supersymmetric tubular D2-brane, which is
supported against collapse by the angular momentum generated by crossed
electric and magnetic Born-Infeld (BI) fields. In this paper we show how the
multiple, smaller tubes with relative angular momentum could condense to a
single, larger tube to stabilize the system. Such a phenomena could also be
shown in the systems under the Melvin magnetic tube or uniform magnetic field
background. However, depending on the magnitude of field strength, a tube in
the uniform magnetic field background may split into multiple, smaller tubes
with relative angular momentum to stabilize the system.",0405192v4
2005-03-02,Spectral and Propagation Results for Magnetic Schroedinger Operators; a C*-Algebraic Framework,"We study generalised magnetic Schroedinger operators of the form
H(A,V)=h(P^A)+V, where h is an elliptic symbol, P^A is the generator of the
magnetic translations, with A a vector potential defining a variable magnetic
field B, and V is a scalar potential. We are mainly interested in anisotropic
functions B and V. The first step is to show that these operators are
affiliated to suitable C*-algebras of (magnetic) pseudodifferential operators.
A study of the quotient of these C*-algebras by the ideal of compact operators
leads to formulae for the essential spectrum of H(A,V), expressed as a union of
spectra of some asymptotic operators, supported by the quasi-orbits of a
suitable dynamical system. The quotient of the same C*-algebras by other ideals
give localization results on the functional calculus of the operators H(A,V),
which can be interpreted as non-propagation properties of their unitary groups.",0503046v1
2002-01-09,Magnetic edge states,"Magnetic edge states are responsible for various phenomena of
magneto-transport. Their importance is due to the fact that, unlike the bulk of
the eigenstates in a magnetic system, they carry electric current along the
boundary of a confined domain. Edge states can exist both as interior (quantum
dot) and exterior (anti-dot) states. In the present report we develop a
consistent and practical spectral theory for the edge states encountered in
magnetic billiards. It provides an objective definition for the notion of edge
states, is applicable for interior and exterior problems, facilitates efficient
quantization schemes, and forms a convenient starting point for both the
semiclassical description and the statistical analysis. After elaborating these
topics we use the semiclassical spectral theory to uncover nontrivial spectral
correlations between the interior and the exterior edge states. We show that
they are the quantum manifestation of a classical duality between the
trajectories in an interior and an exterior magnetic billiard.",0201012v1
2005-04-14,Control of stochasticity in magnetic field lines,"We present a method of control which is able to create barriers to magnetic
field line diffusion by a small modification of the magnetic perturbation. This
method of control is based on a localized control of chaos in Hamiltonian
systems. The aim is to modify the perturbation locally by a small control term
which creates invariant tori acting as barriers to diffusion for Hamiltonian
systems with two degrees of freedom. The location of the invariant torus is
enforced in the vicinity of the chosen target. Given the importance of
confinement in magnetic fusion devices, the method is applied to two examples
with a loss of magnetic confinement. In the case of locked tearing modes, an
invariant torus can be restored that aims at showing the current quench and
therefore the generation of runaway electrons. In the second case, the method
is applied to the control of stochastic boundaries allowing one to define a
transport barrier within the stochastic boundary and therefore to monitor the
volume of closed field lines.",0504033v2
1997-03-07,Nonlinear Dynamics of a Single Ferrofluid-Peak in an Oscillating Magnetic Field,"If a magnetic field normal to the surface of a magnetic fluid is increased
beyond a critical value a spontaneous deformation of the surface arises (normal
field instability). The instability is subcritical and leads to peaks of a
characteristic shape. We investigate the neighborhood of this instability
experimentally under the influence of a temporal modulation of the magnetic
field. We use a small vessel, where only one peak arises. The modulation can
either be stabilizing or destabilizing, depending on the frequency and
amplitude. We observe a cascade of odd-numbered response-periods up to period
11, and also a domain of even-numbered periods. We propose a minimal model
involving a cutoff-condition which captures the essence of the experimental
observations.
  PACS: 47.20.-k, 47.20.Ky, 75.50.Mm
  Keywords: magnetic fluid; nonlinear oscillator; subharmonic response; surface
instability;",9703003v1
1999-05-22,Excitation of nonlinear one-dimensional wake waves in underdense and overdense magnetized plasma by a relativistic electron bunch,"The excitation of wake waves by a relativistic homogeneous electron bunch
passing through cold magnetized plasma at equilibrium is studied for arbitrary
values of the ratio of bunch density to plasma density. The analysis is based
on the assumption that the magnetic field is sufficiently strong ($\omega_B\gg
\omega_p$, where $\omega_p$ and $\omega_B$ are the electron plasma and
cyclotron frequences respectively). The periodic and nonperiodic solutions for
the momentum of plasma electrons inside and outside the bunch are analyzed. It
was shown that the presence of strong external magnetic field may increase the
amplitude of wake waves and the maximum transformer ratio, the latter being
reached at densities of bunch lower than that in the absence of magnetic field.
The optimum conditions for obtaining the maximum values of the wake field
amplitude and of the transformer ratio are found.",9905046v1
2004-09-07,Contactless inductive flow tomography,"The three-dimensional velocity field of a propeller driven liquid metal flow
is reconstructed by a contactless inductive flow tomography (CIFT). The
underlying theory is presented within the framework of an integral equation
system that governs the magnetic field distribution in a moving electrically
conducting fluid. For small magnetic Reynolds numbers this integral equation
system can be cast into a linear inverse problem for the determination of the
velocity field from externally measured magnetic fields. A robust
reconstruction of the large scale velocity field is already achieved by
applying the external magnetic field alternately in two orthogonal directions
and measuring the corresponding sets of induced magnetic fields. Kelvin's
theorem is exploited to regularize the resulting velocity field by using the
kinetic energy of the flow as a regularizing functional. The results of the new
technique are shown to be in satisfactory agreement with ultrasonic
measurements.",0409036v1
2004-11-02,Temperature dependent magnetic properties of FePt: effective spin Hamiltonian model,"A model of magnetic interactions in the ordered ferromagnetic FePt is
proposed on the basis of first-principles calculations of non-collinear
magnetic configurations and shown to be capable of explaining recent
measurements of magnetic anisotropy energy (MAE). The site (Fe,Pt) resolved
contributions to the MAE have been distinguished with small Fe easy-plane and
large Pt easy-axis terms. This model has been tested against available
experimental data on the temperature dependence of MAE showing scaling of
uniaxial MAE (K$_{1}$(T)) with magnetization (M(T)) $K_{1}(T) \sim
M(T)^{\gamma}$ characterized by the unusual exponent of $\gamma= 2.1$. It is
shown that this unusual behavior of the FePt can be quantitatively explained
within the proposed model and originates from an effective anisotropic exchange
mediated by the induced Pt moment. The latter is expected to be a common
feature of 3d-5d(4d) alloys having 5d/4d elements with large spin-orbit
coupling and exchange enhanced Stoner susceptibility.",0411020v1
2004-11-05,Scaling and singularity characteristics of solar wind and magnetospheric fluctuations,"Preliminary results are presented which suggest that scaling and singularity
characteristics of solar wind and ground based magnetic fluctuations appear to
be a significant component in the solar wind - magnetosphere interaction
processes. Of key importance is the intermittence of the ""magnetic turbulence""
as seen in ground based and solar wind magnetic data. The methods used in this
paper (estimation of flatness and multifractal spectra) are commonly used in
the studies of fluid or MHD turbulence. The results show that single
observatory characteristics of magnetic fluctuations are different from those
of the multi-observatory AE-index. In both data sets, however, the influence of
the solar wind fluctuations is recognizable. The correlation between the
scaling/singularity features of solar wind magnetic fluctuations and the
corresponding geomagnetic response is demonstrated in a number of cases. The
results are also discussed in terms of patchy reconnection processes in
magnetopause and forced or/and self-organized criticality (F/SOC) of internal
magnetosphere dynamics.",0411055v1
2005-05-26,"Shell to shell energy transfer in MHD, Part I: steady state turbulence","We investigate the transfer of energy from large scales to small scales in
fully developed forced three-dimensional MHD-turbulence by analyzing the
results of direct numerical simulations in the absence of an externally imposed
uniform magnetic field. Our results show that the transfer of kinetic energy
from the large scales to kinetic energy at smaller scales, and the transfer of
magnetic energy from the large scales to magnetic energy at smaller scales, are
local, as is also found in the case of neutral fluids, and in a way that is
compatible with Kolmogorov (1941) theory of turbulence. However, the transfer
of energy from the velocity field to the magnetic field is a highly non-local
process in Fourier space. Energy from the velocity field at large scales can be
transfered directly into small scale magnetic fields without the participation
of intermediate scales. Some implications of our results to MHD turbulence
modeling are also discussed.",0505183v1
2005-11-03,Observation of a Turbulence-Induced Large Scale Magnetic Field,"An axisymmetric magnetic field is applied to a spherical, turbulent flow of
liquid sodium. An induced magnetic dipole moment is measured which cannot be
generated by the interaction of the axisymmetric mean flow with the applied
field, indicating the presence of a turbulent electromotive force. It is shown
that the induced dipole moment should vanish for any axisymmetric laminar flow.
Also observed is the production of toroidal magnetic field from applied
poloidal magnetic field (the omega-effect). Its potential role in the
production of the induced dipole is discussed.",0511029v2
2006-06-28,Intermittent magnetic field excitation by a turbulent flow of liquid sodium,"The magnetic field measured in the Madison Dynamo Experiment shows
intermittent periods of growth when an axial magnetic field is applied. The
geometry of the intermittent field is consistent with the fastest growing
magnetic eigenmode predicted by kinematic dynamo theory using a laminar model
of the mean flow. Though the eigenmodes of the mean flow are decaying, it is
postulated that turbulent fluctuations of the velocity field change the flow
geometry such that the eigenmode growth rate is temporarily positive.
Therefore, it is expected that a characteristic of the onset of a turbulent
dynamo is magnetic intermittency.",0606239v2
2006-12-12,Structure of the Wake of a Magnetic Obstacle,"We use a combination of numerical simulations and experiments to elucidate
the structure of the flow of an electrically conducting fluid past a localized
magnetic field, called magnetic obstacle. We demonstrate that the stationary
flow pattern is considerably more complex than in the wake behind an ordinary
body. The steady flow is shown to undergo two bifurcations (rather than one)
and to involve up to six (rather than just two) vortices. We find that the
first bifurcation leads to the formation of a pair of vortices within the
region of magnetic field that we call inner magnetic vortices, whereas a second
bifurcation gives rise to a pair of attached vortices that are linked to the
inner vortices by connecting vortices.",0612103v3
2007-01-06,Generation of magnetic field by dynamo action in a turbulent flow of liquid sodium,"We report the observation of dynamo action in the VKS experiment, i.e., the
generation of magnetic field by a strongly turbulent swirling flow of liquid
sodium. Both mean and fluctuating parts of the field are studied. The dynamo
threshold corresponds to a magnetic Reynolds number Rm \sim 30. A mean magnetic
field of order 40 G is observed 30% above threshold at the flow lateral
boundary. The rms fluctuations are larger than the corresponding mean value for
two of the components. The scaling of the mean square magnetic field is
compared to a prediction previously made for high Reynolds number flows.",0701075v1
1997-02-08,Non--locality of particle spin: a consequence of interaction energy?,"Neutron interference measurements with macroscopic beam separation allow to
study the influence of magnetic fields on spin properties. By calculating the
interaction energy with a dynamic and deterministic model, we are able to
establish that the phase shift on one component of the neutron beam is linear
with magnetic intensity, and equally, that interaction energy as well as phase
shifts do not depend on the orientation of the magnetic field. The theoretical
treatment allows the conclusion that the non-local properties of particle spin
derive from the classical equation for interaction energy and the fact, that
interaction energy does not depend on magnetic field orientation. Additionally,
it can be established that the 4 pi symmetry of spinors in this case depends on
the scaling of magnetic fields.",9702023v1
2007-04-27,Flow instabilities of magnetic flux tubes II. Longitudinal flow,"Flow-induced instabilities are relevant for the storage and dynamics of
magnetic fields in stellar convection zones and possibly also in other
astrophysical contexts. We continue the study started in the first paper of
this series by considering the stability properties of longitudinal flows along
magnetic flux tubes. A linear stability analysis was carried out to determine
criteria for the onset of instability in the framework of the approximation of
thin magnetic flux tubes. In the non-dissipative case, we find Kelvin-Helmholtz
instability for flow velocities exceeding a critical speed that depends on the
Alfv{\'e}n speed and on the ratio of the internal and external densities.
Inclusion of a friction term proportional to the relative transverse velocity
leads to a friction-driven instability connected with backward (or negative
energy) waves. We discuss the physical nature of this instability. In the case
of a stratified external medium, the Kelvin-Helmholtz instability and the
friction-driven instability can set in for flow speeds significantly lower than
the Alfv{\'e}n speed. Dissipative effects can excite flow-driven instability
below the thresholds for the Kelvin-Helmholtz and the undulatory (Parker-type)
instabilities. This may be important for magnetic flux storage in stellar
convection zones and for the stability of astrophysical jets.",0704.3685v1
2007-05-04,Discovery of inward moving magnetic enhancements in sunspot penumbrae,"Sunspot penumbrae show a fine structure in continuum intensity that displays
considerable dynamics. The magnetic field, in contrast, although also highly
structured, has appeared to be relatively static. Here we report the discovery
of inward moving magnetic enhancements in the penumbrae of two regular sunspots
based on time series of SOHO/MDI magnetograms. Local enhancements of the LOS
component of the magnetic field in the inner part of the penumbral region move
inward to the umbra-penumbra boundary with a radial speed of about 0.3 km
s$^{-1}$. These local inward-moving enhancements of the LOS component of the
magnetic fields appear to be relatively common. They are associated with dark
structures and tend to display downflows relatively to the penumbral
background. Possible explanations are discussed.",0705.0604v1
2007-06-07,Influence of an external magnetic field on the decoherence of a central spin coupled to an antiferromagnetic environment,"Using the spin wave approximation, we study the decoherence dynamics of a
central spin coupled to an antiferromagnetic environment under the application
of an external global magnetic field. The external magnetic field affects the
decoherence process through its effect on the antiferromagnetic environment. It
is shown explicitly that the decoherence factor which displays a Gaussian decay
with time depends on the strength of the external magnetic field and the
crystal anisotropy field in the antiferromagnetic environment. When the values
of the external magnetic field is increased to the critical field point at
which the spin-flop transition (a first-order quantum phase transition) happens
in the antiferromagnetic environment, the decoherence of the central spin
reaches its highest point. This result is consistent with several recent
quantum phase transition witness studies. The influences of the environmental
temperature on the decoherence behavior of the central spin are also
investigated.",0706.0934v1
2007-08-26,Geodesic plasma flows instabilities of Riemann twisted solar loops,"Riemann and sectional curvatures of magnetic twisted flux tubes in Riemannian
manifold are computed to investigate the stability of the plasma astrophysical
tubes. The geodesic equations are used to show that in the case of thick
magnetic tubes, the curvature of planar (Frenet torsion-free) tubes have the
effect ct of damping the flow speed along the tube. Stability of geodesic flows
in the Riemannian twisted thin tubes (almost filaments), against constant
radial perturbations is investigated by using the method of negative sectional
curvature for unstable flows. No special form of the flow like Beltrami flows
is admitted, and the proof is general for the case of thin magnetic flux tubes.
In the magnetic equilibrium state, the twist of the tube is shown to display
also a damping effect on the toroidal velocity of the plasma flow. It is found
that for positive perturbations and angular speed of the flow, instability is
achieved, since the sectional Ricci curvature of the magnetic twisted tube
metric is negative. Solar flare production may appear from these geometrical
instabilities of the twisted solar loops.",0708.3473v1
2007-09-13,"Electric-Magnetic Struggle in QGP, Deconfinement and Baryons","We argue that quite unusual properties of Quark-Gluon Plasma in the RHIC
temperature domain $T=(1-2)T_c$ are consequences of the approximate equilibrium
between electric and magnetic sectors reached above the deconfinement
temperature. Already classical study of few body motion in a electric-magnetic
plasma shows abnormally large scattering rate due to the so called ``magnetic
bottle'' effect. Molecular dynamics simulation have found that equal mixture of
electric and magnetic quasiparticles do produce plasmas of small viscosity and
even smaller diffusion rate, comparable to what is needed to explain RHIC data
and also to what follows from AdS/CFT.
  As a separate issue, we point out that right above $T_c$ there should be
surviving s-wave baryons made of quarks ($N,\Delta$), as well as adjoint
objects, 3-gluon and 3-monopole chains (the latter being nothing else but
``calorons'' or finite-T instantons).",0709.2175v1
2007-10-24,Time-dependent magnetohydrodynamic self-similar extragalactic jets,"Extragalactic jets are visualized as dynamic erruptive events modelled by
time-dependent magnetohydrodynamic (MHD) equations. The jet structure comes
through the temporally self-similar solutions in two-dimensional axisymmetric
spherical geometry. The two-dimensional magnetic field is solved in the finite
plasma pressure regime, or finite $\beta$ regime, and it is described by an
equation where plasma pressure plays the role of an eigenvalue. This allows a
structure of magnetic lobes in space, among which the polar axis lobe is
strongly peaked in intensity and collimated in angular spread comparing to the
others. For this reason, the polar lobe overwhelmes the other lobes, and a jet
structure arises in the polar direction naturally. Furthermore, within each
magnetic lobe in space, there are small secondary regions with closed
two-dimensional field lines embedded along this primary lobe. In these embedded
magnetic toroids, plasma pressure and mass density are much higher accordingly.
These are termed as secondary plasmoids. The magnetic field lines in these
secondary plasmoids circle in alternating sequence such that adjacent plasmoids
have opposite field lines. In particular, along the polar primary lobe, such
periodic plasmoid structure happens to be compatible with radio observations
where islands of high radio intensities are mapped.",0710.4513v1
2007-11-06,Magnetic dynamo action in helical turbulence,"We investigate magnetic field amplification in a turbulent velocity field
with nonzero helicity, in the framework of the kinematic Kazantsev-Kraichnan
model. We present the numerical solution of the model for the practically
important case of Kolmogorov distribution of velocity fluctuations, with a
large magnetic Reynolds number. We find that in contrast to the nonhelical case
where growing magnetic fields are described by a few bound eigenmodes
concentrated inside the inertial interval of the velocity field, in the helical
case the number of bound eigenmodes considerably increases; moreover, new
unbound eigenmodes appear. Both bound and unbound eigenmodes contribute to the
large-scale magnetic field. This indicates a limited applicability of the
conventional alpha model of a large-scale dynamo action, which captures only
unbound modes.",0711.0973v2
2007-12-20,The role of the disk magnetization on the hysteresis behavior of X-ray binaries,"We present a framework for understanding the dynamical and spectral
properties of X-ray Binaries, where the presence of an organized large scale
magnetic field plays a major role. Such a field is threading the whole
accretion disk with an amplitude measured by the disk magnetization $\mu(r,t)
=B_z^2/(\mu_o P_{tot})$, where $P_{tot}$ is the total, gas and radiation,
pressure.
  Below a transition radius $r_J$, a jet emitting disk (the JED) is settled and
drives self-collimated non relativistic jets. Beyond $r_J$, no jet is produced
despite the presence of the magnetic field and a standard accretion disc (the
SAD) is established. The radial distribution of the disk magnetization $\mu$
adjusts itself to any change of the disk accretion rate $\dot m$, thereby
modifying the transition radius $r_J$.
  We propose that a SAD-to-JED transition occurs locally, at a given radius, in
a SAD when $\mu=\mu_{max} \simeq 1$ while the reverse transition occurs in a
JED only when $\mu=\mu_{min}\simeq 0.1$. This bimodal behavior of the accretion
disk provides a promising way to explain the hysteresis cycles followed by
X-ray binaries during outbursts.",0712.3388v1
2008-01-07,Magnetization reversal driven by spin-injection : a mesoscopic spin-transfer effect,"A mesoscopic description of spin-transfer effect is proposed, based on the
spin-injection mechanism occurring at the junction with a ferromagnet. The
effect of spin-injection is to modify locally, in the ferromagnetic
configuration space, the density of magnetic moments. The corresponding
gradient leads to a current-dependent diffusion process of the magnetization.
In order to describe this effect, the dynamics of the magnetization of a
ferromagnetic single domain is reconsidered in the framework of the
thermokinetic theory of mesoscopic systems. Assuming an Onsager
cross-coefficient that couples the currents, it is shown that spin-dependent
electric transport leads to a correction of the Landau-Lifshitz-Gilbert
equation of the ferromagnetic order parameter with supplementary diffusion
terms. The consequence of spin-injection in terms of activation process of the
ferromagnet is deduced, and the expressions of the effective energy barrier and
of the critical current are derived. Magnetic fluctuations are calculated: the
correction to the fluctuations is similar to that predicted for the activation.
These predictions are consistent with the measurements of spin-transfer
obtained in the activation regime and for ferromagnetic resonance under
spin-injection.",0801.1019v1
2008-01-11,Charge pumping in magnetic tunnel junctions: Scattering theory,"We study theoretically the charge transport pumped by magnetization dynamics
through epitaxial FIF and FNIF magnetic tunnel junctions (F: Ferromagnet, I:
Insulator, N: Normal metal). We predict a small but measurable DC pumping
voltage under ferromagnetic resonance conditions for collinear magnetization
configurations, which may change sign as function of barrier parameters. A much
larger AC pumping voltage is expected when the magnetizations are at right
angles. Quantum size effects are predicted for an FNIF structure as a function
of the normal layer thickness.",0801.1779v3
2008-02-18,Diamagnetic pumping near the base of a stellar convection zone,"The property of inhomogeneous turbulence in conducting fluids to expel
large-scale magnetic fields in the direction of decreasing turbulence intensity
is shown as important for the magnetic field dynamics near the base of a
stellar convection zone. The downward diamagnetic pumping confines a fossil
internal magnetic field in the radiative core so that the field geometry is
appropriate for formation of the solar tachocline. For the stars of solar age,
the diamagnetic confinement is efficient only if the ratio of turbulent
magnetic diffusivity of the convection zone to the (microscopic or turbulent)
diffusivity of the radiative interiour is larger than 10^5. Confinement in
younger stars require still larger diffusivity ratio. The observation of
persistent magnetic structures on young solar-type stars can thus provide
evidences for the nonexistence of tachoclines in stellar interiors and on the
level of turbulence in radiative cores.",0802.2415v1
2008-02-19,Temperature memory and resistive glassy behaviors of a perovskite manganite,"This paper reports the observations of long-time relaxation, aging, and
temperature memory behaviors of resistance and magnetization in the
ferromagnetic state of a polycrystalline La0.7Ca0.3Mn0.925Ti0.075O3 compound.
The observed glassy dynamics of the electrical transport appears to be
magnetically originated and has a very close association with the magnetic
glassiness of the sample. Phase separation and strong correlation between
magnetic interactions and electronic conduction play the essential roles in
producing such a resistive glassiness. We explain the observed effects in terms
of a coexistence of two competing thermomagnetic processes, domain growth and
magnetic freezing, and propose that hole-doped perovskite manganites can be
considered as ""resistive glasses"".",0802.2729v1
2008-02-24,Dynamics of the Solar Magnetic Network. II. Heating the Magnetized Chromosphere,"We consider recent observations of the chromospheric network, and argue that
the bright network grains observed in the Ca II H & K lines are heated by an as
yet unidentified quasi-steady process. We propose that the heating is caused by
dissipation of short-period magnetoacoustic waves in magnetic flux tubes
(periods less than 100 s). Magnetohydrodynamic (MHD) models of such waves are
presented. We consider wave generation in the network due to two separate
processes: (a) by transverse motions at the base of the flux tube; and (b) by
the absorption of acoustic waves generated in the ambient medium. We find that
the former mechanism leads to an efficient heating of the chromosphere by slow
magnetoacoustic waves propagating along magnetic field lines. This heating is
produced by shock waves with a horizontal size of a few hundred kilometers. In
contrast, acoustic waves excited in the ambient medium are converted into
transverse fast modes that travel rapidly through the flux tube and do not form
shocks, unless the acoustic sources are located within 100 km from the tube
axis. We conclude that the magnetic network may be heated by magnetoacoustic
waves that are generated in or near the flux tubes.",0802.3509v1
2008-03-19,Transport magnetic currents driven by moving kink crystal in chiral helimagnets,"We show that the bulk transport magnetic current is generated by the moving
magnetic kink crystal (chiral soliton lattice) formed in the chiral helimagnet
under the static magnetic field applied perpendicular to the helical axis. The
current is caused by the non-equilibrium transport momentum with the kink mass
being determined by the spin fluctuations around the kink crystal state. An
emergence of the transport magnetic currents is then a consequence of the
dynamical off-diagonal long range order along the helical axis. We derive an
explicit formula for the inertial mass of the kink crystal and the current in
the weak field limit.",0803.2750v1
2008-03-19,Strong linewidth variation for spin-torque nano-oscillators as a function of in-plane magnetic field angle,"We measure the microwave signals produced by spin-torque-driven magnetization
dynamics in patterned magnetic multilayer devices at room temperature, as a
function of the angle of a magnetic field applied in the sample plane. We find
strong variations in the frequency linewidth of the signals, with a decrease by
more than a factor of 20 as the field is rotated from the magnetic easy axis to
the in-plane hard axis. Based on micromagnetic simulations, we identify these
variations as due to a transition from spatially incoherent to coherent
precession.",0803.2871v1
2008-04-16,Quantum Zeno Effect Underpinning the Radical-Ion-Pair Mechanism of Avian Magnetoreception,"The intricate biochemical processes underlying avian magnetoreception, the
sensory ability of migratory birds to navigate using earths magnetic field,
have been narrowed down to spin-dependent recombination of radical-ion pairs to
be found in avian species retinal proteins. The avian magnetic field detection
is governed by the interplay between magnetic interactions of the radicals
unpaired electrons and the radicals recombination dynamics. Critical to this
mechanism is the long lifetime of the radical-pair spin coherence, so that the
weak geomagnetic field will have a chance to signal its presence. It is here
shown that a fundamental quantum phenomenon, the quantum Zeno effect, is at the
basis of the radical-ion-pair magnetoreception mechanism. The quantum Zeno
effect naturally leads to long spin coherence lifetimes, without any
constraints on the systems physical parameters, ensuring the robustness of this
sensory mechanism. Basic experimental observations regarding avian magnetic
sensitivity are seamlessly derived. These include the magnetic sensitivity
functional window and the heading error of oriented bird ensembles, which so
far evaded theoretical justification. The findings presented here could be
highly relevant to similar mechanisms at work in photosynthetic reactions. They
also trigger fundamental questions about the evolutionary mechanisms that
enabled avian species to make optimal use of quantum measurement laws.",0804.2646v1
2008-05-19,Impact of impellers on the axisymmetric magnetic mode in the VKS2 dynamo experiment,"In the VKS2 (von K\'arm\'an Sodium 2) successful dynamo experiment of
September 2006, the magnetic field that was observed showed a strong
axisymmetric component, implying that non axisymmetric components of the flow
field were acting. By modeling the induction effect of the spiraling flow
between the blades of the impellers in a kinematic dynamo code, we find that
the axisymmetric magnetic mode is excited and becomes dominant in the vicinity
of the dynamo threshold. % The control parameters are the magnetic Reynolds
number of the mean flow, the coefficient measuring the induction effect,
$\alpha$, and the type of boundary conditions (vacuum for steel impellers and
normal field for soft iron impellers). We show that using realistic values of
$\alpha$, the observed critical magnetic Reynolds number, $Rm^c \approx 32$,
can be reached easily with ferromagnetic boundary conditions. We conjecture
that the dynamo action achieved in this experiment may not be related to the
turbulence in the bulk of the flow, but rather to the alpha effect induced by
the impellers.",0805.2805v2
2008-05-24,Solar convection and oscillations in magnetic regions,"The goal of this research is to investigate how magnetic field affects the
dynamics of granular convection and excitation of solar oscillations by means
of realistic numerical simulations. We have used a 3D, compressible, non-linear
radiative magnetohydrodynamics code developed at the NASA Ames Research Center.
This code takes into account several physical phenomena: compressible fluid
flow in a highly stratified medium, sub-grid scale turbulence models, radiative
energy transfer between the fluid elements, and a real-gas equation of state.
We have studied the influence of the magnetic field of various strength on the
convective cells and on the excitation mechanisms of the acoustic oscillations
by calculating spectral properties of the convective motions and oscillations.
The results reveal substantial changes of the granulation structure with
increased magnetic field, and a frequency-dependent reduction in the
oscillation power in a good agreement with solar observations. These
simulations suggest that the enhanced high-frequency acoustic emission at the
boundaries of active region (""acoustic halo"" phenomenon) is caused by the
changes of the spatial-temporal spectrum of the turbulent convection in
magnetic field, resulting in turbulent motions of smaller scales and higher
frequencies than in quiet Sun regions.",0805.3741v1
2008-05-28,Photonic Dipole Contours of Ferrofluid Hele-Shaw Cell,"This investigation describes and demonstrates a novel technique for the
visualization of magnetic fields. Two ferrofluid Hele-Shaw cells have been
constructed to facilitate the imaging of magnetic field lines. We deduce that
magnetically induced photonic band gap arrays similar to electrostatic liquid
crystal operation are responsible for the photographed images and seek to
mathematically prove the images are of dipole nature.
  A simple way of explaining this work is to think of the old magnetic iron
filling experiments; but now each iron filling is a molecule floating in a
liquid. Each molecule has the freedom to act as an independent lens that can be
aligned by an external magnetic field. Because each lens directs light, the
external field can be analyzed by following the light paths captured in the
photographs.",0805.4364v2
2008-06-05,Theory of spin current in chiral helimagnet,"We give detailed description of the transport spin current in the chiral
helimagnet. Under the static magnetic field applied perpendicular to the
helical axis, the magnetic kink crystal (chiral soliton lattice) is formed.
Once the kink crystal begins to move under the Galilean boost, the spin-density
accumulation occurs inside each kink and there emerges periodic arrays of the
induced magnetic dipoles carrying the transport spin current. The coherent
motion of the kink crystal dynamically generates the spontaneous
demagnetization field. This mechanism is analogous to the
D\""{o}ring-Becker-Kittel mechanism of the domain wall motion in ferromagnets.
To describe the kink crystal motion, we took account of not only the tangential
$\phi$-fluctuations but the longitudinal $\theta$-fluctuations around the
helimagnetic configuration. Based on the collective coordinate method and the
Dirac's canonical formulation for the singular Lagrangian system, we derived
the closed formulae for the mass, spin current and induced magnetic dipole
moment accompanied with the kink crystal motion. To materialize the theoretical
model presented here, symmetry-adapted material synthesis would be required,
where the interplay of crystallographic and magnetic chirality plays a key role
there.",0806.0968v2
2008-07-02,Magnetohydrodynamic experiments on cosmic magnetic fields,"It is widely known that cosmic magnetic fields, i.e. the fields of planets,
stars, and galaxies, are produced by the hydromagnetic dynamo effect in moving
electrically conducting fluids. It is less well known that cosmic magnetic
fields play also an active role in cosmic structure formation by enabling
outward transport of angular momentum in accretion disks via the
magnetorotational instability (MRI). Considerable theoretical and computational
progress has been made in understanding both processes. In addition to this,
the last ten years have seen tremendous efforts in studying both effects in
liquid metal experiments. In 1999, magnetic field self-excitation was observed
in the large scale liquid sodium facilities in Riga and Karlsruhe. Recently,
self-excitation was also obtained in the French ""von Karman sodium"" (VKS)
experiment. An MRI-like mode was found on the background of a turbulent
spherical Couette flow at the University of Maryland. Evidence for MRI as the
first instability of an hydrodynamically stable flow was obtained in the
""Potsdam Rossendorf Magnetic Instability Experiment"" (PROMISE). In this review,
the history of dynamo and MRI related experiments is delineated, and some
directions of future work are discussed.",0807.0299v2
2008-07-22,Exponential equilibration by slow cooling in the planar random-anisotropy magnet: experiments and simulations,"Neutron measurements of the equilibration of the staggered magnetization in
DyAs(0.35)V(0.65)O4 are compared with Monte Carlo simulations of spin dynamics
in a planar random-anisotropy magnet. The simulation results are in agreement
with striking observed relaxation phenomena: when cooled rapidly to a low
temperature no magnetic ordering is observed, but when cooled in small steps an
ordered magnetic moment appears which is found to equilibrate exponentially
with time at temperatures through and below the transition temperature. In
contrast to the freezing of spins in other random systems, the time scale of
the relaxation in this system does not increase significantly even at the
lowest temperatures.",0807.3536v1
2008-09-04,Properties of anisotropic magnetic impurities on surfaces,"Using numerical renormalization group techniques, we study static and dynamic
properties of a family of single-channel Kondo impurity models with axial
magnetic anisotropy $DS_z^2$ terms; such models are appropriate to describe
magnetic impurity atoms adsorbed on non-magnetic surfaces, which may exhibit
surface Kondo effect. We show that for positive anisotropy $D$ and for any spin
$S$, the systems behave at low temperatures as regular Fermi liquids with fully
compensated impurity spin. The approach to the stable fixed point depends on
the value of the spin $S$ and on the ratio $D/T_K^{(0)}$, where $T_K^{(0)}$ is
the Kondo temperature in the absence of the anisotropy. For S=1, the screening
occurs in two stages if $D<T_K^{(0)}$; the second Kondo temperature is
exponentially reduced in this case. More generally, there is an effective
spin-1/2 Kondo effect for any integer $S$ if $D<T_K^{(0)}$ and for any
half-integer $S$ if $D>T_K^{(0)}$. For negative anisotropy $D$, the system is a
non-Fermi liquid with residual anisotropic exchange interaction. However, the
presence of transverse magnetic anisotropy $E(S_x^2-S_y^2)$ restores
Fermi-liquid behavior in real systems.",0809.0759v1
2008-10-31,"Anisotropy, Itineracy, and Magnetic Frustration in High-Tc Iron Pnictides","Using first-principle density functional theory calculations combined with
insight from a tight-binding representation, dynamical mean field theory, and
linear response theory, we have extensively investigated the electronic
structures and magnetic interactions of nine ferropnictides representing three
different structural classes. The calculated magnetic interactions are found to
be short-range, and the nearest ($J_{1a}$) and next-nearest ($J_{2}$) exchange
constants follow the universal trend of J_{1a}/2J_{2}\sim 1, despite their
itinerant origin and extreme sensitivity to the z-position of As. These results
bear on the discussion of itineracy versus magnetic frustration as the key
factor in stabilizing the superconducting ground state. The calculated spin
wave dispersions show strong magnetic anisotropy in the Fe plane, in contrast
to cuprates.",0811.0034v3
2008-12-19,Spin-bias driven magnetization reversal and nondestructive detection in a single molecular magnet,"The magnetization reversal in a single molecular magnet (SMM) weakly coupled
to an electrode with spin-dependent splitting of chemical potentials (spin
bias) is theoretically investigated by means of the rate equation. A
microscopic mechanism for the reversal is demonstrated by the avalanche
dynamics at the reversal point. The magnetization as a function of the spin
bias shows hysteresis loops tunable by the gate voltage and varying with
temperature. The nondestructive measurement to the orientation of giant spin in
SMM is presented by measuring the fully polarized electric current in the
response to a small spin bias. For Mn$_{12}$ac molecule, its small transverse
anisotropy only slightly violates the results above. The situation when there
is an angle between the easy axis of the SMM and the spin quantization
direction of the electrode is also studied.",0812.3713v2
2009-01-12,3D MHD simulations of magnetic fields and radio polarization of barred galaxies,"We present results of three-dimensional, fully nonlinear MHD simulations of a
large-scale magnetic field evolution in a barred galaxy. The model does not
take into consideration the dynamo process. We find that the obtained magnetic
field configurations are highly similar to the observed maps of the polarized
intensity of barred galaxies, because the modeled vectors form coherent
structures along the bar and spiral arms. Due to the dynamical influence of the
bar the gas forms spiral waves which go radially outward. Each spiral arm forms
the magnetic arm which stays much longer in the disk, than the gaseous spiral
structure. Additionally the modeled total energy of magnetic field grows due to
strong compression and shear of non-axisymmetrical bar flows and differential
rotation, respectively.",0901.1564v1
2009-03-13,Dirac-Neutrino Magnetic Moment and the Dynamics of a Supernova Explosion,"The double conversion of the neutrino helicity $\nu_L \to \nu_R \to \nu_L$
has been analyzed for supernova conditions, where the first stage is due to the
interaction of the neutrino magnetic moment with plasma electrons and protons
in the supernova core, and the second stage, due to the resonance spin flip of
the neutrino in the magnetic field of the supernova envelope. It is shown that,
in the presence of the neutrino magnetic moment in the range $10^{-13} \mu_{\rm
B} < \mu_\nu < 10^{-12} \mu_{\rm B}$ and a magnetic field of $\sim 10^{13}$ G
between the neutrinosphere and the shock-stagnation region, an additional
energy of about $10^{51}$ erg, which is sufficient for a supernova explosion,
can be injected into this region during a typical shock-stagnation time.",0903.2321v1
2009-05-26,Scattering Theory of Charge-Current Induced Magnetization Dynamics,"In ferromagnets, charge currents can excite magnons via the spin-orbit
coupling. We develop a novel and general scattering theory of charge current
induced macrospin magnetization torques in normal metal$|$ferromagnet$|$normal
metal layers. We apply the formalism to a dirty GaAs$|$(Ga,Mn)As$|$GaAs system.
By computing the charge current induced magnetization torques and solving the
Landau-Lifshitz-Gilbert equation, we find magnetization switching for current
densities as low as $ 5\times 10^{6}$~A/cm$^2$. Our results are in agreement
with a recent experimental observation of charge-current induced magnetization
switching in (Ga,Mn)As.",0905.4170v2
2009-07-14,Las ondas viajeras son los ojos nuevos de la resonancia en medicina,"Magnetic Resonance Imaging is one of the most versatile experimental
techniques in chemistry, physics and biology, providing insight into the
structure and dynamics of matter at the molecular scale. A group led by Klaas
Pruessmann at ETH Zurich (Brunner, D.O. et al. Travelling-wave nuclear magnetic
resonance. Nature 457;994:2009) has now demonstrated an alternative approach
that radically departs from the conventional view of MRI as a near-field,
inductive technique. This approach also required that radiofre-quency wave is
propagated from an antenna positioned at one end of the cylindrical magnet;
this wave travels through the specimen within the bore of the magnet, producing
signals that are received either by the same antenna or by another one
positioned at the opposite end. The resulting instrument considerably extends
the scale of imaging possible with Magnetic Resonance Imaging, and initial
tests demonstrated uniform high-field imaging of a variety of samples, both
inorganic and inorganic.",0907.2446v1
2009-10-06,Cold and Ultracold Rydberg Atoms in Strong Magnetic Fields,"Cold Rydberg atoms exposed to strong magnetic fields possess unique
properties which open the pathway for an intriguing many-body dynamics taking
place in Rydberg gases consisting of either matter or anti-matter systems. We
review both the foundations and recent developments of the field in the cold
and ultracold regime where trapping and cooling of Rydberg atoms have become
possible. Exotic states of moving Rydberg atoms such as giant dipole states are
discussed in detail, including their formation mechanisms in a strongly
magnetized cold plasma. Inhomogeneous field configurations influence the
electronic structure of Rydberg atoms, and we describe the utility of
corresponding effects for achieving tightly trapped ultracold Rydberg atoms. We
review recent work on large, extended cold Rydberg gases in magnetic fields and
their formation in strongly magnetized ultracold plasmas through collisional
recombination. Implications of these results for current antihydrogen
production experiments are pointed out, and techniques for trapping and cooling
of such atoms are investigated.",0910.1037v1
2009-10-06,First Detection of the Zeeman Effect in the 36 GHz Class I methanol maser line with the EVLA,"We report the first detection of the Zeeman effect in the 36 GHz Class I
methanol maser line. The observations were carried out with 13 antennas of the
EVLA toward the high mass star forming region M8E. Based on our adopted Zeeman
splitting factor of $z = 1.7 Hz/mG, we detect a line of sight magnetic field of
-31.3 +/- 3.5 mG and 20.2 +/- 3.5 mG to the northwest and southeast of the
maser line peak respectively. This change in sign over a 1300 AU size scale may
indicate that the masers are tracing two regions with different fields, or that
the same field curves across the regions where the masers are being excited.
The detected fields are not significantly different from the magnetic fields
detected in the 6.7 GHz Class II methanol maser line, indicating that methanol
masers may trace the large scale magnetic field, or that the magnetic field
remains unchanged during the early evolution of star forming regions. Given
what is known about the densities at which 36 GHz methanol masers are excited,
we find that the magnetic field is dynamically significant in the star forming
region.",0910.1081v1
2009-11-02,Response of spin polarized neutron matter under the presence of a strong magnetic field with Skyrme interactions,"The effects of a strong magnetic field in the dynamical response of a pure
neutron system to a weak neutrino probe are studied within the framework of the
Landau Fermi Liquid Theory in the non-relativistic Hartree-Fock approximation.
We use Skyrme forces to parametrize the partially magnetized nuclear
interacting plasma and describe its collective modes. We study the vector and
vector-axial matter response functions and the neutrino mean free path in this
two-component fermion system with net spin polarization and density dependent
in-medium correlations. We find a decrease in the neutrino opacity of
magnetized matter with respect to the non-magnetized case for fields $log_{10}
B(G) \gtrsim 17$.",0911.0378v1
2009-11-09,Role of dipole-dipole interactions in multiple quantum transitions in magnetic nanoparticles,"In order to better understand the origin of multiple quantum transitions
observed in superparamagnetic nanoparticles, electron magnetic resonance (EMR)
studies have been performed on iron oxide nanoparticles assembled inside the
anodic alumina membrane. The positions of both the main resonance and
""forbidden"" (double-quantum, 2Q) transitions observed at the half-field
demonstrate the characteristic angular dependence with the line shifts
proportional to 3cos2q-1, where q is the angle between the channel axis and
external magnetic field B. This result can be attributed to the interparticle
dipole-dipole interactions within elongated aggregates inside the channels. The
angular dependence of the 2Q intensity is found to be proportional to
sin2qcos2q, that is consistent with the predictions of quantum-mechanical
calculations with the account for the mixing of states by non-secular
inter-particle dipole-dipole interactions. Good agreement is demonstrated
between different kinds of measurements (magnetization curves, line shifts and
2Q intensity), evidencing applicability of the quantum approach to the
magnetization dynamics of superparamagnetic objects.",0911.1752v1
2009-11-16,On the magnetic field of off-limb spicules,"Determining the magnetic field related to solar spicules is vital for
developing adequate models of these plasma jets, which are thought to play a
key role in the thermal, dynamic and magnetic structure of the Chromosphere.
Here we report on the magnetic properties of off-limb spicules in a very quiet
region of the solar atmosphere, as inferred from new spectropolarimetric
observations in the He I 10830 A triplet obtained with the Tenerife Infrared
Polarimeter. We have used a novel inversion code for Stokes profiles caused by
the joint action of atomic level polarization and the Hanle and Zeeman effects
(HAZEL) to interpret the observations. Magnetic fields as strong as ~40G were
detected in a very localized area of the slit, which could represent a possible
lower value of the field strength of organized network spicules.",0911.3149v1
2009-11-23,Ligand exchange in gold-coated FePt nanoparticles,"In this work we present the magnetic properties of gold coated FePt
nanoparticles and the study of stable aqueous dispersions of FePt@Au and FePt
synthesized after ligand exchange with mercaptoundecanoic acid. The particle
size determined from TEM micrographs goes from 4 nm for the uncoated
nanoparticles to a maximum of 10 nm for the gold coated ones indicating that
the thickness of the shell ranges from 1 to 3 nm. The magnetic characterization
consists in hysteresis cycles at 10 and 300 K. The results show that, at low
field and room temperature, the magnetic behavior of uncoated and coated
nanoparticles are surprisingly quite similar. Since the gold coated
nanoparticles keep the magnetic properties of FePt and the presence of gold
improves the functionalization of nanoparticles, the system is suitable for
biological application. Mercaptoundecanoic ligand transfer was used to render
water stable nanoparticles in a wide pH range. Transmission electron microscopy
and dynamic light scattering results show the nanoparticles slightly
agglomerate after ligand exchange. Fourier transform infrared spectroscopy
results suggest that thiol bind to the gold atoms of the surface.",0911.4374v1
2010-01-07,Extracting Nucleon Magnetic Moments and Electric Polarizabilities from Lattice QCD in Background Electric Fields,"Nucleon properties are investigated in background electric fields. As the
magnetic moments of baryons affect their relativistic propagation in constant
electric fields, electric polarizabilities cannot be determined without
knowledge of magnetic moments. This is analogous to the experimental situation,
for which determination of polarizabilities from the Compton amplitude requires
subtraction of Born terms. With the background field method, we devise
combinations of nucleon correlation functions in constant electric fields that
isolate magnetic moments and electric polarizabilities. Using an ensemble of
anisotropic gauge configurations with dynamical clover fermions, we demonstrate
how both observables can be determined from lattice QCD simulations in
background electric fields. We obtain results for the neutron and proton,
however, our study is currently limited to electrically neutral sea quarks. The
value we extract for the nucleon isovector magnetic moment is comparable to
those obtained from measuring lattice three-point functions at similar pion
masses.",1001.1131v2
2010-01-11,Dynamics of braided coronal loops - I. Onset of magnetic reconnection,"The response of the solar coronal magnetic field to small-scale photospheric
boundary motions including the possible formation of current sheets via the
Parker scenario is one of open questions of solar physics. Here we address the
problem via a numerical simulation. The three-dimensional evolution of a
braided magnetic field which is initially close to a force-free state is
followed using a resistive MHD code. A long-wavelength instability takes place
and leads to the formation of two thin current layers. Magnetic reconnection
occurs across the current sheets with three-dimensional features shown,
including an elliptic magnetic field structure about the reconnection site, and
results in an untwisting of the global field structure.",1001.1717v3
2010-01-13,Numerical code SELFAS-3 and electrodynamic aggregation of magnetized nanodust,"The principles of the parallel numerical code SELFAS-3 are presented. The
code modifies previous version of the code to enable parallel computations of
electrodynamic aggregation in a many-body system of basic blocks which are
taken as strongly magnetized thin rods (i.e., one-dimensional static magnetic
dipoles), with electric conductivity and static electric charge, screened with
its own static plasma sheath. The aggregation modelling includes the electric
current dynamics in a complicated evolving network to describe the processes of
external and internal electric short-circuiting. The code enables the
continuous modelling of a transition between the following states: randomly
situated ensemble of solitary basic blocks; electric current-carrying
filamentary system; restructured filamentary network with a trend towards a
fractal skeletal structuring. The latter trend is illustrated with generation
of a bigger magnetic dipole in (i) homogeneous random ensemble between the
biased electrodes in the presence of a plasma electric current filament and
(ii) random ensemble around a straight linear nanodust filament with
inhomogeneous distribution of the trapped magnetic flux along the filament.",1001.2328v1
2010-01-28,Static and Dynamical Susceptibility of LaO1-xFxFeAs,"The mechanism of superconductivity and magnetism and their possible interplay
have recently been under debate in pnictides. A likely pairing mechanism
includes an important role of spin fluctuations and can be expressed in terms
of the magnetic susceptibility chi. The latter is therefore a key quantity in
the determination of both the magnetic properties of the system in the normal
state, and of the contribution of spin fluctuations to the pairing potential. A
basic ingredient to obtain chi is the independent-electron susceptibility chi0.
Using LaO1-xFxFeAs as a prototype material, in this report we present a
detailed ab-initio study of chi0(q,omega), as a function of doping and of the
internal atomic positions. The resulting static chi0(q,0) is consistent with
both the observed M-point related magnetic stripe phase in the parent compound,
and with the existence of incommensurate magnetic structures predicted by
ab-initio calculations upon doping.",1001.5154v1
2010-02-02,Model for Dynamic Self-Assembled Magnetic Surface Structures,"We propose a first-principles model for self-assembled magnetic surface
structures on the water-air interface reported in earlier experiments
\cite{snezhko2,snezhko4}. The model is based on the Navier-Stokes equation for
liquids in shallow water approximation coupled to Newton equations for
interacting magnetic particles suspended on the water-air interface. The model
reproduces most of the observed phenomenology, including spontaneous formation
of magnetic snake-like structures, generation of large-scale vortex flows,
complex ferromagnetic-antiferromagnetic ordering of the snake, and
self-propulsion of bead-snake hybrids. The model provides valuable insights
into self-organization phenomena in a broad range of non-equilibrium magnetic
and electrostatic systems with competing interactions.",1002.0328v1
2010-02-18,Hamilton--Jacobi theory for continuation of magnetic field across a toroidal surface supporting a plasma pressure discontinuity,"The vanishing of the divergence of the total stress tensor (magnetic plus
kinetic) in a neighborhood of an equilibrium plasma containing a toroidal
surface of discontinuity gives boundary and jump conditions that strongly
constrain allowable continuations of the magnetic field across the surface. The
boundary conditions allow the magnetic fields on either side of the
discontinuity surface to be described by surface magnetic potentials, reducing
the continuation problem to that of solving a Hamilton--Jacobi equation. The
characteristics of this equation obey Hamiltonian equations of motion, and a
necessary condition for the existence of a continued field across a general
toroidal surface is that there exist invariant tori in the phase space of this
Hamiltonian system. It is argued from the Birkhoff theorem that existence of
such an invariant torus is also, in general, sufficient for continuation to be
possible. An important corollary is that the rotational transform of the
continued field on a surface of discontinuity must, generically, be irrational.",1002.3499v1
2010-04-05,Abrikosov Gluon Vortices in Color Superconductors,"In this talk I will discuss how the in-medium magnetic field can influence
the gluon dynamics in a three-flavor color superconductor. It will be shown how
at field strengths comparable to the charged gluon Meissner mass a new phase
can be realized, giving rise to Abrikosov's vortices of charged gluons. In that
phase, the inhomogeneous gluon condensate anti-screens the magnetic field due
to the anomalous magnetic moment of these spin-1 particles. This paramagnetic
effect can be of interest for astrophysics, since due to the gluon vortex
antiscreening mechanism, compact stars with color superconducting cores could
have larger magnetic fields than neutron stars made up entirely of nuclear
matter. I will also discuss a second gluon condensation phenomenon connected to
the Meissner instability attained at moderate densities by two-flavor color
superconductors. In this situation, an inhomogeneous condensate of charged
gluons emerges to remove the chromomagnetic instability created by the pairing
mismatch, and as a consequence, the charged gluonic currents induce a magnetic
field. Finally, I will point out a possible relation between glitches in
neutron stars and the existence of the gluon vortices.",1004.0733v1
2010-05-07,Importance of Itinerancy and Quantum Fluctuations for the Magnetism in Iron Pnictides,"By applying density functional theory, we find strong evidence for an
itinerant nature of magnetism in two families of iron pnictides. Furthermore,
by employing dynamical mean field theory with continuous time quantum Monte
Carlo as an impurity solver, we observe that the antiferromagnetic metal with
small magnetic moment naturally arises out of coupling between unfrustrated and
frustrated bands. Our results point to a possible scenario for magnetism in
iron pnictides where magnetism originates from a strong instability at the
momentum vector ($\pi$, $\pi$, $\pi$) while it is reduced by quantum
fluctuations due to the coupling between weakly and strongly frustrated bands.",1005.1170v2
2010-06-04,The effect of spin-orbit interaction and attractive Coulomb potential on the magnetic properties of Ga$_{1-x}$Mn$_{x}$As,"We employ the dynamical mean-field approximation to study the magnetic
properties of a model relevant for the dilute magnetic semiconductors. Our
model includes the spin-orbit coupling on the hole bands, the exchange
interaction, and the attractive Coulomb potential between the negatively
charged magnetic ions and the itinerant holes. The inclusion of the Coulomb
potential significantly renormalizes the exchange coupling and enhances the
ferromagnetic transition temperature for a wide range of couplings. We also
explore the effect of the spin-orbit interaction by using two different values
of the ratio of the effective masses of the heavy and light holes. We show that
in the regime of small $J_{c}$-$V$ the spin-orbit interaction enhances $T_{c}$,
while for large enough values of $J_{c}$-$V$ magnetic frustration reduces $T_c$
to values comparable to the previously calculated strong coupling limit.",1006.0998v2
2010-06-07,Roles of adiabatic and nonadiabatic spin transfer torques on magnetic domain wall motion,"Electric current exerts torques-so-called spin transfer torques (STTs)-on
magnetic domain walls (DWs), resulting in DW motion. At low current densities,
the STTs should compete against disorders in ferromagnetic nanowires but the
nature of the competition remains poorly understood. By achieving
two-dimensional contour maps of DW speed with respect to current density and
magnetic field, here we visualize unambiguously distinct roles of the two
STTs-adiabatic and nonadiabatic-in scaling behaviour of DW dynamics arising
from the competition. The contour maps are in excellent agreement with
predictions of a generalized scaling theory, and all experimental data collapse
onto a single curve. This result indicates that the adiabatic STT becomes
dominant for large current densities, whereas the nonadiabatic STT-playing the
same role as a magnetic field-subsists at low current densities required to
make emerging magnetic nanodevices practical.",1006.1216v1
2010-07-22,Strong reduction of V4+ amount in vanadium oxide/hexadecylamine nanotubes by doping with Co2+ and Ni2+ ions: EPR and magnetic studies,"In this work we present a complete characterization and magnetic study of
vanadium oxide/hexadecylamine nanotubes (VOx/Hexa NT's) doped with Co2+ and
Ni2+ ions. The morphology of the NT's has been characterized by Transmission
Electron Microscopy (TEM) while the metallic elements have been quantified by
Instrumental Neutron Activation Analysis (INAA) technique. The static and
dynamic magnetic properties were studied collecting data of magnetization as a
function of magnetic field and temperature and by Electron Paramagnetic
Resonance (EPR). We observed that the incorporation of metallic ions (Co2+,
S=3/2 and Ni2+, S=1) decrease notably the amount of V4+ ions in the system,
from 14-16% (non-doped case) to 2-4%, with respect to the total vanadium atoms
into the tubular nanostructure, improving considerably their potential
technological applications as Li-ion batteries cathodes.",1007.3968v1
2010-08-02,Instabilities of Shercliff and Stewartson layers in spherical Couette flow,"We explore numerically the flow induced in a spherical shell by
differentially rotating the inner and outer spheres. The fluid is also taken to
be electrically conducting (in the low magnetic Reynolds number limit), and a
magnetic field is imposed parallel to the axis of rotation. If the outer sphere
is stationary, the magnetic field induces a Shercliffe layer on the tangent
cylinder, the cylinder just touching the inner sphere and parallel to the
field. If the magnetic field is absent, but a strong overall rotation is
present, Coriolis effects induce a Stewartson layer on the tangent cylinder.
The non-axisymmetric instabilities of both types of layer separately have been
studied before; here we consider the two cases side by side, as well as the
mixed case, and investigate how magnetic and rotational effects interact. We
find that if the differential rotation and the overall rotation are in the same
direction, the overall rotation may have a destabilizing influence, whereas if
the differential rotation and the overall rotation are in the opposite
direction, the overall rotation always has a stabilizing influence.",1008.0328v1
2010-08-04,Magnetic aspects of QCD and compact stars,"Magnetic properties of quark matter are discussed. The possibility of
ferromagnetic transition is studied by using the one-gluon-exchange
interaction. Magnetic susceptibility is evaluated within Landau Fermi liquid
theory, and the important roles of the screening for the gluon propagation are
elucidated. Static screening for the longitudinal gluons improves the infrared
singularities, while the transverse gluons receive only dynamic screening. The
latter property gives rise to a novel non-Fermi-liquid behaviour in the
magnetic susceptibility. The critical density is estimated to be around the
nuclear density and the Curie temperature several tens MeV. The spin density
wave is also discussed at moderate densities, where chiral transition becomes
important. Pseudoscalar condensate as well as scalar one takes a spatially
non-uniform form in a chirally invariant way. Accordingly magnetization
oscillates like spin density wave. These results should have some implications
on compact star phenomena.",1008.0709v1
2010-08-04,Spin relaxation due to electron-electron magnetic interaction in high Lande g-factor semiconductors,"We investigate spin transport in InSb/InAlSb heterostructure using the Monte
Carlo approach, generalized by including density matrix description of spin for
taking spin dynamics into account. In addition to the dominant Dyakonov-Perel
(DP) mechanism for spin control and relaxation, we consider magnetic
interaction between electrons which assumes importance due to high electronic
Lande g-factor in the material. It is found that while the effect of magnetic
interaction is not important at low densities, it reduces the spin relaxation
length by as much as 50% at higher densities. We also present a calculation
which elucidates the suppression of decoherence attributed to wave vector
dependent magnetic field in the DP relaxation mechanism. We note that magnetic
interaction is a general relaxation mechanism which may assume importance in
materials with high electronic Lande g-factor.",1008.0739v1
2010-08-23,Magnetic aspects of QCD at finite density and temperature,"Some magnetic aspects of QCD are discussed at finite density and temperature.
Possibility of spontaneous magnetization is studied within Landau Fermi-liquid
theory, and the important roles of the screening effects for gluon propagation
are elucidated. Static screening for the longitudinal gluons improves the
infrared singularities, while the transverse gluons receive only dynamic
screening. The latter property gives rise to a novel non-Fermi-liquid behaviour
for the magnetic susceptibility. Appearance of a density-wave state is also
discussed in relation to chiral transition, where pseudoscalar condensate as
well as scalar one takes a spatially non-uniform form in a chirally invariant
way. Accordingly magnetization of quark matter oscillates like spin density
wave. A hadron-quark continuity is suggested in this aspect, remembering pion
condensation in hadronic phase.",1008.3753v1
2010-08-29,Stochastic Flux-Freezing and Magnetic Dynamo,"We argue that magnetic flux-conservation in turbulent plasmas at high
magnetic Reynolds numbers neither holds in the conventional sense nor is
entirely broken, but instead is valid in a novel statistical sense associated
to the ""spontaneous stochasticity"" of Lagrangian particle tra jectories. The
latter phenomenon is due to the explosive separation of particles undergoing
turbulent Richardson diffusion, which leads to a breakdown of Laplacian
determinism for classical dynamics. We discuss empirical evidence for
spontaneous stochasticity, including our own new numerical results. We then use
a Lagrangian path-integral approach to establish stochastic flux-freezing for
resistive hydromagnetic equations and to argue, based on the properties of
Richardson diffusion, that flux-conservation must remain stochastic at infinite
magnetic Reynolds number. As an important application of these results we
consider the kinematic, fluctuation dynamo in non-helical, incompressible
turbulence at unit magnetic Prandtl number. We present results on the
Lagrangian dynamo mechanisms by a stochastic particle method which demonstrate
a strong similarity between the Pr = 1 and Pr = 0 dynamos. Stochasticity of
field-line motion is an essential ingredient of both. We finally consider
briefly some consequences for nonlinear MHD turbulence, dynamo and reconnection",1008.4959v1
2010-09-14,Anisotropy-axis orientation effect on the magnetization of γ-Fe2O3 frozen ferrofluid,"The effect of magnetic anisotropy-axis alignment on the superparamagnetic
(SPM) and superspin glass (SSG) states in a frozen ferrofluid has been
investigated. The ferrofluid studied here consists of maghemite nanoparticles
(\gamma-Fe2O3, mean diameter = 8.6 nm) dispersed in glycerine at a volume
fraction of ~15%. In the high temperature SPM state, the magnetization of
aligned ferrofluid increased by a factor varying between 2 and 4 with respect
to that in the randomly oriented state. The negative interaction energy
obtained from the Curie-Weiss fit to the high temperature susceptibility in the
SPM states as well as the SSG phase onset temperature determined from the
linear magnetization curves were found to be rather insensitive to the
anisotropy axis alignment. The low temperature aging behaviour, explored via
""zero-field cooled magnetization"" (ZFCM) relaxation measurements, however, show
distinct difference in the aging dynamics in the anisotropy-axis aligned and
randomly oriented SSG states.",1009.2610v1
2010-09-18,Cascades and dissipation ratio in rotating MHD turbulence at low magnetic Prandtl number,"A phenomenology of isotropic magnetohydrodynamic turbulence subject to both
rotation and applied magnetic field is presented. It is assumed that the triple
correlations decay-time is the shortest between the eddy turn-over time and the
ones associated to the rotating frequency and Alfv\'en wave period. For $Pm=1$
it leads to four kinds of piecewise spectra, depending on the four parameters,
injection rate of energy, magnetic diffusivity, rotation rate and applied
field. With a shell model of MHD turbulence (including rotation and applied
magnetic field), spectra for $Pm \le 1$ are presented, together with the ratio
between magnetic and viscous dissipation.",1009.3549v2
2010-10-08,Time-resolved detection of spin-transfer-driven ferromagnetic resonance and spin torque measurement in magnetic tunnel junctions,"Several experimental techniques have been introduced in recent years in
attempts to measure spin transfer torque in magnetic tunnel junctions (MTJs).
The dependence of spin torque on bias is important for understanding
fundamental spin physics in magnetic devices and for applications. However,
previous techniques have provided only indirect measures of the torque and
their results to date for the bias dependence are qualitatively and
quantitatively inconsistent. Here we demonstrate that spin torque in MTJs can
be measured directly by using time-domain techniques to detect resonant
magnetic precession in response to an oscillating spin torque. The technique is
accurate in the high-bias regime relevant for applications, and because it
detects directly small-angle linear-response magnetic dynamics caused by spin
torque it is relatively immune to artifacts affecting competing techniques. At
high bias we find that the spin torque vector differs markedly from the simple
lowest-order Taylor series approximations commonly assumed.",1010.1777v1
2010-10-24,Formation of Accretion Disks in Close-Binary Systems with Magnetic Fields,"We have developed a three-dimensional numerical model and applied it to
simulate plasma flows in semi-detached binary systems whose accretor possesses
a strong intrinsic magnetic field. The model is based on the assumption that
the plasma dynamics are determined by the slow mean flow, which forms a
backdrop for the rapid propagation of MHD waves. The equations describing the
slow motion of matter were obtained by averaging over rapidly propagating
pulsations. The numerical model includes the diffusion of magnetic field by
current dissipation in turbulent vortices, magnetic buoyancy, and wave MHD
turbulence. A modified three-dimensional, parallel, numerical code was used to
simulate the flow structure in close binary systems with various accretor
magnetic fields, from $10^5$ to $10^8$ G. The conditions for the formation of
the accretion disk and the criteria distinguishing the two types of flow
corresponding to intermediate polars and polars are discussed.",1010.4949v1
2010-11-04,Spin polarization phenomena in dense neutron matter at a strong magnetic field,"Spin polarized states in neutron matter at strong magnetic fields up to
$10^{18}$ G are considered in the model with the Skyrme effective interaction.
Analyzing the self-consistent equations at zero temperature, it is shown that a
thermodynamically stable branch of solutions for the spin polarization
parameter as a function of density corresponds to the negative spin
polarization when the majority of neutron spins are oriented oppositely to the
direction of the magnetic field. Besides, it is found that in a strong magnetic
field the state with the positive spin polarization can be realized as a
metastable state at the high density region in neutron matter. At finite
temperature, the entropy of the thermodynamically stable branch demonstrates
the unusual behavior being larger than that for the nonpolarized state (at
vanishing magnetic field) above certain critical density which is caused by the
dependence of the entropy on the effective masses of neutrons in a spin
polarized state.",1011.1244v1
2010-11-25,Energy and system-size dependence of the Chiral Magnetic Effect,"The energy dependence of the local ${\cal P}$ and ${\cal CP}$ violation in
Au+Au and Cu+Cu collisions in a large energy range is estimated within a simple
phenomenological model. It is expected that at LHC the chiral magnetic effect
will be about 20 times weaker than at RHIC. At lower energy range, covered by
the low-energy scan at RHIC and future NICA/FAIR facilities, the created
magnetic field strength and energy density of deconfined matter are rather high
providing necessary conditions for the chiral magnetic effect. However, the
particular model for the chiral magnetic effect predicts that this effect
should vanish sharply at energy somewhere above the top SPS one. To elucidate
CME background effects the Hadron-String-Dynamics (HSD) transport model
including electromagnetic fields is put forward. Importance of new planning
experiments at LHC and for the low-energy RHIC scan program is emphasized.",1011.5589v1
2010-12-14,"Modeling the Galactic Magnetic Field using Rotation Measure Observations in the Galactic Disk from the CGPS, SGPS, and the VLA","Interstellar magnetic fields play critical roles in many astrophysical
processes. Yet despite their importance, our knowledge about magnetic fields in
our Galaxy remains limited. For the field within the Milky Way, much of what we
do know comes from observations of polarisation and Faraday rotation measures
(RMs) of extragalactic sources and pulsars. A high angular density of RM
measurements in several critical areas of the Galaxy is needed to clarify the
Galactic magnetic field structure. Using observations made with the VLA, we
have determined RMs for sources in regions of the Galactic plane not covered by
the Canadian Galactic Plane Survey (CGPS) and Southern Galactic Plane Survey
(SGPS). We have combined these new RMs with those determined from the CGPS and
SGPS and have produced a new model for the magnetic field of the Galactic disk.",1012.2936v1
2011-01-17,Research into Orbital Motion Stability in System of Two Magnetically Interacting Bodies,"The stability of the orbital motion of two long cylindrical magnets
interacting exclusively with magnetic forces is described. To carry out
analytical studies a model of magnetically interacting symmetric tops [1] is
used. The model was previously developed within the quasi-stationary approach
for an electromagnetic field based on the general expression of the energy of
interacting magnetic bodies [2]. A special role in the investigation of the
stability of orbital motions is played by the so-called relative equilibria
[3], i.e. the trajectories of the system dynamics which are at the same time
one-parameter subgroups of the system invariance group. Nowadays their
stability is normally investigated using two similar approaches --
energy-momentum and energy-Casimir methods. The most suitable criterion for the
system stability investigation was formulated in the theorem of [4]; this
stability criterion successfully generalizes both the methods mentioned above
and covers the Hamiltonian formalism based on Poisson structures [1]. The
necessary and sufficient conditions for the circular orbit stability are
derived from this theorem.",1101.3237v1
2011-01-24,Detailed Analysis of Fan-Shaped jets in Three Dimensional Numerical Simulation,"We performed three dimensional resistive magnetohydrodynamic simulations to
study the magnetic reconnection using an initially shearing magnetic field
configuration (force free field with a current sheet in the middle of the
computational box). It is shown that there are two types of reconnection jets:
the ordinary reconnection jets and fan-shaped jets, which are formed along the
guide magnetic field. The fan-shaped jets are much different from the ordinary
reconnection jets which are ejected by magnetic tension force. There are two
driving forces for accelerating the fan-shaped jets. The one is the Lorentz
force which dominates the motion of fluid elements at first and then the gas
pressure gradient force accelerates the fluid elements in the later stage. The
dependence on magnetic reconnection angle and resistivity value has also been
studied. The formation and evolution of these jets provide a new understanding
of dynamic magnetohydrodynamic jets.",1101.4598v1
2011-02-12,Electrical control of ferromagnetism in Mn-doped semiconductor heterostructures,"The interplay of tunneling transport and carrier-mediated ferromagnetism in
narrow semiconductor multi-quantum well structures containing layers of GaMnAs
is investigated within a self-consistent Green's function approach, accounting
for disorder in the Mn--doped regions and unwanted spin-flips at
heterointerfaces on phenomenological ground. We find that the magnetization in
GaMnAs layers can be controlled by an external electric bias. The underlying
mechanism is identified as spin-selective hole tunneling in and out of the
Mn-doped quantum wells, whereby the applied bias determines both hole
population and spin polarization in these layers. In particular we predict
that, near resonance, ferromagnetic order in the Mn doped quantum wells is
destroyed. The interplay of both magnetic and transport properties combined
with structural design potentially leads to several interrelated physical
phenomena, such as dynamic spin filtering, electrical control of magnetization
in individual magnetic layers, and, under specific bias conditions, to
self-sustained current and magnetization oscillations
(magneticmulti-stability). Relevance to recent experimental results is
discussed.",1102.2507v2
2011-02-15,Non-equilibrium two-fluid plasmas can generate magnetic fields and flows simultaneously,"A new analytical solution of the set of highly nonlinear two-fluid equations
is presented to explain the mechanism for the generation of ""seed"" magnetic
field and plasma flow by assuming the density n to have a profile like an
exponential in xy-plane and temperature profiles of electrons (ions) to be
linear in yz-plane. {It is shown that the baroclinic vectors -
$\nabla\Psi\times\nabla T_{j}$ (where $\Psi = ln\bar{n}, \bar{n}$ is normalized
density, and $T_{j}$ denote the temperatures of electrons and ions for j = e,
i) can generate not only the magnetic field but the plasma flow as well.} It is
also pointed out that the electron magnetohydrodynamics (EMHD) model has
inconsistencies because it does not take into account the ion dynamics while
the magnetic field is produced on slow time scale. The estimate of the
magnitude of the magnetic field in a classical laser plasma using this model is
in agreement with the experimental observations.",1102.2968v1
2011-02-25,Mechanism of magnetic field effect in cryptochrome,"Creatures as varied as mammals, fish, insects, reptiles, and migratory birds
have an intriguing `sixth' sense that allows them to distinguish north from
south by using the Earth's intrinsic magnetic field. Yet despite decades of
study, the physical basis of this magnetic sense remains elusive. A likely
mechanism is furnished by magnetically sensitive radical pair reactions
occurring in the retina, the light-sensitive part of the eyes. A photoreceptor,
cryptochrome, has been suggested to endow birds with magnetoreceptive abilities
as the protein has been shown to exhibit the biophysical properties required
for an animal magnetoreceptor to operate properly. Here, we propose a concrete
light-driven reaction cycle in cryptochrome that lets a magnetic field
influence the signaling state of the photoreceptor. The reaction cycle ties
together transient absorption and electron-spin-resonance observations with
known facts on avian magnetoreception. Our analysis establishes the feasibility
of cryptochrome to act as a geomagnetic compass in birds, gives insight into
structural and dynamic design features required for optimal detection of the
geomagnetic field direction, and suggests further theoretical and experimental
studies.",1102.5359v2
2011-03-05,Injection and extraction magnets: septa,"An accelerator has limited dynamic range: a chain of accelerators is required
to reach high energy. A combination of septa and kicker magnets is frequently
used to inject and extract beam from each stage. The kicker magnets typically
produce rectangular field pulses with fast rise- and/or fall-times, however the
field strength is relatively low. To compensate for their relatively low field
strength, the kicker magnets are generally combined with electromagnetic septa.
The septa provide relatively strong field strength but are either DC or slow
pulsed. This paper discusses injection and extraction systems with particular
emphasis on the hardware required for the septa.",1103.1062v1
2011-03-19,SDO Observations of Magnetic Reconnection at Coronal Hole Boundaries,"With the observations from the Atmospheric Imaging Assembly (AIA) and the
Helioseismic and Magnetic Imager (HMI) aboard the \emph{Solar Dynamics
Observatory}, we investigate the coronal hole boundaries (CHBs) of an
equatorial extension of polar coronal hole. At the CHBs, lots of
extreme-ultraviolet (EUV) jets, which appear to be the signatures of magnetic
reconnection, are observed in the 193 {\AA} images, and some jets occur
repetitively at the same sites. The evolution of the jets is associated with
the emergence and cancelation of magnetic fields. We notice that both the east
and the west CHBs shift westward, and the shift velocities are close to the
velocities of rigid rotation compared with those of the photospheric
differential rotation. This indicates that magnetic reconnection at CHBs
results in the evolution of CHBs and maintains the rigid rotation of coronal
holes.",1103.3751v1
2011-03-26,Paraelectricity in Magnetized Massless QED,"We show that the chiral-symmetry-broken phase of massless QED in the presence
of a magnetic field exhibits strong paraelectricity. A large anisotropic
electric susceptibility develops in the infrared region, where most of the
fermions are confined to their lowest Landau level, and dynamical mass and
anomalous magnetic moment are generated via the magnetic catalysis mechanism.
The nonperturbative nature of this effect is reflected in the dependence of the
electric susceptibility on the fine-structure constant. The strong
paraelectricity is linked to the electric dipole moments of the
particle/anti-particle pairs that form the chiral condensate. The significant
electric susceptibility can be used as a probe to detect the realization of the
magnetic catalysis of chiral symmetry breaking in physical systems.",1103.5152v2
2011-04-27,Coupled spin models for magnetic variation of planets and stars,"Geomagnetism is characterized by intermittent polarity reversals and rapid
fluctuations. We have recently proposed a coupled macro-spin model to describe
these dynamics based on the idea that the whole dynamo mechanism is described
by the coherent interactions of many small dynamo elements. In this paper, we
further develop this idea and construct a minimal model for magnetic
variations. This simple model naturally yields many of the observed features of
geomagnetism: its time evolution, the power spectrum, the frequency
distribution of stable polarity periods, etc. This model has coexistent two
phases; i.e. the cluster phase which determines the global dipole magnetic
moment and the expanded phase which gives random perpetual perturbations that
yield intermittent polarity flip of the dipole moment. This model can also
describe the synchronization of the spin oscillation. This corresponds to the
case of sun and the model well describes the quasi-regular cycles of the solar
magnetism. Furthermore, by analyzing the relevant terms of MHD equation based
on our model, we have obtained a scaling relation for the magnetism for
planets, satellites, sun, and stars. Comparing it with various observations, we
can estimate the scale of the macro-spins.",1104.5093v1
2011-05-02,Chiral magnetic effect in lattice QCD with a chiral chemical potential,"We perform a first lattice QCD simulation including two-flavor dynamical
fermion with a chiral chemical potential. Because the chiral chemical potential
gives rise to no sign problem, we can exactly analyze a chirally imbalanced QCD
matter by the Monte Carlo simulation. By applying an external magnetic field to
this system, we obtain a finite induced current along the magnetic field, which
corresponds to the chiral magnetic effect. The obtained induced current is
proportional to the magnetic field and to the chiral chemical potential, which
is consistent with an analytical prediction.",1105.0385v3
2011-05-17,Magnetic Resonance Force Microscopy of paramagnetic electron spins at millikelvin temperatures,"Magnetic Resonance Force Microscopy (MRFM) is a powerful technique to detect
a small number of spins that relies on force-detection by an ultrasoft
magnetically tipped cantilever and selective magnetic resonance manipulation of
the spins. MRFM would greatly benefit from ultralow temperature operation,
because of lower thermomechanical noise and increased thermal spin
polarization. Here, we demonstrate MRFM operation at temperatures as low as 30
mK, thanks to a recently developed SQUID-based cantilever detection technique
which avoids cantilever overheating. In our experiment, we detect dangling bond
paramagnetic centers on a silicon surface down to millikelvin temperatures.
Fluctuations of such kind of defects are supposedly linked to 1/f magnetic
noise and decoherence in SQUIDs as well as in several superconducting and
single spin qubits. We find evidence that spin diffusion plays a key role in
the low temperature spin dynamics.",1105.3395v3
2011-05-30,Dynamics of rising magnetized cavities and UHECR acceleration in clusters of galaxies,"We study the expansion of low density cavities produced by Active Galactic
Nuclei jets in clusters of galaxies. The long term stability of these cavities
requires the presence of linked magnetic fields. We find solutions describing
the self-similar expansion of structures containing large-scale electromagnetic
fields. Unlike the force-free spheromak-like configurations, these solutions
have no surface currents and, thus, are less susceptible to resistive decay.
The cavities are internally confined by external pressure, with zero gradient
at the surface. If the adiabatic index of the plasma within the cavity is
$\Gamma>4/3$, the expansion ultimately leads to the formation of large-scale
current sheets. The resulting dissipation of the magnetic field can only
partially offset the adiabatic and radiative losses of radio emitting
electrons. We demonstrate that if the formation of large-scale current sheets
is accompanied by explosive reconnection of the magnetic field, the resulting
reconnection layer can accelerate cosmic rays to ultra high energies. We
speculate that the enhanced flux of UHECRs towards Centaurus A originates at
the cavities due to magnetic reconnection.",1105.6092v1
2011-07-01,"High frequency magnetic behavior through the magnetoimpedance effect in CoFeB/(Ta, Ag, Cu) multilayered ferromagnetic thin films","We studied the dynamics of magnetization through an investigation of the
magnetoimpedance effect in CoFeB/(Ta, Ag, Cu) multilayered thin films grown by
magnetron sputtering. Impedance measurements were analyzed in terms of the
mechanisms responsible for their variations at different frequency intervals
and the magnetic and structural properties of the multilayers. Analysis of the
mechanisms responsible for magnetoimpedance according to frequency and external
magnetic field showed that for the CoFeB/Cu multilayer, ferromagnetic resonance
(FMR) contributes significantly to the magnetoimpedance effect at frequencies
close to 470 MHz. This frequency is low when compared to the results obtained
for CoFeB/Ta and CoFeB/Ag multilayers and is a result of the anisotropy
distribution and non-formation of regular bilayers in this sample. The MImax
values occurred at different frequencies according to the used non-magnetic
metal. Variations between 25% and 30% were seen for a localized frequency band,
as in the case of CoFeB/Ta and CoFeB/Ag, as well as for a wide frequency range,
in the case of CoFeB/Cu.",1107.0204v1
2011-07-06,Cross-helicity effects and turbulent transport in magnetohydrodynamic flow,"In the presence of large-scale vortical motions and/or magnetic-field
strains, the turbulent cross helicity (velocity--magnetic-field correlation in
fluctuations) may contribute to the turbulent electromotive force and the
Reynolds stress. These effects of cross helicity are considered to balance the
primary effects of turbulence such as the turbulent magnetic diffusivity in
magnetic-field evolution and the eddy viscosity in the momentum transport. The
cross-helicity effects may suppress the enhanced transports due to turbulence.
Physical interpretation of the effects is presented with special emphasis on
the difference between the cross-helicity effect and the usual $\alpha$ or
helicity effect in the dynamo action. The relative importance of the
cross-helicity effect in dynamo action is validated with the aid of a direct
numerical simulation (DNS) of the Kolmogorov flow with an imposed magnetic
field. Several mechanisms that provide turbulence with the cross helicity are
also discussed.",1107.1154v1
2011-08-01,"Softening the ""crystal-scaffold"" for life's emergence","Del Giudice's group study how water can organize on hydrophilic surfaces
forming coherent domains (loaning energy from the quantum vacuum), plus
quasi-free electrons, whose excitations produce cold vortices, aligning to
ambient fields. Their electric and magnetic dipolar modes can couple to
oscillatory (electric-organic-dipoles), and/or rotary
(magnetic-mineral-dipoles), besides responding to magnetic potentials. Thus,
imprinted electromagnetic patterns of catalytic colloids - c.f. Cairns-Smith's
""crystal-scaffold""- on their structured water partners could have equipped the
latter with a selection-basis for 'choosing' their context-based ""soft-matter""
(de Gennes) replacements. We consider the potential of the scenario of an
external control on magnetic colloids forming in the Hadean hydrothermal
setting (of Russell and coworkers) - via a magnetic-rock-field - conceptually
enabling self-assembly, induction of asymmetries, response effects towards
close-to-equilibrium dynamics, associative-networks, besides providing a
coherent environment for stabilizing associated symmetry-broken quanta, and
their feedback-interactions with those of coherent water-domains, to address
the emergence of metabolism and replication.",1108.0309v2
2011-08-04,Energy and Angular Momentum Storage in a Rotating Magnet,"We consider a cylindrical metallic magnet that is set into rotation about a
horizontal axis by a falling mass. In such a system the magnetic field will
cause a radial current which is non-solenoidal. This leads to charge
accumulation and a partial attenuation of radial current. The magnetic field
acting on the radially flowing current slows down the acceleration. Using
newtonian dynamics we evaluate the angular velocity and displacement. We
explicitly show that the electromagnetic angular momentum must be taken into
consideration in order to account for the change in angular momentum due to the
external torque on the system. Further the loss in potential energy of the
falling mass can be accounted for only after taking into consideration the
electrostatic energy and the Joule loss. We suggest that this example will be
of pedagogical value to intermediate physics students. A version of this is
scheduled to appear in American Journal of Physics 2011.
  Keywords: Rotating magnet, Charge accumulation",1108.1231v1
2011-08-31,Quantum impurity coupled to Majorana edge fermions,"We study a quantum impurity coupled to the edge states of a two-dimensional
helical topological superconductor, i.e., to a pair of counterpropagating
Majorana fermion edge channels with opposite spin polarizations. For an
impurity described by the Anderson impurity model, we show that the problem
maps onto a variant of the interacting resonant two-level model which, in turn,
maps onto the ferromagnetic Kondo model. Both magnetic and non-magnetic
impurities are considered. For magnetic impurities, the bosonization and
numerical renormalization group analyses show that the system flows to a fixed
point with residual ln 2 entropy and we find characteristically anisotropic
static and dynamic impurity magnetic susceptibilities. For non-magnetic
impurities, the system flows to a fixed point with no residual entropy and we
find diamagnetic response at low temperatures. We comment on the
Schrieffer-Wolff transformation for problems with non-standard conduction band
continua and on the issues related to the differences in describing the
impurities by either Anderson or Kondo impurity models.",1108.6142v2
2011-09-06,Detection of negative effective magnetic pressure instability in turbulence simulations,"We present the first demonstration of the negative effective magnetic
pressure instability in direct numerical simulations of stably stratified,
externally forced, isothermal hydromagnetic turbulence in the regime of large
plasma beta. By the action of this instability, an initially uniform horizontal
magnetic field forms flux concentrations whose scale is large compared to the
turbulent scale. We further show that the magnetic energy of these large-scale
structures is only weakly dependent on the magnetic Reynolds number, provided
its value is large enough for the instability to be excited. Our results
support earlier mean-field calculations and analytic work which identified this
instability. Applications to the formation of active regions in the Sun are
discussed.",1109.1270v3
2011-09-13,Interplay between non equilibrium and equilibrium spin torque using synthetic ferrimagnets,"We discuss the current induced magnetization dynamics of spin valves F0|N|SyF
where the free layer is a synthetic ferrimagnet SyF made of two ferromagnetic
layers F1 and F2 coupled by RKKY exchange coupling. In the interesting
situation where the magnetic moment of the outer layer F2 dominates the
magnetization of the ferrimagnet, we find that the sign of the effective spin
torque exerted on the free middle layer F1 is controlled by the strength of the
RKKY coupling: for weak coupling one recovers the usual situation where spin
torque tends to, say, anti-align the magnetization of F1 with respect to the
pinned layer F0. However for large coupling the situation is reversed and the
spin torque tends to align F1 with respect to F0. Careful numerical simulations
in the intermediate coupling regime reveal that the competition between these
two incompatible limits leads generically to spin torque oscillator (STO)
behavior. The STO is found in the absence of magnetic field, with very
significant amplitude of oscillations and frequencies up to 50 GHz or higher.",1109.2705v2
2011-09-19,Axial dipolar dynamo action in the Taylor-Green vortex,"We present a numerical study of the magnetic field generated by the
Taylor-Green vortex. We show that periodic boundary conditions can be used to
mimic realistic boundary conditions by prescribing the symmetries of the
velocity and magnetic fields. This gives insight in some problems of central
interest for dynamos: the possible effect of velocity fluctuations on the
dynamo threshold, the role of boundary conditions on the threshold and on the
geometry of the magnetic field generated by dynamo action. In particular, we
show that an axial dipolar dynamo similar to the one observed in a recent
experiment can be obtained with an appropriate choice of the symmetries of the
magnetic field. The nonlinear saturation is studied and a simple model
explaining the magnetic Prandtl number dependence of the super/sub critical
nature of the dynamo transition is given.",1109.4063v2
2011-09-22,Universal characteristics of ion-acoustic wave dynamics in magnetized plasmas with emphasis on Tsallis distribution,"Using the extended Poincar\'{e}-Lighthill-Kuo (PLK) reductive perturbation
method, which incorporates the phase-shift variations, it is shown that common
features on propagation and head-on collisions of ion-acoustic waves exist for
a magnetized plasmas of different inertial-less particle distributions. For
instance it is remarked that, the soliton amplitude is always independent of
magnetic field strength while strictly depends on its angle regarding the
propagation direction. Both types of solitons (compressive or rarefactive) are
shown to exist which are defined through the critical angle $\gamma=\pi/2$ or
other critical values depending on plasma fractional parameters. These critical
plasma parameter values also define the sign of head-on collision phase shift.
Furthermore, it is proved that for a given set of plasma parameters there is
always a relative angle of propagation regarding to that of the magnetic-field
for which the soliton width is maximum. Current findings applies to a wide
range of magnetized plasmas including those containing background dust
ingredients or two-temperature inertial-less particles and may be used to study
laboratory or astrophysical magnetoplasmas.",1109.4746v1
2011-09-24,Interaction of a highly magnetized impulsive relativistic flow with an external medium,"Important astrophysical sources, such as gamma-ray bursts (GRBs) or tidal
disruption events, are impulsive -- strongly varying with time. These outflows
are likely highly magnetized near the central source, but their interaction
with the external medium is not yet fully understood. Here I consider the
combined impulsive magnetic acceleration of an initially highly magnetized
shell of plasma and its deceleration by the external medium. I find four main
dynamical regimes, that (for a given outflow) depend on the external density.
(I) For small enough external densities the shell becomes kinetically dominated
before it is significantly decelerated, thus reverting to the familiar
unmagnetized ""thin shell"" case, which produces bright reverse shock emission
that peaks well after the prompt GRB. (II) For larger external densities the
shell remains highly magnetized and the reverse shock is strongly suppressed.
It eventually transfers most of its energy through pdV work to the shocked
external medium, whose afterglow emission peaks on a timescale similar to the
prompt GRB duration. (III) For even larger external densities there is no
initial impulsive acceleration phase. (IV) For the highest external densities
the flow remains Newtonian.",1109.5314v2
2011-10-16,Spin Torque in Anisotropic Tunneling Junctions,"Spin transport in magnetic tunnel junctions comprising a single magnetic
layer in the presence of interfacial spin-orbit interaction (SOI) is
investigated theoretically. Due to the presence of interfacial SOI, a
current-driven spin torque can be generated at the second order in SOI, even in
the absence of an external spin polarizer. This torque possesses two
components, in-plane and perpendicular to the plane of rotation, that can
induce either current-driven {\em magnetization switching} from in-plane to
out-of-plane configuration or {\em magnetization precessions}, similarly to
Spin Transfer Torque in spin-valves. Consequently, it appears that it is
possible to control the magnetization steady state and dynamics by either
varying the bias voltage or electrically modifying the SOI at the interface.",1110.3488v1
2011-11-04,Magnetospheres of massive stars across the EM spectrum,"Magnetic massive stars -- which are being discovered with increasing
frequency -- represent a new category of wind-shaping mechanism for O and B
stars. Magnetic channeling of these stars' radiation-driven winds, the
Magnetically Confined Wind Shock paradigm, leads to the formation of a
shock-heated magnetosphere, which can radiate X-rays, modify UV resonance
lines, and create disks of Halpha emitting material. The dynamical properties
of these magnetospheres are well understood from a theoretical point of view as
an interplay between the magnetic wind confinement and rotation. However, the
manifestations of magnetospheres across the spectrum may be more complex and
various than first anticipated. On the other hand, recent advances in modeling
these magnetospheres provide a key to better understand massive star winds in
general. We will summarize the coordinated observational, theoretical, and
modeling efforts from the Magnetism in Massive Star Project, addressing key
outstanding questions regarding magnetosphere manifestations across the
spectral domain.",1111.1238v1
2011-12-01,Dangling bonds and magnetism of grain boundaries in graphene,"Grain boundaries with dangling bonds (DBGB) in graphene are studied by
atomistic Monte Carlo and molecular dynamics simulations in combination with
density functional (SIESTA) calculations. The most stable configurations are
selected and their structure is analyzed in terms of grain boundary
dislocations. It is shown that the grain boundary dislocation with the core
consisting of pentagon, octagon and heptagon (5-8-7 defect) is a typical
structural element of DBGB with relatively low energies. Electron energy
spectrum and magnetic properties of the obtained DBGB are studied by density
functional calculations. It is shown that the 5-8-7 defect is magnetic and that
its magnetic moment survives after hydrogenation. The effects of hydrogenation
and of out of plane deformations on the magnetic properties of DBGB are
studied.",1112.0160v2
2011-12-05,Magnetic anisotropy of elongated thin ferromagnetic nano-islands for artifical spin ice arrays,"The energetics of thin elongated ferromagnetic nano-islands is considered for
some different shapes, aspect ratios, and applied magnetic field directions.
These nano-island particles are important for artificial spin-ice materials.
For low temperature, the magnetic internal energy of an individual particle is
evaluated numerically as a function of the direction of a particle's net
magnetization. This leads to estimations of effective anisotropy constants for
(1) the easy axis along the particle's long direction, and (2) the hard axis
along the particle's thin direction. A spin relaxation algorithm together with
fast Fourier transform for the demagnetization field is used to solve the
micromagnetics problem for a thin system. The magnetic hysteresis is also
found. The results indicate some possibilities for controlling the equilibrium
and dynamics in spin-ice materials by using different island geometries.",1112.0913v2
2011-12-08,Magnetic fields and gas flows around circumnuclear starbursts,"Radio continuum observations of barred galaxies revealed strong magnetic
fields of >= 50-100 muG in the circumnuclear starbursts. Such fields are
dynamically important and give rise to magnetic stress that causes inflow of
gas towards the center at a rate of several solar masses per year, possibly
along the spiral field seen in radio polarization and as optical dust lanes.
This may solve the long-standing question of how to feed active nuclei, and
explain the relation between the bolometric luminosity of AGN nuclei and the
star-formation rate of their hosts. The strong magnetic fields generated in
young galaxies may serve as the link between star formation and accretion onto
supermassive black holes. -- Magnetic fields of >= 160 muG strength were
measured in the central region of the almost edge-on starburst galaxy NGC 253.
Four filaments emerging from the inner disk delineate the boundaries of the
central outflow cone of hot gas. Strong Faraday rotation of the polarized
emission from the background disk indicates a large-scale helical field in the
outflow walls.",1112.1823v1
2011-12-12,Magnetic and Kinetic Power Spectra as a Tool to Probe the Turbulent Dynamo,"Generation and diffusion of the magnetic field on the Sun is a key mechanism
responsible for solar activity on all spatial and temporal scales - from the
solar cycle down to the evolution of small-scale magnetic elements in the quiet
Sun. The solar dynamo operates as a non-linear dynamical process and is thought
to be manifest in two types: as a global dynamo responsible for the solar cycle
periodicity, and as a small-scale turbulent dynamo responsible for the
formation of magnetic carpet in the quiet Sun. Numerous MHD simulations of the
solar turbulence did not yet reach a consensus as to the existence of a
turbulent dynamo on the Sun. At the same time, high-resolution observations of
the quiet Sun from Hinode instruments suggest possibilities for the turbulent
dynamo. Analysis of characteristics of turbulence derived from observations
would be beneficial in tackling the problem. We analyse magnetic and velocity
energy spectra as derived from Hinode/SOT, SOHO/MDI, SDO/HMI and the New Solar
Telescope (NST) of Big Bear Solar Observatory (BBSO) to explore the
possibilities for the small-scale turbulent dynamo in the quiet Sun.",1112.2750v1
2011-12-20,In-plane magnetic anisotropy of Fe atoms on Bi$_2$Se$_3$(111),"The robustness of the gapless topological surface state hosted by a 3D
topological insulator against perturbations of magnetic origin has been the
focus of recent investigations. We present a comprehensive study of the
magnetic properties of Fe impurities on a prototypical 3D topological insulator
Bi$_2$Se$_3$ using local low temperature scanning tunneling microscopy and
integral x-ray magnetic circular dichroism techniques. Single Fe adatoms on the
Bi$_2$Se$_3$ surface, in the coverage range $\approx 1%$ are heavily relaxed
into the surface and exhibit a magnetic easy axis within the surface-plane,
contrary to what was assumed in recent investigations on the opening of a gap.
Using \textit{ab initio} approaches, we demonstrate that an in-plane easy axis
arises from the combination of the crystal field and dynamic hybridization
effects.",1112.4621v2
2011-12-23,Temperature gradient assisted magnetodynamics in a ferromagnetic nanowire,"The dynamics of the low energy excitations in a ferromagnet is studied in
case a temperature gradient is coupled to the local magnetization. Due to the
different time scales of changing temperature and magnetization it is argued
that only the coupling between the spatially varying part of the temperature
field and the magnetization is relevant. Using variational principles the
evolution equation for the magnetic system is found which is strongly
influenced by the local temperature profile. The system offers damped spin wave
excitations where the strength of damping is determined by the magneto-thermal
coupling. Applying the model to nanowires it is demonstrated that the energy
spectrum is significantly affected by the boundary conditions as well as the
initial temperature distribution. In particular, the coupling between
temperature and magnetization is expected to be several orders stronger for the
open as for the isolated wire.",1112.5562v1
2012-01-02,Localized domain-wall excitations in patterned magnetic dots probed by broadband ferromagnetic resonance,"We investigate the magnetization dynamics in circular Permalloy dots with
spatially separated magnetic vortices interconnected by domain walls (double
vortex state). We identify a novel type of quasi one-dimensional (1D) localised
spin wave modes confined along domain walls, connecting each of two vortex
cores with two edge half-antivortices. Variation of the mode eigenfrequencies
with the dot size is in quantitative agreement with the developed model, which
considers a dipolar origin of the localized 1D spin waves or so-called
Winter\'s magnons [J.M. Winter, Phys.Rev. 124, 452 (1961)]. These spin waves
are analogous to the displacement waves of strings, and could be excited in a
wide class of patterned magnetic nanostructures possessing domain walls, namely
in triangular, square, circular or elliptic magnetic dots.",1201.0482v1
2012-01-09,"""Skew"" scattering of cold unpolarized neutrons in ferromagnetic crystal","The problem of neutron scattering by the single magnetic atom is
theoretically considered in the second order perturbation theory. It is
demonstrated that elastic scattering of unpolarized neutron by magnetic atom is
skewed, i.e. contains the term with the symmetry of mixed product of the atom
magnetic moment and wave vectors of incident and scattered neutrons. The
problem of dynamical diffraction of unpolarized neutrons by the perfect
ferromagnetic crystal is investigated. The case is considered when the Bragg
condition is satisfied for two reciprocal lattice vectors. In this case neutron
skew scattering manifests itself as the dependence of the diffracted beam
intensity on the sign of the crystal magnetization. The calculations have been
done for the diffraction of unpolarized neutrons by Dy crystal. In case of the
diffraction by Dy the change of the intensity under the magnetization reversal
achieves 50%.",1201.1758v2
2012-01-27,Fermi Surface Reconstruction by Dynamic Magnetic Fluctuations,"We demonstrate that nearly critical quantum magnetic fluctuations in strongly
correlated electron systems can change the Fermi surface topology and also lead
to spin charge separation (SCS) in two dimensions. To demonstrate these effects
we consider a small number of holes injected into the bilayer antiferromagnet.
The system has a quantum critical point (QCP) which separates magnetically
ordered and disordered phases. We demonstrate that in the physically
interesting regime there is a magnetically driven Lifshitz point (LP) inside
the magnetically disordered phase. At the LP the topology of the hole Fermi
surface is changed. We also demonstrate that in this regime the hole spin and
charge necessarily separate when approaching the QCP. The considered model
sheds light on generic problems concerning the physics of the cuprates.",1201.5700v2
2012-02-10,A model of magnetic order in hexagonal HoMnO3,"Symmetry arguments are used to develop a spin Hamiltonian for the description
of the complex magnetic ordering in HoMnO$_3$. Using a novel application of the
Landau Lifshitz Gilbert dynamic torque equations to this model of the
frustrated Mn ions on an $AB$ stacked triangular antiferromagnetic, it is shown
that the four principal spin configurations observed in this compound are
stabilized. Ho-Mn coupling is found to be a consequence of an unusual trigonal
anisotropy term which is responsible for simultaneous Mn spin reorientation and
onset of Ho magnetic order. Based on these microscopic considerations, a
mean-field Landau-type free energy is derived which reproduces the succession
of observed temperature driven magnetic phase transitions at zero field,
including re-entrant behavior. In addition, our analysis suggests that the
basal-plane magnetic order should be slightly incommensurate with the lattice.",1202.2321v1
2012-02-10,Conceptual designs of dipole magnet for muon collider ring,"Conceptual designs of a superconducting dipole magnet for a Storage Ring of a
Muon Collider with a 1.5 TeV center of mass (c.o.m.) energy and an average
luminosity of 10 34 cm-2s-1 are presented. In contrast to proton machines, the
dipoles for the Muon Collider should be able to handle ~0.5 kW/m of dynamic
heat load from the muon beam decays. The magnets are based on Nb3Sn
superconductor and designed to provide an operating field of 10 T in the 20-mm
aperture with the critical current margin required for reliable machine
operation. The magnet cross-sections were optimized to achieve the best
possible field quality in the aperture occupied by beams. The developed
mechanical structures provide adequate coil prestress and support at the
maximum level of Lorentz forces in the coil. Magnet parameters are reported and
compared with the requirements.",1202.2380v1
2012-03-02,Investigation of Optical Spin Transfer Torque in Ferromagnetic Semiconductor GaMnAs by Magneto-Optical Pump-and-Probe Method,"We report on magnetization precession induced in (Ga,Mn)As by an optical spin
transfer torque (OSTT). This phenomenon, which corresponds to a transfer of
angular momentum from optically-injected spin-polarized electrons to
magnetization, was predicted theoretically in 2004 and observed experimentally
by our group in 2012. In this paper we provide experimental details about the
observation of OSTT by a time-resolved pump-and-probe magneto-optical
technique. In particular, we show that the precession of magnetization due to
OSTT can be experimentally separated from that induced by the well known
magnetic-anisotropy-related mechanism in a hybrid structure
piezo-stressor/(Ga,Mn)As with an in situ electrical control of the magnetic
anisotropy. We also illustrate that OSTT is clearly apparent in the measured
dynamical magneto-optical signal in a large variety of (Ga,Mn)As samples with a
Mn concentration from 3% to 9%.",1203.0483v1
2012-03-07,Photon Self-Energy and Electric Susceptibility in a Magnetized Three-flavor Color Superconductor,"We study the photon self-energy for the in-medium photon in a three-flavor
color superconductor in the presence of a magnetic field. At strong magnetic
field, the quark dynamics becomes $(1+1)$-dimensional and the self-energy
tensor only has longitudinal components. In this approximation there is no
Debye or Meissner screenings at zero temperature, but the electric
susceptibility is nonzero and highly anisotropic. In the direction transverse
to the applied field, the electric susceptibility is the same as in vacuum,
while in the longitudinal direction it depends on the magnitude of the magnetic
field. Such a behavior is a realization in cold-dense QCD of the
magnetoelectric effect, which was first discovered in condensed matter physics.
The magnetic permeability remains equal to that in vacuum for both transverse
and longitudinal components. We discuss the importance of the Pauli-Villars
regularization to get meaningful physical results in the infrared limit of the
polarization operator. We also find the covariant form of the polarization
operator in the reduced (1+1)-D space of the lowest Landau level and proves its
transversality.",1203.1630v1
2012-03-20,Magnetic catalysis in hot and dense quark matter and quantum fluctuations,"We analyze chiral symmetry breaking in quark matter in an external magnetic
field at zero and finite temperature and quark chemical potential. We first
give a brief overview of analytic results within the mean-field approximation.
There the critical temperature for chiral restoration is increased by the
magnetic field effect. Then we investigate the effects of matter and quantum
fluctuations on the Magnetic Catalysis. More specifically, we compute the
critical coupling as a function of the magnetic field and the temperature for
zero and finite quark chemical potential in the presence of quantum
fluctuations. As soon as a non-zero temperature and/or density is turned on,
long-range correlations are screened and the critical coupling is no longer
vanishing. We extend our dynamical results beyond the leading-order bubble
resummation which results in a non-local four-Fermi coupling. This includes
in-medium meson effects on the more quantitative level.",1203.4330v1
2012-04-12,Evidence for an exotic magnetic transition in the triangular spin system FeGa2S4,"We report positive muon spin relaxation measurements on the triangular
lattice magnetic system FeGa2S4. A magnetic transition not previously detected
by specific heat and magnetic susceptibility measurements is found in zero
field at T^* \simeq 30 K. It is observed through the temperature dependencies
of the signal amplitude and the spin-lattice relaxation rate. This transition
is therefore not a conventional magnetic phase transition. Since persistent
spin dynamics is observed down to 0.1 K, the ground state cannot be of the
canonical spin-glass type, which could be suggested from hysteresis effects in
the bulk susceptibility below T_f \simeq 16 K. These results are compared to
those found for the isomorph NiGa2S4. It is argued that the fate of the
transition, which has been interpreted in terms of the Z_2 topological
transition in this latter system, is probably different in FeGa2S4.",1204.2739v1
2012-05-02,Heavy-Fermion Instability in Double-Degenerate Plasmas,"In this work we study the propagations of normal frequency modes for quantum
hydrodynamic (QHD) waves in the linear limit and introduce a new kind of
instability in a double-degenerate plasma. Three different regimes, namely,
low, intermediate and high magnetic field strengths are considered which span
the applicability of the work to a wide variety of environments. Distinct
behavior is observed for different regimes, for instance, in the
laboratory-scale field regime no frequency-mode instability occurs unlike those
of intermediate and high magnetic-field strength regimes. It is also found that
the instability of this kind is due to the heavy-fermions which appear below a
critical effective-mass parameter ($\mu_{cr}=\sqrt{3}$) and that the responses
of the two (lower and upper frequency) modes to fractional effective-mass
change in different effective-mass parameter ranges (below and above the
critical value) are quite opposite to each other. It is shown that, the
heavy-fermion instability due to extremely high magnetic field such as that
encountered for a neutron-star crust can lead to confinement of stable
propagations in both lower and upper frequency modes to the magnetic poles.
Current study can have important implications for linear wave dynamics in both
laboratory and astrophysical environments possessing high magnetic fields.",1205.0566v1
2012-05-16,Exact Nonlinear Excitations in Double-Degenerate Plasmas,"In this work we use the conventional hydrodynamics (HD) formalism and
incorporate the Chew-Goldberger-Low (CGL) double-adiabatic theory to evaluate
the nonlinear electrostatic ion excitations in double-degenerate (electron
spin-orbit degenerate) magnetized quantum plasmas. Based on the Sagdeev
pseudopotential method an exact general pseudopotential is calculated which
leads to the allowed Mach-number range criteria for such localized density
structures in an anisotropic magnetized plasma. We employ the criteria on the
Mach-number range for diverse magnetized quantums plasma with different
equations of state (EoS). It is remarked that various plasma fractional
parameters such as the system dimensionality, ion-temperature,
relativistic-degeneracy, Zeeman-energy, and plasma composition are involved in
the stability of an obliquely propagating nonlinear ion-acoustic wave in a
double-degenerate quantum plasma. Current study is most appropriate for
nonlinear wave analysis in the dense astrophysical magnetized plasma
environments such as white-dwarfs and neutron-star crusts where the strong
magnetic fields can be present.",1205.3818v1
2012-06-09,Spin-Electromagnetic Hydrodynamics and Magnetization Induced by Spin-Magnetic Interaction,"The hydrodynamic model including the spin degree of freedom and the
electromagnetic field was discussed. In this derivation, we applied
electromagnetism for macroscopic medium proposed by Minkowski. For the equation
of motion of spin, we assumed that the hydrodynamic representation of the Pauli
equation is reproduced when the many-body effect is neglected. Then the
spin-magnetic interaction in the Pauli equation was converted to a part of the
magnetization. The fluid and spin stress tensors induced by the many-body
effect were obtained by employing the algebraic positivity of the entropy
production in the framework of the linear irreversible thermodynamics,
including the mixing effect of the irreversible currents. We further
constructed the constitutive equation of the polarization and the
magnetization. Our polarization equation is more reasonable compared to another
result obtained using electromagnetism for macroscopic medium proposed by de
Groot-Mazur.",1206.1976v3
2012-06-11,Enhanced Dissipation Rate of Magnetic Field in Striped Pulsar Winds by the Effect of Turbulence,"In this letter we report on turbulent acceleration of the dissipation of
magnetic field in the postshock re- gion of a Poynting flux-dominated flow,
such as the Crab pulsar wind nebula. We have performed two- dimensional
resistive relativistic magnetohydrodynamics simulations of subsonic turbulence
driven by the Richtmyer-Meshkov instability at the shock fronts of the Poynting
flux-dominated flows in pulsar winds. We find that turbulence stretches current
sheets which substantially enhances the dissipation of magnetic field, and that
most of the initial magnetic field energy is dissipated within a few
eddy-turnover times. We also develop a simple analytical model for turbulent
dissipation of magnetic field that agrees well with our simulations. The
analytical model indicates that the dissipation rate does not depend on
resistivity even in the small resistivity limit. Our findings can possibly
alleviate the {\sigma}-problem in the Crab pulsar wind nebulae.",1206.2087v1
2012-06-25,A mechanism to pin skyrmions in chiral magnets,"We propose a mechanism to pin skyrmions in chiral magnets by introducing
local maximum of magnetic exchange strength, which can be realized in chiral
magnetic thin films by engineering the local density of itinerate electrons.
Thus we find a way to artificially control the position of a single skyrmion in
chiral magnetic thin films. The stationary properties and the dynamical pinning
and depinning processes of an isolated skyrmion around a pinning center are
studied. We do a series of simulations to show that the critical current to
depin a skyrmion has linearly dependence on the pinning strength. We also
estimate the critical current to have order of magnitude
10^{7}\sim10^{8}A/m^{2} .",1206.5661v1
2012-08-31,"Influence of Mg, Ag and Al substitutions on the magnetic excitations in the triangular-lattice antiferromagnet CuCrO2","Magnetic excitations in CuCrO$_{2}$, CuCr$_{0.97}$Mg$_{0.03}$O$_{2}$,
Cu$_{0.85}$Ag$_{0.15}$CrO$_{2}$, and CuCr$_{0.85}$Al$_{0.15}$O$_{2}$ have been
studied by powder inelastic neutron scattering to elucidate the element
substitution effects on the spin dynamics in the Heisenberg triangular-lattice
antiferromagnet CuCrO$_{2}$. The magnetic excitations in
CuCr$_{0.97}$Mg$_{0.03}$O$_{2}$ consist of a dispersive component and a flat
component. Though this feature is apparently similar to CuCrO$_{2}$, the energy
structure of the excitation spectrum shows some difference from that in
CuCrO$_{2}$. On the other hand, in Cu$_{0.85}$Ag$_{0.15}$CrO$_{2}$ and
CuCr$_{0.85}$Al$_{0.15}$O$_{2}$ the flat components are much reduced, the
low-energy parts of the excitation spectra become intense, and additional
low-energy diffusive spin fluctuations are induced. We argued the origins of
these changes in the magnetic excitations are ascribed to effects of the doped
holes or change of the dimensionality in the magnetic correlations.",1208.6353v1
2012-09-24,Transport blocking and topological phases using ac magnetic fields,"We analyze electron dynamics and topological properties of open double
quantum dots (DQDs) driven by circularly polarized ac-magnetic fields. In
particular we focus on the system symmetries which can be tuned by the
ac-magnetic field. Remarkably, we show that in the electron spin resonance
(ESR) configuration, where the magnetic fields in each dot oscillate with a
phase difference of $\pi$, charge localization occurs giving rise to transport
blocking at arbitrary intensities of the ac field. The conditions for charge
localization are obtained by means of Floquet theory and related with
quasienergies degeneracy. We also demonstrate that a topological phase
transition can be induced in the adiabatic regime for a phase difference of
$\pi$, either by tuning the coupling between dots or by modifying the intensity
of the driving magnetic field.",1209.5301v1
2012-10-08,Inelastic magnetic scattering effect on LDOS of topological insulators,"Magnetic ions such as Fe, Mn and Co with localized spins may be adsorbed on
the surface of topological insulators like \Bi. They form scattering centers
for the helical surface states which have a Dirac cone dispersion as long as
the local spins are disordered. However, the local density of states (LDOS) may
be severely modified by the formation of bound states. Commonly only elastic
scattering due to normal and exchange potentials of the adatom is assumed.
Magnetization measurements show, however, that considerable magnetic single ion
anisotropies exist which lead to a splitting of the local impurity spin states
resulting in a singlet ground state. Therefore inelastic scattering processes
of helical Dirac electrons become possible as described by a dynamical local
self energy of second order in the exchange interaction. The self energy
influences bound state formation and leads to significant new anomalies in the
LDOS at low energies and low temperatures which we calculate within T-matrix
approach. We propose that they may be used for spectroscopy of local impurity
spin states by appropriate tuning of chemical potential and magnetic field.",1210.2222v2
2012-10-10,On the magnetic quenching of mean-field effects in supersonic interstellar turbulence,"The emergence of large-scale magnetic fields observed in the diffuse
interstellar medium is explained by a turbulent dynamo. The underlying
transport coefficients have previously been extracted from numerical
simulations. So far, this was restricted to the kinematic regime, but we aim to
extend our analysis into the realm of dynamically important fields. This marks
an important step on which derived mean-field models rely to explain observed
equipartition strength fields. As in previous work, we diagnose turbulent
transport coefficients by means of the test-field method. We derive quenching
functions for the dynamo {\alpha} effect, diamagnetic pumping, and turbulent
diffusivity, which are compared with theoretical expectations. At late times,
we observe the suppression of the vertical wind. Because this potentially
affects the removal of small-scale magnetic helicity, new concerns arise about
circumventing constraints imposed by the conservation of magnetic helicity at
high magnetic Reynolds numbers. While present results cannot safely rule out
this possibility, the issue only becomes important at late stages and is absent
when the dynamo is quenched by the wind itself.",1210.2928v1
2012-10-24,Coherent structures and the saturation of a nonlinear dynamo,"Eulerian and Lagrangian tools are used to detect coherent structures in the
velocity and magnetic fields of a mean--field dynamo, produced by direct
numerical simulations of the three--dimensional compressible
magnetohydrodynamic equations with an isotropic helical forcing and moderate
Reynolds number. Two distinct stages of the dynamo are studied, the kinematic
stage, where a seed magnetic field undergoes exponential growth, and the
saturated regime. It is shown that the Lagrangian analysis detects structures
with greater detail, besides providing information on the chaotic mixing
properties of the flow and the magnetic fields. The traditional way of
detecting Lagrangian coherent structures using finite--time Lyapunov exponents
is compared with a recently developed method called function M. The latter is
shown to produce clearer pictures which readily permit the identification of
hyperbolic regions in the magnetic field, where chaotic transport/dispersion of
magnetic field lines is highly enhanced.",1210.6637v1
2012-11-19,Symmetry breaking for ratchet transport in presence of interactions and magnetic field,"We study the microwave induced ratchet transport of two-dimensional electrons
on an oriented semidisk Galton board. The magnetic field symmetries of ratchet
transport are analyzed in presence of electron-electron interactions. Our
results show that a magnetic field asymmetric ratchet current can appear due to
two contributions, a Hall drift of the rectified current that depends only
weakly on electron-electron interactions and a breaking of the time reversal
symmetry due to the combined effects of interactions and magnetic field. In the
latter case, the asymmetry between positive and negative magnetic fields
vanishes in the weak interaction limit. We also discuss the recent experimental
results on ratchet transport in asymmetric nanostructures.",1211.4411v1
2012-11-22,Gravity induced evolution of a magnetized fermion gas with finite temperature,"We examine the near collapse dynamics of a self-gravitating magnetized
electron gas at finite temperature, taken as the source of a Bianchi-I
spacetime described by the Kasner metric. The set of Einstein-Maxwell field
equations reduces to a complete and self-consistent system of non-linear
autonomous ODE's. By considering a representative set of initial conditions,
the numerical solutions of this system show the gas collapsing into both,
isotropic (""point--like"") and anisotropic (""cigar-like"") singularities,
depending on the intensity of the magnetic field. We also examined the behavior
during the collapse stage of all relevant state and kinematic variables: the
temperature, the expansion scalar, the magnetic field, the magnetization and
energy density. We notice a significant qualitative difference in the behavior
of the gas for a range of temperatures between the values
$\hbox{T}\sim10^{3}\hbox{K}$ and $\hbox{T}\sim 10^{7}\hbox{K}$.",1211.5980v1
2012-11-28,Cosmic-ray-induced filamentation instability in collisionless shocks,"We used unprecedentedly large 2D and 3D hybrid (kinetic ions - fluid
electrons) simulations of non-relativistic collisionless strong shocks in order
to investigate the effects of self-consistently accelerated ions on the overall
shock dynamics. The current driven by suprathermal particles streaming ahead of
the shock excites modes transverse to the background magnetic field. The
Lorentz force induced by these self-amplified fields tends to excavate tubular,
underdense, magnetic-field-depleted cavities that are advected with the fluid
and perturb the shock surface, triggering downstream turbulent motions. These
motions further amplify the magnetic field, up to factors of 50-100 in
knot-like structures. Once downstream, the cavities tend to be filled by hot
plasma plumes that compress and stretch the magnetic fields in elongated
filaments; this effect is particularly evident if the shock propagates parallel
to the background field. Highly-magnetized knots and filaments may provide
explanations for the rapid X-ray variability observed in RX J1713.7-3946 and
for the regular pattern of X-ray bright stripes detected in Tycho's supernova
remnant.",1211.6765v2
2012-11-30,Sensitive Magnetic Control of Ensemble Nuclear Spin Hyperpolarisation in Diamond,"Dynamic nuclear polarisation, which transfers the spin polarisation of
electrons to nuclei, is routinely applied to enhance the sensitivity of nuclear
magnetic resonance; it is also critical in spintronics, particularly when spin
hyperpolarisation can be produced and controlled optically or electrically.
Here we show the complete polarisation of nuclei located near the
optically-polarised nitrogen-vacancy (NV) centre in diamond. When approaching
the ground-state level anti-crossing condition of the NV electron spins, 13C
nuclei in the first-shell are polarised in a pattern that depends sensitively
and sharply upon the magnetic field. Based on the anisotropy of the hyperfine
coupling and of the optical polarisation mechanism, we predict and observe a
complete reversal of the nuclear spin polarisation with a few-mT change in the
magnetic field. The demonstrated sensitive magnetic control of nuclear
polarisation at room temperature will be useful for sensitivity-enhanced NMR,
nuclear-based spintronics, and quantum computation in diamond.",1212.0035v1
2012-12-26,Epitaxial strain induced magnetic transitions and phonon instabilities of the tetragonal SrRuO3,"Using density-functional theory calculations, we investigate the magnetic as
well as the dynamical properties of tetragonal SrRuO3 (SRO) under the influence
of epitaxial strain. It is found that both the tensile and compressive strain
in the xy-plane could induce the abrupt change in the magnetic moment of Ru
atom. In particular, under the in-plane ~4% compressive strain, a ferromagnetic
to nonmagnetic transition is induced. Whereas for the tensile strain larger
than 3%, the Ru magnetic moment drops gradually with the increase of the
strain, exhibiting a weak ferromagnetic state. We find that such magnetic
transitions could be qualitatively explained by the Stoner model. In addition,
frozen phonon calculations at {\Gamma} point reveal structural instabilities
could occur under both compressive and tensile strains. Such instabilities are
very similar to those of the ferroelectric perovskite oxides, even though SRO
remains to be metallic in the range we studied. These might have influence on
the physical properties of oxide supercells taking SRO as constituent.",1212.6132v1
2013-02-17,Effect of Self-Magnetic Fields on the Nonlinear Dynamics of Relativistic Electron Beam with Virtual Cathode,"The report is devoted to the results of the numerical study of the virtual
cathode formation conditions in the relativistic electron beam under the
influence of the self-magnetic and external axial magnetic fields. The
azimuthal instability of the relativistic electron beam leading to the
formation of the vortex electron structure in the system was found out. This
instability is determined by the influence of the self-magnetic fields of the
relativistic electron beam and it leads to the decrease of the critical value
of the electron beam current (current when the non-stationary virtual cathode
is formed in the drift space). The typical dependencies of the critical current
on the external uniform magnetic field value were discovered. The effect of the
beam thickness on the virtual cathode formation conditions was also analyzed.",1302.4044v1
2013-02-27,Weak magnetic transitions in pyrochlore Bi2Ir2O7,"Muon spin relaxation measurements on Bi2Ir2O7 show that it undergoes a bulk
magnetic transition at 1.84(3) K. This is accompanied by increases in the muon
spin relaxation rate and the amplitude of the non-relaxing part of the signal.
The magnetic field experienced by muons is estimated to be 0.7 mT at
low-temperature, around two orders of magnitude smaller than that in other
pyrochlore iridates. These results suggest that the low-temperature state
represents ordering of exceptionally small magnetic moments with persistent
weak dynamics. The relaxation rate increases further below 0.23(4) K,
coincident with a growth in the specific heat, suggesting the existence of a
second low-temperature magnetic transition.",1302.6905v2
2013-03-04,Numerical Relativity Simulations of Magnetized Black Hole-Neutron Star Mergers,"We present new numerical techniques we developed for launching the first
parameter study of {\it magnetized} black hole--neutron star (BHNS) mergers,
varying the magnetic fields seeded in the initial neutron star. We found that
magnetic fields have a negligible impact on the gravitational waveforms and
bulk dynamics of the system during merger, regardless of magnetic field
strength or BH spin. In a recent simulation, we seeded the remnant disk from an
unmagnetized BHNS merger simulation with large-scale, purely poloidal magnetic
fields, which are otherwise absent in the full simulation. The outcome appears
to be a viable sGRB central engine.",1303.0837v1
2013-03-05,The Induced Electric Field Distribution in Solar Atmosphere,"A method of calculating induced electric field is presented in this paper.
Induced electric field in solar atmosphere is derived by the time variation of
magnetic field when the charged particle accumulation is neglected. In order to
get the spatial distribution of magnetic field, several extrapolation methods
are introduced. With observational data from Helioseismic and Magnetic Imager
(HMI) aboard the NASA's Solar Dynamics Observatory (SDO) on May 20th, 2010, we
extrapolate the magnetic field to the upper atmosphere from the photosphere. By
calculating the time variation of magnetic field, we can get the induced
electric field. The derived induced electric field can reach a value of 100
V/cm and the average electric field has a maximum point at the layer of 360 km
above the photosphere. The Monte Carlo statistics method is used to compute the
triple integration of induced electric field.",1303.0987v1
2013-03-07,Theory of atomistic simulation of spin-transfer torque in nanomagnets,"In spin-transfer torque (STT) for technological applications, the
miniaturization of the magnet may reach the stage of requiring a fully
quantum-mechanical treatment. We present an STT theory which uses the quantum
macrospin ground and excited (magnon) states of the nanomagnet. This allows for
energy and angular momentum exchanges between the current electron and the
nano-magnet. We develop a method of magnetization dynamics simulation which
captures the heating effect on the magnet by the spin-polarized current and the
temperature-dependence in STT. We also discuss the magnetostatics effect on
magnon scattering for ferromagnetic relaxation in a thin film. Our work
demonstrates a realistic step towards simulation of quantum spin-transfer
torque physics in nano-scale magnets.",1303.1775v1
2013-03-12,Global Simulations of Magnetorotational Instability in The Collapsed Core of A Massive Star,"We performed the first numerical simulations of magnetorotational instability
from a sub-magnetar-class seed magnetic field in core collapse supernovae. As a
result of axisymmetric ideal MHD simulations, we found that the magnetic field
is greatly amplified to magnetar-class strength. In saturation phase, a
substantial part of the core is dominated by turbulence, and the magnetic field
possesses dominant large scale components, comparable to the size of the
proto-neutron star. A pattern of coherent chanel flows, which generally appears
during exponential growth phase in previous local simulations, is not observed
in our global simulations. While the approximate convergence in the exponential
growth rate is attained by increasing spatial resolution, that of the
saturation magnetic field is not achieved due to still large numerical
diffusion. Although the effect of magnetic field on the dynamics is found to be
mild, a simulation with a high-enough resolution might result in a larger
impact.",1303.2856v1
2013-03-26,Enhanced pinning in high-temperature superconducting cuprate single crystals at low DC magnetic field,"Low density flux dynamics in Nd based cuprate (NdBa2Cu3O7-x) HTSC single
crystals trapped in 1 Oe range DC magnetic fields were investigated for the
first time. Complex measurements of time, temperature and field dependence of
magnetization were performed. Using the results obtained and the Anderson-Kim
model the normalized magnetization relaxation rate S and the averaged effective
pinning potential U were calculated. We show a significant (one or even two
orders) increase in the effective pinning potential in these samples in
comparison with measurements performed in high magnetic fields for similar
cuprate single crystals formerly. In a wide enough range of temperatures close
to Tc (0.72 < T/Tc < 0.93) it has been shown that the doping of NdBa2Cu3O7-x
single crystals with calcium atoms leads to the increase of the averaged
effective pinning potential with an insignificant decreasing of the critical
temperature.",1303.6501v1
2013-04-29,Polarimetric Properties of Flux-Ropes and Sheared Arcades in Coronal Prominence Cavities,"The coronal magnetic field is the primary driver of solar dynamic events.
Linear and circular polarization signals of certain infrared coronal emission
lines contain information about the magnetic field, and to access this
information, either a forward or an inversion method must be used. We study
three coronal magnetic configurations that are applicable to polar-crown
filament cavities by doing forward calculations to produce synthetic
polarization data. We analyze these forward data to determine the
distinguishing characteristics of each model. We conclude that it is possible
to distinguish between cylindrical flux ropes, spheromak flux ropes, and
sheared arcades using coronal polarization measurements. If one of these models
is found to be consistent with observational measurements, it will mean
positive identification of the magnetic morphology that surrounds certain
quiescent filaments, which will lead to a greater understanding of how they
form and why they erupt.",1304.7594v1
2013-06-10,Attosecond Magnetic Field Pulse Generation by Intense Few Cycle Circularly Polarized UV Pulses,"Intense attosecond magnetic field pulses are predicted to be produced by
intense few cycle circularly polarized UV pulses. Numerical solutions of the
time dependent Schr\""{o}dinger equation for H$_2^+$ are used to study the
dynamical process. Spiralling attosecond circular electron wave packets are
created with nanometer molecular dimensions, thus generating magnetic fields of
several tens of Teslas ($10^5$ Gauss). Simulations show that the induced
magnetic field is critically dependent on the pulse wavelength $\lambda$ and
pulse duration $n\tau$ ($n$ number of cycle) as predicted by a classical model.
For ultrashort few cycle circularly polarized attosecond pulses, molecular
orientation influences the generation of the induced magnetic fields as a
result of preferential ionization perpendicular to the molecular axis.",1306.2209v2
2013-06-12,Atomic spin-orbit coupling synthesized with magnetic-field-gradient pulses,"We discuss a general scheme for creating atomic spin-orbit coupling (SOC)
such as the Rashba or Dresselhaus types using magnetic-field-gradient pulses.
In contrast to conventional schemes based on adiabatic center-of-mass motion
with atomic internal states restricted to a dressed-state subspace, our scheme
works for the complete subspace of a hyperfine-spin manifold by utilizing the
coupling between the atomic magnetic moment and external magnetic fields. A
spatially dependent pulsed magnetic field acts as an internal-state-dependent
impulse, thereby coupling the atomic internal spin with its orbital
center-of-mass motion, as in the Einstein-de Haas effect. This effective
coupling can be dynamically manipulated to synthesize SOC of any type (Rashba,
Dresselhaus, or any linear combination thereof). Our scheme can be realized
with most experimental setups of ultracold atoms and is especially suited for
atoms with zero nuclear spins.",1306.2829v1
2013-06-20,Ultracold molecular collisions in combined electric and magnetic fields,"We consider collisions of electric and magnetic polar molecules, taking the
OH radical as an example, subject to combined electric and magnetic static
fields. We show that the relative orientation of the fields has an important
effect on the collision processes for different fields magnitude at different
collision energies. This is due to the way the molecules polarize in the
combined electric and magnetic fields and hence the way the electric
dipole-dipole interaction rises. If OH molecules are confined in magnetic
quadrupole traps and if an electric field is applied, molecular collisions will
strongly depend on the position as well as the velocity of the molecules, and
consequences on the molecular dynamics are discussed.",1306.4846v1
2013-06-21,Magnetic properties of a family of quinternary oxalates,"We report on the magnetic properties of four isomorphous compounds of a
family of quinternary oxalates down to 60 mK. In all these materials, the
magnetic FeII ions with a strong magneto-crystalline anisotropy form a
distorted kagome lattice, topologically equivalent to a perfect kagome one if
nearest-neighbor interactions only are considered. All the compounds order at
low temperature in an antiferromagnetic arrangement with magnetic moments at
120{\deg}. A remarkable magnetic behavior emerges below the N\'eel temperature
in three compounds (with inter-kagome-layer Zr, Sn, Fe but not with Al): the
spin anisotropy combined with a low exchange path network connectivity lead to
domain walls intersecting the kagome planes through strings of free spins.
These produce an unfamiliar slow spin dynamics in the ordered phase observed by
AC susceptibility, evolving from exchange-released spin-flips towards a
cooperative behavior on decreasing the temperature.",1306.5188v1
2013-07-10,RF amplification property of the MgO-based magnetic tunnel junction using field-induced ferromagnetic resonance,"The radio-frequency (RF) voltage amplification property of a tunnel
magnetoresistance device driven by an RF external-magnetic-field-induced
ferromagnetic resonance was studied. The proposed device consists of a magnetic
tunnel junction (MTJ) and an electrically isolated coplanar waveguide. The
input RF voltage applied to the waveguide can excite the resonant dynamics in
the free layer magnetization, leading to the generation of an output RF voltage
under a DC bias current. The dependences of the RF voltage gain on the static
external magnetic field strength and angle were systematically investigated.
The design principles for the enhancement of the gain factor are also
discussed.",1307.2745v1
2013-07-15,Extracting scaling laws from numerical dynamo models,"Earth's magnetic field is generated by processes in the electrically
conducting, liquid outer core, subsumed under the term `geodynamo'. In the last
decades, great effort has been put into the numerical simulation of core
dynamics following from the magnetohydrodynamic (MHD) equations. However, the
numerical simulations are far from Earth's core in terms of several control
parameters. Different scaling analyses found simple scaling laws for quantities
like heat transport, flow velocity, magnetic field strength and magnetic
dissipation time.
  We use an extensive dataset of 116 numerical dynamo models compiled by
Christensen and co-workers to analyse these scalings from a rigorous model
selection point of view. Our method of choice is leave-one-out cross-validation
which rates models according to their predictive abilities. In contrast to
earlier results, we find that diffusive processes are not negligible for the
flow velocity and magnetic field strength in the numerical dynamos. Also the
scaling of the magnetic dissipation time turns out to be more complex than
previously suggested. Assuming that the processes relevant in the numerical
models are the same as in Earth's core, we use this scaling to estimate an
Ohmic dissipation of 3-8 TW for the core. This appears to be consistent with
recent high CMB heat flux scenarios.",1307.3938v1
2013-07-19,Damping and non-linearity of a levitating magnet in rotation above a superconductor,"We study the dissipation of moving magnets in levitation above a
superconductor. The rotation motion is analyzed using optical tracking
techniques. It displays a remarkable regularity together with long damping time
up to several hours. The magnetic contribution to the damping is investigated
in detail by comparing 14 distinct magnetic configurations, and points towards
amplitude-dependent dissipation mechanisms. The non-linear dynamics of the
mechanical rotation motion is also revealed and described with an effective
Duffing model. The obtained picture of the coupling of levitating magnets to
their environment sheds light on their potential as ultra-low dissipation
mechanical oscillators for high precision physics.",1307.5155v1
2013-08-02,Skyrmion ground state and gyration of skyrmions in magnetic nanodisks without the Dzyaloshinsky-Moriya interaction,"We show by micromagnetic simulations that spontaneous skyrmion ground state
can exist in Co/Ru/Co nanodisks without the Dzyaloshinsky-Moriya interaction
(DMI), which can remain stable in the applied magnetic field along +z direction
even up to 0.44 T. The guiding center ($R_x$,$R_y$) of skyrmion defined by the
moments of the topological density presents a novel gyration with a star-like
trajectory in a pulsed magnetic field and a hexagonal trajectory after the
field is switched off, which is different from that of vortex or bubble. One of
the coupled skyrmions could move without an external magnetic field, but only
induced by the motion of the other one due to strong inter-layer magnetostatic
interactions. This work sheds light on how novel skyrmions can be discovered in
various (not limited to magnetic) systems with competing energies and
contributes to the understanding of the dynamical properties of skyrmion.",1308.0412v2
2013-08-02,Bimodal Distribution of Magnetic Fields and Areas of Sunspots,"We applied automatic identification of sunspot umbrae and penumbrae to daily
observations from the Helioseismic Magnetic Imager (HMI) on board the Solar
Dynamics Observatory (SDO) to study their magnetic flux (B) and area (A). The
results confirm a previously known logarithmic relationship between the area of
sunspots and their maximum flux density. In addition, we find that the relation
between average magnetic flux (Bavg) and sunspot area shows a bimodal
distribution: for small sunspots and pores (A < 20 millionth of solar
hemisphere, MSH), Bavg = 800 G (gauss), and for large sunspots (A > 100 MSH),
Bavg is about 600 G. For intermediate sunspots, average flux density linearly
decreases from about 800 G to 600 G. A similar bimodal distribution was found
in several other integral parameters of sunspots. We show that this bimodality
can be related to different stages of sunspot penumbra formation and can be
explained by the difference in average inclination of magnetic fields at the
periphery of small and large sunspots.",1308.0535v1
2013-08-09,"The Hour-Glass Magnetic Spectrum Arising from a Striped, Cluster Spin Glass Ground State in La1.75Sr0.25CoO4","We report inelastic neutron scattering results that reveal an hour-glass
magnetic excitation spectrum in La1.75Sr0.25CoO4. The magnetic spectrum is
similar to that observed previously in La1.67Sr0.33CoO4, but the spectral
features are broader. We show that the spectrum of La1.75Sr0.25CoO4 can be
modeled by the spin dynamics of a system with a disordered cluster spin glass
ground state. Bulk magnetization measurements are presented which support the
proposed glassy ground state. The observations reiterate the importance of
quasi-one-dimensional magnetic correlations and disorder for the hour-glass
spectrum, and suggest that disordered spin and charge stripes exist at lower
doping in La2-xSrxCoO4 than previously thought.",1308.2141v1
2013-09-09,Quantitative stray field imaging of a magnetic vortex core,"Thin-film ferromagnetic disks present a vortex spin structure whose dynamics,
added to the small size (~10 nm) of their core, earned them intensive study.
Here we use a scanning nitrogen-vacancy (NV) center microscope to
quantitatively map the stray magnetic field above a 1 micron-diameter disk of
permalloy, unambiguously revealing the vortex core. Analysis of both
probe-to-sample distance and tip motion effects through stroboscopic
measurements, allows us to compare directly our quantitative images to
micromagnetic simulations of an ideal structure. Slight perturbations with
respect to the perfect vortex structure are clearly detected either due to an
applied in-plane magnetic field or imperfections of the magnetic structures.
This work demonstrates the potential of scanning NV microscopy to map tiny
stray field variations from nanostructures, providing a nanoscale,
non-perturbative detection of their magnetic texture.",1309.2171v2
2013-09-17,Quantum Nature of Edge Magnetism in Graphene,"It is argued that the subtle crossover from decoherence-dominated classical
magnetism to fluctuation-dominated quantum magnetism is experimentally
accessible in graphene nanoribbons. We show that the width of a nanoribbon
determines whether the edge magnetism is on the classical side, on the quantum
side, or in between. In the classical regime, decoherence is dominant and leads
to static spin polarizations at the ribbon edges, which are well described by
mean-field theories. The quantum Zeno effect is identified as the basic
mechanism which is responsible for the spin polarization and thereby enables
the application of graphene in spintronics. On the quantum side, however, the
spin polarization is destroyed by dynamical processes. The great tunability of
graphene magnetism thus offers a viable route for the study of the
quantum-classical crossover.",1309.4446v2
2013-09-18,Numerical study of broadband spectra caused by internal shocks in magnetized relativistic jets of blazars,"The internal-shocks scenario in relativistic jets has been used to explain
the variability of blazars' outflow emission. Recent simulations have shown
that the magnetic field alters the dynamics of these shocks producing a whole
zoo of spectral energy density patterns. However, the role played by
magnetization in such high-energy emission is still not entirely understood.
With the aid of \emph{Fermi}'s second LAT AGN catalog, a comparison with
observations in the $\gamma$-ray band was performed, in order to identify the
effects of the magnetic field.",1309.4612v1
2013-09-21,Comparison of physical properties of quiet and active regions through the analysis of MHD simulations of the solar photosphere,"Recent observations have shown that the photometric and dynamic properties of
granulation and of small-scale magnetic features depend on the amount of
magnetic flux of the region they are embedded in. We analyze results from
numerical Hydro and Magneto Hydrodynamic simulations characterized by different
amount of average magnetic flux and find qualitatively the same differences as
those reported from observations. We show that these different physical
properties result from the inhibition of convection induced by the presence of
the magnetic field, which changes the temperature stratification of both quiet
and magnetic regions. Our results are relevant for solar irradiance variations
studies, as such differences are still not properly taken into account in
irradiance reconstruction models.",1309.5494v1
2013-10-18,Quantitative characterization of the spin orbit torque using harmonic Hall voltage measurements,"Solid understanding of current induced torques is key to the development of
current and voltage controlled magnetization dynamics in ultrathin magnetic
heterostructures. To evaluate the size and direction of such torques, or
effective fields, a number of methods have been employed. Here we examine the
adiabatic (low frequency) harmonic Hall voltage measurement that has been used
to study the effective field. We derive an analytical formula for the harmonic
Hall voltages to evaluate the effective field for both out of plane and
in-plane magnetized systems. The formula agrees with numerical calculations
based on a macrospin model. Two different in-plane magnetized films,
Pt|CoFeB|MgO and CuIr|CoFeB|MgO are studied using the formula developed. The
effective field obtained for the latter system shows relatively good agreement
with that estimated using a spin torque switching phase diagram measurements
reported previously. Our results illustrate the versatile applicability of
harmonic Hall voltage measurement for studying current induced torques in
magnetic heterostructures.",1310.4879v1
2013-11-01,Coherent Magnetism and Spin-Orbit Interaction in Garnet Films Probed with Femtosecond Magneto-Optical Four Wave Mixing,"We report about the dephasing of the spins states in a Garnet film excited by
femtosecond laser pulses. It is shown that magneto-optical four-wave-mixing
signals are efficiently generated and controlled with an external static
magnetic field. The self-diffraction of either two or three femtosecond pulses
allows us determining independently the coherent response time ($T_2$ < 50 fs)
and the population relaxation, which occurs in several steps. After the laser
induced demagnetization, the spin-lattice relaxation is the dominant mechanism
($T_{1,spin-lat}$ < 1.35 ps) and the precession of the magnetization takes
place at later times ($T_{1,prec}$ < 150 ps). The coherent response of the
spins states is modeled with a multi-level hydrogen-like system, taking into
account the fundamental relativistic quantum dynamics of the spins. Our
experimental approach is general and opens the way to investigating quantum
magnetism in ordered magnetic systems useful for applications in information
and communications technologies.",1311.0080v1
2013-11-06,Magnetization dynamics of cobalt grown on graphene,"Ferromagnetic resonance (FMR) spin pumping is a rapidly growing field which
has demonstrated promising results in a variety of material systems. This
technique utilizes the resonant precession of magnetization in a ferromagnet to
inject spin into an adjacent non-magnetic material. Spin pumping into graphene
is attractive on account of its exceptional spin transport properties. This
article reports on FMR characterization of cobalt grown on CVD graphene and
examines the validity of linewidth broadening as an indicator of spin pumping.
In comparison to cobalt samples without graphene, direct contact
cobalt-on-graphene exhibits increased FMR linewidth--an often used signature of
spin pumping. Similar results are obtained in Co/MgO/graphene structures, where
a 1nm MgO layer acts as a tunnel barrier. However, SQUID, MFM, and Kerr
microscopy measurements demonstrate increased magnetic disorder in cobalt grown
on graphene, perhaps due to changes in the growth process and an increase in
defects. This magnetic disorder may account for the observed linewidth
enhancement due to effects such as two-magnon scattering or mosaicity. As such,
it is not possible to conclude successful spin injection into graphene from FMR
linewidth measurements alone.",1311.1441v1
2013-11-17,On magnetic reconnection and flux rope topology in solar flux emergence,"We present an analysis of the formation of atmospheric flux ropes in a
magnetohydrodynamic (MHD) solar flux emergence simulation. The simulation
domain ranges from the top of the solar interior to the low corona. A twisted
magnetic flux tube emerges from the solar interior and into the atmosphere
where it interacts with the ambient magnetic field. By studying the
connectivity of the evolving magnetic field, we are able to better understand
the process of flux rope formation in the solar atmosphere. In the simulation,
two flux ropes are produced as a result of flux emergence. Each has a different
evolution resulting in different topological structures. These are determined
by plasma flows and magnetic reconnection. As the flux rope is the basic
structure of the coronal mass ejection (CME), we discuss the implications of
our findings for solar eruptions.",1311.4225v1
2013-11-18,Prediction of superconductivity of $3d$ transition-metal based antiperovskites via magnetic phase diagram,"We theoretically studied the electronic structure, magnetic properties, and
lattice dynamics of a series of $3d$ transition-metal antiperovskite compounds
AXM$_{3}$ by density function theory. Based on the Stoner criterion, we drew
the magnetic phase diagram of carbon-based antiperovskites ACM$_{3}$. In the
phase diagram, compounds with non-magnetic ground state but locating near the
ferromagnetic boundary are suggested to yield sizeable electron-phonon coupling
and behave superconductivity. To approve this deduction, we systematically
calculated the phonon spectra and electron-phonon coupling of a series of
Cr-based antiperovskites ACCr$_{3}$ and ANCr$_{3}$. The results show that
AlCCr$_{3}$, GaCCr$_{3}$, and ZnNCr$_{3}$ could be moderate coupling BCS
superconductors. The influence of spin fluctuation on superconductivity are
discussed. Furthermore, other potential superconducting AXM$_{3}$ including
some new Co-base and Fe-based antiperovskite superconductors are predicted from
the magnetic phase diagram.",1311.4240v3
2013-12-11,Two-dimensional nonlinear travelling waves in magnetohydrodynamic channel flow,"The present study is concerned with the stability of a flow of viscous
conducting liquid driven by pressure gradient in the channel between two
parallel walls subject to a transverse magnetic field. Although the magnetic
field has a strong stabilizing effect, this flow, similarly to its hydrodynamic
counterpart -- plane Poiseuille flow, is known to become turbulent
significantly below the threshold predicted by linear stability theory. We
investigate the effect of the magnetic field on 2D nonlinear travelling-wave
states which are found at substantially subcritical Reynolds numbers starting
from $Re_n=2939$ without the magnetic field and from $Re_n\sim6.50\times10^3Ha$
in a sufficiently strong magnetic field defined by the Hartmann number $Ha.$
Although the latter value is by a factor of seven lower than the linear
stability threshold $Re_l\sim4.83\times10^4Ha$,it is still more by an order of
magnitude higher than the experimentally observed value for the onset of
turbulence in the MHD channel flow.",1312.3264v3
2013-12-17,Non-Hermitian quantum rings,"We investigate the spectral and dynamical properties of a quantum particle
constrained on a ring threaded by a magnetic flux in presence of a complex
(non-Hermitian) potential. For a static magnetic flux, the quantum states of
the particle on the ring can be mapped into the Bloch states of a complex
crystal, and magnetic flux tuning enables to probe the spectral features of the
complex crystal, including the appearance of exceptional points. For a
time-varying (linearly-ramped) magnetic flux, Zener tunneling among energy
states is realized owing to the induced electromotive force. As compared to the
Hermitian case, striking effects are observed in the non-Hermitian case, such
as a highly asymmetric behavior of particle motion when reversing the direction
of the magnetic flux and field-induced delayed transparency.",1312.4693v1
2013-12-30,Impurity entanglement through electron scattering in a magnetic field,"We study the entanglement of magnetic impurities in an environment of
electrons through successive scattering while an external magnetic field is
applied. We show that the dynamics of the problem can be approximately
described by a reduced model of three interacting spins, which reveals an
intuitive view on how spins can be entangled by controlled electron scattering.
The role of the magnetic field is rather crucial. Depending on the initial
state configuration, the magnetic field can either increase or decrease the
resulting entanglement but more importantly it can allow the impurities to be
maximally entangled.",1401.0005v1
2014-01-05,Analytic form of head-to-head domain walls in thin ferromagnetic cylinders,"The one-dimensional problem of a static head-to-head domain wall structure in
a thin soft-magnetic nanowire with circular cross-section is treated within the
framework of micromagnetic theory. A radius-dependent analytic form of the
domain wall profile is derived by decomposing the magnetostatic energy into a
monopolar and a dipolar term. We present a model in which the dipolar term of
the magnetostatic energy resulting from the transverse magnetization in the
center of the domain wall is calculated with Osborn's formulas for
homogeneously magnetized ellipsoids [Phys. Rev. 67, 351 (1945)]. The analytic
results agree almost perfectly with simulation data as long as the wire
diameter is sufficiently small to prevent inhomogeneities of the magnetization
along the cross-section. Owing to the recently demonstrated negligible D\""oring
mass of these walls, our results should also apply to the dynamic case, where
domain walls are driven by spin-transfer toque effects and/or an axial magnetic
field.",1401.0909v1
2014-02-10,Determination of Intrinsic Magnetic Response from Local Measurements of Fringing Fields,"Micron-sized Hall bars and micro-SQUIDs are now used routinely to measure the
local static and dynamic magnetic response with micron-scale spatial
resolution. While this provides a powerful new tool, determining the intrinsic
magnetization presents new challenges, as it requires correcting for
demagnetization fields that vary widely with position on a sample. In this
paper we develop a method to correct for the demagnetization effect at local
points of a rectangular prism shaped sample using a finite element analysis of
Maxwell's equation applied to local Hall sensor measurements calibrated by bulk
measurements of the magnetization. This method can be generalized to other
geometric shapes to analyze data obtained with local magnetic probes.",1402.2247v1
2014-03-02,Effect of magnetic field on the velocity autocorrelation and the caging of particles in two-dimensional Yukawa liquids,"We investigate the effect of an external magnetic field on the velocity
autocorrelation function and the ""caging"" of the particles in a two-dimensional
strongly coupled Yukawa liquid, via numerical simulations. The influence of the
coupling strength on the position of the dominant peak in the frequency
spectrum of the velocity autocorrelation function confirms the onset of a joint
effect of the magnetic field and strong correlations at high coupling. Our
molecular dynamics simulations quantify the decorrelation of the particles'
surroundings - the magnetic field is found to increase significantly the caging
time, which reaches values well beyond the timescale of plasma oscillations.
The observation of the increased caging time is in accordance with findings
that the magnetic field decreases diffusion in similar systems.",1403.0232v1
2014-04-08,The Link between Magnetic Fields and Cloud/Star Formation,"The question whether magnetic fields play an important role in the processes
of molecular cloud and star formation has been debated for decades. Recent
observations have revealed a simple picture that may help illuminate these
questions: magnetic fields have a tendency to preserve their orientation at all
scales that have been probed - from 100-pc scale inter-cloud media down to
sub-pc scale cloud cores. This ordered morphology has implications for the way
in which self-gravity and turbulence interact with magnetic fields: both
gravitational contraction and turbulent velocities should be anisotropic, due
to the influence of dynamically important magnetic fields. Such anisotropy is
now observed. Here we review these recent observations and discuss how they can
improve our understanding of cloud/star formation.",1404.2024v1
2014-04-08,Resonant Spin Wave Excitation by Terahertz Magnetic Near-field Enhanced with Split Ring Resonator,"Excitation of antiferromagnetic spin waves in HoFeO$_{3}$ crystal combined
with a split ring resonator (SRR) is studied using terahertz (THz)
electromagnetic pulses. The magnetic field in the vicinity of the SRR induced
by the incident THz electric field component excites and the Faraday rotation
of the polarization of a near-infrared probe pulse directly measures
oscillations that correspond to the antiferromagnetic spin resonance mode. The
good agreement of the temperature-dependent magnetization dynamics with the
calculation using the two-lattice Landau-Lifshitz-Gilbert equation confirms
that the spin wave is resonantly excited by the THz magnetic near-field
enhanced at the LC resonance frequency of the SRR, which is 20 times stronger
than the incident magnetic field.",1404.2179v1
2014-04-23,Anisotropic Magnetic Response in Kondo Lattice with Antiferromagnetic Order,"Magnetic properties are investigated for the Kondo lattice by using the
continuous time quantum Monte Colro (CT-QMC) and the dynamical mean field
theory (DMFT). The DMFT+CT-QMC approach is extended so as to derive the
anisotropic magnetic response in the antiferromagnetic phase. The longitudinal
and transverse magnetic susceptibilities are numerically derived in the
antiferromagnetic phase. For the RKKY regime with a small Kondo coupling, the
transverse susceptibility does not decrease below the transition temperature
while the longitudinal susceptibility decreases as expected from the mean field
picture. In the competing region between the RKKY interaction and the Kondo
effect, however, both longitudinal and transverse susceptibilities decrease
below the transition temperature. The results obtained naturally explain the
temperature dependence of the magnetic susceptibility observed in
CeT$_2$Al$_{10}$ ($T$=Ru,Os,Fe) family.",1404.5718v1
2014-05-05,Mimetic Methods for Lagrangian Relaxation of Magnetic Fields,"We present a new code that performs a relaxation of a magnetic field towards
a force-free state (Beltrami field) using a Lagrangian numerical scheme.
Beltrami fields are of interest for the dynamics of many technical and
astrophysical plasmas as they are the lowest energy states that the magnetic
field can reach. The numerical method strictly preserves the magnetic flux and
the topology of magnetic field lines. In contrast to other implementations we
use mimetic operators for the spatial derivatives in order to improve accuracy
for high distortions of the grid. Compared with schemes using direct
derivatives we find that the final state of the simulation approximates a
force-free state with a significantly higher accuracy. We implement the scheme
in a code which runs on graphical processing units (GPU), which leads to an
enhanced computing speed compared to previous relaxation codes.",1405.0942v2
2014-05-25,Clusters of small eruptive flares produced by magnetic reconnection in the sun,"We report on the formation of small solar flares produced by patchy magnetic
reconnection between interacting magnetic loops. A three-dimensional (3D)
magnetohydrodynamic (MHD) numerical experiment was performed, where a uniform
magnetic flux sheet was injected into a fully developed convective layer. The
gradual emergence of the field into the solar atmosphere results in a network
of magnetic loops, which interact dynamically forming current layers at their
interfaces. The formation and ejection of plasmoids out of the current layers
leads to patchy reconnection and the spontaneous formation of several small
(size ? 1-2Mm) flares. We find that these flares are short-lived (30 s - 3 min)
bursts of energy in the range O(10^25 - 10^27) ergs, which is basically the
nanoflare-microflare range. Their persistent formation and co-operative action
and evolution leads to recurrent emission of fast EUV/X-ray jets and
considerable plasma heating in the active corona.",1405.6420v1
2014-06-15,Direct and inverse spin-orbit torques,"In collinear magnets lacking inversion symmetry application of electric
currents induces torques on the magnetization and conversely magnetization
dynamics induces electric currents. The two effects, which both rely on
spin-orbit interaction (SOI), are reciprocal to each other and denoted direct
spin-orbit torque (SOT) and inverse spin-orbit torque (ISOT), respectively. We
derive expressions for SOT and ISOT within the Kubo linear response formalism.
We show that expressions suitable for density-functional theory calculations
can be derived either starting from a Kohn-Sham Hamiltonian with time-dependent
exchange field or by expressing general susceptibilities in terms of the
Kohn-Sham susceptibilities. For the case of magnetic bilayer systems we derive
the general form of the ISOT current induced under ferromagnetic resonance.
Using \textit{ab initio} calculations within density-functional theory we
investigate SOT and ISOT in Co/Pt(111) magnetic bilayers. We determine the
spatial distribution of spin and charge currents as well as torques in order to
expose the mechanisms underlying SOT and ISOT and to highlight their
reciprocity on the microscopic level. We find that the spin Hall effect is
position-dependent close to interfaces.",1406.3866v2
2014-08-27,The thickness of a weakly-magnetized accretion flow inside the last stable orbit of a Kerr black hole,"If accretion disc contains weak frozen-in entangled magnetic fields, their
dynamical effect may be important inside the last stable orbit because of the
decompression near the sonic point. Here, I consider the radial and vertical
structure of a nearly free-falling flow inside the last stable orbit of a thin
disc around a Kerr black hole. The thickness of such a flow is determined
primarily by the vertical stress created by radial and azimuthal magnetic
fields. The thickness is predicted to oscillate vertically around its
equilibrium value determined by the magnetic field balance with gravity. For
thin discs, this thickness is much larger than that of the accretion disc
itself. Numerical simulations with HARM2d show the vertical structure is more
complicated. In particular, magnetically supported disc seems to be unstable to
segregation of matter into thinner streams with the vertical scale determined
by thermal pressure or other processes.",1408.6449v1
2014-09-03,Pair separation of magnetic elements in the quiet Sun,"The dynamic properties of the quiet Sun photosphere can be investigated by
analyzing the pair dispersion of small-scale magnetic fields (i.e., magnetic
elements).
  By using $25$ hr-long Hinode magnetograms at high spatial resolution
($0"".3$), we tracked $68,490$ magnetic element pairs within a supergranular
cell near the disk center.
  The computed pair separation spectrum, calculated on the whole set of
particle pairs independently of their initial separation, points out what is
known as a super-diffusive regime with spectral index $\gamma=1.55\pm0.05$, in
agreement with the most recent literature, but extended to unprecedented
spatial and temporal scales (from granular to supergranular). Furthermore, for
the first time, we investigated here the spectrum of the mean square
displacement of pairs of magnetic elements, depending on their initial
separation $r_0$. We found that there is a typical initial distance above
(below) which the pair separation is faster (slower) than the average. A
possible physical interpretation of such a typical spatial scale is also
provided.",1409.1010v2
2014-09-05,Magneto-ionic Control of Interfacial Magnetism,"In metal/oxide heterostructures, rich chemical, electronic, magnetic and
mechanical properties can emerge from interfacial chemistry and structure. The
possibility to dynamically control interface characteristics with an electric
field paves the way towards voltage control of these properties in solid-state
devices. Here we show that electrical switching of the interfacial oxidation
state allows for voltage control of magnetic properties to an extent never
before achieved through conventional magnetoelectric coupling mechanisms. We
directly observe, for the first time, in situ voltage driven O$^{2-}$ migration
in a Co/metal-oxide bilayer, which we use to toggle the interfacial magnetic
anisotropy energy by >0.6 erg/cm$^2$. We exploit the thermally-activated nature
of ion migration to dramatically increase the switching efficiency and to
demonstrate reversible patterning of magnetic properties through local
activation of ionic migration. These results suggest a path towards
voltage-programmable materials based on solid-state switching of interface
oxygen chemistry.",1409.1843v1
2014-09-17,Coupled magnetic and electric hysteresis in the multiferroic double perovskite Lu2MnCoO6,"We investigate a magnetic hysteresis loop with a remanent moment that couples
to electric polarization to create coupled hysteretic multiferroic behavior in
Lu2MnCoO6. Measurements of elastic neutron diffraction, muon spin relaxation,
and micro-Hall magnetometry demonstrate an unusual mechanism for the magnetic
hysteresis, namely the hysteretic evolution of a microscopic magnetic order,
and not classic ferromagnetic domain effects. We show how the frustrated spin
system evolves from antiferromagnetism with an incommensurate long-wavelength
modulation and strong fluctuations towards a net magnetism. We also clarify the
different temperature scales for the onset of ordering, dynamics, and
hysteresis.",1409.5072v2
2014-11-07,Magnetic exchange in $α$-iron from the ab initio calculations in the paramagnetic phase,"Applying the local density approximation (LDA) and dynamical mean field
theory (DMFT) to paramagnetic $\alpha $-iron, we revisit a problem of
theoretical description of its magnetic properties. The analysis of local
magnetic susceptibility shows that at sufficiently low temperatures $T<1500K$,
both, $e_{g}$ and $t_{2g}$ states equally contribute to the formation of the
effective magnetic moment with spin S=1. The self-energy of t_{2g} states shows
sizable deviations from Fermi-liquid form, which accompanies earlier found
non-quasiparticle form of e_{g} states. By considering the non-uniform magnetic
susceptibility we find that the non-quasiparticle form of $e_{g}$ states is
crucial for obtaining ferromagnetic instability in $\alpha $-iron. The main
contribution to the exchange interaction, renormalized by the effects of
electron interaction, comes from the hybridization between $t_{2g}$ and $e_{g}$
states. We furthermore suggest the effective spin-fermion model for $\alpha
$-iron, which allows us to estimate the exchange interaction from paramagnetic
phase, which is in agreement with previous calculations in the ordered state
within the LDA approaches.",1411.1888v2
2014-12-04,(Inverse) Magnetic Catalysis in Bose-Einstein Condensation of Neutral Bound Pairs,"The Bose-Einstein condensation of bound pairs made of oppositely charged
fermions in a magnetic field is investigated. We find that the condensation
temperature shows the magnetic catalysis effect in weak coupling and the
inverse magnetic catalysis effect in strong coupling. The different responses
to the magnetic field can be attributed to the competition between the
dimensional reduction by Landau orbitals in pairing dynamics and the anisotropy
of the kinetic spectrum of fluctuations (bound pairs in the normal phase)",1412.1647v3
2014-12-11,Non-collinear Magnetic Configurations at Finite Temperature in Thin Films,"The finite-temperature magnetism of a monolayer on a bcc (110) surface was
examined using a model Hamiltonian containing ferromagnetic or
antiferromagnetic exchange interactions, Dzyaloshinsky-Moriya interactions and
easy-axis on-site anisotropy. We examined the competition between the collinear
ground state parallel to the easy axis and the spin spiral state in the plane
perpendicular to this axis preferred by the Dzyaloshinsky-Moriya interaction.
Using approximative methods to calculate the magnon spectrum at finite
temperatures, it was found that even if the ground state is collinear,
increasing the Dzyaloshinsky-Moriya interaction strongly decreases the critical
temperature where this collinear order disappears. Using atomistic spin
dynamics simulations it was found that at this critical temperature the system
transforms into the non-collinear state. Including external magnetic field
helps stabilising the ferromagnetic state. An effect due to the finite size of
the magnetic monolayer was included in the model by considering a different
value for the anisotropy at the edges of the monolayer. This effect was shown
to stabilize the spin spiral state by fixing the phase at the ends of the
stripe.",1412.3591v1
2014-12-31,Dissipation of magnetic fields in star-forming clouds with different metallicities,"We study dissipation process of magnetic fields in the metallicity range $0-1
Z_{\odot}$ for contracting prestellar cloud cores. By solving non-equilibrium
chemistry for important charged species including charged grains, we evaluate
the drift velocity of the magnetic-field lines with respect to the gas. We find
that the magnetic flux dissipates in the density range $10^{12}{\rm cm^{-3}}
\lesssim n_{\rm H} \lesssim 10^{17}{\rm cm^{-3}}$ for the solar-metallicity
case at the scale of the core, which is assumed to be the Jeans scale. The
dissipation density range becomes narrower for lower metallicity. The magnetic
field is always frozen to the gas below metallicity $\lesssim
10^{-7}-10^{-6}Z_\odot$, depending on the ionization rate by cosmic rays and/or
radioactivity. With the same metallicity, the dissipation density range becomes
wider for lower ionization rate. The presence of such a dissipative regime is
expected to cause various dynamical phenomena in protostellar evolution such as
the suppression of jet/outflow launching and fragmentation of the circumstellar
disks depending on the metallicity.",1501.00087v1
2015-01-08,What flows in the chiral magnetic effect? -- Simulating the particle production with CP-breaking backgrounds,"To address a question of whether the chiral magnetic current is a static
polarization or a genuine flow of charged particles, we elucidate the numerical
formulation to simulate the net production of right-handed particles and
anomalous currents with CP-breaking background fields which cause an imbalance
between particles and anti-particles. For a concrete demonstration we
numerically impose pulsed electric and magnetic fields to confirm our answer to
the question that the produced net particles flow in the dynamical chiral
magnetic effect. The rate for the particle production and the chiral magnetic
current generation is quantitatively consistent with the axial anomaly, while
they appear with a finite response time. We emphasize the importance to
quantify the response time that would suppress observable effects of the
anomalous current.",1501.01940v4
2015-01-26,Vortex Polarity Switching in Magnets with Surface Anisotropy,"Vortex core reversal in magnetic particle is essentially influenced by a
surface anisotropy. Under the action of a perpendicular static magnetic field
the vortex core undergoes a shape deformationof pillow- or barrel-shaped type,
depending on the type of the surface anisotropy. This deformation plays a key
point in the switching mechanism: We predict that the vortex polarity switching
is accompanied (i) by a linear singularity in case of Heisenberg magnet with
bulk anisotropy only and (ii) by a point singularities in case of surface
anisotropy or exchange anisotropy. We study in details the switching process
using spin-lattice simulations and propose a simple analytical description
using a wired core model, which provides an adequate description of the Bloch
point statics, its dynamics and the Bloch point mediated switching process. Our
analytical predictions are confirmed by spin-lattice simulations for Heisenberg
magnet and micromagnetic simulations for nanomagnet with account of a dipolar
interaction.",1501.06548v1
2015-01-29,Energy Deposition Studies for the Hi-Lumi LHC Inner Triplet Magnets,"A detailed model of the High Luminosity LHC inner triplet region with new
large-aperture Nb3Sn magnets, field maps, corrector packages, and segmented
tungsten inner absorbers was built and implemented into the FLUKA and MARS15
codes. In the optimized configuration, the peak power density averaged over the
magnet inner cable width is safely below the quench limit. For the integrated
luminosity of 3000 fb -1, the peak dose in the innermost magnet insulator
ranges from 20 to 35 MGy. Dynamic heat loads to the triplet magnet cold mass
are calculated to evaluate the cryogenic capability. In general, FLUKA and MARS
results are in a very good agreement.",1501.07623v1
2015-02-25,Observation of room temperature magnetic skyrmions and their current-driven dynamics in ultrathin Co films,"Magnetic skyrmions are topologically-protected spin textures that exhibit
fascinating physical behaviors and large potential in highly energy efficient
spintronic device applications. The main obstacles so far are that skyrmions
have been observed in only a few exotic materials and at low temperatures, and
manipulation of individual skyrmions has not yet been achieved. Here, we report
the observation of stable magnetic skyrmions at room temperature in ultrathin
transition metal ferromagnets with magnetic transmission soft x-ray microscopy.
We demonstrate the ability to generate stable skyrmion lattices and drive
trains of individual skyrmions by short current pulses along a magnetic
racetrack. Our findings provide experimental evidence of recent predictions and
open the door to room-temperature skyrmion spintronics in robust thin-film
heterostructures.",1502.07376v1
2015-02-27,Numerical simulations of the internal shock model in magnetized relativistic jets of blazars,"The internal shocks scenario in relativistic jets is used to explain the
variability of the blazar emission. Recent studies have shown that the magnetic
field significantly alters the shell collision dynamics, producing a variety of
spectral energy distributions and light-curves patterns. However, the role
played by magnetization in such emission processes is still not entirely
understood. In this work we numerically solve the magnetohydodynamic evolution
of the magnetized shells collision, and determine the influence of the
magnetization on the observed radiation. Our procedure consists in
systematically varying the shell Lorentz factor, relative velocity, and viewing
angle. The calculations needed to produce the whole broadband spectral energy
distributions and light-curves are computationally expensive, and are achieved
using a high-performance parallel code.",1502.07882v1
2015-04-02,Energy deposition studies for the High-Luminosity Large Hadron Collider inner triplet magnets,"A detailed model of the High Luminosity LHC inner triplet region with new
large-aperture Nb3Sn magnets, field maps, corrector packages, and segmented
tungsten inner absorbers was built and implemented into the FLUKA and MARS15
codes. In the optimized configuration, the peak power density averaged over the
magnet inner cable width is safely below the quench limit. For the integrated
luminosity of 3000 fb-1, the peak dose in the innermost magnet insulator ranges
from 20 to 35 MGy. Dynamic heat loads to the triplet magnet cold mass are
calculated to evaluate the cryogenic capability. In general, FLUKA and MARS
results are in a very good agreement.",1504.00594v1
2015-04-14,Magnetic charges and magnetoelectricity in hexagonal rare-earth manganites and ferrites,"Magnetoelectric (ME) materials are of fundamental interest and show broad
potential for technological applications. Commonly the dominant contribution to
the ME response is the lattice-mediated one, which is proportional to both the
Born electric charge $Z^{\rm e}$ and its analogue, the dynamical magnetic
charge $Z^{\rm m}$. Our previous study has shown that exchange striction acting
on noncollinear spins induces much larger magnetic charges than those that
depend on spin-orbit coupling. The hexagonal manganites $R$MnO$_3$ and ferrites
$R$FeO$_3$ ($R$ = Sc, Y, In, Ho-Lu) exhibit strong couplings between electric,
magnetic and structural degrees of freedom, with the transition-metal ions in
the basal plane antiferromagnetically coupled through super-exchange so as to
form a 120$^\circ$ noncollinear spin arrangement. Here we present a theoretical
study of the magnetic charges, and of the spin-lattice and spin-electronic ME
constants, in these hexagonal manganites and ferrites, clarifying the
conditions under which exchange striction leads to an enhanced $Z^{\rm m}$
values and anomalously large in-plane spin-lattice ME effects.",1504.03405v1
2015-05-04,Phase Diagrams of Forced Magnetic Reconnection in Taylor's Model,"Recent progress in the understanding of how externally driven magnetic
reconnection evolves is organized in terms of parameter space diagrams. These
diagrams are constructed using four pivotal dimensionless parameters: the
Lundquist number $S$, the magnetic Prandtl number $P_m$, the amplitude of the
boundary perturbation $\hat \Psi_0$, and the perturbation wave number $\hat k$.
This new representation highlights the parameters regions of a given system in
which the magnetic reconnection process is expected to be distinguished by a
specific evolution. Contrary to previously proposed phase diagrams, the
diagrams introduced here take into account the dynamical evolution of the
reconnection process and are able to predict slow or fast reconnection regimes
for the same values of $S$ and $P_m$, depending on the parameters that
characterize the external drive, never considered so far. These features are
important to understand the onset and evolution of magnetic reconnection in
diverse physical systems",1505.00726v1
2015-05-06,Hydromagnetics of advective accretion flows around black holes: Removal of angular momentum by large scale magnetic stresses,"We show that the removal of angular momentum is possible in the presence of
large scale magnetic stresses in geometrically thick, advective, sub-Keplerian
accretion flows around black holes in steady-state, in the complete absence of
alpha-viscosity. The efficiency of such an angular momentum transfer could be
equivalent to that of alpha-viscosity with alpha=0.01-0.08. Nevertheless,
required field is well below its equipartition value, leading to a magnetically
stable disk flow. This is essentially important in order to describe the hard
spectral state of the sources, when the flow is non/sub-Keplerian. We show in
our simpler 1.5-dimensional, vertically averaged disk model that larger the
vertical-gradient of azimuthal component of magnetic field, stronger the rate
of angular momentum transfer is, which in turn may lead to a faster rate of
outflowing matter. Finding efficient angular momentum transfer, in black hole
disks, via magnetic stresses alone is very interesting, when the generic origin
of alpha-viscosity is still being explored.",1505.01281v1
2015-05-12,Getting through to a qubit by magnetic solitons,"We propose a method for acting on the spin state of a spin-1/2 localized
particle, or `qubit', by means of a magnetic signal effectively generated by
the nearby transit of a magnetic soliton, there conveyed through a transmission
line. We first introduce the specific magnetic soliton of which we will make
use, and briefly review the properties that make it apt to represent a signal.
We then show that a Heisenberg spin chain can serve as transmission line, and
propose a method for injecting a soliton into the chain by acting just on one
of its ends. We finally demonstrate that the resulting magnetic pulse can
indeed cause, just passing by the spin-1/2 localized particle embodying the
qubit, a permanent change in its spin state, thus realizing the possibility of
getting through to a single, localized qubit, and manipulate its state. A
thorough analysis of how the overall dynamical system operates depending on the
setting of its parameters demonstrates that fine tuning is not necessary as it
exists an extended region in the parameters space that corresponds to an
effective functioning. Moreover, we show that possible noise on the
transmission line does not invalidate the scheme.",1505.02954v1
2015-05-15,Electric-field guiding of magnetic skyrmions,"We theoretically study equilibrium and dynamic properties of nanosized
magnetic skyrmions in thin magnetic films with broken inversion symmetry, where
electric field couples to magnetization via spin-orbit coupling. Based on a
symmetry-based phenomenology and micromagnetic simulations we show that this
electric-field coupling, via renormalizing the micromagnetic energy, modifies
the equilibrium properties of the skyrmion. This change, in turn, results in a
significant alteration of the current-induced skyrmion motion. Particularly,
speed and direction of the skyrmion can be manipulated by designing a desired
energy landscape electrically, which we describe within Thiele's analytical
model and demonstrate in micromagnetic simulations including
electric-field-controlled magnetic anisotropy. We additionally use this
electric-field control to construct gates for controlling skyrmion motion
exhibiting a transistor-like and multiplexer-like function. The proposed
electric-field effect can thus provide a low energy electrical knob to extend
the reach of information processing with skyrmions.",1505.03972v1
2015-06-02,Nonlocal Magnetoresistance Mediated by Spin Superfluidity,"The electrical response of two diffusive metals is studied when they are
linked by a magnetic insulator hosting a topologically stable (superfluid) spin
current. We discuss how charge currents in the metals induce a spin
supercurrent state, which in turn generates a magnetoresistance that depends on
the topology of the electrical circuit. This magnetoresistance relies on phase
coherence over the entire magnet and gives direct evidence for spin
superfluidity. We show that driving the magnet with an ac current allows
coherent spin transport even in the presence of U(1)-breaking magnetic
anisotropy that can preclude dc superfluid transport. Spin transmission in the
ac regime shows a series of resonance peaks as a function of frequency. The
peak locations, heights, and widths can be used to extract static interfacial
properties, e.g., the spin-mixing conductance and effective spin Hall angle,
and to probe dynamic properties such as the spin-wave dispersion. Thus, ac
transport may provide a simpler route to realizing nonequilbrium coherent spin
transport and a useful way to characterize the magnetic system, serving as a
precursor to the realization of dc superfluid spin transport.",1506.01059v2
2015-06-11,Regular Reduction of Controlled Magnetic Hamiltonian System with Symmetry of the Heisenberg Group,"A controlled magnetic Hamiltonian (CMH) system is a regular controlled
Hamiltonian (RCH) system with magnetic symplectic form, it is an important
special case of RCH system. Note that there is a magnetic term on the cotangent
bundle of the Heisenberg group, such that we can define a CMH system with
symmetry of the Heisenberg group. Since the set of the CMH systems with
symmetries is a subset of the RCH systems with symmetries, and it is not
complete under the regular point reduction of RCH system, in this paper, then
we give the regular point reduction of a CMH system with symmetry of the
Heisenberg group, and discuss the M-CH-equivalence and MR-CH-equivalence, and
prove the regular point reduction theorem for such CMH system. In particular,
we deduce the regular point reduced CMH system on the generalization of
coadjoint orbit of the Heisenberg group by calculation in detail. As an
application, we consider the motion of the Heisenberg particle in a magnetic
field.",1506.03640v4
2015-07-13,Magnetic reconnection in plasma under inertial confinement fusion conditions driven by heat flux effects in Ohm's law,"In the interaction of high-power laser beams with solid density plasma there
are a number of mechanisms that generate strong magnetic fields. Such fields
subsequently inhibit or redirect electron flows, but can themselves be advected
by heat fluxes, resulting in complex interplay between thermal transport and
magnetic fields.We show that for heating by multiple laser spots reconnection
of magnetic field lines can occur, mediated by these heat fluxes, using a fully
implicit 2D Vlasov-Fokker-Planck code. Under such conditions, the reconnection
rate is dictated by heat flows rather than Alfv\`enic flows. We find that this
mechanism is only relevant in a high $\beta$ plasma. However, the Hall
parameter $\omega_c \tau_{ei}$ can be large so that thermal transport is
strongly modified by these magnetic fields, which can impact longer time scale
temperature homogeneity and ion dynamics in the system.",1507.03629v1
2015-07-23,Nanomagnet coupled to quantum spin Hall edge: An adiabatic quantum motor,"The precessing magnetization of a magnetic islands coupled to a quantum spin
Hall edge pumps charge along the edge. Conversely, a bias voltage applied to
the edge makes the magnetization precess. We point out that this device
realizes an adiabatic quantum motor and discuss the efficiency of its operation
based on a scattering matrix approach akin to Landauer-B""uttiker theory.
Scattering theory provides a microscopic derivation of the
Landau-Lifshitz-Gilbert equation for the magnetization dynamics of the device,
including spin-transfer torque, Gilbert damping, and Langevin torque. We find
that the device can be viewed as a Thouless motor, attaining unit efficiency
when the chemical potential of the edge states falls into the
magnetization-induced gap. For more general parameters, we characterize the
device by means of a figure of merit analogous to the ZT value in
thermoelectrics.",1507.06505v2
2015-10-24,"On the ""force-free surface "" of the magnetized celestial bodies","The field of a uniformly magnetized rotating sphere is studied with special
attention to the surface where the electric and magnetic fields are orthogonal
to each other. The equation of this surface, valid at arbitrary distances from
the rotating magnetized sphere, is obtained. Inside the light cylinder this
surface can be considered as a force-free surface, i.e. as a place where the
particles with strong radiation damping can be trapped due to their energy
loss. Outside the light cylinder this surface makes just a geometric locus
which moves with a superlight velocity around the axis of rotation. The 2- and
3-dimensional plots of the force-free surface are constructed. Estimation of
influence of the centrifugal force on the particle dynamics is made. It is
shown, that in case of strong magnetic field the centrifugal force is
negligible small everywhere except a narrow neighbourhood of the light
cylinder.",1510.07122v1
2015-10-29,Self-Organization and Heating by Inward Diffusion in Magnetospheric Plasmas,"Through the process of inward diffusion, a strongly localized clump of plasma
is created in a magnetosphere. The creation of the density gradient, instead of
the usual flattening by a diffusion process, can be explained by the
topological constraints given by the adiabatic invariants of magnetized
particles. After developing a canonical formalism for the standard guiding
center dynamics in a dipole magnetic field, we complete our attempt to build a
statistical mechanics on a constrained phase space by discussing the
construction principles of the associated diffusion operator. We then
investigate the heating mechanism associated with inward diffusion: as
particles move toward regions of higher magnetic field, they experience
preferential heating of the perpendicular (with respect to the magnetic field)
temperature in order to preserve the magnetic moment. A relationship between
conservation of bounce action and temperature isotropy emerged. We further show
that this behavior is scaled by the diffusion parameter of the Fokker-Planck
equation. These results are confirmed by numerical simulations.",1510.08571v2
2015-10-29,Indirect switching of vortex polarity through magnetic dynamic coupling,"Magnetic vortex cores exhibit a gyrotropic motion, and may reach a critical
velocity, at which point they invert their z-component of the magnetization. We
performed micromagnetic simulations to describe this vortex core polarity
reversal in magnetic nanodisks presenting a perpendicular anisotropy. We found
that the critical velocity decreases with increasing perpendicular anisotropy,
therefore departing from a universal criterion, that relates this velocity only
to the exchange stiffness of the material. This leads to a critical velocity
inversely proportional to the vortex core radius. We have also shown that in a
pair of interacting disks, it is possible to switch the core vortex polarity
through a non-local excitation; exciting one disk by applying a rotating
magnetic field, one is able to switch the polarity of a neighbor disk, with a
larger perpendicular anisotropy.",1510.08783v1
2015-11-12,Magnetic Rigid Rotor in the Quantum Regime: Theoretical Toolbox,"We describe the quantum dynamics of a magnetic rigid rotor in the mesoscopic
scale where the Einstein-De Haas effect is predominant. In particular, we
consider a single-domain magnetic nanoparticle with uniaxial anisotropy in a
magnetic trap. Starting from the basic Hamiltonian of the system under the
macrospin approximation, we derive a bosonized Hamiltonian describing the
center-of-mass motion, the total angular momentum, and the macrospin degrees of
freedom of the particle treated as a rigid body. This bosonized Hamiltonian can
be approximated by a simple quadratic Hamiltonian that captures the rich
physics of a nanomagnet tightly confined in position, nearly not spinning, and
with its macrospin anti-aligned to the magnetic field. The theoretical tools
derived and used here can be applied to other quantum mechanical rigid rotors.",1511.04022v4
2015-11-12,Thermally-driven spin torques in layered magnetic insulators,"Thermally-driven spin-transfer torques have recently been reported in
electrically insulating ferromagnet$|$normal-metal heterostructures. In this
paper, we propose two physically distinct mechanisms for such torques. The
first is a local effect: out-of-equilibrium, thermally-activated magnons in the
ferromagnet, driven by a spin Seebeck effect, exert a torque on the
magnetization via magnon-magnon scattering with coherent dynamics. The second
is a nonlocal effect which requires an additional magnetic layer to provide the
symmetry breaking necessary to realize a thermal torque. The simplest structure
in which to induce a nonlocal thermal torque is a spin valve composed of two
insulating magnets separated by a normal metal spacer; there, a thermal flux
generates a pure spin current through the spin valve, which results in a torque
when the magnetizations of the layers are misaligned.",1511.04104v1
2015-11-24,Ferromagnetic Resonance of a YIG film in the Low Frequency Regime,"An improved method for characterizing the magnetic anisotropy of films with
cubic symmetry is described and is applied to an yttrium iron garnet (111)
film. Analysis of the FMR spectra performed both in-plane and out-of-plane from
0.7 to 8 GHz yielded the magnetic anisotropy constants as well as the
saturation magnetization. The field at which FMR is observed turns out to be
quite sensitive to anisotropy constants (by more than a factor ten) in the low
frequency (< 2 GHz) regime and when the orientation of the magnetic field is
nearly normal to the sample plane; the restoring force on the magnetization
arising from the magnetocrystalline anisotropy fields is then comparable to
that from the external field, thereby allowing the anisotropy constants to be
determined with greater accuracy. In this region, unusual dynamical behaviors
are observed such as multiple resonances and a switching of FMR resonance with
only a 1 degree change in field orientation at 0.7 GHz.",1511.07892v1
2015-11-27,RD-NMR spectra of the crystal states of the two-dimensional electron gas in a quantizing magnetic field,"Transport experiments on the two-dimensional electron gas (2DEG) confined
into a semiconductor quantum well and subjected to a quantizing magnetic field
have uncovered a rich variety of uniform and nonuniform phases such as the
Laughlin liquids, the Wigner, bubble and Skyrme crystals and the quantum Hall
stripe state. Optically pumped nuclear magnetic resonance (OP-NMR) has also
been extremely useful in studying the magnetization and dynamics of electron
solids with exotic spin textures such as the Skyrme crystal. Recently, it has
been demonstrated that a related technique, resistively-detected nuclear
magnetic resonance (RD-NMR), could be a good tool to study the topography of
the electron solids in the fractional and integer quantum Hall regimes. In this
work, we compute theoretically the RD-NMR line shapes of various crystal phases
of the 2DEG and study the relation between their spin density and texture and
their NMR spectra. This allows us to evaluate the ability of the RD-NMR to
discriminate between the various types of crystal states.",1511.08702v1
2015-12-10,Magnetic ordering and non-Fermi-liquid behavior in the multichannel Kondo-lattice model,"Scaling equations for the Kondo lattice in the paramagnetic and magnetically
ordered phases are derived to two-loop order with account of spin dynamics. The
results are applied to describe various mechanisms of the non-Fermi-liquid
(NFL) behavior in the multichannel Kondo-lattice model where a fixed point
occurs in the weak-coupling region. The corresponding temperature dependences
of electronic and magnetic properties are discussed. The model describes
naturally formation of a magnetic state with soft boson mode and small moment
value. An important role of Van Hove singularities in the magnon spectral
function is demonstrated. The results are rather sensitive to the type of
magnetic ordering and space dimensionality, the conditions for NFL behavior
being more favorable in the antiferromagnetic and 2D cases.",1512.03161v3
2015-12-22,Observation of Unusual Magnetoelastic Effects in a Quasi-1D Spiral Magnet,"We present a systematic study of spin and lattice dynamics in the
quasi-one-dimensional spiral magnet CuBr2, using Raman scattering in
conjunction with infrared and neutron spectroscopy. Along with the development
of spin correlations upon cooling, we observe a rich set of broad Raman bands
at energies that correspond to phonon-dispersion energies near the
one-dimensional magnetic wave vector. The low-energy bands further exhibit a
distinct intensity maximum at the spiral magnetic ordering temperature. We
attribute these unusual observations to two possible underlying mechanisms: (1)
formation of hybrid spin-lattice excitations, and/or (2) ""quadrumerization"" of
the lattice caused by spin-singlet entanglement in competition with the spiral
magnetism.",1512.07120v2
2016-01-04,Influence of magnetic quantum confined Stark effect on the spin lifetime of indirect excitons,"We report on the unusual and counter-intuitive behaviour of spin lifetime of
excitons in coupled semiconductor quantum wells (CQWs) in the presence of
in-plane magnetic field. Instead of conventional acceleration of spin
relaxation due to the Larmor precession of electron and hole spins we observe a
strong increase of the spin relaxation time at low magnetic fields followed by
saturation and decrease at higher fields. We argue that this non-monotonic spin
relaxation dynamics is a fingerprint of the magnetic quantum confined Stark
effect. In the presence of electric field along the CQW growth axis, an applied
magnetic field efficiently suppresses the exciton spin coherence, due to
inhomogeneous broadening of the $g$-factor distribution.",1601.00427v1
2016-02-25,Nonequilibrium current-carrying steady states in the anisotropic $XY$ spin chain,"Out-of-equilibrium behavior is explored in the one-dimensional anisotropic
$XY$ model. Initially preparing the system in the isotropic $XX$ model with a
linearly varying magnetic field to create a domain-wall magnetization profile,
dynamics is generated by rapidly changing the exchange interaction anisotropy
and external magnetic field. Relaxation to a nonequilibrium steady state is
studied analytically at the critical transverse Ising point, where correlation
functions may be computed in closed form. For arbitrary values of anisotropy
and external field, an effective generalized Gibbs' ensemble is shown to
accurately describe observables in the long-time limit. Additionally, we find
spatial oscillations in the exponentially decaying, transverse spin-spin
correlation functions with wavelength set by the magnetization jump across the
initial domain wall. This wavelength depends only weakly on anisotropy and
magnetic field in contrast to the current, which is highly dependent on these
parameters.",1602.08027v2
2016-03-03,"Two dimensional, electronic particle tracking in liquids with a graphene-based magnetic sensor array","The investigation of liquid flow at the nanoscale is a key area of applied
research with high relevance to Physics, Chemistry and Biology. We introduce a
method and a device that allows to spatially resolve liquid flow by integrating
an array of graphene-based magnetic sensors used for tracking the movement of
magnetic nanoparticles in the liquid under investigation. With a novel device
conception based on standard integration processes and experimentally verified
material parameters, we simulate the performance of a single sensor pixel, as
well as the whole array, for tracking magnetic nanoparticles. The results
demonstrate the ability (a) to detect individual nanoparticles in the liquid
and (b) to reconstruct particle trajectories across the sensor array as a
function of time, in what we call ""Magnetic Nanoparticle Velocimetry""
technique. Being a non-optical detection method, potential applications include
particle tracking and flow analysis in opaque media at sub-micron scales.",1603.01298v1
2016-03-18,A Comparison between Abraham and Minkowski Momenta,"In this paper I compare the Abraham and the Minkowski forms for the momentum
pertaining to an electromagnetic wave inside a dielectric or a magnetic
material. The discussion is based on a careful treatment of the surface charges
and currents and of the forces acting on them. While in the dielectric case the
Abraham momentum is certainly more appealing from the physical point of view,
for a magnetic material it suggests an interpretation in terms of magnetic
charges and re- lated magnetic currents. The Minkowski momentum for magnetic
non conducting materials, on the contrary, has a natural interpretation in
terms of an amperian model, in which the dynamics is determined by the Lorentz
force acting on bulk and surface electric currents.",1603.05812v1
2016-04-02,3D Global Coronal Density Structure and Associated Magnetic Field near Solar Maximum,"Measurement of the coronal magnetic field is a crucial ingredient in
understanding the nature of solar coronal dynamic phenomena at all scales. We
employ STEREO/COR1 data obtained near maximum of solar activity in December
2012 (Carrington rotation, CR 2131) to retrieve and analyze the
three-dimensional (3D) coronal electron density in the range of heights from
$1.5$ to $4\ \mathrm{R}_\odot$ using a tomography method and qualitatively
deduce structures of the coronal magnetic field. The 3D electron density
analysis is complemented by the 3D STEREO/EUVI emissivity in 195 \AA \ band
obtained by tomography for the same CR period. We find that the magnetic field
configuration during CR 2131 has a tendency to become radially open at
heliocentric distances below $\sim 2.5 \ \mathrm{R}_\odot$. We compared the
reconstructed 3D coronal structures over the CR near the solar maximum to the
one at deep solar minimum. Results of our 3D density reconstruction will help
to constrain solar coronal field models and test the accuracy of the magnetic
field approximations for coronal modeling.",1604.00535v1
2016-04-18,Slipping magnetic reconnections with multiple flare ribbons during an X-class solar flare,"With the observations of the Solar Dynamics Observatory, we present the
slipping magnetic reconnections with multiple flare ribbons (FRs) during an
X1.2 eruptive flare on 2014 January 7. A center negative polarity was
surrounded by several positive ones, and there appeared three FRs. The three
FRs showed apparent slipping motions, and hook structures formed at their ends.
Due to the moving footpoints of the erupting structures, one tight
semi-circular hook disappeared after the slippage along its inner and outer
edge, and coronal dimmings formed within the hook. The east hook also faded as
a result of the magnetic reconnection between the arcades of a remote filament
and a hot loop that was impulsively heated by the under flare loops. Our
results are accordant with the slipping magnetic reconnection regime in 3D
standard model for eruptive flares. We suggest that complex structures of the
flare is likely a consequence of the more complex flux distribution in the
photosphere, and the eruption involves at least two magnetic reconnections.",1604.04982v1
2016-04-21,Inverse Magnetic Catalysis in Bottom-Up Holographic QCD,"We explore the effect of magnetic field on chiral condensation in QCD via a
simple bottom up holographic model which inputs QCD dynamics through the
running of the anomalous dimension of the quark bilinear. Bottom up holography
is a form of effective field theory and we use it to explore the dependence on
the coefficients of the two lowest order terms linking the magnetic field and
the quark condensate. In the massless theory, we identify a region of parameter
space where magnetic catalysis occurs at zero temperature but inverse magnetic
catalysis at temperatures of order the thermal phase transition. The model
shows similar non-monotonic behaviour in the condensate with B at intermediate
T as the lattice data. This behaviour is due to the separation of the meson
melting and chiral transitions in the holographic framework. The introduction
of quark mass raises the scale of B where inverse catalysis takes over from
catalysis until the inverse catalysis lies outside the regime of validity of
the effective description leaving just catalysis.",1604.06307v1
2016-04-26,Tuning the resonant frequencies of a drop by a magnetic field,"We report an experimental study of a magnetic liquid drop deposited on a
superhydrophobic substrate and subjected to vertical vibrations in presence of
a static magnetic field. It is well-known that a flattened drop of usual liquid
displays oscillating lobes at its periphery when vibrated. By adding
ferromagnetic nanoparticles to a water drop and varying the strength of the
magnetic field, we are experimentally able to efficiently tune the resonant
frequencies of the drop. By using conservation energy arguments, we show that
the magnetic field contribution is equivalent to adding an effective negative
surface tension to the drop. Our model is found in good agreement with the
experiments with no fitting parameter.",1604.07686v1
2016-04-27,History dependence of the magnetic properties of single-crystal Fe$_{1-x}$Co$_{x}$Si,"We report the magnetization, ac susceptibility, and specific heat of
optically float-zoned single crystals of Fe$_{1-x}$Co$_{x}$Si, $0.20 \leq x
\leq 0.50$. We determine the magnetic phase diagrams for all major
crystallographic directions and cooling histories. After zero-field cooling,
the phase diagrams resemble that of the archetypal stoichiometric cubic chiral
magnet MnSi. Besides the helical and conical state, we observe a pocket of
skyrmion lattice phase just below the helimagnetic ordering temperature. At the
phase boundaries between these states evidence for slow dynamics is observed.
When the sample is cooled in small magnetic fields, the phase pocket of
skyrmion lattice may persist metastably down to lowest temperatures. Taken
together with the large variation of the transition temperatures, transition
fields, and the helix wavelength as a function of composition, this hysteresis
identifies Fe$_{1-x}$Co$_{x}$Si as an ideal material for future experiments
exploring, for instance, the topological unwinding of the skyrmion lattice.",1604.08025v1
2016-05-06,Observations of an X-shaped Ribbon Flare in the Sun and Its Three-dimensional Magnetic Reconnection,"We report evolution of an atypical X-shaped flare ribbon which provides novel
observational evidence of three-dimensional (3D) magnetic reconnection at a
separator. The flare occurred on 2014 November 9. High-resolution slit-jaw 1330
A images from the Interface Region Imaging Spectrograph reveal four
chromospheric flare ribbons that converge and form an X-shape. Flare
brightening in the upper chromosphere spreads along the ribbons toward the
center of the ""X"" (the X-point), and then spreads outward in a direction more
perpendicular to the ribbons. These four ribbons are located in a quadrupolar
magnetic field. Reconstruction of magnetic topology in the active region
suggests the presence of a separator connecting to the X-point outlined by the
ribbons. The inward motion of flare ribbons in the early stage therefore
indicates 3D magnetic reconnection between two sets of non-coplanar loops that
approach laterally, and reconnection proceeds downward along a section of
vertical current sheet. Coronal loops are also observed by the Atmospheric
Imaging Assembly on board the Solar Dynamics Observatory confirming the
reconnection morphology illustrated by ribbon evolution.",1605.01833v1
2016-06-01,Magnetic field strength in an intermediate-velocity ionized filament in the First Galactic Quadrant,"We investigate the magnetic field in an intermediate-velocity filament for
which the H$\alpha$ intensity in the WHAM survey correlates with excess Faraday
rotation of extragalactic radio sources over the length of the filament from b
~ 20 degr. to b ~ 55 degr. The density-weighted mean magnetic field is 2.8 +/-
0.8 microgauss, derived from rotation measures and an empirical relation
between H-alpha emission measure and dispersion measure from Berkhuijsen et al.
(2006). In view of the uncertainties in the derived magnetic field strength, we
propose an alternative use of the available data, rotation measure and emission
measure, to derive a lower limit to the Alfven speed, weighted by electron
density n_e^1.5. We find lower limits to the Alfven speed that are comparable
to, or larger than the sound speed in a 10^4 K plasma, and conclude that the
magnetic field is dynamically important. We discuss the role of
intermediate-velocity gas as a locus of Faraday rotation in the interstellar
medium, and propose this lower limit to the Alfven speed may also be applicable
to Faraday rotation by galaxy clusters.",1606.00415v1
2016-06-12,Using SDO/HMI magnetograms as a source of the solar mean magnetic field data,"The solar mean magnetic field (SMMF) provided by the Wilcox Solar Observatory
(WSO) is compared with the SMMF acquired by the \textit{Helioseismic and
Magnetic Imager} (HMI) onboard the \textit{Solar Dynamic Observatory} (SDO). We
found that despite the different spectral lines and measurement techniques used
in both instruments the Pearson correlation coefficient between these two
datasets equals 0.86 while the conversion factor is very close to unity: B(HMI)
= 0.99(2)B(WSO). We also discuss artifacts of the SDO/HMI magnetic field
measurements, namely the 12 and 24-hour oscillations in SMMF and in sunspots
magnetic fields that might be caused by orbital motions of the spacecraft. The
artificial harmonics of SMMF reveal significant changes in amplitude and the
nearly stable phase. The connection between the 24-hour harmonic amplitude of
SMMF and the presence of sunspots is examined. We also found that opposite
phase artificial 12 and/or 24-hour oscillations exist in sunspots of opposite
polarities.",1606.03710v1
2016-06-20,Fundamental Implications of Intergalactic Magnetic Field Observations,"Helical intergalactic magnetic fields at the $\sim 10^{-14}~{\rm G}$ level on
$\sim 10~{\rm Mpc}$ length scales are indicated by current gamma ray
observations. The existence of magnetic fields in cosmic voids and their
non-trivial helicity suggest that they must have originated in the early
universe and thus have implications for the fundamental interactions. I combine
present knowledge of the observational constraints and the dynamics of
cosmological magnetic fields to derive characteristics that would need to be
explained by the magnetic field generation mechanism. The importance of CP
violation and a possible crucial role for chiral effects in the early universe
are pointed out.",1606.06186v2
2016-06-23,QCD Phase-transition and chemical freezeout in nonzero magnetic field at NICA,"Because of relativistic off-center motion of the charged spectators and the
local momentum-imbalance experienced by the participants, a huge magnetic field
is likely generated in high-energy collisions. The influence of such
short-lived magnetic field on the QCD phase-transition(s) shall be analysed.
From Polyakov linear-sigma model, we study the chiral phase-transition and the
magnetic response and susceptibility in dependence on temperature, density and
magnetic field strength. The systematic measurements of the phase-transition
characterizing signals, such as the fluctuations, the dynamical correlations
and the in-medium modifications of rho-meson, for instance, in different
interacting systems and collision centralities are conjectured to reveal an
almost complete description for the QCD phase-structure and the chemical
freezeout. We limit the discussion to NICA energies.",1606.09153v1
2016-06-30,Landau levels of cold dense quark matter in a strong magnetic field,"The occupied Landau levels of strange quark matter are investigated in the
framework of the SU(3) NJL model with a conventional coupling and a
magnetic-field dependent coupling respectively. At lower density, the Landau
levels are mainly dominated by u and d quarks. Threshold values of the chemical
potential for the s quark onset are shown in the $\mu$-$B$ plane. The
magnetic-field-dependent running coupling can broaden the region of
three-flavor matter by decreasing the dynamical masses of $s$ quarks. Before
the onset of $s$ quarks, the Landau level number of light quarks is directly
dependent on the magnetic field strength $B$ by a simple inverse proportional
relation $k_{i,\mathrm{max}}\approx B_i^0/B$ with $B_d^0=5\times 10^{19}$ G,
which is approximately 2 times $B_u^0$ of $u$ quarks at a common chemical
potential. When the magnetic field increases up to $B^0_d$, almost all three
flavors are lying in the lowest Landau level.",1606.09336v2
2016-07-18,A low upper limit on the subsurface rise speed of solar active regions,"Magnetic field emerges at the surface of the Sun as sunspots and active
regions. This process generates a poloidal magnetic field from a rising
toroidal flux tube, it is a crucial but poorly understood aspect of the solar
dynamo. The emergence of magnetic field is also important because it is a key
driver of solar activity. We show that measurements of horizontal flows at the
solar surface around emerging active regions, in combination with numerical
simulations of solar magnetoconvection, can constrain the subsurface rise speed
of emerging magnetic flux. The observed flows imply that the rise speed of the
magnetic field is no larger than 150 m/s at a depth of 20 Mm, that is, well
below the prediction of the (standard) thin flux tube model but in the range
expected for convective velocities at this depth. We conclude that convective
flows control the dynamics of rising flux tubes in the upper layers of the Sun
and cannot be neglected in models of flux emergence.",1607.05250v1
2016-08-08,Low-energy magnetoelectric control of domain states in exchange-coupled heterostructures,"The electric manipulation of antiferromagnets has become an area of great
interest recently for zero-stray-field spintronic devices, and for their rich
spin dynamics. Generally, the application of antiferromagnetic media for
information memories and storage requires a heterostructure with a
ferromagnetic layer for readout through the exchange-bias field. In
magnetoelectric and multiferroic antiferromagnets, the exchange coupling exerts
an additional impediment (energy barrier) to magnetization reversal by the
applied magnetoelectric energy. We proposed and verified a method to overcome
this barrier. We controlled the energy required for switching the magnetic
domains in magnetoelectric \cro films by compensating the exchange-coupling
energy from the ferromagnetic layer with the Zeeman energy of a small
volumetric spontaneous magnetization found for the sputtered \cro films. Based
on a simplified phenomenological model of the field-cooling process, the
magnetic and electric fields required for switching could be tuned. As an
example, the switching of antiferromagnetic domains around a zero-threshold
electric field was demonstrated at a magnetic field of 2.6 kOe.",1608.02390v2
2016-08-24,Maximizing Influence in an Ising Network: A Mean-Field Optimal Solution,"Influence maximization in social networks has typically been studied in the
context of contagion models and irreversible processes. In this paper, we
consider an alternate model that treats individual opinions as spins in an
Ising system at dynamic equilibrium. We formalize the \textit{Ising influence
maximization} problem, which has a natural physical interpretation as
maximizing the magnetization given a budget of external magnetic field. Under
the mean-field (MF) approximation, we present a gradient ascent algorithm that
uses the susceptibility to efficiently calculate local maxima of the
magnetization, and we develop a number of sufficient conditions for when the MF
magnetization is concave and our algorithm converges to a global optimum. We
apply our algorithm on random and real-world networks, demonstrating,
remarkably, that the MF optimal external fields (i.e., the external fields
which maximize the MF magnetization) shift from focusing on high-degree
individuals at high temperatures to focusing on low-degree individuals at low
temperatures. We also establish a number of novel results about the structure
of steady-states in the ferromagnetic MF Ising model on general graph
topologies, which are of independent interest.",1608.06850v2
2016-09-09,The effect of magnetic field on mean flow generation by rotating two-dimensional convection,"Motivated by the significant interaction of convection, rotation and magnetic
field in many astrophysical objects, we investigate the interplay between
large-scale flows driven by rotating convection and an imposed magnetic field.
We utilise a simple model in two dimensions comprised of a plane layer that is
rotating about an axis inclined to gravity. It is known that this setup can
result in strong mean flows; we numerically examine the effect of an imposed
horizontal magnetic field on such flows. We show that increasing the field
strength in general suppresses the time-dependent mean flows, but in some cases
it organises them leading to stronger time-averaged flows. Further, we discuss
the effect of the field on the correlations responsible for driving the flows
and the competition between Reynolds and Maxwell stresses. A change in
behaviour is observed when the (fluid and magnetic) Prandtl numbers are
decreased. In the smaller Prandtl number regime, it is shown that significant
mean flows can persist even when the quenching of the overall flow velocity by
the field is relatively strong.",1609.02693v1
2016-09-12,Spin decoherence of magnetic atoms on surfaces,"We review the problem of spin decoherence of magnetic atoms deposited on a
surface. Recent breakthroughs in scanning tunnelling microscopy (STM) make it
possible to probe the spin dynamics of individual atoms, either isolated or
integrated in nanoengineered spin structures. Transport pump and probe
techniques with spin polarized tips permit measuring the spin relaxation time
$T_1$, while novel demonstration of electrically driven STM single spin
resonance has provided a direct measurement of the spin decoherence time $T_2$
of an individual magnetic adatom. Here we address the problem of spin
decoherence from the theoretical point of view. First we provide a short
general overview of decoherence in open quantum systems and we discuss with
some detail ambiguities that arise in the case of degenerate spectra, relevant
for magnetic atoms. Second, we address the physical mechanisms that allows
probing the spin coherence of magnetic atoms on surfaces. Third, we discuss the
main spin decoherence mechanisms at work on a surface, most notably, Kondo
interaction, but also spin-phonon coupling and dephasing by Johnson noise.
Finally, we propose some schemes to engineer spin decoherence.",1609.03389v2
2016-09-16,Heat and momentum transfer for magnetoconvection in a vertical external magnetic field,"The scaling theory of Grossmann and Lohse (J. Fluid Mech. 407, 27 (2000)) for
the turbulent heat and momentum transfer is extended to the magnetoconvection
case in the presence of a (strong) vertical magnetic field. The comparison with
existing laboratory experiments and direct numerical simulations in the
quasistatic limit allows to restrict the parameter space to very low Prandtl
and magnetic Prandtl numbers and thus to reduce the number of unknown
parameters in the model. Also included is the Chandrasekhar limit for which the
outer magnetic induction field B is large enough such that convective motion is
suppressed and heat is transported by diffusion. Our theory identifies four
distinct regimes of magnetoconvection which are distinguished by the strength
of the outer magnetic field and the level of turbulence in the flow,
respectively.",1609.05031v1
2016-09-20,Static and Dynamic Magnetic Response of Fragmented Haldane-like Spin Chains in Layered Li3Cu2SbO6,"The structure and the magnetic properties of layered Li3Cu2SbO6 are
investigated by powder X-ray diffraction, static susceptibility, and electron
spin resonance studies up to 330 GHz. The XRD data experimentally verify the
space group C2/m with halved unit cell volume in contrast to previously
reported C2/c. In addition, the data show significant Li/Cu-intersite exchange.
Static magnetic susceptibility and ESR measurements show two magnetic
contributions, i.e. quasi-free spins at low-temperature and a spin-gapped
magnetic subsystem, with about half of the spins being associated to each
subsystem. The data suggest ferromagnetic-antiferromagnetic alternating chains
with JFM = -285 K and JAFM = 160 K with a significant amount of Li-defects in
the chains. The results are discussed in the scenario of fragmented 1D S = 1
AFM chains with a rather high defect concentration of about 17% and associated
S = 1/2 edge states of the resulting finite Haldane chains.",1609.06091v1
2016-10-04,Magnetomechanical coupling and ferromagnetic resonance in magnetic nanoparticles,"We address the theory of the coupled lattice and magnetization dynamics of
freely suspended single-domain nanoparticles. Magnetic anisotropy generates
low-frequency satellite peaks in the microwave absorption spectrum and a
blueshift of the ferromagnetic resonance (FMR) frequency. The low-frequency
resonances are very sharp with maxima exceeding that of the FMR, because their
magnetic and mechanical precessions are locked, thereby suppressing Gilbert
damping. Magnetic nanoparticles can operate as nearly ideal motors that convert
electromagnetic into mechanical energy. The Barnett/Einstein-de Haas effect is
significant even in the absence of a net rotation.",1610.01072v2
2016-10-10,"Spin-Wave Spectrum in Magnetic Nanodot with Continuous Transition between Vortex, Bloch-type Skyrmion and Néel-type Skyrmion States","We study spin-wave excitations in a circular ferromagnetic nanodot in
different inhomogeneous, topologically non-trivial magnetization states,
specifically, vortex and skyrmion states. Gradual change in the strength of the
out-of-plane magnetic anisotropy and the Dzyaloshinskii-Moriya exchange
interaction leads to continuous phase transitions between different stable
magnetic configurations and allows for mapping of dynamic spin modes in and
between the vortex, Bloch-type skyrmion and N\'eel-type skyrmion states. Our
study elucidates the connections between gyrotropic modes, azimuthal spin waves
and breathing modes in various stable magnetization states and helps to
understand the rich spin excitation spectrum on the skyrmion background.",1610.02859v1
2016-10-28,On the ions acceleration via collisionless magnetic reconnection in laboratory plasmas,"This work presents an analysis of the ion outflow from magnetic reconnection
throughout fully kinetic simulations with typical laboratory plasmas values. A
symmetric initial configuration for the density and magnetic field is
considered across the current sheet. After analyzing the behavior of a set of
nine simulations with a reduced mass ratio and with a permuted value of three
initial electron temperature and magnetic field intensity, the best ion
acceleration scenario is further studied with a realistic mass ratio in terms
of the ion dynamics and energy budget. Interestingly, a series of shock waves
structures are observed in the outflow, resembling the shock discontinuities
found in recent magnetohydrodynamic (MHD) simulations. An analysis of the ion
outflow at several distances from the reconnection point is presented, in light
of possible laboratory applications. The analysis suggests that magnetic
reconnection could be used as a tool for plasma acceleration, with applications
ranging from electric propulsion to production of ion thermal beams.",1610.09104v1
2016-12-17,$Ξ_c γ\rightarrowΞ^\prime_c$ transition in lattice QCD,"We evaluate the electromagnetic $\Xi_c \gamma \rightarrow\Xi_c^\prime$
transition on 2+1 flavor lattices corresponding to a pion mass of $\sim 156$
MeV. We extract the magnetic Sachs and Pauli form factors which give the
$\Xi_c$-$\Xi_c^\prime$ transition magnetic moment and the decay rates of
$\Xi_c^\prime$ baryons. We did not find a signal for the magnetic form factor
of the neutral transition $\Xi_c^0 \gamma \rightarrow\Xi_c^{\prime 0}$, which
is suppressed by the U-spin flavor symmetry. As a byproduct, we extract the
magnetic form factors and the magnetic moments of $\Xi_c$ and $\Xi_c^\prime$
baryons, which give an insight to the dynamics of $u/d$, $s$ and $c$ quarks
having masses at different scales.",1612.05722v2
2016-12-19,On the fragmentation boundary in magnetised self-gravitating discs,"We investigate the role of magnetic fields in the fragmentation of
self-gravitating discs using 3D global ideal magnetohydrodynamic simulations
performed with the ""phantom"" smoothed particle hydrodynamics code.
  For initially toroidal fields, we find two regimes. In the first, where the
cooling time is greater than five times the dynamical time, magnetic fields
reduce spiral density wave amplitudes, which in turn suppresses fragmentation.
This is the case even if the magnetic pressure is only a tenth of the thermal
pressure. The second regime occurs when the cooling time is sufficiently short
that magnetic fields cannot halt fragmentation.
  We find that magnetised discs produce more massive fragments, due to both the
additional pressure exerted by the magnetic field, and the additional angular
momentum transport induced by Maxwell stresses. The fragments are confined to a
narrower range of initial semimajor axes than those in unmagnetised discs. The
orbital eccentricity and inclination distributions of unmagnetised and
magnetised disc fragments are similar. Our results suggest the fragmentation
boundary could be at cooling times a factor of two lower than predicted by
purely hydrodynamical models.",1612.06145v1
2017-01-10,"Dynamos and Differential Rotation: Advances at the Crossroads of Analytics, Numerics, and Observations","The recent observational, theoretical, and numerical progress made in
understanding stellar magnetism is discussed. Particularly, this review will
cover the physical processes thought to be at the origin of these magnetic
fields and their variability, namely dynamo action arising from the interaction
between convection, rotation, radiation and magnetic fields. Some care will be
taken to cover recent analytical advances regarding the dynamics and magnetism
of radiative interiors, including some thoughts on the role of a tachocline.
Moreover, recent and rapidly advancing numerical modeling of convective dynamos
will be discussed, looking at rapidly rotating convective systems, grand minima
and scaling laws for magnetic field strength. These topics are linked to
observations or their observational implications.",1701.02591v1
2017-01-17,Corotating Magnetic Reconnection Site in Saturn's Magnetosphere,"Using measurements from the Cassini spacecraft in Saturn's magnetosphere, we
propose a 3D physical picture of a corotating reconnection site, which can only
be driven by an internally generated source. Our results demonstrate that the
corotating magnetic reconnection can drive an expansion of the current sheet in
Saturn's magnetosphere and, consequently, can produce Fermi acceleration of
electrons. This reconnection site lasted for longer than one of Saturn's
rotation period. The long-lasting and corotating natures of the magnetic
reconnection site at Saturn suggest fundamentally different roles of magnetic
reconnection in driving magnetospheric dynamics (e.g., the auroral
precipitation) from the Earth. Our corotating reconnection picture could also
potentially shed light on the fast rotating magnetized plasma environments in
the solar system and beyond.",1701.04559v2
2017-02-02,In-vivo biomagnetic characterisation of the American cockroach,"We present a quantitative method, utilising a highly sensitive quantum
sensor, that extends applicability of magnetorelaxometry to biological samples
at physiological temperature. The observed magnetic fields allow for
non-invasive determination of physical properties of magnetic materials and
their surrounding environment inside the specimen. The method is applied to
American cockroaches and reveals magnetic deposits with strikingly different
behaviour in alive and dead insects. We discuss consequences of this finding to
cockroach magneto-reception. To our knowledge, this work represents the first
characterisation of the magnetisation dynamics in live insects and helps to
connect results from behavioural experiments on insects in magnetic fields with
characterisation of magnetic materials in their corpses.",1702.00538v2
2017-02-02,A majority gate with chiral magnetic solitons,"In magnetic materials nontrivial spin textures may emerge owing to the
competition among different types of magnetic interactions. Among such spin
textures, chiral magnetic solitons represent topologically protected spin
configurations with particle-like properties. Based on atomistic spin dynamics
simulations, we demonstrate that these chiral magnetic solitons are ideal to
use for logical operations, and we demonstrate the functionality of a
three-input majority gate, in which the input states can be controlled by
applying an external electromagnetic field or spin-polarized currents. One of
the main advantages of the proposed device is that the input and output signals
are encoded in the chirality of solitons, that may be moved, allowing to
perform logical operations using only minute electric currents. As an example
we illustrate how the three input majority gate can be used to perform logical
relations, such as Boolean AND and OR.",1702.00579v1
2017-02-07,Propulsion and controlled steering of magnetic nanohelices,"Externally controlled motion of micro and nanomotors in a fluid environment
constitutes a promising tool in biosensing, targeted delivery and environmental
remediation. In particular, recent experiments have demonstrated that fuel-free
propulsion can be achieved through the application of external magnetic fields
on magnetic helically shaped structures. The magnetic interaction between
helices and the rotating field induces a torque that rotates and propels them
via the coupled rotational-translational motion. Recent works have shown that
there exist certain optimal geometries of helical shapes for propulsion.
However, experiments show that controlled motion remains a challenge at the
nanoscale due to Brownian motion that interferes with the deterministic motion
and makes it difficult to achieve controlled steering. In the present work we
employ quantitatively accurate simulation methodology to design a setup for
which magnetic nanohelices of 30 nm in radius, with and without cargo, can be
accurately propelled and steered in the presence of thermal fluctuations. In
particular, we demonstrate fast transport of such nanomotors and devise
protocols in manipulating external fields to achieve directionally controlled
steering at biologically relevant temperatures.",1702.01989v1
2017-02-16,Microscopic theory of electrically induced spin torques in magnetic Weyl semimetals,"We theoretically study electrical responses of magnetization in Weyl
semimetals. The Weyl semimetal is a new class of topological semimetals,
possessing hedgehog type spin textures in momentum space. Because of this
peculiar spin texture, an interplay of electron transport and spin dynamics
might provide new method to electrical control of magnetization. In this paper,
we consider the magnetically doped Weyl semimetals, and systematically study
current- and charge-induced spin torque exerted on the local magnetization in
three-dimensional Dirac-Weyl metals. We determine all current-induced spin
torques including spin-orbit torque, spin-transfer torque, and the so-called
$\beta$-term, up to first order with respect to spatial and temporal derivation
and electrical currents. We find that spin-transfer torque and $\beta$-term are
absent while spin-orbit torque is proportional to the axial current density. We
also calculate the charge-induced spin torque microscopically. We find the
charge-induced spin torque originates from the chiral anomaly due to the
correspondence between spin operators and axial current operators in our model.",1702.04918v1
2017-03-08,Spin dynamics of antiferromagnets in the presence of a homogeneous magnetization,"We use general hydrodynamic equations to determine the long-wavelength spin
excitations in isotropic antiferromagnets in the presence of a homogeneous
magnetization. The latter may be induced, such as in antiferromagnets in an
external magnetic field, or spontaneous, such as in ferrimagnetic or canted
phases that are characterized by the coexistence of antiferromagnetic and
ferromagnetic order. Depending on the physical situation, we find propagating
spin waves that are gapped in some cases and gapless in others, diffusive
modes, or relaxational modes. The excitation spectra turn out to be
qualitatively different depending on whether or not the homogeneous
magnetization is a conserved quantity. The results lay the foundation for a
description of a variety of quantum phase transitions, including the transition
from a ferromagnetic metal to an antiferromagnetic one, and the spin-flop
transitions that are observed in some antiferromagnets. They also are crucial
for incorporating weak-localization and Altshuler-Aronov effects into the
descriptions of quantum phases in both clean and disordered magnetic metals.",1703.03009v1
2017-03-15,Model-Free Based Digital Control for Magnetic Measurements,"This paper presents a novel digital control strategy successfully implemented
for a soft magnetic material characterization bench (Epstein frame type). The
main objective is to control the magnetic induction waveform whatever the
applied excitation and the material under study. Given the nonlinear nature of
the magnetization curves of magnetic materials, an original model-free based
control technique is considered. Special mention should be made of the
interesting dynamic properties in closed-loop against the changes of the
operating point related basically to the hysteresis form. The operation and the
performances of the digital control method are illustrated in different working
conditions through both simulation and experimental measurements.",1703.05395v1
2017-03-20,Vortices in two-dimensional nanorings studied by means of the dynamical matrix method,"This paper concerns an investigation of the spin wave excitations in magnetic
nanoparticles. We provide a detailed derivation of the theoretical method for
the determination of the normal modes of confined magnetic systems based on a
discrete lattice of magnetic moments. The method is based on the damping-free
Landau-Lifshitz equation and general enough to be utilized for the magnetic
system of any dimensionality, magnetic structure, shape, and size. As an
example we explore the influence of the competition between exchange and
dipolar interactions on the spectrum of normal modes as well as on the
stability of the vortex state in two-dimensional nanorings. We show the
lowest-frequency mode to be indicative of the dipolar-to-exchange iterations
ratio. We also study behavior of the fundamental mode and present the influence
of both, the discreteness of the lattice and the dipolar-to-exchange iterations
ratio, on its hybridization with azimuthal modes. We complete the paper with a
selective review of the spin wave excitations in circular dots to compare with
the results obtained for the rings.",1703.06678v1
2017-03-24,Evolution of magnetic and dielectric properties in Sr-substituted high temperature multiferroic YBaCuFeO5,"We report the evolution of structural, magnetic and dielectric properties due
to partial substitution of Ba by Sr in the high temperature multiferroic
YBaCuFeO5. This compound exhibits ferroelectric and antiferromagnetic
transitions around 200 K and these two phenomena are presumed to be coupled
with each other. Our studies on magnetic and dielectric properties of the
YBa1-xSrxCuFeO5 (x = 0.0, 0.25 and 0.5) show that substitution of Sr shifts
magnetic transition towards higher temperature whereas dielectric transition to
lower temperature. These results points to the fact that magnetic and
dielectric transitions get decoupled as a result of chemical pressure in form
of Sr substitution. The nature of magnetodielectric coupling changes across the
series with the presence of higher order coupling terms. Additionally in these
compounds glassy dynamics of electric dipoles is observed at low temperatures.",1703.08319v1
2017-04-07,Terahertz spectroscopy of crystal-field transitions in magnetoelectric TmAl3(BO3)4,"Dynamic magnetic properties of magnetoelectric TmAl3(BO3)4 borate have been
investigated by terahertz spectroscopy. Crystal field (CF) transitions within
the ground multiplet 3H6 of Tm3+ ions are observed and they are identified as
magnetic-dipole transitions from the ground singlet A1 to the next excited
doublet E of Tm3+ ions. Unexpected fine structure of the transitions is
detected at low temperatures. The new modes are assigned to local distortions
of the sites with D3 symmetry by Bi3+ impurities, which resulted in the
splitting of A1 -> E transition. Two types of locally distorted sites are
identified and investigated. The main contribution to the static magnetic
susceptibility is shown to be determined by the matrix elements of the observed
magnetic transitions. We demonstrate that even in case of local distortions the
symmetry of the undistorted crystal is recovered for magnetic and for quadratic
magnetoelectric susceptibilities.",1704.02198v1
2017-04-11,Flux noise in a superconducting transmission line,"We study a superconducting transmission line (TL) formed by distributed LC
oscillators and excited by external magnetic fluxes which are aroused from
random magnetization (A) placed in substrate or (B) distributed at interfaces
of a two-wire TL. Low-frequency dynamics of a random magnetic field is
described based on the diffusion Langevin equation with a short-range source
caused by (a) random amplitude or (b) gradient of magnetization. For a TL
modeled as a two-port network with open and shorted ends, the effective
magnetic flux at the open end has non-local dependency on noise distribution
along the TL. The flux-flux correlation function is evaluated and analyzed for
the regimes (Aa), (Ab). (Ba), and (Bb). Essential frequency dispersion takes
place around the inverse diffusion time of random flux along the TL. Typically,
noise effect increases with size faster than the area of TL. The flux-flux
correlator can be verified both via the population relaxation rate of the
qubit, which is formed by the Josephson junction shunted by the TL with flux
noises, and via random voltage at the open end of the TL.",1704.03408v2
2017-04-13,Majorana flipping of quarkonium spin states in transient magnetic field,"We demonstrate that spin flipping transitions occur between various
quarkonium spin states due to transient magnetic field produced in non central
heavy ion collisions (HICs). The inhomogeneous nature of the magnetic field
results in \textit{non adiabatic evolution} of (spin)states of quarkonia moving
inside the transient magnetic environment. Our calculations explicitly show
that the consideration of azimuthal inhomogeneity gives rise to dynamical
mixing between different spin states owing to Majorana spin flipping. Notably,
this effect of non-adiabaticity is novel and distinct from previously predicted
mixing of the singlet and one of the triplet states of quarkonia in the
presence of a static and homogeneous magnetic field.",1704.04094v2
2017-04-14,"Structures, phase transitions, and magnetic properties of Co3Si from first-principles calculations","Co3Si was recently reported to exhibit remarkable magnetic properties in the
nanoparticle form [Appl. Phys. Lett. 108, 152406 (2016)], yet better
understanding of this material is to be promoted. Here we report a study on the
crystal structures of Co3Si using adaptive genetic algorithm, and discuss its
electronic and magnetic properties from first-principles calculations. Several
competing phases of Co3Si have been revealed from our calculations. We show
that the hexagonal Co3Si structure reported in experiments has lower energy in
non-magnetic state than ferromagnetic state at zero temperature. The
ferromagnetic state of the hexagonal structure is dynamically unstable with
imaginary phonon modes and transforms to a new orthorhombic structure, which is
confirmed by our structure searches to have the lowest energy for both Co3Si
and Co3Ge. Magnetic properties of the experimental hexagonal structure and the
lowest-energy structures obtained from our structure searches are investigated
in detail.",1704.04514v2
2017-04-26,Experimental investigation of surface instability of a thin layer of a magnetic fluid,"In the present work the instability of a flat horizontal thin layer of a
magnetic fluid (the depth of no more than 50 \mum) under the action of a
uniform magnetic field is studied experimentally. It was revealed that the
development of instability under the action of tilted magnetic field can lead
to the formation of parallel ridges on a fluid surface; the ridges undergo a
transformation into hexagonal system of conical peaks with the magnetic field
increasing. The necessary conditions for the formation of these surface
patterns are studied. It was found that the development of instability of quite
thin layers may result in layers breakup. The dependencies of instability wave
number on the system physical parameters are obtained. The time for the
development of instability is measured. The experimental results are compared
with the existing theory and discussed.",1704.08304v1
2017-05-23,Magnetic field dynamos and magnetically triggered flow instabilities,"The project A2 of the LIMTECH Alliance aimed at a better understanding of
those magnetohydrodynamic instabilities that are relevant for the generation
and the action of cosmic magnetic fields. These comprise the hydromagnetic
dynamo effect and various magnetically triggered flow instabilities, such as
the magnetorotational instability and the Tayler instability. The project was
intended to support the experimental capabilities to become available in the
framework of the DREsden Sodium facility for DYNamo and thermohydraulic studies
(DRESDYN). An associated starting grant was focused on the dimensioning of a
liquid metal experiment on the newly found magnetic destabilization of rotating
flows with positive shear. In this paper, the main results of these two
projects are summarized.",1705.08189v1
2017-05-29,Double Hall instability: A catalyzer of magnetic energy release,"A pictorial explanation for shear-Hall instability is suggested and shows
that the shear flow is not necessary for the instability because its role can
be played by the Hall effect of an inhomogeneous background magnetic field.
Linear stability analysis for a simple model of magnetic field varying
periodically in space confirms such a 'double Hall' instability. Numerical
computations show a considerable increase in Ohmic dissipation rate at the
nonlinear stage of instability development. Field dissipation has a spiky
character associated with magnetic reconnection in current sheets and X-points.
Double Hall instability can be significant for magnetic field dissipation in
neutron star crusts and, possibly, in the solar corona.",1705.10077v3
2017-06-11,Magnetization spin dynamics in a (LuBi)3Fe5O12 (BLIG) epitaxial film,"Bismuth substituted lutetium iron garnet (BLIG) films exhibit larger Faraday
rotation, and have a higher Curie temperature than yttrium iron garnet. We have
observed magnetic stripe domains and measured domain widths of 1.4 {\mu}{\mu}m
using Fourier domain polarization microscopy, Faraday rotation experiments
yield a coercive field of 5 Oe. These characterizations form the basis of
micromagnetic simulations that allow us to estimate and compare spin wave
excitations in BLIG films. We observed that these films support thermal magnons
with a precessional frequency of 7 GHz with a line width of 400 MHz. Further,
we studied the dependence of precessional frequency on the externally applied
magnetic field. Brillouin light scattering experiments and precession
frequencies predicted by simulations show similar trend with increasing field.",1706.03340v1
2017-08-02,Realistic finite temperature simulations of magnetic systems using quantum statistics,"We have performed realistic atomistic simulations at finite temperatures
using Monte Carlo and atomistic spin dynamics simulations incorporating quantum
(Bose-Einstein) statistics. The description is much improved at low
temperatures compared to classical (Boltzmann) statistics normally used in
these kind of simulations, while at higher temperatures the classical
statistics are recovered. This corrected low-temperature description is
reflected in both magnetization and the magnetic specific heat, the latter
allowing for improved modeling of the magnetic contribution to free energies. A
central property in the method is the magnon density of states at finite
temperatures and we have compared several different implementations for
obtaining it. The method has no restrictions regarding chemical and magnetic
order of the considered materials. This is demonstrated by applying the method
to elemental ferromagnetic systems, including Fe and Ni, as well as Fe-Co
random alloys and the ferrimagnetic system GdFe$_3$ .",1708.00709v1
2017-08-16,Magnetoconductance signatures of chiral domain-wall bound states in magnetic topological insulators,"Recent magnetoconductance measurements performed on magnetic topological
insulator candidates have revealed butterfly-shaped hysteresis. This hysteresis
has been attributed to the formation of gapless chiral domain-wall bound states
during a magnetic field sweep. We treat this phenomenon theoretically,
providing a link between microscopic magnetization dynamics and butterfly
hysteresis in magnetoconductance. Further, we illustrate how a spatially
resolved conductance measurement can probe the most striking feature of the
domain-wall bound states: their chirality. This work establishes a regime where
a definitive link between butterfly hysteresis in longitudinal
magneto-conductance and domain-wall bound states can be made. This analysis
provides an important tool for the identification of magnetic topological
insulators.",1708.05032v2
2017-08-18,Antiferromagnetic nano-oscillator in external magnetic fields,"We describe the dynamics of an antiferromagnetic nano-oscillator in an
external magnetic field of any given time distribution. The oscillator is
powered by a spin current originating from spin-orbit effects in a neighboring
heavy metal layer, and is capable of emitting a THz signal in the presence of
an additional easy-plane anisotropy. We derive an analytical formula describing
the interaction between such a system and an external field, which can affect
the output signal character. Interactions with magnetic pulses of different
shapes, with a sinusoidal magnetic field and with a sequence of rapidly
changing magnetic fields are discussed. We also perform numerical simulations
based on the Landau-Lifshitz-Gilbert equation with spin-transfer torque effects
to verify the obtained results and find a very good quantitative agreement
between analytical and numerical predictions.",1708.05590v2
2017-09-08,Dynamics of trapped magnetic flux in superconducting FeTe0.65Se0.35,"The magnetic moment in the superconducting and normal state of a crystalline
FeTe0.65Se0.35 superconductor, grown by the Bridgman's method with relatively
high growth rate, was measured. The temperature and magnetic field dependences
of magnetization and its relaxation time were determined. Studied crystal,
being non-uniform due to high growth rate of 5 mm/h, exhibits smaller width of
superconducting transition in comparison with an ideal crystal grown with
velocity of 1 mm/h, and the difference in magnetic properties of crystals grown
with various growth rate, related to their microstructure, is discussed.",1709.02627v1
2017-09-28,Monolayer phosphorene under time-dependent magnetic field,"We obtain the exact wave function of a monolayer phosphorene under a low-
intensity time-dependent magnetic field using the dynamical invariant method.
We calculate the quantum-mechanical energy expectation value and the transition
probability for a constant and an oscillatory magnetic field. For the former we
observe that the Landau level energy varies linearly with the quantum numbers n
and m and the magnetic field intensity B_0. No transition takes place. For the
latter, we observe that the energy oscillates in time, increasing linearly with
the Landau level n and m and nonlinearly with the magnetic field. The (k,l) to
(n,m) transitions take place only for l=m. We investigate the (0,0) to (n,0)
and (1,l) and (2,l) probability transitions.",1709.10188v1
2017-10-03,Nonlinear Phononic Control and Emergent Magnetism in Mott Insulating Titanates,"Optical control of structure-driven magnetic order offers a platform for
magneto-optical terahertz devices. We control the magnetic phases of $d^1$ Mott
insulating titanates using nonlinear phononics to transiently perturb the
atomic structure based on density functional theory (DFT) simulations and
solutions to a lattice Hamiltonian including nonlinear multi-mode interactions.
We show that magnetism is tuned by indirect excitation of a Raman-active phonon
mode, which affects the amplitude of the TiO$_6$ octahedral rotations that
couple to static Ti--O Jahn-Teller distortions, through infrared-active phonon
modes of LaTiO$_3$ and YTiO$_3$. The mode excitation reduces the rotational
angle, driving a magnetic phase transition from ferromagnetic (FM) to $A$-type
antiferromagnetic (AFM), and finally a $G$-type AFM state. This novel $A$-AFM
state arises from a change in the exchange interactions and is absent in the
bulk equilibrium phase diagram, but it emerges as a dynamically accessible
optically induced state under multi-mode excitations. Our work shows nonlinear
phononic coupling is able to stabilize phases inaccessible to static chemical
pressure or epitaxial strain.",1710.00993v2
2017-10-09,Weak magnetic anisotropy in GdRh$_2$Si$_2$ studied by magnetic resonance,"The antiferromagnetically (AFM) ordered state of GdRh$_{2}$Si$_{2}$ which
consists of AFM-stacked ferromagnetic layers is investigated by magnetic
resonance spectroscopy. The almost isotropic Gd$^{3+}$ paramagnetic resonance
becomes anisotropic in the AFM ordered region below 107 K. The emerging
internal anisotropic exchange-fields are still small enough to allow an
investigation of their magnetization dynamics by using a standard
microwave-frequency magnetic resonance technique. We could characterize this
anisotropy in detail in the ferromagnetic layers of the excitation at 9 and 34
GHz. We derived a resonance condition for the AFM ordered phase to describe the
weak in-plane anisotropic behaviour in combination with a mean-field analysis.",1710.03079v1
2017-11-01,Far-field theory for trajectories of magnetic ellipsoids in rectangular and circular channels,"We report a method to control the positions of ellipsoidal magnets in flowing
channels of rectangular or circular cross section at low Reynolds number.A
static uniform magnetic field is used to pin the particle orientation, and the
particles move with translational drift velocities resulting from hydrodynamic
interactions with the channel walls which can be described using Blake's image
tensor.Building on his insights, we are able to present a far-field theory
predicting the particle motion in rectangular channels, and validate the
accuracy of the theory by comparing to numerical solutions using the boundary
element method.We find that, by changing the direction of the applied magnetic
field, the motion can be controlled so that particles move either to a curved
focusing region or to the channel walls.We also use simulations to show that
the particles are focused to a single line in a circular channel.Our results
suggest ways to focus and segregate magnetic particles in lab-on-a-chip
devices.",1711.00376v1
2017-11-03,Heteronuclear magnetisms with ultracold spinor bosonic gases in optical lattices,"Motivated by recent realizations of spin-1 NaRb mixtures in the experiments,
here we investigate heteronuclear magnetism in the Mott-insulating regime.
Different from the identical mixtures where the boson (fermion) statistics only
admits even (odd) parity states from angular momentum composition, for
heteronuclear atoms in principle all angular momentum states are allowed, which
can give rise to new magnetic phases. Various magnetic phases can be developed
over these degenerate spaces, however, the concrete symmetry breaking phases
depend not only on the degree of degeneracy, but also the competitions from
many-body interactions. We unveil these rich phases using the bosonic dynamical
mean-field theory approach. These phases are characterized by various orders,
including spontaneous magnetization order, spin magnitude order, singlet
pairing order and nematic order, which may coexist, especially in the regime
with odd parity. Finally we address the possible parameter regimes for
observing these spin-ordered Mott phases.",1711.01203v1
2017-11-19,Oblique propagation of electrostatic waves in a magnetized EPI plasma,"A theoretical investigation is carried out to understand the basic features
of nonlinear propagation of heavy ion-acoustic (HIA) waves subjected to an
external magnetic field in an electron-positron-ion (EPI) plasma which consists
of cold magnetized positively charged heavy ion fluids, and superthermal
distributed electrons and positrons. In the nonlinear regime, the Korteweg-de
Vries (K-dV) and modified K-dV(MK-dV) equations describing the propagation of
HIA waves are derived. The latter admits a solitary wave solution with both
positive and negative potentials (for K-dV equation) and only positive
potential (for mK-dV equation) in the weak amplitude limit. It is observed that
the effects of external magnetic field (obliqueness), superthermal electrons
and positrons, different plasma species concentration, heavy ion dynamics and
temperature ratio significantly modify the basic features of solitary waves
(SWs). The application of the results in a magnetized EPI plasma, which occurs
in many astrophysical objects (e.g., pulsars, cluster explosions, and active
galactic nuclei) is briefly discussed.",1711.06975v1
2017-11-20,"Azimuthal Spin Wave Excitations in Magnetic Nanodots over the Soliton Background: Vortex, Bloch and Néel-like skyrmions","We study azimuthal spin-wave (SW) excitations in a circular ferromagnetic
nanodot in different inhomogeneous, topologically non-trivial magnetization
states, specifically, vortex, Bloch-type skyrmion and N\'eel-type skyrmion
states. Continuous mapping of the SW spectrum between these states is realized
with gradual change of the out-of-plane magnetic anisotropy and
Dzyaloshinskii-Moriya exchange interaction (DMI). Our study shows lifting of
the SW frequencies degeneracy and change in systematics of the frequency
levels. The change is induced by a geometrical Berry phase, that is present for
the dot-edge localized SWs in a vortex state and vanishes in skyrmion states.
Furthermore, channeling of the azimuthal SWs localized at the skyrmion edge is
present and induces large frequency splitting. This is attributed to DMI
induced nonreciprocity, while coupling of the breathing and gyrotropic modes is
related to soliton motion. Finally, an efficient coupling of the dynamic
magnetization in the skyrmion state to uniform magnetic field in nanodots with
non-circular symmetry is shown.",1711.07186v1
2017-12-02,Coherent transport structures in magnetized plasmas II: Numerical results,"In a pair of linked articles (called Article I and II respectively) we apply
the concept of Lagrangian Coherent Structures borrowed from the study of
Dynamical Systems to magnetic field configurations in order to separate regions
where field lines have different kind of behavior. In the present article,
article II, by means of a numerical procedure we investigate the Lagrangian
Coherent Structures in the case of a two-dimensional magnetic configuration
with two island chains that are generated by magnetic reconnection and evolve
nonlinearly in time. The comparison with previous results, obtained by assuming
a fixed magnetic field configuration, allows us to explore the dependence of
transport barriers on the particle velocity.",1712.00591v1
2017-12-07,Enhancement of Pressure Perturbations in Ablation due to Kinetic Magnetised Transport Effects under Direct-Drive ICF relevant conditions,"We present for the first time kinetic 2D Vlasov-Fokker-Planck simulations,
including both self-consistent magnetic fields and ablating ion outflow, of a
planar ablating foil subject to nonuniform laser irradiation. Even for small
hall parameters ($\omega \tau_{ei} \lesssim 0.05$) self-generated magnetic
fields are sufficient to invert and enhance pressure perturbations. The mode
inversion is caused by a combination of the Nernst advection of the magnetic
field and the Righi-Leduc heat-flux. Non-local effects modify these processes.
The mechanism is robust under plasma conditions tested; it is amplitude
independent and occurs for a broad spectrum of perturbation wavelengths,
$\lambda_p = 10-100\,$\mu m$. The ablating plasma response to a dynamically
evolving speckle pattern perturbation, analogous to an optically smoothed beam,
is also simulated. Similar to the single mode case, self-generated magnetic
fields increase the degree of nonuniformity at the ablation surface by up to an
order of magnitude and are found to preferentially enhance lower modes due to
the resistive damping of high mode number magnetic fields.",1712.02663v3
2017-12-18,Origin and dynamics of the Primordial Magnetic field in a parity violating plasma,"In this thesis, we have studied the generation and evolution of the magnetic
field in the early Universe. We investigated the generation of magnetic fields
in the presence of chiral symmetry and gravitational anomaly. We have used
modified kinetic theory by Berry curvature and derived expressions for magnetic
and vortical conductivities. We have also shown that there are two length
scales: kinetic and viscous, which is one of the important results. In the
latter part of the thesis, we have shown the effect of second-order viscous
effects on the modes of the magnetic fields.",1712.06291v2
2018-01-08,Low frequency spin dynamics in XY quantum spin ice Yb$_2$Pt$_2$O$_7$,"The XY pyrochlore Yb$_2$Ti$_2$O$_7$, with pseudo spin 1/2 at the Yb$^{3+}$
site, has been celebrated as potential host for the quantum spin ice state. The
substitution of non-magnetic Ti with Pt gives Yb$_2$Pt$_2$O$_7$, a system with
remarkably similar magnetic properties. The large nuclear gyromagnetic ratio
($\gamma_{n}/2 \pi = 9.15$~MHz/T) of $^{195}$Pt makes Yb$_2$Pt$_2$O$_7$ an
ideal material for NMR investigation of its unconventional magnetic properties.
Based on the $^{195}$Pt nuclear spin-lattice relaxation rate $1/T_1$ and the
magnetic specific heat $C_{p}$ measured in a broad range of magnetic field
$B_{ext}$, we demonstrate that the field-induced magnon gap linearly decreases
with $B_{ext}$ but additional low energy mode of spin excitations emerge below
$\sim 0.5$~T.",1801.02724v2
2018-01-15,Incompressible inviscid resistive MHD surface waves in 2D,"We consider the dynamics of a layer of an incompressible electrically
conducting fluid interacting with the magnetic field in a two-dimensional
horizontally periodic setting. The upper boundary is in contact with the
atmosphere, and the lower boundary is a rigid flat bottom. We prove the global
well-posedness of the inviscid and resistive problem with surface tension
around a non-horizontal uniform magnetic field; moreover, the solution decays
to the equilibrium almost exponentially. One of the key observations here is an
induced damping structure for the fluid vorticity due to the resistivity and
transversal magnetic field.",1801.04694v2
2018-01-18,The Of?p stars of the Magellanic Clouds: Are they strongly magnetic?,"All known Galactic Of?p stars have been shown to host strong, organized,
magnetic fields. Recently, five Of?p stars have been discovered in the
Magellanic Clouds. They posses photometric \citep{Naze} and spectroscopic
\citep{Walborn} variability compatible with the Oblique Rotator Model (ORM).
However, their magnetic fields have yet to be directly detected. We have
developed an algorithm allowing for the synthesis of photometric observables
based on the Analytic Dynamical Magnetosphere (ADM) model of \citet{Owocki}. We
apply our model to OGLE photometry in order to constrain their magnetic
geometries and surface dipole strengths. We predict that the field strengths
for some of these candidate extra-Galactic magnetic stars may be within the
detection limits of the FORS2 instrument.",1801.06152v1
2018-01-25,Magnetic orders in a Fermi gas induced by cavity-field fluctuations,"We study magnetic orders of fermions under cavity-assisted Raman couplings in
a one-dimensional lattice at half filling. The cavity-enhanced atom-photon
coupling introduces a dynamic long-range interaction between the fermions,
which competes with the short-range on-site interaction and leads to a variety
of magnetic orders. Adopting a numerical density-matrix-renormalization-group
method, we investigate the various magnetic orders and map out the steady-state
phase diagram. Interestingly, as all the phase transitions take place outside
the superradiant regime, the magnetic orders are associated with cavity-field
fluctuations with a vanishing number of photons on the mean-field level.",1801.08254v1
2018-01-29,Current-induced spin-orbit torques in ferromagnetic and antiferromagnetic systems,"Spin-orbit coupling in inversion-asymmetric magnetic crystals and structures
has emerged as a powerful tool to generate complex magnetic textures,
interconvert charge and spin under applied current, and control magnetization
dynamics. Current-induced spin-orbit torques mediate the transfer of angular
momentum from the lattice to the spin system, leading to sustained magnetic
oscillations or switching of ferromagnetic as well as antiferromagnetic
structures. The manipulation of magnetic order, domain walls and skyrmions by
spin-orbit torques provides evidence of the microscopic interactions between
charge and spin in a variety of materials and opens novel strategies to design
spintronic devices with potentially high impact in data storage, nonvolatile
logic, and magnonic applications. This paper reviews recent progress in the
field of spin-orbitronics, focusing on theoretical models, material properties,
and experimental results obtained on bulk noncentrosymmetric conductors and
multilayer heterostructures, including metals, semiconductors, and topological
insulator systems. Relevant aspects for improving the understanding and
optimizing the efficiency of nonequilibrium spin-orbit phenomena in future
nanoscale devices are also discussed.",1801.09636v2
2018-03-02,Laser induced phase transition in epitaxial FeRh layers studied by pump-probe valence band photoemission,"We use time-resolved X-ray photoelectron spectroscopy to probe the electronic
and magnetization dynamics in FeRh films after ultrafast laser excitations. We
present experimental and theoretical results which investigate the electronic
structure of the FeRh during the first-order phase transition identifying a
clear signature of the magnetic phase. We find that a spin polarized feature at
the Fermi edge is a fingerprint of the magnetic status of the system that is
independent of the long-range ferromagnetic alignment of the magnetic domains.
We use this feature to follow the phase transition induced by a laser pulse in
a pump-probe experiment and find that the magnetic transition occurs in less
than 50 ps, and reaches its maximum in 100 ps.",1803.00780v1
2018-03-04,Latitude dependence of convection and magnetic field generation in the cube,"The 3D thermal convection in the Boussinesq approximation with heating from
below and dynamo in the cube are considered. We study dependence of the
convection intensity and magnetic field generation on the latitude in
$\beta$-plane approximation. It is shown that kinetic energy gradually
increases from the poles to the equator more than order of magnitude. The model
predicts the strong azimuthal thermal wind, which direction depends on the sign
of the thermal convective fluctuations. The spatial scale of the arising flow
is comparable to the scale of the physical domain. The magnetic energy
increases as well, however dynamo efficiency, i.e., the ratio of the magnetic
energy to the kinetic one decreases to the equator. This effect can explain
predominance of the dipole configuration of the magnetic field observed in the
planets and stars. The approach is useful for modeling of the
magnetohydrodynamic turbulence in planetary cores and stellar convective zones.",1803.01344v1
2018-03-07,Magnetic-field-induced crossover from the inverse Faraday effect to the optical orientation in EuTe,"A time-resolved optical pump-probe technique has been applied for studying
the ultrafast dynamics in the magnetic semiconductor EuTe near the absorption
band gap. We show that application of external magnetic field up to 6 T results
in crossover from the inverse Faraday effect taking place on the femtosecond
time scale to the optical orientation phenomenon with an evolution in the
picosecond time domain. We propose a model which includes both these processes
possessing different spectral and temporal properties. The circularly polarized
optical pumping induces the optical electronic transition $4f^75d^0 \rightarrow
4f^65d^1$ forming the absorption band gap in EuTe. The observed crossover is
related to a strong magnetic-field shift of the band gap in EuTe at low
temperatures. It was found that manipulation of spin states on intrinsic defect
levels takes place on a time scale of 19 ps in the applied magnetic field of 6
T.",1803.02633v1
2018-03-12,Properties of Confined Ammonium Nitrate Ionic Liquids,"Ethylammonium nitrate (EAN) and propylammonium nitrate ionic liquids confined
between polar glass plates and exposed to a strong magnetic field demonstrate
gradually slowing diffusivity, a process that can be reversed by removing the
sample from the magnetic field. The process can be described well by the Avrami
equation, which is typical for autocatalytic (particularly, nucleation
controlled) processes. The transition can be stopped by freezing the sample.
Cooling and heating investigations showed differences in the freezing and
melting behavior of the sample depending on whether it had been exposed to the
magnetic field. After exposure to the magnetic field, the sample demonstrated a
change in the state of residual water. Generally, our findings confirm our
previous suggestion that alteration of the dynamic properties of confined
ammonium nitrate ionic liquids exposed to a magnetic field is related to the
alteration of real physical-chemical phases.",1803.04372v1
2018-03-29,Giant resonant nonlinear damping in nanoscale ferromagnets,"Magnetic damping is a key metric for emerging technologies based on magnetic
nanoparticles, such as spin torque memory and high-resolution biomagnetic
imaging. Despite its importance, understanding of magnetic dissipation in
nanoscale ferromagnets remains elusive, and the damping is often treated as a
phenomenological constant. Here we report the discovery of a giant
frequency-dependent nonlinear damping that strongly alters the response of a
nanoscale ferromagnet to spin torque and microwave magnetic field. This novel
damping mechanism originates from three-magnon scattering that is strongly
enhanced by geometric confinement of magnons in the nanomagnet. We show that
the giant nonlinear damping can invert the effect of spin torque on a
nanomagnet leading to a surprising current-induced enhancement of damping by an
antidamping torque. Our work advances understanding of magnetic dynamics in
nanoscale ferromagnets and spin torque devices.",1803.10925v1
2018-06-02,Ultra-low damping insulating magnetic thin films get perpendicular,"A magnetic material combining low losses and large Perpendicular Magnetic
Anisotropy (PMA) is still a missing brick in the magnonic and spintronic
fields. We report here on the growth of ultrathin Bismuth doped
Y$_{3}$Fe$_{5}$O$_{12}$ (BiYIG) films on Gd$_{3}$Ga$_{5}$O$_{12}$ (GGG) and
substituted GGG (sGGG) (111) oriented substrates. A fine tuning of the PMA is
obtained using both epitaxial strain and growth induced anisotropies. Both
spontaneously in-plane and out-of-plane magnetized thin films can be
elaborated. Ferromagnetic Resonance (FMR) measurements demonstrate the high
dynamic quality of these BiYIG ultrathin films, PMA films with Gilbert damping
values as low as 3 10$^{-4}$ and FMR linewidth of 0.3 mT at 8 GHz are achieved
even for films that do not exceed 30 nm in thickness. Moreover, we measure
Inverse Spin Hall Effect (ISHE) on Pt/BiYIG stacks showing that the magnetic
insulator$'$s surface is transparent to spin current making it appealing for
spintronic applications.",1806.00658v1
2018-06-04,Two-sided-loop jets associated with magnetic reconnection between emerging loops and twisted filament threads,"Coronal jets are always produced by magnetic reconnection between emerging
flux and pre-existing overlying magnetic fields. When the overlying field is
vertical/obilique or horizontal, the coronal jet will appear as anemone type or
two-sided-loop type. Most of observational jets are of the anemone type, and
only a few of two-sided-loop jets have been reported. Using the high-quality
data from New Vacuum Solar Telescope, Interface Region Imaging Spectrograph,
and Solar Dynamics Observatory, we present an example of two-sided-loop jets
simultaneously observed in the chromosphere, transition region, and corona. The
continuous emergence of magnetic flux brought in successively emerging of
coronal loops and the slowly rising of an overlying horizontal filament
threads. Sequentially, there appeared the deformation of the loops, the
plasmoids ejection from the loop top, and pairs of loop brightenings and jet
moving along the untwisting filament threads. All the observational results
indicate there exist magnetic reconnection between the emerging loops and
overlying horizontal filament threads, and it is the first example of
two-sided-loop jets associated with ejected plasmoids and twisted overlying
fields.",1806.00957v1
2018-06-29,Magnetic impurities in Kondo insulators: An application to samarium hexaboride,"Impurities and defects in Kondo insulators can have an unusual impact on
dynamics that blends with effects of intrinsic electron correlations. Such
crystal imperfections are difficult to avoid, and their consequences are
incompletely understood. Here we study magnetic impurities in Kondo insulators
via perturbation theory of the s-d Kondo impurity model adapted to small
bandgap insulators. The calculated magnetization and specific heat agree with
recent thermodynamic measurements in samarium hexaboride (SmB$_6$). This
qualitative agreement supports the physical picture of multi-channel Kondo
screening of local moments by electrons and holes involving both intrinsic and
impurity bands. Specific heat is thermally activated in zero field by Kondo
screening through sub-gap impurity bands and exhibits a characteristic upturn
as the temperature is decreased. In contrast, magnetization obtains a dominant
quantum correction from partial screening by virtual particle-hole pairs in
intrinsic bands. We point out that magnetic impurities could impact de Haas-van
Alphen quantum oscillations in SmB$_6$, through the effects of Landau
quantization in intrinsic bands on the Kondo screening of impurity moments.",1807.00005v2
2018-08-21,Recent advances in spin-orbit torques: Moving towards device applications,"The ability of spintronic devices to utilize an electric current for
manipulating the magnetization has resulted in large-scale developments, such
as, magnetic random access memories and boosted the spintronic research area.
In this regard, over the last decade, magnetization manipulation using
spin-orbit torque has been devoted a lot of research attention as it shows a
great promise for future ultrafast and power efficient magnetic memories. In
this review, we summarize the latest advancements in spin-orbit torque research
and highlight some of the technical challenges for practical spin-orbit torque
devices. We will first introduce the basic concepts and highlight the latest
material choices for spin-orbit torque devices. Then, we will summarize the
important advancements in the study of magnetization switching dynamics using
spin-orbit torque, which are important from scientific as well as technological
aspect. The final major section focuses on the concept of external assist field
free spin-orbit torque switching which is a requirement for practical
spin-orbit torque devices.",1808.06829v1
2018-08-25,Twisted magnetization states and inhomogeneous resonance modes in a Fe/Gd ferrimagnetic multilayer,"Static and dynamic magnetic properties of a ferrimagnetic
[Fe(35A)/Gd(50A)]x12 superlattice were investigated in a wide 4-300 K
temperature range using magneto-optical Kerr effect (MOKE) and ferromagnetic
resonance (FMR) techniques. The multilayer structure was sputtered on a
transparent glass substrate which made it possible to perform MOKE measurements
on both Fe and Gd terminated sides of the superlattice. These experiments
allowed us to detect a transition between field-aligned and canted magnetic
states on both sides of the film and to distinguish between the bulk and
surface twisted phases of the superlattice. As a result, the experimental H-T
magnetic phase diagram of the system was obtained. FMR studies at frequencies
7-36 GHz demonstrated a complex evolution of absorption spectra as temperature
decreased from room down to 4 K. Two spectral branches were detected in the
sample. Theoretical simulations show that the observed spectral branches
correspond to different types of inhomogeneous resonance modes in the
multilayer with non-uniform magnetization precession inside Gd layers.",1808.08466v1
2018-09-19,A hypercritical accretion scenario in Central Compact Objects accompanied with an expected neutrino burst,"The measurement of the period and period derivative, and the canonical model
of dipole radiation have provided a method to estimate the low superficial
magnetic fields in the so-called Central Compact Objects (CCOs). In the present
work, a scenario is introduced in order to explain the magnetic behavior of
such CCOs. Based on magnetohydrodynamic simulations of the post core-collapse
supernova phase during the hypercritical accretion episode, we argue that the
magnetic field of a newborn neutron star could have been early buried. During
this phase, thermal neutrinos are created mainly by the pair annihilation,
plasmon decay, photo-neutrino emission and other processes. We study the
dynamics of these neutrinos in this environment and also estimate the number
expected of the neutrino events with their flavor ratios on Earth. The neutrino
burst is the only viable observable that could provide compelling evidence of
the hypercritical phase and therefore, the hidden magnetic field mechanism as
the most favorable scenario to explain the anomalous low magnetic fields
estimated for CCOs.",1809.07057v2
2018-09-27,Decay of turbulence in a liquid metal duct flow with transverse magnetic field,"Decay of honeycomb-generated turbulence in a duct with a static transverse
magnetic field is studied via direct numerical simulations. The simulations
follow the revealing experimental study of Sukoriansky et al. (1986), in
particular the paradoxical observation of high-amplitude velocity fluctuations,
which exist in the downstream portion of the flow when the strong transverse
magnetic field is imposed in the entire duct including the honeycomb exit, but
not in other configurations. It is shown that the fluctuations are caused by
the large-scale quasi-two-dimensional structures forming in the flow at the
initial stages of the decay and surviving the magnetic suppression. Statistical
turbulence properties, such as the energy decay curves, two-point correlations
and typical length scales are computed. The study demonstrates that turbulence
decay in the presence of a magnetic field is a complex phenomenon critically
depending on the state of the flow at the moment the field is introduced.",1809.10777v2
2018-09-28,Observation of magnetic-field-sweep-direction-dependent dynamic nuclear polarization under periodic optical electron spin pumping,"Optical pump-probe techniques are used to generate and measure electron spin
polarization in a gallium arsenide epilayer in which the electron spin
coherence time exceeds the mode-locked laser repetition period. Resonant spin
amplification occurs at magnetic fields at which the electron spin polarization
excited by successive laser pulses add constructively. Measurements of Kerr
rotation as a function of applied magnetic field reveal nuclear spin
polarization that aligns either with or against the external magnetic field
depending on whether the applied magnetic field is being decreased or
increased. Furthermore, the nuclear spin polarization magnitude varies in
proportion to the perpendicular net electron spin polarization as the latter
changes due to resonant spin amplification and other causes. We also report an
experimental technique that reveals a minutes-long memory of precise field
history in the electron-nuclear spin system.",1810.00055v2
2018-10-31,Theory of chiral effects in magnetic textures with spin-orbit coupling,"We present a theoretical study of two-dimensional spatially and temporally
varying magnetic textures in the presence of spin-orbit coupling (SOC) of both
the Rashba and Dresselhaus types. We show that the effective gauge field due to
these SOCs, contributes to the dissipative and reactive spin torques in exactly
the same way as in electromagnetism. Our calculations reveal that Rashba
(Dresselhaus) SOC induces a chiral dissipation in interfacial (bulk) inversion
asymmetric magnetic materials. Furthermore, we show that in addition to chiral
dissipation $\alpha_c$, these SOCs also produce a chiral renormalization of the
gyromagnetic ratio $\tilde{\gamma}_c$, and show that the latter is
intrinsically linked to the former via a simple relation $\alpha_c =
(\tau/\tau_{\rm ex}) \tilde{\gamma}_c$, where $\tau_{\rm ex}$ and $\tau$ are
the exchange time and the electron relaxation time, respectively. Finally, we
propose a theoretical scheme based on the Scattering theory to calculate and
investigate the properties of damping in chiral magnets. Our findings should in
principle provide a guide for material engineering of effects related to chiral
dynamics in magnetic textures with SOC.",1810.13065v1
2018-12-20,Unidirectional spin-wave channeling along magnetic domain walls of Bloch type,"From the pioneering work of Winter [Phys. Rev. 124, 452 (1961)], a magnetic
domain wall of Bloch type is known to host a special wall-bound spin-wave mode,
which corresponds to spin-waves being channeled along the magnetic texture.
Using micromagnetic simulations, we investigate spin-waves travelling inside
Bloch walls formed in thin magnetic media with perpendicular-to-plane magnetic
anisotropy and we show that their propagation is actually strongly
nonreciprocal, as a result of dynamic dipolar interactions. We investigate
spin-wave non-reciprocity effects in single Bloch walls, which allows us to
clearly pinpoint their origin, as well as in arrays of parallel walls in stripe
domain configurations. For such arrays, a complex domain-wall-bound spin-wave
band structure develops, some aspects of which can be understood qualitatively
from the single-wall picture by considering that a wall array consists of a
sequence of up/down and down/up walls with opposite non-reciprocities.
Circumstances are identified in which the non-reciprocity is so extreme that
spin-wave propagation inside individual walls becomes unidirectional.",1812.08741v1
2018-12-21,"Local magnetism, magnetic order and spin freezing in the 'nonmetallic metal' FeCrAs","We present the results of x-ray scattering and muon-spin relaxation
($\mu^{+}$SR) measurements on the iron-pnictide compound FeCrAs. Polarized
non-resonant magnetic x-ray scattering results reveal the 120$^\circ$
periodicity expected from the suggested three-fold symmetric, non-collinear
antiferromagnetic structure. $\mu^+$SR measurements indicate a magnetically
ordered phase throughout the bulk of the material below $T_\mathrm{N}$=105(5)
K. There are signs of fluctuating magnetism in a narrow range of temperatures
above $T_\mathrm{N}$ involving low-energy excitations, while at temperatures
well below $T_\mathrm{N}$ behaviour characteristic of freezing of dynamics is
observed, likely reflecting the effect of disorder in our polycrystalline
sample. Using density functional theory we propose a distinct muon stopping
site in this compound and assess the degree of distortion induced by the
implanted muon.",1812.09252v3
2018-12-22,Diamagnetic field states in cosmological plasmas,"Using a generally covariant Electro-Vortic (magnetofluid) formalism for
relativistic plasmas, the dynamical evolution of a generalized vorticity (a
combination of the magnetic and kinematic parts) is studied in a cosmological
context. We derive macroscopic vorticity and magnetic field structures that can
emerge in spatial equilibrium configurations of the relativistic plasma. These
fields, however, evolve in time. These magnetic and velocity fields fields are
self-consistently sustained in a diamagnetic state in the expanding Universe,
and do not require an external seed for their existence. In particular, we
explore a special class of magnetic/velocity field structures supported by a
plasma in which the generalized vorticity vanishes. We derive a highly
interesting characteristic of such ""superconductor--like"" fields in a
cosmological plasmas in the radiation--era in early Universe. In that case, the
fields grow proportional to the scale factor, establishing a deep connection
between the expanding universe and the primordial magnetic fields.",1812.09425v1
2018-12-26,Self-diffusion in two-dimensional quasi-magnetized rotating dusty plasmas,"The self-diffusion phenomenon in a two-dimensional dusty plasma at extremely
strong (effective) magnetic fields is studied experimentally and by means of
molecular dynamics simulations. In the experiment the high magnetic field is
introduced by rotating the particle cloud and observing the particle
trajectories in a co-rotating frame, which allows reaching effective magnetic
fields up to 3000 Tesla. The experimental results confirm the predictions of
the simulations: (i) super-diffusive behavior is found at intermediate
time-scales and (ii) the dependence of the self-diffusion coefficient on the
magnetic field is well reproduced.",1812.10253v1
2018-12-31,Electric field switching of the uniaxial magnetic anisotropy of an antiferromagnet,"Electric field control of magnetic anisotropy in ferromagnets has been
intensively pursued in spintronics to achieve efficient memory and computing
devices with low energy consumption. Compared with ferromagnets,
antiferromagnets hold huge potential in high-density information storage for
their ultrafast spin dynamics and vanishingly small stray field. However, the
switching of magnetic anisotropy of antiferromagnets via electric field remains
elusive. Here we use ferroelastic strain from piezoelectric materials to switch
the uniaxial magnetic anisotropy and the N\'eel order reversibly in
antiferromagnetic Mn2Au films with an electric field of only a few kV/cm at
room temperature. Owing to the uniaxial magnetic anisotropy, a ratchet-like
switching behavior driven by the N\'eel spin-orbit torque is observed in the
Mn2Au, which can be reversed by electric fields.",1812.11868v1
2019-01-02,Exploring the thermodynamics of spin-1 $^{87}$Rb Bose Gases with synthetic magnetization,"In this work, we study the thermodynamic properties of a spin-1 Bose gas
across the Bose-Einstein condensation transition. We present the theoretical
description of the thermodynamics of a trapped ideal spin-1 Bose gas and we
describe the phases that can be obtained in this system as a function of the
temperature and of the populations in the different spin components. We propose
a simple way to realize a ""synthetic magnetization"" that can be used to probe
the entire phase diagram while keeping the real magnetization of the system
fixed. We experimentally demonstrate the use of such method to explore
different phases in a sample with zero total magnetization. Our work opens up
new perspectives to study isothermal quenching dynamics through different
magnetic phases in spinor condensates.",1901.00427v2
2019-02-04,Solitary wave excitations of skyrmion strings in chiral magnets,"Chiral magnets possess topological line excitations where the magnetization
within each cross section forms a skyrmion texture. We study analytically and
numerically the low-energy, non-linear dynamics of such a skyrmion string in a
field-polarized cubic chiral magnet, and we demonstrate that it supports
solitary waves. Theses waves are in general non-reciprocal, i.e., their
properties depend on the sign of their velocity $v$, but this non-reciprocity
diminishes with decreasing $|v|$. An effective field-theoretical description of
the solitary waves is derived that is valid in the limit $v \to 0$ and gives
access to their profiles and their existence regime. Our analytical results are
quantitatively confirmed with micromagnetic simulations for parameters
appropriate for the chiral magnet FeGe. Similarities with solitary waves found
in vortex filaments of fluids are pointed out.",1902.01420v1
2019-02-08,Magnetization beyond the Ising limit of Ho$_2$Ti$_2$O$_7$,"We report that the local Ising anisotropy in pyrochlore oxides - the crucial
requirement for realizing the spin-ice state - can be broken by means of high
magnetic fields. For the case of the well-established classical spin-ice
compound Ho$_2$Ti$_2$O$_7$ the magnetization exceeds the angle-dependent
saturation value of the Ising limit using ultra-high fields up to 120 T.
However, even under such extreme magnetic fields full saturation cannot be
achieved. Crystal-electric-field calculations reveal that a level crossing for
two of the four ion positions leads to magnetization steps at 55 and 100 T. In
addition, we show that by using a field sweep rate in the range of the
spin-relaxation time the dynamics of the spin system can be probed. Exclusively
at 25 ns/T a new peak of the susceptibility appears around 2 T. We argue, this
signals the cross-over between spin-ice and polarized correlations.",1902.02990v1
2019-02-12,Constraining Jupiter's internal flows using Juno magnetic and gravity measurements,"Deciphering the flow below the cloud-level of Jupiter remains a critical
milestone in understanding Jupiter's internal structure and dynamics. The
expected high-precision Juno measurements of both the gravity field and the
magnetic field might help to reach this goal. Here we propose a method that
combines both fields to constrain the depth dependent flow field inside
Jupiter. This method is based on a mean-field electrodynamic balance that
relates the flow field to the anomalous magnetic field, and geostrophic balance
that relates the flow field to the anomalous gravity field. We find that the
flow field has two distinct regions of influence - an upper region in which the
flow affects mostly the gravity field, and a lower region in which the flow
affects mostly the magnetic field. An optimization procedure allows to reach a
unified flow structure that is consistent with both the gravity and the
magnetic fields.",1902.04329v1
2019-02-12,Hydrodynamic and Magnetohydrodynamic Simulations of Wire Turbulence,"We report on simulations of laboratory experiments in which magnetized
supersonic flows are driven through a wire mesh. The goal of the study was to
investigate the ability of such a configuration to generate supersonic, MHD
turbulence. We first report on the morphological structures that develop in
both magnetized and non-magnetized cases. We then analyze the flow using a
variety of statistical measures, including power spectra and probability
distribution functions of the density. Using these results we estimate the
sonic mach number in the flows downstream of the wire mesh. We find the
initially hypersonic (M=20) planar shock through the wire mesh does lead to
downstream turbulent conditions. However, in both magnetized and non-magnetized
cases, the resultant turbulence was marginally supersonic to transonic (M~1),
and highly anisotropic in structure.",1902.05063v2
2019-02-19,Electric quadrupole moment and the tensor magnetic polarizability of twisted electrons and a potential for their measurements,"For a twisted (vortex) Dirac particle in nonuniform electric and magnetic
fields, the relativistic Foldy-Wouthuysen Hamiltonian is derived including high
order terms describing new effects. The result obtained shows for the first
time that a twisted spin-1/2 particle possesses a tensor magnetic
polarizability and a measurable (spectroscopic) electric quadrupole moment. We
have calculated the former parameter and have evaluated the latter one for a
twisted electron. The tensor magnetic polarizability of the twisted electron
can be measured in a magnetic storage ring because a beam with an initial
orbital tensor polarization acquires a horizontal orbital vector polarization.
The electric quadrupole moment is rather large and strongly influences the
dynamics of the intrinsic orbital angular momentum. Three different methods of
its measurements, freezing the intrinsic orbital angular momentum and two
resonance methods, are proposed. The existence of the quadrupole moment of
twisted electrons can lead to practical applications.",1902.06882v1
2019-03-04,Thermoelectric microscopy of magnetic skyrmions,"The magnetic skyrmion is a nanoscale topological object characterized by the
winding of the magnetic moments, appearing in magnetic materials with broken
inversion symmetry. Because of its low current threshold for driving, the
skyrmions have been intensely studied toward novel storage applications by
using electron-beam, X-ray, and visible light microscopies. Here, we show that
the skyrmions can be imaged via thermoelectric signals of spin-caloritronic
phenomena in combination with focused heating. We applied a local temperature
gradient to a CoFeB/Ta/W multilayer film with scanning a heating position and,
by exploiting a Hall bar structure, mapped the magnitude as well as the
direction of the resultant thermoelectric current distributions, which link to
skyrmions' inner magnetic textures. This method also enables the observation of
skyrmion dynamics under driving current pulses, being a useful imaging
technique for the development of skyrmion devices.",1903.01037v2
2019-03-06,Magneto-Rayleigh-Taylor instability in an elastic finite-width medium overlying an ideal fluid,"We present the linear theory of two-dimensional incompressible
magneto-Rayleigh-Taylor instability in a system composed of a linear elastic
(Hookean) layer above a lighter semi-infinite ideal fluid with magnetic fields
present, above and below the layer. As expected, magnetic field effects and
elasticity effects together enhance the stability of thick layers. However, the
situation becomes more complicated for relatively thin slabs, and a number of
new and unexpected phenomena are observed. In particular, when the magnetic
field beneath the layer dominates, its effects compete with effects due to
elasticity, and counteract the elasticity stabilising effects. As a
consequence, the layer can become more unstable than when only one of these
stabilising mechanism is acting. This somewhat unexpected result is explained
by the different physical mechanisms for which elasticity and magnetic fields
stabilise the system. Implications for experiments on magnetically driven
accelerated plates and implosions are discussed. Moreover, the relevance for
triggering of crust-quakes in strongly magnetised neutron stars is also pointed
out.",1903.02257v1
2019-03-06,Anomalous Magnetic Behavior in Ba2CoO4 with Isolated CoO4 Tetrahedra,"The dimensionality of the electronic and magnetic structure of a given
material is generally predetermined by its crystal structure. Here, using
elastic and inelastic neutron scattering combined with magnetization
measurements, we find unusual magnetic behavior in three-dimensional (3D)
Ba2CoO4. In spite of isolated CoO4 tetrahedra, the system exhibits a 3D
noncollinear antiferromagnetic order in the ground state with an anomalously
large Curie-Weiss temperature of 110 K compared to TN = 26 K. More
unexpectedly, spin dynamics displays quasi-2D spin wave dispersion with an
unusually large spin gap, and 1D magnetoelastic coupling. Our results indicate
that Ba2CoO4 is a unique system for exploring the interplay between isolated
polyhedra, low-dimensional magnetism, and novel spin states in oxides.",1903.02490v1
2019-03-22,On possible types of magnetospheres of hot Jupiters,"We show that the orbits of exoplanets of the ""hot Jupiter"" type, as a rule,
are located close to the Alf\'{v}en point of the stellar wind of the parent
star. At this, many hot Jupiters can be located in the sub-Alf\'{v}en zone in
which the magnetic pressure of the stellar wind exceeds its dynamic pressure.
Therefore, magnetic field of the wind must play an extremely important role for
the flow of the stellar wind around the atmospheres of the hot Jupiters. This
factor must be considered both in theoretical models and in the interpretation
of observational data. The analysis shows that many typical hot Jupiters should
have shock-less intrinsic magnetospheres, which, apparently, do not have
counterparts in the Solar System. Such magnetospheres are characterized,
primarily, by the absence of the bow shock, while the magnetic barrier
(ionopause) is formed by the induced currents in the upper layers of the
ionosphere. We confirmed this inference by the three-dimensional numerical
simulation of the flow of the parent star stellar wind around the hot Jupiter
HD 209458b in which we took into account both proper magnetic field of the
planet and magnetic field of the wind.",1903.09459v1
2019-03-25,The supernova-regulated ISM -- VI. Magnetic effects on the structure of the interstellar medium,"We explore the effect of magnetic fields on the vertical distribution and
multiphase structure of the supernova-driven interstellar medium (ISM) in
simulations that admit dynamo action. As the magnetic field is amplified to
become dynamically significant, gas becomes cooler and its distribution in the
disc becomes more homogeneous. We attribute this to magnetic quenching of
vertical velocity, which leads to a decrease in the cooling length of hot gas.
A non-monotonic vertical distribution of the large-scale magnetic field
strength, with the maximum at |z| $\approx$ 300 pc causes a downward pressure
gradient below the maximum which acts against outflow driven by SN explosions,
while it provides pressure support above the maximum.",1903.10263v2
2019-04-09,Permanent mean spin source of the chiral magnetic effect in neutron stars,"We suggest the generalization of the Anomalous Magneto-Hydro-Dynamics (AMHD)
in the chiral plasma of a neutron star (NS) accounting for the mean spin in the
ultrarelativistic degenerate electron gas within the magnetized NS core as a
continuing source of the chiral magnetic effect. Using the mean field dynamo
model generalized in AMHD, one can obtain the growth of a seed magnetic field
up to $10^{18}\,\text{G}$ for an old non-superfluid NS at its neutrino cooling
era $t < 10^6\,\text{yr}$, while neglecting any matter turbulence within its
core and assuming the rigid NS rotation. The application of the suggested
approach to the evolution of magnetic fields observed in magnetars, $B\sim
10^{15}\,\text{G}$, should be self-consistent with all approximations used in
the suggested laminar dynamo, at least, up to the jumps of growing fields.",1904.05768v2
2019-04-25,Low damping magnetic properties and perpendicular magnetic anisotropy with strong volume contribution in the Heusler alloy Fe1.5CoGe,"We present a study of the dynamic magnetic properties of TiN-buffered
epitaxial thin films of the Heusler alloy Fe$_{1.5}$CoGe. Thickness series
annealed at different temperatures are prepared and the magnetic damping is
measured, a lowest value of $\alpha=2.18\times 10^{-3}$ is obtained. The
perpendicular magnetic anisotropy properties in Fe$_{1.5}$CoGe/MgO are also
characterized. The evolution of the interfacial perpendicular anisotropy
constant $K^{\perp}_{\rm S}$ with the annealing temperature is shown and
compared with the widely used CoFeB/MgO interface. A large volume contribution
to the perpendicular anisotropy of $(4.3\pm0.5)\times 10^{5}$ $\rm J/m^3$ is
also found, in contrast with vanishing bulk contribution in common Co- and
Fe-based Heusler alloys.",1904.11247v1
2019-06-04,Magnetoresistance in Metallic Ferroelectrics,"Polar metals with ferroelectric-like displacements in metals have been
achieved recently, half century later than Anderson and Blount's prediction.
However, the genuine ferroelectricity with electrical dipolar switching has not
yet been attained experimentally in the conducting materials, especially the
ones possessing magnetic responses. Here we report the coexistence of
ferroelectricity and magnetoresistance (MR) in the metallic PbNb0.12Ti0.88O3
(PNTO) thin films. We found that the conducting and magnetic responses of PNTO
films are highly asymmetric. Negative MR up to 50% is observed under an
in-plane magnetic field; the MR switches to positive with the magnetic field
applied parallel to the surface normal. Such unique behavior is attributed to
the moving electron caused effective magnetic field which couples with the
spins of electrons, which form a dynamic multiferroic state in the metallic
PNTO. These findings break a path to multiferroic metal and offer a great
potential to the multi-functional devices.",1906.01257v1
2019-06-18,Interplay between disorder driven Non-Fermi-liquid behavior and magnetism in Ce0.24La0.76Ge compound,"In this work, we investigate the magnetic, heat capacity and electrical
transport properties of Ce0.6La0.4Ge and Ce0.24La0.76Ge compounds. Our results
show that two antiferromagnetic transitions (~ at 4.7 and 2.7 K) exhibited by
Ce0.6La0.4Ge are suppressed below 1.8 K for Ce0.24La0.76Ge. Interestingly, for
Ce0.24La0.76Ge, susceptibility, heat capacity and electrical resistivity vary
with temperature as: T0.75, T0.5 and T1.6 respectively. The observation of such
anomalous temperature variation suggests to the Non-Fermi-liquid (NFL) behavior
due to the presence of disordered 4f spins due to Ce-site dilution. Under the
application of magnetic field, it is noted that a crossover from the NFL to a
magnetic state occurs around 2 Tesla, where, short-range correlations among the
spins is prevalent due to the dominance of coupling between the magnetic
moments via conduction electrons. Magnetoresistance scaling indicates that
behavior of disorder driven NFL state is described by the dynamical mean field
theory of the spin glass quantum critical point.",1906.07482v1
2019-07-01,"Structural, magnetostatic and magnetodynamic studies of Co/Mo-based uncompensated synthetic antiferromagnets","In this work, we comprehensively investigate and discuss the structural,
magnetostatic, dynamic, and magnetoresistive properties of epitaxial Co/Mo
superlattices. The magnetization of the Co sublayers is coupled
antiferromagnetically with a strength that depends on the thickness of the
nonmagnetic Mo spacer. The magnetization and magnetoresistance hysteresis loops
clearly reflect interlayer exchange coupling and the occurrence of uniaxial
magnetic anisotropy induced by the strained Co sublayers. Upon accounting for a
deviation of the sublayer thicknesses from the nominal value, theoretical
modeling, including both micromagnetic and macrospin approaches, precisely
reproduces experimental magnetic hysteresis loops, magnetoresistance curves,
and ferromagnetic resonance dispersion relations. The Mo spacer thickness as a
function of the interlayer magnetic coupling is determined as a fitting
parameter by modeling the experimental results.",1907.00592v2
2019-07-17,Low Barrier Nanomagnet Design for Binary Stochastic Neurons: Design Challenges for Real Nanomagnets with Fabrication Defects,"Much attention has been focused on the design of low barrier nanomagnets
(LBM), whose magnetizations vary randomly in time owing to thermal noise, for
use in binary stochastic neurons (BSN) which are hardware accelerators for
machine learning. The performance of BSNs depend on two important parameters:
the correlation time associated with the random magnetization dynamics in a
LBM, and the spin-polarized pinning current which stabilizes the magnetization
of a LBM in a chosen direction within a chosen time. Here, we show that common
fabrication defects in LBMs make these two parameters unpredictable since they
are strongly sensitive to the defects. That makes the design of BSNs with real
LBMs very challenging. Unless the LBMs are fabricated with extremely tight
control, the BSNs which use them could be unreliable or suffer from poor yield.",1907.07525v1
2019-08-02,Dense Quark-Gluon Plasma in strong magnetic fields,"A non-perturbative (np) method of Field Correlators (FCM) was applied to
study QCD at temperatures above the deconfinement transition
($1<T/T_c<3,~T_c\sim0.16~GeV$) and nonzero baryon densities (baryon chemical
potential $\mu_B<0.5~GeV$) in an external uniform magnetic field
($eB<0.5~GeV^2$). Within FCM, the np high-temperature dynamics is embodied in
the Polyakov loop and in the Debye mass due to the Color-Magnetic Confinement.
Analytic expressions for quark pressure and magnetic susceptibility were
obtained. The expressions were represented as series and in integral form.
Magnetic susceptibility was found to increase rapidly with temperature and
slowly with density. The results at the zero density limit are in agreement
with lattice data.",1908.00800v2
2019-08-20,Comparing all-optical switching in synthetic-ferrimagnetic multilayers and alloys,"We present an experimental and theoretical investigation of all-optical
switching by single femtosecond laser pulses. Our experimental results
demonstrate that, unlike rare earth-transition metal ferrimagnetic alloys,
Pt/Co/[Ni/Co]$_N$/Gd can be switched in the absence of a magnetization
compensation temperature, indicative for strikingly different switching
conditions. In order to understand the underlying mechanism, we model the
laser-induced magnetization dynamics in Co/Gd bilayers and GdCo alloys on an
equal footing, using an extension of the microscopic three-temperature model to
multiple magnetic sublattices and including exchange scattering. In agreement
with our experimental observations, the model shows that Co/Gd bilayers can be
switched for an arbitrary thickness of the Co layer, i.e, even far away from
compensating the total Co and Gd magnetic moment. We identify the switching
mechanism in Co/Gd bilayers as a front of reversed Co magnetization that
nucleates at the Co/Gd interface and propagates through the Co layer driven by
exchange scattering.",1908.07292v2
2019-09-01,Magnetic field-dependent low-energy magnon dynamics in $α$-RuCl3,"Revealing the spin excitations of complex quantum magnets is key to
developing a minimal model that explains the underlying magnetic correlations
in the ground state. We investigate the low-energy magnons in $\alpha$-RuCl$_3$
by combining time-domain terahertz spectroscopy under an external magnetic
field and model Hamiltonian calculations. We observe two absorption peaks
around 2.0 and 2.4 meV, which we attribute to zone-center spin waves. Using
linear spin-wave theory with only nearest-neighbor terms of the exchange
couplings, we calculate the antiferromagnetic resonance frequencies and reveal
their dependence on an external field applied parallel to the nearest-neighbor
Ru-Ru bonds. We find that the magnon behavior in an applied magnetic field can
be understood only by including an off-diagonal $\Gamma$ exchange term to the
minimal Heisenberg-Kitaev model. Such an anisotropic exchange interaction that
manifests itself as a result of strong spin-orbit coupling can naturally
account for the observed mixing of the modes at higher fields strengths.",1909.00462v1
2019-09-12,Single pulse all-optical toggle switching of magnetization without Gd: The example of Mn2RuxGa,"Energy-efficient control of magnetization without the help of a magnetic
field is a key goal of spintronics. Purely heat-induced single-pulse
all-optical toggle switching has been demonstrated, but so far only in Gd based
amorphous ferrimagnet films. In this work, we demonstrate toggle switching in
the half-metallic compensated ferrimagnetic Heusler alloys Mn2RuxGa, which have
two crystallographically-inequivalent Mn sublattices. Moreover, we observe the
switching at room temperature in samples that are immune to external magnetic
fields in excess of 1 T, provided they exhibit compensation above room
temperature. Observations of the effect in compensated ferrimagnets without Gd
challenges our understanding of all-optical switching. The dynamic behavior
indicates that Mn2RuxGa switches in 2 ps or less. Our findings widen the basis
for fast optical switching of magnetization and break new ground for engineered
materials that can be used for nonvolatile ultrafast switches using ultrashort
pulses of light.",1909.05809v2
2019-09-27,Effective description of non-equilibrium currents in cold magnetized plasma,"The dynamics of cold strongly magnetized plasma -- traditionally the domain
of force-free electrodynamics -- has recently been reformulated in terms of
symmetries and effective field theory, where the degrees of freedom are the
momentum and magnetic flux carried by a fluid of cold strings. In physical
applications where the electron mass can be neglected one might expect the
presence of extra light charged modes -- electrons in the lowest Landau level
-- propagating parallel to the magnetic field lines. We construct an effective
description of such electric charges, describing their interaction with plasma
degrees of freedom in terms of a new collective mode that can be thought of as
a bosonization of the electric charge density along each field line. In this
framework QED phenomena such as charged pair production and the axial anomaly
are described at the classical level. Formally, our construction corresponds to
gauging a particular part of the higher form symmetry associated with magnetic
flux conservation. We study some simple applications of our effective theory,
showing that the scattering of magnetosonic modes generically creates particles
and that the rotating Michel monopole is now surrounded by a cloud of electric
charge.",1909.12609v1
2019-10-14,Magnetic helicity dissipation and production in an ideal MHD code,"We study a turbulent helical dynamo in a periodic domain by solving the ideal
magnetohydrodynamic (MHD) equations with the FLASH code using the
divergence-cleaning eight-wave method and compare our results with direct
numerical simulations (DNS) using the Pencil Code. At low resolution, FLASH
reproduces the DNS results qualitatively by developing the large-scale magnetic
field expected from DNS, but at higher resolution, no large-scale magnetic
field is obtained. In all those cases in which a large-scale magnetic field is
generated, the ideal MHD results yield too little power at small scales. As a
consequence, the small-scale current helicity is too small compared with that
of the DNS. The resulting net current helicity has then always the wrong sign,
and its statistical average also does not approach zero at late times, as
expected from the DNS. Our results have implications for astrophysical dynamo
simulations of stellar and galactic magnetism using ideal MHD codes.",1910.06074v3
2019-11-12,Large-scale dynamo action of magnetized Taylor-Couette flows,"A conducting Taylor-Couette flow with quasi-Keplerian rotation law containing
a toroidal magnetic field serves as a mean-field dynamo model of the
Tayler-Spruit-type. The flows are unstable against nonaxisymmetric
perturbations which form electromotive forces defining $\alpha$ effect and eddy
diffusivity. If both degenerated modes with $m=\pm 1$ are excited with the same
power then the global $\alpha$ effect vanishes and a dynamo cannot work. It is
shown, however, that the Tayler instability produces finite $\alpha$ effects if
only an isolated mode is considered but this intrinsic helicity of the
single-mode is too low for an $\alpha^2$ dynamo. Moreover, an $\alpha\Om$
dynamo model with quasi-Keplerian rotation requires a minimum magnetic Reynolds
number of rotation of ${\rm Rm}\simeq 2.000$ to work. Whether it really works
depends on assumptions about the turbulence energy. For a
steeper-than-quadratic dependence of the turbulence intensity on the magnetic
field, however, dynamos are only excited if the resulting magnetic eddy
diffusivity approximates its microscopic value, $\eta_{\rm T}\simeq \eta$. By
basically lower or larger eddy diffusivities the dynamo instability is
suppressed.",1911.05105v2
2019-12-16,Generic properties of Mañé's set of exact magnetic Lagrangians,"Let $M$ be a closed manifold and $L$ an exact magnetic Lagrangian. In this
paper we proved that there exists a residual $\mathcal{G}$ of $H^{1}\left(
M;\mathbb{R}\right)$ such that the property: \begin{equation*}
{\widetilde{\mathcal{M}}}\left( c\right) ={\widetilde{\mathcal{A}}}\left(
c\right) ={\widetilde{\mathcal{N}}}\left( c\right), \forall c\in \mathcal{G}
\end{equation*} with ${\widetilde{\mathcal{M}}}\left( c\right)$ supports on a
uniquely ergodic measure, is generic in the family of exact magnetic
Lagrangians. We also prove that, fixed the cohomology class $c$, there exists a
residual set of exact magnetic Lagrangians such that when this unique measure
is supported on a periodic orbit, this orbit is hyperbolic and its stable and
unstable manifolds intersect transversally. This result is a version of Theorem
D of \cite{gon5} for the exact magnetic Lagrangian case.",1912.07535v1
2019-12-18,Quantitative Modeling of the UV Line Profiles of Magnetic Massive Stars,"Recent spectropolarimetric surveys (MiMeS, BOB) have revealed that
approximately 7% of massive stars host stable, surface dipolar magnetic fields
with strengths on the order of kG. These fields channel the dense radiatively
driven stellar wind into a circumstellar magnetosphere. Wind-sensitive UV
spectral lines can probe the density and velocity structure of massive star
magnetospheres, providing insight into wind-field interactions. To date,
large-scale magnetohydrodynamic modeling of this phenomenon has been limited by
the associated computational cost. Our analysis, using the Analytic Dynamical
Magnetosphere model, solves this problem by applying a simple analytic
prescription to efficiently calculate synthetic UV spectral lines. It can
therefore be applied in the context of a larger parameter study to derive the
wind properties for the population of known magnetic O stars. We also present
the latest UV spectra of the magnetic O star NGC 1624-2 obtained with HST/COS,
which test the limits of our models and suggest a particularly complex
magnetospheric structure for this archetypal object.",1912.08748v1
2020-01-08,Simultaneous Wide-field Imaging of Phase and Magnitude of AC Magnetic Signal Using Diamond Quantum Magnetometry,"Spectroscopic analysis of AC magnetic signal using diamond quantum
magnetometry is a promising technique for inductive imaging. Conventional
dynamic decoupling like XY8 provides a high sensitivity of an oscillating
magnetic signal with intricate dependence on magnitude and phase, complicating
high throughput detection of each parameter. In this study, a simple
measurement scheme for independent and simultaneous detection of magnitude and
phase is demonstrated by a sequential measurement protocol. Wide-field imaging
experiment was performed for an oscillating magnetic field with approximately
100$\mu$m-squared observation area. Single pixel phase precision was
$2.1^\circ$ for 0.76$\mu$T AC magnetic signal. Our method enables potential
applications including inductive inspection and impedance imaging.",2001.02374v1
2020-01-10,Switching time of spin-torque-driven magnetization in biaxial ferromagnets,"We analytically model the magnetization switching time of a biaxial
ferromagnet driven by an antidamping-like spin torque. The macrospin
magnetization dynamics is mapped to an energy-flow equation, wherein a
rational-function approximation of the elliptic integrals for moderate spin
current and small damping results in a closed-form expression of the switching
time. Randomness in the initial angle of magnetization gives the distribution
function of the switching time. The analytic model conforms to the results
obtained from Monte Carlo simulation for a broad range of material parameters.
Our results can ameliorate design and benchmarking of in-plane spin torque
magnetic memory by obviating expensive numerical computation.",2001.03300v2
2020-01-13,High Frequency Microwave Emission of a Tri-layer Magnetic Tunnel Junction in the Absence of External Bias-Magnetic Field,"We perform an experimental study of DC current induced microwave emission in
a magnetic tunnel junction (MTJ) consisting of three active magnetic layers.
For this tri-layer structure, in addition to a conventional bilayer orthogonal
MTJ containing a perpendicular free layer and an in-plane fixed layer, a second
perpendicular layer has been introduced. We found that the microwave emission
frequency induced by spin-transfer torque (STT) can reach as high as 6 GHz in
the absence of any applied magnetic field. Moreover, microwave emission is
observed for both current polarizations where a redshift is seen with increase
in magnitude of current. We discuss spin-dynamics of the observed
bi-directional high-frequency emission and the physical origin of the
red-shift. The distinct microwave emission properties exhibited in this
tri-layer MTJ structure could potentially be useful for future applications in
nanoscale spintronics devices such as microwave communication and neuromorphic
computing.",2001.04539v1
2020-02-07,Probability Distribution of Magnetic Field Strengths through the Cyclotron Lines in High-Mass X-ray Binaries,"The study of variation of measured cyclotron lines is of fundamental
importance to understand the physics of the accretion process in magnetized
neutron star systems. We investigate the magnetic field formation, evolution
and distribution for several High- Mass X-ray Binaries (HMXBs). We focus our
attention on the cyclotron lines that have been detected in HMXB classes in
their X-ray spectra. As has been correctly pointed out, several sources show
variation in cyclotron lines, this can result due to the effect of accretion
dynamics, and hence that would reflect the magnetic field characteristics.
Besides, the difference in time scales of variation of accretion rate and
different type of companion can be used to distinguish between magnetized
neutron stars.",2002.03011v3
2020-02-19,Photon Self-energy in Magnetized Chiral Plasma from Kinetic Theory,"We study the photon self-energy in magnetized chiral plasma by solving the
response of electromagnetic field perturbations in chiral kinetic theory with
Landau level states. With lowest Landau level approximation and in
collisionless limit, we find solutions for three particular perturbations:
parallel electric field, static perpendicular electric and magnetic field,
corresponding to chiral magnetic wave, drift state and tilted state, from which
we extract components of photon self-energy in different kinematics. We show no
solution is possible for more general field perturbations. We argue this is an
artifact of the collisionless limit: while static solution corresponding to
drift state and tilted state can be found, they cannot be realized dynamically
without interaction between Landau levels. We also discuss possible
manifestation of side-jump effect due to both boost and rotation, with the
latter due to the presence of background magnetic field.",2002.07959v1
2020-03-13,Efficient acceleration of cylindrical jets: effects of radiative cooling and tangled magnetic field,"Diverging supersonic flows are accelerating, as in the case of a de Laval
nozzle, and the same concept has been applied for acceleration of
magnetohydrodynamic flows in the universe. Here, we study the dynamics of
""non-diverging"" cylindrical supersonic flows and show that they can be
accelerated by effects of radiative cooling and the tangled magnetic field. In
addition to radiative cooling of the jet materials (cooling effect), conversion
of the ordered magnetic field into the turbulent one (conversion effect) and
dissipation of the turbulent magnetic field (dissipation effect) are formulated
according to our study on pulsar wind nebulae. Although each of the cooling and
conversion effects is an ineffective acceleration process, the terminal
velocity of magnetized cylindrical jets attains about half of the maximum
possible value when the cooling, conversion and dissipation effects work
simultaneously. The radiation efficiency is also about half of the total
luminosity of the jet in the case of maximal acceleration. The concept for flow
acceleration by the non-ideal MHD effects may be useful for studying
relativistic jets in active galactic nuclei, in which the region near the jet
axis is expected to be cylindrical and kink unstable.",2003.06140v1
2020-03-18,Directed transport of suspended ferromagnetic nanoparticles under both gradient and uniform magnetic fields,"The suspended ferromagnetic particles subjected to the gradient and uniform
magnetic fields experience both the translational force generated by the field
gradient and the rotational torque generated by the fields strengths. Although
the uniform field does not contribute to the force, it nevertheless influences
the translational motion of these particles. This occurs because the
translational force depends on the direction of the particle magnetization,
which in turn depends on the fields strengths. To study this influence, a
minimal set of equations describing the coupled translational and rotational
motions of nanosized ferromagnetic particles is introduced and solved in the
low Reynolds number approximation. Trajectory analysis reveals that, depending
on the initial positions of nanoparticles, there exist four regimes of their
directed transport. The intervals of initial positions that correspond to
different dynamical regimes are determined, their dependence on the uniform
magnetic field is established, and strong impact of this field on the directed
transport is demonstrated. The ability and efficiency of the uniform magnetic
field to control the separation of suspended ferromagnetic nanoparticles is
also discussed.",2003.08498v2
2020-03-27,Modulation instability in the nonlinear Schrödinger equation with a synthetic magnetic field: gauge matters,"We theoretically investigate the phenomenon of modulation instability for
systems obeying nonlinear Schr\""odinger equation, which are under the influence
of an external homogeneous synthetic magnetic field. For an initial condition,
the instability is detected numerically by comparing dynamics with and without
a small initial perturbation; the perturbations are characterized in a standard
fashion by wavevectors in momentum space. We demonstrate that the region of
(in)stability in momentum space, as well as time-evolution in real space, for
identical initial conditions, depend on the choice of the gauge (i.e., vector
potential) used to describe the homogeneous synthetic magnetic field. This
superficially appears as if the gauge invariance is broken, but this is not
true. When the system is evolved from an identical initial condition in two
different gauges, it is equivalent to suddenly turning on the synthetic
magnetic field at $t=0$. This gives rise, via Faraday's law, to an initial
instantaneous kick of a synthetic electric field to the wavepacket, which can
differ for gauges yielding an identical uniform magnetic field at $t>0$.",2003.12620v1
2020-04-08,Deconfinement in the presence of a strong magnetic field,"We study the impact of a finite magnetic field on the deconfinement phase
transition for heavy quarks by computing the fluctuations of the Polyakov
loops. It is demonstrated that the explicit Z(3) breaking field increases with
the magnetic field, leading to a decrease in the (pseudo) critical temperatures
and a shrinking first-order region in the phase diagram. Phenomenological
equations that capture the behaviors of the Z(3) breaking field at strong and
weak magnetic fields for massive and massless quarks are given. Lastly, we
explore the case of dynamical light quarks and demonstrate how an improved
constituent quark mass function can enforce the correct magnetic field
dependence of the deconfinement temperature in an effective model, as observed
in Lattice QCD calculations.",2004.04138v2
2020-04-17,Magnetic-field evolution with large-scale velocity circulation in a neutron-star crust,"We examine the effects of plastic flow that appear in a neutron-star crust
when a magnetic stress exceeds the threshold. The dynamics involved are
described using the Navier--Stokes equation comprising the viscous-flow term,
and the velocity fields for the global circulation are determined using
quasi-stationary approximation.
  We simulate the magnetic-field evolution by taking into consideration the
Hall drift, Ohmic dissipation, and fluid motion induced by the Lorentz force.
The decrease in the magnetic energy is enhanced, as the energy converts to the
bulk motion energy and heat. It is found that the bulk velocity induced by the
Lorentz force has a significant influence in the low-viscosity and
strong-magnetic-field regimes. This effect is crucial near magnetar surfaces.",2004.08006v1
2020-05-07,Thermal Activation Barriers for Creation and Annihilation of Magnetic Droplet Solitons in the Presence of Spin Transfer Torque,"We study noise-induced creation and annihilation of magnetic droplet solitons
in experimental parameter regions in which they are linearly stable against
drift. Exploiting the rotational symmetry of the problem, we transform to the
reference frame rotating with the droplet soliton and introduce an effective
potential energy that accounts for the work done against spin-transfer torque
to rotate the magnetization between two different orientations. We use this
function to compute the activation barrier in both directions between the
uniform magnetization state and the droplet soliton state for a variety of
nanocontact radii and currents. We investigate droplet soliton structures with
both zero and nonzero spin-torque asymmetry parameter. Our approach can be
applied to estimate activation barriers for dynamical systems where
non-gradient terms can be absorbed by changes of reference frames, and suggests
a technique applicable to extended systems that may not be uniformly
magnetized.",2005.03416v1
2020-05-11,Manipulating 1-dimensinal skyrmion motion by external magnetic field gradient,"We have investigated an analytic formula of the 1-dimensional magnetic
skyrmion dynamics under external magnetic field gradient. We find excellent
agreement between the analytical model and micromagnetic simulation results for
various magnetic parameters such as the magnetic field gradient, Gilbert
damping constant. We also observe much faster velocity of the chiral domain
wall (DW) motion. The chiral DW is exist with smaller interfacial
Dzyaloshinskii-Moriya interaction energy density cases. These results provide
to develop efficient control of skyrmion for spintronic devices.",2005.05011v1
2020-05-16,All-optical switching of magnetic domains in Co/Gd heterostructures with Dzyaloshinskii-Moriya Interaction,"Given the development of hybrid spintronic-photonic devices and chiral
magnetic structures, a combined interest in all-optical switching (AOS) of
magnetization and current-induced domain wall motion in synthetic ferrimagnetic
structures with strong Dzyaloshinskii-Moriya Interaction (DMI) is emerging. In
this study, we report a study on single-pulse all-optical toggle switching and
asymmetric bubble expansion in specially engineered Co/Gd-based multilayer
structures. In the absence of any external magnetic fields, we look into the
AOS properties and the potential role of the DMI on the AOS process as well as
the stability of optically written micro-magnetic domains. Particularly,
interesting dynamics are observed in moon-shaped structures written by two
successive laser pulses. The stability of domains resulting from an interplay
of the dipolar interaction and domain-wall energy are compared to simple
analytical models and micromagnetic simulations.",2005.07969v2
2020-06-07,Anomalous magnetic suppression of spin relaxation in a two-dimensional electron gas in a GaAs/AlGaAs quantum well,"We study the spin dynamics in a high-mobility two-dimensional electron gas
confined in a GaAs/AlGaAs quantum well. An unusual magnetic field dependence of
the spin relaxation is found: as the magnetic field becomes stronger, the spin
relaxation time first increases quadratically but then changes to a linear
dependence, before it eventually becomes oscillatory, whereby the longitudinal
and transverse times reach maximal values at even and odd filling Landau level
factors, respectively. We show that the suppression of spin relaxation is due
to the effect of electron gyration on the spin-orbit field, while the
oscillations correspond to oscillations of the density of states appearing at
low temperatures and high magnetic fields. The transition from quadratic to
linear dependence can be related to a transition from classical to Bohm
diffusion and reflects an anomalous behavior of the two-dimensional electron
gas analogous to that observed in magnetized plasmas.",2006.04110v1
2020-06-09,Estimating a fluctuating magnetic field with a continuously monitored atomic ensemble,"We study the problem of estimating a time dependent magnetic field by
continuous optical probing of an atomic ensemble. The magnetic field is assumed
to follow a stochastic Ornstein-Uhlenbeck process and it induces Larmor
precession of the atomic ground state spin, which is read out by the Faraday
polarization rotation of a laser field probe. The interactions and the
measurement scheme are compatible with a hybrid quantum-classical Gaussian
description of the unknown magnetic field, and the atomic and field variables.
This casts the joint conditional quantum dynamics and classical parameter
estimation problem in the form of update formulas for the first and second
moments of the classical and quantum degrees of freedom. Our hybrid
quantum-classical theory is equivalent with the classical theory of Kalman
filtering and with the quantum theory of Gaussian states. By reference to the
classical theory of smoothing and with the quantum theory of past quantum
states, we show how optical probing after time $t$ improves our estimate of the
value of the magnetic field at time $t$, and we present numerical simulations
that analyze and explain the improvement over the conventional filtering
approach.",2006.05516v2
2020-07-02,Magnetic-field-induced reorientation in the SDW and the spin-stripe phases of the frustrated spin-1/2 chain compound $β$-TeVO$_4$,"$\beta$-TeVO$_4$ is a frustrated spin 1/2 zig-zag chain system,where
spin-density-wave (SDW), vector chiral (VC)and an exotic dynamic spin-stripe
phase compete at low temperatures. Here we use torque magnetometry to study the
anisotropy of these phases in magnetic fields of up to 5 T. Our results show
that the magnetic-field-induced spin reorientation occurs in the SDW and in the
spin stripe phases for $\mu_0 H \geq 2$~T. The observed spin reorientation is a
new element of the anisotropic phase diagram for the field directions in the
$ac$ and $a^*b$ crystallographic planes. The presented results should help
establishing the model of anisotropic magnetic interactions, which are
responsible for the formation of complex magnetic phases in $\beta$-TeVO$_4$
and similar quantum systems.",2007.00932v1
2020-07-07,Nonequilibrium Multiple Transitions in the Core-shell Ising Nanoparticles Driven by Randomly Varying Magnetic Fields,"The nonequilibrium behaviour of a core-shell nanoparticle has been studied by
Monte- Carlo simulation. The core consists of Ising spins of $\sigma=1/2$ and
the shell contains Ising spins of $S=1$. The interactions within the core and
in the shell are considered ferromagnetic but the interfacial interaction
between core and shell is antiferromagnetic. The nanoparticle system is kept in
open boundary conditions and is driven by randomly varying (in time but uniform
over the space) magnetic field. Depending on the width of the randomly varying
field and the temperature of the system, the core, shell and total
magnetization varies in such a manner that the time averages vanish for higher
magnitude of the width of random field, exhibiting a dynamical symmetry
breaking transitions. The susceptibilities get peaked at two different
temperatures indicating nonequilibrium multiple transitions. The phase
boundaries of the nonequilibrium multiple transitions are drawn in the plane
formed by the axes of temperature and the width of the randomly varying field.
Furthermore, the effects of the core and shell thicknesses on the multiple
transitions have been discussed.",2007.03280v2
2020-07-10,Magnetic effects of QCD parameters from finite energy sum rules,"One of the advantages of the finite energy sum rules is the fact that every
operator in the operator product expansion series can be selected individually
by the use of an appropriate kernel function which removes other operator
poles. This characteristic is maintained by QCD systems in the presence of
external homogeneous magnetic field, providing interesting information about
the magnetic evolution of QCD and hadronic parameters. In this work finite
energy sum rules are applied on QCD in the light quark sector, combining axial
and pseudoscalar channels in the presence of an external homogeneous magnetic
field, obtaining the magnetic evolution of the light quark masses, pion mass,
the pion decay constant, the gluon condensate and the continuum hadronic
threshold.",2007.05642v1
2020-07-18,Adsorption transition of a grafted ferromagnetic filament controlled by external magnetic fields,"Extensive Langevin dynamics simulations are used to characterize the
adsorption transition of a flexible magnetic filament grafted onto an
attractive planar surface. Our results identify different structural
transitions at different ratios of the thermal energy to the surface attraction
strength: filament straightening, adsorption and the magnetic flux closure. The
adsorption temperature of a magnetic filament is found to be higher in
comparison to an equivalent nonmagnetic chain. The adsorption has been also
investigated under the application of a static homogeneous external magnetic
field. We found that the strength and the orientation of the field can be used
to control the adsorption process, providing a precise switching mechanism.
Interestingly, we have observed that the characteristic field strength and tilt
angle at the adsorption point are related by a simple power law.",2007.09489v1
2020-07-21,Giant effective magnetic fields from optically driven chiral phonons in 4$f$ paramagnets,"We present a mechanism by which optically driven chiral phonon modes in
rare-earth trihalides generate giant effective magnetic fields acting on the
paramagnetic $4f$ spins. With cerium trichloride (CeCl$_3$) as our example
system, we calculate the coherent phonon dynamics in response to the excitation
by an ultrashort terahertz pulse using a combination of phenomenological
modeling and first-principles calculations. We find that effective magnetic
fields of over 100 tesla can possibly be generated that polarize the spins for
experimentally accessible pulse energies. The direction of induced
magnetization can be reversed by changing the handedness of circular
polarization of the laser pulse. The underlying process is a phonon analog of
the inverse Faraday effect in optics that has been described recently, and
which enables novel ways of achieving control over and switching of magnetic
order at terahertz frequencies.",2007.10556v3
2020-08-13,Comparison of measured and simulated spin-wave mode spectra of magnetic nanostructures,"Motivated by the importance of magnetization dynamics in nanomagnets for the
development and optimization of magnetic devices and sensors, we measured and
modeled spin wave spectra in patterned elliptical nanomagnets. Ferromagnetic
resonance spectra for multiple nanomagnets of Ni80Fe20 , fabricated by
electron-beam lithography to have nominal short-axes of 200 nm or 100 nm, were
measured by use of heterodyne magneto-optical microwave microscopy. Scanning
electron microscope images taken of the same nanomagnets were used to define
element shapes for micromagnetic simulations. Measured spectra show significant
differences between nominally identical nanomagnets, which could be only
partially attributed to uncontrolled shape variations in the patterning
process, as evidence by the limited agreement between the measured and
simulated spectra. Agreement between measurements and simulations was improved
by including a zone of reduced magnetization and exchange at the edges of the
nanomagnets in the simulations. Our results show that the reduction of shape
variations between individual magnetic random-access memory elements can
potentially improve their performance. However, unambiguous determination of
materials parameters in nanomagnets based on analysis and modeling of spin wave
spectra remains problematic.",2008.05662v1
2020-09-22,Magnetic mixed valent semimetal EuZnSb$_2$ with Dirac states in the band structure,"We report discovery of new antiferromagnetic semimetal EuZnSb$_2$, obtained
and studied in the form of single crystals. Electric resistivity, magnetic
susceptibility and heat capacity indicate antiferromagnetic order of Eu with
$T_N$ = 20 K. The effective moment of Eu$^{2+}$ inferred from the magnetization
and specific heat measurement is 3.5 $\mu_B$, smaller than the theoretical
value of Eu$^{2+}$ due to presence of both Eu$^{3+}$ and Eu$^{2+}$. Magnetic
field-dependent resistivity measurements suggest dominant quasi two dimensional
Fermi surfaces whereas the first-principle calculations point to the presence
of Dirac fermions. Therefore, EuZnSb$_2$ could represent the first platform to
study the interplay of dynamical charge fluctuations, localized magnetic 4$f$
moments and Dirac states with Sb orbital character.",2009.10720v1
2020-10-06,"Trigonal layered rosiaite-related antiferromagnet MnSnTeO6: ion-exchange preparation, structure and magnetic properties","Ion-exchange treatment of Na2SnTeO6 in molten salt mixtures resulted in
rosiaite (PbSb2O6)-related MnSnTeO6. Its crystal structure was refined by the
Rietveld method. Of the three possible models of Sn/Te distribution, the
disordered model ((P3 Im, a = 5.23093(11) A, c = 4.62430(16) A was preferred
based on bond distances. However, it is supposed that each individual
(SnTeO6)2- layer retains complete ordering of the precursor and the apparent
disorder is only due to stacking faults. The magnetic studies have shown that
MnSnTeO6 orders antiferromagnetically at Neel temperature TN = 9.8 K. The
effective magnetic moment reasonably agrees with theoretical estimations
assuming high-spin configuration of Mn2+ (S = 5/2). Electron spin resonance
reveals spin dynamics in accordance with aniferromagnetic ordering with
critical broadening of ESR linewidth indicating the low-dimentional type of
exchange interactions. Based on the temperature and field-dependent
magnetization studies, the magnetic phase diagram of the new compound was
constructed.",2010.03003v1
2020-10-12,Line-drag damping of Alfvén waves in radiatively driven winds of magnetic massive stars,"Line-driven stellar winds from massive (OB) stars are subject to a strong
line-deshadowing instability. Recently, spectropolarimetric surveys have
collected ample evidence that a subset of Galactic massive stars hosts strong
surface magnetic fields. We investigate here the propagation and stability of
magneto-radiative waves in such a magnetised, line-driven wind. Our analytic,
linear stability analysis includes line-scattering from the stellar radiation,
and accounts for both radial and non-radial perturbations. We establish a
bridging law for arbitrary perturbation wavelength after which we analyse
separately the long- and short-wavelength limits. While long-wavelength
radiative and magnetic waves are found to be completely decoupled, a key result
is that short-wavelength, radially propagating Alfv\'en waves couple to the
scattered radiation field and are strongly damped due to the line-drag effect.
This damping of magnetic waves in a scattering-line-driven flow could have
important effects on regulating the non-linear wind dynamics, and so might also
have strong influence on observational diagnostics of the wind structure and
clumping of magnetic line-driven winds.",2010.05650v1
2020-10-23,Impulsive coronal heating during the interaction of surface magnetic fields in the lower solar atmosphere,"Coronal plasma in the cores of solar active regions is impulsively heated to
more than 5 MK. The nature and location of the magnetic energy source
responsible for such impulsive heating is poorly understood. Using observations
of seven active regions from the Solar Dynamics Observatory, we found that a
majority of coronal loops hosting hot plasma have at least one footpoint rooted
in regions of interacting mixed magnetic polarity at the solar surface. In
cases when co-temporal observations from the Interface Region Imaging
Spectrograph space mission are available, we found spectroscopic evidence for
magnetic reconnection at the base of the hot coronal loops. Our analysis
suggests that interactions of magnetic patches of opposite polarity at the
solar surface and the associated energy release during reconnection are key to
impulsive coronal heating.",2010.12560v3
2020-11-04,Turbulent transport of fast ions in tokamak plasmas in the presence of resonant magnetic perturbations,"The effects of resonant magnetic perturbations on the turbulent transport of
fast ions in tokamak devices are investigated using a theoretical transport
model of test-particle type. The direct numerical simulation method is used to
compute, via the transport model, the diffusion coefficients. The numerical
results are in good agreement with other, analytically derived, estimations. It
is found that finite Larmor radius effects decrease algebraically the
transport, while the amplitude of magnetic perturbations has an opposite
effect. In the presence of stochastic dynamics, the asymmetric toroidal
magnetic field induces a small, radial, outward pinch. A synergistic mechanism
of non-linear coupling between turbulence and magnetic perturbations enhances
the radial diffusion. General scaling laws are proposed for the transport
coefficients.",2011.02302v1
2020-11-11,Role of impurity clusters for the current-driven motion of magnetic Skyrmions,"We study how impurities influence the current-induced dynamics of magnetic
Skyrmions moving in a racetrack geometry. For this, we solve numerically the
generalized Landau-Lifshitz-Gilbert equation extended by the current-induced
spin transfer torque. In particular, we investigate two classes of impurities,
non-conducting and magnetic impurities. The former are magnetically rigid
objects and yield to an inhomogeneous current density over the racetrack which
we determine separately by solving the fundamental electrostatic equations. In
contrast, magnetic impurities leave the applied current density homogeneous
throughout the stripe. Depending on parameters, we observe four different
scenarios of Skyrmion motions in the presence of disorder, the Skyrmion decay,
the pinning, the creation of additional Skyrmions, and ordinary Skyrmion
passage. We calculate and discuss phase diagrams in dependence of the impurity
concentration and radii of the impurities.",2011.06027v1
2020-11-16,Spin Entanglement and Magnetic Competition via Long-range Interactions in Spinor Quantum Optical Lattices,"Quantum matter at ultra-low temperatures offers a testbed for analyzing and
controlling desired properties in strongly correlated systems. Under typical
conditions the nature of the atoms fixes the magnetic character of the system.
Beyond classical light potentials leading to optical lattices and short range
interactions, high-Q cavities introduce novel dynamics into the system via the
quantumness of light. Here we propose a theoretical model and we analyze it
using exact diagonalization and density matrix renormalization group
simulations. We explore the effects of cavity mediated long range magnetic
interactions and optical lattices in ultracold matter. We find that global
interactions modify the underlying magnetic character of the system while
introducing competition scenarios. Antiferromagnetic correlated bosonic matter
emerges in conditions beyond to what nature typically provides. These allow new
alternatives toward the design of robust mechanisms for quantum information
purposes, exploiting the properties of magnetic phases of strongly correlated
quantum matter.",2011.07765v3
2020-11-25,Evolution of axions in the presence of primordial magnetic fields,"We study the evolution of axions interacting with primordial magnetic fields
(PMFs) starting just from the QCD phase transition in the expanding universe.
This interaction is owing to the Primakoff effect. Adopting the zero mode
approximation for axions, we derive the system of equations for axions and
magnetic fields, where the expansion of the universe and the spectra of
magnetic fields are accounted for exactly. We find that the contribution of the
Primakoff effect to the dynamics of axions and magnetic fields is rather weak.
It confirms some previous estimates leading to analogous conclusions, when
accounting here for the Hubble expansion both for an uniform axion field and
non-uniform PMFs using Fourier spectra for their energy and helicity densities.
We solve the corresponding system of the evolution equations and find that the
axion zero mode, when evolving during radiation era, has its amplitude at the
level sufficient for that axion to be a good candidate for the cold dark
matter.",2011.12712v1
2020-11-26,Chaotic exits from a weakly magnetized Schwarzschild black hole,"A charged particle kicked from an initial circular orbit around a weakly
magnetized Schwarzschild black hole undergoes transient chaotic motion before
either getting captured by the black hole or escaping upstream or downstream
with respect to the direction of the magnetic field. These final states form
basins of attraction in the space of initial states. We provide a detailed
numerical study of the basin structure of this initial state space. We find it
to possess the peculiar Wada property: each of its basin boundaries is shared
by all three basins. Using basin entropy as a measure, we show that uncertainty
in predicting the final exit state increases with stronger magnetic
interaction. We also present an approximate analytic expression of the critical
escape energy for a vertically-kicked charged particle, and discuss how this
depends on the strength of the magnetic interaction.",2011.13402v2
2020-12-07,Free-fall velocities and heat transport enhancement in liquid metal magneto-convection,"In geo- and astrophysics, low Prandtl number convective flows often interact
with magnetic fields. Although a static magnetic field acts as a stabilizing
force on such flow fields, we find that self-organized convective flow
structures reach an optimal state where the heat transport significantly
increases and convective velocities reach the theoretical free-fall limit, i.e.
the maximum possible velocity a fluid parcel can achieve when its potential
buoyant energy is fully converted into kinetic energy. Our measurements show
that the application of a static magnetic field leads to an anisotropic, highly
ordered flow structure and a decrease of the turbulent fluctuations. When the
magnetic field strength is increased beyond the optimum, Hartmann braking
becomes dominant and leads to a reduction of the heat and momentum transport.
The results are relevant for the understanding of magneto-hydrodynamic
convective flows in planetary cores and stellar interiors in regions with
strong toroidal magnetic fields oriented perpendicular to temperature
gradients.",2012.03905v2
2020-12-10,Magnetic properties of poly(trimethylene terephthalate-block-poly(tetramethylene oxide) copolymer nanocomposites reinforced by graphene oxide-Fe3O4 hybrid nanoparticles,"Thermoplastic elastomeric nanocomposites based on poly(trimethylene
terephthalate-block-poly(tetramethylene oxide) copolymer (PTT-PTMO) and
graphene oxide-Fe3O4 nanoparticle hybrid were prepared by in situ
polymerization. Superparamagnetic GO-Fe3O4 hybrid nanoparticles before
introducing to elastomeric matrix were characterized by X-ray photoelectron
spectroscopy (XPS), X-ray diffraction (XRD), thermogravimetric analysis (TGA)
and scanning electron microscopy (SEM). The effect of loading (0.3 and 0.5 wt%)
of GO-Fe3O4 nanoparticle hybrid on the phase structure, tensile and magnetic
properties of synthesized nanocomposites was investigated. The phase structure
of nanocomposites was evaluated by differential scanning calorimetry (DSC) and
dynamic mechanical thermal analysis (DMTA). Dispersion of GO-Fe3O4
nanoparticles in elastomeric matrix was evaluated by transmission electron
microscopy (TEM). Magnetic properties of GO-Fe3O4 nanoparticle hybrid and
nanocomposites with their content were characterized by using two different
techniques: dc SQUID magnetization measurements as a function of temperature
(from 2 to 300 K) and external magnetic field and ferromagnetic resonance (FMR)
at microwave frequency.",2012.05819v1
2020-12-20,Inverse transfer of magnetic helicity in direct numerical simulations of compressible isothermal turbulence: helical transfers,"The role of the different helical components of the magnetic and velocity
fields in the inverse spectral transfer of magnetic helicity is investigated
through Fourier shell-to-shell transfer analysis. Both magnetic helicity and
energetic transfer analysis are performed on chosen data from direct numerical
simulations of homogeneous isothermal compressible magnetohydrodynamic
turbulence, subject to both a large-scale mechanical forcing and a small-scale
helical electromotive driving. The root mean square Mach number of the
hydrodynamic turbulent steady-state taken as initial condition varies from 0.1
to about 11. Three physical phenomena can be distinguished in the general
picture of the spectral transfer of magnetic helicity towards larger spatial
scales: local inverse transfer, non-local inverse transfer and local direct
transfer. A shell decomposition allows to associate these three phenomena with
clearly distinct velocity scales while the helical decomposition allows to
establish the role of the different helical components and the compressive part
of the velocity field on the different transfer processes. The locality and
relative strength of the different helical contributions are mainly determined
by the triad helical geometric factor.",2012.10855v1
2021-01-05,Fast Quasi-Geostrophic Magneto-Coriolis Modes in the Earth's core,"Fast changes of Earth's magnetic field could be explained by inviscid and
diffusion-less quasi-geostrophic (QG) Magneto-Coriolis modes. We present a
hybrid QG model with columnar flows and three-dimensional magnetic fields and
find modes with periods of a few years at parameters relevant to Earth's core.
For the simple poloidal magnetic field that we consider here they show a
localization of kinetic and magnetic energy in the equatorial region. This
concentration of energy near the equator and the high frequency make them a
plausible mechanism to explain similar features observed in recent geomagnetic
field observations. Our model potentially opens a way to probe the otherwise
inaccessible magnetic field structure in the Earth's outer core.",2101.01493v2
2021-01-20,Efficient highly-subsonic turbulent dynamo and growth of primordial magnetic fields,"We present the first study on the amplification of magnetic fields by the
turbulent dynamo in the highly subsonic regime, with Mach numbers ranging from
$10^{-3}$ to $0.4$. We find that for the lower Mach numbers the saturation
efficiency of the dynamo, $(E_{\mathrm{mag}}/E_{\mathrm{kin}})_{\mathrm{sat}}$,
increases as the Mach number decreases. Even in the case when injection of
energy is purely through longitudinal forcing modes,
$(E_{\mathrm{mag}}/E_{\mathrm{kin}})_{\mathrm{sat}}$ $\gtrsim 10^{-2}$ at a
Mach number of $10^{-3}$. We apply our results to magnetic field amplification
in the early Universe and predict that a turbulent dynamo can amplify
primordial magnetic fields to $\gtrsim$ $10^{-16}$ Gauss on scales up to 0.1 pc
and $\gtrsim$ $10^{-13}$ Gauss on scales up to 100 pc. This produces fields
compatible with lower limits of the intergalactic magnetic field inferred from
blazar $\gamma$-ray observations.",2101.08256v1
2021-02-03,"Formation of magnetized spatial structures in the Beta Lyrae system III. Reflection of magnetically controlled matter in circumbinary structures in helium lines, in particular arising from metastable levels","Spatial gaseous structures in the Beta Lyrae system have been studied with
the fact of change in the longitudinal component of the donor's magnetic field
during the orbital period in mind. The investigation was based primarily on the
study of the dynamics of the circumstellar structures surrounding the binary
system as a whole. The special emphasis was placed on the study of complex
helium lines, in particular those arising from metastable levels. A number of
different observable facts from the ultraviolet to the red spectral region were
analyzed. The configuration of the donor magnetic field is a factor that not
only enhances mass transfer and influences the formation of spatial gas
structures between stellar components but, to some extent, also affects the
outflow of matter and the formation of external gas structures around this
interacting binary system. Together with previous articles
\citep{Skulskyy2020a, Skulskyy2020b}, the pieces of evidence of this work,
confirming the reflection of magnetically controlled matter in circumbinary
structures, define the basis for a coherent picture of the mass exchange
between components and outflows of matter outwards",2102.01964v1
2021-02-03,"Magnetic dipole moments of the hidden-charm pentaquark states: $P_c(4440)$, $P_c(4457)$ and $P_{cs}(4459)$","In this work, we employ the light-cone QCD sum rule to calculate the magnetic
dipole moments of the $P_c(4440)$, $P_c(4457)$ and $P_{cs}(4459)$ pentaquark
states by considering them as the diquark-diquark-antiquark and molecular
pictures with quantum numbers $J^P = \frac{3}{2}^-$, $J^P = \frac{1}{2}^-$ and
$J^P = \frac{1}{2}^-$, respectively. In the analyses, we use the
diquark-diquark-antiquark and molecular form of interpolating currents, and
photon distribution amplitudes to obtain the magnetic dipole moment of
pentaquark states. Theoretical examinations on magnetic dipole moments of the
hidden-charm pentaquark states, are essential as their results can help us
better figure out their substructure and the dynamics of the QCD as the theory
of the strong interaction. As a by product, we extract the electric quadrupole
and magnetic octupole moments of the $P_c(4440)$ pentaquark. These values show
a non-spherical charge distribution.",2102.01996v2
2021-02-15,"Angular-Dependent Dynamic Response and Magnetization Reversal in Fibonacci-Distorted, Kagome Artificial Spin Ice","We have measured the angular dependence of ferromagnetic resonance (FMR)
spectra for Fibonacci-distorted, Kagome artificial spin ice (ASI). The number
of strong modes in the FMR spectra depend on the orientation of the applied DC
magnetic field. In addition, discontinuities observed in the FMR
field-frequency dispersion curves also depend on DC field orientation, and
signal a multi-step DC magnetization reversal, which is caused by the reduced
energy degeneracy of Fibonacci-distorted vertices. The results suggest the
orientation of applied magnetic field and severity of Fibonacci distortion
constitute control variables for FMR modes and multi-step reversal in future
magnonic devices and magnetic switching systems.",2102.07532v1
2021-03-15,Nonlinear magnetoelectric effect in atomic vapor,"Magnetoelectric (ME) effect refers to the coupling between electric and
magnetic fields in a medium resulting in electric polarization induced by
magnetic fields and magnetization induced by electric fields. The linear ME
effect in certain magnetoelectric materials such as multiferroics has been of
great interest due to its application in the fabrication of spintronics
devices, memories, and magnetic sensors. However, the exclusive studies on the
nonlinear ME effect are mostly centered on the investigation of second-harmonic
generation in chiral materials. Here, we report the demonstration of nonlinear
wave mixing of optical electric fields and radio-frequency (rf) magnetic fields
in thermal atomic vapor, which is the consequence of the higher-order nonlinear
ME effect in the medium. The experimental results are explained by comparing
with density matrix calculations of the system. We also experimentally verify
the expected dependence of the generated field amplitudes on the rf field
magnitude as evidence of the magnetoelectric effect. This study can open up the
possibility for precision rf-magnetometry due to its advantage in terms of
larger dynamic range and arbitrary frequency resolution.",2103.08168v1
2021-03-24,Evolution of localized magnetic field perturbations and the nature of turbulent dynamo,"Kinematic dynamo in incompressible isotropic turbulent flows with high
magnetic Prandtl number is considered. The approach interpreting an arbitrary
magnetic field distribution as a superposition of localized perturbations
(blobs) is proposed. We derive a relation between stochastic properties of a
blob and a stochastically homogenous distribution of magnetic field advected by
the same stochastic flow. This relation allows to investigate the evolution of
a localized blob at late stage when its size exceeds the viscous scale. It is
shown that in 3-dimansional flows, the average magnetic field of the blob
increases exponentially in the inertial range of turbulence, as opposed to the
late-Batchelor stage when it decreases. Our approach reveals the mechanism of
dynamo generation in the inertial range both for blobs and homogenous
contributions. It explains the absence of dynamo in the two-dimensional case
and its efficiency in three dimensions. We propose the way to observe the
mechanism in numerical simulations.",2103.13091v1
2021-03-25,Transport efficiency of biofunctionalized magnetic particles tailored by surfactant concentration,"Controlled transport of surface functionalized magnetic beads in a liquid
medium is a central requirement for the handling of captured biomolecular
targets in microfluidic lab-on-chip biosensors. Here, the influence of the
physiological liquid medium on the transport characteristics of functionalized
magnetic particles and on the functionality of the coupled protein is studied.
These aspects are theoretically modeled and experimentally investigated for
prototype superparamagnetic beads, surface functionalized with green
fluorescent protein immersed in buffer solution with different concentrations
of a surfactant. The model reports on the tunability of the steady-state
particle substrate separation distance to prevent their surface sticking via
the choice of surfactant concentration. Experimental and theoretical average
velocities are discussed for a ratchet like particle motion induced by a
dynamic external field superposed on a static locally varying magnetic field
landscape. The developed model and experiment may serve as a basis for
quantitative forecasts on the functionality of magnetic particle transport
based lab-on-chip devices.",2103.13934v1
2021-04-06,Diffusion of a magnetic skyrmion in 2-dimensional space,"Two-dimensional magnetic skyrmions are particle-like magnetic domains in
magnetic thin films. The kinetic property of the magnetic skyrmions at finite
temperature is well described by the Thiele equation, including a stochastic
field and a finite mass. In this paper, the validity of the constant-mass
approximation is examined by comparing the Fourier spectrum of Brownian motions
described by the Thiele equation and the Landau-Lifshitz-Gilbert equation.
Then, the 4-dimensional Fokker-Planck equation is derived from the Thiele
equation with a mass-term. Consequently, an expression of the diffusion flow
and diffusion constant in a tensor form is derived, extending Chandrasekhar's
method for Thiele dynamics.",2104.02345v2
2021-04-21,Generation of magnetic fields by thermomagnetic effects in a nonuniformly rotating layer of an electrically conductive fluid,"In this paper, the generation of magnetic fields in a nonuniformly rotating
layer of finite thickness of an electrically conducting fluid by thermomagnetic
(TM) instability. This instability arises due to the temperature gradient
$\nabla T_0$ and thermoelectromotive coefficient gradient $\nabla\alpha $. The
influence of the generation of a toroidal magnetic field by TM instability on
convective instability in a nonuniformly rotating layer of an electrically
conductive fluid in the presence of a vertical constant magnetic field
${\bf{B}}_0 \| {\rm OZ}$ is established. As a result of applying the method of
perturbation theory for the small parameter $ \epsilon = \sqrt {(\textrm
{Ra}-\textrm {Ra}_c) / \textrm {Ra}_c} $ of supercriticality of the stationary
Rayleigh number $\textrm {Ra}_c$ a nonlinear equation of the Ginzburg-Landau
type was obtained. This equation describes the evolution of the finite
amplitude of perturbations. Numerical solutions of this equation made it
possible to determine the heat transfer in the fluid layer with and without TM
effects. It is shown that the amplitude of the stationary toroidal magnetic
field noticeably increases with allowance for TM effects.",2104.11068v1
2021-06-03,Resonant Nonlinear Hall Effect in Two-Dimensional Electron Systems,"We study the Hall conductivity of a two-dimensional electron gas under an
inhomogeneous magnetic field $B(x)$. First, we prove using the quantum kinetic
theory that an odd magnetic field can lead to a purely nonlinear Hall response.
Second, considering a real-space magnetic dipole consisting of a sign-changing
magnetic field and based on numerical semiclassical dynamics, we unveil a
parametric resonance involving the cyclotron ratio and a characteristic width
of $B(x)$, which can greatly enhance the Hall response. Different from previous
mechanisms that rely on the bulk Berry curvature dipole, here, the effect
largely stems from boundary states associated with the real-space magnetic
dipole. Our findings pave a new way to engineer current rectification and
higher harmonic generation in two-dimensional materials having or not crystal
inversion symmetry.",2106.02001v2
2021-06-13,Floquet Engineering of Magnetism in Topological Insulator Thin Films,"Dynamic manipulation of magnetism in topological materials is demonstrated
here via a Floquet engineering approach using circularly polarized light.
Increasing the strength of the laser field, besides the expected topological
phase transition, the magnetically doped topological insulator thin film also
undergoes a magnetic phase transition from ferromagnetism to paramagnetism,
whose critical behavior strongly depends on the quantum quenching. In sharp
contrast to the equilibrium case, the non-equilibrium Curie temperatures vary
for different time scale and experimental setup, not all relying on change of
topology. Our discoveries deepen the understanding of the relationship between
topology and magnetism in the non-equilibrium regime and extend optoelectronic
device applications to topological materials.",2106.06977v1
2021-07-05,Kapitsa pendulum effects in Josephson junction + nanomagnet under external periodic drive,"We investigate reorientation effects under external periodic drive in the
nanomagnet dynamics coupled to a Josephson junction. The Kapitsa pendulum is
introduced as a mechanical analog to this system and we demonstrate the
reorientation of the easy axis of the nanomagnet. The magnetic field generated
by the Josephson junction and external drive plays the role of the oscillating
force of the suspension point in the Kapitsa pendulum. The high frequency
oscillations change the orientation of the magnetic moment. The magnetic field
of the quasiparticle current determines the frequency dependence of the
magnetic moment's orientation. We obtain simple analytical formulas for the
stable position of the magnetic moment, both under the external periodic drive
and without it. The influence of external periodic drive on the voltage of
complete reorientation have been demonstrated.",2107.01882v2
2021-07-09,Magnetic moments of spin--1/2 triply-heavy baryons: A study of Light-cone QCD and Quark-diquark model,"In this study, the magnetic moments of the spin-1/2 triply-heavy baryons have
been calculated using both light-cone QCD sum rules and Quark-diquark model.
Theoretical investigations on magnetic moments of the triply-heavy baryons, are
crucial as their results can help us better understand their internal structure
and the dynamics of the QCD as the theory of the strong interaction. We compare
the results extracted for the magnetic moment with the existing theoretical
predictions. It is seen that the obtained magnetic moment values are quite
compatible with the results in the literature.",2107.04361v2
2021-07-27,Measuring magnetic flux suppression in high-power laser-plasma interactions,"Biermann battery magnetic field generation driven by high power laser-solid
interactions is explored in experiments performed with the OMEGA EP laser
system. Proton deflectometry captures changes to the strength, spatial profile,
and temporal dynamics of the self-generated magnetic fields as the target
material or laser intensity is varied. Measurements of the magnetic flux during
the interaction are used to help validate extended magnetohydrodynamic (MHD)
simulations. Results suggest that kinetic effects cause suppression of the
Biermann battery mechanism in laser-plasma interactions relevant to both direct
and indirect-drive inertial confinement fusion. Experiments also find that more
magnetic flux is generated as the target atomic number is increased, which is
counter to a standard MHD understanding.",2107.12864v1
2021-08-07,Normal Forms and Near-Axis Expansions for Beltrami Magnetic Fields,"A formal series transformation to Birkhoff-Gustavson normal form is obtained
for toroidal magnetic field configurations in the neighborhood of a magnetic
axis. Bishop's rotation-minimizing coordinates are used to obtain a local
orthogonal frame near the axis in which the metric is diagonal, even if the
curvature has zeros. We treat the cases of vacuum and force-free (Beltrami)
fields in a unified way, noting that the vector potential is essentially the
Poincar\'e-Liouville one-form of Hamiltonian dynamics, and the resulting
magnetic field corresponds to the canonical two-form of a nonautonomous
one-degree-of-freedom system. Canonical coordinates are obtained and Floquet
theory is used to transform to a frame in which the lowest-order Hamiltonian is
autonomous. The resulting magnetic axis can be elliptic or hyperbolic, and
resonant elliptic cases are treated. The resulting expansion for the field is
shown to be well-defined to all orders, and is explicitly computed to degree
four. An example is given for an axis with constant torsion near a 1:3
resonance.",2108.03337v1
2021-08-26,Quantized electrical conductivity in binary neutron star mergers,"We examine nature of longitudinal electrical conductivity in magnetized
electron-ion plasma in the context of binary neutron star mergers. In presence
of strong magnetic field, high density and temperature, quantum oscillatory
behaviour for electrons emerge due to breakdown of the classical description.
For pronounced thermodynamic effects, we consider zeroth Landau level
population of electrons for electrical conductivity. We solve Boltzmann
equation in presence of magnetic field to obtain the dissipative component of
the conductivity. The conductivity is formulated considering dynamically
scattering centres in the medium with magnetically modified screening.
Numerical estimations show that the effect of magnetically modified screening
mass on electrical conductivity is less. On the other hand, we observe that
frequency dependent screening reduces electrical conductivity leading to a
reduction in the Ohmic decay time scale to become of the order of the
characteristic timescale of the merger process in the low density regime. This
indicates the relevance of dissipative process for the merger simulation in the
above mentioned domain.",2108.11878v1
2021-08-26,Coupling of magnetic and optomechanical structuring in cold atoms,"Self-organized phases in cold atoms as a result of light-mediated
interactions can be induced by coupling to internal or external degrees of the
atoms. There has been growing interest in the interaction of internal spin
degrees of freedom with the optomechanical dynamics of the external
centre-of-mass motion. We present a model for the coupling between magnetic and
optomechanical structuring in a $J=1/2 \to J'=3/2$ system in a single-mirror
feedback scheme, being representative for a larger class of diffractively
coupled systems such as longitudinally pumped cavities and counter-propagating
beam schemes. For negative detunings, a linear stability analysis demonstrates
that optical pumping and optomechanical driving cooperate to create magnetic
ordering. However, for long-period transmission gratings the magnetic driving
will strongly dominate the optomechanical driving, unless one operates very
close to the existence range of the magnetic instability. At small lattice
periods, in particular at wavelength-scale periods, the optomechanical driving
will dominate.",2108.12064v3
2021-09-05,Ion Temperature Effects on Plasma Flow in the Magnetic Mirror Configuration,"Effects of finite ion temperature on plasma flow in the converging-diverging
magnetic field, the magnetic mirror, or equivalently, magnetic nozzle
configuration, are studied using a quasineutral paraxial two-fluid MHD model
with isothermal electrons and warm magnetized ions. The ion acceleration was
studied with an emphasis on the role of the singularity at the sonic point
transition. It is shown that the regularity of the sonic point defines a global
solution describing plasma acceleration from subsonic to supersonic velocity.
Stationary accelerating solutions were obtained and compared with the time
dependent dynamics, confirming that the solutions of the time-dependent
equations converge to the stationary solutions and therefore are stable. The
effects of the ion pressure anisotropy were analyzed using Chew-Goldberger-Low
model and its generalization. It is shown that the mirror force (manifested by
the perpendicular ion pressure) enhances plasma acceleration. The role of
ionization and charge exchange on plasma flow acceleration have been
investigated.",2109.02006v3
2021-09-12,Evolution of neutron star magnetic fields,"Neutron stars are natural physical laboratories allowing us to study a
plethora of phenomena in extreme conditions. In particular, these compact
objects can have very strong magnetic fields with non-trivial origin and
evolution. In many respects its magnetic field determines the appearance of a
neutron star. Thus, understanding the field properties is important for
interpretation of observational data. Complementing this, observations of
diverse kinds of neutron stars enable us to probe parameters of
electro-dynamical processes at scales unavailable in terrestrial laboratories.
In this review we first briefly describe theoretical models of formation and
evolution of magnetic field of neutron stars, paying special attention to field
decay processes. Then we present important observational results related to
field properties of different types of compact objects: magnetars, cooling
neutron stars, radio pulsars, sources in binary systems. After that, we discuss
which observations can shed light on obscure characteristics of neutron star
magnetic fields and their behaviour. We end the review with a subjective list
of open problems.",2109.05584v1
2021-09-13,Andreev Reflections in NbN/graphene Junctions under Large Magnetic Fields,"Hybrid superconductor/graphene (SC/g) junctions are excellent candidates for
investigating correlations between Cooper pairs and quantum Hall (QH) edge
modes. Experimental studies are challenging as Andreev reflections are
extremely sensitive to junction disorder and high magnetic fields are required
to form QH edge states. We fabricated low-resistance SC/g interfaces, composed
of graphene edge contacted with NbN with a barrier strength of $Z\approx 0.4$,
that remain superconducting under magnetic fields larger than $18$ T. We
establish the role of graphene's Dirac band structure on zero-field Andreev
reflections and demonstrate dynamic tunability of the Andreev reflection
spectrum by moving the boundary between specular and retro Andreev reflections
with parallel magnetic fields. Through the application of perpendicular
magnetic fields, we observe an oscillatory suppression of the 2-probe
conductance in the $\nu = 4$ Landau level attributed to the reduced efficiency
of Andreev processes at the NbN/g interface, consistent with theoretical
predictions.",2109.06285v1
2021-10-25,A resolution criterion based on characteristic timescales for MHD simulations of molecular clouds,"We investigate the effect of numerical magnetic diffusion in
magnetohydrodynamic simulations of magnetically supported molecular clouds. To
this end, we have performed numerical studies on adaptive mesh isothermal
simulations of marginally sub-critical molecular clouds. We find that
simulations with low and intermediate resolutions collapse, contrary to what is
theoretically expected. However, the simulation with the highest numerical
resolution oscillates around an equilibrium state without collapsing. In order
to quantify the numerical diffusion of the magnetic field, we ran a second suit
of current-sheet simulations in which the numerical magnetic diffusion
coefficient can be directly measured, and computed the corresponding diffusion
times at various numerical resolutions. On this basis, we propose a criterion
for the resolution of magnetic fields in MHD simulations based on requiring
that the diffusion time to be larger than the characteristic timescale of the
physical process responsible for the dynamic evolution of the structure.",2110.13281v2
2021-11-03,Primordial Helical Magnetic Fields from Inflation?,"We revisit the mechanism of helical magnetogenesis during inflation with a
parity violating interaction using the formalism of stochastic inflation. One
of the polarization of the gauge field undergoes tachyonic growth leading to
the generation of helical magnetic fields. We obtain the Langevin equations
associated with the electromagnetic fields which are in the form of
Ornstein-Uhlenbeck stochastic differential equations. Consequently, the
tachyonic growth of the helical magnetic fields is balanced by a mean-reverting
process of stochastic dynamics such that the magnetic fields settle down to an
equilibrium state with the amplitude smaller than what is obtained in the
absence of the stochastic noises. Working in the parameter space of the model
where both the backreaction and the strong coupling problems are under control
the model does not provide large enough seed to be amplified by the galactic
dynamo as the source of the magnetic fields observed on cosmological scales.",2111.02147v2
2021-11-29,Density perturbation and cosmological evolution in the presence of magnetic field in $f(R)$ gravity models,"In this paper, we have investigated the density perturbations and
cosmological evolution in the FLRW universe in presence of a cosmic magnetic
field, which may be assumed to mimic primordial magnetic fields. Such magnetic
fields have sufficient strength to influence galaxy formation and cluster
dynamics, thereby leaving an imprint on the CMB anisotropies. We have
considered the FLRW universe as a representative of the isotropic cosmological
model in the 1+3 covariant formalism for $f(R)$ gravity. The propagation
equations have been determined and analyzed, where we have assumed that the
magnetic field is aligned uniformly along the $x$-direction, resulting in a
diagonal shear tensor. Subsequently,the density perturbation evolution
equations have been studied and the results have been interpreted. We have also
indicated how these results change in the general relativistic case and briefly
mentioned the expected change in higher-order gravity theories.",2111.14898v2
2021-12-07,Interaction effects of pseudospin-based magnetic monopoles and kinks in a doped dipolar superlattice gas,"Magnetic monopoles and kinks are topological excitations extensively
investigated in quantum spin systems, but usually they are studied in different
setups. We explore the conditions for the coexistence and the interaction
effects of these quasiparticles in the pseudospin chain of the atomic dipolar
superlattice gas. In this chain, the magnetic kink is the intrinsic
quasiparticle, and the particle/hole defect takes over the role of the
north/south magnetic monopole, exerting monopolar magnetic fields to
neighboring spins. A confinement effect between the monopole and kink is
revealed, which renormalizes the dispersion of the kink. The corresponding
dynamical deconfinement process is observed and arises due to the kink-antikink
annihilation. The rich interaction effects of the two quasiparticles could
stimulate corresponding investigations in bulk spin systems.",2112.03496v2
2021-12-13,Causal Interaction between the subsurface rotation rate residuals and radial magnetic field in different timescales,"We studied the presence and spatiotemporal characteristics and evolution of
the variations in the differential rotation rates and radial magnetic fields in
the Schwabe and Quasi-biennial-oscillation (QBO) timescales. To achieve these
objectives, we used rotation rate residuals and radial magnetic field data from
the Michelson Doppler Imager on the Solar and Heliospheric Observatory and the
Helioseismic and Magnetic Imager on the Solar Dynamics Observatory, extending
from May 1996 to August 2020, covering solar cycles 23 and 24, respectively.
Under the assumption that the radial surface magnetic field is non-local and
the differential rotation is symmetric around the equator, our results suggest
that the source region of the Schwabe cycle is confined between
$\sim$30$^{\circ}$ N and S throughout the convection zone. As for the source
region of the QBO, our results suggest that it is below 0.78R$_{\odot}$.",2112.06499v1
2021-12-13,Long-Term Variation of Helioseismic Far-Side Images and What Causes It,"A new time--distance far-side imaging technique was recently developed by
utilizing multiple multi-skip acoustic waves. The measurement procedure is
applied to 11 years of Doppler observations from the Solar Dynamics Observatory
/ Helioseismic and Magnetic Imager, and over 8000 far-side images of the Sun
have been obtained with a 12-hour temporal cadence. The mean travel-time shifts
in these images unsurprisingly vary with the solar cycle. However, the temporal
variation does not show good correlations with the magnetic activity in their
respective northern or southern hemisphere, but show very good anti-correlation
with the global-scale magnetic activity. We investigate four possible causes of
this travel-time variation. Our analysis demonstrates that the acoustic waves
that are used for mapping the Sun's far side experience surface reflections
around the globe, where they may interact with surface or near-surface magnetic
field, and carry travel-time deficits with them. The mean far-side travel-time
shifts from these acoustic waves therefore vary in phase with the Sun's
magnetic activity.",2112.06985v1
2021-12-20,Failure of the mean-field description of magnetic fluctuations in the superconducting quantum dot,"The zero-temperature physics of interacting quantum dots attached to
superconducting leads is now well understood. The overall qualitative picture
is obtained from the static mean-field approximation. The situation drastically
changes at non-zero temperatures. No reliable solutions apart from numerical
simulations exist there. We show that any static mean-field approximation fails
at non-zero temperatures since magnetic fluctuations induce dynamical
corrections that lead to broadening of the in-gap state energies to energy
bands. Spin-symmetric equilibrium state at non-zero temperatures is unstable
with respect to magnetic fluctuations and the zero magnetic field can be
reached only as a weak limit of the spin-polarized solution like in a
magnetically ordered phase.",2112.10397v1
2021-12-26,Field-controlling patterns of sheared ferrofluid droplets,"We investigate how ferrofluid droplets suspended in a wall-bounded shear flow
can organise when subjected to an external magnetic field. By tuning the
magnitude of the external magnetic field, we find that the ferrofluid droplets
form chain-like structures in the flow direction when the magnetic field is
weak, while forming a crystal-like pattern in a strong magnetic field. We
provide the phase diagram and the critical conditions for this chain-to-crystal
transition, by applying both numerical simulations and analytic calculations.
We also examine how the organised patterns of the ferrofluid droplets can be
controlled by simply changing the direction of the magnetic field. This work
demonstrates new aspects of field-controllable ferrofluid droplets as a
configurable and reprocessable metamaterial.",2112.13362v2
2022-01-14,The Magnetic Field versus Density relation in Star-Forming Molecular Clouds,"We study the magnetic field to density ($B-\rho$) relation in turbulent
molecular clouds with dynamically important magnetic fields using nonideal
three-dimensional magnetohydrodynamic simulations. Our simulations show that
there is a distinguishable break density $\rho_{\rm T}$ between the relatively
flat low density regime and a power-law regime at higher densities. We present
an analytic theory for $\rho_{\rm T}$ based on the interplay of the magnetic
field, turbulence, and gravity. The break density $\rho_{\rm T}$ scales with
the strength of the initial Alfv\'en Mach number $\mathcal{M}_{\rm A0}$ for
sub-Alfv\'enic ( $\mathcal{M}_{\rm A0}<1$) and trans-Alfv\'enic
($\mathcal{M}_{\rm A0} \sim 1$) clouds. We fit the variation of $\rho_{\rm T}$
for model clouds as a function of $\mathcal{M}_{\rm A0}$, set by different
values of initial sonic Mach number $\mathcal{M_{\rm 0}}$ and the initial ratio
of gas pressure to magnetic pressure $\beta_{\rm 0}$. This implies that
$\rho_{\rm T}$, which denotes the transition in mass-to-flux ratio from the
subcritical to supercritical regime, is set by the initial turbulent
compression of the molecular cloud.",2201.05620v2
2022-01-19,On the limitations of magneto-frictional relaxation,"The magneto-frictional method is used in solar physics to compute both static
and quasi-static models of the Sun's coronal magnetic field. Here, we examine
how accurately magneto-friction (without fluid pressure) is able to predict the
relaxed state in a one-dimensional test case containing two magnetic null
points. Firstly, we show that relaxation under the full ideal
magnetohydrodynamic equations in the presence of nulls leads necessarily to a
non-force-free state, which could not be reached exactly by magneto-friction.
Secondly, the magneto-frictional solutions are shown to lead to breakdown of
magnetic flux conservation, whether or not the friction coefficient is scaled
with magnetic field strength. When this coefficient is constant, flux is
initially conserved, but only until discontinuous current sheets form at the
null points. In the ensuing weak solution, we show that magnetic flux is
dissipated at these current sheets. The breakdown of flux conservation does not
occur for an alternative viscous relaxation scheme.",2201.07485v1
2022-01-25,Hydrodynamics of interacting spinons in the magnetized spin-$1/2$ chain with the uniform Dzyaloshinskii-Moriya interaction,"We use a hydrodynamic approach to investigate dynamic spin susceptibility of
the antiferromagnetic spin-$1/2$ Heisenberg chain with a uniform
Dzyaloshinskii-Moriya (DM) interaction in the presence of an external magnetic
field. We find that transverse (with respect to the magnetic field) spin
susceptibility harbors two (respectively, three) spin excitation modes when the
magnetic field is parallel (respectively, orthogonal) to the DM axis. In all
cases, the marginally irrelevant backscattering interaction between the spinons
creates a finite energy splitting between optical branches of excitations at $k
= 0$. Additionally, for the orthogonal geometry, the two lower spin branches
exhibit avoided crossing at finite momentum which is determined by the total
magnetic field (the sum of the external and internal molecular fields) acting
on spinons. Our approximate analytical calculations compare well with numerical
results obtained using matrix-product-state (MPS) techniques. Physical
consequences of our findings for the electron spin resonance experiments are
discussed in detail.",2201.10570v2
2022-02-01,Control of sensitivity in vortex-type magnetic tunnel junction magnetometer sensors by the pinned layer geometry,"The tuning of sensitivity and dynamic range in linear magnetic sensors is
required in various applications. We demonstrate the control and design of the
sensitivity in magnetic tunnel junction (MTJ) sensors with a vortex-type
sensing layer. In this work, we develop sensor MTJs with NiFe sensing layers
having a vortex magnetic configuration. We demonstrate that by varying the
pinned layer size, the sensitivity to magnetic field is tuned linearly. We
obtain a high magnetoresistance ratio of 140 %, and we demonstrate a
controllable sensitivity from 0.85 to 4.43 %/Oe, while keeping the vortex layer
fixed in size. We compare our experimental results with micromagnetic
simulations. We find that the linear displacement of vortex core by an applied
field makes the design of vortex sensors simple. The control of the pinned
layer geometry is an effective method to increase the sensitivity, without
affecting the vortex state of the sensing layer. Furthermore, we propose that
the location of the pinned layer can be used to realize more sensing
functionalities from a single sensor.",2202.00207v1
2022-02-09,First-principles calculations for transient absorption of laser-excited magnetic materials,"We investigate the modification in the optical properties of laser-excited
bulk cobalt and nickel using the time-dependent density functional theory at a
finite electron temperature. As a result of the first-principles simulation, a
complex change in the photoabsorption of the magnetic materials is observed
around the $M_{2,3}$ absorption edge. Based on the microscopic analysis, we
clarify that this complex absorption change consists of the two following
components: (i) the decrease in the photoabsorption in a narrow energy range
around the $M_{2,3}$ edge, which reflects the blue shift of the absorption edge
due to the light-induced demagnetization, and (ii) the increase in the
photoabsorption in a wider range around the $M_{2,3}$ edge, which reflects the
modification in the local-field effect due to the light-induced electron
localization. The relation between the transient optical and magnetic
properties may open a way to monitor ultrafast (de)magnetization and spin
dynamics in magnetic materials via transient absorption spectroscopy.",2202.04226v1
2022-02-18,Chiral plasma instability and inverse cascade from nonequilibrium left-handed neutrinos in core-collapse supernovae,"We show that the backreaction of left-handed neutrinos out of equilibrium on
the matter sector induces an electric current proportional to a magnetic field
even without a chiral imbalance for electrons in core-collapse supernovae. We
derive the transport coefficient of this effect based on the recently
formulated chiral radiation transport theory for neutrinos. This chiral
electric current generates a strong magnetic field via the so-called chiral
plasma instability, which could provide a new mechanism for the strong and
stable magnetic field of magnetars. We also numerically study the physical
origin of the inverse cascade of the magnetic energy in the
magnetohydrodynamics including this current. Our results indicate that
incorporating the chiral effects of neutrinos would drastically modify the
hydrodynamic evolutions of supernovae, which may also be relevant to the
explosion dynamics.",2202.09205v2
2022-02-19,Configuration of magnetotail current sheet prior to magnetic reconnection onset,"The magnetotail current sheet configuration determines magnetic reconnection
properties that control the substorm onset, one of the most energetic phenomena
in the Earth's magnetosphere. The quiet-time current sheet is often
approximated as a two-dimensional (2D) magnetic field configuration balanced by
isotropic plasma pressure gradients. However, reconnection onset is preceded by
the current sheet thinning and the formation of a nearly one-dimensional (1D)
magnetic field configuration. In this study, using particle-in-cell
simulations, we investigate the force balance of such thin current sheets when
they are driven by plasma inflow. We demonstrate that the magnetic field
configuration transitions from 2D to 1D thanks to the formation of plasma
pressure nongyrotropy and reveal its origin in the nongyrotropic terms of the
ion distributions. We show that substorm onset may be controlled by the
instability and dynamics of such nongyrotropic current sheets, having
properties much different from the most commonly investigated 2D isotropic
configuration.",2202.09539v2
2022-03-01,Excitations in the ordered and paramagnetic states of honeycomb magnet Na2Co2TeO6,"Na2Co2TeO6 is a proposed approximate Kitaev magnet, yet its actual magnetic
interactions are elusive due to a lack of knowledge on the full excitation
spectrum. Here, using inelastic neutron scattering and single crystals, we
determine the system's temperature-dependent magnetic excitations over the
entire Brillouin zone. Without committing to specific models, we unveil a
distinct signature of the third-nearest-neighbor coupling in the spin waves,
which signifies the associated distance as an emerging ""soft link"" in the
ordered state. The presence of at least six non-overlapping spin-wave branches
is at odds with all models proposed to date. Above the ordering temperature,
persisting dynamic correlations can be described by equal-time magnetic
structure factors of a hexagonal cluster, which reveal the leading
instabilities. Our result sets definitive constraint on theoretical models for
Na2Co2TeO6 and provides new insight for the materialization of the Kitaev
model.",2203.00282v1
2022-03-28,Role of a magnetic field in the context of inhomogeneous gravitational collapse,"Magnetic fields have been found to have an inherent capability of acting
against gravity. An important question posed in the literature is whether
presence of a magnetic field can alter the dynamics of a gravitational collapse
and prevent the final formation of a singularity. Inhomogeneous models of
collapse have not been explored significantly in this context. In the present
work we investigate the role of magnetic fields in the evolution of
inhomogeneous cylindrically symmetric models. We use an approach based on the
Raychaudhuri equation for such an analysis. We show that it is quite possible
for the magnetic field to avert the gravitational collapse in these models.",2203.14877v2
2022-05-03,"Carrier Doping Modulates 2D Intrinsic Ferromagnetic Mn2Ge2Te6 Monolayer High Curie Temperature, Large Magnetic Crystal Anisotropy","The Mn2Ge2Te6 shows intrinsic ferromagnetic (FM) order, with Curie
temperature (Tc) of 316 K. The FM order origins from superexchange interaction
between Mn and Te atoms. Mn2Ge2Te6 is half-metal (HM), and spin-\b{eta}
electron is a semiconductor with gap of 1.462 eV. Mn2Ge2Te6 tends in-plane
anisotropy (IPA), with magnetic anisotropy energy (MAE) of -13.2 meV/f.u.. The
Mn2Ge2Te6 shows good dynamical and thermal stability. Moreover, Mn2Ge2Te6
presents good ferromagnetic and half-metallic stability under charge doping.
The carriers doping could effectively tune magnetic and electronic properties.
Specifically, the magnetic moment, exchange parameter, and MAE could be
efficiently tuned. The total magnetic moment changes linearly with charges
doping. The exchange parameters could be controlled by the doping carriers. The
carriers doping could modulate MAE to -18.4 (+0.4 e), -0.85 (-1.6 e), 1.31
(-2.4 e) meV/f.u., by changing hybridization between Te atoms' py and pz
orbitals. Mn2Ge2Te6 with intrinsic ferromagnetism, high tunable MAE, good
stability of ferromagnetism and half-metallicity could help researchers to
investigate its wide application in the electronics and spintronics.",2205.01498v1
2022-06-15,Giant Rashba electrical control of magnetism in band models,"It is of considerable technological importance to achieve an electrical
control of magnetism of sufficient magnitude. To overcome the in-plane shape
anisotropy, needed is the electrical control of a perpendicular magnetic
anisotropy (PMA). It is known, within a free electron model, the Rashba
spin-orbit coupling provides such a control. Surprisingly, this same Rashba PMA
is enhanced by two to three orders of magnitude when a periodic potential is
added. Usually spin Berry phase physics reflects time dependent magnetic
fields. Here it is shown, within a time independent model, such physics arises
because the Rashba effective magnetic field has texture within the unit cell.
Predicted are electrical controllable band-structure gaps, linear in the
applied electric field $E$, that can result in a truly giant linear PMA. Also
possible is a Peierls mechanism, in which the magnetisation tilts from the
vertical, shifting these gaps to the Fermi level. As a consequence there are
low dissipation electric field driven dynamics, an alternative to the more
dissipative spin torque transfer (STT) effect. The theory requires the
introduction of an intrinsic spin Berry connection $\vec A_s$, an effective
vector potential, and is incompatible with current density functional theories
(DFT).",2206.07399v1
2022-06-30,Ce and Dy substitutions in Nd$_{2}$Fe$_{14}$B: site-specific magnetic anisotropy from first-principles,"A first-principles approach combining density functional and dynamical
mean-field theories in conjunction with a quasi-atomic approximation for the
strongly localized 4$f$ shell is applied to Nd$_{2}$Fe$_{14}$B-based hard
magnets in order to evaluate crystal-field and exchange-field parameters at
rare-earth sites and their corresponding single-ion contribution to the
magnetic anisotropy. In pure Nd$_2$Fe$_{14}$B, our calculations reproduce the
easy-cone to easy axis transition; theoretical magnetization curves agree
quantitatively with experiment. Our study reveals that the rare-earth
single-ion anisotropy in the ""2-14-1"" structure is strongly site-dependent,
with the $g$ rare-earth site exhibiting a larger value. In particular, we
predict that increased $f$ and $g$-site occupancy of $R=$ Ce and Dy,
respectively, leads to an increase of the magnetic anisotropy of the
corresponding (Nd,$R$)$_{2}$Fe$_{14}$B substituted compounds.",2206.15093v1
2022-07-07,Exciton-assisted low-energy magnetic excitations in a photoexcited Mott insulator on a square lattice,"The photoexcitation of a Mott insulator on a square lattice weakens the
intensity of both single- and two-magnon excitations as observed in
time-resolved resonant-inelastic X-ray scattering and time-resolved Raman
scattering, respectively. However, the spectral changes in the low-energy
regions below the magnons have not yet been clearly understood. To uncover the
nature of the photoinduced low-energy magnetic excitations of the Mott
insulator, we numerically investigate the transient magnetic dynamics in a
photoexcited half-filled Hubbard model on a square lattice. After turning off a
pump pulse tuned for an absorption edge, new magnetic signals clearly emerge
well below the magnon energy in both single- and two-magnon excitations. We
find that the low-energy excitations are predominantly created via excitonic
states at the absorption edge. These exciton-assisted magnetic excitations may
provide a possible explanation for the low-energy spectral weight in a recent
time-resolved two-magnon Raman scattering experiment for insulating
YBa$_2$Cu$_3$O$_{6.1}$.",2207.03141v2
2022-07-14,Spin relaxation dynamics of radical-pair processes at low magnetic fields,"We report measurements of room-temperature spin-relaxation times $T_1$ and
$T_2$ of charge-carrier spins in a $\pi$-conjugated polymer thin film under
bipolar injection and low ($1\mbox{ mT}\lesssim B_0\lesssim 10\mbox{ mT}$)
static magnetic fields, using electrically detected magnetic resonant Hahn-echo
and inversion-recovery pulse sequences. The experiments confirm the correlation
between the magnetic-field sensitive observables of radical-pair processes,
which include both the spin-dependent recombination currents in organic
semiconductors and the associated spin-relaxation times when random local
hyperfine fields and external magnetic fields compete in magnitude. Whereas a
striking field dependence of spin-lattice relaxation exists in the low-field
regime, the apparent spin decoherence time remains field independent as the
distinction between the two is lifted at low fields.",2207.07086v1
2022-08-10,Spin and valley effects on the quantum phase transition in two dimensions,"Using several independent methods, we find that the metal-insulator
transition occurs in the strongly-interacting two-valley two-dimensional
electron system in ultra-high mobility SiGe/Si/SiGe quantum wells in zero
magnetic field. The transition survives in this system in parallel magnetic
fields strong enough to completely polarize the electrons' spins, thus making
the electron system ""spinless"". In both cases, the resistivity on the metallic
side near the transition increases with decreasing temperature, reaches a
maximum at a temperature $T_{\text{max}}$, and then decreases. The decrease
reaches more than an order of magnitude in zero magnetic field. The value of
$T_{\text{max}}$ in zero magnetic field is found to be close to the
renormalized Fermi temperature. However, rather than increasing along with the
Fermi temperature, the value $T_{\text{max}}$ decreases appreciably for
spinless electrons in spin-polarizing magnetic fields. The observed behavior of
$T_{\text{max}}$ cannot be described by existing theories. The results indicate
the spin-related origin of the effect. At the same time, the low-temperature
resistivity drop in both spin-unpolarized and spinless electron systems is
described quantitatively by the dynamical mean-field theory.",2208.05356v1
2022-10-17,Ultrafast laser-driven topological spin textures on a 2D magnet,"Ultrafast laser excitations provide an efficient and low-power consumption
alternative since different magnetic properties and topological spin states can
be triggered and manipulated at the femtosecond (fs) regime. However, it is
largely unknown whether laser excitations already used in data information
platforms can manipulate the magnetic properties of recently discovered
two-dimensional (2D) van der Waals (vdW) materials. Here we show that
ultrashort laser pulses (30$-$85 fs) can not only manipulate magnetic domains
of 2D-XY CrCl$_3$ ferromagnets, but also induce the formation and control of
topological nontrivial meron and antimeron spin textures. We observed that
these spin quasiparticles are created within $\sim$100 ps after the excitation
displaying rich dynamics through motion, collision and annihilation with
emission of spin waves throughout the surface. Our findings highlight
substantial opportunities of using photonic driving forces for the exploration
of spin textures on 2D magnetic materials towards magneto-optical topological
applications.",2210.09210v1
2022-10-19,Generalised form of the magnetic anisotropy field in micromagnetic and atomistic spin models,"We present a general approach to the derivation of the effective anisotropy
field which determines the dynamical behaviour of magnetic spins according to
the Landau-Lifshitz-Gilbert equation. The approach is based on the gradient in
spherical polar coordinates with the final results being expressed in Cartesian
coordinates as usually applied in atomistic and micromagnetic model
calculations. The approach is generally valid for all orders of anisotropies
including higher order combinations of azimuthal and rotational anisotropies
often found in functional magnetic materials such as permanent magnets and an
emerging class of antiferromagnetic materials with applications in spintronics.
Anisotropies are represented in terms of spherical harmonics which have the
important property of rational temperature scaling. Effective field vectors are
given for anisotropies up to sixth order, presenting a unified framework for
implementing higher order magnetic anisotropies in numerical simulations.",2210.10916v4
2022-10-28,Stripe order and spin dynamics in triangular-lattice antiferromagnet KErSe$_{2}$: A single-crystal study with a theoretical description,"The rare-earth triangular-lattice chalcogenide is a great platform for
exploring both spin liquids and novel magnetic orders with anisotropic spin
interactions and magnetic frustrations. Here, we report the thermodynamic and
neutron scattering measurements of rare-earth triangular-lattice chalcogenide
KErSe$_{2}$, using single-crystal samples. Our experiments revealed a
long-range stripe order below 0.2 K. Although the magnetic order was
three-dimensional, magnetic excitations exhibited negligible modulation along
the z direction, indicating very weak interlayer coupling. Furthermore,
magnetic excitation developed a well-defined spin-wave dispersion with a gap of
$\sim$0.03 meV at M points. Both the stripe order and spin-wave excitations
could be quantitatively understood from the anisotropic spin interactions of
the Er$^{3+}$ Kramers doublets.",2210.15867v1
2022-10-28,Self-similar collapse in a circular magnetic field and electron beam jets by hybrid transverse plasmon,"Based on the set of nonlinear coupling equations describing the interaction
of the high-frequency field, the self-generated magnetic field and the
ion-acoustic field, the dispersion relation for the circular magnetic field is
obtained. The numerical results indicate that the strength of the magnetic
field have influence on the growth rate of modulation instability. The
self-generated magnetic field has the tendency to self-similar collapse which
makes the electron escapes along the axial region and form collimated jets. The
velocity of jets is calculated and the results are consistent with experimental
observations. The research may be applied to understand the dynamic process of
electron beam jets in laboratory and space plasma.",2210.15899v1
2022-11-04,Picosecond x-ray magnetic circular dichroism spectroscopy at the Fe L-edges with a laser-driven plasma source,"Time-resolved x-ray magnetic circular dichroism (XMCD) enables a unique
spectroscopic view on complex spin and charge dynamics in multi-elemental
magnetic materials. So far, its application in the soft-x-ray range has been
limited to synchrotron-radiation sources and free-electron lasers. By combining
a laser-driven plasma source with a magnetic thin-film polarizer, we generate
circularly polarized photons in the soft x-ray regime, enabling the first XMCD
spectroscopy at the Fe L edges in a laser laboratory. Our approach can be
readily adapted to other transition metal L and rare earth M absorption edges
and with a temporal resolution of < 10 ps, a wide range of ultrafast
magnetization studies can be realized.",2211.02777v1
2022-11-15,Intrinsic and tunable quantum anomalous Hall effect and magnetic topological phases in XYBi2Te5,"By first-principles calculations, we study the magnetic and topological
properties of XYBi2Te5-family (X, Y = Mn, Ni, V, Eu) compounds. The strongly
coupled double magnetic atom-layers can significantly enhance the magnetic
ordering temperature while keeping the topologically nontrivial properties.
Particularly, NiVBi2Te5 is found to be a magnetic Weyl semimetal in bulk and a
Chern insulator in thin film with both the Curie temperature (~150 K) and full
gap well above 77 K. Ni2Bi2Te5, MnNiBi2Te5, NiVBi2Te5 and NiEuBi2Te5 exhibits
intrinsic dynamic axion state. Among them, MnNiBi2Te5 has a Neel temperature
over 200 K and Ni2Bi2Te5 even demonstrates antiferromagnetic order above room
temperature. These results indicate an approach to realize high temperature
quantum anomalous Hall effect and other topological quantum effects for
practical applications.",2211.08163v2
2022-11-15,Far-from-equilibrium universality in two-dimensional Heisenberg antiferromagnets,"We study the far-from-equilibrium dynamics of isolated two-dimensional
Heisenberg antiferromagnets. We consider spin spiral initial conditions which
imprint a position-dependent staggered-magnetization (or Neel order) in the
two-dimensional lattice. Remarkably, we find a long-lived prethermal regime
characterized by self-similar behavior of staggered magnetization fluctuations,
although the system has no long-range order at finite energy and the staggered
magnetization does not couple with conserved charges. Exploiting the separation
of length scales introduced by the initial condition, we derive a simplified
analytical model that allow us to compute the spatial-temporal scaling
exponents and power-law distribution of the staggered magnetization
fluctuations analytically, and find excellent agreement with numerical
simulations using phase space methods. The scaling exponents are insensitive to
details of the initial condition, in particular, no fine-tuning of energy is
required to trigger the self-similar scaling regime. Compared with recent
results on far-from-equilibrium universality on the Heisenberg ferromagnet, we
find quantitatively distinct spatial-temporal scaling exponents, therefore
suggesting that the same model with ferromagnetic and antiferromagnetic initial
conditions can host different universal regimes. Our predictions are relevant
to ultra-cold atoms simulators of Heisenberg magnets and driven
antiferromagnetic insulators.",2211.08392v1
2022-11-23,Pressure-Strain Interaction: III. Particle-in-Cell Simulations of Magnetic Reconnection,"How energy is converted into thermal energy in weakly collisional and
collisionless plasma processes such as magnetic reconnection and plasma
turbulence has recently been the subject of intense scrutiny. The
pressure-strain interaction has emerged as an important piece, as it describes
the rate of conversion between bulk flow and thermal energy density. In two
companion studies, we presented an alternate decomposition of the
pressure-strain interaction to isolate the effects of converging/diverging flow
and flow shear instead of compressible and incompressible flow, and we derived
the pressure-strain interaction in magnetic field-aligned coordinates. Here, we
use these results to study pressure-strain interaction during two-dimensional
anti-parallel magnetic reconnection. We perform particle-in-cell simulations
and plot the decompositions in both Cartesian and magnetic field-aligned
coordinates. We identify the mechanisms contributing to positive and negative
pressure-strain interaction during reconnection. This study provides a roadmap
for interpreting numerical and observational data of the pressure-strain
interaction, which should be important for studies of reconnection, turbulence,
and collisionless shocks.",2211.13159v1
2022-11-26,Spontaneous superconducting diode effect in non-magnetic Nb/Ru/Sr$_2$RuO$_4$ topological junctions,"Non-reciprocal electronic transport in a material occurs if both time
reversal and inversion symmetries are broken. The superconducting diode effect
(SDE) is an exotic manifestation of this type of behavior where the critical
current for positive and negative currents are mismatched, as recently observed
in some non-centrosymmetric superconductors with a magnetic field. Here, we
demonstrate a SDE in non-magnetic Nb/Ru/Sr$_2$RuO$_4$ Josephson junctions
without applying an external magnetic field. The cooling history dependence of
the SDE suggests that time-reversal symmetry is intrinsically broken by the
superconducting phase of Sr$_2$RuO$_4$. Applied magnetic fields modify the SDE
dynamically by randomly changing the sign of the non-reciprocity. We propose a
model for such a topological junction with a conventional superconductor
surrounded by a chiral superconductor with broken time reversal symmetry.",2211.14626v2
2023-01-18,Exploring the magnetic dipole moments of $T_{QQ \bar q \bar s}$ and $T_{QQ \bar s \bar s}$ states in the framework of QCD light-cone sum rules,"Motivated by the recent observation of the tetraquark $ T_{cc}^{+}$, we
investigate the magnetic dipole moments of the possible single and double
strange partners, $T_{QQ \bar q \bar s}$ and $T_{QQ \bar s \bar s}$, with the
spin-parity $ J^{P} = 1^{+}$ by means of the QCD light-cone sum rules. To this
end, we model these states as diquark-antidiquark states with different
organizations and interpolating currents. The results of magnetic dipole
moments obtained using different diquark-antidiquark structures differ from
each other, considerably. The magnetic dipole moment is the leading-order
response of a bound system to a soft external magnetic field. Therefore, it
provides an excellent platform for investigation of the inner structures of
hadrons governed by the quark-gluon dynamics of QCD.",2301.07713v2
2023-02-05,On semiconductor--metal transition in FeSi induced by ultrahigh magnetic field,"At low temperatures, iron monosilicide is a strongly correlated narrow-gap
semiconductor. A first order transition to metal state induced by magnetic
field was observed for the first time at 355 T in Ref. [Yu. B. Kudasov et al.,
JETP Lett. 68 (1998) 350]. However, recently a smooth transition from 230 T to
270 T was found under similar conditions in Ref. [D. Nakamura et al., Phys.
Rev. Lett. 127 (2021) 156601]. This discrepancy goes far beyond experimental
errors and deserves a careful study. A methodological analysis of inductive and
RF techniques of conductivity measurements shows that the difference of these
critical magnetic field estimations stems from a divergence in dynamic ranges
of the techniques. In fact, the above mentioned methods supplement each other.
The semiconductor-metal transition under magnetic field in FeSi is a complex
phenomenon which occurs at the wide range of magnetic fields.",2302.02448v1
2022-12-24,Particle-level Simulation of Magnetorheological Fluids: A Fully-Resolved Solver,"Magnetorheological fluids (MRFs) are smart materials consisting of
micro-scale magnetizable particles suspended in a carrier fluid. The
rheological properties of a MRF can be changed from a fluid-state to a
solid-state upon the application of an external magnetic field. This study
reports the development of a particle-level simulation code for magnetic solid
spheres moving through an incompressible Newtonian carrier fluid. The numerical
algorithm is implemented within an open-source finite-volume solver coupled
with an immersed boundary method (FVM-IBM) to perform fully-resolved
simulations. The particulate phase of the MRF is modeled using the discrete
element method (DEM). The resultant force acting on the particles due to the
external magnetic field is computed based on the Clausius-Mossotti
relationship. The fixed and mutual dipole magnetic models are then used to
account for the magnetic (MAG) interactions between particles. Several
benchmark flows were simulated using the newly-developed FVM-IBM-DEM-MAG
algorithm to assess the accuracy and robustness of the calculations.",2302.03755v1
2023-02-15,The effect of weak magnetic photon emission from quark-gluon plasma,"We propose a novel effect that accounts for the photon emission from a
quark-gluon plasma in the presence of a weak external magnetic field. Although
the weak magnetic photon emission from quark-gluon plasma only leads to a small
correction to the photon production rate, the induced photon spectrum can be
highly azimuthally anisotropic, as a consequence of the coupled effect of the
magnetic field and the longitudinal dynamics in the background medium. With
respect to a realistic medium evolution containing a tilted fireball
configuration, the direct photon elliptic flow from experiments is reproduced.
In comparison to the experimental data of direct photon elliptic flow, in
heavy-ion collisions the magnitude of the magnetic field before 1 fm/c can be
extracted. For the top energy of RHIC collisions, right after the
pre-equilibrium evolution, $|eB|$ is found no larger than a few percent of the
pion mass square.",2302.07696v3
2023-04-17,The impact of local pinning sites in magnetic tunnel junctions with non-homogeneous free layers,"Pinning at local defects is a significant road block for the successful
implementation of technological paradigms which rely on the dynamic properties
of non-trivial magnetic textures. In this report a comprehensive study of the
influence of local pinning sites for non-homogeneous magnetic layers integrated
as the free layer of a magnetic tunnel junction is presented, both
experimentally and with corresponding micromagnetic simulations. The pinning
sites are found to be extremely detrimental to the frequency controllability of
the devices, a key requirement for their use as synapses in a frequency
multiplexed artificial neural networks. In addition to describing the impact of
the local pinning sites in the more conventional NiFe, a vortex-based magnetic
tunnel junction with an amorphous free layer is presented which shows
significantly improved frequency selectivity, marking a clear direction for the
design of future low power devices.",2304.08228v1
2023-04-18,Efficient characteristics of exchange coupling and spin-flop transition in Py/Gd bilayer using anisotropic magnetoresistance,"The interlayer antiferromagnetic coupling rare-earth/transition-metal bilayer
ferrimagnet systems have attracted much attention because they present
variously unusual temperature-and field-dependent nontrivial magnetic states
and dynamics. These properties and the implementation of their applications in
spintronics highly depend on the significant temperature dependence of the
magnetic exchange stiffness constant A. Here, we quantitatively determine the
temperature dependence of magnetic exchange stiffness A_{Py-Gd} and A_{Gd} in
the artificially layered ferrimagnet consisting of a Py/Gd bilayer, using a
measurement of anisotropic magnetoresistance (AMR) of the bilayer thin film at
different temperatures and magnetic fields. The obtained temperature dependence
of A_{Py-Gd} and A_{Gd} exhibit a scaling power law with the magnetization of
Gd. The critical field of spin-flop transition and its temperature dependence
can also be directly obtained by this method. Additionally, the experimental
results are well reproduced by micromagnetic simulations with the obtained
parameters A_{Py-Gd} and A_{Gd}, which further confirms the reliability of this
easily accessible technique.",2304.08858v1
2023-04-18,Electron correlation effects in paramagnetic cobalt,"We study the influence of Coulomb correlations on spectral and magnetic
properties of fcc cobalt using a combination of density functional theory and
dynamical mean-field theory. The computed uniform and local magnetic
susceptibilities obey the Curie-Weiss law, which, as we demonstrate, occurs due
to the partial formation of local magnetic moments. We find that the lifetime
of these moments in cobalt is significantly less than in bcc iron, suggesting a
more itinerant magnetism in cobalt. In contrast to the bcc iron, the obtained
electronic self-energies exhibit a quasiparticle shape with the quasiparticle
mass enhancement factor ${m^*/m}\sim$1.8, corresponding to moderately
correlated metal. Finally, our calculations reveal that the static magnetic
susceptibility of cobalt is dominated by ferromagnetic correlations, as
evidenced by its momentum dependence.",2304.08980v1
2023-05-03,Statistics of Pressure Fluctuations in Turbulent Kinetic Plasmas,"In this study we explore the statistics of pressure fluctuations in kinetic
collisionless turbulence. A 2.5D kinetic particle-in-cell (PIC) simulation of
decaying turbulence is used to investigate pressure balance via the evolution
of thermal and magnetic pressure in a plasma with beta of order unity. We also
discuss the behavior of thermal, magnetic and total pressure structure
functions and their corresponding wavenumber spectra. The total pressure
spectrum exhibits a slope of -7/3 extending for about a decade in the
ion-inertial range. In contrast, shallower -5/3 spectra are characteristic of
the magnetic pressure and thermal pressure. The steeper total pressure spectrum
is a consequence of cancellation caused by density-magnetic field magnitude
anticorrelation. Further, we evaluate higher order total pressure structure
functions in an effort to discuss intermittency and compare the power exponents
with higher order structure functions of velocity and magnetic fluctuations.
Finally, applications to astrophysical systems are also discussed.",2305.02068v1
2023-05-08,Pickup Ion-Mediated Magnetic Reconnection in the Outer Heliosphere,"Pickup ions (PUIs) play a crucial role in the heliosphere, contributing to
the mediation of large-scale structures such as the distant solar wind, the
heliospheric termination shock (HTS), and the heliopause. While magnetic
reconnection is thought to be a common process in the heliosphere due to the
presence of heliospheric current sheets, it is poorly understood how PUIs might
affect the evolution of magnetic reconnection. Although it is reasonable to
suppose that PUIs decrease the reconnection rate since the plasma beta becomes
much larger than $1$ when PUIs are included, we show for the first time that
such a supposition is invalid and that PUI-induced turbulence, heat conduction,
and viscosity can preferentially boost magnetic reconnection in heliospheric
current sheets in the distant solar wind. This suggests that it is critical to
include the effect of the turbulence, heat conduction, and viscosity caused by
PUIs to understand the dynamics of magnetic reconnection in the outer
heliosphere.",2305.04910v1
2023-05-23,"Combined funnel, concentrator, and particle valve functional element for magnetophoretic bead transport based on engineered magnetic domain patterns","Controlled actuation of superparamagnetic beads (SPBs) within a microfluidic
environment using tailored dynamic magnetic field landscapes (MFLs) is a potent
approach for the realization of point-of-care diagnostics within Lab-on-a-chip
(LOC) systems. Making use of an engineered magnetic domain pattern as the MFL
source, a functional LOC-element with combined magnetophoretic funnel,
concentrator, and valve functions for micron-sized SPBs is presented. A
parallel-stripe domain pattern design with periodically increasing/decreasing
stripe lengths has been fabricated in a topographically flat continuous
exchange biased (EB) thin film system by ion bombardment induced magnetic
patterning (IBMP). It is demonstrated that, upon application of external
magnetic field pulses, a fully reversible concentration of SPBs at the domain
pattern focal point occurs. In addition, it is shown that this functionality
may be used as an SPB funnel, allowing only a maximum number of particles to
pass through the focal point. Adjusting the pulse time length, the focal point
can be clogged up for incoming SPBs, resembling an on-and-off switchable
particle valve. The observations are supported by quantitative theoretical
force considerations.",2305.14278v1
2023-06-05,Dynamic Calculations of Magnetic Field and Implications on Spin Polarization and Spin Alignment in Heavy Ion Collisions,"Magnetic field plays a crucial role in various novel phenomena in heavy-ion
collisions. We solve the Maxwell equations numerically in a medium with
time-dependent electric conductivity by using the Finite-Difference Time-Domain
(FDTD) algorithm. We investigate the time evolution of magnetic fields in two
scenarios with different electric conductivities at collision energies ranging
from $\sqrt{s_\text{NN}}=$ 7.7 to 200 GeV. Our results suggest that the
magnetic field may not persist long enough to induce a significant splitting
between the global spin polarizations of $\Lambda$ and $\bar{\Lambda}$ at
freeze-out stage. However, our results do not rule out the possibility of the
magnetic field influencing the spin (anti-)alignment of vector mesons.",2306.02829v1
2023-06-06,Magnetic reconnection-driven turbulence and turbulent reconnection acceleration,"This paper employs an MHD-PIC method to perform numerical simulations of
magnetic reconnection-driven turbulence and turbulent reconnection acceleration
of particles. Focusing on the dynamics of the magnetic reconnection, the
properties of self-driven turbulence, and the behavior of particle
acceleration, we find that: (1) when reaching a statistically steady state of
the self-driven turbulence, the magnetic energy is almost released by 50\%,
while the kinetic energy of the fluid increases by no more than 15\%. (2) the
properties of reconnection-driven turbulence are more complex than the
traditional turbulence driven by an external force. (3) the strong magnetic
field tends to enhance the turbulent reconnection efficiency to accelerate
particles more efficiently, resulting in a hard spectral energy distribution.
Our study provides a particular perspective on understanding turbulence
properties and turbulent reconnection-accelerated particles.",2306.03418v1
2023-06-09,Generation of sub-ion scale magnetic holes from electron shear flow instabilities in plasma turbulence,"Magnetic holes (MHs) are coherent structures associated with strong magnetic
field depressions in magnetized plasmas. They are observed in many
astrophysical environments at a wide range of scales but their origin is still
under debate. In this work we investigate the formation of sub-ion scale MHs
using a fully kinetic 2D simulation of plasma turbulence initialized with
parameters typical of the Earth's magnetosheath. Our analysis shows that the
turbulence is capable of generating sub-ion scale MHs from large scale
fluctuations via the following mechanism: first, the nonlinear large scale
dynamics spontaneously leads to the development of thin and elongated electron
velocity shears; these structures then become unstable to the electron
Kelvin-Helmholtz instability and break up into small scale electron vortices;
the electric current carried by these vortices locally reduces the magnetic
field, inducing the formation of sub-ion scale MHs. The MHs thus produced
exhibit features consistent with satellite observations and with previous
numerical studies. We finally discuss the kinetic properties of the observed
sub-ion scale MHs, showing that they are characterized by complex
non-Maxwellian electron velocity distributions exhibiting anisotropic and
agyrotropic features.",2306.05874v1
2023-07-02,Ferromagnetic filament shapes in a rotating field reveal their magnetoelastic properties,"Flexible ferromagnetic filaments can be used to control the flow on the
micro-scale with external magnetic field. To accurately model them, it is
crucial to know their parameters such as their magnetization and bending
modulus, the latter of which is hard to determine precisely. We present a
method how the ferromagnetic filament's shape in a rotating field can be used
to determine the magnetoelastic number $Cm$ - the ratio of magnetic to elastic
forces. Then once the magnetization of the filament is known, it is possible to
determine its bending modulus. The main idea of the method is that $Cm$ is the
only parameter that determines whether the filament is straight or whether its
tips are bent towards the magnetic field direction. Comparing with numerical
solutions, we show that the method results in an error of $15...20\%$ for the
determined $Cm$, what is more precise than estimations from other methods. This
method will allow to improve the comparability between theoretical filament
models and experimental measurements.",2307.00685v1
2023-07-07,Weak-Coupling Theory of Neutron Scattering as a Probe of Altermagnetism,"Inelastic neutron scattering provides a powerful probe of the magnetic
excitations of quantum magnets. Altermagnets have recently emerged as a new
class of magnets with vanishing net magnetization characteristic of
antiferromagnets and with a spin-split electronic structure typical of
ferromagnets. Here we introduce a minimal Hubbard model with two-sublattice
orthorhombic anisotropy as a framework to study altermagnetism. Using
unrestricted Hartree-Fock calculations, we find an altermagnetic state for this
model that evolves from a metallic state to an insulating state with increasing
Hubbard-$U$ Coulomb repulsion. We then examine the inelastic neutron scattering
response in these states using random-phase approximation calculations of the
dynamic spin susceptibility $\chi''({\bf q}, \omega)$. We find that the
magnetic excitation spectrum depends on its chirality for ${\bf q}$ along
certain directions in reciprocal space, an observation that may be used in
inelastic neutron scattering experiments as a probe of altermagnetism.",2307.03793v2
2023-07-10,Graph Representation of the Magnetic Field Topology in High-Fidelity Plasma Simulations for Machine Learning Applications,"Topological analysis of the magnetic field in simulated plasmas allows the
study of various physical phenomena in a wide range of settings. One such
application is magnetic reconnection, a phenomenon related to the dynamics of
the magnetic field topology, which is difficult to detect and characterize in
three dimensions. We propose a scalable pipeline for topological data analysis
and spatiotemporal graph representation of three-dimensional magnetic vector
fields. We demonstrate our methods on simulations of the Earth's magnetosphere
produced by Vlasiator, a supercomputer-scale Vlasov theory-based simulation for
near-Earth space. The purpose of this work is to challenge the machine learning
community to explore graph-based machine learning approaches to address a
largely open scientific problem with wide-ranging potential impact.",2307.09469v2
2023-07-23,Simulations of Weakly Magnetized Turbulent Mixing Layers,"Radiative turbulent mixing layers are expected to form pervasively at the
phase boundaries in multiphase astrophysical systems. This inherently small
scale structure is dynamically crucial because it directly regulates the mass,
momentum and energy exchanges between adjacent phases. Previous studies on
hydrodynamic turbulent mixing layers have revealed the interactions between
cold and hot phases in the context of the circumgalactic medium, offering
important insight into the fate of cold clouds traveling through hot galactic
winds. However, the role of magnetic field has only been sparsely investigated.
We perform a series of 3D magnetohydrodynamics (MHD) simulations of such mixing
layers in the presence of weak to modest background magnetic field. We find
that due to field amplification, even relatively weak background magnetic
fields can significantly reduce the surface brightness and inflow velocity of
the hot gas in the mixing layer. This reduction is attributed to a combination
of magnetic pressure support and direct suppression of turbulent mixing, both
of which alter the phase structures. Our results are largely independent of
thermal conduction and converged with resolution, offering insights on the
survival of cold gas in multiphase systems.",2307.12355v2
2023-08-21,Shape deformation of magnetically levitated fluid droplets,"Diamagnetic levitation can provide a completely passive method to support
materials against the pull of gravity, and researchers have levitated both
solids and fluids. Such levitation can be assisted by increasing the magnetic
susceptibility contrast by using a surrounding paramagnetic medium and through
buoyancy forces, known as magneto-Archimedean levitation. The
magneto-Archimedean levitation of solids has proved useful in chemistry and
biology. However, the levitation of fluid droplets has an additional interest
because the fluid droplet's shape can deform. We perform experiments and
simulations to gauge the squashing or eccentricity of the static magnetically
levitated fluid droplet. By carefully characterizing all the parameters
affecting the droplet's levitation, using image analysis to estimate the
droplet's eccentricity, and using finite element adaptive simulations to find
the lowest energy droplet shape, we find good agreement between the simulations
and experimental results. As a potential application, we show that the
droplet's eccentricity can be used to perform magnetic gradiometry with a
potential resolution of $S\sim 8\,{\rm nT/cm}$, over a volume of 10 mm$^3$,
which is competitive with other room-temperature magnetic gradiometer
techniques.",2308.10503v1
2023-08-27,Magnetostatic modes and criticality in uniaxial magnetic materials,"We analyze modes in a dipole-dipole coupled quantum magnetic material, taking
into consideration domain structure and other shape effects present in any real
magnet. We find that the soft mode governing quantum criticality in a
non-ellipsoidal sample is an inhomogeneous magnetostatic mode which negates
dynamic, demagnetization-field effects. The demagnetization field is analyzed
from a microscopic perspective. Furthermore, we find a magnetostatic mode
originating from variations in the magnetization of the sample, caused by
domain structure and quantum or thermal fluctuations, will be lower in energy
than the soft mode governing quantum criticality in the bulk of the magnet.
Experimental evidence for these theoretical results is provided by analysis of
electronuclear modes in LiHoF$_4$, an archetypal dipolar quantum Ising
material, in a microwave resonator.",2308.14169v1
2023-09-06,Scissors Modes of a Bose-Einstein Condensate in a Synthetic Magnetic Field,"We study the scissors modes of a harmonically trapped Bose-Einstein
condensate under the influence of a synthetic magnetic field, which induces
rigid rotational components in the velocity field. Our investigation reveals
that the scissors mode, excited in the plane perpendicular to the synthetic
magnetic field, becomes coupled to the quadrupole modes of the condensate,
giving rise to typical beating effects. Moreover, the two scissors modes
excited in the vertical planes are also coupled together by the synthetic
magnetic field, resulting in intriguing gyroscope dynamics. Our analytical
results, derived from a spinor hydrodynamic theory, are further validated
through numerical simulations of the three-dimensional Gross-Pitaevskii
equation. These predictions for the condensates subject to a synthetic magnetic
field are experimentally accessible with current cold-atom setups and hold
promise for potential applications in quantum sensing.",2309.02627v1
2023-09-12,Reconstruction Formulae for 3D Field-Free Line Magnetic Particle Imaging,"Magnetic Particle Imaging (MPI) is a promising noninvasive in vivo imaging
modality that makes it possible to map the spatial distribution of
superparamagnetic nanoparticles by exposing them to dynamic magnetic fields. In
the Field-Free Line (FFL) scanner topology, the spatial encoding of the
particle distribution is performed by applying magnetic fields vanishing on
straight lines. The voltage induced in the receiving coils by the particles
when exposed to the magnetic fields constitute the signal from which the
particle distribution is to be reconstructed. To avoid lengthy calibration,
model-based reconstruction formulae have been developed for the 2D FFL scanning
topology. In this work we develop reconstruction formulae for 3D FFL. Moreover,
we provide a model-based reconstruction algorithm for 3D FFL and we validate it
with a numerical experiment.",2309.06254v1
2023-09-13,Looking at the magnetic properties of half metal from the paramagnetic phase: DFT+DMFT study of exchange interactions in CrO$_2$,"We study magnetic properties of CrO$_2$ within the density functional theory
plus dynamical mean-field theory (DFT+DMFT) approach in the paramagnetic phase.
We consider the 3-orbital (per Cr site) model, containing only $t_{2g}$ states,
the $5$-orbital model, including all $d$-states, as well as the model including
also the oxygen $p$-states. Using the recently proposed approach of calculation
of exchange interactions in paramagnetic phase, we extract exchange interaction
parameters and magnon dispersions for these models. While the magnon dispersion
in the 3-orbital model possesses negative branches in accordance with previous
studies in ferromagnetic phase, this drawback is removed in the $5$-orbital
model. The model containing oxygen states overestimates the exchange
interactions and spin stiffness. Therefore, the $5$-orbital model appears as
most adequate for describing magnetic properties of CrO$_2$, in accordance with
the recent experimental data of fixed valence of chromium in this compound. The
possibility of describing magnetic properties of this material starting from
paramagnetic phase points to the correspondence of magnetic properties in this
phase and ferromagnetic phase and possible important contributions of RKKY
exchange interactions in paramagnetic phase of CrO$_2$.",2309.06892v1
2023-09-14,Magnetic flutter effect on validated edge turbulence simulations,"Small magnetic fluctuations ($B_1/B_0 \sim 10^{-4}$) are intrinsically
present in a magnetic confinement plasma due to turbulent currents. While the
perpendicular transport of particles and heat is typically dominated by
fluctuations of the electric field, the parallel stream of plasma is affected
by fluttering magnetic field lines. In particular through electrons, this
indirectly impacts the turbulence dynamics. Even in low beta conditions, we
find that $E\times B$ turbulent transport can be reduced by more than a factor
2 when magnetic flutter is included in our validated edge turbulence
simulations of L-mode ASDEX Upgrade. The primary reason for this is the
stabilization of drift-Alfv\'en-waves, which reduces the phase shifts of
density and temperature fluctuations with respect to potential fluctuations.
This stabilization can be qualitatively explained by linear analytical theory,
and appreciably reinforced by the flutter nonlinearity. As a secondary effect,
the steeper temperature gradients and thus higher $\eta_i$ increase the impact
of the ion-temperature-gradient mode on overall turbulent transport. With
increasing beta, the stabilizing effect on $E\times B$ turbulence increases,
balancing the destabilization by induction, until direct electromagnetic
perpendicular transport is triggered. We conclude that including flutter is
crucial for predictive edge turbulence simulations.",2309.07763v1
2023-10-16,Magnetic response of a two-dimensional viscous electron fluid,"It has been established that the Coulomb interactions can transform the
electron gas into a viscous fluid. This fluid is realized in a number of
platforms, including graphene and two-dimensional semiconductor
heterostructures. The defining characteristic of the electron fluid is the
formation of layers of charge carriers that are in local thermodynamic
equilibrium, as in classical fluids. In the presence of nonuniformities,
whirlpools and nontrivial flow profiles are formed, which have been directly
imaged in recent experiments. In this paper, we theoretically study the
response of the electron fluid to localized magnetic fields. We find that the
electric current is suppressed by viscous vortices in regions where magnetic
field is sharply varying, causing strong transport signatures. Experimentally,
our considerations are relevant since local magnetic fields can be applied to
the system through implanting adatoms or embedding micromagnets in the
top-gate. Our theory is essential for the characterization and future
applications of electron fluids in hydrodynamic spin transport.",2310.10032v1
2023-11-13,Chiral Symmetry Restoration and the Ultraquantum limit of Axionic Charge Density Waves in Weyl Semimetals,"A new mechanism for chiral symmetry restoration at extreme high magnetic
fields is proposed in the context of the Magnetic Catalysis scenario in Weyl
Semimetals. Contrary to previous proposals, here we show that, at very large
magnetic fields, the transverse velocity of the axion field, the phase mode of
the chiral condensate $\langle \bar{\Psi}\Psi\rangle$, becomes effectively
one-dimensional and its fluctuations destroy a possible nonzero value of this
fermionic condensate. We also show that, despite of the $U(1)$ chiral symmetry
not being broken at extremely large magnetic fields, the spectrum of the system
is comprised by a well defined gapless bosonic excitation, connected to the
axion mode, and a correlated insulating fermionic liquid that is neutral to
$U(1)$ chiral transformations. When the theory is supplemented with the
inclusion of dynamical electromagnetic fields, the chiral symmetry is broken
again, and the conventional scenario of magnetic catalysis can be recovered.",2311.07644v3
2023-11-17,Phononic dynamical axion in magnetic Dirac insulators,"In cosmology, the axion is a hypothetical particle that is currently
considered as candidate for dark matter. In condensed matter, a counterpart of
the axion (the ""axion quasiparticle"") has been predicted to emerge in
magnetoelectric insulators with fluctuating magnetic order and in
charge-ordered Weyl semimetals. To date, both the cosmological and
condensed-matter axions remain experimentally elusive or unconfirmed. Here, we
show theoretically that ordinary lattice vibrations can form an axion
quasiparticle in Dirac insulators with broken time- and space-inversion
symmetries, even in the absence of magnetic fluctuations. The physical
manifestation of the phononic axion is a magnetic-field-induced phonon
effective charge, which can be probed in optical spectroscopy. By replacing
magnetic fluctuations with lattice vibrations, our theory widens the scope for
the observability of the axion quasiparticle in condensed matter.",2311.10674v1
2023-11-27,Case study of the validity of truncation schemes of kinetic equations of motion: few magnetic impurities in a semiconductor quantum ring,"We carry out a study on the validity and limitations of truncation schemes
customarily employed to treat the quantum kinetic equations of motion of
complex interacting systems. Our system of choice is a semiconductor quantum
ring with one electron interacting with few magnetic impurities via a
Kondo-like Hamiltonian. This system is an interesting prototype which displays
the necessary complexity when suitably scaled (large number of magnetic
impurities) but can also be solved exactly when few impurities are present. The
complexity in this system comes from the indirect electron-mediated
impurity-impurity interaction and is reflected in the Heisenberg equations of
motion, which form an infinite hierarchy. For the cases of two and three
magnetic impurities, we solve for the quantum dynamics of our system both
exactly and following a truncation scheme developed for diluted magnetic
semiconductors in the bulk. We find an excellent agreement between the two
approaches when physical observables like the impurities' spin angular momentum
are computed for times that well exceed the time window of validity of
perturbation theory. On the other hand, we find that within time ranges of
physical interest, the truncation scheme introduces negative populations which
represents a serious methodological drawback.",2311.15872v1
2023-12-14,"Self-Diffusion and Structure of a Quasi Two-Dimensional, Classical Coulomb Gas Under Increasing Magnetic Field and Temperature","The influence of a magnetic field applied perpendicularly to the plane of a
quasi two dimensional, low density classical Coulomb gas, with interparticle
potential U of r as 1 over r, is studied using momentum conserving dissipative
particle dynamics simulations. The self diffusion and structure of the gas are
studied as functions of temperature and strength of the magnetic field. It is
found that the gas undergoes a topological phase transition when the
temperature is varied, in accord with the Bohr van Leeuwen BvL theorem, the
structural properties being unaffected, resembling those of the strictly two
dimensional Kosterlitz Thouless transition, with U of r as varying as ln r.
Consistent with the BvL theorem, the transition temperature and the melting
process of the condensed phase are unchanged by the field. Conversely, the self
diffusion coefficient of the gas is strongly reduced by the magnetic field. At
the largest values of the cyclotron frequency, the self diffusion coefficient
is inversely proportional to the applied magnetic field. The implications of
these results are discussed.",2312.09318v1
2024-01-03,"Quasi-two-dimensionality of three-dimensional, magnetically dominated, decaying turbulence","Decaying magnetohydrodynamic (MHD) turbulence is important in various
astrophysical contexts, including early universe magnetic fields, star
formation, turbulence in galaxy clusters, magnetospheres and solar corona.
Previously known in the nonhelical case of magnetically dominated decaying
turbulence, we show that magnetic reconnection is important also in the fully
helical case and is likely the agent responsible for the inverse transfer of
energy. Again, in the fully helical case, we find that there is a similarity in
power law decay exponents in both 2.5D and 3D simulations. To understand this
intriguing similarity, we investigate the possible quasi-two-dimensionalization
of the 3D system. We perform Minkowski functional analysis and find that the
characteristic length scales of a typical magnetic structure in the system are
widely different, suggesting the existence of local anisotropies. Finally, we
provide a quasi-two-dimensional hierarchical merger model which recovers the
relevant power law scalings. In the nonhelical case, we show that a
helicity-based invariant cannot constrain the system, and the best candidate is
still anastrophy or vector potential squared, which is consistent with the
quasi-two-dimensionalization of the system.",2401.01965v1
2024-01-08,So You Want to Image Myelin Using MRI: Magnetic Susceptibility Source Separation for Myelin Imaging,"In MRI, researchers have long endeavored to effectively visualize myelin
distribution in the brain, a pursuit with significant implications for both
scientific research and clinical applications. Over time, various methods such
as myelin water imaging, magnetization transfer imaging, and relaxometric
imaging have been developed, each carrying distinct advantages and limitations.
Recently, an innovative technique named as magnetic susceptibility source
separation has emerged, introducing a novel surrogate biomarker for myelin in
the form of a diamagnetic susceptibility map. This paper comprehensively
reviews this cutting-edge method, providing the fundamental concepts of
magnetic susceptibility, susceptibility imaging, and the validation of the
diamagnetic susceptibility map as a myelin biomarker that indirectly measure
myelin content. Additionally, the paper explores essential aspects of data
acquisition and processing, offering practical insights for readers. A
comparison with established myelin imaging methods is also presented, and both
current and prospective clinical and scientific applications are discussed to
provide a holistic understanding of the technique. This work aims to serve as a
foundational resource for newcomers entering this dynamic and rapidly expanding
field.",2401.03690v2
2024-01-10,Self-generated spin-orbit torque driven by anomalous Hall current,"Spin-orbit torques enable energy-efficient manipulation of magnetization by
electric current and hold promise for applications ranging from nonvolatile
memory to neuromorphic computing. Here we report the discovery of a giant
spin-orbit torque induced by anomalous Hall current in ferromagnetic
conductors. This anomalous Hall torque is self-generated as it acts on
magnetization of the ferromagnet that engenders the torque. The magnitude of
the anomalous Hall torque is sufficiently large to fully negate magnetic
damping of the ferromagnet, which allows us to implement a microwave spin
torque nano-oscillator driven by this torque. The peculiar angular symmetry of
the anomalous Hall torque favors its use over the conventional spin Hall torque
in coupled nano-oscillator arrays. The universal character of the anomalous
Hall torque makes it an integral part of the description of coupled spin
transport and magnetization dynamics in magnetic nanostructures.",2401.05006v1
2024-01-14,Robust Parahydrogen-Induced Polarization at High Concentrations,"Parahydrogen-Induced Polarization (PHIP) is a potent technique for generating
target molecules with high nuclear spin polarization. The PHIP process involves
a chemical reaction between parahydrogen and a target molecule, followed by the
transformation of nuclear singlet spin order into magnetization of a designated
nucleus through magnetic field manipulations. Although the
singlet-to-magnetization polarization transfer process works effectively at
moderate concentrations, it is observed to become much less efficient at high
molar polarization, defined as the product of polarization and concentration.
This strong dependence on the molar polarization is attributed to interference
from the field produced by the sample's magnetization during polarization
transfer, which leads to complex dynamics and can severely impact the
scalability of the technique. We address this challenge with a pulse sequence
that negates the influence of the distant dipolar field, while simultaneously
achieving singlet-to-magnetization polarization transfer to the desired target
spins, free from restrictions on the molar polarization.",2401.07243v1
2024-01-23,Vanishing vortex creep at the transition from ordered to disordered vortex phases in Ba$_{0.64}$K$_{0.36}$Fe$_2$As$_2$,"By measuring the dynamical and conventional magnetization relaxation of the
Ba$_{0.64}$K$_{0.36}$Fe$_2$As$_2$ single crystals, we found strong second peak
effect on the magnetization hysteresis loops. It is found that there is a kink
of magnetization at a field between the valley and maximum magnetization.
Interestingly, the magnetization relaxation rate has a deep minimum at the
field with the kink, indicating a diminished vortex creep. The relaxation rate
at this field is clearly smaller than the so-called universal lower limit of
the relaxation rate characterized by ($S_0\approx \ Gi^{1/2}(T/T_\mathrm{c}$).
This diminished vortex creep is associated with the origin of the SMP effect
and attributed to the strongly hindered flux motion when experiencing the
transition from the quasi-ordered to disordered vortex phases.",2401.12551v1
2024-01-31,Heavy flavor production under a strong magnetic field,"The magnetic field created in high energy nuclear collisions will affect the
dynamical processes in the QCD medium, especially the heavy quark production
that happens in the initial stage of the collisions. We calculate in a strong
magnetic field the heavy quark production cross section for the elementary
process gg {\rightarrow} Q{\bar Q} at leading order and the corresponding
transverse momentum distribution in nucleus-nucleus collisions. In comparison
to the QED process, the heavy quark production is dominated by the unique QCD
channel with gluon self-interaction. Due to the dimension reduction of quark
phase space in a strong magnetic field, the production is concentrated in a
very narrow energy region above the threshold. Since the translation invariance
is broken, the production becomes anisotropic in magnetic field.",2401.17559v1
2024-01-31,Magnetically tunable electrokinetic instability and structuring of non-equilibrium nanoparticle gradients,"Inspired by emergent behaviors of living matter, there is increasing interest
in developing approaches to create dynamic patterns and structures in synthetic
materials with controllable complexity to enable functionalities that are not
possible in thermodynamic equilibrium. Here we show that electrophoretically
driven and maintained non-equilibrium gradients of magnetic nanoparticles in
non-polar solvent can undergo electrokinetic instabilities (EKI), leading to
various electrically controllable spatiotemporally patterned states. These
electrokinetic instabilities and patterns can be tuned with a magnetic field
via magnetostatic energy reduction mechanism to both increase and decrease the
pattern complexity. We reflect the experimental observations on the theoretical
electrokinetic and magnetostatic arguments. We further show that small amounts
of polar water in the otherwise non-polar system are critical enablers for the
electrophoretic mobility of the nanoparticles. Since functionalities of
magnetic nanoparticles are widely tunable, we foresee that the combination of
dissipative electrokinetic driving and magnetic energy reduction can lead to
novel functional dissipative materials.",2401.17956v1
2024-04-01,Effect of magnetic field on the Bose-Einstein condensation of quantum well exciton-polaritons,"We theoretically examine the effect of the magnetic field on the relaxation
process of the exciton-polariton toward the Bose-Einstein condensation in GaAs
quantum wells. As a result of the modification of the exciton's effective mass,
Rabi splitting, and dispersion, the relaxation rate of polaritons toward
condensation has been significantly modified. Using a continuous pumping, the
dynamics of total and condensated polariton versus magnetic field has been
clarified. It has been shown that for low-energy pumping, the condensation is
significantly reduced with a magnetic field, and this arises from the reduction
of the scattering rate between energy levels when the steepness of the
dispersion at higher energy is reduced. Increasing of pump energy together with
the magnetic field could lead to a more effective relaxation and a much higher
number of condensated polariton is obtained.",2404.00970v1
1996-02-23,Rotation And Magnetic Evolution Of Superconducting Strange Stars,"Is pulsar make up of strange matter? The magnetic field decay of a pulsar may
be able to give us an answer. Since Cooper pairing of quarks occurs inside a
sufficiently cold strange star, the strange stellar core is superconducting. In
order to compensate the effect of rotation, different superconducting species
inside a rotating strange star try to set up different values of London fields.
Thus, we have a frustrated system. Using Ginzburg-Landau formalism, I solved
the problem of rotating a superconducting strange star: Instead of setting up a
global London field, vortex bundles carrying localized magnetic fields are
formed. Moreover, the number density of vortex bundles is directly proportional
to the angular speed of the star. Since it is energetically favorable for the
vortex bundles to pin to magnetic flux tubes, the rotational dynamics and
magnetic evolution of a strange star are coupled together, leading to the
magnetic flux expulsion as the star slows down. I investigate this effect
numerically and find that the characteristic field decay time is much less than
20~Myr in all reasonable parameter region. On the other hand, the
characteristic magnetic field decay time for pulsars is $\geq 20$~Myr. Thus, my
finding cast doubt on the hypothesis that pulsars are strange stars.",9602130v1
1997-09-10,Magnetic Field Effects on the Head Structure of Protostellar Jets,"We present the results of 3-D SPMHD numerical simulations of
supermagnetosonic, overdense, radiatively cooling jets. Two initial magnetic
configurations are considered: (i) a helical and (ii) a longitudinal field. We
find that magnetic fields have important effects on the dynamics and structure
of radiative cooling jets, especially at the head. The presence of a helical
field suppresses the formation of the clumpy structure which is found to
develop at the head of purely hydrodynamical jets. On the other hand, a cooling
jet embedded in a longitudinal magnetic field retains clumpy morphology at its
head. This fragmented structure resembles the knotty pattern commonly observed
in HH objects behind the bow shocks of HH jets. This suggests that a strong
(equipartition) helical magnetic field configuration is ruled out at the jet
head. Therefore, if strong magnetic fields are present, they are probably
predominantly longitudinal in those regions. In both magnetic configurations,
we find that the confining pressure of the cocoon is able to excite
short-wavelength MHD K-H pinch modes that drive low-amplitude internal shocks
along the beam. These shocks are not strong however, and it likely that they
could only play a secondary role in the formation of the bright knots observed
in HH jets.",9709092v1
1997-11-21,Effects of Cooling on the Propagation of Magnetized Jets,"We present multi-dimensional simulations of magnetized radiative jets
appropriate to Young Stellar Objects. Magnetized jets subject to collisionally
excited radiative losses have not, as yet, received extensive scrutiny. The
purpose of this letter is to articulate the propagation dynamics of radiative
MHD jets in the context of the extensive jet literature. Most importantly, we
look for morphological and kinematic diagnostics that may distinguish
hydrodynamic protostellar jets from their magnetically dominated cousins. Our
simulations are axisymmetric (2.5-D). A toroidal B_phi field geometry is used.
Our models have high sonic Mach numbers (Ms approx 10), but lower fast mode
Mach number (Mf approx 5). This is approximately the case for jets formed via
disk-wind or X-wind models - currently the consensus choice for launching and
collimating YSO jets. Time-dependent radiative losses are included via a
coronal cooling curve. Our results demonstrate that the morphology and
propagation characteristics of strongly magnetized radiative jets can differ
significantly from jets with weak fields. In particular the formation of
nose-cones via post-shock hoop stresses leads to narrow bow shocks and enhanced
bow shock speeds. In addition, the hoop stresses produce strong shocks in the
jet beam which constrasts with the relatively unperturbed beam in radiative
hydrodynamic jets. Our simulations show that pinch modes produced by magnetic
tension can strongly effect magnetized protostellar jets.",9711250v1
1997-12-11,On the difficulty of launching an outflow from an accretion disk,"We solve for the local vertical structure of a thin accretion disk threaded
by a poloidal magnetic field. The angular velocity deviates from the Keplerian
value as a result of the radial Lorentz force, but is constant on magnetic
surfaces. Angular momentum transport and energy dissipation in the disk are
parametrized by an alpha-prescription, and a Kramers opacity law is assumed to
hold. We also determine the stability of the equilibria with respect to the
magnetorotational (or Balbus-Hawley) instability. If the magnetic field is
sufficiently strong, stable equilibria can be found in which the angle of
inclination, i, of the magnetic field to the vertical at the surface of the
disk has any value in the range [0,90 degrees). By analyzing the dynamics of a
transonic outflow in the corona of the disk, we show that a certain potential
difference must be overcome even when i > 30 degrees. We determine this
potential difference as a function of i for increasing values of the vertical
magnetic field strength. For magnetorotationally stable equilibria, the
potential difference increases faster than the fourth power of the magnetic
field strength, quickly exceeding a value corresponding to the central
temperature of the disk, and is minimized with respect to i at i approximately
equal to 38 degrees. We show that this property is relatively insensitive to
the form of the opacity law. Our results suggest that an additional source of
energy, such as coronal heating, may be required for the launching of an
outflow from a magnetized disk.",9712150v1
2000-01-12,Regular magnetic fields in the dwarf irregular galaxy NGC 4449,"We present a high-resolution VLA study of the total power and polarized radio
continuum emission at 8.46 and 4.86 GHz of the irregular galaxy NGC 4449, known
for its weak rotation and non-systematic gas motions. We found strong
galaxy-scale regular magnetic fields, which is surprising because of a lack of
ordered rotation required for the dynamo action. The strength of the regular
field reaches 8 $\mu$G and that of the total field 14 $\mu$G, comparable to
that of the total magnetic field strength in radio-bright spirals. The magnetic
vectors in NGC 4449 form radial ``fans'' in the central region and fragments of
a spiral pattern in the galaxy's outskirts. These structures are associated
with large regions of systematic Faraday rotation, implying genuine
galaxy-scale magnetic fields rather than random ones compressed and stretched
by gas flows. The observed pattern of polarization B-vectors is similar to
dynamo-type fields in normal spirals. Nonstandard, fast dynamo concepts are
required to explain the observed field strengths, though it is unknown what
kind of magnetic field geometry can be produced in slowly and chaotically
rotating objects. The so far neglected role of magnetic fields for the dynamics
and star formation in dwarf irregulars also needs to be revised.",0001205v1
2001-05-24,"Transport of Magnetic Fields in Convective, Accreting Supernova Cores","We consider the amplification and transport of a magnetic field in the
collapsed core of a massive star, including both the region between the
neutrinosphere and the shock, and the central, opaque core. An analytical
argument explains why rapid convective overturns persist within a newly formed
neutron star for roughly 10 seconds ($> 10^3$ overturns), consistent with
recent numerical models. A dynamical balance between turbulent and magnetic
stresses within this convective layer corresponds to flux densities in excess
of $10^{15}$G. Material accreting onto the core is heated by neutrinos and also
becomes strongly convective. We compare the expected magnetic stresses in this
convective `gain layer' with those deep inside the neutron core.
  Buoyant motions of magnetized fluid are greatly aided by the intense neutrino
flux. We calculate the transport rate through a medium containing free neutrons
protons, and electrons, in the limiting cases of degenerate or non-degenerate
nucleons. Fields stronger than $\sim 10^{13}$ G are able to rise through the
outer degenerate layers of the neutron core during the last stages of
Kelvin-Helmholtz cooling (up to 10 seconds post-collapse), even though these
layers have become stable to convection. We also find the equilibrium shape of
a thin magnetic flux rope in the dense hydrostatic atmosphere of the neutron
star, along with the critical separation of the footpoints above which the rope
undergoes unlimited expansion against gravity. The implications of these
results for pulsar magnetism are summarized, and applied to the case of late
fallback over the first 1,000-10,000 s of the life of a neutron star",0105425v1
2002-01-25,"Nonaxisymmetric Evolution of Magnetically Subcritical Clouds: Bar Growth, Core Elongation, and Binary Formation","We have begun a systematic numerical study of the nonlinear growth of
nonaxisymmetric perturbations during the ambipolar diffusion-driven evolution
of initially magnetically subcritical molecular clouds, with an eye on the
formation of binaries, multiple stellar systems and small clusters. In this
initial study, we focus on the $m=2$ (or bar) mode, which is shown to be
unstable during the dynamic collapse phase of cloud evolution after the central
region has become magnetically supercritical. We find that, despite the
presence of a strong magnetic field, the bar can grow fast enough that for a
modest initial perturbation (at 5% level) a large aspect ratio is obtained
during the isothermal phase of cloud collapse. The highly elongated bar is
expected to fragment into small pieces during the subsequent adiabatic phase.
Our calculations suggest that the strong magnetic fields observed in some
star-forming clouds and envisioned in the standard picture of single star
formation do not necessarily suppress bar growth and fragmentation; on the
contrary, they may actually promote these processes, by allowing the clouds to
have more than one (thermal) Jeans mass to begin with without collapsing
promptly. Nonlinear growth of the bar mode in a direction perpendicular to the
magnetic field, coupled with flattening along field lines, leads to the
formation of supercritical cores that are triaxial in general. It removes a
longstanding objection to the standard scenario of isolated star formation
involving subcritical magnetic field and ambipolar diffusion based on the
likely prolate shape inferred for dense cores. Continuted growth of the bar
mode in already elongated starless cores, such as L1544, may lead to future
binary and multiple star formation.",0201419v1
2002-08-26,Magnetized Accretion-Ejection Structures: 2.5D MHD simulations of continuous Ideal Jet launching from resistive accretion disks,"We present numerical magnetohydrodynamic (MHD) simulations of a magnetized
accretion disk launching trans-Alfvenic jets. These simulations, performed in a
2.5 dimensional time-dependent polytropic resistive MHD framework, model a
resistive accretion disk threaded by an initial vertical magnetic field. The
resistivity is only important inside the disk, and is prescribed as eta =
alpha_m V_AH exp(-2Z^2/H^2), where V_A stands for Alfven speed, H is the disk
scale height and the coefficient alpha_m is smaller than unity. By performing
the simulations over several tens of dynamical disk timescales, we show that
the launching of a collimated outflow occurs self-consistently and the ejection
of matter is continuous and quasi-stationary. These are the first ever
simulations of resistive accretion disks launching non-transient ideal MHD
jets. Roughly 15% of accreted mass is persistently ejected. This outflow is
safely characterized as a jet since the flow becomes super-fastmagnetosonic,
well-collimated and reaches a quasi-stationary state. We present a complete
illustration and explanation of the `accretion-ejection' mechanism that leads
to jet formation from a magnetized accretion disk. In particular, the magnetic
torque inside the disk brakes the matter azimuthally and allows for accretion,
while it is responsible for an effective magneto-centrifugal acceleration in
the jet. As such, the magnetic field channels the disk angular momentum and
powers the jet acceleration and collimation. The jet originates from the inner
disk region where equipartition between thermal and magnetic forces is
achieved. A hollow, super-fastmagnetosonic shell of dense material is the
natural outcome of the inwards advection of a primordial field.",0208459v1
2005-02-07,"Quiescent Cores and the Efficiency of Turbulence-Accelerated, Magnetically Regulated Star Formation","The efficiency of star formation, defined as the ratio of the stellar to
total (gas and stellar) mass, is observed to vary from a few percent in regions
of dispersed star formation to about a third in cluster-forming cores. This
difference may reflect the relative importance of magnetic fields and
turbulence in controlling star formation. We investigate the interplay between
supersonic turbulence and magnetic fields using numerical simulations, in a
sheet-like geometry. We demonstrate that star formation with an efficiency of a
few percent can occur over several gravitational collapse times in moderately
magnetically subcritical clouds that are supersonically turbulent. The
turbulence accelerates star formation by reducing the time for dense core
formation. The dense cores produced are predominantly quiescent, with subsonic
internal motions. These cores tend to be moderately supercritical. They have
lifetimes long compared with their local gravitational collapse time. Some of
the cores collapse to form stars, while others disperse away without star
formation. In turbulent clouds that are marginally magnetically supercritical,
the star formation efficiency is higher, but can still be consistent with the
values inferred for nearby embedded clusters. If not regulated by magnetic
fields at all, star formation in a multi-Jeans mass cloud endowed with a strong
initial turbulence proceeds rapidly, with the majority of cloud mass converted
into stars in a gravitational collapse time. The efficiency is formally higher
than the values inferred for nearby cluster-forming cores, indicating that
magnetic fields are dynamically important even for cluster formation.",0502130v1
2005-07-07,Simulating field-aligned diffusion of a cosmic ray gas,"The macroscopic behaviour of cosmic rays in turbulent magnetic fields is
discussed. An implementation of anisotropic diffusion of cosmic rays with
respect to the magnetic field in a non-conservative, high-order,
finite-difference magnetohydrodynamic code is discussed. It is shown that the
standard implementation fails near singular X-points of the magnetic field,
which are common if the field is random. A modification to the diffusion model
for cosmic rays is described and the resulting telegraph equation (implemented
by solving a dynamic equation for the diffusive flux of cosmic rays) is used;
it is argued that this modification may better describe the physics of cosmic
ray diffusion. The present model reproduces several processes important for the
propagation and local confinement of cosmic rays, including spreading
perpendicular to the local large-scale magnetic field, controlled by the
random-to-total magnetic field ratio, and the balance between cosmic ray
pressure and magnetic tension. Cosmic ray diffusion is discussed in the context
of a random magnetic field produced by turbulent dynamo action. It is argued
that energy equipartition between cosmic rays and other constituents of the
interstellar medium do not necessarily imply that cosmic rays play a
significant role in the balance of forces.",0507176v2
2005-11-01,Magnetic Reynolds number dependence of reconnection rate and flow structure of the self-similar evolution model of fast magnetic reconnection,"This paper investigates Magnetic Reynolds number dependence of the
``self-similar evolution model'' (Nitta et al. 2001) of fast magnetic
reconnection. I focused my attention on the flow structure inside and around
the reconnection outflow, which is essential to determine the entire
reconnection system (Nitta et al. 2002). The outflow is consist of several
regions divided by discontinuities, e.g., shocks, and it can be treated by a
shock-tube approximation (Nitta 2004). By solving the junction conditions
(e.g., Rankine-Hugoniot condition), the structure of the reconnection outflow
is obtained. Magnetic reconnection in most astrophysical problems is
characterized by a huge dynamic range of its expansion ($sim 10^7$ for typical
solar flares) in a free space which is free from any influence of external
circumstances. Such evolution results in a spontaneous self-similar expansion
which is controlled by two intrinsic parameters: the plasma-$beta$ and the
magnetic Reynolds number. The plasma-$beta$ dependence had been investigated in
our previous paper. This paper newly clarifies the relation between the
reconnection rate and the inflow structure just outside the Petschek-like slow
shock: As the magnetic Reynolds number increases, strongly converging inflow
toward the Petschek-like slow shock forms, and it significantly reduces the
reconnection rate.",0511013v1
2006-12-29,Dynamic confinement of jets by magneto-torsional oscillations,"Many quasars and active galactic nuclei (AGN) appear in radio, optical, and
X-ray maps, as a bright nuclear sources from which emerge single or double
long, thin jets (Thomson et al., 1993). When observed with high angular
resolution these jets show structure with bright knots separated by relatively
dark regions. High percentages of polarization, sometimes more then 50% in some
objects, indicates the nonthermal nature of the radiation which is well
explained as the synchrotron radiation of the relativistic electrons in an
ordered magnetic field.
  A strong collimation of jets is most probably connected with ordered magnetic
fields. The mechanism of magnetic collimation, first suggested by
Bisnovatyi-Kogan et al. (1969), was based on the initial charge separation,
leading to creation of oscillating electrical current, which produces azimuthal
magnetic field, preventing jet expansion and disappearance. Here we consider
magnetic collimation, connected with torsional oscillations of a cylinder with
elongated magnetic field. Instead of initial blobs with charge separation, we
consider a cylinder with a periodically distributed initial rotation around the
cylinder axis. The stabilizing azimuthal magnetic field is created here by
torsional oscillations, where charge separation is not necessary. Approximate
simplified model is developed. Ordinary differential equation is derived, and
solved numerically, what gives a possibility to estimate quantitatively the
range of parameters where jets may be stabilized by torsional oscillations.",0612781v1
2007-02-05,Magnetic Helicity and the Relaxation of Fossil Fields,"In the absence of an active dynamo, purely poloidal magnetic field
configurations are unstable to large-scale dynamical perturbations, and decay
via reconnection on an Alfvenic timescale. Nevertheless, a number of classes of
dynamo-free stars do exhibit significant, long-lived, surface magnetic fields.
Numerical simulations suggest that the large-scale poloidal field in these
systems is stabilized by a toroidal component of the field in the stellar
interior. Using the principle of conservation of total helicity, we develop a
variational principle for computing the structure of the magnetic field inside
a conducting sphere surrounded by an insulating vacuum. We show that, for a
fixed total helicity, the minimum energy state corresponds to a force-free
configuration. We find a simple class of axisymmetric solutions, parametrized
by angular and radial quantum numbers. However, these solutions have a
discontinuity in the toroidal magnetic field at the stellar surface which will
exert a toroidal stress on the surface of the star. We then describe two other
classes of solutions, the standard spheromak solutions and ones with fixed
surface magnetic fields, the latter being relevant for neutron stars with rigid
crusts. We discuss the implications of our results for the structure of neutron
star magnetic fields, the decay of fields, and the origin of variability and
outbursts in magnetars.",0702128v2
2007-03-09,Burial of the polar magnetic field of an accreting neutron star. II. Hydromagnetic stability of axisymmetric equilibria,"The theory of polar magnetic burial in accreting neutron stars predicts that
a mountain of accreted material accumulates at the magnetic poles of the star,
and that, as the mountain spreads equatorward, it is confined by, and
compresses, the equatorial magnetic field. Here, we extend previous,
axisymmetric, Grad-Shafranov calculations of the hydromagnetic structure of a
magnetic mountain up to accreted masses as high as $\Ma = 6\times
10^{-4}\Msun$, by importing the output from previous calculations (which were
limited by numerical problems and the formation of closed bubbles to $\Ma <
10^{-4}\Msun$) into the time-dependent, ideal-magnetohydrodynamic code ZEUS-3D
and loading additional mass onto the star dynamically. The rise of buoyant
magnetic bubbles through the accreted layer is observed in these experiments.
We also investigate the stability of the resulting hydromagnetic equilibria by
perturbing them in ZEUS-3D. Surprisingly, it is observed that the equilibria
are marginally stable for all $\Ma\leq 6\times 10^{-4}\Msun$; the mountain
oscillates persistently when perturbed, in a combination of Alfv\'en and
acoustic modes, without appreciable damping or growth, and is therefore not
disrupted (apart from a transient Parker instability initially, which expels $<
1 %$ of the mass and magnetic flux).",0703203v1
2001-03-21,Asymptotic properties of turbulent magnetohydrodynamics,"The dynamic renormalization group (RG) is used to study the large-distance
and long-time limits of viscous and resistive incompressible
magnetohydrodynamics subject to random forces and currents. The scale-dependent
viscosity and magnetic resistivity are derived and used for carrying out
RG-improved perturbation theory. This is applied to derive both the asymptotic
scaling and the overall proportionality coefficients for both the velocity and
magnetic field correlation functions as well as the kinetic and magnetic energy
density spectral functions. The Kolmogorov, Iroshnikov-Kraichnan, as well as
other energy spectra, formally can be obtained by suitable choice of injected
noise, although the method limits the validity of these energy spectra only to
the asymptotic regime . Injection of a random magnetic helicity is considered,
its RG-improved spectral density derived, and its contribution to the velocity
and magnetic field correlation functions determined. The RG scaling solutions
are used to determine information at asymptotic scales about energy and
helicity cascade directions and mixing between magnetic and kinetic energy.
Some of the results found here also are shown to be valid for the Navier-Stokes
hydrodynamic equation. The results have applicability to geomagnetism as well
as cosmic magnetic fields at astrophysical and cosmological scales.",0103447v3
2003-11-18,"Field-induced magnetic order in La$sub {2-x}$Sr$sub x$CuO$sub 4$ (x = 0.10, 0.115, 0.13) studied by in-plane thermal conductivity measurements","We have measured the thermal conductivity in the $ab$-plane of
La$_{2-x}$Sr$_x$CuO$_4$ ($x =$ 0.10, 0.115, 0.13) in magnetic fields up to 14 T
parallel to the c-axis and also parallel to the $ab$-plane. By the application
of magnetic fields parallel to the c-axis, the thermal conductivity has been
found to be suppressed at low temperatures below the temperature $T_\kappa$
which is located above the superconducting transition temperature and is almost
independent of the magnitude of the magnetic field. The suppression is marked
in $x =$ 0.10 and 0.13, while it is small in $x =$ 0.115. Furthermore, no
suppression is observed in the 1 % Zn-substituted
La$_{2-x}$Sr$_x$Cu$_{0.99}$Zn$_{0.01}$O$_4$ with $x =$ 0.115. Taking into
account the experimental results that the temperature dependence of the
relative reduction of the thermal conductivity is quite similar to the
temperature dependence of the intensity of the incommensurate magnetic Bragg
peak corresponding to the static stripe order and that the Zn substitution
tends to stabilize the static order, it is concluded that the suppression of
the thermal conductivity in magnetic fields is attributed to the development of
the static stripe order. The present results suggest that the field-induced
magnetic order in La$_{2-x}$Sr$_x$CuO$_4$ originates from the pinning of the
dynamical stripes of spins and holes by vortex cores.",0311402v3
2005-02-23,Phonon Bottleneck Effect Leads to Observation of Quantum Tunneling of the Magnetization and Butterfly Hysteresis Loops in (Et4N)3Fe2F9,"A detailed investigation of the unusual dynamics of the magnetization of
(Et4N)3Fe2F9 (Fe2), containing isolated [Fe2F9]3- dimers, is presented and
discussed. Fe2 possesses an S=5 ground state with an energy barrier of 2.40 K
due to an axial anisotropy. Poor thermal contact between sample and bath leads
to a phonon bottleneck situation, giving rise to butterfly-shaped hysteresis
loops below 5 K concomitant with slow decay of the magnetization for magnetic
fields Hz applied along the Fe--Fe axis. The butterfly curves are reproduced
using a microscopic model based on the interaction of the spins with resonant
phonons. The phonon bottleneck allows for the observation of resonant quantum
tunneling of the magnetization at 1.8 K, far above the blocking temperature for
spin-phonon relaxation. The latter relaxation is probed by AC magnetic
susceptibility experiments at various temperatures and bias fields. At H=0, no
out-of-phase signal is detected, indicating that at T smaller than 1.8 K Fe2
does not behave as a single-molecule magnet. At 1 kG, relaxation is observed,
occurring over the barrier of the thermally accessible S=4 first excited state
that forms a combined system with the S=5 state.",0502548v1
2006-02-09,Magnetization damping in polycrystalline Co ultra-thin films: Evidence for non-local effects,"The magnetic properties and magnetization dynamics of polycrystalline
ultra-thin Co layers were investigated using a broadband ferromagnetic
resonance (FMR) technique at room temperature. A variable thickness (1 nm $\leq
t \leq$ 10 nm) Co layer is sandwiched between 10 nm thick Cu layers (10 nm Cu|
t Co|10 nm Cu), while materials in contact with the Cu outer interfaces are
varied to determine their influence on the magnetization damping. The resonance
field and the linewidth were studied for in-plane magnetic fields in field
swept experiments at a fixed frequency, from 4 to 25 GHz. The Co layers have a
lower magnetization density than the bulk, and an interface contribution to the
magnetic anisotropy normal to the film plane. The Gilbert damping, as
determined from the frequency dependence of the linewidth, increases with
decreasing Co layer thickness for films with outer Pt layers. This enhancement
is not observed in structures without Pt layers. The result can be understood
in terms of a non-local contribution to the damping due to spin pumping from Co
through the Cu layer and spin relaxation in Pt layers. Pt layers just 1.5 nm
thick are found to be sufficient to enhance the damping and thus act as
efficient ""spin-sinks"". In structures with Pt outer layers, this non-local
contribution to the damping becomes predominant when the Co layer is thinner
than 4 nm.",0602243v2
2006-05-06,Three-dimensionality of field-induced magnetism in a high-temperature superconductor,"Many physical properties of high-temperature (high-Tc) superconductors are
two-dimensional phenomena derived from their square planar CuO2 building
blocks. This is especially true of the magnetism from the copper ions. As
mobile charge carriers enter the CuO2 layers, the antiferromagnetism of the
parent insulators, where each copper spin is antiparallel to its nearest
neighbours1, evolves into a fluctuating state where the spins show tendencies
towards magnetic order of a longer periodicity. For certain charge carrier
densities, quantum fluctuations are sufficiently suppressed to yield static
long-period order2,3,4,5,6, and external magnetic fields also induce such
order7,8,9,10,11,12. Here we show that in contrast to the chemically-controlled
order in superconducting samples, the field-induced order in these same samples
is actually three-dimensional, implying significant magnetic linkage between
the CuO2 planes. The results are important because they show that there are
three-dimensional magnetic couplings which survive into the superconducting
state, and coexist with the crucial inter-layer couplings responsible for
three-dimensional superconductivity. Both types of coupling will straighten the
vortex lines, implying that we have finally established a direct link between
technical superconductivity, which requires zero electrical resistance in an
applied magnetic field and depends on vortex dynamics, and the underlying
antiferromagnetism of the cuprates.",0605164v1
2006-08-10,The origin of the spin glass transition in a model geometrically frustrated magnet,"Highly frustrated systems have macroscopically degenerate ground states that
lead to novel properties. In magnetism its consequences underpin exotic and
technologically important effects, such as, high temperature superconductivity,
colossal magnetoresistence, and the anomalous Hall effect. One of the enduring
mysteries of frustrated magnetism is why certain experimental systems have a
spin glass transition and its exact nature, given that it is not determined by
the strength of the dominant magnetic interactions. There have been some
suggestions that real systems possess disorder of the magnetic sites or bonds
that are responsible. We show that the spin glass transition in the model
kagome antiferromagnet hydronium jarosite arises from a spin anisotropy. This
weaker energy scale is much smaller than that of the magnetic exchange, yet it
is responsible for the energy barriers that are necessary to stabilise a glassy
magnetic phase at finite temperature. The resultant glassy phase is quite
unlike those found in conventional disordered spin glasses as it is based on
complex collective rearrangements of spins called ""spin folds"". This simplifies
hugely theoretical treatment of both the complex dynamics characteristic of a
spin glass and the microscopic nature of the spin glass transition itself.",0608234v1
2003-03-17,Constituent-Quark Model and New Particles,"An elementary constituent-quark (CQ) model by Mac Gregor is reviewed with
currently published data from light meson spectroscopy. It was previously shown
in the CQ model that there existed several mass quanta m = 70 MeV, B = 140 MeV
and X = 420 MeV, which were responsible for the quantization of meson yrast
levels. The existence of a 70-MeV quantum was postulated by Mac Gregor and was
shown to fit the Nambu empirical mass formula mn = (n/2)137me, n a positive
integer. The 70-MeV quantum can be derived in three different ways: 1) pure
electric coupling, 2) pure magnetic coupling, and 3) mixed electric and
magnetic charges (dyons). Schwinger first introduced dyons in a magnetic model
of matter. It is shown in this paper that recent data of new light mesons fit
into the CQ model (a pure electric model) without the introduction of magnetic
charges. However, by introducing electric and magnetic quarks (dyons) into the
CQ model, new dynamical forces can be generated by the presence of magnetic
fields internal to the quarks (dyons). The laws of angular momentum and of
energy conservation are valid in the presence of magnetic charge. With the
introduction of the Russell-Saunders coupling scheme into the CQ model, several
new meson particles are predicted to exist. The existence of the f0(560) meson
is predicted and is shown to fit current experimental data from the Particle
Data Group listing. The existence of meson partners or groupings is shown.",0303139v3
2002-02-06,From U(1) Maxwell Chern-Simons to Azbel-Hofstadter: Testing Magnetic Monopoles and Gravity to $\sim 10^{-15}$\textit{m}?,"It is built a map between an Abelian Topological Quantum Field Theory, $2+1D$
compact U(1) gauge Maxwell Chern-Simons Theory and the nonrelativistic quantum
mechanics Azbel-Hofstadter model of Bloch electrons. The $U_q(sl_2)$ quantum
group and the magnetic translations group of the Azbel-Hofstadter model
correspond to discretized subgroups of U(1) with linear gauge parameters. The
magnetic monopole confining and condensate phases in the Topological Quantum
Field Theory are identified with the extended (energy bands) and localized
(gaps) phases of the Bloch electron. The magnetic monopole condensate is
associated, at the nonrelativistic level, with gravitational white holes due to
deformed classical gauge fields. These gravitational solutions render the
existence of finite energy pure magnetic monopoles possible. This mechanism
constitutes a dynamical symmetry breaking which regularizes the solutions on
those localized phases allowing physical solutions of the Shr\""odinger equation
which are chains of electron filaments connecting several monopole-white
holes.To test these results would be necessary a strong external magnetic field
$B\sim 5 T$ at low temperature $T<1 K$. To be accomplished, it would test the
existence of magnetic monopoles and classical gravity to a scale of $\sim
10^{-15}$ \textit{meters}, the dimension of the monopole-white hole. A proper
discussion of such experiment is out of the scope of this theoretical work.",0202033v3
2004-11-11,The magnetized dusty plasma discharge : negative and positive space charge modes,"The structure of a discharge across a magnetic field in a dusty plasma is
analysed. The dust macroparticles are negatively charged, but are unmagnetized
because of their high mass. The electrons are highly magnetized, and the ions
have intermediate magnetization. This results in different transport rates of
the different species across the magnetic field. Depending on the size of the
magnetic field, and the relative charge on the different species, the dust
grains can be the dominant current carrier. The space charge clouds near the
electrodes will then be determined by the relative mobility of the different
species. The discharge can operate in one of two modes, a positive space charge
(PSC) mode, characterized by a strong cathode fall, and a negative space charge
(NSC) mode, characterized by a broad anode fall. Features unique to the dust
particles can also play a role in the structure of the discharge, such as the
variable equilibrium charge on the grains, dependent on the local potential and
species temperatures, the effect of gravity on the grain dynamics, and the rate
of charging of the grains. The dust grains can also form an ordered structure,
the dust-plasma crystal. A fluid model of the different species is used to
calculate the structure of the resulting discharge, incorporating the above
effects. The transition from the PSC mode to the NSC mode as the magnetic
field, pressure and dust properties are varied is demonstrated.",0411116v1
2006-06-14,Paradox of inductionless magnetorotational instability in a Taylor-Couette flow with a helical magnetic field,"We consider the magnetorotational instability (MRI) of a hydrodynamically
stable Taylor-Couette flow with a helical external magnetic field in the
inductionless approximation defined by a zero magnetic Prandtl number
($\Pm=0)$. This leads to a considerable simplification of the problem
eventually containing only hydrodynamic variables. First, we point out that the
energy of any perturbation growing in the presence of magnetic field has to
grow faster without the field. This is a paradox because the base flow is
stable without the magnetic while it is unstable in the presence of a helical
magnetic field without being modified by the latter as it has been found
recently by Hollerbach and Rudiger [Phys. Rev. Lett. 95, 124501 (2005)]. We
revisit this problem by using a Chebyshev collocation method to calculate the
eigenvalue spectrum of the linearized problem. In this way, we confirm that MRI
with helical magnetic field indeed works in the inductionless limit where the
destabilization effect appears as an effective shift of the Rayleigh line.
Second, we integrate the linearized equations in time to study the transient
behavior of small amplitude perturbations, thus showing that the energy
arguments are correct as well. However, there is no real contradiction between
both facts. The linear stability theory predicts the asymptotic development of
an arbitrary small-amplitude perturbation, while the energy stability theory
yields the instant growth rate of any particular perturbation, but it does not
account for the evolution of this perturbation.",0606122v1
2007-08-21,Magnetogenesis from Cosmic String Loops,"Large-scale coherent magnetic fields are observed in galaxies and clusters,
but their ultimate origin remains a mystery. We reconsider the prospects for
primordial magnetogenesis by a cosmic string network. We show that the magnetic
flux produced by long strings has been overestimated in the past, and give
improved estimates. We also compute the fields created by the loop population,
and find that it gives the dominant contribution to the total magnetic field
strength on present-day galactic scales. We present numerical results obtained
by evolving semi-analytic models of string networks (including both one-scale
and velocity-dependent one-scale models) in a Lambda-CDM cosmology, including
the forces and torques on loops from Hubble redshifting, dynamical friction,
and gravitational wave emission. Our predictions include the magnetic field
strength as a function of correlation length, as well as the volume covered by
magnetic fields. We conclude that string networks could account for magnetic
fields on galactic scales, but only if coupled with an efficient dynamo
amplification mechanism.",0708.2901v3
2007-12-20,Magnetized CMB observables: a dedicated numerical approach,"Large-scale magnetic fields affect the scalar modes of the geometry whose
ultimate effect is to determine the anisotropies of the Cosmic Microwave
Background (CMB in what follows). For the first time, a consistent numerical
approach to the magnetized CMB anisotropies is pursued with the aim of
assessing the angular power spectra of temperature and polarization when the
scalar modes of the geometry and a stochastic background of inhomogeneous
magnetic fields are simultaneously present in the plasma. The effects related
to the magnetized nature of the plasma are taken into account both at the level
of the dynamical equations and at the level of the initial conditions of the
Einstein-Boltzmann hierarchy. The temperature and polarization observables are
exploited to infer the peculiar signatures of a pre-equality magnetic field.
Using the extrapolated best fit to the three year WMAP data the increase and
distortions of the first seven peaks in the TT autocorrelations are monitored
for different values of the regularized magnetic field intensity and for the
physical range of spectral indices. Similar analyses are also conducted for the
first few anticorrelation (and corrrelation) peaks of the TE power spectra.
Possible interesting degeneracies and stimulating perspectives are pointed out
and explored.",0712.3483v1
2008-04-01,Magnetic Component of Quark-Gluon Plasma is also a Liquid!,"The so called magnetic scenario recently suggested in \cite{Liao_ES_mono}
emphasizes the role of monopoles in strongly coupled quark-gluon plasma (sQGP)
near/above the deconfinement temperature, and specifically predicts that they
help reduce its viscosity by the so called ""magnetic bottle"" effect. Arguments
for ""magnetic liquid"" in 1-2$T_c$ based on lattice measurement of monopole
density were provided in \cite{Chernodub}. Here we present results for
monopole-(anti)monopole correlation functions from the same classical molecular
dynamics simulations, which are found to be in very good agreement with recent
lattice results \cite{D'Alessandro:2007su}. We show that the magnetic Coulomb
coupling does run in the direction $opposite$ to the electric one, as expected,
and it is roughly inverse of the asymptotic freedom formula for the electric
one. However, as $T$ decreases to $T_c$, the magnetic coupling never gets weak,
with the plasma parameter always large enough ($\Gamma>1$). This nicely agrees
with empirical evidences from RHIC experiments, implying that magnetic objects
cannot have large mean free path and should also form a good liquid with low
viscosity.",0804.0255v4
2009-03-02,"Ferromagnetic properties of p-(Cd,Mn)Te quantum wells: Interpretation of magneto-optical measurements by Monte Carlo simulations","In order to single out dominant phenomena that account for carrier-controlled
magnetism in p-(Cd,Mn)Te quantum wells we have carried out magneto-optical
measurements and Monte Carlo simulations of time dependent magnetization. The
experimental results show that magnetization relaxation is faster than 20 ns in
the paramagnetic state. Decreasing temperature below the Curie temperature Tc
results in an increase of the relaxation time but to less than 10 micro
seconds. This fast relaxation may explain why the spontaneous spin splitting of
electronic states is not accompanied by the presence of non-zero macroscopic
magnetization below Tc. Our Monte Carlo results reproduce the relative change
of the relaxation time on decreasing temperature. At the same time, the
numerical calculations demonstrate that antiferromagnetic spin-spin
interactions, which compete with the hole-mediated long-range ferromagnetic
coupling, play an important role in magnetization relaxation of the system. We
find, in particular, that magnetization dynamics is largely accelerated by the
presence of antiferromagnetic couplings to the Mn spins located outside the
region, where the holes reside. This suggests that macroscopic spontaneous
magnetization should be observable if the thickness of the layer containing
localized spins will be smaller than the extension of the hole wave function.
Furthermore, we study how a spin-independent part of the Mn potential affects
Tc. Our findings show that the alloy disorder potential tends to reduce Tc, the
effect being particularly strong for the attractive potential that leads to
hole localization.",0903.0406v1
2009-05-04,Magnetic field structure due to the global velocity field in spiral galaxies,"We present a set of global, self-consistent N-body/SPH simulations of the
dynamic evolution of galactic discs with gas and including magnetic fields. We
have implemented a description to follow the evolution of magnetic fields with
the ideal induction equation in the SPH part of the Vine code. Results from a
direct implementation of the field equations are compared to a representation
by Euler potentials, which pose a div(B)-free description, an constraint not
fulfilled for the direct implementation. All simulations are compared to an
implementation of magnetic fields in the Gadget code which includes also
cleaning methods for div(B).
  Starting with a homogeneous seed field we find that by differential rotation
and spiral structure formation of the disc the field is amplified by one order
of magnitude within five rotation periods of the disc. The amplification is
stronger for higher numerical resolution. Moreover, we find a tight connection
of the magnetic field structure to the density pattern of the galaxy in our
simulations, with the magnetic field lines being aligned with the developing
spiral pattern of the gas. Our simulations clearly show the importance of
non-axisymmetry for the evolution of the magnetic field.",0905.0351v1
2009-07-12,Magnetic field evolution in simulations with Euler potentials,"Using two- and three-dimensional hydromagnetic simulations for a range of
different flows, including laminar and turbulent ones, it is shown that
solutions expressing the field in terms of Euler potentials (EP) are in general
incorrect if the EP are evolved with an artificial diffusion term. In three
dimensions, standard methods using the magnetic vector potential are found to
permit dynamo action when the EP give decaying solutions. With an imposed
field, the EP method yields excessive power at small scales. This effect is
more exaggerated in the dynamic case, suggesting an unrealistically reduced
feedback from the Lorentz force. The EP approach agrees with standard methods
only at early times when magnetic diffusivity did not have time to act. It is
demonstrated that the usage of EP with even a small artificial magnetic
diffusivity does not converge to a proper solution of hydromagnetic turbulence.
The source of this disagreement is not connected with magnetic helicity or the
three-dimensionality of the magnetic field, but is simply due to the fact that
the nonlinear representation of the magnetic field in terms of EP that depend
on the same coordinates is incompatible with the linear diffusion operator in
the induction equation.",0907.1906v2
2009-07-22,Quantized spin wave modes in magnetic tunnel junction nanopillars,"We present an experimental and theoretical study of the magnetic field
dependence of the mode frequency of thermally excited spin waves in rectangular
shaped nanopillars of lateral sizes 60x100, 75x150, and 105x190 nm2, patterned
from MgO-based magnetic tunnel junctions. The spin wave frequencies were
measured using spectrally resolved electrical noise measurements. In all
spectra, several independent quantized spin wave modes have been observed and
could be identified as eigenexcitations of the free layer and of the synthetic
antiferromagnet of the junction. Using a theoretical approach based on the
diagonalization of the dynamical matrix of a system of three coupled, spatially
confined magnetic layers, we have modeled the spectra for the smallest pillar
and have extracted its material parameters. The magnetization and exchange
stiffness constant of the CoFeB free layer are thereby found to be
substantially reduced compared to the corresponding thin film values. Moreover,
we could infer that the pinning of the magnetization at the lateral boundaries
must be weak. Finally, the interlayer dipolar coupling between the free layer
and the synthetic antiferromagnet causes mode anticrossings with gap openings
up to 2 GHz. At low fields and in the larger pillars, there is clear evidence
for strong non-uniformities of the layer magnetizations. In particular, at zero
field the lowest mode is not the fundamental mode, but a mode most likely
localized near the layer edges.",0907.3792v2
2009-07-27,Observations of The Magnetic Reconnection Signature of An M2 Flare on 2000 March 23,"Multi-wavelength observations of an M 2.0 flare event on 2000 March 23 in
NOAA active region 8910 provide us a good chance to study the detailed
structure and dynamics of the magnetic reconnection region. In the process of
the flare, extreme ultraviolet (EUV) loops displayed two times of sideward
motions upon a loop-top hard X-ray source with average velocities of 75 and
25.6 km/s, respectively. We consider these two motions to be the observational
evidence of reconnection inflow, and find an X-shaped structure upon the
post-flare loops during the period of the second motion. Two separations of the
flare ribbons are associated with these two sideward motions, with average
velocities of 3.3 and 1.3 km/s, separately. Using the observation of
photospheric magnetic field, the velocities of the sideward motions and the
separations, we deduce the corresponding coronal magnetic field strength to be
about 13.2-15.2 G, and estimate the reconnection rates to be 0.05 and 0.02 for
these two magnetic reconnection processes, respectively. We also observe
motions of bright points upward and downward along the EUV loops with
velocities ranging from 45.4 to 556.7 km/s. A cloud of bright material flowing
outward from the loop-top hard X-ray source with an average velocity of 51 km/s
in the process of the flare may be accelerated by the tension force of the
newly reconnected magnetic field lines. All the observations can be explained
by schematic diagrams of magnetic reconnection.",0907.4541v1
2009-08-17,Parameters of the Magnetic Flux inside Coronal Holes,"Parameters of magnetic flux distribution inside low-latitude coronal holes
(CHs) were analyzed. A statistical study of 44 CHs based on Solar and
Heliospheric Observatory (SOHO)/MDI full disk magnetograms and SOHO/EIT 284\AA
images showed that the density of the net magnetic flux, $B_{{\rm net}}$, does
not correlate with the associated solar wind speeds, $V_x$. Both the area and
net flux of CHs correlate with the solar wind speed and the corresponding
spatial Pearson correlation coefficients are 0.75 and 0.71, respectively. A
possible explanation for the low correlation between $B_{{\rm net}}$ and $V_x$
is proposed. The observed non-correlation might be rooted in the structural
complexity of the magnetic field. As a measure of complexity of the magnetic
field, the filling factor, $ f(r)$, was calculated as a function of spatial
scales. In CHs, $f(r)$ was found to be nearly constant at scales above 2 Mm,
which indicates a monofractal structural organization and smooth temporal
evolution. The magnitude of the filling factor is 0.04 from the Hinode SOT/SP
data and 0.07 from the MDI/HR data. The Hinode data show that at scales smaller
than 2 Mm, the filling factor decreases rapidly, which means a mutlifractal
structure and highly intermittent, burst-like energy release regime. The
absence of necessary complexity in CH magnetic fields at scales above 2 Mm
seems to be the most plausible reason why the net magnetic flux density does
not seem to be related to the solar wind speed: the energy release dynamics,
needed for solar wind acceleration, appears to occur at small scales below 1
Mm.",0908.2460v1
2009-12-28,Universality between current- and field-driven domain wall dynamics in ferromagnetic nanowires,"Spin-polarized electric current exerts torque on local magnetic spins,
resulting in magnetic domain-wall (DW) motion in ferromagnetic nanowires. Such
current-driven DW motion opens great opportunities toward next-generation
magnetic devices controlled by current instead of magnetic field. However, the
nature of the current-driven DW motion--considered qualitatively different from
magnetic-field-driven DW motion--remains yet unclear mainly due to the
painfully high operation current densities J_OP, which introduce uncontrollable
experimental artefacts with serious Joule heating. It is also crucial to reduce
J_OP for practical device operation. By use of metallic Pt/Co/Pt nanowires with
perpendicular magnetic anisotropy, here we demonstrate DW motion at current
densities down to the range of 10^9 A/m^2--two orders smaller than existing
reports. Surprisingly the current-driven motion exhibits a scaling behaviour
identical to the field-driven motion and thus, belongs to the same universality
class despite their qualitative differences. Moreover all DW motions driven by
either current or field (or by both) collapse onto a single curve, signalling
the unification of the two driving mechanisms. The unified law manifests
non-vanishing current efficiency at low current densities down to the practical
level, applicable to emerging magnetic nanodevices.",0912.5127v2
2010-03-12,Turbulent magnetic reconnection in 2D and 3D,"Magnetic field embedded in a perfectly conducting fluid preserves its
topology for all time. Although ionized astrophysical objects, like stars and
galactic disks, are almost perfectly conducting, they show indications of
changes in topology, `magnetic reconnection', on dynamical time scales.
Reconnection can be observed directly in the solar corona, but can also be
inferred from the existence of large scale dynamo activity inside stellar
interiors. Solar flares and gamma ray busts are usually associated with
magnetic reconnection. Previous work has concentrated on showing how
reconnection can be rapid in plasmas with very small collision rates. Here we
present numerical evidence, based on three dimensional simulations, that
reconnection in a turbulent fluid occurs at a speed comparable to the rms
velocity of the turbulence, regardless of the value of the resistivity. In
particular, this is true for turbulent pressures much weaker than the magnetic
field pressure so that the magnetic field lines are only slightly bent by the
turbulence. These results are consistent with the proposal by Lazarian and
Vishniac (1999) that reconnection is controlled by the stochastic diffusion of
magnetic field lines, which produces a broad outflow of plasma from the
reconnection zone. This work implies that reconnection in a turbulent fluid
typically takes place in approximately a single eddy turnover time, with broad
implications for dynamo activity and particle acceleration throughout the
universe. In contrast, the reconnection in 2D configurations in the presence of
turbulence depends on resistivity, i.e. is slow.",1003.2656v1
2010-04-16,Gravitational Wave Signatures of Magnetohydrodynamically-Driven Core-Collapse Supernova Explosions,"By performing a series of two-dimensional, special relativistic
magnetohydrodynamic (MHD) simulations, we study signatures of gravitational
waves (GWs) in the magnetohydrodynamically-driven core-collapse supernovae. In
order to extract the gravitational waveforms, we present a stress formula
including contributions both from magnetic fields and special relativistic
corrections. By changing the precollapse magnetic fields and initial angular
momentum distributions parametrically, we compute twelve models. As for the
microphysics, a realistic equation of state is employed and the neutrino
cooling is taken into account via a multiflavor neutrino leakage scheme.
  With these computations, we find that the total GW amplitudes show a
monotonic increase after bounce for models with a strong precollapse magnetic
field ($10^{12}$G) also with a rapid rotation imposed.
  We show that this trend stems both from the kinetic contribution of MHD
outflows with large radial velocities and also from the magnetic contribution
dominated by the toroidal magnetic fields that predominantly trigger MHD
explosions. For models with weaker initial magnetic fields, the total GW
amplitudes after bounce stay almost zero, because the contribution from the
magnetic fields cancels with the one from the hydrodynamic counterpart.
  These features can be clearly understood with a careful analysis on the
explosion dynamics. We point out that the GW signals with the increasing trend,
possibly visible to the next-generation detectors for a Galactic supernova,
would be associated with MHD explosions with the explosion energies exceeding
$10^{51}$ erg.",1004.2896v2
2010-08-24,Protostellar collapse of magneto-turbulent cloud cores: shape during collapse and outflow formation,"We investigate protostellar collapse of molecular cloud cores by numerical
simulations, taking into account turbulence and magnetic fields. By using the
adaptive mesh refinement technique, the collapse is followed over a wide
dynamic range from the scale of a turbulent cloud core to that of the first
core. The cloud core is lumpy in the low density region owing to the
turbulence, while it has a smooth density distribution in the dense region
produced by the collapse. The shape of the dense region depends mainly on the
mass of the cloud core; a massive cloud core tends to be prolate while a less
massive cloud core tends to be oblate. In both cases, anisotropy of the dense
region increases during the isothermal collapse. The minor axis of the dense
region is always oriented parallel to the local magnetic field. All the models
eventually yield spherical first cores supported mainly by the thermal
pressure. Most of turbulent cloud cores exhibit protostellar outflows around
the first cores. These outflows are classified into two types, bipolar and
spiral flows, according to the morphology of the associated magnetic field.
Bipolar flow often appears in the less massive cloud core. The rotation axis of
the first core is oriented parallel to the local magnetic field for bipolar
flow, while the orientation of the rotation axis from the global magnetic field
depends on the magnetic field strength. In spiral flow, the rotation axis is
not aligned with the local magnetic field.",1008.3984v2
2010-09-23,Magnetic loops in the quiet Sun,"We investigate the fine structure of magnetic fields in the atmosphere of the
quiet Sun. We use photospheric magnetic field measurements from {\sc
Sunrise}/IMaX with unprecedented spatial resolution to extrapolate the
photospheric magnetic field into higher layers of the solar atmosphere with the
help of potential and force-free extrapolation techniques. We find that most
magnetic loops which reach into the chromosphere or higher have one foot point
in relatively strong magnetic field regions in the photosphere. $91%$ of the
magnetic energy in the mid chromosphere (at a height of 1 Mm) is in field
lines, whose stronger foot point has a strength of more than 300 G, i.e. above
the equipartition field strength with convection. The loops reaching into the
chromosphere and corona are also found to be asymmetric in the sense that the
weaker foot point has a strength $B < 300$ G and is located in the
internetwork. Such loops are expected to be strongly dynamic and have short
lifetimes, as dictated by the properties of the internetwork fields.",1009.4715v1
2011-01-01,Signature effects of spin clustering and distribution of spin couplings on magnetization behaviour in Ni-Fe-Mo and Ni-Fe-W alloys,"The spontaneous magnetization as a function of temperature is investigated
for a number of disordered Ni-Fe-Mo and Ni-Fe-W alloys using superconducting
quantum interference device magnetometry, with a focus on the low-T behavior as
well as the critical exponents associated with the magnetic phase transition.
While the low-T magnetization is found to be well described by Bloch's T^{3/2}
law, an extraordinary enhancement of the spin-wave parameter B and the reduced
coefficient B_{3/2}=BT_C ^{3/2} are observed with increasing Fe dilution as
compared to conventional 3d ferromagnets, whereas the critical amplitudes are
found to decrease systematically. Recent locally self-consistent calculations
of finite-temperature spin dynamics in a generic diluted magnet provide an
understanding in terms of two distinct energy scales associated with weakly
coupled bulk spins in the FM matrix and strongly coupled cluster spins. In view
of similar behaviour observed in diluted magnetic semiconductors and other
ferromagnetic alloys, it is proposed that these distinctive features
corresponding to the three important temperature regimes provide macroscopic
indicators of signature effects of spin clustering on magnetization behaviour
in disordered ferromagnets.",1101.0343v2
2011-08-23,Buoyant Magnetic Loops in a Global Dynamo Simulation of a Young Sun,"The current dynamo paradigm for the Sun and sun-like stars places the
generation site for strong toroidal magnetic structures deep in the solar
interior. Sunspots and star-spots on sun-like stars are believed to arise when
sections of these magnetic structures become buoyantly unstable and rise from
the deep interior to the photosphere. Here we present the first 3-D global
magnetohydrodynamic (MHD) simulation in which turbulent convection,
stratification, and rotation combine to yield a dynamo that self-consistently
generates buoyant magnetic loops. We simulate stellar convection and dynamo
action in a spherical shell with solar stratification, but rotating three times
faster than the current solar rate. Strong wreaths of toroidal magnetic field
are realized by dynamo action in the convection zone. By turning to a dynamic
Smagorinsky model for subgrid-scale turbulence, we here attain considerably
reduced diffusion in our simulation. This permits the regions of strongest
magnetic field in these wreaths to rise toward the top of the convection zone
via a combination of magnetic buoyancy instabilities and advection by
convective giant cells. Such a global simulation yielding buoyant loops
represents a significant step forward in combining numerical models of dynamo
action and flux emergence.",1108.4697v1
2012-07-31,Circular Ribbon Flares and Homologous Jets,"Solar flare emissions in the chromosphere often appear as elongated ribbons
on both sides of the magnetic polarity inversion line (PIL), which has been
regarded as evidence of a typical configuration of magnetic reconnection.
However, flares having a circular ribbon have rarely been reported, although it
is expected in the fan--spine magnetic topology involving reconnection at a
three-dimensional (3D) coronal null point. We present five circular ribbon
flares with associated surges, using high-resolution and high-cadence halpha
blue wing observations obtained from the recently digitized films of Big Bear
Solar Observatory. In all the events, a central parasitic magnetic field is
encompassed by the opposite polarity, forming a circular PIL traced by filament
material. Consequently, a flare kernel at the center is surrounded by a
circular flare ribbon. The four homologous jet-related flares on 1991 March 17
and 18 are of particular interest, as (1) the circular ribbons brighten
sequentially, with co-spatial surges, rather than simultaneously, (2) the
central flare kernels show an intriguing ""round-trip"" motion and become
elongated, and (3) remote brightenings occur at a region with the same magnetic
polarity as the central parasitic field and are co-temporal with a separate
phase of flare emissions. In another flare on 1991 February 25, the circular
flare emission and surge activity occur successively, and the event could be
associated with magnetic flux cancellation across the circular PIL. We discuss
the implications of these observations combining circular flare ribbons,
homologous jets, and remote brightenings for understanding the dynamics of 3D
magnetic restructuring.",1207.7345v2
2012-08-10,Ion energy distribution functions behind the sheaths of magnetized and non magnetized radio frequency discharges,"The effect of a magnetic field on the characteristics of capacitively coupled
radio frequency discharges is investigated and found to be substantial. A
one-dimensional particle-in-cell simulation shows that geometrically symmetric
discharges can be asymmetrized by applying a spatially inhomogeneous magnetic
field. This effect is similar to the recently discovered electrical asymmetry
effect. Both effects act independently, they can work in the same direction or
compensate each other. Also the ion energy distribution functions at the
electrodes are strongly affected by the magnetic field, although only
indirectly. The field influences not the dynamics of the sheath itself but
rather its operating conditions, i.e., the ion flux through it and voltage drop
across it. To support this interpretation, the particle-in-cell results are
compared with the outcome of the recently proposed ensemble-in-spacetime
algorithm. Although that scheme resolves only the sheath and neglects
magnetization, it is able to reproduce the ion energy distribution functions
with very good accuracy, regardless of whether the discharge is magnetized or
not.",1208.2248v2
2012-08-16,Multiple views of magnetism in cool stars,"Magnetic fields are regarded as a crucial element for our understanding of
stellar physics. They can be studied with a variety of methods which provide
complementary - and sometimes contradictory - information about the structure,
strength and dynamics of the magnetic field and its role in the evolution of
stars. Stellar magnetic fields can be investigated either with direct methods
based on the Zeeman effect or through the observation of activity phenomena
resulting from the interaction of the field with the stellar atmosphere. In
this Cool Stars XVII Splinter Session we discussed the results obtained by the
many ongoing studies of stellar activity and direct studies of surface magnetic
fields, as well as the state- of-the-art techniques on which they are based. We
show the strengths and limitations of the various approaches currently used and
to point out their evolution as well as the interest of coupling various
magnetism and activity proxies.",1208.3338v1
2012-10-29,Small-Scale Dynamo Action in Primordial Halos,"The first galaxies form due to gravitational collapse of primordial halos.
During this collapse, weak magnetic seed fields get amplified exponentially by
the small-scale dynamo - a process converting kinetic energy from turbulence
into magnetic energy. We use the Kazantsev theory, which describes the
small-scale dynamo analytically, to study magnetic field amplification for
different turbulent velocity correlation functions. For incompressible
turbulence (Kolmogorov turbulence), we find that the growth rate is
proportional to the square root of the hydrodynamic Reynolds number, Re^(1/2).
In the case of highly compressible turbulence (Burgers turbulence) the growth
rate increases proportional to Re^(1/3). With a detailed chemical network we
are able to follow the chemical evolution and determine the kinetic and
magnetic viscosities (due to Ohmic and ambipolar diffusion) during the collapse
of the halo. This way, we can calculate the growth rate of the small-scale
dynamo quantitatively and predict the evolution of the small-scale magnetic
field. As the magnetic energy is transported to larger scales on the local
eddy-timescale, we obtain an estimate for the magnetic field on the Jeans
scale. Even there, we find that equipartition with the kinetic energy is
reached on small timescales. Dynamically relevant field structures can thus be
expected already during the formation of the first objects in the Universe.",1210.7751v1
2012-11-30,Global Dynamics of Subsurface Solar Active Regions,"We present three-dimensional numerical simulations of a magnetic loop
evolving in either a convectively stable or unstable rotating shell. The
magnetic loop is introduced in the shell in such a way that it is buoyant only
in a certain portion in longitude, thus creating an \Omega-loop. Due to the
action of magnetic buoyancy, the loop rises and develops asymmetries between
its leading and following legs, creating emerging bipolar regions whose
characteristics are similar to the ones of observed spots at the solar surface.
In particular, we self-consistently reproduce the creation of tongues around
the spot polarities, which can be strongly affected by convection. We moreover
emphasize the presence of ring-shaped magnetic structures around our simulated
emerging regions, which we call ""magnetic necklace"" and which were seen in a
number of observations without being reported as of today. We show that those
necklaces are markers of vorticity generation at the periphery and below the
rising magnetic loop. We also find that the asymmetry between the two legs of
the loop is crucially dependent on the initial magnetic field strength. The
tilt angle of the emerging regions is also studied in the stable and unstable
cases and seems to be affected both by the convective motions and the presence
of a differential rotation in the convective cases.",1211.7251v1
2013-01-26,Role of uniform horizontal magnetic field on convective flow,"The effect of uniform magnetic field applied along a fixed horizontal
direction in Rayleigh-B\'enard convection in low-Prandtl-number fluids has been
studied using a low dimensional model. The model shows the onset of convection
(primary instability) in the form of two dimensional stationary rolls in the
absence of magnetic field, when the Rayleigh number $R$ is raised above a
critical value $R_c$. The flow becomes three dimensional at slightly higher
values of Rayleigh number via wavy instability. These wavy rolls become chaotic
for slightly higher values of $R$ in low-Prandtl-number ($P_r$) fluids. A
uniform magnetic field along horizontal plane strongly affects all kinds of
convective flows observed at higher values of $R$ in its absence. As the
magnetic field is raised above certain value, it orients the convective rolls
in its own direction. Although the horizontal magnetic field does not change
the threshold for the primary instability, it affects the threshold for
secondary (wavy) instability. It inhibits the onset of wavy instability. The
critical Rayleigh number $R_o (Q,P_r)$ at the onset of wavy instability, which
depends on Chandrasekhar's number $Q$ and $P_r$, increases monotonically with
$Q$ for a fixed value of $P_r$. The dimensionless number $R_o (Q, P_r)/(R_c Q
P_r)$ scales with $Q$ as $Q^{-1}$. A stronger magnetic field suppresses chaos
and makes the flow two dimensional with roll pattern aligned along its
direction.",1301.6220v1
2013-02-26,Resonant state selection in synthetic ferrimagnets,"Resonant activation of a synthetic antiferromagnet (SAF) is known to result
in a dynamic running state, where the SAF's symmetric spin-flop pair
continuously rotates between the two antiparallel ground states of the system,
with the two magnetic moments in-phase in the so-called acoustical
spin-resonance mode. The symmetry of an ideal SAF does not allow, however, to
deterministically select a particular ground state using a resonant excitation.
In this work, we study asymmetric SAF's, or synthetic ferrimagnets (SFi), in
which the two magnetic particles are different in thickness or are biased
asymmetrically with an external field. We show how the magnetic phase space of
the system can be reversibly tuned, post-fabrication, between the antiferro-
and ferri-magnetic behavior by exploiting these two asymmetry parameters and
applying a uniform external field. We observe a splitting of the optical
spin-resonance for the two ground states of the SFi system, with a frequency
spacing that can be controlled by a quasistatic uniform external field. We
demonstrate how the tunable magnetic asymmetry in SFi allows to
deterministically select a particular ground state using the splitting of the
optical spin-resonance. These results offer a new way of controlling the
magnetic state of a spin-flop bilayer, currently used in such large scale
applications as magnetic memory.",1302.6483v2
2013-04-29,Magnetic Structure Producing X- and M-Class Solar Flares in Solar Active Region 11158,"We study the three-dimensional magnetic structure of solar active region
11158, which produced one X-class and several M-class flares on 2011 February
13$-$16. We focus on the magnetic twist in four flare events, M6.6, X2.2, M1.0,
and M1.1. The magnetic twist is estimated from the nonlinear force-free field
extrapolated from the vector fields obtained from the Helioseismic and Magnetic
Imager on board the Solar Dynamic Observatory using magnetohydrodynamic
relaxation method developed by \cite{2011ApJ...738..161I}. We found that
strongly twisted lines ranging from half-turn to one-turn twist were built up
just before the M6.6- and X2.2 flares and disappeared after that. Because most
of the twist remaining after these flares was less than half-turn twist, this
result suggests that the buildup of magnetic twist over the half-turn twist is
a key process in the production of large flares. On the other hand, even though
these strong twists were also built up just before the M1.0 and M1.1 flares,
most of them remained afterwords. Careful topological analysis before the M1.0
and M1.1 flares shows that the strongly twisted lines were surrounded mostly by
the weakly twisted lines formed in accordance with the clockwise motion of the
positive sunspot, whose footpoints are rooted in strong magnetic flux regions.
These results imply that these weakly twisted lines might suppress the activity
of the strongly twisted lines in the last two M-class flares.",1304.8073v1
2013-05-22,"Measuring the Magnetic Field Strength of the Quiet Solar Corona Using ""EIT Waves""","Variations in the propagation of globally-propagating disturbances (commonly
called ""EIT waves"") through the low solar corona offer a unique opportunity to
probe the plasma parameters of the solar atmosphere. Here, high-cadence
observations of two ""EIT wave"" events taken using the Atmospheric Imaging
Assembly (AIA) instrument onboard the Solar Dynamics Observatory (SDO) are
combined with spectroscopic measurements from the Extreme ultraviolet Imaging
Spectrometer (EIS) onboard the Hinode spacecraft and used to examine the
variability of the quiet coronal magnetic-field strength. The combination of
pulse kinematics from SDO/AIA and plasma density from Hinode/EIS is used to
show that the magnetic-field strength is in the range ~2-6 G in the quiet
corona. The magnetic-field estimates are then used to determine the height of
the pulse, allowing a direct comparison with theoretical values obtained from
magnetic-field measurements from the Helioseismic and Magnetic Imager (HMI)
onboard SDO using PFSS and local-domain extrapolations. While local-scale
extrapolations predict heights inconsistent with prior measurements, the
agreement between observations and the PFSS model indicates that ""EIT waves""
are a global phenomenon influenced by global-scale magnetic field.",1305.5169v1
2013-05-28,"DR 21(OH): a highly fragmented, magnetized, turbulent dense core","We present high-angular-resolution observations of the massive star forming
core DR21(OH) at 880 mum using the Submillimeter Array (SMA). The dense core
exhibits an overall velocity gradient in a Keplerian-like pattern, which breaks
at the center of the core where SMA 6 and SMA 7 are located. The dust
polarization shows a complex magnetic field, compatible with a toroidal
configuration. This is in contrast with the large, parsec-scale filament that
surrounds the core, where there is a smooth magnetic field. The total magnetic
field strengths in the filament and in the core are 0.9 and 2.1 mG,
respectively. We found evidence of magnetic field diffusion at the core scales,
far beyond the expected value for ambipolar diffusion. It is possible that the
diffusion arises from fast magnetic reconnection in the presence of turbulence.
The dynamics of the DR 21(OH) core appear to be controlled energetically in
equal parts by the magnetic field, magneto-hydrodynamic (MHD) turbulence and
the angular momentum. The effect of the angular momentum (this is a fast
rotating core) is probably causing the observed toroidal field configuration.
Yet, gravitation overwhelms all the forces, making this a clear supercritical
core with a mass-to-flux ratio of ~6 times the critical value. However,
simulations show that this is not enough for the high level of fragmentation
observed at 1000 AU scales. Thus, rotation and outflow feedback is probably the
main cause of the observed fragmentation.",1305.6509v2
2013-08-05,Axisymmetry vs. nonaxisymmetry of a Taylor-Couette flow with azimuthal magnetic fields,"The instability of a supercritical Taylor-Couette flow of a conducting fluid
with resting outer cylinder under the influence of a uniform axial electric
current is investigated for magnetic Prandtl number Pm=1. In the linear theory
the critical Reynolds number for axisymmetric perturbations is not influenced
by the current-induced axisymmetric magnetic field but all axisymmetric
magnetic perturbations decay. The nonaxisymmetric perturbations with m=1 are
excited even without rotation for large enough Hartmann numbers (""Tayler
instability""). For slow rotation their growth rates scale with the Alfv\'en
frequency of the magnetic field but for fast rotation they scale with the
rotation rate of the inner cylinder. In the nonlinear regime the ratio of the
energy of the magnetic m=1 modes and the toroidal background field is very low
for the non-rotating Tayler instability but it strongly grows if differential
rotation is present. For super-Alfv\'enic rotation the energies in the m=1
modes of flow and field do not depend on the molecular viscosity, they are
almost in equipartition and contain only 1.5% of the centrifugal energy of the
inner cylinder. The geometry of the excited magnetic field pattern is strictly
nonaxisymmetric for slow rotation but it is of the mixed-mode type for fast
rotation -- contrary to the situation which has been observed at the surface of
Ap stars.",1308.1026v2
2014-02-11,Chaotic motion of charged particles in toroidal magnetic configurations,"We study the motion of a charged particle in a tokamak magnetic field and
discuss its chaotic nature. Contrary to most of recent studies, we do not make
any assumption on any constant of the motion and solve numerically the
cyclotron gyration using Hamiltonian formalism. We take advantage of a
symplectic integrator allowing us to make long-time simulations. First
considering an idealized magnetic configuration, we add a non generic
perturbation corresponding to a magnetic ripple, breaking one of the invariant
of the motion. Chaotic motion is then observed and opens questions about the
link between chaos of magnetic field lines and chaos of particle trajectories.
Second, we return to a axi-symmetric configuration and tune the safety factor
(magnetic configuration) in order to recover chaotic motion. In this last
setting with two constants of the motion, the presence of chaos implies that no
third global constant exists, we highlight this fact by looking at variations
of the first order of the magnetic moment in this chaotic setting. We are
facing a mixed phase space with both regular and chaotic regions and point out
the difficulties in performing a global reduction such as gyrokinetics.",1402.2625v2
2014-03-13,Orbital Magnetism of Graphene Nanostructures: Bulk and Confinement Effects,"We consider the orbital magnetic properties of non-interacting charge
carriers in graphene-based nanostructures in the low-energy regime. The
magnetic response of such systems results both, frombulk contributions and from
confinement effects that can be particularly strong in ballistic quantum dots.
First we provide a comprehensive study of the magnetic susceptibility $\chi$ of
bulk graphene in a magnetic field for the different regimes arising from the
relative magnitudes of the energy scales involved, i.e. temperature, Landau
level spacing and chemical potential. We show that for finite temperature or
chemical potential, $\chi$ is not divergent although the diamagnetic
contribution $\chi_{0}$ from the filled valance band exhibits the well-known
$-B^{-1/2}$ dependence. We further derive oscillatory modulations of $\chi$,
corresponding to de Haas-van Alphen oscillations of conventional
two-dimensional electron gases. These oscillations can be large in graphene,
thereby compensating the diamagnetic contribution $\chi_{0}$ and yielding a net
paramagnetic susceptibility for certain energy and magnetic field regimes.
Second, we predict and analyze corresponding strong, confinement-induced
susceptibility oscillations in graphene-based quantum dots with amplitudes
distincly exceeding the corresponding bulk susceptibility. Within a
semiclassical approach we derive generic expressions for orbital magnetism of
graphene quantum dots with regular classical dynamics. Graphene-specific
features can be traced back to pseudospin interference along the underlying
periodic orbits. We demonstrate the quality of the semiclassical approximation
by comparison with quantum mechanical results for two exemplary mesoscopic
systems, a graphene disk with infinite mass-type edges and a rectangular
graphene structure with armchair and zigzag edges, using numerical
tight-binding calculations in the latter case.",1403.3688v3
2014-04-07,Lattice strain accompanying the colossal magnetoresistance effect in EuB$_6$,"The coupling of magnetic and electronic degrees of freedom to the crystal
lattice in the ferromagnetic semimetal EuB$_6$, which exhibits a complex
ferromagnetic order and a colossal magnetoresistance (CMR) effect, %, very
likely involving magnetic polarons, is studied by high-resolution thermal
expansion and magnetostriction experiments. EuB$_6$ may be viewed as a model
system, where pure magnetism-tuned transport and the response of the crystal
lattice can be studied in a comparatively simple environment,i.e., not
influenced by strong crystal-electric field effects and Jahn-Teller
distortions. We find a very large lattice response, quantified by (i) the
magnetic Gr\""uneisen parameter, (ii) the spontaneous strain when entering the
ferromagnetic region and (iii) the magnetostriction in the paramagnetic
temperature regime. Our analysis reveals that a significant part of the lattice
effects originates in the magnetically-driven delocalization of charge
carriers, consistent with the scenario of percolating magnetic polarons. A
strong effect of the formation and dynamics of local magnetic clusters on the
lattice parameters is suggested to be a general feature of CMR materials.",1404.1693v1
2014-04-16,Manipulation of edge magnetism in hexagonal graphene nanoflake,"We explore possible ways to manipulate the intrinsic edge magnetism in
hexagonal graphene nanoflake with zigzag edges, using density functional theory
supplemented with on-site Coulomb interaction. The effect of carrier doping,
chemical modification at the edge, and finite temperature on the edge magnetism
has been studied. The magnetic phase diagram with varied carrier doping, and
on-site Coulomb interaction is found to be complex. In addition to the
intrinsic antiferromag- netic solution, as predicted for charge neutral
hexagonal nanoflake, fully polarized ferromagnetic, and mixed phase solutions
are obtained depending on the doped carrier concentration, and on-site Coulomb
interaction. The complexity arises due to the competing nature of local Coulomb
in- teraction and carrier doping, favoring antiferromagnetic and ferromagnetic
coupling, respectively. Chemical modification of the edge atoms by hydrogen
leads to partial quenching of local moments, giving rise to a richer phase
diagram consisting of antiferromagnetic, ferromagnetic, mixed, and nonmagnetic
phases. We further report the influence of temperature on the long-range
magnetic ordering at the edge using ab initio molecular dynamics. In agreement
with the recent experimental observations, we find that temperature can also
alter the magnetic state of neutral nanoflake, which is otherwise
antiferromagnetic at zero temperature. These findings will have important
implications in controlling magnetism in graphene based low dimensional
structures for technological purpose, and in understanding varied experimental
reports.",1404.4143v2
2014-07-28,Microscopic electronic configurations after ultrafast magnetization dynamics,"We provide a model for the prediction of the electronic and magnetic
configurations of ferromagnetic Fe after an ultrafast decrease or increase of
magnetization. The model is based on the well-grounded assumption that, after
the ultrafast magnetization change, the system achieves a partial thermal
equilibrium. With statistical arguments it is possible to show that the
magnetic configurations are qualitatively different in the case of reduced or
increased magnetization. The predicted magnetic configurations are then used to
compute the dielectric response at the 3p (M) absorption edge, which can be
related to the changes observed in the experimental T-MOKE data. The good
qualitative agreement between theory and experiment offers a substantial
support to the existence of an ultrafast increase of magnetisation, which has
been fiercely debated in the last years.",1407.7411v2
2014-08-06,Non-Axisymmetric Flows on Hot Jupiters with Oblique Magnetic Fields,"Giant planets that reside in close proximity to their host stars are subject
to extreme irradiation, which gives rise to thermal ionization of trace Alkali
metals in their atmospheres. On objects where the atmospheric electrical
conductivity is substantial, the global circulation couples to the background
magnetic field, inducing supplementary fields and altering the nature of the
flow. To date, a number of authors have considered the influence of a spin-pole
aligned dipole magnetic field on the dynamical state of a weakly-ionized
atmosphere and found that magnetic breaking may lead to significantly slower
winds than predicted within a purely hydrodynamical framework. Here, we
consider the effect of a tilted dipole magnetic field on the circulation and
demonstrate that in addition to regulating wind velocities, an oblique field
generates stationary non-axisymmetric structures that adhere to the geometry of
the magnetic pole. Using a kinematic perturbative approach, we derive a
closed-form solution for the perturbed circulation and show that the fractional
distortion of zonal jets scales as the product of the field obliquity and the
Elsasser number. The results obtained herein suggest that on planets with
oblique magnetic fields, advective shifts of dayside hotspots may have
substantial latitudinal components. This prediction may be tested
observationally using the eclipse mapping technique.",1408.1386v1
2014-10-01,Evidence for magnetic clusters in Ni$_{1-x}$V$_{x}$ close to the quantum critical concentration,"The d-metal alloy Ni$_{1-x}$V$_{x}$ undergoes a quantum phase transition from
a ferromagnetic ground state to a paramagnetic ground state as the vanadium
concentration $x$ is increased. We present magnetization, ac-susceptibility and
muon-spin relaxation data at several vanadium concentrations near the critical
concentration $x_c \approx11.6%$ at which the onset of ferromagnetic order is
suppressed to zero temperature. Below $x_c$, the muon data reveal a broad
magnetic field distribution indicative of long-range ordered ferromagnetic
state with spatial disorder. We show evidence of magnetic clusters in the
ferromagnetic phase and close to the phase boundary in this disordered
itinerant system as an important generic ingredient of a disordered quantum
phase transition. In contrast, the temperature dependence of the magnetic
susceptibility above $x_c$ is best described in terms of a magnetic quantum
Griffiths phase with a power-law distribution of fluctuation rates of dynamic
magnetic clusters. At the lowest temperatures, the onset of a short-range
ordered cluster-glass phase is recognized by an increase in the muon
depolarization in transverse fields and maxima in ac-susceptibility.",1410.0094v1
2014-12-26,"A Laboratory Experiment of Magnetic Reconnection: Outflows, Heating and Waves in Chromospheric Jets","Hinode observations have revealed intermittent recurrent plasma
ejections/jets in the chromosphere. These are interpreted as a result of
non-perfectly anti-parallel magnetic reconnection, i.e. component reconnection,
between a twisted magnetic flux tube and the pre-existing coronal/chromospheric
magnetic field, though the fundamental physics of component reconnection is
unrevealed. In this paper, we experimentally reproduced the magnetic
configuration and investigated the dynamics of plasma ejections, heating and
wave generation triggered by component reconnection in the chromosphere. We set
plasma parameters as in the chromosphere (density 10^14 cm^-3, temperature 5-10
eV, i.e. (5-10)x10^4 K, and reconnection magnetic field 200 G) using argon
plasma. Our experiment shows bi-directional outflows with the speed of 5 km/s
at maximum, ion heating in the downstream area over 30 eV and magnetic
fluctuations mainly at 5-10 us period. We succeeded in qualitatively
reproducing chromospheric jets, but quantitatively we still have some
differences between observations and experiments such as jet velocity, total
energy and wave frequency. Some of them can be explained by the scale gap
between solar and laboratory plasma, while the others probably by the
difference of microscopy and macroscopy, collisionality and the degree of
ionization, which have not been achieved in our experiment.",1412.7903v1
2015-03-09,Magnetic field measurements at milliarcsecond resolution around massive young stellar objects,"Magnetic fields have only recently been included in theoretical simulations
of high-mass star formation. The simulations show that magnetic fields can play
a crucial role not only in the formation and dynamics of molecular outflows,
but also in the evolution of circumstellar disks. Therefore, new measurements
of magnetic fields at milliarcsecond resolution close to massive young stellar
objects (YSOs) are fundamental for providing new input for numerical
simulations and for understanding the formation process of massive stars. The
polarized emission of 6.7 GHz CH3OH masers allows us to investigate the
magnetic field close to the massive YSO where the outflows and disks are
formed. Recently, we have detected with the EVN CH3OH maser polarized emission
towards 10 massive YSOs. From a first statistical analysis we have found
evidence that magnetic fields are primarily oriented along the molecular
outflows. To improve our statistics we are carrying on a large observational
EVN campaign for a total of 19 sources, the preliminary results of the first
seven sources are presented in this contribution. Furthermore, we also describe
our efforts to estimate the Lande' g-factors of the CH3OH maser transition to
determine the magnetic field strength from our Zeeman-splitting measurements.",1503.02403v1
2015-03-12,Parallelization of the SIR code for the investigation of small-scale features in the solar photosphere,"Magnetic fields are one of the most important drivers of the highly dynamic
processes that occur in the lower solar atmosphere. They span a broad range of
sizes, from large- and intermediate-scale structures such as sunspots, pores
and magnetic knots, down to the smallest magnetic elements observable with
current telescopes. On small scales, magnetic flux tubes are often visible as
Magnetic Bright Points (MBPs). Apart from simple $V/I$ magnetograms, the most
common method to deduce their magnetic properties is the inversion of
spectropolarimetric data. Here we employ the SIR code for that purpose. SIR is
a well-established tool that can derive not only the magnetic field vector and
other atmospheric parameters (e.g., temperature, line-of-sight velocity), but
also their stratifications with height, effectively producing 3-dimensional
models of the lower solar atmosphere. In order to enhance the runtime
performance and the usability of SIR we parallelized the existing code and
standardized the input and output formats. This and other improvements make it
feasible to invert extensive high-resolution data sets within a reasonable
amount of computing time. An evaluation of the speedup of the parallel SIR code
shows a substantial improvement in runtime.",1503.03710v1
2015-03-16,Observation of the zero Hall plateau in a quantum anomalous Hall insulator,"Quantum anomalous Hall (QAH) effect in magnetic topological insulator (TI) is
a novel transport phenomenon in which the Hall resistance reaches the quantum
plateau in the absence of external magnetic field. Recently, this exotic effect
has been discovered experimentally in an ultrathin film of the Bi2Te3 family TI
with spontaneous ferromagnetic (FM) order. An important question concerning the
QAH state is whether it is simply a zero-magnetic-field version of the quantum
Hall (QH) effect, or if there is new physics beyond the conventional paradigm.
Here we report experimental investigations on the quantum phase transition
between the two opposite Hall plateaus of a QAH insulator caused by
magnetization reversal. We observe a well-defined plateau with zero Hall
conductivity over a range of magnetic field around coercivity, consistent with
a recent theoretical prediction. The features of the zero Hall plateau are
shown to be closely related to that of the QAH effect, but its temperature
evolution exhibits quantitative differences from the network model for
conventional QH plateau transition. We propose that the chiral edge states
residing at the magnetic domain boundaries, which are unique to a QAH
insulator, are responsible for the zero Hall plateau. The rich magnetic domain
dynamics makes the QAH effect a distinctive class of quantum phenomenon that
may find novel applications in spintronics.",1503.04569v1
2015-06-21,Fast magnetic field amplification in the early Universe: growth of collisionless plasma instabilities in turbulent media,"In this work we report a numerical study of the cosmic magnetic field
amplification due to collisionless plasma instabilities. The collisionless
magnetohydrodynamic equations derived account for the pressure anisotropy that
leads, in specific conditions, to the firehose and mirror instabilities. We
study the time evolution of seed fields in turbulence under the influence of
such instabilities. An approximate analytical time evolution of magnetic field
is provided. The numerical simulations and the analytical predictions are
compared. We found that i) amplification of magnetic field was efficient in
firehose unstable turbulent regimes, but not in the mirror unstable models, ii)
the growth rate of the magnetic energy density is much faster than the
turbulent dynamo, iii) the efficient amplification occurs at small scales. The
analytical prediction for the correlation between the growth timescales with
pressure anisotropy ratio is confirmed by the numerical simulations. These
results reinforce the idea that pressure anisotropies - driven naturally in a
turbulent collisionless medium, e.g. the intergalactic medium -, could
efficiently amplify the magnetic field in the early Universe
(post-recombination era), previous to the collapse of the first large-scale
gravitational structures. This mechanism, though fast for the small scale
fields ($\sim$kpc scales), is however unable to provide relatively strong
magnetic fields at large scales. Other mechanisms that were not accounted here
(e.g., collisional turbulence once instabilities are quenched, velocity shear,
or gravitationally induced inflows of gas into galaxies and clusters) could
operate afterwards to build up large scale coherent field structures in the
long time evolution.",1506.06398v1
2015-08-28,Control of magnetic relaxation by electric-field-induced ferroelectric phase transition and inhomogeneous domain switching,"Electric-field modulation of magnetism in strain-mediated multiferroic
heterostructures is considered a promising scheme for enabling memory and
magnetic microwave devices with ultralow power consumption. However, it is not
well understood how electric-field-induced strain influences magnetic
relaxation, an important physical process for device applications. Here we
investigate resonant magnetization dynamics in ferromagnet/ferrolectric
multiferroic heterostructures, FeGaB/PMN-PT and NiFe/PMN-PT, in two distinct
strain states provided by electric-field-induced ferroelectric phase
transition. The strain not only modifies magnetic anisotropy but also magnetic
relaxation. In FeGaB/PMN-PT, we observe a nearly two-fold change in intrinsic
Gilbert damping by electric field, which is attributed to strain-induced tuning
of spin-orbit coupling. By contrast, a small but measurable change in extrinsic
linewidth broadening is attributed to inhomogeneous ferroelastic domain
switching during the phase transition of the PMN-PT substrate.",1508.07290v2
2015-10-13,Critical decay index at the onset of solar eruptions,"Magnetic flux ropes are topological structures consisting of twisted magnetic
field lines that globally wrap around an axis. The torus instability model
predicts that a magnetic flux rope of major radius $R$ undergoes an eruption
when its axis reaches a location where the decay index $-d(\ln B_{ex})/d(\ln
R)$ of the ambient magnetic field $B_{ex}$ is larger than a critical value. In
the current-wire model, the critical value depends on the thickness and
time-evolution of the current channel. We use magneto-hydrodynamic (MHD)
simulations to investigate if the critical value of the decay index at the
onset of the eruption is affected by the magnetic flux rope's internal current
profile and/or by the particular pre-eruptive photospheric dynamics. The
evolution of an asymmetric, bipolar active region is driven by applying
different classes of photospheric motions. We find that the critical value of
the decay index at the onset of the eruption is not significantly affected by
either the pre-eruptive photospheric evolution of the active region or by the
resulting different magnetic flux ropes. As in the case of the current-wire
model, we find that there is a `critical range' $ [1.3-1.5]$, rather than a
`critical value' for the onset of the torus instability. This range is in good
agreement with the predictions of the current-wire model, despite the inclusion
of line-tying effects and the occurrence of tether-cutting magnetic
reconnection.",1510.03713v1
2015-10-19,Filament formation in wind-cloud interactions. I. Spherical clouds in uniform magnetic fields,"Filamentary structures are ubiquitous in the interstellar medium, yet their
formation, internal structure, and longevity have not been studied in detail.
We report the results from a comprehensive numerical study that investigates
the characteristics, formation, and evolution of filaments arising from
magnetohydrodynamic interactions between supersonic winds and dense clouds.
Here we improve on previous simulations by utilising sharper density contrasts
and higher numerical resolutions. By following multiple density tracers, we
find that material in the envelopes of the clouds is removed and deposited
downstream to form filamentary tails, while the cores of the clouds serve as
footpoints and late-stage outer layers of these tails. Aspect ratios >12,
subsonic velocity dispersions ~0.1-0.3 of the wind sound speed, and magnetic
field amplifications ~100 are found to be characteristic of these filaments. We
also report the effects of different magnetic field strengths and orientations.
The magnetic field strength regulates vorticity production: sinuous filamentary
towers arise in non-magnetic environments, while strong magnetic fields inhibit
small-scale perturbations at boundary layers making tails less turbulent.
Magnetic field components aligned with the direction of the flow favour the
formation of pressure-confined flux ropes inside the tails, whilst transverse
components tend to form current sheets. Softening the equation of state to
nearly isothermal leads to suppression of dynamical instabilities and further
collimation of the tail. Towards the final stages of the evolution, we find
that small cloudlets and distorted filaments survive the break-up of the clouds
and become entrained in the winds, reaching velocities ~0.1 of the wind speed.",1510.05356v1
2015-10-23,Soft spin-amplitude fluctuations in a Mott-insulating ruthenate,"Magnetism in transition-metal compounds (TMCs) has traditionally been
associated with spin degrees of freedom, because the orbital magnetic moments
are typically largely quenched. On the other hand, magnetic order in 4f- and
5d-electron systems arises from spin and orbital moments that are rigidly tied
together by the large intra-atomic spin-orbit coupling (SOC). Using inelastic
neutron scattering on the archetypal 4d-electron Mott insulator Ca$_2$RuO$_4$,
we report a novel form of excitonic magnetism in the intermediate-strength
regime of the SOC. The magnetic order is characterized by ``soft'' magnetic
moments with large amplitude fluctuations manifested by an intense, low-energy
excitonic mode analogous to the Higgs mode in particle physics. This mode
heralds a proximate quantum critical point separating the soft magnetic order
driven by the superexchange interaction from a quantum-paramagnetic state
driven by the SOC. We further show that this quantum critical point can be
tuned by lattice distortions, and hence may be accessible in epitaxial
thin-film structures. The unconventional spin-orbital-lattice dynamics in
Ca$_2$RuO$_4$ identifies the SOC as a novel source of quantum criticality in
TMCs.",1510.07011v1
2015-11-07,Do magnetic fields enhance turbulence at low magnetic Reynolds number ?,"Imposing a magnetic field on a turbulent flow of electrically conducting
fluid incurs the Joule effect. A current paradigm is that the corresponding
dissipation increases with the intensity of the magnetic field, and as a result
turbulent fluctuations are all the more damped as the magnetic field is strong.
While this idea finds apparent support in the phenomenology of decaying
turbulence, measurements of turbulence in duct flows and other, more complex
configurations have produced seemingly contradicting results. The root of the
controversy is that magnetic fields promote sufficient scale-dependent
anisotropy to profoundly reorganise the structure of turbulence, so their net
effect cannot be understood in terms of the additional dissipation only. Here
we show that when turbulence is forced in a magnetic field that acts on
turbulence itself rather than on the mechanisms that generate it, the field
promotes large, nearly 2D structures capturing sufficient energy to offset the
loss due to Joule dissipation, with the net effect of increasing the intensity
of turbulent fluctuations. This change of paradigm potentially carries
important consequences for systems as diverse as the liquid cores of planets,
accretion disks and a wide range of metallurgical and nuclear engineering
applications.",1511.02364v4
2015-12-04,Observations of a Series of Flares and Associated Jet-like Eruptions Driven by the Emergence of Twisted Magnetic Fields,"We studied temporal changes of morphological and magnetic properties of a
succession of four confined flares followed by an eruptive flare using the
high-resolution New Solar Telescope (NST) operating at the Big Bear Solar
Observatory (BBSO), Helioseismic and Magnetic Imager (HMI) magnetograms and
Atmospheric Image Assembly (AIA) EUV images provided by Solar Dynamics
Observatory (SDO). From the NST/Halpha and the SDO/AIA~304 A observations we
found that each flare developed a jet structure that evolved in a manner
similar to evolution of the blowout jet : 1) an inverted-Y shape jet appeared
and drifted away from its initial position; 2) jets formed a curtain-like
structure that consisted of many fine threads accompanied with subsequent
brightenings near the footpoints of the fine threads; and finally 3) the jet
showed a twisted structure visible near the flare maximum. Analysis of the HMI
data showed that both the negative magnetic flux and the magnetic helicity have
been gradually increasing in the positive polarity region indicating the
continuous injection of magnetic twist before and during the series of flares.
Based on these results, we suggest that the continuous emergence of twisted
magnetic flux played an important role in producing a successive flares and
developing a series of blowout jets.",1512.01330v1
2015-12-05,Kinetic theory of spin-polarized systems in electric and magnetic fields with spin-orbit coupling: II. RPA response functions and collective modes,"The spin and density response functions in the random phase approximation
(RPA) are derived by linearizing the kinetic equation including a magnetic
field, the spin-orbit coupling, and mean fields with respect to an external
electric field. Different polarization functions appear describing various
precession motions showing Rabi satellites due to an effective Zeeman field.
The latter turns out to consist of the mean-field magnetization, the magnetic
field, and the spin-orbit vector. The collective modes for charged and neutral
systems are derived and a threefold splitting of the spin waves dependent on
the polarization and spin-orbit coupling is shown. The dielectric function
including spin-orbit coupling, polarization and magnetic fields is presented
analytically for long wave lengths and in the static limit. The dynamical
screening length as well as the long-wavelength dielectric function shows an
instability in charge modes, which are interpreted as spin segregation and
domain formation. The spin response describes a crossover from damped
oscillatory behavior to exponentially damped behavior dependent on the
polarization and collision frequency. The magnetic field causes ellipsoidal
trajectories of the spin response to an external electric field and the
spin-orbit coupling causes a rotation of the spin axes. The spin-dephasing
times are extracted and discussed in dependence on the polarization, magnetic
field, spin-orbit coupling and single-particle relaxation times.",1512.01661v1
2015-11-25,Magneto-optical extinction trend inversion in ferrofluids,"Effects of pulse magnetic field on the optical transmission properties of
thin ferrofluid (FF) layers were experimentally investigated. It was observed
that, under an influence of an external uniform magnetic field, pulses applied
to the samples surfaces in normal direction decrease the optical transmission
with further returning it to its original state, even before the end of the
field pulse. The dependencies of the observed effects on the magnetic pulse
magnitude and the samples thickness were investigated. The experimental results
are explained using FF columnar aggregates growth and lateral coalescence under
influence of a magnetic field, leading to a light scattering type
Rayleigh-to-Mie transition. Further evolution of this process comes to a
geometrical optics scale and respective macroscopic observable opaque FF
columnar aggregates emergence. These changes of optical transmission are
non-monotonic during the magnetic field pulse duration with minimal value in
the case of Mie scattering, which is known as a magneto-optical extinction
trend inversion. The residual inversion was detected after the external
magnetic field pulse falling edge. Using molecular dynamics simulation, we
showed that a homogeneous external magnetic field is enough for the formation
of columnar aggregates and their fusion. The results clarify the known Li
theory (Li et al., J. Phys. D: Appl. Phys. 37 (2004) 3357, and Sci. Technol.
Adv. Mate. 8 (2007) 448), implying an inhomogeneous field as a required
prerequisite for the magneto-optical extinction trend inversion phenomenon.",1512.04572v3
2015-12-15,Local Magnetism and Spin Dynamics of the Frustrated Honeycomb Rhodate Li2RhO3,"We report magnetization, heat capacity, 7Li - nuclear magnetic resonance
(NMR), and muSR (muon spin roration) measurements on the honeycomb 4d5 spin
liquid candidate Li2RhO3. The magnetization in small magnetic fields provides
evidence of the partial spin-freezing of a small fraction of Rh4+ -moments at 6
K, whereas the Curie-Weiss behavior above 100 K suggests a pseudo-spin-1/2
paramagnet with a moment of about 2.2 muB. The magnetic specific heat (Cm)
exhibits no field dependence and demonstrates the absence of long range
magnetic order down to 0.35 K. Cm/T passes through a broad maximum at about 10
K and Cm=T^2 at low temperatures. Measurements of the spin-lattice relaxation
rate (1/T1) reveal a gapless slowing down of spin fluctuations upon cooling
with 1/T1=T^2.2. The results from NMR and muSR are consistent with a scenario
in which a minority of Rh4+ moments are in a ahort-range correlated frozen
state and coexist with a majority of moments in a liquid-like state that
continue to fluctuate at low temperatures.",1512.04904v4
2016-02-09,Measuring nanoscale magnetic write head fields using a hybrid quantum register,"The generation and control of nanoscale magnetic fields are of fundamental
interest in material science and a wide range of applications. Nanoscale
magnetic resonance imaging quantum spintronics for example require single spin
control with high precision and nanoscale spatial resolution using fast
switchable magnetic fields with large gradients. Yet, characterizing those
fields on nanometer length scales at high band width with arbitrary orientation
has not been possible so far. Here we demonstrate single electron and nuclear
spin coherent control using the magnetic field of a hard disc drive write head.
We use single electron spins for measuring fields with high spatial resolution
and single nuclear spins for large band width measurements. We are able to
derive field profiles from coherent spin Rabi oscillations close to GHz in
fields with gradients of up to 10 mT/nm and measure all components of a static
and dynamic magnetic field independent of its orientation. Our method paves the
way for precision measurement of the magnetic fields of nanoscale write heads
important for future miniaturization of the devices.",1602.02948v1
2016-02-26,"Spin dynamics, electronic and thermal transport properties of two-dimensional CrPS4 single crystal","2-Dimensional (2D) CrPS4 single crystals have been grown by the chemical
vapor transport method. The crystallographic, magnetic, electronic and thermal
transport properties of the single crystals were investigated by the
room-temperature X-ray diffraction, electrical resistivity \r{ho}(T), specific
heat CP(T) and the electronic spin response (ESR) measurements. CrPS4 crystals
crystallize into a monoclinic structure. The electrical resistivity \r{ho}(T)
shows a semiconducting behavior with an energy gap Ea=0.166 eV. The
antiferromagntic (AFM) transition temperature is about TN=36 K. The spin
flipping induced by the applied magnetic field is observed along the c axis.
The magnetic phase diagram of CrPS4 single crystal has been discussed. The
extracted magnetic entropy at TN is about 10.8 J/mol K, which is consistent
with the theoretical value R ln(2S + 1) for S = 3/2 of the Cr3+ ion. Based on
the mean-field theory, the magnetic exchange constants J1 and Jc corresponding
to the interactions of the intralayer and between layers are about 0.143 meV
and -0.955 meV are obtained based on the fitting of the susceptibility above
TN, which agree with the results obtained from the ESR measurements. With the
help of the strain for tuning the magnetic properties, monolayer CrPS4 may be a
promising candidate to explore 2D magnetic semiconductors.",1602.08174v1
2016-03-15,Kinetic simulations of the lowest-order unstable mode of relativistic magnetostatic equilibria,"We present the results of particle-in-cell numerical pair plasma simulations
of relativistic 2D magnetostatic equilibria known as the 'ABC' fields. In
particular, we focus on the lowest-order unstable configuration consisting of
two minima and two maxima of the magnetic vector potential. Breaking of the
initial symmetry leads to exponential growth of the electric energy and to the
formation of two current layers, which is consistent with the picture of
'X-point collapse' first described by Syrovatskii. Magnetic reconnection within
the layers heats a fraction of particles to very high energies. After the
saturation of the linear instability, the current layers are disrupted and the
system evolves chaotically, diffusing the particle energies in a stochastic
second-order Fermi process leading to the formation of power-law energy
distributions. The power-law slopes harden with the increasing mean
magnetization, but they are significantly softer than those produced in
simulations initiated from Harris-type layers. The maximum particle energy is
proportional to the mean magnetization, which is attributed partly to the
increase of the effective electric field and partly to the increase of the
acceleration time scale. We describe in detail the evolving structure of the
dynamical current layers, and report on the conservation of magnetic helicity.
These results can be applied to highly magnetized astrophysical environments,
where ideal plasma instabilities trigger rapid magnetic dissipation with
efficient particle acceleration and flares of high-energy radiation.",1603.04850v1
2016-04-08,Binary neutron star mergers: a jet engine for short gamma-ray bursts,"We perform magnetohydrodynamic simulations in full general relativity (GRMHD)
of quasi-circular, equal-mass, binary neutron stars that undergo merger. The
initial stars are irrotational, $n=1$ polytropes and are magnetized. We explore
two types of magnetic-field geometries: one where each star is endowed with a
dipole magnetic field extending from the interior into the exterior, as in a
pulsar, and the other where the dipole field is initially confined to the
interior. In both cases the adopted magnetic fields are initially dynamically
unimportant. The merger outcome is a hypermassive neutron star that undergoes
delayed collapse to a black hole (spin parameter $a/M_{\rm BH} \sim 0.74$)
immersed in a magnetized accretion disk. About $4000M \sim 60(M_{\rm
NS}/1.625M_\odot)$ ms following merger, the region above the black hole poles
becomes strongly magnetized, and a collimated, mildly relativistic outflow ---
an incipient jet --- is launched. The lifetime of the accretion disk, which
likely equals the lifetime of the jet, is $\Delta t \sim 0.1 (M_{\rm
NS}/1.625M_\odot)$ s. In contrast to black hole--neutron star mergers, we find
that incipient jets are launched even when the initial magnetic field is
confined to the interior of the stars.",1604.02455v2
2016-07-13,Sunspot Rotation as a Driver of Major Solar Eruptions in NOAA Active Region 12158,"We studied the developing conditions of sigmoid structure under the influence
of magnetic non-potential characteristics of a rotating sunspot in the active
region (AR) 12158. Vector magnetic field measurements from Helioseismic
Magnetic Imager and coronal EUV observations from Atmospheric Imaging Assembly
reveal that the erupting inverse-S sigmoid had roots in the location of the
rotating sunspot. Sunspot rotates at a rate of 0-5deg/h with increasing trend
in the first half followed by a decrease. Time evolution of many non-potential
parameters had a well correspondence with the sunspot rotation. The evolution
of the AR magnetic structure is approximated by a time series of force free
equilibria. The NLFFF magnetic structure around the sunspot manifests the
observed sigmoid structure. Field lines from the sunspot periphery constitute
the body of the sigmoid and those from interior overly the sigmoid similar to a
fluxrope structure. While the sunspot is being rotating, two major CME
eruptions occurred in the AR. During the first (second) event, the coronal
current concentrations enhanced (degraded) consistent with the photospheric net
vertical current, however the magnetic energy is released during both the
cases. The analysis results suggest that the magnetic connections of the
sigmoid are driven by slow motion of sunspot rotation, which transforms to a
highly twisted flux rope structure in a dynamical scenario. An exceeding
critical twist in the flux rope probably leads to the loss of equilibrium and
thus triggering the onset of two eruptions.",1607.03806v1
2016-08-13,Detection and optical imaging of induced convection under the action of static gradient magnetic field in a non-conducting diamagnetic fluid,"The report elaborates for the first time visual observations of induced
convections in a non-conducting diamagnetic fluid under the action of static
gradient magnetic field in the absence of thermal gradients and the techniques
employed to observe and record them. Suspension of Deionized (DI) and double
distilled water and Lycopodium pollen grains was used as the fluid in a test
tube. Permanent magnets having field strength of 0.12T each were used to
provide the static gradient magnetic field. The suspension filled test tube was
kept in a room temperature water bath. The convections were visually observed
and recorded using travelling microscope attached with a web camera. Various
geometrical configurations of magnets in the vicinity of test tube provided
different types of magnetic gradient shapes. These gradients were responsible
for the occurrence of different types of orientations in the convective flows
in the test tube. Convections were observed over a range of fluid volumes from
0.2ml-10ml. The experimentally observed results provide proof of concept that
irrespective of the weak interactions of diamagnetic fluids with magnetic
fields, these effects can be easily observed and recorded with the use of low
tech laboratory equipments.",1608.03973v2
2016-08-18,Turbulent dynamo in a conducting fluid and partially ionized gas,"By following the Kazantsev theory and taking into account both microscopic
and turbulent diffusion of magnetic fields, we develop a unified treatment of
the kinematic and nonlinear stages of turbulent dynamo, and study the dynamo
process for a full range of magnetic Prandtl number Pm and ionization
fractions. We find a striking similarity between the dependence of dynamo
behavior on Pm in a conducting fluid and R (a function of ionization fraction)
in partially ionized gas. In a weakly ionized medium, the kinematic stage is
largely extended, including not only exponential growth but a new regime of
dynamo characterized by linear-in-time growth of magnetic field strength, and
the resulting magnetic energy is much higher than the kinetic energy carried by
viscous-scale eddies. Unlike the kinematic stage, the subsequent nonlinear
stage is unaffected by microscopic diffusion processes and has a universal
linear-in-time growth of magnetic energy with the growth rate as a constant
fraction $3/38$ of the turbulent energy transfer rate, showing good agreement
with earlier numerical results. Applying the analysis to the first stars and
galaxies, we find that the kinematic stage is able to generate a field strength
only an order of magnitude smaller than the final saturation value. But the
generation of large-scale magnetic fields can only be accounted for by the
relatively inefficient nonlinear stage and requires longer time than the
free-fall time. It suggests that magnetic fields may not have played a
dynamically important role during the formation of the first stars.",1608.05161v2
2016-09-15,Crossover from magnetostatic to exchange coupling in La0.67Ca0.33MnO3/YBa2Cu3O7/La0.67Ca0.33MnO3 heterostructures,"The influence of YBa2Cu3O4 (YBCO) superconductor layer (S-layer) with varying
thickness d-YBCO = 20 to 50 nm on the magnetic coupling between two
La0.67Ca0.33MnO3 (LCMO) ferromagnet layers (F-layer, thickness d-LCMO = 50 nm)
in F/S/F heterostructures (HSs) was investigated by measuring global
magnetization (M) in a temperature (T) range = 2 - 300 K and magnetic field (H)
range = 0 - 10 kOe. All the HSs were superconducting with critical temperature
(Tc) decreasing from = 78 to 36 K with decrease in d-YBCO, whereas the
ferromagnetic ordering temperature Tm = 250 K did not change much.
Systematically measured M-H loops of all HSs at both T > Tc and T < Tc show
three main results- (a) the two step magnetic reversal above Tc converts into a
four step reversal below Tc in HSs with d-YBCO >= 30 nm, (b) the magnetic field
corresponding to the additional two switching steps and their magnitude show
characteristic evolution with T and d-YBCO and (c) the HS with d-YBCO = 20 nm
shows radically different behaviour, where the two step magnetic reversal above
Tc continues to persist below Tc and converts into a single step reversal at T
<< Tc. The first two results indicate magnetostatic coupling between the
magnetic domains and the vortices across the two F/S interfaces resulting in
reversal dynamics different from that deep within the LCMO layers. Whereas, the
result c reveals indirect exchange coupling between LCMO layers through the
superconducting YBCO layer, which is a clear experimental evidence of
coexistence of ferromagnetism and superconductivity in nm scale F/S/F HSs
expected theoretically by C.A.R. Sa de Melo (Physica C 387, 17-25 (2003)).",1609.04519v1
2016-10-17,Vector magnetic field measurements along a cooled stereo-imaged coronal loop,"The variation of the vector magnetic field along structures in the solar
corona remains unmeasured. Using a unique combination of spectropolarimetry and
stereoscopy, we infer and compare the vector magnetic field structure and
three-dimensional morphology of an individuated coronal loop structure
undergoing a thermal instability. We analyze spectropolarimetric data of the He
I 10830 {\AA} triplet ($1s2s{\ }^{3}S_{1} - 1s2p{\ }^{3}P_{2,1,0}$) obtained at
the Dunn Solar Telescope with the Facility Infrared Spectropolarimeter on 19
September 2011. Cool coronal loops are identified by their prominent drainage
signatures in the He I data (redshifts up to 185 km sec$^{-1}$). Extinction of
EUV background radiation along these loops is observed by both the Atmospheric
Imaging Assembly onboard the Solar Dynamics Observatory and the Extreme
Ultraviolet Imager onboard spacecraft A of the Solar Terrestrial Relations
Observatory, and is used to stereoscopically triangulate the loop geometry up
to heights of 70 Mm ($0.1$ $R_{sun}$) above the solar surface. The He I
polarized spectra along this loop exhibit signatures indicative of atomic-level
polarization as well as magnetic signatures through the Hanle and Zeeman
effects. Spectropolarimetric inversions indicate that the magnetic field is
generally oriented along the coronal loop axis, and provide the height
dependence of the magnetic field intensity. The technique we demonstrate is a
powerful one that may help better understand the thermodynamics of coronal fine
structure magnetism.",1610.05332v1
2017-03-22,Magnetization induced dynamics of a Josephson junction coupled to a nanomagnet,"We study the superconducting current of a Josephson junction (JJ) coupled to
an external nanomagnet driven by a time dependent magnetic field both without
and in the presence of an external AC drive. We provide an analytic, albeit
perturbative, solution for the Landau-Lifshitz (LL) equations governing the
coupled JJ-nanomagnet system in the presence of a magnetic field with arbitrary
time-dependence oriented along the easy axis of the nanomagnet's magnetization
and in the limit of weak dimensionless coupling $\epsilon_0$ between the JJ and
the nanomagnet. We show the existence of Shapiro-like steps in the I-V
characteristics of the JJ subjected to a voltage bias for a constant or
periodically varying magnetic field and explore the effect of rotation of the
magnetic field and the presence of an external AC drive on these steps. We
support our analytic results with exact numerical solution of the LL equations.
We also extend our results to dissipative nanomagnets by providing a
perturbative solution to the Landau-Lifshitz-Gilbert (LLG) equations for weak
dissipation. We study the fate of magnetization-induced Shapiro steps in the
presence of dissipation both from our analytical results and via numerical
solution of the coupled LLG equations. We discuss experiments which can test
our theory.",1703.07717v3
2017-03-26,Sausage Instabilities on Top of Kinking Lengthening Current-Carrying Magnetic Flux Tubes,"We theoretically explore the possibility of sausage instabilities developing
on top of a kink instability in lengthening current-carrying magnetic flux
tubes. Observations indicate that the dynamics of magnetic flux tubes in our
cosmos and terrestrial experiments can involve topological changes faster than
time scales predicted by resistive magnetohydrodynamics. Recent laboratory
experiments suggest that hierarchies of instabilities, such as kink and
Rayleigh-Taylor, could be responsible for initiating fast topological changes
by locally accessing two fluid and kinetic regimes. Sausage instabilities can
also provide this coupling mechanism between disparate scales. Flux tube
experiments can be classified by the flux tube's evolution in a configuration
space described by a normalized inverse aspect-ratio $\bar{k}$ and
current-to-magnetic flux ratio $\bar{\lambda}$. A lengthening current-carrying
magnetic flux tube traverses this $\bar{k}$ - $\bar{\lambda}$ space and crosses
stability boundaries. We derive a single general criterion for the onset of the
sausage and kink instabilities in idealized magnetic flux tubes with core and
skin currents. The criterion indicates a dependence of the stability boundaries
on current profiles and shows overlapping kink and sausage unstable regions in
the $\bar{k}$ - $\bar{\lambda}$ space with two free parameters. Numerical
investigation of the stability criterion reduces the number of free parameters
to a single one that describes the current profile, and confirms the
overlapping sausage and kink unstable regions in $\bar{k}$ - $\bar{\lambda}$
space. A lengthening, ideal current-carrying magnetic flux tube can therefore
become sausage unstable after it becomes kink unstable.",1703.08771v2
2017-04-24,Magnetic nulls and super-radial expansion in the solar corona,"Magnetic fields in the sun's outer atmosphere -- the corona -- control both
solar-wind acceleration and the dynamics of solar eruptions. We present the
first clear observational evidence of coronal magnetic nulls in off-limb
linearly polarized observations of pseudostreamers, taken by the Coronal
Multichannel Polarimeter (CoMP) telescope. These nulls represent regions where
magnetic reconnection is likely to act as a catalyst for solar activity. CoMP
linear-polarization observations also provide an independent, coronal proxy for
magnetic expansion into the solar wind, a quantity often used to parameterize
and predict the solar wind speed at Earth. We introduce a new method for
explicitly calculating expansion factors from CoMP coronal linear-polarization
observations, which does not require photospheric extrapolations. We conclude
that linearly-polarized light is a powerful new diagnostic of critical coronal
magnetic topologies and the expanding magnetic flux tubes that channel the
solar wind.",1704.07470v2
2017-05-17,The Properties of Reconnection Current Sheets in GRMHD Simulations of Radiatively Inefficient Accretion Flows,"Non-ideal MHD effects may play a significant role in determining the
dynamics, thermal properties, and observational signatures of radiatively
inefficient accretion flows onto black holes. In particular, particle
acceleration during magnetic reconnection events may influence black hole
spectra and flaring properties. We use representative GRMHD simulations of
black hole accretion flows to identify and explore the structures and
properties of current sheets as potential sites of magnetic reconnection. In
the case of standard and normal (SANE) disks, we find that, in the reconnection
sites, the plasma beta ranges from $0.1$ to $1000$, the magnetization ranges
from $10^{-4}$ to $1$, and the guide fields are weak compared to the
reconnecting fields. In magnetically arrested (MAD) disks, we find typical
values for plasma beta from $10^{-2}$ to $10^3$, magnetizations from $10^{-3}$
to $10$, and typically stronger guide fields, with strengths comparable to or
greater than the reconnecting fields. These are critical parameters that govern
the electron energy distribution resulting from magnetic reconnection and can
be used in the context of plasma simulations to provide microphysics inputs to
global simulations. We also find that ample magnetic energy is available in the
reconnection regions to power the fluence of bright X-ray flares observed from
the black hole in the center of the Milky Way.",1705.06293v1
2017-05-17,Magnetic tension and instabilities in the Orion A integral shaped filament,"The Orion nebula is a prime example of a massive star-forming region in our
galaxy. Observations have shown that gravitational and magnetic energy are
comparable in its integral shaped filament (ISF) on a scale of ~1 pc, and that
the population of pre-main sequence stars appears dynamically heated compared
to the protostars. These results have been attributed to a slingshot mechanism
resulting from the oscillation of the filament (Stutz & Gould 2016). In this
paper, we show that radially contracting filaments naturally evolve toward a
state where gravitational, magnetic, and rotational energy are comparable.
While the contraction of the filament will preferentially amplify the axial
component of the magnetic field, the presence of rotation leads to a helical
field structure. We show how magnetic tension can give rise to a filament
oscillation, and estimate a typical timescale of 0.7 million years for the
motion of the filament to the position of maximum displacement, consistent with
the characteristic timescale of the ejected stars. Furthermore, the presence of
helical magnetic fields is expected to give rise to magneto-hydrodynamical
instabilities. We show here that the presence of a magnetic field significantly
enhances the overall instability, which operates on a characteristic scale of
about 1 pc. We expect the physics discussed here to be generally relevant in
massive star forming regions, and encourage further investigations in the
future.",1705.06302v2
2017-06-19,Spin order and dynamics in the diamond-lattice Heisenberg antiferromagnets CuRh2O4 and CoRh2O4,"Antiferromagnetic insulators on the diamond lattice are candidate materials
to host exotic magnetic phenomena ranging from spin-orbital entanglement to
degenerate spiral ground-states and topological paramagnetism. Compared to
other three-dimensional networks of magnetic ions, such as the geometrically
frustrated pyrochlore lattice, the investigation of diamond-lattice magnetism
in real materials is less mature. In this work, we characterize the magnetic
properties of model A-site spinels CoRh2O4 (cobalt rhodite) and CuRh2O4 (copper
rhodite) by means of thermo-magnetic and neutron scattering measurements and
perform group theory analysis, Rietveld refinement, mean-field theory, and spin
wave theory calculations to analyze the experimental results. Our investigation
reveals that cubic CoRh2O4 is a canonical S=3/2 diamond-lattice Heisenberg
antiferromagnet with a nearest neighbor exchange J = 0.63 meV and a Neel
ordered ground-state below a temperature of 25 K. In tetragonally distorted
CuRh2O4, competiting exchange interactions between up to third nearest-neighbor
spins lead to the development of an incommensurate spin helix at 24 K with a
magnetic propagation vector k = (0,0,0.79). Strong reduction of the ordered
moment is observed for the S=1/2 spins in CuRh2O4 and captured by our 1/S
corrections to the staggered magnetization. Our work identifies CoRh2O4 and
CuRh2O4 as reference materials to guide future work searching for exotic
quantum behavior in diamond-lattice antiferromagnets.",1706.05881v1
2017-08-14,Solar Eruption and Local Magnetic Parameters,"It is now a common practice to use local magnetic parameters such as magnetic
decay index for explaining solar eruptions from active regions, but there can
be an alternative view that the global properties of the source region should
be counted as a more important factor. We discuss this issue based on {\it
Solar Dynamics Observatory} (SDO) observations of the three successive
eruptions within 1.5 hours from the NOAA active region 11444 and the magnetic
parameters calculated using the nonlinear force-free field (NLFFF) model. Two
violent eruptions occurred in the regions with relatively high magnetic twist
number (0.5--1.5) and high decay index (0.9--1.1) at the nominal height of the
filament (12$''$) and otherwise a mild eruption occurred, which supports the
local parameter paradigm. Our main point is that the time sequence of the
eruptions did not go with these parameters. It is argued that an additional
factor, in the form of stabilizing force, should operate to determine the onset
of the first eruption and temporal behaviors of subsequent eruptions. As
supporting evidence, we report that the heating and fast plasma flow continuing
for a timescale of an hour was the direct cause for the first eruption, and
that the unidirectional propagation of the disturbance determined the timing of
subsequent eruptions. Both of these factors are associated with the overall
magnetic structure rather than local magnetic properties of the active region.",1708.04055v1
2017-09-18,Cobalt substitution induced magnetodielectric enhancement in multiferroic Bi2Fe4O9,"Antiferromagnetic Bi_{2}Fe_{4}O_{9} (BFO), lightly substituted by cobalt is
studied for magnetodielectricity. The substitution causes a substantial
decrease in the Neel temperature (T_N) from 250 K (in parent sample, BFO) to
152 K (in 2% Co substituted sample). At the same time, the substituted samples
display a pronounced irreversibility in the ZFC-FC magnetization data for T $<$
370 K and opening of hysteresis in the M-H plot, thus signifying the onset of
weak ferromagnetism (FM) and magnetic glassiness. The induced magnetic
glassiness is found to slow down the dynamics such that the magnetization decay
follows. The dielectric measurement in the same temperature window shows
unusual oppression in, for T$\sim$T_N and contrasting nature of tan loss for
temperatures above and below T_N, thus hinting a plausible coupling between the
magnetic and electric order parameters. A confirmation to this coupling is seen
in the magnetodielectric (MD) results, in which it is found that the
substitution induces an additional component in the MD, apart from the usual
components in BFO. This additional component of MD is found to obey behaviour,
with the n values being comparable to 1-p of magnetization. The temperature
variation of MD also shows a contrasting behaviour for the parent and 2% Co
substituted sample with an enhancement of two times in MD value. In summary,
our study shows ME coupling introduced by the magnetic glassiness and its
behaviour is very much different from the intrinsic one.",1709.05854v1
2017-11-24,Magneto-Rayleigh-Taylor instability driven by a rotating magnetic field: Cylindrical liner configuration,"We propose using a directional time-varying (rotating) driving magnetic field
to suppress magneto-Rayleigh-Taylor (MRT) instability in dynamic Z-pinches. A
rotational drive magnetic field is equivalent to two magnetic-field components,
{\Theta} and Z, that alternate in time, referred to as an alternate
Theta-Z-pinch configuration. We consider the finitely thick cylindrical liner
configuration in this paper. We numerically integrate the perturbation equation
to stagnation time based on the optimal background unperturbed trajectories. We
assess the cumulative growth of the dominant mode selected by some mechanism at
the beginning of an implosion. The maximum e-folding number at stagnation of
the dominant mode of an optimized alternate Theta-Z-pinch is significantly
lower than that of the standard Theta- or Z-pinch. The directional rotation of
the magnetic field contributes to suppress the instabilities, independent of
the finite thickness. The finite thickness effect plays a role only when the
orientation of the magnetic field varies in time whereas it does not appear in
the standard Theta- or Z-pinch. The rotating frequency of the magnetic field
and the thickness of liner are both having a monotonic effect on suppression.
Their synergistic effect can enhance the suppression on MRT instability.
Because the MRT instability can be well suppressed in this way, the alternate
Theta-Z-pinch configuration has potential applications in liner inertial
fusion. This work is supported by the NSFC (Grant Nos. 11405167, 51407171,
11571293, 11605188, and 11605189) and the Foundation of the China Academy of
Engineering Physics (No. 2015B0201023).",1711.08891v2
2018-01-22,Role of quantum fluctuations on spin liquids and ordered phases in the Heisenberg model on the honeycomb lattice,"Motivated by the rich physics of honeycomb magnetic materials, we obtain the
phase diagram and analyze magnetic properties of the spin-1/2 and spin-1
J1-J2-J3 Heisenberg model on the honeycomb lattice. Based on the SU(2) and
SU(3) symmetry representations of the Schwinger boson approach, which treats
disordered spin liquids and magnetically ordered phases on an equal footing, we
obtain the complete phase diagrams in the (J2,J3)plane. This is achieved using
a fully unrestricted approach which does not assume any pre-defined Ansatze.
For S=1/2, we find a quantum spin liquid (QSL) stabilized between the N\'eel,
spiral and collinear antiferromagnetic phases in agreement with previous
theoretical work. However, by increasing S from 1/2 to 1, the QSL is quickly
destroyed due to the weakening of quantum fluctuations indicating that the
model already behaves as a quasi-classical system. The dynamical structure
factors and temperature dependence of the magnetic susceptibility are obtained
in order to characterize all phases in the phase diagrams. Moreover, motivated
by the relevance of the single-ion anisotropy, D, to various S=1 honeycomb
compounds, we have analyzed the destruction of magnetic order based on a SU(3)
representation of the Schwinger bosons. Our analysis provides a unified
understanding of the magnetic properties of honeycomb materials realizing the
J1-J2-J3 Heisenberg model from the strong quantum spin regime at S=1/2 to the
S=1 case. Neutron scattering and magnetic susceptibility experiments can be
used to test the destruction of the QSL phase when replacing S=1/2 by S=1
localized moments in certain honeycomb compounds.",1801.07042v1
2018-01-23,A Statistical Comparison between Photospheric Vector Magnetograms Obtained by SDO/HMI and Hinode/SP,"Since May 1, 2010, we have been able to study (almost) continuously the
vector magnetic field in the Sun, thanks to two space-based observatories: the
Solar Dynamics Observatory (SDO) and Hinode. Both are equipped with instruments
able to measure the Stokes parameters of Zeeman-induced polarization of
photospheric line radiation. But the observation modes, the spectral lines, the
spatial, spectral and temporal sampling, and even the inversion codes used to
recover magnetic and thermodynamic information from the Stokes profiles are
different. We compare the vector magnetic fields derived from observations with
the HMI instrument on board SDO, with those observed by the SP instrument on
Hinode. We have obtained relationships between components of magnetic vectors
in the umbra, penumbra and plage observed in 14 maps of NOAA AR 11084.
Importantly, we have transformed SP data into observables comparable to those
of HMI, to explore possible influences of the different modes of operation of
the two instruments, and the inversion schemes used to infer the magnetic
fields. The assumed filling factor (fraction of each pixel containing a Zeeman
signature) produces the most significant differences in derived magnetic
properties, especially in the plage. The spectral and angular samplings have
the next largest effects. We suggest to treat the disambiguation in the same
way in the data provided by HMI and SP. That would make the relationship
between the vector magnetic field recovered from these data stronger, what
would favor the simultaneous or complementary use of both instruments.",1801.07374v1
2018-01-23,"Similarity solution for the flow behind a magnetogasdynamic exponential shock wave in a perfect gas with varying density, heat conduction and radiation heat flux","Similarity solutions are obtained for one dimensional, unsteady, adiabatic
propagation of an exponential shock wave in a perfect gas with heat conduction
and radiation heat flux, in the presence of azimuthal magnetic field. The shock
wave is driven out by a piston moving with time according to an exponential
law. The equilibrium flow conditions are maintained. The heat conduction is
expressed in terms of Fourier's law and the radiation is considered to be of
the diffusion type for an optically thick grey gas model. The thermal
conductivity and the absorption coefficient are assumed to vary with
temperature and density according to power law. The density and magnetic field
ahead of the shock front, are assumed to vary as an exponential law. The
effects of the variation of the strength of ambient magnetic field, heat
transfer parameters, adiabatic exponent, ambient density variation index on the
shock strength, the distance between the piston and the shock front, and on the
flow variables are studied out in detail. The similarity solution exists only
when the sum of shock radius and ambient magnetic field exponent is equal to
the half of the ambient density exponent. It is manifested that the shock
strength decreases by increasing the strength of ambient magnetic field but it
is independent from the heat transfer parameters. The total energy of the flow
field behind the shock front is not constant but varies as power of shock
radius. The compressibility of the medium is increased in the non-magnetic
field. Also, the presence of the magnetic field have significant effects on the
shock wave.",1801.07697v1
2018-02-09,"Relations between variability of the photospheric and interplanetary magnetic fields, solar wind and geomagnetic characteristics","Large scale solar magnetic field topology has a great influence on the
structure of the corona, heliosphera and geomagnetic perturbations.
  Data obtained over the last three solar cycles have been analysed to reveal
the relationships between the photospheric field measured along the line of
sight by the WSO group at 30 levels of heliolatitudes from -75 to 75 degrees
and the interplanetary magnetic field The main aim of this first paper is to
make a direct comparison between the basic structure and dynamics of the
photospheric magnetic field and components and intensity of the interplanetary
magnetic field % solar wind and geomagnetic parameters without using
theoretical assumptions, models, physical expectations, etc.
  The second paper by Gavryuseva, 2018d presents the raports between different
characteristics of the solar wind at the Earth orbit, and geomagnetic
parameters provided by the OMNI team. % Data obtained over the last three solar
cycles have been analysed % to reveal the relationships % between the
photospheric field measured along the line of sight % by the WSO group % at
heliolatitudes from -75 to 75 degrees averaged over one year % and the
interplanetary magnetic field, different characteristics % of the solar wind at
the Earth orbit, and geomagnetic parameters. % provided by the OMNI team.
  The heliospheric and geomagnetic data are found to be divided into two groups
characterized by their response to variability of the solar magnetic field
latitudinal structures on short and on long time scales.",1802.03135v1
2018-02-13,Inverted hysteresis and negative remanence in a homogeneous antiferromagnet,"Magnetic remanence - found in bar magnets or magnetic storage devices - is
probably the oldest and most ubiquitous phenomenon underpinning technological
applications of magnetism. It is a macroscopic non-equilibrium phenomenon: a
remanent magnetisation appears when a magnetic field is applied to an initially
unmagnetised ferromagnet, and then taken away. Here, we present an inverted
magnetic hysteresis loop in the pyrochlore compound Nd$_2$Hf$_2$O$_7$: the
remanent magnetisation points in a direction opposite to the applied field.
This phenomenon is exquisitely tunable as a function of the protocol in field
and temperature, and it is reproducible as in a quasi-equilibrium setting. We
account for this phenomenon in considerable detail in terms of the properties
of non-equilibrium population of domain walls which exhibit a magnetic moment
between domains of an ordered antiferromagnetic state which itself has zero net
magnetisation. Properties and (non-equilibrium) dynamics of topological defects
play an important role in modern spintronics, and our study adds an instance
where a uniform field couples selectively to domain walls rather than the bulk.",1802.04610v1
2018-03-22,Rotation and magnetism in intermediate mass stars,"Rotation and magnetism are increasingly recognized as important phenomena in
stellar evolution. Surface magnetic fields from a few to $20{,}000\,$G have
been observed and models have suggested that magnetohydrodynamic transport of
angular momentum and chemical composition could explain the peculiar
composition of some stars. Stellar remnants such as white dwarfs have been
observed with fields from a few to more than $10^{9}\,$G. We investigate the
origin of and the evolution, on thermal and nuclear rather than dynamical
time-scales, of an averaged large-scale magnetic field throughout a star's life
and its coupling to stellar rotation. Large-scale magnetic fields sustained
until late stages of stellar evolution with conservation of magnetic flux could
explain the very high fields observed in white dwarfs. We include these effects
in the Cambridge stellar evolution code using three time-dependant
advection-diffusion equations coupled to the structural and composition
equations of stars to model the evolution of angular momentum and the two
components of the magnetic field. We present the evolution in various cases for
a $3\rm\,M_{\odot}$ star from the beginning to the late stages of its life. Our
particular model assumes that turbulent motions, including convection, favour
small-scale field at the expense of large-scale field. As a result the
large-scale field concentrates in radiative zones of the star and so is
exchanged between the core and the envelope of the star as it evolves. The
field is sustained until the end of the asymptotic giant branch, when it
concentrates in the degenerate core.",1803.08270v1
2018-04-30,Field-Free Switching of Perpendicular Magnetic Tunnel Junction via Voltage-Gated Spin Hall Effect for Low-Power Spintronic Memory,"Spin Hall effect (SHE) and voltage-controlled magnetic anisotropy (VCMA) are
two promising methods for low-power electrical manipulation of magnetization.
Recently, magnetic field-free switching of perpendicular magnetization through
SHE has been reported with the aid of an exchange bias from an
antiferromagnetic IrMn layer. In this letter, we experimentally demonstrate
that the IrMn/CoFeB/MgO structure exhibits a VCMA effect of 39 fJ/Vm, which is
comparable to that of the Ta/CoFeB/MgO structure. Magnetization dynamics under
a combination of the SHE and VCMA are modeled and simulated. It is found that,
by applying a voltage of 1.5 V, the critical SHE switching current can be
decreased by 10 times owing to the VCMA effect, leading to low-power
operations. Furthermore, a high-density spintronic memory structure can be
built with multiple magnetic tunnel junctions (MTJs) located on a single IrMn
strip. Through hybrid CMOS/MTJ simulations, we demonstrate that fast-speed
write operations can be achieved with power consumption of only 8.5 fJ/bit.
These findings reveal the possibility to realize high-density and low-power
spintronic memory manipulated by voltage-gated SHE.",1804.11025v1
2018-06-11,Intermediate gapless phase and topological phase transition of Kitaev model in a uniform magnetic field,"We study the Kitaev spin liquid (KSL) in a [001] magnetic field employing the
mean field theory (MFT/MF) in the Majorana fermion representation. The MF
Hamiltonian of the system has the Bogoliubov de-Gennes (BdG) form of a 2D Weyl
superconductor. We discover a robust gapless regime in intermediate magnetic
field for both gapless and gapped AFM KSL with Jx = Jy before the system is
polarized in high magnetic field. A topological phase transition (PT)
connecting two gapless phases with nodal lines takes place at a critical
magnetic field hc1 in this regime. While the nodal lines at hc1 is protected by
the mirror symmetry with Jx = Jy and disappear at Jx 6= Jy with a gap opening
near the critical field, the nodal points at E = 0 can exist at intermediate
fields even without mirror symmetry. We reveal that the phase evolution of the
KSL in the magnetic field is driven by the competition between the magnetic
field and the particle-hole (p-h) asymmetry of the normal state of the BdG
Hamiltonian, which results in the robust intermediate gapless phase for AFM
KSL. For FM KSL, there is no intermediate PT before polarization. The above
phase diagrams are confirmed by dynamical MFT (DMFT) results.",1806.04184v2
2018-08-14,A Survey of Changes in Magnetic Helicity Flux on the Photosphere During Relatively Low Class Flares,"Using the 135-second cadence of the photospheric vector data provided by the
Helioseismic and Magnetic Imager telescope on board the Solar Dynamic
Observatory, we examined the time-evolution of magnetic helicity fluxes across
the photosphere during 16 flares with the energy class lower than M5.0. During
the flare in 4 out of 16 events, we found impulsive changes in the helicity
fluxes. This indicates that even the flare with less energy could be associated
with anomalistic transportation of the magnetic helicity across the
photosphere. Accompanying the impulsive helicity fluxes, the poynting fluxes
across the photosphere evolved from positive to negative. As such, the
transportations of magnetic energy across the photosphere were toward solar
interior during these flares. In each of the 4 events, the impulsive change in
the helicity flux was always mainly contributed by abrupt change in horizontal
velocity field on a sunspot located near the flaring polarity inversion line.
The velocity field on each sunspot shows either an obvious vortex patten or an
shearing patten relative to the another magnetic polarity, which tended to
relax the magnetic twist or shear in the corona. During these flares, abrupt
change in the Lorentz force acting on these sunspots were found. The rotational
motions and shearing motions of these sunspots always had the same directions
with the resultant Lorentz forces. These results support the view that the
impulsive helicity transportation during the flare could be driven by the
change in the Lorentz force applied on the photosphere.",1808.04591v1
2018-08-30,Accelerating the calculation of dipolar interactions in particle based simulations with open boundary conditions by means of the P2NFFT method,"Magnetic gels are soft elastic materials consisting of magnetic particles
embedded in a polymer network. Their shape and elasticity can be controlled by
an external magnetic field, which gives rise to both, engineering and
biomedical applications. Computer simulations are a commonly used tool to study
these materials. A well-known bottleneck of these simulations is the demanding
calculation of dipolar interactions. Under periodic boundary conditions
established algorithms are available for doing this, however, at the expense of
restricting the way in which the gels can deform in an external magnetic field.
Moreover, the magnetic properties depend on the sample shape, ruling out
periodic boundary conditions entirely for some research questions. In this
article we will employ the recently developed dipolar variant of the P$^2$NFFT
method that is able to calculate dipolar interactions under open boundary
conditions with an $N \log N$ scaling in the number of particles, rather than
the expensive $N^2$ scaling of a direct summation of pair forces. The dipolar
P$^2$NFFT method has been implemented within the ScaFaCoS library. The
molecular dynamics software ESPResSo has been extended to make use of the
library.
  After a short summary of the method, we will discuss its value for studying
magnetic soft matter systems. A particular focus is put on developing a tuning
strategy to reach the best performance of the method at a predefined accuracy,
and lastly applying the method to a magnetic gel model. Here, adapting to the
gel's change in shape during the course of a simulation is of particular
interest.",1808.10341v1
2018-09-09,High-speed shear driven dynamos. Part 1. Asymptotic analysis,"Rational large Reynolds number matched asymptotic expansions of
three-dimensional nonlinear magneto-hydrodynamic (MHD) states are concerned.
The nonlinear MHD states, assumed to be predominantly driven by a
unidirectional shear, can be sustained without any linear instability of the
base flow and hence are responsible for subcritical transition to turbulence.
Two classes of nonlinear MHD states are found. The first class of nonlinear
states emerged out of a nice combination of the purely hydrodynamic vortex/wave
interaction theory by Hall \& Smith (1991) and the resonant absorption theories
on Alfv\'en waves, developed in the solar physics community (e.g. Sakurai et
al. 1991; Goossens et al. 1995). Similar to the hydrodynamic theory, the
mechanism of the MHD states can be explained by the successive interaction of
the roll, streak, and wave fields, which are now defined both for the
hydrodynamic and magnetic fields. The derivation of this `vortex/Alfv\'en wave
interaction' state is rather straightforward as the scalings for both of the
hydrodynamic and magnetic fields are identical. It turns out that the leading
order magnetic field of the asymptotic states appears only when a small
external magnetic field is present. However, it does not mean that purely
shear-driven dynamos are not possible. In fact, the second class of
`self-sustained shear driven dynamo theory' shows the magnetic generation that
is slightly smaller size in the absence of any external field. Despite small
size, the magnetic field causes the novel feedback mechanism in the velocity
field through resonant absorption, wherein the magnetic wave becomes more
strongly amplified than the hydrodynamic counterpart.",1809.03853v2
2018-11-26,The overdamped chiral magnetic wave,"About eight years ago it was predicted theoretically that a charged chiral
plasma could support the propagation of the so-called chiral magnetic waves,
which are driven by the anomalous chiral magnetic and chiral separation
effects. This prompted intensive experimental efforts in search of signatures
of such waves in relativistic heavy-ion collisions. In fact, several
experiments have already reported a tentative detection of the predicted
signal, albeit with a significant background contribution. Here, we critically
reanalyze the theoretical foundations for the existence of the chiral magnetic
waves. We find that the commonly used background-field approximation is not
sufficient for treating the waves in hot chiral plasmas in the long-wavelength
limit. Indeed, the back-reaction from dynamically induced electromagnetic
fields turns the chiral magnetic wave into a diffusive mode. While the
situation is slightly better in the strongly-coupled near-critical regime of
quark-gluon plasma created in heavy-ion collisions, the chiral magnetic wave is
still strongly overdamped due to the effects of electrical conductivity and
charge diffusion.",1811.10635v2
2019-01-14,Magnetized interstellar molecular clouds: II. The Large-Scale Structure and Dynamics of Filamentary Molecular Clouds,"We perform ideal MHD high resolution AMR simulations with driven turbulence
and self-gravity and find that long filamentary molecular clouds are formed at
the converging locations of large-scale turbulence flows and the filaments are
bounded by gravity. The magnetic field helps shape and reinforce the long
filamentary structures. The main filamentary cloud has a length of ~4.4 pc.
Instead of a monolithic cylindrical structure, the main cloud is shown to be a
collection of fiber/web-like sub-structures similar to filamentary clouds such
as L1495. Unless the line-of-sight is close to the mean field direction, the
large-scale magnetic field and striations in the simulation are found roughly
perpendicular to the long axis of the main cloud, similar to 1495. This
provides strong support for a large-scale moderately strong magnetic field
surrounding L1495. We find that the projection effect from observations can
lead to incorrect interpretations of the true three-dimensional physical shape,
size, and velocity structure of the clouds. Helical magnetic field structures
found around filamentary clouds that are interpreted from Zeeman observations
can be explained by a simple bending of the magnetic field that pierces through
the cloud. We demonstrate that two dark clouds form a T-shape configuration
which are strikingly similar to the Infrared dark cloud SDC13 leading to the
interpretation that SDC13 results from a collision of two long filamentary
clouds. We show that a moderately strong magnetic field (M_A ~ 1) is crucial
for maintaining a long and slender filamentary cloud for a long period of time
~0.5 million years.",1901.04593v2
2019-02-28,To Heat or not to Heat: a Study of the Performances of Iron Carbide Nanoparticles in Magnetic Heating,"Heating magnetic nanoparticles with high frequency magnetic fields is a topic
of interest for biological applications (magnetic hyperthermia) as well as for
heterogeneous catalysis. This study shows why FeC NPs of similar structures and
static magnetic properties display radically different heating power (SAR from
0 to 2 kW.g-1). By combining results from Transmission Electron Microscopy
(TEM), Dynamic Light Scattering (DLS) and static and time-dependent
high-frequency magnetic measurements, we propose a model describing the heating
mechanism in FeC nanoparticles. Using, for the first time, time-dependent
high-frequency hysteresis loop measurements, it is shown that in the samples
displaying the larger heating powers, the hysteresis is strongly time
dependent. More precisely, the hysteresis area increases by a factor 10 on a
timescale of a few tens of seconds. This effect is directly related to the
ability of the nanoparticles to form chains under magnetic excitation, which
depends on the presence or not of strong dipolar couplings. These differences
are due to different ligand concentrations on the surface of the particles. As
a result, this study allows the design of a scalable synthesis of nanomaterials
displaying a controllable and reproducible SAR.",1902.10965v1
2019-03-11,Vacancy mediated magnetization and healing of a graphene monolayer,"Vacancy-induced magnetization of a graphene layer is investigated by means of
a first principle DFT method. Calculations of the formation energy and the
magnetization by creating the different number of vacancies in a supercell show
that a clustering with big number of vacancies in the cluster is rather
favorable than that of isolated vacancies, homogeneously distributed in the
layer. The magnetic moment of a cluster with big number of vacancies is shown
to be not proportional with the vacancy concentration, which is in good
agreement with the recent experimental results. Our studies support the idea
that although the vacancies in a graphene create a magnetic moment, they do not
produce a magnetic ordering. It is shown that although the Lieb's rule for the
magnetization in a hexagonal structure violates, two-vacancies, including a
di-vacancy, in the supercell generate quasi-localized state when they belong to
the different sublattices, and instead two-vacancies generate an extended state
when they belong to the same sublattices. Analytical investigation of the
dynamics of carbon atom- and vacancy-concentrations according to the non-linear
continuity equations shows that the vacancies, produced by irradiation at the
middle of a graphene layer, migrate to the edge of the sample resulting in a
specific {\it 'segregation'} of the vacancy concentration and self-healing of
the graphene.",1903.04180v1
2019-03-13,Wide-field Magnetic Field and Temperature Imaging using Nanoscale Quantum Sensors,"The simultaneous imaging of magnetic fields and temperature (MT) is important
in a range of applications, including studies of carrier transport, solid-state
material dynamics, and semiconductor device characterization. Techniques exist
for separately measuring temperature (e.g., infrared (IR) microscopy,
micro-Raman spectroscopy, and thermo-reflectance microscopy) and magnetic
fields (e.g., scanning probe magnetic force microscopy and superconducting
quantum interference devices). However, these techniques cannot measure
magnetic fields and temperature simultaneously. Here, we use the exceptional
temperature and magnetic field sensitivity of nitrogen vacancy (NV) spins in
conformally-coated nanodiamonds to realize simultaneous wide-field MT imaging.
Our ""quantum conformally-attached thermo-magnetic"" (Q-CAT) imaging enables (i)
wide-field, high-frame-rate imaging (100 - 1000 Hz); (ii) high sensitivity; and
(iii) compatibility with standard microscopes. We apply this technique to study
the industrially important problem of characterizing multifinger gallium
nitride high-electron-mobility transistors (GaN HEMTs). We spatially and
temporally resolve the electric current distribution and resulting temperature
rise, elucidating functional device behavior at the microscopic level. The
general applicability of Q-CAT imaging serves as an important tool for
understanding complex MT phenomena in material science, device physics, and
related fields.",1903.05717v1
2019-03-25,Self-gravitating magnetised tori around black holes in general relativity,"We investigate stationary, self-gravitating, magnetised disks (or tori)
around black holes. The models are obtained by numerically solving the coupled
system of the Einstein equations and the equations of ideal
general-relativistic magnetohydrodynamics. The mathematical formulation and
numerical aspects of our approach are similar to those reported in previous
works modeling stationary self-gravitating perfect-fluid tori, but the
inclusion of magnetic fields represents a new ingredient. Following previous
studies of purely hydrodynamical configurations, we construct our models
assuming Keplerian rotation in the disks and both spinning and spinless black
holes. We focus on the case of a toroidal distribution of the magnetic field
and build a large set of models corresponding to a wide range of values of the
magnetisation parameter, starting with weakly magnetised disks and ending at
configurations in which the magnetic pressure dominates over the thermal one.
In all our models, the magnetic field affects the equilibrium structure of the
torus mainly due to the magnetic pressure. In particular, an increasing
contribution of the magnetic field shifts the location of the maximum of the
rest-mass density towards inner regions of the disk. The total mass of the
system and the angular momentum are affected by the magnetic field in a complex
way, that depends on the black hole spin and the location of the inner radius
of the disk. The non-linear dynamical stability of the solutions presented in
this paper will be reported elsewhere.",1903.10408v1
2019-03-29,Modulated rotating waves in the magnetized spherical Couette system,"We present a study devoted to a detailed description of modulated rotating
waves (MRW) in the magnetized spherical Couette system. The set-up consists of
a liquid metal confined between two differentially rotating spheres and
subjected to an axially applied magnetic field. When the magnetic field
strength is varied, several branches of MRW are obtained by means of three
dimensional direct numerical simulations (DNS). The MRW originate from parent
branches of rotating waves (RW) and are classified according to Rand's (Arch.
Ration. Mech. Anal 79:1-37, 182) and Coughling & Marcus (J. Fluid Mech.
234:1-18,1992) theoretical description. We have found relatively large
intervals of multistability of MRW at low magnetic field, corresponding to the
radial jet instability known from previous studies. However, at larger magnetic
field, corresponding to the return flow regime, the stability intervals of MRW
are very narrow and thus they are unlikely to be found without detailed
knowledge of their bifurcation point. A careful analysis of the spatio-temporal
symmetries of the most energetic modes involved in the different classes of MRW
will allow in the future a comparison with the HEDGEHOG experiment, a
magnetized spherical Couette device hosted at the Helmholtz-Zentrum
Dresden-Rossendorf.",1904.00056v2
2019-04-03,The impact of magnetic fields on cold streams feeding galaxies,"High redshift, massive halos are observed to have sustained, high star
formation rates, which require that the amount of cold gas in the halo is
continuously replenished. The cooling time scale for the hot virialized halo
gas is too long to provide the source of cold gas. Supersonic, cold streams
have been invoked as a mechanism for feeding massive halos at high redshift and
deliver the cold gas required for continued star formation at the rates
observed. This mechanism for replenishing the cold gas reservoir is motivated
by some cosmological simulations. However, the cold streams are likely to be
subject to the supersonic version of the Kelvin-Helmholtz instability (KHI),
which eventually leads to stream disruption. Cosmological simulations have yet
to obtain the spatial resolution required for understanding the detailed
stability properties of cold streams. In this paper, we consider instead an
idealized model of magnetized cold streams that we spatially resolve. Using
linear theory we show how magnetic fields with dynamically important field
strengths do not inhibit the KHI but rather enhance its growth rate. We perform
nonlinear simulations of magnetized stream disruption and find that magnetic
fields can nevertheless increase stream survival times by suppressing the
mixing rate of cold gas with the circumgalactic medium. We find that magnetic
fields can allow streams to survive $\sim 2-8$ times longer and, consequently,
that streams $\sim 2-8$ times thinner can reach the central galaxy if the
magnetic field strength is $\sim 0.3-0.8 \mu$G.",1904.02167v2
2019-05-22,Ultra-low magnetic damping in Co 2 Mn-based Heusler compounds: promising materials for spintronic,"The prediction of ultra-low magnetic damping in Co 2 MnZ Heusler half-metal
thin-film magnets is explored in this study and the damping response is shown
to be linked to the underlying electronic properties. By substituting the Z
elements in high crystalline quality films (Co 2 MnZ with Z=Si, Ge, Sn, Al, Ga,
Sb), electronic properties such as the minority spin band gap, Fermi energy
position in the gap and spin polarization can be tuned and the consequence on
magnetization dynamics analyzed. The experimental results allow us to directly
explore the interplay of spin polarization, spin gap, Fermi energy position and
the magnetic damping obtained in these films, together with ab initio
calculation predictions. The ultra-low magnetic damping coefficients measured
in the range 4.1 10-4-9 10-4 for Co 2 MnSi, Ge, Sn, Sb are the lowest values
obtained on a conductive layer and offers a clear experimental demonstration of
theoretical predictions on Half-Metal Magnetic Heusler compounds and a pathway
for future materials design.",1905.08987v1
2019-05-29,The effect of internal magnetic field on collective flow in heavy ion collisions at intermediate energies,"The properties of nuclear matter under extreme conditions of high
temperature, density and isospin-asymmetry have attracted wide attentions in
recent years. At present, heavy ion reactions in combination with corresponding
model simulations are one of the most important ways to investigate this
subject. It is known that a strong magnetic field can be created in heavy ion
collisions. However, its effect on the motion of charged particles is usually
neglected in previous transport model simulations. In this work, within the
Ultra-relativistic Quantum Molecular Dynamics (UrQMD) model, the temporal
evolution and spatial distribution of the internal magnetic field are
calculated. The magnetic field strength is found to reach about $eB\approx470$
MeV$^{2}$ ($B\approx8\times10^{16}$ G) for Au+Au collisions at
$E_{\text{lab}}$=1 GeV/nucleon with impact parameter of 7 fm. The magnetic
field in Cu+Au collisions exhibits somewhat different spatial distribution from
that in Au+Au collisions. The magnetic field is found to affect the directed
flow of pions at forward and backward rapidities to some extent, dependent of
the impact parameter and beam energy while the effect on the elliptic flow is
small. This suggests that, because $\pi$ mesons produced in heavy ion
collisions at intermediate energies are considered as a sensitive probe for the
nuclear symmetry energy, it is necessary to consider the effect of the internal
magnetic field.",1905.12492v1
2019-06-18,Nuclear magnetic resonance spectroscopy with a superconducting flux qubit,"We theoretically analyze the performance of the nuclear magnetic resonance
(NMR) spectroscopy with a superconducting flux qubit (FQ). Such NMR with the FQ
is attractive because of the possibility to detect the relatively small number
of nuclear spins in a local region ($\sim\mu$m) with low temperatures ($\sim$
mK) and low magnetic fields ($\sim$ mT), in which other types of quantum
sensing schemes cannot easily access. A sample containing nuclear spins is
directly attached on the FQ, and the FQ is used as a magnetometer to detect
magnetic fields from the nuclear spins. Especially, we consider two types of
approaches to NMR with the FQ. One of them is to use spatially inhomogeneous
excitations of the nuclear spins, which are induced by a spatially asymmetric
driving with radio frequency~(RF) pulses. Such an inhomogeneity causes a change
in the DC magnetic flux penetrating a loop of the FQ, which can be detected by
a standard Ramsey measurement on the FQ. The other approach is to use a
dynamical decoupling on the FQ to measure AC magnetic fields induced by Larmor
precession of the nuclear spins. In this case, neither a spin excitation nor a
spin polarization is required since the signal comes from fluctuating magnetic
fields of the nuclear spins. We calculate the minimum detectable density
(number) of the nuclear spins for the FQ with experimentally feasible
parameters. We show that the minimum detectable density (number) of the nuclear
spins with these approaches is around $10^{21}$ /cm$^3$ ($10^8$) with an
accumulation time of a second.",1906.07386v1
2019-07-11,Analysis of azimuthal magnetorotational instability of rotating MHD flows and Tayler instability via an extended Hain-Lust equation,"We consider a differentially rotating flow of an incompressible electrically
conducting and viscous fluid subject to an external axial magnetic field and to
an azimuthal magnetic field that is allowed to be generated by a combination of
an axial electric current external to the fluid and electrical currents in the
fluid itself. In this setting we derive an extended version of the celebrated
Hain-Lust differential equation for the radial Lagrangian displacement that
incorporates the effects of the axial and azimuthal magnetic fields,
differential rotation, viscosity, and electrical resistivity. We apply the
Wentzel-Kramers-Brillouin method to the extended Hain-Lust equation and derive
a new comprehensive dispersion relation for the local stability analysis of the
flow to three-dimensional disturbances. We confirm that in the limit of low
magnetic Prandtl numbers, in which the ratio of the viscosity to the magnetic
diffusivity is vanishing, the rotating flows with radial distributions of the
angular velocity beyond the Liu limit, become unstable subject to a wide
variety of the azimuthal magnetic fields, and so is the Keplerian flow. In the
analysis of the dispersion relation we find an evidence of a new
long-wavelength instability which is caught also by the numerical solution of
the boundary value problem for a magnetized Taylor-Couette flow.",1907.05488v3
2019-07-17,Negative-vector-chirality 120$^\circ$ spin structure in the defect- and distortion-free quantum kagome antiferromagnet YCu$_3$(OH)$_6$Cl$_3$,"The magnetic ground state of the ideal quantum kagome antiferromagnet (QKA)
has been a long-standing puzzle, mainly because perturbations to the
nearest-neighbor isotropic Heisenberg Hamiltonian can lead to various
fundamentally different ground states. Here we investigate a recently
synthesized QKA representative YCu$_3$(OH)$_6$Cl$_3$, where perturbations
commonly present in real materials, like lattice distortion and intersite ion
mixing, are absent. Nevertheless, this compound enters a long-range
magnetically ordered state below $T_N=15$ K. Our powder neutron diffraction
experiment reveals that its magnetic structure corresponds to a coplanar
$120^\circ$ state with negative vector spin chirality. The ordered magnetic
moments are suppressed to $0.42(2)\mu_B$, which is consistent with the
previously detected spin dynamics persisting to the lowest experimentally
accessible temperatures. This indicates either a coexistence of magnetic order
and disorder or the presence of strong quantum fluctuations in the ground state
of YCu$_3$(OH)$_6$Cl$_3$. The origin of the magnetic order is sought in terms
of Dzyaloshinskii-Moriya magnetic anisotropy and further-neighbor isotropic
exchange interactions.",1907.07489v2
2019-09-10,Magnetic phase diagram of a spin S=1/2 antiferromagnetic two-leg ladder in the presence of modulated along legs Dzyaloshinskii-Moriya interaction,"We study the ground-state magnetic phase diagram of a spin S=1/2
antiferromagnetic two-leg ladder in the presence of period two lattice units
modulated, Dzyaloshinskii-Moriya (DM) interaction along the legs. We consider
the case of collinear DM vectors and strong rung exchange and magnetic field.
In this limit we map the initial ladder model onto the effective spin
$\sigma=1/2$ XXZ chain and study the latter using the continuum-limit
bosonization approach. We identified four quantum phase transitions and
corresponding critical magnetic fields, which mark transitions from the spin
gapped regimes into the gapless quantum spin-liquid regimes. In the gapped
phases the magnetization curve of the system shows plateaus at magnetisation
M=0 and to its saturation value per rung M=1. We have shown that the very
presence of alternating DM interaction leads to opening of a gap in the
excitation spectrum at magnetization M=0.5. The width of the magnetization
plateau at M=0.5, is determined by the associated with the dynamical generation
of a gap in the spectrum is calculated and is shown that its length scales as
$(D_{0}D_{1}/J^{2})^{\alpha}$ where $D_{0},D_{1}$ are uniform and staggered
components of the DM term, $J$ is the intraleg exchange and $\alpha \leq 3/4$
and weakly depends on the DM couplings.",1909.04618v1
2019-10-08,Holographic Vector Field Electron Tomography of Three-Dimensional Nanomagnets,"Complex 3D magnetic textures in nanomagnets exhibit rich physical properties,
for example in their dynamic interaction with external fields and currents, and
play an increasing role for current technological challenges such as
energy-efficient memory devices. To study these magnetic nanostructures
including their dependency on geometry, composition and crystallinity, a 3D
characterization of the magnetic field with nanometer spatial resolution is
indispensable. Here we show how holographic vector field electron tomography
can reconstruct all three components of magnetic induction as well as the
electrostatic potential of a Co/Cu nanowire with sub 10\,nm spatial resolution.
We address the workflow from acquisition, via image alignment to holographic
and tomographic reconstruction. Combining the obtained tomographic data with
micromagnetic considerations we derive local key magnetic characteristics, such
as magnetization current or exchange stiffness, and demonstrate how
magnetization configurations, such as vortex states in the Co-disks, depend on
small structural variations of the as-grown nanowire.",1910.03430v1
2019-10-29,Imaging and control of critical spin fluctuations in two-dimensional magnets,"Strong spin fluctuations are expected near the thermodynamic critical point
of a continuous magnetic phase transition. Such critical spin fluctuations are
highly correlated and in principle can occur at any time- and length-scales;
they govern critical phenomena and potentially can drive new phases. Although
theoretical studies have been made for decades, direct observation of critical
spin fluctuations remains elusive. The recent discovery of two-dimensional (2D)
layered magnets, in which spin fluctuations significantly modify magnetic
properties as compared to their bulk counterparts and integration into
heterostructures and devices can be easily achieved, provides an ideal platform
to investigate and harness critical spin phenomena. Here we develop a fast and
sensitive magneto-optical imaging microscope to achieve wide-field, real-time
monitoring of critical spin fluctuations in single-layer CrBr3, which is a 2D
ferromagnetic insulator. We track the critical phenomena directly from the
fluctuation correlations and observe both slowing-down dynamics and enhanced
correlation length. Through real-time feedback control of critical spin
fluctuations, we further achieve switching of magnetic states solely by
electrostatic gating without applying a magnetic field or a Joule current. The
ability to directly image and control critical spin fluctuations in 2D magnets
opens up exciting opportunities to explore critical phenomena and develop
applications in nanoscale engines and information science.",1910.13023v1
2019-12-11,Magnetic field effects on electron transport in nanoring with orbital Rashba coupling,"We study the effects of a Zeeman magnetic field on the electron transport of
one-dimensional quantum rings which are marked by electronic states with
$d-$orbital symmetry in the presence of spin-orbit and orbital Rashba
couplings. By considering phase-coherent propagation, we analyse the geometric
Aharonov-Anandan (AA) phase of the channels which is acquired in a closed path,
by demonstrating that the orbital polarization can influence the electronic
transport when amplitude and magnetic field directions are varied. We explore
all the possible cases for the injection of electrons at various energies in
the regime of low electron filling. The magnetic field can allow the selection
of only one channel where the transmission is uniquely affected by the AA
phase. Conversely, when more orbital channels are involved there is also a
dynamical contribution that lead to oscillations in the transmission as the
magnetic field is varied. In particular, the transmission is chiral when the
energy states are close to the absolute minimum of the energy bands. Instead,
when an interference between the channels occurs the orbital and spin
contributions tend to balance each other with the increasing of the magnetic
field amplitude resulting in a trivial AA phase. This saturation effect does
not occur in the high magnetic field regime when orbital and spin properties of
the channels exhibit sharp variations with direct consequences on the
transport.",1912.05602v1
2020-01-15,Generation of Solenoidal Modes and Magnetic Fields in Turbulence Driven by Compressive Driving,"We perform numerical simulations of hydrodynamic (HD) and magnetohydrodynamic
(MHD) turbulence driven by compressive driving to study generation of
solenoidal velocity component and small-scale magnetic field. We mainly focus
on the effects of mean magnetic field ($B_0$) and the sonic Mach number
($M_s$). We also consider two different driving schemes in terms of correlation
timescale of forcing vectors: a finite-correlated driving and a
delta-correlated driving. The former has a longer correlation timescale of
forcing vectors, which is comparable to large-eddy turnover time, than the
latter. Our findings are as follows. First, when we fix the value of $B_0$, the
level of solenoidal velocity component after saturation increases as $M_s$
increases. A similar trend is observed for generation of magnetic field when
$B_0$ is small. Second, when we fix the value of $M_s$, HD and MHD simulations
result in similar level of the solenoidal component when $B_0$ $\lesssim$ 0.2
(or Alfven Mach number of $\sim$ 5). However, the level increases when $B_0$
$\gtrsim$ 0.2. Roughly speaking, the magnetic energy density after saturation
is a linearly increasing function of $B_0$ irrespective of $M_s$. Third,
generation of solenoidal velocity component is not sensitive to numerical
resolution, but that of magnetic energy density is mildly sensitive. Lastly,
when initial conditions are same, the finite-correlated driving always produces
more solenoidal velocity and small-scale magnetic field components than the
delta-correlated driving. We additionally analyze the vorticity equation to
understand why higher $M_s$ and $B_0$ yield larger quantity of the solenoidal
velocity component.",2001.05154v1
2020-02-20,Ultrafast ratchet dynamics of skyrmion by defect engineering under gigahertz magnetic fields,"The novel ratchet motion of magnetic skyrmions driven by microwave magnetic
fields, with the motion direction and speed tunable by field parameters,
provides a promising route to drive magnetic skyrmions in materials with poor
conductivity. However, as an indirect motion, skyrmion ratchet motion speed is
generally low in comparison with the direct motions driven by current. Toward
practical applications, it is important to ask if there are mechanisms to
realize ultrafast ratchet motion of magnetic skyrmions and how such novel
motion can be integrated into racetrack-type skyrmion devices. In this work, we
explore the effects of defects and edges on the ratchet motion of magnetic
skyrmions under time-varying magnetic fields in GHz. We demonstrate that the
ratchet motion of skyrmion is not only guided along the defect tracks or edges,
but also with a remarkable speed-up (with a factor over ten) compared with that
in the bulk region. The skyrmion ratchet motion speed reaches 100 m/s along a
straight defect track/edge and 10^9 rad/s along a circular edge under a field
of ~50 mT, comparable to those direct motions driven by currents. Moreover, the
skyrmion ratchet motion along the defect track/edge can be facilely controlled
by the field and defect parameters. Analysis based on time-averaged Thiele
equation of skyrmion verifies that such a speed-up effect is due to the
increased time-averaged driving force perpendicular to the skyrmion motion when
it approaches the defect track or edge, analogous to that discovered in direct
motions driven by currents.",2002.08865v2
2020-03-02,Spin-caloritronic transport in hexagonal graphene nanoflakes,"We investigate the spin-dependent thermoelectric effect of graphene flakes
with magnetic edges in the ballistic regime. Employing static, respectively,
dynamic mean-field theory we first show that magnetism appears at the zigzag
edges for a window of Coulomb interactions that increases significantly with
increasing flake size. We then use the Landauer formalism in the framework of
the non-equilibrium Green's function method to calculate the spin and charge
currents in magnetic hexagonal graphene flakes by varying the temperature of
the junction for different flake sizes. While in non-magnetic gated graphene
the temperature gradient drives a charge current, we observe a significant spin
current for hexagonal graphene flakes with magnetic zigzag edges. Specifically,
we show that in the ""meta"" configuration of a hexagonal flake subject to weak
Coulomb interactions, a pure spin current can be driven just by a temperature
gradient in a temperature range that is promising for device applications.
Bigger flakes are found to yield a bigger window of Coulomb interactions where
such spin currents are induced by the magnetic zigzag edges, and larger values
of the current.",2003.01139v2
2020-03-06,Mixed topology ring states for Hall effect and orbital magnetism in skyrmions of Weyl semimetals,"Skyrmion lattices as a novel type of chiral spin states are attracting
increasing attention, owing to their peculiar properties stemming from
real-space topological properties. At the same time, the properties of magnetic
Weyl semimetals with complex $k$-space topology are moving into the focus of
research in spintronics. We consider the Hall transport properties and orbital
magnetism of skyrmion lattices imprinted in topological semimetals, by
employing a minimal model of a 2D mixed Weyl semimetal which, as a function of
the magnetization direction, exhibits two Chern insulator phases separated by a
Weyl state for an an in-plane magnetization direction. We find that while the
orbital magnetization is topologically robust and Hall transport properties are
very sensitive to the details of the spin distribution in accordance to the
behavior expected from the recently discovered chiral Hall effect[1], their
behavior in the region of the Chern insulator gap is largely determined by the
properties of the so-called mixed topology ring states, emerging in domain
walls that separate the skyrmion core from the ferromagnetic background. In
particular, we show that these localized ring states possess a given orbital
chirality which reverses sign as a function of the skyrmion radius, thereby
mediating a smooth switching dynamics of the orbital magnetization of the
skyrmion lattice. We speculate that while the emergent ring states can possibly
play a role in the physics of Majorana states, probing their properties
experimentally can provide insights into the details of skyrmionic spin
structures.",2003.03243v2
2020-04-06,Spin Hall magnetoresistance in antiferromagnetic insulators,"Antiferromagnetic materials promise improved performance for spintronic
applications, as they are robust against external magnetic field perturbations
and allow for faster magnetization dynamics compared to ferromagnets. The
direct observation of the antiferromagnetic state, however, is challenging due
to the absence of a macroscopic magnetization. Here, we show that the spin Hall
magnetoresistance (SMR) is a versatile tool to probe the antiferromagnetic spin
structure via simple electrical transport experiments by investigating the
easy-plane antiferromagnetic insulators $\alpha$-Fe2O3 (hematite) and NiO in
bilayer heterostructures with a Pt heavy metal top electrode. While rotating an
external magnetic field in three orthogonal planes, we record the longitudinal
and the transverse resistivities of Pt and observe characteristic resistivity
modulations consistent with the SMR effect. We analyze both their amplitude and
phase and compare the data to the results from a prototypical collinear
ferrimagnetic Y3Fe5O12/Pt bilayer. The observed magnetic field dependence is
explained in a comprehensive model, based on two magnetic sublattices and
taking into account magnetic field-induced modifications of the domain
structure. Our results show that the SMR allows us to understand the spin
configuration and to investigate magnetoelastic effects in antiferromagnetic
multi-domain materials. Furthermore, in $\alpha$-Fe2O3/Pt bilayers, we find an
unexpectedly large SMR amplitude of $2.5 \times 10^{-3}$, twice as high as for
prototype Y3Fe5O12/Pt bilayers, making the system particularly interesting for
room-temperature antiferromagnetic spintronic applications.",2004.02639v2
2020-05-11,Skyrmion confinement and dynamics in tracks patterned with magnetic anisotropy: theory and simulations,"Skyrmion is a topologically protected spin texture excited in magnetic thin
films. The radii of skyrmions are typically 10-100 nm. Because of the size, the
skyrmion is expected to be a candidate for memory and novel-device usages. To
realize the futuristic devices that will be using the skyrmion circuit, the
tracks which guide the motion of skyrmions are needed. The tracks patterned
with differences in the magnetic-anisotropy energy are well-paved without a
potential pocket, whereas the tracks carved out of magnetic films have the
potential pockets at corners due to the demagnetizing field. Therefore, the
tracks patterned with the magnetic anisotropy plays a key role in making the
skyrmion circuits. The experiment along this idea has been conducted for the
hub and bent tracks. However, we have little known the motion of skyrmions in
these tracks. This work aims to identify the forces acting between skyrmions
and walls of the tracks. The static force on a skyrmion can be expressed as
minus the gradient of the potential energy caused by the magnetic-anisotropy
undulation. The potential can be estimated numerically, modeling the shape of
skyrmions with their radii and domain wall widths. We find that the forces
depend not only on the distance from the wall but also on the shape of
skyrmions. We have also performed micromagnetic simulations where the Magnus
force and the acceleration by the magnetic-anisotropy gradient are taken into
account as well as the force by the walls. The simulation results show good
agreement with those calculated from the modeled skyrmions.",2005.04860v1
2020-07-27,Estimating Satellite Orbital Drag During Historical Magnetic Superstorms,"Understanding extreme space weather events is of paramount importance in
efforts to protect technological systems in space and on the ground.
Particularly in the thermosphere, the subsequent extreme magnetic storms can
pose serious threats to low-Earth orbit (LEO) spacecraft by intensifying errors
in orbit predictions. Extreme magnetic storms (minimum Dst $\leq$ --250 nT) are
extremely rare: only 7 events occurred during the era of spacecraft with
high-level accelerometers such as CHAMP (CHAllenge Mini-satellite Payload) and
GRACE (Gravity Recovery And Climate experiment), and none with minimum Dst
$\leq$ --500 nT, here termed magnetic superstorms. Therefore, current knowledge
of thermospheric mass density response to superstorms is very limited. Thus, in
order to advance this knowledge, four known magnetic superstorms in history,
i.e., events occurring before CHAMP's and GRACE's commission times, with
complete datasets, are used to empirically estimate density enhancements and
subsequent orbital drag. The November 2003 magnetic storm (minimum Dst = --422
nT), the most extreme event observed by both satellites, is used as the
benchmark event. Results show that, as expected, orbital degradation is more
severe for the most intense storms. Additionally, results clearly point out
that the time duration of the storm is strongly associated with storm-time
orbital drag effects, being as important as or even more important than storm
intensity itself. The most extreme storm-time decays during CHAMP/GRACE-like
sample satellite orbits estimated for the March 1989 magnetic superstorm show
that long-lasting superstorms can have highly detrimental consequences for the
orbital dynamics of satellites in LEO.",2007.13892v1
2020-08-17,Magnetorotational core collapse of possible GRB progenitors. III. Three-dimensional models,"We explore the influence of non-axisymmetric modes on the dynamics of the
collapsed core of rotating, magnetized high-mass stars in three-dimensional
simulations of a rapidly rotating star with an initial mass of $M_{ZAMS}$ = 35
solar masses endowed with four different pre-collapse configurations of the
magnetic field, ranging from moderate to very strong field strength and
including the field predicted by the stellar evolution model. The model with
the weakest magnetic field achieves shock revival due to neutrino heating in a
gain layer characterized by a large-scale, hydrodynamic m = 1 spiral mode.
Later on, the growing magnetic field of the proto-neutron star launches weak
outflows into the early ejecta. Their orientation follows the evolution of the
rotational axis of the proto-neutron star, which starts to tilt from the
original orientation due to the asymmetric accretion flows impinging on its
surface. The models with stronger magnetization generate mildly relativistic,
magnetically driven polar outflows propagating over a distance of $10^4$ km
within a few 100 ms. These jets are stabilized against disruptive
non-axisymmetric instabilities by their fast propagation and by the shear of
their toroidal magnetic field. Within the simulation times of around 1 s, the
explosions reach moderate energies and the growth of the proto-neutron star
masses ceases at values substantially below the threshold for black hole
formation, which, in combination with the high rotational energies, might
suggest a possible later proto-magnetar activity.",2008.07205v2
2020-09-04,NV-Diamond Magnetic Microscopy using a Double Quantum 4-Ramsey Protocol,"We introduce a double quantum (DQ) 4-Ramsey measurement protocol that enables
wide-field magnetic imaging using nitrogen vacancy (NV) centers in diamond,
with enhanced homogeneity of the magnetic sensitivity relative to conventional
single quantum (SQ) techniques. The DQ 4-Ramsey protocol employs
microwave-phase alternation across four consecutive Ramsey (4-Ramsey)
measurements to isolate the desired DQ magnetic signal from any residual SQ
signal induced by microwave pulse errors. In a demonstration experiment
employing a 1-$\mu$m-thick NV layer in a macroscopic diamond chip, the DQ
4-Ramsey protocol provides volume-normalized DC magnetic sensitivity of
$\eta^\text{V}=34\,$nTHz$^{-1/2} \mu$m$^{3/2}$ across a $125\,\mu$m$
\,\times\,125\,\mu $m field of view, with about 5$\times$ less spatial
variation in sensitivity across the field of view compared to a SQ measurement.
The improved robustness and magnetic sensitivity homogeneity of the DQ 4-Ramsey
protocol enable imaging of dynamic, broadband magnetic sources such as
integrated circuits and electrically-active cells.",2009.02371v2
2020-09-16,Impact of magnetic field on neutrino-matter interactions in core-collapse supernova,"We explore the impact of magnetic field on neutrino-matter interactions in
core-collapse supernova. We first derive the modified source terms for
neutrino-nucleon scattering and neutrino absorption and emission processes in
the moment formalism. Then we perform full relativistic three-dimensional,
magnetorotational core-collapse supernova simulations of a 20 $M_\odot$ star
with spectral neutrino transport. Our simulations treat self-consistently the
parity violation effects of weak interaction in the presence of external
magnetic field. The result shows a significant global asymmetry, mostly
confined in the proto-neutron star, with clearly reflecting the magnetic field
structure. The asymmetric property arises from two factors: the angle between
the neutrino flux and magnetic field, and the term, which is parallel to the
magnetic field and is also proportional to the deviation of distribution
function of neutrinos from thermal equilibrium. The typical correction value
amounts to $\sim1$ % relative to the total neutrino-matter interaction rate for
the magnetic field strength of $\sim 10^{15-16}$~G. Although these asymmetric
properties do not immediately affect the explosion dynamics, our results imply
that they would be significant once the neutrinos diffuse out the proto-neutron
star core carrying those asymmetries away. We also show that, during our
simulation time of $\sim370$ ms after bounce, our results indicate that the
correction value due to the modified inelastic scattering process dominates
over that of the modified neutrino absorption and emission process.",2009.07733v2
2020-09-20,Inverse transfer of magnetic helicity in direct numerical simulations of compressible isothermal turbulence: scaling laws,"The inverse transfer of magnetic helicity is investigated through direct
numerical simulations of large-scale-mechanically-driven turbulent flows in the
isothermal ideal magnetohydrodynamics (MHD) framework. The mechanical forcing
is either purely solenoidal or purely compressive and the turbulent
steady-states considered exhibit root mean square (RMS) Mach numbers 0.1
$\lesssim$ M $\lesssim$ 11. A continuous small-scale electromotive forcing
injects magnetic helical fluctuations, which lead to the build-up of ever
larger magnetic structures. Spectral scaling exponents are observed which, for
low Mach numbers, are consistent with previous research done in the
incompressible case. Higher compressibility leads to flatter magnetic helicity
scaling exponents. The deviations from the incompressible case are
comparatively small for solenoidally-driven turbulence, even at high Mach
numbers, as compared to those for compressively-driven turbulence, where strong
deviations are already visible at relatively mild RMS Mach numbers M $\gtrsim$
3. Compressible effects can thus play an important role in the inverse transfer
of magnetic helicity, especially when the turbulence drivers are rather
compressive. Theoretical results observed in the incompressible case can,
however, be transferred to supersonic turbulence by an appropriate change of
variables, using the Alfv\'en velocity in place of the magnetic field.",2009.09374v1
2020-09-14,Magnetohydrodynamic Simulation of Magnetic Null-point Reconnections and Coronal dimmings during the X2.1 flare in NOAA AR 11283,"The magnetohydrodynamics of active region NOAA 11283 is simulated using an
initial non-force-free magnetic field extrapolated from its photospheric vector
magnetogram. We focus on the magnetic reconnections at a magnetic null point
that participated in the X2.1 flare on 2011 September 6 around 22:21 UT
(SOL2011-09-06T22:21X2.1) followed by the appearance of circular flare ribbons
and coronal dimmings. The initial magnetic field from extrapolation displays a
three-dimensional (3D) null topology overlying a sheared arcade. Prior to the
flare, magnetic loops rise due to the initial Lorentz force, and reconnect at
the 3D null, leading to expansion and loss of confined plasma that produce the
observed pre-flare coronal dimmings. Further, the simulated dynamics documents
the transfer of twist from the arcade to the overlying loops through
reconnections, developing a flux rope. The non-parallel field lines comprising
the rope and lower-lying arcades form an X-type geometry. Importantly, the
simultaneous reconnections at the 3D null and the X-type geometry can explain
the observed circular and parallel flare ribbons. Reconnections at the 3D null
transform closed inner spine field lines into open field lines of the outer
spine. The footpoints of these open field lines correspond to a ring-shaped
coronal dimming region, tracing the dome. Further, the flux rope bifurcates
because of these reconnections which also results in the generation of open
magnetic field lines. The plasma loss along the open field lines can
potentially explain the observed coronal dimming.",2009.11109v1
2020-10-13,Magnetic Helicity Flux across Solar Active Region Photospheres: I. Hemispheric Sign Preference in Solar Cycle 24,"A hemispheric preference in the dominant sign of magnetic helicity has been
observed in numerous features in the solar atmosphere: i.e.,
left-handed/right-handed helicity in the northern/southern hemisphere. The
relative importance of different physical processes which may contribute to the
observed hemispheric sign preference (HSP) of magnetic helicity is still under
debate. Here, we estimate magnetic helicity flux ($dH/dt$) across the
photospheric surface for 4,802 samples of 1,105 unique active regions (ARs)
that appeared over an 8-year period from 2010 to 2017 during solar cycle 24,
using photospheric vector magnetic field observations by the Helioseismic and
Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO). The
estimates of $dH/dt$ show that 63% and 65% of the investigated AR samples in
the northern and southern hemispheres, respectively, follow the HSP. We also
find a trend that the HSP of $dH/dt$ increases from ~50-60% up to ~70-80% as
ARs (1) appear at the earlier inclining phase of the solar cycle or higher
latitudes; (2) have larger values of $|dH/dt|$, the total unsigned magnetic
flux, and the average plasma flow speed. These observational findings support
the enhancement of the HSP mainly by the Coriolis force acting on a buoyantly
rising and expanding flux tube through the turbulent convection zone. In
addition, the differential rotation on the solar surface as well as the
tachocline $\alpha$-effect of flux-transport dynamo may reinforce the HSP for
ARs at higher latitudes.",2010.06134v1
2020-10-26,Structure and magnetism of the skyrmion hosting family GaV$_4$S$_{8-y}$Se$_y$ with low levels of substitutions between $0 \leq y \leq 0.5$ and $7.5 \leq y\leq 8$,"Polycrystalline members of the GaV$_4$S$_{8-y}$Se$_y$ family of materials
with small levels of substitution between $0 \leq y \leq 0.5$ and $7.5 \leq
y\leq 8$ have been synthesized in order to investigate their magnetic and
structural properties. Substitutions to the skyrmion hosting parent compounds
GaV$_4$S$_8$ and GaV$_4$Se$_8$, are found to suppress the temperature of the
cubic to rhombohedral structural phase transition that occurs in both end
compounds and to create a temperature region around the transition where there
is a coexistence of these two phases. Similarly, the magnitude of the
magnetization and temperature of the magnetic transition are both suppressed in
all substituted compounds until a glassy-like magnetic state is realized. There
is evidence from the $ac$ susceptibility data that skyrmion lattices with
similar dynamics to those in GaV$_4$S$_8$ and GaV$_4$Se$_8$ are present in
compounds with very low levels of substitution, $0 < y< 0.2$ and $7.8 < y < 8$,
however, these states vanish at higher levels of substitution. The magnetic
properties of these substituted materials are affected by the substitution
altering exchange pathways and resulting in the creation of increasingly
disordered magnetic states.",2010.13450v1
2020-11-02,Signatures of magnetostriction and spin-phonon coupling in magnetoelectric hexagonal 15R-BaMnO3,"Spin-phonon coupling, the interaction of spins with surrounding lattice is a
key parameter to understand the underlying physics of multiferroics and
engineer their magnetization dynamics. Elementary excitations in multiferroic
materials are strongly influenced by spin-phonon interaction, making Raman
spectroscopy a unique tool to probe these coupling(s). Recently, it has been
suggested that the dielectric and magnetic properties of 15R-type hexagonal
BaMnO3 are correlated through the spin-lattice coupling. Here, we report the
observation of an extensive renormalization of the Raman spectrum of 15R-BaMnO3
at 230 K, 280 K, and 330 K. Magnetic measurements reveal the presence of a
long-range and a short-range magnetic ordering in 15R-BaMnO3 at 230 K and 330
K, respectively. The Raman spectrum shows the appearance of new Raman modes in
the magnetically ordered phases. Furthermore, an additional Raman phonon
appears below ~ 280 K, possibly arising from a local lattice-distortion due to
the displacement of Mn-ions, that exhibits anomalous shift with temperature.
The origin of the observed renormalization and phonon anomalies in Raman
spectra are discussed based on the evidences from temperature- and
magnetic-field-dependent Raman spectra, temperature-dependent x-ray
diffraction, magnetization, and specific heat measurements. Our results
indicate the presence of magnetostriction and spin-phonon coupling in
15R-BaMnO3 thus suggesting that the optical phonons are strongly correlated to
its magnetoelectric properties.",2011.00757v1
2020-11-05,Large-Scale Vortex Motion and Multiple Plasmoid Ejection Due to Twisting Prominence Threads and Associated Reconnection,"We analyze the characteristics of a quiescent polar prominence using the
Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory
(SDO). Initially, small-scale barb-like structures are evident on the solar
disk, which firstly grow vertically and thereafter move towards the south-west
limb. Later, a spine connects these barbs and we observe apparent rotating
motions in the upper part of the prominence. These apparent rotating motions
might play an important role for the evolution and growth of the filament by
transferring cool plasma and magnetic twist. The large-scale vortex motion is
evident in the upper part of the prominence, and consists of a swirl-like
structure within it. The slow motion of the footpoint twists the legs of the
prominence due to magnetic shear, causing two different kinds of magnetic
reconnection. The internal reconnection is initiated by a resistive
tearing-mode instability, which leads to the formation of multiple plasmoids in
the elongated current sheet. The estimated growth rate was found to be
0.02--0.05. The magnetic reconnection heats the current sheet for a small
duration. However, most of the energy release due to magnetic reconnection is
absorbed by the surrounding cool and dense plasma and used to accelerate the
plasmoid ejection. The multiple plasmoid ejections destroy the current sheet.
Therefore, the magnetic arcades collapse near the X-point. Oppositely directed
magnetic arcades may reconnect with the southern segment of the prominence and
an elongated thin current sheet is formed. This external reconnection drives
prominence eruption.",2011.02950v1
2020-11-19,Coronal heating and solar wind formation in quiet Sun and coronal holes: a unified scenario,"Coronal holes (CHs) are darker than quiet Sun (QS) when observed in coronal
channels. This study aims to understand the similarities and differences
between CHs and QS in the transition region using the \ion{Si}{4}~1394~{\AA}
line recorded by the Interface Region Imaging Spectrograph (IRIS) by
considering the distribution of magnetic field measured by the Helioseismic and
Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO). We find
that \ion{Si}{4} intensities obtained in CHs are lower than those obtained in
QS for regions with identical magnetic flux densities. Moreover, the difference
in intensities between CHs and QS increases with increasing magnetic flux. For
the regions with equal magnetic flux density, QS line profiles are more
redshifted than those measured in CHs. Moreover, the blue shifts measured in
CHs show an increase with increasing magnetic flux density unlike in the QS.
The non-thermal velocities in QS, as well as in CHs, show an increase with
increasing magnetic flux. However, no significant difference was observed in QS
and CHs, albeit a small deviation at small flux densities. Using these results,
we propose a unified model for the heating of the corona in the QS and in CHs
and the formation of the solar wind.",2011.09803v1
2020-11-23,Asymmetric depinning of chiral domain walls in ferromagnetic trilayers,"We show that the coupling between two ferromagnetic layers separated by a
nonmagnetic spacer can be used to control the depinning of domain walls and
induce unidirectional domain wall propagation. We investigated CoFeB/Ti/CoFeB
trilayers where the easy axis of the magnetization of the top CoFeB layer is
out-of-plane and that of the bottom layer is in-plane. Using Magneto-optic Kerr
effect microscopy, we find that the depinning of a domain wall in the
perpendicularly magnetized CoFeB layer is influenced by the orientation of the
magnetization of the in-plane layer, which gives rise to a field-driven
asymmetric domain expansion. This effect occurs due to the magnetic coupling
between the internal magnetization of the domain wall and the magnetization of
the in-plane CoFeB layer, which breaks the symmetry of up-down and down-up
homochiral N\'eel domain walls in the perpendicular CoFeB layer. Micromagnetic
simulations support these findings by showing that the interlayer coupling
either opposes or favors the Dzyaloshinskii-Moriya interaction in the domain
wall, thereby generating an imbalance in the depinning fields. This effect also
allows for artificially controlling the chirality and dynamics of domain walls
in magnetic layers lacking a strong Dzyaloshinskii-Moriya interaction.",2011.11290v1
2021-01-01,Superparamagnetic dynamics and blocking transition in Fe$_3$O$_4$ nanoparticles probed by vibrating sample magnetometry and muon spin relaxation,"The magnetic properties of Fe$_3$O$_4$ nanoparticle assemblies have been
investigated in detail through a combination of vibrating sample magnetometry
and muon spin relaxation ($\mu$SR) techniques. Two samples with average
particle sizes of 5 nm and 20 nm, respectively, were studied. For both samples,
the magnetometry and $\mu$SR results exhibit clear signatures of the
superparmagnetic state at high temperature and the magnetically blocked state
at low temperature. The $\mu$SR data demonstrate that the transition from the
superparamagnetic to the blocked state occurs gradually throughout the sample
volume over a broad temperature range due to the finite particle size
distribution of each sample. The transition occurs between approximately 3 K
and 45 K for the 5 nm sample and 150 K and 300 K for the 20 nm sample. The
magnetometry and $\mu$SR data are further analyzed to yield estimates of
microscopic magnetic parameters including the nanoparticle spin-flip activation
energy $E_A$, magnetic anisotropy $K$, and intrinsic nanoparticle spin reversal
attempt time $\tau_0$. These results highlight the complementary information
about magnetic nanoparticles that can be obtained by bulk magnetic probes such
as magnetometry and local magnetic probes such as $\mu$SR.",2101.00258v1
2021-01-11,Current-driven domain wall dynamics in ferrimagnetic Ni-doped Mn4N films : very large domain wall velocities and reversal of motion direction across the magnetic compensation point,"Spin-transfer torque (STT) and spin-orbit torque (SOT) are spintronic
phenomena allowing magnetization manipulation using electrical currents. Beyond
their fundamental interest, they allow developing new classes of magnetic
memories and logic devices, in particular based on domain wall (DW) motion. In
this work, we report the study of STT driven DW motion in ferrimagnetic
manganese nickel nitride (Mn4-xNixN) films, in which a fine adjustment of the
Ni content allows setting the magnetic compensation at room temperature. The
reduced magnetization, combined with the large spin polarization of conduction
electrons, strongly enhances the STT so that domain wall velocities approaching
3000 m/s can be obtained for Ni compositions close to the compensation point.
In addition, a reversal of the domain wall motion direction is observed when
the magnetic compensation composition is crossed. This striking feature,
related to the change of direction of the spin polarization with respect to
that of the net magnetization, is clarified by ab initio band structure
calculations.",2101.04220v2
2021-02-10,Direct visualization of three-dimensional shape of skyrmion strings in a noncentrosymmetric magnet,"Magnetic skyrmion, i.e. a topologically stable swirling spin texture, appears
as a particle-like object in the two-dimensional (2D) systems, and has recently
attracted attention as a candidate of novel information carrier. In the real
three-dimensional (3D) systems, a skyrmion is expected to form a string
structure along an extra dimension, while its experimental identification has
rarely been achieved. Here, we report the direct visualization of 3D shape of
individual skyrmion strings, for the recently discovered room-temperature
skyrmion-hosting noncentrosymmetric compound Mn1.4Pt0.9Pd0.1Sn. For this
purpose, we have newly developed the magnetic X-ray tomography measurement
system that can apply magnetic field, which plays a key role on the present
achievement. Through the tomographic reconstruction of the 3D magnetization
distribution based on the transmission images taken from various angles, a
genuine skyrmion string running through the entire thickness of the sample, as
well as various defect structures such as the interrupted and Y-shaped strings,
are successfully identified. The observed point defect may represent the
emergent magnetic monopole, as recently proposed theoretically. The present
tomographic approach with tunable magnetic field paves the way for the direct
visualization of the structural dynamics of individual skyrmion strings in the
3D space, which will contribute to the better understanding of the creation,
annihilation and transfer process of these topological objects toward the
potential device applications.",2102.05556v1
2021-03-05,Could switchbacks originate in the lower solar atmosphere? I. Formation mechanisms of switchbacks,"The recent rediscovery of magnetic field switchbacks or deflections embedded
in the solar wind flow by the Parker Solar Probe mission lead to a huge
interest in the modelling of the formation mechanisms and origin of these
switchbacks. Several scenarios for their generation were put forth, ranging
from lower solar atmospheric origins by reconnection, to being a manifestation
of turbulence in the solar wind, and so on. Here we study some potential
formation mechanisms of magnetic switchbacks in the lower solar atmosphere,
using three-dimensional magneto-hydrodynamic (MHD) numerical simulations. The
model is that of an intense flux tube in an open magnetic field region, aiming
to represent a magnetic bright point opening up to an open coronal magnetic
field structure, e.g. a coronal hole. The model is driven with different plasma
flows in the photosphere, such as a fast up-shooting jet, as well as shearing
flows generated by vortex motions or torsional oscillations. In all scenarios
considered, we witness the formation of magnetic switchbacks in regions
corresponding to chromospheric heights. Therefore, photospheric plasma flows
around the foot-points of intense flux tubes appear to be suitable drivers for
the formation of magnetic switchbacks in the lower solar atmosphere.
Nevertheless, these switchbacks do not appear to be able to enter the coronal
heights of the simulation in the present model. In conclusion, based on the
presented simulations, switchbacks measured in the solar wind are unlikely to
originate from photospheric or chromospheric dynamics.",2103.03726v1
2021-03-15,Nature of the ferromagnetic-antiferromagnetic transition in Y$_{1-x}$La$_{x}$TiO$_{3}$,"We explore the magnetically-ordered ground state of the
isovalently-substituted Mott-insulator Y$_{1-x}$La$_{x}$TiO$_{3}$ for $x$
$\leq$ 0.3 via single crystal growth, magnetometry, neutron diffraction, x-ray
magnetic circular dichroism (XMCD), muon spin rotation ($\mu$SR) and
small-angle neutron scattering (SANS). We find that the decrease in the
magnetic transition temperature on approaching the ferromagnetic (FM) -
antiferromagnetic (AFM) phase boundary at the La concentration $x_c$ $\approx$
0.3 is accompanied by a strong suppression of both bulk and local ordered
magnetic moments, along with a volume-wise separation into magnetically-ordered
and paramagnetic regions. The thermal phase transition does not show
conventional second-order behavior, since neither a clear signature of dynamic
critical behavior nor a power-law divergence of the magnetic correlation length
is found for the studied substitution range; this finding becomes increasingly
obvious with substitution. Finally, from SANS and magnetometry measurements, we
discern a crossover from easy-axis to easy-plane magneto-crystalline anisotropy
with increasing La substitution. These results indicate complex changes in
magnetic structure upon approaching the phase boundary.",2103.08565v1
2021-03-24,Magnetic ordering of the distorted kagome antiferromagnet Y$_3$Cu$_9$(OH)$_{18}$[Cl$_8$(OH)] prepared via optimal synthesis,"Experimental studies of high-purity kagome-lattice antiferromagnets (KAFM)
are of great importance in attempting to better understand the predicted
enigmatic quantum spin-liquid ground state of the KAFM model. However,
realizations of this model can rarely evade magnetic ordering at low
temperatures due to various perturbations to its dominant isotropic exchange
interactions. Such a situation is for example encountered due to sizable
Dzyaloshinskii-Moriya magnetic anisotropy in YCu$_3$(OH)$_6$Cl$_3$, which
stands out from other KAFM materials by its perfect crystal structure. We find
evidence of magnetic ordering also in the distorted sibling compound
Y$_3$Cu$_9$(OH)$_{18}$[Cl$_8$(OH)], which has recently been proposed to feature
a spin-liquid ground state arising from a spatially anisotropic kagome lattice.
Our findings are based on a combination of bulk susceptibility, specific heat,
and magnetic torque measurements that disclose a N\'eel transition temperature
of $T_N=11$~K in this material, which might feature a coexistence of magnetic
order and persistent spin dynamics as previously found in
YCu$_3$(OH)$_6$Cl$_3$. Contrary to previous studies of single crystals and
powders containing impurity inclusions, we use high-purity single crystals of
Y$_3$Cu$_9$(OH)$_{18}$[Cl$_8$(OH)] grown via an optimized hydrothermal
synthesis route that minimizes such inclusions. This study thus demonstrates
that the lack of magnetic ordering in less pure samples of the investigated
compound does not originate from the reduced symmetry of spin lattice but is
instead of extrinsic origin.",2103.13254v2
2021-03-29,Nonequilibrium Dynamics of the Chiral Quark Condensate under a Strong Magnetic Field,"Strong magnetic fields impact quantum-chromodynamics (QCD) properties in
several situations; examples include the early universe, magnetars, and
heavy-ion collisions. These examples share a common trait: time evolution. A
prominent QCD property impacted by a strong magnetic field is the quark
condensate, an approximate order parameter of the QCD transition between a
high-temperature quark-gluon phase and a low-temperature hadronic phase. We use
the linear sigma model with quarks to address the quark condensate time
evolution under a strong magnetic field. We use the closed time path formalism
of nonequilibrium quantum field theory to integrate out the quarks and obtain a
mean-field Langevin equation for the condensate. The Langevin equation features
dissipation and noise kernels controlled by a damping coefficient. We compute
the damping coefficient for magnetic field and temperature values achieved in
peripheral relativistic heavy-ion collisions and solve the Langevin equation
for a temperature quench scenario. The magnetic field changes the dissipation
and noise pattern by increasing the damping coefficient compared to the
zero-field case. An increased damping coefficient increases fluctuations and
time scales controlling condensate's short-time evolution, a feature that can
impact hadron formation at the QCD transition. The formalism developed here can
be extended to include other order parameters, hydrodynamic modes, and system's
expansion to address magnetic field effects in complex settings as heavy-ion
collisions, the early universe, and magnetars.",2103.15665v1
2021-04-01,Crystallographic and magnetic structures of the VI$_3$ and LiVI$_3$ van der Waals compounds,"Two-dimensional (2D) layered magnetic materials are generating a great amount
of interest for the next generation of electronic devices thanks to their
remarkable properties associated to spin dynamics. The recently discovered
layered VI$_3$ ferromagnetic phase belongs to this family, although a full
understanding of its properties is limited by an ill-defined crystallographic
structure. This is not any longer true. Here, we investigate the VI$_3$ crystal
structure upon cooling using both synchrotron X-ray and neutron powder
diffraction and provide structural models for the two structural transitions
occurring at 76 K and 32 K. Moreover, we confirm by magnetic measurements that
VI$_3$ becomes ferromagnetic at 50 K and discuss the difficulty of solving its
full magnetic structure by neutrons. We equally determined the magnetic
properties of our recently reported LiVI$_3$ phase, which is alike the
well-known CrI$_3$ ferromagnetic phase in terms of electronic and
crystallographic structures and found to our surprise an antiferromagnetic
behavior with a N\'eel temperature of 12 K. Such a finding provides extra clues
for a better understanding of magnetism in these low dimension compounds.
Finally, the easiness of preparing novel Li-based 2D magnetic materials by
chemical/electrochemical means opens wide the opportunity to design materials
with exotic properties.",2104.00457v1
2021-05-03,Relaxation of a single defect spin by the low-frequency gyrotropic mode of a magnetic vortex,"We excite the gyrotropic mode of a magnetic vortex and observe the resulting
effect on the spin state of a nearby nitrogen-vacancy (NV) defect in diamond.
Thin permalloy disks fabricated on a diamond sample are magnetized in a vortex
state in which the magnetization curls around a central core. The magnetization
dynamics of this configuration are described by a discrete spectrum of confined
magnon modes, as well as a low-frequency gyrotropic mode in which the vortex
core precesses about its equilibrium position. Despite the spin transition
frequencies being far-detuned from the modes of the ferromagnet, we observe
enhanced relaxation of the NV spin when driving the gyrotropic mode. Moreover,
we map the spatial dependence of the interaction between the vortex and the
spin by translating the vortex core within the disk with an applied magnetic
field, resulting in steplike motion as the vortex is pinned and de-pinned. The
strong spin relaxation is observed when the vortex core is within approximately
250 nm of the NV center defect. We attribute this effect to the higher
frequencies in the spectrum of the magnetic fringe field arising from the
soliton-like nature of the gyrotropic mode when driven with sufficiently large
amplitude.",2105.00927v1
2021-05-17,AC susceptometry of 2D van der Waals magnets enabled by the coherent control of quantum sensors,"Precision magnetometry is fundamental to the development of novel magnetic
materials and devices. Recently, the nitrogen-vacancy (NV) center in diamond
has emerged as a promising probe for static magnetism in 2D van der Waals
materials, capable of quantitative imaging with nanoscale spatial resolution.
However, the dynamic character of magnetism, crucial for understanding the
magnetic phase transition and achieving technological applications, has rarely
been experimentally accessible in single 2D crystals. Here, we coherently
control the NV center's spin precession to achieve ultra-sensitive,
quantitative ac susceptometry of a 2D ferromagnet. Combining dc hysteresis with
ac susceptibility measurements varying temperature, field, and frequency, we
illuminate the formation, mobility, and consolidation of magnetic domain walls
in few-layer CrBr3. We show that domain wall mobility is enhanced in ultrathin
CrBr3, with minimal decrease for excitation frequencies exceeding hundreds of
kilohertz, and is influenced by the domain morphology and local pinning of the
flake. Our technique extends NV magnetometry to the multi-functional ac and dc
magnetic characterization of wide-ranging spintronic materials at the
nanoscale.",2105.08030v1
2021-05-25,Low-temperature high-frequency dynamic magnetic susceptibility of classical spin-ice Dy$_2$Ti$_2$O$_7$,"Radio-frequency (14.6 MHz) AC magnetic susceptibility, $\chi^{\prime}_{AC}$,
of \dytio\ was measured using a self-oscillating tunnel-diode resonator.
Measurements were made with the excitation AC field parallel to the
superimposed DC magnetic field up 5 T in a wide temperature range from 50 mK to
100 K. At 14.6 MHz a known broad peak of $\chi^{\prime}_{AC}(T)$ from kHz -
range audio-frequency measurements around 15~K for both [111] and [110]
directions shifts to 45~K, continuing the Arrhenius activated behavior with the
same activation energy barrier of $E_a \approx 230$~K. Magnetic field
dependence of $\chi^{\prime}_{AC}$ along [111] reproduces previously reported
low-temperature two-in-two-out to three-in-one-out spin configuration
transition at about 1~T, and an intermediate phase between 1 and 1.5~T. The
boundaries of the intermediate phase show reasonable overlap with the
literature data and connect at a critical endpoint of the first-order
transition line, suggesting that these low-temperature features are frequency
independent. An unusual upturn of magnetic susceptibility at $T \to 0$ was
observed in magnetic fields between 1.5~T and 2~T for both magnetic field
directions, before fully polarized configuration sets in above 2~T.",2105.12218v1
2021-05-29,Directional field-dependence of tunable magnetic domains in noncentrosymmetric ferromagnetic Weyl semimetal CeAlSi,"Dynamics and textures of magnetic domain walls (DWs) may largely alter the
electronic behaviors in a Weyl semimetal system via emergent gauge fields.
However, very little is known about even the basic properties of these domain
walls in Weyl materials. In this work, we imaged the spontaneous magnetization
and magnetic susceptibility of a ferromagnetic (FM) Weyl semimetal CeAlSi using
scanning SQUID microscopy. We observed the ferromagnetic DWs lined-up with the
[100] direction (or other degenerate directions). We also discovered the
coexistence of stable and metastable domain phases, which arise likely due to
magnetoelastic and magnetostriction effects and are expected to be highly
tunable with small strains. We applied an in-plane external field as the CeAlSi
sample was cooled down to below the magnetic phase transition of 8.3K, showing
that the pattern of FM domains is strongly correlated with both the amplitude
and the orientation of the external field even for weak fields of a few Gauss.
The area of stable domains increases with field and reaches maximum when the
field is parallel to the main crystallographic axes of the CeAlSi crystal. Our
results suggest that the manipulation of these heterogeneous phases can provide
a practical way to study the interplay between magnetism and electronic
properties in Weyl systems, and that these systems can even serve as a new
platform for magnetic sensors.",2105.14384v1
2021-07-02,Scaling of Turbulent Viscosity and Resistivity: Extracting a Scale-dependent Turbulent Magnetic Prandtl Number,"Turbulent viscosity $\nu_t$ and resistivity $\eta_t$ are perhaps the simplest
models for turbulent transport of angular momentum and magnetic fields,
respectively. The associated turbulent magnetic Prandtl number $Pr_t\equiv
\nu_t/\eta_t$ has been well recognized to determine the final magnetic
configuration of accretion disks. Here, we present an approach to determining
these ''effective transport'' coefficients acting at different length-scales
using coarse-graining and recent results on decoupled kinetic and magnetic
energy cascades [Bian & Aluie 2019]. By analyzing the kinetic and magnetic
energy cascades from a suite of high-resolution simulations, we show that our
definitions of $\nu_t$, $\eta_t$, and $Pr_t$ have power-law scalings in the
''decoupled range.'' We observe that $Pr_t\approx1 \text{~to~}2$ at the
smallest inertial-inductive scales, increasing to $\approx 5$ at the largest
scales. However, based on physical considerations, our analysis suggests that
$Pr_t$ has to become scale-independent and of order unity in the decoupled
range at sufficiently high Reynolds numbers (or grid-resolution), and that the
power-law scaling exponents of velocity and magnetic spectra become equal. In
addition to implications to astrophysical systems, the scale-dependent
turbulent transport coefficients offer a guide for large eddy simulation
modeling.",2107.00861v1
2021-08-30,Bright and dark states of two distant macrospins strongly coupled by phonons,"We study the collective dynamics of two distant magnets coherently coupled by
acoustic phonons that are transmitted through an intercalated crystal. By
tuning the ferromagnetic resonances of the two magnets to an acoustic resonance
of the crystal, we control a coherent three levels system. We show that the
parity of the phonon mode governs the nature of the indirect coupling between
the magnets: the resonances with odd / even phonon modes correspond to
out-of-phase / in-phase lattice displacements at the magnets, leading to bright
/ dark states in response to uniform microwave magnetic fields, respectively.
The sample is a tri-layer garnet consisting of two thin magnetic layers
epitaxially grown on both sides of a half-millimeter thick non-magnetic single
crystal. In spite of the relatively weak magneto-elastic interaction, the long
lifetimes of the magnon and phonon modes in the sample are the key to unveil
this long range strong coupling. This demonstrates that garnets are a great
platform to study multi-partite hybridization process between magnon and
phonons at microwave frequencies.",2108.13272v2
2021-09-09,Magnetic field induced quantum spin liquid in the two coupled trillium lattices of K$_2$Ni$_2$(SO$_4$)$_3$,"Quantum spin liquids are exotic states of matter which form when strongly
frustrated magnetic interactions induce a highly entangled quantum paramagnet
far below the energy scale of the magnetic interactions. Three-dimensional
cases are especially challenging due to the significant reduction of the
influence of quantum fluctuations. Here, we report the magnetic
characterization of {\kni} forming a three dimensional network of Ni$^{2+}$
spins. Using density functional theory calculations we show that this network
consists of two interconnected spin-1 trillium lattices. In the absence of a
magnetic field, magnetization, specific heat, neutron scattering and muon spin
relaxation experiments demonstrate a highly correlated and dynamic state,
coexisting with a peculiar, very small static component exhibiting a strongly
renormalized moment. A magnetic field $B \gtrsim 4$ T diminishes the ordered
component and drives the system in a pure quantum spin liquid state. This shows
that a system of interconnected $S=1$ trillium lattices exhibit a significantly
elevated level of geometrical frustration.",2109.04102v1
2021-09-10,Predicting the Magnetic Fields of a Stealth CME Detected by Parker Solar Probe at 0.5 AU,"Stealth coronal mass ejection (CMEs) are eruptions from the Sun that are not
associated with appreciable low-coronal signatures. Because they often cannot
be linked to a well-defined source region on the Sun, analysis of their initial
magnetic configuration and eruption dynamics is particularly problematic. In
this manuscript, we address this issue by undertaking the first attempt at
predicting the magnetic fields of a stealth CME that erupted in 2020 June from
the Earth-facing Sun. We estimate its source region with the aid of off-limb
observations from a secondary viewpoint and photospheric magnetic field
extrapolations. We then employ the Open Solar Physics Rapid Ensemble
Information (OSPREI) modelling suite to evaluate its early evolution and
forward-model its magnetic fields up to Parker Solar Probe, which detected the
CME in situ at a heliocentric distance of 0.5 AU. We compare our hindcast
prediction with in-situ measurements and a set of flux rope reconstructions,
obtaining encouraging agreement on arrival time, spacecraft crossing location,
and magnetic field profiles. This work represents a first step towards reliable
understanding and forecasting of the magnetic configuration of stealth CMEs and
slow, streamer-blowout events.",2109.04933v1
2021-09-12,Zero-field magnetometry using hyperfine-biased nitrogen-vacancy centers near diamond surfaces,"Shallow nitrogen-vacancy (NV) centers in diamond are promising for
nano-magnetometry for they can be placed proximate to targets. To study the
intrinsic magnetic properties, zero-field magnetometry is desirable. However,
for shallow NV centers under zero field, the strain near diamond surfaces would
cause level anti-crossing between the spin states, leading to clock transitions
whose frequencies are insensitive to magnetic signals. Furthermore, the charge
noises from the surfaces would induce extra spin decoherence and hence reduce
the magnetic sensitivity. Here we demonstrate that the relatively strong
hyperfine coupling (130 MHz) from a first-shell 13C nuclear spin can provide an
effective bias field to an NV center spin so that the clock-transition
condition is broken and the charge noises are suppressed. The hyperfine bias
enhances the dc magnetic sensitivity by a factor of 22 in our setup. With the
charge noises suppressed by the strong hyperfine field, the ac magnetometry
under zero field also reaches the limit set by decoherence due to the nuclear
spin bath. In addition, the 130 MHz splitting of the NV center spin transitions
allows relaxometry of magnetic noises simultaneously at two well-separated
frequencies (~2.870 +/- 0.065 GHz), providing (low-resolution) spectral
information of high-frequency noises under zero field. The hyperfine-bias
enhanced zero-field magnetometry can be combined with dynamical decoupling to
enhance single-molecule magnetic resonance spectroscopy and to improve the
frequency resolution in nanoscale magnetic resonance imaging.",2109.05445v1
2021-09-20,Sympathetic Filament Eruptions within a Fan-spine Magnetic System,"It is unclear whether successive filament eruptions at different sites within
a short time interval are physically connected or not. Here, we present the
observations of the successive eruptions of a small and a large filament in a
tripolar magnetic field region whose coronal magnetic field showed as a
fan-spine magnetic system. By analyzing the multi-wavelength observations taken
by the Solar Dynamic Observatory (SDO) and the extrapolated three-dimensional
coronal magnetic field, we find that the two filaments resided respectively in
the two lobes that make up the inner fan structure of the fan-spine magnetic
system. In addition, a small fan-spine system was also revealed by the
squashing factor Q map, which located in the east lobe of the fan structure of
the large fan-spine system. The eruption of the small filament was a failed
filament eruption, which did not cause any coronal mass ejection (CME) except
for three flare ribbons and two post-flare-loop systems connecting the three
magnetic polarities. The eruption of the large filament not only caused similar
post-flare-loop systems and flare ribbons as observed in the small filament
eruption, but also a large-scale CME. Based on our analysis results, we
conclude that the two successive filament eruptions were physically connected,
in which the topology change caused by the small filament eruption is thought
to be the physical linkage. In addition, the eruption of the small fan-spine
structure further accelerated the instability and violent eruption of the large
filament.",2109.09285v1
2021-11-19,Streaming instabilities in accreting and magnetized laminar protoplanetary disks,"The streaming instability is one of the most promising pathways to the
formation of planetesimals from pebbles. Understanding how this instability
operates under realistic conditions expected in protoplanetary disks is
therefore crucial to assess the efficiency of planet formation. Contemporary
models of protoplanetary disks show that magnetic fields are key to driving gas
accretion through large-scale, laminar magnetic stresses. However, the effect
of such magnetic fields on the streaming instability has not been examined in
detail. To this end, we study the stability of dusty, magnetized gas in a
protoplanetary disk. We find the streaming instability can be enhanced by
passive magnetic torques and even persist in the absence of a global radial
pressure gradient. In this case, instability is attributed to the azimuthal
drift between dust and gas, unlike the classical streaming instability, which
is driven by radial drift. This suggests that the streaming instability can
remain effective inside dust-trapping pressure bumps in accreting disks. When a
live vertical field is considered, we find the magneto-rotational instability
can be damped by dust feedback, while the classic streaming instability can be
stabilized by magnetic perturbations. We also find that Alfv\'en waves can be
destabilized by dust-gas drift, but this instability requires nearly ideal
conditions. We discuss the possible implications of these results for dust
dynamics and planetesimal formation in protoplanetary disks.",2111.10381v1
2021-12-08,Magnetic Fields in Primordial Galaxies,"Magnetic fields play a vital role in numerous astrophysical processes such as
star formation and the interstellar medium. In particular, their role in the
formation and evolution of galaxies is not well understood. This paper presents
high-resolution magnetohydrodynamic (MHD) simulations performed with GIZMO to
investigate the effect of magnetic fields on primordial galaxy formation.
Physical processes such as relevant gas physics (e.g., gas cooling and gas
chemistry), star formation, stellar and supernova feedback, and chemical
enrichment were considered in the simulations. The simulation results suggest
that cosmic magnetic fields can be amplified from 1e-13 G to a few microgauss
during cosmic structure evolution and galaxy formation. In the ideal MHD
setting, in primordial galaxies at z>8, the magnetic energy is less than the
thermal and kinetic energy, and therefore, magnetic fields hardly affect the
gas dynamics and star formation in these galaxies. Specifically, the
consideration of micro-physics properties such as metal diffusion, heat
conduction, and viscosity in the MHD simulations, could increase the magnetic
field strength. Notably, metal diffusion reduced gas cooling by decreasing the
metallicity and thereby suppresses star formation in the primordial galaxies.
As a result, the cosmic re-ionization driven by these primordial galaxies may
be delayed.",2112.04512v1
2021-12-13,Fluctuating magnetic droplets immersed in a sea of quantum spin liquid,"The search of quantum spin liquid (QSL), an exotic magnetic state with
strongly-fluctuating and highly-entangled spins down to zero temperature, is a
main theme in current condensed matter physics. However, there is no
smoking-gun evidence for deconfined spinons in any QSL candidate so far. The
disorders and competing exchange interactions may prevent the formation of an
ideal QSL state on frustrated spin lattices. Here we report comprehensive and
systematic measurements of the magnetic susceptibility, ultra-low temperature
specific heat, muon spin relaxation (muSR), nuclear magnetic resonance (NMR),
and thermal conductivity for NaYbSe2 single crystals, in which Yb3+ ions with
effective spin-1/2 form a perfect triangular lattice. All these complementary
techniques find no evidence of long-range magnetic order down to their
respective base temperatures. Instead, specific heat, muSR and NMR measurements
suggest the coexistence of quasi-static and dynamic spins in NaYbSe2. The
scattering from these quasi-static spins may cause the absence of magnetic
thermal conductivity. Thus, we propose a scenario of fluctuating ferrimagnetic
droplets immersed in a sea of QSL. This may be quite common on the way pursuing
an ideal QSL, and provides a brand-new platform to study how a QSL state
survives impurities and coexists with other magnetically ordered states.",2112.06523v1
2021-12-20,Stable and unstable supersonic stagnation of an axisymmetric rotating magnetized plasma,"The Naval Research Laboratory ""Mag Noh problem"", described in this paper, is
a self-similar magnetized implosion flow, which contains a fast MHD outward
propagating shock of constant velocity. We generalize the classic Noh (1983)
problem to include azimuthal and axial magnetic fields as well as rotation. Our
family of ideal MHD solutions is five-parametric, each solution having its own
self-similarity index, gas gamma, magnetization, the ratio of axial to the
azimuthal field, and rotation. While the classic Noh problem must have a
supersonic implosion velocity to create a shock, our solutions have an
interesting three-parametric special case with zero initial velocity in which
magnetic tension, instead of implosion flow, creates the shock at $t=0+$. Our
self-similar solutions are indeed realized when we solve the initial value MHD
problem with finite volume MHD code Athena. We numerically investigated the
stability of these solutions and found both stable and unstable regions in
parameter space. Stable solutions can be used to test the accuracy of numerical
codes. Unstable solutions have also been widely used to test how codes
reproduce linear growth, transition to turbulence, and the practically
important effects of mixing. Now we offer a family of unstable solutions
featuring all three elements relevant to magnetically driven implosions:
convergent flow, magnetic field, and a shock wave.",2112.10828v1
2022-01-06,On-demand augmentation in heat transfer of Taylor bubble flows using ferrofluids,"The thermo-fluidic transport characteristics of ferrofluids can be influenced
by the application of a magnetic field. The magnetic manipulations of
ferrofluids have been useful in augmenting heat transfer, as evident from
recent investigations. In the present study, we examine a novel strategy for
augmenting two-phase heat transfer and show that the magnetic manipulation of
non-boiling Taylor bubble flow (TBF) of ferrofluids can provide on-demand
augmentation. In an earlier investigation (DOI:10.1016/j.colsurfa.2020.124589),
we had shown that the characteristics of the TBF of ferrofluids could be
altered through external magnetic manipulations. As transport characteristics
of TBFs primarily depend on their flow morphology, it was anticipated that such
alteration would affect their thermal transport characteristics, which are
examined in the present work. The generation of smaller bubbles and unit-cells
through magnetic manipulations decreases the void fraction of the resulting
TBF. In addition, a greater number of units participated in the heat exchange
process compared to larger bubble-slug systems at any time instance. Such flow
modifications cause considerable augmentation (which can go up to 100%) in
two-phase heat transfer. The extent of augmentation depends on the applied
magnetic field and induced magnetic force, homogeneous gas fraction, liquid
film thickness/void fraction and flow morphology of the resulting TBF, which
are examined in the present study. The application of ferrofluids in TBFs
provides multiple benefits, such as suspension of nanoparticles with better
thermal properties and additional functionality of the flow manipulations
through external means. The proposed application with the suggested
manipulation technique provides an effective alternative for an on-demand
augmentation in two-phase heat transfer in low Reynolds number flows.",2201.02145v1
2022-04-10,Effect of iron vacancies on the magnetic order and spin dynamics of the spin ladder BaFe$_{2-δ}$S$_{1.5}$Se$_{1.5}$,"Quasi-one-dimensional iron chalcogenides possess various magnetic states
depending on the lattice distortion, electronic correlations, and presence of
defects. We present neutron diffraction and inelastic neutron scattering
experiments on the spin ladder compound BaFe$_{2-\delta}$S$_{1.5}$Se$_{1.5}$
with $\sim$6% iron vacancies. The data reveal that long-range magnetic order is
absent, while the characteristic magnetic excitations that correspond to both
the stripe- and block-type antiferromagnetic correlations are observed.
First-principles calculations support the existence of both stripe and
block-type antiferromagnetic short-range order in the experimental sample. The
disappearance of long-range magnetic order may be due to the competition
between these two magnetic orders, which is greatly enhanced for a certain
concentration of iron vacancies, which we calculate to be about 6%, consistent
with the measured iron vacancy concentration. Our results highlight how iron
vacancies in the iron-based spin ladder system strongly influence the magnetic
ground state.",2204.04697v1
2022-04-19,Electric field-induced domain wall motion in spin spiral multiferroics,"Switching in magnetic materials gives rise to rich physical phenomena and
lies at the heart of their technological applications. Although domain wall
motion in ferro- and antiferromagnets has been studied, in spiral magnets it is
still poorly understood despite 20 years of active research since the discovery
of spiral multiferroics. The problem of the domain wall motion in a spiral
magnet is a compelling one, the more so the magnetic domain walls in cycloidal
spiral phase are also ferroelectric, thus enabling electric control of
magnetism, i.e. domain wall motion under the action of an external electric
field. Phase transition to a spiral phase leads to a formation of chiral
domains with opposite spin rotation senses, that are separated by chiral domain
walls. Spiral order breaks inversion symmetry and induces a ferroelectric
polarization, whose sign is determined by the chirality of the domain. Thus the
spiral order allows for the manipulation of spins via an external electric
field. Here we study domain wall motion in magnets with spiral ground state,
that are the most basic non-collinear magnets. We formulate a simplified
variational model and derive the equation of motion for the domain wall driven
by an external electric field. The results are corroborated with atomistic spin
dynamics simulations. The results suggest a linear dependence of the wall speed
on the external electric field, and a peculiar dependence on the system
geometry and domain structure.",2204.09027v3
2022-05-05,Incommensurate magnetic order in Sm$_3$BWO$_9$ with the distorted kagome lattice,"We investigate the magnetic ground state of Sm$_3$BWO$_9$ with the distorted
kagome lattice. A magnetic phase transition is identified at $T_N=0.75$ K from
the temperature dependence of specific heat. From $^{11}$B nuclear magnetic
resonance (NMR) measurements, an incommensurate magnetic order is shown by the
double-horn type spectra under a $c$-axis magnetic field. While, absence of
line splitting is observed for field oriented within the $ab$-plane, indicating
the incommensurate modulation of the internal field strictly along $c$-axis.
From the spin dynamics, the critical slowing down behavior is observed in the
temperature dependence of $1/T_1$ with $\mu_0H\perp c$-axis, which is
completely absent in that with $\mu_0H||c$-axis. Based on the local symmetry of
$^{11}$B sites, we analyze the hyperfine coupling tensors and propose two
constraints on the possible magnetic structure. The single ion anisotropy
should play an important role in the determination of the contrasting ground
states of Sm$_3$BWO$_9$ and Pr$_3$BWO$_9$.",2205.02448v2
2022-07-25,Spin-orbit torque switching of magnetic tunnel junctions for memory application,"Spin-orbit torques (SOT) provide a versatile tool to manipulate the
magnetization of diverse classes of materials and devices using electric
currents, leading to novel spintronic memory and computing approaches. In
parallel to spin transfer torques (STT), which have emerged as a leading
non-volatile memory technologie, SOT broaden the scope of current-induced
magnetic switching to applications that run close to the clock speed of the
central processing unit and unconventional computing architectures. In this
paper, we review the fundamental characteristics of SOT and their use to switch
magnetic tunnel junction (MTJ) devices, the elementary unit of the
magnetoresistive random access memory (MRAM). In the first part, we illustrate
the physical mechanisms that drive the SOT and magnetization reversal in
nanoscale structures. In the second part, we focus on the SOT-MTJ cell. We
discuss the anatomy of the MTJ in terms of materials and stack development,
summarize the figures of merit for SOT switching, review the field-free
operation of perpendicularly magnetized MTJs, and present options to combine
SOT, STT and voltage-gate assisted switching. In the third part, we consider
SOT-MRAMs in the perspective of circuit integration processes, introducing
considerations on scaling and performance, as well as macro-design
architectures. We thus bridge the fundamental description of SOT-driven
magnetization dynamics with an application-oriented perspective, including
device and system-level considerations, goals, and challenges.",2207.11974v1
2022-08-10,Magnetic field effects on nucleosynthesis and kilonovae from neutron star merger remnants,"We investigate the influence of parametric magnetic field configurations of a
hypermassive neutron star (HMNS) on electromagnetic (EM) observables,
specifically the kilonova lightcurves and nucleosynthesis yields. We perform
three-dimensional (3D) dynamical-spacetime general-relativistic
magnetohydrodynamic (GRMHD) simulations, including a neutrino leakage scheme,
microphysical finite-temperature equation of state (EOS), and an initial
poloidal magnetic field. We find that varying the magnetic field strength and
falloff impacts the formation of magnetized winds or mildy-relativistic jets,
which in turn has profound effects on the outflow properties. All of the
evolved configurations collapse to a black hole (BH) $\sim 21-23$ ms after the
onset of the simulations, however, the ones forming jets may be considerably
more effective at transporting angular momentum out of the system, resulting in
earlier collapse times. Larger mass ejecta rates and radial velocities of
unbound material characterise the systems that form jets. The bolometric light
curves of the kilonovae and $r$-process yields change considerably with
different magnetic field parameters. We conclude that the magnetic field
strength and falloff have robust effects on the outflow properties and
electromagnetic observables. This can be particularly important as the total
ejecta mass from our simulations ($\simeq 10^{-3}\;M_{\odot}$) makes the ejecta
from HMNS a compelling source to power kilonova through radioactive decay of
$r$-process elements.",2208.05330v1
2022-08-24,Complex magnetoelectric effect in PFN-PT/CoFe$_{2-x}$Zn$_x$O$_4$ bulk particulate composites,"The structural, dielectric, magnetic, and magnetoelectric (ME) properties of
particulate composites containing lead-iron niobate and lead titanate
piezoelectric 0.94[PbFe$_{0.5}$Nb$_{0.5}$O$_3$]-0.06[PbTi$_{0.5}$O$_3$]
(PFN-PT) and Zn-substituted cobalt ferrite magnetostrictive
CoFe$_{2-x}$Zn$_{x}$O$_4$ (CF$_{2-x}$Z$_{x}$O);
0.6(PFN-PT)/0.4(CF$_{2-x}$Z$_{x}$O), x=0, 0.025, 0.1, 0.2, 0.3 (with ratio of
60 Wt\% ferroelectric and 40 Wt\% ferrite); have been investigated. We
investigated the ME voltage coefficient as a complex quantity for all composite
samples using the dynamic piezomagnetic coefficient, $q^{ac}$=$\partial
\lambda^{ac}/{\partial H}$. The results reveal that tuning the magnetostrictive
phase has a strong effect on the real part of the ME voltage coefficient.
Doping zinc into cobalt ferrite modified the magnetic properties of the
magnetic phase, such as magnetic anisotropy and coercive field, and hence the
ME properties. The highest ME coefficient value of 12.33 $\frac{mV}{cm. Oe}$
was obtained for x=0.1 at the magnetic field of 755 Oe. In addition, the
magnetic field at which the maximum value of the ME coefficient was observed
($H_{peak}$) strongly depends on the value of Zn substitution. The results were
interpreted using the magnetic field dependence of the CF$_{2-x}$Z$_{x}$O
magnetostriction.",2208.11317v1
2022-09-17,MHD Simulations on Magnetic Compression of Field Reversed Configurations,"The magnetic compression has long been proposed a promising method for the
plasma heating in a field reversed configuration (FRC), however, it remains a
challenge to fully understand the physical mechanisms underlying the
compression process, due to its highly dynamic nature beyond the
one-dimensional (1D) adiabatic theory model [R. L. Spencer et al., Phys. Fluids
26, 1564 (1983)]. In this work, magnetohydrodynamics (MHD) simulations on the
magnetic compression of FRCs using the NIMROD code [C. R. Sovinec et al., J.
Comput. Phys. 195, 355 (2004)] and their comparisons with the 1D theory have
been performed. The effects of the assumptions of the theory on the compression
process have been explored, and the detailed profiles of the FRC during
compression have been investigated. The pressure evolution agrees with the
theoretical prediction under various initial conditions. The axial contraction
of the FRC can be affected by the initial density profile and the ramping rate
of the compression magnetic field, but the theoretical predictions on the FRC's
length in general and the relation $r_s=\sqrt{2}r_o$ in particular hold
approximately well during the whole compression process, where $r_s$ is the
major radius of FRC separatrix and $r_o$ is that of the magnetic axis. The
evolutions of the density and temperature can be affected significantly by the
initial equilibrium profile and the ramping rate of the compression magnetic
field. During the compression, the major radius of the FRC is another parameter
that is susceptible to the ramping rate of the compression field. Basically,
for the same magnetic compression ratio, the peak density is higher and the
FRC's radius $r_s$ is smaller than the theoretical predictions.",2209.08280v1
2022-09-29,Magnetic helicity fluxes from triple correlators,"Fluxes of the magnetic helicity density play an important role in large-scale
turbulent dynamos, allowing the growth of large-scale magnetic fields while
overcoming catastrophic quenching. We show here, analytically, how several
important types of magnetic helicity fluxes can arise from terms involving
triple correlators of fluctuating fields in the helicity density evolution
equation. For this, we assume incompressibility and weak inhomogeneity, and use
a quasinormal closure approximation: fourth-order correlators are replaced by
products of second-order ones, and the effect of the fourth-order cumulants on
the evolution of the third moments is modelled by a strong damping term. First,
we show how a diffusive helicity flux, till now only measured in simulations,
arises from the triple correlation term. This is accompanied by what we refer
to as a `random advective flux', which predominantly transports magnetic
helicity along the gradients of the random fields. We also find that a new
helicity flux contribution, in some aspects similar to that first proposed by
Vishniac, can arise from the triple correlator. This contribution depends on
the gradients of the random magnetic and kinetic energies along the large-scale
vorticity, and thus arises in any rotating, stratified system, even if the
turbulence is predominantly nonhelical. It can source a large-scale dynamo by
itself while spatially transporting magnetic helicity within the system.",2209.14810v2
2022-10-14,An Evolution and Eruption of the Coronal Magnetic Field through a Data-Driven MHD Simulation,"We present a newly developed data-driven magnetohydrodynamics (MHD)
simulation code under a zero-beta approximation based on a method proposed by
Hayashi et al. 2018 and 2019. Although many data-driven MHD simulations have
been developed and conducted, there are not many studies on how accurately
those simulations can reproduce the phenomena observed in the solar corona. In
this study, we investigated the performance of our data-driven simulation
quantitatively using ground-truth data. The ground-truth data was produced by
an MHD simulation in which the magnetic field is twisted by the sunspot
motions. A magnetic flux rope (MFR) is created by the cancellation of the
magnetic flux at the polarity inversion line due to the converging flow on the
sunspot, which eventually leads the eruption of the MFR. We attempted to
reproduce these dynamics using the data-driven MHD simulation. The coronal
magnetic fields are driven by the electric fields, which are obtained from a
time-series of the photospheric magnetic field that is extracted from the
ground-truth data, on the surface. As a result, the data-driven simulation
could capture the subsequent MHD processes, the twisted coronal magnetic field
and formation of the MFR, and also its eruption. We report these results and
compare with the ground-truth data, and discuss how to improve the accuracy and
optimize numerical method.",2210.07492v1
2022-11-02,Unravelling the origin of the peculiar transition in the magnetically ordered phase of the Weyl semimetal Co3Sn2S2,"Recent discovery of topologically non-trivial behavior in Co3Sn2S2 stimulated
a notable interest in this itinerant ferromagnet (Tc = 174 K). The exact
magnetic state remains ambiguous, with several reports indicating the existence
of a second transition in the range 125 -- 130 K, with antiferromagnetic and
glassy phases proposed to coexist with the ferromagnetic phase. Using detailed
angle-dependent DC and AC magnetization measurements on large, high-quality
single crystals we reveal a highly anisotropic behavior of both static and
dynamic response of Co3Sn2S2. It is established that many observations related
to sharp magnetization changes when B || c are influenced by the
demagnetization factor of a sample. On the other hand, a genuine transition has
been found at Tp = 128 K, with the magnetic response being strictly
perpendicular to the c-axis and several orders of magnitude smaller than for B
|| c. Calculations using density-functional theory indicate that the ground
state magnetic structure consist of magnetic moments canted away from the
c-axis by a small angle (~ 1.5deg). We argue that the second transition
originates from a small additional canting of moments within the kagome plane,
with two equivalent orientations for each spin.",2211.01483v1
2022-11-15,Modification of three-magnon splitting in a flexed magnetic vortex,"We present an experimental and numerical study of three-magnon splitting in a
micrometer-sized magnetic disk with the vortex state strongly deformed by
static in-plane magnetic fields. Excited with a large enough power at frequency
$f_\mathrm{RF}$, the primary radial magnon modes of a cylindrical magnetic
vortex can decay into secondary azimuthal modes via spontaneous three-magnon
splitting. This nonlinear process exhibits selection rules leading to
well-defined and distinct frequencies $f_\mathrm{RF}/2\pm \Delta f$ of the
secondary modes. Here, we demonstrate that three-magnon splitting in vortices
can be significantly modified by deforming the magnetic vortex with in-plane
magnetic fields, leading to a much richer three-magnon response. We find that,
with increasing field, an additional class of secondary modes is excited which
are localized to the highly-flexed regions adjacent to the displaced vortex
core. While these modes satisfy the same selection rules of three-magnon
splitting, they exhibit a much lower three-magnon threshold power compared to
regular secondary modes of a centered vortex. The applied static magnetic
fields are small ($\simeq$ 10 mT), providing an effective parameter to control
the nonlinear spectral response of confined vortices. Our work expands the
understanding of nonlinear magnon dynamics in vortices and advertises these for
potential neuromorphic applications based on magnons.",2211.08226v2
2022-11-21,General time-reversal equivariant neural network potential for magnetic materials,"This study introduces time-reversal E(3)-equivariant neural network and
SpinGNN++ framework for constructing a comprehensive interatomic potential for
magnetic systems, encompassing spin-orbit coupling and noncollinear magnetic
moments. SpinGNN++ integrates multitask spin equivariant neural network with
explicit spin-lattice terms, including Heisenberg, Dzyaloshinskii-Moriya,
Kitaev, single-ion anisotropy, and biquadratic interactions, and employs
time-reversal equivariant neural network to learn high-order spin-lattice
interactions using time-reversal E(3)-equivariant convolutions. To validate
SpinGNN++, a complex magnetic model dataset is introduced as a benchmark and
employed to demonstrate its capabilities. SpinGNN++ provides accurate
descriptions of the complex spin-lattice coupling in monolayer CrI$_3$ and
CrTe$_2$, achieving sub-meV errors. Importantly, it facilitates large-scale
parallel spin-lattice dynamics, thereby enabling the exploration of associated
properties, including the magnetic ground state and phase transition.
Remarkably, SpinGNN++ identifies a new ferrimagnetic state as the ground
magnetic state for monolayer CrTe2, thereby enriching its phase diagram and
providing deeper insights into the distinct magnetic signals observed in
various experiments.",2211.11403v3
2022-12-15,Exploration of all-3d Heusler alloys for permanent magnets: an ab initio based high-throughput study,"Heusler alloys have attracted interest in various fields of functional
materials since their properties can quite easily be tuned by composition.
Here, we have investigated the relatively new class of all-3d Heusler alloys in
view of its potential as permanent magnets. To identify suitable candidates, we
performed a high-throughput study using an electronic structure database to
search for X$_2$YZ-type Heusler systems with tetragonal symmetry and high
magnetization. For the alloys which passed our selection filters, we have used
a combination of density functional theory calculations and spin dynamics
modelling to investigate their magnetic properties including the
magnetocrystalline anisotropy energy and exchange interactions. The candidates
which fulfilled all the search criteria served as input for the investigation
of the temperature dependence of the magnetization and determination of Curie
temperature. Based on our results, we suggest that Fe$_2$NiZn, Fe$_2$NiTi and
Ni$_2$CoFe are potential candidates for permanent magnets with large
out-of-plane magnetic anisotropy (1.23, 0.97 and 0.82 MJ/m$^3$ respectively)
and high Curie temperatures lying more than 200 K above the room temperature.
We further show that the magnitude and direction of anisotropy is very
sensitive to the strain by calculating the values of anisotropy energy for
several tetragonal phases. Thus, application of strain can be used to tune the
anisotropy in these compounds.",2212.07845v3
2023-01-20,A Unified Model for Bipolar Outflows from Young Stars: Apparent Magnetic Jet Acceleration,"We explore a new, efficient mechanism that can power toroidally magnetized
jets up to two to three times their original terminal velocity after they enter
a self-similar phase of magnetic acceleration. Underneath the elongated outflow
lobe formed by a magnetized bubble, a wide-angle free wind, through the
interplay with its ambient toroid, is compressed and accelerated around its
axial jet. The extremely magnetic bubble can inflate over its original size,
depending on the initial Alfv\'en Mach number $M_A$ of the launched flow. The
shape-independent slope $\partial{}v_r/\partial{}r=2/3t$ is a salient feature
of the self-similarity in the acceleration phase. Peculiar kinematic signatures
are observable in the position--velocity (PV) diagrams and can combine with
other morphological signatures as probes for the density-collimated jets
arising in toroidally dominated magnetized winds. The apparent second
acceleration is powered by the decrease of the toroidal magnetic field but
operates far beyond the scales of the primary magnetocentrifugal launch region
and the free asymptotic terminal state. Rich implications may connect the jets
arising from the youngest protostellar outflows such as HH 211 and HH 212 and
similar systems with parsec-scale jets across the mass and evolutionary
spectra.",2301.08512v1
2023-03-02,Statistical analysis of the total magnetic flux decay rate in solar active regions,"We used line-of-sight magnetograms acquired by the Helioseismic and Magnetic
Imager on board the Solar Dynamics Observatory to derive the decay rate of
total unsigned magnetic flux for 910 ephemeral and active regions (ARs)
observed between 2010 and 2017. We found that: i) most of the ARs obey the
power law dependence between the peak magnetic flux and the magnetic flux decay
rate, $DR$, so that $DR\sim \Phi^{0.70}$; ii) larger ARs lose smaller fraction
of their magnetic flux per unit of time than the smaller ARs; iii) there exists
a cluster of ARs exhibiting significantly lower decay rate than it would follow
from the power law and all of them are unipolar sunspots with total fluxes in
the narrow range of $(2 - 8) \times 10^{21}$ Mx; iv) a comparison with our
previous results shows that the emergence rate is always higher than the decay
rate. The emergence rate follows a power law with a shallower slope than the
slope of the decay-rate power law. The results allowed us to suggest that not
only the maximum total magnetic flux determines the character of the decaying
regime of the AR, some of the ARs end up as a slowly decaying unipolar sunspot;
there should be certain physical mechanisms to stabilize such a sunspot.",2303.01321v2
2023-03-07,Effects of Magnetic Fields on Gas Dynamics and Star Formation in Nuclear Rings,"Nuclear rings at the centers of barred galaxies are known to be strongly
magnetized. To explore the effects of magnetic fields on star formation in
these rings and nuclear gas flows, we run magnetohydrodynamic simulations in
which there is a temporally-constant magnetized inflow to the ring,
representing a bar-driven inflow. The mass inflow rate is
$1\,M_\odot\,\mathrm{yr}^{-1}$, and we explore models with a range of field
strength in the inflow. We adopt the TIGRESS framework developed by Kim &
Ostriker to handle radiative heating and cooling, star formation, and resulting
supernova (SN) feedback. We find that magnetic fields are efficiently amplified
in the ring due to rotational shear and SN feedback. Within a few
$100\,\mathrm{Myr}$, the turbulent component $B_\mathrm{trb}$ in the ring
saturates at $\sim 35\,\mu\mathrm{G}$ (in rough equipartition with the
turbulent kinetic energy density), while the regular component $B_\mathrm{reg}$
exceeds $50\,\mu\mathrm{G}$. Expanding superbubbles created by clustered SN
explosions vertically drag predominantly-toroidal fields from near the midplane
to produce poloidal fields in high-altitude regions. The growth of magnetic
fields greatly suppresses star formation at late times. Simultaneously, strong
magnetic tension in the ring drives radially inward accretion flows from the
ring to form a circumnuclear disk in the central region; this feature is absent
in the unmagnetized model.",2303.04206v1
2023-03-15,Ultrafast Opto-magnetic Effects in the Extreme Ultraviolet Spectral Range,"Coherent light-matter interactions mediated by opto-magnetic phenomena like
the inverse Faraday effect (IFE) are expected to provide a non-thermal pathway
for ultrafast manipulation of magnetism on timescales as short as the
excitation pulse itself. As the IFE scales with the spin-orbit coupling
strength of the involved electronic states, photo-exciting the strongly
spin-orbit coupled core-level electrons in magnetic materials appears as an
appealing method to transiently generate large opto-magnetic moments. Here, we
investigate this scenario in a ferrimagnetic GdFeCo alloy by using intense and
circularly polarized pulses of extreme ultraviolet radiation. Our results
reveal ultrafast and strong helicity-dependent magnetic effects which are in
line with the characteristic fingerprints of an IFE, corroborated by ab initio
opto-magnetic IFE theory and atomistic spin dynamics simulations.",2303.08564v1
2023-03-15,Particle acceleration by magnetic reconnection in relativistic jets: the transition from small to large scales,"Several MHD works and, in particular, the recent one by Medina-Torrejon et
al. (2021) based on three-dimensional MHD simulations of relativistic jets,
have evidenced that particle acceleration by magnetic reconnection driven by
the turbulence in the flow occurs from the resistive up to the large injection
scale of the turbulence. Particles experience Fermi-type acceleration up to
ultra-high-energies, predominantly of the parallel velocity component to the
local magnetic field, in the reconnection layers in all scales due to the ideal
electric fields of the background fluctuations ($V\times B$, where $V$ and $B$
are the velocity and magnetic field of the fluctuations, respectively). In this
work, we show MHD-particle-in-cell (MHD-PIC) simulations following the early
stages of the particle acceleration in the relativistic jet which confirm these
previous results, demonstrating the strong potential of magnetic reconnection
driven by turbulence to accelerate relativistic particles to extreme energies
in magnetically dominated flows. Our results also show that the dynamical time
variations of the background magnetic fields do not influence the acceleration
of the particles in this process.",2303.08780v3
2023-03-28,Understanding the deflection of the `Cartwheel CME': data analysis and modeling,"We study the low corona evolution of the `Cartwheel' coronal mass ejection
(CME; 2008-04-09) by reconstructing its 3D path and modeling it with
magneto-hydrodynamic simulations. This event exhibits a double-deflection that
has been reported and analyzed in previous works but whose underlying cause
remained unclear. The `Cartwheel CME' travels toward a coronal hole (CH) and
against the magnetic gradients. Using a high-cadence, full trajectory
reconstruction, we accurately determine the location of the magnetic flux rope
(MFR) and, consequently, the magnetic environment in which it is immersed. We
find a pseudostreamer (PS) structure whose null point may be responsible for
the complex evolution of the MFR at the initial phase. From the pre-eruptive
magnetic field reconstruction, we estimate the dynamic forces acting on the MFR
and provide a new physical insight on the motion exhibited by the 2008-04-09
event. By setting up a similar magnetic configuration in a 2.5D numerical
simulation we are able to reproduce the observed behavior, confirming the
importance of the PS null point. We find that the magnetic forces directed
toward the null point cause the first deflection, directing the MFR towards the
CH. Later, the magnetic pressure gradient of the CH produces the reversal
motion of the MFR.",2303.15998v2
2023-04-18,A True Random Number Generator for Probabilistic Computing using Stochastic Magnetic Actuated Random Transducer Devices,"Magnetic tunnel junctions (MTJs), which are the fundamental building blocks
of spintronic devices, have been used to build true random number generators
(TRNGs) with different trade-offs between throughput, power, and area
requirements. MTJs with high-barrier magnets (HBMs) have been used to generate
random bitstreams with $\lesssim$ 200~Mb/s throughput and pJ/bit energy
consumption. A high temperature sensitivity, however, adversely affects their
performance as a TRNG. Superparamagnetic MTJs employing low-barrier magnets
(LBMs) have also been used for TRNG operation. Although LBM-based MTJs can
operate at low energy, they suffer from slow dynamics, sensitivity to process
variations, and low fabrication yield. In this paper, we model a TRNG based on
medium-barrier magnets (MBMs) with perpendicular magnetic anisotropy. The
proposed MBM-based TRNG is driven with short voltage pulses to induce
ballistic, yet stochastic, magnetization switching. We show that the proposed
TRNG can operate at frequencies of about 500~MHz while consuming less than
100~fJ/bit of energy. In the short-pulse ballistic limit, the switching
probability of our device shows robustness to variations in temperature and
material parameters relative to LBMs and HBMs. Our results suggest that
MBM-based MTJs are suitable candidates for building fast and energy-efficient
TRNG hardware units for probabilistic computing.",2304.08808v1
2023-05-02,Low-temperature Magnetic Fluctuations Investigated by $^{125}$Te-NMR on the Uranium-based Superconductor UTe$_{2}$,"To investigate the static and dynamic magnetic properties on the
uranium-based superconductor UTe$_{2}$, we measured the NMR Knight shift $K$
and the nuclear spin-lattice relaxation rate $1/T_{1}$ in $H \parallel a$ by
$^{125}$Te-NMR on a $^{125}$Te-enriched single-crystal sample. $1/T_1T$ in $H
\parallel a$ is much smaller than $1/T_1T$ in $H \parallel b$ and $c$, and
magnetic fluctuations along each axis are derived from the $1/T_1T$ measured in
$H$ parallel to all three crystalline axes. The magnetic fluctuations are
almost identical at two Te sites and isotropic at high temperatures, but become
anisotropic below 40 K, where heavy-fermion state is formed. The character of
magnetic fluctuations in UTe$_2$ is discussed with the comparison to its static
susceptibility and the results on other U-based superconductors. It is
considered that the magnetic fluctuations probed with the NMR measurements are
determined by the magnetic properties inside the two-leg ladder formed by U
atoms, which are dominated by the $q_a$ = 0 ferromagnetic fluctuations.",2305.01218v1
2023-05-05,Are Non-thermal Velocities in Active Region Coronal Loops Anisotropic?,"We have measured line widths in active region coronal loops in order to
determine whether the non-thermal broadening is anisotropic with respect to the
magnetic field direction. These non-thermal velocities are caused by unresolved
fluid motions. Our analysis method combines spectroscopic data and a magnetic
field extrapolation. We analyzed spectra from the Extreme Ultraviolet Imaging
Spectrometer on Hinode. A differential emission measure analysis showed that
many spectral lines that are commonly considered to be formed in the active
region have a substantial contribution from the background quiet Sun. From
these spectra we identified lines whose emission was dominated by the active
region loops rather than background sources. Using these lines, we constructed
maps of the non-thermal velocity. With data from the Helioseismic Magnetic
Imager on the Solar Dynamics Observatory and the Coronal Modeling System
nonlinear force-free magnetic field reconstruction code, we traced several of
the magnetic field lines through the active region. Comparing the spectroscopic
and magnetic data, we looked for correlations of non-thermal velocity with the
viewing angle between the line of sight and the magnetic field. We found that
non-thermal velocities show a weak anti-correlation with the viewing angle.
That is, the tendency is for the non-thermal velocity to be slightly larger in
the parallel direction. This parallel broadening may be due to acoustic waves
or unresolved parallel flows.",2305.03808v1
2023-06-20,Asymmetries of frequency splittings of dipolar mixed modes: a window on the topology of deep magnetic fields,"Space asteroseismology is revolutionizing our knowledge of the internal
structure and dynamics of stars. A breakthrough is ongoing with the recent
discoveries of signatures of strong magnetic fields in the core of red giant
stars. The key signature for such a detection is the asymmetry these fields
induce in the frequency splittings of observed dipolar mixed gravito-acoustic
modes. We investigate the ability of the observed asymmetries of the frequency
splittings of dipolar mixed modes to constrain the geometrical properties of
deep magnetic fields. We use the powerful analytical Racah-Wigner algebra used
in Quantum Mechanics to characterize the geometrical couplings of dipolar mixed
oscillation modes with various possible realistic fossil magnetic fields'
topologies and compute the induced perturbation of their frequencies. First, in
the case of an oblique magnetic dipole, we provide the exact analytical
expression of the asymmetry as a function of the angle between the rotation and
magnetic axes. Its value provides a direct measure of this angle. Second,
considering a combination of axisymmetric dipolar and quadrupolar fields, we
show how the asymmetry is blind to unravel the relative strength and sign of
each component. Finally, in the case of a given multipole, we show that a
negative asymmetry is a signature of non-axisymmetric topologies. Therefore,
asymmetries of dipolar mixed modes provide key but only partial information on
the geometrical topology of deep fossil magnetic fields. Asteroseismic
constraints should therefore be combined with spectropolarimetric observations
and numerical simulations, which aim to predict the more probable stable
large-scale geometries.",2306.11587v1
2023-07-11,"Dzyaloshinskii-Moriya interactions, Néel skyrmions and V$_4$ magnetic clusters in multiferroic lacunar spinel GaV$_4$S$_8$","Using ab initio density functional theory with static mean-field
correlations, we calculate the Heisenberg and Dzyaloshinskii-Moriya
interactions (DMI) for an atomistic spin Hamiltonian for the lacunar spinel,
GaV$_4$S$_8$. The parameters describing these interactions are used in
atomistic spin dynamics and micromagnetic simulations. The magnetic properties
of the lacunar spinel GaV$_4$S$_8$, a material well-known from experiment to
host magnetic skyrmions of N\'eel character, are simulated with these ab initio
calculated parameters. The Dzyaloshinskii-Moriya contribution to the
micromagnetic energy is a sum of two Lifshitz invariants, supporting the
formation of N\'eel skyrmions and its symmetry agrees with what is usually
expected for $C_{3\nu}$-symmetric systems. The are several conclusions one may
draw from this work. One concerns the quantum nature of the magnetism, where we
show that the precise magnetic state of the V$_4$ cluster is crucial for
understanding quantitatively the magnetic phase diagram. In particular we
demonstrate that a distributed-moment state of each V$_4$ cluster explains well
a variety of properties of GaV$_4$S$_8$, such as the band gap, observed Curie
temperature and especially the stability of N\'eel skyrmions in the
experimentally relevant temperature and magnetic-field range. In addition, we
find that electronic correlations change visibly the calculated value of the
DMI.",2307.05733v1
2023-07-17,Confinement of spinons in the XXZ spin-1/2 chain in presence of the transverse magnetic field,"We study the tuning effect of a transverse magnetic field on the confinement
of spinons in the infinite XXZ spin-1/2 chain. The spinon confinement in this
model takes place in the gapped antiferromagnetic phase upon application of a
staggered longitudinal magnetic field. The tuning transverse magnetic field has
mutually orthogonal uniform and staggered components. The energy spectra of the
two-spinon bound states (the `mesons') in the confinement regime are
analytically calculated in this model using two different perturbative schemes.
The first one applies in the extreme anisotropic (Ising) limit and employs the
inverse anisotropy constant as a small parameter. The second perturbative
scheme, which applies at any anisotropy in the gapped antiferromagnetic domain,
exploits the integrability of the XXZ spin chain at zero magnetic field. The
small parameters in the second technique are the components of the transverse,
and staggered longitudinal magnetic fields. It is shown, that the weak
transverse magnetic field mixes the transverse and longitudinal meson modes,
and leads to an avoided crossing of their energies upon increase of its
strength. The explicit formulas for the two-spinon contribution to the
dynamical structure factors of local spin operators are obtained as well in
this model in the weak confinement regime for wave-vectors close to the points
k = 0 and k = \pi.",2307.08328v3
2023-07-28,Magnetic Antiskyrmions in Two-Dimensional van der Waals Magnets Engineered by Layer Stacking,"Magnetic skyrmions and antiskyrmions are topologically protected
quasiparticles exhibiting a whirling spin texture in real space. Antiskyrmions
offer some advantages over skyrmions as they are expected to have higher
stability and can be electrically driven with no transverse motion. However,
unlike the widely investigated skyrmions, antiskyrmions are rarely observed due
to the required anisotropic Dzyaloshinskii-Moriya interaction (DMI). Here we
propose to exploit the recently demonstrated van der Waals (vdW) assembly of
two-dimensional (2D) materials that breaks inversion symmetry and creates
conditions for anisotropic DMI. Using a 2D vdW magnet CrI${}_3$ as an example,
we demonstrate, based on density functional theory (DFT) calculations, that
this strategy is a promising platform to realize antiskyrmions. Polar layer
stacking of two centrosymmetric magnetic monolayers of CrI${}_3$ efficiently
lowers the symmetry, resulting in anisotropic DMI that supports antiskyrmions.
The DMI is reversible by switching the ferroelectric polarization inherited
from the polar layer stacking, offering the control of antiskyrmions by an
electric field. Furthermore, we find that the magnetocrystalline anisotropy and
DMI of CrI${}_3$ can be efficiently modulated by Mn doping, creating a
possibility to control the size of antiskyrmions. Using atomistic spin dynamics
simulations with the parameters obtained from our DFT calculations, we predict
the formation of antiskyrmions in a Cr${}_{0.88}$Mn${}_{0.12}$I${}_3$ bilayer
and switching their spin texture with polarization reversal. Our results open a
new direction to generate and control magnetic antiskyrmions in 2D vdW magnetic
systems.",2307.15769v1
2023-08-07,"Buzdin, Shapiro and Chimera Steps in $\varphi_0$ Josephson Junctions","The unique resonance and locking phenomena in the
superconductor-ferromagnet-superconductor $\varphi_0$ Josephson junction under
external electromagnetic radiation are demonstrated when not just the electric
but also the magnetic component of external radiation is taken into account.
Due to the coupling of superconductivity and magnetism in this system, the
magnetic moment precession of the ferromagnetic layer caused by the magnetic
component of external radiation can lock the Josephson oscillations, which
results in the appearance of a particular type of steps in the current-voltage
characteristics, completely different from the well-known Shapiro steps. We
call these steps the Buzdin steps in the case when the system is driven only by
the magnetic component and the Chimera steps in the case when both magnetic and
electric components are present. Unlike the Shapiro steps where the
magnetization remains constant along the step, here it changes though the
system is locked. The spin-orbit coupling substantially contributes to the
amplitude, i.e., the size of these steps. Dramatic changes in their amplitudes
are also observed at frequencies near the ferromagnetic resonance. Combinations
of the Josephson and Kittel ferromagnetic resonances together with different
types of locking pronounced in dynamics and current-voltage characteristics
make the physics of this system very interesting and open up a series of new
applications.",2308.03701v2
2023-08-10,Budget equations and astrophysical nonlinear mean-field dynamos,"Solar, stellar and galactic large-scale magnetic fields are originated due to
a combined action of non-uniform (differential) rotation and helical motions of
plasma via mean-field dynamos. Usually, nonlinear mean-field dynamo theories
take into account algebraic and dynamic quenching of alpha effect and algebraic
quenching of turbulent magnetic diffusivity. However, the theories of the
algebraic quenching do not take into account the effect of modification of the
source of turbulence by the growing large-scale magnetic field. This phenomenon
is due to the dissipation of the strong large-scale magnetic field resulting in
an increase of the total turbulent energy. This effect has been studied using
the budget equation for the total turbulent energy (which takes into account
the feedback of the generated large-scale magnetic field on the background
turbulence) for (i) a forced turbulence, (ii) a shear-produced turbulence and
(iii) a convective turbulence. As the result of this effect, a nonlinear dynamo
number decreases with increase of the large-scale magnetic field, so that that
the mean-field $\alpha\Omega$, $\alpha^2$ and $\alpha^2\Omega$ dynamo
instabilities are always saturated by the strong large-scale magnetic field.",2308.05590v3
2023-09-11,Dynamics of photospheric magnetic flux distribution and variations in solar RVs -- a study using HARPS-N solar and SDO observations,"The distribution and evolution of photospheric magnetic field in sunspots,
plages and network, and variations in their relative flux content, play key
roles in radial velocity (RV) fluctuations observed in Sun-as-a-star spectra.
Differentiating and disentangling such magnetic contributions to RVs help in
building models to account for stellar activity signals in high precision RV
exoplanet searches. In this work, as earlier authors, we employ high-resolution
images of the solar magnetic field and continuum intensities from SDO/HMI to
understand the activity contributions to RVs from HARPS-N solar observations.
Using well observed physical relationships between strengths and fluxes of
photospheric magnetic fields, we show that the strong fields (spots, plages and
network) and the weak internetwork fields leave distinguishing features in
their contributions to the RV variability. We also find that the fill-factors
and average unsigned magnetic fluxes of different features correlate
differently with the RVs and hence warrant care in employing either of them as
a proxy for RV variations. In addition, we examine disk averaged UV intensities
at 1600 \r{A} and 1700 \r{A} wavelength bands imaged by SDO/AIA and their
performances as proxies for variations in different magnetic features. We find
that the UV intensities provide a better measure of contributions of plage
fields to RVs than the Ca II H-K emission indices, especially during high
activity levels when the latter tend to saturate.",2309.05428v1
2023-09-19,Engineering skyrmion from spin spiral in transition metal multilayers,"Skyrmions having topologically protected field configurations with
particle-like properties play an important role in various field of science.
Our present study focus on the generation of skyrmion from spin spiral in the
magnetic multilayers of 4d-Fe/Ir(111) with 4d = Y, Zr, Nb, Mo, Ru, Rh. Here we
investigate the impact of 4d transition metals on the isotropic Heisenberg
exchanges and anti-symmetric Dzyaloshinskii-Moriya interactions originating
from the broken inversion symmetry at the interface of 4d-Fe/Ir(111)
multilayers. We find a strong exchange frustration due to the hybridization of
the Fe-3d layer with both 4d and Ir-5d layers which modifies due to band
filling effects of the 4d transition metals. We strengthen the analysis of
exchange frustration by shedding light on the orbital decomposition of
isotropic exchange interactions of Fe-3d orbitals. Our spin dynamics and Monte
Carlo simulations indicate that the magnetic ground state of 4d-Fe/Ir(111)
transition multilayers is a spin spiral in the $ab$-plane with a period of 1 to
2.5 nm generated by magnetic moments of Fe atoms and propagating along the
$a$-direction. The spiral wavelengths in Y-Fe/Ir(111) are much larger compared
to Rh-Fe/Ir(111). In order to manipulate the skyrmion phase in 4d-Fe/Ir(111),
we investigate the magnetic ground state of 4d-Fe/Ir(111) transition
multilayers with different external magnetic field. An increasing external
magnetic field of $\sim$ 12 T is responsible for deforming the spin spiral into
a isolated skyrmion which flips into skyrmion lattice phase around $\sim$ 18 T
in Rh-Fe/Ir(111). Our study predict that the stability of magnetic skyrmion
phase in Rh-Fe/Ir(111) against thermal fluctuations is upto temperature T $\leq
90$ K.",2309.10752v1
2023-09-29,Muon Spin Relaxation Study of frustrated Tm$_3$Sb$_3$Mg$_2$O$_{14}$ with kagomé lattice,"The structure and magnetic properties of rare-earth ions Tm$^{3+}$ kagom\'{e}
lattice Tm$_3$Sb$_3$Mg$_2$O$_{14}$ are studied by X-ray diffraction, magnetic
susceptibility and muon spin relaxation ($\mu$SR) experiments. The existence of
a small amount of Tm/Mg site-mixing disorder is revealed. DC magnetic
susceptibility measurement shows that Tm$^{3+}$ magnetic moments are
antiferromagnetically correlated with a negative Curie-Weiss temperature of
-26.3 K. Neither long-range magnetic order nor spin-glass transition is
observed by DC and AC magnetic susceptibility, and confirmed by $\mu$SR
experiment down to 0.1 K. However, the emergence of short-range magnetic order
is indicated by the zero-field $\mu$SR experiments, and the absence of spin
dynamics at low temperatures is evidenced by the longitudinal-field $\mu$SR
technique. Compared with the results of Tm$_3$Sb$_3$Zn$_2$O$_{14}$, another
Tm-based kagom\'{e} lattice with much more site-mixing disorder, the gapless
spin liquid like behaviors in Tm$_3$Sb$_3$Zn$_2$O$_{14}$ can be induced by
disorder effect. Samples with perfect geometrical frustration are in urgent
demand to establish whether QSL exits in this kind of materials with rare-earth
kagom\'{e} lattice.",2309.16947v1
2023-11-03,Magnetic properties and spin dynamics in a spin-orbit driven Jeff= 1/2 triangular lattice antiferromagnet,"Frustration-induced strong quantum fluctuations accompanied by spin-orbit
coupling and crystal electric field can give rise to rich and diverse magnetic
phenomena associated with unconventional low-energy excitations in rare-earth
based quantum magnets. Herein, we present crystal structure, magnetic
susceptibility, specific heat, muon spin relaxation(muSR), and electron spin
resonance (ESR) studies on the polycrystalline samples of Ba6Yb2Ti4O17 in which
Yb3+ ions constitute a perfect triangular lattice in ab-plane without
detectable anti-site disorder between atomic sites. The Curie-Weiss fit of
low-temperature magnetic susceptibility data suggest the spin-orbit entangled
Jeff = 1/2 degrees of freedom of Yb3+ spin with weak antiferromagnetic exchange
interactions in the Kramers doublet ground state. The zero-field specific heat
data reveal the presence of long-range magnetic order at TN = 77 mK which is
suppressed in a magnetic field 1 T. The broad maximum in specific heat is
attributed to the Schottky anomaly implying the Zeeman splitting of the Kramers
doublet ground state. The ESR measurements suggest the presence of anisotropic
exchange interaction between the moments of Yb3+ spins and the well separated
Kramers doublet state. muSR experiments reveal a fluctuating state of Yb3+
spins in the temperature range 0.1 K-100 K owing to depopulation of crystal
electric field levels, which suggests that the Kramers doublets are well
separated consistent with thermodynamic and ESR results. In addition to the
intraplane nearest-neighbor superexchange interaction, the interplane exchange
interaction and anisotropy are expected to stabilize the long-range ordered
state in this triangular lattice antiferromagnet.",2311.01858v1
2024-01-08,Computational approaches to modeling dynamos in galaxies,"Galaxies are observed to host magnetic fields with a typical total strength
of around 15microgauss. A coherent large-scale field constitutes up to a few
microgauss of the total, while the rest is built from strong magnetic
fluctuations over a wide range of spatial scales. This represents sufficient
magnetic energy for it to be dynamically significant. Several questions
immediately arise: What is the physical mechanism that gives rise to such
magnetic fields? How do these magnetic fields affect the formation and
evolution of galaxies? In which physical processes do magnetic fields play a
role, and how can that role be characterized? Numerical modelling of magnetized
flows in galaxies is playing an ever-increasing role in finding those answers.
We review major techniques used for these models. Current results strongly
support the conclusion that field growth occurs during the formation of the
first galaxies on timescales shorter than their accretion timescales due to
small-scale turbulent dynamos. The saturated small-scale dynamo maintains field
strengths at a few percent of equipartition with turbulence. The subsequent
action of large-scale dynamos in differentially rotating discs produces
observed modern field strengths in equipartition with the turbulence and having
power at large scales. The field structure resulting appears consistent with
observations including Faraday rotation and polarisation from synchrotron and
dust thermal emission. Major remaining challenges include scaling numerical
models toward realistic scale separations and Prandtl and Reynolds numbers.",2401.04015v1
2024-01-08,Outflows Driven from a Magnetic Pseudodisk,"Outflows play a pivotal role in star formation as one of its most visible
markers and a means of transporting mass, momentum, and angular momentum from
the infalling gas into the surrounding molecular cloud. Their wide reach (at
least thousands of au) is a contrast to typical disk sizes ($\sim 10-100$ au).
We employ high-resolution three-dimensional nested-grid nonideal
magnetohydrodynamic (MHD) simulations to study outflow properties in the Class
0 phase. We find that no disk wind is driven from the extended centrifugal disk
that has weak magnetic coupling. The low-velocity winds emerge instead from the
infalling magnetic pseudodisk. Much of the disk actually experiences an infall
of matter rather than outflowing gas. Some of the pseudodisk wind (PD-wind)
moves inward to regions above the disk and either falls onto the disk or
proceeds upward. The upward flow gives the impression of a disk wind above a
certain height even if the gas is originally emerging from the pseudodisk. The
PD-wind has the strongest flow coming from a disk interaction zone that lies
just outside the disk and is an interface between the inwardly advected
magnetic field of the pseudodisk and the outwardly diffusing magnetic field of
the disk. The low-velocity wind exhibits the features of a flow driven by the
magnetic pressure gradient force in some regions and those of a
magnetocentrifugal wind in other regions. We interpret the structure and
dynamics of the outflow zone in terms of the basic physics of gravity, angular
momentum, magnetic fields, and nonideal MHD.",2401.04260v3
2024-02-01,Comparing growth of titania and carbonaceous dusty nanoparticles in weakly magnetised capacitively coupled plasmas,"This study investigates the growth cycles of titania and carbonaceous dusty
nanoparticles in capacitively coupled radiofrequency plasmas, focusing on the
impact of weak magnetic field of approximately 480 Gauss. The growth of dusty
nanoparticles in these plasmas is cyclic, with particles reaching their maximum
size and subsequently moving out of the plasma, followed by the initiation of a
new particle growth cycle. We compare the growth of titania and carbonaceous
dusty particles in the presence and absence of a magnetic field. The presence
of the magnetic field fastens the growth cycle in both types of dusty plasmas.
Langmuir probe measurement of the background plasma parameters such as electron
temperature and floating potential reveal radial variations in floating
potential but not electron temperature. Furthermore, the magnetic field changes
the radial variation of floating potential. The magnetic field also influences
the spatial distribution of the two dust cloud differently. It is possible that
these differences arises due to the material's different response to the
magnetic field. These findings shed light on the intricate dynamics of titania
and carbonaceous dusty plasma particle growth, offering insights into the
impact of weak magnetization on dust behavior, which have implications for
various applications, from space science to semiconductor processing.",2402.00951v3
2024-02-06,Ultrafast terahertz field control of the emergent magnetic and electronic interactions at oxide interfaces,"Ultrafast electric-field control of emergent electronic and magnetic states
at oxide interfaces offers exciting prospects for the development of new
generations of energy-efficient devices. Here, we demonstrate that the
electronic structure and emergent ferromagnetic interfacial state in epitaxial
LaNiO3/CaMnO3 superlattices can be effectively controlled using intense
single-cycle THz electric-field pulses. We employ a combination of
polarization-dependent X-ray absorption spectroscopy with magnetic circular
dichroism and X-ray resonant magnetic reflectivity to measure a detailed
magneto-optical profile and thickness of the ferromagnetic interfacial layer.
Then, we use time-resolved and temperature-dependent magneto-optical Kerr
effect, along with transient optical reflectivity and transmissivity
measurements, to disentangle multiple correlated electronic and magnetic
processes driven by ultrafast high-field (~1 MV/cm) THz pulses. These processes
include an initial sub-picosecond electronic response, consistent with
non-equilibrium Joule heating; a rapid (~270 fs) demagnetization of the
ferromagnetic interfacial layer, driven by THz-field-induced nonequilibrium
spin-polarized currents; and subsequent multi-picosecond dynamics, possibly
indicative of a change in the magnetic state of the superlattice due to the
transfer of spin angular momentum to the lattice. Our findings shed light on
the intricate interplay of electronic and magnetic phenomena in this strongly
correlated material system, suggesting a promising avenue for efficient control
of two-dimensional ferromagnetic states at oxide interfaces using ultrafast
electric-field pulses.",2402.04302v1
2024-02-12,Magnetic field of gas giant exoplanets and its influence on the retention of their exomoons,"We study the magnetic and tidal interactions of a gas-giant exoplanet with
its host star and with its exomoons, and focus on their retention. We briefly
revisit the scaling law for planetary dynamo in terms of its mass, radius and
luminosity. Based on the virial theorem, we construct an evolution law for
planetary magnetic field and find that its initial entropy is important for the
field evolution of a high-mass planet. We estimate the magnetic torques on
orbit arising from the star-planet and planet-moon magnetic interactions, and
find that it can compensate tidal torques and bypass frequency valleys where
dynamical-tide response is ineffective. For exomoon's retention we consider two
situations. In the presence of a circumplanetary disk (CPD), by comparison
between CPD's inner and outer radii, we find that planets with too strong
magnetic fields or too small distance from its host star tend not to host
exomoons. During the subsequent CPD-free evolution, we find, by comparison
between planet's spindown and moon's migration timescales, that hot Jupiters
with periods of several days are unlikely to retain large exomoons, albeit they
could be surrounded by rings from the debris of tidally disrupted moons. In
contrast, moons, if formed around warm or cold Jupiters, can be preserved.
Finally, we estimate the radio power and flux density due to the star-planet
and planet-moon magnetic interactions and give the upper limit of detection
distance by FAST.",2402.07387v5
2024-03-12,Noncentrosymmetric Triangular Magnet CaMnTeO$_6$: Strong Quantum Fluctuations and Role of s0 vs. s2 Electronic States in Competing Exchange Interactions,"Noncentrosymmetric triangular magnets offer a unique platform for realizing
strong quantum fluctuations. However, designing these quantum materials remains
an open challenge attributable to a knowledge gap in the tunability of
competing exchange interactions at the atomic level. Here, we create a new
noncentrosymmetric triangular S = 3/2 magnet CaMnTeO$_6$ based on careful
chemical and physical considerations. The model material displays competing
magnetic interactions and features nonlinear optical responses with the
capability of generating coherent photons. The incommensurate magnetic ground
state of CaMnTeO$_6$ with an unusually large spin rotation angle of 127 deg.(1)
indicates that the anisotropic interlayer exchange is strong and competing with
the isotropic interlayer Heisenberg interaction. The moment of 1.39(1) $\mu$B,
extracted from low-temperature heat capacity and neutron diffraction
measurements, is only 46% of the expected value of the static moment 3 $\mu$B.
This reduction indicates the presence of strong quantum fluctuations in the
half-integer spin S = 3/2 CaMnTeO$_6$ magnet, which is rare. By comparing the
spin-polarized band structure, chemical bonding, and physical properties of
AMnTeO$_6$ (A = Ca, Sr, Pb), we demonstrate how quantum-chemical interpretation
can illuminate insights into the fundamentals of magnetic exchange
interactions, providing a powerful tool for modulating spin dynamics with
atomically precise control.",2403.08069v1
2001-05-16,A dynamical mean field theory for the magnetic transition temperature of ferromagnetic semiconductors,"This paper has been withdrawn by the authors due to a numerical error. A
revised version will be posted shortly.",0105331v2
2007-01-25,The anisotropic XY model on the inhomogeneous periodic chain,"The static and dynamic properties of the anisotropic XY-model $(s=1/2)$ on
the inhomogeneous periodic chain, composed of $N$ cells with $n$ different
exchange interactions and magnetic moments, in a transverse field $h,$ are
determined exactly at arbitrary temperatures. The properties are obtained by
introducing the Jordan-Wigner fermionization and by reducing the problem to a
diagonalization of a finite matrix of $nth$ order. The quantum transitions are
determined exactly by analyzing, as a function of the field, the induced
magnetization $1/n\sum_{m=1}^{n}\mu_{m}\left< S_{j,m}^{z}\right>$ ($j$ denotes
the cell, $m$ the site within the cell, $\mu_{m}$ the magnetic moment at site
$m$ within the cell) and the spontaneous magnetization $1/n\sum_{m=1}^{n}\left<
S_{j,m}^{x},\right>$ which is obtained from the correlations $\left<
S_{j,m}^{x}S_{j+r,m}^{x}\right>$ for large spin separations. These results,
which are obtained for infinite chains, correspond to an extension of the ones
obtained by Tong and Zhong(\textit{Physica B} \textbf{304,}91 (2001)). The
dynamic correlations, $\left<
S_{j,m}^{z}(t)S_{j^{\prime},m^{\prime}}^{z}(0)\right>$, and the dynamic
susceptibility, $\chi_{q}^{zz}(\omega),$ are also obtained at arbitrary
temperatures. Explicit results are presented in the limit T=0, where the
critical behaviour occurs, for the static susceptibility $\chi_{q}^{zz}(0)$ as
a function of the transverse field $h$, and for the frequency dependency of
dynamic susceptibility $\chi_{q}^{zz}(\omega)$.",0701630v2
2005-09-22,"Ericson fluctuations in an open, deterministic quantum system: theory meets experiment","We provide numerically exact photoexcitation cross sections of rubidium
Rydberg states in crossed, static electric and magnetic fields, in quantitative
agreement with recent experimental results. Their spectral backbone underpins a
clear transition towards the Ericson regime.",0509189v1
2006-02-24,Gyroelectric cubic-quintic dissipative solitons,"The influence of an externally applied magnetic field upon classic cubic
quintic dissipative solitons is investigated using both exact simulations and a
Lagrangian technique. The basic approach is to use a spatially inhomogeneous
magnetic field and to consider two important geometries, namely the Voigt and
the Faraday effects. A layered structure is selected for the Voigt case with
the principal aim being to demonstrate non-reciprocal behaviour for various
classes of spatial solitons that are known to exist as solutions of the complex
Ginzburg-Landau cubic-quintic envelope equation under dissipative conditions.
The system is viewed as dynamical and an opportunity is taken to display the
behaviour patterns of the spatial solitons in terms of two-dimensional
dynamical plots involving the total energy and the peak amplitude of the
spatial solitons. This action this leads to limit cycle plots that beautifully
reveal the behaviour of the solitons solutions at all points along the
propagation axis. The closed contour that exists in the absence of a magnetic
field is opened up and a limit point is exposed. The onset of chaos is revealed
in a dramatic way and it is clear that detailed control by the external
magnetic field can be exercised. The Lagrangian approach is adjusted to deal
with dissipative systems and through the choice of particular trial functions,
aspects of the dynamic behaviour of the spatial are predicted by this approach.
Finally, some vortex dynamics in the Faraday configuration are investigated.",0602166v1
2007-07-13,Effect of antiferromagnetic exchange interactions on the Glauber dynamics of one-dimensional Ising models,"We study the effect of antiferromagnetic interactions on the single spin-flip
Glauber dynamics of two different one-dimensional (1D) Ising models with spin
$\pm 1$. The first model is an Ising chain with antiferromagnetic exchange
interaction limited to nearest neighbors and subject to an oscillating magnetic
field. The system of master equations describing the time evolution of
sublattice magnetizations can easily be solved within a linear field
approximation and a long time limit. Resonant behavior of the magnetization as
a function of temperature (stochastic resonance) is found, at low frequency,
only when spins on opposite sublattices are uncompensated owing to different
gyromagnetic factors (i.e., in the presence of a ferrimagnetic short range
order). The second model is the axial next-nearest neighbor Ising (ANNNI)
chain, where an antiferromagnetic exchange between next-nearest neighbors (nnn)
is assumed to compete with a nearest-neighbor (nn) exchange interaction of
either sign. The long time response of the model to a weak, oscillating
magnetic field is investigated in the framework of a decoupling approximation
for three-spin correlation functions, which is required to close the system of
master equations. The calculation, within such an approximate theoretical
scheme, of the dynamic critical exponent z, defined as ${1/\tau} \approx ({1/
{\xi}})^z$ (where \tau is the longest relaxation time and \xi is the
correlation length of the chain), suggests that the T=0 single spin-flip
Glauber dynamics of the ANNNI chain is in a different universality class than
that of the unfrustrated Ising chain.",0707.1953v1
2008-07-20,Nonlinear Solar Dynamo Model with Magnetic Helicity,"Variations of the sunspot number are important indicators of the solar
activity cycles. The sunspot formation is a result of a dynamo process inside
the Sun, which is far from being understood. We use simple dynamical models of
the dynamo process to simulate the magnetic field evolution and investigate
general properties of the sunspot number variations during the solar cycles. We
have found that the classical Parker's model with a standard kinetic helicity
quenching cannot represent the typical profiles of the solar-cycle variations
of the sunspot number, and also does not give chaotic solutions. For modeling
of the solar cycle properties we use a nonlinear dynamo model of Kleeorin and
Ruzmaikin (1982), which takes into account dynamics of the turbulent magnetic
helicity. We have obtained a series of periodic and chaotic solutions for
different layers of the convective zone. The solutions qualitatively reproduce
some basic observational features of the solar cycle properties, in particular,
the relationship between the growth time and the cycle amplitude. Also, on the
longer time scale the dynamo model with the magnetic helicity has intermittent
solutions, which may be important for modeling long-term variations of the
solar cycles.",0807.3192v2
2008-09-15,Kinetics of a mixed spin-1/2 and spin-3/2 Ising ferrimagnetic model,"We present a study, within a mean-field approach, of the kinetics of a mixed
ferrimagnetic model on a square lattice in which two interpenetrating square
sublattices have spins that can take two values, $\sigma=\pm1/2$, alternated
with spins that can take the four values, $S=\pm3/2, \pm1/2$. We use the
Glauber-type stochastic dynamics to describe the time evolution of the system
with a crystal-field interaction in the presence of a time-dependent
oscillating external magnetic field. The nature (continuous and discontinuous)
of transition is characterized by studying the thermal behaviors of average
order parameters in a period. The dynamic phase transition points are obtained
and the phase diagrams are presented in the reduced magnetic field amplitude
$(h)$ and reduced temperature $(T)$ plane, and in the reduced temperature and
interaction parameter planes, namely in the $(h, T)$ and $(d, T)$ planes, $d$
is the reduced crystal-field interaction. The phase diagrams always exhibit a
tricritical point in $(h, T)$ plane, but do not exhibit in the $(d, T)$ plane
for low values of $h$. The dynamic multicritical point or dynamic critical end
point exist in the $(d, T)$ plane for low values of $h$. Moreover, phase
diagrams contain paramagnetic $(p)$, ferromagnetic $(f)$, ferrimagnetic $(i)$
phases, two coexistence or mixed phase regions, $(f+p)$ and $(i+p)$, that
strongly depend on interaction parameters.",0809.2450v1
2009-02-17,Helicity and the Mañé critical value,"We establish a relationship between the helicity of a magnetic flow on a
closed surface of genus $\geq 2$ and the Ma\~n\'e critical value.",0902.2903v2
2011-04-24,"Reduced fluid-kinetic equations for low-frequency dynamics, magnetic reconnection and electron heating in low-beta plasmas","A minimal model for magnetic reconnection and, generally, low-frequency
dynamics in low-beta plasmas is proposed. The model combines analytical and
computational simplicity with physical realizability: it is a rigorous limit of
gyrokinetics for plasma beta of order the electron-ion mass ratio. The model
contains collisions and can be used both in the collisional and collisionless
reconnection regimes. It includes gyrokinetic ions (not assumed cold) and
allows for the topological rearrangement of the magnetic field lines by either
resistivity or electron inertia, whichever predominates. The two-fluid dynamics
are coupled to electron kinetics --- electrons are not assumed isothermal and
are described by a reduced drift-kinetic equation. The model therefore allows
for irreversibility and conversion of magnetic energy into electron heat via
parallel phase mixing in velocity space. An analysis of the exchanges between
various forms of free energy and its conversion into electron heat is provided.
It is shown how all relevant linear waves and regimes of the tearing
instability (collisionless, semicollisional and fully resistive) are recovered
in various limits of our model. An efficient way to simulate our equations
numerically is proposed, via the Hermite representation of the velocity space.
It is shown that small scales in velocity space will form, giving rise to a
shallow Hermite-space spectrum, whence it is inferred that, for steady-state or
sufficiently slow dynamics, the electron heating rate will remain finite in the
limit of vanishing collisionality.",1104.4622v2
2012-08-04,Dynamical and stationary critical behavior of the Ising ferromagnet in a thermal gradient,"In this paper we present and discuss results of Monte Carlo numerical
simulations of the two-dimensional Ising ferromagnet in contact with a heat
bath that intrinsically has a thermal gradient. The extremes of the magnet are
at temperatures $T_1<T_c<T_2$, where $T_c$ is the Onsager critical temperature.
In this way one can observe a phase transition between an ordered phase
($T<T_c$) and a disordered one ($T>T_c$) by means of a single simulation. By
starting the simulations with fully disordered initial configurations with
magnetization $m\equiv 0$ corresponding to $T=\infty$, which are then suddenly
annealed to a preset thermal gradient, we study the short-time critical dynamic
behavior of the system. Also, by setting a small initial magnetization $m=m_0$,
we study the critical initial increase of the order parameter. Furthermore, by
starting the simulations from fully ordered configurations, which correspond to
the ground state at T=0 and are subsequently quenched to a preset gradient, we
study the critical relaxation dynamics of the system. Additionally, we perform
stationary measurements ($t\rightarrow\infty$) that are discussed in terms of
the standard finite-size scaling theory. We conclude that our numerical
simulation results of the Ising magnet in a thermal gradient, which are
rationalized in terms of both dynamic and standard scaling arguments, are fully
consistent with well established results obtained under equilibrium conditions.",1208.0965v1
2012-10-01,"Seismology of the Sun : Inference of Thermal, Dynamic and Magnetic Field Structures of the Interior","Recent overwhelming evidences show that the sun strongly influences the
Earth's climate and environment. Moreover existence of life on this Earth
mainly depends upon the sun's energy. Hence, understanding of physics of the
sun, especially the thermal, dynamic and magnetic field structures of its
interior, is very important. Recently, from the ground and space based
observations, it is discovered that sun oscillates near 5 min periodicity in
millions of modes. This discovery heralded a new era in solar physics and a
separate branch called helioseismology or seismology of the sun has started.
Before the advent of helioseismology, sun's thermal structure of the interior
was understood from the evolutionary solution of stellar structure equations
that mimicked the present age, mass and radius of the sun. Whereas solution of
MHD equations yielded internal dynamics and magnetic field structure of the
sun's interior. In this presentation, I review the thermal, dynamic and
magnetic field structures of the sun's interior as inferred by the
helioseismology.",1210.0467v1
2014-12-17,Chromospheric diagnosis with Ca II lines: forward modeling in forward scattering (I),"This paper shows the first synthetic tomography of the quiet solar
chromosphere formed by spatial maps of scattering polarization. It has been
calculated for the CaII 8498, 8542 and 3934 A lines by solving the NLTE
(non-local thermodynamical equilibrium) RT (radiative transfer) problem of the
second kind in a 3D atmosphere model obtained from realistic MHD
(magneto-hydrodynamical) simulations. Maps of circular polarization were
calculated neglecting atomic polarization. Our investigation focuses on the
linear polarization signals induced by kinematics, radiation field anisotropy
and Hanle effect in forward-scattering geometry. Thus, instead of considering
slit profiles at the limb as normally done in the study of the second solar
spectrum, we synthetize and analyze spatial maps of polarization at disk
center. It allows us to understand the spatial signatures of dynamics and
magnetic field in the linear polarization for discriminating them
observationally. Our results suggest new ideas for chromospheric diagnosis that
will be developed throughout a serie of papers. In particular, Hanle Polarity
Inversion Lines and dynamic Hanle diagrams are two concepts introduced in the
present work. We find that chromospheric dynamics and magnetic field topology
create spatial fingerprints in the polarization maps that trace the dynamic
situation of the plasma and the magnetic field. Based on such spatial features
we reconstruct the magnetic field intensity in the middle chromosphere along
grooves of null linear polarization. We finally address the problems of
diagnosing Hanle saturation and kinematic amplification of scattering signals
using Hanle diagrams.",1412.5386v1
2015-03-03,A simple and universal setup of quasi-monocolor gamma-ray source,"Strict classic 3-D dynamics theory reveals that arbitrarily high center
frequency light source can be achieved by flexible application of
not-too-strong static electric field and static magnetic field. The magnitudes
of the fields are not required to be high.",1503.00815v1
2015-05-25,Quantum Dynamical Phase Transition in a Spin-Orbit Coupled Bose Condensate,"Spin-orbit coupled bosons can exhibit rich equilibrium phases at low
temperature and in the presence of particle-particle interactions. In the case
with a 1D synthetic spin-orbit interaction, it has been observed that the
ground state of a Bose gas can be a normal phase, stripe phase, or magnetized
phase in different experimentally controllable parameter regimes. The
magnetized states are doubly degenerate and consist of a many-particle
two-state system. In this work, we investigate the nonequilibrium quantum
dynamics by switching on an external perturbation to induce resonant couplings
between the magnetized phases, and predict the novel quantum spin dynamics
which cannot be obtained in the single-particle systems. In particular, due to
particle-particle interactions, the transition of the Bose condensate from one
magnetized phase to the other is forbidden when the strength of external
perturbation is less than a critical value, and a full transition can occur
only when the perturbation exceeds such critical strength. This phenomenon
manifests itself a quantum dynamical phase transition, with the critical point
behavior being exactly solvable. From the numerical simulations and exact
analytic studies we show that the predicted many-body effects can be well
observed with the current experiments.",1505.06687v1
2015-07-13,Macrospin Dynamics in Antiferromagnets Triggered by Sub-20 femtosecond Injection of Nanomagnons,"The understanding of how the sub-nanoscale exchange interaction evolves in
macroscale correlations and ordered phases of matter, such as magnetism and
superconductivity, requires to bridge the quantum and classical worlds. This
monumental challenge has so far only been achieved for systems close to their
thermodynamical equilibrium. Here we follow in real time the ultrafast dynamics
of the macroscale magnetic order parameter triggered by the impulsive optical
generation of spin excitations with the shortest possible nanometer-wavelength
and femtosecond-period. Our experiments also disclose a possibility for the
coherent control of these femtosecond nanomagnons, which are defined by the
exchange energy. These findings open up novel opportunities for fundamental
research on the role of short-wavelength spin excitations in magnetism and
high-temperature superconductivity, since they provide a macroscopic probe of
the femtosecond dynamics of sub-nanometer spin-spin correlations and,
ultimately, of the exchange energy. With this approach it becomes possible to
trace the dynamics of such short-range magnetic correlations for instance
during phase transitions. Moreover, our work suggests that nanospintronics and
nanomagnonics can employ phase-controllable spin waves with frequencies in the
20 THz domain.",1507.03377v1
2015-09-01,Lattice dynamics and a magnetic-structural phase transition in the nickel orthoborate $Ni_{3}(BO_{3})_{2}$,"Nickel orthoborate $Ni_{3}(BO_{3})_{2}$ having a complex orthorhombic
structure $P_{nnm}$ (#58, Z=2) of the kotoite type is known for quite a long
time as an antiferromagnetic material below $T_{N}$ = 46 K, but up to now its
physical properties including the lattice dynamics have not been explored. Six
magnetic nickel $Ni^{2+}$ ions (S=1) in the unit cell are distributed over the
2a and 4f positions in the centers of distorted $[O_{6}]$ octahedra. The
$[NiO_{6}]$ units are linked via rigid $[BO_{3}]$ groups and these structural
particularities impose restrictions on the lattice dynamics and spin-phonon
interactions. We performed the symmetry analysis of the phonon modes at the
center of the Brillouin zone. The structural parameters and phonon modes were
calculated using Dmol3 program. We report and analyze results of infrared and
Raman studies of phonon spectra measured in all required polarizations. Most of
the even and odd phonons predicted on the basis of the symmetry analysis and
theoretical calculations were reliably identified in the measured spectra.
Absorption measurements in the infrared region showed emergence of several very
narrow and weak phonons at the magnetic ordering temperature $T_{N}$. This
observation proves the existence of a structural phase transition not reported
before which is evidently coupled intrinsically with the magnetic dynamics of
$Ni_{3}(BO_{3})_{2}$. A clear evidence of spin-phonon interaction was observed
for some particular phonons below $T_{N}$.",1509.00344v2
2015-12-14,The dynamics of Wolf numbers based on nonlinear dynamo with magnetic helicity: comparisons with observations,"We investigate the dynamics of solar activity using a nonlinear
one-dimensional dynamo model and a phenomenological equation for the evolution
of Wolf numbers. This system of equations is solved numerically. We take into
account the algebraic and dynamic nonlinearities of the alpha effect. The
dynamic nonlinearity is related to the evolution of a small-scale magnetic
helicity, and it leads to a complicated behavior of solar activity. The
evolution equation for the Wolf number is based on a mechanism of formation of
magnetic spots as a result of the negative effective magnetic pressure
instability (NEMPI). This phenomenon was predicted 25 years ago and has been
investigated intensively in recent years through direct numerical simulations
and mean-field simulations. The evolution equation for the Wolf number includes
the production and decay of sunspots. Comparison between the results of
numerical simulations and observational data of Wolf numbers shows a 70 %
correlation over all intervals of observation (about 270 years). We determine
the dependence of the maximum value of the Wolf number versus the period of the
cycle and the asymmetry of the solar cycles versus the amplitude of the cycle.
These dependencies are in good agreement with observations.",1512.04363v4
2016-01-08,Spin-glass like dynamics of ferromagnetic clusters in La$_{0.75}$Ba$_{0.25}$CoO$_3$,"We report the magnetization study of the compound
La$_{0.75}$Ba$_{0.25}$CoO$_3$ where Ba$^{2+}$ doping is just above the critical
limit for percolation of ferromagnetic clusters. The field cooled (FC) and zero
field cooled (ZFC) magnetization exhibit a thermomagnetic irreversibility and
the ac susceptibility show a frequency dependent peak at the ferromagnetic
ordering temperature (T$_C$$\approx$203~K) of the clusters. These features
indicate about the presence of a non-equilibrium state below T$_C$. In the
non-equilibrium state, the dynamic scaling of the imaginary part of ac
susceptibility and the static scaling of the nonlinear susceptibility clearly
establish a spin-glass like cooperative freezing of ferromagnetic clusters at
200.9(2)~K. The existence of spin-glass like freezing of ferromagnetic clusters
is further substantiated by the ZFC aging and memory experiments. We also
observe certain dynamical features which are not present in a typical
spin-glass, such as, initial magnetization after ZFC aging first increases and
then decreases with the wait time and an imperfect recovery of relaxation in
negative temperature cycling experiments. This imperfect recovery transforms to
perfect recovery on concurrent field cycling. Our analysis suggests that these
additional dynamical features have their origin in inter-cluster exchange
interaction and cluster size distribution. The inter-cluster exchange
interaction above the magnetic percolation gives a superferromagnetic state in
some granular thin films but our results show the absence of typical
superferromagnetic like state in La$_{0.75}$Ba$_{0.25}$CoO$_3$.",1601.02009v1
2017-02-24,Magnetic order and spin dynamics across a ferromagnetic quantum critical point: $μ$SR investigations of YbNi$_4$(P$_{1-x}$As$_x$)$_2$,"In the quasi-1D heavy-fermion system YbNi$_4$(P$_{1-x}$As$_x$)$_2$ the
presence of a ferromagnetic (FM) quantum critical point (QCP) at $x_c$ $\approx
0.1$ with unconventional quantum critical exponents in the thermodynamic
properties has been recently reported. Here, we present muon-spin relaxation
($\mu$SR) experiments on polycrystals of this series to study the magnetic
order and the low energy 4$f$-electronic spin dynamics across the FM QCP. The
zero field $\mu$SR measurements on pure YbNi$_4$(P$_{2}$ proved static long
range magnetic order and suggested a strongly reduced ordered Yb moment of
about 0.04$\mu_B$. With increasing As substitution the ordered moment is
reduced by half at $x = 0.04$ and to less than 0.005 $\mu_B$ at $x=0.08$. The
dynamic behavior in the $\mu$SR response show that magnetism remains
homogeneous upon As substitution, without evidence for disorder effect. In the
paramagnetic state across the FM QCP the dynamic muon-spin relaxation rate
follows 1/$T_{1}T\propto T^{-n}$ with $1.01 \pm 0.04 \leq n \leq 1.13 \pm
0.06$. The critical fluctuations are very slow and are even becoming slower
when approaching the QCP.",1702.07504v2
2017-03-22,Time evolution of a pair of distinguishable interacting spins subjected to controllable and noisy magnetic fields,"The quantum dynamics of a
$\hat{\mathbf{J}}^2=(\hat{\mathbf{j}}_1+\hat{\mathbf{j}}_2)^2$-conserving
Hamiltonian model describing two coupled spins $\hat{\mathbf{j}}_1$ and
$\hat{\mathbf{j}}_2$ under controllable and fluctuating time-dependent magnetic
fields is investigated. Each eigenspace of $\hat{\mathbf{J}}^2$ is dynamically
invariant and the Hamiltonian of the total system restricted to any one of such
$(j_1+j_2)-|j_1-j_2|+1$ eigenspaces, possesses the SU(2) structure of the
Hamiltonian of a single fictitious spin acted upon by the total magnetic field.
We show that such a reducibility holds regardless of the time dependence of the
externally applied field as well as of the statistical properties of the noise,
here represented as a classical fluctuating magnetic field. The time evolution
of the joint transition probabilities of the two spins $\hat{\mathbf{j}}_1$ and
$\hat{\mathbf{j}}_2$ between two prefixed factorized states is examined,
bringing to light peculiar dynamical properties of the system under scrutiny.
When the noise-induced non-unitary dynamics of the two coupled spins is
properly taken into account, analytical expressions for the joint Landau-Zener
transition probabilities are reported. The possibility of extending the
applicability of our results to other time-dependent spin models is pointed
out.",1703.07673v3
2017-05-02,Slow spin tunneling in the paramagnetic phase of the pyrochlore Nd2Sn2O7,"The insulating pyrochlore compound Nd2Sn2O7 has been shown to undergo a
second order magnetic phase transition at Tc ~ 0.91 K to a noncoplanar
all-in--all-out magnetic structure of the Nd3+ magnetic moments. An anomalously
slow paramagnetic spin dynamics has been evidenced from neutron backscattering
and muon spin relaxation (muSR). In the case of muSR this has been revealed
through the strong effect of a 50 mT longitudinal field on the spin-lattice
relaxation rate. Here, motivated by a recent successful work performed for
Yb2Ti2O7 and Yb2Sn2O7, analyzing the shape of the muSR longitudinal
polarization function, we substantiate the existence of extremely slow
paramagnetic spin dynamics in the microsecond time range for Nd2Sn2O7. Between
1.7 and 7 K, this time scale is temperature independent. This suggests a double
spin-flip tunneling relaxation mechanism to be at play, probably involving spin
substructures such as tetrahedra. Unexpectedly, the standard deviation of the
field distribution at the muon site increases as the system is cooled. This
exotic spin dynamics is in sharp contrast with the dynamics above 100 K which
is driven by the Orbach relaxation mechanism involving single Nd3+ magnetic
moments.",1705.01004v1
2017-05-18,Magnetization dynamics and its scattering mechanism in thin CoFeB films with interfacial anisotropy,"Studies of magnetization dynamics have incessantly facilitated the discovery
of fundamentally novel physical phenomena, making steady headway in the
development of magnetic and spintronics devices. The dynamics can be induced
and detected electrically, offering new functionalities in advanced electronics
at the nanoscale. However, its scattering mechanism is still disputed.
Understanding the mechanism in thin films is especially important, because most
spintronics devices are made from stacks of multilayers with nanometer
thickness. The stacks are known to possess interfacial magnetic anisotropy, a
central property for applications, whose influence on the dynamics remains
unknown. Here, we investigate the impact of interfacial anisotropy by adopting
CoFeB/MgO as a model system. Through systematic and complementary measurements
of ferromagnetic resonance (FMR), on a series of thin films, we identify
narrower FMR linewidths at higher temperatures. We explicitly rule out the
temperature dependence of intrinsic damping as a possible cause, and it is also
not expected from existing extrinsic scattering mechanisms for ferromagnets. We
ascribe this observation to motional narrowing, an old concept so far neglected
in the analyses of FMR spectra. The effect is confirmed to originate from
interfacial anisotropy, impacting the practical technology of spin-based
nanodevices up to room temperature.",1705.06624v1
2018-02-20,Ultrafast magnetization dynamics in pure and doped Heusler and inverse Heusler alloys,"By using a multiscale approach based on first-principles density functional
theory combined with atomistic spin dynamics, we investigate the electronic
structure and magnetization dynamics of an inverse Heusler and a Heusler
compound and their alloys, i. e. Mn$_{2-x}Z_x$CoAl and Mn$_{2-x}Z_x$VAl, where
$Z$ = Mo, W, Os and Ru, respectively. A signature of the ferrimagnetic ordering
of Mn$_{2}$CoAl and Mn$_{2}$VAl Heusler alloys is reflected in the calculated
Heisenberg exchange constants. They decay very rapidly with the interatomic
distance and have short range, which is a consequence of the existence of the
finite gap in the minority spin band. The calculated Gilbert damping parameter
of both Mn$_2$CoAl and Mn$_2$VAl is high compared to other half-metals, but
interestingly in the particular case of the inverse Mn$_{2}$CoAl alloys and due
to the spin-gapless semiconducting property, the damping parameters decrease
with the doping concentration in clear contradiction to the general trend.
Atomistic spin dynamics simulations predict ultrafast magnetisation switching
in Mn$_{2}$CoAl and Mn$_{2}$VAl under the influence of an external magnetic
field, starting from a threshold field of $2\text{T}$. Our overall finding
extends with Heusler and inverse Heusler alloys, the class of materials that
exhibits laser induced magnetic switching.",1802.07195v1
2018-06-24,Nanoscopic time crystal obtained by nonergodic spin dynamics,"We study the far-from-equilibrium properties of quenched magnetic nanoscopic
classical spin systems. In particular, we focus on the interplay between
lattice vibrations and magnetic frustrations induced by surface effects typical
of an antiferromagnet. We use a combination of Monte Carlo simulations and
explore the dynamical behaviours by solving the stochastic
Landau-Lifshitz-Gilbert equation at finite temperature. The Monte Carlo
approach treats both the ionic degrees of freedom and spin variables on the
same footing, via an extended Lennard-Jones Hamiltonian with a spin-lattice
coupling. The zero temperature phase diagram of the finite size nanoscopic
systems with respect to the range of the Heisenberg interaction and the
Lennard-Jones coupling constant shows two main structures with non-trivial
magnetisation triggered by antiferromagnetism: a simple cubic and a
body-centred cubic. At non zero temperature, the competition between spins and
the ionic vibrations considerably affects the magnetization of the system.
Exploring the dynamics reveals a non-trivial structural induced behaviour in
the spin relaxation with a concomitant memory of the initially applied
ferromagnetic quench. We report the observation of a non-trivial dynamical
scenario, obtained after a ferromagnetic magnetic quench at low temperature.
Furthermore, we observe long-lived non-thermal states which could open new
avenues for nano-technology.",1806.09130v4
2018-08-01,Spin fluctuations in the light-induced high spin state of Cobalt valence tautomers,"We present a study of the static magnetic properties and spin dynamics in
Cobalt valence tautomers (VT), molecules where a low-spin (LS) to high-spin
(HS) crossover driven by an intramolecular electron transfer can be controlled
by the temperature, by the external pressure or by light irradiation. In the
investigated complex, a LS-Co(III) ion bound to a dinegative organic ligand can
be reversibly converted into the HS-Co(II) bound to a mononegative one. By
combining magnetization measurements with Nuclear Magnetic Resonance (NMR) and
Muon Spin Relaxation ({\mu}SR), we have investigated the static magnetic
properties and the spin dynamics as a function of the temperature. Moreover,
the effect of the external pressure as well as of the infrared light
irradiation have been explored through magnetometry and NMR measurements to
determine the spin dynamics of the HS state. The photoinduced HS state, which
can have a lifetime of several hours below 30 K, is characterized by spin
dynamics in the MHz range, which persist at least down to 10 K. The application
of an external pressure causes a progressive increase of the LS-HS crossover,
which reaches room temperature for pressures around 10 kbar.",1808.04229v1
2019-04-17,Peculiarities of the Dynamics of Solar NOAA Active Region 12673,"The dynamics of active region (AR) 12673 is qualitatively studied using
observational data obtained with the Helioseismic and Magnetic Imager of the
Solar Dynamics Observatory on August 31--September 8, 2017. This AR was
remarkable for its complex structure and extraodinary flare productivity. The
sunspot group in this AR consisted of (1) an old, well-developed and highly
stable, coherent sunspot, which had also been observed two solar rotations
earlier, and (2) a rapidly developing cluster of umbral and penumbral
fragments. Cluster (2) formed two elongated, arc-shaped chains of spot
elements, skirting around the major sunspot (1), with two chains of magnetic
elements spatially coinciding with the arcs. AR components (1) and (2) were in
relative motion, cluster (2) overtaking spot (1), and their relative velocity
agrees in order of magnitude with the velocity jump over the near-surface shear
layer, or leptocline. The pattern of motion of the features about the main spot
bears amazing resemblance to the pattern of a fluid flow about a roundish body.
This suggests that spot (1) was dynamically coupled with the surface layers,
while cluster (2) developed in deeper layers of the convection zone. The
magnetic-flux emergence in cluster (2) appeared to be associated with fluid
motions similar to roll convection. The mutual approach of components (1) and
(2) gave rise to lights bridges in the umbrae of sunspots with the magnetic
field having the same sign on both sides of the bridge.",1904.08367v3
2019-08-12,"Spin beats in the photoluminescence polarization dynamics of charged excitons in InP/(In,Ga)P quantum dots in presence of nuclear quadrupole interaction","The spin dynamics of positively (X$^{+}$) and negatively (X$^{-}$) charged
excitons in InP/In$_{0.48}$Ga$_{0.52}$P quantum dots subject to a magnetic
field is studied. We find that a characteristic feature of the system under
study is the presence of nuclear quadrupole interaction, which leads to
stabilization of the nuclear and electron spins in a quantum dot in zero
external magnetic field. In detail, the nuclear quadrupole interaction leads to
pinning of the Overhauser field along the quadrupole axis, which is close to
the growth axis of the heterostructure. The nuclear effects are observed only
when resident electrons are confined in the quantum dots, i.e. for X$^{-}$
trion photoexcitation. The presence of X$^{-}$ and X$^{+}$ trion contributions
to the photoluminescence together with the quadrupole interaction significantly
affects the dynamics of optical orientation in Voigt magnetic field. In absence
of dynamic nuclear spin polarization the time evolution of the
photoluminescence polarization was fitted by a form which describes the
electron spin relaxation in ""frozen"" nuclear field fluctuations. In relatively
large external magnetic fields exceeding 60 mT good agreement between theory
and experiment is achieved.",1908.04167v1
2020-10-17,Ferromagnetic Gyroscopes for Tests of Fundamental Physics,"A ferromagnetic gyroscope (FG) is a ferromagnet whose angular momentum is
dominated by electron spin polarization and that will precess under the action
of an external torque, such as that due to a magnetic field. Here we model and
analyze FG dynamics and sensitivity, focusing on practical schemes for
experimental realization. In the case of a freely floating FG, we model the
transition from dynamics dominated by libration in relatively high externally
applied magnetic fields, to those dominated by precession at relatively low
applied fields. Measurement of the libration frequency enables in situ
measurement of the magnetic field and a technique to reduce the field below the
threshold for which precession dominates the FG dynamics. We note that evidence
of gyroscopic behavior is present even at magnetic fields much larger than the
threshold field below which precession dominates. We also model the dynamics of
an FG levitated above a type-I superconductor via the Meissner effect, and find
that for FGs with dimensions larger than about 100 nm the observed precession
frequency is reduced compared to that of a freely floating FG. This is akin to
negative feedback that arises from the distortion of the field from the FG by
the superconductor. Finally we assess the sensitivity of an FG levitated above
a type-I superconductor to exotic spin-dependent interactions under practical
experimental conditions, demonstrating the potential of FGs for tests of
fundamental physics.",2010.08731v1
2021-01-10,On the origin of the solar hemispherical helicity rules: Simulations of the rise of magnetic flux concentrations in a background field,"Solar active regions and sunspots are believed to be formed by the emergence
of strong toroidal magnetic flux from the solar interior. Modeling of such
events has focused on the dynamics of compact magnetic entities, colloquially
known as ""flux tubes"", often considered to be isolated magnetic structures
embedded in an otherwise field-free environment. In this paper, we show that
relaxing such idealized assumptions can lead to surprisingly different
dynamics. We consider the rise of tube-like flux concentrations embedded in a
large-scale volume-filling horizontal field in an initially quiescent
adiabatically-stratified compressible fluid. In a previous letter, we revealed
the unexpected major result that concentrations that have their twist aligned
with the background field at the bottom of the tube are more likely to rise
than the opposite orientation (for certain values of the parameters). This bias
leads to a selection rule which, when applied to solar dynamics, is in
agreement with the observations known as the solar hemispheric helicity rule(s)
(SHHR). Here, we examine this selection mechanism in more detail than was
possible in the earlier letter. We explore the dependence on the parameters via
simulations, delineating the Selective Rise Regime (SRR), where the bias
operates. We provide a theoretical model to predict and explain the simulation
dynamics. Furthermore, we create synthetic helicity maps from Monte Carlo
simulations to mimic the SHHR observations and demonstrate that our mechanism
explains the observed scatter in the rule and its variation over the solar
cycle.",2101.03472v1
2021-06-11,Fast collective oscillations and clustering phenomena in an antiferromagnetic mean-field model,"We study the out-of-equilibrium properties of the antiferromagnetic
Hamiltonian Mean-Field model at low energy. In this regime, the Hamiltonian
dynamics exhibits the presence of a stationary state where the rotators are
gathered in a bicluster. This state is not predicted by equilibrium statistical
mechanics in the microcanonical ensemble. Performing a low kinetic energy
approximation, we derive the explicit expression of the magnetization vector as
a function of time. We find that the latter displays coherent oscillations, and
we show numerically that the probability distribution for its phase is bimodal
or quadrimodal. We then look at the individual rotator dynamics as a motion in
an external time-dependent potential, given by the magnetization. This dynamics
exhibits two distinct time scales, with the fast one associated to the
oscillations of the global magnetization vector. Performing an average over the
fast oscillations, we derive an expression for the effective force acting on
the individual rotator. This force is always bimodal, and determines a low
frequency oscillation of the rotators. Our approach leads to a self-consistent
theory linking the time-dependence of the magnetization to the motion of the
rotators, providing a heuristic explanation for the formation of the bicluster.",2106.07392v1
2022-06-20,First-principles calculation of the parameters used by atomistic magnetic simulations,"While the ground state of magnetic materials is in general well described on
the basis of spin density functional theory (SDFT), the theoretical description
of finite-temperature and non-equilibrium properties require an extension
beyond the standard SDFT. Time-dependent SDFT (TD-SDFT), which give for example
access to dynamical properties are computationally very demanding and can
currently be hardly applied to complex solids. Here we focus on the alternative
approach based on the combination of a parameterized phenomenological spin
Hamiltonian and SDFT-based electronic structure calculations, giving access to
the dynamical and finite-temperature properties for example via spin-dynamics
simulations using the Landau-Lifshitz-Gilbert (LLG) equation or Monte Carlo
simulations. We present an overview on the various methods to calculate the
parameters of the various phenomenological Hamiltonians with an emphasis on the
KKR Green function method as one of the most flexible band structure methods
giving access to practically all relevant parameters. Concerning these, it is
crucial to account for the spin-orbit coupling (SOC) by performing relativistic
SDFT-based calculations as it plays a key role for magnetic anisotropy and
chiral exchange interactions represented by the DMI parameters in the spin
Hamiltonian. This concerns also the Gilbert damping parameters characterizing
magnetization dissipation in the LLG equation, chiral multispin interaction
parameters of the extended Heisenberg Hamiltonian, as well as spin-lattice
interaction parameters describing the interplay of spin and lattice dynamics
processes, for which an efficient computational scheme has been developed
recently by the present authors.",2206.09969v1
2022-08-17,The quantum dynamic range of room temperature spin imaging,"Magnetic resonance imaging of spin systems combines scientific applications
in medicine, chemistry and physics. Here, we investigate the pixel-wise
coherent quantum dynamics of spins consisting of a 40 by 40 micron sized region
of interest implanted with nitrogen vacancy centers (NV) coupled to a
nano-magnetic flake of $\mathrm{CrTe_2}$. $\mathrm{CrTe_2}$ is an in-plane van
der Waals ferromagnet, which we can probe quantitatively by the NV electron's
spin signal even at room temperature. First, we combine the nano-scale sample
shapes measured by atomic force microscope with the magnetic resonance imaging
data. We then map out the coherent dynamics of the colour centers coupled to
the van der Waals ferromagnet using pixel-wise coherent Rabi and Ramsey imaging
of the NV sensor layer. Next, we fit the pixel-wise solution of the Hamiltonian
to the quantum sensor data. Combining data and model, we can explore the
detuning range of the spin oscillation with a quantum dynamic range of over
$\left|\Delta_{max}\right|= 60 { }\mathrm{MHz} $ in the Ramsey interferometry
mode. Finally, we show the effect of the $\mathrm{CrTe_2}$ van der Waals magnet
on the coherence of the NV sensor layer and measure a 70 times increase in the
maximum frequency of the quantum oscillation going from the Rabi to the Ramsey
imaging mode.",2208.08146v1
2022-11-07,Multilayer spintronic neural networks with radio-frequency connections,"Spintronic nano-synapses and nano-neurons perform complex cognitive
computations with high accuracy thanks to their rich, reproducible and
controllable magnetization dynamics. These dynamical nanodevices could
transform artificial intelligence hardware, provided that they implement
state-of-the art deep neural networks. However, there is today no scalable way
to connect them in multilayers. Here we show that the flagship nano-components
of spintronics, magnetic tunnel junctions, can be connected into multilayer
neural networks where they implement both synapses and neurons thanks to their
magnetization dynamics, and communicate by processing, transmitting and
receiving radio frequency (RF) signals. We build a hardware spintronic neural
network composed of nine magnetic tunnel junctions connected in two layers, and
show that it natively classifies nonlinearly-separable RF inputs with an
accuracy of 97.7%. Using physical simulations, we demonstrate that a large
network of nanoscale junctions can achieve state-of the-art identification of
drones from their RF transmissions, without digitization, and consuming only a
few milliwatts, which is a gain of more than four orders of magnitude in power
consumption compared to currently used techniques. This study lays the
foundation for deep, dynamical, spintronic neural networks.",2211.03659v1
2022-11-09,Time-resolved measurement of spin excitations in Cu$_2$OSeO$_3$,"Magnetic diffraction in combination with x-ray detected ferromagnetic
resonance (DFMR) is a powerful technique for performing time-resolved
measurements on individual spin textures. Here, we study the ferromagnetic
resonance (FMR) modes of both the conical and field-polarized phases in the
chiral magnet Cu$_2$OSeO$_3$. Following the identification of the FMR modes at
different temperatures using broadband vector network analyzer FMR, we use DFMR
on the crystalline (001) Bragg peak to reveal the time-dependent spin
configurations of the selected FMR modes. By being able to measure both the
amplitude and phase response of the spin system across the resonance, a
continuous phase advance (of 180$^\circ$) in the conical mode, and a phase lag
(of 180$^\circ$) in the field-polarized mode is found. By performing dynamic
measurements in the conical phase as a function of the linear polarization
angle of the x-rays, i.e., successively probing the dynamics of the moments, we
find an inversion of the dynamics along the conical axis upon inverting the
applied field direction. By allowing for time-resolved measurements of the
phase and amplitude of individual magnetic phases, DFMR opens up new
opportunities for obtaining a deeper understanding of the complex dynamics of
chiral magnets.",2211.05195v1
2023-04-26,Charged particle dynamics in parabolic magnetosphere around Schwarzschild black hole,"The study of charged particle dynamics in the combined gravitational and
magnetic field can provide important theoretical insight into astrophysical
processes around black holes. In this paper, we explore the charged particle
dynamics in parabolic magnetic field configuration around Schwarzschild black
hole, since the paraboloidal shapes of magnetic field lines around black holes
are well motivated by the numerical simulations and supported by observations
of relativistic jets. Analysing the stability of bounded orbits and using the
effective potential approach, we show the possibility of existence of stable
circular off-equatorial orbits around the symmetry axis. We also show the
influence of radiation reaction force on the dynamics of charged particles, in
particular on the chaoticity of the motion and Poincar\'{e} sections,
oscillatory frequencies, and emitted electromagnetic spectrum. Applied to
Keplerian accretion disks, we show that in parabolic magnetic field
configuration, the thin accretion configurations can be either destroyed or
transformed into a thick toroidal structure given the radiation reaction and
electromagnetic-disk interactions included. Calculating the Fourier spectra for
radiating charged particle trajectories, we find that the radiation reaction
force does not affect the main frequency peaks, however, it lowers the higher
harmonics making the spectrum more flat and diluted in high frequency range.",2304.13603v1
2023-07-22,Magnetization dynamics due to field interplay in field free spin Hall nano-oscillators,"Spin Hall nano oscillators (SHNOs) have shown applications in unconventional
computing schemes and broadband frequency generation in the presence of applied
external magnetic field. However, under field-free conditions, the oscillation
characteristics of SHNOs display a significant dependence on the effective
field, which can be tuned by adjusting the constriction width, thereby
presenting an intriguing area of study. Here we study the effect of nano
constriction width on the magnetization dynamics in anisotropy assisted field
free SHNOs. In uniaxial anisotropy-based field-free SHNOs, either the
anisotropy field or the demagnetization field can dominate the magnetization
dynamics depending on the constriction width. Our findings reveal distinct
auto-oscillation characteristics in narrower constrictions with 20 nm and 30 nm
constriction width compared to their wider counterpart with 100 nm width. The
observed frequency shift variations with input current and constriction widths
stem from the inherent nonlinearity of the system. The interplay between the
B_demag and B_anis, coupled with changes in constriction width, yields rich
dynamics and offers control over frequency tunability, auto oscillation
amplitude, and threshold current. Notably, the spatial configuration of spin
wave wells within the constriction undergoes transformations in response to
changes in both constriction width and anisotropy. The findings highlight the
significant influence of competing fields at the constriction on the field-free
auto oscillations of SHNOs, with this impact intensifying as the constriction
width is varied.",2307.12068v1
2023-12-12,Sliding Dynamics of Current-Driven Skyrmion Crystal and Helix in Chiral Magnets,"The skyrmion crystal (SkX) and helix (HL) phases, present in typical chiral
magnets, can each be considered as forms of density waves but with distinct
topologies. The SkX exhibits gyrodynamics analogous to electrons under a
magnetic field, while the HL state resembles topological trivial spin density
waves. However, unlike the charge density waves, the theoretical analysis of
the sliding motion of SkX and HL remains unclear, especially regarding the
similarities and differences in sliding dynamics between these two spin density
waves. In this work, we systematically explore the sliding dynamics of SkX and
HL in chiral magnets in the limit of large current density. We demonstrate that
the sliding dynamics of both SkX and HL can be unified within the same
theoretical framework as density waves, despite their distinct microscopic
orders. Furthermore, we highlight the significant role of gyrotropic sliding
induced by impurity effects in the SkX state, underscoring the impact of
nontrivial topology on the sliding motion of density waves. Our theoretical
analysis shows that the effect of impurity pinning is much stronger in HL
compared with SkX, i.e., $\chi^{SkX}/\chi^{HL}\sim \alpha^2$ ($\chi^{SkX}$,
$\chi^{HL}$: susceptibility to the impurity potential, $\alpha$ ($\ll 1$) is
the Gilbert damping). Moreover, the velocity correction is mostly in the
transverse direction to the current in SkX. These results are further
substantiated by realistic Landau-Lifshitz-Gilbert simulations.",2312.07116v2
2023-12-14,Dust Dynamics in Hall-effected Protoplanetary Disks. I. Background Drift Hall Instability,"Recent studies have shown that the large-scale gas dynamics of protoplanetary
disks (PPDs) are controlled by non-ideal magneto-hydrodynamics (MHD), but how
this influences dust dynamics is not fully understood. To this end, we
investigate the stability of dusty, magnetized disks subject to the Hall
effect, which applies to planet-forming regions of PPDs. We find a novel
Background Drift Hall Instability (BDHI) that may facilitate planetesimal
formation in Hall-effected disk regions. Through a combination of linear
analysis and nonlinear simulations, we demonstrate the viability and
characteristics of BDHI. We find it can potentially dominate over the classical
streaming instability (SI) and standard MHD instabilities at low dust-to-gas
ratios and weak magnetic fields. We also identify magnetized versions of the
classic SI, but these are usually subdominant. We highlight the complex
interplay between magnetic fields and dust-gas dynamics in PPDs, underscoring
the need to consider non-ideal MHD like the Hall effect in the broader
narrative of planet formation.",2312.09180v1
2024-03-11,Ultrafast switching of sliding ferroelectricity and dynamical magnetic field in van der Waals bilayer induced by light,"Sliding ferroelectricity is a unique type of polarity recently observed in a
properly stacked van der Waals bilayer. However, electric-field control of
sliding ferroelectricity is hard and could induce large coercive electric
fields and serious leakage currents which corrode the ferroelectricity and
electronic properties, which are essential for modern two-dimensional
electronics and optoelectronics. Here, we proposed laser-pulse deterministic
control of sliding ferroelectricity in bilayer h-BN by first principles and
molecular dynamics simulation with machine-learned force fields. The laser
pulses excite shear modes which exhibit certain directional movements of
lateral sliding between bilayers. The vibration of excited modes under laser
pulses is predicted to overcome the energy barrier and achieve the switching of
sliding ferroelectricity. Furthermore, it is found that three possible sliding
transitions - between AB (BA) and BA (AB) stacking - can lead to the occurrence
of dynamical magnetic fields along three different directions. Remarkably, the
magnetic fields are generated by the simple linear motion of nonmagnetic
species, without any need for more exotic (circular, spiral) pathways. Such
predictions of deterministic control of sliding ferroelectricity and
multi-states of dynamical magnetic field thus expand the potential applications
of sliding ferroelectricity in memory and electronic devices.",2403.06531v2
1996-08-07,Dynamics of the 1D Heisenberg model and optical absorption of spinons in cuprate antiferromagnetic chains,"We use numerical and analytical results to construct a simple ansatz for the
energy dynamical correlation function of the 1D antiferromagnetic Heisenberg
model. This is applied to compute the phonon assisted absorption spectra of
magnetic excitations (spinons) in quasi-one dimensional spin 1/2 insulators and
show to reproduce very well recent infrared measurements in Sr$_2$CuO$_3$.",9608035v1
1998-07-08,Integrability and Quantum Chaos in Spin Glass Shards,"We study spin glass clusters (""shards"") in a random transverse magnetic
field, and determine the regime where quantum chaos and random matrix level
statistics emerge from the integrable limits of weak and strong field.
Relations with quantum phase transition are also discussed.",9807123v1
1998-12-09,Critical Relaxation and Critical Exponents,"Dynamic relaxation of the XY model and fully frustrated XY model quenched
from an initial ordered state to the critical temperature or below is
investigated with Monte Carlo methods. Universal power law scaling behaviour is
observed. The dynamic critical exponent $z$ and the static exponent $\eta$ are
extracted from the time-dependent Binder cumulant and magnetization. The
results are competitive to those measured with traditional methods.",9812147v1
2000-09-25,Spin dynamics in Cuprates and its relation to superconductivity,"The relevance of magnetism for the mechanism responsible for high-temperature
superconductivity remains an open and still interesting issue. The observation
by inelastic neutron scattering of strong antiferromagnetic dynamical
correlations in superconducting cuprates is discussed in relation to the
unusual physical properties of the cuprates as well as in relation to the
superconducting pairing.",0009373v1
2000-11-21,Time resolved nanosecond vortex dynamics in high Tc superconducting films,"We report on the observation of the rearrangement of vortices in the ns time
scale following a fast magnetic field variation due to selective local heating
of a high Tc superconducting film. Detailed simulations describing the measured
photoinduced voltage signals along the film lead to a better understanding of
vortex dynamics in fast regimes, where mean vortex velocities reach 10000 m/s",0011347v1
2001-04-27,Magnetic order in the Ising model with parallel dynamics,"It is discussed how the equilibrium properties of the Ising model are
described by an Hamiltonian with an antiferromagnetic low temperature behavior
if only an heat bath dynamics, with the characteristics of a Probabilistic
Cellular Automaton, is assumed to determine the temporal evolution of the
system.",0104529v1
2001-10-19,Very low-frequency excitations in frustrated two-dimensional $S=1/2$ Heisenberg antiferromagnets,"muSR and 7Li NMR relaxation measurements in frustrated two-dimensional S=1/2
Heisenberg antiferromagnets on a square lattice are presented. It is found that
both in Li2VOSiO4 and Li2VOGeO4, spin dynamics at frequencies well below the
Heisenberg exchange frequency are present. These dynamics are associated with
the motions of walls separating coexisting collinear domains with a magnetic
wave vector rotated by 90 degrees.",0110398v1
2002-08-20,"Comment on ""Critical Dynamics of a Vortex-Loop Model for the Superconducting Transition""","Recently, Aji and Goldenfeldt [Phys. Rev. Lett. 87, 197003 (2001),
cond-mat/0105622] put forward an explanation for the value of the dynamic
critical exponent z observed in certain Monte Carlo simulations of the
superconducting phase transition in zero magnetic field. In this Comment, we
point out that their analysis is based on incorrect assumptions regarding the
scaling dimension of the vortex density.",0208380v1
2003-10-30,Time and length scales in spin glasses,"We discuss the slow, nonequilibrium, dynamics of spin glasses in their glassy
phase. We briefly review the present theoretical understanding of the
spectacular phenomena observed in experiments and describe new numerical
results obtained in the first large-scale simulation of the nonequilibrium
dynamics of the three dimensional Heisenberg spin glass.",0310721v1
2004-02-03,Simulations of Dynamical Ordering in Pinned Vortex Systems,"We model a vortex system in a sample with bulk pinning and superficial
pinning generated by a magnetic decoration. We perform a sequence of finite
temperature numerical experiments in which external forces are applied to
obtain a dynamically ordered vortex lattice. We analyze the final structures
and the behavior of the total energy of the system.",0402098v1
2005-02-10,Mott--Hubbard and Anderson Transitions in Dynamical Mean--Field Theory,"The Anderson--Hubbard Hamiltonian at half--filling is investigated within
dynamical mean--field theory at zero temperature. The local density of states
is calculated by taking the geometric and arithmetic mean, respectively. The
non--magnetic ground state phase diagrams obtained within the different
averaging schemes are compared.",0502257v1
2005-03-03,Quantum dynamics of a nanomagnet in a rotating field,"Quantum dynamics of a two-state spin system in a rotating magnetic field has
been studied. Analytical and numerical results for the transition probability
have been obtained along the lines of the Landau-Zener-Stueckelberg theory. The
effect of various kinds of noise on the evolution of the system has been
analyzed.",0503085v1
2005-08-29,Effect of nuclear spins on the electron spin dynamics in negatively charged InP quantum dots,"Kinetics of polarized photoluminescence of the negatively charged InP quantum
dots in weak magnetic field is studied experimentally. Effect of both the
nuclear spin fluctuations and the dynamical nuclear polarization on the
electron spin orientation is observed.",0508698v1
1993-11-19,Exploring the Dynamics of Three-Dimensional Lattice Gauge Theories by External Fields,"We investigate the dynamics of three-dimensional lattice gauge theories by
means of an external Abelian magnetic field. For the SU(2) lattice gauge theory
we find evidence of the unstable modes.",9311024v1
2004-07-13,Dynamical correlation functions of the XXZ spin-1/2 chain,"We derive a master equation for the dynamical spin-spin correlation functions
of the XXZ spin-1/2 Heisenberg finite chain in an external magnetic field. In
the thermodynamic limit, we obtain their multiple integral representation.",0407108v1
2001-10-18,Topological entropy of a magnetic flow and the growth of the number of trajectories,Final version. To appear in Discrete and Continuous Dynamical Systems - A.,0110194v3
1998-02-02,Muon Dynamics in a Toroidal Sector Magnet,"We present a Hamiltonian formulation of muon dynamics in toroidal sector
solenoids (bent solenoids)",9802002v1
2004-05-11,Dynamic effects in nonlinear magneto-optics of atoms and molecules,"A brief review is given of topics relating to dynamical processes arising in
nonlinear interactions between light and resonant systems (atoms or molecules)
in the presence of a magnetic field.",0405049v1
2006-08-03,Periodically forced ferrofluid pendulum: effect of polydispersity,"We investigate a torsional pendulum containing a ferrofluid that is forced
periodically to undergo small-amplitude oscillations. A homogeneous magnetic
field is applied perpendicular to the pendulum axis. We give an analytical
formula for the ferrofluid-induced ``selfenergy'' in the pendulum's dynamic
response function for monodisperse as well as for polydisperse ferrofluids.",0608037v1
2007-10-17,On the dynamics created by a time--dependent Aharonov-Bohm flux,"We study the dynamics of classical and quantum particles moving in a
punctured plane under the influence of a homogeneous magnetic field and driven
by a time-dependent singular flux tube through the hole.",0710.3270v1
2009-06-22,A tool to estimate the critical dynamics and thickness of superconducting films and interfaces,"We demonstrate that the magnetic field dependence of the conductivity
measured at the transition temperature allows the dynamical critical exponent,
the thickness of thin superconducting films and interfaces, and the limiting
lateral length to be determined. The resulting tool is applied to the
conductivity data of an amorphous Nb0.15 Si0.85 film and a LaAlO3/SrTiO3
interface.",0906.3990v1
2009-10-07,Gauge invariance and classical dynamics of noncommutative particle theory,"We consider a model of classical noncommutative particle in an external
electromagnetic field. For this model, we prove the existence of generalized
gauge transformations. Classical dynamics in Hamiltonian and Lagrangian form is
discussed, in particular, the motion in the constant magnetic field is studied
in detail.",0910.1341v1
2012-07-24,Dynamic aperture and space charge effect studies for the Recycler ring for Project-X,"A simplified Recycler lattice was created to fine tune injection straight,
ring tune, and phase trombone. In this paper, we will present detailed
modifications for further optimization of Recycler lattice which requires the
investigation of tune footprint and dynamic aperture based on higher order
momentum components of the magnetic fields, together with the space charge
effects.",1207.5733v1
2013-02-14,"Bounded analytic maps, Wall fractions and ABC flow","In this work we study the qualitative properties of real analytic bounded
maps defined in the infinite complex strip. The main tool is approximation by
continued g-fractions of Wall. As an application, the ABC flow system is
considered which is essential to the origin of the solar magnetic field.",1302.3279v3
2013-08-25,Open problems and questions about geodesics,"The paper surveys open problems and questions related to geodesics defined by
Riemannian, Finsler, semi Riemannian and magnetic structures on manifolds.",1308.5417v3
2014-06-16,Macroscopic limit of a bipartite Curie-Weiss model: a dynamical approach,"We analyze the Glauber dynamics for a bi-populated Curie-Weiss model. We
obtain the limiting behavior of the empirical averages in the limit of
infinitely many particles. We then characterize the phase space of the model in
absence of magnetic field and we show that several phase transitions in the
inter-groups interaction strength occur.",1406.4044v2
2016-07-07,MHD turbulence and distributed chaos,"It is shown, using results of recent direct numerical simulations, that
spectral properties of distributed chaos in MHD turbulence with zero mean
magnetic field are similar to those of hydrodynamic turbulence. An exception is
MHD spontaneous breaking of space translational symmetry, when the stretched
exponential spectrum $\exp(-k/k_{\beta})^{\beta}$ has $\beta=4/7$.",1607.02073v2
2017-02-12,Vortex Dynamics in Type II Superconductors,"Time dependent Ginzburg-Landau equation is solved for type II superconductors
numerically, and the dynamics of entering vortices, geometric defects and
pinning effects have been investigated. A superconducting wire with ratchet
defects is designed to pump the vortices move in a specific direction and
enhance the supercurrent when applying the periodical magnetic field. Some
properties of this wire have been investigated numerically and analytically.",1702.08842v2
2022-11-02,Light-induced ultrafast dynamics of spin crossovers in LaCoO3,"Ultrafast quantum dynamics relaxation of a photoexcited state in a strongly
correlated spin crossover system LaCoO3 under a sudden perturbation is
considered with the density matrix generalized master equation. The
magnetization and cobalt-oxygen bond length oscillations were found. The
evolution of the electronic band structure during relaxation is calculated in
the framework of the LDA+GTB method.",2211.01300v1
2022-11-02,Drift approximation by the modified Boris algorithm of charged-particle dynamics in toroidal geometry,"In this paper, we study the charged-particle dynamics under strong magnetic
field in a toroidal axi-symmetric geometry. Using modulated Fourier expansions
of the exact and numerical solutions, the long-term drift motion of the exact
solution in toroidal geometry is derived and the error analysis of the
large-stepsize modified Boris algorithm over long time scales is provided.
Numerical experiments illustrate the theoretical results.",2211.01462v1
2023-06-29,Approximate Entropy Analysis for Nonlinear Beam Dynamics,"In this paper, we apply approximate entropy (ApEn) analysis to the nonlinear
beam dynamics in circular accelerators. Due to the presence of strong nonlinear
magnets, chaos of beam motion gradually increases with amplitude. Such chaos
can be quantitatively characterized with ApEn of beam turn-by-turn readings.
Then ApEn, as a chaos indicator, can be used for nonlinear lattice optimization
and analysis.",2306.17297v1
2023-11-07,Dynamics of Carrollian Scalar Fields,"Adopting an intrinsic Carrollian viewpoint, we show that the generic
Carrollian scalar field action is a combination of electric and magnetic
actions, found in the literature by taking the Carrollian limit of the
relativistic scalar field. This leads to non-trivial dynamics: even a single
particle with non-vanishing energy can move in Carrollian physics.",2311.04113v2
1997-11-12,Transonic Magnetic Slim Accretion Disks and kilo-Hertz Quasi-Periodic Oscillations in Low-Mass X-Ray Binaries,"The inner regions of accretion disks of weakly magnetized neutron stars are
affected by general relativity and stellar magnetic fields. Even for field
strengths sufficiently small so that there is no well-defined magnetosphere
surrounding the neutron star, there is still a region in the disk where
magnetic field stress plays an important dynamical role. We construct magnetic
slim disk models appropriate for neutron stars in low-mass X-ray binaries
(LMXBs) which incorporate both effects (GR and magnetic fields). The
B-field--disk interaction is treated in a phenomenological manner, allowing for
both closed and open field configurations. We show that even for surface
magnetic fields as weak as $10^7-10^8$ G, the sonic point of the accretion flow
can be significantly modified from the pure GR value (near $6M$). We derive an
approximate analytical expression for the sonic radius and show that it mainly
depends on the surface field strength $B_0$ and mass accretion rate $\dot M$
through the ratio $b^2\propto B_0^2/\dot M$. The sonic radius thus obtained
approaches the usual Alfven radius for high $b^2$, and asymptotes to $6M$ as
$b^2\to 0$. We therefore suggest that for neutron stars in LMXBs, the
distinction between the disk sonic radius and the magnetospheric radius may not
exist. We apply our theoretical results to the kHz QPOs observed in the X-ray
fluxes of LMXBs. If these QPOs are associated with the orbital frequency at the
inner radius of the disk, then the QPO frequencies and their correlation with
mass accretion rate can provide useful constraints on the nature of the
magnetic field -- disk interactions as well as on the structure of magnetic
fields in LMXBs. Current observational data may suggest that the magnetic
fields in LMXBs have complex topology.",9711142v2
2007-04-07,Magnetic fields in protoplanetary disks,"Magnetic fields likely play a key role in the dynamics and evolution of
protoplanetary discs. They have the potential to efficiently transport angular
momentum by MHD turbulence or via the magnetocentrifugal acceleration of
outflows from the disk surface, and magnetically-driven mixing has implications
for disk chemistry and evolution of the grain population. However, the weak
ionisation of protoplanetary discs means that magnetic fields may not be able
to effectively couple to the matter. I present calculations of the ionisation
equilibrium and magnetic diffusivity as a function of height from the disk
midplane at radii of 1 and 5 AU. Dust grains tend to suppress magnetic coupling
by soaking up electrons and ions from the gas phase and reducing the
conductivity of the gas by many orders of magnitude. However, once grains have
grown to a few microns in size their effect starts to wane and magnetic fields
can begin to couple to the gas even at the disk midplane. Because ions are
generally decoupled from the magnetic field by neutral collisions while
electrons are not, the Hall effect tends to dominate the diffusion of the
magnetic field when it is able to partially couple to the gas.
  For a standard population of 0.1 micron grains the active surface layers have
a combined column of about 2 g/cm^2 at 1 AU; by the time grains have aggregated
to 3 microns the active surface density is 80 g/cm^2. In the absence of grains,
x-rays maintain magnetic coupling to 10% of the disk material at 1 AU (150
g/cm^2). At 5 AU the entire disk thickness becomes active once grains have
aggregated to 1 micron in size.",0704.0970v3
2009-07-05,Magnetic survey of emission line B-type stars with FORS1 at the VLT,"We report the results of our search for magnetic fields in a sample of 16
field Be stars, the binary emission-line B-type star upsilon Sgr, and in a
sample of fourteen members of the open young cluster NGC3766 in the Carina
spiral arm. The sample of cluster members includes Be stars, normal B-type
stars and He-strong/He-weak stars. Nine Be stars have been studied with
magnetic field time series obtained over ~1 hour to get an insight into the
temporal behaviour and the correlation of magnetic field properties with
dynamical phenomena taking place in Be star atmospheres. The
spectropolarimetric data were obtained at the European Southern Observatory
with the multi-mode instrument FORS1 installed at the 8m Kueyen telescope. We
detect weak photospheric magnetic fields in four field Be stars, HD62367, mu
Cen, o Aqr, and epsilon Tuc. The strongest longitudinal magnetic field,
<B_z>=117+-38G, was detected in the Be star HD62367. Among the Be stars studied
with time series, one Be star, lambda Eri, displays cyclic variability of the
magnetic field with a period of 21.12min. The binary star upsilon Sgr, in the
initial rapid phase of mass exchange between the two components with strong
emission lines in the visible spectrum, is a magnetic variable star, probably
on a timescale of a few months. The maximum longitudinal magnetic field
<B_z>=-102+-10G at MJD54333.018 was measured using hydrogen lines. The cluster
NGC3766 seems to be extremely interesting, where we find evidence for the
presence of a magnetic field in seven early B-type stars out of the observed
fourteen cluster members.",0907.0882v1
2010-02-03,"Magnetic transitions induced by tunnelling electrons in individual adsorbed M-Phthalocyanine molecules (M $\equiv$ Fe, Co)","We report on a theoretical study of magnetic transitions induced by
tunnelling electrons in individual adsorbed M-Phthalocyanine (M-Pc) molecules
where M is a metal atom: Fe-Pc on a Cu(110)(2$\times$1)-O surface and Co-Pc
layers on Pb(111) islands. The magnetic transitions correspond to the change of
orientation of the spin angular momentum of the metal ion with respect to the
surroundings and possibly an applied magnetic field. The adsorbed Fe-Pc system
is studied with a Density Functional Theory (DFT) transport approach showing
that i) the magnetic structure of the Fe atom in the adsorbed Fe-Pc is quite
different from that of the free Fe atom or of other adsorbed Fe systems and ii)
that injection of electrons (holes) into the Fe atom in the adsorbed Fe-Pc
molecule dominantly involves the Fe $3d_{z^2}$ orbital. These results fully
specify the magnetic structure of the system and the process responsible for
magnetic transitions. The dynamics of the magnetic transitions induced by
tunnelling electrons is treated in a strong-coupling approach. The Fe-Pc
treatment is extended to the Co-Pc case. The present calculations accurately
reproduce the strength of the magnetic transitions as observed by magnetic IETS
(Inelastic Electron Tunnelling Spectroscopy) experiments; in particular, the
dominance of the inelastic current in the conduction of the adsorbed M-Pc
molecule is accounted for.",1002.0728v1
2010-05-26,Shear-driven and diffusive helicity fluxes in alpha-Omega dynamos,"We present nonlinear mean-field alpha-Omega dynamo simulations in spherical
geometry with simplified profiles of kinematic alpha effect and shear. We take
magnetic helicity evolution into account by solving a dynamical equation for
the magnetic alpha effect. This gives a consistent description of the quenching
mechanism in mean-field dynamo models. The main goal of this work is to explore
the effects of this quenching mechanism in solar-like geometry, and in
particular to investigate the role of magnetic helicity fluxes, specifically
diffusive and Vishniac-Cho (VC) fluxes, at large magnetic Reynolds numbers
(Rm). For models with negative radial shear or positive latitudinal shear, the
magnetic alpha effect has predominantly negative (positive) sign in the
northern (southern) hemisphere. In the absence of fluxes, we find that the
magnetic energy follows an Rm^-1 dependence, as found in previous works. This
catastrophic quenching is alleviated in models with diffusive magnetic helicity
fluxes resulting in magnetic fields comparable to the equipartition value even
for Rm=10^7. On the other hand, models with a shear-driven Vishniac-Cho flux
show an increase of the amplitude of the magnetic field with respect to models
without fluxes, but only for Rm<10^4. This is mainly a consequence of assuming
a vacuum outside the Sun which cannot support a significant VC flux across the
boundary. However, in contrast with the diffusive flux, the VC flux modifies
the distribution of the magnetic field. In addition, if an ill-determined
scaling factor in the expression for the VC flux is large enough, subcritical
dynamo action is possible that is driven by the action of shear and the
divergence of current helicity flux.",1005.4818v1
2010-06-30,Characteristics and Evolution of the Magnetic field and Chromospheric Emission in an Active Region Core Observed by Hinode,"We describe the characteristics and evolution of the magnetic field and
chromospheric emission in an active region core observed by the Solar Optical
Telescope on Hinode. Consistent with previous studies, we find that the moss is
unipolar, the spatial distribution of magnetic flux evolves slowly, and the
magnetic field is only moderately inclined. We show that the field line
inclination and horizontal component are coherent, and that the magnetic field
is mostly sheared in the inter-moss regions where the highest magnetic flux
variability is seen. Using extrapolations from SP magnetograms we show that the
magnetic connectivity in the moss is different than in the quiet Sun because
most of the magnetic field extends to significant coronal heights. The magnetic
flux, field vector, and chromospheric emission in the moss also appear highly
dynamic, but actually show only small scale variations in magnitude on
time-scales longer than the cooling times for hydrodynamic loops computed from
our extrapolations, suggesting high-frequency (continuous) heating events. Some
evidence is found for flux (Ca 2 intensity) changes on the order of 100--200 G
(DN) on time-scales of 20--30 mins that could be taken as indicative of
low-frequency heating. We find, however, that only a small fraction (10%) of
our simulated loops would be expected to cool on these time-scales, and we find
no clear evidence that the flux changes consistently produce intensity changes
in the chromosphere. The magnetic flux and chromospheric intensity in most
individual SOT pixels in the moss vary by less than ~ 20% and ~ 10%,
respectively, on loop cooling time-scales. In view of the high energy
requirements of the chromosphere, we suggest that these variations could be
sufficient for the heating of `warm' EUV loops, but that the high basal levels
may be more important for powering the hot core loops rooted in the moss.",1006.5776v1
2010-07-16,Magnetism and Charge Dynamics in Iron Pnictides,"In a wide variety of materials, such as copper oxides, heavy fermions,
organic salts, and the recently discovered iron pnictides, superconductivity is
found in close proximity to a magnetically ordered state. The character of the
proximate magnetic phase is thus believed to be crucial for understanding the
differences between the various families of unconventional superconductors and
the mechanism of superconductivity. Unlike the AFM order in cuprates, the
nature of the magnetism and of the underlying electronic state in the iron
pnictide superconductors is not well understood. Neither density functional
theory nor models based on atomic physics and superexchange, account for the
small size of the magnetic moment. Many low energy probes such as transport,
STM and ARPES measured strong anisotropy of the electronic states akin to the
nematic order in a liquid crystal, but there is no consensus on its physical
origin, and a three dimensional picture of electronic states and its relations
to the optical conductivity in the magnetic state is lacking. Using a first
principles approach, we obtained the experimentally observed magnetic moment,
optical conductivity, and the anisotropy of the electronic states. The theory
connects ARPES, which measures one particle electronic states, optical
spectroscopy, probing the particle hole excitations of the solid and neutron
scattering which measures the magnetic moment. We predict a manifestation of
the anisotropy in the optical conductivity, and we show that the magnetic phase
arises from the paramagnetic phase by a large gain of the Hund's rule coupling
energy and a smaller loss of kinetic energy, indicating that iron pnictides
represent a new class of compounds where the nature of magnetism is
intermediate between the spin density wave of almost independent particles, and
the antiferromagnetic state of local moments.",1007.2867v1
2011-10-20,Relativistic MHD in dynamical spacetimes: Improved EM gauge condition for AMR grids,"We recently developed a new general relativistic magnetohydrodynamic code
with adaptive mesh refinement that evolves the electromagnetic (EM) vector
potential (A) instead of the magnetic fields directly. Evolving A enables one
to use any interpolation scheme on refinement level boundaries and still
guarantee that the magnetic field remains divergenceless. As in classical EM, a
gauge choice must be made when evolving A, and we chose a straightforward
""algebraic"" gauge condition to simplify the A evolution equation. However,
magnetized black hole-neutron star (BHNS) simulations in this gauge exhibit
unphysical behavior, including the spurious appearance of strong magnetic
fields on refinement level boundaries. This spurious behavior is exacerbated
when matter crosses refinement boundaries during tidal disruption of the NS.
Applying Kreiss-Oliger dissipation to the evolution of the magnetic vector
potential A slightly weakens this spurious magnetic effect, but with undesired
consequences. We demonstrate via an eigenvalue analysis and a numerical study
that zero-speed modes in the algebraic gauge, coupled with the frequency
filtering that occurs on refinement level boundaries, are responsible for the
creation of spurious magnetic fields. We show that the EM Lorenz gauge exhibits
no zero-speed modes, and as a consequence, spurious magnetic effects are
quickly propagated away, allowing for long-term, stable magnetized BHNS
evolutions. Our study demonstrates how the EM gauge degree of freedom can be
chosen to one's advantage, and that for magnetized BHNS simulations the Lorenz
gauge constitutes a major improvement over the algebraic gauge.",1110.4633v3
2012-10-08,Orbital and Mass Ratio Evolution of Protobinaries Driven by Magnetic Braking,"The majority of stars reside in multiple systems, especially binaries. The
formation and early evolution of binaries is a longstanding problem in star
formation that is not fully understood. In particular, how the magnetic field
observed in star-forming cores shapes the binary characteristics remains
relatively unexplored. We demonstrate numerically, using the ENZO-MHD code,
that a magnetic field of the observed strength can drastically change two of
the basic quantities of a binary system: the orbital separation and mass ratio
of the two components. Our calculations focus on the protostellar mass
accretion phase, after a pair of stellar 'seeds' have already formed. We find
that, in dense cores magnetized to a realistic level, the angular momentum of
the gas accreted by the protobinary is greatly reduced by magnetic braking.
Accretion of strongly braked material shrinks the protobinary separation by a
large factor compared to the non-magnetic case. The magnetic braking also
changes the evolution of the mass ratio of unequal-mass protobinaries by
producing gas of low specific angular momentum that accretes preferentially
onto the primary rather than the secondary. This is in contrast with the
preferential mass accretion onto the secondary previously found for
protobinaries accreting from an unmagnetized envelope, which tends to drive the
mass ratio towards unity. In addition, the magnetic field greatly modifies the
morphology and dynamics of the protobinary accretion flow. It suppresses the
circumstellar and circumbinary disks that feed the protobinary in the
non-magnetic case; the binary is fed instead by a fast collapsing pseudodisk
whose rotation is strongly braked. The magnetic braking-driven inward migration
of binaries from their birth locations may be constrained by high-resolution
observations of the orbital distribution of deeply embedded protobinaries,
especially with ALMA.",1210.2308v2
2012-10-29,Exploring Magnetic Field Structure in Star-Forming Cores with Polarization of Thermal Dust Emission,"The configuration and evolution of the magnetic field in star-forming cores
are investigated in order to directly compare simulations and observations. We
prepare four different initial clouds having different magnetic field strengths
and rotation rates, in which magnetic field lines are aligned/misaligned with
the rotation axis. First, we calculate the evolution of such clouds from the
prestellar stage until long after protostar formation. Then, we calculate the
polarization of thermal dust emission expected from the simulation data. We
create polarization maps with arbitrary viewing angles and compare them with
observations. Using this procedure, we confirmed that the polarization
distribution projected on the celestial plane strongly depends on the viewing
angle of the cloud. Thus, by comparing the observations with the polarization
map predicted by the simulations, we can roughly determine the angle between
the direction of the global magnetic field and the line of sight. The
configuration of the polarization vectors also depends on the viewing angle. We
find that an hourglass configuration of magnetic field lines is not always
realized in a collapsing cloud when the global magnetic field is misaligned
with the cloud rotation axis. Depending on the viewing angle, an S-shaped
configuration of the magnetic field (or the polarization vectors) appears early
in the protostellar accretion phase. This indicates that not only the magnetic
field but also the cloud rotation affects the dynamical evolution of such a
cloud. In addition, by comparing the simulated polarization with actual
observations, we can estimate properties of the host cloud such as the
evolutionary stage, magnetic field strength, and rotation rate.",1210.7637v1
2013-08-02,Magnetic arms generated by multiple interfering galactic spiral patterns,"Interfering two- and three-arm spiral patterns have previously been inferred
to exist in many galaxies and also in numerical simulations, and invoked to
explain important dynamical properties, such as lack of symmetry, kinks in
spiral arms, and star formation in armlets. The non-axisymmetric galactic
mean-field dynamo model of Chamandy et al. is generalized to allow for such
multiple co-existing spiral patterns in the kinetic alpha_k effect, leading to
the existence of magnetic spiral arms in the large-scale magnetic field with
several new properties. The large-scale magnetic field produced by an evolving
superposition of two- and three-arm (or two- and four-arm) patterns evolves
with time along with the superposition. Magnetic arms can be stronger and more
extended in radius and in azimuth when produced by two interfering patterns
rather than by one pattern acting alone. Transient morphological features arise
in the magnetic arms, including bifurcations, disconnected armlets, and
temporal and spatial variation in arm strength and pitch angles. Pitch angles
of the large-scale magnetic field and magnetic arm structures (ridges) are
smaller than those typically inferred from observations of spiral galaxies for
model parameters of Chamandy et al., but can become comparable to typically
inferred values for certain (still realistic) parameters. The magnetic field is
sometimes strongest in between the alpha_k-arms, unlike in standard models with
a single pattern, where it is strongest within the alpha_k-arms. Moreover, for
models with a two- and three-arm pattern, some amount of m=1 azimuthal symmetry
is found to be present in the magnetic field, which is generally not the case
for forcing by single two- or three-arm patterns. (abridged)",1308.0432v2
2013-09-19,Radiative accretion shocks along nonuniform stellar magnetic fields in classical T Tauri stars,"(abridged) AIMS. We investigate the dynamics and stability of post-shock
plasma streaming along nonuniform stellar magnetic fields at the impact region
of accretion columns. We study how the magnetic field configuration and
strength determine the structure, geometry, and location of the shock-heated
plasma. METHODS. We model the impact of an accretion stream onto the
chromosphere of a CTTS by 2D axisymmetric magnetohydrodynamic simulations. Our
model takes into account the gravity, the radiative cooling, and the
magnetic-field-oriented thermal conduction. RESULTS. The structure, stability,
and location of the shocked plasma strongly depend on the configuration and
strength of the magnetic field. For weak magnetic fields, a large component of
B may develop perpendicular to the stream at the base of the accretion column,
limiting the sinking of the shocked plasma into the chromosphere. An envelope
of dense and cold chromospheric material may also develop around the shocked
column. For strong magnetic fields, the field configuration determines the
position of the shock and its stand-off height. If the field is strongly
tapered close to the chromosphere, an oblique shock may form well above the
stellar surface. In general, a nonuniform magnetic field makes the distribution
of emission measure vs. temperature of the shocked plasma lower than in the
case of uniform magnetic field. CONCLUSIONS. The initial strength and
configuration of the magnetic field in the impact region of the stream are
expected to influence the chromospheric absorption and, therefore, the
observability of the shock-heated plasma in the X-ray band. The field strength
and configuration influence also the energy balance of the shocked plasma, its
emission measure at T > 1 MK being lower than expected for a uniform field. The
above effects contribute in underestimating the mass accretion rates derived in
the X-ray band.",1309.5038v1
2014-06-12,Coronal magnetic reconnection driven by CME expansion -- the 2011 June 7 event,"Coronal mass ejections (CMEs) erupt and expand in a magnetically structured
solar corona. Various indirect observational pieces of evidence have shown that
the magnetic field of CMEs reconnects with surrounding magnetic fields,
forming, e.g., dimming regions distant from the CME source regions. Analyzing
Solar Dynamics Observatory (SDO) observations of the eruption from AR 11226 on
2011 June 7, we present the first direct evidence of coronal magnetic
reconnection between the fields of two adjacent ARs during a CME. The
observations are presented jointly with a data-constrained numerical
simulation, demonstrating the formation/intensification of current sheets along
a hyperbolic flux tube (HFT) at the interface between the CME and the
neighbouring AR 11227. Reconnection resulted in the formation of new magnetic
connections between the erupting magnetic structure from AR 11226 and the
neighboring active region AR 11227 about 200 Mm from the eruption site. The
onset of reconnection first becomes apparent in the SDO/AIA images when
filament plasma, originally contained within the erupting flux rope, is
re-directed towards remote areas in AR 11227, tracing the change of large-scale
magnetic connectivity. The location of the coronal reconnection region becomes
bright and directly observable at SDO/AIA wavelengths, owing to the presence of
down-flowing cool, dense (10^{10} cm^{-3}) filament plasma in its vicinity. The
high-density plasma around the reconnection region is heated to coronal
temperatures, presumably by slow-mode shocks and Coulomb collisions. These
results provide the first direct observational evidence that CMEs reconnect
with surrounding magnetic structures, leading to a large-scale re-configuration
of the coronal magnetic field.",1406.3153v1
2014-07-30,Impulsive energy release and non-thermal emission in a confined M4.0 flare triggered by rapidly evolving magnetic structures,"We present observations of a confined M4.0 flare from NOAA 11302 on 2011
September 26. Observations at high temporal, spatial, and spectral resolution
from Solar Dynamics Observatory, Reuven Ramaty High Energy Solar Spectroscopic
Imager, and Nobeyama Radioheliograph enabled us to explore the possible
triggering and energy release processes of this flare despite its very
impulsive behavior and compact morphology. The flare light curves exhibit an
abrupt rise of non-thermal emission with co-temporal hard X-ray (HXR) and
microwave (MW) bursts that peaked instantly without any precursor emission.
This stage was associated with HXR emission up to 200 keV that followed a power
law with photon spectral index ($\delta$) $\sim$3. Another non-thermal peak,
observed 32 s later, was more pronounced in the MW flux than the HXR profiles.
Dual peaked structure in the MW and HXR light curves suggest a two-step
magnetic reconnection process. Extreme ultraviolet (EUV) images exhibit a
sequential evolution of the inner and outer core regions of magnetic loop
system while the overlying loop configuration remained unaltered. Combined
observations in HXR, (E)UV, and H$\alpha$ provide support for flare models
involving interaction of coronal loops. The magnetograms obtained from
Helioseismic and Magnetic Imager (HMI) reveal the emergence of magnetic flux
which started $\sim$5 hr before the flare. However, the more crucial changes in
the photospheric magnetic flux occurred about 1 minute prior to the flare onset
with opposite polarity magnetic transients appearing at the early flare
location within the inner core region. The spectral, temporal, and spatial
properties of magnetic transients suggest that the sudden changes in the
small-scale magnetic field have likely triggered the flare by destabilizing the
highly sheared pre-flare magnetic configuration.",1407.8115v3
2014-10-17,Ferromagnetic resonance in $ε$-Co magnetic composites,"We investigate the electromagnetic properties of assemblies of nanoscale
$\epsilon$-cobalt crystals with size range between 5 nm to 35 nm, embedded in a
polystyrene (PS) matrix, at microwave (1-12 GHz) frequencies. We investigate
the samples by transmission electron microscopy (TEM) imaging, demonstrating
that the particles aggregate and form chains and clusters. By using a broadband
coaxial-line method, we extract the magnetic permeability in the frequency
range from 1 to 12 GHz, and we study the shift of the ferromagnetic resonance
with respect to an externally applied magnetic field. We find that the
zero-magnetic field ferromagnetic resonant peak shifts towards higher
frequencies at finite magnetic fields, and the magnitude of complex
permeability is reduced. At fields larger than 2.5 kOe the resonant frequency
changes linearly with the applied magnetic field, demonstrating the transition
to a state in which the nanoparticles become dynamically decoupled. In this
regime, the particles inside clusters can be treated as non-interacting, and
the peak position can be predicted from Kittel's ferromagnetic resonance theory
for non-interacting uniaxial spherical particles combined with the
Landau-Lifshitz-Gilbert (LLG) equation. In contrast, at low magnetic fields
this magnetic order breaks down and the resonant frequency in zero magnetic
field reaches a saturation value reflecting the interparticle interactions as
resulting from aggregation. Our results show that the electromagnetic
properties of these composite materials can be tuned by external magnetic
fields and by changes in the aggregation structure.",1410.4789v2
2015-05-13,A Supercooled Spin Liquid State in the Frustrated Pyrochlore Dy2Ti2O7,"A ""supercooled"" liquid develops when a fluid does not crystallize upon
cooling below its ordering temperature. Instead, the microscopic relaxation
times diverge so rapidly that, upon further cooling, equilibration eventually
becomes impossible and glass formation occurs. Classic supercooled liquids
exhibit specific identifiers including microscopic relaxation times diverging
on a Vogel-Tammann-Fulcher (VTF) trajectory, a Havriliak-Negami (HN) form for
the dielectric function, and a general Kohlrausch-Williams-Watts (KWW) form for
time-domain relaxation. Recently, the pyrochlore Dy2Ti2O7 has become of
interest because its frustrated magnetic interactions may, in theory, lead to
highly exotic magnetic fluids. However, its true magnetic state at low
temperatures has proven very difficult to identify unambiguously. Here we
introduce high-precision, boundary-free magnetization transport techniques
based upon toroidal geometries and gain a fundamentally new understanding of
the time- and frequency-dependent magnetization dynamics of Dy2Ti2O7. We
demonstrate a virtually universal HN form for the magnetic susceptibility, a
general KWW form for the real-time magnetic relaxation, and a divergence of the
microscopic magnetic relaxation rates with precisely the VTF trajectory. Low
temperature Dy2Ti2O7 therefore exhibits the characteristics of a supercooled
magnetic liquid; the consequent implication is that this translationally
invariant lattice of strongly correlated spins is evolving towards an
unprecedented magnetic glass state, perhaps due to many-body localization of
spin.",1505.03484v2
2016-02-05,Irreversible Rapid Changes of Magnetic Field Associated with the 2012 October 23 Circular Near-limb X1.8 Flare,"It has been found that photospheric magnetic fields can change in accordance
with the three-dimensional magnetic field restructuring following solar
eruptions. Previous studies mainly use vector magnetic field data taken for
events near the disk center. In this paper, we analyze the magnetic field
evolution associated with the 2012 October 23 X1.8 flare in NOAA AR 11598 that
is close to the solar limb, using both the 45 s cadence line-of-sight and 12
minute cadence vector magnetograms from the Helioseismic and Magnetic Imager on
board Solar Dynamic Observatory. This flare is classified as a circular-ribbon
flare with spine-fan type magnetic topology containing a null point. In the
line-of-sight magnetograms, there are two apparent polarity inversion lines
(PIL). The PIL closer to the limb is affected more by the projection effect.
Between these two PILs there lie positive polarity magnetic fields, which are
surrounded by negative polarity fields outside the PILs. We find that after the
flare, both the apparent limb-ward and disk-ward negative fluxes decrease,
while the positive flux in-between increases. We also find that the horizontal
magnetic fields have a significant increase along the disk-ward PIL, while in
surrounding area, they decrease. Synthesizing the observed field changes, we
conclude that the magnetic fields collapse toward the surface above the
disk-ward PIL as depicted in the coronal implosion scenario, while the
peripheral field turns to a more vertical configuration after the flare. We
also suggest that this event is an asymmetric circular-ribbon flare: a flux
rope is likely present above the disk-ward PIL. Its eruption causes the
instability of the entire fan-spine structure and the implosion near that PIL.",1602.02080v1
2016-07-04,Large-scale-vortex dynamos in planar rotating convection,"Several recent studies have demonstrated how large-scale vortices may arise
spontaneously in rotating planar convection. Here we examine the dynamo
properties of such flows in rotating Boussinesq convection. For moderate values
of the magnetic Reynolds number ($100 \lesssim Rm \lesssim 550$, with $Rm$
based on the box depth and the convective velocity), a large-scale (i.e.
system-size) magnetic field is generated. The amplitude of the magnetic energy
oscillates in time, nearly out of phase with the oscillating amplitude of the
large-scale vortex. The large-scale vortex is disrupted once the magnetic field
reaches a critical strength, showing that these oscillations are of magnetic
origin. The dynamo mechanism relies on those components of the flow that have
length scales lying between that of the large-scale vortex and the typical
convective cell size; smaller-scale flows are not required. The large-scale
vortex plays a crucial role in the magnetic induction despite being essentially
two-dimensional; we thus refer to this dynamo as a large-scale-vortex dynamo.
For larger magnetic Reynolds numbers, the dynamo is small scale, with a
magnetic energy spectrum that peaks at the scale of the convective cells. In
this case, the small-scale magnetic field continuously suppresses the
large-scale vortex by disrupting the correlations between the convective
velocities that allow it to form. The suppression of the large-scale vortex at
high $Rm$ therefore probably limits the relevance of the large-scale-vortex
dynamo to astrophysical objects with moderate values of $Rm$, such as planets.
In this context, the ability of the large-scale-vortex dynamo to operate at low
magnetic Prandtl numbers is of great interest.",1607.00824v2
2017-01-24,Magnetic field formation in the Milky Way-like disk galaxies of the Auriga project,"The magnetic fields observed in the Milky~Way and nearby galaxies appear to
be in equipartition with the turbulent, thermal, and cosmic ray energy
densities, and hence are expected to be dynamically important. However, the
origin of these strong magnetic fields is still unclear, and most previous
attempts to simulate galaxy formation from cosmological initial conditions have
ignored them altogether. Here, we analyse the magnetic fields predicted by the
simulations of the Auriga Project, a set of 30 high-resolution cosmological
zoom simulations of Milky~Way-like galaxies, carried out with a moving-mesh
magneto-hydrodynamics code and a detailed galaxy formation physics model. We
find that the magnetic fields grow exponentially at early times owing to a
small-scale dynamo with an e-folding time of roughly $100\,\rm{Myr}$ in the
center of halos until saturation occurs around $z=2-3$, when the magnetic
energy density reaches about $10\%$ of the turbulent energy density with a
typical strength of $10-50\,\rm{\mu G}$. In the galactic centers the ratio
between magnetic and turbulent energy remains nearly constant until $z=0$. At
larger radii, differential rotation in the disks leads to linear amplification
that typically saturates around $z=0.5$ to $z=0$. The final radial and vertical
variations of the magnetic field strength can be well described by two joint
exponential profiles, and are in good agreement with observational constraints.
Overall, the magnetic fields have only little effect on the global evolution of
the galaxies as it takes too long to reach equipartition. We also demonstrate
that our results are well converged with numerical resolution.",1701.07028v2
2017-06-28,Pr-magnetism in the quasi-skutterudite compound PrFe$_2$Al$_8$,"The intermetallic compound \PFA\ that possesses a three-dimensional network
structure of Al polyhedra centered at the transition metal element Fe and the
rare earth Pr is investigated through neutron powder diffraction and inelastic
neutron scattering in order to elucidate the magnetic ground state of Pr and Fe
and the crystal field effects of Pr. Our neutron diffraction study confirms
long-range magnetic order of Pr below $T_N $ = 4.5~K in this compound.
Subsequent magnetic structure estimation reveals a magnetic propagation vector
$k$ = ($\frac{1}{2}$~0~$\frac{1}{2}$) with a magnetic moment value of
2.5~$\mu_\mathrm{B}$/Pr along the orthorhombic $c$-axis and evidence the lack
of ordering in the Fe sublattice. The inelastic neutron scattering study
reveals one crystalline electric field excitation near 19~meV at 5~K in \PFA.
The energy-integrated intensity of the 19~meV excitation as a function of |$Q$|
$(A^{-1})$ follows the square of the magnetic form factor of Pr$^{3+}$ thereby
confirming that the inelastic excitation belongs to the Pr sublattice. The
second sum rule applied to the dynamic structure factor indicates only
1.6(2)~$\mu_\mathrm{B}$ evolving at the 19~meV peak compared to the
3.58~$\mu_\mathrm{B}$ for free Pr$^{3+}$, indicating that the crystal field
ground state is magnetic and the missing moment is associated with the
resolution limited quasi-elastic line. The magnetic order occurring in Pr in
\PFA\ is counter-intuitive to the symmetry-allowed crystal field level scheme,
hence, is suggestive of exchange-mediated mechanisms of ordering stemming from
the magnetic ground state of the crystal field levels.",1706.09081v1
2017-08-14,Modeling Coronal Response in Decaying Active Regions with Magnetic Flux Transport and Steady Heating,"We present new measurements of the dependence of the Extreme Ultraviolet
radiance on the total magnetic flux in active regions as obtained from the
Atmospheric Imaging Assembly (AIA) and the Helioseismic and Magnetic Imager on
board the Solar Dynamics Observatory (SDO). Using observations of nine active
regions tracked along different stages of evolution, we extend the known
radiance - magnetic flux power-law relationship ($I\propto\Phi^{\alpha}$) to
the AIA 335 \AA\ passband, and the Fe XVIII 93.93 \AA\ spectral line in the 94
\AA\ passband. We find that the total unsigned magnetic flux divided by the
polarity separation ($\Phi/D$) is a better indicator of radiance for the Fe
XVIII line with a slope of $\alpha=3.22\pm0.03$. We then use these results to
test our current understanding of magnetic flux evolution and coronal heating.
We use magnetograms from the simulated decay of these active regions produced
by the Advective Flux Transport (AFT) model as boundary conditions for
potential extrapolations of the magnetic field in the corona. We then model the
hydrodynamics of each individual field line with the Enthalpy-based Thermal
Evolution of Loops (EBTEL) model with steady heating scaled as the ratio of the
average field strength and the length ($\bar{B}/L$) and render the Fe XVIII and
335 \AA\ emission. We find that steady heating is able to partially reproduce
the magnitudes and slopes of the EUV radiance - magnetic flux relationships and
discuss how impulsive heating can help reconcile the discrepancies. This study
demonstrates that combined models of magnetic flux transport, magnetic topology
and heating can yield realistic estimates for the decay of active region
radiances with time.",1708.04324v1
2017-11-15,Laminar and turbulent dynamos in chiral magnetohydrodynamics. II. Simulations,"Using direct numerical simulations (DNS), we study laminar and turbulent
dynamos in chiral magnetohydrodynamics (MHD) with an extended set of equations
that accounts for an additional contribution to the electric current due to the
chiral magnetic effect (CME). This quantum phenomenon originates from an
asymmetry between left- and right-handed relativistic fermions in the presence
of a magnetic field and gives rise to a chiral dynamo. We show that the
magnetic field evolution proceeds in three stages: (1) a small-scale chiral
dynamo instability; (2) production of chiral magnetically driven turbulence and
excitation of a large-scale dynamo instability due to a new chiral effect
(alpha_mu effect); and (3) saturation of magnetic helicity and magnetic field
growth controlled by a conservation law for the total chirality. The
$\alpha_\mu$ effect becomes dominant at large fluid and magnetic Reynolds
numbers and is not related to kinetic helicity. The growth rate of the
large-scale magnetic field and its characteristic scale measured in the
numerical simulations agree well with theoretical predictions based on
mean-field theory. The previously discussed two-stage chiral magnetic scenario
did not include stage (2) during which the characteristic scale of magnetic
field variations can increase by many orders of magnitude. Based on the
findings from numerical simulations, the relevance of the CME and the chiral
effects revealed in the relativistic plasma of the early universe and of
proto-neutron stars are discussed.",1711.09733v3
2017-11-29,Solar Jet on 2014 April 16 Modeled by Kelvin--Helmholtz Instability,"We study here the arising of Kelvin--Helmholtz Instability (KHI) in one fast
jet of 2014 April 16 observed by the Atmospheric Imaging Assembly (AIA) on
board Solar Dynamics Observatory (SDO) in different UV and EUV wavelengths. The
evolution of jet indicates the blob like structure at its boundary which could
be the observational evidence of the KHI. We model the jet as a moving
cylindrical magnetic flux tube of radius $a$ embedded in a magnetic field B_i
and surrounded by rest magnetized plasma with magnetic field B_e. We explore
the propagation of the kink MHD mode along the jet that can become unstable
against the KHI if its speed exceeds a critical value. Concerning magnetic
fields topology we consider three different configurations, notably of (i)
spatially homogeneous magnetic fields (untwisted magnetic flux tube), (ii)
internal (label `i') twisted magnetic field and external homogeneous one (label
`e') (single-twisted flux tube), and (iii) both internal and external twisted
magnetic fields (double-twisted magnetic flux tube). Plasma densities in the
two media rho_i and rho_e are assumed to be homogeneous. The density contrast
is defined in two ways: first as rho_e/rho_i and second as rho_e/(rho_i +
rho_e). Computations show that the KHI can occur at accessible flow velocities
in all the cases of untwisted and single-twisted flux tubes. It turns out,
however, that in the case of a double-twisted flux tube the KHI can merge at an
accessible jet speed only when the density contrast is calculated from the
ratio rho_e/(rho_i} + rho_e). Evaluated KHI developing times and kink mode wave
phase velocities at wavelength of 4 Mm lie in the ranges of 1--6.2 min and
202--271 km/s, respectively---all being reasonable for the modeled jet.",1711.10734v1
2018-01-21,Magnetic reconnection at the earliest stage of solar flux emergence,"On 2016 September 20, the Interface Region Imaging Spectrograph observed an
active region during its earliest emerging phase for almost 7 hours. The
Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory
observed continuous emergence of small-scale magnetic bipoles with a rate of
$\sim$10$^{16}$ Mx~s$^{-1}$. The emergence of magnetic fluxes and interactions
between different polarities lead to frequent occurrence of ultraviolet (UV)
bursts, which exhibit as intense transient brightenings in the 1400 \AA{}
images. In the meantime, discrete small patches with the same magnetic polarity
tend to move together and merge, leading to enhancement of the magnetic fields
and thus formation of pores (small sunspots) at some locations. The spectra of
these UV bursts are characterized by the superposition of several chromospheric
absorption lines on the greatly broadened profiles of some emission lines
formed at typical transition region temperatures, suggesting heating of the
local materials to a few tens of thousands of kelvin in the lower atmosphere by
magnetic reconnection. Some bursts reveal blue and red shifts of
$\sim$100~km~s$^{-1}$ at neighboring pixels, indicating the spatially resolved
bidirectional reconnection outflows. Many such bursts appear to be associated
with the cancellation of magnetic fluxes with a rate of the order of
$\sim$10$^{15}$ Mx~s$^{-1}$. We also investigate the three-dimensional magnetic
field topology through a magneto-hydrostatic model and find that a small
fraction of the bursts are associated with bald patches (magnetic dips).
Finally, we find that almost all bursts are located in regions of large
squashing factor at the height of $\sim$1 Mm, reinforcing our conclusion that
these bursts are produced through reconnection in the lower atmosphere.",1801.06785v1
2019-03-07,Interferometric observations of magnetic fields in forming stars,"The magnetic field is a key ingredient in the recipe of star formation. Over
the past two decades, millimeter and submillimeter interferometers have made
major strides in unveiling the role of the magnetic field in star formation at
progressively smaller spatial scales. From the kiloparsec scale of molecular
clouds down to the inner few hundred au immediately surrounding forming stars,
the polarization at millimeter and submillimeter wavelengths is dominated by
polarized thermal dust emission, where the dust grains are aligned relative to
the magnetic field. Interferometric studies have focused on this dust
polarization and occasionally on the polarization of spectral-line emission. We
review the current state of the field of magnetized star formation in the
context of several questions that continue to motivate the studies of high- and
low-mass star formation. By aggregating and analyzing the results from
individual studies, we come to several conclusions: (1) Magnetic fields and
outflows from low-mass protostellar cores are randomly aligned, suggesting that
the magnetic field at ~1000 au scales is not the dominant factor in setting the
angular momentum of embedded disks and outflows. (2) Recent measurements of the
thermal and dynamic properties in high-mass star-forming regions reveal small
virial parameters, challenging the assumption of equilibrium star formation.
However, we estimate that a magnetic field strength of a fraction of a mG to
several mG in these objects could bring the dense gas close to a state of
equilibrium. Finally, (3) We find that the small number of sources with
hourglass-shaped magnetic field morphologies at 0.01 -- 0.1 pc scales cannot be
explained purely by projection effects, suggesting that while it does occur
occasionally, magnetically dominated core collapse is not the predominant mode
of low- or high-mass star formation. [Abridged]",1903.03177v1
2020-05-27,High-resolution tunneling spin transport characteristics of topologically distinct magnetic skyrmionic textures from theoretical calculations,"High-resolution tunneling electron spin transport properties (longitudinal
spin current (LSC) and spin transfer torque (STT) maps) of topologically
distinct real-space magnetic skyrmionic textures are reported by employing a
3D-WKB combined scalar charge and vector spin transport theory in the framework
of spin-polarized scanning tunneling microscopy (SP-STM). For our theoretical
investigation metastable skyrmionic spin structures with various topological
charges ($Q=-3,-2,-1,0,1,2$) in the (Pt$_{0.95}$Ir$_{0.05}$)/Fe/Pd(111)
ultrathin magnetic film are considered. Using an out-of-plane magnetized SP-STM
tip it is found that the maps of the LSC vectors acting on the spins of the
magnetic textures and all STT vector components exhibit the same topology as
the skyrmionic objects. In contrast, an in-plane magnetized tip generally does
not result in spin transport vector maps that are topologically equivalent to
the underlying spin structure, except for the LSC vectors acting on the spins
of the skyrmionic textures at a specific relation between the spin
polarizations of the sample and the tip. The magnitudes of the spin transport
vector quantities exhibit close relations to charge current SP-STM images
irrespectively of the skyrmionic topologies. Moreover, we find that the STT
efficiency (torque/current ratio) acting on the spins of the skyrmions can
reach large values up to $\sim$25 meV/$\mu$A ($\sim$0.97 $h/e$) above the rim
of the magnetic objects, but it considerably varies between large and small
values depending on the lateral position of the SP-STM tip above the
topological spin textures. A simple expression for the STT efficiency is
introduced to explain its variation. Our calculated spin transport vectors can
be used for the investigation of spin-polarized tunneling-current-induced spin
dynamics of topologically distinct surface magnetic skyrmionic textures.",2005.13266v2
2020-08-20,Formation of magnetized spatial structures in the Beta Lyrae system II. Reflection of magnetically controlled structures in the visible spectrum,"This article proposes a picture of magnetized accretion structures formed
during the mass transfer in the Beta Lyrae system. It is shown that the
structure of the gaseous flows between the donor and the gainer is due to the
spatial configuration of the donor magnetic field. Its dipole axis is deviated
substantially from the line joining the centers of the components and is
inclined to the orbital plane of the binary system; the center of the magnetic
dipole is displaced from the donor center toward the gainer. The surface around
the donor magnetic pole, which is close to the gainer, is a region of an
additional matter loss from the donor surface. The effective collision of the
magnetized plasma with the accretion disk is enhanced by the fast
counter-rotation of this disk, especially in the secondary quadrature phases,
in which the high-temperature medium and the system of formed accretion flows
are observed. This concept is demonstrated, primarily, in the obvious
correlations between the phase variability of the donor magnetic field and the
corresponding variability of the dynamic and energy characteristics of the
various complex lines. This refers to the behavior of the radial velocity
curves of the emission-absorption lines formed in the gaseous structures of
type H$\alpha$, HeI $\lambda$ 7065, or the variability of their equivalent
width and intensity, and the variability of conventional absorption lines of
the donor atmosphere. This is true for the phase variability of the absolute
flux in the H$\alpha$ emission line and the fast varying of the continuum in
the H$\alpha$ region as certain parameters, which reflect the phase variability
of the donor magnetic field. This approach made it possible to determine the
phase boundaries of the location of the magnetic polar region on the donor
surface above which the matter outflows are formed.",2008.08787v1
2020-11-20,A three-dimensional thermal model of the human cochlea for magnetic cochlear implant surgery,"In traditional cochlear implant surgery, physical trauma may occur during
electrode array insertion. Magnetic guidance of the electrode array has been
proposed to mitigate this medical complication. After insertion, the guiding
magnet attached to the tip of the electrode array must be detached via a
heating process and removed. This heating process may, however, cause thermal
trauma within the cochlea. In this study, a validated three-dimensional finite
element heat transfer model of the human cochlea is applied to perform an
intracochlear thermal analysis necessary to ensure the safety of the magnet
removal phase. Specifically, the maximum safe input power density to detach the
magnet is determined as a function of the boundary conditions, heating
duration, cochlea size, implant electrode array radius and insertion depth,
magnet size, and cochlear fluid. A dimensional analysis and numerical
simulations reveal that the maximum safe input power density increases with
increasing cochlea size and the radius of the electrode array, whereas it
decreases with increasing electrode array insertion depth and magnet size. The
best cochlear fluids from the thermal perspective are perilymph and a soap
solution. Even for the worst case scenario in which the cochlear walls are
assumed to be adiabatic except at the round window, the maximum safe input
power density is larger than that required to melt 1 $\rm{mm^3}$ of paraffin
bonding the magnet to the implant electrode array. By combining the outcome of
this work with other aspects of the design of the magnetic insertion process,
namely the magnetic guidance procedure and medical requirements, it will be
possible to implement a thermally safe patient-specific surgical procedure.",2011.10183v1
2021-01-08,Shear rheology of a dilute emulsion of ferrofluid droplets dispersed in a non-magnetizable carrier fluid under the influence of a uniform magnetic field,"The effect of a spatially uniform magnetic field on the shear rheology of a
dilute emulsion of monodispersed ferrofluid droplets, immersed in a
non-magnetizable immiscible fluid, is investigated using direct numerical
simulations. The direction of the applied magnetic field is normal to the shear
flow direction. The droplets extra stress tensor arising from the presence of
interfacial forces of magnetic nature is modeled on the basis of the seminal
work of G. K. Batchelor, J. Fluid Mech., 41.3 (1970) under the assumptions of a
linearly magnetizable ferrofluid phase and negligible inertia. The results show
that even relatively small magnetic fields can have significant consequences on
the rheological properties of the emulsion due to the magnetic forces that
contribute to deform and orient the droplets towards the direction of the
applied magnetic vector. In particular, we have observed an increase of the
effective (bulk) viscosity and a reversal of the sign of the two normal stress
differences with respect to the case without magnetic field for those
conditions where the magnetic force prevails over the shearing force.
Comparisons between the results of our model with a direct integration of the
viscous stress have provided an indication of its reliability to predict the
effective viscosity of the suspension. Moreover, this latter quantity has been
found to behave as a monotonic increasing function of the applied magnetic
field for constant shearing flows (""magneto-thickening"" behaviour), which
allowed us to infer a simple constitutive equation describing the emulsion
viscosity.",2101.03166v2
2021-02-25,Fully General Relativistic Magnetohydrodynamic Simulations of Accretion Flows onto Spinning Massive Black Hole Binary Mergers,"We perform the first suite of fully general relativistic magnetohydrodynamic
simulations of spinning massive black hole binary mergers. We consider binary
black holes with spins of different magnitudes aligned to the orbital angular
momentum, which are immersed in a hot, magnetized gas cloud. We investigate the
effect of the spin and degree of magnetization (defined through the fluid
parameter $\beta^{-1}\equiv p_{\mathrm{mag}}/p_{\mathrm{fluid}}$) on the
properties of the accretion flow. We find that magnetized accretion flows are
characterized by more turbulent dynamics, as the magnetic field lines are
twisted and compressed during the late inspiral. Post-merger, the polar regions
around the spin axis of the remnant Kerr black hole are magnetically dominated,
and the magnetic field strength is increased by a factor $\sim$10$^2$
(independently from the initial value of $\beta^{-1}$). The magnetized gas in
the equatorial plane acquires higher angular momentum, and settles in a thin
circular structure around the black hole. We find that mass accretion rates of
magnetized configurations are generally smaller than in the unmagnetized cases
by up to a factor $\sim$3. Black hole spins have also a suppressing effect on
the accretion rate, as large as $\sim$48\%. As a potential driver for
electromagnetic emission we follow the evolution of the Poynting luminosity,
which increases after merger up to a factor $\sim2$ with increasing spin,
regardless of the initial level of magnetization of the fluid. Our results
stress the importance of taking into account both spins and magnetic fields
when studying accretion processes onto merging massive black holes.",2102.13166v2
2022-02-21,Detecting deep axisymmetric toroidal magnetic fields in stars. The traditional approximation of rotation for differentially rotating deep spherical shells with a general azimuthal magnetic field,"Asteroseismology has revealed small core-to-surface rotation contrasts in
stars in the whole HR diagram. This is the signature of strong transport of
angular momentum (AM) in stellar interiors. One of the plausible candidates to
efficiently carry AM is magnetic fields with various topologies that could be
present in stellar radiative zones. Among them, strong axisymmetric azimuthal
magnetic fields have received a lot of interest. Indeed, if they are subject to
the so-called Tayler instability, the accompanying triggered Maxwell stresses
can transport AM efficiently. In addition, the electromotive force induced by
the fluctuations of magnetic and velocity fields could potentially sustain a
dynamo action that leads to the regeneration of the initial strong axisymmetric
azimuthal magnetic field. The key question we aim to answer is: can we detect
signatures of these deep strong azimuthal magnetic fields? The only way to
answer this question is asteroseismology and the best laboratories of study are
intermediate-mass and massive stars. Most of these are rapid rotators during
their main-sequence. Therefore, we have to study stellar pulsations propagating
in stably stratified, rotating, and potentially strongly magnetised radiative
zones. We generalise the traditional approximation of rotation by
simultaneously taking general axisymmetric differential rotation and azimuthal
magnetic fields into account in a non-perturbative way. Using this new
formalism, we derive the asymptotic properties of magneto-gravito-inertial
(MGI) waves and their period spacings. We find that toroidal magnetic fields
induce a shift in the period spacings of MGI modes. An equatorial azimuthal
magnetic field with an amplitude of the order of $10^5\,\rm G$ leads to
signatures that can be detectable thanks to modern space photometry. More
complex hemispheric configurations are more difficult to observe.",2202.10026v2
2022-06-03,Turbulent magnetic field in the HII region Sh 2-27,"Magnetic fields in the turbulent interstellar medium (ISM) are a key element
in understanding Galactic dynamics, but there are many observational
challenges. One useful probe for studying the magnetic field component parallel
to the line of sight (LoS) is Faraday rotation of linearly polarized radio
synchrotron emission, combined with H$\alpha$ observations. HII regions are the
perfect laboratories to probe such magnetic fields as they are localized in
space, and are well-defined sources often with known distances and measurable
electron densities. We chose the HII region Sharpless 2-27 (Sh 2-27). By using
a map of the magnetic field strength along the LoS ($B_{\parallel}$) for the
first time, we investigate the basic statistical properties of the turbulent
magnetic field inside Sh 2-27. We study the scaling of the magnetic field
fluctuations, compare it to the Kolmogorov scaling, and attempt to find an
outer scale of the turbulent magnetic field fluctuations. We estimate the
median value of $n_e$ as $7.3\pm0.1$ cm$^{-3}$, and the median value of
$B_{\parallel}$ as $-4.5\pm0.1$ $\mu$G, which is comparable to the magnetic
field strength in diffuse ISM. The slope of the structure function of the
estimated $B_{\parallel}$-map is found to be slightly steeper than Kolmogorov,
consistent with our Gaussian-random-field $B_{\parallel}$ simulations revealing
that an input Kolmogorov slope in the magnetic field results in a somewhat
steeper slope in $B_{\parallel}$. These results suggest that the lower limit to
the outer scale of turbulence is 10 pc in the HII region, which is comparable
to the size of the computation domain. This may indicate that the turbulence
probed here could actually be cascading from the larger scales in the ambient
medium, associated with the interstellar turbulence in the general ISM, which
is illuminated by the presence of Sh 2-27.",2206.01787v1
2022-10-05,Tracking magnetic flux and helicity from Sun to Earth -- Multi-spacecraft analysis of a magnetic cloud and its solar source,"We analyze the complete chain of effects caused by a solar eruptive event in
order to better understand the dynamic evolution of magnetic-field related
quantities in interplanetary space, in particular that of magnetic flux and
helicity. We study a series of connected events (a confined C4.5 flare, a
flare-less filament eruption and a double-peak M-class flare) that originated
in NOAA active region (AR) 12891 on 2021 November 1 and November 2. We deduce
the magnetic structure of AR 12891 using stereoscopy and nonlinear force-free
(NLFF) magnetic field modeling, allowing us to identify a coronal flux rope and
to estimate its axial flux and helicity. Additionally, we compute reconnection
fluxes based on flare ribbon and coronal dimming signatures from remote sensing
imagery. Comparison to corresponding quantities of the associated magnetic
cloud (MC), deduced from in-situ measurements from Solar Orbiter and near-Earth
spacecraft, allows us to draw conclusions on the evolution of the associated
interplanetary coronal mass ejection (ICME). The latter are aided through the
application of geometric fitting techniques (graduated cylindrical shell
modeling; GCS) and interplanetary propagation models (drag based ensemble
modeling; DBEM) to the ICME. NLFF modeling suggests the host AR's magnetic
structure in the form of a left-handed (negative-helicity) sheared arcade/flux
rope reaching to altitudes of 8-10 Mm above photospheric levels, in close
agreement with the corresponding stereoscopic estimate. Revealed from GCS and
DBEM modeling, the ejected flux rope propagated in a self-similar expanding
manner through interplanetary space. Comparison of magnetic fluxes and
helicities processed by magnetic reconnection in the solar source region and
the respective budgets of the MC indicate a considerable contribution from the
eruptive process, though the pre-eruptive budgets appear of relevance too.",2210.02228v1
2022-10-18,Mean opacities of a strongly magnetized high temperature plasma,"Geometry and dynamical structure of emission regions in accreting pulsars are
shaped by the interplay between gravity, radiation, and strong magnetic field,
which significantly affects the opacities of a plasma and radiative pressure
under such extreme conditions. Quantitative consideration of magnetic plasma
opacities is, therefore, an essential ingredient of any self-consistent
modeling of emission region structure of X-ray pulsars. We present results of
computations of the Rosseland and Planck mean opacities of a strongly
magnetized plasma with a simple chemical composition,namely the solar
hydrogen/helium mix. We consider all relevant specific opacities of the
magnetized plasma including vacuum polarization effect and contribution of
electron-positron pairs where the pair number density is computed in the
thermodynamic equilibrium approximation. The magnetic Planck mean opacity
determines the radiative cooling of an optically thin strongly magnetized
plasma. It is by factor of three smaller than non-magnetic Planck opacity at
$k_{\rm B}T < 0.1\,E_{\rm cyc}$ and increases by a factor of $10^2 - 10^4$ at
$k_{\rm B}T > 0.3\,E_{\rm cyc}$ due to cyclotron thermal processes. We propose
a simple approximate expression which has sufficient accuracy for the magnetic
Planck opacity description. We provide the Rosseland opacity in a tabular form
computed in the temperature range 1 - 300 keV, magnetic field range $3 \times
10^{10} - 10^{15}$ G, and a broad range of plasma densities. We demonstrate
that the scattering on the electron-positron pairs increases the Rosseland
opacity drastically at temperatures >50 keV in the case of mass densities
typical for accretion channel in X-ray pulsars.",2210.09995v1
2022-12-06,Application of a magnetic-field-induced transition in Fe X to solar and stellar coronal magnetic field measurements,"Magnetic fields play a key role in driving a broad range of dynamic phenomena
in the atmospheres of the Sun and other stars. Routine and accurate
measurements of the magnetic fields at all the atmospheric layers are of
critical importance to understand these magnetic activities, but in the solar
and stellar coronae such a measurement is still a challenge due to the weak
field strength and the high temperature. Recently, a magnetic-field-induced
transition (MIT) of Fe X at 257.26 {\AA} has been proposed for the magnetic
field measurements in the solar and stellar coronae. In this review, we present
an overview of recent progresses in the application of this method in
astrophysics. We start by introducing the theory underlying the MIT method and
reviewing the existing atomic data critical for the spectral modeling of Fe X
lines. We also discuss the laboratory measurements that verify the potential
capability of the MIT technique as a probe for diagnosing the plasma magnetic
fields. We then continue by investigating the suitability and accuracy of solar
and stellar coronal magnetic field measurements based on the MIT method through
forward modeling. Furthermore, we discuss the application of the MIT method to
the existing spectroscopic observations obtained by the Extreme-ultraviolet
Imaging Spectrometer onboard Hinode. This novel technique provides a possible
way for routine measurements of the magnetic fields in the solar and stellar
coronae, but still requires further efforts to improve its accuracy. Finally,
the challenges and prospects for future research on this topic are discussed.",2212.02873v2
2022-12-14,Competing magnetic fluctuations and orders in a multiorbital model of doped SrCo$_2$As$_2$,"We revisit the intriguing magnetic behavior of the paradigmatic itinerant
frustrated magnet $\rm{Sr}\rm{Co}_2\rm{As}_2$, which shows strong and competing
magnetic fluctuations yet does not develop long-range magnetic order. By
calculating the static spin susceptibility $\chi(\mathbf{q})$ within a
realistic sixteen orbital Hubbard-Hund model, we determine the leading
instability to be ferromagnetic (FM). We then explore the effect of doping and
calculate the critical Hubbard interaction strength $U_c$ that is required for
the development of magnetic order. We find that $U_c$ decreases under electron
doping and with increasing Hund's coupling $J$, but increases rapidly under
hole doping. This suggests that magnetic order could possibly emerge under
electron doping but not under hole doping, which agrees with experimental
findings. We map out the leading magnetic instability as a function of doping
and Hund's coupling and find several antiferromagnetic phases in addition to
FM. We also quantify the degree of itinerant frustration in the model and
resolve the contributions of different orbitals to the magnetic susceptibility.
Finally, we discuss the dynamic spin susceptibility, $\chi(\mathbf{q},
\omega)$, at finite frequencies, where we recover the anisotropy of the peaks
at $\mathbf{Q}_\pi = (\pi, 0)$ and $(0, \pi)$ observed by inelastic neutron
scattering that is associated with the phenomenon of itinerant magnetic
frustration. By comparing results between theory and experiment, we conclude
that the essential experimental features of doped SrCo$_2$As$_2$ are well
captured by a Hubbard-Hund multiorbital model if one considers a small shift of
the chemical potential towards hole doping.",2212.07511v2
2023-03-09,Zonostrophic instabilities in magnetohydrodynamic Kolmogorov flow,"This paper concerns the stability of Kolmogorov flow u = (0, sin x) in the
infinite (x,y)-plane. A mean magnetic field of strength B0 is introduced and
the MHD linear stability problem studied for modes with wave-number k in the
y-direction, and Bloch wavenumber l in the x-direction. The parameters
governing the problem are Reynolds number 1/nu, magnetic Prandtl number P, and
dimensionless magnetic field strength B0. The mean magnetic field can be taken
to have an arbitrary direction in the (x,y)-plane and a mean x-directed flow U0
can be incorporated.
  First the paper considers Kolmogorov flow with y-directed mean magnetic
field, referred to as vertical. Taking l=0, the suppression of the pure
hydrodynamic instability is observed with increasing field strength B0. A
branch of strong-field instabilities occurs for magnetic Prandtl number P less
than unity, as found by A.E. Fraser, I.G. Cresser and P. Garaud (J. Fluid Mech.
949, A43, 2022). Analytical results using eigenvalue perturbation theory in the
limit k->0 support the numerics for both weak- and strong-field instabilities,
and originate in the coupling of large-scale modes with x-wavenumber n=0, to
smaller-scale modes.
  The paper considers the case of horizontal or x-directed mean magnetic field.
The unperturbed state consists of steady, wavey magnetic field lines. As the
magnetic field is increased, the purely hydrodynamic instability is suppressed
again, but for stronger fields a new branch of instabilities appears. Allowing
a non-zero Bloch wavenumber l allows further instability, and in some
circumstances when the system is hydrodynamically stable, arbitrarily weak
magnetic fields can give growing modes. Numerical results are presented
together with eigenvalue perturbation theory in the limits k,l->0. The theory
gives analytical approximations for growth rates and thresholds in good
agreement with those computed.",2303.05212v1
2023-06-19,Onion-like Fe3O4/MgO/CoFe2O4 magnetic nanoparticles: new ways to control magnetic coupling between soft/hard phases,"The control of the magnetization inversion dynamics is one of the main
challenges driving the design of new nanostructured magnetic materials for
magnetoelectronic applications. Nanoparticles with onion-like architecture
offer a unique opportunity to expand the possibilities allowing to combine
different phases at the nanoscale and also modulate the coupling between
magnetic phases by introducing spacers in the same structure. Here we report
the fabrication, by a three-step high temperature decomposition method, of
Fe3O4/MgO/CoFe2O4 onio-like nanoparticles and their detailed structural
analysis, elemental compositional maps and magnetic response. The
core/shell/shell nanoparticles present epitaxial growth and cubic shape with
overall size of (29+-6) nm. These nanoparticles are formed by cubic iron oxide
core of (22+-4) nm covered by two shells, the inner of magnesium oxide and the
outer of cobalt ferrite of ~1 and ~2.5 nm of thickness, respectively. The
magnetization measurements show a single reversion magnetization curve and the
enhancement of the coercivity field, from HC~608 Oe for the Fe3O4/MgO to
HC~5890 Oe to the Fe3O4/MgO/CoFe2O4 nanoparticles at T=5 K, ascribed to the
coupling between both ferrimagnetic phases with a coupling constant of =2
erg/cm2. The system also exhibits exchange bias effect, where the exchange bias
field increases up to HEB~2850 Oe at 5 K accompanied with the broadening of the
magnetization loop of HC~6650 Oe. This exchange bias effect originates from the
freezing of the surface spins below the freezing temperature TF=32 K that
pinned the magnetic moment of the cobalt ferrite shell.",2306.13108v1
2023-07-04,Electrical operation of planar Ge hole spin qubits in an in-plane magnetic field,"In this work we present a comprehensive theory of spin physics in planar Ge
hole quantum dots in an in-plane magnetic field, where the orbital terms play a
dominant role in qubit physics, and provide a brief comparison with
experimental measurements of the angular dependence of electrically driven spin
resonance. We focus the theoretical analysis on electrical spin operation,
phonon-induced relaxation, and the existence of coherence sweet spots. We find
that the choice of magnetic field orientation makes a substantial difference
for the properties of hole spin qubits. Furthermore, although the
Schrieffer-Wolff approximation can describe electron dipole spin resonance
(EDSR), it does not capture the fundamental spin dynamics underlying qubit
coherence. Specifically, we find that: (i) EDSR for in-plane magnetic fields
varies non-linearly with the field strength and weaker than for perpendicular
magnetic fields; (ii) The EDSR Rabi frequency is maximized when the a.c.
electric field is aligned parallel to the magnetic field, and vanishes when the
two are perpendicular; (iii) The Rabi ratio $T_1/T_\pi$, i.e. the number of
EDSR gate operation per unit relaxation time, is expected to be as large as
$5{\times}10^5$ at the magnetic fields used experimentally; (iv) The orbital
magnetic field terms make the in-plane $g$-factor strongly anisotropic in a
squeezed dot, in excellent agreement with experimental measurements; (v) The
coherence sweet spots do not exist in an in-plane magnetic field, as the
orbital magnetic field terms expose the qubit to all components of the defect
electric field. These findings will provide a guideline for experiments to
design ultrafast, highly coherent hole spin qubits in Ge.",2307.01451v1
2023-09-22,Probing the strongly correlated magnetic state of Co$_2$C nanoparticles at low temperatures using $μ$SR,"Co$_2$C nanoparticles (NPs) are amongst transition metal carbides whose
magnetic properties have not been well explored. A recent study by Nirmal Roy
et al. [1] showed that a collection of Co$_2$C NPs exhibit an exchange bias
(EB) effect below T$_{EB}$ = 50 K and also a spin glass (SG) state below
T$_{SG}$ = 5 K. We use magnetic, electrical transport, specific heat, and muon
spin rotation ($\mu$SR) measurements to explore further the magnetic properties
of these NPs. We uncover the onset of Kondo localization at Kondo temperature
T$_K$ (= 40.1 K), near the onset of EB effect. A crossover from the
Kondo-screened scenario to an RKKY interaction-dominated regime is also
observed for T < T$_K$. Specific heat measurements confirm Kondo localization
and heavy fermionic nature in Co$_2$C at low T. At low T, zero field $\mu$SR
spectra reveal a dominant magnetically disordered fraction with slow relaxation
and a smaller fraction with short-range order exhibiting fast relaxation, with
no evidence of long-range magnetic order. We observe an increase in this fast
relaxation rate between T$_{EB}$ and T$_{SG}$, suggesting a slowing down of the
fluctuating local magnetic environment around muons. Transverse field $\mu$SR
spectra show the emergence of a stable, multi-peaked local magnetic field
distribution below T$_{EB}$. Longitudinal field $\mu$SR spectra shows distinct
changes in the dynamics of fluctuations suggesting the presence of a frozen
glassy like state below 6 K. Our results suggest that below T$_{EB}$, Co$_2$C
NPs pellet develops a magnetic interface, separating disordered and short-range
order fractions. The Exchange interaction that sets in below T$_{EB}$ at the
interface couples them and suppresses the fluctuations. With the suppression of
magnetic fluctuations below T$_{EB}$, strong correlation effects in the
electronic state of Co$_2$C lead to Kondo localization.",2309.12797v1
2023-12-01,Exploring kinetically induced bound states in triangular lattices with ultracold atoms: spectroscopic approach,"Quantum simulations with ultracold fermions in triangular optical lattices
have recently emerged as a new platform for studying magnetism in frustrated
systems. Experimental realizations of the Fermi Hubbard model revealed striking
contrast between magnetism in bipartite and triangular lattices. In bipartite
lattices magnetism peaks at half filling, and doped charge carriers tend to
suppress magnetic correlations. In triangular lattices for large $U/t$,
magnetism is enhanced by doping away from $n=1$ because kinetic energy of
dopants can be lowered through developing magnetic correlations. This
corresponds to formation of magnetic polarons, with hole and doublon doping
resulting in antiferro- and ferromagnetic polarons respectively. Snapshots
obtained with quantum gas microscopes revealed formation of magnetic polarons
around dopants at temperatures exceeding the superexchange energy scale. In
this work we discuss theoretically that additional insight into properties of
magnetic polarons can be achieved using spectroscopic experiments with
ultracold atoms. We consider starting from a spin polarized state with small
hole doping and applying a two-photon Raman photoexcitation, which transfers
atoms into a different spin state. We show that such magnon injection spectra
exhibit a separate peak corresponding to formation of a bound state between a
hole and a magnon. This polaron peak is separated from the simple magnon
spectrum by energy proportional to single particle tunneling and can be easily
resolved with currently available experimental techniques. For some momentum
transfer there is an additional peak corresponding to photoexciting a bound
state between two holes and a magnon. We point out that in two component Bose
mixtures in triangular lattices one can also create dynamical magnetic
polarons, with one hole and one magnon forming a repulsive bound state.",2312.00768v1
2023-12-07,On the role of numerical diffusivity in MHD simulations of global accretion disc dynamos,"Observations, mainly of outbursts in dwarf novae, imply that the anomalous
viscosity in highly ionized accretion discs is magnetic in origin, and requires
that the plasma $\beta \sim 1$. Until now most simulations of the magnetic
dynamo in accretion discs have used a local approximation (known as the
shearing box). While these simulations demonstrate the possibility of a
self-sustaining dynamo, the magnetic activity generated in these models
saturates at $\beta \gg 1$. This long-standing discrepancy has previously been
attributed to the local approximation itself. There have been recent attempts
at simulating magnetic activity in global accretion discs with parameters
relevant to the dwarf novae. These too find values of $\beta \gg 1$. We
speculate that the tension between these models and the observations may be
caused by numerical magnetic diffusivity. As a pedagogical example, we present
exact time-dependent solutions for the evolution of weak magnetic fields in an
incompressible fluid subject to linear shear and magnetic diffusivity. We find
that the maximum factor by which the initial magnetic energy can be increased
depends on the magnetic Reynolds number as ${\mathcal R}_{\rm m}^{2/3}$. We
estimate that current global numerical simulations of dwarf nova discs have
numerical magnetic Reynolds numbers around 6 orders of magnitude less than the
physical value found in dwarf nova discs of ${\mathcal R}_{\rm m} \sim
10^{10}$. We suggest that, given the current limitations on computing power,
expecting to be able to compute realistic dynamo action in observable accretion
discs using numerical MHD is, for the time being, a step too far.",2312.04400v1
2023-12-30,"Onset mechanism of an inverted U-shaped solar filament eruption revealed by NVST, SDO, and STEREO-A observations","Utilizing observations from the New Vacuum Solar Telescope (NVST), Solar
Dynamics Observatory (SDO), and Solar Terrestrial Relations Observatory-Ahead
(STEREO-A), we investigate the event from two distinct observational
perspectives: on the solar disk using NVST and SDO, and on the solar limb using
STEREO-A. We employ both a non-linear force-free field model and a potential
field model to reconstruct the coronal magnetic field, aiming to understand its
magnetic properties. Two precursor jet-like activities were observed before the
eruption, displaying an untwisted rotation. The second activity released an
estimated twist of over two turns. During these two jet-like activities,
Y-shaped brightenings, newly emerging magnetic flux accompanied by magnetic
cancellation, and the formation of newly moving fibrils were identified.
Combining these observational features, it can be inferred that these two
precursor jet-like activities released the magnetic field constraining the
filament and were triggered by newly emerging magnetic flux. Before the
filament eruption, it was observed that some moving flows had been ejected from
the site as the onset of two jet-like activities, indicating the same physical
process as two jet-like activities. Extrapolations revealed that the filament
laid under the height of the decay index of 1.0 and had strong magnetic field
(540 Gauss) and a high twisted number (2.4 turns) before the eruption. An
apparent rotational motion was observed during the filament eruption. We deduce
that the solar filament, exhibiting an inverted U-shape, is a significantly
twisted flux rope. The eruption of the filament was initiated by the release of
constraining magnetic fields through continuous magnetic reconnection. This
reconnection process was triggered by the emergence of newly magnetic flux.",2401.00185v1
2020-03-24,Anomalous out-of-equilibrium dynamics in the spin-ice material Dy$_2$Ti$_2$O$_7$ under moderate magnetic fields,"We study experimentally and numerically the dynamics of the spin ice material
Dy$_2$Ti$_2$O$_7$ in the low temperature ($T$) and moderate magnetic field
($\mathbf{B}$) regime ($T\in[0.1,1.7]K$, $B\in[0,0.3]T$). Our objective is to
understand the main physics shaping the out-of-equilibrium magnetisation vs.
temperature curves in two different regimes. Very far from equilibrium, turning
on the magnetic field after having cooled the system in zero field (ZFC) can
increase the concentration of magnetic monopoles; this accelerates the
dynamics. Similarly to electrolytes, this occurs through dissociation of bound
monopole pairs. However, for spin ices the polarisation of the vacuum out of
which the monopole pairs are created is a key factor shaping the magnetisation
curves, with no analog. We observe a threshold field near $0.2T$ for this fast
dynamics to take place, linked to the maximum magnetic force between the
attracting pairs. Surprisingly, within a regime of low temperatures and
moderate fields, an extended Ohm's law can be used to describe the ZFC
magnetisation curve obtained with the dipolar spin-ice model. However, in real
samples the acceleration of the dynamics appears even sharper than in
simulations, possibly due to the presence of avalanches. On the other hand, the
effect of the field nearer equilibrium can be just the opposite to that at very
low temperatures. Single crystals, as noted before for powders, abandon
equilibrium at a blocking temperature $T_B$ which increases with field.
Curiously, this behaviour is present in numerical simulations even within the
nearest-neighbours interactions model. Simulations and experiments show that
the increasing trend in $T_B$ is stronger for $\mathbf{B}\parallel[100]$. This
suggests that the field plays a part in the dynamical arrest through monopole
suppression, which is quite manifest for this field orientation.",2003.11088v2
2006-02-17,Dynamic Alignment in Driven Magnetohydrodynamic Turbulence,"Motivated by recent analytic predictions, we report numerical evidence
showing that in driven incompressible magnetohydrodynamic turbulence the
magnetic- and velocity-field fluctuations locally tend to align the directions
of their polarizations. This dynamic alignment is stronger at smaller scales
with the angular mismatch between the polarizations decreasing with the scale
\lambda approximately as \theta_\lambda ~ \lambda^{1/4}. This can naturally
lead to a weakening of the nonlinear interactions and provide an explanation
for the energy spectrum E(k) ~ k^{-3/2} that is observed in numerical
experiments of strongly magnetized turbulence.",0602382v2
1997-11-14,Comparison of meanfield and Monte Carlo approaches to dynamic hysteresis in Ising ferromagnets,"The dynamical hysteresis is studied in the kinetic Ising model in the
presence of a sinusoidal magnetic field both by Monte Carlo simulation and by
solving the dynamical meanfield equation for the averaged magnetisation. The
frequency variations of the dynamic coercive field are studied below the
critical temperature. In both the cases, it shows a power law frequency
variation however it becomes frequency independent in the low frequency regime
for the meanfield case.",9711132v1
1998-01-26,Dynamic Approach to the Fully Frustrated XY Model,"Using Monte Carlo simulations, we systematically investigate the
non-equilibrium dynamics of the chiral degree of freedom in the two-dimensional
fully frustrated XY model. The critical initial increase of the staggered
chiral magnetization is observed. By means of the short-time dynamics approach,
we estimate the second order phase transition temperature $T_{c}$ and all the
dynamic and static critical exponents $\theta$, z, $\beta$ and $\nu$.",9801253v1
1999-08-31,Dynamic scaling of I-V data for the neutral 2D Coulomb gas,"The value of the dynamic critical exponent z has been studied for
experimental two-dimensional superconducting and Josephson Junction array
systems in zero magnetic field via the Fisher-Fisher-Huse dynamic scaling
analysis. We found z~5.6, a relatively large value indicative of non-diffusive
dynamics. We extend this work here to simulational I-V curves that are also
found to be characterized by the same large value of z.",9908473v1
2000-04-04,Static and dynamic coupling transitions of vortex lattices in disordered anisotropic superconductors,"We use three-dimensional molecular dynamics simulations of magnetically
interacting pancake vortices to study vortex matter in disordered, highly
anisotropic materials such as BSCCO. We observe a sharp 2D-3D transition from
vortex lines to decoupled pancakes as a function of relative interlayer
coupling strength, with an accompanying large increase in the critical current
remniscent of a second peak effect. We find that decoupled pancakes, when
driven, simultaneously recouple and order into a crystalline-like state at high
drives. We construct a dynamic phase diagram and show that the dynamic
recoupling transition is associated with a double peak in dV/dI.",0004054v1
2002-03-14,Spin dynamics in a hole-doped S=1/2 Heisenberg antiferromagnet with a disordered ground state,"Only 3% hole doping by Li is sufficient to suppress the long-range
antiferromagnetic order in La2CuO4. Spin dynamics in such a disordered state
was investigated with measurements of the dynamic magnetic structure factor
S(omega,q), using cold neutron spectroscopy, for La2(Cu0.94Li0.06)O4. The
S(omega,q) is found to sharply peak at (pi,pi), and its dynamics to be
relaxational. Confirming theoretical expectation for the quantum disordered 2D
S=1/2 Heisenberg antiferromagnet, the energy scale saturates at a finite value
at low temperatures. Possible connection to the ``pseudo spin gap'' phenomenon
observed in the NMR/NQR studies on underdoped cuprates is discussed.",0203318v1
2002-04-16,Universality and scaling study of the critical behavior of the two-dimensional Blume-Capel model in short-time dynamics,"In this paper we study the short-time behavior of the Blume-Capel model at
the tricritical point as well as along the second order critical line. Dynamic
and static exponents are estimated by exploring scaling relations for the
magnetization and its moments at early stage of the dynamic evolution. Our
estimates for the dynamic exponents, at the tricritical point, are $z=
2.215(2)$ and $\theta= -0.53(2)$.",0204346v1
2002-11-15,Relationship between dynamical heterogeneities and stretched exponential relaxation,"We identify the dynamical heterogeneities as an essential prerequisite for
stretched exponential relaxation in dynamically frustrated systems. This
heterogeneity takes the form of ordered domains of finite but diverging
lifetime for particles in atomic or molecular systems, or spin states in
magnetic materials. At the onset of the dynamical heterogeneity, the
distribution of time intervals spent in such domains or traps becomes stretched
exponential at long time. We rigorously show that once this is the case, the
autocorrelation function of the renewal process formed by these time intervals
is also stretched exponential at long time.",0211322v3
2003-09-01,Direct-Current Induced Dynamics in Co90Fe10/Ni80Fe20 Point Contacts,"We have directly measured coherent high-frequency magnetization dynamics in
ferromagnet films induced by a spin-polarized DC current. The precession
frequency can be tuned over a range of several gigahertz, by varying the
applied current. The frequencies of excitation also vary with applied field,
resulting in a microwave oscillator that can be tuned from below 5 GHz to above
40 GHz. This novel method of inducing high-frequency dynamics yields
oscillations having quality factors from 200 to 800. We compare our results
with those from single-domain simulations of current-induced dynamics.",0309042v1
2006-02-10,Low-frequency vortex dynamic susceptibility and relaxation in mesoscopic ferromagnetic dots,"Vortex dynamics in a restricted geometry is considered for a magnetic system
consisting of ferromagnetic cylindrical dots. To describe the vortex dynamic
susceptibility and relaxation the equation of motion for the vortex center
position is applied. The dependencies of the vortex dynamic susceptibility and
resonance linewidth on geometrical parameters are calculated. A new method of
extracting damping parameter from the vortex low-frequency resonance peaks is
proposed and applied for interpretation of resonance data on FeNi circular
dots.",0602279v1
2006-05-17,Dynamic Critical Behavior of Percolation Observables in the 2d Ising Model,"We present preliminary results of our numerical study of the critical
dynamics of percolation observables for the two-dimensional Ising model. We
consider the (Monte-Carlo) short-time evolution of the system obtained with a
local heat-bath method and with the global Swendsen-Wang algorithm. In both
cases, we find qualitatively different dynamic behaviors for the magnetization
and Omega, the order parameter of the percolation transition. This may have
implications for the recent attempts to describe the dynamics of the QCD phase
transition using cluster observables.",0605015v1
2002-04-25,N=2-Supersymmetric Dynamics of a Spin-1/2 Particle in an Extended External Field,"One considers the quantum dynamics of a charged spin-1/2 particle in an
extended external eletromagnetic field that arises from the reduction of a
5-dimensional Abelian gauge theory. The non-relativistic regime of the reduced
4D-dynamics is worked out and one identifies the system as a sector of an
N=2-supersymmetric quantum-mechanical dynamics. The full supersymmetric model
is studied and one checks the algebra of the fermionic charges; the conclusion
is that no central charge drops out. The possible r\^ole of the extra external
fields, a scalar and a magnetic-like field, is discussed.",0204220v2
2004-12-20,Dynamical delocalization in random Landau Hamiltonians,"We prove the existence of dynamical delocalization for random Landau
Hamiltonians near each Landau level. Since typically there is dynamical
localization at the edges of each disordered-broadened Landau band, this
implies the existence of at least one dynamical mobility edge at each Landau
band, namely a boundary point between the localization and delocalization
regimes, which we prove to converge to the corresponding Landau level as either
the magnetic field or the disorder goes to zero.",0412070v1
2005-06-29,Singularity dynamics: Action and Reaction,"The interaction between singular and regular fields is considered for
Lorentz-invariant scalar and vector wave equations. The singular field is
generated by a Dirac source term. Its dynamics are deduced from the total field
Lagrangian. At non-relativistic speeds, the resulting equations of motion are
those of a mass in a scalar potential. Using this method we deduce the
relationship between source amplitude (scalar gravitational mass) and dynamic
mass (inertial mass). Generalising this method implies Lorentz forces for
charge singularities in the electromagnetic field and describes the dynamics
and interaction of hypothetical magnetic monopoles.",0506209v1
2007-10-18,Dynamics in One-Dimensional Spin Systems - Density-Matrix Renormalization Group Study,"We study the one-dimensional $S=1/2$ Heisenberg model with a uniform and a
staggered magnetic fields, using the dynamical density-matrix renormalization
group (DDMRG) technique. The DDMRG enables us to investigate the dynamical
properties of chain with lengths up to a few hundreds, and the results are
numerically exact in the same sense as 'exact diagonalization' results are.
Thus, we can analyze the low-energy spectrum almost in the thermodynamic limit.
In this work, we calculate the dynamical spin structure factor and demonstrate
the performance of the DDMRG method applying the open-end boundary condition as
well as the periodic boundary condition.",0710.3506v1
2007-12-05,"Glauber dynamics for the mean-field Ising model: cut-off, critical power law, and metastability","We study the Glauber dynamics for the Ising model on the complete graph, also
known as the Curie-Weiss Model. For beta < 1, we prove that the dynamics
exhibits a cut-off: the distance to stationarity drops from near 1 to near 0 in
a window of order n centered at [2(1-beta)]^{-1} n log n. For beta = 1, we
prove that the mixing time is of order n^{3/2}. For beta > 1, we study
metastability. In particular, we show that the Glauber dynamics restricted to
states of non-negative magnetization has mixing time O(n log n).",0712.0790v2
2007-12-15,Dynamic structure factors of the spin-1/2 XX chain with Dzyaloshinskii-Moriya interaction,"We consider the spin-1/2 isotropic XY chain in a (z) transverse magnetic
field with the Dzyaloshinskii-Moriya interaction directed along the z-axis in
spin space and examine the effects of the latter interaction on the zz, xx (yy)
and xy (yx) dynamic structure factors. The Dzyaloshinskii-Moriya interaction
does not manifest itself in the zz dynamic quantities. In contrast, the xx (yy)
and xy (yx) dynamic structure factors show dramatical changes owing to the
Dzyaloshinskii-Moriya interaction. Implications of our results for electron
spin resonance experiments are briefly discussed.",0712.2507v1
2008-06-06,Quantum-Fluctuation-Driven Coherent Spin Dynamics in Small Condensates,"We have studied quantum spin dynamics of small condensates of cold sodium
atoms. For a condensate initially prepared in a mean field ground state, we
show that coherent spin dynamics are {\em purely} driven by quantum
fluctuations of collective spin coordinates and can be tuned by quadratic
Zeeman coupling and magnetization. These dynamics in small condensates can be
probed in a high-finesse optical cavity where temporal behaviors of excitation
spectra of a coupled condensate-photon system reveal the time evolution of
populations of atoms at different hyperfine spin states.",0806.1220v1
2009-05-11,Non-Markovian dynamics of clusters during nucleation,"Most theories of homogeneous nucleation are based on a Fokker-Planck-like
description of the behavior of the mass of clusters. Here we will show that
these approaches are incomplete for a large class of nucleating systems, as
they assume the effective dynamics of the clusters to be Markovian, i.e.,
memoryless. We characterize these non-Markovian dynamics and show how this
influences the dynamics of clusters during nucleation. Our results are
validated by simulations of a three-dimensional Ising model with locally
conserved magnetization.",0905.1588v1
2010-01-07,Localization of spin mixing dynamics in a spin-1 Bose-Einstein condensate,"We propose to localize spin mixing dynamics in a spin-1 Bose-Einstein
condensate by a temporal modulation of spin exchange interaction, which is
tunable with optical Feshbach resonance. Adopting techniques from coherent
control, we demonstrate the localization/freezing of spin mixing dynamics, and
the suppression of the intrinsic dynamic instability and spontaneous spin
domain formation in a ferromagnetically interacting condensate of $^{87}$Rb
atoms. This work points to a promising scheme for investigating the weak
magnetic spin dipole interaction, which is usually masked by the more dominant
spin exchange interaction.",1001.1035v2
2010-03-30,Disentangling phonons from spins in ion-trap-based quantum spin simulators,"We compute how phonon creation affects the fidelity of the quantum spin
dynamics in trapped ion simulators. A rigorous treatment of the quantum
dynamics is made by employing an exact operator factorization of the evolution
operator. Although it is often assumed that phonon creation modifies the
dynamics of the spin evolution, for an Ising spin-spin interaction in an
external magnetic field, phonons have \textit{no effect} on the probabilities
of spin product states measured in the direction of the Ising model axis.
Phonons play a much more important role in influencing the effective spin
dynamics for Heisenberg or XY model spin simulators or for other observables,
like witness operators in the Ising model.",1003.5901v1
2010-06-01,Coherent control of interacting particles using dynamical and Aharonov-Bohm phases,"A powerful method of manipulating the dynamics of quantum coherent particles
is to control the phase of their tunneling. We consider a system of two
electrons hopping on a quasi one-dimensional lattice in the presence of a
uniform magnetic field, and study the effect of adding a time-periodic driving
potential. We show that the dynamical phases produced by the driving can
combine with the Aharonov-Bohm phases to give precise control of the
localization and dynamics of the particles, even in the presence of strong
particle interactions.",1006.0188v1
2011-03-29,Efficient control of accelerator maps,"Recently, the Hamiltonian Control Theory was used in [Boreux et al.] to
increase the dynamic aperture of a ring particle accelerator having a localized
thin sextupole magnet. In this letter, these results are extended by proving
that a simplified version of the obtained general control term leads to
significant improvements of the dynamic aperture of the uncontrolled model. In
addition, the dynamics of flat beams based on the same accelerator model can be
significantly improved by a reduced controlled term applied in only 1 degree of
freedom.",1103.5631v2
2011-09-29,A new deterministic model for chaotic reversals,"We present a new chaotic system of three coupled ordinary differential
equations, limited to quadratic nonlinear terms. A wide variety of dynamical
regimes are reported. For some parameters, chaotic reversals of the amplitudes
are produced by crisis-induced intermittency, following a mechanism different
from what is generally observed in similar deterministic models. Despite its
simplicity, this system therefore generates a rich dynamics, able to model more
complex physical systems. In particular, a comparison with reversals of the
magnetic field of the Earth shows a surprisingly good agreement, and highlights
the relevance of deterministic chaos to describe geomagnetic field dynamics.",1109.6569v2
2012-02-07,Dynamic effect of overhangs and islands at the depinning transition in two-dimensional magnets,"With the Monte Carlo methods, we systematically investigate the short-time
dynamics of domain-wall motion in the two-dimensional random-field Ising model
with a driving field ?DRFIM?. We accurately determine the depinning transition
field and critical exponents. Through two different definitions of the domain
interface, we examine the dynamics of overhangs and islands. At the depinning
transition, the dynamic effect of overhangs and islands reaches maximum. We
argue that this should be an important mechanism leading the DRFIM model to a
different universality class from the Edwards-Wilkinson equation with quenched
disorder",1202.1349v1
2012-03-03,On perturbations of generalized Landau-Lifshitz dynamics,"We consider deterministic and stochastic perturbations of dynamical systems
with conservation laws in $\R^3$. The Landau-Lifshitz equation for the
magnetization dynamics in ferromagnetics is a special case of our system. The
averaging principle is a natural tool in such problems. But bifurcations in the
set of invariant measures lead to essential modification in classical
averaging. The limiting slow motion in this case, in general, is a stochastic
process even if pure deterministic perturbations of a deterministic system are
considered. The stochasticity is a result of instabilities in the non-perturbed
system as well as of existence of ergodic sets of a positive measure. We
effectively describe the limiting slow motion.",1203.0602v1
2012-03-20,Vortex dynamics in the presence of excess energy for the Landau-Lifschitz-Gilbert equation,"We study the Landau-Lifshitz-Gilbert equation for the dynamics of a magnetic
vortex system. We present a PDE-based method for proving vortex dynamics that
does not rely on strong well-preparedness of the initial data and allows for
instantaneous changes in the strength of the gyrovector force due to bubbling
events. The main tools are estimates of the Hodge decomposition of the
supercurrent and an analysis of the defect measure of weak convergence of the
stress energy tensor. Ginzburg-Landau equations with mixed dynamics in the
presence of excess energy are also discussed.",1203.4426v1
2012-07-27,"Dynamic Phases, Pinning, and Pattern Formation for Driven Dislocation Assemblies","We show that driven dislocation assemblies exhibit a set of dynamical phases
remarkably similar to those of driven systems with quenched disorder such as
vortices in superconductors, magnetic domain walls, and charge density wave
materials. These phases include jammed, fluctuating, and dynamically ordered
states, and each produces distinct dislocation patterns as well as specific
features in the noise fluctuations and transport properties. Our work suggests
that many of the results established for systems with quenched disorder
undergoing depinning transitions can be applied to dislocation systems,
providing a new approach for understanding dislocation pattern formation and
dynamics.",1207.6657v1
2012-08-09,Voter models with conserved dynamics,"We propose a modified voter model with locally conserved magnetization and
investigate its phase ordering dynamics in two dimensions in numerical
simulations. Imposing a local constraint on the dynamics has the surprising
effect of speeding up the phase ordering process. The system is shown to
exhibit a scaling regime characterized by algebraic domain growth, at odds with
the logarithmic coarsening of the standard voter model. A phenomenological
approach based on cluster diffusion and similar to Smoluchowski ripening
correctly predicts the observed scaling regime. Our analysis exposes unexpected
complexity in the phase ordering dynamics without thermodynamic potential.",1208.2050v1
2013-03-26,Dynamics of strain bifurcations in magnetostrictive ribbon,"We develop a coupled nonlinear oscillator model involving magnetization and
strain to explain several experimentally observed dynamical features exhibited
by forced magnetostrictive ribbon. Here we show that the model recovers the
observed period doubling route to chaos as function of the dc field for a fixed
ac field and quasiperiodic route to chaos as a function of the ac field,
keeping the dc field constant. The model also predicts induced and suppressed
chaos under the influence of an additional small-amplitude near resonant ac
field. Our analysis suggests rich dynamics in coupled order parameter systems
like magnetomartensitic and magnetoelectric materials.",1303.6602v1
2013-09-24,Dynamics of the modified Kibble-Żurek mechanism in antiferromagnetic spin-1 condensates,"We investigate the dynamics and outcome of a quantum phase transition from an
antiferromagnetic to phase separated ground state in a spin-1 Bose-Einstein
condensate of ultracold atoms. We explicitly demonstrate double universality in
dynamics within experiments with various quench time. Furthermore, we show that
spin domains created in the nonequilibrium transition constitute a set of
mutually incoherent quasicondensates. The quasicondensates appear to be
positioned in a semi-regular fashion, which is a result of the conservation of
local magnetization during the post-selection dynamics.",1309.6107v1
2013-11-18,Thermoconvective structure dynamics in horizontal Homeotropic Nematics films heated from above,"Homeotropic Nematic Liquid Crystal heated from above present convective
Rayleigh-Benard instability for applied thermal gradients greater than
$\Delta$Tc. This threshold increase with the intensity of applied external
magnetic field parallel to the initial molecular orientation (vertical). We
studied the different patterns which exist near to and away from the threshold,
such as hexagons and squares, and the transitions between them. The study of
patterns evolution away the threshold shows coexistence between patterns and a
global system dynamics. Also we studied how this general dynamics change with
the inclination of the cell, where for very small angles (1 degree) the
hexagons and squares are transformed into rolls.",1311.4271v1
2014-10-14,Wave packet dynamics in various two-dimensional systems: a unified description,"In this article we present an exact and unified description of wave-packet
dynamics in various 2D systems in presence of a transverse magnetic field. We
consider an initial minimum-uncertainty Gaussian wave-packet, and find that its
long term dynamics displays the universal phenomena of spontaneous collapse and
quantum revival. We estimate the timescales associated with these phenomena
based on very general arguments for various materials, whose carrier dynamics
is described either by the Schr\""odinger equation or by the Dirac equation.",1410.3548v1
2015-04-24,Universal coarsening dynamics of a quenched ferromagnetic spin-1 condensate,"We demonstrate that a quasi-two-dimensional spin-1 condensate quenched to a
ferromagnetic phase undergoes universal coarsening in its late time dynamics.
The quench can be implemented by a sudden change in the applied magnetic field
and, depending on the final value, the ferromagnetic phase has easy-axis
(Ising) or easy-plane (XY) symmetry, with different dynamical critical
exponents. Our results for the easy-plane phase reveal a fractal domain
structure and the crucial role of polar-core spin vortices in the coarsening
dynamics.",1504.06404v2
2016-08-19,Nonequilibrium phase transitions in isotropic Ashkin-Teller Model,"Dynamic behaviour of an isotropic Ashkin-Teller model in the presence of a
periodically oscillating magnetic field has been analysed by means of the mean
field approximation. The dynamic equation of motion has been constructed with
the help of a Glauber type stochastic process and solved for a square lattice.
  After defining the possible dynamical phases of the system, phase diagrams
have been given and the behaviour of the hysteresis loops has been investigated
in detail. The hysteresis loop for specific order parameter of isotropic
Ashkin-Teller model has been defined and characteristics of this loop in
different dynamical phases have been given.",1608.05492v1
2016-11-05,Dynamic multiscaling in magnetohydrodynamic turbulence,"We present the first study of the multiscaling of time-dependent velocity and
magnetic-field structure functions in homogeneous, isotropic
magnetohydrodynamic (MHD) turbulence in three dimensions. We generalize the
formalism that has been developed for analogous studies of time-dependent
structure functions in fluid turbulence to MHD. By carrying out detailed
numerical studies of such time-dependent structure functions in a shell model
for three-dimensional MHD turbulence, we obtain both equal-time and dynamic
scaling exponents.",1611.01660v1
2019-04-10,Estimating Lyapunov exponents in billiards,"Dynamical billiards are paradigmatic examples of chaotic Hamiltonian
dynamical systems with widespread applications in physics. We study how well
their Lyapunov exponent, characterizing the chaotic dynamics, and its
dependence on external parameters can be estimated from phase space volume
arguments, with emphasis on billiards with mixed regular and chaotic phase
spaces. We show that in the very diverse billiards considered here the leading
contribution to the Lyapunov exponent is inversely proportional to the chaotic
phase space volume, and subsequently discuss the generality of this
relationship. We also extend the well established formalism by Dellago, Posch,
and Hoover to calculate the Lyapunov exponents of billiards to include external
magnetic fields and provide a software implementation of it.",1904.05108v2
2019-11-12,Coding closed and open quantum systems in MATLAB: applications in quantum optics and condensed matter,"We develop a package of numerical simulations implemented in MATLAB to solve
complex many-body quantum systems. We focus on widely used examples that
include the calculation of the magnetization dynamics for the closed and open
Ising model, dynamical quantum phase transition in cavity QED arrays, Markovian
dynamics for interacting two-level systems, and the non-Markovian dynamics of
the pure-dephasing spin-boson model. These examples will be useful for
undergraduate and graduate students with a medium or high background in MATLAB,
and also for researches interested in numerical studies applied to quantum
optics and condensed matter systems.",1911.04906v2
2023-12-04,Dynamical description of the phase transition into the superconducting state,"Dynamical equations that are valid in the vicinity of the phase transition
into the superconducting state are given. Probable effects of the field of
charge carriers' magnetic interactions and the field of temperature
fluctuations were taken into account. The order of the phase transition is
discussed. Considerations in favor of the applicability of the stochastic
dynamical model A to describe the dynamics of this phase transition are given.",2312.01927v1
1997-12-29,Nonequilibrium phase transition in the kinetic Ising model: Is transition point the maximum lossy point ?,"The nonequilibrium dynamic phase transition, in the kinetic Ising model in
presence of an oscillating magnetic field, has been studied both by Monte Carlo
simulation (in two dimension) and by solving the meanfield dynamical equation
of motion for the average magnetization. The temperature variations of
hysteretic loss (loop area) and the dynamic correlation have been studied near
the transition point. The transition point has been identified as the
minimum-correlation point. The hysteretic loss becomes maximum above the
transition point. An analytical formulation has been developed to analyse the
simulation results. A general relationship among hysteresis loop area, dynamic
order parameter and dynamic correlation has also been developed.",9712309v2
1998-11-06,Dynamic transitions and hysteresis,"When an interacting many-body system, such as a magnet, is driven in time by
an external perturbation, such as a magnetic field,the system cannot respond
instantaneously due to relaxational delay. The response of such a system under
a time-dependent field leads to many novel physical phenomena with intriguing
physics and important technological applications. For oscillating fields, one
obtains hysteresis that would not occur under quasistatic conditions in the
presence of thermal fluctuations. Under some extreme conditions of the driving
field, one can also obtain a non-zero average value of the variable undergoing
such dynamic hysteresis. This non-zero value indicates a breaking of symmetry
of the hysteresis loop, around the origin. Such a transition to the
spontaneously broken symmetric phase occurs dynamically when the driving
frequency of the field increases beyond its threshold value which depends on
the field amplitude and the temperature. Similar dynamic transitions also occur
for pulsed and stochastically varying fields. We present an overview of the
ongoing researches in this not-so-old field of dynamic hysteresis and
transitions.",9811086v1
2000-09-06,Dynamical Properties of the 1/r^2-Type Supersymmetric t-J Model in a Magnetic Field: Manifestation of Spin-Charge Separation,"Quasi-particle picture in a magnetic field is pursued for dynamical spin and
charge correlation functions of the one-dimensional supersymmetric t-J model
with inverse-square interaction. With use of exact diagonalization and the
asymptotic Bethe-ansatz equations for finite systems, excitation contents of
relevant excited states are identified which are valid in the thermodynamic
limit. The excitation contents are composed of spinons, antispinons, holons and
antiholons obeying fractional statistics. Both longitudinal and transverse
components of the dynamical spin structure factor are independent of the
electron density in the region where only quasi-particles with spin degrees of
freedom (spinons and antispinons) contribute. The dynamical charge structure
factor does not depend on the spin-polarization density in the region where
only quasi-particles with charge (holons and antiholons) are excited. These
features indicate the strong spin-charge separation in dynamics, reflecting the
high symmetry of the model.",0009075v1
2003-02-22,On the dynamical anomalies in the Hamiltonian Mean Field model,"We study the N-dependence of the thermodynamical variables and the dynamical
behavior of the well-known Hamiltonian Mean Field model. Microcanonical
analysis revealed a thermodynamic limit which defers from the a priory
traditional assumption of the N-dependence of the coupling constant g as 1/N:
to tend N?infinity keeping constant E/N^3 and g/N, prescription which
guarantees the extensivity of the Boltzmann entropy. The analysis of dynamics
leads to approximate the time evolution of the magnetization density m by means
of a Langevin equation with multiplicative noise. This equation leads to a
Fokker-Planck's equation which is N-independent when the time variable is
scaled by the N-dependent time constant T_{mac}=sqrt(IN/g), which represents
the characteristic time scale for the dynamical evolution of the macroscopic
observables derived from the magnetization density. This results explains the
origin of the slow relaxation regimen observed in microcanonical numerical
computations of dynamics of this model system. Connection with the system
self-similarity is suggested.",0302456v2
2004-09-07,Spin Dynamics of Layered Triangular Antiferromagnets with Uniaxial Anisotropy,"The spin dynamics of the semiclassical Heisenberg model with uniaxial
anisotropy, on the layered triangular lattice with antiferromagnetic coupling
for both intralayer nearest neighbor interaction and interlayer interaction is
studied both in the ordered phase and in the paramagnetic phase, using the
Monte Carlo-molecular dynamics technique. The important quantities calculated
are the full dynamic structure function $S(\bf q, \omega)$, the chiral dynamic
structure function $S_{chi}(\omega)$, the static order parameter and some
thermodynamic quantities. Our results show the existence of propagating modes
corresponding to both $S(\bf q, \omega)$ and $S_{chi}(\omega)$ in the ordered
phase, supporting the recent conjectures. Our results for the static properties
show the magnetic ordering in each layer to be of coplanar 3-sublattice type
deviating from 120 degree structure. In the presence of magnetic trimerization
however, we find the 3-sublattice structure to be weakened alongwith the
tendency towards non-coplanarity of the spins.",0409151v1
2004-09-27,Dynamics at a smeared phase transition,"We investigate the effects of rare regions on the dynamics of Ising magnets
with planar defects, i.e., disorder perfectly correlated in two dimensions. In
these systems, the magnetic phase transition is smeared because static
long-range order can develop on isolated rare regions. We first study an
infinite-range model by numerically solving local dynamic mean-field equations.
Then we use extremal statistics and scaling arguments to discuss the dynamics
beyond mean-field theory. In the tail region of the smeared transition the
dynamics is even slower than in a conventional Griffiths phase: the spin
autocorrelation function decays like a stretched exponential at intermediate
times before approaching the exponentially small equilibrium value following a
power law at late times.",0409688v2
2005-09-12,Spin and Charge Dynamics in a Renormalised Perturbation Theory,"We calculate the spin and charge dynamical susceptibilities of a strongly
correlated impurity model in a renormalised perturbation theory.
  The irreducible for vertices for the quasiparticle scattering are deduced
from the renormalised parameters, which have been calculated by fitting of the
low-lying levels of a numerical renormalisation group (NRG) calculation to
those of an effective Anderson model. The susceptibilities are asymptotically
exact in the low frequency limit and satisfy the Korringa-Shiba relation. By
comparing the results with those calculated from a direct NRG calculation, we
show that the renormalised perturbation theory (RPT) description gives a very
good description of spin dynamics for all values of the local interaction U,
not only at low frequencies but over the whole relevant frequency range.
  In the presence of a magnetic field the approach can be generalised using
field dependent renormalised parameters to calculate the transverse and
parallel spin dynamic susceptibilities. The RPT results give accurate results
for the spin dynamics over the whole frequency range and for all values of the
magnetic field.",0509296v1
2005-11-15,Dynamic response of HTS composite tapes to pulsed currents,"Dynamic voltage-current characteristics of an HTS Ag/BiSCCO composite tape
are studied both experimentally and theoretically. The tape is subjected by
pulsed currents with different shapes and magnitude and voltage traces are
measured using the four-point method with different location of potential taps
on the sample surface. Clockwise and anticlockwise hysteresis loops are
obtained for the same sample depending on location of the potential taps. The
dynamic characteristics deviate substantially from the DC characteristic,
especially in the range of low voltages where a criterion for the critical
current value is usually chosen (1-10 mkV/cm). The critical current determined
from dynamic characteristics and its change with the pulse magnitude depend on
location of the potential taps and on the curve branch chosen for the critical
current determination (ascending or descending). The theoretical analysis is
based on a model of the magnetic flux diffusion into a composite tape for a
superconductor described by the flux creep characteristic. Numerical simulation
based on this model gives the results in good agreement with the experimental
ones and explains the observed peculiarities of the dynamic characteristics of
HTS composite tapes. The difference between the magnetic diffusion into a tape
and a slab is discussed.",0511365v1
2006-03-15,Field momentum and gyroscopic dynamics of classical systems with topological defects,"The standard relation between the field momentum and the force is generalized
for the system with a field singularity: in addition to the regular force,
there appear the singular one. This approach is applied to the description of
the gyroscopic dynamics of the classical field with topological defects. The
collective variable Lagrangian description is considered for gyroscopical
systems with account of singularities. Using this method we describe the
dynamics of two-dimensional magnetic solitons. We establish a relation between
the gyroscopic force and the singular one. An effective Lagrangian description
is discussed for the magnetic soliton dynamics.",0603035v3
2009-05-11,Flux dynamics in NdO1-xFxFeAs bulk sample,"We present data of multi harmonic magneto-dynamic experiments. In particular,
we performed ac magnetic susceptibility experiments on layered pnictide-oxide
quaternary compound NdOFeAs doped with fluorine. The experiments allow measure
the critical temperature and probe the flux dynamic behavior using the third
harmonic component of the ac susceptibility of a NdF0.16FeAsO0.84 bulk sample
as a function of temperature and frequency of the applied ac magnetic fields.
Measured signals are connected with the non-linear superconducting flux dynamic
behavior and are characterized by a flux critical states sustaining a
superconducting critical current. In this framework the irreversibility line
that describes the stable superconducting state has been extracted from the
onset of the third harmonic signal vs. frequency. Finally we present also the
analysis of the flux dynamic dimensionality in the investigated sample.",0905.1633v1
2010-05-30,From Phase Space Representation to Amplitude Equations in a Pattern Forming Experiment,"We describe and demonstrate a method to reconstruct an amplitude equation
from the nonlinear relaxation dynamics in the succession of the Rosensweig
instability. A flat layer of a ferrofluid is cooled such that the liquid has a
relatively high viscosity. Consequently, the dynamics of the formation of the
Rosensweig pattern becomes very slow. By sudden switching of the magnetic
induction, the system is pushed to an arbitrary point in the phase space
spanned by the pattern amplitude and the magnetic induction. Afterwards, it is
allowed to relax to its equilibrium point. From the dynamics of this
relaxation, we reconstruct the underlying fully nonlinear equation of motion of
the pattern amplitude. The measured nonlinear dynamics serves to select the
best weakly nonlinear expansion which describes this hysteretic transition.",1005.5544v1
2011-04-05,Emergent gauge dynamics of highly frustrated magnets,"Condensed matter exhibits a wide variety of exotic emergent phenomena such as
the fractional quantum Hall effect and the low temperature cooperative behavior
of highly frustrated magnets. I consider the classical Hamiltonian dynamics of
spins of the latter phenomena using a method introduced by Dirac in the 1950s
by assuming they are constrained to their lowest energy configurations as a
simplifying measure. Focusing on the kagome antiferromagnet as an example, I
find it is a gauge system with topological dynamics and non-locally connected
edge states for certain open boundary conditions similar to doubled
Chern-Simons electrodynamics expected of a $Z_2$ spin liquid. These dynamics
are also similar to electrons in the fractional quantum Hall effect. The
classical theory presented here is a first step towards a controlled
semi-classical description of the spin liquid phases of many pyrochlore and
kagome antiferromagnets and towards a description of the low energy classical
dynamics of the corresponding unconstrained Heisenberg models.",1104.0721v3
2012-03-03,Effective field investigation of dynamic phase transitions for site diluted Ising ferromagnets driven by a periodically oscillating magnetic field,"Dynamic behavior of a site diluted Ising ferromagnet in the presence of
periodically oscillating magnetic field has been analyzed by means of the
effective field theory (EFT). Dynamic equation of motion have been solved for a
honeycomb lattice ($z=3$) with the help of a Glauber type stochastic process.
The global phase diagrams and the variation of the corresponding dynamic order
parameter as a function of the Hamiltonian parameters and temperature has been
investigated in detail and it has been shown that the system exhibits reentrant
phenomena, as well as a dynamic tricritical point which disappears for
sufficiently weak dilution.",1203.0639v1
2012-08-11,Dynamical Configurations and Bistability of Helical Nanostructures under External Torque,"We study the motion of a ferromagnetic helical nanostructure under the action
of a rotating magnetic field. A variety of dynamical configurations were
observed that depended strongly on the direction of magnetization and the
geometrical parameters, which were also confirmed by a theoretical model, based
on the dynamics of a rigid body under Stokes flow. Although motion at low
Reynolds numbers is typically deterministic, under certain experimental
conditions, the nanostructures showed a surprising bistable behavior, such that
the dynamics switched randomly between two configurations, possibly induced by
thermal fluctuations. The experimental observations and the theoretical results
presented in this letter are general enough to be applicable to any system of
ellipsoidal symmetry under external force or torque.",1208.2381v1
2013-08-20,Dynamic-Symmetry-Breaking Breathing and Spreading Transitions in Ferromagnetic Film Irradiated by Spherical Electromagnetic Wave,"The dynamical responses of a ferromagnetic film to a propagating spherical
electromagnetic wave passing through it are studied by Monte Carlo simulation
of two dimensional Ising ferromagnet. For a fixed set of values of the
frequency and wavelength of the spherical EM wave, and depending on the values
of amplitude of the EM wave and temperature of the system, three different
modes are identified. The static pinned mode, the localised dynamical breathing
mode and extended dynamical spreading mode are observed. The nonequilibrium
dynamical-symmetry-breaking breathing and spreading phase transitions are also
observed and the transition temperatures are obtained as functions of the
amplitude of the magnetic field of EM wave. A comprehensive phase diagram is
drawn. The boundaries of breathing and spreading transitions merge eventually
at the equilibrium transition temperature for two dimensional Ising ferromagnet
as the value of the amplitude of the magnetic field becomes vanishingly small.",1308.4277v1
2013-10-24,Dynamics in spinor condensates controlled by a microwave dressing field,"We experimentally study spin dynamics in a sodium antiferromagnetic spinor
condensate with off-resonant microwave pulses. In contrast to a magnetic field,
a microwave dressing field enables us to explore rich spin dynamics under the
influence of a negative net quadratic Zeeman shift $q_{\rm net}$. We find an
experimental signature to determine the sign of $q_{\rm net}$, and observe
harmonic spin population oscillations at every $q_{\rm net}$ except near each
separatrix in phase space where spin oscillation period diverges. In the
negative and positive $q_{\rm net}$ regions, we also observe a remarkably
different relationship between each separatrix and the magnetization. Our data
confirms an important prediction derived from the mean-field theory:
spin-mixing dynamics in spin-1 condensates substantially depends on the sign of
the ratio of $q_{\rm net}$ and the spin-dependent interaction energy. This work
may thus be the first to use only one atomic species to reveal mean-field spin
dynamics, especially the separatrix, which are predicted to appear differently
in spin-1 antiferromagnetic and ferromagnetic spinor condensates.",1310.6785v1
2016-02-17,Fractional Spin Fluctuation as a Precursor of Quantum Spin Liquids: Majorana Dynamical Mean-Field Study for the Kitaev Model,"Experimental identification of quantum spin liquids remains a challenge, as
the pristine nature is to be seen in asymptotically low temperatures. We here
theoretically show that the precursor of quantum spin liquids appears in the
spin dynamics in the paramagnetic state over a wide temperature range. Using
the cluster dynamical mean-field theory and the continuous-time quantum Monte
Carlo method, which are newly developed in the Majorana fermion representation,
we calculate the dynamical spin structure factor, relaxation rate in nuclear
magnetic resonance, and magnetic susceptibility for the honeycomb Kitaev model
whose ground state is a canonical example of the quantum spin liquid. We find
that dynamical spin correlations show peculiar temperature and frequency
dependence even below the temperature where static correlations saturate. The
results provide the experimentally-accessible symptoms of the fluctuating
fractionalized spins evincing the quantum spin liquids.",1602.05253v2
2016-06-10,Nanoscale confinement of ultrafast spin transfer torque exciting non-uniform spin dynamics by femtosecond spin current pulses,"Spintronics had a widespread impact over the past decades due to transferring
information by spin rather than electric currents. Its further development
requires miniaturization and reduction of characteristic timescales of spin
dynamics combining the sub-nanometer spatial and femtosecond temporal ranges.
These demands shift the focus of interest towards the fundamental open question
of the interaction of femtosecond spin current (SC) pulses with a ferromagnet
(FM). The spatio-temporal properties of the impulsive spin transfer torque
exerted by ultrashort SC pulses on the FM open the time domain for probing
non-uniform magnetization dynamics. Here we employ laser-generated ultrashort
SC pulses for driving ultrafast spin dynamics in FM and analyzing its transient
local source. Transverse spins injected into FM excite inhomogeneous
high-frequency spin dynamics up to 0.6 THz, indicating that the perturbation of
the FM magnetization is confined to 2 nm.",1606.03403v1
2017-03-09,Dynamics and inertia of a skyrmion in chiral magnets and interfaces: A linear response approach based on magnon excitations,"Taking all the magnon modes into account, we derive the skyrmion dynamics in
response to a weak external drive. A skyrmion has rotational symmetry and the
magnon modes can be characterized by an angular momentum. For a weak distortion
of a skyrmion, only the magnon modes with an angular momentum $|m|=1$ govern
the dynamics of skyrmion topological center. The skyrmion inertia is determined
by the magnon modes in the continuum spectrum. For a skyrmion driven by a
magnetic field gradient or by a spin transfer torque generated by a current,
the dynamical response is practically instantaneous. This justifies the rigid
skyrmion approximation used in Thiele's collective coordinate approach. For a
skyrmion driven by a spin Hall torque, the torque couples to the skyrmion
motion through the magnons in the continuum and damping, therefore the skyrmion
dynamics shows sizable inertia in this case. The trajectory of a skyrmion is an
ellipse for an ac drive of spin Hall torque.",1703.03099v2
2017-03-30,Protected ultrastrong coupling regime of the two-photon quantum Rabi model with trapped ions,"We propose a robust realization of the two-photon quantum Rabi model in a
trapped-ion setting based on a continuous dynamical decoupling scheme. In this
manner the magnetic dephasing noise, which is identified as the main obstacle
to achieve long time coherent dynamics in ion-trap simulators, can be safely
eliminated. More specifically, we investigate the ultrastrong coupling regime
of the two-photon quantum Rabi model whose realization in trapped ions involves
second-order sideband processes. Hence, the resulting dynamics becomes
unavoidably slow and more exposed to magnetic noise requiring an appropriate
scheme for its elimination. Furthermore, we discuss how dynamical decoupling
methods take a dual role in our protocol, namely they remove the main source of
decoherence from the dynamics while actively define the parameter regime of the
simulated model.",1703.10539v1
2017-07-13,Simulation of left ventricle fluid dynamics with mitral regurgitation from magnetic resonance images with fictitious elastic structure regularization,"Computer modeling can provide quantitative insight into cardiac fluid
dynamics phenomena that are not evident from standard imaging tools. We propose
a new approach to modeling left ventricle fluid dynamics based on an
image-driven model-based description of ventricular motion. In this approach,
the end-diastolic geometry and time-dependent deformation of the left ventricle
cavity are obtained from cardiac magnetic resonance images and a fictitious
elastic structure is used to impose the contractile behavior of the left
ventricle. This allows seamless treatment of the isovolumic phases. Besides the
ventricular motion, the intracavitary fluid dynamics is controlled by the
mitral valve. Three different mitral valve models are included in the
simulation: an idealized diode (with or without regurgitation) and a lumped
parameter model accounting for the opening dynamics of the valve and including
regurgitation.",1707.03998v1
2018-01-30,Massively parallel symplectic algorithm for coupled magnetic spin dynamics and molecular dynamics,"A parallel implementation of coupled spin-lattice dynamics in the LAMMPS
molecular dynamics package is presented. The equations of motion for both spin
only and coupled spin-lattice dynamics are first reviewed, including a detailed
account of how magneto-mechanical potentials can be used to perform a proper
coupling between spin and lattice degrees of freedom. A symplectic numerical
integration algorithm is then presented which combines the Suzuki-Trotter
decomposition for non-commuting variables and conserves the geometric
properties of the equations of motion. The numerical accuracy of the serial
implementation was assessed by verifying that it conserves the total energy and
the norm of the total magnetization up to second order in the timestep size.
Finally, a very general parallel algorithm is proposed that allows large
spin-lattice systems to be efficiently simulated on large numbers of processors
without degrading its mathematical accuracy. Its correctness as well as scaling
efficiency were tested for realistic coupled spin-lattice systems, confirming
that the new parallel algorithm is both accurate and efficient.",1801.10233v1
2018-01-31,Constructing effective free energies for dynamical quantum phase transitions in the transverse-field Ising chain,"The theory of dynamical quantum phase transitions represents an attempt to
extend the concept of phase transitions to the far from equilibrium regime.
While there are many formal analogies to conventional transitions, it is a
major question to which extent it is possible to formulate a nonequilibrium
counterpart to a Landau-Ginzburg theory. In this work we take a first step in
this direction by constructing an effective free energy for continuous
dynamical quantum phase transitions appearing after quantum quenches in the
transverse-field Ising chain. Due to unitarity of quantum time evolution this
effective free energy becomes a complex quantity transforming the conventional
minimization principle of the free energy into a saddle-point equation in the
complex plane of the order parameter, which as in equilibrium is the
magnetization. We study this effective free energy in the vicinity of the
dynamical quantum phase transition by performing an expansion in terms of the
complex magnetization and discuss the connections to the equilibrium case.
Furthermore, we study the influence of perturbations and signatures of these
dynamical quantum phase transitions in spin correlation functions.",1802.00020v3
2018-09-21,Anomalous transport of magnetic colloids in a liquid crystal-magnetic colloid mixture,"We report an extensive molecular dynamics study on the translational dynamics
of a hybrid system composed of dipolar soft spheres (DSS), representing
ferromagnetic particles, suspended in a liquid crystal (LC) matrix. We observe
that the LC matrix strongly modifies the dynamics of the DSS. In the isotropic
regime, the DSS show a crossover from subdiffusive to normal diffusive behavior
at long times, with an increase of the subdiffusive regime as the dipolar
coupling strength is increased. In the nematic regime, the LC matrix, due to
collective reorientation of LC particles, imposes a cylindrical confinement on
the DSS chains. This leads to a diffusive dynamics of DSS along the nematic
director and a subdiffusive dynamics (with an exponent $\sim 0.5$) in the
perpendicular direction. The confinement provided by the LC matrix is also
reflected by oscillatory behavior of the components of the velocity
autocorrelation function of the DSS in the nematic phase.",1809.08288v2
2018-11-07,"Magnetization and energy dynamics in spin ladders: Evidence of diffusion in time, frequency, position, and momentum","The dynamics of magnetization and energy densities are studied in the two-leg
spin-1/2 ladder. Using an efficient pure-state approach based on the concept of
typicality, we calculate spatio-temporal correlation functions for large
systems with up to 40 lattice sites. In addition, two subsequent Fourier
transforms from real to momentum space as well as from time to frequency domain
yield the respective dynamic structure factors. Summarizing our main results,
we unveil the existence of genuine diffusion both for spin and energy. In
particular, this finding is based on four distinct signatures which can all be
equally well detected: (i) Gaussian density profiles, (ii) time-independent
diffusion coefficients, (iii) exponentially decaying density modes, and (iv)
Lorentzian line shapes of the dynamic structure factor. The combination of (i)
- (iv) provides a comprehensive picture of high-temperature dynamics in
thisarchetypal nonintegrable quantum model.",1811.02806v2
2019-01-04,Collisional N-Body Dynamics Coupled to Self-Gravitating Magnetohydrodynamics Reveals Dynamical Binary Formation,"We describe a star cluster formation model that includes individual star
formation from self-gravitating, magnetized gas, coupled to collisional stellar
dynamics. The model uses the Astrophysical Multi-purpose Software Environment
(AMUSE) to integrate an adaptive-mesh magnetohydrodynamics code (FLASH) with a
fourth order Hermite N-body code (ph4), a stellar evolution code (SeBa), and a
method for resolving binary evolution (multiples). This combination yields
unique star formation simulations that allow us to study binaries formed
dynamically from interactions with both other stars and dense, magnetized gas
subject to stellar feedback during the birth and early evolution of stellar
clusters. We find that for massive stars, our simulations are consistent with
the observed dynamical binary fractions and mass ratios. However, our binary
fraction drops well below observed values for lower mass stars, presumably due
to unincluded binary formation during initial star formation. Further, we
observe a build up of binaries near the hard-soft boundary that may be an
important mechanism driving early cluster contraction.",1901.01132v1
2019-02-14,Coupled dynamics of long-range and internal spin cluster order in Cu$_{2}$OSeO$_{3}$,"Cu$_4$ triplet clusters form the relevant spin entity for the formation of
long-range magnetic order in the cluster magnet Cu$_2$OSeO$_3$. Using
time-resolved Raman spectroscopy, we probed photoinduced spin and lattice
dynamics in this Mott insulator. Multiple ps-decade spin-lattice relaxation
dynamics is observed, evidencing a separation of the order parameter dynamics
into disordering of long-range and internal spin cluster order. Our study
exemplifies the double order parameter dynamics of generalized molecular
crystals of charge, spin, and orbital nature.",1902.05329v1
2019-03-22,Exceptional points in classical spin dynamics,"Non-conservative physical systems admit a special kind of spectral
degeneracy, known as exceptional point (EP), at which eigenvalues and
eigenvectors of the corresponding non-Hermitian Hamiltonian coalesce. Dynamical
parametric encircling of the EP can lead to non-adiabatic evolution associated
with a state flip, a sharp transition between the resonant modes. Physical
consequences of the dynamical encircling of EPs in open dissipative systems
have been explored in optics and photonics. Building on the recent progress in
understanding the parity-time (PT)-symmetric dynamics in spin systems, we use
topological properties of EPs to implement chiral non-reciprocal transmission
of a spin through the material with non-uniform magnetization, like helical
magnet. We consider an exemplary system, spin-torque-driven single spin
described by the time-dependent non-Hermitian Hamiltonian. We show that
encircling individual EPs in parameter space results in non-reciprocal spin
dynamics and find the range of optimal protocol parameters for high-efficiency
asymmetric spin filter based on this effect. Our findings offer a platform for
non-reciprocal spin devices for spintronics and magnonics.",1903.09729v2
2019-04-10,Dynamics of Phase Separated States in the Double Exchange Model,"We present extensive large-scale dynamical simulations of phase-separated
states in the double exchange model. These inhomogeneous electronic states that
play a crucial role in the colossal magnetoresistance phenomenon are composed
of ferromagnetic metallic clusters embedded in an antiferromagnetic insulating
matrix. We compute the dynamical structure factor of these nanoscale textures
using an efficient real-space formulation of coupled spin and electron
dynamics. Dynamical signatures of the various underlying magnetic structures
are identified. At small hole doping, the structure factor exhibits a
dominating signal of magnons from the background N\'eel order and localized
modes from magnetic polarons. A low-energy continuum due to large-size
ferromagnetic clusters emerges at higher doping levels. Implications for
experiments on magnetoresistive manganites are discussed.",1904.05252v2
2019-08-06,Electron Induced Massive Dynamics of Magnetic Domain Walls,"We study the dynamics of domain walls (DWs) in a metallic, ferromagnetic
nanowire. We develop a Keldysh collective coordinate technique to describe the
effect of conduction electrons on rigid magnetic structures. The effective
Lagrangian and Langevin equations of motion for a DW are derived. The DW
dynamics is described by two collective degrees of freedom: position and
tilt-angle. The coupled Langevin equations therefore involve two correlated
noise sources, leading to a generalized fluctuation-dissipation theorem (FDT).
The DW response kernel due to electrons contains two parts: one related to
dissipation via FDT, and another `inertial' part. We prove that the latter term
leads to a mass for both degrees of freedom, even though the intrinsic bare
mass is zero. The electron-induced mass is present even in a clean system
without pinning or specifically engineered potentials. The resulting equations
of motion contain rich dynamical solutions and point toward a new way to
control domain wall motion in metals via the electronic system properties. We
discuss two observable consequences of the mass, hysteresis in the DW dynamics
and resonant response to ac current.",1908.02299v1
2020-10-19,Ultrafast dynamics in relativistic Mott insulators,"We study the photoinduced ultrafast dynamics in relativistic Mott insulators,
i.e., Mott insulators with strong spin-orbit coupling. For this purpose, we
consider a minimal one-band Hubbard model on lattices with square and
triangular symmetries, as relevant for layered transition metal compounds such
as Sr$_2$IrO$_4$. Depending on the lattice and the spin-orbit coupling, the
systems have canted antiferromagnetic or $120^\circ$ order. They are excited by
simulating a short laser pulse, and the dynamics is solved using nonequilibrium
dynamical mean-field theory. The pulse generates hot carriers, which
subsequently perturb the magnetic order due to the coupling between the
collective order and photocarriers. We find that this dynamics, which is known
form regular antiferromagnets, depends sensitively on the spatial structure of
the spin-orbit coupling. On the triangular lattice, in particular, relaxation
times are influenced by the spin-orbit coupling for the chiral $120^\circ$
order, while on the square lattice with canted antiferromagnetic order the
spin-orbit induced canting angle remains unchanged after the excitation. Our
study opens up new possibilities of controlling magnetism and exotic spin
states on the ultrafast timescales.",2010.09253v1
2021-06-30,Influence of Roughening Transition on Magnetic Ordering,"In the literature of magnetic phase transitions, in addition to a critical
point, existence of another special point has been discussed. This is related
to the broadening of interface between two different ordering phases and is
referred to as the point of roughening transition. While there exists good
understanding on equilibrium properties associated with this transition,
influence of this on nonequilibrium dynamics has not been investigated. In this
paper we present comprehensive results, from Monte Carlo simulations, on
coarsening dynamics in a system, over a wide range of temperature, in space
dimension d = 3, for which there exists roughening transition at a nonzero
temperature. State-of-the-art analysis of the simulation data, on structure,
growth and aging, shows that the onset of unexpected glass-like slow dynamics
in this system, that has received much attention in recent times, for quenches
to zero temperature, actually occurs at this transition point. This
demonstrates an important structure-dynamics connection in phase-ordering
dynamics. We compare the key results with those from d = 2, for which there
exists no non-zero roughening transition temperature. Absence of the above
mentioned anomalous features in the latter dimension places our conjecture on
the role of roughening transition on a firmer footing.",2106.16232v1
2021-12-27,Skyrmion based energy efficient straintronic physical reservoir computing,"Physical Reservoir Computing (PRC) is an unconventional computing paradigm,
which exploits nonlinear dynamics of reservoir blocks to perform recognition
and classification tasks. Here we show with simulations that patterned thin
films hosting several skyrmions, particularly one, two, four and nine
skyrmions, can implement energy efficient reservoir computing. This reservoir
computing (RC) block is based on nonlinear breathing dynamics of skyrmions,
which are coupled to each other through dipole interaction and spin waves, in
response to a voltage generated strain. This nonlinear and coupled
magnetization dynamics is exploited to perform temporal pattern recognition.
Two performance metrics, namely Short-Term Memory (STM) and Parity Check (PC)
capacity are studied to demonstrate the potential of such skyrmion based PRC in
addition to showing it can classify sine and square waves with 100% accuracy.
Furthermore, our study demonstrates that nonlinear magnetization dynamics and
interaction through spin waves and dipole coupling have a strong influence on
STM and PC capacity, thus explaining the role of physical interactions in a
dynamical system on its ability to perform Reservoir Computing (RC).",2112.13527v2
2022-12-19,Discrete Laplacian Thermostat for Spin Systems with Conserved Dynamics,"A well-established numerical technique to study the dynamics of spin systems
in which symmetries and conservation laws play an important role is to
microcanonically integrate their reversible equations of motion, obtaining
thermalization through initial conditions drawn with the canonical
distribution. In order to achieve a more realistic relaxation of the magnetic
energy, numerically expensive methods that explicitly couple the spins to the
underlying lattice are normally employed. Here we introduce a stochastic
conservative thermostat that relaxes the magnetic energy while preserving the
constant of motions, thus turning microcanonical spin dynamics into a
conservative canonical dynamics, without actually simulating the lattice. We
test the thermostat on the Heisenberg antiferromagnet in d=3 and show that the
method reproduces the exact values of the static and dynamic critical
exponents, while in the low-temperature phase it yields the correct spin wave
phenomenology. Finally, we demonstrate that the relaxation coefficient of the
new thermostat is quantitatively connected to the microscopic parameters of the
spin-lattice coupling.",2212.09647v2
2023-01-18,Topology in the space-time scaling limit of quantum dynamics,"We investigate the role of topology in the space-time scaling limit of
quantum quench dynamics, where both time and system size tend to infinity at a
constant ratio. There, while the standard topological characterization relying
on local unitary transformations becomes ill defined, we show how a different
dynamical notion of topology naturally arises through a dynamical winding
number encoding the linear response of the Berry phase to a magnetic flux.
Specifically, we find that the presence of a locally invisible constant
magnetic flux is revealed by a dynamical staircase behavior of the Berry phase,
whose topologically quantized plateaus characterize the space-time scaling
limit of a quenched Rice-Mele model. These jumps in the Berry phase are also
shown to be related to the interband elements of the DC current operator. We
outline possible experimental platforms for observing the predicted phenomena
in finite systems.",2301.07752v2
2023-04-19,Interacting quantum walk on a two-leg flux ladder: Emergence of re-entrant dynamics,"We study the quench dynamics of interacting bosons on a two-leg flux ladder
by implementing the continuous-time quantum walk and explore the combined
effect of the magnetic field and onsite interaction in the presence of uniform
flux. We show that in the regime of weak interaction, the magnetic field
substantially slows down the spreading of the particles' wavefunction during
the dynamics. However, in the presence of strong interaction, we obtain an
interesting re-entrant behaviour in the dynamics where the radial velocity
associated to the spreading first increases, then decreases, and increases
again as a function of the flux strength. We also find a re-entrant dynamics in
the chiral motion of the particles as a function of interaction for fixed flux
strengths.",2304.09796v1
2023-06-17,Bloch dynamics in inversion symmetry broken monolayer phosphorene,"We investigate Bloch oscillations of wave packets in monolayer phosphorene
with broken inversion symmetry. We find that the real space trajectories, Berry
and group velocities of Bloch electron undergo Bloch oscillations in the
system. The strong dependence of Bloch dynamics on the crystal momentum is
illustrated. It is shown that the spin-orbit interaction crucially affects the
dynamics of the Bloch electron. We also demonstrate the dynamics in external
electric and magnetic field within the framework of Newton's equations of
motion, leading to the geometric visualization of such an oscillatory motion.
In the presence of both applied in-plane electric and transverse magnetic
fields, the system undergoes a dynamical transition from confined to
de-confined state and vice versa, tuned by the relative strength of the fields.",2306.10314v1
2024-03-19,Kinetically constrained models constructed from dissipative quantum dynamics,"We propose a construction of kinetically constrained models using the
Markovian quantum dynamics under strong dissipation. Using the
Gorini-Kossakowski-Sudarshan-Lindblad (GKSL) formalism, we show that strong
dissipation leads to the emergent decoherence-free subspaces, within which
constrained quantum many-body unitary dynamics can take place. We argue that
the unitary dynamics constructed by the GKSL dynamics is more tightly
constrained than that constructed by the strongly interacting Hamiltonian,
where the interactions have the same form with the GKSL jump operators. As an
example, we demonstrate that a one-dimensional spin system with two-site
dissipation leads to the kinetically constrained ``PXQ"" model, which exhibits
the free domain-wall motion with an additional frozen-block structure. Under
the uniform magnetic field, the PXQ model shows the domain-wall localization,
similar to the Wannier-Stark localization. We then couple two PXQ chains with
the magnetic field and inter-chain interaction. We discover that, while
localization of the domain walls persists despite the interactions for typical
parameter regimes, a non-trivial partial delocalization appears for a certain
parameter line.",2403.12548v1
2001-11-25,Dynamical Quenching of the $α^2$ Dynamo,"We present a two-scale approximation for the dynamics of a nonlinear
$\alpha^2$ dynamo. Solutions of the resulting nonlinear equations agree with
the numerical simulations of Brandenburg (2001), and show that $\alpha$ is
quenched by the buildup of magnetic helicity at the forcing scale $1/k_2$ as
the $\alpha$ effect transfers it from the large scale $1/k_1$. For times $t >
(k_1/k_2)R_{M,2}$ in eddy turnover units (where $R_{M,2}$ is the magnetic
Reynolds number of the forcing scale), $\alpha$ is resistively limited in the
form predicted for the steady-state case. However, for $t << R_{M,2}$, $\alpha$
takes on its kinematic value, independent of $R_{M,2}$, allowing the production
of large-scale magnetic energy equal to $k_1/k_2$ times equipartition. Thus the
dynamic theory of $\alpha$ predicts substantial ""fast"" growth of large-scale
field despite being ""slow"" at large times.",0111470v2
1998-03-16,Probing Physics of Magnetohydrodynamic Turbulence Using Direct Numerical Simulation,"The energy spectrum and the nolinear cascade rates of MHD turbulence is not
clearly understood. We have addressed this problem using direct numerical
simulation and analytical calculations. Our numerical simulations indicate that
Kolmogorov-like phenomenology with $k^{-5/3}$ energy spectrum, rather than
Kraichnan's $k^{-3/2}$, appears to be applicable in MHD turbulence. Here, we
also construct a self-consistent renomalization group procedure in which the
mean magnetic field gets renormalized, which in turns yields $k^{-5/3}$ energy
spectrum. The numerical simulations also show that the fluid energy is
transferred to magnetic energy. This result could shed light on the generation
magnetic field as in dynamo mechanism.",9803022v2
1994-09-07,Diamagnetic Response due to Localization in Chains of Connected Mesoscopic Rings,"A dynamic response to a magnetic field of a chain of connected mesoscopic
rings is considered. We show that the low frequency behavior corresponds to
localization of the elctrons along the chain and to diamagnetic dynamic
currents inside rings. The magnetization density due to these currents does not
vanish even in the limit of the infinitely long chain of strongly connected
rings. showing that this is a macroscopic effect. Being of a dynamic origin the
currents can be destroyed by inelastic scattering, but we argue that the
corresponding decay time can at low enough temperatures be large compared to
all other time scales of the system. We suggest to check our results by
measuring the magnetization of an ensemble of chains of connected rings in the
diffusive regime or alternatively of linear antidot lattices where the mean
free path due to elastic scattering is smaller than the superlattice spacing.
This yields a new contactless method of studying the localization.",9409026v1
1997-02-12,Suppression of chaotic dynamics and localization of two-dimensional electrons by a weak magnetic field,"We study a two-dimensional motion of a charged particle in a weak random
potential and a perpendicular magnetic field. The correlation length of the
potential is assumed to be much larger than the de Broglie wavelength. Under
such conditions, the motion on not too large length scales is described by
classical equations of motion. We show that the phase-space averaged diffusion
coefficient is given by Drude-Lorentz formula only at magnetic fields $B$
smaller than certain value $B_c$. At larger fields, the chaotic motion is
suppressed and the diffusion coefficient becomes exponentially small. In
addition, we calculate the quantum-mechanical localization length as a function
of $B$ in the minima of $\sigma_{xx}$. At $B < B_c$ it is exponentially large
but decreases with increasing $B$. At $B > B_c$, the localization length drops
precipitously, and ceases to be exponentially large at a field $B_\ast$, which
is only slightly above $B_c$. Implications for the crossover from the
Shubnikov-de Haas oscillations to the quantum Hall effect are discussed.",9702121v1
1998-02-05,Coarsening on percolation clusters: out-of-equilibrium dynamics versus non linear response,"We analyze the violations of linear fluctuation-dissipation theorem (FDT) in
the coarsening dynamics of the antiferromagnetic Ising model on percolation
clusters in two dimensions. The equilibrium magnetic response is shown to be
non linear for magnetic fields of the order of the inverse square root of the
number of sites. Two extreme regimes can be identified in the thermoremanent
magnetization: (i) linear response and out-of-equilibrium relaxation for small
waiting times (ii) non linear response and equilibrium relaxation for large
waiting times. The function $X(C)$ characterizing the deviations from linear
FDT cross-overs from unity at short times to a finite positive value for longer
times, with the same qualitative behavior whatever the waiting time. We show
that the coarsening dynamics on percolation clusters exhibits stronger
long-term memory than usual euclidian coarsening.",9802062v1
1999-11-16,Dynamic Radio-Frequency Transverse Susceptibility in Magnetic Nanoparticle Systems,"A novel resonant method based on a tunnel-diode oscillator (TDO) is used to
study the dynamic transverse susceptibility in a Fe nanoparticle system. The
magnetic system consists of an aggregate of nanometer-size core (Au)-shell (Fe)
structure, synthesized by reverse micelle methods. Static and dynamic
magnetization measurements carried out in order to characterize the system
reveal a superparamagnetic behavior at high temperature. The field-dependent
transverse susceptibility at radio-frequencies (RF), for different temperatures
reveals distinct peak structure at characteristics fields (H_k, H_c) which
changes with temperature. It is proposed that relaxation processes could
explain the influence of the temperature on the field dependence of the
transverse susceptibility on the MI.",9911242v1
2000-03-17,Mean field and Monte Carlo studies of the magnetization-reversal transition in the Ising model,"Detailed mean field and Monte Carlo studies of the dynamic
magnetization-reversal transition in the Ising model in its ordered phase under
a competing external magnetic field of finite duration have been presented
here. Approximate analytical treatment of the mean field equations of motion
shows the existence of diverging length and time scales across this dynamic
transition phase boundary. These are also supported by numerical solutions of
the complete mean field equations of motion and the Monte Carlo study of the
system evolving under Glauber dynamics in both two and three dimensions.
Classical nucleation theory predicts different mechanisms of domain growth in
two regimes marked by the strength of the external field, and the nature of the
Monte Carlo phase boundary can be comprehended satisfactorily using the theory.
The order of the transition changes from a continuous to a discontinuous one as
one crosses over from coalescence regime (stronger field) to nucleation regime
(weaker field). Finite size scaling theory can be applied in the coalescence
regime, where the best fit estimates of the critical exponents are obtained for
two and three dimensions.",0003294v2
2001-09-11,Spin dynamics and coherent tunnelling in the molecular magnetic rings Fe_6 and Fe_8,"We present detailed calculations of low-energy spin dynamics in the ``ferric
wheel'' systems Na:Fe_6 and Cs:Fe_8 in a magnetic field. We compute by exact
diagonalisation the low-energy spectra and matrix elements for total-spin and
N'eel-vector components, and thus the time-dependent correlation functions of
these operators. Comparison of our results with the semiclassical theory of
coherent quantum tunnelling of the N'eel vector demonstrates the validity of a
two-state description for the low-energy dynamics of ferric wheels. We discuss
the implications of our results for mesoscopic quantum coherent phenomena, and
for the experimental techniques to observe them, in molecular magnetic rings.",0109201v2
2002-04-29,Dynamic transitions and resonances in Josephson-junction arrays under oscillating magnetic fields,"We investigate dynamic transitions and stochastic resonance phenomena in
two-dimensional fully frustrated Josephson-junction arrays driven by staggered
oscillating magnetic fields. As the temperature is lowered, the dynamic order
parameter, defined to be the average staggered magnetization, is observed to
acquire nonzero values. The resulting transition is found to belong to the same
universality as the equilibrium Z_2 transition for small driving amplitudes
while large driving fields appear to induce deviation from the universality
class. The transition is also manifested by the stochastic resonance peak of
the signal-to-noise ratio, which develops above the transition temperature.",0204598v1
2002-07-24,Vortex dynamics and pinning properties analysis of MgB2 bulk samples by ac susceptibility measurements,"The flux lines dynamics have been investigated on MgB2 bulk superconductors
obtained by reactive liquid infiltration by measuring the ac magnetic
susceptibility. The fundamental and third harmonics have been studied as a
function of temperature, dc magnetic field, ac field amplitude and frequency.
In order to determine the dynamical regimes governing the vortex motion, the
experimental results have been compared with susceptibility curves obtained by
numerical calculations of the non-linear diffusion equation for the magnetic
field. The frequency behaviour of the third harmonic response, that cannot be
explained by frequency dependent critical state models, has been related to the
current dependence of the flux creep activation energy U(J) in the diffusion
coefficient. In this way we have shown that the measured curves are correctly
interpreted within the framework of a vortex glass description.",0207580v1
2003-03-20,"Towards a Fully Ab-Initio Description of the Diluted Magnetic Semiconductor $Ga_{1-x}Mn_{x}As$. Ferromagnetism, Electronic Structure, and Optical Response","There are two competing views of itinerant ferromagnetism, the first viewing
ferromagnetism as resulting from the indirect coupling between local moments
via the itinerant carrier dynamics, the so-called RKKY mechanism, while in the
alternative picture, ferromagnetism results from the spin polarization of
itinerant carriers by the strong atomic Hund interaction - the so-called double
exchange (DE) scenario. Which view describes the ferromagnetism in diluted
magnetic semiconductors, materials with promise for spintronic applications, is
still unclear. Here, we describe the detailed physical response of the
prototype material $Ga_{1-x}Mn_{x}As$ using a combination of first-principles
bandstructure with methods based on dynamical mean field theory to incorporate
strong, dynamical correlations {\it and} intrinsic as well as extrinsic
disorder in one single theoretical picture. We show how ferromagnetism is
driven by DE, in agreement with very recent observations, along with a good
quantitative description of the details of the electronic structure, as probed
by scanning tunnelling microscopy (STM) and optical conductivity. Our results
show how ferromagnetism can be driven by DE even in diluted magnetic
semiconductors with small carrier concentration.",0303427v1
2005-03-09,Magnetic properties of a Kondo insulator with RKKY interaction: Extended dynamical mean field study,"We study the Kondo lattice model with the Heisenberg-type RKKY-exchange
coupling among localized f-spins in the presence of a magnetic field. By means
of an extended dynamical mean field theory combined with the non-crossing
approximation, we investigate the one-particle spectral function and the
dynamical spin correlation function in the Kondo insulating phase. It is shown
that the magnetic field and the RKKY exchange interaction both cause the
instability to the antiferromagnetic order with enhanced transverse spin
fluctuations, which give rise to a strong renormalization of quasi-particles as
the system approaches the quantum critical point. This leads to a tendency to
retain the Kondo insulating gap up to rather large fields.",0503228v1
2005-12-29,Current-induced magnetization dynamics in disordered itinerant ferromagnets,"Current-driven magnetization dynamics in ferromagnetic metals are studied in
a self-consistent adiabatic local-density approximation in the presence of
spin-conserving and spin-dephasing impurity scattering. Based on a quantum
kinetic equation, we derive Gilbert damping and spin-transfer torques entering
the Landau-Lifshitz equation to linear order in frequency and wave vector.
Gilbert damping and a current-driven dissipative torque scale identically and
compete, with the result that a steady current-driven domain-wall motion is
insensitive to spin dephasing in the limit of weak ferromagnetism. A uniform
magnetization is found to be much more stable against spin torques in the
itinerant than in the \textit{s}-\textit{d} model for ferromagnetism. A dynamic
spin-transfer torque reminiscent of the spin pumping in multilayers is
identified and shown to govern the current-induced domain-wall distortion.",0512715v4
2006-02-23,Dynamical magneto-electric coupling in helical magnets,"Collective mode dynamics of the helical magnets coupled to electric
polarization via spin-orbit interaction is studied theoretically. The soft
modes associated with the ferroelectricity are not the transverse optical
phonons, as expected from the Lyddane-Sachs-Teller relation, but are the spin
waves hybridized with the electric polarization. This leads to the Drude-like
dielectric function $\epsilon(\omega)$ in the limit of zero magnetic
anisotropy. There are two more low-lying modes; phason of the spiral and
rotation of helical plane along the polarization axis. The roles of these soft
modes in the neutron scattering and antiferromagnetic resonance are revealed,
and a novel experiment to detect the dynamical magneto-electric coupling is
proposed.",0602547v1
2006-05-17,Short-time Dynamics of Percolation Observables,"We consider the critical short-time evolution of magnetic and
droplet-percolation order parameters for the Ising model in two and three
dimensions, through Monte-Carlo simulations with the (local) heat-bath method.
We find qualitatively different dynamic behaviors for the two types of order
parameters. More precisely, we find that the percolation order parameter does
not have a power-law behavior as encountered for the magnetization, but
develops a scale (related to the relaxation time to equilibrium) in the
Monte-Carlo time. We argue that this difference is due to the difficulty in
forming large clusters at the early stages of the evolution. Our results show
that, although the descriptions in terms of magnetic and percolation order
parameters may be equivalent in the equilibrium regime, greater care must be
taken to interprete percolation observables at short times. In particular, this
concerns the attempts to describe the dynamics of the deconfinement phase
transition in QCD using cluster observables.",0605438v1
2006-09-26,Spin Dynamics in a Tunnel Junction between Ferromagnets,"The dynamics of a single spin embedded in the tunnel junction (quantum point
contact) between ferromagnets is addressed. Using the Keldysh technique, we
derive a quantum Langevin equation. As a consequence of the spin-polarization
in the leads, the spin displays a rich and unusual dynamics. Parallel
configured and equally strong magnetic moments in the leads yield an ordinary
spin precession with a Larmor frequency given by the effective magnetic field.
Unequal and/or non-parallel configured magnetization, however, causes nutation
of the spin in addition to the precession. Our predictions may be directly
tested for macroscopic spin clusters.",0609673v3
2000-07-07,Dynamical Systems Approach to Magnetised Cosmological Perturbations,"Assuming a large-scale homogeneous magnetic field, we follow the covariant
and gauge-invariant approach used by Tsagas and Barrow to describe the
evolution of density and magnetic field inhomogeneities and curvature
perturbations in a matter-radiation universe. We use a two parameter
approximation scheme to linearize their exact non-linear general-relativistic
equations for magneto-hydrodynamic evolution. Using a two-fluid approach we set
up the governing equations as a fourth order autonomous dynamical system.
Analysis of the equilibrium points for the radiation dominated era lead to
solutions similar to the super-horizon modes found analytically by Tsagas and
Maartens. We find that a study of the dynamical system in the dust-dominated
era leads naturally to a magnetic critical length scale closely related to the
Jeans Length. Depending on the size of wavelengths relative to this scale,
these solutions show three distinct behaviours: large-scale stable growing
modes, intermediate decaying modes, and small-scale damped oscillatory
solutions.",0007010v2
1994-05-10,Catalysis of Dynamical Flavor Symmetry Breaking by a Magnetic Field in $2+1$ Dimensions,"It is shown that in $2+1$ dimensions, a constant magnetic field is a strong
catalyst of dynamical flavor symmetry breaking, leading to generating a fermion
dynamical mass even at the weakest attractive interaction between fermions. The
effect is illustrated in the Nambu-Jona-Lasinio model in a magnetic field. The
low-energy effective action in this model is derived and the thermodynamic
properties of the model are established. The relevance of this effect for
planar condensed matter systems is pointed out.",9405262v2
2005-10-06,Gauge independent approach to chiral symmetry breaking in a strong magnetic field,"The gauge independence of the dynamical fermion mass generated through chiral
symmetry breaking in QED in a strong, constant external magnetic field is
critically examined. We show that the bare vertex approximation, in which the
vertex corrections are ignored, is a consistent truncation of the
Schwinger-Dyson equations in the lowest Landau level approximation. The
dynamical fermion mass, obtained as the solution of the truncated
Schwinger-Dyson equations evaluated on the fermion mass shell, is shown to be
manifestly gauge independent. By establishing a direct correspondence between
the truncated Schwinger-Dyson equations and the 2PI (two-particle-irreducible)
effective action truncated at the lowest nontrivial order in the loop expansion
as well as in the 1/N_f expansion (N_f is the number of fermion flavors), we
argue that in a strong magnetic field the dynamical fermion mass can be
reliably calculated in the bare vertex approximation.",0510066v3
2006-04-11,Effect of Electric and Magnetic Fields on Spin Dynamics in the Resonant Electric Dipole Moment Experiment,"A buildup of the vertical polarization in the resonant electric dipole moment
(EDM) experiment [Y. F. Orlov, W. M. Morse, and Y. K. Semertzidis, Phys. Rev.
Lett. 96, 214802 (2006)] is affected by a horizontal electric field in the
particle rest frame oscillating at a resonant frequency. This field is defined
by the Lorentz transformation of an oscillating longitudinal electric field and
a uniform vertical magnetic one. The effect of a longitudinal electric field is
significant, while the contribution from a magnetic field caused by forced
coherent longitudinal oscillations of particles is dominant. The effect of
electric field on the spin dynamics was not taken into account in previous
calculations. This effect is considerable and leads to decreasing the EDM
effect for the deuteron and increasing it for the proton. The formula for
resonance strengths in the EDM experiment has been derived. The spin dynamics
has been calculated.",0604095v2
2001-10-27,Quantum Field Dynamics in a Uniform Magnetic Field: Description using Fields in Oblique Phase Space,"We present a simple field transformation which changes the field arguments
from the ordinary position-space coordinates to the oblique phase-space
coordinates that are linear in position and momentum variables. This is useful
in studying quantum field dynamics in the presence of external uniform magnetic
field: here, the field transformation serves to separate the dynamics within
the given Landau level from that between different Landau levels. We apply this
formalism to both nonrelativistic and relativistic field theories. In the large
external magnetic field our formalism provides an efficient method for
constructing the relevant lower-dimensional effective field theories with the
field degrees defined only on the lowest Landau level.",0110249v1
2003-08-07,Non-integrability of the problem of a rigid satellite in gravitational and magnetic fields,"In this paper we analyse the integrability of a dynamical system describing
the rotational motion of a rigid satellite under the influence of gravitational
and magnetic fields. In our investigations we apply an extension of the Ziglin
theory developed by Morales-Ruiz and Ramis. We prove that for a symmetric
satellite the system does not admit an additional real meromorphic first
integral except for one case when the value of the induced magnetic moment
along the symmetry axis is related to the principal moments of inertia in a
special way.",0308010v2
1997-06-01,How to Probe for Dynamical Structure in the Collapse of Entangled States Using Nuclear Magnetic Resonance,"The spin state of two magnetically inequivalent protons in contiguous atoms
of a molecule becomes entangeled by the indirect spin-spin interaction
(j-coupling). The degree of entanglement oscillates at the beat frequency
resulting from the splitting of a degeneracy. This beating is manifest in NMR
spectroscopy as an envelope of the transverse magnetization and should be
visible in the free induction decay signal. The period (approximately 1 sec) is
long enough for interference between the linear dynamics and collapse of the
wave-function induced by a Stern-Gerlach inhomogeneity to significantly alter
the shape of that envelope. Various dynamical collapse theories can be
distinguished by their observably different predictions with respect to this
alteration. Adverse effects of detuning due to the Stern-Gerlach inhomogeneity
can be reduced to an acceptable level by having a sufficiently thin sample or a
strong rf field.",9706002v2
2005-03-15,Direct versus measurement assisted bipartite entanglement in multi-qubit systems and their dynamical generation in spin systems,"We consider multi-qubit systems and relate quantitatively the problems of
generating cluster states with high value of concurrence of assistance, and
that of generating states with maximal bipartite entanglement. We prove an
upper bound for the concurrence of assistance. We consider dynamics of spin-1/2
systems that model qubits, with different couplings and possible presence of
magnetic field to investigate the appearance of the discussed entanglement
properties. We find that states with maximal bipartite entanglement can be
generated by an XY Hamiltonian, and their generation can be controlled by the
initial state of one of the spins. The same Hamiltonian is capable of creating
states with high concurrence of assistance with suitably chosen initial state.
We show that the production of graph states using the Ising Hamiltonian is
controllable via a single-qubit rotation of one spin-1/2 subsystem in the
initial multi-qubit state. We shown that the property of Ising dynamics to
convert a product state basis into a special maximally entangled basis is
temporally enhanced by the application of a suitable magnetic field. Similar
basis transformations are found to be feasible in the case of isotropic XY
couplings with magnetic field.",0503133v1
2007-04-19,Magnetic induction in a turbulent flow of liquid sodium: mean behaviour and slow fluctuations,"We study the flow response to an externally imposed homogeneous magnetic
field in a turbulent swirling flow of liquid sodium -- the VKS2 experiment in
which magnetic Reynolds numbers Rm up to 50 are reached. Induction effects are
larger than in the former VKS1 experiment. At Rm larger than about 25, the
local amplitude of induced field components supersedes that of the applied
field, and exhibits non-Gaussian fluctuations. Slow dynamical instationarities
and low-frequency bimodal dynamics are observed in the induction, presumably
tracing back to large scale fluctuations in the hydrodynamic flow.",0704.2565v1
2007-11-02,Dynamical feedback of the curvature drift instability on its saturation process,"We investigate the reconstruction of pulsar magnetospheres close to the light
cylinder surface to study the curvature drift instability (CDI) responsible for
the twisting of magnetic field lines in the mentioned zone. The influence of
plasma dynamics on the saturation process of the CDI is studied. On the basis
of the Euler, continuity, and induction equations, we derive the increment of
the CDI and analyze parametrically excited drift modes. The dynamics of the
reconstruction of the pulsar magnetosphere is studied analytically. We show
that there is a possibility of a parametrically excited rotational-energy
pumping-process in the drift modes. It is indicated by the generation of a
toroidal component of the magnetic field that transforms the field lines into
such a configuration, in which plasma particles do not experience any forces.
At this stage, the instability process saturates and the further amplification
of the toroidal component to the magnetic field lines is suspended.",0711.0295v8
2008-03-20,Hidden magnetic frustration by quantum relaxation in anisotropic Nd-langasite,"The static and dynamic magnetic properties of the Nd$_3$Ga$_5$SiO$_{14}$
compound, which appears as the first materialization of a rare-earth
kagome-type lattice, were re-examined, owing to contradictory results in the
previous studies. Neutron scattering, magnetization and specific heat
measurements were performed and analyzed, in particular by fully taking account
of the crystal electric field effects on the Nd$^{3+}$ ions. One of the novel
findings is that the peculiar temperature independent spin dynamics observed
below 10 K expresses single-ion quantum processes. This would short-circuit the
frustration induced cooperative dynamics, which would emerge only at very low
temperature.",0803.3056v2
2008-11-03,Collective spin dynamics in magnetic nanomaterials,"Magnetic nanomaterials are considered, formed by magnetic nanomolecules with
high spins. The problem of spin reversal in these materials is analyzed, which
is of interest for the possible use of such materials for quantum information
processing and quantum computing. The fastest spin reversal can be achieved by
coupling the spin sample to a resonant electric circuit and by an appropriate
choice of the system parameters. A principal point is to choose these
parameters so that to organize coherent spin motion. Dynamics of collective
motion is modelled by computer simulations, which confirm the high level of
dynamical coherence of molecular spins in the process of spin reversal.",0811.0267v1
2009-04-23,Spin dynamics in rare earth single molecule magnets from muSR and NMR in [TbPc$_{2}$]$^{0}$ and [DyPc$_{2}$]$^{0}$,"The spin dynamics in [TbPc$_{2}$]$^{0}$ and [DyPc$_{2}$]$^{0}$ single
molecule magnets have been investigated by means of muon and nuclear
spin-lattice relaxation rate measurements. The correlation time for the spin
fluctuations was found to be close to 0.1 ms already at 50 K, about two orders
of magnitude larger than the one previously found in other lanthanide based
single molecule magnets. In [TbPc$_{2}$]$^{0}$ two different regimes for the
spin fluctuations have been evidenced: a high temperature activated one
involving spin fluctuations across a barrier $\Delta\simeq 880 K$ separating
the ground and first excited states and a low temperature regime involving
quantum fluctuations within the twofold degenerate ground-state. In
[DyPc$_{2}$]$^{0}$ a high temperature activated spin dynamics is also evidenced
which, however, cannot be explained in terms of a single spin-phonon coupling
constant.",0904.3619v1
2009-06-19,Lattice dynamics in magnetic superelastic Ni-Mn-In alloys. Neutron scattering and ultrasonic experiments,"Neutron scattering and ultrasonic methods have been used to study the lattice
dynamics of two single crystals of Ni-Mn-In Heusler alloys close to
Ni$_{50}$Mn$_{34}$In$_{16}$ magnetic superelastic composition. The paper
reports the experimental determination of the low-lying phonon dispersion
curves and the elastic constants for this alloy system. We found that the
frequencies of the TA$_{2}$ branch are relatively low and it exhibits a small
dip anomaly at a wave number $\xi_{0} \approx 1/3$, which softens with
decreasing temperature. Associated with the softening of this phonon, we also
observed the softening of the shear elastic constant $C'=(C_{11}-C_{12})/2$.
Both temperature softenings are typical for bcc based solids which undergo
martensitic transformations and reflect the dynamical instability of the cubic
lattice against shearing of $\{110\}$ planes along $<1\bar{1}0>$ directions.
Additionally, we measured low-lying phonon dispersion branches and elastic
constants in applied magnetic fields aimed to characterize the magnetoelastic
coupling.",0906.3622v1
2009-12-24,Current driven magnetization dynamics in ferromagnetic nanowires with Dzyaloshinskii-Moriya interaction,"We study current induced magnetization dynamics in a long thin ferromagnetic
wire with Dzyaloshinskii-Moriya interaction (DMI). We find a spiral domain wall
configuration of the magnetization and obtain an analytical expression for the
width of the domain wall as a function of the interaction strengths. Our
findings show that above a certain value of DMI a domain wall configuration
cannot exist in the wire. Below this value we determine the domain wall
dynamics for small currents, and calculate the drift velocity of the domain
wall along the wire. We show that the DMI suppresses the minimum value of
current required to move the domain wall. Depending on its sign, the DMI
increases or decreases the domain wall drift velocity.",0912.4732v1
2010-01-22,Dynamics of domain growth driven by dipolar interactions in a perpendicularly magnetized ultrathin film,"Measurements of the ac magnetic susceptibility of perpendicularly magnetized
Fe/2ML Ni/W(110) ultrathin films show a clear signature of the dynamics of
domain growth and domain density changes in the striped domain pattern that
this system supports. The susceptibility peak measured at different constant
heating rates in the range 0.20 K/s < R < 0.70 K/s shifts to higher temperature
as the heating rate is increased. Analysis using a relaxation model
demonstrates quantitatively that the dynamics is driven by a non-equilibrium
domain density at (nearly) zero field (i.e. by dipole interactions), and that
the temperature shift is due to a response time determined by the pinning of
local domain wall segments by structural defects. The fundamental time scale
for relaxation of the domain density driven by dipole interactions is of order
10^5 times slower than the fundamental time scale for an individual Barkhausen
step driven by an applied field. The increase in the fundamental time scale
reflects the relative size of dipole and Zeeman energies, and the need for the
correlated motion of many local domain wall segments to affect domain growth.",1001.4069v1
2010-02-27,Fast magnetic reconnection in three dimensional MHD simulations,"We present a constructive numerical example of fast magnetic reconnection in
a three dimensional periodic box. Reconnection is initiated by a strong,
localized perturbation to the field lines. The solution is intrinsically three
dimensional, and its gross properties do not depend on the details of the
simulations. $\sim 50%$ of the magnetic energy is released in an event which
lasts about one Alfven time, but only after a delay during which the field
lines evolve into a critical configuration. We present a physical picture of
the process. The reconnection regions are dynamical and mutually interacting.
In the comoving frame of these regions, reconnection occurs through an X-like
point, analogous to Petschek reconnection. The dynamics appear to be driven by
global flows, not local processes.",1003.0105v1
2010-03-23,Low frequency spin dynamics in the quantum magnet copper pyrazine dinitrate,"The S=1/2 antiferromagnetic Heisenberg chain exhibits a magnetic field driven
quantum critical point. We study the low frequency spin dynamics in copper
pyrazine dinitrate (CuPzN), a realization of this model system of quantum
magnetism, by means of $^{13}$C-NMR spectroscopy. Measurements of the nuclear
spin-lattice relaxation rate $T_1^{-1}$ in the vicinity of the saturation field
are compared with quantum Monte Carlo calculations of the dynamic structure
factor. Both show a strong divergence of low energy excitations at temperatures
in the quantum regime. The analysis of the anisotropic $T_1^{-1}$-rates and
frequency shifts allows one to disentangle the contributions from transverse
and longitudinal spin fluctuations for a selective study and to determine the
transfer of delocalized spin moments from copper to the neighboring nitrogen
atoms.",1003.4535v1
2010-04-28,Dynamics of atoms in a time-orbiting-potential trap: consequences of the classical description,"The classical model that describes the motion of an atom in a magnetic trap
is solved in order to investigate the relationship between the failure of the
usual adiabatic approximation assumption and the physical parameters of the
trap. This allows to evaluate the effect that reversing of the bias field
rotation produces on the vertical position of the atomic orbit, a displacement
that is closely related to the adiabatic character of the trap motion. The
present investigation has been motivated by a similar experimental test
previously carried out in the actual magnetic time orbiting potential trap. We
find that the non-adiabatic effects provided by the classical model are
extremely small. Thus, we conclude that the theoretical explanation of the
experimental measures, requires a quantum description of the dynamics in
magnetic traps.",1004.4978v1
2010-05-31,Can catastrophic quenching be alleviated by separating shear and alpha effect?,"The small-scale magnetic helicity produced as a by-product of the large-scale
dynamo is believed to play a major role in dynamo saturation. In a mean-field
model the generation of small-scale magnetic helicity can be modelled by using
the dynamical quenching formalism. Catastrophic quenching refers to a decrease
of the saturation field strength with increasing Reynolds number. It has been
suggested that catastrophic quenching only affects the region of non-zero
helical turbulence (i.e. where the kinematic alpha operates) and that it is
possible to alleviate catastrophic quenching by separating the region of strong
shear from the alpha layer. We perform a systematic study of a simple
axisymmetric two-layer alpha-omega dynamo in a spherical shell for Reynolds
numbers in the range 1 < Rm < 10^5. In the framework of dynamical quenching we
show that this may not be the case, suggesting that magnetic helicity fluxes
would be necessary.",1005.5708v1
2010-09-30,Magnetic Cycles and Meridional Circulation in Global Models of Solar Convection,"We review recent insights into the dynamics of the solar convection zone
obtained from global numerical simulations, focusing on two recent developments
in particular. The first is quasi-cyclic magnetic activity in a long-duration
dynamo simulation. Although mean fields comprise only a few percent of the
total magnetic energy they exhibit remarkable order, with multiple polarity
reversals and systematic variability on time scales of 6-15 years. The second
development concerns the maintenance of the meridional circulation. Recent
high-resolution simulations have captured the subtle nonlinear dynamical
balances with more fidelity than previous, more laminar models, yielding more
coherent circulation patterns. These patterns are dominated by a single cell in
each hemisphere, with poleward and equatorward flow in the upper and lower
convection zone respectively. We briefly address the implications of and future
of these modeling efforts.",1009.6184v2
2010-10-18,Chemical spots and their dynamical evolution on HgMn stars,"Our recent studies of late B-type stars with HgMn peculiarity revealed for
the first time the presence of fast dynamical evolution of chemical spots on
their surfaces. These observations suggest a hitherto unknown physical process
operating in the stars with radiative outer envelopes. Furthermore, we have
also discovered existence of magnetic fields on these stars that have up to now
been thought to be non-magnetic. Here we will discuss the dynamical spot
evolution on HD 11753 and our new results on magnetic fields on AR Aur.",1010.3509v1
2011-05-24,Model of bound interface dynamics for coupled magnetic domain walls,"A domain wall in a ferromagnetic system will move under the action of an
external magnetic field. Ultrathin Co layers sandwiched between Pt have been
shown to be a suitable experimental realization of a weakly disordered 2D
medium in which to study the dynamics of 1D interfaces (magnetic domain walls).
The behavior of these systems is encapsulated in the velocity-field response
v(H) of the domain walls. In a recent paper [P.J. Metaxas et al., Phys. Rev.
Lett. 104, 237206 (2010)] we studied the effect of ferromagnetic coupling
between two such ultrathin layers, each exhibiting different v(H)
characteristics. The main result was the existence of bound states over
finite-width field ranges, wherein walls in the two layers moved together at
the same speed. Here, we discuss in detail the theory of domain wall dynamics
in coupled systems. In particular, we show that a bound creep state is expected
for vanishing H and we give the analytical, parameter free expression for its
velocity which agrees well with experimental results.",1105.4728v1
2011-06-19,Anomalous slow dynamics in the metallic helimagnet Gd_{1-x}Y_{x},"Ac-suseptibility measurements were performed in the metallic helimagnet
Gd_{1-x}Y_{x} alloy. A remarkable increase of the imaginary part of the
ac-suseptibility was observed in the temperature range of the helimagnetic
phase. Moreover, a strong nonlinearity of the magnetization was observed at
paramagnetic-helimagnetic transition temperature T_{N}. On the other hand,
These anomalous behavior were not observed in similar rare-earth helimagnets Ho
and Ho_{1-x}Y_{x}. It strongly suggests that the weak magnetic anisotropy of
the Gd-moments is responsible for the anomalous slow dynamics in the
helimagnetic phase and for the strong nonlinearity of the magnetization at
T_{N}. The slow dynamics may result from a chiral-domain motion, or a long-time
variation of the period of the helimagnetic structure.",1106.3730v1
2011-08-24,Microwave state transfer and adiabatic dynamics of magnetically trapped polar molecules,"Cold and ultracold polar molecules with nonzero electronic angular momentum
are of great interest for studies in quantum chemistry and control,
investigations of novel quantum systems, and precision measurement. However, in
mixed electric and magnetic fields, these molecules are generically subject to
a large set of avoided crossings among their Zeeman sublevels; in magnetic
traps, these crossings lead to distorted potentials and trap loss from electric
bias fields. We have characterized these crossings in OH by
microwave-transferring trapped OH molecules from the upper |f; M = +3/2> parity
state to the lower |e; +3/2> state and observing their trap dynamics under an
applied electric bias field. Our observations are very well described by a
simple Landau-Zener model, yielding insight to the rich spectra and dynamics of
polar radicals in mixed external fields.",1108.4871v1
2011-10-06,Equation of state of paramagnetic CrN from ab initio molecular dynamics,"Equation of state for chromium nitride has been debated in the literature in
connection with a proposed collapse of its bulk modulus following the pressure
induced transition from the paramagnetic cubic phase to the antiferromagnetic
orthorhombic phase [F. Rivadulla et al., Nat Mater 8, 974 (2009); B. Alling et
al., Nat Mater 9, 283 (2010)]. Experimentally the measurements are complicated
due to the low transition pressure, while theoretically the simulation of
magnetic disorder represent a major challenge. Here a first-principles method
is suggested for the calculation of thermodynamic properties of magnetic
materials in their high temperature paramagnetic phase. It is based on
ab-initio molecular dynamics and simultaneous redistributions of the disordered
but finite local magnetic moments. We apply this disordered local moments
molecular dynamics method to the case of CrN and simulate its equation of
state. In particular the debated bulk modulus is calculated in the paramagnetic
cubic phase and is shown to be very similar to that of the antiferromagnetic
orthorhombic CrN phase for all considered temperatures.",1110.1331v1
2011-11-09,A dynamical phase transition in ferromagnetic granular materials,"We study, using simulations the dynamical properties of complex ferromagnetic
granular materials. The system of grains is modeled by a disordered
two-dimensional lattice in which the grains are embedded, while the magnitude
and direction of the easy axis are random. Using the monte-carlo method we
track the dynamics of the magnetic moments of the grains. We observe a
transition of the system from a macroscopic blocked (ferromagnetic) phase at
low temperature in which the grain's magnetic moment do not flip to the other
direction to an unblocked (superparamagnetic) phase at high temperature in
which the magnetic moment is free to rotate. Our results suggest that this
transition exhibits the characteristics of a second order phase transition such
as the appearance of a giant cluster of unblocked grains which is fractal at
the critical temperature, a peak in the size of the second largest cluster at
the same temperature and a power law distribution of cluster sizes near the
criticality.",1111.2130v1
2011-11-20,Detection of picosecond magnetization dynamics of 50 nm magnetic dots down to the single dot regime,"We report an all-optical time-domain detection of picosecond magnetization
dynamics of arrays of 50 nm Ni80Fe20 (permalloy) dots down to the single
nanodot regime. In the single nanodot regime the dynamics reveals one dominant
resonant mode corresponding to the edge mode of the 50 nm dot with slightly
higher damping than that of the unpatterned thin film. With the increase in
areal density of the array both the precession frequency and damping increases
significantly due to the increase in magnetostatic interactions between the
nanodots and a mode splitting and sudden jump in apparent damping are observed
at an edge-to-edge separation of 50 nm.",1111.4625v1
2011-11-25,On the dynamics of vortex modes within magnetic islands,"Recent work investigating the interaction of magnetic islands with
micro-turbulence has uncovered the striking observation of large scale vortex
modes forming within the island structure [W.A. Hornsby {\it et al.}, Phys.
Plasmas {\bf 17} 092301 (2010)]. These electrostatic vortices are found to be
the size of the island and are oscillatory. It is this oscillatory behaviour
and the presence of turbulence that leads us to believe that the dynamics are
related to the Geodesic Acoustic Mode (GAM), and it is this link that is
investigated in this paper.
  Here we derive an equation for the GAM in the MHD limit, in the presence of a
magnetic island modified three-dimensional axisymmetric geometry. The
eigenvalues and eigenfunctions are calculated numerically and then utilised to
analyse the dynamics of oscillatory large-scale electrostatic potential
structures seen in both linear and non-linear gyro-kinetic simulations.",1111.5981v2
2011-12-05,Vortex core magnetization dynamics induced by thermal excitation,"We investigate the effect of temperature on the dynamic properties of
magnetic vortices in small disks. Our calculations use a stochastic version of
the Landau-Lifshitz-Gilbert (LLG) equation, valid for finite temperatures well
below the Curie critical temperature. We show that a finite temperature induces
a vortex precession around the center of the disk, even in the absence of other
excitation sources. We discuss the origin and implications of the appearance of
this new dynamics. We also show that a temperature gradient plays a role
similar to that of a small constant magnetic field.",1112.0911v2
2012-03-23,"Scaling, Finite Size Effects, and Crossovers of the Resistivity and Current-Voltage Characteristics in Two-Dimensional Superconductors","We revisit the scaling properties of the resistivity and the current-voltage
characteristics at and below the Berezinskii-Kosterlitz-Thouless transition,
both in zero and nonzero magnetic field. The scaling properties are derived by
integrating the renormalization group flow equations up to a scale where they
can be reliably matched to simple analytic expressions. The vortex fugacity
turns out to be dangerously irrelevant for these quantities below Tc, thereby
altering the scaling behavior. We derive the possible crossover effects as the
current, magnetic field, or system size is varied, and find a strong
multiplicative logarithmic correction near Tc, all of which is necessary to
account for when interpreting experiments and simulation data. Our analysis
clarifies a longstanding discrepancy between the finite size dependence found
in many simulations and the current-voltage characteristics of experiments. We
further show that the logarithmic correction can be avoided by approaching the
transition in a magnetic field, thereby simplifying the scaling analysis. We
confirm our results by large-scale numerical simulations, and calculate the
dynamic critical exponent z, for relaxational Langevin dynamics and for
resistively and capacitively shunted Josephson junction dynamics.",1203.5317v2
2012-05-09,"Spinor Bose gases: Explorations of symmetries, magnetism and quantum dynamics","Spinor Bose gases form a family of quantum fluids manifesting both magnetic
order and superfluidity. This article reviews experimental and theoretical
progress in understanding the static and dynamic properties of these fluids.
The connection between system properties and the rotational symmetry properties
of the atomic states and their interactions are investigated. Following a
review of the experimental techniques used for characterizing spinor gases,
their mean-field and many-body ground states, both in isolation and under the
application of symmetry-breaking external fields, are discussed. These states
serve as the starting point for understanding low-energy dynamics, spin
textures and topological defects, effects of magnetic dipole interactions, and
various non-equilibrium collective spin-mixing phenomena. The paper aims to
form connections and establish coherence among the vast range of works on
spinor Bose gases, so as to point to open questions and future research
opportunities.",1205.1888v2
2012-08-28,A Lagrangian kinetic model for collisionless magnetic reconnection,"A new fully kinetic system is proposed for modeling collisionless magnetic
reconnection. The formulation relies on fundamental principles in Lagrangian
dynamics, in which the inertia of the electron mean flow is neglected in the
expression of the Lagrangian, rather then enforcing a zero electron mass in the
equations of motion. This is done upon splitting the electron velocity into its
mean and fluctuating parts, so that the latter naturally produce the
corresponding pressure tensor. The model exhibits a new Coriolis force term,
which emerges from a change of frame in the electron dynamics. Then, if the
electron heat flux is neglected, the strong electron magnetization limit yields
a hybrid model, in which the electron pressure tensor is frozen into the
electron mean velocity.",1208.5674v2
2012-09-24,The effect of disorder on transverse domain wall dynamics in magnetic nanostrips,"We study the effect of disorder on the dynamics of a transverse domain wall
in ferromagnetic nanostrips, driven either by magnetic fields or spin-polarized
currents, by performing a large ensemble of GPU-accelerated micromagnetic
simulations. Disorder is modeled by including small, randomly distributed
non-magnetic voids in the system. Studying the domain wall velocity as a
function of the applied field and current density reveals fundamental
differences in the domain wall dynamics induced by these two modes of driving:
For the field-driven case, we identify two different domain wall pinning
mechanisms, operating below and above the Walker breakdown, respectively,
whereas for the current-driven case pinning is absent above the Walker
breakdown. Increasing the disorder strength induces a larger Walker breakdown
field and current, and leads to decreased and increased domain wall velocities
at the breakdown field and current, respectively. Furthermore, for adiabatic
spin transfer torque, the intrinsic pinning mechanism is found to be suppressed
by disorder. We explain these findings within the one-dimensional model in
terms of an effective damping parameter $\alpha^*$ increasing with the disorder
strength.",1209.5274v1
2012-10-09,Dynamic Correlation Length Growth in Superspin Glass: Bridging Experiments and Simulations,"Interacting magnetic nanoparticles display a wide variety of magnetic
behaviors that are now being gathered in the emerging field of
'supermagnetism.' We have investigated how the out-of-equilibrium dynamics in
the disordered superspin glass (SSG) state of a frozen ferrofluid sample is
affected by texturation. Via magnetization relaxation experiments at low
temperatures, we were able to estimate superspin correlation lengths for both
textured and non-textured samples. The comparison with simulations and
experiments on atomic spin glasses shows that the dynamic correlations in SSG's
appear to develop in a way reminiscent to those in atomic spin glasses at
intermediate time/length scales.",1210.2524v1
2012-12-15,Non-equilibrium magnetic interactions in strongly correlated systems,"We formulate a low-energy theory for the magnetic interactions between
electrons in the multi-band Hubbard model under non-equilibrium conditions
determined by an external time-dependent electric field which simulates
laser-induced spin dynamics. We derive expressions for dynamical exchange
parameters in terms of non-equilibrium electronic Green functions and
self-energies, which can be computed, e.g., with the methods of time-dependent
dynamical mean-field theory. Moreover, we find that a correct description of
the system requires, in addition to exchange, a new kind of magnetic
interaction, that we name ""twist exchange"", which formally resembles
Dzyaloshinskii-Moriya coupling, but is not due to spin-orbit, and is actually
due to an effective three-spin interaction. Our theory allows the evaluation of
the related time-dependent parameters as well.",1212.3671v1
2013-01-20,Lattice QCD Methods for Hadronic Polarizabilities,"Chiral dynamics makes definitive predictions for the electromagnetic
polarizabilities of hadrons near the chiral limit; but, agreement with
experiment is tenuous in some cases. We provide an overview of lattice QCD
methods to compute the electric and magnetic polarizabilities of hadrons.
Central to these methods is the lattice simulation of quarks in uniform,
classical electromagnetic fields. A long-term goal is the determination of
polarizabilities directly from lattice computations, however, in the near term,
one may need to rely on partially quenched chiral perturbation theory.
Nonetheless the same striking predictions for the pion mass dependence of
electric and magnetic polarizabilities can be made from chiral dynamics, and
tested with lattice QCD. A particular focus is a novel new method to handle
charged particle correlation functions in magnetic fields.",1301.4622v1
2013-03-19,Vortex Wall Dynamics and Pinning in Helical Magnets,"Domain walls formed by one dimensional array of vortex lines have been
recently predicted to exist in disordered helical magnets and multiferroics.
These systems are on one hand analogues to the vortex line lattices in type-II
superconductors while on the other hand they propagate in the magnetic medium
as a domain boundary. Using a long wavelength approach supported by numerical
optimization we lay out detailed theory for dynamics and structure of such
topological fluctuations at zero temperature in presence of weak disorder. We
show the interaction between vortex lines is weak. This is the direct
consequence of the screening of the vorticity by helical background in the
system. We explain how one can use this result to understand the elasticity of
the wall with a vicinal surface approach. Also we show the internal degree of
freedom of this array leads to the enhancement of its mobility. We present
estimates for the interaction and mobility enhancements using the microscopic
parameters of the system. Finally we determine the range of velocities/force
densities in which the internal movement of the vortex wall can be effective in
its dynamics.",1303.4780v4
2013-03-21,Spin dynamics of two bosons in an optical lattice site: a role of anharmonicity and anisotropy of the trapping potential,"We study a spin dynamics of two magnetic Chromium atoms trapped in a single
site of a deep optical lattice in a resonant magnetic field. Dipole-dipole
interactions couple spin degrees of freedom of the two particles to their
motion in the site. The motion is quantized, therefore a trap geometry combined
with two-body contact s-wave interactions influence a spin dynamics through the
energy spectrum of the two atom system. Anharmonicity and anisotropy of the
site results in a `fine' structure of two body eigenenergies. The structure can
be easily resolved by a weak magnetic dipole-dipole interactions. As an example
we examine the effect of anharmonicity and anisotropy of the binding potential
on the Einstein-de Haas effect. We show that the weak dipolar interactions
provide a perfect tool for a precision spectroscopy of the energy spectrum of
the interacting few particle system.",1303.5232v2
2013-04-30,Correlation effects in insulating surface nanostructures,"We study the role of static and dynamical Coulomb correlation effects on the
electronic and magnetic properties of individual Mn, Fe and Co adatoms
deposited on the CuN surface. For these purposes, we construct a realistic
Anderson model, solve it by using finite-temperature exact diagonalization
method and compare the calculated one-particle spectral functions with the
LDA+$U$ densities of states. In contrast to Mn/CuN and Fe/CuN, the cobalt
system tends to form the electronic excitations at the Fermi level. Based on
the calculated magnetic response functions, the relative relaxation times for
the magnetic moments of impurity orbitals are estimated. To study the effect of
the dynamical correlations on the exchange interaction in nanoclusters, we
solve the two-impurity Anderson model for the Mn dimer on the CuN surface. It
is found that the experimental exchange interaction can be well reproduced by
employing $U$=3 eV, which is two times smaller than the value used in static
mean-field LDA+$U$ calculations. This suggests on important role of dynamical
correlations in the interaction between adatoms on a surface.",1304.8044v1
2013-06-13,Speed limit of FePt spin dynamics on femtosecond timescales,"Magnetization manipulation is becoming an indispensable tool for both basic
and applied research. Theory predicts two types of ultrafast demagnetization
dynamics classified as type I and type II. In type II materials, a second
slower process takes place after the initial fast drop of magnetization. In
this letter we investigate this behavior for FePt recording materials with
perpendicular anisotropy. The magnetization dynamics have been simulated using
a thermal micromagnetic model based on the Landau-Lifshitz-Bloch equation. We
identify a transition to type II behavior and relate it to the electron
temperatures reached by the laser heating. This slowing down is a fundamental
limit to reconding speeds in heat assisted reversal.",1306.3112v1
2013-07-03,Universal properties of magnetization dynamics in polycrystalline ferromagnetic films,"We investigate the scaling behavior in the statistical properties of
Barkhausen noise in ferromagnetic films. We apply the statistical treatment
usually employed for bulk materials in experimental Barkhausen noise time
series measured with the traditional inductive technique in polycrystalline
ferromagnetic films having different thickness from 100 up to 1000 nm, and
investigate the scaling exponents. Based on this procedure, we can group the
samples in a single universality class, characterized by exponents \tau \sim
1.5, \alpha \sim 2.0, and 1/\sigma \nu z \sim \vartheta \sim 2.0. We interpret
these results in terms of theoretical models and provide experimental evidence
that a well-known mean-field model for the dynamics of a ferromagnetic domain
wall in three-dimensional ferromagnets can be extended for films. We identify
that the films present an universal three-dimensional magnetization dynamics,
governed by long-range dipolar interactions, even at the smallest thicknesses,
indicating that the two-dimensional magnetic behavior commonly verified for
films cannot be generalized for all thickness ranges.",1307.1154v1
2013-08-15,Detection of the microwave spin pumping using the inverse spin Hall effect,"We report electrical detection of the dynamical part of the spin pumping
current emitted during ferromagnetic resonance (FMR) using the inverse Spin
Hall Effect (ISHE). The experiment is performed on a YIG$|$Pt bilayer. The
choice of YIG, a magnetic insulator, ensures that no charge current flows
between the two layers and only pure spin current produced by the magnetization
dynamics are transferred into the adjacent strong spin-orbit Pt layer via spin
pumping. To avoid measuring the parasitic eddy currents induced at the
frequency of the microwave source, a resonance at half the frequency is induced
using parametric excitation in the parallel geometry. Triggering this nonlinear
effect allows to directly detect on a spectrum analyzer the microwave component
of the ISHE voltage. Signals as large as 30 $\mu$V are measured for precession
angles of a couple of degrees. This direct detection provides a novel efficient
means to study magnetization dynamics on a very wide frequency range with great
sensitivity.",1308.3433v2
2013-10-16,Terahertz out-of-plane resonances due to spin-orbit coupling,"A microscopic kinetic theory is developed which allows to investigate
non-Abelian SU(2) systems interacting with meanfields and spin-orbit coupling
under magnetic fields in one, two, and three dimensions. The coupled kinetic
equations for the scalar and spin components are presented and linearized with
respect to an external electric field. The dynamical classical and quantum Hall
effect are described in this way as well as the anomalous Hall effect where a
new symmetric dynamical contribution to the conductivity is presented. The
coupled density and spin response functions to an electric field are derived
including arbitrary magnetic fields. The magnetic field induces a staircase
structure at frequencies of the Landau levels. It is found that for linear
Dresselhaus and Rashba spin-orbit coupling a dynamical out-of-plane spin
response appears at these Landau level frequencies establishing terahertz
resonances.",1310.4405v1
2013-11-18,The structure and dynamics of self-assembling colloidal monolayers in oscillating magnetic fields,"Many fascinating phenomena such as large-scale collective flows, enhanced
fluid mixing and pattern formation have been observed in so-called active
fluids, which are composed of particles that can absorb energy and dissipate it
into the fluid medium. For active particles immersed in liquids, fluid-mediated
viscous stresses can play an important role on the emergence of collective
behavior. Here, we experimentally investigate their role in the dynamics of
self-assembling magnetically-driven colloidal particles which can rapidly form
organized hexagonal structures. We find that viscous stresses reduce hexagonal
ordering, generate smaller clusters, and significantly decrease the rate of
cluster formation, all while holding the system at constant number density.
Furthermore, we show that time and length scales of cluster formation depend on
the Mason number (Mn), or ratio of viscous to magnetic forces, scaling as t /
Mn and L / Mn^(1/2). Our results suggest that viscous stresses hinder
collective behavior in a self-assembling colloidal system.",1311.4497v1
2014-01-07,Dynamical magnetic charges and linear magnetoelectricity,"Magnetoelectric (ME) materials are of fundamental interest and have been
investigated for their broad potential for technological applications. The
search for, and eventually the theoretical design of, materials with large ME
couplings present challenging issues. First-principles methods have only
recently been developed to calculate the full ME response tensor $\alpha$
including both electronic and ionic (i.e., lattice-mediated) contributions. The
latter is proportional to both the Born dynamical electric charge $Z^{\rm e}$
and its analogue, the dynamical magnetic charge $Z^{\rm m}$. Here we present a
theoretical study of the magnetic charge $Z^{\rm m}$ and the mechanisms that
could enhance it. Using first-principles density-functional methods, we
calculate the atomic $Z^{\rm m}$ tensors in $\rm{Cr_2O_3}$, a prototypical
magnetoelectric, and in KITPite, a fictitious material that has previously been
reported to show a strong ME response arising from exchange striction effects.
Our results confirm that in $\rm{Cr_2O_3}$, the $Z^{\rm m}$ values and
resulting ME responses arise only from spin-orbit coupling (SOC) and are
therefore rather weak. In KITPite, by contrast, the exchange striction acting
on the non-collinear spin structure induces much $Z^{\rm m}$ values that
persist even when SOC is completely absent.",1401.1538v1
2014-03-24,Dynamics and polarization of superparamagnetic chiral nanomotors in a rotating magnetic field,"Externally powered magnetic nanomotors are of particular interest due to the
potential use for \emph{in vivo} biomedical applications. Here we develop a
theory for dynamics and polarization of recently fabricated superparamagnetic
chiral nanomotors powered by a rotating magnetic field. We study in detail
various experimentally observed regimes of the nanomotor dynamic orientation
and propulsion and establish the dependence of these properties on polarization
and geometry of the propellers. Based on the proposed theory we introduce a
novel ""steerability"" parameter $\gamma$ that can be used to rank polarizable
nanomotors by their propulsive capability. The theoretical predictions of the
nanomotor orientation and propulsion speed are in excellent agreement with
available experimental results. Lastly, we apply slender-body approximation to
estimate polarization anisotropy and orientation of the easy-axis of
superparamagnetic helical propellers.",1403.6007v1
2014-05-04,Spin Torque Oscillators with Thermal Noise: A Constant Energy Orbit Approach,"We consider a biaxial macrospin with an easy and hard axis, and study its
dynamical evolution under the combined effects of thermal noise and spin
transfer torque. The spin-torque is associated with both a perpendicularly
magnetized polarizer and an in-plane magnetized reference layer, leading to an
effective tilt between the easy and spin polarization axes. Using techniques
based on energy averaging over the relevant dynamical trajectories, we analyze
the effects of tilt on the dynamics and derive the conditions for the
occurrence of stable out-of-plane precessionary states. The presence of these
states and their predicted stability boundaries can be tested in experiments on
orthogonal spin-transfer devices, and may also serve as a test of the
applicability of the macrospin model to real devices, which have internal
magnetic degrees of freedom.",1405.0731v3
2014-06-04,"Spin-Phonon Coupling, High Pressure Phase Transitions and Thermal Expansion of Multiferroic GaFeO3: A Combined First Principles and Inelastic Neutron Scattering Study","We have carried out an extensive phonon study on multiferroic GaFeO3 to
elucidate its dynamical behavior. Inelastic neutron scattering measurements are
performed over a wide temperature range, 150 to 1198 K. First principles
lattice dynamical calculations are done for the sake of the analysis and
interpretation of the observations. The comparison of the phonon spectra from
magnetic and non-magnetic calculations highlights pronounced differences. The
energy range of the vibrational atomistic contributions of the Fe and O ions
are found to differ significantly in the two calculation types. Therefore,
magnetism induced by the active spin degrees of freedom of Fe cations plays a
key role in stabilizing the structure and dynamics of GaFeO3. Moreover, the
computed enthalpy in various phases of GaFeO3 is used to gain deeper insights
into the high pressure phase stability of this material. Further, the volume
dependence of the phonon spectra is used to determine its thermal expansion
behavior.",1406.1027v1
2014-10-22,Resonating Valence-Bond State in an Orbitally Degenerate Quantum Magnet with Dynamical Jahn-Teller Effect,"Short-range resonating-valence bond states in an orbitally degenerate magnet
on a honeycomb lattice is studied. A quantum-dimer model is derived from the
Hamiltonian which represents the superexchange interaction and the dynamical
Jahn-Teller (JT) effect. We introduce two local units termed ""spin-orbital
singlet dimer"", where two spins in a nearest-neighbor bond form a singlet state
associated with an orbital polarization along the bond, and ""local JT singlet"",
where an orbital polarization is quenched due to the dynamical JT effect. A
derived quantum-dimer model consists of the hopping of the spin-orbital singlet
dimers and the JT singlets, and the chemical potential of the JT singlets. We
analyze the model by the mean-field approximation, and find that a
characteristic phase, termed ""JT liquid phase"", where both the spin-orbital
singlet dimers and the JT singlets move quantum mechanically, is realized.
Possible scenarios for the recently observed non magnetic-ordered state in
Ba$_3$CuSb$_2$O$_9$ are discussed.",1410.6009v1
2015-01-02,"Inertia, diffusion and dynamics of a driven skyrmion","Skyrmions recently discovered in chiral magnets are a promising candidate for
magnetic storage devices because of their topological stability, small size
($\sim 3-100$nm), and ultra-low threshold current density ($\sim
10^{6}$A/m$^2$) to drive their motion. However, the time-dependent dynamics has
hitherto been largely unexplored. Here we show, by combining the numerical
solution of the Landau-Lifshitz-Gilbert equation and the analysis of a
generalized Thiele's equation, that inertial effects are almost completely
absent in skyrmion dynamics driven by a time-dependent current. In contrast,
the response to time-dependent magnetic forces and thermal fluctuations depends
strongly on frequency and is described by a large effective mass and a (anti-)
damping depending on the acceleration of the skyrmion. Thermal diffusion is
strongly suppressed by the cyclotron motion and is proportional to the Gilbert
damping coefficient $\alpha$. This indicates that the skyrmion position is
stable, and its motion responds to the time-dependent current without delay or
retardation even if it is fast. These findings demonstrate the advantages of
skyrmions as information carriers.",1501.00444v1
2015-02-27,Collective dynamical skyrmions excitations in magnonic crystal,"We investigate theoretically skyrmion magnonic crystal, i.e., the dynamics of
the magnetization in a chain of the ferromagnetic nanodots being in skyrmion
magnetic configuration. We show that collective excitations are possible to be
observed in the structure. We present the dispersion relation of the coupled
skyrmions. It exhibit a periodical property in dependence on wave vector,
characteristic feature of the band structure in magnonic crystals. Spatial
analysis of the magnetization amplitude associated with the magnonic bands
confirms type of the excited modes, as breathing and clockwise gyrotropic
dynamical skyrmions. These high and low frequency excitations, propagate with
negative and positive group velocity, respectively, and can be explored for
study fundamental properties and technological applications in spintronics and
magnonics.",1502.08024v1
2015-05-20,Simulating coronal condensation dynamics in 3D,"We present numerical simulations in 3D settings where coronal rain phenomena
take place in a magnetic configuration of a quadrupolar arcade system. Our
simulation is a magnetohydrodynamic simulation including anisotropic thermal
conduction, optically thin radiative losses, and parametrised heating as main
thermodynamical features to construct a realistic arcade configuration from
chromospheric to coronal heights. The plasma evaporation from chromospheric and
transition region heights eventually causes localised runaway condensation
events and we witness the formation of plasma blobs due to thermal instability,
that evolve dynamically in the heated arcade part and move gradually downwards
due to interchange type dynamics. Unlike earlier 2.5D simulations, in this case
there is no large scale prominence formation observed, but a continuous coronal
rain develops which shows clear indications of Rayleigh-Taylor or interchange
instability, that causes the denser plasma located above the transition region
to fall down, as the system moves towards a more stable state. Linear stability
analysis is used in the non-linear regime for gaining insight and giving a
prediction of the system's evolution. After the plasma blobs descend through
interchange, they follow the magnetic field topology more closely in the lower
coronal regions, where they are guided by the magnetic dips.",1505.05333v1
2015-05-28,Dynamic and rate-dependent yielding behavior of Co0.9Ni0.1 nanocluster based magnetorheological fluids,"In this paper we performed steady shear and oscillatory magnetorheological
(MR) studies in magnetic fluids containing CoNi nanoclusters of 450 nm in
diameter. Co-rich nanoclusters were synthesized by conventional homogeneous
nucleation without any external surfactant or reducing agent in liquid polyol
at elevated temperature. The x-ray diffraction, energy dispersive X-Ray
analysis, scanning and transmission electron microscopy studies were done for
analyzing the sample composition and morphology. Two variants of fluid samples
were prepared by dispersing 15 vol% and 20 Vol% of CoNi powders in castor oil.
Room temperature steady magnetoshear studies indicate viscoplastic behavior
with stronger dependence of static yield stress on magnetization than a dipolar
coupling that was operational in the dynamic yield stress. Magnetosweep
measurements at constant shear rate showed interesting relaxation at high
magnetic fields. We also explored dynamical elastic behavior through
oscillatory magnetorheological studies under both strain sweep and frequency
sweep modes, and showed glass transition like phenomenon occurring in them
above critical shear amplitudes.",1505.07591v2
2015-09-10,Ultrafast cooling and heating scenarios for the laser induced phase transition in CuO,"The multiferroic compound CuO exhibits low temperature magnetic properties
similar to antiferromagnetic iron oxides, while the electronic properties have
much more in common with the high $T_c$ cuprate superconductors. This suggests
novel possibilities for the ultrafast optical excitation of magnetism. On the
basis on atomistic spin dynamics simulations, we study the effect of
phonon-assisted multimagnon absorption and photodoping on the spin dynamics in
the vicinity of the first-order phase transition from collinear to spin-spiral
magnetic order. Similar as in recent experiments, we find that for both
excitations the phase transition can proceed on the picosecond timescale.
Interestingly, however, these excitation mechanisms display very distinct
dynamics. Following photodoping, the spin system first cools down on sub-ps
timescales, which we explain as an ultrafast magnetocaloric effect. Opposed to
this, following phonon-assisted multimagnon excitation the spin systems rapidly
heats up and subsequently evolves to the noncollinear phase even under the
influence of isotropic exchange interactions alone.",1509.03202v2
2015-12-16,The Hamiltonian Dynamics of Planar Magnetic Confinement,"Inspired by a question of Colin de Verdi\`{e}re and Truc we study the
dynamics of a classical charged particle moving in a bounded planar domain
$\Omega$ under the influence of a magnetic field $\mathbf{B}$ which blows up at
the boundary of the domain. We prove that under appropriate blow-up conditions
the particle will never reach the boundary. As a corollary we obtain
completeness of the magnetic flow. Our blow-up condition is that $\mathbf{B}$
should not be integrable along normal rays to the boundary, while its
tangential derivative should be integrable along those same rays.",1512.05308v3
2016-01-19,Minimal Radius of Magnetic Skyrmions: Statics and Dynamics,"In a broad range of applied magnetic fields and material parameters isolated
magnetic skyrmions condense into skyrmion lattices. While the geometry of
isolated skyrmions and their lattice counterparts strongly depend on field and
Dzyaloshinski-Moriya interaction, this issue has not been adequately addressed
in previous studies. Meanwhile, this information is extremely important for
applications, because the skyrmion size and the interskyrmion distance have to
be tuned for skyrmion based memory and logic devices. In this investigation we
elucidate the size and density-dependent phase diagram showing traditional
phases in field vs. material parameters space by means of Monte-Carlo
simulations on a discrete lattice. The obtained diagram permits us to establish
that, in contrast to the continuum limit, skyrmions on a discrete lattice
cannot be smaller than some critical size and have a very specific shape. These
minimal skyrmions correspond to the micromagnetic configuration at the energy
barrier between the ferromagnetic and the skyrmionic states. Furthermore, we
use atomistic Landau-Lifshitz-Gilbert simulations to study dynamics of the
skyrmion annihilation. It is shown that this procees consists of two stages:
the continuous skyrmion contraction and its discontinuous annihilation. The
detailed analysis of this dynamical process is given.",1601.04898v1
2016-03-25,Microscopic theory of spin-orbit torques and skyrmion dynamics,"We formulate a general microscopic approach to spin-orbit torques in thin
ferromagnet/heavy-metal bilayers in linear response to electric current or
electric field. The microscopic theory we develop avoids the notion of spin
currents and spin-Hall effect. Instead, the torques are directly related to a
local spin polarization of conduction electrons, which is computed from
generalized Kubo-St\v{r}eda formulas. A symmetry analysis provides a one-to-one
correspondence between polarization susceptibility tensor components and
different torque terms in the Landau-Lifshitz-Gilbert equation for
magnetization dynamics. The spin-orbit torques arising from Rashba or
Dresselhaus type of spin-orbit interaction are shown to have different
symmetries. We analyze these spin-orbit torques microscopically for a generic
electron model in the presence of an arbitrary smooth magnetic texture. For a
model with spin-independent disorder we find a major cancelation of the
torques. In this case the only remaining torque corresponds to the
magnetization-independent Edelstein effect. Furthermore, our results are
applied to analyze the dynamics of a Skyrmion under the action of electric
current.",1603.07994v2
2016-05-10,Dynamical supersymmetry breaking on magnetized tori and orbifolds,"We construct several dynamical supersymmetry breaking (DSB) models within a
single ten-dimensional supersymmetric Yang-Mills (SYM) theory, compactified on
magnetized tori with or without orbifolding. We study the case that the
supersymmetry breaking is triggered by a strong dynamics of $SU(N_C)$ SYM
theory with $N_F$ flavors contained in the four-dimensional effective theory.
We show several configurations of magnetic fluxes and orbifolds, those
potentially yield, below the compactification scale, the field contents and
couplings required for triggering DSB. We especially find a class of
self-complete DSB models on orbifolds, where all the extra fields are
eliminated by the orbifold projection and DSB successfully occurs within the
given framework. Comments on some perspectives for associating the obtained DSB
models with the other sectors, such as the visible sector and another hidden
sector for, e.g., stabilizing moduli, are also given.",1605.02922v1
2016-10-13,Spin-wave spectra in periodically surface-modulated ferromagnetic thin films,"This article presents theoretical results for the dynamic response of
periodically surface-modulated ferromagnetic thin films. For such system, the
role of the periodic dipolar field induced by the modulation is addressed by
using the plane-wave method. By controlling the geometry of the modulated
volumes within the film, the frequency modes and spatial profiles of spin waves
can be manipulated. The angular dependence of the frequency band-gaps unveils
the influence of both dynamic and static magnetic charges, which reside in the
edges of the etching periodic zones, and it is stablished that band-gap widths
created by static magnetic charges are broader than the one created by dynamic
magnetic charges. To corroborate the validity of the model, the theoretical
results are compared with ferromagnetic resonance simulations, where a very
good agreement is achieved between both methods. The theoretical model allows
for a detailed understanding of the physics underlying these kind of systems,
thereby providing an outlook to potential applications associated with magnonic
crystals-based devices.",1610.04176v1
2016-11-23,"Electron interactions, spin-orbit coupling, intersite correlations in pyrochlore iridates","We perform combined density functional and dynamical mean-field calculations
to study the pyrochlore iridates Lu$_2$Ir$_2$O$_7$, Y$_2$Ir$_2$O$_7$ and
Eu$_2$Ir$_2$O$_7$. Both single-site and cluster dynamical mean-field
calculations are performed and spin-orbit coupling is included. Paramagnetic
metallic phases, antiferromagnetic metallic phases with tilted Weyl cones and
antiferromagnetic insulating phases are found. The magnetic phases display
all-in/all-out magnetic ordering, consistent with previous studies. Unusually
for electronically three dimensional materials, the single-site dynamical
mean-field approximation fails to reproduce qualitative material trends,
predicting in particular that the paramagnetic phase properties of
Y$_2$Ir$_2$O$_7$ and Eu$_2$Ir$_2$O$_7$ are almost identical, although in
experiments the Y compound has a much higher resistance than the Eu compound.
This qualitative failure is attributed to the importance of intersite magnetic
correlations in the physics of these materials.",1611.07997v2
2016-11-29,Dynamical Shiba states by precessing magnetic moments in an s-wave superconductor,"We study theoretically the dynamics of a Shiba state forming around
precessing classical spin in an s-wave superconductor. Utilizing a rotating
wave description for the precessing magnetic impurity, we find the resulting
Shiba bound state quasi-energy and the spatial extension of the Shiba
wavefunction. We show that such a precession pertains to dc charge and spin
currents flowing through a normal STM tip tunnel coupled to the superconductor
in the vicinity of the impurity. We calculate these currents and find that they
strongly depend on the magnetic impurity precession frequency, precession
angle, and on the position of the Shiba energy level in the superconducting
gap. The resulting charge current is found to be proportional to the difference
between the electron and hole wavefunctions of the Shiba state, being a direct
measure for such an asymmetry. By dynamically driving the impurity one can
infer the spin dependence of the Shiba states in the absence of a
spin-polarized STM tip",1611.09722v1
2016-12-04,Dynamic Magnetometer Calibration and Alignment to Inertial Sensors by Kalman Filtering,"Magnetometer and inertial sensors are widely used for orientation estimation.
Magnetometer usage is often troublesome, as it is prone to be interfered by
onboard or ambient magnetic disturbance. The onboard soft-iron material
distorts not only the magnetic field, but the magnetometer sensor frame
coordinate and the cross-sensor misalignment relative to inertial sensors. It
is desirable to conveniently put magnetic and inertial sensors information in a
common frame. Existing methods either split the problem into successive
intrinsic and cross-sensor calibrations, or rely on stationary accelerometer
measurements which is infeasible in dynamic conditions. This paper formulates
the magnetometer calibration and alignment to inertial sensors as a state
estimation problem, and collectively solves the magnetometer intrinsic and
cross-sensor calibrations, as well as the gyroscope bias estimation. Sufficient
conditions are derived for the problem to be globally observable, even when no
accelerometer information is used at all. An extended Kalman filter is designed
to implement the state estimation and comprehensive test data results show the
superior performance of the proposed approach. It is immune to acceleration
disturbance and applicable potentially in any dynamic conditions.",1612.01044v1
2016-12-18,Non-equilibrium critical dynamics of the two-dimensional Ashkin-Teller model at the Baxter line,"We investigate the short-time universal behavior of the two dimensional
Ashkin-Teller model at the Baxter line by performing time-dependent Monte Carlo
Simulations. First, as preparatory results, we obtain the critical parameters
by searching the optimal power law decay of the magnetization. Thus, the
dynamic critical exponents $\theta _{m}$ and $\theta _{p}$, related to the
magnetic and electric order parameters, as well as the persistence exponent
$\theta _{g}$, are estimated using heat-bath Monte Carlo simulations. In
addition, we estimate the dynamic exponent $z$ and the static critical
exponents $\beta $ and $\nu $ for both order parameters. We propose a refined
method to estimate the static exponents that considers two different averages:
one that combines an internal average using several seeds with another which is
taken over geographic variations in the power laws. Moreover, we also performed
the bootstrapping method for a complementary analysis. Our results show that
the ratio $\beta /\nu $ exhibits universal behavior along the critical line
corroborating the conjecture for both magnetization and polarization.",1612.05842v1
2017-02-13,Spin Pumping and Inverse Spin Hall Voltages from Dynamical Antiferromagnets,"Dynamical antiferromagnets pump spins efficiently into adjacent conductors as
ferromagnets. The high antiferromagnetic resonance frequencies represent a
challenge for experimental detection, but magnetic fields can reduce these
resonance frequencies. We compute the inverse spin Hall voltages resulting from
dynamical spin excitations as a function of a magnetic field along the easy
axis and the polarization of the driving AC magnetic field perpendicular to the
easy axis. We consider the insulating antiferromagnets MnF$_2$, FeF$_2$, and
NiO. Near the spin-flop transition, there is a significant enhancement of the
DC spin pumping and inverse spin Hall voltage for the uniaxial antiferromagnets
MnF$_2$ and FeF$_2$. In the biaxial NiO, the voltages are much weaker, and
there is no spin-flop enhancement of the DC component.",1702.03779v1
2017-03-16,Generation of droplet arrays with rational number spacing patterns driven by a periodic energy landscape,"The generation of droplets at low Reynolds numbers is driven by non-linear
dynamics that give rise to complex patterns concerning both the
droplet-to-droplet spacing and the individual droplet sizes. Here we
demonstrate an experimental system in which a time-varying energy landscape
provides a periodic magnetic force that generates an array of droplets from an
immiscible mixture of ferrofluid and silicone oil. The resulting droplet
patterns are periodic, owing to the nature of the magnetic force, yet the
droplet spacing and size can vary greatly by tuning a single bias pressure
applied on the ferrofluid phase; for a given cycle period of the magnetic
force, droplets can be generated either at integer multiples (1, 2, etc.), or
at rational fractions (3/2, 5/3, 5/2, etc.) of this period with mono- or
multidisperse droplet sizes. We develop a discrete-time dynamical systems model
not only to reproduce the phenotypes of the observed patterns but also provide
a framework for understanding systems driven by such periodic energy
landscapes.",1703.05489v2
2017-04-13,Fluctuations and Noise Signatures of Driven Magnetic Skyrmions,"Magnetic skyrmions are particle-like objects with topologically-protected
stability which can be set into motion with an applied current. Using a
particle-based model we simulate current-driven magnetic skyrmions interacting
with random quenched disorder and examine the skyrmion velocity fluctuations
parallel and perpendicular to the direction of motion as a function of
increasing drive. We show that the Magnus force contribution to skyrmion
dynamics combined with the random pinning produces an isotropic effective
shaking temperature. As a result, the skyrmions form a moving crystal at large
drives instead of the moving smectic state observed in systems with a
negligible Magnus force where the effective shaking temperature is anisotropic.
We demonstrate that spectral analysis of the velocity noise fluctuations can be
used to identify dynamical phase transitions and to extract information about
the different dynamic phases, and show how the velocity noise fluctuations are
correlated with changes in the skyrmion Hall angle, transport features, and
skyrmion lattice structure.",1704.04272v1
2017-04-25,Chirality-mediated bistability and strong frequency downshifting of the gyrotropic resonance of a dynamically de-stiffened magnetic vortex,"We demonstrate an enhanced, bidirectional, in-plane magnetic field tuning of
the gyrotropic resonance frequency of a magnetic vortex within a disk by
introducing a flat edge. When the core is in its vicinity, the flat edge
locally reduces the core's directional dynamic stiffness for movement parallel
to the edge. This strongly reduces the net dynamic core stiffness, leading to
the gyrotropic frequency being significantly less than when the core is
centered (or located near the round edge). This leads to the measurable range
of gyrotropic frequencies being more than doubled and also results in a clear
chirality-mediated bistability of the gyrotropic resonance frequency due to
what is effectively a chirality-dependence of the core's confining potential.",1704.07748v1
2017-05-09,Nonlinear parametric excitation effect induces stability transitions in swimming direction of flexible superparamagnetic microswimmers,"Microscopic artificial swimmers have recently become highly attractive due to
their promising potential for biomedical applications. The pioneering work of
Dreyfus et al (2005) has demonstrated the motion of a microswimmer with an
undulating chain of superparamagnetic beads, which is actuated by an
oscillating external magnetic field. Interestingly, it has also been
theoretically predicted that the swimming direction of this swimmer will
undergo a $90^\circ$-transition when the magnetic field's oscillations
amplitude is increased above a critical value of $\sqrt{2}$. In this work, we
further investigate this transition both theoretically and experimentally by
using numerical simulations and presenting a novel flexible microswimmer with a
superparamagnetic head. We realize the $90^\circ$-transition in swimming
direction, prove that this effect depends on both frequency and amplitude of
the oscillating magnetic field, and demonstrate the existence of an optimal
amplitude, under which, maximal swimming speed can be achieved. By
asymptotically analyzing the dynamic motion of microswimmer with a minimal
two-link model, we reveal that the stability transitions representing the
changes in the swimming direction are induced by the effect of nonlinear
parametric excitation.",1705.03486v1
2017-05-16,Bloch line dynamics within moving domain walls in 3D ferromagnets,"We study field-driven magnetic domain wall dynamics in garnet strips by
large-scale three-dimensional micromagnetic simulations. The domain wall
propagation velocity as a function of the applied field exhibits a low-field
linear part terminated by a sudden velocity drop at a threshold field
magnitude, related to the onset of excitations of internal degrees of freedom
of the domain wall magnetization. By considering a wide range of strip
thicknesses from 30 nm to 1.89 $\mu$m, we find a non-monotonic thickness
dependence of the threshold field for the onset of this instability, proceeding
via nucleation and propagation of Bloch lines within the domain wall. We
identify a critical strip thickness above which the velocity drop is due to
nucleation of horizontal Bloch lines, while for thinner strips and depending on
the boundary conditions employed, either generation of vertical Bloch lines, or
close-to-uniform precession of the domain wall internal magnetization takes
place. For strips of intermediate thicknesses, the vertical Bloch lines assume
a deformed structure due to demagnetizing fields at the strip surfaces,
breaking the symmetry between the top and bottom faces of the strip, and
resulting in circulating Bloch line dynamics along the perimeter of the domain
wall.",1705.05642v1
2017-05-23,Coherent many-body spin dynamics in a long-range interacting Ising chain,"Coherent many-body quantum dynamics lies at the heart of quantum simulation
and quantum computation. Both require coherent evolution in the exponentially
large Hilbert space of an interacting many-body system. To date, trapped ions
have defined the state of the art in terms of achievable coherence times in
interacting spin chains. Here, we establish an alternative platform by
reporting on the observation of coherent, fully interaction-driven quantum
revivals of the magnetization in Rydberg-dressed Ising spin chains of atoms
trapped in an optical lattice. We identify partial many-body revivals at up to
about ten times the characteristic time scale set by the interactions. At the
same time, single-site-resolved correlation measurements link the magnetization
dynamics with inter-spin correlations appearing at different distances during
the evolution. These results mark an enabling step towards the implementation
of Rydberg atom based quantum annealers, quantum simulations of higher
dimensional complex magnetic Hamiltonians, and itinerant long-range interacting
quantum matter.",1705.08372v1
2017-12-12,Dynamical Negative Differential Resistance in Antiferromagnetically Coupled Few-Atom Spin-Chains,"We present the appearance of negative differential resistance (NDR) in
spin-dependent electron transport through a few-atom spin-chain. A chain of
three antiferromagnetically coupled Fe atoms(Fe trimer) was positioned on a
Cu2N/Cu(100) surface and contacted with the spin-polarized tip of a scanning
tunneling microscope, thus coupling the Fe trimer to one non-magnetic and one
magnetic lead. Pronounced NDR appears at the low bias of 7 mV where inelastic
electron tunneling dynamically locks the atomic spin in a long-lived excited
state. This causes a rapid increase of the magnetoresistance between
spin-polarized tip and Fe trimer and quenches elastic tunneling. By varying the
coupling strength between tip and Fe trimer we find that in this transport
regime the dynamic locking of the Fe trimer competes with magnetic exchange
interaction, which statically forces the Fe trimer into the
high-magnetoresistance state and removes the NDR.",1712.04213v1
2017-12-20,Nonlinear mean-field dynamo and prediction of solar activity,"We apply a nonlinear mean-field dynamo model which includes a budget equation
for the dynamics of Wolf numbers to predict solar activity. This dynamo model
takes into account the algebraic and dynamic nonlinearities of the alpha
effect, where the equation for the dynamic nonlinearity is derived from the
conservation law for the magnetic helicity. The budget equation for the
evolution of the Wolf number is based on a formation mechanism of sunspots
related to the negative effective magnetic pressure instability. This
instability redistributes the magnetic flux produced by the mean-field dynamo.
To predict solar activity on the time scale of one month we use a method based
on a combination of the numerical solution of the nonlinear mean-field dynamo
equations and the artificial neural network. A comparison of the results of the
prediction of the solar activity with the observed Wolf numbers demonstrates a
good agreement between the forecast and observations.",1712.07501v4
2018-01-18,Magpy: A C++ accelerated Python package for simulating magnetic nanoparticle stochastic dynamics,"Magpy is a C++ accelerated Python package for modelling and simulating the
magnetic dynamics of nano-sized particles. Nanoparticles are modelled as a
system of three-dimensional macrospins and simulated with a set of coupled
stochastic differential equations (the Landau-Lifshitz-Gilbert equation), which
are solved numerically using explicit or implicit methods. The results of the
simulations may be used to compute equilibrium states, the dynamic response to
external magnetic fields, and heat dissipation. Magpy is built on a C++
library, which is optimised for serial execution, and exposed through a Python
interface utilising an embarrassingly parallel strategy. Magpy is free,
open-source, and available on github under the 3-Clause BSD License.",1801.06073v2
2018-02-02,Stochastic model for quantum spin dynamics in magnetic nanostructures,"We develop a numerical model that reproduces the thermal equilibrium and the
spin transfer mechanisms in magnetic nanomaterials. We analyze the coherent
two-particle spin exchange interaction and the electron-electron collisions.
Our study is based on a quantum atomistic approach and the particle dynamics is
performed by using a Monte Carlo technique. The coherent quantum evolution of
the atoms is interrupted by instantaneous collisions with itinerant electrons.
The collision processes are associated to the quantum collapse of the local
atomic wave function. We show that particle-particle interactions beyond the
molecular field approximation can be included in this framework. Our model is
able to reproduce the thermal equilibrium and strongly out-of-equilibrium
phenomena such as the ultrafast dynamics of the magnetization in nanomatrials.",1802.00663v1
2018-02-12,Gyrotropic resonance of individual Néel skyrmions in Ir/Fe/Co/Pt multilayers,"Magnetic skyrmions are nanoscale spin structures recently discovered at room
temperature (RT) in multilayer films. Employing their novel topological
properties towards exciting technological prospects requires a mechanistic
understanding of the excitation and relaxation mechanisms governing their
stability and dynamics. Here we report on the magnetization dynamics of RT
N\'eel skyrmions in Ir/Fe/Co/Pt multilayer films. We observe a ubiquitous
excitation mode in the microwave absorption spectrum, arising from the
gyrotropic resonance of topological skyrmions, and robust over a wide range of
temperatures and sample compositions. A combination of simulations and
analytical calculations establish that the spectrum is shaped by the interplay
of interlayer and interfacial magnetic interactions unique to multilayers,
yielding skyrmion resonances strongly renormalized to lower frequencies. Our
work provides fundamental spectroscopic insights on the spatiotemporal dynamics
of topological spin structures, and crucial directions towards their
functionalization in nanoscale devices.",1802.03979v1
2018-03-05,Attenuation of the NMR signal due to hydrodynamic Brownian motion,"Nuclear magnetic resonance (NMR) is a widely used nondestructive method to
study random motion of spin-bearing particles in different systems. In the
long-time limit the theoretical description of the NMR experiments is well
developed and allows proper interpretation of measurements of normal and
anomalous diffusion. The traditional description becomes, however, insufficient
for the shorter-time dynamics of the particles. In the present paper, the
all-time attenuation function of the NMR signal in a magnetic-field gradient
due to the Brownian motion (BM) of particles in incompressible liquids is
calculated by using the method of accumulation of phases by a precessing
magnetic moment, without reference to a concrete model of the stochastic
dynamics. The obtained expressions are then used to evaluate the attenuation
within the hydrodynamic theory of the BM. It is shown that the well-known time
behavior of the formulas corresponding to the Einstein theory of diffusion in
the case of steady gradient and Hahn's echo experiments is reached at times
much larger than the characteristic time of the loss of memory in the particle
dynamics. At shorter times the attenuation function significantly differs from
the classical formulas used to interpret these experiments.",1803.01554v1
2018-04-06,Assessing the Capabilities of Dynamic Coronal Seismology of Alfvénic Waves through Forward Modeling,"Coronal seismology is a diagnostic tool used in solar physics for measuring
parameters that are otherwise hard to measure; of these parameters, magnetic
field values are arguably the most important. The parameters are inferred by
combining observations of waves with magnetohydrodynamic (MHD) wave theory. To
date, coronal seismology has successfully been applied to various
single-oscillation events. Such events are relatively rare, resulting in rare
occasions to use diagnostics. Ubiquitous waves in the solar atmosphere might,
however, allow for the possibility of dynamic coronal seismology, which
involves the continuous inversions of coronal parameters and would constitute a
huge leap forward in many areas of solar physics. In this paper, we investigate
the robustness and accuracy of magnetic field diagnostics applied to
forward-modeled 3D MHD simulations of propagating Alfv\'enic waves. We find
that the seismologically measured magnetic field values are reassuringly close
to the input value (within 20%) for a range of setups studied, providing
encouragement and confidence for the further development of dynamic coronal
seismology.",1804.02175v2
2018-06-29,Intra-cell dynamics and cyclotron motion without magnetic field,"Intra-cell motion endows rich non-trivial phenomena to a wide variety of
quantum materials. The most prominent example is a transverse current in the
absence of a magnetic field (i.e. the anomalous Hall effect). Here we show
that, in addition to a dc Hall effect, anomalous Hall materials possess
circulating currents and cyclotron motion without magnetic field. These are
generated from the intricate wavefunction dynamics within the unit cell, and
correspond to interband transitions (coherences) in much the same way that
cyclotron resonances arise from inter-Landau level transitions in
magneto-optics. Curiously, anomalous cyclotron motion exhibits an intrinsic
decay in time (even in pristine materials) displaying a characteristic power
law decay. This reveals an intrinsic dephasing similar to that of inhomogeneous
broadening of spinors. Circulating currents can manifest as the emission of
circularly polarized light pulses in response to incident linearly polarized
(pulsed) electric field, and provide a direct means of interrogating the
intra-unit-cell dynamics of quantum materials.",1807.00014v1
2018-07-29,Magnetic Polymer Models for Epigenomic Organisation and Phase Separation,"The genetic instructions stored in the genome require an additional layer of
information to robustly determine cell fate. This additional regulation is
provided by the interplay between chromosome-patterning biochemical
(""epigenetic"") marks and three-dimensional genome folding. Yet, the physical
principles underlying the dynamical coupling between three-dimensional genomic
organisation and one-dimensional epigenetic patterns remain elusive. To shed
light on this issue, here we study by mean field theory and Brownian dynamics
simulations a magnetic polymer model for chromosomes, where each monomer
carries a dynamic epigenetic mark. At the single chromosome level, we show that
a first order transition describes the unlimited spreading of epigenetic marks,
a phenomenon that is often observed in vivo. At the level of the whole nucleus,
experiments suggest chromosomes form micro-phase separated compartments with
distinct epigenetic marks. We here discover that for a melt of magnetic
polymers such a morphology is thermodynamically unstable, but can be stabilised
by a non- equilibrium and ATP-mediated epigenetic switch between different
monomer states.",1807.11101v1
2018-09-27,Non-equilibrium Quantum Langevin dynamics of orbital diamagnetic moment,"We investigate the time dependent orbital diamagnetic moment of a charged
particle in a magnetic field in a viscous medium via the Quantum Langevin
Equation. We study how the interplay between the cyclotron frequency and the
viscous damping rate governs the dynamics of the orbital magnetic moment in the
high temperature classical domain and the low temperature quantum domain for an
Ohmic bath. These predictions can be tested via state of the art cold atom
experiments with hybrid traps for ions and neutral atoms. We also study the
effect of a confining potential on the dynamics of the magnetic moment. We
obtain the expected Bohr Van Leeuwen limit in the high temperature, asymptotic
time ($ \gamma t\longrightarrow \infty$, where $ \gamma $ is the viscous
damping coefficient) limit.",1809.10370v1
2018-10-01,Determination of spin Hall angle in heavy metal/CoFeB-based heterostructures with interfacial spin-orbit fields,"Magnetization dynamics in W/CoFeB, CoFeB/Pt and W/CoFeB/Pt multilayers was
investigated using spin-orbit-torque ferromagnetic resonance (SOT-FMR)
technique. An analytical model based on magnetization dynamics due to SOT was
used to fit heavy metal (HM) thickness dependence of symmetric and
antisymmetric components of the SOT-FMR signal. The analysis resulted in a
determination of the properties of HM layers, such as spin Hall angle and spin
diffusion length. The spin Hall angle of -0.36 and 0.09 has been found in the
W/CoFeB and CoFeB/Pt bilayers, respectively, which add up in the case of
W/CoFeB/Pt trilayer. More importantly, we have determined effective interfacial
spin-orbit fields at both W/CoFeB and CoFeB/Pt interfaces, which are shown to
cancel Oersted field for particular thicknesses of the heavy metal layers,
leading to pure spin-current-induced dynamics and indicating the possibility
for a more efficient magnetization switching.",1810.00641v1
2018-11-05,Nonadiabatic dynamics and geometric phase of an ultrafast rotating electron spin,"The spin in a rotating frame has attracted a lot of attentions recently, as
it deeply relates to both fundamental physics such as pseudo-magnetic field and
geometric phase, and applications such as gyroscopic sensors. However, previous
studies only focused on adiabatic limit, where the rotating frequency is much
smaller than the spin frequency. Here we propose to use a levitated
nano-diamond with a built-in nitrogen-vacancy (NV) center to study the dynamics
and the geometric phase of a rotating electron spin without adiabatic
approximation. We find that the transition between the spin levels appears when
the rotating frequency is comparable to the spin frequency at zero magnetic
field. Then we use Floquet theory to numerically solve the spin energy
spectrum, study the spin dynamics and calculate the geometric phase under a
finite magnetic field, where the rotating frequency to fulfill the resonant
transition condition could be greatly reduced.",1811.01641v1
2018-12-05,Single spin sensing of domain wall structure and dynamics in a thin film skyrmion host,"Skyrmions are nanoscale magnetic structures with features promising for
future low-power memory or logic devices. In this work, we demonstrate novel
scanning techniques based on nitrogen vacancy center magnetometry that
simultaneously probe both the magnetic dynamics and structure of room
temperature skyrmion bubbles in a thin film system Ta/CoFeB/MgO. We confirm the
handedness of the Dzyaloshinskii-Moriya interaction in this material and
extract the helicity angle of the skyrmion bubbles. Our measurements also show
that the skyrmion bubbles in this material change size in discrete steps,
dependent on the local pinning environment, with their average size determined
dynamically as their domain walls hop between pinning sites. In addition, an
increase in magnetic field noise is observed near all skyrmion bubble domain
walls. These measurements highlight the importance of interactions between
internal degrees of freedom of skyrmion bubble domain walls and pinning sites
in thin film systems. Our observations have relevance for future devices based
on skyrmion bubbles where pinning interactions will determine important aspects
of current-driven motion.",1812.01764v1
2019-02-01,Collective magnetization dynamics in nano-arrays of thin FePd discs,"We report on the magnetization dynamics of a square array of mesoscopic
discs, fabricated from an iron palladium alloy film. The dynamics properties
were explored using ferromagnetic resonance measurements and micromagnetic
simulations. The obtained spectra exhibit features resulting from the
interactions between the discs, with a clear dependence on both temperature and
the direction of the externally applied field. We demonstrate a qualitative
agreement between the measured and calculated spectra. Furthermore, we
calculated the mode profiles of the standing spin waves excited during a
time-dependent magnetic field excitations. The resulting maps confirm that the
features appearing in the ferromagnetic resonance absorption spectra originate
from the temperature and directional dependent inter-disc interactions.",1902.00403v1
2019-04-02,Scale-invariant spin dynamics and the quantum limits of field sensing,"We describe quantum limits to field sensing that relate noise, geometry and
measurement duration to fundamental constants, with no reference to particle
number. We cast the Tesche and Clarke (TC) bound on dc-SQUID sensitivity as
such a limit, and find analogous limits for volumetric spin-precession
magnetometers. We describe how randomly-arrayed spins, coupled to an external
magnetic field of interest and to each other by the magnetic dipole-dipole
interaction, execute a spin dynamics that depolarizes the spin ensemble even in
the absence of coupling to an external reservoir. We show the resulting spin
dynamics are scale invariant, with a depolarization rate proportional to spin
number density and thus a number-independent quantum limit on the energy
resolution per bandwidth $E_R$. Numerically, we find $E_R \ge \alpha \hbar$,
$\alpha \sim 1$, in agreement with the TC limit, for paradigmatic spin-based
measurements of static and oscillating magnetic fields.",1904.01528v2
2019-04-17,Controlling Dipolar Exchange Interactions in a Dense 3D Array of Large Spin Fermions,"Dipolar interactions are ubiquitous in nature and rule the behavior of a
broad range of systems spanning from energy transfer in biological systems to
quantum magnetism. Here, we study magnetization-conserving dipolar induced
spin-exchange dynamics in dense arrays of fermionic erbium atoms confined in a
deep three-dimensional lattice. Harnessing the special atomic properties of
erbium, we demonstrate control over the spin dynamics by tuning the dipole
orientation and changing the initial spin state within the large 20 spin
hyperfine manifold. Furthermore, we demonstrate the capability to quickly turn
on and off the dipolar exchange dynamics via optical control. The experimental
observations are in excellent quantitative agreement with numerical
calculations based on discrete phase-space methods, which capture entanglement
and beyond-mean field effects. Our experiment sets the stage for future
explorations of rich magnetic behaviors in long-range interacting dipoles,
including exotic phases of matter and applications for quantum information
processing.",1904.08262v1
2019-05-20,Spin dynamics of hot excitons in diluted magnetic semiconductors with spin-orbit interaction,"We explore the impact of a Rashba-type spin-orbit interaction in the
conduction band on the spin dynamics of hot excitons in diluted magnetic
semiconductor quantum wells. In materials with strong spin-orbit coupling, we
identify parameter regimes where spin-orbit effects greatly accelerate the spin
decay and even change the dynamics qualitatively in the form of damped
oscillations. Furthermore, we show that the application of a small external
magnetic field can be used to either mitigate the influence of spin-orbit
coupling or entirely remove its effects for fields above a material-dependent
threshold.",1905.07947v3
2019-05-27,Phase Diagram and Quench Dynamics in a Spinful Interacting Kitaev Chain,"We consider an exact solvable interacting spinful Kitaev chain which is a
generalization of the Mattis-Nam model. A nearest-neighbor dimerized
interaction favoring the production of disjoint molecules drives the quantum
phase into an insulating one. The phases are characterized statically and
dynamically in terms of magnetization and spin-singlet correlations by using
the exact solution. The model is shown to be exactly solvable also in the
presence of boundary interactions which are originated from a spin-singlet
superconducting pairing and a magnetic field. We exploit the exact solution to
investigate the out-of-equilibrium dynamics as due to a quench at the boundary.
The propagation of the disturbance in nearest-neighbor magnetic and
spin-singlet pairing displays a ballistic behavior for long times with
different velocities.",1905.11090v3
2019-06-24,Weak focusing low emittance storage ring with large 6d dynamic aperture based on canted cosine theta magnet technology,"We developed a low emittance electron storage ring with large 6D dynamic
aperture. Contrary to the traditional approach using strong focusing magnetic
cells with optimized (and large) horizontal phase advance, which yields huge
natural chromaticity, we employed a relatively weak focusing lattice with low
chromaticity per cell and, consequently, wide on- and off-momentum dynamic
aperture. Inevitable for weak focusing emittance growth, we compensated by
slicing the lattice into many short, with small bending angle, elementary
periodic cells. To reduce the size, we superimposed focusing gradient and
chromaticity compensating sextupole components over the dipole field utilizing
superconducting magnets based on the Canted Cosine Theta (CCT) winding
technology. The result is a model lattice with 50 pm horizontal emittance at 3
GeV beam energy, with 400-500 m circumference and large 6D aperture.",1906.09692v1
2019-06-27,Magnetic Bloch Oscillations and domain wall dynamics in a near-Ising ferromagnetic chain,"When charged particles in periodic lattices are subjected to a constant
electric field, they respond by oscillating. Here we demonstrate that the
magnetic analogue of these Bloch oscillations are realised in a one-dimensional
ferromagnetic easy axis chain. In this case, the ""particle"" undergoing
oscillatory motion in the presence of a magnetic field is a domain wall.
Inelastic neutron scattering reveals three distinct components of the low
energy spin-dynamics including a signature Bloch oscillation mode. Using
parameter-free theoretical calculations, we are able to account for all
features in the excitation spectrum, thus providing detailed insights into the
complex dynamics in spin-anisotropic chains.",1906.11554v1
2019-07-01,The importance of thermal gradients on the vortex dynamics and magnetic behavior of mesoscopic superconducting samples,"Usually, the measurements of electronic and magnetic properties of
superconducting samples are carried out under a constant temperature bath. On
the other hand, thermal gradients induce local variation of the superconducting
order parameter, and the vortex dynamics can present interesting behaviors. In
this work, we solved the time-dependent Ginzburg-Landau equations simulating
samples under two different thermal gradients, and considering two values of
the Ginzburg-Landau parameter, \k{appa}. We find out that both parameters,
i.e., \k{appa} and thermal gradients, play an important role on the vortex
dynamics and on the magnetization behavior of the samples.",1907.01088v1
2019-07-25,Order out of a Coulomb phase and Higgs transtion: frustrated transverse interactions of Nd2Zr2O7,"The pyrochlore material Nd$_2$Zr$_2$O$_7$ with an ""all-in-all-out"" (AIAO)
magnetic order shows novel quantum moment fragmentation with gapped flat
dynamical spin ice modes. The parameterized spin Hamiltonian with a dominant
frustrated ferromagnetic transverse term reveals a proximity to a U(1) spin
liquid. Here we study magnetic excitations of Nd$_2$Zr$_2$O$_7$ above the
ordering temperature ($T_\text{N}$) using high-energy-resolution inelastic
neutron scattering. We find strong spin ice correlations at zero energy with
the disappearance of gapped magnon excitations of the AIAO order. It seems that
the gap to the dynamical spin ice closes above $T_\text{N}$ and the system
enters a quantum spin ice state competing with and suppressing the AIAO order.
Classical Monte Carlo, molecular dynamics and quantum boson calculations
support the existence of a Coulombic phase above $T_\text{N}$. Our findings
relate the magnetic ordering of Nd$_2$Zr$_2$O$_7$ with the Higgs mechanism and
provide explanations for several previously reported experimental features.",1907.11056v1
2019-09-04,Vortex phases and glassy dynamics in the highly anisotropic superconductor HgBa$_2$CuO$_{4+δ}$,"We present an extensive study of vortex dynamics in a high-quality single
crystal of HgBa$_2$CuO$_{4+\delta}$ (Hg1201), a highly anisotropic
superconductor that is a model system for studying the effects of anisotropy.
From magnetization $M$ measurements over a wide range of temperatures $T$ and
fields $H$, we construct a detailed vortex phase diagram. We find that the
temperature-dependent vortex penetration field $H_p(T)$, second magnetization
peak $H_{smp}(T)$, and irreversibility field $H_{irr}(T)$ all decay
exponentially at low temperatures and exhibit an abrupt change in behavior at
high temperatures $T/T_c \gtrsim 0.5$. By measuring the rates of thermally
activated vortex motion (creep) $S(T,H)=|d \ln M(T,H) / d \ln t|$, we reveal
glassy behavior involving collective creep of bundles of 2D pancake vortices as
well as temperature- and time-tuned crossovers from elastic (collective)
dynamics to plastic flow. Based on the creep results, we show that the second
magnetization peak coincides with the elastic-to-plastic crossover at low $T$,
yet the mechanism changes at higher temperatures.",1909.01618v1
2019-10-25,Nutation Wave as a Platform for Ultrafast Spin Dynamics in Ferromagnets,"At short time scales the inertia term becomes relevant for the magnetization
dynamics of ferromagnets and leads to nutation for the magnetization vector.
For the case of spatially extended magnetic systems, for instance Heisenberg
spin chains with isotropic spin-exchange interaction, this leads to the
appearance of a novel collective excitation, the ""nutation wave"", whose
properties are elucidated by analytic arguments and numerical studies. The
one--particle excitations can be identified as relativistic massive particles.
These particles, the ""nutatons"", acquire their mass via the Brout-Englert-Higgs
mechanism, through the interaction of the wave with an emergent topological
gauge field. This spin excitation would appear as a peak in the spectrum of the
scattering structure factor in inelastic neutron scattering experiments. The
high frequency and speed of the nutation wave can open new paths for realizing
ultrafast spin dynamics.",1910.11897v3
2019-11-05,Engineering the dynamics of topological spin textures by anisotropic spin-orbit torques,"Integrating topologically stabilized magnetic textures such as skyrmions as
nanoscale information carriers into future technologies requires the reliable
control by electric currents. Here, we uncover that the relevant skyrmion Hall
effect, which describes the deflection of moving skyrmions from the current
flow direction, acquires important corrections owing to anisotropic spin-orbit
torques that alter the dynamics of topological spin structures. Thereby, we
propose a viable means for manipulating the current-induced motion of skyrmions
and antiskyrmions. Based on these insights, we demonstrate by first-principles
calculations and symmetry arguments that the motion of spin textures can be
tailored by materials design in magnetic multilayers of Ir/Co/Pt and Au/Co/Pt.
Our work advances the understanding of the current-induced dynamics of these
magnetic textures, which underlies a plethora of memory and logic applications.",1911.01987v1
2019-11-30,Microwave dynamics of pure and doped anisotropic S=1 chain antiferromagnet NiCl2-4SC(NH2)2,"We studied electron spin resonance in a quantum magnet NiCl2-4SC(NH2)2,
demonstrating a field-induced quantum phase transition from a
quantum-disordered phase to an antiferromagnet. We observe two branches of the
antiferromagnetic resonance of the ordered phase, one of them has a gap and the
other is a Goldstone mode with zero frequency at a magnetic field along the
four-fold axis. This zero frequency mode acquires a gap at a small tilting of
the magnetic field with respect to this direction. The upper gap was found to
be reduced in the doped compound Ni(Cl(1-x)Br(x))2-4SC(NH2)2 with $x=0.21$.
This reduction is unexpected because of the previously reported rise of the
main exchange constant in a doped compound. Further, a nonresonant diamagnetic
susceptibility $\chi^{\prime}$ was found for the ordered phase in a wide
frequency range above the quasi-Goldstone mode. This dynamic diamagnetism is as
large as the dynamic susceptibility of the paramagnetic resonance. We speculate
that it originates from a two-magnon absorption band of low-frequency
dispersive magnon branch.",1912.00172v1
2020-03-10,Global Evolution of Solar Magnetic Fields and Prediction of Activity Cycles,"Prediction of solar activity cycles is challenging because physical processes
inside the Sun involve a broad range of multiscale dynamics that no model can
reproduce and because the available observations are highly limited and cover
mostly surface layers. Helioseismology makes it possible to probe solar
dynamics in the convective zone, but variations in differential rotation and
meridional circulation are currently available for only two solar activity
cycles. It has been demonstrated that sunspot observations, which cover over
400 years, can be used to calibrate the Parker-Kleeorin-Ruzmaikin dynamo model,
and that the Ensemble Kalman Filter (EnKF) method can be used to link the
modeled magnetic fields to sunspot observations and make reliable predictions
of a following activity cycle. However, for more accurate predictions, it is
necessary to use actual observations of the solar magnetic fields, which are
available only for the last four solar cycles. In this paper I briefly discuss
the influence of the limited number of available observations on the accuracy
of EnKF estimates of solar cycle parameters, the criteria to evaluate the
predictions, and application of synoptic magnetograms to the prediction of
solar activity.",2003.04563v1
2020-07-15,Effect of pseudospin polarization on wave packet dynamics in graphene antidot lattices (GALs) in the presence of a normal magnetic field,"We have investigated the role of pseudospin polarization in electron wave
packet dynamics in pristine graphene and in a graphene antidot lattice subject
to an external magnetic field. Employing a Green's function formalism, we show
that the electron dynamics can be controlled by tuning pseudospin polarization.
We find that in Landau quantized pristine graphene both the propagation of an
electron wave packet and Zitterbewegung oscillations strongly depend on
pseudospin polarization. The electron wave packet is found to propagate in the
direction of initial pseudospin polarization. We also show that, in this
system, the propagation of an electron can be enhanced in any desired direction
by carving a one dimensional antidot lattice in that direction. The study
suggests that a graphene antidot lattice can serve as a channel for electron
transport with the possibility of tunability by means of pseudospin
polarization, antidot potential and applied normal magnetic field strength.",2007.07836v2
2020-08-28,Optical Magnetic Lens: towards actively tunable terahertz optics,"As we read this text, our eyes dynamically adjust the focal length to keep
the line image in focus on the retina. Similarly, in many optics applications
the focal length must be dynamically tunable. In the quest for compactness and
tunability, flat lenses based on metasurfaces were introduced. However, their
dynamic tunability is still limited because their functionality mostly relies
upon fixed geometry. In contrast, we put forward an original concept of a
tunable Optical Magnetic Lens (OML) that focuses photon beams using a
subwavelength-thin layer of a magneto-optical material in a non-uniform
magnetic field. We applied the OML concept to a wide range of materials and
found out that the effect of OML is present in a broad frequency range from
microwaves to visible light. For terahertz light, OML can allow 50% relative
tunability of the focal length on the picosecond time scale, which is of
practical interest for ultrafast shaping of electron beams in microscopy. The
OML based on magneto-optical natural bulk and 2D materials may find broad use
in technologies such as 3D optical microscopy and acceleration of charged
particle beams by THz beams.",2008.12640v1
2020-10-11,Beyond the gyrotropic motion: dynamic C-state in vortex spin torque oscillators,"In the present study, we investigate a dynamical mode beyond the gyrotropic
(G) motion of a magnetic vortex core in a confined magnetic disk of a
nano-pillar spin torque nano oscillator. It is characterized by the in-plane
circular precession associated to a C-shaped magnetization distribution. We
show a transition between G and C-state mode which is found to be purely
stochastic in a current-controllable range. Supporting our experimental
findings with micromagnetic simulations, we believe that the results provide
novel opportunities for the dynamic and stochastic control of STOs, which could
be interesting to be implemented for example in neuromorphic networks.",2010.06437v1
2021-01-17,Dynamics of self-accelerating electron beams in a homogeneous magnetic field,"We examine the dynamics of electron beams that, in free space, are
self-accelerating, in the presence of an additional magnetic field. We focus
our attention in the case of Airy beams that follow parabolic trajectories and
in generalized classes of beams associated with power-law trajectories. We
study the interplay between beam self-acceleration and the circular motion
caused by the magnetic field. In the case of Airy beams, using an integral
representation, we find closed-form solutions for the electron wavefunction. We
also derive asymptotic formulas for the beam trajectories both for Airy beams
and for self-accelerating power-law beams. A ray optics description is rather
useful for the interpretation of the beam dynamics. Our results are in
excellent comparison with direct numerical simulations.",2101.06754v1
2021-02-28,Motion of classical charged particles with magnetic moment in external plane-wave electromagnetic fields,"We study the motion of a charged particle with magnetic moment in external
electromagnetic fields utilizing covariant unification of Gilbertian and
Amperian descriptions of particle magnetic dipole moment. Considering the case
of a current loop, our approach is verified by comparing classical dynamics
with the classical limit of relativistic quantum dynamics. We obtain motion of
a charged particle in the presence of an external linearly polarized EM (laser)
plane wave field incorporating the effect of spin dynamics. For specific
laser-particle initial configurations, we determine that the Stern-Gerlach
force can have a cumulative effect on the trajectory of charged particles.",2103.02594v2
2021-03-27,Zeeman term for the Néel vector in a two sublattice antiferromagnet using Dzyaloshinsky-Moriya interaction and magnetic field,"We theoretically investigate the dynamics of solitons in two sublattice
antiferromagnets under external perturbations, focusing on the effect of
Dzyaloshinsky-Moriya (DM) interactions. To this end, we construct a
micromagnetic field theory for the antiferromagnet in the presence of the
external magnetic field, DM interaction, and spin-transfer torque. In
particular, we show external magnetic field and spin current couple to N\'eel
vector in a Zeeman-like manner when DM interactions present, which can be used
to efficiently drive antiferromagnetic solitons of different dimensions.
Besides, we study the effect of straining the local lattice. It can serve as an
external handle on the N\'eel field inertia and thus dynamical properties. Our
findings may find applications in antiferromagnetic spintronics.",2103.14982v2
2021-05-05,Spin dynamical decoupling for generating macroscopic superpositions of a free-falling nanodiamond,"Levitated nanodiamonds containing negatively charged nitrogen-vacancy centers
(${\text{NV}}^{-}$) have been proposed as a platform to generate macroscopic
spatial superpositions. Requirements for this include having a long
${\text{NV}}^{-}$ spin coherence time, which necessitates formulating a
dynamical decoupling strategy in which the regular spin flips do not cancel the
growth of the superposition through the Stern-Gerlach effect in an
inhomogeneous magnetic field. Here, we propose a scheme to place a
$250$-nm-diameter diamond in a superposition with spatial separation of over
$250$ nm, while incorporating dynamical decoupling. We achieve this by letting
a diamond fall for $2.4$ m through a magnetic structure, including $1.13$ m in
an inhomogeneous region generated by magnetic teeth.",2105.02105v3
2021-05-31,"Dynamic polarization of electron spins in indirect band gap (In,Al)As/AlAs quantum dots in weak magnetic field: experiment and theory","A novel spin orientation mechanism - dynamic electron spin polarization has
been recently suggested in Phys. Rev. Lett. $\mathbf{125}$, 156801 (2020). It
takes place for unpolarized optical excitation in weak magnetic fields of the
order of a few millitesla. In this paper we demonstrate experimentally and
theoretically that the dynamic electron spin polarization degree changes sign
as a function of time, strength of the applied magnetic field and its
direction. The studies are performed on indirect band-gap (In,Al)As/AlAs
quantum dots and their results are explained in the framework of a theoretical
model developed for our experimental setting.",2106.00044v1
2021-07-02,Dynamic impurities in two-dimensional topological insulator-edge states,"Helical edge states of two-dimensional topological insulators show a gap in
the density of states (DOS) and suppressed conductance in the presence of
ordered magnetic impurities. Here we will consider the dynamical effects on the
DOS and transmission when the magnetic impurities are driven periodically.
Using the Floquet formalism and Green's functions, the system properties are
studied as a function of the driving frequency and the potential energy
contribution of the impurities. We see that increasing the potential part
closes the DOS gap for all driving regimes. The transmission gap is also
closed, showing a pronounced asymmetry as a function of energy. These features
indicate that the dynamical transport properties could yield valuable
information about the magnetic impurities.",2107.00994v2
2021-10-01,Thermalization of nuclear spins in lanthanide molecular magnets,"Single molecule magnets distinguish themselves in the field of quantum
magnetism through the ability to combine fundamental research with promising
applications, the evolution of quantum spintronics in the last decade
exemplifying the potential held by molecular based quantum devices. Notably,
the read-out and manipulation of the embedded nuclear spin states was used in
proof of principle studies of quantum computation at the single molecule level.
In this paper we study the relaxation dynamics of the $^{159}$Tb nuclear spins
in a diluted molecular crystal by using recently acquired understanding of the
nonadiabatic dynamics of TbPc$_2$ molecules. We find that phonon modulated
hyperfine interaction opens a direct relaxation channel between the nuclear
spins and the phonon bath. We highlight the potential importance of the
discovered mechanism for the theory of spin bath and the relaxation dynamics of
the molecular spins at crossover temperatures.",2110.00161v1
2021-12-12,Rodlike Heisenberg nanomagnet driven by propagating magnetic field: Nonequilibrium phase transition,"The dynamical responses of a rodlike anisotropic Heisenberg ferromagnet,
irradiated by the propagating magnetic field wave, have been studied by Monte
Carlo simulation using Metropolis single spin-flip algorithm. A nonequilibrium
dynamical phase transition has been observed. The transition temperature has
been obtained from the peak position of the variance of the dynamic order
parameter plotted as a function of the temperature. The transition has been
found to occur at a lower temperature for the higher value of the amplitude of
the propagating magnetic field wave. A comprehensive phase boundary was drawn.
The phase boundary has been found to be insensitive to the wavelength of the
propagating field wave. The boundary encloses more area of the region of the
ordered phase for stronger anisotropy.",2112.06138v2
2022-01-22,Microwave-free dynamic nuclear polarization via sudden thermal jumps,"Dynamic Nuclear Polarization (DNP) presently stands as the preferred strategy
to enhance the sensitivity of nuclear magnetic resonance measurements, but its
application relies on the use of high-frequency microwave to manipulate
electron spins, an increasingly demanding task as the applied magnetic field
grows. Here we investigate the dynamics of a system hosting a polarizing agent
formed by two distinct paramagnetic centers near a level anti-crossing. We
theoretically show that nuclear spins polarize efficiently under a cyclic
protocol that combines alternating thermal jumps and radio-frequency pulses
connecting hybrid states with opposite nuclear and electronic spin alignment.
Central to this process is the difference between the spin-lattice relaxation
times of either electron spin species, transiently driving the electronic spin
bath out of equilibrium after each thermal jump. Without the need for microwave
excitation, this route to enhanced nuclear polarization may prove convenient,
particularly if the polarizing agent is designed to feature electronic level
anti-crossings at high magnetic fields.",2201.09092v1
2022-02-06,Magnetic monopole relaxation effects probed by modulation calorimetry in small spin-ice samples,"We use modulation calorimetry to study the heat capacity of small samples (30
nano gram-10 micro gram) of the classical spin-ice compounds Dy2Ti2O7 and
Ho2Ti2O7 at low temperature (0.5-3 K). Using modulation frequencies of 0.1-200
Hz we find a strong frequency dependence in the measured heat capacity and are
able to study thermal relaxation effects on the corresponding timescales.
Performing dynamic Monte Carlo simulations we verify that the specific heat
frequency response has its origin in the slow magnetic monopole dynamics
indigenous to spin ice. We find a timescale of 20ms per Monte Carlo step at 4K
in contrast to 2.5ms mentioned in previous studies by other 1 techniques. Our
study establishes modulation calorimetry as a versatile experimental probe of
dynamic processes in frustrated magnetic materials.",2202.02801v1
2022-03-21,Advances and Challenges in Observations and Modeling of the Global-Sun Dynamics and Dynamo,"Computational heliophysics has shed light on the fundamental physical
processes inside the Sun, such as the differential rotation, meridional
circulation, and dynamo-generation of magnetic fields. However, despite the
substantial advances, the current results of 3D MHD simulations are still far
from reproducing helioseismic inferences and surface observations. The reason
is the multi-scale nature of the solar dynamics, covering a vast range of
scales, which cannot be solved with the current computational resources. In
such a situation, significant progress has been achieved by the mean-field
approach, based on the separation of small-scale turbulence and large-scale
dynamics. The mean-field simulations can reproduce solar observations,
qualitatively and quantitatively, and uncover new phenomena. However, they do
not reveal the complex physics of large-scale convection, solar magnetic
cycles, and the magnetic self-organization that causes sunspots and solar
eruptions. Thus, developing a synergy of these approaches seems to be a
necessary but very challenging task.",2203.10721v1
2022-05-15,Phase coexistence and associated non-equilibrium dynamics under simultaneously applied magnetic field and pressure,"A quantitative estimation of the effect of simultaneously applied external
pressure (P) and magnetic field (H) on the phase coexistence has been presented
for Pr0.5Ca0.5Mn0.975Al0.025O3 and La0.5Ca0.5MnO3, where the ferromagnetic
(FM)-metal and antiferromagnetic (AFM)-insulator phases compete in real space.
We found that the nonequilibrium dynamics across the FM-AFM transition is
primarily dictated by the effect of P and H on the supercooling, superheating
temperatures, and the nucleation and growth rate of the equilibrium phase.
These effects across the transition is also responsible for the relative volume
fraction of the competing phases at low temperature. Importantly in the entire
magnetic field-pressure-temperature range of phase coexistence, the interface
between the two competing phases having different spin and structural order
plays a very important role in controlling the non-equilibrium dynamics.",2205.07192v2
2022-05-27,Quantum Simulation of the Two-Dimensional Weyl Equation in a Magnetic Field,"Quantum simulation of 1D relativistic quantum mechanics has been achieved in
well-controlled systems like trapped ions, but properties like spin dynamics
and response to external magnetic fields that appear only in higher dimensions
remain unexplored. Here we simulate the dynamics of a 2D Weyl particle. We show
the linear dispersion relation of the free particle and the discrete Landau
levels in a magnetic field, and we explicitly measure the spatial and spin
dynamics from which the conservation of helicity and properties of
antiparticles can be verified. Our work extends the application of an ion trap
quantum simulator in particle physics with the additional spatial and spin
degrees of freedom.",2205.14038v1
2022-06-09,Reconfigurable Reservoir Computing in a Magnetic Metamaterial,"In-materia reservoir computing (RC) leverages the intrinsic physical
responses of functional materials to perform complex computational tasks.
Magnetic metamaterials are exciting candidates for RC due to their huge state
space, nonlinear emergent dynamics, and non-volatile memory. However, to be
suitable for a broad range of tasks, the material system is required to exhibit
a broad range of properties, and isolating these behaviours experimentally can
often prove difficult. By using an electrically accessible device consisting of
an array of interconnected magnetic nanorings -- a system shown to exhibit
complex emergent dynamics -- here we show how reconfiguring the reservoir
architecture allows exploitation of different aspects the system's dynamical
behaviours. This is evidenced through state-of-the-art performance in diverse
benchmark tasks with very different computational requirements, highlighting
the additional computational configurability that can be obtained by altering
the input/output architecture around the material system.",2206.04446v3
2022-08-02,Entanglement dynamics and phase transitions of the Floquet cluster spin chain,"Cluster states were introduced in the context of measurement based quantum
computing. In one dimension, the cluster Hamiltonian possesses topologically
protected states. We investigate the Floquet dynamics of the cluster spin chain
in an external field, interacting with a particle. We explore the entanglement
properties of the topological and magnetic phases, first in the integrable spin
lattice case, and then in the interacting quantum walk case. We find, in
addition to thermalization, dynamical phase transitions separating low- and
high-entanglement nonthermal states, reminiscent of the ones present in the
integrable case, but differing in their magnetic properties. The nonergodic
phases are characterized by the emergence of magnetic order, persistent at long
times.",2208.01706v2
2022-10-01,Nonlinear features of the superconductor--ferromagnet--superconductor $\varphi_0$ Josephson junction in ferromagnetic resonance region,"We demonstrate the manifestations of the nonlinear features in magnetic
dynamics and IV-characteristics of the $\varphi_0$ Josephson junction in the
ferromagnetic resonance region. We show that at small values of system
parameters, namely, damping, spin-orbit interaction, and Josephson to magnetic
energy ratio, the magnetic dynamics is reduced to the dynamics of the scalar
Duffing oscillator, driven by the Josephson oscillations. The role of
increasing superconducting current in the resonance region is clarified.
Shifting of the ferromagnetic resonant frequency and the reversal of its
damping dependence due to nonlinearity are demonstrated by the full
Landau-Lifshitz-Gilbert-Josephson system of equations, and in its different
approximations. Finally, we demonstrate the negative differential resistance in
the IV--characteristics, and its correlation with the foldover effect.",2210.00366v1
2022-10-21,Emergent many-body composite excitations of interacting spin-1/2 trimers,"Understanding exotic forms of magnetism in quantum spin systems is an
emergent topic of modern condensed matter physics. Quantum dynamics can be
described by particle-like carriers of information, known-as quasiparticles
that appear from the collective behaviour of the underlying system. Spinon
excitations, governing the excitations of quantum spin-systems, have been
accurately calculated and precisely verified experimentally for the
antiferromagnetic chain model. However, identification and characterization of
novel quasiparticles emerging from the topological excitations of the spin
system having periodic exchange interactions are yet to be obtained. Here, we
report the identification of emergent composite excitations of the novel
quasiparticles doublons and quartons in spin-1/2 trimer-chain antiferromagnet
Na2Cu3Ge4O12 (having periodic intrachain exchange interactions J1-J1-J2) and
its topologically protected quantum 1/3 magnetization-plateau state. The
characteristic energies, dispersion relations, and dynamical structure factor
of neutron scattering as well as macroscopic quantum 1/3 magnetization-plateau
state are in good agreement with the state-of-the-art dynamical density matrix
renormalization group calculations.",2210.11765v1
2022-11-02,Pump--probe x-ray microscopy of photo-induced magnetization dynamics at MHz repetition rates,"We present time-resolved scanning x-ray microscopy measurements with
picosecond photo-excitation via a tailored infrared pump laser at a scanning
transmission x-ray microscope. Specifically, we image the laser-induced
demagnetization and remagnetization of thin ferrimagnetic GdFe films proceeding
on a few nanoseconds time scale. Controlling the heat load on the sample via
additional reflector and heat-sink layers allows us to conduct destruction-free
measurements at a repetition rate of \SI{50}{\mega\hertz}. Near-field
enhancement of the photo-excitation and controlled annealing effects lead to
laterally heterogeneous magnetization dynamics which we trace with
\SI{30}{\nano\meter} spatial resolution. Our work opens new opportunities to
study photo-induced dynamics on the nanometer scale, with access to picosecond
to nanosecond timescales, which is of technological relevance, especially in
the field of magnetism.",2211.01230v2
2023-01-01,Spin Hall Induced Magnetization Dynamics in Multiferroic Tunnel Junction,"The combination of spin-orbit coupling driven effects and multiferroic
tunneling properties was explored experimentally in thin Pt/Co/BTO/LSMO
multilayers. The presence of a Pt heavy metal allows for the spin
current-induced magnetization precession of Co upon radio-frequency charge
current injection. The utilization of a BTO ferroelectric tunnel barrier
separating the Co and LSMO ferromagnetic electrodes gives rise to both
tunneling-magnetoresistance and electroresistance. Using the spin-orbit torque
ferromagnetic resonance, the maganetization dynamics of the Co/Pt bilayers was
studied at room temperature. Unexpectedly the magnetization dynamics study in
the same geometry performed at low temperature reveals the existence of both Co
and LSMO resonance peaks indicating efficient spin current generation both
using the spin Hall effect in Pt and spin pumping in LSMO that tunnel via the
BTO barrier.",2301.00459v1
2023-03-10,Dynamic Exchange Coupling between Magnets Mediated by Attenuating Elastic Waves,"Coupling between spatially separated magnets can be mediated by excitations
such as photons and phonons, which can be characterized as coherent coupling
and dissipative coupling with real and imaginary coupling rate. We
theoretically predict the existence of dynamic exchange coupling in a closed
magneto-elastic system mediated by attenuating elastic waves and whose coupling
rate is complex in general, leading to alternating repulsive or attractive
spectrum depending on thickness of the elastic media. The presence of dynamic
exchange coupling and its competition with coherent coupling are numerically
verified according to the generalized Hooke's law in magneto-elastic systems.
The predicted mechanism provides a new strategy to synchronize precessing
magnets as well as other excitations over long distance and pave the way for
non-Hermitian engineering of collective modes in hybrid magnonics, phononics
and photonics.",2303.05778v1
2023-03-10,Ultrafast magnetization reversal in ferromagnetic spin-valves: an s-d model perspective,"We present an extension to simple s-d models, aiming at simulating ultrafast
magnetization dynamics and spin transport in metallic heterostructures. In
particular, we consider an alternative spin dissipation channel due to a finite
exchange splitting of the s band. From this theory, we show three different
mechanisms governing the dynamics of spin accumulation. On top of the already
widely discussed ""-dM/dt"" electron-magnon mechanism, we study the role of a
dynamic change of exchange splitting (of conduction electrons) as well as the
rotation of spins reflected at an interface with a ferromagnet. Finally, we use
the presented theory to explain the recent observation of subpicosecond
reversal of a ferromagnet in rare-earth free spin-valves. Our conclusion agrees
with the one of reference [1] favoring magnetization reversal due to the
rotation of the spin polarization of a reflected spin current.",2303.05907v1
2023-03-16,Ultrafast dynamics of electrons excited by femtosecond laser pulses: spin polarization and spin-polarized currents,"Laser radiation incident on a ferromagnetic sample produces excited electrons
and currents whose spin polarization must not be aligned with the magnetization
-- an effect due to spin-orbit coupling that is ubiquitous in spin- and
angle-resolved photoemission. In this Paper, we report on a systematic
investigation of the dynamics of spin polarization and spin-polarized currents
produced by femtosecond laser pulses, modeled within our theoretical framework
EVOLVE. The spin polarization depends strongly on the properties of the laser
pulse and on the sample composition, as is shown by comparing results for
Cu(100), Co(100), and a Co/Cu heterostructure. We find a transition from
coherence before the laser pulse's maximum to incoherence thereafter. Moreover,
the time dependence of the spin-polarization components induced by spin-orbit
coupling differ significantly in Cu and Co: in Cu, we find long-period
oscillations with tiny rapid modulations, whereas in Co prominent rapid
oscillations with long period ones are superimposed. The pronounced spatial
dependencies of the signals underline the importance of inhomogeneities, in
particular magnetic/non-magnetic interfaces `act as source' for ultrafast
spin-polarization effects. Our investigation provides detailed insight into
electron dynamics during and shortly after a femtosecond laser excitation.",2303.09291v1
2023-03-25,Muon spin relaxation and emergence of disorder-induced unconventional dynamic magnetic fluctuations in Dy$_{2}$Zr$_{2}$O$_{7}$,"The disordered pyrochlore oxide Dy$_{2}$Zr$_{2}$O$_{7}$ shows the signatures
of field-induced spin freezing with remnant zero-point spin-ice entropy at 5
kOe magnetic field. We have performed zero-field and longitudinal field Muon
spin relaxation ($\mu$SR) studies on Dy$_{2}$Zr$_{2}$O$_{7}$. Our zero field
studies reveal the absence of both long-range ordering and spin freezing down
to 62 mK. The $\mu$SR relaxation rate exhibits a temperature-independent
plateau below 4 K, indicating a dynamic ground state of fluctuating spins
similar to the well-known spin ice system Dy$_{2}$Ti$_{2}$O$_{7}$. The
low-temperature spin fluctuations persist in the longitudinal field of 20 kOe
as well and show unusual field dependence of the relaxation rate, which is
uncommon for a spin-liquid system. Our results, combined with the previous
studies do not show any evidence of spin ice or spin glass ground state, rather
point to a disorder-induced dynamic magnetic ground state in the
Dy$_{2}$Zr$_{2}$O$_{7}$ material.",2303.14426v1
2023-03-29,Dynamical criticality of magnetization transfer in integrable spin chains,"Recent studies have found that fluctuations of magnetization transfer in
integrable spin chains violate the central limit property. Here we revisit the
problem of anomalous counting statistics in the Landau-Lifshitz field theory by
specializing to two distinct anomalous regimes featuring a dynamical critical
point. By performing optimized numerical simulations using an integrable
space-time discretization we extract the algebraic growth exponents of
time-dependent cumulants which attain their threshold values. The distinctly
non-Gaussian statistics of magnetization transfer in the easy-axis regime is
found to converge towards the universal distribution of charged single-file
systems. At the isotropic point we infer a weakly non-Gaussian distribution,
corroborating the view that superdiffusive spin transport in integrable spin
chains does not belong to any known dynamical universality class.",2303.16691v5
2023-04-07,A two-color dual-comb system for time-resolved measurements of ultrafast magnetization dynamics using triggerless asynchronous optical sampling,"We report on an Er-doped fiber (EDF)-laser-based dual-comb system that allows
us to perform triggerless asynchronous optical sampling (ASOPS) pump-probe
measurements of ultrafast demagnetization and spin precession in magnetic
materials. Because the oscillation frequencies of the two frequency-comb light
sources are highly stabilized, the pulse-to-pulse timing jitter is sufficiently
suppressed and data accumulation without any trigger signals is possible. To
effectively induce spin precession in ferromagnetic thin films, the spectral
bandwidth of the output of one of the EDF frequency comb sources is broadened
by a highly nonlinear fiber and then amplified at a wavelength of about 1030 nm
by a Yb-doped fiber amplifier. The output of the other frequency comb source is
converted to about 775 nm by second harmonic generation. We used this system to
observe the ultrafast demagnetization and spin precession dynamics on the
picosecond and nanosecond time scales in a permalloy thin film. This
time-domain spectroscopy system is promising for the rapid characterization of
spin-wave generation and propagation dynamics in magnetic materials.",2304.03666v1
2023-05-01,Coherent and incoherent magnons induced by strong ultrafast demagnetization in thin permalloy films,"Understanding spin dynamics on femto- and picosecond timescales offers new
opportunities for faster and more efficient spintronic devices. Here, we
experimentally investigate the coherent spin dynamics after ultrashort laser
excitation by time-resolved magneto optical Kerr effect (TR-MOKE) in thin
Ni80Fe20 films. We provide a detailed study of the magnetic field and pump
fluence dependence of the coherent precessional dynamics. We show that the
coherent precession lifetime increases with the applied external magnetic field
which cannot be understood by viscous Gilbert damping of the coherent magnons.
Instead, it can be explained by nonlinear magnon interactions and by the change
in the fraction of incoherent magnons. This interpretation is in agreement with
the observed trends of the coherent magnon amplitude and lifetime as a function
of the exciting laser fluence. Our results provide a new insight into the
magnetization relaxation processes in ferromagnetic thin films, which is of
great importance for further spintronic applications.",2305.00814v2
2023-06-10,Statistical Analysis of Magnetic Domain Wall Dynamics to Quantify Dzyaloshinskii-Moriya Interaction,"We utilize statistical tools to analyze the magnetic domain wall dynamics in
a nanostrip, which can quantify the magnitude and reveal the effects of
interfacial Dzyaloshinskii-Mariya interaction. We find that there exist two
peaks in the velocity frequency spectrum, the magnitude ratio of which can be
used to determine the DMI strength. Our approach is validated using a
collective-coordinate model, and is demonstrated to be robust against thermal
noise and material impurities. Moreover, third-order cumulant and third-order
time-dependent correlation function of velocity are calculated and yield
valuable information regarding the asymmetry induced by DMI. Our findings offer
novel and efficient analysis tools to understand physical process of domain
wall dynamics under DMI and exotic magnetic phenomena.",2306.06451v1
2023-06-22,Ultrafast Dynamics of Orbital Angular Momentum of Electrons Induced by Femtosecond Laser Pulses: Generation and Transfer Across Interfaces,"The orbital angular momenta (OAM) of electrons play an increasingly important
role in ultrafast electron and magnetization dynamics. In this theoretical
study, we investigate the electron dynamics induced by femtosecond laser pulses
in a normal metal, a ferromagnet, and a ferromagnet/normal metal
heterostructure. We analyze the spatio-temporal distributions of the
laser-induced OAM and their respective currents. Our findings demonstrate that
a circularly polarized laser pulse can induce a sizable and long-lasting OAM
component in a normal metal. Furthermore, an interface between a ferromagnet
and a normal metal facilitates the demagnetization of the magnet by the OAM
contribution to the total magnetization. Finally, to transfer OAM from a
ferromagnet into a normal metal, it is advantageous to use a laser setup that
induces the desired OAM component in the ferromagnet, but not in the normal
metal.",2306.12810v1
2023-07-24,Super narrow peaks in excitation spectrum of alkali spin polarization: non-adiabatic case of spin dynamics,"We theoretically describe the phenomenon of non-adiabatic spin dynamics,
which occurs in a gas cell filled by alkali vapor in presence of a strong
alternating magnetic field and pump light. Steep increase of the spin
polarization occurs if frequency of the magnetic field is equal to the certain
value. Although, the observable effect relies on the periodic field that
consists of two perpendicular components defined by harmonics with the same
amplitudes and different frequencies. Considered spin effect cannot be
explained by a resonance, because the own Larmor frequency of spin precession
is absent without a constant component of magnetic field. Moreover, there are
some clearly visible peaks in the excitation spectrum of spin polarization, and
they are super narrow in comparison to relaxation rate. Detailed analysis
according to proposed quantum model results in the reasoning of the effect via
qualitative properties of non-adiabatic dynamics of atomic spin.",2307.12647v2
2023-07-29,Single-spin spectroscopy of spontaneous and phase-locked spin torque oscillator dynamics,"We employ N-$V$ magnetometry to measure the stray field dynamics of a
ferromagnetic permalloy nanowire driven by spin-orbit torques. Specifically, we
observe the optically detected magnetic resonance (ODMR) signatures of both
spontaneous DC-driven magnetic oscillations and phase-locking to a second
harmonic drive, developing a simple macrospin model that captures the salient
features. We also observe signatures of dynamics beyond the macrospin model,
including an additional ODMR feature (associated with a second SW mode) and one
mode sapping power from another. Our results provide additional insight into
N-$V$-spin wave coupling mechanisms, and represent a new modality for
sub-wavelength N-$V$ scanned probe microscopy of nanoscale magnetic
oscillators.",2307.16049v1
2023-08-21,Three-dimensional Turbulent Reconnection within Solar Flare Current Sheet,"Solar flares can release coronal magnetic energy explosively and may impact
the safety of near-earth space environments. Their structures and properties on
macroscale have been interpreted successfully by the generally-accepted
two-dimension standard model invoking magnetic reconnection theory as the key
energy conversion mechanism. Nevertheless, some momentous dynamical features as
discovered by recent high-resolution observations remain elusive. Here, we
report a self-consistent high-resolution three-dimension magnetohydrodynamical
simulation of turbulent magnetic reconnection within a flare current sheet. It
is found that fragmented current patches of different scales are spontaneously
generated with a well-developed turbulence spectrum at the current sheet, as
well as at the flare loop-top region. The close coupling of tearing-mode and
Kelvin-Helmholtz instabilities plays a critical role in developing turbulent
reconnection and in forming dynamical structures with synthetic observables in
good agreement with realistic observations. The sophisticated modeling makes a
paradigm shift from the traditional to three-dimension turbulent reconnection
model unifying flare dynamical structures of different scales.",2308.10494v1
2023-09-04,Quantum Barkhausen Noise Induced by Domain Wall Co-Tunneling,"Most macroscopic magnetic phenomena (including magnetic hysteresis) are
typically understood classically. Here, we examine the dynamics of a uniaxial
rare-earth ferromagnet deep within the quantum regime, so that domain wall
motion, and the associated hysteresis, is dominated by large-scale quantum
tunneling of spins, rather than classical thermal activation over a potential
barrier. The domain wall motion is found to exhibit avalanche dynamics,
observable as an unusual form of Barkhausen noise. We observe non-critical
behavior in the avalanche dynamics that only can be explained by going beyond
traditional renormalization group methods or classical domain wall models. We
find that this ``quantum Barkhausen noise'' exhibits two distinct mechanisms
for domain wall movement, each of which is quantum-mechanical, but with very
different dependences on an external magnetic field applied transverse to the
spin (Ising) axis. These observations can be understood in terms of the
correlated motion of pairs of domain walls, nucleated by co-tunneling of
plaquettes (sections of domain wall), with plaquette pairs correlated by
dipolar interactions; this correlation is suppressed by the transverse field.
Similar macroscopic correlations may be expected to appear in the hysteresis of
other systems with long-range interactions.",2309.01799v1
2023-09-10,Resonant excitation of vortex gyrotropic mode via surface acoustic waves,"Finding new energy-efficient methods for exciting magnetization dynamics is
one of the key challenges in magnonics. In this work, we present an approach to
excite the gyrotropic dynamics of magnetic vortices through the phenomenon of
inverse magnetostriction, also known as the Villari effect. We develop an
analytical model based on the Thiele formalism that describes the gyrotropic
motion of the vortex core including the energy contributions due to inverse
magnetostriction. Based on this model, we predict excitations of the vortex
core resonances by surface acoustic waves whose frequency is resonant with the
frequency of the vortex core. We verify the model's prediction using
micromagnetic simulations, and show the dependence of the vortex core's
oscillation radius on the surface acoustic wave amplitude and the static bias
field. Our study contributes to the advancement of energy-efficient magnetic
excitations by relying on voltage-induced driven dynamics, which is an
alternative to conventional current-induced excitations.",2309.04987v1
2023-09-18,Nonlinear dynamics and magneto-elasticity of nanodrums near the phase transition,"Nanomechanical resonances of two-dimensional (2D) materials are sensitive
probes for condensed-matter physics, offering new insights into magnetic and
electronic phase transitions. Despite extensive research, the influence of the
spin dynamics near a second-order phase transition on the nonlinear dynamics of
2D membranes has remained largely unexplored. Here, we investigate nonlinear
magneto-mechanical coupling to antiferromagnetic order in suspended
FePS$_3$-based heterostructure membranes. By monitoring the motion of these
membranes as a function of temperature, we observe characteristic features in
both nonlinear stiffness and damping close to the N\'{e}el temperature
$T_{\rm{N}}$. We account for these experimental observations with an analytical
magnetostriction model in which these nonlinearities emerge from a coupling
between mechanical and magnetic oscillations, demonstrating that
magneto-elasticity can lead to nonlinear damping. Our findings thus provide
insights into the thermodynamics and magneto-mechanical energy dissipation
mechanisms in nanomechanical resonators due to the material's phase change and
magnetic order relaxation.",2309.09672v1
2023-09-29,SpinView: General Interactive Visual Analysis Tool for Multiscale Computational Magnetism,"Multiscale magnetic simulations, including micromagnetic and atomistic spin
dynamics simulations, are widely used in the study of complex magnetic systems
over a wide range of spatial and temporal scales. The advances in these
simulation technologies have generated considerable amounts of data. However, a
versatile and general tool for visualization, filtering, and denoising this
data is largely lacking. To overcome these limitations, we have developed
SpinView, a general interactive visual analysis tool for graphical exploration
and data distillation. Combined with dynamic filters and a built-in database,
it is possible to generate reproducible publication-quality images, videos, or
portable interactive webpages within seconds. Since the basic input to SpinView
is a vector field, it can be directly integrated with any spin dynamics
simulation tool. With minimal effort on the part of the user, SpinView delivers
a simplified workflow, speeds up analysis of complex datasets and trajectories,
and enables new types of analysis and insight. SpinView is available from
https://mxjk851.github.io/SpinView/",2309.17367v2
2023-11-14,Magnetic dynamics driven integer and fractional high harmonic generation arising from highly nonlinear instantaneous energy levels,"We demonstrate that the strongly nonlinear regime of the magnetic dynamics
driven high harmonic generation (HHG) can be captured through an adiabatic
treatment. Within this approximation, the instantaneous energy levels reveal a
highly nonlinear term that mirrors the high-frequency excitations present in
non-equilibrium transport quantities. Therefore, we find that the revelation of
nonlinear dynamics in the instantaneous energy levels offers a simplified
understanding of HHG in magnetic systems. Additionally, leveraging the highly
nonlinear nature of these instantaneous bands, we predict the emergence of
tunable integer and fractional high harmonics in the presence of a
time-modulated spin-orbit interaction. Our findings initiate tantalizing
prospects for harnessing HHG in condensed matter systems.",2311.08368v3
2024-01-09,Data-driven scale identification in oscillatory dynamos,"Parker's mean-field model includes two processes generating large-scale
oscillatory dynamo waves: stretching of magnetic field lines by small-scale
helical flows, and by differential rotation. In this work, we investigate the
capacity of data-driven modal analysis, Dynamic Mode Decomposition, to identify
coherent magnetic field structures of this model. In its canonical form, the
only existing field scale corresponds to the dynamo instability. To take into
account multi-scale nature of the dynamo, the model was augmented with coherent
in time flow field, forcing small-scale magnetic field with a faster temporal
evolution. Two clusters of DMD modes were obtained: the ``slow"" cluster,
located near the dynamo wave frequency and associated with its nonlinear
self-interaction, and the ``fast"" cluster, centered around the forcing
frequency and resulting from the interaction between the wave and the flow.
Compared to other widely used methods of data analysis, such as Fourier
transform, DMD provides a natural spatiotemporal basis for the dynamo, related
to its nonlinear dynamics. We assess how the parameters of the DMD model, rank
and delay, influence its accuracy, and finally discuss the limitations of this
approach when applied to randomly forced, more complex dynamo flows.",2401.04505v1
2024-03-30,Chaotic dynamics under the influence of synthetic magnetic field in optomechanical system,"The optomechanical systems produce chaotic behaviour due to nonlinear
interaction between photons and phonons, and the same systems are used to
understand the synthetic fields as well. Here, we report on the study of
chaotic behaviour in the presence of a phononic synthetic magnetic field in a
closed loop configuration consisting of a single optical mode and two
mechanical modes. The modulation phase of the mechanical coupling between the
two mechanical modes plays a critical role in determining the mechanical and
optical intensity dynamics in the nonlinear regime. Our study shows the dark
mode breaking effect in the presence of a synthetic magnetic field, which
brings about a complex way of mechanical energy exchange that causes the cavity
field to alternate between chaotic and regular behaviour periodically in
temporal domain. However in the stronger nonlinear regime the temporal dynamics
demonstrate predominantly chaotic behaviour. Besides, with the advent of
advanced fabrication technologies, this study holds promises in developing
phase tunable integrated low-power chaotic light sources to support efficient
optical secure communication systems.",2404.00472v2
2017-03-22,Fast domain wall motion induced by antiferromagnetic spin dynamics at the angular momentum compensation temperature of ferrimagnets,"Antiferromagnetic spintronics is an emerging research field which aims to
utilize antiferromagnets as core elements in spintronic devices. A central
motivation toward this direction is that antiferromagnetic spin dynamics is
expected to be much faster than ferromagnetic counterpart because
antiferromagnets have higher resonance frequencies than ferromagnets. Recent
theories indeed predicted faster dynamics of antiferromagnetic domain walls
(DWs) than ferromagnetic DWs. However, experimental investigations of
antiferromagnetic spin dynamics have remained unexplored mainly because of the
immunity of antiferromagnets to magnetic fields. Furthermore, this immunity
makes field-driven antiferromagnetic DW motion impossible despite rich physics
of field-driven DW dynamics as proven in ferromagnetic DW studies. Here we show
that fast field-driven antiferromagnetic spin dynamics is realized in
ferrimagnets at the angular momentum compensation point TA. Using rare-earth
3d-transition metal ferrimagnetic compounds where net magnetic moment is
nonzero at TA, the field-driven DW mobility remarkably enhances up to 20 km/sT.
The collective coordinate approach generalized for ferrimagnets and atomistic
spin model simulations show that this remarkable enhancement is a consequence
of antiferromagnetic spin dynamics at TA. Our finding allows us to investigate
the physics of antiferromagnetic spin dynamics and highlights the importance of
tuning of the angular momentum compensation point of ferrimagnets, which could
be a key towards ferrimagnetic spintronics.",1703.07515v1
2020-09-14,Non-Hamiltonian dynamics of indirectly coupled classical impurity spins,"We discuss the emergence of an effective low-energy theory for the real-time
dynamics of two classical impurity spins within the framework of a prototypical
and purely classical model of indirect magnetic exchange: Two classical
impurity spins are embedded in a host system which consists of a finite number
of classical spins localized on the sites of a lattice and interacting via a
nearest-neighbor Heisenberg exchange. An effective low-energy theory for the
slow impurity-spin dynamics is derived for the regime, where the local exchange
coupling between impurity and host spins is weak. To this end we apply the
recently developed adiabatic spin dynamics (ASD) theory. Besides the
Hamiltonian-like classical spin torques, the ASD additionally accounts for a
novel topological spin torque that originates as a holonomy effect in the
close-to-adiabatic-dynamics regime. It is shown that the effective low-energy
precession dynamics cannot be derived from an effective Hamilton function and
is characterized by a non-vanishing precession frequency even if the initial
state deviates only slightly from a ground state. The effective theory is
compared to the fully numerical solution of the equations of motion for the
whole system of impurity and host spins to identify the parameter regime where
the adiabatic effective theory applies. Effective theories beyond the adiabatic
approximation must necessarily include dynamic host degrees of freedom and go
beyond the idea of a simple indirect magnetic exchange. We discuss an example
of a generalized constrained spin dynamics which does improve the description
but also fails for certain geometrical setups.",2009.06296v2
2005-05-31,"The Core-Collapse Supernova with ""Non-Uniform"" Magnetic Fields","We perform two-dimensional numerical simulations on the core-collapse of a
massive star with strong magnetic fields and differential rotations using a
numerical code ZEUS-2D. Changing field configurations and laws of differential
rotation parametrically, we compute 14 models and investigate effects of these
parameters on the dynamics. In our models, we do not solve the neutrino
transport and instead employ a phenomenological parametric EOS that takes into
account the neutrino emissions. As a result of the calculations, we find that
the field configuration plays a significant role in the dynamics of the core if
the initial magnetic field is large enough. Models with initially concentrated
fields produce more energetic explosions and more prolate shock waves than the
uniform field. Quadrapole-like fields produce remarkably collimated and fast
jet, which might be important for gamma-ray bursts(GRB). The Lorentz forces
exerted in the region where the plasma-beta is less than unity are responsible
for these dynamics. The pure toroidal field, on the other hand, does not lead
to any explosion or matter ejection. This suggests the presupernova models of
Heger et al.(2003), in which toroidal fields are predominant, is
disadvantageous for the magnetorotation-induced supernova considered here.
Models with initially weak magnetic fields do not lead to explosion or matter
ejection, either. In these models magnetic fields play no role as they do not
grow on the timescale considered in this paper so that the magnetic pressure
could be comparable to the matter pressure. This is because the exponential
field growth as expected in MRI is not seen in our models. The magnetic field
is amplified mainly by field-compression and field-wrapping in our simulations.",0505611v3
2008-05-01,Improved ring potential of QED at finite temperature and in the presence of weak and strong magnetic field,"Using the general structure of the vacuum polarization tensor
$\Pi_{\mu\nu}(k_{0},\mathbf{k})$ in the infrared (IR) limit, $k_{0}\to 0$, the
ring contribution to QED effective potential at finite temperature and non-zero
magnetic field is determined beyond the static limit, $(k_{0}\to
0,\mathbf{k}\to \mathbf{0})$. The resulting ring potential is then studied in
weak and strong magnetic field limit. In the limit of weak magnetic field, at
high temperature and for $\alpha\to 0$, the improved ring potential consists of
a term proportional to $T^{4}\alpha^{5/2}$, in addition to the expected
$T^{4}\alpha^{3/2}$ term arising from the static limit. Here, $\alpha$ is the
fine structure constant. In the limit of strong magnetic field, where QED
dynamics is dominated by the lowest Landau level (LLL), the ring potential
includes a novel term consisting of dilogarithmic function
$(eB){Li}_{2}(-\frac{2\alpha}{\pi}\frac{eB}{m^{2}})$. Using the ring improved
(one-loop) effective potential including the one-loop effective potential and
ring potential in the IR limit, the dynamical chiral symmetry breaking of QED
is studied at finite temperature and in the presence of strong magnetic field.
The gap equation, the dynamical mass and the critical temperature of QED in the
regime of LLL dominance are determined in the improved IR as well as in the
static limit. For a given value of magnetic field, the improved ring potential
is shown to be more efficient in decreasing the critical temperature arising
from one-loop effective potential.",0805.0078v2
2012-06-28,Poloidal-Field Instability in Magnetized Relativistic Stars,"We investigate the instability of purely poloidal magnetic fields in
nonrotating neutron stars by means of three-dimensional general-relativistic
magnetohydrodynamics simulations, extending the work presented in Ciolfi et al.
(2011). Our aim is to draw a clear picture of the dynamics associated with the
instability and to study the final configuration reached by the system, thus
obtaining indications on possible equilibria in a magnetized neutron star.
Furthermore, since the internal rearrangement of magnetic fields is a highly
dynamical process, which has been suggested to be behind magnetar giant flares,
our simulations can provide a realistic estimate of the electromagnetic and
gravitational-wave emission which should accompany the flare event. Our main
findings are the following: (i) the initial development of the instability
meets all the expectations of perturbative studies in terms of the location of
the seed of the instability, the timescale for its growth and the generation of
a toroidal component; (ii) in the subsequent nonlinear reorganization of the
system, ~90% of magnetic energy is lost in few Alfven timescales mainly through
electromagnetic emission, and further decreases on a much longer timescale;
(iii) all stellar models tend to achieve a significant amount of magnetic
helicity and the equipartition of energy between poloidal and toroidal magnetic
fields, and evolve to a new configuration which does not show a subsequent
instability on dynamical or Alfven timescales; (iv) the electromagnetic
emission matches the duration of the initial burst in luminosity observed in
giant flares, giving support to the internal rearrangement scenario; (v) only a
small fraction of the energy released during the process is converted into
f-mode oscillations and in the consequent gravitational-wave emission, thus
resulting in very low chances of detecting this signal with present and..",1206.6604v2
2019-04-23,Spin injection and pumping generated by a direct current flowing through a magnetic tunnel junction,"A charge flow through a magnetic tunnel junction (MTJ) leads to the
generation of a spin-polarized current which exerts a spin-transfer torque
(STT) on the magnetization. When the density of applied direct current exceeds
some critical value, the STT excites high-frequency magnetization precession in
the ""free"" electrode of MTJ. Such precession gives rise to microwave output
voltage and, furthermore, can be employed for spin pumping into adjacent normal
metal or semiconductor. Here we describe theoretically the spin dynamics and
charge transport in the CoFeB/MgO/CoFeB/Au tunneling heterostructure connected
to a constant-current source. The magnetization dynamics in the free CoFeB
layer with weak perpendicular anisotropy is calculated by numerical integration
of the Landau-Lifshitz-Gilbert-Slonczewski equation accounting for both STT and
voltage controlled magnetic anisotropy associated with the CoFeB|MgO interface.
It is shown that a large-angle magnetization precession, resulting from
electrically induced dynamic spin reorientation transition, can be generated in
a certain range of relatively low current densities. An oscillating spin
current, which is pumped into the Au overlayer owing to such precession, is
then evaluated together with the injected spin current. Considering both the
driving spin-polarized charge current and the pumped spin current, we also
describe the charge transport in the CoFeB/Au bilayer with the account of
anomalous and inverse spin Hall effects. An electric potential difference
between the lateral sides of the CoFeB/Au bilayer is calculated as a function
of distance from the CoFeB|MgO interface. It is found that this transverse
voltage signal in Au is large enough for experimental detection, which
indicates significant efficiency of the proposed current-driven spin injector.",1904.10361v1
2020-10-20,Ultrafast current and field driven domain-wall dynamics in van der Waals antiferromagnet MnPS3,"The discovery of magnetism in two-dimensional (2D) van der Waals (vdW)
materials has flourished a new endeavour of fundamental problems in magnetism
as well as potential applications in computing, sensing and storage
technologies. Of particular interest are antiferromagnets, which due to their
intrinsic antiferromagnetic exchange coupling show several advantages in
relation to ferromagnets such as robustness against external magnetic
perturbations. This property is one of the cornerstones of antiferromagnets and
implies that information stored in antiferromagnetic domains is invisible to
applied magnetic fields preventing it from being erased or manipulated. Here we
show that, despite this fundamental understanding, the magnetic domains of
recently discovered vdW MnPS3 antiferromagnet can be controlled via external
magnetic fields and currents. We realize ultrafast domain-wall dynamics with
velocities up to 1500 m/s and 3000 m/s respectively to a broad range of fields
and current densities. Both domain wall dynamics are determined by the edge
terminations which generated uncompensated spins following the underlying
symmetry of the honeycomb structure. We find that edge atoms belonging to
different magnetic sublattices function as geometrical constrictions preventing
the displacement of the wall, whereas having atoms of the same sublattice at
both edges of the material allows for the field-driven domain wall motion which
is only limited by the spin-flop transition of the antiferromagnet beyond 25 T.
Conversely, electric currents can induce motion of domain walls in most of the
edges except those where the two sublattices are present at the borders (e.g.
armchair edges). Our results indicate that the implementation of 2D vdW
antiferromagnets in real applications requires the engineering of the layer
edges which enables an unprecedented functional feature in ultrathin device
platforms.",2010.10466v2
2021-03-28,Magnetic field-induced softening of spin waves and hard-axis order in Kondo-lattice ferromagnet CeAgSb$_{2}$,"A significant number of Kondo-lattice ferromagnets order perpendicular to the
easy magnetization axis dictated by the crystalline electric field. The nature
of this phenomenon has attracted considerable attention, but remains poorly
understood. In the present paper we use inelastic neutron scattering supported
by magnetization and specific heat measurements to study the spin dynamics in
the hard-axis ferromagnet CeAgSb2. In the zero field state we observed two
sharp magnon modes, which are associated with Ce ordering and extended up to
$\approx 3 meV with a considerable spin gap of 0.6 meV. Application of a
magnetic field perpendicular to the moment direction reduces the spectral
intensity and suppresses the gap and significantly enhances the low-temperature
specific heat at a critical field of Bc ~ 2.8 T via a mean-field-like
transition. Above the transition, in the field polarized state, the gap
eventually reopens due to the Zeeman effect. We modeled the observed dispersion
using linear spin-wave theory (LSWT) taking into account the ground state Gamma
6 doublet and exchange anisotropy. Our model correctly captures the essential
features of the spin dynamics including magnetic dispersion, distribution of
the spectral intensity as well as the field-induced behavior, although several
minor features remain obscure. The observed spectra do not show significant
broadening due to the finite lifetime of the quasiparticles. Along with a
moderate electronic specific heat coefficient gamma = 46 mJ/mol K2 this
indicates that the Kondo coupling is relatively weak and the Ce moments are
well localized. Altogether, our results provide profound insight into the spin
dynamics of the hard-axis ferromagnet CeAgSb2 and can be used as solid ground
for studying magnetic interactions in isostructural compounds including
CeAuSb2, which exhibits nematicity and unusual mesoscale magnetic textures.",2103.15137v2
2021-12-15,Turbulent magnetic field amplification in binary neutron star mergers,"Magnetic fields are expected to play a key role in the dynamics and the
ejection mechanisms that accompany the merger of two neutron stars. General
relativistic magnetohydrodynamic (MHD) simulations offer a unique opportunity
to unravel the details of the ongoing physical processes. Nevertheless, current
numerical studies are severely limited by the fact that any affordable
resolution remains insufficient to fully capture the small-scale dynamo,
initially triggered by the Kelvin-Helmholtz instability, and later sourced by
several MHD processes involving differential rotation. Here, we alleviate this
limitation by using explicit large-eddy simulations, a technique where the
unresolved dynamics occurring at the sub-grid scales (SGS) is modeled by extra
terms, which are functions of the resolved fields and their derivatives. The
combination of high-order numerical schemes, high resolutions, and the gradient
SGS model allow us to capture the small-scale dynamos produced during the
binary neutron star mergers. Here we follow the first 50 milliseconds after the
merger and, for the first time, we find numerical convergence on the magnetic
field amplification, in terms of integrated energy and spectral distribution
over spatial scales. We also find that the average intensity of the magnetic
field in the remnant saturates at $\sim 10^{16}$~G around $5$~ms after the
merger. After $20-30$~ms, both toroidal and poloidal magnetic field components
grow continuously, fed by the winding mechanism that provides a slow inverse
cascade. We find no clear hints for magneto-rotational instabilities, and no
significant impact of the magnetic field on the redistribution of angular
momentum in the remnant in our simulations, probably due to the very turbulent
and dynamical topology of the magnetic field at all stages, with small-scale
components largely dominating over the large-scale ones.",2112.08413v2
2022-10-27,Fast broadband cluster spin-glass dynamics in PbFe$_{1/2}$Nb$_{1/2}$O$_{3}$,"PbFe$_{1/2}$Nb$_{1/2}$O$_{3}$ (PFN) is a relaxor ferroelectric (T$_{c}$
$\sim$ 400 K) consisting of disordered magnetic Fe$^{3+}$ (S=${5\over2}$,
L$\approx$0) ions resulting in a low temperature ``cluster glass"" phase (W.
Kleemann $\textit{et al.}$ Phys. Rev. Lett. ${\bf{105}}$, 257202 (2010)). We
apply neutron scattering to investigate the dynamic magnetism of this phase in
a large single crystal which displays a low temperature spin glass transition
(T$_{g} \sim$ 15 K), but no observable spatially long-range antiferromagnetic
order. The static response in the cluster glass phase (sampled on the timescale
set by our resolution) is found to be characterized by an average magnetic spin
direction that lacks any preferred direction. The dynamics that drive this
phase are defined by a magnetic correlation length that gradually increases
with decreasing temperature. However, below $\sim$ 50 K the spatial
correlations gradually becoming more short range indicative of increasing
disorder on cooling, thereby unravelling magnetism, until the low temperature
glass phase sets in at T$_{g}$ $\sim$ 15 K. Neutron spectroscopy is used to
characterize the spin fluctuations in the cluster glass phase and are found to
be defined by a broadband of frequencies on the scale of $\sim$ THz, termed
here ``fast"" fluctuations. The frequency bandwidth driving the magnetic
fluctuations mimics the correlation length and decreases until $\sim$ 50 K, and
then increases again until the glass transition. Through investigating the
low-energy acoustic phonons we find evidence of multiple distinct structural
regions which form the basis of the clusters, generating a significant amount
of local disorder. We suggest that random molecular fields originating from
conflicting interactions between clusters is important for the destruction of
magnetic order and the eventual formation of the cluster glass in PFN.",2210.15706v1
2023-06-12,"Transition from small-scale to large-scale dynamo in a supernova-driven, multiphase medium","Magnetic fields are widely recognised as critical at many scales to galactic
dynamics and structure, including multiphase pressure balance, dust processing,
and star formation. Using imposed magnetic fields cannot reliably model the
interstellar medium's (ISM) dynamical structure nor phase interactions. Dynamos
must be modelled. ISM models exist of turbulent magnetic fields using
small-scale dynamo (SSD). Others model the large-scale dynamo (LSD) organising
magnetic fields at scale of the disc or spiral arms. Separately, neither can
fully describe the galactic magnetic field dynamics nor topology. We model the
LSD and SSD together at sufficient resolution to use the low explicit
Lagrangian resistivity required. The galactic SSD saturates within 20 Myr. We
show that the SSD is quite insensitive to the presence of an LSD and is even
stronger in the presence of a large-scale shear flow. The LSD grows more slowly
in the presence of SSD, saturating after 5 Gyr vs. 1--2 Gyr in studies where
the SSD is weak or absent. The LSD primarily grows in warm gas in the galactic
midplane. Saturation of the LSD occurs due to ${\alpha}$-quenching near the
midplane as the growing mean field produces a magnetic ${\alpha}$ that opposes
the kinetic ${\alpha}$. The magnetic energy in our models of the LSD shows
slightly sublinear response to increasing resolution, indicating that we are
converging towards the physical solution at 1 pc resolution. Clustering
supernovae in OB associations increases the growth rates for both the SSD and
the LSD, compared to a horizontally uniform supernova distribution.",2306.07051v3
2010-03-22,Coherent Control of Quantum Dynamics with Sequences of Unitary Phase-Kick Pulses,"Coherent optical control schemes exploit the coherence of laser pulses to
change the phases of interfering dynamical pathways in order to manipulate
dynamical processes. These active control methods are closely related to
dynamical decoupling techniques, popularized in the field of Quantum
Information. Inspired by Nuclear Magnetic Resonance (NMR) spectroscopy,
dynamical decoupling methods apply sequences of unitary operations to modify
the interference phenomena responsible for the system dynamics thus also
belonging to the general class of coherent control techniques. Here we review
related developments in the fields of coherent optical control and dynamical
decoupling, with emphasis on control of tunneling and decoherence in general
model systems. Considering recent experimental breakthroughs in the
demonstration of active control of a variety of systems, we anticipate that the
reviewed coherent control scenarios and dynamical decoupling methods should
raise significant experimental interest.",1003.4269v1
2017-05-09,Dynamic finite-size scaling at first-order transitions,"We investigate the dynamic behavior of finite-size systems close to a
first-order transition (FOT). We develop a dynamic finite-size scaling (DFSS)
theory for the dynamic behavior in the coexistence region where different
phases coexist. It is characterized by an exponentially large time scale
related to the tunneling between the two phases. We show that, when considering
time scales of the order of the tunneling time, the dynamic behavior can be
described by a two-state coarse-grained dynamics. This allows us to obtain
exact predictions for the dynamical scaling functions. To test the general DFSS
theory at FOTs, we consider the two-dimensional Ising model in the
low-temperature phase, where the external magnetic field drives a FOT, and the
20-state Potts model, which undergoes a thermal FOT. Numerical results for a
purely relaxational dynamics fully confirm the general theory.",1705.03198v2
2020-12-19,Unconsciousness reconfigures modular brain network dynamics,"The dynamic core hypothesis posits that consciousness is correlated with
simultaneously integrated and differentiated assemblies of transiently
synchronized brain regions. We represented time-dependent functional
interactions using dynamic brain networks, and assessed the integrityof the
dynamic core by means of the flexibility and largest multilayer module of these
networks. As a first step, we constrained parameter selection using a newly
developed benchmark for module detection in heterogeneous temporal networks.
Next, we applied a multilayer modularity maximization algorithm to dynamic
brain networks computed from functional magnetic resonance imaging (fMRI) data
acquired during deep sleep and under propofol anesthesia. We found that
unconsciousness reconfigured network flexibility and reduced the size of the
largest spatiotemporal module, which we identified with the dynamic core. Our
results present a first characterization of modular brain network dynamics
during states of unconsciousness measured with fMRI, adding support to the
dynamic core hypothesis of human consciousness.",2012.10785v1
2022-08-03,Ultrafast Demagnetization Dynamics Due to Electron-Electron Scattering and Its Relation to Momentum Relaxation in Ferromagnets,"We analyze theoretically the demagnetization dynamics in a ferromagnetic
model system due to the interplay of spin-orbit coupling and electron-electron
Coulomb scattering. We compute the $k$-resolved electronic reduced spin-density
matrix including precessional dynamics around internal spin-orbit and exchange
fields as well as the electron-electron Coulomb scattering for densities and
spin coherences. Based on a comparison with numerical solutions of the full
Boltzmann scattering integrals, we establish that the $k$-resolved reduced
spin-density matrix dynamics are well described using a simpler generalized
relaxation-time ansatz for the reduced spin-density matrix. This ansatz allows
one to relate the complicated scattering dynamics underlying the
demagnetization dynamics to a physically meaningful momentum relaxation time
$\tau$. Our approach reproduces the behaviors of the demagnetization time
$\tau_{m} \propto 1/\tau$ and $\tau_{m} \propto \tau$ for the limits of short
and long $\tau$, respectively, and is also valid for the intermediate regime.
The ansatz thus provides a tool to include the correct demagnetization behavior
in approaches that treat other contributions to the magnetization dynamics such
as transport or magnon/phonon dynamics.",2208.02356v1
1999-09-02,The MHD Kelvin-Helmholtz Instability III: The Role of Sheared Magnetic Field in Planar Flows,"We have carried out simulations of the nonlinear evolution of the
magnetohydrodynamic (MHD) Kelvin-Helmholtz (KH) instability for compressible
fluids in $2\frac{1}{2}$-dimensions, extending our previous work by Frank et al
(1996) and Jones \etal (1997). In the present work we have simulated flows in
the x-y plane in which a ``sheared'' magnetic field of uniform strength
``smoothly'' rotates across a thin velocity shear layer from the z direction to
the x direction, aligned with the flow field. We focus on dynamical evolution
of fluid features, kinetic energy dissipation, and mixing of the fluid between
the two layers, considering their dependence on magnetic field strength for
this geometry. The introduction of magnetic shear can allow a Cat's Eye-like
vortex to form, even when the field is stronger than the nominal linear
instability limit given above. For strong fields that vortex is asymmetric with
respect to the preliminary shear layer, however, so the subsequent dissipation
is enhanced over the uniform field cases of comparable field strength. In fact,
so long as the magnetic field achieves some level of dynamical importance
during an eddy turnover time, the asymmetries introduced through the magnetic
shear will increase flow complexity, and, with that, dissipation and mixing.
The degree of the fluid mixing between the two layers is strongly influenced by
the magnetic field strength. Mixing of the fluid is most effective when the
vortex is disrupted by magnetic tension during transient reconnection, through
local chaotic behavior that follows.",9909033v1
2001-03-21,Spectra and Growth Rates of Fluctuating Magnetic Fields in the Kinematic Dynamo Theory with Large Magnetic Prandtl Numbers,"The existence of a weak galactic magnetic field has been repeatedly confirmed
by observational data. The origin of this field has not as yet been explained
in a fully satisfactory way and represents one of the main challenges of the
astrophysical dynamo theory. In both the galactic dynamo theory and the
primordial-origin theory, a major influence is exerted by the small-scale
magnetic fluctuations. This article is devoted to constructing a systematic
second-order statistical theory of such small-scale fields. The statistics of
these fields are studied in the kinematic approximation and for the case of
large Prandtl numbers, which is relevant for the galactic and protogalactic
plasma. The advecting velocity field is assumed to be Gaussian and short-time
correlated. Theoretical understanding of this kinematic dynamo model is a
necessary prerequisite for any prospective nonlinear dynamo theory. The theory
is developed for an arbitrary degree of compressibility and formally in d
dimensions, which generalizes the previously known results, elicits the
structure of the solutions, and uncovers a number of new effects. The magnetic
energy spectra are studied as they grow and spread over scales during the
initial stage of the field amplification. Exact Green's-function solutions are
obtained. The spectral theory is supplemented by the study of magnetic-field
correlation functions in the configuration space, where the dynamo problem can
be mapped onto a particular one-dimensional quantum-mechanical problem. The
latter approach is most suitable for the description of the kinematic dynamo in
the long-time limit, i.e. when the magnetic excitation has spread over all
scales present in the system. A simple way of calculating the growth rates of
the magnetic fields in this long-time limit is proposed.",0103333v2
2005-01-21,Magnetic Flux Tube Reconnection: Tunneling Versus Slingshot,"The discrete nature of the solar magnetic field as it emerges into the corona
through the photosphere indicates that it exists as isolated flux tubes in the
convection zone, and will remain as discrete flux tubes in the corona until it
collides and reconnects with other coronal fields. Collisions of these flux
tubes will in general be three dimensional, and will often lead to
reconnection, both rearranging the magnetic field topology in fundamental ways,
and releasing magnetic energy. With the goal of better understanding these
dynamics, we carry out a set of numerical experiments exploring fundamental
characteristics of three dimensional magnetic flux tube reconnection. We first
show that reconnecting flux tubes at opposite extremes of twist behave very
differently: in some configurations, low twist tubes slingshot while high twist
tubes tunnel. We then discuss a theory explaining these differences: by
assuming helicity conservation during the reconnection one can show that at
high twist, tunneled tubes reach a lower magnetic energy state than slingshot
tubes, whereas at low twist the opposite holds. We test three predictions made
by this theory. 1) We find that the level of twist at which the transition from
slingshot to tunnel occurs is about two to three times higher than predicted on
the basis of energetics and helicity conservation alone, probably because the
dynamics of the reconnection play a large role as well. 2) We find that the
tunnel occurs at all flux tube collision angles predicted by the theory. 3) We
find that the amount of magnetic energy a slingshot or a tunnel reconnection
releases agrees reasonably well with the theory, though at the high
resistivities we have to use for numerical stability, a significant amount of
magnetic energy is lost to diffusion, independent of reconnection.",0501473v1
2007-06-21,Bubble-Wrap for Bullets: The Stability Imparted By A Thin Magnetic Layer,"There has been significant recent work which examines a situation where a
thin magnetic layer is `draped' over a core merging into a larger cluster; the
same process also appears to be at work at a bubble rising from the cluster
centre. Such a thin magnetic layer could thermally isolate the core from the
cluster medium, but only if the same shear process which generates the layer
does not later disrupt it. On the other hand, if the magnetized layer can
stabilize against the shear instabilities, then the magnetic layer can have the
additional dynamical effect of reducing the shear-driven mixing of the core's
material during the merger process. These arguments could equally well to
underdense cluster bubbles, which would be even more prone to disruption. While
it is well known that magnetic fields can suppress instabilities, it is less
clear that a thin layer can suppress instabilities on scales significantly
larger than its thickness. Here we consider the stability imparted by a thin
magnetized layer. Such a layer can have a significant stabilizing effect even
on modes with wavelengths much larger than the thickness of the layer l; to
stabilize modes ten times larger requires only that the Alfv\'en speed in the
magnetized layer is comparable to the relevant destabilizing velocity -- the
shear velocity in the case of pure Kelvin-Helmholtz like instability, or a
typical buoyancy velocity in the case of pure Rayleigh-Taylor. We confirm our
calculations with two-dimensional numerical experiments using the Athena code.",0706.3216v3
2008-04-23,Three Dimensional Magnetohydrodynamical Simulations of Core Collapse Supernova,"We show three-dimensional magnetohydrodynamical simulations of core collapse
supernova in which the progenitor has magnetic fields inclined to the rotation
axis. The simulations employed a simple empirical equation of state in which
the pressure of degenerate gas is approximated by piecewise polytropes for
simplicity. Neither energy loss due to neutrino is taken into account for
simplicity. The simulations start from the stage of dynamical collapse of an
iron core. The dynamical collapse halts at $ t $ = 189 ms by the pressure of
high density gas and a proto-neutron star (PNS) forms. The evolution of PNS was
followed about 40 milli-seconds in typical models. When the initial rotation is
mildly fast and the initial magnetic fields are mildly strong, bipolar jets are
launched from an upper atmosphere ($ r \sim 60 {\rm km} $) of the PNS. The jets
are accelerated to $ \sim 3 \times 10 ^4 $ km s$^{-1}$, which is comparable to
the escape velocity at the foot point. The jets are parallel to the initial
rotation axis. Before the launch of the jets, magnetic fields are twisted by
rotation of the PNS. The twisted magnetic fields form torus-shape multi-layers
in which the azimuthal component changes alternately. The formation of magnetic
multi-layers is due to the initial condition in which the magnetic fields are
inclined with respect to the rotation axis. The energy of the jet depends only
weakly on the initial magnetic field assumed. When the initial magnetic fields
are weaker, the time lag is longer between the PNS formation and jet ejection.
It is also shown that the time lag is related to the Alfv\'en transit time.
Although the nearly spherical prompt shock propagates outward in our
simulations, it is ...",0804.3700v3
2010-01-12,Nonlinear Processes in Coronal Heating and Slow Solar Wind Acceleration,"This work consists of two parts: the first devoted to the study of the
heating of the magnetically confined Solar Corona, and the second to the
acceleration of the Slow Solar Wind.
  Direct 3D reduced MHD simulations are presented. They model the heating of
coronal loops in the solar atmosphere via the tangling of coronal field lines
by photospheric footpoints motions within the framework of the ""Parker
scenario"". We have derived scalings of physical quantities with loop length,
and the ratio of photospheric to coronal Alfven velocities. The development of
a turbulent dynamics makes the dissipation rate independent of the Reynolds
number. The dynamics in physical space are desribed by weak turbulence, which
develops when an MHD system is embedded in a strong axial magnetic field.
  The slow wind originates in and around the coronal streamer belt. The LASCO
instrument onboard the SOHO spacecraft has observed plasma density enhancements
forming beyond the cusp of a helmet streamer. Previous theoretical models for
the formation and initial motion of these density enhancements are improved.
  The average expansion suffered by a parcel of plasma propagating outward, and
the diamagnetic force due to the overall magnetic field radial gradients are
now included. It is found that the magnetized wake configuration is resistively
unstable, that an outward accelerating magnetic island develops at the center
of the streamer, and that density enhancements occur within the magnetic
islands. The values of the acceleration and density contrasts can be in good
agreement with LASCO observations, provided the spherical divergence of the
magnetic lines starts beyond a critical distance from the Sun. This result
provides a constraint on the topology of the magnetic field.",1001.1770v1
2012-07-25,Bipolar jets launched from magnetically diffusive accretion disks. I. Ejection efficiency vs field strength and diffusivity,"We investigate the launching of jets and outflows from magnetically diffusive
accretion disks. Using the PLUTO code we solve the time-dependent resistive MHD
equations taking into account the disk and jet evolution simultaneously. The
main question we address is which kind of disks do launch jets and which kind
of disks do not? In particular, we study how the magnitude and distribution of
the (turbulent) magnetic diffusivity affect mass loading and jet acceleration.
We have applied a turbulent magnetic diffusivity based on \alpha-prescription,
but have also investigate examples where the scale height of diffusivity is
larger than that of the disk gas pressure. We further investigate how the
ejection efficiency is governed by the magnetic field strength. Our simulations
last for up to 5000 dynamical time scales corresponding to 900 orbital periods
of the inner disk. As a general result we observe a continuous and robust
outflow launched from the inner part of the disk, expanding into a collimated
jet of super fast magneto-sonic speed. For long time scales the disk internal
dynamics changes, as due to outflow ejection and disk accretion the disk mass
decreases. For magneto-centrifugally driven jets we find that for i) less
diffusive disks, ii) a stronger magnetic field, iii) a low poloidal
diffusivity, or a iv) lower numerical diffusivity (resolution), the mass
loading of the outflow is increased - resulting in more powerful jets with high
mass flux. For weak magnetization the (weak) outflow is driven by the magnetic
pressure gradient. We further investigate the jet asymptotic velocity and the
jet rotational velocity in respect of the different launching scenarios. We
find a lower degree of jet collimation than previous studies, most probably due
to our revised outflow boundary condition.",1207.6086v1
2013-05-02,Magnetic monopole field exposed by electrons,"Magnetic monopoles have provided a rich field of study, leading to a wide
area of research in particle physics, solid state physics, ultra-cold gases,
superconductors, cosmology, and gauge theory. So far, no true magnetic
monopoles were found experimentally. Using the Aharonov-Bohm effect, one of the
central results of quantum physics, shows however, that an effective monopole
field can be produced. Understanding the effects of such a monopole field on
its surroundings is crucial to its observation and provides a better grasp of
fundamental physical theory. We realize the diffraction of fast electrons at a
magnetic monopole field generated by a nanoscopic magnetized ferromagnetic
needle. Previous studies have been limited to theoretical semiclassical optical
calculations of the motion of electrons in such a monopole field. Solid state
systems like the recently studied 'spin ice' provide a constrained system to
study similar fields, but make it impossible to separate the monopole from the
material. Free space diffraction helps to understand the dynamics of the
electron-monopole system without the complexity of a solid state system. The
use of a simple object such as a magnetized needle will allow various areas of
physics to use the general dynamical effects of monopole fields without
requiring a monopole particle or specific solids which have internal
monopole-like properties. The experiment performed here shows that even without
a true magnetic monopole particle, the theoretical background on monopoles
serves as a basis for experiments and can be applied to efficiently create
electron vortices. Various predictions about angular momentum and general field
effects can readily be studied using the available equipment. This realization
provides insights for the scientific community on how to detect magnetic
monopoles in high energy collisions, cosmological processes, or novel
materials.",1305.0570v2
2013-06-10,Interpreting Eruptive Behavior in NOAA AR 11158 via the Region's Magnetic Energy and Relative Helicity Budgets,"In previous works we introduced a nonlinear force-free method that
self-consistently calculates the instantaneous budgets of free magnetic energy
and relative magnetic helicity in solar active regions (ARs). Calculation is
expedient and practical, using only a single vector magnetogram per
computation. We apply this method to a timeseries of 600 high-cadence vector
magnetograms of the eruptive NOAA AR 11158 acquired by the Helioseismic and
Magnetic Imager onboard the Solar Dynamics Observatory over a five-day
observing interval. Besides testing our method extensively, we use it to
interpret the dynamical evolution in the AR, including eruptions. We find that
the AR builds large budgets of both free magnetic energy and relative magnetic
helicity, sufficient to power many more eruptions than the ones it gave within
the interval of interest. For each of these major eruptions, we find
eruption-related decreases and subsequent free-energy and helicity budgets that
are consistent with the observed eruption (flare and coronal-mass-ejection
[CME]) sizes. In addition, we find that (1) evolution in the AR is consistent
with the recently proposed (free) energy - (relative) helicity diagram of solar
ARs, (2) eruption-related decreases occur {\it before} the flare and the
projected CME-launch times, suggesting that CME progenitors precede flares, and
(3) self-terms of free energy and relative helicity most likely originate from
respective mutual-terms, following a progressive mutual-to-self conversion
pattern that most likely stems from magnetic reconnection. This results in the
non-ideal formation of increasingly helical pre-eruption structures and
instigates further research on the triggering of solar eruptions with magnetic
helicity firmly placed in the eruption cadre.",1306.2135v2
2014-08-24,Thickness dependence of dynamic and static magnetic properties of pulsed laser deposited La$_{0.7}$Sr$_{0.3}$MnO$_3$ films on SrTiO$_3$(001),"We present a comprehensive study of the thickness dependence of static and
magneto-dynamic magnetic properties of La$_{0.7}$Sr$_{0.3}$MnO$_3$. Epitaxial
pulsed laser deposited La$_{0.7}$Sr$_{0.3}$MnO$_3$ / SrTiO$_3$(001) thin films
in the range from 3 unit cells (uc) to 40 uc (1.2 - 16 nm) have been
investigated through ferromagnetic resonance spectroscopy (FMR) and SQUID
magnetometry at variable temperature. Magnetodynamically, three different
thickness, $d$, regimes are identified: 20 uc $\lesssim d$ uc where the system
is bulk like, a transition region 8 uc $\le d \lesssim 20$ uc where the FMR
line width and position depend on thickness and $d=6$ uc which displays
significantly altered magnetodynamic properties, while still displaying bulk
magnetization. Magnetization and FMR measurements are consistent with a
nonmagnetic volume corresponding to $\sim$ 4 uc. We observe a reduction of
Curie temperature ($T_C$) with decreasing thickness, which is coherent with a
mean field model description. The reduced ordering temperature also accounts
for the thickness dependence of the magnetic anisotropy constants and resonance
fields. The damping of the system is strongly thickness dependent, and is for
thin films dominated by thickness dependent anisotropies, yielding both a
strong 2-magnon scattering close to $T_c$ and a low temperature broadening. For
the bulk like samples a large part of the broadening can be linked to spread in
magnetic anisotropies attributed to crystal imperfections/domain boundaries of
the bulk like film.",1408.5631v1
2015-01-22,A Measure of Monopole Inertia in the Quantum Spin Ice Yb$_2$Ti$_2$O$_7$,"An important and continuing theme of modern solid state physics is the
realization of exotic excitations in materials (e.g. quasiparticles) that have
no analogy (or have not yet been observed) in the actual physical vacuum of
free space. Although they are not fundamental particles, such quasiparticles do
constitute the most basic description of the excited states of the ""vacuum"" in
which they reside. In this regard the magnetic textures of the excited states
of spin ices, magnetic pyrochlore oxides with dominant Ising interactions, are
proposed to be modeled as effective magnetic charge monopoles. Recent inelastic
neutron scattering experiments have established the pyrochlore material
Yb$_2$Ti$_2$O$_7$ (YbTO) as a quantum spin ice, where in addition to the Ising
interactions there are substantial transverse terms that may induce quantum
dynamics and - in principle - coherent monopole motion. Here we report a
combined time domain terahertz spectroscopy (TDTS) and microwave cavity study
of YbTO to probe its complex dynamic magnetic susceptibility. We find that the
form of the susceptibility is consistent with monopole motion and a magnetic
monopole conductivity can be defined and measured. Using the unique phase
sensitive capabilities of these techniques, we observe a sign change in the
reactive part of the magnetic response. In generic models of monopole motion
this is only possible through introducing inertial effects, e.g. a mass
dependent term, to the equations of motion. Analogous to conventional electric
charge systems, measurement of the conductivity's spectral weight allows us to
derive a value for the magnetic monopole mass, which we find to be
approximately 1800 electron masses. Our results establish the magnetic
monopoles of quantum spin ice as true coherently propagating quasiparticles of
this system.",1501.05638v1
2015-02-24,Ultra-high-resolution Observations of MHD Waves in Photospheric Magnetic Structures,"Here we review the recent progress made in the detection, examination,
characterisation and interpretation of oscillations manifesting in small-scale
magnetic elements in the solar photosphere. This region of the Sun's atmosphere
is especially dynamic, and importantly, permeated with an abundance of magnetic
field concentrations. Such magnetic features can span diameters of hundreds to
many tens of thousands of km, and are thus commonly referred to as the
`building blocks' of the magnetic solar atmosphere. However, it is the smallest
magnetic elements that have risen to the forefront of solar physics research in
recent years. Structures, which include magnetic bright points, are often at
the diffraction limit of even the largest of solar telescopes. Importantly, it
is the improvements in facilities, instrumentation, imaging techniques and
processing algorithms during recent years that have allowed researchers to
examine the motions, dynamics and evolution of such features on the smallest
spatial and temporal scales to date. It is clear that while these structures
may demonstrate significant magnetic field strengths, their small sizes make
them prone to the buffeting supplied by the ubiquitous surrounding convective
plasma motions. Here, it is believed that magnetohydrodynamic waves can be
induced, which propagate along the field lines, carrying energy upwards to the
outermost extremities of the solar corona. Such wave phenomena can exist in a
variety of guises, including fast and slow magneto-acoustic modes, in addition
to Alfven waves. Coupled with rapid advancements in magnetohydrodynamic wave
theory, we are now in an ideal position to thoroughly investigate how wave
motion is generated in the solar photosphere, which oscillatory modes are most
prevalent, and the role that these waves play in supplying energy to various
layers of the solar atmosphere.",1502.06960v1
2016-05-23,Dynamical model for spindown of solar-type stars,"Since their formation, stars slow down their rotation rates by the removal of
angular momentum from their surfaces, e.g. via stellar winds. Despite the
complexity of the processes involved, a traditional model, where the removal of
angular momentum loss by magnetic fields is prescribed, has provided a useful
framework to understand observational relations between stellar rotation and
age and magnetic field strength. Here, a spindown model is proposed where loss
of angular momentum by magnetic fields is evolved dynamically, instead of being
kinematically prescribed. To this end, we evolve the stellar rotation and
magnetic field simultaneously over stellar evolution time by extending our
previous work on a dynamo model which incorporates the nonlinear feedback
mechanisms on rotation and magnetic fields. Our extended model reproduces key
observations and explains the presence of the two branches of (fast and slow
rotating) stars which have different relations between rotation rate $\Omega$
vs. time (age), magnetic field strength $|B|$ vs. rotation rate, and frequency
of magnetic field $\omega_{cyc}$ vs. rotation rate. For fast rotating stars we
find: (i) an exponential spindown $\Omega \propto e^{-1.35t}$, with $t$
measured in Gyrs, (ii) $|B|$ saturates for higher rotation rate, (iii)
$\omega_{cyc} \propto \Omega^{0.85}$. For slow rotating stars we obtain: (i) a
power law spindown $\Omega \propto t^{-0.52}$, (ii) $|B|$ scales almost
linearly with rotation rate, (iii) $\omega_{cyc} \propto \Omega^{1.16}$. The
results obtained are in good agreement with observations. The Vaughan-Preston
gap is consistently explained in our model by the shortest spindown timescale
in this transition from fast to slow rotators. Our results highlight the
importance of self-regulation of magnetic fields and rotation by direct and
indirect interactions involving nonlinear feedback in stellar evolution.",1605.07125v2
2017-04-30,Laminar and turbulent dynamos in chiral magnetohydrodynamics-I: Theory,"The magnetohydrodynamic (MHD) description of plasmas with relativistic
particles necessarily includes an additional new field, the chiral chemical
potential associated with the axial charge (i.e., the number difference between
right- and left-handed relativistic fermions). This chiral chemical potential
gives rise to a contribution to the electric current density of the plasma
(\emph{chiral magnetic effect}). We present a self-consistent treatment of the
\emph{chiral MHD equations}, which include the back-reaction of the magnetic
field on a chiral chemical potential and its interaction with the plasma
velocity field. A number of novel phenomena are exhibited. First, we show that
the chiral magnetic effect decreases the frequency of the Alfv\'{e}n wave for
incompressible flows, increases the frequencies of the Alfv\'{e}n wave and of
the fast magnetosonic wave for compressible flows, and decreases the frequency
of the slow magnetosonic wave. Second, we show that, in addition to the
well-known laminar chiral dynamo effect, which is not related to fluid motions,
there is a dynamo caused by the joint action of velocity shear and chiral
magnetic effect. In the presence of turbulence with vanishing mean kinetic
helicity, the derived mean-field chiral MHD equations describe turbulent
large-scale dynamos caused by the chiral alpha effect, which is dominant for
large fluid and magnetic Reynolds numbers. The chiral alpha effect is due to an
interaction of the chiral magnetic effect and fluctuations of the small-scale
current produced by tangling magnetic fluctuations (which are generated by
tangling of the large-scale magnetic field by sheared velocity fluctuations).
These dynamo effects may have interesting consequences in the dynamics of the
early universe, neutron stars, and the quark--gluon plasma.",1705.00378v3
2017-12-22,"Dynamic, magnetic and electronic properties of the C14 Laves phase Nb0.975Fe2.025 compound: Mössbauer-effect study","C14 Laves phase Nb0.975Fe2.025 compound was investigated by means of the
M\""ossbauer spectroscopy. Spectra were recorded in the temperature range of
5-300K. Their analysis in terms of three sub spectra yielded information on
magnetic and lattice dynamical properties of Fe atoms regularly occupying 2a
and 6h lattice sites and, excessively, 4f sites. No indication of magnetism was
observed down to the temperature of T=ca.50K, and spectral parameters viz.
center shift, CS, and the main component of the electric field gradient, Vzz,
behave regularly. In particular, analysis of CS(T) in terms of the Debye model
yielded the following values of the Debye temperature, T_D: 453(5) K for the
site 6h, 544(10) K for the site 2a, 479(4) K for the weighted average over 6h
and 2a sites, and 363(35) K for the site 4f. Below ca.50K anomalous behavior
was observed: a broadening of the spectrum appeared indicating thereby a
transition into a magnetic phase. Analysis of a temperature dependence of the
hyperfine field, B, associated with the 6h and 2a sub spectra yielded the
magnetic ordering temperature T_C1=ca.50K for the former and T_C2=ca.31K for
the latter. The maximum values of the hyperfine field at 5K, Bo, were 8.2 kGs
and 3.3 kGs, respectively. The Bo-values were used to estimate values the
underlying magnetic moments, m_Fe(6h)=0.055-0.065 Bohr magneton and
m_Fe(2a)=0.02-0.025 Bohr magneton. Noteworthy, they are over one order of
magnitude lower than those theoretically calculated. The CS and Vzz parameters
showed anomalies in the magnetic phase, in particular the former exhibited a
strong departure from the Debye model prediction testifying to a significant
effect of magnetism on the lattice vibrations.",1712.08514v1
2018-02-15,Characterization of methanol as a magnetic field tracer in star-forming regions,"Magnetic fields play an important role during star formation. Direct magnetic
field strength observations have proven specifically challenging in the
extremely dynamic protostellar phase. Because of their occurrence in the
densest parts of star forming regions, masers, through polarization
observations, are the main source of magnetic field strength and morphology
measurements around protostars. Of all maser species, methanol is one of the
strongest and most abundant tracers of gas around high-mass protostellar disks
and in outflows. However, as experimental determination of the magnetic
characteristics of methanol has remained largely unsuccessful, a robust
magnetic field strength analysis of these regions could hitherto not be
performed. Here we report a quantitative theoretical model of the magnetic
properties of methanol, including the complicated hyperfine structure that
results from its internal rotation. We show that the large range in values of
the Land\'{e} g-factors of the hyperfine components of each maser line lead to
conclusions which differ substantially from the current interpretation based on
a single effective g-factor. These conclusions are more consistent with other
observations and confirm the presence of dynamically important magnetic fields
around protostars. Additionally, our calculations show that (non-linear) Zeeman
effects must be taken into account to further enhance the accuracy of
cosmological electron-to-proton mass ratio determinations using methanol.",1802.05764v1
2018-05-17,Large magneto-optical Kerr effect and imaging of magnetic octupole domains in an antiferromagnetic metal,"When a polarized light beam is incident upon the surface of a magnetic
material, the reflected light undergoes a polarization rotation. This
magneto-optical Kerr effect (MOKE) has been intensively studied in a variety of
ferro- and ferrimagnetic materials because it provides a powerful probe for
electronic and magnetic properties as well as for various applications
including magneto-optical recording. Recently, there has been a surge of
interest in antiferromagnets (AFMs) as prospective spintronic materials for
high-density and ultrafast memory devices, owing to their vanishingly small
stray field and orders of magnitude faster spin dynamics compared to their
ferromagnetic counterparts. In fact, the MOKE has proven useful for the study
and application of the antiferromagnetic (AF) state. Although limited to
insulators, certain types of AFMs are known to exhibit a large MOKE, as they
are weak ferromagnets due to canting of the otherwise collinear spin structure.
Here we report the first observation of a large MOKE signal in an AF metal at
room temperature. In particular, we find that despite a vanishingly small
magnetization of $M \sim$0.002 $\mu_{\rm B}$/Mn, the non-collinear AF metal
Mn$_3$Sn exhibits a large zero-field MOKE with a polar Kerr rotation angle of
20 milli-degrees, comparable to ferromagnetic metals. Our first-principles
calculations have clarified that ferroic ordering of magnetic octupoles in the
non-collinear Neel state may cause a large MOKE even in its fully compensated
AF state without spin magnetization. This large MOKE further allows imaging of
the magnetic octupole domains and their reversal induced by magnetic field. The
observation of a large MOKE in an AF metal should open new avenues for the
study of domain dynamics as well as spintronics using AFMs.",1805.06758v1
2018-08-10,Hot Grain Dynamics by Electric Charging and Magnetic Trapping in Debris Disks,"The recent discovery of hot dust grains in the vicinity of main-sequence
stars has become a hot issue among the scientific community of debris disks.
Hot grains must have been enormously accumulated near their sublimation zones,
but it is a mystery how such a high concentration of hot grains is sustained.
The most difficult conundrum is that the size of hot dust grains is estimated
to lie in the submicrometer range, while submicrometer-sized grains are
instantly swept away from near-stellar environments by stellar radiation
pressure. One and only mechanism proposed for prolonging the residence time of
hot grains in the near-stellar environments is trapping of charged
nanoparticles by stellar magnetic fields. We revisit the model of magnetic
grain trapping around main-sequence stars of various spectral classes by taking
into account sublimation and electric charging of the grains. The model of
magnetic grain trapping predicts that hot dust grains are present in the
vicinity of main-sequence stars with high rotation velocities and intermediate
magnetic-field strengths. On the contrary, we find that the detection of hot
dust grains has no correlation with the rotation velocities of central stars
nor the magnetic field strengths of the stars. Our numerical evaluation of
electric grain charging indicates that the surface potential of
submicrometer-sized grains in the vicinity of main-sequence stars is typically
4-5 V, which is one order of magnitude smaller than the value assumed by the
model of magnetic grain trapping. On the basis of our numerical simulation on
sublimation of dust grains in the vicinity of a star, it turns out that their
lives end due to sublimation in a timescale much shorter than the period of one
revolution at the gyroradius. It is, therefore, infeasible to dynamically
extend the dwell time of hot grains inside the sublimation zone by magnetic
trapping, ...",1808.03389v1
2020-01-11,Comparative Study of Data-driven Solar Coronal Field Models Using a Flux Emergence Simulation as a Ground-truth Data Set,"For a better understanding of magnetic field in the solar corona and dynamic
activities such as flares and coronal mass ejections, it is crucial to measure
the time-evolving coronal field and accurately estimate the magnetic energy.
Recently, a new modeling technique called the data-driven coronal field model,
in which the time evolution of magnetic field is driven by a sequence of
photospheric magnetic and velocity field maps, has been developed and revealed
the dynamics of flare-productive active regions. Here we report on the first
qualitative and quantitative assessment of different data-driven models using a
magnetic flux emergence simulation as a ground-truth (GT) data set. We compare
the GT field with those reconstructed from the GT photospheric field by four
data-driven algorithms. It is found that, at least, the flux rope structure is
reproduced in all coronal field models. Quantitatively, however, the results
show a certain degree of model dependence. In most cases, the magnetic energies
and relative magnetic helicity are comparable to or at most twice of the GT
values. The reproduced flux ropes have a sigmoidal shape (consistent with GT)
of various sizes, a vertically-standing magnetic torus, or a packed structure.
The observed discrepancies can be attributed to the highly non-force-free input
photospheric field, from which the coronal field is reconstructed, and to the
modeling constraints such as the treatment of background atmosphere, the bottom
boundary setting, and the spatial resolution.",2001.03721v1
2020-07-30,Large-scale magnetic field in the Monoceros OB-1 East molecular cloud,"We study the large-scale magnetic field structure and its interplay with the
gas dynamics in the Monoceros OB1 East molecular cloud. We combine observations
of dust polarised emission from the Planck telescope and CO molecular line
emission observations from the Taeduk Radio Astronomy Observatory 14-metre
telescope. We calculate the strength of the plane-of-the-sky magnetic field
using a modified Chandrasekhar-Fermi method and estimate mass over flux ratios
in different regions of the cloud. We use the comparison of the velocity and
intensity gradients of the molecular line observations with the polarimetric
observations to trace dynamically active regions. The molecular complex shows
an ordered large-scale plane-of-the-sky magnetic field structure. In the
Northern part, it is mostly orientated along the filamentary structures while
the Southern part shows at least two regions with distinct magnetic field
orientations. We find that in the Northern filaments the magnetic field is
unlikely to provide support against fragmentation at large scales. Our analysis
reveals a shock region in the Northern part of the complex right in-between two
filamentary clouds which were previously suggested to be in collision.
Moreover, the shock seems to extend farther towards the Western part of the
complex. In the Southern part, we find that either the magnetic field guides
the accretion of interstellar matter towards the cloud or it was dragged by the
matter towards the densest regions. The large-scale magnetic field in Monoceros
OB-1 East molecular clouds is tightly connected to the global structure of the
complex and, in the Northern part, it seems to be dominated by gravity and
turbulence, while in the Southern part it influences the structuring of matter.",2007.15344v2
2020-12-02,Synthesis and Characterization of Sodium Iron Antimonate Na2FeSbO5 One-Dimensional Antiferromagnetic Chain Compound with a Spin Glass Ground State,"A new oxide, sodium iron antimonate, Na2FeSbO5, was synthesized and
structurally characterized, and its static and dynamic magnetic properties were
comprehensively studied both experimentally by dc and ac magnetic
susceptibility, magnetization, specific heat, electron spin resonance (ESR) and
Moessbauer measurements, and theoretically by density functional calculations.
The resulting single-crystal structure (a = 15.6991(9) A; b = 5.3323 (4) A; c =
10.8875(6) A; S.G. Pbna) consists of edge-shared SbO6 octahedral chains, which
alternate with vertex-linked, magnetically active FeO4 tetrahedral chains. The
57Fe Moessbauer spectra confirmed the presence of high-spin Fe3+ (3d5) ions in
a distorted tetrahedral oxygen coordination. The magnetic susceptibility and
specific heat data show the absence of a long-range magnetic ordering in
Na2FeSbO5 down to 2 K, but ac magnetic susceptibility unambigously demonstrates
spin-glass-type behavior with a unique two-step freezing at Tf1 about 80 K and
Tf2 about 35 K. Magnetic hyperfine splitting of 57Fe Moessbauer spectra was
observed below T* about 104 K (Tf1 < T*). The spectra just below T* (Tf1 < T <
T*) exhibit a relaxation behavior caused by critical spin fluctuations,
indicating the existence of short-range correlations. The stochastic model of
ionic spin relaxation was used to account for the shape of the Moessbauer
spectra below the freezing temperature. A complex slow dynamics is further
supported by ESR data revealing two different absorption modes presumably
related to ordered and disordered segments of spin chains. The data imply a
spin-cluster ground state for Na2FeSbO5.",2012.01106v1
2021-10-22,Jet Launching from Merging Magnetized Binary Neutron Stars with Realistic Equations of State,"We perform general relativistic, magnetohydrodynamic (GRMHD) simulations of
binary neutron stars in quasi-circular orbit that merge and undergo delayed or
prompt collapse to a black hole (BH). The stars are irrotational and modeled
using an SLy or an H4 nuclear equation of state. To assess the impact of the
initial magnetic field configuration on jet launching, we endow the stars with
a purely poloidal magnetic field that is initially unimportant dynamically and
is either confined to the stellar interior or extends from the interior into
the exterior as in typical pulsars. Consistent with our previous results, we
find that only the BH + disk remnants originating from binaries that form
hypermassive neutron stars (HMNSs) and undergo delayed collapse can drive
magnetically-powered jets. We find that the closer the total mass of the binary
is to the threshold value for prompt collapse, the shorter is the time delay
between the gravitational wave peak amplitude and jet launching. This time
delay also strongly depends on the initial magnetic field configuration. We
also find that seed magnetic fields confined to the stellar interior can launch
a jet over $\sim 25\,\rm ms$ later than those with pulsar-like magnetic fields.
The lifetime of the jet [$\Delta t\lesssim 150\,\rm ms$] and its outgoing
Poynting luminosity [$L_{\rm EM}\sim 10^{52\pm 1}\rm erg/s$] are consistent
with typical short gamma-ray burst central engine lifetimes, as well as with
the Blandford--Znajek mechanism for launching jets and their associated
Poynting luminosities. Our numerical results also suggest that the dynamical
ejection of matter can be enhanced by the magnetic field. Therefore, GRMHD
studies are required to fully understand kilonova signals from GW170818-like
events.",2110.11968v2
2022-02-25,Energy balance and Alfvén Mach numbers in compressible magnetohydrodynamic turbulence with a large-scale magnetic field,"Energy equipartition is a powerful theoretical tool for understanding
astrophysical plasmas. It is invoked, for example, to measure magnetic fields
in the interstellar medium (ISM), as evidence for small-scale turbulent dynamo
action, and, in general, to estimate the energy budget of star-forming
molecular clouds. In this study we motivate and explore the role of the
volume-averaged root-mean-squared (rms) magnetic coupling term between the
turbulent, $\delta\mathbf{B}$ and large-scale, $\mathbf{B}_0$ fields, $\left<
(\delta\mathbf{B}\cdot\mathbf{B}_0)^{2} \right>^{1/2}_{\mathcal{V}}$. By
considering the second moments of the energy balance equations we show that the
rms coupling term is in energy equipartition with the volume-averaged turbulent
kinetic energy for turbulence with a sub-Alfv\'enic large-scale field. Under
the assumption of exact energy equipartition between these terms, we derive
relations for the magnetic and coupling term fluctuations, which provide
excellent, parameter-free agreement with time-averaged data from 280 numerical
simulations of compressible MHD turbulence. Furthermore, we explore the
relation between the turbulent, mean-field and total Alfv\'en Mach numbers, and
demonstrate that sub-Alfv\'enic turbulence can only be developed through a
strong, large-scale magnetic field, which supports an extremely
super-Alfv\'enic turbulent magnetic field. This means that the magnetic field
fluctuations are significantly subdominant to the velocity fluctuations in the
sub-Alfv\'enic large-scale field regime. Throughout our study, we broadly
discuss the implications for observations of magnetic fields and understanding
the dynamics in the magnetised ISM.",2202.13020v2
2022-05-30,Sustaining Rabi oscillations by using a phase-tunable image drive,"Recent electron spin resonance experiments on CaWO$_4$:Gd$^{3+}$ and on other
magnetic impurities have demonstrated that sustained Rabi oscillations can be
created by driving a magnetic moment with a microwave field frequency slightly
larger than the Larmor frequency and tuned to the Floquet resonance together
with another microwave field (image drive). These observations are confirmed by
the new experimental results reported in this paper. We investigate several
mechanisms of decoherence and dissipation by using a combination of numerical
and analytical techniques. The first microscopic model describes a magnetic
moment in external magnetic fields, interacting with a bath of two-level
systems acting as a source of decoherence and dissipation. The second model
describes a collection of the identical, interacting magnetics moments, all
subject to the same magnetic fields. In this case, the many-body interactions
causes a decay of the Rabi oscillations. In addition, we also study the effect
of the inhomogeneity of the microwave radiation on the decay of the Rabi
oscillations. Our simulation results show that the dynamics of a magnetic
moment subject to the two microwave fields with different frequencies and in
contact with an environment is highly nontrivial. We show that under
appropriate conditions, and in particular at the Floquet resonance, the
magnetization exhibits sustained Rabi oscillations, in some cases with
additional beatings. Although these two microscopic models separately describe
the experimental data well, a simulation study that simultaneously accounts for
both types of interactions is currently prohibitively costly. To gain further
insight into the microscopic dynamics of these two different models, we study
the time dependence of the bath and system energy and of the correlations of
the spins, data that is not readily accessible experimentally.",2205.14863v1
2023-11-01,"Misaligned magnetized accretion flows onto spinning black holes: magneto-spin alignment, outflow power and intermittent jets","Magnetic fields regulate black hole (BH) accretion, governing both the inflow
and outflow dynamics. When a BH becomes saturated with large-scale vertical
magnetic flux, it enters a magnetically-arrested disk (MAD) state. The
dynamically-important BH magnetic flux powers highly efficient relativistic
outflows (or jets) and sporadically erupts from the BH into the disk midplane.
Here we explore the evolution of MADs when the BH and gas angular momentum are
misaligned, which is expected to be more common. Using numerical simulations,
we find that jets from rapidly spinning, prograde BHs force the inner accretion
flow into alignment with the BH spin via the magneto-spin alignment mechanism
for disks initially misaligned at $\mathcal{T}\lesssim 60^{\circ}$. Extremely
misaligned MAD disks, on the other hand, exhibit intermittent jets that blow
out parts of the disk to $\approx 100$ gravitational radii before collapsing,
leaving behind hot cavities and magnetized filaments. These intermittent jet
mechanism forms a mini-feedback cycle and could explain some cases of X-ray and
radio quasi-periodic eruptions observed in dim AGN. Further, we find that (i)
for BHs with low power jets, the BH spin and initial disk tilt angle changes
the amount of horizon magnetic flux, and (ii) geometrically-thick, misaligned
accretion flows do not undergo sustained Lense-Thirring (LT) precession.
Thereby, we suggest that low-luminosity accreting BHs ($\dot{M}\ll 10^{-3}
\dot{M}_{\rm Edd}$) are not likely to exhibit quasi-periodic oscillations in
lightcurves due to LT precession, in agreement with observations of BH X-ray
binaries and AGN in the low-hard/quiescent state. Instead, we suggest that
magnetic flux eruptions can mimic precession-like motion, such as observed in
the M87 jet, by driving large-scale surface waves in the jets.",2311.00432v1
2023-12-04,Magnetic Fields in the Central Molecular Zone Influenced by Feedback and Weakly Correlated with Star Formation,"Magnetic fields of molecular clouds in the Central Molecular Zone (CMZ) have
been relatively underobserved at sub-parsec resolution. Here we report
JCMT/POL2 observations of polarized dust emission in the CMZ, which reveal
magnetic field structures in dense gas at ~0.5 pc resolution. The eleven
molecular clouds in our sample including two in the western part of the CMZ
(Sgr C and a far-side cloud candidate), four around the Galactic longitude 0
(the 50 km s-1 cloud, CO0.02-0.02, the `Stone' and the `Sticks & Straw' among
the Three Little Pigs), and five along the Dust Ridge (G0.253+0.016, clouds b,
c, d, and e/f), for each of which we estimate the magnetic field strength using
the angular dispersion function method. The morphologies of magnetic fields in
the clouds suggest potential imprints of feedback from expanding H II regions
and young massive star clusters. A moderate correlation between the total viral
parameter versus the star formation rate and the dense gas fraction of the
clouds is found. A weak correlation between the mass-to-flux ratio and the star
formation rate, and a weak anti-correlation between the magnetic field and the
dense gas fraction are also found. Comparisons between magnetic fields and
other dynamic components in clouds suggest a more dominant role of self-gravity
and turbulence in determining the dynamical states of the clouds and affecting
star formation at the studied scales.",2312.01776v2
2007-05-12,"A case for variational geomagnetic data assimilation: insights from a one-dimensional, nonlinear, and sparsely observed MHD system","Secular variations of the geomagnetic field have been measured with a
continuously improving accuracy during the last few hundred years, culminating
nowadays with satellite data. It is however well known that the dynamics of the
magnetic field is linked to that of the velocity field in the core and any
attempt to model secular variations will involve a coupled dynamical system for
magnetic field and core velocity. Unfortunately, there is no direct observation
of the velocity. Independently of the exact nature of the above-mentioned
coupled system -- some version being currently under construction -- the
question is debated in this paper whether good knowledge of the magnetic field
can be translated into good knowledge of core dynamics. Furthermore, what will
be the impact of the most recent and precise geomagnetic data on our knowledge
of the geomagnetic field of the past and future? These questions are cast into
the language of variational data assimilation, while the dynamical system
considered in this paper consists in a set of two oversimplified
one-dimensional equations for magnetic and velocity fields. This toy model
retains important features inherited from the induction and Navier-Stokes
equations: non-linear magnetic and momentum terms are present and its linear
response to small disturbances contains Alfv\'en waves. It is concluded that
variational data assimilation is indeed appropriate in principle, even though
the velocity field remains hidden at all times; it allows us to recover the
entire evolution of both fields from partial and irregularly distributed
information on the magnetic field. This work constitutes a first step on the
way toward the reassimilation of historical geomagnetic data and geomagnetic
forecast.",0705.1777v1
2008-07-30,The solar chromosphere at high resolution with IBIS. II. Acoustic shocks in the quiet internetwork and the role of magnetic fields,"(Abridged)
  Aims: We characterize the dynamics of the quiet inter-network chromosphere by
studying the occurrence of acoustic shocks and their relation with the
concomitant photospheric structure and dynamics.
  Methods: We analyze a comprehensive data set that includes high resolution
chromospheric and photospheric spectra obtained with the IBIS imaging
spectrometer in two quiet-Sun regions. This is complemented by high-resolution
sequences of MDI magnetograms of the same targets. From the chromospheric
spectra we identify the spatio-temporal occurrence of the acoustic shocks. We
compare it with the photospheric dynamics by means of both Fourier and wavelet
analysis, and study the influence of magnetic structures.
  Results: Mid-chromospheric shocks occur as a response to underlying powerful
photospheric motions at periodicities nearing the acoustic cut-off, consistent
with 1-D hydrodynamical modeling. However, their spatial distribution within
the supergranular cells is highly dependent on the local magnetic topology,
both at the network and internetwork scale. Large portions of the internetwork
regions undergo very few shocks, as ""shadowed"" by the horizontal component of
the magnetic field. The latter is betrayed by the presence of chromospheric
fibrils, observed in the core of the CaII line as slanted structures with
distinct dynamical properties. The shadow mechanism appears to operate also on
the very small scales of inter-network magnetic elements, and provides for a
very pervasive influence of the magnetic field even in the quietest region
analyzed.",0807.4966v2
2014-05-15,Dynamic properties of bright points in an active region,"Context. Bright points (BPs) are small-scale, magnetic features ubiquitous
across the solar surface. Previously, we have observed and noted their
properties for quiet Sun regions. Here, we determine the dynamic properties of
BPs using simultaneous quiet Sun and active region data. Methods. High spatial
and temporal resolution G-band observations of active region AR11372 were
obtained with the Rapid Oscillations in the Solar Atmosphere instrument at the
Dunn Solar Telescope. Three subfields of varying polarity and magnetic flux
density were selected with the aid of magnetograms obtained from the
Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory.
Bright points within these subfields were subsequently tracked and analysed.
Results. It is found that BPs within active regions display attenuated velocity
distributions with an average horizontal velocity of ~0.6 km/s, compared to the
quiet region which had an average velocity of 0.9 km/s. Active region BPs are
also ~21% larger than quiet region BPs and have longer average lifetimes
(~132s) than their quiet region counterparts (88 s). No preferential flow
directions are observed within the active region subfields. The diffusion index
(gamma) is estimated at ~1.2 for the three regions. Conclusions. We confirm
that the dynamic properties of BPs arise predominately from convective motions.
The presence of stronger field strengths within active regions is the likely
reason behind the varying properties observed. We believe that larger amounts
of magnetic flux will attenuate BP velocities by a combination of restricting
motion within the intergranular lanes and by increasing the number of
stagnation points produced by inhibited convection. Larger BPs are found in
regions of higher magnetic flux density and we believe that lifetimes increase
in active regions as the magnetic flux stabilises the BPs.",1405.3923v1
2017-10-09,Magnetization dynamics of weakly interacting sub-100 nm square artificial spin ices,"Artificial Spin Ice (ASI), consisting of a two dimensional array of nanoscale
magnetic elements, provides a fascinating opportunity to observe the physics of
out of equilibrium systems. Initial studies concentrated on the static, frozen
state, whilst more recent studies have accessed the out-of-equilibrium dynamic,
fluctuating state. This opens up exciting possibilities such as the observation
of systems exploring their energy landscape through monopole quasiparticle
creation, potentially leading to ASI magnetricity, and to directly observe
unconventional phase transitions. In this work we have measured and analysed
the magnetic relaxation of thermally active ASI systems by means of SQUID
magnetometry. We have investigated the effect of the interaction strength on
the magnetization dynamics at different temperatures in the range where the
nanomagnets are thermally active and have observed that they follow an
Arrhenius-type N\'eel-Brown behaviour. An unexpected negative correlation of
the average blocking temperature with the interaction strength is also
observed, which is supported by Monte Carlo simulations. The magnetization
relaxation measurements show faster relaxation for more strongly coupled
nanoelements with similar dimensions. The analysis of the stretching exponents
obtained from the measurements suggest 1-D chain-like magnetization dynamics.
This indicates that the nature of the interactions between nanoelements lowers
the dimensionality of the ASI from 2-D to 1-D. Finally, we present a way to
quantify the effective interaction energy of a square ASI system, and compare
it to the interaction energy calculated from a simple dipole model and also to
the magnetostatic energy computed with micromagnetic simulations.",1710.03018v2
2018-02-02,"Stellar dynamics: rotation, convection, and magnetic fields","Stars are changing entities in a constant evolution during their lives. At
non-secular time scales (from seconds to years) the effect of dynamical
processes such as convection, rotation, and magnetic fields can modify the
stellar oscillations. Convection excites acoustic modes in solar-like stars,
while rotation and magnetic fields can perturb the oscillation frequencies
lifting the degeneracy in the azimuthal component m of the eigenfrequencies.
Moreover, the interaction between rotation, convection, and magnetic fields can
produce magnetic dynamos, which sometimes yield to regular magnetic activity
cycles. In this chapter we review how stellar dynamics can be studied and
explain what long-term seismic observations can bring to the understanding of
this field. Thus, we show how we can study some properties of the convective
time scales operating in a star like the Sun. We also compare the stratified
information we can obtain on the internal (radial) differential rotation from
main sequence solar-like stars, to the Sun, and to more evolved sub giants, and
giants. We complement this information on the internal rotation with the
determination of the surface (latitudinal differential) rotation obtained
directly from the light curves. Indeed, when stars are active there can be
spots on their surfaces dimming the light emitted. When the star rotates, the
emitted light will be modulated by the presence of these spots with a period
corresponding to the rotation rate at the active latitudes (where the spots
develop). We conclude this chapter by discussing the seismology of fast
rotating stars and, from a theoretical point of view, what are the current
challenges to infer properties of the internal structure and dynamics of
intermediate- and high-mass stars.",1802.00666v2
2019-08-28,Masses and Implications for Ages of Low-Mass Pre-Main Sequence Stars in Taurus and Ophiuchus,"The accuracy of masses of pre-main sequence (PMS) stars derived from their
locations on the Hertzsprung-Russell Diagram (HRD) can be tested by comparison
with accurate and precise masses determined independently. We present 29 single
stars in the Taurus star-forming region (SFR) and 3 in the Ophiuchus SFR with
masses measured dynamically to a precision of at least $10 \%$. Our results
include 9 updated mass determinations and 3 that have not had their dynamical
masses published before. This list of stars with fundamental, dynamical masses,
M$_{dyn}$, is drawn from a larger list of 39 targets in the Taurus SFR and 6 in
the Ophiuchus SFR. Placing the stars with accurate and precise dynamical masses
on HRDs that do not include internal magnetic fields underestimates the mass
compared to M$_{dyn}$ by about $30 \%$. Placing them on an HRD that does
include magnetic fields yields mass estimates in much better agreement with
M$_{dyn}$, with an average difference between M$_{dyn}$ and the estimated track
mass of $0.01\pm0.02$~\msun. The ages of the stars, 3--10 MY on tracks that
include magnetic fields, is older than the 1--3 MY indicated by the
non-magnetic models. The older ages of T Tauri stars predicted by the magnetic
models increase the time available for evolution of their disks and formation
of the giant gas exoplanets. The agreement between our M$_{dyn}$ values and the
masses on the magnetic field tracks provides indirect support for these older
ages.",1908.10952v1
2019-10-30,Large-scale dynamics of winds originated from black hole accretion flows: (II) Magnetohydrodynamics,"Winds from black hole accretion disks are essential ingredients in
understanding the coevolution between the supermassive black hole and its host
galaxy. The great difference of dynamical ranges from small-scale accretion
disk simulations to large-scale or cosmological simulations places barriers to
track wind kinematics. In the first paper of this series, we have studied the
dynamics of disk winds from the outer edge of the accretion disk toward galaxy
scales in the hydrodynamical framework. In this paper, we further incorporate
magnetic fields to understand the wind dynamics by adopting one-dimensional
magnetohydrodynamical (MHD) model, with boundary conditions set for hot
accretion flows. The geometry of poloidal magnetic field is prescribed as a
straight line with an angle $\theta=45^\circ$ from the rotational axis, and the
strength satisfies the divergence free condition. The wind solution is achieved
through requesting gas to pass through the slow, Alfv\'{e}n and fast
magneto-sonic points smoothly. Physical quantities are found to show a
power-law dependence on cylindrical radius $R$ beyond the fast magneto-sonic
point, for which $\rho \propto R^{-2}, v_{\rm p}\propto {\rm const.}, v_{\rm
\phi}\propto R^{-1}, B_{\rm \phi}\propto R^{-1},$ and $ \beta \propto
\rho^{\gamma-1}$. The magnetization of wind is dominant in determining the wind
properties. The wind is accelerated to a greater terminal velocity with strong
magnetization ($v_{\rm Ap0}>1$) compared to the hydrodynamical case, which the
magnetic pressure gradient dominates and the centrifugal potential converts to
the kinetic energy. The dependance of wind physical quantities on
magnetization, temperature, field line angular velocity, and adiabatic index is
also discussed.",1910.13779v4
2020-03-21,First-principles perspective on magnetic second sound,"The fluctuations of the magnetic order parameter, or longitudinal spin
excitations, are investigated theoretically in the ferromagnetic Fe and Ni as
well as in the antiferromagnetic phase of the pnictide superconductor FeSe. The
charge and spin dynamics of these systems is described by evaluating the
generalized charge and spin density response function calculated from
first-principles linear response time dependent density functional theory
within adiabatic local spin density approximation. We observe that the formally
non-interacting Kohn-Sham system features strong coupling between the
magnetization and charge dynamics in the longitudinal channel and that the
coupling is effectively removed upon the inclusion of the Coulomb interaction
in the charge channel and the resulting appearance of plasmons. The
longitudinal spin fluctuations acquire a collective character without the
emergence of the Goldstone boson, similar to the case of paramagnon excitations
in non-magnetic metals like Pd. In ferromagnetic Fe and Ni the longitudinal
spin dynamics is governed by interactions between low-energy intraband
electron-hole pairs while in quasi two dimensional antiferromagnet FeSe it is
dominated by the interband transitions with energies of the order of exchange
splitting. In the later material, the collective longitudinal magnetization
fluctuations feature well defined energies and long life times for small
momenta and appear below the particle-hole continuum. The modes become strongly
Landau-damped for growing wave-vectors. We relate our theoretical findings to
existing experimental spin-polarized electron energy loss spectroscopy results.
In bulk bcc Fe, the longitudinal magnetic modes appear above the typical
energies of transverse spin-waves, have energies comparable with the Stoner
spin-flip excitation continuum, and are order of magnitude less energetic than
the charge dynamics.",2003.09735v3
2021-04-20,Unusual spin dynamics in the low-temperature magnetically ordered state of Ag$_{3}$LiIr$_{2}$O$_{6}$,"Recently, there have been contrary claims of Kitaev spin-liquid behaviour and
ordered behavior in the honeycomb compound Ag$_3$LiIr$_2$O$_6$ based on various
experimental signatures. Our investigations on this system reveal a
low-temperature ordered state with persistent dynamics down to the lowest
temperatures. Magnetic order is confirmed by clear oscillations in the muon
spin relaxation ($\mu$SR) time spectrum below 9 K till 52 mK. Coincidentally in
$^7$Li nuclear magnetic resonance, a wipe-out of the signal is observed below
$\sim$ 10 K which again strongly indicates magnetic order in the low
temperature regime. This is supported by our density functional theory
calculations which show an appreciable Heisenberg exchange term in the spin
Hamiltonian that favors magnetic ordering. The $^7$Li shift and spin-lattice
relaxation rate also show anomalies at $\sim$ 50 K. They are likely related to
the onset of dynamic magnetic correlations, but their origin is not completely
clear. Detailed analysis of our $\mu$SR data is consistent with a co-existence
of incommensurate N\'eel and striped environments. A significant and
undiminished dynamical relaxation rate ($\sim 5$ MHz) as seen in $\mu$SR deep
into the ordered phase indicates enhanced quantum fluctuations in the ordered
state.",2104.10057v2
2023-05-12,Improving the Understanding of Subsurface Structure and Dynamics of Solar Active Regions (A white paper submitted to the decadal survey for solar and space Physics (Heliophysics) -- SSPH 2024-2033),"The goal of helioseismology is to provide accurate information about the
Sun's interior from the observations of the wave field at its surface. In the
last three decades, both global and local helioseismology studies have made
significant advances and breakthroughs in solar physics. However, 3-d mapping
of the structure and dynamics of sunspots and active regions below the surface
has been a challenging task and are among the longest standing and intriguing
puzzles of solar physics due to the complexity of the turbulent and dynamic
nature of sunspots. Thus the key problems that need to be addressed during the
next decade are: (i) Understanding the wave excitation mechanisms in the quiet
Sun and magnetic regions, (ii) Characterizing the wave propagation and
transformation in strong and inclined magnetic field regions and understanding
the magnetic portals in the chromosphere, (iii) Improving helioseismology
techniques and investigating the whole life cycle of active regions, from
magnetic flux emergence to dissipation, and (iv) Detecting helioseismic
signature of the magnetic flux of active regions before it becomes visible on
the surface so as to provide warnings several days before the emergence. For a
transformative progress on these problems require full disk, simultaneous
Doppler and vector magnetic field measurements of the photosphere up to the
chromosphere with a spatial resolution of about 2 arc-sec as well as
large-scale radiative MHD simulations of the plasma dynamics from the
sub-photosphere to the chromosphere.",2305.07585v1
2017-01-05,Turbulent geodynamo simulations: a leap towards Earth's core,"We present an attempt to reach realistic turbulent regime in direct numerical
simulations of the geodynamo. We rely on a sequence of three convection-driven
simulations in a rapidly rotating spherical shell. The most extreme case
reaches towards the Earth's core regime by lowering viscosity (magnetic Prandtl
number Pm=0.1) while maintaining vigorous convection (magnetic Reynolds number
Rm>500) and rapid rotation (Ekman number E=1e-7), at the limit of what is
feasible on today's supercomputers. A detailed and comprehensive analysis
highlights several key features matching geomagnetic observations or dynamo
theory predictions -- all present together in the same simulation -- but it
also unveils interesting insights relevant for Earth's core dynamics.In this
strong-field, dipole-dominated dynamo simulation, the magnetic energy is one
order of magnitude larger than the kinetic energy. The spatial distribution of
magnetic intensity is highly heterogeneous, and a stark dynamical contrast
exists between the interior and the exterior of the tangent cylinder (the
cylinder parallel to the axis of rotation that circumscribes the inner core).In
the interior, the magnetic field is strongest, and is associated with a
vigorous twisted polar vortex, whose dynamics may occasionally lead to the
formation of a reverse polar flux patch at the surface of the shell.
Furthermore, the strong magnetic field also allows accumulation of light
material within the tangent cylinder, leading to stable stratification there.
Torsional Alfv{\'e}n waves are frequently triggered in the vicinity of the
tangent cylinder and propagate towards the equator.Outside the tangent
cylinder, the magnetic field inhibits the growth of zonal winds and the kinetic
energy is mostly non-zonal. Spatio-temporal analysis indicates that the
low-frequency, non-zonal flow is quite geostrophic (columnar) and predominantly
large-scale: an m=1 eddy spontaneously emerges in our most extreme simulations,
without any heterogeneous boundary forcing.Our spatio-temporal analysis further
reveals that (i) the low-frequency, large-scale flow is governed by a balance
between Coriolis and buoyancy forces -- magnetic field and flow tend to align,
minimizing the Lorentz force; (ii) the high-frequency flow obeys a balance
between magnetic and Coriolis forces; (iii) the convective plumes mostly live
at an intermediate scale, whose dynamics is driven by a 3-term 1 MAC balance --
involving Coriolis, Lorentz and buoyancy forces. However, small-scale (E^{1/3})
quasi-geostrophic convection is still observed in the regions of low magnetic
intensity.",1701.01299v3
1995-03-30,Coarsening in the q-State Potts Model and the Ising Model with Globally Conserved Magnetization,"We study the nonequilibrium dynamics of the $q$-state Potts model following a
quench from the high temperature disordered phase to zero temperature. The time
dependent two-point correlation functions of the order parameter field satisfy
dynamic scaling with a length scale $L(t)\sim t^{1/2}$. In particular, the
autocorrelation function decays as $L(t)^{-\lambda(q)}$. We illustrate these
properties by solving exactly the kinetic Potts model in $d=1$. We then analyze
a Langevin equation of an appropriate field theory to compute these correlation
functions for general $q$ and $d$. We establish a correspondence between the
two-point correlations of the $q$-state Potts model and those of a kinetic
Ising model evolving with a fixed magnetization $(2/q-1)$. The dynamics of this
Ising model is solved exactly in the large q limit, and in the limit of a large
number of components $n$ for the order parameter. For general $q$ and in any
dimension, we introduce a Gaussian closure approximation and calculate within
this approximation the scaling functions and the exponent $\lambda (q)$. These
are in good agreement with the direct numerical simulations of the Potts model
as well as the kinetic Ising model with fixed magnetization. We also discuss
the existing and possible experimental realizations of these models.",9503163v1
2005-01-21,The isotropic XY model on the inhomogeneous periodic chain,"The static and dynamic properties of the isotropic XY-model $(s=1/2)$ on the
inhomogeneous periodic chain, composed of \emph{N} segments with \emph{n}
different exchange interactions and magnetic moments, in a transverse field
\emph{h} are obtained exactly at arbitrary temperatures. The properties are
determined by introducing the generalized Jordan-Wigner transformation and by
reducing the problem to a diagonalization of a finite matrix of \emph{n-th}
order. The diagonalization procedure is discussed in detail and the critical
behaviour induced by the transverse field, at T=0, is presented. The quantum
transitions are determined by analyzing the behaviour of the induced
magnetization, defined as $(1/n)\sum_{m=1}^{n}\mu_{m}<S_{j,m}^{z}>$ where
$\mu_{m}$ is the magnetic moment at site \emph{m} within the segment \emph{j},
as a function of the field, and the critical fields determined exactly. The
dynamic correlations, $<S_{j,m}^{z}(t)S_{j^{'},m^{'}}^{z}(0)>$, and the dynamic
susceptibility $\chi_{q}^{zz}(\omega)$ are also obtained at arbitrary
temperatures. Explicit results are also presented in the limit T=0, where the
critical behaviour occurs, for the static susceptibility $\chi_{q}^{zz}(0)$ as
a function of the transverse field \emph{h}, and for the frequency dependency
of dynamic susceptibility $\chi_{q}^{zz}(\omega)$. Also in this limit, the
transverse time-correlation $<S_{j,m}^{x}(t)S_{j^{'},m^{'}}^{x}(0)>$, the
dynamic and isothermal susceptibilities, $\chi_{q}^{xx}(\omega)$ and
$\chi_{T}^{xx}$, are obtained for the transverse field greater or equal than
the saturation field.",0501525v1
2006-03-14,Critical Behavior of an Ising System on the Sierpinski Carpet: A Short-Time Dynamics Study,"The short-time dynamic evolution of an Ising model embedded in an infinitely
ramified fractal structure with noninteger Hausdorff dimension was studied
using Monte Carlo simulations. Completely ordered and disordered spin
configurations were used as initial states for the dynamic simulations. In both
cases, the evolution of the physical observables follows a power-law behavior.
Based on this fact, the complete set of critical exponents characteristic of a
second-order phase transition was evaluated. Also, the dynamic exponent $\theta
$ of the critical initial increase in magnetization, as well as the critical
temperature, were computed. The exponent $\theta $ exhibits a weak dependence
on the initial (small) magnetization. On the other hand, the dynamic exponent
$z$ shows a systematic decrease when the segmentation step is increased, i.e.,
when the system size becomes larger. Our results suggest that the effective
noninteger dimension for the second-order phase transition is noticeably
smaller than the Hausdorff dimension. Even when the behavior of the
magnetization (in the case of the ordered initial state) and the
autocorrelation (in the case of the disordered initial state) with time are
very well fitted by power laws, the precision of our simulations allows us to
detect the presence of a soft oscillation of the same type in both magnitudes
that we attribute to the topological details of the generating cell at any
scale.",0603387v1
2006-08-02,Magnetocatalytic Adiabatic Spin Torque Orbital Transformations for Novel Chemical and Catalytic Reaction Dynamics: The Little Effect,"In this manuscript the theory and phenomena associated with the Little Effect
are introduced as the spin induced orbital dynamics of confined fermions under
strong magnetic and thermal environments. This Little Effect is considered in
details for the electron transfer reactions associated with redox processes of
Cu-Ag alloy within deionized water and for the orbital dynamics during the iron
catalyzed covalent bond rearrangements associated with amorphous carbon
conversion to diamond. Furthermore, prolong extreme conditions of 74,000 amps,
403 V, strong Lorentz compression, and thermal stresses upon this Cu-Ag- H2O
system on the basis of the Little Effect of high spin, thermally induced
orbital dynamics are predicted and demonstrated to cause the magnetically
organized reverse beta, electron capture, proton capture and neutron capture
processes for various infrequent pycnonuclear transmutations within the Cu-Ag
coil. The general experimental verification and the broad implications of this
Little Effect on chemistry are demonstrated within these two ideal systems: an
ionic case and a molecular case. The Little Effect is contrasted with the
Hedvall Effect as a dynamical phenomenon causing the kinematics of the Hedvall
Effect. The compatibility of the Little Effect with the Woodward-Hoffmann Rule
is demonstrated. The Little Effect provides greater understanding of order in
systems far from equilibrium. The implications of the Little Effect for other
interesting phenomena such as ferromagnetism, unconventional magnetism,
superparamagnetism, superconductivity, and pycnonuclear effects are concluded.",0608071v1
2000-05-09,Dynamical Symmetry Breaking in Supersymmetric SU(n_c) and USp(2n_c) Gauge Theories,"We find the phase and flavor symmetry breaking pattern of each N=1
supersymmetric vacuum of SU(n_c) and USp(2 n_c) gauge theories, constructed
from the exactly solvable N=2 theories by perturbing them with small adjoint
and generic bare hypermultiplet (quark) masses. In SU(n_c) theories with n_f
\leq n_c the vacua are labelled by an integer r, in which the flavor U(n_f)
symmetry is dynamically broken to U(r) \times U(n_f-r) in the limit of
vanishing bare hyperquark masses. In the r=1 vacua the dynamical symmetry
breaking is caused by the condensation of magnetic monopoles in the n_f
representation. For general r, however, the monopoles in the {}_{n_f}C_r
representation, whose condensation could explain the flavor symmetry breaking
but would produce too-many Nambu--Goldstone multiplets, actually ""break up""
into ""magnetic quarks"": the latter with nonabelian interactions condense and
induce confinement and dynamical symmetry breaking. In USp(2n_c) theories with
n_f \leq n_c + 1, the flavor SO(2n_f) symmetry is dynamically broken to U(n_f),
but with no description in terms of a weakly coupled local field theory. In
both SU(n_c) and USp(2 n_c) theories, with larger numbers of quark flavors,
besides the vacua with these properties, there exist also vacua in free
magnetic phase, with unbroken global symmetry.",0005076v2
2006-09-07,"Magnetic Monopole Dynamics, Supersymmetry, and Duality","We review the properties of BPS, or supersymmetric, magnetic monopoles, with
an emphasis on their low-energy dynamics and their classical and quantum bound
states. After an overview of magnetic monopoles, we discuss the BPS limit and
its relation to supersymmetry. We then discuss the properties and construction
of multimonopole solutions with a single nontrivial Higgs field. The low-energy
dynamics of these monopoles is most easily understood in terms of the moduli
space and its metric. We describe in detail several known examples of these.
This is then extended to cases where the unbroken gauge symmetry include a
non-Abelian factor. We next turn to the generic supersymmetric Yang-Mills (SYM)
case, in which several adjoint Higgs fields are present. Working first at the
classical level, we describe the effects of these additional scalar fields on
the monopole dynamics, and then include the contribution of the fermionic zero
modes to the low-energy dynamics. The resulting low-energy effective theory is
itself supersymmetric. We discuss the quantization of this theory and its
quantum BPS states, which are typically composed of several loosely bound
compact dyonic cores. We close with a discussion of the D-brane realization of
${\cal N}=4$ SYM monopoles and dyons and explain the ADHMN construction of
monopoles from the D-brane point of view.",0609055v2
2010-04-23,Time domain study of frequency-power correlation in spin-torque oscillators,"This paper describes a numerical experiment, based on full micromagnetic
simulations of current-driven magnetization dynamics in nanoscale spin valves,
to identify the origins of spectral linewidth broadening in spin torque
oscillators. Our numerical results show two qualitatively different regimes of
magnetization dynamics at zero temperature: regular (single-mode precessional
dynamics) and chaotic. In the regular regime, the dependence of the oscillator
integrated power on frequency is linear, and consequently the dynamics is well
described by the analytical theory of current-driven magnetization dynamics for
moderate amplitudes of oscillations. We observe that for higher oscillator
amplitudes, the functional dependence of the oscillator integrated power as a
function of frequency is not a single-valued function and can be described
numerically via introduction of nonlinear oscillator power. For a range of
currents in the regular regime, the oscillator spectral linewidth is a linear
function of temperature. In the chaotic regime found at large current values,
the linewidth is not described by the analytical theory. In this regime we
observe the oscillator linewidth broadening, which originates from sudden jumps
of frequency of the oscillator arising from random domain wall nucleation and
propagation through the sample. This intermittent behavior is revealed through
a wavelet analysis that gives superior description of the frequency jumps
compared to several other techniques.",1004.4184v1
2010-10-28,"The Interplay of Magnetic Fields, Fragmentation and Ionization Feedback in High-Mass Star Formation","Massive stars disproportionately influence their surroundings. How they form
has only started to become clear recently through radiation gas dynamical
simulations. However, until now, no simulation has simultaneously included both
magnetic fields and ionizing radiation. Here we present the results from the
first radiation-magnetohydrodynamical (RMHD) simulation including ionization
feedback, comparing an RMHD model of a 1000 M_sol rotating cloud to earlier
radiation gas dynamical models with the same initial density and velocity
distributions. We find that despite starting with a strongly supercritical mass
to flux ratio, the magnetic field has three effects. First, the field offers
locally support against gravitational collapse in the accretion flow,
substantially reducing the amount of secondary fragmentation in comparison to
the gas dynamical case. Second, the field drains angular momentum from the
collapsing gas, further increasing the amount of material available for
accretion by the central, massive, protostar, and thus increasing its final
mass by about 50% from the purely gas dynamical case. Third, the field is wound
up by the rotation of the flow, driving a tower flow. However, this flow never
achieves the strength seen in low-mass star formation simulations for two
reasons: gravitational fragmentation disrupts the circular flow in the central
regions where the protostars form, and the expanding H II regions tend to
further disrupt the field geometry. Therefore, outflows driven by ionization
heating look likely to be more dynamically important in regions of massive star
formation.",1010.5905v2
2011-03-09,"First-Principles Dynamical Coherent-Potential Approximation Approach to the Ferromagnetism of Fe, Co, and Ni","Magnetic properties of Fe, Co, and Ni at finite temperatures have been
investigated on the basis of the first-principles dynamical CPA (Coherent
Potential Approximation) combined with the LDA (Local Density Approximation) +
$U$ Hamiltonian in the Tight-Binding Linear Muffintin Orbital (TB-LMTO)
representation. The Hamiltonian includes the transverse spin fluctuation terms.
Numerical calculations have been performed within the harmonic approximation
with 4th-order dynamical corrections. Calculated single-particle densities of
states in the ferromagnetic state indicate that the dynamical effects reduce
the exchange splitting, suppress the band width of the quasi-particle state,
and causes incoherent excitations corresponding the 6 eV satellites. Results of
the magnetization vs temperature curves, paramagnetic spin susceptibilities,
and the amplitudes of local moments are presented. Calculated Curie
temperatures ($T_{\rm C}$) are reported to be 1930K for Fe, 2550K for Co, and
620K for Ni; $T_{\rm C}$ for Fe and Co are overestimated by a factor of 1.8,
while $T_{\rm C}$ in Ni agrees with the experimental result. Effective Bohr
magneton numbers calculated from the inverse susceptibilities are 3.0 $\mu_{\rm
B}$ (Fe), 3.0 $\mu_{\rm B}$ (Co), and 1.6 $\mu_{\rm B}$ (Ni), being in
agreement with the experimental ones. Overestimate of $T_{\rm C}$ in Fe and Co
is attributed to the neglects of the higher-order dynamical effects as well as
the magnetic short range order.",1103.1683v1
2014-06-18,Anisotropic transport and early dynamical impact of Cosmic Rays around Supernova remnants,"We present a novel implementation of cosmic rays (CR) in the
magneto-hydrodynamic code FLASH. CRs are described as separate fluids with
different energies. CR advection, energy dependent anisotropic diffusion with
respect to the magnetic field and adiabatic losses to follow the evolution of
spectra are taken into account. We present a first study of the transport and
immediate (~150 kyr) dynamical impact of CRs on the turbulent magnetised
interstellar medium around supernova remnants on scales up to 80 pc. CR
diffusion quickly leads to an efficient acceleration of low-density gas (mainly
perpendicular to the magnetic field) with accelerations up to two orders of
magnitude above the thermal values. Peaked (at 1 GeV) CR injection spectra have
a stronger impact on the dynamics than power-law spectra. For realistic
magnetic field configurations low energy CRs (with smaller diffusion
coefficients) distribute anisotropically with large spatial variations of a
factor of ten and more. Adiabatic losses can change the local spectra
perceptibly but do not have an integral effect on the dynamics at the spatial
and temporal scales considered here. We discuss the potential global impact of
CRs and anisotropic transport on the dynamical structure of the ISM and also
detail the limitations of the model.",1406.4861v1
2014-10-22,Spin dynamics of Mn impurities and their bound acceptors in GaAs,"We present results of tight-binding spin-dynamics simulations of individual
and pairs of substitutional Mn impurities in GaAs. Our approach is based on the
mixed quantum-classical scheme for spin dynamics, with coupled equations of
motions for the quantum subsystem, representing the host, and the localized
spins of magnetic dopants, which are treated classically. In the case of a
single Mn impurity, we calculate explicitly the time evolution of the Mn spin
and the spins of nearest-neighbors As atoms, where the acceptor (hole) state
introduced by the Mn dopant resides. We relate the characteristic frequencies
in the dynamical spectra to the two dominant energy scales of the system,
namely the spin-orbit interaction strength and the value of the p-d exchange
coupling between the impurity spin and the host carriers. For a pair of Mn
impurities, we find signatures of the indirect (carrier-mediated) exchange
interaction in the time evolution of the impurity spins. Finally, we examine
temporal correlations between the two Mn spins and their dependence on the
exchange coupling and spin-orbit interaction strength, as well as on the
initial spin-configuration and separation between the impurities. Our results
provide insight into the dynamic interaction between localized magnetic
impurities in a nano-scaled magnetic-semiconductor sample, in the extremely
dilute (solotronics) regime.",1410.5978v1
2017-03-20,Spin-Wave Dynamics in the Presence of Magnetic Vortices,"This chapter describes spin-wave excitations in nanosized dots and rings in
the presence of the vortex state. The special attention is paid to the
manifestation of the competition between exchange and dipolar interactions in
the spin-wave spectrum as well as the correlation between the spectrum and the
stability of the vortex. The calculation method uses the dynamic matrix for an
all-discrete system, the numerical diagonalization of which yields the spectrum
of frequencies and spin-wave profiles of normal modes of the dot. We study
in-plane vortices of two types: a circular magnetization in circular dots and
rings and the Landau state in square rings. We examine the influence of the
dipolar-exchange competition and the geometry of the dot on the stability of
the vortex and on the spectrum of spin waves. We show that the lowest-frequency
mode profile proves to be indicative of the dipolar-to-exchange interaction
ratio and the vortex stability is closely related to the spin-wave profile of
the soft mode. The negative dispersion relation is also shown. Our results
obtained for in-plane vortices are in qualitative agreement with results for
core-vortices obtained from experiments, micromagnetic simulations, and
analytical calculations.",1703.06668v1
2021-07-06,Transverse Linear Beam Dynamics,"The subject of this introductory course is transverse dynamics of charged
particle beams in linear approximation. Starting with a discussion of the most
important types of magnets and defining their multipole strengths, the
linearized equations of motion of charged particles in static magnetic fields
are derived using an orthogonal reference frame following the design orbit.
Analytical solutions are determined for linear elements of a typical beam
transfer line (drift, dipole and quadrupole magnets), and stepwise combined by
introducing the matrix formalism in which each element's contribution is
represented by a single transfer matrix. Application of this formalism allows
to calculate single particle's trajectories in linear approximation. After
introducing the beam emittance as the area occupied by a particle beam in phase
space, a linear treatment of transverse beam dynamics based on appropriately
defined optical functions is introduced. The formalism is applied to the
concepts of both weak and strong focusing, in particular discussing the
properties of the widely-used FODO cell. Specific characteristics of transverse
beam dynamics in periodic systems like circular accelerators are studied in
detail, emphazising the effects of linear field errors on orbit stability and
introducing the phenomena of optical resonances. Finally, the dynamics of
off-momentum particles is presented, introducing dispersion functions and
explaining effects like chromaticity.",2107.02614v2
2021-09-08,Dynamics of spin relaxation in nonequilibrium magnetic nanojunctions,"We investigate nonequilibrium phenomena in magnetic nano-junctions using a
numerical approach that combines classical spin dynamics with the hierarchical
equations of motion technique for quantum dynamics of conduction electrons. Our
focus lies on the spin dynamics, where we observe non-monotonic behavior in the
spin relaxation rates as a function of the coupling strength between the
localized spin and conduction electrons. Notably, we identify a distinct
maximum at intermediate coupling strength, which we attribute to a competition
that involves the increasing influence of the coupling between the classical
spin and electrons, as well as the influence of decreasing local density of
states at the Fermi level. Furthermore, we demonstrate that the spin dynamics
of a large open system can be accurately simulated by a short chain coupled to
semi-infinite metallic leads. In the case of a magnetic junction subjected to
an external DC voltage, we observe resonant features in the spin relaxation,
reflecting the electronic spectrum of the system. The precession of classical
spin gives rise to additional side energies in the electronic spectrum, which
in turn leads to a broadened range of enhanced damping in the voltage.",2109.03476v3
2022-09-23,Realization of high-dynamic-range broadband magnetic-field sensing with ensemble nitrogen-vacancy centers in diamond,"We present a new magnetometry method integrating an ensemble of
nitrogen-vacancy (NV) centers in a single-crystal diamond with an extended
dynamic range for monitoring the fast changing magnetic-field. The NV-center
spin resonance frequency is tracked using a closed-loop frequency locked
technique with fast frequency hopping to achieve a 10 kHz measurement
bandwidth, thus, allowing for the detection of fast changing magnetic signals
up to 0.723 T/s.This technique exhibits an extended dynamic range subjected to
the working bandwidth of the microwave source. This extended dynamic range can
reach up to 4.3 mT, which is 86 times broader than the intrinsic dynamic range.
The essential components for NV spin control and signal processing such as
signal generation, microwave frequency control, data processing and readout are
integrated in a board-level system. With this platform, we demonstrate
broadband magnetometry with an optimized sensitivity of 4.2 nT-Hz-1/2. This
magnetometry method has the potential to be implemented in a multichannel
frequency locked vector magnetometer suitable for a wide range of practical
applications such as magnetocardiography and high-precision current sensors.",2209.11360v1
2023-05-15,Order parameter dynamics in Mn$_3$Sn driven by DC and pulsed spin-orbit torques,"We numerically investigate and develop analytic models for both the DC and
pulsed spin-orbit-torque (SOT)-driven response of order parameter in
single-domain Mn$_3$Sn, which is a metallic antiferromagnet with an anti-chiral
120$^\circ$ spin structure. We show that DC currents above a critical threshold
can excite oscillatory dynamics of the order parameter in the gigahertz to
terahertz frequency spectrum. Detailed models of the oscillation frequency
versus input current are developed and found to be in excellent agreement with
the numerical simulations of the dynamics. In the case of pulsed excitation,
the magnetization can be switched from one stable state to any of the other
five stable states in the Kagome plane by tuning the duration or the amplitude
of the current pulse. Precise functional forms of the final switched state
versus the input current are derived, offering crucial insights into the
switching dynamics of Mn$_3$Sn. The readout of the magnetic state can be
carried out via either the anomalous Hall effect, or the recently demonstrated
tunneling magnetoresistance in an all-Mn$_3$Sn junction. We also discuss
possible disturbance of the magnetic order due to heating that may occur if the
sample is subject to large currents. Operating the device in pulsed mode or
using low DC currents reduces the peak temperature rise in the sample due to
Joule heating. Our predictive modeling and simulation results can be used by
both theorists and experimentalists to explore the interplay of SOT and the
order dynamics in Mn$_3$Sn, and to further benchmark the device performance.",2305.08728v2
2014-01-08,Dynamic exchange via spin currents in acoustic and optical modes of ferromagnetic resonance in spin-valve structures,"Two ferromagnetic layers magnetically decoupled by a thick normal metal
spacer layer can be, nevertheless, dynamically coupled via spin currents
emitted by the spin-pump and absorbed through the spin-torque effects at the
neighboring interfaces. A decrease of damping in both layers due to a partial
compensation of the angular momentum leakage in each layer was previously
observed at the coincidence of the two ferromagnetic resonances. In case of
non-zero magnetic coupling, such a dynamic exchange will depend on the mutual
precession of the magnetic moments in the layers. A difference in the linewidth
of the resonance peaks is expected for the acoustic and optical regimes of
precession. However, the interlayer coupling hybridizes the resonance responses
of the layers and therefore can also change their linewidths. The interplay
between the two mechanisms has never been considered before. In the present
work, the joint influence of the hybridization and non-local damping on the
linewidth has been studied in weakly coupled NiFe/CoFe/Cu/CoFe/MnIr spin-valve
multilayers. It has been found that the dynamic exchange by spin currents is
different in the optical and acoustic modes, and this difference is dependent
on the interlayer coupling strength. In contrast to the acoustic precession
mode, the dynamic exchange in the optical mode works as an additional damping
source. A simulation in the framework of the Landau-Lifshitz-Gilbert formalism
for two ferromagnetic layers coupled magnetically and by spin currents has been
done to separate the effects of the non-local damping from the resonance modes
hybridization. In our samples both mechanisms bring about linewidth changes of
the same order of magnitude, but lead to a distinctly different angular
behavior. The obtained results are relevant for a broad class of coupled
magnetic multilayers with ballistic regime of the spin transport.",1401.1672v1
2003-10-17,Morphology of Rising Hydrodynamic and Magneto-hydrodynamic Bubbles from Numerical Simulations,"Recent Chandra and XMM-Newton observations of galaxy cluster cooling flows
have revealed X-ray emission voids of up to 30 kpc in size that have been
identified with buoyant, magnetized bubbles. Motivated by these observations,
we have investigated the behavior of rising bubbles in stratified atmospheres
using the FLASH adaptive-mesh simulation code. We present results from 2-D
simulations with and without the effects of magnetic fields, and with varying
bubble sizes and background stratifications. We find purely hydrodynamic
bubbles to be unstable; a dynamically important magnetic field is required to
maintain a bubble's integrity. This suggests that, even absent thermal
conduction, for bubbles to be persistent enough to be regularly observed, they
must be supported in large part by magnetic fields. Thermal conduction
unmitigated by magnetic fields can dissipate the bubbles even faster. We also
observe that the bubbles leave a tail as they rise; the structure of these
tails can indicate the history of the dynamics of the rising bubble.",0310517v2
1998-10-28,Energy transfer in two-dimensional magnetohydrodynamic turbulence,"In an earlier paper (physics/0006012) we had developed a method for computing
the effective energy transfer between any two Fourier modes in fluid or
magnetohydrodynamic (MHD) flows. This method is applied to a pseudo-spectral,
direct numerical simulation (DNS) study of energy transfer in the quasi-steady
state of 2-D MHD turbulence with large scale kinetic forcing. Two aspects of
energy transfer are studied: the energy fluxes, and the energy transfer between
different wavenumber regions ({\it shells}). The picture of energy fluxes that
emerges is quite complex - there is a forward cascade of magnetic energy, an
inverse cascade of kinetic energy, a flux of energy from the kinetic to the
magnetic field, and a reverse flux which transfers the energy back to the
kinetic from the magnetic. The energy transfer between different wave number
shells is also complex - local and nonlocal transfers often possess opposing
features, i.e., energy transfer between some wave number shells occurs from
kinetic to magnetic, and between other wave number shells this transfer is
reversed. The net transfer of energy is from kinetic to magnetic. The results
obtained from the flux studies and the shell-to-shell energy transfer studies
are consistent with each other.",9810032v2
1999-05-11,The Radiative Kicked Oscillator: A Stochastic Web or Chaotic Attractor ?,"A relativistic charged particle moving in a uniform magnetic field and kicked
by an electric field is considered. Under the assumption of small magnetic
field, an iterative map is developed. We consider both the case in which no
radiation is assumed and the radiative case, using the Lorentz-Dirac equation
to describe the motion. Comparison between the non-radiative case and the
radiative case shows that in both cases one can observe a stochastic web
structure for weak magnetic fields, and, although there are global differences
in the result of the map, that both cases are qualitatively similar in their
small scale behavior. We also develop an iterative map for strong magnetic
fields. In that case the web structure no longer exists; it is replaced by a
rich chaotic behavior. It is shown that the particle does not diffuse to
infinite energy; it is limited by the boundaries of an attractor (the
boundaries are generally much smaller than light velocity). Bifurcation occurs,
converging rapidly to Feigenbaum's universal constant. The chaotic behavior
appears to be robust.
  For intermediate magnetic fields, it is more difficult to observe the web
structure, and the influence of the unstable fixed point is weaker.",9905013v1
1998-09-07,Non-Dissipative Logic Device NOT Based on Two Coupled Quantum Dots,"Non-dissipative dynamics of interacting electrons in two tunnel-coupled
quantum dots is studied theoretically within the framework of the Hubbard
model. Various values of intra-dot Coulomb repulsion energy $U$ and inter-dot
tunneling energy $V$ are considered, which correspond to various size of the
dots and to various distance between them. In the ground state, the average
value of the spin projection (magnetic moment) at each dot is zero. The input
signal (the local external magnetic field $H$) applied to one of the dots at a
time $t=0$ causes the electronic subsystem to evolve in such a way that
magnetic moments of quantum dots become oriented in the opposite directions at
any time $t>0$. For any set of $U$ and $V$, there exist optimal values of $H$
and $t$ which maximize the absolute values of magnetic moments at both dots,
and magnetic moments become almost saturated. Thus, the antiferromagnetic-like
spin ordering can be realized at the stage of coherent temporal evolution, well
before the relaxation to a new ground state at the sacrifice of inelastic
processes. This effect (""dynamical antiferromagnetism"") may be used for
implementation of a logic function NOT in an extremely short time. A
possibility to use the arrays of quantum dots as high-speed single-electron
devices of new generation is discussed.",9809112v1
1999-01-07,^{139}La Spectrum and Spin-Lattice Relaxation Measurements of La_{2/3}Ca_{1/3}MnO_3 in the Paramagnetic State,"^{139}La NMR measurements of La_{2/3}Ca_{1/3}MnO_3 performed in the
paramagnetic state and high magnetic fields are reported. Analysis of the high
temperature spectrum measured establishes that the spectrum it is a standard
powder pattern broadened by a variation in lattice distortions. At lower
temperatures, the spectrum shifts and broadens. Both the shift and broadening
exhibit Curie-Weiss behavior, indicating that the shift measures the
polarization of the electron spin system, and the broadening reflects a
distribution of magnetic susceptibilities. This distribution may result from
variations of local susceptibility in the bulk of the sample or from
differences in demagnetizing factors among powder grains. The spectrum also
indicates that the lattice distortions do not change as the temperature lowers.
Spectral diffusion measurements suggest that the temperature dependence of the
spectrum shape does not result from freezing out of motion of magnetic
polarons. Variation of the nuclear spin-lattice relaxation across the spectrum
indicate that magnetic fluctuations, not lattice vibrations, dominate nuclear
relaxation. Nuclear spin-lattice relaxation therefore measures electron spin
dynamics in this system. The magnetic field dependence of the spin-lattice
relaxation indicates that the electron spin-spin correlation function adopts
simple single exponential behavior with a slow field-independent correlation
time of 10^{-8} seconds near Tc. The spin-spin correlation function changes
form at higher temperatures, but remains slow. The spin-lattice relaxation
therefore indicates the presence of extremely slow dynamics above Tc.",9901048v1
2000-10-12,Thermal Magnetization Reversal in Arrays of Nanoparticles,"The results of large-scale simulations investigating the dynamics of
magnetization reversal in arrays of single-domain nanomagnets after a rapid
reversal of the applied field at nonzero temperature are presented. The
numerical micromagnetic approach uses the Landau-Lifshitz-Gilbert equation
including contributions from thermal fluctuations and long-range dipole-dipole
demagnetizing effects implemented using a fast-multipole expansion. The
individual model nanomagnets are 9 nm x 9 nm x 150 nm iron pillars similar to
those fabricated on a surface with STM-assisted chemical vapor deposition [S.
Wirth, et al., J. Appl. Phys {\bf 85}, 5249 (1999)]. Nanomagnets oriented
perpendicular to the surface and spaced 300 nm apart in linear arrays are
considered. The applied field is always oriented perpendicular to the surface.
When the magnitude of the applied field is less than the coercive value, about
2000 Oe for an individual nanomagnet, magnetization reversal in the nanomagnets
can only occur by thermally activated processes. Even though the interaction
from the dipole moment of neighboring magnets in this geometry is only about 1
Oe, less than 1% of the coercive field, it can have a large impact on the
switching dynamics. What determines the height of the free-energy barrier is
the difference between the coercive and applied fields, and 1 Oe can be a
significant fraction of that. The magnetic orientations of the neighbors are
seen to change the behavior of the nanomagnets in the array significantly.",0010184v2
2000-10-25,Spin and charge dynamics of the ferromagnetic and antiferromagnetic two-dimensional half-filled Kondo lattice model,"We present a detailed numerical study of spin and charge dynamics of the
two-dimensional Kondo lattice model with hopping t and exchange J. At T=0 and J
> 0, the competition between the RKKY interaction and Kondo effect triggers a
quantum phase transition between magnetically ordered and disordered
insulators: J_c/t = 1.45(5). The quasiparticle gap scales as |J|. S(q,\omega),
evolves smoothly from its strong coupling form with spin gap at q = (\pi,\pi)
to a spin wave form. At J>0, A(\vec{k},\omega) shows a dispersion relation
following that of hybridized bands. For J < J_c this feature is supplemented by
shadows thus pointing to a coexistence of Kondo screening and magnetism. For J
< 0 A(\vec{k},\omega) is similar to that of non-interacting electrons in a
staggered magnetic field. Spin, T_S, and charge, T_C, scales are defined. For
weak to intermediate couplings, T_S marks the onset of antiferromagnetic
fluctuations and follows a J^2 law. At strong couplings T_S scales as J. T_C
scales as J both at weak and strong couplings. At and slightly below T_C we
observe i) a rise in the resistivity as a function of decreasing temperature,
ii) a dip in the integrated density of states at the Fermi energy and iii) the
occurrence of hybridized bands in A(k,\omega). It is shown that in the weak
coupling limit, the charge gap of order J is of magnetic origin. The specific
heat shows a two peak structure, the low temperature peak being of magnetic
origin. Our results are compared to various mean-field theories.",0010393v1
2002-08-09,Persistent magnetoresistive memory in phase separated manganites,"We have studied magnetic and transport properties on different manganese
oxide based compounds exhibiting phase separation: polycrystalline
La5/8-yPryCa3/8MnO3 (y=0.3) and La1/2Ca1/2Mn1-zFezO3 (z = 0.05), and single
crystals of La5/8-yPryCa3/8MnO3 (y=0.35). Time dependent effects indicate that
the fractions of the coexisting phases are dynamically changing in a definite
temperature range. We found that in this range the ferromagnetic fraction f can
be easily tuned by application of low magnetic fields (< 1 T). The effect is
persistent after the field is turned off, thus the field remains imprinted in
the actual value of f and can be recovered through transport measurements. This
effect is due both to the existence of a true phase separated equilibrium state
with definite equilibrium fraction f0, and to the slow growth dynamics. The
fact that the same global features were found on different compounds and in
polycrystalline and single crystalline samples, suggests that the effect is a
general feature of some phase separated media.",0208199v1
2006-08-31,FFLO State and Peak Effect Dynamics in CeCoIn_5: Magnetization Studies,"Magnetization measurements were performed on CeCoIn_5 at temperature down to
30 mK with the magnetic field applied in three different orientations:
parallel, near parallel ($\sim$ 10$^{0}$ rotated) and perpendicular to the
ab-plane. For these three orientations we have observed crossover features in
the torque/magnetization traces at fields just below $H_{c2}$, giving further
evidence for the formation of a high field Fulde-Ferrel-Larkin-Ovchinnikov
(FFLO) superconducting state in CeCoIn_5 for $H \parallel ab$-plane and newly
indicates that the FFLO state persists for out-of-plane field orientations.
Furthermore, for the (near) parallel to $ab$-plane field configurations and
$T\leq \mathrm{50 mK}$, we have found an anomalous peak effect(APE) just below
the crossover field when the magnetic field is sweeping down from normal to
superconducting state. The dynamics of this peak suggests the existence of a
metastable phase occurring in the vicinity of the FFLO phase and raises
questions about the order (first or second) of the transition from FFLO to the
superconducting state. None of the above features were found in a
Ce_{0.98}Gd_{0.02}CoIn_{5} crystal.",0608722v1
2007-06-18,Understanding of complex periodic transformations of moving domain walls in magnetic nanostripes,"The magnetic field (or electric current) driven domain-wall motion in
magnetic nanostripes is of considerable interest because it is essential to the
performance of information storage and logic devices. One of the currently key
problems is to unveil the complex behaviors of oscillatory domain-wall motions
under applied magnetic fields stronger than the so-called Walker field, beyond
which the velocity of domain walls markedly drops. Here, we provide not only
considerably better understandings but also new details of the complex
domain-wall motions. In a certain range just above the Walker field, the
motions are not chaotic but rather periodic with different unique periodicities
of dynamic transformations of a moving domain wall between the different types
of its internal structure. Three unique periodicities found, which consist of
different types of domain wall that are transformed from type one to another.
The transformation periods vary with the field strength and the nanostripe
width. This novel phenomenon can be described by the dynamic motion of a
limited number of magnetic topological solitons such as vortex and antivortex
in nanostripes.",0706.2542v1
2007-10-16,A Tale Of Two Spicules: The Impact of Spicules on the Magnetic Chromosphere,"We use high-resolution observations of the Sun in Ca II H 3968 A from the
Solar Optical Telescope on Hinode to show that there are at least two types of
spicules that dominate the structure of the magnetic solar chromosphere. Both
types are tied to the relentless magnetoconvective driving in the photosphere,
but have very different dynamic properties. ``Type-I'' spicules are driven by
shock waves that form when global oscillations and convective flows leak into
the upper atmosphere along magnetic field lines on 3-7 minute timescales.
``Type-II'' spicules are much more dynamic: they form rapidly (in ~10s), are
very thin (<200km wide), have lifetimes of 10-150s (at any one height) and seem
to be rapidly heated to (at least) transition region temperatures, sending
material through the chromosphere at speeds of order 50-150 km/s. The
properties of Type II spicules suggest a formation process that is a
consequence of magnetic reconnection, typically in the vicinity of magnetic
flux concentrations in plage and network. Both types of spicules are observed
to carry Alfven waves with significant amplitudes of order 20 km/s.",0710.2934v2
2008-12-27,Bistability of vortex core dynamics in a single perpendicularly magnetized nano-disk,"Microwave spectroscopy of individual vortex-state magnetic nano-disks in a
perpendicular bias magnetic field, $H$, is performed using a magnetic resonance
force microscope (MRFM). It reveals the splitting induced by $H$ on the
gyrotropic frequency of the vortex core rotation related to the existence of
the two stable polarities of the core. This splitting enables spectroscopic
detection of the core polarity. The bistability extends up to a large negative
(antiparallel to the core) value of the bias magnetic field $H_r$, at which the
core polarity is reversed. The difference between the frequencies of the two
stable rotational modes corresponding to each core polarity is proportional to
$H$ and to the ratio of the disk thickness to its radius. Simple analytic
theory in combination with micromagnetic simulations give quantitative
description of the observed bistable dynamics.",0812.4733v2
2009-10-20,Flow instabilities of magnetic flux tubes IV. Flux storage in the solar overshoot region,"We consider the effects of material flows on the dynamics of toroidal
magnetic flux tubes located close to the base of the solar convection zone,
initially within the overshoot region. The problem is to find the physical
conditions in which magnetic flux can be stored for periods comparable to the
dynamo amplification time, which is of the order of a few years. We carry out
nonlinear numerical simulations to investigate the stability and dynamics of
thin flux tubes subject to perpendicular and longitudinal flows. We compare the
simulations with the results of simplified analytical approximations. We
determine ranges of the flow parameters for which a linearly Parker-stable
magnetic flux tube is stored in the middle of the overshoot region for a period
comparable to the dynamo amplification time. The residence time for magnetic
flux tubes with fluxes of 2x10^{21} Mx in the convective overshoot layer is
comparable to the dynamo amplification time, provided that the average speed
and the duration of the downflow do not exceed about 50 m/s and 100 days,
respectively, and that the lateral extension of the flow is smaller than about
10 degrees.",0910.3858v1
2010-07-16,Relativistic magnetohydrodynamics in dynamical spacetimes: A new AMR implementation,"We have written and tested a new general relativistic magnetohydrodynamics
(GRMHD) code, capable of evolving MHD fluids in dynamical spacetimes with
adaptive-mesh refinement (AMR). Our code solves the Einstein-Maxwell-MHD system
of coupled equations in full 3+1 dimensions, evolving the metric via the
Baumgarte-Shapiro Shibata-Nakamura (BSSN) formalism and the MHD and magnetic
induction equations via a conservative, high-resolution shock-capturing scheme.
The induction equations are recast as an evolution equation for the magnetic
vector potential, which exists on a grid that is staggered with respect to the
hydrodynamic and metric variables. The divergenceless constraint div(B)=0 is
enforced by the curl of the vector potential. Our MHD scheme is fully
compatible with AMR, so that fluids at AMR refinement boundaries maintain
div(B)=0. In simulations with uniform grid spacing, our MHD scheme is
numerically equivalent to a commonly used, staggered-mesh constrained-transport
scheme. We present code validation test results, both in Minkowski and curved
spacetimes. They include magnetized shocks, nonlinear Alfv\'en waves,
cylindrical explosions, cylindrical rotating disks, magnetized Bondi tests, and
the collapse of a magnetized rotating star. Some of the more stringent tests
involve black holes. We find good agreement between analytic and numerical
solutions in these tests, and achieve convergence at the expected order.",1007.2848v1
2010-11-26,Generation of electric currents in the chromosphere via neutral-ion drag,"We consider the generation of electric currents in the solar chromosphere
where the ionization level is typically low. We show that ambient electrons
become magnetized even for weak magnetic fields (30 G); that is, their
gyrofrequency becomes larger than the collision frequency while ion motions
continue to be dominated by ion-neutral collisions. Under such conditions, ions
are dragged by neutrals, and the magnetic field acts as if it is frozen-in to
the dynamics of the neutral gas. However, magnetized electrons drift under the
action of the electric and magnetic fields induced in the reference frame of
ions moving with the neutral gas. We find that this relative motion of
electrons and ions results in the generation of quite intense electric
currents. The dissipation of these currents leads to resistive electron heating
and efficient gas ionization. Ionization by electron-neutral impact does not
alter the dynamics of the heavy particles; thus, the gas turbulent motions
continue even when the plasma becomes fully ionized, and resistive dissipation
continues to heat electrons and ions. This heating process is so efficient that
it can result in typical temperature increases with altitude as large as
0.1-0.3 eV/km. We conclude that this process can play a major role in the
heating of the chromosphere and corona",1011.5834v1
2011-02-01,Kinetic description of quasi-stationary axisymmetric collisionless accretion disk plasmas with arbitrary magnetic field configurations,"A kinetic treatment is developed for collisionless magnetized plasmas
occurring in high-temperature, low-density astrophysical accretion disks, such
as are thought to be present in some radiatively-inefficient accretion flows
onto black holes. Quasi-stationary configurations are investigated, within the
framework of a Vlasov-Maxwell description. The plasma is taken to be
axisymmetric and subject to the action of slowly time-varying gravitational and
electromagnetic fields. The magnetic field is assumed to be characterized by a
family of locally nested but open magnetic surfaces. The slow collisionless
dynamics of these plasmas is investigated, yielding a reduced gyrokinetic
Vlasov equation for the kinetic distribution function. For doing this, an
asymptotic quasi-stationary solution is first determined, represented by a
generalized bi-Maxwellian distribution expressed in terms of the relevant
adiabatic invariants. The existence of the solution is shown to depend on
having suitable kinetic constraints and conditions leading to particle trapping
phenomena. With this solution one can treat temperature anisotropy, toroidal
and poloidal flow velocities and finite Larmor-radius effects. An asymptotic
expansion for the distribution function permits analytic evaluation of all of
the relevant fluid fields. Basic theoretical features of the solution and their
astrophysical implications are discussed. As an application, the possibility of
describing the dynamics of slowly time-varying accretion flows and the
self-generation of magnetic field by means of a \textquotedblleft kinetic
dynamo effect\textquotedblright\ is discussed. Both effects are shown to be
related to intrinsically-kinetic physical mechanisms.",1102.0179v2
2011-04-04,Computer simulations of colloidal transport on a patterned magnetic substrate,"We study the transport of paramagnetic colloidal particles on a patterned
magnetic substrate with kinetic Monte Carlo and Brownian dynamics computer
simulations. The planar substrate is decorated with point dipoles in either
parallel or zigzag stripe arrangements and exposed to an additional external
magnetic field that oscillates in time. For the case of parallel stripes we
find that the magnitude and direction of the particle current is controlled by
the tilt angle of the external magnetic field. The effect is reliably obtained
in a wide range of ratios between temperature and magnetic permeability.
Particle transport is achieved only when the period of oscillation of the
external field is greater than a critical value. For the case of zigzag stripes
a current is obtained using an oscillating external field normal to the
substrate. In this case, transport is only possible in the vertex of the
zigzag, giving rise to a narrow stream of particles. The magnitude and
direction of the particle current are found to be controlled by a combination
of the zigzag angle and the distance of the colloids from the substrate.
Metropolis Monte Carlo and Brownian dynamics simulations predict results that
are in good agreement with each other. Using kinetic Monte Carlo we find that
at high density the particle transport is hindered by jamming.",1104.0544v1
2011-07-18,Quasi-static magnetohydrodynamic turbulence at high Reynolds number,"We analyse the anisotropy of homogeneous turbulence in an electrically
conducting fluid submitted to a uniform magnetic field, for low magnetic
Reynolds number, in the quasi- static approximation. We interpret disagreeing
previous predictions between linearized theory and simulations: in the linear
limit, the kinetic energy of transverse velocity components, normal to the
magnetic field, decays faster than the kinetic energy of the axial component,
along the magnetic field (Moffatt (1967)); whereas many numerical studies
predict a final state characterised by dominant energy of transverse velocity
components. We investigate the corresponding nonlinear phenomenon using Direct
Numerical Simulations of freely-decaying turbulence, and a two-point
statistical spectral closure based on the Eddy Damped Quasi-Normal Markovian
model. The transition from the three-dimensional turbulent flow to a
""two-and-a-half-dimensional"" flow (Montgomery & Turner (1982)) is a result of
the combined effects of short-time linear Joule dissipation and longer time
nonlinear creation of polarisation anisotropy. It is this combination of linear
and nonlinear effects which explains the disagreement between predictions from
linearized theory and results from numerical simulations. The transition is
characterized by the elongation of turbulent structures along the applied
magnetic field, and by the strong anisotropy of directional two-point
correlation spectra, in agreement with experimental evidence. Inertial
equatorial transfers in both DNS and the model are presented to describe in
detail the most important equilibrium dynamics. Spectral scalings are
maintained in high Reynolds number turbulence attainable only with the EDQNM
model, which also provides simplified modelling of the asymptotic state of
quasi-static MHD turbulence.",1107.3435v1
2011-09-17,Magnetic clouds in the solar wind: A numerical assessment study of analytical models,"Magnetic clouds (MCs) are ""magnetized plasma clouds"" moving in the solar
wind. MCs transport magnetic flux and helicity away from the Sun. These
structures are not stationary but feature temporal evolution as they propagate
in the solar wind. Simplified analytical models are frequently used for the
description of MCs, and fit certain observational data well. The goal of the
present study is to investigate numerically the validity of an analytical model
which is widely used for the description of MCs, and to determine under which
conditions this model's implied assumptions cease to be valid. A numerical
approach is applied. Analytical solutions that have been derived in previous
studies are implemented in a \textbf{3-D magnetohydrodynamic} simulation code
as initial conditions. Initially, the analytical model represents the main
observational features of the MCs. However, these characteristics prevail only
if the structure moves with a velocity close to the velocity of the background
flow. In this case an MC's evolution can quite accurately be described using an
analytic, self-similar approach. The dynamics of the magnetic structures which
move with a velocity significantly above or below that of the velocity of the
solar wind is investigated in detail. Besides the standard case in which MCs
only expand and propagate in the solar wind, the case of an MC rotating around
its axis of symmetry is also considered, and the resulting influence on the
MC's dynamics is studied.",1109.3790v1
2012-03-27,High frequency permeability of magnonic metamaterials with magnetic inclusions of complex shape,"We present a method of calculation of the effective magnetic permeability of
magnonic metamaterials containing periodically arranged magnetic inclusions of
arbitrary shapes. The spectrum of spin wave modes confined in the inclusions is
fully taken into account. Within the scope of the proposed method, we compare
two approaches. The first approach is based on a simple semi-analytical theory
that uses the numerically calculated susceptibility tensor of an isolated
inclusion as input data. Within the second approach, micromagnetic packages
with periodic boundary conditions (PBC) are used to calculate the
susceptibility of a single 2D periodic array of such inclusions, with the whole
3D metamaterial consisting of a stack of such arrays. To calculate the
susceptibility tensor of an isolated inclusion, we have implemented and
compared two different methods: (a) a micromagnetic method, in which we have
employed three different micromagnetic packages: the finite element package
NMAG and the two finite differences packages OOMMF and MicroMagus; and (b) the
modified dynamical matrix method. To illustrate the methodology, we have
calculated the effective permeability of a metamaterial consisting of a stack
of hexagonal arrays of magnetic nanodisks in a non-magnetic matrix. The range
of geometrical parameters for which such a metamaterial is characterized by the
negative permeability has been identified. The critical comparison of the
different micromagnetic packages and the dynamical matrix method (based on the
calculation of the susceptibility tensor of an isolated inclusion) has
demonstrated that their results agree to within 3 %.",1203.6116v1
2012-05-11,Simulated Tempering and Magnetizing: An Application of Two-Dimensional Simulated Tempering to Two-Dimensional Ising Model and Its Crossover,"We performed two-dimensional simulated tempering (ST) simulations of the
two-dimensional Ising model with different lattice sizes in order to
investigate the two-dimensional ST's applicability to dealing with phase
transitions and to study the crossover of critical scaling behavior. The
external field, as well as the temperature, was treated as a dynamical variable
updated during the simulations. Thus, this simulation can be referred to as
""Simulated Tempering and Magnetizing (STM)."" We also performed the ""Simulated
Magnetizing"" (SM) simulations, in which the external field was considered as a
dynamical variable and temperature was not. As has been discussed by previous
studies, the ST method is not always compatible with first-order phase
transitions. This is also true in the magnetizing process. Flipping of the
entire magnetization did not occur in the SM simulations under $T_\mathrm{c}$
in large lattice-size simulations. However, the phase changed through the high
temperature region in the STM simulations. Thus, the dimensional extension let
us eliminate the difficulty of the first-order phase transitions and study wide
area of the phase space. We then discuss how frequently parameter-updating
attempts should be made for optimal convergence. The results favor frequent
attempts. We finally study the crossover behavior of the phase transitions with
respect to the temperature and external field. The crossover behavior was
clearly observed in the simulations in agreement with the theoretical
implications.",1205.2523v1
2012-06-08,Electron cyclotron resonance near the axis of the gas-dynamic trap,"Propagation of an extraordinary electromagnetic wave in the vicinity of
electron cyclotron resonance surface in an open linear trap is studied
analytically, taking into account inhomogeneity of the magnetic field in
paraxial approximation. Ray trajectories are derived from a reduced dispersion
equation that makes it possible to avoid the difficulty associated with a
transition from large propagation angles to the case of strictly longitudinal
propagation. Our approach is based on the theory, originally developed by the
Zvonkov and Timofeev [1], who used the paraxial approximation for the magnetic
field strength, but did not consider the slope of the magnetic field lines,
which led to considerable error, as has been recently noted by Gospodchikov and
Smolyakova [2]. We have found ray trajectories in analytic form and
demonstrated that the inhomogeneity of both the magnetic field strength and the
field direction can qualitatively change the picture of wave propagation and
significantly affect the efficiency of electron cyclotron heating of a plasma
in a linear magnetic trap. Analysis of the ray trajectories has revealed a
criterion for the resonance point on the axis of the trap to be an attractor
for the ray trajectories. It is also shown that a family of ray trajectories
can still reach the resonance point on the axis if the latter generally repels
the ray trajectories.
  As an example, results of general theory are applied to the electron
cyclotron resonance heating experiment which is under preparation on the Gas
Dynamic Trap in the Budker Institute of Nuclear Physics [3].",1206.1670v1
2012-07-31,Surface Acoustic Wave-Driven Ferromagnetic Resonance in Nickel Thin Films: Theory and Experiment,"We present an extensive experimental and theoretical study of surface
acoustic wave-driven ferromagnetic resonance. In a first modeling approach
based on the Landau-Lifshitz-Gilbert equation, we derive expressions for the
magnetization dynamics upon magnetoelastic driving that are used to calculate
the absorbed microwave power upon magnetic resonance as well as the spin
current density generated by the precessing magnetization in the vicinity of a
ferromagnet/normal metal interface. In a second modeling approach, we deal with
the backaction of the magnetization dynamics on the elastic wave by solving the
elastic wave equation and the Landau-Lifshitz-Gilbert equation
selfconsistently, obtaining analytical solutions for the acoustic wave phase
shift and attenuation. We compare both modeling approaches with the complex
forward transmission of a LiNbO$_3$/Ni surface acoustic wave hybrid device
recorded experimentally as a function of the external magnetic field
orientation and magnitude, rotating the field within three different planes and
employing three different surface acoustic wave frequencies. We find
quantitative agreement of the experimentally observed power absorption and
surface acoustic wave phase shift with our modeling predictions using one set
of parameters for all field configurations and frequencies.",1208.0001v1
2012-10-01,51V-NMR study of antiferromagnetic state and spin dynamics in the quasi-one-dimensional BaCo2V2O8,"We report on our 51V-NMR study of static and dynamical magnetic properties in
the quasi-one-dimensional antiferromagnet BaCo2V2O8. Although the NMR spectrum
shows well-defined antiferromagnetic (AF) order in the N\'eel ground state, the
AF characteristic from the NMR spectrum is incomplete between 3.5 K and TN= 5.4
K, which could be affected by quantum spin fluctuations. The AF NMR spectrum
indicates two V sites experiencing different magnetic field magnitudes, HA1=2.1
kOe and HA2=3.8 kOe. These internal fields could be explained by accounting for
the classical and the pseudo-dipolar fields from Co2+ spins with a proposed
magnetic structure based on the neutron diffraction measurements. In the
paramagnetic state, the nuclear spin relaxation is dominated by AF spin
fluctuations through the dipolar-type coupling between V and surrounding Co2+
ions. The linear relation between the nuclear spin-lattice relaxation rate
1/T1T and the magnetic susceptibility chi indicates that the Q component of
magnetic susceptibility chi(Q) is roughly proportional to chi, where Q is the
AF wave number. A change in slope of 1/T1T with respect to \chi around 150 K
suggests a change in the AF spin fluctuation spectrum.",1210.0272v1
2013-01-21,Frustration effect on magnetic excitations in a two-leg spin-ladder system,"We theoretically study the magnetic excitations in a frustrated two-leg
spin-ladder system, in which antiferromagnetic exchange interactions act on the
nearest-neighbor and next-nearestneighbor bonds in the leg direction, and on
the nearest-neighbor bonds in the rung direction. A dynamical spin correlation
function at zero temperature is calculated by using the dynamical
density-matrix renormalization-group method for possible magnetic phases, i.e.,
columnar-dimer and rung-singlet phases. The columnar-dimer phase is
characterized by multi-spinon excitations with spin gap, while the rung-singlet
phase is dominated by the triplet excitation in the rung direction. It is found
that a major difference between these two phases appears in the spectral weight
of magnetic excitations, in particular, of the bonding and anti-bonding modes
in the rung direction. Therefore, we can distinguish one phase from the other
by the difference of the spectral weight. Furthermore, we examine frustration
effect on both modes in the rung-singlet phase with a perturbation theory from
the strong coupling limit. It is shown that the anti-bonding mode is stable
against the frustration, and a wavenumber with minimum excitation energy is
shifted from the commensurate to incommensurate ones. On the other hand, the
bonding mode is merged into the continuum excitation of multiple triplet
excitations by increasing the frustration. By comparing our results with
inelastic neutron scattering experiments for BiCu$_2$PO$_6$, the magnitude of
the magnetic exchange interactions and the ground state will be determined.",1301.4740v3
2013-02-14,Influence of electromagnetic fields on the evolution of initially homogeneous and isotropic universe,"Simple exact solutions presented here describe the universes which spatial
geometries are asymptotically homogeneous and isotropic near the initial
singularity, but which evolution goes under the influence of primordial
magnetic fields. In all these ""deformed"" Friedmann models (spatially flat, open
or closed), the initial magnetic fields are concentrated near some axis of
symmetry and their lines are the circles -- the lines of the azimuthal
coordinate $\varphi$. Caused by the expansion of the universe, the
time-dependence of a magnetic field induces (in accordance with the Faraday
law) the emergence of source-free electric fields. In comparison with the
Friedmann models, the cosmological expansion goes with acceleration in spatial
directions across the magnetic field, and with deceleration along the magnetic
lines, so that in flat and open models, in fluid comoving coordinates, the
lengths of $\varphi$-circles of large enough radius or for late enough times
decrease and vanish for $t\to\infty$. This means that in flat and open models,
we have a partial dynamical closure of space-time at large distances from the
axis, i.e. from the regions where the electromagnetic fields in our solutions
are concentrated. To get simple exact solutions of the Einstein-Maxwell and
perfect fluid equations, we assume for the perfect fluid (which supports the
isotropic and homogeneous ""background"" Friedmann geometries) rather exotic,
stiff matter equation of state $\varepsilon=p$. However, it seems reasonable to
expect that similar effects might take place in the mutual dynamics of geometry
and strong electromagnetic fields in the universes with more realistic matter
equations of state.",1302.3338v1
2013-02-18,Local support against gravity in magneto-turbulent fluids,"Comparisons of the integrated thermal pressure support of gas against its
gravitational potential energy lead to critical mass scales for gravitational
instability such as the Jeans and the Bonnor-Ebert masses, which play an
important role in analysis of many physical systems, including the heuristics
of numerical simulations. In a strict theoretical sense, however, neither the
Jeans nor the Bonnor-Ebert mass are meaningful when applied locally to
substructure in a self-gravitating turbulent medium. For this reason, we
investigate the local support by thermal pressure, turbulence, and magnetic
fields against gravitational compression through an approach that is
independent of these concepts. At the centre of our approach is the dynamical
equation for the divergence of the velocity field. We carry out a statistical
analysis of the source terms of the local compression rate (the negative time
derivative of the divergence) for simulations of forced self-gravitating
turbulence in periodic boxes with zero, weak, and moderately strong mean
magnetic fields (measured by the averages of the magnetic and thermal
pressures). We also consider the amplification of the magnetic field energy by
shear and by compression. Thereby, we are able to demonstrate that the support
against gravity is dominated by thermal pressure fluctuations, although
magnetic pressure also yields a significant contribution. The net effect of
turbulence in the highly supersonic regime, however, is to enhance compression
rather than supporting overdense gas even if the vorticity is very high. This
is incommensurate with the support of the highly dynamical substructures in
magneto-turbulent fluids being determined by local virial equilibria of volume
energies without surface stresses.",1302.4292v2
2013-04-10,Magnetization and spin dynamics of the spin S=1/2 hourglass nanomagnet Cu5(OH)2(NIPA)4*10H2O,"We report a combined experimental and theoretical study of the spin S=1/2
nanomagnet Cu5(OH)2(NIPA)4*10H2O (Cu5-NIPA). Using thermodynamic, electron spin
resonance and 1H nuclear magnetic resonance measurements on one hand, and ab
initio density-functional band-structure calculations, exact diagonalizations
and a strong coupling theory on the other, we derive a microscopic magnetic
model of Cu5-NIPA and characterize the spin dynamics of this system. The
elementary five-fold Cu2+ unit features an hourglass structure of two
corner-sharing scalene triangles related by inversion symmetry. Our microscopic
Heisenberg model comprises one ferromagnetic and two antiferromagnetic exchange
couplings in each triangle, stabilizing a single spin S=1/2 doublet ground
state (GS), with an exactly vanishing zero-field splitting (by Kramer's
theorem), and a very large excitation gap of \Delta~68 K. Thus, Cu5-NIPA is a
good candidate for achieving long electronic spin relaxation (T1) and coherence
(T2) times at low temperatures, in analogy to other nanomagnets with low-spin
GS's. Of particular interest is the strongly inhomogeneous distribution of the
GS magnetic moment over the five Cu2+ spins. This is a purely
quantum-mechanical effect since, despite the non-frustrated nature of the
magnetic couplings, the GS is far from the classical collinear ferrimagnetic
configuration. Finally, Cu5-NIPA is a rare example of a S=1/2 nanomagnet
showing an enhancement in the nuclear spin-lattice relaxation rate 1/T1 at
intermediate temperatures.",1304.2826v1
2013-04-27,Quantum motion and level quantization of a skyrmion in a pinning potential in chiral magnets,"A new topological excitation called skyrmion has been observed experimentally
in chiral magnets without spatial inversion symmetry. The dynamics of a
skyrmion is equivalent to an electron moving in a strong magnetic field. As a
skyrmion involves large number of spins, it is not clear whether there exist
observable quantum effects. In this work, we study the quantum dynamics of a
skyrmion in a pinning potential. Without a pinning potential, the skyrmion
performs cyclotron motion due to the strong emergent magnetic field originating
from the Berry phase of spins, and all skyrmions occupy the lowest Landau
level. Their wave functions are strongly localized in a region with radius less
than $1\ \AA$ when no driving force is applied. Thus in most circumstances, the
quantum effects of a skyrmion are weak. In the presence of a pinning potential,
the lowest Landau level for skyrmions is split into quantized levels,
characterized by the orbital moments. The transition between quantized levels
can be observed experimentally by microwave absorption measurements in low
temperature region. The quantum effects are more prominent for a skyrmion with
a small size, which can be realized in magnets with a large
Dzyaloshinskii-Moriya interaction.",1304.7322v2
2013-08-14,A strong magnetic field around the supermassive black hole at the centre of the Galaxy,"The centre of our Milky Way harbours the closest candidate for a supermassive
black hole. The source is thought to be powered by radiatively inefficient
accretion of gas from its environment. This form of accretion is a standard
mode of energy supply for most galactic nuclei. X-ray measurements have already
resolved a tenuous hot gas component from which it can be fed. However, the
magnetization of the gas, a crucial parameter determining the structure of the
accretion flow, remains unknown. Strong magnetic fields can influence the
dynamics of the accretion, remove angular momentum from the infalling gas,
expel matter through relativistic jets and lead to the observed synchrotron
emission. Here we report multi-frequency measurements with several radio
telescopes of a newly discovered pulsar close to the Galactic Centre and show
that its unusually large Faraday rotation indicates a dynamically relevant
magnetic field near the black hole. If this field is accreted down to the event
horizon it provides enough magnetic flux to explain the observed emission from
the black hole, from radio to X-rays.",1308.3147v1
2013-12-11,Electric and magnetic response of hot QCD matter,"We study the electric conductivity as well as the magnetic response of hot
QCD matter at various temperatures $T$ and chemical potentials $\mu_q$ within
the off-shell Parton-Hadron-String Dynamics (PHSD) transport approach for
interacting partonic systems in a finite box with periodic boundary conditions.
The response of the strongly-interacting system in equilibrium to an external
electric field defines the electric conductivity $\sigma_0$ whereas the
response to a moderate external magnetic field defines the induced diamagnetic
moment $\mu_L$ ($T, \mu_q$) as well as the spin susceptibility $\chi_S(T,
\mu_q)$. We find a sizeable temperature dependence of the dimensionless ratio
$\sigma_0/T$ well in line with calculations in a relaxation time approach for
$T_c \! < \! T < \! 2.5 \!\, T_c$ as well as an increase of $\sigma_0$ with
$\mu_q^2/T^2$. Furthermore, the frequency dependence of the electric
conductivity $\sigma(\Omega)$ shows a simple functional form well in line with
results from the Dynamical QuasiParticle Model (DQPM). The spin susceptibility
$\chi_S(T,\mu_q)$ is found to increase with temperature $T$ and to rise $\sim
\mu_q ^2/T^2$, too. The actual values for the magnetic response of the QGP in
the temperature range below 250 MeV show that the QGP should respond
diamagnetically in actual ultra-relativistic heavy-ion collisions since the
maximal magnetic fields created in these collisions are smaller than $B_c(T)$
which defines a boundary between diamagnetism and paramagnetism.",1312.3189v2
2014-06-25,"Surface acoustic wave driven ferromagnetic resonance in (Ga,Mn)(As,P) epilayers","Interdigitated transducers were used to generate and detect surface acoustic
waves on a thin layer of (Ga,Mn)(As,P). The out-of-plane uniaxial magnetic
anisotropy of this dilute magnetic semiconductor is very sensitive to the
strain of the layer, making it an ideal test material for the dynamic control
of magnetization via magneto-striction. The time-domain measurement of the
amplitude and phase of the transmitted SAW during magnetic field sweeps
indicated a clear resonant behavior at a field close to the one calculated to
give a precession frequency equal to the SAW frequency. A resonance was
observed from 5K to 85K, just below the Curie temperature of the layer. A full
analytical treatment of the coupled magnetization/acoustic dynamics showed that
the magneto-strictive coupling modifies the elastic constants of the material
and accordingly the wave-vector solution to the elastic wave equation. The
shape and position of the resonance were well reproduced by the calculations,
in particular the fact that velocity (phase) variations resonated at lower
fields than the acoustic attenuation variations.",1406.6483v1
2014-10-04,Reconnection Properties of Large-Scale Current Sheets During Coronal Mass Ejection Eruptions,"We present a detailed analysis of the properties of magnetic reconnection at
large-scale current sheets in a high cadence version of the Lynch & Edmondson
(2013) 2.5D MHD simulation of sympathetic magnetic breakout eruptions from a
pseudostreamer source region. We examine the resistive tearing and breakup of
the three main current sheets into chains of X- and O-type null points and
follow the dynamics of magnetic island growth, their merging, transit, and
ejection with the reconnection exhaust. For each current sheet, we quantify the
evolution of the length-to-width aspect ratio (up to $\sim$100:1), Lundquist
number ($\sim$10$^3$), and reconnection rate (inflow-to-outflow ratios reaching
$\sim$0.40). We examine the statistical and spectral properties of the
fluctuations in the current sheets resulting from the plasmoid instability,
including the distribution of magnetic island area, mass, and flux content. We
show that the temporal evolution of the spectral index of the
reconnection-generated magnetic energy density fluctuations appear to reflect
global properties of the current sheet evolution. Our results are in excellent
agreement with recent, high resolution reconnection-in-a-box simulations even
though our current sheets' formation, growth, and dynamics are intrinsically
coupled to the global evolution of sequential sympathetic CME eruptions.",1410.1089v2
2015-01-19,Negative magnetic eddy diffusivities from test-field method and multiscale stability theory,"The generation of large-scale magnetic field in the kinematic regime in the
absence of an alpha-effect is investigated by following two different
approaches, namely the test-field method and multiscale stability theory
relying on the homogenisation technique. We show analytically that the former,
applied for the evaluation of magnetic eddy diffusivities, yields results that
fully agree with the latter. Our computations of the magnetic eddy diffusivity
tensor for the specific instances of the parity-invariant flow-IV of G.O.
Roberts and the modified Taylor-Green flow in a suitable range of parameter
values confirm the findings of previous studies, and also explain some of their
apparent contradictions. The two flows have large symmetry groups; this is used
to considerably simplify the eddy diffusivity tensor. Finally, a new analytic
result is presented: upon expressing the eddy diffusivity tensor in terms of
solutions to auxiliary problems for the adjoint operator, we derive relations
between magnetic eddy diffusivity tensors that arise for opposite small-scale
flows v(x) and -v(x).",1501.04465v3
2015-01-29,Mitigating Radiation Impact on Superconducting Magnets of the Higgs Factory Muon Collider,"Recent discovery of a Higgs boson boosted interest in a low-energy
medium-luminosity Muon Collider as a Higgs Factory (HF). A preliminary design
of the HF storage ring (SR) is based on cos-theta Nb3Sn superconducting (SC)
magnets with the coil inner diameter ranging from 50 cm in the interaction
region to 16 cm in the arc. The coil cross-sections were chosen based on the
operation margin, field quality and quench protection considerations to provide
an adequate space for the beam pipe, helium channel and inner absorber (liner).
With the 62.5-GeV muon energy and 2 x 10^12 muons per bunch, the electrons from
muon decays deposit about 300 kW in the SC magnets, or unprecedented 1 kW/m
dynamic heat load, which corresponds to a multi-MW room temperature equivalent.
Based on the detailed MARS15 model built and intense simulations, a
sophisticated protection system was designed for the entire SR to bring the
peak power density in the SC coils safely below the quench limit and reduce the
dynamic heat load to the cold mass by a factor of 100. The system consists of
tight tungsten masks in the magnet interconnect regions and elliptical tungsten
liners optimized for each magnet.",1501.07624v1
2015-03-12,"Classification of magnetized star--planet interactions: bow shocks, tails, and inspiraling flows","Close-in exoplanets interact with their host stars gravitationally as well as
via their magnetized plasma outflows. The rich dynamics that arises may result
in distinct observable features. Our objective is to study and classify the
morphology of the different types of interaction that can take place between a
giant close-in planet (a Hot Jupiter) and its host star, based on the physical
parameters that characterize the system. We perform 3D magnetohydrodynamic
numerical simulations to model the star--planet interaction, incorporating a
star, a Hot Jupiter, and realistic stellar and planetary outflows. We explore a
wide range of parameters and analyze the flow structures and magnetic
topologies that develop. Our study suggests the classification of star--planet
interactions into four general types, based on the relative magnitudes of three
characteristic length scales that quantify the effects of the planetary
magnetic field, the planetary outflow, and the stellar gravitational field in
the interaction region. We describe the dynamics of these interactions and the
flow structures that they give rise to, which include bow shocks, cometary-type
tails, and inspiraling accretion streams. We point out the distinguishing
features of each of the classified cases and discuss some of their
observationally relevant properties. The magnetized interactions of
star--planet systems can be categorized, and their general morphologies
predicted, based on a set of basic stellar, planetary, and orbital parameters.",1503.03551v1
2015-03-16,Spontaneous decay of periodic magnetostatic equilibria,"In order to understand the conditions which lead a highly magnetized,
relativistic plasma to become unstable, and in such cases how the plasma
evolves, we study a prototypical class of magnetostatic equilibria where the
magnetic field satisfies $\nabla \times\mathbf B = \alpha \mathbf B$, where
$\alpha$ is spatially uniform, on a periodic domain. Using numerical solutions
we show that generic examples of such equilibria are unstable to ideal modes
(including incompressible ones) which are marked by exponential growth in the
linear phase. We characterize the unstable mode, showing how it can be
understood in terms of merging magnetic and current structures, and explicitly
demonstrate its instability using the energy principle. Following the nonlinear
evolution of these solutions, we find that they rapidly develop regions with
relativistic velocities and electric fields of comparable magnitude to the
magnetic field, liberating magnetic energy on dynamical timescales and
eventually settling into a configuration with the largest allowable wavelength.
These properties make such solutions a promising setting for exploring the
mechanisms behind extreme cosmic sources of gamma rays.",1503.04793v3
2015-05-04,Spatially resolved ultrafast magnetic dynamics launched at a complex-oxide hetero-interface,"Static strain in complex oxide heterostructures has been extensively used to
engineer electronic and magnetic properties at equilibrium. In the same spirit,
deformations of the crystal lattice with light may be used to achieve
functional control across hetero-interfaces dynamically. Here, by exciting
large amplitude infrared-active vibrations in a LaAlO3 substrate we induce
magnetic order melting in a NdNiO3 film across a hetero-interface. Femtosecond
Resonant Soft X-ray Diffraction is used to determine the spatial and temporal
evolution of the magnetic disordering. We observe a magnetic melt front that
grows from the substrate interface into the film, at a speed that suggests
electronically driven propagation. Light control and ultrafast phase front
propagation at hetero-interfaces may lead to new opportunities in
optomagnetism, for example by driving domain wall motion to transport
information across suitably designed devices.",1505.00601v1
2015-05-20,Three dimensional MHD Modeling of Vertical Kink Oscillations in an Active Region Plasma Curtain,"Observations on 2011 August 9 of an X6.9-class flare in active region (AR)
11263 by the Atmospheric Imaging Assembly (AIA) on-board the Solar Dynamics
Observatory (SDO), were followed by a rare detection of vertical kink
oscillations in a large-scale coronal active region plasma curtain in EUV
coronal lines. The damped oscillations with periods in the range 8.8-14.9 min
were detected and analyzed recently. Our aim is to study the generation and
propagation of the MHD oscillations in the plasma curtain taking into account
realistic 3D magnetic and density structure of the curtain. We also aim at
testing and improving coronal seismology for more accurate determination of the
magnetic field than with standard method. We use the observed morphological and
dynamical conditions, as well as plasma properties of the coronal curtain based
on Differential Emission Measure (DEM) analysis to initialize a 3D MHD model of
its vertical and transverse oscillations by implementing the impulsively
excited velocity pulse mimicking the flare generated nonlinear fast
magnetosonic propagating disturbance interacting with the curtain obliquely.
The model is simplified by utilizing initial dipole magnetic field, isothermal
energy equation, and gravitationally stratified density guided by observational
parameters. Using the 3D MHD model, we are able to reproduce the details of the
vertical oscillations and study the process of their excitation by nonlinear
fast magnetosonic pulse, propagation, and damping, finding agreement with the
observations. We estimate the accuracy of simplified slab-based coronal
seismology by comparing the determined magnetic field strength to actual values
from the 3D MHD modeling results and demonstrate the importance of taking into
account more realistic magnetic geometry and density for improving coronal
seismology.",1505.05427v1
2015-06-23,Layer-resolved magnetic exchange interactions of surfaces of late 3d elements: effects of electronic correlations,"We present the results of an ab initio study of magnetic properties of Fe, Co
and Ni surfaces. In particular, we discuss their electronic structure and
magnetic exchange interactions (Jij), as obtained by means of a combination of
density functional theory and dynamical mean-field theory. All studied systems
have a pronounced tendency to ferromagnetism both for bulk and surface atoms.
The presence of narrow-band surface states is shown to enhance the magnetic
moment as well as the exchange couplings. The most interesting results were
obtained for the Fe surface where the atoms have a tendency to couple
antiferromagnetically with each other. This interaction is relatively small,
when compared to interlayer ferromagnetic interaction, and strongly depends on
the lattice parameter. Local correlation effects are shown to lead to strong
changes of the overall shape of the spectral functions. However, they seem to
not play a decisive role on the overall picture of the magnetic couplings
studied here. We have also investigated the influence of correlations on the
spin and orbital moments of the bulk-like and surface atoms. We found that
dynamical correlations in general lead to enhanced values of the orbital
moment.",1506.07126v2
2015-07-09,Validity of the Néel-Arrhenius model for highly anisotropic Co_xFe_{3-x}O_4 nanoparticles,"We report a systematic study on the structural and magnetic properties of
Co_{x}Fe_{3-x}O_{4} magnetic nanoparticles with sizes between $5$ to $25$ nm,
prepared by thermal decomposition of Fe(acac)_{3} and Co(acac)_{2}. The large
magneto-crystalline anisotropy of the synthesized particles resulted in high
blocking temperatures ($42$ K \leqq $T_B$ $\leqq 345$ K for $5 \leqq$ d $\leqq
13$ nm ) and large coercive fields ($H_C \approxeq 1600$ kA/m for $T = 5$ K).
The smallest particles ($<d>=5$ nm) revealed the existence of a magnetically
hard, spin-disordered surface. The thermal dependence of static and dynamic
magnetic properties of the whole series of samples could be explained within
the N\'{e}el-Arrhenius relaxation framework without the need of ad-hoc
corrections, by including the thermal dependence of the magnetocrystalline
anisotropy constant $K_1(T)$ through the empirical Br\""{u}khatov-Kirensky
relation. This approach provided $K_1(0)$ values very similar to the bulk
material from either static or dynamic magnetic measurements, as well as
realistic values for the response times ($\tau_0 \simeq 10^{-10}$ s).
Deviations from the bulk anisotropy values found for the smallest particles
could be qualitatively explained based on Zener\'{}s relation between $K_1(T)$
and M(T).",1507.02711v1
2016-01-27,On the use of financial analysis tools for the study of Dst time series in the frame of complex systems,"Technical analysis is considered the oldest, currently omnipresent, method
for financial markets analysis, which uses past prices aiming at the possible
short-term forecast of future prices. In the frame of complex systems, methods
used to quantitatively analyze specific dynamic phenomena are often used to
analyze phenomena from other disciplines on the grounds that are governed by
similar dynamics. An interesting task is the forecast of a magnetic storm. The
hourly Dst is used as a global index for the monitoring of Earth's
magnetosphere, which could be either in quiet (normal) or in magnetic storm
(pathological) state. This work is the first attempt to apply technical
analysis tools on Dst time series, aiming at the identification of indications
which could be used for the study of the temporal evolution of Earth's
magnetosphere state. We focus on the analysis of Dst time series around the
occurrence of magnetic storms, discussing the possible use of the resulting
information in the frame of multidisciplinary efforts towards extreme events
forecasting. We employ the following financial analysis tools: simple moving
average (SMA), Bollinger bands, and relative strength index (RSI). Using these
tools, we formulate a methodology based on all indications that could be
revealed in order to infer the onset, duration and recovery phase of a magnetic
storm, focusing on the temporal sequence they occur. The applicability of the
proposed methodology is examined on characteristic cases of magnetic storms
with encouraging results for space weather forecasting.",1601.07334v1
2016-03-03,"Cantilever detected ferromagnetic resonance in thin Fe$_{50}$Ni$_{50}$, Co$_2$FeAl$_{0.5}$Si$_{0.5}$ and Sr$_2$FeMoO$_6$ films using a double modulation technique","In this work we introduce a new method of a ferromagnetic resonance (FMR)
detection from thin, nm-size, films. Our setup is based on the commercial
piezo-cantilever, used for atomic force microscopy. It has an option to rotate
the sample in the magnetic field and it operates up to the high microwave
frequencies of 160 GHz. Using our cantilever based FMR spectrometer we have
investigated a set of samples, namely quasi-bulk and 84 nm film
Co$_2$FeAl$_{0.5}$Si$_{0.5}$ samples, 16 nm Fe$_{50}$Ni$_{50}$ film and 150 nm
Sr$_2$FeMoO$_6$ film. The high frequency ferromagnetic resonance (FMR) response
from an extremely thin Fe$_{50}$Ni$_{50}$ film we have fitted with the
conventional model for the magnetization dynamics. The cantilever detected FMR
experiments on Sr$_2$FeMoO$_6$ film reveal an inability of the conventional
model to fit frequency and angular dependences with the same set of parameters,
which suggests that one has to take into account much more complicated nature
of the magnetization precession in the Sr$_2$FeMoO$_6$ at low temperatures and
high frequencies. Moreover, the complicated dynamics of the magnetization
apparent in all investigated samples is suggested by a drastic increase of the
linewidths with increasing microwave frequency, and by an emergence of the
second line with an opposite angular dependence.",1603.01142v1
2016-03-09,Subarcsecond Bright Points and Quasi-periodic Upflows Below a Quiescent Filament Observed by the IRIS,"Using UV spectra and SJIs from the IRIS, and coronal images and magnetograms
from the Solar Dynamics Observatory (SDO), we present the new features in a
quiescent filament channel: subarcsecond bright points (BPs) and quasi-periodic
upflows. The BPs in the TR have a spatial scale of about 350$-$580 km and
lifetime of more than several tens of minutes. They are located at stronger
magnetic structures in the filament channel, with magnetic flux of about
10$^{17}$$-$10$^{18}$ Mx. Quasi-periodic brightenings and upflows are observed
in the BPs and the period is about 4$-$5 min. The BP and the associated
jet-like upflow comprise a ""tadpole-shaped"" structure. The upflows move along
bright filament threads and their directions are almost parallel to the spine
of the filament. The upflows initiated from the BPs with opposite polarity
magnetic fields have opposite directions. The velocity of the upflows in plane
of sky is about 5$-$50 km s$^{-1}$. The emission line of Si IV 1402.77 {\AA} at
the locations of upflows exhibits obvious blueshifts of about 5$-$30 km
s$^{-1}$, and the line profile is broadened with the width of more than 20 km
s$^{-1}$. The BPs seem to be the bases of filament threads and the upflows are
able to convey mass for the dynamic balance of the filament. The
""counter-streaming"" flows in previous observations may be caused by the
propagation of bi-directional upflows initiated from opposite polarity magnetic
fields. We suggest that quasi-periodic brightenings of BPs and quasi-periodic
upflows result from small-scale oscillatory magnetic reconnections, which are
modulated by solar p-mode waves.",1603.02809v1
2016-03-31,Numerical optimization of writer and media for bit patterned magnetic recording,"In this work we present a micromagnetic study of the performance potential of
bit-patterned (BP) magnetic recording media via joint optimization of the
design of the media and of the magnetic write heads. Because the design space
is large and complex, we developed a novel computational framework suitable for
parallel implementation on compute clusters. Our technique combines advanced
global optimization algorithms and finite-element micromagnetic solvers.
Targeting data bit densities of $4\mathrm{Tb}/\mathrm{in}^2$, we optimize
designs for centered, staggered, and shingled BP writing. The magnetization
dynamics of the switching of the exchange-coupled composite BP islands of the
media is treated micromagnetically. Our simulation framework takes into account
not only the dynamics of on-track errors but also of the thermally induced
adjacent-track erasure. With co-optimized write heads, the results show
superior performance of shingled BP magnetic recording where we identify two
particular designs achieving write bit-error rates of $1.5\mathrm{x}10^{-8}$
and $8.4\mathrm{x}10^{-8}$, respectively. A detailed description of the key
design features of these designs is provided and contrasted with centered and
staggered BP designs which yielded write bit error rates of only
$2.8\mathrm{x}10^{-3}$ (centered design) and $1.7\mathrm{x}10^{-2}$ (staggered
design) even under optimized conditions.",1603.09493v4
2016-04-18,Relationship between chromospheric evaporation and magnetic field topology in M-class solar flare,"Chromospheric evaporation is observed as Doppler blueshift during solar
flares. It plays one of key roles in dynamics and energetics of solar flares,
however, its mechanism is still unknown. In this paper we present a detailed
analysis of spatially-resolved multi-wavelength observations of chromospheric
evaporation during an M 1.0 class solar flare (SOL2014-06-12T21:12) using data
from the NASA's IRIS (Interface Region Imaging Spectrograph) and HMI/SDO
(Helioseismic and Magnetic Imager onboard Solar Dynamics Observatory)
telescopes, and VIS/NST (Visible Imaging Spectrometer at New Solar Telescope)
high-resolution observations, covering the temperature range from 10^4 K to
10^7 K. The results show that the averaged over the region Fe XXI blueshift of
the hot evaporating plasma is delayed relative to the C II redshift of the
relatively cold chromospheric plasma by about 1 min. The spatial distribution
of the delays is not uniform across the region and can be as long as 2 min in
several zones. Using vector magnetograms from HMI we reconstruct the magnetic
field topology and the quasi-separatrix layer (QSL) and find that the blueshift
delay regions as well as the H-alpha flare ribbons are connected to the region
of magnetic polarity inversion line (PIL) and an expanding flux rope via a
system of low-lying loop arcades with height < ~4.5 Mm. This allows us to
propose an interpretation of the chromospheric evaporation based on the
geometry of local magnetic fields, and the primary energy source associated
with the PIL.",1604.05346v1
2016-05-29,Heisenberg Necklace Model in Magnetic Field,"Motivated by the experimental realizations of nearly one-dimensional spin-1/2
Heisenberg model found in chain cuprates SrCuO$_2$ and Sr$_2$CuO$_3$, which
remain in a quantum-critical Luttinger liquid state down to temperatures that
are much lower than in-chain exchange coupling, we consider the perturbation to
this state caused by interactions with nuclear spins on the same sites. We
study the low-energy sector of the Heisenberg Necklace model and estimate the
effect of the coupling between the nuclear and the electronic spins on the
overall spins dynamics and its dependence on the magnetic field. We find that
the Necklace model has a characteristic energy scale, $\Lambda \sim
J^{1/3}(\gamma I)^{2/3}$, at which the coupling between spins of the necklace
and the spins of the Heisenberg chain becomes strong. In the strong magnetic
field $\mu_B B > \Lambda$ the low energy spectrum is gapless, but two gapless
bosonic modes have different velocities whose ratio at strong fields approaches
a universal number, $\sqrt 2 +1$. In the case of Sr$_2$CuO$_3$ the energy scale
$\Lambda $ is sizable and comparable to the Neel ordering temperature induced
by the inter-chain coupling, and thus could noticeably modify the low
temperature magnon dynamics. We further find that the above energy scale is
insensitive to strong magnetic field, $\mu_B B \gg \Lambda \sim J^{1/3}(\gamma
I)^{2/3}$, and therefore the interaction with nuclear spins cannot lead to
unusually strong magnetic field dependence of the magnon spectrum observed by
ESR in Sr$_2$CuO$_3$, which has been attributed to the magnon interaction with
the Higgs mode.",1605.09014v2
2016-06-02,Dynamical structure of magnetized dissipative accretion flow around black holes,"We study the global structure of optically thin, advection dominated,
magnetized accretion flow around black holes. We consider the magnetic field to
be turbulent in nature and dominated by the toroidal component. With this, we
obtain the complete set of accretion solutions for dissipative flows where
bremsstrahlung process is regarded as the dominant cooling mechanism. We show
that rotating magnetized accretion flow experiences virtual barrier around
black hole due to centrifugal repulsion that can trigger the discontinuous
transition of the flow variables in the form of shock waves. We examine the
properties of the shock waves and find that the dynamics of the post-shock
corona (PSC) is controlled by the flow parameters, namely viscosity, cooling
rate and strength of the magnetic field, respectively. We separate the
effective region of the parameter space for standing shock and observe that
shock can form for wide range of flow parameters. We obtain the critical
viscosity parameter that allows global accretion solutions including shocks. We
estimate the energy dissipation at the PSC from where a part of the accreting
matter can deflect as outflows and jets. We compare the maximum energy that
could be extracted from the PSC and the observed radio luminosity values for
several super-massive black hole sources and the observational implications of
our present analysis are discussed.",1606.00526v1
2016-08-29,A proposed paradigm for solar cycle dynamics mediated via turbulent pumping of magnetic flux in Babcock-Leighton type solar dynamos,"At present, Babcock-Leighton flux transport solar dynamo models appear as the
most promising model for explaining diverse observational aspects of the
sunspot cycle. The success of these flux transport dynamo models is largely
dependent upon a single-cell meridional circulation with a deep equatorward
component at the base of the Sun's convection zone. However, recent
observations suggest that the meridional flow may in fact be very shallow
(confined to the top 10 % of the Sun) and more complex than previously thought.
Taken together these observations raise serious concerns on the validity of the
flux transport paradigm. By accounting for the turbulent pumping of magnetic
flux as evidenced in magnetohydrodynamic simulations of solar convection, we
demonstrate that flux transport dynamo models can generate solar-like magnetic
cycles even if the meridional flow is shallow. Solar-like periodic reversals is
recovered even when meridional circulation is altogether absent, however, in
this case the solar surface magnetic field dynamics does not extend all the way
to the polar regions. Very importantly, our results demonstrate that the
Parker-Yoshimura sign rule for dynamo wave propagation can be circumvented in
Babcock-Leighton dynamo models by the latitudinal component of turbulent
pumping -- which can generate equatorward propagating sunspot belts in the
absence of a deep, equatorward meridional flow. We also show that variations in
turbulent pumping coefficients can modulate the solar cycle amplitude and
periodicity. Our results suggest the viability of an alternate magnetic flux
transport paradigm -- mediated via turbulent pumping -- for sustaining
solar-stellar dynamo action.",1608.08167v1
2016-11-29,Critical quench dynamics of random quantum spin chains: Ultra-slow relaxation from initial order and delayed ordering from initial disorder,"By means of free fermionic techniques combined with multiple precision
arithmetic we study the time evolution of the average magnetization,
$\overline{m}(t)$, of the random transverse-field Ising chain after global
quenches. We observe different relaxation behaviors for quenches starting from
different initial states to the critical point. Starting from a fully ordered
initial state, the relaxation is logarithmically slow described by
$\overline{m}(t) \sim \ln^a t$, and in a finite sample of length $L$ the
average magnetization saturates at a size-dependent plateau $\overline{m}_p(L)
\sim L^{-b}$; here the two exponents satisfy the relation $b/a=\psi=1/2$.
Starting from a fully disordered initial state, the magnetization stays at zero
for a period of time until $t=t_d$ with $\ln t_d \sim L^{\psi}$ and then starts
to increase until it saturates to an asymptotic value $\overline{m}_p(L) \sim
L^{-b'}$, with $b'\approx 1.5$. For both quenching protocols, finite-size
scaling is satisfied in terms of the scaled variable $\ln t/L^{\psi}$.
Furthermore, the distribution of long-time limiting values of the magnetization
shows that the typical and the average values scale differently and the average
is governed by rare events. The non-equilibrium dynamical behavior of the
magnetization is explained through semi-classical theory.",1611.09495v2
2017-01-27,Application of Spin-Exchange Relaxation-Free Magnetometry to the Cosmic Axion Spin Precession Experiment,"The Cosmic Axion Spin Precession Experiment (CASPEr) seeks to measure
oscillating torques on nuclear spins caused by axion or axion-like-particle
(ALP) dark matter via nuclear magnetic resonance (NMR) techniques. A sample
spin-polarized along a leading magnetic field experiences a resonance when the
Larmor frequency matches the axion/ALP Compton frequency, generating precessing
transverse nuclear magnetization. Here we demonstrate a Spin-Exchange
Relaxation-Free (SERF) magnetometer with sensitivity $\approx 1~{\rm
fT/\sqrt{Hz}}$ and an effective sensing volume of 0.1 $\rm{cm^3}$ that may be
useful for NMR detection in CASPEr. A potential drawback of
SERF-magnetometer-based NMR detection is the SERF's limited dynamic range. Use
of a magnetic flux transformer to suppress the leading magnetic field is
considered as a potential method to expand the SERF's dynamic range in order to
probe higher axion/ALP Compton frequencies.",1701.08082v5
2017-03-11,Spin-wave dynamics in Permalloy/Cobalt magnonic crystals in the presence of a non-magnetic spacer,"In this paper, we theoretically study the influence of a non-magnetic spacer
between ferromagnetic dots and ferromagnetic matrix on the frequency dispersion
of the spin wave excitations in two-dimensional bi-component magnonic crystals.
By means of the dynamical matrix method we investigate structures inhomogeneous
across the thickness represented by square arrays of Cobalt or Permalloy dots
in a Permalloy matrix. We show that the introduction of a non-magnetic spacer
significantly modifies the total internal magnetic field especially at the
edges of the grooves and dots. This permits the manipulation of the magnonic
band structure of spin waves localized either at the edges of the dots or in
matrix material at the edges of grooves. According to the micromagnetic
simulations two types of end modes were found. The corresponding frequencies
are significantly influenced by the end modes localization region. We also show
that, with the use of a single ferromagnetic material, it is possible to design
a magnonic crystal preserving properties of bi-component magnonic crystals and
magnonic antidot lattices. Finally, the influence of the non-magnetic spacers
on the technologically relevant parameters like group velocity and magnonic
band width are discussed.",1703.03978v1
2017-03-23,Mott Transition and Magnetism in Rare Earth Nickelates and its Fingerprint on the X-ray Scattering,"The metal-insulator transition (MIT) remains among the most thoroughly
studied phenomena in solid state physics, but the complexity of the phenomena,
which usually involves cooperation of many degrees of freedom including
orbitals, fluctuating local moments, magnetism, and the crystal structure, have
resisted predictive ab-initio treatment. Here we develop ab-initio theoretical
method for correlated electron materials, based on Dynamical Mean Field Theory,
which can predict the change of the crystal structure across the MIT at finite
temperature. This allows us to study the coupling between electronic, magnetic
and orbital degrees of freedom with the crystal structure across the MIT in
rare-earth nickelates. We predict the free energy profile of the competing
states, and the theoretical magnetic ground state configuration, which is in
agreement with neutron scattering data, but is different from the magnetic
models proposed before. The resonant elastic X-ray response at the K-edge,
which was argued to be a direct probe of the charge order, is theoretically
modelled within the Dynamical Mean Field Theory, including the core-hole
interaction. We show that the line-shape of the measured resonant elastic X-ray
response can be explained with the ""site-selective"" Mott scenario without real
charge order on Ni sites.",1703.08196v2
2017-06-05,Quantum Kinetic Theory of the Chiral Anomaly,"We present a general quantum kinetic theory of low-field magnetotransport in
weakly disordered crystals that accounts fully for the interplay between
electric-field induced interband coherence, Bloch-state scattering, and an
external magnetic field. The quantum kinetic equation we derive for the
Bloch-state density matrix naturally incorporates the momentum-space Berry
phase effects whose influence on Bloch-state wavepacket dynamics is normally
incorporated into transport theory in an ad hoc manner. The Berry phase
correction to the momentum-space density of states in the presence of an
external magnetic field implied by semiclassical wavepacket dynamics is
captured by our theory as an intrinsic density-matrix response to a magnetic
field. We propose a simple and general procedure for expanding the linear
response of the Bloch-state density matrix to an electric field in powers of
magnetic field. As an illustration, we apply our theory to magnetotransport in
Weyl semimetals. We show that the chiral anomaly (positive magnetoconductivity
quadratic in magnetic field) that appears when separate Fermi surface pockets
surround distinct Weyl points survives only when intervalley scattering is very
weak compared to intravalley scattering.",1706.01200v3
2017-06-06,Coronal rain in magnetic bipolar weak fields,"We intend to investigate the underlying physics for the coronal rain
phenomenon in a representative bipolar magnetic field, including the formation
and the dynamics of coronal rain blobs. With the MPI-AMRVAC code, we performed
three dimensional radiative magnetohydrodynamic (MHD) simulation with strong
heating localized on footpoints of magnetic loops after a relaxation to quiet
solar atmosphere. Progressive cooling and in-situ condensation starts at the
loop top due to radiative thermal instability. The first large-scale
condensation on the loop top suffers Rayleigh-Taylor instability and becomes
fragmented into smaller blobs. The blobs fall vertically dragging magnetic
loops until they reach low beta regions and start to fall along the loops from
loop top to loop footpoints. A statistic study of the coronal rain blobs finds
that small blobs with masses of less than 10^10 g dominate the population. When
blobs fall to lower regions along the magnetic loops, they are stretched and
develop a non-uniform velocity pattern with an anti-parallel shearing pattern
seen to develop along the central axis of the blobs. Synthetic images of
simulated coronal rain with Solar Dynamics Observatory Atmospheric Imaging
Assembly well resemble real observations presenting dark falling clumps in hot
channels and bright rain blobs in a cool channel. We also find density
inhomogeneities during a coronal rain ""shower"", which reflects the observed
multi-stranded nature of coronal rain.",1706.01804v1
2017-09-15,I. Jet Formation and Evolution due to 3D Magnetic Reconnection,"Using simulated data-driven three-dimensional resistive MHD simulations of
the solar atmosphere, we show that magnetic reconnection can be responsible of
the formation of jets with characteristic of Type II spicules. For this, we
numerically model the photosphere-corona region using the C7 equilibrium
atmosphere model. The initial magnetic configuration is a 3D potential magnetic
field, extrapolated up to the solar corona region from a dynamic realistic
simulation of solar photospheric magnetoconvection model which is mimicking
quiet-Sun. In this case we consider a uniform and constant value of the
magnetic resistivity of $12.56 ~\Omega~{\rm m}$. We have found that formation
of the jets depends on the Lorentz force, which helps to accelerate the plasma
upwards. Analyzing various properties of the jet dynamics, we found that the
jet structure shows Doppler shift near to regions with high vorticity. The
morphology, upward velocity, covering a range up to 100 $\rm km$ $\rm s^{-1}$,
and life-time of the estructure, bigger than 100 ${\rm s}$, are similar to
those expected for Type II spicules.",1709.05066v2
2017-11-07,Negative magnetic eddy diffusivity due to oscillogenic $α$-effect,"We study large-scale kinematic dynamo action of steady mirror-antisymmetric
flows of incompressible fluid, that involve small spatial scales only, by
asymptotic methods of the multiscale stability theory. It turns out that, due
to the magnetic $\alpha$-effect in such flows, the large-scale mean field
experiences harmonic oscillations in time on the scale O($\varepsilon t$)
without growth or decay. Here $\varepsilon$ is the spatial scale ratio and $t$
is the fast time of the order of the flow turnover time. The interaction of the
accompanying fluctuating magnetic field with the flow gives rise to an
anisotropic magnetic eddy diffusivity, whose dependence on the direction of the
large-scale wave vector generically exhibits a singular behaviour, and thus to
negative eddy diffusivity for whichever molecular magnetic diffusivity.
Consequently, such flows always act as kinematic dynamos on the time scale
O($\varepsilon^2t$); for the directions at which eddy diffusivity is infinite,
the large-scale mean-field growth rate is finite on the scale
O($\varepsilon^{3/2}t$). We investigate numerically this dynamo mechanism for
two sample flows.",1711.02390v4
2018-01-08,Dynamic magnetic-transformation-induced exchange bias in (Fe2O3)0.1-(FeTiO3)0.9,"Up to now, for the conventional exchange bias (EB) systems there has been one
pinning phase and one pinned phase, and the pinning and pinned phases are
inherent to the material and do not mutually transform into each other.
Interestingly, we show here that EB is observed in a special system
(Fe2O3)0.1(FeTiO3)0.9 (HI9) different from the conventional EB system. Neutron
powder diffraction and magnetic measurement confirm that for HI9: i) two types
of short range antiferromagnetic ordering coexist, ii) there are two pinning
phases and one pinned phase, iii) the pinned phase is not intrinsic to the
structure but can be dynamically produced from the pinning phase with the help
of an external magnetic field. Consequently, two anomalous EB behaviors are
observed: i) both the coercivity (HC) and the exchange bias field (HE)
simultaneously decrease to zero at 30 K, ii) for a high cooling field (Hcool)
HE decreases logarithmically with increasing Hcool. Using Arrott plots it is
confirmed that the first order magnetic phase transformation (FOMPT) from the
AFM Fe2+ to ferromagnetic (FM) Fe2+ and the second order magnetic phase
transformation (SOMPT) for the process whereby the FM Fe2+ aligns with the
external field direction coexist in HI-9. The Morin transition and FOMPT cause
the anomalous EB behaviors. This work may provide fresh ideas for research into
EB behavior.",1801.02420v1
2018-01-29,Spin dynamics and exchange interactions in CuO measured by neutron scattering,"The magnetic properties of CuO encompass several contemporary themes in
condensed matter physics, including quantum magnetism, magnetic frustration,
magnetically-induced ferroelectricity and orbital currents. Here we report
polarized and unpolarized neutron inelastic scattering measurements which
provide a comprehensive map of the cooperative spin dynamics in the low
temperature antiferromagnetic (AFM) phase of CuO throughout much of the
Brillouin zone. At high energies ($E \gtrsim 100$\,meV) the spectrum displays
continuum features consistent with the des Cloizeax--Pearson dispersion for an
ideal $S=\frac{1}{2}$ Heisenberg AFM chain. At lower energies the spectrum
becomes more three-dimensional, and we find that a linear spin-wave model for a
Heisenberg AFM provides a very good description of the data, allowing for an
accurate determination of the relevant exchange constants in an effective spin
Hamiltonian for CuO. In the high temperature helicoidal phase, there are
features in the measured low-energy spectrum that we could not reproduce with a
spin-only model. We discuss how these might be associated with the
magnetically-induced multiferroic behavior observed in this phase.",1801.09473v2
2018-04-30,Spin-fluctuation and spin-relaxation effects of single adatoms from first principles,"Single adatoms offer an exceptional playground for studying magnetism and its
associated dynamics at the atomic scale. Here we review recent results on
single adatoms deposited on metallic substrates, based on time-dependent
density functional theory. First we analyze quantum zero-point
spin-fluctuations (ZPSF) as calculated from the fluctuation-dissipation
theorem, and show how they affect the magnetic stability by modifying the
magnetic anisotropy energy. We also assess the impact of ZPSF in the limit of
small hybridization to the substrate characteristic of semi-insulating
substrates, connecting to recent experimental investigations where magnetic
stability of a single adatom was achieved for the first time. Secondly, we
inspect further the dynamics of single adatoms by considering the longitudinal
and transverse spin-relaxation processes, whose time-scales are analyzed and
related to the underlying electronic structure of both the adatom and the
substrate. Thirdly, we analyze spin-fluctuation modes of paramagnetic adatoms,
i.e., adatoms where the Stoner criterion for magnetism is almost fulfilled.
Interestingly, such modes can develop well-defined peaks in the meV range,
their main characteristics being determined by two fundamental electronic
properties, namely the Stoner parameter and the density of states at the Fermi
level. Furthermore, simulated inelastic scanning tunneling spectroscopy curves
reveal that these spin-fluctuation modes can be triggered by tunneling
electrons, opening up potential applications also for paramagnetic adatoms.
Lastly, an overview of the outstanding issues and future directions is given.",1805.00347v1
2018-06-26,Terahertz magnetic field enhancement in an asymmetric spiral metamaterial,"We use finite element simulations in both the frequency and the time-domain
to study the terahertz resonance characteristics of a metamaterial (MM)
comprising a spiral connected to a straight arm. The MM acts as a RLC circuit
whose resonance frequency can be precisely tuned by varying the characteristic
geometrical parameters of the spiral: inner and outer radius, width and number
of turns. We provide a simple analytical model that uses these geometrical
parameters as input to give accurate estimates of the resonance frequency.
Finite element simulations show that linearly polarized terahertz radiation
efficiently couples to the MM thanks to the straight arm, inducing a current in
the spiral, which in turn induces a resonant magnetic field enhancement at the
center of the spiral. We observe a large (approximately 20 times) and uniform
(over an area of $\sim 10~\mu m^{2}$) enhancement of the magnetic field for
narrowband terahertz radiation with frequency matching the resonance frequency
of the MM. When a broadband, single-cycle terahertz pulse propagates towards
the metamaterial, the peak magnetic field of the resulting band-passed waveform
still maintains a 6-fold enhancement compared to the peak impinging field.
Using existing laser-based terahertz sources, our metamaterial design allows to
generate magnetic fields of the order of 2 T over a time scale of several
picoseconds, enabling the investigation of non-linear ultrafast spin dynamics
in table-top experiments. Furthermore, our MM can be implemented to generate
intense near-field narrowband, multi-cycle electromagnetic fields to study
generic ultrafast resonant terahertz dynamics in condensed matter.",1806.10100v1
2018-06-29,Sequential eruptions triggered by flux emergence - observations and modeling,"We describe and analyze observations by the Solar Dynamics Observatory of the
emergence of a small, bipolar active region within an area of unipolar magnetic
flux that was surrounded by a circular, quiescent filament. Within only eight
hours of the start of the emergence, a partial splitting of the filament and
two consecutive coronal mass ejections took place. We argue that all three
dynamic events occurred as a result of particular magnetic-reconnection
episodes between the emerging bipole and the pre-existing coronal magnetic
field. In order to substantiate our interpretation, we consider
three-dimensional magnetohydrodynamic simulations that model the emergence of
magnetic flux in the vicinity of a large-scale coronal flux rope. The
simulations qualitatively reproduce most of the reconnection episodes suggested
by the observations; as well as the filament-splitting, the first eruption, and
the formation of sheared/twisted fields that may have played a role in the
second eruption. Our results suggest that the position of emerging flux with
respect to the background magnetic configuration is a crucial factor for the
resulting evolution, while previous results suggest that parameters such as the
orientation or the amount of emerging flux are important as well. This poses a
challenge for predicting the onset of eruptions that are triggered by flux
emergence, and it calls for a detailed survey of the relevant parameter space
by means of numerical simulations.",1807.00020v1
2018-07-06,Spin wave modes in a cylindrical nanowire in crossover dipolar-exchange regime,"Nanoscale magnetic systems have been studied extensively in various
geometries, such as wires of different cross-sections, arrays of wires, dots,
rings, etc. Such systems have interesting physical properties and promising
applications in advanced magnetic devices. Uniform magnetic nanowires are the
basic structures which were broadly investigated. However, some of their
dynamical properties, like: (anti)crossing between the spin wave modes and
impact of the magnetic field on spin wave spectrum, still need to be exploited.
We continue this research by investigation of the spin wave dynamics in solid
Ni nanowire of the circular cross-section. We use two approaches:
semi-analytical calculations and numerical computations based on finite element
method. We solve coupled Landau-Lifshitz and Maxwell equations and consider
both magnetostatic and exchange interactions. We identify the dispersion
brunches and its (anti)crossing by plotting the spatial profiles of spin wave
amplitudes and magnetostatic potential. We also check how we can tune the
spectrum of the modes by application of the external magnetic field and how it
affects the modes and their dominating type of interaction.",1807.02580v3
2018-08-15,The SILCC project - V. The impact of magnetic fields on the chemistry and the formation of molecular clouds,"Magnetic fields are ubiquitously observed in the interstellar medium (ISM) of
present-day star-forming galaxies with dynamically relevant energy densities.
Using three-dimensional magneto-hydrodynamic (MHD) simulations of the supernova
(SN) driven ISM in the flux-freezing approximation (ideal MHD) we investigate
the impact of the magnetic field on the chemical and dynamical evolution of the
gas, fragmentation and the formation of molecular clouds. We follow the
chemistry with a network of six species (H$^{+}$, H, H$_2$, C$^+$, CO, free
electrons) including local shielding effects. We find that magnetic fields
thicken the disc by a factor of a few to a scale height of
$\sim100\,\mathrm{pc}$, delay the formation of dense (and molecular) gas by
$\sim25\,\mathrm{Myr}$ and result in differently shaped gas structures. The
magnetised gas fragments into fewer clumps, which are initially at subcritical
mass-to-flux ratios, $M/\Phi\approx0.3(M/\Phi)_\mathrm{crit}$, and accrete gas
preferentially parallel to the magnetic field lines until supercritial
mass-to-flux ratios of up to order 10 are reached. The accretion rates onto
molecular clouds scale with $\dot{M}\propto M^{1.5}$. The median of the
inter-cloud velocity dispersion is $\sim2-5\,\mathrm{km\,s}^{-1}$ and lower
than the internal velocity dispersion in the clouds
($\sim3-7\,\mathrm{km\,s}^{-1}$). However, individual cloud-cloud collisions
occur at speeds of a few $10\,\mathrm{km\,s}^{-1}$.",1808.05222v1
2018-11-01,Competition between static and dynamic magnetism in the Kitaev spin liquid material Cu2IrO3,"Anyonic excitations emerging from a Kitaev spin liquid can form a basis for
quantum computers. Searching for such excitations motivated intense research on
the honeycomb iridate materials. However, access to a spin liquid ground state
has been hindered by magnetic ordering. Cu2IrO3 is a new honeycomb iridate
without thermodynamic signatures of a long-range order. Here, we use muon spin
relaxation to uncover the magnetic ground state of Cu2IrO3. We find a
two-component depolarization with slow and fast relaxation rates corresponding
to distinct regions with dynamic and static magnetism, respectively. X-ray
absorption spectroscopy and first principles calculations identify a mixed
copper valence as the origin of this behavior. Our results suggest that a
minority of Cu2+ ions nucleate regions of static magnetism whereas the majority
of Cu+/Ir4+ on the honeycomb lattice give rise to a Kitaev spin liquid.",1811.00565v1
2018-11-13,Ferromagnetic resonance and magnetic precessions in $\varphi_0$ junction,"The Josephson $\varphi_0$ junctions with the current-phase relation $I = I_c
\sin (\varphi-\varphi_0)$, where the phase shift $\varphi_0$ is proportional to
the magnetic moment perpendicular to the gradient of the asymmetric spin-orbit
potential, demonstrate a number of unique features important for
superconducting spintronics and modern informational technologies. Here we show
that a current sweep along IV-characteristic of the $\varphi_0$ junction may
lead to regular magnetization dynamics with a series of specific phase
trajectories. The origin of these trajectories is related to a direct coupling
between the magnetic moment and the Josephson oscillations in these junctions,
and ferromagnetic resonance when Josephson frequency coincides with the
ferromagnetic one. We demonstrate that an external electromagnetic field can
control the dynamics of magnetic moment within a current interval corresponding
to a Shapiro step and produce topological transformation of specific precession
trajectories. We demonstrate the appearance of the DC component of
superconducting current and clarify its role in the transformation of
IV-characteristics in the resonance region. Good agreement between numerical
and analytical results has been found in the ferromagnetic resonance region.
The presented results might be used for developing novel resonance methods of
determination of the spin-orbit coupling parameter in the non-centrosymmetric
materials. We discuss experiments which can test our results.",1811.05282v2
2019-02-08,Subnanosecond Fluctuations in Low-Barrier Nanomagnets,"Fast magnetic fluctuations due to thermal torques have useful technological
functionality ranging from cryptography to probabilistic computing. The
characteristic time of fluctuations in typical uniaxial anisotropy magnets
studied so far is bounded from below by the well-known energy relaxation
mechanism. This time scales as $\alpha^{-1}$, where $\alpha$ parameterizes the
strength of dissipative processes. Here, we theoretically analyze the
fluctuating dynamics in easy-plane and antiferromagnetically coupled
nanomagnets. We find in such magnets, the dynamics are strongly influenced by
fluctuating intrinsic fields, which give rise to an additional dephasing-type
mechanism for washing out correlations. In particular, we establish two time
scales for characterizing fluctuations (i) the average time for a nanomagnet to
reverse|which for the experimentally relevant regime of low damping is governed
primarily by dephasing and becomes independent of $\alpha$, (ii) the time scale
for memory loss of a single nanomagnet|which scales as $\alpha^{-1/3}$ and is
governed by a combination of energy dissipation and dephasing mechanism. For
typical experimentally accessible values of intrinsic fields, the resultant
thermal-fluctuation rate is increased by multiple orders of magnitude when
compared with the bound set solely by the energy relaxation mechanism in
uniaxial magnets. This could lead to higher operating speeds of emerging
devices exploiting magnetic fluctuations.",1902.03312v3
2019-04-30,Magnetism trends in doped Ce-Cu intermetallics in the vicinity of quantum criticality: realistic Kondo lattice models based on dynamical mean-field theory,"The quantum critical point (QCP) in the archetypical heavy-fermion compound
CeCu$_6$ doped by Au is described, accounting for the localized $4f$-electron
of Ce, using realistic electronic structure calculations combined with
dynamical mean-field theory (DMFT). Magnetism trends in
Ce(Cu$_{1-\epsilon}$Au$_\epsilon$)$_6$ are compared with those in Co-doped
CeCu$_{5}$, which resides on the non-ferromagnetic side of the composition
space of one of the earliest rare-earth permanent magnet compounds,
Ce(Co,Cu)$_5$. The construction of a realistic Doniach phase diagram shows that
the system crosses over a magnetic quantum critical point in the Kondo lattice
in $0.2<x<0.4$ of Ce(Cu$_{1-x}$Co$_x$)$_5$. Comparison between Au-doped
CeCu$_6$ and Co-doped CeCu$_5$ reveals that the swept region in the vicinity of
QCP for the latter thoroughly covers that of the former. The implications of
these trends on the coercivity of the bulk rare-earth permanent magnets are
discussed.",1904.13160v5
2019-05-16,Element-specific visualization of dynamic magnetic coupling in a Co/Py bilayer microstructure,"We present the element-specific and time resolved visualization of uniform
ferromagnetic resonance excitations of a Permalloy (Py) disk - Cobalt (Co)
stripe bilayer microstructure. The transverse high frequency component of the
resonantly excited magnetization is sampled in the ps regime by a combination
of ferromagnetic resonance (FMR) and scanning transmission X-ray microscopy
(STXM- FMR) recording snapshots of the local magnetization precession of Py and
Co with nanometer spatial resolution. The approach allows us to individually
image the resonant dynamic response of each element, and we find that angular
momentum is transferred from the Py disk to the Co stripe and vice versa at
their respective resonances. The integral (cavity) FMR spectrum of our sample
shows an unexpected additional third resonance. This resonance is observed in
the STXM-FMR experiments as well and our microscopic findings suggest that it
is governed by magnetic exchange between Py and Co, showing for the Co stripe a
difference in relative phase of the magnetization due to stray field influence.",1905.06772v6
2019-05-18,Characterising Jupiter's dynamo radius using its magnetic energy spectrum,"Jupiter's magnetic field is generated by the convection of liquid metallic
hydrogen in its interior. The transition from molecular hydrogen to metallic
hydrogen as temperature and pressure increase is believed to be a smooth one.
As a result, the electrical conductivity in Jupiter varies continuously from
being negligible at the surface to a large value in the deeper region. Thus,
unlike the Earth where the upper boundary of the dynamo---the dynamo
radius---is definitively located at the core-mantle boundary, it is not clear
at what depth dynamo action becomes significant in Jupiter. In this paper,
using a numerical model of the Jovian dynamo, we examine the magnetic energy
spectrum at different depth and identify a dynamo radius below which (and away
from the deep inner core) the shape of the magnetic energy spectrum becomes
invariant. We find that this shift in the behaviour of the magnetic energy
spectrum signifies a change in the dynamics of the system as electric current
becomes important. Traditionally, a characteristic radius derived from the
Lowes--Mauersberger spectrum---the Lowes radius---gives a good estimate to the
Earth's core-mantle boundary. We argue that in our model, the Lowes radius
provides a lower bound to the dynamo radius. We also compare the
Lowes--Mauersberger spectrum in our model to that obtained from recent Juno
observations. The Lowes radius derived from the Juno data is significantly
lower than that obtained from our models. The existence of a stably stratified
region in the neighbourhood of the transition zone might provide an explanation
of this result.",1905.07661v2
2019-06-18,Noninertial and spin effects on the 2D Dirac oscillator in the magnetic cosmic string background,"In this work, we analyze the influence of noninertial and spin effects on the
dynamics of the 2D Dirac oscillator in the magnetic cosmic string background.
To model this background, we consider a uniform magnetic field, the
Aharonov-Bohm effect, and a parameter $\eta$ generated by a cosmic string.
Posteriorly, we determine the bound-state solutions of the system: the Dirac
spinor and the relativistic energy spectrum. We verified that this spinor is
written in terms of the generalized Laguerre polynomials and this spectrum
depends on the effective quantum number $N_r$, angular velocity $\Omega$ and
parameter $s$ associated to the noninertial and spin effects, magnetic flux
$\Phi$, cyclotron frequency $\omega_c$, zero-point energy $E_0$, and on the
deficit angle $\eta$. In particular, we note that besides this spectrum to be a
periodic function and asymmetric, its values infinitely increase when $\eta\to
0$ or $N_r=\omega_c=\Omega\to\infty$. We also note that the energies of the
antiparticle with spin down are larger than of the particle with spin up or
down. In the nonrelativistic limit, we get the Schr\""{o}dinger-Pauli oscillator
with two types of couplings: the spin-orbit coupling and the spin-rotation
coupling, and two Hamiltonians: one quantum harmonic oscillator-type and other
Zeeman-type. Finally, we compare our results with other works, where we
verified that our problem generalizes some particular cases of the literature
when $\Omega$, $\omega_c$, $\Phi$, $s$ or $\eta$ are excluded from the system.",1906.07369v2
2019-09-04,Global dynamics of the interstellar medium in magnetised disc galaxies,"Magnetic fields are an elemental part of the interstellar medium in galaxies.
However, their impact on gas dynamics and star formation in galaxies remains
controversial. We use a suite of global magnetohydrodynamical simulations of
isolated disc galaxies to study the influence of magnetic fields on the diffuse
and dense gas in the discs. We find that the magnetic field acts in multiple
ways. Stronger magnetised discs fragment earlier due to the shorter growth time
of the Parker instability. Due to the Parker instability in the magnetised
discs we also find cold ($T<50\,\mathrm{K}$) and dense
($n\sim10^3-10^4\,\mathrm{cm}^{-3}$) gas several hundred pc above/below the
midplane without any form of stellar feedback. In addition, magnetic fields
change the fragmentation pattern. While in the hydrodynamical case, the disc
breaks up into ring-like structures, magnetised discs show the formation of
filamentary entities that extent both in the azimuthal and radial direction.
These kpc scale filaments become magnetically (super-)critical very quickly and
allow for the rapid formation of massive giant molecular clouds. Our
simulations suggest that major differences in the behaviour of star formation -
due to a varying magnetisation - in galaxies could arise.",1909.01623v1
2019-10-18,Comparison of the Scaling Properties of EUV Intensity Fluctuations in Coronal Hole and Quiet-Sun Regions,"Using detrended fluctuation analysis (DFA) and rescaled range (R/S) analysis,
we investigate the scaling properties of EUV intensity fluctuations of
low-latitude coronal holes (CHs) and neighboring quiet-Sun (QS) regions in
signals obtained with the Solar Dynamics Observatory/Atmospheric Imaging
Assembly (SDO/AIA) instrument. Contemporaneous line-of-sight SDO/Helioseismic
and Magnetic Imager (HMI) magnetic fields provide a context for the physical
environment. We find that the intensity fluctuations in the time series of EUV
images present at each spatial point a scaling symmetry over the range $\sim
20$ min to $\sim$ 1 hour. Thus we are able to calculate a generalized Hurst
exponent and produce image maps, not of physical quantities like intensity or
temperature, but of a single dynamical parameter that sums up the statistical
nature of the intensity fluctuations at each pixel. In quiet-Sun (QS) regions
and in coronal holes (CHs) with magnetic bipoles, the scaling exponent ($1.0 <
\alpha \leq 1.5$) corresponds to anti-correlated turbulent-like processes. In
coronal holes, and in quiet-Sun regions primarily associated with (open)
magnetic field of dominant polarity, the generalized exponent (0.5 $< \alpha <$
1) corresponds to positively-correlated (persistent) processes. We identify a
tendency for $\alpha$ $\sim$ $1$ near coronal hole boundaries and in other
regions in which open and closed magnetic fields are in proximity. This is a
signature of an underlying $1/f$ type process that is characteristic for
self-organized criticality and shot-noise models.",1910.09541v1
2019-11-17,Expansion of Solar Coronal Hot Electrons in an Inhomogeneous Magnetic Field: 1-D PIC Simulation,"The expansion of hot electrons in flaring magnetic loops is crucial to
understanding the dynamics of solar flares. In this paper we investigate, for
the first time, the transport of hot electrons in a magnetic mirror field based
on a 1-D particle-in-cell (PIC) simulation. The hot electrons with small pitch
angle transport into the cold plasma, which leads to the generation of Langmuir
waves in the cold plasma and ion acoustic waves in the hot plasma. The large
pitch angle electrons can be confined by the magnetic mirror, resulting in the
different evolution time scale between electron parallel and perpendicular
temperature. This will cause the formation of electron temperature anisotropy,
which can generate the whistler waves near the interface between hot electrons
and cold electrons. The whistler waves can scatter the large pitch angle
electrons to smaller value through the cyclotron resonance, leading to
electrons escaping from the hot region. These results indicate that the
whistler waves may play an important role in the transport of electrons in
flaring magnetic loops. The findings from this study provide some new insights
to understand the electron dynamics of solar flares.",1911.07207v1
2019-11-18,The Formation and Evolution of Wide-Orbit Stellar Multiples In Magnetized Clouds,"Stars rarely form in isolation. Nearly half of the stars in the Milky Way
have a companion, and this fraction increases in star-forming regions. However,
why some dense cores and filaments form bound pairs while others form single
stars remains unclear. We present a set of three-dimensional,
gravo-magnetohydrodynamic simulations of turbulent star-forming clouds, aimed
at understanding the formation and evolution of multiple-star systems formed
through large scale (>~$10^3$ AU) turbulent fragmentation. We investigate three
global magnetic field strengths, with global mass-to-flux ratios of
$\mu_\phi$=2, 8, and 32. The initial separations of protostars in multiples
depends on the global magnetic field strength, with stronger magnetic fields
(e.g., $\mu_\phi$=2) suppressing fragmentation on smaller scales. The overall
multiplicity fraction (MF) is between 0.4-0.6 for our strong and intermediate
magnetic field strengths, which is in agreement with observations. The weak
field case has a lower fraction. The MF is relatively constant throughout the
simulations, even though stellar densities increase as collapse continues.
While the MF rarely exceeds 60% in all three simulations, over 80% of all
protostars are part of a binary system at some point. We additionally find that
the distribution of binary spin mis-alignment angles is consistent with a
randomized distribution. In all three simulations, several binaries originate
with wide separations and dynamically evolve to <~ $10^2$ AU separations. We
show that a simple model of mass accretion and dynamical friction with the gas
can explain this orbital evolution.",1911.07863v1
2020-01-13,Hydrodynamics of magnetic fluid droplets on superhydrophobic surfaces,"The study reports the aspects of postimpact hydrodynamics of ferrofluid
droplets on superhydrophobic SH surfaces in the presence of a horizontal
magnetic field. A wide gamut of dynamics was observed by varying the impact
Weber number We, the Hartmann number Ha and the magnetic field strength
manifested through the magnetic Bond number Bom. For a fixed We 60, we observed
that at moderately low Bom 300, droplet rebound off the SH surface is
suppressed. The noted We is chosen to observe various impact outcomes and to
reveal the consequent ferrohydrodynamic mechanisms. We also show that
ferrohydrodynamic interactions leads to asymmetric spreading, and the droplet
spreads preferentially in a direction orthogonal to the magnetic field lines.
We show analytically that during the retraction regime, the kinetic energy of
the droplet is distributed unequally in the transverse and longitudinal
directions due to the Lorentz force. This ultimately leads to suppression of
droplet rebound. We study the role of Bom at fixed We 60, and observed that the
liquid lamella becomes unstable at the onset of retraction phase, through
nucleation of holes, their proliferation and rupture after reaching a critical
thickness only on SH surfaces, but is absent on hydrophilic surfaces. We
propose an analytical model to predict the onset of instability at a critical
Bom. The analytical model shows that the critical Bom is a function of the
impact We, and the critical Bom decreases with increasing We. We illustrate a
phase map encompassing all the post impact ferrohydrodynamic phenomena on SH
surfaces for a wide range of We and Bom.",2001.04273v1
2020-01-23,Global simulations of self-gravitating magnetized protoplanetary disks,"In the early stages of a protoplanetary disk, when its mass is a significant
fraction of its star's, turbulence generated by gravitational instability (GI)
should feature significantly in the disk's evolution. At the same time, the
disk may be sufficiently ionised for magnetic fields to play some role in the
dynamics. Though usually neglected, the impact of magnetism on the GI may be
critical, with consequences for several processes: the efficiency of accretion,
spiral structure formation, fragmentation, and the dynamics of solids. In this
paper, we report on global three-dimensional magnetohydrodynamical simulations
of a self-gravitating protoplanetary disk using the meshless finite mass (MFM)
Lagrangian technique. We confirm that GI spiral waves trigger a dynamo that
amplifies an initial magnetic field to nearly thermal amplitudes (plasma beta <
10), an order of magnitude greater than that generated by the
magneto-rotational instability alone. We also determine the dynamo's nonlinear
back reaction on the gravitoturbulent flow: the saturated state is
substantially hotter, with an associated larger Toomre parameter and weaker,
more 'flocculent' spirals. But perhaps of greater import is the dynamo's
boosting of accretion via a significant Maxwell stress; mass accretion is
enhanced by factors of several relative to either pure GI or pure MRI. Our
simulations use ideal MHD, an admittedly poor approximation in protoplanetary
disks, and thus future studies should explore the full gamut of non-ideal MHD.
In preparation for that, we exhibit a small number of Ohmic runs that reveal
that the dynamo, if anything, is stronger in a non-ideal environment. This work
confirms that magnetic fields are a potentially critical ingredient in
gravitoturbulent young disks, possibly controlling their evolution, especially
via their enhancement of (potentially episodic) accretion.",2001.08693v1
2020-02-14,"Comparative ab initio study of the structural, electronic, magnetic, and dynamical properties of LiOsO$_3$ and NaOsO$_3$","Despite similar chemical compositions, LiOsO$_3$ and NaOsO$_3$ exhibit
remarkably distinct structural, electronic, magnetic, and spectroscopic
properties. At low temperature, LiOsO$_3$ is a polar bad metal with a
rhombohedral $R3c$ structure without the presence of long-range magnetic order,
whereas NaOsO$_3$ is a $G$-type antiferromagnetic insulator with an
orthorhombic $Pnma$ structure. By means of comparative first-principles DFT+$U$
calculations with the inclusion of the spin-orbit coupling, we ($i$) identify
the origin of the different structural ($R3c$ vs. $Pnma$) properties using a
symmetry-adapted soft mode analysis, ($ii$) provide evidence that all
considered exchange-correlation functionals (LDA, PBE, PBEsol, SCAN, and HSE06)
and the spin disordered polymorphous descriptions are unsatisfactory to
accurately describe the electronic and magnetic properties of both systems
simultaneously, and ($iii$) clarify that the distinct electronic (metallic vs.
insulating) properties originates mainly from a cooperative steric and magnetic
effect. Finally, we find that although at ambient pressure LiOsO$_3$ with a
$Pnma$ symmetry and NaOsO$_3$ with a $R\bar{3}c$ symmetry are energetically
unfavorable, they do not show soft phonons and therefore are dynamically
stable. A pressure-induced structural phase transition from $R3c$ to $Pnma$ for
LiOsO$_3$ is predicted, whereas for NaOsO$_3$ no symmetry change is discerned
in the considered pressure range.",2002.06081v1
2020-05-07,Effect of interfacial oxidation layer in spin pumping experiments on Ni$_{80}$Fe$_{20}$/SrIrO$_3$ heterostructures,"SrIrO$_3$ with its large spin-orbit coupling and low charge conductivity has
emerged as a potential candidate for efficient spin-orbit torque magnetization
control in spintronic devices. We here report on the influence of an
interfacial oxide layer on spin pumping experiments in Ni$_{80}$Fe$_{20}$
(NiFe)/SrIrO$_3$ bilayer heterostructures. To investigate this scenario we have
carried out broadband ferromagnetic resonance (BBFMR) measurements, which
indicate the presence of an interfacial antiferromagnetic oxide layer. We
performed in-plane BBFMR experiments at cryogenic temperatures, which allowed
us to simultaneously study dynamic spin pumping properties (Gilbert damping)
and static magnetic properties (such as the effective magnetization and
magnetic anisotropy). The results for NiFe/SrIrO$_3$ bilayer thin films were
analyzed and compared to those from a NiFe/NbN/SrIrO$_3$ trilayer reference
sample, where a spin-transparent, ultra-thin NbN layer was inserted to prevent
oxidation of NiFe. At low temperatures, we observe substantial differences in
the magnetization dynamics parameters of these samples, which can be explained
by an antiferromagnetic interfacial layer in the NiFe/SrIrO$_3$ bilayers.",2005.03727v1
2020-05-30,Magnetization dynamics in proximity-coupled superconductor/ferromagnet/superconductor multilayers,"In this work, magnetization dynamics is studied in
superconductor/ferromagnet/superconductor three-layered films in a wide
frequency, field, and temperature ranges using the broad-band ferromagnetic
resonance measurement technique. It is shown that in presence of both
superconducting layers and of superconducting proximity at both
superconductor/ferromagnet interfaces a massive shift of the ferromagnetic
resonance to higher frequencies emerges. The phenomenon is robust and
essentially long-range: it has been observed for a set of samples with the
thickness of ferromagnetic layer in the range from tens up to hundreds of
nanometers. The resonance frequency shift is characterized by proximity-induced
magnetic anisotropies: by the positive in-plane uniaxial anisotropy and by the
drop of magnetization. The shift and the corresponding uniaxial anisotropy grow
with the thickness of the ferromagnetic layer. For instance, the anisotropy
reaches 0.27~T in experiment for a sample with 350~nm thick ferromagnetic
layer, and about 0.4~T in predictions, which makes it a ferromagnetic film
structure with the highest anisotropy and the highest natural resonance
frequency ever reported. Various scenarios for the superconductivity-induced
magnetic anisotropy are discussed. As a result, the origin of the phenomenon
remains unclear. Application of the proximity-induced anisotropies in
superconducting magnonics is proposed as a way for manipulations with a
spin-wave spectrum.",2006.00348v1
2020-06-20,Momentum-dependent magnon lifetime in the metallic non-collinear triangular antiferromagnet CrB2,"Non-collinear magnetic order arises for various reasons in several magnetic
systems and exhibits interesting spin dynamics. Despite its ubiquitous
presence, little is known of how magnons, otherwise stable quasiparticles,
decay in these systems, particularly in metallic magnets. Using inelastic
neutron scattering, we examine the magnetic excitation spectra in a metallic
non-collinear antiferromagnet CrB$_{2}$, in which Cr atoms form a triangular
lattice and display incommensurate magnetic order. Our data show intrinsic
magnon damping and continuum-like excitations that cannot be explained by
linear spin wave theory. The intrinsic magnon linewidth $\Gamma(q,E_{q})$ shows
very unusual momentum dependence, which our analysis shows to originate from
the combination of two-magnon decay and the Stoner continuum. By comparing the
theoretical predictions with the experiments, we identify where in the momentum
and energy space one of the two factors becomes more dominant. Our work
constitutes a rare comprehensive study of the spin dynamics in metallic
non-collinear antiferromagnets. It reveals, for the first time, definite
experimental evidence of the higher-order effects in metallic antiferromagnets.",2006.11530v2
2020-09-22,"Dynamics evolution of a solar active-region filament from quasi-static state to eruption: rolling motion, untwisting motion, material transfer, and chirality","To better understand magnetic structure and eruptive process of solar
filaments, a solar active-region filament (labeled F2) eruption associated with
a B-class flare was investigated by using high-resolution H$\alpha$ data from
the 1 m New Vacuum Solar Telescope (NVST), combined with EUV observations of
the Solar Dynamical Observatory (SDO). The filament F2 was disturbed by another
filament (labeled F1) eruption that experienced a whip-like motion. Before the
filament F2 eruption, the Dopplergrams show that the southern and the northern
parts of the filament F2 body exhibit blueshift and redshift along the filament
spine, simultaneously. It implies that the filament F2 was rolling from one
side to the other. During the filament F2 eruption, the Doppler velocity shifts
of the filament body are opposite to that before its eruption. It demonstrates
that the filament body exhibits an untwisting motion, which can be also
identified by tracing the movement of the eruptive filament threads. Moreover,
it is found that the material of the filament F2 was transferred to the
surrounding magnetic field loops, which is caused by magnetic reconnection
between the filament F2 and the surrounding magnetic loops. According to the
right-bearing threads of the filament F2 before its eruption, it can be deduced
that the filament F2 is initially supported by a sheared arcade. The following
observations reveal that the twisted magnetic structure of the filament F2
formed in the eruption phase.",2009.10345v1
2020-10-14,Dynamic detection of current-induced spin-orbit magnetic fields: a phase independent approach,"Current induced spin-orbit torques (SOTs) in ferromagnet/non-magnetic metal
heterostructures open vast possibilities to design spintronic devices to store,
process and transmit information in a simple architecture. It is a central task
to search for efficient SOT-devices, and to quantify the magnitude as well as
the symmetry of current-induced spin-orbit magnetic fields (SOFs). Here, we
report a novel approach to determine the SOFs based on magnetization dynamics
by means of time-resolved magneto-optic Kerr microscopy. A microwave current in
a narrow Fe/GaAs (001) stripe generates an Oersted field as well as SOFs due to
the reduced symmetry at the Fe/GaAs interface, and excites standing spin wave
(SSW) modes because of the lateral confinement. Due to their different
symmetries, the SOFs and the Oersted field generate distinctly different mode
patterns. Thus it is possible to determine the magnitude of the SOFs from an
analysis of the shape of the SSW patterns. Specifically, this method, which is
conceptually different from previous approaches based on lineshape analysis, is
phase independent and self-calibrated. It can be used to measure the current
induced SOFs in other material systems, e.g., ferromagnetic metal/non-magnetic
metal heterostructures.",2010.06960v1
2021-03-04,Canted antiferromagnetic order and spin dynamics in the honeycomb-lattice Tb2Ir3Ga9,"Single crystal neutron diffraction, inelastic neutron scattering, bulk
magnetization measurements, and first-principles calculations are used to
investigate the magnetic properties of the honeycomb lattice $\rm
Tb_2Ir_3Ga_9$. While the $R\ln2$ magnetic contribution to the low-temperature
entropy indicates a $\rm J_{eff}=1/2$ moment for the lowest-energy
crystal-field doublet, the Tb$^{3+}$ ions form a canted antiferromagnetic
structure below 12.5 K. Due to the Dzyalloshinskii-Moriya interactions, the Tb
moments in the $ab$ plane are slightly canted towards $b$ by $6^\circ$ with a
canted moment of 1.22 $\mu_{\rm B} $ per formula unit. A minimal $xxz$ spin
Hamiltonian is used to simultaneously fit the spin-wave frequencies along the
high symmetry directions and the field dependence of the magnetization along
the three crystallographic axes. Long-range magnetic interactions for both
in-plane and out-of-plane couplings up to the second nearest neighbors are
needed to account for the observed static and dynamic properties. The $z$
component of the exchange interactions between Tb moments are larger than the
$x$ and $y$ components. This compound also exhibits bond-dependent exchange
with negligible nearest exchange coupling between moments parallel and
perpendicular to the 4$f$ orbitals. Despite the $J_{{\rm eff}}=1/2$ moments,
the spin Hamiltonian is denominated by a large in-plane anisotropy $K_z \sim
-1$ meV. DFT calculations confirm the antiferromagnetic ground state and the
substantial inter-plane coupling at larger Tb-Tb distances.",2103.03157v1
2021-03-10,"Spin texture induced by non-magnetic doping and spin dynamics in 2D triangular lattice antiferromagnet h-Y(Mn,Al)O3","Novel effects induced by nonmagnetic impurities in frustrated magnets and
quantum spin liquid represent a highly nontrivial and interesting problem. A
theoretical proposal of extended modulated spin structures induced by doping of
such magnets, distinct from the well-known skyrmions has attracted significant
interest. Here, we demonstrate that nonmagnetic impurities can produce such
extended spin structures in h-YMnO3, a triangular antiferromagnet with
noncollinear magnetic order. Using inelastic neutron scattering (INS), we
measured the full dynamical structure factor in Al-doped h-YMnO3 and confirmed
the presence of magnon damping with a clear momentum dependence. Our
theoretical calculations can reproduce the key features of the INS data,
supporting the formation of the proposed spin textures. As such, our study
provides the first experimental confirmation of the impurity-induced spin
textures. It offers new insights and understanding of the impurity effects in a
broad class of noncollinear magnetic systems.",2103.05936v2
2021-05-10,The global structure of magnetic fields and gas in simulated Milky Way-analogue galaxies,"We simulate an isolated, magnetised Milky Way-like disc galaxy using a
self-consistent model of unresolved star formation and feedback, evolving the
system until it reaches statistical steady state. We show that the
quasi-steady-state structure is distinctly layered in galactocentric height
$z$, with an innermost region having comparable gas and magnetic pressures
(plasma beta $\beta \sim 1$), an outermost region having dominant gas pressures
($\beta \gg 1$), and an intermediate region between $300$ pc $\lesssim |z|
\lesssim 3$ kpc that is dynamically dominated by magnetic fields ($\beta \ll
1$). We find field strengths, gas surface densities, and star formation rates
that agree well with those observed both in the Galactic centre and in the
Solar neighbourhood. The most significant dynamical effect of magnetic fields
on the global properties of the disc is a reduction of the star formation rate
by a factor of 1.5-2 with respect to an unmagnetised control simulation. At
fixed star formation rate, there is no significant difference in the mass
outflow rates or profiles between the magnetised and non-magnetised
simulations. Our results for the global structure of the magnetic field have
significant implications for models of cosmic ray-driven winds and cosmic-ray
propagation in the Galaxy, and can be tested against observations with the
forthcoming Square Kilometre Array and other facilities. Finally, we report the
discovery of a physical error in the implementation of neutral gas heating and
cooling in the popular GIZMO code, which may lead to qualitatively incorrect
phase structures if not corrected.",2105.04136v1
2021-06-21,Crystallization of Bosonic Quantum Hall States,"The dominance of interactions over kinetic energy lies at the heart of
strongly correlated quantum matter, from fractional quantum Hall liquids, to
atoms in optical lattices and twisted bilayer graphene. Crystalline phases
often compete with correlated quantum liquids, and transitions between them
occur when the energy cost of forming a density wave approaches zero. A prime
example occurs for electrons in high magnetic fields, where the instability of
quantum Hall liquids towards a Wigner crystal is heralded by a roton-like
softening of density modulations at the magnetic length. Remarkably,
interacting bosons in a gauge field are also expected to form analogous liquid
and crystalline states. However, combining interactions with strong synthetic
magnetic fields has been a challenge for experiments on bosonic quantum gases.
Here, we study the purely interaction-driven dynamics of a Landau gauge
Bose-Einstein condensate in and near the lowest Landau level (LLL). We observe
a spontaneous crystallization driven by condensation of magneto-rotons,
excitations visible as density modulations at the magnetic length. Increasing
the cloud density smoothly connects this behaviour to a quantum version of the
Kelvin-Helmholtz hydrodynamic instability, driven by the sheared internal flow
profile of the rapidly rotating condensate. At long times the condensate
self-organizes into a persistent array of droplets, separated by vortex
streets, which are stabilized by a balance of interactions and effective
magnetic forces.",2106.11300v1
2021-06-29,The sub-millimetre non-uniformity measurement of residual and coil-generated field in the magnetic shield using atomic vapor cell,"Magnetic field source localization and imaging happen at different scales.
The sensing baseline ranges from meter scale such as magnetic anomaly
detection, centimeter scale such as brain field imaging to nanometer scale such
as the imaging of magnetic skyrmion and single cell. Here we show how atomic
vapor cell can be used to realize a baseline of 109.6 {\mu}m with a magnetic
sensitivity of 10pT/sqrt(Hz)@0.6-100Hz and a dynamic range of 2062-4124nT.We
use free induction decay (FID) scheme to suppress low-frequency noise and avoid
scale factor variation for different domains due to light non-uniformity. The
measurement domains are scanned by digital micro-mirror device (DMD). The
currents of 22mA, 30mA, 38mA and 44mA are applied in the coils to generate
different fields along the pumping axis which are measured respectively by
fitting the FID signals of the probe light. The residual fields of every domain
are obtained from the intercept of linearly-fitting of the measurement data
corresponding to these four currents. The coil-generated fields are calculated
by deducting the residual fields from the total fields. The results demonstrate
that the hole of shield affects both the residual and the coil-generated field
distribution. The potential impact of field distribution measurement with an
outstanding comprehensive properties of spatial resolution, sensitivity and
dynamic range is far-reaching. It could lead to capability of 3D magnetography
for small stuffs and/or organs in millimeter or even smaller scale.",2106.15310v2
2021-07-05,Two Classes of Eruptive Events During Solar Minimum,"During solar minimum, the Sun is relatively inactive with few sunspots
observed on the solar surface. Consequently, we observe a smaller number of
highly energetic events such as solar flares or coronal mass ejections (CMEs),
which are often associated with active regions on the photosphere. Nonetheless,
our magnetofrictional simulations during the minimum period suggest that the
solar corona is still dynamically evolving in response to the large-scale
shearing velocities on the solar surface. The non-potential evolution of the
corona leads to the accumulation of magnetic free energy and helicity, which is
periodically shed in eruptive events. We find that these events fall into two
distinct classes: One set of events are caused by eruption and ejection of
low-lying coronal flux ropes, and they could explain the origin of occasional
CMEs during solar minimum. The other set of events are not driven by
destabilisation of low-lying structures but rather by eruption of overlying
sheared arcades. These could be associated with streamer blowouts or stealth
CMEs. The two classes differ significantly in the amount of magnetic flux and
helicity shed through the outer coronal boundary. We additionally explore how
other measurables such as current, open magnetic flux, free energy, coronal
holes, and the horizontal component of the magnetic field on the outer model
boundary vary during the two classes of event. This study emphasises the
importance and necessity of understanding the dynamics of the coronal magnetic
field during solar minimum.",2107.01941v1
2021-09-10,Thermally-induced magnetic order from glassiness in elemental neodymium,"Temperature in thermodynamics is synonymous with disorder, and responsible
for ultimately destroying ordered phases. Here, we show an unusual magnetic
transition where, with increasing the temperature of elemental neodymium,
long-range multi-Q magnetic order emerges from a self-induced spin glass. Using
temperature-dependent spin-polarized scanning tunneling microscopy, we
characterize the local Q order in the spin-Q glass phase and quantify the
emergence of long-range multi-Q order with increasing temperature. We develop
two distinct analysis tools, which enable the quantification of the glass
transition temperature, based on measured spatially-dependent magnetization. We
compare these observations with atomic spin dynamics simulations, which
reproduce the qualitative observation of a phase transition from a
low-temperature spin glass phase to an intermediate ordered multi-Q phase.
These simulations trace the origin of the unexpected high temperature order in
weakened frustration driven by temperature-dependent sublattice correlations.
These findings constitute an example of order from disorder and provide a rich
platform to study magnetization dynamics in a self-induced spin glass.",2109.04815v1
2021-11-17,Small-scale magnetic flux ropes and their properties based on in-situ measurements from Parker Solar Probe,"We report small-scale magnetic flux ropes via the Parker Solar Probe in situ
measurements during the first six encounters and present additional analyses to
supplement our prior work in Chen et al. 2021. These flux ropes are detected by
the Grad-Shafranov-based algorithm with the duration and scale size ranging
from 10 seconds to $\lesssim$1 hour and from a few hundred kilometers to
10$^{-3}$ au, respectively. They include both static structures and those with
significant field-aligned plasma flows. Most structures tend to possess large
cross helicity, while the residual energy distributes in wide ranges. We find
that these dynamic flux ropes mostly propagate anti-sunward, with no
preferential sign of magnetic helicity. The magnetic flux function follows a
power law and is proportional to scale size. We also present case studies
showing reconstructed two-dimensional (2D) configurations, which confirm that
the static and dynamic flux ropes have the common configuration of spiral
magnetic field lines (also streamlines). Moreover, the existence of such events
hints at the interchange reconnection as a possible mechanism to generate flux
rope-like structures near the Sun. Lastly, we summarize the major findings and
discuss the possible correlation between these flux rope-like structures and
turbulence due to the process of local Alfvenic alignment.",2111.09261v1
2021-12-22,Untangling magnetic massive star properties with linear polarization variability and the Analytic Dynamical Magnetosphere model,"Light scattered off particles can become linearly polarized. Stars surrounded
by oblique, co-rotating envelopes are therefore expected to manifest periodic
linear polarimetric variations. The electron scattering magnetospheres of
magnetic massive stars are expected to be suitable candidates to observe this
effect. In this paper, we present the first semi-analytical model capable of
synthesizing the continuum polarimetric signatures of magnetic O-type stars in
an optically thin, single electron scattering limit. The purpose of this
investigation is to improve our general understanding of magnetic hot stars by
characterizing their polarimetric behaviour. Our linear polarization model is
constructed by combining the analytical expressions for the polarimetric
variations of an obliquely rotating envelope with the Analytic Dynamical
Magnetosphere model to represent a physical model for the envelope density
structure. We compute grids of model Stokes $Q$ and $U$ curves and show that
their shapes are unique to the choice of inclination and obliquity angles. We
apply our model to HD 191612, a prototypical Of?p-type star, having both
polarimetric and photometric observations. We find that the polarimetric
modulations are best reproduced with $i=19^{+12}_{-3}$$^\circ$,
$\beta=71^{+3}_{-9}$$^\circ$, and $\log \dot{M}_{B=0}=-6.11^{+0.12}_{-0.06}$
[M$_{\odot}$ yr$^{-1}$]. These results agree with previous investigations of
this star. By combining both polarimetric and photometric synthesis tools, we
simultaneously model the observations thus adding further refinement of the
wind and magnetic properties of HD 191612.",2112.11942v1
2022-04-28,"Low-energy spin excitations of the frustrated ferromagnetic $J_1$-$J_2$ chain material linarite, PbCuSO$_4$(OH)$_2$, in applied magnetic fields $\mathbf{H} \parallel b$ axis","We report a study of the spin dynamics of the frustrated ferromagnetic
$J_1$-$J_2$ chain compound linarite, PbCuSO$_4$(OH)$_2$, in applied magnetic
fields up to field polarization. By means of an extreme-environment inelastic
neutron scattering experiment, we have measured the low-energy spin excitations
of linarite in fields up to 8.8 T for $\mathbf{H} \parallel b$ axis. We have
recorded the spin excitation spectra along $h$, $k$ and $l$ for the
field-induced magnetic phases IV, V and the field polarized state close to
saturation. By employing first-principles calculations, we estimate the leading
magnetic exchanges out of the $bc$ plane and model the dispersion relations
using linear spin-wave theory. In this way, we find evidence for a (very weak)
residual magnetic exchange coupling out of the $bc$ plane. Together with the
previously established dominant intrachain couplings $J_1$ and $J_2$ and the
interchain coupling $J_3$, we derive an effective set of exchange couplings for
a microscopic description of linarite. Further, we find that the peculiar
character of phase V manifests itself in the measured spin dynamics.",2204.13482v2
2022-05-12,"Investigation of lattice dynamics, magnetism and electronic transport in $β$-Na$_{0.33}$V$_2$O$_{5}$","We investigate the electronic and magnetic properties as well as lattice
dynamics and spin-phonon coupling of $\beta$-Na$_{0.33}$V$_2$O$_5$ using
temperature-dependent Raman scattering, dc-magnetization and dc-resistivity,
x-ray photoemission, and absorption spectroscopy. The Rietveld refinement of
XRD pattern with space group C2/m confirms the monoclinic structure. The
analysis of temperature-dependent Raman spectra in a temperature range of
13--673~K reveals an anharmonic dependence of the phonon frequency and full
width at half maximum, which is accredited to the symmetric phonon decay.
However, below about 40 K, the hardening of the phonon frequency beyond
anharmonicity is attributed to the spin-phonon coupling. Interestingly, the
estimated effective magnetic moment $\mu_{\rm eff}=$ 0.63~$\mu_B$ from the
magnetization data manifests a mixed-valence state of V ions in 4+ (18$\pm$1\%)
and 5+ (82$\pm$1\%). A similar ratio of V$^{4+}$ to V$^{5+}$ is also observed
in the x-ray photoemission and x-ray absorption near-edge spectra and that is
found to be consistent with the sample stoichiometry. In addition, the V$^{4+}$
ions are distributed between different vanadium (V1 and V3) sites. The analysis
of extended x-ray absorption fine structure at different V-sites gives the
corresponding V--O bond lengths, which are utilized in the assignment of Raman
modes. Moreover, the temperature-dependent resistivity resembles a
semiconducting behavior where the charge carrier transport is facilitated by
the band conduction at higher temperatures and via hopping $\le$260~K.",2205.06019v2
2022-05-31,Dualities of Adjoint QCD$_3$ from Branes,"We consider an 'electric' $U(N)$ level $k$ QCD$_3$ theory with one adjoint
Majorana fermion. Inspired by brane dynamics, we suggest that for $k \ge N/2$
the massive $m<0$ theory, in the vicinity of the supersymmetric point, admits a
$U(k-\frac{N}{2})_{-(\frac{1}{2}k+\frac{3}{4}N),-(k+\frac{N}{2})}$ 'magnetic'
dual with one adjoint Majorana fermion. The magnetic theory flows in the IR to
a topological $U(k-\frac{N}{2})_{-N,-(k+\frac{N}{2})}$ pure Chern-Simons theory
in agreement with the dynamics of the electric theory. When $k<N/2$ the
magnetic dual is $U(\frac{N}{2}-k)_{\frac{1}{2}k+\frac{3}{4}N,N}$ with one
adjoint Majorana fermion. Depending on the sign of the fermion mass, the
magnetic theory flows to either $U(\frac{N}{2}-k)_{N,N}$ or
$U(\frac{N}{2}-k)_{\frac{1}{2}N+k,N}$ TQFT. A second magnetic theory,
$U(N/2+k)_{\frac{1}{2}k-\frac{3}{4}N,-N}$, flows to either
$U(\frac{N}{2}+k)_{-N,-N}$ or $U(\frac{N}{2}+k)_{-(\frac{1}{2}N-k),-N}$ TQFT.
Dualities for $SO$ and $USp$ theories with one adjoint fermion are also
discussed.",2205.15706v4
2022-06-06,Quantitative theory of magnetic interactions in solids,"In this report we review the method of explicit calculations of interatomic
exchange interactions of magnetic materials. This involves exchange mechanisms
normally referred to as Heisenberg exchange, Dzyaloshinskii-Moriya interaction
and anisotropic symmetric exchange. The connection between microscopic theories
of the electronic structure, such as density functional theory or dynamical
mean field theory, and interatomic exchange, is given in detail. The different
aspects of extracting information for an effective spin Hamiltonian that
involves thousands of atoms, from electronic structure calculations considering
significantly fewer atoms (1-50) is highlighted. Examples of exchange
interactions of a large group of materials is presented, which involves heavy
elements of the 3d period, alloys between transition metals, Heusler compounds,
multilayer systems as well as overlayers and adatoms on a substrate, transition
metal oxides, 4f elements, magnetic materials in two dimensions and molecular
magnets. Where possible, a comparison to experimental data is made, that
naturally becomes focused on the magnon dispersion. The influence of relativity
is reviewed for a few cases, as is the importance of dynamical correlations.
Development to theories that handle out of equilibrium conditions is also
described here. The review ends with a short description of extensions of the
theories behind explicit calculations of interatomic exchange, to non-magnetic
situations, e.g. that describe chemical (charge) order and superconductivity.",2206.02415v2
2022-11-07,A Census of Outflow to Magnetic Field Orientations in Nearby Molecular Clouds,"We define a sample of 200 protostellar outflows showing blue and redshifted
CO emission in the nearby molecular clouds Ophiuchus, Taurus, Perseus and Orion
to investigate the correlation between outflow orientations and local, but
relatively large-scale, magnetic field directions traced by Planck 353 GHz dust
polarization. At high significance (p~1e-4), we exclude a random distribution
of relative orientations and find that there is a preference for alignment of
projected plane of sky outflow axes with magnetic field directions. The
distribution of relative position angles peaks at ~30deg and exhibits a broad
dispersion of ~50deg. These results indicate that magnetic fields have
dynamical influence in regulating the launching and/or propagation directions
of outflows. However, the significant dispersion around perfect alignment
orientation implies that there are large measurement uncertainties and/or a
high degree of intrinsic variation caused by other physical processes, such as
turbulence or strong stellar dynamical interactions. Outflow to magnetic field
alignment is expected to lead to a correlation in the directions of nearby
outflow pairs, depending on the degree of order of the field. Analyzing this
effect we find limited correlation, except on relatively small scales < 0.5 pc.
Furthermore, we train a convolutional neural network to infer the inclination
angle of outflows with respect to the line of sight and apply it to our outflow
sample to estimate their full 3D orientations. We find that the angles between
outflow pairs in 3D space also show evidence of small-scale alignment.",2211.03781v1
2022-11-13,Non-perturbative diffusion of Heavy Quark moving in a hot and magnetised Quark Gluon Plasma,"Heavy Quarks (HQs) serve as excellent probes to understand various
characteristics of deconfined hot QCD medium, comprising light quarks and
gluons, created in the Heavy Ion Collisions (HICs). Strong magnetic fields in
non-central HICs may significantly affect HQ dynamics in this medium. Exploring
the impact of the magnetic field on the perturbative and non-perturbative
transport coefficients of HQ is an intriguing endeavor. This necessitates the
development of a comprehensive theoretical framework accommodating the
non-perturbative Quantum Chromo Dynamics(npQCD) description alongside
perturbative QCD(pQCD). The present work provides such a formulation in which
Quarkonium potential in the hot and magnetic QCD medium is implemented as the
effective gluon propagator to calculate the rate of elastic scattering between
HQ and the light partons inside a medium of hot Quark Gluon Plasma(QGP) in the
presence of a strong but uniform magnetic field. Diffusion coefficients of a
charm quark have been computed for a short-range Yukawa potential ( pQCD) as
well as a long-range confining/non-perturbative potential (npQCD) for two cases
in which the velocity of the charm quark is parallel and perpendicular to the
magnetic field respectively. Non-perturbative contribution is seen to dominate
over the perturbative one in the regime of low temperatures and low to
intermediate momenta of charm quark. As the momentum of charm quark as well as
the temperature of the medium increase pQCD starts to gain on the npQCD
contribution, ultimately prevailing at higher temperatures and charm momenta.",2211.06985v3
2022-12-14,Magnetic fluxes of solar active regions of different magneto-morphological classes: I. Cyclic variations,"Data for 3046 solar active regions (ARs) observed since May 12, 1996 to
December 27, 2021 were utilized to explore how the magnetic fluxes from ARs of
different complexity follow the solar cycle. Magnetograms from the Michelson
Doppler Imager instrument on the Solar and Heliospheric Observatory and from
the Helioseismic and Magnetic Imager instrument on the Solar Dynamics
Observatory were utilized. Each AR was classified as a regular bipolar AR
(classes A1 or A2), or as an irregular bipolar AR (class B1), or as a
multipolar AR (classes B2 or B3). Unipolar ARs were segregated into a specific
class U. We found the following results. Unsigned magnetic fluxes from ARs of
different classes evolve synchronously following the cycle, the correlation
coefficient between the flux curves varies in a range of (0.70 - 0.99). The
deepest solar minimum is observed simultaneously for all classes. Only the most
simple ARs were observed during a deepest minimum: A1- and B1-class ARs. The
overall shape of a cycle is governed by the regular ARs, whereas the fine
structure of a solar maximum is determined by the most complex irregular ARs.
Approximately equal amount of flux (45$-$50% of the total flux) is contributed
by the A-class and B-class ARs during a solar maximum. Thus, observations allow
us to conclude that the appearance of ARs with the magnetic flux above
10$^{21}$ Mx is caused by the solar dynamo that operates as a unique process
displaying the properties of a non-linear dynamical dissipative system with a
cyclic behaviour and unavoidable fluctuations.",2212.07115v1
2023-01-02,Tailoring the escape rate of a Brownian particle by combining a vortex flow with a magnetic field,"The probability per unit time for a thermally activated Brownian particle to
escape over a potential well is in general well-described by Kramers theory.
Kramers showed that the escape time decreases exponentially with increasing
barrier height. The dynamics slow down when the particle is charged and
subjected to a Lorentz force due to an external magnetic field. This is evident
via a rescaling of the diffusion coefficient entering as a prefactor in the
Kramers escape rate without any impact on the barrier-height-dependent
exponent. Here we show that the barrier height can be effectively changed when
the charged particle is subjected to an external vortex flow. While the
external vortex alone does not affect the mean escape time of the particle,
when combined with a magnetic field it effectively pushes the fluctuating
particle either radially outside or inside depending on its sign relative to
that of the magnetic field. In particular, the effective potential over which
the particle escapes can be changed to a flat, a stable, and an unstable
potential by tuning the signs and magnitudes of the external vortex and the
applied magnetic field. Notably, the last case corresponds to enhanced escape
dynamics.",2301.00589v1
2023-02-01,A Brillouin light scattering study of the spin-wave magnetic field dependence in a magnetic hybrid system made of an artificial spin-ice structure and a film underlayer,"We present a combined Brillouin light scattering and micromagnetic simulation
investigation of the magnetic-field dependent spin-wave spectra in a hybrid
structure made of permalloy (NiFe) artificial spin-ice (ASI) systems, composed
of stadium-shaped nanoislands, deposited on the top of an unpatterned permalloy
film with a nonmagnetic spacer layer. The thermal spin-wave spectra were
recorded by Brillouin light scattering (BLS) as a function of the magnetic
field applied along the symmetry direction of the ASI sample. Magneto-optic
Kerr effect magnetometry was used to measure the hysteresis loops in the same
orientation as the BLS measurements. The frequency and intensity of several
spin-wave modes detected by BLS were measured as a function of the applied
magnetic field. Micromagnetic simulations enabled us to identify the modes in
terms of their frequency and spatial symmetry and to extract information about
the existence and strength of the dynamic coupling, relevant only to a few
modes of a given hybrid system. Using this approach, we suggest a way to
understand if dynamic coupling between ASI and film modes is present or not,
with interesting implications for the development of future three-dimensional
magnonic applications and devices.",2302.00483v1
2023-04-07,A Numerical Study of the Impact of Jet Magnetic Topology on Radio Galaxy Evolution,"The propagation of active galactic nucleus jets depends both on the
environment into which they propagate and on their internal structure. To test
the impact that different magnetic topologies have on the observable properties
of radio galaxies on kpc scales, we conducted a series of magneto-hydrodynamic
simulations of jets injected with different magnetic field configurations
propagating into a gaseous atmosphere modeled on the Perseus cluster. The
simulations show that the structure of the field affects the collimation and
propagation of the jets on cluster scales and thus the morphology of the radio
lobes inflated by the jets, due both to magnetic collimation and the
development of dynamical instabilities in jets with different magnetic
topologies. In all cases, the simulations show a distinct reversal of the
sychrotron spectral age gradient in the radio lobes about a dynamical time
after the jets turn off due to large scale circulation inside the radio lobe,
driven primarily by buoyancy, which could provide a way to constrain the age of
radio sources in cluster environments without the need for detailed spectral
modeling and thus constrain the radio mode feedback efficiency. We suggest a
robust diagnostic to search for such age gradients in multi-frequency radio
data.",2304.03863v1
2023-06-27,"Three-dimensional spin-wave dynamics, localization and interference in a synthetic antiferromagnet","Spin waves are collective perturbations in the orientation of the magnetic
moments in magnetically ordered materials. Their rich phenomenology is
intrinsically three dimensional, from the trajectory of the spin precession
during their propagation, to the profiles of the spin-wave mode throughout the
volume of the magnetic system. This gives rise to novel complex phenomena with
high potential for applications in the field of magnonics. However, the
three-dimensional imaging of spin waves, key to understanding and harnessing
these phenomena, has so far not been possible. Here, we image the
three-dimensional dynamics of spin waves excited in a synthetic
antiferromagnet, with nanoscale spatial resolution and sub-ns temporal
resolution, using time-resolved magnetic laminography. In this way, we map the
distribution of the spin-wave modes throughout the volume of the structure,
revealing unexpected depth-dependent profiles originating from the interlayer
dipolar interaction. We experimentally demonstrate the existence of complex
three-dimensional interference patterns, and analyze them via micromagnetic
modelling. We find that these patterns are generated by the superposition of
spin waves with non-uniform amplitude profiles, and that their features can be
controlled by tuning the composition and structure of the magnetic system. Our
results open unforeseen possibilities for the study of complex spin-wave modes
and their interaction within nanostructures, and for the generation and
manipulation of three-dimensional spin-wave landscapes for the design of novel
functions in magnonic devices.",2306.15404v1
2023-06-28,Dipolar Ultrastrong Magnon-Magnon Coupling in a 3D Multilayered Artificial Spin-Vortex Ice,"Strongly-interacting nanomagnetic arrays are ideal systems for exploring
reconfigurable magnonics. They provide huge microstate spaces and integrated
solutions for storage and neuromorphic computing alongside GHz functionality.
These systems may be broadly assessed by their range of reliably accessible
states and the strength of magnon coupling phenomena and nonlinearities.
  Increasingly, nanomagnetic systems are expanding into three-dimensional
architectures. This has enhanced the range of available magnetic microstates
and functional behaviours, but engineering control over 3D states and dynamics
remains challenging.
  Here, we introduce a 3D magnonic metamaterial composed from multilayered
artificial spin ice nanoarrays. Comprising two magnetic layers separated by a
non-magnetic spacer, each nanoisland may assume four macrospin or vortex states
per magnetic layer. This creates a system with a rich $16^N$ microstate space
and intense static and dynamic dipolar magnetic coupling.
  The system exhibits a broad range of emergent phenomena driven by the strong
inter-layer dipolar interaction, including ultrastrong magnon-magnon coupling
with normalised coupling rates of $\frac{\Delta \omega}{\gamma} = 0.57$, GHz
mode shifts in zero applied field and chirality-selective magneto-toroidal
microstate programming and corresponding magnonic spectral control.",2306.16159v2
2023-10-03,Controlled Quasi-Latitudinal Solutions for ultra-fast Spin-Torque Precessional Magnetization Switching,"The aim of the paper is to present a novel class of time-dependent controls
to realize ultra-fast magnetization switching in nanomagnets driven by
spin-torques produced by spin-polarized electric currents. Magnetization
dynamics in such systems is governed by the Landau-Lifshitz-Slonczewski
equation which describes the precessional motion of (dimensionless)
magnetization vector on the unit-sphere. The relevant case of nanoparticles
with uniaxial anisotropy having in-plane easy and intermediate axes and
out-of-plane hard axis is considered. By exploiting the characteristic
smallness of damping and spin-torque intensity, the aforementioned controls are
constructed via suitable perturbative tools in a way to realise approximate
\emph{latitudinal solutions} (i.e. motions on a sphere in which the
out-of-plane magnetization component stays constant) with the effect to fast
``switch'' the system from one stationary state to another. The possibility to
keep a (``small'') bounded value of the out-of-plane coordinate throughout this
process of ``transfer'', turns out to be advantageous in the applications as it
sensibly reduces the post-switching relaxation oscillations that may cause the
failure of switching in real samples. Further relevant quantitative results on
the behaviour of the solutions during the pre- and post-switching stages
(termed ``expulsion'' and ``attraction'', respectively), are given as a
byproduct. A selection of validating numerical experiments is presented
alongside the corresponding theoretical results.",2310.02070v1
2023-10-17,Winds and Disk Turbulence Exert Equal Torques on Thick Magnetically Arrested Disks,"The conventional accretion disk lore is that magnetized turbulence is the
principal angular momentum transport process that drives accretion. However,
when dynamically important magnetic fields thread an accretion disk, they can
produce mass and angular momentum outflows that also drive accretion. Yet, the
relative importance of turbulent and wind-driven angular momentum transport is
still poorly understood. To probe this question, we analyze a long-duration
($1.2 \times 10^5 r_{\rm g}/c$) simulation of a rapidly rotating ($a=0.9$)
black hole (BH) feeding from a thick ($H/r\sim0.3$), adiabatic, magnetically
arrested disk (MAD), whose dynamically-important magnetic field regulates mass
inflow and drives both uncollimated and collimated outflows (e.g., ""winds"" and
""jets"", respectively). By carefully disentangling the various angular momentum
transport processes occurring within the system, we demonstrate the novel
result that both disk winds and disk turbulence extract roughly equal amounts
of angular momentum from the disk. We find cumulative angular momentum and mass
accretion outflow rates of $\dot{L}\propto r^{0.9}$ and $\dot{M}\propto
r^{0.4}$, respectively. This result suggests that understanding both turbulent
and laminar stresses is key to understanding the evolution of systems where
geometrically thick MADs can occur, such as the hard state of X-ray binaries,
low-luminosity active galactic nuclei, some tidal disruption events, and
possibly gamma ray bursts.",2310.11490v1
2023-10-29,"Double-loop hysteresis of multisite dilute Sr(Y$_{1-x}$Dy$_x$)$_2$O$_4$ single crystal Kramers paramagnets: electron-phonon interaction, quantum tunneling and cross-relaxation","Experimental and theoretical studies of the dynamic magnetization in swept
magnetic fields of the orthorhombic SrY$_2$O$_4$ single-crystals doped with the
Dy$^{3+}$ Kramers ions (0.01 and 0.5 at.%) with natural abundances of even and
odd Dy isotopes are presented. Impurity ions substitute for Y$^{3+}$ ions at
two nonequivalent crystallographic sites with the same local $C_s$ symmetry but
strongly different crystal fields. Well pronounced double-loop hysteresis is
observed at temperatures 2, 4, 5 and 6 K for sweeping rates of 5 and 1 mT/s.
The microscopic model of spectral, magnetic and kinetic properties of Dy$^{3+}$
ions is developed based on the results of EPR, site selective optical spectra
and magnetic relaxation measurements. The derived approach to the dynamic
magnetization in the sweeping field based on the numerical solution of
generalized master equations with time-dependent transition probabilities
induced by the electron-phonon interaction, quantum tunneling and
cross-relaxation allowed us to reproduce successfully the evolution of the
hysteresis loop shape with temperature, sweeping rate and concentration of
paramagnetic ions.",2310.18947v1
2023-12-14,In situ Remnants of Solar Surface Structures from Jensen-Shannon Scalogram,"The heliosphere is permeated with highly structured solar wind originating
from the sun. One of the primary science objectives of Parker Solar Probe (PSP)
is to determine the structures and dynamics of the plasma and magnetic fields
at the sources of the solar wind. However, establishing the connection between
in situ measurements and structures and dynamics in the solar atmosphere is
challenging: most of the magnetic footpoint mapping techniques have significant
uncertainties in the source localization of a plasma parcel observed in situ,
and the PSP plasma measurements suffer from a limited field of view. Therefore
it lacks a universal tool to self-contextualize the in situ measurements. Here
we develop a novel time series visualization method named Jensen-Shannon
Scalogram. Utilizing this method, by analyzing the magnetic magnitude data from
both PSP and Ulysses, we successfully identify in situ remnants of solar
atmospheric and magnetic structures spanning more than seven orders of
magnitude, from years to seconds, including polar and mid-latitude coronal
holes, as well as structures compatible with super-granulation, ``jetlets'' and
very small scale flaring activity. Furthermore, computer simulations of
Alfv\'enic turbulence support key features of the observed magnetic magnitude
distribution. Building upon these discoveries, the Jensen-Shannon Scalogram
therefore not only enables us to reveal the fractal fine structures in the
solar wind time series from both PSP and decades-old data archive, but will
also serve as a general-purpose data visualization method applicable to all
time series.",2312.08669v1
2002-12-20,Disk Accretion Flow Driven by Large-Scale Magnetic Fields: Solutions with Constant Specific Energy,"(Abridged) We study the dynamical evolution of a stationary, axisymmetric,
and perfectly conducting cold accretion disk containing a large-scale magnetic
field around a Kerr black hole, trying to understand the relation between
accretion and the transportation of angular momentum and energy. We solve the
radial momentum equation for solutions corresponding to an accretion flow that
starts from a subsonic state at infinity, smoothly passes the fast critical
point, then supersonically falls into the horizon of the black hole. The
solutions always have the following features: 1) The specific energy of fluid
particles remains constant but the specific angular momentum is effectively
removed by the magnetic field. 2) At large radii, where the disk motion is
dominantly rotational, the energy density of the magnetic field is
equipartitioned with the rotational energy density of the disk. 3) Inside the
fast critical point, where radial motion becomes important, the ratio of the
electromagnetic energy density to the kinetic energy density drops quickly. The
results indicate that: 1) Disk accretion does not necessarily imply energy
dissipation since magnetic fields do not have to transport or dissipate a lot
of energy as they effectively transport angular momentum. 2) When resistivity
is small, the large-scale magnetic field is amplified by the shearing rotation
of the disk until the magnetic energy density is equipartitioned with the
rotational energy density, ending up with a geometrically thick disk. This is
in contrast with the evolution of small-scale magnetic fields where if the
resistivity is nonzero the magnetic energy density is likely to be
equipartitioned with the kinetic energy density associated with local random
motions (e.g., turbulence), making a thin Keplerian disk possible.",0212503v2
2003-08-10,Meissner Effect and Vortex Dynamics in Quark Stars -- A Model for Soft Gamma-Ray Repeaters,"We present a new model for soft gamma-ray repeaters based on a quark star
born with temperatures above the critical value (T_c) for the onset of the
colour-flavor locked superconductivity. The quark star then quickly cools below
T_c, expelling a fraction of the surface magnetic field via the Meissner
effect. We show that if a small fraction (\leq 10%) of the surface magnetic
field (10^{14} - 10^{15} {\rm G}) is expelled, it quickly decays via magnetic
reconnection and heats up the quark star surface to temperatures > 10^9 {\rm
K}. Created (e^{+},e^{-}) pairs annihilate into gamma rays emitted in a giant
burst (the first burst in our model), with a luminosity of \sim 10^{45} {\rm
ergs} {\rm s}^{-1}. Subsequent bursts result from the restructuring of the
surface magnetic field following the formation and relaxation of a vortex
lattice which confines the internal magnetic field. During this phase, energy
is sporadically released as a consequence of magnetic reconnection events in
the entangled surface magnetic field as it evolves into a smooth, more stable,
configuration. The star eventually enters a quiescent phase in which energy is
continuously supplied by vortex annihilation at the surface. As the star spins
down, the outermost vortex lines will be pushed to the surface where they
annihilate and release their confined magnetic field. We show that the
corresponding luminosity is L_v \sim 10^{36} {\rm ergs} {\rm s}^{-1} for a
typical soft gamma-ray repeater spinning with a period of 8 {\rm s} and a
surface magnetic field not exceeding 10^{15} {\rm G}. Our model can be applied
to any situation where a T>T_{\rm c} quark star is generated. We discuss the
connection between anomalous X-ray pulsars and soft gamma-ray repeaters in the
context of our model.",0308166v2
2004-09-15,Observations of magnetic fields in the Milky Way and in nearby galaxies with a Square Kilometre Array,"The role of magnetic fields in the dynamical evolution of galaxies and of the
interstellar medium (ISM) is not well understood, mainly because such fields
are difficult to directly observe. Radio astronomy provides the best tools to
measure magnetic fields: synchrotron radiation traces fields illuminated by
cosmic-ray electrons, while Faraday rotation and Zeeman splitting allow us to
detect fields in all kinds of astronomical plasmas, from lowest to highest
densities. Here we describe how fundamental new advances in studying magnetic
fields, both in our own Milky Way and in other nearby galaxies, can be made
through observations with the proposed Square Kilometre Array. Underpinning
much of what we propose is an all-sky survey of Faraday rotation, in which we
will accumulate tens of millions of rotation measure measurements toward
background radio sources. This will provide a unique database for studying
magnetic fields in individual Galactic supernova remnants and HII regions, for
characterizing the overall magnetic geometry of our Galaxy's disk and halo, and
for understanding the structure and evolution of magnetic fields in galaxies.
Also of considerable interest will be the mapping of diffuse polarized emission
from the Milky Way in many narrow bands over a wide frequency range. This will
allow us to carry out Faraday tomography of the Galaxy, yielding a
high-resolution three-dimensional picture of the magnetic field within a few
kpc of the Sun, and allowing us to understand its coupling to the other
components of the ISM. Finally, direct synchrotron imaging of a large number of
nearby galaxies, combined with Faraday rotation data, will allow us to
determine the magnetic field structure in these sources, and to test both the
dynamo and primordial field theories for field origin and amplification.",0409368v2
2010-03-05,Small magnetic loops connecting the quiet surface and the hot outer atmosphere of the Sun,"Sunspots are the most spectacular manifestation of solar magnetism, yet, 99%
of the solar surface remains 'quiet' at any time of the solar cycle. The quiet
sun is not void of magnetic fields, though; they are organized at smaller
spatial scales and evolve relatively fast, which makes them difficult to
detect. Thus, although extensive quiet Sun magnetism would be a natural driver
to a uniform, steady heating of the outer solar atmosphere, it is not clear
what the physical processes involved would be due to lack of observational
evidence. We report the topology and dynamics of the magnetic field in very
quiet regions of the Sun from spectropolarimetric observations of the Hinode
satellite, showing a continuous injection of magnetic flux with a well
organized topology of Omega-loop from below the solar surface into the upper
layers. At first stages, when the loop travels across the photosphere, it has a
flattened (staple-like) geometry and a mean velocity ascent of $\sim3$ km/s.
When the loop crosses the minimum temperature region, the magnetic fields at
the footpoints become almost vertical and the loop topology ressembles a
potential field. The mean ascent velocity at chromospheric height is $\sim12$
km/s. The energy input rate of these small-scale loops in the lower boundary of
the chromosphere is (at least) of 1.4x10^6-2.2x10^7 erg cm-2 s-1. Our findings
provide empirical evidence for solar magnetism as a multi-scale system, in
which small-scale low-flux magnetism plays a crucial role, at least as
important as active regions, coupling different layers of the solar atmosphere
and being an important ingredient for chromospheric and coronal heating models.",1003.1255v1
2010-07-12,Simple models for dynamic hysteresis loops calculation: Application to hyperthermia optimization,"To optimize the heating properties of magnetic nanoparticles (MNPs) in
magnetic hyperthermia applications, it is necessary to calculate the area of
their hysteresis loops in an alternating magnetic field. The three types of
theories suitable for describing the hysteresis loops of MNPs are presented and
compared to numerical simulations: equilibrium functions, Stoner-Wohlfarth
model based theories (SWMBTs) and linear response theory (LRT). Suitable
formulas to calculate the hysteresis area of major cycles are deduced from
SWMBTs and from numerical simulations; the domain of validity of the analytical
formula is explicitly studied. In the case of minor cycles, the hysteresis area
calculations are based on the LRT. A perfect agreement between LRT and
numerical simulations of hysteresis loops is obtained. The domain of validity
of the LRT is explicitly studied. Formulas to calculate the hysteresis area at
low field valid for any anisotropy of the MNP are proposed. Numerical
simulations of the magnetic field dependence of the area show it follows
power-laws with a large range of exponents. Then, analytical expressions
derived from LRT and SWMBTs are used for a theoretical study of magnetic
hyperthermia. It is shown that LRT is only pertinent for MNPs with strong
anisotropy and that SWMBTs should be used for weak anisotropy MNPs. The optimum
volume of MNPs for magnetic hyperthermia as function of material and
experimental parameters is derived. The maximum specific absorption rate (SAR)
achievable is calculated versus the MNP anisotropy. It is shown that an optimum
anisotropy increases the SAR and reduces the detrimental effects of size
distribution. The optimum anisotropy is simple to calculate and depends on the
magnetic field used in the hyperthermia experiments and on the MNP
magnetization only. The theoretical optimum parameters are compared to the one
of several magnetic materials.",1007.2009v2
2011-08-03,Dynamical evolution of a magnetic cloud from the Sun to 5.4 AU,"Magnetic Clouds (MCs) are a particular subset of Interplanetary Coronal Mass
Ejections (ICMEs), forming large scale magnetic flux ropes. In this work we
analyze the evolution of a particular MC (observed on March 1998) using {\it in
situ} observations made by two spacecraft approximately aligned with the Sun,
the first one at 1 AU from the Sun and the second one at 5.4 AU. We study the
MC expansion, its consequent decrease of magnetic field intensity and mass
density, and the possible evolution of the so-called global ideal-MHD
nvariants. We describe the magnetic configuration of the MC at both spacecraft
using different models and compute relevant global quantities (magnetic fluxes,
helicity and energy) at both helio-distances. We also track back this structure
to the Sun, in order to find out its solar source. We find that the flux rope
is significantly distorted at 5.4 AU. However, we are able to analyze the data
before the flux rope center is over-passed and compare it with observations at
1 AU. From the observed decay of magnetic field and mass density, we quantify
how anisotropic is the expansion, and the consequent deformation of the flux
rope in favor of a cross section with an aspect ratio at 5.4 AU of $\approx
1.6$ (larger in the direction perpendicular to the radial direction from the
Sun). We quantify the ideal-MHD invariants and magnetic energy at both
locations, and find that invariants are almost conserved, while the magnetic
energy decays as expected with the expansion rate found. The use of MHD
invariants to link structures at the Sun and the interplanetary medium is
supported by the results of this multispacecraft study. We also conclude that
the local dimensionless expansion rate, that is computed from the velocity
profile observed by a single spacecraft, is very accurate for predicting the
evolution of flux ropes in the solar wind.",1108.0951v1
2011-08-30,Morphology and magnetism of multifunctional nanostructured $γ$-Fe$_2$O$_3$ films: Simulation and experiments,"This paper introduces a new approach for simulating magnetic properties of
nanocomposites comprising magnetic particles embedded in a non-magnetic matrix,
taking into account the 3D structure of the system in which particles'
positions correctly mimic real samples. The proposed approach develops a
multistage simulation procedure in which the size and distribution of particles
within the hosting matrix is firstly attained by means of the Cell Dynamic
System (CDS) model. The 3D structure provided by the CDS step is further
employed in a Monte Carlo (MC) simulation of zero-field-cooled/field-cooled
(ZFC/FC) and magnetic hysteresis loops ($M \times H$ curves) for the system.
Simulations are aimed to draw a realistic picture of the as-produced ultra-thin
films comprising maghemite nanoparticles dispersed in polyaniline. Comparison
(ZFC/FC and $M \times H$ curves) between experiments and simulations regarding
the maximum of the ZFC curve ($T_{\scriptsize MAX}$), remanence ($M_R/M_s$) and
coercivity ($H_C$) revealed the great accuracy of the multistage approach
proposed here while providing information about the system's morphology and
magnetic properties. For a typical sample the value we found experimentally for
$T_{\scriptsize MAX}$ (54 K) was very close to the value provided by the
simulation (53 K). For the parameters depending on the nanoparticle clustering
the experimental values were consistently lower ($M_R/M_s$ = 0.32 and $H_C$ =
210 Oe) than the values we found in the simulation ($M_R/M_s$ = 0.53 and $H_C$
= 274 Oe). Indeed, the approach introduced here is very promising for the
design of real magnetic nanocomposite samples with optimized features.",1108.6099v2
2012-01-12,A Magnetic Calibration of Photospheric Doppler Velocities,"The zero point of measured photospheric Doppler shifts is uncertain for at
least two reasons: instrumental variations (from, e.g., thermal drifts), and
the convective blueshift, a known correlation between intensity and upflows.
Calibrated Doppler velocities would be useful for (i) improving estimates of
the Poynting flux of magnetic energy across the photosphere, and (ii)
constraining processes underlying flux cancellation, the mutual apparent loss
of magnetic flux in closely spaced, opposite-polarity magnetogram features. We
present a method to absolutely calibrate line-of-sight (LOS) velocities in
solar active regions (ARs) near disk center using three successive vector
magnetograms and one Dopplergram coincident with the central magnetogram. It
exploits the fact that Doppler shifts measured along polarity inversion lines
(PILs) of the LOS magnetic field determine one component of the velocity
perpendicular to the magnetic field, and optimizes consistency between changes
in LOS flux near PILs and the transport of transverse magnetic flux by LOS
velocities, assuming ideal electric fields govern the magnetic evolution. We
apply our method to vector magnetograms of AR 11158, observed by the
Helioseismic and Magnetic Imager (HMI) aboard the Solar Dynamics Observatory,
and find clear evidence of offsets in the Doppler zero point, in the range of
50 -- 550 m s$^{-1}$. In addition, we note that a simpler calibration can be
determined from an LOS magnetogram and Dopplergram pair from the median Doppler
velocity among all near-disk-center PIL pixels. We briefly discuss shortcomings
in our initial implementation, and suggest ways to address these. In addition,
as a step in our data reduction, we discuss use of temporal continuity in the
transverse magnetic field direction to correct apparently spurious fluctuations
in resolution of the 180$^\circ$ ambiguity.",1201.2451v4
2014-07-15,Magnetic Fields and Massive Star Formation,"Massive stars ($M > 8$ \msun) typically form in parsec-scale molecular clumps
that collapse and fragment, leading to the birth of a cluster of stellar
objects. We investigate the role of magnetic fields in this process through
dust polarization at 870 $\mu$m obtained with the Submillimeter Array (SMA).
The SMA observations reveal polarization at scales of $\lsim$ 0.1 pc. The
polarization pattern in these objects ranges from ordered hour-glass
configurations to more chaotic distributions. By comparing the SMA data with
the single dish data at parsec scales, we found that magnetic fields at dense
core scales are either aligned within $40^\circ$ of or perpendicular to the
parsec-scale magnetic fields. This finding indicates that magnetic fields play
an important role during the collapse and fragmentation of massive molecular
clumps and the formation of dense cores. We further compare magnetic fields in
dense cores with the major axis of molecular outflows. Despite a limited number
of outflows, we found that the outflow axis appears to be randomly oriented
with respect to the magnetic field in the core. This result suggests that at
the scale of accretion disks ($\lsim 10^3$ AU), angular momentum and dynamic
interactions possibly due to close binary or multiple systems dominate over
magnetic fields. With this unprecedentedly large sample massive clumps, we
argue on a statistical basis that magnetic fields play an important role during
the formation of dense cores at spatial scale of 0.01 - 0.1 pc in the context
of massive star and cluster star formation.",1407.3984v1
2015-01-28,Spin and Magnetism of White Dwarfs,"The magnetism and rotation of white dwarf (WD) stars are investigated in
relation to a hydromagnetic dynamo operating in the progenitor during shell
burning phases. The downward pumping of angular momentum in the convective
envelope, in combination with the absorption of a planet or tidal spin-up from
a binary companion, can trigger strong dynamo action near the core-envelope
boundary. Several arguments point to the outer core as the source for a
magnetic field in the WD remnant: the outer third of a $\sim 0.55\,M_\odot$ WD
is processed during the shell burning phase(s) of the progenitor; the escape of
magnetic helicity through the envelope mediates the growth of (compensating)
helicity in the core, as is needed to maintain a stable magnetic field in the
remnant; and the intense radiation flux at the core boundary facilitates
magnetic buoyancy within a relatively thick tachocline layer. The helicity flux
into the growing core is driven by a dynamical imbalance with a
latitude-dependent rotational stress. The magnetic field deposited in an
isolated massive WD is concentrated in an outer shell of mass $\lesssim
0.1\,M_\odot$ and can reach $\sim 10\,$MG. A buried toroidal field experiences
moderate ohmic decay above an age $\sim 0.3$ Gyr, which may lead to growth or
decay of the external magnetic field. The final WD spin period is related to a
critical spin rate below which magnetic activity shuts off, and core and
envelope decouple; it generally sits in the range of hours to days. WD periods
ranging up to a year are possible if the envelope re-expands following a late
thermal pulse.",1501.07197v3
2015-02-20,Magnetic properties of (Fe$_{1-x}$Co$_x$)$_2$B alloys and the effect of doping by 5$d$ elements,"We have explored, computationally and experimentally, the magnetic properties
of \fecob{} alloys. Calculations provide a good agreement with experiment in
terms of the saturation magnetization and the magnetocrystalline anisotropy
energy with some difficulty in describing Co$_2$B, for which it is found that
both full potential effects and electron correlations treated within dynamical
mean field theory are of importance for a correct description. The material
exhibits a uniaxial magnetic anisotropy for a range of cobalt concentrations
between $x=0.1$ and $x=0.5$. A simple model for the temperature dependence of
magnetic anisotropy suggests that the complicated non-monotonous temperature
behaviour is mainly due to variations in the band structure as the exchange
splitting is reduced by temperature. Using density functional theory based
calculations we have explored the effect of substitutional doping the
transition metal sublattice by the whole range of 5$d$ transition metals and
found that doping by Re or W elements should significantly enhance the
magnetocrystalline anisotropy energy. Experimentally, W doping did not succeed
in enhancing the magnetic anisotropy due to formation of other phases. On the
other hand, doping by Ir and Re was successful and resulted in magnetic
anisotropies that are in agreement with theoretical predictions. In particular,
doping by 2.5~at.\% of Re on the Fe/Co site shows a magnetocrystalline
anisotropy energy which is increased by 50\% compared to its parent
(Fe$_{0.7}$Co$_{0.3}$)$_2$B compound, making this system interesting, for
example, in the context of permanent magnet replacement materials or in other
areas where a large magnetic anisotropy is of importance.",1502.05916v3
2015-04-20,The Helioseismic and Magnetic Imager (HMI) Vector Magnetic Field Pipeline: Magnetohydrodynamics Simulation Module for the Global Solar Corona,"Time-dependent three-dimensional magnetohydrodynamics (MHD) simulation
modules are implemented at the Joint Science Operation Center (JSOC) of Solar
Dynamics Observatory (SDO). The modules regularly produce three-dimensional
data of the time-relaxed minimum-energy state of the solar corona using global
solar-surface magnetic-field maps created from Helioseismic Magnetic Imager
(HMI) full-disk magnetogram data. With the assumption of polytropic gas with
specific heat ratio of 1.05, three types of simulation products are currently
generated: i) simulation data with medium spatial resolution using the
definitive calibrated synoptic map of the magnetic field with a cadence of one
Carrington rotation, ii) data with low spatial resolution using the definitive
version of the synchronic frame format of the magnetic field, with a cadence of
one day, and iii) low-resolution data using near-real-time (NRT) synchronic
format of the magnetic field on daily basis. The MHD data available in the JSOC
database are three-dimensional, covering heliocentric distances from 1.025 to
4.975 solar radii, and contain all eight MHD variables: the plasma density,
temperature and three components of motion velocity, and three components of
the magnetic field. This article describes details of the MHD simulations as
well as the production of the input magnetic-field maps, and details of the
products available at the JSOC database interface. In order to assess the
merits and limits of the model, we show the simulated data in early 2011 and
compare with the actual coronal features observed by the Atmospheric Imaging
Assembly (AIA) and the near-Earth in-situ data.",1504.05217v1
2015-08-05,North-South asymmetry in the magnetic deflection of polar coronal hole jets,"Measurements of the magnetic field in the interplanetary medium, of the
sunspots area, and of the heliospheric current sheet position, reveal a
possible North-South asymmetry in the magnetic field of the Sun. We study the
North-South asymmetry as inferred from measurements of the deflection of polar
coronal hole jets when they propagate throughout the corona. Since the corona
is an environment where the magnetic pressure is greater than the kinetic
pressure, we can assume that magnetic field controls the dynamics of plasma. On
average, jets during their propagation follow the magnetic field lines,
highlighting its local direction. The average jet deflection is studied both in
the plane perpendicular to the line of sight, and, for a reduced number of
jets, in three dimensional space. The observed jet deflection is studied in
terms of an axisymmetric magnetic field model comprising dipole We measured the
position angles at 1 rs and at 2 rs of the 79 jets from the catalogue of
Nistico et al 2009., based on the STEREO ultraviolet and white-light
coronagraph observations during the solar minimum period March 2007-April 2008.
We found that the propagation is not radial, in agreement with the deflection
due to magnetic field lines. Moreover, the amount of the deflection is
different between jets over the north and those from the south pole. Comparison
of jet deflections and field line tracing shows that a ratio g2/g1 ~ -0.5 for
the quadrupole and a ratio g3/g1 ~ 1.6-2.0 for the esapole can describe the
field. The presence of a non-negligible quadrupole moment. We find that the
magnetic deflection of jets is larger in the North than in the South of the
order of 25-40%, with an asymmetry which is consistent with a southward
deflection of the heliospheric current sheet of the order of 10 deg, consistent
with that inferred from other, independent, datasets and instruments.",1508.01072v1
2016-08-05,Upper Chromospheric Magnetic Field of a Sunspot Penumbra: Observations of Fine Structure,"The fine-structure of magnetic field of a sunspot penumbra in the upper
chromosphere is to be explored and compared to that in the photosphere. High
spatial resolution spectropolarimetric observations were recorded with the
1.5-meter GREGOR telescope using the GREGOR Infrared Spectrograph (GRIS). The
observed spectral domain includes the upper chromospheric He I triplet at
1083.0 nm and the photospheric Si I 1082.7 nm and Ca I 1083.3 nm spectral
lines. The upper chromospheric magnetic field is obtained by inverting the He I
triplet assuming a Milne-Eddington type model atmosphere. A height dependent
inversion was applied to the Si I 1082.7 nm and Ca I 1083.3 nm lines to obtain
the photospheric magnetic field. We find that the inclination of the magnetic
field shows variations in the azimuthal direction both in the photosphere, but
also in the upper chromosphere. The chromospheric variations remarkably well
coincide with the variations in the inclination of the photospheric field and
resemble the well-known spine and inter-spine structure in the photospheric
layers of penumbrae. The typical peak-to-peak variations in the inclination of
the magnetic field in the upper chromosphere is found to be 10-15 degree, i.e.,
roughly half the variation in the photosphere. In contrast, the magnetic field
strength of the observed penumbra does not show variations on small spatial
scales in the upper chromosphere. Thanks to the high spatial resolution
observations possible with the GREGOR telescope at 1.08 microns, we find that
the prominent small-scale fluctuations in the magnetic field inclination, which
are a salient part of the property of sunspot penumbral photospheres, also
persist in the chromosphere, although at somewhat reduced amplitudes. Such a
complex magnetic configuration may facilitate penumbral chromospheric dynamic
phenomena, such as penumbral micro-jets or transient bright dots.",1608.01988v1
2016-09-27,Report on imaging of individual skyrmions of MML systems made by different techniques,"Deliverable 1.1 is a report on imaging of individual skyrmions of MML systems
made by different techniques. This deliverable falls within work package 1
(WP1:Static equilibrium properties of interface induced skyrmions in magnetic
films and multilayers) of the MAGicSky research proposal whose objectives are:
the growth of magnetic multilayer systems with large interfacial chiral
interaction, the determination of the amplitude of the Dzyaloshinskii--Moriya
interactions (DMi), the detailed structural and magnetic characterization of
interfaces, the magnetic imaging of individual skyrmions and the thermal
stability and interaction of skyrmions with intrinsic and/or artificial
defects. It should be emphasized that imaging magnetic skyrmionic structures
forms the basis for all the planned subsequent material optimization,
electrical transport experiments and devices preparation in the MAGicSky
research program. In particular, imaging is paramount as a technique since it
enables us to unequivocally demonstrate chiral magnetic skyrmions by directly
imaging i) the effects of engineering multilayer samples with large DMis and
ii) the skyrmionic structure and properties. In order to get complementary
physical insight on the properties of magnetic skyrmions and to corroborate our
research results, we utilise a plethora of key state-of-the-art advanced
magnetic imaging techniques, namely: i) Spin-polarised Scanning Tunnelling
Microscopy (SP-STM), Scanning Transmission X-ray Microscopy (STXM) Lorentz
Transmission Electron Microscopy (Lorentz TEM) as well as, Magnetic Force
Microscopy (MFM). These imaging experiments can therefore stand as the base for
all the other objectives in WP1 as well as WP2 that will study Dynamic
properties: nucleation, propagation and detection of skyrmions. Imaging has
been aimed to test and get feedback from the theoretical and numerical
investigations of skyrmion systems of WP3. Finally, for WP4: Towards skyrmion
based devices for information storage and processing, some of the techniques
detailed here in D1.1 will enable us to image and test the first devices and
their functionality. In short, the MAGicSky consortium research results in D1.1
constitute a major milestone that acts as a stepping-stone to every other
experimental effort and we have achieved it. The results relevant to the D1.1
deliverable were presented in 5 publications and several invited talks at
international workshop and conferences:",1609.08415v1
2016-11-15,Three-dimensional structure of a sunspot light bridge,"Active regions are the most prominent manifestations of solar magnetic
fields; their generation and dissipation are fundamental problems in solar
physics. Light bridges are commonly present during sunspot decay, but a
comprehensive picture of their role in the removal of photospheric magnetic
field is still missing. We study the three dimensional configuration of a
sunspot and in particular its light bridge during one of the last stages of its
decay. We present the magnetic and thermodynamical stratification inferred from
full Stokes inversions of the photospheric Si I 10827 \AA\ and Ca I 10839 \AA\
lines obtained with the GREGOR Infrared Spectrograph of the GREGOR telescope at
Observatorio del Teide, Tenerife, Spain. The analysis is complemented by a
study of continuum images covering the disk passage of the active region, which
are provided by the Helioseismic and Magnetic Imager on board the Solar
Dynamics Observatory. The sunspot shows a light bridge with penumbral continuum
intensity that separates the central umbra from a smaller umbra. We find that
in this region the magnetic field lines form a canopy with lower magnetic field
strength in the inner part. The photospheric light bridge is dominated by gas
pressure (high-$\beta$), as opposed to the surrounding umbra where the magnetic
pressure is higher. A convective flow is observed in the light bridge. This
flow is able to bend the magnetic field lines and to produce field reversals.
The field lines close above the light bridge and become as vertical and strong
as in the surrounding umbra. We conclude that it develops because of two highly
magnetized regions which come closer during the sunspot evolution.",1611.04803v1
2017-03-02,On flare-CME characteristics from Sun to Earth combining remote-sensing image data with in-situ measurements supported by modeling,"We analyze the well observed flare-CME event from October 1, 2011
(SOL2011-10-01T09:18) covering the complete chain of action - from Sun to Earth
- for a better understanding of the dynamic evolution of the CME and its
embedded magnetic field. We study the solar surface and atmosphere associated
with the flare-CME from SDO and ground-based instruments, and also track the
CME signature off-limb from combined EUV and white-light data with STEREO. By
applying 3D reconstruction techniques (GCS, total mass) to stereoscopic
STEREO-SoHO coronagraph data, we track the temporal and spatial evolution of
the CME in interplanetary space and derive its geometry and 3D-mass. We combine
the GCS and Lundquist model results to derive the axial flux and helicity of
the MC from in-situ measurements (Wind). This is compared to nonlinear
force-free (NLFF) model results as well as to the reconnected magnetic flux
derived from the flare ribbons (flare reconnection flux) and the magnetic flux
encompassed by the associated dimming (dimming flux). We find that magnetic
reconnection processes were already ongoing before the start of the impulsive
flare phase, adding magnetic flux to the flux rope before its final eruption.
The dimming flux increases by more than 25% after the end of the flare,
indicating that magnetic flux is still added to the flux rope after eruption.
Hence, the derived flare reconnection flux is most probably a lower limit for
estimating the magnetic flux within the flux rope. We find that the magnetic
helicity and axial magnetic flux are reduced in interplanetary space by ~50%
and 75%, respectively, possibly indicating to an erosion process. A mass
increase of 10% for the CME is observed over the distance range from ~4-20 Rs.
The temporal evolution of the CME associated core dimming regions supports the
scenario that fast outflows might supply additional mass to the rear part of
the CME.",1703.00694v1
2017-07-25,The Non-Linear Growth of the Magnetic Rayleigh-Taylor Instability,"This work examines the effect of the embedded magnetic field strength on the
non-linear development of the magnetic Rayleigh-Taylor Instability (RTI) (with
a field-aligned interface) in an ideal gas close to the incompressible limit in
three dimensions. Numerical experiments are conducted in a domain sufficiently
large so as to allow the predicted critical modes to develop in a physically
realistic manner. The ratio between gravity, which drives the instability in
this case (as well as in several of the corresponding observations), and
magnetic field strength is taken up to a ratio which accurately reflects that
of observed astrophysical plasma, in order to allow comparison between the
results of the simulations and the observational data which served as
inspiration for this work. This study finds reduced non-linear growth of the
rising bubbles of the RTI for stronger magnetic fields, and that this is
directly due to the change in magnetic field strength, rather than the indirect
effect of altering characteristic length scales with respect to domain size. By
examining the growth of the falling spikes, the growth rate appears to be
enhanced for the strongest magnetic field strengths, suggesting that rather
than affecting the development of the system as a whole, increased magnetic
field strengths in fact introduce an asymmetry to the system. Further
investigation of this effect also revealed that the greater this asymmetry, the
less efficiently the gravitational energy is released. By better understanding
the under-studied regime of such a major phenomenon in astrophysics, deeper
explanations for observations may be sought, and this work illustrates that the
strength of magnetic fields in astrophysical plasmas influences observed RTI in
subtle and complex ways.",1707.07987v1
2017-08-15,Comparing UV/EUV line parameters and magnetic field in a quiescent prominence with tornadoes,"Context. Understanding the close relationship between the plasma and the
magnetic field is important to describe and explain the observed complex
dynamics of solar prominences. Aims. We determine if a close relationship
between plasma and magnetic field parameters measured in a well-observed solar
prominence with high spatial resolution can be found. Methods. We select a
prominence observed on 15 July 2014 from space (IRIS, Hinode, SDO) and from the
ground (THEMIS). We perform a robust co-alignment of the data sets using a 2D
cross-correlation technique. We derive the magnetic field parameters from
spectropolarimetric measurements of the He I D3 line taken by THEMIS. Line
ratios and line-of-sight velocities from the Mg II h and k lines observed by
IRIS are compared with magnetic field strength, inclination and azimuth.
Electron densities are calculated using Hinode/EIS Fe XII line ratios and also
compared with THEMIS and IRIS data. Results. We find Mg II k/h ratios of around
1.4 everywhere, similar to values found previously in prominences. We also find
that the magnetic field is strongest (around 30 G) and predominantly horizontal
in the tornado-like legs of the prominence. The k3 Doppler shift is found to be
between +/- 10 km/s everywhere. Electron densities at a temperature of 1.5e6 K
are found to be around 1e9 /cm3. No significant correlations are found between
the magnetic field parameters, and any of the other plasma parameters inferred
from EUV spectroscopy, which may be explained by the large differences in the
temperatures of the lines used in this study. Conclusions. This is the first
time that a detailed statistical study of plasma and magnetic field parameters
has been carried out at high spatial resolution in a prominence. Our results
provide important constraints on future models of the plasma and magnetic field
in these structures.",1708.04606v1
2018-05-29,Emergence of Weyl metals driven by doped magnetic impurities in spin-orbit coupled semiconductors,"Constructing an effective field theory in terms of doped magnetic impurities
(described by an O(3) vector model with a random mass term), itinerant
electrons of spin-orbit coupled semiconductors (given by a Dirac theory with a
relatively large mass term), and effective interactions between doped magnetic
ions and itinerant electrons (assumed by an effective Zeeman coupling term), we
perform the perturbative renormalization group analysis in the one-loop level
based on the dimensional regularization technique. As a result, we find that
the mass renormalization in dynamics of itinerant electrons acquires negative
feedback effects due to quantum fluctuations involved with the Zeeman coupling
term, in contrast with that of the conventional problem of quantum
electrodynamics, where such interaction effects enhance the fermion mass more
rapidly. Recalling that the applied magnetic field decreases the band gap in
the presence of spin-orbit coupling, this renormalization group analysis shows
that the external magnetic field overcomes the renormalized band gap, allowed
by doped magnetic impurities even without ferromagnetic ordering. In other
words, the Weyl metal physics can be controlled by doping magnetic impurities
into spin-orbit coupled semiconductors, even if the external magnetic field
alone cannot realize the Weyl metal phase due to relatively large band gaps of
semiconductors. Furthermore, we emphasize that quasiparticles do not exist in
this emergent disordered Weyl metal phase due to correlations with strong
magnetic fluctuations. This non-Fermi liquid type Weyl metal state may be
regarded to be a novel metallic phase in the respect that a topologically
nontrivial band structure appears in the vicinity of quantum criticality.",1805.11209v3
2018-09-27,Electrodynamics of double neutron star mergers,"We consider electromagnetic interaction and precursor emission of merging
neutron stars. Orbital motion of the magnetized neutron stars may revive pair
production within the common magnetosphere years before the merger, igniting
pulsar-like magnetospheric dynamics. We identify two basic scenarios: (i) only
one star is magnetized (1M-DNS scenario) and (ii) both stars are magnetized
(2M-DNS scenario). Inductively created electric fields can have component along
the total magnetic field (gaps) and/or the electric field may exceed the value
of the local magnetic field. The key to the detection is orbital modulation of
the emission. If only one star is magnetized (1M-DNS scenario) the emission is
likely to be produced along the direction of the magnetic field at the location
of the secondary; then, if the magnetic axis is misaligned with the orbital
spin, this direction is modulated on the orbital period. For the 2M-DNS
scenario, the structure of the common magnetosphere of the non-rotating neutron
stars is complicated, with gaps, but no $E>B$ regions; there is strong orbital
variations for the case of misaligned magnetic moments. For the same parameters
of neutron stars the 2M-DNS scenario has intrinsically higher potential than
the 1M-DNS one. The overall powers are not very high, $\leq 10^{45} $ erg
s$^{-1}$; the best chance to detect electromagnetic precursors to the merging
neutron stars is if the interaction of their magnetospheres leads to the
production of pulsar-like coherent radio emission modulated at the orbital
period, with luminosity of up to $\sim 1$ Jankys at the time the merger.",1809.10478v4
2018-10-31,Magnetohydrodynamic Simulation of the X9.3 Flare on 2017 September 6: Evolving Magnetic Topology,"Three-dimensional magnetic topology is crucial to understanding the explosive
release of magnetic energy in the corona during solar flares. Much attention
has been given to the pre-flare magnetic topology to identify candidate sites
of magnetic reconnection, yet it is unclear how the magnetic reconnection and
its attendant topological changes shape the eruptive structure and how the
topology evolves during the eruption. Here we employed a realistic,
data-constrained magnetohydrodynamic simulation to study the evolving magnetic
topology for an X9.3 eruptive flare that occurred on 2017 September 6. The
simulation successfully reproduces the eruptive features and processes in
unprecedented detail. The numerical results reveal that the pre-flare corona
contains multiple twisted flux systems with different connections, and during
the eruption, these twisted fluxes form a coherent flux rope through
tether-cutting-like magnetic reconnection below the rope. Topological analysis
shows that the rising flux rope is wrapped by a quasi-separatrix layer, which
intersects itself below the rope, forming a topological structure known as
hyperbolic flux tube, where a current sheet develops, triggering the
reconnection. By mapping footpoints of the newly-reconnected field lines, we
are able to reproduce both the spatial location and, for the first time, the
temporal separation of the observed flare ribbons, as well as the dynamic
boundary of the flux rope's feet. Futhermore, the temporal profile of the total
reconnection flux is comparable to the soft X-ray light curve. Such a
sophisticated characterization of the evolving magnetic topology provides
important insight into the eventual understanding and forecast of solar
eruptions.",1810.13095v1
2018-11-08,Solar Sources of Interplanetary Magnetic Clouds Leading to Helicity Prediction,"This study identifies the solar origins of magnetic clouds that are observed
at 1 AU and predicts the helical handedness of these clouds from the solar
surface magnetic fields. We started with the magnetic clouds listed by the
Magnetic Field Investigation (MFI) team supporting NASA's WIND spacecraft in
what is known as the MFI table and worked backwards in time to identify solar
events that produced these clouds. Our methods utilize magnetograms from the
Helioseismic and Magnetic Imager (HMI) instrument on the Solar Dynamics
Observatory (SDO) spacecraft so that we could only analyze MFI entries after
the beginning of 2011. This start date and the end date of the MFI table gave
us 37 cases to study. Of these we were able to associate only eight surface
events with clouds detected by WIND at 1 AU. We developed a simple algorithm
for predicting the cloud helicity which gave the correct handedness in all
eight cases. The algorithm is based on the conceptual model that an ejected
flux tube has two magnetic origination points at the positions of the strongest
radial magnetic field regions of opposite polarity near the places where the
ejected arches end at the solar surface. We were unable to find events for the
remaining 29 cases: lack of a halo or partial halo CME in an appropriate time
window, lack of magnetic and/or filament activity in the proper part of the
solar disk, or the event was too far from disk center. The occurrence of a
flare was not a requirement for making the identification but in fact flares,
often weak, did occur for seven of the eight cases.",1811.03560v1
2019-01-03,Turbulent viscosity and effective magnetic Prandtl number from simulations of isotropically forced turbulence,"(abridged) Context: Turbulent diffusion of large-scale flows and magnetic
fields play major roles in many astrophysical systems. Aims: Our goal is to
compute turbulent viscosity and magnetic diffusivity, relevant for diffusing
large-scale flows and magnetic fields, respectively, and their ratio, the
turbulent magnetic Prandtl number, ${\rm Pm}_{\rm t}$, for isotropically forced
homogeneous turbulence. Methods: We use simulations of forced turbulence in
fully periodic cubes composed of isothermal gas with an imposed large-scale
sinusoidal shear flow. Turbulent viscosity is computed either from the
resulting Reynolds stress or from the decay rate of the large-scale flow.
Turbulent magnetic diffusivity is computed using the test-field method. The
scale dependence of the coefficients is studied by varying the wavenumber of
the imposed sinusoidal shear and test fields. Results: We find that turbulent
viscosity and magnetic diffusivity are in general of the same order of
magnitude. Furthermore, the turbulent viscosity depends on the fluid Reynolds
number (${\rm Re}$) and scale separation ratio of turbulence. The scale
dependence of the turbulent viscosity is found to be well approximated by a
Lorentzian. The results for the turbulent transport coefficients appear to
converge at sufficiently high values of ${\rm Re}$ and the scale separation
ratio. However, a weak decreasing trend is found even at the largest values of
${\rm Re}$. The turbulent magnetic Prandtl number converges to a value that is
slightly below unity for large ${\rm Re}$ whereas for small ${\rm Re}$, we find
values between 0.5 and 0.6. Conclusions: The turbulent magnetic diffusivity is
in general consistently higher than the turbulent viscosity. The actual value
of ${\rm Pm}_{\rm t}$ found from the simulations ($\approx0.9\ldots0.95$) at
large ${\rm Re}$ and scale separation ratio is higher than any of the analytic
predictions.",1901.00787v2
2019-09-12,Spectral decimation of the magnetic Laplacian on the Sierpinski gasket: Solving the Hofstadter-Sierpinski butterfly,"The magnetic Laplacian (also called the line bundle Laplacian) on a connected
weighted graph is a self-adjoint operator wherein the real-valued adjacency
weights are replaced by unit complex-valued weights $\{\omega_{xy}\}_{xy\in
E}$, satisfying the condition that $\omega_{xy}=\overline{\omega_{yx}}$ for
every directed edge $xy$. When properly interpreted, these complex weights give
rise to magnetic fluxes through cycles in the graph.
  In this paper we establish the spectrum of the magnetic Laplacian, as a set
of real numbers with multiplicities, on the Sierpinski gasket graph ($SG$)
where the magnetic fluxes equal $\alpha$ through the upright triangles, and
$\beta$ through the downright triangles. This is achieved upon showing the
spectral self-similarity of the magnetic Laplacian via a 3-parameter map
$\mathcal{U}$ involving non-rational functions, which takes into account
$\alpha$, $\beta$, and the spectral parameter $\lambda$. In doing so we provide
a quantitative answer to a question of Bellissard [Renormalization Group
Analysis and Quasicrystals (1992)] on the relationship between the dynamical
spectrum and the actual magnetic spectrum.
  Our main theorems lead to two applications. In the case $\alpha=\beta$, we
demonstrate the approximation of the magnetic spectrum by the filled Julia set
of $\mathcal{U}$, the Sierpinski gasket counterpart to Hofstadter's butterfly.
Meanwhile, in the case $\alpha,\beta\in \{0,\frac{1}{2}\}$, we can compute the
determinant of the magnetic Laplacian determinant and the corresponding
asymptotic complexity.",1909.05662v4
2020-01-15,"Emergence of weak pyrochlore phase and signature of field induced spin ice ground state in Dy$_{2-x}$La$_{x}$Zr$_{2}$O$_{7}$; x = 0, 0.15, 0.3","The pyrochlore oxides Dy$_{2}$Ti$_{2}$O$_{7}$ and Ho$_{2}$Ti$_{2}$O$_{7}$ are
well studied spin ice systems and have shown the evidences of magnetic monopole
excitations. Unlike these, Dy$_{2}$Zr$_{2}$O$_{7}$ is reported to crystallize
in a distorted fluorite structure. We present here the magnetic and heat
capacity studies of La substituted Dy$_{2}$Zr$_{2}$O$_{7}$. Our findings
suggest the absence of spin ice state in Dy$_{2}$Zr$_{2}$O$_{7}$ but the
emergence of the magnetic field induced spin freezing near T $\approx$ 10 K in
ac susceptibility measurements which is similar to Dy$_{2}$Ti$_{2}$O$_{7}$. The
magnetic heat capacity of Dy$_{2}$Zr$_{2}$O$_{7}$ shows a shift in the peak
position from 1.2 K in zero field to higher temperatures in the magnetic field,
with the corresponding decrease in the magnetic entropy. The low temperature
magnetic entropy at 5 kOe field is Rln2 - (1/2)Rln(3/2) which is same as for
the spin ice state. Substitution of non-magnetic, isovalent La$^{3+}$ for
Dy$^{3+}$ gradually induces the structural change from highly disordered
fluorite to weakly ordered pyrochlore phase. The La$^{3+}$ substituted
compounds with less distorted pyrochlore phase show the spin freezing at lower
field which strengthens further on the application of magnetic field. Our
results suggest that the spin ice state can be stabilized in
Dy$_{2}$Zr$_{2}$O$_{7}$ either by slowing down of the spin dynamics or by
strengthening the pyrochlore phase by suitable substitution in the system.",2001.05244v2
2020-04-08,"Magnetic, thermal, and electronic-transport properties of EuMg2Bi2 single crystals","The trigonal compound EuMg2Bi2 has recently been discussed in terms of its
topological band properties. These are intertwined with its magnetic
properties. Here detailed studies of the magnetic, thermal, and electronic
transport properties of EuMg2Bi2 single crystals are presented. The Eu{+2}
spins-7/2 in EuMg2Bi2 exhibit an antiferromagnetic (AFM) transition at a
temperature TN = 6.7 K, as previously reported. By analyzing the anisotropic
magnetic susceptibility chi data below TN in terms of molecular-field theory
(MFT), the AFM structure is inferred to be a c-axis helix, where the ordered
moments in the hexagonal ab-plane layers are aligned ferromagnetically in the
ab plane with a turn angle between the moments in adjacent moment planes along
the c axis of about 120 deg. The magnetic heat capacity exhibits a lambda
anomaly at TN with evidence of dynamic short-range magnetic fluctuations both
above and below TN. The high-T limit of the magnetic entropy is close to the
theoretical value for spins-7/2. The in-plane electrical resistivity rho(T)
data indicate metallic character with a mild and disorder-sensitive upturn
below Tmin = 23 K. An anomalous rapid drop in rho(T) on cooling below TN as
found in zero field is replaced by a two-step decrease in magnetic fields. The
rho(T) measurements also reveal an additional transition below TN in applied
fields of unknown origin that is not observed in the other measurements and may
be associated with an incommensurate to commensurate AFM transition. The
dependence of TN on the c-axis magnetic field Hperp was derived from the
field-dependent chi(T), Cp(T), and rho(T) measurements. This TN(Hperp) was
found to be consistent with the prediction of MFT for a c-axis helix with S =
7/2 and was used to generate a phase diagram in the Hperp-T plane.",2004.03753v2
2020-05-21,ALMA reveals the coherence of the magnetic field geometry in OH 231.8+4.2,"In a continuing effort to investigate the role of magnetic fields in evolved
low and intermediate mass stars (principally regarding the shaping of their
envelopes), we present new ALMA high resolution polarization data obtained for
the nebula OH 231.8+4.2. We found that the polarized emission likely arises
from aligned grains in the presence of magnetic fields rather than radiative
alignment and self scattering. The ALMA data show well organized electric field
orientations in most of the nebula and the inferred magnetic field vectors
(rotated by 90 degrees) trace an hourglass morphology centred on the central
system of the nebula. One region in the southern part of OH 231.8+4.2 shows a
less organized distribution probably due to the shocked environment. These
findings, in conjunction with earlier investigations (maser studies and dust
emission analysis at other scales and wavelengths) suggest an overall magnetic
hourglass located inside a toroidal field. We propose the idea that the
magnetic field structure is closely related to the architecture of a magnetic
tower and that the outflows were therefore magnetically launched. While the
current dynamical effect of the fields might be weak in the equatorial plane
principally due to the evolution of the envelope, it would still be affecting
the outflows. In that regard, the measurement of the magnetic field at the
stellar surface, which is still missing, combined with a full MHD treatment are
required to better understand and constrain the events occurring in OH
231.8+4.2.",2005.10398v1
2020-07-09,Magnetic resonance of collective paramagnets with gapped excitations spectrum,"Some magnets due to particular geometry of the exchange bonds do not undergo
transition to the conventional magnetically ordered state despite of the
presence of significant exchange couplings. Instead, a collective paramagnetic
state is formed. The later state can remain stable down to $T=0$ if the ground
state of this magnet turns out to be nonmagnetic singlet separated from the
excited triplet states by an energy gap. Low-temperature spin dynamics of the
collective paramagnets with gapped excitations spectrum (or spin-gap magnets)
can be described in terms of a dilute gas of the triplet excitations. Applied
magnetic field can suppress the energy gap, resulting in the formation of the
gapless spin-liquid state or even leading to the unusual phenomenon of
field-induced antiferromagnetic order. Introduction of defects in the
crystallographic structure of the spin-gap magnet can result either in the
formation of multi-spin paramagnetic center or in the formation of randomly
distributed modified exchange bonds in the crystal. This review includes
results of electron spin resonance (ESR) spectroscopy study of several
representative quantum paramagnets with gapped excitations spectrum:
quasy-two-dimensional magnet \phcc{}, quasy-one-dimensional magnets of
""spin-tube"" type \sul{} and ""spin-ladder"" type \dimpy{}. We will demonstrate
that ESR absorption spectra reveal common features of these systems: ESR
spectroscopy allows to observe and characterize fine structure if the triplet
energy levels, to identify many-particles relaxation processes in the gas of
triplet excitations and to observe collective spin-wave oscillations in the
field induced antiferromagnetically ordered state, as well as to observe some
individual features of the studied systems.",2007.04885v1
2020-08-21,Radiation and Polarization Signatures from Magnetic Reconnection in Relativistic Jets--I. A Systematic Study,"Blazars are relativistic magnetized plasma outflows from supermassive black
holes that point very close to our line of sight. Their emission is nonthermal
dominated and highly variable across the entire electromagnetic spectrum.
Relativistic magnetic reconnection has been proposed as the driver of particle
acceleration during blazar flares. While recent particle-in-cell simulations
have self-consistently studied the evolution of magnetic reconnection and
particle acceleration therein, the resulting radiation signatures have not been
systematically explored. In particular, the polarization signatures, which
directly reflect the characteristic strongly dynamical magnetic field evolution
during reconnection, have not been carefully investigated. In this paper, we
present a systematic study of radiation and polarization signatures arising
from magnetic reconnection in blazars, based on combined PIC and polarized
radiation transfer simulations with various physical parameters. We identify a
harder-when-brighter trend in the spectral evolution. Moreover,
higher-frequency bands tend to flare earlier than lower-frequency bands in the
synchrotron spectral component. Most importantly, polarization signatures
appear more variable with higher frequencies. We find that the temporal
polarization variations strongly depends on the guide field strength.
Specifically, reconnection with significant guide field component leads to very
high polarization degree that contradict to typical blazar observations, while
large polarization angle rotations are unique signatures of magnetic
reconnection between nearly anti-parallel magnetic field lines. These rotations
are at least $90^o$ and can extend to $>180^o$, and they may rotate in both
directions. These results imply that blazars that have shown large polarization
angle rotations intrinsically have more nearly anti-parallel magnetic field
morphology.",2008.09444v1
2020-08-30,Time Series Analysis of Photospheric Magnetic Parameters of Flare-quiet versus Flaring Active Regions: Scaling Properties of Fluctuations,"Time series of photospheric magnetic parameters of solar active regions (ARs)
are used to answer whether scaling properties of fluctuations embedded in such
time series help to distinguish between flare-quiet and flaring ARs. We examine
a total of 118 flare-quiet and 118 flaring AR patches (called HARPs), which
were observed from 2010 to 2016 by the Helioseismic and Magnetic Imager (HMI)
on board the Solar Dynamics Observatory (SDO). Specifically, the scaling
exponent of fluctuations is derived applying the Detrended Fluctuation Analysis
(DFA) method to a dataset of 8-day time series of 18 photospheric magnetic
parameters at 12-min cadence for all HARPs under investigation. We first find a
statistically significant difference in the distribution of the scaling
exponent between the flare-quiet and flaring HARPs, in particular for some
space-averaged, signed parameters associated with magnetic field line twist,
electric current density, and current helicity. The flaring HARPs tend to show
higher values of the scaling exponent compared to those of the flare-quiet
ones, even though there is considerable overlap between their distributions. In
addition, for both the flare-quiet and flaring HARPs the DFA analysis indicates
that (1) time series of most of various magnetic parameters under consideration
are non-stationary, and (2) time series of the total unsigned magnetic flux and
the mean photospheric magnetic free energy density in general present a
non-stationary, persistent property, while the total unsigned flux near
magnetic polarity inversion lines and parameters related to current density
show a non-stationary, anti-persistent trend in their time series.",2008.13085v1
2020-09-15,Experimental validation of a three-dimensional heat transfer model within the scala tympani with application to magnetic cochlear implant surgery,"Magnetic guidance of cochlear implants is a promising technique to reduce the
risk of physical trauma during surgery. In this approach, a magnet attached to
the tip of the implant electrode array is guided within the scala tympani using
a magnetic field. After surgery, the magnet must be detached from the implant
electrode array via localized heating and removed from the scala tympani which
may cause thermal trauma. Objectives: The objective of this work is to
experimentally validate a three-dimensional (3D) heat transfer model of the
scala tympani which will enable accurate predictions of the maximum safe input
power to avoid localized hyperthermia when detaching the magnet from the
implant electrode array. Methods: Experiments are designed using a rigorous
scale analysis and performed by measuring transient temperatures in a
3D-printed scala tympani phantom subjected to a sudden change in its thermal
environment and localized heating via a small heat source. Results: The
measured and predicted temperatures are in good agreement with an error less
than 6$\%$. Conclusions: The validated 3D heat transfer model of the scala
tympani is finally applied to evaluate the maximum safe input power to avoid
localized hyperthermia when detaching the magnet. For the most conservative
case where all boundaries except the insertion opening are adiabatic, the power
required to release the magnet attached to the implant electrode array by 1
mm$^3$ of paraffin is approximately half of the predicted maximum safe input
power. Significance: This work will enable the design of a thermally safe
magnetic cochlear implant surgery procedure.",2009.07199v1
2021-03-03,Long-Timescale Magnetization Ordering Induced by an Adsorbed Chiral Monolayer on Ferromagnets,"When an electron passes through a chiral molecule there is a high probability
for a correlation between the momentum and spin of the charge, thus leading to
spin polarized current. This phenomenon is known as the chiral induced spin
selectivity (CISS) effect. One of the most surprising experimental results
recently demonstrated is that magnetization reversal in a ferromagnet (FM) with
perpendicular anisotropy can be realized solely by chemisorbing a chiral
molecular monolayer without applying any current or external magnetic field.
This result raises the currently open question of whether this effect is due to
the bonding event, held by the ferromagnet, or a long timescale effect
stabilized by exchange interactions. In this work we have performed vectorial
magnetic field measurements of the magnetization reorientation of a
ferromagnetic layer exhibiting perpendicular anisotropy due to CISS using
nitrogen-vacancy centers in diamond, and followed the time dynamics of this
effect. In parallel, we have measured the molecular monolayer tilt angle in
order to find a correlation between the time dependence of the magnetization
re-orientation and the change of the tilt angle of the molecular monolayer. We
have identified that changes in the magnetization direction correspond to
changes of the molecular monolayer tilt angle, providing evidence for a
long-timescale characteristic of the induced magnetization reorientation. This
suggests that the CISS effect has an effect over long-timescales which we
attribute to exchange interactions. These results offer significant insights
into the fundamental processes underlying the CISS effect, contributing to the
implementation of CISS in state-of-the-art applications such as spintronic and
magnetic memory devices.",2103.04772v1
2021-03-12,The Alfvénic nature of chromospheric swirls,"We investigate the evolution and origin of small-scale chromospheric swirls
by analyzing numerical simulations of the quiet solar atmosphere, using the
radiative magnetohydrodynamic code CO$5$BOLD. We are interested in finding
their relation with magnetic field perturbations and in the processes driving
their evolution. For the analysis, the swirling strength criterion and its
evolution equation are applied in order to identify vortical motions and to
study their dynamics. We introduce a new criterion, the magnetic swirling
strength, which allows us to recognize torsional perturbations in the magnetic
field. We find a strong correlation between swirling strength and magnetic
swirling strength, in particular in intense magnetic flux concentrations, which
suggests a tight relation between vortical motions and torsional magnetic field
perturbations. Furthermore, we find that swirls propagate upward with the local
Alfv\'en speed as unidirectional swirls, in the form of pulses, driven by
magnetic tension forces alone. In the photosphere and low chromosphere, the
rotation of the plasma co-occurs with a twist in the upwardly directed magnetic
field that is in the opposite direction of the plasma flow. All together, these
are characteristics of torsional Alfv\'en waves. We also find indications of an
imbalance between the hydrodynamic and magnetohydrodynamic baroclinic effects
being at the origin of the swirls. At the base of the chromosphere, we find a
net upwardly directed Poynting flux, which is mostly associated with large and
complex swirling structures that we interpret as the superposition of various
small-scale vortices. We conclude that the ubiquitous swirling events observed
in simulations are tightly correlated with perturbations of the magnetic field.
At photospheric and chromospheric levels, they form Alfv\'en pulses that
propagate upward and may contribute to chromospheric heating.",2103.07366v1
2021-04-15,MHD Modeling of Solar Coronal Magnetic Evolution Driven by Photospheric Flow,"It is well known that magnetic fields dominate the dynamics in the solar
corona, and new generation of numerical modelling of the evolution of coronal
magnetic fields, as featured with boundary conditions driven directly by
observation data, are being developed. This paper describes a new approach of
data-driven magnetohydrodynamic (MHD) simulation of solar active region (AR)
magnetic field evolution, which is for the first time that a data-driven
full-MHD model utilizes directly the photospheric velocity field from DAVE4VM.
We constructed a well-established MHD equilibrium based on a single vector
magnetogram by employing an MHD-relaxation approach with sufficiently small
kinetic viscosity, and used this MHD equilibrium as the initial conditions for
subsequent data-driven evolution. Then we derived the photospheric surface
flows from a time series of observed magentograms based on the DAVE4VM method.
The surface flows are finally inputted in time sequence to the bottom boundary
of the MHD model to self-consistently update the magnetic field at every time
step by solving directly the magnetic induction equation at the bottom
boundary. We applied this data-driven model to study the magnetic field
evolution of AR 12158 with SDO/HMI vector magnetograms. Our model reproduced a
quasi-static stress of the field lines through mainly the rotational flow of
the AR's leading sunspot, which makes the core field lines to form a coherent S
shape consistent with the sigmoid structure as seen in the SDO/AIA images. The
total magnetic energy obtained in the simulation matches closely the
accumulated magnetic energy as calculated directly from the original vector
magnetogram with the DAVE4VM derived flow field. Such a data-driven model will
be used to study how the coronal field, as driven by the slow photospheric
motions, reaches a unstable state and runs into eruptions.",2104.07229v1
2021-06-05,Naked emergence of an anti-Hale active region I. Overall evolution and magnetic properties,"{In order to understand the emergence of the active region, we investigate
the emerging process and magnetic properties of a naked anti-Hale active region
during the period between August 24 to 25, 2018.}
  {Using the data from Helioseismic and Magnetic Imager on board the Soar
Dynamic Observatory and the New Vacuum Solar Telescope, we calculated different
evolving parameters (such as pole separation, tilt angle) and magnetic
parameters (such as vertical electric current, force-free parameter, relative
magnetic helicity) during the emergence of the active region. With these
calculated parameters and some reasonable assumptions, we use two different
methods to estimate the twist of the active region.}
  {The magnetic flux and pole separation continue increasing while the tilt
angle exhibits a decreasing pattern during the emergence of the active region.
The increase of the pole separation is mainly contributed as a result of the
enhancement in the longitude direction. A power-law relationship between pole
separation and total flux is found during the emergence of the active region.
On the other hand, it is found that both the positive and negative electric
currents increased equivalently and the average flux-weighted force-free
parameter $\tilde \alpha$ remains almost consistently positive, on the order of
$\sim$ 10$^{-8}$ m$^{-1}$. The relative magnetic helicity is mainly contributed
by the shear term, while the relative magnetic helicity injection flux of the
shear term changes its sign at the latter stage of the emergence. The twist
number of the whole active region remains on the order of 10$^{-1}$ turns
during the emergence of the active region.}
  {We find that the magnetic flux tube with low twist also could emerge into
the solar atmosphere.}",2106.02786v1
2022-01-18,Using small-angle scattering to guide functional magnetic nanoparticle design,"Magnetic nanoparticles offer unique potential for various technological,
biomedical, or environmental applications thanks to the size-, shape- and
material-dependent tunability of their magnetic properties. To optimize
particles for a specific application, it is crucial to interrelate their
performance with their structural and magnetic properties. This review presents
the advantages of small-angle X-ray and neutron scattering techniques for
achieving a detailed multiscale characterization of magnetic nanoparticles and
their ensembles in a mesoscopic size range from 1 to a few hundred nanometers
with nanometer resolution. Both X-rays and neutrons allow the ensemble-averaged
determination of structural properties, such as particle morphology or particle
arrangement in multilayers and 3D assemblies. Additionally, the magnetic
scattering contributions enable retrieving the internal magnetization profile
of the nanoparticles as well as the inter-particle moment correlations caused
by interactions within dense assemblies. Most measurements are used to
determine the time-averaged ensemble properties, in addition advanced
small-angle scattering techniques exist that allow accessing particle and spin
dynamics on various timescales. In this review, we focus on conventional
small-angle X-ray and neutron scattering (SAXS and SANS), X-ray and neutron
reflectometry, gracing-incidence SAXS and SANS, X-ray resonant magnetic
scattering, and neutron spin-echo spectroscopy techniques. For each technique,
we provide a general overview, present the latest scientific results, and
discuss its strengths as well as sample requirements. Finally, we give our
perspectives on how future small-angle scattering experiments, especially in
combination with micromagnetic simulations, could help to optimize the
performance of magnetic nanoparticles for specific applications.",2201.06905v1
2022-04-26,Evidence of Ferromagnetic Clusters and Griffiths Singularity in Magnetic Weyl Semimetal Co$ _{3} $Sn$ _{2} $S$ _{2} $,"Cobalt based sulphides of compositional formula Co$ _{3} $A$ _{2} $S$ _{2} $
(A = Sn and In) are endowed with frustrated kagome lattice structure and a
plethora of novel phenomena determined from the topological band structure.
Here-in, we report on the detailed exploration of anisotropic magnetic
properties of single crystals of ferromagnetic compound Co$ _{3} $Sn$ _{2} $S$
_{2} $ under magnetic field $H$ applied along the c-axis $(H\parallel c)$ and
the ab-plane $(H\parallel ab)$. A low temperature clustered-glassy magnetic
behaviour is revealed in field-cooled and zero field-cooled magnetization and
memory effect measurement protocols. The sharp downturn and non-linearity
observed in the inverse susceptibility above the critical temperature $T
_{\text{C}} $ in the paramagnetic region corroborates to the presence of
short-range ferromagnetic clusters above $ T _{\text{C}} $ in Co$ _{3} $Sn$
_{2} $S$ _{2} $. The deviation from linear Curie-Weiss behaviour in the
paramagnetic state signifies the strong Griffiths singularity in the material.
The slow spin dynamics behaviour and zero spontaneous magnetization above $ T
_{\text{C}} $ give an evidence of Griffiths phase owing to the ferromagnetic
clusters. The Arrott plots derived from magnetization reveal convex type
curvature at low fields and linear positive behavior in the high field region,
confirming the second order magnetic phase transition in Co$ _{3} $Sn$ _{2} $S$
_{2} $. The Takahashi spin fluctuation theory analysis provides sufficient
evidence for itinerant ferromagnetism in Co$ _{3} $Sn$ _{2} $S$ _{2} $. A large
magneto-crystalline anisotropy concomitant with a high anisotropy field
suggests the dominance of strong spin orbit coupling phenomenon. Our
experimental results emphasize an intuitive understanding of the complex nature
of magnetism present in Co-based shandite systems.",2204.12252v1
2022-07-08,The Role of Magnetic Fields in the Formation of the Filamentary Infrared Dark Cloud G11.11-0.12,"We report on the near-infrared polarimetric observations of G11.11-0.12
(hereafter G11) obtained with SIRPOL on the 1.4 m IRSF telescope. The starlight
polarisation of the background stars reveals the on-sky component of magnetic
fields in G11, and these are consistent with the field orientation observed
from polarised dust emission at $850\,\mu$m. The magnetic fields in G11 are
perpendicular to the filament, and are independent of the filament's
orientation relative to the Galactic plane. The field strength in the envelope
of G11 is in the range $50-100\,\mu$G, derived from two methods. The analyses
of the magnetic fields and gas velocity dispersion indicate that the envelope
of G11 is supersonic but sub-Alfv{\'e}nic. The critical mass-to-flux ratio in
the envelope of G11 is close to 1 and increases to $\gtrsim 1$ on the spine of
G11. The relative weights on the importance of magnetic fields, turbulence and
gravity indicate that gravity dominates the dynamical state of G11, but with
significant contribution from magnetic fields. The field strength,
$|\mathbf{B}|$, increases slower than the gas density, $n$, from the envelope
to the spine of G11, characterized by $|\mathbf{B}|\propto n^{0.3}$. The
observed strength and orientation of magnetic fields in G11 imply that
supersonic and sub-Alfv{\'e}nic gas flow is channelled by the strong magnetic
fields and is assembled into filaments perpendicular to the magnetic fields.
The formation of low-mass stars is enhanced in the filaments with high column
density, in agreement with the excess of low-mass protostars detected in the
densest regions of G11.",2207.03695v3
2022-10-26,Magnetic field extrapolation in active region well comparable with observations in multiple layers,"Magnetic field extrapolation is a fundamental tool to reconstruct the
three-dimensional magnetic field above the solar photosphere. However, the
prevalently used force-free field model might not be applicable in the lower
atmosphere with non-negligible plasma \b{eta}, where the crucial process of
flux rope formation and evolution could happen. In this work, we perform
extrapolation in active region (AR) 12158, based on an recently developed
magnetohydrostatic (MHS) method which takes plasma forces into account. By
comparing the results with those from the force-free field extrapolation
methods, we find that the overall properties, which are characterized by the
magnetic free energy and helicity, are roughly the same. The major differences
lie in the magnetic configuration and the twist number of magnetic flux rope
(MFR). Unlike previous works either obtained sheared arcades or one coherent
flux rope, the MHS method derives two sets of MFR, which are highly twisted and
slightly coupled. Specifically, the result in the present work is more
comparable with the high-resolution observations from the chromosphere, through
the transition region to the corona, such as the filament fibrils, pre-eruptive
braiding characteristics and the eruptive double-J shaped hot channel. Overall,
our work shows that the newly developed MHS method is more promising to
reproduce the magnetic fine structures that can well match the observations at
multiple layers, and future data-driven simulation based on such extrapolation
will benefit in understanding the critical and precise dynamics of flux rope
before eruption.",2210.15074v3
2022-11-19,Nonlinear evolution of magnetorotational instability in a magnetized Taylor-Couette flow: scaling properties and relation to upcoming DRESDYN-MRI experiment,"Magnetorotational instability (MRI) is the most likely mechanism driving
angular momentum transport in astrophysical disks. However, despite many
efforts, a conclusive experimental evidence of MRI is still missing. Recently,
performing 1D linear analysis of the standard MRI (SMRI) in a cylindrical
Taylor-Couette (TC) flow with an axial magnetic field, we showed that SMRI can
be detected in the upcoming DRESDYN-MRI experiment based on a magnetized TC
flow of liquid sodium. In this study, also related to DRESDYN-MRI experiments,
we focused on the nonlinear evolution and saturation properties of SMRI and
analyzed its scaling behavior with respect to the main parameters of the TC
flow. We did a detailed analysis over the extensive ranges of magnetic Reynolds
number $Rm\in [8.5, 37.1]$, Lundquist number $Lu\in[1.5, 15.5]$ and Reynolds
number, $Re\in[10^3, 10^5]$. We considered small magnetic Prandtl numbers, $Pm
\ll 1$, down to $Pm\sim 10^{-4}$, aiming at values typical of liquid sodium in
the experiments. In the saturated state, the magnetic energy of SMRI and torque
due to perturbations on the cylinders, which characterizes angular momentum
transport, both increase with $Rm$ for fixed $(Lu, Re)$, while for fixed $(Lu,
Rm)$, the magnetic energy decreases and torque increases with increasing $Re$.
We studied the scaling of the magnetic energy and torque in the saturated state
as a function of $Re$ and find a power law dependence $Re^{-0.6...-0.5}$ for
the magnetic energy and $Re^{0.4...0.5}$ for the torque at all $(Lu, Rm)$ and
high $Re\geq 4000$. We also explored the dependence on Lundquist number and
angular velocity of the cylinders. These scaling laws will be instrumental in
the subsequent analysis of more realistic finite-length TC flows and comparison
of numerical results with those obtained from the DRESDYN-MRI experiments to
unambiguously identify SMRI in laboratory.",2211.10811v3
2022-11-21,Comparison of magnetic energy and helicity in coronal jet simulations,"While free/non-potential magnetic energy is a necessary element of any active
phenomenon in the solar corona, its role as a marker of the trigger of eruptive
process remains elusive. Based on the unique decomposition of the magnetic
field into potential and non-potential components, magnetic energy and helicity
can also both be uniquely decomposed into two quantities. Using two 3D MHD
parametric simulations of a configuration that can produce coronal jets, we
compare the dynamics of the magnetic energies and of the relative magnetic
helicities. Both simulations share the same initial set-up and line-tied
bottom-boundary driving profile. However, they differ by the duration of the
forcing. In one simulation the system is driven sufficiently so that an helical
jet is induced. The generation of the jet is however markedly delayed: a
relatively long phase of lower-intensity reconnection takes place before the
jet is eventually induced. In the other reference simulation, the system is
driven during a shorter time, and no jet is produced. As expected, we observe
that the Jet producing simulation contains a higher value of non-potential
energy and non-potential helicity. Focusing on the phase between the end of the
driving-phase and the jet generation, we note that magnetic energies remain
relatively constant, while magnetic helicities have a noticeable evolution.
During this post-driving phase, the ratio of the non-potential to total
magnetic energy very slightly decreases while the helicity eruptivity index
significantly increases. The jet is generated when the system is at the highest
value of this helicity eruptivity index. This proxy critically decreases during
the jet generation phase. The free energy also decreases but does not present
any peak when the jet is being generated.",2211.11265v1
2022-11-21,Room Temperature Optically and Magnetically Active Edges in Phosphorene Nanoribbons,"Nanoribbons - nanometer wide strips of a two-dimensional material - are a
unique system in condensed matter physics. They combine the exotic electronic
structures of low-dimensional materials with an enhanced number of exposed
edges, where phenomena including ultralong spin coherence times, quantum
confinement and topologically protected states can emerge. An exciting prospect
for this new material concept is the potential for both a tunable
semiconducting electronic structure and magnetism along the nanoribbon edge.
This combination of magnetism and semiconducting properties is the first step
in unlocking spin-based electronics such as non-volatile transistors, a route
to low-energy computing, and has thus far typically only been observed in doped
semiconductor systems and/or at low temperatures. Here, we report the magnetic
and semiconducting properties of phosphorene nanoribbons (PNRs). Static (SQUID)
and dynamic (EPR) magnetization probes demonstrate that at room temperature,
films of PNRs exhibit macroscopic magnetic properties, arising from their edge,
with internal fields of ~ 250 to 800 mT. In solution, a giant magnetic
anisotropy enables the alignment of PNRs at modest sub-1T fields. By leveraging
this alignment effect, we discover that upon photoexcitation, energy is rapidly
funneled to a dark-exciton state that is localized to the magnetic edge and
coupled to a symmetry-forbidden edge phonon mode. Our results establish PNRs as
a unique candidate system for studying the interplay of magnetism and
semiconducting ground states at room temperature and provide a stepping-stone
towards using low-dimensional nanomaterials in quantum electronics.",2211.11374v1
2023-05-03,Reconnection generated plasma flows in the quasi-separatrix layer in localised solar corona,"Multiwavelength observations of the propagating disturbances (PDs),
discovered by Atmospheric Imaging Assembly (AIA) onboard Solar Dynamics
Observatory (SDO), are analyzed to determine its driving mechanism and physical
nature. Two magnetic strands in the localised corona are observed to approach
and merge with each other followed by the generation of brightening, which
further propagates in a cusp-shaped magnetic channel. Differential emission
measure analysis shows an occurrence of heating in this region-of-interest
(ROI). We extrapolate potential magnetic field lines at coronal heights from
observed Helioseismic and Magnetic Imager (HMI) vector magnetogram via Green's
function method using MPI-AMRVAC. We analyze the field to locate magnetic nulls
and quasi-separatrix layers (QSLs) which are preferential locations for
magnetic reconnection. Dominant QSLs including a magnetic null are found to
exist and match the geometry followed by PDs, therefore, it provides conclusive
evidence of magnetic reconnection. In addition, spectroscopic analysis of
Interface Region Imaging Spectrograph (IRIS) Si IV 1393.77 {\AA} line profiles
show a rise of line-width in the same time range depicting presence of mass
motion in the observed cusp-shaped region. PDs are observed to exhibit
periodicities of around four minutes. The speeds of PDs measured by Surfing
Transform Technique are almost close to each other in four different SDO/AIA
bandpasses, i.e., 304, 171, 193 and 131 {\AA} excluding the interpretation of
PDs in terms of slow magnetoacoustic waves. We describe comprehensively the
observed PDs as quasi-periodic plasma flows generated due to periodic
reconnection in vicinity of a coronal magnetic null.",2305.02277v1
2023-08-30,Energy distribution and substructure formation in astrophysical MHD simulations,"During substructure formation in magnetized astrophysical plasma, dissipation
of magnetic energy facilitated by magnetic reconnection affects the system
dynamics by heating and accelerating the ejected plasmoids. Numerical
simulations are a crucial tool for investigating such systems. In astrophysical
simulations, the energy dissipation, reconnection rate and substructure
formation critically depend on the onset of reconnection of numerical or
physical origin. In this paper, we hope to assess the reliability of the
state-of-the-art numerical codes, PLUTO and KORAL by quantifying and discussing
the impact of dimensionality, resolution, and code accuracy on magnetic energy
dissipation, reconnection rate, and substructure formation. We quantitatively
compare results obtained with relativistic and non-relativistic, resistive and
non-resistive, as well as two- and three-dimensional setups performing the
Orszag-Tang test problem. We find the sufficient resolution in each model, for
which numerical error is negligible and the resolution does not significantly
affect the magnetic energy dissipation and reconnection rate. The
non-relativistic simulations show that at sufficient resolution, magnetic and
kinetic energies convert to internal energy and heat up the plasma. The results
show that in the relativistic system, energy components undergo mutual
conversion during the simulation time, which leads to a substantial increase in
magnetic energy at 20\% and 90\% of the total simulation time of $10$
light-crossing times -- the magnetic field is amplified by a factor of five due
to relativistic shocks. We also show that the reconnection rate in all our
simulations is higher than $0.1$, indicating plasmoid-mediated regime. It is
shown that in KORAL simulations magnetic energy is slightly larger and more
substructures are captured than in PLUTO simulations.",2308.16062v2
2023-10-25,Fundamental MHD scales -- II: the kinematic phase of the supersonic small-scale dynamo,"The small-scale dynamo (SSD) amplifies weak magnetic fields exponentially
fast via kinetic motions. While there exist well-established theories for SSDs
in incompressible flows, many astrophysical SSDs operate in supersonic
turbulence. To understand the impact of compressibility on amplified magnetic
fields, we perform an extensive set of visco-resistive SSD simulations,
covering a wide range of sonic Mach number $\mathcal{M}$, hydrodynamic Reynolds
number Re, and magnetic Prandtl number Pm. We develop robust methods for
measuring kinetic and magnetic energy dissipation scales $\ell_\nu$ and
$\ell_\eta$, as well as the scale at which magnetic fields are strongest
$\ell_p$ during the kinematic phase of these simulations. We show that
$\ell_\nu/\ell_\eta \sim$ Pm$^{1/2}$ is a universal feature in the kinematic
phase of Pm $\geq 1$ SSDs, regardless of $\mathcal{M}$ or Re, and we confirm
earlier predictions that SSDs operating in incompressible plasmas (either
$\mathcal{M} \leq 1$ or Re $<$ Re$_{\rm crit} \approx 100$) concentrate
magnetic energy at the smallest scales allowed by magnetic dissipation, $\ell_p
\sim \ell_\eta$, and produce fields organised with field strength and
field-line curvature inversely correlated. However, we show that these
predictions fail for compressible SSDs ($\mathcal{M} > 1$ and Re $>$ Re$_{\rm
crit}$), where shocks concentrate magnetic energy in large-scale, over-dense,
coherent structures, with size $\ell_p \sim (\ell_{\rm turb} / \ell_{\rm
shock})^{1/3} \ell_\eta \gg \ell_\eta$, where $\ell_{\rm shock} \sim
\mathcal{M}^2 / [$Re $ (\mathcal{M} - 1)^2]$ is shock width, and $\ell_{\rm
turb}$ is the turbulent outer scale; magnetic field-line curvature becomes
almost independent of the field strength. We discuss the implications for
galaxy mergers and for cosmic-ray transport models in the interstellar medium
that are sensitive to field-line curvature statistics.",2310.17036v1
2024-01-02,A study of the magnetocaloric behavior of Dy-substituted YMn$_2$O$_5$ compounds,"In this paper, we report on the magnetic and magnetocaloric features of
Dy-substituted YMn$_2$O$_5$ (Y$_{1-x}$Dy$_x$Mn$_2$O$_5$) with $x=$ 0.6, 0.8,
and 1 series elaborated by sol-gel method. X-ray diffraction and Raman
measurements attest well the high quality of our polycrystalline samples that
crystallize in an orthorhombic structure with the Pbam space group. The Raman
phonon frequencies were carried out and compared with the lattice dynamics
calculations to identify the vibrational properties of all detected modes at
room temperature. As expected, our magnetic study reveals that the
magnetization was enhanced by the substitution of Y$^{3+}$ by Dy$^{3+}$. The
Dy-substituted YMn$_2$O$_5$ sets the N\'eel transition [TN (Mn)] in the
temperature range going from 40 to 45 K favoring the emergence of a transition
at a very low temperature due to the long-range ordering of the Dy3+ magnetic
moments below 13K [TN (Dy)]. Dual peaks in the magnetic entropy change curve
are also observed being in good agreement with magnetization data, which
enlarges the range of application of these materials. On the other hand, a
large magnetocaloric effect is observed close to 13 K which is mainly due the
ordering of Dy$^{3+}$ magnetic moments. Also, the incommensurate
antiferromagnetic transition of Mn magnetic moment taking place around 40 K
affects slightly the entropy change. Our refrigerant capacity (RC) findings are
higher compared to the average of RC for a, b, c axis of single crystal samples
as HoMn$_2$O$_5$ and TbMn$_2$O$_5$, which confirms that our polycrystalline
materials stand for promising magnetic refrigerant candidates that can be
invested in space technology, hydrogen and helium liquefaction at cryogenic
temperature.",2401.01431v1
2024-01-31,"Formation Mechanism of Laser-Driven Magnetized ""Pillars of Creation""","Pillars of Creation, one of the most recognized objects in the sky, are
believed to be associated with the formation of young stars. However, so far,
the formation and maintenance mechanism for the pillars are still not fully
understood due to the complexity of the nonlinear radiation
magneto-hydrodynamics (RMHD). Here, assuming laboratory laser-driven
conditions, we studied the self-consistent dynamics of pillar structures in
magnetic fields by means of two-dimensional (2D) and three-dimensional (3D)
RMHD simulations, and these results also support our proposed experimental
scheme. We find only when the magnetic pressure and ablation pressure are
comparable, the magnetic field can significantly alter the plasma
hydrodynamics. For medium magnetized cases ($\beta_{initial} \approx 3.5$),
{the initial magnetic fields undergo compression and amplification. This
amplification results in the magnetic pressure inside the pillar becoming large
enough to support the sides of the pillar against radial collapse due to
pressure from the surrounding hot plasma. This effect is particularly
pronounced for the parallel component ($B_y$), which is consistent with
observational results.} In contrast, a strong perpendicular ($B_x, B_z$)
magnetic field ($\beta_{initial} < 1$) almost remains its initial distribution
and significantly suppresses the expansion of blow-off gas plasma, leading to
the inability to form pillar-like structures. The 3D simulations suggest that
the bending at the head of `Column \uppercase\expandafter{\romannumeral1}' in
pillars of creation may be due to the non-parallel magnetic fields. After
similarity scaling transformation, our results can be applied to explain the
formation and maintenance mechanism of the pillars, and can also provide useful
information for future experimental designs.",2401.17561v1
2024-02-09,Characterization of the Clinically Approved MRI Tracer Resotran for Magnetic Particle Imaging in a Comparison Study,"Objective. The availability of magnetic nanoparticles with medical approval
for human intervention is fundamental to the clinical translation of magnetic
particle imaging (MPI). In this work, we thoroughly evaluate and compare the
magnetic properties of an magnetic resonance imaging (MRI) approved tracer to
validate its performance for MPI in future human trials. Approach. We analyze
whether the recently approved MRI tracer Resotran is suitable for MPI. In
addition, we compare Resotran with the previously approved and extensively
studied tracer Resovist, with Ferrotran, which is currently in a clinical phase
III study, and with the tailored MPI tracer Perimag. Main results. Initial
magnetic particle spectroscopy measurements indicate that Resotran exhibits
performance characteristics akin to Resovist, but below Perimag. We provide
data on four different tracers using dynamic light scattering, transmission
electron microscopy, vibrating sample magnetometry measurements, magnetic
particle spectroscopy to derive hysteresis, point spread functions, and a
serial dilution, as well as system matrix based MPI measurements on a
preclinical scanner (Bruker 25/20 FF), including reconstructed images.
Significance. Numerous approved magnetic nanoparticles used as tracers in MRI
lack the necessary magnetic properties essential for robust signal generation
in MPI. The process of obtaining medical approval for dedicated MPI tracers
optimized for signal performance is an arduous and costly endeavor, often only
justifiable for companies with a well-defined clinical business case. Resotran
is an approved tracer that has become available in Europe for MRI. In this
work, we study the eligibility of Resotran for MPI in an effort to pave the way
for human MPI trials.",2402.06350v1
2024-03-21,The dependence of the magnetism of a near-limb sunspot on height,"The physical parameters of the sunspot are not fully understood, especially
the height dependence of the magnetic field. So far, it is also an open
question as to which heights the He I 1083 nm spectral line is formed at. Our
aim is to investigate the magnetic and dynamical properties in the atmosphere
above a sunspot, from the photosphere to the chromosphere. We analyzed the
photospheric and chromospheric magnetic field properties of a stable sunspot in
AR 12553 on June 20, 2016 using spectropolarimetric observations obtained with
GRIS at GREGOR. A spectral-line inversion technique was used to infer the
magnetic field vector and Doppler velocities from the full Stokes profiles. In
total, three spectral lines were inverted and the variation of the magnetic
properties was qualified using the average values of the radial circles. The
sunspot is located close to the solar limb, and thus this allows us to make a
geometrical determination of the height of the spectral line He I 1083 nm. We
find the height of helium spectral line to be 970 km above the photospheric
spectral lines directly from observation at a stable sunspot. The total
magnetic field strength decreases with height over the sunspot; the rates are
-0.34 G/km for the umbra and -0.28 G/km for the penumbra. The inclination
increases with increasing height in the umbra, but decreases in the penumbra.
In the umbra, the vertical component ($B_z$) decreases with height, while the
horizontal component ($B_{hor}$) remains almost constant. In the penumbra this
is reversed, as $B_z$ remains nearly constant over height, while $B_{hor}$
decreases. We also observe fast velocities with 30 km/s in small chromospheric
patches on the central side of the spot. The key parameters depending on height
in the sunspot are the $B_{z}$ component of the magnetic field for the umbra
and the $B_{hor}$ component of the magnetic field for the penumbra.",2403.14532v1
2000-01-07,Theory of the spin bath,"The quantum dynamics of mesoscopic or macroscopic systems is always
complicated by their coupling to many ""environmental"" modes.At low T these
environmental effects are dominated by localised modes, such as nuclear and
paramagnetic spins, and defects (which also dominate the entropy and specific
heat). This environment, at low energies, maps onto a ""spin bath"" model. This
contrasts with ""oscillator bath"" models (originated by Feynman and Vernon)
which describe {\it delocalised} environmental modes such as electrons,
phonons, photons, magnons, etc. One cannot in general map a spin bath to an
oscillator bath (or vice-versa); they constitute distinct ""universality
classes"" of quantum environment. We show how the mapping to spin bath models is
made, and then discuss several examples in detail, including moving particles,
magnetic solitons, nanomagnets, and SQUIDs, coupled to nuclear and paramagnetic
spin environments. We show how to average over spin bath modes, using an
operator instanton technique, to find the system dynamics, and give analytic
results for the correlation functions, under various conditions. We then
describe the application of this theory to magnetic and superconducting
systems.Particular attention is given to recent work on tunneling magnetic
macromolecules, where the role of the nuclear spin bath in controlling the
tunneling is very clear; we also discuss other magnetic systems in the quantum
regime, and the influence of nuclear and paramagnetic spins on flux dynamics in
SQUIDs.",0001080v2
2001-07-24,Diluted quantum antiferromagnets: spin excitations and long-range order,"We have studied the static and dynamic magnetic properties of two-dimensional
(2D) and quasi-two-dimensional, spin-S, quantum Heisenberg antiferromagnets
(QHAF) diluted with spinless vacancies. Using spin-wave theory and T-matrix
approximation we have calculated the staggered magnetization, M(x,T), the
neutron scattering dynamical structure factor, S(k,omega), the 2D magnetic
correlation length, \xi(x,T), and the Neel temperature, T_N(x), for the
quasi-2D case. We find that in 2D the hydrodynamic description of excitations
in terms of spin-waves breaks down at the wavelength larger than \ell/a ~
exp(\pi/4x), x being impurity concentration and ""a"" the lattice spacing. The
density of states acquires a constant term and exhibits an anomalous peak at
omega omega_0 associated with the low-energy localized excitations. These
anomalies lead to a substantial enhancement of the magnetic specific heat, C_M,
at low temperatures. Although the dynamical properties are significantly
modified we show that 2D is not the lower critical dimension for this problem.
Our results are in quantitative agreement with the recent Monte Carlo
simulations and experimental data for S=1/2, S=1, and S=5/2.",0107488v1
2007-03-21,Dynamics of the Magnetic Flux Trapped in Fractal Clusters of a Normal Phase in Percolative Superconductors,"The effect of the fractal clusters of a normal phase, which act as pinning
centers, on the dynamics of magnetic flux in percolative type-II superconductor
is considered. The main features of these clusters are studied in detail: the
cluster statistics is analyzed; the fractal dimension of their boundary is
estimated; the distribution of critical currents is obtained, and its
peculiarities are explored. It is found that there is the range of fractal
dimension where this distribution has anomalous statistical properties,
specifically, its dispersion becomes infinite. It is examined how the finite
resolution capacity of the cluster geometric size measurement affects the
estimated value of fractal dimension. The effect of fractal properties of the
normal phase clusters on the electric field arisen from magnetic flux motion is
investigated for the cluster area distribution of different kinds. The
voltage-current characteristics of fractal superconducting structures in the
resistive state are obtained for an arbitrary fractal dimension. It is revealed
that the fractality of the boundaries of the normal phase clusters intensifies
the magnetic flux trapping and thereby raises the critical current of a
superconductor.",0703541v1
2007-05-24,Optical investigations of quantum-dot spin dynamics,"We have performed all-optical measurements of spin relaxation in single
self-assembled InAs/GaAs quantum dots (QD) as a function of static external
electric and magnetic fields. To study QD spin dynamics we measure the degree
of resonant absorption which results from a competition between optical spin
pumping induced by the resonant laser field and spin relaxation induced by
reservoirs. Fundamental interactions that determine spin dynamics in QDs are
hyperfine coupling to QD nuclear spin ensembles, spin-phonon coupling and
exchange-type interactions with a nearby Fermi sea of electrons. We show that
the strength of spin relaxation generated by the three fundamental interactions
can be changed by up to five orders of magnitude upon varying the applied
electric and magnetic fields. We find that the strength of optical spin pumping
that we use to study the spin relaxation is determined predominantly by
hyperfine-induced mixing of single-electron spin states at low magnetic fields
and heavy-light hole mixing at high magnetic fields. Our measurements allow us
to determine the rms value of the hyperfine (Overhauser) field to be ~15 mTesla
with an electron g-factor of g_e=0.6 and a hole mixing strength of |epsilon|^2
= 0.0005.",0705.3557v1
2007-07-24,Dynamics of wave packets in two-dimensional crystals under external magnetic and electric fields: Vortices formation,"In the present work we deal with the dynamics of wave packets in a
two-dimensional crystal under the action of magnetic and electric fields. The
magnetic field is perpendicular to the plane and the electric field is on the
plane. In the simulations we considered a symmetric gauge for the vector
potential while the initial wave packet was assumed to have a gaussian
structure with given velocities. The parameters that control the kind of time
evolution of the packets are: the width of the gaussian, its velocity, and, the
intensity and direction of the electric field as well as the magnitude of the
magnetic field. In order to characterize the kind of propagation we evaluated
the mean-square displacement (MSD), the participation function and which is
more important we were able to follow the wave at different times, which
allowed us to see the time evolution of the centroid of the wave packets. A
novel effect was observed, namely, the dynamics is such that the wave function
\emph{splits} into two or more components and \emph{reconstructs} successively
as time goes, vortices are forming. To our understanding this is the first time
such an effect is reported. As for the inclusion of the electric field, we
observe a complex behavior of the wave packet as well as note that the vortices
propagate in direction perpendicular to the applied electric field. A similar
behavior presented by the classical treatment, In our case we give a quantum
mechanics explanation for that",0707.3554v1
2008-04-17,Dynamical effects of self-generated magnetic fields in cosmic ray modified shocks,"Recent observations of greatly amplified magnetic fields ($\delta B/B\sim
100$) around supernova shocks are consistent with the predictions of the
non-linear theory of particle acceleration (NLT), if the field is generated
upstream of the shock by cosmic ray induced streaming instability. The high
acceleration efficiencies and large shock modifications predicted by NLT need
however to be mitigated to confront observations, and this is usually assumed
to be accomplished by some form of turbulent heating. We show here that
magnetic fields with the strength inferred from observations have an important
dynamical role on the shock, and imply a shock modification substantially
reduced with respect to the naive unmagnetized case. The effect appears as soon
as the pressure in the turbulent magnetic field becomes comparable with the
pressure of the thermal gas. The relative importance of this unavoidable effect
and of the poorly known turbulent heating is assessed. More specifically we
conclude that even in the cases in which turbulent heating may be of some
importance, the dynamical reaction of the field cannot be neglected, as instead
is usually done in most current calculations.",0804.2884v2
2008-06-26,Ferromagnetic resonance force spectroscopy of individual sub-micron size samples,"We review how a magnetic resonance force microscope (MRFM) can be applied to
perform ferromagnetic resonance (FMR) spectroscopy of \emph{individual}
sub-micron size samples. We restrict our attention to a thorough study of the
spin-wave eigen-modes excited in permalloy (Py) disks patterned out of the same
43.3 nm thin film. The disks have a diameter of either 1.0 or $0.5 \mu$m and
are quasi-saturated by a perpendicularly applied magnetic field. It is shown
that \emph{quantitative} spectroscopic information can be extracted from the
MRFM measurements. In particular, the data are extensively compared with
complementary approximate models of the dynamical susceptibility: i) a 2D
analytical model, which assumes an homogeneous magnetization dynamics along the
thickness and ii) a full 3D micromagnetic simulation, which assumes an
homogeneous magnetization dynamics below a characteristic length scale $c$ and
which approximates the cylindrical sample volume by a discretized
representation with regular cubic mesh of lateral size $c=3.9$ nm. In our
analysis, the distortions due to a breaking of the axial symmetry are taken
into account, both models incorporating the possibility of a small misalignment
between the applied field and the normal of the disks.",0806.4244v1
2008-08-22,Development of Anisotropy in Incompressible Magnetohydrodynamic Turbulence,"We present a set of three-dimensional (3D) direct numerical simulations of
incompressible decaying magnetohydrodynamic turbulence in which we investigate
the influence of an external uniform magnetic field B_0. A parametric study in
terms of B_0 intensity is made where, in particular, we distinguish the shear-
from the pseudo-Alfven waves dynamics. The initial kinetic and magnetic
energies are equal with a negligible cross-correlation. Both the temporal and
spectral effects of B_0 are discussed. A sub-critical balance is found between
the Alfven and nonlinear times with both a global and a spectral definition.
The nonlinear dynamics of strongly magnetized flows is characterized by a
different k_perp-spectrum (where B_0 defines the parallel direction) if it is
plotted at a fixed k_// (2D spectrum) or if it is integrated (averaged) over
all k_// (1D spectrum). In the former case a much wider inertial range is found
with a steep power law, closer to the wave turbulence prediction than the
Kolmogorov one like in the latter case. It is believed that the averaging
effect may be a source of difficulty to detect the transition towards wave
turbulence in natural plasmas. For the first time, the formation of filaments
is reported within current and vorticity sheets in strongly magnetized flows
which modifies our classical picture of dissipative sheets in conductive flows.",0808.3061v1
2010-02-11,Forward and adjoint quasi-geostrophic models of the geomagnetic secular variation,"We introduce a quasi-geostrophic model of core dynamics, which aims at
describing core processes on geomagnetic secular variation timescales. It
extends the formalism of Alfv\'en torsional oscillations by incorporating
non-zonal motions. Within this framework, the magnetohydrodynamics takes place
in the equatorial plane; it involves quadratic magnetic quantities, which are
averaged along the direction of rotation of the Earth. In addition, the
equatorial flow is projected on the core-mantle boundary. It interacts with the
magnetic field at the core surface, through the radial component of the
magnetic induction equation. That part of the model connects the dynamics and
the observed secular variation, with the radial component of the magnetic field
acting as a passive tracer. We resort to variational data assimilation to
construct formally the relationship between model predictions and observations.
Variational data assimilation seeks to minimize an objective function, by
computing its sensitivity to its control variables. The sensitivity is
efficiently calculated after integration of the adjoint model. We illustrate
that framework with twin experiments, performed first in the case of the
kinematic core flow inverse problem, and then in the case of Alfv\'en torsional
oscillations. In both cases, using the adjoint model allows us to retrieve core
state variables which, while taking part in the dynamics, are not directly
sampled at the core surface. We study the effect of several factors on the
solution (width of the assimilation time window, amount and quality of data),
and we discuss the potential of the model to deal with real geomagnetic
observations.",1002.2323v1
2010-04-22,Critical current and vortex dynamics in single crystals of Ca(Fe$_{1-x}$Co$_{x}$)$_2$As$_2$,"We investigate the critical current density and vortex dynamics in single
crystals of Ca(Fe$_{1-x}$Co$_{x}$)$_2$As$_2$ ($x$ = 0.051, 0.056, 0.065, and
0.073). The samples exhibit different critical temperatures and superconducting
phase fractions. We show that in contrast to their Ba-based counterpart, the
crystals do not exhibit a second peak in the field dependence of magnetization.
The calculated composition-dependent critical current density ($j_{\rm c}$)
increases initially with Co doping, maximizing at $x$ = 0.065, and then
decreases. This variation in $j_{\rm c}$ follows the superconducting phase
fractions in this series. The calculated $j_{\rm c}$ shows strong temperature
dependence, decreasing rapidly upon heating. Magnetic relaxation measurements
imply a nonlogarithmic dependence on time. We find that the relaxation rate is
large, reflecting weak characteristic pinning energy. The analysis of
temperature- and field-dependent magnetic relaxation data suggests that vortex
dynamics in these compounds is consistent with plastic creeping rather than the
collective creep model, unlike other 122 pnictide superconductors. This
difference may cause the absence of the second peak in the field dependent
magnetization of Ca(Fe$_{1-x}$Co$_{x}$)$_2$As$_2$.",1004.3910v1
2010-05-29,"Confinement of the Sun's interior magnetic field, with implications for lithium burning","The simplest interior magnetic field B_i that can explain the observed
uniform rotation of the Sun's radiative envelope is an axial dipole stabilized
by a deep toroidal field. It can explain the uniform rotation only if confined
in the polar caps. The field must be prevented from diffusing up into the
high-latitude convection zone, whose slower rotation must remain decoupled from
the radiative interior. This paper describes new analytical and numerical
solutions of the relevant magnetohydrodynamic equations showing that such
confinement and decoupling is dynamically possible by means of a laminar
""magnetic confinement layer"" at the bottom of the tachocline. With realistic
values of the microscopic diffusivities, a weak laminar downwelling flow
U~10^{-5}cm/s over the poles is enough to enforce exponential decay of B_i with
altitude, in a confinement layer only a fraction of a megameter thick.
Downwelling in the polar tachocline is implied both by helioseismic
observations, combined with elementary dynamics, and by theoretical arguments
about the ""gyroscopic pumping"" that would spread differential rotation downward
in the absence of B_i. Our confinement-layer solutions are the first to take
account of all the relevant physical effects in a self-consistent mathematical
model. The effects include magnetic diffusion, baroclinicity and stable
stratification (thermal and compositional), Coriolis effects, and thermal
relaxation. We discuss how the confinement layers at each pole might fit into a
global dynamical picture of the solar tachocline. That picture, in turn,
suggests a new insight into the early Sun and solar lithium depletion.",1005.5481v2
2012-06-26,Transport and semiclassical dynamics of coupled quantum dots interacting with a local magnetic moment,"We present a theory of magnetotransport through a system of two coupled
electronic orbitals, where the electron spin interacts with a (large) local
magnetic moment via an exchange interaction. For the physical realization of
such a set-up we have in mind, for example, semiconductor quantum dots coupled
to an ensemble of nuclear spins in the host material or molecular orbitals
coupled to a local magnetic moment. Using a semiclassical approximation, we
derive a set of Ehrenfest equations of motion for the electron density matrix
and the mean value of the external spin (Landau equations): Due to the spin
coupling they turn out to be nonlinear and, importantly, also coherences
between electron states with different spin directions need to be considered.
The electronic spin-polarized leads are implemented in form of a Lindblad-type
dissipator in the infinite bias limit. We have solved this involved dynamical
system numerically for various isotropic and anisotropic coupling schemes. For
isotropic spin coupling and spin-polarized leads we study the effect of
current-induced magnetization of the attached spin and compare this with a
single quantum dot set-up. We further demonstrate that an anisotropic coupling
can lead to a rich variety of parametric oscillations in the average current
reflecting the complicated interplay between the Larmor precession of the
external spin and the dissipative coherent dynamics of the electron spin.",1206.5994v2
2012-07-04,The chromosphere and prominence magnetometer,"The Chromosphere and Prominence Magnetometer (ChroMag) is conceived with the
goal of quantifying the intertwined dynamics and magnetism of the solar
chromosphere and in prominences through imaging spectro-polarimetry of the full
solar disk. The picture of chromospheric magnetism and dynamics is rapidly
developing, and a pressing need exists for breakthrough observations of
chromospheric vector magnetic field measurements at the true lower boundary of
the heliospheric system. ChroMag will provide measurements that will enable
scientists to study and better understand the energetics of the solar
atmosphere, how prominences are formed, how energy is stored in the magnetic
field structure of the atmosphere and how it is released during space weather
events like flares and coronal mass ejections. An integral part of the ChroMag
program is a commitment to develop and provide community access to the
""inversion"" tools necessary for the difficult interpretation of the
measurements and derive the magneto-hydrodynamic parameters of the plasma.
Measurements of an instrument like ChroMag provide critical physical context
for the Solar Dynamics Observatory (SDO) and Interface Region Imaging
Spectrograph (IRIS) as well as ground-based observatories such as the future
Advanced Technology Solar Telescope (ATST).",1207.0969v1
2013-02-25,Monte Carlo investigation of a spherical ferrimagnetic core-shell nanoparticle under a time dependent magnetic field,"Monte Carlo simulation based on Metropolis algorithm has been used with a
great success to analyze the dynamic phase transition properties of a single
spherical core-shell nanoparticle system with a spin-3/2 core surrounded by a
spin-1 shell layer with antiferromagnetic interface coupling under the
influence of a time dependent oscillating magnetic filed. It has been found
that the dynamic phase boundaries strongly depend on the Hamiltonian parameters
such as for the high amplitude and period values of the external field, the
phase transition temperature sharply changes whereas it tends to slowly alter
as the reduced magnitude of interlayer parameter $\mathrm{J_{int}/J_{sh}}$
increases. Moreover, it is observed that the magnetization curves of the
particle have been found to obey P-type, N-type and Q-type classification
schemes under certain conditions. Much effort has also been paid to the
influence of the particle size on the thermal and magnetic properties of the
particle. Finally, a comparison of our observations with those of recently
published study including dynamic treatments of a nanocubic core-shell system
is represented and the findings indicate that there exists a qualitatively good
agreement with some relatively distinct differences.",1302.6227v2
2015-02-26,The decay of wall-bounded MHD turbulence at low Rm,"We present Direct Numerical Simulations of decaying Magnetohydrodynamic (MHD)
turbulence at low magnetic Reynolds number. The domain considered is bounded by
periodic boundary conditions in the two directions perpendicular to the
magnetic field and by two plane Hartmann walls in the third direction. High
magnetic fields (Hartmann number of up to 896) are considered thanks to a
numerical method based on a spectral code using the eigenvectors of the
dissipation operator. It is found that the decay proceeds through two phases:
first, energy and integral lengthscales vary rapidly during a
two-dimensionalisation phase extending over about one Hartmann friction time.
During this phase, the evolution of the former appears significantly more
impeded by the presence of walls than that of the latter. Once the large scales
are close to quasi-two dimensional, the decay results from the competition of a
two-dimensional dynamics driven by dissipation in the Hartmann boundary layers
and the three-dimensional dynamics of smaller scales. In the later stages of
the decay, three-dimensionality subsists under the form of barrel-shaped
structures. A purely quasi-two dimensional decay dominated by friction in the
Hartmann layers is not reached, because of residual dissipation in the bulk.
However, this dissipation is not generated by the three-dimensionality that
subsists, but by residual viscous friction due to horizontal velocity
gradients. Also, the energy in the velocity component aligned with the magnetic
field is found to be strongly suppressed, as is transport in this direction.
This results reproduces the experimental findings of Kolesnikov & Tsinober
(1974).",1502.07631v2
2015-03-11,Current-Driven Dynamics of Skyrmions Stabilized in MnSi Nanowires Revealed by Topological Hall Effect,"Skyrmions, novel topologically stable spin vortices, hold promise for
next-generation magnetic storage due to their nanoscale domains to enable high
information storage density and their low threshold for current-driven motion
to enable ultralow energy consumption. One-dimensional (1D) nanowires are ideal
hosts for skyrmions since they not only serve as a natural platform for
magnetic racetrack memory devices but also can potentially stabilize skyrmions.
Here we use the topological Hall effect (THE) to study the phase stability and
current-driven dynamics of the skyrmions in MnSi nanowires. The THE was
observed in an extended magnetic field-temperature window (15 to 30 K),
suggesting stabilization of skyrmion phase in nanowires compared with the bulk
(27 to 29.5 K). Furthermore, for the first time, we study skyrmion dynamics in
this extended skyrmion phase region and found that under the high
current-density of $10^{8}-10^{9} Am^{-2}$ enabled by nanowire geometry, the
THE decreases with increasing current densities, which demonstrates the
current-driven motion of skyrmions generating the emergent electric field.
These results open up the exploration of nanowires as an attractive platform
for investigating skyrmion physics in 1D systems and exploiting skyrmions in
magnetic storage concepts.",1503.03523v2
2015-04-28,Microwave soft x-ray microscopy for nanoscale magnetization dynamics in the 5-10 GHz frequency range,"We present a scanning transmission x-ray microscopy setup combined with a
novel microwave synchronization scheme in order to study high frequency
magnetization dynamics at synchrotron light sources. The sensitivity necessary
to detect small changes of the magnetization on short time scales and nanometer
spatial dimensions is achieved by combination of the developed excitation
mechanism with a single photon counting electronics that is locked to the
synchrotron operation frequency. The required mechanical stability is achieved
by a compact design of the microscope. Our instrument is capable of creating
direct images of dynamical phenomena in the 5-10 GHz range, with 35 nm
resolution. When used together with circularly polarized x-rays, the above
capabilities can be combined to study magnetic phenomena at microwave
frequencies, such as ferromagnetic resonance (FMR) and spin waves. We
demonstrate the capabilities of our technique by presenting phase resolved
images of a 6 GHz nanoscale spin wave generated by a spin torque oscillator, as
well as the uniform ferromagnetic precession with ~0.1 deg amplitude at 9 GHz
in a micrometer-sized cobalt strip.",1504.07561v1
2015-08-02,Soliton-like magnetic domain wall motion induced by the interfacial Dzyaloshinskii-Moriya interaction,"Topological defects such as magnetic solitons, vortices, Bloch lines, and
skyrmions have started to play an important role in modern magnetism because of
their extraordinary stability, which can be exploited in the production of
memory devices. Recently, a novel type of antisymmetric exchange interaction,
namely the Dzyaloshinskii-Moriya interaction (DMI), has been uncovered and
found to influence the formation of topological defects. Exploring how the DMI
affects the dynamics of topological defects is therefore an important task.
Here we investigate the dynamic domain wall (DW) under a strong DMI and find
that the DMI induces an annihilation of topological vertical Bloch lines (VBLs)
by lifting the four-fold degeneracy of the VBL. As a result, velocity reduction
originating from the Walker breakdown is completely suppressed, leading to a
soliton-like constant velocity of the DW. Furthermore, the strength of the DMI,
which is the key factor for soliton-like DW motion, can be quantified without
any side effects possibly arising from current-induced torques or extrinsic
pinnings in magnetic films. Our results therefore shed light on the physics of
dynamic topological defects, which paves the way for future work in
topology-based memory applications.",1508.00197v1
2015-08-22,Helical mode interactions and spectral transfer processes in magnetohydrodynamic turbulence,"Spectral transfer processes in magnetohydrodynamic (MHD) turbulence are
investigated analytically by decomposition of the velocity and magnetic fields
in Fourier space into helical modes. Steady solutions of the dynamical system
which governs the evolution of the helical modes are determined, and a
stability analysis of these solutions is carried out. The interpretation of the
analysis is that unstable solutions lead to energy transfer between the
interacting modes while stable solutions do not. From this, a dependence of
possible interscale energy and helicity transfers on the helicities of the
interacting modes is derived. As expected from the inverse cascade of magnetic
helicity in 3D MHD turbulence, mode interactions with like helicities lead to
transfer of energy and magnetic helicity to smaller wavenumbers. However, some
interactions of modes with unlike helicities also contribute to an inverse
energy transfer. As such, an inverse energy cascade for nonhelical magnetic
fields is shown to be possible. Furthermore, it is found that high values of
the cross-helicity may have an asymmetric effect on forward and reverse
transfer of energy, where forward transfer is more quenched in regions of high
cross-helicity than reverse transfer. This conforms with recent observations of
solar wind turbulence. For specific helical interactions the relation to dynamo
action is established.",1508.05528v2
2015-11-05,Spinor Bose-Einstein gases,"In a spinor Bose-Einstein gas, the non-zero hyperfine spin of the gas becomes
an accessible degree of freedom. At low temperature, such a gas shows both
magnetic and superfluid order, and undergoes both density and spin dynamics.
These lecture notes present a general overview of the properties of spinor
Bose-Einstein gases. The notes are divided in five sections. In the first, we
summarize basic properties of multi-component quantum fluids, focusing on the
specific case of spinor Bose-Einstein gases and the role of rotational symmetry
in defining their properties. Second, we consider the magnetic state of a
spinor Bose-Einstein gas, highlighting effects of thermodynamics and
Bose-Einstein statistics and also of spin-dependent interactions between atoms.
In the third section, we discuss methods for measuring the properties of
magnetically ordered quantum gases and present newly developed schemes for
spin-dependent imaging. We then discuss the dynamics of spin mixing in which
the spin composition of the gas evolves through the spin-dependent interactions
within the gas. We discuss spin mixing first from a microscopic perspective,
and then advance to discussing collective and beyond-mean-field dynamics. The
fifth section reviews recent studies of the magnetic excitations of
quantum-degenerate spinor Bose gases. We conclude with some perspectives on
future directions for research.",1511.01575v1
2016-09-02,Abnormal oscillation modes in a waning light bridge,"A sunspot acts as a waveguide in response to the dynamics of the solar
interior; the trapped waves and oscillations could reveal its thermal and
magnetic structures.
  We study the oscillations in a sunspot intruded by a light bridge, the
details of the oscillations could reveal the fine structure of the magnetic
topology.
  We use the Solar Dynamics Observatory/Atmospheric Imaging Assembly data to
analyse the oscillations in the emission intensity of light bridge plasma at
different temperatures and investigate their spatial distributions.
  The extreme ultraviolet emission intensity exhibits two persistent
oscillations at five-minute and sub-minute ranges. The spatial distribution of
the five-minute oscillation follows the spine of the bridge; whereas the
sub-minute oscillations overlap with two flanks of the bridge. Moreover, the
sub-minute oscillations are highly correlated in spatial domain, however, the
oscillations at the eastern and western flanks are asymmetric with regard to
the lag time. In the meanwhile, jet-like activities are only found at the
eastern flank.
  Asymmetries in forms of oscillatory pattern and jet-like activities
\textbf{are} found between two flanks of a granular light bridge. Based on our
study and recent findings, we propose a new model of twisted magnetic field for
a light bridge and its dynamic interactions with the magnetic field of a
sunspot.",1609.00596v1
2016-10-28,Insulating nanomagnets driven by spin torque,"Magnetic insulators, such as yttrium iron garnet (Y$_3$Fe$_5$O$_{12}$), are
ideal materials for ultra-low power spintronics applications due to their low
energy dissipation and efficient spin current generation and transmission.
Recently, it has been realized that spin dynamics can be driven very
effectively in micrometer-sized Y$_3$Fe$_5$O$_{12}$/Pt heterostructures by
spin-Hall effects. We demonstrate here the excitation and detection of spin
dynamics in Y$_3$Fe$_5$O$_{12}$/Pt nanowires by spin-torque ferromagnetic
resonance. The nanowires defined via electron-beam lithography are fabricated
by conventional room temperature sputtering deposition on Gd$_3$Ga$_5$O$_{12 }$
substrates and lift-off. We observe field-like and anti-damping-like torques
acting on the magnetization precession, which are due to simultaneous
excitation by Oersted fields and spin-Hall torques. The Y$_3$Fe$_5$O$_{12}$/Pt
nanowires are thoroughly examined over a wide frequency and power range. We
observe a large change in the resonance field at high microwave powers, which
is attributed to a decreasing effective magnetization due to microwave
absorption. These heating effects are much more pronounced in the investigated
nanostructures than in comparable micron-sized samples. By comparing different
nanowire widths, the importance of geometrical confinements for magnetization
dynamics becomes evident: quantized spin-wave modes across the width of the
wires are observed in the spectra. Our results are the first stepping stones
toward the realization of integrated magnonic logic devices based on
insulators, where nanomagnets play an essential role.",1610.09360v1
2017-03-13,Quantum kinetic equations for the ultrafast spin dynamics of excitons in diluted magnetic semiconductor quantum wells after optical excitation,"Quantum kinetic equations of motion for the description of the exciton spin
dynamics in II-VI diluted magnetic semiconductor quantum wells with laser
driving are derived. The model includes the magnetic as well as the nonmagnetic
carrier-impurity interaction, the Coulomb interaction, Zeeman terms, and the
light-matter coupling, allowing for an explicit treatment of arbitrary
excitation pulses. Based on a dynamics-controlled truncation scheme,
contributions to the equations of motion up to second order in the generating
laser field are taken into account. The correlations between the carrier and
the impurity subsystems are treated within the framework of a correlation
expansion. For vanishing magnetic field, the Markov limit of the quantum
kinetic equations formulated in the exciton basis agrees with existing theories
based on Fermi's golden rule. For narrow quantum wells excited at the $1s$
exciton resonance, numerical quantum kinetic simulations reveal pronounced
deviations from the Markovian behavior. In particular, the spin decays
initially with approximately half the Markovian rate and a non-monotonic decay
in the form of an overshoot of up to $10\,\%$ of the initial spin polarization
is predicted.",1703.04435v3
2017-03-03,Correlations and dynamics of spins in an XY-like spin-glass (Ni0.4Mn0.6)TiO3 single crystal system,"Elastic and inelastic neutron scattering (ENS and INS) experiments were
performed on a single crystal of (Ni0.4Mn0.6)TiO3 (NMTO) to study the spatial
correlations and dynamics of spins in the XY-like spin-glass (SG) state.
Magnetization measurements reveal signatures of SG behavior in NMTO with a
freezing temperature of TSG ~ 9.1 K. The ENS experiments indicated that the
intensity of magnetic diffuse scattering starts to increase around 12 K, which
is close to TSG. Also, spin-spin correlation lengths (zeta) at 1.5 K are
approximately (21) and (73) angstrom in the interlayer and the in-plane
directions, respectively, demonstrating that magnetic correlations in NMTO
exhibit quasi two-dimensional antiferromagnetic order. In addition, critical
exponent (beta) is determined to be 0.37 from the intensity of magnetic diffuse
scattering confirms the XY-like SG state of NMTO. INS results show
quasi-elastic neutron scattering (QENS) profiles below TSG. The life-time of
dynamic correlations obtained from the half width at half maximum of the
Lorentzian QENS profiles, are approximately 16 and 16 ps at 10 K for two
positions (0.00, 0.00, 1.52) and (0.01, 0.01, 1.50), respectively. Therefore,
our experimental findings demonstrate that short-range-ordered
antiferromagnetic clusters with short-lived spin correlations are present in
the XY-like SG state of NMTO.",1703.09773v1
2017-04-02,Electron-magnetohydrodynamic simulations of electron scale current sheet dynamics in the Vineta.II guide field reconnection experiment,"Three dimensional electron-magnetohydrodynamic (EMHD) simulations of electron
current sheet dynamics in a background of stationary and unmagnetized ions and
the subsequent generation of electromagnetic fluctuations are carried out. The
physical parameters and initial magnetic configuration in the simulations are
chosen to be similar to those in the \textsc{Vineta}.II magnetic reconnection
experiment. Consistent with the experimental results, our 3D EMHD simulations
show the formation of an elongated electron scale current sheet together with
the excitation of electromagnetic fluctuations within this sheet. The
fluctuations in the simulations are generated by an electron shear flow
instability growing on the in-plane (perpendicular to the direction of the main
current in the sheet) electron shear flow (or current) developed during the
current sheet evolution. Similar to the experiments, the magnetic field
fluctuations perpendicular to the guide magnetic field exhibit a broadband
frequency spectrum following a power law and a positive correlation with the
axial current density. Although the experimental results show that ions
influence the spectral properties of the fluctuations, the simulations suggest
that the electron dynamics, even in the absence of ion motion, primarily
determines the formation of the current sheet and the generation of
electromagnetic fluctuations observed in the experiments.",1704.00287v2
2018-04-10,"Effects of a strong magnetic field on internal gravity waves: trapping, phase mixing, reflection and dynamical chaos","The spectrum of oscillation modes of a star provides information not only
about its material properties (e.g. mean density), but also its symmetries.
Spherical symmetry can be broken by rotation and/or magnetic fields. It has
been postulated that strong magnetic fields in the cores of some red giants are
responsible for their anomalously weak dipole mode amplitudes (the ""dipole
dichotomy"" problem), but a detailed understanding of how gravity waves interact
with strong fields is thus far lacking. In this work, we attack the problem
through a variety of analytical and numerical techniques, applied to a
localised region centred on a null line of a confined axisymmetric magnetic
field which is approximated as being cylindrically symmetric. We uncover a rich
variety of phenomena that manifest when the field strength exceeds a critical
value, beyond which the symmetry is drastically broken by the Lorentz force.
When this threshold is reached, the spatial structure of the g-modes becomes
heavily altered. The dynamics of wave packet propagation transitions from
regular to chaotic, which is expected to fundamentally change the organisation
of the mode spectrum. In addition, depending on their frequency and the
orientation of field lines with respect to the stratification, waves impinging
on different parts of the magnetised region are found to undergo either
reflection or trapping. Trapping regions provide an avenue for energy loss
through Alfven wave phase mixing. Our results may find application in various
astrophysical contexts, including the dipole dichotomy problem, the solar
interior, and compact star oscillations.",1804.03664v1
2018-04-26,Large voltage tuning of Dzyaloshinskii-Moriya Interaction: a route towards dynamic control of skyrmion chirality,"Electric control of magnetism is a prerequisite for efficient and low power
spintronic devices. More specifically, in heavy metal/ ferromagnet/ insulator
heterostructures, voltage gating has been shown to locally and dynamically tune
magnetic properties like interface anisotropy and saturation magnetization.
However, its effect on interfacial Dzyaloshinskii-Moriya Interaction (DMI),
which is crucial for the stability of magnetic skyrmions, has been challenging
to achieve and has not been reported yet for ultrathin films. Here, we
demonstrate 130% variation of DMI with electric field in Ta/FeCoB/TaOx
trilayers through Brillouin Light Spectroscopy (BLS). Using polar-
Magneto-Optical-Kerr-Effect microscopy, we further show a monotonic variation
of DMI and skyrmionic bubble size with electric field, with an unprecedented
efficiency. We anticipate through our observations that a sign reversal of DMI
with electric field is possible, leading to a chirality switch. This dynamic
manipulation of DMI establishes an additional degree of control to engineer
programmable skyrmion based memory or logic devices.",1804.09955v2
2018-08-23,Relativity and Synchrotron Radiation: Critical Reexamination of Existing Theory,"Maxwell's equations are valid only in Lorentz frame i.e. in inertial frame
where the Einstein synchronization procedure is used to assign values of the
time coordinate. Einstein time order must be applied and kept in consistent way
in both dynamics and electrodynamics. However, the usual for accelerator
engineering non-covariant treatment of relativistic particle dynamics in a
constant magnetic field looks precisely the same as in non-relativistic
Newtonian dynamics. According to both treatments, the magnetic field is only
capable of altering the direction of motion, but not the speed of an electron.
However, the non-covariant trajectory does not include relativistic kinematics
effects. The covariant electron trajectory is viewed from the Lorentz lab frame
as a result of successive infinitesimal Lorentz transformations. One of the
consequences of non-commutativity of non-collinear Lorentz boosts is a
difference between covariant and non-covariant single particle trajectories in
a constant magnetic field. One can see that this essential point has never
received attention in the physical community. As a result a correction of the
conventional radiation theory is required. In this paper we present a critical
reexamination of existing synchrotron radiation theory. The main emphasis of
this paper is on spontaneous synchrotron radiation from bending magnets and
undulators.",1808.07808v1
2019-02-21,Effects of the removal of Ta capping layer on the magnetization dynamics of Permalloy thin films,"We have investigated the spin wave dynamics of Permalloy (Py) thin films with
and without a Ta capping layer for varying Py thickness (15 nm, 20 nm and 30
nm) using all optical time-resolved magneto-optical Kerr effect measurements.
XPS measurements confirm the oxidation of the originally-prepared samples and
also that the removal of the Ta capping layer is achievable by a few sputtering
cycles. The magnetic field strength dependencies of the spin wave modes with
the variation of the Py film thickness for the samples are studied. We observe
that the presence of the Ta capping layer reduces the precessional frequencies
of the samples while the samples without a Ta capping layer enhance the role of
Py thickness. We also observe that the decay time of spin waves is highly
dependent on the top layer of the samples. The decay time increases with
increasing Py thicknesses for Ta/Py/Ta samples implying that the enhancement of
decay time is caused by the Ta/Py/Ta interfaces. Whereas, for Ta/Py samples the
decay time decreases with increasing Py thickness. The results of this work
extend the knowledge on the magnetization dynamics of Py thin films giving
information on how to resume and even enhance the spin mobility after a
deleterious oxidation process. This can open new scenarios on the building
process and on the maintenance of fast magnetic switching devices.",1902.07832v1
2019-03-15,The dynamics of AR 12700 in its early emerging phase I: interchange reconnection,"The emergence of active regions (ARs) leads to various dynamic activities.
Using high-resolution and long-lasting H$\alpha$ observations from the New
Vacuum Solar Telescope, we report the dynamics of NOAA AR 12700 in its emerging
phase on 26 February 2018 in detail. In this AR, constant interchange
reconnections between emerging fibrils and preexisting ones were detected.
Driven by the flux emergence, small-scale fibrils observed in H$\alpha$
wavelength continuously emerged at the center of the AR and reconnected with
the ambient preexisting fibrils, forming new longer fibrils. We investigate
three scenarios of such interchange reconnection in two hours. Specially, the
third scenario of reconnection resulted in the formation of longer fibrils that
show pronounced rotation motion. To derive the evolution of the magnetic
structure during the reconnections, we perform nonlinear force-free field
extrapolations. The extrapolated three-dimensional magnetic fields clearly
depict a set of almost potential emerging loops, two preexisting flux ropes at
03:00 UT before the second reconnection scenario, and a set of newly formed
loops with less twist at 03:48 UT after the third reconnection scenario. All of
these extrapolated structures are consistent with the fibrils detected in
H$\alpha$ wavelength. The aforementioned observations and extrapolation results
suggest that the constant interchange reconnections resulted in that the
magnetic twist was redistributed from preexisting flux ropes towards the
newly-formed system with longer magnetic structure and weaker twist.",1903.06352v1
2019-10-02,Stability and dynamics of in-plane skyrmions in collinear ferromagnets,"We study the emergence and dynamics of in-plane skyrmions in collinear
ferromagnetic heterostructures. We present a minimal energy model for this
class of magnetic textures, determine the crystal symmetries compatible with it
and propose material candidates, based on symmetries only, for the observation
of these topological solitons. We calculate exact solutions of the energy model
for in-plane skyrmions in the absence of dipolar interactions at critical
coupling, the latter defined by the relations $H = K$ and $D = \sqrt{AK}$ for
the strength of the external magnetic field and the Dzyaloshinskii coupling
constant, respectively, with $K$ and $A$ being the anisotropy constant and the
exchange stiffness of the material. Through micromagnetic simulations, we
demonstrate the possibility of in-plane skyrmion production via i) the motion
of domain walls through a geometrical constriction and ii) shedding from a
magnetic impurity driven both by spin-transfer torques. In-plane skyrmion
dynamics triggered by spin-orbit torques are also investigated analytically and
numerically. Our findings point towards the possibility of designing racetracks
for in-plane skyrmions, whose speed could be tuned by adjusting the angle
between the charge current and the uniform background magnetization; in
particular, the speed is maximum for currents parallel to the easy axis and
becomes zero for currents transverse to it.",1910.00987v2
2019-11-23,Dynamics of interacting particle systems: Modeling implications of the repulsive interactions and experiments on magnetic prototypes,"In this work, we investigate the dynamics of interacting particle systems
subjected to repulsive forces, such as lattices of magnetized particles. To
this end, we first develop a general model capable of capturing the complete
dynamical behavior of interacting particle systems governed by arbitrary
potentials. The model elucidates the important role played by the static
repulsive forces exchanged between particles in the initial equilibrium
configuration, which is distilled and mathematically captured by a dedicated
component of the stiffness matrix. The implications of the model are then
examined through the simple illustrative example of a magnetic particle
oscillator, by which we show that the effect associated with the initial static
forces is germane to two- or higher-dimensional particle systems and vanishes
for 1D chains. In the context of wave propagation, we show that this type of
effect manifests as modal-selective corrections of the dispersion relation of
2D repulsive lattices. To corroborate these findings, we perform laser
vibrometry experiments on a lattice prototype consisting of a triangular grid
of magnets supported by an elastic foundation of thin pillars. The tests
unequivocally confirm the emergence of distinctive dispersive regimes in
quantitative accordance to the model.",1911.10410v3
2020-04-08,Spin dynamics and a nearly continuous magnetic phase transition in an entropy-stabilized oxide antiferromagnet,"The magnetic order and the spin dynamics in the antiferromagnetic
entropy-stabilized oxide (Mg$_{0.2}$Co$_{0.2}$Ni$_{0.2}$Cu$_{0.2}$Zn$_{0.2}$)O
(MgO-ESO) have been studied using muon spin relaxation ($\mu$SR) and inelastic
neutron scattering. We find that antiferromagnetic order develops gradually in
the sample volume as it is cooled below 140 K, becoming fully ordered around
100 K. The spin dynamics show a critical slowing down in the vicinity of the
transition, and the magnetic order parameter grows continuously in the ordered
state. These results indicate that the antiferromagnetic transition is
continuous but proceeds with a Gaussian distribution of ordering temperatures.
The magnetic contribution to the specific heat determined from inelastic
neutron scattering likewise shows a broad feature centered around 120 K.
High-resolution inelastic neutron scattering further reveals an initially
gapped spectrum at low temperature which sees an increase in a quasielastic
contribution upon heating until the ordering temperature.",2004.04218v2
2020-08-31,Dynamic State Analysis of a Driven Magnetic Pendulum using Ordinal Partition Networks and Topological Data Analysis,"The use of complex networks for time series analysis has recently shown to be
useful as a tool for detecting dynamic state changes for a wide variety of
applications. In this work, we implement the commonly used ordinal partition
network to transform a time series into a network for detecting these state
changes for the simple magnetic pendulum. The time series that we used are
obtained experimentally from a base-excited magnetic pendulum apparatus, and
numerically from the corresponding governing equations.The magnetic pendulum
provides a relatively simple, non-linear example demonstrating transitions from
periodic to chaotic motion with the variation of system parameters. For our
method, we implement persistent homology, a shape measuring tool from
Topological Data Analysis (TDA), to summarize the shape of the resulting
ordinal partition networks as a tool for detecting state changes. We show that
this network analysis tool provides a clear distinction between periodic and
chaotic time series. Another contribution of this work is the successful
application of the networks-TDA pipeline, for the first time, to signals from
non-autonomous nonlinear systems. This opens the door for our approach to be
used as an automatic design tool for studying the effect of design parameters
on the resulting system response. Other uses of this approach include fault
detection from sensor signals in a wide variety of engineering operations.",2009.02149v1
2020-09-18,Floquet dynamical quantum phase transition in the extended XY model: nonadiabatic to adiabatic topological transition,"We investigate both pure and mixed states Floquet dynamical quantum phase
transition (DQPT) in the periodically time-dependent extended XY model. We
exactly show that the proposed Floquet Hamiltonian of interacting spins can be
expressed as a sum of noninteracting quasi-spins imposed by an effective time
dependent magnetic field (Schwinger-Rabi model). The calculated Chern number
indicates that there is a topological transition from nonadiabatic to adiabatic
regime. In the adiabatic regime, the quasi-spins trace the time dependent
effective magnetic field and then oscillate between spin up and down states.
While in the nonadiabatic regime, the quasi-spins cannot follow the time
dependent effective magnetic field and feel an average magnetic field. We find
the range of driving frequency over which the quasi-spins experience adiabatic
cyclic processes. Moreover, we obtain the exact expression of the Loschmidt
amplitude and generalized Loschmidt amplitude of the proposed Floquet system.
The results represent that both pure and mixed states dynamical phase
transition occurs when the system evolves adiabatically. In other words, the
minimum required driving frequency for the appearance of Floquet DQPT is equal
to the threshold frequency needed for transition from nonadiabatic to adiabatic
regime.",2009.09008v1
2020-10-15,Spin injection characteristics of Py/graphene/Pt by gigahertz and terahertz magnetization dynamics driven by femtosecond laser pulse,"Spin transport characteristics of graphene has been extensively studied so
far. The spin transport along c-axis is however reported by rather limited
number of papers. We have studied spin transport characteristics through
graphene along c-axis with permalloy(Py)/graphene(Gr)/Pt by gigahertz (GHz) and
terahertz (THz) magnetization dynamics driven by femtosecond laser pulses. The
relatively simple sample structure does not require electrodes on the sample.
The graphene layer was prepared by chemical vapor deposition and transferred on
Pt film. The quality of graphene layer was characterized by Raman microscopy.
Time resolved magneto-optical Kerr effect is used to characterize gigahertz
magnetization dynamics. Magnetization precession is clearly observed both for
Pt/Py and Pt/Gr/Py. The Gilbert damping constant of Pt/Py was 0.015, indicates
spin pumping effect from Py to Pt. The Gilbert damping constant of Pt/Gr/Py is
found to be 0.011, indicates spin injection is blocked by graphene layer. We
have also performed the measurement of THz emission for Pt/Py and Pt/Gr/Py.
While the THz emission is clearly observed for Pt/Py, a strong reduction of THz
emission is observed for Pt/Gr/Py. With these two different experiments, and
highly anisotropic resistivity of graphite, we conclude that the vertical spin
transport is strongly suppressed by the graphene layer.",2010.07694v1
2020-11-18,Domain Wall Motion in Magnetic Nanostrips,"Domain walls are the transition regions between two magnetic domains. These
objects have been very relevant during the last decade, not only due to their
intrinsic interest in the development of novel spintronics devices but also
because of their fundamental interest. The study of domain wall has been linked
to the research on novel spin-orbit coupling phenomena such as the
Dzyaloshinskii-Moriya interaction and the spin Hall effect amount others.
Domain walls can be nucleated in ferromagnetic nanostrips and can be driven by
conventional magnetic fields and spin currents due to the injection of
electrical pulses, which make them very promising for technological
applications of recording and logic devices. In this review, based on full
micromagnetic simulations supported by extended one-dimensional models, we
describe the static and dynamic properties of domain walls in thin
ferromagnetic and ferrimagnetic wires with perpendicular magnetic anisotropy.
The present chapter aims to provide a fundamental theoretical description of
the fundaments of domain walls, and the numerical tools and models which allow
describing the DW dynamics in previous and future experimental setups.",2011.09423v2
2020-12-08,Stable and unstable trajectories in a dipolar chain,"In classical mechanics, solutions can be classified according to their
stability. Each of them is part of the possible trajectories of the system.
However, the signatures of unstable solutions are hard to observe in an
experiment, and most of the times if the experimental realization is adiabatic,
they are considered just a nuisance. Here we use a small number of XY magnetic
dipoles subject to an external magnetic field for studying the origin of their
collective magnetic response. Using bifurcation theory we have found all the
possible solutions being stable or unstable, and explored how those solutions
are naturally connected by points where the symmetries of the system are lost
or restored. Unstable solutions that reveal the symmetries of the system are
found to be the culprit that shape hysteresis loops in this system. The
complexity of the solutions for the nonlinear dynamics is analyzed using the
concept of boundary basin entropy, finding that the damping time scale is
critical for the emergence of fractal structures in the basins of attraction.
Furthermore, we numerically found domain wall solutions that are the smallest
possible realizations of transverse walls and vortex walls in magnetism. We
experimentally confirmed their existence and stability showing that our system
is a suitable platform to study domain wall dynamics at the macroscale.",2012.04173v1
2020-12-14,Homologous flaring activity over a sunspot light bridge in an emerging active region,"Sunspot light bridges are known to exhibit a variety of dynamic and
persistent phenomena such as surges, small-scale jets etc. in the chromosphere
and transition region. While it has generally been proposed that magnetic
reconnection is responsible for this small-scale dynamism, persistent flaring
activity lasting several hours from the same spatial location on a sunspot
light bridge, has rarely been reported. We combine observations from the
Atmospheric Imaging Assembly and the Helioseismic Magnetic Imager on board the
Solar Dynamics Observatory to investigate homologous flaring activity over a
small sunspot light bridge in an emerging flux region. The homologous flares
all produced broad, collimated jets, including a B6.4 class flare. The jets
rise at a speed of about 200 km/s, reach projected heights of about 98 Mm, and
emerge from the same spatial location for nearly 14 hrs, after which they cease
completely. A non-linear force free extrapolation of the photospheric magnetic
field shows a low-lying flux rope connecting the light bridge to a remote
opposite-polarity network. The persistent flares occur as a result of the rapid
horizontal motion of the leading sunspot that causes the relatively vertical
magnetic fields in the adjacent umbra to reconnect with the low-lying flux rope
in the light bridge. Our results indicate that the flaring ceases once the flux
rope has lost sufficient twist through repeated reconnections.",2012.07454v1
2021-03-30,Photoinduced Transient States of Antiferromagnetic Orderings in La${}_{1/3}$Sr${}_{2/3}$FeO${}_{3}$ and SrFeO${}_{3}$ Thin Films Observed through Time-resolved Resonant Soft X-ray Scattering,"The relationship between the magnetic interaction and photoinduced dynamics
in antiferromagnetic perovskites is investigated in this study. In
La${}_{1/3}$Sr${}_{2/3}$FeO${}_{3}$ thin films, commensurate spin ordering is
accompanied by charge disproportionation, whereas SrFeO${}_{3}$ thin films show
incommensurate helical antiferromagnetic spin ordering due to increased
ferromagnetic coupling compared to La${}_{1/3}$Sr${}_{2/3}$FeO${}_{3}$. To
understand the photoinduced spin dynamics in these materials, we investigate
the spin ordering through time-resolved resonant soft X-ray scattering. In
La${}_{1/3}$Sr${}_{2/3}$FeO${}_{3}$, ultrafast quenching of the magnetic
ordering within 130 fs through a nonthermal process is observed, triggered by
charge transfer between the Fe atoms. We compare this to the photoinduced
dynamics of the helical magnetic ordering of SrFeO${}_{3}$. We find that the
change in the magnetic coupling through optically induced charge transfer can
offer an even more efficient channel for spin-order manipulation.",2103.16038v1
2021-03-30,Magnetic Fluctuations in Gyrokinetic Simulations of Tokamak Scrape-Off Layer Turbulence,"Understanding turbulent transport physics in the tokamak edge and scrape-off
layer (SOL) is critical to developing a successful fusion reactor. The dynamics
in these regions plays a key role in achieving high fusion performance by
determining the edge pedestal that suppresses turbulence in H-mode.
Additionally, the survivability of a reactor is set by the heat load to the
vessel walls, making it important to understand turbulent spreading of heat as
it flows along open magnetic field lines in the SOL. Large-amplitude
fluctuations, magnetic X-point geometry, and plasma interactions with material
walls make simulating turbulence in the edge/SOL more challenging than in the
core region, necessitating specialized gyrokinetic codes. Further, the
inclusion of electromagnetic effects in gyrokinetic simulations that can handle
the unique challenges of the boundary plasma is critical to the understanding
of phenomena such as the pedestal and ELMs, for which electromagnetic dynamics
are expected to be important.
  In this thesis, we develop the first capability to simulate electromagnetic
gyrokinetic turbulence on open magnetic field lines. This is an important step
towards comprehensive electromagnetic gyrokinetic simulations of the coupled
edge/SOL system. By using a continuum full-f approach via an energy-conserving
discontinuous Galerkin (DG) discretization scheme that avoids the Ampere
cancellation problem, we show that electromagnetic fluctuations can be handled
in a robust, stable, and efficient manner in the gyrokinetic module of the
Gkeyll code. We then present results which roughly model the scrape-off layer
of the National Spherical Torus Experiment (NSTX), and show that
electromagnetic effects can affect blob dynamics and transport. We also
formulate the gyrokinetic system in field-aligned coordinates for modeling
realistic edge and scrape-off layer geometries in experiments.",2103.16062v1
2021-06-09,Spin dynamics slowdown near the antiferromagnetic critical point in atomically thin FePS3,"Two-dimensional (2D) magnetic materials have attracted much recent interest
with unique properties emerging at the few-layer limit. Beyond the reported
impacts on the static magnetic properties, the effects of reducing the
dimensionality on the magnetization dynamics are also of fundamental interest
and importance for 2D device development. In this report, we investigate the
spin dynamics in atomically-thin antiferromagnetic FePS3 of varying layer
numbers using ultrafast pump-probe spectroscopy. Following the absorption of an
optical pump pulse, the time evolution of the antiferromagnetic order parameter
is probed by magnetic linear birefringence. We observe a strong divergence in
the demagnetization time near the N\'eel temperature. The divergence can be
characterized by a power-law dependence on the reduced temperature, with an
exponent decreasing with sample thickness. We compare our results to
expectations from critical slowing down and a two-temperature model involving
spins and phonons, and discuss the possible relevance of spin-substrate phonon
interactions.",2106.05159v1
2021-06-23,Alignment of the magnetic field in star forming regions and why it might be difficult to observe,"Magnetic fields are an important component of the interstellar medium (ISM)
and exhibit strongly varying field strengths and a non-trivial correlation with
the gas density. Its dynamical impact varies between individual regions of the
ISM and correlates with the orientation of the field with respect to the gas
structures. Using high-resolution magneto-hydrodynamical simulations of the ISM
we explore the connection between the orientation of the field and the
dynamical state of the gas. We find that the onset of gravitational instability
in molecular gas above a density of $\rho\sim10^{-21}\,\mathrm{g\,cm^{-3}}$
$(n\sim400\,\mathrm{cm^{-3}})$ coincides with an alignment of the magnetic
field lines and the gas flow. At this transition the gradient of the density
changes from mainly perpendicular to preferentially parallel to the field
lines. A connection between the three-dimensional alignment and projected
two-dimensional observables is non-trivial, because of a large dispersion of
the magnetic field orientation along the line of sight. The turbulent
correlation lengths can be small compared to the typical integration lengths.
As a consequence the small scale signal of the orientation can sensitively
depend on the line of sight or the dynamical state of the cloud, can fluctuate
stochastically or be completely averaged out. With higher spatial resolution
more small scale structures are resolved, which aggravates the link between
magneto-hydrodynamical quantities and projected observables.",2106.12596v3
2021-06-25,"Ultraviolet Line Profiles of Slowly Rotating Massive Star Winds Using the ""Analytic Dynamical Magnetosphere"" Formalism","Recent large-scale spectropolarimetric surveys have established that a small
but significant percentage of massive stars host stable, surface dipolar
magnetic fields with strengths on the order of kG. These fields channel the
dense, radiatively driven stellar wind into circumstellar magnetospheres, whose
density and velocity structure can be probed using ultraviolet (UV)
spectroscopy of wind-sensitive resonance lines. Coupled with appropriate
magnetosphere models, UV spectroscopy provides a valuable way to investigate
the wind-field interaction, and can yield quantitative estimates of the wind
parameters of magnetic massive stars. We report a systematic investigation of
the formation of UV resonance lines in slowly rotating magnetic massive stars
with dynamical magnetospheres. We pair the Analytic Dynamical Magnetosphere
(ADM) formalism with a simplified radiative transfer technique to produce
synthetic UV line profiles. Using a grid of models, we examine the effect of
magnetosphere size, the line strength parameter, and the cooling parameter on
the structure and modulation of the line profile. We find that magnetic massive
stars uniquely exhibit redshifted absorption at most viewing angles and
magnetosphere sizes, and that significant changes to the shape and variation of
the line profile with varying line strengths can be explained by examining the
individual wind components described in the ADM formalism. Finally, we show
that the cooling parameter has a negligible effect on the line profiles.",2106.13676v1
2021-07-23,Generalized Bloch model: a theory for pulsed magnetization transfer,"Purpose: The paper introduces a classical model to describe the dynamics of
large spin-1/2 ensembles associated with nuclei bound in large molecule
structures, commonly referred to as the semi-solid spin pool, and their
magnetization transfer (MT) to spins of nuclei in
  Theory and Methods: Like quantum-mechanical descriptions of spin dynamics and
like the original Bloch equations, but unlike existing MT models, the proposed
model is based on the algebra of angular momentum in the sense that it
explicitly models the rotations induced by radio-frequency (RF) pulses. It
generalizes the original Bloch model to non-exponential decays, which are,
e.g., observed for semi-solid spin pools. The combination of rotations with
non-exponential decays is facilitated by describing the latter as Green's
functions, comprised in an integro-differential equation.
  Results: Our model describes the data of an inversion-recovery
magnetization-transfer experiment with varying durations of the inversion pulse
substantially better than established models. We made this observation for all
measured data, but in particular for pulse durations small than 300$\mu$s.
Furthermore, we provide a linear approximation of the generalized Bloch model
that reduces the simulation time by approximately a factor 15,000, enabling
simulation of the spin dynamics caused by a rectangular RF-pulse in roughly
2$\mu$s.
  Conclusion: The proposed theory unifies the original Bloch model, Henkelman's
steady-state theory for magnetization transfer, and the commonly assumed
rotation induced by hard pulses (i.e., strong and infinitesimally short
applications of RF fields) and describes experimental data better than previous
models.",2107.11000v3
2022-10-04,Terahertz electric-field driven dynamical multiferroicity in SrTiO$_3$,"The emergence of collective order in matter is among the most fundamental and
intriguing phenomena in physics. In recent years, the ultrafast dynamical
control and creation of novel ordered states of matter not accessible in
thermodynamic equilibrium is receiving much attention. Among those, the
theoretical concept of dynamical multiferroicity has been introduced to
describe the emergence of magnetization by means of a time-dependent electric
polarization in non-ferromagnetic materials. In simple terms, a large amplitude
coherent rotating motion of the ions in a crystal induces a magnetic moment
along the axis of rotation. However, the experimental verification of this
effect is still lacking. Here, we provide evidence of room temperature
magnetization in the archetypal paraelectric perovskite SrTiO$_3$ due to this
mechanism. To achieve it, we resonantly drive the infrared-active soft phonon
mode with intense circularly polarized terahertz electric field, and detect a
large magneto-optical Kerr effect. A simple model, which includes two coupled
nonlinear oscillators whose forces and couplings are derived with ab-initio
calculations using self-consistent phonon theory at a finite temperature,
reproduces qualitatively our experimental observations on the temporal and
frequency domains. A quantitatively correct magnitude of the effect is obtained
when one also considers the phonon analogue of the reciprocal of the Einsten -
de Haas effect, also called the Barnett effect, where the total angular
momentum from the phonon order is transferred to the electronic one. Our
findings show a new path for designing ultrafast magnetic switches by means of
coherent control of lattice vibrations with light.",2210.01690v1
2022-11-17,Delayed Hopf bifurcation and control of a ferrofluid interface via a time-dependent magnetic field,"A ferrofluid droplet confined in a Hele-Shaw cell can be deformed into a
stably spinning ``gear,'' using crossed magnetic fields. Previously, fully
nonlinear simulation revealed that the spinning gear emerges as a stable
traveling wave along the droplet's interface bifurcates from the trivial
(equilibrium) shape. In this work, a center manifold reduction is applied to
show the geometrical equivalence between a two-harmonic-mode coupled system of
ordinary differential equations arising from a weakly nonlinear analysis of the
interface shape and a Hopf bifurcation. The rotating complex amplitude of the
fundamental mode saturates to a limit circle as the periodic traveling wave
solution is obtained. An amplitude equation is derived from a
multiple-time-scale expansion as a reduced model of the dynamics. Then,
inspired by the well-known delay behavior of time-dependent Hopf bifurcations,
we design a slowly time-varying magnetic field such that the timing and
emergence of the interfacial traveling wave can be controlled. The proposed
theory allows us to determine the time-dependent saturated state resulting from
the dynamic bifurcation and delayed onset of instability. The amplitude
equation also reveals hysteresis-like behavior upon time reversal of the
magnetic field. The state obtained upon time reversal differs from the state
obtained during the initial (forward-time) period, yet it can still be
predicted by the proposed reduced-order theory.",2211.09758v3
2023-09-20,Magic Angle Spinning Effects on Longitudinal NMR Relaxation: 15N in L-Histidine,"Solid-state magnetic resonance is a unique technique that can reveal the
dynamics of complex biological systems with atomic resolution. Longitudinal
relaxation is a mechanism that returns longitudinal nuclear magnetization to
its thermal equilibrium by incoherent processes. The measured longitudinal
relaxation rate constant however represents the combination of both incoherent
and coherent contributions to the change of nuclear magnetization. This work
demonstrates the effect of magic angle spinning rate on the longitudinal
relaxation rate constant in two model compounds: L-histidine hydrochloride
monohydrate and glycine serving as proxies for isotopically-enriched biological
materials. Most notably, it is demonstrated that the longitudinal 15N
relaxation of the two nitrogen nuclei in the imidazole ring in histidine is
reduced by almost three orders of magnitude at the condition of rotational
resonance with the amine, while the amine relaxation rate constant is increased
at these conditions. The observed phenomenon may have radical implications for
the solid-state magnetic resonance in biophysics and materials, especially in
the proper measurement of dynamics and as a selective serial transfer step in
dynamic nuclear polarization.",2310.03029v3
2023-12-01,Volkov solutions for relativistic magnetized plasma in strong field quantum electrodynamics regime,"This study shows the dynamics of relativistic electrons in terms of Dirac
equation solutions when an ultra-intense short laser pulse of intensity $\ge
10^{23} {W.cm^{-2}}$ propagates through magnetized dense plasma ($B_0\approx
{1MG})$. The interaction dynamics is analyzed near the strong-field quantum
electrodynamics (SF-QED) regime. Our study finds new solutions in plasma media
considering the effects of the re-normalized mass of relativistic electrons and
the nonzero effective mass of accelerated photons. We have provided a general
method for constructing exact solutions of the Dirac relativistic equation that
correctly explains the dynamics of electrons in the strongly magnetized plasma
medium. The modified solutions of the Dirac equation for one electron are
derived and compared to the Volkov solutions. The new solutions are a basis for
a feasible explanation of quantum attributes of relativistic electrons in a
strong electromagnetic field of very short ultra-intense laser pulses with
intensity near Schwinger field intensity. The solutions are called new Volkov
solutions in a plasma medium. These solutions can be used to understand better
the theory of quantum radiation reaction for the next-generation laser-plasma
accelerator. Our results show that the Volkov solutions are not applicable in a
magnetized plasma medium",2312.00493v1
2024-02-01,"Element-specific and high-bandwidth ferromagnetic resonance spectroscopy with a coherent, extreme ultraviolet (EUV) source","We developed and applied a tabletop, ultrafast, high-harmonic generation
(HHG) source to measure the element-specific ferromagnetic resonance (FMR) in
ultra-thin magnetic alloys and multilayers on an opaque Si substrate. We
demonstrate a continuous wave bandwidth of 62 GHz, with promise to extend to
100 GHz or higher. This laboratory-scale instrument detects the FMR using
ultrafast, extreme ultraviolet (EUV) light, with photon energies spanning the
M-edges of most relevant magnetic elements. An RF frequency comb generator is
used to produce a microwave excitation that is intrinsically synchronized to
the EUV pulses with a timing jitter of 1.4 ps or better. We apply this system
to measure the dynamics in a multilayer system as well as Ni-Fe and Co-Fe
alloys. Since this instrument operates in reflection-mode, it is a milestone
toward measuring and imaging the dynamics of the magnetic state and spin
transport of active devices on arbitrary and opaque substrates. The higher
bandwidth also enables measurements of materials with high magnetic anisotropy,
as well as ferrimagnets, antiferromagnets, and short-wavelength (high
wavevector) spinwaves in nanostructures or nanodevices. Furthermore, the
coherence and short wavelength of the EUV will enable extending these studies
using dynamic nanoscale lensless imaging techniques such as coherent
diffractive imaging, ptychography, and holography.",2402.00783v1
2024-03-16,Nonequilibrium tricritical behaviour in anisotropic XY ferromagnet driven by elliptically polarised propagating magnetic field wave,"Three dimensional anisotropic XY ferromagnet driven by elliptically polarized
propagating magnetic field wave has been extensively investigated by Monte
Carlo simulation with Metropolis single spin flip algorithm. Both the effects
of the bilinear exchange type and the single site anisotropies are thoroughly
investigated. The time average magnetisation (over the complete cycle of the
elliptically polarized propagating magnetic field wave) components play the
role of dynamic order parameter. For fixed set of values of the strength of
anisotropy and the field amplitudes, the system has been found to get
dynamically ordered at a pseudocritical temperature. The pseudocritical
temperature of such dynamic nonequilibrium phase transition has been found to
depend on both the strength of anisotropy and the amplitudes of the
elliptically polarized propagating magnetic field wave. A comprehensive phase
diagram is represented here in the form of image plot of the pseudocritical
temperature in the plane formed by the strength of anisotropy and field
amplitudes. Interestingly, this nonequilibrium phase transition has been found
discontinuous (first order) for higher values of the field amplitude and lower
values of the anisotropy. On the other hand, the continuous (second order)
transition has been noticed for lower values of the field amplitude and higher
values of the anisotropy.",2403.10975v2
1999-03-02,Dynamics of Vortices in Two-Dimensional Magnets,"Theories, simulations and experiments on vortex dynamics in
quasi-two-dimensional magnetic materials are reviewed. These materials can be
modelled by the classical two-dimensional anisotropic Heisenberg model with XY
(easy-plane) symmetry. There are two types of vortices, characterized by their
polarization (a second topological charge in addition to the vorticity): Planar
vortices have Newtonian dynamics (even-order equations of motion) and exhibit
strong discreteness effects, while non-planar vortices have non-Newtonian
dynamics (odd-order equations of motion) and smooth trajectories. The influence
of thermal fluctuations on single vortices is investigated. Different types of
noise and damping are discussed and implemented into the microscopic equations
which yields stochastic equations of motion for the vortices. The solutions of
the these equations are compared with Langevin dynamics simulations. Moreover,
noise-induced transitions between opposite polarizations of a vortex are
investigated. For temperatures above the Kosterlitz-Thouless vortex-antivortex
unbinding transition, a phenomenological theory, namely the vortex gas
approach, yields central peaks in the dynamic form factors for the spin
correlations. Such peaks are observed both in combined Monte Carlo- and Spin
Dynamics-Simulations and in inelastic neutron scattering experiments. However,
the assumption of ballistic vortex motion appears questionable.",9903037v1
2014-06-26,Blume-Capel model on cylindrical Ising nanowire with core/shell structure: Existence of a dynamic compensation temperatures,"We present a study, within a mean-field approach, of the kinetics of the
spin-1 Blume-Capel model on cylindrical Ising nanowire in the presence of a
time-dependent oscillating external magnetic field. We employ the Glauber
transition rates to construct the mean-field dynamical equations. We
investigate the thermal behavior of the dynamic order parameters. From these
studies, we obtain the dynamic phase transition (DPT) points. Then, we study
the temperature dependence of the dynamic total magnetization to find the
dynamic compensation points as well as to determine the type of behavior. We
also investigate the effect of a crystal-field interaction and the exchange
couplings between the nearest-neighbor pairs of spins on the compensation
phenomenon and construct the phase diagrams in four different planes. The
dynamic phase diagrams contain paramagnetic (P), ferromagnetic (F), the
antiferromagnetic (AF), and two coexistence or mixed phase regions, namely, the
F + P and AF + P that strongly depend on interaction parameters. The system
also exhibits the compensation temperatures, or the N-, P-, Q-, S- type
behaviors. Furthermore, we also observed two compensation temperatures, namely
W-type behaviors, which this result is compared with some experimental works
and a good overall agreement is found.",1406.6926v1
2017-04-10,Majorana dynamical mean-field study of spin dynamics at finite temperatures in the honeycomb Kitaev model,"A prominent feature of quantum spin liquids is fractionalization of the spin
degree of freedom. Fractionalized excitations have their own dynamics in
different energy scales, and hence, affect finite-temperature ($T$) properties
in a peculiar manner even in the paramagnetic state harboring the quantum spin
liquid state. We here present a comprehensive theoretical study of the spin
dynamics in a wide $T$ range for the Kitaev model on a honeycomb lattice, whose
ground state is such a quantum spin liquid. In this model, the
fractionalization occurs to break up quantum spins into itinerant matter
fermions and localized gauge fluxes, which results in two crossovers at very
different $T$ scales. Extending the previous study for the isotropic coupling
case [J. Yoshitake, J. Nasu, and Y. Motome, Phys. Rev. Lett. ${\textbf 117}$,
157203 (2016)], we calculate the dynamical spin structure factor, the NMR
relaxation rate, and the magnetic susceptibility while changing the anisotropy
in the exchange coupling constants, by using the dynamical mean-field theory
and the continuous-time quantum Monte Carlo method based on a Majorana fermion
representation. We find that the system exhibits peculiar behaviors below the
high-$T$ crossover whose temperature is comparable to the average of the
exchange constants, reflecting the spin fractionalization in the paramagnetic
region. Among them, the dichotomy between the static and dynamical spin
correlations is unusual behavior hardly seen in conventional magnets. We
discuss the relation between the dichotomy and the spatial configuration of
gauge fluxes. Our results could stimulate further experimental and theoretical
analyses of candidate materials for the Kitaev quantum spin liquids.",1704.02707v1
2019-01-03,High Performance Direct-Current Generator Based on Dynamic PN Junctions,"After the electromagnetic generator, searching for novel electric generators
without strong magnetic field is highly demanded. The generator without strong
magnetic field calls for a physical picture distinct from the traditional
generators. As the counterpart of the static PN junction has been widely used
in the integrated circuits, we develop an electric generator named dynamic PN
generator with a high current density and voltage output, which converts
mechanical energy into electricity by sliding two semiconductors with different
Fermi level. A dynamic N-GaAs/SiO2/P-Si generator with the open-circuit voltage
of 3.1 V and short-circuit density of 1.0 A/m2 have been achieved. The physical
mechanism of the dynamic PN generator is proposed based on the built-in
electric field bounding back diffusing carriers in dynamic PN junctions, which
breaks the equilibrium between drift and diffusion current in the PN junction.
Moreover, the dynamic MoS2/AlN/Si generator with the open-circuit voltage of
5.1 V and short-circuit density of 112 A/m2 (11.2 mA/cm2) have also been
achieved, which can effectively output a direct-current and light up a blue
light-emitting diode directly. This dynamic MoS2/AlN/Si generator can
continuously work for hours without obvious degradation, demonstrating its
unique mechanism and potential applications in many fields where the mechanical
energy is available.",1901.00701v1
2019-06-19,Large dynamical axion field in topological antiferromagnetic insulator Mn$_2$Bi$_2$Te$_5$,"The dynamical axion field is a new state of quantum matter where the
magnetoelectric response couples strongly to its low-energy magnetic
fluctuations. It is fundamentally different from an axion insulator with a
static quantized magnetoelectric response. The dynamical axion field exhibits
many exotic phenomena such as axionic polariton and axion instability. However,
these effects have not been experimentally confirmed due to the lack of proper
topological magnetic materials. Here by combining analytic models and
first-principles calculations, we predict a series of van der Waal layered
Mn$_2$Bi$_2$Te$_5$-related topological antiferromagnetic materials could host
the long-sought dynamical axion field with a topological origin. We also show a
large dynamical axion field can be achieved in antiferromagnetic insulating
states close to the topological phase transition. We further propose the
optical and transport experiments to detect such a dynamical axion field. Our
results could directly aid and facilitate the search for topological-origin
large dynamical axion field in realistic materials.",1906.07891v3
2021-09-23,Dynamics in an exact solvable quantum magnet: benchmark for quantum computer,"Quantum magnets are never short of novel and fascinating dynamics, yet its
simulation by classical computers requires exponentially-scaled computation
resources, which renders the research on large-scale many-body dynamics
fiendishly difficult. In this letter, we explore the dynamic behavior of 2D
large-scale ferromagnetic J1-J2 Heisenberg model both theoretically and
experimentally. First, the analytical solution of magnon dynamics is obtained
to show an obvious ballistic propagation of magnon, which is typical for
quantum walk. Then, we verify the dynamic behavior of the system through
numerical approach of exact diagonalization and tensor network method. We also
calculate out-of-time ordered correlators and butterfly velocities among
different lattice points, finding that they can well depict the competition
between different couplings. Finally, a quantum walk experiment is designed and
conducted on the basis of IBM programmable quantum processors, and the
experimental results are in consistence with our theoretical predictions. Since
the analytical results can be used, in principle, to predict the behavior of
large-scale quantum many-body systems and even those infinitely large, this
work will help facilitate further research on quantum walk and quantum
many-body dynamics in large-scale lattice systems, guide future design of
quantum computers, as well as popularize quantum computers until they are known
and available to every household in the world.",2109.11371v5
2023-08-23,State-transition dynamics of resting-state functional magnetic resonance imaging data: Model comparison and test-to-retest analysis,"Electroencephalogram (EEG) microstate analysis entails finding dynamics of
quasi-stable and generally recurrent discrete states in multichannel EEG time
series data and relating properties of the estimated state-transition dynamics
to observables such as cognition and behavior. While microstate analysis has
been widely employed to analyze EEG data, its use remains less prevalent in
functional magnetic resonance imaging (fMRI) data, largely due to the slower
timescale of such data. In the present study, we extend various data clustering
methods used in EEG microstate analysis to resting-state fMRI data from healthy
humans to extract their state-transition dynamics. We show that the quality of
clustering is on par with that for various microstate analyses of EEG data. We
then develop a method for examining test-retest reliability of the
discrete-state transition dynamics between fMRI sessions and show that the
within-participant test-retest reliability is higher than between-participant
test-retest reliability for different indices of state-transition dynamics,
different networks, and different data sets. This result suggests that
state-transition dynamics analysis of fMRI data could discriminate between
different individuals and is a promising tool for performing fingerprinting
analysis of individuals.",2308.11910v2
2023-08-24,Dynamics of quantum spin-nematics: Comparisons with canted antiferromagnets,"The identification of spin-nematic states is challenging due to the absence
of Bragg peaks. However, the study of dynamical physical quantities provides a
promising avenue for characterizing these states. In this study, we investigate
the dynamical properties of spin-nematic states in three-dimensional quantum
spin systems in a magnetic field, using a two-component boson theory that
incorporates magnons and bi-magnons. Our particular focus lies on the dynamical
spin structure factor at zero temperature and the nuclear magnetic resonance
(NMR) relaxation rate at finite temperatures. Our findings reveal that the
dynamical structure factor does not exhibit any diverging singularity across
momentum and frequency while providing valuable information about the form
factor of bi-magnon states and the underlying structure of spin-nematic order.
Furthermore, we find a temperature dependence in the NMR relaxation rate
proportional to $T^3$ at low temperatures, similar to canted antiferromagnets.
A clear distinction arises as there is no critical divergence of the NMR
relaxation rate at the spin-nematic transition temperature. Our theoretical
framework provides a comprehensive understanding of the excitation spectrum and
the dynamical properties of spin-nematic states, covering arbitrary spin values
$S$ and encompassing site and bond nematic orders. Additionally, we apply the
same methodology to analyze these dynamical quantities in a canted
antiferromagnetic state and compare the results with those in the spin-nematic
states.",2308.12569v2
1994-07-21,The Dynamics of Flux Tubes in a High Beta Plasma,"We suggest a new model for the structure of a magnetic field embedded high
$\beta$ turbulent plasma, based on the popular notion that the magnetic field
will tend to separate into individual flux tubes. We point out that
interactions between the flux tubes will be dominated by coherent effects
stemming from the turbulent wakes created as the fluid streams by the flux
tubes. Balancing the attraction caused by shielding effects with turbulent
diffusion we find that flux tubes have typical radii comparable to the local
Mach number squared times the large scale eddy length, are arranged in a one
dimensional fractal pattern, have a radius of curvature comparable to the
largest scale eddies in the turbulence, and have an internal magnetic pressure
comparable to the ambient pressure. When the average magnetic energy density is
much less than the turbulent energy density the radius, internal magnetic field
and curvature scale of the flux tubes will be smaller than these estimates.
Realistic resistivity does not alter the macroscopic properties of the fluid or
the large scale magnetic field. In either case we show that the Sweet-Parker
reconnection rate is much faster than an eddy turnover time. Realistic stellar
plasmas are expected to either be in the ideal limit (e.g. the solar
photosphere) or the resistive limit (most of the solar convection zone). All
current numerical simulations of three dimensional MHD turbulence are in the
viscous regime and are inapplicable to stars or accretion disks.",9407063v1
1997-08-15,Instability of toroidal magnetic field in jets and plerions,"Jets and pulsar-fed supernova remnants (plerions) tend to develop highly
organized toroidal magnetic field. Such a field structure could explain the
polarization properties of some jets, and contribute to their lateral
confinement. A toroidal field geometry is also central to models for the Crab
Nebula - the archetypal plerion - and leads to the deduction that the Crab
pulsar's wind must have a weak magnetic field. Yet this `Z-pinch' field
configuration is well known to be locally unstable, even when the magnetic
field is weak and/or boundary conditions slow or suppress global modes. Thus,
the magnetic field structures imputed to the interiors of jets and plerions are
unlikely to persist.
  To demonstrate this, I present a local analysis of Z-pinch instabilities for
relativistic fluids in the ideal MHD limit. Kink instabilities dominate,
destroying the concentric field structure and probably driving the system
toward a more chaotic state in which the mean field strength is independent of
radius (and in which resistive dissipation of the field may be enhanced). I
estimate the timescales over which the field structure is likely to be
rearranged and relate these to distances along relativistic jets and radii from
the central pulsar in a plerion.
  I conclude that a concentric toroidal field is unlikely to exist well outside
the Crab pulsar's wind termination shock. There is thus no dynamical reason to
conclude that the magnetic energy flux carried by the pulsar wind is much
weaker than the kinetic energy flux. Abandoning this inference would resolve a
long-standing puzzle in pulsar wind theory.",9708142v1
2003-01-22,Understanding Helical Magnetic Dynamo Spectra with a Nonlinear Four-Scale Theory,"Recent MHD dynamo simulations for magnetic Prandtl number $>1$ demonstrate
that when MHD turbulence is forced with sufficient kinetic helicity, the
saturated magnetic energy spectrum evolves from having a single peak below the
forcing scale to become doubly peaked with one peak at the system (=largest)
scale and one at the forcing scale. The system scale field growth is well
modeled by a recent nonlinear two-scale nonlinear helical dynamo theory in
which the system and forcing scales carry magnetic helicity of opposite sign.
But a two-scale theory cannot model the shift of the small-scale peak toward
the forcing scale. Here I develop a four-scale helical dynamo theory which
shows that the small-scale helical magnetic energy first saturates at very
small scales, but then successively saturates at larger values at larger
scales, eventually becoming dominated by the forcing scale. The transfer of the
small scale peak to the forcing scale is completed by the end of the kinematic
growth regime of the large scale field, and does not depend on magnetic
Reynolds number $R_M$ for large $R_M$. The four-scale and two-scale theories
subsequently evolve almost identically, and both show significant field growth
on the system and forcing scales that is independent of $R_M$. In the present
approach, the helical and nonhelical parts of the spectrum are largely
decoupled. Implications for fractionally helical turbulence are discussed.",0301432v2
2003-05-22,Polarization of prompt GRB emission: evidence for electromagnetically-dominated outflow,"Observations by the {\RHESSI} satellite of large polarization of the prompt
$\gamma$-ray emission from the Gamma Ray Burst GRB021206 \citep{coburn03} imply
that the magnetic field coherence scale is larger than the size of the visible
emitting region, $\sim R/\Gamma$, where $R$ is the radius of the flow, $\Gamma$
is the associated Lorentz factor.
  Such fields cannot be generated in a causally disconnected, hydrodynamically
dominated outflow. Electromagnetic models of GRBs \citep{lyutikov02}, in which
large scale, dynamically dominant, magnetic fields are present in the outflow
from the very beginning, provide a natural explanation of this large reported
linear polarization. We derive Stokes parameters of synchrotron emission of a
relativistically moving plasma with a given magnetic field configuration and
calculate the pulse averaged polarization fraction of the emission from a
relativistically expanding shell carrying global toroidal magnetic field. For
viewing angles larger than $1/\Gamma$ the observed patch of the emitting shell
has almost homogeneous magnetic field, producing a large fractional
polarization ($56% $ for a power-law energy distribution of relativistic
particles $dn/d\epsilon \propto \epsilon^{-3}$). The maximum polarization is
smaller than the theoretical upper limit for a stationary plasma in uniform
magnetic field due to relativistic kinematic effects.",0305410v2
2003-07-16,Magnetohydrodynamical Accretion Flows: Formation of Magnetic Tower Jet and Subsequent Quasi-Steady State,"We present three-dimensional (3-D) magnetohydrodynamical (MHD) simulations of
radiatively inefficient accretion flow around black holes. General relativistic
effects are simulated by using the pseudo-Newtonian potential. We start
calculations with a rotating torus threaded by localized poloidal magnetic
fields with plasma beta, a ratio of the gas pressure to the magnetic pressure,
$\beta =10$ and 100. When the bulk of torus material reaches the innermost
region close to a central black hole, a magnetically driven jet emerges. This
magnetic jet is derived by vertically inflating toroidal fields (`magnetic
tower') and has a two-component structure: low-$\beta$ ($\lsim 1$) plasmas
threaded with poloidal (vertical) fields are surrounded by that with toroidal
fields. The collimation width of the jet depends on external pressure, pressure
of ambient medium; the weaker the external pressure is, the wider and the
longer-lasting becomes the jet. Unless the external pressure is negligible, the
bipolar jet phase ceases after several dynamical timescales at the original
torus position and a subsequent quasi-steady state starts. The black hole is
surrounded by quasi-spherical zone with highly inhomogeneous structure in which
toroidal fields are dominant except near the rotation axis. Mass accretion
takes place mainly along the equatorial plane. Comparisons with other MHD
simulation results and observational implications are discussed.",0307306v2
2003-10-24,Synthetic Observations of Simulated Radio Galaxies I: Radio and X-ray Analysis,"We present an extensive synthetic observational analysis of numerically-
simulated radio galaxies designed to explore the effectiveness of conventional
observational analyses at recovering physical source properties. These are the
first numerical simulations with sufficient physical detail to allow such a
study. The present paper focuses on extraction of magnetic field properties
from nonthermal intensity information. Synchrotron and inverse-Compton
intensities provided meaningful information about distributions and strengths
of magnetic fields, although considerable care was called for. Correlations
between radio and X-ray surface brightness correctly revealed useful dynamical
relationships between particles and fields. Magnetic field strength estimates
derived from the ratio of X-ray to radio intensity were mostly within about a
factor of two of the RMS field strength along a given line of sight. When
emissions along a given line of sight were dominated by regions close to the
minimum energy/equipartition condition, the field strengths derived from the
standard power-law-spectrum minimum energy calculation were also reasonably
close to actual field strengths, except when spectral aging was evident.
Otherwise, biases in the minimum- energy magnetic field estimation mirrored
actual differences from equipartition. The ratio of the inverse-Compton
magnetic field to the minimum-energy magnetic field provided a rough measure of
the actual total energy in particles and fields in most instances, within an
order of magnitude. This may provide a practical limit to the accuracy with
which one may be able to establish the internal energy density or pressure of
optically thin synchrotron sources.",0310719v1
2003-12-01,"Magnetic Braking in Differentially Rotating, Relativistic Stars","We study the magnetic braking and viscous damping of differential rotation in
incompressible, uniform density stars in general relativity. Differentially
rotating stars can support significantly more mass in equilibrium than
nonrotating or uniformly rotating stars. The remnant of a binary neutron star
merger or supernova core collapse may produce such a ""hypermassive"" neutron
star. Although a hypermassive neutron star may be stable on a dynamical
timescale, magnetic braking and viscous damping of differential rotation will
ultimately alter the equilibrium structure, possibly leading to delayed
catastrophic collapse. Here we consider the slow-rotation, weak-magnetic field
limit in which E_rot << E_mag << W, where E_rot is the rotational kinetic
energy, E_mag is the magnetic energy, and W is the gravitational binding energy
of the star. We assume the system to be axisymmetric and solve the MHD
equations in both Newtonian gravitation and general relativity. Toroidal
magnetic fields are generated whenever the angular velocity varies along the
initial poloidal field lines. We find that the toroidal fields and angular
velocities oscillate independently along each poloidal field line, which
enables us to transform the original 2+1 equations into 1+1 form and solve them
along each field line independently. The incoherent oscillations on different
field lines stir up turbulent-like motion in tens of Alfven timescales (""phase
mixing""). In the presence of viscosity, the stars eventually are driven to
uniform rotation, with the energy contained in the initial differential
rotation going into heat. Our evolution calculations serve as qualitative
guides and benchmarks for future, more realistic MHD simulations in full 3+1
general relativity.",0312038v1
2004-04-06,Estimating galaxy cluster magnetic fields by the classical and hadronic minimum energy criterion,"We wish to estimate magnetic field strengths of radio emitting galaxy
clusters by minimising the non-thermal energy density contained in cosmic ray
electrons (CRe), protons (CRp), and magnetic fields. The classical minimum
energy estimate can be constructed independently of the origin of the radio
synchrotron emitting CRe yielding thus an absolute minimum of the non-thermal
energy density. Provided the observed synchrotron emission is generated by a
CRe population originating from hadronic interactions of CRp with the ambient
thermal gas of the intra-cluster medium, the parameter space of the classical
scenario can be tightened by means of the hadronic minimum energy criterion.
For both approaches, we derive the theoretically expected tolerance regions for
the inferred minimum energy densities. Application to the radio halo of the
Coma cluster and the radio mini-halo of the Perseus cluster yields
equipartition between cosmic rays and magnetic fields within the expected
tolerance regions. In the hadronic scenario, the inferred central magnetic
field strength ranges from 2.4 muG (Coma) to 8.8 muG (Perseus), while the
optimal CRp energy density is constrained to 2% +/- 1% of the thermal energy
density (Perseus). We discuss the possibility of a hadronic origin of the Coma
radio halo while current observations favour such a scenario for the Perseus
radio mini-halo. Combining future expected detections of radio synchrotron,
hard X-ray inverse Compton, and hadronically induced gamma-ray emission should
allow an estimate of volume averaged cluster magnetic fields and provide
information about their dynamical state.",0404119v2
2005-04-04,Amplification of Interstellar Magnetic Fields and Turbulent Mixing by Supernova-Driven Turbulence Part II - The Role of Dynamical Chaos,"In this paper we further advance the study of magnetic field amplification in
the interstellar medium that was started in Balsara et al (2004, Paper I). We
show that the flux growth rate is comparable to the rate of magnetic energy
growth found in Paper I. We also demonstrate the role of intermittency in field
amplification. The density shows a double-peaked PDF, consistent with the
cooling curve that was used. The PDF of the magnetic field shows a high-end
tail, providing a tell-tale signature of the operation of the small scale
dynamo. The magnetic field strength correlates positively with the density. As
a result, the field amplification takes place more vigorously in the lower
temperature, denser gas. The Lagrangian chaos in the simulated turbulent flows
is studied in substantial detail. It is shown that the stretching rate of
material lines as well as the Lyapunov exponents can be used to gain important
insights into the growth of magnetic field. The cancellation exponent for the
small scale supernova-driven dynamo is derived and it is shown that
constructive folding of field lines in the dynamo is very inefficient. We also
show that our Lagrangian approach can yield actual measures of the turbulent
diffusivity in the simulated ISM. The turbulent diffusivity provides insights
into the mixing of elements from supernova ejecta on macroscopic scales. The
high rates of line stretching in interstellar turbulence suggests that the
eventual diffusion of elements at the molecular level is very efficient. Many
of the diagnostics of turbulence that are presented here can be used to make
direct connections between simulations and observations.",0504065v1
2005-12-16,The Double Helix Nebula: a magnetic torsional wave propagating out of the Galactic centre,"Radioastronomical studies have indicated that the magnetic field in the
central few hundred parsecs of our Milky Way Galaxy has a dipolar geometry and
a strength substantially larger than elsewhere in the Galaxy, with estimates
ranging up to a milligauss. A strong, large-scale magnetic field can affect the
Galactic orbits of molecular clouds by exerting a drag on them, it can inhibit
star formation, and it can guide a wind of cosmic rays away from the central
region, so a characterization of the magnetic field at the Galactic center is
important for understanding much of the activity there. Here, we report Spitzer
Space Telescope observations of an unprecedented infrared nebula having the
morphology of an intertwined double helix. This feature is located about 100 pc
from the Galaxy's dynamical centre toward positive Galactic latitude, and its
axis is oriented perpendicular to the Galactic plane. The observed segment is
about 25 pc in length, and contains about 1.25 full turns of each of the two
continuous, helically wound strands. We interpret this feature as a torsional
Alfven wave propagating vertically away from the Galactic disk, driven by
rotation of the magnetized circumnuclear gas disk. As such, it offers a new
morphological probe of the Galactic center magnetic field. The direct
connection between the circumnuclear disk and the double helix is ambiguous,
but the MSX images show a possible meandering channel that warrants further
investigation.",0512452v1
2006-01-11,"Turbulence, magnetic fields and plasma physics in clusters of galaxies","Observations of galaxy clusters show that the intracluster medium (ICM) is
likely to be turbulent and is certainly magnetized. The properties of this
magnetized turbulence are determined both by fundamental nonlinear
magnetohydrodynamic interactions and by the plasma physics of the ICM, which
has very low collisionality. Cluster plasma threaded by weak magnetic fields is
subject to firehose and mirror instabilities. These saturate and produce
fluctuations at the ion gyroscale, which can scatter particles, increasing the
effective collision rate and, therefore, the effective Reynolds number of the
ICM. A simple way to model this effect is proposed. The model yields a
self-accelerating fluctuation dynamo whereby the field grows explosively fast,
reaching the observed, dynamically important, field strength in a fraction of
the cluster lifetime independent of the exact strength of the seed field. It is
suggested that the saturated state of the cluster turbulence is a combination
of the conventional isotropic magnetohydrodynamic turbulence, characterized by
folded, direction-reversing magnetic fields and an Alfv\'en-wave cascade at
collisionless scales. An argument is proposed to constrain the reversal scale
of the folded field. The picture that emerges appears to be in qualitative
agreement with observations of magnetic fields in clusters.",0601246v2
2006-04-03,A Revised Prescription for the Tayler-Spruit Dynamo: Magnetic Angular Momentum Transport in Stars,"Angular momentum transport by internal magnetic fields is an important
ingredient for stellar interior models. In this paper we critically examine the
basic heuristic assumptions in the model of the Tayler-Spruit dynamo, which
describes how a pinch-type instability of a toroidal magnetic field in
differentially rotating stellar radiative zones may result in large-scale fluid
motion. We agree with prior published work both on the existence of the
instability and its nearly horizontal geometry for perturbations. However, the
approximations in the original Acheson dispersion relation are valid only for
small length scales, and we disagree that the dispersion relation can be
extrapolated to horizontal length scales of order the radius of the star. We
contend that dynamical effects, in particular angular momentum conservation,
limit the maximum horizontal length scale. We therefore present transport
coefficients for chemical mixing and angular momentum redistribution by
magnetic torques that are significantly different from previous published
values. The new magnetic viscosity is reduced by 2 to 3 orders of magnitude
compared to the old one, and we find that magnetic angular momentum transport
by this mechanism is very sensitive to gradients in the mean molecular weight.
The revised coefficients are more compatible with empirical constraints on the
timescale of core-envelope coupling in young stars than the previous ones.
However, solar models including only this mechanism possess a rapidly rotating
core, in contradiction with helioseismic data. (abridged)",0604045v2
2006-04-04,Which jet launching mechanism(s) in TTauri stars?,"AIMS: We examine whether ejection phenomena from accreting T Tauri stars can
be described by only one type of self-collimated jet model. METHODS: We present
analytical kinematic predictions valid soon after the Alfv\'en surface for all
types of steady magnetically self-confined jets. RESULTS: We show that extended
disc winds, X-winds, and stellar winds occupy distinct regions in the poloidal
speed vs. specific angular momentum plane. Comparisons with current
observations of T Tauri jets yield quantitative constraints on the range of
launching radii, magnetic lever arms, and specific energy input in disc and
stellar winds. Implications on the origin of jet asymmetries and disc magnetic
fields are outlined. CONCLUSIONS: We argue that ejection phenomena from
accreting T Tauri stars most likely include three dynamical components: (1) an
outer self-collimated steady disc wind carrying most of the mass-flux in the
optical jet (when present), confining (2) a pressure-driven coronal stellar
wind and (3) a hot inner flow made of blobs sporadically ejected from the
magnetopause. If the stellar magnetic moment is parallel to the disc magnetic
field, then the highly variable inner flow resembles a ""Reconnection X-wind"",
that has been proven to efficiently brake down an accreting and contracting
young star. If the magnetic moment is anti-parallel, then larger versions of
the solar coronal mass ejections are likely to occur. The relative importance
of these three components in the observed outflows and the range of radii
involved in the disc wind are expected to vary with time, from the stage of
embedded source to the optically revealed T Tauri star phase.",0604053v1
2006-05-24,Synchrotron emission in small scale magnetic field as possible explanation for prompt emission spectra of gamma-ray bursts,"Synchrotron emission is believed to be a major radiation mechanism during
gamma-ray bursts' (GRBs) prompt emission phase. A significant drawback of this
assumption is that the theoretical predicted spectrum, calculated within the
framework of the ``internal shocks'' scenario using the standard assumption
that the magnetic field maintains a steady value throughout the shocked region,
leads to a slope F_\nu \propto \nu^{-1/2} below 100 keV, which is in
contradiction to the much harder spectra observed. This is due to the electrons
cooling time being much shorter than the dynamical time. In order to overcome
this problem, we propose here that the magnetic field created by the internal
shocks decays on a length scale much shorter than the comoving width of the
plasma. We show that under this assumption synchrotron radiation can reproduce
the observed prompt emission spectra of the majority of the bursts. We
calculate the required decay length of the magnetic field, and find it to be
\~10^4 - 10^5 cm (equivalent to 10^5 - 10^6 skin depths), much shorter than the
characteristic comoving width of the plasma, ~3*10^{9} cm. We implement our
model to the case of GRB050820A, where a break at <~ 4 keV was observed, and
show that this break can be explained by synchrotron self absorption. We
discuss the consequences of the small scale magnetic field scenario on current
models of magnetic field generation in shock waves.",0605641v2
2006-11-09,Grain Alignment and Polarized Emission from Magnetized T Tauri Disks,"The structure of magnetic fields within protostellar disks may be studied via
polarimetry provided that grains are aligned in respect to magnetic field
within the disks. We explore alignment of dust grains by radiative torque in T
Tauri disks and provide predictions for polarized emission for disks viewed at
different wavelengths and viewing angles. We show that the alignment is
especially efficient in outer part of the disks. In the presence of magnetic
field, these aligned grains produce polarized emission in infrared wavelengths.
We consider a simple model of an accretion disk and provide predictions for
polarization that should be available to both instruments that do not resolve
the disks and future instruments that will resolve the disks. As the surface
magnetic field and the bulk magnetic field play different roles for the disk
dynamics, we consider separately the contributions that arises from the surface
areas of the disk and its interior. We find that the polarized emission drops
for wavelengths shorter than $\sim 10 \mu m$. Between $\sim 10 \mu m$ and $\sim
100 \mu m$, the polarized emission is dominated by the emission from the
surface layer of the disks and the degree of polarization can be as large as
$\sim 10%$ for unresolved disks. The degree of polarization is around 2-3 %
level at wavelengths larger than $\sim100\mu m$.",0611280v2
2006-11-14,Paramagnetism in color superconductivity and compact stars,"It is quite plausible that color superconductivity occurs in the inner
regions of neutron stars. At the same time, it is known that strong magnetic
fields exist in the interior of these compact objects. In this paper we discuss
some important effects that can occur in the color superconducting core of
compact stars due to the presence of the stars' magnetic field. In particular,
we consider the modification of the gluon dynamics for a color superconductor
with three massless quark flavors in the presence of an external magnetic
field. We show that the long-range component of the external magnetic field
that penetrates the color-flavor locked phase produces an instability for field
values larger than the charged gluons' Meissner mass. As a consequence, the
ground state is restructured forming a vortex state characterized by the
condensation of charged gluons and the creation of magnetic flux tubes. In the
vortex state the magnetic field outside the flux tubes is equal to the applied
one, while inside the tubes its strength increases by an amount that depends on
the amplitude of the gluon condensate. This paramagnetic behavior of the color
superconductor can be relevant for the physics of compact stars.",0611460v2
2007-01-21,Cosmological magnetic fields from nonlinear effects,"In the standard cosmological model, magnetic fields and vorticity are
generated during the radiation era via second-order density perturbations. In
order to clarify the complicated physics of this second-order magnetogenesis,
we use a covariant approach and present the electromagneto-dynamical equations
in the nonlinear regime. We use the tight-coupling approximation to analyze
Thomson and Coulomb scattering. At the zero-order limit of exact
tight-coupling, we show that the vorticity is zero and no magnetogenesis takes
place at any nonlinear order. We show that magnetogenesis also fails at all
orders if either protons or electrons have the same velocity as the radiation,
and momentum transfer is neglected. Then we prove a key no-go result: at
first-order in the tight-coupling approximation, magnetic fields and vorticity
still cannot be generated even via nonlinear effects. The tight-coupling
approximation must be broken at first order, for the generation of vorticity
and magnetic fields, and we derive a closed set of nonlinear evolution
equations that governs this generation. We estimate that the amplitude of the
magnetic field at recombination on the horizon scale is $\sim 10^{-27} $G.",0701596v2
2007-02-20,Polarization of the Crab Nebula with disordered magnetic components,"In this paper, we present an expanding disc model to derive polarization
properties of the Crab nebula. The distribution function of the plasma and the
energy density of the magnetic field are prescribed as function of the distance
from the pulsar by using the model by Kennel and Coroniti (1984) with $\sigma =
0.003$, where $\sigma$ is the ratio of Poynting flux to the kinetic energy flux
in the bulk motion just before the termination shock. Unlike previous models,
we introduce disordered magnetic field, which is parameterized by the
fractional energy density of the disordered component. Flow dynamics is not
solved. The mean field is toroidal. Averaged polarization degree over the disc
is obtained as a function of inclination angle and fractional energy density of
the disordered magnetic field. It is found for the Crab that the disordered
component has about 60 percent of the magnetic field energy. This value is also
supported by the facts that the disc appears not `lip-shape' but as `rings' in
the intensity map as was observed, and that the highest polarization degree of
$\sim 40$ percent is reproduced for rings, being consistent with the
observation. We suggest that because the disordered field contributes rather
pressure than tension, the pinch force may be over-estimated in previous
relativistic magnetohydrodynamic simulations. Disruption of the toroidal
magnetic field with alternating direction, which is proposed by Lyubarski
(2003), may actually takes place. The relativistic flow speed, which is
indicated by the front-back contrast, can be detected in asymmetry in
distributions of the position angle and depolarization.",0702512v2
1994-12-02,Statistical Mechanics of Nonuniform Magnetization Reversal,"The magnetization reversal rate via thermal creation of soliton pairs in
quasi-1D ferromagnetic systems is calculated. Such a model describes e.g. the
time dependent coercivity of elongated particles as used in magnetic recording
media. The energy barrier that has to be overcome by thermal fluctuations
corresponds to a soliton-antisoliton pair whose size depends on the external
field. In contrast to other models of first order phase transitions such as the
phi^4 model, an analytical expression for this energy barrier is found for all
values of the external field. The magnetization reversal rate is calculated
using a functional Fokker-Planck description of the stochastic magnetization
dynamics. Analytical results are obtained in the limits of small fields and
fields close to the anisotropy field. In the former case the hard-axis
anisotropy becomes effectively strong and the magnetization reversal rate is
shown to reduce to the nucleation rate of soliton-antisoliton pairs in the
overdamped double sine-Gordon model. The present theory therefore includes the
nucleation rate of soliton-antisoliton pairs in the double sine-Gordon chain as
a special case. These results demonstrate that for elongated particles, the
experimentally observed coercivity is significantly lower than the value
predicted by the standard theories of N\'eel and Brown.",9412010v2
1996-08-29,Towards a Fermi Liquid Theory of the $ν$=1/2 State : Magnetized Composite Fermions,"The Fermionic Chern-Simons approach has had remarkable success in the
description of quantum Hall states at even denominator filling fractions
$\nu=\frac{1}{2m}$. In this paper we review a number of recent works concerned
with modeling this state as a Landau-Silin Fermi liquid. We will then focus on
one particular problem with constructing such a Landau theory that becomes
apparent in the limit of high magnetic field, or equivalently the limit of
small electron band mass $m_b$. In this limit, the static response of electrons
to a spatially varying magnetic field is largely determined by kinetic energy
considerations. We then remedy this problem by attaching an orbital
magnetization to each fermion to separate the current into magnetization and
transport contributions, associated with the cyclotron and guiding center
motions respectively. This leads us to a description of the $\nu=\frac{1}{2m}$
state as a Fermi liquid of magnetized composite fermions which correctly
predicts the $m_b$ dependence of the static and dynamic response in the limit
$m_b \rightarrow 0$. As an aside, we derive a sum rule for the Fermi liquid
coefficients for the Chern-Simons Fermi liquid. This paper is intended to be
readable by people who may not be completely familiar with this field.",9608147v1
1999-11-22,Spectral and transport properties of doped Mott-Hubbard systems with incommensurate magnetic order,"We present spectral and optical properties of the Hubbard model on a
two-dimensional square lattice using a generalization of dynamical mean-field
theory to magnetic states in finite dimension. The self-energy includes the
effect of spin fluctuations and screening of the Coulomb interaction due to
particle-particle scattering. At half-filling the quasiparticles reduce the
width of the Mott-Hubbard `gap' and have dispersions and spectral weights that
agree remarkably well with quantum Monte Carlo and exact diagonalization
calculations. Away from half-filling we consider incommensurate magnetic order
with a varying local spin direction, and derive the photoemission and optical
spectra. The incommensurate magnetic order leads to a pseudogap which opens at
the Fermi energy and coexists with a large Mott-Hubbard gap. The quasiparticle
states survive in the doped systems, but their dispersion is modified with the
doping and a rigid band picture does not apply. Spectral weight in the optical
conductivity is transferred to lower energies and the Drude weight increases
linearly with increasing doping. We show that incommensurate magnetic order
leads also to mid-gap states in the optical spectra and to decreased scattering
rates in the transport processes, in qualitative agreement with the
experimental observations in doped systems. The gradual disappearence of the
spiral magnetic order and the vanishing pseudogap with increasing temperature
is found to be responsible for the linear resistivity. We discuss the possible
reasons why these results may only partially explain the features observed in
the optical spectra of high temperature superconductors.",9911355v1
2000-02-06,Local magnetic structures induced by inhomogeneities of the lattice in S=1/2 bond-alternating chains and response to time-dependent magnetic field with a random noise,"We study effect of inhomogeneities of the lattice in the S=1/2
bond-alternating chain by using a quantum Monte Carlo method and an exact
diagonalization method. We adopt a defect in the alternating order as the
inhomogeneity and we call it `bond impurity'. Local magnetic structures induced
by the bond impurities are investigated both in the ground state and at very
low temperatures. The local magnetic structure can be looked on as an effective
S=1/2 spin and the weakness of the interaction between the local structures
causes the quasi-degenerate states in the low energy. We also investigate the
force acting between bond impurities and find that the force is generally
attractive. We also study the dynamical property of the local magnetic
structure. While the local magnetic structure behaves as an isolated S=1/2 spin
in the response to a time-dependent uniform field, it is found to be robust
against the effect of a random noise applied at each site individually in the
sweeping filed.",0002082v2
2004-08-05,Spectroscopy of spontaneous spin noise as a probe of spin dynamics and magnetic resonance,"Not all noise in experimental measurements is unwelcome. Certain fundamental
noise sources contain valuable information about the system itself -- a notable
example being the inherent voltage fluctuations that exist across any resistor
(Johnson noise), from which temperature may be determined[1,2]. In magnetic
systems, fundamental noise can exist in the form of random spin
fluctuations[3,4]. Felix Bloch noted in 1946 that statistical fluctuations of N
paramagnetic spins should generate measurable noise of order \sqrt{N} spins,
even in zero magnetic field[5,6]. Here we address precisely these same spin
fluctuations, using off-resonant Faraday rotation to passively[7,8] ""listen"" to
the magnetization noise in an equilibrium ensemble of paramagnetic alkali
atoms. These random fluctuations generate spontaneous spin coherences which
precess and decay with the same characteristic energy and time scales as the
macroscopic magnetization of an intentionally polarized or driven ensemble.
Correlation spectra of the measured spin noise reveals g-factors, nuclear spin,
isotope abundance ratios, hyperfine splittings, nuclear moments, and spin
coherence lifetimes -- without having to excite, optically pump, or otherwise
drive the system away from thermal equilibrium. These noise signatures scale
inversely with interaction volume, suggesting routes towards non-perturbative,
sourceless magnetic resonance of small systems.",0408107v1
2005-12-20,Theory of Spin Torque in a nanomagnet,"We present a complete theory of the spin torque phenomena in a ultrasmall
nanomagnet coupled to non-collinear ferromagnetic electrodes through tunnelling
junctions. This model system can be described by a simple microscopic model
which captures many physical effects characteristic of spintronics: tunneling
magneto resistance, intrinsic and transport induced magnetic relaxation,
current induced magnetization reversal and spin accumulation. Treating on the
same footing the magnetic and transport degrees of freedom, we arrive at a
closed equation for the time evolution of the magnetization. This equation is
very close to the Landau-Lifshitz-Gilbert equation used in spin valves
structures. We discuss how the presence of the Coulomb blockade phenomena and
the discretization of the one-body spectrum gives some additional features to
the current induced spin torque. Depending on the regime, the dynamic induced
by the coupling to electrode can be viewed either as a spin torque or as a
relaxation process. In addition to the possibility of stabilizing uniform spin
precession states, we find that the system is highly hysteretic: up to three
different magnetic states can be simultaneously stable in one region of the
parameter space (magnetic field and bias voltage).We also discuss how the
magneto-resistance can be used to provide additional information on the
non-equilibrium peaks present in the nanomagnet spectroscopy experiments.",0512508v1
2006-08-29,Collective modes for an array of magnetic dots in the vortex state,"The dispersion relations for collective magnon modes for square-planar arrays
of vortex-state magnetic dots, having closure magnetic flux are calculated. The
array dots have no direct contact between each other, and the sole source of
their interaction is the magnetic dipolar interaction. The magnon formalism
using Bose operators along with translational symmetry of the lattice, with the
knowledge of mode structure for the isolated dot, allows the diagonalization of
the system Hamiltonian giving the dispersion relation. Arrays of vortex-state
dots show a large variety of collective mode properties, such as positive or
negative dispersion for different modes. For their description, not only
dipolar interaction of effective magnetic dipoles, but non-dipolar terms common
to higher multipole interaction in classical electrodynamics can be important.
The dispersion relation is shown to be non-analytic as the value of the
wavevector approaches zero for all dipolar active modes of the single dot. For
vortex-state dots the interdot interaction is not weak, because, the dynamical
part (in contrast to the static magnetization of the vortex state) dot does not
contain the small parameter, the ratio of vortex core size to the dot radius.
This interaction can lead to qualitative effects like the formation of modes of
angular standing waves instead of modes with definite azimuthal number known
for the insolated vortex state dot.",0608638v1
2004-07-27,Domain wall dynamics driven by adiabatic spin transfer torques,"In a first approximation, known as the adiabatic process, the direction of
the spin polarization of currents is parallel to the local magnetization vector
in a domain wall. Thus the spatial variation of the direction of the spin
current inside the domain wall results in an adiabatical spin transfer torque
on the magnetization. We show that domain wall motion driven by this spin
torque has many unique features that do not exist in the conventional wall
motion driven by a magnetic field. By analytically and numerically solving the
Landau-Lifshitz-Gilbert equation along with the adiabatic spin torque in
magnetic nanowires, we find the domain wall has its maximum velocity at the
initial application of the current but the velocity decreases to zero as the
domain wall begins to deform during its motion. We have computed domain wall
displacement and domain wall deformation of nanowires, and concluded that the
spin torque based on the adiabatic propagation of the spin current in the
domain wall is unable to maintain wall movement. We also introduce a novel
concept of domain wall inductance to characterize the capacity of the
spin-torque induced magnetic energy stored in a domain wall. In the presence of
domain wall pinning centers, we construct a phase diagram for the domain wall
depinning by the combined action of the magnetic field and the spin current.",0407064v1
2006-11-26,A new class of quantum bound states: diprotons in extreme magnetic fields,"This paper considers the possibility that two charged particles with an
attractive short-ranged potential between them which is not strong enough to
form bound states in free space, may bind in uniform magnetic fields. It is
shown that in the formal limit where Coulomb repulsion is negligible (q -> 0
and B_0 -> \infty with q B_0 fixed where q is the charge and B_0 the field
strength) there always exists a bound state for a system of two identical
charged particles in a constant magnetic field, provided that there is a
short-range uniformly attractive potential between them. Moreover, it is shown
that in this limit {\it any} potential with an attractive s-wave scattering
length will posses bound states provided that the range of the potential is
much smaller than the characteristic magnetic length, r_0 = (\frac{q
B_0}{4})^{-1/2}. For this case, the binding is computed numerically. We
estimate the size of the magnetic field needed to approximately reach a regime
where the formal limit considered becomes a good approximation to the dynamics.
These numerical estimates indicate that two protons in an extremely strong
magnetic field such as might be found in a magnetar will bind to form a
diproton.",0611092v1
2001-04-23,Axisymmetric equilibria of a gravitating plasma with incompressible flows,"It is found that the ideal magnetohydrodynamic equilibrium of an axisymmetric
gravitating magnetically confined plasma with incompressible flows is governed
by a second-order elliptic differential equation for the poloidal magnetic flux
function containing five flux functions coupled with a Poisson equation for the
gravitation potential, and an algebraic relation for the pressure. This set of
equations is amenable to analytic solutions. As an application, the
magnetic-dipole static axisymmetric equilibria with vanishing poloidal plasma
currents derived recently by Krasheninnikov, Catto, and Hazeltine [Phys. Rev.
Lett. {\bf 82}, 2689 (1999)] are extended to plasmas with finite poloidal
currents, subject to gravitating forces from a massive body (a star or black
hole) and inertial forces due to incompressible sheared flows. Explicit
solutions are obtained in two regimes: (a) in the low-energy regime
$\beta_0\approx \gamma_0\approx \delta_0 \approx\epsilon_0\ll 1$, where
$\beta_0$, $\gamma_0$, $\delta_0$, and $\epsilon_0$ are related to the thermal,
poloidal-current, flow and gravitating energies normalized to the
poloidal-magnetic-field energy, respectively, and (b) in the high-energy regime
$\beta_0\approx \gamma_0\approx \delta_0 \approx\epsilon_0\gg 1$. It turns out
that in the high-energy regime all four forces, pressure-gradient,
toroidal-magnetic-field, inertial, and gravitating contribute equally to the
formation of magnetic surfaces very extended and localized about the symmetry
plane such that the resulting equilibria resemble the accretion disks in
astrophysics.",0104072v1
2002-05-03,Molecular gyroscopes and biological effects of weak ELF magnetic fields,"Extremely-low-frequency magnetic fields are known to affect biological
systems. In many cases, biological effects display `windows' in biologically
effective parameters of the magnetic fields: most dramatic is the fact that
relatively intense magnetic fields sometimes do not cause appreciable effect,
while smaller fields of the order of 10--100 $\mu$T do. Linear resonant
physical processes do not explain frequency windows in this case. Amplitude
window phenomena suggest a nonlinear physical mechanism. Such a nonlinear
mechanism has been proposed recently to explain those `windows'. It considers
quantum-interference effects on protein-bound substrate ions. Magnetic fields
cause an interference of ion quantum states and change the probability of
ion-protein dissociation. This ion-interference mechanism predicts specific
magnetic-field frequency and amplitude windows within which biological effects
occur. It agrees with a lot of experiments. However, according to the
mechanism, the lifetime $\Gamma^{-1}$ of ion quantum states within a protein
cavity should be of unrealistic value, more than 0.01 s for frequency band
10--100 Hz. In this paper, a biophysical mechanism has been proposed that (i)
retains the attractive features of the ion interference mechanism and (ii) uses
the principles of gyroscopic motion and removes the necessity to postulate
large lifetimes. The mechanism considers dynamics of the density matrix of the
molecular groups, which are attached to the walls of protein cavities by two
covalent bonds, i.e., molecular gyroscopes. Numerical computations have shown
almost free rotations of the molecular gyros. The relaxation time due to van
der Waals forces was about 0.01 s for the cavity size of 28 angstr\""{o}ms.",0205008v2
2003-12-11,Magnetically induced anions,"The main focus of this review is on magnetically induced anions. Before
discussing these new anionic states which exclusively exist in the presence of
magnetic fields, we review in some detail the anionic physics without external
field. For completeness, we also outline the properties of field-free existing
anions when exposed to magnetic fields. The magnetically induced states
constitute an infinite manifold assuming that the nucleus of the anion is
infinitely heavy. At laboratory field strengths the corresponding binding
energies show two different scaling properties belonging to the ground
($\propto B^2$) and the excited ($\propto B^3$) magnetically induced states. We
provide a detailed discussion of the physics of the moving anion taking into
account the coupling between the anionic centre of mass and its electronic
degrees of freedom. A number of field-adapted techniques exploiting exact
constants of motion and the adiabatic separation of motions are applied to
simplify the Hamiltonian that describes the effective interaction between the
centre of mass and the electronic degrees of freedom. Employing classical
simulations the autodetachment process of the anions in the field is observed
and a rich variety of spectral properties of moving anions is predicted:
depending on the parameters, such as the mass and polarizability of the neutral
system as well as the field strength, induced bound states, resonances and
detaching states of the corresponding anions are to be expected. An outline of
an {\it{ab initio}} quantum approach is provided that allows quantum dynamical
investigations of the moving anion.",0312076v1
2006-06-01,In situ evidence for the structure of the magnetic null in a 3D reconnection event in the Earth's magnetotail,"Magnetic reconnection is one of the most important processes in
astrophysical, space and laboratory plasmas. Identifying the structure around
the point at which the magnetic field lines break and subsequently reform,
known as the magnetic null point, is crucial to improving our understanding
reconnection. But owing to the inherently three-dimensional nature of this
process, magnetic nulls are only detectable through measurements obtained
simultaneously from at least four points in space. Using data collected by the
four spacecraft of the Cluster constellation as they traversed a diffusion
region in the Earth's magnetotail on 15 September, 2001, we report here the
first in situ evidence for the structure of an isolated magnetic null. The
results indicate that it has a positive-spiral structure whose spatial extent
is of the same order as the local ion inertial length scale, suggesting that
the Hall effect could play an important role in 3D reconnection dynamics.",0606014v2
2008-02-01,Collapse of magnetized hypermassive neutron stars in general relativity: Disk evolution and outflows,"We study the evolution in axisymmetry of accretion disks formed
self-consistently through collapse of magnetized hypermassive neutron stars to
black holes. Such stars can arise following the merger of binary neutron stars.
They are differentially rotating, dynamically stable, and have rest masses
exceeding the mass limit for uniform rotation. However, hypermassive neutron
stars are secularly unstable to collapse due to MHD-driven angular momentum
transport. The rotating black hole which forms in this process is surrounded by
a hot, massive, magnetized torus and a magnetic field collimated along the spin
axis. This system is a candidate for the central engine of a short-hard
gamma-ray burst (GRB). Our code integrates the coupled Einstein-Maxwell-MHD
equations and is used to follow the collapse of magnetized hypermassive neutron
star models in full general relativity until the spacetime settles down to a
quasi-stationary state. We then employ the Cowling approximation, in which the
spacetime is frozen, to track the subsequent evolution of the disk. This
approximation allows us to greatly extend the disk evolutions and study the
resulting outflows, which may be relevant to the generation of a GRB. We find
that outflows are suppressed when a stiff equation of state is assumed for low
density disk material and are sensitive to the initial magnetic field
configuration.",0802.0200v1
2008-04-08,Frequent Occurrence of High-speed Local Mass Downflows on the Solar Surface,"We report on new spectro-polarimetric measurements with simultaneous filter
imaging observation, revealing the frequent appearance of polarization signals
indicating high-speed, probably supersonic, downflows that are associated with
at least three different configurations of magnetic fields in the solar
photosphere. The observations were carried out with the Solar Optical Telescope
onboard the {\em Hinode} satellite. High speed downflows are excited when a
moving magnetic feature is newly formed near the penumbral boundary of
sunspots. Also, a new type of downflows is identified at the edge of sunspot
umbra that lack accompanying penumbral structures. These may be triggered by
the interaction of magnetic fields sweeped by convection with well-concentrated
magnetic flux. Another class of high speed downflows are observed in quiet sun
and sunspot moat regions. These are closely related to the formation of small
concentrated magnetic flux patches. High speed downflows of all types are
transient time-dependent mass motions. These findings suggest that the
excitation of supersonic mass flows are one of the key observational features
of the dynamical evolution occurring in magnetic-field fine structures on the
solar surface.",0804.1167v1
2008-05-05,Sub-Alfvenic Non-Ideal MHD Turbulence Simulations with Ambipolar Diffusion: I. Turbulence Statistics,"Most numerical investigations on the role of magnetic fields in turbulent
molecular clouds (MCs) are based on ideal magneto-hydrodynamics (MHD). However,
MCs are weakly ionized, so that the time scale required for the magnetic field
to diffuse through the neutral component of the plasma by ambipolar diffusion
(AD) can be comparable to the dynamical time scale. We have performed a series
of 256^3 and 512^3 simulations on supersonic but sub-Alfvenic turbulent systems
with AD using the Heavy-Ion Approximation developed in Li, McKee, & Klein
(2006). Our calculations are based on the assumption that the number of ions is
conserved, but we show that these results approximately apply to the case of
time-dependent ionization in molecular clouds as well. Convergence studies
allow us to determine the optimal value of the ionization mass fraction when
using the heavy-ion approximation for low Mach number, sub-Alfvenic turbulent
systems. We find that ambipolar diffusion steepens the velocity and magnetic
power spectra compared to the ideal MHD case. Changes in the density PDF, total
magnetic energy, and ionization fraction are determined as a function of the AD
Reynolds number. The power spectra for the neutral gas properties of a strongly
magnetized medium with a low AD Reynolds number are similar to those for a
weakly magnetized medium; in particular, the power spectrum of the neutral
velocity is close to that for Burgers turbulence.",0805.0597v1
2008-05-10,Modelling of magnetic field structure of the Ergodic Divertor of Tore Supra and comparison to the Dynamic Ergodic Divertor of TEXTOR,"An analytical model previously developed to study the structure of the
magnetic field for the TEXTOR-DED [S.S. Abdullaev et al. Phys. Plasmas, 6, 153
(1999)] is applied to the similar study of the Ergodic Divertor of Tore Supra
tokamak [Ph. Ghendrih, Plasma Phys. Control. Fusion, 38, 1653 (1996)]. The coil
configuration of ED Tore Supra consists of six modules equidistantly located
along the toroidal direction on the low-field-side of the torus with given
toroidal and poloidal extensions. The Hamiltonian formulation of field line
equations in straight-field-line coordinates (Boozer coordinates) and the
computationally efficient mapping method for integration of the Hamiltonian
field line equations are used to study the magnetic field structure in the ED.
Asymptotical formulas for the perturbation magnetic field created by the ED
coils are obtained and the spectrum of magnetic perturbations is analyzed and
compared with the one of the TEXTOR-DED. The structure of ergodic and laminar
zones are studied by plotting Poincare sections, so-called laminar plots
(contour plots of wall to wall connection lengths) and magnetic footprints. The
radial profiles of field line diffusion coefficients are calculated for
different perturbation currents and it is found that for the Tore Supra case in
the ergodic zone the numerical field line diffusion coefficients perfectly
follow the quasilinear formula for smaller perturbation currents although the
situation is different for the maximum perturbation current.",0805.1477v1
2008-06-26,The magnetic field of the proto-planetary nebula candidate IRAS 19296+2227,"Context: Magnetic fields are thought to be one of the possible mechanisms
responsible for shaping the generally spherical outflow of evolved stars into
often aspherical planetary nebulae. However, direct measurements of magnetic
fields during the transition to the planetary nebula phase are rare.
  Aims: The aim of this project is to expand the number of magnetic field
measurements of stars in the (proto-)planetary nebula phase and find if the
magnetic field strength is sufficient to affect the stellar outflow.
  Methods: We used Very Long Baseline Array observations to measure the
circular polarization due to the Zeeman splitting of 22 GHz water masers in the
envelope of the proto-planetary nebula candidate star IRAS 19296+2227 and the
planetary nebula K3-35.
  Results: A strong magnetic field of B||=-135+-28 is detected in the water
maser region of the proto-planetary nebula candidate IRAS 19296+2227. The water
masers of K3-35 are too weak to detect circular polarization although we do
present the measurements of weak linear polarization in those masers.
  Conclusions: The field measured in the masers of IRAS 19296+2227 is
dynamically important and, if it is representative of the large scale field, is
an important factor in driving the stellar mass loss and shaping the stellar
outflow.",0806.4261v1
2008-12-19,Magnetic control of large room-temperature polarization,"Numerous authors have referred to room-temperature magnetic switching of
large electric polarizations as The Holy Grail of magnetoelectricity.We report
this long-sought effect using a new physical process of coupling between
magnetic and ferroelectric relaxor nano-regions. Here we report magnetic
switching between the normal ferroelectric state and the ferroelectric relaxor
state. This gives both a new room-temperature, single-phase, multiferroic
magnetoelectric, PbZr0.46Ti0.34Fe0.13W0.07O3, with polarization, loss (<4%),
and resistivity (typically 108 -109 ohm.cm) equal to or superior to BiFeO3, and
also a new and very large magnetoelectric effect: switching not from +Pr to
negative Pr with applied H, but from Pr to zero with applied H of less than a
Tesla. This switching of the polarization occurs not because of a conventional
magnetically induced phase transition, but because of dynamic effects:
Increasing H lengthens the relaxation time by x500 from <200 ns to >100 ?s, and
it couples strongly the polarization relaxation and spin relaxations. The
diverging polarization relaxation time accurately fits a modified Vogel-Fulcher
Equation in which the freezing temperature Tf is replaced by a critical
freezing field Hf that is 0.92 positive/negative 0.07 Tesla. This field
dependence and the critical field Hc are derived analytically from the
spherical random bond random field (SRBRF) model with no adjustable parameters
and an E2H2 coupling. This device permits 3-state logic (+Pr,0,negative Pr) and
a condenser with >5000% magnetic field change in its capacitance.",0812.3875v2
2008-12-29,Measuring interstellar magnetic fields by radio synchrotron emission,"Radio synchrotron emission, its polarization and its Faraday rotation are
powerful tools to study the strength and structure of interstellar magnetic
fields. The total intensity traces the strength and distribution of total
magnetic fields. Total fields in gas-rich spiral arms and bars of nearby
galaxies have strengths of 20-30 $\mu$Gauss, due to the amplification of
turbulent fields, and are dynamically important. In the Milky Way, the total
field strength is about 6 $\mu$G near the Sun and several 100 $\mu$G in
filaments near the Galactic Center. -- The polarized intensity measures ordered
fields with a preferred orientation, which can be regular or anisotropic
fields. Ordered fields with spiral structure exist in grand-design, barred,
flocculent and even in irregular galaxies. The strongest ordered fields are
found in interarm regions, sometimes forming ""magnetic spiral arms"" between the
optical arms. Halo fields are X-shaped, probably due to outflows. -- The
Faraday rotation of the polarization vectors traces coherent regular fields
which have a preferred direction. In some galaxies Faraday rotation reveals
large-scale patterns which are signatures of dynamo fields. However, in most
galaxies the field has a complicated structure and interacts with local gas
flows. In the Milky Way, diffuse polarized radio emission and Faraday rotation
of the polarized emission from pulsars and background sources show many
small-scale and large-scale magnetic features, but the overall field structure
in our Galaxy is still under debate.",0812.4925v1
2009-03-03,Detecting and Switching Magnetization of Stoner Nanograin in Non-local Spin Valve,"The magnetization detection and switching of an ultrasmall Stoner nanograin
in a non-local spin valve (NLSV) device is studied theoretically. With the help
of the rate equations, a unified description can be presented on the same
footing for the NLSV signal that reads out the magnetization, and for the
switching process. The setup can be viewed as that the grain is connected to
two non-magnetic leads via sequential tunneling. In one lead, the chemical
potentials for spin-up and -down electrons are split due to the spin injection
in the NLSV. This splitting (or the spin bias) is crucial to the NLSV signal
and the critical condition to the magnetization switching. By using the
standard spin diffusion equation and parameters from recent NLSV device, the
magnitude of the spin bias is estimated, and found large enough to drive the
magnetization switching of the cobalt nanograin reported in earlier
experiments. A microscopic interpretation of NLSV signal in the sequential
tunneling regime is thereby raised, which show properties due to the ultrasmall
size of the grain. The dynamics at the reversal point shows that there may be a
spin-polarized current instead of the anticipated pure spin current flowing
during the reversal due to the electron accumulation in the floating lead used
for the readout of NLSV signal.",0903.0456v2
2009-03-12,Field-induced suppression of the pi-band superconductivity and magnetic hysteresis in the microwave surface resistance of MgB_2 at temperatures near T_c,"We report on the magnetic-field-induced variations of the microwave surface
resistance, R_s, in a polycrystalline MgB_2 sample, at different values of
temperature. We have detected a magnetic hysteresis in R_s, which exhibits an
unexpected plateau on decreasing the DC magnetic field below a certain value.
In particular, at temperatures near T_c the hysteresis manifests itself only
through the presence of the plateau. Although we do not quantitatively justify
the anomalous shape of the magnetic hysteresis, we show that the results
obtained in the reversible region of the R_s(H) curve can be quite well
accounted for by supposing that, in this range of magnetic field, the pi-gap is
almost suppressed by the applied field and, consequently, all the pi-band
charge carriers are quasiparticles. On this hypothesis, we have calculated
R_s(H) supposing that fluxons assume a conventional (single core) structure and
the flux dynamics can be described in the framework of conventional models.
  From the fitting of the experimental results, we determine the values of
H_{c2}^pi(T) at temperatures near T_c. In our opinion, the most important
result of our investigation is that, at least at temperatures near T_c, the
value of the applied field that separates the reversible and irreversible
regions of the R_s(H) curves is just H_{c2}^pi(T); a qualitative discussion of
the possible reason of this finding is given.",0903.2162v1
2009-05-13,Chiral MHD description of a perfect magnetized QGP using the effective NJL model in a strong magnetic field,"To study the effect of a strong magnetic field $B$ on the sound velocity
$v_{s}$ of plane waves propagating in a strongly magnetized quark-gluon plasma
(QGP), a chiral magnetohydrodynamical (MHD) description of a perfect
(non-dissipative) QGP exhibiting dynamical chiral symmetry breaking (D$\chi$SB)
is developed using the effective action of the Nambu-Jona-Lasinio (NJL) model
of QCD at finite temperature, finite baryon chemical potential and in the
presence of a strong magnetic field. Here, the D$\chi$SB arises due to the
phenomenon of magnetic catalysis. Apart from an interesting frequency
dependence, for plane waves propagating in the transverse or longitudinal
direction with respect to the $B$ field, the sound velocity is anisotropic and
depends on the angle between the corresponding wave vectors and the direction
of the $B$ field. Moreover, for plane waves propagating in the transverse
(longitudinal) direction to the external $B$ field, the sound velocity has a
maximum (minimum) at $T<T_{c}$, reaches a local minimum (maximum) at $T\sim
T_{c}$ and remains constant at $T\gtrsim T_{c}$. Here, $T_{c}$ is the critical
temperature of the chiral phase transition. Thus, the constant value $v_{s}\sim
1.5 c_{s}$ at $T \gtrsim T_{c}$ turns out to be a lower (upper) bound for waves
propagating in the transverse (longitudinal) direction with respect to the
external $B$ field. Here, $c_{s}=1/\sqrt{3}$ is the sound velocity in an ideal
gas.",0905.2097v1
2009-08-31,Cobalt nano-clusters on metal supported Xe monolayers: influence of the substrate on cluster formation kinetics and magnetism,"The growth dynamics of submonolayer coverages of Cobalt during buffer layer
assisted growth on Ag(111) and Pt(111) substrates is investigated by variable
temperature scanning tunneling microscopy in the temperature range between 80
and 150 Kelvin. It is found that attractive cluster-substrate interactions can
govern the cluster formation on the Xe buffer layer, if the Xe layer is
sufficiently thin. The interpretation of the microscopy results are supported
by x-ray magnetic circular dichroism which monitors the effect of
cluster-substrate interactions on the formation of magnetic moments and
magnetic anisotropy of Co nanocluster during the different stages of growth.
{\it Ab-initio} calculations show that the cluster magnetism is controlled by
the interface anisotropy, leading to perpendicular magnetization for Co on
Pt(111). Limits of and new potential for nanocluster fabrication by buffer
layer assisted growth are discussed.",0908.4365v2
2009-11-18,Kelvin-Helmholtz Instability of the Magnetopause of Disc-Accreting Stars,"This work investigates the short wavelength stability of the magnetopause
between a rapidly-rotating, supersonic, dense accretion disc and a
slowly-rotating low-density magnetosphere of a magnetized star. The
magnetopause is a strong shear layer with rapid changes in the azimuthal
velocity, the density, and the magnetic field over a short radial distance and
thus the Kelvin-Helmholtz (KH) instability may be important. The plasma
dynamics is treated using non-relativistic, compressible (isentropic)
magnetohydrodynamics. It is necessary to include the displacement current in
order that plasma wave velocities remain less than the speed of light. We focus
mainly on the case of a star with an aligned dipole magnetic field so that the
magnetic field is axial in the disc midplane and perpendicular to the disc flow
velocity. However, we also give results for cases where the magnetic field is
at an arbitrary angle to the flow velocity. For the aligned dipole case the
magnetopause is most unstable for KH waves propagating in the azimuthal
direction perpendicular to the magnetic field which tends to stabilize waves
propagating parallel to it. The wave phase velocity is that of the disc matter.
A quasi-linear theory of the saturation of the instability leads to a
wavenumber ($k$) power spectrum $\propto k^{-1}$ of the density and temperature
fluctuations of the magnetopause, and it gives the mass accretion and angular
momentum inflow rates across the magnetopause. For self-consistent conditions
this mass accretion rate will be equal to the disc accretion rate at large
distances from the magnetopause.",0911.3590v1
2010-01-20,Theory of Magnetic Seed-Field Theory of Magnetic Seed-Field Generation during the Cosmological First-Order Electroweak Phase Transition,"We present a theory of the generation of magnetic seed fields in bubble
collisions during a first-order electroweak phase transition (EWPT) possible
for some choices of parameters in the minimal supersymmetric Standard Model.
The theory extends earlier work and is formulated to assess the importance of
surface dynamics in such collisions. We are led to linearized equations of
motion with O(3) symmetry appropriate for examining collisions in which the
Higgs field is relatively unperturbed from its mean value in the collision
volume. Coherent evolution of the charged $W$ fields within the bubbles is the
main source of the electromagnetic current for generating the seed fields, with
fermions also contributing through the conductivity terms. We present numerical
simulations within this formulation to quantify the role of the surface of the
colliding bubbles, particularly the thickness of the surface, and to show how
conclusions drawn from earlier work are modified. The main sensitivity arises
such that the steeper the bubble surface the more enhanced the seed fields
become. Consequently, the magnetic seed fields may be several times larger and
smoother over the collision volume than found in earlier studies. Our work thus
provides additional support to the supposition that magnetic fields produced
during the EWPT in the early universe seed the galactic and extra-galactic
magnetic fields observed today.",1001.3694v1
2010-02-03,Magnetic fields and radiative feedback in the star formation process,"Star formation is a complex process involving the interplay of many physical
effects, including gravity, turbulent gas dynamics, magnetic fields and
radiation. Our understanding of the process has improved substantially in
recent years, primarily as a result of our increased ability to incorporate the
relevant physics in numerical calculations of the star formation process. In
this contribution we present an overview of our recent studies of star cluster
formation in turbulent, magnetised clouds using self-gravitating
radiation-magnetohydrodynamics calculations (Price and Bate 2008, 2009). Our
incorporation of magnetic fields and radiative transfer into the Smoothed
Particle Hydrodynamics method are discussed. We highlight how magnetic fields
and radiative heating of the gas around newborn stars can solve several of the
key puzzles in star formation, including an explanation for why star formation
is such a slow and inefficient process. However, the presence of magnetic
fields at observed strengths in collapsing protostellar cores also leads to
problems on smaller scales, including a difficulty in forming protostellar
discs and binary stars (Price and Bate 2007, Hennebelle and Teyssier 2008),
which suggests that our understanding of the role of magnetic fields in star
formation is not yet complete.",1002.0650v2
2010-04-20,Edge pinch instability of oblate liquid metal drops in a transverse AC magnetic field,"This paper considers the stability of liquid metal drops subject to a
high-frequency AC magnetic field. An energy variation principle is derived in
terms of the surface integral of the scalar magnetic potential. This principle
is applied to a thin perfectly conducting liquid disk, which is used to model
the drops constrained in a horizontal gap between two parallel insulating
plates. Firstly, the stability of a circular disk is analysed with respect to
small-amplitude harmonic edge perturbations. Analytical solution shows that the
edge deformations with the azimuthal wavenumbers m=2,3,4... start to develop as
the magnetic Bond number exceeds the critical threshold Bm_c=3pi(m+1)/2. The
most unstable is m=2 mode, which corresponds to an elliptical deformation.
Secondly, strongly deformed equilibrium shapes are modelled numerically by
minimising the associated energy in combination with the solution of a surface
integral equation for the scalar magnetic potential on an unstructured
triangular mesh. The edge instability is found to result in the equilibrium
shapes of either two- or threefold rotational symmetry depending on the
magnetic field strength and the initial perturbation. The shapes of higher
rotational symmetries are unstable and fall back to one of these two basic
states. The developed method is both efficient and accurate enough for
modelling of strongly deformed drop shapes.",1004.3467v3
2010-06-02,Interaction of a magnetized shell with an ambient medium: limits on impulsive magnetic acceleration,"The interaction of relativistic magnetized ejecta with an ambient medium is
studied for a range of structures and magnetization of the unshocked ejecta. We
particularly focus on the effect of the ambient medium on the dynamics of an
impulsive, high-sigma shell. It is found that for sufficiently high values of
the initial magnetization $\sigma_0$ the evolution of the system is
significantly altered by the ambient medium well before the shell reaches its
coasting phase. The maximum Lorentz factor of the shell is limited to values
well below $\sigma_0$; for a shell of initial energy $E=10^{52}E_{52}$ ergs and
size $r_0=10^{12}T_{30}$ cm expelled into a medium having a uniform density
$n_i$ we obtain $\Gamma_{\rm max}\simeq180(E_{52}/T_{30}^3 n_i)^{1/8}$ in the
high sigma limit. The reverse shock and any internal shocks that might form if
the source is fluctuating are shown to be very weak. The restriction on the
Lorentz factor is more severe for shells propagating in a stellar wind.
Intermittent ejection of small sub-shells doesn't seem to help, as the shells
merge while still highly magnetized. Lower sigma shells start decelerating
after reaching the coasting phase and spreading away. The properties of the
reverse shock then depend on the density profiles of the coasting shell and the
ambient medium. For a self-similar cold shell the reverse shock becomes strong
as it propagates inwards, and the system eventually approaches the self-similar
solution derived recently by Nakamura \& Shigeyama.",1006.0336v3
2010-11-04,"Magnetic order, metamagnetic transitions, and low-temperature spin freezing in Ca3Co2O6: an NMR study","We report on a 59Co NMR investigation of the trigonal cobaltate Ca3Co2O6
carried out on a single crystal, providing precise determinations of the
electric field gradient and chemical shift tensors, and of the internal
magnetic fields at the non-magnetic Co I sites, unavailable from former studies
on powders. The magnetic field-induced ferri- and ferromagnetic phases at
intermediate temperature (e.g. 10 K) are identified by distinct internal
fields, well accounted for by purely dipolar interactions. The vanishing
transferred hyperfine field at the Co I site indicates that the Co3+(I)
orbitals do not participate in the intra-chain superexchange, in disagreement
with a previous theoretical model. The strong Ising character of the system is
confirmed experimentally by the field dependence of the resonance lines,
indicating that local moments are saturated even at the phase boundaries. In
the vicinity of the critical fields, nuclear spin-spin relaxations detect the
spin reversal dynamics of large magnetic assemblies, either Ising chain
fragments or finite size domains, which drive the metamagnetic transitions.
Such collective excitations exhibit a glassy behavior, slowing down to
subacoustic frequencies and freezing at low temperature. The relevance of such
slow fluctuation modes for the low-temperature multi-step behavior reported in
the magnetization is discussed.",1011.1142v2
2010-11-25,Calculation of the Expected Zero Field Muon Relaxation Rate in the Geometrically Frustrated Rare Earth Pyrochlore Gd2Sn2O7 Antiferromagnet,"The magnetic insulator Gd2Sn2O7 is one of many geometrically frustrated
magnetic materials known to exhibit a nonzero muon spin polarization relaxation
rate, $\lambda(T)$, down to the lowest temperature (T) studied. Such behaviour
is typically interpreted as a significant level of persisting spin dynamics
(PSD) of the host material. In the case of Gd2Sn2O7, such PSD comes as a
surprise since magnetic specific heat measurements suggest conventional gapped
magnons, which would naively lead to an exponentially vanishing $\lambda(T)$ as
$T \rightarrow 0$. In contrast to most materials that display PSD, the ordered
phase of Gd2Sn2O7 is well characterized and both the nature and the magnitude
of the interactions have been inferred from the magnetic structure and the
temperature dependence of the magnetic specific heat. Based on this
understanding, the temperature dependence of the muon spin polarization
relaxation through the scattering of spin waves (magnons) is calculated. The
result explicitly shows that, despite the unusual extensive number of weakly
dispersive (gapped)excitations characterizing Gd2Sn2O7, a remnant of the zero
modes of the parent frustrated pyrochlore Heisenberg antiferromagnet, the
temperature dependence of the calculated $\lambda(T)$ differs dramatically from
the experimental one. Indeed, the calculation conforms to the naive expectation
of an exponential collapse of $\lambda(T)$ at temperatures below ~ 0.7K. This
result, for the first time, illustrates crisply and quantitatively the paradox
that presents itself with the pervasive occurrence of PSD in highly frustrated
magnetic systems as evinced by muon spin relaxation measurements.",1011.5689v1
2010-12-01,Effect of Randomness on Quantum Data Buses of Heisenberg Spin Chains,"A strongly coupled spin chain can mediate long-distance effective couplings
or entanglement between remote qubits, and can be used as a quantum data bus.
We study how the fidelity of a spin-1/2 Heisenberg chain as a spin bus is
affected by static random exchange couplings and magnetic fields. We find that,
while non-uniform exchange couplings preserve the isotropy of the qubit
effective couplings, they cause the energy levels, the eigenstates, and the
magnitude of the couplings to vary locally. On the other hand, random local
magnetic fields lead to an avoided level crossing for the bus ground state
manifold, and cause the effective qubit couplings to be anisotropic.
Interestingly, the total magnetic moment of the ground state of an odd-size bus
may not be parallel to the average magnetic field. Its alignment depends on
both the direction of the average field and the field distribution, in contrast
with the ground state of a single spin which always aligns with the applied
magnetic field to minimize the Zeeman energy. Lastly, we calculate
sensitivities of the spin bus to such local variations, which are potentially
useful for evaluating decoherence when dynamical fluctuations in the exchange
coupling or magnetic field are considered.",1012.0288v2
2011-01-18,Simulation of dark lanes in post--flare supra--arcades,"Using two simulations of 1.5D, for the first time, in Costa et al. (2009) and
Shulz et al. (2010) we numerically reproduce the observational dark inflows
described in Verwichte et al. (2005). We show that the dark tracks can be
explained as hot plasma vacuums generated upstream of a slow magnetoacoustic
shock wave produced by a localized deposition of energy. In this work, we show
that the overall 2D results are in agreement with the observational behaviour.
However they show a slight shift in the characteristic parameter with respect
to those found previously. Also, we confirm qualitatively the behaviour found
in the previous papers, i.e. for a given numerical domain the period of the
kink--like structure is a function of the magnetic field intensity: larger
periods are associated with lower magnetic field intensities. Contrary to the
1D result -where the sunward dynamic is independent of the magnetic field
intensity due to its exclusively waveguide role- in the 2D simulation the
sunward speed is larger for larger values of the magnetic field. This can be
interpreted as the capability of the low coronal plasma to collimate the
deposition of energy into the magnetic field direction. The moving features
consistent of low--density and high--temperature plasma cavities have larger
inside values of the structuring parameter beta than the neighboring media.
Thus, the voids seem to be the emergence structures of a whole nonlinear
interacting plasma context of shocks and waves more than voided plasma loops
magnetically structured.",1101.3530v1
2011-01-30,Dynamics of entanglement in a two-dimensional spin system,"We consider the time evolution of entanglement in a finite two dimensional
transverse Ising model. The model consists of a set of 7 localized spin-1/2
particles in a two dimensional triangular lattice coupled through nearest
neighbor exchange interaction in presence of an external time dependent
magnetic field. The magnetic field is applied in different function forms:
step, exponential, hyperbolic and periodic. We found that the time evolution of
the entanglement shows an ergodic behavior under the effect of the time
dependent magnetic fields. Also we found that while the step magnetic field
causes great disturbance to the system creating rabid oscillations, the system
shows great controllability under the effect of the other magnetic fields where
the entanglement profile follows closely the shape of the applied field even
with the same frequency for periodic fields. This follow up trend breaks down
as the strength of the field, the transition constant for exponential and
hyperbolic, or frequency for periodic field increase leading to rapid
oscillations. We observed that the entanglement is very sensitive to the
initial value of the applied periodic field, the smaller the initial value the
less distorted is the entanglement profile. Furthermore, the effect of thermal
fluctuations is very devastating to the entanglement which decays very rapidly
as the temperature increases. Interestingly, although large value of the
magnetic field strength may yield small entanglement, it was found to be more
persistent against thermal fluctuations than the small field strengths.",1101.5784v1
2011-02-06,LES of turbulent convection in solar-type stars and formation of large-scale magnetic structures,"In this study we investigate the effects of turbulent convection on formation
of large-scale inhomogeneous magnetic structures by means of Large-Eddy
Simulation (LES) for convection in solar-type stars. The main idea of this
study is the implementation of a new subgrid-scale model for the effective
Lorentz force in a three-dimensional nonlinear radiative magnetohydrodynamics
(MHD) code developed for simulating the upper solar convection zone and lower
atmosphere. To this end we derived the energy budget equations, which include
the effects of the subgrid-scale turbulence on the Lorentz-force, and
implemented the new subgrid-scale turbulence model (TELF-Model) in a
three-dimensional nonlinear MHD LES code. Using imposed initial vertical and
horizontal uniform magnetic fields in LES with the TELF-Model, we have shown
that the magnetic flux tubes formation is started when the initial mean
magnetic field is larger than a threshold value (about 100 G). This is in
agreement with the theoretical studies by Rogachevskii and Kleeorin (2007). We
have determined the vertical profiles of the velocity and magnetic
fluctuations, total MHD energy and anisotropy of turbulent magneto-convection,
kinetic and current and cross helicities.",1102.1206v1
2011-04-07,The magnetic phase diagram of an extended J1-J2 model on a modulated square lattice and its implications for the antiferromagnetic phase of KyFexSe2,"Motivated by the experimentally observed $\sqrt{5} \times \sqrt{5}$ iron
vacancy order and a block spin antiferromagnetic phase with large magnetic
moment in $\mathrm{K}_{0.8}\mathrm{Fe}_{1.6}\mathrm{Se}_2$, we study the
magnetic phase diagram of an extended $J_1-J_2$ model on a 1/5-depleted square
lattice with $\sqrt{5} \times \sqrt{5}$ vacancy order, using a classical Monte
Carlo analysis. The magnetic phase diagram involves various
antiferromagnetically ordered phases, and most of them have higher order
commensuration. We find that the experimentally relevant block-spin state
occupies a significant portion of the phase diagram, and we discuss the spin
dynamics of this phase using a linear spin-wave analysis. Based on our spin
wave calculations in different parameter regimes corresponding to the block
spin antiferromagnetic phase, we show how spin-wave degeneracy along the high
symmetry directions of the magnetic Brillouin zone can provide information
regarding the underlying exchange couplings. We have also analyzed the magnetic
phase diagram of a $J_1-J_2$ model on two different modulated square lattices
relevant for $\mathrm{K}_y\mathrm{Fe}_{1.5}\mathrm{Se}_2$, which respectively
exhibit 1/4-depleted $2\times2$ and $4\times2$ vacancy ordering.",1104.1445v2
2011-05-10,Numerical Experiments on the Two-step Emergence of Twisted Magnetic Flux Tubes in the Sun,"We present the new results of the two-dimensional numerical experiments on
the cross-sectional evolution of a twisted magnetic flux tube rising from the
deeper solar convection zone (-20,000 km) to the corona through the surface.
The initial depth is ten times deeper than most of previous calculations
focusing on the flux emergence from the uppermost convection zone. We find that
the evolution is illustrated by the two-step process described below: the
initial tube rises due to its buoyancy, subject to aerodynamic drag due to the
external flow. Because of the azimuthal component of the magnetic field, the
tube maintains its coherency and does not deform to become a vortex roll pair.
When the flux tube approaches the photosphere and expands sufficiently, the
plasma on the rising tube accumulates to suppress the tube's emergence.
Therefore, the flux decelerates and extends horizontally beneath the surface.
This new finding owes to our large scale simulation calculating simultaneously
the dynamics within the interior as well as above the surface. As the magnetic
pressure gradient increases around the surface, magnetic buoyancy instability
is triggered locally and, as a result, the flux rises further into the solar
corona. We also find that the deceleration occurs at a higher altitude than in
our previous experiment using magnetic flux sheets (Toriumi and Yokoyama). By
conducting parametric studies, we investigate the conditions for the two-step
emergence of the rising flux tube: field strength > 1.5x10^4 G and the twist >
5.0x10^-4 km^-1 at -20,000 km depth.",1105.1904v1
2011-05-19,Driving magnetic order in a manganite by ultrafast lattice excitation,"Optical control of magnetism, of interest for high-speed data processing and
storage, has only been demonstrated with near-infrared excitation to date.
However, in absorbing materials, such high photon energies can lead to
significant dissipation, making switch back times long and miniaturization
challenging. In manganites, magnetism is directly coupled to the lattice, as
evidenced by the response to external and chemical pressure, or to
ferroelectric polarization. Here, femtosecond mid-infrared pulses are used to
excite the lattice in La0.5Sr1.5MnO4 and the dynamics of electronic order are
measured by femtosecond resonant soft x-ray scattering with an x-ray free
electron laser. We observe that magnetic and orbital orders are reduced by
excitation of the lattice. This process, which occurs within few picoseconds,
is interpreted as relaxation of the complex charge-orbital-spin structure
following a displacive exchange quench - a prompt shift in the equilibrium
value of the magnetic and orbital order parameters after the lattice has been
distorted. A microscopic picture of the underlying unidirectional lattice
displacement is proposed, based on nonlinear rectification of the
directly-excited vibrational field, as analyzed in the specific lattice
symmetry of La0.5Sr1.5MnO4. Control of magnetism through ultrafast lattice
excitation has important analogies to the multiferroic effect and may serve as
a new paradigm for high-speed optomagnetism.",1105.3866v1
2011-05-31,Jet rotation driven by MHD shocks in helical magnetic fields,"In this paper we present a detailed numerical investigation of the hypothesis
that a rotation of astrophysical jets can be caused by magnetohydrodynamic
shocks in a helical magnetic field. Shock compression of the helical magnetic
field results in a toroidal Lorentz force component which will accelerate the
jet material in toroidal direction. This process transforms magnetic angular
momentum (magnetic stress) carried along the jet into kinetic angular momentum
(rotation). The mechanism proposed here only works in a helical magnetic field
configuration. We demonstrate the feasibility of this mechanism by axisymmetric
MHD simulations in 1.5D and 2.5D using the PLUTO code. In our setup the jet is
injected into the ambient gas with zero kinetic angular momentum (no rotation).
Different dynamical parameters for jet propagation are applied such as the jet
internal Alfven Mach number and fast magnetosonic Mach number, the density
contrast of jet to ambient medium, or the external sonic Mach number of the
jet. The mechanism we suggest should work for a variety of jet applications,
e.g. protostellar or extragalactic jets, and internal jet shocks (jet knots) or
external shocks between the jet and ambient gas (entrainment). For typical
parameter values for protostellar jets, the numerically derived rotation
feature looks consistent with the observations, i.e. rotational velocities of
0.1-1 percent of the jet bulk velocity.",1105.6232v1
2011-06-07,Towards Unquenched Holographic Magnetic Catalysis,"We propose a string dual to the SU(Nc) N=4 SYM coupled to Nf massless
fundamental flavors in an external magnetic field. The flavors are introduced
by homogeneously smeared Nf D7-branes and the external magnetic field via a
non-trivial Kalb-Ramond B-field. Our solution is perturbative in a parameter
that counts the number of internal flavor loops. In the limit of vanishing
B-field the background reduces to the supersymmetric one obtained in
hep-th/0612118. We introduce an additional probe D7--brane and in the
supersymmetric limit of vanishing B-field perform a holographic renormalization
of its ""on-shell"" action. We consider also non-supersymmetric probes with fixed
worldvolume gauge field corresponding to a magnetic field coupled only to the
fundamental fields of the probe brane. We study the influence of the
backreacted flavors on the effect of dynamical mass generation. Qualitatively
the physical picture remains unchanged. In the next step we consider the case
when the magnetic field couples to both the backreacted and the probe
fundamental degrees of freedom. At sufficiently strong magnetic field the meson
spectrum signals an instability of the probe D7-brane, which we interpret as
reflecting an instability of the supergravity background.",1106.1330v2
2011-06-08,Kinematics and Fine Structure of An Unwinding Polar Jet Observed by SDO/AIA,"We present an observational study of the kinematics and fine structure of an
unwinding polar jet, with high temporal and spatial observations taken by the
Atmospheric Imaging Assembly (AIA) on board the Solar Dynamic Observatory (SDO)
and the Solar Magnetic Activity Research Telescope (SMART). During the rising
period, the shape of the jet resembled a cylinder with helical structures on
the surface, while the mass of the jet was mainly distributed on the cylinder's
shell. In the radial direction, the jet expanded successively at its western
side. The radial expansion presented three distinct phases: the gradually
expanding phase, the fast expanding phase, and the steady phase. Each phase
lasted for about 12 minutes. The angular speed of the unwinding jet and the
twist transferred into the outer corona during the eruption are estimated to be
11.1 \times 10{-3} rad/s (period = 564 s) and 1.17 to 2.55 turns (or 2.34 to
5.1{\pi}) respectively. On the other hand, by calculating the azimuthal
component of the magnetic field in the jet and comparing the free energy stored
in the non-potential magnetic field with the jet's total energy, we find that
the non-potential magnetic field in the jet is enough to supply the energy for
the ejection. These new observational results strongly support the scenario
that the jets are driven by the magnetic twist, which is stored in the twisted
closed field of a bipole, and released through magnetic reconnection between
the bipole and its ambient open field.",1106.1489v2
2011-06-22,Spin correlations in the extended kagome system YBaCo3FeO7,"The transition metal based oxide YBaCo3FeO7 is structurally related to the
mineral Swedenborgite SbNaBe4O7, a polar non-centrosymmetric crystal system.
The magnetic Co3Fe sublattice consists of a tetrahedral network containing
kagome-like layers with trigonal interlayer sites. This geometry causes
frustration effects for magnetic ordering, which were investigated by
magnetization measurements, M\""ossbauer spectroscopy, polarized neutron
diffraction, and neutron spectroscopy. Magnetization measurement and neutron
diffraction do not show long range ordering even at low temperature (1 K)
although a strong antiferromagnetic coupling (~2000 K) is deduced from the
magnetic susceptibility. Below 590 K, we observe two features, a spontaneous
weak anisotropic magnetization hysteresis along the polar crystallographic axis
and a hyperfine field on the Fe kagome sites, whereas the Fe spins on the
interlayer sites remain idle. Below ~50 K, the onset of a hyperfine field shows
the development of moments static on the M\""ossbauer time scale also for the Fe
interlayer sites. Simultaneously, an increase of spin correlations is found by
polarized neutron diffraction. The relaxation part of the dynamic response has
been further investigated by high-resolution neutron spectroscopy, which
reveals that the spin correlations start to freeze in below ~50 K. Monte Carlo
simulations show that the neutron scattering results at lower temperatures are
compatible with a recent proposal that the particular geometric frustration in
the Swedenborgite structure promotes quasi one dimensional partial order.",1106.4465v1
2011-07-28,Identification and selection rules of the spin-wave eigen-modes in a normally magnetized nano-pillar,"We report on a spectroscopic study of the spin-wave eigen-modes inside an
individual normally magnetized two layers circular nano-pillar
(Permalloy$|$Copper$|$Permalloy) by means of a Magnetic Resonance Force
Microscope (MRFM). We demonstrate that the observed spin-wave spectrum
critically depends on the method of excitation. While the spatially uniform
radio-frequency (RF) magnetic field excites only the axially symmetric modes
having azimuthal index $\ell=0$, the RF current flowing through the
nano-pillar, creating a circular RF Oersted field, excites only the modes
having azimuthal index $\ell=+1$. Breaking the axial symmetry of the
nano-pillar, either by tilting the bias magnetic field or by making the pillar
shape elliptical, mixes different $\ell$-index symmetries, which can be excited
simultaneously by the RF current. Experimental spectra are compared to
theoretical prediction using both analytical and numerical calculations. An
analysis of the influence of the static and dynamic dipolar coupling between
the nano-pillar magnetic layers on the mode spectrum is performed.",1107.5699v2
2011-07-29,Large-scale magnetic field generation via the kinetic Kelvin-Helmholtz instability in unmagnetized scenarios,"Collisionless plasma instabilities are fundamental in magnetic field
generation in astrophysical scenarios, but their role has been addressed in
scenarios where velocity shear is absent. In this work we show that velocity
shears must be considered when studying realistic astrophysical scenarios,
since these trigger the collisionless Kelvin-Helmholtz instability (KHI). We
present the first self-consistent three-dimensional (3D) particle-in-cell (PIC)
simulations of the KHI in conditions relevant for unmagnetized relativistic
outflows with velocity shear, such as active galactic nuclei (AGN) and
gamma-ray bursts (GRBs). We show the generation of a strong large-scale DC
magnetic field, which extends over the entire shear-surface, reaching
thicknesses of a few tens of electron skin depths, and persisting on
time-scales much longer than the electron time scale. This DC magnetic field is
not captured by MHD models since it arises from intrinsically kinetic effects.
Our results indicate that the KHI can generate intense magnetic fields yielding
equipartition values up to \epsilon_B/\epsilon_p ~ 10^-3 in the electron
time-scale. The KHI-induced magnetic fields have a characteristic structure
that will lead to a distinct radiation signature, and can seed the turbulent
dynamo amplification process. The dynamics of the KHI are relevant for
non-thermal radiation modeling and can also have a strong impact on the
formation of relativistic shocks in presence of velocity shears.",1107.6037v2
2011-09-02,Thermally Activated Magnetization and Resistance Decay during Near Ambient Temperature Aging of Co Nanoflakes in a Confining Semi-metallic Environment,"We report the observation of magnetic and resistive aging in a self assembled
nanoparticle system produced in a multilayer Co/Sb sandwich. The aging decays
are characterized by an initial slow decay followed by a more rapid decay in
both the magnetization and resistance. The decays are large accounting for
almost 70% of the magnetization and almost 40% of the resistance for samples
deposited at 35 $^oC$. For samples deposited at 50 $^oC$ the magnetization
decay accounts for $\sim 50%$ of the magnetization and 50% of the resistance.
During the more rapid part of the decay, the concavity of the slope of the
decay changes sign and this inflection point can be used to provide a
characteristic time. The characteristic time is strongly and systematically
temperature dependent, ranging from $\sim1$x$10^2 s$ at 400K to $\sim3$x$10^5
s$ at 320K in samples deposited at $35 ^oC$. Samples deposited at 50 $^oC$
displayed a 7-8 fold increase in the characteristic time (compared to the $35
^oC$ samples) for a given aging temperature, indicating that this timescale may
be tunable. Both the temperature scale and time scales are in potentially
useful regimes. Pre-Aging, Scanning Tunneling Microscopy (STM) reveals that the
Co forms in nanoscale flakes. During aging the nanoflakes melt and migrate into
each other in an anisotropic fashion forming elongated Co nanowires. This aging
behavior occurs within a confined environment of the enveloping Sb layers. The
relationship between the characteristic time and aging temperature fits an
Arrhenius law indicating activated dynamics.",1109.0506v2
2011-10-03,Small-scale dynamo action in rotating compressible convection,"We study dynamo action in a convective layer of electrically-conducting,
compressible fluid, rotating about the vertical axis. At the upper and lower
bounding surfaces, perfectly-conducting boundary conditions are adopted for the
magnetic field. Two different levels of thermal stratification are considered.
If the magnetic diffusivity is sufficiently small, the convection acts as a
small-scale dynamo. Using a definition for the magnetic Reynolds number $R_M$
that is based upon the horizontal integral scale and the horizontally-averaged
velocity at the mid-layer of the domain, we find that rotation tends to reduce
the critical value of $R_M$ above which dynamo action is observed. Increasing
the level of thermal stratification within the layer does not significantly
alter the critical value of $R_M$ in the rotating calculations, but it does
lead to a reduction in this critical value in the non-rotating cases. At the
highest computationally-accessible values of the magnetic Reynolds number, the
saturation levels of the dynamo are similar in all cases, with the mean
magnetic energy density somewhere between 4 and 9% of the mean kinetic energy
density. To gain further insights into the differences between rotating and
non-rotating convection, we quantify the stretching properties of each flow by
measuring Lyapunov exponents. Away from the boundaries, the rate of stretching
due to the flow is much less dependent upon depth in the rotating cases than it
is in the corresponding non-rotating calculations. It is also shown that the
effects of rotation significantly reduce the magnetic energy dissipation in the
lower part of the layer. We also investigate certain aspects of the saturation
mechanism of the dynamo.",1110.0374v1
2012-01-03,Reversible room-temperature ferromagnetism in Nb-doped SrTiO3 single crystals,"The search for oxide-based room-temperature ferromagnetism has been one of
the holy grails in condensed matter physics. Room-temperature ferromagnetism
observed in Nb-doped SrTiO3 single crystals is reported in this Rapid
Communication. The ferromagnetism can be eliminated by air annealing (making
the samples predominantly diamagnetic) and can be recovered by subsequent
vacuum annealing. The temperature dependence of magnetic moment resembles the
temperature dependence of carrier density, indicating that the magnetism is
closely related to the free carriers. Our results suggest that the
ferromagnetism is induced by oxygen vacancies. In addition, hysteretic
magnetoresistance was observed for magnetic field parallel to current,
indicating that the magnetic moments are in the plane of the samples. The x-ray
photoemission spectroscopy, the static time-of-flight and the dynamic secondary
ion mass spectroscopy and proton induced x-ray emission measurements were
performed to examine magnetic impurities, showing that the observed
ferromagnetism is unlikely due to any magnetic contaminant.",1201.0579v4
2012-01-03,The alpha-effect in a turbulent liquid-metal plane Couette flow,"We calculate the mean electromotive force in plane Couette flows of a
nonrotating conducting fluid under the influence of a large-scale magnetic
field for driven turbulence. A vertical stratification of the turbulence
intensity results in an alpha effect owing to the presence of horizontal shear.
Here we discuss the possibility of an experimental determination of the
components of the alpha tensor using both quasilinear theory and nonlinear
numerical simulations. For magnetic Prandtl numbers of the order of unity, we
find that in the high-conductivity limit the alpha effect in the direction of
the flow clearly exceeds the component in spanwise direction. In this limit,
alpha runs linearly with the magnetic Reynolds number Rm while in the
low-conductivity limit it runs with the product Rm*Re, where Re is the kinetic
Reynolds number so that for given Rm the alpha effect grows with decreasing
magnetic Prandtl number.
  For the small magnetic Prandtl numbers of liquid metals, a common value for
the horizontal elements of the alpha tensor appears, which makes it unimportant
whether the alpha effect is measured in the spanwise or streamwise directions.
The resulting effect should lead to an observable voltage in both directions of
about 0.5 mV for magnetic fields of 1 kgauss and velocity fluctuations of about
1 m/s in a channel of 50 cm height (independent of its width).",1201.0652v2
2012-02-04,Directional dependence of color superconducting gap in two-flavor QCD in a magnetic field,"We study the effect of a magnetic field on the pairing dynamics in two-flavor
color superconducting dense quark matter. The study is performed in the weakly
coupled regime of QCD at asymptotically high density, using the framework of
the Schwinger-Dyson equation in the improved rainbow approximation. We show
that the superconducting gap function develops a directional dependence in
momentum space. Quasiparticles with momenta perpendicular to the direction of
the magnetic field have the largest gaps, while quasiparticles with momenta
parallel to the field have the smallest gaps. We argue that the directional
dependence is a consequence of a long-range interaction in QCD. The
quantitative measure of the ellipticity of the gap function is determined by a
dimensionless ratio, proportional to the square of the magnetic field and
inversely proportional to the fourth power of the quark chemical potential. For
magnetic fields in stars, $B\lesssim 10^{18} G$, the corresponding ratio is
estimated to be less than about $10^{-2}$, justifying the use of the weak
magnetic field limit in all stellar applications.",1202.0872v2
2012-02-13,Collisionless magnetic reconnection in a plasmoid chain,"The kinetic features of plasmoid chain formation and evolution are
investigated by two dimensional Particle-in-Cell simulations. Magnetic
reconnection is initiated in multiple X points by the tearing instability.
Plasmoids form and grow in size by continuously coalescing. Each chain plasmoid
exhibits a strong out-of plane core magnetic field and an out-of-plane electron
current that drives the coalescing process. The disappearance of the X points
in the coalescence process are due to anti-reconnection, a magnetic
reconnection where the plasma inflow and outflow are reversed with respect to
the original reconnection flow pattern. Anti-reconnection is characterized by
the Hall magnetic field quadrupole signature. Two new kinetic features, not
reported by previous studies of plasmoid chain evolution, are here revealed.
First, intense electric fields develop in-plane normally to the separatrices
and drive the ion dynamics in the plasmoids. Second, several bipolar electric
field structures are localized in proximity of the plasmoid chain. The analysis
of the electron distribution function and phase space reveals the presence of
counter-streaming electron beams, unstable to the two stream instability, and
phase space electron holes along the reconnection separatrices.",1202.2663v1
2012-03-15,Two-color QCD with staggered fermions at finite temperature under the influence of a magnetic field,"In this paper we investigate the influence of a constant external magnetic
field on the finite-temperature phase structure and the chiral properties of a
simplified lattice model for QCD. We assume an SU(2) gauge symmetry and employ
dynamical staggered fermions of identical mass without rooting, corresponding
to Nf=4 flavors of identical electric charge. For fixed mass (given in lattice
units) the critical temperature is seen to rise with the magnetic field
strength. For three fixed beta-values, selected such that we stay (i) within
the chirally broken phase, (ii) within the transition region or (iii) within
the chirally restored phase, we study the approach to the chiral limit for
various values of the magnetic field. Within the chirally broken (confinement)
phase the chiral condensate is found to increase monotonically with a growing
magnetic field strength. In the chiral limit the increase starts linear in
agreement with a chiral model studied by Shushpanov and Smilga. Within the
chirally restored (deconfinement) phase the chiral condensate tends to zero in
the chiral limit, irrespective of the strength of the magnetic field.",1203.3360v2
2012-05-16,Self-cancellation of ephemeral regions in the quiet Sun,"With the observations from the Helioseismic and Magnetic Imager aboard the
Solar Dynamics Observatory, we statistically investigate the ephemeral regions
(ERs) in the quiet Sun. We find that there are two types of ERs: normal ERs
(NERs) and self-cancelled ERs (SERs). Each NER emerges and grows with
separation of its opposite polarity patches which will cancel or coalesce with
other surrounding magnetic flux. Each SER also emerges and grows and its
dipolar patches separate at first, but a part of magnetic flux of the SER will
move together and cancel gradually, which is described with the term
""self-cancellation"" by us. We identify 2988 ERs among which there are 190 SERs,
about 6.4% of the ERs. The mean value of self-cancellation fraction of SERs is
62.5%, and the total self-cancelled flux of SERs is 9.8% of the total ER flux.
Our results also reveal that the higher the ER magnetic flux is, (i) the easier
the performance of ER self-cancellation is, (ii) the smaller the
self-cancellation fraction is, and (iii) the more the self-cancelled flux is.
We think that the self-cancellation of SERs is caused by the submergence of
magnetic loops connecting the dipolar patches, without magnetic energy release.",1205.3716v1
2012-07-17,Thermal Shielding of an Emerging Active Region,"The interaction between emerging active regions and the pre-existing coronal
magnetic field is important to understand better the mechanisms of storage and
release of magnetic energy from the convection zone to the high corona. We are
aiming at describing the first steps of the emergence of an active region
within a pre-existing quiet-Sun corona in terms of the thermal and magnetic
structure. We use unprecedented spatial, temporal and spectral coverage from
the Atmospheric Imager Assembly (AIA) and from the Helioseismic and Magnetic
Imager (HMI) on board the Solar Dynamics Observatory (SDO). Starting on 30 May
2010 at 17:00 UT and for 8 hours, we follow the emergence of the active region
AR11076 within a quiet-Sun region. Using several SDO/AIA filters covering
temperatures from 50000K to 10 MK, we show that the emerging process is
characterised by a thermal shield at the interface between the emerging flux
and pre-existing quiet-Sun corona. The active region 11076 can be considered as
a peculiar example of emerging active region as (i) the polarities emerge in a
photospheric quiet-Sun region near a supergranular-like distribution, (ii) the
polarities forming the bipolar emerging structure do not rotate with respect to
each other indicating a small amount of twist in the emerging flux bundle.
There is a thermal shield formed at the interface between the emerging active
region and the pre-existing quiet-Sun region. The thermal shielding structure
deduced from all SDO/AIA channels exhibits a strong asymmetry between the two
polarities of the active region suggesting that the heating mechanism for one
polarity is more likely to be magnetic reconnection, whilst it is due to
increasing magnetic pressure for the opposite polarity.",1207.3992v1
2012-08-06,A Non-radial Eruption in a Quadrupolar Magnetic Configuration with a Coronal Null,"We report one of several homologous non-radial eruptions from NOAA active
region (AR) 11158 that are strongly modulated by the local magnetic field as
observed with the Solar Dynamic Observatory (SDO). A small bipole emerged in
the sunspot complex and subsequently created a quadrupolar flux system.
Non-linear force-free field (NLFFF) extrapolation from vector magnetograms
reveals its energetic nature: the fast-shearing bipole accumulated ~2e31 erg
free energy (10% of AR total) over just one day despite its relatively small
magnetic flux (5% of AR total). During the eruption, the ejected plasma
followed a highly inclined trajectory, over 60 degrees with respect to the
radial direction, forming a jet-like, inverted-Y shaped structure in its wake.
Field extrapolation suggests complicated magnetic connectivity with a coronal
null point, which is favorable of reconnection between different flux
components in the quadrupolar system. Indeed, multiple pairs of flare ribbons
brightened simultaneously, and coronal reconnection signatures appeared near
the inferred null. Part of the magnetic setting resembles that of a
blowout-type jet; the observed inverted-Y structure likely outlines the open
field lines along the separatrix surface. Owing to the asymmetrical
photospheric flux distribution, the confining magnetic pressure decreases much
faster horizontally than upward. This special field geometry likely guided the
non-radial eruption during its initial stage.",1208.1097v1
2012-08-28,Instrumentation status of the low-b magnet systems at the Large Hadron Collider (LHC),"The low-beta magnet systems are located in the Large Hadron Collider (LHC)
insertion regions around the four interaction points. They are the key elements
in the beams focusing/defocusing process allowing proton collisions at
luminosity up to 10**34/cm**2s. Those systems are a contribution of the US-LHC
Accelerator project. The systems are mainly composed of the quadrupole magnets
(triplets), the separation dipoles and their respective electrical feed-boxes
(DFBX). The low-beta magnet systems operate in an environment of extreme
radiation, high gradient magnetic field and high heat load to the cryogenic
system due to the beam dynamic effect. Due to the severe environment, the
robustness of the diagnostics is primordial for the operation of the triplets.
The hardware commissioning phase of the LHC was completed in February 2010. In
the sake of a safer and more user-friendly operation, several consolidations
and instrumentation modifications were implemented during this commissioning
phase. This paper presents the instrumentation used to optimize the engineering
process and operation of the final focusing/defocusing quadrupole magnets for
the first years of operation.",1208.5779v1
2012-09-07,General relativistic simulations of black hole-neutron star mergers: Effects of tilted magnetic fields,"Black hole--neutron star (BHNS) binary mergers can form disks in which
magnetorotational instability (MRI)-induced turbulence may drive accretion onto
the remnant BH, supporting relativistic jets and providing the engine for a
short-hard gamma-ray burst (SGRB). Our earlier study of magnetized BHNSs showed
that NS tidal disruption winds the magnetic field into a toroidal
configuration, with poloidal fields so weak that capturing MRI with full-disk
simulations would require $\sim 10^8$ CPU-hours. In that study we imposed
equatorial symmetry, suppressing poloidal magnetic fields that might be
generated from plasma crossing the orbital plane. Here we show that initial
conditions that break this symmetry (i.e., {\it tilted} poloidal magnetic
fields in the NS) generate much stronger poloidal fields in the disk,
indicating that asymmetric initial conditions may be necessary for establishing
BHNS mergers as SGRB progenitors via fully general relativistic MHD
simulations. We demonstrate that BHNS mergers may form an SGRB engine under the
right conditions by seeding the remnant disk from an unmagnetized BHNS
simulation with purely poloidal fields dynamically unimportant initially, but
strong enough to resolve MRI. Magnetic turbulence occurs in the disk, driving
accretion and supporting Poynting-dominated jet outflows sufficient to power an
SGRB.",1209.1632v2
2012-11-26,Nonlinear Evolution of the Radiation-Driven Magneto-Acoustic Instability (RMI),"We examine the nonlinear development of unstable magnetosonic waves driven by
a background radiative flux -- the Radiation-Driven Magneto-Acoustic
Instability (RMI, a.k.a. the ""photon bubble"" instability). The RMI may serve as
a persistent source of density, radiative flux, and magnetic field fluctuations
in stably-stratified, optically-thick media. The conditions for instability are
present in a variety of astrophysical environments, and do not require the
radiation pressure to dominate or the magnetic field to be strong. Here we
numerically study the saturation properties of the RMI, covering three orders
of magnitude in the relative strength of radiation, magnetic field, and gas
energies. Two-dimensional, time-dependent radiation-MHD simulations of local,
stably-stratified domains are conducted with Zeus-MP in the optically-thick,
highly-conducting limit. Our results confirm the theoretical expectations of
Blaes and Socrates (2003) in that the RMI operates even in gas
pressure-dominated environments that are weakly magnetized. The saturation
amplitude is a monotonically increasing function of the ratio of radiation to
gas pressure. Keeping this ratio constant, we find that the saturation
amplitude peaks when the magnetic pressure is comparable to the radiation
pressure. We discuss the implications of our results for the dynamics of
magnetized stellar envelopes, where the RMI should act as a source of
sub-photospheric perturbations.",1211.6107v2
2012-12-21,Buoyant Magnetic Loops Generated by Global Convective Dynamo Action,"Our global 3D simulations of convection and dynamo action in a Sun-like star
reveal that persistent wreaths of strong magnetism can be built within the bulk
of the convention zone. Here we examine the characteristics of buoyant magnetic
structures that are self-consistently created by dynamo action and turbulent
convective motions in a simulation with solar stratification but rotating at
three times the current solar rate. These buoyant loops originate within
sections of the magnetic wreaths in which turbulent flows amplify the fields to
much larger values than is possible through laminar processes. These amplified
portions can rise through the convective layer by a combination of magnetic
buoyancy and advection by convective giant cells, forming buoyant loops. We
measure statistical trends in the polarity, twist, and tilt of these loops.
Loops are shown to preferentially arise in longitudinal patches somewhat
reminiscent of active longitudes in the Sun, although broader in extent. We
show that the strength of the axisymmetric toroidal field is not a good
predictor of the production rate for buoyant loops or the amount of magnetic
flux in the loops that are produced.",1212.5612v1
2013-04-15,"Collisions, magnetization, and transport coefficients in the lower solar atmosphere","The lower solar atmosphere is an intrinsically multi-component and
collisional environment with electron and proton collision frequencies in the
range $10^{8}-10^{10}$ Hz, which may be considerably higher than the
gyro-frequencies for both species. We aim to provide a reliable quantitative
set of data for collision frequencies, magnetization, viscosity, and thermal
conductivity for the most important species in the lower solar atmosphere.
Having such data at hand is essential for any modeling that is aimed at
describing realistic properties of the considered environment.
  We describe the altitude dependence of the parameters and the different
physics of collisions between charged species, and between charged and neutrals
species. Regions of dominance of each type of collisions are clearly
identified. We determine the layers within which either electrons or ions or
both are unmagnetized. Protons are shown to be un-magnetized in the lower
atmosphere in a layer that is at least 1000 km thick even for a kilo-Gauss
magnetic field that decreases exponentially with altitude. In these layers the
dynamics of charged species cannot be affected by the magnetic field, and this
fact is used in our modeling. Viscosity and thermal conductivity coefficients
are calculated for layers where ions are unmagnetized. We compare viscosity and
friction and determine the regions of dominance of each of the phenomena.
  We provide the most reliable quantitative values for most important
parameters in the lower solar atmosphere to be used in analytical modeling and
numerical simulations of various phenomena such as waves, transport and
magnetization of particles, and the triggering mechanism of coronal mass
ejections.",1304.4010v1
2013-04-29,Spatially periodic domain wall pinning potentials: Asymmetric pinning and dipolar biasing,"Domain wall propagation has been measured in continuous, weakly disordered,
quasi-two-dimensional, Ising-like magnetic layers that are subject to spatially
periodic domain wall pinning potentials. The potentials are generated
non-destructively using the stray magnetic field of ordered arrays of
magnetically hard [Co/Pt]$_m$ nanoplatelets which are patterned above and are
physically separated from the continuous magnetic layer. The effect of the
periodic pinning potentials on thermally activated domain wall creep dynamics
is shown to be equivalent, at first approximation, to that of a uniform,
effective retardation field, $H_{ret}$, which acts against the applied field,
$H$. We show that $H_{ret}$ depends not only on the array geometry but also on
the relative orientation of $H$ and the magnetization of the nanoplatelets. A
result of the latter dependence is that wall-mediated hysteresis loops obtained
for a set nanoplatelet magnetization exhibit many properties that are normally
associated with ferromagnet/antiferromagnet exchange bias systems. These
include a switchable bias, coercivity enhancement and domain wall roughness
that is dependent on the applied field polarity.",1304.7560v1
2013-05-16,Out-of-Equilibrium Chiral Magnetic Effect at Strong Coupling,"We study the charge transports originating from triangle anomaly in
out-of-equilibrium conditions in the framework of AdS/CFT correspondence at
strong coupling, to gain useful insights on possible charge separation effects
that may happen in the very early stages of heavy-ion collisions. We first
construct a gravity background of a homogeneous mass shell with a finite
(axial) charge density gravitationally collapsing to a charged blackhole, which
serves as a dual model for out-of-equilibrium charged plasma undergoing
thermalization. We find that a finite charge density in the plasma slows down
the thermalization. We then study the out-of-equilibrium properties of Chiral
Magnetic Effect and Chiral Magnetic Wave in this background. As the medium
thermalizes, the magnitude of chiral magnetic conductivity and the response
time delay grow. We find a dynamical peak in the spectral function of retarded
current correlator, which we identify as an out-of-equilibrium chiral magnetic
wave. The group velocity of the out-of-equilibrium chiral magnetic wave is
shown to receive a dominant contribution from a non-equilibrium effect, making
the wave moving much faster than in the equilibrium, which may enhance the
charge transports via triangle anomaly in the early stage of heavy-ion
collisions.",1305.3949v2
2013-06-20,"Magnetic orders, excitations, and phase transitions in Fe_1+yTe","We study the magnetic properties of Fe_1+y Te, a parent compound of the
iron-based high-temperature superconductors. Motivated by recent neutron
scattering experiments, we show that a spin-1 exchange model, supplemented by a
single-ion spin anisotropy, accounts well for the experimentally observed low
temperature magnetic phase diagram, that exhibits a commensurate bicollinear
order at low Fe dopings (y < 0.12) and an incommensurate spin-spiral order at
high Fe dopings (y > 0.12). We suggest that the commensurate-incommensurate
transition at y = 0.12 is due to the competition between the exchange
interaction and the local spin anisotropy. At low Fe dopings, the single-ion
spin anisotropy is strong and pins the spins along the easy axis, which,
together with the spatially anisotropic exchanges, induces a unusual
bicollinear commensurate magnetic order. The low-energy spin-wave excitation is
gapped due to the explict breaking of spin-rotational symmetry by the local
spin anisotropy. At high Fe dopings, the single-ion anisotropy is weak, and the
exchange favors an incommensurate coplanar state. The incommensurate magnetic
wavevector averages out the spin anisotropy so that a gapless low-energy
spin-wave excitation is obtained. We also analyze the low-energy hydrodynamic
model and use it to describe the magneto-structural transition and the static
and dynamical spin structure factors across the magnetic ordering transitions.",1306.4730v2
2013-08-01,Quark confinement via magnetic color-flavor locking,"The color-flavor locking phenomenon in the magnetic picture can be the
microscopic description of the quark confinement in QCD. We demonstrate it in
an N=2 supersymmetric SU(Nc)xSU(Nc) quiver gauge theory coupled to Nf flavors
of quarks (Nf<Nc). This model reduces to SU(Nc) gauge theory with Nf flavors
when the vacuum expectations value of the link field is much larger than the
dynamical scales, and thus provides a continuous deformation of the N=2
supersymmetric QCD. We study a vacuum which survives upon adding a
superpotential term to reduce to N=1 while preserving the vectorial SU(Nf)
flavor symmetry. We find a region of the parameter space where the confinement
is described by the Higgsing of a weakly coupled magnetic SU(Nf)xU(1) gauge
theory. The Higgsing locks the quantum numbers of SU(Nf) magnetic color to
those of SU(Nf) flavor symmetry, and thus the massive magnetic gauge bosons
become the singlet and adjoint representations of the flavor group, i.e, the
vector mesons. If the qualitative picture remains valid in non-supersymmetric
QCD, one can understand the Hidden Local Symmetry as the magnetic dual
description of QCD, and the confining string is identified as the vortex of
vector meson fields.",1308.0093v2
2013-09-10,Artificial magnetic field induced by an evanescent wave,"Cold atomic gases are perfect laboratories for realization of quantum
simulators. In order to simulate solid state systems in the presence of
magnetic fields special effort has to be made because atoms are charge neutral.
There are different methods for realization of artificial magnetic fields, that
is the creation of specific conditions so that the motion of neutral particles
mimics the dynamics of charged particles in an effective magnetic field. Here,
we consider adiabatic motion of atoms in the presence of an evanescent wave.
Theoretical description of the adiabatic motion involves artificial vector and
scalar potentials related to the Berry phases. Due to the large gradient of the
evanescent field amplitude, the potentials can be strong enough to induce
measurable effects in cold atomic gases. We show that the resulting artificial
magnetic field is able to induce vortices in a Bose-Einstein condensate trapped
close to a surface of a prism where the evanescent wave is created. We also
analyze motion of an atomic cloud released from a magneto-optical trap that
falls down on the surface of the prism. The artificial magnetic field is able
to reflect falling atoms that can be observed experimentally.",1309.2674v4
2013-09-13,Laminar and transitional liquid metal duct flow near a magnetic point dipole,"The flow transformation and the generation of vortex structures by a strong
magnetic dipole field in a liquid metal duct flow is studied by means of
three-dimensional direct numerical simulations. The dipole is considered as the
paradigm for a magnetic obstacle which will deviate the streamlines due to
Lorentz forces acting on the fluid elements. The duct is of square
cross-section. The dipole is located above the top wall and is centered in
spanwise direction. Our model uses the quasi-static approximation which is
applicable in the limit of small magnetic Reynolds numbers. The analysis covers
the stationary flow regime at small hydrodynamic Reynolds numbers Re as well as
the transitional time-dependent regime at higher values which may generate a
turbulent flow in the wake of the magnetic obstacle. We present a systematic
study of these two basic flow regimes and their dependence on Re and on the
Hartmann number Ha, a measure of the strength of the magnetic dipole field.
Furthermore, three orientations of the dipole are compared: streamwise,
spanwise and wall-normal oriented dipole axes. The most efficient generation of
turbulence at a fixed distance above the duct follows for the spanwise
orientation, which is caused by a certain configuration of Hartmann layers and
reversed flow at the top plate. The enstrophy in the turbulent wake grows
linearly with Ha which is connected with a dominance of the wall-normal
z-derivative of the streamwise velocity.",1309.3506v2
2013-11-25,A study of crossover from 3D ferrimagnetic Bulk $NiCr_{2}O_{4}$ compound into 2D spin-glass like nanophase,"In this report, the magnetic behaviour of $NiCr_{2}O_{4}$ bulk and
nanoparticle samples under different applied magnetic field has been
investigated extensively. Nanoparticles of $NiCr_{2}O_{4}$ were obtained by
mechanical milling of polycrystalline powder prepared by polyol method. FC-ZFC
measurement of bulk at different applied magnetic field has revealed the
existence of a ferrimagnetic transition around 66K followed by an
antiferromagnetic transition close to 30K. However, its nano counterpart has
shown remarkable change in magnetic properties - a suppression of ferrimagnetic
transition accompanied by strengthening low temperature magnetic phase and
observation of a new transition at 90K ($T_P$), which is weakly magnetic in
nature. The frequency dependent ac susceptibility data of nanoparticle have
been fitted to the well known de Almedia-Thouless equation and a $H^{2/3}$
dependence of the low temperature peak is observed with a resulting zero field
freezing temperature ($T_f^0$) equal to 10.1K. Further, the dynamical behaviour
near freezing temperature has been analysed in terms of critical behaviour and
the obtained fitted parameters values being as $\tau_0$(relaxation time
constant) = $3.6 X 10^{-6}s$, $T_f^0=8.7$K and $z\nu = 11.1$. Moreover,
Vogel-Fulcher law has been used to understand the nature of freezing transition
and the parameter after fitting are obtained as $E_a/k_B = 58.9$K, $\tau_0 =
5.22 \times 10^{-8}$ and $T_0 = 8.03$K. Finally, the spin-glass phase is
concluded. Moreover, in contrast to bulk, the $H^{2/3}$ dependence of freezing
temperature of nanoparticle sample (75h) does support the 2D surface like spin
glass nature.",1311.6269v1
2014-01-23,Spin-correlations and magnetic structure in an Fe monolayer on 5d transition metal surfaces,"We present a detailed first principles study on the magnetic structure of an
Fe monolayer on different surfaces of 5d transition metals. We use the
spin-cluster expansion technique to obtain parameters of a spin model, and
predict the possible magnetic ground state of the studied systems by employing
the mean field approach and in certain cases by spin dynamics calculations. We
point out that the number of shells considered for the isotropic exchange
interactions plays a crucial role in the determination of the magnetic ground
state. In the case of Ta substrate we demonstrate that the out-of-plane
relaxation of the Fe monolayer causes a transition from ferromagnetic to
antiferromagnetic ground state. We examine the relative magnitude of nearest
neighbour Dzyaloshinskii-Moriya (D) and isotropic (J) exchange interactions in
order to get insight into the nature of magnetic pattern formations. For the
Fe/Os(0001) system we calculate a very large D/J ratio, correspondingly, a spin
spiral ground state. We find that, mainly through the leading isotropic
exchange and Dzyaloshinskii-Moriya interactions, the inward layer relaxation
substantially influences the magnetic ordering of the Fe monolayer. For the
Fe/Re(0001) system characterized by large antiferromagnetic interactions we
also determine the chirality of the $120^{\circ}$ N\'eel-type ground state.",1401.6073v2
2014-01-24,A search for magnetic fields on central stars in planetary nebulae,"One of the possible mechanisms responsible for the panoply of shapes in
planetary nebulae is the presence of magnetic fields that drive the ejection of
ionized material during the proto-planetary nebula phase. Therefore, detecting
magnetic fields in such objects is of key importance for understanding their
dynamics. Still, magnetic fields have not been detected using polarimetry in
the central stars of planetary nebulae. Circularly polarized light spectra have
been obtained with the Focal Reducer and Low Dispersion Spectrograph at the
Very Large Telescope of the European Southern Observatory and the Intermediate
dispersion Spectrograph and Imaging System at the William Herschel Telescope.
Nineteen planetary nebulae spanning very different morphology and evolutionary
stages have been selected. Most of central stars have been observed at
different rotation phases to point out evidence of magnetic variability. In
this paper, we present the result of two observational campaigns aimed to
detect and measure the magnetic field in the central stars of planetary nebulae
on the basis of low resolution spectropolarimetry. In the limit of the adopted
method, we can state that large scale fields of kG order are not hosted on the
central star of planetary nebulae.",1401.6282v1
2014-02-12,Properties of sunspot umbrae observed in Cycle 24,"We analyzed the size, intensity, and magnetic field strength of sunspot
umbrae to compare the present cycle 24 with the previous one. We used data of
the Helioseismic and Magnetic Imager onboard the Solar Dynamics Observatory and
selected all sunspots between May 2010 and October 2012, using one image per
day. We created two subsets of this data with a manual tracking algorithm, both
without duplication. One is containing each sunspot (910 umbrae within 488
spots) and was used to analyze the distribution of umbral areas, selected with
an automated thresholding method. The other one contains 205 fully evolved
sunspots. We find nonlinear relations between umbral minimum intensity and size
and between maximum magnetic field strength and size. The field strength scales
linear with the intensity and the umbral size scales roughly linear with the
total magnetic flux, while the size and field strength level off with stronger
flux. When separated in hemisphere and averaged temporally, the southern umbrae
show a temporal increase in size and the northern umbrae stay constant. There
is no temporal variation in the umbral mean intensity detectable. The
probability density function of the umbral area in the ascending phase of the
current solar cycle is similar to that of the last solar cycle. From our
investigation of umbral area, magnetic field, magnetic flux and umbral
intensity of the sunspots of the rising phase of cycle 24, we do not find a
significant difference to the previous cycle, and hence no indication for a
long-term decline of solar activity.",1402.2881v1
2014-03-12,Deciphering Solar Magnetic Activity I: On The Relationship Between The Sunspot Cycle And The Evolution Of Small Magnetic Features,"Sunspots are a canonical marker of the Sun's internal magnetic field which
flips polarity every ~22-years. The principal variation of sunspots, an
~11-year variation in number, modulates the amount of magnetic field that
pierces the solar surface and drives significant variations in our Star's
radiative, particulate and eruptive output over that period. This paper
presents observations from the Solar and Heliospheric Observatory and Solar
Dynamics Observatory indicating that the 11-year sunspot variation is
intrinsically tied it to the spatio-temporal overlap of the activity bands
belonging to the 22-year magnetic activity cycle. Using a systematic analysis
of ubiquitous coronal brightpoints, and the magnetic scale on which they appear
to form, we show that the landmarks of sunspot cycle 23 can be explained by
considering the evolution and interaction of the overlapping activity bands of
the longer scale variability.",1403.3071v3
2014-03-18,Two-Fluid Description of Wave-Particle Interactions in Strong Buneman Turbulence,"To understand the nature of anomalous resistivity in magnetic reconnection,
we investigate turbulence-induced momentum transport and energy dissipation
while a plasma is unstable to the Buneman instability in force-free current
sheets. Using 3D particle-in-cell simulations, we find that the macroscopic
effects generated by wave-particle interactions in Buneman instability can be
approximately described by a set of electron fluid equations. We show that both
energy dissipation and momentum transport along electric current in the current
layer are locally quasi-static, but globally dynamic and irreversible.
Turbulent drag dissipates both the streaming energy of the current sheet and
the associated magnetic energy. The net loss of streaming energy is converted
into the electron component heat conduction parallel to the magnetic field and
increases the electron Boltzmann entropy. The growth of self-sustained Buneman
waves satisfies a Bernoulli-like equation that relates the turbulence-induced
convective momentum transport and thermal momentum transport. Electron trapping
and de-trapping drive local momentum transports, while phase mixing converts
convective momentum into thermal momentum. The drag acts like a micro-macro
link in the anomalous heating processes. The decrease of magnetic field
maintains an inductive electric field that re-accelerates electrons, but most
of the magnetic energy is dissipated and converted into the component heat of
electrons perpendicular to the magnetic field. This heating process is
decoupled from the heating of Buneman instability in the current sheets. Ion
heating is weak but ions plays an important role in assisting energy exchanges
between waves and electrons. Cold ion fluid equations together with our
electron fluid equations form a complete set of equations that describes the
occurrence, growth, saturation and decay of the Buneman instability.",1403.4552v2
2014-03-31,The magnetic field configuration of a solar prominence inferred from spectropolarimetric observations in the He I 10830 A triplet,"Context: The determination of the magnetic field vector in quiescent solar
prominences is possible by interpreting the Hanle and Zeeman effects in
spectral lines. However, observational measurements are scarce and lack high
spatial resolution. Aims: To determine the magnetic field vector configuration
along a quiescent solar prominence by interpreting spectropolarimetric
measurements in the He I 1083.0 nm triplet obtained with the Tenerife Infrared
Polarimeter installed at the German Vacuum Tower Telescope of the Observatorio
del Teide. Methods. The He I 1083.0 nm triplet Stokes profiles are analyzed
with an inversion code that takes into account the physics responsible of the
polarization signals in this triplet. The results are put into a solar context
with the help of extreme ultraviolet observations taken with the Solar Dynamic
Observatory and the Solar Terrestrial Relations Observatory satellites.
Results: For the most probable magnetic field vector configuration, the
analysis depicts a mean field strength of 7 gauss. We do not find local
variations in the field strength except that the field is, in average, lower in
the prominence body than in the prominence feet, where the field strength
reaches 25 gauss. The averaged magnetic field inclination with respect to the
local vertical is 77 degrees. The acute angle of the magnetic field vector with
the prominence main axis is 24 degrees for the sinistral chirality case and 58
degrees for the dextral chirality. These inferences are in rough agreement with
previous results obtained from the analysis of data acquired with lower spatial
resolutions.",1403.7976v1
2014-05-07,Dynamic Evolution of an X-shaped Structure above a Trans-equatorial Quadrupole Solar Active Region Group,"In the solar corona, magnetic reconnection usually takes place at the
singular configuration of magnetic field, in particular near a magnetic null
owing to its high susceptibility to perturbations. In this Letter, we report a
rare X-shaped structure, encompassing a magnetic null, above a trans-equatorial
quadrupole active region group that is well observed by the Atmospheric Imaging
Assembly (AIA). The observations show that this X-shaped structure is visible
in all AIA EUV passbands and stably exists for days. However, possibly induced
by flare activities at the northern part of the quadrupole active region group,
the X-shaped structure starts to destabilize and meanwhile a jet erupted near
its center at 15:05 UT on 2013 October 7. Through the non-linear force-free
field modeling, we identify a magnetic null, which is above the quadrupole
polarities and well corresponds to the X-shaped structure. After the jet
eruption, the temperature and emission measure of the plasma near the X-shaped
structure rise from 2.3 MK and 1.2x10^27 cm-5 at 15:01 UT to 5.4 MK and
3.7x10^27 cm-5 at 15:36 UT, respectively, revealed by the differential emission
measure analysis, indicating that magnetic reconnection most likely takes place
there to heat the plasma. Moreover, the height of the null has an increase of
10 Mm, which is most likely due to the partial opening of the field lines near
the fan surface that makes the null underneath rise to seek for a new
equilibrium.",1405.1563v1
2014-06-06,High speed photospheric material flow observed at the polarity inversion line of a delta-type sunspot producing an X5.4 flare on 7 March 2012,"Solar flares abruptly release the free energy stored as a non-potential
magnetic field in the corona and may be accompanied by eruptions of the coronal
plasma. Formation of a non-potential magnetic field and the mechanisms for
triggering the onset of flares are still poorly understood. In particular,
photospheric dynamics observed near those polarity inversion lines that are
sites of major flare production have not been well observed with high spatial
resolution spectro-polarimetry. This paper reports on a remarkable high-speed
material flow observed along the polarity inversion line located between flare
ribbons at the main energy release side of an X5.4 flare on 7 March 2012.
Observations were carried out by the spectro-polarimeter of the Solar Optical
Telescope onboard Hinode. The high-speed material flow was observed in the
horizontally-oriented magnetic field formed nearly parallel to the polarity
inversion line. This flow persisted from at least 6 hours before the onset of
the flare, and continued for at least several hours after the onset of the
flare. Observations suggest that the observed material flow represents neither
the emergence nor convergence of the magnetic flux. Rather, it may be
considered to be material flow working both to increase the magnetic shear
along the polarity inversion line and to develop magnetic structures favorable
for the onset of the eruptive flare.",1406.1617v1
2014-09-24,Modelling MHD accretion-ejection - episodic ejections of jets triggered by a mean-field disk dynamo,"We present MHD simulations exploring the launching, acceleration and
collimation of jets and disk winds. The evolution of the disk structure is
consistently taken into account. Extending our earlier studies, we now consider
the self-generation of the magnetic field by an $\alpha^2\Omega$ mean-field
dynamo. The disk magnetization remains on a rather low level, that helps to
evolve the simulations for $T > 10,000$ dynamical time steps on a domain
extending 1500 inner disk radii. We find a magnetic field of the inner disk
similar to the commonly found open field structure, favoring
magneto-centrifugal launching. The outer disk field is highly inclined and
predominantly radial. Here, differential rotation induces a strong toroidal
component that plays a key role in outflow launching. These outflows from the
outer disk are slower, denser, and less collimated. If the dynamo action is not
quenched, magnetic flux is continuously generated, diffuses outward the disk,
and fills the entire disk. We have invented a toy model triggering a
time-dependent mean-field dynamo. The duty cycles of this dynamo lead to
episodic ejections on similar timescales. When the dynamo is suppressed as the
magnetization falls below a critical value, the generation of the outflows and
also accretion is inhibited. The general result is that we can steer episodic
ejection and {\em large-scale jet knots} by a {\em disk-intrinsic dynamo} that
is time-dependent and regenerates the jet-launching magnetic field.",1409.7003v1
2014-11-07,Coronal magnetic field and the plasma beta determined from radio and multiple satellite observations,"We derived the coronal magnetic field, plasma density, and temperature from
the observation of polarization and intensity of radio thermal free-free
emission using the Nobeyama Radioheliograph (NoRH) and extreme ultraviolet
(EUV) observations. We observed a post-flare loop on the west limb 11 April
2013. The line-of-sight magnetic field was derived from the circularly
polarized free-free emission observed by NoRH. The emission measure and
temperature were derived from the Atmospheric Imaging Assembly (AIA) onboard
Solar Dynamics Observatory (SDO). The derived temperature was used to estimate
the emission measure from the NoRH radio free-free emission observations. The
derived density from NoRH was larger than that determined using AIA, which can
be explained by the fact that the low temperature plasma is not within the
temperature coverage of the AIA filters used in this study. We also discuss the
other observation of the post-flare loops by the EUV Imager onboard the Solar
Terrestrial Relations Observatory (STEREO), which can be used in future studies
to reconstruct the coronal magnetic field strength. The derived plasma
parameters and magnetic field were used to derive the plasma beta, which is a
ratio between the magnetic pressure and the plasma pressure. The derived plasma
beta is about 5.7*10^(-4) to 7.6*10^(-4) at the loop top region.",1411.1913v1
2014-11-18,The Turbulent Dynamo in Highly Compressible Supersonic Plasmas,"The turbulent dynamo may explain the origin of cosmic magnetism. While the
exponential amplification of magnetic fields has been studied for
incompressible gases, little is known about dynamo action in
highly-compressible, supersonic plasmas, such as the interstellar medium of
galaxies and the early Universe. Here we perform the first quantitative
comparison of theoretical models of the dynamo growth rate and saturation level
with three-dimensional magnetohydrodynamical simulations of supersonic
turbulence with grid resolutions of up to 1024^3 cells. We obtain numerical
convergence and find that dynamo action occurs for both low and high magnetic
Prandtl numbers Pm = nu/eta = 0.1-10 (the ratio of viscous to magnetic
dissipation), which had so far only been seen for Pm >= 1 in supersonic
turbulence. We measure the critical magnetic Reynolds number, Rm_crit = 129
(+43, -31), showing that the compressible dynamo is almost as efficient as in
incompressible gas. Considering the physical conditions of the present and
early Universe, we conclude that magnetic fields need to be taken into account
during structure formation from the early to the present cosmic ages, because
they suppress gas fragmentation and drive powerful jets and outflows, both
greatly affecting the initial mass function of stars.",1411.4707v2
2015-01-26,Primordial magnetic fields from self-ordering scalar fields,"A symmetry-breaking phase transition in the early universe could have led to
the formation of cosmic defects. Because these defects dynamically excite not
only scalar and tensor type cosmological perturbations but also vector type
ones, they may serve as a source of primordial magnetic fields. In this study,
we calculate the time evolution and the spectrum of magnetic fields that are
generated by a type of cosmic defects, called global textures, using the
non-linear sigma (NLSM) model. Based on the standard cosmological perturbation
theory, we show, both analytically and numerically, that a vector-mode relative
velocity between photon and baryon fluids is induced by textures, which
inevitably leads to the generation of magnetic fields over a wide range of
scales. We find that the amplitude of the magnetic fields is given by
$B\sim{10^{-9}}{((1+z)/10^3)^{-2.5}}({v}/{m_{\rm pl}})^2({k}/{\rm
Mpc^{-1}})^{3.5}/{\sqrt{N}}$ Gauss in the radiation dominated era for
$k\lesssim 1$ Mpc$^{-1}$, with $v$ being the vacuum expectation value of the
O(N) symmetric scalar fields. By extrapolating our numerical result toward
smaller scales, we expect that $B\sim {10^{-17}}((1+z)/1000)^{-1/2}({v}/{m_{\rm
pl}})^2({k}/{\rm Mpc^{-1}})^{1/2}/{\sqrt{N}}$ Gauss on scales of $k\gtrsim 1$
Mpc$^{-1}$ at redshift $z\gtrsim 1100$. This might be a seed of the magnetic
fields observed on large scales today.",1501.06304v1
2015-02-26,Laws of volume elasticity of the physical processes and the parameter effect,"We consider the mechanism of elastic strains and stresses as the main
controlling factor of structure change under the influence of temperature,
magnetic field, hydrostatic pressure. We should take into account that the
energy of elastic deformation is commensurate to the energy of electric
interactions and that is much higher than the rest of the bonds of lower energy
value. Besides, the energy elastic stresses are of long range, so it forms the
linearity in magnetization and bulk change. These regularities requires a
fundamental understanding of the laws of interaction with respect to accepted
interpretation of quantum mechanical forces of short range that are attributes
of magnetism formation. Due to the high sensitivity of electronic and resonance
properties with respect to small changes of the structure, we were able to
define the direct relation between elastic stresses and field-frequency
dependencies, as well as to analyze the evolution of the dynamics of phase
transitions and phase states. A cycle of studies of the influence of
hydrostatic pressure on the resonance properties are presented also. The
analysis of the effect of magnetic, magneto-elastic and elastic energy allowed
us to define the combinations of magneto-elastic interactions. The role of
elastic stresses in the linear changes of the magnetostriction, magnetization,
magnetoelasticity of single-crystal magnetic semiconductors is described in
details.",1502.07473v1
2015-03-15,Stability of toroidal magnetic fields in stellar interiors,"We present 3D MHD simulations of purely toroidal and mixed poloidal-toroidal
magnetic field configurations to study the behavior of the Tayler instability.
For the first time the simultaneous action of rotation and magnetic diffusion
are taken into account and the effects of a poloidal field on the dynamic
evolution of unstable toroidal magnetic fields is included. In the absence of
diffusion, fast rotation (rotation rate compared to Alfv\'en frequency) is able
to suppress the instability when the rotation and magnetic axes are aligned and
when the radial field strength gradient p < 1.5. When diffusion is included,
this system turns unstable for diffusion dominated and marginally diffusive
dominated regions. If the magnetic and rotation axes are perpendicular to each
other the stabilizing effect induced by the Coriolis force is scale dependent
and decreases with increasing wavenumber. In toroidal fields with radial field
gradients bigger than p > 1.5, rapid rotation does not suppress the instability
but instead introduces a damping factor to the growth rate in agreement with
the analytic predictions. For the mixed poloidal-toroidal fields we find an
unstable axisymmetric mode, not predicted analytically, right at the stability
threshold for the non-axisymmetric modes; it has been argued that an
axisymmetric mode is necessary for the closure of the Tayler-Spruit dynamo
loop.",1503.04450v1
2015-03-19,Consistent generation of magnetic fields in axion inflation models,"There has been a growing evidence for the existence of magnetic fields in the
extra-galactic regions, while the attempt to associate their origin with the
inflationary epoch alone has been found extremely challenging. We therefore
take into account the consistent post-inflationary evolution of the magnetic
fields that are originated from vacuum fluctuations during inflation. In the
model of our interest, the electromagnetic (EM) field is coupled to a
pseudo-scalar inflaton $\phi$ through the characteristic term $\phi F\tilde F$,
breaking the conformal invariance. This interaction dynamically breaks the
parity and enables a continuous production of only one of the polarization
states of the EM field through tachyonic instability. The produced magnetic
fields are thus helical. We find that the dominant contribution to the observed
magnetic fields in this model comes from the modes that leave the horizon near
the end of inflation, further enhanced by the tachyonic instability right after
the end of inflation. The EM field is subsequently amplified by parametric
resonance during the period of inflaton oscillation. Once the thermal plasma is
formed (reheating), the produced helical magnetic fields undergo a turbulent
process called inverse cascade, which shifts their peak correlation scales from
smaller to larger scales. We consistently take all these effects into account
within the regime where the perturbation of $\phi$ is negligible and obtain
$B_{\rm eff} \sim 10^{-19}$G, indicating the necessity of additional mechanisms
to accommodate the observations.",1503.05802v2
2015-03-27,Magnetic Correlations in the Quasi-2D Semiconducting Ferromagnet CrSiTe$_3$,"Intrinsic, two-dimensional ferromagnetic semiconductors are an important
class of materials for overcoming the limitations of dilute magnetic
semiconductors for spintronics applications. CrSiTe$_3$ is a particularly
interesting member of this class, since it can likely be exfoliated down to
single layers, where T$_C$ is predicted to increase dramatically. Establishing
the nature of the magnetism in the bulk is a necessary precursor to
understanding the magnetic behavior in thin film samples and the possible
applications of this material. In this work, we use elastic and inelastic
neutron scattering to measure the magnetic properties of single crystalline
CrSiTe$_3$. We find that there is a very small single ion anisotropy favoring
magnetic ordering along the $c$-axis and that the measured spin waves fit well
to a model where the moments are only weakly coupled along that direction.
Finally, we find that both static and dynamic correlations persist within the
$ab$-plane up to at least 300 K, strong evidence of this material's
two-dimensional characteristics that are relevant for future studies on thin
film and monolayer samples.",1503.08199v2
2015-04-01,Evidence of the Dynamics of Relativistic Jet Launching in Quasars,"Hubble Space Telescope (HST) spectra of the extreme ultraviolet (EUV), the
optically thick emission from the innermost accretion flow onto the central
supermassive black hole, indicate that RLQs tend to be EUV weak compared to the
radio quiet quasars (RQQs); yet the remainder of the optically thick thermal
continuum is indistinguishable. The deficit of EUV emission in RLQs has a
straightforward interpretation as a missing or a suppressed innermost region of
local energy dissipation in the accretion flow. This article is an examination
of the evidence for a distribution of magnetic flux tubes in the innermost
accretion flow that results in magnetically arrested accretion (MAA) and
creates the EUV deficit. These same flux tubes and possibly the interior
magnetic flux that they encircle are the source of the jet power as well. In
the MAA scenario, islands of large scale magnetic vertical flux perforate the
innermost accretion flow of RLQs. The first prediction of the theory that is
supported by the HST data is that the strength of the (large scale poloidal
magnetic fields) jets in the MAA region is regulated by the ram pressure of the
accretion flow in the quasar environment. The second prediction that is
supported by the HST data is that the rotating magnetic islands remove energy
from the accretion flow as a Poynting flux dominated jet in proportion to the
square of the fraction of the EUV emitting gas that is displaced by these
islands.",1504.00228v1
2015-04-17,Chiral damping of magnetic domain walls,"Structural symmetry breaking in magnetic materials is responsible for a
variety of outstanding physical phenomena. Examples range from the existence of
multiferroics, to current induced spin orbit torques (SOT) and the formation of
topological magnetic structures. In this letter we bring into light a novel
effect of the structural inversion asymmetry (SIA): a chiral damping mechanism.
This phenomenon is evidenced by measuring the field driven domain wall (DW)
motion in perpendicularly magnetized asymmetric Pt/Co/Pt trilayers. The
difficulty in evidencing the chiral damping is that the ensuing DW dynamics
exhibit identical spatial symmetry to those expected from the
Dzyaloshinskii-Moriya interaction (DMI). Despite this fundamental resemblance,
the two scenarios are differentiated by their time reversal properties: while
DMI is a conservative effect that can be modeled by an effective field, the
chiral damping is purely dissipative and has no influence on the equilibrium
magnetic texture. When the DW motion is modulated by an in-plane magnetic
field, it reveals the structure of the internal fields experienced by the DWs,
allowing to distinguish the physical mechanism. The observation of the chiral
damping, not only enriches the spectrum of physical phenomena engendered by the
SIA, but since it can coexists with DMI it is essential for conceiving DW and
skyrmion devices.",1504.04411v1
2015-06-24,Ultrafast quantum spin-state switching in the Co-octaethylporphyrin molecular magnet with a terahertz pulsed magnetic field,"Molecular spin crossover switches are the objects of intense theoretical and
experimental studies in recent years. This interest is due to the fact that
these systems allow one to control their spin state by applying an external
photo-, thermo-, piezo-, or magnetic stimuli. The greatest amount of research
is currently devoted to the study the effect of the photoexcitation on the
bi-stable states of spin crossover single molecular magnets (SMMs). The main
limitation of photo-induced bi-stable states is their short lifetime. In this
paper we present the results of a study of the spin dynamics of the CoOEP
molecule in the Low Spin (LS) state and the High Spin (HS) state induced by
applying the magnetic pulse of 36.8 T. We show that the spin switching in case
of the HS state of the CoOEP molecule is characterized by a long lifetime and
is dependent on the magnitude and duration of the applied field. Thus, after
applying an external stimuli the system in the LS state after the spin
switching reverts to its ground state, whereas the system in the HS state
remains in the excited state for a long time. We found that the temperature
dependency of magnetic susceptibility shows an abrupt thermal spin transition
between two spin states at 40K. The proposed here theoretical approach opens
the way to create modern devices for spintronics with the controllable spin
switching process.",1506.07315v1
2015-06-30,Realistic modeling of local dynamo processes on the Sun,"Magnetic fields are usually observed in the quiet Sun as small-scale elements
that cover the entire solar surface (the `salt and pepper' patterns in
line-of-sight magnetograms). By using 3D radiative MHD numerical simulations we
find that these fields result from a local dynamo action in the top layers of
the convection zone, where extremely weak 'seed' magnetic fields (e.g., from a
$10^{-6}$ G) can locally grow above the mean equipartition field, to a stronger
than 2000~G field localized in magnetic structures. Our results reveal that the
magnetic flux is predominantly generated in regions of small-scale helical
downflows. We find that the local dynamo action takes place mostly in a
shallow, about 500~km deep, subsurface layer, from which the generated field is
transported into the deeper layers by convective downdrafts. We demonstrate
that the observed dominance of vertical magnetic fields at the photosphere and
horizontal fields above the photosphere can be explained by small-scale
magnetic loops produced by the dynamo. Such small-scale loops play an important
role in the structure and dynamics of the solar atmosphere and that their
detection in observations is critical for understanding the local dynamo action
on the Sun.",1506.08924v1
2015-07-02,Confined Flares in Solar Active Region 12192 from 2014 October 18 to 29,"Using the observations from the Atmospheric Imaging Assembly (AIA) and
Helioseismic and Magnetic Imager (HMI) aboard the Solar Dynamics Observatory
(SDO), we investigate six X-class and twenty-nine M-class flares occurring in
solar active region (AR) 12192 from October 18 to 29. Among them, thirty
(including six X- and twenty-four M-class) flares originated from the AR core
and the other five M-flares appeared at the AR periphery. Four of the X-flares
exhibited similar flaring structures, indicating they were homologous flares
with analogous triggering mechanism. The possible scenario is: photospheric
motions of emerged magnetic fluxes lead to shearing of the associated coronal
magnetic field, which then yields a tether-cutting favorable configuration.
Among the five periphery M-flares, four were associated with jet activities.
The HMI vertical magnetic field data show that the photospheric fluxes of
opposite magnetic polarities emerged, converged and canceled with each other at
the footpoints of the jets before the flares. Only one M-flare from the AR
periphery was followed by a coronal mass ejection (CME). From October 20 to 26,
the mean decay index of the horizontal background field within the height range
of 40-105 Mm is below the typical threshold for torus instability onset. This
suggests that a strong confinement from the overlying magnetic field might be
responsible for the poor CME production of AR 12192.",1507.00651v1
2015-07-13,Enhanced Acoustic Emission in Relation to the Acoustic Halo Surrounding Active Region 11429,"The use of acoustic holography in the high-frequency $p$-mode spectrum can
resolve the source distributions of enhanced acoustic emissions within halo
structures surrounding active regions. In doing so, statistical methods can
then be applied to ascertain relationships with the magnetic field. This is the
focus of this study. The mechanism responsible for the detected enhancement of
acoustic sources around solar active regions has not yet been explained.
Furthermore the relationship between the magnetic field and enhanced acoustic
emission has not yet been comprehensively examined. We have used vector
magnetograms from the \Helioseismic and Magnetic Imager (HMI) on-board the
Solar Dynamics Observatory (SDO) to image the magnetic-field properties in the
halo. We have studied the acoustic morphology of an active region, with a
complex halo and ""glories,"" and we have linked some acoustic properties to the
magnetic-field configuration. In particular, we find that acoustic sources are
significantly enhanced in regions of intermediate field strength with
inclinations no different from the distributions found in the quiet Sun.
Additionally we have identified a transition region between the active region
and the halo, in which the acoustic source power is hindered by inclined fields
of intermediate field strength. Finally, we have compared the results of
acoustic emission maps, calculated from holography, and the commonly used local
acoustic maps, finding that the two types of maps have similar properties with
respect to the magnetic field but lack spatial correlation when examining the
highest-powered regions.",1507.03447v1
2015-08-19,Termination of the magnetorotational instability via parasitic instabilities in core-collapse supernovae,"The magnetorotational instability (MRI) can be a powerful mechanism
amplifying the magnetic field in core-collapse supernovae. Whether initially
weak magnetic fields can be amplified by this instability to dynamically
relevant strengths is still a matter of debate. One of the main uncertainties
concerns the process that terminates the growth of the instability. Parasitic
instabilities of both Kelvin-Helmholtz and tearing-mode type have been
suggested to play a crucial role in this process, disrupting MRI channel flows
and quenching magnetic field amplification. We perform two-dimensional and
three-dimensional sheering-disc simulations of a differentially rotating
protoneutron star layer in non-ideal magnetohydrodynamics with unprecedented
high numerical accuracy, finding that Kelvin-Helmholtz parasitic modes dominate
tearing modes in the regime of large hydrodynamic and magnetic Reynolds
numbers, as encountered close to the surface of protoneutron stars. They also
determine the maximum magnetic field stress achievable during the exponential
growth of the MRI. Our results are consistent with the theory of parasitic
instabilities based on a local stability analysis. To simulate the
Kelvin-Helmholtz instabilities properly, a very high numerical resolution is
necessary. Using ninth-order spatial reconstruction schemes, we find that at
least eight grid zones per MRI channel are necessary to simulate the growth
phase of the MRI and reach an accuracy of ~10 per cent in the growth rate,
while more than ~60 zones per channel are required to achieve convergent
results for the value of the magnetic stress at MRI termination.",1508.04799v2
2015-09-01,Light Bridge in a Developing Active Region. II. Numerical Simulation of Flux Emergence and Light Bridge Formation,"Light bridges, the bright structure dividing umbrae in sunspot regions, show
various activity events. In Paper I, we reported on analysis of
multi-wavelength observations of a light bridge in a developing active region
(AR) and concluded that the activity events are caused by magnetic reconnection
driven by magnetconvective evolution. The aim of this second paper is to
investigate the detailed magnetic and velocity structures and the formation
mechanism of light bridges. For this purpose, we analyze numerical simulation
data from a radiative magnetohydrodynamics model of an emerging AR. We find
that a weakly-magnetized plasma upflow in the near-surface layers of the
convection zone is entrained between the emerging magnetic bundles that appear
as pores at the solar surface. This convective upflow continuously transports
horizontal fields to the surface layer and creates a light bridge structure.
Due to the magnetic shear between the horizontal fields of the bridge and the
vertical fields of the ambient pores, an elongated cusp-shaped current layer is
formed above the bridge, which may be favorable for magnetic reconnection. The
striking correspondence between the observational results of Paper I and the
numerical results of this paper provides a consistent physical picture of light
bridges. The dynamic activity phenomena occur as a natural result of the bridge
formation and its convective nature, which has much in common with those of
umbral dots and penumbral filaments.",1509.00205v1
2015-09-01,Investigation of Umbral Dots with the New Vacuum Solar Telescope,"Umbral dots (UDs) are small isolated brightenings observed in sunspot umbrae.
They are convective phenomena existing inside umbrae. UDs are usually divided
into central UDs (CUDs) and peripheral UDs (PUDs) according to their positions
inside an umbra. Our purpose is to investigate UD properties and analyze their
relationships, and further to find whether or not the properties depend on
umbral magnetic field strengths. Thus, we selected high-resolution TiO images
of four active regions (ARs) taken under the best seeing conditions with the
\textit{New Vacuum Solar Telescope} in the Fuxian Solar Observatory of the
Yunnan Astronomical Observatory, China. The four ARs (NOAA 11598, 11801, 12158,
and 12178) include six sunspots. A total of 1220 CUDs and 603 PUDs were
identified. Meanwhile, the radial component of the vector magnetic field of the
sunspots taken with the \textit{Helioseismic and Magnetic Imager} on-board the
\textit{Solar Dynamics Observatory} was used to analyze relationships between
UD properties and umbral magnetic field strengths. We find that diameters and
lifetimes of UDs exhibit an increasing trend with the brightness, but
velocities do not. Moreover, diameters, intensities, lifetimes and velocities
depend on the surrounding magnetic field. A CUD diameter was found larger, the
CUD brighter, its lifetime longer, and its motion slower in a weak umbral
magnetic field environment than in a strong one.",1509.00312v4
2015-09-21,On the current solar magnetic activity in the light of its behaviour during the Holocene,"Solar modulation potential (SMP) reconstructions based on cosmogenic nuclide
records reflect changes in the open solar magnetic field and can therefore help
us obtain information on the behaviour of the open solar magnetic field over
the Holocene period. We aim at comparing the Sun's large-scale magnetic field
behaviour over the last three solar cycles with variations in the SMP
reconstruction through the Holocene epoch. To achieve these objectives, we use
the IntCal13 $^{14}$C data to investigate distinct patterns in the occurrences
of grand minima and maxima during the Holocene period. We then check whether
these patterns might mimic the recent solar magnetic activity by investigating
the evolution of the energy in the Sun's large-scale dipolar magnetic field
using the Wilcox Solar Observatory data. The cosmogenic radionuclide data
analysis shows that $\sim$71\% of grand maxima during the period from 6600 BC
to 1650 AD were followed by a grand minimum. The occurrence characteristics of
grand maxima and minima are consistent with the scenario in which the dynamical
non-linearity induced by the Lorentz force leads the Sun to act as a relaxation
oscillator. This finding implies that the probability for these events to occur
is non-uniformly distributed in time, as there is a memory in their driving
mechanism, which can be identified via the back reaction of the Lorentz force.",1509.06182v1
2015-10-06,"Role of boundary conditions, topology and disorder in the chiral magnetic effect in Weyl semimetals","Quantum field theory predicts Weyl semimetals to possess a peculiar response
of the longitudinal current density to the application of a DC magnetic field.
Such a response function has been shown to be at odds with a general result
showing the vanishing of the bulk current in an equilibrium system on any real
material with a lattice in an external magnetic field. Here we resolve this
apparent contradiction by introducing a model where a current flows in response
to a magnetic field even without Weyl nodes. We point out that the previous
derivation of a vanishing CME in the limit of vanishing real frequency is a
consequence of the assumption of periodic boundary conditions of the system. A
more realistic system with open boundary conditions is not subject to these
constraints and can have a non-vanishing CME. Consistent with recent work, we
found the finite frequency CME to be non-vanishing in general when there was a
non-vanishing Berry curvature on the Fermi surface. This does not necessitate
having a topological Berry flux as in the case of a Weyl node. Finally, we
study how the perturbation theory in magnetic field might be more stable in the
presence of disorder. Using the standard diagrammatic treatment of disorder
within the Born approximation, we have found that in a realistic disordered
system, the chiral magnetic response is really a dynamical phenomena and
vanishes in the DC limit.",1510.01767v2
2015-10-21,Automatic Detection of Magnetic delta in Sunspot Groups,"Large and magnetically complex sunspot groups are known to be associated with
flares. To date, the Mount Wilson scheme has been used to classify sunspot
groups based on their morphological and magnetic properties. The most flare
prolific class, the delta sunspot-group, is characterised by opposite polarity
umbrae within a common penumbra, separated by less than 2 degrees. In this
article, we present a new system, called the Solar Monitor Active Region
Tracker - Delta Finder (SMART-DF), that can be used to automatically detect and
classify magnetic deltas in near-realtime. Using continuum images and
magnetograms from the Helioseismic and Magnetic Imager (HMI) onboard NASA's
Solar Dynamics Observatory (SDO), we first estimate distances between opposite
polarity umbrae. Opposite polarity pairs having distances of less that 2
degrees are then identified, and if these pairs are found to share a common
penumbra, they are identified as a magnetic delta configuration. The algorithm
was compared to manual delta detections reported by the Space Weather
Prediction Center (SWPC), operated by the National Oceanic and Atmospheric
Administration (NOAA). SMART-DF detected 21 out of 23 active regions (ARs) that
were marked as delta spots by NOAA during 2011 - 2012 (within +/- 60 degrees
longitude). SMART-DF in addition detected five ARs which were not announced as
delta spots by NOAA. The near-relatime operation of SMART-DF resulted in many
deltas being identified in advance of NOAA's daily notification. SMART-DF will
be integrated with SolarMonitor (www.solarmonitor.org) and the near-realtime
information will be available to the public.",1510.06413v1
2015-11-04,Magnetic fields in primordial accretion disks,"Magnetic fields are considered as a vital ingredient of contemporary star
formation, and may have been important during the formation of the first stars
in the presence of an efficient amplification mechanism. Initial seed fields
are provided via plasma fluctuations, and are subsequently amplified by the
small-scale dynamo, leading to a strong tangled magnetic field. Here we explore
how the magnetic field provided by the small-scale dynamo is further amplified
via the $\alpha-\Omega$ dynamo in a protostellar disk and assess its
implications. For this purpose, we consider two characteristic cases, a typical
Pop.~III star with $10$~M$_\odot$ and an accretion rate of
$10^{-3}$~M$_\odot$~yr$^{-1}$, and a supermassive star with $10^5$~M$_\odot$
and an accretion rate of $10^{-1}$~M$_\odot$~yr$^{-1}$. For the $10$~M$_\odot$
Pop.~III star, we find that coherent magnetic fields can be produced on scales
of at least $100$~AU, which are sufficient to drive a jet with a luminosity of
$100$~L$_\odot$ and a mass outflow rate of $10^{-3.7}$~M$_\odot$~yr$^{-1}$. For
the supermassive star, the dynamical timescales in its environment are even
shorter, implying smaller orbital timescales and an efficient magnetization out
to at least $1000$~AU. The jet luminosity corresponds to
$\sim10^{6.0}$~L$_\odot$, and a mass outflow rate of
$10^{-2.1}$~M$_\odot$~yr$^{-1}$. We expect that the feedback from the
supermassive star can have a relevant impact on its host galaxy.",1511.01317v1
2015-11-05,Polarization Evolution of Early Optical Afterglows of Gamma-Ray Bursts,"The central engine and jet composition of gamma-ray bursts (GRBs) remain
mysterious. Here we suggest that observations on polarization evolution of
early optical afterglows may shed light on these questions. We first study the
dynamics of a reverse shock and a forward shock that are generated during the
interaction of a relativistic jet and its ambient medium. The jet is likely
magnetized with a globally large-scale magnetic field from the central engine.
The existence of the reverse shock requires that the magnetization degree of
the jet should not be high ($\sigma\leq 1$), so that the jet is mainly composed
of baryons and leptons. We then calculate the light curve and polarization
evolution of an early optical afterglow, and find that when the polarization
position angle changes by $90^\circ$ during the early afterglow, the
polarization degree is zero for a toroidal magnetic field but is very likely to
be non-zero for an aligned magnetic field. This result would be expected to
provide a probe for the central engine of GRBs, because an aligned field
configuration could originate from a magnetar central engine and a toroidal
field configuration could be produced from a black hole via the
Blandford-Znajek mechanism. Finally, for such two kinds of magnetic field
configurations, we fit the observed data of the early optical afterglow of GRB
120308A equally well.",1511.01582v2
2015-11-26,Theoretical formalism for collective spin-wave edge excitations in arrays of dipolarly interacting magnetic nanodots,"A general theory of edge spin wave excitations in semi-infinite and finite
periodic arrays of magnetic nanodots existing in a spatially uniform
magnetization ground state is developed. The theory is formulated using a
formalism of multi-vectors of magnetization dynamics, which allows one to study
edge excitations in arrays having arbitrary complex primitive cells and lattice
geometry. The developed formalism can describe edge excitations localized both
at the physical edges of the array and at the internal ""domain walls""
separating array regions existing in different static magnetization states.
Using a perturbation theory in the framework of the developed formalism it is
possible to calculate damping of edge modes and their excitation by external
variable magnetic fields. The theory is illustrated on the following
practically important examples: (i) calculation of the FMR absorption in a
finite nanodot array having the shape of a right triangle, (ii) calculation of
nonreciprocal spin wave spectra of edge modes, including modes at the physical
edges of an array and modes at the domain walls inside an array, (iii) study of
the influence of the domain wall modes on the FMR spectrum of an array existing
in a non-ideal chessboard antiferromagnetic ground state.",1511.08483v3
2015-11-30,Near field magnetostatics and Neel Brownian interactions mediated magnetorheological characteristics of highly stable nano ferrocolloids,"Magnetic nanocolloids with synthesized super paramagnetic Fe3O4 nanoparticles
(SPION) (5 to 15 nm) dispersed in insitu developed Polyethylene Glycol (PEG
400) and nano silica complex have been synthesized. The PEG nano Silica complex
physically encapsulates the SPIONs, ensuring no phase separation under high
magnetic fields (1.2 T). Exhaustive magnetorheological investigations have been
performed to comprehend the structural behavior and response of the
ferrocolloids. Remarkable stability and reversibility have been observed under
magnetic field for concentrated systems. The results exhibit the impact of
particle concentration, size and encapsulation efficiency on parameters such as
shear viscosity, yield stress, viscoelastic moduli, magnetoviscous hysteresis
etc. Analytical models to reveal the system mechanism and mathematically
predict the magnetoviscosity and magneto yield stress has been theorized. The
mechanistic approach based on near field magnetostatics and Neel Brownian
interactivities can predict the colloidal properties under the effect of field
accurately. The colloid exhibits amplified storage and loss moduli alongside
highly augmented linear viscoelastic region under magnetic stimuli. The
transition of the colloidal state from fluidic phase to soft condensed phase
and its viscoelastic stimuli under the influence of magnetic field has been
explained based on the mathematical analysis. The remarkable stability,
magnetic properties and accurate physical models reveal good promise for the
colloids in transient situations viz. magneto MEMS/ NEMS devices, antiseismic
damping, biomedical invasive treatments etc.",1512.00081v1
2015-12-01,Magnetization reversal of an individual exchange biased permalloy nanotube,"We investigate the magnetization reversal mechanism in an individual
permalloy (Py) nanotube (NT) using a hybrid magnetometer consisting of a
nanometer-scale SQUID (nanoSQUID) and a cantilever torque sensor. The Py NT is
affixed to the tip of a Si cantilever and positioned in order to optimally
couple its stray flux into a Nb nanoSQUID. We are thus able to measure both the
NT's volume magnetization by dynamic cantilever magnetometry and its stray flux
using the nanoSQUID. We observe a training effect and temperature dependence in
the magnetic hysteresis, suggesting an exchange bias. We find a low blocking
temperature $T_B = 18 \pm 2$ K, indicating the presence of a thin
antiferromagnetic native oxide, as confirmed by X-ray absorption spectroscopy
on similar samples. Furthermore, we measure changes in the shape of the
magnetic hysteresis as a function of temperature and increased training. These
observations show that the presence of a thin exchange-coupled native oxide
modifies the magnetization reversal process at low temperatures. Complementary
information obtained via cantilever and nanoSQUID magnetometry allows us to
conclude that, in the absence of exchange coupling, this reversal process is
nucleated at the NT's ends and propagates along its length as predicted by
theory.",1512.00199v1
2015-12-03,Orbitally induced hierarchy of exchange interactions in zigzag antiferromagnetic state of honeycomb silver delafossite Ag3Co2SbO6,"We report the revised crystal structure, static and dynamic magnetic
properties of quasi-two dimensional honeycomb-lattice silver delafossite
Ag3Co2SbO6. The magnetic susceptibility and specific heat data are consistent
with the onset of antiferromagnetic long range order at low temperatures with
N\'eel temperature TN ~ 21.2 K. In addition, the magnetization curves revealed
a field-induced (spin-flop type) transition below TN in moderate magnetic
fields. The GGA+U calculations show the importance of the orbital degrees of
freedom, which maintain a hierarchy of exchange interaction in the system. The
strongest antiferromagnetic exchange coupling was found in the shortest Co-Co
pairs and is due to direct and superexchange interactions between the
half-filled xz+yz orbitals pointing directly to each other. The other four out
of six nearest neighbor exchanges within the cobalt hexagon are suppressed,
since for these bonds active half-filled orbitals turned out to be parallel and
do not overlap. The electron spin resonance (ESR) spectra reveal a Gaussian
shape line attributed to Co2+ ion in octahedral coordination with average
effective g-factor g=2.3+/-0.1 at room temperature and shows strong divergence
of ESR parameters below 120 K, which imply an extended region of short-range
correlations. Based on the results of magnetic and thermodynamic studies in
applied fields, we propose the magnetic phase diagram for the new
honeycomb-lattice delafossite.",1512.01016v1
2015-12-15,Ginzburg-Landau theory for skyrmions in inversion-symmetric magnets with competing interactions,"Magnetic skyrmions have attracted considerable attention recently for their
huge potential in spintronic applications. Generally skyrmions are big compared
to the atomic lattice constant, which allows for the Ginzburg-Landau type
description in the continuum limit. Such a description successfully captures
the main experimental observations on skyrmions in B20 compound without
inversion symmetry. Skyrmions can also exist in inversion-symmetric magnets
with competing interactions. Here we derive a general Ginzburg-Landau theory
for skyrmions in these magnets valid in the long wavelength limit. We study the
unusual static and dynamical properties of skyrmions based on the derived
Ginzburg-Landau theory. We show that an easy axis spin anisotropy is sufficient
to stabilize a skyrmion lattice. Interestingly, the skyrmion in
inversion-symmetric magnets has a new internal degree of freedom associated
with the rotation of helicity, i.e. the ""spin"" of the skyrmion as a particle,
in addition to the usual translational motion of skyrmions (orbital motion).
The orbital and spin degree of freedoms of an individual skyrmion can couple to
each other, and give rise to unusual behavior that is absent for the skyrmions
stabilized by the Dzyaloshinskii-Moriya interaction. The derived
Ginzburg-Landau theory provides a convenient and general framework to discuss
skyrmion physics and will facilitate the search for skyrmions in
inversion-symmetric magnets.",1512.05012v1
2016-01-05,Quantum revivals and magnetization tunneling in effective spin systems,"Quantum mechanical objects or nanoobjects have been proposed as bits for
information storage. While time-averaged properties of magnetic,
quantum-mechanical particles have been extensively studied experimentally and
theoretically, experimental investigations of the real time evolution of
magnetization in the quantum regime were not possible until recent developments
in pump-probe techniques. Here we investigate the quantum dynamics of effective
spin systems by means of analytical and numerical treatments. Particular
attention is paid to the quantum revival time and its relation to the
magnetization tunneling. The quantum revival time has been initially defined as
the recurrence time of a total wave-function. Here we show that the quantum
revivals of wave-functions and expectation values in spin systems may be quite
different which gives rise to a more sophisticated definition of the quantum
revival within the realm of experimental research. Particularly, the revival
times for integer spins coincide which is not the case for half-integer spins.
Furthermore, the quantum revival is found to be shortest for integer ratios
between the on-site anisotropy and an external magnetic field paving the way to
novel methods of anisotropy measurements. We show that the quantum tunneling of
magnetization at avoided level crossing is coherent to the quantum revival time
of expectation values, leading to a connection between these two fundamental
properties of quantum mechanical spins.",1601.00803v1
2016-02-07,Interaction of microwave photons with nanostructured magnetic metasurfaces,"A theoretical formalism for the description of the interaction of microwave
photons with a thin (compared to the photon wavelength) magnetic metasurface
comprised of dipolarly interacting nano-scale magnetic elements is developed. A
scattering matrix describing the processes of photon transmission and
reflection at the metasurface boundary is derived. As an example of the use of
the developed formalism, it is demonstrated, that the introduction of a
magnetic metasurface inside a microstrip electromagnetic waveguide
quantitatively changes the dispersion relation of the fundamental waveguide
mode, opening a non-propagation frequency band gap in the waveguide spectrum.
The frequency position and the width of the band gap are dependent on the
waveguide thickness, and can be controlled dynamically by switching the
magnetic ground state of the metasurface. For sufficiently thin waveguides the
position of the band gap is shifted from the resonance absorption frequency of
the metasurface. In such a case, the magnetic metasurface inside a waveguide
works as an efficient reflector, as the energy absorption in the metasurface is
small, and most of the electromagnetic energy inside the non-propagation band
gap is reflected.",1602.02340v2
2016-02-18,Spin Liquid State in the 3D Frustrated Antiferromagnet PbCuTe2O6: NMR and muSR Studies,"PbCuTe2O6 is a rare example of a spin liquid candidate featuring a three
dimensional magnetic lattice. Strong geometric frustration arises from the
dominant antiferromagnetic interaction which generates a hyperkagome network of
Cu2+ ions although additional interactions enhance the magnetic lattice
connectivity. Through a combination of magnetization measurements and local
probe investigation by NMR and muSR down to 20 mK, we provide a robust evidence
for the absence of magnetic freezing in the ground state. The local spin
susceptibility probed by the NMR shift hardly deviates from the macroscopic one
down to 1 K pointing to a homogeneous magnetic system with a low defect
concentration. The saturation of the NMR shift and the sublinear power law
temperature (T) evolution of the 1/T1 NMR relaxation rate at low T point to a
non-singlet ground state favoring a gapless fermionic description of the
magnetic excitations. Below 1 K a pronounced slowing down of the spin dynamics
is witnessed, which may signal a reconstruction of spinon Fermi surface.
Nonetheless, the compound remains in a fluctuating spin liquid state down to
the lowest temperature of the present investigation.",1602.05818v1
2016-02-22,Electromagnetic spectral properties and Debye screening of a strongly magnetized hot medium,"We have evaluated the electromagnetic spectral function and its spectral
properties by computing the one-loop photon polarization tensor in presence of
magnetic field, particularly in a strong field approximation compared to the
thermal scale. When the magnetic scale is higher than the thermal scale the
lowest Landau level (LLL) becomes effectively (1+1) dimensional strongly
correlated system that provides a kinematical threshold based on the mass
scale. Beyond this threshold the photon strikes the LLL and the spectral
strength starts with a high value due to the dimensional reduction and then
falls off with increase of the photon energy due to LLL dynamics in a strong
field approximation. This strongly enhances the dilepton rate over the thermal
perturbative leading order (Born) rate at very low invariant mass. We have also
investigated the electromagnetic screening by computing the Debye screening
mass and it depends distinctively on three different scales (mass of the
quasiquark, temperature and the magnetic field strength) of a hot magnetized
system. The mass dependence of the Debye screening supports the occurrence of a
magnetic catalysis effect in the strong field approximation.",1602.06769v3
2016-04-19,Formation of the penumbra and start of the Evershed flow,"We studied the variations of line-of-sight photospheric plasma flows during
the formation phase of the penumbra around a pore in Active Region NOAA 11490.
We used a high spatial, spectral, and temporal resolution data set acquired by
the Interferometric BIdimensional Spectrometer (IBIS) operating at the NSO/Dunn
Solar Telescope as well as data taken by the Helioseismic and Magnetic Imager
onboard the Solar Dynamics Observatory satellite (SDO/HMI). Before the penumbra
formed we observed a redshift of the spectral line in the inner part of the
annular zone surrounding the pore as well as a blueshift of material associated
with opposite magnetic polarity further away from the pore. We found that the
onset of the classical Evershed flow occurs in a very short time scale -- 1-3
hours -- while the penumbra is forming. During the same time interval we found
changes in the magnetic field inclination in the penumbra, with the vertical
field actually changing sign near the penumbral edge, while the total magnetic
field showed a significant increase, about 400 G. To explain these and other
observations related to the formation of the penumbra and the onset of the
Evershed flow we propose a scenario in which the penumbra is formed by magnetic
flux dragged down from the canopy surrounding the initial pore. The Evershed
flow starts when the sinking magnetic field dips below the solar surface and
magnetoconvection sets in.",1604.05610v1
2016-04-20,Resonance-based Detection of Magnetic Nanoparticles and Microbeads Using Nanopatterned Ferromagnets,"Biosensing with ferromagnet-based magnetoresistive devices has been dominated
by electrical detection of particle-induced changes to the devices' static
magnetic configuration. There are however potential advantages to be gained
from using field dependent, high frequency magnetization dynamics for magnetic
particle detection. Here we demonstrate the use of nano-confined ferromagnetic
resonances in periodically patterned magnetic films for the detection of
adsorbed magnetic particles with diameters ranging from 6 nm to 4 $\mu$m. The
nanopatterned films contain arrays of holes which can act as preferential
adsorption sites for small particles. Hole-localized particles act in unison to
shift the resonant frequencies of the various modes of the patterned layer with
shift polarities determined by the localization of each mode within the
nanopattern's repeating unit cell. The same polarity shifts are observed for a
large range of coverages, even when hole-localized particles are covered by
quasi-continuous particle sheets. For large particles however, preferential
adsorption no longer occurs, leading to resonance shifts with polarities which
are independent of the mode localization. Analogous shifts are seen in
continuous layers where, for small particles, the shift of the layer's
fundamental mode is typically about 10 times less than in patterned systems and
induced by relatively weak fields emanating beyond the particle in the
direction of the static applied field. This highlights the importance of having
confined modes consistently positioned with respect to nearby particles.",1604.05835v1
2016-07-28,On the Rotation of Sunspots and Their Magnetic Polarity,"The rotation of sunspots of 2 yr in two different solar cycles is studied
with the data from the Helioseismic and Magnetic Imager on board the \it Solar
Dynamics Observatory \rm and the Michelson Doppler Imager instrument on board
the \it Solar and Heliospheric Observataory.\rm We choose the $\alpha$ sunspot
groups and the relatively large and stable sunspots of complex active regions
in our sample. In the year of 2003, the $\alpha$ sunspot groups and the
preceding sunspots tend to rotate counterclockwise and have positive magnetic
polarity in the northern hemisphere. In the southern hemisphere, the magnetic
polarity and rotational tendency of the $\alpha$ sunspot groups and the
preceding sunspots are opposite to the northern hemisphere. The average
rotational speed of these sunspots in 2003 is about $0^{\circ}.65 \rm \
hr^{-1}$. From 2014 January to 2015 February, the $\alpha$ sunspot groups and
the preceding sunspots tend to rotate clockwise and have negative magnetic
polarity in the northern hemisphere. The patterns of rotation and magnetic
polarity of the southern hemisphere are also opposite to those of the northern
hemisphere. The average rotational speed of these sunspots in 2014/2015 is
about $1^{\circ}.49 \rm \ hr^{-1}$. The rotation of the relatively large and
stable preceding sunspots and that of the $\alpha$ sunspot groups located in
the same hemisphere have opposite rotational direction in 2003 and 2014/2015.",1607.08343v1
2016-07-29,The relevance of ambipolar diffusion for neutron star evolution,"We study ambipolar diffusion in strongly magnetised neutron stars, with
special focus on the effects of neutrino reaction rates and the impact of a
superfluid/superconducting transition in the neutron star core. For
axisymmetric magnetic field configurations, we determine the deviation from
$\beta-$equilibrium induced by the magnetic force and calculate the velocity of
the slow, quasi-stationary, ambipolar drift. We study the temperature
dependence of the velocity pattern and clearly identify the transition to a
predominantly solenoidal flow. For stars without superconducting/superfluid
constituents and with a mixed poloidal-toroidal magnetic field of typical
magnetar strength, we find that ambipolar diffusion proceeds fast enough to
have a significant impact on the magnetic field evolution only at low core
temperatures, $T \lesssim 1-2\times10^8$ K. The ambipolar diffusion timescale
becomes appreciably shorter when fast neutrino reactions are present, because
the possibility to balance part of the magnetic force with pressure gradients
is reduced. We also find short ambipolar diffusion timescales in the case of
superconducting cores for $T \lesssim 10^9$ K, due to the reduced interaction
between protons and neutrons. In the most favourable scenario, with fast
neutrino reactions and superconducting cores, ambipolar diffusion results in
advection velocities of several km/kyr. This velocity can substantially
reorganize magnetic fields in magnetar cores, in a way that can only be
confirmed by dynamical simulations.",1608.00001v2
2016-08-08,Magnetically self-regulated formation of early protoplanetary discs,"The formation of protoplanetary discs during the collapse of molecular dense
cores is significantly influenced by angular momentum transport, notably by the
magnetic torque. In turn, the evolution of the magnetic field is determined by
dynamical processes and non-ideal MHD effects such as ambipolar diffusion.
Considering simple relations between various timescales characteristic of the
magnetized collapse, we derive an expression for the early disc radius, $ r
\simeq 18 \, {\rm AU} \, \left({\eta_{\rm AD} / 0.1 \, {\rm s}} \right)^{2/9}
\left({B_z / 0.1\, {\rm G}} \right) ^{-4/9} \left({M / 0.1 \msol} \right)
^{1/3},$ where $M$ is the total disc plus protostar mass, $\eta_\mathrm{AD}$ is
the ambipolar diffusion coefficient and $B_z$ is the magnetic field in the
inner part of the core. This is about significantly smaller than the discs that
would form if angular momentum was conserved.
  The analytical predictions are confronted against a large sample of 3D,
non-ideal MHD collapse calculations covering variations of a factor 100 in core
mass, a factor 10 in the level of turbulence, a factor 5 in rotation, and
magnetic mass-to-flux over critical mass-to-flux ratios 2 and 5. The disc
radius estimates are found to agree with the numerical simulations within less
than a factor 2.
  A striking prediction of our analysis is the weak dependence of circumstellar
disc radii upon the various relevant quantities, suggesting weak variations
among class-0 disc sizes. In some cases, we note the onset of large spiral arms
beyond this radius.",1608.02525v1
2016-08-21,Manipulating antiferromagnets with magnetic fields: ratchet motion of multiple domain walls induced by asymmetric field pulses,"Future applications of antiferromagnets (AFs) in many spintronics devices
rely on the precise manipulation of domain walls. The conventional approach
using static magnetic fields is inefficient due to the low susceptibility of
AFs. Recently proposed electrical manipulation with spin-orbit torques is
restricted to metals with a specific crystal structure.
  Here we propose an alternative, broadly applicable approach: using asymmetric
magnetic field pulses to induce controlled ratchet motion of AF domain walls.
  The efficiency of this approach is based on three peculiarities of AF
dynamics. First, a time-dependent magnetic field couples with an AF order
parameter stronger than a static magnetic field, which leads to higher mobility
of the domain walls. Second, the rate of change of the magnetic field couples
with the spatial variation of the AF order parameter inside the domain and this
enables synchronous motion of multiple domain walls with the same structure.
Third, tailored asymmetric field pulses in combination with static friction can
prevent backward motion of domain walls and thus lead to the desired controlled
ratchet effect.
  The proposed use of an external field, rather than internal spin-orbit
torques, avoids any restrictions on size, conductivity, and crystal structure
of the AF material.
  We believe that our approach paves a way for the development of new AF-based
devices based on controlled motion of AF domain walls.",1608.05967v1
2016-09-26,Mechanical force involved multiple fields switching of both local ferroelectric and magnetic domain in a Bi5Ti3FeO15 thin film,"Multiferroics have received intense attention due to their great application
potential in multi-state information storage devices and new types of sensors.
Coupling among ferroic orders such as ferroelectricity, (anti-)ferromagnetism,
ferroelasticity, etc. will enable dynamic interaction between these ordering
parameters. Direct visualization of such coupling behaviour in single phase
multiferroic materials is highly desirable for both applications and
fundamental study. Manipulation of both ferroelectric and magnetic domains of
Bi5Ti3FeO15 thin film using electric field and external mechanical force is
reported, which confirms the magnetoelectric coupling in Bi5Ti3FeO15, indicates
the electric and magnetic orders are coupled through ferroelasticity. Due to
the anisotropic relaxation of ferroelastic strain, the back-switching of
out-of-plane electric domains is not as obvious as in-plane. An inevitable
destabilization of the coupling between elastic and magnetic ordering happens
because of the elastic strain relaxation, which result in a subsequent decay of
magnetic domain switching. Mechanical force applied on the surface of
Bi5Ti3FeO15 film generates by an AFM tip will effectively drive a transition of
the local ferroelastic strain state, reverse both the polarization and
magnetization in a way similar to an electric field. Current work provides a
framework for exploring cross-coupling among multiple orders and potential for
developing novel nanoscale functional devices.",1609.07831v1
2016-10-27,Spin Currents and Magnon Dynamics in Insulating Magnets,"Nambu-Goldstone theorem provides gapless modes to both relativistic and
nonrelativistic systems. The Nambu-Goldstone bosons in insulating magnets are
called magnons or spin-waves and play a key role in magnetization transport. We
review here our past works on magnetization transport in insulating magnets and
also add new insights, with a particular focus on magnon transport. We
summarize in detail the magnon counterparts of electron transport, such as the
Wiedemann-Franz law, the Onsager reciprocal relation between the Seebeck and
Peltier coefficients, the Hall effects, the superconducting state, the
Josephson effects, and the persistent quantized current in a ring to list a
few. Focusing on the electromagnetism of moving magnons, i.e., magnetic
dipoles, we theoretically propose a way to directly measure magnon currents. As
a consequence of the Mermin-Wagner-Hohenberg theorem, spin transport is
drastically altered in one-dimensional antiferromagnetic (AF) spin-1/2 chains;
where the N\'eel order is destroyed by quantum fluctuations and a quasiparticle
magnon-like picture breaks down. Instead, the low-energy collective excitations
of the AF spin chain are described by a Tomonaga-Luttinger liquid (TLL) which
provides the spin transport properties in such antiferromagnets some universal
features at low enough temperature. Finally, we enumerate open issues and
provide a platform to discuss the future directions of magnonics.",1610.08901v2
2016-10-31,Validation of a Scaling Law for the Coronal Magnetic Field Strengths and Loop Lengths of Solar and Stellar Flares,"Shibata & Yokoyama (1999, 2002) proposed a method of estimating the coronal
magnetic field strengths ($B$) and magnetic loop lengths ($L$) of solar and
stellar flares, on the basis of magnetohydrodynamic simulations of the magnetic
reconnection model. Using the scaling law provided by Shibata & Yokoyama (1999,
2002), $B$ and $L$ are obtained as functions of the emission measure
($EM=n^2L^3$) and temperature ($T$) at the flare peak. Here, $n$ is the coronal
electron density of the flares. This scaling law enables the estimation of $B$
and $L$ for unresolved stellar flares from the observable physical quantities
$EM$ and $T$, which is helpful for studying stellar surface activities. To
apply this scaling law to stellar flares, we discuss its validity for spatially
resolved solar flares. $EM$ and $T$ were calculated from GOES soft X-ray flux
data, and $B$ and $L$ are theoretically estimated using the scaling law. For
the same flare events, $B$ and $L$ were also observationally estimated with
images taken by Solar Dynamics Observatory (SDO)/ Helioseismic and Magnetic
Imager (HMI) Magnetogram and Atmospheric Imaging Assembly (AIA) 94{\AA} pass
band. As expected, a positive correlation was found between the theoretically
and observationally estimated values. We interpret this result as indirect
evidence that flares are caused by magnetic reconnection. Moreover, this
analysis makes us confident in the validity of applying this scaling law to
stellar flares as well as solar flares.",1610.09811v1
2016-10-31,"Kinetic analysis of spin current contribution to spectrum of electromagnetic waves in spin-1/2 plasma, Part II: Dispersion dependencies","The dielectric permeability tensor for spin polarized plasmas derived in
terms of the spin-1/2 quantum kinetic model in six-dimensional phase space in
Part I of this work is applied for study of spectra of high-frequency
transverse and transverse-longitudinal waves propagating perpendicular to the
external magnetic field. Cyclotron waves are studied at consideration of waves
with electric field directed parallel to the external magnetic field. It is
found that the separate spin evolution modifies the spectrum of cyclotron
waves. These modifications increase with the increase of the spin polarization
and the number of the cyclotron resonance. Spin dynamics with no account of the
anomalous magnetic moment gives a considerable modification of spectra either.
The account of anomalous magnetic moment leads to a fine structure of each
cyclotron resonance. So, each cyclotron resonance splits on three waves.
Details of this spectrum and its changes with the change of spin polarization
are studied for the first and second cyclotron waves. A cyclotron resonance
existing at $\omega\approx0.001\mid\Omega_{e}\mid$ due to the anomalous
magnetic moment is also described, where $\mid\Omega_{e}\mid$ is the cyclotron
frequency. The ordinary waves does not have any considerable modification. The
electrostatic and electromagnetic Berstein modes are studied during the
analysis of waves propagating perpendicular to the external magnetic field with
the electric field perturbation directed perpendicular to the external field. A
modification of the oscillatory structure caused by the equilibrium spin
polarization is found in both regimes. Similar modification is found for the
extraordinary wave spectrum.",1611.00046v1
2016-12-02,Unveiling Magnetic Interactions of Ruthenium Trichloride via Constraining Direction of Orbital moments: Potential Routes to Realize Quantum Spin Liquid,"Recent experiments reveal that the honeycomb ruthenium trichloride
{\alpha}-RuCl3 is a prime candidate of the Kitaev quantum spin liquid (QSL).
However, there is no theoretical model which can properly describe its
experimental dynamical response, due to the lack of a full understanding of its
magnetic interactions. Here, we propose a general scheme to calculate the
magnetic interactions in systems (e.g., {\alpha}-RuCl3) with non-negligible
orbital moments by constraining the directions of orbital moments. With this
scheme, we put forward a minimal J1-K1-{\Gamma}1-J3-K3 model for {\alpha}-RuCl3
and find that: (I) The third nearest neighbor (NN) antiferromagnetic Heisenberg
interaction J3 stabilizes the zigzag antiferromagnetic order; (II) The NN
symmetric off-diagonal exchange {\Gamma}1 plays a pivotal role in determining
the preferred direction of magnetic moments and generating the spin wave gap.
Exact diagonalization study on this model shows that the Kitaev QSL can be
realized by suppressing the NN symmetric off-diagonal exchange {\Gamma}1 and
the third NN Heisenberg interaction J3. Thus, we not only propose a powerful
general scheme for investigating the intriguing magnetism of Jeff=1/2 magnets,
but also point out future directions for realizing the Kitaev QSL in the
honeycomb ruthenium trichloride {\alpha}-RuCl3.",1612.00761v1
2017-02-03,Physical properties of a candidate quantum spin-ice system Pr2Hf2O7,"Physical properties of a pyrohafnate compound Pr2Hf2O7 have been investigated
by ac magnetic susceptibility \chi_ ac(T), dc magnetic susceptibility \chi(T),
isothermal magnetization M(H) and heat capacity C_p(T) measurements on
polycrystalline as well as single crystal samples combined with high-resolution
synchrotron x-ray diffraction (XRD) for structural characterization and
inelastic neutron scattering (INS) to determine the crystal field energy level
scheme and wave functions. Synchrotron XRD data confirm the ordered cubic
pyrochlore (Fd-3m) structure without any noticeable site mixing or oxygen
deficiency. No clear evidence of long range magnetic ordering is observed down
to 90 mK, however the \chi_ac(T) evinces slow spin dynamics revealed by a
frequency dependent broad peak associated with spin freezing. The INS data
reveal the expected five well defined magnetic excitations due to crystal field
splitting of the J = 4 ground state multiplet of the Pr^3+. The crystal field
parameters and ground state wavefunction of Pr^3+ have been determined. The
Ising anisotropic nature of magnetic ground state is inferred from the INS as
well as \chi(T) and M(H) data. Together these properties make Pr2Hf2O7 a
candidate compound for quantum spin-ice behavior.",1702.01098v1
2017-02-09,Laboratory evidence of dynamo amplification of magnetic fields in a turbulent plasma,"Magnetic fields are ubiquitous in the Universe. Extragalactic disks, halos
and clusters have consistently been shown, via diffuse radio-synchrotron
emission and Faraday rotation measurements, to exhibit magnetic field strengths
ranging from a few nG to tens of $\mu$G. The energy density of these fields is
typically comparable to the energy density of the fluid motions of the plasma
in which they are embedded, making magnetic fields essential players in the
dynamics of the luminous matter. The standard theoretical model for the origin
of these strong magnetic fields is through the amplification of tiny seed
fields via turbulent dynamo to the level consistent with current observations.
Here we demonstrate, using laser-produced colliding plasma flows, that
turbulence is indeed capable of rapidly amplifying seed fields to near
equipartition with the turbulent fluid motions. These results support the
notion that turbulent dynamo is a viable mechanism responsible for the observed
present-day magnetization of the Universe.",1702.03016v1
2017-02-16,Spectral Analysis of Non-Ideal MRI Modes: The effect of Hall diffusion,"The effect of magnetic field diffusion on the stability of accretion disks is
a problem that has attracted considerable interest of late. In particular, the
Hall effect has the potential to bring about remarkable changes in the
dynamical behavior of disks that are without parallel. In this paper, we
conduct a systematic examination of the linear eigenmodes in a weakly
magnetized differentially rotating gas with special focus on Hall diffusion. We
first develop a geometrical representation of the eigenmodes and provide a
detailed quantitative description of the polarization properties of the
oscillatory modes under the combined influence of the Coriolis and Hall
effects. We also analyze the effects of magnetic diffusion on the structure of
the unstable modes and derive analytical expressions for the kinetic and
magnetic stresses and energy densities associated with the non-ideal MRI. Our
analysis explicitly demonstrates that, if the dissipative effects are
relatively weak, the kinetic stresses and energies make up the dominant
contribution to the total stress and energy density when the equilibrium
angular momentum and magnetic field vectors are anti-parallel. This is in sharp
contrast to what is observed in the case of the ideal or dissipative MRI. We
conduct shearing box simulations and find very good agreement with the results
derived from linear analysis. As the modes in consideration are also exact
solutions of the non-linear equations, the unconventional nature of the kinetic
and magnetic stresses may have significant implications for the non-linear
evolution in some regions of protoplanetary disks.",1702.04979v1
2017-02-22,Testing the Accuracy of Data Driven MHD Simulations of Active Region Evolution,"Models for the evolution of the solar coronal magnetic field are vital for
understanding solar activity, yet the best measurements of magnetic field lie
at the photosphere, necessitating the development of coronal models which are
\textit{""data-driven""} at the photosphere. We present an investigation to
determine the feasibility and accuracy of such methods. Our validation
framework uses a simulation of active region (AR) formation, modeling the
emergence of magnetic flux from the convection zone to the corona, as a
ground-truth dataset, to supply both the photospheric information, and to
perform the validation of the data-driven method. We focus our investigation on
how the data-driven model accuracy depends on the temporal frequency of the
driving data. The Helioseismic and Magnetic Imager on NASA's Solar {\jl
Dynamics} Observatory produces full-disk vector magnetic field measurements at
a 12 minute cadence. Using our framework we show that ARs that emerge over 25
hours can be modeled by the data-driving method with only $\sim$1\% error in
the free magnetic energy, assuming the photospheric information is specified
every 12 minutes. However, for rapidly evolving features, under-sampling of the
dynamics at this cadence leads to a strobe effect, generating large electric
currents and incorrect coronal morphology and energies. We derive a sampling
condition for the driving cadence based on the evolution of these small-scale
features, and show that higher-cadence driving can lead to acceptable errors.
Future work will investigate the source of errors associated with deriving
plasma variables from the photospheric magnetograms as well as other sources of
errors such as reduced resolution and instrument bias and noise.",1702.06808v1
2017-02-27,Single shot ultrafast all optical magnetization switching of ferromagnetic Co/Pt multilayers,"In a number of recent experiments, it has been shown that femtosecond laser
pulses can control magnetization on picosecond timescales, which is at least an
order of magnitude faster compared to conventional magnetization dynamics.
Among these demonstrations, one material system (GdFeCo ferromagnetic films) is
particularly interesting, as deterministic toggle-switching of the magnetic
order has been achieved without the need of any symmetry breaking magnetic
field. This phenomenon is often referred to as all optical switching (AOS).
However, so far, GdFeCo remains the only material system where such
deterministic switching has been observed. When extended to ferromagnetic
systems, which are of greater interest in many technological applications, only
a partial effect can be achieved, which in turn requires repeated laser pulses
for full switching. However, such repeated pulsing is not only energy hungry,
it also negates the speed advantage of AOS. Motivated by this problem, we have
developed a general method for single-shot, picosecond timescale, complete all
optical switching of ferromagnetic materials. We demonstrate that in
exchange-coupled layers of Co/Pt and GdFeCo, single shot, switching of the
ferromagnetic Co/Pt layer is achieved within 7 picoseconds after irradiation by
a femtosecond laser pulse. We believe that this approach will greatly expand
the range of materials and applications for ultrafast magnetic switching.",1702.08491v3
2017-03-23,Confinement of rotating convection by a laterally varying magnetic field,"Spherical shell dynamo models based on rotating convection show that the flow
within the tangent cylinder is dominated by an off-axis plume that extends from
the inner core boundary to high latitudes and drifts westward. Earlier studies
explained the formation of such a plume in terms of the effect of a uniform
axial magnetic field that significantly increases the lengthscale of convection
in a rotating plane layer. However, rapidly rotating dynamo simulations show
that the magnetic field within the tangent cylinder has severe lateral
inhomogeneities that may influence the onset of an isolated plume. Increasing
the rotation rate in our dynamo simulations (by decreasing the Ekman number
$E$) produces progressively thinner plumes that appear to seek out the location
where the field is strongest. Motivated by this result, we examine the linear
onset of convection in a rapidly rotating fluid layer subject to a laterally
varying axial magnetic field. A cartesian geometry is chosen where the finite
dimensions $(x,z)$ mimic $(\phi,z)$ in cylindrical coordinates. The lateral
inhomogeneity of the field gives rise to a unique mode of instability where
convection is entirely confined to the peak-field region. The localization of
the flow by the magnetic field occurs even when the field strength (measured by
the Elsasser number $\varLambda$) is small and viscosity controls the smallest
lengthscale of convection. The lowest Rayleigh number at which an isolated
plume appears within the tangent cylinder in spherical shell dynamo simulations
agrees closely with the viscous-mode Rayleigh number in the plane layer linear
magnetoconvection model. The localized excitation of viscous-mode convection by
a laterally varying magnetic field provides a mechanism for the formation of
isolated plumes within Earth's tangent cylinder.",1703.08051v2
2017-03-29,Measuring the magnetic field of a trans-equatorial loop system using coronal seismology,"""EIT waves"" are freely-propagating global pulses in the low corona which are
strongly associated with the initial evolution of coronal mass ejections
(CMEs). They are thought to be large-amplitude, fast-mode magnetohydrodynamic
waves initially driven by the rapid expansion of a CME in the low corona. An
""EIT wave"" was observed on 6 July 2012 to impact an adjacent trans-equatorial
loop system which then exhibited a decaying oscillation as it returned to rest.
Observations of the loop oscillations were used to estimate the magnetic field
strength of the loop system by studying the decaying oscillation of the loop,
measuring the propagation of ubiquitous transverse waves in the loop and
extrapolating the magnetic field from observed magnetograms. Observations from
the Atmospheric Imaging Assembly onboard the Solar Dynamics Observatory
(SDO/AIA) and the Coronal Multi-channel Polarimeter (CoMP) were used to study
the event. An Empirical Mode Decomposition analysis was used to characterise
the oscillation of the loop system in CoMP Doppler velocity and line width and
in AIA intensity. The loop system was shown to oscillate in the 2nd harmonic
mode rather than at the fundamental frequency, with the seismological analysis
returning an estimated magnetic field strength of ~5.5+/-1.5 G. This compares
to the magnetic field strength estimates of ~1-9 G and ~3-9 G found using the
measurements of transverse wave propagation and magnetic field extrapolation
respectively.",1703.10020v1
2017-04-18,"Study on Precursor Activity of the X1.6 Flare in the Great AR 12192 with SDO, IRIS, and Hinode","The physical properties and its contribution to the onset of solar flare are
still unclear although chromospheric brightening is considered a precursor
phenomenon of flare. Many studies suggested that photospheric magnetic field
changes cause destabilization of large-scale coronal structure. We aim to
understand how a small photospheric change contributes to a flare and to reveal
how the intermediary chromosphere behaves in the precursor phase. We analyzed
the precursor brightening of the X1.6 flare on 2014 October 22 in the AR 12192
using the Interface Region Imaging Spectrograph (IRIS) and Hinode/EUV Imaging
Spectrometer (EIS) data. We investigated a localized jet with the strong
precursor brightening, and compared the intensity, Doppler velocity, and line
width in C II, Mg II k, Si IV lines by IRIS and He II, Fe XII, Fe XV lines by
Hinode/EIS. We also analyzed photospheric magnetic field and
chromospheric/coronal structures using Solar Dynamics Observatory
(SDO)/Helioseismic and Magnetic Imager (HMI) and Atmospheric Imaging Assembly
(AIA). We found a significant blueshift (~ 100 km/s), which is related to the
strong precursor brightening over a characteristic magnetic field structure,
and the blueshift was observed at all the temperature. This might indicate that
the flow is accelerated by Lorentz force. Moreover, the large-scale coronal
loop that connects the foot-points of the flare ribbons was destabilized just
after the precursor brightening with the blueshift. It suggests that magnetic
reconnection locally occurred in the lower chromosphere and it triggered
magnetic reconnection of the X1.6 flare in the corona.",1704.05158v1
2017-04-27,Novel self-similar rotating solutions of non-ideal transverse magnetohydrodynamics,"The evolution of electromagnetic and thermodynamic fields in a non-ideal
fluid are studied in the framework of ultrarelativistic transverse
magnetohydrodynamics (MHD), which is essentially characterized by electric and
magnetic fields being transverse to the fluid velocity. Extending the method of
self-similar solutions of relativistic hydrodynamics to the case of
non-conserved charges, the differential equations of non-ideal transverse MHD
are solved, and two novel sets of self-similar solutions are derived. The first
set turns out to be a boost-invariant and exact solution, which is
characterized by non-rotating electric and magnetic fields. The second set is a
non-boost-invariant solution, which is characterized by rotating electric and
magnetic fields. The rotation occurs with increasing rapidity $\eta$, as the
angular velocity is defined by
$\omega_{0}\equiv\frac{\partial\zeta}{\partial\eta}=\frac{\partial\phi}{\partial\eta}$,
with $\zeta$ and $\phi$ being the angles of electric and magnetic vectors with
respect to a certain axis in the local rest frame of the fluid. For both sets
of solutions, the electric and magnetic fields are either parallel or
anti-parallel to each other. Performing a complete numerical analysis, the
effects of finite electric conductivity as well as electric and magnetic
susceptibilities of the medium on the evolution of rotating and non-rotating
MHD solutions are explored, and the interplay between the angular velocity
$\omega_{0}$ and these quantities is scrutinized. The lifetime of
electromagnetic fields and the evolution of the temperature of the
electromagnetized fluid are shown to be affected by $\omega_{0}$.",1705.00536v1
2017-05-07,Non-potential field formation in the X-shaped quadrupole magnetic field configuration,"Some types of solar flares are observed in X-shaped quadrupolar field
configuration. To understand the magnetic energy storage in such a region, we
studied non-potential field formation in an X-shaped quadrupolar field region
formed in the active region NOAA 11967, which produced three X-shaped M-class
flares on February 2, 2014. Nonlinear force-free field modeling was applied to
a time series of vector magnetic field maps from the Solar Optical Telescope on
board Hinode and Helioseismic and Magnetic Imager on board Solar Dynamics
Observatory. Our analysis of the temporal three-dimensional magnetic field
evolution shows that the sufficient free energy had already been stored more
than 10 hours before the occurrence of the first M-class flare and that the
storage was observed in a localized region. In this localized region,
quasi-separatrix layers (QSLs) started to develop gradually from 9 hours before
the first M-class flare. One of the flare ribbons that appeared in the first
M-class flare was co-spatial with the location of the QSLs, suggesting that the
formation of the QSLs is important in the process of energy release. These QSLs
do not appear in the potential field calculation, indicating that they were
created by the non-potential field. The formation of the QSLs was associated
with the transverse photospheric motion of the pre-emerged flux and the
emergence of a new flux. This observation indicates that the occurrence of the
flares requires the formation of QSLs in the non-potential field in which free
magnetic energy is stored in advance.",1705.02560v2
2017-07-14,Successive Two-sided loop Jets Caused by Magnetic Reconnection between Two adjacent Filamentary Threads,"We present observational analysis of two successive two-sided loop jets
observed by the ground-based New Vacuum Solar Telescope (NVST) and the
space-borne Solar Dynamics Observatory ( SDO). The two successive two-sided
loop jets manifested similar evolution process and both were associated with
the interaction of two small-scale adjacent filamentary threads, magnetic
emerging and cancellation processes at the jet's source region. High temporal
and high spatial resolution observations reveal that the two adjacent ends of
the two filamentary threads are rooted in opposite magnetic polarities within
the source region. The two threads approached to each other, and then an
obvious brightening patch is observed at the interaction position.
Subsequently, a pair of hot plasma ejections are observed heading to opposite
directions along the paths of the two filamentary threads, and with a typical
speed of two-sided loop jets of the order 150 km/s. Close to the end of the
second jet, we report the formation of a bright hot loop structure at the
source region, which suggests the formation of new loops during the
interaction. Based on the observational results, we propose that the observed
two-sided loop jets are caused by the magnetic reconnection between the two
adjacent filamentary threads, largely different from the previous scenario that
a two-sided loop jet is generated by magnetic reconnection between an emerging
bipole and the overlying horizontal magnetic fields.",1707.04421v1
2017-07-17,Coupling of structure to magnetic and superconducting orders in quasi-one-dimensional $\text{K}_2\text{Cr}_3\text{As}_3$,"Quasi-one-dimensional $A_2\text{Cr}_3\text{As}_3$ (with $A = \text{K, Cs,
Rb}$) is an intriguing new family of superconductors which exhibit many similar
features to the cuprate and iron-based unconventional superconductor families.
Yet in contrast to these systems, no charge or magnetic ordering has been
observed which could provide the electronic correlations presumed necessary for
an unconventional superconducting pairing mechanism - an absence which defies
predictions of first principles models. We report the results of neutron
scattering experiments on polycrystalline $\text{K}_2\text{Cr}_3\text{As}_3$
$(T_c \sim 7\text{K})$ which probed the low temperature dynamics near $T_c$ .
Neutron diffraction data evidence a strong response of the nuclear lattice to
the onset of superconductivity while inelastic scattering reveals a highly
dispersive column of intensity at the commensurate wavevector $q =
(00\frac{1}{2})$ which loses intensity beneath $T_c$ - indicative of
short-range magnetic fluctuations. Using linear spin-wave theory we model the
observed scattering and suggest a possible structure to the short-range
magnetic order. These observations suggest that
$\text{K}_2\text{Cr}_3\text{As}_3$ is in close proximity to a magnetic
instability and that the incipient magnetic order both couples strongly to the
lattice and competes with superconductivity - in direct analogy with the
iron-based superconductors.",1707.05286v1
2017-07-19,Engineering elliptical spin-excitations by complex anisotropy fields in Fe adatoms and dimers on Cu(111),"We investigate the dynamics of Fe adatoms and dimers deposited on the Cu(111)
metallic surface in the presence of spin-orbit coupling, within time-dependent
density functional theory. The \textit{ab initio} results provide
material-dependent parameters that can be used in semiclassical approaches,
which are used for insightful interpretations of the excitation modes. By
manipulating the surroundings of the magnetic elements, we show that elliptical
precessional motion may be induced through the modification of the magnetic
anisotropy energy. We also demonstrate how different kinds of spin precession
are realized, considering the symmetry of the magnetic anisotropy energy, the
ferro- or antiferromagnetic nature of the exchange coupling between the
impurities, and the strength of the magnetic damping. In particular, the normal
modes of a dimer depend on the initial magnetic configuration, changing
drastically by going from a ferromagnetic metastable state to the
antiferromagnetic ground state. By taking into account the effect of the
damping into their resonant frequencies, we reveal that an important
contribution arises for strongly biaxial systems and specially for the
antiferromagnetic dimers with large exchange couplings. Counter intuitively,
our results indicate that the magnetic damping influences the quantum
fluctuations by decreasing the zero-point energy of the system.",1707.06087v2
2017-07-31,Lattice Quantum Monte Carlo Study of Chiral Magnetic Effect in Dirac Semimetals,"In this paper Chiral Magnetic Effect (CME) in Dirac semimetals is studied by
means of lattice Monte Carlo simulation. We measure conductivity of Dirac
semimetals as a function of external magnetic field in parallel
$\sigma_{\parallel}$ and perpendicular $\sigma_{\perp}$ to the external field
directions. The simulations are carried out in three regimes: semimetal phase,
onset of the insulator phase and deep in the insulator phase. In the semimetal
phase $\sigma_{\parallel}$ grows whereas $\sigma_{\perp}$ drops with magnetic
field. Similar behaviour was observed in the onset of the insulator phase but
conductivity is smaller and its dependence on magnetic field is weaker. Finally
in the insulator phase conductivities $\sigma_{\parallel, \perp}$ are close to
zero and do not depend on magnetic field. In other words, we observe
manifestation of the CME current in the semimetal phase, weaker manifestation
of the CME in the onset of the insulator phase. We do not observe signatures of
CME in the insulator phase. We believe that the suppression of the CME current
in the insulator phase is connected to chiral symmetry breaking and generation
of dynamical fermion mass which take place in this phase.",1707.09810v2
2017-08-07,Field dependence of non-reciprocal magnons in chiral MnSi,"Spin waves in chiral magnetic materials are strongly influenced by the
Dzyaloshinskii-Moriya interaction resulting in intriguing phenomena like
non-reciprocal magnon propagation and magnetochiral dichroism. Here, we study
the non-reciprocal magnon spectrum of the archetypical chiral magnet MnSi and
its evolution as a function of magnetic field covering the field-polarized and
conical helix phase. Using inelastic neutron scattering, the magnon energies
and their spectral weights are determined quantitatively after deconvolution
with the instrumental resolution. In the field-polarized phase the imaginary
part of the dynamical susceptibility $\chi''(\varepsilon, {\bf q})$ is shown to
be asymmetric with respect to wavevectors ${\bf q}$ longitudinal to the applied
magnetic field ${\bf H}$, which is a hallmark of chiral magnetism. In the
helimagnetic phase, $\chi''(\varepsilon, {\bf q})$ becomes increasingly
symmetric with decreasing ${\bf H}$ due to the formation of helimagnon bands
and the activation of additional spinflip and non-spinflip scattering channels.
The neutron spectra are in excellent quantitative agreement with the low-energy
theory of cubic chiral magnets with a single fitting parameter being the
damping rate of spin waves.",1708.02098v2
2017-10-09,Global solar magnetic field organization in the outer corona: influence on the solar wind speed and mass flux over the cycle,"The dynamics of the solar wind depends intrinsically on the structure of the
global solar magnetic field, which undergoes fundamental changes over the 11-yr
solar cycle. For instance, the wind terminal velocity is thought to be
anti-correlated with the expansion factor, a measure of how the magnetic field
varies with height in the solar corona, usually computed at a fixed height
($\approx 2.5 R_{\odot}$, the source surface radius which approximates the
distance at which all magnetic field lines become open). However, the magnetic
field expansion affects the solar wind in a more detailed way, its influence on
the solar wind properties remaining significant well beyond the source surface.
We demonstrate this using 3D global MHD simulations of the solar corona,
constrained by surface magnetograms over half a solar cycle (1989-2001). A
self-consistent expansion beyond the solar wind critical point (even up to
$10R_{\odot}$) makes our model comply with observed characteristics of the
solar wind, namely, that the radial magnetic field intensity becomes latitude
independent at some distance from the Sun, and that the mass flux is mostly
independent of the terminal wind speed. We also show that near activity
minimum, the expansion in the higher corona has more influence on the wind
speed than the expansion below $2.5 R_{\odot}$.",1710.02908v1
2017-10-12,Magnetic monolayer Li$_{2}$N: Density Functional Theory Calculations,"Density functional theory (DFT) calculations are used to investigate the
electronic and magnetic structures of a two-dimensional (2D) monolayer
Li$_{2}$N. It is shown that bulk Li$_{3}$N is a non-magnetic semiconductor. The
non-spinpolarized DFT calculations show that $p$ electrons of N in 2D Li$_{2}$N
form a narrow band at the Fermi energy $E_{\rm{F}}$ due to a low coordination
number, and the density of states at the Fermi energy ($g(E_{\rm{F}}$)) is
increased as compared with bulk Li$_{3}$N. The large $g(E_{\rm{F}}$) shows
instability towards magnetism in Stoner's mean field model. The spin-polarized
calculations reveal that 2D Li$_{2}$N is magnetic without intrinsic or impurity
defects. The magnetic moment of 1.0\,$\mu_{\rm{B}}$ in 2D Li$_{2}$N is mainly
contributed by the $p_{z}$ electrons of N, and the band structure shows
half-metallic behavior. {Dynamic instability in planar Li$_{2}$N monolayer is
observed, but a buckled Li$_{2}$N monolayer is found to be dynamically stable.}
The ferromagnetic (FM) and antiferromagnetic (AFM) coupling between the N atoms
is also investigated to access the exchange field strength. {We found that
planar (buckled) 2D Li$_{2}$N is a ferromagnetic material with Curie
temperature $T_{c}$ of 161 (572) K.}",1710.04440v1
2017-10-18,Valence Bonds in Random Quantum Magnets: Theory and Application to YbMgGaO4,"We analyze the effect of quenched disorder on spin-1/2 quantum magnets in
which magnetic frustration promotes the formation of local singlets. Our
results include a theory for 2d valence-bond solids subject to weak bond
randomness, as well as extensions to stronger disorder regimes where we make
connections with quantum spin liquids. We find, on various lattices, that the
destruction of a valence-bond solid phase by weak quenched disorder leads
inevitably to the nucleation of topological defects carrying spin-1/2 moments.
This renormalizes the lattice into a strongly random spin network with
interesting low-energy excitations. Similarly when short-ranged valence bonds
would be pinned by stronger disorder, we find that this putative glass is
unstable to defects that carry spin-1/2 magnetic moments, and whose residual
interactions decide the ultimate low energy fate. Motivated by these results we
conjecture Lieb-Schultz-Mattis-like restrictions on ground states for
disordered magnets with spin-1/2 per statistical unit cell. These conjectures
are supported by an argument for 1d spin chains. We apply insights from this
study to the phenomenology of YbMgGaO$_4$, a recently discovered triangular
lattice spin-1/2 insulator which was proposed to be a quantum spin liquid. We
instead explore a description based on the present theory. Experimental
signatures, including unusual specific heat, thermal conductivity, and
dynamical structure factor, and their behavior in a magnetic field, are
predicted from the theory, and compare favorably with existing measurements on
YbMgGaO$_4$ and related materials.",1710.06860v2
2017-10-26,Time-reversed magnetically controlled perturbation (TRMCP) optical focusing inside scattering media,"Manipulating and focusing light deep inside biological tissue and tissue-like
complex media has been desired for long yet considered challenging. One
feasible strategy is through optical wavefront engineering, where the optical
scattering-induced phase distortions are time reversed or pre-compensated so
that photons travel along different optical paths interfere constructively at
the targeted position within a scattering medium. To define the targeted
position, an internal guidestar is needed to guide or provide a feedback for
wavefront engineering. It could be injected or embedded probes such as
fluorescence or nonlinear microspheres, ultrasonic modulation, as well as
absorption perturbation. Here we propose to use a magnetically controlled
optical absorbing microsphere as the internal guidestar. Using a digital
optical phase conjugation system, we obtained sharp optical focusing within
scattering media through time-reversing the scattered light perturbed by the
magnetic microshpere. Since the object is magnetically controlled, dynamic
optical focusing is allowed with a relatively large field-of-view by scanning
the magnetic field externally. Moreover, the magnetic microsphere can be
packaged with an organic membrane, using biological or chemical means to serve
as a carrier. Therefore the technique may find particular applications for
enhanced targeted drug delivery, and imaging and photoablation of angiogenic
vessels in tumours.",1710.09603v3
2017-10-30,Enhancement of intrinsic magnetic damping in defect-free epitaxial Fe3O4 thin films,"We have investigated the magnetic damping of precessional spin dynamics in
defect-controlled epitaxial grown Fe$_3$O$_4$(111)/Yttria-stabilized Zirconia
(YSZ) nanoscale films by all-optical pump-probe measurements. The intrinsic
damping constant of the defect-free Fe$_3$O$_4$ film is found to be strikingly
larger than that of the as-grown Fe$_3$O$_4$ film with structural defects. We
demonstrate that the population of the first-order perpendicular standing spin
wave (PSSW) mode, which is exclusively observed in the defect-free film under
sufficiently high external magnetic fields, leads to the enhancement of the
magnetic damping of the uniform precession (Kittel) mode. We propose a physical
picture in which the PSSW mode acts as an additional channel for the extra
energy dissipation of the Kittel mode. The energy transfer from Kittel mode to
PSSW mode increases as in-plane magnetization precession becomes more uniform,
resulting in the unique intrinsic magnetic damping enhancement in the
defect-free Fe$_3$O$_4$ film.",1710.10938v2
2017-11-03,"Relativistic MHD modeling of magnetized neutron stars, pulsar winds, and their nebulae","Neutron stars are among the most fascinating astrophysical sources, being
characterized by strong gravity, densities about the nuclear one or even above,
and huge magnetic fields. Their observational signatures can be extremely
diverse across the electromagnetic spectrum, ranging from the periodic and
low-frequency signals of radio pulsars, up to the abrupt high-energy gamma-ray
flares of magnetars, where energies of ~10^46 erg are released in a few
seconds. Fast-rotating and highly magnetized neutron stars are expected to
launch powerful relativistic winds, whose interaction with the supernova
remnants gives rise to the non-thermal emission of pulsar wind nebulae, which
are known cosmic accelerators of electrons and positrons up to PeV energies. In
the extreme cases of proto-magnetars (magnetic fields of ~10^15 G and
millisecond periods), a similar mechanism is likely to provide a viable engine
for the still mysterious gamma-ray bursts. The key ingredient in all these
spectacular manifestations of neutron stars is the presence of strong magnetic
fields in their constituent plasma. Here we will present recent updates of a
couple of state-of-the-art numerical investigations by the high-energy
astrophysics group in Arcetri: a comprehensive modeling of the steady-state
axisymmetric structure of rotating magnetized neutron stars in general
relativity, and dynamical 3-D MHD simulations of relativistic pulsar winds and
their associated nebulae.",1711.01120v1
2017-11-29,Plasma Beta Stratification in the Solar Atmosphere: A Possible Explanation for the Penumbra Formation,"Plasma beta is an important and fundamental physical quantity in order to
understand plasma dynamics, particularly in the context of magnetically active
stars, because it tells about the domination of magnetic versus thermodynamic
processes on the plasma motion. We estimate the value ranges of plasma beta in
different regions within the solar atmosphere and we describe a possible
mechanism that helps forming a penumbra. For that we evaluate data from a 3D
magnetohydrodynamic model of the solar corona above a magnetically active
region. We compare our results with previously established data that is based
on magnetic field extrapolations and that was matched for some observations.
Our model data suggest that plasma beta in the photosphere should be considered
to be larger than unity outside of sunspots. However, in the corona we also
find that the beta value range reaches lower than previously thought, which
coincides with a recent observation. We present an idea based on a
gravity-driven process in a high-beta regime that might be responsible for the
formation of the penumbra around sunspot umbra, where the vertical field
strength reaches a given threshold. This process would also explain
counter-Evershed flows. Regarding the thermal and magnetic pressure within the
mixed-polarity solar atmosphere, including non-vertical magnetic field and
quiet regions, plasma beta may reach unity at practically any height from the
photosphere to the outer corona.",1711.10965v1
2017-12-07,Signatures of Star-planet interactions,"Planets interact with their host stars through gravity, radiation and
magnetic fields, and for those giant planets that orbit their stars within
$\sim$10 stellar radii ($\sim$0.1 AU for a sun-like star), star-planet
interactions (SPI) are observable with a wide variety of photometric,
spectroscopic and spectropolarimetric studies. At such close distances, the
planet orbits within the sub-alfv\'enic radius of the star in which the
transfer of energy and angular momentum between the two bodies is particularly
efficient. The magnetic interactions appear as enhanced stellar activity
modulated by the planet as it orbits the star rather than only by stellar
rotation. These SPI effects are informative for the study of the internal
dynamics and atmospheric evolution of exoplanets. The nature of magnetic SPI is
modeled to be strongly affected by both the stellar and planetary magnetic
fields, possibly influencing the magnetic activity of both, as well as
affecting the irradiation and even the migration of the planet and rotational
evolution of the star. As phase-resolved observational techniques are applied
to a large statistical sample of hot Jupiter systems, extensions to other
tightly orbiting stellar systems, such as smaller planets close to M dwarfs
become possible. In these systems, star-planet separations of tens of stellar
radii begin to coincide with the radiative habitable zone where planetary
magnetic fields are likely a necessary condition for surface habitability.",1712.02814v1
2017-12-12,Critical phenomena in the general spherically symmetric Einstein-Yang-Mills system,"We study critical behavior in gravitational collapse of a general spherically
symmetric Yang-Mills field coupled to the Einstein equations. Unlike the
magnetic ansatz used in previous numerical work, the general Yang-Mills
connection has two degrees of freedom in spherical symmetry. This fact changes
the phenomenology of critical collapse dramatically. The magnetic sector
features both type I and type II critical collapse, with universal critical
solutions. In contrast, in the general system type I disappears and the
critical behavior at the threshold between dispersal and black hole formation
is always type II. We obtain values of the mass scaling and echoing exponents
close to those observed in the magnetic sector, however we find some
indications that the critical solution differs from the purely magnetic
discretely self-similar attractor and exact self-similarity and universality
might be lost. The additional ""type III"" critical phenomenon in the magnetic
sector, where black holes form on both sides of the threshold but the
Yang-Mills potential is in different vacuum states and there is a mass gap,
also disappears in the general system. We support our dynamical numerical
simulations with calculations in linear perturbation theory; for instance, we
compute quasi-normal modes of the unstable attractor (the Bartnik-McKinnon
soliton) in type I collapse in the magnetic sector.",1712.04458v2
2017-12-19,Active Region Photospheric Magnetic Properties Derived from Line-of-sight and Radial Fields,"The effect of using two representations of the normal-to-surface magnetic
field to calculate photospheric measures that are related to active region (AR)
potential for flaring is presented. Several AR properties were computed using
line-of-sight ($B_{\rm los}$) and spherical-radial ($B_{r}$) magnetograms from
the Space-weather HMI Active Region Patch (SHARP) products of the \emph{Solar
Dynamics Observatory}, characterizing the presence and features of magnetic
polarity inversion lines, fractality, and magnetic connectivity of the AR
photospheric field. The data analyzed corresponds to $\approx$ 4,000 AR
observations, achieved by randomly selecting 25 % of days between September
2012 and May 2016 for analysis at 6-hr cadence. Results from this statistical
study include: i) the $B_{r}$ component results in a slight upwards shift of
property values in a manner consistent with a field-strength underestimation by
the $B_{\rm los}$ component; ii) using the $B_{r}$ component results in
significantly lower inter-property correlation in one-third of the cases,
implying more independent information about the state of the AR photospheric
magnetic field; iii) flaring rates for each property vary between the field
components in a manner consistent with the differences in property-value ranges
resulting from the components; iv) flaring rates generally increase for higher
values of properties, except Fourier spectral power index that has flare rates
peaking around a value of $5/3$. These findings indicate that there may be
advantages in using $B_{r}$ rather than $B_{\rm los}$ in calculating
flare-related AR magnetic properties, especially for regions located far from
central meridian.",1712.06902v1
2018-01-04,Synchronized Observations of Bright Points from the Solar Photosphere to Corona,"One of the most important features in the solar atmosphere is magnetic
network and its rela- tionship with the transition region (TR), and coronal
brightness. It is important to understand how energy is transported into the
corona and how it travels along the magnetic-field lines be- tween deep
photosphere and chromosphere through the TR and corona. An excellent proxy for
transportation is the Interface Region Imaging Spectrograph (IRIS) raster scans
and imaging observations in near-ultraviolet (NUV) and far-ultraviolet (FUV)
emission channels with high time-spatial resolutions. In this study, we focus
on the quiet Sun as observed with IRIS. The data with high signal to noise
ratio in Si IV, C II and Mg II k lines and with strong emission intensities
show a high correlation in TR bright network points. The results of the IRIS
intensity maps and dopplergrams are compared with those of Atmo- spheric
Imaging Assembly (AIA) and Helioseismic and Magnetic Imager (HMI) instruments
onboard the Solar Dynamical Observatory (SDO). The average network intensity
profiles show a strong correlation with AIA coronal channels. Furthermore, we
applied simultaneous observations of magnetic network from HMI and found a
strong relationship between the network bright points in all levels of the
solar atmosphere. These features in network elements exhibited high doppler
velocity regions and large mag- netic signatures. A dominative fraction of
corona bright points emission, accompanied by the magnetic origins in
photosphere, suggest that magnetic-field concentrations in the network rosettes
could help couple between inner and outer solar atmosphere.",1801.01307v1
2018-01-29,Magnetism of a Co monolayer on Pt(111) capped by overlayers of $5d$ elements: a spin-model study,"Using first-principles calculations, we study the magnetic properties of a Co
monolayer on a Pt(111) surface with a capping monolayer of selected $5d$
elements (Re, Os, Ir, Pt and Au). First we determine the tensorial exchange
interactions and magnetic anisotropies characterizing the Co monolayer for all
considered systems. We find a close relationship between the magnetic moment of
the Co atoms and the nearest-neighbor isotropic exchange interaction, which is
attributed to the electronic hybridization between the Co and the capping
layers, in spirit of the Stoner picture of ferromagnetism. The
Dzyaloshinskii-Moriya interaction is decreased for all overlayers compared to
the uncapped Co/Pt(111) system, while even the sign of the
Dzyaloshinskii-Moriya interaction changes in case of the Ir overlayer. We
conclude that the variation of the Dzyaloshinskii-Moriya interaction is well
correlated with the change of the magnetic anisotropy energy and of the orbital
moment anisotropy. The unique influence of the Ir overlayer on the
Dzyaloshinskii-Moriya interaction is traced by scaling the strength of the
spin--orbit coupling of the Ir atoms in Ir/Co/Pt(111) and by changing the Ir
concentration in the Au$_{1-x}$Ir$_{x}$/Co/Pt(111) system. Our spin dynamics
simulations indicate that the magnetic ground state of Re/Co/Pt(111) thin film
is a spin spiral with a tilted normal vector, while the other systems are
ferromagnetic.",1801.09426v1
2018-02-20,Implosive collapse about magnetic null points: A quantitative comparison between 2D and 3D nulls,"Null collapse is an implosive process whereby MHD waves focus their energy in
the vicinity of a null point, forming a current sheet and initiating magnetic
reconnection. We consider, for the first time, the case of collapsing 3D
magnetic null points in nonlinear, resistive MHD using numerical simulation,
exploring key physical aspects of the system as well as performing a detailed
parameter study. We find that within a particular plane containing the 3D null,
the plasma and current density enhancements resulting from the collapse are
quantitatively and qualitatively as per the 2D case in both the linear and
nonlinear collapse regimes. However, the scaling with resistivity of the 3D
reconnection rate - which is a global quantity - is found to be less favourable
when the magnetic null point is more rotationally symmetric, due to the action
of increased magnetic back-pressure. Furthermore, we find that with increasing
ambient plasma pressure the collapse can be throttled, as is the case for 2D
nulls. We discuss this pressure-limiting in the context of fast reconnection in
the solar atmosphere and suggest mechanisms by which it may be overcome. We
also discuss the implications of the results in the context of null collapse as
a trigger mechanism of Oscillatory Reconnection, a time-dependent reconnection
mechanism, and also within the wider subject of wave-null point interactions.
We conclude that, in general, increasingly rotationally-asymmetric nulls will
be more favourable in terms of magnetic energy release via null collapse than
their more symmetric counterparts.",1802.07076v1
2018-03-07,Spheroidal and conical shapes of ferrofluid-filled capsules in magnetic fields,"We investigate the deformation of soft spherical elastic capsules filled with
a ferrofluid in external uniform magnetic fields at fixed volume by a
combination of numerical and analytical approaches. We develop a numerical
iterative solution strategy based on nonlinear elastic shape equations to
calculate the stretched capsule shape numerically and a coupled finite element
and boundary element method to solve the corresponding magnetostatic problem,
and employ analytical linear response theory, approximative energy
minimization, and slender-body theory. The observed deformation behavior is
qualitatively similar to the deformation of ferrofluid droplets in uniform
magnetic fields. Homogeneous magnetic fields elongate the capsule, and a
discontinuous shape transition from a spheroidal shape to a conical shape takes
place at a critical field strength. We investigate how capsule elasticity
modifies this hysteretic shape transition. We show that conical capsule shapes
are possible but involve diverging stretch factors at the tips, which gives
rise to rupture for real capsule materials. In a slender-body approximation we
find that the critical susceptibility above which conical shapes occur for
ferrofluid capsules is the same as for droplets. At small fields capsules
remain spheroidal, and we characterize the deformation of spheroidal capsules
both analytically and numerically. Finally, we determine whether wrinkling of a
spheroidal capsule occurs during elongation in a magnetic field and how it
modifies the stretching behavior. We find the nontrivial dependence between the
extent of the wrinkled region and capsule elongation. Our results can be
helpful in order to quantitatively determine capsule or ferrofluid material
properties from magnetic deformation experiments. All results also apply to
elastic capsules filled with a dielectric liquid in an external uniform
electric field.",1803.02607v2
2018-03-22,A Statistical Study of the Magnetic Imprints of X-Class Solar Flares,"Magnetic imprints, the rapid and irreversible evolution of photospheric
magnetic fields as a feedback from flares in the corona, have been confirmed by
many previous studies. These studies showed that the horizontal field will
permanently increase at the polarity inversion line (PIL) after eruptions,
indicating that a more horizontal geometry of photospheric magnetic field is
produced. In this study, we analyze 20 X-class flares since the launch of the
Solar Dynamics Observatory (SDO) in 15 active regions (ARs) with heliographic
angle no greater than 45 degrees. We observe clear magnetic imprints in 16
flares, whereas 4 flares are exceptional. The imprint regions of the horizontal
field are located not only at the PIL but also at sunspot penumbra with strong
vertical fields. Making use of the observed mass and speed of the corresponding
coronal mass ejections (CMEs) , we find that the CMEs with larger momenta are
associated with stronger magnetic imprints. Furthermore, a linear relationship,
with a Kendall's Tau-b coefficient 0.54, between the CME momentum and the
change of Lorentz force is revealed. Based on that, we quantify the back
reaction time to be around 70 s, with a 90% confidence interval from about 50 s
to 90 s.",1803.08310v2
2018-04-30,The effect of cosmic-ray acceleration on supernova blast wave dynamics,"Non-relativistic shocks accelerate ions to highly relativistic energies
provided that the orientation of the magnetic field is closely aligned with the
shock normal (quasi-parallel shock configuration). In contrast,
quasi-perpendicular shocks do not efficiently accelerate ions. We model this
obliquity-dependent acceleration process in a spherically expanding blast wave
setup with the moving-mesh code {\sc arepo} for different magnetic field
morphologies, ranging from homogeneous to turbulent configurations. A
Sedov-Taylor explosion in a homogeneous magnetic field generates an oblate
ellipsoidal shock surface due to the slower propagating blast wave in the
direction of the magnetic field. This is because of the efficient cosmic ray
(CR) production in the quasi-parallel polar cap regions, which softens the
equation of state and increases the compressibility of the post-shock gas. We
find that the solution remains self-similar because the ellipticity of the
propagating blast wave stays constant in time. This enables us to derive an
effective ratio of specific heats for a composite of thermal gas and CRs as a
function of the maximum acceleration efficiency. We finally discuss the
behavior of supernova remnants expanding into a turbulent magnetic field with
varying coherence lengths. For a maximum CR acceleration efficiency of about 15
per cent at quasi-parallel shocks (as suggested by kinetic plasma simulations),
we find an average efficiency of about 5 per cent, independent of the assumed
magnetic coherence length.",1805.00128v1
2018-05-02,Magnetohydrodynamic simulation of magnetic null-point reconnections in NOAA AR12192 initiated with an extrapolated non-force-free-field,"Magnetohydrodynamics of the solar corona is simulated numerically. The
simulation is initialized with an extrapolated non-force-free magnetic field
using the vector magnetogram of the active region (AR) NOAA 12192 obtained on
the solar photosphere. Particularly, we focus on the magnetic reconnections
occurring close to a magnetic null-point that resulted in appearance of
circular chromospheric flare ribbons on October 24, 2014 around 21:21 UT, after
peak of an X3.1 flare. The extrapolated field lines show the presence of the
three-dimensional (3D) null near one of the polarity inversion lines---where
the flare was observed. In the subsequent numerical simulation, we find
magnetic reconnections occurring near the null point, where the magnetic field
lines from the fan-plane of the 3D null form a X-type configuration with
underlying arcade field lines. The footpoints of the dome-shaped field lines,
inherent to the 3D null, show high gradients of the squashing factor. We find
slipping reconnections at these quasi-separatrix layers, which are co-located
with the post-flare circular brightening observed at the chromospheric heights.
This demonstrates the viability of the initial non-force-free field along with
the dynamics it initiates. Moreover, the initial field and its simulated
evolution is found to be devoid of any flux rope, which is in congruence with
the confined nature of the flare.",1805.00635v1
2018-05-30,Low-frequency Alfven Waves Produced by Magnetic Reconnection in the Sun's Magnetic Carpet,"The solar corona is a hot, dynamic, and highly magnetized plasma environment
whose source of energy is not yet well understood. One leading contender for
that energy source is the dissipation of magnetohydrodynamic (MHD) waves or
turbulent fluctuations. Many wave-heating models for the corona and the solar
wind presume that these fluctuations originate at or below the Sun's
photosphere. However, this paper investigates the idea that magnetic
reconnection may generate an additional source of MHD waves over a gradual
range of heights in the low corona. A time-dependent Monte Carlo simulation of
the mixed-polarity magnetic field is used to predict the properties of
reconnection-driven coronal MHD waves. The total power in these waves is
typically small in comparison to that of photosphere-driven waves, but their
frequencies are much lower. Reconnection-driven waves begin to dominate the
total power spectrum at periods longer than about 30 minutes. Thus, they may
need to be taken into account in order to understand the low-frequency
power-law spectra observed by both coronal spectropolarimetry and in-situ
particle/field instruments. These low-frequency Alfven waves should carry more
magnetic energy than kinetic energy, and thus they may produce less nonthermal
Doppler broadening (in comparison to photosphere-driven high-frequency waves)
in emission lines observed above the solar limb.",1805.12184v1
2018-06-20,Evaluation of noise limits of a precision ADC for direct digital signal integration of magnetic measurements,"The characterization of laboratory plasma instabilities, magnetic
reconnection and turbulence associated phenomena, require the simultaneous
signal sampling from arrays of magnetic sensors (hundreds or even thousands) to
obtain spatial resolution, with several hundred kHz for time resolution.
Magnetic measurements based on pick-up (Mirnov) coil are quite common in
experimental pulsed devices for plasma research, thanks to their simplicity and
reliability. Being the signal from this type of sensor proportional to the time
variation of the magnetic field ($dB/dt$), it has to be time-integrated to
recover the instant value of magnetic field. Depending on the required
integration time usually either analog integrators or chopped integrators are
used. However these solutions tend to limit the frequency bandwidth in the kHz
range, they are not easy to design and build, and require additional fast
channels to directly acquire the $dB/dt$ signal to recover the plasma dynamic
features at higher frequencies. In this paper we evaluate the feasibility of
using a direct single channel precision ADC to allow the simultaneous
acquisition of $dB/dt$ measurements and to provide the integrated B measurement
by means of digital integration on the new RFX-mod2 device. On this purpose we
interfaced an existing ADC-module to a Xilinx Zynq FPGA, in order to evaluate
the intrinsic noise and to investigate the feasible integration window of this
configuration. The result opens the door to a compact, cost-effective and
reliable acquisition system, usable for simultaneous real-time control and
transient signal recording, scalable from tenths to thousands channels,
applicable to a broad class of pulsed plasma experimental devices.",1806.07886v1
2018-07-30,Standing shocks in magnetized advection accretion flows onto a rotating black hole,"We present the global structure of magnetized advective accretion flow around
the rotating black holes in presence of dissipation. By considering accretion
flow to be threaded by toroidal magnetic fields and by assuming synchrotron
radiative mechanism to be the dominant cooling process, we obtain global
transonic accretion solutions in terms of dissipation parameters, such as
viscosity ($\alpha_B$), accretion rate (${\dot m}$) and plasma-$\beta$,
respectively. In the rotating magnetized accretion flow, centrifugal barrier is
developed in the nearby region of the black hole that triggers the
discontinuous shock transition in the flow variables. Evidently, the shock
properties and the dynamics of the post-shock flow (hereafter post-shock corona
(PSC)) are being governed by the flow parameters. We study the role of
dissipation parameters in the formation of standing shock wave and find that
global shocked accretion solutions exist both in gas pressure dominated flows
and in magnetic pressure dominated flows. In addition, we observe that standing
shock continues to form around the rapidly rotating black holes as well. We
identify the range of dissipation parameters that permits shocked accretion
solutions and find that standing shocks continue to form even in presence of
high dissipation limit, although the likelihood of shock formation diminishes
with the increase of dissipation. Further, we compute the critical accretion
rate (${\dot m}^{\rm cri}$) that admits shock and observe that standing shock
exists in a magnetically dominated accretion flow when the accretion rate lies
in general in the sub-Eddington domain. At the end, we calculate the maximum
dissipated energy that may be escaped from the PSC and indicate its possible
implication in the astrophysical context.",1807.11417v1
2018-08-10,Diamond magnetic microscopy of malarial hemozoin nanocrystals,"Magnetic microscopy of malarial hemozoin nanocrystals was performed using
optically detected magnetic resonance imaging of near-surface diamond
nitrogen-vacancy centers. Hemozoin crystals were extracted from
$Plasmodium$-$falciparum$-infected human blood cells and studied alongside
synthetic hemozoin crystals. The stray magnetic fields produced by individual
crystals were imaged at room temperature as a function of applied field up to
350 mT. More than 100 nanocrystals were analyzed, revealing the distribution of
their magnetic properties. Most crystals ($96\%$) exhibit a linear dependence
of stray field magnitude on applied field, confirming hemozoin's paramagnetic
nature. A volume magnetic susceptibility $\chi=3.4\times10^{-4}$ is inferred
using a magnetostatic model informed by correlated scanning electron microscopy
measurements of crystal dimensions. A small fraction of nanoparticles (4/82 for
$Plasmodium$-produced and 1/41 for synthetic) exhibit a saturation behavior
consistent with superparamagnetism. Translation of this platform to the study
of living malaria-infected cells may shed new light on hemozoin formation
dynamics and their interaction with antimalarial drugs.",1808.03636v2
2018-09-09,High-speed shear driven dynamos. Part 2. Numerical analysis,"This paper aims to numerically verify the large Reynolds number asymptotic
theory of magneto-hydrodynamic (MHD) flows proposed in the companion paper
Deguchi (2019). To avoid any complexity associated with the chaotic nature of
turbulence and flow geometry, nonlinear steady solutions of the
viscous-resistive magneto-hydrodynamic equations in plane Couette flow have
been utilised. Two classes of nonlinear MHD states, which convert kinematic
energy to magnetic energy effectively, have been determined. The first class of
nonlinear states can be obtained when a small spanwise uniform magnetic field
is applied to the known hydrodynamic solution branch of the plane Couette flow.
The nonlinear states are characterised by the hydrodynamic/magnetic roll-streak
and the resonant layer at which strong vorticity and current sheets are
observed. These flow features, and the induced strong streamwise magnetic
field, are fully consistent with the vortex/Alfv\'en wave interaction theory
proposed in Deguchi (2019). When the spanwise uniform magnetic field is
switched off, the solutions become purely hydrodynamic. However, the second
class of `self-sustained shear driven dynamos' at the zero-external magnetic
field limit can be found by homotopy via the forced states subject to a
spanwise uniform current field. The discovery of the dynamo states has
motivated the corresponding large Reynolds number matched asymptotic analysis
in Deguchi (2019). Here, the reduced equations derived by the asymptotic theory
have been solved numerically. The asymptotic solution provides remarkably good
predictions for the finite Reynolds number dynamo solutions.",1809.03855v2
2018-09-11,Magnetic field resilient superconducting coplanar waveguide resonators for hybrid cQED experiments,"Superconducting coplanar waveguide resonators that can operate in strong
magnetic fields are important tools for a variety of high frequency
superconducting devices. Magnetic fields degrade resonator performance by
creating Abrikosov vortices that cause resistive losses and frequency
fluctuations, or suppressing superconductivity entirely. To mitigate these
effects we investigate lithographically defined artificial defects in
resonators fabricated from NbTiN superconducting films. We show that by
controlling the vortex dynamics the quality factor of resonators in
perpendicular magnetic fields can be greatly enhanced. Coupled with the
restriction of the device geometry to enhance the superconductors critical
field, we demonstrate stable resonances that retain quality factors $\simeq
10^5$ at the single photon power level in perpendicular magnetic fields up to
$B_\perp \simeq$ 20 mT and parallel magnetic fields up to $B_\parallel \simeq$
6 T. We demonstrate the effectiveness of this technique for hybrid systems by
integrating an InSb nanowire into a field resilient superconducting resonator,
and use it to perform fast charge readout of a gate defined double quantum dot
at $B_\parallel =$ 1 T.",1809.03932v1
2018-10-05,Magnetic field direction dependent antiskyrmion motion with microwave electric fields,"Magnetic skyrmions are regarded as promising information candidates in future
spintronic devices, which have been investigated theoretically and
experimentally in isotropic system. Recently, the sta- bilization of
antiskyrmions in the presence of anisotropic Dzyaloshinskii-Moriya interaction
and its dynamics driven by current have been investigated. Here, we report the
antiskyrmion motion with the combined action of the in-plane magnetic field and
microwave electric fields. The in-plane mag- netic field breaks the rotation
symmetry of the antiskyrmion, and perpendicular microwave electric field
induces the pumping of magnetic anisotropy, leading to antiskyrmion breathing
mode. With above two effects, the antiskyrmion propagates with a desired
trajectory. Antiskyrmion propagation velocity depends on the frequency,
amplitude of anisotropy pumping, and damping constant as well as strength of
in-plane field, which reaches the maximum value when the frequency of microwave
electric field is in consist with the resonance frequency of antiskyrmion.
Moreover, we show that the antiskyrmion propagation depends on the direction of
magnetic field, where the antiskyrmion Hall angle can be suppressed or
enhanced. At a critical direction of magnetic field, the Hall angle is zero.
Our results introduce a possible application of antiskyrmion in
antiskyrmion-based spintronic devices with lower energy consumption.",1810.02464v1
2018-10-15,Magnetic interactions and spin dynamics in the bond-disordered pyrochlore fluoride NaCaCo$_2$F$_7$,"We report high-frequency/high-field electron spin resonance (ESR) and
high-field magnetization studies on single crystals of the bond-disordered
pyrochlore NaCaCo$_2$F$_7$. Frequency- and temperature-dependent ESR
investigations above the freezing temperature $T_f \sim 2.4$ K reveal the
coexistence of two distinct magnetic phases. A cooperative paramagnetic phase,
evidenced by a gapless excitation mode, is found as well as a spin-glass phase
developing below 20 K which is associated with a gapped low-energy excitation.
Effective $g$-factors close to 2 are obtained for both modes in line with
pulsed high-field magnetization measurements which show an unsaturated
isotropic behavior up to 58 T at 2 K. In order to describe the field-dependent
magnetization in high magnetic fields, we propose an empirical model accounting
for highly anisotropic ionic $g$-tensors expected for this material and taking
into account the strongly competing interactions between the spins which lead
to a frustrated ground state. As a detailed quantitative relation between
effective $g$-factors as determined from ESR and the local $g$-tensors obtained
by neutron scattering [Ross et al., Phys. Rev. B 93, 014433 (2016)] is still
sought after, our work motivates further theoretical investigations of the
low-energy excitations in bond-disordered pyrochlores.",1810.06399v2
2018-10-22,Nonlinear Evolution of the Resonant Drag Instability in Magnetized Gas,"We investigate, for the first time, the nonlinear evolution of the magnetized
""resonant drag instabilities"" (RDIs). We explore magnetohydrodynamic (MHD)
simulations of gas mixed with (uniform) dust grains subject to Lorentz and drag
forces, using the GIZMO code. The magnetized RDIs exhibit fundamentally
different behaviour than the purely acoustic RDIs. The dust organizes into
coherent structures and the system exhibits strong dust-gas separation. In the
linear and early nonlinear regime, the growth rates agree with linear theory
and the dust self-organizes into two-dimensional planes or ""sheets."" Eventually
the gas develops fully nonlinear, saturated Alfv\'enic and compressible
fast-mode turbulence, which fills the under-dense regions with a small amount
of dust, and drives a dynamo which saturates at equipartition of kinetic and
magnetic energy. The dust density fluctuations exhibit significant
non-Gaussianity, and the power spectrum is strongly weighted towards the
largest (box-scale) modes. The saturation level can be understood via
quasi-linear theory, as the forcing and energy input via the instabilities
becomes comparable to saturated tension forces and dissipation in turbulence.
The magnetized simulation presented here is just one case; it is likely that
the magnetic RDIs can take many forms in different parts of parameter space.",1810.09491v1
2018-11-08,Magnetoelastics of High Field Phenomena in Antiferromagnets UO2 and CeRhIn5,"We use a recently developed optical fiber Bragg grating technique, in
continuous and pulsed magnetic fields in excess of 90T, to study magnetoelastic
correlations in magnetic materials at cryogenic temperatures. Both insulating
UO2 and metallic CeRhIn5 present antiferromagnetic ground states, at T_N =
30.3K and T_N = 3.85K respectively. A strong coupling of the magnetism to the
crystal lattice degrees of freedom in UO2 is found, revealing piezomagnetism as
well as the dynamics of antiferromagnetic domain switching between spin
arrangements connected by time reversal. The AFM domains become harder to
switch as the temperature is reduced, reaching a record value H_PZ(T = 4K) =
18T. The effect of strong magnetic fields is also studied in CeRhIn5, where an
anomaly in the sample crystallographic c-axis of magnitude Delta_c/c = 2 ppm is
found associated to a recently proposed electronic nematic state at H_en = 30T
applied 11o off the c-axis. Here we show that while this anomaly is absent when
the magnetic field is applied 18o off the a-axis, strong magnetoelastic quantum
oscillations attest to the high quality of the single crystal samples.",1811.03701v3
2018-11-28,Effect of magnetic anisotropy on Skyrmions with a high topological number in itinerant magnets,"We report our numerical results for the effect of magnetic anisotropy on a
Skyrmion crystal with a high topological number of two, which was recently
discovered in an itinerant electron model [R. Ozawa, S. Hayami, and Y. Motome,
Phys. Rev. Lett. 118, 147205 (2017)]. By performing numerical simulations based
on the kernel polynomial method and the Langevin dynamics for the Kondo lattice
model on a triangular lattice, we find that the topological property remains
robust against the single-ion anisotropy, while the magnetic texture is
deformed continuously. The resultant spin structure is characterized by three
wave numbers (triple-$Q$ state), in which the $xy$ component of spins forms a
magnetic vortex crystal and the $z$ component of spins behaves a sinusoidal
wave. For larger anisotropy, we show that the system exhibits a phase
transition from the Skyrmion crystal to topologically trivial phases with
vanishing scalar chirality: a single-$Q$ collinear and double-$Q$ noncoplanar
states for the easy-axis and easy-plane anisotropy, respectively. We also
examine the effect of the single-ion anisotropy in an external magnetic field,
and find that the field range of the Skyrmion crystal is rather insensitive to
the anisotropy, in contrast to another Skyrmion crystal with the topological
number of one whose field range is considerably extended (reduced) by the
easy-axis (easy-plane) anisotropy.",1811.11336v1
2018-12-21,Dynamo saturation down to vanishing viscosity: strong-field and inertial scaling regimes,"We present analytical examples of fluid dynamos that saturate through the
action of the Coriolis and inertial terms of the Navier-Stokes equation. The
flow is driven by a body force and is subject to global rotation and uniform
sweeping velocity. The model can be studied down to arbitrarily low viscosity
and naturally leads to the strong-field scaling regime for the magnetic energy
produced above threshold: the magnetic energy is proportional to the global
rotation rate and independent of the viscosity. Depending on the relative
orientations of global rotation and large-scale sweeping, the dynamo
bifurcation is either supercritical or subcritical. In the supercritical case,
the magnetic energy follows the scaling-law for supercritical strong-field
dynamos predicted on dimensional grounds by Petrelis & Fauve (2001). In the
subcritical case, the system jumps to a finite-amplitude dynamo branch. The
magnetic energy obeys a magneto-geostrophic scaling-law (Roberts & Soward
1972), with a turbulent Elsasser number of the order of unity, where the
magnetic diffusivity of the standard Elsasser number appears to be replaced by
an eddy diffusivity. In the absence of global rotation, the dynamo bifurcation
is subcritical and the saturated magnetic energy obeys the equipartition
scaling regime. We consider both the vicinity of the dynamo threshold and the
limit of large distance from threshold to put these various scaling behaviors
on firm analytical ground.",1812.09166v1
2018-12-24,Non-Reciprocal Spin Pumping in Asymmetric Magnetic Trilayers,"In magnetic trilayer systems, spin pumping is generally addressed as a
reciprocal mechanism characterized by one unique spin mixing conductance common
to both interfaces. However, this assumption is questionable in cases where
different types of interfaces are present in the material. Here, we present a
general theory for analyzing spin pumping in cases with more than one unique
interface. The theory is applied to analyze layer-resolved ferromagnetic
resonance experiments on the trilayer system
Ni$_{20}$Fe$_{80}$/Ru/Fe$_{49}$Co$_{49}$V$_2$ where the Ru spacer thickness is
varied to tune the indirect exchange coupling. The results show that the spin
pumping in trilayer systems with dissimilar magnetic layers is non-reciprocal,
with a surprisingly large difference between spin-pumping induced damping of
different interfaces. Our findings have importance on dynamics of spintronic
devices based on magnetic multilayer materials.",1812.09978v1
2019-01-16,Quantum oscillations in strongly correlated topological Kondo insulators,"The observation of quantum oscillations in topological Kondo insulators SmB6
and YbB12 is a recent puzzling experimental discovery. Quantum oscillations
observed in the resistivity and the magnetization are usually explained by the
existence of the Fermi surface. However, Kondo insulators do not have a Fermi
surface and thus should not show quantum oscillations. By performing dynamical
mean-field calculations for topologically nontrivial Kondo insulators in a
magnetic field, we analyze the effect of correlations on the emergence of
quantum oscillations in narrow-gap topological Kondo insulators and demonstrate
that the interplay between correlations and nonlocal hybridization,
ubiquitously occurring in topological Kondo insulators, can lead to observable
quantum oscillations without the necessity of a Fermi surface. Particularly, we
show that correlations make it easier to observe quantum oscillations in the
magnetization and the resistivity of the bulk material. The fundamental
mechanism for these quantum oscillations is a combination of correlation
effects and Landau levels coming very close to the Fermi energy. We furthermore
demonstrate that quantum oscillations in a three-dimensional system can be
understood by analyzing the physics on the two-dimensional planes in the
momentum space for which the hybridization in direction of the magnetic field
vanishes. We believe that this scenario is relevant to understanding the
observation of quantum oscillations in the magnetic torque for SmB6 as well as
oscillations in the resistivity and the magnetic torque of YbB12.",1901.05099v2
2019-02-13,The Role of a Tiny Brightening in a Huge Geo-effective Solar Eruption Leading to the St Patrick's Day Storm,"The largest magnetic storm in solar cycle 24 was caused by a fast coronal
mass ejection (CME) that was related to a small C9.1 flare that occurred on 15
March 2015 in solar active region (AR) NOAA 12297. The purpose of this study is
to understand the onset mechanism of the geo-effective huge solar eruption. We
focused on the C2.4 flare that occurred prior to the C9.1 flare of the filament
eruption. The magnetic field structure in the AR was complicated: there were
several filaments including the one that erupted and caused the CME. We hence
carefully investigated the photospheric magnetic field, brightenings observed
in the solar atmosphere, and the three-dimensional coronal magnetic field
extrapolated from nonlinear force-free field modeling, using data from Hinode
and Solar Dynamics Observatory. We found three intriguing points : (1) There
was a compact but noticeably highly twisted magnetic field structure that is
represented by a small filament in the C2.4 flaring region, where a tiny
precursor brightening was observed before the C2.4 flare. (2) The C2.4 flaring
region is located in the vicinity of a foot point of the closed field that
prohibits the filament from erupting. (3) The filament shows a sudden eruption
after the C2.4 flare and accompanying small filament eruption. From our
analysis, we suggest that a small magnetic disturbance that was represented by
the tiny precursor brightening at the time of the C2.4 flare is related to the
trigger of the huge filament eruption.",1902.04871v1
2019-03-07,Transient magnetic domain wall AC dynamics by means of MOKE microscopy,"The domain wall response under constant external magnetic fields reveals a
complex behavior where sample disorder plays a key role. Furthermore, the
response to alternating magnetic fields has only been explored in limited cases
and analyzed in terms of the constant field solution. Here we unveil phenomena
in the evolution of magnetic domain walls under the application of alternating
magnetic fields within the creep regime, well beyond a small fuctuation limit
of the domain wall position. Magnetic field pulses were applied in ultra-thin
ferromagnetic films with perpendicular anisotropy, and the resulting domain
wall evolution was characterized by polar magneto-optical Kerr effect
microscopy. Whereas the DC characterization is well predicted by the elastic
interface model, striking unexpected features are observed under the
application of alternating square pulses: magneto-optical images show that
after a transient number of cycles, domain walls evolve toward strongly
distorted shapes concomitantly with a modification of domain area. The
morphology of domain walls is characterized with a roughness exponent when
possible and contrasted with alternative observables which result to be more
suitable for the characterization of this transient evolution. The final
stationary convergence as well as the underlying physics is discussed.",1903.03006v1
2019-03-10,The decisive role of magnetic anisotropy in honeycomb layered Li$_3$Ni$_2$SbO$_6$ and Na$_3$Ni$_2$SbO$_6$,"The decisive role of magnetic anisotropy even in systems with small
anisotropy is illustrated for the honeycomb-layered antiferromagnets The
decisive role of magnetic anisotropy in honeycomb layered Li$_3$Ni$_2$SbO$_6$
and Na$_3$Ni$_2$SbO$_6$ with $A$ = Li and Na. Both systems evolve long range
magnetic order below $T_{\rm N}$ = 14 and 16.5~K, respectively. The magnetic
phase diagrams obtained from static magnetisation studies up to 15~T imply
competing antiferromagnetic phases and a tricritical point at $T_{\rm N}$. The
phase boundaries are visible in the dynamic response of the antiferromagnetic
resonance modes, too, which investigation by means of high frequency/high field
electron spin resonance enables precise determination of magnetic anisotropy.
The anisotropy gap amounts to $\Delta = 360 \pm 2$~GHz in Na$_3$Ni$_2$SbO$_6$
while in Li$_3$Ni$_2$SbO$_6$ orthorhombicity is associated with $\Delta = 198
\pm 4$ and $218 \pm 4$~GHz. Above $T_{\rm N}$, the data imply short-range
antiferromagnetic order up to at least 80~K. The data suggest a crucial role of
anisotropy for selecting the actual spin structure at $B=0$~T.",1903.04041v1
2019-03-26,"Partial invariants, large-scale dynamo action, and the inverse transfer of magnetic helicity","The existence of partially conserved enstrophy-like quantities is conjectured
to cause inverse energy transfers to develop embedded in magnetohydrodynamical
(MHD) turbulence, in analogy to the influence of enstrophy in two-dimensional
nonconducting turbulence. By decomposing the velocity and magnetic fields in
spectral space onto helical modes, we identify subsets of three-wave (triad)
interactions conserving two new enstrophy-like quantities which can be mapped
to triad interactions recently identified with facilitating large-scale
$\alpha$-type dynamo action and the inverse transfer of magnetic helicity. Due
to their dependence on interaction scale locality, the invariants suggest that
the inverse transfer of magnetic helicity might be facilitated by both local-
and nonlocal-scale interactions, and is a process more local than the
$\alpha$-dynamo. We test the predicted embedded (partial) energy fluxes by
constructing a shell model (reduced wave-space model) of the minimal set of
triad interactions (MTI) required to conserve the ideal MHD invariants.
Numerically simulated MTIs demonstrate that, for a range of forcing
configurations, the partial invariants are, with some exceptions, indeed useful
for understanding the embedded contributions to the total spectral energy flux.
Furthermore, we demonstrate that strictly inverse energy transfers may develop
if enstrophy-like conserving interactions are favoured, a mechanism recently
attributed to the energy cascade reversals found in nonconducting
three-dimensional turbulence subject to strong rotation or confinement. The
presented results have implications for the understanding of the physical
mechanisms behind large-scale dynamo action and the inverse transfer of
magnetic helicity, processes thought to be central to large-scale magnetic
structure formation.",1903.10725v2
2019-05-17,Mapping Dynamical Magnetic Responses of Ultra-thin Micron-size Superconducting Films using Nitrogen-vacancy Centers in Diamond,"Two-dimensional superconductors have attracted growing interest because of
their scientific novelty, structural tunability, and useful properties. Studies
of their magnetic responses, however, are often hampered by difficulties to
grow large-size samples of high quality and uniformity. We report here an
imaging method that employed NV- centers in diamond as sensor capable of
mapping out the microwave magnetic field distribution on an ultrathin
superconducting film of micron size. Measurements on a 33nm-thick film and a
125nm-thick bulk-like film of $Bi_2Sr_2CaCu_2O_{8+\delta}$ revealed that the ac
Meissner effect (or repulsion of ac magnetic field) set in at 78K and 91K,
respectively; the latter was the superconducting transition temperature (Tc) of
both films. The unusual ac magnetic response of the thin film presumably was
due to thermally excited vortex-antivortex diffusive motion in the film.
Spatial resolution of our ac magnetometer was limited by optical diffraction
and the noise level was at 14 ${\mu}T/Hz^{1/2}$. The technique could be
extended with better detection sensitivity to extract local ac
conductivity/susceptibility of ultrathin or monolayer superconducting samples
as well as ac magnetic responses of other two-dimensional exotic thin films of
limited lateral size.",1905.07083v1
2019-05-20,Orbitally Dominated Rashba-Edelstein Effect in Noncentrosymmetric Antiferromagnets,"Efficient manipulation of magnetic order with electric current pulses is
desirable for achieving fast spintronic devices. The Rashba-Edelstein effect,
wherein a spin polarization is electrically induced in noncentrosymmetric
systems, provides a mean to achieve current-induced staggered spin-orbit
torques. Initially predicted for spin, the orbital counterpart of this effect
has been disregarded up to now. Here, we present a generalized Rashba-Edelstein
effect, which generates not only spin polarization but also orbital
polarization, which we find to be far from being negligible and could play a
crucial role in the magnetization dynamics. We show that the orbital
Rashba-Edelstein effect does not require spin-orbit coupling to exist. We
present first-principles calculations of the frequency-dependent spin and
orbital Rashba-Edelstein susceptibility tensors for the noncentrosymmetric
antiferromagnets CuMnAs and Mn2Au. We show that the electrically induced local
magnetization has both staggered in-plane components and non-staggered
out-of-plane components, and can exhibit Rashba-like or Dresselhaus-like
symmetries, depending on the magnetic configuration. Furthermore, there is an
induced local magnetization on the nonmagnetic atoms as well, that is smaller
in Mn2Au than in CuMnAs. We compute sizable induced magnetizations at optical
frequencies, which suggest that electric-field driven switching could be
achieved at much higher frequencies.",1905.08279v1
2019-05-22,Ultradense Tailored Vortex Pinning Arrays in Superconducting YBa$_2$Cu$_3$O$_{7-δ}$ Thin Films Created by Focused He Ion Beam Irradiation for Fluxonics Applications,"Magnetic fields penetrate a type-II superconductor as magnetic flux quanta,
called vortices. In a clean superconductor they arrange in a hexagonal lattice,
while by adding periodic artificial pinning centers many other arrangements can
be realized. Using the focused beam of a helium ion microscope we have
fabricated periodic patterns of dense pinning centers with spacings as small as
70 nm in thin films of the cuprate superconductor
YBa$_{2}$Cu$_{3}$O$_{7-\delta}$. In these ultradense kagom\'e-like patterns,
the voids lead to magnetic caging of vortices, resulting in unconventional
commensurability effects that manifest themselves as peaks in the critical
current and minima in the resistance versus applied magnetic field up to $\sim
0.4\,$T. The various vortex patterns at different magnetic fields are analyzed
by molecular dynamics simulations of vortex motion, and the magnetic field
dependence of the critical current is confirmed. These findings open the way
for a controlled manipulation of vortices in cuprate superconductors by
artificial sub-100 nm pinning landscapes.",1905.09070v2
2019-08-06,"Tunable magnetization in nanoscale LuFeO3: Role of morphology, ortho-hexa phase ratio and local structure","We have observed enhancement and shift in the spin reorientation transition
temperature as a consequence of coexistence of orthorhombic and hexagonal
phases and higher aspect ratio in nanoscale LuFeO3. Nanoparticles and
nanofibers of LuFeO3 are considered for this work. Nanoparticles have 75 %
orthorhombic phase and 25 % hexagonal phase, while nanofibers have 23%
orthorhombic phase and 77%-hexagonal phase. Larger aspect ratio in case of
nanofibers is seen to help strain-stabilize the hexagonal phase in the
material. Magnetic measurements show significant difference in the magnetic
behavior and spin reorientation temperature; 183K for the nanoparticle case and
150K for the case of nanofibers. Moreover, the ferromagnetic moment is two
order of magnitude higher for nanofibers than that of nanoparticles, In
hexagonal phase, frustration of triangular lattice, works against the long
range ordering while magnetic anisotropy works in favor of the long range
ordering, which contributes towards the enhanced and anomalous magnetic
behavior in case of fibers. X -ray absorption near edge spectroscopy (XANES) at
the Fe K-edge has been used to probe the symmetry driven dynamics of Fe 3d- 4p
orbitals. It established that due to noncentrocymmetry of the Fe atom, the
nanofibers have decreased 3d-4p orbital mixing and reduced crystal field
splitting energy, which are also contributing factor for the enhanced magnetic
behaviour.",1908.02073v1
2019-10-21,Energy origination and triggering mechanism of a series of homologous confined flares,"Using the H$\alpha$ data from the New Vacuum Solar Telescope (NVST) at the
Fuxian Solar Observatory together with multi-wavelength images and magnetograms
obtained by Solar Dynamics Observatory (SDO), we study the detailed process of
three homologous confined flares in active-region (AR) NOAA 11861 on 2013
October 12. All of the three flares occurred at same location, with similar
morphologies and comparable class. Through analyzing the evolution of magnetic
field and flow field, we found an emergence of magnetic flux and a strong
shearing motion between two opposite polarities near the following sunspot. The
magnetic flux and the average transverse field strength exhibited a decrease
before each eruption and reached the lowest point at the onset of each
eruption. By calculating the shearing and the emergence energy in the
photosphere, we found that the integral of energy injected from the
photosphere, for a few hours, could provide enough energy for the flares. The
reconnection between different loops was observed in H$\alpha$ images during
the occurrence of each flare. These results suggest that the emerging magnetic
flux and the shearing motion in the photosphere can inject the energy to the
sheared magnetic loops and the energy finally was released via magnetic
reconnection to power the solar flares.",1910.09147v1
2019-10-23,Giant room temperature anomalous Hall effect and magnetically tuned topology in the ferromagnetic Weyl semimetal Co2MnAl,"Weyl semimetals (WSM) have been extensively studied due to their exotic
properties such as topological surface states and anomalous transport
phenomena. Their band structure topology is usually predetermined by material
parameters and can hardly be manipulated once the material is formed. Their
unique transport properties appear usually at very low temperature, which sets
challenges for practical device applications. In this work, we demonstrate a
way to modify the band topology via a weak magnetic field in a ferromagnetic
topological semimetal, Co2MnAl, at room temperature. We observe a tunable,
giant anomalous Hall effect, which is induced by the transition between Weyl
points and nodal rings as rotating the magnetization axis. The anomalous Hall
conductivity is as large as that of a 3D quantum anomalous Hall effect (QAHE),
with the Hall angle reaching a record value (21%) at the room temperature among
magnetic conductors. Furthermore, we propose a material recipe to generate the
giant anomalous Hall effect by gaping nodal rings without requiring the
existence of Weyl points. Our work reveals an ideal intrinsically magnetic
platform to explore the interplay between magnetic dynamics and topological
physics for the development of a new generation of spintronic devices.",1910.10378v1
2019-10-25,Numerical simulations of the emerging plasma blob into a solar coronal hole,"We numerically simulate emergence of a magnetic plasma blob into a solar
coronal hole. This blob may be associated with granulation and therefore it has
a weak magnetic field. Two-dimensional simulations are performed using the
MAGNUS code which solves magnetohydrodynamic equations, taking into account
magnetic resistivity and thermal conduction. As a result of the interaction of
the emerging blob with the ambient plasma, the magnetic lines experience
reconnection with the blob getting flattened and deformed with time.
Additionally, this process launches a vertical outflow of hot plasma and the
chromosphere in its response increases its temperature. We perform parametric
studies by varying the magnitude of the magnetic field of the blob and
observing the net heating of the chromosphere. These studies are inspired by
realistic simulations of granulation made with the use of two-fluid JOANNA
code. In these simulations a number of magnetic blobs are detected in the
convection zone and in the photosphere. From the numerical results, we conclude
that as a result of granulation operating in a solar quiet region the emerging
blob may trigger very complex dynamics in the upper regions of the solar
atmosphere, and the associated outflows may be a source of heating of the
chromosphere and possibly the solar corona.",1910.11814v1
2019-11-07,Properties of two-temperature magnetized advective accretion flow around rotating black hole,"We study the two-temperature magnetized advective accretion flow around the
Kerr black holes. During accretion, ions are heated up due to viscous
dissipation, and when Coulomb coupling becomes effective, they transfer a part
of their energy to the electrons. On the contrary, electrons lose energy due to
various radiative cooling processes, namely bremsstrahlung, synchrotron, and
Comtonization processes, respectively. To account for the magnetic contribution
inside the disc, we consider the toroidal magnetic fields which are assumed to
be dominant over other components. Moreover, we adopt the relativistic equation
of state to describe the thermal characteristics of the flow. With this, we
calculate the global transonic accretion solutions around the rotating black
holes. We find that accretion solution containing multiple critical points may
harbor shock wave provided the standing shock conditions are satisfied.
Further, we investigate the shock properties, such as shock location ($x_s$)
and compression ratio ($R$) that delineate the post-shock corona (hereafter
PSC) and find that the dynamics of PSC is controlled by the flow parameters,
such as accretion rate (${\dot m}$) and magnetic fields ($\beta$, defined as
the ratio of gas pressure to the magnetic pressure), etc. Finally, we calculate
the emission spectra of the accretion flows containing PSC and indicate that
both ${\dot m}$ and $\beta$ play the pivotal roles in explaining the spectral
state transitions commonly observed for black hole X-ray binaries.",1911.02757v2
2019-11-08,Spatio-temporal behavior of magnetohydrodynamic fluctuations with cross-helicity and background magnetic field,"We study the spatio-temporal behavior of the Els\""asser variables describing
magnetic and velocity field fluctuations, using direct numerical simulations of
three-dimensional magnetohydrodynamic turbulence. We consider cases with
relatively small, intermediate, and large values of a mean background magnetic
field, and with null, small, and high cross-helicity (correlations between the
velocity and the magnetic field). Wavenumber-dependent time correlation
functions are computed for the different simulations. From these correlation
functions, the decorrelation time is computed and compared with different
theoretical characteristic times: the local non-linear time, the
random-sweeping time, and the Alfv\'enic time. It is found that decorrelation
times are dominated by sweeping effects for low values of the mean magnetic
field and for low values of the cross-helicity, while for large values of the
background field or of the cross-helicity and for wave vectors sufficiently
aligned with the guide field, decorrelation times are controlled by Alfv\'enic
effects. Finally, we observe counter-propagation of Alfv\'enic fluctuations due
to reflections produced by inhomogeneities in the total magnetic field. This
effect becomes more prominent in flows with large cross-helicity, strongly
modifying the propagation of waves in turbulent magnetohydrodynamic flows.",1911.03400v1
2019-11-11,Global axisymmetric simulations of photoevaporation and magnetically driven protoplanetary disk winds,"Photoevaporation and magnetically driven winds are two independent mechanisms
to remove mass from protoplanetary disks. In addition to accretion, the effect
of these two principles acting concurrently could be significant and the
transition between those two has not been extensively studied and quantified in
the literature yet. In order to contribute to the understanding of disk winds,
we present the phenomena emerging in the framework of two-dimensional
axisymmetric, non-ideal magnetohydrodynamic simulations including EUV-/ X-ray
driven photoevaporation. Of particular interest are the examination of the
transition region between photoevaporation and magnetically driven wind, the
possibility of emerging magneto-centrifugal wind effects, as well as the
morphology of the wind itself depending on the strength of the magnetic field.
We use the PLUTO code in a 2.5D axisymmetric configuration with additional
treatment of EUV-/ X-ray heating and dynamic ohmic diffusion based on a
semi-analytical chemical model. We identify the transition between both outflow
types to occur for values of the initial plasma beta $\beta \geq 10^7$, while
magnetically driven winds generally outperform photoevaporation for stronger
fields. In our simulations we observe irregular and asymmetric outflows for
stronger magnetic fields. In the weak field regime the photoevaporation rates
are slightly lowered by perturbations of the gas density in the inner regions
of the disk. Overall, our results predict a wind with a lever arm smaller than
1.5, consistent with a hot magneto-thermal wind. Stronger accretion flows are
present for values of $\beta < 10^7$.",1911.04510v2
2019-11-25,Magnetic response of FeRh to static and dynamic disorder,"Changes of the magnetic and crystal structure on the microscopic scale in 40
nm FeRh thin films have been applied to investigate the phenomena of a disorder
induced ferromagnetism at room temperature initiated through light
ion-irradiation with fluences up to 0.125 Ne$^+$/nm$^{-2}$. Magnetometry shows
an increase of magnetic ordering at low temperatures and a decrease of the
transition temperature combined with a broadening of the hysteresis with rising
ion fluence. $^{57}$Fe M\""ossbauer spectroscopy reveals the occurrence of an
additional magnetic contributions with an hyperfine splitting of 27.2 T -
identical to that of ferromagnetic B2-FeRh. The appearance of an anti-site
Fe-contribution can be assumed to be lower than 0.6 Fe-at%, indicating that no
change of the chemical composition is evident. The investigation of the local
structure shows an increase of the static mean square relative displacement
determined by X-ray absorption fine structure spectroscopy, while an increase
of the defect-concentration has been determined by positron annihilation
spectroscopy. From the changes of the microscopic magnetic structure a
similarity between the temperature induced and the structural disorder induced
ferromagnetic phase can be observed. These findings emphasize the relationship
between magnetic ordering and the microscopic defect structure in FeRh.",1911.11256v1
2019-12-03,The dynamo-wind feedback loop: Assessing their non-linear interplay,"Though generated deep inside the convection zone, the solar magnetic field
has a direct impact on the Earth space environment via the Parker spiral. It
strongly modulates the solar wind in the whole heliosphere, especially its
latitudinal and longitudinal speed distribution over the years. However the
wind also influences the topology of the coronal magnetic field by opening the
magnetic field lines in the coronal holes, which can affect the inner magnetic
field of the star by altering the dynamo boundary conditions. This coupling is
especially difficult to model because it covers a large variety of
spatio-temporal scales. Quasi-static studies have begun to help us unveil how
the dynamo-generated magnetic field shapes the wind, but the full interplay
between the solar dynamo and the solar wind still eludes our understanding.
  We use the compressible magnetohydrodynamical (MHD) code PLUTO to compute
simultaneously in 2.5D the generation and evolution of magnetic field inside
the star via an alpha-Omega dynamo process and the corresponding evolution of a
polytropic coronal wind over several activity cycles for a young Sun. A
multi-layered boundary condition at the surface of the star connects the inner
and outer stellar layers, allowing both to adapt dynamically. Our continuously
coupled dynamo-wind model allows us to characterize how the solar wind
conditions change as a function of the cycle phase, and also to quantify the
evolution of integrated quantities such as the Alfv\'en radius. We further
assess the impact of the solar wind on the dynamo itself by comparing our
results with and without wind feedback.",1912.01271v1
2019-12-04,MnSnTeO6: a Chiral Antiferromagnet Prepared by a Two-Step Topotactic Transformation,"MnSnTeO6, a new chiral antiferromagnet, was prepared both by topotactic
transformation of the metastable rosiaite-type polymorph and by direct
synthesis from coprecipitated hydroxides. Its structure, static and dynamic
magnetic properties were studied comprehensively both experimentally (through
X-ray and neutron powder diffraction, magnetization, specific heat, dielectric
permittivity and ESR techniques) and theoretically (by means of ab initio
density functional theory (DFT) calculations within the spin-polarized
generalized gradient approximation). MnSnTeO6 is isostructural with MnSb2O6
(space group P321) and does not show any structural transition between 3 and
300 K. The magnetic susceptibility and specific heat exhibit an
antiferromagnetic ordering at TN=9.8 K, which is confirmed by low-temperature
neutron data. At the same time, the thermodynamic parameters demonstrate an
additional anomaly on the temperature dependences of magnetic susceptibility
chi(T), specific heat Cp(T) and dielectric permittivity epsilon(T) at T*=4.9 K,
which is characterized by significant temperature hysteresis. Clear enhancement
of the dielectric permittivity at T* is most likely to reflect the coupling of
dielectric and magnetic subsystems leading to development of electric
polarization. It was established that the ground state of MnSnTeO6 is
stabilized by seven exchange parameters, and neutron diffraction revealed
incommensurate magnetic structure with propagation vector k= (0, 0, 0.183)
analogous to that of MnSb2O6. Ab initio DFT calculations demonstrate that the
strongest exchange coupling occurs between planes along diagonals. All exchange
parameters are antiferromagnetic and reveal moderate frustration.",1912.02623v1
2019-12-12,A Curious Case of Circular Polarization in the 25 GHz Methanol Maser Line toward OMC-1,"We report the detection of a circular polarization signature in the Stokes
$V$ profile of a 25 GHz Class I CH$_3$OH maser toward the high mass star
forming region OMC-1. Such a feature usually constitutes a detection of the
Zeeman effect. If due to a magnetic field in OMC-1, this would represent the
first detection and discovery of the Zeeman effect in the 25 GHz Class I
CH$_3$OH maser. The feature in Stokes $V$ is detected in two observations with
different angular resolutions taken eight years apart with the Very Large Array
(VLA); for our 2009 D-configuration observations, the fitted value for $z
B_\text{los}$ is $152 \pm 12$ Hz, where $z$ is the Zeeman splitting factor and
$B_\text{los}$ is the line-of-sight magnetic field. For our 2017
C-configuration observations, the fitted value for $zB_\text{los} = 149 \pm
19~$Hz, likely for the same maser spot. These correspond to $B_\text{los}$ in
the range 171-214 mG, depending on which hyperfine transition is responsible
for the maser line. While these $B_\text{los}$ values are high, they are not
implausible. If the magnetic field increases in proportion to the molecular
hydrogen density in shocked regions, then our detected fields predict values
for the pre-shock magnetic field that are in agreement with observations. With
$B_\text{los}$ =171-214 mG, the magnetic energy in the post-shocked regions
where these 25 GHz Class I CH$_3$OH masers occur would dominate over the
kinetic energy density and be at least of the order of the pressure in the
shock, implying that the magnetic field would exert significant influence over
the dynamics of these regions.",1912.06235v1
2020-02-18,Quantum dynamics in strongly driven random dipolar magnets,"The random dipolar magnet LiHo$_x$Y$_{1-x}$F$_4$ enters a strongly frustrated
regime for small Ho$^{3+}$ concentrations with $x<0.05$. In this regime, the
magnetic moments of the Ho$^{3+}$ ions experience small quantum corrections to
the common Ising approximation of LiHo$_x$Y$_{1-x}$F$_4$, which lead to a
$Z_2$-symmetry breaking and small, degeneracy breaking energy shifts between
different eigenstates. Here we show that destructive interference between two
almost degenerate excitation pathways burns spectral holes in the magnetic
susceptibility of strongly driven magnetic moments in LiHo$_x$Y$_{1-x}$F$_4$.
Such spectral holes in the susceptibility, microscopically described in terms
of Fano resonances, can already occur in setups of only two or three frustrated
moments, for which the driven level scheme has the paradigmatic
$\Lambda$-shape. For larger clusters of magnetic moments, the corresponding
level schemes separate into almost isolated many-body $\Lambda$-schemes, in the
sense that either the transition matrix elements between them are negligibly
small or the energy difference of the transitions is strongly off-resonant to
the drive. This enables the observation of Fano resonances, caused by many-body
quantum corrections to the common Ising approximation also in the thermodynamic
limit. We discuss its dependence on the driving strength and frequency as well
as the crucial role that is played by lattice dissipation.",2002.07834v1
2020-02-20,Numerical simulation of the wave turbulence on the surface of a ferrofluid in a horizontal magnetic field,"The turbulence of capillary waves on the surface of a ferrofluid with a high
permeability in a horizontal magnetic field is considered in the framework of a
one-dimensional weakly nonlinear model. In the limit of a strong magnetic
field, the surface waves under study can propagate without distortions along or
against the direction of external field, i.e., similar to Alfv\'en waves in a
perfectly conducting fluid. The interaction of counter-propagating nonlinear
waves leads to the development of wave turbulence on the surface of the liquid.
The computational data show that the spectrum of turbulence is divided into two
parts: a low-frequency dispersionless region, where the magnetic forces
dominate and a high-frequency dispersive one, in which the influence of
capillary forces becomes significant. In the first region, the spectrum of the
surface elevation has the same exponent in $k$ and $\omega$ domains and its
value is close to $-3.5$, what is in a good agreement with the estimation
obtained from the dimensional analysis of the weak turbulence spectra. At the
high frequencies, the computed spatial spectrum of the surface waves is close
to $k^{-5/2}$ which corresponds to $\omega^{-5/3}$ in terms of the frequency.
This spectrum does not coincide with the Zakharov-Filonenko spectrum obtained
for pure capillary waves. A possible explanation of this fact is in the
influence of coherent structures (like shock waves) usually arising in media
with weak dispersion.",2002.08698v1
2020-02-29,Magnetoelectric polarizability: A microscopic perspective,"We extend a field theoretic approach for the investigation of the electronic
charge-current density response of crystalline systems to arbitrary applied
electromagnetic fields. The approach leads to the introduction of microscopic
polarization and magnetization fields, as well as free charge and current
densities, the dynamics of which are described by a lattice gauge theory. The
spatial averages of such quantities constitute the fields of macroscopic
electrodynamics. We implement this formalism to study the orbital electronic
response of a class of insulators to applied uniform dc electric and magnetic
fields at zero temperature. To first-order in the applied fields, the free
charge and current densities vanish; thus the response of the system is
characterized by the first-order modifications to the microscopic polarization
and magnetization fields. Associated with the dipole moment of the microscopic
polarization (magnetization) field is a macroscopic polarization
(magnetization), for which we extract various response tensors. We focus on the
orbital magnetoelectric polarizability (OMP) tensor, and find the accepted
expression as derived from the ""modern theory of polarization and
magnetization."" Since our results are based on the spatial averages of
microscopic fields, we can identify the distinct contributions to the OMP
tensor from the perspective of this microscopic theory, and we establish the
general framework in which extensions to finite frequency can be made.",2003.00313v2
2020-04-12,Hybridized quadrupolar excitations in the spin-anisotropic frustrated magnet FeI2,"Magnetic order is usually associated with well-defined magnon excitations.
Exotic magnetic fluctuations with fractional, topological or multipolar
character, have been proposed for radically different forms of magnetic matter
such as spin-liquids. As a result, considerable efforts have searched for, and
uncovered, low-spin materials with suppressed dipolar order at low
temperatures. Here, we report neutron-scattering experiments and quantitative
theoretical modeling of an exceptional spin-1 system -- the uniaxial triangular
magnet FeI2 -- where a bright and dispersive band of mixed dipolar-quadrupolar
fluctuations emerges just above a dipolar ordered ground-state. This excitation
arises from anisotropic exchange interactions that hybridize overlapping modes
carrying fundamentally different quantum numbers. Remarkably, a generalization
of spin-wave theory to local SU(3) degrees of freedom accounts for all details
of the low-energy dynamical response of FeI2 without going beyond quadratic
order. Our work highlights that quantum excitations without classical
counterparts can be realized even in presence of fully developed magnetic
order.",2004.05623v1
2020-05-01,Magnetic catalysis and the chiral condensate in holographic QCD,"We investigate the effect of a non-zero magnetic field on the chiral
condensate using a holographic QCD approach. We extend the model proposed by
Iatrakis, Kiritsis and Paredes in arXiv:1010.1364 that realises chiral symmetry
breaking dynamically from 5d tachyon condensation. We calculate the chiral
condensate, magnetisation and susceptibilities for the confined and deconfined
phases. The model leads, in the probe approximation, to magnetic catalysis of
chiral symmetry breaking in both confined and deconfined phases. In the chiral
limit, $m_q=0$, we find that in the deconfined phase a sufficiently strong
magnetic field leads to a second order phase transition from the chirally
restored phase to a chirally broken phase. The transition becomes a crossover
as the quark mass increases. Due to a scaling in the temperature, the chiral
transition will also be interpreted as a transition in the temperature for
fixed magnetic field. We elaborate on the relationship between the chiral
condensate, magnetisation and the (magnetic) free energy density. We compare
our results at low and moderate temperatures temperatures with lattice QCD
results.",2005.00500v3
2020-05-04,Current-driven skyrmionium in a frustrated magnetic system,"Magnetic skyrmionium can be used as a nanometer-scale non-volatile
information carrier, which shows no skyrmion Hall effect due to its special
structure carrying zero topological charge. Here, we report the static and
dynamic properties of an isolated nanoscale skyrmionium in a frustrated
magnetic monolayer, where the skyrmionium is stabilized by competing
interactions. The frustrated skyrmionium has a size of about $10$ nm, which can
be further reduced by tuning perpendicular magnetic anisotropy or magnetic
field. It is found that the nanoscale skyrmionium driven by the damping-like
spin-orbit torque shows directional motion with a favored Bloch-type helicity.
A small driving current or magnetic field can lead to the transformation of an
unstable N\'eel-type skyrmionium to a metastable Bloch-type skyrmionium. A
large driving current may result in the distortion and collapse of the
Bloch-type skyrmionium. Our results are useful for the understanding of
frustrated skyrmionium physics, which also provide guidelines for the design of
spintronic devices based on topological spin textures.",2005.01403v2
2020-05-14,Field Cooled Annular Josephson Tunnel Junctions,"We investigate the physics of planar annular Josephson tunnel junctions
quenched through their transition temperature in the presence of an external
magnetic field. Experiments carried out with long Nb/Al-AlOx/Nb annular
junctions showed that the magnetic flux trapped in the high-quality
doubly-connected superconducting electrodes forming the junction generates a
persistent current whose associated magnetic field affects the both the static
and dynamics properties of the junctions. More specifically, the field trapped
in the hole of one electrode combined with a d.c. bias current induces a
viscous flow of dense trains of Josephson vortices which manifests itself
through the sequential appearance of displaced linear slopes, Fiske step
staircases and Eck steps in the junction's current-voltage characteristic.
Furthermore, a field shift is observed in the first lobe of the magnetic
diffraction pattern. The effects of the persistent current can be mitigated or
even canceled by an external magnetic field perpendicular to the junction
plane. The radial field associated with the persistent current can be
accurately modeled with the classical phenomenological sine-Gordon model for
extended one-dimensional Josephson junctions. Extensive numerical simulations
were carried out to disclose the basic flux-flow mechanism responsible for the
appearance of the magnetically induced steps and to elucidate the role of
geometrical parameters. It was found that the imprint of the field cooling is
enhanced in confocal annular junctions which are the natural generalization of
the well studied circular annular junctions.",2005.06816v1
2020-05-18,The Einstein - de Haas effect at radio frequencies in and near magnetic equilibrium,"The Einstein-de Haas (EdH) effect and its reciprocal the Barnett effect are
fundamental to magnetism and uniquely yield measures of the ratio of magnetic
moment to total angular momentum. These effects, small and generally difficult
to observe, are enjoying a resurgence of interest as contemporary techniques
enable new approaches to their study. The high mechanical resonance frequencies
in nanomechanical systems offer a tremendous advantage for the observation of
EdH torques in particular. At radio frequencies, the EdH effect can become
comparable to or even exceed in magnitude conventional cross-product magnetic
torques. In addition, the RF-EdH torque is expected to be phase-shifted by 90
degrees relative to cross-product torques, provided the magnetic system remains
in quasi-static equilibrium, enabling separation in quadratures when both
sources of torque are operative. Radio frequency EdH measurements are
demonstrated through the full hysteresis range of micrometer scale,
monocrystalline yttrium iron garnet (YIG) disks. Equilibrium behavior is
observed in the vortex state at low bias field. Barkhausen-like features emerge
in the in-plane EdH torque at higher fields in the vortex state, revealing
magnetic disorder too weak to be visible through the in-plane cross-product
torque. Beyond vortex annihilation, peaks arise in the EdH torque versus bias
field, and these together with their phase signatures indicate additional
utility of the Einstein-de Haas effect for the study of RF-driven spin
dynamics.",2005.08406v1
2020-04-14,An experimental design for the control and assembly of magnetic microwheels,"Superparamagnetic colloidal particles can be reversibly assembled into
wheel-like structures called microwheels ($\mu$wheels) which roll on surfaces
due to friction and can be driven at user-controlled speeds and directions
using rotating magnetic fields. Here, we describe the hardware and software to
create and control the magnetic fields that assemble and direct wheel motion
and the optics to visualize them. Motivated by portability, adaptability and
low-cost, an extruded aluminum heat dissipating frame incorporating open optics
and audio speaker coils outfitted with high magnetic permeability cores was
constructed. Open-source software was developed to define the magnitude,
frequency, and orientation of the magnetic field, allowing for real time
joystick control of $\mu$wheels through two-dimensional (2D) and
three-dimensional (3D) fluidic environments. With this combination of hardware
and software, $\mu$wheels translate at speeds up to 50 $\mu$m/s through sample
sizes up to 5 cm x 5 cm x 5 cm using 0.75-2.5 mT magnetic fields with rotation
frequencies of 5-40 Hz. Heat dissipation by aluminum coil clamps maintained
sample temperatures within 3 C of ambient temperature, a range conducive for
biological applications. With this design, $\mu$wheels can be manipulated and
imaged in 2D and 3D networks at length scales of micrometers to centimeters",2006.02524v1
2020-06-08,Transverse Chiral Magnetic Photocurrent Induced by Linearly Polarized Light in Mirror-Symmetric Weyl Semimetals,"A new class of photocurrents is predicted to occur in both type-I and type-II
Weyl semimetals. Unlike the previously studied photocurrents in chiral
materials, the proposed current requires neither circularly polarized light,
nor an absence of symmetry with respect to a plane of reflection. We show that
if a Weyl semimetal has a broken inversion symmetry then linearly polarized
light can induce a photocurrent transverse to the direction of an applied
magnetic field, in spite of the symmetry with respect to a reflection plane and
the time reversal symmetry. The class of materials in which we expect this to
occur is sufficiently broad and includes the transition metal monopnictides
such as TaAs. The effect stems from the dynamics of Weyl chiral quasi-particles
in a magnetic field, restricted by the symmetries described above; because the
resulting current is transverse to the direction of magnetic field, we call it
the transverse chiral magnetic photocurrent. The magnitude of the resulting
photocurrent is predicted to be significant in the THz frequency range, about
$0.75\; \mathrm{\mu A}$ for type-I and $2.5\; \mathrm{\mu A}$ for type-II Weyl
semimetals. This opens the possibility to utilize the predicted transverse
chiral magnetic photocurrent for sensing unpolarized THz radiation.",2006.04857v2
2020-06-26,Elasticity of tangled magnetic fields,"The fundamental difference between incompressible ideal magnetohydrodynamics
and the dynamics of a non-conducting fluid is that magnetic fields exert a
tension force that opposes their bending; magnetic fields behave like elastic
strings threading the fluid. It is natural, therefore, to expect that a
magnetic field tangled at small length scales should resist a large-scale shear
in an elastic way, much as a ball of tangled elastic strings responds
elastically to an impulse. Furthermore, a tangled field should support the
propagation of `magnetoelastic waves', the isotropic analogue of Alfv\'en waves
on a straight magnetic field. Here, we study magnetoelasticity in the idealised
context of an equilibrium tangled field configuration. In contrast to previous
treatments, we explicitly account for intermittency of the Maxwell stress, and
show that this intermittency necessarily decreases the frequency of
magnetoelastic waves in a stable field configuration. We develop a mean-field
formalism to describe magnetoelastic behaviour, retaining leading-order
corrections due to the coupling of large- and small-scale motions, and solve
the initial-value problem for viscous fluids subjected to a large-scale shear,
showing that the development of small-scale motions results in anomalous
viscous damping of large-scale waves. Finally, we test these analytic
predictions using numerical simulations of standing waves on tangled, linear
force-free magnetic-field equilibria.",2006.15141v1
2020-07-03,Inconsistencies Between Local and Global Measures of CME Radial Expansion as Revealed by Spacecraft Conjunctions,"The radial expansion of coronal mass ejections (CMEs) is known to occur from
remote observations; from the variation of their properties with radial
distance; and from local in situ plasma measurements showing a decreasing speed
profile throughout the magnetic ejecta (ME). However, little is known on how
local measurements compare to global measurements of expansion. Here, we
present results from the analysis of 42 CMEs measured in the inner heliosphere
by two spacecraft in radial conjunction. The magnetic field decrease with
distance provides a measure of their global expansion. Near 1 au, the decrease
in their bulk speed provides a measure of their local expansion. We find that
these two measures have little relation with each other. We also investigate
the relation between characteristics of CME expansion and CME properties. We
find that the expansion depends on the initial magnetic field strength inside
the ME, but not significantly on the magnetic field inside the ME measured near
1 au. This is an indirect evidence that CME expansion in the innermost
heliosphere is driven by the high magnetic pressure inside the ME, while by the
time the MEs reach 1 au, they are expanding due to the decrease in the solar
wind dynamic pressure with distance. We also determine the evolution of the ME
tangential and normal magnetic field components with distance, revealing
significant deviations as compared to the expectations from force-free field
configurations as well as some evidence that the front half of MEs expand at a
faster rate than the back half.",2007.01699v1
2020-07-09,Intertwined Magnetic Sub-Lattices in the Double Perovskite Compound LaSrNiReO6,"We report a muon spin rotation ($\mu^{+}$SR) study of the magnetic properties
of the double perovskite compound LaSrNiReO$_{6}$. Using the unique length and
time scales of the $\mu^{+}$SR technique, we successfully clarify the magnetic
ground state of LaSrNiReO$_{6}$, which was previously deemed as a spin glass
state. Instead, our $\mu^{+}$SR results point towards a long-range dynamically
ordered ground state below $T_{\rm C}= 23$ K, for which a static limit is
foreseen at $T=0$. Furthermore, between 23 K$<T\leq$300 K, three different
magnetic phases are identified: a dense ($23$ K$<T\leq75$ K), a dilute ($75$
K$<T\leq250$ K), and a paramagnetic ($T>250$ K) state. Our results reveal how
two separate, yet intertwined magnetic lattices interact within the unique
double perovskite structure and the importance of using complementary
experimental techniques to obtain a complete understanding of the microscopic
magnetic properties of complex materials.",2007.04616v3
2020-08-14,Analysis of quiet-sun turbulence on the basis of SDO/HMI and Goode Solar Telescope data,"We analysed line-of-sight magnetic fields and magnetic power spectra of an
undisturbed photosphere using magnetograms acquired by the Helioseismic and
Magnetic Imager (HMI) on-board the Solar Dynamic Observatory (SDO) and the Near
InfraRed Imaging Spectrapolarimeter (NIRIS) operating at the Goode Solar
Telescope (GST) of the Big Bear Solar Observatory. In the NIRIS data revealed
the presence of thin flux tubes of 200-400~km in diameter and of field strength
of 1000-2000~G. The HMI power spectra determined for a coronal hole, a quiet
sun and a plage areas exhibit the same spectral index of -1 on a broad range of
spatial scales from 10-20~Mm down to 2.4~Mm. This implies that the same
mechanism(s) of magnetic field generation operate everywhere in the undisturbed
photosphere. The most plausible one is the local turbulent dynamo. When
compared to the HMI spectra, the -1.2 slope of the NIRIS spectrum appears to be
more extended into the short spatial range until the cutoff at 0.8-0.9~Mm,
after which it continues with a steeper slope of -2.2. Comparison of the
observed and Kolmogorov-type spectra allowed us to infer that the Kolmogorov
turbulent cascade cannot account for more than 35\% of the total magnetic
energy observed in the scale range of 3.5-0.3~Mm. The energy excess can be
attributed to other mechanisms of field generation such as the local turbulent
dynamo and magnetic super-diffusivity observed in an undisturbed photosphere
that can slow down the rate of the Kolmogorov cascade leading to a shallower
resulting spectrum.",2008.06264v1
2020-08-16,Magnetic Excitations of the Hybrid Multiferroic (ND4)2FeCl5D2O,"We report a comprehensive inelastic neutron scattering study of the hybrid
molecule-based multiferroic compound (ND4)2FeCl5D2O in the zero-field
incommensurate cycloidal phase and the high-field quasi-collinear phase. The
spontaneous electric polarization changes its direction concurrently with the
field-induced magnetic transition, from mostly aligned with the
crystallographic a-axis to the c-axis. To account for such change of
polarization direction, the underlying multiferroic mechanism was proposed to
switch from the spin-current model induced via the inverse
Dzyalloshinskii-Moriya interaction to the p-d hybridization model. We perform a
detailed analysis of the inelastic neutron data of (ND4)2FeCl5D2O using linear
spin-wave theory to quantify magnetic interaction strengths and investigate
possible impact of different multiferroic mechanisms on the magnetic couplings.
Our result reveals that the spin dynamics of both multiferroic phases can be
well-described by a Heisenberg Hamiltonian with an easy-plane anisotropy. We do
not find notable differences between the optimal model parameters of the two
phases. The hierarchy of exchange couplings and the balance among frustrated
interactions remain the same between two phases, suggesting that magnetic
interactions in (ND4)2FeCl5D2O are much more robust than the electric
polarization in response to delicate reorganizations of the electronic degrees
of freedom in an applied magnetic field.",2008.06827v2
2020-08-26,Nature of the magnetic stripes in fully oxygenated La$_{2}$CuO$_{4+y}$,"We present triple-axis neutron scattering studies of static and dynamic
magnetic stripes in an optimally oxygen-doped cuprate superconductor,
La$_{2}$CuO$_{4+y}$, which exhibits a clean superconducting transition at
$T_{\rm c}=42$ K. Polarization analysis reveals that the magnetic stripe
structure is equally represented along both of the tetragonal crystal axes and
that the fluctuating stripes display significant weight for in-plane as well as
out-of-plane spin components. Both static magnetic order as well as low-energy
fluctuations are fully developed in zero applied magnetic field and the
low-energy spin fluctuations at $\hbar \omega = 0.3-10$ meV intensify upon
cooling. We interpret this as an indication that superconductivity and
low-energy spin fluctuations co-exist microscopically in spatial regions which
are separated from domains with static magnetic order.",2008.11527v1
2020-09-06,Adiabatic and Nonadiabatic Spin-transfer Torques in Antiferromagnets,"Electron transport in magnetic orders and the magnetic orders dynamics have a
mutual dependence, which provides the key mechanisms in spin-dependent
phenomena. Recently, antiferromagnetic orders are focused on as the magnetic
order, where current-induced spin-transfer torques, a typical effect of
electron transport on the magnetic order, have been debatable mainly because of
the lack of an analytic derivation based on quantum field theory. Here, we
construct the microscopic theory of spin-transfer torques on the slowly-varying
staggered magnetization in antiferromagnets with weak canting. In our theory,
the electron is captured by bonding/antibonding states, each of which is the
eigenstate of the system, doubly degenerates, and spatially spreads to
sublattices because of electron hopping. The spin of the eigenstates depends on
the momentum in general, and a nontrivial spin-momentum locking arises for the
case with no site inversion symmetry, without considering any spin-orbit
couplings. The spin current of the eigenstates includes an anomalous component
proportional to a kind of gauge field defined by derivatives in momentum space
and induces the adiabatic spin-transfer torques on the magnetization.
Unexpectedly, we find that one of the nonadiabatic torques has the same form as
the adiabatic spin-transfer torque, while the obtained forms for the adiabatic
and nonadiabatic spin-transfer torques agree with the phenomenological
derivation based on the symmetry consideration. This finding suggests that the
conventional explanation for the spin-transfer torques in antiferromagnets
should be changed. Our microscopic theory provides a fundamental understanding
of spin-related physics in antiferromagnets.",2009.02686v1
2020-09-21,Influence of ambipolar and Hall effects on vorticity in 3D simulations of magneto-convection,"This paper presents the results of the analysis of 3D simulations of solar
magneto-convection that include the joint action of the ambipolar diffusion and
the Hall effect. Three simulation-runs are compared: one including both
ambipolar diffusion and Hall effect; one including only ambipolar diffusion;
and one without any of these two effects. The magnetic field is amplified from
initial field to saturation level by the action of turbulent local dynamo. In
each of these cases, we study 2 hours of simulated solar time after the local
dynamo reaches the saturation regime. We analyze the power spectra of
vorticity, of magnetic field fluctuations and of the different components of
the magnetic Poynting flux responsible for the transport of vertical or
horizontal perturbations. Our preliminary results show that the ambipolar
diffusion produces a strong reduction of vorticity in the upper chromospheric
layers and that it dissipates the vortical perturbations converting them into
thermal energy. The Hall effect acts in the opposite way, strongly enhancing
the vorticity. When the Hall effect is included, the magnetic field in the
simulations becomes, on average, more vertical and long-lived flux tube-like
structures are produced. We trace a single magnetic structure to study its
evolution pattern and the magnetic field intensification, and their possible
relation to the Hall effect.",2009.09753v1
2020-09-28,Nanoscale transient magnetization gratings excited and probed by femtosecond extreme ultraviolet pulses,"We utilize coherent femtosecond extreme ultraviolet (EUV) pulses derived from
a free electron laser (FEL) to generate transient periodic magnetization
patterns with periods as short as 44 nm. Combining spatially periodic
excitation with resonant probing at the dichroic M-edge of cobalt allows us to
create and probe transient gratings of electronic and magnetic excitations in a
CoGd alloy. In a demagnetized sample, we observe an electronic excitation with
50 fs rise time close to the FEL pulse duration and ~0.5 ps decay time within
the range for the electron-phonon relaxation in metals. When the experiment is
performed on a sample magnetized to saturation in an external field, we observe
a magnetization grating, which appears on a sub-picosecond time scale as the
sample is demagnetized at the maxima of the EUV intensity and then decays on
the time scale of tens of picoseconds via thermal diffusion. The described
approach opens prospects for studying dynamics of ultrafast magnetic phenomena
on nanometer length scales.",2009.13330v1
2020-09-29,"Ferrofluidic aqueous two-phase system with ultralow interfacial tension, instabilities and pattern formation","Ferrofluids are strongly magnetic fluids consisting of magnetic nanoparticles
dispersed in a carrier fluid. Besides their technological applications, they
have a tendency to form beautiful and intriguing patterns when subjected to
external static and dynamic magnetic fields. Most of the patterns occur in
systems consisting of two fluids: one ferrofluidic and one non-magnetic (oil,
air, etc.), wherein the fluid-fluid interface deforms as a response to magnetic
fields. Usually, the fluids are completely immiscible and so the interfacial
energy in this systems is very large. Here we show that it is possible to
design a fully aqueous ferrofluid system by using phase separation of
incompatible polymers. This continuous aqueous system allows an ultralow
interfacial tension (down to 1 $\mu$N/m) and nearly vanishing pinning at three
phase contact lines. We demonstrate the normal-field instability with the
system and focus on the miniaturization of the pattern length from the typical
$\sim$10 mm size down to $\sim$200 $\mu$m. The normal-field instability is
characterized in glass capillaries of thickness comparable to the pattern
length. This system paves way towards interesting physics such as the
interaction between magnetic instabilities and thermal capillary waves and
offers a way to evaluate extremely small interfacial tensions.",2009.13945v1
2020-10-01,On the rate of crustal failures in young magnetars,"The activity of magnetars is powered by their intense and dynamic magnetic
fields and has been proposed as the trigger to extragalactic Fast Radio Bursts.
Here we estimate the frequency of crustal failures in young magnetars, by
computing the magnetic stresses in detailed magneto-thermal simulations
including Hall drift and Ohmic dissipation. The initial internal topology at
birth is poorly known but is likely to be much more complex than a dipole.
Thus, we explore a wide range of initial configurations, finding that the
expected rate of crustal failures varies by orders of magnitude depending on
the initial magnetic configuration. Our results show that this rate scales with
the crustal magnetic energy, rather than with the often used surface value of
the dipolar component related to the spin-down torque. The estimated frequency
of crustal failures for a given dipolar component can vary by orders of
magnitude for different initial conditions, depending on how much magnetic
energy is distributed in the crustal non-dipolar components, likely dominant in
newborn magnetars. The quantitative reliability of the expected event rate
could be improved by a better treatment of the magnetic evolution in the core
and the elastic/plastic crustal response, here not included. Regardless of
that, our results are useful inputs in modelling the outburst rate of young
Galactic magnetars, and their relation with the Fast Radio Bursts in our and
other galaxies.",2010.00617v1
2020-10-07,Orbital isotropy of magnetic fluctuations in correlated electron materials induced by Hund's exchange coupling,"Characterizing non-local magnetic fluctuations in materials with strong
electronic Coulomb interactions remains one of the major outstanding challenges
of modern condensed matter theory. In this work we address the spatial symmetry
and orbital structure of magnetic fluctuations in perovskite materials. To this
aim, we develop a consistent multi-orbital diagrammatic extension of dynamical
mean field theory, which we apply to an anisotropic three-orbital model of
cubic $t_{2g}$ symmetry. We find that the form of spatial spin fluctuations is
governed by the local Hund's coupling. For small values of the coupling,
magnetic fluctuations are anisotropic in orbital space, which reflects the
symmetry of the considered $t_{2g}$ model. Large Hund's coupling enhances
collective spin excitations, which mixes orbital and spatial degrees of
freedom, and magnetic fluctuations become orbitally isotropic. Remarkably, this
effect can be seen only in two-particle quantities; single-particle observables
remain anisotropic for any value of the Hund's coupling. Importantly, we find
that the orbital isotropy can be induced both, at half-filling and for the case
of $4$ electrons per lattice site, where the magnetic instability is associated
with different, antiferromagnetic and ferromagnetic modes, respectively.",2010.03433v4
2020-10-14,Exact hybrid-kinetic equilibria for magnetized plasmas with shearing flows,"Context. Magnetized plasmas characterized by shearing flows are present in
many natural contexts, such as the Earth's magnetopause and the solar wind. The
collisionless nature of involved plasmas requires a kinetic description. When
the width of the shear layer is of the order of ion scales, the Hybrid
Vlasov-Maxwell approach can be adopted. Aims. The aim of the paper is to derive
explicit forms for stationary configurations of magnetized plasmas with planar
shearing flows,within the Hybrid Vlasov-Maxwell description. Two configurations
are considered: the first with a uniform magnetic field obliquely directed with
respect to the bulk velocity; and the second with a uniform-magnitude
variable-direction magnetic field. Methods. Stationary ion distribution
functions are obtained by combining single-particle constant of motions, which
are derived studying particle dynamics. Preliminary information about the form
of the distribution functions are analytically derived considering a local
approximation for the background electromagnetic field. Then, a numerical
method is set up to obtain a solution for general profiles. Results. The
explicit distribution functions that are found allow to obtain profiles of
density, bulk velocity, temperature and heat flux. Anisotropy and agyrotropy in
the distribution function are also evaluated. Stationarity of the solution
during numerical simulations is checked in the uniform oblique magnetic field
case. Conclusions. The considered configurations can be used as models for the
Earth's magnetopause in simulations of the Kelvin-Helmholtz instability.",2010.06929v1
2020-10-20,On the deceleration of Fanaroff-Riley Class I jets: mass loading of magnetized jets by stellar winds,"In this paper we present steady-state RMHD simulations that include a
mass-load term to study the process of jet deceleration. The mass-load mimics
the injection of a proton-electron plasma from stellar winds within the host
galaxy into initially pair plasma jets, with mean stellar mass-losses ranging
from $10^{-14}$ to $10^{-9}\,{M_\odot\,yr^{-1}}$. The spatial jet evolution
covers $\sim 500\,{\rm pc}$ from jet injection in the grid at 10~pc from the
jet nozzle. Our simulations use a relativistic gas equation of state and a
pressure profile for the ambient medium. We compare these simulations with
previous dynamical simulations of relativistic, non-magnetised jets. Our
results show that toroidal magnetic fields can prevent fast jet expansion and
the subsequent embedding of further stars via magnetic tension. In this sense,
magnetic fields avoid a runaway deceleration process. Furthermore, when the
mass-load is large enough to increase the jet density and produce fast,
differential jet expansion, the conversion of magnetic energy flux into kinetic
energy flux (i.e., magnetic acceleration), helps to delay the deceleration
process with respect to non-magnetised jets. We conclude that the typical
stellar population in elliptical galaxies cannot explain jet deceleration in
classical FRI radio galaxies. However, we observe a significant change in the
jet composition, thermodynamical parameters and energy dissipation along its
evolution, even for moderate values of the mass-load.",2010.10234v1
2020-11-02,Subcritical dynamos in rapidly-rotating planar convection,"We study dynamo action using numerical simulations of planar Boussinesq
convection at rapid rotation (low Ekman numbers, Ek), focusing on subcritical
dynamo action in which the dynamo is sustained for Rayleigh numbers, Ra, below
the critical Rayleigh number for the onset of nonmagnetic convection, Ra$_c$.
These solutions are found by first investigating the supercritical regime, in
which the dynamo is able to generate a large-scale magnetic field that
significantly influences the convective motions, with an associated Elsasser
number of order Ek$^{1/3}$. Subcritical solutions are then found by tracking
this solution branch into the subcritical regime, taking a supercritical
solution and then gradually lowering the corresponding Rayleigh number. We show
that decreasing the Ekman number leads to an extension of the subcritical range
of Ra/Ra$_c$, down to an optimal value of Ek$=10^{-5}$. For magnetic Prandtl
numbers of order unity, subcriticality is then hampered by the emergence of a
large-scale mode at lower Ekman numbers when the dynamo driven by the smaller
scale convection generates relatively stronger large-scale magnetic field. The
inability of the large-scale mode to sustain dynamo action leads to an
intermittent behaviour that appears to inhibit subcriticality. The subcritical
solutions are also sensitive to the value of the magnetic Reynolds number (or
equivalently, the magnetic Prandtl number, Pm), as values of the magnetic
Reynolds number greater than 70 are required to produce dynamo action, but
large values lead to fluctuations that are able to push the system too far from
the subcritical branch and towards the trivial conducting state.",2011.00852v2
2020-11-24,The rotation rate of solar active and ephemeral regions -- I. Dependence on morphology and peak magnetic flux,"Using magnetic field maps acquired by the Helioseismic and Magnetic Imager on
board the Solar Dynamics Observatory we measured rotation rates of 864 active
and 322 ephemeral regions observed between 2010 and 2016. We found smaller
magnetic tracers to show a tendency to rotate faster as compared to larger
ones. Thus, ephemeral regions exhibit on average the fastest rotation rate. We
further divided active regions into three classes. Class A comprised magnetic
bipoles obeying Hale's polarity law, Joy's law, and exhibiting more coherent
leading polarity in comparison with the following one. The second class B
included active regions violating at least one of the aforementioned empirical
laws. The third class U comprised unipolar active regions. We found no
significant difference between the rotation rates of active regions of classes
A and B. In contrast, unipolar active regions exhibited on average lower
rotation rate and narrower distribution of the rotation rate differences.
Assuming the rotation rate to indicate the anchoring depth of the magnetic
structure within the convection zone, we supposed that active regions of
classes A and B might be anchored throughout the entire convective envelope
while unipolar active regions a rooted within a thin layer located either near
the base of the convection zone or at a shallow near-surface depth.",2011.12060v1
2021-01-04,Influence of the martensitic transformation kinetics on the magnetocaloric effect in Ni-Mn-In,"The inverse magnetocaloric effect (MCE) in Ni-Mn-based Heusler compounds
occurs during the magnetostructural transition between low-temperature,
low-magnetization martensite and high-temperature, high-magnetization
austenite. In this study, we analyze the metamagnetic transformation of a
$Ni_{49.8}Mn_{35}In_{15.2}$ compound by simultaneous adiabatic temperature
change and strain measurements in pulsed magnetic fields up to 10 T. We observe
an adiabatic temperature change of -10 K and a strain of -0.22 % when the
reverse martensitic transition is fully induced at a starting temperature of
285 K. By a variation of the magnetic field-sweep rates between 316 Ts$^{-1}$,
865 Ts$^{-1}$ and 1850 Ts$^{-1}$, the transitional dynamics of the reverse
martensitic transformation have been investigated. Our experiments reveal an
apparent delay upon the end of the reverse martensitic transformation at field
rates exceeding 865 Ts$^{-1}$ which is related to the annihilation of retained
martensite. As a consequence, the field hysteresis increases and higher fields
are required to saturate the transition. In contrast, no time-dependent effects
on the onset of the reverse martensitic transformation were observed in the
studied field-sweep range. Our results demonstrate that kinetic effects in
Heusler compounds strongly affect the magnetic cooling cycle, especially when
utilising a multicaloric ""exploiting-hysteresis cycle"" where high magnetic
field-sweep rates are employed.",2101.00840v1
2021-01-14,Laser-induced THz magnetism of antiferromagnetic CoF$_2$,"Excitation, detection and control of coherent THz magnetic excitation in
antiferromagnets are challenging problems that can be addressed using ever
shorter laser pulses. We study experimentally excitation of magnetic dynamics
at THz frequencies in an antiferromagnetic insulator CoF$_2$ by sub-10 fs laser
pulses. Time-resolved pump-probe polarimetric measurements at different
temperatures and probe polarizations reveal laser-induced transient circular
birefringence oscillating at the frequency of 7.45 THz and present below the
N\'eel temperature. The THz oscillations of circular birefringence are ascribed
to oscillations of the magnetic moments of Co$^{2+}$ ions induced by the
laser-driven coherent E$_g$ phonon mode via the THz analogue of the transverse
piezomagnetic effect. It is also shown that the same pulse launches coherent
oscillations of the magnetic linear birefringence at the frequency of 3.4 THz
corresponding to the two-magnon mode. Analysis of the probe polarization
dependence of the transient magnetic linear birefringence at the frequency of
the two-magnon mode enables identifying its symmetry.",2101.05622v5
2021-02-02,Distinct magnetic ground states of $R_2$ZnIrO$_6$ ($R$ = La and Nd) determined by neutron powder diffraction,"Double perovskite iridates $A_2$ZnIrO$_6$ ($A$ = alkaline or lanthanide) show
complex magnetic behaviors ranging from weak ferromagnetism to successive
antiferromagnetic transitions. Here we report the static ($dc$) and dynamic
($ac$) magnetic susceptibility, and neutron powder diffraction measurements for
$A$ = La and Nd compounds to elucidate the magnetic ground state. Below 10~K,
the $A$ = La compound is best described as canted iridium moments in an
antiferromagnet arrangement with a propagation vector \textbf{k} = 0 and a net
ferromagnetic component along the $c$-axis. On the other hand, Nd$_2$ZnIrO$_6$
is described well as an antiferromagnet with a propagation vector \textbf{k} =
(1/2~1/2~0) below $T_\mathrm{N} \sim$ 17 K. Scattering from both the Nd and Ir
magnetic sublattices were required to describe the data and both were found to
lie almost completely within the $ab$-plane. $Dc$ susceptibility revealed a
bifurcation between the zero-field-cooled and field-cooled curves below
$\sim$13 K in Nd$_2$ZnIrO$_6$. A glassy state was ruled out by $ac$
susceptibility but detailed magnetic isotherms revealed the opening of the loop
below 13~K. These results suggest a delicate balance exists between the
Dzyaloshinskii-Moriya, crystal field schemes, and $d$-$f$ interaction in this
series of compounds.",2102.01276v1
2021-02-04,Evaluating the reliability of a simple method to map the magnetic field azimuth in the solar chromosphere,"The Zeeman effect is of limited utility for probing the magnetism of the
quiet solar chromosphere. The Hanle effect in some spectral lines is sensitive
to such magnetism, but the interpretation of the scattering polarization
signals requires taking into account that the chromospheric plasma is highly
inhomogeneous and dynamic (i.e., that the magnetic field is not the only cause
of symmetry breaking). Here we investigate the reliability of a well-known
formula for mapping the azimuth of chromospheric magnetic fields directly from
the scattering polarization observed in the \ion{Ca}{2}~8542~\AA\, line, which
is typically in the saturation regime of the Hanle effect. To this end, we use
the Stokes profiles of the \ion{Ca}{2}~8542~\AA\, line computed with the PORTA
radiative transfer code in a three-dimensional (3D) model of the solar
chromosphere, degrading them to mimic spectropolarimetric observations for a
range of telescope apertures and noise levels. The simulated observations are
used to obtain the magnetic field azimuth at each point of the field of view,
which we compare with the actual values within the 3D model. We show that,
apart from intrinsic ambiguities, the method provides solid results. Their
accuracy depends more on the noise level than on the telescope diameter.
Large-aperture solar telescopes, like DKIST and EST, are needed to achieve the
required noise-to-signal ratios using reasonable exposure times.",2102.02880v1
2021-02-09,Chiral Magnetic Effect and Three-point Function from AdS/CFT Correspondence,"The chiral magnetic effect with a fluctuating chiral imbalance is more
realistic in the evolution of quark-gluon plasma, which reflects the random
gluonic topological transition. Incorporating this dynamics, we calculate the
chiral magnetic current in response to space-time dependent axial gauge
potential and magnetic field in AdS/CFT correspondence. In contrast to
conventional treatment of constant axial chemical potential, the response
function here is the AVV three-point function of the $\mathcal{N}=4$ super
Yang-Mills at strong coupling. Through an iterative solution of the nonlinear
equations of motion in Schwarzschild-AdS$_5$ background, we are able to express
the AVV function in terms of two Heun functions and prove its UV/IR finiteness,
as expected for $\mathcal{N}=4$ super Yang-Mills theory. We found that the
dependence of the chiral magnetic current on a non-constant chiral imbalance is
non-local, different from hydrodynamic approximation, and demonstrates the
subtlety of the infrared limit discovered in field theoretic approach. We
expect our results enrich the understanding of the phenomenology of the chiral
magnetic effect in the context of relativistic heavy ion collisions.",2102.04851v2
2021-02-28,Topological excitations in quasi two-dimensional quantum magnets with weak interlayer interactions,"The study of topological magnetic excitations has attracted widespread
attention in the past few years. In this thesis, I have studied some examples
of novel topological magnonic phases/phenomena in low-dimensional quantum
magnets. The first chapter motivates the research based on the research gap in
this field of study. The second chapter is written to make the thesis
self-sufficient and the concepts are explained through examples. In the second
chapter, the following formalisms and physical observables are described:
Holstein-Primakoff, bond operator, Schwinger boson, Bogoliubov-Valatin, Group
theory, Berry-phase, Berry-curvature, Chern number, thermal Hall conductance,
Nernst conductivity, dynamical spin structure factor, edge-current. The main
results of the thesis are shown in the third, fourth, and fifth chapters. In
the third chapter, I have shown that anti-chiral edge states (co-propagating
edge states) arise in the ferromagnetic Heisenberg model on the honeycomb
lattice with Dzyaloshinskii-Moriya (DM) interactions. My results suggest that
such anti-chiral edge states may be induced in certain realistic models of
quantum magnets. In the fourth chapter, I have found the emergence of many
magnon band-topological phases in the flux state of the Shastry-Sutherland
model. I have derived a simple analytical form of the temperature dependence of
derivative of thermal Hall conductivity near the band topological transition
point which I propose to be experimentally useful. In the fifth chapter, I have
investigated the emergence of Weyl triplons due to inter-layer DM-interaction
in a microscopic model of SrCu2(BO3)2, a widely studied frustrated quantum
magnet. I have shown that the thermal magnon Hall conductivity has a
quasi-linear dependence as a function of the magnetic field in a Weyl-triplon
region.",2103.00401v1
2021-03-13,What Causes Faint Solar Coronal Jets from Emerging Flux Regions in Coronal Holes?,"Using EUV images and line-of-sight magnetograms from Solar Dynamics
Observatory, we examine eight emerging bipolar magnetic regions (BMRs) in
central-disk coronal holes for whether the emerging magnetic arch made any
noticeable coronal jets directly, via reconnection with ambient open field as
modeled by Yokoyama and Shibata (1995). During emergence, each BMR produced no
obvious EUV coronal jet of normal brightness, but each produced one or more
faint EUV coronal jets that are discernible in AIA 193 {\AA} images. The spires
of these jets are much fainter and usually narrower than for typical EUV jets
that have been observed to be produced by minifilament eruptions in quiet
regions and coronal holes. For each of 26 faint jets from the eight emerging
BMRs, we examine whether the faint spire was evidently made a la Yokoyama and
Shibata (1995). We find: (1) 16 of these faint spires evidently originate from
sites of converging opposite-polarity magnetic flux and show base brightenings
like those in minifilament-eruption-driven coronal jets, (2) the 10 other faint
spires maybe were made by a burst of the external-magnetic-arcade-building
reconnection of the emerging magnetic arch with the ambient open field,
reconnection directly driven by the arch's emergence, but (3) none were
unambiguously made by such emergence-driven reconnection. Thus, for these eight
emerging BMRs the observations indicate that emergence-driven external
reconnection of the emerging magnetic arch with ambient open field at most
produces a jet spire that is much fainter than in previously-reported, much
more obvious coronal jets driven by minifilament eruptions.",2103.07813v1
2021-05-04,Energy-gap driven low-temperature magnetic and transport properties in Cr$_{1/3}M$S$_2$ ($M$ = Nb or Ta),"The helimagnets Cr$_{1/3}M$S$_2$ ($M$ = Nb or Ta) have attracted renewed
attention due to the discovery of a chiral soliton lattice (CSL) stabilized in
an applied magnetic field, but reports of unusual low-temperature transport and
magnetic properties in this system lack a unifying explanation. Here we present
electronic structure calculations that demonstrate the materials are
half-metals. There is also a gap-like feature (width in range 40-100 meV) in
the density of states of one spin channel. This electronic structure explains
the low-temperature electronic and magnetic properties of Cr$_{1/3}M$S$_2$ ($M$
= Nb or Ta), with the gap-like feature particularly important for explaining
the magnetic behavior. Our magnetometry measurements confirm the existence of
this gap. Dynamic spin fluctuations driven by excitations across this gap are
seen over a wide range of frequencies (0.1 Hz to MHz) with AC susceptibility
and muon-spin relaxation ($\mu^+$SR) measurements. We show further how effects
due to the CSL in Cr$_{1/3}$NbS$_2$, as detected with $\mu^+$SR, dominate over
the gap-driven magnetism when the CSL is stabilized as the majority phase.",2105.01393v3
2021-05-14,Magnetic field and thermal radiation induced entropy generation in a multiphase non-isothermal plane Poiseuille flow,"The effect of radiative heat transfer on the entropy generation in a
two-phase non-isothermal fluid flow between two infinite horizontal parallel
plates under the influence of a constant pressure gradient and transverse
non-invasive magnetic field have been explored. Both the fluids are considered
to be viscous, incompressible, immiscible, Newtonian, and electrically
conducting. The governing equations in Cartesian coordinate are solved
analytically with the help of appropriate boundary conditions to obtain the
velocity and temperature profile inside the channel. Application of transverse
magnetic field is found to reduce the throughput and the temperature
distribution of the fluids in a pressure-driven flow. The temperature and fluid
flow inside the channel can also be non-invasively altered by tuning the
magnetic field intensity, the temperature difference between the channel walls
and the fluids, and several intrinsic fluid properties. The entropy generation
due to the heat transfer, magnetic field, and fluid flow irreversibilities can
be controlled by altering the Hartmann number, radiation parameter, Brinkmann
number, filling ratio, and the ratios of fluid viscosities, thermal and
electrical conductivities. The surfaces of the channel wall are found to act as
a strong source of entropy generation and heat transfer irreversibility. The
rate of heat transfer at the channel walls can also be tweaked by the magnetic
field intensity, temperature differences, and fluid properties. The proposed
strategies in the present study can be of significance in the design and
development of gen-next microscale reactors, micro heat exchangers, and energy
harvesting devices.",2105.06682v2
2021-05-20,Spin dynamics of the quantum dipolar magnet Yb$_3$Ga$_5$O$_{12}$ in an external field,"We investigate ytterbium gallium garnet Yb$_{3}$Ga$_{5}$O$_{12}$ in the
paramagnetic phase above the supposed magnetic transition at $T_{\lambda}
\approx 54$ mK. Our study combines susceptibility and specific heat
measurements with neutron scattering experiments and theoretical calculations.
Below 500 mK, the elastic neutron response is strongly peaked in the momentum
space. Along with that the inelastic spectrum develops flat excitation modes.
In magnetic field, the lowest energy branch follows a Zeeman shift in
accordance with the field-dependent specific heat data. An intermediate state
with spin canting away from the field direction is evidenced in small magnetic
fields. In the field of 2 T, the total magnetization almost saturates and the
measured excitation spectrum is well reproduced by the spin-wave calculations
taking into account solely the dipole-dipole interactions. The small positive
Curie-Weiss temperature derived from the susceptibility measurements is also
accounted for by the dipole spin model. Altogether, our results suggest that
Yb$_{3}$Ga$_{5}$O$_{12}$ is a quantum dipolar magnet.",2105.09817v1
2021-06-05,Optimization of Magnetic Flux Ropes Modeled with the RBSL method,"The so-called regularized Biot-Savart laws (RBSLs) provide an efficient and
flexible method for modeling pre-eruptive magnetic configurations of coronal
mass ejections (CMEs) whose characteristics are constrained by observational
images and magnetic-field data. This method allows one to calculate the field
of magnetic flux ropes (MFRs) with small circular cross-sections and an
arbitrary axis shape. The field of the whole configuration is constructed as a
superposition of (1) such a flux-rope field and (2) an ambient potential field
derived, for example, from an observed magnetogram. The RBSL kernels are
determined from the requirement that the MFR field for a straight cylinder must
be exactly force-free. For a curved MFR, however, the magnetic forces are
generally unbalanced over the whole path of the MFR. To minimize these forces,
we apply a modified Gauss-Newton method to find optimal MFR parameters. This is
done by iteratively adjusting the MFR axis path and axial current. We then try
to relax the resulting optimized configuration in a subsequent line-tied
zero-beta magnetohydrodynamic simulation toward a force-free equilibrium. By
considering two models of the sigmoidal pre-eruption configuration for the 2009
February 13 CME, we demonstrate how this approach works and what it is capable
of. We show, in particular, that the building blocks of the core magnetic
structure described by these models match to morphological features typically
observed in such type of configurations. Our method will be useful for both the
modeling of particular eruptive events and theoretical studies of idealized
pre-eruptive MFR configurations.",2106.02789v1
2021-06-28,Laminar flow characterization using low-field magnetic resonance techniques,"Laminar flow velocity profiles depend heavily on fluid rheology. Developing
methods of laminar flow characterization, based on low-field magnetic resonance
(MR), contributes to the widespread industrial application of the MR technique
in rheology. In this paper, we designed a low-cost, palm-sized permanent magnet
with a 1H resonance frequency of 20.48 MHz to measure laminar flow. The magnet
consists of two disk magnets, which were each tilted at an angle of 1{\deg}
from a starting separation of 1.4 cm to generate a constant gradient, 65
gauss/cm, in the direction of flow. Subsequently, a series of process methods,
for MR measurements, were proposed to characterize Newtonian and non-Newtonian
fluid flows in a pipe, including phase-based method, magnitude-based method,
and velocity spectrum method. The accuracies of the proposed methods were
validated by simulations, and experiments of Poiseuille flow and shear-thinning
flow on the designed magnet. The new velocity profile methods proposed are
advantageous because the MR instrumentation and measurement methods are simple
and portable. The sophistication is found in the analysis although the physical
principles are straight forward.",2106.14967v2
2021-08-03,Comparative study of magnetic properties of Mn$^{3+}$ magnetic clusters in GaN using classical and quantum mechanical approach,"Currently, simulations of many-body quantum systems are known to be
computationally too demanding to be solved on classical computers. The main
problem is that the computation time and memory necessary for performing the
calculations usually grow exponentially with the number of particles $N$. An
efficient approach to simulate many-body quantum systems is the use of
classical approximation. However, it is known that at least at low
temperatures, the allowed spin fluctuations in this approach are overestimated
what results in enhanced thermal fluctuations. It is therefore timely and
important to assess the validity of the classical approximation. To this end,
in this work, we compare the results of numerical calculations of small
Mn$^{3+}$ magnetic clusters in GaN, where the Mn spins are treated classically
with those where they are treated quantum-mechanically (crystal field model).
In the first case, we solve the Landau-Lifshitz-Gilbert (LLG) equation that
describes the precessional dynamics of spins represented by classical vectors.
On the other hand, in the crystal field model, the state of Mn$^{3+}$ ion
($d^4$ configuration with $S=2$, $L=2$) is characterized by the set of orbital
and spin quantum numbers $|m_s,m_L>$. Particular attention is paid to use
numerical parameters that ensure the same single ion magnetic anisotropy in
both classical and quantum approximation. Finally, a detailed comparative study
of magnetization $\mathbf{M}(\mathbf{H}, T)$ as a function of the magnetic
field $\mathbf{H}$, temperature $T$, number of ions in a given cluster $N$ and
the strength of super-exchange interaction $J$, obtained from both approaches
will be presented.",2108.01474v1
2021-09-28,Evolution of primordial magnetic fields during large-scale structure formation,"Primordial magnetic fields could explain the large-scale magnetic fields
present in the Universe. Inflation and phase transitions in the early Universe
could give rise to such fields with unique characteristics. We investigate the
magneto-hydrodynamic evolution of these magnetogenesis scenarios with
cosmological simulations. We evolve inflation-generated magnetic fields either
as (i) uniform (homogeneous) or as (ii) scale-invariant stochastic fields, and
phase transition-generated ones either as (iii) helical or as (iv) non-helical
fields from the radiation-dominated epoch. We find that the final distribution
of magnetic fields in the simulated cosmic web shows a dependence on the
initial strength and the topology of the seed field. Thus, the observed field
configuration retains information on the initial conditions at the moment of
the field generation. If detected, primordial magnetic field observations would
open a new window for indirect probes of the early universe. The differences
between the competing models are revealed on the scale of galaxy clusters,
bridges, as well as filaments and voids. The distinctive spectral evolution of
different seed fields produces imprints on the correlation length today. We
discuss how the differences between rotation measures from highly ionized
regions can potentially be probed with forthcoming surveys.",2109.13520v2
2021-09-29,Convectively coupled equatorial trapped waves in stars and planets,"In this paper, we have studied the convectively coupled equatorially trapped
waves in rotating stars, with and without magnetic field. The equatorial
trapped HD and MHD Poincar\'e, Rossby, mixed Rossby-Poincar\'e, and Kelvin
waves were identified. The effects of stratification and non-traditional
Coriolis force terms have been investigated. When the flow is strongly
stratified, the wave frequencies of the convectively coupled model are almost
the same as those of shallow water model. However, when the flow is weakly
stratified, the wave frequencies are constrained by the buoyancy frequency. The
non-traditional Coriolis terms affect the widths and phases of the equatorial
waves. The width increases with the increasing non-traditional Coriolis
parameter. Phase shift occurs when the non-traditional Coriolis parameter is
included. Magnetic effect is significant when the magnetic field is strong. We
have applied the model in the solar atmosphere and solar tachocline to explain
the Rieger type periodicities. For the solar atmosphere, when magnetic effect
is taken into account, we find that the magnetic field should be smaller than
$5G$ in the solar photosphere. Otherwise, the Rieger type periodicities can be
only attributed to long Rossby waves. For the solar tachocline, we find that
magnetic field of the solar tachocline should be smaller than $50kG$ to observe
the 160 days Rieger period. In addition, we find that the effect of the
non-traditional Coriolis terms is not obvious in the solar photosphere, but its
effect on the tachocline is significant.",2109.14283v1
2021-10-12,Vortex dynamics and second magnetization peak in the iron-pnictide superconductor Ca$_{0.82}$La$_{0.18}$Fe$_{0.96}$Ni$_{0.04}$As$_2$,"We report the studies of detailed magnetic relaxation and isothermal
magnetization measurements in the vortex state of the 112-type iron-pnictide
Ca$_{0.82}$La$_{0.18}$Fe$_{0.96}$Ni$_{0.04}$As$_2$ superconductor with $T_c$
$\sim$ 22 K. In the isothermal $M(H)$, a well defined second magnetization peak
(SMP) feature is observed in the entire temperature range below $T_c$ for
measurements with $H$ $\parallel$ $c$-axis. However, for $H$ $\parallel$
$ab$-planes, the SMP feature is suppressed at low temperatures, which might be
due to 2D Josephson vortices forming at low temperatures and high magnetic
fields in such an anisotropic system. A rigorous analysis considering the
magnetic relaxation data for $H$ $\parallel$ $c$-axis suggests an elastic to
plastic pinning crossover across $H_p$, which also seems accompanied with a
possible phase transition in vortex lattice near $H_p$. Moreover, point
disorder and surface defects are likely to be the dominant sources of pinning,
which contribute to the $\delta l$-type of pinning in the sample. A high $J_c$,
in access of 10$^5$ A/cm$^2$ observed could potentially make this material
technologically important.",2110.05677v1
2021-11-17,Dynamics of relativistic electrons in non-uniform magnetic fields and its applications in quantum computing and astrophysics,"We explore the two-dimensional motion of relativistic electrons when they are
trapped in magnetic fields having spatial power-law variation. Its impacts
include lifting of degeneracy that emerged in the case of the constant magnetic
field, special alignment of Landau levels of spin-up and spin-down electrons
depending on whether the magnetic field is increasing or decreasing from the
centre, splitting of Landau levels of electrons with zero angular momentum from
that of positive one and the change in the equation of state of matter. Landau
quantization (LQ) in variable magnetic fields has interdisciplinary
applications in a variety of disciplines ranging from condensed matter to
quantum information. As examples, we discuss the increase in quantum speed of
the electron in presence of spatially increasing magnetic field; and the
attainment of super Chandrasekhar mass of white dwarfs by taking into account
LQ and Lorentz force simultaneously.",2111.09334v1
2021-12-01,Low power continuous-wave all-optical magnetic switching in ferromagnetic nanoarrays,"All-optical magnetic switching promises ultrafast, high-resolution
magnetisation control with the technological attraction of requiring no
magnetic field. Existing all-optical switching schemes are driven by ultrafast
transient effects, typically requiring power-hungry femtosecond-pulsed lasers
and complex magnetic materials. Here, we demonstrate deterministic, all-optical
magnetic switching in simple ferromagnetic nanomagnets (Ni$_{81}$Fe$_{19}$,
Ni$_{50}$Fe$_{50}$) with sub-diffraction limit dimensions using a focused
low-power, linearly-polarised continuous-wave laser. Isolated nanomagnets are
switched across a range of dimensions, laser wavelengths and powers. All
square-geometry artificial spin ice vertex configurations are written,
including ground-state and energetically-unfavourable `monopole-like' states at
powers as low as 2.74 mW. Usually, magnetic switching with linearly polarised
light is symmetry-forbidden; however, here the laser spot has a similar size to
the nanomagnets, producing an absorption distribution dependent on the relative
nanoisland-spot displacement. We attribute the observed deterministic switching
to the transient dynamics of this asymmetric absorption. No switching is
observed in Co samples, suggesting the multi-species nature of NiFe alloys
plays a role in reversal. The results presented here usher in cheap, low-power
optically-controlled devices with impact across data storage, neuromorphic
computation and reconfigurable magnonics.",2112.00697v2
2022-01-11,Resonant Precession of Magnetization and Precession -- Induced DC voltages in FeGaB Thin Films,"Measurements of frequency dependent ferromagnetic resonance (FMR) and spin
pumping driven dc voltage (V_{dc}) are reported for amorphous films of
Fe_{78}Ga_{13}B_{9} (FeGaB) alloy to address the phenomenon of self-induced
inverse spin Hall effect (ISHE) in plain films of metallic ferromagnets. The
V_{dc} signal, which is antisymmetric on field reversal, comprises of symmetric
and asymmetric Lorentzians centered around the resonance field. Dominant role
of thin film size effects is seen in setting the magnitude of static
magnetization, V_{dc} and dynamics of magnetization precession in thinner films
(\leq 8 nm). The film thickness dependence of magnetization parameters
indicates the presence of a magnetically disordered region at the
film-substrate interface, which may promote preferential flow of spins
generated by the precessing magnetization towards the substrate. However, the
V_{dc} signal also draws contributions from rectification effects of a \approx
0.4 \% anisotropic magnetoresistance and a large (\approx 54 n\Omega.m)
anomalous Hall resistivity (AHR) of these films which ride over the effect of
spin-orbit coupling driven spin-to-charge conversion near the film-substrate
interface. We have addressed these data in the framework of the existing
theories of electrodynamics of a ferromagnetic film subjected to
radio-frequency field in a coplanar waveguide geometry. Our estimation of the
self-induced ISHE for the sample with 54 n\Omega.m AHR shows that it may
contribute significantly (\approx 90\%) to the measured symmetric voltage. This
study is expected to be very useful for fully understanding the spin pumping
induced dc voltages in metallic ferromagnets with disordered interfaces and
large anomalous Hall effect.",2201.03739v1
2022-01-17,Magnetic imprints of eruptive and non-eruptive Solar flares as observed by Solar Dynamics Observatory,"The abrupt and permanent changes of photospheric magnetic field in the
localized regions of active regions during solar flares called magnetic
imprints (MIs), have been observed for the past nearly three decades. The well
known ""coronal implosion"" model is assumed to explain such flare associated
changes but the complete physical understanding is still missing and debatable.
In this study, we made a systematic analysis of flare-related changes of
photospheric magnetic field during 21 flares (14 eruptive and 7 non-eruptive)
using the high-cadence (\texttt{135s}) vector-magnetogram data obtained from
Helioseismic and Magnetic Imager. The MI regions for eruptive flares are found
to be strongly localised, whereas the majority of non-eruptive events
($>70~\%$) have scattered imprint regions. To quantify the strength of the MIs,
we derived the integrated change of horizontal field and total change of
Lorentz force over an area. These quantities correlate well with the flare
strength, irrespective of whether flares being eruptive or not, short or long
duration. Further, the free-energy (FE), determined from virial-theorem
estimates, exhibits statistically significant downward trend which starts
around the flare time is observed in majority of flares. The change of FE
during flares do not depend on eruptivity but have a strong positive
correlation ($\approx 0.8$) with the Lorentz force change, indicating that the
part of FE released would penetrate into the photosphere. While these results
strongly favor the idea of significant feedback from corona on the photospheric
magnetic field, the characteristics of MIs are quite indistinguishable for
flares being eruptive or not.",2201.06550v1
2022-01-27,Slow spin dynamics in the hyper-honeycomb Lattice [(C2H5)3NH]2Cu2(C2O4)3 revealed by 1H NMR studies,"We report the results of magnetic susceptibility $\chi$ and $^1$H nuclear
magnetic resonance (NMR) measurements on a three-dimensional hyper-honeycomb
lattice compound [(C$_2$H$_5$)$_3$NH]$_2$Cu$_2$(C$_2$O$_4$)$_3$ (CCCO). The
average value of the antiferromagnetic (AFM) exchange coupling between the
Cu$^{2+}$ ($S$ = 1/2) spins was determined to be $J$~$\sim$~50 K from the
$\chi$ measurements. No long-range magnetic ordering has been observed down to
$T$ = 50 mK, although NMR lines become slightly broader at low temperatures
below 1 K. The broadening of the NMR spectrum observed below 1 K reveals that
the Cu spin moments remain at this temperature, suggesting a non-spin-singlet
ground state. The temperature and magnetic field dependence of 1/$T_1$ at
temperatures above 20 K is well explained by paramagnetic thermal spin
fluctuations where the fluctuation frequency of Cu$^{2+}$ spins is higher than
the NMR frequency of the order of MHz. However, a clear signature of the
slowing down of the Cu$^{2+}$ spin fluctuations was observed at low
temperatures where 1/$T_1$ shows a thermally-activated behavior. The magnetic
field dependence of the magnitude of the spin excitation gap suggests that the
magnetic behaviors of CCCO are characterized as an AFM chain at low
temperatures.",2201.11243v2
2022-02-07,Sr$_2$IrO$_4$/Sr$_3$Ir$_2$O$_7$ superlattice for a model 2D quantum Heisenberg antiferromagnet,"Spin-orbit entangled pseudospins hold promise for a wide array of exotic
magnetism ranging from a Heisenberg antiferromagnet to a Kitaev spin liquid
depending on the lattice and bonding geometry, but many of the host materials
suffer from lattice distortions and deviate from idealized models in part due
to inherent strong pseudospin-lattice coupling. Here, we report on the
synthesis of a magnetic superlattice comprising the single ($n$=1) and the
double ($n$=2) layer members of the Ruddlesden-Popper series iridates
Sr$_{n+1}$Ir$_{n}$O$_{3n+1}$ alternating along the $c$-axis, and provide a
comprehensive study of its lattice and magnetic structures using scanning
transmission electron microscopy, resonant elastic and inelastic x-ray
scattering, third harmonic generation measurements and Raman spectroscopy. The
superlattice is free of the structural distortions reported for the parent
phases and has a higher point group symmetry, while preserving the magnetic
orders and pseudospin dynamics inherited from the parent phases, featuring two
magnetic transitions with two symmetry-distinct orders. We infer weaker
pseudospin-lattice coupling from the analysis of Raman spectra and attribute it
to frustrated magnetic-elastic couplings. Thus, the superlattice expresses a
near ideal network of effective spin-one-half moments on a square lattice.",2202.02982v2
2022-02-11,Disorder-induced broadening of the spin waves in a triangular-lattice quantum-spin-liquid candidate YbZnGaO$_4$,"Disorder is important in the study of quantum spin liquids, but its role on
the spin dynamics remains elusive. Here, we explore the disorder effect by
investigating the magnetic-field dependence of the low-energy magnetic
excitations in a triangular-lattice frustrated magnet YbZnGaO$_4$ with
inelastic neutron scattering. With an intermediate field of 2.5 T applied along
the $c$-axis, the broad continuum at zero field becomes more smeared both in
energy and momentum. With a field up to 10 T, which fully polarizes the
magnetic moments, we observe clear spin-wave excitations with a gap of
$\sim$1.4 meV comparable to the bandwidth. However, the spectra are
significantly broadened. The excitation spectra both at zero and high fields
can be reproduced by performing classical Monte Carlo simulations which take
into account the disorder effect arising from the random site mixing of
nonmagnetic Zn$^{2+}$ and Ga$^{3+}$ ions. These results elucidate the critical
role of disorder in broadening the magnetic excitation spectra and mimicking
the spin-liquid features in frustrated quantum magnets.",2202.05462v1
2022-02-15,Assessing the Influence of Input Magnetic Maps on Global Modeling of the Solar Wind and CME-driven Shock in the 2013 April 11 Event,"In the past decade, significant efforts have been made in developing
physics-based solar wind and coronal mass ejection (CME) models, which have
been or are being transferred to national centers (e.g., SWPC, CCMC) to enable
space weather predictive capability. However, the input data coverage for space
weather forecasting is extremely limited. One major limitation is the solar
magnetic field measurements, which are used to specify the inner boundary
conditions of the global magnetohydrodynamic (MHD) models. In this study, using
the Alfven wave solar model (AWSoM), we quantitatively assess the influence of
the magnetic field map input (synoptic/diachronic vs. synchronic magnetic maps)
on the global modeling of the solar wind and the CME-driven shock in the 2013
April 11 solar energetic particle (SEP) event. Our study shows that due to the
inhomogeneous background solar wind and dynamical evolution of the CME, the
CME-driven shock parameters change significantly both spatially and temporally
as the CME propagates through the heliosphere. The input magnetic map has a
great impact on the shock connectivity and shock properties in the global MHD
simulation. Therefore this study illustrates the importance of taking into
account the model uncertainty due to the imperfect magnetic field measurements
when using the model to provide space weather predictions.",2202.07214v1
2022-03-17,Anticollinear order and degeneracy lifting in square lattice antiferromagnet LaSrCrO4,"We report the static and dynamic magnetic properties of LaSrCrO$_4$, a
seemingly canonical spin-3/2 square-lattice antiferromagnet that exhibits
frustration between magnetic layers -- owing to their AB stacking -- and offers
a rare testbed to investigate accidental-degeneracy lifting in magnetism.
Neutron diffraction experiments on single-crystal samples uncover a remarkable
anticollinear magnetic order below $T_N$ = 170 K characterized by a N\'eel
arrangement of the spins within each layer and an orthogonal arrangement
between adjacent layers. To understand the origin of this unusual magnetic
structure, we analyze the spin-wave excitation spectrum by means of inelastic
neutron scattering and bulk measurements. A spectral gap of 0.5 meV, along with
a spin-flop transition at 3.2\, T, reflect the energy scale associated with the
degeneracy-lifting. A minimal model to explain these observations requires both
a positive biquadratic interlayer exchange and dipolar interactions, both of
which are on the order of 10$^{-4}$ meV, only a few parts per million of the
dominant exchange interaction $J_1 \approx 11$ meV. These results provide
direct evidence for the selection of a non-collinear magnetic structure by the
combined effect of two distinct degeneracy lifting interactions.",2203.09049v1
2022-04-14,Piezoelectric ferromagnetism in Janus monolayer YBrI: a first-principle prediction,"Coexistence of intrinsic ferromagnetism and piezoelectricity, namely
piezoelectric ferromagnetism (PFM), is crucial to advance multifunctional
spintronic technologies. In this work, we demonstrate that Janus monolayer YBrI
is a PFM, which is dynamically, mechanically and thermally stable. Electronic
correlation effects on physical properties of YBrI are investigated by using
generalized gradient approximation plus $U$ (GGA+$U$) approach. For
out-of-plane magnetic anisotropy, YBrI is a ferrovalley (FV) material, and the
valley splitting is larger than 82 meV in considered $U$ range. The anomalous
valley Hall effect (AVHE) can be achieved under an in-plane electric field.
However, for in-plane magnetic anisotropy, YBrI is a common ferromagnetic (FM)
semiconductor. When considering intrinsic magnetic anisotropy, the easy axis of
YBrI is always in-plane with magnetic anisotropy energy (MAE) from 0.309 meV to
0.237 meV ($U$=0.0 eV to 3.0 eV). However, the magnetization can be adjusted
from the in-plane to off-plane direction by external magnetic field, and then
lead to the occurrence of valley polarization. Moreover, missing centrosymmetry
along with mirror symmetry breaking results in both in-plane and out-of-plane
piezoelectricity in YBrI monolayer. At a typical $U$=2.0 eV, the $d_{11}$ is
predicted to be -5.61 pm/V, which is higher than or compared with ones of other
two-dimensional (2D) known materials. The electronic and piezoelectric
properties of YBrI can be effectively tuned by applying a biaxial strain. For
example, tensile strain can enhance valley splitting and $d_{11}$ (absolute
value). The predicted Curie temperature of YBrI is higher than those of
experimentally synthesized 2D ferromagnetic materials $\mathrm{CrI_3}$ and
$\mathrm{Cr_2Ge_2Te_6}$.",2204.06713v1
2022-05-14,Strain effects on topological and valley properties of Janus monolayer $\mathrm{VSiGeN_4}$,"Strain is an effective method to tune the electronic properties of
two-dimension (2D) materials, and can induce novel phase transition. Recently,
2D $\mathrm{MA_2Z_4}$ family materials are of interest because of their
emerging topological, magnetic and superconducting properties. Here, we
investigate the impact of strain effects ($a/a_0$:0.96$\sim$1.04) on the
physical properties of Janus monolayer $\mathrm{VSiGeN_4}$ as a derivative of
$\mathrm{VSi_2N_4}$ or $\mathrm{VGe_2N_4}$, which possesses dynamical,
mechanical and thermal stabilities. For out-of-plane magnetic anisotropy, with
increasing strain, $\mathrm{VSiGeN_4}$ undergoes transition between ferrovalley
semiconductor (FVS), half-valley-metal (HVM), valley-polarized quantum
anomalous Hall insulator (VQAHI), HVM and FVS. These imply twice topological
phase transitions, which are related with sign-reversible Berry curvature and
band inversion between $d_{xy}$+$d_{x^2-y^2}$ and $d_{z^2}$ orbitals for K or
-K valley. The band inversion also leads to transformation of valley splitting
strength between valence and conduction bands. However, for in-plane magnetic
anisotropy, no special quantum anomalous Hall (QAH) states and valley
polarization exist within the considered strain range. The actual magnetic
anisotropy energy (MAE) shows no special QAH and HVM states in monolayer
$\mathrm{VSiGeN_4}$. Fortunately, these can be easily achieved by external
magnetic field, which adjusts the easy magnetization axis of
$\mathrm{VSiGeN_4}$ from in-plane one to out-of-plane one. Our findings shed
light on how strain can be employed to engineer the electronic states of
$\mathrm{VSiGeN_4}$, which may open new perspectives for multifunctional
quantum devices in valleytronics and spintronics.",2205.07012v2
2022-05-26,The competition between the hydrodynamic instability from noise and magnetorotational instability in the Keplerian disks,"We venture for the comparison between growth rates for magnetorotational
instability (MRI) and hydrodynamics instability in the presence of an extra
force in the local Keplerian accretion flow. The underlying model is described
by the Orr-Sommerfeld and Squire equations in the presence of rotation,
magnetic field and an extra force, plausibly noise with a nonzero mean. We
obtain MRI using Wentzel-Kramers-Brillouin (WKB) approximation without extra
force for purely vertical magnetic field and vertical wavevector of the
perturbations. Expectedly, MRI is active within a range of magnetic field,
which changes depending on the perturbation wavevector magnitude. Next, to
check the effect of noise on the growth rates, a quartic dispersion relation
has been obtained. Among those four solutions for growth rate, the one that
reduces to MRI growth rate at the limit of vanishing mean of noise in the MRI
active region of the magnetic field, is mostly dominated by MRI. However, in
MRI inactive region, in the presence of noise the solution turns out to be
unstable, which are almost independent of the magnetic field. Another growth
rate, which is almost complementary to the previous one, leads to stability at
the limit of vanishing noise. The remaining two growth rates, which correspond
to the hydrodynamical growth rates at the limit of the vanishing magnetic
field, are completely different from the MRI growth rate. More interestingly,
the latter growth rates are larger than that of the MRI. If we consider
viscosity, the growth rates decrease depending on the Reynolds number.",2205.13230v1
2022-06-12,Kagome lattice promotes chiral spin fluctuations,"Magnetic materials with tilted electron spins often exhibit conducting
behavior that cannot be explained from semiclassical theories without invoking
fictitious (emergent) electromagnetic fields. Quantum-mechanical models
explaining such phenomena are rooted in the concept of a moving quasiparticle's
Berry phase, driven by a chiral (left- or right-handed) spin-habit. Dynamical
and nearly random spin fluctuations, with a slight bent towards left- or
right-handed chirality, represent a promising route to realizing Berry-phase
phenomena at elevated temperatures, but little is known about the effect of
crystal lattice geometry on the resulting macroscopic observables. Here, we
report thermoelectric and electric transport experiments on two metals with
large magnetic moments on a triangular and on a slightly distorted kagom\'e
lattice, respectively. We show that the impact of chiral spin fluctuations is
strongly enhanced for the kagom\'e lattice. Both these spiral magnets have
similar magnetic phase diagrams including a periodic array of magnetic
skyrmions. However, our modelling shows that the geometry of the kagom\'e
lattice, with corner-sharing spin-trimers, helps to avoid cancellation of
Berry-phase contributions; spin fluctuations are endowed with a net chiral
habit already in the thermally disordered (paramagnetic) state. Hence, our
observations for the kagom\,e material contrast with theoretical models
treating magnetization as a continuous field, and emphasize the role of lattice
geometry on emergent electrodynamic phenomena.",2206.05756v2
2022-06-22,Tracing the magnetic field topology of the quiet corona using propagating disturbances,"The motion of faint propagating disturbances (PD) in the solar corona reveals
an intricate structure which must be defined by the magnetic field. Applied to
quiet Sun observations by the Atmospheric Imaging Assembly (AIA)/Solar Dynamics
Observatory (SDO), a novel method reveals a cellular network, with cells of
typical diameters 50\arcsec\ in the cool 304\AA\ channel, and 100\arcsec\ in
the coronal 193\AA\ channel. The 193\AA\ cells can overlie several 304\AA\
cells, although both channels share common source and sink regions. The sources
are points, or narrow corridors, of divergence that occupy the centres of
cells. They are significantly aligned with photospheric network features and
enhanced magnetic elements. This shows that the bright network is important to
the production of PDs, and confirms that the network is host to the source
footpoint of quiet coronal loops. The other footpoint, or the sinks of the PDs,
form the boundaries of the coronal cells. These are not significantly aligned
with the photospheric network - they are generally situated above the dark
internetwork photosphere. They form compact points or corridors, often without
an obvious signature in the underlying photosphere. We argue that these sink
points can either be concentrations of closed field footpoints associated with
minor magnetic elements in the internetwork, or concentrations of
upward-aligned open field. The link between the coronal velocity and magnetic
fields is strengthened by a comparison with a magnetic extrapolation, which
shows several general and specific similarities, thus the velocity maps offer a
valuable additional constraint on models.",2206.11077v1
2022-06-24,Transition from Pauli paramagnetism to Curie-Weiss behaviour in vanadium,"We study electron correlations and their impact on magnetic properties of bcc
vanadium by a combination of density functional and dynamical mean-field
theory. The calculated uniform magnetic susceptibility {in bcc structure} is of
Pauli type at low temperatures, while it obeys the Curie-Weiss law at higher
temperatures. Thus, we qualitatively reproduce the experimental temperature
dependence of magnetic susceptibility without introducing the martensitic phase
transition. Our results for local spin-spin correlation function and local
susceptibility reveal that the Curie-Weiss behavior appears due to partial
formation of local magnetic moments, which originate from $t_{2g}$ states and
occur due to local spin correlations caused by Hund's rule coupling. At the
same time, the fermionic quasiparticles remain well-defined, while the
formation of local moments is accompanied by a deviation from the Fermi-liquid
behavior. In particular, the self-energy of the $t_{2g}$ states shows the
non-analytic frequency dependence, which is a characteristic of the
spin-freezing behavior, while the quasiparticle damping changes approximately
linearly with temperature in the intermediate temperature range $200$--$700$~K.
By analyzing the momentum dependence of static magnetic susceptibility, we find
incommensurate magnetic correlations, which may provide a mechanism for
unconventional superconductivity at low temperatures.",2206.12383v3
2022-07-20,Numerical modelling of imposed magnetohydrodynamic effects in hypersonic flows,"Weakly ionised plasmas, formed in high enthalpy hypersonic flows, can be
actively manipulated via imposed magnetic fields - a concept termed
magnetohydrodynamic (MHD) flow control. Imposed MHD effects, within flows which
exhibit multiple shock interactions, are consequential for emerging aerospace
technologies: including aerodynamic control via magnetic actuation. However,
numerical modelling of this flow type remains challenging due to the
sensitivity of feature formation and the real gas modelling of weakly ionised,
electrically conductive, air plasma. In this work, numerical simulation
capabilities have been developed for the study of MHD affected, hypersonic
flows, around 2D axisymmetric non-simple geometries. The validated numerical
methodology, combined with an advanced 19 species equation of state for air
plasma, achieves quantitative agreement between simulation and experiment for a
Mach 5.6 double cone geometry with applied magnetic field. Numerical studies
are conducted for varied conical surface angle and magnetic field
configuration. This paper demonstrates how, for hypersonic flows with complex
shock interactions, the MHD affected flow is not only augmented in terms of
shock position, but may exhibit topological adaptations in the fundamental flow
structure. A classification system is introduced for the emergent flow
topologies. The applied numerical studies examine the mechanisms by which the
magnetic field configuration influences the MHD augmented shock structure,
leading to: (1) differences in magnitude of MHD enhancement effect, and (2)
structural adaptations of the flow topology. Most critically, classes of
conditions are identified which produce topological equivalence between the
magnetic interaction effects and a generalised mechanical control surface.",2207.09857v1
2022-08-05,Nature of helicity injection in non-erupting solar active regions,"Using time-sequence vector magnetic field and coronal observations from
\textit{Solar Dynamics Observatory}, we report the observations of the magnetic
field evolution and coronal activity in four emerging active regions (ARs). The
ARs emerge with leading polarity being the same as for the majority of ARs in a
hemisphere of solar cycle 24. After emergence, the magnetic polarities separate
each other without building a sheared polarity inversion line. In all four ARs,
the magnetic fields are driven by foot point motions such that the sign of the
helicity injection ($dH/dt$) in the first half of the evolution is changed to
the opposite sign in the later part of the observation time. This successive
injection of opposite helicity is also consistent with the sign of mean
force-free twist parameter ($\alpha_{av}$). Further, the EUV light curves of
the ARs in 94\AA~and GOES X-ray flux reveal flaring activity below C-class
magnitude. Importantly, the white-light coronagraph images in conjunction with
the AR images in AIA 94 \AA~delineate the absence of associated CMEs with the
studied ARs. These observations imply that the ARs with successive injection of
opposite sign magnetic helicity are not favorable to twisted flux rope
formation with excess coronal helicity, and therefore are unable to launch
CMEs, according to recent reports. This study provides the characteristics of
helicity flux evolution in the ARs referring to the conservative property of
magnetic helicity and more such studies would help to quantify the eruptive
capability of a given AR.",2208.03228v1
2022-08-19,Revealing nanoscale disorder in W/CoFeB/MgO ultra-thin films using domain wall motion,"Disorder in ultra-thin magnetic films can significantly hinder domain wall
motion. One of the main issues on the path towards efficient domain wall based
devices remains the characterization of the pinning landscape at the nanoscale.
In this paper, we study domain wall motion in W/CoFeB/MgO thin films with
perpendicular magnetic anisotropy crystallized by annealing at 400$^{\circ}$C
and a process based on He$^{+}$ irradiation combined with elevated
temperatures. Magnetic properties are similar for the whole series of samples,
while the magnetic domain wall mobility is critically improved in the
irradiated samples. By using an analytical model to extract nanoscale pinning
parameters, we reveal important variations in the disorder of the crystallized
samples. This work offers a unique opportunity to selectively analyze the
effects of disorder on the domain wall dynamics, without the contribution of
changes in the magnetic properties. Our results highlight the importance of
evaluating the nanoscale pinning parameters of the material when designing
devices based on domain wall motion, which in return can be a powerful tool to
probe the disorder in ultra-thin magnetic films.",2208.09280v1
2022-10-08,Magnetohydrodynamics evolution of three-dimensional magnetic null in NOAA active region 11515 initiated using non-force-free field extrapolation,"Magnetohydrodynamics simulation of active region NOAA 11515 is performed to
examine the initiation of the M5.6 flaring event that starts around 10:43 UT on
2012 July 2. The simulation is conducted using an extrapolated non-force-free
magnetic field generated from the photospheric vector magnetogram of the active
region as the initial magnetic field. The magnetic field shows the presence of
a three-dimensional (3D) magnetic null with the corresponding dome overlying a
filament and a low-lying magnetic flux rope, observed in 304~\AA~ and 131~\AA~
respectively.
  The simulated dynamics, triggered by the initial Lorentz force, lead to the
bifurcations of the flux rope, which is similar to the observed bifurcation in
the 131 \AA~ brightenings. Additionally, the rope exhibits a rise and
reconnects at the 3D null. These reconnections convert field lines of the rope
into the anchored outer spine of the 3D null -- explaining the occurrence of a
nearby confined C-class flare. Further, the results show that the field lines
of the flux rope reach the vicinity of the filament and become non-parallel to
the field lines of the filament. This initiates the reconnections between the
rope and the field lines of the filament -- activating the filament for the
eruption. This interesting interaction of the flux rope and filament seems to
contribute to the onset of the M-class flare.",2210.03957v1
2022-10-29,On the specific energy and pressure in near-Earth magnetic clouds,"The pressure and energy density of the gas and magnetic field inside solar
coronal mass ejections (in relation to that in the ambient solar wind) is
thought to play an important role in determining their dynamics as they
propagate through the heliosphere. We compare the specific energy (${\rm
erg\,g^{-1}}$) [comprising kinetic ($H_{\rm k}$), thermal ($H_{\rm th }$) and
magnetic field ($H_{\rm mag}$) contributions] inside MCs and the solar wind
background. We examine if the excess thermal + magnetic pressure and specific
energy inside MCs (relative to the background) is correlated with their
propagation and internal expansion speeds. We ask if the excess thermal +
magnetic specific energy inside MCs might make them resemble rigid bodies in
the context of aerodynamic drag. We use near-Earth in-situ data from the WIND
spacecraft to identify a sample of 152 well observed interplanetary coronal
mass ejections and their MC counterparts. We compute various metrics using
these data to address our questions. We find that the total specific energy
($H$) inside MCs is approximately equal to that in the background solar wind.
We find that the the excess (thermal + magnetic) pressure and specific energy
are not well correlated with the near-Earth propagation and expansion speeds.
We find that the excess thermal+magnetic specific energy $\gtrsim$ the specific
kinetic energy of the solar wind incident on 81--89 \% of the MCs we study.
This might explain how MCs retain their structural integrity and resist
deformation by the solar wind bulk flow.",2210.16571v1
2022-11-09,Modelling the magnetic structure of a large-scale horse-shoe-like filament in a decaying and diffuse active region,"A large-scale, horse-shoe-like filament was investigated and the magnetic
field around it was reconstructed. This is an intermediate filament (IF) that
appeared on the solar disk for the first time at 02:00 UT on 2015 November 7,
and took 8 days to move to the central median on the solar disk. The active
region AR 12452 around which the filament occurred was diffuse so that the
magnetic field nearby was weak, the average field strength is 106 G. Therefore,
the existing approaches to extrapolating the coronal magnetic field and to
constructing the filament configuration in the region with strong background
field do not work well here. On the basis of the regularized Biot-Savart laws
method, we successfully constructed a data-constrained, non-linear force-free
field configuration for this IF observed on 2015 November 14. The overall IF
configuration obtained in this way matches well the morphology suggested by a
304~\AA \ image taken by the Atmospheric Imaging Assembly on board Solar
Dynamics Observatory. Magnetic dips in the configuration were coincident in
space with the H$\alpha$ features of the filament, which is lower in altitude
than the features seen in 304~\AA. This suggests that the cold plasma fills the
lower part of the filament, and hot plasma is situated in the higher region. A
quasi-separatrix layer wraps the filament, and both the magnetic field and the
electric current are stronger near the inner edge of the filament.",2211.04842v1
2022-11-11,Frequency and Amplitude Optimizations for Magnetic Particle Spectroscopy Applications,"Nowadays, there is a growing interest in the field of magnetic particle
spectroscopy (MPS)-based bioassays. MPS monitors the dynamic magnetic response
of surface-functionalized magnetic nanoparticles (MNPs) upon excitation by an
alternating magnetic field (AMF) to detect various target analytes. This
technology has flourished in the past decade due to its low cost, low
background magnetic noise interference from biomatrix, and fast response time.
A large number of MPS variants have been reported by different groups around
the world, with applications ranging from disease diagnosis to foodborne
pathogen detection, and virus detection. However, there is an urgent need for
guidance on how to optimize the sensitivity of MPS detection by choosing
different types of MNPs, AMF modalities, and MPS assay strategies (i.e.,
volume- and surface-based assays). In this work, we systematically study the
effect of AMF frequencies and amplitudes on the responses of single- and
multi-core MNPs under two extreme conditions, namely, the bound and unbound
states. Our results show that some modalities such as dual-frequency MPS
utilizing multicore MNPs are more suitable for surface-based bioassay
applications, whereas, single-frequency MPS systems using single- or multi-core
MNPs are better suited for volumetric bioassay applications. Furthermore, the
bioassay sensitivities for these modalities can be further improved by careful
selection of AMF frequencies and amplitudes.",2211.05936v1
2022-11-17,A simplified lattice Boltzmann implementation of the quasi-static approximation in pipe flows under the presence of non-uniform magnetic fields,"We propose a single-step simplified lattice Boltzmann algorithm capable of
performing magnetohydrodynamic (MHD) flow simulations in pipes for very small
values of magnetic Reynolds numbers $R_m$. In some previous works, most lattice
Boltzmann simulations are performed with values of $R_m$ close to the Reynolds
numbers for flows in simplified rectangular geometries. One of the reasons is
the limitation of some traditional lattice Boltzmann algorithms in dealing with
situations involving very small magnetic diffusion time scales associated with
most industrial applications in MHD, which require the use of the so-called
quasi-static (QS) approximation. Another reason is related to the significant
dependence that many boundary conditions methods for lattice Boltzmann have on
the relaxation time parameter. In this work, to overcome the mentioned
limitations, we introduce an improved simplified algorithm for velocity and
magnetic fields which is able to directly solve the equations of the QS
approximation, among other systems, without preconditioning procedures. In
these algorithms, the effects of solid insulating boundaries are included by
using an improved explicit immersed boundary algorithm, whose accuracy is not
affected by the values of $R_m$. Some validations with classic benchmarks and
the analysis of the energy balance in examples including uniform and
non-uniform magnetic fields are shown in this work. Furthermore, a progressive
transition between the scenario described by the QS approximation and the MHD
canonical equations in pipe flows is visualized by studying the evolution of
the magnetic energy balance in examples with unsteady flows.",2211.09370v3
2022-11-29,Magnetic interactions of 4$f$ electrons in the topological insulator chalcogenide Bi$_{2}$Se$_{3}$,"The gap opening mechanism of a topological insulator, the quantum anomalous
Hall effect and the axion physics are still pressing open questions and a
microscopic viewpoint to further understand the role of magnetism in topology
is highly desirable. In this work we have performed a microscopic
investigation, by means of electron spin resonance (ESR) along with
complementary bulk measurements, on the chalcogenide
(Bi$_{1-x}$Gd$_{x}$)$_{2}$Se$_{3}$ ($x$ = 0, 0.001, 0.002 and 0.006). Our
analysis of the Gd$^{3+}$ spin dynamics reveal no significant change of the
Fermi surface as a function of Gd$^{3+}$ concentration, which indicates that
the 4$f$ magnetism is different from the non-local effects induced by
transition metals ($d$ electrons) substitutions. Additionally, we observe an
unusual evolution of the Gd$^{3+}$ ESR spectra as a function of the applied
magnetic field, which we discuss considering the magnetic interaction between
Gd$^{3+}$ 4$f$ electrons and impurity centers such as Se vacancies. This
interaction would give rise to a local weak antilocalization effect surrounding
the Gd$^{3+}$ ions. Such mechanism is observable due to particular details of
the Gd$^{3+}$ 4$f$ electrons magnetism in this system compared to $d$
electrons. Our work points out that rare earth substitutions in this model
topological insulator is a promising path to explore the axion insulating
systems.",2211.16048v1
2023-01-10,Magnetic anisotropy and low-energy spin dynamics in the van der Waals compounds Mn$_{2}$P$_{2}$S$_{6}$ and MnNiP$_{2}$S$_{6}$,"We report the detailed high-field and high-frequency electron spin resonance
(HF-ESR) spectroscopic study of the single-crystalline van der Waals compounds
Mn$_{2}$P$_{2}$S$_{6}$ and MnNiP$_{2}$S$_{6}$. Analysis of magnetic excitations
shows that in comparison to Mn$_{2}$P$_{2}$S$_{6}$ increasing the Ni content
yields a larger magnon gap in the ordered state and a larger g-factor value and
its anisotropy in the paramagnetic state. The studied compounds are found to be
strongly anisotropic having each the unique ground state and type of magnetic
order. Stronger deviation of the g-factor from the free electron value in the
samples containing Ni suggests that the anisotropy of the exchange is an
important contributor to the stabilization of a certain type of magnetic order
with particular anisotropy. At temperatures above the magnetic order, we have
analyzed the spin-spin correlations resulting in a development of slowly
fluctuating short-range order. They are much stronger pronounced in
MnNiP$_{2}$S$_{6}$ compared to Mn$_{2}$P$_{2}$S$_{6}$. The enhanced spin
fluctuations in MnNiP$_{2}$S$_{6}$ are attributed to the competition of
different types of magnetic order. Finally, the analysis of the temperature
dependent critical behavior of the magnon gaps below the ordering temperature
in Mn$_{2}$P$_{2}$S$_{6}$ suggests that the character of the spin wave
excitations in this compound undergoes a field induced crossover from a 3D-like
towards 2D XY regime.",2301.04239v2
2023-01-22,Responses of quark-antiquark interaction and heavy quark dynamics to magnetic field,"We study how a magnetic field induced by colliding nuclei influences both
heavy quark (HQ) potential and HQ momentum diffusion coefficients beyond the
lowest Landau level approximation. By means of real-time hard thermal loop
resummed technique combined with dimension two gluon condensate, the effective
gluon propagator describing both perturbative and nonperturbative QCD nature at
finite temperature and magnetic field are obtained. We find that HQ momentum
diffusion coefficients in magnetized QCD medium become anisotropic, and with
increasing temperature, the higher Landau levels become significant, which
leads to the reduction of the anisotropic ratio ($>$ 1) and even overturn the
behavior ($<$ 1) at high temperature. The nonperturbative (perturbative)
contribution of HQ momentum diffusion coefficient in the low (high) temperature
is dominant. And the anisotropy feature of the real part of the potential is
essentially encoded in angular dependent Coulomb coupling and string tension.
Whereas, the imaginary part of the potential from quark-loop in this work
displays a significant anisotropy even though using the angular-independent
Coulomb coupling and string tension. Furthermore, we study the magnetic
response of viscous quark matter, which is manifested in the non-equilibrium
distribution function of (anti-)quark by solving Boltzmann equation within the
relaxation time approximation. We find the anisotropy ratio is almost
insensitive to the magnetized bulk viscosity, although HQ momentum diffusion
coefficient and HQ potential itself have visible changes.",2301.09110v1
2023-02-22,Magnetized fingering convection in stars,"Fingering convection (also known as thermohaline convection) is a process
that drives the vertical transport of chemical elements in regions of stellar
radiative zones where the mean molecular weight increases with radius.
Recently, Harrington & Garaud (2019) used three-dimensional direct numerical
simulations to show that a vertical magnetic field can dramatically enhance the
rate of chemical mixing by fingering convection. Furthermore, they proposed a
so-called ""parasitic saturation"" theory to model this process. Here, we test
their model over a broad range of parameter space, using a suite of direct
numerical simulations of magnetized fingering convection varying the magnetic
Prandtl number, magnetic field strength, and composition gradient. We find that
the rate of chemical mixing measured in the simulations is not always predicted
accurately by their existing model, in particular when the magnetic diffusivity
is large. We then present an extension of the Harrington & Garaud (2019) model
which resolves this issue. When applied to stellar parameters, it recovers the
results of Harrington & Garaud (2019) except in the limit where fingering
convection becomes marginally stable, where the new model is preferred. We
discuss the implications of our findings for stellar structure and evolution.",2302.11610v3
2023-03-07,Tetrahedral triple-Q magnetic ordering and large spontaneous Hall conductivity in the metallic triangular antiferromagnet Co1/3TaS2,"The triangular lattice antiferromagnet (TLAF) has been the standard paradigm
of frustrated magnetism for several decades. The most common magnetic ordering
in insulating TLAFs is the 120 structure. However, a new triple-Q chiral
ordering can emerge in metallic TLAFs, representing the short wavelength limit
of magnetic skyrmion crystals. We report the metallic TLAF Co1/3TaS2 as the
first example of tetrahedral triple-Q magnetic ordering with the associated
topological Hall effect (non-zero {\sigma}_{xy}(H=0)). We also present a
theoretical framework that describes the emergence of this magnetic ground
state, which is further supported by the electronic structure measured by
angle-resolved photoemission spectroscopy. Additionally, our measurements of
the inelastic neutron scattering cross section are consistent with the
calculated dynamical structure factor of the tetrahedral triple-Q state.",2303.03760v5
2023-03-09,Eruption of a Magnetic Flux Rope in a Comprehensive Radiative Magnetohydrodynamic Simulation of flare-productive active regions,"Radiative magnetohydrodynamic simulation includes sufficiently realistic
physics to allow for the synthesis of remote sensing observables that can be
quantitatively compared with observations. We analyze the largest flare in a
simulation of the emergence of large flare-productive active regions described
by Chen et al. The flare releases $4.5\times10^{31}$ erg of magnetic energy and
is accompanied by a spectacular coronal mass ejection. Synthetic soft X-ray
flux of this flare reaches M2 class. The eruption reproduces many key features
of observed solar eruptions. A pre-existing magnetic flux rope is formed along
the highly sheared polarity inversion line between a sunspot pair and is
covered by an overlying multi-pole magnetic field. During the eruption, the
progenitor flux rope actively reconnects with the canopy field and evolves to
the large-scale multi-thermal flux rope that is observed in the corona.
Meanwhile, the magnetic energy released via reconnection is channeled down to
the lower atmosphere and gives rise to bright soft X-ray post-flare loops and
flare ribbons that reproduce the morphology and dynamic evolution of observed
flares. The model helps to shed light on questions of where and when the a flux
rope may form and how the magnetic structures in an eruption are related to
observable emission properties.",2303.05405v2
2023-03-22,Physical Models for Solar Cycle Predictions,"The dynamic activity of stars such as the Sun influences (exo)planetary space
environments through modulation of stellar radiation, plasma wind, particle and
magnetic fluxes. Energetic stellar phenomena such as flares and coronal mass
ejections act as transient perturbations giving rise to hazardous space
weather. Magnetic fields -- the primary driver of stellar activity -- are
created via a magnetohydrodynamic dynamo mechanism within stellar convection
zones. The dynamo mechanism in our host star -- the Sun -- is manifest in the
cyclic appearance of magnetized sunspots on the solar surface. While sunspots
have been directly observed for over four centuries, and theories of the origin
of solar-stellar magnetism have been explored for over half a century, the
inability to converge on the exact mechanism(s) governing cycle to cycle
fluctuations and inconsistent predictions for the strength of future sunspot
cycles have been challenges for models of solar cycle forecasts. This review
discusses observational constraints on the solar magnetic cycle with a focus on
those relevant for cycle forecasting, elucidates recent physical insights which
aid in understanding solar cycle variability, and presents advances in solar
cycle predictions achieved via data-driven, physics-based models. The most
successful prediction approaches support the Babcock-Leighton solar dynamo
mechanism as the primary driver of solar cycle variability and reinforces the
flux transport paradigm as a useful tool for modelling solar-stellar magnetism.",2303.12648v1
2023-03-25,The gap-size influence on the excitation of magnetorotational instability in cylindrical Couette flows,"The excitation conditions of the magnetorotational instability are studied
for axially unbounded Taylor-Couette flows of various gap widths between the
cylinders. The cylinders are considered as made from both perfect-conducting or
insulating material and the conducting fluid with a finite but small magnetic
Prandtl number rotates with a quasi-Keplerian velocity profile. The solutions
are optimized with respect to the wave number and the Reynolds number of the
rotation of the inner cylinder. For the axisymmetric modes we find the critical
Lundquist number of the applied axial magnetic field the lower the wider the
gap between the cylinders. A similar result is obtained for the induced cell
structure: the wider the gap the more spherical the cells are. The marginal
rotation rate of the inner cylinder -- for fixed size of the outer cylinder --
always possesses a minimum for not too wide and not too narrow gap widths. For
perfect-conducting walls the minimum lies at $r_{\rm in}\simeq 0.4$ while it is
at $r_{\rm in}\simeq 0.5$ for insulating walls where $r_{\rm in}$ is the
normalized radius of the inner cylinder. The lowest magnetic field amplitudes
to excite the instability are required for Taylor-Couette flows between
perfect-conducting cylinders with gaps corresponding to $r_{\rm in}\simeq 0.2$.
For even wider and also for very thin gaps the needed magnetic fields and
rotation frequencies are shown to become rather huge.
  Also the nonaxisymmetric modes with $|m|=1$ have been considered. Their
excitation generally requires stronger magnetic fields and higher magnetic
Reynolds numbers in comparison to those for the axisymmetric modes which is
true for wide-gap containers with $r_{\rm in} \lesssim 0.3$.",2303.14547v1
2023-06-07,The magnetic dual chiral density wave phase in a rotating cold quark matter,"The effect of rotation on the formation of the magnetic dual chiral density
wave (\MD) in a dense and magnetized cold quark matter is studied. This phase
is supposed to exist in the extreme conditions prevailing, e.g., in a neutron
star. These conditions are, apart from high densities and strong magnetic
fields, a relatively large angular velocity. To answer the question of whether
the rotation enhances or suppresses the formation of this phase, we first
determine the effect of rotation on the energy dispersion relation of a
fermionic system in the presence of a constant magnetic field and then focus on
the thermodynamic potential of the model at low temperature $T$ and finite
chemical potential $\mu$. The thermodynamic potential consists, in particular,
of an anomalous part leading to certain topological effects. We show that in
comparison with the nonrotating case, a term proportional to the angular
velocity appears in this anomalous potential. We then solve the corresponding
gap equations to the chiral and spatial modulation condensates, and study the
dependence of these dynamical variables on the chemical potential ($\mu$),
magnetic field ($eB$), and angular velocity ($\Omega$). It turns out that the
interplay between these parameters suppresses the formation of the \MD~phase in
relevant regimes for cold neutron stars. This is interpreted as the
manifestation of the inverse magnetorotational catalysis, which is also
reflected in the phase portraits $eB$-$\mu$, $eB$-$\Omega R$, and $\mu$-$\Omega
R$, explored in this work.",2306.04472v2
2023-06-16,Probing the Anomalous Hall Transport and Magnetic Reversal of Chiral-Lattice Antiferromagnet Co$_{1/3}$NbS$_2$,"Antiferromagnets exhibiting giant anomalous Hall effect (AHE) and anomalous
Nernst effect (ANE) have recently aroused broad interest, not only for their
potential applications in future electronic devices, but also because of the
rich physics arising from the Berry curvature near the Fermi level.
$\rm{Co_{1/3}NbS_2}$, by intercalating $\rm{Co^{2+}}$ ions between $\rm{NbS_2}$
layers, is a quasi-two-dimensional layered antiferromagnet with a chiral
lattice. A large AHE has been observed in $\rm{Co_{1/3}NbS_2}$, but its origin
is under debate. In this letter, we report the large AHE and ANE in exfoliated
$\rm{Co_{1/3}NbS_2}$ flakes. By analyzing the thermoelectric data via the Mott
relation, we determined that the observed large AHE and ANE primarily result
from the intrinsic Berry curvature. We also observed the magnetic domains in
$\rm{Co_{1/3}NbS_2}$ by reflective magnetic circular dichroism measurements.
Combined with electrical transport measurements, we confirmed that the magnetic
reversal in $\rm{Co_{1/3}NbS_2}$ is determined by domain wall motion, and the
critical field ($H_c$) exhibits a memory effect of consecutive magnetic sweeps.
Our work provides insight into the topological properties of
$\rm{Co_{1/3}NbS_2}$ and paves the way to studying the spin configuration and
magnetic domain dynamics in this fascinating antiferromagnet.",2306.09616v1
2023-07-10,Periodicity staircase in a Fe/Gd magnetic thin film,"Presence of multiple competing periodicities may result in a system to go
through states with modulated periodicities, an example of which is the
self-similar staircase-like structure called the Devil's staircase. Herein we
report on a novel staircase structure of domain periodicity in an amorphous and
achiral Fe/Gd magnetic thin film wherein the reciprocal space wavevector
\textbf{Q} due to the ordered stripe domains does not evolve continuously,
rather exhibits a staircase structure. Resonant X-ray scattering experiments
show jumps in the periodicity of the stripe domains as a function of an
external magnetic field. When resolved in components, the step change along
Q$_x$ was found to be an integral multiple of a minimum step height of 7 nm,
which resembles closely to the exchange length of the system. Modeling the
magnetic texture in the Fe/Gd thin film as an achiral spin arrangement, we have
been able to reproduce the steps in the magnetization using a Landau-Lifshitz
spin dynamics calculation. Our results indicate that anisotropy and not the
dipolar interaction is the dominant cause for the staircase pattern, thereby
revealing the effect of achiral magnetism.",2307.04938v3
2023-08-12,Avalanches and the Distribution of Reconnection Events in Magnetized Circumstellar Disks,"Cosmic rays produced by young stellar objects can potentially alter the
ionization structure, heating budget, chemical composition, and accretion
activity in circumstellar disks. The inner edges of these disks are truncated
by strong magnetic fields, which can reconnect and produce flaring activity
that accelerates cosmic radiation. The resulting cosmic rays can provide a
source of ionization and produce spallation reactions that alter the
composition of planetesimals. This reconnection and particle acceleration are
analogous to the physical processes that produce flaring in and heating of
stellar coronae. Flaring events on the surface of the Sun exhibit a power-law
distribution of energy, reminiscent of those measured for Earthquakes and
avalanches. Numerical lattice-reconnection models are capable of reproducing
the observed power-law behavior of solar flares under the paradigm of
self-organized criticality. One interpretation of these experiments is that the
solar corona maintains a nonlinear attractor -- or ``critical'' -- state by
balancing energy input via braided magnetic fields and output via reconnection
events. Motivated by these results, we generalize the lattice-reconnection
formalism for applications in the truncation region of magnetized disks. Our
numerical experiments demonstrate that these nonlinear dynamical systems are
capable of both attaining and maintaining criticality in the presence of
Keplerian shear and other complications. The resulting power-law spectrum of
flare energies in the equilibrium attractor state is found to be nearly
universal in magnetized disks. This finding indicates that magnetic
reconnection and flaring in the inner regions of circumstellar disks occur in a
manner similar to activity on stellar surfaces.",2308.06650v1
2023-09-08,Magnetic Fields and Velocity Gradients in L1551: The Role of Stellar Feedback,"Magnetic fields play a crucial role in star formation, yet tracing them
becomes particularly challenging, especially in the presence of outflow
feedback in protostellar systems. We targeted the star-forming region L1551,
notable for its apparent outflows, to investigate the magnetic fields. These
fields were probed using polarimetry observations from the Planck satellite at
353 GHZ/849 $\mu$m, the SOFIA/HAWC+ measurement at 214 $\mu$m, and the
JCMT/SCUPOL 850 $\mu$m survey. Consistently, all three measurements show that
the magnetic fields twist towards the protostar IRS 5. Additionally, we
utilized the Velocity Gradients Technique (VGT) on the $^{12}$CO (J = 1-0)
emission data to distinguish the magnetic fields directly associated with the
protostellar outflows. These were then compared with the polarization results.
Notably, in the outskirts of the region, these measurements generally align.
However, as one approaches the center of IRS 5, the measurements tend to yield
mostly perpendicular relative orientations. This suggests that the outflows
might be dynamically significant from a scale of approximately $\sim0.2$~pc,
causing the velocity gradient to change direction by 90 degrees. Furthermore,
we discovered that the polarization fraction $p$ and the total intensity $I$
correlate as $p \propto I^{-\alpha}$. Specifically, $\alpha$ is approximately
$1.044\pm0.06$ for SCUPOL and around $0.858\pm0.15$ for HAWC+. This indicates
that the outflows could significantly impact the alignment of dust grains and
magnetic fields in the L1551 region.",2309.04173v2
2023-09-20,Evaluating Gilbert Damping in Magnetic Insulators from First Principles,"Magnetic damping has a significant impact on the performance of various
magnetic and spintronic devices, making it a long-standing focus of research.
The strength of magnetic damping is usually quantified by the Gilbert damping
constant in the Landau-Lifshitz-Gilbert equation. Here we propose a
first-principles based approach to evaluate the Gilbert damping constant
contributed by spin-lattice coupling in magnetic insulators. The approach
involves effective Hamiltonian models and spin-lattice dynamics simulations. As
a case study, we applied our method to Y$_3$Fe$_5$O$_{12}$, MnFe$_2$O$_4$ and
Cr$_2$O$_3$. Their damping constants were calculated to be $0.8\times10^{-4}$,
$0.2\times10^{-4}$, $2.2\times 10^{-4}$, respectively at a low temperature. The
results for Y$_3$Fe$_5$O$_{12}$ and Cr$_2$O$_3$ are in good agreement with
experimental measurements, while the discrepancy in MnFe$_2$O$_4$ can be
attributed to the inhomogeneity and small band gap in real samples. The
stronger damping observed in Cr$_2$O$_3$, compared to Y$_3$Fe$_5$O$_{12}$,
essentially results from its stronger spin-lattice coupling. In addition, we
confirmed a proportional relationship between damping constants and the
temperature difference of subsystems, which had been reported in previous
studies. These successful applications suggest that our approach serves as a
promising candidate for estimating the Gilbert damping constant in magnetic
insulators.",2309.11152v1
2023-10-04,A Database of Magnetic and Thermodynamic Properties of Confined And Eruptive Solar Flares,"Solar flares sometimes lead to coronal mass ejections that directly affect
the Earth's environment. However, a large fraction of flares, including on
solar-type stars, are confined flares. What are the differences in physical
properties between confined and eruptive flares? For the first time, we
quantify thermodynamic and magnetic properties of hundreds of confined and
eruptive flares of GOES class C5.0 and above, 480 flares total. We first
analyze large flares of GOES class M1.0 and above observed by the Solar
Dynamics Observatory (SDO): 216 flares total, including 103 eruptive and 113
confined flares, from 2010 until 2016 April, we then look at the entire dataset
above C5.0 of 480 flares. We compare GOES X-ray thermodynamic flare properties,
including peak temperature and emission measure, and active-region and
flare-ribbon magnetic field properties, including reconnected magnetic flux and
peak reconnection rate. We find that for fixed peak X-ray flux, confined and
eruptive flares have similar reconnection fluxes; however, for fixed peak X-ray
flux confined flares have on average larger peak magnetic reconnection rates,
are more compact, and occur in larger active regions than eruptive flares.
These findings suggest that confined flares are caused by reconnection between
more compact, stronger, lower lying magnetic-fields in larger active regions
that reorganizes smaller fraction of these regions' fields. This reconnection
proceeds at faster rates and ends earlier, potentially leading to more
efficient flare particle acceleration in confined flares.",2310.02878v3
2023-10-06,An Analytic Model For Magnetically-Dominated Accretion Disks,"Recent numerical cosmological
radiation-magnetohydrodynamic-thermochemical-star formation simulations have
resolved the formation of quasar accretion disks with Eddington or
super-Eddington accretion rates onto supermassive black holes (SMBHs) down to a
few hundred gravitational radii. These 'flux-frozen' and hyper-magnetized disks
appear to be qualitatively distinct from classical $\alpha$ disks and
magnetically-arrested disks: the midplane pressure is dominated by toroidal
magnetic fields with plasma $\beta \ll 1$ powered by advection of magnetic flux
from the interstellar medium (ISM), and they are super-sonically and
trans-Alfvenically turbulent with cooling times short compared to dynamical
times yet remain gravitationally stable owing to magnetic support. In this
paper, we present a simple analytic similarity model for such disks. For
reasonable assumptions, the model is entirely specified by the boundary
conditions (inflow rate at the BH radius of influence [BHROI]). We show that
the scalings from this model are robust to various detailed assumptions, agree
remarkably well with the simulations (given their simplicity), and demonstrate
the self-consistency and gravitational stability of such disks even in the
outer accretion disk (approaching the BHROI) at hyper-Eddington accretion
rates.",2310.04507v2
2023-10-09,Mapping of Spin-Wave Transport in Thulium Iron Garnet Thin Films Using Diamond Quantum Microscopy,"Spin waves, collective dynamic magnetic excitations, offer crucial insights
into magnetic material properties. Rare-earth iron garnets offer an ideal
spin-wave (SW) platform with long propagation length, short wavelength,
gigahertz frequency, and applicability to magnon spintronic platforms. Of
particular interest, thulium iron garnet (TmIG) has attracted a huge interest
recently due to its successful growth down to a few nanometers, observed
topological Hall effect and spin orbit torque-induced switching effects.
However, there is no direct spatial measurement of its SW properties. This work
uses diamond nitrogen vacancy (NV) magnetometry in combination with SW
electrical transmission spectroscopy to study SW transport properties in TmIG
thin films. NV magnetometry allows probing spin waves at the sub-micrometer
scale, seen by the amplification of the local microwave magnetic field due to
the coupling of NV spin qubits with the stray magnetic field produced by the
microwave-excited spin waves. By monitoring the NV spin resonances, the SW
properties in TmIG thin films are measured as function of the applied magnetic
field, including their amplitude, decay length (~ 50 um), and wavelength (0.8 -
2 um). These results pave the way for studying spin qubit-magnon interactions
in rare-earth magnetic insulators, relevant to quantum magnonics applications.",2310.06188v4
2023-10-29,Bridging Scales in Black Hole Accretion and Feedback: Magnetized Bondi Accretion in 3D GRMHD,"Fueling and feedback couple supermassive black holes (SMBHs) to their host
galaxies across many orders of magnitude in spatial and temporal scales, making
this problem notoriously challenging to simulate. We use a multi-zone
computational method based on the general relativistic magneto-hydrodynamic
(GRMHD) code KHARMA that allows us to span $7$ orders of magnitude in spatial
scale, to simulate accretion onto a non-spinning SMBH from an external medium
with Bondi radius $R_B\approx 2\times 10^5 \,GM_\bullet/c^2$, where $M_\bullet$
is the SMBH mass. For the classic idealized Bondi problem, spherical gas
accretion without magnetic fields, our simulation results agree very well with
the general relativistic analytic solution. Meanwhile, when the accreting gas
is magnetized, the SMBH magnetosphere becomes saturated with a strong magnetic
field. The density profile varies as $\sim r^{-1}$ rather than $r^{-3/2}$ and
the accretion rate $\dot{M}$ is consequently suppressed by over 2 orders of
magnitude below the Bondi rate $\dot{M}_B$. We find continuous energy feedback
from the accretion flow to the external medium at a level of
$\sim10^{-2}\dot{M}c^2 \sim 5\times 10^{-5} \dot{M}_B c^2$. Energy transport
across these widely disparate scales occurs via turbulent convection triggered
by magnetic field reconnection near the SMBH. Thus, strong magnetic fields that
accumulate on horizon scales transform the flow dynamics far from the SMBH and
naturally explain observed extremely low accretion rates compared to the Bondi
rate, as well as at least part of the energy feedback.",2310.19135v2
2023-11-30,Induced quantum magnetism in CEF singlet ground state models: Thermodynamics and excitations,"We present a comparative investigation of singlet ground state induced
magnetism for singlet, doublet and triplet excited CEF states of non-Kramers
f-electrons relevant primarily for Pr- and U- based compounds. This type of
magnetic order is of intrinsic quantum nature because it requires the
superposition of singlet ground state with excited states due to non-diagonal
matrix elements of the effective intersite exchange to generate local moments.
In contrast to conventional magnets the local moments and their ordering appear
simultaneously at the transition temperature. It is finite only if the control
parameter proportional to the ratio of exchange strength to level splitting
exceeds a critical value marking the quantum critical point of the models. We
determine the dependence of transition temperature, saturation moment,
renormalised level splitting, specific heat jumps and low-temperature
susceptibility as function of control parameter. Furthermore the temperature
dependence of these quantities is calculated for control parameters above and
below the quantum critical point and the distinction to conventional magnetism
is discussed. In addition we investigate the dynamical properties of the three
models, deriving the magnetic exciton dispersion and their critical behaviour.
In particular the conditions for true and arrested soft-mode behaviour at the
ordering wave vector are identified.",2311.18416v2
2024-01-09,Microwave magnetic excitations in U-type hexaferrite Sr$_4$CoZnFe$_{36}$O$_{60}$ ceramics,"Microwave (MW) spectra of ferrimagnetic U-hexaferrite
Sr$_4$CoZnFe$_{36}$O$_{60}$ ceramics were studied using several experimental
techniques from 100 MHz to 50 GHz at temperatures between 10 and 390 K. They
revealed 9 excitations, which exhibit remarkable temperature dependences near
the magnetic phase transitions at 145 K and 305 K. Some of them also change
under the application of a weak bias magnetic field at room temperature. Three
lowest-frequency modes seen between 200 MHz and 3 GHz were assigned to the
dynamics of the magnetic domains. The mode attributed to the natural
ferromagnetic resonance exhibits a dramatic critical slowing-down from 25 GHz
at 390 K to 5 GHz near the 305 K phase transition, and again a hardening to ~11
GHz on further cooling. The higher-frequency excitations are most likely spin
wave (magnon) modes arising from the complex ferrimagnetic structure of
Sr$_4$CoZnFe$_{36}$O$_{60}$. The high sensitivity of the MW spectra to the weak
magnetic bias field $H$ < 700 Oe at room temperature is shown to be caused by
the transformation of the polydomain magnetic structure in randomly oriented
ceramic grains to a monodomain one. The MW response measured above 2 GHz using
coplanar and microstrip lines with different electromagnetic field distribution
and sample coupling revealed the same excitations with similar temperature and
bias field dependences. It confirms the reliability of the results and proves
the effectiveness of the used MW techniques.",2401.04678v1
2024-01-25,Nucleosynthesis in magnetorotational supernovae: impact of the magnetic field configuration,"The production of heavy elements is one of the main by-products of the
explosive end of massive stars. A long sought goal is finding differentiated
patterns in the nucleosynthesis yields, which could permit identifying a number
of properties of the explosive core. Among them, the traces of the magnetic
field topology are particularly important for \emph{extreme} supernova
explosions, most likely hosted by magnetorotational effects. We investigate the
nucleosynthesis of five state-of-the-art magnetohydrodynamic models with fast
rotation that have been previously calculated in full 3D and that involve an
accurate neutrino transport (M1). One of the models does not contain any
magnetic field and synthesizes elements around the iron group, in agreement
with other CC-SNe models in literature. All other models host a strong magnetic
field of the same intensity, but with different topology. For the first time,
we investigate the nucleosynthesis of MR-SNe models with a quadrupolar magnetic
field and a 90 degree tilted dipole. We obtain a large variety of ejecta
compositions reaching from iron nuclei to nuclei up to the third r-process
peak. We assess the robustness of our results by considering the impact of
different nuclear physics uncertainties such as different nuclear masses,
$\beta^{-}$-decays and $\beta^{-}$-delayed neutron emission probabilities,
neutrino reactions, fission, and a feedback of nuclear energy on the
temperature. We find that the qualitative results do not change with different
nuclear physics input. The properties of the explosion dynamics and the
magnetic field configuration are the dominant factors determining the ejecta
composition.",2401.14402v1
2024-02-02,Skyrmions: A review on materials perspective for future electronic devices,"Recent years have witnessed an enormous rise in research interest in magnetic
skyrmions owing to their capability to improve over contemporary spintronic
devices. An overview of the various magnetic interactions responsible for the
formation of skyrmion together with distinct noncentrosymmetric and
centrosymmetric skyrmion candidates is given in this review article. The
magnetic interactions known as Dzyaloshinskii-Moriya interactions (DMI) have
been extensively studied over the years to better understand the mechanism of
skyrmions in chiral magnets that have larger skyrmion sizes. Because of their
low skyrmion size, the centrosymmetric frustrated magnets are dwelling to
skyrmions controlled by long-range interactions such as the
Ruderman-Kittel-Kasuya-Yosida interaction (RKKY), which may be useful in the
development of high-density memory devices. To lay a solid foundation for the
magnetic interactions involved in skyrmion formations and many other special
physical properties, more research in the field of centrosymmetric skyrmions is
required. Apart from studying candidates with low skyrmion sizes, one of the
main goals for the future is to better understand the dynamics of skyrmion
using polarized magnons, which has the potential to be extremely beneficial for
spintronic applications.",2402.01503v3
2024-03-12,Fragmentation of Dense Rotation-Dominated Structures Fed by Collapsing Gravomagneto-Sheetlets and Origin of Misaligned 100 au-Scale Binaries and Multiple Systems,"The majority of stars are in binary/multiple systems. How such systems form
in turbulent, magnetized cores of molecular clouds in the presence of non-ideal
MHD effects remains relatively under-explored. Through ATHENA++-based non-ideal
MHD AMR simulations with ambipolar diffusion, we show that the collapsing
protostellar envelope is dominated by dense gravomagneto-sheetlets, a
turbulence-warped version of the classic pseuodisk produced by anisotropic
magnetic resistance to the gravitational collapse, in agreement with previous
simulations of turbulent, magnetized single-star formation. The sheetlets feed
mass, magnetic fields, and angular momentum to a Dense ROtation-Dominated
(DROD) structure, which fragments into binary/multiple systems. This DROD
fragmentation scenario is a more dynamic variant of the traditional disk
fragmentation scenario for binary/multiple formation, with dense spiral
filaments created by inhomogeneous feeding from the highly structured
larger-scale sheetlets rather than the need for angular momentum transport,
which is dominated by magnetic braking. Collisions between the dense spiraling
filaments play a key role in pushing the local magnetic Toomre parameter
$Q_\mathrm{m}$ below unity, leading to gravitational collapse and stellar
companion formation provided that the local material is sufficiently
demagnetized, with a plasma-$\beta$ of order unity or more. This mechanism can
naturally produce {\it in situ} misaligned systems on the 100-au scale, often
detected with high-resolution ALMA observations. Our simulations also highlight
the importance of non-ideal MHD effects, which affect whether fragmentation
occurs and, if so, the masses and orbital parameters of the stellar companions
formed.",2403.07777v1
2024-04-04,Dynamics and Emission Properties of Flux Ropes from Two-Temperature GRMHD Simulations with Multiple Magnetic Loops,"Flux ropes erupting from the vicinity of the black hole are thought to be a
potential model for the flares observed in Sgr\,A$^*$. In this study, we
examine the radiative properties of flux ropes that emerged from the vicinity
of the black hole. We have performed three-dimensional two-temperature General
Relativistic Magnetohydrodynamic (GRMHD) simulations of magnetized accretion
flows with alternating multiple magnetic loops, and General Relativistic
Radiation Transfer (GRRT) calculations. In GRMHD simulations, two different
sizes of initial magnetic loops are implemented. In the small loop case,
magnetic dissipation leads to a weaker excitement of magneto-rotational
instability inside the torus which generates a lower accretion rate compared to
the large loop case. However, it makes more generation of flux ropes due to
frequent reconnection by magnetic loops with different polarities. By
calculating the thermal synchrotron emission, we found that the variability of
light curves and emitting region are tightly related. At $230\,\rm GHz$ and
higher frequency, the emission from the flux ropes is relatively stronger
compared with the background, which is responsible for the filamentary
structure in the images. At lower frequencies, e.g. $43\,\rm GHz$, emission
comes from more extended regions, which have a less filamentary structure in
the image. Our study shows self-consistent electron temperature models are
essential for the calculation of thermal synchrotron radiation and the
morphology of the GRRT images. Flux ropes contribute considerable emission at
frequencies $\gtrsim 230\,\rm GHz$.",2404.03237v1
1998-02-13,Dynamics on the Edge: Star-Disk Interaction,"Our understanding of the dynamical processes which control the structure and
evolution of the interaction region between an accretion disk and the central
star is reviewed. If the central star is unmagnetized, this interaction is in
the form of a classical boundary layer. However, if the central star is
strongly magnetized, it is expected that the inner disk is truncated by the
stellar field, and the accretion flow follows the stellar field lines to the
magnetic poles. Several outstanding questions remain regarding this picture. It
is likely that numerical MHD simulations will prove essential to answering
these questions. However, in order to model the MHD turbulence, angular
momentum transport, and possible dynamo action associated with the dynamics of
star-disk interaction, fully three-dimensional simulations spanning long
dynamical times are required.",9802190v1
1997-09-29,"Spin conductance, dynamic spin stiffness and spin diffusion in itinerant magnets","We discuss analogies between the charge- and spin response functions of
itinerant magnets. We show that the spin-analog of the charge stiffness is not
given by the usual spin stiffness rho_s, but by the dynamic spin stiffness D_s,
which is obtained from the dynamic spin conductance G_s (omega) in the limit of
vanishing frequency omega. The low-frequency behavior of G_s (omega) is used to
define ideal spin conductors, normal spin conductors, and spin insulators.
Assuming diffusive spin dynamics, we show that the spin diffusion coefficient
is proportional to lim_{omega rightarrow 0} {Re} {G}_s (omega). We exploit this
fact to develop a new extrapolation scheme for the spin diffusion coefficient
in the paramagnetic phase of the Hubbard model.",9709316v2
1997-11-23,Dynamical universality classes of the superconducting phase transition,"We present a finite temperature Monte Carlo study of the XY-model in the
vortex representation, and study its dynamical critical behavior in two limits.
The first neglects magnetic field fluctuations, corresponding to the absence of
screening, which should be a good approximation in high $T_c$ superconductors
($\kappa\to \infty$) except extremely close to the critical point. Here, from
finite size scaling of the linear resistivity we find the dynamical critical
exponent of the vortex motion to be $z\approx 1.5$. The second limit includes
magnetic field fluctuations in the strong screening limit ($\kappa\to 0$)
corresponding to the true asymptotic inverted XY critical regime, where we find
the unexpectedly large value $z\approx 2.7$. We compare these results, obtained
from dissipative dynamics in the vortex representation, with the universality
class of the corresponding model in the phase representation with propagating
(spin wave) modes. We also discuss the effect of disorder and the relevance of
our results for experiments.",9711236v1
1998-04-07,Peierls Dimerization with Non-Adiabatic Spin-Phonon Coupling,"We study the magnetic properties of a frustrated Heisenberg spin chain with a
dynamic spin-phonon interaction. By Lanczos diagonalization, preserving the
full lattice dynamics, we explore the non-adiabatic regime with phonon
frequencies comparable to the exchange coupling energy which is e.g. the
relevant limit for the spin-Peierls compound $CuGeO_3$. When compared to the
static limit of an alternating spin chain the magnetic properties are strongly
renormalized due to the coupled dynamics of spin and lattice degrees of
freedom. The magnitude of the spin triplet excitation gap changes from a strong
to a weak dimerization dependence with increasing phonon frequencies implying
the necessity to include dynamic effects in an attempt for a quantitative
description of the spin-Peierls state.",9804085v2
1999-12-20,Spin dynamics simulations - a powerful method for the study of critical dynamics,"Spin-dynamics techniques can now be used to study the deterministic
time-dependent behavior of magnetic systems containing over 10^5 spins with
quite good accuracy. This approach will be introduced, including the
theoretical foundations of the methods of analysis. Then newly developed,
improved techniques based upon Suzuki-Trotter decomposition methods will be
described. The current ``state-of-the-art'' will be evaluated with specific
examples drawn from data on simple magnetic models. The examination of dynamic
critical behavior will be highlighted but the extraction of information about
excitations at low temperatures will be included.",9912375v1
2000-04-05,Low-frequency spin dynamics in the orthorombic phase of La_2CuO_4,"An effective field theory is derived that describes the low-frequency spin
dynamics in the low-temperature orthorombic phase of La_2CuO_4. Restricted to a
single CuO_2 layer the effective theory is a simple generalization of the
relativistic nonlinear sigma model to include all spin interactions allowed by
symmetry. Incorporating a weak interlayer interaction leads to two coupled
nonlinear sigma models which provide an efficient description of the complete
bilayer dynamics. Particular attention is paid to the weak-ferromagnetic and
spin-flop transitions induced by external magnetic fields. The main features of
the observed (covert) weak ferromagnetism are thus accounted for in a
straightforward manner but some of the finer theoretical predictions would
require further experimental investigation. The derived framework is also
suitable for the study of the structure and dynamics of magnetic domains in
undoped La_2CuO_4.",0004071v1
2002-02-15,Novel dynamical effects and persistent memory in phase separated manganites,"The time dependent response of the magnetic and transport properties of
Fe-doped phase separated (PS) manganite La$_{0.5}$Ca$_{0.5}$MnO$_{3}$ is
reported. The nontrivial coexistence of ferromagnetic (FM) and non FM regions
induces a slow dynamics which leads to time relaxation and cooling rate
dependence within the PS regime. This dynamics influences drastically on
physical properties. On one hand, metallic like behavior, assumed to be a
fingerprint of percolation, can be also observed before the FM phase percolates
as a result of dynamical contributions. On the other one, two novel effects for
the manganites are reported, namely the rejuvenation of the resistivity after
ageing, and a persistent memory of low magnetic fields ($<$ 1 T), imprinted in
the amount of the FM phase. As a distinctive fact, this memory can be recovered
through transport measurements.",0202263v1
2003-02-13,Optimization and self-organized criticality in a magnetic system,"We propose a kind of Bak-Sneppen dynamics as a general optimization technique
to treat magnetic systems. The resulting dynamics shows self-organized
criticality with power law scaling of the spatial and temporal correlations. An
alternative method of the extremal optimization is also analyzed here. We
provided a numerical confirmation that, for any possible value of its free
parameter $\tau$, the extremal optimization dynamics exhibits a non-critical
behavior with an infinite spatial range and exponential decay of the
avalanches. Using the chiral clock model as our test system, we compare the
efficiency of the two dynamics with regard to their abilities to find the
system's ground state.",0302260v1
2003-02-22,The dynamics of quantum phases in a spinor condensate,"We discuss the quantum phases and their diffusion dynamics in a spinor-1
atomic Bose-Einstein condensate. For ferromagnetic interactions, we obtain the
exact ground state distribution of the phases associated with the total atom
number ($N$), the total magnetization (${\cal M}$), and the alignment (or
hypercharge) ($Y$) of the system. The mean field ground state is stable against
fluctuations of atom numbers in each of the spin components, and the phases
associated with the order parameter for each spin components diffuse while
dynamically recover the two broken continuous symmetries [U(1) and SO(2)] when
$N$ and ${\cal M}$ are conserved as in current experiments. We discuss the
implications to the quantum dynamics due to an external (homogeneous) magnetic
field. We also comment on the case of a spinor-1 condensate with
anti-ferromagnetic interactions.",0302464v1
2003-10-15,Quasiparticle Relaxation Dynamics in Two Distinct Gap Structures: La$_{0.67}$Ca$_{0.33}$MnO$_{3}$ and LaMnO$_{3}$,"The spin and quasiparticle relaxation dynamics in
La$_{0.67}$Ca$_{0.33}$MnO$_{3}$ and LaMnO$_{3}$ single crystals and thin films
is investigated as a function of temperature and magnetic field by pump-probe
spectroscopy. The unusually slow spin-lattice relaxation dynamics is governed
by the temperature- and magnetic field- dependent pseudogap in
La$_{0.67}$Ca$_{0.33}$MnO$_{3}$, and by the temperature-independent Jahn-Teller
gap in LaMnO$_{3}$. Our results show that the coupled dynamics of charge, spin
and lattice is strongly correlated with the distinct gap structures in these
manganites.",0310359v1
2005-01-06,Electron Spin Dynamics and Hyperfine Interactions in Fe/Al_0.1Ga_0.9As/GaAs Spin Injection Heterostructures,"We have studied hyperfine interactions between spin-polarized electrons and
lattice nuclei in Al_0.1Ga_0.9As/GaAs quantum well (QW) heterostructures. The
spin-polarized electrons are electrically injected into the semiconductor
heterostructure from a metallic ferromagnet across a Schottky tunnel barrier.
The spin-polarized electron current dynamically polarizes the nuclei in the QW,
and the polarized nuclei in turn alter the electron spin dynamics. The
steady-state electron spin is detected via the circular polarization of the
emitted electroluminescence. The nuclear polarization and electron spin
dynamics are accurately modeled using the formalism of optical orientation in
GaAs. The nuclear spin polarization in the QW is found to depend strongly on
the electron spin polarization in the QW, but only weakly on the electron
density in the QW. We are able to observe nuclear magnetic resonance (NMR) at
low applied magnetic fields on the order of a few hundred Oe by electrically
modulating the spin injected into the QW. The electrically driven NMR
demonstrates explicitly the existence of a Knight field felt by the nuclei due
to the electron spin.",0501096v1
2005-06-24,Critical dynamics and effective exponents of magnets with extended impurities,"We investigate the asymptotic and effective static and dynamic critical
behavior of (d=3)-dimensional magnets with quenched extended defects,
correlated in $\epsilon_d$ dimensions (which can be considered as the
dimensionality of the defects) and randomly distributed in the remaining
$d-\epsilon_d$ dimensions. The field-theoretical renormalization group
perturbative expansions being evaluated naively do not allow for the reliable
numerical data. We apply the Chisholm-Borel resummation technique to restore
convergence of the two-loop expansions and report the numerical values of the
asymptotic critical exponents for the model A dynamics. We discuss different
scenarios for static and dynamic effective critical behavior and give values
for corresponding non-universal exponents.",0506644v1
2006-10-17,Inhomogeneous phase separation and domain wall dynamics in the orthorhombically distorted La_{0.87}MnO_x,"We observe the interesting magnetization and resistivity results in the
orthorhombically distorted self doped manganite, La_{0.87}MnO_x. A significant
irreversibility between zero-field-cooled and field-cooled resistivities as
well as magnetization data is clearly observed up to a temperature which is
well above the both paramagnetic to ferromagnetic (T_c) and metal to insulator
transition (T_{MI}) temperatures. In the ferromagnetically ordered and metallic
states the magnetotransport results suggest the influence of domain wall
dynamics. Interesting scenario of memory effects through the domain wall
dynamics is strikingly observed in the relaxation of resistivity in a limited
temperature region. The characteristic features of magnetotransport behaviour
are suggested duo to the co-existence of structurally driven inhomogeneous
phase separation and domain wall dynamics.",0610464v1
2007-03-21,Ultrafast Vortex-Core Reversal Dynamics in Ferromagnetic Nanodots,"To verify the exact underlying mechanism of ultrafast vortex-core reversal as
well as the vortex state stability we conducted numerical calculations of the
dynamic evolution of magnetic vortices in Permalloy cylindrical nanodots under
an oscillating in-plane magnetic field over a wide range of the frequency and
amplitude. The calculated results reveal different kinds of the non-trivial
dynamic responses of vortices to the driving external field. In particular, the
results offer insight into the 10 ps scale underlying physics of the ulrafast
vortex-core reversals driven by small amplitude (~10 Oe) in-plane fields. This
work also provides fundamentals of how to manipulate effectively the dynamical
switching of the vortex-core orientation.",0703538v1
2000-06-07,Dynamical Symmetry Breaking and Magnetic Confinement in QCD,"We present a gauge independent method to construct the effective action of
QCD, and calculate the one loop effective action of $SU(2)$ QCD in an arbitrary
constant background field. Our result establishes the existence of a dynamical
symmetry breaking by demonstrating that the effective potential develops a
unique and stable vacuum made of the monopole condensation in one loop
approximation. This provides a strong evidence for the magnetic confinement of
color through the dual Meissner effect in the non-Abelian gauge theory. The
result is obtained by separating the topological degrees which describe the
non-Abelian monopoles from the dynamical degrees of the gauge potential, and
integrating out all the dynamical degrees of QCD. We present three independent
arguments to support our result.",0006051v3
2005-01-01,Equatorial and related non-equilibrium states in magnetization dynamics of ferromagnets: Generalization of Suhl's spin-wave instabilities,"We investigate the nonlinear dynamics underlying the evolution of a 2-D
nanoscale ferromagnetic film with uniaxial anisotropy in the presence of
perpendicular pumping. Considering the associated Landau-Lifshitz spin
evolution equation with Gilbert damping together with Maxwell equation for the
demagnetization field, we study the dynamics in terms of the stereographic
variable. We identify several new fixed points for suitable choice of external
field in a rotating frame of reference. In particular, we identify explicit
equatorial and related fixed points of the spin vector in the plane transverse
to the anisotropy axis when the pumping frequency coincides with the amplitude
of the static parallel field. We then study the linear stability of these novel
fixed points under homogeneous and spin wave perturbations and obtain a
generalized Suhl's instability criterion, giving the condition for exponential
growth of P-modes under spin wave perturbations. Two parameter phase diagrams
(in terms of amplitudes of static parallel and oscillatory perpendicular
magnetic fields) for stability are obtained, which differ qualitatively from
those for the conventional ferromagnetic resonance near thermal equilibrium and
are amenable to experimental tests.",0501002v2
2007-08-15,Density-Profile Processes Describing Biological Signaling Networks: Almost Sure Convergence to Deterministic Trajectories,"We introduce jump processes in R^k, called density-profile process, to model
biological signaling networks. They describe the macroscopic evolution of
finite-size spin-flip models with k types of spins interacting through a
non-reversible Glauber dynamics. We focus on the the k-dimensional
empirical-magnetization vector in the thermodynamic limit, and prove that,
within arbitrary finite time-intervals, its path converges almost surely to a
deterministic trajectory determined by a first-order (non-linear) differential
equation. As parameters of the spin-flip dynamics change, the associated
dynamical system may go through bifurcations, associated to phase transitions
in the statistical mechanical setting. We present a simple example of spin-flip
stochastic model leading to a dynamical system with Hopf and pitchfork
bifurcations; depending on the parameter values, the magnetization random path
can either converge to a unique stable fixed point, converge to one of a pair
of stable fixed points, or asymptotically evolve close to a deterministic orbit
in R^k.",0708.2044v1
2007-11-27,Non-linear dynamics and two-dimensional solitons for spin $ S=1$ ferromagnets with biquadratic exchange,"We develop a consistent semiclassical theory of spin dynamics for an
isotropic ferromagnet with a spin $ S=1$ taking into consideration both
bilinear and biquadratic over spin operators exchange interaction. For such
non-Heisenberg magnets, a peculiar class of spin oscillations and waves, for
which the quantum spin expectation value $ {\rm {\bf m}}=<{\rm {\bf S}}>$ does
not change it direction, but changes in length, is presented. Such
``longitudinal'' excitations do not exist in regular magnets, dynamics of which
are described in terms of the Landau-Lifshitz equation or by means of the spin
Heisenberg Hamiltonian. We demonstrate the presence of non-linear uniform
oscillations and waves, as well as self-localized dynamical excitations
(solitons) with finite energy. A possibility of excitation of such oscillations
by ultrafast laser pulse is discussed.",0711.4285v1
2008-12-08,A Diffusive Strategic Dynamics for Social Systems,"We propose a model for the dynamics of a social system, which includes
diffusive effects and a biased rule for spin-flips, reproducing the effect of
strategic choices. This model is able to mimic some phenomena taking place
during marketing or political campaigns. Using a cost function based on the
Ising model defined on the typical quenched interaction environments for social
systems (Erdos-Renyi graph, small-world and scale-free networks), we find, by
numerical simulations, that a stable stationary state is reached, and we
compare the final state to the one obtained with standard dynamics, by means of
total magnetization and magnetic susceptibility. Our results show that the
diffusive strategic dynamics features a critical interaction parameter strictly
lower than the standard one. We discuss the relevance of our findings in social
systems.",0812.1435v3
2008-12-29,Vortex dynamics in confined stratified conditions,"We report on linear spin dynamics in the vortex state of the Permalloy dots
subjected to stratified (magnetic) field. We demonstrate experimentally and by
simulations the existence of two distinct dynamic regimes corresponding to the
vortex stable and metastable states. Breaking cylindrical symmetry leads to
unexpected eigenmodes frequency splitting in the stable state and appearance of
new eigenmodes in the metastable state above the vortex nucleation field.
Dynamic response in the metastable state strongly depends on relative
orientation of the external rf pumping and the bias magnetic fields. These
findings may be relevant for different vortex states in confined and stratified
conditions.",0812.4954v1
2009-01-30,Theory of magnetism with temporal disorder applied to magnetically doped ZnO,"A dynamic model of the asymmetric Ising glass is presented: an Ising model
with antiferromagnet bonds with probabilities q arranged at random in a
ferromagnetic matrix. The dynamics is introduced by changing the arrangement of
the antiferromagnetic bonds after n Monte Carlo steps but keeping the same
value of q and spin configuration. In the region where there is a second order
transition between the ferromagnetic and paramagnetic states the dynamic
behaviour follows that expected for motional narrowing and reverts to the
static behaviour only for large n. There is a different dynamic behaviour where
there is a first order transition between the ferromagnetic and spin glass
states where it shows no effects of motional narrowing. The implications of
this are discussed. This model is devised to explain the properties of doped
ZnO where the magnetisation is reduced when the exchange interactions change
with time.",0901.4947v1
2009-03-06,Kondo effect and conductance of nanocontacts with magnetic impurities,"We study the impact of dynamical correlations on the electronic structure and
coherent transport properties of Cu nanocontacts hosting a single magnetic
impurity (Ni,Co,Fe) in the contact region. The strong dynamical correlations of
the impurity 3d-electrons are fully taken into account by combining density
functional calculations with a dynamical treatment of the impurity 3d-shell in
the one-crossing approximation. We find that dynamical correlations give rise
to the Kondo effect and lead to Fano features in the coherent transport
characteristics similar to those observed in related experiments.",0903.1274v2
2009-05-14,Dynamical signature of a domain phase transition in a perpendicularly-magnetized ultrathin film,"Domain phases in ultrathin Fe/Ni/W(110) films with perpendicular anisotropy
have been studied using the ac magnetic susceptibility. Dynamics on time scales
of minutes to hours were probed by quenching the system from high temperature
to the stripe phase region, and varying the constant rate of temperature
increase as the susceptibility traces were measured. The entire susceptibility
peak is observed to relax slowly along the temperature axis, with the peak
temperature increasing as the rate of heating is decreased. This is precisely
opposite to what would happen if this slow relaxation was driven by changes in
the domain density within the stripe phase. The data are instead consistent
with a simple model for the removal of a significant density of pattern defects
and curvature trapped in the quench from high temperature. A quantitative
analysis confirms that the relaxation dynamics are consistent with the
mesoscopic rearrangement of domains required to remove pattern defects, and
that the experiment constitutes a ""dynamical"" observation of the phase
transition from a high temperature, positionally disordered phase to the low
temperature, ordered stripe phase.",0905.2339v2
2009-10-27,Contribution of a time-dependent metric on the dynamics of an interface between two immiscible electro-magnetically controllable Fluids,"We consider the case of a deformable material interface between two
immiscible moving media, both of them being magnetiable. The time dependence of
the metric at the interface introduces a non linear term, proportional to the
mean curvature, in the surface dynamical equations of mass momentum and angular
momentum. We take into account the effects of that term also in the singular
magnetic and electric fields inside the interface which lead to the existence
of currents and charges densities through the interface, from the derivation of
the Maxwell equations inside both bulks and the interface. Also, we give the
expression for the entropy production and of the different thermo-dynamical
fluxes. Our results enlarge previous results from other theories where the
specific role of the time dependent surface metric was insufficiently stressed.",0910.5020v1
2010-07-13,Asymmetric optical nuclear spin pumping in a single uncharged quantum dot,"A highly asymmetric dynamic nuclear spin pumping is observed in a single self
assembled InGaAs quantum dot subject to resonant optical pumping of the neutral
exciton transition leading to a large maximum polarization of 54%. This dynamic
nuclear polarization is found to be much stronger following pumping of the
higher energy Zeeman state. Time-resolved measurements allow us to directly
monitor the buildup of the nuclear spin polarization in real time and to
quantitatively study the dynamics of the process. A strong dependence of the
observed dynamic nuclear polarization on the applied magnetic field is found,
with resonances in the pumping efficiency being observed for particular
magnetic fields. We develop a model that fully accounts for the observed
behaviour, where the pumping of the nuclear spin system is due to
hyperfine-mediated spin flip transitions between the states of the neutral
exciton manifold.",1007.2145v2
2010-09-01,Single- and few-electron dynamic quantum dots in a perpendicular magnetic field,"We present experimental studies of the current pumped through a dynamic
quantum dot over a wide range of magnetic fields. At low fields we observe
repeatable structure indicating increased confinement of the electrons in the
dynamic dot. At higher fields (B>3T), we observe structure which changes
markedly from device to device suggesting that in this regime the transport is
sensitive to local disorder. The results are significant for the development of
dynamic quantum dot pumps as quantum standards of electrical current.",1009.0203v1
2010-09-24,Photoinduced dynamics in quantum rings,"We investigate the spin-dependent dynamical response of a semiconductor
quantum ring with a spin orbit interaction (SOI) upon the application of a
single and two linearly polarized, picosecond, asymmetric electromagnetic
pulses in the presence of a static magnetic flux. We find that the
pulse-generated electric dipole moment is spin dependent. It is also shown that
the SOI induces an extra SU(2) effective flux in addition to the static
external magnetic flux which is reflected in an additional periodicity of the
spin-dependent dipole moment. Furthermore, the pulses may induce a net
dynamical charge currents (CC) and dynamical spin currents (SC) when the
clockwise and anti-clockwise symmetry of the carrier is broken upon the pulse
application.",1009.4809v1
2010-11-12,Irradiated bilayer graphene,"We describe the gated bilayer graphene system when it is subjected to intense
terahertz frequency electromagnetic radiation. We examine the electron band
structure and density of states via exact diagonalization methods within
Floquet theory. We find that dynamical states are induced which lead to
modification of the band structure. We first examine the situation where there
is no external magnetic field. In the unbiased case, dynamical gaps appear in
the spectrum which manifest as dips in the density of states. For finite
interlayer bias (where a static gap is present in the band structure of
unirradiated bilayer graphene), dynamical states may be induced in the static
gap. These states can show a high degree of valley polarization. When the
system is placed in a strong magnetic field, the radiation induces coupling
between the Landau levels which allows dynamical levels to exist. For strong
fields, this means the Landau levels are smeared to form a near-continuum of
states.",1011.3043v1
2011-03-16,Collective beating of artificial microcilia,"We combine technical, experimental and theoretical efforts to investigate the
collective dynamics of artificial microcilia in a viscous fluid. We take
advantage of soft-lithography and colloidal self-assembly to devise microcapets
made of hundreds of slender magnetic rods. This novel experimental setup is
used to investigate the dynamics of extended cilia arrays driven by a
precessing magnetic field. Whereas the dynamics of an isolated cilium is a
rigid body rotation, collective beating results in a symmetry breaking of the
precession patterns. The trajectories of the cilia are anisotropic and
experience a significant structural evolution as the actuation frequency
increases. We present a minimal model to account for our experimental findings
and demonstrate how the global geometry of the array imposes the shape of the
trajectories via long range hydrodynamic interactions.",1103.3122v1
2011-06-06,Simulating cyclotron-Bloch dynamics of a charged particle in a 2D lattice by means of cold atoms in driven quasi 1D optical lattices,"Quantum dynamics of a charged particle in a 2D lattice subject to magnetic
and electric fields is a rather complicated interplay between cyclotron
oscillations (the case of vanishing electric field) and Bloch oscillations
(zero magnetic field), details of which has not yet been completely understood.
In the present work we suggest to study this problem by using cold atoms in
optical lattices. We introduce a 1D model which can be easily realized in
laboratory experiments with quasi 1D optical lattices and show that this model
captures many features of the cyclotron-Bloch dynamics of the quantum particle
in 2D square lattices.",1106.0945v1
2011-06-16,Non-Arrhenius relaxation of the Heisenberg model with dipolar and anisotropic interactions,"The dynamical properties of a 2D Heisenberg model with dipolar interactions
and perpendicular anisotropy are studied using Monte Carlo simulations in two
different ordered regions of the equilibrium phase diagram. We find a
temperature defining a dynamical transition below which the relaxation suddenly
slows down and the system aparts from the typical Arrhenius relaxation to a
Vogel-Fulcher-Tamann law. This anomalous behavior is observed in the scaling of
the magnetic relaxation and may eventually lead to a freezing of the system.
Through the analysis of the domain structures we explain this behavior in terms
of the domains dynamics. Moreover, we calculate the energy barriers
distribution obtained from the data of the magnetic viscosity. Its shape
supports our comprehension of both, the Vogel-Fulcher-Tamann dynamical slowing
down and the freezing mechanism.",1106.3301v1
2011-07-21,A nonstationary nonparametric Bayesian approach to dynamically modeling effective connectivity in functional magnetic resonance imaging experiments,"Effective connectivity analysis provides an understanding of the functional
organization of the brain by studying how activated regions influence one
other. We propose a nonparametric Bayesian approach to model effective
connectivity assuming a dynamic nonstationary neuronal system. Our approach
uses the Dirichlet process to specify an appropriate (most plausible according
to our prior beliefs) dynamic model as the ""expectation"" of a set of plausible
models upon which we assign a probability distribution. This addresses model
uncertainty associated with dynamic effective connectivity. We derive a Gibbs
sampling approach to sample from the joint (and marginal) posterior
distributions of the unknowns. Results on simulation experiments demonstrate
our model to be flexible and a better candidate in many situations. We also
used our approach to analyzing functional Magnetic Resonance Imaging (fMRI)
data on a Stroop task: our analysis provided new insight into the mechanism by
which an individual brain distinguishes and learns about shapes of objects.",1107.4181v1
2012-02-06,Spin Dynamics at Very Low Temperature in Spin Ice Dy$_2$Ti$_2$O$_7$,"We have performed AC susceptibility and DC magnetic relaxation measurements
on the spin ice system Dy$_2$Ti$_2$O$_7$ down to 0.08 K. The relaxation time of
the magnetization has been estimated below 2 K down to 0.08 K. The spin
dynamics of Dy$_2$Ti$_2$O$_7$ is well described by using two relaxation times
($\tau_{\rm S}$ (short time) and $\tau_{\rm L}$ (long time)). Both $\tau_{\rm
S}$ and $\tau_{\rm L}$ increase on cooling. Assuming the Arrhenius law in the
temperature range 0.5-1 K, we obtained an energy barrier of 9 K. Below 0.5 K,
both $\tau_{\rm S}$ and $\tau_{\rm L}$ show a clear deviation from the thermal
activated dynamics toward temperature independent relaxation, suggesting a
quantum dynamics.",1202.1107v1
2012-03-29,Theory of spin dynamics of magnetic adatoms traced by time-resolved scanning tunneling spectroscopy,"The inelastic scanning tunneling microscopy (STM) has been shown recently
(Loth et al. Science 329, 1628 (2010)) to be extendable as to access the
nanosecond, spin-resolved dynamics of magnetic adatoms and molecules. Here we
analyze theoretically this novel tool by considering the time-resolved spin
dynamics of a single adsorbed Fe atom excited by a tunneling current pulse from
a spin-polarized STM tip. The adatom spin-configuration can be controlled and
probed by applying voltage pulses between the substrate and the spin-polarized
STM tip. We demonstrate how, in a pump-probe manner, the relaxation dynamics of
the sample spin is manifested in the spin-dependent tunneling current. Our
model calculations are based on the scattering theory in a wave-packet
formulation. The scheme is nonpertubative and hence, is valid for all voltages.
The numerical results for the tunneling probability and the conductance are
contrasted with the prediction of simple analytical models and compared with
experiments.",1203.6476v1
2012-06-22,Internal and external dynamics of antihydrogen in electric and magnetic fields of arbitrary orientation,"We have studied the motion of antihydrogen atoms in electric and magnetic
fields of arbitrary orientation. Our aim was to find an exact model of external
(centre-of-charge) and internal dynamics which would allow us to describe
antihydrogen motion in a realistic way. After analysing the kinematics of the
charge exchange reaction producing antihydrogen and of antihydrogen
deexcitation, we discuss the dynamics in cases of special fields
configurations. Finally, we propose an expression for the force acting on an
antihydrogen atom when the field configuration is arbitrary. The general
formula -- when reduced in the special field configurations -- matches the
results which we obtained independently before. The focus is both on the centre
of charge dynamics and on the accurate internal quantum characteristics.",1206.5171v1
2012-08-16,"The effect of the size of the system, aspect ratio and impurities concentration on the dynamic of emergent magnetic monopoles in artificial spin ice systems","In this work we study the dynamical properties of a finite array of
nanomagnets in artificial kagome spin ice at room temperature. The dynamic
response of the array of nanomagnets is studied by implementing a ""frustrated
celular aut\'omata"" (FCA), based in the charge model and dipolar model. The FCA
simulations, allow us to study in real-time and deterministic way, the dynamic
of the system, with minimal computational resource. The update function is
defined according to the coordination number of vertices in the system. Our
results show that for a set geometric parameters of the array of nanomagnets,
the system exhibits high density of Dirac strings and high density emergent
magnetic monopoles. A study of the effect of disorder in the arrangement of
nanomagnets is incorporated in this work.",1208.3482v1
2012-08-22,Transport through single-level systems: Spin dynamics in the nonadiabatic regime,"We investigate the Fano-Anderson model coupled to a large ensemble of spins
under the influence of an external magnetic field. The interaction between the
two spin systems is treated within a meanfield-approach and we assume an
anisotropic coupling between these two systems. By using a nonadiabatic
approach, we make no further approximations in the theoretical description of
our system, apart from the semiclassical treatment. Therewith, we can include
the short-time dynamics as well as the broadening of the energy levels arising
due to the coupling to the external electronic reservoirs. We study the spin
dynamics in the regime of low and high bias. For the infinite bias case, we
compare our results to those obtained from a simpler rate equation approach,
where higher-order transitions are neglected. We show, that these higher-order
terms are important in the range of low magnetic field. Additionally, we
analyze extensively the finite bias regime with methods from nonlinear
dynamics, and we discuss the possibility of switching of the large spin.",1208.4574v2
2012-08-31,Dynamics and hysteresis in square lattice artificial spin-ice,"Dynamical effects under geometrical frustration are considered in a model for
artificial spin ice on a square lattice in two dimensions. Each island of the
spin ice has a three-component Heisenberg-like dipole moment subject to shape
anisotropies that influence its direction. The model has real dynamics,
including rotation of the magnetic degrees of freedom, going beyond the
Ising-type models of spin ice. The dynamics is studied using a Langevin
equation solved via a second order Heun algorithm. Thermodynamic properties
such as the specific heat are presented for different couplings. A peak in
specific heat is related to a type of melting-like phase transition present in
the model. Hysteresis in an applied magnetic field is calculated for model
parameters where the system is able to reach thermodynamic equilibrium.",1208.6557v2
2012-10-01,Staggered Dynamics in Antiferromagnets by Collective Coordinates,"Antiferromagnets can be used to store and manipulate spin information, but
the coupled dynamics of the staggered field and the magnetization are very
complex. We present a theory which is conceptually much simpler and which uses
collective coordinates to describe staggered field dynamics in
antiferromagnetic textures. The theory includes effects from dissipation,
external magnetic fields, as well as reactive and dissipative current-induced
torques. We conclude that, at low frequencies and amplitudes, currents induce
collective motion by means of dissipative rather than reactive torques. The
dynamics of a one-dimensional domain wall, pinned at 90$^{\circ}$ at its ends,
are described as a driven harmonic oscillator with a natural frequency
inversely proportional to the length of the texture.",1210.0429v2
2012-10-23,Out-of-equilibrium dynamics in systems with long-range interactions: characterizing quasi-stationary states,"Systems with long-range interactions (LRI) display unusual thermodynamical
and dynamical properties that stem from the non-additive character of the
interaction potential. We focus in this work on the lack of relaxation to
thermal equilibrium when a LRI system is started out-of-equilibrium. Several
attempts have been made at predicting the so-called quasi-stationary state
(QSS) reached by the dynamics and at characterizing the resulting transition
between magnetized and non-magnetized states. We review in this work recent
theories and interpretations about the QSS. Several theories exist but none of
them has provided yet a full account of the dynamics found in numerical
simulations.",1210.6316v1
2013-04-17,Viscosity control of the dynamic self-assembly in ferromagnetic suspensions,"Recent studies of dynamic self-assembly in ferromagnetic colloids suspended
in liquid-air or liquid-liquid interfaces revealed a rich variety of dynamic
structures ranging from linear snakes to axisymmetric asters, which exhibit
novel morphology of the magnetic ordering accompanied by large-scale
hydrodynamic flows. Based on controlled experiments and first principle theory,
we argue that the transition from snakes to asters is governed by the viscosity
of the suspending liquid where less viscous liquids favor snakes and more
viscous, asters. By obtaining analytic solutions of the time-averaged
Navier-Stokes equations, we gain insights into the role of mean hydrodynamic
flows and an overall balance of forces governing the self-assembly. Our results
illustrate that the viscosity can be used to control the outcome of the dynamic
self-assembly in magnetic colloidal suspensions.",1304.4968v1
2013-04-30,A Dynamical Systems Approach to a Bianchi Type I Viscous Magnetohydrodynamic Model,"We use the expansion-normalized variables approach to study the dynamics of a
non-tilted Bianchi Type I cosmological model with both a homogeneous magnetic
field and a viscous fluid. In our model the perfect magnetohydrodynamic
approximation is made, and both bulk and shear viscous effects are retained.
The dynamical system is studied in detail through a fixed-point analysis which
determines the local sink and source behavior of the system. We show that the
fixed points may be associated with Kasner-type solutions, a flat universe FLRW
solution, and interestingly, a new solution to the Einstein Field equations
involving non-zero magnetic fields, and non-zero viscous coefficients. It is
further shown that for certain values of the bulk and shear viscosity and
equation of state parameters, the model isotropizes at late times.",1304.8042v5
2013-09-03,Detection and measurement of spin-dependent dynamics in random telegraph signals,"A quantum point contact was used to observe single-electron fluctuations of a
quantum dot in a GaAs heterostructure. The resulting random telegraph signals
(RTS) contain statistical information about the electron spin state if the
tunneling dynamics are spin-dependent. We develop a statistical method to
extract information about spin-dependent dynamics from RTS and use it to
demonstrate that these dynamics can be studied in the thermal energy regime.
The tunneling rates of each spin state are independently measured in a finite
external magnetic field. We confirm previous findings of a decrease in overall
tunneling rates for the spin excited state compared to the ground state as an
external magnetic field is increased.",1309.0567v2
2013-12-19,On Dynamics of G-Band Bright Points,"Various parameters describing the dynamics of G-band bright points (GBPs)
were derived from G-band images, acquired by the Dutch Open Telescope (DOT), of
a quiet region close to the disk center. Our study is based on four commonly
used diagnostics (effective velocity, change in the effective velocity, change
in the direction angle, and centrifugal acceleration) and two new ones (rate of
motion and time lag between recurrence of GBPs). The results concerning the
commonly used parameters are in agreement with previous studies for a
comparable spatial and temporal resolution of the used data.To broaden our
understanding of the dynamics of GBPs, two new parameters were defined: the
real displacement between their appearance and disappearance (rate of motion)
and the frequency of their recurrence at the same locations (time lag). Results
for both new parameters indicate that the locations of different dynamical
types of GBPs (stable/farther traveling or with short/long lifetimes) are bound
to the locations of more stable and long-living magnetic field concentrations.
Thus, the disappearance/reappearance of the tracked GBPs cannot be perceived as
the disappearance/reappearance of their corresponding magnetic field
concentrations.",1312.5464v1
2014-01-11,Dynamics of mobile interacting ferromagnetic films: theory and numerical implementation,"Coating two nearby bodies with thin ferromagnetic films one obtains, below
the Curie temperature, two interacting sets of magnetic domains. The dynamical
properties of the bodies in presence of this domain interaction have never been
investigated so far. In this work I derive a set of equations to simultaneously
describe both the domain evolution within the two films and the dynamics of the
coated bodies. The shape, size and mobility of the domains can be easily
controlled with an external magnetic field or properly choosing the material
properties, thus unravelling how the domain characteristics influence the
system dynamics. This can be thus of great technological relevance, providing
new means to control and actuate mechanical motion at the micro- and
nano-scale.",1401.2513v1
2014-07-16,Spin dynamics and magneto-optical response in charge-neutral tunnel-coupled quantum dots,"We model the electron and hole spin dynamics in an undoped double quantum dot
structure, considering the carrier tunneling between quantum dots. Taking into
account also the presence of an in-plane or tilted magnetic field, we provide
the simulation of magneto-optical experiments, like the time resolved Kerr
rotation measurement, which are performed currently on such structures to probe
the temporal spin dynamics. With our model, we reproduce the experimentally
observed effect of the extension of the spin polarization life time caused by
the spatial charge separation, which may occur in structures of this type.
Moreover, we provide a number of qualitative predictions concerning the
necessary conditions for observation of this effect as well as about possible
channels of its suppression, including the spin-orbit coupling, which leads to
tunneling of carriers accompanied by a spin-flip. We consider also the impact
of the magnetic field tilting, which results in an interesting spin
polarization dynamics.",1407.4529v4
2014-10-02,Stability and dynamics of magnetocapillary interactions,"Recent experiments have shown that floating ferromagnetic beads, under the
influence of an oscillating background magnetic field, can move along a
liquid-air interface in a sustained periodic locomotion [Lumay et al., Soft
Matter, 2013, 9, 2420]. Dynamic activity arises from a periodically induced
dipole-dipole repulsion between the beads acting in concert with capillary
attraction. We investigate analytically and numerically the stability and
dynamics of this magnetocapillary swimming, and explore other related topics
including the steady and periodic equilibrium configurations of two and three
beads, and bead collisions. The swimming speed and system stability depend on a
dimensionless measure of the relative repulsive and attractive forces which we
term the magnetocapillary number. An oscillatory magnetic field may stabilize
an otherwise unstable collinear configuration, and striking behaviors are
observed in fast transitions to and from locomotory states, offering insight
into the behavior and self-assembly of interface-bound micro-particles.",1410.0429v2
2014-11-21,"The Sun's interior structure and dynamics, and the solar cycle","The Sun's internal structure and dynamics can be studied with
helioseismology, which uses the Sun's natural acoustic oscillations to build up
a profile of the solar interior. We discuss how solar acoustic oscillations are
affected by the Sun's magnetic field. Careful observations of these effects can
be inverted to determine the variations in the structure and dynamics of the
Sun's interior as the solar cycle progresses. Observed variations in the
structure and dynamics can then be used to inform models of the solar dynamo,
which are crucial to our understanding of how the Sun's magnetic field is
generated and maintained.",1411.5941v2
2015-01-19,Numerical study of the depinning transition of a ferromagnetic magnetic domain wall in films,"We report first principle numerical study of domain wall (DW) depinning in
two-dimensional magnetic film, which is modeled by 2D random-field Ising system
with the dipole-dipole interaction. We observe nonconventional activation-type
motion of DW and reveal its fractal structure of DW near the depinning
transition. We determine scaling functions describing critical dynamics near
the transition and obtain universal exponents establishing connection between
thermal softening of pinning potential and critical dynamics. We observe that
tuning the strength of the dipole-dipole interaction switches DW dynamics
between two different universality classes corresponding to two distinct
dynamic regimes, motion in the random potential and that in the random force.",1501.04436v1
2015-02-10,Numerical code for multi-component galaxies: from N-body to chemistry and magnetic fields,"We present a numerical code for multi-component simulation of the galactic
evolution. Our code includes the following parts: $N$-body is used to evolve
dark matter, stellar dynamics and dust grains, gas dynamics is based on
TVD-MUSCL scheme with the extra modules for thermal processes, star formation,
magnetic fields, chemical kinetics and multi-species advection. We describe our
code in brief, but we give more details for the magneto-gas dynamics. We
present several tests for our code and show that our code have passed the tests
with a reasonable accuracy. Our code is parallelized using the MPI library. We
apply our code to study the large scale dynamics of galactic discs.",1502.02935v1
2015-03-14,Dynamic nuclear polarization as kinetically constrained diffusion,"Dynamic nuclear polarization (DNP) is a promising strategy for generating a
significantly increased non-thermal spin polarization in nuclear magnetic
resonance (NMR) applications thereby circumventing the need for strong magnetic
fields. Although much explored in recent experiments, a detailed theoretical
understanding of the precise mechanism behind DNP is so far lacking. We address
this issue by theoretically investigating solid effect DNP in a system where a
single electron is coupled to an ensemble of interacting nuclei and which can
be microscopically modelled by a quantum master equation. By deriving effective
equations of motion that govern the polarization dynamics we show analytically
that DNP can be understood as kinetically constrained spin diffusion. On the
one hand this approach provides analytical insights into the mechanism and
timescales underlying DNP. On the other hand it permits the numerical study of
large ensembles which are typically intractable from the perspective of a
quantum master equation. This paves the way for a detailed exploration of DNP
dynamics which might form the basis for future NMR applications.",1503.04357v1
2015-04-21,Tailoring the magnetodynamic properties of nanomagnets using magnetocrystalline and shape anisotropies,"Magnetodynamical properties of nanomagnets are affected by the demagnetizing
fields created by the same nanoelements. In addition, magnetocrystalline
anisotropy produces an effective field that also contributes to the spin
dynamics. In this article we show how the dimensions of magnetic elements can
be used to balance crystalline and shape anisotropies, and that this can be
used to tailor the magnetodynamic properties. We study ferromagnetic ellipses
patterned from a 10 nm thick epitaxial Fe film with dimensions ranging from 50
x 150 nm to 150 x 450 nm. The study combines ferromagnetic resonance (FMR)
spectroscopy with analytical calculations and micromagnetic simulations, and
proves that the dynamical properties can be effectively controlled by changing
the size of the nanomagnets. We also show how edge defects in the samples
influence the magnetization dynamics. Dynamical edge modes localized along the
sample edges are strongly influenced by edge defects, and this needs to be
taken into account in understanding the full FMR spectrum",1504.05404v2
2015-04-26,Amplitude modes and dynamic coexistence of competing orders in multicomponent superconductors,"We study the nonequilibrium dynamics of an electronic model with competing
spin-density-wave and unconventional superconductivity in the context of iron
pnictides. Focusing on the collisionless regime, we find that magnetic and
superconducting order parameters may coexist dynamically after a sudden quench,
even though the equilibrium thermodynamic state supports only one order
parameter. We consider various initial conditions concomitant with the phase
diagram and in a certain regime identify different oscillatory amplitude modes
with incommensurate frequencies for magnetic and superconducting responses. At
the technical level we solve the equations of motion for the electronic Green's
functions and self-consistency conditions by reducing the problem to a closed
set of Bloch equations in a pseudospin representation. For certain quench
scenarios the nonadiabatic dynamics of the pairing amplitude is completely
integrable and in principle can be found exactly.",1504.06874v2
2015-06-09,Strain induced coherent dynamics of coupled carriers and Mn spins in a quantum dot,"We report on the coherent dynamics of the spin of an individual magnetic atom
coupled to carriers in a semiconductor quantum dot which has been investigated
by resonant photoluminescence of the positively charged exciton (X+). We
demonstrate that a positively charged CdTe/ZnTe quantum dot doped with a single
Mn atom forms an ensemble of optical Lambda systems which can be addressed
independently. We show that the spin dynamics of the X+Mn complex is dominated
by the electron-Mn exchange interaction and report on the coherent dynamics of
the electron-Mn spin system that is directly observed in the time domain.
Quantum beats reflecting the coherent transfer of population between
electron-Mn spin states, which are mixed by an anisotropic strain in the plane
of the quantum dot, are clearly observed. We finally highlight that this strain
induced coherent coupling is tunable with an external magnetic field.",1506.02821v1
2015-12-01,Electric voltage generation by antiferromagnetic dynamics,"We theoretically demonstrate dc and ac electric voltage generation due to
spinmotive forces originating from domain wall motion and magnetic resonance,
respectively, in two-sublattice antiferromagnets. Our theory accounts for the
canting between the sublattice magnetizations, the nonadiabatic electron spin
dynamics, and the Rashba spin-orbit coupling, with the inter-sublattice
electron dynamics treated as a perturbation. This work suggests a new way to
observe and explore the dynamics of antiferromagnetic textures by electrical
means, an important aspect in the emerging field of antiferromagnetic
spintronics, where both manipulation and detection of antiferromagnets are
needed.",1512.00509v6
2016-03-17,Formation and plasma circulation of solar prominences,"Solar prominences are long-lived cool and dense plasma curtains in the hot
and rarefied outer solar atmosphere or corona. The physical mechanism
responsible for their formation and especially for their internal plasma
circulation has been uncertain for decades. The observed ubiquitous down flows
in quiescent prominences are difficult to interpret as plasma with high
conductivity seems to move across horizontal magnetic field lines. Here we
present three-dimensional numerical simulations of prominence formation and
evolution in an elongated magnetic flux rope as a result of in-situ plasma
condensations fueled by continuous plasma evaporation from the solar
chromosphere. The prominence is born and maintained in a fragmented, highly
dynamic state with continuous reappearance of multiple blobs and thread
structures that move mainly downward dragging along mass-loaded field lines.
The prominence plasma circulation is characterized by the dynamic balance
between the drainage of prominence plasma back to the chromosphere and the
formation of prominence plasma via continuous condensation. Plasma evaporates
from the chromosphere, condenses into the prominence in the corona, and drains
back to the chromosphere, establishing a stable chromosphere-corona plasma
cycle. Synthetic images of the modeled prominence with the Solar Dynamics
Observatory Atmospheric Imaging Assembly closely resemble actual observations,
with many dynamical threads underlying an elliptical coronal cavity.",1603.05397v1
2016-04-06,Post Markovian Dynamics of Quantum Correlations: Entanglement vs. Discord,"Dynamics of an open two-qubit system is investigated in the post-Markovian
regime, where the environments have a short-term memory. Each qubit is coupled
to separate environment which is held in its own temperature. The inter-qubit
interaction is modeled by XY-Heisenberg model in the presence of spin-orbit
interaction and inhomogeneous magnetic field. The dynamical behavior of
entanglement and discord has been considered. The results show that, quantum
discord is more robust than quantum entanglement, during the evolution. Also
the asymmetric feature of quantum discord can be monitored by introducing the
asymmetries due to inhomogeneity of magnetic field and temperature difference
between the reservoirs. By employing proper parameters of the model, it is
possible to maintain non-vanishing quantum correlation at high degree of
temperature. The results can provide a useful recipe for studying of dynamical
behavior of two-qubit systems such as trapped spin-electrons in coupled quantum
dots.",1604.01786v2
2016-06-16,Spin dynamics of counterrotating Kitaev spirals via duality,"Incommensurate spiral order is a common occurrence in frustrated magnetic
insulators. Typically, all magnetic moments rotate uniformly, through the same
wavevector. However the honeycomb iridates family Li2IrO3 shows an
incommensurate order where spirals on neighboring sublattices are
counter-rotating, giving each moment a different local environment.
Theoretically describing its spin dynamics has remained a challenge: the Kitaev
interactions proposed to stabilize this state, which arise from strong
spin-orbit effects, induce magnon umklapp scattering processes in spin-wave
theory. Here we propose an approach via a (Klein) duality transformation into a
conventional spiral of a frustrated Heisenberg model, allowing a direct
derivation of the dynamical structure factor. We analyze both Kitaev and
Dzyaloshinskii-Moriya based models, both of which can stabilize counterrotating
spirals, but with different spin dynamics, and we propose experimental tests to
identify the origin of counterrotation.",1606.05356v2
2016-09-05,Coarsening dynamics of topological defects in thin Permalloy films,"We study the dynamics of topological defects in the magnetic texture of
rectangular Permalloy thin film elements during relaxation from random
magnetization initial states. Our full micromagnetic simulations reveal complex
defect dynamics during relaxation towards the stable Landau closure domain
pattern, manifested as temporal power-law decay, with a system-size dependent
cut-off time, of various quantities. These include the energy density of the
system, and the number densities of the different kinds of topological defects
present in the system. The related power-law exponents assume non-trivial
values, and are found to be different for the different defect types. The
exponents are robust against a moderate increase in the Gilbert damping
constant and introduction of quenched structural disorder. We discuss details
of the processes allowed by conservation of the winding number of the defects,
underlying their complex coarsening dynamics.",1609.01094v1
2016-10-03,Quench dynamics of spin-imbalanced Fermi-Hubbard model in one dimension,"We study a nonequilibrium dynamics of a one-dimensional spin-imbalanced
Fermi-Hubbard model following a quantum quench of on-site interaction,
realizable, for example, in Feshbach-resonant atomic Fermi gases. We focus on
the post-quench evolution starting from the initial BCS and
FuldeFerrell-Larkin-Ovchinnikov (FFLO) ground states and analyze the
corresponding spin-singlet, spin-triplet, density-density, and
magnetization-magnetization correlation functions. We find that beyond a
light-cone crossover time, rich post-quench dynamics leads to thermalized and
pre-thermalized stationary states that display strong dependence on the initial
ground state. For initially gapped BCS state, the long-time stationary state
resembles thermalization with the effective temperature set by the initial
value of the Hubbard interaction. In contrast, while the initial gapless FFLO
state reaches a stationary pre-thermalized form, it remains far from
equilibrium. We suggest that such post-quench dynamics can be used as a
fingerprint for identification and study of the FFLO phase.",1610.00670v2
2016-12-19,Dynamical Multiferroicity,"An appealing mechanism for inducing multiferroicity in materials is the
generation of electric polarization by a spatially varying magnetization that
is coupled to the lattice through the spin-orbit interaction. Here we describe
the reciprocal effect, in which a time-dependent electric polarization induces
magnetization even in materials with no existing spin structure. We develop a
formalism for this dynamical multiferroic effect in the case for which the
polarization derives from optical phonons, and compute the strength of the
phonon Zeeman effect, which is the solid-state equivalent of the
well-established vibrational Zeeman effect in molecules, using density
functional theory. We further show that a recently observed behavior -- the
resonant excitation of a magnon by optically driven phonons -- is described by
the formalism. Finally, we discuss examples of scenarios that are not driven by
lattice dynamics and interpret the excitation of Dzyaloshinskii-Moriya-type
electromagnons and the inverse Faraday effect from the viewpoint of dynamical
multiferroicity.",1612.06331v2
2017-01-20,Fast vortex wall motion in wide Permalloy strips from double switching of the vortex core,"We study vortex domain wall dynamics in wide Permalloy strips driven by
applied magnetic fields and spin-polarized electric currents. As recently
reported [V. Est\'evez and L. Laurson, Phys. Rev. B, 93, 064403 (2016)], for
sufficiently wide strips and above a threshold field, periodic dynamics of the
vortex core are localized in the vicinity of one of the strip edges, and the
velocity drop typically observed for narrow strips is replaced by a
high-velocity plateau. Here, we analyze this behavior in more detail by means
of micromagnetic simulations. We show that the high-velocity plateau originates
from a repeated double switching of the magnetic vortex core, underlying the
periodic vortex core dynamics in the vicinity of the strip edge, i.e., the
""attraction-repulsion"" effect. We also discuss the corresponding dynamics
driven by spin-polarized currents, as well as the effect of including quenched
random structural disorder to the system.",1701.05732v2
2017-02-20,Spin-resolved dynamical conductance of correlated large-spin magnetic molecules,"The finite-frequency transport properties of a large-spin molecule attached
to ferromagnetic contacts are studied theoretically in the Kondo regime. The
focus is on the behavior of the dynamical conductance in the linear response
regime, which is determined by using the numerical renormalization group
method. It is shown that the dynamical conductance depends greatly on the
magnetic configuration of the device and intrinsic parameters of the molecule.
In particular, conductance exhibits characteristic features for frequencies
corresponding to the dipolar and quadrupolar exchange fields resulting from the
presence of spin-dependent tunneling. Moreover, a dynamical spin accumulation
in the molecule, associated with the off-diagonal-in-spin component of the
conductance, is predicted. This spin accumulation becomes enhanced with
increasing the spin polarization of the leads, and it results in a nonmonotonic
dependence of the conductance on frequency, with local maxima occurring for
characteristic energy scales.",1702.05935v1
2017-03-13,Dynamical amplification of magnetoresistances and Hall currents up to the THz regime,"Spin-orbit-related effects offer a highly promising route for reading and
writing information in magnetic units of future devices. These phenomena rely
not only on the static magnetization orientation but also on its dynamics to
achieve fast switchings that can reach the THz range. In this work, we consider
Co/Pt and Fe/W bilayers to show that accounting for the phase difference
between different processes is crucial to the correct description of the
dynamical currents. By tuning each system towards its ferromagnetic resonance,
we reveal that dynamical spin Hall angles can non-trivially change sign and be
boosted by over 500%, reaching giant values. We demonstrate that charge and
spin pumping mechanisms can greatly magnify or dwindle the currents flowing
through the system, influencing all kinds of magnetoresistive and Hall effects,
thus impacting also dc and second harmonic experimental measurements.",1703.04493v2
2017-04-09,Test particle energization and the anisotropic effects of dynamical MHD turbulence,"In this paper we analyze the effect of dynamical three-dimensional MHD
turbulence on test particle acceleration, and compare how this evolving system
affects particle energization by current sheets interaction, against
frozen-in-time fields. To do this we analize the ensamble particle acceleration
for static electromagnetic fields extracted from direct numerical simulations
of the MHD equations, and compare with the dynamical fields. We show that a
reduction in particle acceleration in the dynamical model results from the
particle trapping in the field lines, which forces the particles to remain in a
moving current sheet that suppress the longer exposure at the strong electric
field gradients located between structures, which is an efficient particle
acceleration mechanism. In addition, we analize the effect of anisotropy caused
by the mean magnetic field. It is well known that for sufficiently strong
external fields, the system suffers a transition towards a two-dimensional
flow. This causes an increment in the size of the coherent structures,
resulting in a magnetized state of the particles and the reduction of the
particle energization.",1704.02642v1
2018-03-16,Dynamically enhanced magnetic incommensurability: Effects of local dynamics on non-local spin-correlations in a strongly correlated metal,"We compute the spin susceptibility of the two-dimensional Hubbard model away
from half-filling, and analyze the impact of frequency dependent vertex
corrections as obtained from the dynamical mean field theory (DMFT). We find
that the local dynamics captured by the DMFT vertex strongly affects non-local
spin correlations, and thus the momentum dependence of the spin susceptibility.
While the widely used random phase approximation yields commensurate
N\'eel-type antiferromagnetism as the dominant instability over a wide doping
range, the vertex corrections favor incommensurate ordering wave vectors away
from $(\pi,\pi)$. Our results indicate that the connection between the magnetic
ordering wave vector and the Fermi surface geometry, familiar for weakly
interacting systems, can hold in a strongly correlated metal, too.",1803.06175v1
2018-06-13,Dynamical response and dimensional crossover for spatially anisotropic antiferromagnets,"Theoretically challenging, the understanding of the dynamical response in
quantum antiferromagnets is of great interest, in particular for both inelastic
neutron scattering (INS) and nuclear magnetic resonance (NMR) experiments. In
such a context, we theoretically address this question for
quasi-one-dimensional quantum magnets, e.g. weakly coupled spin chains for
which many compounds are available in Nature. In this class of systems, the
dimensional crossover between a three-dimensional ordered regime at low
temperature towards one-dimensional physics at higher temperature is a
non-trivial issue, notably difficult concerning dynamical properties. Here we
present a comprehensive theoretical study based on both analytical calculations
(bosonization + random phase and self-consistent harmonic approximations) and
numerical simulations (quantum Monte Carlo + stochastic analytic continuation)
which allows us to describe the full temperature crossover for the NMR
relaxation rate $1/T_1$, from one-dimensional Tomonaga-Luttinger liquid physics
to the three-dimensional ordered regime, as a function of inter-chain
couplings. The dynamical structure factor, directly probing the INS intensity,
is also computed in the different regimes.",1806.04913v2
2018-07-18,Observation of Transient Overcritical Currents in YBCO Thin Films using High-Speed Magneto-Optical Imaging and Dynamic Current Mapping,"The dynamics of transient current distributions in superconducting YBCO thin
films were investigated during and immediately following an external field
ramp, using high-speed (real-time) Magneto-Optical Imaging and calculation of
dynamic current profiles. A number of qualitatively unique and previously
unobserved features are seen in this novel analysis of the evolution of
supercurrent during penetration. As magnetic field ramps up from zero, the
dynamic current profile is characterized by strong peaks, the magnitude of
which exceed the conventional critical current density (as determined from
static current profiles). These peaks develop close to the sample edges,
initially resembling screening currents but quickly growing in intensity as the
external field increases. A discontinuity in field and current behaviour is
newly observed, indicating a novel transition from increasing peak current
toward relaxation behaviour. After this transition, the current peaks move
toward the centre of the sample while reducing in intensity as magnetic
vortices penetrate inward. This motion slows exponentially with time, with the
current distribution in the long-time limit reducing to the expected Kim-model
profile.",1807.06745v1
2018-11-14,Microscopic Theory of Ultrafast Out-of-Equilibrium Dynamics in Magnetic Insulators. Unraveling the Magnon-Phonon Coupling,"The interaction between lattice and spins is at the heart of an extremely
intriguing ultrafast dynamics in magnetic materials. In this work we formulate
a general non-equilibrium theory that disentangles the complex interplay
between them in a THz laser-excited antiferromagnetic insulator. The theory
provides a quantitative description of the transient energy flow between the
spin and lattice sub-systems, subject to magnon-phonon and phonon-phonon
scatterings, giving rise to finite life-times of the quasiparticles and to the
equilibration time of the system. We predict a novel kind of scattering process
where two magnons of opposite polarizations decay into a phonon, previously
omitted in the literature. The theory is combined with first-principle
calculations and then applied to simulate a realistic dynamics of NiO. The main
relaxation channels and hot spots in the reciprocal space, giving the strongest
contribution to the energy transfer between phonons and magnons are identified.
The diverse interaction strengths lead to distinct coupled dynamics of the
lattice and spin systems and subsequently to different equilibration
timescales.",1811.05841v1
2018-11-21,The high-frequency dynamics of domain walls with strong Dzyaloshinskii-Moriya interaction,"Domain walls (DWs) in perpendicularly magnetized nanotracks (PMNTs) with
interfacial Dzyaloshinskii-Moriya interaction (DMI) have become the primary
objects of theoretical and experimental interest due to their technological
suitability in spintronic nanodevices. Chiral DWs in PMNTs can be driven
efficiently by the spin-orbit torque. However, the high-frequency dynamic
behavior of the chiral DW has not been explored. In this work, using
micromagnetic calculation, we have discovered a novel dynamic mode, the sway
mode, of DWs under an out-of-plane high-frequency alternating current (AC)
magnetic field in a PMNT with strong DMI. This dynamic phenomenon is strictly
related with DMI-related boundary effect and can be understood in terms of the
propagation of an amplitude-tuned spin wave in the DW plane. The spin wave
exhibits some characteristic frequencies due to the space-confinement of DW.
This work offers the possibility of a visual route for characterizing DMI.",1811.08590v1
2019-03-16,Can the dynamical Lamb effect be observed in a superconducting circuit?,"The dynamical Lamb effect is predicted to arise in superconducting circuits
when the coupling of a superconducting qubit with a resonator is periodically
switched ""on"" and ""off"" nonadiabatically. We show that by using a
superconducting circuit which allows to switch between longitudinal and
transverse coupling of a qubit to a resonator, it is possible of to observe the
dynamical Lamb effect. {The switching between longitudinal and transverse
coupling can be achieved by modulating the magnetic flux through the circuit
loops.} By solving the Schr\""{o}dinger equation for a qubit coupled to a
resonator, we calculate the time evolution of the probability of excitation of
the qubit and the creation of $n$ photons in the resonator due to the dynamical
Lamb effect. The probability is maximum when the coupling is periodically
switched between longitudinal and transverse using a square-wave or sinusoidal
modulation of the magnetic flux with frequency equal to the sum of the average
qubit and photon transition frequencies.",1903.06880v2
2019-03-25,Quench dynamics of spin in quantum dots coupled to spin-polarized leads,"We investigate the quench dynamics of a quantum dot strongly coupled to
spin-polarized ferromagnetic leads. The real-time evolution is calculated by
means of the time-dependent density-matrix numerical renormalization group
method implemented within the matrix product states framework. We examine the
system's response to a quench in the spin-dependent coupling strength to
ferromagnetic leads as well as in the position of the dot's orbital level. The
spin dynamics is analyzed by calculating the time-dependent expectation values
of the quantum dot's magnetization and occupation. Based on these, we determine
the time-dependence of a ferromagnetic-contact-induced exchange field and
predict its nonmonotonic build-up. In particular, two time scales are
identified, describing the development of the exchange field and the dot's
magnetization sign change. Finally, we study the effects of finite temperature
on the dynamical behavior of the system.",1903.10381v1
2019-08-05,The dynamic critical exponent $z$ of the three-dimensional Ising universality class: Monte Carlo simulations of the improved Blume-Capel model,"We study purely dissipative relaxational dynamics in the three-dimensional
Ising universality class. To this end, we simulate the improved Blume-Capel
model on the simple cubic lattice by using local algorithms. We perform a
finite size scaling analysis of the integrated autocorrelation time of the
magnetic susceptibility in equilibrium at the critical point. We obtain
$z=2.0245(15)$ for the dynamic critical exponent. As a complement, fully
magnetized configurations are suddenly quenched to the critical temperature,
giving consistent results for the dynamic critical exponent. Furthermore, our
estimate of $z$ is fully consistent with recent field theoretic results.",1908.01702v2
2020-04-17,Scaling properties of the dynamics at first-order quantum transitions when boundary conditions favor one of the two phases,"We address the out-of-equilibrium dynamics of a many-body system when one of
its Hamiltonian parameters is driven across a first-order quantum transition
(FOQT). In particular, we consider systems subject to fixed boundary
conditions, favoring one of the two phases separated by the FOQT: more
precisely, boundary conditions that favor the same magnetized phase (EFBC) or
opposite phases (OFBC) at the two ends of the chain. These issues are
investigated within the paradigmatic one-dimensional quantum Ising model, in
which FOQTs are driven by the longitudinal magnetic field h. We study the
dynamic behavior for an instantaneous quench and for a protocol in which h is
slowly varied across the FOQT. We develop a dynamic finite-size scaling theory
for both EFBC and OFBC, which displays some remarkable differences with respect
to the case of neutral boundary conditions. The corresponding relevant time
scale shows a qualitative different size dependence in the two cases: it
increases exponentially with the size in the case of EFBC, and as a power of
the size in the case of OFBC.",2004.08360v1
2020-07-03,Dynamic spin-triplet order induced by alternating electric fields in superconductor-ferromagnet-superconductor Josephson junctions,"Dynamic states offer extended possibilities to control the properties of
quantum matter. Recent efforts are focused on studying the ordered states which
appear exclusively under the time-dependent drives. Here we demonstrate a class
of systems which feature dynamic spin-triplet superconducting order stimulated
by the alternating electric field. The effect is based on the interplay of
ferromagnetism, interfacial spin-orbital coupling (SOC) and the condensate
motion driven by the field, which converts hidden static p-wave order, produced
by the joint action of the ferromagnetism and the SOC, into dynamical s-wave
equal-spin triplet correlations. We demonstrate that the critical current of
Josephson junctions hosting these states is proportional to the electromagnetic
power, supplied either by the external irradiation or by the ac current source.
Based on these unusual properties we propose the scheme of a Josephson
transistor which can be switched by the ac voltage and demonstrates an
even-numbered sequence of Shapiro steps. Combining the photo-active Josephson
junctions with recently discovered Josephson phase batteries we find
photo-magnetic SQUID devices which can generate spontaneous magnetic fields
while being exposed to irradiation.",2007.01805v2
2021-02-24,Finite-momentum energy dynamics in a Kitaev magnet,"We study the energy-density dynamics at finite momentum of the
two-dimensional Kitaev spin-model on the honeycomb lattice. Due to
fractionalization of magnetic moments, the energy relaxation occurs through
mobile Majorana matter, coupled to a static $\mathbb{Z}_2$ gauge field. At
finite temperatures, the $\mathbb{Z}_2$ flux excitations act as an emergent
disorder, which strongly affects the energy dynamics. We show that sufficiently
far above the flux proliferation temperature, but not yet in the classical
regime, gauge disorder modifies the coherent low-temperature energy-density
dynamics into a form which is almost diffusive, with hydrodynamic momentum
scaling of a diffusion-kernel, which however remains retarded, primarily due to
the presence of two distinct relaxation channels of particle-hole and
particle-particle nature. Relations to thermal conductivity are clarified. Our
analysis is based on complementary calculations in the low-temperature
homogeneous gauge and a mean-field treatment of thermal gauge fluctuations,
valid at intermediate and high temperatures.",2102.12492v1
2021-03-03,THz Field-induced Spin Dynamics in Ferrimagnetic Iron Garnets,"THz magnetization dynamics is excited in ferrimagnetic thulium iron garnet
with a picosecond, single-cycle magnetic field pulse and seen as a
high-frequency modulation of the magneto-optical Faraday effect. Data analysis
combined with numerical modelling and evaluation of the effective Lagrangian
allow us to conclude that the dynamics corresponds to the exchange mode excited
by Zeeman interaction of the THz field with the antiferromagnetically coupled
spins. We argue that THz-pump IR-probe experiments on ferrimagnets offer a
unique tool for quantitative studies of dynamics and mechanisms to control
antiferromagnetically coupled spins.",2103.02449v1
2021-05-24,Response Dynamics of Alkali Metal-Noble Gas Hybrid Trispin System,"With numerical calculation of coupled Bloch equations, we have simulated the
spin dynamics of nuclear magnetic resonance gyroscope based on alkali
metal-noble gas hybrid trispin system. From the perspective of damping harmonic
oscillator, a thorough analysis of the response dynamics is demonstrated. The
simulation results shows a linear increasing response of gyroscope signal while
the noblge gas nuclear spin magnetization and alkali atomic spin lifetime
parameters are at the over damping condition. An upper limit of response is
imposed on the NMR gyroscope signal due to the inherent dynamics of the hybrid
trispin system. The results agrees with present available experimental results
and provide useful guidings for future experiments.",2105.11124v2
2021-06-15,Ultrafast Optical Control of Magnetic Order and Fermi Surface Topology at a Quantum Critical Point,"Designing material properties on demand has important implications to
potential future technological applications. While theoretically it is always
possible to tune various intrinsic energy scales, there are fundamental
limitations to this approach in experiment. In recent years, ultrafast
spectroscopy has evolved as a promising tool to use light to dynamically induce
non-trivial electronic states of matter. Here we theoretically investigate
light pulse driven dynamics in a Kondo system close to quantum criticality. We
show, that light can dehybridize the local Kondo screening and induce magnetic
order out of a previously paramagnetic state. We demonstrate that, depending on
the laser pulse field parameters, it is possible to deconfine the Kondo singlet
and thereby induce second order phase transition to a dynamically ordered
state, as well as a dynamic Lifshitz transition that changes the Fermi surface
topology from hole- to electron-like.",2106.08220v1
2021-07-05,Dynamical signatures of point-gap Weyl semimetal,"We demonstrate a few unique dynamical properties of point-gap Weyl semimetal,
an intrinsic non-Hermitian topological phase in three dimensions. We consider a
concrete model where a pair of Weyl points reside on the imaginary axis of the
complex energy plane, opening up a point gap characterized by a topological
invariant, the three-winding number $W_3$. This gives rise to surface spectra
and dynamical responses that differ fundamentally from those in Hermitian Weyl
semimetals. First, we predict a time-dependent current flow along the magnetic
field in the absence of an electric field, in sharp contrast to the current
driven by the chiral anomaly, which requires both electric and magnetic fields.
Second, we reveal a novel type of boundary-skin mode in the wire geometry which
becomes localized at two corners of the wire cross section. We explain its
origin and show its experimental signatures in wave-packet dynamics.",2107.02135v3
2021-11-18,Floquet dynamical quantum phase transitions under synchronized periodic driving,"We study a generic class of fermionic two-band models under synchronized
periodic driving, i.e., with the different terms in a Hamiltonian subject to
periodic drives with the same frequency and phase. With all modes initially in
a maximally mixed state, the synchronized drive is found to produce nonperiodic
patterns of dynamical quantum phase transitions, with their appearance
determined by an interplay of the band structure and the frequency of the
drive. A case study of the anisotropic XY chain in a transverse magnetic field,
transcribed to an effective two-band model, shows that the modes come with
quantized geometric phases, allowing for the construction of an effective
dynamical order parameter. Numerical studies in the limit of a strong magnetic
field reveal distinct signals of precursors of dynamical quantum phase
transitions also when the initial state of the XY chain is perturbed slightly
away from maximal mixing, suggesting that the transitions may be accessible
experimentally. A blueprint for an experiment built around laser-trapped
circular Rydberg atoms is proposed.",2111.09926v3
2021-12-22,Machine learning nonequilibrium electron forces for adiabatic spin dynamics,"We present a generalized potential theory of nonequilibrium torques for the
Landau-Lifshitz equation. The general formulation of exchange forces in terms
of two potential energies allows for the implementation of accurate machine
learning models for adiabatic spin dynamics of out-of-equilibrium itinerant
magnetic systems. To demonstrate our approach, we develop a deep-learning
neural network that successfully learns the forces in a driven s-d model
computed from the nonequilibrium Green's function method. We show that the
Landau-Lifshitz dynamics simulations with forces predicted from the neural-net
model accurately reproduce the voltage-driven domain-wall propagation. Our work
opens a new avenue for multi-scale modeling of nonequilibrium dynamical
phenomena in itinerant magnets and spintronics based on machine-learning
models.",2112.12124v1
2022-02-09,A Nonlinear Proportional Integral Disturbance Observer and Motion Control Technique for Permanent Magnet Synchronous Motors,"In this paper, we present a Nonlinear-Proportional Integrator (N-PI)
disturbance observer (DOB) to enhance the motion tracking of the performance of
a surface-mounted Permanent Magnet Synchronous Motor (SPMSM) in rapidly speed
varying regions. By presenting an N-PI-DOB for load torque estimation with
torque modulation technique, we show that the tracking error dynamics of
angular position/velocity are coupled with tracking errors of currents loop and
estimation errors. After analyzing disturbances of currents tracking error
dynamics, we design the N-PI-DOB and Lyapunov-based nonlinear currents
controller to enhance the motion tracking performances. With these N-PI-DOBs
and motion controllers, we analyze the stability of motion tracking error
dynamics and estimation error dynamics. We experimentally perform a comparative
study with/without the N-PI-DOB to verify the effectiveness of the proposed
method in the condition of the unknown load torque and rapidly speed-varying.",2202.04594v1
2022-02-10,Dynamical quantum phase transition in diamond: applications in quantum metrology,"Nonequilibrium dynamics is a paramount scenario for studying quantum systems.
The emergence of new features with no equilibrium counterpart, such as
dynamical quantum phase transition (DQPT), has attracted wide attention. In
this work, we depart from the well known Ising model and showcase an
experimentally accessible configuration of a negatively charged
Nitrogen-Vacancy center that interacts with nearby Carbon-13 nuclear spins. We
provide new insights into this system in the context of DQPT. We show that
nuclear spins undergo DQPT by appropriately choosing the relation between the
transverse and longitudinal components of an external magnetic field.
Furthermore, we can steer the DQPT via a time-dependent longitudinal magnetic
field and apply this control to enhance the estimation of the coupling strength
between the nuclear spins. Moreover, we propose a novel quenched dynamics that
originates from the rotation of the central electron spin, which controls the
DQPT relying on the anisotropy of the hyperfine coupling.",2202.05216v1
2022-02-22,Dynamical scaling as a signature of multiple phase competition in Yb$_2$Ti$_2$O$_7$,"$\rm Yb_2Ti_2O_7$ is a celebrated example of a pyrochlore magnet with
highly-frustrated, anisotropic exchange interactions. To date, attention has
largely focused on its unusual, static properties, many of which can be
understood as coming from the competition between different types of magnetic
order. Here we use inelastic neutron scattering with exceptionally high energy
resolution to explore the dynamical properties of $\rm Yb_2Ti_2O_7$. We find
that spin correlations exhibit dynamical scaling, analogous to behavior found
near to a quantum critical point. We show that the observed scaling collapse
can be explained within a phenomenological theory of multiple--phase
competition, and confirm that a scaling collapse is also seen in
semi--classical simulations of a microscopic model of $\rm Yb_2Ti_2O_7$. These
results suggest a general picture for dynamics in systems with competing ground
states.",2202.11085v1
2022-06-17,Dynamics of a pair of magnetic dipoles with nonreciprocal interactions due to a moving conductor,"Recently it was demonstrated theoretically and experimentally that the
presence of a moving conductor can break the reciprocity in the interactions
between magnetic dipoles. In this article we investigate the influence of
nonreciprocity on the dynamics of a pair of rigid XY dipoles, which have been
realized in experiments. In particular, we focus on the energy nonconservation,
which is a consequence of the nonreciprocity. We find that the dynamics indeed
has regimes, wherein the kinetic energy grows quadratically. However, whether
energy absorption occurs depends strongly on the initial conditions on the
dipoles. Simulations for various initial conditions reveal an intricate
dependence, resulting in a rich structure of the energy absorbing regime in the
initial condition space. Nevertheless, we provide a qualitative explanation of
these observations, interpreting the absence of energy absorption as a
confinement of the dynamics in phase space.",2206.08765v2
2022-07-06,Field-driven side-by-side magnetic domain wall dynamics in ferromagnetic nanostrips,"There has been a plethora of studies on domain wall dynamics in magnetic
nanostrips, mainly because of its versatile non-linear physics and potential
applications in data storage devices. However, most of the studies focus on
out-of-plane domain walls or in-plane head-to-head (tail-to-tail) domain walls.
Here, we numerically study the field-driven dynamics of in-plane side-by-side
domain walls in ferromagnetic strips, which can be stable in the presence of an
in-plane easy-axis anisotropy transverse to the strip. The domain walls move in
a rigid-body manner at low field, and show complex Walker breakdown behavior at
high field. We observe a multi-step Walker breakdown through vortex nucleation
in wide strips. In the presence of Dzyaloshinskii-Moriya interaction (DMI), the
first Walker breakdown field first decreases then increases with interfacial
DMI, while keeps increasing with bulk DMI. These findings complement the
current understanding on domain wall dynamics.",2207.02494v1
2022-11-10,Waves in planetary dynamos,"This Special Topic focuses on magnetohydrodynamic (MHD) processes in deep
interiors of planets, in which their fluid dynamos are in operation. The
dynamo-generated, global, magnetic fields provide a background for our
solar-terrestrial environment. Probing the processes within the dynamos is a
significant theoretical and computational challenge and any window into
interior dynamics greatly increases our understanding. Such a window is
provided by exploring rapid dynamics, particularly MHD waves about the
dynamo-defined basic state. This field is the subject of current attention as
geophysical observations and numerical modellings advance. We give here
particular attention to torsional Alfv\'{e}n waves/oscillations and magnetic
Rossby waves, which may be regarded as typical axisymmetric and nonaxisymmetric
modes, respectively, amongst a wide variety of wave classes of the
rapidly-rotating MHD fluids. The excitation of those waves is being evidenced
for the geodynamo, whilst also being suggested for Jupiter. We shall overview
their dynamics, summarise our current understanding, and give open questions
for future perspectives.",2211.05270v1
2022-12-16,Relaxation dynamics of an unlike spin pair system,"Redfield master equation was applied to study the dynamics of an ensemble of
interacting pair of unlike spins at room temperature. This spin quantum system
is a workbench quantum model to analyze the relaxation dynamics of a
heteronuclear two-level spin system interacting by a pure dipole-dipole
coupling. Expressions for the density matrix elements and their relaxation rate
constants of each coherence order were computed. In addition, the solutions
were evaluated considering three initial quantum states, and the theoretical
predictions, such as multi-exponential evolutions and enhancement, are
behaviors that the solutions preserve and agree with previous studies performed
for magnetization time evolutions. Moreover, the solutions computed to predict
the dynamics of the longitudinal magnetization avoid the disagreement reported
by I. Solomon.",2212.08747v2
2023-02-06,Accelerated Dynamic Magnetic Resonance Imaging from Spatial-Subspace Reconstructions (SPARS),"Dynamic contrast-enhanced (DCE) magnetic resonance imaging (MRI) ideally
requires a high spatial and high temporal resolution, but hardware limitations
prevent acquisitions from simultaneously achieving both. Existing image
reconstruction techniques can artificially create spatial resolution at a given
temporal resolution by estimating data that is not acquired, but, ultimately,
spatial details are sacrificed at very high acceleration rates. The purpose of
this paper is to introduce the concept of spatial subspace reconstructions
(SPARS) and demonstrate its ability to reconstruct high spatial resolution
dynamic images from as few as one acquired radial spoke per dynamic frame.
Briefly, a low-temporal-high-spatial resolution organization of the acquired
raw data is used to estimate a spatial subspace in which the
high-temporal-high-spatial ground truth data resides. This subspace is then
used to estimate entire images from single k-space spokes. In both simulated
and human in-vivo data, the proposed SPARS reconstruction method outperformed
standard GRASP and GRASP-Pro reconstruction, providing a shorter reconstruction
time and yielding higher accuracy from both a spatial and temporal perspective.",2302.02564v3
2023-03-23,Dualities of 3D $\mathcal{N}=1$ SQCD from Branes and non-SUSY deformations,"We study the dynamics of an 'electric' $\mathcal{N}=1$ 3D
$U(N_c)_{k,k+\frac{N_c}{2}}$ SQCD theory. By embedding the theory in string
theory, we propose that the theory admits a 'magnetic' dual and analyse the low
energy dynamics of the theory using its dual. When $\frac{N_f}{2}
\ge\frac{N_c}{2}-k$ the IR dynamics is described by either a TQFT for large
quark masses, or a Grassmanian and a Wess-Zumino (WZ) term for small masses. We
also consider non-supersymmetric mass deformations and RG flows in the vicinity
of the SUSY point and find agreement between the IR of the electric and its
magnetic dual. When $\frac{N_f}{2} < \frac{N_c}{2}-k$ supersymmetry is broken
and the IR dynamics is a described by a TQFT accompanied by a Goldstino. We
also discuss SQCD theories based on $SO$/$USp$ gauge groups.",2303.13597v2
2023-03-31,Characterization and Coherent Control of Spin Qubits with Modulated Electron Beam and Resonator,"The coherent dynamics and control of spin qubits are essential requirements
for quantum technology. A prominent challenge for coherent control of a spin
qubit in a set of qubits is the destructive effect of the applied magnetic
field on the coherent dynamics of neighbouring qubits due to its spatial
extension. We propose a novel scheme to characterize the coherent dynamics of
these quantum systems and to coherently control them using a magnetic field.
Our scheme consists of a resonator that encompasses the desired quantum system
and a modulated electron beam that passes through the resonator in close
proximity to the quantum system of interest. The dynamics of the system is
obtained by solving the Lindblad master equation. To verify the reliability of
our model, we tested the model on a Potassium atom, $^{41}$K and NV$^-$ centre
in Diamond. The results show that by properly controlling the parameters of the
resonator and the electron beam, the coherence and decoherence rates of these
quantum systems can be improved. Our model has the potential to be used for
characterizing different types of spin-based quantum systems, and implementing
quantum logic gates for quantum computation.",2303.17952v1
2023-05-03,An overview of the spin dynamics of antiferromagnetic Mn$_5$Si$_3$,"The metallic compound Mn$_5$Si$_3$ hosts a series of antiferromagnetic phases
which can be controlled by external stimuli such as temperature and magnetic
field. In this work, we investigate the spin-excitation spectrum of bulk
Mn$_5$Si$_3$ by combining inelastic neutron scattering measurements and density
functional theory calculations. We study the evolution of the dynamical
response under external parameters and demonstrate that the spin dynamics of
each phase is robust against any combination of temperature and magnetic field.
In particular, the high-energy spin dynamics is very characteristic of the
different phases consisting of either spin waves or broad fluctuations
patterns.",2305.02409v2
2023-05-04,Dynamics of magnetoelastic robots in water-saturated granular beds,"We investigate the dynamics of a magnetoelastic robot with a dipolar magnetic
head and a slender elastic body as it performs undulatory strokes and burrows
through water-saturated granular beds. The robot is actuated by an oscillating
magnetic field and moves forward when the stroke amplitude increases above a
critical threshold. By visualizing the medium, we show that the undulating body
fluidizes the bed, resulting in the appearance of a dynamic burrow, which
rapidly closes in behind the moving robot as the medium loses energy. We
investigate the applicability of Lighthill's elongated body theory of fish
locomotion, and estimate the contribution of thrust generated by the undulating
body and the drag incorporating the granular volume fraction-dependent
effective viscosity of the medium. The projected speeds are found to be
consistent with the measured speeds over a range of frequencies and amplitudes
above the onset of forward motion. However, systematic deviations are found to
grow with increasing driving, pointing to a need for further sophisticated
modelling of the medium-structure interactions.",2305.02903v2
2023-05-17,Substantial reduction of write-error rate for voltage-controlled magnetoresistive random access memory by in-plane demagnetizing field and voltage-induced negative out-of-plane anisotropy field,"Voltage-controlled magnetoresistive random access memory (VC-MRAM) based on
voltage-induced dynamic switching in magnetic tunnel junctions (MTJs) is a
promising ultimate non-volatile memory with ultralow power consumption.
However, the dynamic switching in a conventional MTJ is accompanied by a
relatively high write error rate (WER), hindering the reliable operation of
VC-MRAM. Here, we propose a reliable writing scheme using the in-plane
demagnetizing field (IDF) and voltage-induced negative out-of-plane anisotropy
field (NOAF). Numerical simulations based on macrospin model demonstrate that
the voltage-induced NOAF modifies the switching dynamics and increases the
torque due to the IDF, thereby reducing the switching time. The IDF and
voltage-induced NOAF also reduce the mean energy difference between the
magnetization direction at the end of the pulse and the equilibrium direction.
As a result, an appropriate combination of the IDF and voltage-induced NOAF
reduces the WER by one order of magnitude compared with that of the dynamic
switching in a conventional MTJ.",2305.09883v1
2023-06-20,"Magnetoelectric fractals, Magnetoelectric parametric resonance and Hopf bifurcation","In the present work, we study the dynamics of a magnetic nanoparticle coupled
through the magnetoelectric coupling to the ferroelectric crystal. The model of
our interest is nonlinear, and we explore the problem under different limits of
weak and strong linearity. By applying two electric fields with different
frequencies, we control the form of the confinement potential of the
ferroelectric subsystem and realize different types of dynamics. We proved that
the system is more sensitive to magnetoelectric coupling in the case of
double-well potential. In particular, in the case of strong nonlinearity,
arbitrary small values of magnetoelectric coupling lead to chaotic dynamics. In
essence, magnetoelectric coupling plays a role akin to the small perturbations
destroying invariant tors according to the KAM theorem. We showed that
bifurcations in the system are of Hopf's type. We observed the formation of
magnetoelectric fractals in the system. In the limit of weak nonlinearity, we
studied a problem of parametric nonlinear resonance and enhancement of magnetic
oscillations through magnetoelectric coupling.",2306.13110v1
2023-08-23,Magnetic-order-mediated carrier and phonon dynamics in MnBi2Te4,"We investigate the quasiparticle dynamics in MnBi2Te4 single crystal using
the ultrafast optical spectroscopy. Our results show that there exist anomalous
dynamical optical responses below the antiferromagnetic (AFM) ordering
temperature TN. In specific, we reveal that both the initial carrier decay and
recombination processes can be modulated via introducing the AFM order in
sub-picosecond and picosecond timescales, respectively. We also discover a long
relaxation process emerging below TN with a timescale approaching to the
nanosecond regime, and can be attributed to the T-dependent spin-lattice
interaction. There also emerges an unusual phonon energy renormalization below
TN , which is found to arise from its coupling the spin degree via the exchange
interaction and magnetic anisotropy. Our findings provide key information for
understanding the dynamical properties of non-equilibrium carrier, spin and
lattice in MnBi2Te4.",2308.11935v1
2023-08-28,Spacetime dynamics of chiral magnetic currents in a hot non-Abelian plasma,"The correlations of electric currents in hot non-Abelian plasma are
responsible for the experimental manifestations of the chiral magnetic effect
(CME) in heavy-ion collisions. We evaluate these correlations using holography,
and show that they are driven by large-scale topological fluctuations. In a
non-Abelian plasma with chiral fermions, local axial charge can be generated
either by topological fluctuations (creating domains with nonzero Chern-Simons
number) or by thermal fluctuations. Within holography, we investigate the
dynamical creation of the axial charge and isolate the imprint of the
topological dynamics on the spatial correlations of electric current. In
particular, we show that the spatial extent of the current correlation is quite
large ($\sim 1\ {\rm fm}$) and grows with time, which is consistent with
sphaleronlike dynamics. We provide numerical estimates for this spatial size
that can be used as an input in phenomenological analyses.",2308.14825v2
2023-09-22,Dynamical defects in a two-dimensional Wigner crystal: self-doping and kinetic magnetism,"We study the quantum dynamics of interstitials and vacancies in a
two-dimensional Wigner crystal (WC) using a semi-classical instanton method
that is asymptotically exact at low density, i.e., in the $r_s\to \infty$
limit. The dynamics of these point defects mediates magnetism with much higher
energy scales than the exchange energies of the pure WC. Via exact
diagonalization of the derived effective Hamiltonians in the single-defect
sectors, we find the dynamical corrections to the defect energies. The
resulting expression for the interstitial (vacancy) energy extrapolates to 0 at
$r_s = r_{\rm mit} \approx 70$ ($r_s \approx 30$), suggestive of a self-doping
instability to a partially melted WC for some range of $r_s$ below $r_{\rm
mit}$. We thus propose a ""metallic electron crystal'' phase of the
two-dimensional electron gas at intermediate densities between a low density
insulating WC and a high density Fermi fluid.",2309.13121v2
2023-11-01,Coupled spin-lattice dynamics from the tight-binding electronic structure,"We developed a method which performs the coupled adiabatic spin and lattice
dynamics based on the tight-binding electronic structure model, where the
intrinsic magnetic field and ionic forces are calculated from the converged
self-consistent electronic structure at every time step. By doing so, this
method allows us to explore limits where the physics described by a
parameterized spin-lattice Hamiltonian is no longer accurate. We demonstrate
how the lattice dynamics is strongly influenced by the underlying magnetic
configuration, where disorder is able to induce significant lattice
distortions. The presented method requires significantly less computational
resources than ab initio methods, such as time-dependent density functional
theory (TD-DFT). Compared to parameterized Hamiltonian-based methods, it also
describes more accurately the dynamics of the coupled spin and lattice degrees
of freedom, which becomes important outside of the regime of small lattice and
spin fluctuations.",2311.00765v1
2024-01-19,Diversity of Ultrafast Spin Dynamics Near the Tricritical Point in a Ferrimagnetic Gd/FeCo Multilayer,"It is found that subtle changes in the external magnetic field and
temperature result in dramatic changes in the ultrafast response of spins to a
femtosecond laser excitation in a ferrimagnetic Gd/FeCo multilayer. A total of
six distinct types of spin dynamics were observed and explained by considering
the spin-flop transition to the noncollinear phase and the concept of a
tricritical point in the $H$-$T$ phase diagram. A particularly interesting type
of dynamics is the exchange-driven reversal. These exchange-driven dynamics
provide new insights into the tricritical point, which is shown to separate two
thermodynamically distinct noncollinear phases with the transition-metal
magnetization pointing on adjacent sides of the anisotropy plane.",2401.10671v1
2024-01-25,Dynamic image reconstruction in MPI with RESESOP-Kaczmarz,"In Magnetic Particle Imaging (MPI), it is typically assumed that the studied
specimen is stationary during the data acquisition. In practical applications
however, the searched-for 3D distribution of the magnetic nanoparticles might
show a dynamic behavior, caused by e.g. breathing or movement of the blood.
Neglecting those dynamics during the reconstruction step results in motion
artifacts and a reduced image quality.
  This article addresses the challenge of capturing high quality images in the
presence of motion. A promising technique provides the Regularized Sequential
Subspace Optimization (RESESOP) algorithm, which takes dynamics as model
inexactness into account, significantly improving reconstruction compared to
standard static algorithms like regularized Kaczmarz. Notably, this algorithm
operates with minimal prior information and the method allows for subframe
reconstruction, making it suitable for scenarios with rapid particle movement.
The performance of the proposed method is demonstrated on both simulated and
real data sets.",2401.14202v1
2024-02-01,Longtime dynamics for the Landau Hamiltonian with a time dependent magnetic field,"We consider a modulated magnetic field, $B(t) = B_0 +\varepsilon f(\omega
t)$, perpendicular to a fixed plane, where $B_0$ is constant, $\varepsilon>0$
and $f$ a periodic function on the torus ${\mathbb T}^n$. Our aim is to study
classical and quantum dynamics for the corresponding Landau Hamiltonian. It
turns out that the results depend strongly on the chosen gauge. For the Landau
gauge the position observable is unbounded for ""almost all"" non resonant
frequencies $\omega$. On the contrary, for the symmetric gauge we obtain that,
for ""almost all"" non resonant frequencies $\omega$, the Landau Hamiltonian is
reducible to a two dimensional harmonic oscillator and thus gives rise to
bounded dynamics. The proofs use KAM algorithms for the classical dynamics.
Quantum applications are given. In particular, the Floquet spectrum is
absolutely continuous in the Landau gauge while it is discrete, of finite
multiplicity, in symmetric gauge.",2402.00428v1
1996-09-23,Gravitational Dynamics of Cold Matter using the Deformation Tensor,"In this paper we present a new local Lagrangian approximation to the
gravitational dynamics of cold matter. We describe the dynamics of a Lagrangian
fluid element through only one quantity, the deformation tensor. We show that
this tensor is clearly suited to the study of gravitational dynamics and,
moreover, that knowing its evolution is enough to completely describe a fluid
element. Our approximate dynamics treat exactly the conservation of mass, of
the velocity divergence and of the shear and is exact in the case of planar,
cylindrical and spherical collapses. It also reproduces very accurately the
evolution of all dynamical quantities for a very wide class of initial
conditions as illustrated by a detailed comparison with the homogeneous
ellipsoid model. Beside, we highlight, for the first time, the important
dynamical role played by the Newtonian counterpart of the magnetic part of the
Weyl tensor.",9609156v1
1996-11-23,Nonequilibrium Phase Transition in the Kinetic Ising model: Critical Slowing Down and Specific-heat Singularity,"The nonequilibrium dynamic phase transition, in the kinetic Ising model in
presence of an oscillating magnetic field, has been studied both by Monte Carlo
simulation and by solving numerically the mean field dynamic equation of motion
for the average magnetisation. In both the cases, the Debye 'relaxation'
behaviour of the dynamic order parameter has been observed and the 'relaxation
time' is found to diverge near the dynamic transition point. The Debye
relaxation of the dynamic order parameter and the power law divergence of the
relaxation time have been obtained from a very approximate solution of the mean
field dynamic equation. The temperature variation of appropiately defined
'specific-heat' is studied by Monte Carlo simulation near the transition point.
The specific-heat has been observed to diverge near the dynamic transition
point.",9611181v1
1997-01-24,Nonequilibrium phase transition in the kinetic Ising model: Divergences of fluctuations and responses near the transition point,"The nonequilibrium dynamic phase transition, in the kinetic Ising model in
presence of an oscillating magnetic field, has been studied by Monte Carlo
simulation. The fluctuation of dynamic order parameter has been studied as a
function of temperature near the dynamic transition point. The temperature
variation of appropriately defined `susceptibility' has also been studied near
the dynamic transition point. Similarly, fluctuation of energy and
appropriately defined `specific-heat' have been studied as a function of
temperature near the dynamic transition point. In both the cases, the
fluctuations (of dynamic order parameter and energy) and the corresponding
responses diverge (as power law fashion) near the dynamic transition point with
similar critical behavior (with identical exponent values).",9701180v2
1997-05-20,Current-induced vortex dynamics in Josephson-junction arrays: Imaging experiments and model simulations,"We study the dynamics of current-biased Josephson-junction arrays with a
magnetic penetration depth smaller than the lattice spacing. We compare the
dynamics imaged by low-temperature scanning electron microscopy to the vortex
dynamics obtained from model calculations based on the resistively-shunted
junction model, in combination with Maxwell's equations. We find three bias
current regions with fundamentally different array dynamics. The first region
is the subcritical region, i.e. below the array critical current I_c. The
second, for currents I above I_c, is a ""vortex region"", in which the response
is determined by the vortex degrees of freedom. In this region, the dynamics is
characterized by spatial domains where vortices and antivortices move across
the array in opposite directions in adjacent rows and by transverse voltage
fluctuations. In the third, for still higher currents, the dynamics is
dominated by coherent-phase motion, and the current-voltage characteristics are
linear.",9705194v1
1999-02-10,Low Temperature Static and Dynamic Behavior of the Two-Dimensional Easy-Axis Heisenberg Model,"We apply the self-consistent harmonic approximation (SCHA) to study static
and dynamic properties of the two-dimensional classical Heisenberg model with
easy-axis anisotropy. The static properties obtained are magnetization and spin
wave energy as functions of temperature, and the critical temperature as a
function of the easy-axis anisotropy. We also calculate the dynamic correlation
functions using the SCHA renormalized spin wave energy. Our analytical results,
for both static properties and dynamic correlation functions, are compared to
numerical simulation data combining cluster-Monte Carlo algorithms and Spin
Dynamics. The comparison allows us to conclude that far below the transition
temperature, where the SCHA is valid, spin waves are responsible for all
relevant features observed in the numerical simulation data; topological
excitations do not seem to contribute appreciably. For temperatures closer to
the transition temperature, there are differences between the dynamic
correlation functions from SCHA theory and Spin Dynamics; these may be due to
the presence of domain walls and solitons.",9902149v1
2006-03-20,Local and cluster critical dynamics of the 3d random-site Ising model,"We present the results of Monte Carlo simulations for the critical dynamics
of the three-dimensional site-diluted quenched Ising model. Three different
dynamics are considered, these correspond to the local update Metropolis scheme
as well as to the Swendsen-Wang and Wolff cluster algorithms. The lattice sizes
of L=10-96 are analysed by a finite-size-scaling technique. The site dilution
concentration p=0.85 was chosen to minimize the correction-to-scaling effects.
We calculate numerical values of the dynamical critical exponents for the
integrated and exponential autocorrelation times for energy and magnetization.
As expected, cluster algorithms are characterized by lower values of dynamical
critical exponent than the local one: also in the case of dilution critical
slowing down is more pronounced for the Metropolis algorithm. However, the
striking feature of our estimates is that they suggest that dilution leads to
decrease of the dynamical critical exponent for the cluster algorithms. This
phenomenon is quite opposite to the local dynamics, where dilution enhances
critical slowing down.",0603521v1
2008-12-26,Thermalization dynamics close to a quantum phase transition,"We investigate the dissipative dynamics of a quantum critical system in
contact with a thermal bath. In analogy with the standard protocol employed to
analyze aging, we study the response of a system to a sudden change of the bath
temperature. The specific example of the XY model in a transverse magnetic
field whose spins are locally coupled to a set of bosonic baths is considered.
The peculiar nature of the dynamics is encoded in the correlations developing
out of equilibrium. By means of a kinetic equation we analyze the spin-spin
correlations and block correlations. We identify some universal features in the
out-of-equilibrium dynamics. Two distinct regimes, characterized by different
time and length scales, emerge. During the initial transient the dynamics is
characterized by the same critical exponents as those of the equilibrium
quantum phase transition and resembles the dynamics of thermal phase
transitions. At long times equilibrium is reached through the propagation along
the chain of a thermal front in a manner similar to the classical Glauber
dynamics.",0812.4717v2
2010-12-15,A message-passing scheme for non-equilibrium stationary states,"We study stationary states in a diluted asymmetric (kinetic) Ising model. We
apply the recently introduced dynamic cavity method to compute magnetizations
of these stationary states. Depending on the update rule, different versions of
the dynamic cavity method apply. We here study synchronous updates and random
sequential updates, and compare local properties computed by the dynamic cavity
method to numerical simulations. Using both types of updates, the dynamic
cavity method is highly accurate at high enough temperatures. At low enough
temperatures, for sequential updates the dynamic cavity method tends to a fixed
point, but which does not agree with numerical simulations, while for parallel
updates, the dynamic cavity method may display cyclic behavior. When it
converges and is accurate, the dynamic cavity method offers a huge speed-up
compared to Monte Carlo, particularly for large systems.",1012.3388v2
2012-12-01,Time evolution of autocorrelation function in dynamical replica theory,"Asynchronous dynamics given by the master equation in the
Sherrington--Kirkpatrick (SK) spin-glass model is studied based on dynamical
replica theory (DRT) with an extension to take into account the autocorrelation
function. The dynamical behaviour of the system is approximately described by
dynamical equations of the macroscopic quantities: magnetization, energy
contributed by randomness, and the autocorrelation function. The dynamical
equations under the replica symmetry assumption are derived by introducing the
subshell equipartitioning assumption and exploiting the replica method. The
obtained dynamical equations are compared with Monte Carlo (MC) simulations,
and it is demonstrated that the proposed formula describes well the time
evolution of the autocorrelation function in some parameter regions. The study
offers a reasonable description of the autocorrelation function in the SK
spin-glass system.",1212.0058v1
2017-08-27,Symmetry Protected Dynamical Symmetry in the Generalized Hubbard Models,"In this letter we present a theorem on the dynamics of the generalized
Hubbard models. This theorem shows that the symmetry of the single particle
Hamiltonian can protect a kind of dynamical symmetry driven by the
interactions. Here the dynamical symmetry refers to that the time evolution of
certain observables are symmetric between the repulsive and attractive Hubbard
models. We demonstrate our theorem with three different examples in which the
symmetry involves bipartite lattice symmetry, reflection symmetry and
translation symmetry, respectively. Each of these examples relates to one
recent cold atom experiment on the dynamics in the optical lattices where such
a dynamical symmetry is manifested. These experiments include expansion
dynamics of cold atoms, chirality of atomic motion within a synthetic magnetic
field and melting of charge-density-wave order. Therefore, our theorem provides
a unified view of these seemingly disparate phenomena.",1708.08070v2
2018-01-30,Chaos in Homeostatically Regulated Neural Systems,"Low-dimensional yet rich dynamics often emerge in the brain. Examples include
oscillations and chaotic dynamics during sleep, epilepsy, and voluntary
movement. However, a general mechanism for the emergence of low dimensional
dynamics remains elusive. Here, we consider Wilson-Cowan networks and
demonstrate through numerical and analytical work that a type of homeostatic
regulation of the network firing rates can paradoxically lead to a rich
dynamical repertoire. The dynamics include mixed-mode oscillations, mixed-mode
chaos, and chaotic synchronization. This is true for single recurrently coupled
node, pairs of reciprocally coupled nodes without self-coupling, and networks
coupled through experimentally determined weights derived from functional
magnetic resonance imaging data. In all cases, the stability of the homeostatic
set point is analytically determined or approximated. The dynamics at the
network level are directly determined by the behavior of a single node system
through synchronization in both oscillatory and non-oscillatory states. Our
results demonstrate that rich dynamics can be preserved under homeostatic
regulation or even be caused by homeostatic regulation.",1801.09997v1
2019-01-18,CRDN: Cascaded Residual Dense Networks for Dynamic MR Imaging with Edge-enhanced Loss Constraint,"Dynamic magnetic resonance (MR) imaging has generated great research
interest, as it can provide both spatial and temporal information for clinical
diagnosis. However, slow imaging speed or long scanning time is still one of
the challenges for dynamic MR imaging. Most existing methods reconstruct
Dynamic MR images from incomplete k-space data under the guidance of compressed
sensing (CS) or low rank theory, which suffer from long iterative
reconstruction time. Recently, deep learning has shown great potential in
accelerating dynamic MR. Our previous work proposed a dynamic MR imaging method
with both k-space and spatial prior knowledge integrated via multi-supervised
network training. Nevertheless, there was still a certain degree of smooth in
the reconstructed images at high acceleration factors. In this work, we propose
cascaded residual dense networks for dynamic MR imaging with edge-enhance loss
constraint, dubbed as CRDN. Specifically, the cascaded residual dense networks
fully exploit the hierarchical features from all the convolutional layers with
both local and global feature fusion. We further utilize the total variation
(TV) loss function, which has the edge enhancement properties, for training the
networks.",1901.06111v1
2019-05-08,Diffusive transport in a quasiperiodic Fibonacci chain: absence of many-body localization at small interactions,"We study high-temperature magnetization transport in a many-body spin-1/2
chain with on-site quasiperiodic potential governed by the Fibonacci rule. In
the absence of interactions it is known that the system is critical with the
transport described by a continuously varying dynamical exponent (from
ballistic to localized) as a function of the on-site potential strength. Upon
introducing weak interactions, we find that an anomalous noninteracting
dynamical exponent becomes diffusive for any potential strength. This is borne
out by a boundary-driven Lindblad dynamics as well as unitary dynamics, with
agreeing diffusion constants. This must be contrasted to random potential where
transport is subdiffusive at such small interactions. Mean-field treatment of
the dynamics for small U always slows down the non-interacting dynamics to
subdiffusion, and is therefore unable to describe diffusion in an interacting
quasiperiodic system. Finally, briefly exploring larger interactions we find a
regime of interaction-induced subdiffusive dynamics, despite the on-site
potential itself having no ""rare-regions"".",1905.03128v2
2019-05-21,Quantum quench and non-equilibrium dynamics in lattice-confined spinor condensates,"We present an experimental study on non-equilibrium dynamics of a spinor
condensate after it is quenched across a superfluid to Mott insulator (MI)
phase transition in cubic lattices. Intricate dynamics consisting of
spin-mixing oscillations at multiple frequencies are observed in time
evolutions of the spinor condensate localized in deep lattices after the
quantum quench. Similar spin dynamics also appear after spinor gases in the MI
phase are suddenly moved away from their ground states via quenching magnetic
fields. We confirm these observed spectra of spin-mixing dynamics can be
utilized to reveal atom number distributions of an inhomogeneous system, and to
study transitions from two-body to many-body dynamics. Our data also imply the
non-equilibrium dynamics depend weakly on the quench speed but strongly on the
lattice potential. This enables precise measurements of the spin-dependent
interaction, a key parameter determining the spinor physics.",1905.08738v1
2020-06-22,The dynamics of a driven harmonic oscillator coupled to independent Ising spins in random fields,"We aim at an understanding of the dynamical properties of a periodically
driven damped harmonic oscillator coupled to a Random Field Ising Model (RFIM)
at zero temperature, which is capable to show complex hysteresis. The system is
a combination of a continuous (harmonic oscillator) and a discrete (RFIM)
subsystem, which classifies it as a hybrid system. In this paper we focus on
the hybrid nature of the system and consider only independent spins in quenched
random local fields, which can already lead to complex dynamics such as chaos
and multistability. We study the dynamic behavior of this system by using the
theory of piecewise-smooth dynamical systems and discontinuity mappings.
Specifically, we present bifurcation diagrams, Lyapunov exponents as well as
results for the shape and the dimensions of the attractors and the
self-averaging behavior of the attractor dimensions and the magnetization.
Furthermore we investigate the dynamical behavior of the system for an
increasing number of spins and the transition to the thermodynamic limit, where
the system behaves like a driven harmonic oscillator with an additional
nonlinear smooth external force.",2006.12588v2
2022-06-21,Markov property of the Super-MAG Auroral Electrojet Indices,"The dynamics of the Earth's magnetosphere exhibits strongly fluctuating
patterns as well as non-stationary and non-linear interactions, more pronounced
during magnetospheric substorms and magnetic storms. This complex dynamics
comprises both stochastic and deterministic features occurring at different
time scales. Here we investigate the stochastic nature of the magnetospheric
substorm dynamics by analysing the Markovian character of SuperMAG SME and SML
geomagnetic indices. By performing the Chapman-Kolmogorov test, the SME/SML
dynamics appears to satisfy the Markov condition at scales below 60 minutes.
The Kramers-Moyal analysis instead highlights that a purely diffusive process
is not representative of the magnetospheric dynamics, thus a model that
includes both diffusion and Poisson-jump processes is used to reproduce the SME
dynamical features at small scales. A discussion of the similarities and
differences between this model and the SME properties is provided with a
special emphasis on the metastability of the Earth's magnetospheric dynamics.
Finally, the relevance of our results in the framework of Space Weather is also
addressed.",2206.10378v3
2022-06-30,Experimental quantum simulation of non-Hermitian dynamical topological states using stochastic Schrödinger equation,"Noise is ubiquitous in real quantum systems, leading to non-Hermitian quantum
dynamics, and may affect the fundamental states of matter. Here we report in
experiment a quantum simulation of the two-dimensional non-Hermitian quantum
anomalous Hall (QAH) model using the nuclear magnetic resonance processor.
Unlike the usual experiments using auxiliary qubits, we develop a stochastic
average approach based on the stochastic Schr\""odinger equation to realize the
non-Hermitian dissipative quantum dynamics, which has advantages in saving the
quantum simulation sources and simplifies implementation of quantum gates. We
demonstrate the stability of dynamical topology against weak noise, and observe
two types of dynamical topological transitions driven by strong noise.
Moreover, a region that the emergent topology is always robust regardless of
the noise strength is observed. Our work shows a feasible quantum simulation
approach for dissipative quantum dynamics with stochastic Schr\""odinger
equation and opens a route to investigate non-Hermitian dynamical topological
physics.",2206.15127v1
2023-11-30,Dynamic Phase Transition in 2D Ising Systems: Effect of Anisotropy and Defects,"We investigate the dynamic phase transition in two-dimensional Ising models
whose equilibrium characteristics are influenced by either anisotropic
interactions or quenched defects. The presence of anisotropy reduces the
dynamical critical temperature, leading to the expected result that the
critical temperature approaches zero in the full-anisotropy limit. We show that
a comprehensive understanding of the dynamic behavior of systems with quenched
defects requires a generalized definition of the dynamic order parameter. By
doing so, we demonstrate that the inclusion of quenched defects lowers the
dynamic critical temperature as well, with a linear trend across the range of
defect fractions considered. We also explore if and how it is possible to
predict the dynamic behavior of specific magnetic systems with quenched
randomness. Various geometric quantities, such as a defect potential index, the
defect dipole moment, and the properties of the defect Delaunay triangulation,
prove useful for this purpose.",2312.00153v2
2024-03-14,Quantum analog of Landau-Lifshitz-Gilbert dynamics,"The Landau-Lifshitz-Gilbert (LLG) and Landau-Lifshitz (LL) equations play an
essential role for describing the dynamics of magnetization in solids. While a
quantum analog of the LL dynamics has been proposed in [Phys. Rev. Lett. 110,
147201 (2013)], the corresponding quantum version of LLG remains unknown. Here,
we propose such a quantum LLG equation that inherently conserves purity of the
quantum state. We examine the quantum LLG dynamics of a dimer consisting of two
interacting spin-1/2 particles. Our analysis reveals that, in the case of
ferromagnetic coupling, the evolution of initially uncorrelated spins mirrors
the classical LLG dynamics. However, in the antiferromagnetic scenario, we
observe pronounced deviations from classical behavior, underscoring the unique
dynamics of becoming a spinless state, which is non-locally correlated.
Moreover, when considering spins that are initially correlated, our study
uncovers an unusual form of transient quantum correlation dynamics, which
differ significantly from what is typically seen in open quantum systems.",2403.09255v1
2007-09-24,The Spin Density Matrix II: Application to a system of two quantum dots,"This work is a sequel to our work ""The Spin Density Matrix I: General Theory
and Exact Master Equations"" (eprint arXiv:0708.0644 [cond-mat]). Here we
compare pure- and pseudo-spin dynamics using as an example a system of two
quantum dots, a pair of localized conduction-band electrons in an n-doped GaAs
semiconductor. Pure-spin dynamics is obtained by tracing out the orbital
degrees of freedom, whereas pseudo-spin dynamics retains (as is conventional)
an implicit coordinate dependence. We show that magnetic field inhomogeneity
and spin-orbit interaction result in a non-unitary evolution in pure-spin
dynamics, whereas these interactions contribute to the effective pseudo-spin
Hamiltonian via terms that are asymmetric in spin permutations, in particular,
the Dzyaloshinskii-Moriya (DM) spin-orbit interaction. We numerically
investigate the non-unitary effects in the dynamics of the triplet states
population, purity, and Lamb energy shift, as a function of interdot distance
and magnetic field difference. The spin-orbit interaction is found to produce
effects of roughly four orders of magnitude smaller than those due to magnetic
field difference in the pure-spin model. We estimate the spin-orbit interaction
magnitude in the DM-interaction term. Our estimate gives a smaller value than
that recently obtained by Kavokin [Phys. Rev. B 64, 075305 (2001)], who did not
include double occupancy effects. We show that a necessary and sufficient
condition for obtaining a universal set of quantum logic gates, involving only
two spins, in both pure- and pseudo-spin models is that the magnetic field
inhomogeneity and the Heisenberg interaction are both non-vanishing. We also
briefly analyze pure-spin dynamics in the electron on liquid helium system
recently proposed by Lyon [Phys. Rev. A 74, 052338 (2006)].",0709.3685v1
2016-12-22,Disruption of sheetlike structures in Alfvénic turbulence by magnetic reconnection,"We propose a mechanism whereby the intense, sheet-like structures naturally
formed by dynamically aligning Alfv\'enic turbulence are destroyed by magnetic
reconnection at a scale $\hat{\lambda}_{\rm D}$, larger than the dissipation
scale predicted by models of intermittent, dynamically aligning turbulence. The
reconnection process proceeds in several stages: first, a linear tearing mode
with $N$ magnetic islands grows and saturates, and then the $X$-points between
these islands collapse into secondary current sheets, which then reconnect
until the original structure is destroyed. This effectively imposes an upper
limit on the anisotropy of the structures within the perpendicular plane, which
means that at scale $\hat{\lambda}_{\rm D}$ the turbulent dynamics change: at
scales larger than $\hat{\lambda}_{\rm D}$, the turbulence exhibits
scale-dependent dynamic alignment and a spectral index approximately equal to
$-3/2$, while at scales smaller than $\hat{\lambda}_{\rm D}$, the turbulent
structures undergo a succession of disruptions due to reconnection, limiting
dynamic alignment, steepening the effective spectral index and changing the
final dissipation scale. The scaling of $\hat{\lambda}_{\rm D}$ with the
Lundquist (magnetic Reynolds) number $S_{L_\perp}$ depends on the order of the
statistics being considered, and on the specific model of intermittency; the
transition between the two regimes in the energy spectrum is predicted at
approximately $\hat{\lambda}_{\rm D} \sim S_{L_\perp}^{-0.6}$. The spectral
index below $\hat{\lambda}_{\rm D}$ is bounded between $-5/3$ and $-2.3$. The
final dissipation scale is at $\hat{\lambda}_{\eta,\infty}\sim
S_{L_\perp}^{-3/4}$, the same as the Kolmogorov scale arising in theories of
turbulence that do not involve scale-dependent dynamic alignment.",1612.07604v3
2005-02-16,Global Dynamical Evolution of the ISM in Star Forming Galaxies - I. High Resolution 3D HD and MHD Simulations: Effect of the Magnetic Field,"In star forming disk galaxies, matter circulation between stars and the
interstellar gas, and, in particular the energy input by random and clustered
supernova explosions, determine the dynamical and chemical evolution of the
ISM, and hence of the galaxy as a whole. Using a 3D MHD code with adaptive mesh
refinement developed for this purpose, we have investigated the r\^ole of
magnetized matter circulation between the gaseous disk and the surrounding
galactic halo. Special emphasis has been put on the effect of the magnetic
field with respect to the volume and mass fractions of the different ISM
``phases'', the relative importance of ram, thermal and magnetic pressures, and
whether the field can prevent matter transport from the disk into the halo. The
simulations were performed on a grid with an area of 1 kpc$^{2}$, centered on
the solar circle, extending $\pm 10$ kpc perpendicular to the galactic disk
with a resolution as high as 1.25 pc. The simulations were run for a time scale
of 400 Myr, sufficiently long to avoid memory effects of the initial setup, and
to allow for a global dynamical equilibrium to be reached in case of a constant
energy input rate. (...) We find that in general gas transport into the halo in
3D is not prevented by an initial disk parallel magnetic field, but only
delayed initially, for as long as it is needed to punch holes into the thick
magnetized gas disk. The mean volume filling factor of the hot phase in the
disk is similar in HD and MHD (the latter with a total field strength of 4.4
$\mu$G) runs, amounting to $\sim 17-21%$ for the Galactic supernova rate.",0502327v2
2007-01-10,Magnetic fields in barred galaxies. V. Modelling NGC 1365,"We present a model of the global magnetic field in the barred galaxy NGC 1365
based jointly on the large-scale velocity field of interstellar gas fitted to
HI and CO observations of this galaxy and on mean-field dynamo theory. The aim
of the paper is to present a detailed quantitative comparison of a galactic
dynamo model with independent radio observations. We consider several gas
dynamical and nonlinear dynamo models that include plausible variations of
parameters that are poorly known. Models of cosmic ray distribution in the
galaxy are introduced to produce synthetic radio polarization maps allowing
direct comparison with those observed at 3.5cm and 6.2cm. We show that the
dynamo model is robust in that the most important magnetic features are
controlled by the relatively well established properties of the density
distribution and gas velocity field. The optimal agreement between the
synthetic polarization maps and observations is obtained when a uniform cosmic
ray distribution is adopted. We find some indirect evidence for enhanced
turbulence in the regions of strong velocity shear and within 1-2kpc of the
galactic centre. We confirm that magnetic stresses can drive an inflow of gas
into the inner 1kpc of the galaxy at a rate of a few solar masses per year. The
dynamo models are successful to some extent in modelling the large scale
regular magnetic field in this galaxy. Our results demonstrate that dynamo
models and synthetic polarization maps can provide information about both the
gas dynamical models and conditions in the interstellar medium. We demonstrate
that the dynamical effects of magnetic fields cannot be everywhere ignored in
galaxy modelling.",0701277v1
2007-12-17,Dynamical Simulations of Magnetically Channeled Line-Driven Stellar Winds: II. The Effects of Field-Aligned Rotation,"Building upon our previous MHD simulation study of magnetic channeling in
radiatively driven stellar winds, we examine here the additional dynamical
effects of stellar {\em rotation} in the (still) 2-D axisymmetric case of an
aligned dipole surface field. In addition to the magnetic confinement parameter
$\eta_{\ast}$ introduced in Paper I, we characterize the stellar rotation in
terms of a parameter $W \equiv V_{\rm{rot}}/V_{\rm{orb}}$ (the ratio of the
equatorial surface rotation speed to orbital speed), examining specifically
models with moderately strong rotation $W =$ 0.25 and 0.5, and comparing these
to analogous non-rotating cases. Defining the associated Alfv\'{e}n radius
$R_{\rm{A}} \approx \eta_{\ast}^{1/4} \Rstar$ and Kepler corotation radius
$R_{\rm{K}} \approx W^{-2/3} \Rstar$, we find rotation effects are weak for
models with $R_{\rm{A}} < R_{\rm{K}}$, but can be substantial and even dominant
for models with $R_{\rm{A}} \gtwig R_{\rm{K}}$. In particular, by extending our
simulations to magnetic confinement parameters (up to $\eta_{\ast} = 1000$)
that are well above those ($\eta_{\ast} = 10$) considered in Paper I, we are
able to study cases with $R_{\rm{A}} \gg R_{\rm{K}}$; we find that these do
indeed show clear formation of the {\em rigid-body} disk predicted in previous
analytic models, with however a rather complex, dynamic behavior characterized
by both episodes of downward infall and outward breakout that limit the buildup
of disk mass. Overall, the results provide an intriguing glimpse into the
complex interplay between rotation and magnetic confinement, and form the basis
for a full MHD description of the rigid-body disks expected in strongly
magnetic Bp stars like $\sigma$ Ori E.",0712.2780v1
2011-02-10,A Constraint on the Organization of the Galactic Center Magnetic Field Using Faraday Rotation,"We present new 6 and 20 cm Very Large Array (VLA) observations of polarized
continuum emission of roughly 0.5 square degrees of the Galactic center (GC)
region. The 6 cm observations detect diffuse linearly-polarized emission
throughout the region with a brightness of roughly 1 mJy per 15""x10"" beam. The
Faraday rotation measure (RM) toward this polarized emission has structure on
degree size scales and ranges from roughly +330 rad/m2 east of the dynamical
center (Sgr A) to -880 rad/m2 west of the dynamical center. This RM structure
is also seen toward several nonthermal radio filaments, which implies that they
have a similar magnetic field orientation and constrains models for their
origin. Modeling shows that the RM and its change with Galactic longitude are
best explained by the high electron density and strong magnetic field of the GC
region. Considering the emissivity of the GC plasma shows that while the
absolute RM values are indirect measures of the GC magnetic field, the RM
longitude structure directly traces the magnetic field in the central
kiloparsec of the Galaxy. Combining this result with previous work reveals a
larger RM structure covering the central ~2 degrees of the Galaxy. This RM
structure is similar to that proposed by Novak and coworkers, but is shifted
roughly 50 pc west of the dynamical center of the Galaxy. If this RM structure
originates in the GC region, it shows that the GC magnetic field is organized
on ~300 pc size scales. The pattern is consistent with a predominantly poloidal
field geometry, pointing from south to north, that is perturbed by the motion
of gas in the Galactic disk.",1102.2204v1
2014-11-11,Magnonic Charge Pumping via Spin-Orbit Coupling,"The interplay between spin, charge, and orbital degrees of freedom has led to
the development of spintronic devices like spin-torque oscillators, spin-logic
devices, and spin-transfer torque magnetic random-access memories. In this
development spin pumping, the process where pure spin-currents are generated
from magnetisation precession, has proved to be a powerful method for probing
spin physics and magnetisation dynamics. The effect originates from direct
conversion of low energy quantised spin-waves in the magnet, known as magnons,
into a flow of spins from the precessing magnet to adjacent normal metal leads.
The spin-pumping phenomenon represents a convenient way to electrically detect
magnetisation dynamics, however, precessing magnets have been limited so far to
pump pure spin currents, which require a secondary spin-charge conversion
element such as heavy metals with large spin Hall angle or multi-layer layouts
to be detectable. Here, we report the experimental observation of charge
pumping in which a precessing ferromagnet pumps a charge current, demonstrating
direct conversion of magnons into high-frequency currents via the relativistic
spin-orbit interaction. The generated electric current, differently from spin
currents generated by spin-pumping, can be directly detected without the need
of any additional spin to charge conversion mechanism and amplitude and phase
information about the relativistic current-driven magnetisation dynamics. The
charge-pumping phenomenon is generic and gives a deeper understanding of the
recently observed spin-orbit torques, of which it is the reciprocal effect and
which currently attract interest for their potential in manipulating magnetic
information. Furthermore, charge pumping provides a novel link between
magnetism and electricity and may find application in sourcing alternating
electric currents.",1411.2779v1
2017-04-05,"Interplay of transitions between oscillations with emergence of fireballs and quantification of phase coherence, scaling index in a magnetized glow discharge plasma of toroidal assembly","Interplay of transition of floating potential fluctuations in a glow
discharge plasma in the toroidal vacuum vessel of SINP tokamak has been
observed. With variation in the strength of the vertical and toroidal magnetic
fields, regular and inverted relaxation oscillations as well as sinusoidal
oscillations are observed with the slow and fast time scale of the relaxation
oscillations reversing their nature at a high value of vertical magnetic field
strength. However for small value of toroidal magnetic field the transitions
follow relaxation $\rightarrow$ chaotic oscillations with the chaotic nature
prevailing at higher values of toroidal magnetic field. Evolution of associated
anode fireball dynamics under the action of increasing vertical, toroidal as
well as increasing vertical field at a fixed toroidal field (mixed field) of
different strength has been studied. Estimation of phase coherence index for
each case has been carried out to examine the evidence of finite nonlinear
interaction. A comprehensive study of the dynamics of the fireball is found to
be associated with the values of phase coherence index. The index is found to
take maximum values for the case of toroidal, mixed field when there is an
existence of power/energy concentration in a large region of frequency band. A
detailed study of the scaling region using detrended fluctuation analysis (DFA)
by estimating the scaling exponent has been carried out for increasing values
of discharge voltage, vertical, toroidal as well as the mixed field (toroidal
plus vertical). A persistence long range behaviour associated with the nature
of the anode glow has been investigated in case of higher values of toroidal,
mixed field whereas increasing DV, vertical magnetic field leads to a perfectly
correlated dynamics with values of scaling exponent greater than unity",1704.01325v1
2019-01-14,A 3D kinematic Babcock Leighton solar dynamo model sustained by dynamic magnetic buoyancy and flux transport processes,"Magnetohydrodynamic interactions between plasma flows and magnetic fields is
fundamental to the origin and sustenance of the 11-year sunspot cycle. These
processes are intrinsically three-dimensional (3D) in nature. Our goal is to
construct a 3D solar dynamo model that on the one hand captures the buoyant
emergence of tilted bipolar sunspot pairs, and on the other hand produces
cyclic large-scale field reversals mediated via surface flux-transport
processes -- that is, the Babcock-Leighton mechanism. We perform kinematic
dynamo simulations where the prescribed velocity field is a combination of
solar-like differential rotation and meridional circulation, along with a
parametrized turbulent diffusivity. We use a novel methodology for modeling
magnetic buoyancy through field-strength-dependent 3D helical up-flows that
results in the formation of tilted bipolar sunspots. The bipolar spots produced
in our simulations participate in the process of poloidal-field generation
through the Babcock-Leighton mechanism, resulting in self-sustained and
periodic large-scale magnetic field reversal. Our parameter space study varying
the amplitude of the meridional flow, the convection zone diffusivity, and
parameters governing the efficiency of the magnetic buoyancy mechanism reveal
their relative roles in determining properties of the sunspot cycle such as
amplitude, period, and dynamical memory relevant to solar cycle prediction. We
also derive a new dynamo number for the Babcock-Leighton solar dynamo mechanism
which reasonably captures our model dynamics. This study elucidates the
relative roles of different flux-transport processes in the Sun's convection
zone in determining the properties and physics of the sunspot cycle and could
potentially lead to realistic, data-driven 3D dynamo models for solar-activity
predictions and exploration of stellar magnetism and starspot formation in
other stars.",1901.04251v1
2020-07-06,Spin-canting effects in GMR sensors with wide dynamic field range,"Magnetoresistive (xMR) sensors find extensive application in science and
industry, replacing Hall sensors in various low field environments. While there
have been some efforts in increasing the dynamic field range of xMR sensors,
Hall sensors remain to dominate high field applications due to their wide
linear range. Using a perpendicular magnetized reference system and an in-plane
free layer allows us to overcome this disadvantage of xMR sensors, and,
furthermore, investigate spin-canting effects in interlayer exchange coupled
perpendicular synthetic antiferromagnets (p-SAF). We created p-SAFs with
exchange coupling fields of up to 10 kOe, based on magnetic Co/Pt multilayer
systems. The p-SAFs are either designed as ""single"" p-SAFs, where two Co/Pt
multilayers are interlayer exchange coupled via a 4 {\AA} thick Ru spacer, or
as ""double"" p-SAFs, where an additional Co layer is interlayer exchange coupled
to the top multilayer. These p-SAFs are used for giant magnetoresistance (GMR)
sensors with wide dynamic field range. By using a p-SAF as the reference system
and employing an in-plane magnetic layer as the GMR's free layer, the linear
range can be effectively increased limited only by the p-SAF's switching
fields. Additionally, the magnetic anisotropy of the in-plane free layer is
fully controlled, which allows saturation fields by design. Different
configurations were investigated, ranging from free layer magnetic saturation
at lower to far higher fields than the p-SAF's switching fields. We can show
through micromagnetic simulations that certain GMR transfer curves are
dominated by spin-canting effects in the interlayer exchange coupled reference
system. Finally, our simulation results lay out the correlation of the p-SAF's
design parameters and its magnetization reversal behavior.",2007.02640v1
2020-08-17,A global model of particle acceleration at pulsar wind termination shocks,"Pulsar wind nebulae are efficient particle accelerators, and yet the
processes at work remain elusive. Self-generated, microturbulence is too weak
in relativistic magnetized shocks to accelerate particles over a wide energy
range, suggesting that the global dynamics of the nebula may be involved in the
acceleration process instead. In this work, we study the role played by the
large-scale anisotropy of the transverse magnetic field profile on the shock
dynamics. We performed large two-dimensional particle-in-cell simulations for a
wide range of upstream plasma magnetizations. A large-scale velocity shear and
current sheets form in the equatorial regions and at the poles, where they
drive strong plasma turbulence via Kelvin-Helmholtz vortices and kinks. The
mixing of current sheets in the downstream flow leads to efficient nonthermal
particle acceleration. The power-law spectrum hardens with increasing
magnetization, akin to those found in relativistic reconnection and kinetic
turbulence studies. The high end of the spectrum is composed of particles
surfing on the wake produced by elongated spearhead-shaped cavities forming at
the shock front and piercing through the upstream flow. These particles are
efficiently accelerated via the shear-flow acceleration mechanism near the Bohm
limit. Magnetized relativistic shocks are very efficient particle accelerators.
Capturing the global dynamics of the downstream flow is crucial to
understanding them, and therefore local plane parallel studies may not be
appropriate for pulsar wind nebulae and possibly other astrophysical
relativistic magnetized shocks. A natural outcome of such shocks is a variable
and Doppler-boosted synchrotron emission at the high end of the spectrum
originating from the shock-front cavities, reminiscent of the mysterious Crab
Nebula gamma-ray flares.",2008.07253v1
2020-10-26,Dynamics of test particles around renormalization group improved Schwarzschild black holes,"In this paper we have investigated the dynamics of neutral, electrically
charged and magnetized particles around renormalized group improved (RGI)
Schwarzschild black hole in the presence of external asymptotically uniform
magnetic field. We have analyzed the spacetime structure around RGI black hole
by investigating Ricci, the square of Ricci tensor and Kretschmann curvature
scalars and shown that only in the case when the parameter $\gamma=0$ the
curvature becomes infinite at the center of the black hole, while for non-zero
values of $\gamma$ parameter the black hole curvature reflects the properties
of regular black hole. Analyzing the innermost stable circular orbits of test
neutral particles around RGI black hole and comparing with the results for
rotating Kerr black hole we have shown that RGI black hole parameters can mimic
the rotation parameter of Kerr black hole upto $a/M \lesssim 0.31$ providing
the same ISCO radius. Since according to the astronomical observations of the
accretion disks confirm that the astrophysical black holes are rapidly rotating
with the spin parameter upto $a/M \sim 0.99$ one may conclude that the effects
of parameters of RGI Schwarzschild black hole on the circular orbits of the
neutral particles can not mimic the Kerr black hole. Then the Hamilton-Jacobi
equation has been used to analyze the charged and magnetized particles motion
near the RGI black hole in the presence of the strong interaction between
external asymptotically uniform magnetic (electromagnetic) field and magnetized
(electrically charged) particle. We have shown that RGI black hole parameters
quantitatively change the dynamics of the charged and magnetized particles, in
particular ISCO radius of the particles decreases with increasing the parameter
$\lambda$, while the increase of the parameter $\gamma$ causes to increase of
it.",2010.15079v1
2021-08-10,Gravity Versus Magnetic Fields in Forming Molecular Clouds,"Magnetic fields are dynamically important in the diffuse interstellar medium.
Understanding how gravitationally bound, star-forming clouds form requires
modeling of the fields in a self-consistent, supernova-driven, turbulent,
magnetized, stratified disk. We employ the FLASH magnetohydrodynamics code to
follow the formation and early evolution of clouds with final masses of 3-8
$\times 10^3 M_{\odot}$ within such a simulation. We use the code's adaptive
mesh refinement capabilities to concentrate numerical resolution in zoom-in
regions covering single clouds, allowing us to investigate the detailed
dynamics and field structure of individual self-gravitating clouds in a
consistent background medium. Our goal is to test the hypothesis that dense
clouds are dynamically evolving objects far from magnetohydrostatic
equilibrium. We find that the cloud envelopes are magnetically supported with
field lines parallel to density gradients and flow velocity, as indicated by
the histogram of relative orientations and other statistical measures. In
contrast, the dense cores of the clouds are gravitationally dominated, with
gravitational energy exceeding internal, kinetic, or magnetic energy and
accelerations due to gravity exceeding those due to magnetic or thermal
pressure gradients. In these regions field directions vary strongly, with a
slight preference towards being perpendicular to density gradients, as shown by
three-dimensional histograms of relative orientation.",2108.04967v2
2003-07-27,Magnetohydrodynamic Density Waves in a Composite Disk System of Interstellar Medium and Cosmic-Ray Gas,"Multi-wavelength observations from radio to soft X-ray bands of large-scale
galactic spiral structures offer synthesized and comprehensive views of nearby
disk galaxies. In the presence of a massive dark-matter halo, the density-wave
dynamics on galactic scales involves the stellar disk, the gas disk of
interstellar medium (ISM), the magnetic field, and the cosmic-ray gas (CRG). In
this paper, we explore the dynamic and electromagnetic interplay between the
magnetized ISM disk and CRG disk so that structural and diagnostic features of
optical, infrared, and synchrotron radio-continuum emissions from a spiral
galaxy can be physically understood. On timescales of galactic density waves,
cosmic rays collectively may be treated as a relativistically hot tenuous gas
fluid that is tied to the large-scale mean magnetic field in transverse bulk
motions but moves otherwise differently along the magnetic field relative to
the ISM. For both fast and slow magnetohydrodynamic (MHD) density waves in a
composite disk system of magnetized ISM and CRG, the minute CRG mass density
enhancement is phase shifted relative to the enhancement of parallel magnetic
field. Owing to an extremely small number of cosmic rays, the large-scale
magnetic field enhancement dominates in synchrotron radio-continuum emissions
(as if the CRG is almost unperturbed) for spiral structural manifestations. In
addition to the fast and slow MHD density waves, there also exists a
suprathermal MHD wave mode by which CRG adjusts itself with an effective
suprathermal sound speed close to the speed of light c.",0307466v1
2004-10-22,Development of supersonic plasma flows by use of a magnetic nozzle and an ICRF heating,"A high-beta, supersonic plasma flow plays a crucial role in MHD phenomena in
space and fusion plasmas. There are a few experimental researches on production
and control of a fast flowing plasma in spite of a growing significance in the
magnetized-plasma flow dynamics. A magneto-plasma-dynamic arcjet (MPDA) is one
of promising devices to produce a supersonic plasma flow and has been utilized
as an electric propulsion device with a higher specific impulse and a
relatively larger thrust. We have improved the performance of an MPDA to
produce a quasi-steady plasma flow with a transonic and supersonic Mach number
in a highly-ionized state. There are two methods in order to control an
ion-acoustic Mach number of the plasma flow exhausted from an MPDA: one is to
use a magnetic Laval nozzle to convert a thermal energy to a flow energy and
the other is a combined system of an ion heating and a divergent magnetic
nozzle. The former is an analogous method to a compressible air flow and the
latter is the method proposed in an advanced thruster for a manned
interplanetary space mission. We have clarified the plasma flow characteristics
in various shapes of a magnetic field configuration. It was demonstrated that
the Mach number of the plasma flow could increase up to almost 3 in a divergent
magnetic nozzle field. This paper reports recent results on the flow field
improvements: one is on a magnetic-Laval-nozzle effects observed at the muzzle
region of the MPDA, and the other is on ICRF (ion-cyclotron-range of frequency)
heating of a supersonic plasma by use of a helical antenna.",0410205v1
2008-06-03,Euler-Lagrange models with complex currents of three-phase electrical machines and observability issues,"A new Lagrangian formulation with complex currents is developed and yields a
direct and simple method for modeling three-phase permanent-magnet and
induction machines. The Lagrangian is the sum a mechanical one and of a
magnetic one. This magnetic Lagrangian is expressed in terms of rotor angle,
complex stator and rotor currents. A complexification procedure widely used in
quantum electrodynamic is applied here in order to derive the Euler-Lagrange
equations with complex stator and rotor currents. Such complexification process
avoids the usual separation into real and imaginary parts and simplifies
notably the calculations. Via simple modifications of such magnetic Lagrangians
we derive new dynamical models describing permanent-magnet machines with both
saturation and saliency, and induction machines with both magnetic saturation
and space harmonics. For each model we also provide its Hamiltonian thus its
magnetic energy. This energy is also expressed with complex currents and can be
directly used in Lyapunov and/or passivity based control. Further, we briefly
investigate the observability of this class of Euler-Lagrange models, in the
so-called sensorless case when the measured output is the stator current and
the load torque is constant but unknown. For all the dynamical models obtained
via such variational principles, we prove that their linear tangent systems are
unobservable around a one-dimensional family of steady-states attached to the
same constant stator voltage and current. This negative result explains why
sensorless control of three-phase electrical machines around zero stator
frequency remains yet a difficult control problem.",0806.0387v4
2008-09-19,"Magnetism, FeS colloids, and Origins of Life","A number of features of living systems: reversible interactions and weak
bonds underlying motor-dynamics; gel-sol transitions; cellular connected
fractal organization; asymmetry in interactions and organization; quantum
coherent phenomena; to name some, can have a natural accounting via $physical$
interactions, which we therefore seek to incorporate by expanding the horizons
of `chemistry-only' approaches to the origins of life. It is suggested that the
magnetic 'face' of the minerals from the inorganic world, recognized to have
played a pivotal role in initiating Life, may throw light on some of these
issues. A magnetic environment in the form of rocks in the Hadean Ocean could
have enabled the accretion and therefore an ordered confinement of
super-paramagnetic colloids within a structured phase. A moderate H-field can
help magnetic nano-particles to not only overcome thermal fluctuations but also
harness them. Such controlled dynamics brings in the possibility of accessing
quantum effects, which together with frustrations in magnetic ordering and
hysteresis (a natural mechanism for a primitive memory) could throw light on
the birth of biological information which, as Abel argues, requires a
combination of order and complexity. This scenario gains strength from
observations of scale-free framboidal forms of the greigite mineral, with a
magnetic basis of assembly. And greigite's metabolic potential plays a key role
in the mound scenario of Russell and coworkers-an expansion of which is
suggested for including magnetism.",0809.3316v5
2011-02-01,A new Jeans resolution criterion for (M)HD simulations of self-gravitating gas: Application to magnetic field amplification by gravity-driven turbulence,"Cosmic structure formation is characterized by the complex interplay between
gravity, turbulence, and magnetic fields. The processes by which gravitational
energy is converted into turbulent and magnetic energies, however, remain
poorly understood. Here, we show with high-resolution, adaptive-mesh
simulations that MHD turbulence is efficiently driven by extracting energy from
the gravitational potential during the collapse of a dense gas cloud.
Compressible motions generated during the contraction are converted into
solenoidal, turbulent motions, leading to a natural energy ratio of E_sol/E_tot
of approximately 2/3. We find that the energy injection scale of gravity-driven
turbulence is close to the local Jeans scale. If small seeds of the magnetic
field are present, they are amplified exponentially fast via the small-scale
dynamo process. The magnetic field grows most efficiently on the smallest
scales, for which the stretching, twisting, and folding of field lines, and the
turbulent vortices are sufficiently resolved. We find that this scale
corresponds to about 30 grid cells in the simulations. We thus suggest a new
minimum resolution criterion of 30 cells per Jeans length in
(magneto)hydrodynamical simulations of self-gravitating gas, in order to
resolve turbulence on the Jeans scale, and to capture minimum dynamo
amplification of the magnetic field. Due to numerical diffusion, however, any
existing simulation today can at best provide lower limits on the physical
growth rates. We conclude that a small, initial magnetic field can grow to
dynamically important strength on time scales significantly shorter than the
free-fall time of the cloud.",1102.0266v2
2014-04-07,The Helioseismic and Magnetic Imager (HMI) Vector Magnetic Field Pipeline: Overview and Performance,"The Helioseismic and Magnetic Imager (HMI) began near-continuous full-disk
solar measurements on 1 May 2010 from the Solar Dynamics Observatory (SDO). An
automated processing pipeline keeps pace with observations to produce
observable quantities, including the photospheric vector magnetic field, from
sequences of filtergrams. The primary 720s observables were released in mid
2010, including Stokes polarization parameters measured at six wavelengths as
well as intensity, Doppler velocity, and the line-of-sight magnetic field. More
advanced products, including the full vector magnetic field, are now available.
Automatically identified HMI Active Region Patches (HARPs) track the location
and shape of magnetic regions throughout their lifetime.
  The vector field is computed using the Very Fast Inversion of the Stokes
Vector (VFISV) code optimized for the HMI pipeline; the remaining 180 degree
azimuth ambiguity is resolved with the Minimum Energy (ME0) code. The
Milne-Eddington inversion is performed on all full-disk HMI observations. The
disambiguation, until recently run only on HARP regions, is now implemented for
the full disk. Vector and scalar quantities in the patches are used to derive
active region indices potentially useful for forecasting; the data maps and
indices are collected in the SHARP data series, hmi.sharp_720s. Patches are
provided in both CCD and heliographic coordinates.
  HMI provides continuous coverage of the vector field, but has modest spatial,
spectral, and temporal resolution. Coupled with limitations of the analysis and
interpretation techniques, effects of the orbital velocity, and instrument
performance, the resulting measurements have a certain dynamic range and
sensitivity and are subject to systematic errors and uncertainties that are
characterized in this report.",1404.1881v1
2015-02-27,Blowing Magnetic Skyrmion Bubbles,"Soap bubbles form when blowing air through a suspended thin film of soapy
water and this phenomenon entertains children and adults alike. The formation
of soap bubbles from thin films is accompanied by topological transitions, and
thus the natural question arises whether this concept is applicable to the
generation of other topological states. Here we show how a magnetic topological
structure, namely a skyrmion bubble, can be generated in a solid state system
in a similar manner. Beyond enabling the investigation of complex
surface-tension driven dynamics in a novel physical system, this observation
has also practical implications, since the topological charge of magnetic
skyrmions has been envisioned as an information carrier for new data processing
technologies. A main goal towards this end is the experimental creation and
manipulation of individual mobile skyrmions at room temperature. By utilizing
an inhomogeneous in-plane current in a system with broken inversion asymmetry,
we experimentally blow magnetic skyrmion bubbles through a geometrical
constriction. The presence of a spatially divergent spin-orbit torque gives
rise to instabilities of the magnetic domain structures that are reminiscent of
Rayleigh-Plateau instabilities in fluid flows. Experimentally we can determine
the electric current versus magnetic field phase diagram for skyrmion formation
and we reveal the efficient manipulation of these dynamically created
skyrmions, including depinning and motion. The demonstrated current-driven
transformation from stripe domains to magnetic skyrmion bubbles could provide
additional avenues for implementing skyrmion-based spintronics.",1502.08028v1
2015-12-20,Turbulent dynamo in a collisionless plasma,"Magnetic fields pervade the entire Universe and affect the formation and
evolution of astrophysical systems from cosmological to planetary scales. The
generation and dynamical amplification of extragalactic magnetic fields through
cosmic times, up to $\mu$Gauss levels reported in nearby galaxy clusters, near
equipartition with kinetic energy of plasma motions and on scales of at least
tens of kiloparsecs, is a major puzzle largely unconstrained by observations. A
dynamo effect converting kinetic flow energy into magnetic energy is often
invoked in that context, however extragalactic plasmas are weakly collisional
(as opposed to magnetohydrodynamic fluids), and whether magnetic-field growth
and sustainment through an efficient turbulent dynamo instability is possible
in such plasmas is not established. Fully kinetic numerical simulations of the
Vlasov equation in a six-dimensional phase space necessary to answer this
question have until recently remained beyond computational capabilities. Here,
we show by means of such simulations that magnetic-field amplification via a
dynamo instability does occur in a stochastically-driven, non-relativistic
subsonic flow of initially unmagnetized collisionless plasma. We also find that
the dynamo self-accelerates and becomes entangled with kinetic instabilities as
magnetization increases. The results suggest that such a plasma dynamo may be
realizable in laboratory experiments, support the idea that intracluster medium
(ICM) turbulence may have significantly contributed to the amplification of
cluster magnetic fields up to near-equipartition levels on a timescale shorter
than the Hubble time, and emphasize the crucial role of multiscale kinetic
physics in high-energy astrophysical plasmas.",1512.06455v2
2017-11-10,Artificial Magnetic Field Quenches in Synthetic Dimensions,"Recent cold atom experiments have realized models where each hyperfine state
at an optical lattice site can be regarded as a separate site in a synthetic
dimension. In such synthetic ribbon configurations, manipulation of the
transitions between the hyperfine levels provide direct control of the hopping
in the synthetic dimension. This effect was used to simulate a magnetic field
through the ribbon. Precise control over the hopping matrix elements in the
synthetic dimension makes it possible to change this artificial magnetic field
much faster than the time scales associated with atomic motion in the lattice.
In this paper, we consider such a magnetic flux quench scenario in synthetic
dimensions. Sudden changes have not been considered for real magnetic fields as
such changes in a conducting system would result in large induced currents.
Hence, we first study the difference between a time varying real magnetic field
and an artificial magnetic field using a minimal six site model. This minimal
model clearly shows the connection between gauge dependence and the lack of on
site induced scalar potential terms. We then investigate the dynamics of a
wavepacket in an infinite two or three leg ladder following a flux quench and
find that the gauge choice has a dramatic effect on the packet dynamics.
Specifically, a wavepacket splits into a number of smaller packets. Both the
weights and the number of packets depend on the implemented gauge. If an
initial packet, prepared under zero flux in a n--leg ladder, is quenched to
Hamiltonian with a vector potential parallel to the ladder; it splits into at
most $n$ smaller wavepackets. The same initial packet splits up to $n^2$
packets if the vector potential is implemented to be along the rungs. Finally,
we show that edge states in a thick ribbon are robust under the quench only
when the same gap supports an edge state for the final Hamiltonian.",1711.03777v2
2017-12-15,Optimization of Photospheric Electric Field Estimates for Accurate Retrieval of Total Magnetic Energy Injection,"Estimates of the photospheric magnetic, electric and plasma velocity fields
are essential for studying the dynamics of the solar atmosphere, for example
through the derivative quantities of Poynting and relative helicity flux and by
using of the fields to obtain the lower boundary condition for data-driven
coronal simulations. In this paper we study the performance of a data
processing and electric field inversion approach that requires only
high-resolution and high-cadence line-of-sight or vector magnetograms -- which
we obtain from Helioseismic and Magnetic Imager (HMI) onboard Solar Dynamics
Observatory (SDO). The approach does not require any photospheric velocity
estimates, and the lacking velocity information is compensated using ad hoc
assumptions. We show that the free parameters of these assumptions can be
optimized to reproduce the time evolution of the total magnetic energy
injection through the photosphere in NOAA AR 11158, when compared to the recent
estimates for this active region. However, we find that the relative magnetic
helicity injection is reproduced poorly reaching at best a modest
underestimation. We discuss also the effect of some of the data processing
details on the results, including the masking of the noise-dominated pixels and
the tracking method of the active region, both of which have not received much
attention in the literature so far. In most cases the effect of these details
is small, but when the optimization of the free parameters of the ad hoc
assumptions is considered a consistent use of the noise mask is required. The
results found in this paper imply that the data processing and electric field
inversion approach that uses only the photospheric magnetic field information
offers a flexible and straightforward way to obtain photospheric magnetic and
electric field estimates suitable for practical applications such as coronal
modeling studies.",1712.05757v1
2018-07-15,LiZn$_{2}$V$_{3}$O$_{8}$: A new geometrically frustrated cluster spin-glass,"We have investigated the structural and magnetic properties of a new cubic
spinel LiZn$_{2}$V$_{3}$O$_{8}$ (LZVO) through x-ray diffraction, dc and ac
susceptibility, magnetic relaxation, aging, memory effect, heat capacity and
$^{7}$Li nuclear magnetic resonance (NMR) measurements. A Curie-Weiss fit of
the dc susceptibility $\chi_{\mathrm{dc}}$($\mathit{T}$) yields a Curie-Weiss
temperature $\mathrm{\theta}_{\mathrm{CW}}$ = -185 K. This suggests strong
antiferromagnetic (AFM) interactions among the magnetic vanadium ions. The dc
and ac susceptibility data indicate the spin-glass behavior below a freezing
temperature $T_{f}$ $\simeq$ 3 K. The frequency dependence of the $T_{f}$ is
characterized by the Vogel-Fulcher law and critical dynamic scaling behavior or
power law. From both fitting, we obtained the value of the characteristic
angular frequency $\omega_{0}$ $\approx$ 3.56$\times$10$^{6}$ Hz, the dynamic
exponent $\mathit{zv}$ $\approx$ 2.65, and the critical time constant
$\tau_{0}$ $\approx$ 1.82$\times$10$^{-6}$ s, which falls in the conventional
range for typical cluster spin-glass (CSG) systems. The value of relative shift
in freezing temperature $\delta T_{f}$ $\simeq$ 0.039 supports a CSG ground
states. We also found aging phenomena and memory effects in LZVO. The
asymmetric response of the magnetic relaxation below $T_{f}$ supports the
hierarchical model. Heat capacity data show no long-range or short-range
ordering down to 2 K. Only about 25% magnetic entropy change $(\Delta
S_{\mathrm{m}})$ signifies the presence of strong frustration in the system.
The $^{7}$Li NMR spectra show a shift and broadening with decreasing
temperature. The spin-lattice and spin-spin relaxation rates show anomalies due
to spin freezing around 3 K as the bulk magnetization.",1807.05612v3
2019-09-23,Does magnetic field impact tidal dynamics inside the convective zone of low-mass stars along their evolution?,"The dissipation of the kinetic energy of wave-like tidal flows within the
convective envelope of low-mass stars is one of the key physical mechanisms
that shapes the orbital and rotational dynamics of short-period exoplanetary
systems. Although low-mass stars are magnetically active objects, the question
of how the star's magnetic field impacts large-scale tidal flows and the
excitation, propagation and dissipation of tidal waves still remains open. Our
goal is to investigate the impact of stellar magnetism on the forcing of tidal
waves, and their propagation and dissipation in the convective envelope of
low-mass stars as they evolve. We have estimated the amplitude of the magnetic
contribution to the forcing and dissipation of tidally induced magneto-inertial
waves throughout the structural and rotational evolution of low-mass stars
(from M to F-type). For this purpose, we have used detailed grids of rotating
stellar models computed with the stellar evolution code STAREVOL. The amplitude
of dynamo-generated magnetic fields is estimated via physical scaling laws at
the base and the top of the convective envelope. We find that the large-scale
magnetic field of the star has little influence on the excitation of tidal
waves in the case of nearly-circular orbits and coplanar hot-Jupiter planetary
systems, but that it has a major impact on the way waves are dissipated. Our
results therefore indicate that a full magneto-hydrodynamical treatment of the
propagation and dissipation of tidal waves is needed to properly assess the
impact of star-planet tidal interactions throughout the evolutionary history of
low-mass stars hosting short-period massive planets.",1909.10490v1
2019-09-30,First-principles theory of proximity spin-orbit torque on a two-dimensional magnet: Current-driven antiferromagnet-to-ferromagnet reversible transition in bilayer CrI$_3$,"The recently discovered two-dimensional (2D) magnetic insulator CrI$_3$ is an
intriguing case for basic research and spintronic applications since it is a
ferromagnet in the bulk, but an antiferromagnet in bilayer form, with its
magnetic ordering amenable to external manipulations. Using first-principles
quantum transport approach, we predict that injecting unpolarized charge
current parallel to the interface of bilayer-CrI$_3$/monolayer-TaSe$_2$ van der
Waals heterostructure will induce spin-orbit torque (SOT) and thereby driven
dynamics of magnetization on the first monolayer of CrI$_3$ in direct contact
with TaSe$_2$. By combining calculated complex angular dependence of SOT with
the Landau-Lifshitz-Gilbert equation for classical dynamics of magnetization,
we demonstrate that current pulses can switch the direction of magnetization on
the first monolayer to become parallel to that of the second monolayer, thereby
converting CrI$_3$ from antiferromagnet to ferromagnet while not requiring any
external magnetic field. We explain the mechanism of this reversible
current-driven nonequilibrium phase transition by showing that first monolayer
of CrI$_3$ carries current due to evanescent wavefunctions injected by metallic
transition metal dichalcogenide TaSe$_2$, while concurrently acquiring strong
spin-orbit coupling (SOC) via such proximity effect, whereas the second
monolayer of CrI$_3$ remains insulating. The transition can be detected by
passing vertical read current through the vdW heterostructure, encapsulated by
bilayer of hexagonal boron nitride and sandwiched between graphite electrodes,
where we find tunneling magnetoresistance of $\simeq 240$%.",1909.13876v2
2020-10-23,"An observational correlation between magnetic field, angular momentum and fragmentation in the envelopes of Class 0 protostars?","To assess the potential role of magnetic fields in regulating the envelope
rotation and the fragmentation of Class 0 protostars, we carried out
observations of the dust polarized emission at 0.87 mm with the SMA, in the
envelopes of a large sample of 20 Class 0 protostars. We estimate the mean
magnetic field orientation over the central 1000 au envelope scales and
compared it to that of the protostellar outflow in order to study the relation
between their misalignment and the kinematics of the circumstellar gas. We
discover a strong relationship between the misalignment of the magnetic field
orientation with the outflow and the amount of angular momentum observed at
similar scales in the protostellar envelope, revealing a potential link between
the kinetic and the magnetic energy at envelope scales. The relation could be
driven by favored B misalignments in more dynamical envelopes or a dependence
of the envelope dynamics with the large-scale B initial configuration.
Comparing the trend with the presence of fragmentation, we observe that single
sources are mostly associated with conditions of low angular momentum in the
inner envelope and good alignment of the magnetic field with protostellar
outflows, at intermediate scales. Our results suggest that the properties of
the magnetic field in protostellar envelopes bear a tight relationship with the
rotating-infalling gas directly involved in the star and disk formation: we
find that it may not only influence the fragmentation of protostellar cores
into multiple stellar systems, but also set the conditions establishing the
pristine properties of planet-forming disks.",2010.12466v2
2020-12-27,Exploring scalar-photon interactions in energetic astrophysical events,"Scalar fields like dilaton appear in quantum field theory (QFT) due to scale
symmetry breaking. Their appeal also extends to modified theories of gravity,
like $F(R)$ gravity, Horva Lifshitz gravity etc. In unified theories they make
their appearance through compactification of the extra dimension. Apart from
resolving the issues of compactification scale and size, the particles of their
fields can also turn out to be excellent candidate to solve the dark energy
(DE) and dark matter (DM) problem of the universe. In this work we study their
mixing dynamics with photons in a magnetized media, by incorporating the effect
of parity violating part of the photon polarization tensor, evaluated in a
finite density magnetized media. This piece, though in general is odd in the
external magnetic field strength $eB$; in this work we however have retained
terms to $O$($eB$). We are able to demonstrate in this work that, in magnetized
medium a dilatonic scalar field $(\phi)$ can excite the two transverse degrees
of freedom (DOF) of the photons. One due to direct coupling and the other
indirectly through the parity violating term originating due to magnetized
medium effects. This results in the mixing dynamics being governed by, $3\times
3$ mixing matrices. This mixing results in making the underlying media
optically active. In this work we focus on the spectro-polarimetric imprints of
these particles, on the spectra of the electromagnetic (EM) fields of Gamma Ray
Bursters (GRB). Focusing on a range of parameters (i.e., magnetic field
strength, plasma frequency $(\omega_{p})$, size of the magnetized volume,
coupling strength to photons and their mass) we make an attempt to point out
how space-borne detectors should be designed to optimise their detection
possibility.",2012.13903v1
2021-04-16,Evolution of large-amplitude Alfvén waves and generation of switchbacks in the expanding solar wind,"Motivated by recent Parker Solar Probe (PSP) observations of ""switchbacks""
(abrupt, large-amplitude reversals in the radial magnetic field, which exhibit
Alfv\'enic correlations) we examine the dynamics of large-amplitude Alfv\'en
waves in the expanding solar wind. We develop an analytic model which makes
several predictions: switchbacks should preferentially occur in regions where
the solar wind plasma has undergone a greater expansion, the switchback
fraction at radii comparable to PSP should be an increasing function of radius,
and switchbacks should have their gradients preferentially perpendicular to the
mean magnetic field direction. The expansion of the plasma generates small
compressive components as part of the wave's nonlinear evolution: these are
maximized when the normalized fluctuation amplitude is comparable to
$\sin\theta$, where $\theta$ is the angle between the propagation direction and
the mean magnetic field. These compressive components steepen the primary
Alfv\'enic waveform, keeping the solution in a state of nearly constant
magnetic field strength as its normalized amplitude $\delta B/B$ grows due to
expansion. The small fluctuations in the magnetic-field-strength are minimized
at a particular $\theta$-dependent value of $\beta$, usually of order unity,
and the density and magnetic-field-strength fluctuations can be correlated or
anticorrelated depending on $\beta$ and $\theta$. Example solutions of our
dynamical equation are presented; some do indeed form magnetic-field reversals.
Our predictions appear to match some previously unexplained phenomena in
observations and numerical simulations, providing evidence that the observed
switchbacks result from the nonlinear evolution of the initially
small-amplitude Alfv\'en waves already known to be present at the coronal base.",2104.08321v1
2022-09-30,A tabletop Optically Pumped Magnetometer setup for the monitoring of magnetic nanoparticle clustering and immobilization using Thermal Noise Magnetometry,"Many characterization techniques for magnetic nanoparticles depend on the
usage of external fields. This is not the case in Thermal Noise Magnetometry
(TNM), where thermal fluctuations in the magnetic signal of magnetic
nanoparticle ensembles are measured without any external excitation. This can
provide valuable information about the fundamental dynamical properties of the
particles, due to the purely observative experiments of this relatively new
technique. Until now, TNM signals have been detected only by a superconducting
quantum interference device (SQUID) sensor. We present a tabletop setup using
Optically Pumped Magnetometers (OPMs) in a small magnetic shield, offering a
flexible and accessible alternative and show the agreement between both
measurement systems for two different commercially available nanoparticle
samples. We argue that the OPM setup with high accessibility complements the
SQUID setup with high sensitivity and bandwidth. Furthermore, because of its
excellent sensitivity in the lower frequencies, the OPM tabletop setup is well
suited to monitor aggregation processes where the magnetization dynamics of the
particles tend to slow down, e.g. in biological processes. As a proof of
concept, we show for three different immobilization and clustering processes
the changes in the noise spectrum measured in the tabletop setup: 1) the
aggregation of particles due to the addition of ethanol, 2) the formation of
polymer structures in the sample due to UV exposure, and 3) the cellular uptake
of the particles by THP-1 cells. From our results we conclude that the tabletop
setup offers a flexible and widely adoptable sensor measurement unit to monitor
the immobilization and clustering of magnetic nanoparticles over time for
different applications.",2209.15360v1
2024-03-18,Tuning of the ultrafast demagnetization by ultrashort spin polarized currents in multi-sublattice ferrimagnets,"Femtosecond laser pulses can be used to induce ultrafast changes of the
magnetization in magnetic materials. Several microscopic mechanisms have been
proposed to explain the observations, including the transport of ultrashort
spin-polarized hot-electrons (SPHE). Such ultrafast spin currents find growing
interest because of the recent challenges in ultrafast spintronics however they
are only poorly characterized. One of the key challenges is to characterize the
spin-polarized ultrafast currents and the microscopic mechanisms behind SPHE
induced manipulation of the magnetization, especially in the case of
technologically relevant ferrimagnetic alloys. Here, we have used a combined
approach using time- and element-resolved X-ray magnetic circular dichroism and
theoretical calculations based on atomistic spin-dynamics simulations to
address the ultrafast transfer of the angular momentum from spin-polarized
currents into ferrimagnetic Fe74Gd26 films and the concomitant reduction of
sub-lattice magnetization. Our study shows that using a Co/Pt multilayer as a
polarizer in a spin-valve structure, the SPHE drives the demagnetization of the
two sub-lattices of the Fe74Gd26 film. This behaviour is explained based on two
physical mechanisms, i.e., spin transfer torque and thermal fluctuations
induced by the SPHE. We provide a quantitative description of the heat transfer
of the ultrashort SPHE pulse to the Fe74Gd26 films, as well as the degree of
spin-polarization of the SPHE current density responsible for the observed
magnetization dynamics. Our work finally characterizes the spin-polarization of
the SPHEs revealing unexpected opposite spin polarization to the Co
magnetization, explaining our experimental results.",2403.11739v2
2013-02-22,Magnetic Fields in the Milky Way and in Galaxies (revised version of September 2023),"Most of the visible matter in the Universe is ionized, so that cosmic
magnetic fields are quite easy to generate and due to the lack of magnetic
monopoles hard to destroy. Magnetic fields have been measured in or around
practically all celestial objects, either by in-situ measurements of
spacecrafts or by the electromagnetic radiation of embedded cosmic rays, gas,
or dust. The Earth, the Sun, solar planets, stars, pulsars, the Milky Way,
nearby galaxies, distant radio galaxies, quasars, and even intergalactic space
in clusters of galaxies have significant magnetic fields. Even larger volumes
of the Universe may be permeated by so-far invisble magnetic fields.
Information on cosmic magnetic fields has increased enormously as the result of
the rapid development of observational methods, especially in radio astronomy.
In the Milky Way, a wealth of magnetic phenomena was discovered that are only
partly related to objects visible in other spectral ranges. The large-scale
structure of the Milky Way's magnetic field is still under debate. The
available data for external spiral galaxies can well be explained by field
amplification and ordering via the dynamo mechanism. The measured field
strengths and the similarity of field patterns and flow patterns of the diffuse
ionized gas give strong indication that galactic magnetic fields are
dynamically important. They may affect the formation of spiral arms, outflows,
and the general evolution of galaxies. In spite of our increasing knowledge on
magnetic fields, many important questions on the origin and evolution of
magnetic fields, like their first occurrence in young galaxies or the existence
of large-scale intergalactic fields, remain unanswered. Progress can be
expected from new-generation radio telescopes like LOFAR and the SKA, for which
""cosmic magnetism"" is a key science project.",1302.5663v11
2017-06-12,Unveiling the Role of the Magnetic Field at the Smallest Scales of Star Formation,"We report ALMA observations of polarized dust emission from the protostellar
source Ser-emb 8 at a linear resolution of 140 au. Assuming models of
dust-grain alignment hold, the observed polarization pattern gives a projected
view of the magnetic field structure in this source. Contrary to expectations
based on models of strongly magnetized star formation, the magnetic field in
Ser-emb 8 does not exhibit an hourglass morphology. Combining the new ALMA data
with previous observational studies, we can connect magnetic field structure
from protostellar core (~80,000 au) to disk (~100 au) scales. We compare our
observations with four magnetohydrodynamic gravo-turbulence simulations made
with the AREPO code that have initial conditions ranging from super-Alfv\'enic
(weakly magnetized) to sub-Alfv\'enic (strongly magnetized). These simulations
achieve the spatial dynamic range necessary to resolve the collapse of
protostars from the parsec scale of star-forming clouds down to the ~100 au
scale probed by ALMA. Only in the very strongly magnetized simulation do we see
both the preservation of the field direction from cloud to disk scales and an
hourglass-shaped field at < 1000 au scales. We conduct an analysis of the
relative orientation of the magnetic field and the density structure in both
the Ser-emb 8 ALMA observations and the synthetic observations of the four
AREPO simulations. We conclude that the Ser-emb 8 data are most similar to the
weakly magnetized simulations, which exhibit random alignment, in contrast to
the strongly magnetized simulation, where the magnetic field plays a role in
shaping the density structure in the source. In the weak-field case, it is
turbulence -- not the magnetic field -- that shapes the material that forms the
protostar, highlighting the dominant role that turbulence can play across many
orders of magnitude in spatial scale.",1706.03806v1
1995-08-19,Extragalactic Magnetic Field and the Highest Energy Cosmic Rays,"The strength and spectrum of the extragalactic magnetic field are still
unknown. Its measurement would help answer the question of whether galactic
fields are purely a primordial relic or were dynamically enhanced from a much
smaller cosmological seed field. In this letter, we show that the composition,
spectrum, and directional distribution of extragalactic ultrahigh energy cosmic
rays with energies above $\simeq 10^{18}\ev$ can probe the large scale
component of the extragalactic magnetic field below the present observational
upper limit of $10^{-9}$ Gauss. Cosmic ray detectors under construction or
currently in the proposal stage should be able to test the existence of the
extragalactic magnetic fields on scales of a few to tens of Mpc and strengths
in the range $\simeq 10^{-10} - 10^{-9}$ Gauss.",9508088v2
1997-02-03,Magnetohydrodynamics in Superconducting-Superfluid Neutron Stars,"MHD equations are presented for the mixture of superfluid neutrons,
superconducting protons, and normal electrons believed to exist in the outer
cores of neutron stars. The dissipative effects of electron viscosity and
mutual friction due to electron-vortex scattering are also included. It is
shown that Alfven waves are replaced by cyclotron- vortex waves that have not
been previously derived from MHD theory. The cyclotron- vortex waves are
analogous to Alfven waves with the tension due to the magnetic energy density
replaced by the vortex energy density. The equations are then put into a
simplified form useful for studying the effects of the interior magnetic field
on the dynamics. Of particular interest is the crust-core coupling time which
can be inferred from pulsar glitch observations. The hypothesis that
cyclotron-vortex waves play a significant role in the core spin-up during a
glitch is used to place limits on the interior magnetic field. The results are
compared with those of other studies.",9702032v3
1997-08-01,Magnetic Fields from Phase Transitions,"The generation of primordial magnetic fields from cosmological phase
transitions is discussed, paying particular attention to the electroweak
transition and to the various definitions of the `average' field that have been
put forward. It is emphasised that only the volume average has dynamical
significance as a seed for galactic dynamos. On rather general grounds of
causality and energy conservation, it is shown that, in the absence of MHD
effects that transfer power in the magnetic field from small to large scales,
processes occurring at the electroweak transition cannot generate fields
stronger than $10^{-20}$ Gauss on a scale of 0.5 Mpc. However, it is
implausible that this upper bound could ever be reached, as it would require
all the energy in the Universe to be turned into a magnetic field coherent at
the horizon scale. Non-linear MHD effects seem therefore to be necessary if the
electroweak transition is to create a primordial seed field.",9708004v2
1998-02-27,Dynamics of Magnetic Flux Elements in the Solar Photosphere,"The interaction of magnetic fields and convection is investigated in the
context of the coronal heating problem. We study the motions of photospheric
magnetic elements using filtergrams obtained at the Swedish Vacuum Solar
Telescope at La Palma. We use potential-field modeling to extrapolate the
magnetic and velocity fields to larger height. We find that the velocity in the
chromosphere can be locally enhanced at the separatrix surfaces between
neighboring flux tubes. The predicted velocities are several km/s,
significantly larger than those of the photospheric flux tubes, which may have
important implications for coronal heating. s",9802359v1
1999-01-26,Magnetic CVs in Globular Clusters,"Cataclysmic variables (CVs) are tracers of dynamical evolution in globular
clusters. A significant sample has been found by the relatively shallow \Halpha
surveys conducted with HST thus far in just three globulars: NGC 6397 (3), NGC
6752 (2), and $\omega$-Cen (2). These are identified with the low luminosity
x-ray sources discovered in these globulars with similarly moderate-depth ROSAT
surveys. Follow-up spectroscopy of the initial 3 CV candidates in NGC 6397 with
the FOS on HST suggests they may be magnetic CVs of the intermediate polar (IP)
type, and follow-up deep imaging with both HST and ROSAT yield a 4th confirmed
and 5th strong candidate CV near the center of this collapsed core cluster. The
optical vs. x-ray properties of the 4 confirmed IP candidates in NGC 6397 are
compared with disk IPs, and arguments for and against this interpretation are
presented. The possible strong excess of magnetic CVs in globulars will be
tested with much deeper HST/AXAF surveys.",9901356v1
1999-04-02,Scale-free Equilibria of Magnetized Polytropic Clouds,"We investigate the equilibrium properties of self-gravitating magnetized
clouds with polytropic equations of state with negative index n. In particular,
we consider scale-free isopedic configurations that have constant dimensionless
spherical mass-to-flux ratio lambda_r and that may constitute ``pivotal''
states for subsequent dynamical collapse to form groups or clusters of stars.
For given Gamma = 1 + 1/n, equilibria with smaller values of lambda_r are more
flattened, ranging from spherical configurations with lambda_r = infinity to
completely flattened states for lambda_r=1. For a given amount of support
provided by the magnetic field as measured by the dimensionless parameter H_0,
equilibria with smaller values of Gamma are more flattened. However, logatropic
(defined by Gamma = 0) disks do not exist. The only possible scale-free
isopedic equilibria with logatropic equation of state are spherical uniformly
magnetized clouds.",9904028v1
1999-04-26,Magnetic fields in the early universe in the string approach to MHD,"There is a reformulation of magnetohydrodynamics in which the fundamental
dynamical quantities are the positions and velocities of the lines of magnetic
flux in the plasma, which turn out to obey equations of motion very much like
ideal strings. We use this approach to study the evolution of a primordial
magnetic field generated during the radiation-dominated era in the early
Universe. Causality dictates that the field lines form a tangled random
network, and the string-like equations of motion, plus the assumption of
perfect reconnection, inevitably lead to a self-similar solution for the
magnetic field power spectrum. We present the predicted form of the power
spectrum, and discuss insights gained from the string approximation, in
particular the implications for the existence or not of an inverse cascade.",9904358v1
1999-10-20,Enhanced Cloud Disruption by Magnetic Field Interaction,"We present results from the first three-dimensional numerical simulations of
moderately supersonic cloud motion through a tenuous, magnetized medium. We
show that the interaction of the cloud with a magnetic field perpendicular to
its motion has a great dynamical impact on the development of instabilities at
the cloud surface. Even for initially spherical clouds, magnetic field lines
become trapped in surface deformations and undergo stretching. The consequent
field amplification that occurs there and particularly its variation across the
cloud face then dramatically enhance the growth rate of Rayleigh-Taylor
unstable modes, hastening the cloud disruption.",9910357v1
2000-03-13,"Cosmic magnetism, curvature and the expansion dynamics","We discuss how a cosmological magnetic field could affect the expansion of
the universe, through its interaction with the spacetime geometry. The tension
of the field lines means that the magneto-curvature coupling tends to
accelerate positively curved regions and decelerate those with negative
curvature. Depending on the equation of state of the matter, the effect varies
from mimicking that of a cosmological constant to resembling a time-decaying
quintessence. Most interestingly, the coupling between magnetism and geometry
implies that even weak fields have a significant impact if the curvature
contribution is strong. This leads to kinematical complications that can
inhibit the onset of the accelerated phase in spatially open inflationary
models. We employ a simple cosmological model to illustrate these effects and
examine the conditions necessary for them to have a appreciable impact.",0003181v3
2000-12-02,How astrophysical mean field dynamos can circumvent existing quenching constraints,"Mean field dynamo theory is a leading candidate to explain the observed large
scale magnetic fields of galaxies and stars. However, controversy arises over
the extent of premature quenching by the backreaction of the growing field. We
distinguish between rapid mean field dynamo action, which is required by
astrophysical systems, and resistively limited action. We show how the flow of
magnetic helicity is important for rapid action. Existing numerical and
analytic work suggesting that mean field dynamos are prematurely quenched and
resistively limited include approximations or boundary conditions which
suppress the magnetic helicity flow from the outset. Thus they do not
unambiguously reveal whether real astrophysical mean field dynamos are
dynamically suppressed when the helicity flow is allowed. An outflow of
helicity also implies an outflow of magnetic energy and so active coronae or
winds should accompany mean field dynamos. Open boundaries alone may not be
sufficient for rapid dynamo action and the additional physics of buoyancy and
outflows may be required. Possible simulation approaches to test some of the
principles are briefly discussed. Some limitations of the ``Zeldovich
relation'' are also addressed.",0012048v1
2001-01-19,Rapid-Process Nucleosynthesis in Neutrino-Magneto-Centrifugally Driven Winds,"We have studied whether the rotation and magnetic fields in neutrino-driven
winds can be key processes for the rapid-process (r-process) nucleosynthesis.
We have examined the features of a steady and subsonic wind solutions which
extend the model of Weber and Davis (1967), which is a representative solar
wind model. As a result, we found that the entropy per baryon becomes lower and
the dynamical timescale becomes longer as the angular velocity becomes higher.
These results are inappropriate for the production of the r-process nuclei. As
for the effects of magnetic fields, we found that a solution as a steady wind
from the surface of the proto-neutron star can not be obtained when the
strength of the magnetic field becomes $\ge$ $10^{11}$ G. Since the magnetic
field in normal pulsars is of order $10^{12}$ G, a steady wind solution might
not be realized there, which means that the models in this study may not be
adopted for normal proto-neutron stars. In this situation, we have little
choice but to conclude that it is difficult to realize a successful r-process
nucleosynthesis in the wind models in this framework.",0101347v1
2001-02-02,Magnetohydrodynamic simulations of black hole accretion,"We discuss the results of three-dimensional magnetohydrodynamic simulations,
using a pseudo-Newtonian potential, of thin disk (h/r ~ 0.1) accretion onto
black holes. We find (i) that magnetic stresses persist within the marginally
stable orbit, and (ii) that the importance of those stresses for the dynamics
of the flow depends upon the strength of magnetic fields in the disk outside
the last stable orbit. Strong disk magnetic fields (alpha > 0.1) lead to a
gross violation of the zero-torque boundary condition at the last stable orbit,
while weaker fields (alpha ~ 0.01) produce results more akin to traditional
models for thin disk accretion onto black holes. Fluctuations in the magnetic
field strength in the disk could lead to changes in the radiative efficiency of
the flow on short timescales.",0102045v1
2001-03-30,A Magnetohydrodynamic Nonradiative Accretion Flow in Three Dimensions,"We present a global magnetohydrodynamic (MHD) three dimensional simulation of
a nonradiative accretion flow originating in a pressure supported torus. The
evolution is controlled by the magnetorotational instability which produces
turbulence. The flow forms a nearly Keplerian disk. The total pressure scale
height in this disk is comparable to the vertical size of the initial torus.
Gas pressure dominates only near the equator; magnetic pressure is more
important in the surrounding atmosphere. A magnetically dominated bound outflow
is driven from the disk. The accretion rate through the disk exceeds the final
rate into the hole, and a hot torus forms inside 10 r_g. Hot gas, pushed up
against the centrifugal barrier and confined by magnetic pressure, is ejected
in a narrow, unbound, conical outflow. The dynamics are controlled by magnetic
turbulence, not thermal convection, and a hydrodynamic alpha model is
inadequate to describe the flow. The limitations of two dimensional MHD
simulations are also discussed.",0103522v1
2001-06-06,New circumstellar magnetic field diagnostics,"In this paper I will discuss new magnetic field diagnostics and
instrumentation for an area of astrophysics where magnetic field observations
have been difficult - circumstellar material. Such diagnostics would be
particularly relevant to star formation and evolution. Stellar photosphere
diagnostics include the Zeeman effect and atomic scattering diagnostics like
the Hanle Effect and atomic alignment. The Zeeman Effect is in general not
sensitive enough for the field strengths expected for circumstellar material,
and it is easily defeated by Doppler broadening in a dynamic envelope. Atomic
scattering diagnostics, pioneered recently for the Sun, are promising, but have
never been applied outside the Sun. For the study of unresolved envelopes, the
Hanle Effect may be applicable particularly in the ultraviolet. A medium
resolution UV spectropolarimeter, for instance, would serve for such studies.
Atomic alignment effects could utilize a ground-based, high signal-to-noise
spectropolarimeter, with profile information from high spectral resolution. I
will briefly mention several instrumentation development efforts in these
directions.",0106114v1
2002-04-17,Finite size scaling in the solar wind magnetic field energy density as seen by WIND,"Statistical properties of the interplanetary magnetic field fluctuations can
provide an important insight into the solar wind turbulent cascade. Recently,
analysis of the Probability Density Functions (PDF) of the velocity and
magnetic field fluctuations has shown that these exhibit non-Gaussian
properties on small time scales while large scale features appear to be
uncorrelated. Here we apply the finite size scaling technique to explore the
scaling of the magnetic field energy density fluctuations as seen by WIND. We
find a single scaling sufficient to collapse the curves over the entire
investigated range. The rescaled PDF follow a non Gaussian distribution with
asymptotic behavior well described by the Gamma distribution arising from a
finite range Levy walk. Such mono scaling suggests that a Fokker-Planck
approach can be applied to study the PDF dynamics. These results strongly
suggest the existence of a common, nonlinear process onthe time scale up to ~26
hours.",0204287v1
2003-03-26,Radial evolution of solar wind intermittency in the inner heliosphere,"We analyzed intermittency in the solar wind, as observed on the ecliptic
plane, looking at magnetic field and velocity fluctuations between 0.3 and 1
AU, for both fast and slow wind and for compressive and directional
fluctuations. Our analysis focused on the property that probability
distribution functions of a fluctuating field affected by intermittency become
more and more peaked at smaller and smaller scales. Since the peakedness of a
distribution is measured by its flatness factor we studied the behavior of this
parameter for different scales to estimate the degree of intermittency of our
time series. We confirmed that both magnetic field and velocity fluctuations
are rather intermittent and that compressive magnetic fluctuations are
generally more intermittent than the corresponding velocity fluctuations. In
addition, we observed that compressive fluctuations are always more
intermittent than directional fluctuations and that while slow wind
intermittency does not depend on the radial distance from the sun, fast wind
intermittency of both magnetic field and velocity fluctuations clearly
increases with the heliocentric distance....",0303578v1
2003-03-31,Interaction of a magnetized pulsar wind with its surroundings. MHD simulations of Pulsar Wind Nebulae,"Magnetohydrodynamical simulations are presented of a magnetized pulsar wind
interacting directly with the interstellar medium, or, in the case of a
surrounding supernova remnant, with the associated freely expanding ejecta of
the progenitor star. In both cases the simulations show that the pulsar wind
nebula will be elongated due to the dynamical influence of the toroidal
magnetic fields, which confirm predictions from a semi-analytical model
presented by Begelman & Li. The simulations follow the expansion of the pulsar
wind nebula when the latter is bounded by a strong shock and show that the
expansion can be modeled with a standard power-law expansion rate. By
performing different simulations with different magnetization parameters, I
show that the latter weakly correlates with the elongation of the pulsar wind
nebula. The results from the simulations are applied to determine the nature of
the expansion rate of the pulsar wind nebula 3C58. It is shown that there is
both observational and theoretical evidence which supports the scenario in
which the pulsar wind nebula 3C58 has caught up with the reverse shock of the
associated (but undetected) supernova remnant.",0303661v1
2003-08-14,Saturated State of the Nonlinear Small-Scale Dynamo,"We consider the problem of incompressible, forced, nonhelical, homogeneous
and isotropic MHD turbulence with no mean magnetic field and large magnetic
Prandtl number. This type of MHD turbulence is the end state of the turbulent
dynamo, which generates folded fields with small-scale direction reversals. We
propose a model in which saturation is achieved as a result of the velocity
statistics becoming anisotropic with respect to the local direction of the
magnetic folds. The model combines the effects of weakened stretching and
quasi-two-dimensional mixing and produces magnetic-energy spectra in remarkable
agreement with numerical results at least in the case of a one-scale flow. We
conjecture that the statistics seen in numerical simulations could be explained
as a superposition of these folded fields and Alfv\'en-like waves that
propagate along the folds.",0308252v2
2003-08-29,The Last Hurrah: PPN Formation by a Magnetic Explosion,"We discuss a mechanism by which a giant star can expel its envelope in an
outburst, leaving its core exposed. The outburst is powered by rotational
kinetic energy of the core, transferred to the envelope via the twisting of
magnetic fields. We show that, if the core is magnetized, and if it has
sufficient angular momentum, this mechanism may be triggered at the end of the
asymptotic giant branch phase, and drive a proto-planetary nebula (pPN)
outflow. This explosion of magnetic energy self-consistently explains some of
the asymmetries and dynamics of pPNe.",0308548v2
2003-11-10,Semi-analytical homologous solutions of the gravo-magnetic contraction,"We propose an extension of the semi-analytical solutions derived by Lin et
al. (1965) describing the two-dimensional homologous collapse of a
self-gravitating rotating cloud having uniform density and spheroidal shape,
which includes magnetic field (with important restrictions) and thermal
pressure. The evolution of the cloud is reduced to three time dependent
ordinary equations allowing to conduct a quick and preliminary investigation of
the cloud dynamics during the precollapse phase, for a wide range of
parameters. We apply our model to the collapse of a rotating and magnetized
oblate and prolate isothermal core. Hydrodynamical numerical simulations are
performed and comparison with the semi-analytical solutions is discussed. Under
the assumption that all cores are similar, an apparent cloud axis ratio
distribution is calculated from the sequence of successive evolutionary states
for each of a large set of initial conditions. The comparison with the
observational distribution of the starless dense cores belonging to the catalog
of Jijina et al. (1999) shows a good agreement for the rotating and initially
prolate cores (aspect ratio $\simeq 0.5$) permeated by an helical magnetic
field ($\simeq 17-20 \mu$G for a density of $\simeq 10^4$ cm$^{-3}$).",0311221v1
2003-12-24,High-Energy Emission from Presupernovae,"The non-thermal emission in the magnetospheres of presupernova collapsing
stars with initial dipole magnetic fields and a certain initial energy
distribution of the charged particles in a magnetosphere is considered. The
analysis of particle dynamics and its emission in the stellar magnetosphere
under collapse show that the collapsing stars can be the powerful sources of a
non-thermal radiation produced by the interaction of charged particles with the
magnetic field. The radiation flux grows with decreasing stellar radius. This
flux can be observed in the form of radiation burst with duration equal to the
stellar collapse time. The radiation flux depends on the distance to the stars,
the particle spectrum and magnetic field in the magnetosphere. The radiation
flux is calculated for various collapsing stars with initial dipole magnetic
fields and an initial power series, relativistic Maxwell, and Boltzmann
particle energy distribution in the magnetosphere. This radiation can be
observed by means of modern astronomical instruments.",0312615v1
2004-09-08,Applying Magnetized Accretion-Ejection Models to Microquasars: a preliminary step,"We present in this proceeding some aspects of a model that should explain the
spectral state changes observed in microquasars. In this model, ejection is
assumed to take place only in the innermost disc region where a large scale
magnetic field is anchored. Then, in opposite to conventional ADAF models, the
accretion energy can be efficiently converted in ejection and not advected
inside the horizon. We propose that changes of the disc physical state (e.g.
transition from optically thick to optically thin states) can strongly modify
the magnetic accretion-ejection structure resulting in the spectral
variability. After a short description of our scenario, we give some details
concerning the dynamically self-consistent magnetized accretion-ejection model
used in our computation. We also present some preliminary results of spectral
energy distribution.",0409194v1
2004-09-21,Coronal heating through braiding of magnetic field lines,"Cool stars like our Sun are surrounded by a million degree hot outer
atmosphere, the corona. Since more than 60 years the physical nature of the
processes heating the corona to temperatures well in excess of those on the
stellar surface remain puzzling. Recent progress in observational techniques
and numerical modeling now opens a new window to approach this problem. We
present the first coronal emission line spectra synthesized from
three-dimensional numerical models describing the evolution of the dynamics and
energetics as well as of the magnetic field in the corona. In these models the
corona is heated through motions on the stellar surface that lead to a braiding
of magnetic field lines inducing currents which are finally dissipated. These
forward models enable us to synthesize observed properties like (average)
emission line Doppler shifts or emission measures in the outer atmosphere,
which until now have not been understood theoretically, even though many
suggestions have been made in the past. As our model passes these observational
tests, we conclude that the flux braiding mechanism is a prime candidate for
being the dominant heating process of the magnetically closed corona of the Sun
and solar-like stars.",0409504v1
2004-10-11,Magnetohydrodynamic Simulations of Shock Interactions with Radiative Clouds,"We present results from two-dimensional numerical simulations of the
interactions between magnetized shocks and radiative clouds. Our primary goal
is to characterize the dynamical evolution of the shocked clouds. We perform
runs in both the strong and weak magnetic field limits and consider three
different field orientations. For the geometries considered, we generally find
that magnetic fields external to, but concentrated near, the surface of the
cloud suppress the growth of destructive hydrodynamic instabilities. External
fields also increase the compression of the cloud by effectively acting as a
confinement mechanism driven by the interstellar flow and local field
stretching. This can have a dramatic effect on both the efficiency of radiative
cooling, which tends to increase with increasing magnetic field strength, and
on the size and distribution of condensed cooled fragments. In contrast, fields
acting predominately internally to the cloud tend to resist compression,
thereby inhibiting cooling. We observe that, even at modest strengths, internal
fields can completely suppress low-temperature cooling.",0410285v1
2005-10-24,Collapse of magnetized hypermassive neutron stars in general relativity,"Hypermassive neutron stars (HMNSs) -- equilibrium configurations supported
against collapse by rapid differential rotation -- are possible transient
remnants of binary neutron star mergers. Using newly developed codes for
magnetohydrodynamic simulations in dynamical spacetimes, we are able to track
the evolution of a magnetized HMNS in full general relativity for the first
time. We find that secular angular momentum transport due to magnetic braking
and the magnetorotational instability results in the collapse of an HMNS to a
rotating black hole, accompanied by a gravitational wave burst. The nascent
black hole is surrounded by a hot, massive torus undergoing quasistationary
accretion and a collimated magnetic field. This scenario suggests that HMNS
collapse is a possible candidate for the central engine of short gamma-ray
bursts.",0510653v2
2005-11-10,Spectrum of magnetohydrodynamic turbulence,"We propose a phenomenological theory of strong incompressible
magnetohydrodynamic turbulence in the presence of a strong large-scale external
magnetic field. We argue that in the inertial range of scales, magnetic-field
and velocity-field fluctuations tend to align the directions of their
polarizations. However, the perfect alignment cannot be reached, it is
precluded by the presence of a constant energy flux over scales. As a
consequence, the directions of fluid and magnetic-field fluctuations at each
scale $\lambda$ become effectively aligned within the angle
$\phi_{\lambda}\propto \lambda^{1/4}$, which leads to scale-dependent depletion
of nonlinear interaction and to the field-perpendicular energy spectrum
$E(k_{\perp})\propto k_{\perp}^{-3/2}$. Our results may be universal, i.e.,
independent of the external magnetic field, since small-scale fluctuations
locally experience a strong field produced by large-scale eddies.",0511290v2
2006-05-12,Physical Origin of the Quadrupole Out-of-Plane Magnetic Field in Hall-MHD Reconnection,"A quadrupole pattern of the out-of-plane component of the magnetic field
inside a reconnection region is seen as an important signature of the
Hall-magnetohydrodynamic (Hall-MHD) regime of reconnection. It has been first
observed in numerical simulations and just recently confirmed in the MRX
(Magnetic Reconnection Experiment) [M. Yamada, H. Ji, S. Hsu, T. Carter, R.
Kulsrud, N. Bertz, F. Jobes, Y. Ono, and F. Perkins, Phys. Plasmas 4, 1936
(1997)] and also seen in spacecraft observations of Earth's magnetosphere. In
this study, the physical origin of the quadrupole field is analyzed and traced
to a current of electrons that flows along the lines in and out of the inner
reconnection region to maintain charge neutrality. The role of the quadrupole
magnetic field in the overall dynamics of the reconnection process is
discussed. In addition, the bipolar poloidal electric field is estimated and
its effect on ion motions is emphasized.",0605309v1
2006-06-06,Helical Magnetorotational Instability in Magnetized Taylor-Couette Flow,"Hollerbach and Rudiger have reported a new type of magnetorotational
instability (MRI) in magnetized Taylor-Couette flow in the presence of combined
axial and azimuthal magnetic fields. The salient advantage of this ""helical''
MRI (HMRI) is that marginal instability occurs at arbitrarily low magnetic
Reynolds and Lundquist numbers, suggesting that HMRI might be easier to realize
than standard MRI (axial field only). We confirm their results, calculate HMRI
growth rates, and show that in the resistive limit, HMRI is a weakly
destabilized inertial oscillation propagating in a unique direction along the
axis. But we report other features of HMRI that make it less attractive for
experiments and for resistive astrophysical disks. Growth rates are small and
require large axial currents. More fundamentally, instability of highly
resistive flow is peculiar to infinitely long or periodic cylinders: finite
cylinders with insulating endcaps are shown to be stable in this limit. Also,
keplerian rotation profiles are stable in the resistive limit regardless of
axial boundary conditions. Nevertheless, the addition of toroidal field lowers
thresholds for instability even in finite cylinders.",0606125v1
2006-06-12,"The mean electro-motive force, current- and cross-helicity under the influence of rotation, magnetic field and shear","The mean electromotive force (MEMF) in a rotating stratified
magnetohydrodynamical turbulence is studied.Our study rests on the mean-field
magnetohydrodynamics framework and $\tau$ approximation. We compute the effects
of the large-scale magnetic fields (LSMF), global rotation and large-scale
shear flow on the different parts of the MEMF (such as $\alpha$ - effect,
turbulent diffusion, turbulent transport, etc.) in an explicit form. The
influence of the helical magnetic fluctuations which stem from the small-scale
dynamo is taken into account, as well. In the paper, we derive the equation
governing the current helicity evolution. It is shown that the joint effect of
the differential rotation and magnetic fluctuations in the stratified media can
be responsible for the generation, maintenance and redistribution of the
current helicity. The implication of the obtained results to astrophysical
dynamos is considered as well.",0606265v6
2006-11-21,Exact scaling laws and the local structure of isotropic magnetohydrodynamic turbulence,"This paper examines the consistency of the exact scaling laws for isotropic
MHD turbulence in numerical simulations with large magnetic Prandtl numbers Pm
and with Pm=1. The exact laws are used to elucidate the structure of the
magnetic and velocity fields. Despite the linear scaling of certain third-order
correlation functions, the situation is not analogous to the case of Kolmogorov
turbulence. The magnetic field is adequately described by a model of stripy
(folded) field with direction reversals at the resistive scale. At currently
available resolutions, the cascade of kinetic energy is short-circuited by the
direct exchange of energy between the forcing-scale motions and the stripy
magnetic fields. This nonlocal interaction is the defining feature of isotropic
MHD turbulence.",0611692v1
2007-02-05,On the intermittent character of interplanetary magnetic field fluctuations,"Interplanetary magnetic field magnitude fluctuations are notoriously more
intermittent than velocity fluctuations in both fast and slow wind. This
behaviour has been interpreted in terms of the anomalous scaling observed in
passive scalars in fully developed hydrodynamic turbulence. In this paper, the
strong intermittent nature of the interplanetary magnetic field is briefly
discussed comparing results performed during different phases of the solar
cycle. The scaling properties of the interplanetary magnetic field magnitude
show solar cycle variation that can be distinguished in the scaling exponents
revealed by structure functions. The scaling exponents observed around solar
maximum coincide, within the errors, to those measured for passive scalars in
hydrodynamic turbulence. However, it is also found that the values are not
universal in the sense that the solar cycle variation may be reflected in
dependence on the structure of the velocity field.",0702114v1
2007-02-14,Quantum Electrodynamical Photon Splitting in Magnetized Nonlinear Pair Plasmas,"We present for the first time the nonlinear dynamics of quantum
electrodynamic (QED) photon splitting in a strongly magnetized
electron-positron (pair) plasma. By using a QED corrected Maxwell equation, we
derive a set of equations that exhibit nonlinear couplings between
electromagnetic (EM) waves due to nonlinear plasma currents and QED
polarization and magnetization effects. Numerical analyses of our coupled
nonlinear EM wave equations reveal the possibility of a more efficient decay
channel, as well as new features of energy exchange among the three EM modes
that are nonlinearly interacting in magnetized pair plasmas. Possible
applications of our investigation to astrophysical settings, such as magnetars,
are pointed out.",0702364v1
2007-02-15,Seething Horizontal Magnetic Fields in the Quiet Solar Photosphere,"The photospheric magnetic field outside of active regions and the network has
a ubiquitous and dynamic line-of-sight component that strengthens from disk
center to limb as expected for a nearly horizontal orientation. This component
shows a striking time variation with an average temporal rms near the limb of
1.7 G at ~3"" resolution. In our moderate resolution observations the nearly
horizontal component has a frequency variation power law exponent of -1.4 below
1.5 mHz and is spatially patchy on scales up to ~15 arcsec. The field may be a
manifestation of changing magnetic connections between eruptions and evolution
of small magnetic flux elements in response to convective motions. It shows no
detectable latitude or longitude variations.",0702415v1
1995-05-04,What happens to the quantum Hall effect when magnetic-field-induced spin-density wave moves,"The influence of the motion of a magnetic-field-induced spin-density wave
(FISDW) on the quantum Hall effect in a quasi-one-dimensional conductor is
studied theoretically. In the ideal case of a free FISDW, it is found that the
counterflow of the FISDW precisely cancels the quantum Hall current, so the
resultant Hall conductivity is zero. In real systems, the Hall conductivity
should vanish at the high frequencies, where the pinning and the damping can be
neglected, and the dynamics of the FISDW is dominated by inertia.",9505016v1
1996-10-09,Superconducting transition induced by columnar disorder in strong magnetic field,"The superconducting transition in presence of strong columnar disorder
parallel to the magnetic field is considered. A solvable model appropriate for
description of the broad crossover regime towards the true ""glassy"" critical
behavior is constructed, and the behavior of the thermodynamic quantities and
of the Edwards-Anderson order-parameter is obtained. The critical exponents for
the correlation lengths orthogonal and parallel to the magnetic field are equal
and in agreement with the experimental values. The dynamical critical exponent
is $z=2$, also in agreement with the measured value. Several perturbations to
the solvable model are considered and shown to be irrelevant for the critical
behavior. It is argued that there exists an optimal density of defects at which
the transition temperature at given magnetic field reaches its maximum.",9610083v1
1996-11-27,Micromagnetic Simulation of Nanoscale Films with Perpendicular Anisotropy,"A model is studied for the theoretical description of nanoscale magnetic
films with high perpendicular anisotropy. In the model the magnetic film is
described in terms of single domain magnetic grains with Ising-like behavior,
interacting via exchange as well as via dipolar forces. Additionally, the model
contains an energy barrier and a coupling to an external magnetic field.
Disorder is taken into account in order to describe realistic domain and domain
wall structures. The influence of a finite temperature as well as the dynamics
can be modeled by a Monte Carlo simulation.
  Many of the experimental findings can be investigated and at least partly
understood by the model introduced above. For thin films the magnetisation
reversal is driven by domain wall motion. The results for the field and
temperature dependence of the domain wall velocity suggest that for thin films
hysteresis can be described as a depinning transition of the domain walls
rounded by thermal activation for finite temperatures.",9611218v1
1997-09-17,Theory of Hysteresis Loop in Ferromagnet,"A theory of the hysteresis loop in ferromagnets controlled by the domain wall
motion is presented. Domain walls are considered as plane or linear interfaces
moving in a random medium under the action of the external ac magnetic field
$H=H_0\sin\omega t$. We introduce important characteristics of the hysteresis
loop, such as dynamic threshold fields, reversal field etc. together with well
known characteristics as coercive field and hysteresis loop area (HLA) $\cal
A$. We show that all these characteristics are regulated by two dimensionless
combinations of the $H_0$ and $\omega$and intrinsic characteristics of the
ferromagnet. The moving domain wall can create magnetic bubbles playing the
role of pre-existing nuclei of the reversed magnetization. We discuss a simple
model of this process. For magnetization reversal determined by domain
inflation we predict that HLA scales as ${\cal A}\propto
\omega^{\beta}H_0^{\alpha}$ with $\alpha =1/2$ and $\beta =1/2$. Numerical
simulations confirmthis result.",9709194v2
1998-08-03,Coupled quantum dots as quantum gates,"We consider a new quantum gate mechanism based on electron spins in coupled
semiconductor quantum dots. Such gates provide a general source of spin
entanglement and can be used for quantum computers. We determine the exchange
coupling J in the effective Heisenberg model as a function of magnetic (B) and
electric fields, and of the inter-dot distance (a) within the Heitler-London
approximation of molecular physics. This result is refined by using
sp-hybridization, and by the Hund-Mulliken molecular-orbit approach which leads
to an extended Hubbard description for the two-dot system that shows a
remarkable dependence on B and a due to the long-range Coulomb interaction. We
find that the exchange J changes sign at a finite field (leading to a
pronounced jump in the magnetization) and then decays exponentially. The
magnetization and the spin susceptibilities of the coupled dots are calculated.
We show that the dephasing due to nuclear spins in GaAs can be strongly
suppressed by dynamical nuclear spin polarization and/or by magnetic fields.",9808026v2
1998-12-31,Anisotropic spin freezing in the S=1/2 zigzag ladder compound SrCuO2,"Using magnetic neutron scattering we characterize an unusual low temperature
phase in orthorhombic SrCuO2. The material contains zigzag spin ladders formed
by pairs of S=1/2 chains (J=180 meV) coupled through a weak frustrated
interaction |J'|<0.1J. At T<Tc1=5.0(4)K an elastic peak develops in a gapless
magnetic excitation spectrum indicating spin freezing on a time scale larger
than 200 picoseconds. While the frozen state has long range commensurate
antiferromagnetic order along the chains with the correlation length exceeding
200 lattice periods along the c-axis and a substantial correlation length of
60(25) spacings along the a-axis perpendicular to the zigzag plane, only 2
lattice units are correlated along the b-axis which is the direction of the
frustrated interactions. The frozen magnetic moment of each Cu ion is very
small, 0.033(7) Bohr magneton even at T=0.35K, and has unusual temperature
dependence with a cusp at Tc2=1.5K reminiscent of a phase transition. We argue
that slow dynamics of stripe-like cooperative magnetic defects in tetragonal
a-c planes yield this anisotropic frozen state.",9812440v3
1999-05-18,From ferromagnetism to spin-density wave: Magnetism in the two channel periodic Anderson model,"The magnetic properties of the two-channel periodic Anderson model for
uranium ions, comprised of a quadrupolar and a magnetic doublet are
investigated through the crossover from the mixed-valent to the stable moment
regime using dynamical mean field theory. In the mixed-valent regime
ferromagnetism is found for low carrier concentration on a hyper-cubic lattice.
The Kondo regime is governed by band magnetism with small effective moments and
an ordering vector $\q$ close to the perfect nesting vector. In the stable
moment regime nearest neighbour anti-ferromagnetism dominates for less than
half band filling and a spin density wave transition for larger than half
filling. $T_m$ is governed by the renormalized RKKY energy scale $\mu_{eff}^2
<X_\sigma>^2 J^2\rho_0(\mu)$.",9905260v1
1999-08-10,Magnetic relaxation in a classical spin chain as model for nanowires,"With decreasing particle size, different mechanisms dominate the thermally
activated magnetization reversal in ferromagnetic particles. We investigate
some of these mechanisms for the case of elongated, single-domain nanoparticles
which we describe by a classical Heisenberg spin chain driven by an external
magnetic field. For sufficiently small system size the magnetic moments rotate
coherently. With increasing size a crossover to a reversal due to
soliton-antisoliton nucleation sets in. For even larger systems many of these
soliton-antisoliton pairs nucleate at the same time. These effects give rise to
a complex size dependence of the energy barriers and characteristic time scales
of the relaxation. We study these quantities using Monte Carlo simulations as
well as a direct integration of the Landau-Lifshitz-Gilbert equation of motion
with Langevin dynamics and we compare our results with asymptotic solutions for
the escape rate following from the Fokker-Planck equation. Also, we investigate
the crossover from coherent rotation to soliton-antisoliton nucleation and
multi-droplet nucleation, especially its dependence on the system size, the
external field and the anisotropy of the system.",9908150v1
1999-08-26,Parallel flow in Hele-Shaw cells with ferrofluids,"Parallel flow in a Hele-Shaw cell occurs when two immiscible liquids flow
with relative velocity parallel to the interface between them. The interface is
unstable due to a Kelvin-Helmholtz type of instability in which fluid flow
couples with inertial effects to cause an initial small perturbation to grow.
Large amplitude disturbances form stable solitons. We consider the effects of
applied magnetic fields when one of the two fluids is a ferrofluid. The
dispersion relation governing mode growth is modified so that the magnetic
field can destabilize the interface even in the absence of inertial effects.
However, the magnetic field does not affect the speed of wave propagation for a
given wavenumber. We note that the magnetic field creates an effective
interaction between the solitons.",9908377v2
2000-06-22,Quantum and classical dynamics in complex one-dimensional antiferromagnets,"Of great recent interest in condensed matter physics are phenomena of
coexistence of quantum and classical properties in the same material. Such
duality occurs in certain mixed-spin antiferromagnets composed of quantum spin
chains interacting through ``auxiliary'' magnetic ions. In this category are
linear-chain rare earth nickelates that exhibit a seemingly paradoxical
co-existence of long-range magnetic order and one-dimensional gapped quantum
spin excitations at low temperatures. In the present paper we propose a unified
physical description of these compounds, supported by recent neutron
diffraction and inelastic scattering studies. Our basic concept is the
effective separation between low-frequency classical and high-frequency quantum
spin correlations. This interpretation enables experimental measurements of
some previously inaccessible fundamental properties of quantum spin chains, and
predicts new exotic magnetic excitations and mechanisms of long-range ordering
in complex quantum magnets.",0006350v1
2000-07-17,Fine-grid Simulations of Thermally Activated Switching in Nanoscale Magets,"Numerical integration of the Landau-Lifshitz-Gilbert equation with thermal
fluctuations is used to study the dynamic response of single-domain nanomagnets
to rapid changes in the applied magnetic field. The simulation can resolve
magnetization patterns within nanomagnets and uses the Fast Multipole method to
calculate dipole-dipole interactions efficiently. The thermal fluctuations play
an essential part in the reversal process whenever the applied field is less
than the zero-temperature coercive field. In this situation pillar-shaped
nanomagnets are found to reverse through a local curling mode that involves the
formation and propagation of a domain wall. Tapering the ends of the pillars to
reduce pole-avoidance effects changes the energies involved but not the
fundamental process. The statistical distribution of switching times is well
described by the independent nucleation and subsequent growth of regions of
reversed magnetization at both ends of the pillar.",0007279v1
2000-09-06,Mixing of magnetic and phononic excitations in incommensurate Spin-Peierls systems,"We analyze the excitation spectra of a spin-phonon coupled chain in the
presence of a soliton. This is taken as a microscopic model of a Spin-Peierls
material placed in a high magnetic field. We show, by using a semiclassical
approximation in the bosonized representation of the spins that a trapped
magnetic state obtained in the adiabatic approximation is destroyed by
dynamical phonons. Low energy states are phonons trapped by the soliton. When
the magnetic gap is smaller than the phonon frequencies the only low energy
state is a mixed magneto-phonon state with the energy of the gap. We emphasize
that our results are relevant for the Raman spectra of the inorganic
Spin-Peierls material CuGeO$_3$.",0009091v1
2000-10-06,Magnetic relaxation phenomena and cluster glass properties of La{0.7-x}Y{x}Ca{0.3}MnO{3} manganites,"The dynamic magnetic properties of the distorted perovskite system
La{0.7-x}Y{x}Ca{0.3}MnO{3} (0 <= x <= 0.15) have been investigated by
ac-susceptibility and dc magnetization measurements, with emphasis on
relaxation and aging studies. They evidence for x >= 0.10 the appearance of a
metallic cluster glass phase, that develops just below the ferromagnetic
transition temperature. The clusters grow with decreasing temperature down to a
temperature T(f0) at which they freeze due to severe intercluster frustration.
The formation of these clusters is explained by the presence of yttrium induced
local structural distortions that create localized spin disorder in a magnetic
lattice where double-exchange ferromagnetism is dominant.",0010110v2
2000-10-24,Rotating Hele-Shaw cells with ferrofluids,"We investigate the flow of two immiscible, viscous fluids in a rotating
Hele-Shaw cell, when one of the fluids is a ferrofluid and an external magnetic
field is applied. The interplay between centrifugal and magnetic forces in
determining the instability of the fluid-fluid interface is analyzed. The
linear stability analysis of the problem shows that a non-uniform, azimuthal
magnetic field, applied tangential to the cell, tends to stabilize the
interface. We verify that maximum growth rate selection of initial patterns is
influenced by the applied field, which tends to decrease the number of
interface ripples. We contrast these results with the situation in which a
uniform magnetic field is applied normally to the plane defined by the rotating
Hele-Shaw cell.",0010374v1
2000-11-20,Spin correlations in the two-leg antiferromagnetic ladder in a magnetic field,"We study the ground-state spin correlations in the gapless incommensurate
regime of a S=1/2 XXZ chain and a two-leg antiferromagnetic ladder under a
magnetic field, in which the gapless excitations form a Tomonaga-Luttinger (TL)
liquid. We calculate numerically the two-spin correlation functions and the
local magnetization in the two models using the density-matrix
renormalization-group method. By fitting the numerical results for an open XXZ
chain of 100 spins to correlation functions of a Gaussian model, we determine
the TL-liquid parameter K and the amplitudes of the correlation functions. The
value of K estimated from the fits is in excellent agreement with the exact
value obtained from the Bethe ansatz. We apply the same method to the open
ladder consisting of 200 spins and determine the dependence of K on the
magnetization M. The K-M relation changes drastically depending on the ratio of
the coupling constants in the leg and rung directions. We also discuss
implications of these results to experiments on the nuclear spin relaxation
rate 1/T_1 and dynamical spin structure factors.",0011320v1
2000-11-23,Quantum Computing in Molecular Magnets,"Shor and Grover demonstrated that a quantum computer can outperform any
classical computer in factoring numbers and in searching a database by
exploiting the parallelism of quantum mechanics. Whereas Shor's algorithm
requires both superposition and entanglement of a many-particle system, the
superposition of single-particle quantum states is sufficient for Grover's
algorithm. Recently, the latter has been successfully implemented using Rydberg
atoms. Here we propose an implementation of Grover's algorithm that uses
molecular magnets, which are solid-state systems with a large spin; their spin
eigenstates make them natural candidates for single-particle systems. We show
theoretically that molecular magnets can be used to build dense and efficient
memory devices based on the Grover algorithm. In particular, one single crystal
can serve as a storage unit of a dynamic random access memory device. Fast
electron spin resonance pulses can be used to decode and read out stored
numbers of up to 10^5, with access times as short as 10^{-10} seconds. We show
that our proposal should be feasible using the molecular magnets Fe8 and Mn12.",0011415v2
2001-03-19,Time decay of the remanent magnetization in the $\pm J$ spin glass model at T=0,"Using the zero-temperature Metropolis dynamics, the time decay of the
remanent magnetization in the $\pm J$ Edward-Anderson spin glass model with a
uniform random distribution of ferromagnetic and antiferromagnetic interactions
has been investigated. Starting from the saturation, the magnetization per spin
$m$ reveals a slow decrease with time, which can be approximated by a power
law:$m(t)=m_{\infty}+ ({t\over a_{0}})^{a_{1}}$, $a_{1} < 0$. Moreover, its
relaxation does not lead it into one of the ground states, and therefore the
system is trapped in metastable isoenergetic microstates remaining magnetized.
Such behaviour is discussed in terms of a random walk the system performs on
its available configuration space.",0103382v4
2001-06-12,Theory of combined exciton-cyclotron resonance in a two-dimensional electron gas: The strong magnetic field regime,"I develop a theory of combined exciton-cyclotron resonance (ExCR) in a
low-density two-dimensional electron gas in high magnetic fields. In the
presence of excess electrons an incident photon creates an exciton and
simultaneously excites one electron to higher-lying Landau levels. I derive
exact ExCR selection rules that follow from the existing dynamical symmetries,
magnetic translations and rotations about the magnetic field axis. The nature
of the final states in the ExCR is elucidated. The relation between ExCR and
shake-up processes is discussed. The double-peak ExCR structure for transitions
to the first electron Landau level is predicted.",0106229v2
2001-11-20,Semiclassical analysis of a two-electron quantum dot in a magnetic field: dimensional phenomena,"While the dynamics for three-dimensional axially symmetric two-electron
quantum dots with parabolic confinement potentials is in general non-separable
we have found an exact separability with three quantum numbers for specific
values of the magnetic field. Furthermore, it is shown that the magnetic
properties such as the magnetic moment and the susceptibility are sensitive to
the presence and strength of a vertical confinement. Using a semiclassical
approach the calculation of the eigenvalues reduces to simple quadratures
providing a transparent and almost analytical quantization of the quantum dot
energy levels which differ from the exact energies only by a few percent.",0111381v2
2001-12-10,Unusual magneto-elastic forces stabilize the insulating ferromagnetic state of manganites: the La(0.8)Ca(0.2)$MnO(3) case,"The ferromagnetic and insulating state observed in La$_{1-x}$Ca$_{x}$MnO$_3$,
0.125$<$x$<$0.2, is characterized by structural and magnetic anomalies below
T$_C$, similar to those observed in the x$_{Sr}$$\approx$1/8. A neutron
scattering study of the superlattice {\bf Q$_0$}= (0,0,1+/4)$_{cub}$ peak, and
of the magnetic excitations are reported in the x$_{Ca}$=0.2 sample. The
occurrence of this superstructure is associated with the observation of a gap
in the spin dynamics, at a {\bf q$_0$} wave-vector ({\bf q$_0$}={\bf
Q$_0$}-$\tau$) with the same modulus $|${\bf q$_0$}$|$ in all directions, which
divides the dispersion into two regimes. For $|${\bf q}$|$$<$$|${\bf q$_0$}$|$
the dispersion is splitted into two or three curves. For $|${\bf
q}$|$$>$$|${\bf q$_0$}$|$, magnetic excitations lock on acoustic and optic
phonon energies, revealing a new kind of magneto-vibrational coupling.
  We suggest an analysis in terms of two distinct magnetic couplings,
associated with two ferromagnetic media involved into a collective state.",0112159v1
2002-01-09,Monte-Carlo simulation of nucleation in the two-dimensional Potts model,"Nucleation in the two-dimensional q-state Potts model has been studied by
means of Monte-Carlo simulations using the heat-bath dynamics. The initial
metastable state has been prepared by magnetic quench of the ordered
low-temperature phase. The magnetic field dependence of the nucleation time has
been measured as the function of the magnetic field for different q and lattice
sizes at T=0.5 Tc. A size-dependent crossover from the coalescence to
nucleation region is observed at all q. The magnetic field dependence of the
nucleation time is roughly described by the classical nucleation theory. Our
data show increase of the anisotropy in the shape of the critical droplets with
increase of q.",0201128v1
2002-03-10,Thermal Conductivity in 3D NJL Model Under External Magnetic Field,"The thermal conductivity of the (2+1)-dimensional NJL model in the presence
of a constant magnetic field is calculated in the mean-field approximation and
its different asymptotic regimes are analyzed. Taking into account the
dynamical generation of a fermion mass due to the magnetic catalysis
phenomenon, it is shown that for certain relations among the theory's
parameters (particle width, temperature and magnetic field), the profile of the
thermal conductivity versus the applied field exhibits kink- and plateau-like
behaviors. We point out possible applications to planar condensed matter.",0203217v2
2002-06-10,"Prediction of huge X-ray Faraday rotation at the Gd N_4,5 threshold","X-ray absorption spectra in a wide energy range around the 4d-4f excitation
threshold of Gd were recorded by total electron yield from in-plane magnetized
Gd metal films. Matching the experimental spectra to tabulated absorption data
reveals unprecedented short light absorption lengths down to 3 nm. The
associated real parts of the refractive index for circularly polarized light
propagating parallel or antiparallel to the Gd magnetization, determined
through the Kramers-Kronig transformation, correspond to a magneto-optical
Faraday rotation of 0.7 degrees per atomic layer. This finding shall allow the
study of magnetic structure and magnetization dynamics of lanthanide elements
in nanosize systems and dilute alloys.",0206154v2
2002-09-23,Magnetic flux penetration and motion in antiferromagnetic superconductors,"The paper reviews a concept of induced spin-flop domain inside vortices in an
antiferromagnetic superconductor. Such phenomenon may occur when an external
magnetic field is strong enough to flip over magnetic moments in the core of
the vortex from their ground state configuration. The formation of the domain
structure inside vortices modifies the surface energy barrier of the
superconductor. During this process the entrance of the flux is stopped and a
newly created state exhibits perfect shielding. Such behavior should be visible
as a plateau on the dependence of flux density as a function of the external
magnetic field. The end of the plateau determines the critical field, which has
been called the second critical field for flux penetration. Moreover, it is
predicted and described how this phenomenon modifies flux creep in layered
superconductors. The various scenarios of changing the creep regime from
thermal to quantum and vice versa at constant temperature are discussed.",0209526v2
2002-11-05,Spectral density functional approach to electronic correlations and magnetism in crystals,"A novel approach to electronic correlations and magnetism of crystals based
on realistic electronic structure calculations is reviewed. In its simplest
form it is a combination of the ``local density approximation'' (LDA) and the
dynamical mean field theory (DMFT) approaches. Using numerically exact QMC
solution to the effective DMFT multi-orbital quantum-impurity problem, a
successful description of electronic structure and finite temperature magnetism
of transition metals has been achieved. We discuss a simplified perturbation
LDA+DMFT scheme which combines the T-matrix and fluctuation-exchange
approximation (TM-FLEX). We end with a discussion of cluster generalization of
the non-local DMFT scheme and its applications to the magnetism and
superconductivity of high-Tc superconductors.",0211076v1
2002-11-25,Spin-polarized current oscillations in diluted magnetic semiconductor multiple quantum wells,"We study the spin and charge dynamics of electrons in n-doped II--VI
semiconductor multiple quantum wells when one or more quantum wells are doped
with Mn. The interplay between strongly nonlinear inter-well charge transport
and the large tunable spin-splitting induced by exchange interactions with
spin-polarized Mn ions produces interesting new spin dependent features. The
tunneling current between quantum wells can be strongly spin polarized and,
under certain conditions, can develop self sustained oscillations under a
finite dc voltage. The spin polarization oscillates in both magnetic and
nonmagnetic quantum wells and the time average in magnetic wells can differ
from its zero-voltage value. Our numerical simulations demonstrate that the
amplitude of the spin polarization oscillations depends on the distribution of
magnetic wells within the sample. We discuss how the spin polarized current and
the spin polarization of the quantum wells can be tailored experimentally.",0211557v1
2003-03-18,Extended quantum critical phase in a magnetized spin-1/2 antiferromagnetic chain,"Measurements are reported of the magnetic field dependence of excitations in
the quantum critical state of the spin S=1/2 linear chain Heisenberg
antiferromagnet copper pyrazine dinitrate (CuPzN). The complete spectrum was
measured at k_B T/J <= 0.025 for H=0 and H=8.7 Tesla where the system is ~30%
magnetized. At H=0, the results are in quantitative agreement with exact
calculations of the dynamic spin correlation function for a two-spinon
continuum. At high magnetic field, there are multiple overlapping continua with
incommensurate soft modes. The boundaries of these continua confirm
long-standing predictions, and the intensities are consistent with exact
diagonalization and Bethe Ansatz calculations.",0303364v1
2003-04-07,Separation Between Antiferromagnetic and Ferromagnetic Transitions in Ru_1-xCu_xSr_2EuCu_2O_8+d,"The macroscopic magnetizations of Ru_1-xCu_xSr_2EuCu_2O_8+d with x between 0
and 0.15 were investigated. A ferromagnet-like transition as well as an
antiferromagnet-like transition appear around T_M in the low-field
magnetization and around T_AM in the high-field differential susceptibility,
respectively. The separation between them, which is accompanied by a flat
plateau in the magnetic C_p, increases with x. Superparamagnetic M(H) and slow
spin dynamics, i.e. characteristics of nanomagnetic clusters, were observed far
above T_M. A comparison with RuSr_2(Eu_1-yCe_y)Cu_2O_10+d and some manganites
further suggests that a phase separation occurs, which can describe well the
conflicting magnetic-superconductivity data previously reported.",0304163v1
2003-04-24,Ferromagnetism in a dilute magnetic semiconductor -- Generalized RKKY interaction and spin-wave excitations,"Carrier-mediated ferromagnetism in a dilute magnetic semiconductor has been
studied using i) a single-impurity based generalized RKKY approach which goes
beyond linear response theory, and ii) a mean-field-plus-spin-fluctuation
(MF+SF) approach within a (purely fermionic) Hubbard-model representation of
the magnetic impurities, which incorporates dynamical effects associated with
finite frequency spin correlations in the ordered state. Due to a competition
between the magnitude of the carrier spin polarization and its oscillation
length scale, the ferromagnetic spin coupling is found to be optimized with
respect to both hole doping concentration and impurity-carrier spin coupling
energy $J$ (or equivalently $U$). The ferromagnetic transition temperature
$T_c$, deteremined within the spin-fluctuation theory, corresponds closely with
the observed $T_c$ values. Positional disorder of magnetic impurities causes
significant stiffening of the high-energy magnon modes. We also explicitly
study the stability/instability of the mean-field ferromagnetic state, which
highlights the role of competing AF interactions causing spin twisting and
noncollinear ferromagnetic ordering.",0304557v2
2003-04-27,Nanomechanical Analog of a Laser: Amplification of Mechanical Oscillations by Stimulated Zeeman Transitions,"We propose a magnetomechanical device that exhibits many properties of a
laser. The device is formed by a nanocantilever and dynamically polarized
paramagnetic nuclei of a solid sample in a strong external magnetic field. The
corresponding quantum oscillator and effective two-level systems are coupled by
the magnetostatic dipole-dipole interaction between a permanent magnet on the
cantilever tip and the magnetic moments of the spins, so that the entire system
is effectively described by the Jaynes-Cummings model. We consider the
possibility of observing transient and cw lasing in this system, and show how
these processes can be used to improve the sensitivity of magnetic resonance
force microscopy.",0304605v2
2003-06-06,The kinetic spherical model in a magnetic field,"The long-time kinetics of the spherical model in an external magnetic field
and below the equilibrium critical temperature is studied. The solution of the
associated stochastic Langevin equation is reduced exactly to a single
non-linear Volterra equation. For a sufficiently small external field, the
kinetics of the magnetization-reversal transition from the metastable to the
ground state is compared to the ageing behaviour of coarsening systems quenched
into the low-temperature phase. For an oscillating magnetic field and below the
critical temperature, we find evidence for the absence of the
frequency-dependent dynamic phase transition, which was observed previously to
occur in Ising-like systems.",0306171v2
2003-06-24,Optical bandedge of diluted magnetic semiconductors: difference between II-VI and III-V-based DMSs,"Applying the dynamical coherent potential approximation to a simple model, we
have theoretically studied the behavior of the optical bandedge in diluted
magnetic semiconductors (DMSs). For $A^{\rm II}_{1-x}$Mn$_{x}B^{\rm VI}$-type
DMS, the present study reveals that the linear relationship between
exchange-spitting $\mit\Delta E_{ex} $ and the averaged magnetization $|x < S_z
>|$ widely holds for different values of $x$. The ratio, $\mit\Delta E_{ex}/x <
S_z>$, however, depends not only the exchange strength but also the band
offset. Furthermore, the present study reveals that in the low dilution of
Ga$_{1-x}$Mn$_x$As the optical bandedge exists not at the bandedge of the
impurity band but near the bottom of host band. The optical bandedge behaves as
if the exchange interaction is ferromagnetic although the antiferromagnetic
exchange interaction actually operates at Mn site. We conclude that the
spin-dependent shift of the carrier states between the impurity band and host
band accompanying with the change of magnetization causes the apparently
ferromagnetic behavior of the optical bandedge which was reported in the
magnetoreflection measurement of Ga$_{1-x}$Mn$_x$As.",0306588v1
2003-06-25,Ultrafast Auger spectroscopy of quantum well excitons in a strong magnetic field,"We study theoretically the ultrafast nonlinear optical response of quantum
well excitons in a perpendicular magnetic field. We address the role of
many-body correlations originating from the electron scattering between Landau
levels (LL). In the linear optical response, the processes involving inter-LL
transitions are suppressed provided that the magnetic field is sufficiently
strong. However, in the nonlinear response, the Auger processes involving
inter-LL scattering of two photoexcited electrons remain unsuppressed. We show
that Auger scattering plays the dominant role in the coherent exciton dynamics
in strong magnetic field. We perform numerical calculations for the third-order
four-wave-mixing (FWM) polarization which incorporate the Auger processes
nonperturbatively. We find that inter-LL scattering leads to a strong
enhancement and to oscillations of the FWM signal at negative time delays.
These oscillations represent quantum beats between optically-inactive
two-exciton states related to each other via Auger processes.",0306630v2
2003-10-18,Experiment and Dynamic Simulations of Radiation Damping of Laser-polarized liquid 129Xe at low magnetic field in a flow system,"Radiation damping is generally observed when the sample with high spin
concentration and high gyro-magnetic ratio is placed in a high magnetic field.
However, we firstly observed liquid state 129Xe radiation damping using
laser-enhanced nuclear polarization at low magnetic field in a flow system in
which the polarization enhancement factor for the liquid state 129Xe was
estimated to be 5000, and furthermore theoretically simulated the envelopes of
the 129Xe FID and spectral lineshape in the presence of both relaxation and
radiation damping with different pulse flip angles and ratios of T2*/Trd. The
radiation damping time constant Trd of 5 ms was derived based on the
simulations. The reasons of depolarization and the further possible
improvements were also discussed.",0310435v1
2004-03-18,"Electrons in 2D periodic magnetic and electric fields: Avoided band crossings, metal-insulator transitions, and changes in the spreading direction","We study the energy spectrum and wave packet dynamics of electrons in a
two-dimensional periodic potential subject to a perpendicular spatially
modulated magnetic field. We observe avoided band crossings, metal-insulator
transitions, and changes in the spreading direction of wave packets in two new
situations: (i) for a purely magnetic modulation and (ii) in the limit of weak
electric and magnetic modulations, where the Landau band coupling can be
neglected. In the second case the corresponding classical system is no longer
chaotic but integrable, whereby these phenomena are not expected. We argue that
they are due to tunneling.",0403462v2
2004-03-19,Charge dynamics and Kondo effect in single electron traps in field effect transistors,"We study magneto-electric properties of single electron traps in
metal-oxide-semiconductor field effect transistors. Using a microscopic
description of the system based on the single-site Anderson-Holstein model, we
derive an effective low energy action for the system. The behavior of the
system is characterized by simultaneous polaron tunneling (corresponding to the
charging and discharging of the trap) and Kondo screening of the trap spin in
the singly occupied state. Hence, the obtained state of the system is a hybrid
between the Kondo regime, typically associated with single electron occupancy,
and the mixed valence regime, associated with large charge fluctuations. In the
presence of a strong magnetic field, we demonstrate that the system is
equivalent to a two level-level system coupled to an Ohmic bath, with a bias
controlled by the applied magnetic field. Due to the Kondo screening, the
effect of the magnetic field is significantly suppressed in the singly occupied
state. We claim that this suppression can be responsible for the experimentally
observed anomalous magnetic field dependence of the average trap occupancy in
${\rm Si-Si0_2}$ field effect transistors.",0403491v1
2004-07-29,Kondo Effect on Mesoscopic Scale (Review),"Following the discovery of the Kondo effect the bulk transport and magnetic
behavior of the dilute magnetic alloys have been successfully described. In the
last fifteen years new directions have been developed as the study of the
systems of reduced dimensions and the artificial atoms so called quantum dots.
In this review the first subject is reviewed starting with the scanning
tunneling microscope (STM) study of a single magnetic impurity. The next
subject is the reduction of the amplitude of the Kondo effect in samples of
reduced dimension which was explained by the surface magnetic anisotropy which
blocks the motion of the integer spin nearby the surface. The electron
dephasing and energy relaxation experiments are discussed with the possible
explanation including the surface anisotropy, where the situation in cases of
integer and half-integer spins is very different. Finally, the present
situation of the theory of dynamical structural defects is briefly presented
which may lead to two-channel Kondo behavior.",0407766v1
2004-08-26,Exchange constants and spin dynamics in Mn12-acetate,"We have obtained new inelastic neutron scattering (INS) data for the
molecular magnet Mn12-acetate which exhibit at least six magnetic peaks in the
energy range 5-35 meV. These are compared with a microscopic Heisenberg model
for the 12 quantum spins localised on the Mn ions, coupled by four inequivalent
magnetic exchange constants. A fit to the magnetic susceptibility under the
constraint that the spin of the ground state be S=10 yields two dominant
exchange constants of very similar value, J1 ~ J2 ~ 65 K (~ 5.5 meV), and two
smaller exchange constants J3 and J4. We compute the low-lying excitations by
exact numerical diagonalisation and demonstrate that the parameters determined
from the ground state and susceptibility fit provide qualitative agreement with
the excitations observed by INS.",0408576v1
2004-08-27,Long-range dynamics of magnetic impurities coupled to a two-dimensional Heisenberg antiferromagnet,"We consider a two-dimensional Heisenberg antiferromagnet on a square lattice
with weakly coupled impurities, i.e. additional spins interacting with the host
magnet by a small dimensionless coupling constant g<<1. Using linear spin-wave
theory, we find that the magnetization disturbance at distance r from a single
impurity behaves as g/r for 1<<r<<1/g and as 1/(gr^3) for r>>1/g. Surprisingly
the disturbance is inversely proportional to the coupling constant! The
interaction between two impurities separated by a distance r is proportional to
g^2/r for 1<<r<<1/g and to 1/r^3 for r>>1/g. Hence at large distances, the
interaction is universal and independent of the coupling constant. We also find
that the frequency of Rabi oscillations between two impurities is proportional
to g^2 ln(gr) at 1<<r<<1/g, logarithmically enhanced compared to the spin-wave
width. This leads to a new mechanism for NMR, NQR and EPR line broadening. All
these astonishing results are due to the gapless spectrum of the magnetic
excitations in the quantum antiferromagnet.",0408598v2
2004-11-01,Optical Orientation of Excitons in CdSe Self-Assembled Quantum Dots,"We study spin dynamics of excitons confined in self-assembled CdSe quantum
dots by means of optical orientation in magnetic field. At zero field the
exciton emission from QDs populated via LO phonon-assisted absorption shows a
circular polarization of 14%. The polarization degree of the excitonic emission
increases dramatically when a magnetic field is applied. Using a simple model,
we extract the exciton spin relaxation times of 100 ps and 2.2 ns in the
absence and presence of magnetic field, respectively. With increasing
temperature the polarization of the QD emission gradually decreases.
Remarkably, the activation energy which describes this decay is independent of
the external magnetic field, and, therefore, of the degeneracy of the exciton
levels in QDs. This observation implies that the temperature-induced
enhancement of the exciton spin relaxation is insensitive to the energy level
degeneracy and can be attributed to the same excited state distribution.",0411036v1
2005-02-18,Non-equilibrium Magnetization Dynamics in the Fe_8 Single-Molecule Magnet Induced by High-Intensity Microwave Radiation,"Resonant microwave radiation applied to a single crystal of the molecular
magnet Fe_8 induces dramatic changes in the sample's magnetization. Transitions
between excited states are found even though at the nominal system temperature
these levels have negligible population. We find evidence that the sample heats
significantly when the resonance condition is met. In addition, heating is
observed after a short pulse of intense radiation has been turned off,
indicating that the spin system is out of equilibrium with the lattice.",0502424v2
2005-02-24,Non-exponential spin relaxation in magnetic field in quantum wells with random spin-orbit coupling,"We investigate the spin dynamics of electrons in quantum wells where the
Rashba type of spin-orbit coupling is present in the form of random nanosize
domains. We study the effect of magnetic field on the spin relaxation in these
systems and show that the spatial randomness of spin-orbit coupling limits the
minimum relaxation rate and leads to a Gaussian time-decay of spin polarization
due to memory effects. In this case the relaxation becomes faster with increase
of the magnetic field in contrast to the well known magnetic field suppression
of spin relaxation.",0502593v3
2005-03-25,Bose-Einstein Condensation on a Permanent-Magnet Atom Chip,"We have produced a Bose-Einstein condensate on a permanent-magnet atom chip
based on periodically magnetized videotape. We observe the expansion and
dynamics of the condensate in one of the microscopic waveguides close to the
surface. The lifetime for atoms to remain trapped near this dielectric material
is significantly longer than above a metal surface of the same thickness. These
results illustrate the suitability of microscopic permanent-magnet structures
for quantum-coherent preparation and manipulation of cold atoms.",0503619v2
2005-05-30,Mechanisms limiting the coherence time of spontaneous magnetic oscillations driven by DC spin-polarized currents,"The spin-transfer torque from a DC spin-polarized current can generate
highly-coherent magnetic precession in nanoscale magnetic-multilayer devices.
By measuring linewidths of spectra from the resulting resistance oscillations,
we argue that the coherence time can be limited at low temperature by thermal
deflections about the equilibrium magnetic trajectory, and at high temperature
by thermally-activated transitions between dynamical modes. Surprisingly, the
coherence time can be longer than predicted by simple macrospin simulations.",0505733v2
2005-06-17,Magnetic density of states at low energy in geometrically frustrated systems,"Using muon-spin-relaxation measurements we show that the pyrochlore compound
Gd2Ti2O7, in its magnetically orderered phase below ~ 1 K, displays persistent
spin dynamics down to temperatures as low as 20 mK. The characteristics of the
induced muon relaxation can be accounted for by a scattering process involving
two magnetic excitations, with a density of states characterized by an upturn
at low energy and a small gap depending linearly on the temperature. We propose
that such a density of states is a generic feature of geometrically frustrated
magnetic materials.",0506451v1
2005-09-27,Memory and aging effect in hierarchical spin orderings of stage-2 CoCl_{2} graphite intercalation compound,"Stage-2 CoCl$_{2}$ graphite intercalation compound undergoes two magnetic
phase transitions at $T_{cl}$ (= 7.0 K) and $T_{cu}$ (= 8.9 K). The aging
dynamics of this compound is studied near $T_{cl}$ and $T_{cu}$. The
intermediate state between $T_{cl}$ and $T_{cu}$ is characterized by a spin
glass phase extending over ferromagnetic islands. A genuine thermoremnant
magnetization (TRM) measurement indicates that the memory of the specific spin
configurations imprinted at temperatures between $T_{cl}$ and $T_{cu}$ during
the field-cooled (FC) aging protocol can be recalled when the system is
re-heated at a constant heating rate. The zero-field cooled (ZFC) and TRM
magnetization is examined in a series of heating and reheating process. The
magnetization shows both characteristic memory and rejuvenation effects. The
time $(t)$ dependence of the relaxation rate
$S_{ZFC}(t)=(1/H)$d$M_{ZFC}(t)$/d$\ln t$ after the ZFC aging protocol with a
wait time $t_{w}$, exhibits two peaks at characteristic times $t_{cr1}$ and
$t_{cr2}$ between $T_{cl}$ and $T_{cu}$. An aging process is revealed as the
strong $t_{w}$ dependence of $t_{cr2}$. The observed aging and memory effect is
discussed in terms of the droplet model.",0509696v1
2005-11-11,"Spin amplification, reading, and writing in transport through anisotropic magnetic molecules","Inelastic transport through a single magnetic molecule weakly coupled to
metallic leads is studied theoretically. We consider dynamical processes that
are relevant for writing, storing, and reading spin information in molecular
memory devices. Magnetic anisotropy is found to be crucial for slow spin
relaxation. In the presence of anisotropy we find giant spin amplification: The
spin accumulated in the leads if a bias voltage is applied to a molecule
prepared in a spin-polarized state can be made exponentially large in a
characteristic energy divided by temperature. For one ferromagnetic and one
paramagnetic lead the molecular spin can be reversed by applying a bias voltage
even in the absence of a magnetic field. We propose schemes for reading and
writing spin information based on our findings.",0511291v2
2006-03-14,Topological Effects caused by the Fractal Substrate on the Nonequilibrium Critical Behavior of the Ising Magnet,"The nonequilibrium critical dynamics of the Ising magnet on a fractal
substrate, namely the Sierpinski carpet with Hausdorff dimension $d_H$ =1.7925,
has been studied within the short-time regime by means of Monte Carlo
simulations. The evolution of the physical observables was followed at
criticality, after both annealing ordered spin configurations (ground state)
and quenching disordered initial configurations (high temperature state), for
three segmentation steps of the fractal. The topological effects become evident
from the emergence of a logarithmic periodic oscillation superimposed to a
power law in the decay of the magnetization and its logarithmic derivative and
also from the dependence of the critical exponents on the segmentation step.
These oscillations are discussed in the framework of the discrete scale
invariance of the substrate and carefully characterized in order to determine
the critical temperature of the second-order phase transition and the critical
exponents corresponding to the short-time regime. The exponent $\theta $ of the
initial increase in the magnetization was also obtained and the results suggest
that it would be almost independent of the fractal dimension of the susbstrate,
provided that $d_H$ is close enough to d=2.",0603386v1
2006-05-05,Bose-Einstein condensation and entanglement in magnetic systems,"We present a study of magnetic field induced quantum phase transitions in
insulating systems. A generalized scaling theory is used to obtain the
temperature dependence of several physical quantities along the quantum
critical trajectory ($H=H_{C}$, $T\to0$) where $H$ is a longitudinal external
magnetic field and $H_{C}$ the critical value at which the transition occurs.
We consider transitions from a spin liquid at a critical field $H_{C1}$ and
from a fully polarized paramagnet, at $H_{C2}$, into phases with long range
order in the transverse components. The transitions at $H_{C1}$ and $H_{C2}$
can be viewed as Bose-Einstein condensations of magnons which however belong to
different universality classes since they have different values of the dynamic
critical exponent $z$. Finally, we use that the magnetic susceptibility is an
entanglement witness to discuss how this type of correlation sets in as the
system approaches the quantum critical point along the critical trajectory,
$H=H_{C2}$, $T\to0$.",0605148v2
2006-05-15,Vortex fluctuations and freezing of dipolar-coupled granular moments in thin ferromagnetic films,"Below the Curie temperature T_c of a Heusler-alloy film, consisting of
densely packed, but exchange-decoupled nanograins, the spontaneous
magnetization M_s(T) and static in-plane susceptibility \chi_{||}(T) increase
very slowly signalizing a suppression of magnetization fluctuations. The
unpredicted variation \chi_{||}(T) ~ G_d^2(T), where G_d ~ M_s^2 is the
intergranular dipolar coupling, and also the magnetic freezing observed in the
dynamic susceptibility at lower temperatures is quantitatively reproduced by
Monte Carlo (MC) simulations with 10^4 dipolar-coupled moments on a disordered
triangular lattice. At high temperatures, the MC spin configurations clearly
reveal a dense gas of local vortex structures, which at low temperatures
condense in regions with stronger disorder. This vortex depletion upon
decreasing temperature seems to be responsible for the observed
\textit{increase} of the magnetic relaxation time. For weak disorder, the
temperature dependence of the MC vorticity and a singularity of the specific
heat at T_v=1/2 G_d(T_v)/k_B indicate a thermal transition from a vortex gas to
a state with a single vortex center plus linear vortex structures.",0605377v1
2006-05-23,Dynamics of a trapped domain wall in a current perpendicular to the plane spin valve nano-structure,"A study of transverse tail-to-tail magnetic domain walls (DW) in novel
current perpendicular to the plane (CPP) spin valves (SV) of various dimensions
is presented. For films with dimensions larger than the DW width, we find that
DW motion can give rise to a substantial low frequency noise. For dimensions
comparable to the DW width, we show that the DW can be controlled by an
external field or by a spin momentum torque as opposed to the case of CPP-SV
with uniform magnetization. It is shown that in a single domain biased CPP-SV,
the spin torque can give rise to 1/f-type noise. The dipolar field, the spin
torque and the Oersted field are all accounted for in this work. Our proposed
SV requires low current densities to move DW and can simulate devices for
logical operation or magnetic sensing without having to switch the
magnetization in the free layer.",0605586v1
2006-05-30,Electric Dipole Induced Spin Resonance in Disordered Semiconductors,"One of the hallmarks of spintronics is the control of magnetic moments by
electric fields enabled by strong spin-orbit interaction (SOI) in
semiconductors. A powerful way of manipulating spins in such structures is
electric dipole induced spin resonance (EDSR), where the radio-frequency fields
driving the spins are electric, and not magnetic like in standard paramagnetic
resonance. Here, we present a theoretical study of EDSR for a two-dimensional
electron gas in the presence of disorder where random impurities not only
determine the electric resistance but also the spin dynamics via SOI.
Considering a specific geometry with the electric and magnetic fields parallel
and in-plane, we show that the magnetization develops an out-of-plane component
at resonance which survives the presence of disorder. We also discuss the spin
Hall current generated by EDSR. These results are derived in a diagrammatic
approach with the dominant effects coming from the spin vertex correction, and
the optimal parameter regime for observation is identified.",0605735v1
2006-08-04,Multiferroicity and colossal magneto-capacitance in Cr-thiospinels,"The sulfur based Cr-spinels RCr2S4 with R = Cd and Hg exhibit the coexistence
of ferromagnetic and ferroelectric properties together with a pronounced
magnetocapacitive coupling. While in CdCr2S4 purely ferromagnetic order is
established, in HgCr2S4 a bond-frustrated magnetic ground state is realized,
which, however, easily can be driven towards a ferromagnetic configuration in
weak magnetic fields. This paper shall review our recent investigation for both
compounds. Besides the characterization of the magnetic properties, the complex
dielectric permittivity was studied by means of broadband dielectric
spectroscopy as well as measurements of polarization hysteresis and
pyro-currents. The observed colossal magneto-capacitive effect at the magnetic
transition seems to be driven by an enormous variation of the relaxation
dynamics.",0608113v1
2006-08-13,Observation of superspin-glass behavior in Fe$_{3}$O$_{4}$ nanoparticles,"The aging and memory effects of Fe$_{3}$O$_{4}$ nanoparticles have been
studied using a series of zero-field cooled (ZFC) and field-cooled (FC)
magnetization measurements at various aging protocols. The genuine ZFC
magnetization after the ZFC procedure with a single stop and wait process shows
an aging dip at the stop temperature on reheating. The depth of the aging dip
is dependent on the wait time. The frequency dependence of the AC magnetic
susceptibility is indicative of critical slowing down at a freezing temperature
$T_{f}$ ($= 30.6 \pm 1.6$ K). The relaxation time $\tau$ is described by a
power law form with a dynamic critical exponent $x$ ($= 8.2 \pm 1.0$) and a
microscopic relaxation time $\tau_{0}$ [$=(1.33 \pm 0.05) \times 10^{-9}$ sec].
The ZFC-peak temperature decreases with increasing magnetic field ($H$),
forming a critical line with an exponent $p = 1.78 \pm 0.26$, close to the de
Almeida-Thouless exponent ($p = 3/2$). These results indicate that the
superspin glass phase occurs below $T_{f}$.",0608297v4
2006-11-22,Magnetization damping in a local-density approximation,"The linear response of itinerant transition metal ferromagnets to transverse
magnetic fields is studied in a self-consistent adiabatic local-density
approximation. The susceptibility is calculated from a microscopic Hamiltonian,
including spin-conserving impurities, impurity induced spin-orbit interaction
and magnetic impurities using the Keldysh formalism. The Gilbert damping
constant in the Landau-Lifshitz-Gilbert equation is identified, parametrized by
an effective transverse spin dephasing rate, and is found to be inversely
proportional to the exchange splitting. Our result justify the phenomenological
treatment of transverse spin dephasing in the study of current-induced
magnetization dynamics in weak, itinerant ferromagnets by Tserkovnyak
\textit{et al.}. We show that neglect of gradient corrections in the
quasiclassical transport equations leads to incorrect results when the exchange
potential becomes of the order of the Fermi energy.",0611588v1
2006-12-11,Size Distribution of Superparamagnetic Particles Determined by Magnetic Sedimentation,"We report on the use of magnetic sedimentation as a means to determine the
size distribution of dispersed magnetic particles. The particles investigated
here are i) single anionic and cationic nanoparticles of diameter D = 7 nm and
ii) nanoparticle clusters resulting from electrostatic complexation with
polyelectrolytes and polyelectrolyte-neutral copolymers. A theoretical
expression of the sedimentation concentration profiles at the steady state is
proposed and it is found to describe accurately the experimental data. When
compared to dynamic light scattering, vibrating sample magnetometry and
cryogenic transmission electron microscopy, magnetic sedimentation exhibits a
unique property : it provides the core size and core size distribution of
nanoparticle aggregates.",0612255v1
2007-02-01,Quantum Magnetism and Superconductivity,"The spin of the neutron allows neutron scattering to reveal the magnetic
structure and dynamics of materials over nanometre length scales and picosecond
timescales. Neutron scattering is particularly in demand in order to understand
high-temperature superconductors, which lie close to magnetically ordered
phases, and highly correlated metals with giant effective fermion masses, which
lie close to magnetic order or pass through a mysterious phase of hidden order
before becoming superconducting. Neutron scattering also is the probe of choice
for revealing new phases of matter and new particles, as seen in the surprising
behaviour of quantum spin chains and ladders where mass gaps and excited
triplons replace conventional spin waves. Examples are given of quantum
phenomena where neutron scattering has played a defining role that challenges
current understanding of condensed matter.",0702024v1
2007-03-22,Spin-Torque Ferromagnetic Resonance Measurements of Damping in Nanomagnets,"We measure the magnetic damping parameter a in thin film CoFeB and permalloy
(Py) nanomagnets at room temperature using ferromagnetic resonance driven by
microwave frequency spin-transfer torque. We obtain $\alpha_{CoFeB} = 0.014 \pm
0.003$ and $\alpha_{Py}=0.010 \pm 0.002$, values comparable to measurements for
extended thin films, but significantly less than the effective damping
determined previously for similar nanomagnets by fits to time-domain studies of
large-angle magnetic excitations and magnetic reversal. The greater damping
found for the large amplitude nanomagnet dynamics is attributed to the
nonlinear excitation of non-uniform magnetic modes.",0703577v1
1999-05-07,On magnetic catalysis in even-flavor QED3,"In this paper, we discuss the role of an external magnetic field on the
dynamically generated fermion mass in even-flavor QED in three space-time
dimensions. Based on some reasonable approximations, we present analytic
arguments on the fact that, for weak fields, the magnetically-induced mass
increases quadratically with increasing field, while at strong fields one
crosses over to a mass scaling logarithmically with the external field. We also
confirm this type of scaling behavior through quenched lattice calculations
using the non-compact version for the gauge field. Both the zero and finite
temperature cases are examined. A preliminary study of the fermion condensate
in the presence of magnetic flux tubes on the lattice is also included.",9905272v3
2000-04-17,QED in a Strong External Magnetic Field: Beyond the Constant Mass Approximation,"We solve the Schwinger-Dyson equations for QED in 2+1 or 3+1 dimensions in
the presence of a strong homogeneous external magnetic field. The magnetic
field is assumed strong enough, so that the lowest Landau level approximation
holds, but the usual assumption of a momentum-independent self-energy is not
made. In 2+1 dimensions, the scaling with logarithm changes to a square root
dependence on the magnetic field, but the most spectacular result takes place
in 3+1 dimensions, where the constant mass approximation turns out to be
unreliable and the (momentum-dependent) dynamical mass is larger by several
orders of magnitude compared to what has been found till now using the constant
mass approximation.",0004165v2
2000-12-06,Charged spin 1/2 particle in an arbitrary magnetic field in two spatial dimensions: a supersymmetric quantum mechanical system,"It is shown that the 2 X 2 matrix Hamiltonian describing the dynamics of a
charged spin 1/2 particle with g-factor 2 moving in an arbitrary, spatially
dependent, magnetic field in two spatial dimensions can be written as the
anticommuator of a nilpotent operator and its hermitian conjugate.
Consequently, the Hamiltonians for the two different spin projections form
partners of a supersymmetric quantum mechanical system. The resulting
supersymmetry algebra can then be exploited to explicitly construct the exact
zero energy ground state wavefunction for the system. Modulo this ground state,
the remainder of the eigenstates and eigenvalues of the two partner
Hamiltonians form positive energy degenerate pairs. We also construct the
spatially asymptotic form of the magnetic field which produces a finite
magnetic flux and associated zero energy normalizable ground state
wavefunction.",0012074v1
2005-03-23,Exotic and nonexotic magnetic transitions in the context of the SELEX and GRAAL experiments,"We calculate magnetic transition moments in the chiral quark-soliton model,
with explicit SU(3)-symmetry breaking taken into account. The dynamical model
parameters are fixed by experimental data for the magnetic moments of the
baryon octet and from the recent measurements of $\Theta^{+}$ mass. Known
magnetic transition moments $\mu_{\Lambda\Sigma}$, $\mu_{N\Delta}$ are
reproduced and predictions for other octet-decuplet and octet-antidecuplet
transitions are given. In particular $\mu_{\Sigma\Sigma^{\ast}}$ recently
constrained by SELEX is shown to be below $0.82 \mu_N$. The recent GRAAL data
on $\eta$ photoproduction off the nucleon are explained in terms of a new
narrow antidecuplet neutron-like resonance.",0503237v2
2006-11-09,Strongly coupled plasma with electric and magnetic charges,"A number of theoretical and lattice results lead us to believe that
Quark-Gluon Plasma not too far from $T_c$ contains not only electrically
charged quasiparticles -- quarks and gluons -- but magnetically charged ones --
monopoles and dyons -- as well. Although binary systems like charge-monopole
and charge-dyon were considered in details before in both classical and quantum
settings, it is the first study of coexisting electric and magnetic particles
in many-body context. We perform Molecular Dynamics study of strongly coupled
plasmas with $\sim 1000$ particles and different fraction of magnetic charges.
Correlation functions and Kubo formulae lead to such transport properties as
diffusion constant, shear viscosity and electric conductivity: we compare the
first two with empirical data from RHIC experiments as well as results from
AdS/CFT correspondence. We also study a number of collective excitations in
these systems.",0611131v2
1995-08-29,Adiabatic Motion of a Quantum Particle in a Two-Dimensional Magnetic Field,"The adiabatic motion of a charged, spinning, quantum particle in a two -
dimensional magnetic field is studied. A suitable set of operators generalizing
the cinematical momenta and the guiding center operators of a particle moving
in a homogeneous magnetic field is constructed. This allows us to separate the
two degrees of freedom of the system into a {\sl fast} and a {\sl slow} one, in
the classical limit, the rapid rotation of the particle around the guiding
center and the slow guiding center drift. In terms of these operators the
Hamiltonian of the system rewrites as a power series in the magnetic length
$\lb=\sqrt{\hbar c\over eB}$ and the fast and slow dynamics separates. The
effective guiding center Hamiltonian is obtained to the second order in the
adiabatic parameter $\lb$ and reproduces correctly the classical limit.",9508160v1
1998-11-25,Dynamical Generation of the Primordial Magnetic Field by Ferromagnetic Domain Walls,"The spontaneous generation of uniform magnetic condensate in $QED_3$ gives
rise to ferromagnetic domain walls at the electroweak phase transition. These
ferromagnetic domain walls are caracterized by vanishing effective surface
energy density avoiding, thus, the domain wall problem. Moreover we find that
the domain walls generate a magnetic field $B \simeq 10^{24} Gauss$ at the
electroweak scale which account for the seed field in the so called dynamo
mechanism for the cosmological primordial magnetic field. We find that the
annihilation processes of walls with size $R \simeq 10^5 Km$ could release an
energy of order $10^{52} erg$ indicating the invisible ferromagnetic walls as
possible compact sources of Gamma Ray Bursts.",9811221v1
1999-10-05,On Magnetic Catalysis and Gauge Symmetry Breaking,"Non-perturbative effects of constant magnetic fields in a Higgs-Yukawa gauge
model are studied using the extremum equations of the effective action for
composite operators. It is found that the magnetic field induces a Higgs
condensate, a fermion-antifermion condensate, and a fermion dynamical mass,
hence breaking the discrete chiral symmetry of the theory. The results imply
that for a non-simple group extension of the present model, the external
magnetic field would either induce or reinforce gauge symmetry breaking.
Possible cosmological applications of these results in the electroweak phase
transition are suggested.",9910035v2
2004-12-08,Non-Abelian Meissner Effect in Yang--Mills Theories at Weak Coupling,"We present a weak-coupling Yang--Mills model supporting non-Abelian magnetic
flux tubes and non-Abelian confined magnetic monopoles. In the dual description
the magnetic flux tubes are prototypes of the QCD strings. Dualizing the
confined magnetic monopoles we get gluelumps which convert a ""QCD string"" in
the excited state to that in the ground state. Introducing a mass parameter m
we discover a phase transition between the Abelian and non-Abelian confinement
at a critical value m=m_* of order of Lambda. Underlying dynamics are governed
by a Z_N symmetry inherent to the model under consideration. At m>m_* the Z_N
symmetry is spontaneously broken, resulting in N degenerate Z_N (Abelian)
strings. At m<m_* the Z_N symmetry is restored, the degeneracy is lifted, and
the strings become non-Abelian. We calculate tensions of the non-Abelian
strings, as well as the decay rates of the metastable strings, at N >> 1.",0412082v3
2006-02-02,Dynamics of O(N) Model in a Strong Magnetic Background Field as a Modified Noncommutative Field Theory,"In the presence of a strong magnetic field, the effective action of a
composite scalar field in an scalar O(N) model is derived using two different
methods. First, in the framework of worldline formalism, the 1PI n-point vertex
function for the composites is determined in the limit of strong magnetic
field. Then, the n-point effective action of the composites is calculated in
the regime of lowest Landau level dominance. It is shown that in the limit of
strong magnetic field, the results coincide and an effective field theory
arises which is comparable with the conventional noncommutative field theory.
In contrast to the ordinary case, however, the UV/IR mixing is absent in this
modified noncommutative field theory.",0602023v1
2004-04-26,The Gauss-Landau-Hall problem on Riemannian surfaces,"We introduce the notion of Gauss-Landau-Hall magnetic field on a Riemannian
surface. The corresponding Landau-Hall problem is shown to be equivalent to the
dynamics of a massive boson. This allows one to view that problem as a globally
stated, variational one. In this framework, flowlines appear as critical points
of an action with density depending on the proper acceleration. Moreover, we
can study global stability of flowlines. In this equivalence, the massless
particle model correspond with a limit case obtained when the force of the
Gauss-Landau-Hall increases arbitrarily. We also obtain new properties related
with the completeness of flowlines for a general magnetic fields. The paper
also contains new results relative to the Landau-Hall problem associated with a
uniform magnetic field. For example, we characterize those revolution surfaces
whose parallels are all normal flowlines of a uniform magnetic field.",0404060v1
2001-03-22,Field theoretic calculation of energy cascade rates in nonhelical magnetohydrodynamic turbulence,"Energy cascade rates and Kolmogorov's constant for nonhelical steady
magnetohydrodynamic turbulence have been calculated by solving the flux
equations to the first order in perturbation. For zero cross helicity and space
dimension $d=3$, magnetic energy cascades from large length-scales to small
length-scales (forward cascade). In addition, there are energy fluxes from
large-scale magnetic field to small-scale velocity field, large-scale velocity
field to small-scale magnetic field, and large-scale velocity field to
large-scale magnetic field. Kolmogorov's constant for magnetohydrodynamics is
approximately equal to that for fluid turbulence ($\approx 1.6$) for Alfv\'{e}n
ratio $0.5 \le r_A \le \infty$. For higher space-dimensions, the energy fluxes
are qualitatively similar, and Kolmogorov's constant varies as $d^{1/3}$. For
the normalized cross helicity $\sigma_c \to 1$, the cascade rates are
proportional to $(1-\sigma_c)/ (1+\sigma_c)$, and the Kolmogorov's constants
vary significantly with $\sigma_c$.",0103033v4
2002-08-07,Dynamo effect beyond the kinematic approximation: suppression of linear instabilities by non-linear feedback,"The turbulent dynamo effect, which describes the generation of magnetic
fields in astrophysical objects, is described by the dynamo equation. This, in
the kinematic (linear) approximation gives an unbounded exponential growth of
the long wavelength part of the magnetic fields. Here we, in a systematic
diagrammatic, perturbation theory, show how non-linear effects suppress the
linear instability and bring down the growth rate to zero in the large time
limit. We work with diffferent background velicity spectrum and initial
magnetic field correlations. Our results indicate the robustness and very
general nature of dynamo growth: It is qualitatively independent of the
background velicty and intial magnetic field spectra. We also argue that our
results can be justified within the framework of the first order smoothing
approximation, as applicable for the full non-linear problem. We discuss our
results from the view points of renormalisation group analysis.",0208010v1
2002-09-20,The Onset of Dissipation in the Kinematic Dynamo,"The kinematic regime of the magnetic dynamo neglects the backreaction of the
magnetic field on the flow. For small magnetic diffusivity, in the early stage
of evolution, there is an ideal phase where dissipative effects can also be
neglected. We estimate the magnitude of the energy dissipation term (Ohmic
heating), taking into account differential constraints on chaotic flows. We
find that the period of ideal evolution is roughly doubled over an estimate
without constraints. The helicity generation terms are exponentially smaller
than the energy dissipation, so that large quantities of energy are dissipated
before any helicity can be created. Helicity flow is exponentially larger than
net helicity generation. The constraints also lead to the existence of a
singular initial condition for the magnetic field for which sizable amounts of
helicity can potentially be created.",0209042v1
2006-11-16,Magnetic field gradients in solar wind plasma and geophysics periods,"Using recent data obtained by Advanced Composition Explorer (ACE) the pumping
scale of the magnetic field gradients of the solar wind plasma has been
calculated. This pumping scale is found to be equal to 24h $\pm$ 2h. The ACE
spacecraft orbits at the L1 libration point which is a point of Earth-Sun
gravitational equilibrium about 1.5 million km from Earth. Since the Earth's
magnetosphere extends into the vacuum of space from approximately 80 to 60,000
kilometers on the side toward the Sun the pumping scale cannot be a consequence
of the 24h-period of the Earth's rotation. Vise versa, a speculation is
suggested that for the very long time of the coexistence of Earth and of the
solar wind the weak interaction between the solar wind and Earth could lead to
stochastic synchronization between the Earth's rotation and the pumping scale
of the solar wind magnetic field gradients. This synchronization could
transform an original period of the Earth's rotation to the period close to the
pumping scale of the solar wind magnetic field gradients.",0611034v1
2003-05-09,Magnetic moments of the N(1535) resonance in the chiral unitary model,"We calculate the magnetic moments of the N(1535) resonance using the chiral
unitary model, where the resonance is dynamically generated in the scatterings
of the lowest-lying mesons and baryons. We obtain the magnetic moments of the
resonance as +1.1 and -0.25 for p(1535) and n(1535), respectively, in units of
the nuclear magneton. We discuss the origin of these numbers within the chiral
unitary model, then we compare the present results with those of the quark
model and the chiral doublet model. The possibility to observe the magnetic
moments in experiments is also investigated.",0305023v1
2001-09-25,Weak magnetohydrodynamic turbulence of magnetized plasma,"Weak turbulence of magnetohydrodynamic (MHD) waves in strongly magnetized
plasma is studied when the thermal pressure is less than the magnetic field
pressure. In this situation the main nonlinear mechanism is the resonance
scattering of fast magneto-acoustic and Alfvenic waves on slow magneto-acoustic
waves. As a result, the former waves serve as the high-frequency waves with
respect to the latter ones so that the total number of HF waves - an adiabatic
invariant - conserves additionally. In the weak turbulence regime this
invariant is shown to generate the Kolmogorov type spectrum with a constant
flux of HF waves towards large-scale region. In the short-wave region another
Kolmogorov spectrum can be realized with a constant energy flux. The explicit
angle dependences for both types of turbulent spectra are found for the
propagation angles close to the direction of a mean magnetic field.",0109066v1
2002-12-20,Orbital magnetism in axially deformed sodium clusters: From scissors mode to dia-para magnetic anisotropy,"Low-energy orbital magnetic dipole excitations, known as scissors mode (SM),
are studied in alkali metal clusters. Subsequent dynamic and static effects are
explored. The treatment is based on a self-consistent microscopic approach
using the jellium approximation for the ionic background and the Kohn-Sham mean
field for the electrons. The microscopic origin of SM and its main features
(structure of the mode in light and medium clusters, separation into low- and
high-energy plasmons, coupling high-energy M1 scissors and E2 quadrupole
plasmons, contributions of shape isomers, etc) are discussed. The scissors M1
strength acquires large values with increasing cluster size. The mode is
responsible for the van Vleck paramagnetism of spin-saturated clusters. Quantum
shell effects induce a fragile interplay between Langevin diamagnetism and van
Vleck paramagnetism and lead to a remarkable dia-para anisotropy in magnetic
susceptibility of particular light clusters. Finally, several routes for
observing the SM experimentally are discussed.",0212084v1
2003-01-15,Numerical study of homogeneous dynamo based on experimental von Karman type flows,"A numerical study of the magnetic induction equation has been performed on
von Karman type flows. These flows are generated by two co-axial
counter-rotating propellers in cylindrical containers. Such devices are
currently used in the von Karman sodium (VKS) experiment designed to study
dynamo action in an unconstrained flow. The mean velocity fields have been
measured for different configurations and are introduced in a periodic
cylindrical kinematic dynamo code. Depending on the driving configuration, on
the poloidal to toroidal flow ratio and on the conductivity of boundaries, some
flows are observed to sustain growing magnetic fields for magnetic Reynolds
numbers accessible to a sodium experiment. The response of the flow to an
external magnetic field has also been studied: The results are in excellent
agreement with experimental results in the single propeller case but can differ
in the two propellers case.",0301032v1
2004-10-07,Numerical study of dynamo action at low magnetic Prandtl numbers,"We present a three--pronged numerical approach to the dynamo problem at low
magnetic Prandtl numbers $P_M$. The difficulty of resolving a large range of
scales is circumvented by combining Direct Numerical Simulations, a
Lagrangian-averaged model, and Large-Eddy Simulations (LES). The flow is
generated by the Taylor-Green forcing; it combines a well defined structure at
large scales and turbulent fluctuations at small scales. Our main findings are:
(i) dynamos are observed from $P_M=1$ down to $P_M=10^{-2}$; (ii) the critical
magnetic Reynolds number increases sharply with $P_M^{-1}$ as turbulence sets
in and then saturates; (iii) in the linear growth phase, the most unstable
magnetic modes move to small scales as $P_M$ is decreased and a Kazantsev
$k^{3/2}$ spectrum develops; then the dynamo grows at large scales and modifies
the turbulent velocity fluctuations.",0410046v1
2005-09-08,On the inverse cascade of magnetic helicity,"We study the inverse cascade of magnetic helicity in conducting fluids by
investigating the detailed transfer of helicity between different spherical
shells in Fourier space in direct numerical simulations of three-dimensional
magnetohydrodynamics (MHD). Two different numerical simulations are used, one
where the system is forced with an electromotive force in the induction
equation, and one in which the system is forced mechanically with an ABC flow
and the magnetic field is solely sustained by a dynamo action. The magnetic
helicity cascade at the initial stages of both simulations is observed to be
inverse and local (in scale space) in the large scales, and direct and local in
the small scales. When saturation is approached most of the helicity is
concentrated in the large scales and the cascade is non-local. Helicity is
transfered directly from the forced scales to the largest scales. At the same
time, a smaller in amplitude direct cascade is observed from the largest scale
to small scales.",0509069v1
2005-11-28,Magnetic field induced by elliiptical instability in a rotating tidally distorded sphere,"It is usually believed that the geo-dynamo of the Earth or more generally of
other planets, is created by the convective fluid motions inside their molten
cores. An alternative to this thermal or compositional convection can however
be found in the inertial waves resonances generated by the eventual precession
of these planets or by the possible tidal distorsions of their liquid cores. We
will review in this paper some of our experimental works devoted to the
elliptical instability and present some new results when the experimental fluid
is a liquid metal. We show in particular that an imposed magnetic field is
distorted by the spin- over mode generated by the elliptical instability. In
our experiment, the field is weak (20 Gauss) and the Lorenz force is negligible
compared to the inertial forces, therefore the magnetic field does not modify
the fluid flow and the pure hydrodynamics growth rates of the instability are
recovered through magnetic measurements.",0511226v1
2002-01-22,Macroscopic and mesoscopic matter waves,"It has been shown earlier that matter waves which are known to lie typically
in the range of a few Angstrom, can also manifest in the macrodomain with a
wave length of a few centimeters, for electrons propagating along a magnetic
field. This followed from the predictions of a probability amplitude theory by
the author in the classical macrodomain of the dynamics of charged particles in
a magnetic field. It is shown in this paper that this case constitutes only a
special case of a generic situation whereby composite systems such as atoms and
molecules in their highly excited internal states,can exhibit matter wave
manifestation in macro and mesodomains. The wave length of these waves is
determined, not by the mass of the particle as in the case of the de Broglie
wave, but by the frequency, associated with the internal state of excitation,
and is given by a nonquantal expression, $\lambda =2\pi v/\omega$, $v$ being
the velocity of the particle. For the electrons in a magnetic field the
frequency corresponds to the gyrofrequency, $\Omega$ and the nonquantal wave
length is given by $\lambda = 2\pi v_{\parallel}/\Omega$; $v_{\parallel}$ being
the velocity of electrons along the magnetic field.",0201101v1
2003-04-04,Creation of effective magnetic fields in optical lattices: The Hofstadter butterfly for cold neutral atoms,"We investigate the dynamics of neutral atoms in a 2D optical lattice which
traps two distinct internal states of the atoms in different columns. Two Raman
lasers are used to coherently transfer atoms from one internal state to the
other, thereby causing hopping between the different columns. By adjusting the
laser parameters appropriately we can induce a non vanishing phase of particles
moving along a closed path on the lattice. This phase is proportional to the
enclosed area and we thus simulate a magnetic flux through the lattice. This
setup is described by a Hamiltonian identical to the one for electrons on a
lattice subject to a magnetic field and thus allows us to study this equivalent
situation under very well defined controllable conditions. We consider the
limiting case of huge magnetic fields -- which is not experimentally accessible
for electrons in metals -- where a fractal band structure, the Hofstadter
butterfly, characterizes the system.",0304038v1
2004-09-28,Magnetometry with entangled atomic samples,"We present a theory for the estimation of a scalar or a vector magnetic field
by its influence on an ensemble of trapped spin polarized atoms. The atoms
interact off-resonantly with a continuous laser field, and the measurement of
the polarization rotation of the probe light, induced by the dispersive
atom-light coupling, leads to spin-squeezing of the atomic sample which enables
an estimate of the magnetic field which is more precise than that expected from
standard counting statistics. For polarized light and polarized atoms, a
description of the non-classical components of the collective spin angular
momentum for the atoms and the collective Stokes vectors of the light-field in
terms of effective gaussian position and momentum variables is practically
exact. The gaussian formalism describes the dynamics of the system very
effectively and accounts explicitly for the back-action on the atoms due to
measurement and for the estimate of the magnetic field. Multi-component
magnetic fields are estimated by the measurement of suitably chosen atomic
observables and precision and efficiency is gained by dividing the atomic gas
in two or more samples which are entangled by the dispersive atom-light
interaction.",0409202v1
2007-04-04,Ising-like dynamics and frozen states in systems of ultrafine magnetic particles,"We use Monte-Carlo simulations to study aging phenomena and the occurence of
spinglass phases in systems of single-domain ferromagnetic nanoparticles under
the combined influence of dipolar interaction and anisotropy energy, for
different combinations of positional and orientational disorder. We find that
the magnetic moments oriente themselves preferably parallel to their anisotropy
axes and changes of the total magnetization are solely achieved by 180 degree
flips of the magnetic moments, as in Ising systems. Since the dipolar
interaction favorizes the formation of antiparallel chain-like structures,
antiparallel chain-like patterns are frozen in at low temperatures, leading to
aging phenomena characteristic for spin-glasses. Contrary to the intuition,
these aging effects are more pronounced in ordered than in disordered
structures.",0704.0580v1
2007-04-26,On the MHD load and the MHD metage,"In analogy with the load and the metage in hydrodynamics, we define
magnetohydrodynamic load and magnetohydrodynamic metage in the case of
magnetofluids. They can be used to write the magnetic field in MHD in Clebsch's
form. We show how these two concepts can be utilised to derive the magnetic
analogue of the Ertel's theorem and also, how in the presence of non-trivial
topology of the magnetic field in the magnetofluid one may associate the
linking number of the magnetic field lines with the invariant MHD loads. The
paper illustrates that the symmetry translation of the MHD metage in the
corresponding label space generates the conservation of cross helicity.",0704.3482v1
2007-05-04,Applicability of layered sine-Gordon models to layered superconductors: II. The case of magnetic coupling,"In this paper, we propose a quantum field theoretical renormalization group
approach to the vortex dynamics of magnetically coupled layered
superconductors, to supplement our earlier investigations on the
Josephson-coupled case. We construct a two-dimensional multi-layer sine-Gordon
type model which we map onto a gas of topological excitations. With a special
choice of the mass matrix for our field theoretical model, vortex dominated
properties of magnetically coupled layered superconductors can be described.
The well known interaction potentials of fractional flux vortices are
consistently obtained from our field-theoretical analysis, and the physical
parameters (vortex fugacity and temperature parameter) are also identified. We
analyse the phase structure of the multi-layer sine--Gordon model by a
differential renormalization group method for the magnetically coupled case
from first principles. The dependence of the transition temperature on the
number of layers is found to be in agreement with known results based on other
methods.",0705.0578v3
2007-05-09,Low-lying magnetic loops in the solar internetwork,"The aim of this work is to study the structure of the magnetic field vector
in the internetwork and search for the presence of small-scale loops. We invert
1.56 micron spectropolarimetric observations of internetwork regions at disc
centre by applying the SIR code. This allows us to recover the atmospheric
parameters that play a role in the formation of these spectral lines. We are
mainly interested in the structure of the magnetic field vector. We find that
many opposite polarity elements of the internetwork are connected by short
(2-6''), low-lying (photospheric) loops. These loops connect at least the 10-20
% of the internetwork flux visible in our data. Also we have some evidence that
points towards a dynamic scenario which can be produced by the emergence of
internetwork magnetic flux.",0705.1319v1
2007-05-15,Nonequilibrium critical relaxation at a first-order phase transition point,"We study numerically the nonequilibrium dynamical behavior of an Ising model
with mixed two-spin and four-spin interactions after a sudden quench from the
high-temperature phase to the first-order phase transition point. The
autocorrelation function is shown to approach its limiting value, given by the
magnetization in the ordered phase at the transition point, m_c, through a
stretched exponential decay. On the other hand relaxation of the magnetization
starting with an uncorrelated initial state with magnetization, m_i, approaches
either m_c, for m_i>0.5, or zero, for m_i<0.5. For small m_i and for m_i
slightly larger than 0.5 the relaxation of the magnetization shows an
asymptotic power-law time dependence, thus from a nonequilibrium point of view
the transition is continuous.",0705.2142v2
2007-06-01,Influence of the Magnetic Coupling Process on the Advection Dominated Accretion Flows around Black Holes,"A large-scale closed magnetic field can transfer angular momentum and energy
between a black hole (BH) and its surrounding accretion flow. We investigate
the effects of this magnetic coupling (MC) process on the dynamics of a hot
accretion flow (e.g., an advection dominated accretion flow, hereafter ADAF).
The energy and angular momentum fluxes transported by the magnetic field are
derived by an equivalent circuit approach. For a rapidly rotating BH, it is
found that the radial velocity and the electron temperature of the accretion
flow decrease, whereas the ion temperature and the surface density increase.
The significance of the MC effects depends on the value of the viscous
parameter \alpha. The effects are obvious for \alpha=0.3 but nearly ignorable
for \alpha=0.1. For a BH with specific angular momentum, a_*=0.9, and
\alpha=0.3, we find that for reasonable parameters the radiative efficiency of
a hot accretion flow can be increased by about 30%.",0706.0124v2
2007-06-18,Propagation of UHE protons through a magnetized large scale structure,"The propagation of UHECRs is affected by the intergalactic magnetic fields
that were produced during the course of the large scale structure formation of
the universe. We adopt a novel model where the large scale extragalactic
magnetic fields (EGMF) are estimated from local dynamic properties of the gas
flows in hydrodynamic simulations of a concordance LambdaCDM universe. With the
model magnetic fields, we calculate the deflection angle, time delay and energy
spectrum of protons with E > 10^{19} eV that are injected at cosmological
sources and then travel through the large scale structure of the universe,
losing the energy due to interactions with the cosmic background radiation.
Implications of this study on the origin of UHECRs are discussed.",0706.2597v2
2007-06-26,Molecular cloud formation and magnetic fields in spiral galaxies,"We present ongoing hydrodynamic and MHD simulations of molecular cloud
formation in spiral galaxies. The hydrodynamic results show the formation of
molecular gas clouds where spiral shocks compress atomic gas to high densities.
The spiral shocks also produce structure in the spiral arms, provided the gas
is cold (< 1000 K). When both hot and cold components of the ISM are modeled,
this structure is enhanced. Properties such as the clump mass spectra and
spatial distribution will be compared from clouds identified in these
simulations. In particular the multiphase simulations predict the presence of
much more interarm molecular gas than when a single phase is assumed. We also
discuss very recent results from galactic-scale MHD calculations. From
observational comparisons of the magnetic and thermal pressure, magnetic fields
are expected to be a major factor in explaining the dynamics of the ISM, from
kpc scales to those of star formation. We describe the difference in structure
of the spiral arms, and the evolution of the global magnetic field for a range
of field strengths.",0706.3862v1
2007-07-31,Coherent spin rotations in open driven double quantum dots,"We analyze the charge and spin dynamics in a DC biased double quantum dot
driven by crossed DC and AC magnetic fields. In this configuration, spatial
delocalization due to inter-dot tunnel competes with intra-dot spin rotations
induced by the time dependent magnetic field, giving rise to a complicated time
dependent behavior of the tunnelling current. When the Zeeman splitting has the
same value in both dots and spin flip is negligible, the electrons remain in
the triplet subspace (dark subspace) performing coherent spin rotations and the
current does not flow. This electronic trapping is removed either by finite
spin relaxation or when the Zeeman splitting is different in each quantum dot.
In the first case, our results show that measuring the current will allow to
get information on the spin relaxation time. In the last case, we will show
that applying a resonant bichromatic magnetic field, the electrons become
trapped in a coherent superposition of states and electronic transport is
blocked. Then, manipulating AC magnetic fields, electrons are driven to perform
coherent spin rotations which can be unambiguously detected by direct
measurement of the tunneling current.",0707.4636v2
2007-08-03,Equilibrium Low Temperature Heat Capacity of the Spin Density Wave compound (TMTTF)2 Br: effect of a Magnetic Field,"We have investigated the effect of the magnetic field (B) on the very
low-temperature equilibrium heat capacity ceq of the quasi-1 D organic compound
(TMTTF)2Br, characterized by a commensurate Spin Density Wave (SDW) ground
state. Below 1K, ceq is dominated by a Schottky-like contribution, very
sensitive to the experimental time scale, a property that we have previously
measured in numerous DW compounds. Under applied field (in the range 0.2- 7 T),
the equilibrium dynamics, and hence ceq extracted from the time constant,
increases enormously. For B = 2-3 T, ceq varies like B2, in agreement with a
magnetic Zeeman coupling. Another specific property, common to other
Charge/Spin density wave (DW) compounds, is the occurrence of metastable
branches in ceq, induced at very low temperature by the field exceeding a
critical value. These effects are discussed within a generalization to SDWs in
a magnetic field of the available Larkin-Ovchinnikov local model of strong
pinning. A limitation of the model when compared to experiments is pointed out.",0708.0540v1
2007-08-22,Pump-Probe Experiments on the Single-Molecule Magnet Fe8 : Measurement of Excited Level Lifetimes,"We present magnetization measurements on the single molecule magnet Fe8 in
the presence of pulsed microwave radiation. A pump-probe technique is used with
two microwave pulses with frequencies of 107 GHz and 118 GHz and pulse lengths
of several nanoseconds to study the spin dynamics via time-resolved
magnetization measurements using a Hall probe magnetometer. We find evidence
for short spin-phonon relaxation times of the order of one microsecond. The
temperature dependence of the spin-phonon relaxation time in our experiments is
in good agreement with previously published theoretical results. We also
established the presence of very short energy diffusion times, that act on a
timescale of about 70 ns.",0708.3024v1
2007-08-27,Neutrino spin-flavor oscillations in electromagnetic fields of various configurations,"We study spin-flavor oscillations of Dirac neutrinos with mixing and having
non-zero matrix of magnetic moments in magnetic fields of various
configurations. We discuss constant transversal and twisting magnetic fields.
To describe the dynamics of Dirac neutrinos we use relativistic quantum
mechanics approach based on the exact solutions to the Dirac-Pauli equation in
an external electromagnetic field. We derive transition probabilities for
different neutrino magnetic moments matrices.",0708.3572v1
2007-09-06,Microscopic Theory for the Markovian Decay of Magnetization Fluctuations in Nanomagnets,"We present a microscopic theory for the phonon-driven decay of the
magnetization fluctuations in a wide class of nanomagnets where the dominant
energy is set by isotropic exchange and/or uniaxial anisotropy. Based on the
Zwanzig-Mori projection formalism, the theory reveals that the magnetization
fluctuations are governed by a single decay rate $\omega_c$, which we further
identify with the zero-frequency portion of the associated self-energy. This
dynamical decoupling from the remaining slow degrees of freedom is attributed
to a conservation law and the discreteness of the energy spectrum, and explains
the omnipresent mono-exponential decay of the magnetization over several
decades in time, as observed experimentally. A physically transparent
analytical expression for $\omega_c$ is derived which highlights the three
specific mechanisms of the slowing down effect which are known so far in
nanomagnets.",0709.0914v2
2007-09-17,Nature of the spin dynamics and 1/3 magnetization plateau in azurite,"We present a specific heat and inelastic neutron scattering study in magnetic
fields up into the 1/3 magnetization plateau phase of the diamond chain
compound azurite Cu$_3$(CO$_3$)$_2$(OH)$_2$. We establish that the
magnetization plateau is a dimer-monomer state, {\it i.e.}, consisting of a
chain of $S = 1/2$ monomers, which are separated by $S = 0$ dimers on the
diamond chain backbone. The effective spin couplings $J_{mono}/k_B = 10.1(2)$ K
and $J_{dimer}/k_B = 1.8(1)$ K are derived from the monomer and dimer
dispersions. They are associated to microscopic couplings $J_1/k_B = 1(2)$ K,
$J_2/k_B = 55(5)$ K and a ferromagnetic $J_3/k_B = -20(5)$ K, possibly as
result of $d_{z^2}$ orbitals in the Cu-O bonds providing the superexchange
pathways.",0709.2560v1
2007-09-23,Quantum dynamics and entanglement of spins on a square lattice,"Bulk magnetism in solids is fundamentally quantum mechanical in nature. Yet
in many situations, including our everyday encounters with magnetic materials,
quantum effects are masked, and it often suffices to think of magnetism in
terms of the interaction between classical dipole moments. Whereas this
intuition generally holds for ferromagnets, even as the size of the magnetic
moment is reduced to that of a single electron spin (the quantum limit), it
breaks down spectacularly for antiferromagnets, particularly in low dimensions.
Considerable theoretical and experimental progress has been made in
understanding quantum effects in one-dimensional quantum antiferromagnets, but
a complete experimental description of even simple two-dimensional
antiferromagnets is lacking. Here we describe a comprehensive set of neutron
scattering measurements that reveal a non-spin-wave continuum and strong
quantum effects, suggesting entanglement of spins at short distances in the
simplest of all two-dimensional quantum antiferromagnets, the square lattice
Heisenberg system.",0709.3632v1
2007-10-01,Manipulation with Andreev states in spin active mesoscopic Josephson junctions,"We investigate manipulation with Andreev bound states in Josephson quantum
point contacts with magnetic scattering. Rabi oscillations in the two-level
Andreev subsystems are excited by resonant driving the direction of magnetic
moment of the scatterer, and by modulating the superconducting phase difference
across the contact. The Andreev level dynamics is manifested by temporal
oscillation of the Josephson current, accompanied, in the case of magnetic
manipulation, also by oscillation of the Andreev states spin polarization. The
interlevel transitions obey a selection rule that forbids manipulations in a
certain region of external parameters, and results from specific properties of
Andreev bound states in magnetic contacts: 4$\pi$-periodicity with respect to
the superconducting phase, and strong spontaneous spin polarization.",0710.0320v2
2007-10-18,Nonlinear NMR dynamics in hyperpolarized liquid 3He,"In a highly polarized liquid (laser-polarized 3He-4He mixtures in our
experiment), dipolar magnetic interactions within the liquid introduce a
significant nonlinear and nonlocal contribution to the Bloch equation that
leads to instabilities during NMR evolution. We have launched a study of these
instabilities using spin echo techniques. At high magnetizations, a simple 180
degree rf pulse fails to refocus magnetization, so we use a standard
solid-state NMR pulse sequence: the magic sandwich. We report an experimental
and numerical investigation of the effect of this sequence on unstable NMR
evolution. Using a series of repeated magic sandwich sequences, the transverse
magnetization lifetime can be increased by up to three orders of magnitude.",0710.3567v2
2007-11-12,Landau Analog Levels for Dipoles in the Noncommutative Space and Phase Space,"In the present contribution we investigate the Landau analog energy
quantization for neutral particles, that possesses a nonzero permanent magnetic
and electric dipole moments, in the presence of an homogeneous electric and
magnetic external fields in the context of the noncommutative quantum
mechanics. Also, we analyze the Landau--Aharonov--Casher and
Landau--He--McKellar--Wilkens quantization due to noncommutative quantum
dynamics of magnetic and electric dipoles in the presence of an external
electric and magnetic fields and the energy spectrum and the eigenfunctions are
obtained. Furthermore, we have analyzed Landau quantization analogs in the
noncommutative phase space, and we obtain also the energy spectrum and the
eigenfunctions in this context.",0711.1773v2
2007-11-14,Emergent singular solutions of non-local density-magnetization equations in one dimension,"We investigate the emergence of singular solutions in a non-local model for a
magnetic system. We study a modified Gilbert-type equation for the
magnetization vector and find that the evolution depends strongly on the length
scales of the non-local effects. We pass to a coupled density-magnetization
model and perform a linear stability analysis, noting the effect of the length
scales of non-locality on the system's stability properties. We carry out
numerical simulations of the coupled system and find that singular solutions
emerge from smooth initial data. The singular solutions represent a collection
of interacting particles (clumpons). By restricting ourselves to the
two-clumpon case, we are reduced to a two-dimensional dynamical system that is
readily analyzed, and thus we classify the different clumpon interactions
possible.",0711.2177v1
2007-11-28,Magnetically Arrested Disks and Origin of Poynting Jets: Numerical Study,"The dynamics and structure of accretion disks, which accumulate the vertical
magnetic field in the centers, are investigated using two- and
three-dimensional MHD simulations. The central field can be built up to the
equipartition level and disrupts a nearly axisymmetric outer accretion disk
inside a magnetospheric radius, forming a magnetically arrested disk (MAD). In
the MAD, the mass accretes in a form of irregular dense spiral streams and the
vertical field, split into separate bundles, penetrates through the disk plane
in low-density magnetic islands. The accreting mass, when spiraling inward,
drags the field and twists it around the axis of rotation, resulting in
collimated Poynting jets in the polar directions. These jets are powered by the
accretion flow with the efficiency up to ~1.5% (in units \dot{M}c^2). The
spiral flow pattern in the MAD is dominated by modes with low azimuthal
wavenumbers m~1-5 and can be a source of quasi-periodic oscillations in the
outgoing radiation. The formation of MAD and Poynting jets can naturally
explain the observed changes of spectral states in Galactic black hole
binaries. Our study is focused on black hole accretion flows; however, the
results can also be applicable to accretion disks around nonrelativistic
objects, such as young stellar objects and stars in binary systems.",0711.4391v1
2007-12-13,Electromagnon excitations in modulated multiferroics,"The phenomenological theory of ferroelectricity in spiral magnets presented
in [M. Mostovoy, Phys. Rev. Lett. 96, 067601 (2006)] is generalized to describe
consistently states with both uniform and modulated-in-space ferroelectric
polarizations. A key point in this description is the symmetric part of the
magnetoelectric coupling since, although being irrelevant for the uniform
component, it plays an essential role for the non-uniform part of the
polarization. We illustrate this importance in generic examples of modulated
magnetic systems: longitudinal and transverse spin-density wave states and
planar cycloidal phase. We show that even in the cases with no uniform
ferroelectricity induced, polarization correlation functions follow to the soft
magnetic behavior of the system due to the magnetoelectric effect. Our results
can be easily generalized for more complicated types of magnetic ordering, and
the applications may concern various natural and artificial systems in
condensed matter physics (e.g., magnon properties could be extracted from
dynamic dielectric response measurements).",0712.2138v2
2007-12-17,Easy-Axis Kagomé Antiferromagnet: Local-Probe Study of Nd$_3$Ga$_5$SiO$_{14}$,"We report a local-probe investigation of the magnetically anisotropic
kagom\'e compound Nd$_3$Ga$_5$SiO$_{14}$. Our zero-field $\mu$SR results
provide a direct evidence of a fluctuating collective paramagnetic state down
to 60 mK, supported by a wipe-out of the Ga nuclear magnetic resonance (NMR)
signal below 25 K. At 60 mK a dynamics crossover to a much more static state is
observed by $\mu$SR in magnetic fields above 0.5 T. Accordingly, the NMR signal
is recovered at low $T$ above a threshold field, revealing a rapid temperature
and field variation of the magnetic fluctuations.",0712.2660v2
2008-01-06,Voltage control of magnetocrystalline anisotropy in ferromagnetic - semiconductor/piezoelectric hybrid structures,"We demonstrate dynamic voltage control of the magnetic anisotropy of a
(Ga,Mn)As device bonded to a piezoelectric transducer. The application of a
uniaxial strain leads to a large reorientation of the magnetic easy axis which
is detected by measuring longitudinal and transverse anisotropic
magnetoresistance coefficients. Calculations based on the mean-field
kinetic-exchange model of (Ga,Mn)As provide microscopic understanding of the
measured effect. Electrically induced magnetization switching and detection of
unconventional crystalline components of the anisotropic magnetoresistance are
presented, illustrating the generic utility of the piezo voltage control to
provide new device functionalities and in the research of micromagnetic and
magnetotransport phenomena in diluted magnetic semiconductors.",0801.0886v2
2008-01-23,Helical Magnetic Fields from Sphaleron Decay and Baryogenesis,"Many models of baryogenesis rely on anomalous particle physics processes to
give baryon number violation. By numerically evolving the electroweak equations
on a lattice, we show that baryogenesis in these models creates helical cosmic
magnetic fields. After a transitory period, electroweak dynamics is found to
conserve the Chern-Simons number and the total electromagnetic helicity. We
argue that baryogenesis could lead to magnetic fields of nano-Gauss strength
today on astrophysical length scales. In addition to being astrophysically
relevant, such helical magnetic fields can provide an independent probe of
baryogenesis and CP violation in particle physics.",0801.3653v2
2008-01-30,Time-Resolved X-ray Microscopy of Spin-Torque-Induced Magnetic Vortex Gyration,"Time-resolved X-ray microscopy is used to image the influence of alternating
high-density currents on the magnetization dynamics of ferromagnetic vortices.
Spin-torque induced vortex gyration is observed in micrometer-sized permalloy
squares. The phases of the gyration in structures with different chirality are
compared to an analytical model and micromagnetic simulations, considering both
alternating spinpolarized currents and the current's Oersted field. In our case
the driving force due to spin-transfer torque is about 70% of the total
excitation while the remainder originates from the current's Oersted field.
This finding has implications to magnetic storage devices using spin-torque
driven magnetization switching and domain-wall motion.",0801.4719v1
2008-02-14,Light-induced magnetization precession in GaMnAs,"We report dynamics of the transient polar Kerr rotation (KR) and of the
transient reflectivity induced by femtosecond laser pulses in ferromagnetic
(Ga,Mn)As with no external magnetic field applied. It is shown that the
measured KR signal consist of several different contributions, among which only
the oscillatory signal is directly connected with the ferromagnetic order in
(Ga,Mn)As. The origin of the light-induced magnetization precession is
discussed and the magnetization precession damping (Gilbert damping) is found
to be strongly influenced by annealing of the sample.",0802.2043v2
2008-02-15,Spontaneous vortex phases in ferromagnet-superconductor nanocomposites,"The interplay between superconductivity and magnetism gives rise to many
intriguing and exciting phenomena. In this Letter we report about a novel
manifestation of this interplay: a temperature induced phase transition between
different spontaneous vortex phases in lead superconducting films with embedded
magnetic nanoparticles. Unlike common vortices in superconductors the vortex
phase appears without any applied magnetic field. The vortices nucleate
exclusively due to the stray field of the magnetic nanoparticles, which serve
the dual role of providing the internal field and simultaneously acting as
pinning centers. As in usual superconductors, one can move the spontaneous
vortices with an applied electric current. Transport measurements reveal
dynamical phase transitions that depend on temperature (T) and applied field
(H) and support the obtained (H-T) phase diagram. In particular, we used a
scaling analysis to characterize a transition from a liquid to a novel
disordered solid resembling a vortex glass.",0802.2252v2
2008-03-21,Solar and Stellar Active Regions:Cosmic laboratories for the study of Complexity,"Solar active regions are driven dissipative dynamical systems. The turbulent
convection zone forces new magnetic flux tubes to rise above the photosphere
and shuffles the magnetic fields which are already above the photosphere. The
driven 3D active region responds to the driver with the formation of Thin
Current Sheets in all scales and releases impulsively energy, when special
thresholds are met, on the form of nano-, micro-, flares and large scale
coronal mass ejections. It has been documented that active regions form self
similar structures with area Probability Distribution Functions (PDF's)
following power laws and with fractal dimensions ranging from $1.2-1.7$. The
energy release on the other hand follows a specific energy distribution law
$f(E_T)\sim E_T^{-a}$, where $a \sim 1.6-1.8$ and $E_T$ is the total energy
released. A possible explanation for the statistical properties of the
magnetogrms and the energy release by the active region is that the magnetic
field formation follows rules analogous to \textbf{percolating models}, and the
3D magnetic fields above the photosphere reach a \textbf{Self Organized
Critical (SOC) state}. The implications of these findings on the acceleration
of energetic particles during impulsive phenomena will briefly be outlined.",0803.3158v1
2008-04-01,Dipolar spinor Bose-Einstein condensates,"Under many circumstances, the only important two-body interaction between
atoms in ultracold dilute atomic vapors is the short-ranged isotropic s-wave
collision. Recent studies have shown, however, that situations may arise where
the dipolar interaction between atomic magnetic or electric dipole moments can
play a significant role. The long-range anisotropic nature of the dipolar
interaction greatly enriches the static and dynamic properties of ultracold
atoms. In the case of dipolar spinor condensates, the interplay between the
dipolar interaction and the spin exchange interaction may lead to nontrivial
spin textures. Here we pay particular attention to the spin vortex state that
is analogous to the magnetic vortex found in thin magnetic films.",0804.0191v1
2008-05-07,Simulations of 2D magnetic electron drift vortex mode turbulence in plasmas,"Simulations are performed to investigate turbulent properties of nonlinearly
interacting two-dimensional (2D) magnetic electron drift vortex (MEDV) modes in
a nonuniform unmagnetized plasma. The relevant nonlinear equations governing
the dynamics of the MEDV modes are the wave magnetic field and electron
temperature perturbations in the presence of the equilibrium density and
temperature gradients. The important nonlinearities come from the advection of
the electron fluid velocity perturbation and the electron temperature, as well
as from the nonlinear electron Lorentz force. Computer simulations of the
governing equations for the nonlinear MEDV modes reveal the generation of
streamer-like electron flows, such that the corresponding gradients in the
direction of the inhomogeneities tend to flatten out. By contrast, the
gradients in an orthogonal direction vary rapidly. Consequently, the inertial
range energy spectrum in decaying MEDV mode turbulence exhibits a much steeper
anisotropic spectral index. The magnetic structures in the MEDV mode turbulence
produce nonthermal electron transport in our nonuniform plasma.",0805.1047v1
2008-05-19,Intrinsic magnetic properties of NdFeAsO$_{0.9}$F$_{0.1}$ superconductor from local and global measurements,"Magneto-optical imaging was used to study the local magnetization in
polycrystalline NdFeAsO$_{0.9}$F$_{0.1}$ (NFAOF). Individual crystallites up to
$\sim200\times100\times30$ $\mu m^{3}$ in size could be mapped at various
temperatures. The in-grain, persistent current density is about $j\sim10^{5}$
A/cm$^{2}$ and the magnetic relaxation rate in a remanent state peaks at about
$T_{m}\sim38$ K. By comparison with with the total magnetization measured in a
bar-shaped, dense, polycrystalline sample, we suggest that
NdFeAsO$_{0.9}$F$_{0.1}$ is similar to a layered high-$T_{c}$, compound such as
Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+x}$ and exhibits a $3D\to2D$ crossover in the
vortex structure. The 2D Ginzburg parameter is about $Gi^{2D}% \simeq10^{-2}$
implying electromagnetic anisotropy as large as $\epsilon \sim1/30$. Below
$T_{m}$, the static and dynamic behaviors are consistent with collective
pinning and creep.",0805.2783v2
2008-05-23,Magnetism in Gallium doped CeFe_2: The martensitic scenario,"Ce(Fe_{1-x}Ga_x)_2 compounds with x = 0, 0.01, 0.025 and 0.05 have been
investigated to unravel the effect of Ga on the magnetic state of CeFe_2. For
the first time, we find that the dynamic antiferromagnetic phase present in
CeFe_2 gets stabilized with Ga substitution. The hysteresis loops show that
while the compounds with x = 0 and 0.01 show normal behavior, the other two
show multiple magnetization steps across the antiferromagnetic-ferromagnetic
transition region. The virgin curve is found to lie outside the envelope curve
in these two compounds, similar to the observations made in Ru and Re
substituted CeFe_2 compounds. Temperature, sweep rate and time dependences of
the magnetization show that the compounds with x >=0.025 possess glassy
behavior at low temperatures. Various results obtained reveal that these two
compounds belong to the martensite family.",0805.3589v1
2008-05-30,Langevin dynamics in crossed magnetic and electric fields: Hall and diamagnetic fluctuations,"Based on the classical Langevin equation, we have re-visited the problem of
orbital motion of a charged particle in two dimensions for a normal magnetic
field crossed with or without an in-plane electric bias. We are led to two
interesting fluctuation effects: First, we obtain not only a longitudinal
""work-fluctuation"" relation as expected for a barotropic type system, but also
a transverse work-fluctuation relation perpendicular to the electric bias. This
""Hall fluctuation"" involves the product of the electric and the magnetic
fields. And second, for the case of harmonic confinement without bias, the
calculated probability density for the orbital magnetic moment gives non-zero
even moments, not derivable as field derivatives of the classical free energy.",0806.0022v3
2008-06-16,Interplay of Local-Moment Ferromagnetism and Superconductivity in ErRh$_4$B$_4$ Single Crystals,"Tunnel-diode resonator technique was used to study crystals of ferromagnetic
re-entrant superconductor, ErRh$_{4}$B$_{4}$. At the boundary between
ferromagnetism (FM) and superconductivity (SC), dynamic magnetic
susceptibility, $\chi(T,H)$, exhibits highly asymmetric behavior upon warming
and cooling as well as enhanced diamagnetism on the SC side. SC phase nucleates
upon warming in a cascade of discontinuous jumps in magnetic susceptibility
$\chi(T,H)$, whereas FM phase develops gradually as reported in detail in
\cite{prozorov2008}. Here we further investigate enhanced diamagnetism. We find
that when a magnetic field is applied along the magnetic easy axes, a region of
enhanced diamagnetic screening is smaller than in the perpendicular
orientation. A discussion of possible causes of this effect is provided.",0806.2479v1
2008-07-16,Equilibrium tuned by a magnetic field in phase separated manganite,"We present magnetic and transport measurements on La5/8-yPryCa3/8MnO3 with y
= 0.3, a manganite compound exhibiting intrinsic multiphase coexistence of
sub-micrometric ferromagnetic and antiferromagnetic charge ordered regions.
Time relaxation effects between 60 and 120K, and the obtained magnetic and
resistive viscosities, unveils the dynamic nature of the phase separated state.
An experimental procedure based on the derivative of the time relaxation after
the application and removal of a magnetic field enables the determination of
the otherwise unreachable equilibrium state of the phase separated system. With
this procedure the equilibrium phase fraction for zero field as a function of
temperature is obtained. The presented results allow a correlation between the
distance of the system to the equilibrium state and its relaxation behavior.",0807.2631v1
2008-07-25,"Spin, orbital ordering and magnetic dynamics of LaVO3: magnetization, heat capacity and neutron scattering studies","We report the results of magnetization, heat capacity and neutron scattering
studies of LaVO3 single crystals. From the neutron diffraction studies, it was
found that the compound is magnetically ordered with a C-type antiferromagnetic
spin structure at about 136 K. In the vicinity of the ordering temperature, we
also observed hysteresis in the neutron diffraction data measured on cooling
and heating which indicates the first order nature of the phase transition. In
the antiferromagnetically ordered phase, the inelastic neutron scattering
studies reveal the presence of a temperature independent c axis spin-wave gap
of about 6 meV which is similar to that previously reported for the sister
compound YVO3.",0807.4195v1
2008-08-22,Weak quasistatic magnetism in the frustrated Kondo lattice Pr_2Ir_2O_7,"Muon spin relaxation experiments have been performed in the pyrochlore
iridate Pr_2Ir_2O_7 for temperatures in the range 0.025-250 K. Kubo-Toyabe
relaxation functions are observed up to > 200 K, indicating static magnetism
over this temperature range. The T -> 0 static muon spin relaxation rate
Delta(0) ~ 8 mus^-1 implies a weak quasistatic moment (~0.1 mu_B). The
temperature dependence of Delta is highly non-mean-field-like, decreasing
smoothly by orders of magnitude but remaining nonzero below ~150 K. The data
rule out ordering of the full Pr^3+ CEF ground-state moment (3.0 mu_B) down to
0.025 K. The weak static magnetism is most likely due to hyperfine-enhanced
^141Pr nuclear magnetism. The dynamic relaxation rate lambda increases markedly
below ~20 K, probably due to slowing down of spin fluctuations in the
spin-liquid state. At low temperatures lambda is strong and
temperature-independent, indicative of a high density of low-lying spin
excitations as is common in frustrated antiferromagnets.",0808.3142v1
2008-08-27,Entanglement control in one-dimensional $s=1/2$ random XY spin chain,"The entanglement in one-dimensional random XY spin systems where the
impurities of exchange couplings and the external magnetic fields are
considered as random variables is investigated by solving the different
spin-spin correlation functions and the average magnetization per spin. The
entanglement dynamics near particular locations of the system is also studied
when the exchange couplings (or the external magnetic fields) satisfy three
different distributions(the Gaussian distribution, double-Gaussian
distribution, and bimodal distribution). We find that the entanglement can be
controlled by varying the strength of external magnetic field and the different
distributions of impurities. Moreover, the entanglement of some
nearest-neighboring qubits can be increased for certain parameter values of the
three different distributions.",0808.3678v1
2008-09-08,Anisotropic three-dimensional magnetism in CaFe2As2,"Inelastic neutron scattering measurements on the low energy spin waves in
CaFe2As2 show that the magnetic exchange interactions in the Fe layers are
exceptionally large and similar to the cuprates. However, the exchange between
layers is ~10% of the coupling in the layers and the magnetism is more
appropriately categorized as anisotropic three-dimensional, in contrast to the
two-dimensional cuprates. Band structure calculations of the spin dynamics and
magnetic exchange interactions are in good agreement with the experimental
data.",0809.1410v1
2008-11-28,Dynamic non-null magnetic reconnection in three dimensions - II. Composite solutions,"In this series of papers we examine magnetic reconnection in a domain where
the magnetic field does not vanish and the non-ideal region is localised in
space. In a previous paper we presented a technique for obtaining analytical
solutions to the stationary resistive MHD equations in such a situation and
examined specific examples of non-ideal reconnective solutions. Here we further
develop the model, noting that certain ideal solutions may be superimposed onto
the fundamental non-ideal solutions and examining the effect of imposing
various such flows. Significant implications are found for the evolution of
magnetic flux in the reconnection process. It is shown that, in contrast to the
two-dimensional case, in three-dimensions there is a very wide variety of
physically different steady reconnection solutions.",0811.4621v1
2009-01-06,Non-stationary magnetized axially symmetric equilibrium from the fluid equations of motion,"The equations of motion for a fully ionized hydrogenic plasma in applied
coaxial electric and magnetic fields are analyzed, where the term for the Hall
effect in the generalized Ohm's law equation picks up a factor of 1/2 relative
to its usual expression. Magnetization of the medium is incorporated through
the decomposition of the Hall term and the inclusion of the magnetization
force, which is found to equal or exceed the gradient of the scalar pressure. A
limit on the kinetic pressure obtains which corresponds to the usual limit of
unity for a certain selection of parameters. Solutions of these equations for
the free motion of the charges in the case of an infinite column with azimuthal
symmetry are compared for various prescribed pressure profiles, where one finds
that the profile near the outer edge plays an important role in the feasibility
of the equilibrium.",0901.0732v2
2009-01-23,Itinerant magnetic excitations in antiferromagnetic CaFe2As2,"Neutron scattering measurements of the magnetic excitations in single
crystals of antiferromagnetic CaFe2As2 reveal steeply dispersive and
well-defined spin waves up to an energy of 100 meV. Magnetic excitations above
100 meV and up to the maximum energy of 200 meV are however broader in energy
and momentum than the experimental resolution. While the low energy modes can
be fit to a Heisenberg model, the total spectrum cannot be described as arising
from excitations of a local moment system. Ab-initio calculations of the
dynamic magnetic susceptibility suggest that the high energy behavior is
dominated by the damping of spin waves by particle-hole excitations.",0901.3784v2
2009-01-27,Realistic Numerical Modeling of Solar Magnetoconvection and Oscillations,"We have developed 3D, compressible, non-linear radiative MHD simulations to
study the influence of the magnetic field of various strength and geometry on
the turbulent convective cells and on the excitation mechanisms of the acoustic
oscillations. The results reveal substantial changes of the granulation
structure with increased magnetic field, and a frequency-dependent reduction in
the oscillation power. These simulation results reproduce the enhanced
high-frequency acoustic emission observed at the boundaries of active region
(""acoustic halo"" phenomenon). In the presence of inclined magnetic field the
solar convection develops filamentary structure with flows concentrated along
magnetic filaments, and also exhibits behavior of running magnetoconvective
waves, resembling recent observations of the sunspot penumbra dynamics from
Hinode/SOT.",0901.4369v2
2009-02-20,Understanding of the role of magnetic fields: Galactic perspective,"A combination of observation, theory, modeling, and laboratory plasma
experiments provides a multifaceted approach to develop a much greater
understanding of how magnetic fields arise in galactic settings and how these
magnetic fields mediate important processes that affect the dynamics,
distribution, and composition of galactic plasmas. An important emphasis below
is the opportunity to connect laboratory experiments to astrophysics. This
approach is especially compelling for the galactic neighborhood, where the
distribution and character of magnetic fields can be observed with greater
detail than what is possible elsewhere in the universe. The ability to produce
laboratory plasmas with unparalleled accessibility permits an even greater
level of detail to be assessed and exposed. Theory and modeling provide
fundamental ways to understand important processes, and they act as the bridge
to connect experimental validation to astronomical observations. In many cases
the studies that utilize this approach can make use of existing laboratory
facilities, resulting in a cost that is quite small compared to the cost of
measurements in dedicated space missions.",0902.3618v1
2009-02-26,Low energy spin dynamics in the antiferromagnetic phase of CaFe2As2,"We present 75As nuclear magnetic resonance data in the paramagnetic and
magnetic states of single crystal CaFe2As2. The electric field gradient and the
internal magnetic field at the As sites change discontinuously below the first
order structural transition at T0 = 169 K. In the magnetic state, we find a
single value of the internal hyperfine field consistent with commensurate
antiferromagnetic order of Fe moments pointing in the ab plane. The spin
lattice relaxation rate shows Korringa behavior for T<T0/3, reflecting the
metallic nature of the ordered state. Surprisingly, 1/T1 exhibits a small peak
at 10 K, revealing the presence of slow spin fluctuations that may be
associated with domain wall motion.",0902.4492v1
2009-03-06,"Lagrangian relaxation schemes for calculating force-free magnetic fields, and their limitations","Force-free magnetic fields are important in many astrophysical settings.
Determining the properties of such force-free fields -- especially smoothness
and stability properties -- is crucial to understanding many key phenomena in
astrophysical plasmas, for example energy release processes that heat the
plasma and lead to dynamic or explosive events. Here we report on a serious
limitation on the computation of force-free fields that has the potential to
invalidate the results produced by numerical force-free field solvers even for
cases in which they appear to converge (at fixed grid resolution) to an
equilibrium magnetic field. In the present work we discuss this problem within
the context of a Lagrangian relaxation scheme that conserves magnetic flux and
div(B) identically. Error estimates are introduced to assess the quality of the
calculated equilibrium. We go on to present an algorithm, based on re-writing
the curl operation via Stokes' theorem, for calculating the current which holds
great promise for improving dramatically the accuracy of the Lagrangian
relaxation procedure.",0903.1226v1
2009-03-11,Magnetic Properties of the Superconducting State of Iron-Based Superconductors,"We show that features of the dynamical spin susceptibility can unambiguously
distinguish between different pairing symmetries of the superconducting state
in iron-based superconductors. A magnetic resonance is a unique feature for the
extended $s_{x^2y^2}$-wave $\cos k_x\cos k_y$ pairing symmetry. It is present
in the pure superconducting (SC) state, but weakens in the mixed SC and
magnetically ordered state. We calculate the the RPA correction to the NMR spin
relaxation rate $1/T_1$ and the Knight shift in the above states and show a
good agreement with experimental results. Moreover, we argue that the energy
dispersion of the magnetic resonance along c axis observed in neutron
scattering experiments is also an indirect evidence supporting the $s_{x^2y^2}$
pairing symmetry.",0903.2025v1
2009-03-12,"Magnetic Seed Field Generation from Electroweak Bubble Collisions, with Bubble Walls of Finite Thickness","Building on earlier work, we develop an equation-of-motion method for
calculating magnetic seed fields generated from currents arising from charged W
fields in bubble collisions during a first-order primordial electroweak phase
transition allowed in some proposed extensions of the standard model. The novel
feature of our work is that it takes into account, for the first time, the
dynamics of the bubble walls in such collisions. We conclude that for bubbles
with sufficiently thin surfaces the magnetic seed fields may be comparable to,
or larger than, those found in earlier work. Thus, our results strengthen the
conclusions of previous studies that cosmic magnetic fields observed today may
originate from seeds created during the electroweak phase transition, and
consequently that these fields may offer a clue relevant to extensions of the
standard model.",0903.2227v2
2009-03-18,Magnetic field resonantly enhanced free spins in underdoped YBa$_{2}$Cu$_{3}$O$_{6+x}$,"Using neutron scattering, we investigate the effect of a magnetic field on
the static and dynamic spin response in heavily underdoped superconducting
YBa$_{2}$Cu$_{3}$O$_{6+x}$ (YBCO$_{6+x}$) with x=0.33 (T$_{c}$=8 K) and 0.35
(T$_{c}$=18 K). In contrast to the heavily doped and superconducting monolayer
cuprates, the elastic central peak characterizing static spin correlations does
not respond observably to a magnetic field which suppresses superconductivity.
Instead, we find a magnetic field induced resonant enhancement of the spin
fluctuations. The energy scale of the enhanced fluctuations matches the Zeeman
energy within both the normal and vortex phases while the momentum dependence
is the same as the zero field bilayer response. The magnitude of the
enhancement is very similar in both phases with a fractional intensity change
of $(I/I_{0}-1) \sim 0.1$. We suggest that the enhancement is not directly
correlated with superconductivity but is the result of almost free spins
located near hole rich regions.",0903.3172v2
2009-05-01,Collisionless plasma expansion in the presence of a dipole magnetic field,"The collisionless interaction of an expanding high-energy plasma cloud with a
magnetized background plasma in the presence of a dipole magnetic field is
examined in the framework of a 2D3V hybrid (kinetic ions and massless fluid
electrons) model. The retardation of the plasma cloud and the dynamics of the
perturbed electromagnetic fields and the background plasma are studied for high
Alfven-Mach numbers using the particle-in-cell method. It is shown that the
plasma cloud expands excluding the ambient magnetic field and the background
plasma to form a diamagnetic cavity which is accompanied by the generation of a
collisionless shock wave. The energy exchange between the plasma cloud and the
background plasma is also studied and qualitative agreement with the analytical
model suggested previously is obtained.",0905.0051v1
2009-05-01,Huge ac magnetoresistance in La0.7Sr0.3MnO3 in sub- kilo gauss magnetic fields,"We report dynamical magnetotransport in a ferromagnetic metallic oxide,
La0.7Sr0.3MnO3 using the ac impedance technique. The temperature dependence of
the ac resistance(R) and the inductive reactance (X) of the complex impedance
(Z = R+jX) under different dc bias magnetic fields (Hdc = 0-1 kOe) were studied
for different frequencies f = 0.1 to 5 MHz of alternating current. The zero
field R, which decreases smoothly around the Curie temperature TC for f = 100
kHz, transforms into a peak for f = 0.5-5 MHz. The peak decreases in amplitude,
broadens and shifts downward in temperature as the bias field increases. A huge
ac magnetoresistance (= 45 % at f = 2 MHz) in a field of Hdc = 1 kOe is found
and we attribute it to the magnetic field- induced enhancement in the skin
depth and concomitant suppression of magnetic fluctuations near TC. Our study
suggests that radio frequency magnetotransport provides an alternative strategy
to enhance the magnetoresistance and probe the spin-charge coupling in
manganites.",0905.0090v1
2009-05-16,Local-moment ferromagnetism and unusual magnetic domains in Fe$_{1/4}$TaS$_{2}$ crystals,"Single crystals of Fe$_{1/4}$TaS$_{2}$ have been studied by using
magneto-optical (MO) imaging and radio-frequency (rf) magnetic susceptibility,
$\chi$. Real time MO images reveal unusual, slow dynamics of dendritic domain
formation, the details of which are strongly dependent upon magnetic and
thermal history. Measurements of $\chi(T)$ show well-defined, local moment
ferromagnetic transition at $T\approx 155$ K as well as thermal hysteresis for
50 K$<T<$60 K. This temperature range corresponds to the domain formation
temperature as determined by MO. Together these observations provide strong
evidence for local moment ferromagnetism in Fe$_{1/4}$TaS$_{2}$ crystals with
large, temperature dependent magnetic anisotropy.",0905.2684v1
2009-05-22,Metal-insulator transition and magnetism in correlated band insulator: FeSi and Fe$_{1-x}$Co$_{x}$Si,"The LDA+DMFT (local density approximation combined with dynamical mean-field
theory) computation scheme has been used to study spectral and magnetic
properties of FeSi and Fe$_{1-x}$Co$_{x}$Si. Having compared different models
we conclude that a correlated band insulator scenario in contrast to Kondo
insulator model agrees with FeSi band structure as well as experimental data.
Coulomb correlation effects lead to band narrowing of the states near the Fermi
level with mass renormalization parameter $m^*\approx 2$ in agreement with the
results of angle-resolved photoemission spectroscopy (ARPES). Temperature
dependence of spectral functions and magnetic susceptibility calculated in DMFT
reproduces transition from nonmagnetic semiconductor to metal with local
magnetic moments observed experimentally. Cobalt doping leads to ferromagnetism
that has itinerant nature and can be successfully described by LDA+DMFT method.",0905.3653v1
2009-05-28,Emerging nonequilibrium bound state in spin-current-local-spin scattering,"Magnetization reversal is a well-studied problem with obvious applicability
in computer hard-drives. One can accomplish a magnetization reversal in at
least one of two ways: application of a magnetic field, or through a spin
current. The latter is more amenable to a fully quantum mechanical analysis. We
formulate and solve the problem whereby a spin current interacts with a
ferromagnetic Heisenberg spin chain, to eventually reverse the magnetization of
the chain. Spin-flips are accomplished through both elastic and inelastic
scattering. A consequence of the inelastic scattering channel, when it is no
longer energetically possible, is the occurrence of a new entity: a
non-equilibrium bound state (NEBS), which is an emergent property of the
coupled local plus itinerant spin system. For certain definite parameter values
the itinerant spin lingers near the local spins for some time, before
eventually leaking out as an outwardly diffusing state. This phenomenon results
in novel spin-flip dynamics and filtering properties for this type of system.",0905.4708v2
2009-07-09,Excitation spectrum and magnetic field effects in a quantum critical spin-orbital system:FeSc$_2$S$_4$,"The orbitally degenerate A-site spinel compound FeSc2S4 has been
experimentally identified as a ``spin-orbital liquid'', with strong
fluctuations of both spins and orbitals. Assuming that the second neighbor spin
exchange J2 is the dominant one, we argued in a recent theoretical study [Chen
et al., Phys. Rev. Lett. 102, 096406 (2009)] that FeSc2S4 is in a local
``spin-orbital singlet'' state driven by spin orbit coupling, close to a
quantum critical point, which separates the ``spin-orbital singlet'' phase from
a magnetically and orbitally ordered phase. In this paper, we refine further
and develop this theory of FeSc2S4 . First, we show that inclusion of a small
first neighbor exchange J1 induces a narrow region of incommensurate phase near
the quantum critical point. Next, we derive the phase diagram in the presence
of an external magnetic field B, and show that the latter suppresses the
ordered phase. Lastly, we compute the field dependent dynamical magnetic
susceptibility $\chi(k, \omega; B)$, from which we extract a variety of
physical quantities. Comparison with and suggestions for experiment are
discussed.",0907.1692v1
2009-07-20,Computational study of microwave oscillations in absence of external field in nonstandard spin valves in the diffusive transport limit,"An anomalous (inverse) spin accumulation in the nonmagnetic spacer may build
up when the spin valve consists of magnetic films having different spin
symmetries. This leads to wavy-like dependence of spin-transfer torque on the
angle between magnetizations, as predicted by spin-dependent diffusive
transport model, and also confirmed experimentally. Making use of these
predictions, we have numerically studied the magnetization dynamics in presence
of such a wavy-torque in Co(8 nm)/Cu(10 nm)/Py(8 nm) nanopillar, considering
geometry with extended and etched Co layer. In both cases we specify conditions
for the out-of-plane precession to appear in absence of external magnetic field
and neglecting thermal fluctuations. We prove the assumption of wavy-like
torque angular dependence to be fully consistent with experimental
observations. We also show that some features reported experimentally, like
nonlinear slope of frequency vs. current behavior, are beyond the applicability
range of macrospin approximation and can be explained only by means of full
micromagnetic analysis.",0907.3384v1
2009-08-12,Small-Scale Kinematic Dynamo and Non-Dynamo in Inertial-Range Turbulence,"We investigate the Lagrangian mechanism of the kinematic ``fluctuation''
magnetic dynamo in turbulent plasma flow at small magnetic Prandtl numbers. The
combined effect of turbulent advection and plasma resistivity is to carry
infinitely many field lines to each space point, with the resultant magnetic
field at that point given by the average over all the individual line vectors.
As a consequence of the roughness of the advecting velocity, this remains true
even in the limit of zero resistivity. We show that the presence of dynamo
effect requires sufficient angular correlation of the passive line-vectors that
arrive simultaneously at the SAME space point. We demonstrate this in detail
for the Kazantsev-Kraichnan model of kinematic dynamo with a Gaussian advecting
velocity that is spatially rough and white-noise in time. In the regime where
dynamo action fails, we also obtain the precise rate of decay of the magnetic
energy. These exact results for the model are obtained by a generalization of
the ``slow-mode expansion'' of Bernard, Gawedzki and Kupiainen to non-Hermitian
evolution. Much of our analysis applies also to magnetohydrodynamic turbulence.",0908.1598v1
2009-08-27,Reciprocal and real space maps for EMCD experiments,"Electron magnetic chiral dichroism (EMCD) is an emerging tool for
quantitative measurements of magnetic properties using the transmission
electron microscope (TEM), with the possibility of nanometer resolution. The
geometrical conditions, data treatment and electron gun settings are found to
influence the EMCD signal. In this article, particular care is taken to obtain
a reliable quantitative measurement of the ratio of orbital to spin magnetic
moment using energy filtered diffraction patterns. For this purpose, we
describe a method for data treatment, normalization and selection of mirror
axis. The experimental results are supported by theoretical simulations based
on dynamical diffraction and density functional theory. Special settings of the
electron gun, so called telefocus mode, enable a higher intensity of the
electron beam, as well as a reduction of the influence from artifacts on the
signal. Using these settings, we demonstrate the principle of acquiring real
space maps of the EMCD signal. This enables advanced characterization of
magnetic materials with superior spatial resolution.",0908.3963v1
2009-09-03,Simulating and detecting artificial magnetic fields in trapped atoms,"A Bose-Einstein condensate exhibiting a nontrivial phase induces an
artificial magnetic field in immersed impurity atoms trapped in a stationary,
ring-shaped optical lattice. We present an effective Hamiltonian for the
impurities for two condensate setups: the condensate in a rotating ring and in
an excited rotational state in a stationary ring. We use Bogoliubov theory to
derive analytical formulas for the induced artificial magnetic field and the
hopping amplitude in the limit of low condensate temperature where the impurity
dynamics is coherent. As methods for observing the artificial magnetic field we
discuss time of flight imaging and mass current measurements. Moreover, we
compare the analytical results of the effective model to numerical results of a
corresponding two-species Bose-Hubbard model. We also study numerically the
clustering properties of the impurities and the quantum chaotic behavior of the
two-species Bose-Hubbard model.",0909.0728v2
2009-09-11,Magnetism of one-dimensional strongly repulsive spin-1 bosons with antiferromagnetic spin exchange interaction,"We investigate magnetism and quantum phase transitions in a one-dimensional
system of integrable spin-1 bosons with strongly repulsive density-density
interaction and antiferromagnetic spin exchange interaction via the
thermodynamic Bethe ansatz method. At zero temperature, the system exhibits
three quantum phases: (i) a singlet phase of boson pairs when the external
magnetic field $H$ is less than the lower critical field $H_{c1}$; (ii) a
ferromagnetic phase of atoms in the hyperfine state $|F=1, m_{F}=1>$ when the
external magnetic field exceeds the upper critical field $H_{c2}$; and (iii) a
mixed phase of singlet pairs and unpaired atoms in the intermediate region
$H_{c1}<H<H_{c2}$. At finite temperatures, the spin fluctuations affect the
thermodynamics of the model through coupling the spin bound states to the
dressed energy for the unpaired $m_{F}=1$ bosons. However, such spin dynamics
is suppressed by a sufficiently strong external field at low temperatures. Thus
the singlet pairs and unpaired bosons may form a two-component Luttinger liquid
in the strong coupling regime.",0909.2097v1
2009-09-11,Amplification of whistler waves in a thin Keplerian disc embedded in an axisymmetric magnetic field,"The linear stability of thin Keplerian discs of weakly ionized polytropic
plasma embedded in a mixed (toroidal-poloidal) axisymmetric magnetic field is
studied. The effects of the central body on the equilibrium state of the discs
are modeled by an axial total current and a dipole magnetic field. Studying the
amplification of high-frequency waves shows that Keplerian discs can be
destabilized due to combined effects of equilibrium density stratification and
poloidal magnetic field. By considering a boundary value problem in the limit
of strong Hall effect, it is shown that whistler waves which are free of direct
influence of compressibility, rotation and gravity, are amplified along their
trajectory while other modes of wave propagation do not change their amplitude.",0909.2135v1
2009-10-14,Pseudospin excitations in coaxial nanotubes,"In a 2DEG confined to two coaxial tubes the `tube degree of freedom' can be
described in terms of pseudospin-1/2 dynamics. The presence of tunneling
between the two tubes leads to a collective oscillation known as pseudospin
resonance. We employ perturbation theory to examine the dependence of the
frequency of this mode with respect to a coaxial magnetic field for the case of
small intertube distances. Coulomb interaction leads to a shift of the
resonance frequency and to a finite lifetime of the pseudospin excitations. The
presence of the coaxial magnetic field gives rise to pronounced peaks in the
shift of the resonance frequency. For large magnetic fields this shift vanishes
due to the effects of Zeeman splitting. Finally, an expression for the
linewidth of the resonance is derived. Numerical analysis of this expression
suggests that the linewidth strongly depends on the coaxial magnetic field,
which leads to several peaks of the linewidth as well as regions where damping
is almost completely suppressed.",0910.2552v2
2009-10-23,Prandtl number dependence of convection driven dynamos in rotating spherical fluid shells,"The value of the Prandtl number $P$ exerts a strong influence on
convection-driven dynamos in rotating spherical shells filled with electrically
conducting fluids. Low Prandtl numbers promote dynamo action through the shear
provided by differential rotation, while the generation of magnetic fields is
more difficult to sustain in high-Prandtl-number fluids where higher values of
the magnetic Prandtl number $P_m$ are required. The magnetostrophic
approximation often used in dynamo theory appears to be valid only for
relatively high values of $P$ and $P_m$. Dynamos with a minimum value of $P_m$
seem to be most readily realizable in the presence of convection columns at
moderately low values of $P$. The structure of the magnetic field varies
strongly with $P$ in that dynamos with a strong axial dipole field are found
for high values of $P$ while the energy of this component is exceeded by that
of the axisymmetric toroidal field and by that of the non-axisymmetric
components at low values of $P$. Some conclusions are discussed in relation to
the problem of the generation of planetary magnetic fields by motions in their
electrically conducting liquid cores.",0910.4493v1
2009-11-06,Chiral magnetic effect in 2+1 flavor QCD+QED,"The exciting possibility of direct observation of QCD instantons in heavy-ion
collisions has recently been proposed by Kharzeev. The underlying phenomenon,
known as the chiral magnetic effect, may have been observed recently at RHIC,
and a first principles calculation is needed to confirm and understand the
results. The chiral magnetic effect is thought to be visible in the symmetric
phase, at temperatures above the QCD critical temperature, and in the presence
of an external magnetic field. We report on first 2+1 flavor, domain wall
fermion, QCD+QED dynamical simulations above the critical temperature, in a
fixed topological sector(s), which are used to study the electric charge
separation produced by the effect.",0911.1348v1
2009-11-18,Screened perturbation theory for 3d Yang-Mills theory and the magnetic modes of hot QCD,"Perturbation theory for non-abelian gauge theories at finite temperature is
plagued by infrared divergences which are caused by magnetic soft modes ~g^2T,
corresponding to gluon fields of a 3d Yang-Mills theory. While the divergences
can be regulated by a dynamically generated magnetic mass on that scale, the
gauge coupling drops out of the effective expansion parameter requiring
summation of all loop orders for the calculation of observables. Some gauge
invariant possibilities to implement such infrared-safe resummations are
reviewed. We use a scheme based on the non-linear sigma model to estimate some
of the contributions ~g^6 of the soft magnetic modes to the QCD pressure
through two loops. The NLO contribution amounts to ~10% of the LO, suggestive
of a reasonable convergence of the series.",0911.3595v2
2009-11-24,Low temperature study of field induced antiferro-ferromagnetic transition in Pd doped FeRh,"The first order antiferromagnetic (AFM) to ferromagnetic (FM) transition in
the functional material Fe49(Rh0.93Pd0.07)51 has been studied at low
temperatures and high magnetic fields. We have addressed the non-monotonic
variation of lower critical field required for FM to AFM transition. It is
shown that critically slow dynamics of the transition dominates below 50 K. At
low temperature and high magnetic field, state of the system depends on the
measurement history resulting in tunable coexistence of AFM and FM phases. By
following cooling and heating in unequal magnetic field (CHUF) protocol it is
shown that equilibrium state at 6 Tesla magnetic field is AFM state. Glass like
FM state at 6 T (obtained after cooling in 8 T) shows reentrant transition with
increasing temperature; viz. devitrification to AFM state followed by melting
to FM state.",0911.4552v1
2009-12-15,"Dynamics of Uniform Quantum Gases, II: Magnetic Susceptibility","A general expression for temperature-dependent magnetic susceptibility of
quantum gases composed of particles possessing both charge and spin degrees of
freedom has been obtained within the framework of the generalized random-phase
approximation. The conditions for the existence of dia-, para-, and
ferro-magnetism have been analyzed in terms of a parameter involving
single-particle charge and spin. The zero-temperature limit retrieves the
expressions for the Landau and the Pauli susceptibilities for an electron gas.
It is found for a Bose gas that on decreasing the temperature, it passes either
through a diamagnetic incomplete Meissner-effect regime or through a
paramagnetic-ferromagnetic large magnetization fluctuation regime before going
to the Meissner phase at BEC critical temperature.",0912.2841v1
2010-01-12,Dynamic pinning at a Py/Co interface measured using inductive magnetometry,"Broadband FMR responses for metallic single-layer and bi-layer magnetic films
with total thicknesses smaller than the microwave magnetic skin depth have been
studied. Two different types of microwave transducers were used to excite and
detect magnetization precession: a narrow coplanar waveguide and a wide
microstrip line. Both transducers show efficient excitation of higher-order
standing spin wave modes. The ratio of amplitudes of the first standing spin
wave to the fundamental resonant mode is independent of frequency for single
films. In contrast, we find a strong variation of the amplitudes with frequency
for bi-layers and the ratio is strongly dependent on the ordering of layers
with respect to a stripline transducer. Most importantly, cavity FMR
measurements on the same samples show considerably weaker amplitudes for the
standing spin waves. All experimental data are consistent with expected effects
due to screening by eddy currents in films with thicknesses below the microwave
magnetic skin depth. Finally, conditions for observing eddy current effects in
different types of experiments are critically examined.",1001.1837v1
2010-02-04,Neutron scattering and muSR investigations of the spin liquid state with quenched disorder in LuCuGaO4,"LuCuGaO4 has magnetic Cu2+ and diamagnetic Ga3+ ions distributed on a
triangular bilayer and is suggested to undergo a spin-glass transition with T_g
approximately 0.4 K. Using muSR and neutron scattering measurements, we show
that at low temperature the spins form a short-range correlated state with spin
fluctuations detectable over a wide range of timescales: at 0.05 K magnetic
fluctuations can be detected in both the muSR time window and also extending
beyond 7 meV in the inelastic neutron scattering response, indicating magnetic
fluctuations spanning timescales between 1E-5 and 1E-10 seconds. The dynamical
susceptibility scales according to the form \chi''(\omega)T^\alpha, with
\alpha=1, throughout the measured temperature range (0.05 - 50 K). These
effects are associated with quantum fluctuations and some degree of structural
disorder in ostensibly quite different materials, including certain heavy
fermion alloys, kagome spin liquids, quantum spin glasses, and valence bond
glasses. We therefore suggest LuCuGaO4 is an interesting model compound for the
further examination of disorder and quantum magnetism.",1002.0975v2
2010-02-12,The Parker Scenario for Coronal Heating as an MHD Turbulence Problem,"The Parker or field line tangling model of coronal heating is investigated
through long-time high-resolution simulations of the dynamics of a coronal loop
in cartesian geometry within the framework of reduced magnetohydrodynamics
(RMHD). Slow photospheric motions induce a Poynting flux which saturates by
driving an anisotropic turbulent cascade dominated by magnetic energy and
characterized by current sheets elongated along the axial magnetic field.
Increasing the value of the axial magnetic field different regimes of MHD
turbulence develop with a bearing on coronal heating rates. In physical space
magnetic field lines at the scale of convection cells appear only slightly
bended in agreement with observations of large loops of current (E)UV and X-ray
imagers.",1002.2631v1
2010-03-11,On the Chiral Magnetic Effect in Soft-Wall AdS/QCD,"The essence of the chiral magnetic effect is generation of an electric
current along an external magnetic field. Recently it has been studied by
Rebhan et al. within the Sakai--Sugimoto model, where it was shown to be zero.
As an alternative, we calculate the chiral magnetic effect in soft-wall AdS/QCD
and find a non-zero result with the natural boundary conditions. The mechanism
of the dynamical neutralization of the chiral chemical potential via the string
production is discussed in the dual two-form representation.",1003.2293v3
2010-03-11,Electric Charge Separation in Strong Transient Magnetic Fields,"We discuss various mechanisms for the creation of an asymmetric charge
fluctuation with respect to the reaction plane among hadrons emitted in
relativistic heavy-ion collisions. We show that such mechanisms exist in both,
the hadronic gas and the partonic phases of QCD. The mechanisms considered here
all require the presence of a strong magnetic field (the ``chiral magnetic
effect''), but they do not involve parity or charge-parity violations. We
analyze how a transient local electric current fluctuation generated by the
chiral magnetic effect can dynamically evolve into an asymmetric charge
distribution among the final-state hadrons in momentum space. We estimate the
magnitude of the event-by-event fluctuations of the final-state charge
asymmetry due to the partonic and hadronic mechanisms.",1003.2436v3
2010-03-18,Self-organization of dissipative and coherent vortex structures in non-equilibrium magnetized two-dimensional plasmas,"The properties of non-equilibrium magnetized plasmas confined in planar
geometry are studied on the basis of the first principle microscopic Langevin
dynamics computer simulations. The non-equilibrium state of plasmas is
maintained due to the recombination and generation of charges.The intrinsic
microscopic structure of non-equilibrium steady-state magnetized plasmas, in
particular, the inter-particle correlations and self-organization of vortex
structures are examined. The simulations have been performed for a wide range
of parameters including strong plasma coupling, high charge recombination and
generation rates, and intense magnetic field. As is shown in simulations, the
non-equilibrium recombination and generation processes trigger the formation of
ordered dissipative or coherent drift vortex states in 2D plasmas with
distinctly spatially separated components, which are far from thermal
equilibrium. This is evident from the unusual properties of binary
distributions and behavior of the Coulomb energy of the system, which turn out
to be quite different from the ones typical for the equilibrium state of
plasmas under the same conditions.",1003.3666v2
2010-03-22,Nontrivial in-plane-magnetic-field dependence of THz wave emission from intrinsic Josephson junctions controlled by surface impedance,"In THz wave emission from intrinsic Josephson junctions in in-plane magnetic
fields, emission intensity strongly depends on the surface impedance $Z$
similarly to the case without external magnetic fields. Cavity resonance modes
are stabilized for $Z \ge 3$, and the fundamental mode gives the strongest
emission. As the in-plane magnetic field increases for a fixed number of
junctions, dynamical phase transitions seem to occur between the
$\pi$-phase-kink state, various incommensurate phase-kink states, and in-phase
state. As $Z$ varies, a crossover of the field profile of maximum intensity
takes place for $Z \approx 50$ between characteristic peaks for smaller $Z$
(typically $Z \approx 30$) and monotonic decrease for larger $Z$ (typically $Z
\approx 70$). The double-peak structure reported in experiments can be
explained for Z=30 by finite-size analysis with respect to number of junctions.",1003.4045v2
2010-03-30,Shock-triggered formation of magnetically-dominated clouds. II. Weak shock-cloud interaction in three dimensions,"To understand the formation of a magnetically dominated molecular cloud from
an atomic cloud, we study the interaction of a weak, radiative shock with a
magnetised cloud. The thermally stable warm atomic cloud is initially in static
equilibrium with the surrounding hot ionised gas. A shock propagating through
the hot medium then interacts with the cloud. We follow the dynamical evolution
of the shocked cloud with a time-dependent ideal magnetohydrodynamic code. By
performing the simulations in 3D, we investigate the effect of different
magnetic field orientations including parallel, perpendicular and oblique to
the shock normal. We find that the angle between the shock normal and the
magnetic field must be small to produce clouds with properties similar to
observed molecular clouds.",1003.5843v1
2010-04-08,Divergent effects of static disorder and hole doping in geometrically frustrated b-CaCr2O4,"The gallium substituted and calcium deficient variants of geometrically
frustrated b-CaCr2O4, b-CaCr2-2xGa2xO4 (0.02<= x<= 0.25) and b-Ca1-yCr2O4
(0.075<= y<= 0.15), have been investigated by x-ray powder diffraction,
magnetization and specific heat measurements. This allows for a direct
comparison of the effects, in a geometrically frustrated magnet, of the static
disorder that arises from non-magnetic substitution and the dynamic disorder
that arises from hole doping. In both cases, disturbing the Cr3+ lattice
results in a reduction in the degree of magnetic frustration. On substitution
of Ga, which introduces disorder without creating holes, a gradual release of
spins from ordered antiferromagnetic states is observed. In contrast, in the
calcium deficient compounds the introduction of holes induces static
ferrimagnetic ordering and much stronger perturbations of the b-CaCr2O4 host.",1004.1390v1
2010-04-14,Simple waves in relativistic fluids,"We consider the Riemann problem for relativistic flows of polytropic fluids
and find relations for the flow characteristics. Evolution of physical
quantities take especially simple form for the case of cold magnetized plasmas.
We find exact, explicit analytical solutions for one dimensional expansion of
magnetized plasma into vacuum, valid for arbitrary magnetization. We also
consider expansion into cold unmagnetized external medium both for stationary
initial conditions and for initially moving plasma, as well as reflection of
rarefaction wave from a wall. We also find self-similar structure of
three-dimensional magnetized outflows into vacuum, valid close to the
plasma-vacuum interface.
  The key results of this work, the self-similar solutions, were incorporated
post-initial submission into appendices of the published version of Granot et
al. (2010).",1004.2428v4
2010-05-10,"Exchange constant and domain wall width in (Ga,Mn)(As,P) films with self-organization of magnetic domains","The incorporation of Phosphorus into (Ga,Mn)As epilayers allows for the
tuning of the magnetic easy axis from in-plane to perpendicular-to-plane
without the need for a (Ga,In)As template. For perpendicular easy axis, using
magneto-optical imaging a self-organized pattern of up- and down-magnetized
domains is observed for the first time in a diluted magnetic semiconductor.
Combining Kerr microscopy, magnetometry and ferromagnetic resonance
spectroscopy, the exchange constant and the domain wall width parameter are
obtained as a function of temperature. The former quantifies the effective
Mn-Mn ferromagnetic interaction. The latter is a key parameter for domain wall
dynamics. The comparison with results obtained for (Ga,Mn)As/(Ga,In)As reveals
the improved quality of the (Ga,Mn)As$_{1-y}$P$_y$ layers regarding domain wall
pinning, an increase of the domain wall width parameter and of the effective
Mn-Mn spin coupling. However, at constant Mn doping, no significant increase of
this coupling is found with increasing P concentration in the investigated
range.",1005.1494v1
2010-05-11,Zero-field magnetization reversal of two-body Stoner particles with dipolar interaction,"Nanomagnetism has recently attracted explosive attention, in particular,
because of the enormous potential applications in information industry, e.g.
new harddisk technology, race-track memory[1], and logic devices[2]. Recent
technological advances[3] allow for the fabrication of single-domain magnetic
nanoparticles (Stoner particles), whose magnetization dynamics have been
extensively studied, both experimentally and theoretically, involving magnetic
fields[4-9] and/or by spin-polarized currents[10-20]. From an industrial point
of view, important issues include lowering the critical switching field $H_c$,
and achieving short reversal times. Here we predict a new technological
perspective: $H_c$ can be dramatically lowered (including $H_c=0$) by
appropriately engineering the dipole-dipole interaction (DDI) in a system of
two synchronized Stoner particles. Here, in a modified Stoner-Wohlfarth (SW)
limit, both of the above goals can be achieved. The experimental feasibility of
realizing our proposal is illustrated on the example of cobalt nanoparticles.",1005.1828v1
2010-05-26,Dispersive magnetized waves in the solar wind plasma,"We derive a generalized linear dispersion relation of waves in a strongly
magnetized, compressible, homogeneous and isotropic quasineutral plasma.
Starting from a two fluid model, describing distinguishable electron and ion
fluids, we obtain a six order linear dispersion relation of magnetized waves
that contains effects due to electron and ion inertia, finite plasma beta and
angular dependence of phase speed. We investigate propagation characteristics
of these magnetized waves in a regime where scale lengths are comparable with
electron and ion inertial length scales. This regime corresponds essentially to
the solar wind plasma where length scales, comparable with ion cyclotron
frequency, lead to dispersive effects. These scales in conjunction with linear
waves present a great deal of challenges in understanding the high frequency,
small scale dynamics of turbulent fluctuations in the solar wind plasma.",1005.4902v1
2010-05-30,Smooth double critical state theory for type-II superconductors,"Several aspects of the general theory for the critical states of a vortex
lattice and the magnetic flux dynamics in type-II superconductors are examined
by a direct variational optimisation method and widespread physical principles.
Our method allows to unify a number of conventional models describing the
complex vortex configurations in the critical state regime. Special attention
is given to the discussion of the relation between the flux-line cutting
mechanism and the depinning threshold limitation. This is done by using a
smooth double critical state concept which incorporates the so-called
isotropic, elliptical, T and CT models as well-defined limits of our general
treatment. Starting from different initial configurations for a superconducting
slab in a 3D magnetic field, we show that the predictions of the theory range
from the collapse to zero of transverse magnetic moments in the isotropic
model, to nearly force free configurations in which paramagnetic values can
arbitrarily increase with the applied field for magnetically anisotropic
current voltage laws. Noteworthily, the differences between the several model
predictions are minimal for the low applied field regime.",1005.5540v1
2010-05-31,Driven weak to strong pinning crossover in partially nanopatterned 2H-NbSe2 single crystal,"Investigations into the heterogeneous pinning properties of the vortex state
created by partially nano-patterning single crystals of 2H-NbSe2 reveal an
atypical magnetization response which is significantly drive dependent.
Analysis of the magnetization response shows non-monotonic behavior of the
magnetization relaxation rate with varying magnetic field sweep rate. With all
the patterned pinning centers saturated with vortices, we find that the pinning
force experienced by the vortices continues to increase with increasing drive.
Our studies reveal an unconventional dynamic weak to strong pinning crossover
where the flow of the vortex state appears to be hindered or jammed as it is
driven harder through the interstitial voids in the patterned pinning lattice.",1005.5641v1
2010-06-03,Peristaltic Transport of a Physiological Fluid in an Asymmetric Porous Channel in the Presence of an External Magnetic Field,"The paper deals with a theoretical investigation of the peristaltic transport
of a physiological fluid in a porous asymmetric channel under the action of a
magnetic field. The stream function, pressure gradient and axial velocity are
studied by using appropriate analytical and numerical techniques. Effects of
different physical parameters such as permeability, phase difference, wave
amplitude and magnetic parameter on the velocity, pumping characteristics,
streamline pattern and trapping are investigated with particular emphasis. The
computational results are presented in graphical form. The results are found to
be in perfect agreement with those of a previous study carried out for a
non-porous channel in the absence of a magnetic field.",1006.0616v1
2010-06-29,Magnetic properties of exchange biased and of unbiased oxide/permalloy thin layers: a ferromagnetic resonance and Brillouin scattering study,"Microstrip ferromagnetic resonance and Brillouin scattering are used to
provide a comparative determination of the magnetic parameters of thin
permalloy layers interfaced with a non-magnetic (Al2O3) or with an
antiferromagnetic oxide (NiO). It is shown that the perpendicular anisotropy is
monitored by an interfacial surface energy term which is practically
independent of the nature of the interface. In the investigated interval of
thicknesses (5-25 nm) the saturation magnetisation does not significantly
differ from the reported one in bulk permalloy. In-plane uniaxial anisotropy
and exchange-bias anisotropy are also derived from this study of the dynamic
magnetic excitations and compared to our independent evaluations using
conventional magnetometry",1006.5598v1
2010-07-01,Magnetic dynamics of simple collective modes in a two-sphere plasma model,"A plasma blob is modeled as consisting of two homogeneous spheres of equal
radius and equal but opposite charge densities that can move relative to each
other. Relative translational and rotational motion are considered separately.
Magnetic effects from the current density caused by the relative motion are
included. Magnetic interaction is seen to cause an inductive inertia. In the
relative translation case the Coulomb attraction, approximately a linear force
for small amplitudes, causes an oscillation. For a large number of particles
the corresponding oscillation frequency will not be the Langmuir plasma
frequency, because of the large inductive inertia. For rotation an external
magnetic field is included and the energy and diamagnetism of the plasma in the
model is calculated. Finally it is noted how the neglect of resistivity is
motivated by the results.",1007.0216v1
2010-07-07,Flow and Heat Transfer of a MHD Viscoelastic Fluid in a Channel with Stretching Walls: Some Applications to Haemodynamics,"Of concern in the paper is a study of steady incompressible viscoelastic and
electrically conducting fluid flow and heat transfer in a parallel plate
channel with stretching walls in the presence of a magnetic field applied
externally. The flow is considered to be governed by Walter's liquid B fluid.
The problem is solved by developing a suitable numerical method. The results
are found to be in good agrement with those of earlier investigations reported
in existing scientific literatures. The study reveals that a back flow occurs
near the central line of the channel due to the stretching walls and further
that this flow reversal can be stopped by applying a strong external magnetic
field. The study also shows that with the increase in the strength of the
magnetic field, the fluid velocity decreases but the temperature increases.
Thus the study bears potential applications in the study of the haemodynamic
flow of blood in the cardiovascular system when subjected to an external
magnetic field.",1007.1228v1
2010-08-06,Recovering magnetization distributions from their noisy diffraction data,"We study, using simulated experiments inspired by thin film magnetic domain
patterns, the feasibility of phase retrieval in X-ray diffractive imaging in
the presence of intrinsic charge scattering given only photon-shot-noise
limited diffraction data. We detail a reconstruction algorithm to recover the
sample's magnetization distribution under such conditions, and compare its
performance with that of Fourier transform holography. Concerning the design of
future experiments, we also chart out the reconstruction limits of diffractive
imaging when photon- shot-noise and the intensity of charge scattering noise
are independently varied. This work is directly relevant to the time-resolved
imaging of magnetic dynamics using coherent and ultrafast radiation from X-ray
free electron lasers and also to broader classes of diffractive imaging
experiments which suffer noisy data, missing data or both.",1008.1136v1
2010-08-22,Local analysis of the magnetic instability in rotating magneto-hydrodynamics with the short-wavelength approximation,"We investigate analytically the magnetic instability in a rotating and
electrically conducting fluid induced by an imposed magnetic field with its
associated electric current. The short-wavelength approximation is used in the
linear stability analysis, i.e. the length scale of the imposed field is much
larger than the wavelength of perturbations. The dispersion relationship is
derived and then simplified to give the criteria for the onset of the magnetic
instability in three cases of imposed field, namely the axial dependence, the
radial dependence and the mixed case. The orientation of rotation, imposed
field and imposed current is important for this instability.",1008.3706v7
2010-08-28,Modelling stellar coronal magnetic fields,"Our understanding of the structure and dynamics of stellar coronae has
changed dramatically with the availability of surface maps of both star spots
and also magnetic field vectors. Magnetic field extrapolations from these
surface maps reveal surprising coronal structures for stars whose masses and
hence internal structures and dynamo modes may be very different from that of
the Sun. Crucial factors are the fraction of open magnetic flux (which
determines the spin-down rate for the star as it ages) and the location and
plasma density of closed-field regions, which determine the X-ray and radio
emission properties. There has been recent progress in modelling stellar
coronae, in particular the relative contributions of the field detected in the
bright surface regions and the field that may be hidden in the dark star spots.
For the Sun, the relationship between the field in the spots and the large
scale field is well studied over the solar cycle. It appears, however, that
other stars can show a very different relationship.",1008.4885v1
2010-09-06,Energy oscillations and a possible route to chaos in a modified Riga dynamo,"Starting from the present version of the Riga dynamo experiment with its
rotating magnetic eigenfield dominated by a single frequency we ask for those
modifications of this set-up that would allow for a non-trivial magnetic field
behaviour in the saturation regime. Assuming an increased ratio of azimuthal to
axial flow velocity, we obtain energy oscillations with a frequency below the
eigenfrequency of the magnetic field. These new oscillations are identified as
magneto-inertial waves that result from a slight imbalance of Lorentz and
inertial forces. Increasing the azimuthal velocity further, or increasing the
total magnetic Reynolds number, we find transitions to a chaotic behaviour of
the dynamo.",1009.1051v1
2010-09-09,Simulation of RF Cavity Dark Current in Presence of Helical Magnetic Field,"In order to produce muon beam of high enough quality to be used for a Muon
Collider, its large phase space must be cooled several orders of magnitude.
This task can be accomplished by ionization cooling. Ionization cooling
consists of passing a high-emittance muon beam alternately through regions of
low Z material, such as liquid hydrogen, and very high accelerating RF cavities
within a multi-Tesla solenoidal focusing channel. But first high power tests of
RF cavity with beryllium windows in solenoidal magnetic field showed a dramatic
drop in accelerating gradient due to RF breakdowns. It has been concluded that
external magnetic fields parallel to RF electric field significantly modifies
the performance of RF cavities. However, magnetic field in Helical Cooling
Channel has a strong dipole component in addition to solenoidal one. The dipole
component essentially changes electron motion in a cavity compare to pure
solenoidal case, making dark current less focused at field emission sites. The
simulation of dark current dynamic in HCC performed with CST Studio Suit is
presented in this paper.",1009.1857v1
2010-09-22,Long-term decaying evolution of MHD turbulence,"The free decay of MHD turbulence at large Reynolds numbers is studied
numerically using a shell model. We study the statistical properties based on
representative sample of realisations (128 realisations for each type of
initial conditions) over the period of $10^5$ large-scale turnover times. The
performed simulations show that the force-free non-helical MHD turbulence can
demonstrate two different scenarios of evolution in spite of similar initial
conditions. Within the first scenario, the cross-helicity accumulation is so
fast that the energy cascade vanishes before significant magnetic energy
dissipates. Then the system approaches the state of maximal cross-helicity.
Within the second scenario, the cascade process continues to remain active
until time $10^4$ in units of large-scale turnover time. Then the magnetic
field becomes vastly helical due to magnetic helicity conservation. Thus the
magnetic energy does not dissipate with kinetic energy.",1009.4337v1
2010-09-27,On the Stability of Non Force-Free Magnetic Equilibria in Stars,"The existence of stable magnetic configurations in white dwarfs, neutron
stars and various non-convective stellar {regions} is now well recognized. It
has recently been shown numerically that various families of equilibria,
including axisymmetric mixed poloidal-toroidal configurations, are stable. Here
we test the stability of an analytically-derived non force-free magnetic
equilibrium, using three-dimensional magnetohydrodynamic simulations: the mixed
configuration is compared with the dynamical evolution of its purely poloidal
and purely toroidal components, both known to be unstable. The mixed
equilibrium shows no sign of instability under white noise perturbations. {This
configuration therefore provides a good description of magnetic equilibrium
topology inside non-convective stellar objects and will be useful to initialize
magneto-rotational transport in stellar evolution codes.",1009.5384v1
2010-09-28,Magneto-acoustic wave propagation and mode conversion in a magnetic solar atmosphere: comparing results from the CO5BOLD code with ray theory,"We present simulations of magneto-acoustic wave propagation in a magnetic,
plane-parallel stratified solar model atmosphere, employing the CO5BOLD-code.
The tests are carried out for two models of the solar atmosphere, which are
similar to the ones used by Cally (2007) and Schunker & Cally (2006). The two
models differ only in the orientation of the magnetic field. A qualitative
comparison shows good agreement between the numerical results and the results
from ray theory. The tests are done in view of the application of the present
numerical code for the computation of energy fluxes of propagating acoustic
waves into a dynamically evolving magnetic solar atmosphere. For this, we
consider waves with frequencies above the acoustic cut-off frequency.",1009.5586v1
2010-09-28,Role of non-collinear magnetization: from ferromagnetic nano wires to quantum rings,"We consider interaction effects related to a nonuniform magnetization in
ferromagnetic nanowires and their possible generalization to nanorings. First
we show that the electric current in a ferromagnetic nanowire with more than
one domain wall induces an exchange coupling between the walls mediated by the
spin-dependent interference of scattered carriers. This interaction reveals a
complex behavior as a function of mutual orientations and separation of the
domain walls, thus affecting the domain wall dynamics. Then we consider how the
theory should be modified in the case of a magnetized quantum ring. In this
case the situation is more complicated because the vortex and onion
magnetization states in the ring are not truly ferromagnetic.",1009.5688v1
2010-09-30,Existence of a phase transition under finite magnetic field in the long-range RKKY Ising spin glass Dy$_{x}$Y$_{1-x}$Ru$_{2}$Si$_{2}$,"A phase transition of a model compound of the long-range Ising spin glass
(SG) Dy$_{x}$Y$_{1-x}$Ru$_{2}$Si$_{2}$, where spins interact via the RKKY
interaction, has been investigated. The static and the dynamic scaling analyses
reveal that the SG phase transition in the model magnet belongs to the
mean-field universality class. Moreover, the characteristic relaxation time in
finite magnetic fields exhibits a critical divergent behavior as well as in
zero field, indicating a stability of the SG phase in finite fields. The
presence of the SG phase transition in field in the model magnet strongly
syggests that the replica symmetry is broken in the long-range Ising SG.",1009.6115v2
2010-10-01,Magnetic fields and the outer rotation curve of M31,"Recent observations of the rotation curve of M31 show a rise of the outer
part that can not be understood in terms of standard dark matter models or
perturbations of the galactic disc by M31's satellites. Here, we propose an
explanation of this dynamical feature based on the influence of the magnetic
field within the thin disc. We have considered standard mass models for the
luminous mass distribution, a NFW model to describe the dark halo, and we have
added up the contribution to the rotation curve of a magnetic field in the
disc, which is described by an axisymmetric pattern. Our conclusion is that a
significant improvement of the fit in the outer part is obtained when magnetic
effects are considered. The best-fit solution requires an amplitude of ~4
microG with a weak radial dependence between 10 and 38 kpc.",1010.0270v1
2010-10-08,A Lagrangian model for the evolution of turbulent magnetic and passive scalar fields,"In this paper we present an extension of the \emph{Recent Fluid Deformation
(RFD)} closure introduced by Chevillard and Meneveau (2006) which was developed
for modeling the time evolution of Lagrangian fluctuations in incompressible
Navier-Stokes turbulence. We apply the RFD closure to study the evolution of
magnetic and passive scalar fluctuations. This comparison is especially
interesting since the stretching term for the magnetic field and for the
gradient of the passive scalar are similar but differ by a sign such that the
effect of stretching and compression by the turbulent velocity field is
reversed. Probability density functions (PDFs) of magnetic fluctuations and
fluctuations of the gradient of the passive scalar obtained from the RFD
closure are compared against PDFs obtained from direct numerical simulations.",1010.1598v2
2010-10-18,An extension of gas-kinetic BGK Navier-Stokes scheme to multidimensional astrophysical magnetohydrodynamics,"The multidimensional gas-kinetic scheme for the Navier-Stokes equations under
gravitational fields [J. Comput. Phys. 226 (2007) 2003-2027] is extended to
resistive magnetic flows. The non-magnetic part of the magnetohydrodynamics
equations is calculated by a BGK solver modified due to magnetic field. The
magnetic part is treated by the flux splitting method based gas-kinetic theory
[J. Comput. Phys. 153 (1999) 334-352 ], using a particle distribution function
constructed in the BGK solver. To include Lorentz force effects into gas
evolution stage is very important to improve the accuracy of the scheme. For
some multidimensional problems, the deviations tangential to the cell interface
from equilibrium distribution are essential to keep the scheme robust and
accurate. Besides implementation of a TVD time discretization scheme, enhancing
the dynamic dissipation a little bit is a simply and efficient way to stabilize
the calculation. One-dimensional and two-dimensional shock waves tests are
calculated to validate this new scheme. A three-dimensional turbulent
magneto-convection simulation is used to show the applicability of current
scheme to complicated astrophysical flows.",1010.3594v1
2010-11-03,Existence of vertical spin stiffness in Landau-Lifshitz-Gilbert equation in ferromagnetic semiconductors,"We calculate the magnetization torque due to the spin polarization of the
itinerant electrons by deriving the kinetic spin Bloch equations based on the
$s$-$d$ model. We find that the first-order gradient of the magnetization
inhomogeneity gives rise to the current-induced torques, which are consistent
to the previous works. At the second-order gradient, we find an effective
magnetic field perpendicular to the spin stiffness filed. This field is
proportional to the nonadiabatic parameter $\beta$. We show that this vertical
spin stiffness term can significantly modify the domain-wall structure in
ferromagnetic semiconductors and hence should be included in the
Landau-Lifshitz-Gilbert equation in studying the magnetization dynamics.",1011.0871v1
2010-12-13,Cold and ultracold NH--NH collisions in magnetic fields,"Elastic and spin-changing inelastic collision cross sections are presented
for cold and ultracold magnetically trapped NH. The cross sections are obtained
from coupled-channel scattering calculations as a function of energy and
magnetic field. We specifically investigate the influence of the intramolecular
spin-spin, spin-rotation, and intermolecular magnetic dipole coupling on the
collision dynamics. It is shown that $^{15}$NH is a very suitable candidate for
evaporative cooling experiments. The dominant trap-loss mechanism in the
ultracold regime originates from the intermolecular dipolar coupling term. At
higher energies and fields, intramolecular spin-spin coupling becomes
increasingly important. Our qualitative results and conclusions are fairly
independent of the exact form of the potential and of the size of the channel
basis set.",1012.2814v1
2010-12-16,The magneto-ionic medium in the Milky Way,"One way in which the Canadian Galactic Plane Survey has made an important
contribution to the understanding of the Galactic interstellar medium is
through its polarization surveys. Investigation of these data has enabled a big
step in the study of magnetic fields in the interstellar medium and a range of
discrete, extended, interstellar objects. In this review, I will discuss the
role that the magnetic field plays in the interstellar medium, summarizing the
ways in which magnetic field interacts with the other components in the Milky
Way. Magnetic fields in the Galactic halo are discussed, and an outlook to a
number of successor surveys of the polarized CGPS in the near future is given.",1012.3755v1
2010-12-23,Scaling behavior of current- voltage characteristics of Y(1-x)CaxBa2Cu3O(7-δ) polycrystalline samples,"I-V characteristics of polycrystalline Y(1-x)CaxBa2Cu3O(7-{\delta}) samples
(x=0.025 and 0.20) have been measured at different temperatures and magnetic
fields in the range 0.1 T-6.9 T. The scaling behavior has been established for
both samples at all magnetic fields. The dynamic exponent z displays some
morphology dependence with higher value for small grain size sample
Y0.8Ca0.2Ba2Cu3O(7-{\delta}). The static exponent {\nu} has been determined
from {\rho} vs. T dependence at given magnetic field. The critical exponents
are field independent with one only exception ({\nu} - for
Y0.975Ca0.025Ba2Cu3O(7-{\delta}) sample). This is connected with the special
interrelation between the vortex correlation length, {\xi}, and intervortex
spacing {\alpha} ({\xi} \leq {\alpha}) at all magnetic fields above Tg for this
sample and its better pinning.",1012.5267v1
2011-01-06,Confined Magnetic Monopoles in Dense QCD,"Non-Abelian strings exist in the color-flavor locked phase of dense QCD. We
show that kinks appearing in the world-sheet theory on these strings, in the
form of the kink-antikink bound pairs, are the magnetic monopoles ---
descendants of the 't Hooft-Polyakov monopoles surviving in such a special form
in dense QCD. Our consideration is heavily based on analogies and inspiration
coming from certain supersymmetric non-Abelian theories. This is the first ever
analytic demonstration that objects unambiguously identifiable as the magnetic
monopoles are ""native"" to non-Abelian Yang-Mills theories (albeit our analysis
extends only to the phase of the monopole confinement and has nothing to say
about their condensation). Technically our demonstration becomes possible due
to the fact that low-energy dynamics of the non-Abelian strings in dense QCD is
that of the orientational zero modes. It is described by an effective
two-dimensional CP(2) model on the string world sheet. The kinks in this model
representing confined magnetic monopoles are in a highly quantum regime.",1101.1120v1
2011-01-30,Spin transport in magnetically ordered systems: effect of the lattice relaxation time,"Spin resistivity $R$ has been shown to result mainly from the scattering of
itinerant spins with magnetic impurities and lattice spins. $R$ is proportional
to the spin-spin correlation so that its behavior is very complicated near and
at the magnetic phase transition of the lattice spins. For the time being there
are many new experimental data on the spin resistivity going from
semiconductors to superconductors. Depending on materials, various behaviors
have been observed. There is however no theory so far which gives a unified
mechanism for spin resistivity in magnetic materials. Recently, we have showed
Monte Carlo results for different systems. We found that the spin resistivity
is very different from one material to another. In this paper, we show for the
first time how the dynamic relaxation time of the lattice spins affects the
resistivity of itinerant spins observed in Monte Carlo simulation.",1101.5789v1
2011-02-07,Vortex dynamics and irreversibility line in optimally doped SmFeAsO$_{0.8}$F$_{0.2}$ from ac susceptibility and magnetization measurements,"Ac susceptibility and static magnetization measurements were performed in the
optimally doped SmFeAsO$_{0.8}$F$_{0.2}$ superconductor. The field -
temperature phase diagram of the superconducting state was drawn and, in
particular, the features of the flux line lattice derived. The dependence of
the intra-grain depinning energy on the magnetic field intensity was derived in
the thermally-activated flux creep framework, enlightening a typical 1/H
dependence in the high-field regime. The intra-grain critical current density
was extrapolated in the zero temperature and zero magnetic field limit, showing
a remarkably high value $J_{c0}(0) \sim 2 \cdot 10^{7}$ A/cm$^{2}$, which
demonstrates that this material is rather interesting for the potential future
technological applications.",1102.1404v1
2011-02-10,"Hour-glass magnetic spectrum in an insulating, hole-doped antiferromagnet","Superconductivity in layered copper-oxide compounds emerges when charge
carriers are added to antiferromagnetically-ordered CuO2 layers. The carriers
destroy the antiferromagnetic order, but strong spin fluctuations persist
throughout the superconducting phase and are intimately linked to
super-conductivity. Neutron scattering measurements of spin fluctuations in
hole-doped copper oxides have revealed an unusual `hour-glass' feature in the
momentum-resolved magnetic spectrum, present in a wide range of superconducting
and non-superconducting materials. There is no widely-accepted explanation for
this feature. One possibility is that it derives from a pattern of alternating
spin and charge stripes, an idea supported by measurements on stripe-ordered
La1.875Ba0.125CuO4. However, many copper oxides without stripe order also
exhibit an hour-glass spectrum$. Here we report the observation of an
hour-glass magnetic spectrum in a hole-doped antiferromagnet from outside the
family of superconducting copper oxides. Our system has stripe correlations and
is an insulator, which means its magnetic dynamics can conclusively be ascribed
to stripes. The results provide compelling evidence that the hour-glass
spectrum in the copper-oxide superconductors arises from fluctuating stripes.",1102.2205v1
2011-02-13,Kagome Approximation for 3He on Husimi Lattice with Two- and Three-Site Exchange Interactions,"The Ising approximation of the Heisenberg model in a strong magnetic field,
with two-, and three-spin exchange interactions are studied on a Husimi
lattice. This model can be considered as an approximation of the third layer of
3He absorbed on the surface of graphite (kagome lattice). Using dynamic
approach we have found exact recursion relation for the partition function. For
different values of exchange parameters and temperature the diagrams of
magnetization are plotted and showed that magnetization properties of the model
vary from ferromagnetic to antiferromagnetic depending from the value of model
parameters. For antiferromagnetic case magnetization plateau at 1/3 of
saturation field is obtained. Lyapunov exponent for recursion relation are
considered and showed absents of bifurcation points in thermodynamic limit. The
Yang-Lee zeros are analyzed in terms of neutral fixed points and showed that
Yang-Lee zeros of the model are located on the arcs of the circle with the
radius R=1.",1102.2607v1
2011-02-17,Theory of flux cutting and flux transport at the critical current of a type-II superconducting cylindrical wire,"I introduce a critical-state theory incorporating both flux cutting and flux
transport to calculate the magnetic-field and current-density distributions
inside a type-II superconducting cylinder at its critical current in a
longitudinal applied magnetic field. The theory is an extension of the elliptic
critical-state model introduced by Romero-Salazar and Perez-Rodriguez. The
vortex dynamics depend in detail upon two nonlinear effective resistivities for
flux cutting (\rho_\parallel) and flux flow (\rho_\perp), and their ratio r =
\rho_\parallel/\rho_\perp. When r < 1, the low relative efficiency of flux
cutting in reducing the magnitude of the internal magnetic-flux density leads
to a paramagnetic longitudinal magnetic moment. As a model for understanding
the experimentally observed interrelationship between the critical currents for
flux cutting and depinning, I calculate the forces on a helical vortex arc
stretched between two pinning centers when the vortex is subjected to a current
density of arbitrary angle \phi. Simultaneous initiation of flux cutting and
flux transport occurs at the critical current density J_c(\phi) that makes the
vortex arc unstable.",1102.3678v1
2011-02-25,MHD turbulence-Star Formation Connection: from pc to kpc scales,"The transport of magnetic flux to outside of collapsing molecular clouds is a
required step to allow the formation of stars. Although ambipolar diffusion is
often regarded as a key mechanism for that, it has been recently argued that it
may not be efficient enough. In this review, we discuss the role that MHD
turbulence plays in the transport of magnetic flux in star forming flows. In
particular, based on recent advances in the theory of fast magnetic
reconnection in turbulent flows, we will show results of three-dimensional
numerical simulations that indicate that the diffusion of magnetic field
induced by turbulent reconnection can be a very efficient mechanism, especially
in the early stages of cloud collapse and star formation. To conclude, we will
also briefly discuss the turbulence-star formation connection and feedback in
different astrophysical environments: from galactic to cluster of galaxy
scales.",1102.5140v1
2011-03-05,Dynamic and thermodynamic properties of the generalised diamond chain model for azurite,"The natural mineral azurite Cu3(CO3)2(OH)2 is an interesting spin-1/2 quantum
antiferromagnet. Recently, a generalised diamond chain model has been
established as a good description of the magnetic properties of azurite with
parameters placing it in a highly frustrated parameter regime. Here we explore
further properties of this model for azurite. First, we determine the inelastic
neutron scattering spectrum in the absence of a magnetic field and find good
agreement with experiments, thus lending further support to the model.
Furthermore, we present numerical data for the magnetocaloric effect and
predict that strong cooling should be observed during adiabatic
(de)magnetisation of azurite in magnetic fields slightly above 30T. Finally,
the presence of a dominant dimer interaction in azurite suggests the use of
effective Hamiltonians for an effective low-energy description and we propose
that such an approach may be useful to fully account for the three-dimensional
coupling geometry.",1103.1080v1
2011-03-18,Cotunneling theory of inelastic STM spin spectroscopy,"We propose cotunneling as the microscopic mechanism that makes possible
inelastic electron spectroscopy of magnetic atoms in surfaces for a wide range
of systems, including single magnetic adatoms, molecules and molecular stacks.
We describe electronic transport between the scanning tip and the conducting
surface through the magnetic system (MS) with a generalized Anderson model,
without making use of effective spin models. Transport and spin dynamics are
described with an effective cotunneling Hamiltonian in which the correlations
in the magnetic system are calculated exactly and the coupling to the
electrodes is included up to second order in the tip-MS and MS-substrate. In
the adequate limit our approach is equivalent to the phenomenological Kondo
exchange model that successfully describe the experiments . We apply our method
to study in detail inelastic transport in two systems, stacks of Cobalt
Phthalocyanines and a single Mn atom on Cu$_2$N. Our method accounts both, for
the large contribution of the inelastic spin exchange events to the conductance
and the observed conductance asymmetry.",1103.3676v1
2011-03-22,Inductive determination of the optimum tunnel barrier thickness in magnetic tunnelling junction stacks for spin torque memory applications,"We use pulsed inductive microwave magnetometry to study the precessional
magnetization dynamics of the free layer in CoFeB/MgO/CoFeB based magnetic
tunnelling junction stacks with varying MgO barrier thickness. From the field
dependence of the precession frequency we are able to derive the uniaxial
anisotropy energy and the exchange coupling between the free and the pinned
layer. Furthermore the field dependence of the effective damping parameter is
derived. Below a certain threshold barrier thickness we observe an increased
effective damping for antiparallel orientation of free and pinned layer which
would inhibit reversible low current density spin torque magnetization
reversal. Such inductive measurements, in combination with wafer probe station
based magneto transport experiments, allow a fast determination of the optimum
tunnel barrier thickness range for spin torque memory applications in a
lithography free process.",1103.4248v1
2011-03-30,Numerical simulations of high Lundquist number relativistic magnetic reconnection,"We present the results of two-dimensional and three-dimensional
magnetohydrodynamical numerical simulations of relativistic magnetic
reconnection, with particular emphasis on the dynamics of the plasma in a
Petschek-type configuration with high Lundquist numbers, S\sim 10^5-10^8. The
numerical scheme adopted, allowing for unprecedented accuracy for this type of
calculations, is based on high order finite volume and discontinuous Galerkin
methods as recently proposed by \citet{Dumbser2009}. The possibility of
producing high Lorentz factors is discussed, showing that Lorentz factors close
to \sim 4 can be produced for a plasma parameter \sigma_m=20. Moreover, we find
that the Sweet-Parker layers are unstable, generating secondary magnetic
islands, but only for S > S_c = 10^8, much larger than what is reported in the
Newtonian regime. Finally, the effects of a mildly anisotropic Ohm law are
considered in a configuration with a guide magnetic field. Such effects produce
only slightly faster reconnection rates and Lorentz factors of about 1% larger
with respect to the perfectly isotropic Ohm law.",1103.5924v2
2011-03-30,Intermittent heating in the solar corona employing a 3D MHD model,"We investigate the spatial and temporal evolution of the heating of the
corona of a cool star such as our Sun in a three-dimensional
magneto-hydrodynamic (3D MHD) model. We solve the 3D MHD problem numerically in
a box representing part of the (solar) corona. The energy balance includes
Spitzer heat conduction along the magnetic field and optically thin radiative
losses. The self-consistent heating mechanism is based on the braiding of
magnetic field lines rooted in the convective photosphere. Magnetic stress
induced by photospheric motions leads to currents in the atmosphere which heat
the corona through Ohmic dissipation. While the horizontally averaged
quantities, such as heating rate, temperature or density, are relatively
constant in time, the simulated corona is highly variable and dynamic, on
average reaching temperatures and densities as found in observations. The
strongest heating per particle is found in the transition region from the
chromosphere to the corona. The heating is concentrated in current sheets
roughly aligned with the magnetic field and is transient in time and space.
This supports the idea that numerous small heating events heat the corona,
often referred to a nanoflares.",1103.6042v2
2011-03-31,On the two-dimensionalization of quasistatic magnetohydrodynamic turbulence,"We analyze the anisotropy of turbulence in an electrically conducting fluid
in the presence of a uniform magnetic field, for low magnetic Reynolds number,
using the quasi-static approximation. In the linear limit, the kinetic energy
of velocity components normal to the magnetic field decays faster than the
kinetic energy of component along the magnetic field [Moffatt, JFM 28, 1967].
However, numerous numerical studies predict a different behaviour, wherein the
final state is characterized by dominant horizontal energy. We investigate the
corresponding nonlinear phenomenon using Direct Numerical Simulations. The
initial temporal evolution of the decaying flow indicates that the turbulence
is very similar to the so-called ""two-and-a-half-dimensional"" flow [Montgomery
& Turner, Phys. Fluids 25(2), 1982] and we offer an explanation for the
dominance of horizontal kinetic energy.",1103.6207v1
2011-04-26,A multi megawatt ring cyclotron to search for CP violation in the neutrino sector,"A new approach to search for CP violation in the neutrino sector [1,2] is
proposed by the experiment called DAE{\delta}ALUS (Decay At rest Experiment for
{\delta}cp At Laboratory for Underground Science). DAE{\delta}ALUS needs three
sources of neutrino fluxes, each one located at 1.5, 8 and 20 km from the
underground detector. Here we present the study for a Superconducting Ring
Cyclotron able to accelerate the H2+ molecules and to deliver proton beam with
maximum energy of 800 MeV and the required high power. The magnetic field
produced by the proposed superconducting magnetic sector, simulated by the code
TOSCA, the isochronous magnetic field, some preliminary feature on the beam
dynamic and the magnetic forces acting on the coils are here presented.",1104.4985v1
2011-05-09,Ground state structures of superparamagnetic 2D dusty plasma crystals,"Ground state structures of finite, cylindrically confined two-dimensional
Yukawa systems composed of charged superparamagnetic dust grains in an external
magnetic field are investigated numerically, using molecular dynamic
simulations and lattice summation methods. The ground state configuration of
the system is identified using, as an approximation, the experimentally
obtained shape of the horizontal confinement potential in a classical single
layer dusty plasma experiment with non-magnetic grains. Results are presented
for the dependence of the number density and lattice parameters of the dust
layer on (1) the ratio of the magnetic dipole-dipole force to electrostatic
force between the grains and (2) the orientation of the grain magnetic moment
with respect to the layer.",1105.1664v1
2011-05-18,InMnO3 : a fully frustrated multiferroic,"InMnO$_3$ is a peculiar member of the hexagonal manganites h-RMnO$_3$ (where
R is a rare earth metal element), showing crystalline, electronic and magnetic
properties at variance with the other compounds of the family. We have studied
high quality samples synthesized at high pressure and temperature by powder
neutron diffraction. The position of the Mn ions is found to be close to the
threshold $\it{x}=1/3$ where superexchange Mn-Mn interactions along the
$\it{c}$ axis compensate. Magnetic long range order occurs below $T_{\rm N}$=
120(2) K with a magnetic unit cell doubled along $\it{c}$, whereas short range
two dimensional dynamical spin correlations are observed above $T_{\rm N}$. We
propose that pseudo-dipolar interactions are responsible for the long period
magnetic structure.",1105.3640v1
2011-05-19,Charged Magnetic Brane Correlators and Twisted Virasoro Algebras,"Prior work using gauge/gravity duality has established the existence of a
quantum critical point in the phase diagram of 3+1-dimensional gauge theories
at finite charge density and background magnetic field. The critical theory,
obtained by tuning the dimensionless charge density to magnetic field ratio,
exhibits nontrivial scaling in its thermodynamic properties, and an associated
nontrivial dynamical critical exponent. In the present work, we analytically
compute low energy correlation functions in the background of the charged
magnetic brane solution to 4+1-dimensional Einstein-Maxwell-Chern-Simons
theory, which represents the bulk description of the critical point. Results
are obtained for neutral scalar operators, the stress tensor, and the
U(1)-current. The theory is found to exhibit a twisted Virasoro algebra,
constructed from a linear combination of the original stress tensor and chiral
U(1)-current. The effective speed of light in the IR is renormalized downward
for one chirality, but not the other, by finite density, a behavior that is
consistent with a Luttinger liquid description of fermions in the lowest Landau
level. The results obtained here do not directly shed light on the mechanism
driving the phase transition, and we comment on why this is so.",1105.3998v1
2011-06-01,Remarkable paramagnetic features of Fermi-Dirac plasmas,"In this paper by using the relativistic magnetic susceptibility of a
Fermi-Dirac (relativistically degenerate) plasma, quantum magnetohydrodynamics
(QMHD) model is used to investigate the propagation of spin-induced (SI)
magnetosonic nonlinear excitations in a normally and relativistically
degenerate dense electron-ion plasma in the presence of the spin magnetization
effect. Based on the conventional pseudopotential method the matching criterion
for the evolution of SI solitary structures is evaluated. It is found that, the
plasma mass density and strength of the magnetic field have significant effects
on excitation and evolution of magnetosonic nonlinear structures in Fermi-Dirac
plasmas. Only rarefactive SI magnetosonic solitary structures are found to
excite in such plasmas. Furthermore, fundamental differences are shown to exist
in magnetosonic soliton dynamics in the two distinct plasma degeneracy regimes,
which is due to interplay between the negative spin paramagnetism pressure-like
and positive relativistic degeneracy pressure of electrons. Current
investigation can help better understand the electron spin effects on nonlinear
wave propagations in strongly magnetized dense astrophysical objects such as
white dwarfs and pulsar magnetospheres.",1106.0226v1
2011-06-01,Instabilities in magnetized spherical Couette flow,"We report 3D numerical simulations of the flow of an electrically conducting
fluid in a spherical shell when a magnetic field is applied. Different
spherical Couette configurations are investigated, by varying the rotation
ratio between the inner and the outer sphere, the geometry of the imposed
field, and the magnetic boundary conditions on the inner sphere. Either a
Stewartson layer or a Shercliff layer, accompanied by a radial jet, can be
generated depending on the rotation speeds and the magnetic field strength, and
various non-axisymmetric destabilizations of the flow are observed. We show
that instabilities arising from the presence of boundaries present striking
similarities with the magnetorotational instability (MRI). To this end, we
compare our numerical results to experimental observations of the Maryland
experiment, who claimed to observe MRI in a similar setup.",1106.0319v1
2011-06-08,Relating field-induced shift in transition temperature to the kinetics of coexisting phases in magnetic shape memory alloys,"In a magnetic shape memory alloy system, we vary composition following
phenomenological arguments to tune macroscopic properties. We achieve
significantly higher shift in austenite to martensitic phase transition
temperature with magnetic field. This enhancement is accompanied by significant
broadening of the transition and by field-induced arrest of kinetics, both of
which are related to the dynamics of the coexisting phases. This reveals
hitherto unknown interrelationship between different length-scales. This may
serve as an effective route for comprehensive understanding of similar
multicomponent systems which show considerable variation in physical properties
by minor change in microscopic parameters.",1106.1532v1
2011-06-14,A time-dependent model for magnetic reconnection in the presence of a separator,"We present a model for separator reconnection due to an isolated reconnection
process. Separator reconnection is a process which occurs in the neighbourhood
of a distinguished field line (the separator) connecting two null points of a
magnetic field. It is, for example, important for the dynamics of magnetic flux
at the dayside magnetopause and in the solar corona. We find that, above a
certain threshold, such a reconnection process generates new separators which
leads to a complex system of magnetic flux tubes connecting regions of
previously separated flux. Our findings are consistent with the findings of
large numbers of separators in numerical simulations. We discuss how to measure
and interpret the reconnection rate in a configuration with multiple
separators.",1106.2672v3
2011-06-17,Quantum magneto-optics of graphite with trigonal warping,"The optical conductivity of graphite in quantizing magnetic fields is
studied. Both the dynamical conductivities, longitudinal as well as Hall's, are
analytically evaluated. The conductivity peaks are explained in terms of
electron transitions. We have shown that the trigonal warping in graphite can
be considered within the perturbation theory at the strong magnetic field
larger than 1 T approximately. The main optical transitions obey the selection
rule with $\Delta n=1$ for the Landau number $n$, however the $\Delta n=2$
transitions due to the trigonal warping with the small probability are also
essential. The Kerr rotation and reflectivity in graphite in the quantizing
magnetic fields are calculated. Parameters of the Slonczewski--Weiss--McClure
model are used in the fit taking into account the previous dHvA measurements
and correcting some of them for the case of the strong magnetic fields.",1106.3401v2
2011-06-21,Velocity and Magnetic Transients Driven by the X2.2 White-Light Flare of 2011 February 15 in NOAA 11158,"The first X-class flare of the current solar cycle 24 occurred in Active
Region NOAA 11158 during its central meridian passage on 2011 February 15. This
two ribbon white-light flare was well observed by the Helioseismic and Magnetic
Imager (HMI) on board Solar Dynamics Observatory (SDO). During the peak phase
of the flare, we detected magnetic and Doppler velocity transients appearing
near the umbral boundary of the main sunspot. These transients persisted for a
few minutes and showed spatial and temporal correspondence with the flare
kernels. The observed magnetic polarity at the transients' locations underwent
sign reversal, together with large enhancement in Doppler velocities. We
explain this observational phenomena using the HMI spectral data obtained
before, during and after the flare. These changes were reflected in the maps of
the AR in all the Stokes parameters. Association of the transient features with
various signatures of the flare and the cause and effects of their appearance
are also presented on the basis of present theoretical models.",1106.4166v3
2011-06-22,Tunable Magnonic Frequency and Damping in [Co/Pd]8 Multilayers with Variable Co Layer Thickness,"We report the experimental observation of collective picosecond magnetization
dynamics in [Co/Pd]8 multilayers with perpendicular magnetic anisotropy. The
precession frequency shows large and systematic variation from about 5 GHz to
about 90 GHz with the decrease in the Co layer thickness from 1.0 nm to 0.22 nm
due to the linear increase in the perpendicular magnetic anisotropy. The
damping coefficient 'alpha' is found to be inversely proportional to the Co
layer thickness and a linear relation between the perpendicular magnetic
anisotropy and 'alpha' is established. We discuss the possible reasons behind
the enhanced damping as the d-d hybridization at the interface and spin
pumping. These observations are significant for the applications of these
materials in spintronics and magnonic crystals.",1106.4491v1
2011-06-23,The scale-free texture of the fast solar wind,"The higher order statistics of magnetic field magnitude fluctuations in the
fast quiet solar wind are quantified systematically scale-by-scale for the
first time. We find a single global non-Gaussian scale free behaviour from
minutes to over 5 hours. This spans the signature of an inertial range of
magnetohydrodynamic (MHD) turbulence and a ~1/f range in magnetic field
components. This global scaling in field magnitude fluctuations is an intrinsic
component of the underlying texture of the solar wind and it suggests a single
stochastic process for magnetic field magnitude fluctuations operating across
the full range of MHD time scales supported by the solar wind. Intriguingly,
the magnetic field and velocity components show scale dependent dynamic
alignment outside of the inertial range.",1106.4705v2
2011-07-02,Energy dissipation and switching delay in spin-transfer torque switching of nanomagnets with low-saturation magnetization in the presence of thermal fluctuations,"A common ploy to reduce the switching current and energy dissipation in
spin-transfer-torque driven magnetization switching of shape-anisotropic
single-domain nanomagnets is to employ magnets with low saturation
magnetization $M_s$ and high shape-anisotropy. The high shape-anisotropy
compensates for low $M_s$ to keep the static switching error rate constant.
However, this ploy increases the switching delay, its variance in the presence
of thermal noise, and the dynamic switching error rate. Using the stochastic
Landau-Lifshitz-Gilbert equation with a random torque emulating thermal noise,
we show that pumping some excess spin-polarized current into the nanomagnet
during switching will keep the mean switching delay and its variance constant
as we reduce $M_s$, while still reducing the energy dissipation significantly.",1107.0387v2
2011-07-04,Influence of randomness and retardation on the FMR-linewidth,"The theory predicts that the spin-wave lifetime $\tau_L$ and the linewidth of
ferromagnetic resonance $\Delta B$ can be governed by random fields and spatial
memory. To that aim the effective field around which the magnetic moments
perform a precession is superimposed by a stochastic time dependent magnetic
field with finite correlation time. The magnetization dynamics is altered by
inclusion of a spatial memory effect monitoring a non-local interaction of size
$\xi$. The underlying Landau-Lifshitz-Gilbert equation (LLG) is modified
accordingly. The stochastic LLG is equivalent to a Fokker-Planck equation which
enables to calculate the mean values of the magnetization vector. Within the
spin-wave approximation we present an analytical solution for the excitation
energy and its damping. The lifetime and the linewidth are analyzed depending
on the strength of the random field $D$ and its correlation time $\tau_c$ as
well as the retardation strength $\Gamma_0$ and the size $\xi$. Whereas
$\tau_L$ decreases with increasing $D$, retardation strength $\Gamma_0$ and
$\tau_c$, the lifetime is enhanced for growing width $\xi$ of the spatial
retardation kernel. In the same manner we calculate the experimentally
measurable linewidth $\Delta B$ is increased strongly when the correlation time
$\tau_c$ ranges in the nanosecond interval.",1107.0638v1
2011-07-08,Time-resolved study of the magnetic field effects on electroluminescence in tri-(8-hydroxyquinoline)- aluminum based organic light emitting devices,"We investigated the magnetic field effects (MFEs) in organic light-emitting
diodes (OLEDs) through the transient electroluminescence (EL) method. The
time-resolved MFEs on the emission were obtained for the first time, which
would be a useful method to clarify the underlying mechanisms of the MFEs. The
fluorescent dye doped tri-(8-hydroxyquinoline)-aluminum (Alq3) based OLEDs were
fabricated. Then, the transient EL was measured both with and without a
magnetic field. To explore the time-resolved MFEs on the emission of the
device, the excitons population dynamics in the device have been analyzed by a
kinetic model. Our results suggest that both the intersystem crossing between
the singlet and triplet electron-hole pairs and the triplet-triplet
annihilation perturbed by the external magnetic field cause the time-resolved
MFEs.",1107.1550v1
2011-07-17,Spin current induced magnetization oscillations in a paramagnetic disc,"When electron spins are injected uniformly into a paramagnetic disc, they can
precess along the demagnetizing field induced by the resulting magnetic moment.
Normally this precession damps out by virtue of the spin relaxation which is
present in paramagnetic materials. We propose a new mechanism to excite a
steady-state form of this dynamics by injecting a constant spin current into
this paramagnetic disc. We show that the rotating magnetic field generated by
the eddy currents provide a torque which makes this possible. Unlike the
ferromagnetic equivalent, the spin-torque-oscillator, the oscillation frequency
is fixed and determined by the dimensions and intrinsic parameters of the
paramagnet. The system possesses an intrinsic threshold for spin injection
which needs to be overcome before steady-state precession is possible. The
additional application of a magnetic field lowers this threshold. We discuss
the feasibility of this effect in modern materials. Transient analysis using
pump-probe techniques should give insight in the physical processes which
accompany this effect.",1107.3288v1
2011-07-27,Debye-Hückel theory for spin ice at low temperature,"At low temperatures, spin ice is populated by a finite density of magnetic
monopoles-pointlike topological defects with a mutual magnetic Coulomb
interaction. We discuss the properties of the resulting magnetic Coulomb liquid
in the framework of Debye H\""{u}ckel theory, for which we provide a detailed
context-specific account. We discuss both thermodynamical and dynamical
signatures, and compare Debye H\""{u}ckel theory to experiment as well as
numerics, including data for specific heat and AC susceptibility. We also
evaluate the entropic Coulomb interaction which is present in addition to the
magnetic one and show that it is quantitatively unimportant in the current
compounds. Finally, we address the role of bound monopole anti-monopole pairs
and derive an expression for the monopole mobility.",1107.5482v2
2011-08-18,Relativistic quantum dynamics of a charged particle in cosmic string spacetime in the presence of magnetic field and scalar potential,"In this paper we analyze the relativistic quantum motion of charged spin-0
and spin-1/2 particles in the presence of a uniform magnetic field and scalar
potentials in the cosmic string spacetime. In order to develop this analysis,
we assume that the magnetic field is parallel to the string and the scalar
potentials present a cylindrical symmetry with their center on the string. Two
distinct configurations for the scalar potential, $S(r)$, are considered: $(i)$
the potential proportional to the inverse of the polar distance, i.e.,
$S\propto1/r$, and $(ii)$ the potential proportional to this distance, i.e.,
$S\propto r$. The energy spectra are explicitly computed for different physical
situations and presented their dependences on the magnetic field strength and
scalar coupling constants.",1108.3786v2
2011-09-09,On the 2D behavior of 3D MHD with a strong guiding field,"The Magneto-hydrodynamic (MHD) equations in the presence of a guiding
magnetic field are investigated by means of direct numerical simulations. The
basis of the investigation consists of 9 runs forced at the small scales. The
results demonstrate that for a large enough uniform magnetic field the large
scale flow behaves as a two dimensional (non-MHD) fluid exhibiting an inverse
cascade of energy in the direction perpendicular to the magnetic field, while
the small scales behave like a three dimensional MHD-fluid cascading the energy
forwards. The amplitude of the inverse cascade is sensitive to the magnetic
field amplitude, the domain size, the forcing mechanism, and the forcing scale.
All these dependencies are demonstrated by the varying parameters of
simulations. Furthermore, in the case that the system is forced anisotropically
in the small parallel scales an inverse cascade in the parallel direction is
observed that is feeding the 2D modes.",1109.2039v1
2011-09-12,Microfluidic propulsion by the metachronal beating of magnetic artificial cilia: a numerical analysis,"In this work we study the effect of metachronal waves on the flow created by
magnetically-driven plate-like artificial cilia in microchannels using
numerical simulations. The simulations are performed using a coupled
magneto-mechanical solid-fluid computational model that captures the physical
interactions between the fluid flow, ciliary deformation and applied magnetic
field. When a rotating magnetic field is applied to super-paramagnetic
artificial cilia, they mimic the asymmetric motion of natural cilia, consisting
of an effective and recovery stroke. When a phase-difference is prescribed
between neighbouring cilia, metachronal waves develop. Due to the discrete
nature of the cilia, the metachronal waves change direction when the phase
difference becomes sufficiently large, resulting in antiplectic as well as
symplectic metachrony. We show that the fluid flow created by the artificial
cilia is significantly enhanced in the presence of metachronal waves and that
the fluid flow becomes unidirectional. Antiplectic metachrony is observed to
lead to a considerable enhancement in flow compared to symplectic metachrony,
when the cilia spacing is small. Obstruction of flow in the direction of the
effective stroke for the case of symplectic metachrony was found to be the key
mechanism that governs this effect.",1109.2547v1
2011-09-13,Thickness-dependent magnetic structure of ultrathin Fe/Ir(001) films: from spin-spiral states towards ferromagnetic order,"We present a detailed study of the ground-state magnetic structure of
ultrathin Fe films on the surface of fcc Ir(001). We use the spin-cluster
expansion technique in combination with the relativistic disordered local
moment scheme to obtain parameters of spin models and then determine the
favored magnetic structure of the system by means of a mean field approach and
atomistic spin dynamics simulations. For the case of a single monolayer of Fe
we find that layer relaxations very strongly influence the ground-state spin
configurations, whereas Dzyaloshinskii-Moriya (DM) interactions and biquadratic
couplings also have remarkable effects. To characterize the latter effect we
introduce and analyze spin collinearity maps of the system. While for two
monolayers of Fe we find a single-q spin spiral as ground state due to DM
interactions, for the case of four monolayers the system shows a noncollinear
spin structure with nonzero net magnetization. These findings are consistent
with experimental measurements indicating ferromagnetic order in films of four
monolayers and thicker.",1109.2721v1
2011-09-14,Antiferromagnetic coupling across silicon regulated by tunneling currents,"We report on the enhancement of antiferromagnetic coupling in epitaxial
Fe/Si/Fe structures by voltage-driven spin-polarized tunneling currents. Using
the ballistic electron magnetic microscopy we established that the hot-electron
collector current reflects magnetization alignment and the magnetocurrent
exceeds 200% at room temperature. The saturation magnetic field for collector
current corresponding to parallel alignment of magnetizations rises up with the
tunneling current, thus demonstrating stabilization of the antiparallel
alignment and increasing antiferromagnetic coupling. We connect the enhancement
of antiferromagnetic coupling with local dynamic spin torques mediated by
tunneling electrons",1109.3106v1
2011-09-16,An MHD Model of the December 13 2006 Eruptive Flare,"We present a 3D MHD simulation that qualitatively models the coronal mag-
netic field evolution associated with the eruptive flare that occurred on
December 13, 2006 in the emerging {\delta}-sunspot region NOAA 10930 observed
by the Hinode satellite. The simulation is set up where we drive the emergence
of an east-west oriented magnetic flux rope at the lower boundary into a
pre-existing coronal field constructed from the SOHO/MDI full-disk magnetogram
at 20:51:01 UT on December 12, 2006. The resulting coronal flux rope embedded
in the ambi- ent coronal magnetic field first settles into a stage of
quasi-static rise, and then undergoes a dynamic eruption, with the leading edge
of the flux rope cavity accel- erating to a steady speed of about 830 km/s. The
pre-eruption coronal magnetic field shows morphology that is in qualitative
agreement with that seen in the Hinode soft X-ray observation in both the
magnetic connectivity as well as the development of an inverse-S shaped X-ray
sigmoid. We examine the properties of the erupting flux rope and the morphology
of the post-reconnection loops, and compare them with the observations.",1109.3734v1
2011-09-21,Itinerant and local-moment magnetism in strongly correlated electron systems,"Detailed analysis of the magnetic properties of the Hubbard model within
dynamical mean-field theory (DMFT) is presented. Using a RPA-like decoupling of
two-particle propagators we derive a universal form for susceptibilities, which
captures essential aspects of localized and itinerant pictures. This expression
is shown to be quantitatively valid whenever long-range coherence of
particle-hole excitations can be neglected, as is the case in large parts of
the phase diagram where antiferromag- netism is dominant. The applicability of
an interpretation in terms of the two archetypical pictures of magnetism is
investigated for the Hubbard model on a body-centered cubic lattice with
additional next-nearest neighbor hopping t'. For large values of the Coulomb
interaction, local-moment mag- netism is found to be dominant, while for weakly
interacting band electrons itinerant quasiparticle magnetism prevails. In the
intermediate regime and for finite t' an re-entrant behavior is discovered,
where antiferromagnetism only exists in a finite temperature interval.",1109.4509v2
2011-09-23,Ubiquitous rotating network magnetic fields and EUV cyclones in the quiet Sun,"We present the {\it Solar Dynamics Observatory} (SDO) Atmospheric Imaging
Assembly (AIA) observations of EUV cyclones in the quiet Sun. These cyclones
are rooted in the Rotating Network magnetic Fields (RNFs). Such cyclones can
last several to more than ten hours, and, at the later phase, they are found to
be associated with EUV brightenings (microflares) and even EUV waves. SDO
Helioseismic and Magnetic Imager (HMI) observations show an ubiquitous presence
of the RNFs. Using HMI line-of-sight magnetograms on 2010 July 8, we find 388
RNFs in an area of 800$\times$980 square arcseconds near the disk center where
no active region is present. The sense of rotation shows a weak hemisphere
preference. The unsigned magnetic flux of the RNFs is about 4.0$\times10^{21}$
Mx, or 78% of the total network flux. These observational phenomena at small
scale reported in this letter are consistent with those at large scale in
active regions. The ubiquitous RNFs and EUV cyclones over the quiet Sun may
suggest an effective way to heat the corona.",1109.5003v1
2011-09-23,Chiral Magnetic Effect and evolution of electromagnetic field,"The energy dependence of observable two particle correlator in search for the
local strong parity violation in Au+Au collisions is estimated within a simple
phenomenological model. The model reproduces available RHIC data but at LHC
predicts that the Chiral Magnetic effect (CME) will be about 20 times weaker
than at RHIC, contrary to the first LHC measurements. In the lower energy range
this effect should vanish sharply at energy somewhere above the top SPS one
with agreement with the preliminary results of the Beam Energy Scan program. To
elucidate CME background effects a transport HSD model including magnetic field
evolution is put forward and electromagnetic dynamics at the RHIC energy is
investigated. It is observed that the electromagnetic field included into the
hadronic model does not influence on observables due to mutual compensation of
effects of electric and magnetic fields.",1109.5015v3
2011-10-10,Magnetic order and frustrated dynamics in Li(Ni0.8Co0.1Mn0.1)O2: a study by μ+SR and SQUID magnetometry,"Recently, the mixed transition metal oxides of the form Li(Ni1-y-zCoyMnz)O2,
have become the center of attention as promising candidates for novel battery
material. These materials have also revealed very interesting magnetic
properties due to the alternate stacking of planes of metal oxides on a 2D
triangular lattice and the Li-layers. The title compound,
Li(Ni0.8Co0.1Mn0.1)O2, has been investigated by both magnetometry and
measurements and {\mu}+SR. We find the evolution of localized magnetic moments
with decreasing temperature below 70 K. The magnetic ground state (T = 2 K) is,
however, shown to be a frustrated system in 3D, followed by a transition into a
possible 2D spinglass above 22 K. With further increasing temperature the
compound show the presence of remaining correlations with increasing effective
dimensionality all the way up to the ferrimagnetic transition at TC = 70 K.",1110.2071v1
2011-10-10,Magnetic order and transitions in the spin-web compound Cu3TeO6,"The spin-web compound Cu3TeO6, belongs to an intriguing group of materials
where magnetism is governed by 3d9 copper Cu2+ ions. This compound has been
sparsely experimentally studied and we here present the first investigation of
its local magnetic properties using muon-spin relaxation/rotation ({\mu}+SR).
Our results show a clear long-range 3D magnetic order below TN as indicated by
clear zero-field (ZF) muon-precessions. At TN = 61.7 K a very sharp transition
is observed in the weak transverse-field (wTF) as well as ZF data. Contrary to
suggestions by susceptibility measurements and inelastic neutron scattering, we
find no evidence for either static or dynamic (on the time-scale of {\mu}+SR)
spin-correlations above TN.",1110.2073v1
2011-10-12,Spin supercurrent in Josephson contacts with noncollinear ferromagnets,"We present a theoretical study of the Josephson coupling of two s-wave
superconductors which are connected through a diffusive contact consisting of
noncollinear ferromagnetic domains. First, we consider a contact with two
domains with magnetization vectors misoriented by an angle $\theta$. Using the
quantum circuit theory, we find that in addition to the charge supercurrent, a
spin supercurrent, which is even in $\phi$ and odd in $\theta$, with a spin
polarization normal to the magnetization vectors flows between the domains.
Furthermore, with asymmetric insulating barriers at the interfaces of the
junction, the system may experience an antiferromagnetic-ferromagnetic phase
transition for $\phi=\pi$. Secondly, we discuss the spin supercurrent in an
extended magnetic texture with multiple domainwalls. We find the
position-dependent spin supercurrent. The magnitude of the spin supercurrent
strongly depends on the phase difference between the superconductors and the
number of domain walls. Our results demonstrate the possibility to couple the
superconducting phase to the magnetization dynamics.",1110.2568v1
2011-10-19,Current-induced switching in transport through anisotropic magnetic molecules,"Anisotropic single-molecule magnets may be thought of as molecular switches,
with possible applications to molecular spintronics. In this paper, we consider
current-induced switching in single-molecule junctions containing an
anisotropic magnetic molecule. We assume that the carriers interact with the
magnetic molecule through the exchange interaction and focus on the regime of
high currents in which the molecular spin dynamics is slow compared to the time
which the electrons spend on the molecule. In this limit, the molecular spin
obeys a non-equilibrium Langevin equation which takes the form of a generalized
Landau-Lifshitz-Gilbert equation and which we derive microscopically by means
of a non-equilibrium Born-Oppenheimer approximation. We exploit this Langevin
equation to identify the relevant switching mechanisms and to derive the
current-induced switching rates. As a byproduct, we also derive S-matrix
expressions for the various torques entering into the Landau-Lifshitz-Gilbert
equation which generalize previous expressions in the literature to
non-equilibrium situations.",1110.4270v2
2011-11-08,Analysis of the Collective Behavior of a 10 by 10 Array of Fe3O4 Dots in a Large Micromagnetic Simulation,"We report a full (3D) micromagnetic simulation of a set of 100 ferrite
(Fe$_3$O$_4$) cylindrical dots, arranged in a 10 by 10 square (planar) array of
side 3.27 $\mu$m, excited by an external in-plane magnetic field. The resulting
power spectrum of magnetic excitations and the dynamical magnetization field at
the resulting resonance modes were investigated. The absorption spectrum
deviates considerably from that of a single particle reference simulation,
presenting a mode-shifting and splitting effect. We found an inversion symmetry
through the center of the array, in the sense that each particle and its
inversion counterpart share approximately the same magnetization mode behavior.
Magnonic designs aiming at synchronous or coherent tunings of spin-wave
excitations at given spatially separated points within a regular square array
may benefit from the new effects here described.",1111.1866v1
2011-11-18,Charge and Spin Transport in Magnetic Tunnel Junctions: Microscopic Theory,"We study the charge and spin currents passing through a magnetic tunnel
junction (MTJ) on the basis of a tight-binding model. The currents are
evaluated perturbatively with respect to the tunnel Hamiltonian. The charge
current has the form $A[\bm M_1(t)\times\dot{\bm M}_1(t)]\cdot\bm M_2+B\dot{\bm
M}_1(t)\cdot\bm M_2$, where $\bm M_1(t)$ and $\bm M_2$ denote the directions of
the magnetization in the free layer and fixed layer, respectively. The constant
$A$ vanishes when one or both layers are insulators, {while the constant $B$
disappears when both layers are insulators or the same ferromagnets.} The first
term in the expression for charge current represents dissipation driven by the
effective electric field induced by the dynamic magnetization. In addition,
from an investigation of the spin current, we obtain the microscopic expression
for the enhanced Gilbert damping constant $\varDelta \alpha$. We show that
$\varDelta\alpha$ is proportional to the tunnel conductance and depends on the
bias voltage.",1111.4295v2
2011-12-01,Compressible hydromagnetic nonlinearities in the predecoupling plasma,"The adiabatic inhomogeneities of the scalar curvature lead to a compressible
flow affecting the dynamics of the hydromagnetic nonlinearities. The influence
of the plasma on the evolution of a putative magnetic field is explored with
the aim of obtaining an effective description valid for sufficiently large
scales. The bulk velocity of the plasma, computed in the framework of the
LambdaCDM scenario, feeds back into the evolution of the magnetic power spectra
leading to a (nonlocal) master equation valid in Fourier space and similar to
the ones discussed in the context of wave turbulence. Conversely, in physical
space, the magnetic power spectra obey a Schroedinger-like equation whose
effective potential depends on the large-scale curvature perturbations.
Explicit solutions are presented both in physical space and in Fourier space.
It is argued that curvature inhomogeneities, compatible with the WMAP 7yr data,
shift to lower wavenumbers the magnetic diffusivity scale.",1112.0191v1
2011-12-01,Cosmological Simulations using GCMHD+,"Radio observations of galaxy clusters show that the intra cluster medium is
permeated by \mu G magnetic fields. The origin and evolution of these
cosmological magnetic fields is currently not well understood and so their
impact on the dynamics of structure formation is not known. Numerical
simulations are required to gain a greater understanding and produce
predictions for the next generation of radio telescopes. We present the
galactic chemodynamics smoothed particle magnetohydrodynamic (SPMHD) code
(GCMHD+), which is an MHD implementation for the cosmological smoothed particle
hydrodynamic code GCD+. The results of 1, 2 and 3 dimensional tests are
presented and the performance of the code is shown relative to the ATHENA grid
code. GCMHD+ shows good agreement with the reference solutions produced by
ATHENA. The code is then used to simulate the formation of a galaxy cluster
with a simple primordial magnetic field embedded in the gas. A homogeneous seed
field of 10^-11 G is amplified by a factor of 10^3 during the formation of the
cluster. The results show good agreement with the profiles found in other
magnetic cluster simulations of similar resolution.",1112.0340v1
2011-12-30,Ultrafast Magneto-Acoustics in Nickel Films,"We report about the existence of magneto-acoustic pulses propagating in a
200-nm-thick ferromagnetic nickel film excited with 120 fs laser pulses. They
result from the coupling between the magnetization of the ferromagnetic film
and the longitudinal acoustic waves associated to the propagation of the
lattice deformation induced by the femtosecond laser pulses. The
magneto-acoustic pulses are detected from both the front and back sides of the
film, using the time-resolved magneto-optical Kerr technique, measuring both
the time dependent rotation and ellipticity. We show that the propagating
acoustic pulse couples efficiently to the magnetization and is strong enough to
induce a precession of the magnetization. It is due to a transient change of
the crystalline anisotropy associated to the lattice deformation. It is shown
that the results can be interpreted by combining the concepts of acoustic pulse
propagation and ultrafast magnetization dynamics.",1201.0170v1
2012-01-06,Magnetic field decay with Hall drift in neutron star crusts,"The dynamics of magnetic field decay with Hall drift is investigated.
Assuming that axisymmetric magnetic fields are located in a spherical crust
with uniform conductivity and electron number density, long-term evolution is
calculated up to Ohmic dissipation. The nonlinear coupling between poloidal and
toroidal components is explored in terms of their energies and helicity.
Nonlinear oscillation by the drift in strongly magnetized regimes is clear only
around the equipartition between two components. Significant energy is
transferred to the poloidal component when the toroidal component initially
dominates. However, the reverse is not true. Once the toroidal field is less
dominant, it quickly decouples due to a larger damping rate. The polar field at
the surface is highly distorted from the initial dipole during the Hall drift
timescale, but returns to the initial dipole in a longer dissipation timescale,
since it is the least damped one.",1201.1346v1
2012-02-03,Magnetic topology of a naked sunspot: Is it really naked?,"The high spatial, temporal and spectral resolution achieved by Hinode
instruments give much better understanding of the behavior of some elusive
solar features, such as pores and naked sunspots. Their fast evolution and, in
some cases, their small sizes have made their study difficult. The moving
magnetic features, despite being more dynamic structures, have been studied
during the last 40 years. They have been always associated with sunspots,
especially with the penumbra. However, a recent observation of a naked sunspot
(one with no penumbra) has shown MMF activity. The authors of this reported
observation expressed their reservations about the explanation given to the
bipolar MMF activity as an extension of the penumbral filaments into the moat.
How can this type of MMFs exist when a penumbra does not? In this paper, we
study the full magnetic and (horizontal) velocity topology of the same naked
sunspot, showing how the existence of a magnetic field topology similar to that
observed in sunspots can explain these MMFs, even when the intensity map of the
naked sunspot does not show a penumbra.",1202.0591v1
2012-02-16,Two-dimensional incommensurate magnetic fluctuations in Sr$_2$(Ru$_{0.99}$Ti$_{0.01}$)O$_4$,"We investigate the imaginary part of the wave vector dependent dynamic spin
susceptibility in Sr$_2$(Ru$_{0.99}$Ti$_{0.01}$)O$_4$ as a function of
temperature using neutron scattering. At T=5 K, two-dimensional incommensurate
(IC) magnetic fluctuations are clearly observed around
$\mathbf{Q}_\text{c}=(0.3,0.3,L)$ up to approximately 60 meV energy transfer.
We find that the IC excitations disperse to ridges around the $(\pi,\pi)$
point. Below 50 K, the energy and temperature dependent excitations are well
described by the phenomenological response function for a Fermi liquid system
with a characteristic energy of 4.0(1) meV. Although the wave vector dependence
of the IC magnetic fluctuations in Sr$_2$(Ru$_{0.99}$Ti$_{0.01}$)O$_4$ is
similar to that in the Fermi liquid state of the parent compound,
Sr$_2$RuO$_4$, the magnetic fluctuations are clearly suppressed by the
Ti-doping.",1202.3654v1
2012-02-20,Semiclassical Dynamics and Magnetic Weyl Calculus,"Weyl quantization and related semiclassical techniques can be used to study
conduction properties of crystalline solids subjected to slowly-varying,
external electromagnetic fields. The case where the external magnetic field is
constant, is not covered by existing theory as proofs involving usual Weyl
calculus break down. This is the regime of the so-called quantum Hall effect
where quantization of transverse conductance is observed. To rigorously derive
semiclassical equations of motion, one needs to systematically develop a
magnetic Weyl calculus which contains a semiclassical parameter.
  Mathematically, the operators involved in the analysis are magnetic
pseudodifferential operators, a topic which by itself is of interest for the
mathematics and mathematical physics community alike. Hence, we will devote two
additional chapters to further understanding of properties of those operators.",1202.4668v2
2012-02-23,Multiscale methods for Levitron Problems: Theory and Applications,"In this paper, we describe a multiscale model based on magneto-static traps
of neutral atoms or ion traps. The idea is to levitate a magnetic spinning top
in the air repelled by a base magnet.
  For such a problem, we have to deal with different time and spatial scales
and we propose a novel splitting method for solving the levitron problem.
  We focus on the multiscale problem, which we obtain by coupling the kinetic T
and the potential U part of our equation. The kinetic and potential parts, can
be seen as generators of flows.
  The main problem is based on the accurate computation of the Hamiltonian
equation and we propose a novel higher order splitting scheme to obtain stable
states near the relative equilibrium. To improve the splitting scheme we apply
a novel method so called MPE (multiproduct expansion method), which include
higher order extrapolation schemes.
  In numerical studies, we discuss the stability near this relative equilibrium
with our improved time-integrators. Best results are obtained by iterative and
extrapolated Verlet schemes in comparison to higher order explicit Runge-Kutta
schemes. Experiments are applied to a magnetic top in an axisymmetric magnetic
field (i.e. the Levitron) and we discuss the future applications to quantum
computations.",1202.5169v1
2012-03-16,Turbulence and magnetic field amplification from spiral SASI modes in core-collapse supernovae,"The stationary accretion shock instability (SASI) plays a central role in
modern simulations of the explosion phase of core-collapse supernovae (CCSNe).
It may be key to realizing neutrino powered explosions, and possibly links
birth properties of pulsars (e.g., kick, spin, and magnetic field) to supernova
dynamics. Using high-resolution magnetohydrodynamic simulations, we study the
development of turbulence, and subsequent amplification of magnetic fields in a
simplified model of the post-bounce core-collapse supernova environment.
Turbulence develops from secondary instabilities induced by the SASI. Our
simulations suggest that the development of turbulence plays an important role
for the subsequent evolution of the SASI. The turbulence also acts to amplify
weak magnetic fields via a small-scale dynamo.",1203.3748v1
2012-04-05,Dynamical magnetic anisotropy and quantum phase transitions in a vibrating spin-1 molecular junction,"We study the electronic transport through a spin-1 molecule in which
mechanical stretching produces a magnetic anisotropy. In this type of device, a
vibron mode along the stretching axis will couple naturally to the molecular
spin. We consider a single molecular vibrational mode and find that the
electron-vibron interaction induces an effective correction to the magnetic
anisotropy that shifts the ground state of the device toward a non-Fermi liquid
phase. A transition into a Fermi liquid phase could then be achieved, by means
of mechanical stretching, passing through an underscreened spin-1 Kondo regime.
We present numerical renormalization group results for the differential
conductance, the spectral density, and the magnetic susceptibility across the
transition.",1204.1108v2
2012-04-10,Observation of spin-glass behavior in nickel adsorbed few layer graphene,"Nickel-adsorbed graphene was prepared by first synthesizing graphite oxide
(GO) by modified Hummers' method and then reducing a solution containing both
GO and $Ni^{2+}$. EDX analysis showed 31 atomic percent nickel was present.
Magnetization measurements under both dc and ac magnetic fields were carried
out in the temperature range 2 K to 300 K. The zero field cooled and field
cooled magnetization data showed a pronounced irreversibility at a temperature
around 20 K. The analysis of the ac susceptibility data were carried out by
both Vogel-Fulcher as well as power law. From dynamic scaling analysis the
microscopic flipping time $\tau_{0}\sim 10^{-13} s$ and critical exponent
$z\nu=5.9\pm0.1$ were found, indicating presence of conventional spin glass in
the system. The spin glass transition temperature was estimated as 19.5 K.
  Decay of thermoremanent magnetization (TRM) was explained by stretched
exponential function with a value of the exponent as 0.6 .
  From the results it is concluded that nickel adsorbed graphene behaves like a
spin-glass.",1204.2123v2
2012-04-18,Electronic properties and magnetism of iron at the Earth's inner core conditions,"We employ state-of-the-art ab initio simulations within the dynamical
mean-field theory to study three likely phases of iron (hexogonal close-packed,
hcp, face centered cubic, fcc, and body centered cubic, bcc) at the Earth's
core conditions. We demonstrate that the correction to the electronic free
energy due to correlations can be significant for the relative stability of the
phases. The strongest effect is observed in bcc Fe, which shows a non-Fermi
liquid behaviour, and where a Curie-Weiss behaviour of the uniform
susceptbility hints at a local magnetic moment still existing at 5800 K and 300
GPa. We predict that all three structures have sufficiently high magnetic
susceptibility to stabilize the geodynamo.",1204.3954v2
2012-04-19,Plasma Pressure Driven Asymmetric Supernovae and Highly Collimated Gamma-Ray Bursts,"During the process of collapse of a massive star, a cavity is generated
between the central iron core and an outer stellar envelope. The dynamics of
this cavity, filled with plasma and magnetic field of the rapidly rotating
proto-magnetar's magnetosphere, is believed to be very relevant in
understanding supernovae and gamma-ray bursts. The interactions of the
pressurized conducting plasma and the magnetic fields are described by a set of
magnetohydrodynamic (MHD) equations with poloidal and toroidal plasma flows not
aligned with magnetic fields. A sequence of MHD equilibria in response to the
increasing plasma pressure in the cavity, by continuous filling from the
rotating magnetosphere, is solved to account for asymmetric supernovae, highly
collimated gamma-ray burst jets, and also active galactic nucleus plasma torus.
It is shown that the magnetosphere of the central compact star is likely the
central engine of supernova and gamma-ray burst by feeding them plasma,
magnetic energy, and rotational energy.",1204.4360v2
2012-04-27,Prediction of polaron-like vortices and dissociation depinning transition in magnetic superconductors: The example of ErNi2B2C,"In borocarbide ErNi$_2$B$_2$C the phase transition to the commensurate spin
density wave at 2.3 K leaves 1/20 part of Ising-like Er spins practically free.
Vortices polarize these spins nonuniformly and repolarize them when moving. At
a low spin relaxation rate and at low bias currents vortices carrying magnetic
polarization clouds become polaron-like and their velocities are determined by
the effective drag coefficient which is significantly bigger than the
Bardeen-Stephen (BS) one. As current increases, at a critical current $J_c$
vortices release polarization clouds and the velocity as well as the voltage in
the I-V characteristics jump to values corresponding to the BS drag
coefficient. The nonuniform components of the magnetic field and magnetization
drop as velocity increases resulting in weaker polarization and discontinuous
dynamic dissociation depinning transition. As current decreases, on the way
back, vortices are retrapped by polarization clouds at the current $J_r<J_c$.",1204.6348v2
2012-05-13,Voltage-Induced Ferromagnetic Resonance in Magnetic Tunnel Junctions,"We demonstrate excitation of ferromagnetic resonance in CoFeB/MgO/CoFeB
magnetic tunnel junctions (MTJs) by the combined action of voltage-controlled
magnetic anisotropy (VCMA) and spin transfer torque (ST). Our measurements
reveal that GHz-frequency VCMA torque and ST in low-resistance MTJs have
similar magnitudes, and thus that both torques are equally important for
understanding high-frequency voltage-driven magnetization dynamics in MTJs. As
an example, we show that VCMA can increase the sensitivity of an MTJ-based
microwave signal detector to the sensitivity level of semiconductor Schottky
diodes.",1205.2835v2
2012-05-22,Unraveling Orbital Correlations via Magnetic Resonant Inelastic X-ray Scattering,"Although orbital degrees of freedom are a factor of fundamental importance in
strongly correlated transition metal compounds, orbital correlations and
dynamics remain very difficult to access, in particular by neutron scattering.
Via a direct calculation of scattering amplitudes we show that instead magnetic
resonant inelastic x-ray scattering (RIXS) does reveal orbital correlations. In
contrast to neutron scattering, the intensity of the magnetic excitations in
RIXS depends very sensitively on both the symmetry of the orbitals that spins
occupy, and on photon polarizations. We show in detail how this effect allows
magnetic RIXS to distinguish between alternating orbital ordered and
ferro-orbital (or orbital liquid) states.",1205.4940v3
2012-05-22,Quantum magneto-optics of graphite family,"The optical conductivity of graphene, bilayer graphene, and graphite in
quantizing magnetic fields is studied. Both dynamical conductivities,
longitudinal and Hall's, are analytically evaluated. The conductivity peaks are
explained in terms of electron transitions. We have shown that trigonal warping
can be considered within the perturbation theory for strong magnetic fields
larger than 1 T and in the semiclassical approach for weak fields when the
Fermi energy is much larger than the cyclotron frequency. The main optical
transitions obey the selection rule with \Deltan = 1 for the Landau number n,
however the \Deltan = 2 transitions due to the trigonal warping are also
possible. The Faraday/Kerr rotation and light transmission/reflection in the
quantizing magnetic fields are calculated. Parameters of the
Slonczewski-Weiss-McClure model are used in the fit taking into account the
previous dHvA measurements and correcting some of them for the case of strong
magnetic fields.",1205.4952v1
2012-06-13,Scanning superconducting quantum interference device on a tip for magnetic imaging of nanoscale phenomena,"We describe a new type of scanning probe microscope based on a
superconducting quantum interference device (SQUID) that resides on the apex of
a sharp tip. The SQUID-on-tip is glued to a quartz tuning fork which allows
scanning at a tip-sample separation of a few nm. The magnetic flux sensitivity
of the SQUID is 1.8 {\mu}_0/Hz^{1/2} and the spatial resolution is about 200
nm, which can be further improved. This combination of high sensitivity,
spatial resolution, bandwidth, and the very close proximity to the sample
provides a powerful tool for study of dynamic magnetic phenomena on the
nanoscale. The potential of the SQUID-on-tip microscope is demonstrated by
imaging of the vortex lattice and of the local AC magnetic response in
superconductors.",1206.2853v1
2012-07-04,Negative-μ regime in the ac magnetic response of superconductor nanoshells,"The time-dependent Ginzburg-Landau formalism is applied to analyze the vortex
states and vortex dynamics in superconducting spherical nanoshells, subjected
to mutually perpendicular strong dc and weak ac magnetic fields. We demonstrate
that nonuniformity of the shell thickness can dramatically affect the ac
magnetic response of a 3D array of superconducting nanoshells. Remarkably, this
response is strongly influenced not only by the relevant geometric and material
parameters and the ac-field frequency but also by the magnitude of the applied
dc field: by changing this field the real part of the effective ac magnetic
permeability can be tuned from positive values significantly larger than one
down to negative values.",1207.0971v1
2012-07-05,Out-of-phase oscillation between superfluid and thermal components for a trapped Bose condensate under oscillatory excitation,"The vortex nucleation and the emergence of quantum turbulence induced by
oscillating magnetic fields, introduced by Henn E A L, et al. 2009 (Phys. Rev.
A 79, 043619) and Henn E A L, et al. 2009 (Phys. Rev. Lett. 103, 045301), left
a few open questions concerning the basic mechanisms causing those interesting
phenomena. Here, we report the experimental observation of the slosh dynamics
of a magnetically trapped $^{87}$Rb Bose-Einstein condensate (BEC) under the
influence of a time-varying magnetic field. We observed a clear relative
displacement in between the condensed and the thermal fraction center-of-mass.
We have identified this relative counter move as an out-of-phase oscillation
mode, which is able to produce ripples on the condensed/thermal fractions
interface. The out-of-phase mode can be included as a possible mechanism
involved in the vortex nucleation and further evolution when excited by time
dependent magnetic fields.",1207.1146v2
2012-07-26,On the structure of MHD shock waves in a viscous gas,"The exact solutions for MHD shock waves in an ideal gas are obtained taking
into consideration only the viscosity of the gas. In view of an axial magnetic
field, the analytical expressions for the particle velocity, temperature,
pressure and change-in-entropy within the shock transition region are obtained.
The flow variables are numerical analysed to explore the influence of static
magnetic field, shock strength, specific heat ratio, initial pressure, initial
density and coefficient of viscosity on the flow variables. The findings
confirm that thickness of MHD shock front increases with increase in the
viscosity of the gas and the change in thickness is more noticeable for large
values of the strength of magnetic field. The results provided a clear picture
of whether and how the viscosity of gas and the magnetic field affect the
thickness of shock front.",1207.6166v4
2012-08-11,Contribution of the magnetic resonance to the third harmonic generation from a fishnet metamaterial,"We investigate experimentally and theoretically the third harmonic generated
by a double-layer fishnet metamaterial. To unambiguously disclose most notably
the influence of the magnetic resonance, the generated third harmonic was
measured as a function of the angle of incidence. It is shown experimentally
and numerically that when the magnetic resonance is excited by pump beam, the
angular dependence of the third harmonic signal has a local maximum at an
incidence angle of {\theta} \simeq 20{\deg}. This maximum is shown to be a
fingerprint of the antisymmetric distribution of currents in the gold layers.
An analytical model based on the nonlinear dynamics of the electrons inside the
gold shows excellent agreement with experimental and numerical results. This
clearly indicates the difference in the third harmonic angular pattern at
electric and magnetic resonances of the metamaterial.",1208.2370v2
2012-09-13,Structural and magnetic properties of the new cobaltate series (BaSr)4-xLa2xCo4O15,"We report the structural and magnetic properties of a new class of cobaltates
with the chemical formula (BaSr)4-xLa2xCo4O15 (x = 0, 0.5 and 1). These
compounds crystallize in a hexagonal structure in which cobalt ions are
distributed among two distinct crystallographic sites with different oxygen
coordination. Three Co-O tetrahedra and one octahedron are linked by shared
oxygen atoms to form Co4O15 clusters, which are packed together into a
honeycomb-like network. Partial substitution of Sr and/or Ba atoms by La allows
one to adjust the degree of Co valence mixing, but all compositions remain
subject to a random distribution of charge. Magnetic susceptibility together
with neutron scattering measurements reveal that all studied specimens are
characterized by competing ferro- and antiferro-magnetic exchange interactions
that give rise to a three dimensional Heisenberg spin-glass state. Neutron
spectroscopy shows a clear trend of slowing down of spin-dynamics upon
increasing La concentration, suggesting a reduction in charge randomness in the
doped samples.",1209.2986v1
2012-09-18,Nonrelativistic parallel shocks in unmagnetized and weakly magnetized plasmas,"We present results of 2D3V particle-in-cell simulations of non-relativistic
plasma collisions with absent or parallel large-scale magnetic field for
parameters applicable to the conditions at young supernova remnants. We study
the collision of plasma slabs of different density, leading to two different
shocks and a contact discontinuity. Electron dynamics play an important role in
the development of the system. While non-relativistic shocks in both
unmagnetized and magnetized plasmas can be mediated by Weibel-type
instabilities, the efficiency of shock-formation processes is higher when a
large-scale magnetic field is present. The electron distributions downstream of
the forward and reverse shocks are generally isotropic, whereas that is not
always the case for the ions. We do not see any significant evidence of
pre-acceleration, neither in the electron population nor in the ion
distribution.",1209.3938v1
2012-10-04,Multiscale Gyrokinetics for Rotating Tokamak Plasmas II: Reduced Models for Electron Dynamics,"In this paper, we extend the multiscale approach developed in [Abel et. al.,
Rep. Prog. Phys., submitted] by exploiting the scale separation between ions
and the electrons. The gyrokinetic equation is expanded in powers of the
electron to ion mass ratio, which provides a rigorous method for deriving the
reduced electron model. We prove that ion-scale electromagnetic turbulence
cannot change the magnetic topology, and argue that to lowest order the
magnetic field lies on fluctuating flux surfaces. These flux surfaces are used
to construct magnetic coordinates, and in these coordinates a closed system of
equations for the electron response to ion-scale turbulence is derived. All
fast electron timescales have been eliminated from these equations. We also use
these magnetic surfaces to construct transport equations for electrons and for
electron heat in terms of the reduced electron model.",1210.1417v3
2012-10-05,MRI: a possible mechanism for funnel flows?,"The magnetorotational instability (MRI) has been suggested to have an
important role on the dynamics of accretion disks. We investigate MRI as an
alternative way for guiding the plasma from the disk to the funnel flow at the
disk-magnetosphere boundary of classical T Tauri stars (CTTSs) by considering
the diamagnetic effects. We solve the magnetohydrodynamic equations by
including the effect of both the magnetic field gradient and the perpendicular
(to the field) velocity gradient produced by the magnetization current at the
disk-magnetosphere boundary for the first time. Diamagnetic current modified
MRI produces a non-propagating mode which may lift the plasma from the disk
towards the vertical magnetic field lines. Our model also shows that the
diamagnetic effects play an important role in triggering the MRI. The
instability becomes more powerful with the inclusion of the gradient in the
magnetic field and the perpendicular velocity.",1210.1733v1
2012-10-17,The power-law spectra of energetic particles during multi-island magnetic reconnection,"Power-law distributions are a near universal feature of energetic particle
spectra in the heliosphere. Anomalous Cosmic Rays (ACRs), super-Alfv\'enic ions
in the solar wind and the hardest energetic electron spectra in flares all have
energy fluxes with power-laws that depend on energy $E$ approximately as
$E^{-1.5}$. We present a new model of particle acceleration in systems with a
bath of merging magnetic islands that self-consistently describes the
development of velocity-space anisotropy parallel and perpendicular to the
local magnetic field and includes the self-consistent feedback of pressure
anisotropy on the merging dynamics. By including pitch-angle scattering we
obtain an equation for the omni-directional particle distribution $f(v,t)$ that
is solved in closed form to reveal $v^{-5}$ (corresponding to an energy flux
varying as $E^{-1.5}$) as a near-universal solution as long as the
characteristic acceleration time is short compared with the characteristic loss
time. In such a state the total energy in the energetic particles reaches
parity with the remaining magnetic free energy. More generally, the resulting
transport equation can serve as the basis for calculating the distribution of
energetic particles resulting from reconnection in large-scale inhomogeneous
systems.",1210.4830v1
2012-10-21,Reshaping and Capturing Leidenfrost drops with a magnet,"Liquid oxygen, which is paramagnetic, also undergoes Leidenfrost effect at
room temperature. In this article, we first study the deformation of oxygen
drops in a magnetic field and show that it can be described via an effective
capillary length, which includes the magnetic force. In a second part, we
describe how these ultra-mobile drops passing above a magnet significantly slow
down and can even be trapped. The critical velocity below which a drop is
captured is determined from the deformation induced by the field.",1210.5718v3
2012-10-29,Thermally-Assisted Spin-Transfer Torque Magnetization Reversal of Uniaxial Nanomagnets in Energy Space,"The asymptotic behavior of switching time as a function of current for a
uniaxial macrospin under the effects of both spin-torque and thermal noise is
explored analytically by focusing on its diffusive energy space dynamics. The
scaling dependence ($I\rightarrow 0$, $<\tau\propto\exp(-\xi(1-I)^2)$) is shown
to confirm recent literature results. The analysis shows the mean switching
time to be functionally independent of the angle between the spin current and
magnet's uniaxial axes. These results have important implications for modeling
the energetics of thermally assisted magnetization reversal of spin transfer
magnetic random access memory bit cells.",1210.7682v2
2012-10-30,Quantum Tunneling in Half-Integer-Spin Kagome-Lattice Langasites,"Employing the muon spin relaxation technique we evidence temperature
independent magnetic fluctuations persisting down to the lowest temperatures in
the samarium-based ($J=5/2$) kagome-lattice Langasite. A detailed
bulk-magnetization characterization and comparison to the neodymium-based
($J=9/2$) compound allow us to assign the persistent spin dynamics to a quantum
tunneling process. This is facilitated by pairwise anisotropic magnetic
interactions, leading to a universal scaling of the muon relaxation. Our study
reveals a remarkable analogy between weakly interacting half-integer-spin
rare-earth magnets and molecular nanomagnets.",1210.8187v1
2012-11-18,Simulation of relativistically colliding laser-generated electron flows,"The plasma dynamics resulting from the simultaneous impact, of two equal,
ultra-intense laser pulses, in two spatially separated spots, onto a dense
target is studied via particle-in-cell (PIC) simulations. The simulations show
that electrons accelerated to relativistic speeds, cross the target and exit at
its rear surface. Most energetic electrons are bound to the rear surface by the
ambipolar electric field and expand along it. Their current is closed by a
return current in the target, and this current configuration generates strong
surface magnetic fields. The two electron sheaths collide at the midplane
between the laser impact points. The magnetic repulsion between the
counter-streaming electron beams separates them along the surface normal
direction, before they can thermalize through other beam instabilities. This
magnetic repulsion is also the driving mechanism for the beam-Weibel
(filamentation) instability, which is thought to be responsible for magnetic
field growth close to the internal shocks of gamma-ray burst (GRB) jets. The
relative strength of this repulsion compared to the competing electrostatic
interactions, which is evidenced by the simulations, suggests that the
filamentation instability can be examined in an experimental setting.",1211.4195v1
2012-12-21,Superconductivity and magnetic fluctuations in electron-doped cobaltate superconductors,"We study the interplay between superconductivity and antiferromagnetic spin
fluctuations in electron-doped cobaltate Na$_{x}$CoO$_{2}\cdot y$H$_{2}$O based
on the kinetic energy driven superconductivity mechanism. We show that the
superconducting state is governed by both charge carrier pairing and
quasiparticle coherent, and displays a common dome-shaped phase diagram in
agreement with experimental results. By calculating the dynamical spin
structure factor, we theoretically find that the magnetic excitation shows a
commensurate resonance peak, which locates at antiferromagnetic wave vector $Q
(\frac{2\pi}{3},\frac{2\pi}{\sqrt{3}})$ for a broad range of low energies, then
evolves outward into six incommensurate magnetic scattering peaks with
increasing energy. Such commensurate-incommensurate spin resonance excitation
should be measured by the inelastic neutron scattering technique (INS). Our
present results strongly suggest that magnetic resonance can indeed be one of
the fundamental features in doped Mott insulators.",1212.5367v1
2012-12-31,Ubiquitous Solar Eruptions Driven by Magnetized Vortex Tubes,"The solar surface is covered by high-speed jets transporting mass and energy
into the solar corona and feeding the solar wind. The most prominent of these
jets have been known as spicules. However, the mechanism initiating these
eruptions events is still unknown. Using realistic numerical simulations we
find that small-scale eruptions are produced by ubiquitous magnetized vortex
tubes generated by the Sun's turbulent convection in subsurface layers. The
swirling vortex tubes (resembling tornadoes) penetrate into the solar
atmosphere, capture and stretch background magnetic field, and push surrounding
material up, generating quasiperiodic shocks. Our simulations reveal a
complicated high-speed flow patterns, and thermodynamic and magnetic structure
in the erupting vortex tubes. We found that the eruptions are initiated in the
subsurface layers and are driven by the high-pressure gradients in the
subphotosphere and photosphere, and by the Lorentz force in the higher
atmosphere layers.",1301.0018v1
2013-01-02,Plasmoid Instability in High-Lundquist-Number Magnetic Reconnection,"Our understanding of magnetic reconnection in resistive magnetohydrodynamics
has gone through a fundamental change in recent years. The conventional wisdom
is that magnetic reconnection mediated by resistivity is slow in laminar high
Lundquist ($S$) plasmas, constrained by the scaling of the reconnection rate
predicted by Sweet-Parker theory. However, recent studies have shown that when
$S$ exceeds a critical value $\sim10^{4}$, the Sweet-Parker current sheet is
unstable to a super-Alfv\'enic plasmoid instability, with a linear growth rate
that scales as $S^{1/4}$. In the fully developed statistical steady state of
two-dimensional resistive magnetohydrodynamic simulations, the normalized
average reconnection rate is approximately 0.01, nearly independent of $S$, and
the distribution function $f(\psi)$ of plasmoid magnetic flux $\psi$ follows a
power law $f(\psi)\sim\psi^{-1}$. When Hall effects are included, the plasmoid
instability may trigger onset of Hall reconnection even when the conventional
criterion for onset is not satisfied. The rich variety of possible reconnection
dynamics is organized in the framework of a phase diagram.",1301.0331v2
2013-01-29,A brief report on statistical study of net electric current in solar active regions with longitudinal fields of opposite polarity,"Dynamic processes occurring in solar active regions are dominated by the
solar magnetic field. As of now, observations using a solar magnetograph have
supplied us with the vector components of a solar photospheric magnetic field.
The two transverse components of a photospheric magnetic field allow us to
compute the amount of electric current. We found that the electric current in
areas with positive (negative) polarity due to the longitudinal magnetic field
have both positive and negative signs in an active region, however, the net
current is found to be an order-of-magnitude less than the mean absolute
magnitude and has a preferred sign. In particular, we have statistically found
that there is a systematic net electric current from areas with negative
(positive) polarity to areas with positive (negative) polarity in solar active
regions in the northern (southern) hemisphere, but during the solar minimum
this tendency is reversed over time at some latitudes. The result indicates
that there is weak net electric current in areas of solar active regions with
opposite polarity, thus providing further details about the hemispheric
helicity rule found in a series of previous studies.",1301.6812v2
2013-02-07,Identification of intermittent multi-fractal turbulence in fully kinetic simulations of magnetic reconnection,"Recent fully nonlinear, kinetic three-dimensional simulations of magnetic
reconnection [Daughton et al. 2011] evolve structures and exhibit dynamics on
multiple scales, in a manner reminiscent of turbulence. These simulations of
reconnection are among the first to be performed at sufficient spatio-temporal
resolution to allow formal quantitative analysis of statistical scaling which
we present here. We find that the magnetic field fluctuations generated by
reconnection are anisotropic, have non-trivial spatial correlation and exhibit
the hallmarks of finite range fluid turbulence; they have non-Gaussian
distributions, exhibit Extended Self-Similarity in their scaling and are
spatially multifractal. Furthermore, we find that the field J.E is also
multifractal, so that magnetic energy is converted to plasma kinetic energy in
a manner that is spatially intermittent. This suggests that dissipation in this
sense in collisionless reconnection on kinetic scales has an analogue in
fluid-like turbulent phenomenology, in that it proceeds via multifractal
structures generated by an intermittent cascade.",1302.1749v1
2013-02-18,Early stages of magnetization relaxation in superconductors,"Magnetic flux dynamics in type-II superconductors is studied within the model
of a viscous nonlinear diffusion of vortices for various sample geometries. We
find that time dependence of magnetic moment relaxation after the field is
switched off can be accurately approximated by $m(t)\propto
1-\sqrt{t/\tilde\tau}$ in the narrow initial time interval and by $m(t)\propto
(1+t/\tau)^{-1}$ at later times before the flux creep sets in. The
characteristic times $\tilde\tau$ and $\tau$ are proportional to the viscous
drag coefficient $\eta$. Quantitative agreement with available experimental
data is obtained for both conventional and high-temperature superconductors
with $\eta$ exceeding by many orders of magnitude the Bardeen-Stephen
coefficient for free vortices. Huge enhancement of the drag, as well as its
exponential temperature dependence, indicates a strong influence of pinning
centers on the flux diffusion. Notwithstanding complexity of the vortex motion
in the presence of pinning and thermal agitation, we argue that the initial
relaxation of magnetization can still be considered as a viscous flux flow with
an effective drag coefficient.",1302.4312v2
2013-02-19,Magnetoelectric susceptibility tensor of multiferroic TbMnO3 with cycloidal antiferromagnetic structure in external field,"Magnetoelectric, dielectric and magnetic susceptibility tensors of
multiferroic TbMnO3 with cycloidal antiferromagnetic structure in external
electric and magnetic fields have been investigated with taking into account
dynamics of spin, electro-dipole and acoustic subsystems. All components of
tensors depend on values of external electric and magnetic fields. The
possibility of control of electrodynamic properties of multiferroic TbMnO3 with
cycloidal antiferromagnetic structure by external electric and magnetic fields
has been shown. The resonant interaction of spin, electro-dipole,
electromagnetic and acoustic waves in such material is observed.",1302.4515v1
2013-03-18,Vortex Lattices and Crystalline Geometries,"We consider $AdS_2 \times R^2$ solutions supported by a magnetic field, such
as those which arise in the near-horizon limit of magnetically charged $AdS_4$
Reissner-Nordstrom black branes. In the presence of an electrically charged
scalar field, such magnetic solutions can be unstable to spontaneous formation
of a vortex lattice. We solve the coupled partial differential equations which
govern the charged scalar, gauge field, and metric degrees of freedom to lowest
non-trivial order in an expansion around the critical point, and discuss the
corrections to the free energy and thermodynamic functions arising from the
formation of the lattice. We describe how such solutions can also be
interpreted, via S-duality, as characterizing infrared crystalline phases of
conformal field theories doped by a chemical potential, but in zero magnetic
field; the doped conformal field theories are dual to geometries that exhibit
dynamical scaling and hyperscaling violation.",1303.4390v2
2013-03-29,Pseudo-magnetic fields in graphene in excess of 300T: theoretical framework,"The experimental demonstration of pseudo-magnetic fields exceeding 300 T in
graphene [2] nanobubbles represents considerable challenge for the present
theory connecting the emergence of gauge fields due to strain in the underlying
lattice. Here we propose a theoretical framework within which the magnitude of
the pseudo-magnetic fields can be computed more accurately. The basic feature
of this framework is that the carriers in graphene are considered with their
three dimensional wave function which is then gradually constrained to the
graphene surface. In the process, a geometrically induced gauge field emerges
in the two dimensional equation for the surface dynamics. The computation of
the magnetic field associated with this gauge potential reproduces the measured
field strength.",1303.7373v1
2013-04-21,Superconducting flux-flow type ultra-low-noise magnetic sensors - An alternative to dc-SQUIDs,"A superconducting magnetometers based on the magnetic field dependence of the
Eck step voltage in long Josephson tunnel junctions (LJTJs) is demonstrated.
The field to be measured is applied perpendicular to a continuous
superconducting pickup loop. Wherever the loop has a narrow constriction, the
density of the flux-restoring circulating currents will become relatively high
and will locally create a magnetic field large enough to bring a biased LJTJ in
the flux-flow state, i.e., at a finite voltage proportional to the field
strength. This method allows the realization of a novel family of robust and
general-purpose superconducting devices which, despite their simplicity,
function as ultra-low-noise, wide-band and high-dynamics magnetometers. The
performances of low-T$_c$ sensor prototypes, among which a highly linear
voltage responsivity and a magnetic spectral density $S_B^{1/2}<
3\,fT/Hz^{1/2}$, promise to be competitive with those of the best
superconducting quantum interference devices.",1304.5761v2
2013-05-02,Magnetohydrodynamic equilibria in barotropic stars,"Although barotropic matter does not constitute a realistic model for magnetic
stars, it would be interesting to confirm a recent conjecture that states that
magnetized stars with a barotropic equation of state would be dynamically
unstable (Reisenegger 2009). In this work we construct a set of barotropic
equilibria, which can eventually be tested using a stability criterion. A
general description of the ideal MHD equations governing these equilibria is
summarized, allowing for both poloidal and toroidal magnetic field components.
A new finite-difference numerical code is developed in order to solve the
so-called Grad-Shafranov equation describing the equilibrium of these
configurations, and some properties of the equilibria obtained are briefly
discussed.",1305.0592v1
2013-05-27,Time-resolved magnetic sensing with electronic spins in diamond,"Quantum probes can measure time-varying fields with high sensitivity and
spatial resolution, enabling the study of biological, material, and physical
phenomena at the nanometer scale. In particular, nitrogen-vacancy centers in
diamond have recently emerged as promising sensors of magnetic and electric
fields. Although coherent control techniques have measured the amplitude of
constant or oscillating fields, these techniques are not suitable for measuring
time-varying fields with unknown dynamics. Here we introduce a coherent
acquisition method to accurately reconstruct the temporal profile of
time-varying fields using Walsh sequences. These decoupling sequences act as
digital filters that efficiently extract spectral coefficients while
suppressing decoherence, thus providing improved sensitivity over existing
strategies. We experimentally reconstruct the magnetic field radiated by a
physical model of a neuron using a single electronic spin in diamond and
discuss practical applications. These results will be useful to implement
time-resolved magnetic sensing with quantum probes at the nanometer scale.",1305.6082v2
2013-06-04,Filament eruption on 2010 October 21 from three viewpoints,"A filament eruption on 2010 October 21 observed from three different
viewpoints by the Solar Terrestrial Relations Observatory (STEREO) and the
Solar Dynamic Observatory (SDO) is analyzed with invoking also data from the
Solar and Heliospheric Observatory (SOHO) and the Kanzelhoehe Solar
Observatory. The position of the filament just before the eruption at the
central meridian not far from the center of the solar disk was favorable for
photospheric magnetic field measurements in the area below the filament.
Because of this, we were able to calculate with high precision the distribution
of the coronal potential magnetic field near the filament. We found that the
filament began to erupt when it approached the height in the corona where the
magnetic field decay index was greater than one. We determined also that during
the initial stage of the eruption the filament moved along the magnetic neutral
surface.",1306.0679v1
2013-06-10,Using Coronal Cells to Infer the Magnetic Field Structure and Chirality of Filament Channels,"Coronal cells are visible at temperatures of ~ 1.2 MK in Fe XII coronal
images obtained from the Solar Dynamics Observatory (SDO) and Solar Terrestrial
Relations Observatory (STEREO) spacecraft. We show that near a filament
channel, the plumelike tails of these cells bend horizontally in opposite
directions on the two sides of the channel like fibrils in the chromosphere.
Because the cells are rooted in magnetic flux concentrations of majority
polarity, these observations can be used with photospheric magnetograms to
infer the direction of the horizontal field in filament channels and the
chirality of the associated magnetic field. This method is similar to the
procedure for inferring the direction of the magnetic field and the chirality
of the fibril pattern in filament channels from H-alpha observations. However,
the coronal cell observations are easier to use and provide clear inferences of
the horizontal field direction for heights up to ~ 50 Mm into the corona.",1306.2273v3
2013-06-17,Magnetic excitations in the spin-1/2 triangular-lattice antiferromagnet Cs$_2$CuBr$_4$,"We report on high-field electron spin resonance (ESR) studies of magnetic
excitations in the spin-1/2 triangular-lattice antiferromagnet Cs$_2$CuBr$_4$.
Frequency-field diagrams of ESR excitations are measured for different
orientations of magnetic fields up to 25 T. We show that the substantial
zero-field energy gap, $\Delta\approx9.5$ K, observed in the low-temperature
excitation spectrum of Cs$_2$CuBr$_4$ [Zvyagin $et~al.$, Phys. Rev. Lett. 112,
077206 (2014)], is present well above $T_N$. Noticeably, the transition into
the long-range magnetically ordered phase does not significantly affect the
size of the gap, suggesting that even below $T_N$ the high-energy spin dynamics
in Cs$_2$CuBr$_4$ is determined by short-range-order spin correlations. The
experimental data are compared with results of model spin-wave-theory
calculations for spin-1/2 triangle-lattice antiferromagnet.",1306.3887v3
2013-06-18,Magnetic Dynamics of a Multiferroic with an Antiferromagnetic Layer,"Shape effects in magnetic particles are widely studied, because of the
ability of the shape and the size to control the parameters of a sample during
its production. Experiments with nano-sized samples show that the shape can
affect also the properties of antiferromagnetic (AFM) materials. However, the
theoretical interpretation of these effects is under discussion. We propose a
model to study the shape-induced effects in AFM particles at the AFM resonance
frequency. The Lagrange function method is used to calculate the spectrum of
resonance oscillations of the AFM vector in a synthetic multiferroic
(piezoelectric + antiferromagnet). The influence of the specimen shape on the
AFM resonance frequency in the presence of an external magnetic field is
studied. Conditions for a resonance under the action of an external force or
for a parametric resonance to arise in the magnetic subsystem are considered.",1306.4300v1
2013-06-21,Spinodal decomposition of a binary magnetic fluid confined to a surface,"In our previous work [J. Chem. Phys. \textbf{136}, 024502 (2012)], we
reported a demixing phase transition of a two-dimensional, binary Heisenberg
fluid mixture driven by the ferromagnetic interactions of the magnetic species.
Here, we present a theoretical study for the \textit{time-dependent} coarsening
occuring within the two-phase region in the density-concentration plane, also
known as spinodal decomposition. Our investigations are based on Dynamical
Density Functional Theory (DDFT). The particles in the mixture are modelled as
Gaussian soft spheres on a two-dimensional surface, where one component carries
a classical spin of Heisenberg type. To investigate the two-phase region, we
first present a linear stability analysis with respect to small, harmonic
density perturbations. Second, to capture non-linear effects, we calculate
time-dependent structure factors by combining DDFT with Percus' test particle
method. For the growth of the average domain size $l$ during spinodal
decomposition with time $t$, we observe a power-law behavior $l\propto
t^{\delta_\alpha}$ with $\delta_m\simeq 0.333$ for the magnetic species and
$\delta_n\simeq 0.323$ for the non-magnetic species.",1306.5171v2
2013-07-03,Asymmetric Magnetic Domain-Wall Motion by the Dzyaloshinskii-Moriya Interaction,"We demonstrate here that ultrathin ferromagnetic Pt/Co/Pt films with
perpendicular magnetic anisotropy exhibit a sizeable Dzyaloshinskii-Moriya
interaction (DMI) effect. Such a DMI effect modifies the domain-wall (DW)
energy density and consequently, results in an asymmetric DW expansion driven
by an out-of-plane magnetic field under an in-plane magnetic field bias. From
an analysis of the asymmetry, the DMI effect is estimated to be strong enough
for the DW to remain in the N\'eel-type configuration in contrast to the
general expectations of these materials. Our findings emphasize the critical
role of the DMI effect on the DW dynamics as the underlying physics of the
asymmetries that are often observed in spin-transfer-related phenomena.",1307.0984v1
2013-07-12,Intergalactic magnetic fields in Stephan's Quintet,"We present results of the VLA radio continuum total power and polarised
intensity observations of Stephan's Quintet at 1.43 and 4.86 GHz along with
complementary 4.85 and 8.35 GHz Effelsberg observations. Our study shows a
large envelope of radio emission encompassing all the member galaxies and hence
a large volume of intergalac- tic matter. Infall of the galaxy NGC 7318B
produces a ridge of intergalactic, polarised emission, for which the magnetic
field strength was estimated as 11.0 \pm 2.2 {\mu}G, with an ordered component
of 2.6 \pm 0.8 {\mu}G. The energy density of the field within the ridge area is
of the same order as estimates of the thermal component, implying a significant
role of the magnetic field in the dynamics of the intergalactic matter. We also
report that the tidal dwarf galaxy candidate SQ-B possesses a strong and highly
anisotropic magnetic field with the total strength being equal to 6.5 \pm 1.9
{\mu}G and an ordered component reaching 3.5 \pm 1.2 {\mu}G, which is
comparable to that found in normal-sized galaxies.",1307.3447v1
2013-07-20,Probing the anharmonicity of the potential well for magnetic vortex core in nanodot,"The anharmonicity of the potential well confining the position of the
magnetic vortex core is measured dynamically with a Magnetic Resonance Force
Microscope (MRFM). The stray field of the MRFM tip is used to displace the core
position away from the well minimum. Anharmonicity is then inferred from the
relative frequency shift induced on the eigen-frequency of the vortex core
translational mode. Traces of these shifts are recorded while scanning the tip
above an isolated nanodot, patterned out of a single crystal FeV film. An
analytical framework is proposed to analyze the data. It results in a
quantitative measurement of the anharmonic coefficient found to be positive and
50% of the parabolic contribution. This calibrates the tunability of the
gyrotropic mode by external magnetic fields. In our sample, we observe a
variation of the eigen-frequency as high as +10% for a displacement of the
vortex core to about one third of the nanodot radius.",1307.5429v2
2013-07-20,Magnetic properties of Sn/1-x/Cr/x/Te diluted magnetic semiconductors,"We present the studies of Sn/1-x/Cr/x/Te semimagnetic semiconductors with
chemical composition x ranging from 0.004 to 0.012. The structural
characterization indicates that even at low average Cr-content x < ?0.012, the
aggregation into micrometer size clusters appears in our samples. The magnetic
properties are affected by the presence of clusters. In all our samples we
observe the transition into the ordered state at temperatures between 130 and
140 K. The analysis of both static and dynamic magnetic susceptibility data
indicates that the spin-glass-like state is observed in our samples. The
addition of Cr to the alloy seems to shift the spin-glass-like transition from
130 K for x = 0.004 to 140 K for x = 0.012.",1307.5464v1
2013-08-06,Chirality from interfacial spin-orbit coupling effects in magnetic bilayers,"As nanomagnetic devices scale to smaller sizes, spin-orbit coupling due to
the broken structural inversion symmetry at interfaces becomes increasingly
important. Here we study interfacial spin-orbit coupling effects in magnetic
bilayers using a simple Rashba model. The spin-orbit coupling introduces
chirality into the behavior of the electrons and through them into the
energetics of the magnetization. In the derived form of the magnetization
dynamics, all of the contributions that are linear in the spin-orbit coupling
follow from this chirality, considerably simplifying the analysis. For these
systems, an important consequence is a correlation between the
Dzyaloshinskii-Moriya interaction and the spin-orbit torque. We use this
correlation to analyze recent experiments.",1308.1198v2
2013-08-22,Confined dissipative droplet solitons in spin-valve nanowires with perpendicular magnetic anisotropy,"Magnetic dissipative droplets are localized, strongly nonlinear dynamical
modes excited in nano-contact spin valves with perpendicular magnetic
anisotropy. These modes find potential application in nanoscale structures for
magnetic storage and computation, but dissipative droplet studies have so far
been limited to extended thin films. Here, numerical and asymptotic analyses
are used to demonstrate the existence and properties of novel solitons in
confined structures. As a nanowire's width is decreased with a nano-contact of
fixed size at its center, the observed modes undergo transitions from a fully
localized two-dimensional droplet into a two-dimensional droplet edge mode and
then a pulsating one-dimensional droplet. These solitons are interpreted as
dissipative versions of classical, conservative solitons, allowing for an
analytical description of the modes and the mechanisms of bifurcation. The
presented results open up new possibilities for the study of low-dimensional
solitons and droplet applications in nanostructures.",1308.4812v1
2013-08-27,Growth rate degeneracies in kinematic dynamos,"We consider the classical problem of kinematic dynamo action in simple steady
flows. Due to the adjointness of the induction operator, we show that the
growth rate of the dynamo will be exactly the same for two types of magnetic
boundary conditions: the magnetic field can be normal (infinite magnetic
permeability, also called pseudo-vacuum) or tangent (perfect electrical
conductor) to the boundaries of the domain. These boundary conditions
correspond to well-defined physical limits often used in numerical models and
relevant to laboratory experiments. The only constraint is for the velocity
field u to be reversible, meaning there exists a transformation changing u into
-u. We illustrate this surprising property using S2T2 type of flows in
spherical geometry inspired by Dudley and James (1989). Using both types of
boundary conditions, it is shown that the growth rates of the dynamos are
identical, although the corresponding magnetic eigenmodes are drastically
different.",1308.5966v2
2013-09-05,Asymmetric Energy Transfer Between Coupled Magnetic Vortices by Asymmetric Gyration Amplification,"We present a numerical exploration of the possibility of sustained
amplification of magnetic vortex gyration by controlling the relative
polarities of a coupled vortices in short vortex chains. First, we numerically
establish the asymmetry in gyration of a single vortex based on its polarity by
use of external magnetic field rotating at the gyrotropic frequency. This
phenomena can be used to design logical adapters if vortex core switching is
avoided. The criteria to obtain a good gyration amplitude ratio to easily
observe true or false output has been examined further. The cases of coupled
magnetic vortices and short vortex chains have been studied with different
polarity configurations to reveal other desirable aspects of vortex dynamics
including, but not limited to, highly efficient signal transfer. These findings
are important in applications for information signal processing.",1309.1367v2
2013-09-20,"Ground state and Spin-Wave dynamics in Brownmillerite SrCoO2.5, A combined Hybrid Functional and LSDAU study","We theoretically investigate the ground state magnetic properties of the
brownmillerite phase of SrCoO2.5. Strong correlations within Co d electrons are
treated within the local spin density approximations of Density Functional
theory (DFT) with Hubbard U corrections (LSDAU) and results are compared with
the Heyd Scuzeria Ernzerhof (HSE) functional. The parameters computed with a U
value of 7.5 eV are found to match closely to those computed within the HSE
functional. A G type antiferromagnetic structure is found to be the most stable
one, consistent with experimental observation. By mapping the total energies of
different magnetic configurations onto a Heisenberg Hamiltonian we compute the
magnetic exchange interaction parameters, J, between the nearest neighbor Co
atoms. The J s obtained are then used to compute the spin wave frequencies and
inelastic neutron scattering intensities. Among four spin wave branches, the
lowest energy mode was found to have the largest scattering intensity at the
magnetic zone center, while the other modes becomes dominant at different
momenta. These predictions can be tested by experimentally.",1309.5287v1
2013-09-29,Coupled magnetic and ferroelectric excitations in PbFe_{1/2}Nb_{1/2}O_{3},"A neutron scattering investigation of the magnetoelectric coupling in
PbFe_{1/2}Nb_{1/2}O_{3} (PFN) has been undertaken. Ferroelectric order occurs
below 400 K, as evidenced by the softening with temperature and subsequent
recovery of the zone center transverse optic phonon mode energy (\hbar
\Omega_{0}). Over the same temperature range, magnetic correlations become
resolution limited on a terahertz energy scale. In contrast to the behavior of
nonmagnetic disordered ferroelectrics (namely Pb(Mg,Zn)_{1/3}Nb_{2/3}O_{3}), we
report the observation of a strong deviation from linearity in the temperature
dependence of (\hbar \Omega_{0})^{2}. This deviation is compensated by a
corresponding change in the energy scale of the magnetic excitations, as probed
through the first moment of the inelastic response. The coupling between the
short-range ferroelectric and antiferromagnetic correlations is consistent with
calculations showing that the ferroelectricity is driven by the displacement of
the body centered iron site, illustrating the multiferroic nature of magnetic
lead based relaxors in the dynamical regime.",1309.7587v1
2013-10-09,Interchange reconnection between an active region and a corona hole,"With the data from the Atmospheric Imaging Assembly (AIA) and the
Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory
(SDO), we present a magnetic interaction between an isolated coronal hole (CH)
and an emerging active region (AR). The AR emerged nearby the CH and interacted
with it. Bright loops constantly formed between them, which led to a continuous
retreat of the CH boundaries (CHBs). Meanwhile, two coronal dimmings
respectively appeared at the negative polarity of the AR and the east boundary
of the bright loops, and the AR was partly disturbed. Loop eruptions followed
by a flare occurred in the AR. The interaction was also accompanied by many
jets and an arc-shaped brightening that appeared to be observational signatures
of magnetic reconnection at the CHBs. By comparing the observations with the
derived coronal magnetic configuration, it is suggested that the interaction
between the CH and the AR excellently fitted in with the model of interchange
reconnection. It appears that our observations provide obvious evidences for
interchange reconnection.",1310.2395v1
2013-10-16,Domain wall motion in magnetic nanowires: An asymptotic approach,"We develop a systematic asymptotic description for domain wall motion in
one-dimensional magnetic nanowires under the influence of small applied
magnetic fields and currents and small material anisotropy. The magnetization
dynamics, as governed by the Landau--Lifshitz--Gilbert equation, is
investigated via a perturbation expansion. We compute leading-order behaviour,
propagation velocities, and first-order corrections of both travelling waves
and oscillatory solutions, and find bifurcations between these two types of
solutions. This treatment provides a sound mathematical foundation for numerous
results in the literature obtained through more ad hoc arguments.",1310.4442v1
2013-10-23,Kinematic dynamo action in square and hexagonal patterns,"We consider kinematic dynamo action in rapidly rotating Boussinesq convection
just above onset. The velocity is constrained to have either a square or a
hexagonal pattern. For the square pattern, large-scale dynamo action is
observed at onset, with most of the magnetic energy being contained in the
horizontally-averaged component. As the magnetic Reynolds number increases,
small-scale dynamo action becomes possible, reducing the overall growth rate of
the dynamo. For the hexagonal pattern, the breaking of symmetry between up and
down flows results in an effective pumping velocity. For intermediate rotation
rates, this additional effect can prevent the growth of any mean-field dynamo,
so that only a small-scale dynamo is eventually possible at large enough
magnetic Reynolds number. For very large rotation rates, this pumping term
becomes negligible, and the dynamo properties of square and hexagonal patterns
are qualitatively similar. These results hold for both perfectly conducting and
infinite magnetic permeability boundary conditions.",1310.6255v1
2013-11-06,Observation of zero-point quantum fluctuations of a single-molecule magnet through the relaxation of its nuclear spin bath,"A single-molecule magnet placed in a magnetic field perpendicular to its
anisotropy axis can be truncated to an effective two-level system, with easily
tunable energy splitting. The quantum coherence of the molecular spin is
largely determined by the dynamics of the surrounding nuclear spin bath. Here
we report the measurement of the nuclear spin--lattice relaxation in a single
crystal of the single-molecule magnet Mn$_{12}$-ac, at $T \approx 30$ mK in
perpendicular fields $B_{\perp}$ up to 9 T. Although the molecular spin is in
its ground state, we observe an increase of the nuclear relaxation rates by
several orders of magnitude up to the highest $B_{\perp}$. This unique finding
is a consequence of the zero-point quantum fluctuations of the Mn$_{12}$-ac
spin, which allow it to efficiently transfer energy from the excited nuclear
spin bath to the lattice. Our experiment highlights the importance of quantum
fluctuations in the interaction between an `effective two-level system' and its
surrounding spin bath.",1311.1373v1
2013-11-25,Notes on analytical study of holographic superconductors with Lifshitz scaling in external magnetic field,"We employ the matching method to analytically investigate the holographic
superconductors with Lifshitz scaling in an external magnetic field. We discuss
systematically the restricted conditions for the matching method and find that
this analytic method is not always powerful to explore the effect of external
magnetic field on the holographic superconductors unless the matching point is
chosen in an appropriate range and the dynamical exponent $z$ satisfies the
relation $z=d-1$ or $z=d-2$. From the analytic treatment, we observe that
Lifshitz scaling can hinder the condensation to be formed, which can be used to
back up the numerical results. Moreover, we study the effect of Lifshitz
scaling on the upper critical magnetic field and reproduce the well-known
relation obtained from Ginzburg-Landau theory.",1311.6260v3
2013-11-29,Resonant optical pumping of a Mn spin in a strain free quantum dot,"We report on the spin properties of individual Mn atoms in II-VI
semiconductor strain free quantum dots. Strain free Mn-doped CdTe quantum dots
are formed by width fluctuations in thin quantum wells lattice matched on a
CdTe substrate. These quantum dots permit to optically probe and address any
spin state of a Mn atom in a controlled strain environment. The absence of
strain induced magnetic anisotropy prevents an optical pumping of the Mn spin
at zero magnetic field. Thus, a large photoluminescence is obtained under
resonant optical excitation of the exciton-Mn complex. An efficient optical
pumping of the coupled electronic and nuclear spins of the Mn is restored under
a weak magnetic field. The observed reduction of the resonant photoluminescence
intensity under magnetic field is well described by a model including the
hyperfine coupling and a residual crystal field splitting of the Mn atom.
Finally, we show that the second order correlation function of the resonant
photoluminescence presents a large photon bunching at short delay which is a
probe of the dynamics of coupled electronic and nuclear spins of the Mn atom.",1311.7622v1
2013-12-11,Electric current generation in distorted graphene,"Graphene-like materials can be effectively described by Quantum
Electrodynamics in (2+1)-dimensions. In a pristine state, these systems exhibit
a symmetry between the nonequivalent Dirac points in the honeycomb lattice.
Realistic samples which include distortions and crystalline anisotropies are
considered through mass gaps of topological and dynamical nature. In this work
we show that the incorporation of an in-plane uniform external magnetic field
on this pseudochiral asymmetric configuration generates a non-dissipative
electric current aligned with the magnetic field: The pseudo chiral magnetic
effect. This scenario resembles the chiral magnetic effect in Quantum
Chromodynamics.",1312.3274v3
2014-01-08,Successive filament eruptions within one solar breakout event,"The magnetic breakout model has been widely used to explain solar eruptive
activities. Here, we apply it to explain successive filament eruptions occurred
in a quadrupolar magnetic source region. Based on the high temporal and spatial
resolution, multi-wavelengths observations taken by the Atmospheric Imaging
Assembly (AIA) on board the Solar Dynamic Observatory (SDO), we find some
signatures that support the occurrence of breakout-like external reconnection
just before the start of the successive filament eruptions. Furthermore, the
extrapolated three-dimensional coronal field also reveals that the magnetic
topology above the quadrupolar source region resembles that of the breakout
model. We propose a possible mechanism within the framework of the breakout
model to interpret the successive filament eruptions, in which the so-called
magnetic implosion mechanism is firstly introduced to be the physical linkage
of successive filament eruptions. We conclude that the structural properties of
coronal fields are important for producing successive filament eruptions.",1401.1694v1
2014-01-22,Long-term memory in experiments and numerical simulations of hydrodynamic and magnetohydrodynamic turbulence,"We analyze time series stemming from experiments and direct numerical
simulations of hydrodynamic and magnetohydrodynamic turbulence. Simulations are
done in periodic boxes, but with a volumetric forcing chosen to mimic the
geometry of the flow in the experiments, the von K\'arm\'an swirling flow
between two counter-rotating impellers. Parameters in the simulations are
chosen to (within computational limitations) allow comparisons between the
experiments and the numerical results. Conducting fluids are considered in all
cases. Two different configurations are considered: a case with a weak
externally imposed magnetic field, and a case with self-sustained magnetic
fields. Evidence of long-term memory and $1/f$ noise is observed in experiments
and simulations, in the case with weak magnetic field associated with the
hydrodynamic behavior of the shear layer in the von K\'arm\'an flow, and in the
dynamo case associated with slow magnetohydrodynamic behavior of the large
scale magnetic field.",1401.5792v1
2014-03-03,Faraday rotation echo spectroscopy and detection of quantum fluctuations,"Central spin decoherence is useful for detecting many-body physics in
environments and moreover, the spin echo control can remove the effects of
static thermal fluctuations so that the quantum fluctuations are revealed. The
central spin decoherence approach, however, is feasible only in some special
configurations and often requires uniform coupling between the central spin and
individual spins in the baths, which are very challenging in experiments. Here,
by making analogue between central spin decoherence and depolarization of
photons, we propose a scheme of Faraday rotation echo spectroscopy (FRES) for
studying quantum fluctuations in interacting spin systems. The echo control of
the photon polarization is realized by flipping the polarization with a
birefringence crystal. The FRES, similar to spin echo in magnetic resonance
spectroscopy, can suppress the effects of the static magnetic fluctuations and
therefore reveal dynamical magnetic fluctuations. We apply the scheme to a
rare-earth compound LiHoF4 and calculate the echo signal, which is related to
the quantum fluctuations of the system. We observe enhanced signals at the
phase boundary. The FRES should be useful for studying quantum fluctuations in
a broad range of spin systems, including cold atoms, quantum dots, solid-state
impurities, and transparent magnetic materials.",1403.0420v1
2014-03-14,The Helioseismic and Magnetic Imager (HMI) Vector Magnetic Field Pipeline: Optimization of the Spectral Line Inversion Code,"The Very Fast Inversion of the Stokes Vector (VFISV) is a Milne-Eddington
spectral line inversion code used to determine the magnetic and thermodynamic
parameters of the solar photosphere from observations of the Stokes vector in
the 6173 A Fe I line by the Helioseismic and Magnetic Imager (HMI) onboard the
Solar Dynamics Observatory (SDO). We report on the modifications made to the
original VFISV inversion code in order to optimize its operation within the HMI
data pipeline and provide the smoothest solution in active regions. The changes
either sped up the computation or reduced the frequency with which the
algorithm failed to converge to a satisfactory solution. Additionally, coding
bugs which were detected and fixed in the original VFISV release, are reported
here.",1403.3677v1
2014-03-22,Lifshitz Effects on Vector Condensate Induced by a Magnetic Field,"By numerical and analytical methods, we study in detail the effects of the
Lifshitz dynamical exponent $z$ on the vector condensate induced by an applied
magnetic field in the probe limit. Concretely, in the presence of the magnetic
field, we obtain the Landau level independent of $z$, and also find the
critical value by coupling a Maxwell complex vector field and SU(2) field into
a (3+1)-dimensional Lifshitz black hole, respectively. The research results
show that for both two models with the lowest Landau level, the increasing $z$
improves the response of the critical temperature to the applied magnetic field
even without the charge density, and the analytical results uphold the
numerical results. In addition, we find even in the Lifshitz black hole, the
Maxwell complex vector model is still a generalization of the SU(2) Yang-Mills
model. Furthermore, we construct the square vortex lattice and discuss the
implications of these results.",1403.5649v2
2014-03-25,Long-term Evolution of Large-scale Magnetic Fields in Rotating Stratified Convection,"Convective dynamo simulations are performed in local Cartesian geometry. We
report the first successful simulation of a large-scale oscillatory dynamo in
rigidly rotating convection without stably stratified layers. A key requirement
for exciting the large-scale dynamo is a sufficiently long integration time
comparable to the ohmic diffusion time. By comparing two models with and
without stably stratified layers, their effect on the large-scale dynamo is
also studied. The spatiotemporal evolution of the large-scale magnetic field is
similar in both models. However, it is intriguing that the magnetic cycle is
much shorter in the model without the stable layer than with the stable layer.
This suggests that the stable layer impedes the cyclic variations of the
large-scale magnetic field.",1403.6221v1
2014-04-01,"Magnetic order and spin dynamics in La2O2Fe2OSe2 probed by 57Fe-Moessbauer, 139La-NMR, and muon spin relaxation spectroscopy","We present a detailed local probe study of the magnetic order in the
oxychalcogenide La2O2Fe2OSe2 utilizing 57Fe Moessbauer, 139La NMR, and muon
spin relaxation spectroscopy. This system can be regarded as an insulating
reference system of the Fe arsenide and chalcogenide superconductors. From the
combination of the local probe techniques we identify a non-collinear magnetic
structure similar to Sr2F2Fe2OS2. The analysis of the magnetic order parameter
yields an ordering temperature TN = 90.1 K and a critical exponent of beta =
0.133, which is close to the 2D Ising universality class as reported in the
related oxychalcogenide family.",1404.0249v2
2014-04-15,Molding the flow of light with a magnetic field: plasmonic cloaking and directional scattering,"We investigate electromagnetic scattering and plasmonic cloaking in a system
composed by a dielectric cylinder coated with a magneto-optical shell. In the
long-wavelength limit we demonstrate that the application of an external
magnetic field can not only switch on and off the cloaking mechanism but also
mitigate losses, as the absorption cross-section is shown to be minimal
precisely at the cloaking operation frequency band. We also show that the
angular distribution of the scattered radiation can be effectively controlled
by applying an external magnetic field, allowing for a swift change in the
scattering pattern. By demonstrating that these results are feasible with
realistic, existing magneto-optical materials, such as graphene epitaxially
grown on SiC, we suggest that magnetic fields could be used as an effective,
versatile external agent to tune plasmonic cloaks and to dynamically control
electromagnetic scattering in an unprecedented way, we hope that these results
may find use in disruptive photonic technologies.",1404.3979v1
2014-05-02,Weyl fermions induced Magnon electrodynamics in Weyl semimetal,"Weyl fermions, which are fermions with definite chiralities, can give rise to
anomalous breaking of the symmetry of the physical system which they are a part
of. In their (3+1)-dimensional realizations in condensed matter systems, i.e.,
the so-called Weyl semimetals, this anomaly gives rise to topological
electromagnetic response of magnetic fluctuations, which takes the form of
non-local interaction between magnetic fluctuations and electromagnetic fields.
We study the physical consequences of this non-local interaction, including
electric field assisted magnetization dynamics, an extra gapless magnon
dispersion, and polariton behaviors that feature ""sibling"" bands in small
magnetic fields.",1405.0491v2
2014-05-15,Reducing error rates in straintronic multiferroic dipole-coupled nanomagnetic logic by pulse shaping,"Dipole-coupled nanomagnetic logic (NML), where nanomagnets with bistable
magnetization states act as binary switches and information is transferred
between them via dipole coupling and Bennett clocking, is a potential
replacement for conventional transistor logic since magnets dissipate less
energy than transistors when they switch in response to the clock. However,
dipole-coupled NML is much more error-prone than transistor logic because
thermal noise can easily disrupt magnetization dynamics. Here, we study a
particularly energy-efficient version of dipole-coupled NML known as
straintronic multiferroic logic (SML) where magnets are clocked/switched with
electrically generated mechanical strain. By appropriately shaping the voltage
pulse that generates strain, the error rate in SML can be reduced to tolerable
limits. In this paper, we describe the error probabilities associated with
various stress pulse shapes and discuss the trade-off between error rate and
switching speed in SML.",1405.4000v1
2014-05-17,Noise-enhanced synchronization of stochastic magnetic oscillators,"We present an experimental study of phase-locking in a stochastic magnetic
oscillator. The system comprises a magnetic tunnel junction with a
superparamagnetic free layer, whose magnetization dynamics is driven with spin
torques through an external periodic driving current. We show that
synchronization of this stochastic oscillator to the input current is possible
for current densities below $3 \times 10^6$ A/cm$^2$, and occurs for input
frequencies lower than the natural mean frequency of the stochastic oscillator.
We show that such injection-locking is robust and leads to a drastic reduction
in the phase diffusion of the stochastic oscillator, despite the presence of a
frequency mismatch between the oscillator and the excitation.",1405.4360v1
2014-05-17,Attitude dynamics and control of spacecraft using geomagnetic Lorentz force,"The attitude stabilization of a charged rigid spacecraft in Low Earth Orbit
(LEO) using torques due to Lorentz force in pitch and roll directions is
considered. A spacecraft that generates an electrostatic charge on its surface
in the Earth magnetic field will be subject to perturbations from Lorentz
force. The Lorentz force acting on an electrostatically charged spacecraft may
provide a useful thrust for controlling a spacecraft's orientation. We assume
that the spacecraft is moving in the Earth's magnetic field in an elliptical
orbit under the effects of the gravitational, geomagnetic and Lorentz torques.
The magnetic field of the Earth is modeled as a non-tilted dipole. A model
incorporating all Lorentz torques as a function of orbital elements has been
developed on the basis of electric and magnetic fields. The stability of the
spacecraft orientation is investigated both analytically and numerically. The
existence and stability of equilibrium positions is investigated for different
values of the charge to mass ratio ($\alpha^*$). Stable orbits are identified
for various values of $\alpha^*$. The main parameters for stabilization of the
spacecraft are $\alpha^*$ and the difference between the components of the
moment of inertia of spacecraft.",1405.4411v1
2014-05-18,Ultracold Spin-Orbit Coupled Bose-Einstein Condensate in a Cavity: Route to Magnetic Phases Through Cavity Transmission,"We study the spin orbit coupled ultra cold Bose-Einstein condensate placed in
a single mode Fabry-P\'erot cavity. The cavity introduces a quantum optical
lattice potential which dynamically couples with the atomic degrees of freedom
and realizes a generalized extended Bose Hubbard model whose zero temperature
phase diagram can be controlled by tuning the cavity parameters. In the
non-interacting limit, where the atom-atom interaction is set to zero, the
resulting atomic dispersion shows interesting features such as bosonic analogue
of Dirac points, cavity controlled Hofstadter spectrum which bears the hallmark
of pseudo-spin-1/2 bosons in presence of Abelian and non-Abelian gauge field
(the later due to spin-orbit coupling) in a cavity induced optical lattice
potential. In the presence of atom-atom interaction, using a mapping to a
generalized Bose Hubbard model of spin-orbit coupled bosons in classical
optical lattice, we show that the system realizes a host of quantum magnetic
phases whose magnetic order can be be detected from the cavity transmission.
This provides an alternative approach for detecting quantum magnetism in ultra
cold atoms. We discuss the effect of cavity induced optical bistability on this
phases and their experimental consequences.",1405.4500v1
2014-06-02,Tracking Temperature Dependent Relaxation Times of Individual Ferritin Nanomagnets with a Wide-band Quantum Spectrometer,"We demonstrate the tracking of the spin dynamics of ensemble and individual
magnetic ferritin proteins from cryogenic up to room temperature using the
nitrogen-vacancy color center in diamond as magnetic sensor. We employ
different detection protocols to probe the influence of the ferritin
nanomagnets on the longitudinal and transverse relaxation of the
nitrogen-vacancy center, which enables magnetic sensing over a wide frequency
range from Hz to GHz. The temperature dependence of the observed spectral
features can be well understood by the thermally induced magnetization
reversals of the ferritin and enables the determination of the anisotropy
barrier of single ferritin molecules.",1406.0362v1
2014-06-25,Ion and electron heating during magnetic reconnection in weakly collisional plasmas,"Magnetic reconnection and associated heating of ions and electrons in
strongly magnetised, weakly collisional plasmas are studied by means of
gyrokinetic simulations. It is shown that an appreciable amount of the released
magnetic energy is dissipated to yield (irreversible) electron and ion heating
via phase mixing. Electron heating is mostly localized to the magnetic island,
not the current sheet, and occurs after the dynamical reconnection stage. Ion
heating is comparable to electron heating only in high-beta plasmas, and
results from both parallel phase mixing and perpendicular phase mixing due to
finite Larmor radius effects; in space, ion heating is localized to the
interior of a secondary island (plasmoid) that arises from the instability of
the current sheet.",1406.6456v2
2014-09-16,Phonon-modulated magnetic interactions and spin Tomonaga-Luttinger liquid in the p-orbital antiferromagnet CsO2,"The magnetic response of antiferromagnetic CsO2, coming from the p-orbital
S=1/2 spins of anionic O2- molecules, is followed by 133Cs nuclear magnetic
resonance across the structural phase transition occuring at Ts1=61 K on
cooling. Above Ts1, where spins form a square magnetic lattice, we observe a
huge, nonmonotonic temperature dependence of the exchange coupling originating
from thermal librations of O2- molecules. Below Ts1, where antiferromagnetic
spin chains are formed as a result of p-orbital ordering, we observe a spin
Tomonaga-Luttinger-liquid behavior of spin dynamics. These two interesting
phenomena, which provide rare simple manifestations of the coupling between
spin, lattice and orbital degrees of freedom, establish CsO2 as a model system
for molecular solids.",1409.4818v2
2014-09-18,Decoherence imaging of spin ensembles using a scanning single-electron spin in diamond,"The nitrogen-vacancy (NV) defect center in diamond has demonstrated great
capability for nanoscale magnetic sensing and imaging for both static and
periodically modulated target fields. However, it remains a challenge to detect
and image randomly fluctuating magnetic fields. Recent theoretical and
numerical works have outlined detection schemes that exploit changes in
decoherence of the detector spin as a sensitive measure for fluctuating fields.
Here we experimentally monitor the decoherence of a scanning NV center in order
to image the fluctuating magnetic fields from paramagnetic impurities on an
underlying diamond surface. We detect a signal corresponding to roughly 800
${\mu}_B$ in 2 s of integration time, without any control on the target spins,
and obtain magnetic-field spectral information using dynamical decoupling
techniques. The extracted spatial and temporal properties of the surface
paramagnetic impurities provide insight to prolonging the coherence of
near-surface qubits for quantum information and metrology applications.",1409.5418v1
2014-09-23,A strategy for the design of skyrmion racetrack memories,"Magnetic storage based on racetrack memory is very promising for the design
of ultra-dense, low-cost and low-power storage technology. Information can be
coded in a magnetic region between two domain walls or, as predicted recently,
in topological magnetic objects known as skyrmions. Here, we show the
technological advantages and limitations of using Bloch and Neel skyrmions
manipulated by spin current generated within the ferromagnet or via the
spin-Hall effect arising from a non-magnetic heavy metal underlayer. We found
that the Neel skyrmion moved by the spin-Hall effect is a very promising
strategy for technological implementation of the next generation of skyrmion
racetrack memories (zero field, high thermal stability, and ultra-dense
storage). We employed micromagnetics reinforced with an analytical formulation
of skyrmion dynamics that we developed from the Thiele equation. We identified
that the excitation, at high currents, of a breathing mode of the skyrmion
limits the maximal velocity of the memory.",1409.6491v2
2014-09-30,Towards the stabilization of extra dimensions by brane dynamics,"All the models of elementary particles and their interactions derived from
String Theory involve a compact six-dimensional internal space. Its volume and
shape should be fixed or stabilized, since otherwise massless scalar fields
(moduli) reflecting their deformations appear in our four-dimensional
space-time, with sizable effects on known particles and fields. We propose a
strategy towards stabilizing the compact space without fluxes of three-form
fields from closed strings. Our main motivation and goal is to proceed insofar
as possible within conventional string world-sheet theory. As we shall see,
D-branes with magnetic flux (""magnetized D-branes"") and the forces between them
can be used to this end. We investigate here some necessary ingredients: open
string one-loop vacuum amplitudes between magnetized D-branes, magnetized
D-branes fixed at orbifold singularities, and potential energies among such
D-branes in the compact space that result from tree-level closed string
exchanges.",1409.8401v3
2014-10-03,Vortices at the magnetic equator generated by hybrid Alfven resonant waves,"We performed three-dimensional magnetohydrodynamic simulations of shear
Alfven waves in a full field line system with magnetosphere-ionosphere coupling
and plasma non-uniformities. Feedback instability of the Alfven resonant modes
showed various nonlinear features under the field line cavities: i) a secondary
flow shear instability occurs at the magnetic equator, ii) trapping of the
ionospheric Alfven resonant modes facilitates deformation of field-aligned
current structures, and iii) hybrid Alfven resonant modes grow to cause
vortices and magnetic oscillations around the magnetic equator. Essential
features in the initial brightening of auroral arc at substorm onsets could be
explained by the dynamics of Alfven resonant modes, which are the nature of the
field line system responding to a background rapid change.",1410.0764v1
2014-10-15,Magneto-optical properties of paramagnetic superrotors,"We study the dynamics of paramagnetic molecular superrotors in an external
magnetic field. Optical centrifuge is used to create dense ensembles of oxygen
molecules in ultra-high rotational states. It is shown for the first time, that
the gas of rotating molecules becomes optically birefringent in the presence of
magnetic field. The discovered effect of ""magneto-rotational birefringence""
indicates preferential alignment of molecular axes along the field direction.
We provide an intuitive qualitative model, in which the influence of the
applied magnetic field on the molecular orientation is mediated by the
spin-rotation coupling. This model is supported by the direct imaging of the
distribution of molecular axes, the demonstration of the magnetic reversal of
the rotational Raman signal, and by numerical calculations.",1410.4257v4
2014-10-24,Rectification of radio frequency current in giant magnetoresistance spin valve,"We report on a highly efficient spin diode effect in an exchange-biased
spin-valve giant magnetoresistance (GMR) strips. In such multilayer structures,
symmetry of the current distribution along the vertical direction is broken
and, as a result, a non-compensated Oersted field acting on the magnetic free
layer appears. This field, in turn, is a driving force of magnetization
precessions. Due to the GMR effect, resistance of the strip oscillates
following the magnetization dynamics. This leads to rectification of the
applied radio frequency current and induces a direct current voltage $V_{DC}$.
We present a theoretical description of this phenomenon and calculate the spin
diode signal, $V_{DC}$, as a function of frequency, external magnetic field,
and angle at which the external field is applied. A satisfactory quantitative
agreement between theoretical predictions and experimental data has been
achieved. Finally, we show that the spin diode signal in GMR devices is
significantly stronger than in the anisotropic magnetoresistance
permalloy-based devices.",1410.6672v1
2014-10-24,Non-linear spin-wave excitation at low bias fields,"Non-linear magnetization dynamics is essential for the operation of many
spintronics devices. For microwave assisted switching of magnetic elements the
low field regime is of particular interest. In addition a large number of
experiments uses high amplitude FMR in order to generate d.c. currents via spin
pumping mechanism. Here we use time resolved X-ray magnetic circular dichroism
experiments to determine the number density of excited magnons in magnetically
soft Ni_80Fe_20 thin films at small bias fields and large rf-excitation
amplitudes. Our data shows that the common model of non-linear ferromagnetic
resonance is not suitable to describe the low bias field limit. Here we derive
a new model of parametric spin-wave excitation which correctly predicts
threshold amplitudes and decay rates also at low bias fields. In fact a new
series of critical modes with amplitude phase oscillations is found,
generalizing the theory of parametric spin-wave excitation.",1410.6693v1
2014-10-26,Short-range correlations in the magnetic ground state of Na4Ir3O8,"The magnetic ground state of the hyper-kagome lattice in Na4Ir3O8 is explored
via combined bulk magnetization, muon spin relaxation, and neutron scattering
measurements. A short-range, frozen, state comprised of quasi-static moments
develops below a characteristic temperature of T_F=6 K, revealing an
inhomogeneous distribution of spins occupying the entirety of the sample
volume. Quasi-static, short-range, spin correlations persist until at least 20
mK and differ substantially from the nominally dynamic response of a quantum
spin liquid. Our data demonstrate that an inhomogeneous magnetic ground state
arises in Na4Ir3O8 driven either by disorder inherent to the creation of the
hyper-kagome lattice itself or stabilized via quantum fluctuations.",1410.7072v1
2014-11-12,Electric-Field Control over Spin-Wave and Current Induced Domain Wall Motion and Magnonic Torques in Multiferroics,"We discover that the way spin-waves exert magnetic torques in multiferroic
materials can cause not only domain wall motion, but also magnetization
dynamics for homogeneous magnetization textures. Interestingly, the domain wall
motion can be controlled via purely electrical means with the spin-waves being
generated by an ac electric field $E$ while the direction of the wall motion
also is sensitive to an applied dc $E$ field. Moreover, we determine the
interaction between spin-transfer torque from an electric current and a
magnetic domain wall in multiferroics and show that the Walker breakdown
threshold scales with the magnitude of a perpendicular electric field, offering
a way to control the properties of domain wall propagation via electric gating.",1411.3327v2
2014-11-03,Magnetic Critical Solutions in Holography,"The AdS/CFT correspondence is a realization of the holographic principle in
the context of string theory. It is a map between a quantum field theory and a
string theory living in one or more extra dimensions. Holography provides new
tools to study strongly-coupled quantum field theories. It has important
applications in quantum chromodynamics (QCD) and condensed matter (CM) systems,
which are usually complicated and strongly coupled. Quantum critical CM
theories have scaling symmetries and can be connected to higher-dimensional
scale invariant space-times. The Effective Holographic Theory paradigm may be
used to describe the low-energy (IR) holographic dynamics of quantum critical
systems by the Einstein-Maxwell-Dilaton (EMD) theory. We find the magnetic
critical scaling solutions of an EMD theory containing an extra parity-odd term
$F\wedge F$. Previous studies in the absence of magnetic fields have shown the
existence of quantum critical lines separated by quantum critical points. We
find this is also true in the presence of a magnetic field. The critical
solutions are characterized by the triplet of critical exponents
($\theta,z,\zeta$), the first two describing the geometry, while the latter
describes the charge density.",1411.3579v3
2014-11-26,Electrical manipulation of orbital occupancy and magnetic anisotropy in manganites,"Electrical manipulation of lattice, charge, and spin has been realized
respectively by the piezoelectric effect, field-effect transistor, and electric
field control of ferromagnetism, bringing about dramatic promotions both in
fundamental research and industrial production. However, it is generally
accepted that the orbital of materials are impossible to be altered once they
have been made. Here we use electric-field to dynamically tune the electronic
phase transition in (La,Sr)MnO3 films with different Mn^4+/(Mn^3+ + Mn^4+)
ratios. The orbital occupancy and corresponding magnetic anisotropy of these
thin films are manipulated by gate voltage in a reversible and quantitative
manner. Positive gate voltage increases the proportion of occupancy of the
orbital and magnetic anisotropy that were initially favored by strain
(irrespective of tensile and compressive), while negative gate voltage reduces
the concomitant preferential orbital occupancy and magnetic anisotropy. Besides
its fundamental significance in orbital physics, our findings might advance the
process towards practical oxide-electronics based on orbital.",1411.7128v1
2014-11-27,Inverse Magnetic Catalysis in the three-flavor NJL model with axial-vector interaction,"In this paper we explore the chiral phase transition in QCD within the
three-flavor Nambu-Jona-Lasinio (NJL) model with a negative coupling constant
in the isoscalar axial-vector channel, which is associated with a polarized
instanton--anti-instanton molecule background. The QCD phase diagram described
in this scenario shows a new first order phase transition around the transition
temperature $T_c$ toward a phase without chiral condensates, but with
nontrivial dynamic chiral chemical potentials for the light quarks,
spontaneously giving rise to local $\mathcal {CP}$ violation and local
chirality imbalance. The corresponding critical temperature $T_{5c}$ for this
phase transition decreases with the magnetic field and it gives a natural
explanation to the inverse magnetic catalysis effect for light quarks when
incorporating a reasonable value of the coupling constant in the isoscalar
axial-vector channel. Furthermore, when the isoscalar axial-vector interaction
is dominant in light quark sector and suppressed in strange quark sector, it is
found that there is no inverse magnetic catalysis for strange quark condensate,
which agrees with lattice results.",1411.7552v2
2014-12-23,Super-elastic Collision of Large-scale Magnetized Plasmoids in The Heliosphere,"Super-elastic collision is an abnormal collisional process, in which some
particular mechanisms cause the kinetic energy of the system increasing. Most
studies in this aspect focus on solid-like objects, but they rarely consider
gases or liquids, as the collision of the latter is primarily a mixing process.
With cross-field diffusion being effectively prohibited, magnetized plasmoids
are different from ordinary gases. But it remains unclear how they act during a
collision. Here we present the global picture of a unique collision between two
coronal mass ejections in the heliosphere, which are the largest magnetized
plasmoids erupting from the Sun. Our analysis for the first time reveals that
these two magnetized plasmoids collided like solid-like objects with a 73%
likelihood of being super-elastic. Their total kinetic energy surprisingly
increased by about 6.6% through the collision, which significantly influenced
the dynamics of the plasmoids.",1412.7375v1
2015-01-06,Design and simulation of a hybrid controller for a multi-input multi-output magnetic suspension system,"In this paper we present a Fuzzy Logic control approach designed to stabilize
a multi-input multi-output magnetic suspension system. The system has four
cubic floaters and four actuators that apply magnetic forces on the floaters,
the suspension is performed by changing the voltages applied on the actuators,
hence changing their currents, producing vertical magnetic forces that balance
with the gravitational force. A fuzzy logic controller is used to control each
actuator; the system is nonlinear and sensitive to initial conditions. Another
fuzzy logic controller is used as a supervisory controller in order to increase
the dynamic range of the system, enabling it to stabilize the floaters when the
initial displacements are relatively big. Another design consideration was to
keep the four floaters in the same plane as much as possible, to perform that
task, a PD controller was set to modulate the currents of the four actuators in
order to minimize an error signal measuring the relative vertical displacement
of all the four floaters. Simulation results show that the designed control
scheme stabilized the system for the design constrains.",1501.01294v1
2015-01-12,Magnetic correlations beyond the Heisenberg model in an Fe monolayer on Rh(001),"Motivated by a recent experimental observation of a complex magnetic
structure [Takada et al. 2013 J. Magn. Magn. Mater. 329 95] we present a
theoretical study of the magnetic structure of an Fe monolayer deposited on
Rh(001). We use a classical spin Hamiltonian with parameters obtained from ab
initio calculations and go beyond the usual anisotropic Heisenberg model by
including isotropic biquadratic interactions. Zero-temperature
Landau--Lifshitz--Gilbert spin dynamics simulations lead to a complex collinear
spin configuration that, however, contradicts experimental finding. We thus
conclude that higher order multi-spin interactions are likely needed to account
for the magnetic ordering of the system.",1501.02657v1
2015-01-12,Multiply periodic states and isolated skyrmions in an anisotropic frustrated magnet,"Multiply periodic states appear in a wide variety of physical contexts, such
as the Rayleigh-Benard convection, Faraday waves, liquid crystals, domain
patterns in ferromagnetic films and skyrmion crystals recently observed in
chiral magnets. Here we study a simple model of an anisotropic frustrated
magnet and show that its zero-temperature phase diagram contains numerous
multi-q states including the skyrmion crystal. We clarify the mechanism for
stabilization of these states, discuss their multiferroic properties and
formulate rules for finding new skyrmion materials. In addition to skyrmion
crystal, we find stable isolated skyrmions with topological charge 1 and 2.
Physics of isolated skyrmions in frustrated magnets is very rich. Their
statical and dynamical properties are strongly affected by the new zero mode -
skyrmion helicity.",1501.02757v1
2015-01-27,Wave Heating of the Solar Atmosphere,"Magnetic waves are a relevant component in the dynamics of the solar
atmosphere. Their significance has increased because of their potential as a
remote diagnostic tool and their presumed contribution to plasma heating
processes. We discuss our current understanding on coronal heating by magnetic
waves, based on recent observational evidence and theoretical advances. The
discussion starts with a selection of observational discoveries that have
brought magnetic waves to the forefront of the coronal heating discussion.
Then, our theoretical understanding on the nature and properties of the
observed waves and the physical processes that have been proposed to explain
observations are described. Particular attention is given to the sequence of
processes that link observed wave characteristics with concealed energy
transport, dissipation, and heat conversion. We conclude with a commentary on
how the combination of theory and observations should help us understanding and
quantifying magnetic wave heating of the solar atmosphere.",1501.06708v1
2015-02-12,Integrated information storage and transfer with a coherent magnetic device,"Quantum systems are inherently dissipation-less, making them excellent
candidates even for classical information processing. We propose to use an
array of large-spin quantum magnets for realizing a device which has two modes
of operation: memory and data-bus. While the weakly interacting low-energy
levels are used as memory to store classical information (bits), the
high-energy levels strongly interact with neighboring magnets and mediate the
spatial movement of information through quantum dynamics. Despite the fact that
memory and data-bus require different features, which are usually prerogative
of different physical systems -- well isolation for the memory cells, and
strong interactions for the transmission -- our proposal avoids the notorious
complexity of hybrid structures. The proposed mechanism can be realized with
different setups. We specifically show that molecular magnets, as the most
promising technology, can implement hundreds of operations within their
coherence time, while adatoms on surfaces probed by a scanning tunneling
microscope is a future possibility.",1502.03857v2
2015-02-19,Exciton-polaron complexes in pulsed electrically-detected magnetic resonance,"Several microscopic pathways have been proposed to explain the large magnetic
effects observed in organic semiconductors, but identifying and characterising
which microscopic process actually influences the overall magnetic field
response is challenging. Pulsed electrically-detected magnetic resonance
provides an ideal platform for this task as it intrinsically monitors the
charge carriers of interest and provides dynamical information which is
inaccessible through conventional magnetoconductance measurements. Here we
develop a general time domain theory to describe the spin-dependent reaction of
exciton-charge complexes following the coherent manipulation of paramagnetic
centers through electron spin resonance. A general Hamiltonian is treated, and
it is shown that the transition frequencies and resonance positions of the
exciton-polaron complex can be used to estimate inter-species coupling. This
work also provides a general formalism for analysing multi-pulse experiments
which can be used to extract relaxation and transport rates.",1502.05470v1
2015-02-23,Quantum metrology for the Ising Hamiltonian with transverse magnetic field,"We consider quantum metrology for unitary evolutions generated by
parameter-dependent Hamiltonians. We focus on the unitary evolutions generated
by the Ising Hamiltonian that describes the dynamics of a one-dimensional chain
of spins with nearest-neighbour interactions and in the pres- ence of a global,
transverse, magnetic field. We analytically solve the problem and show that the
precision with which one can estimate the magnetic field (interaction strength)
given one knows the interaction strength (magnetic field) scales at the
Heisenberg limit, and can be achieved by a linear superposition of the vacuum
and N free fermion states. In addition, we show that GHZ-type states exhibit
Heisenberg scaling in precision throughout the entire regime of parameters.
Moreover, we numerically observe that the optimal precision using a product
input state scales at the standard quantum limit.",1502.06459v3
2015-02-26,Coils and the Electromagnet Used in the Joule Balance at the NIM,"In the joule balance developed at National Institute of Metrology (NIM), the
dynamic phase of a watt balance is replaced by the mutual inductance
measurement in an attempt to provide an alternative method for the kg
redefinition. But for this method a rather large current in the exciting coil,
is needed to offer the necessary magnetic field in the force weighing phase,
and the coil heating becomes an important uncertainty source. To reduce coil
heating, a new coil system, in which a ferromagnetic material is used to
increase the magnetic field was designed recently. But adopting the
ferromagnetic material brings the difficulty from the nonlinear characteristic
of material. This problem can be removed by measuring the magnetic flux linkage
difference of the suspended coil at two vertical positions directly to replace
the mutual inductance parameter. Some systematic effects of this magnet are
discussed.",1502.07456v1
2015-02-28,Distinct magnetic signatures of fractional vortex configurations in multiband superconductors,"Vortices carrying fractions of a flux quantum are predicted to exist in
multiband superconductors, where vortex core can split between multiple
band-specific components of the superconducting condensate. Using the
two-component Ginzburg-Landau model, we examine such vortex configurations in a
two-band superconducting slab in parallel magnetic field. The fractional
vortices appear due to the band-selective vortex penetration caused by
different thresholds for vortex entry within each band-condensate, and
stabilize near the edges of the sample. We show that the resulting fractional
vortex configurations leave distinct fingerprints in the static measurements of
the magnetization, as well as in ac dynamic measurements of the magnetic
susceptibility, both of which can be readily used for the detection of these
fascinating vortex states in several existing multiband superconductors.",1503.00047v2
2015-03-02,Dynamical chiral symmetry breaking in the NJL model with a constant external magnetic field,"In this paper we develop a new method that is different from Schwinger proper
time method to deduce the fermion propagator with a constant external magnetic
field. In the NJL model, we use this method to find out the gap equation at
zero and non-zero temperature, and give the numerical results and phase diagram
between magnetic field and temperature. Beside these, we also introduce current
mass to study the susceptibilities, because there is a new parameter (the
strength of external magnetic field) in this problem, corresponding this new
parameter, we have defined a new susceptibility $\chi_B$ to compare with the
other two susceptibilities $\chi_c$ (chiral susceptibility) and $\chi_T$
(thermal susceptibility), and all of the three susceptibilities show than when
current mass is not zero, the phase transition is a crossover, while for
comparison, in the chiral limit, the susceptibilities show a second order phase
transition. At last, we give out the critical coefficients of different
susceptibilities in the chiral limit.",1503.00452v1
2015-03-03,Magnetic Fields of Uranus and Neptune: Metallic Fluid Hydrogen,"The magnetic fields of the Ice Giant Planets Uranus and Neptune (U/N) are
unique in the solar system. Based on a substantial database measured on Earth
for representative planetary fluids at representative dynamic pressures up to
200 GPa (2 Mbar) and a few 1000 K, the complex magnetic fields of U/N are (i)
probably made primarily by degenerate metallic fluid H (MFH) at or near the
crossover from the H-He envelopes to Ice cores at ~100 GPa (Mbar) pressures and
normalized radii of ~90% of the radii of U/N; (ii) because those magnetic
fields are made relatively close to the surfaces of U/N, non-dipolar fields can
be expected; (iii) the Ice cores are most probably a heterogeneous fluid
mixture of H, N, O, C, Fe/Ni and silicate-oxides and their mutual reaction
products at high pressures and temperatures,as discussed elsewhere. Ironically,
there is probably little nebular Ice in the Ice Giant Planets.",1503.01042v2
2015-03-30,Broadband Setup for Magnetic-Field-Induced Domain Wall Motion in Cylindrical Nanowires,"In order to improve the precision of domain wall dynamics measurements, we
develop a coplanar waveguide-based setup where the domain wall motion should be
triggered by pulses of magnetic field. The latter are produced by the Oersted
field of the waveguide as a current pulse travels toward its termination, where
it is dissipated. Our objective is to eliminate a source of bias in domain wall
speed estimation while optimizing the field amplitude. Here, we present
implementations of this concept for magnetic force microscopy (MFM) and
synchrotron-based investigation.",1503.08557v2
2015-04-01,Direct observation and imaging of a spin-wave soliton with $p-$like symmetry,"The prediction and realization of magnetic excitations driven by electrical
currents via the spin transfer torque effect, enables novel magnetic
nano-devices where spin-waves can be used to process and store information. The
functional control of such devices relies on understanding the properties of
non-linear spin-wave excitations. It has been demonstrated that spin waves can
show both an itinerant character, but also appear as localized solitons. So
far, it was assumed that localized solitons have essentially cylindrical,
$s-$like symmetry. Using a newly developed high-sensitivity time-resolved
magnetic x-ray microscopy, we instead observe the emergence of a novel
localized soliton excitation with a nodal line, i.e. with $p-$like symmetry.
Micromagnetic simulations identify the physical mechanism that controls the
transition from $s-$ to $p-$like solitons. Our results suggest a potential new
pathway to design artificial atoms with tunable dynamical states using
nanoscale magnetic devices.",1504.00144v1
2015-04-01,All-magnetic control of skyrmions in nanowires by a spin wave,"Magnetic skyrmions are topologically protected nanoscale objects, which are
promising building blocks for novel magnetic and spintronic devices. Here, we
investigate the dynamics of a skyrmion driven by a spin wave in a magnetic
nanowire. It is found that (i) the skyrmion is first accelerated and then
decelerated exponentially; (ii) it can turn L-corners with both right and left
turns; and (iii) it always turns left (right) when the skyrmion number is
positive (negative) in the T- and Y-junctions. Our results will be the basis of
skyrmionic devices driven by a spin wave.",1504.00409v2
2015-04-09,Low-field microwave absorption and magnetoresistance in iron nanostructures grown by electrodeposition on n-type lightly-doped silicon substrates,"In this study we investigate magnetic properties, surface morphology and
crystal structure in iron nanoclusters electrodeposited on lightly-doped (100)
n-type silicon substrates. Our goal is to investigate the spin injection and
detection in the Fe/Si lateral structures. The samples obtained under electric
percolation were characterized by magnetoresistive and magnetic resonance
measurements with cycling the sweeping applied field in order to understand the
spin dynamics in the as-produced samples. The observed hysteresis in the
magnetic resonance spectra, plus the presence of a broad peak in the
non-saturated regime confirming the low field microwave absorption (LFMA), were
correlated to the peaks and slopes found in the magnetoresistance curves. The
results suggest long range spin injection and detection in low resistive
silicon and the magnetic resonance technique is herein introduced as a
promising tool for analysis of electric contactless magnetoresistive samples.",1504.02508v1
2015-04-15,Buckling of paramagnetic chains in soft gels,"We study the magneto-elastic coupling behavior of paramagnetic chains in soft
polymer gels exposed to external magnetic fields. To this end, a laser scanning
confocal microscope is used to observe the morphology of the paramagnetic
chains together with the deformation field of the surrounding gel network. The
paramagnetic chains in soft polymer gels show rich morphological shape changes
under oblique magnetic fields, in particular a pronounced buckling deformation.
The details of the resulting morphological shapes depend on the length of the
chain, the strength of the external magnetic field, and the modulus of the gel.
Based on the observation that the magnetic chains are strongly coupled to the
surrounding polymer network, a simplified model is developed to describe their
buckling behavior. A coarse-grained molecular dynamics simulation model
featuring an increased matrix stiffness on the surfaces of the particles leads
to morphologies in agreement with the experimentally observed buckling effects.",1504.03884v1
2015-04-20,"Electric field-induced magnetization switching in interface-coupled multiferroic heterostructures: A highly-dense, non-volatile, and ultra-low-energy computing paradigm","Electric-field induced magnetization switching in multiferroic
magnetoelectric devices is promising for beyond Moore's law computing. We show
here that interface-coupled multiferroic heterostructures, i.e., a
ferroelectric layer coupled with a ferromagnetic layer, are particularly
suitable for highly-dense, non-volatile, and ultra-low-energy computing. By
solving stochastic Landau-Lifshitz-Gilbert equation of magnetization dynamics
in the presence of room-temperature thermal fluctuations, we demonstrate that
error-resilient switching of magnetization is possible in sub-nanosecond delay
while expending a minuscule amount of energy of $\sim$1 attojoule. Such devices
can be operated by drawing energy from the environment without the need for an
external battery.",1504.05572v1
2015-04-22,From pulsar scintillations to coronal heating: discontinuities in magnetohydrodynamics,"From pulsar scintillations we infer the presence of sheet-like structures in
the ISM; it has been suggested that these are current sheets. Current sheets
probably play an important role in heating the solar corona, and there is
evidence for their presence in the solar wind. Such magnetic discontinuities
have been found in numerical simulations with particular boundary conditions,
as well as in simulations using an incompressible equation of state. Here, I
investigate their formation under more general circumstances by means of
topological considerations as well as numerical simulations of the relaxation
of an arbitrary smoothly-varying magnetic field. The simulations are performed
with a variety of parameters and boundary conditions: in low, high and
of-order-unity plasma-$\beta$ regimes, with periodic and fixed boundaries, with
and without a friction force, at various resolutions and with various
diffusivities. Current sheets form, over a dynamical timescale, under {\it all}
conditions explored. At higher resolution they are thinner, and there is a
greater number of weaker current sheets. The magnetic field eventually relaxes
into a smooth minimum energy state, the energy of which depends on the magnetic
helicity, as well as on the nature of the boundaries.",1504.05952v1
2015-04-23,The Tayler instability at low magnetic Prandtl numbers: between chiral symmetry breaking and helicity oscillations,"The Tayler instability is a kink-type, current driven instability that plays
an important role in plasma physics but might also be relevant in liquid metal
applications with high electrical currents. In the framework of the
Tayler-Spruit dynamo model of stellar magnetic field generation, the question
of spontaneous helical (chiral) symmetry breaking during the saturation of the
Tayler instability has received considerable interest. Focusing on fluids with
low magnetic Prandtl numbers, for which the quasistatic approximation can be
applied, we utilize an integro-differential equation approach in order to
investigate the saturation mechanism of the Tayler instability. Both the
exponential growth phase and the saturated phase are analyzed in terms of the
action of the alpha and beta effects of mean-field magnetohydrodynamics. In the
exponential growth phase we always find a spontaneous chiral symmetry breaking
which, however, disappears in the saturated phase. For higher degrees of
supercriticality, we observe helicity oscillations in the saturated regime. For
Lundquist numbers in the order of one we also obtain chiral symmetry breaking
of the saturated magnetic field.",1504.06120v1
2015-05-01,Real-time simulation of non-equilibrium transport of magnetization in large open quantum spin systems driven by dissipation,"Using quantum Monte Carlo, we study the non-equilibrium transport of
magnetization in large open strongly correlated quantum spin $\frac{1}{2}$
systems driven by purely dissipative processes that conserve the uniform or
staggered magnetization. We prepare both a low-temperature Heisenberg
ferromagnet and an antiferromagnet in two parts of the system that are
initially isolated from each other. We then bring the two subsystems in contact
and study their real-time dissipative dynamics for different geometries. The
flow of the uniform or staggered magnetization from one part of the system to
the other is described by a diffusion equation that can be derived
analytically.",1505.00135v2
2015-06-01,The Antisymmetry of Distortions,"Distortions are ubiquitous in nature. Under perturbations such as stresses,
fields, or other changes, a physical system reconfigures by following a path
from one state to another; this path, often a collection of atomic
trajectories, describes a distortion. Here we introduce an antisymmetry
operation called distortion reversal, 1*, that reverses a distortion pathway.
The symmetry of a distortion pathway is then uniquely defined by a distortion
group involving 1*; it has the same form as a magnetic group that involves time
reversal, 1'. Given its isomorphism to magnetic groups, distortion groups could
potentially have commensurate impact in the study of distortions as the
magnetic groups have had in the study of magnetic structures. Distortion
symmetry has important implications for a range of phenomena such as structural
and electronic phase transitions, diffusion, molecular conformational changes,
vibrations, reaction pathways, and interface dynamics.",1506.00325v1
2015-06-01,Closing the hierarchy for non-Markovian magnetization dynamics,"We propose a stochastic approach for the description of the time evolution of
the magnetization of nanomagnets, that interpolates between the
Landau--Lifshitz--Gilbert and the Landau--Lifshitz--Bloch approximations, by
varying the strength of the noise. In addition, we take into account the
autocorrelation time of the noise and explore the consequences, when it is
finite, on the scale of the response of the magnetization, i.e. when it may be
described as colored, rather than white, noise and non-Markovian features
become relevant. We close the hierarchy for the moments of the magnetization,
by introducing a suitable truncation scheme, whose validity is tested by direct
numerical solution of the moment equations and compared to the average deduced
from a numerical solution of the corresponding stochastic Langevin equation. In
this way we establish a general framework, that allows both coarse-graining
simulations and faster calculations beyond the truncation approximation used
here.",1506.00544v1
2015-06-10,Emergent Coulombic criticality and Kibble-Zurek scaling in a topological magnet,"When a classical system is driven through a continuous phase transition, its
nonequilibrium response is universal and exhibits Kibble-Zurek scaling. We
explore this dynamical scaling in the novel context of a three-dimensional
topological magnet with fractionalized excitations, namely the liquid-gas
transition of the emergent mobile magnetic monopoles in dipolar spin ice. Using
field-mixing and finite-size scaling techniques, we place the critical point of
the liquid-gas line in the three-dimensional Ising universality class. We then
demonstrate Kibble-Zurek scaling for sweeps of the magnetic field through the
critical point. Unusually slow microscopic time scales in spin ice offer a
unique opportunity to detect this universal nonequilibrium physics in current
experimental setups.",1506.03472v2
2015-06-25,Critical entropies and magnetic-phase-diagram analysis of ultracold three-component fermionic mixtures in optical lattices,"We study theoretically many-body equilibrium magnetic phases and
corresponding thermodynamic characteristics of ultracold three-component
fermionic mixtures in optical lattices described by the SU(3)-symmetric
single-band Hubbard model. Our analysis is based on the generalization of the
exact diagonalization solver for multicomponent mixtures that is used in the
framework of the dynamical mean-field theory. It allows us to obtain a
finite-temperature phase diagram with the corresponding transition lines to
magnetically ordered phases at filling one particle per site (1/3 band filling)
in simple cubic lattice geometry. Based on the developed theoretical approach,
we also attain the necessary accuracy to study the entropy dependence in the
vicinity of magnetically ordered phases that allows us to make important
predictions for ongoing and future experiments aiming to approach and study
long-range-order phases in ultracold atomic mixtures.",1506.07642v2
2015-06-18,Separating read and write units in multiferroic devices,"Strain-mediated multiferroic composites, i.e., piezoelectric-magnetostrictive
heterostructures, hold profound promise for energy-efficient computing in
beyond Moore's law era. While reading a bit of information stored in the
magnetostrictive nanomagnets using a magnetic tunnel junction (MTJ), a material
selection issue crops up since magnetostrictive materials in general cannot be
utilized as the free layer of the MTJ. This is an important issue since we need
to achieve a high magnetoresistance for technological applications. We show
here that magnetically coupling the magnetostrictive nanomagnet and the free
layer e.g., utilizing the magnetic dipole coupling between them can circumvent
this issue. By solving stochastic Landau-Lifshitz-Gilbert equation of
magnetization dynamics in the presence of room-temperature thermal
fluctuations, we show that such design can eventually lead to a superior
energy-delay product.",1506.07865v1
2015-07-11,Coherent nonhelical shear dynamos driven by magnetic fluctuations at low Reynolds numbers,"Nonhelical shear dynamos are studied with a particular focus on the
possibility of coherent dynamo action. The primary results -- serving as a
follow up to the results of Squire & Bhattacharjee [arXiv:1506.04109 (2015)] --
pertain to the ""magnetic shear-current effect"" as a viable mechanism to drive
large-scale magnetic field generation. This effect raises the interesting
possibility that the saturated state of the small-scale dynamo could drive
large-scale dynamo action, and is likely to be important in the unstratified
regions of accretion disk turbulence. In this paper, the effect is studied at
low Reynolds numbers, removing the complications of small-scale dynamo
excitation and aiding analysis by enabling the use of quasi-linear statistical
simulation methods. In addition to the magnetically driven dynamo, new results
on the kinematic nonhelical shear dynamo are presented. These illustrate the
relationship between coherent and incoherent driving in such dynamos,
demonstrating the importance of rotation in determining the relative dominance
of each mechanism.",1507.03154v1
2015-08-04,Magnetic properties of light nuclei from lattice QCD,"After a short review of Lattice QCD methodology and techniques, I summarize
recent results of Lattice QCD calculations of the interactions of nucleons and
light nuclei with magnetic fields at pion masses of 805 MeV and 450 MeV.
Interestingly, the magnetic moments are found to be consistent with the
experimental values when given in terms of natural nuclear magnetons. The very
low-energy cross section for $np\rightarrow d\gamma$ is calculated and found to
agree with the experimental measurement. First calculations of the magnetic
polarizabilities of light nuclei are presented, with a large isovector
polarizability observed for the nucleon at these heavier pion masses.",1508.00892v1
2015-08-04,Acoustic signatures of the phases and phase transitions in Yb$_2$Ti$_2$O$_7$,"We report on measurements of the sound velocity and attenuation in a single
crystal of the candidate quantum- spin-ice material Yb$_2$Ti$_2$O$_7$ as a
function of temperature and magnetic field. The acoustic modes couple to the
spins magneto-elastically and, hence, carry information about the spin
correlations that sheds light on the intricate magnetic phase diagram of
Yb$_2$Ti$_2$O$_7$ and the nature of spin dynamics in the material.
Particularly, we find a pronounced thermal hysteresis in the acoustic data with
a concomitant peak in the specific heat indicating a possible first-order phase
transition at about $0.17$ K. At low temperatures, the acoustic response to
magnetic field saturates hinting at the development of magnetic order.
Furthermore, mean-field calculations suggest that Yb$_2$Ti$_2$O$_7$ undergoes a
first-order phase transition from a cooperative paramagnetic phase to a
ferromagnet below $T\approx 0.17$ K.",1508.00925v1
2015-08-06,Electromotive force due to magnetohydrodynamic fluctuations in sheared rotating turbulence,"This article presents a calculation of the mean electromotive force arising
from general small-scale magnetohydrodynamical turbulence, within the framework
of the second-order correlation approximation. With the goal of improving
understanding of the accretion disk dynamo, effects arising through small-scale
magnetic fluctuations, velocity gradients, density and turbulence
stratification, and rotation, are included. The primary result, which
supplements numerical findings, is that an off-diagonal turbulent resistivity
due to magnetic fluctuations can produce large-scale dynamo action -- the
magnetic analogue of the ""shear-current"" effect. In addition, consideration of
$\alpha$ effects in the stratified regions of disks gives the puzzling result
that there is no strong prediction for a sign of $\alpha$, since the effects
due to kinetic and magnetic fluctuations, as well as those due to shear and
rotation, are each of opposing signs and tend to cancel each other.",1508.01566v1
2015-09-03,Spin-mediated dissipation and frequency shifts of a cantilever at milliKelvin temperatures,"We measure the dissipation and frequency shift of a magnetically coupled
cantilever in the vicinity of a silicon chip, down to $25$ mK. The dissipation
and frequency shift originates from the interaction with the unpaired
electrons, associated with the dangling bonds in the native oxide layer of the
silicon, which form a two dimensional system of electron spins. We approach the
sample with a $3.43$ $\mu$m-diameter magnetic particle attached to an ultrasoft
cantilever, and measure the frequency shift and quality factor as a function of
temperature and the distance. Using a recent theoretical analysis [J. M. de
Voogd et al., arXiv:1508.07972 (2015)] of the dynamics of a system consisting
of a spin and a magnetic resonator, we are able to fit the data and extract the
relaxation time $T_1=0.39\pm0.08$ ms and spin density $\sigma=0.14\pm0.01$
spins per nm$^2$. Our analysis shows that at temperatures $\leq500$ mK magnetic
dissipation is an important source of non-contact friction.",1509.01251v2
2015-09-16,Variational principle for magnetisation dynamics in a temperature gradient,"By applying a variational principle on a magnetic system within the framework
of extended irreversible thermodynamics, we find that the presence of a
temperature gradient in a ferromagnet leads to a generalisation of the
Landau-Lifshitz equation with an additional magnetic induction field
proportional to the temperature gradient. This field modulates the damping of
the magnetic excitation. It can increase or decrease the damping, depending on
the orientation of the magnetisation wave-vector with respect to the
temperature gradient. This variational approach confirms the existence of the
Magnetic Seebeck effect which was derived from thermodynamics and provides a
quantitative estimate of the strength of this effect.",1509.04825v2
2015-09-16,A spin dynamics approach to solitonics,"It is spatial dispersion which is exclusively responsible for the emergence
of exchange interaction and magnetic ordering. In contrast, magneto-crystalline
anisotropy present in any realistic material brings in a certain non-linearity
to the equation of motion. Unlike homogeneous ferromagnetic ordering a variety
of non-collinear ground state configurations emerge as a result of competition
among exchange, anisotropy, and dipole-dipole interaction. These particle-like
states, e.g. magnetic soliton, skyrmion, domain wall, form a spatially
localised clot of magnetic energy. In this paper we explore topologically
protected magnetic solitons that might potentially be applied for logical
operations and/or information storage in the rapidly advancing filed of
solitonics (and skyrmionics). An ability to easily create, address, and
manipulate such structures is among the prerequisite forming a basis of -onics
technology, and is investigated in detail here using numerical and analytical
tools.",1509.04860v2
2015-09-23,Dirac Spin-Orbit Torques at the Surface of Topological Insulators,"We address the nature of spin-orbit torques at the magnetic surfaces of
topological insulators, using both Kubo formula and Keldysh formalism. Through
the analysis of the derived spin-charge equations, we find that the so-called
Dirac torques possess a different symmetry compared to their Rashba
counterpart, as well as a high anisotropy as a function of the magnetization
direction. In particular, the damping torque vanishes when the magnetization
lies in the plane of the topological insulator surface. This peculiarity has
important consequences in terms of magnetization dynamics and switching, as
demonstrated numerically via a macrospin model.",1509.06929v2
2015-10-05,Continuous dynamical decoupling of a single diamond nitrogen-vacancy center spin with a mechanical resonator,"Inhomogeneous dephasing from uncontrolled environmental noise can limit the
coherence of a quantum sensor or qubit. For solid state spin qubits such as the
nitrogen-vacancy (NV) center in diamond, a dominant source of environmental
noise is magnetic field fluctuations due to nearby paramagnetic impurities and
instabilities in a magnetic bias field. In this work, we use ac stress
generated by a diamond mechanical resonator to engineer a dressed spin basis in
which a single NV center qubit is less sensitive to its magnetic environment.
For a qubit in the thermally isolated subspace of this protected basis, we
prolong the dephasing time $T_2^*$ from $2.7\pm0.1$ $\mu$s to $15\pm1$ $\mu$s
by dressing with a $\Omega=581\pm2$ kHz mechanical Rabi field. Furthermore, we
develop a model that quantitatively predicts the relationship between $\Omega$
and $T_2^*$ in the dressed basis. Our model suggests that a combination of
magnetic field fluctuations and hyperfine coupling to nearby nuclear spins
limits the protected coherence time over the range of $\Omega$ accessed here.
We show that amplitude noise in $\Omega$ will dominate the dephasing for larger
driving fields.",1510.01194v1
2015-10-13,Entropy produced by magnetic energy density fluctuations in the quiet Sun,"D.J. Evans, et al., [Phys. Rev. Lett. 71, 2401 (1993)] discovered a relation,
subsequently known as the Fluctuation Theorem (FT), which quantifies the
probability of observing fluctuations violating the second law of
thermodynamics in thermostatted dissipative non-equilibrium systems. The FT has
been confirmed experimentally for small systems. We analyze fluctuations of the
total entropy production related to the distribution of the magnetic energy
density in the quiet Sun lower atmosphere inferred by means of
spectro-polarimetric observations. We show that the magnetic energy density is
intermittent, at non-equilibrium and has the Markov property. The applicability
of the FT is proven for the first time in an astronomical environment in spite
of the seemingly large scales involved. We also found a non-linear behavior of
the probability current, indicating that the dynamics of the magnetic field
depends on the field strength. On average, however, the observed
non-equilibrium behavior is governed by the second law of thermodynamics for
open systems. Our results provide novel constraints for testing the
hypothesizes of the solar local dynamo, which should be consistent with the
inferred statistics of the fluctuations.",1510.03566v2
2015-10-21,Strong field dynamo action in rapidly rotating convection with no inertia,"The Earth's magnetic field is generated by dynamo action driven by convection
in the outer core. For numerical reasons, inertial and viscous forces play an
important role in geodynamo models; however, the primary dynamical balance in
the Earth's core is believed to be between buoyancy, Coriolis and magnetic
forces. The hope has been that by setting the Ekman number to be as small as
computationally feasible, an asymptotic regime would be reached in which the
correct force balance is achieved. However, recent analyses of geodynamo models
suggest that the desired balance has still not yet been attained. Here we adopt
a complementary approach consisting of a model of rapidly rotating convection
in which inertial forces are neglected from the outset. Within this framework
we are able to construct a new branch of solutions in which the dynamo
generates a strong magnetic field that satisfies the expected force balance.
The resulting strongly magnetized convection is dramatically different to the
corresponding solutions in which the field is weak.",1510.06220v2
2015-10-23,Laser-induced THz magnetization precession for a tetragonal Heusler-like nearly compensated ferrimagnet,"Laser-induced magnetization precessional dynamics was investigated in
epitaxial films of Mn$_3$Ge, which is a tetragonal Heusler-like nearly
compensated ferrimagnet. The ferromagnetic resonance (FMR) mode was observed,
the precession frequency for which exceeded 0.5 THz and originated from the
large magnetic anisotropy field of approximately 200 kOe for this ferrimagnet.
The effective damping constant was approximately 0.03. The corresponding
effective Landau-Lifshitz constant of approximately 60 Mrad/s and is comparable
to those of the similar Mn-Ga materials. The physical mechanisms for the
Gilbert damping and for the laser-induced excitation of the FMR mode were also
discussed in terms of the spin-orbit-induced damping and the laser-induced
ultrafast modulation of the magnetic anisotropy, respectively.",1510.06793v1
2015-10-23,"Fast, nanoscale addressability of nitrogen-vacancy spins via coupling to a dynamic ferromagnetic vortex","The core of a ferromagnetic vortex domain creates a strong, localized
magnetic field which can be manipulated on nanosecond timescales, providing a
platform for addressing and controlling individual nitrogen-vacancy center
spins in diamond at room temperature, with nanometer-scale resolution. First,
we show that the ferromagnetic vortex can be driven into proximity with a
nitrogen-vacancy defect using small applied magnetic fields, inducing
significant nitrogen-vacancy spin splitting. Second, we find that the magnetic
field gradient produced by the vortex is sufficient to address spins separated
by nanometer length scales. By applying a microwave-frequency magnetic field,
we drive both the vortex and the nitrogen-vacancy spins, resulting in enhanced
coherent rotation of the spin state. Finally we demonstrate that by driving the
vortex on fast timescales, sequential addressing and coherent manipulation of
spins is possible on $\sim100$ ns timescales.",1510.07073v2
2015-10-26,A publicly available simulation of an enhanced network region of the Sun,"Context. The solar chromosphere is the interface between the solar surface
and the solar corona. Modelling of this region is difficult because it
represents the transition from optically thick to thin radiation escape, from
gas-pressure domination to magnetic-pressure domination, from a neutral to an
ionised state, from MHD to plasma physics, and from near-equilibrium (LTE) to
non-equilibrium conditions.
  Aims. Our aim is to provide the community with realistic simulations of the
magnetic solar outer atmosphere. This will enable detailed comparison of
existing and upcoming observations with synthetic observables from the
simulations, thereby elucidating the complex interactions of magnetic fields
and plasma that are crucial for our understanding of the dynamic outer
atmosphere.
  Methods. We used the radiation magnetohydrodynamics code Bifrost to perform
simulations of a computational volume with a magnetic field topology similar to
an enhanced network area on the Sun.
  Results. The full simulation cubes are made available online. The general
properties of the simulation are discussed, and limitations are discussed.",1510.07581v1
2015-11-03,Luminosity of ultrahigh energy cosmic rays and bounds on magnetic luminosity of radio-loud active galactic nuclei,"We investigate the production of magnetic flux from rotating black holes in
active galactic nuclei (AGNs) and compare it with the upper limit of ultrahigh
energy cosmic ray (UHECR) luminosities, calculated from observed integral flux
of GeV-TeV gamma rays for nine UHECR AGN sources. We find that, for the
expected range of black hole rotations (0.44<a<0.80), the corresponding bounds
of theoretical magnetic luminosities from AGNs coincides with the calculated
UHECR luminosity. We argue that such result possibly can contribute to
constrain AGN magnetic and dynamic properties as phenomenological tools to
explain the requisite conditions to proper accelerate the highest energy cosmic
rays.",1511.01003v1
2015-11-13,The interface between heavy fermions and normal electrons investigated by spatially-resolved nuclear magnetic resonance,"We have studied the superlattices with alternating block layers (BLs) of
heavy-fermion superconductor CeCoIn5 and conventional-metal YbCoIn5 by
site-selective nuclear magnetic resonance(NMR) spectroscopy, which uniquely
offers spatially-resolved dynamical magnetic information. We find that the
presence of antiferromagnetic fluctuations is confined to the Ce-BLs,
indicating that magnetic degrees of freedom of f-electrons are quenched inside
the Yb-BLs. Contrary to simple expectations that the two-dimensionalization
enhances fluctuations, we observe that antiferromagnetic fluctuations are
rapidly suppressed with decreasing Ce-BL thickness. Moreover, the suppression
is more prominent near the interfaces between the BLs. These results imply
significant effects of local inversion-symmetry breaking at the interfaces.",1511.04209v1
2015-11-19,Inverse problem in Parker's dynamo,"The inverse solution of the 1D Parker dynamo equations is considered. The
method is based on minimization of the cost-function, which characterize
deviation of the model solution properties from the desired ones. The output is
the latitude distribution of the magnetic field generation sources: the
$\alpha$- and $\omega$-effects. Minimization is made using the Monte-Carlo
method. The details of the method, as well as some applications, which can be
interesting for the broad dynamo community, are considered: conditions when the
invisible for the observer at the surface of the planet toroidal part of the
magnetic field is much larger than the poloidal counterpart. It is shown that
at some particular distributions of $\alpha$ and $\omega$ the well-known thesis
that sign of the dynamo-number defines equatorial symmetry of the magnetic
field to the equator plane, is violated. It is also demonstrated in what
circumstances magnetic field in the both hemispheres have different properties,
and simple physical explanation of this phenomenon is proposed.",1511.06243v1
2015-11-27,Magnetic properties of Fe$_{1-x}$Ni$_x$ alloy from CPA+DMFT perspectives,"We use a combination of the coherent potential approximation and dynamical
mean field theory to study magnetic properties of the Fe$_{1-x}$Ni$_x$ alloy
from a first principles. Calculated uniform magnetic susceptibilities have a
Curie-Weiss-like behavior and extracted effective temperatures are in agreement
with the experimental results. The individual squared magnetic moments obtained
as function of nickel concentration follow the same trends as experimental
data. An analysis of the ionic and spin weights shows a possibility of a
high-spin to intermediate- and low-spin states transitions at high
temperatures.",1511.08625v2
2016-01-15,An electron motion induced by magnetic field pulse in bi-layer quantum wire,"We consider theoretically the possibility of an electron acceleration in
quantum wire by short magnetic pulses lasted bewteen several to few tens of
picoseconds. We show that such possibility exists provided that, the electron
is initially localized in part of nanowire that consists of two vertically
aligned layers which are tunnel coupled. When a horizontally directed magnetic
field, changeable in time, is also perpendicular to the main axis of a wire, it
generates a rotational electric field in it which pushes the upper and the
lower parts of the electron wavepacket in opposite directions. We have found
however, that for an asymmetric vertical confinement, the majority part of
charge density starts to move in the direction of local electric field in its
layer but it also drags the minority part in the same direction what results in
coherent motion of an entire wavepacket. We discuss the dynamics of this motion
in dependence on the time characteristics of the magnetic pulse.",1601.03880v1
2016-02-04,The Global Solar Dynamo,"A brief summary of the various observations and constraints that underlie
solar dynamo research are presented. The arguments that indicate that the solar
dynamo is an alpha-omega dynamo of the Babcock-Leighton type are then shortly
reviewed. The main open questions that remain are concerned with the subsurface
dynamics, including why sunspots emerge at preferred latitudes as seen in the
familiar butterfly wings, why the cycle is about 11 years long, and why the
sunspot groups emerge tilted with respect to the equator (Joy's law). Next, we
turn to magnetic helicity, whose conservation property has been identified with
the decline of large-scale magnetic fields found in direct numerical
simulations at large magnetic Reynolds numbers. However, magnetic helicity
fluxes through the solar surface can alleviate this problem and connect theory
with observations, as will be discussed.",1602.01754v1
2016-02-12,Spin dynamics in the high-field phases of volborthite,"We report single-crystal 51V NMR studies on volborthite Cu3V2O7(OH)2 2H2O,
which is regarded as a quasi-two-dimensional frustrated magnet with competing
ferromagnetic and antiferromagnetic interactions. In the 1/3 magnetization
plateau above 28 T, the nuclear spin-lattice relaxation rate 1/T1 indicates an
excitation gap with a large effective g factor in the range of 4.6-5.9,
pointing to magnon bound states. Below 26 T where the gap has closed, the NMR
spectra indicate small internal fields with a Gaussian-like distribution,
whereas 1/T1 shows a power-law-like temperature dependence in the paramagnetic
state, which resembles a slowing down of spin fluctuations associated with
magnetic order. We discuss the possibility of an exotic spin state caused by
the condensation of magnon bound states below the magnetization plateau.",1602.04028v2
2016-02-14,Ultra-sensitive nanoscale magnetic field sensors based on resonant spin filtering,"Solid state magnetic field sensors based on magneto-resistance modulation
find direct applications in communication devices, specifically in proximity
detection, rotational reference detection and current sensing. In this work, we
propose sensor structures based on the magneto-resistance physics of resonant
spin-filtering and present device designs catered toward exceptional magnetic
field sensing capabilities. Using the non-equilibrium Green's function spin
transport formalism self consistently coupled to the Poisson's equation, we
present highly-tunable pentalayer magnetic tunnel junction structures that are
capable of exhibiting an ultra-high peak tunnel magneto resistance $(\approx
2500 \%$). We show how this translates to device designs featuring an
ultra-high current sensitivity enhancement of over 300\% in comparison with
typical trilayer MTJ sensors, and a wider tunable range of field sensitivity.
We also demonstrate that a dynamic variation in sensor functionalities with the
structural landscape enables a superior design flexibility over typical
trilayer sensors. An optimal design exhibiting close to a 700\% sensitivity
increase as a result of angle dependent spin filtering is then presented.This
work sets a stage to engineer spintronic building blocks via the design of
functional structures tailored to exhibit ultra-sensitive spin filtering.",1602.04438v1
2016-02-17,Fermionic response from fractionalization in an insulating two-dimensional magnet,"Conventionally ordered magnets possess bosonic elementary excitations, called
magnons. By contrast, no magnetic insulators in more than one dimension are
known whose excitations are not bosons but fermions. Theoretically, some
quantum spin liquids (QSLs) -- new topological phases which can occur when
quantum fluctuations preclude an ordered state -- are known to exhibit Majorana
fermions as quasiparticles arising from fractionalization of spins. Alas,
despite much searching, their experimental observation remains elusive. Here,
we show that fermionic excitations are remarkably directly evident in
experimental Raman scattering data across a broad energy and temperature range
in the two-dimensional material $\alpha$-RuCl$_3$. This shows the importance of
magnetic materials as hosts of Majorana fermions. In turn, this first
systematic evaluation of the dynamics of a QSL at finite temperature emphasizes
the role of excited states for detecting such exotic properties associated with
otherwise hard-to-identify topological QSLs.",1602.05277v1
2016-02-23,Experimental Investigation of Temperature-Dependent Gilbert Damping in Permalloy Thin Films,"The Gilbert damping of ferromagnetic materials is arguably the most important
but least understood phenomenological parameter that dictates real-time
magnetization dynamics. Understanding the physical origin of the Gilbert
damping is highly relevant to developing future fast switching spintronics
devices such as magnetic sensors and magnetic random access memory. Here, we
report an experimental study of temperature-dependent Gilbert damping in
permalloy (Py) thin films of varying thicknesses by ferromagnetic resonance.
From the thickness dependence, two independent contributions to the Gilbert
damping are identified, namely bulk damping and surface damping. Of particular
interest, bulk damping decreases monotonically as the temperature decreases,
while surface damping shows an enhancement peak at the temperature of ~50 K.
These results provide an important insight to the physical origin of the
Gilbert damping in ultrathin magnetic films.",1602.07325v1
2016-03-14,Merging droplets in double nano-contact spin torque oscillators,"We demonstrate how magnetic droplet soliton pairs, nucleated by two separated
nano-contact (NC) spin torque oscillators, can merge into a single droplet
soliton. A detailed description of the magnetization dynamics of this merger
process is obtained by micromagnetic simulations: A droplet pair with a
steady-state in-phase spin precession is generated through the spin-transfer
torque effect underneath two separate NCs, followed by a gradual expansion of
the droplets volume and the out phase of magnetization on the inner side of the
two droplets, resulting in the droplets merging into a larger droplet. This
merger occurs only when the NC separation is smaller than a critical value. A
transient breathing mode is observed before the merged droplet stabilizes into
a steady precession state. The precession frequency of the merged droplet is
lower than that of the droplet pair, consistent with its larger size. Merged
droplets can again break up into droplet pairs at high enough magnetic field
with a strong hysteretic response.",1603.04133v1
2016-03-16,Is the magnetic field making the inner crust of neutron stars more heterogeneous?,"We study the effect of strong magnetic fields, of the order of
$10^{15}-10^{17}$ G, on the extension of the crust of magnetized neutron stars.
The dynamical instability region of neutron-proton-electron ($npe$) matter at
subsaturation densities and the mode with the largest growth rate are
determined within a relativistic mean field model. It is shown that a strong
magnetic field has a large effect on the instability region, defining the
crust-core transition as a wide density range.",1603.05000v2
2016-05-04,Magnetic moments of the hidden-charm pentaquark states,"The magnetic moment of a baryon state is an equally important dynamical
observable as its mass, which encodes crucial information of its underlying
structure. According to the different color-flavor structure, we have
calculated the magnetic moments of the hidden-charm pentaquark states with
$J^P={\frac{1}{2}}^{\pm}$, ${\frac{3}{2}}^{\pm}$, ${\frac{5}{2}}^{\pm}$ and
${\frac{7}{2}}^{+}$ in the molecular model, the diquark-triquark model and the
diquark-diquark-antiquark model respectively. Although a good description for
the pentaquark mass spectrum and decay patterns has been obtained in all the
three models, different color-flavor structures lead to different magnetic
moments, which can be used to pin down their inner structures and distinguish
various models.",1605.01337v3
2016-05-19,Spatially Assisted Schwinger Mechanism and Magnetic Catalysis,"Using the worldline formalism we compute an effective action for fermions
under a temporally modulated electric field and a spatially modulated magnetic
field. It is known that the former leads to an enhanced Schwinger Mechanism,
while we find that the latter can also result in enhanced particle production
and even cause a reorganization of the vacuum to acquire a larger dynamical
mass in equilibrium which spatially assists the Magnetic Catalysis.",1605.05957v3
2016-06-03,General planar transverse domain walls realized by optimized transverse magnetic field pulses in magnetic biaxial nanowires,"We report the realization of a planar transverse domain wall (TDW) with
arbitrary tilting angle in a magnetic biaxial nanowire under a transverse
magnetic field (TMF) pulse with fixed strength and optimized orientation
profile. We smooth any twisting in azimuthal angle plane of a TDW and thus
completely decouple the polar and azimuthal degrees of freedom. The analytical
differential equation that describes the polar angle distribution is then
derived and the resulting solution is not a Walker-ansatz form. With this
optimized TMF pulse comoving, the field-driven dynamics of the planar TDW is
investigated. It turns out the comoving TMF pulse increases the wall velocity
under the same axial driving field. These results will help to design a series
of modern logic and memory nanodevices based on general planar TDWs.",1606.01046v2
2016-06-06,Controlled Capillary Assembly of Magnetic Janus Particles at Fluid-Fluid Interfaces,"Capillary interactions can be used to direct assembly of particles adsorbed
at fluid-fluid interfaces. Precisely controlling the magnitude and direction of
capillary interactions to assemble particles into favoured structures for
materials science purposes is desirable but challenging. In this paper, we
investigate capillary interactions between magnetic Janus particles adsorbed at
fluid-fluid interfaces. We develop a pair-interaction model that predicts that
these particles should arrange into a side--side configuration, and carry out
simulations that confirm the predictions of our model. Finally, we investigate
the monolayer structures that form when many magnetic Janus particles adsorb at
the interface. We find that the particles arrange into long, straight chains
exhibiting little curvature, in contrast with capillary interactions between
ellipsoidal particles. We further find a regime in which highly ordered,
lattice-like monolayer structures form, which can be tuned dynamically using an
external magnetic field.",1606.01646v2
2016-06-08,Characterization of switching field distributions in Ising-like magnetic arrays,"The switching field distribution within arrays of single-domain ferromagnetic
islands incorpo- rates both island-island interactions and quenched disorder in
island geometry. Separating these two contributions is important for
disentangling the effects of disorder and interactions in the magnetization
dynamics of island arrays. Using sub-micron, spatially resolved Kerr imaging in
an external magnetic field for islands with perpendicular magnetic anisotropy,
we map out the evolution of island arrays during hysteresis loops. Resolving
and tracking individual islands across four different lattice types and a range
of inter-island spacings, we extract the individual switching fields of every
island and thereby determine the relative contributions of interactions and
quenched disorder in the arrays. The width of the switching field distribution
is well explained by a simple model comprising the sum of an array-independent
contribution (interpreted as disorder-induced), and a term proportional to the
maximum field the fully polarized array could exert on a single island. We
conclude that disorder in these arrays is primarily a single-island property.",1606.02770v1
2016-06-17,Mechanical Actuation of Magnetic Domain-Wall Motion,"We theoretically study the motion of a magnetic domain wall induced by
transverse elastic waves in a one-dimensional magnetic wire, which respects
both rotational and translational symmetries. By invoking the conservation of
the associated total angular and linear momenta, we are able to derive the
torque and the force on the domain wall exerted by the waves. We then show how
ferromagnetic and antiferromagnetic domain walls can be driven by circularly-
and linear-polarized waves, respectively. We envision that elastic waves may
provide effective means to drive the dynamics of magnetic solitons in
insulators.",1606.05430v3
2016-06-19,Viscoelasticity of model surfactant solutions determined by rotational magnetic spectroscopy,"Being able to reduce the size of a rheometer down to the micron scale is a
unique opportunity to explore the mechanical response of expensive and/or
confined liquids and gels. To this aim, we synthesize micron size wires with
magnetic properties and examine the possibility of using them as microrheology
probes. In this work, we exploit the technique of rotational magnetic
spectroscopy by placing a wire in a rotating magnetic field and monitor its
temporal evolution by time-lapse microscopy. The wire-based microrheology
technique is tested on wormlike micellar surfactant solutions showing very
different relaxation dynamics and viscosities. A model for the wire rotation is
also developed and used to predict the wire behavior. It is shown that the
rheological parameters of the surfactant solutions including the static shear
viscosity, the entangled micellar network relaxation time and the elastic
modulus are in good agreement with those of conventional rheometry.",1606.06667v1
2016-07-06,Symmetries and Couplings of Non-Relativistic Electrodynamics,"We examine three versions of non-relativistic electrodynamics, known as the
electric and magnetic limit theories of Maxwell's equations and Galilean
electrodynamics (GED) which is the off-shell non-relativistic limit of Maxwell
plus a free scalar field. For each of these three cases we study the couplings
to non-relativistic dynamical charged matter (point particles and charged
complex scalars). The GED theory contains besides the electric and magnetic
potentials a so-called mass potential making the mass parameter a local
function. The electric and magnetic limit theories can be coupled to twistless
torsional Newton-Cartan geometry while GED can be coupled to an arbitrary
torsional Newton-Cartan background. The global symmetries of the electric and
magnetic limit theories on flat space consist in any dimension of the infinite
dimensional Galilean conformal algebra and a $U(1)$ current algebra. For the
on-shell GED theory this symmetry is reduced but still infinite dimensional,
while off-shell only the Galilei algebra plus two dilatations remain. Hence one
can scale time and space independently, allowing Lifshitz scale symmetries for
any value of the critical exponent $z$.",1607.01753v2
2016-07-08,Supercurrent-induced Skyrmion dynamics and Tunable Weyl points in Chiral Magnet with Superconductivity,"Recent studies show superconductivity provides new perspectives on
spintronics. We study a heterostructure composed of an s-wave superconductor
and a cubic chiral-magnet that can stabilize a topological spin texture, a
skyrmion. We propose a supercurrent-induced spin torque that originates from
the spin-orbit coupling, and we show that the spin torque can drive a skyrmion
in an efficient way that reduces Joule heating. We also study the band
structure of Bogoliubov quasiparticles and show the existence of Weyl points,
whose positions can be controlled by the direction of the magnetization. This
results in an effective magnetic field acting on the quasiparticles in the
presence spin textures. Furthermore, the tilt of the Weyl cones can also be
tuned by the strength of the spin-orbit coupling, and we propose a possible
realization of type-II Weyl points.",1607.02336v4
2016-07-10,On the motion of free interface in ideal incompressible MHD,"For the free boundary problem of the plasma-vacuum interface to
three-dimensional ideal incompressible magnetohydrodynamics (MHD), the a priori
estimates of smooth solutions are proved in Sobolev norms by adopting a
geometrical point of view and some quantities such as the second fundamental
form and the velocity of the free interface are estimated. In the vacuum
region, the magnetic fields are described by the div-curl system of pre-Maxwell
dynamics, while at the interface the total pressure is continuous and the
magnetic fields are tangent to the interface, but we do not need any
restrictions on the size of the magnetic fields on the free interface. We
introduce the ""virtual particle"" endowed with a virtual velocity field in
vacuum to reformulate the problem to a fixed boundary problem under the
Lagrangian coordinates. The $L^2$-norms of any order covariant derivatives of
the magnetic fields both in vacuum and on the boundaries are bounded in terms
of initial data and the second fundamental forms of the free interface and the
rigid wall. The estimates of the curl of the electric fields in vacuum are also
obtained, which are also indispensable in elliptic estimates.",1607.02731v2
2016-07-11,Boosting Magnetic Reconnection by Viscosity and Thermal Conduction,"Nonlinear evolution of magnetic reconnection is investigated by means of
magnetohydrodynamic simulations including uniform resistivity, uniform
viscosity, and anisotropic thermal conduction. When viscosity exceeds
resistivity (the magnetic Prandtl number Prm > 1), the viscous dissipation
dominates outflow dynamics and leads to the decrease in the plasma density
inside a current sheet. The low-density current sheet supports the excitation
of the vortex. The thickness of the vortex is broader than that of the current
for Prm > 1. The broader vortex flow more efficiently carries the upstream
magnetic flux toward the reconnection region, and consequently boosts the
reconnection. The reconnection rate increases with viscosity provided that
thermal conduction is fast enough to take away the thermal energy increased by
the viscous dissipation (the fluid Prandtl number Pr < 1). The result suggests
the need to control the Prandtl numbers for the reconnection against the
conventional resistive model.",1607.02839v2
2016-07-11,Magnetic Catalysis in Graphene Effective Field Theory,"We report on the first observation of magnetic catalysis at zero temperature
in a fully nonperturbative simulation of the graphene effective field theory.
Using lattice gauge theory, a nonperturbative analysis of the theory of
strongly-interacting, massless, (2+1)-dimensional Dirac fermions in the
presence of an external magnetic field is performed. We show that in the
zero-temperature limit, a nonzero value for the chiral condensate is obtained
which signals the spontaneous breaking of chiral symmetry. This result implies
a nonzero value for the dynamical mass of the Dirac quasiparticle. This in turn
has been posited to account for the quantum-Hall plateaus that are observed at
large magnetic fields.",1607.03137v3
2016-07-25,Magnetic-field effects on photon-induced quantum transport in a single dot-cavity system,"In this study, we show how a static magnetic field can control photon-induced
electron transport through a quantum dot system coupled to a photon cavity. The
quantum dot system is connected to two electron reservoirs and exposed to an
external perpendicular static magnetic field. The propagation of electrons
through the system is thus influenced by the static magnetic and the dynamic
photon fields. It is observed that the photon cavity forms photon replica
states controlling electron transport in the system. IF the photon field has
more energy than the cyclotron energy, then the photon field is dominant in the
electron transport. Consequently, the electron transport is enhanced due to
activation of photon replica states. By contrast, the electron transport is
suppressed in the system when the photon energy is smaller than the cyclotron
energy.",1607.07201v1
2016-07-27,Magnetic Collapse and the Behavior of Transition Metal Oxides at High Pressure,"We report a detail theoretical study of the electronic structure and phase
stability of transition metal oxides MnO, FeO, CoO, and NiO in their
paramagnetic cubic B1 structure by employing dynamical mean-field theory of
correlated electrons combined with \emph{ab initio} band structure methods
(DFT+DMFT). Our calculations reveal that under pressure these materials exhibit
a Mott insulator-metal transition (IMT) which is accompanied by a simultaneous
collapse of local magnetic moments and lattice volume, implying a complex
interplay between chemical bonding and electronic correlations. Moreover, our
results for the transition pressure show a monotonous decrease from ~ 145 GPa
to 40 GPa, upon moving from MnO to CoO. In contrast to that, in NiO, magnetic
collapse is found to occur at remarkably higher pressure of ~ 429 GPa. We
provide a unified picture of such a behavior and suggest that it is primary a
localized to itinerant moment behavior transition at the IMT that gives rise to
magnetic collapse in transition metal oxides.",1607.08261v1
2016-08-02,Lattice Field Theory Study of Magnetic Catalysis in Graphene,"We discuss the simulation of the low-energy effective field theory (EFT) for
graphene in the presence of an external magnetic field. Our fully
nonperturbative calculation uses methods of lattice gauge theory to study the
theory using a hybrid Monte Carlo approach. We investigate the phenomenon of
magnetic catalysis in the context of graphene by studying the chiral condensate
which is the order parameter characterizing the spontaneous breaking of chiral
symmetry. In the EFT, the symmetry breaking pattern is given by $U(4) \to U(2)
\times U(2)$. We also comment on the difficulty, in this lattice formalism, of
studying the time-reversal-odd condensate characterizing the ground state in
the presence of a magnetic field. Finally, we study the mass spectrum of the
theory, in particular the Nambu-Goldstone (NG) mode as well as the Dirac
quasiparticle, which is predicted to obtain a dynamical mass.",1608.00666v1
2016-08-10,The multiplicity problem for periodic orbits of magnetic flows on the 2-sphere,"We consider magnetic Tonelli Hamiltonian systems on the cotangent bundle of
the 2-sphere, where the magnetic form is not necessarily exact. It is known
that, on very low and on high energy levels, these systems may have only
finitely many periodic orbits. Our main result asserts that almost all energy
levels in a precisely characterized intermediate range $(e_0,e_1)$ possess
infinitely many periodic orbits. Such a range of energies is non-empty, for
instance, in the physically relevant case where the Tonelli Lagrangian is a
kinetic energy and the magnetic form is oscillating (in which case, $e_0=0$ is
the minimal energy of the system).",1608.03144v1
2016-08-11,Effective perihelion advance and potentials in a conformastatic background with magnetic field,"An Exact solution of the Einstein-Maxwell field equations for a
conformastatic metric with magnetized sources is study. In this context,
effective potential are studied in order to understand the dynamics of the
magnetic field in galaxies. We derive the equations of motion for neutral and
charged particles in a spacetime background characterized by this class of
solutions. In this particular case, we investigate the main physical properties
of equatorial circular orbits and related effective potentials. In addition, we
obtain an effective analytic expression for the perihelion advance of test
particles. Our theoretical predictions are compared with the observational data
calibrated with the ephemerides of the planets of the Solar system and the Moon
(EPM2011). We show that, in general, the magnetic punctual mass predicts values
that are in better agreement with observations than the values predicted in
Einstein gravity alone.",1608.03813v1
2016-08-16,Magnetic field generation by intermittent convection,"Magnetic field generation in three-dimensional Rayleigh-B\'enard convection
of an electrically conducting fluid is studied numerically by fixing the
Prandtl number at $P=0.3$ and varying the Rayleigh number (Ra) as a control
parameter. A recently reported route to hyperchaos involving quasiperiodic
regimes, crises and chaotic intermittent attractors is followed, and the
critical magnetic Prandtl number ($P_m^c$) for dynamo action is determined as a
function of Ra. A mechanism for the onset of intermittency in the magnetic
energy is described, the most beneficial convective regimes for dynamo action
in this transition to weak turbulence are identified, and the impact of
intermittency on the dependence of $P_m^c$ on Ra is discussed.",1608.04716v2
2016-08-25,The mimetic finite difference method for the Landau-Lifshitz equation,"The Landau-Lifshitz equation describes the dynamics of the magnetization
inside ferromagnetic materials. This equation is highly nonlinear and has a
non-convex constraint (the magnitude of the magnetization is constant) which
pose interesting challenges in developing numerical methods. We develop and
analyze explicit and implicit mimetic finite difference schemes for this
equation. These schemes work on general polytopal meshes which provide enormous
flexibility to model magnetic devices with various shapes. A projection on the
unit sphere is used to preserve the magnitude of the magnetization. We also
provide a proof that shows the exchange energy is decreasing in certain
conditions. The developed schemes are tested on general meshes that include
distorted and randomized meshes. The numerical experiments include a test
proposed by the National Institute of Standard and Technology and a test
showing formation of domain wall structures in a thin film.",1608.07309v1
2016-08-29,High-Energy Neutrino Emission from White Dwarf Mergers,"The merger of two white dwarfs is expected to result in a central fast
rotating core surrounded by a debris disk, in which magnetorotational
instabilities give rise to a hot magnetized corona and a magnetized outflow.
The dissipation of magnetic energy via reconnection could lead to the
acceleration of cosmic-rays in the expanding material, which would result in
high energy neutrinos. We discuss the possibility of using these neutrino
signals as probes of the outflow dynamics, magnetic energy dissipation rate and
cosmic-ray acceleration efficiency. Importantly, the accompanying high-energy
gamma-rays are absorbed within these sources because of the large optical
depth, so these neutrino sources can be regarded as hidden cosmic-ray
accelerators that are consistent with the non-detection of gamma-rays with
Fermi-LAT. While the cosmic-ray generation rate is highly uncertain, if it
reaches $\sim10^{45}\,\rm erg\,Mpc^{-3}\,yr^{-1}$, the diffuse neutrino flux
could contribute a substantial fraction of the IceCube observations. We also
evaluate the prospect of observing individual merger events, which provides a
means for testing such sources in the future.",1608.08150v2
2016-09-06,Chiral charge pumping in graphene deposited on a magnetic insulator,"We demonstrate that a sizable chiral charge pumping can be achieved at room
temperature in graphene/Yttrium Iron Garnet (YIG) bilayer systems. The effect,
which cannot be attributed to the ordinary spin pumping, reveals itself in the
creation of a dc electric field/voltage in graphene as a response to the
dynamic magnetic excitations (spin waves) in an adjacent out-of-plane
magnetized YIG film. We show that the induced voltage changes its sign when the
orientation of the static magnetization is reversed, clearly indicating the
broken spatial inversion symmetry in the studied system. The strength of effect
shows a non-monotonous dependence on the spin-wave frequency, in agreement with
the proposed theoretical model.",1609.01613v2
2016-09-21,Level anti-crossing spectra of nitrogen-vacancy centers in diamond detected by using modulation of the external magnetic field,"We report a study of the magnetic field dependence of the photo-luminescence
of NV$^-$ centers (negatively charged nitrogen-vacancy centers) in diamond
single crystals. In such a magnetic field dependence characteristic lines are
observed, which are coming from Level Anti-Crossings (LACs) in the coupled
electron-nuclear spin system. For enhancing the sensitivity, we used lock-in
detection to measure the photo-luminescence intensity and observed a remarkably
strong dependence of the LAC-derived lines on the modulation frequency. Upon
decreasing of the modulation frequency from 12 kHz to 17 Hz the amplitude of
the lines increases by approximately two orders of magnitude. To take a
quantitative account for such effects, we developed a theoretical model, which
describes the spin dynamics in the coupled electron-nuclear spin system under
the action of an oscillating external magnetic field. Good agreement between
experiments and theory allows us to conclude that the observed effects are
originating from coherent spin polarization exchange in the NV$^-$ center. Our
results are of great practical importance allowing one to optimize the
experimental conditions for probing LAC-derived lines in diamond defect
centers.",1609.06661v1
2016-10-06,Spin-torque switching and control using chirped oscillating currents,"We propose to use oscillating spin currents with slowly varying frequency
(chirp) to manipulate and control the magnetization dynamics in a nanomagnet.
By recasting the Landau-Lifshitz-Slonczewski equation in a quantum-like
two-level formalism, we show that a chirped spin current polarized in the
direction normal to the anisotropy axis can induce a stable precession of the
magnetic moment at any angle (up to $90^\circ$) with respect to the anisotropy
axis. The drive current can be modest ($10^6\,\rm A/cm^2$ or lower) provided
the chirp rate is sufficiently slow. The induced precession is stable against
thermal noise, even for small nano-objects at room temperature. Complete
reversal of the magnetization can be achieved by adding a small external
magnetic field antiparallel to the easy axis. Alternatively, a combination of
chirped ac and dc currents with different polarization directions can also be
used to trigger the reversal.",1610.01819v2
2016-10-08,Transitions in a magnetized quasi-laminar spherical Couette Flow,"First results of a new spherical Couette experiment are presented. The liquid
metal flow in a spherical shell is exposed to a homogeneous axial magnetic
field. For a Reynolds number Re=1000, we study the effect of increasing
Hartmann number Ha. The resulting flow structures are inspected by ultrasound
Doppler velocimetry. With a weak applied magnetic field, we observe an
equatorially anti-symmetric jet instability with azimuthal wave number m=3. As
the magnetic field strength increases, this instability vanishes. When the
field is increased further, an equatorially symmetric return flow instability
arises. Our observations are shown to be in good agreement with linear
stability analysis and non-linear flow simulations.",1610.02571v3
2016-10-10,Magnetic concentrations in stratified turbulence: the negative effective magnetic pressure instability,"In the presence of strong density stratification, hydromagnetic turbulence
attains qualitatively new properties: the formation of magnetic flux
concentrations. We review here the theoretical foundations of this mechanism in
terms of what is now called the negative effective magnetic pressure
instability. We also present direct numerical simulations of forced turbulence
in strongly stratified layers and discuss the qualitative and quantitative
similarities with corresponding mean-field simulations. Finally, the relevance
to sunspot formation is discussed.",1610.03459v3
2016-10-30,Time-Resolved Studies of the Spin-Transfer Reversal Mechanism in Perpendicularly Magnetized Magnetic Tunnel Junctions,"Pulsed spin-torque switching has been studied using single-shot time-resolved
electrical measurements in perpendicularly magnetized magnetic tunnel junctions
as a function of pulse amplitude and junction size in 50 to 100 nm diameter
circular junctions. The mean switching time depends inversely on pulse
amplitude for all junctions studied. However, the switching dynamics is found
to be strongly dependent on junction size and pulse amplitude. In 50 nm
diameter junctions the switching onset is stochastic but the switching once
started, is fast; after being initiated it takes less than 2 ns to switch. In
larger diameter junctions the time needed for complete switching is strongly
dependent on the pulse amplitude, reaching times less than 2 ns at large pulse
amplitudes. Anomalies in the switching rate versus pulse amplitude are shown to
be associated with the long lived (> 2 ns) intermediate junction resistance
states.",1610.09710v1
2016-11-02,Current-induced skyrmion generation and dynamics in symmetric bilayers,"Magnetic skyrmions are textures behaving as quasiparticles which are
topologically different from other states. Their discovery in systems with
broken inversion symmetry sparked the search for materials containing such
magnetic phase at room temperature. Their topological properties combined with
the chirality-related spin-orbit torques make them interesting objects to
control the magnetization at nanoscale. Here we show that a pair of coupled
skyrmions with the same topological charge and opposite chiralities can be
stabilized in a symmetric magnetic bilayer system by combining
Dzyaloshinskii-Moriya interaction (DMI) and dipolar coupling effects. This
effect opens a new path for skyrmion stabilization with much lower DMI. We then
demonstrate in a single device with two different electrodes that such
skyrmions can be efficiently and independently written and shifted by electric
current at large velocities. The skyrmionic nature of the observed
quasiparticles is further confirmed by using the gyrotropic force as a
topological filter. These results set the ground for emerging spintronic
technologies where issues concerning skyrmion stability, nucleation, and
propagation are paramount.",1611.00647v2
2016-11-14,Off-equilibrium scaling driven by a time-dependent magnetic field in O(N) vector models,"We investigate the off-equilibrium dynamics of a spin system with O($N$)
symmetry in $2 < d < 4$ spatial dimensions arising by the presence of a slowly
varying time-dependent magnetic field $h(t,t_s) \sim t/t_s$, $t_s$ is a time
scale, at the critical temperature $T = T_c$ and below it $T < T_c$. After
showing the general theory, we demonstrate the off-equilibrium scaling and we
formally compute the correlation functions in the limit of large $N$. We derive
the off-equilibrium scaling relations for the hysteresis loop area and for the
magnetic work done by the system when the magnetic field $h(t,t_s)$ is varied
across the phase transitions cyclically in time. We also investigate the first
deviations from the equilibrium behavior in the correlation functions checking
the consistence for an exponential approach.",1611.04367v1
2016-11-15,On the formation of a stable penumbra in a region of flux emergence in the Sun,"We studied the formation of the first penumbral sector around a pore in the
following polarity of the Active Region (AR) NOAA 11490. We used a high
spatial, spectral, and temporal resolution data set acquired by the
Interferometric BIdimensional Spectrometer operating at the NSO/Dunn Solar
Telescope as well as data taken by the Helioseismic and Magnetic Imager onboard
the Solar Dynamics Observatory satellite. On the side towards the leading
polarity, elongated granules in the photosphere and an arch filament system
(AFS) in the chromosphere are present, while the magnetic field shows a
sea-serpent configuration, indicating a region of magnetic flux emergence. We
found that the formation of a stable penumbra in the following polarity of the
AR begins in the area facing the opposite polarity located below the AFS in the
flux emergence region, differently from what found by Schlichenmaier and
colleaguestbf. Moreover, during the formation of the first penumbral sector,
the area characterized by magnetic flux density larger than 900 G and the area
of the umbra increase.",1611.04749v1
2016-11-22,Azimuthal Magnetorotational Instability at low and high magnetic Prandtl numbers,"The magnetorotational instability (MRI) is considered to be one of the most
powerful sources of turbulence in hydrodynamically stable quasi-Keplerian
flows, such as those governing accretion disk flows. Although the linear
stability of these flows with applied external magnetic field has been studied
for decades, the influence of the instability on the outward angular momentum
transport, necessary for the accretion of the disk, is still not well known. In
this work we model Keplerian rotation with Taylor-Couette flow and imposed
azimuthal magnetic field using both linear and nonlinear approaches. We present
scalings of instability with Hartmann and Reynolds numbers via linear analysis
and direct numerical simulations (DNS) for the two magnetic Prandtl numbers of
$1.4 \cdot 10^{-6}$ and $1$. Inside of the instability domains modes with
different axial wavenumbers dominate, resulting in sub-domains of
instabilities, which appear different for each $Pm$. The DNS show the emergence
of 1- and 2-frequency spatio-temporally oscillating structures for $Pm=1$ close
the onset of instability, as well as significant enhancement of angular
momentum transport for $Pm=1$ as compared to $Pm=1.4 \cdot 10^{-6}$.",1611.07296v1
2016-11-23,MESL: Proposal for a Non-volatile Cascadable Magneto-Electric Spin Logic,"In the quest for novel, scalable and energy-efficient computing technologies,
many non-charge based logic devices are being explored. Recent advances in
multi-ferroic materials have paved the way for electric field induced low
energy and fast switching of nano-magnets using the magneto-electric (ME)
effect. In this paper, we propose a voltage driven logic-device based on the ME
induced switching of nano-magnets. We further demonstrate that the proposed
logic-device, which exhibits decoupled read and write paths, can be used to
construct a complete logic family including XNOR, NAND and NOR gates. The
proposed logic family shows good scalability with a quadratic dependence of
switching energy with respect to the switching voltage. Further, the proposed
logic-device has better robustness against the effect of thermal noise as
compared to the conventional current driven switching of nano-magnets. A
device-to-circuit level coupled simulation framework, including magnetization
dynamics and electron transport model, has been developed for analyzing the
present proposal. Using our simulation framework, we present energy and delay
results for the proposed Magneto-Electric Spin Logic (MESL) gates.",1611.07738v1
2016-11-27,Positive Quantum Magnetoresistance in Tilted Magnetic Field,"Transport properties of highly mobile 2D electrons are studied in symmetric
GaAs quantum wells placed in titled magnetic fields. Quantum positive
magnetoresistance (QPMR) is observed in magnetic fields perpendicular to the 2D
layer. Application of in-plane magnetic field produces a dramatic decrease of
the QPMR. This decrease correlates strongly with the reduction of the amplitude
of Shubnikov de Haas resistance oscillations due to modification of the
electron spectrum via enhanced Zeeman splitting. Surprisingly no quantization
of the spectrum is detected when the Zeeman energy exceeds the half of the
cyclotron energy suggesting an abrupt transformation of the electron dynamics.
Observed angular evolution of QPMR implies strong mixing between spin subbands.
Theoretical estimations indicate that in the presence of spin-orbital
interaction the elastic impurity scattering provides significant contribution
to the spin mixing in GaAs quantum wells at high filling factors.",1611.08890v1
2016-11-30,Energy Conservation and the Chiral Magnetic Effect,"We analyze the chiral magnetic effect in a homogeneous neutral plasma from
the point of view of energy conservation, and construct an effective potential
for the growth of maximally helical perturbations of the electromagnetic field.
We show that a negative curvature at the origin of the potential, indicating
instability of the plasma, is induced by a chiral asymmetry in electron Fermi
energy, as opposed to number density, while the potential grows at large field
value. It follows that the ground state for a plasma has zero magnetic
helicity; a nonzero electron mass will allow an excited state of a plasma with
nonzero helicity to relax to that ground state quickly. We conclude that a
chiral plasma instability triggered by weak interactions is not a viable
mechanism for explaining magnetic fields in stars except possibly when dynamics
drives the system far from equilibrium.",1612.00032v2
2017-01-07,Torsion effects on Condensed Matter: like a magnetic field but not so much,"In this work, we study the effects of torsion due to a uniform distribution
of topological defects (screw dislocations) on free spin/carrier dynamics in
elastic solids. When a particle moves in such a medium, the effect of the
torsion associated to the defect distribution is analogous to that of an
applied magnetic field but with subtle differences. Analogue Landau levels are
present in this system but they cannot be confined to two dimensions. In the
case of spinless carriers, zero modes, which do not appear in the magnetic
Landau levels, show up for quantized values of the linear momentum projected on
the defects axis. Particles with spin are subjected to a Zeeman-like coupling
between spin and torsion, which is insensitive to charge. This suggests the
possibility of spin resonance experiments without a magnetic field for charged
carriers or quasiparticles without electrical charge, like triplet excitons,
for instance.",1701.01829v1
2017-01-07,A quantitative description of Nernst effect in high-temperature superconductors,"A quantitative vortex-fluid model for flux-flow resistivity $\rho$ and Nernst
signal $e_N$ in high-temperature superconductors (HTSC) is proposed. Two kinds
of vortices, magnetic and thermal, are considered, and the damping viscosity
$\eta$ is modeled by extending the Bardeen-Stephen model to include the
contributions of flux pinning at low temperature and in weak magnetic fields,
and vortex-vortex collisions in strong magnetic fields. Remarkably accurate
descriptions for both Nernst signal of six samples and flux flow resistivity
are achieved over a wide range of temperature $T$ and magnetic field $B$. A
discrepancy of three orders of magnitude between data and Anderson's model of
Nernst signal is pointed out and revised using experimental values of $\eta$
from magnetoresistance. Furthermore, a two-step procedure is developed to
reliably extract, from the Nernst signal, a set of physical parameters
characterizing the vortex dynamics, which yields predictions of local
superfluid density $n_s$, the Kosterlitz coefficient $b$ of thermal vortices,
and upper critical field and temperature. Application of the model and
systematic measurement of relevant physical quantities from Nernst signal in
other HTSC samples are discussed.",1701.01832v1
2017-01-26,Precision determination of a fluxoid quantum's magnetic moment in a superconducting micro-ring,"Using dynamic cantilever magnetometry and experimentally determining the
cantilever's vibrational mode shape, we precisely measured the magnetic moment
of a lithographically defined micron-sized superconducting Nb ring, a key
element for the previously proposed subpiconewton force standard. The magnetic
moments due to individual magnetic fluxoids and a diamagnetic response were
independently determined at T = 4.3 K, with a subfemtoampere-square-meter
resolution. The results show good agreement with the theoretical estimation
yielded by the Brandt and Clem model within the spring constant determination
accuracy.",1701.07598v1
2017-02-09,Second-order chiral kinetic theory: Chiral magnetic and pseudomagnetic waves,"The consistent chiral kinetic theory accurate to the second order in
electromagnetic and pseudoelectromagnetic fields is derived for a relativistic
matter with two Weyl fermions. By making use of such a framework, the
properties of longitudinal collective excitations, which include both chiral
magnetic and chiral pseudomagnetic waves, are studied. It is shown that the
proper treatment of dynamical electromagnetism transforms these gapless waves
into chiral (pseudo)magnetic plasmons, whose Langmuir (plasma) gap receives
corrections quadratic in both magnetic and pseudomagnetic fields.",1702.02950v2
2017-02-26,Chiral Magnetic Effect in a Lattice Model,"We study analytically the one-loop contribution to the Chiral Magnetic
Effect(CME) using lattice regularization with a Wilson fermion field. In the
continuum limit, we find that the chiral magnetic current vanishes at nonzero
temperature but emerges at zero temperature consistent with that found by
Pauli-Villas regularization. For finite lattice size, however, the chiral
magnetic current is nonvanishing at nonzero temperature. But the numerical
vaule of the coefficient of CME current is very small compared with that
extracted from the full QCD simulation for the same lattice parameters. The
possibility of higher order corrections from QCD dynamics is also assessed.",1702.07980v1
2017-03-03,Magnetic field induced polarization enhancement in monolayers of tungsten dichalcogenides: Effects of temperature,"Optical orientation of localized/bound excitons is shown to be effectively
enhanced by the application of magnetic fields as low as 20 mT in monolayer
WS$_2$. At low temperatures, the evolution of the polarization degree of
different emission lines of monolayer WS$_2$ with increasing magnetic fields is
analyzed and compared to similar results obtained on a WSe$_2$ monolayer. We
study the temperature dependence of this effect up to $T=60$ K for both
materials, focusing on the dynamics of the valley pseudospin relaxation. A rate
equation model is used to analyze our data and from the analysis of the width
of the polarization deep in magnetic field we conclude that the competition
between the dark exciton pseudospin relaxation and the decay of the dark
exciton population into the localized states are rather different in these two
materials which are representative of the two extreme cases for the ratio of
relaxation rate and depolarization rate.",1703.01129v1
2017-03-18,A thermodynamically consistent model of magneto-elastic materials under diffusion at large strains and its analysis,"The theory of elastic magnets is formulated under possible diffusion and heat
flow governed by Fick's and Fourier's laws in the deformed (Eulerian)
configuration, respectively. The concepts of nonlocal nonsimple materials and
viscous Cahn-Hilliard equations are used. The formulation of the problem uses
Lagrangian (reference) configuration while the transport processes are pulled
back. Except the static problem, the demagnetizing energy is ignored and only
local non-selfpenetration is considered. The analysis as far as existence of
weak solutions of the (thermo)dynamical problem is performed by a careful
regularization and approximation by a Galerkin method, suggesting also a
numerical strategy. Either ignoring or combining particular aspects, the model
has numerous applications as ferro-to-paramagnetic transformation in elastic
ferromagnets, diffusion of solvents in polymers possibly accompanied by
magnetic effects (magnetic gels), or metal-hydride phase transformation in some
intermetalics under diffusion of hydrogen accompanied possibly by magnetic
effects (and in particular ferro-to-antiferromagnetic phase transformation),
all in the full thermodynamical context under large strains.",1703.06267v2
2017-04-01,Ferrofluids and magnetically guided superparamagnetic particles in flows: A review of simulations and modeling,"Ferrofluids are typically suspensions of magnetite nanoparticles, and behave
as a homogeneous continuum. The production of nanoparticles with a narrow size
distribution and the achievement of colloidal stability are important
technological issues. The ability of the ferrofluid to respond to an external
magnetic field in a controllable manner has made it emerge as a smart material
in a variety of applications, such as seals, lubricants, electronics cooling,
shock absorbers and adaptive optics. Magnetic nanoparticle suspensions have
also gained attraction recently in a range of biomedical applications, such as
cell separation, hyperthermia, MRI, drug targeting and cancer diagnosis. In
this review, we provide an introduction to mathematical modeling of three
problems: motion of superparamagnetic nanoparticles in magnetic drug targeting,
the motion of a ferrofluid drop consisting of chemically bound nanoparticles
without a carrier fluid, and the breakage of a thin film of a ferrofluid.",1704.00104v1
2017-04-05,The role of boundary conditions on helicoidal flow collimation: consequences for the Von-Kármán-Sodium dynamo experiment,"We present hydrodynamic and magneto-hydrodynamic simulations of liquid sodium
flow with the PLUTO compressible MHD code to investigate influence of magnetic
boundary conditions on the collimation of helicoidal motions. We use a
simplified cartesian geometry to represent the flow dynamics in the vicinity of
one cavity of a multi-blades impeller inspired by those used in the
Von-K\'{a}rm\'{a}n-Sodium (VKS) experiment. We show that the impinging of the
large scale flow upon the impeller generates a coherent helicoidal vortex
inside the blades, located at a distance from the upstream blade piloted by the
incident angle of the flow. This vortex collimates any existing magnetic field
lines leading to an enhancement of the radial magnetic field that is stronger
for ferromagnetic than for conducting blades. The induced magnetic field
modifies locally the velocity fluctuations, resulting in an enhanced helicity.
This process possibly explains why dynamo action is more easily triggered in
the VKS experiment when using soft iron impellers.",1704.01637v1
2017-04-11,Gluon Spectrum in Quark-Gluon Plasma under Strong Magnetic Fields,"We obtain the general expression of the gluon propagator at finite
temperature ($T$) and in a magnetic field ($B$), for the case that the four
transverse tensor structures appear in the gluon self-energy. By using this
expression and a specific form of the one-loop gluon self-energy in the lowest
Landau level approximation, we analyze the gluon spectrum in the strong
magnetic field limit. As a result, we find that there exist two collective
excitations of which the energies are of the order of $p\sim gT$ with $g$ being
the coupling constant. One of the two excitations enjoys properties quite
different from those of the collective excitations at $B=0$ which have been
discussed by using the hard thermal loop approximation. We also discuss the
static and dynamical screening effects, which are expected to be important for
computation of transport coefficients in strong magnetic fields.",1704.03191v3
2017-04-23,Photonic orbital angular momentum transfer and magnetic skyrmion rotation,"Magnetic skyrmions are chiral quasiparticles that show promise for future
spintronic applications such as skyrmion racetrack memories and logic devices
because of their topological stability, small size (typically $\sim3-500$ nm),
and ultralow threshold force to drive their motion. On the other hand, the
ability of light to carry and deliver orbital angular momentum (OAM) in the
form of optical vortices has attracted a lot of interest. In this work, we
predict a photonic OAM transfer effect, by studying the dynamics of magnetic
skyrmions subject to Laguerre-Gaussian optical vortices, which manifests a
rotational motion of the skyrmionic quasiparticle around the beam axis. The
topological charge of the optical vortex determines both the magnitude and the
handedness of the rotation velocity of skyrmions. In our proposal, the twisted
light beam acts as an optical tweezer to enable us displacing skyrmions over
large-scale defects in magnetic films to avoid being captured.",1704.06926v3
2017-05-05,Kinetic orbital moments and nonlocal transport in disordered metals with nontrivial geometry,"We study the effects of spatial dispersion in disordered noncentrosymmetric
metals. These include the kinetic magnetoelectric effect, natural optical
activity of metals, as well as the so-called dynamic chiral magnetic effect as
a particular case of the latter. These effects are determined by the linear in
the wave vector of an electromagnetic perturbation contribution to the
conductivity tensor of a material, and stem from the magnetic moments of
quasiparticles near the Fermi surface. We identify new disorder-induced
contributions to these magnetic moments that come from the skew scattering and
side jump processes, familiar from the theory of anomalous Hall effect. We show
that at low frequencies the spatial dispersion of the conductivity tensor comes
mainly either from the skew scattering or intrinsic contribution, and there is
always a region of frequencies in which the intrinsic mechanism dominates. Our
results imply that in clean three-dimensional metals, current-induced
magnetization is in general determined by impurity skew scattering, rather than
intrinsic contributions. Intrinsic effects are expected to dominate in cubic
enantiomorphic crystals with point groups $T$ and $O$, and in polycrystalline
samples, regardless of their mobility.",1705.02367v1
2017-05-09,Low spin wave damping in the insulating chiral magnet Cu$_{2}$OSeO$_{3}$,"Chiral magnets with topologically nontrivial spin order such as Skyrmions
have generated enormous interest in both fundamental and applied sciences. We
report broadband microwave spectroscopy performed on the insulating chiral
ferrimagnet Cu$_{2}$OSeO$_{3}$. For the damping of magnetization dynamics we
find a remarkably small Gilbert damping parameter of about $1\times10^{-4}$ at
5 K. This value is only a factor of 4 larger than the one reported for the best
insulating ferrimagnet yttrium iron garnet. We detect a series of sharp
resonances and attribute them to confined spin waves in the mm-sized samples.
Considering the small damping, insulating chiral magnets turn out to be
promising candidates when exploring non-collinear spin structures for high
frequency applications.",1705.03416v1
2017-05-10,Long-time coherence in fourth-order spin correlation functions,"We study the long-time decay of fourth-order electron spin correlation
functions for an isolated singly charged semi-conductor quantum dot. The
electron spin dynamics is governed by the applied external magnetic field as
well as the hyperfine interaction. While the long-time coherent oscillations in
the correlation functions can be understood within an semi-classical approach
treating the Overhauser field as frozen, the field dependent decay of its
amplitude reported in different experiments cannot be explained by the
central-spin model indicating the insufficiency of such a description. By
incorporating the nuclear Zeeman splitting and the strain induced
nuclear-electric quadrupolar interaction, we find the correct crossover from a
fast decay in small magnetic fields to a slow exponential asymptotic in large
magnetic fields. It originates from a competition between the quadrupolar
interaction inducing an enhanced spin decay and the nuclear Zeeman term that
suppressed the spin-flip processes. We are able to explain the magnetic field
dependency of the characteristic long-time decay time $T_2$ depending on the
experimental setups. The calculated asymptotic values of $T_2 = 3 -4\,\mu$s
agree qualitatively well with the experimental data.",1705.03712v2
2017-05-15,Momentum-dependent susceptibilities and magnetic exchange in bcc iron from supercell DMFT calculations,"We analyze the momentum- and temperature dependences of the magnetic
susceptibilities and magnetic exchange interaction in paramagnetic bcc iron by
a combination of density functional theory and dynamical mean-field theory
(DFT+DMFT). By considering a general derivation of the orbital-resolved
effective model for spin degrees of freedom for Hund's metals, we relate
momentum-dependent susceptibilities in the paramagnetic phase to the magnetic
exchange. We then calculate non-uniform orbital-resolved susceptibilities at
high-symmetry wave vectors by constructing appropriate supercells in the DMFT
approach. Extracting the irreducible parts of susceptibilities with respect to
Hund's exchange interaction, we determine the corresponding orbital-resolved
exchange interactions, which are then interpolated to the whole Brillouin zone.
Using the spherical model we estimate the temperature dependence of the
resulting exchange between local moments.",1705.05296v2
2017-06-01,Dynamo enhancement and mode selection triggered by high magnetic permeability,"We present results from consistent dynamo simulations, where the electrically
conducting and incompressible flow inside a cylinder vessel is forced by moving
impellers numerically implemented by a penalization method. The numerical
scheme models jumps of magnetic permeability for the solid impellers,
resembling various configurations tested experimentally in the von-Karman
Sodium experiment. The most striking experimental observations are reproduced
in our set of simulations. In particular, we report on the existence of an
axisymmetric dynamo mode, self-consistently generated when the magnetic
permeability of the impellers exceeds a threshold. We describe a possible
scenario involving both the turbulent flow in the vicinity of the impellers and
the high magnetic permeability of the impellers.",1706.00260v4
2017-06-13,Experimental Observation of Bethe Strings,"Almost one century ago, string states - complex bound states (Wellenkomplexe)
of magnetic excitations - have been predicted to exist in one-dimensional
quantum magnets and since then become a subject of intensive theoretical study.
However, experimental realization and identification of string states in
condensed-matter systems remains an unsolved challenge up to date. Here we use
high-resolution terahertz spectroscopy to identify string states in the
antiferromagnetic Heisenberg-Ising chain SrCo2V2O8 in strong longitudinal
magnetic fields. We observe complex bound states (strings) and fractional
magnetic excitations (psinons and antipsinons) in the field-induced critical
regime, which are precisely described by the Bethe ansatz. Our study reveals
that two-string and three-string states govern the quantum spin dynamics close
to the quantum criticality, while the fractional excitations are dominant at
low energies, reflecting the antiferromagnetic quantum fluctuations.",1706.04181v1
2017-06-09,Chiral edge mode in the coupled dynamics of magnetic solitons in a honeycomb lattice,"Motivated by a recent experimental demonstration of a chiral edge mode in an
array of spinning gyroscopes, we theoretically study the coupled gyration modes
of topological magnetic solitons, vortices and magnetic bubbles, arranged as a
honeycomb lattice. The soliton lattice under suitable conditions is shown to
support a chiral edge mode like its mechanical analogue, the existence of which
can be understood by mapping the system to the Haldane model for an electronic
system. The direction of the chiral edge mode is associated with the
topological charge of the constituent solitons, which can be manipulated by an
external field or by an electric-current pulse. The direction can also be
controlled by distorting the honeycomb lattice. Our results indicate that the
lattices of magnetic solitons can serve as reprogrammable topological
metamaterials.",1706.04187v2
2017-06-14,Characteristics of Bi(Pb)SrCaCuO superconducting ceramics samples properties by method of dynamic magnetic susceptibility measurements,"Literature and experimental data about practical determination of phase
composition and structure of BiHTSC ceramics from results of measurements of
temperature dependence of magnetic susceptibility are presented. It is shown
that Bi2223 to Bi2212 phases ratio, calculated by XRD-analysis results, does
not correlate precisely with magnetic susceptibility - temperature dependence.
Phase, which is present in smaller amount, especially Bi2212 phase with lower
critical temperature, appears less pronouncedly at the magnetic susceptibility
- temperature dependence. Besides, limiting parts of Meissner effect
manifestation are peculiarities of contacts of superconducting grains.
Different kinds of such contacts lead to lowering the critical temperature
together with conservation of narrow transition interval, to division of
transition into stages, and to increasing of transition interval. Among
negative consequences of this influence there are hindrances in realization of
grain oriented Bi ceramics synthesis scheme which imply powdering of samples
consisting of chaotically evolved grains of Bi2223 phase, ordered placement of
obtained plate grains of this phase - for example, through vibration
compacting, and sintering.",1706.04420v1
2017-06-15,Exploring the Statistics of Magnetic Reconnection X-points in Kinetic Particle-in-Cell (PIC) Turbulence,"Magnetic reconnection is a ubiquitous phenomenon in turbulent plasmas. It is
an important part of the turbulent dynamics and heating of space and
astrophysical plasmas. We examine the statistics of magnetic reconnection using
a quantitative local analysis of the magnetic vector potential, previously used
in magnetohydrodynamics simulations, and now generalized to fully kinetic PIC
simulations. Different ways of reducing the particle noise for analysis
purposes including multiple smoothing techniques are explored. We find that a
Fourier filter applied at the Debye scale is an optimal choice for analyzing
PIC data. Finlay, we find a broader distribution of normalized reconnection
rates compared to the MHD limit with rates as large as 0.5 but with an average
of approximately 0.1.",1706.04905v1
2017-07-03,Anisotropic chiral magnetic effect from tilted Weyl cones,"We determine the antisymmetric current-current response for a pair of (type
I) tilted Weyl cones with opposite chirality. We find that the dynamical chiral
magnetic effect depends on the magnitude of the tilt and on the angle between
the tilting direction and the wave vector of the magnetic field. Additionally,
the chiral magnetic effect is shown to be closely related to the presence of an
intrinsic anomalous Hall effect with a current perpendicular to the tilting
direction and the electric field. We investigate the nonanalytic
long-wavelength limit of the corresponding transport coefficients.",1707.00598v2
2017-07-06,Strong angular dependent transmission ratio due to interplay between spin waves and the domain wall in perpendicular magnetic anisotropy materials,"Spin waves (SW) can induce rich domain wall (DW) motion in a
perpendicular-magnetic anisotropy nanostrip. In-plane magnetization tilt angle
resulting from the fluctuation of the effective field of the magnetization
response in the DW region plays an essential role in the dynamics of SW
interacting with a DW. We performed simulation and found that the transmission
ratio of the propagating SW across the DW depends strongly on the tilt angle in
the low-frequency regime. The material parameters and the geometrical
configuration can be fine-tuned for practical devices.",1707.01617v1
2017-07-18,Solitary potentials in a three component space plasma,"The heavy ion acoustic solitary waves (HIASWs) in a magnetized,
collisionless, three component space plasma system in the presence of dynamical
heavy particles and bi-kappa distributed two distinct temperature of electrons
has been carried out. The Korteweg-de Vries (K-dV), the modified K-dV (mK-dV),
and the further mK-dV (fmK-dV) equations are derived by employing the well
known reductive perturbation method. The basic features of HIASWs (speed,
amplitude, width etc.) are found to be significantly modified by the effects of
number density of plasma species, superthermal electrons with two distinct
temperatures and the magnetic field (obliqueness). The K-dV and fm-KdV solitons
exhibit both compressive and rarefactive structures, whereas the mK-dV solitons
support only compressive structures. In order to trace the vicinity of the
critical heavy ion density, neither the magnetized KdV nor magnetized mKdV
equation is suitable for describing the HIASWs. Therefore, we have considered a
higher order nonlinearity to describe the solitary profiles by deriving fmK-dV
equation. The implication of our results in some space and laboratory plasma
situations are concisely discussed.",1707.05571v1
2017-07-23,Investigating different $Λ$ and $\barΛ$ polarizations in relativistic heavy-ion collisions,"Based on the chiral kinetic equations of motion, spin polarizations of
various quarks, due to the magnetic field induced by spectator protons as well
as the quark-antiquark vector interaction, are studied within a partonic
transport approach. Although the magnetic field in QGP enhances the splitting
of the spin polarizations of partons compared to the results under the magnetic
field in vacuum, the spin polarizations of $s$ and $\bar s$ quarks are also
sensitive to the quark-antiquark vector interaction, challenging that the
different $\Lambda$ and $\bar \Lambda$ spin polarization is a good measure of
the magnetic field in relativistic heavy-ion collisions. It is also found that
there is no way to obtain the large splitting of the spin polarization between
$\Lambda$ and $\bar \Lambda$ at $\sqrt{s_{NN}}=7.7$ GeV with partonic dynamics.",1707.07262v3
2017-07-30,Multi-second magnetic coherence in a single domain spinor Bose-Einstein condensate,"We describe a compact, robust and versatile system for studying magnetic
dynamics in a spinor Bose-Einstein condensate. Condensates of 87 Rb are
produced by all-optical evaporation in a 1560 nm optical dipole trap, using a
non-standard loading sequence that employs an auxiliary 1529 nm beam for
partial compensation of the strong differential light shift induced by the
dipole trap itself. We use near-resonance Faraday rotation probing to
non-destructively track the condensate magnetization, and demonstrate
few-Larmor-cycle tracking with no detectable degradation of the spin
polarization. In the ferromagnetic F = 1 ground state, we observe magnetic T1
and T2$^*$ coherence times limited only by the several-second residence time of
the atoms in the trap.",1707.09607v1
2017-08-02,Non-equilibrium 2D Ising model with stationary uphill diffusion,"Usually, in a non-equilibrium setting, a current brings mass from the highest
density regions to the lowest density ones. Although rare, the opposite
phenomenon (known as ""uphill diffusion"") has also been observed in
multicomponent systems, where it appears as an artificial effect of the
interaction among components. We show here that uphill diffusion can be a
substantial effect, i.e. it may occur even in single component systems as a
consequence of some external work. To this aim we consider the 2D ferromagnetic
Ising model in contact with two reservoirs that fix, at the left and the right
boundaries, magnetizations of the same magnitude but of opposite signs. We
provide numerical evidence that a class of non-equilibrium steady states exists
in which, by tuning the reservoir magnetizations, the current in the system
changes from ""downhill"" to ""uphill"". Moreover, we also show that, in such
non-equilibrium set-up, the current vanishes precisely when the reservoir
magnetizations equal the magnetization of the corresponding equilibrium
dynamics, thus establishing a novel relation between equilibrium and
non-equilibrium properties.",1708.00751v3
2017-08-12,Effects of equatorial chorus wave normal azimuthal distribution on wave propagation,"The non-ducted propagation characteristics of the VLF waves in the inner
magnetosphere were studied with respect to their frequency, source
localization, and initial wave normal angle, between the wave-normal and the
background magnetic field. The ray tracing software based on multi-components
cold plasma approach was developed by use of the IGRF magnetic field model and
the GCPM model of plasma density. We described dynamics of the wave-normals
direction during its propagation and magnetospheric reflection. We showed that
whistler waves can be reflected when lower hybrid resonance frequency becomes
greater than the wave frequency: w_LH>w. It corresponds to the magnetic
latitude of ~50 degrees. The simulation results confirmed the inapplicability
of the quasi-longitudinal approximation to describe the propagation of
magnetospheric whistlers. The simulation results of chorus emissions
propagation, which used realistic distributions of waves on the initial
parameters were presented. Particularly, we obtained distributions of chorus
emission waves in dependence on the wave-normal directions for different
magnetic latitudes. It is required for studying diffusive processes in the
radiation belts. The results were found to be in a good agreement with the
CLUSTER STAFF-SA measurements.",1708.03819v1
2017-08-22,Direct observation of domain wall surface tension by deflating or inflating a magnetic bubble,"The surface energy of a magnetic Domain Wall (DW) strongly affects its static
and dynamic behaviours. However, this effect was seldom directly observed and
many related phenomena have not been well understood. Moreover, a reliable
method to quantify the DW surface energy is still missing. Here, we report a
series of experiments in which the DW surface energy becomes a dominant
parameter. We observed that a semicircular magnetic domain bubble could
spontaneously collapse under the Laplace pressure induced by DW surface energy.
We further demonstrated that the surface energy could lead to a geometrically
induced pinning when the DW propagates in a Hall cross or from a nanowire into
a nucleation pad. Based on these observations, we developed two methods to
quantify the DW surface energy, which could be very helpful to estimate
intrinsic parameters such as Dzyaloshinskii-Moriya Interactions (DMI) or
exchange stiffness in magnetic ultra-thin films.",1708.06719v1
2017-08-29,Spin Dynamics in a Stripe-ordered Buckled Honeycomb Lattice Antiferromagnet Ba$_{2}$NiTeO$_{6}$,"We carried out inelastic neutron scattering experiments on a buckled
honeycomb lattice antiferromagnet Ba$_{2}$NiTeO$_{6}$ exhibiting a stripe
structure at a low temperature. Magnetic excitations are observed in the energy
range of $\hbar \omega \lesssim 10$ meV having an anisotropy gap of 2 meV at 2
K. We perform spin-wave calculations to identify the spin model. The obtained
microscopic parameters are consistent with the location of the stripe structure
in the classical phase diagram. Furthermore, the Weiss temperature
independently estimated from a bulk magnetic susceptibility is consistent with
the microscopic parameters. The results reveal that a competition between the
NN and NNN interactions that together with a relatively large single ion
magnetic anisotropy stabilize the stripe magnetic structure.",1708.08717v1
2017-08-29,Ultrafast hot-electron induced quenching of Tb4f magnetic order,"We have investigated ultrafast quenching of the Tb4f magnetic order in
Co74Tb26 alloys, induced by femtosecond hot-electrons pulses. The hot-electron
pulses were produced in specific non-magnetic capping layers by infrared fs
laser pulses. Our experimental results show that sub-picosecond dynamics of
Tb4f magnetic moments can be induced by non-thermal and thermal hot-electrons.
We further demonstrate that the demagnetization efficiencies of non-thermal and
thermal hot electrons are similar. However, the characteristic demagnetization
times show values of 0.35 ps for non-thermal hot-electrons excitations and 1.2
ps for thermal hot-electrons excitations. We explain this temporal elongation
by the propagation time of thermal hot-electrons through the 15 nm thick CoTb
film.",1708.08919v2
2017-09-07,Tunable spin pumping in exchange coupled magnetic trilayers,"Magnetic thin films at ferromagnetic resonance (FMR) leak angular momentum,
which may be absorbed by adjacent layers. This phenomenon, known as spin
pumping, is manifested by an increase in the resonance linewidth ($\Delta H$),
and the closely related Gilbert damping. Another effect of this transfer of
spin currents is a dynamical and long-range coupling that can drive two
magnetic layers into a collective precession when their FMR frequencies
coincide. A collective behavior is also found in magnetic trilayers with
interlayer exchange coupling (IEC). In this study we investigate the interplay
between IEC and spin pumping, using Co/Cu/Py pseudo-spin values. We employ
broadband FMR spectroscopy to explore both the frequency and coupling-strength
dependence of $\Delta H$. Our observations show that there exists a cut-off
frequency, set by the IEC strength, below which the precession is truly
collective and the spin pumping is suppressed. These results demonstrate that
it is possible to control the spin pumping efficiency by varying the frequency
or the interlayer exchange coupling.",1709.02295v1
2017-09-11,Manipulating magnetism by ultrafast control of the exchange interaction,"In recent years, the optical control of exchange interactions has emerged as
an exciting new direction in the study of the ultrafast optical control of
magnetic order. Here we review recent theoretical works on antiferromagnetic
systems, devoted to i) simulating the ultrafast control of exchange
interactions, ii) modeling the strongly nonequilibrium response of the magnetic
order and iii) the relation with relevant experimental works developed in
parallel. In addition to the excitation of spin precession, we discuss examples
of rapid cooling and the control of ultrafast coherent longitudinal spin
dynamics in response to femtosecond optically induced perturbations of exchange
interactions. These elucidate the potential for exploiting the control of
exchange interactions to find new scenarios for both faster and more
energy-efficient manipulation of magnetism.",1709.03343v2
2017-09-28,Magnetic monopoles and nonassociative deformations of quantum theory,"We examine certain nonassociative deformations of quantum mechanics and
gravity in three dimensions related to the dynamics of electrons in uniform
distributions of magnetic charge. We describe a quantitative framework for
nonassociative quantum mechanics in this setting, which exhibits new effects
compared to ordinary quantum mechanics with sourceless magnetic fields, and the
extent to which these theoretical consequences may be experimentally testable.
We relate this theory to noncommutative Jordanian quantum mechanics, and show
that its underlying algebra can be obtained as a contraction of the alternative
algebra of octonions. The uncontracted octonion algebra conjecturally describes
a nonassociative deformation of three-dimensional quantum gravity induced by
magnetic monopoles, which we propose is realised by a non-geometric
Kaluza-Klein monopole background in M-theory.",1709.10080v2
2017-10-07,Thermodynamics of dyonic black holes with Thurston horizon geometries,"In five dimensions, we consider a model described by the Einstein gravity
with a source given by a scalar field and various Abelian gauge fields with
dilatonic-like couplings. For this model, we are able to construct two dyonic
black holes whose three-dimensional horizons are modeled by two nontrivial
homogeneous Thurston's geometries. The dyonic solutions are of Lifshitz type
with an arbitrary value of the dynamical exponent. In fact, the first gauge
field ensures the anisotropy asymptotic while the remaining Abelian fields
sustain the electric and magnetic charges. Using the Hamiltonian formalism, the
mass, the electric and magnetic charges are explicitly computed. Interestingly
enough, the dyonic solutions behave like Chern-Simons vortices in the sense
that their electric and magnetic charges turn to be proportional. The extension
with an hyperscaling violating factor is also scrutinized where we notice that
for specific values of the violating factor, purely magnetic solutions are
possible.",1710.02720v2
2017-10-26,"Coherent transport structures in magnetized plasmas, I : Theory","In a pair of linked articles (called Article I and II respectively) we apply
the concept of Lagrangian Coherent Structures (LCSs) borrowed from the study of
Dynamical Systems to magnetic field configurations in order to separate regions
where field lines have different kind of behaviour. In the present article,
article I, after recalling the definition and the properties of the LCSs, we
show how this conceptual framework can be applied to the study of particle
transport in a magnetized plasma. Futhermore we introduce a simplified model
that allows us to consider explicitly the case where the magnetic configuration
evolves in time on timescales comparable to the particle transit time through
the configuration. In contrast with previous works on this topic, this analysis
requires that a system that is aperiodic in time be investigated. In this case
the Poincar\'e map technique cannot be applied and LCSs remain the only viable
tool.",1710.09815v2
2017-10-27,Single shot all optical switching of intrinsic micron size magnetic domains of a Pt/Co/Pt ferromagnetic stack,"We demonstrate that the magnetization reversal in a ferromagnetic Pt/Co/Pt
stack can be induced by a single femtosecond laser pulse. We find that the size
of the switched spot is comparable to the size of the intrinsic magnetic
domains. It requires an absorbed energy density of $\sim$4 mJ.cm$^{-2}$, beyond
which the excited spot fragments into a multidomain structure. The switching
process occurs back and forth with subsequent laser pulses and it is
helicity-independent. Furthermore, the dynamics of the magnetization reversal
occurs in a timescale less than one microsecond. These results suggest that all
optical switching in ferromagnetic films requires to match the laser spot with
the specific domain sizes.",1710.10341v1
2017-11-03,A Study of Acoustic Halos in Active Region NOAA 11330 using Multi-Height SDO Observations,"We analyze data from the Helioseismic Magnetic Imager (HMI) and the
Atmospheric Imaging Assembly (AIA) instruments on board the Solar Dynamics
Observatory (SDO) to characterize the spatio-temporal acoustic power
distribution in active regions as a function of the height in the solar
atmosphere. For this, we use Doppler velocity and continuum intensity observed
using the magnetically sensitive line at 6173 \AA\ as well as intensity at 1600
\AA\ and 1700 \AA. We focus on the power enhancements seen around AR 11330 as a
function of wave frequency, magnetic field strength, field inclination and
observation height. We find that acoustic halos occur above the acoustic cutoff
frequency and extends up to 10 mHz in HMI Doppler and AIA 1700 \AA\
observations. Halos are also found to be strong functions of magnetic field and
their inclination angle. We further calculate and examine the spatially
averaged relative phases and cross-coherence spectra and find different wave
characteristics at different heights.",1711.01259v1
2017-11-06,"First-principles study of magnetism, lattice dynamics, and superconductivity in LaFeSiH$_x$","The structural, electronic, magnetic, and vibrational properties of
LaFeSiH$_x$ for $x$ between 0 and 1 are investigated using density functional
calculations. We find that the electronic and magnetic properties are strongly
controlled by the hydrogen concentration $x$ in LaFeSiH$_x$. While fully
hydrogenated LaFeSiH has a striped antiferromagnetic ground state, the
underdoped LaFeSiH$_x$ for $x\leq0.75$ is not magnetic within the virtual
crystal approximation or with explicit doping of supercells. The
antiferromagnetic configuration breaks the symmetry of Fe $d$ orbitals and
increases electron-phonon coupling up to $50\%$, especially for modes in the
20-50 meV range that are associated with Fe atomic movement. We find competing
nearest and next-nearest neighbor exchange interactions and significant
spin-phonon coupling, qualitatively similar but smaller in magnitude compared
those found in LaOFeAs superconductors. The superconducting $T_c$ for
antiferromagnetic LaFeSiH$_x$, assuming conventional superconductivity via
McMillan's equation, therefore is computed to be 2-10 K, in contrast to
$T_c\approx0$ for the nonmagnetic material. We also predict that the
LaFeSiH$_x$ could be a good proton conductor due to phase stability with a wide
range of hydrogen concentration $x < 1$.",1711.01764v1
2017-11-14,Observation of vortex-nucleated magnetization reversal in individual ferromagnetic nanotubes,"The reversal of a uniform axial magnetization in a ferromagnetic nanotube
(FNT) has been predicted to nucleate and propagate through vortex domains
forming at the ends. In dynamic cantilever magnetometry measurements of
individual FNTs, we identify the entry of these vortices as a function of
applied magnetic field and show that they mark the nucleation of magnetization
reversal. We find that the entry field depends sensitively on the angle between
the end surface of the FNT and the applied field. Micromagnetic simulations
substantiate the experimental results and highlight the importance of the ends
in determining the reversal process. The control over end vortex formation
enabled by our findings is promising for the production of FNTs with tailored
reversal properties.",1711.05164v1
2017-11-16,Direct URCA Processes in Supernovae and Accretion Disks with Arbitrary Magnetic Field,"An effect of a magnetic field of an arbitrary strength on the beta-decay and
reactions related with it by the crossing symmetry (the beta-processes) in
supernovae and accretion disks around black holes is analyzed. Rates of the
beta-processes and the energy and momentum transfered through them to an
optically transparent matter are calculated. It is shown that the macroscopic
momentum transferred to the medium increases linearly with the magnetic field
strength and can substantially affect the dynamics of supernovae and accretion
disks especially when a matter inside is degenerate. It is also demonstrated
that the rates of the beta-processes and the energy deposition in these
reactions for the magnetic field strength $B \lesssim 10^{15}$ G, which is
assumed to be typical in supernovae and accretion disks, are lower than in the
absence of the field. This suppression is more pronounced for reactions with
neutrinos.",1711.06083v1
2017-11-16,Ideal GLM-MHD: About the entropy consistent nine-wave magnetic field divergence diminishing ideal magnetohydrodynamics equations,"The paper presents two contributions in the context of the numerical
simulation of magnetized fluid dynamics. First, we show how to extend the ideal
magnetohydrodynamics (MHD) equations with an inbuilt magnetic field divergence
cleaning mechanism in such a way that the resulting model is consistent with
the second law of thermodynamics. As a byproduct of these derivations, we show
that not all of the commonly used divergence cleaning extensions of the ideal
MHD equations are thermodynamically consistent. Secondly, we present a
numerical scheme obtained by constructing a specific finite volume
discretization that is consistent with the discrete thermodynamic entropy. It
includes a mechanism to control the discrete divergence error of the magnetic
field by construction and is Galilean invariant. We implement the new
high-order MHD solver in the adaptive mesh refinement code FLASH where we
compare the divergence cleaning efficiency to the constrained transport solver
available in FLASH (unsplit staggered mesh scheme).",1711.06269v3
2017-12-05,Surface plasmon-mediated nanoscale localization of laser-driven sub-THz spin dynamics in magnetic dielectrics,"Ultrafast all-optical control of spins with femtosecond laser pulses is one
of the hot topics at the crossroads of photonics and magnetism with a direct
impact on future magnetic recording. Unveiling light-assisted recording
mechanisms for an increase of the bit density beyond the diffraction limit
without excessive heating of the recording medium is an open challenge. Here we
show that surface plasmon-polaritons in hybrid metal-dielectric structures can
provide spatial confinement of the inverse Faraday effect, mediating the
excitation of localized coherent spin precession with 0.41 THz frequency. We
demonstrate a two orders of magnitude enhancement of the excitation efficiency
at the surface plasmon resonance within the 100 nm layer in dielectric garnet.
Our findings broaden the horizons of ultrafast spin-plasmonics and open
pathways towards non-thermal opto-magnetic recording at the nano-scale.",1712.01617v1
2017-12-13,Co$_{2}$PtGa: A promising magnetic shape memory alloy with high martensite transition temperature,"In the present work, a combined theoretical and experimental study on
Co$_{2}$PtGa Heusler alloy shows that it exhibits a martensite transition
around 1320 K with a small thermal hysteresis (10 K). Dynamical stability of
Co$_2$PtGa in the tetragonal phase has been established by the theoretically
calculated phonon dispersion curves. Magnetization measurements suggest that
this alloy is a ferromagnetic material with a saturation magnetic moment of
2.83 $\mu_B$/f.u. at 2 K, which is in excellent agreement with the value
obtained from \textit{ab-initio} calculations (2.87 $\mu_B$/f.u.). Our present
study demonstrates that Co$_{2}$PtGa is a promising material for high
temperature magnetic shape memory application.",1712.04765v4
2017-12-19,Evolution of magnetic Dirac bosons in a honeycomb lattice,"We examine the presence and evolution of magnetic Dirac nodes in the
Heisenberg honeycomb lattice. Using linear spin theory, we evaluate the
collinear phase diagram as well as the change in the spin dynamics with various
exchange interactions. We show that the ferromagnetic structure produces
bosonic Dirac and Weyl points due to the competition between superexchange
interactions. Furthermore, it is shown that the criteria for magnetic Dirac
nodes are coupled to the magnetic structure and not the overall crystal
symmetry, where the breaking of inversion symmetry greatly affects the
antiferromagnetic configurations. The tunability of the nodal points through
variation of the exchange parameters leads to the possibility of controlling
Dirac symmetries through an external manipulation of the orbital interactions.",1712.06775v1
2017-12-19,Standing Shocks in Magnetized Dissipative Accretion Flow around Black Holes,"We explore the global structure of the accretion flow around a Schwarzschild
black hole where the accretion disc is threaded by toroidal magnetic fields.
The accretion flow is optically thin and advection dominated. Synchrotron
radiation is considered to be the active cooling mechanism in the flow. With
this, we obtain the global transonic accretion solutions and show that
centrifugal barrier in the rotating magnetized accretion flow causes a
discontinuous transition of the flow variables in the form of shock waves. The
shock properties and the dynamics of the post-shock corona are affected by the
flow parameters such as viscosity, cooling rate and strength of the magnetic
fields. The shock properties are investigated against these flow parameters. We
further show that for given set of boundary parameters at the outer edge of the
disc, accretion flow around a black hole admits shock when the flow parameters
are tuned for a considerable range.",1712.07588v1
2018-01-24,Massive star winds interacting with magnetic fields on various scales,"One of the defining processes which govern massive star evolution is their
continuous mass loss via dense, supersonic line-driven winds. In the case of
those OB stars which also host a surface magnetic field, the interaction
between that field and the ionized outflow leads to complex circumstellar
structures known as magnetospheres. In this contribution, we review recent
developments in the field of massive star magnetospheres, including current
efforts to characterize the largest magnetosphere surrounding an O star: that
of NGC 1624-2. We also discuss the potential of the `analytic dynamical
magnetosphere' (ADM) model to interpret multi-wavelength observations. Finally,
we examine the possible effects of -- heretofore undetected -- small-scale
magnetic fields on massive star winds and compare their hypothetical
consequences to existing, unexplained observations.",1801.08123v1
2018-01-25,Theory of high-resolution tunneling spin transport on a magnetic skyrmion,"Tunneling spin transport characteristics of a magnetic skyrmion are described
theoretically in magnetic scanning tunneling microscopy (STM). The
spin-polarized charge current in STM (SP-STM) and tunneling spin transport
vector quantities, the longitudinal spin current and the spin transfer torque,
are calculated in high spatial resolution within the same theoretical
framework. A connection between the conventional charge current SP-STM image
contrasts and the magnitudes of the spin transport vectors is demonstrated that
enables the estimation of tunneling spin transport properties based on
experimentally measured SP-STM images. A considerable tunability of the spin
transport vectors by the involved spin polarizations is also highlighted. These
possibilities and the combined theory of tunneling charge and vector spin
transport pave the way for gaining deep insight into electric-current-induced
tunneling spin transport properties in SP-STM and to the related dynamics of
complex magnetic textures at surfaces.",1801.08375v2
2018-02-05,Quantum many-body dynamics of the Einstein-de Haas effect,"In 1915, Einstein and de Haas and Barnett demonstrated that changing the
magnetization of a magnetic material results in mechanical rotation, and vice
versa. At the microscopic level, this effect governs the transfer between
electron spin and orbital angular momentum, and lattice degrees of freedom,
understanding which is key for molecular magnets, nano-magneto-mechanics,
spintronics, and ultrafast magnetism. Until now, the timescales of
electron-to-lattice angular momentum transfer remain unclear, since modeling
this process on a microscopic level requires addition of an infinite amount of
quantum angular momenta. We show that this problem can be solved by
reformulating it in terms of the recently discovered angulon quasiparticles,
which results in a rotationally invariant quantum many-body theory. In
particular, we demonstrate that non-perturbative effects take place even if the
electron--phonon coupling is weak and give rise to angular momentum transfer on
femtosecond timescales.",1802.01638v2
2018-02-26,Fractal Universality in Near-Threshold Magnetic Lanthanide Dimers,"Ergodic quantum systems are often quite alike, whereas nonergodic, fractal
systems are unique and display characteristic properties. We explore one of
these fractal systems, weakly bound dysprosium lanthanide molecules, in an
external magnetic field. As recently shown, colliding ultracold magnetic
dysprosium atoms display a soft chaotic behavior with a small degree of
disorder. We broaden this classification by investigating the generalized
inverse participation ratio and fractal dimensions for large sets of molecular
wave functions. Our exact close-coupling simulations reveal a dynamic phase
transition from partially localized states to totally delocalized states and
universality in its distribution by increasing the magnetic field strength to
only a hundred Gauss (or 10 mT). Finally, we prove the existence of nonergodic
delocalized phase in the system and explain the violation of ergodicity by
strong coupling between near-threshold molecular states and the nearby
continuum.",1802.09586v1
2018-03-03,Stable spin domains in a non-degenerate ultra-cold gas,"We study the stability of two-domain spin structures in an ultra-cold gas of
magnetically trapped $^{87}$Rb atoms above quantum degeneracy. Adding a small
effective magnetic field gradient stabilizes the domains via coherent
collective spin rotation effects, despite negligibly perturbing the potential
energy relative to the thermal energy. We demonstrate that domain stabilization
is accomplished through decoupling the dynamics of longitudinal magnetization,
which remains in time-independent domains, from transverse magnetization, which
undergoes a purely transverse spin wave trapped within the domain wall. We
explore the effect of temperature and density on the steady-state domains, and
compare our results to a hydrodynamic solution to a quantum Boltzmann equation.",1803.01096v1
2018-03-07,Spin excitations and thermodynamics of the t-J model on the honeycomb lattice,"We present a spin-rotation-invariant Green-function theory for the dynamic
spin susceptibility in the spin-1/2 antiferromagnetic t-J Heisenberg model on
the honeycomb lattice. Employing a generalized mean-field approximation for
arbitrary temperatures and hole dopings, the electronic spectrum of
excitations, the spin-excitation spectrum and thermodynamic quantities
(two-spin correlation functions, staggered magnetization, magnetic
susceptibility, correlation length) are calculated by solving a coupled system
of self-consistency equations for the correlation functions. The temperature
and doping dependence of the magnetic (uniform static) susceptibility is
ascribed to antiferromagnetic short-range order. Our results on the doping
dependencies of the magnetization and susceptibility are analyzed in comparison
with previous results for the t_J model on the square lattice.",1803.03140v1
2018-03-11,Skyrmions Driven by Intrinsic Magnons,"We study the dynamics of a skyrmion in a magnetic insulating nanowire in the
presence of time-dependent oscillating magnetic field gradients. These ac
fields act as a net driving force on the skyrmion via its own intrinsic
magnetic excitations. In a microscopic quantum field theory approach we include
the unavoidable coupling of the external field to the magnons, which gives rise
to time-dependent dissipation for the skyrmion. We demonstrate that the
magnetic ac field induces a super-Ohmic to Ohmic crossover behavior for the
skyrmion dissipation kernels with time-dependent Ohmic terms. The ac driving of
the magnon bath at resonance results in a unidirectional helical propagation of
the skyrmion in addition to the otherwise periodic bounded motion.",1803.04001v2
2018-03-12,Pre-Eruptive Magnetic Reconnection within a Multi-Flux-Rope System in the Solar Corona,"The solar corona is frequently disrupted by coronal mass ejections (CMEs),
whose core structure is believed to be a flux rope made of helical magnetic
field. This has become a ""standard"" picture although it remains elusive how the
flux rope forms and evolves toward eruption. While 1/3 of the ejecta passing
through spacecrafts demonstrate a flux-rope structure, the rest have complex
magnetic fields. Are they originating from a coherent flux rope, too? Here we
investigate the source region of a complex ejecta, focusing on a flare
precursor with definitive signatures of magnetic reconnection, i.e., nonthermal
electrons, flaring plasma, and bi-directional outflowing blobs. Aided by
nonlinear force-free field modeling, we conclude that the reconnection occurs
within a system of multiple braided flux ropes with different degree of
coherency. The observation signifies the importance of internal structure and
dynamics in understanding CMEs and in predicting their impacts on Earth.",1803.04088v1
2018-03-12,Hydrogenation driven formation of local magnetic moments in FeO$_2$H$_x$,"The electronic and magnetic properties of recently discovered new important
constituent of the Earth's lower mantle FeO2H were investigated by means of the
density functional theory combined with the dynamical mean field theory
(DFT+DMFT). Addition of the hydrogen to the parent FeO2 compound, which is an
uncorrelated bad metal, destroys the most important ingredient of its
electronic structure - O-O molecular orbitals. In effect physical properties of
FeO2 and FeO2H turn to be completely different, FeO2H is a correlated metal
with a mass renormalization, m*/m ~ 2, and magnetic moments on Fe ions become
well localized with the Curie-Weiss type of uniform magnetic susceptibility.",1803.04255v2
2018-03-19,Geometric phase magnetometry using a solid-state spin,"Magnetometry is a powerful technique for the non-invasive study of biological
and physical systems. A key challenge lies in the simultaneous optimization of
magnetic field sensitivity and maximum field range. In interferometry-based
magnetometry, a quantum two-level system acquires a dynamic phase in response
to an applied magnetic field. However, due to the 2{\pi} periodicity of the
phase, increasing the coherent interrogation time to improve sensitivity
results in reduced field range. Here we introduce a route towards both large
magnetic field range and high sensitivity via measurements of the geometric
phase acquired by a quantum two-level system. We experimentally demonstrate
geometric-phase magnetometry using the optically addressable electronic spin
associated with the nitrogen vacancy (NV) color center in diamond. Our approach
enables unwrapping of the 2{\pi} phase ambiguity, decoupling of magnetic field
range from sensitivity, and enhancement of the field range by about 400 times.
We also find additional improvement in sensitivity in the nonadiabatic regime,
and study how geometric-phase decoherence depends on adiabaticity. Our results
show that the geometric phase can be a versatile tool for quantum sensing
applications.",1803.07176v1
2018-03-26,Development of a SEMPA system for magnetic imaging with ns time resolution and phase-sensitive detection,"Scanning electron microscopy with polarization analysis is a powerful
lab-based magnetic imaging technique offering parallel imaging of multiple
magnetization components and a very high spatial resolution. However, one
drawback of the technique is the long required acquisition time resulting from
the low inherent efficiency of spin detection, which has limited the
applicability of the technique to certain quasi-static measurement schemes and
materials with strong contrast. Here we demonstrate the ability to improve the
signal-to-noise ratio for particular classes of measurement involving periodic
excitation of the magnetic structure via the integration of a time-to-digital
converter to the system and a digital lock-in detection scheme. The modified
setup provides dynamic imaging capabilities using selected time windows and
finally full time-resolved imaging with a demonstrated time resolution of
better than 2 ns.",1803.09775v1
2018-04-08,Mixed convection in a downward flow in a vertical duct with strong transverse magnetic field,"The downward flow in a vertical duct with one heated and three thermally
insulated walls is analyzed numerically using the two-dimensional approximation
valid in the asymptotic limit of an imposed strong transverse magnetic field.
The work is motivated by the design of liquid metal blankets with poloidal
ducts for future nuclear fusion reactors, in which the main component of the
magnetic field is perpendicular to the flow direction and very strong heating
is applied at the wall facing the reaction chamber. The flow is found to be
steady-state or oscillating depending on the strengths of the heating and
magnetic field. A parametric study of the instability leading to the
oscillations is performed. It is found among other results that the flow is
unstable and develops high-amplitude temperature oscillations at the conditions
typical for a fusion reactor blanket.",1804.02659v1
2018-04-13,Magnetic field topology from non-force free extrapolation and magnetohydrodynamic simulation of its eventual dynamics,"Magnetic reconnections (MRs) for various magnetic field line (MFL) topologies
are believed to be the initiators of various solar eruptive events like flares
and coronal mass ejections (CMEs). Consequently, important is a thorough
understanding and quantification of the MFL topology and their evolution which
leads to MRs. Contemporary standard is to extrapolate the coronal MFLs using
equilibrium models where the Lorentz force on the coronal plasma is zero
everywhere, because either there is no current or the current is parallel to
the magnetic field. In tandem, a non-force-free-field (NFFF) extrapolation
scheme has evolved and allows for a Lorentz force which is non-zero only at the
photosphere but asymptotically vanishes with height. The paper reports
magnetohydrodynamic (MHD)- simulations initiated by NFFF extrapolation of the
coronal MFLs for the active region NOAA 11158. Interestingly, quasi-separatrix
layers (QSLs) which facilitate MRs are detected in the extrapolated MFLs. The
AR 11158 is flare producing and, the paper makes an attempt to asses the role
of QSLs in the flare onsets.",1804.04814v1
2018-04-22,Atom-by-atom construction of attractors in a tunable finite size spin array,"We demonstrate that a two-dimensional finite and periodic array of Ising
spins coupled via RKKY-like exchange can exhibit tunable magnetic states
ranging from three distinct magnetic regimes: (1) a conventional ferromagnetic
regime, (2) a glass-like regime, and (3) a new multi-well regime. These
magnetic regimes can be tuned by one gate-like parameter, namely the ratio
between the lattice constant and the oscillating interaction wavelength. We
characterize the various magnetic regimes, quantifying the distribution of low
energy states, aging relaxation dynamics, and scaling behavior. The glassy and
multi-well behavior results from the competing character of the oscillating
long-range exchange interactions. The multi-well structure features multiple
attractors, each with a sizable basin of attraction. This may open the possible
application of such atomic arrays as associative memories.",1804.08093v2
2018-04-30,Gravity effects on mixing with magnetic micro-convection in microfluidics,"Mixing remains an important problem for development of successful
microfluidic and lab-on-a-chip devices, where simple and predictable systems
are particularly interesting. One is magnetic micro-convection, an instability
happening on the interface of miscible magnetic and non-magnetic fluids in a
Hele-Shaw cell under applied field. Previous work proved that Brinkman model
quantitatively explains the experiments. However, a gravity caused convective
motion complicated the tests. Here we first improve the experimental system to
exclude the gravitational convective motion. Afterwards, we observe and
quantify how gravity and laminar flow play an important role in stabilizing the
perturbations that create the instability. Accordingly, we improve our
theoretical model and perform linear analysis. Two dimensionless quantities
explain the experimental observations of change in critical field needed for
instability and characteristic size of the emerging pattern. Finally, we
discuss the conditions at which gravity plays an important role in microfluidic
systems.",1805.00101v2
2018-05-03,Sculpting the Spin-Wave Response of Artificial Spin Ice via Microstate Selection,"The spin-wave dynamics of the ferromagnetic nanoarrays termed artificial spin
ice (ASI) are known to vary depending on their magnetic microstate. However,
little work has been done to characterise this relationship. Recent advances in
control over the magnetic configuration of ASI bring designs harnessing the
interplay between spin-wave eigenmodes and the microstate within reach,
offering diverse applications including reconfigurable magnonic crystals,
microwave filters and microstate read-out probes. These designs hinge on a
strong understanding of the underlying spin wave-microstate correspondence.
  Here, we analyse the effects of the magnetic microstate on spin-wave spectra
of honeycomb ASI systems via micromagnetic simulation. We find the spin-wave
spectrum to be highly-tunable via the microstate to an enhanced degree relative
to existing magnonic crystals, with mode shifting and (de)activation realised
by reversing individual nanoislands. Symmetries of ASI systems and the
chirality of magnetic defects are found to play important roles in determining
the high-frequency response.",1805.01397v3
2018-05-23,Turbulence in Magnetized Pair Plasmas,"Alfv\'enic-type turbulence in strongly magnetized, low-beta pair plasmas is
investigated. A coupled set of equations for the evolution of the magnetic and
flow potentials are derived, covering both fluid and kinetic scales. In the
fluid (MHD) range those equations are the same as for electron-ion plasmas, so
turbulence at those scales is expected to be of the Alfv\'enic nature,
exhibiting critical balance, dynamic alignment, and transition to a tearing
mediated regime at small scales. The critical scale at which a transition to a
tearing-mediated range occurs is derived, and the spectral slope in that range
is predicted to be $k_\perp^{-8/3}$ (or $k_\perp^{-3}$, depending on details of
the reconnecting configuration assumed). At scales below the electron (and
positron) skin depth, it is argued that turbulence is dictated by a cascade of
the inertial Alfv\'en wave, which we show to result in the magnetic energy
spectrum $\propto k_\perp^{-11/3}$.",1805.09224v1
2018-06-05,Backscattering immunity of dipole-exchange magnetostatic surface spin waves,"The existence of backscattering-immune spin-wave modes is demonstrated in
magnetic thin films of nano-scale thickness. Our results reveal that chiral
Magneto Static Surface Waves (cMSSWs), which propagate perpendicular to the
magnetization direction in an in-plane magnetized thin film, are robust against
backscattering from surface defects. cMSSWs are protected against various types
of surface inhomogeneities and defects as long as their frequency lies inside
the gap of the volume modes. Our explanation is independent of the topology of
the modes and predicts that this robustness is a consequence of symmetry
breaking of the dynamic magnetic fields of cMSSWs due to the off-diagonal part
of the dipolar interaction tensor, which is present both for long- (dipole
dominated) and short-wavelength (exchange dominated) spin waves. Micromagnetic
simulations confirm the robust character of the cMSSWs. Our results open a new
direction in designing highly efficient magnonic logic elements and devices
employing cMSSWs in nano-scale thin films.",1806.01554v3
2018-06-05,Galvano- and thermo-magnetic effects at low and high temperatures within non-Markovian quantum Langevin approach,"The quantum Langevin formalism is used to study the charge carrier transport
in a twodimensional sample. The center of mass of charge carriers is visualized
as a quantum particle, while an environment acts as a heat bath coupled to it
through the particle-phonon interaction. The dynamics of the charge carriers is
limited by the average collision time which takes effectively into account the
two-body effects. The functional dependencies of particle-phonon interaction
and average collision time on the temperature and magnetic field are
phenomenologically treated. The galvano-magnetic and thermo-magnetic effects in
the quantum system appear as the result of the transitional processes at low
temperatures.",1806.01685v1
2018-06-11,Breaking Kelvin: Circulation conservation and vortex breakup in MHD at low Magnetic Prandtl Number,"In this paper we examine the role of weak magnetic fields in breaking
Kelvin's circulation theorem and in vortex breakup in two-dimensional
magnetohydrodynamics for the physically important case of a low magnetic
Prandtl number (low $Pm$) fluid. We consider three canonical inviscid solutions
for the purely hydrodynamical problem, namely a Gaussian vortex, a circular
vortex patch and an elliptical vortex patch. We examine how magnetic fields
lead to an initial loss of circulation $\Gamma$ and attempt to derive scaling
laws for the loss of circulation as a function of field strength and diffusion
as measured by two non-dimensional parameters. We show that for all cases the
loss of circulation depends on the integrated effects of the Lorentz force,
with the patch cases leading to significantly greater circulation loss. For the
case of the elliptical vortex the loss of circulation depends on the total area
swept out by the rotating vortex and so this leads to more efficient
circulation loss than for a circular vortex.",1806.03922v1
2018-06-11,Quantum Spin Fragmentation in Kagome Ice Ho3Mg2Sb3O14,"A promising route to realize entangled magnetic states combines geometrical
frustration with quantum-tunneling effects. Spin-ice materials are canonical
examples of frustration, and Ising spins in a transverse magnetic field are the
simplest many-body model of quantum tunneling. Here, we show that the tripod
kagome lattice material Ho3Mg2Sb3O14 unites an ice-like magnetic degeneracy
with quantum-tunneling terms generated by an intrinsic splitting of the Ho3+
ground-state doublet, realizing a frustrated transverse Ising model. Using
neutron scattering and thermodynamic experiments, we observe a
symmetry-breaking transition at T*~0.32 K to a remarkable quantum state with
three peculiarities: a continuous magnetic excitation spectrum down to T~0.12K;
a macroscopic degeneracy of ice-like states; and a fragmentation of the spin
into periodic and aperiodic components strongly affected by quantum
fluctuations. Our results establish that Ho3Mg2Sb3O14 realizes a
spin-fragmented state on the kagome lattice, with intrinsic quantum dynamics
generated by a homogeneous transverse field.",1806.04081v1
2018-06-14,Planar Superconductor-Ferromagnet-Superconductor Josephson junctions as scanning probe sensors,"We propose a novel type of magnetic scanning probe sensor, based on a single
planar Josephson junction with a magnetic barrier. The planar geometry together
with high magnetic permeability of the barrier helps to focus flux in the
junction and thus enhance the sensitivity of the sensor. As a result, it may
outperform equally sized SQUID both in terms of the magnetic field sensitivity
and the spatial resolution in one scanning direction. We fabricate and analyze
experimentally sensor prototypes with a superparamagnetic CuNi and a
ferromagnetic Ni barrier. We demonstrate that the planar geometry allows easy
miniaturization to nm-scale, facilitates an effective utilization of the
self-field phenomenon for amplification of sensitivity and a simple
implementation of a control line for feed-back operation in a broad dynamic
range.",1806.05582v1
2018-06-21,"Testing Alfvén wave propagation in a ""realistic"" set-up of the solar atmosphere","We present a radiative magneto-hydrodynamic simulation set-up using the
PENCIL CODE to study the generation, propagation and dissipation of Alfv\'en
waves in the solar atmosphere which includes a convective layer, photosphere
below and chromosphere, transition region and the corona above. We prepare a
setup of steady-state solar convection where the imposed external magnetic
field also has reached the final value gradually starting from a very small
value. From that state, we start several simulations by varying the magnetic
Prandtl number and the forcing strengths. We find the propagation
characteristics of waves excited in this simulation run depend strongly on the
magnetic Prandtl number and the wave number of the forcing. For magnetic
Prandtl number of unity, we obtain localized heating in the corona due to shock
dissipation.",1806.08166v2
2018-06-26,Instabilities of MHD flows driven by traveling magnetic fields,"The flow of an electrically conducting fluid driven by a traveling magnetic
field imposed at the endcaps of a cylindrical annulus is numerically studied.
At sufficiently large magnetic Reynolds number, the system undergoes a
transition from synchronism with the traveling field to a stalled flow, similar
to the one observed in electromagnetic pumps. A new type of boundary layer is
identified for such electromagnetically-driven flows, that can be understood as
a combination of Hartmann and Shercliff layers generated by the spatio-temporal
variations of the magnetic field imposed at the boundaries. An energy budget
calculation shows that energy dissipation mostly occurs within these boundary
layers and we observe that the ohmic dissipation Db always overcomes the
viscous dissipation Dv, suggesting the existence of an upper bound for the
efficiency of electromagnetic pumps. Finally, we show that the destabilization
of the flow occurs when both dissipations are nearly equal.",1806.09970v1
2018-06-26,Fully resolved array of simulations investigating the influence of the magnetic Prandtl number on magnetohydrodynamic turbulence,"We explore the effect of the magnetic Prandtl number Pm on energy and
dissipation in fully resolved direct numerical simulations of steady-state,
mechanically forced homogeneous magnetohydrodynamic turbulence in the range Pm
= 1/32 to 32. We compare the spectra and show that if the simulations are not
fully resolved, the steepness of the scaling of the kinetic-to-magnetic
dissipation ratio with Pm is overestimated. We also present results of decaying
turbulence with helical and nonhelical magnetic fields, where we find
nonhelical reverse spectral transfer for Pm < 1. The results of this systematic
analysis have applications including stars, planetary dynamos, and accretion
disks.",1806.10202v2
2018-07-05,Mesonic excited states of magnetic monopoles in quantum spin ice,"Spin ice magnetic monopoles are fractionalized emergent excitations in a
class of frustrated magnets called spin ices. The classical spin ice model has
an extensive number of ground state spin configurations, whereas magnetic
monopoles can be thought of as the endpoints of string operators applied to
these ground states. Introducing quantum fluctuations into the model induces
monopoles with dynamics, which would normally lift the degeneracy of the
two-monopole energy level at the linear order of the perturbation. Contrary to
this expectation, we find that quantum fluctuations in the form of a locally
transverse field term partially preserve the extensive degeneracy of the
monopole pairs up to the much weaker splitting of the background monopole-free
spin ice configurations. Each of these approximately degenerate excited states,
termed mesons, can be represented as a bound monopole-antimonopole pair
delocalized in a classical spin ice background.",1807.02193v1
2018-07-18,Microfluidic pumping using artificial magnetic cilia,"One of the vital functions of naturally occurring cilia is fluid transport.
Biological cilia use spatially asymmetric strokes to generate a net fluid flow
that can be utilized for feeding, swimming, and other functions. Biomimetic
synthetic cilia with similar asymmetric beating can be useful for fluid
manipulations in lab-on-chip devices. In this paper, we demonstrate the
microfluidic pumping by magnetically actuated synthetic cilia arranged in
multi-row arrays. We use a microchannel loop to visualize flow created by the
ciliary array and to examine pumping for a range of cilia and microchannel
parameters. We show that magnetic cilia can achieve flow rates of up to 11
{\mu}l/min with the pressure drop of~ 1 Pa. Such magnetic ciliary array can be
useful in microfluidic applications requiring rapid and controlled fluid
transport.",1807.06765v1
2018-07-25,Programmable collective behavior in dynamically self-assembled mobile microrobotic swarms,"Collective control of mobile microrobotic swarms is indispensable for their
potential high-impact applications in targeted drug delivery, medical
diagnostics, parallel micromanipulation, and environmental sensing and
remediation. Lack of on-board computational and sensing capabilities in current
microrobotic systems necessitates use of physical interactions among individual
microrobots for local physical communication and cooperation. Here, we show
that mobile microrobotic swarms with well-defined collective behavior can be
designed by engineering magnetic interactions among individual units.
Microrobots, consisting of a linear chain of self-assembled magnetic
microparticles, locomote on surfaces in response to a precessing magnetic
field. Control over the direction of precessing magnetic field allows
engineering attractive and repulsive interactions among microrobots and, thus,
collective order with well-defined spatial organization and parallel operation
over macroscale distances (~1 cm). These microrobotic swarms can be guided
through confined spaces, while preserving microrobot morphology and function.
These swarms can further achieve directional transport of large cargoes on
surfaces and small cargoes in bulk fluids. Described design approach,
exploiting physical interactions among individual robots, enables facile and
rapid formation of self-organized and reconfigurable microrobotic swarms with
programmable collective order.",1807.09702v1
2018-07-31,Terahertz Spin Transfer Torque Oscillator Based on a Synthetic Antiferromagnet,"Bloch-Bloembergen-Slonczewski equation is adopted to simulate magnetization
dynamics in spin-valve based spin-transfer torque oscillator with synthetic
antiferromagnet acting as a free magnetic layer. High frequency up to the
terahertz scale is predicted in synthetic antiferromagnet spin-transfer torque
oscillator with no external magnetic field if the following requirements are
fulfilled: antiferromagnetic coupling between synthetic antiferromagnetic
layers is sufficiently strong, and the thickness of top (bottom) layer of
synthetic antiferromagnet is sufficiently thick (thin) to achieve a wide
current density window for the high oscillation frequency. Additionally, the
transverse relaxation time of the free magnetic layer should be sufficiently
larger compared with the longitudinal relaxation time. Otherwise, stable
oscillation cannot be sustained or scenarios similar to regular spin
valve-based spin-transfer torque oscillator with relatively low frequency will
occur. Our calculations pave a new way for exploring THz spintronics devices.",1807.11633v1
2018-08-03,"Magnetic structure, dipole reversals, and 1/f noise in resistive MHD spherical dynamos","A parametric study of the magnetic dipole behavior in resistive
incompressible MHD inside a rotating sphere is performed, using direct
numerical simulations and considering Reynolds and Ekman numbers as controlling
parameters. The tendency is to obtain geodynamo-like magnetic dipole reversal
regimes for sufficiently small Ekman and large Reynolds numbers. The typical
dipole latitude obtained in the reversal regime is around 40 degrees (with
respect to the rotation axis of the sphere). A statistical analysis of waiting
times between dipole reversals is also performed, obtaining a non-Poissonian
distribution of waiting times, indicating long-term memory effects. We also
report the presence of a $1/f$ frequency power spectrum in the magnetic dipole
time-series, which also shows a tendency to grow toward lower frequencies as
the Ekman number is decreased.",1808.01370v2
2018-08-30,Influence of spin glass-like magnetic relaxation on the zero-field-cooled exchange bias effect,"The zero-field-cooled exchange bias (ZEB) effect is a remarkable phenomenon
recently reported for some reentrant spin glass-like compounds. In this work,
the time-evolution of magnetization is thoroughly investigated for two ZEB
materials in order to figure out the role played by the spin glass-like phase
on such effect. La$_{1.5}$Sr$_{0.5}$CoMnO$_{6}$ and
La$_{1.5}$Ca$_{0.5}$CoMnO$_{6}$ were chosen as representative samples of ZEB
systems, since the former compound presents the largest ZEB reported so far,
while the second has a much smaller effect, despite being
structurally/chemically similar. Comprehensive magnetic measurements were
carried on both samples, and the results are discussed in terms of the amount
and time-evolution of the spin glass-like phase under the influence of a
varying field. We also propose a phenomenological model, based on the pinning
of spin glass-like moments and on the dynamics of their magnetic relaxation, to
explain the asymmetry observed in the hysteresis loops. The good agreement
between the simulated and experimental results confirms our hypothesis that the
spin glass-like phase is key to the ZEB effect.",1808.10305v1
2018-09-04,Joint Instability and Abrupt Nonlinear Transitions in a Differentially Rotating Plasma,"Global magnetohydrodynamic (MHD) instabilities are investigated in a
computationally tractable two-dimensional model of the solar tachocline. The
model's differential rotation yields stability in the absence of a magnetic
field, but if a magnetic field is present, a joint instability is observed. We
analyze the nonlinear development of the instability via fully nonlinear direct
numerical simulation, the generalized quasilinear approximation (GQL), and
direct statistical simulation (DSS) based upon low-order expansion in
equal-time cumulants. As the magnetic diffusivity is decreased, the nonlinear
development of the instability becomes more complicated until eventually a set
of parameters are identified that produce a previously unidentified long-term
cycle in which energy is transformed from kinetic energy to magnetic energy and
back. We find that the periodic transitions, which mimic some aspects of solar
variability -- for example, the quasiperiodic seasonal exchange of energy
between toroidal field and waves or eddies -- are unable to be reproduced when
eddy-scattering processes are excluded from the model.",1809.00921v2
2018-09-04,Twisted domain walls and skyrmions in perpendicularly magnetized multilayers,"We present an analytical theory to describe three-dimensional magnetic
textures in perpendicularly magnetized magnetic multilayers that arise in the
presence of magnetostatic interactions and the Dzyaloshinskii-Moriya
interaction (DMI). We demonstrate that domain walls in multilayers develop a
complex twisted structure, which persists even for films with strong DMI. The
origin of this twist is surface-volume stray field interactions that manifest
as a depth-dependent effective field whose form mimics the DMI effective field.
We find that the wall twist has a minor impact on the equilibrium skyrmion or
domain size, but can significantly affect current-driven dynamics. Our
conclusions are based on the derived analytical expressions for the
magnetostatic energy and confirmed by micromagnetic simulations.",1809.01247v1
2018-09-06,Growth rate distribution and intermittency in kinematic turbulent dynamos : which moment predicts the dynamo onset?,"We consider the generation of magnetic field by a turbulent flow. For the
linear induction equation (i.e. the kinematic dynamo problem), we show that the
statistical moments of the magnetic field display multiscaling and in
particular moments of different order turn unstable for different values of the
control parameter. On a canonical example, we map the problem onto the
calculation of the injected power by a time correlated fluctuating force acting
on a Brownian particle. We are then able to calculate analytically the growth
rate of the moments of the magnetic field and explain the origin of this
intermittency. We finally show that the onset for the nonlinear problem is
predicted by the linear onset of the moment of order 0 + (i.e. the logarithm of
the magnetic field)",1809.01877v1
2018-09-07,Anisotropy-driven collisional separation of impurities in magnetized compressing and expanding cylindrical plasmas,"When a cylindrically-symmetric magnetized plasma compresses or expands,
velocity-space anisotropy is naturally generated as a result of the different
adiabatic conservation laws parallel and perpendicular to the magnetic field.
When the compression timescale is comparable to the collision timescale, and
both are much longer than the gyroperiod, this pressure anisotropy can become
significant. We show that this naturally-generated anisotropy can dramatically
affect the transport of impurities in the compressing plasma, even in the
absence of scalar temperature or density gradients, by modifying the azimuthal
frictions that give rise to radial particle transport. Although the impurity
transport direction depends only on the sign of the pressure anisotropy, the
anisotropy itself depends on the pitch magnitude of the magnetic field and the
sign of the radial velocity. Thus, pressure anisotropy effects can drive
impurities either towards or away from the plasma core. These
anisotropy-dependent terms represent a qualitatively new effect, influencing
transport particularly in the sparse edge regions of dynamically-compressing
screw pinch plasmas. Such plasmas are used for both X-ray generation and
magneto-inertial fusion, applications which are sensitive to impurity
concentrations.",1809.02528v1
2018-09-27,Spin wave radiation from vortices in $^3$He-B,"We consider a vortex line in the B phase of superfluid $^3$He under uniformly
precessing magnetization. The magnetization exerts torque on the vortex,
causing its order parameter to oscillate. These oscillations generate spin
waves, which is analogous to an oscillating charge generating electromagnetic
radiation. The spin waves carry energy, causing dissipation in the system.
Solving the equations of spin dynamics, we calculate the energy dissipation
caused by spin wave radiation for arbitrary tipping angles of the magnetization
and directions of the magnetic field, and for both vortex types of $^3$He-B.
For the double-core vortex we also consider the anisotropy of the radiation and
the dependence of the dissipation on twisting of the half cores. The radiated
energy is compared with experiments in the mid-temperature range $T \sim 0.5
T_c$. The dependence of the calculated dissipation on several parameters is in
good agreement with the experiments. Combined with numerically calculated
vortex structure, the radiation theory produces the order of magnitude of the
experimental dissipation. The agreement with the experiments indicates that
spin wave radiation is the dominant dissipation mechanism for vortices in
superfluid $^3$He-B in the mid-temperature range.",1809.10484v1
2018-09-27,Crystalline anisotropy induces a second antiferromagnetic phase in the absence of SDW in the heavily hydrogen-doped LaFeAsO$_{1-x}$H$_x$ $(x\sim0.5 )$,"Electronic and magnetic properties of the heavily H-doped
LaFeAsO$_{1-x}$H$_x$ $(x\sim0.5 )$ were studied in the framework of the density
functional theory combined with the dynamical mean field theory (DFT+DMFT). We
found a stripe-like-ordered structure of hydrogen and oxygen atoms, as a ground
state, with the same configuration as the antiferromagnetic (AF) order. The new
configuration could explain the existing experimental results related to the
heavily H-doped LaFeAsO$_{1-x}$H$_x$, such as an in-plane electronic anisotropy
and a non-uniform magnetic behavior. A significant anisotropy was observed
between Fe- 3d$_{xz}$ (xz) and Fe-3d$_{yz}$ (yz) orbitals in the ground state
in the absence of the pseudogap resulting from the spin density wave phase,
which was found to originate from the crystalline anisotropy. Magnetic moments
were not spatially uniform and were sensitive to the crystal configuration. We
found that a non-uniform magnetic behavior is associated with the As-Fe-As bond
angle in the structure. Our findings would clarify the importance of crystal
details and orbital degrees of freedom in iron-based superconductors.",1809.10641v1
2018-10-12,"Quadrupolar Superexchange Interactions, Multipolar Order and Magnetic Phase Transition in UO$_2$","The origin of non-collinear magnetic order in UO$_{2}$ is studied by an ab
initio dynamical-mean-field-theory framework in conjunction with a
linear-response approach for evaluating inter-site superexchange interactions
between U 5$f^{2}$ shells. The calculated quadrupole-quadruple superexchange
interactions are found to unambiguously resolve the frustration of
face-centered-cubic U sublattice toward stabilization of the experimentally
observed non-collinear 3k-magnetic order. Therefore, the exotic 3k
antiferromagnetic order in UO$_{2}$ can be accounted for by a purely electronic
exchange mechanism acting in the undistorted cubic lattice structure. The
quadrupolar short-range order above magnetic ordering temperature $T_N$ is
found to qualitatively differ from the long-range order below $T_N$.",1810.05484v2
2018-12-05,Spin-imbalance-induced transverse magnetization in the Hofstadter-Hubbard model,"The fermionic, time-reversal invariant Hofstadter-Hubbard model with a
population difference between the two spin states is investigated. In the
strongly interacting regime, where the system can be described by an effective
spin model, we find an exotic spin structure by means of classical Monte-Carlo
calculations. Remarkably, this spin structure exhibits a transverse net
magnetization perpendicular to the magnetization induced by the population
imbalance. It is thus inherently different from canted antiferromagnetism. We
further investigate effects of quantum fluctuations within the dynamical
mean-field approximation and obtain a rich phase diagram including
ferromagnetic, anti-ferromagnetic, ferrimagnetic, and transverse magnetization
phases.",1812.02001v2
2019-01-11,Open quantum system in external magnetic field within non-Markovian quantum Langevin approach,"The non-Markovian dynamics of a charged particle linearly coupled to a
neutral bosonic heat bath is investigated in an external uniform magnetic
field. The analytical expressions for the time-dependent and asymptotic
friction and diffusion coefficients, cyclotron frequencies, variances of the
coordinate and momentum, and orbital magnetic moments are derived. The role of
magnetic field in the dissipation and diffusion processes is illustrated by
several examples in the low- and high-temperature regimes. The localization
phenomenon for a charged particle is observed. The orbital diamagnetism of
quantum system in a dissipative environment is studied. The quantization
conditions are found for the angular momentum.",1901.03562v1
2019-01-25,"Low temperature magnetic and dielectric properties of LnBaCuFeO5 (Ln = Nd, Eu, Gd, Ho and Yb)","The layered perovskite compounds are interesting due to their intriguing
physical properties. In this article we report the structural, magnetic and
dielectric properties of LnBaCuFeO5 (Ln=Nd, Eu, Gd, Ho and Yb). The structural
parameters decrease from Nd to Yb due to the decrease in the ionic radii of the
rare earth ions. An antiferromagnetic transition is observed for EuBaCuFeO5
near 120 K along with the glassy dynamics of the electric dipoles below 100 K.
The magnetic transition is absent in other compounds, which may be due to the
dominance of the magnetic moment of the rare earth ions. The dielectric
constant does not show any anomaly, except in the case of HoBaCuFeO5 where it
shows a weak frequency dependence around 54 K. These compounds show a
significant enhancement of dielectric constant at high temperatures which have
been attributed to Maxwell-Wagner effect. However, no significant
magneto-dielectric coupling has been observed in these layered perovskites.",1901.08835v1
2019-01-28,Topological structures of velocity and electric field in the vicinity of a cusp-type magnetic null point,"Topological characteristics reveal important physical properties of plasma
structures and astrophysical processes. Physical parameters and constraints are
linked with topological invariants, which are important for describing magnetic
reconnection scenarios. We analyze stationary non-ideal Ohm's law concerning
the Poincare classes of all involved physical fields in 2D by calculating the
corresponding topological invariants of their Jacobian (here: particularly the
eigenvalues) or Hessian matrices. The magnetic field is assumed to have a cusp
structure, and the stagnation point of the plasma flow coincides with the cusp.
We find that the stagnation point must be hyperbolic. Furthermore, the
functions describing both the resistivity and the Ohmic heating have a saddle
point structure, being displaced with respect to the cusp point. These results
imply that there is no monotonous relation between current density and
anomalous resistivity in the case of a 2D standard magnetic cusp.",1901.09913v1
2019-02-07,Triplet excitations in the frustrated spin ladder Li$_2$Cu$_2$O(SO$_4$)$_2$,"Magnetic excitations of the recently discovered frustrated spin-1/2 two-leg
ladder system Li$_2$Cu$_2$O(SO$_4$)$_2$ are investigated using inelastic
neutron scattering, magnetic susceptibility and infrared absorption
measurements. Despite the presence of a magnetic dimerization concomitant with
the tetragonal-to-triclinic structural distortion occurring below 125 K,
neutron scattering experiments reveal the presence of dispersive triplet
excitations above a spin gap of $\Delta = 10.6$ meV at 1.5 K, a value
consistent with the estimates extracted from magnetic susceptibility. The
likely detection of these spin excitations in infrared spectroscopy is
explained by invoking a dynamic Dzyaloshinskii-Moriya mechanism in which light
is coupled to the dimer singlet-to-triplet transition through an optical
phonon. These results are qualitatively explained by exact diagonalization and
higher-order perturbation calculations carried out on the basis of the
dimerized spin Hamiltonian derived from first-principles.",1902.02798v1
2019-02-10,Spin dynamics of anisotropic azimuthal modes in heterogeneous magnetic nanodisks,"It is well known that azimuthal spin wave modes of magnetic vortex state in
permalloy nanodisks have circular symmetry. Intuitively, magnetic materials
having magnetocrystalline anisotropy is not compatible with the circular
symmetry of the azimuthal modes. In this article, however, we report cubic
azimuthal modes in heterogeneous nanodisks consisting of a permalloy core and a
Fe shell. The fourfold symmetry of azimuthal modes is due to the exchange, and
magneto-static, interactions between the permalloy core and the Fe shell. In
comparison to results of circular azimuthal mode, the vortex switching occurs
considerably faster under the excitation of cubic azimuthal mode. The gyration
path of vortex core turns into square under the influence of induced cubic
anisotropy in the Py region. We find out periodic oscillation of the vortex
core size and the gyration speed as well. Our findings may offer a new route
for spintronic applications using heterogeneous magnetic nanostructures.",1902.03536v1
2019-02-13,Controlling injection using a magnetic field to produce sub-femtosecond bunches in the laser wakefield accelerator,"It is shown that electron injection into a laser-driven plasma bubble can be
manipulated by applying an external magnetic field in the presence of a plasma
density gradient. The down-ramp of the density-tailored plasma locally reduces
the plasma wave phase velocity, which triggers injection. The longitudinal
magnetic field dynamically induces an expanding hole in the electron density
distribution at the rear of the wake bubble, which reduces the peak electron
velocity in its vicinity. Electron injection is suppressed when the electron
velocity drops below the phase velocity, which depends on the size of the
density hole. This enables the start and end of electron injection to be
independently controlled, which allows generation of sub-femtosecond electron
bunches with peak currents of a few kilo-Ampere, for an applied magnetic field
of $\sim 10$ Tesla.",1902.04771v2
2019-02-15,Magnetization and entanglement after a geometric quench in the XXZ chain,"We investigate the dynamics of the XXZ spin chain after a geometric quench,
which is realized by connecting two half-chains prepared in their ground states
with zero and maximum magnetizations, respectively. The profiles of
magnetization after the subsequent time evolution are studied numerically by
density-matrix renormalization group methods, and a comparison to the
predictions of generalized hydrodynamics yields a very good agreement. We also
calculate the profiles of entanglement entropy and propose an ansatz for the
noninteracting XX case, based on arguments from conformal field theory. In the
general interacting case, the propagation of the entropy front is studied
numerically both before and after the reflection from the chain boundaries.
Finally, our results for the magnetization fluctuations indicate a leading
order proportionality relation to the entanglement entropy.",1902.05834v2
2019-03-03,Theory for spin torque in Weyl semimetal with magnetic texture,"The spin-transfer torque is a fundamental physical quantity to operate the
spintronics devices such as racetrack memory. We theoretically study the
spin-transfer torque and analyze the dynamics of the magnetic domain walls in
magnetic Weyl semimetals. Owing to the strong spin-orbit coupling in Weyl
semimetals, the spin-transfer torque can be significantly enhanced, because of
which they can provide a more efficient means of controlling magnetic textures.
We derive the analytical expression of the spin-transfer torque and find that
the velocity of the domain wall is one order of magnitude greater than that of
conventional ferromagnetic metals. Furthermore, due to the suppression of
longitudinal conductivity in the thin domain-wall configuration, the
dissipation due to Joule heating for the spin-transfer torque becomes much
smaller than that in bulk metallic ferromagnets. Consequently, the fast-control
of the domain wall can be achieved with smaller dissipation from Joule heating
in the Weyl semimetals as required for application to low-energy-consumption
spintronics devices.",1903.00849v1
2019-03-11,Effects of small-scale dynamo and compressibility on the $Λ$ effect,"The $\Lambda$ effect describes a rotation-induced non-diffusive contribution
to the Reynolds stress. It is commonly held responsible for maintaining the
observed differential rotation of the Sun and other late-type stars. Here the
sensitivity of the $\Lambda$ effect to small-scale magnetic fields and
compressibility is studied by means of forced turbulence simulations either
with anisotropic forcing in fully periodic cubes or in density-stratified
domains with isotropic forcing. Effects of small-scale magnetic fields are
studied in cases where the magnetic fields are self-consistently generated by a
small-scale dynamo. The results show that small-scale magnetic fields lead to a
quenching of the $\Lambda$ effect which is milder than in cases where also a
large-scale field is present. The effect of compressibility on the $\Lambda$
effect is negligible in the range of Mach numbers from 0.015 to 0.8. Density
stratification induces a marked anisotropy in the turbulence and a vertical
$\Lambda$ effect if the forcing scale is roughly two times larger than the
density scale height.",1903.04363v2
2019-03-15,Stellar physics with high-resolution UV spectropolarimetry,"Current burning issues in stellar physics, for both hot and cool stars,
concern their magnetism. In hot stars, stable magnetic fields of fossil origin
impact their stellar structure and circumstellar environment, with a likely
major role in stellar evolution. However, this role is complex and thus poorly
understood as of today. It needs to be quantified with high-resolution UV
spectropolarimetric measurements. In cool stars, UV spectropolarimetry would
provide access to the structure and magnetic field of the very dynamic upper
stellar atmosphere, providing key data for new progress to be made on the role
of magnetic fields in heating the upper atmospheres, launching stellar winds,
and more generally in the interaction of cool stars with their environment
(circumstellar disk, planets) along their whole evolution.",1903.06509v1
2019-03-28,Magnetic-Resonance-Induced Pseudo-electric Field and Giant Current Response in Axion Insulators,"A quantized version of the magnetoelectric effect, known as the topological
magnetoelectric effect, can exist in a time-reversal invariant topological
insulator with all its surface states gapped out by magnetism. This topological
phase, called the axion insulator phase, has been theoretically proposed but is
still lack of conclusive experimental evidence due to the small signal of
topological magnetoelectric effect. In this work, we propose that the dynamical
in-plane magnetization in an axion insulator can generate a ""pseudo-electric
field"", which acts on the surface state of topological insulator films and
leads to the non-zero response current. Strikingly, we find that the current at
magnetic resonance (either ferromagnetic or anti-ferromagnetic) is larger than
that of topological magnetoelectric effect by several orders of magnitude, and
thereby serves as a feasible smoking gun to confirm the axion insulator phase
in the candidate materials.",1903.12068v2
2019-03-29,Electrical Néel-order switching in magnetron-sputtered CuMnAs thin films,"Antiferromagnetic materials as active components in spintronic devices
promise insensitivity against external magnetic fields, the absence of own
magnetic stray fields, and ultrafast dynamics at the picosecond time scale.
Materials with certain crystal-symmetry show an intrinsic N\'eel-order
spin-orbit torque that can efficiently switch the magnetic order of an
antiferromagnet. The tetragonal variant of CuMnAs was shown to be electrically
switchable by this intrinsic spin-orbit effect and its use in memory cells with
memristive properties has been recently demonstrated for high-quality films
grown with molecular beam epitaxy. Here, we demonstrate that the magnetic order
of magnetron-sputtered CuMnAs films can also be manipulated by electrical
current pulses. The switching efficiency and relaxation as a function of
temperature, current density, and pulse width can be described by a
thermal-activation model. Our findings demonstrate that CuMnAs can be
fabricated with an industry-compatible deposition technique, which will
accelerate the development cycle of devices based on this remarkable material.",1903.12387v2
2019-04-01,51V-NMR study on the S=1/2 square lattice antiferromagnet K2V3O8,"Static and dynamic properties of the quasi-two-dimensional antiferromagnet
K$_2$V$_3$O$_8$ have been investigated by $^{51}$V-NMR experiments on
nonmagnetic V$^{5+}$ sites. Above the structural transition temperature
$T_{\rm{S}}$ = 115 K, NMR spectra are fully compatible with the $P4bm$ space
group symmetry. The formation of superstructure below $T_{\rm{S}}$ causes
splitting of the NMR lines, which get broadened at lower temperatures so that
individual peaks are not well resolved. Evolution of NMR spectra with magnetic
field along $c$-axis below the magnetic transition temperature $T_{\rm{N}} \sim
4$ K is qualitatively consistent with a simple N\'{e}el order and a spin flop
transition. However, broad feature of the spectra does not rule out possible
incommensurate spin structure. The spin-lattice relaxation rate $1/T_1$ below
$T_{\rm{N}}$ shows huge enhancement for a certain range of magnetic field,
which is independent of temperature and attributed to cross relaxation due to
anomalously large nuclear spin-spin coupling between V$^{5+}$ and magnetic
V$^{4+}$ sites. The results indicate strong gapless spin fluctuations, which
could arise from incommesurate orders or complex spin textures.",1904.00909v1
2019-04-03,Simulating chiral anomalies with spin dynamics,"Considering that the chiral kinetic equations of motion (CEOM) can be derived
from the spin kinetic equations of motion (SEOM) for massless particles with
approximations, we simulate the chiral anomalies by using the latter in a box
system with the periodic boundary condition under a uniform external magnetic
field. We found that the chiral magnetic effect is weaker while the damping of
the chiral magnetic wave is stronger from the SEOM compared with that from the
CEOM. In addition, effects induced by chiral anomalies from the SEOM are less
sensitive to the decay of the magnetic field than from the CEOM due to the spin
relaxation process.",1904.01834v2
2019-05-10,Magnetoplasmons for the $α$-T$_3$ model with filled Landau levels,"Using the $\alpha$-T$_3$ model, we carried out analytical and numerical
calculations for the static and dynamic polarization functions in the presence
of a perpendicular magnetic field. These results were employed to determine the
longitudinal dielectric function and the magnetoplasmon dispersion relation.
The magnetic field splits the continuous valence, conduction and flat energy
subband into discrete Landau levels which present significant effects on the
polarization function and magnetoplasmon dispersion. We present results for a
doped layer in the integer quantum Hall regime for fixed hopping parameter
$\alpha$ and various magnetic fields as well as chosen magnetic field and
different $\alpha$ in the random phase approximation.",1905.04387v3
2019-05-15,Background-subtracted Solar Activity Maps,"We introduce the concept of a Background-subtracted Solar Activity Map
(BaSAM) as a new quantitative tool to assess and visualize the temporal
variation of the photospheric magnetic field and the UV 160 nm intensity. The
method utilizes data of the Solar Dynamics Observatory (SDO) and is applicable
to both full-disk observations and regions-of-interest. We illustrate and
discuss the potential of BaSAM resorting to datasets representing solar minimum
and maximum conditions: (1) Contributions of quiet-Sun magnetic fields, i.e.
the network and (decaying) plage, to solar activity can be better determined
when their variation is measured with respect to the background given by ""deep""
magnetograms. (2) Flaring and intermittent brightenings are easily appraised in
BaSAMs of the UV intensity. (3) Both magnetic-field and intensity variations
demonstrated that the flux system of sunspots is well connected to the
surrounding supergranular cells. In addition, producing daily full-disk BaSAMs
for the entire mission time of SDO provides a unique tool to analyze solar
cycle variations, showing how vigorous or frail are the variations of
magnetic-field and intensity features.",1905.06057v1
2019-05-16,Magnetogenesis from a rotating scalar: à la scalar chiral magnetic effect,"The chiral magnetic effect (CME) is a phenomenon in which an electric current
is induced parallel to an external magnetic field in the presence of chiral
asymmetry in a fermionic system. In this paper, we show that the electric
current induced by the dynamics of a pseudo-scalar field which anomalously
couples to electromagnetic fields can be interpreted as closely analogous to
the CME. In particular, the velocity of the pseudo-scalar field, which is the
phase of a complex scalar, indicates that the system carries a global U(1)
number asymmetry as the source of the induced current. We demonstrate that an
initial kick to the phase-field velocity and an anomalous coupling between the
phase-field and gauge fields are naturally provided, in a set-up such as the
Affleck-Dine mechanism. The resulting asymmetry carried by the Affleck-Dine
field can give rise to instability in the (electro)magnetic field. Cosmological
consequences of this mechanism are also investigated.",1905.06966v2
2019-05-22,Current-Driven Dynamics of Magnetic Hopfions,"Topological magnetic textures have attracted considerable interest since they
exhibit new properties and might be useful in information technology. Magnetic
hopfions are three-dimensional (3D) spatial variations in the magnetization
with a non-trivial Hopf index. We find that in ferromagnetic materials, two
types of hopfions, Bloch-type and N\'{e}el-type hopfions, can be excited as
metastable states in the presence of bulk and interfacial Dzyaloshinskii-Moriya
interactions, respectively. We further investigate how hopfions can be driven
by currents via spin-transfer torques (STTs) and spin-Hall torques (SHTs).
Distinct from 2D ferromagnetic skyrmions, hopfions have a vanishing gyrovector.
Consequently, there are no undesirable Hall effects. N\'{e}el-type hopfions
move along the current direction via both STT and SHT, while Bloch-type
hopfions can be moved either transverse to the current direction by SHT or
parallel to the current direction by STT. Our findings open the door to
utilizing hopfions as information carriers.",1905.09154v5
2019-06-01,Polarized inelastic neutron scattering of non-reciprocal spin waves in MnSi,"We report spin-polarized inelastic neutron scattering of the dynamical
structure factor of the conical magnetic helix in the cubic chiral magnet MnSi.
We find that the spectral weight of spin-flip scattering processes is
concentrated on single branches for wavevector transfer parallel to the helix
axis as inferred from well-defined peaks in the neutron spectra. In contrast,
for wavevector transfers perpendicular to the helix the spectral weight is
distributed among different branches of the magnon band structure as reflected
in broader features of the spectra. Taking into account the effects of
instrumental resolution, our experimental results are in excellent quantitative
agreement with parameter-free theoretical predictions. Whereas the dispersion
of the spin waves in MnSi appears to be approximately reciprocal at low
energies and small applied fields, the associated spin-resolved spectral weight
displays a pronounced non-reciprocity that implies a distinct non-reciprocal
response in the limit of vanishing uniform magnetization at zero magnetic
field.",1906.00221v3
2019-06-01,Magnetic fluctuations in the itinerant ferromagnet LaCrGe3 studied by 139La NMR,"LaCrGe$_3$ is an itinerant ferromagnet with a Curie temperature of $T_{\rm
c}$ = 85 K and exhibits an avoided ferromagnetic quantum critical point under
pressure through a modulated antiferromagnetic phase as well as tri-critical
wing structure in its temperature-pressure-magnetic field ($T$-$p$-$H$) phase
diagram. In order to understand the static and dynamical magnetic properties of
LaCrGe$_3$, we carried out $^{139}$La nuclear magnetic resonance (NMR)
measurements. Based on the analysis of NMR data, using the
self-consistent-renomalization (SCR) theory, the spin fluctuations in the
paramagnetic state are revealed to be isotropic ferromagnetic and three
dimensional (3D) in nature. Moreover, the system is found to follow the
generalized Rhodes-Wohfarth relation which is expected in 3D itinerant
ferromagnetic systems. As compared to other similar itinerant ferromagnets, the
Cr 3$d$ electrons and their spin fluctuations are characterized to have a
relatively high degree of localization in real space.",1906.00249v1
2019-06-12,Orbital Selectivity and Magnetic Ordering in Fe intercalated Dirac Semimetal Bi$_2$Se$_3$,"In this paper we investigate the intercalation effects of Iron (Fe) in the
van der Waals gap of Bi$_2$Se$_3$ on the magnetic and transport properties
using first-principles band structure estimations combined using dynamical
mean-field theory. Inclusion of electronic correlations and spin-orbit coupling
effects are found to result in the emergence of ferromagnetic properties in an
intercalated Bi$_2$Se$_3$. Further the Dirac cone in the band structure of
Bismuth Selenide is modified via Fe intercalation at moderate densities.
Accompanied by novel structural effects, the onset of an orbital selective
metal insulator transition in the Fe 3$d$ orbitals brings about a magnetic
phase transition in the Fe intercalated Bi$_2$Se$_3$. Further we have explored
the dependency of the inter-orbital electron-electron correlations on the
magnetic ordering and the effects of intercalation in establishing new physical
properties.",1906.05249v2
2019-06-16,Kondo impurity at the edge of a superconducting wire,"Quantum impurity models are prevalent throughout many body physics, providing
some prime examples of strongly correlated systems. Aside from being of great
interest in themselves they can provide deep insight into the effects of strong
correlations in general. The classic example is the Kondo model wherein a
magnetic impurity is screened at low energies by a non interacting metallic
bath. Here we consider a magnetic impurity coupled to a quantum wire with
pairing interaction which dynamically generates a mass gap. Using Bethe Ansatz
we solve the system exactly finding that it exhibits both screened and
unscreened phases for an antiferromagnetic impurity. We determine the ground
state density of states and magnetization in both phases as well as the
excitations. In contrast to the well studied case of magnetic impurities in
superconductors we find that there are no intragap bound states in the
spectrum. The phase transition is not associated to a level crossing but with
quantum fluctuations.",1906.06728v1
2019-06-19,Edge localization of spin waves in antidot multilayers with perpendicular magnetic anisotropy,"We study the spin-wave dynamics in nanoscale antidot lattices based on Co/Pd
multilayers with perpendicular magnetic anisotropy. Using time-resolved
magneto-optical Kerr effect measurements we demonstrate that the variation of
the antidot shape introduces significant change in the spin-wave spectra,
especially in the lower frequency range. By employing micromagnetic simulations
we show that additional peaks observed in the measured spectra are related to
narrow shell regions around the antidots, where the magnetic anisotropy is
reduced due to the Ga+ ion irradiation during the focused ion beam milling
process of the antidot fabrication. The results point at new possibilities for
exploitation of localized spin waves in out-of-plane magnetized thin films,
which are easily tunable and suitable for magnonics applications.",1906.08109v2
2019-06-21,Entanglement-based 3D magnetic gradiometry with an ultracold atomic scattering halo,"Ultracold collisions of Bose-Einstein condensates can be used to generate a
large number of counterpropagating pairs of entangled atoms, which collectively
form a thin spherical shell in momentum space, called a scattering halo. Here
we generate a scattering halo composed almost entirely of pairs in a symmetric
entangled state and observe a scattering angle-dependent mixing to the
anti-symmetric state due to the presence of an inhomogeneous magnetic field. We
report on a proof-of-principle application of the observed parity dynamics to
demonstrate magnetic gradiometry, insensitive to common-mode fluctuations of
the background magnetic field. Furthermore, the highly multimode nature and
narrow radial width of the scattering halos enable 3D tomography of an
interrogated field without the need for a scanning probe.",1906.08958v1
2019-07-01,DC spinmotive force from microwave-active resonant dynamics of skyrmion crystal under a tilted magnetic field,"We theoretically show that a temporally oscillating spin-driven electromotive
force or voltage with a large DC component can be generated by exciting
microwave-active spin-wave modes of a skyrmion crystal confined in a quasi
two-dimensional magnet under a tilted magnetic field. The DC component and the
AC amplitude of the oscillating electric voltage is significantly enhanced when
the microwave frequency is tuned to an eigenfrequency of the peculiar spin-wave
modes of the skyrmion crystal called ``rotation modes"" and ``breathing mode"",
whereas the sign of the DC component depends on the microwave polarization and
on the spin-wave mode. These results provide an efficient means to convert
microwaves to a DC electric voltage by using skyrmion-hosting magnets and to
switch the sign of the voltage via tuning the microwave frequency, which are
important capabilities for spintronic devices.",1907.00621v1
2019-07-01,Nonlinear theory of magnetohydrodynamic flows of stratified rotating plasma in two-layer shallow water approximation. Rossby waves and their three-wave interactions,"This article deals with rotating magnetohydrodynamic flows of a thin
stratified layer of astrophysical plasma in a gravitational field with a
free-surface in a vertical external magnetic field. Magnetohydrodynamic
equations are obtained in the two-layer shallow water approximation in an
external magnetic field when plasma is divided into two layers of different
densities. In the beta-plane approximation a system of shallow water equations
for rotating stratified plasma in an external magnetic field is obtained. For
stationary initial conditions in the form of vertical or horizontal magnetic
field a linear theory has been developed and solutions have been found in the
form of magneto-Rossby waves with modifications to them describing the effects
of stratification. A qualitative analysis of the dispersion curves shows the
presence of three-wave nonlinear interactions of magneto-Rossby waves for each
of the stationary solutions. The appearance of parametric instabilities is
shown and their increments are found.",1907.00743v1
2019-07-01,Spinon Confinement and a Sharp Longitudinal Mode in Yb$_2$Pt$_2$Pb in Magnetic Fields,"The fundamental excitations in an antiferromagnetic chain of spins-1/2 are
spinons, de-confined fractional quasiparticles that when combined in pairs,
form a triplet excitation continuum. In an Ising-like spin chain the continuum
is gapped and the ground state is N{\'e}el ordered. Here, we report high
resolution neutron scattering experiments, which reveal how a magnetic field
closes this gap and drives the spin chains in \YPP\ to a critical, disordered
Luttinger-liquid state. In \YPP\ the effective spins-1/2 describe the dynamics
of large, Ising-like Yb magnetic moments, ensuring that the measured
excitations are exclusively longitudinal, which we find to be well described by
time-dependent density matrix renormalization group calculations. The
inter-chain coupling leads to the confinement of spinons, a condensed matter
analog of quark confinement in quantum chromodynamics. Insensitive to
transverse fluctuations, our measurements show how a gapless, dispersive
longitudinal mode arises from confinement and evolves with magnetic order.",1907.01067v1
2019-07-04,Self-induced spin glass state in elemental and crystalline neodymium,"Spin glasses are a highly complex magnetic state of matter, intricately
linked to spin frustration and structural disorder. They exhibit no long-range
order and exude aging phenomena, distinguishing them from quantum spin liquids.
We report a new type of spin glass state, the spin-Q glass, observable in
bulk-like crystalline metallic neodymium thick films. Using spin-polarized
scanning tunneling microscopy combined with ab initio calculations and
atomistic spin-dynamics simulations, we visualize the variations in
atomic-scale non-collinear order and its response to magnetic field and
temperature. We quantify the aging phenomena relating the glassiness to
crystalline symmetry and the energy landscape. This result not only resolves
the long-standing debate of the magnetism of neodymium, but also suggests that
glassiness may arise in other magnetic solids lacking extrinsic disorder.",1907.02295v2
2019-07-04,Dynamics of the magnetoelastic phase transition and adiabatic temperature change in Mn1.3Fe0.7P0.5Si0.55,"The adiabatic temperature change DTad of a Mn1.3Fe0.7P0.5Si0.55 Fe2P-type
alloy was measured under different magnetic field-sweep rates from 0.93 Ts-1 to
2870 Ts-1. We find a field-sweep-rate independent magnetocaloric effect due to
a partial alignment of magnetic moments in the paramagnetic region overlapping
with the magnetocaloric effect of the first-order phase transition.
Additionally, the first-order phase transition is not completed even in fields
up to 20 T leading to a non-saturating behavior of DTad. Measurements in
different pulsed fields reveal that the first-order phase transition cannot
follow the fast field changes as previously assumed, resulting in a distinct
field-dependent hysteresis in DTad.",1907.02307v1
2019-07-09,Synthetic dimensions and topological chiral currents in mesoscopic rings,"The recently-introduced concept of ""synthetic dimensions"" allows for the
realization of higher-dimensional topological phenomena in lower-dimensional
systems. In this work we study the complementary aspect that synthetic
dimensions provide a natural route to topological states in mesoscopic hybrid
devices. We demonstrate this for the current induced into a closed
one-dimensional Aharonov-Bohm ring by the interaction with a dynamic mesoscopic
magnet. The quantization of the magnetic moment provides a synthetic dimension
that complements the charge motion around the ring. We present a direct mapping
that places the combined ring-magnet system into the class of quantum Hall
models, and demonstrate that topological features, combined with the magnet's
anisotropy, can lead to clear signatures in the persistent current of the
single-particle ground state.",1907.04231v1
2019-07-10,Non-diffusive angular momentum transport in rotating $\bf z$-pinches,"The stability of conducting Taylor-Couette flows under the presence of
toroidal magnetic background fields is considered. For strong enough magnetic
amplitudes such magnetohydrodynamic flows are unstable against nonaxisymmetric
perturbations which may also transport angular momentum. In accordance with the
often used diffusion approximation one expects the angular momentum transport
vanishing for rigid rotation. In the sense of a nondiffusive $\Lambda$ effect,
however, even for {\em rigidly} rotating $z$-pinches an axisymmetric angular
momentum flux appears which is directed outward (inward) for large (small)
magnetic Mach numbers. The internal rotation in a magnetized rotating tank can
thus never be uniform. Those particular rotation laws are used to estimate the
value of the instability-induced eddy viscosity for which the nondiffusive
$\Lambda$ effect and the diffusive shear-induced transport compensate each
other. The results provide the Shakura-Sunyaev viscosity ansatz leading to
numerical values linearly growing with the Reynolds number of rotation.",1907.08291v1
2019-07-30,Oriented propagation of magnetization due to chiral edge modes in Kitaev-type models,"Detecting chiral edge modes in topological materials has been intensively
pursued in experiments. However, the phenomena caused by the modes are not yet
elucidated theoretically. We study the dynamics of chiral spinon wave packets
at the edge in Kitaev-type magnets. More precisely, by relying on the exact
solvability of the models, we construct a spinon wave packet, localized edge
magnetization, which shows oriented propagation along the edge, whose behavior
is expected from the chiral character of the dispersion relation of the chiral
edge modes. In general, this approach enables us to study not only spin
transport in anisotropic magnets but also charge transport in Bogoliubov-de
Gennes-type superconductors because it does not rely on a conserved quantity.",1907.12863v2
2019-08-12,On a simple derivation of the very low damping escape rate for classical spins by modifying the method of Kramers,"The original perturbative Kramers' method (starting from the phase space
coordinates) (Kramers, 1940) of determining the energy-controlled-diffusion
equation for Newtonian particles with separable and additive Hamiltonians is
generalized to yield the energy-controlled diffusion equation and thus the very
low damping (VLD) escape rate including spin-transfer torque for classical
giant magnetic spins with two degrees of freedom. These have dynamics governed
by the magnetic Langevin and Fokker-Planck equations and thus are generally
based on non-separable and non-additive Hamiltonians. The derivation of the VLD
escape rate directly from the (magnetic) Fokker-Planck equation for the surface
distribution of magnetization orientations in the configuration space of the
polar and azimuthal angles $(\vartheta, \varphi)$ is much simpler than those
previously used.",1908.06747v1
2019-09-12,"Giant Vesicles with Encapsulated Magnetic Nanowires as Versatile Carriers, Transported via Rotating and Non-Homogeneous Magnetic Fields","In this work, two real-time methods to transport magnetic nanowires confined
in giant hybrid vesi-cles upon the application of different strategies are
studied. The microscale carriers are either mag-netically guided through the
viscous medium by a non-homogenous field or advected by precisely monitored
hydrodynamic flows. The slender geometry of the magnetic component enables the
ap-plication of large torques, the in-situ characterization of the rotational
dynamics, as well as the guided propulsion via the continuous thrust of the
nanowire tip on the confining bilayer. The flexi-bility of the vesicles,
required to deform along the steering direction during passage through
mi-croscale openings, is enhanced via the adsorption of non-ionic surfactants
on the lipid membrane. The resulting integrated system is an excellent
candidate to transport colloidal cargos or flu-id amounts of some picoliters in
microfluidic platforms, even in physiological environments, since it combines
the maneuverability of propelled microscopic systems and the protection
conferred by the vesicle.",1909.05502v1
2019-09-15,The Flyby Anomaly and the Gravitational-Magnetic Field Induced Frame-Dragging Effect around the Earth,"The anomalous energy difference observed during the Earth flybys is modelled
here as a dynamical effect resulting from the coupling of the gravitational and
the magnetic fields of the Earth. The theoretical analysis shows that general
relativistic frame-dragging can become modified under the Earth's magnetic
field by orders of magnitude. For twelve flyby cases, including the null
results reported in some recent flybys, the predicted velocities correspond to
the observed velocities within the observational error. The gravito-magnetic
effect is also shown to account for the linear distance relation,
time-variation of the anomalous energy, and the reduction in the anomalous
velocity for high altitude flybys near the Earth.",1909.08083v1
2019-09-27,Thermally driven two-magnet nano-oscillator with large spin-charge conversion,"Next-generation spintronic applications require material properties that can
be hardly met by one material candidate. Here we demonstrate that by combining
insulating and metallic magnets, enhanced spin-charge conversion and
energy-efficient thermal spin currents can be realized. We develop a nanowire
device consisting of an yttrium iron garnet and permalloy bi-layer. An
interfacial temperature gradient drives the nanowire magnetization into
auto-oscillations at gigahertz frequencies. Interfacial spin coupling and
magnetoresistance of the permalloy layer translate spin dynamics into sizable
microwave signals. The results show prospect for energy-efficient spintronic
devices and present an experimental realization of magnon condensation in a
heterogeneous magnetic system.",1909.12445v1
2019-09-29,Perfect optical absorption-enhanced magneto-optic Kerr effect microscopy,"Magnetic and spintronic media have offered fundamental scientific subjects
and technological applications. Magneto-optic Kerr effect (MOKE) microscopy
provides the most accessible platform to study the dynamics of spins, magnetic
quasi-particles, and domain walls. However, in the research of nanoscale spin
textures and state-of-the-art spintronic devices, optical techniques are
generally restricted by the extremely weak magneto-optical activity and
diffraction limit. Highly sophisticated, expensive electron microscopy and
scanning probe methods thus have come to the forefront. Here, we show that
perfect optical absorption (POA) dramatically improves the performance and
functionality of MOKE microscopy. For 1-nm-thin Co film, we demonstrate a Kerr
amplitude as large as 20 degree and magnetic domain imaging visibility of 0.47.
Especially, POA-enhanced MOKE microscopy enables real-time detection and
statistical analysis of sub-wavelength magnetic domain reversals. Furthermore,
we exploit enhanced magneto-optic birefringence and demonstrate analyser-free
MOKE microscopy. The POA technique is promising for optical investigations and
applications of nanomagnetic systems.",1909.13275v1
2019-09-29,Magnetic Dissipation of Near-Wall Turbulent Coherent Structures in Magnetohydrodynamic Pipe Flows,"Relaminarization of wall-bounded turbulent flows by means of external static
magnetic fields is a long-known phenomenon in the physics of electrically
conducting fluids at low magnetic Reynolds numbers. Despite the large
literature on the subject, it is not yet completely clear what combination of
the Hartmann ($M$) and the Reynolds number has to be used to predict the
laminar-turbulent transition in channel or pipe flows fed by upstream turbulent
flows free of magnetic perturbations. Relying upon standard phenomenological
approaches related to mixing length and structural concepts, we put forward
that $M/R_\tau$, where $R_\tau$ is the friction Reynolds number, is the
appropriate controlling parameter for relaminarization, a proposal which finds
good support from available experimental data.",1909.13289v3
2019-10-18,Lattice study of electromagnetic conductivity of quark-gluon plasma in external magnetic field,"We study the electromagnetic (e.m.) conductivity of QGP in a magnetic
background by lattice simulations with $N_f = 2+1$ dynamical rooted staggered
fermions at the physical point. We study the correlation functions of the
e.m.~currents at $T=200,\,250$\,MeV and use the Tikhonov approach to extract
the conductivity. This is found to rise with the magnetic field in the
direction parallel to it and to decrease in the transverse direction, giving
evidence for both the Chiral Magnetic Effect and the magnetoresistance
phenomenon in QGP. We also estimate the chiral charge relaxation time in QGP.",1910.08516v1
2019-10-21,Quantum Depinning of a Magnetic Skyrmion,"We investigate the quantum depinning of a weakly driven skyrmion out of an
impurity potential in a mesoscopic magnetic insulator. For small barrier
height, the Magnus force dynamics dominates over the inertial one, and the
problem is reduced to a massless charged particle in a strong magnetic field.
The universal form of the WKB exponent, the rate of tunneling, and the
crossover temperature between thermal and quantum tunneling is provided,
independently of the detailed form of the pinning potential. The results are
discussed in terms of macroscopic parameters of the insulator Cu2OSeO3 and
various skyrmion radii. We demonstrate that small enough magnetic skyrmions,
with a radius of ~ 10 lattice sites, consisting of some thousands of spins, can
behave as quantum objects at low temperatures in the mK regime.",1910.09585v2
2019-10-24,Electron spin resonance in spiral antiferromagnet linarite: theory and experiment,"We present combined experimental and theoretical investigation of the
low-frequency ESR dynamics in the ordered phases of magnetic mineral linarite.
This material consists of weakly coupled spin-1/2 chains of copper ions with
frustrated ferro- and antiferromagnetic interactions. In zero magnetic field,
linarite orders into a spiral structure and exhibits a peculiar magnetic phase
diagram sensitive to the field orientation. The resonance frequencies and their
field dependence are analyzed combining microscopic and macroscopic theoretical
approaches and precise values of magnetic anisotropy constants are obtained. We
conclude that possible realization of exotic multipolar quantum states in this
material is greatly influenced by the biaxial anisotropy.",1910.11056v1
2019-10-24,Spin waves in ferromagnetic thin films,"A spin wave is the disturbance of intrinsic spin order in magnetic materials.
In this paper, a spin wave in the Landau-Lifshitz-Gilbert equation is obtained
based on the assumption that the spin wave maintains its shape while it
propagates at a constant velocity. Our main findings include: (1) in the
absence of Gilbert damping, the spin wave propagates at a constant velocity
with the increment proportional to the strength of the magnetic field; (2) in
the absence of magnetic field, at a given time the spin wave converges
exponentially fast to its initial profile as the damping parameter goes to zero
and in the long time the relaxation dynamics of the spin wave converges
exponentially fast to the easy-axis direction with the exponent proportional to
the damping parameter; (3) in the presence of both Gilbert damping and magnetic
field, the spin wave converges to the easy-axis direction exponentially fast at
a small timescale while propagates at a constant velocity beyond that. These
provides a comprehensive understanding of spin waves in ferromagnetic
materials.",1910.11200v1
2019-10-30,Moiré pattern of spin liquid and Neel magnet in a Kitaev chain,"A moir\'{e} pattern occurs when two periodic structures in a system have a
slight mismatch period, resulting the coexistence of distinct phases in
different large-scale spacial regions of the same system. Two periodic
structures can arise from periodic electric and magnetic fields, respectively.
We investigated the moir\'{e} pattern via a dimerized Kitaev spin chain with a
periodic transverse field, which can be mapped onto the system of dimerized
spinless fermions with p-wave superconductivity. The exact solution for
staggered field demonstrated that the ground state has two distinct phases: (i)
Neel magnetic phase for nonzero field, (ii) Spin liquid phase due to the
emergence of isolated flat Bogoliubov--de Gennes band for vanishing field. We
computed the staggered magnetization and local density of states
(\textrm{LDOS}) for the field with a slight difference period to the chain
lattice. Numerical simulation demonstrated that such two phases appear
alternatively along the chain with a long beat period. Additionally, we
proposed a dynamic scheme to detect the Moir\'{e} fringes based on the
measurement of Loschmidt echo (\textrm{LE}) in the presence of local
perturbation.",1910.13682v1
2019-11-05,Spin-selective transmission through a single-stranded magnetic helix,"Magnetic helix (MH) structure can be a role model for future spintronic
devices. Utilizing the advantage of constructing possible magnetic
configurations, in the present work first time we investigate spintronic
behavior, to the best of our knowledge, in a helical geometry with finite
magnetic ordering. The interplay between short-range and long-range hopping of
electrons yields many non-trivial features which are thoroughly studied. Quite
interestingly we see that the MH exhibits the strong chiral-induced spin
selectivity effect, like what is observed in chiral molecules. Finally, to make
the model more realistic we also examine the effect of helical dynamics. All
the results are valid for a wide range of physical parameters, which prove the
robustness of our analysis.",1911.01630v1
2019-11-05,Impurity Moments Conceal Low-Energy Relaxation of Quantum Spin Liquids,"We scrutinize the magnetic properties of $\kappa$-(BEDT-TTF)$_2$Hg(SCN)$_2$Cl
through its first-order metal-insulator transition at $T_{\rm CO}=30$ K by
means of $^1$H nuclear magnetic resonance (NMR). While in the metal we find
Fermi-liquid behavior with temperature-independent $(T_1T)^{-1}$, the
relaxation rate exhibits a pronounced enhancement when charge order sets in.
The NMR spectra remain unchanged through the transition and no magnetic order
stabilizes down to 25 mK. Similar to the isostructural spin-liquid candidates
$\kappa$-(BEDT-TTF)$_2$Cu$_2$(CN)$_3$ and
$\kappa$-(BEDT-TTF)$_2$Ag$_2$(CN)$_3$, $T_1^{-1}$ acquires a dominant maximum
(here around 5 K). Field-dependent experiments identify the low-temperature
feature as a dynamic inhomogeneity contribution that is typically dominant over
the intrinsic relaxation but gets suppressed with magnetic field.",1911.02057v1
2019-11-12,Iridium Enabled Field-free Spin-orbit Torque Switching of Perpendicular Magnetic Tunnel Junction Device,"Writing magnetic bits by spin-orbit torques (SOTs) arising from spin Hall
effect creates new possibilities for ultrafast and low-power magnetoresistive
random access memory (MRAM). For perpendicular MRAM, an extra in-plane field is
required to break the symmetry for the deterministic SOT writing of the
perpendicular storage layer. Although schemes have been demonstrated in
external-field-free SOT switching of a perpendicular layer, practically
integrating them with perpendicular MTJs still appears to be challenging. Here,
we present experimental demonstration of spin-orbit torques (SOTs) switching a
perpendicular magnetic tunnel junction (MTJ) device without applying an
external magnetic field. An Ir layer is used to serve dual-purpose of both
injecting the pure spin current via spin Hall effect and mediating an in-plane
exchange field to the perpendicular free layer of the MTJ. Robust field-free
SOT switching with pulsed write path current is demonstrated for various MTJ
sizes ranging from 50 nm to 500 nm. The effect of MTJ size and pulse width on
the critical switching current is studied. Combined micromagnetic simulations
are carried out to provide in-depth analysis of the switching dynamics as well
as the thermal effect on the switching.",1911.05007v1
2019-11-19,Unidirectional Seebeck effect in magnetic topological insulators,"We theoretically investigate the temperature gradient-dependent or
unidirectional Seebeck effect (USE) in a magnetic/ nonmagnetic topological
insulator (TI) heterostructure with in-plane magnetization in terms of the
semiclassical electron dynamics and Fermi golden rule. The USE has a quantum
origin arising from the magnon asymmetric scattering of surface Dirac electrons
on TI. We discuss the USE in the heterostructures,
Cr$_{x}$(Bi$_{1-y}$Sb$_y$)$_{2-x}$Te$_{3}$/ (Bi$_{1-y}$Sb$_{y})_2$Te$_{3}$. The
USE exhibits $\cos\phi$-dependence (measured from $y$-direction) on the
orientation of magnetization. It's found that the sign of USE stays unchanged
when the system is transferred from $p$-doping to $n$-doping. The USE shows on
inverse-linearly temperature dependent at high temperature.",1911.08122v1
2019-11-27,Radial flow induced by inhomogeneous magnetic field in heavy ion collisions,"In this paper, we study the effects of an in-homogeneous magnetic field on
the quark gluon plasma dynamics, within the Magneto-Hydrodynamic framework. We
have investigated the effect of an inhomogeneous external magnetic field on the
transverse expansion of in-viscid fluid created in high energy nuclear
collisions. Transverse velocity and energy density are modified by the presence
of the magnetic field. This effects can also influence the transverse momentum
spectrum for particles at the freeze-out surface. In this context we obtained
an interesting comparison with data extracted from heavy-ion collisions.",1911.13209v1
2019-12-10,Husimi function for electrons moving in magnetic fields,"Husimi functions allow one to obtain sensible and useful phase space
probability distributions from quantumechanical wavefunctions or classical wave
fields, linking them to (semi-)classical methods and intuition. They have been
used in several fields of physics, including electronic transport. We show that
applying Husimi functions to ballistic electron dynamics in magnetic fields
needs special consideration in order for them to obey gauge invariance and
energy conservation. We therefore extend the Husimi function formalism to allow
for magnetic fields making use of magnetic translation operators. We
demonstrate the application in tight-binding magneto-transport calculations in
graphene nanodevices, highlighting connections with Klein tunneling. In
continuation of recent work, with this paper we further pave the way for using
the Husimi function to unravel quantum transport phenomena in nanodevices.",1912.04622v1
2019-12-10,Electron-Acoustic Solitons in Magnetized Collisional Nonthermal Plasmas,"The properties of obliquely propagating dissipative electron-acoustic
solitary waves (OPdEASWs) have been investigated in a magnetized collisional
superthermal plasma consisting of inertial cold electrons, inertialess hot
electrons featuring kappa-distribution and static ions via the fluid dynamical
approach. By using the reductive perturbation technique, a nonlinear Schamel
equation that governs the nonlinear features of OPdEASWS is obtained. The
solitary wave solution of the Schamel equation is used to examine the basic
features of small, but finite amplitude OPdEASWs in such a magnetized
collisional superthermal plasma. The basic features (width, amplitude, speed,
etc.) of OPdEASWs are found to be modified by the different plasma
configuration parameters, such as the superthermality index, obliquity angle,
collisional parameter, trapping parameter, and external magnetic field. The
nature of electrostatic disturbances, that may propagate in different realistic
space and laboratory plasma systems (e.g., in Saturn ring), are briefly
discussed.",1912.04756v5
2019-12-12,Robust spin and charge excitations throughout high-$T_c$-cuprate phase diagram from incipient Mottness,"The generic phase diagram of lightly hole-doped high-$T_c$-cuprates hosts
antiferromagnetic insulating phase with well-defined spin-wave excitations.
Contrary to the weak-coupling prediction, these modes persist up to the
overdoped metallic regime as intense and dispersive paramagnons. Here we report
on our study of the low-energy magnetic and charge excitations within the
extended Hubbard model at strong-coupling, using a modified $1/N$ expansion
method with a variational state serving as the saddle point solution. Despite
clear separation of magnetic and Hubbard-$U$ energy scales, we find that
incipient Mottness affects qualitatively dispersions and widths of magnetic
modes throughout entire phase diagram. The obtained magnetic and charge
dynamical structure factors agree semi-quantitatively with recent resonant
$X$-ray and neutron scattering data for
$\mathrm{La_{2-\mathit{x}}Sr_\mathit{x}CuO_4}$ and
$\mathrm{(Bi,Pb)_2(Sr,La)_2CuO_{6+\delta}}$ at all available doping levels. The
weak-coupling random-phase-approximation fails already for underdoped samples,
pointing to the non-trivial intertwining of distinct energy scales in cuprate
superconductors. The existence of a discrete charge mode which splits off the
electron-hole continuum is also predicted.",1912.06232v2
2019-12-13,The physics of helical electron beam in a uniform magnetic field as a testing ground of gauge principle,"According to Bliokh et al., allowing free propagation along the direction of
a uniform magnetic field, the familiar Landau electron state can be regarded as
a non-diffracting version of the helical electron beam propagating along the
magnetic field. Based on this observation, they argued that, while propagating
along the magnetic field, the Landau electrons receive characteristic rotation
with three different angular velocities, depending on the eigen-value $m$ of
the canonical OAM operator, which is generally gauge-variant, and this
splitting was in fact experimentally confirmed. Through complete analyses of
highly mysterious $m$-dependent rotational dynamics of the quantum Landau
states, we try to make clear how and why their observation does not contradict
the widely-believed gauge principle.",1912.06297v2
2019-12-26,Arbitrary amplitude ion acoustic solitary structures in a collisionless magnetized plasma consisting of nonthermal and isothermal electrons,"We have used the Sagdeev pseudo-potential technique to investigate the
arbitrary amplitude ion acoustic solitons, double layers and supersolitons in a
collisionless magnetized plasma consisting of adiabatic warm ions, isothermal
cold electrons and nonthermal hot electrons immersed in an external uniform
static magnetic field. We have used the phase portraits of the dynamical system
describing the nonlinear behaviour of ion acoustic waves to confirm the
existence of different solitary structures. We have also investigated the
transition of different solitary structures: soliton (before the formation of
double layer) $\rightarrow$ double layer $\rightarrow$ supersoliton
$\rightarrow$ soliton (soliton after the formation of double layer) by
considering the variation of $\theta$ only, where $\theta$ is the angle between
the direction of the external uniform static magnetic field and the direction
of propagation of the wave.",1912.11881v1
2019-12-04,Relativistic and magnetic effects in photo ionization of hydrogen-like ions,"We investigate the role of relativistic and magnetic corrections in photo
ionization of hydrogen-like ions. For hydrogen, the wavelengths of the laser
resides in the weakly ultra violet region. For higher nuclear charges, the
laser parameters are scaled in a manner which renders the time-dependent
Schr{\""o}dinger equation in the dipole approximation independent of nuclear
charge. By comparing with numerical solutions of both the relativistic
time-dependent Dirac equation and the non-relativistic Schr{\""o}dinger equation
-- with and without magnetic interactions, the influence of these interactions
are revealed. Moreover, we investigate to what extent relativistic effects are
induced by the relativistic shift in the structure of the ions or by the strong
external field. In agreement with a recent work, Ivanova et al., Phys. Rev. A
{\bf 98}, 063402 (2018), we find that the dominant relativistic correction is
the former, i.e., the relativistic shift in the binding energy of the ions.
However, dynamical relativistic corrections induced by the laser field are also
seen -- in addition to a significant influence of the magnetic field for the
longer wavelengths.",2001.00646v1
2020-01-14,Superintegrable Bertrand magnetic geodesic flows,"The problem of description of superintegrable systems (i.e., systems with
closed trajectories in a certain domain) in the class of rotationally symmetric
natural mechanical systems goes back to Bertrand and Darboux. We describe all
superintegrable (in a domain of slow motions) systems in the class of
rotationally symmetric magnetic geodesic flows. We show that all sufficiently
slow motions in a central magnetic field on a two-dimensional manifold of
revolution are periodic if and only if the metric has a constant scalar
curvature and the magnetic field is homogeneous, i.e. proportional to the area
form.",2001.05067v1
2020-01-20,Magnetic skyrmion interactions in the micromagnetic framework,"Magnetic skyrmions are localized swirls of magnetization with a non-trivial
topological winding number. This winding increases their robustness to
superparamagnetism and gives rise to a myriad of novel dynamical properties,
making them attractive as next-generation information carriers. Recently the
equation of motion for a skyrmion was derived using the approach pioneered by
Thiele, allowing for macroscopic skyrmion systems to be modeled efficiently.
This powerful technique suffers from the prerequisite that one must have a
priori knowledge of the functional form of the interaction between a skyrmion
and all other magnetic structures in its environment. Here we attempt to
alleviate this problem by providing a simple analytic expression which can
generate arbitrary repulsive interaction potentials from the micromagnetic
Hamiltonian. We also discuss a toy model of the radial profile of a skyrmion
which is accurate for a wide range of material parameters.",2001.07193v1
2020-02-06,Simulation of plasmaspheric plume impact on dayside magnetic reconnection,"During periods of strong magnetic activity, cold dense plasma from the
plasmasphere typically forms a plume extending towards the dayside
magnetopause, eventually reaching it. In this work, we present a large-scale
two-dimensional fully kinetic Particle-In-Cell simulation of a reconnecting
magnetopause hit by a propagating plasmaspheric plume. The simulation is
designed so that it undergoes four distinct phases: initial unsteady state,
steady state prior to plume arrival at the magnetospause, plume interaction and
steady state once the plume is well engulfed in the reconnection site. We show
the evolution of the magnetopause's dynamics subjected to the modification of
the inflowing plasma. Our main result is that the change in the plasma
temperature (cold protons in the plume) have no effects on the magnetic
reconnection rate, which on average depends only on the inflowing magnetic
field and total ion density, before, during and after the impact.",2002.02243v1
2020-03-17,Domain wall dynamics in stepped magnetic nanowire with perpendicular magnetic anisotropy,"Micromagnetic simulation is carried out to investigate the current-driven
domain wall (DW) in a nanowire with perpendicular magnetic anisotropy (PMA). A
stepped nanowire is proposed to pin DW and achieve high information storage
capacity based on multi-bit per cell scheme. The DW speed is found to increase
for thicker and narrower nanowires. For depinning DW from the stepped region,
the current density Jdep is investigated with emphasis on device geometry and
materials intrinsic properties. The Jdep could be analytically determined as a
function of the nanocontriction dimension and the thickness of the nanowire.
Furthermore, Jdep is found to exponential dependent on the anisotropy energy
and saturation magnetization, offering thus more flexibility in adjusting the
writing current for memory applications.",2003.07721v1
2020-04-01,The influence of an applied magnetic field on the self-assembly of magnetic nanogels,"Using Langevin dynamics simulations, we investigate the self-assembly of
magnetic nanogels in the presence of applied magneticfields of moderate
strength. We find that even weak fields lead to drastic changes in the
structure factors of both, the embeddedmagnetic nanoparticles and of whole
nanogel particles. Nanogels assemble by uniting magnetic particle clusters
forming inter-gelbridges. At zero field the average amount of such bridges for
a pair of nanogels is close to one, whereas even for weak fields itfastly
doubles. Rapid growth of cluster size at low values of the applied field is
followed by a broad region of slow increase, causedby the mechanical
constraints imposed the polymer matrix. The influence of the latter manifests
itself in both, the slow growth ofthe magnetisation curve at intermediate
fields and the slow decay of the total Zeeman energy.",2004.00725v1
2020-04-09,Nanoscale electrometry based on a magnetic-field-resistant spin sensor,"The nitrogen-vacancy (NV) center is a potential atomic-scale spin sensor for
electric field sensing. However, its natural susceptibility to the magnetic
field hinders effective detection of the electric field. Here we propose a
robust electrometric method utilizing continuous dynamic decoupling (CDD)
technique. During the CDD period, the NV center evolves in a dressed-state
space, where the sensor is resistant to magnetic fields but remains sensitive
to electric fields. As an example, we use this method to isolate the electric
noise from a complex electro-magnetical environment near diamond surface via
measuring the dephasing rate between dressed states. By reducing the surface
electric noise with different covered liquids, we observe an unambiguous
relation between the dephasing rate and the dielectric permittivity of the
liquid, which enables a quantitative investigation of electric noise model near
diamond surface.",2004.04385v1
2020-04-14,Probing photo-induced rearrangements in the NdNiO$_{3}$ magnetic spiral with polarization-sensitive ultrafast resonant soft x-ray scattering,"We use resonant soft X-ray diffraction to track the photo-induced dynamics of
the antiferromagnetic structure in a NdNiO$_{3}$ thin film. Femtosecond laser
pulses with a photon energy of 0.61 eV, resonant with electron transfer between
long-bond and short-bond nickel sites, are used to excite the material and
drive an ultrafast insulator-metal transition. Polarization sensitive soft
X-ray diffraction, resonant to the nickel L$_{3}$-edge, then probes the
evolution of the underlying magnetic spiral as a function of time delay with 80
picosecond time resolution. By modelling the azimuthal dependence of the
scattered intensity for different linear X-ray polarizations, we benchmark the
changes of the local magnetic moments and the spin alignment. The measured
changes are consistent with a reduction of the long-bond site magnetic moments
and an alignment of the spins towards a more collinear structure at early time
delays.",2004.06752v2
2020-04-20,"Optical detection of electron spin dynamics driven by fast variations of a magnetic field: a simple method to measure $T_1$, $T_2$, and $T_2^*$ in semiconductors","We develop a simple method for measuring the electron spin relaxation times
$T_1$, $T_2$ and $T_2^*$ in semiconductors and demonstrate its exemplary
application to $n$-type GaAs. Using an abrupt variation of the magnetic field
acting on electron spins, we detect the spin evolution by measuring the Faraday
rotation of a short laser pulse. Depending on the magnetic field orientation,
this allows us to measure either the longitudinal spin relaxation time $T_1$ or
the inhomogeneous transverse spin dephasing time $T_2^*$. In order to determine
the homogeneous spin coherence time $T_2$, we apply a pulse of an oscillating
radiofrequency (rf) field resonant with the Larmor frequency and detect the
subsequent decay of the spin precession. The amplitude of the rf-driven spin
precession is significantly enhanced upon additional optical pumping along the
magnetic field.",2004.09408v1
2020-05-06,Sub-Nanosecond Spin-Transfer Torque in an Ensemble of Superparamagnetic-Like Nanomagnets,"Spin currents can exert spin-transfer torques on magnetic systems even in the
limit of vanishingly small net magnetization, as is the case for
antiferromagnets. Here, we experimentally show that a spin-transfer torque is
operative in a material with weak, short-range magnetic order -- namely, a
macroscopic ensemble of superparamagnetic-like Co nanomagnets. We employ
element- and time-resolved X-ray ferromagnetic resonance (XFMR) spectroscopy to
directly detect sub-ns dynamics of the Co nanomagnets, excited into precession
with cone angle $\geq$0.003$^{\circ}$ by an oscillating spin current. XFMR
measurements reveal that as the net moment of the ensemble decreases, the
strength of the spin-transfer torque increases relative to those of magnetic
field torques. Our findings point to spin-transfer torque as an effective way
to manipulate the state of nanomagnet ensembles at sub-ns timescales.",2005.03081v1
2020-05-21,Temporal Memory with Magnetic Racetracks,"Race logic is a relative timing code that represents information in a
wavefront of digital edges on a set of wires in order to accelerate dynamic
programming and machine learning algorithms. Skyrmions, bubbles, and domain
walls are mobile magnetic configurations (solitons) with applications for
Boolean data storage. We propose to use current-induced displacement of these
solitons on magnetic racetracks as a native temporal memory for race logic
computing. Locally synchronized racetracks can spatially store relative timings
of digital edges and provide non-destructive read-out. The linear kinematics of
skyrmion motion, the tunability and low-voltage asynchronous operation of the
proposed device, and the elimination of any need for constant skyrmion
nucleation make these magnetic racetracks a natural memory for low-power,
high-throughput race logic applications.",2005.10704v1
2020-06-16,Antiferromagnetic long-range order in $5d^1$ double-perovskite Sr$_2$MgReO$_6$,"The double-perovskite A$_2$BB'O$_6$ with heavy transition metal ions on the
ordered B' sites is an important family of compounds to study the interplay
between electron correlation and spin-orbit coupling (SOC). Here we prepared
high-quality Sr$_2$MgReO$_6$ powder and single-crystal samples and performed
non-resonant and resonant synchrotron x-ray diffraction experiments to
investigate its magnetic ground state. By combining the magnetic susceptibility
and heat capacity measurements, we conclude that Sr$_2$MgReO$_6$ exhibits a
layered antiferromagnetic (AF) order at temperatures below $\sim$ 55 K with a
propagation vector q = (001), which contrasts the previously suspected spin
glass state. Our works clarify the magnetic order in Sr$_2$MgReO$_6$ and
demonstrate it as a candidate system to look for magnetic octupolar orders and
exotic spin dynamics.",2006.08865v1
2020-06-19,Tidal Love numbers of Proca stars,"Proca stars (arXiv:1508.05395) are everywhere regular, asymptotically flat
self-gravitating solitons composed of complex, massive vector bosons, forming a
macroscopic, star-sized, Bose-Einstein condensate. They have been suggested as
a classical, dynamical model of exotic compact objects, black hole mimickers
and dark matter candidates. In spherical symmetry, they are qualitatively
similar to their scalar cousins, the standard (scalar) boson stars in all
respects studied so far. Here we study the tidal deformability and the
quadrupolar tidal Love numbers, both electric-type and magnetic-type, of
spherically symmetric Proca stars. The equations for the perturbations are
derived and the numerical values of the Love numbers for some concrete
background solutions are computed. We observe that both Love numbers are
qualitatively similar to the ones of the scalar boson stars; in particular, the
electric-type (magnetic-type) quadrupolar Love numbers are positive (negative)
for Proca stars. Quantitatively, for the same compactness, the electric and
magnetic Love numbers in the Proca case are closer (in magnitude) than in the
scalar case, with the electric Love numbers having a slightly larger magnitude
than the magnetic ones.",2006.11083v1
2020-06-22,Spin-orbit torque induced magnetisation dynamics and switching in CoFeB/Ta/CoFeB system with mixed magnetic anisotropy,"Spin-orbit torque (SOT) induced magnetisation switching in CoFeB/Ta/CoFeB
trilayer with two CoFeB layers exhibiting in-plane magnetic anisotropy (IPMA)
and perpendicular magnetic anisotropy (PMA) is investigated. Interlayer
exchange coupling (IEC), measured using ferromagnetic resonance technique is
modified by varying thickness of Ta spacer. The evolution of the IEC leads to
different orientation of the magnetic anisotropy axes of two CoFeB layers: for
thicker Ta layer where magnetisation prefers antiferromagnetic ordering and for
thinner Ta layer where ferromagnetic coupling exists. Magnetisation state of
the CoFeB layer exhibiting PMA is controlled by the spin-polarized current
originating from SOT in $\mu m$ sized Hall bars. The evolution of the critical
SOT current density with Ta thickness is presented, showing an increase with
decreasing $t_\mathrm{Ta}$, which coincides with the coercive field dependence.
In a narrow range of $t_\mathrm{Ta}$ corresponding to the ferromagnetic IEC,
the field-free SOT-induced switching is achieved.",2006.12068v1
2020-06-24,Magnetic order and ballistic spin transport in a sine-Gordon spin chain,"We report the results of muon-spin spectroscopy ($\mu^+$SR) measurements on
the staggered molecular spin chain [pym-Cu(NO$_3$)$_2$(H$_2$O)$_2$] (pym =
pyrimidine), a material previously described using sine-Gordon field theory.
Zero-field $\mu^+$SR reveals a long range magnetically-ordered ground state
below a transition temperature $T_\mathrm{N}=0.22(1)$ K. Using
longitudinal-field (LF) $\mu^+$SR we investigate the dynamic response in
applied magnetic fields $0< B < 500$ mT and find evidence for ballistic spin
transport. Our LF $\mu^+$SR measurements on the chiral spin chain
[Cu(pym)(H$_2$O)$_4$]SiF$_6 \cdot $H$_2$O instead demonstrate one-dimensional
spin diffusion and the distinct spin transport in these two systems likely
reflects differences in their magnetic excitations.",2006.13743v1
2020-06-30,Operando Control of Skyrmion Density in a Lorentz Transmission Electron Microscope with Current Pulses,"Magnetic skyrmions hold promise for spintronic devices. To explore the
dynamical properties of skyrmions in devices, a nanoscale method to image spin
textures in response to a stimulus is essential. Here, we apply a technique for
operando electrical current pulsing of chiral magnetic devices in a Lorentz
transmission electron microscope. In ferromagnetic multilayers with interfacial
Dzyaloshinskii-Moriya interaction (DMI), we study the creation and annihilation
of skyrmions localized by point-like pinning sites due to defects. Using a
combination of experimental and micromagnetic techniques, we establish a
thermal contribution for the creation and annihilation of skyrmions in our
study. Our work reveals a mechanism for controlling skyrmion density, which
enables an examination of skyrmion magnetic field stability as a function of
density. We find that high-density skyrmion states are more stable than
low-density states or isolated skyrmions resisting annihilation over a magnetic
field range that increases monotonically with density.",2006.16780v1
2020-07-04,High Entropy Alloys as a new metaphor in sociophysics,"Most of the opinion dynamics models in sociophysics have their historic
origin in studies two-dimensional magnetization phenomena. This metaphor has
proven quite useful, as it allowed to use well known techniques, relating
individual behaviours of single spins and their interactions, to large scale
properties, such as magnetization or magnetic domains creation. These physical
properties were then ``mapped'' to social concepts: spin orientation to a
person's views on a specific issue, magnetization to global opinion on the
issue, etc. During the past 20 years, the models were significantly expanded,
using more complex individual agent characteristics and even more complex types
of the interactions, but the power of the metaphor remained unchanged. In the
current paper we propose to use a new physical system as the basis for new
ideas in sociophysics. We shall argue that the concepts and tools devoted to
studies of High Entropy Alloys (HEAs) could significantly broaden the range of
social concepts ``addressable'' by sociophysics, by focusing on a wider range
of global phenomena, arising from atomic properties, interactions and
arrangements. We illustrate the new idea by calculating a few characteristics
of a simple HEA system and their possible ``mapping'' into social concepts.",2007.02154v1
2020-07-28,Electron correlation effects on exchange interactions and spin excitations in 2D van der Waals materials,"Despite serious effort, the nature of the magnetic interactions and the role
of electron-correlation effects in magnetic two-dimensional (2D) van der Waals
materials remain elusive. Using CrI$_3$ as a model system, we show that the
calculated electronic structure including nonlocal electron correlations yields
spin excitations consistent with inelastic neutron scattering measurements.
Remarkably, this approach identifies an unreported correlation-enhanced
interlayer super-superexchange, which rotates the magnon Dirac lines off, and
introduces a gap along, the high-symmetry $\Gamma$-$K$-$M$ path. This discovery
provides a different perspective on the gap opening mechanism observed in
CrI$_3$, which was previously associated with spin-orbit coupling through the
Dzyaloshinskii-Moriya interaction or Kitaev interaction. Our observation
elucidates the critical role of electron correlations on the spin ordering and
spin dynamics in magnetic van der Waals materials and demonstrates the
necessity of explicit treatment of electron correlations in the broad family of
2D magnetic materials.",2007.14518v2
2020-08-06,Artificial magnetic field for synthetic quantum matter without dynamical modulation,"We propose an all-static method to realize an artificial magnetic field for
charge neutral particles without introducing any time modulation. Our proposal
consists of one-dimensional tubes subject to harmonic trapping potentials with
shifted centers. We show that this setup realizes an artificial magnetic field
in a hybrid real-momentum space. We discuss how characteristic features of
particles in a magnetic field, such as chiral edge states and the quantized
Hall response, can be observed in this setup. We find that the mean-field
ground state of bosons in this setup in the presence of long-range interactions
in physical real space can have quantized vortices in the hybrid real-momentum
space; such a state with vortices exhibits a supersolid structure in the
physical real space. Our method can be applied to a variety of synthetic
quantum matter, including ultracold atomic gases, coupled photonic cavities,
coupled waveguides, and exciton-polariton lattices.",2008.02412v2
2020-08-07,Cavity magnon polariton based precision magnetometry,"A photon-magnon hybrid system can be realised by coupling the electron spin
resonance of a magnetic material to a microwave cavity mode. The quasiparticles
associated with the system dynamics are the cavity magnon polaritons, which
arise from the mixing of strongly coupled magnons and photons. We illustrate
how these particles can be used to probe the magnetisation of a sample with a
remarkable sensitivity, devising suitable spin-magnetometers which ultimately
can be used to directly assess oscillating magnetic fields. Specifically, the
capability of cavity magnon polaritons of converting magnetic excitations to
electromagnetic ones, allows for translating to magnetism the quantum-limited
sensitivity reached by state-of-the-art electronics. Here we employ hybrid
systems composed of microwave cavities and ferrimagnetic spheres, to
experimentally implement two types of novel spin-magnetometers.",2008.03062v2
2020-08-13,Dynamics of the Parker-Jeans instability of gaseous disks including the effect of cosmic rays,"The effect of cosmic rays on the Parker-Jeans instability in magnetized
self-gravitating gaseous disks is studied by three-dimensional
magnetohydrodynamic (MHD) simulations with cosmic rays taken as a massless
fluid with notable pressure. Cosmic ray diffusion along the magnetic field is
taken into account in the simulation. The initial configuration of the disk is
a magnetized cold gas slab sandwiched between hot corona (on top and bottom).
We show that cosmic rays play an important role in the formation of filaments
or clumps in some parameter regimes. In a certain range of the thickness of the
gas slab, the cosmic ray diffusion coefficient plays a decisive role in
determining whether the filaments lie along or perpendicular to the magnetic
field. We also consider the effect of rotation on the system.",2008.06117v1
2020-08-25,Helical magnetic fields from Riemann coupling,"We study the inflationary generation of helical magnetic fields from the
Riemann coupling with the electromagnetic field. Most models in the literature
introduce non-minimal coupling to the electromagnetic fields with a scalar
field, hence, breaking the conformal invariance. In this work, we show that
non-minimal coupling to the Riemann tensor generates sufficient primordial
helical magnetic fields at all observable scales. We explicitly show that one
of the helical states decay while the other helical mode increases, leading to
a net non-zero helicity. Our model has three key features:~(i) the helical
power-spectrum has a slight red-tilt for slow-roll inflation consistent with
bounds from observations and free from backreaction problem, (ii) the energy
density of the helical fields generated is at least one order of magnitude
larger than the scalar-field coupled models, and (iii) unlike the scalar field
coupled models, the generated helical fields are insensitive to the reheating
dynamics. We show that our model generates the magnetic field of strength 0.01
PicoGauss over Mpc scale.",2008.10825v2
2020-08-29,A $j_\mathrm{eff} = 1/2$ pseudospinon continuum in CaIrO$_3$,"In so-called $j_\mathrm{eff} = 1/2$ systems, including some iridates and
ruthenates, the coherent superposition of $t_\mathrm{2g}$ orbitals in the
ground state gives rise to hopping processes that strongly depend on the bond
geometry. Resonant inelastic x-ray scattering (RIXS) measurements on CaIrO$_3$
reveal a prototypical $j_\mathrm{eff} = 1/2$ pseudospinon continuum, a hallmark
of one-dimensional (1D) magnetic systems despite its three-dimensional crystal
structure. The experimental spectra compare very well to the calculated
magnetic dynamical structure factor of weakly coupled spin-1/2 chains. We
attribute the onset of such quasi-1D magnetism to the fundamental difference in
the magnetic interactions between the $j_\mathrm{eff} = 1/2$ pseudospins along
the corner- and edge-sharing bonds in CaIrO$_3$.",2008.12921v1
2020-09-09,Symmetric set of transport coefficients for collisional magnetized plasma,"In Braginskii extended magneto-hydrodynamics (ExMHD), applicable to
collisional astrophysical and high energy density plasmas, the electric field
and heat flow are described by the $\alpha$, $\beta$ and $\kappa$ transport
coefficients. We show that magnetic transport relies primarily on
$\beta_\parallel-\beta_\perp$ and $\alpha_\perp-\alpha_\parallel$, rather than
$\alpha_\perp$ and $\beta_\perp$ themselves. However, commonly used coefficient
fit functions [Epperlein and Haines, Phys. Fluids 29, 1029 (1986)] cannot
accurately calculate these quantities. This means that many ExMHD simulations
have significantly over-estimated the cross-gradient Nernst advection,
resulting in artificial magnetic dissipation and discontinuities. We repeat the
kinetic analysis to provide fits that rectify this problem. Use of these in the
Gorgon ExMHD code resolves the known discrepancies with kinetic simulations in
the literature. Recognizing the fundamental importance of
$\alpha_\perp-\alpha_\parallel$ and $\beta_\parallel-\beta_\perp$, we re-cast
the set of coefficients to find that each of them now shares the same
underlying properties. This makes explicit the symmetry of the magnetic and
thermal transport equations, as well as the symmetry of the coefficients
themselves.",2009.04562v1
2020-09-21,"Controllable Capillary Assembly of Magnetic Ellipsoidal Janus Particles into Tunable Rings, Chains and Hexagonal Lattices","Colloidal assembly at fluid interfaces has a great potential for the
bottom-up fabrication of novel structured materials. However, challenges remain
in realizing controllable and tunable assembly of particles into diverse
structures. Herein, we report the capillary assembly of magnetic ellipsoidal
Janus particles at a fluid-fluid interface. Depending on their tilt angle, i.e.
the angle the particle main axis forms with the fluid interface, these
particles deform the interface and generate capillary dipoles or hexapoles.
Driven by capillary interactions, multiple particles thus assemble into chain-,
hexagonal lattice- and ring-like structures, which can be actively controlled
by applying an external magnetic field. We predict a field-strength phase
diagram in which various structures are present as stable states. Owing to the
diversity, controllability, and tunability of assembled structures, magnetic
ellipsoidal Janus particles at fluid interfaces could therefore serve as
versatile building blocks for novel materials.",2009.09804v2
2020-10-01,Interband plasmon polaritons in magnetized charge-neutral graphene,"Studying the collective excitations in charge neutral graphene (CNG) has
recently attracted a great interest because of unusual mechanisms of the charge
carrier dynamics. The latter can play a crucial role, for instance, for
superconducting phases in the periodically strained CNG and the magic angle
twisted bilayer graphene or for formation of graphene plasmon polaritons (GPPs)
associated with interband transitions due to the strain-induced pseudomagnetic
field. Importantly, GPP in CNG can be a tool providing new insights into
various intriguing quantum phenomena in CNG via optical experiments. However,
interband GPPs in CNG are barely investigated, even in the simplest
configurations. Here, we show that magnetically biased single layer CNG
(particularly, at zero temperature) can support interband GPPs of both
transverse magnetic and transverse electric polarizations. They exist inside
the narrow absorption bands originating from the electronic transitions between
Landau levels and are tunable by the external magnetic field. We put our study
into the context of potential near-field and far-field optical experiments,
thus opening the door to the exploration of CNG for optics and nanophotonics.",2010.00306v1
2020-10-13,Origins of anomalously low Raman exponents in single-molecule magnets,"The Raman exponent of single-molecular magnetic relaxation may take various
unexpected values because of rich phonon spectrum and spin-phonon coupling of
molecular crystals. We systematically examine the origins of different
abnormalities, and clarify misunderstandings in the past, particularly the
appropriateness of the fitting procedures for the exponents. We find that
exponential laws raised by optical phonons can yield spurious power laws with
low exponents. This observation indicates long-standing misunderstandings for
origins of low Raman exponents in a large bulk of single-molecule magnets.
Resulting from spin-lattice coupling with optical modes, presence of these
exponents suggests the importance of the local dynamical environment for the
magnetic relaxation in this regime.",2010.09486v2
2020-10-21,Spontaneous Chiral-Spin Ordering in Spin-Orbit Coupled Honeycomb Magnets,"Frustrated magnets with highly degenerate ground states are at the heart of
hunting exotic states of matter. Recent studies in spin-orbit coupled honeycomb
magnets have generated immense interest in bond-dependent interactions,
appreciating a symmetric off-diagonal $\Gamma$ interaction which exhibits a
macroscopic degeneracy in the classical limit. Here, we study a generic spin
model and discover a novel chiral-spin ordering with spontaneously broken
time-reversal symmetry near the dominant $\Gamma$ region. The chiral-spin phase
is demonstrated to possess a staggered chirality relation in different
sublattices, and it exhibits gapless excitations as revealed by the vanishing
energy gap and the finite central charge on cylinders. Although there is a
vestige of a tiny peak in the corner of the second Brillouin zone, the magnetic
order is likely to vanish as the system size increases. Finally, we also
attempt to gain insight into the possible topological signature of the
chiral-spin phase by calculating the dynamic structure factor and the modular
$\mathcal{S}$ matrix.",2010.11233v2
2020-10-28,Thermomagnetic instability of plasma composition gradients,"We show that, under Braginskii magneto-hydrodynamics, anti-parallel gradients
in average ion charge state and electron temperature can be unstable to the
growth of self-generated magnetic fields. The instability is analogous to the
field-generating thermomagnetic instability, although it is driven by the
collisional thermal force magnetic source term rather than the Biermann battery
term. The gradient in ion charge state causes a gradient in collisionality,
which couples with temperature perturbations to create a self-generated
magnetic field. This magnetic field deflects the electron heat flux in a way
that reinforces the temperature perturbation. The derived linearized growth
rate, typically on hydrodynamic timescales, includes the resistive and thermal
smoothing. It increases with large ion composition gradients and electron heat
flux, conditions typical of the hohlraum walls or contaminant mix jets in
inertial confinement fusion implosions. However, extended magneto-hydrodynamic
simulations indicate that the instability is usually dominated and stabilized
by the nonlinear Nernst advection, in a similar manner to the standard
thermomagnetic instability.",2010.15072v1
2020-11-02,Spin-orbital magnetic response of relativistic fermions with band hybridization,"Spins of relativistic fermions are related to their orbital degrees of
freedom. In order to quantify the effect of hybridization between relativistic
and nonrelativistic degrees of freedom on spin-orbit coupling, we focus on the
spin-orbital (SO) crossed susceptibility arising from spin-orbit coupling. The
SO crossed susceptibility is defined as the response function of their spin
polarization to the ""orbital"" magnetic field, namely the effect of magnetic
field on the orbital motion of particles as the vector potential. Once
relativistic and nonrelativistic fermions are hybridized, their SO crossed
susceptibility gets modified at the Fermi energy around the band hybridization
point, leading to spin polarization of nonrelativistic fermions as well. These
effects are enhanced under a dynamical magnetic field that violates thermal
equilibrium, arising from the interband process permitted by the band
hybridization. Its experimental realization is discussed for Dirac electrons in
solids with slight breaking of crystalline symmetry or doping, and also for
quark matter including dilute heavy quarks strongly hybridized with light
quarks, arising in a relativistic heavy-ion collision process.",2011.00882v2
2020-11-03,Relativistic non-resistive viscous magnetohydrodynamics from the kinetic theory:a relaxation time approach,"We derive the relativistic non-resistive, viscous second-order
magnetohydrodynamic equations for the dissipative quantities using the
relaxation time approximation. The Boltzmann equation is solved for a system of
particles and antiparticles using Chapman-Enskog like gradient expansion of the
single-particle distribution function truncated at second order. In the first
order, the transport coefficients are independent of the magnetic field. In the
second-order, new transport coefficients that couple magnetic field and the
dissipative quantities appear which are different from those obtained in the
14-moment approximation \cite{Denicol:2018rbw} in the presence of a magnetic
field. However, in the limit of the weak magnetic field, the form of these
equations are identical to the 14-moment approximation albeit with a different
values of these coefficients. We also derive the anisotropic transport
coefficients in the Navier-Stokes limit.",2011.01606v1
2020-11-13,Classification of Magnetic Vortices by Angular Momentum Conservation,"Superfluid vortices are quantum excitations carrying quantized amount of
orbital angular momentum in a phase where global symmetry is spontaneously
broken. We address a question of whether magnetic vortices in superconductors
with dynamical gauge fields can carry nonzero orbital angular momentum or not.
We discuss the angular momentum conservation in several distinct classes of
examples from crossdisciplinary fields of physics across condensed matter,
dense nuclear systems, and cosmology. The angular momentum carried by gauge
field configurations around the magnetic vortex plays a crucial role in
satisfying the principle of the conservation law. Based on various ways how the
angular momentum conservation is realized, we provide a general scheme of
classifying magnetic vortices in different phases of matter.",2011.06981v1
2020-11-18,"On the photo-evaporation, dust polarization, and kinematics of two nebulae in Sh2-236","In the work presented here, the impact of magnetic field on the dynamical
evolution of cometary globules Sim 129 and Sim 130 in Sh2-236 H\,II region and
the ionized gas streaming out of their surfaces is investigated. The magnetic
field morphology in the region associated with these globules is inferred using
optical polarization measurements with the Sampurnanand Telescope at ARIES. The
nebular emission is probed through radio continuum mapping at 1.4~GHz with the
archival National Radio Astronomy Observatory (NRAO) very large array (VLA) Sky
Survey (NVSS) data. The correlation of these measurements suggest that the
photoevaporated gas from the surfaces of Sim 129 and Sim 130 is accumulated in
clouds and starts streaming along the magnetic field lines. The $\rm ^{12}CO$
(J=1-0) molecular line observations are performed towards NGC\,1893 from 14-m
single dish radio telescope in Taeduk Radio Astronomy Observatory (TRAO). The
velocity dispersion in $\rm ^{12}CO$ (J=1-0) molecular line and the dispersion
in polarization angles are used in Davis-Chandrasekhar-Fermi formulation to
estimate the magnetic field strength towards two sim nebulae. The average value
of field strength is found to be $\sim$60$\mu$G with uncertainty of 0.5 times
the estimated value.",2011.09572v1
2020-11-20,Neutron scattering by magnetic octupoles of a quantum liquid,"Neutron scattering is a powerful tool to study magnetic structures and
dynamics, benefiting from a precisely established theoretical framework. The
neutron dipole moment interacts with electrons in materials via their magnetic
field, which can have spin and orbital origins. Yet in most experimentally
studied cases the individual degrees of freedom are well described within the
dipole approximation, sometimes accompanied by further terms of a multipolar
expansion that usually act as minor corrections to the dipole form factor. Here
we report a unique example of neutrons diffracted mainly by magnetic octupoles.
This unusual situation arises in a quantum spin ice where the electronic
wavefunction becomes essentially octupolar under the effect of correlations.
The discovery of such a new type of quantum spin liquid that comes with a
specific experimental signature in neutron scattering is remarkable, because
these topical states of matter are notoriously difficult to detect.",2011.10418v1
2020-11-23,Asymmetric spin wave dispersion due to a saturation magnetization gradient,"We demonstrate using micromagnetic simulations and a theoretical model that a
gradient in the saturation magnetization ($M_s$) of a perpendicularly
magnetized ferromagnetic film induces a non-reciprocal spin wave propagation
and, consequently an asymmetric dispersion relation. The $M_s$ gradient adds a
linear potential to the spin wave equation of motion consistent with the
presence of a force. We consider a transformation from an inertial reference
frame in which the $M_s$ is constant to an accelerated reference frame where
the resulting inertial force corresponds to the force from the $M_s$ gradient.
As in the Doppler effect, the frequency shift leads to an asymmetric dispersion
relation. Additionally, we show that under certain circumstances,
unidirectional propagation of spin waves can be achieved which is essential for
the design of magnonic circuits. Our results become more relevant in light of
recent experimental works in which a suitable thermal landscape is used to
dynamically modulate the saturation magnetization.",2011.11148v2
2020-11-23,Electron vortex beams in non-uniform magnetic fields,"We consider the quantum theory of paraxial non-relativistic electron beams in
non-uniform magnetic fields, such as the Glaser field. We find the wave
function of an electron from such a beam and show that it is a joint eigenstate
of two ($z$-dependent) commuting gauge-independent operators. This generalized
Laguerre-Gaussian vortex beam has a phase that is shown to consist of two
parts, each being proportional to the eigenvalue of one of the two conserved
operators and each having different symmetries. We also describe the dynamics
of the angular momentum and cross-sectional area of any mode and how a varying
magnetic field can split a mode into a superposition of modes. By a suitable
change in frame of reference all of our analysis also applies to an electron in
a quantum Hall system with a time-dependent magnetic field.",2011.11729v2
2020-11-26,Optically-induced frequency up-conversion of the ferromagnetic resonance in an ultrathin garnet,"We perform ultrafast pump-probe measurements on a nanometer-thick crystalline
Bi-doped yttrium iron garnet film with perpendicular magnetic anisotropy.
Tuning the photon energy of the pump laser pulses above and below the
material's bandgap, we trigger ultrafast optical and spin dynamics via both
one- and two-photon absorption. Contrary to the common scenario, the
optically-induced heating of the system induces an increase up to 20% of the
ferromagnetic resonance frequency. We explain this unexpected result in terms
of a photo-induced modification of the magnetic anisotropy, i.e. of the
effective field, identifying the necessary conditions to observe this effect.
Our results disclose the possibility to optically increase the magnetic
eigenfrequency in nanometer-thick magnets.",2011.13403v1
2020-12-02,Magneto-active elastic shells with tunable buckling strength,"Shell buckling is central in many biological structures and advanced
functional materials, even if, traditionally, this elastic instability has been
regarded as a catastrophic phenomenon to be avoided for engineering structures.
Either way, predicting critical buckling conditions remains a long-standing
challenge. The subcritical nature of shell buckling imparts extreme sensitivity
to material and geometric imperfections. Consequently, measured critical loads
are inevitably lower than classic theoretical predictions. Here, we present a
robust mechanism to dynamically tune the buckling strength of shells,
exploiting the coupling between mechanics and magnetism. Our experiments on
pressurized spherical shells made of a magnetorheological elastomer demonstrate
the tunability of their buckling pressure via magnetic actuation. We develop a
theoretical model for thin magnetic elastic shells, which rationalizes the
underlying mechanism, in excellent agreement with experiments. A dimensionless
magneto-elastic buckling number is recognized as the key governing parameter,
combining the geometrical, mechanical, and magnetic properties of the system.",2012.01163v2
2020-12-08,Decoupled Strain Response of Ferroic Properties in Multiferroic VOCl2 Monolayer,"Two-dimensional (2D) magnetoelectric multiferroics are promising
multifunctional materials for miniaturized logic and memory devices. Herein, we
explore the effectiveness of strain-engineering for tuning the properties of a
recently predicted 2D antiferromagnetic-ferroelectric, VOCl2 monolayer.
Interestingly, we find that magnetic-ordering and electric polarization can be
tuned independently using uniaxial tensile strain along different in-plane
lattice vectors. A 4% tensile strain along lattice vector b induces a
transition from an antiferromagnetic (AFM) ground state with an out-of-plane
magnetization to a ferromagnetic (FM) ground state with in-plane magnetization.
On the other hand, tensile strain along lattice vector a enhances spontaneous
electric polarization, without affecting the magnetic ordering. The monolayers
remain dynamically stable under tensile strain, which further helps to raise
the Curie temperature of ferromagnetism, as well as ferroelectricity. Such a
strain-tunable multiferroic material holds great promises for future generation
nanoelectronic devices.",2012.04592v2
2020-12-29,"Spin dynamics in the pyrochlore iridate, Er2Ir2O7, investigated by muSR spectroscopy","Building a full understanding of the complex magnetism in the rare-earth
pyrochlore iridates, A2Ir2O7, is an ongoing issue in condensed matter physics.
The possibility of two interpenetrating and interacting frustrated magnetic
pyrochlore sublattices, connected with the strong spin-orbit coupling of the 5d
Ir4+ ion, lends itself to a wide array of potential electronic and magnetic
states. In this paper, we present longitudinal field and zero field muSR
measurements of Er2Ir2O7. The muSR response of Er2Ir2O7 is dominated by the
strong fluctuating fields of the Er3+ (J = 15/2) moments, showing a peak in
fitted relaxation rate, {\lambda}_2, at T = 15 K . The muSR spectra show just
weak signatures of the Ir transition, that were easily suppressed in applied
longitudinal field, however full decoupling of the muons from local magnetic
fields was not achieved even in applied fields up to 3 kG.",2012.14730v1
2021-01-10,A Thermodynamic Core using Voltage-Controlled Spin-Orbit-Torque Magnetic Tunnel Junctions,"We present a magnetic implementation of a thermodynamic computing fabric.
Magnetic devices within computing cores harness thermodynamics through its
voltage-controlled thermal stability; while the evolution of network states is
guided by the spin-orbit-torque effect. We theoretically derive the dynamics of
the cores and show that the computing fabric can successfully compute ground
states of a Boltzmann Machine. Subsequently, we demonstrate the physical
realization of these devices based on a CoFeB-MgO magnetic tunnel junction
structure. The results of this work pave the path towards the realization of
highly efficient, high-performance thermodynamic computing hardware. Finally,
this paper will also give a perspective of computing beyond thermodynamic
computing.",2101.03448v3
2021-01-14,Magnetic field-induced thermal emission tuning of InSb-based metamaterials in the terahertz frequency regime,"This work theoretically and analytically demonstrates the magnetic
field-induced spectral radiative properties of photonic metamaterials
incorporating both Indium Antimonide (InSb) and Tungsten (W) in the terahertz
(THz) frequency regime. We have varied multiple factors of the nanostructures,
including composite materials, layer thicknesses and surface grating fill
factors, which impact the light-matter interactions and in turn modify the
thermal emission of the metamaterials. We have proposed and validated a method
for determining the spectral properties of InSb under an applied direct current
(DC) magnetic field, and have employed this method to analyze how these
properties can be dynamically tuned by modulating the magnitude of the field.
For the first time, we have designed an InSb-W metamaterial exhibiting unity
narrowband emission which can serve as an emitter for wavelengths around 55
$\mu$m (approximately 5.5 THz). Additionally, the narrowband emission of this
metamaterial can be magnetically tuned in both bandwidth and peak wavelength
with a normal emissivity close to unity.",2101.05852v2
2021-01-16,Universal Critical Exponents of the Magnetic Domain Wall Depinning Transition,"Magnetic field driven domain wall dynamics in a ferrimagnetic GdFeCo thin
film with perpendicular magnetic anisotropy is studied using low temperature
magneto-optical Kerr microscopy. Measurements performed in a practically
athermal condition allow for the direct experimental determination of the
velocity ($ \beta = 0.30 \pm 0.03 $) and correlation length ($ \nu = 1.3 \pm
0.3 $) exponents of the depinning transition. The whole family of exponents
characterizing the transition is deduced, providing evidence that the depinning
of magnetic domain walls is better described by the quenched Edwards-Wilkinson
universality class.",2101.06555v3
2021-01-25,Raman Spectroscopy and Aging of the Low-Loss Ferrimagnet Vanadium Tetracyanoethylene,"Vanadium tetracyanoethylene (V[TCNE]$_{x}$, $x\approx 2$) is an organic-based
ferrimagnet with a high magnetic ordering temperature $\mathrm{T_C>600 ~K}$,
low magnetic damping, and growth compatibility with a wide variety of
substrates. However, similar to other organic-based materials, it is sensitive
to air. Although encapsulation of V[TCNE]$_{x}$ with glass and epoxy extends
the film lifetime from an hour to a few weeks, what is limiting its lifetime
remains poorly understood. Here we characterize encapsulated V[TCNE]$_{x}$
films using confocal microscopy, Raman spectroscopy, ferromagnetic resonance
and SQUID magnetometry. We identify the relevant features in the Raman spectra
in agreement with \textit{ab initio} theory, reproducing $\mathrm{C=C,C\equiv
N}$ vibrational modes. We correlate changes in the effective dynamic
magnetization with changes in Raman intensity and in photoluminescence. Based
on changes in Raman spectra, we hypothesize possible structural changes and
aging mechanisms in V[TCNE]$_x$. These findings enable a local optical probe of
V[TCNE]$_{x}$ film quality, which is invaluable in experiments where assessing
film quality with local magnetic characterization is not possible.",2101.10240v1
2021-02-16,Spin-glass behavior in Shastry-Sutherland lattice of Tm$_\textbf{2}$Cu$_\textbf{2}$In,"The spin-glass behavior in the ferromagnetic phase of $\mathrm{Tm_2Cu_2In}$
was investigated by dc-, ac-magnetization, and non-equilibrium dynamics
characterizations. Arc-melted polycrystalline compound of $\mathrm{Tm_2Cu_2In}$
crystallizes in $\mathrm{Mo_2FeB_2}$-type tetragonal structure (space group
$P_4/mbm$). The temperature variation dc-magnetization exhibits a ferromagnetic
behavior with Curie temperature $T_C$ = 32.5 K. The zero-field cooled (ZFC) and
field cooled (FC) magnetic curves show a thermomagnetic irreversible behavior
below $T_C$. The field dependence of irreversible temperature follows
Almeida-Thouless line. The frequency and ac-driven field-dependent anomalies in
ac-susceptibility results indicate the existence of spin-glass state in
$\mathrm{Tm_2Cu_2In}$. The time dependence of magnetization further supports
the spin-glass behavior observed in $\mathrm{Tm_2Cu_2In}$. The frequency
dependence of the freezing temperature in the real part of ac-susceptibility
has been analyzed on the basis power-law divergence and Vogel-Fulcher law. The
obtained results revealed that $\mathrm{Tm_2Cu_2In}$ belongs to a ferromagnetic
canonical spin-glass class of compounds.",2102.08230v2
2021-02-19,Critical Energy Dissipation in a Binary Superfluid Gas by a Moving Magnetic Obstacle,"We study the critical energy dissipation in an atomic superfluid gas with two
symmetric spin components by an oscillating magnetic obstacle. Above a certain
critical oscillation frequency, spin-wave excitations are generated by the
magnetic obstacle, demonstrating the spin superfluid behavior of the system.
When the obstacle is strong enough to cause density perturbations via local
saturation of spin polarization, half-quantum vortices (HQVs) are created for
higher oscillation frequencies, which reveals the characteristic evolution of
critical dissipative dynamics from spin-wave emission to HQV shedding. Critical
HQV shedding is further investigated using a pulsed linear motion of the
obstacle, and we identify two critical velocities to create HQVs with different
core magnetization.",2102.09826v2
2021-02-24,Spin waves and high-frequency response in layered superconductors with helical magnetic structure,"We evaluate the spin-wave spectrum and dynamic susceptibility in a layered
superconductors with helical interlayer magnetic structure. We especially focus
on the structure in which the moments rotate 90$^{\circ}$ from layer to layer
realized in the iron pnictide RbEuFe$_{4}$As$_{4}$. The spin-wave spectrum in
superconductors is strongly renormalized due to the long-range electromagnetic
interactions between the oscillating magnetic moments. This leads to strong
enhancement of the frequency of the mode coupled with uniform field and this
enhancement exists only within a narrow range of the c-axis wave vectors of the
order of the inverse London penetration depth. The key feature of materials
like RbEuFe$_{4}$As$_{4}$ is that this uniform mode corresponds to the maximum
frequency of the spin-wave spectrum with respect to c-axis wave vector. As a
consequence, the high-frequency surface resistance acquires a very distinct
asymmetric feature spreading between the bare and renormalized frequencies. We
also consider excitation of spin waves with Josephson effect in a tunneling
contact between helical-magnetic and conventional superconductors and study the
interplay between the spin-wave features and geometrical cavity resonances in
the current-voltage characteristics.",2102.12445v2
2021-03-01,Electron-induced nuclear magnetic ordering in n-type semiconductors,"Nuclear magnetism in n-doped semiconductors with positive hyperfine constant
is revisited. Two kinds of nuclear magnetic ordering can be induced by resident
electrons in a deeply cooled nuclear spin system. At positive nuclear spin
temperature below a critical value, randomly oriented nuclear spin polarons
similar to that predicted by I. Merkulov [I. Merkulov, Physics of the Solid
State 40, 930 (1998)] should emerge. These polarons are oriented randomly and
within each polaron nuclear and electron spins are aligned
antiferromagnetically. At negative nuclear spin temperature below a critical
value we predict another type of magnetic ordering - dynamically induced
nuclear ferromagnet. This is a long-range ferromagnetically ordered state
involving both electrons and nuclei. It can form if electron spin relaxation is
dominated by the hyperfine coupling, rather than by the spin-orbit interaction.
Application of the theory to the n-doped GaAs suggests that the ferromagnetic
order may be reached at experimentally achievable nuclear spin temperature
$\Theta_N \approx 0.5$ $\mu$K and lattice temperature $T_L \approx 5$ K.",2103.01120v1
2021-03-09,A compact and fast magnetic coil for the interaction manipulation of quantum gases with Feshbach resonances,"Cold atom experiments commonly use broad magnetic Feshbach resonances to
manipulate the interaction between atoms. In order to induce quantum dynamics
by a change of the interaction strength, rapid ($\sim\mu s$) magnetic field
changes over several tens of Gauss are required. Here we present a compact
design of a coil and its control circuit for a change of the magnetic field up
to $36G$ in $3\mu s$. The setup comprises two concentric solenoids with minimal
space requirements, which can be readily added to existing apparatuses. This
design makes the observation of non-equilibrium physics with broad Feshbach
resonances accessible.",2103.05273v1
2021-03-12,Mott transition and magnetism in a fragile topological insulator,"We study the effects of electronic correlations on fragile topology using
dynamical mean-field theory. Fragile topological insulators (FTIs) offer
obstruction to the formation of exponentially localized Wannier functions, but
they can be trivialized by adding certain trivial degrees of freedom. For the
same reason, FTIs do not host symmetry-protected flow of edge states between
bulk bands in cylindrical boundary conditions but are expected to have a
spectral flow between the fragile bands and other bands under certain twisted
boundary conditions. We here analyze commonly observed effects of strong
correlations, such as the Mott-insulator transition and magnetism, on a known
model hosting fragile topology. We show that in the nonmagnetic case, fragile
topology, along with the twisted boundary states, is stable with interactions
below a critical interaction strength. Above this interaction strength, a
transition to the Mott insulating phase occurs, and the twisted boundary states
disappear. Furthermore, by applying a homogeneous magnetic field, the fragile
topology is destroyed. However, we show that a magnetic field can induce a
topological phase transition which converts a fragile topological insulator to
a Chern insulator. Finally, we study ferromagnetic solutions of the fragile
topological model.",2103.07203v2
2021-03-17,Micromagnetic description of twisted spin spirals in the B20 chiral magnet FeGe from first principles,"Using the model of classical Heisenberg exchange and Dzyaloshinskii-Moriya
(DM) interaction we show that the ground state of B20 FeGe chiral magnet is a
superposition of twisted helical spin-density waves formed by different
sublattices of the crystal. Such twisted spin-density waves propagate in the
same direction but with different phases and different directions of the
rotation axes. We derive an advanced micromagnetic expression describing the
exchange and DM interaction for such magnetic structures. By employing
first-principles calculations based on density functional theory and using our
micromagnetic model we show that the magnitude of the spin-spiral twist in B20
FeGe is of the same order as global spiraling. While the energy difference
between the ground state of twisted spirals and the FM state is in good
agreement with the experimental results, for the spin spirals without a twist
it is smaller by factor three. In addition, we verify our results by employing
spin dynamics simulations. This calls for new experiments exploring the ground
state properties of B20 chiral magnets.",2103.09800v2
2021-03-22,Hardware realization of the multiply and accumulate operation on radio-frequency signals with magnetic tunnel junctions,"Artificial neural networks are a valuable tool for radio-frequency (RF)
signal classification in many applications, but digitization of analog signals
and the use of general purpose hardware non-optimized for training make the
process slow and energetically costly. Recent theoretical work has proposed to
use nano-devices called magnetic tunnel junctions, which exhibit intrinsic RF
dynamics, to implement in hardware the Multiply and Accumulate (MAC) operation,
a key building block of neural networks, directly using analogue RF signals. In
this article, we experimentally demonstrate that a magnetic tunnel junction can
perform multiplication of RF powers, with tunable positive and negative
synaptic weights. Using two magnetic tunnel junctions connected in series we
demonstrate the MAC operation and use it for classification of RF signals.
These results open the path to embedded systems capable of analyzing RF signals
with neural networks directly after the antenna, at low power cost and high
speed.",2103.11993v2
2021-03-24,Effective strain manipulation of the antiferromagnetic state of polycrystalline NiO,"As a candidate material for applications such as magnetic memory,
polycrystalline antiferromagnets offer the same robustness to external magnetic
fields, THz spin dynamics, and lack of stray field as their single crystalline
counterparts, but without the limitation of epitaxial growth and lattice
matched substrates. Here, we first report the detection of the average Neel
vector orientiation in polycrystalline NiO via spin Hall magnetoresistance
(SMR). Secondly, by applying strain through a piezo-electric substrate, we
reduce the critical magnetic field required to reach a saturation of the SMR
signal, indicating a change of the anisotropy. Our results are consistent with
polycrystalline NiO exhibiting a positive sign of the in-plane
magnetostriction. This method of anisotropy-tuning offers an energy efficient,
on-chip alternative to manipulate a polycrystalline antiferromagnets magnetic
state.",2103.13105v1
2021-03-29,Surface Acoustic Wave induced modulation of tunneling magnetoresistance in magnetic tunnel junctions,"We show that a surface acoustic wave (SAW) applied across the terminals of a
magnetic tunnel junction (MTJ) decreases both the (time-averaged) parallel and
antiparallel resistances of the MTJ, with the latter decreasing much more than
the former. This results in a decrease of the tunneling magnetoresistance (TMR)
ratio. The coercivities of the free and fixed layer of the MTJ, however, are
not affected significantly, suggesting that the SAW does not cause large-angle
magnetization rotation in the magnetic layers through the inverse
magnetostriction (Villari) effect at the power levels used. This study sheds
light on the dynamical behavior of an MTJ under periodic compressive and
tensile strain.",2103.15303v1
2021-03-31,Itinerant magnetism of chromium under pressure: a DFT+DMFT study,"We consider electronic and magnetic properties of chromium, a well-known
itinerant antiferromagnet, by a combination of density functional theory (DFT)
and dynamical mean-field theory (DMFT). We find that electronic correlation
effects in chromium, in contrast to its neighbours in the periodic table, are
weak, leading to the quasiparticle mass enhancement factor ${m^*/m \approx
1.2}$. Our results for local spin-spin correlation functions and distribution
of weigths of atomic configurations indicate that the local magnetic moments
are not formed. Similarly to previous results of DFT at ambient pressure, the
non-uniform magnetic susceptibility as a function of momentum possesses close
to the wave vector ${{\mathbf Q}_{\rm H}=(0,0,2\pi/a)}$ ($a$ is the lattice
constant) sharp maxima, corresponding to Kohn anomalies. We find that these
maxima are preserved by the interaction and are not destroyed by pressure. Our
calculations qualitatively capture a decrease of the N\'eel temperature with
pressure and a breakdown of itinerant antiferomagnetism at pressure of $\sim$9
GPa in agreement with experimental data, although the N\'eel temperature is
significantly overestimated because of the mean-field nature of DMFT.",2103.17133v2
2021-04-08,Out-of-plane magnetization oscillation in spin Hall device assisted by field-like torque,"An excitation of a large-amplitude out-of-plane magnetization oscillation in
a ferromagnet by the spin Hall effect is of great interest for practical
applications such as microwave generator and neuromorphic computing. However,
both experimental and theoretical works have revealed that only small-amplitude
oscillation around an in-plane easy axis can be excited via the spin Hall
effect. Here, we propose that an out-of-plane oscillation can be excited due to
an assistance of field-like torque. We focus on an in-plane magnetized
ferromagnet with an easy axis parallel to current direction. We notice that the
field-like torque with an appropriate sign provides an additional field
modifying the dynamic trajectory of the magnetization and drives the
auto-oscillation. The condition on the sign of the field-like torque is
satisfied for typical nonmagnet used in spin Hall devices such as tungsten.",2104.04057v1
2021-04-19,Infrared phonon spectroscopy on the Cairo pentagonal antiferromagnet Bi2Fe4O9: a study through the pressure induced structural transition,"Magnetic and crystallographic transitions in the Cairo pentagonal magnet
Bi2Fe4O9 are investigated by means of infrared synchrotron-based spectroscopy
as a function of temperature (20 - 300 K) and pressure (0 - 15.5 GPa). One of
the phonon modes is shown to exhibit an anomalous softening as a function of
temperature in the antiferromagnetic phase below 240 K, highlighting
spin-lattice coupling. Moreover, under applied pressure at 40 K, an even larger
softening is observed through the pressure induced structural transition.
Lattice dynamical calculations reveal that this mode is indeed very peculiar as
it involves a minimal bending of the strongest superexchange path in the
pentagonal planes, as well as a decrease of the distances between second
neighbor irons. The latter confirms the hypothesis made by Friedrich et al.,1
about an increase in the oxygen coordination of irons being at the origin of
the pressure-induced structural transition. As a consequence, one expects a new
magnetic superexchange path that may alter the magnetic structure under
pressure.",2104.09384v1
2021-04-21,Magnetism of magic-angle twisted bilayer graphene,"We investigate magnetic instabilities in charge-neutral twisted bilayer
graphene close to so-called ""magic angles"" using a combination of real-space
Hartree-Fock and dynamical mean-field theories. In view of the large size of
the unit cell close to magic angles, we examine a previously proposed rescaling
that permits to mimic the same underlying flat minibands at larger twist
angles. We find that localized magnetic states emerge for values of the Coulomb
interaction $U$ that are significantly smaller than what would be required to
render an isolated layer antiferromagnetic. However, this effect is
overestimated in the rescaled system, hinting at a complex interplay of
flatness of the minibands close to the Fermi level and the spatial extent of
the corresponding localized states. Our findings shed new light on perspectives
for experimental realization of magnetic states in charge-neutral twisted
bilayer graphene.",2104.10694v3
2021-05-04,A drift kinetic model for the expander region of a magnetic mirror,"We present a drift kinetic model for the free expansion of a thermal plasma
out of a magnetic nozzle. This problem relates to plasma space propulsion
systems, natural environments such as the solar wind, and end losses from the
expander region of mirror magnetically confined fusion concepts such as the Gas
Dynamic Trap. The model incorporates trapped and passing orbit types
encountered in the mirror expander geometry and maps to an upstream thermal
distribution. This boundary condition and quasineutrality require the
generation of an ambipolar potential drop of $\sim5 T_e/e$, forming a thermal
barrier for the electrons. The model for the electron and ion velocity
distributions and fluid moments is confirmed with data from a fully kinetic
simulation. Finally, the model is extended to account for a population of fast
sloshing ions arising from neutral beam heating within a magnetic mirror, again
resulting in good agreement with a corresponding kinetic simulation.",2105.01572v1
2021-05-05,Heavy quark dynamics in a strongly magnetized quark-gluon plasma,"We present a calculation of the heavy quark momentum diffusion coefficients
in a quark-gluon plasma under the presence of a strong external magnetic field,
within the Lowest Landau Level (LLL) approximation. In particular, we apply the
Hard Thermal Loop (HTL) technique for the resummed effective gluon propagator,
generalized for a hot and magnetized medium. Using the derived effective HTL
gluon propagator and the LLL quark propagator we analytically derive the full
results for the longitudinal and transverse momentum diffusion coefficients as
well as the energy losses for charm and bottom quarks beyond the static limit.
We also show numerical results for these coefficients in two special cases
where the heavy quark is moving either parallel or perpendicular to the
external magnetic field.",2105.02167v2
2021-05-07,Magnetic properties of the itinerant ferromagnet LaCrGe3 under pressure studied by 139La NMR,"$^{139}$La nuclear magnetic resonance (NMR) measurements under pressure ($p =
0-2.64$ GPa) have been carried out to investigate the static and dynamic
magnetic properties of the itinerant ferromagnet LaCrGe$_3$. $^{139}$La-NMR
spectra for all measured pressures in the ferromagnetically ordered state show
a large shift due to the internal field induction $|$$B_{\rm int}$$|$ $\sim$ 4
T at the La site produced by Cr ordered moments. The change in $B_{\rm int}$ by
less than 5\% with $p$ up to 2.64~GPa indicates that the Cr 3$d$ moments are
robust under pressure. The temperature dependence of NMR shift and $B_{\rm
int}$ suggest that the ferromagnetic order develops below $\sim$ 50~K under
higher pressures in a magnetic field of $\sim$ 7.2 T. Based on the analysis of
NMR data using the self-consistent-renormalization (SCR) theory, the spin
fluctuations in the paramagnetic state well above $T_{\rm C}$ are revealed to
be three dimensional ferromagnetic throughout the measured $p$ region.",2105.03479v1
2021-05-16,Measurement of magnetic cavitation driven by heat flow in a plasma,"We describe the direct measurement of the expulsion of a magnetic field from
a plasma driven by heat flow. Using a laser to heat a column of gas within an
applied magnetic field, we isolate Nernst advection and show how it changes the
field over a nanosecond timescale. Reconstruction of the magnetic field map
from proton radiographs demonstrates that the field is advected by heat flow in
advance of the plasma expansion. This changes the dynamics of high energy
density plasmas, in which heat flows and fields are strongly coupled, and may
disrupt magnetised inertial confinement fusion schemes.",2105.07414v3
2021-05-20,Rate equations model for multiple magnetic mirrors in various thermodynamic scenarios,"Axial particle loss is one of the main challenges for fusion aimed, linear
magnetic mirror plasma configurations. One way to mitigate this disadvantage
and increase the confinement time is to use a multiple mirrors setup. The idea
is to reduce the outgoing flux by collisions in the outer magnetic cells. Here,
we develop a semi-kinetic rate equation model for the ions' density dynamics,
including scattering within the magnetic cell and the transmission between
neighboring cells. The dominant parameter is the ions' mean free path, which
depends on the temperature and density in each cell. The steady-state flow is
studied analytically and numerically for three thermodynamic scenarios:
isothermal plasma, adiabatic expansion, and constant diffusion. It is found
that the confinement time varies about five-fold over the different scenarios,
where the adiabatic cooling is the best confining scenario.",2105.09614v2
2021-06-04,Topological superconducting domain walls in magnetic Weyl semimetals,"Recent experimental breakthrough in magnetic Weyl semimetals have inspired
exploration on the novel effects of various magnetic structures in these
materials. Here we focus on a domain wall structure which connects two uniform
domains with different magnetization directions. We study the topological
superconducting state in presence of an s-wave superconducting pairing
potential. By tuning the chemical potential, we can reach a topological state,
where a chiral Majorana mode or zero-energy Majorana bound state is localized
at the edges of the domain walls. This property allows a convenient braiding
operation of Majorana modes by controlling the dynamics of domain walls.",2106.02215v1
2021-06-07,Voltage-control of damping constant in magnetic-insulator/topological-insulator bilayers,"The magnetic damping constant is a critical parameter for magnetization
dynamics and the efficiency of memory devices and magnon transport. Therefore,
its manipulation by electric fields is crucial in spintronics. Here, we
theoretically demonstrate the voltage-control of magnetic damping in ferro- and
ferrimagnetic-insulator (FI)/topological-insulator (TI) bilayers. Assuming a
capacitor-like setup, we formulate an effective dissipation torque induced by
spin-charge pumping at the FI/TI interface as a function of an applied voltage.
By using realistic material parameters, we find that the effective damping for
a FI with 10nm thickness can be tuned by one order of magnitude under the
voltage with 0.25V. Also, we provide perspectives on the voltage-induced
modulation of the magnon spin transport on proximity-coupled FIs.",2106.03332v1
2021-06-09,A Compact Model for Scalable MTJ Simulation,"This paper presents a physics-based modeling framework for the analysis and
transient simulation of circuits containing Spin-Transfer Torque (STT) Magnetic
Tunnel Junction (MTJ) devices. The framework provides the tools to analyze the
stochastic behavior of MTJs and to generate Verilog-A compact models for their
simulation in large VLSI designs, addressing the need for an industry-ready
model accounting for real-world reliability and scalability requirements.
Device dynamics are described by the Landau-Lifshitz-Gilbert-Slonczewsky
(s-LLGS ) stochastic magnetization considering Voltage-Controlled Magnetic
Anisotropy (VCMA) and the non-negligible statistical effects caused by thermal
noise. Model behavior is validated against the OOMMF magnetic simulator and its
performance is characterized on a 1-Mb 28 nm Magnetoresistive-RAM (MRAM) memory
product.",2106.04976v1
2021-06-30,Quantum magnetism and topological superconductivity in Yu-Shiba-Rusinov chains,"Chains of magnetic adatoms on superconductors have been discussed as
promising systems for realizing Majorana end states. Here, we show that dilute
Yu-Shiba-Rusinov (YSR) chains are also a versatile platform for quantum
magnetism and correlated electron dynamics, with widely adjustable spin values
and couplings. Focusing on subgap excitations, we derive an extended $t-J$
model for dilute quantum YSR chains and use it to study the phase diagram as
well as tunneling spectra. We explore the implications of quantum magnetism for
the formation of a topological superconducting phase, contrasting it to
existing models assuming classical spin textures.",2107.00031v1
2021-07-11,Hysteretic Magnetoresistance in a Non-Magnetic SrSnO3 Film via Thermal Coupling to Dynamic Substrate Behavior,"Hysteretic magnetoresistance (MR) is often used as a signature of
ferromagnetism in conducting oxide thin films and heterostructures. Here,
magnetotransport is investigated in a non-magnetic uniformly La-doped SrSnO3
film grown using hybrid molecular beam epitaxy. A 12 nm La:SrSnO3/2 nm
SrSnO3/GdScO3 (110) film with insulating behavior exhibited a robust hysteresis
loop in the MR at T < 5 K accompanied by an anomaly at ~ +/- 3 T at T < 2.5 K.
Furthermore, MR with the field in-plane yielded a value exceeded 100% at 1.8 K.
Using detailed temperature-, angle- and magnetic field-dependent resistance
measurements, we illustrate the origin of hysteresis is not due to magnetism in
the film but rather is associated with the magnetocaloric effect of the GdScO3
substrate. Given GdScO3 and similar substrates are commonly used in complex
oxide research, this work highlights the importance of thermal coupling to
processes in the substrates which must be carefully accounted for in the data
interpretation for thin films and heterostructures utilizing these substrates.",2107.04965v1
2021-08-08,Exchange scaling of ultrafast angular momentum transfer in 4$\it{f}$ antiferromagnets,"Ultrafast manipulation of the magnetic state of matter bears great potential
for future information technologies. While demagnetisation in ferromagnets is
governed by dissipation of angular momentum, materials with multiple spin
sublattices, e.g. antiferromagnets, can allow direct angular momentum transfer
between opposing spins, promising faster functionality. In lanthanides,
4$\it{f}$ magnetic exchange is mediated indirectly through the conduction
electrons (the Ruderman-Kittel-Kasuya-Yosida interaction, RKKY), and the effect
of such conditions on direct spin transfer processes is largely unexplored.
Here, we investigate ultrafast magnetization dynamics in 4f antiferromagnets,
and systematically vary the 4$\it{f}$ occupation, thereby altering the
magnitude of RKKY. By combining time-resolved soft x-ray diffraction with
ab-initio calculations, we find that the rate of direct transfer between
opposing moments is directly determined by the magnitude of RKKY. Given the
high sensitivity of RKKY to the conduction electrons, our results offer a novel
approach for fine-tuning the speed of magnetic devices.",2108.03714v1
2021-08-10,Metallic nature and site-selective magnetic collapse in iron oxide Fe4O5 at the extreme conditions of Earth's deep interior,"Properties of iron oxides at the extreme conditions are of essential
importance in condensed matter physics and Geophysics. The recent discovery of
a new type of iron oxide, Fe4O5, at high pressure and high temperature of
Earth's deep interior attracts great interests. In this paper, we report the
electronic structure and the magnetic properties of Fe4O5 predicted by the
density functional theory plus dynamic mean field theory (DFT+DMFT) approach.
We find that Fe4O5 stays metallic from ambient pressure to high pressure. The
magnetic moments of iron atoms at the three different crystallographic
positions of Fe4O5 undergo position-dependent collapse as being compressed.
Such site-selective magnetic moment collapse originates from the shift of
energy levels and the consequent charge transfer among the Fe-3d orbits under
compression.",2108.04477v1
2021-08-11,Q Dependence of Magnetic Resonance Mode on FeTe$_{0.5}$Se$_{0.5}$ Studied by Inelastic Neutron Scattering,"Inelastic neutron scattering measurements have been performed on a
superconducting single crystal FeTe$_{0.5}$Se$_{0.5}$ to examine the ${\bf
Q}$-dependent enhancement of the dynamical structure factor, $S({\bf Q},E)$,
from ${\bf Q}$ = (0, 0) to ($\pi$, $\pi$), including ($\pi$, 0) in the
superconducting state. In most of iron-based superconductors, $S({\bf Q},E)$ is
enhanced at ${\bf Q}$ = ($\pi$, 0), where the ""magnetic resonance mode"" is
commonly observed in the unfolded Brillouin zone. Constant-$E$ cuts of $S({\bf
Q},E)$ suggest that the enhancement is not uniform in the magnetic excitation,
and limited around ${\bf Q}$ = ($\pi$, 0). This result is consistent with the
theoretical simulation of the magnetic resonance mode due to the
Bardeen$-$Cooper$-$Schrieffer coherence factor with the sign-reversing order
parameter of s$_{\pm}$ wave.",2108.05051v1
2021-08-11,Holographic Schwinger effect in the dynamical AdS/QCD model,"In this paper, we discuss the potential analysis of the holographic Schwinger
effect in the bottom up AdS/QCD model. We study the effect of the magnetic
field on the critical field and total potential in finite chemical potential
case. By evaluating the critical electric field from the DBI action, one can
observe that magnetic field decreases critical electric field Ec. From the
results of potential analysis, we find the magnetic field reduces the potential
barrier and favor the Schwinger effect which agrees with the results of the
critical electric field. Moreover, the Schwinger effect is more obvious when
pairs are parallel to the magnetic field than that in perpendicular case in
this Einstein-Maxwell-dilaton model.",2108.05148v1
2021-08-12,Flexibility and rigidity of free boundary MHD equilibria,"We study stationary free boundary configurations of an ideal incompressible
magnetohydrodynamic fluid possessing nested flux surfaces. In 2D simply
connected domains, we prove that if the magnetic field and velocity field are
never commensurate, the only possible domain for any such equilibria is a disk,
and the velocity and magnetic field are circular. We give examples of
non-symmetric equilibria occupying a domain of any shape by imposing an
external magnetic field generated by a singular current sheet charge
distribution (external coils). Some results carry over to 3D axisymmetric
solutions. These results highlight the importance of external magnetic fields
for the existence of asymmetric equilibria.",2108.05977v2
2021-08-16,Frustration-induced diffusive scattering anomaly and dimension change in $\rm FeGe_2$,"Magnetic frustration, arising from the competition of exchange interactions,
has received great attention because of its relevance to exotic quantum
phenomena in materials. In the current work, we report an unusual
checkerboard-shaped scattering anomaly in $\rm FeGe_2$, far from the known
incommensurate magnetic satellite peaks, for the first time by inelastic
neutron scattering. More surprisingly, such phenomenon appears as spin dynamics
at low temperature, but it becomes prominent above N\'eel transition as elastic
scattering. A new model Hamiltonian that includes an intraplane next-nearest
neighbor was proposed and attributes such anomaly to the near-perfect magnetic
frustration and the emergence of unexpected two-dimensional magnetic order in
the quasi-one-dimensional $\rm FeGe_2$.",2108.06882v2
2021-08-21,Electron dynamics in noncommutative geometry with magnetic field and Zitterbewegung phenomenon,"Starting from a gauge invariant Dirac Hamiltonian with noncommutativity of
space sector in the presence of an external uniform magnetic field, the
resulting Dirac equation has been solved for electrons and its corresponding
zitterbewegung (ZBW) phenomenon has been studied. The corresponding energy
spectrum is shown to be different from previous studies wherein the non-gauge
invariant Dirac Hamiltonian has been used. The effects of noncommutativity
alter the amplitude as well as the cyclotron and the ZBW frequencies of the
average velocity of charge carriers. This result is contrary to previous
studies wherein there was no magnetic field and hence, neither the amplitude
nor the frequency of the motion was affected. Moreover, all of the ZBW
frequencies of the Landau energy-levels appear in the results. Also, in weak
magnetic fields, we have calculated the average velocity of charge carriers for
two initially localized spin-up and spin-down cases. The plotted trajectories
reveal difference between these two cases. In addition, the ZBW phenomenon has
been shown that manifests itself as a circular motion whose direction is spin
dependent while accompanied by the cyclotron motion.",2108.09557v1
2021-08-22,Nonlinear and Thermal Effects in the Absorption of Microwaves by Random Magnets,"Abstract We study the temperature dependence of the absorption of microwaves
by random-anisotropy magnets. It is governed by strong metastability due to the
broad distribution of energy barriers separating different spin configurations.
At a low microwave power, when the heating is negligible, the spin dynamics is
close to linear. It corresponds to the precession of ferromagnetically ordered
regions that are in resonance with the microwave field. Previously we have
shown (doi.org/10.1103/PhysRevB.103.214414) that in this regime a dielectric
substance packed with random magnets would be a strong microwave absorber in a
broad frequency range. Here we demonstrate that on increasing the power,
heating and over barrier spin transitions come into play, resulting in the
nonlinear behavior. At elevated temperatures the absorption of microwave power
decreases dramatically, making the dielectric substance with random magnets
transparent for the microwaves.",2108.09804v1
2021-09-02,Magnetic Field Dependent Piezoelectricity in Atomically Thin Co$_2$Te$_3$,"Two dimensional (2D) materials have received a surge in research interest due
to their exciting range of properties. Here we show that 2D cobalt telluride
(Co2Te3), successfully synthesized via liquid-phase exfoliation in an organic
solvent, exhibits weak ferromagnetism and semiconducting behavior at room
temperature. The magnetic field-dependent piezoelectric properties of 2D Co2Te3
sample show magneto-electric response of the material and a linear relationship
between the output voltage and applied magnetic field. First-principles density
functional theory (DFT) and ab initio molecular dynamics are used to explain
these experimental results. Our work could pave the way for the development of
2D materials with coupled magnetism and piezoelectricity, leading to new
applications in electromagnetics.",2109.02781v1
2021-09-14,Infinite critical boson non-Fermi liquid on heterostructure interfaces,"We study the emergence of non-Fermi liquid on heterostructure interfaces
where there exists an infinite number of critical boson modes accounting for
the magnetic fluctuations in two spatial dimensions. The interfacial
Dzyaloshinskii-Moriya interaction naturally arises in magnetic interactions due
to the absence of inversion symmetry, resulting in a degenerate contour for the
low-energy bosonic modes in the momentum space which simultaneously becomes
critical near the magnetic phase transition. The itinerant electrons are
scattered by the critical boson contour via the Yukawa coupling. When the boson
contour is much smaller than the Fermi surface, it is shown that, there exists
a regime with a dynamic critical exponent z=3 while the boson contour still
controls the low-energy magnetic fluctuations. Using a self-consistent
renormalization calculation for this regime, we uncover a prominent non-Fermi
liquid behavior in the resistivity with a characteristic temperature scaling
power. These findings open up new avenues for understanding boson-fermion
interactions and the novel fermionic quantum criticality.",2109.06594v2
2021-09-28,Direct numerical simulations of optimal thermal convection in rotating plane layer dynamos,"The heat transfer behavior of convection-driven dynamos in a rotating plane
layer between two parallel plates, heated from below and cooled from the top,
is investigated. At a fixed rotation rate (Ekman Number, $E=10^{-6}$) and fluid
properties (thermal and magnetic Prandtl numbers, $Pr=Pr_m=1$), both dynamo
convection (DC) and non-magnetic rotating convection (RC) simulations are
performed to demarcate the effect of magnetic field on heat transport at
different thermal forcings (Rayleigh Number,
$Ra=3.83\times10^{9}-3.83\times10^{10}$). In this range, our turbulence
resolving simulations demonstrate the existence of an optimum thermal forcing,
at which heat transfer between the plates in DC exhibits maximum enhancement,
as compared to the heat transport in the RC simulations. Unlike any global
force balance reported in the literature, the present simulations reveal an
increase in the Lorentz force in the \textit{thermal boundary layer}, due to
stretching of magnetic field line by the vortices near the walls with no-slip
boundary condition. This increase in Lorentz force mitigates turbulence
suppression owing to the Coriolis force, resulting in enhanced turbulence and
heat transfer",2109.13455v2
2021-10-05,Probing modified gravity with magnetically levitated resonators,"We present an experimental procedure, based on Meissner effect levitation of
neodymium ferromagnets, as a method of measuring the gravitational interactions
between mg masses. The scheme consists of two superconducting lead traps, with
a magnet levitating in each trap. The levitating magnets behave as harmonic
oscillators, and by carefully driving the motion of one magnet on resonance
with the other, we find that it should be easily possible to measure the
gravitational field produced by a 4~mg sphere, with the gravitational
attraction from masses as small as 30~$\mu$g predicted to be measurable within
realistic a realistic measurement time frame. We apply this acceleration
sensitivity to one concrete example and show the ability of testing models of
modified Newtonian dynamics.",2110.02263v1
2021-10-21,Micromagnetic simulations of clusters of nanoparticles with internal structure: Application to magnetic hyperthermia,"Micromagnetic simulation results on dynamic hysteresis loops of clusters of
iron oxide nanoparticles (NPs) with internal structure composed of nanorods are
compared with the widely used macrospin approximation. Such calculations
allowing for nanorod-composed NPs is facilitated by a previously developed
coarse-graining method based on the renormalization group approach. With a
focus on applications to magnetic hyperthermia, we show that magnetostatic
interactions improve the heating performance of NPs in chains and triangles,
and reduce heating performance in fcc arrangements. Hysteresis loops of
triangular and fcc systems of complex NPs are not recovered within the
macrospin approximation, especially at smaller interparticle distances. For
triangular arrangements, the macrospin approximation predicts that
magnetostatic interactions reduce loop area, in contrast to the complex NP
case. An investigation of the local hysteresis loops of individual NPs and
macrospins in clusters reveals the impact of the geometry of their neighbours
on individual versus collective magnetic response, inhomogenous heating within
clusters, and further differences between simulating NPs with internal
structure and the use of the macrospin approximation. Capturing the internal
physical and magnetic structure of NPs is thus important for some applications.",2110.11178v2
2021-11-03,Lattice QED in external electromagnetic fields,"We study QED in external electromagnetic fields using methods developed for
simulating lattice QCD. Our first project is to simulate QED in a constant (in
space and time) external magnetic field on a euclidean space-time lattice using
the Rational Hybrid Monte Carlo (RHMC) method. Observables we measure include
the condensate $\langle\bar{\psi}\psi\rangle$ and the effective electron action
after integrating out the fermion fields. We look for evidence that the
combined effect of the magnetic field and the electron-positron attraction from
QED produces a non-zero condensate in the limit of zero electron mass, a
non-perturbative effect analogous to spontaneous chiral symmetry breaking. Very
preliminary evidence is that such a condensate exists, at least for strong
external magnetic fields and unphysically large electric charge. In addition,
we are storing field configurations to measure the expected distortions and
screenings of the coulomb field of a charged particle due to the vacuum
polarization asymmetries produced by the magnetic field. We hope also to
measure the dynamical contribution to the electron mass produced by the same
mechanism that produces a finite condensate in the zero input mass limit.",2111.01990v1
2021-11-15,Model-free approach to the interpretation of restricted and anisotropic self-diffusion in magnetic resonance of biological tissues,"Magnetic resonance imaging (MRI) is the method of choice for noninvasive
studies of micrometer-scale structures in biological tissues via their effects
on the time/frequency-dependent (""restricted"") and anisotropic self-diffusion
of water. Traditional MRI relies on pulsed magnetic field gradients to encode
the signal with information about translational motion in the direction of the
gradient, which convolves fundamentally different aspects-such as bulk
diffusivity, restriction, anisotropy, and flow-into a single effective
observable lacking specificity to distinguish between biologically plausible
microstructural scenarios. To overcome this limitation, we introduce a formal
analogy between measuring rotational correlation functions and interaction
tensor anisotropies in nuclear magnetic resonance (NMR) spectroscopy and
investigating translational motion in MRI, which we utilize to convert data
acquisition and analysis strategies from NMR of rotational dynamics in
macromolecules to MRI of diffusion in biological tissues, yielding
model-independent quantitative metrics reporting on relevant microstructural
properties with unprecedented specificity. Our model-free approach advances the
state-of-the-art in microstructural MRI, thereby enabling new applications to
complex multi-component tissues prevalent in both tumors and healthy brain.",2111.07827v1
2021-11-16,Helicity transfer in strong laser fields via the electron anomalous magnetic moment,"Electron beam longitudinal polarization during the interaction with
counterpropagating circularly-polarized ultraintense laser pulses is
investigated, while accounting for the anomalous magnetic moment of the
electron. Although it is known that the helicity transfer from the laser
photons to the electron beam is suppressed in linear and nonlinear Compton
scattering processes, we show that the helicity transfer nevertheless can
happen via an intermediate step of the electron radiative transverse
polarization, phase-matched with the driving field, followed up by spin
rotation into the longitudinal direction as induced by the anomalous magnetic
moment of the electron. With spin-resolved QED Monte Carlo simulations, we
demonstrate the consequent helicity transfer from laser photons to the electron
beam with a degree up to 10%, along with an electron radial polarization up to
65% after multiple photon emissions in a femtosecond timescale. This effect is
detectable with currently achievable laser facilities, evidencing the role of
the leading QED vertex correction to the electron anomalous magnetic moment in
the polarization dynamics in ultrastrong laser fields.",2111.08167v2
2021-11-24,Local magnetic moment formation and Kondo screening in the half filled two dimensional single band Hubbard model,"We study formation of local magnetic moments in strongly correlated Hubbard
model within dynamical mean field theory and associate peculiarities of
temperature dependence of local charge $\chi_c$ and spin $\chi_s$
susceptibilities with different stages of local moment formation. Local maximum
of temperature dependence of the charge susceptibility $\chi_c$ is associated
with beginning of local magnetic moments formation, while the minimum of the
susceptibility $\chi_c$ and double occupation, as well as low temperature
boundary of the plateau of effective local magnetic moment $\mu_{\rm
eff}^2=T\chi_s$ temperature dependence are connected with full formation of
local moments. We also obtain interaction dependence of the Kondo temperature
$T_K$, which is compared to the fingerprint criterion of Phys. Rev. Lett. 126,
056403 (2021). Near the Mott transition the two criteria coincide, while
further away from Mott transition the fingerprint criterion somewhat
overestimates Kondo temperature. The relation of the observed features to the
behavior of eigenvectors/eigenvalues of fermionic frequency-resolved charge
susceptibility and divergences of irreducible vertices is discussed.",2111.12441v2
2021-11-28,Parametrically driven THz magnon-pairs: predictions towards ultimately fast and minimally dissipative switching,"Findings ways to achieve switching between magnetic states at the fastest
possible time scale that simultaneously dissipates the least amount of energy
is one of the main challenges in magnetism. Antiferromagnets exhibit intrinsic
dynamics in the THz regime, the highest among all magnets and are therefore
ideal candidates to address this energy-time dilemma. Here we study
theoretically THz-driven parametric excitation of antiferromagnetic
magnon-pairs at the edge of the Brillouin zone and explore the potential for
switching between two stable oscillation states. Using a semi-classical theory,
we predict that switching can occur at the femtosecond time scale with an
energy dissipation down to a few zepto Joule. This result touches the
thermodynamical bound of the Landauer principle, and approaches the quantum
speed limit up to 5 orders of magnitude closer than demonstrated with magnetic
systems so far.",2111.14202v1
2021-11-29,Charging up Boosted Black Holes,"Contrary to a prevailing assumption that black holes would swiftly discharge,
we argue that black holes can charge preferentially when boosted through an
ambient magnetic field. Though the details are very different, the preference
for charge is related to the precipitation of the Wald charge on a spinning
black hole in an ambient magnetic field. The gravito-electrodynamics upstage
naive arguments about screening electric fields in determining the value of the
charge accrued. Charged test particles, which build up the black hole charge,
exhibit chaotic behavior as evidenced by fractal basin boundaries between
dynamical regions. Charged, boosted black holes will generate their own
electromagnetic fields and thereby their own luminous signatures, even if they
are initially bare. We therefore add boosted black holes to the growing list of
potentially observable black hole signatures, alongside black hole batteries
and black hole pulsars. The implications should be relevant for supermassive
black holes that are boosted relative to a galactic magnetic field as well as
black holes merging with magnetized neutron stars.",2111.15027v3
2021-12-02,Equation of State of dense QCD in external magnetic field,"In this proceeding we present our first results of the study of the QCD
Equation of State at non-zero baryon density and in external magnetic field. We
focused on the first three non-vanishing expansion coefficients of pressure in
chemical potential and their dependence on magnetic field. The study is carried
out within lattice simulations with $N_f=2+1$ dynamical quarks with physical
quark masses. To overcome the sign problem, the simulations are carried out at
imaginary baryon chemical potential. Our results suggest that external magnetic
field considerably enhances the expansion coefficients and modifies their
dependence on temperature.",2112.01032v3
2021-12-02,Antiferromagnetic spin pumping via hyperfine interaction,"Spin pumping is an interfacial spin current generation from the ferromagnetic
layer to the non-magnetic metal at its interface. The polarization of the
pumped spin current $\textbf{J}_s \propto \textbf{m}\times \dot{\textbf{m}}$
depends on the dynamics of the magnetic moment $\textbf{m}$. When the materials
are based on light transition metals, mechanism behind the spin current
transfer is dominated by the exchange interaction between spin of localized
d-electrons and itinerant conduction electrons. In heavier transition metals,
however, the interaction is not limited to the exchange interaction. The spin
of the conduction electron can interact to its nuclear spin by means of
hyperfine interaction, as observed in the shift of NMR frequency. By studying
the spin polarization of conduction electron of the non-magnetic metallic layer
due to a nuclear magnetic moment $\textbf{I}$ of the ferromagnetic layer, we
show that the hyperfine interaction can mediate the spin pumping. The
polarization of the spin current generation is shown to have a similar form
$J_s\propto \textbf{I}\times\dot{\textbf{I}}$.",2112.01111v1
2021-12-10,Magnetic energy spectrum produced by turbulent dynamo: effect of time irreversibility,"We consider the kinematic stage of evolution of magnetic field advected by
turbulent hydrodynamic flow. We use a generalization of the Kazantsev-Kraichnan
model to investigate time irreversible flows. In the viscous range of scales,
the infinite-time limit of the spectrum is a power law but its slope is more
flat than that predicted by Kazantsev model. This result agrees with numerical
simulations. The rate of magnetic energy growth is slower than that in the
time-symmetric case. We show that for high magnetic Prandtl turbulent plasma,
the formation of the power-law spectrum shape takes very long time and may
never happen because of the nonlinearity. We propose another ansatz to describe
the spectrum shape at finite time.",2112.05736v2
2021-12-16,Decoupling of Spin Decoherence Paths near Zero Magnetic Field,"We demonstrate a method to quantify and manipulate nuclear spin decoherence
mechanisms that are active in zero to ultralow magnetic fields. These include:
(i) non-adiabatic switching of spin quantization axis, due to residual
background fields; (ii) scalar pathways due to through-bond couplings between
$^1$H and heteronuclear spin species, such as $^2$H used partially as an
isotopic substitute for $^1$H. Under conditions of free evolution, scalar
relaxation due to $^2$H can significantly limit nuclear spin polarization
lifetimes and thus the scope of magnetic resonance procedures near zero field.
It is shown that robust trains of pulsed dc magnetic fields that apply $\pi$
flip angles to one or multiple spin species may switch effective symmetry of
the nuclear spin Hamiltonian, imposing decoupled or coupled dynamic regimes on
demand. The method should broaden the spectrum of hyperpolarized biomedical
contrast-agent compounds and hyperpolarization procedures that are used near
zero field.",2112.08536v1
2021-12-30,Identification of active magnetic reconnection using magnetic flux transport in plasma turbulence,"Magnetic reconnection has been suggested to play an important role in the
dynamics and energetics of plasma turbulence by spacecraft observations,
simulations and theory over the past two decades, and recently, by
magnetosheath observations of MMS. A new method based on magnetic flux
transport (MFT) has been developed to identify reconnection activity in
turbulent plasmas. This method is applied to a gyrokinetic simulation of
two-dimensional (2D) plasma turbulence. Results on the identification of three
active reconnection X-points are reported. The first two X-points have
developed bi-directional electron outflow jets. Beyond the category of
electron-only reconnection, the third X-point does not have bi-directional
electron outflow jets because the flow is modified by turbulence. In all cases,
this method successfully identifies active reconnection through clear inward
and outward flux transport around the X-points. This transport pattern defines
reconnection and produces a new quadrupolar structure in the divergence of MFT.
This method is expected to be applicable to spacecraft missions such as MMS,
Parker Solar Probe, and Solar Orbiter.",2112.14878v1
2022-01-17,Synthetic Hall ladder with tunable magnetic flux,"We describe a synthetic three-leg Hall ladder system with a tunable magnetic
flux for neutral $^{173}$Yb atoms in a one-dimensional optical lattice. The
ladder legs are formed by three hyperfine ground spin states of the atoms, and
the complex interleg links are generated through Raman couplings between the
spin states using multiple laser beams. The effective magnetic flux through a
ladder plaquette, $\phi$, is controlled by the angles of the Raman laser beams
with the lattice axis. We investigate the quench dynamics of the Hall ladder
system for $\phi\approx\frac{\pi}{3}, \frac{\pi}{2},$ and $\frac{2\pi}{3}$
after a sudden application of the Raman coupling in various interleg link
configurations. The semi-classical trajectory of the atoms in the plane of the
spin composition and lattice position exhibits the characteristic motion for
the effective magnetic field. In a tube configuration with the three legs
cyclically linked, the quench evolution was observed to be substantially
damped, which is attributed to the random flux threading the Hall tube.",2201.06462v1
2022-01-27,Ab-initio study of the electronic structure and magnetic properties of Ce$_2$Fe$_{17}$,"The Ce$_2$Fe$_{17}$ intermetallic compound has been studied intensely for
several decades; its low-temperature state is reported experimentally either as
ferromagnetic or antiferromagnetic by different authors, with a measured
ordering temperature ranging within a hundred Kelvin. The existing theoretical
investigations overestimate the experimental total magnetic moment of
Ce$_2$Fe$_{17}$ by 20-40 % and predict a ferromagnetic ground state. By means
of first-principle electronic structure calculations, we show that the total
magnetic moment of Ce$_2$Fe$_{17}$ can be reproduced within the Local Density
Approximation while functionals based on the Generalized Gradient Approximation
fail. Atomistic spin dynamics simulations are shown to capture the change in
the magnetic state of Ce$_2$Fe$_{17}$ with temperature, and closely replicate
the reported helical structure that appears in some of the experimental
investigations.",2201.11573v1
2022-02-06,Longitudinally Modulated Dynamo Action in Simulated M-Dwarf Stars,"M-dwarf stars are well known for the intense magnetic activity that many of
them exhibit. In cool stars with near-surface convection zones, this magnetic
activity is thought to be driven largely by the interplay of convection and the
large scale differential rotation and circulations it establishes. The highly
nonlinear nature of these flows yields a fascinatingly sensitive and diverse
parameter space, with a wide range of possible dynamics. We report here on a
set of three global MHD simulations of rapidly rotating M2 (0.4 $M_\odot$)
stars. Each of these three models established nests of vigorous convection that
were highly modulated in longitude at low latitudes. Slight differences in
their magnetic parameters led each model to disparate dynamo states, but the
effect of the convective nest was a unifying feature. In each case, the action
of longitudinally modulated convection led to localized (and in one case,
global) reversals of the toroidal magnetic field, as well as the formation of
an active longitude, with enhanced poloidal field amplitudes and flux
emergence.",2202.02869v1
2022-02-23,Ferroelectric control of magnetic skyrmions in two-dimensional van der Waals heterostructures,"Magnetic skyrmions are chiral nanoscale spin textures which are usually
induced by Dzyaloshinskii-Moriya interaction (DMI). Recently, magnetic
skyrmions have been observed in two-dimensional (2D) van der Waals (vdW)
ferromagnetic materials, such as Fe$_{3}$GeTe$_{2}$. The electric control of
skyrmions is important for their potential application in low-power memory
technologies. Here, we predict that DMI and magnetic skyrmions in a
Fe$_{3}$GeTe$_{2}$ monolayer can be controlled by ferroelectric polarization of
an adjacent 2D vdW ferroelectric In$_{2}$Se$_{3}$. Based on density functional
theory and atomistic spin-dynamics modeling, we find that the interfacial
symmetry breaking produces a sizable DMI in a
Fe$_{3}$GeTe$_{2}$/In$_{2}$Se$_{3}$ vdW heterostructure. We show that the
magnitude of DMI can be controlled by ferroe-lectric polarization reversal,
leading to creation and annihilation of skyrmions. Furthermore, we find that
the sign of DMI in a In$_{2}$Se$_{3}$/Fe$_{3}$GeTe$_{2}$/In$_{2}$Se$_{3}$
heterostructure changes with ferroelectric switching reversing the skyrmion
chirality. The predicted electrically controlled skyrmion formation may be
interesting for spintronic applications.",2202.11348v2
2022-03-07,A dynamo simulation generating Saturn-like small magnetic dipole tilts,"Among planetary dynamos, the magnetic field of Saturn stands out in its
exceptional level of axisymmetry. One of its peculiar features is that the
magnetic dipole mode is tilted with respect to the planetary rotation axis by
only $\approx 0.007^{\circ}$ or less. Numerical dynamo simulations performed in
this context have had great difficulty in producing such small dipole tilt
angles without introducing ad hoc ingredients such as a latitudinally varying
heat flux pattern in the outer layers or stably stratified layers (SSL). Here
we present a numerical dynamo simulation that generates a highly axisymmetric
dynamo with a dipole tilt of about $\approx 0.0008^{\circ}$ on average. The
model consists of a deep dynamo layer and an overlying low-conductivity layer
but without any SSL. We highlight a novel mechanism where strong differential
rotation generated in the atmospheric layer penetrates into the dynamo region,
helping to maintain a very small magnetic dipole tilt.",2203.03520v1
2022-03-07,Cosmic-void observations reconciled with primordial magnetogenesis,"It has been suggested that the weak magnetic field hosted by the
intergalactic medium in cosmic voids could be a relic from the early Universe.
However, accepted models of turbulent magnetohydrodynamic decay predict that
the present-day strength of fields originally generated at the electroweak
phase transition (EWPT) without parity violation would be too low to explain
the observed scattering of $\gamma$-rays from TeV blazars. Here, we propose
that the decay is mediated by magnetic reconnection and conserves the mean
square fluctuation level of magnetic helicity. We find that the relic fields
would be stronger by several orders of magnitude under this theory than was
indicated by previous treatments, which restores the consistency of the
EWPT-relic hypothesis with the observational constraints. Moreover, efficient
EWPT magnetogenesis would produce relics at the strength required to resolve
the Hubble tension via magnetic effects at recombination and seed
galaxy-cluster fields close to their present-day strength.",2203.03573v3
2022-03-08,Accretion of the magnetized neutrino-cooled torus,"Neutrino-cooled accretion flow around a black hole, produced by a compact
binary merger, is a promising scenario for jet formation and magnetic-driven
winds to explain short duration gamma ray bursts (GRBs) central engine and
kilonovae based on GW170817 gravitational wave observation. Magnetorotational
instability (MRI) turbulence and Blandford-Znajek (BZ) mechanism are expected
to play key roles in the thermal equilibrium of the disk (balancing neutrino
cooling) and in driving accretion and creating jets. Using the open-source
GRMHD HARM-COOL code, we study the magnetically-driven evolution of an
accretion disk with realistic equation of state in the fixed curved space-time
background. We identify the effects of the neutrino cooling and the magnetic
field, paying particular attention to the dynamical, thermal and composition
evolution of the disk and outflows.",2203.04344v1
2022-03-10,Dynamics of the collapse of a ferromagnetic skyrmion in a centrosymmetric lattice,"Time dependence of the size and chirality of a ferromagnetic skyrmion in a
Heisenberg model with the magnetic field on a square lattice has been studied
analytically and numerically. The lattice and the magnetic field generate
strong time dependence of the skyrmion chirality. Due to nonlinearity, the
lattice alone also generates strong intrinsic damping that leads to the
skyrmion collapse via the emission of spin waves. In the absence of the
magnetic field the collapse is slow for a large skyrmion but it becomes
exponentially fast in the presence of the Landau-Lifshitz damping when the
field is turned on. Magnons emitted by a collapsing skyrmion must have a
discrete spectrum due to the quantization of the skyrmion magnetic moment.",2203.05342v1
2022-03-25,Magnetic domain walls of the van der Waals material Fe$_3$GeTe$_2$,"Among two-dimensional materials, Fe$_3$GeTe$_2$ has come to occupy a very
important place owing to its ferromagnetic nature with one of the highest Curie
temperatures among known van der Waals materials and the potential for hosting
skyrmions. In this combined experimental and theoretical work, we investigate
the magnetic bubble domains as well as the microscopic domain wall profile
using spin-polarized scanning tunneling microscopy in combination with
atomistic spin-dynamics simulations performed with parameters from density
functional theory calculations. We find a weak magneto-electric effect
influencing the domain wall width by the electric field in the tunneling
junction and determine the critical magnetic field for the collapse of the
bubble domains. Our findings shed light on the origins of complex magnetism
that Fe$_3$GeTe$_2$ exhibits.",2203.13562v1
2022-05-18,Two ultracold highly magnetic atoms in a one-dimensional harmonic trap,"We theoretically investigate the properties of two interacting ultracold
highly magnetic atoms trapped in a one-dimensional harmonic potential. The
atoms interact via an anisotropic long-range dipole-dipole interaction, which
in one dimension effectively can be modeled by the contact interaction. We
investigate the interplay of the external magnetic field, the spin-spin
interaction, and the trapping potential and how they affect the magnetization
of the system. We show the role of indistinguishability and symmetries in the
dynamics by studying the time evolution of the observables that could be
measured experimentally. The presented model may depict the on-site interaction
of the extended Hubbard models, therefore giving a better understanding of the
fundamental building block of the respective many-body quantum simulators.",2205.08965v3
2022-05-30,Self-biased SAW Magnetic Field Sensors Based on Angle Dependent Magneto-acoustic Coupling,"Surface-acoustic-wave (SAW) based devices have emerged as a promising
technology in magnetic field sensing by integrating a magnetostrictive layer
with the giant {\Delta}E/{\Delta}G effect. However, almost all SAW magnetic
field sensors require a bias field to obtain high sensitivity. In addition, the
true nature of magneto-acoustic coupling still presents a major challenge in
understanding and designing of this kind of devices. In current work, a dynamic
magnetoelastic model for the {\Delta}E/{\Delta}G effect is established in
consideration of the important role of the dipole-dipole interaction. The model
is also implemented into a FEM software to calculate the resonance frequency
responses of multiple fabricated sensors with different {\psi} angles between
of the acoustic wave vector and the induced uniaxial magnetic anisotropy. The
measured results are in excellent agreement with the simulated ones. A strong
resonance frequency sensitivity (RFS) of 630.4 kHz/Oe was achieved at zero bias
field for the device with optimized {\psi} angle. Furthermore, the RFS
measurements along different directions verify its vector-sensing capability.",2205.14933v2
2022-05-31,TIC: A Stokes inversion code for scattering polarization with partial frequency redistribution and arbitrary magnetic fields,"We present the Tenerife Inversion Code (TIC), which has been developed to
infer the magnetic and plasma properties of the solar chromosphere and
transition region via full-Stokes inversion of polarized spectral lines. The
code is based on the HanleRT forward engine, which takes into account many of
the physical mechanisms that are critical for a proper modeling of the Stokes
profiles of spectral lines originating in the tenuous and highly dynamic
plasmas of the chromosphere and transition region: quantum level population
imbalance and interference (atomic polarization), frequency coherence effects
in polarized resonance scattering (partial frequency redistribution), and the
impact of arbitrary magnetic fields on the atomic polarization and the
radiation field. We present first results of atmospheric and magnetic
inversions, and discuss future developments for the project.",2205.15666v1
2022-06-02,Skyrmion formation in Ni-based Janus dihalide monolayers: Interplay between magnetic frustration and Dzyaloshinskii-Moriya interaction,"We present a comprehensive theory of the magnetic phases in monolayer
nickel-based Janus dihalides (NiIBr, NiICl, NiBrCl) through a combination of
first-principles calculations and atomistic simulations. Phonon band structure
calculations and finite temperature molecular dynamics simulations show that
Ni-dihalide Janus monolayers are stable. The parameters of the interacting spin
Hamiltonian extracted from ab initio calculations show that nickel-dihalide
Janus monolayers exhibit varying degrees of magnetic frustration together with
a large Dzyaloshinskii-Moriya interaction due to their inherent inversion
symmetry breaking. The atomistic simulations reveal a delicate interplay
between these two competing magnetic interactions in giving rise to skyrmion
formation.",2206.01325v1
2022-06-19,Enhancement of spin mixing conductance by $s$-$d$ orbital hybridization in heavy metals,"In a magnetic multilayer, the spin transfer between localized magnetization
dynamics and itinerant conduction spin arises from the interaction between a
normal metal and an adjacent ferromagnetic layer. The spin-mixing conductance
then governs the spin-transfer torques and spin pumping at the magnetic
interface. Theoretical description of spin-mixing conductance at the magnetic
interface often employs a single conduction-band model. However, there is
orbital hybridization between conduction $s$ electron and localized $d$
electron of the heavy transition metal, in which the single conduction-band
model is insufficient to describe the $s$-$d$ orbital hybridization. In this
work, using the generalized Anderson model, we estimate the spin-mixing
conductance that arises from the $s$-$d$ orbital hybridization. We find that
the orbital hybridization increases the magnitude of the spin-mixing
conductance.",2206.09347v2
2022-06-27,Quantitative and realistic description of the magnetic potential energy of spin-torque vortex oscillators,"Understanding the dynamics of magnetic vortices has emerged as an important
challenge regarding the recent development of spin-torque vortex oscillators.
Either micromagnetic simulations or the analytical Thiele equation approach are
typically used to study such systems theoretically. This work focuses on the
precise description of the restoring forces exerted on the vortex when it is
displaced from equilibrium. In particular, the stiffness parameters related to
a modification of the magnetic potential energy terms are investigated. A
method is proposed to extract exchange, magnetostatic and Zeeman stiffness
expressions from micromagnetic simulations. These expressions are then compared
to state-of-the-art analytical derivations. Furthermore, it is shown that the
stiffness parameters depend not only on the vortex core position but also on
the injected current density. This phenomenon is not predicted by commonly used
analytical ans\""atze. We show that these findings result from a deformation of
the theoretical magnetic texture caused by the current induced Amp\`ere-Oersted
field.",2206.13438v2
2022-06-28,Spin-down by dynamo action in simulated radiative stellar layers,"The evolution of a star is influenced by its internal rotation dynamics
through transport and mixing mechanisms, which are poorly understood. Magnetic
fields can play a role in transporting angular momentum and chemical elements,
but the origin of magnetism in radiative stellar layers is unclear. Using
global numerical simulations, we identify a subcritical transition to
turbulence due to the generation of a magnetic dynamo. Our results have many of
the properties of the theoretically-proposed Tayler-Spruit dynamo mechanism,
which strongly enhances transport of angular momentum in radiative zones. It
generates deep toroidal fields that are screened by the stellar outer layers.
This mechanism could produce strong magnetic fields inside radiative stars,
without an observable field on their surface.",2206.13819v2
2022-06-28,Phase structure of electroweak vacuum in a strong magnetic field: the lattice results,"Using first-principle lattice simulations, we demonstrate that in the
background of a strong magnetic field (around $10^{20}$ T), the electroweak
sector of the vacuum experiences two consecutive crossover transitions
associated with dramatic changes in the zero-temperature dynamics of the vector
$W$ bosons and the scalar Higgs particles, respectively. Above the first
crossover, we observe the appearance of large, inhomogeneous structures
consistent with a classical picture of the formation of $W$ and $Z$ condensates
pierced by vortices. The presence of the $W$ and $Z$ condensates supports the
emergence of the exotic superconducting and superfluid properties induced by a
strong magnetic field in the vacuum. We find evidence that the vortices form a
disordered solid or a liquid rather than a crystal. The second transition
restores the electroweak symmetry. Such conditions can be realized in the
near-horizon region of the magnetized black holes.",2206.14008v2
2022-07-08,Light-induced magnetization driven by interorbital charge motion in a spin-orbit assisted Mott insulator alpha-RuCl3,"In a honeycomb-lattice spin-orbit assisted Mott insulator {\alpha}-RuCl3, an
ultrafast magnetization is induced by circularly polarized excitation below the
Mott gap. Photo-carriers play an important role, which are generated by turning
down the synergy of the on-site Coulomb interaction and the spin-orbit
interaction realizing the insulator state. An ultrafast 6- fs measurement of
photo-carrier dynamics and a quantum mechanical analysis clarify the mechanism,
according to which the magnetization emerges from a coherent charge motion
between different t2g orbitals (dyz-dxz-dxy) of Ru3+ ions. This ultrafast
magnetization is weakened in the antiferromagnetic (AF) phase, which is
opposite to the general tendency that the inverse Faraday effect is larger in
AF compounds than in paramagnetic ones. This temperature dependence indicates
that the interorbital charge motion is affected by pseudo-spin rotational
symmetry breaking in the AF phase.",2207.03877v2
2022-07-14,Dzyaloshinskii-Moriya spin density by skew scattering,"Anisotropic exchange couplings, such as the Dzyaloshinskii-Moriya interaction
(DMI), have played a vital role in the formation and dynamics of spin textures.
This work predicts an anisotropic conduction electron spin density in metals
with heavy magnetic impurities. The polarization of this
$Dzyaloshinskii$-$Moriya$ $spin$ $density$ (DM-SD) is not collinear to the
localized magnetic moments but rotated by the spin-dependent skew scattering of
heavy atoms. The DM-SD induces the DMI between magnetic moments in metals and,
therefore, it is the anisotropic extension of the
Rutherman-Kittel-Kasuya-Yoshida spin density. Our model consists of two
localized magnetic moments, one with a large spin-orbit coupling (a lanthanide
or rare earth), in a free electron gas. The lanthanide spin controls the DM-SD
strength and polarization, promising a flexible control mechanism for
anisotropic couplings.",2207.06842v2
2022-07-21,Energy-scale competition in the Hall resistivity of a strange metal,"Anomalous transport behavior -- both longitudinal and Hall -- is the defining
characteristic of the strange-metal state of High-Tc cuprates. The temperature,
frequency, and magnetic field dependence of the resistivity is understood
within strange metal phenomenology as resulting from energy-scale competition
to set the inelastic relaxation rate. The anomalously strong temperature
dependence of the Hall coefficient, however, is at odds with this
phenomenology. Here we report measurements of the Hall resistivity in the
strange metal state of cuprates over a broad range of magnetic fields and
temperatures. The observed field and temperature dependent Hall resistivity at
very high magnetic fields reveals a distinct high-field regime which is
controlled by energy-scale competition. This extends the strange metal
phenomenology in the cuprates to include the Hall resistivity and suggests, in
particular, that the direct effect of magnetic field on the relaxation dynamics
of quantum fluctuations may be at least partially responsible for the anomalous
Hall resistivity of the strange metal state.",2207.10244v1
2022-07-21,Influence of nonuniform magnetization reorientation on spin-orbit torque measurements,"Measurements of spin-orbit torques in a ferromagnetic/nonmagnetic multilayer
are typically based on an assumption that the entire ferromagnetic layer
uniformly responds to the spin-orbit torque. This assumption breaks down when
the thickness of the ferromagnetic layer is comparable to the dynamic exchange
coupling length, which can be as short as a few nanometers in certain
measurement geometries. The nonuniform magnetization reorientation coupled with
nonuniform contribution of each magnetic sublayer to the magnetoresistance or
the Kerr effect may impact the accuracy in the extrapolation of spin-orbit
torque, particularly if a thick ferromagnetic layer is used. In this paper, we
use numerical models to investigate such an impact in three different
techniques: the magneto-optic-Kerr-effect method, the second-harmonic method
and the spin torque ferromagnetic resonance method. We show that the
second-harmonic and magneto-optic-Kerr-effect methods are prone to be
influenced by the nonuniform magnetization reorientation, while the spin torque
ferromagnetic resonance method is much less impacted.",2207.10349v1
2022-07-26,Measuring Lateral Capillary Forces on Floating Particles using the Moses Effect,"This study presents a novel and user-friendly technique for detecting the
lateral capillary force on a floating spherical particle. The technique
leverages the interplay between the capillary attracting forces, hydrostatic
pressure forces, and magnetic repulsion forces. A magnetic field is applied to
induce a surface curvature in the liquid, resulting in a non-uniform
distribution of capillary and hydrostatic pressure forces across the particle's
surface. This leads to a stable equilibrium position of the particle at a
specific distance from the magnet. The study analyzes the equilibrium position
and other relevant parameters in comparison with the developed theory.
Classical mechanics and intermolecular forces are applied to establish the
theoretical basis for the method, modeling the behavior of the particle in
response to the magnetic field, surface curvature, and hydrostatic pressure.
The equilibrium position of the particle is determined by numerically solving
the balance of forces equation.",2207.13134v3
2022-08-11,Tunable electronic and magnetic phases in layered ruthenates: SrRuO3-SrTiO3 heterostructure upon strain,"Layered ruthenates are a unique class of systems which manifests a variety of
electronic and magnetic features emerged from competing energy scales. At the
heart of such features lies the multi-orbital physics, especially, the
orbital-selective behavior. Here, we propose that the SrRuO3-SrTiO3
heterostructure is a highly tunable platform to obtain the various emergent
properties. Employing the density functional theory plus dynamical mean-field
theory, we thoroughly investigate the orbital-dependent physics of the system
and identify the competing magnetic fluctuations. We show that the epitaxial
strain drives the system towards multi-orbital or orbital selective Mott phases
from the Hund metal regime. At the same time, the two different types of static
magnetism are stabilized, ferromagnetism and checkerboard antiferromagnetism,
from the competition with the spin-density wave instability.",2208.05702v1
2022-08-10,Effect of interlayer exchange coupling in spin-torque nano oscillator,"The dynamics of the magnetization of the free layer in a spin-torque nano
oscillator (STNO) influenced by a noncollinear alignment between the
magnetizations of the free and pinned layers due to an interlayer exchange
coupling has been investigated theoretically. The orientations of the
magnetization of the free layer with that of the pinned layer have been
computed through the macrospin model and they are found to match well with
experimental results. The bilinear and biquadratic coupling strengths make the
current to switch the magnetization between two states or oscillate steadily.
The expressions for the critical currents between which oscillations are
possible and the critical bilinear coupling strength below which oscillations
are not possible are derived. The frequency of the oscillations is shown to be
tuned and increased to or above 300 GHz by the current which is the largest to
date among nanopillar-shaped STNOs.",2208.07223v1
2022-08-25,Ultrashort spin-orbit torque generated by femtosecond laser pulses,"To realize the very objective of spintronics, namely the development of
ultra-high frequency and energy-efficient electronic devices, an ultrafast and
scalable approach to switch magnetic bits is required. Magnetization switching
with spin currents generated by the spin-orbit interaction at
ferromagnetic/non-magnetic interfaces is one of such scalable approaches, where
the ultimate switching speed is limited by the Larmor precession frequency.
Understanding the magnetization precession dynamics induced by spin-orbit
torques (SOTs) is therefore of great importance. Here we demonstrate generation
of ultrashort SOT pulses that excite Larmor precession at an epitaxial Fe/GaAs
interface by converting femtosecond laser pulses into high-amplitude current
pulses in an electrically biased p-i-n photodiode. We control the polarity,
amplitude, and duration of the current pulses and, most importantly, also their
propagation direction with respect to the crystal orientation. The SOT origin
of the excited Larmor precession was revealed by a detailed analysis of the
precession phase and amplitude at different experimental conditions.",2208.11974v1
2022-09-07,Acoustic attenuation in magnetic insulator films: effects of magnon polaron formation,"A magnon and a phonon are the quanta of spin wave and lattice wave,
respectively, and they can hybridize into a magnon polaron when their
frequencies and wavenumbers match close enough the values at the exceptional
point. Guided by an analytically calculated magnon polaron dispersion,
dynamical phase-field simulations are performed to investigate the effects of
magnon polaron formation on the attenuation of a bulk acoustic wave in a
magnetic insulator film. It is shown that a stronger magnon-phonon coupling
leads to a larger attenuation. The simulations also demonstrate the existence
of a minimum magnon-phonon interaction time required for the magnon polaron
formation, which is found to decrease with the magnetoelastic coupling
coefficient but increase with the magnetic damping coefficient. These results
deepen the understanding of the mechanisms of acoustic attenuation in magnetic
crystals and provide insights into the design of new-concept spin interconnects
that operate based on acoustically driven magnon propagation.",2209.03481v2
2022-09-08,Inflationary magnetogenesis of primordial magnetic fields with multiple vector fields,"In this paper, we discussed the multiple vector fields during the inflation
era and the inflationary magnetogenesis with multiple vector fields. Instead of
a single coupling function in single vector field models, the coupling matrix
between vector fields and scalar field which drive the inflation is introduced.
The dynamical equations for multiple vector fields are obtained and applied to
the inflation era. We discussed three cases for the double-field model. In no
mutual-coupling case, one can find that both electric and magnetic spectrum can
be scale-invariant at the end of inflation, meanwhile, the strong coupling
problem can be avoided. The effect of mutual-coupling between different vector
fields is also discussed. We found that weak mutual-coupling can lead to the
slightly blue spectrum of the magnetic field. On the other hand, in the strong
mutual-coupling case, the scale-invariant magnetic spectrum can also be
obtained but the energy density of electromagnetic fields either lead to the
backreaction problem or is diluted by inflation.",2209.03545v3
2022-09-15,Field-free high-frequency exchange-spring spin-torque nano-oscillators,"Spin-torque nano-oscillators (STNOs) are a type of nanoscale microwave
auto-oscillators utilizing spin-torque to generate magnetodynamics with great
promise for applications in microwaves, magnetic memory, and neuromorphic
computing. Here, we report the first demonstration of exchange-spring STNOs,
with an exchange-spring ([Co/Pd]-Co) reference layer and a perpendicular
([Co/Ni]) free layer. This magnetic configuration results in high-frequency
(>10 GHz) microwave emission at a zero magnetic field and exchange-spring
dynamics in the reference layer and the observation of magnetic droplet
solitons in the free layer at different current polarities. Our demonstration
of bipolar and field-free exchange-spring-based STNOs operating over a 20 GHz
frequency range greatly extends the design freedom and functionality of the
current STNO technology for energy-efficient high-frequency spintronic and
neuromorphic applications.",2209.06996v1
2022-09-17,Identification of a non-axisymmetric mode in laboratory experiments searching for standard magnetorotational instability,"The standard magnetorotational instability (SMRI) is a promising mechanism
for turbulence and rapid accretion in astrophysical disks. It is a
magnetohydrodynamic (MHD) instability that destabilizes otherwise
hydrodynamically stable disk flow. Due to its microscopic nature at
astronomical distances and stringent requirements in laboratory experiments,
SMRI has remained unconfirmed since its proposal, despite its astrophysical
importance. Here we report a nonaxisymmetric MHD instability in a modified
Taylor-Couette experiment. To search for SMRI, a uniform magnetic field is
imposed along the rotation axis of a swirling liquid-metal flow. The
instability initially grows exponentially, becoming prominent only for
sufficient flow shear and moderate magnetic field. These conditions for
instability are qualitatively consistent with SMRI, but at magnetic Reynolds
numbers below the predictions of linear analyses with periodic axial
boundaries. Three-dimensional numerical simulations, however, reproduce the
observed instability, indicating that it grows linearly from the primary
axisymmetric flow modified by the applied magnetic field.",2209.08410v1
2022-10-01,Centrifugal barriers in magnetospheric accretion,"We reconsider the dynamics of accretion flows onto magnetized central star.
For dipolar magnetically aligned case, the centrifugal barrier is at $R_{cb} =
(2/3)^{1/3} R_c = 0.87 R_c$, where $R_c= ( G M/\Omega^2)^{1/3}$ is the
corotation radius. For oblique dipole direct accretion from the corotation
radius $R_c$ is possible only for magnetic obliquity satisfying $\tan
\theta_\mu \geq 1/( 2 \sqrt{3}) $ ($\theta_\mu \geq 16.1^\circ $). The
accretion proceeds in a form of funnel flows - along two streams centered on
the $\mu-\Omega$ plane, with azimuthal opening angle $ \cos (\Delta \phi) = {
\cot^ 2 {\theta_\mu} }/{12} $. For the magnetosphere distorted by the
diamagnetic disk, the centrifugal barrier can be at as small radius as $R_{cb}=
0.719 R_c$ for the fully confined dipole, extending out to $R_{cb} \sim R_c$
for the magnetically balanced case. Type-II X-ray bursts in accreting neutron
stars may be mediated by the centrifugal barrier; this requires nearly aligned
configuration. Centrifugally-barriered material trapped in the magnetosphere
may lead to periodic obscuration (""dips"") in the light curve of the host star,
e.g., as observed in accreting young stellar objects and X-ray binaries.",2210.00300v3
2022-10-05,Controlled Ordering of Room-Temperature Magnetic Skyrmions in a Polar Van der Waals Magnet,"Control and understanding of ensembles of skyrmions is important for
realization of future technologies. In particular, the order-disorder
transition associated with the 2D lattice of magnetic skyrmions can have
significant implications for transport and other dynamic functionalities. To
date, skyrmion ensembles have been primarily studied in bulk crystals, or as
isolated skyrmions in thin film devices. Here, we investigate the condensation
of the skyrmion phase at room temperature and zero field in a polar, Van der
Waals magnet. We demonstrate that we can engineer an ordered skyrmion crystal
through structural confinement on the $\mu$m scale, showing control over this
order-disorder transition on scales relevant for device applications.",2210.02546v1
2022-10-12,"Spin Diffusion in Spin Glasses Require Two Magnetic Variables, $\vec{M}$ and $\vec{m}$","Experiment has established that spin-glasses can support a steady-state spin
current $\vec{j}_{i}$. However, the accepted theory of spin glass dynamics
permits oscillations but no steady-state spin current. Onsager's irreversible
thermodynamics implies that the spin current is proportional to the gradient of
a magnetization. We argue, however, that the magnon distribution function
associated with the local equilibrium magnetization $\vec{M}$ cannot diffuse
because it represents $10^{23}$ variables. We therefore invoke the
non-equilibrium magnetization $\vec{m}$, which in spintronics is called the
{\it spin accumulation}. Applying the theory of irreversible thermodynamics we
indeed find that it predicts spin diffusion, and we consider other experimental
consequences of the theory, including a wavelength-dependent coupling between
the reactive and the diffusive degrees of freedom.",2210.05865v2
2022-10-14,Magnetic-field-induced cavity protection for intersubband polaritons,"We analyse the effect of a strong perpendicular magnetic field on an
intersubband transition in a disordered doped quantum well strongly coupled to
an optical cavity. The magnetic field changes the lineshape of the intersubband
optical transition due to the interface roughness of the quantum well from a
Lorentzian to a Gaussian one. In this regime, a novel form of
magnetic-field-induced cavity protection sets in, which strongly reduces the
polariton linewidth to the cavity contribution only. Implications of our
results for fundamental studies of nonlinear polariton dynamics and for
technological applications to polariton lasers are finally highlighted.",2210.08026v2
2022-10-27,Gravitational and electromagnetic radiation from binary black holes with electric and magnetic charges: Hyperbolic orbits on a cone,"We derive the hyperbolic orbit of binary black holes with electric and
magnetic charges. In the low-velocity and weak-field regime, by using the
Newtonian method, we calculate the total emission rate of energy due to
gravitational and electromagnetic radiation from binary black holes with
electric and magnetic charges in hyperbolic orbits. Moreover, we develop a
formalism to derive the merger rate of binary black holes with electric and
magnetic charges from the two-body dynamical capture. We apply the formalism to
investigate the effects of the charges on the merger rate for the near-extremal
case and find that the effects cannot be ignored.",2210.15564v2
2022-11-03,Deformative Magnetic Marked Length Spectrum Rigidity,"Let $M$ be a closed surface and let $\{g_s \ | \ s \in (-\epsilon,
\epsilon)\}$ be a smooth one-parameter family of Riemannian metrics on $M$.
Also let $\{\kappa_s : M \rightarrow \mathbb{R} \ | \ s \in (-\epsilon,
\epsilon)\}$ be a smooth one-parameter family of functions on $M$. Then the
family $\{(g_s, \kappa_s) \ | \ s \in (-\epsilon, \epsilon)\}$ gives rise to a
family of magnetic flows on $TM$. We show that if the magnetic curvatures are
negative for $s \in (-\epsilon, \epsilon)$ and the lengths of each periodic
orbit remains constant as the parameter $s$ varies, then there exists a smooth
family of diffeomorphisms $\{f_s : M \rightarrow M \ | \ s \in (-\epsilon,
\epsilon)\}$ such that $f_s^*(g_s) = g_0$ and $f_s^*(\kappa_s) = \kappa_0$.
This generalizes a result of Guillemin and Kazhdan to the setting of magnetic
flows.",2211.01865v2
2022-11-07,"Magnetic excitations, phase diagram and order-by-disorder in the extended triangular-lattice Hubbard model","The dynamical structure factor is an important observable of quantum magnets
but due to numerical and theoretical limitations, it remains a challenge to
make predictions for Hubbard-like models beyond one dimension. In this work, we
study the magnetic excitations of the triangular lattice Hubbard model
including next-nearest neighbor hopping. Starting from the 120$^{\circ}$ and
stripe magnetic orders we compute the magnon spectra within a self-consistent
random phase approximation. In the stripe phase, we generically find accidental
zero modes related to a classical degeneracy known from the corresponding
$J_1$-$J_2$ Heisenberg model. We extend the order-by-disorder mechanism to
Hubbard systems and show how quantum fluctuations stabilize the stripe order.
In addition, the frustration-induced condensation of magnon modes allows us to
map out the entire phase diagram which is in remarkable agreement with recent
numerical works. We discuss connections to experiments on triangular lattice
compounds and the relation of our results to the proposed chiral spin liquid
phase.",2211.03654v2
2022-11-14,Spin wave diffraction model for perpendicularly magnetized films,"We present a near-field diffraction model for spin waves in perpendicularly
magnetized films applicable in any geometries of excitation fields. This model
relies on Kalinikos-Slavin formalism to express the dynamic susceptibility
tensor in k-space, and calculate the diffraction patterns via inverse
2D-Fourier transform of the response functions. We show an excellent
quantitative agreement between our model and MuMax3 micro-magnetic simulations
on two different geometries of antennas. Our method benchmarks spin wave
diffraction in perpendicularly magnetized films, and is readily applicable for
future designs of magnon beamforming and interferometric devices.",2211.07231v2
2022-11-15,Turbulent reconnection acceleration,"The ubiquitous turbulence in astrophysical plasmas is important for both
magnetic reconnection and reconnection acceleration. We study the particle
acceleration during fast 3D turbulent reconnection with reconnection-driven
turbulence. Particles bounce back and forth between the reconnection-driven
inflows due to the mirror reflection and convergence of strong magnetic fields.
Via successive head-on collisions, the kinetic energy of the inflows is
converted into the accelerated particles. Turbulence not only regulates the
inflow speed but also introduces various inflow obliquities with respect to the
local turbulent magnetic fields. As both the energy gain and escape probability
of particles depend on the inflow speed, the spectral index of particle energy
spectrum is not universal. We find it in the range from $\approx 2.5$ to $4$,
with the steepest spectrum expected at a strong guide field, i.e. a small angle
between the total incoming magnetic field and the guide field. Without
scattering diffusion needed for confining particles, the reconnection
acceleration can be very efficient at a large inflow speed and a weak guide
field.",2211.08444v1
2022-11-21,Radon-based Image Reconstruction for MPI using a continuously rotating FFL,"Magnetic particle imaging is a relatively new tracer-based medical imaging
technique exploiting the non-linear magnetization response of magnetic
nanoparticles to changing magnetic fields. If the data are generated by using a
field-free line, the sampling geometry resembles the one in computerized
tomography. Indeed, for an ideal field-free line rotating only in between
measurements it was shown that the signal equation can be written as a
convolution with the Radon transform of the particle concentration. In this
work, we regard a continuously rotating field-free line and extend the forward
operator accordingly. We obtain a similar result for the relation to the Radon
data but with two additive terms resulting from the additional
time-dependencies in the forward model. We jointly reconstruct particle
concentration and corresponding Radon data by means of total variation
regularization yielding promising results for synthetic data.",2211.11683v2
2022-12-03,Magnetoplasmonics: current challenges and future opportunities,"Plasmonics represents a unique approach to confine and enhance
electromagnetic radiation well below the diffraction limit, bringing a huge
potential for novel applications, for instance in energy harvesting,
optoelectronics, and nanoscale biochemistry. To achieve novel functionalities,
the combination of plasmonic properties with other material functions has
become increasingly attractive. In this Perspective, we review the current
state of the art, challenges, and future opportunities in nanoscale
magnetoplasmonics, an emerging area aiming to merge magnetism and plasmonics in
confined geometries to control either plasmons, electromagnetic-induced
collective electronic excitations, using magnetic properties, or magnetic
phenomena with plasmons. We begin by highlighting the cornerstones of the
history and principles of this research field. We then provide our vision of
its future development by showcasing raising research directions in
mid-infrared light-driven spintronics and novel materials for
magnetoplasmonics, such as transparent conductive oxides and hyperbolic
metamaterials. As well, we provide an overview of recent developments in
plasmon-driven magnetization dynamics, nanoscale optomagnetism and
acousto-magnetoplasmonics. We conclude by giving our personal vision of the
future of this thriving research field.",2212.01684v1
2022-12-12,Magnon-Plasmon Hybridization Mediated by Spin-Orbit Interaction in Magnetic Materials,"We propose a mechanism for magnon-plasmon coupling and hybridization in
ferromagnetic (FM) and antiferromagnetic (AFM) systems. The electric field
associated with plasmon oscillations creates a non-equilibrium spin density via
the inverse spin galvanic effect. This plasmon-induced spin density couples to
magnons by an exchange interaction. The strength of magnon-plasmon coupling
depends on the magneto-electric susceptibility of the system and the wavevector
at which the level repulsion is happened. This wavevector may be tuned by an
applied magnetic field. In AFM systems, the degeneracy of two chiral magnons is
broken in the presence of a magnetic field, and we find two separate hybrid
modes for left-handed and right-handed AFM magnons. Furthermore, we show that
magnon-plasmon coupling in AFM systems is enhanced because of strong
intra-sublattice spin dynamics. We argue that the recently discovered
two-dimensional magnetic systems are ideal platforms to investigate proposed
magnon-plasmon hybrid modes.",2212.06105v1
2022-12-15,Observation of a new light-induced skyrmion phase in the Mott insulator Cu2OSeO3,"We report the discovery of a novel skyrmion phase in the multiferroic
insulator Cu2OSeO3 for magnetic fields below the equilibrium skyrmion pocket.
This phase can be accessed by exciting the sample out of equilibrium with
near-infrared (NIR) femtosecond laser pulses but can not be reached by any
conventional field cooling protocol. From the strong wavelength dependence of
the photocreation process and via spin dynamics simulations, we identify the
magnetoelastic effect as the most likely photocreation mechanism. This effect
results in a transient modification of the magnetic interaction extending the
equilibrium skyrmion pocket to lower magnetic fields. Once created, the
skyrmions rearrange and remain stable over a long time, reaching minutes. The
presented results are relevant for designing high-efficiency non-volatile data
storage based on magnetic skyrmions.",2212.07878v1
2022-12-21,Chirality-dependent spin current generation in a helimagnet: zero-field probe of chirality,"In a magnetic texture, the spin of a conduction electron is forced to be
aligned to the localized moment. As a result, the topology of the magnetic
texture affects the electron dynamics in nontrivial ways. A representative
example is the topological Hall effect in noncoplanar spin textures with finite
spin chirality. While propagating in the noncoplanar spin texture, electrons
acquire Berry phase, and their motion is deflected as if they were in a
magnetic field. Here, we report a distinct Berry phase effect in a coplanar
helimagnet: the spin moment of the conduction electron is polarized under
electric currents depending on the chirality of the helimagnet. The accumulated
spin polarization works as a source of spin current, and the chirality can be
detected by the inverse spin Hall mechanism. The functionality allows us to
read out the chirality without magnetic fields, and therefore paves the way to
future helimagnet-based spintronics.",2212.10980v1
2023-01-07,Saturn's Magnetic Field at Unprecedented Detail Achieved by Cassini's Close Encounters,"The last 22.5 orbits of the Cassini mission brought the spacecraft to less
than 3000 km from Saturn's 1-bar surface. These close encounters offered an
unprecedented view of Saturn's magnetic field, including contributions from the
internal dynamo, the ionosphere, and the magnetosphere. In this chapter, we
highlight the new picture of Saturn's magnetic field from the Cassini mission
including the persistent yet time-varying low-latitude field-aligned currents,
Alfv\'en waves planet-ward of the D-ring, extreme axisymmetry, and high-degree
magnetic moments. We then discuss the implications and new questions raised for
Saturn's innermost magnetosphere, equatorial ionosphere, and interior. We
conclude this chapter with an outlook for the future exploration of Saturn and
other giant planets.",2301.02756v1
2023-01-17,"Observation of field-induced single-ion magnetic anisotropy in a multiorbital Kondo alloy $\mathrm{(Lu,Yb)}\mathrm{Rh}_{2}\mathrm{Zn}_{20}$","We demonstrate field-induced single-ion magnetic anisotropy resulting from
the multiorbital Kondo effect on the diluted ytterbium alloy
$(\mathrm{Lu}_{1-x}\mathrm{Yb}_x)\mathrm{Rh}_2\mathrm{Zn}_{20}$. Single-ion
anisotropic metamagnetic behavior is revealed in low-temperature regions where
the local Fermi-liquid state is formed. Specific hea, low-field magnetic
susceptibility, and resistivity indicate reproduction of the ground-state
properties by the $\mathrm{SU}(N = 8)$ Kondo model with a relatively large
$c$-$f$ hybridization of $T_{\mathrm{K}} = 60.9 \ \mathrm{K}$. Dynamical
susceptibility measurements on $\mathrm{Yb}\mathrm{Rh}_2\mathrm{Zn}_{20}$
support realizing the multiorbital Kondo ground state in
$(\mathrm{Lu}_{1-x}\mathrm{Yb}_x)\mathrm{Rh}_2\mathrm{Zn}_{20}$. The single-ion
magnetic anisotropy becomes evident above $\sim5 \ \mathrm{T}$, which is lower
than the isotropic Kondo crossover field of 22.7 T, verifying blurred low-lying
crystal field states through the multiorbital Kondo effect.",2301.07038v3
2023-02-09,Single-shot laser-induced switching of an exchange biased antiferromagnet,"Ultrafast manipulation of magnetic order has challenged our understanding the
fundamental and dynamic properties of magnetic materials. So far single shot
magnetic switching has been limited to ferrimagnetic alloys and multilayers. In
ferromagnetic (FM)/antiferromagnetic (AFM) bilayers, exchange bias (He) arises
from the interfacial exchange coupling between the two layers and reflects the
microscopic orientation of the antiferromagnet. Here we demonstrate the
possibility of single shot switching of the antiferromagnet (change of the sign
and amplitude of He) with a single femtosecond laser pulse in IrMn/CoGd
bilayers. We demonstrate the switching for a wide range of fluences for
different layer thicknesses and compositions. Atomistic simulations predict
ultrafast switching and recovery of the AFM magnetization on a timescale of 2
ps. These results provide the fastest and the most energy-efficient method to
set the exchange bias and pave the way to potential applications for ultrafast
spintronic devices.",2302.04510v1
2023-02-13,Helical dynamo growth at modest versus extreme magnetic Reynolds numbers,"Understanding large-scale magnetic field growth in astrophysical objects is a
persistent challenge. We tackle the long-standing question of how much helical
large-scale dynamo growth occurs independent of the magnetic Reynolds number
(Rm) in a closed volume. From modest-Rm numerical simulations, we identify a
pre-saturation regime when the large-scale field grows independently of Rm, but
to an Rm-dependent magnitude. For plausible magnetic spectra however, the
analysis predicts the magnitude to be Rm-independent and substantial as
Rm$\to\infty$. This gives renewed optimism for the relevance of closed dynamos
and pinpoints how modest Rm and hyper-diffusive simulations can cause
misapprehension of the Rm$\to\infty$ behavior.",2302.06042v2
2023-02-23,Giant supermagnonic Bloch point velocities by jet propulsion effect in cylindrical ferromagnetic nanowires,"Achieving high velocities of magnetic domain walls is a crucial factor for
their use as information carriers in modern nanoelectronic applications. In
nanomagnetism and spintronics, these velocities are often limited either by
internal domain wall instabilities, known as the Walker breakdown phenomenon,
or by spin wave emission, known as the magnonic regime. In the rigid domain
wall model, the maximum magnon velocity acts as an effective ""speed of light"",
providing a relativistic analogy for the domain wall speed limitation.
Cylindrical magnetic nanowires are an example of systems with the absence of
the Walker breakdown phenomenon. Here we demonstrate that in cylindrical
nanowires with high magnetization such as Iron, also the magnonic limit could
be outstandingly surpassed. Our numerical modelling shows the Bloch point
domain wall velocities as high as 14 km/s, well above the magnonic limit
estimated in the interval 1.7-2.0 km/s. The key ingredient is the conical shape
of the domain wall which elongates and breaks during the dynamics, leading to
domain wall acceleration due to the jet propulsion effect. This effect will be
very important for three-dimensional spintronic networks based on cylindrical
magnetic nanowires.",2302.12329v1
2023-02-28,How closely does transient magnetic linear dichroism follow the spin moment?,"In highly out-of-equilibrium states of matter, such as those induced by a
pump laser, the applicability of well established spectroscopic probes of
magnetic order are called into question. Here we address the validity of x-ray
absorption techniques in pump laser conditions, focusing on magnetic linear
dichroism (MLD), a crucial probe of antiferromagnetic (AFM) order. We directly
compute the dynamics of the square of the spin moment and compare to those
obtained via the MLD response. For AFM FePd the agreement between these
distinct routes to the magnetic moment severely degrades at pulse fluences
greater than 1 mJ/cm$^2$, indicating a breakdown of the MLD response as an
accurate probe of the transient moment. This contrasts with the MLD for
ferromagnetic FePt which reliably tracks the moment for fluences (and absorbed
energies) up to an order of magnitude greater than the breakdown threshold for
AFM FePd. The underlying microscopic reason for this we find to be increased
laser induced excitations out of the $d$-band in AFM FePd, where this increase
is made possible by the AFM pseudogap.",2302.14856v2
2023-03-01,On the instability of the magnetohydrodynamic pipe flow subject to a transverse magnetic field,"The linear stability of a fully-developed liquid-metal MHD pipe flow subject
to a transverse magnetic field is studied numerically. Because of the lack of
axial symmetry in the mean velocity profile, we need to perform a BiGlobal
stability analysis. For that purpose, we develop a two-dimensional complex
eigenvalue solver relying on a Chebyshev-Fourier collocation method in physical
space. By performing an extensive parametric study, we show that in contrast to
Hagen-Poiseuille flow known to be linearly stable for all Reynolds numbers, the
MHD pipe flow with transverse magnetic field is unstable to three-dimensional
disturbances at sufficiently high values of the Hartmann number and wall
conductance ratio. The instability observed in this regime is attributed to the
presence of velocity overspeeds in the so-called Roberts layers and the
corresponding inflection points in the mean velocity profile. The nature and
characteristics of the most unstable modes are investigated, and we show that
they vary significantly depending on the wall conductance ratio. A major result
of this paper is that the global critical Reynolds number for the MHD pipe flow
with transverse magnetic field is $Re=45230$, and it occurs for a perfectly
conducting pipe wall and the Hartmann number $Ha=19.7$.",2303.00395v1
2023-03-15,Extended magnetic reconnection in kinetic plasma turbulence,"Magnetic reconnection and plasma turbulence are ubiquitous processes
important for laboratory, space and astrophysical plasmas. Reconnection has
been suggested to play an important role in the energetics and dynamics of
turbulence by observations, simulations and theory for two decades. The
fundamental properties of reconnection at kinetic scales, essential to
understanding the general problem of reconnection in magnetized turbulence,
remain largely unknown at present. Here we present an application of the
magnetic flux transport method that can accurately identify reconnection in
turbulence to a three-dimensional simulation. Contrary to ideas that
reconnection in turbulence would be patchy and unpredictable, highly extended
reconnection X-lines, on the same order of magnitude as the system size, form
at kinetic scales. Extended X-lines develop through bi-directional reconnection
spreading. They satisfy critical balance characteristic of turbulence, which
predicts the X-line extent at a given scale. These results present a picture of
fundamentally extended reconnection in kinetic-scale turbulence.",2303.08642v2
2023-03-21,Helitronics for classical and unconventional computing,"Magnetic textures are promising candidates for unconventional computing due
to their non-linear dynamics. We propose to investigate the rich variety of
seemingly trivial lamellar magnetic phases, e.g., helical, spiral, stripy
phase, or other one-dimensional soliton lattices. These are the natural stray
field-free ground states of almost every magnet. The order parameters of these
phases may be of potential interest for both classical and unconventional
computing, which we refer to as helitronics. For the particular case of a
chiral magnet and its helical phase, we use micromagnetic simulations to
demonstrate the working principles of all-electrical (i) classical binary
memory cells and (ii) memristor and neuron cells, based on the orientation of
the helical stripes.",2303.11688v1
2023-03-25,On Improved Commutation for Moving-Magnet Planar Actuators,"The demand for high-precision and high-throughput motion control systems has
increased significantly in recent years. The use of moving-magnet planar
actuators (MMPAs) is gaining popularity due to their advantageous
characteristics, such as complete environmental decoupling and reduction of
stage mass. Nonetheless, model-based commutation techniques for MMPAs are
compromised by misalignment between the mover and coil array and mismatch
between the ideal electromagnetic model and the physical system, often leading
to decreased system performance. To address this issue, a novel improved
commutation approach is proposed in this paper\YB{, which is applicable for
general planar motor applications,} by means of dynamic regulation of the
position dependence of the ideal model-based commutation algorithm, which
allows for attenuation of magnetic misalignment, manufacturing inaccuracies and
other unmodelled phenomena. The effectiveness of the proposed approach is
validated through experiments using a state-of-the-art moving-magnet planar
actuator prototype.",2303.14392v2
2023-04-19,Cavity magnomechanical coupling with coupled magnon modes in a synthetic antiferromagnet,"On-chip cavity magnomechanics is an emerging field exploring acoustic and
magnonic functionalities of various ferromagnetic materials and structures
using strongly confined phonons. It is expected that such cavity magnomechanics
can be extended to multilayer ferromagnets, especially synthetic
antiferromagnets (SAFs) that exhibit zero net magnetization through interlayer
exchange coupling. However, the conventional theoretical framework for a single
ferromagnet cannot be used directly because of the antiferromagnetic
magnetization dynamics associated with the interlayer exchange coupling. In
this paper, we theoretically investigate phonon-magnon coupling with a
three-layer SAF. Our formulation of the phonon-magnon coupling constants
reveals that the acoustic (optical) magnon mode dominantly couples to the
cavity phonon when the magnetization angles in the two ferromagnetic layers are
antiparallel (orthogonal). Moreover, numerical calculations including the
effects of dipole-dipole interactions and in-plane uniaxial magnetic anisotropy
allow us to predict phonon frequency shifts and linewidth broadening that can
be detected in experiments. These theoretical insights would greatly help us to
make a strategy for bringing the system into the strong coupling regime and to
devise novel control protocols in analogy to cavity quantum electrodynamics and
cavity optomechanics.",2304.09458v2
2023-04-20,Heliknoton in a film of cubic chiral magnet,"Cubic chiral magnets exhibit a remarkable diversity of two-dimensional
topological magnetic textures, including skyrmions. However, the experimental
confirmation of topological states localized in all three spatial dimensions
remains challenging. In this paper, we investigate a three-dimensional
topological state called a heliknoton, which is a hopfion embedded into a helix
or conic background. We explore the range of parameters at which the heliknoton
can be stabilized under realistic conditions using micromagnetic modeling,
harmonic transition state theory, and stochastic spin dynamics simulations. We
present theoretical Lorentz TEM images of the heliknoton, which can be used for
experimental comparison. Additionally, we discuss the stability of the
heliknoton at finite temperatures and the mechanism of its collapse. Our study
offers a pathway for future experimental investigations of three-dimensional
topological solitons in magnetic crystals.",2304.10181v1
2023-04-25,Simulations of Magnetization Reversal in FM/AFM Bilayers With THz Frequency Pulses,"It is widely known that antiferromagnets (AFMs) display a high frequency
response in the terahertz (THz) range, which opens up the possibility for
ultrafast control of their magnetization for next generation data storage and
processing applications. However, because the magnetization of the different
sublattices cancel, their state is notoriously difficult to read. One way to
overcome this is to couple AFMs to ferromagnets - whose state is trivially read
via magneto-resistance sensors. Here we present conditions, using theoretical
modelling, that it is possible to switch the magnetization of an AFM/FM bilayer
using THz frequency pulses with moderate field amplitude and short durations,
achievable in experiments. Consistent switching is observed in the phase
diagrams for an order of magnitude increase in the interface coupling and a
tripling in the thickness of the FM layer. We demonstrate a range of reversal
paths that arise due to the combination of precession in the materials and the
THz-induced fields. Our analysis demonstrates that the AFM drives the switching
and results in a much higher frequency dynamics in the FM due to the exchange
coupling at the interface. The switching is shown to be robust over a broad
range of temperatures relevant for device applications.",2304.12969v1
2023-05-13,Surface magnetism in the pulsating RV Tauri star R Scuti,"We present the surface magnetic field conditions of the brightest pulsating
RV Tauri star, R Sct. Our investigation is based on the longest
spectropolarimetric survey ever performed on this variable star. The analysis
of high resolution spectra and circular polarization data give sharp
information on the dynamics of the atmosphere and the surface magnetism,
respectively. Our analysis shows that surface magnetic field can be detected at
different phases along a pulsating cycle, and that it may be related to the
presence of a radiative shock wave periodically emerging out of the photosphere
and propagating throughout the stellar atmosphere.",2305.07849v1
2023-05-16,Solar Active Region Magnetogram Image Dataset for Studies of Space Weather,"In this dataset we provide a comprehensive collection of magnetograms (images
quantifying the strength of the magnetic field) from the National Aeronautics
and Space Administration's (NASA's) Solar Dynamics Observatory (SDO). The
dataset incorporates data from three sources and provides SDO Helioseismic and
Magnetic Imager (HMI) magnetograms of solar active regions (regions of large
magnetic flux, generally the source of eruptive events) as well as labels of
corresponding flaring activity. This dataset will be useful for image analysis
or solar physics research related to magnetic structure, its evolution over
time, and its relation to solar flares. The dataset will be of interest to
those researchers investigating automated solar flare prediction methods,
including supervised and unsupervised machine learning (classical and deep),
binary and multi-class classification, and regression. This dataset is a
minimally processed, user configurable dataset of consistently sized images of
solar active regions that can serve as a benchmark dataset for solar flare
prediction research.",2305.09492v3
2023-05-18,Magnetic catalysis in weakly interacting hyperbolic Dirac materials,"Due to linearly vanishing density of states, emergent massless Dirac
quasiparticle resulting from the free fermion motion on a family of
two-dimensional half-filled bipartite hyperbolic lattices feature dynamic mass
generation through quantum phase transitions only for sufficiently strong
finite-range Coulomb repulsion. As such, strong nearest-neighbor Coulomb
repulsion ($V$) is conducive to the nucleation of a charge-density-wave (CDW)
order with a staggered pattern of average fermionic density between two
sublattices of bipartite hyperbolic lattices. Considering a collection of
spinless fermions (for simplicity), here we show that application of strong
external magnetic fields by virtue of producing a \emph{finite} density of
states near the zero energy triggers the condensation of the CDW order even for
\emph{infinitesimal} $V$. The proposed magnetic catalysis mechanism is
operative for uniform as well as inhomogeneous (bell-shaped) magnetic fields.
We present scaling of the CDW order with the total flux enclosed by hyperbolic
Dirac materials for a wide range of (especially subcritical) $V$.",2305.11174v1
2023-05-29,Light-induced weak ferromagnetism through nonlinear magnonic rectification,"Rectification describes the generation of a quasistatic component from an
oscillating field, such as an electric polarization in optical rectification,
or a structural distortion in nonlinear phononic rectification. Here, we
present a third fundamental process for magnetization, in which spin precession
is rectified along the coordinates of a nonlinearly driven magnon mode in an
antiferromagnet. We demonstrate theoretically that a quasistatic magnetization
can be induced by transient spin canting in response to the coherent excitation
of a chiral phonon mode that produces an effective magnetic field for the
spins. This mechanism, which we call nonlinear magnonic rectification, is
generally applicable to magnetic systems that exhibit infrared-active chiral
phonon modes. Our results serve as an example of light-induced weak
ferromagnetism and open a promising avenue towards creating dynamical spin
configurations that are not accessible in equilibrium.",2305.18656v2
2023-05-31,Pressure evolution of electronic structure and magnetism in the layered van der Waals ferromagnet CrGeTe$_3$,"Layered van der Waals ferromagnets, which preserve their magnetic properties
down to exfoliated monolayers, are fueling an abundance of fundamental research
and nanoscale device demonstration. CrGeTe$_3$ is a prime example for this
class of materials. Its temperature-pressure phase diagram features an
insulator-to-metal transition and a significant increase of ferromagnetic
Curie-Weiss temperatures upon entering the metallic state. We use density
functional theory to understand the magnetic exchange interactions in
CrGeTe$_3$ at ambient and elevated pressure. We calculate Heisenberg exchange
couplings, which provide the correct ferromagnetic ground state and explain the
experimentally observed pressure dependence of magnetism in CrGeTe$_3$.
Furthermore, we combine density functional theory with dynamical mean field
theory to investigate the effects of electronic correlations and the nature of
the high pressure metallic state in CrGeTe$_3$.",2306.00081v2
2023-06-12,Magnetically tunable exciton valley coherence in monolayer WS$_2$ mediated by the electron-hole exchange and exciton-phonon interactions,"We develop a model, which incorporates both intra- and intervalley
scatterings to master equation, to explore exciton valley coherence in
monolayer WS$_2$ subjected to magnetic field. For linearly polarized (LP)
excitation accompanied with an initial coherence, our determined valley
dynamics manifests the coherence decay being faster than the exciton population
relaxation, and agrees with experimental data by Hao et al.[Nat. Phys. 12, 677
(2016)]. Further, we reveal that magnetic field may quench the electron-hole
(e-h) exchange induced pure dephasing -- a crucial decoherence source -- as a
result of lifting of valley degeneracy, allowing to magnetically regulate
valley coherence. In particular, at low temperatures for which the
exciton-phonon (ex-ph) interaction is weak, we find that the coherence time is
expected to attain ${\tau}_{\mathcal{C}}\sim 1$ ps, facilitating full control
of qubits based on the valley pseudospin. For dark excitons, we demonstrate an
emerging coherence even in the absence of initial coherent state, which has a
long coherence time ($\sim 15$ ps) at low temperature. Our work provides an
insight into tunable valley coherence and coherent valley control based on dark
excitons.",2306.06977v1
2023-06-26,Variational Model-Based Reconstruction Techniques for Multi-Patch Data in Magnetic Particle Imaging,"Magnetic Particle Imaging is an emerging imaging modality through which it is
possible to detect tracers containing superparamagnetic nanoparticles. The
exposure of the particles to dynamic magnetic fields generates a non-linear
response that is used to locate the particles and produce an image of their
distribution. The bounding box that can be covered by a single scan curve
depends on the strength of the gradients of the magnetic fields applied, which
is limited due to the risk of causing peripheral nerve stimulation (PNS) in the
patients. To address this issue, multiple scans are performed by practitioners.
The scan data must be merged together to produce reconstructions of larger
regions of interest. In this paper we propose a mathematical framework which
generalizes the current multi-patch scanning by utilizing transformations. We
show the flexibility of this framework in a variety of different scanning
approaches. Moreover, we describe an iterative reconstruction algorithm, show
its convergence to a minimizer and perform numerical experiments on simulated
data.",2306.14454v1
2023-07-03,Emergence of Elastic Softening Featuring Ultra-Slow Dynamics Around Magnetic Critical Endpoint in UCoAl,"We conducted an investigation on the temperature and magnetic field
dependence of the elastic properties of UCoAl. The longitudinal elastic
stiffness, $C_{33}$, exhibits significant softening as the system approaches
the critical endpoint (CEP). This softening is indicative of an ultrasonic
dispersion phenomenon, where the anomaly in the elastic constants diminishes
with increasing measurement frequency. Fine structures were observed near the
CEP in higher frequencies. The magnetic field dependence of $C_{33}$ can be
explained by assuming a specific field dependence of the relaxation time.
Remarkably, we recorded a relaxation time of 3.5$\times$10$^{-8}$ s in the
vicinity of the CEP, which is the longest observed value among solids. These
peculiar ultrasonic properties cannot be explained solely by Ising-like
ferromagnetic fluctuations, suggesting the involvement of the quadrupole
(orbital) degree of freedom in the formation of the CEP. We discussed the
origin of these observed phenomena in relation to the magnetic ground state of
the UCoAl system.",2307.00703v1
2023-07-27,Chiral anomaly and dynamos from inhomogeneous chemical potential fluctuations,"In the standard model of particle physics, the chiral anomaly can occur in
relativistic plasmas and plays a role in the early Universe, protoneutron
stars, heavy-ion collisions, and quantum materials. It gives rise to a magnetic
instability if the number densities of left- and right-handed electrically
charged fermions are unequal. Using direct numerical simulations, we show this
can result just from spatial fluctuations of the chemical potential, causing a
chiral dynamo instability, magnetically driven turbulence, and ultimately a
large-scale magnetic field through the magnetic alpha effect.",2307.15118v4
2023-08-22,Theory of defect-induced crystal field perturbations in rare earth magnets,"We present a theory describing the single-ion anisotropy of rare earth (RE)
magnets in the presence of point defects. Taking the RE-lean 1:12 magnet class
as a prototype, we use first-principles calculations to show how the
introduction of Ti substitutions into SmFe$_{12}$ perturbs the crystal field,
generating new coefficients due to the lower symmetry of the RE environment. We
then demonstrate that these perturbations can be described extremely
efficiently using a screened point charge model. We provide analytical
expressions for the anisotropy energy which can be straightforwardly
implemented in atomistic spin dynamics simulations, meaning that such
simulations can be carried out for an arbitrary arrangement of point defects.
The significant crystal field perturbations calculated here demonstrate that a
sample which is single-phase from a structural point of view can nonetheless
have a dramatically varying anisotropy profile at the atomistic level if there
is compositional disorder, which may influence localized magnetic objects like
domain walls or skyrmions.",2308.11303v1
2023-08-31,DFT+DMFT study of the magnetic susceptibility and the correlated electronic structure in transition-metal intercalated NbS$_2$,"The Co-intercalated NbS$_2$ (Co$_{1/3}$NbS$_2$) compound exhibits large
anomalous Hall conductance, likely due to the non-coplanar magnetic ordering of
Co spins. In this work, we study the relation between this novel magnetism and
the correlated electronic structure of Co$_{1/3}$NbS$_2$ by adopting dynamical
mean field theory (DMFT) to treat the correlation effect of Co $d$ orbitals. We
find that the hole doping of Co$_{1/3}$NbS$_2$ can tune the size of the Nb hole
pocket at the DMFT Fermi surface, producing features consistent with those
observed in angle resolved photoemission spectra [Phys. Rev. B 105, L121102
(2022)]. We also compute the momentum-resolved spin susceptibility, and
correlate it with the Fermi surface shape. We find that the magnetic ordering
wavevector of Co$_{1/3}$NbS$_2$ obtained from the peak in spin susceptibility
agrees with the one observed experimentally by neutron scattering; it is
compatible with commensurate non-coplanar $3q$ spin structure. We also discuss
how results change if some other than Co transition metal intercalations are
used.",2309.00112v1
2023-09-04,Suppression of small-scale dynamo in time irreversible turbulence,"The conventional theory of small-scale magnetic field generation in a
turbulent flow considers time-reversible random flows. However, real turbulent
flows are known to be time irreversible: the presence of energy cascade is an
intrinsic property of turbulence. We generalize the 'standard' model to account
for the irreversibility. We show that even small time asymmetry leads to
significant suppression of the dynamo effect at low magnetic Prandtl numbers,
increases the generation threshold and may even make generation impossible for
any magnetic Reynolds number. We calculate the magnetic energy growth rate as a
function of the parameters of the flow.",2309.01693v3
2023-09-07,Nonreciprocal phonon dichroism induced by Fermi pocket anisotropy in two-dimensional Dirac materials,"Electrons in two-dimensional (2D) Dirac materials carry local band geometric
quantities, such as the Berry curvature and orbital magnetic moments, which,
combined with electron-phonon coupling, may affect the phonon dynamics in an
unusual way. Here, we propose intrinsic nonreciprocal linear and circular
phonon dichroism in magnetic 2D Dirac materials, which originate from nonlocal
band geometric quantities of electrons and reduce to pure Fermi-surface
properties for acoustic phonons. We find that to acquire the nonreciprocity,
the Fermi pocket anisotropy rather than the chirality of electrons is crucial.
Two possible mechanisms of Fermi pocket anisotropy are suggested: (i) trigonal
warping and out-of-plane magnetization or (ii) Rashba spin-orbit interaction
and in-plane magnetization. As a concrete example, we predict appreciable and
tunable nonreciprocal phonon dichroism in 2H-MoTe 2 on a EuO substrate. Our
finding points to a different route towards electrical control of phonon
nonreciprocity for acoustoelectronics applications based on 2D quantum
materials.",2309.03540v1
2023-09-15,Linear and nonlinear eccentric mode evolution in unstratified MHD discs,"In this paper we develop a framework for studying unstratified, magnetised
eccentric discs and compute uniformly precessing eccentric modes in a
cylindrical annulus which provide convenient initial conditions for numerical
simulations. The presence of a magnetic field in an eccentric disc can be
described by an effective gas with a modified equation of state. At magnetic
field strengths relevant to the magneto-rotational instability the magnetic
field has negligible influence on the evolution of the eccentric disc, however
the eccentric disc can significantly enhance the magnetic field strength over
that in the a circular disc. We verify the suitability of these eccentric disc
solutions by carrying out 2D simulations in RAMSES. Our simulated modes (in 2D)
follow a similar evolution to the purely hydrodynamical modes, matching
theoretical expectations, provided they are adequately resolved. Such solutions
will provide equilibrium states for studies of the eccentric magneto-rotational
instability and magnetised parametric instability in unstratified discs and are
useful for exploring the response of disc turbulence on top of a fluid flow
varying on the orbital timescale.",2309.08261v1
2023-09-15,Charge pumping with strong spin-orbit coupling: Fermi surface breathing and higher harmonic generation,"Spin and charge pumping induced by a precessing magnetization has been
instrumental to the development of spintronics. Nonetheless, most theoretical
studies so far treat the spin-orbit coupling as a perturbation, which
disregards the dynamical competition between exchange and spin-orbit fields. In
this work, based on Keldysh formalism and Wigner expansion, we develop an
adiabatic theory of spin and charge pumping adapted to multiorbital systems
with arbitrary spin-orbit coupling. We apply this theory to the magnetic Rashba
gas and magnetic graphene cases and discuss the pumped ac and dc current. We
show that the pumped current possesses both intrinsic and extrinsic
contributions, akin to the magnetic damping. In addition, we find that higher
harmonics can be generated under large angle precession and we propose a couple
of experimental setups where such an effect could be experimentally observed.",2309.08597v1
2023-10-04,Metadynamics calculations of the effect of thermal spin fluctuations on skyrmion stability,"The stability of magnetic skyrmions has been investigated in the past, but
mostly in the absence of thermal fluctuations. However, thermal spin
fluctuations modify the magnetic properties (exchange stiffness,
Dzyaloshinskii-Moriya interaction (DMI) and anisotropy) that define skyrmion
stability. Thermal magnons also excite internal skrymion dynamics, deforming
the skyrmion shape. Entropy has also been shown to modify skyrmion lifetimes in
experiments, but is absent or approximated in previous studies. Here we use
metadynamics to calculate the free energy surface of a magnetic thin film in
terms of the topological charge and magnetization. We identify the free energy
minima corresponding to different spin textures and the lowest energy paths
between the ferromagnetic and single skyrmion states. We show that at low
temperatures the lowest free energy barrier is a skyrmion collapse process.
However, this energy barrier increases with temperature. An alternative path,
where a singularity forms on the skrymion edge, has a larger free energy
barrier at low temperatures but decreases with increasing temperature and
eventually becomes the lowest energy barrier.",2310.03169v1
2023-10-09,Toward interpreting the IBEX ribbon with mirror diffusion in interstellar turbulent magnetic fields,"We investigate the role of the magnetohydrodynamic (MHD) turbulence measured
by Voyager in the very local interstellar medium (VLISM) in modeling the
Interstellar Boundary Explorer (IBEX) ribbon. We demonstrate that the mirroring
by compressible modes of MHD turbulence dominates over that by the mean
magnetic field. Based on the new mirror diffusion mechanism identified by
Lazarian and Xu for particles with large pitch angles in MHD turbulence, we
find that the mirror diffusion can both confine pickup ions and preserve their
initial pitch angles, and thus accounts for the enhanced intensity of energetic
neutral atoms that return to the heliosphere. The ribbon width is determined by
both the range of pitch angles for effective turbulent mirroring and the field
line wandering induced by Alfv\'{e}nic modes. It in turn provides a constraint
on the amplitude of magnetic fluctuations of fast modes. The field line
wandering also affects the coherence of the ribbon structure across the sky. By
extrapolating the magnetic energy spectrum measured by Voyager, we find that
the injection scale of the turbulence in the VLISM is less than $\sim 500$ au
for the ribbon structure to be coherent.",2310.06032v1
2023-10-16,Axionic Instability of Periodic Weyl-Semimetal Superstructures,"Weyl-semimetal superstructures with a spiraling position of a pair of Weyl
nodes of opposite chirality can host a chiral-symmetry preserving Fermi-arc
metal state, where the chirality is carried by cylindrical Fermi surfaces,
electron- and hole-like depending on the chirality. The Fermi surfaces nest at
vanishing momentum separation (zero nesting vector) at the
electron-hole-compensation energy because the nesting is topologically
protected by vanishing spatial overlap of any pair of equal-momentum
opposite-chirality states. In this work we show that the nesting and Coulomb
interaction drive a spontaneous chiral symmetry breaking in such a Fermi arc
metal, which leads to a dynamical axion insulator state but without breaking
translational symmetry (no charge-density-wave order) as in a conventional Weyl
semimetal. As for material realization, we discuss magnetically doped
Bi$_2$Se$_3$, for which the Weyl-node positions depend on the order of the
magnetic dopands. In this case, the axionic condensation can itself stabilize a
spiral order of the magnetization, and hence the spiraling node positions, even
if the magnetic interaction is intrinsically ferromagnetic.",2310.10345v1
2023-11-01,Towards Ultimate Memory with Single-Molecule Multiferroics,"The demand for high-density storage is urgent in the current era of data
explosion. Recently, several single-molecule (-atom) magnets/ferroelectrics
have been reported to be promising candidates for high-density storage. As
another promising candidate, single-molecule multiferroics are not only small
but also possess ferroelectric and magnetic orderings, which can sometimes be
strongly coupled and used as data storages to realize the combination of
electric writing and magnetic reading. However, they have been rarely proposed,
and never been experimentally reported. Here, by building Hamiltonian models,
we propose a new model of single-molecule multiferroic in which electric
dipoles and magnetic moments are parallel and can rotate with the rotation of
the single molecule. Furthermore, with performing spin-lattice dynamics
simulations, we reveal the conditions (e.g., large enough single-ion anisotropy
and appropriate electric field) under which the new single-molecule
multiferroic can arise. Based on this model, as well as first-principles
calculations, a realistic example Co(NH3)4N@SWCNT is constructed and
numerically confirmed to demonstrate the feasibility of the new single-molecule
multiferroic model. Our work not only sheds light on the discovery of
single-molecule multiferroics but also provides a new guideline to design
multifunctional materials for ultimate memory devices.",2311.00255v1
2023-11-07,A pseudofermion functional renormalization group study of dipolar-octupolar pyrochlore magnets,"Motivated by recent experiments on Ce$_2$Zr$_2$O$_7$ that reveal a dynamic,
liquid-like ground state, we study the nearest neighbor XYZ Hamiltonian of
dipolar-octupolar pyrochlore magnets with the pseudofermion functional
renormalization group (PFFRG), which is numerically implemented by the
SpinParser software. Taking the interaction between the octupolar components to
be dominant and antiferromagnetic, we map out the phase diagram demarcating the
quantum disordered and magnetically ordered states. We identify four distinct
phases, namely the $0$-flux and $\pi$-flux quantum spin ices, and the
all-in-all-out magnetic orders along the local $z$ and $x$ axes. We further use
the static two-spin correlations output by the PFFRG algorithm to compute the
polarized neutron scattering cross-sections, which are able to capture several
qualitative features observed experimentally, in the materially relevant
parameter regime that stabilizes the $\pi$-flux quantum spin ice. Our results
provide support for a quantum spin liquid ground state in Ce$_2$Zr$_2$O$_7$.",2311.04269v1
2023-11-15,Amplitude Higgs mode in superconductors with magnetic impurities,"We study nonlinear response of conventional superconducting alloys with weak
magnetic impurities to an external alternating electromagnetic field. In
particular, we calculate a correction to superconducting order parameter up to
the second order in external vector potential. We show that frequency
dependence of the order parameter amplitude has characteristic resonant shape
with a maximum at frequency which is smaller than twice the magnitude of the
pairing amplitude in equilibrium and at the same time exceeds the
single-particle threshold energy. Our results suggest that in the presence of
magnetic impurities the dynamics of the pairing amplitude in the collisionless
regime will remain robust with respect to dissipative processes. We also
evaluate the third harmonic contribution to the current as a function of the
probe frequency and for various concentrations of magnetic impurities.",2311.09310v1
2023-11-17,Critical ultrasonic propagation in magnetic fields,"Effect of an external magnetic field on the critical sound attenuation and
velocity of the longitudinal wave is studied in ferromagnets. We derive a
parametric model that incorporates a crossover from the asymptotic critical
behavior to the Landau-Ginzburg regular behavior far away from the critical
point. The dynamics is based on the time dependent Ginzburg-Landau model with
non conserved order parameter (model A). The variations of the sound
attenuation coefficient and velocity have been obtained for arbitrary values of
the magnetic field and reduced temperature. The scaling functions are given
within the renormalization group formalism at one-loop order. Using MnP as an
example, we show that such parametric crossover model yields an accurate
description of ultrasonic data in a large region of temperatures and magnetic
fields around the critical point.",2311.10654v1
2023-11-19,Analysis of the $Z_b(10650)$ state based on electromagnetic properties,"In this study, the magnetic and quadrupole moments of the $Z_b(10650)$ state
are determined using the compact diquark-antidiquark interpolating current
through the QCD light-cone sum rule. The values that are obtained as a result
of the analysis are as follows: $\mu_{Z_b} = 2.35^{+0.34}_{-0.33}~\mu_N$ and
$\mathcal{D}_{Z_b} =(1.82^{+0.35}_{-0.31})\times 10^{-2}~\mbox{fm}^2$.
Examining the results obtained, it can be seen that the magnetic moments are
large enough to be measured experimentally, while the quadrupole moment is
obtained as a small but non-zero value, corresponding to a prolate charge
distribution. The magnetic moment is the leading-order response of a bound
system to a weak external magnetic field. It therefore provides an excellent
platform to probe the internal structures of hadrons governed by the
quark-gluon dynamics of QCD.",2311.11327v2
2023-11-22,Quantum Sensing of Magnetic Fields with Molecular Spins,"Spins are prototypical systems with the potential to probe magnetic fields
down to the atomic scale limit. Exploiting their quantum nature through
appropriate sensing protocols allows to enlarge their applicability to fields
not always accessible by classical sensors. Here we first show that quantum
sensing protocols for AC magnetic fields can be implemented on molecular spin
ensembles embedded into hybrid quantum circuits. We then show that, using only
echo detection at microwave frequency and no optical readout, Dynamical
Decoupling protocols synchronized with the AC magnetic fields can enhance the
sensitivity up to $S = 10^{-10}-10^{-9}T/\sqrt{Hz}$ with a low (4-5) number of
applied pulses. These results paves the way for the development of strategies
to exploit molecular spins as quantum sensors.",2311.13504v1
2023-11-25,Imaging a Semi-Analytical Jet model Generated by 3D GRMHD Simulation,"Employing 3D GRMHD simulation, we study the images of a geometrically thin
jet, whose emissions concentrate on its surface, for accretion system
surrounding a central spinning BH. By introducing a strong magnetic field, we
observe three phases of BH accretion evolution: (a) initially, both the
accretion rate and the magnetic flux on the horizon gradually increase; (b) at
an intermediate stage, the magnetic flux approximately reaches saturation, and
a jet forms via the Blandford-Znajek (BZ) mechanism; (c) ultimately, the entire
system achieves a dynamic equilibrium, and a magnetically arrested disk (MAD)
forms. We carefully study the jet images during the saturation and MAD regimes
at various frequencies and from different observational angles. We reveal the
presence of U-shaped brighter lines near the jet surface boundaries, which can
be attributed to the photons whose trajectories skim over the jet surface. The
existence of these brighter lines is a unique feature of a geometrically thin
jet. Moreover, we notice that the jet images are relatively insensitive to the
observed frequencies of interest. Additionally, we observe that the
time-averaged images for the highly oscillating MAD regime show only slight
differences from those of the saturation regime.",2311.14954v1
2023-11-28,A Coarse-grained Model for Aqueous Two-phase Systems: Application to Ferrofluids,"Aqueous two-phase systems (ATPSs), that is, phase-separating solutions of
water soluble but mutually immiscible molecular species, offer fascinating
prospects for selective partitioning, purification, and extraction. Here, we
formulate a general Brownian dynamics based coarse-grained simulation model for
a polymeric ATPS comprising two water soluble but mutually immiscible polymer
species. A third solute species, representing, e.g., nanoparticles (NPs),
additional macromolecular species, or impurities can readily be incorporated
into the model. We demonstrate that the model captures satisfactorily the phase
separation, partitioning, and interfacial properties of a model ATPS composed
of a polymer mixture of dextran and polyethylene glycol (PEG) in which magnetic
NPs selectively partition into one of the two polymeric phases. The NP
partitioning is characterized both via the computational model and
experimentally under different conditions. The simulation model captures the
trends observed in the experiments and quantitatively links the partitioning
behavior to the component species interactions. Finally, the response of the
simulation model to external magnetic field, with the magnetic NPs as the
additional partitioned component, shows that the ATPS interface fluctuations
can be controlled by the magnetic field at length scales much smaller than
those probed experimentally to date.",2311.16906v1
2023-11-29,Subcritical transition to turbulence triggered by a magnetic dynamo,"It has recently been shown that a significant slowdown of many stars can be
attributed to the emergence of a strong magnetic field within the radiative
region, where heat is transferred through radiation in a stably stratified
layer. Here, we describe how this transition can be understood as a subcritical
bifurcation to small-scale turbulence in linearly stable flows. The turbulence
is sustained by a nonlinear mean-field dynamo and can be observed down to
relatively small differential rotation, arbitrarily far from the linear onset
of any hydrodynamic instability. In this regime, turbulent fluctuations provide
diffusivity-free transfer of angular momentum that increases the transport
generated by the magnetic field triggering the turbulence. Finally, we present
a simple nonlinear model that captures this scenario and can be used as a
general description of the transition to turbulence in astrophysical flows, as
long as it involves a competition between a large-scale dynamo, and a
small-scale magnetic instability.",2311.17654v1
2023-12-01,Incommensurate Spiral Spin Order in CaMn$_2$Bi$_2$ observed via High Pressure Neutron Diffraction,"High pressure neutron diffraction is employed to investigate the magnetic
behavior of CaMn$_2$Bi$_2$ in extreme conditions. In contrast to
antiferromagnetic ordering on Mn atoms reported at ambient pressure, our
results reveal that at high pressure, incommensurate spiral spin order emerges
due to the interplay between magnetism on the Mn atoms and strong spin-orbit
coupling on the Bi atoms: sinusoidal spin order is observed at pressures as
high as 7.4 GPa. Competing antiferromagnetic order is observed at different
temperatures in the partially frustrated lattice. This research provides a
unique toolbox for conducting experimental magnetic and spin dynamics studies
on magnetic quantum materials via high pressure neutron diffraction.",2312.00397v1
2023-12-07,Energy dissipation in astrophysical simulations: results of the Orszag-Tang test problem,"The magnetic field through the magnetic reconnection process affects the
dynamics and structure of astrophysical systems. Numerical simulations are the
tools to study the evolution of these systems. However, the resolution,
dimensions, resistivity, and turbulence of the system are some important
parameters to take into account in the simulations. In this paper, we
investigate the evolution of magnetic energy in astrophysical simulations by
performing a standard test problem for MHD codes, Orszag-Tang. We estimate the
numerical dissipation in the simulations using state-of-the-art numerical
simulation code in astrophysics, PLUTO. The estimated numerical resistivity in
2D simulations corresponds to the Lundquist number $\approx 10^{4}$ in the
resolution of $512\times512$ grid cells. It is also shown that the plasmoid
unstable reconnection layer can be resolved with sufficient resolutions. Our
analysis demonstrates that in non-relativistic magnetohydrodynamics
simulations, magnetic and kinetic energies undergo conversion into internal
energy, resulting in plasma heating.",2312.06675v1
2023-12-14,Tuning the spontaneous exchange bias effect in La1.5Sr0.5CoMnO6 with sintering temperature,"Here, we present a study of the influence of microstructure on the magnetic
properties of polycrystalline samples of the La1.5Sr0.5CoMnO6 double
perovskite, with primary attention to the spontaneous exchange bias effect, a
fascinating recently discovered phenomena for which some materials exhibit
unidirectional magnetic anisotropy after being cooled in zero magnetic field.
By sintering La1.5Sr0.5CoMnO6 at different temperatures, we obtained samples
with distinct average grain sizes, ranging from 0.76(21) to 4.95(1.73) mu_m. A
detailed investigation of the material's structural, morphologic, electronic,
and magnetic properties using X-ray powder diffraction, powder neutron
diffraction, X-ray absorption near edge structure spectroscopy, scanning
electron microscopy, and AC and DC magnetometry has revealed a systematic
enhancement of the exchange bias effect with increasing the grain size and the
intergrain coalescence. Our results regarding the strengthening of the exchange
coupling at the grain interfaces and its influence on the dynamics of the spin
glass-like moments present in the samples are discussed.",2312.09149v1
2023-12-19,Emergence of a potential charge disproportionated insulating state in SrCrO$_{3}$,"We use a combination of density functional theory (DFT) and dynamical
mean-field theory (DMFT) to investigate the potential emergence of a
charge-disproportionated insulating phase in SrCrO$_{3}$, whereby the Cr
cations disproportionate according to $3\text{Cr}^{4+} \rightarrow
2\text{Cr}^{3+} + \text{Cr}^{6+}$ and arrange in ordered planes perpendicular
to the cubic [111] direction. We show that the charge disproportionation
couples to a structural distortion where the oxygen octahedra around the
nominal Cr$^{6+}$ sites contract, while the octahedra surrounding the Cr$^{3+}$
sites expand and distort. Our results indicate that the
charge-disproportionated phase can be stabilized for realistic values of the
Hubbard $U$ and Hund's $J$ parameters, but this requires a scaling of the
DFT+DMFT double counting correction by at least 35%. The disproportionated
state can also be stabilized in DFT+U calculations for a specific magnetic
configuration with antiparallel magnetic moments on adjacent Cr$^{3+}$ sites
and no magnetic moment on the Cr$^{6+}$ sites. While this phase is higher in
energy than other magnetic states that can arise in SrCrO$_{3}$, the presence
of an energy minimum suggests that the charge disproportionated state
represents a metastable state of SrCrO$_{3}$.",2312.12033v2
2023-12-28,Statistical domain wall roughness analysis through correlations,"The geometry and morphology of magnetic domain walls (DWs) are closely
related to their dynamics when driven by external forces. Under some reliable
approximations DWs can be considered self-affine interfaces, so universal laws
govern their behavior. On the other hand, large-scale DW structure has been
less explored so far. Recently, it has been shown that bubble-like magnetic
domains can be strongly deformed on a large scale by applying alternating (ac)
magnetic field pulses. In the present work, we conduct a comprehensive analysis
of DW structure at both small and large length scales in bubble-like domains
present in ferromagnetic thin films with perpendicular anisotropy, focusing on
its initial evolution under the application of ac magnetic pulses. Results
obtained from the widely used roughness correlation function B(r) and its
corresponding structure factor, are consistent with those obtained from the
spatial autocorrelation function of DW fluctuations. Whereas the roughness
exponent slightly increases during the ac evolution, a strong deformation is
observed at a large scale, where a striking periodicity (statistically
speaking) is observed. This period is probably determined by the boundary
conditions and a characteristic intrinsic length.",2312.16802v1
2023-12-13,Relativistic spin-0 Duffin-Kemmer-Petiau equation in Bonnor-Melvin-Lambda solution,"In this paper, we conduct a comprehensive exploration of the relativistic
quantum dynamics of spin-0 scalar particles, as described by the
Duffin-Kemmer-Petiau (DKP) equation, within the framework of a magnetic
space-time. Our focus is on the Bonnor-Melvin-Lambda (BML) solution, a
four-dimensional magnetic universe characterized by a magnetic field that
varies with axial distance. To initiate this investigation, we derive the
radial equation using a suitable wave function ansatz and subsequently employ
special functions to solve it. Furthermore, we extend our analysis to include
Duffin-Kemmer-Petiau oscillator fields within the same BML space-time
background. We derive the corresponding radial equation and solve it using
special functions. Significantly, our results show that the geometry's topology
and the cosmological constant (both are related with the magnetic field
strength) influences the eigenvalue solution of spin-0 DKP fields and
DKP-oscillator fields, leading to substantial modifications in the overall
outcomes",2401.01359v2
2024-01-25,Magnon dispersion and spin transport in CrCl$_3$ bilayers under different strain-induced magnetic states,"Atomically-thin van der Waals magnetic materials offer exceptional
opportunities to mechanically and electrically manipulate magnetic states and
spin textures. The possibility of efficient spin transport in these materials
makes them promising for the development of novel nanospintronics technology.
Using atomistic spin dynamics simulations, we investigate magnetic ground
state, magnon dispersion, critical temperature, and magnon spin transport in
CrCl$_3$ bilayers in the absence and presence of compressive and tensile
strains. We show that in the presence of mechanical strain, the magnon band gap
at the $\Gamma$ point and the critical temperature of the bilayer are
increased. Furthermore, our simulations show that the magnon diffusion length
is reduced in the presence of strain. Moreover, by exciting magnons through the
spin Seebeck effect and spin Hall-induced torque, we illustrate distinctions
between magnon spin transport in the antiferromagnetic state, under compressive
strains, and ferromagnetic states, under tensile strains or in the unstrained
case.",2401.13984v1
2024-02-05,Interconnected Renormalization of Hubbard Bands and Green's Function Zeros in Mott Insulators Induced by Strong Magnetic Fluctuations,"We analyze the role of spatial electronic correlations and, in particular, of
the magnetic fluctuations in Mott insulators. A half-filled Hubbard model is
solved at large strength of the repulsion U on a two-dimensional square lattice
using an advanced diagrammatic non-perturbative approach capable of going
beyond Hartree-Fock and single-site dynamical mean-field theories. We show that
at high temperatures the magnetic fluctuations are weak, and the electronic
self-energy of the system is mainly local and is well reproduced by the atomic
(Hubbard-I) approximation. Lowering the temperature toward the low-temperature
magnetically ordered phase, the non-locality of the self-energy becomes crucial
in determining the momentum-dispersion of the Hubbard bands and the Green's
function zeros. We therefore establish a precise link between Luttinger
surface, non-local correlations and spectral properties of the Hubbard bands.",2402.02814v2
2024-02-05,Amorphous quantum magnets in a two-dimensional Rydberg atom array,"Amorphous solids, i.e., systems which feature well-defined short-range
properties but lack long-range order, constitute an important research topic in
condensed matter. While their microscopic structure is known to differ from
their crystalline counterpart, there are still many open questions concerning
the emergent collective behavior in amorphous materials. This is particularly
the case in the quantum regime, where the numerical simulations are extremely
challenging. In this article, we instead propose to explore amorphous quantum
magnets with an analog quantum simulator. To this end, we first present an
algorithm to generate amorphous quantum magnets, suitable for Rydberg
simulators of the Ising model. Subsequently, we use semiclassical approaches to
get a preliminary insight of the physics of the model. In particular, we
calculate mean-field phase diagrams, and use the linear-spin-wave theory to
study localization properties and dynamical structure factors of the
excitations. Finally, we outline an experimental proposal based on Rydberg
atoms in programmable tweezer arrays, thus opening the road towards the study
of amorphous quantum magnets in regimes difficult to simulate classically.",2402.02852v1
2024-02-06,Elucidating the nature of hidden-charm pentaquark states with spin-$\frac{3}{2}$ through their electromagnetic form factors,"We perform a systematic study of the electromagnetic properties of exotic
states to shed light on their nature, which is still controversial and not
fully understood. The magnetic dipole and higher multipole moments of a
hadronic state are as fundamental a dynamical quantity as its mass, and they
contain valuable information about the deep structure underlying it. In the
present work, we have explored the magnetic dipole and higher multipole moments
of the hidden-charm pentaquarks with quantum number $J^P = 3/2^-$ using the QCD
light-cone sum rule method and different interpolating currents. The obtained
results show that different interpolating currents employed to probe
pentaquarks with the same quark content produce varying results for their
magnetic dipole and higher multipole moments at all. This can be interpreted to
mean that there is more than one hidden-charm pentaquark with identical quark
content but with different magnetic dipole and higher multipole moments. The
nature, internal structure, and quark-gluon configurations of these states can
be better understood by studying their electromagnetic properties.",2402.03802v2
2024-02-06,Probing magnetism in moiré heterostructures with quantum twisting microscopes,"Spin-ordered states close to metal-insulator transitions are poorly
understood theoretically and challenging to probe in experiments. Here, we
propose that the quantum twisting microscope, which provides direct access to
the energy-momentum resolved spectrum of single-particle and collective
excitations, can be used as a novel tool to distinguish between different types
of magnetic order. To this end, we calculate the single-particle spectral
function and the dynamical spin-structure factor for both a ferromagnetic and
antiferromagnetic generalized Wigner crystal formed in fractionally filled
moir\'e superlattices of transition metal dichalcogenide heterostructures. We
demonstrate that magnetic order can be clearly identified in these response
functions. Furthermore, we explore signatures of quantum phase transitions in
the quantum twisting microscope response. We focus on the specific case of
triangular moir\'e lattices at half filling, which have been proposed to host a
topological phase transition between a chiral spin liquid and a 120 degree
ordered state. Our work demonstrates the potential for quantum twisting
microscopes to characterize quantum magnetism in moir\'e heterostructures.",2402.04311v1
2024-02-09,Supernova dust destruction in the magnetized turbulent ISM,"Dust in the interstellar medium (ISM) is critical to the absorption and
intensity of emission profiles used widely in astronomical observations, and
necessary for star and planet formation. Supernovae (SNe) both produce and
destroy ISM dust. In particular the destruction rate is difficult to assess.
Theory and prior simulations of dust processing by SNe in a uniform ISM predict
quite high rates of dust destruction, potentially higher than the supernova
dust production rate in some cases. Here we show simulations of
supernova-induced dust processing with realistic ISM dynamics including
magnetic field effects and demonstrate how ISM inhomogeneity and magnetic
fields inhibit dust destruction. Compared to the non-magnetic homogeneous case,
the dust mass destroyed within 1 Myr per SNe is reduced by more than a factor
of two, which can have a great impact on the ISM dust budget.",2402.06543v1
2024-02-25,Effect of Magnetic Anisotropy Constants on Skyrmion Formation in Co/Pt,"Skyrmions, which are topologically stable magnetic structures, have
manifested promising features to be used as an information carrier in new-age,
non-volatile data storage devices. In this article, we show how the creation
and stability of skyrmion can be manipulated by external stimuli (here,
nano-second current pulse). Co/Pt square nano-structure with Co free layer
thickness in the range 1 nm to 5 nm and first and second-order anisotropy
constants are taken to study the controlled creation of skyrmions. The
magnetization dynamics controlled by the current-induced spin transfer torque
help to nucleate skyrmions by transformation from perpendicularly magnetized
ground state to a stable state of isolated skyrmions via complex transformation
of Neel wall following its image inversion. Compared with the first-order
anisotropy alone, how the higher-order anisotropy constants (up to second
order) impact the relaxed state of a system has been discussed.",2402.16060v1
2024-03-02,Zero-field magnetic skyrmions in exchange-biased ferromagnetic-antiferromagnetic bilayers,"We report on the stabilization of ferromagnetic skyrmions in zero external
magnetic fields, in exchange-biased systems composed of
ferromagnetic-antiferromagnetic (FM-AFM) bilayers. By performing atomistic spin
dynamics simulations, we study cases of compensated, uncompensated, and partly
uncompensated FM-AFM interfaces, and investigate the impact of important
parameters such as temperature, inter-plane exchange interaction,
Dzyaloshinskii-Moria interaction, and magnetic anisotropy on the skyrmions
appearance and stability. The model with an uncompensated FM-AFM interface
leads to the stabilization of individual skyrmions and skyrmion lattices in the
FM layer, caused by the effective field from the AFM instead of an external
magnetic field. Similarly, in the case of a fully compensated FM-AFM interface,
we show that FM skyrmions can be stabilized. We also demonstrate that
accounting for interface roughness leads to stabilization of skyrmions both in
compensated and uncompensated interfaces. Moreover, in bilayers with a rough
interface, skyrmions in the FM layer are observed for a wide range of exchange
interaction values through the FM-AFM interface, and the chirality of the
skyrmions depends critically on the exchange interaction.",2403.01175v1
2024-03-04,Realization of a Spin Glass in a two-dimensional van der Waals material,"Recent advances in van der Waals (vdW) materials have sparked renewed
interest in the impact of dimensionality on magnetic phase transitions. While
ordered magnetic phases have been demonstrated to survive in the
two-dimensional (2D) limit, the quest for a spin-glass with quenched magnetic
disorder in lower dimensions has proven elusive. Here we show evidence of a
spin-glass emerging from randomly distributed Fe atoms in Fe3GeTe2, the first
time such a state has been reported in a vdW material. AC magnetic
susceptibility displays a strong frequency dependence indicative of slow spin
dynamics. Additional distinctive phenomena, including ageing, chaos, and memory
effects, further substantiate the existence of a glassy state. Remarkably, we
find that this state persists even in single-unit-cell thick Fe3GeTe2, thereby
confirming the existence of a 2D spin-glass. The formation of spin-glass states
via intercalation in vdW systems allows for highly tunable spin-glass states
that are otherwise difficult to realize.",2403.02088v1
2024-03-12,QCD Equation of State at nonzero baryon density in external magnetic field,"This paper is devoted to the study of QCD equation of state in external
magnetic field and nonzero baryon density. Our study is carried out by means of
lattice simulation with 2+1 dynamical staggered quarks at the physical masses.
The simulation is conducted at imaginary baryon chemical potential what allowed
us to overcome the sign problem. We expand the pressure in the baryon imaginary
chemical potential and study three leading nonzero coefficients in this
expansion. These coefficients were calculated for the following values of
magnetic field: $eB=0.3$, $0.6$, $1.2$ GeV$^2$ with the lattice sizes
$8\times32^3$, $10\times40^3$, $12\times48^3$. Using these data we take
continuum limit for the coefficients. Our results indicate considerable
enhancement of the expansion coefficients by the magnetic field.",2403.07783v1
2024-03-13,Spin-resolved counting statistics as a sensitive probe of spin correlation in transport through a quantum dot spin valve,"We investigate the noise in spin transport through a single quantum dot (QD)
tunnel coupled to ferromagnetic electrodes with noncollinear magnetizations.
Based on a spin-resolved quantum master equation, auto- and cross-correlations
of spin-resolved currents are analyzed to reveal the underlying spin transport
dynamics and characteristics for various polarizations. We find the currents of
majority and minority spins could be strongly autocorrelated despite
uncorrelated charge transfer. The interplay between tunnel coupling and the
Coulomb interaction gives rise to an exchange magnetic field, leading to the
precession of the accumulated spin in the QD. It strongly suppresses the
bunching of spin tunneling events and results in a unique double-peak structure
in the noise of the net spin current. The spin autocorrelation is found to be
susceptible to magnetization alignments, which may serve as a sensitive tool to
measure the magnetization directions between the ferromagnetic electrodes.",2403.08621v1
2024-03-30,Investigation of self-focusing of Gaussian laser beams within magnetized plasma via source-dependent expansion method,"Self-focusing emerges as a nonlinear optical phenomenon resulting from an
intense laser field and plasma interaction. This study investigates the
self-focusing behavior of Gaussian laser beams within magnetized plasma
environments utilizing a novel approach, source-dependent expansion. By
employing source-dependent expansion, we explore the intricate dynamics of
laser beam propagation, considering the influence of plasma density and
external magnetic fields. The interplay between the beam's Gaussian profile and
the self-focusing mechanism through rigorous mathematical analysis and
numerical simulations, particularly in the presence of plasma-induced
nonlinearities are elucidated here. Our findings reveal crucial insights into
the evolution of laser beams under diverse parameters, the ponderomotive force,
relativistic factors, plasma frequency, polarization states, external magnetic
field, wavelength, and laser intensity. This research not only contributes to
advancing our fundamental understanding of laser-plasma interactions but also
holds promise for optimizing laser-driven applications.",2404.00331v1
2024-04-01,A novel seamless magnetic-based actuating mechanism for end-effector-based robotic rehabilitation platforms,"In this pioneering study, we unveiled a groundbreaking approach for actuating
rehabilitation robots through the innovative use of magnetic technology as a
seamless haptic force generator, offering a leap forward in enhancing user
interface and experience, particularly in end-effector-based robots for
upper-limb extremity motor rehabilitation. We employed the Extended Kalman
Filter to meticulously analyze and formalize the robotic system's nonlinear
dynamics, showcasing the potential of this sophisticated algorithm in
accurately tracking and compensating for disturbances, thereby ensuring
seamless and effective motor training. The proposed planar robotic system
embedded with magnetic technology was evaluated with the recruitment of human
subjects. We reached a minimum RMS value of 0.2 and a maximum of 2.06 in our
estimations, indicating our algorithm's capability for tracking the system
behavior. Overall, the results showed significant improvement in smoothness,
comfort, and safety during execution and motor training. The proposed novel
magnetic actuation and advanced algorithmic control opens new horizons for the
development of more efficient and user-friendly rehabilitation technologies.",2404.01441v1
2001-04-11,Dynamic disquantization of Dirac equation,"Classical model S_dc of Dirac particle S_D is constructed. S_D is the dynamic
system described by the Dirac equation. Its classical analog S_dc is described
by a system of ordinary differential equations, containing the quantum constant
h as a parameter. Dynamic equations for S_dc are determined by the Dirac
equation uniquely. Both dynamic systems S_D and S_dc appear to be
nonrelativistic. One succeeded in transforming nonrelativistic dynamic system
S_dc into relativistic one S_dcr. The dynamic system S_dcr appears to be a
two-particle structure (special case of a relativistic rotator). It explains
freely such properties of S_D as spin and magnetic moment, which are strange
for pointlike structure. The rigidity function f_r(a), describing rotational
part of total mass as a function of the radius $a$ of rotator, has been
calculated for S_dcr. For investigation of S_D and construction of S_dc one
uses new dynamic methods: dynamic quantization and dynamic disquantization.
These relativistic pure dynamic procedures do not use principles of quantum
mechanics (QM). They generalize and replace conventional quantization and
transition to classical approximation. Totality of these methods forms the
model conception of quantum phenomena (MCQP). Technique of MCQP is more subtle
and effective, than conventional methods of QM. MCQP relates to conventional
QM, much as the statistical physics relates to thermodynamics.",0104060v3
2023-07-24,Exploring the equilibrium and dynamic phase transition properties of Ising ferromagnet on a decorated triangular lattice,"We study the equilibrium and dynamic phase transition properties of
two-dimensional Ising model on a decorated triangular lattice under the
influence of a time-dependent magnetic field composed of a periodic square wave
part plus a time independent bias term. Using Monte Carlo simulations with
standard Metropolis algorithm, we determine the equilibrium critical behavior
in zero field. At a fixed temperature corresponding to the multidroplet regime,
we locate the relaxation time and the dynamic critical half-period at which a
dynamic phase transition takes place between ferromagnetic and paramagnetic
states. Benefiting from finite-size scaling theory, we estimate the dynamic
critical exponent ratios for the dynamic order parameter and its scaled
variance, respectively. The response function of the average energy is found to
follow a logarithmic scaling as a function of lattice size. At the critical
half-period and in the vicinity of small bias field regime, average of the
dynamic order parameter obeys a scaling relation with a dynamic scaling
exponent which is very close to the equilibrium critical isotherm value.
Finally, in the slow critical dynamics regime, investigation of metamagnetic
fluctuations in the presence of bias field revels a symmetric double-peak
behavior for the scaled variance contours of dynamic order parameter and
average energy. Our results strongly resemble those previously reported for
kinetic Ising models.",2307.12684v1
1997-03-16,Dynamics and Structure of Three-Dimensional Poloidally Magnetized Supermagnetosonic Jets,"A set of 3D MHD simulations of magnetized jets has been performed. The jets
contain an equipartition primarily poloidal magnetic field and the effect of
jet density on jet dynamics and structure is evaluated. The jet is precessed at
the origin to excite Kelvin-Helmholtz unstable helical modes. We extensively
compare the structure in these simulations with linear stability theory. The
jet that is dense with respect to the external medium develops a high speed
core surrounded by a less dense sheath consisting of slower moving jet fluid.
These simulations suggest that extended extragalactic jets propagate to such
large distances because they are surrounded by a lobe or cocoon whose density
is less than the jet density. (Abridged abstract.)",9703098v1
1997-12-02,A dynamical model for the penumbral fine structure and the Evershed effect in sunspots,"Relying on the assumption that the interchange convection of magnetic flux
tubes is the physical cause for the existence of sunspot penumbrae, we propose
a model in which the dynamical evolution of a thin magnetic flux tube
reproduces the Evershed effect and the penumbral fine structure such as bright
and dark filaments and penumbral grains.
  According to our model, penumbral grains are the manifestation of the
footpoints of magnetic flux tubes, along which hot subphotospheric plasma flows
upwards with a few km/s. Above the photosphere the hot plasma inside the tube
is cooled by radiative losses as it flows horizontally outwards. As long as the
flowing plasma is hotter than the surroundings, it constitutes a bright radial
filament. The flow confined to a thin elevated channel reaches the temperature
equilibrium with the surrounding atmosphere and becomes optically thin near the
outer edge of the penumbra.
  Here, the tube has a height of approximately 100 km above the continuum and
the flow velocity reaches up to 14 km/s. Such a flow channel can reproduce the
observed signatures of the Evershed effect.",9712029v1
1999-01-27,Non-linear dynamics of Kelvin-Helmholtz unstable magnetized jets: three-dimensional effects,"A numerical study of the Kelvin-Helmholtz instability in compressible
magnetohydrodynamics is presented. The three-dimensional simulations consider
shear flow in a cylindrical jet configuration, embedded in a uniform magnetic
field directed along the jet axis. The growth of linear perturbations at
specified poloidal and axial mode numbers demonstrate intricate non-linear
coupling effects. The physical mechanims leading to induced secondary
Kelvin-Helmholtz instabilities at higher mode numbers are identified. The
initially weak magnetic field becomes locally dominant in the non-linear
dynamics before and during saturation. Thereby, it controls the jet deformation
and eventual breakup. The results are obtained using the Versatile Advection
Code [G. Toth, Astrophys. Lett. Comm. 34, 245 (1996)], a software package
designed to solve general systems of conservation laws. An independent
calculation of the same Kelvin-Helmholtz unstable jet configuration using a
three-dimensional pseudo-spectral code gives important insights into the
coupling and excitation events of the various linear mode numbers.",9901383v1
1999-04-03,The Galactic Center Isolated Nonthermal Filaments as Analogs of Cometary Plasma Tails,"We propose a model for the origin of the isolated nonthermal filaments
observed at the Galactic center based on an analogy to cometary plasma tails.
We invoke the interaction between a large scale magnetized galactic wind and
embedded molecular clouds. As the advected wind magnetic field encounters a
dense molecular cloud, it is impeded and drapes around the cloud, ultimately
forming a current sheet in the wake. This draped field is further stretched by
the wind flow into a long, thin filament whose aspect ratio is determined by
the balance between the dynamical wind and amplified magnetic field pressures.
The key feature of this cometary model is that the filaments are dynamic
configurations, and not static structures. As such, they are local
amplifications of an otherwise weak field and not directly connected to any
static global field. The derived field strengths for the wind and wake are
consistent with observational estimates. Finally, the observed synchrotron
emission is naturally explained by the acceleration of electrons to high energy
by plasma and MHD turbulence generated in the cloud wake.",9904048v1
2000-12-01,Singular Isothermal Disks and the Formation of Multiple Stars,"A crucial missing ingredient in previous theoretical studies of fragmentation
is the inclusion of dynamically important levels of magnetic fields. As a
minimal model for a candidate presursor to the formation of binary and multiple
stars, we therefore consider the equilibrium configuration of isopedically
magnetized, scale-free, singular isothermal disks, without the assumption of
axial symmetry. We find that lopsided (M=1) configurations exist at any
dimensionless rotation rate, including zero. Multiple-lobed (M = 2, 3, 4, ...)
configurations bifurcate from an underlying axisymmetric sequence at
progressively higher dimensionless rates of rotation, but such nonaxisymmetric
sequences always terminate in shockwaves before they have a chance to fission
into separate bodies. We advance the hypothesis that binary and multiple
star-formation from smooth (i.e., not highly turbulent) starting states that
are supercritical but in unstable mechanical balance requires the rapid (i.e.,
dynamical) loss of magnetic flux at some stage of the ensuing gravitational
collapse.",0012019v1
2000-12-11,Sheared helical turbulence and the helicity constraint in large-scale dynamos,"The effect of shear on the growth of large scale magnetic fields in helical
turbulence is investigated. The resulting large-scale magnetic field is also
helical and continues to evolve, after saturation of the small scale field, on
a slow resistive time scale. This is a consequence of magnetic helicity
conservation. Because of shear, the time scale needed to reach an
equipartition-strength large scale field is shortened proportionally to the
ratio of the resulting toroidal to poloidal large scale fields.",0012240v1
2001-12-17,Modeling the Galactic Center Nonthermal Filaments as Magnetized Wake,"We simulate the Galactic Center nonthermal filaments as magnetized wakes
formed dynamically from amplification of a weak (tens of $\mu$G) global
magnetic field through the interaction of molecular clouds with a Galactic
Center wind. One of the key issues in this cometary model is the stability of
the filament against dynamical disruption. Here we show 2-dimensional MHD
simulations for interstellar conditions that are appropriate for the Galactic
Center. The structures eventually disrupt through a shear driven nonlinear
instability but maintain coherence for lengths up to 100 times their width as
observed. The final instability, which destroys the filament through shredding
and plasmoid formation, grows quickly in space (and time) and leads to an
abrupt end to the structure, in accord with observations. As a by-product, the
simulation shows that emission should peak well downstream from the cloud-wind
interaction site.",0112378v1
2004-06-28,Dynamics and Heating of the Magnetic Network on the Sun: Efficiency of mode transformation,"We aim to identify the physical processes which occur in the magnetic network
of the chromosphere and which contribute to its dynamics and heating.
Specifically, we study the propagation of transverse (kink) MHD waves which are
impulsively excited in flux tubes through footpoint motions. When these waves
travel upwards, they get partially converted to longitudinal waves through
nonlinear effects (mode coupling). By solving the nonlinear, time-dependent MHD
equations we find that significant longitudinal wave generation occurs in the
photosphere typically for Mach numbers as low as 0.2 and that the onset of
shock formation occurs at heights of about 600 km above the photospheric base.
We also investigate the compressional heating due to longitudinal waves and the
efficiency of mode coupling for various values of the plasma $\beta$, that
parameterises the magnetic field strength in the network. We find that this
efficiency is maximum for field strengths corresponding to $\beta\approx 0.2$,
when the kink and tube wave speeds are almost identical. This can have
interesting observational implications. Furthermore, we find that even when the
two speeds are different, once shock formation occurs, the longitudinal and
transverse shocks exhibit strong mode coupling.",0406626v1
2005-09-23,The Effect of Coherent Structures on Stochastic Acceleration in MHD Turbulence,"We investigate the influence of coherent structures on particle acceleration
in the strongly turbulent solar corona. By randomizing the Fourier phases of a
pseudo-spectral simulation of isotropic MHD turbulence (Re $\sim 300$), and
tracing collisionless test protons in both the exact-MHD and phase-randomized
fields, it is found that the phase correlations enhance the acceleration
efficiency during the first adiabatic stage of the acceleration process. The
underlying physical mechanism is identified as the dynamical MHD alignment of
the magnetic field with the electric current, which favours parallel
(resistive) electric fields responsible for initial injection. Conversely, the
alignment of the magnetic field with the bulk velocity weakens the acceleration
by convective electric fields $- \bfu \times \bfb$ at a non-adiabatic stage of
the acceleration process. We point out that non-physical parallel electric
fields in random-phase turbulence proxies lead to artificial acceleration, and
that the dynamical MHD alignment can be taken into account on the level of the
joint two-point function of the magnetic and electric fields, and is therefore
amenable to Fokker-Planck descriptions of stochastic acceleration.",0509717v1
2005-09-29,Are Neutral Sunspot Winds Important for Penumbral Dynamics and the First Ionization Potential Effect?,"The low ionization state in parts of a sunspot may play an important role in
its evolution and dynamical state. The cool magnetic interior region of the
sunspot develops a substantial neutral atomic and molecular hydrogen osmotic
pressure which can drive a wind outward from the umbra. Ambipolar diffusion
against the magnetically pinned ionized plasma component can also distort the
umbral magnetic field into a filamentary penumbral structure. This may be
important for explaining the development of the sunspot penumbra and the
Evershed flow. This fractionation process may also be important for the ``First
Ionization Potential'' (FIP) effect seen in the solar wind. In support of this
mechanism we find evidence for such ionization fractionization in UV
observations of molecular hydrogen in a sunspot umbra and penumbra.",0509862v1
1997-07-23,Experimental Study of Bifurcations in A Parametrically Forced Pendulum,"An experimental study of bifurcations associated with stability of stationary
points (SP's) in a parametrically forced magnetic pendulum and a comparison of
its results with numerical results are presented. The critical values for which
the SP's lose or gain their stability are experimentally measured by varying
the two parameters $\Omega$ (the normalized natural frequency) and $A$ (the
normalized driving amplitude). It is observed that, when the amplitude $A$
exceeds a critical value, the normal SP with $\theta=0$ ($\theta$ is the angle
between the permanent magnet and the magnetic field) becomes unstable either by
a period-doubling bifurcation or by a symmetry-breaking pitchfork bifurcation,
depending on the values of $\Omega$. However, in contrast with the normal SP
the inverted SP with $\theta=\pi$ is observed to become stable as $A$ is
increased above a critical value by a pitchfork bifurcation, but it also
destabilizes for a higher critical value of $A$ by a period-doubling
bifurcation. All of these experimental results agree well with numerical
results obtained using the Floquet theory.",9707017v1
1995-12-12,Magnetic Properties of the t-J Model in the Dynamical Mean-Field Theory,"We present a theory for the spin correlation function of the t-J model in the
framework of the dynamical mean-field theory. Using this mapping between the
lattice and a local model we are able to obtain an intuitive expression for the
non-local spin susceptibility, with the corresponding local correlation
function as input. The latter is calculated by means of local Goldstone
diagrams following closely the procedures developed and successfully applied
for the (single impurity) Anderson model.We present a systematic study of the
magnetic susceptibility and compare our results with those of a Hubbard model
at large U. Similarities and differences are pointed out and the magnetic phase
diagram of the t-J model is discussed.",9512096v1
1997-06-25,Domain Dynamics of Magnetic Films with Perpendicular Anisotropy,"We study the magnetic properties of nanoscale magnetic films with large
perpendicular anisotropy comparing polarization microscopy measurements on
Co_28Pt_72 alloy samples based on the magneto-optical Kerr effect with Monte
Carlo simulations of a corresponding micromagnetic model. We focus on the
understanding of the dynamics especially the temperature and field dependence
of the magnetisation reversal process. The experimental and simulational
results for hysteresis, the reversal mechanism, domain configurations during
the reversal, and the time dependence of the magnetisation are in very good
qualitative agreement. The results for the field and temperature dependence of
the domain wall velocity suggest that for thin films the hysteresis can be
described as a depinning transition of the domain walls rounded by thermal
activation for finite temperatures.",9706253v1
1997-10-20,Row-switched states in two-dimensional underdamped Josephson junction arrays,"When magnetic flux moves across layered or granular superconductor
structures, the passage of vortices can take place along channels which develop
finite voltage, while the rest of the material remains in the zero-voltage
state. We present analytical studies of an example of such mixed dynamics: the
row-switched (RS) states in underdamped two-dimensional Josephson arrays,
driven by a uniform DC current under external magnetic field but neglecting
self-fields. The governing equations are cast into a compact
differential-algebraic system which describes the dynamics of an assembly of
Josephson oscillators coupled through the mesh current. We carry out a formal
perturbation expansion, and obtain the DC and AC spatial distributions of the
junction phases and induced circulating currents. We also estimate the interval
of the driving current in which a given RS state is stable. All these
analytical predictions compare well with our numerics. We then combine these
results to deduce the parameter region (in the damping coefficient versus
magnetic field plane) where RS states can exist.",9710204v1
1997-11-13,NMR study of the S=1/2 Heisenberg Ladder Cu2(C5H12N2)2Cl4 : Quantum phase transition and critical dynamics,"We present an extensive NMR study of the spin-1/2 antiferromagnetic
Heisenberg ladder Cu2(C5H12N2)2Cl4 in a magnetic field range 4.5 - 16.7 T. By
measuring the proton NMR relaxation rate 1/T_1 and varying the magnetic field
around the critical field H_c1 = Delta / g\mu_B = 7.5 T, we have studied the
transition from a gapped spin liquid ground state to a gapless magnetic regime
which can be described as a Luttinger liquid. We identify an intermediate
regime T > |H-H_c1|, where the spin dynamics is (possibly) only controlled by
the T=0 critical point H_c1.",9711124v1
1998-01-21,Nucleation and hysteresis in Ising model: Classical theory versus computer simulation,"We have studied the nucleation in the nearest neighbour ferromagnetic Ising
model, in different (d) dimensions, by extensive Monte Carlo simulation using
the heat-bath dynamics. The nucleation time (tau) has been studied as a
function of the magnetic field (h) for various system sizes in different
dimensions (d=2,3,4). The logarithm of the nucleation time is found to be
proportional to the power (-(d-1)) of the magnetic field (h) in d dimensions.
The size dependent crossover from coalescence to nucleation regime is observed
in all dimensions. The distribution of metastable lifetimes are studied in both
regions. The numerical results are compared and found to be consistent with the
classical theoretical predictions. In two dimensions, we have also studied the
dynamical response to a sinusoidally oscillating magnetic field. The reversal
time is studied as a function of the inverse of the coercive field. The
applicability of the classical nucleation theory to study the hysteresis and
coercivity has been discussed.",9801213v2
1998-06-11,Dynamical properties of a two-dimensional electron gas in a magnetic field within the composite fermion model,"We investigate the response of a two-dimensional electron gas, in the
fractional quantum Hall regime, to the sudden appearance of a localised charged
probe using the Chern-Simons theory of composite fermions. The dynamic
structure factor of the electron gas is found to have a major influence on the
spectral function of the probe. In particular, there is an orthogonality
catastrophe when the filling factor is an even-denominator filling fraction due
to the compressibility of the state, but there is no catastrophe at
odd-denominator filling factors because these states have a gap to excitations.
The catastrophe is found to be more severe for composite fermions in zero
effective magnetic field than it is for electrons in zero real magnetic field.
Oscillations in the spectral function, arising when the composite fermions are
at integer filling, have a period equal to the composite fermion cyclotron
energy. We propose a tunneling experiment which directly measures the spectral
function from which one could determine the composite fermion effective mass.",9806149v1
1998-11-06,Combining dynamical mean-field theory and realistic bandstructure of V2O3,"Recent neutron scattering experiments on V2O3 show that the magnetic
fluctuations on the metallic side of the antiferromagnetic metal-insulator
transition are not related to the spin structure of the insulator, but rather
to the bandstructure-driven spin-density wave phase of the doped system
V(2-y)O3. We calculate these magnetic fluctuations starting from a
Slater-Koster bandstructure and incorporating the correlation effects through
the dynamical mean-field theory (DMFT). Our results demonstrate that the
magnetic properties of the paramagnetic metallic phase are dominated by the
Fermi surface topology. On the other hand, the electron-electron interaction
drives the paramagnetic metal-insulator transition in V2O3. The transition to
the antiferromagnetic insulator by virtue of orbital ordering is discussed in
the framework of the DMFT.",9811095v1
1999-04-06,Non-Fermi-liquid behavior in the Kondo lattices induced by peculiarities of magnetic ordering and spin dynamics,"A scaling consideration of the Kondo lattices is performed with account of
singularities in the spin excitation spectral function. It is shown that a
non-Fermi-liquid (NFL) behavior between two critical values of the bare $s-f$
coupling constant occurs naturally for complicated magnetic structures with
several magnon branches. This may explain the fact that a NFL behavior takes
place often in the heavy-fermion systems with peculiar spin dynamics. Another
kind of a NFL-like state (with different critical exponents) can occur for
simple antiferromagnets with account of magnon damping, and for paramagnets,
especially with two-dimensional character of spin fluctuations. The mechanisms
proposed lead to some predictions about behavior of specific heat, resistivity,
magnetic susceptibility, and anisotropy parameter, which can be verified
experimentally.",9904072v3
1999-07-19,Renormalization of the Spin-Peierls Transition due to Phonon Dynamics,"We report results from a systematic strong-coupling expansion of a spin-1/2
Heisenberg chain coupled to Einstein phonons. In the non-adiabatic regime this
model is used to describe zero temperature properties of CuGeO3. The linked
cluster expansion allows the determination of observables in the thermodynamic
limit preserving the full lattice dynamics without a truncation of the phononic
Hilbert space. In particular, the spin gap and the dispersion of the elementary
triplet excitation are calculated up to 10th order in a dimer expansion. The
magnetic structure factor of the ground state is evaluated up to 6th order. We
show that the spin-phonon coupling leads to a renormalization of the elementary
triplet dispersion. Surprisingly in the non-adiabatic regime a substantial
renormalization of the spin gap only sets in at much larger couplings than
those proposed for CuGeO3. The ground state magnetic correlations are found to
be hardly effected by the spin-phonon coupling, but dominated by the
frustrating magnetic interaction in the parameter regime relevant for CuGeO3.",9907266v1
1999-07-26,Magnetic susceptibility of the double exchange model,"Previously a many-body coherent potential approximation (CPA) was used to
study the double exchange (DE) model with quantum local spins S, both for S=1/2
and for general S in the paramagnetic state. This approximation, exact in the
atomic limit, was considered to be a many-body extension of Kubo's one-electron
dynamical CPA for the DE model. We now extend our CPA treatment to the case of
general S and spin polarization. We show that Kubo's one-electron CPA is always
recovered in the empty-band limit and that our CPA is equivalent to dynamical
mean field theory in the classical spin limit. We then solve our CPA equations
self-consistently to obtain the static magnetic susceptibility chi in the
strong-coupling limit. As in the case of the CPA for the Hubbard model we find
unphysical behaviour in chi at half-filling and no magnetic transition for any
finite S. We identify the reason for this failure of our approximation and
propose a modification which gives the correct Curie-law behaviour of chi at
half-filling and a transition to ferromagnetism for all S.",9907393v1
1999-12-06,Coherent spin dynamics in diluted-magnetic quantum wells,"Spins of charge carriers, paramagnetic centers, and nuclei in semiconductor
structures are known to be oriented if circular-polarized light is shined upon
the structure. This is due to transfer of angular momentum from photons to
various excitations in the semiconductor. What would happen if the structure is
anisotropic, so that the angular momentum is not conserved? Experiments
performed on diluted-magnetic II-VI quantum wells (such as CdMnTe/CdMgTe) show
that at some conditions the effect of light upon the spin state of the
semiconductor structure can be greatly amplified. Exchange field of
photoexcited heavy holes initiates a coherent evolution of a great number of
localized spins of Mn-ions. In the simplest case, it is just a precession in an
effective field, but generally the spin dynamics is more intricate. It shows
up, for instance, in long sequences of spin-flip peaks in Raman spectra, in
enhanced magnetic-polaron effect, and in modulation of optical response within
the picosecond time scale.",9912084v1
2000-01-19,Precision microwave dielectric and magnetic susceptibility measurements of correlated electronic materials using superconducting cavities,"We analyze microwave cavity perturbation methods, and show that the technique
is an excellent, precision method to study the dynamic magnetic and dielectric
response in the $GHz$ frequency range. Using superconducting cavities, we
obtain exceptionally high precision and sensitivity for measurements of
relative changes. A dynamic electromagnetic susceptibility
$\tilde{\zeta}(T)=\zeta ^{\prime}+i\zeta ^{\prime \prime}$ is introduced, which
is obtained from the measured parameters: the shift of cavity resonant
frequency $\delta f$ and quality factor $Q$. We focus on the case of a
spherical sample placed at the center of a cylindrical cavity resonant in the
$TE_{011}$ mode. Depending on the sample characteristics, the magnetic
permeability $\tilde{\mu}$, the dielectric permittivity $\tilde{\epsilon}$ and
the complex conductivity $\tilde{\sigma}$ can be extracted from
$\tilde{\zeta}_{H}$. A full spherical wave analysis of the cavity perturbation
is given. This analysis has led to the observation of new phenomena in novel
low dimensional materials.",0001280v1
2000-05-10,Lineshape predictions via Bethe ansatz for the one-dimensional spin-1/2 Heisenberg antiferromagnet in a magnetic field,"The spin fluctuations parallel to the external magnetic field in the ground
state of the one-dimensional (1D) s=1/2 Heisenberg antiferromagnet are
dominated by a two-parameter set of collective excitations. In a cyclic chain
of N sites and magnetization 0<M_z<N/2, the ground state, which contains 2M_z
spinons, is reconfigured as the physical vacuum for a different species of
quasi-particles, identifiable in the framework of the coordinate Bethe ansatz
by characteristic configurations of Bethe quantum numbers. The dynamically
dominant excitations are found to be scattering states of two such
quasi-particles. For N -> \infty, these collective excitations form a continuum
in (q,\omega)-space with an incommensurate soft mode. Their matrix elements in
the dynamic spin structure factor S_{zz}(q,\omega) are calculated directly from
the Bethe wave functions for finite N. The resulting lineshape predictions for
N -> \infty complement the exact results previously derived via algebraic
analysis for the exact 2-spinon part of S_{zz}(q,\omega) in the zero-field
limit. They are directly relevant for the interpretation of neutron scattering
data measured in nonzero field on quasi-1D antiferromagnetic compounds.",0005174v1
2001-03-19,Magnetism of correlated systems: beyond LDA,"A novel approach to electronic correlations in magnetic crystals which takes
into account a dynamical many-body effects is present. In order to to find a
frequency dependence of the electron self energy, an effective quantum-impurity
many-particle problem need to be solved within the dynamical mean-field theory.
The numerically exact QMC-scheme and the spin-polarized fluctuation exchange
approximation are used for the self-consistent solution of this single-site
many-particle problem. The calculations of effective exchange interaction
parameters based on the realistic electronic structure of correlated magnetic
crystals have been discussed.",0103389v1
2001-03-30,Thermal magnetization fluctuations in thin films and a new physical form for magnetization damping,"The effect of thermal fluctuations on a thin film magnetoresistive element
has been calculated. The technique involves adding to the basic spin dynamics a
general form of interaction with a thermal bath. For a general anisotropic
magnetic system the resulting equation can be written as a Langevin equation
for a harmonic oscillator. Our approach predicts two times smaller noise power
at low frequencies than the conventional stochastic Landau-Lifshitz-Gilbert
equation. It is shown that equivalent results can be obtained by introducing a
tensor phenomenological damping term to the gyromagnetic dynamics driven by a
thermal fluctuating field.",0103624v3
2001-08-19,Magnetic relaxation and critical current density of $MgB_2$ thin films,"Magnetic relaxation and critical current density have been measured on a
$MgB_2$ thin film in a wide region of temperature with the magnetic field up to
8 T. The irreversibility line has also been determined. It is found that the
relaxation rate has a very weak temperature dependence below $1/2 T_c$ showing
a clear residual relaxation rate at zero K, which cannot be easily explained as
due to thermally activated flux creep. Furthermore the relaxation rate has a
strong field dependence. The flux dynamics of thin films are very similar to
that of high pressure synthesized bulks although the relaxation rate in thin
film is systematically higher than that of a bulk sample. All the results here
together with those from the bulk samples suggest that the flux dynamics may be
dominated by the quantum effects, such as quantum fluctuation and tunneling.",0108294v1
2002-05-22,Quantum Spin Dynamics in Molecular Magnets,"The detailed theoretical understanding of quantum spin dynamics in various
molecular magnets is an important step on the roadway to technological
applications of these systems. Quantum effects in both ferromagnetic and
antiferromagnetic molecular clusters are, by now, theoretically well
understood. Ferromagnetic molecular clusters allow one to study the interplay
of incoherent quantum tunneling and thermally activated transitions between
states with different spin orientation. The Berry phase oscillations found in
Fe_8 are signatures of the quantum mechanical interference of different
tunneling paths. Antiferromagnetic molecular clusters are promising candidates
for the observation of coherent quantum tunneling on the mesoscopic scale.
Although challenging, applications of molecular magnetic clusters for data
storage and quantum data processing are within experimental reach already with
present day technology.",0205457v1
2002-08-27,Semi-analytical solution of the Kondo model in a magnetic field,"The single impurity Kondo model at zero temperature in a magnetic field is
solved by a semi-analytical approach based on the flow equation method. The
resulting problem is shown to be equivalent to a resonant level model with a
non-constant hybridization function. This nontrivial effective hybridization
function encodes the quasiparticle interaction in the Kondo limit, while the
magnetic field enters as the impurity orbital energy. The evaluation of static
and dynamic quantities of the strong-coupling Kondo model becomes very simple
in this effective model. We present results for thermodynamic quantities and
the dynamical spin-structure factor and compare them with NRG calculations.",0208539v2
2003-04-02,Continuous quantum phase transition in a Kondo lattice model,"We study the magnetic quantum phase transition in an anisotropic Kondo
lattice model. The dynamical competition between the RKKY and Kondo
interactions is treated using an extended dynamic mean field theory (EDMFT)
appropriate for both the antiferromagnetic and paramagnetic phases. A quantum
Monte Carlo approach is used, which is able to reach very low temperatures, of
the order of 1% of the bare Kondo scale. We find that the finite-temperature
magnetic transition, which occurs for sufficiently large RKKY interactions, is
first order. The extrapolated zero-temperature magnetic transition, on the
other hand, is continuous and locally critical.",0304033v3
2003-07-05,Anisotropic splitting of intersubband spin plasmons in quantum wells with bulk and structural inversion asymmetry,"In semiconductor heterostructures, bulk and structural inversion asymmetry
and spin-orbit coupling induce a k-dependent spin splitting of valence and
conduction subbands, which can be viewed as being caused by momentum-dependent
crystal magnetic fields. This paper studies the influence of these effective
magnetic fields on the intersubband spin dynamics in an asymmetric n-type
GaAs/AlGaAs quantum well. We calculate the dispersions of intersubband spin
plasmons using linear response theory. The so-called D'yakonov-Perel'
decoherence mechanism is inactive for collective intersubband excitations,
i.e., crystal magnetic fields do not lead to decoherence of spin plasmons.
Instead, we predict that the main signature of bulk and structural inversion
asymmetry in intersubband spin dynamics is a three-fold, anisotropic splitting
of the spin plasmon dispersion. The importance of many-body effects is pointed
out, and conditions for experimental observation with inelastic light
scattering are discussed.",0307124v1
2003-12-30,"Dynamical Systems, Topology and Conductivity in Normal Metals in strong magnetic fields","We represent here the full description of all asymptotic regimes of
conductivity behavior in the so-called ""Geometric Strong Magnetic Field limit""
in the 3D single crystal normal metals with topologically complicated Fermi
surfaces. In particular, new observable integer-valued characteristics of
conductivity of the topological origin were introduced by the present authors
few years ago; they are based on the Topological Resonance found by the present
authors and play the basic role in the total picture. Our investigation is
based on the study of dynamical systems on Fermi surfaces for the
semi-classical motion of electron in magnetic field realized by the Moscow
topological group.",0312708v1
2004-03-29,Two regimes in the magnetic field response of superconducting MgB$_2$,"Using Scanning Tunneling Microscope at low temperature we explore the
superconducting phase diagram in the $\pi$-band of the two-band superconductor
MgB$_2$. In this band the peculiar shape of the local tunneling spectra and
their dynamics in the magnetic field reveal the complex character of the
quasiparticle density of states (DOS). The gap in the DOS is first rapidly
filled with states in raising the magnetic field up to 0.5 T and then slowly
approaches the normal state value : The gap is observed up to 2 T. Such a
change in the DOS dynamics suggests the existence of two terms in the DOS of
the $\pi$-band: a first one, reflecting an intrinsic superconductivity in the
band and a second one, originating from an inter-band coupling to the
$\sigma$-band. Our findings allow a deeper understanding of the unique phase
diagram of MgB$_2$.",0403700v1
2004-03-30,"Finite-Size effects in ""Single Chain Magnets"": an experimental and theoretical study","The problem of finite size effects in s=1/2 Ising systems showing slow
dynamics of the magnetization is investigated introducing diamagnetic
impurities in a Co$^{2+}$-radical chain. The static magnetic properties have
been measured and analyzed considering the peculiarities induced by the
ferrimagnetic character of the compound. The dynamic susceptibility shows that
an Arrhenius law is observed with the same energy barrier for the pure and the
doped compounds while the prefactor decreases, as theoretically predicted.
Multiple spins reversal has also been investigated.",0403731v1
2004-04-04,Critical dynamics of the Potts model: short-time Monte Carlo simulations,"We calculate the new dinamic exponent $\theta $ of the 4-state Potts model,
using short-time simulations. Our estimates $\theta_{1}=-0.0471(33)$ and $%
\theta_{2}=$ $-0.0429(11)$ obtained by following the behavior of the
magnetization or measuring the evolution of the time correlation function of
the magnetization corroborate the conjecture by Okano et. al. In addition,
these values agree with previous estimate of the same dynamic exponent for the
two-dimensional Ising model with three-spin interactions in one direction, that
is known to belong to the same universality class as the 4-state Potts model.
The anomalous dimension of initial magnetization $% x_{0}=z\theta +\beta /\nu $
is calculated by an alternative way that mixes two different initial
conditions. We have also estimated the values of the static exponents $\beta $
and $\nu $. They are in complete agreement with the pertinent results of the
literature.",0404083v1
2004-10-07,Quantum Fluctuations and Excitations in Antiferromagnetic Quasicrystals,"We study the effects of quantum fluctuations and the excitation spectrum for
the antiferromagnetic Heisenberg model on a two-dimensional quasicrystal, by
numerically solving linear spin-wave theory on finite approximants of the
octagonal tiling. Previous quantum Monte Carlo results for the distribution of
local staggered magnetic moments and the static spin structure factor are
reproduced well within this approximate scheme. Furthermore, the magnetic
excitation spectrum consists of magnon-like low-energy modes, as well as
dispersionless high-energy states of multifractal nature. The dynamical spin
structure factor, accessible to inelastic neutron scattering, exhibits
linear-soft modes at low energies, self-similar structures with bifurcations
emerging at intermediate energies, and flat bands in high-energy regions. We
find that the distribution of local staggered moments stemming from the
inhomogeneity of the quasiperiodic structure leads to a characteristic energy
spread in the local dynamical spin susceptibility, implying distinct nuclear
magnetic resonance spectra, specific for different local environments.",0410180v1
2005-09-14,Reentrance during nonequilibrium relaxation,"We consider nonequilibrium critical dynamics of the two-dimensional Ising
model for which the initial state is prepared by switching on random fields
with zero mean and variance $H$. In the initial state there is no magnetic
order but the clusters of parallel spins have a percolation transition for
small enough $H$. Using heath-bath dynamics we measure the relaxation of the
magnetization which shows a reentrance in time. Due to cluster dissolution in
the early time regime there is a decrease of the magnetization, followed by an
increase due to nonequilibrium domain growth which itself turns to a decrease
due to equilibrium relaxation. The power law decay of the nonequilibrium
autocorrelation function is not influenced by the percolation in the initial
state.",0509372v1
2005-11-02,Magneto-optical imaging of magnetic flux patterns in superconducting films with antidots,"Superconducting YBaCuO thin films were equipped with a special arrangement of
antidots (holes) of 1 micron radius in order to guide the stream of magnetic
flux moving in (or out of) the sample. The flux distribution and its dynamics
were visualized using real-time magneto-optical imaging. It is clearly
demonstrated that one-dimensional antidot arrays strongly facilitate
propagation of magnetic flux. We also demonstrate a possibility to alter the
direction of flux motion in a controlled way by special arrangement of
intercepting antidot arrays. Our resolution was sufficient for observation of
flux in particular antidots, which allows a more detailed dynamic analysis of
such systems.",0511060v1
2005-12-01,Nano-magnetic droplets and implications to orbital ordering in La1-xSrxCoO3,"Inelastic cold neutron scattering on LaCoO3 provided evidence for a distinct
low energy excitation at 0.6 meV coincident with the thermally induced magnetic
transition. Coexisting strong ferromagnetic (FM) and weaker antiferromagnetic
(AFM) correlations that are dynamic follow the activation to the excited state,
identified as the intermediate S=1 spin triplet. This is indicative of
dynamical orbital ordering favoring the observed magnetic interactions. With
hole doping as in La1-xSrxCoO3, the FM correlations between Co spins become
static and isotropically distributed due to the formation of FM droplets. The
correlation length and condensation temperature of these droplets increase
rapidly with metallicity due to the double exchange mechanism.",0512030v1
2006-07-04,Influence of the driving mechanism on the response of systems with athermal dynamics: the example of the random-field Ising model,"We investigate the influence of the driving mechanism on the hysteretic
response of systems with athermal dynamics. In the framework of local-mean
field theory at finite temperature (but neglecting thermallly activated
processes), we compare the rate-independent hysteresis loops obtained in the
random field Ising model (RFIM) when controlling either the external magnetic
field $H$ or the extensive magnetization $M$. Two distinct behaviors are
observed, depending on disorder strength. At large disorder, the $H$-driven and
$M$-driven protocols yield identical hysteresis loops in the thermodynamic
limit. At low disorder, when the $H$-driven magnetization curve is
discontinuous (due to the presence of a macroscopic avalanche), the $M$-driven
loop is re-entrant while the induced field exhibits strong intermittent
fluctuations and is only weakly self-averaging. The relevance of these results
to the experimental observations in ferromagnetic materials, shape memory
alloys, and other disordered systems is discussed.",0607069v1
2006-10-10,Slow dynamics in critical ferromagnetic vector models relaxing from a magnetized initial state,"Within the universality class of ferromagnetic vector models with O(n)
symmetry and purely dissipative dynamics, we study the non-equilibrium critical
relaxation from a magnetized initial state. Transverse correlation and response
functions are exactly computed for Gaussian fluctuations and in the limit of
infinite number n of components of the order parameter. We find that the
fluctuation-dissipation ratios (FDRs) for longitudinal and transverse modes
differ already at the Gaussian level. In these two exactly solvable cases we
completely describe the crossover from the short-time to the long-time
behavior, corresponding to a disordered and a magnetized initial condition,
respectively. The effects of non-Gaussian fluctuations on longitudinal and
transverse quantities are calculated in the first order in the
epsilon-expansion and reliable three-dimensional estimates of the two FDRs are
obtained.",0610266v1
2006-10-11,Computational Experiment on Glassy Dynamic Nature of the Field-Cooled Magnetization in an Ising Spin-Glass Model,"The field-cooled (FC) process on an Ising spin-glass model is investigated by
a standard Monte Calro (MC) method on one hand, and the equilibrium
magnetization of the same system is evaluated by the exchang MC method on the
other hand. The two types of simulation reveal intriguing glassy
(nonequilibrium) dynamic properties of the FC magnetization (FCM) of the
system. Particularly, the FCM decrease is observed when the FC process is
halted at a low temperater, although its value is smaller than the
corresponding equilibrium value. We present a comprehensive interpretation of
such peculiar phenomena based on the scenario for the FCM process recently
propsed by J\""onsson and one of us (HT).",0610284v1
2007-02-01,Adiabatic Domain Wall Motion and Landau-Lifshitz Damping,"Recent theory and measurements of the velocity of current-driven domain walls
in magnetic nanowires have re-opened the unresolved question of whether
Landau-Lifshitz damping or Gilbert damping provides the more natural
description of dissipative magnetization dynamics. In this paper, we argue that
(as in the past) experiment cannot distinguish the two, but that
Landau-Lifshitz damping nevertheless provides the most physically sensible
interpretation of the equation of motion. From this perspective, (i) adiabatic
spin-transfer torque dominates the dynamics with small corrections from
non-adiabatic effects; (ii) the damping always decreases the magnetic free
energy, and (iii) microscopic calculations of damping become consistent with
general statistical and thermodynamic considerations.",0702020v3
1999-04-04,The Physical Interpretation of the Lanczos Tensor,"The field equations of general relativity can be written as first order
differential equations in the Weyl tensor, the Weyl tensor in turn can be
written as a first order differential equation in a three index tensor called
the Lanczos tensor. The Lanczos tensor plays a similar role in general
relativity to that of the vector potential in electro-magnetic theory. The
Aharonov-Bohm effect shows that when quantum mechanics is applied to
electro-magnetic theory the vector potential is dynamically significant, even
when the electro-magnetic field tensor $F_{ab}$ vanishes. Here it is assumed
that in the quantum realm the Lanczos tensor is dynamically significant, and
this leads to an attempt to quantize the gravitational field by pursuing the
analogy between the vector field and the Lanczos tensor.",9904006v1
2001-10-22,On the tidally induced gravitational collapse of a particle cluster,"An important issue in the dynamics of neutron star binaries is whether tidal
interaction can cause the individual stars to collapse into black holes during
inspiral. To understand this issue better, we study the dynamics of a cluster
of collisionless particles orbiting a non-rotating black hole, which is part of
a widely separated circular binary. The companion body's electric- and
magnetic-type tidal fields distort the black hole and perturb the cluster,
eventually causing the cluster to collapse into the hole as the companion
spirals in under the influence of gravitational radiation reaction. We find
that magnetic-type tidal forces do not significantly influence the evolution of
the cluster as a whole. However, individual orbits can be strongly affected by
these forces. For example, some orbits are destabilized due to the addition of
magnetic-type tidal forces. We find that the most stable orbits are close to
the companion's orbital plane and retrograde with respect to the companion's
orbit.",0110091v1
1998-02-27,Dynamical Flavour Symmetry Breaking by a Magnetic Field in Lattice QED_3,"We perform a lattice study, in the quenched approximation, of dynamical mass
generation in a system of relativistic (Dirac) fermions, coupled to an Abelian
gauge field in (2+1)-dimensions, in the presence of an external (constant)
magnetic field, perpendicular to the spatial planes. It is shown that a strong
magnetic field catalyzes chiral symmetry breaking, in agreement with results in
the continuum. The r\^ole of the higher-Landau poles in inducing a critical
temperature above which the phenomenon disappears is pointed out. We also
discuss the implications of this model on the opening of a gap in doped
antiferromagnetic superconductors.",9802037v2
1997-08-12,Primordial Magnetic Fields and Their Development,"In this talk I discuss the non-linear development of magnetic fields in the
early universe. I very briefly mention a number of particle physics mechanisms
for generating magnetic fields in the early universe, but the emphasis is on a
review of the field theoretic aspects of the developments of these fields,
mainly on the occurence of inverse cascades, i.e. generation of order from
disorder in the sense of transfer of energy from smaller to larger scales. In
this connection it is also briefly reviewed how the Silk effect (photon
diffusion) is counteracted by the inverse cascade.",9708320v1
1997-09-15,Chiral Symmetry Breaking in the weakly coupled QED in a Magnetic Field,"The catalysis of chiral symmetry breaking in the massless weakly coupled QED
in a magnetic field is studied. It is shown that the effect is due to the
dimensional reduction $D\to D-2$ in the dynamics of fermion pairing in a
magnetic field. The dynamical mass of fermions (energy gap in the fermion
spectrum) is determined. The temperature of the symmetry restoration is
estimated analytically.",9709340v1
1998-07-14,Dynamical symmetry restoration for a higher-derivative four-fermion model in an external electromagnetic field,"A four-fermion model with additional higher-derivative terms is investigated
in an external electromagnetic field. The effective potential in the leading
order of large-N expansion is calculated in external constant magnetic and
electric fields.
  It is shown that, in contrast to the former results concerning the universal
character of ""magnetic catalysis"" in dynamical symmetry breaking, in the
present higher-derivative model the magnetic field restores chiral symmetry
broken initially on the tree level. Numerical results describing a second-order
phase transition that accompanies the symmetry restoration at the quantum level
are presented.",9807368v1
2000-04-12,Magnetic Catalysis in the Presence of Scalar Fields,"The influence of scalar field interactions in the magnetic catalysis
phenomenom is studied in a gauge theory with scalar and fermion fields. It is
shown that the external magnetic field catalyzes the appearance of a
fermion-antifermion condensate along with a non-zero scalar vev, hence
generating a fermion dynamical mass and breaking the discrete chiral symmetry
of the theory. The model does not require the introduction of a scalar mass
term with a wrong sign in the original Lagrangian to generate the scalar vev,
nor does it exhibit dimensional transmutation ''a la Coleman-Weinberg.'' The
scalar vev noticeably enhances the dynamically generated fermion mass. Possible
cosmological applications of these results are suggested.",0004113v1
2001-06-05,"Testing Non-commutative QED, Constructing Non-commutative MHD","The effect of non-commutativity on electromagnetic waves violates Lorentz
invariance: in the presence of a background magnetic induction field b, the
velocity for propagation transverse to b differs from c, while propagation
along b is unchanged. In principle, this allows a test by the Michelson-Morley
interference method. We also study non-commutativity in another context, by
constructing the theory describing a charged fluid in a strong magnetic field,
which forces the fluid particles into their lowest Landau level and renders the
fluid dynamics non-commutative, with a Moyal product determined by the
background magnetic field.",0106044v3
2003-06-16,Can branes travel beyond CTC horizon in Godel Universe?,"Godel universe in M-theory is a supersymmetric and homogeneous background
with rotation and four-form magnetic flux. It is known that, as seen in
inertial frame of co-moving observer, all geodesics with zero orbital angular
momentum orbit inside `surface of light velocity' (CTC horizon). To learn if
other probes can travel beyond the CTC horizon, we study dynamics of M-graviton
and, in particular, M2-brane, whose motion is affected by Lorentz force exerted
by the four-form magnetic flux and by nonzero orbital angular momentum.
Classically, we find that both probes gyrate closed orbits, but diameter and
center of gyration depends on sign and magnitude of probe's energy, charge and
orbital angular momentum. For M2-brane, orbits in general travel outside the
CTC horizon. Quantum-mechanically, we find that wave function and excitation
energy levels are all self-similar. We draw analogy of probe's dynamics with
Landau problem for charged particle in magnetic field.",0306148v2
2006-10-19,The Noncommutative Anandan's Quantum Phase,"In this work we study the noncommutative nonrelativistic quantum dynamics of
a neutral particle, that possesses permanent magnetic and electric dipole
momenta, in the presence of an electric and magnetic fields. We use the
Foldy-Wouthuysen transformation of the Dirac spinor with a non-minimal coupling
to obtain the nonrelativistic limit. In this limit, we will study the
noncommutative quantum dynamics and obtain the noncommutative Anandan's
geometric phase. We analyze the situation where magnetic dipole moment of the
particle is zero and we obtain the noncommutative version of the
He-McKellar-Wilkens effect. We demonstrate that this phase in the
noncommutative case is a geometric dispersive phase. We also investigate this
geometric phase considering the noncommutativity in the phase space and the
Anandan's phase is obtained.",0610222v2
2005-07-15,Energetic and dynamic properties of a quantum particle in a spatially random magnetic field with constant correlations along one direction,"We consider an electrically charged particle on the Euclidean plane subjected
to a perpendicular magnetic field which depends only on one of the two
Cartesian co-ordinates. For such a ``unidirectionally constant'' magnetic field
(UMF), which otherwise may be random or not, we prove certain spectral and
transport properties associated with the corresponding one-particle
Schroedinger operator (without scalar potential) by analysing its ``energy-band
structure''. In particular, for an ergodic random UMF we provide conditions
which ensure that the operator's entire spectrum is almost surely absolutely
continuous. This implies that, along the direction in which the random UMF is
constant, the quantum-mechanical motion is almost surely ballistic, while in
the perpendicular direction in the plane one has dynamical localisation. The
conditions are verified, for example, for Gaussian and Poissonian random UMF's
with non-zero mean-values. These results may be viewed as ``random analogues''
of results first obtained by A. Iwatsuka [Publ. RIMS, Kyoto Univ. 21 (1985)
385] and (non-rigorously) by J. E. Mueller [Phys. Rev. Lett. 68 (1992) 385].",0507035v1
2003-06-28,Self-similar turbulent dynamo,"The amplification of magnetic fields in a highly conducting fluid is studied
numerically. During growth, the magnetic field is spatially intermittent: it
does not uniformly fill the volume, but is concentrated in long thin folded
structures. Contrary to a commonly held view, intermittency of the folded field
does not increase indefinitely throughout the growth stage if diffusion is
present. Instead, as we show, the probability-density function (PDF) of the
field strength becomes self-similar. The normalized moments increase with
magnetic Prandtl number in a powerlike fashion. We argue that the
self-similarity is to be expected with a finite flow scale and system size. In
the nonlinear saturated state, intermittency is reduced and the PDF is
exponential. Parallels are noted with self-similar behavior recently observed
for passive-scalar mixing and for map dynamos.",0306059v3
2006-11-06,Numerical simulations on cleaning the neutron trap for measuring the neutron lifetime,"We present the results of numerical simulations of the dynamical behavior of
trajectories of ultra cold neutrons (UCN) in a magnetic trap. The main goal of
our simulations was to optimize the trap parameters in order to minimize the
characteristic times for removing from the trap those untrapped neutrons with
relatively long escape times (cleaning the trap). Our results demonstrate that,
by a proper choice of the trap parameters cleaning times can be reduced to 15
sec, or even less. Many other dynamical characteristics of the neutrons in the
trap, including the conservation of the adiabatic invariant which characterizes
the orientation of the magnetic moment along the local magnetic field, are also
discussed.",0611005v1
1997-12-04,Nonadiabatic Dynamics of Atoms in Nonuniform Magnetic Fields,"Dynamics of neutral atoms in nonuniform magnetic fields, typical of
quadrupole magnetic traps, is considered by applying an accurate method for
solving nonlinear systems of differential equations. This method is more
general than the adiabatic approximation and, thus, permits to check the limits
of the latter and also to analyze nonadiabatic regimes of motion. An unusual
nonadiabatic regime is found when atoms are confined from one side of the
z-axis but are not confined from another side. The lifetime of atoms in a trap
in this semi-confining regime can be sufficiently long for accomplishing
experiments with a cloud of such atoms. At low temperature, the cloud is
ellipsoidal being stretched in the axial direction and moving along the z-axis.
The possibility of employing the semi-confining regime for studying the
relative motion of one component through another, in a binary mixture of gases
is discussed.",9712009v1
1998-03-15,On the Dynamical Stability of the Hovering Magnetic Top,"In this paper we analyze the dynamic stability of the hovering magnetic top
from first principles without using any preliminary assumptions. We write down
the equations of motion for all six degrees of freedom and solve them
analytically around the equilibrium solution. Using this solution we then find
conditions which the height of the hovering top above the base, its total mass,
and its spinning speed have to satisfy for stable hovering. The calculation
presented in this paper can be used as a guide to the analysis and synthesis of
magnetic traps for neutral particles.",9803020v1
1998-12-10,Evidence for a singularity in ideal magnetohydrodynamics: implications for fast reconnection,"Numerical evidence for a finite-time singularity in ideal 3D
magnetohydrodynamics (MHD) is presented. The simulations start from two
interlocking magnetic flux rings with no initial velocity. The magnetic
curvature force causes the flux rings to shrink until they come into contact.
This produces a current sheet between them. In the ideal compressible
calculations, the evidence for a singularity in a finite time $t_c$ is that the
peak current density behaves like $|J|_\infty \sim 1/(t_c-t)$ for a range of
sound speeds (or plasma betas). For the incompressible calculations consistency
with the compressible calculations is noted and evidence is presented that
there is convergence to a self-similar state. In the resistive reconnection
calculations the magnetic helicity is nearly conserved and energy is
dissipated.",9812017v2
1999-10-27,Atomic dynamics in evaporative cooling of trapped alkali atoms in strong magnetic fields,"We investigate how the nonlinearity of the Zeeman shift for strong magnetic
fields affects the dynamics of rf field induced evaporative cooling in magnetic
traps. We demonstrate for the 87-Rb and 23-Na F=2 trapping states with wave
packet simulations how the cooling stops when the rf field frequency goes below
a certain limit (for the 85-Rb F=2 trapping state the problem does not appear).
We examine the applicability of semiclassical models for the strong field case
as an extension of our previous work [Phys. Rev. A 58, 3983 (1998)]. Our
results verify many of the aspects observed in a recent $^{87}$Rb experiment
[Phys. Rev. A 60, R1759 (1999)].",9910043v1
2001-08-06,Quantum Measurement of a Single Spin using Magnetic Resonance Force Microscopy,"Single-spin detection is one of the important challenges facing the
development of several new technologies, e.g. single-spin transistors and
solid-state quantum computation. Magnetic resonance force microscopy with a
cyclic adiabatic inversion, which utilizes a cantilever oscillations driven by
a single spin, is a promising technique to solve this problem. We have studied
the quantum dynamics of a single spin interacting with a quasiclassical
cantilever. It was found that in a similar fashion to the Stern-Gerlach
interferometer the quantum dynamics generates a quantum superposition of two
quasiclassical trajectories of the cantilever which are related to the two spin
projections on the direction of the effective magnetic field in the rotating
reference frame. Our results show that quantum jumps will not prevent a
single-spin measurement if the coupling between the cantilever vibrations and
the spin is small in comparison with the amplitude of the radio-frequency
external field.",0108025v1
2007-04-11,Preparation and detection of magnetic quantum phases in optical superlattices,"We describe a novel approach to prepare, detect and characterize magnetic
quantum phases in ultra-cold spinor atoms loaded in optical superlattices. Our
technique makes use of singlet-triplet spin manipulations in an array of
isolated double well potentials in analogy to recently demonstrated quantum
control in semiconductor quantum dots. We also discuss the many-body
singlet-triplet spin dynamics arising from coherent coupling between nearest
neighbor double wells and derive an effective description for such system. We
use it to study the generation of complex magnetic states by adiabatic and
non-equilibrium dynamics.",0704.1413v3
2007-05-24,Nonequilibrium Dynamics of Anisotropic Large Spins in the Kondo Regime: Time-Dependent Numerical Renormalization Group Analysis,"We investigate the time-dependent Kondo effect in a single-molecule magnet
(SMM) strongly coupled to metallic electrodes. Describing the SMM by a Kondo
model with large spin S > 1/2, we analyze the underscreening of the local
moment and the effect of anisotropy terms on the relaxation dynamics of the
magnetization. Underscreening by single-channel Kondo processes leads to a
logarithmically slow relaxation, while finite uniaxial anisotropy causes a
saturation of the SMM's magnetization. Additional transverse anisotropy terms
induce quantum spin tunneling and a pseudospin-1/2 Kondo effect sensitive to
the spin parity.",0705.3654v2
2007-05-30,Structure and Dynamics of the Sun's Open Magnetic Field,"The solar magnetic field is the primary agent that drives solar activity and
couples the Sun to the Heliosphere. Although the details of this coupling
depend on the quantitative properties of the field, many important aspects of
the corona - solar wind connection can be understood by considering only the
general topological properties of those regions on the Sun where the field
extends from the photosphere out to interplanetary space, the so-called open
field regions that are usually observed as coronal holes. From the simple
assumptions that underlie the standard quasi-steady corona-wind theoretical
models, and that are likely to hold for the Sun, as well, we derive two
conjectures on the possible structure and dynamics of coronal holes: (1)
Coronal holes are unique in that every unipolar region on the photosphere can
contain at most one coronal hole. (2) Coronal holes of nested polarity regions
must themselves be nested. Magnetic reconnection plays the central role in
enforcing these constraints on the field topology. From these conjectures we
derive additional properties for the topology of open field regions, and
propose several observational predictions for both the slowly varying and
transient corona/solar wind.",0705.4430v1
2007-09-04,Dynamic nuclear polarisation in biased quantum wires with spin-orbit interaction,"We propose a new method for dynamic nuclear polarisation in a quasi
one-dimensional quantum wire utilising the spin-orbit interaction, the
hyperfine interaction, and a finite source-drain potential difference. In
contrast with current methods, our scheme does not rely on external magnetic or
optical sources which makes independent control of closely placed devices much
more feasible. Using this method, a significant polarisation of a few per cent
is possible in currently available InAs wires which may be detected by
conductance measurements. This may prove useful for nuclear-magnetic-resonance
studies in nanoscale systems as well as in spin-based devices where external
magnetic and optical sources will not be suitable.",0709.0333v2
2007-09-04,Vortices on a superconducting nanoshell: phase diagram and dynamics,"In superconductors, the search for special vortex states such as giant
vortices focuses on laterally confined or nanopatterned thin superconducting
films, disks, rings, or polygons. We examine the possibility to realize giant
vortex states and states with non-uniform vorticity on a superconducting
spherical nanoshell, due to the interplay of the topology and the applied
magnetic field. We derive the phase diagram and identify where, as a function
of the applied magnetic field, the shell thickness and the shell radius, these
different vortex phases occur. Moreover, the curved geometry allows these
states (or a vortex lattice) to coexist with a Meissner state, on the same
curved film. We have examined the dynamics of the decay of giant vortices or
states with non-uniform vorticity into a vortex lattice, when the magnetic
field is adapted so that a phase boundary is crossed.",0709.0463v1
2007-09-07,Linear response subordination to intermittent energy release in off-equilibrium aging dynamics,"The interpretation of experimental and numerical data describing
off-equilibrium aging dynamics crucially depends on the connection between
spontaneous and induced fluctuations. The hypothesis that linear response
fluctuations are statistically subordinated to irreversible outbursts of
energy, so-called quakes, leads to predictions for averages and fluctuations
spectra of physical observables in reasonable agreement with experimental
results [see e.g. Sibani et al., Phys. Rev. B74:224407, 2006]. Using
simulational data from a simple but representative Ising model with plaquette
interactions, direct statistical evidence supporting the hypothesis is
presented and discussed in this work.
  A strict temporal correlation between quakes and intermittent magnetization
fluctuations is demonstrated. The external magnetic field is shown to bias the
pre-existent intermittent tails of the magnetic fluctuation distribution, with
little or no effect on the Gaussian part of the latter. Its impact on energy
fluctuations is shown to be negligible.
  Linear response is thus controlled by the quakes and inherits their temporal
statistics. These findings provide a theoretical basis for analyzing
intermittent linear response data from aging system in the same way as thermal
energy fluctuations, which are far more difficult to measure.",0709.1085v2
2007-09-11,Finite Temperature Large N Gauge Theory with Quarks in an External Magnetic Field,"Using a ten dimensional dual string background, we study aspects of the
physics of finite temperature large N four dimensional SU(N) gauge theory,
focusing on the dynamics of fundamental quarks in the presence of a background
magnetic field. At vanishing temperature and magnetic field, the theory has N=2
supersymmetry, and the quarks are in hypermultiplet representations. In a
previous study, similar techniques were used to show that the quark dynamics
exhibit spontaneous chiral symmetry breaking. In the present work we begin by
establishing the non-trivial phase structure that results from finite
temperature. We observe, for example, that above the critical value of the
field that generates a chiral condensate spontaneously, the meson melting
transition disappears, leaving only a discrete spectrum of mesons at any
temperature. We also compute several thermodynamic properties of the plasma.",0709.1547v2
2007-10-17,Generation of Magnetic Field by Combined Action of Turbulence and Shear,"The feasibility of a mean-field dynamo in nonhelical turbulence with
superimposed linear shear is studied numerically in elongated shearing boxes.
Exponential growth of magnetic field at scales much larger than the outer scale
of the turbulence is found. The charateristic scale of the field is l_B ~
S^{-1/2} and growth rate is gamma ~ S, where S is the shearing rate. This newly
discovered shear dynamo effect potentially represents a very generic mechanism
for generating large-scale magnetic fields in a broad class of astrophysical
systems with spatially coherent mean flows.",0710.3359v2
2007-10-18,Toward theory of quantum Hall effect in graphene,"We analyze a gap equation for the propagator of Dirac quasiparticles and
conclude that in graphene in a magnetic field, the order parameters connected
with the quantum Hall ferromagnetism dynamics and those connected with the
magnetic catalysis dynamics necessarily coexist (the latter have the form of
Dirac masses and correspond to excitonic condensates). This feature of graphene
could lead to important consequences, in particular, for the existence of
gapless edge states. Solutions of the gap equation corresponding to recently
experimentally discovered novel plateaus in graphene in strong magnetic fields
are described.",0710.3527v2
2007-11-10,SU(2) Symmetry and Conservation of Helicity for a Dirac Particle in a Static Magnetic Field at First Order,"We investigate the spin dynamics and the conservation of helicity in the
first order $S-$matrix of a Dirac particle in any static magnetic field. We
express the dynamical quantities using a coordinate system defined by the three
mutually orthogonal vectors; the total momentum
$\mathbf{k}=\mathbf{p_f}+\mathbf{p_i}$, the momentum transfer
$\mathbf{q}=\mathbf{p_f-p_i}$, and $\mathbf{l}=\mathbf{k\times q}$. We show
that this leads to an alternative symmetric description of the conservation of
helicity in a static magnetic field at first order. In particular, we show that
helicity conservation in the transition can be viewed as the invariance of the
component of the spin along $\mathbf{k}$, and the flipping of its component
along $\mathbf{q}$, just as what happens to the momentum vector of a ball
bouncing off a wall. We also derive a ""plug and play"" formula for the
transition matrix element where the only reference to the specific field
configuration, and the incident and outgoing momenta is through the kinematical
factors multiplying a general matrix element that is independent of the
specific vector potential present.",0711.1583v2
2007-11-27,Nonequilibrium interacting electrons in a ferromagnet,"Dynamics of the magnetization in ferromagnets is examined in the presence of
transport electrons allowing the latter to interact. It is found that the
existence of inhomogeneities such as domain wall (DW) structures, leads to
changes that affect the dynamical structure of the equations of motion for the
magnetization. Only in the limit of uniform magnetizations or sufficiently wide
DW's, the equations of motion maintain the form they have in the noninteracting
case. In this limit, results like the spin torques, the Gilbert parameter, and
the DW velocities become renormalized. However the length scale that defines
such a limit depends on the strength of the interaction. It is shown that if
large ferromagnetic fluctuations exist in the metallic band then the range for
which conformity with the noninteracting case holds extends to the limit of
arbitrarily narrow DW's.",0711.4170v2
2007-12-10,Chirality tunneling and quantum dynamics for domain walls in mesoscopic ferromagnets,"We studied the quantum dynamics of ferromagnetic domain walls (topological
kink-type solitons) in one dimensional ferromagnetic spin chains. We show that
the tunneling probability does not depend on the number of spins in a domain
wall; thus, this probability can be large even for a domain wall containing a
large number of spins. We also predict that there is a strong interplay between
the tunneling of a wall from one lattice site to another (tunneling of the kink
coordinate) and the tunneling of the kink topological charge (so-called
chirality). Both of these elementary processes are suppressed for kinks in
one-dimensional ferromagnets with half-integer spin. The dispersion law (i.e.,
the domain wall energy versus momentum) is essentially different for chains
with either integer or half-integer spins. The predicted quantum effects could
be observed for mesoscopic magnetic structures, e.g., chains of magnetic
clusters, large-spin molecules, or nanosize magnetic dots.",0712.1392v1
2008-01-14,Hartmann flow with Braginsky viscosity: a test problem for intercluster plasma,"We consider a Hartmann layer, stationary flow of a viscose and resistive
fluid between two plates with superimposed transverse magnetic field, in the
limit of gyrotropic plasma, when viscosity across the field is strongly
suppressed. For zero cross-field viscosity, the problem is not well posed,
since viscosity then vanishes on the boundaries and in the middle of the layer,
where there is no longitudinal field. An additional arbitrarily small isotropic
viscosity allows one to find magnetic field and velocity profiles which are
independent of this viscosity floor and different from flows with isotropic
viscosity. Velocity sharply rises in a thin boundary layer, which thickness
depends both on the Hartmann number and on the Lundquist number of the flow.
  The implication of the work is that, in simulating ICM dynamics it is
imperative to use numerical schemes which take into account anisotropic
viscosity. Although magnetic fields are dynamically subdominant in the ICM they
do determine its the dissipative properties, stability of embedded structures
and the transition to turbulence.",0801.2166v1
2008-01-28,Dynamical correlations in the spin-half two-channel Kondo model,"Dynamical correlations of various local operators are studied in the
spin-half two-channel Kondo (2CK) model in the presence of channel anisotropy
or external magnetic field. A conformal field theory-based scaling approach is
used to predict the analytic properties of various spectral functions in the
vicinity of the two-channel Kondo fixed point. These analytical results compare
well with highly accurate density matrix numerical renormalization group
results. The universal cross-over functions interpolating between
channel-anisotropy or magnetic field-induced Fermi liquid regimes and the
two-channel Kondo, non-Fermi liquid regimes are determined numerically. The
boundaries of the real 2CK scaling regime are found to be rather restricted,
and to depend both on the type of the perturbation and on the specific operator
whose correlation function is studied. In a small magnetic field, a universal
resonance is observed in the local fermion's spectral function. The dominant
superconducting instability appears in the composite superconducting channel.",0801.4272v1
2008-02-16,"Hysteresis, transient oscillations, and nonhysteretic switching in resonantly modulated large-spin systems","We study the classical dynamics of resonantly modulated large-spin systems in
a strong magnetic field. We show that these systems have special symmetry. It
leads to characteristic nonlinear effects. They include abrupt switching
between magnetization branches with varying modulating field without hysteresis
and a specific pattern of switching in the presence of multistability and
hysteresis. Along with steady forced vibrations the transverse spin components
can display transient vibrations at a combination of the Larmor frequency and a
slower frequency determined by the anisotropy energy. The analysis is based on
a microscopic theory that takes into account relaxation mechanisms important
for single-molecule magnets and other large-spin systems. We find how the
Landau-Lifshitz model should be modified in order to describe the classical
spin dynamics. The occurrence of transient oscillations depends on the
interrelation between the relaxation parameters.",0802.2304v1
2008-04-14,Quantum critical dynamics of a S = 1/2 antiferromagnetic Heisenberg chain studied by 13C-NMR spectroscopy,"We present a 13C-NMR study of the magnetic field driven transition to
complete polarization of the S=1/2 antiferromagnetic Heisenberg chain system
copper pyrazine dinitrate Cu(C_4H_4N_2)(NO_3)_2 (CuPzN). The static local
magnetization as well as the low-frequency spin dynamics, probed via the
nuclear spin-lattice relaxation rate 1/T_1, were explored from the low to the
high field limit and at temperatures from the quantum regime (k_B T << J) up to
the classical regime (k_B T >> J). The experimental data show very good
agreement with quantum Monte Carlo calculations over the complete range of
parameters investigated. Close to the critical field, as derived from static
experiments, a pronounced maximum in 1/T_1 is found which we interpret as the
finite-temperature manifestation of a diverging density of zero-energy magnetic
excitations at the field-driven quantum critical point.",0804.2170v2
2008-06-22,"Electronic structures of ternary iron arsenides AFe$_2$As$_2$ (A=Ba, Ca, or Sr)","We have studied the electronic and magnetic structures of the ternary iron
arsenides AFe$_2$As$_2$ (A = Ba, Ca, or Sr) using the first-principles density
functional theory. The ground states of these compounds are in a collinear
antiferromagnetic order, resulting from the interplay between the nearest and
the next-nearest neighbor superexchange antiferromagnetic interactions bridged
by As $4p$ orbitals. The correction from the spin-orbit interaction to the band
structure is small. The pressure can reduce dramatically the magnetic moment
and diminish the collinear antiferromagnetic order. Based on the calculations,
we propose that the low energy dynamics of these materials is described
effectively by a $t-J_H-J_1-J_2$-type model.",0806.3526v3
2008-06-24,Spin Glass Order Induced by Dynamic Frustration,"Spin glasses are systems whose magnetic moments freeze at low temperature
into random orientations without long-range order. It is generally accepted
that both frustration and disorder are essential ingredients in all spin
glasses, so it was surprising that PrAu2Si2, a stoichiometric compound with a
well-ordered crystal structure, was reported to exhibit spin glass freezing. In
this article, we report on inelastic neutron scattering measurements of the
crystal field excitations, which show that PrAu2Si2 has a singlet ground state
and that the exchange coupling is very close to the critical value to induce
magnetic order. We propose that spin glass freezing results from dynamic
fluctuations of the crystal field levels that destabilize the induced moments
and frustrate the development of long-range magnetic correlations. This novel
mechanism for producing a frustrated ground state could provide a method of
testing the concept of `avoided criticality' in glassy systems.",0806.3935v1
2008-06-24,Excitability in the vicinity of a saddle-node bifurcation: a mechanism for reversals,"We present a new mechanism for oscillatory or random reversals of the
magnetic field that occur from a stationary dynamo state. The basic ingredient
is the existence of two nearly critical dynamo eigenmodes, a stable and an
unstable one, that collide during a saddle-node bifurcation. Above this
bifurcation, a finite amplitude limit cycle is generated. In the neighbourhood
of the bifurcation, even a small amount of turbulent fluctuations is enough to
generate random reversals of the magnetic field. When the two competing
eigenmodes have different symmetries, this scenario requires a broken symmetry
of the flow that generates the dynamo. This is in good agreement with the
observations of the VKS experiment. It can also explain the dynamics of
planetary or stellar magnetic fields and can be used to understand reversals of
large scale flows that often develop on a turbulent background.",0806.3937v2
2008-07-31,Optical probing of spin fluctuations of a single magnetic atom,"We analyzed the photoluminescence intermittency generated by a single
paramagnetic spin localized in an individual semiconductor quantum dot. The
statistics of the photons emitted by the quantum dot reflect the quantum
fluctuations of the localized spin interacting with the injected carriers.
Photon correlation measurements which are reported here reveal unique
signatures of these fluctuations. A phenomenological model is proposed to
quantitatively describe these observations, allowing a measurement of the spin
dynamics of an individual magnetic atom at zero magnetic field. These results
demonstrate the existence of an efficient spin relaxation channel arising from
a spin-exchange with individual carriers surrounding the quantum dot. A
theoretical description of a spin-flip mechanism involving spin exchange with
surrounding carriers gives relaxation times in good agreement with the measured
dynamics.",0807.5002v1
2008-09-17,Fermi condensates for dynamic imaging of electro-magnetic fields,"Ultracold gases provide micrometer size atomic samples whose sensitivity to
external fields may be exploited in sensor applications. Bose-Einstein
condensates of atomic gases have been demonstrated to perform excellently as
magnetic field sensors \cite{Wildermuth2005a} in atom chip
\cite{Folman2002a,Fortagh2007a} experiments. As such, they offer a combination
of resolution and sensitivity presently unattainable by other methods
\cite{Wildermuth2006a}. Here we propose that condensates of Fermionic atoms can
be used for non-invasive sensing of time-dependent and static magnetic and
electric fields, by utilizing the tunable energy gap in the excitation spectrum
as a frequency filter. Perturbations of the gas by the field create both
collective excitations and quasiparticles. Excitation of quasiparticles
requires the frequency of the perturbation to exceed the energy gap. Thus, by
tuning the gap, the frequencies of the field may be selectively monitored from
the amount of quasiparticles which is measurable for instance by
RF-spectroscopy. We analyse the proposed method by calculating the
density-density susceptibility, i.e. the dynamic structure factor, of the gas.
We discuss the sensitivity and spatial resolution of the method which may, with
advanced techniques for quasiparticle observation \cite{Schirotzek2008a}, be in
the half a micron scale.",0809.2959v1
2008-09-20,Mottness scenario for non-Fermi liquid behavior in the periodic Anderson model within Dynamical Mean Field Theory,"We study the Mott metal-insulator transition in the Periodic Anderson Model
within Dynamical Mean Field Theory (DMFT). Near the quantum transition, we find
a non-Fermi liquid metallic state down to a vanishing temperature scale. We
identify the origin of the non-Fermi liquid behavior as due to magnetic
scattering of the doped carriers by the localized moments. The non-Fermi liquid
state can be tuned by either doping or external magnetic field. Our results
show that the coupling to spatial magnetic fluctuations (absent in DMFT) is not
a prerequisite to realize a non-Fermi liquid scenario for heavy fermion
systems.",0809.3520v1
2008-09-30,Investigation of the magnetic fluctuations in Tb$_2$Sn$_2$O$_7$ ordered spin ice by high resolution energy-resolved neutron scattering,"We have studied magnetically frustrated Tb$_2$Sn$_2$O$_7$ by neutron
diffraction and high resolution energy-resolved neutron scattering. At 0.1 K,
we observe short range magnetic correlations with a typical scale of 4 \AA,
close to the near neighbor distance between Tb$^{3+}$ ions. %(3.686 \AA), This
short range order coexists with ferromagnetic correlations and long range spin
ice order at the scales of 18 and 190 \AA, respectively. Spin dynamics was
investigated at a time scale down to 10$^{-9}$s, by energy-resolved experiments
on a backscattering spectrometer. We observe a freezing of the spin dynamics
for all length-scales, with a strong slowing down of the spin fluctuations when
long range order settles in. We discuss the spin fluctuations remaining in the
ground state in comparison with previous data obtained by muon spectroscopy.",0809.5241v1
2008-10-20,Magnetically-induced buckling of a whirling conducting rod with applications to electrodynamic space tethers,"We study the effect of a magnetic field on the behaviour of a slender
conducting elastic structure, motivated by stability problems of electrodynamic
space tethers. Both statical (buckling) and dynamical (whirling) instability
are considered and we also compute post-buckling configurations. The equations
used are the geometrically exact Kirchhoff equations. Magnetic buckling of a
welded rod is found to be described by a surprisingly degenerate bifurcation,
which is unfolded when both transverse anisotropy of the rod and angular
velocity are considered. By solving the linearised equations about the (quasi-)
stationary solutions we find various secondary instabilities. Our results are
relevant for current designs of electrodynamic space tethers and potentially
for future applications in nano- and molecular wires.",0810.3668v1
2008-10-29,Three-Layer Magnetoconvection,"It is believed that some stars have two or more convection zones in close
proximity near to the stellar photosphere. These zones are separated by
convectively stable regions that are relatively narrow. Due to the close
proximity of these regions it is important to construct mathematical models to
understand the transport and mixing of passive and dynamic quantities. One key
quantity of interest is a magnetic field, a dynamic vector quantity, that can
drastically alter the convectively driven flows, and have an important role in
coupling the different layers. In this paper we present the first investigation
into the effect of an imposed magnetic field in such a geometry. We focus our
attention on the effect of field strength and show that, while there are some
similarities with results for magnetic field evolution in a single layer, new
and interesting phenomena are also present in a three layer system.",0810.5279v2
2008-12-22,Criterion for transformation of transverse domain wall to vortex or antivortex wall in soft magnetic thin-film nanostripes,"We report on the criterion for the dynamic transformation of the internal
structure of moving domain walls (DWs) in soft magnetic thin-film nanostripes
above the Walker threshold field, Hw. In order for the process of
transformation from transverse wall (TW) to vortex wall (VW) or antivortex wall
(AVW) occurs, the edge-soliton core of the TW-type DW should grow sufficiently
to the full width at half maximum of the out-of-plane magnetizations of the
core area of the stabilized vortex (or antivortex) by moving inward along the
transverse (width) direction. Upon completion of the nucleation of the vortex
(antivortex) core, the VW (AVW) is stabilized, and then its core accompanies
the gyrotropic motion in a potential well (hill) of a given nanostripe. Field
strengths exceeding the Hw, which is the onset field of DW velocity breakdown,
are not sufficient but necessary conditions for dynamic DW transformation.",0812.4083v1
2009-01-13,Vortex-Antivortex annihilation dynamics in a square mesoscopic superconducting cylinder,"The dynamics of the annihilation of a vortex-antivortex pair is investigated.
The pair is activated magnetically during the run of a simulated hysteresis
loop on a square mesoscopic superconducting cylinder with an antidot inserted
at its center. We study the nucleation of vortices and antivortices by first
increasing the magnetic field, applied parallel to the axis of the sample, from
zero until the first vortex is created. A further increase of the field pulls
the vortex in, until it reaches the antidot. As the polarity of the field is
reversed, an antivortex enters the scene, travels toward the center of the
sample and eventually the pair is annihilated. Depending on the sample size,
its temperature, and Ginzburg-Landau parameter, the vortex-antivortex encounter
takes place at the antidot or at the superconducting sea around it. The
position and velocity of the vortex and antivortex singularities were evaluated
as a function of time. The current density, magnetization and order parameter
topology were also calculated.",0901.1829v1
2009-01-19,Hysteresis and noise in ferromagnetic materials with parallel domain walls,"We investigate dynamic hysteresis and Barkhausen noise in ferromagnetic
materials with a huge number of parallel and rigid Bloch domain walls.
Considering a disordered ferromagnetic system with strong in-plane uniaxial
anisotropy and in-plane magnetization driven by an external magnetic field, we
calculate the equations of motion for a set of coupled domain walls,
considering the effects of the long-range dipolar interactions and disorder. We
derive analytically an expression for the magnetic susceptivity, related to the
effective demagnetizing factor, and show that it has a logarithmic dependence
on the number of domains. Next, we simulate the equations of motion and study
the effect of the external field frequency and the disorder on the hysteresis
and noise properties. The dynamic hysteresis is very well explained by means of
the loss separation theory.",0901.2918v1
2009-05-19,The Effect of Quantized Magnetic Flux Lines on the Dynamics of Superfluid Neutron Star Cores,"We investigate dynamical coupling timescales of a neutron star's superfluid
core, taking into account the interactions of quantized neutron vortices with
quantized flux lines of the proton superconductor in addition to the previously
considered scattering of the charged components against the spontaneous
magnetization of the neutron vortex line. We compare the cases where vortex
motion is constrained in different ways by the array of magnetic flux tubes
associated with superconducting protons. This includes absolute pinning to and
creep across a uniform array of flux lines. The effect of a toroidal
arrangement of flux lines is also considered. The inclusion of a uniform array
of flux tubes in the neutron star core significantly decreases the timescale of
coupling between the neutron and proton fluid constituents in all cases. For
the toroidal component, creep response similar to that of the inner crust
superfluid is possible.",0905.3097v1
2009-05-22,Some Unusual Properties of Turbulent Convection and Dynamos in Rotating Spherical Shells,"The dynamics of convecting fluids in rotating spherical shells is governed at
Prandtl numbers of the order unity by the interaction between differential
rotation and roll-like convection eddies. While the differential rotation is
driven by the Reynolds stresses of the eddies, its shearing action inhibits
convection and causes phenomena such as localized convection and turbulent
relaxation oscillations. The response of the system is enriched in the case of
dynamo action. Lorentz forces may brake either entirely or partially the
geostrophic differential rotation and give rise to two rather different dynamo
states. Bistability of turbulent dynamos exists for magnetic Prandtl numbers of
the order unity. While the ratios between mean magnetic and kinetic energies
differ by a factor of 5 or more for the two dynamo states, the mean convective
heat transports are nearly the same. They are much larger than in the absence
of a magnetic field.",0905.3723v1
2009-06-04,Magnetic trapping of ultracold Rydberg atoms in low angular momentum states,"We theoretically investigate the quantum properties of nS, nP, and nD Rydberg
atoms in a magnetic Ioffe-Pritchard trap. In particular, it is demonstrated
that the two-body character of Rydberg atoms significantly alters the trapping
properties opposed to point-like particles with identical magnetic moment.
Approximate analytical expressions describing the resulting Rydberg trapping
potentials are derived and their validity is confirmed for experimentally
relevant field strengths by comparisons to numerical solutions of the
underlying Schroedinger equation. In addition to the electronic properties, the
center of mass dynamics of trapped Rydberg atoms is studied. In particular, we
analyze the influence of a short-time Rydberg excitation, as required by
certain quantum-information protocols, on the center of mass dynamics of
trapped ground state atoms. A corresponding heating rate is derived and the
implications for the purity of the density matrix of an encoded qubit are
investigated.",0906.0946v2
2009-06-16,Phase Diagram and Spin Dynamics in Volborthite with a Distorted Kagome Lattice,"We report 51V-NMR study on a high-quality powder sample of volborthite
Cu3V2O7(OH)2/2H2O, a spin-1/2 Heisenberg antiferromagnet on a distorted kagome
lattice formed by isosceles triangles. In the magnetic fields below 4.5 T, a
sharp peak in the nuclear spin-lattice relaxation rate 1/T1 accompanied with
line broadening revealed a magnetic transition near 1 K. The low temperature
phase shows anomalies such as a Lorentzian line shape, a 1/T1 propto T behavior
indicating dense low energy excitations, and a large spin-echo decay rate 1/T2
pointing to unusually slow fluctuations. Another magnetic phase appears above
4.5 T with less anomalous spectral shape and dynamics.",0906.2846v2
2009-06-30,Low-Prandtl-number Bénard-Marangoni convection in a vertical magnetic field,"The effect of a homogeneous magnetic field on surface-tension-driven
B\'{e}nard convection is studied by means of direct numerical simulations. The
flow is computed in a rectangular domain with periodic horizontal boundary
conditions and the free-slip condition on the bottom wall using a
pseudospectral Fourier-Chebyshev discretization. Deformations of the free
surface are neglected. Two- and three-dimensional flows are computed for either
vanishing or small Prandtl number, which are typical of liquid metals. The main
focus of the paper is on a qualitative comparison of the flow states with the
non-magnetic case, and on the effects associated with the possible
near-cancellation of the nonlinear and pressure terms in the momentum equations
for two-dimensional rolls. In the three-dimensional case, the transition from a
stationary hexagonal pattern at the onset of convection to three-dimensional
time-dependent convection is explored by a series of simulations at zero
Prandtl number.",0906.5466v1
2009-07-04,One-dimensional magnetic fluctuations in the spin-2 triangular lattice α-NaMnO2,"The S=2 anisotropic triangular lattice alpha-NaMnO2 is studied by neutron
inelastic scattering. Antiferromagnetic order occurs at T ~ 45 K with opening
of a spin gap. The spectral weight of the magnetic dynamics above the gap
(Delta ~ 7.5 meV) has been analysed by the single-mode approximation. Excellent
agreement with the experiment is achieved when a dominant exchange interaction
(|J|/k_B ~ 73 K), along the monoclinic b-axis and a sizeable easy-axis magnetic
anisotropy (|D|/k_B ~ 3 K) are considered. Despite earlier suggestions for
two-dimensional spin interactions, the dynamics illustrate strongly coupled
antiferromagnetic S=2 chains and cancellation of the interchain exchange due to
the lattice topology. alpha-NaMnO2 therefore represents a model system where
the geometric frustration is resolved through the lowering of the
dimensionality of the spin interactions.",0907.0742v1
2009-10-03,Cross-relaxation and phonon bottleneck effects on magnetization dynamics in LiYF4:Ho3+,"Frequency and dc magnetic field dependences of dynamic susceptibility in
diluted paramagnets LiYF$_4$:Ho$^{3+}$ have been measured at liquid helium
temperatures in the ac and dc magnetic fields parallel to the symmetry axis of
a tetragonal crystal lattice. Experimental data are analyzed in the framework
of microscopic theory of relaxation rates in the manifold of 24
electron-nuclear sublevels of the lowest non-Kramers doublet and the first
excited singlet in the Ho$^{3+}$ ground multiplet $^5I_8$ split by the crystal
field of S$_4$ symmetry. The one-phonon transition probabilities were computed
using electron-phonon coupling constants calculated in the framework of
exchange charge model and were checked by optical piezospectroscopic
measurements. The specific features observed in field dependences of the in-
and out-of-phase susceptibilities (humps and dips, respectively) at the
crossings (anti-crossings) of the electron-nuclear sublevels are well
reproduced by simulations when the phonon bottleneck effect and the cross-spin
relaxation are taken into account.",0910.0519v1
2009-10-09,Anomalous Hanle Effect in Quantum Dots : Evidence for Strong Dynamical Nuclear Polarization in Transverse Magnetic Field,"Hanle effect is ubiquitous in the study of spin-related phenomena and has
been used to determine spin lifetime, precession and transport in
semiconductors. Here, we report an experimental observation of anomalous Hanle
effect in individual self-assembled InAs/GaAs quantum dots where we find that a
sizeable photo-created electron spin polarization can be maintained in
transverse fields as high as 1T until it abruptly collapses. The striking
broadening of the Hanle curve by a factor of ~20 and its bistability upon
reversal of the magnetic sweep direction points to a novel dynamical nuclear
spin polarization mechanism where the effective nuclear magnetic field
compensates the transverse applied field. This interpretation is further
supported by the measurement of actual electron Zeeman splitting which exhibits
an abrupt increase at the Hanle curve collapse. Strong inhomogeneous
quadrupolar interactions typical for strained quantum dots are likely to play a
key role in polarizing nuclear spins perpendicular to the optically injected
spin orientation.",0910.1744v1
2009-10-28,Study of the ground state properties of $LiHo_xY_{1-x}F_4$ using $μ$SR,"$LiHo_xY_{1-x}F_4$ is an insulating system where the magnetic Ho$^{3+}$ ions
have an Ising character, and interact mainly through magnetic dipolar fields.
We used the muon spin relaxation technique to study the nature of the ground
state for samples with x=0.25, 0.12, 0.08, 0.045 and 0.018. In contrast with
some previous works, we have not found any signature of canonical spin glass
behavior down to $\approx$15mK. Instead, below $\approx$300mK we observed
dynamic magnetism characterized by a single correlation time with a temperature
independent fluctuation rate. We observed that this low temperature fluctuation
rate increases with x up to 0.08, above which it levels off. The 300mK energy
scale corresponds to the Ho3+ hyperfine interaction strength, suggesting that
the hyperfine interaction may be intimately involved with the spin dynamics in
this system.",0910.5486v1
2009-12-10,Spin dynamics in the geometrically frustrated multiferroic CuCrO2,"The spin dynamics of the geometrically frustrated triangular antiferromagnet
multiferroic CuCrO2 have been mapped out using inelastic neutron scattering.
The relevant spin Hamiltonian parameters modelling the incommensurate modulated
helicoid have been determined, and correspond to antiferromagnetic nearest and
next-nearest neighbour interactions in the ab plane, with a strong planar
anisotropy. The weakly dispersive excitation along c reflects the essentially
two-dimensional character of the magnetic interactions and according to
classical energy calculations it is weakly ferromagnetic. Our results clearly
point out the relevance of the balance between a ferromagnetic coupling between
adjacent planes and a weakly antiferromagnetic next-nearest neighbour
interaction in stabilising the three-dimensional ferroelectric magnetic order
in CuCrO2. This novel insight on the interplay between magnetic interactions in
CuCrO2 should provide a useful basis in the design of new delafossite-based
multiferroic materials.",0912.1962v1
2009-12-23,Nonexistence of classical diamagnetism and nonequilibrium fluctuation theorems for charged particles on a curved surface,"We show that the classical Langevin dynamics for a charged particle on a
closed curved surface in a time-independent magnetic field leads to the
canonical distribution in the long time limit. Thus the Bohr-van Leeuwen
theorem holds even for a finite system without any boundary and the average
magnetic moment is zero. This is contrary to the recent claim by Kumar and
Kumar (EPL, {\bf 86} (2009) 17001), obtained from numerical analysis of
Langevin dynamics, that a classical charged particle on the surface of a sphere
in the presence of a magnetic field has a nonzero average diamagnetic moment.
We extend our analysis to a many-particle system on a curved surface and show
that the nonequilibrium fluctuation theorems also hold in this geometry.",0912.4697v2
2009-12-25,Doping Dependence of Spin Dynamics in Electron-Doped Ba(Fe1-xCox)2As2,"The spin dynamics in single crystal, electron-doped Ba(Fe1-xCox)2As2 has been
investigated by inelastic neutron scattering over the full range from undoped
to the overdoped regime. We observe damped magnetic fluctuations in the normal
state of the optimally doped compound (x=0.06) that share a remarkable
similarity with those in the paramagnetic state of the parent compound (x=0).
In the overdoped superconducting compound (x=0.14), magnetic excitations show a
gap-like behavior, possibly related to a topological change in the hole Fermi
surface (Lifshitz transition), while the imaginary part of the spin
susceptibility prominently resembles that of the overdoped cuprates. For the
heavily overdoped, non-superconducting compound (x=0.24) the magnetic
scattering disappears, which could be attributed to the absence of a hole
Fermi-surface pocket observed by photoemission.",0912.4945v2
2010-01-18,Dynamical nuclear spin polarization induced by electronic current through double quantum dots,"We analyze electron spin relaxation in electronic transport through
coherently coupled double quantum dots in the spin blockade regime. In
particular, we focus on hyperfine interaction as the spin relaxation mechanism.
We pay special attention to the effect of the dynamical nuclear spin
polarization induced by the electronic current on the nuclear environment. We
discuss the behaviour of the electronic current and the induced nuclear spin
polarization versus an external magnetic field for different hyperfine coupling
intensities and interdot tunnelling strengths. We take into account, for each
magnetic field, all hyperfine mediated spin relaxation processes coming from
the different opposite spin levels approaches. We find that the current as a
function of the external magnetic field shows a peak or a dip, and that the
transition from a current dip to a current peak behaviour is obtained by
decreasing the hyperfine coupling or by increasing the interdot tunnelling
strength. We give a physical picture in terms of the interplay between the
electrons tunnelling out of the double quantum dot and the spin flip processes
due to the nuclear environment.",1001.3104v2
2010-02-16,Dynamic Nuclear Polarization and Nuclear Magnetic Resonance in the Simplest Pseudospin Quantum Hall Ferromagnet,"We present dynamic nuclear polarization (DNP) in the simplest pseudospin
quantum Hall ferromagnet (QHF) of an InSb two-dimensional electron gas with a
large g factor using tilted magnetic fields. The DNP-induced amplitude change
of a resistance spike of the QHF at large current enables observation of the
resistively detected nuclear magnetic resonance of the high nuclear spin
isotope 115In with nine quadrupole splittings. Our results demonstrate the
importance of domain structures in the DNP process. The nuclear spin relaxation
time T1 in this QHF was relatively short (~ 120 s), and almost temperature
independent.",1002.3087v3
2010-06-28,Creating artificial magnetic fields for cold atoms by photon-assisted tunneling,"This paper proposes a simple setup for introducing an artificial magnetic
field for neutral atoms in 2D optical lattices. This setup is based on the
phenomenon of photon-assisted tunneling and involves a low-frequency periodic
driving of the optical lattice. This low-frequency driving does not affect the
electronic structure of the atom and can be easily realized by the same means
which employed to create the lattice. We also address the problem of detecting
this effective magnetic field. In particular, we study the center of mass
wave-packet dynamics, which is shown to exhibit certain features of cyclotron
dynamics of a classical charged particle.",1006.5270v2
2010-08-21,Dynamical Corrections to Spin Wave Excitations in Quantum Wells due to Coulomb Interactions and Magnetic Ions,"We have measured dispersions of spin-flip waves and spin-flip single-particle
excitations of a spin polarized two-dimensional electron gas in a CdMnTe
quantum well using resonant Raman scattering. We find the energy of the
spin-flip wave to be below the spin-flip single particle excitation continuum,
a contradiction to the theory of spin waves in diluted magnetic semiconductors
put forth in [Phys. Rev. B 70, 045205 (2004)]. We show that the inclusion of
terms accounting for the Coulomb interaction between carriers in the spin wave
propagator leads to an agreement with our experimental results. The dominant
Coulomb contribution leads to an overall red shift of the mixed electron-Mn
spin modes while the dynamical coupling between Mn ions results in a small blue
shift. We provide a simulated model system which shows the reverse situation
but at an extremely large magnetic field.",1008.3663v1
2010-09-17,Coronal mass ejections as expanding force-free structures,"We mode Solar coronal mass ejections (CMEs) as expanding force-fee magnetic
structures and find the self-similar dynamics of configurations with spatially
constant \alpha, where {\bf J} =\alpha {\bf B}, in spherical and cylindrical
geometries, expanding spheromaks and expanding Lundquist fields
correspondingly. The field structures remain force-free, under the conventional
non-relativistic assumption that the dynamical effects of the inductive
electric fields can be neglected. While keeping the internal magnetic field
structure of the stationary solutions, expansion leads to complicated internal
velocities and rotation, induced by inductive electric field. The structures
depends only on overall radius R(t) and rate of expansion \dot{R}(t) measured
at a given moment, and thus are applicable to arbitrary expansion laws. In case
of cylindrical Lundquist fields, the flux conservation requires that both axial
and radial expansion proceed with equal rates. In accordance with observations,
the model predicts that the maximum magnetic field is reached before the
spacecraft reaches the geometric center of a CME.",1009.3463v2
2010-09-29,Dynamical polarization of monolayer graphene in a magnetic field,"The one-loop dynamical polarization function of graphene in an external
magnetic field is calculated as a function of wavevector and frequency at
finite chemical potential, temperature, band gap, and width of Landau levels.
The exact analytic result is given in terms of digamma functions and
generalized Laguerre polynomials, and has the form of double sum over Landau
levels. Various limits (static, clean, etc) are discussed. The Thomas-Fermi
inverse length $q_F$ of screening of the Coulomb potential is found to be an
oscillating function of a magnetic field and a chemical potential. At zero
temperature and scattering rate, it vanishes when the Fermi level lies between
the Landau levels.",1009.5980v1
2011-01-27,A pseudogap term in the magnetic response of the cuprate superconductors,"We combine neutron scattering (INS) and NMR/NQR nuclear spin lattice
relaxation rate data to deduce the existence of a new contribution to the
magnetic response (dynamic susceptibility) in cuprate superconductors. This
contribution, which has yet to be observed with INS, is shown to embody the
magnetic pseudogap effects. As such, it explains the long-standing puzzle of
pseudogap effects missing from cuprate INS data, dominated by stripe
fluctuations, for the dynamic susceptibility at low energies. For
La_(1.86)Sr_(0.14)CuO_(4) and YBa_(2)Ba_(3)O_(6.5), the new term is the chief
contributor to nuclear spin lattice relaxation at T >> T_(c).",1101.5360v1
2011-02-08,Dynamics of the Magnetic Susceptibility Deep in the Coulomb Phase of the Dipolar Spin Ice Material Ho2Ti2O7,"Low temperature measurements of the ac magnetic susceptibility along the
[110] direction of a single crystal of the dipolar spin ice material Ho2Ti2O7,
in zero static field, are presented. While behavior that is qualitatively
consistent with previous work on Ho2Ti2O7 and the related material Dy2Ti2O7 is
observed, this work extends measurements to appreciably lower temperatures and
frequencies. In the freezing regime, below 1 K, the dynamics are found to be
temperature activated, thus well described by an Arrhenius law with an
activation energy close to 6J_eff, a result that is not easily explained with
the current model of magnetic monopole excitations in dipolar spin ice. The
form and temperature dependence of the ac susceptibility spectra are found to
be nontrivial and distinct from standard glassy relaxation. Particular
attention has been paid to correcting for the demagnetization effect, which is
quite significant in these materials and has important, even qualitative,
effects on the susceptibility spectra.",1102.1703v2
2011-02-16,Magnetization dynamics and Majorana fermions in ferromagnetic Josephson junctions along the quantum spin Hall edge,"We investigate the interplay between ferromagnetic and superconducting order
at the edge of a quantum spin Hall insulator. Using complementary analytical
and self-consistent numerical approaches, we study a ferromagnetic Josephson
junction and show how the direct coupling between magnetism and the
superconducting U(1) phase gives rise to several unusual phenomena which
distinguishes the present system from its non-topological equivalent. In
particular, we demonstrate how the anomalous current-phase relation triggers
supercurrent-induced magnetization dynamics and also study the spatial
localization of the Majorana fermions appearing in the junction.",1102.3403v2
2011-03-02,Switching dynamics of a magnetostrictive single-domain nanomagnet subjected to stress,"The temporal evolution of the magnetization vector of a single-domain
magnetostrictive nanomagnet, subjected to in-plane stress, is studied by
solving the Landau-Lifshitz-Gilbert equation. The stress is ramped up linearly
in time and the switching delay, which is the time it takes for the
magnetization to flip, is computed as a function of the ramp rate. For high
levels of stress, the delay exhibits a non-monotonic dependence on the ramp
rate, indicating that there is an {\it optimum} ramp rate to achieve the
shortest delay. For constant ramp rate, the delay initially decreases with
increasing stress but then saturates showing that the trade-off between the
delay and the stress (or the energy dissipated in switching) becomes less and
less favorable with increasing stress. All of these features are due to a
complex interplay between the in-plane and out-of-plane dynamics of the
magnetization vector induced by stress.",1103.0352v1
2011-03-09,Gravitational collapse and entropy of Black Holes with magnetic sources,"This thesis is divided in two parts, each one addressing problems that can be
relevant in the study of compact objects. The first part deals with the study
of a magnetized and self-gravitating gas of degenerated fermions (electrons and
neutrons) as sources of a Bianchi-I space-time. We solve numerically the
Einstein-Maxwell field equations for a large set of initial conditions of the
dynamical variables. The collapsing singularity is isotropic for the neutron
gas and can be anisotropic for the electron gas. This result is consistent with
the fact that electrons exhibit a stronger coupling with the magnetic field,
which is the source of anisotropy in the dynamical variables. In the second
part we calculate the entropy of extremal black holes in 4 and 5 dimensions,
using the entropy function formalism of Sen and taking into account higher
order derivative terms that come from the complete set of Riemann invariants.
The resulting entropies show the deviations from the well know
Bekenstein-Hawking area law.",1103.1828v1
2011-04-14,The magnetic excitation spectra in BaFe$_{2}$As$_{2}$: a two-particle approach within DFT+DMFT,"We study the magnetic excitation spectra in the paramagnetic state of
BaFe$_{2}$As$_{2}$ from the \textit{ab initio} perspective. The one-particle
excitation spectrum is determined within the combination of the density
functional theory and the dynamical mean-field theory method. The two-particle
response function is extracted from the local two-particle vertex function,
also computed by the dynamical mean field theory, and the polarization
function. This method reproduces all the experimentally observed features in
inelastic neutron scattering (INS), and relates them to both the one particle
excitations and the collective modes. The magnetic excitation dispersion is
well accounted for by our theoretical calculation in the paramagnetic state
without any broken symmetry, hence nematic order is not needed to explain the
INS experimental data.",1104.2942v1
2011-05-26,Non-equilibrium quantum dynamics after local quenches,"We study the quantum dynamics resulting from preparing a one-dimensional
quantum system in the ground state of initially two decoupled parts which are
then joined together (local quench). Specifically we focus on the transverse
Ising chain and compute the time-dependence of the magnetization profile,
m_l(t), and correlation functions at the critical point, in the
ferromagnetically ordered phase and in the paramagnetic phase. At the critical
point we find finite size scaling forms for the nonequilibrium magnetization
and compare predictions of conformal field theory with our numerical results.
In the ferromagnetic phase the magnetization profiles are well matched by our
predictions from a quasi-classical calculation.",1105.5317v1
2011-06-15,Routh Reduction by Stages,"This paper deals with the Lagrangian analogue of symplectic or point
reduction by stages. We develop Routh reduction as a reduction technique that
preserves the Lagrangian nature of the dynamics. To do so we heavily rely on
the relation between Routh reduction and cotangent symplectic reduction. The
main results in this paper are: (i) we develop a class of so called magnetic
Lagrangian systems and this class has the property that it is closed under
Routh reduction; (ii) we construct a transformation relating the magnetic
Lagrangian system obtained after two subsequent Routh reductions and the
magnetic Lagrangian system obtained after Routh reduction w.r.t. to the full
symmetry group.",1106.2950v2
2011-08-05,MHD Turbulence Simulation in a Cosmic Structure Context,"The gaseous media of galaxy clusters and cosmic filaments, which constitute
most of the baryonic matter in the universe, is highly dynamic. It is also
probably turbulent, although the turbulence properties are poorly known. The
gas is highly rarefied, essentially fully ionized plasma. Observational
evidence suggests intracluster media (ICMs) are magnetized at some level. There
are several possible origins for ICM seed fields; the observed fields are
likely the result of turbulence in the ICM. We are engaged in a simulation
study designed to understand in this context how very weak initial magnetic
fields evolve in driven turbulence. We find that the magnetic fields eventually
evolve towards equipartition levels with the vortical, solenoidal kinetic
energy in the turbulence. As they do so the topology of the field structures
transition from filamentary forms into ribbon-like structures in which the
field orientations are laminated with vorticity structures.",1108.1369v1
2011-08-25,Driving Force of Ultrafast Magnetization Dynamics,"Irradiating a ferromagnetic material with an ultrashort laser pulse leads to
demagnetization on a femtosecond timescale. We implement Elliott-Yafet type
spin-flip scattering, mediated by electron-electron and electron-phonon
collisions, into the framework of a spin-resolved Boltzmann equation.
Considering three mutually coupled reservoirs, (i) spin-up electrons, (ii)
spin-down electrons and (iii) phonons, we trace non-equilibrium electron
distributions during and after laser excitation. We identify the driving force
for ultrafast magnetization dynamics as the equilibration of temperatures and
chemical potentials between the electronic subsystems. This principle can be
used to easily predict the maximum quenching of magnetization upon ultrashort
laser irradiation in any material, as we show for the example of
3d-ferromagnetic nickel.",1108.5170v1
2011-08-29,Domain Wall Spin Dynamics in Kagome Antiferromagnets,"We report magnetization and neutron scattering measurements down to 60 mK on
a new family of Fe based kagome antiferromagnets, in which a strong local spin
anisotropy combined with a low exchange path network connectivity lead to
domain walls intersecting the kagome planes through strings of free spins.
These produce unfamiliar slow spin dynamics in the ordered phase, evolving from
exchange-released spin-flips towards a cooperative behavior on decreasing the
temperature, probably due to the onset of long-range dipolar interaction. A
domain structure of independent magnetic grains is obtained that could be
generic to other frustrated magnets.",1108.5540v2
2011-09-14,Domain-wall dynamics in translationally noninvariant nanowires: theory and applications,"We generalize domain-wall dynamics to the case of translationally
noninvariant ferromagnetic nanowires. The obtained equations of motion make the
description of the domain-wall propagation more realistic by accounting for the
variations along the wire, such as disorder or change in the wire shape. We
show that the effective equations of motion are very general and do not depend
on the model details. As an example of their use, we consider an
hourglass-shaped nanostrip in detail. A transverse domain wall is trapped in
the middle and has two stable magnetization directions. We study the switching
between the two directions by short current pulses. We obtain the exact time
dependence of the current pulses required to switch the magnetization with the
minimal Ohmic losses per switching. Furthermore, we find how the Ohmic losses
per switching depend on the switching time for the optimal current pulse. As a
result, we show that as a magnetic memory this nanodevice can be $10^5$ times
more energy efficient than the best modern devices.",1109.2996v2
2011-10-02,Metastable magnetic domain wall dynamics,"The dynamics of metastable magnetic domain walls in straight ferromagnetic
nanowires under spin waves, external magnetic fields, and current induced spin
transfer torque are studied by micromagnetic simulations. It is found that in
contrast to a stable wall, it is possible to displace a metastable domain wall
in the absence of any external excitation. In addition, independent of the
domain wall excitation method, the velocity of a metastable wall is much
smaller than a stable wall and their displacement direction could be different
from the stable wall depending on the structure of metastable walls. Under the
current induced spin transfer torque excitation, the direction of domain wall
displacement is directly related to the intensity of nonadiabatic spin transfer
torque. In a rough nanowire, it is found that the displacement of a metastable
wall could happen much below the critical excitation of a stable wall.
Furthermore, we show that it is possible to have either a forward or backward
displacement of a metastable domain wall by changing the pulse width of the
excitation.",1110.0175v1
2011-10-16,Spin Hall Effect-driven Spin Torque in Magnetic Texture,"Current-induced spin torque and magnetization dynamics in the presence of
spin Hall effect in magnetic textures is studied theoretically. The local
deviation of the charge current gives rise to a current-induced spin torque of
the form (1-\beta{\bf M})x[({\bf u}_0+\alpha_H{\bf u}_0x\bf M})\cdot{\bm
\nabla}]{\bf M}, where {\bf u}_0 is the direction of the injected current,
\alpha_H is the Hall angle and \beta is the non-adiabaticity parameter due to
spin relaxation. Since \alpha_H and \beta can have a comparable order of
magnitude, we show that this torque can significantly modify the
current-induced dynamics of both transverse and vortex walls.",1110.3485v1
2011-10-19,Time Dependence of Advection Dominated Accretion Flow with a Toroidal Magnetic Field,"The present study examines self-similarity evolution of advection dominated
accretion flow (ADAF) in the presence of a toroidal magnetic field. In this
research, it was assumed that the angular momentum transport is due to viscous
turbulence and $\alpha$-prescription was used for kinematics coefficient of
viscosity. In order to solve the integrated equations which govern the
dynamical behavior of the accretion flow, self-similar solution was used. The
solutions show that the behavior of physical quantities in a dynamical ADAF are
different from steady accretion flow and a disk with polytropic approach. The
solution indicates a transonic point in the accretion flow for all selected
amounts of advection parameter. Also, by adding strength of the magnetic field
and the degree of advection, the radial-thickness of the disk decreased and the
disk compressed. The model implies that the flow has differential rotation and
is sub-Keplerian at small radii and is super-Keplerian in large radii.",1110.4191v1
2011-11-15,Surprises in relativistic matter in a magnetic field,"A short review of recent advances in understanding the dynamics of
relativistic matter in a magnetic field is presented. The emphasis is on the
dynamics related to the generation of the chiral shift parameter in the normal
ground state. We argue that the chiral shift parameter contributes to the axial
current density, but does not modify the conventional axial anomaly relation.
The analysis based on gauge invariant regularization schemes in the
Nambu-Jona-Lasinio model suggests that these findings should be valid also in
gauge theories. It is pointed out that the chiral shift parameter can affect
observable properties of compact stars and modify the key features of the
chiral magnetic effect in heavy ion collisions.",1111.3401v2
2011-12-01,Dynamic RKKY interaction in graphene,"The growing interest in carbon-based spintronics has stimulated a number of
recent theoretical studies on the RKKY interaction in graphene, based on which
the energetically favourable alignment between magnetic moments embedded in
this material can be calculated. The general consensus is that the strength of
the RKKY interaction in graphene decays as 1/D3 or faster, where D is the
separation between magnetic moments. Such an unusually fast decay for a
2-dimensional system suggests that the RKKY interaction may be too short ranged
to be experimentally observed in graphene. Here we show in a mathematically
transparent form that a far more long ranged interaction arises when the
magnetic moments are taken out of their equilibrium positions and set in
motion. We not only show that this dynamic version of the RKKY interaction in
graphene decays far more slowly but also propose how it can be observed with
currently available experimental methods.",1112.0205v2
2012-01-20,"Vortex Dynamics in Ferromagnetic Superconductors: Vortex Clusters, Domain Walls and Enhanced Viscosity","We demonstrate that there is a long-range vortex-vortex attraction in
ferromagnetic superconductors due to polarization of the magnetic moments.
Vortex clusters are then stabilized in the ground state for low vortex
densities. The motion of vortex clusters driven by the Lorentz force excites
magnons. This regime becomes unstable at a threshold velocity above which
domain walls are generated for slow relaxation of the magnetic moments and the
vortex configuration becomes modulated. This dynamics of vortices and magnetic
moments can be probed by transport measurements.",1201.4195v3
2012-01-23,Influence of the magnetoelectric coupling on the electric field induced magnetization reversal in a composite non-strained multiferroic chain,"We study theoretically the multiferroic dynamics in a composite
one-dimensional system consisting of BaTiO3 multiferroically coupled to an iron
chain. The method treats the magnetization and the polarization as
thermodynamic quantities describable via a combination of the Landau-Lifshits-
Gilbert and the Ginzburg-Landau dynamics coupled via an additional term in the
total free energy density. This term stems from the multiferroic interaction at
the interface. For a wide range of strengths of this coupling we predict the
possibility of obtaining a well-developed hysteresis in the ferromagnetic part
of the system induced by an external electric field. The dependence of the
reversal modes on the electric field frequency is also investigated and we
predict a considerable stability of the magnetization reversal for frequencies
in the range of 0.5 - 12 [GHz].",1201.4740v1
2012-01-30,Helical buckling of a whirling conducting rod in a uniform magnetic field,"We study the effect of a magnetic field on the behaviour of a conducting
elastic rod subject to a novel set of boundary conditions that, in the case of
a transversely isotropic rod, give rise to exact helical post-buckling
solutions. The equations used are the geometrically exact Kirchhoff equations
and both static (buckling) and dynamic (whirling) instability are considered.
Critical loads are obtained explicitly and are given by a surprisingly simple
formula. By solving the linearised equations about the (quasi-)stationary
solutions we also find secondary instabilities described by (Hamiltonian-)Hopf
bifurcations, the usual signature of incipient `breathing' modes. The boundary
conditions can also be used to generate and study helical solutions through
traditional non-magnetic buckling due to compression, twist or whirl.",1201.6351v1
2012-01-31,Field-Induced Degeneracy Regimes in Quantum Plasmas,"It is shown that in degenerate magnetized Fermi-Dirac plasma where the
electron-orbital are quantized distinct quantum hydrodynamic (QHD) limits exist
in which the nonlinear density waves behave differently. The Coulomb
interaction among degenerate electrons affect the electrostatic nonlinear wave
dynamics more significant in the ground-state Landau quantization or the
so-called quantum-limit ($l=0$) rather than in the classical-limit
($l=\infty$). It is also remarked that the effective electron quantum potential
unlike the number-density and degeneracy pressure is independent of the applied
magnetic field in the classical-limit plasma, while, it depends strongly on the
field strength in the quantum-limit. Current findings are equally important in
the study of wave dynamics in arbitrarily-high magnetized astrophysical and
laboratory dense plasmas.",1201.6571v2
2012-02-15,Residual Energy in Weak and Strong MHD Turbulence,"Recent numerical and observational studies revealed that spectra of magnetic
and velocity fluctuations in MHD turbulence have different scaling indexes.
This intriguing feature has been recently explained in the case of weak MHD
turbulence, that is, turbulence consisting of weakly interacting Alfven waves.
However, astrophysical turbulence is strong in majority of cases. In the
present work, we propose a unifying picture that allows one to address weak and
strong MHD turbulence on the same footing. We argue that magnetic and kinetic
energies are different in both weak and strong MHD turbulence. Their
difference, the so-called residual energy, is spontaneously generated by
turbulence, it has the Fourier spectrum E_r(k)=E_v(k)-E_b(k) \propto
-f_w(k_||/k_perp) k_perp^{-2} in weak turbulence, and E_r(k) \propto
-f_s(k_||/k_perp) k_perp^{-3} in strong turbulence. Here f_w,s(x) are functions
declining fast for x>C_w,s and not significantly varying for $x<C_w,s$ with
some constants C_w,s, and k_|| and k_perp the field-parallel and
field-perpendicular wave vectors with respect to the applied strong uniform
magnetic field.",1202.3453v1
2012-02-17,Muon Collider interaction region and machine-detector interface design,"One of the key systems of a Muon Collider (MC) - seen as the most exciting
option for the energy frontier machine in the post-LHC era - is its interaction
region (IR). Designs of its optics, magnets and machine-detector interface are
strongly interlaced and iterative. As a result of recent comprehensive studies,
consistent solutions for the 1.5-TeV c.o.m. MC IR have been found and are
described here. To provide the required momentum acceptance, dynamic aperture
and chromaticity, an innovative approach was used for the IR optics. Conceptual
designs of large-aperture high-field dipole and high-gradient quadrupole
magnets based on Nb3Sn superconductor were developed and analyzed in terms of
the operating margin, field quality, mechanics, coil cooling and quench
protection. Shadow masks in the interconnect regions and liners inside the
magnets are used to mitigate the unprecedented dynamic heat deposition due to
muon decays (~0.5 kW/m). It is shown that an appropriately designed
machine-detector interface (MDI) with sophisticated shielding in the detector
has a potential to substantially suppress the background rates in the MC
detector.",1202.3979v1
2012-02-28,Collisionless reconnection: The sub-microscale mechanism of magnetic field line interaction,"Magnetic field lines are quantum objects carrying one quantum
$\Phi_0=2\pi\hbar/e$ of magnetic flux and have finite radius $\lambda_m$. Here
we argue that they possess a very specific dynamical interaction. Parallel
field lines reject each other. When confined to a certain area they form
two-dimensional lattices of hexagonal structure. We estimate the filling factor
of such an area. Antiparallel field lines, on the other hand, attract each
other. We identify the physical mechanism as being due to the action of the
gauge potential field which we determine quantum mechanically for two parallel
and two antiparallel field lines. The distortion of the quantum electrodynamic
vacuum causes a cloud of virtual pairs. We calculate the virtual pair
production rate from quantum electrodynamics and estimate the virtual pair
cloud density, pair current and Lorentz force density acting on the field lines
via the pair cloud. These properties of field line dynamics become important in
collisionless reconnection, consistently explaining why and how reconnection
can spontaneously set on in the field-free centre of a current sheet below the
electron-inertial scale.",1202.6209v1
2012-06-28,The Landau-Lifshitz-Bloch equation for ferrimagnetic materials,"We derive the Landau-Lifshitz-Bloch (LLB) equation for a two-component
magnetic system valid up to the Curie temperature. As an example, we consider
disordered GdFeCo ferrimagnet where the ultrafast optically induced
magnetization switching under the action of heat alone has been recently
reported. The two-component LLB equation contains the longitudinal relaxation
terms responding to the exchange fields from the proper and the neighboring
sublattices. We show that the sign of the longitudinal relaxation rate at high
temperatures can change depending on the dynamical magnetization value and a
dynamical polarisation of one material by another can occur. We discuss the
differences between the LLB and the Baryakhtar equation, recently used to
explain the ultrafast switching in ferrimagnets. The two-component LLB equation
forms basis for the largescale micromagnetic modeling of nanostructures at high
temperatures and ultrashort timescales.",1206.6672v1
2012-07-12,Cosmic magnetization: from spontaneously emitted aperiodic turbulent to ordered equipartition fields,"It is shown that an unmagnetized nonrelativistic thermal electron-proton
plasma spontaneously emits aperiodic turbulent magnetic field fluctuations of
strength $|\delta B|=9\beta_eg^{1/3}W_e^{1/2}$ G, where $\beta_e$ is the
normalized thermal electron temperature, $W_e$ the thermal plasma energy
density and $g$ the plasma parameter. Aperiodic modes fluctuate only in space,
but are not propagating. For the unmagnetized intergalactic medium, immediately
after the reionization onset, the field strength from this mechanism is about
$4.7\cdot 10^{-16}$ G, too weak to affect the dynamics of the plasma. The shear
and/or compression of the intergalactic medium exerted by the first supernova
explosions amplify these seed fields and make them anisotropic, until the
magnetic restoring forces affect the gas dynamics at ordered plasma betas near
unity.",1207.2963v1
2012-08-17,Detection of domain wall eigenfrequency in infinity-shaped magnetic nanostructures,"The dynamics of a magnetic infinity-shaped nanostructure has been
experimentally studied by two different techniques such as the sinusoidal
resonance excitation and the damped short pulse excitation to measure the
eigenfrequency of domain walls. Direct observation of the magnetic domain wall
nucleation has been measured in the frequency domain. Electrical measurements
of the domain wall dynamics in the frequency domain reveal the existence of
multi-eigenmodes for large excitation amplitudes. The time-resolved
measurements show that the frequency of the damped gyration is similar to that
of the frequency domain and coexistence of spin wave excitations.",1208.3527v1
2012-08-29,Persistent spin dynamics intrinsic to amplitude-modulated long-range magnetic order,"An incommensurate elliptical helical magnetic structure in the frustrated
coupled-spin-chain system FeTe2O5Br is surprisingly found to persist down to
53(3) mK (T/T_N ~ 1/200), according to neutron scattering and muon spin
relaxation. In this state, finite spin fluctuations at T -> 0 are evidenced by
muon depolarization, which is in agreement with specific-heat data indicating
the presence of both gapless and gapped excitations. We thus show that the
amplitude-modulated magnetic order intrinsically accommodates contradictory
persistent spin dynamics and long-range order and can serve as a model
structure to investigate their coexistence.",1208.5843v2
2012-09-29,Brownian Motion and Quantum Dynamics of Magnetic Monopoles in Spin Ice,"Spin ice illustrates many unusual magnetic properties, including zero point
entropy, emergent monopoles and a quasi liquid-gas transition. To reveal the
quantum spin dynamics that underpin these phenomena is an experimental
challenge. Here we show how crucial information is contained in the frequency
dependence of the magnetic susceptibility and in its high frequency or
adiabatic limit. These measures indicate that monopole diffusion is strictly
Brownian but is underpinned by spin tunnelling and is influenced by collective
monopole interactions. We also find evidence of driven monopole plasma
oscillations in weak applied field, and unconventional critical behaviour in
strong applied field. Our results resolve contradictions in the present
understanding of spin ice, reveal unexpected physics and establish adiabatic
susceptibility as a revealing characteristic of exotic spin systems.",1210.0106v1
2012-10-03,Torsional Oscillations in a Global Solar Dynamo,"We characterize and analyze rotational torsional oscillations developing in a
large-eddy magnetohydrodynamical simulation of solar convection (Ghizaru,
Charbonneau, and Smolarkiewicz, Astrophys. J. Lett., 715, L133 (2010); Racine
et al., Astrophys. J., 735, 46 (2011)) producing an axisymmetric large-scale
magnetic field undergoing periodic polarity reversals. Motivated by the many
solar-like features exhibited by these oscillations, we carry out an analysis
of the large-scale zonal dynamics. We demonstrate that simulated torsional
oscillations are not driven primarily by the periodically-varying large-scale
magnetic torque, as one might have expected, but rather via the magnetic
modulation of angular-momentum transport by the large-scale meridional flow.
This result is confirmed by a straightforward energy analysis. We also detect a
fairly sharp transition in rotational dynamics taking place as one moves from
the base of the convecting layers to the base of the thin tachocline-like shear
layer formed in the stably stratified fluid layers immediately below. We
conclude by discussing the implications of our analyses with regards to the
mechanism of amplitude saturation in the global dynamo operating in the
simulation, and speculate on the possible precursor value of torsional
oscillations for the forecast of solar cycle characteristics.",1210.1209v1
2012-10-16,"Dynamics of self-generated, large amplitude magnetic fields following high-intensity laser matter interaction","The dynamics of magnetic fields with amplitude of several tens of Megagauss,
generated at both sides of a solid target irradiated with a high intensity (?
1019W/cm2) picosecond laser pulse, has been spatially and temporally resolved
using a proton imaging technique. The amplitude of the magnetic fields is
sufficiently large to have a constraining effect on the radial expansion of the
plasma sheath at the target surfaces. These results, supported by numerical
simulations and simple analytical modeling, may have implications for ion
acceleration driven by the plasma sheath at the rear side of the target as well
as for the laboratory study of self-collimated high-energy plasma jets.",1210.4366v1
2012-11-13,Atomistic spin dynamics of low-dimensional magnets,"We investigate the magnetic properties of a range of low-dimensional
ferromagnets using a combination of first-principles calculations and atomistic
spin dynamics simulations. This approach allows us to evaluate the ground state
and finite temperature properties of experimentally well characterized systems
such as Co/Cu(111), Co/Cu(001), Fe/Cu(001) and Fe/W(110), for different
thicknesses of the magnetic layer. We compare our calculated spin wave spectra
with experimental data available in the literature, and find a good
quantitative agreement. We also predict magnon spectra for systems for which no
experimental data exist at the moment, and estimate the role of temperature
effects.",1211.2964v2
2012-12-18,Magnetoelasticity in ACr2O4 spinel oxides,"Dynamical properties of the lattice structure was studied by optical
spectroscopy in ACr2O4 chromium spinel oxide magnetic semiconductors over a
broad temperature region of T=10-335K. The systematic change of the A-site ions
(A=Mn, Fe, Co, Ni and Cu) showed that the occupancy of 3d orbitals on the
A-site, has strong impact on the lattice dynamics. For compounds with orbital
degeneracy (FeCr2O4, NiCr2O4 and CuCr2O4), clear splitting of infrared-active
phonon modes and/or activation of silent vibrational modes have been observed
upon the Jahn-Teller transition and at the onset of the subsequent long-range
magnetic order. Although MnCr2O4 and CoCr2O4 show multiferroic and
magnetoelectric character, no considerable magnetoelasticity was found in
spinel compounds without orbital degeneracy as they closely preserve the
high-temperature cubic spinel structure even in their magnetic ground state.
Besides lattice vibrations, intra-atomic 3d-3d transitions of the A$^{2+}$ ions
were also investigated to determine the crystal field and Racah parameters and
the strength of the spin-orbit coupling.",1212.4301v1
2013-01-09,"Quantum reactive scattering of ultracold NH($X\,^3Σ^-$) radicals in a magnetic trap","We investigate the ultracold reaction dynamics of magnetically trapped NH($X
^3\Sigma^-$) radicals using rigorous quantum scattering calculations involving
three coupled potential energy surfaces. We find that the reactive NH + NH
cross section is driven by a short-ranged collisional mechanism, and its
magnitude is only weakly dependent on magnetic field strength. Unlike most
ultracold reactions observed so far, the NH + NH scattering dynamics is
non-universal. Our results indicate that chemical reactions can cause more trap
loss than spin-inelastic NH + NH collisions, making molecular evaporative
cooling more difficult than previously anticipated.",1301.1931v1
2013-02-04,Robust synchronization of an arbitrary number of spin-torque driven vortex nanooscillators,"Non-linear magnetization dynamics in ferromagnetic nanoelements excited by
the spin-polarized dc-current is one of the most intensively studied phenomena
in solid state magnetism. Despite immense efforts, synchronization of
oscillations induced in several such nanoelements (spin-torque driven
nanooscillators, or STNO) still represents a major challenge both from the
fundamental and technological points of view. In this paper we propose a system
where synchronization of any number of STNOs, represented by magnetization
vortices inside squared nanoelements, can be easily achieved. Using full-scale
micromagnetic simulations we show that synchronization of these STNOs is
extremely dynamically stable due to their very large coupling energy provided
by the magnetodipolar interaction. Finally, we demonstrate that our concept
allows robust synchronization of an arbitrary number of STNOs (arranged either
as a 1D chain or as a 2D array), even when current supplying nanocontacts have
a broad size distribution.",1302.0659v3
2013-04-25,Critical behavior and magnetic relaxation dynamics of Nd0.4Sr0.6MnO3 nanoparticles,"Detailed dc and ac magnetic properties of chemically synthesized
Nd0.4Sr0.6MnO3 with different particle size (down to 27 nm) have been studied
in details. We have found ferromagnetic state in the nanoparticles, whereas,
the bulk Nd0.4Sr0.6MnO3 is known to be an A-type antiferromagnet. A
Griffiths-like phase has also been identified in the nanoparticles. Further,
critical behavior of the nanoparticles has been studied around the second order
ferromagnetic-paramagnetic transition region (|(T-TC)/TC|{\pounds} 0.04) in
terms of modified Arrott plot, Kouvel-Fisher plot and critical isotherm
analysis. The estimated critical exponents (b,g,d) are quite different from
those predicted according to three-dimensional mean-field, Heisenberg and Ising
models. This signifies a quite unusual nature of the size-induced ferromagnetic
state in Nd0.4Sr0.6MnO3. The nanoparticles are found to be interacting and do
not behave like ideal superparamagnet. Interestingly, we find spin glass like
slow relaxation of magnetization, aging and memory effect in the nanometric
samples. These phenomena have been attributed to very broad distribution of
relaxation time as well as to inter-particle interaction. Experimentally, we
have found out that the dynamics of the nanoparticle systems can be best
described by hierarchical model of spin glasses.",1304.6964v1
2013-05-16,Size-dependent magnetization switching characteristics and spin wave modes of FePt nanostructures,"We present a comprehensive investigation of the size-dependent switching
characteristics and spin wave modes of FePt nanoelements. Curved nanomagnets
(""caps"") are compared to flat disks of identical diameter and volume over a
size range of 100 to 300nm. Quasi-static magnetization reversal analysis using
first-order reversal curves (FORC) shows that spherical caps have lower vortex
nucleation and annihilation fields than the flat disks. As the element diameter
decreases, the reversal mechanism in the caps crosses over sooner to coherent
rotation than in the disks. The magnetization dynamics are studied using
optically induced small angle precession and reveal a strong size dependence
that differs for the two shapes. Flat disks exhibit well-known center and edge
modes at all sizes, but as the diameter of the caps increases from 100 to 300
nm, additional oscillation modes appear in agreement with dynamic micromagnetic
simulations. In addition, we show that the three-dimensional curvature of the
cap causes a much greater sensitivity to the applied field angle which provides
an additional way for controlling the ultrafast response of nanomagnetic
elements.",1305.3686v1
2013-05-18,Stress due to Electric and Magnetic fields in Viscoelastic Fluids,"A clear understanding of body force densities due to external electromagnetic
fields is necessary to study flow and deformation of materials exposed to the
fields. In this paper, we derive an expression for stress in continua with
viscous and elastic properties in presence of external, static electric or
magnetic fluids. Our derivation follows from fundamental thermodynamic
principles. We demonstrate the soundness of our results by showing that they
reduce to known expressions for Newtonian fluids and elastic solids. We point
out the extra care to be taken while applying these techniques to permanently
polarized or magnetized materials and derive an expression for stress in a
ferro-fluid. Lastly, we derive expressions for ponderomotive forces in several
situations of interest to fluid dynamics and rheology.",1305.4233v1
2013-05-23,Spin pumping and interlayer exchange coupling through palladium,"The magnetic behaviour of ultrathin ferromagnetic films deposited on
substrates is strongly affected by the properties of the substrate. We
investigate the spin pumping rate, interlayer exchange coupling and dynamic
exchange coupling between ultrathin ferromagnetic films through palladium, a
non-magnetic substrate that displays strong Stoner enhancement. We find that
the interlayer exchange coupling, both in the static and dynamic versions, is
qualitatively affected by the substrate's Stoner enhancement. For instance, the
oscillatory behavior that is a hallmark property of the RKKY exchange coupling
is strongly suppressed by Stoner enhancement. Although the spin pumping rate of
ferromagnetic films atop palladium is only mildly changed by Stoner enhancement
the change is large enough to be detected experimentally. The qualitative
aspects of our results for palladium are expected to remain valid for any
non-magnetic substrate where Coulomb repulsion is large.",1305.5459v1
2013-06-05,Kinetic Simulations of Plasmoid Chain Dynamics,"The dynamics of a plasmoid chain is studied with three dimensional
Particle-in-Cell simulations. The evolution of the system with and without a
uniform guide field, whose strength is 1/3 the asymptotic magnetic field, is
investigated. The plasmoid chain forms by spontaneous magnetic reconnection:
the tearing instability rapidly disrupts the initial current sheet generating
several small-scale plasmoids, that rapidly grow in size coalescing and
kinking. The plasmoid kink is mainly driven by the coalescence process. It is
found that the presence of guide field strongly influences the evolution of the
plasmoid chain. Without a guide field, a main reconnection site dominates and
smaller reconnection regions are included in larger ones, leading to an
hierarchical structure of the plasmoid-dominated current sheet. On the contrary
in presence of a guide field, plasmoids have approximately the same size and
the hierarchical structure does not emerge, a strong core magnetic field
develops in the center of the plasmoid in the direction of the existing guide
field, and bump-on-tail instability, leading to the formation of electron
holes, is detected in proximity of the plasmoids.",1306.1050v1
2013-06-25,Phase Winding a Two-Component BEC in an Elongated Trap: Experimental Observation of Moving Magnetic Orders and Dark-bright Solitons,"We experimentally investigate the phase winding dynamics of a harmonically
trapped two-component BEC subject to microwave induced Rabi oscillations
between two pseudospin components. While the single particle dynamics can be
explained by mapping the system to a two-component Bose-Hubbard model,
nonlinearities due to the interatomic repulsion lead to new effects observed in
the experiments: In the presence of a linear magnetic field gradient, a
qualitatively stable moving magnetic order that is similar to antiferromagnetic
order is observed after critical winding is achieved. We also demonstrate how
the phase winding can be used as a new tool to generate copious dark-bright
solitons in a two-component BEC, opening the door for new experimental studies
of these nonlinear features.",1306.6102v2
2013-06-26,Spin dynamics in p-doped semiconductor nanostructures subject to a magnetic field tilted from the Voigt geometry,"We develop a theoretical description of the spin dynamics of resident holes
in a p-doped semiconductor quantum well (QW) subject to a magnetic field tilted
from the Voigt geometry. We find the expressions for the signals measured in
time-resolved Faraday rotation (TRFR) and resonant spin amplification (RSA)
experiments and study their behavior for a range of system parameters. We find
that an inversion of the RSA peaks can occur for long hole spin dephasing times
and tilted magnetic fields. We verify the validity of our theoretical findings
by performing a series of TRFR and RSA experiments on a p-modulation doped
GaAs/Al_{0.3}Ga_{0.7}As single QW and showing that our model can reproduce
experimentally observed signals.",1306.6363v2
2013-07-31,Freezing a Quantum Magnet by Repeated Quantum Interference: An Experimental Realization,"We experimentally demonstrate the phenomenon of dynamical many-body freezing
in a periodically driven Ising chain. Theoretically [Phys. Rev. B 82, 172402
(2010)], for certain values of the drive parameters all fundamental degrees of
freedom contributing to the response dynamics freeze for all time and for
arbitrary initial states. Also, since the condition of freezing involves only
the drive parameters and not on the quantization of the momentum (i.e., the
system-size), our simulation with a small (3-spin) chain captures all salient
features of the freezing phenomenon predicted for the infinite chain. Using
optimal control techniques, we realize high-fidelity cosine modulated drive,
and observe non-monotonic freezing of magnetization at specific frequencies of
modulation. Time-evolution of the excitations in momentum space has been
tracked directly through magnetization measurements.",1307.8219v3
2013-09-08,Magnetohydrodynamic simulations of the elliptical instability in triaxial ellipsoids,"The elliptical instability can take place in planetary cores and stars
elliptically deformed by gravitational effects, where it generates large-scale
three-dimensional flows assumed to be dynamo capable. In this work, we present
the first magneto-hydrodynamic numerical simulations of such flows, using a
finite-element method. We first validate our numerical approach by comparing
with kinematic and dynamic dynamos benchmarks of the literature. We then
systematically study the magnetic field induced by various modes of the
elliptical instability from an imposed external field in a triaxial ellipsoidal
geometry, relevant in a geo- and astrophysical context. Finally, in tidal
induction cases, the external magnetic field is suddenly shut down and the
decay rates of the magnetic field are systematically reported.",1309.1929v1
2013-09-09,Characterization of the International Linear Collider damping ring optics,"A method is presented for characterizing the emittance dilution and dynamic
aperture for an arbitrary closed lattice that includes guide field magnet
errors, multipole errors and misalignments. This method, developed and tested
at the Cornell Electron Storage Ring Test Accelerator (CesrTA), has been
applied to the damping ring lattice for the International Linear Collider
(ILC). The effectiveness of beam based emittance tuning is limited by beam
position monitor (BPM) measurement errors, number of corrector magnets and
their placement, and correction algorithm. The specifications for damping ring
magnet alignment, multipole errors, number of BPMs, and precision in BPM
measurements are shown to be consistent with the required emittances and
dynamic aperture. The methodology is then used to determine the minimum number
of position monitors that is required to achieve the emittance targets, and how
that minimum depends on the location of the BPMs. Similarly, the maximum
tolerable multipole errors are evaluated. Finally, the robustness of each BPM
configuration with respect to random failures is explored.",1309.2248v3
2013-10-28,Maximal Rabi frequency of an electrically driven spin in a disordered magnetic field,"We present a theoretical study of the spin dynamics of a single electron
confined in a quantum dot. Spin dynamics is induced by the interplay of
electrical driving and the presence of a spatially disordered magnetic field,
the latter being transverse to a homogeneous magnetic field. We focus on the
case of strong driving, i.e., when the oscillation amplitude $A$ of the
electron's wave packet is comparable to the quantum dot length $L$. We show
that electrically driven spin resonance can be induced in this system by
subharmonic driving, i.e., if the excitation frequency is an integer fraction
(1/2, 1/3, etc) of the Larmor frequency. At strong driving we find that (i) the
Rabi frequencies at the subharmonic resonances are comparable to the Rabi
frequency at the fundamental resonance, and (ii) at each subharmonic resonance,
the Rabi frequency can be maximized by setting the drive strength to an
optimal, finite value. Our simple model is applied to describe electrical
control of a spin-valley qubit in a weakly disordered carbon nanotube.",1310.7350v2
2013-11-02,"Attraction, Merger, Reflection, and Annihilation in Magnetic Droplet Soliton Scattering","The interaction behavior of solitons are defining characteristics of these
nonlinear, coherent structures. Due to recent experimental observations, thin
ferromagnetic films offer a promising medium in which to study the scattering
properties of two-dimensional magnetic droplet solitons, particle-like,
precessing dipoles. Here, a rich set of two-droplet interaction behaviors are
classified through micromagnetic simulations. Repulsive and attractive
interaction dynamics are generically determined by the relative phase and
speeds of the two droplets and can be classified into four types: (1) merger
into a breather bound state, (2) counterpropagation trapped along the axis of
symmetry, (3) reflection, and (4) violent droplet annihilation into spin wave
radiation and a breather. Utilizing a nonlinear method of images, it is
demonstrated that these dynamics describe repulsive/attractive scattering of a
single droplet off of a magnetic boundary with pinned/free spin boundary
conditions, respectively. These results explain the mechanism by which
propagating and stationary droplets can be stabilized in a confined
ferromagnet.",1311.0349v3
2013-12-05,Hysteresis behavior of the anisotropic quantum Heisenberg model driven by periodic magnetic field,"Dynamic behavior of a quantum Heisenberg ferromagnet in the presence of a
periodically oscillating magnetic field has been analyzed by means of the
effective field theory with two spin cluster. The dynamic equation of motion
has been constructed with the help of a Glauber type stochastic process and
solved for a simple cubic lattice. After the phase diagrams given, the behavior
of the hysteresis loop area, coercive field and remanent magnetization with the
anisotropy in the exchange interaction has been investigated in detail.
Especially, by comparing of the magnitudes of the hysteresis loop area in the
high anisotropy limit (i.e. Ising model) and low anisotropy limit (i.e.
isotropic Heisenberg model), detailed description of the hysteresis loop area
with the anisotropy in the exchange interaction given. Some interesting
features have been obtained about this behavior as well as in phase diagrams
such as tricritical points.",1312.1647v1
2014-01-16,Electric field driven magnetic domain wall motion in ferromagnetic-ferroelectric heterostructures,"We investigate magnetic domain wall (MDW) dynamics induced by applied
electric fields in ferromagnetic-ferroelectric thin-film heterostructures. In
contrast to conventional driving mechanisms where MDW motion is induced
directly by magnetic fields or electric currents, MDW motion arises here as a
result of strong pinning of MDWs onto ferroelectric domain walls (FDWs) via
local strain coupling. By performing extensive micromagnetic simulations, we
find several dynamical regimes, including instabilities such as spin wave
emission and complex transformations of the MDW structure. In all cases, the
time-averaged MDW velocity equals that of the FDW, indicating the absence of
Walker breakdown.",1401.3959v1
2014-01-22,Superconducting Spintronics with Magnetic Domain Walls,"The recent experimental demonstration of spin-polarized supercurrents offer a
venue for establishment of a superconducting analogue to conventional
spintronics. Whereas domain wall motion in purely magnetic structures is a
well-studied topic, it is not clear how domain wall dynamics may influence
superconductivity and if some functional property can be harnessed from such a
scenario. Here, we demonstrate that domain wall motion in superconducting
systems offers a unique way of controlling the quantum state of the
superconductor. Considering both the diffusive and ballistic limits, we show
that moving the domain wall to different locations in a Josephson junction will
change the quantum ground state from being in a 0 state to a $\pi$ state.
Remarkably, we also show that domain wall motion can be used to turn on and off
superconductivity: the position of the domain wall determines the critical
temperature $T_c$ and thus if the system is in a resistive state or not,
causing even a quantum phase transition between the dissipationless and normal
state at $T=0$. In this way, one achieves dynamical control over the
superconducting state within a single sample by utilizing magnetic domain wall
motion.",1401.5806v1
2014-01-23,The suppression of electron correlations in the collapsed tetragonal phase of CaFe2As2 under ambient pressure demonstrated by 75As NMR-NQR measurements,"The static and the dynamic spin correlations in the low temperature collapsed
tetragonal and the high temperature tetragonal phase in CaFe2As2 have been
investigated by 75As nuclear magnetic resonance (NMR) and nuclear quadrupole
resonance (NQR) measurements.
  Through the temperature (T) dependence of the nuclear spin lattice relaxation
rates (1/T1) and the Knight shifts, although stripe-type antiferromagnetic
(AFM) spin correlations are realized in the high temperature tetragonal phase,
no trace of the AFM spin correlations can be found in the non-superconducting,
low temperature, collapsed tetragonal (cT) phase.
  Given that there is no magnetic broadening in 75As NMR spectra, together with
the T-independent behavior of magnetic susceptibility (x) and the T dependence
of 1/T1Tx, we conclude that Fe spin correlations are completely quenched
statically and dynamically in the non-superconducting cT phase in CaFe2As2.",1401.6191v1
2014-01-24,Extended Order Parameter and Conjugate Field for the Dynamic Phase Transition in a Ginzburg-Landau Mean-Field Model in an Oscillating Field,"We present numerical evidence for an extended order parameter and conjugate
field for the dynamic phase transition in a Ginzburg-Landau mean-field model
driven by an oscillating field. The order parameter, previously taken to be the
time-averaged magnetization, comprises the deviations of the Fourier components
of the magnetization from their values at the critical period. The conjugate
field, previously taken to be the time-averaged magnetic field, comprises the
even Fourier components of the field. The scaling exponents beta and delta
associated with the extended order parameter and conjugate field are shown
numerically to be consistent with their values in the equilibrium mean-field
model.",1401.6465v1
2014-01-30,Langevin spin dynamics based on ab initio calculations: numerical schemes and applications,"A method is proposed to study the finite-temperature behaviour of small
magnetic clusters based on solving the stochastic Landau-Lifshitz-Gilbert
equations, where the effective magnetic field is calculated directly during the
solution of the dynamical equations from first principles instead of relying on
an effective spin Hamiltonian. Different numerical solvers are discussed in the
case of a one-dimensional Heisenberg chain with nearest-neighbour interactions.
We performed detailed investigations for a monatomic chain of ten Co atoms on
top of Au(001) surface. We found a spiral-like ground state of the spins due to
Dzyaloshinsky-Moriya interactions, while the finite-temperature magnetic
behaviour of the system was well described by a nearest-neighbour Heisenberg
model including easy-axis anisotropy.",1401.7885v2
2014-01-31,Far from equilibrium monopole dynamics in spin ice,"Condensed matter in the low temperature limit reveals much exotic physics
associated with unusual orders and excitations, examples ranging from helium
superfluidity to magnetic monopoles in spin ice. The far-from-equilibrium
physics of such low temperature states may be even more exotic, yet to access
it in the laboratory remains a challenge. Here we demonstrate a simple and
robust technique, the 'magnetothermal avalanche quench', and its use in the
controlled creation of nonequilibrium populations of magnetic monopoles in spin
ice at millikelvin temperatures. These populations are found to exhibit
spontaneous dynamical effects that typify far-from-equilibrium systems, yet are
captured by simple models. Our method thus opens the door to the study of
far-from-equilibrium states in spin ice and other exotic magnets.",1401.8194v1
2014-02-12,Dynamics of a Charged Particle Around a Slowly Rotating Kerr Black Hole Immersed in Magnetic Field,"The dynamics of a charged particle moving around a slowly rotating Kerr black
hole in the presence of an external magnetic field is investigated. We are
interested to explore the conditions under which the charged particle can
escape from the gravitational field of the black hole after colliding with
another particle. The escape velocity of the charged particle in the innermost
stable circular orbit is calculated. The effective potential and escape
velocity of the charged particle with angular momentum in the presence of
magnetic field is analyzed. This work serves as an extension of a preceding
paper dealing with the Schwarzschild black hole [Zahrani {\it et al}, Phys.
Rev. D 87, 084043 (2013)].",1402.2731v2
2014-05-26,Magnetic helicity and the evolution of decaying magnetohydrodynamic turbulence,"Ensemble averaged high resolution direct numerical simulations of reverse
spectral transfer are presented, extending on the many single realization
numerical studies done up to now. This identifies this type of spectral
transfer as a statistical property of magnetohydrodynamic turbulence and thus
permits reliable numerical exploration of its dynamics. The magnetic energy
decay exponent from these ensemble runs has been determined to be $n_E = (0.47
\pm 0.03) + (13.9 \pm 0.8)/R_{\lambda}$ for initially helical magnetic fields.
We show for the first time that even after removing the Lorentz force term in
the momentum equation, thus decoupling it from the induction equation, reverse
spectral transfer still persists. The induction equation is now linear with an
externally imposed velocity field, thus amenable to numerous analysis
techniques. A new door has opened for analyzing reverse spectral transfer, with
various ideas discussed.",1405.6756v3
2014-06-14,Negative magnetoresistance dynamics in expanded graphite under hydrostatic pressure up to 1.8 GPa,"Basal plane resistivity of expanded graphite was studied under simultaneous
influence of hydrostatic pressure up to 1.8 GPa and magnetic field 0.8 T in the
77-300 K temperature region. Magnetic field induces negative magnetoresistance
in the sample within all temperature and pressure range studied. A change in
resistivity of the sample under maximum pressure reaches 80%. Significant
change in resistivity dependence on temperature under the pressure of 0.6 GPa
suggests for ordering transition in the sample studied. Negative
magnetoresistance in the graphite reaches about 15% at 0.6 GPa. Magnetic field
acts in the same way as pressure and potentiates the transition formation and
further magnetoresistance dynamics. The effects observed are mostly of elastic
character according to resistivity of the unloaded sample.",1406.3770v1
2014-06-30,Effect of laser intensity and dynamics of plasma on laser induced breakdown spectroscopy,"Laser-induced breakdown spectroscopy (LIBS) show enhancement in its signal,
when the laser-induced plasma is confined/decelerated under the effect of an
external steady magnetic field or in a small cavity. An enhancement in LIBS
signal has been observed ~2 times in the case of magnetic confinement.
Combination of magnetic and spatial confinement provide enhancement by an order
of magnitude. Theoretical analysis of the decelerated plasma has been found in
agreement with the experimental observations. The enhancement in LIBS signal is
found dependent on the efficiency of plasma confinement as well as on the time
duration of laser. The saturation in LIBS signal at higher laser intensity is
found correlated with electron-ion collision frequency as well as on the
dynamics and instability of plasma plume. Possibility of further enhancement in
emission has also been discussed.",1406.7703v1
2014-07-29,Motion of domain walls and the dynamics of kinks in the magnetic Peierls potential,"We study the dynamics of magnetic domain walls in the Peierls potential due
to the discreteness of the crystal lattice. The propagation of a narrow domain
wall (comparable to the lattice parameter) under the effect of a magnetic field
proceeds through the formation of kinks in its profile. We predict that,
despite the discreteness of the system, such kinks can behave like sine-Gordon
solitons in thin films of materials such as yttrium iron garnets, and we derive
general conditions for other materials. In our simulations we also observe
long-lived breathers. We provide analytical expressions for the effective mass
and limiting velocity of the kink in excellent agreement with our numerical
results.",1407.7754v1
2014-10-27,Dynamics of a dipolar Bose-Einstein condensate in the vicinity of a superconductor,"We study the dynamics of a dipolar Bose-Einstein condensate, like for example
a $^{52}$Cr or $^{164}$Dy condensate, interacting with a superconducting
surface. The magnetic dipole moments of the atoms in the Bose-Einstein
condensate induce eddy currents in the superconductor. The magnetic field
generated by eddy currents modifies the trapping potential such that the
center-of-mass oscillation frequency is shifted. We numerically solve the
Gross-Pitaevskii equation for this system and compare the results with
analytical approximations. We present an approximation that gives excellent
agreement with the numerical results. The eddy currents give rise to anharmonic
terms, which leads to the excitation of shape fluctuations of the condensate.
We discuss how the strength of the excitation of such modes can be increased by
exploiting resonances, and we examine the strength of the resonances as a
function of the center-of-mass oscillation amplitude of the condensate.
Finally, we study different orientations of the magnetic dipoles and discuss
favorable conditions for the experimental observation of the eddy current
effect.",1410.7153v1
2015-01-22,Relativistic dynamical spin excitations of magnetic adatoms,"We present a first-principles theory of dynamical spin excitations in the
presence of spin-orbit coupling. The broken global spin rotational invariance
leads to a new sum rule. We explore the competition between the magnetic
anisotropy energy and the external magnetic field, as well as the role of
electron-hole excitations, through calculations for 3$d$-metal adatoms on the
Cu(111) surface. The spin excitation resonance energy and lifetime display
non-trivial behavior, establishing the strong impact of relativistic effects.
We legitimate the use of the Landau-Lifshitz-Gilbert equation down to the
atomic limit, but with parameters that differ from a stationary theory.",1501.05509v1
2015-01-26,Flows and Waves in Braided Solar Coronal Magnetic Structures,"We study the high frequency dynamics in the braided magnetic structure of an
active region (AR 11520) moss as observed by High-Resolution Coronal Imager
(Hi-C). We detect quasi periodic flows and waves in these structures. We search
for high frequency dynamics while looking at power maps of the observed region.
We find that shorter periodicites (30 - 60 s) are associated with small spatial
scales which can be resolved by Hi-C only. We detect quasi periodic flows with
wide range of velocities from 13 - 185 km/s associated with braided regions.
This can be interpreted as plasma outflows from reconnection sites. We also
find presence of short period and large amplitude transverse oscillations
associated with braided magnetic region. Such oscillations could be triggered
by reconnection or such oscillation may trigger reconnection.",1501.06507v1
2015-02-26,Dynamics of magnetic shells and information loss problem,"We investigate dynamics of magnetic thin-shells in three dimensional anti de
Sitter background. Because of the magnetic field, an oscillatory solution is
possible. This oscillating shell can tunnel to a collapsing shell or a bouncing
shell, where both of tunnelings induce an event horizon and a singularity. In
the entire path integral, via the oscillating solution, there is a non-zero
probability to maintain a trivial causal structure without a singularity.
Therefore, due to the path integral, the entire wave function can conserve
information. Since an oscillating shell can tunnel after a number of
oscillations, in the end, it will allow an infinite number of different
branchings to classical histories. This system can be a good model of the
effective loss of information, where information is conserved by a solution
that is originated from gauge fields.",1502.07471v2
2015-03-05,Feshbach molecule formation through an oscillating magnetic field: subharmonic resonances,"The conversion of ultracold atoms to molecules via a magnetic Feshbach
resonance with a sinusoidal modulation of the field is studied. Different
practical realizations of this method in Bose atomic gases are analyzed. Our
model incorporates many-body effects through an effective reduction of the
complete microscopic dynamics. Moreover, we simulate the experimental
conditions corresponding to the preparation of the system as a thermal gas and
as a condensate. Some of the experimental findings are clarified. The origin of
the observed dependence of the production efficiency on the frequency,
amplitude, and application time of the magnetic modulation is elucidated. Our
results uncover also the role of the atomic density in the dynamics,
specifically, in the observed saturation of the atom-molecule conversion
process.",1503.01700v1
2015-03-06,Spin-Motive Forces and Current-Induced Torques in Ferromagnets,"In metallic ferromagnets, the spin-transfer torque and spin-motive force are
known to exhibit a reciprocal relationship. Recent experiments on ferromagnets
with strong spin-orbit coupling have revealed a rich complexity in the
interaction between itinerant charge carriers and magnetization, but a full
understanding of this coupled dynamics is lacking. Here, we develop a general
phenomenology of the two reciprocal processes of charge pumping by spin-motive
forces and current-driven magnetization dynamics. The formalism is valid for
spin-orbit coupling of any strength and presents a systematic scheme for
deriving all possible torque and charge-pumping terms that obey the symmetry
requirements imposed by the point group of the system. We demonstrate how the
different charge pumping and torque contributions are connected via the Onsager
reciprocal relations. The formalism is applied to two important classes of
systems: isotropic ferromagnets with non-uniform magnetization and homogeneous
ferromagnets described by the point group $C_{2v}$.",1503.01899v2
2015-03-19,Half-Quantum Vortices in an Antiferromagnetic Spinor Bose-Einstein Condensate,"We report on the observation of half-quantum vortices (HQVs) in the
easy-plane polar phase of an antiferromagnetic spinor Bose-Einstein condensate.
Using in situ magnetization-sensitive imaging, we observe that pairs of HQVs
with opposite core magnetization are generated when singly charged quantum
vortices are injected into the condensate. The dynamics of HQV pair formation
is characterized by measuring the temporal evolutions of the pair separation
distance and the core magnetization, which reveals the short-range nature of
the repulsive interactions between the HQVs. We find that spin fluctuations
arising from thermal population of axial magnon excitations do not
significantly affect the HQV pair formation dynamics. Our results demonstrate
the instability of a singly charged vortex in the antiferromagnetic spinor
condensate.",1503.05648v3
2015-04-01,Multiscale modeling of ultrafast element-specific magnetization dynamics of ferromagnetic alloys,"A hierarchical multiscale approach to model the magnetization dynamics of
ferromagnetic ran- dom alloys is presented. First-principles calculations of
the Heisenberg exchange integrals are linked to atomistic spin models based
upon the stochastic Landau-Lifshitz-Gilbert (LLG) equation to calculate
temperature-dependent parameters (e.g., effective exchange interactions,
damping param- eters). These parameters are subsequently used in the
Landau-Lifshitz-Bloch (LLB) model for multi-sublattice magnets to calculate
numerically and analytically the ultrafast demagnetization times. The developed
multiscale method is applied here to FeNi (permalloy) as well as to copper-
doped FeNi alloys. We find that after an ultrafast heat pulse the Ni sublattice
demagnetizes faster than the Fe sublattice for the here-studied FeNi-based
alloys.",1504.00199v1
2015-04-27,Transition to magnetorotational turbulence in Taylor--Couette flow with imposed azimuthal magnetic field,"The magnetorotational instability (MRI) is thought to be a powerful source of
turbulence and momentum transport in astrophysical accretion discs, but
obtaining observational evidence of its operation is challenging. Recently,
laboratory experiments of Taylor--Couette flow with externally imposed axial
and azimuthal magnetic fields have revealed the kinematic and dynamic
properties of the MRI close to the instability onset. While good agreement was
found with linear stability analyses, little is known about the transition to
turbulence and transport properties of the MRI. We here report on a numerical
investigation of the MRI with an imposed azimuthal magnetic field. We show that
the laminar Taylor--Couette flow becomes unstable to a wave rotating in the
azimuthal direction and standing in the axial direction via a supercritical
Hopf bifurcation. Subsequently, the flow features a catastrophic transition to
spatio-temporal defects which is mediated by a subcritical subharmonic Hopf
bifurcation. Our results are in qualitative agreement with the PROMISE
experiment and dramatically extend their realizable parameter range. We find
that as the Reynolds number increases defects accumulate and grow into
turbulence, yet the momentum transport scales weakly.",1504.07208v2
2015-05-20,Effect of Transverse Magnetic Field on Dynamics of Current Driven Domain Wall Motion in the Presence of Spin-Hall Effect,"Theoretically, we study the dynamics of a current induced domain wall in the
bi-layer structure consists of a ferromagnetic layer and a non-magnetic metal
layer with strong spin-orbit coupling in the presence of spin-Hall effect. The
analytical expressions for the velocity and width of the domain wall interms of
excitation angle are obtained by solving the Landau-Lifshitz-Gilbert equation
with adiabatic, nonadiabatic and spin Hall effect-spin transfer torques using
Schryers and Walker's method. Numerical results show that the occurance of
polarity switching in the domain wall is observed only above the threshold
current density. The presence of transverse magnetic field along with spin Hall
effect-spin transfer torque enchances the value of the threshold current
density, and the corresponding saturated velocity at the threshold current
density is also increased.",1505.05249v1
2015-05-22,Effective Hamiltonian and dynamics of edge states in two-dimensional topological insulators under magnetic fields,"The magnetic field opens a gap in the edge state spectrum of two-dimensional
topological insulators thereby destroying protection of these states against
backscattering. To relate properties of this gap to parameters of the system
and to study dynamics of electrons in edge states in the presence of
inhomogeneous potentials, the effective Hamiltonian theory is developed. Using
this analytical theory, quantum-mechanical problems of edge-state electron
transmission through potential steps and barriers, and of motion in constant
electric field are considered. The influence of magnetic field on the
resistance of two-dimensional topological insulators based on HgTe quantum
wells is discussed together with comparison to experimental data.",1505.06144v1
2015-06-18,Dynamical structure factor of magnetic Bloch oscillations at finite temperatures,"Domain-walls in one-dimensional Ising ferromagnets can undergo Bloch
oscillations when subjected to a skew magnetic field. Such oscillations imply
finite temperature non-dispersive low-frequency peaks in the dynamical
structure factor which can be probed in neutron scattering. We study in detail
the spectral weight of these peaks. Using an analytical approach based on an
approximate treatment of a gas of spin-cluster excitations we give an explicit
expression for the momentum- and temperature-dependence of the spectral
weights. Generally the spectral weights increase with temperature and
approaches the same order of magnitude as the spin-wave spectral weights at
high temperatures. We compare the analytical expression to numerical exact
diagonalizations and find that it can, without any adjustable parameters,
account for the temperature and momentum-transfer dependence of the numerically
obtained spectral weights in the parameter regime where the ratio of magnetic
fields $h_x/h_z \ll 1$ and the temperature is $h_x < T <\sim J_z/2$. We also
carry out numerical calculations pertinent to the material CoNb$_2$O$_6$, and
find qualitatively similar results.",1506.05614v1
2015-07-08,Phase-sensitive imaging of ferromagnetic resonance using ultrafast heat pulses,"Measuring local magnetization dynamics and its spatial variation is essential
for advancements in spintronics and relevant applications. Here we demonstrate
a phase-sensitive imaging technique for studying patterned magnetic structures
based on picosecond laser heating. With the time-resolved anomalous Nernst
effect (TRANE) and extensions, we simultaneously image the dynamic
magnetization and RF driving current density. The stroboscopic detection
implemented in TRANE microscopy provides access to both amplitude and phase
information of ferromagnetic resonance (FMR) and RF current. Using this
approach, we measure the spatial variation of the Oersted driving field angle
across a uniform channel. In a spatially nonuniform sample with a cross shape,
a strong spatial variation for the RF current as well as FMR precession is
observed. We find that both the amplitude and the phase of local FMR precession
are closely related to those of the RF current.",1507.02303v1
2015-08-06,Large spin-wave bullet in a ferrimagnetic insulator driven by spin Hall effect,"Due to its transverse nature, spin Hall effects (SHE) provide the possibility
to excite and detect spin currents and magnetization dynamics even in magnetic
insulators. Magnetic insulators are outstanding materials for the investigation
of nonlinear phenomena and for novel low power spintronics applications because
of their extremely low Gilbert damping. Here, we report on the direct imaging
of electrically driven spin-torque ferromagnetic resonance (ST-FMR) in the
ferrimagnetic insulator Y$_3$Fe$_5$O$_{12}$ based on the excitation and
detection by SHEs. The driven spin dynamics in Y$_3$Fe$_5$O$_{12}$ is directly
imaged by spatially-resolved microfocused Brillouin light scattering (BLS)
spectroscopy. Previously, ST-FMR experiments assumed a uniform precession
across the sample, which is not valid in our measurements. A strong spin-wave
localization in the center of the sample is observed indicating the formation
of a nonlinear, self-localized spin-wave `bullet'.",1508.01427v1
2015-08-13,On the dynamics of the Meissner effect,"The question of how a metal becoming superconducting expels a magnetic field
is addressed. It is argued that the conventional theory of superconductivity
has not answered this question despite its obvious importance. We argue that
the growth of the superconducting into the normal region and associated
expulsion of magnetic field from the superconducting region can only be
understood if it is accompanied by motion of charge from the superconducting
into the normal region. From a microscopic point of view it is shown that the
perfect diamagnetism of superconductors requires that superconducting electrons
reside in orbits of spatial extent $2\lambda_L$, with $\lambda_L$ the London
penetration depth. Associated with this physics, the spin-orbit interaction of
the electron magnetic moment and the positively charged ionic background gives
rise to a ""Spin Meissner"" effect, the generation of a macroscopic spin current
near the surface of superconductors. We point out that both the Meissner and
the Spin Meissner effect can be understood dynamically under the assumption
that the superfluid condensate wavefunction $\Psi(\vec{r})$ does not screen
itself, just like the $\Psi(\vec{r})$ for an electron in a hydrogen atom. We
argue that the conventional theory of superconductivity cannot explain the
Meissner effect because it does not contain the physical elements discussed
here.",1508.03307v1
2015-09-06,Study of spin dynamics and damping on the magnetic nanowire arrays with various nanowire widths,"We investigate the spin dynamics including Gilbert damping in the
ferromagnetic nanowire arrays. We have measured the ferromagnetic resonance of
ferromagnetic nanowire arrays using vector-network analyzer ferromagnetic
resonance (VNA-FMR) and analyzed the results with the micromagnetic
simulations. We find excellent agreement between the experimental VNA-FMR
spectra and micromagnetic simulations result for various applied magnetic
fields. We find that the demagnetization factor for longitudinal conditions, Nz
(Ny) increases (decreases) as decreasing the nanowire width in the
micromagnetic simulations. For the transverse magnetic field, Nz (Ny) increases
(decreases) as increasing the nanowire width. We also find that the Gilbert
damping constant increases from 0.018 to 0.051 as the increasing nanowire width
for the transverse case, while it is almost constant as 0.021 for the
longitudinal case.",1509.01807v1
2015-09-14,Arnold diffusion of charged particles in ABC magnetic fields,"We prove the existence of diffusing solutions in the motion of a charged
particle in the presence of an ABC magnetic field. The equations of motion are
modeled by a 3DOF Hamiltonian system depending on two parameters. For small
values of these parameters, we obtain a normally hyperbolic invariant manifold
and we apply the so-called geometric methods for a priori unstable systems
developed by A. Delshams, R. de la Llave, and T.M. Seara. We characterize
explicitly sufficient conditions for the existence of a transition chain of
invariant tori having heteroclinic connections, thus obtaining global
instability (Arnold diffusion). We also check the obtained conditions in a
computer assisted proof. ABC magnetic fields are the simplest force-free type
solutions of the magnetohydrodynamics equations with periodic boundary
conditions, so our results are of potential interest in the study of the motion
of plasma charged particles in a tokamak.",1509.04141v1
2015-10-19,Oscillatory instability and routes to chaos in Rayleigh-Bénard convection: effect of external magnetic field,"We investigate oscillatory instability and routes to chaos in
Rayleigh-B\'enard convection of electrically conducting fluids in presence of
external horizontal magnetic field. Three dimensional direct numerical
simulations (DNS) of the governing equations are performed for the
investigation. DNS shows that oscillatory instability is inhibited by the
magnetic field. The supercritical Rayleigh number for the onset of oscillation
is found to scale with the Chandrasekhar number $\mathrm{Q}$ as
$\mathrm{Q}^{\alpha}$ in DNS with $\alpha = 1.8$ for low Prandtl numbers
($\mathrm{Pr}$). Most interestingly, DNS shows $\mathrm{Q}$ dependent routes to
chaos for low Prandtl number fluids like mercury ($\mathrm{Pr} = 0.025$). For
low $\mathrm{Q}$, period doubling routes are observed, while, quasiperiodic
routes are observed for high $\mathrm{Q}$. The bifurcation structure associated
with $\mathrm{Q}$ dependent routes to chaos is then understood by constructing
a low dimensional model from the DNS data. The model also shows similar scaling
laws as DNS. Bifurcation analysis of the low dimensional model shows that
origin of different routes are associated with the bifurcation structure near
the primary instability. These observations show similarity with the previous
results of convection experiments performed with mercury.",1510.05339v1
2015-11-12,"Dynamics of a linear magnetic ""microswimmer molecule""","In analogy to nanoscopic molecules that are composed of individual atoms, we
consider an active ""microswimmer molecule"". It is built up from three
individual magnetic colloidal microswimmers that are connected by harmonic
springs and hydrodynamically interact with each other. In the ground state,
they form a linear straight molecule. We analyze the relaxation dynamics for
perturbations of this straight configuration. As a central result, with
increasing self-propulsion, we observe an oscillatory instability in accord
with a subcritical Hopf bifurcation scenario. It is accompanied by a
corkscrew-like swimming trajectory of increasing radius. Our results can be
tested experimentally, using for instance magnetic self-propelled Janus
particles, supposably linked by DNA molecules.",1511.03918v1
2015-12-01,Nonequilibrium low temperature phase in pyrochlore iridate Y$_2$Ir$_2$O$_7$: Possibility of glass-like dynamics,"Geometrical frustration and spin-orbit coupling effect together play vital
role to influence properties in pyrochlore based iridium oxides. Here we have
investigated detailed structural, magnetic, thermodynamic and transport
properties of pyrochlore iridate Y$_2$Ir$_2$O$_7$. Magnetization data show
onset of magnetic irreversibility around temperature $T_{irr}$ $\sim$ 160 K,
however, no sign of long-range type ferromagnetic ordering is observed below
$T_{irr}$. Specific heat data show no visible anomaly across $T_{irr}$, and the
analysis of data indicate sizable density of states across Fermi level.
Temperature dependent x-ray diffraction measurements show no change in
structural symmetry down to low temperature. The material, on the other hand,
shows significant relaxation and aging behavior similar to glassy dynamics. The
electronic charge transport in this highly insulating system is found to follow
power law dependence with temperature. The material shows negative
magnetoresistance which is explained with quantum interference effect.",1512.00233v1
2016-02-18,Signatures of Topological Phase Transition in 3d Topological Insulators from Dynamical Axion Response,"Axion electrodynamics, first proposed in the context of particle physics,
manifests itself in condensed matter physics in the topological field theory
description of 3d topological insulators and gives rise to magnetoelectric
effect, where applying magnetic (electric) field $\mathbf{B}(\mathbf{E})$
induces polarization (magnetization) $\mathbf{p}(\mathbf{m})$. We use linear
response theory to study the associated topological current using the
Fu-Kane-Mele model of 3d topological insulators in the presence of
time-dependent uniform weak magnetic field. By computing the dynamical current
susceptibility $\chi^{\mathbf{j}_p\mathbf{j}_p}_{ij}(\omega)$, we discover from
its static limit an `order parameter' of the topological phase transition
between weak topological (or ordinary) insulator and strong topological
insulator, found to be continuous. The
$\chi^{\mathbf{j}_p\mathbf{j}_p}_{ij}(\omega)$ shows a sign-changing
singularity at a critical frequency with suppressed strength in the topological
insulating state. Our results can be verified in current noise experiment on 3d
TI candidate materials for the detection of such topological phase transition.",1602.05878v2
2016-03-11,Time-dependent spin and transport properties of a single-molecule magnet in a tunnel junction,"In single-molecule magnets, the exchange between a localized spin moment and
the electronic background provides a suitable laboratory for studies of
dynamical aspects of both local spin and transport properties. Here we address
the time evolution of a localized spin moment coupled to an electronic level in
a molecular quantum dot embedded in a tunnel junction between metallic leads.
The interactions between the localized spin moment and the electronic level
generate an effective interaction between the spin moment at different
instances in time. Therefore, we show that, despite being a single-spin system,
there are effective contributions of isotropic Heisenberg, and anisotropic
Ising and Dzyaloshinski-Moriya character acting on the spin moment. The
interactions can be controlled by gate voltage, voltage bias, the spin
polarization in the leads, in addition to external magnetic fields. Signatures
of the spin dynamics are found in the transport properties of the tunneling
system, and we demonstrate that measurements of the spin current may be used
for read-out of the local spin moment orientation.",1603.03628v2
2016-04-04,Hindered magnetic dipole transitions between P-wave bottomonia and coupled-channel effects,"In the hindered magnetic dipole transitions of heavy quarkonia, the
coupled-channel effects originating from the coupling of quarkonia to a pair of
heavy and anti-heavy mesons can play a dominant role. Here, we study the
hindered magnetic dipole transitions between two $P$-wave bottomonia, $\chi_b(n
P)$ and $h_b(n^\prime P)$, with $n\neq n^\prime$. In these processes the
coupled-channel effects are expected to lead to partial widths much larger than
the quark model predictions. We estimate these partial widths which, however,
are very sensitive to unknown coupling constants related to the vertices
$\chi_{b0}(nP)B\bar B$. A measurement of the hindered M1 transitions can shed
light on the coupled-channel dynamics in these transitions and hence on the
size of the coupling constants. We also suggest to check the coupled-channel
effects by comparing results from quenched and fully dynamical lattice QCD
calculations.",1604.00770v2
2016-04-07,Tunable magneto-granular phononic crystals,"This paper reports on the study of the dynamics of 1D magneto-granular
phononic crystals composed of a chain of spherical steel beads inside a
properly designed magnetic field. This field is induced by an array of
permanent magnets, located in a holder at a given distance from the chain. The
theoretical and experimental results of the band gap structure are displayed,
including all six degrees of freedom for the beads, i.e. three translations and
three rotations. Experimental evidence of transverse-rotational modes of
propagation is pre- sented; moreover, by changing the strength of the magnetic
field, the dynamic response of the granular chain is tuned. The combination of
non-contact tunability with the potentially strong nonlinear behavior of gran-
ular systems ensures the suitability of magneto-granular phononic crystals as
nonlinear, tunable mechanical metamaterials for use in controlling elastic wave
propagation.",1604.01973v1
2016-06-13,Tridimensional to bidimensional transition in magnetohydrodynamic turbulence with a guide field and kinetic helicity injection,"We study the transition in dimensionality of a three-dimensional
magnetohydrodynamic flow forced only mechanically, when the strength of a
magnetic guiding field is gradually increased. We use numerical simulations to
consider cases in which the mechanical forcing injects (or not) helicity in the
flow. As the guiding field is increased, the strength of the magnetic field
fluctuations decrease as a power law of the guiding field intensity. We show
that for strong enough guiding fields, the helical magnetohydrodynamic flow can
become almost two-dimensional. In this case, the mechanical energy can undergo
a process compatible with an inverse cascade, being transferred preferentially
towards scales larger than the forcing scale. The presence of helicity changes
the spectral scaling of the small magnetic field fluctuations, and affects the
statistics of the velocity field and of the velocity gradients. Moreover, at
small scales the dynamics of the flow becomes dominated by a direct cascade of
helicity, which can be used to derive scaling laws for the velocity field.",1606.04026v3
2016-06-27,Supercurrent-Induced Spin-Orbit Torques,"We theoretically investigate the supercurrent-induced magnetization dynamics
of a two-dimensional lattice of ferromagnetically ordered spins placed on a
conventional superconductor with broken spatial inversion symmetry and strong
spin-orbit coupling. We develop a phenomenological description of the coupled
dynamics of the superconducting condensate and the spin system, and demonstrate
that supercurrents produce a reactive spin-orbit torque on the magnetization.
By performing a microscopic self-consistent calculation, we show that the
spin-orbit torque originates from a spin-polarization of the Cooper pairs due
to current-induced spin-triplet correlations. Interestingly, we find that there
exists an intrinsic limitation for the maximum achievable spin-orbit torque,
which is determined by the coupling strength between the condensate and the
spin system. In proximitized hole-doped semiconductors, the maximum achievable
spin-orbit torque field is estimated to be on the order of $0.16$ mT, which is
comparable to the critical field for current-induced magnetization switching in
ferromagnetic semiconductors.",1606.08470v1
2016-06-29,Interaction-Induced Topological and Magnetic Phases in the Hofstadter-Hubbard Model,"Interaction effects have been a subject of contemporary interest in
topological phases of matter. But in the presence of interactions, the accurate
determination of topological invariants in their general form is difficult due
to their dependence on multiple integrals containing Green's functions and
their derivatives. Here we employ the recently proposed ""effective topological
Hamiltonian"" approach to explore interaction-induced topological phases in the
time-reversal-invariant Hofstadter-Hubbard model. Within this approach, the
zero-frequency part of the self-energy is sufficient to determine the correct
topological invariant. We combine the topological Hamiltonian approach with the
local self-energy approximation, both for the static and the full dynamical
self-energy evaluated using dynamical mean field theory (DMFT), and present the
resulting phase diagram in the presence of many-body interactions. We
investigate the emergence of quantum spin Hall (QSH) states for different
interaction strengths by calculating the $\mathbb{Z}_2$ invariant. The
interplay of strong correlations and a staggered potential also induces
magnetic long-range order with an associated first order transition. We present
results for the staggered magnetization ( $m_s$ ), staggered occupancy ( $n_s$
), and double occupancy across the transition.",1606.09161v2
2016-07-15,Electrified magnetic catalysis in three-dimensional topological insulators,"The gap equations for the surface quasiparticle propagators in a slab of
three-dimensional topological insulator in external electric and magnetic
fields perpendicular to the slab surfaces are analyzed and solved. A different
type of magnetic catalysis is revealed with the dynamical generation of both
Haldane and Dirac gaps. Its characteristic feature manifests itself in the
crucial role that the electric field plays in dynamical symmetry breaking and
the generation of a Dirac gap in the slab. It is argued that, for a
sufficiently large external electric field, the ground state of the system is a
phase with a homogeneous surface charge density.",1607.04649v2
2016-09-29,Energy dynamics in a generalized compass chain,"We investigate the energy dynamics in a generalized compass chain under an
external magnetic field. We show that the energy current operators act on three
contiguous sites in the absence of the magnetic field, and they are
incorporated with inhomogenous Dzyaloshinskii-Moriya interactions in the
presence of the magnetic field. These complex interactions remain the
Hamiltonian be an exactly solvable spin model. We study the effects of the
three-site interactions and the Dzyaloshinskii-Moriya interactions on the
energy spectra and phase diagram. The results have revealed that the energy
current of the pristine quantum compass model is conserved due to the
associated intermediate symmetries, and for other general cases such
characteristic does not exist.",1609.09203v1
2016-10-04,Effect of temperature on spin-transfer torque induced magnetic solitons,"Spin-transfer torques in a nanocontact to an extended magnetic film can
create spin waves that condense to form dissipative droplet solitons. Here we
report an experimental study of the temperature dependence of the current and
applied field thresholds for droplet soliton formation, as well as the
nanocontact's electrical characteristics associated with droplet dynamics.
Nucleation of droplet solitons requires higher current densities at higher
temperatures, in contrast to typical spin-transfer torque induced switching
between static magnetic states. Magnetoresistance and electrical noise
measurements show that soliton instabilities become more pronounced with
increasing temperature. These results are of fundamental interest in
understanding the influence of thermal noise on droplet solitons, and in
controlling their dynamics.",1610.00931v1
2016-10-11,Self-Consistent Field Theory studies of the thermodynamics and quantum spin dynamics of magnetic Skyrmions,"A self-consistent field theory is introduced and used to investigate the
thermodynamics and spin dynamics of an $S = 1$ quantum spin system with a
magnetic Skyrmion. The temperature dependence of the Skyrmion profile as well
as the phase diagram are calculated. It is shown that the Skyrmion carries a
phase transition to the ferromagnetic phase of first order with increasing
temperature, while the magnetization of the surrounding ferromagnet undergoes a
phase transition of second order when changing to the paramagnetic phase.
Furthermore, the electric field driven annihilation process of the Skyrmion is
described quantum mechanical by solving the time dependent Schr\""odinger
equation. The results are compared with the trajectories of the semi-classical
description of the spin expectation values using a differential equation
similar to the classical Landau-Lifshitz-Gilbert equation.",1610.03191v2
2016-10-12,Signatures of Chaos in Thermal Switching of Nanomagnets,"Thermally-activated magnetization dynamics of small nanoparticles subject to
microwave (AC) external fields is studied. It is shown that, under sufficiently
strong microwave excitations, chaotic magnetization dynamics may occur close to
saddle-type heteroclinic connections, and this heteroclinic chaos is
responsible for the erosion of the safe basin around stable magnetization
states. The erosion phenomenon is then connected to the escape problem from the
energy well surrounding a stable equilibrium. It is shown that escape times
follow a generalized Arrhenius' law governed by temperature, microwave field
amplitude, frequency and heteroclinic chaos threshold.",1610.03755v3
2016-10-21,Spin transport and dynamics in all-oxide perovskite La$_{2/3}$Sr$_{1/3}$MnO$_3$/SrRuO$_3$ bilayers probed by ferromagnetic resonance,"Thin films of perovskite oxides offer the possibility of combining emerging
concepts of strongly correlated electron phenomena and spin current in magnetic
devices. However, spin transport and magnetization dynamics in these complex
oxide materials are not well understood. Here, we experimentally quantify spin
transport parameters and magnetization damping in epitaxial perovskite
ferromagnet/paramagnet bilayers of La$_{2/3}$Sr$_{1/3}$MnO$_3$/SrRuO$_3$
(LSMO/SRO) by broadband ferromagnetic resonance spectroscopy. From the SRO
thickness dependence of Gilbert damping, we estimate a short spin diffusion
length of $\lesssim$1 nm in SRO and an interfacial spin-mixing conductance
comparable to other ferromagnet/paramagnetic-metal bilayers. Moreover, we find
that anisotropic non-Gilbert damping due to two-magnon scattering also
increases with the addition of SRO. Our results demonstrate LSMO/SRO as a
spin-source/spin-sink system that may be a foundation for examining
spin-current transport in various perovskite heterostructures.",1610.06661v1
2016-12-20,Eigenmodes of Neel skyrmions in ultrathin magnetic films,"The static and dynamic states of N\'eel skyrmions in ultrathin ferromagnetic
films with interfacial Dzyaloshinskii-Moriya interaction (DMI) have been
micromagnetically simulated as functions of the interfacial DMI strength and
applied static magnetic field. Findings reveal that while the breathing,
counterclockwise (CCW) and clockwise (CW) rotational eigenmodes exist in the
skyrmion lattice (SkL) phase, only the first two modes are present in the
isolated skyrmion (ISk) phase. Additionally, the eigenfrequency of the CCW mode
is insensitive to the magnetic-field driven SkL-ISk phase transition, and the
inter-skyrmion interaction is largely responsible for exciting the CW mode in
the SkL phase. The findings provide physical insight into the dynamics of the
phase transition and would be of use to potential skyrmion-based microwave
applications.",1612.06622v3
2017-03-06,Topological Properties of the Chiral Magnetic Effect in Multi-Weyl Semimetals,"We compute the chiral magnetic effect (CME) in multi-Weyl semimetals
(multi-WSMs) based on the chiral kinetic theory. Multi-WSMs are WSMs with
multiple monopole charges that have nonlinear and anisotropic dispersion
relations near Weyl points, and we need to extend conventional computation of
CME in WSMs with linear dispersion relations. Topological properties of CME in
multi-WSMs are investigated in details for not only static magnetic fields but
also time-dependent (dynamic) ones. We propose an experimental setup to measure
the multiple monopole charge via the topological nature hidden in the dynamic
CME.",1703.02040v2
2017-03-08,Vortex dynamics in lattice Bose gases in a synthesized magnetic field with a random noise and a dissipation: Study by the stochastic Gross-Pitaevskii equation,"In this paper, we investigate vortex dynamics in a two-dimensional
Bose-Hubbard model coupled with a weak artificial magnetic field, a random
white noise and a dissipation. Origin of the noise and dissipation is
considered as thermal fluctuations of atoms that do not participate the
Bose-Einstein condensation (BEC). Solving a stochastic Gross-Pitaevskii
equation to this system, we show that the interplay of the magnetic field and
the white noise generates vortices in the bulk of the BEC and stable steady
states of vortices form after a transition period. We calculate the
incompressible part of the kinetic-energy spectrum of the BEC. In the
transition period, a Kolmogorov $k^{-5/3}$ spectrum appears in the infrared
regime with the wave number $k$, $k<\zeta^{-1}$, where $\zeta$ is the healing
length, whereas in the ultraviolet region, $k>\zeta^{-1}$, the spectrum behaves
as $k^{-3}$. On the other hand in the steady states, another scaling low
appears. We find a relationship between the above mentioned kinetic-energy
spectra and the velocity of vortices. By an inverse cascade, the large velocity
of a few created vortices develops the Kolmogorov $k^{-5/3}$ spectrum.",1703.02750v1
2017-03-16,Rectification and One-Way Street for the Energy Current in Boundary-Driven Asymmetric Quantum Spin Chains,"Motivated by the demand of efficient quantum devices to engineer the energy
transport, we analyze some inhomogeneous quantum spin systems, including the
XXZ chains, with magnetization baths at the ends. Aimed at finding general
properties, we study the effects of suitable transformations on the
boundary-driven Lindblad master equation associated to the dynamics of the
systems. For asymmetric models with target polarization at the edges or twisted
XY boundary gradients, we show properties of the steady state which establish
features of the energy current, irrespective of the system size and of the
regime of transport. We show the ubiquitous occurrence of energy rectification
and, more interestingly, of an unusual phenomenon: in the absence of external
magnetic field, there is an one-way street for the energy current, i.e., the
direction of the energy current does not change as we invert the magnetization
baths at the boundaries. Given the extensiveness of the procedures, which
essentially involve properties of the Lindblad master equation, our results
certainly follow for other interactions and other boundary conditions.
Moreover, our results indicate graded spin chains as genuine quantum
rectifiers.",1703.05567v1
2017-03-26,Relativistic magnetohydrodynamics,"We present the equations of relativistic hydrodynamics coupled to dynamical
electromagnetic fields, including the effects of polarization, electric fields,
and the derivative expansion. We enumerate the transport coefficients at
leading order in derivatives, including electrical conductivities, viscosities,
and thermodynamic coefficients. We find the constraints on transport
coefficients due to the positivity of entropy production, and derive the
corresponding Kubo formulas. For the neutral state in a magnetic field, small
fluctuations include Alfven waves, magnetosonic waves, and the dissipative
modes. For the state with a non-zero dynamical charge density in a magnetic
field, plasma oscillations gap out all propagating modes, except for
Alfven-like waves with a quadratic dispersion relation. We relate the transport
coefficients in the ""conventional"" magnetohydrodynamics (formulated using
Maxwell's equations in matter) to those in the ""dual"" version of
magnetohydrodynamics (formulated using the conserved magnetic flux).",1703.08757v1
2017-03-27,Anomalous Hall effect and topological defects in antiferromagnetic Weyl semimetals: Mn$_3$Sn/Ge,"We theoretically study the interplay between bulk Weyl electrons and magnetic
topological defects, including magnetic domains, domain walls, and
$\mathbb{Z}_6$ vortex lines, in the antiferromagnetic Weyl semimetals \mnsn\
and Mn$_3$Ge with negative vector chirality. We argue that these materials
possess a hierarchy of energies scales which allows a description of the spin
structure and spin dynamics using a XY model with $\mathbb{Z}_6$ anisotropy. We
propose a dynamical equation of motion for the XY order parameter, which
implies the presence of $\mathbb{Z}_6$ vortex lines, the double-domain pattern
in the presence of magnetic fields, and the ability to control domains with
current. We also introduce a minimal electronic model which allows efficient
calculation of the electronic structure in the antiferromagnetic configuration,
unveiling Fermi arcs at domain walls, and sharp quasi-bound states at
$\mathbb{Z}_6$ vortices. Moreover, we have shown how these materials may allow
electronic-based imaging of antiferromagnetic microstructure, and propose a
possible device based on domain-dependent anomalous Hall effect.",1703.08910v1
2017-03-30,Multidimensional relativistic MHD simulations of Pulsar Wind Nebulae: dynamics and emission,"Pulsar Wind Nebulae, and the Crab nebula in particular, are the best cosmic
laboratories to investigate the dynamics of magnetized relativistic outflows
and particle acceleration up to PeV energies. Multidimensional MHD modeling by
means of numerical simulations has been very successful at reproducing, to the
very finest details, the innermost structure of these synchrotron emitting
nebulae, as observed in the X-rays. Therefore, the comparison between the
simulated source and observations can be used as a powerful diagnostic tool to
probe the physical conditions in pulsar winds, like their composition,
magnetization, and degree of anisotropy. However, in spite of the wealth of
observations and of the accuracy of current MHD models, the precise mechanisms
for magnetic field dissipation and for the acceleration of the non-thermal
emitting particles are mysteries still puzzling theorists to date. Here we
review the methodologies of the computational approach to the modeling of
Pulsar Wind Nebulae, discussing the most relevant results and the recent
progresses achieved in this fascinating field of high-energy astrophysics.",1703.10442v1
2017-04-30,Angular Dependent Magnetization Dynamics of Kagome Artificial Spin Ice Incorporating Topological Defects,"We report angular-dependent spin-wave spectroscopy on kagome artificial spin
ice made of large arrays of interconnected Ni80Fe20 nanobars. Spectra taken in
saturated and disordered states exhibit a series of resonances with
characteristic in-plane angular dependencies. Micromagnetic simulations allow
us to interpret characteristic resonances of a two-step magnetization reversal
of the nanomagnets. The dynamic properties are consistent with topological
defects that are provoked via a magnetic field applied at specific angles.
Simulations that we performed on previously investigated kagome artificial spin
ice consisting of isolated nanobars show characteristic discrepancies in the
spin wave modes which we explain by the absence of vertices.",1705.00270v1
2017-06-05,Terahertz-driven magnetism dynamics in the orthoferrite DyFeO3,"Terahertz driven magnetization dynamics are explored in the orthoferrite
DyFeO3. A high-field, single cycle THz pulse is used to excite magnon modes in
the crystal together with other resonances. Both quasi-ferromagnetic and
quasi-antiferromagnetic magnon modes are excited and appear in time-resolved
measurements of the Faraday rotation. Other modes are also observed in the
measurements of the time-resolved linear birefringence. Analysis of the
excitation process reveals that despite larger than expected electro-optical
susceptibility it is mainly the THz magnetic field that couples to the
quasi-ferromagnetic and quasi-antiferromagnetic magnon branches.",1706.01545v1
2017-06-09,Magnetically Activated Rotational Vacuum Friction,"We predict the existence of a torque acting on an isotropic neutral
nanosphere activated by a static magnetic field when the particle temperature
differs from the surrounding vacuum. This phenomenon originates in
time-reversal symmetry breaking of the particle interaction with the vacuum
electromagnetic field. We present a rigorous quantum treatment of photons and
particle excitations that leads to a nonzero torque even in a motionless
particle. We also find that the dynamical evolution of the particle temperature
and rotation frequency follow an exotic dynamics, including spontaneous changes
in the rotation direction. Magnetically activated thermal vacuum torques open a
unique avenue for the investigation of the effect of time-reversal
symmetry-breaking in thermal and Casimir physics.",1706.02924v3
2017-06-29,Three-dimensional oscillatory magnetic reconnection,"Here we detail the dynamic evolution of localised reconnection regions about
three-dimensional (3D) magnetic null points by using numerical simulation. We
demonstrate for the first time that reconnection triggered by the localised
collapse of a 3D null point due to an external MHD wave involves a
self-generated oscillation, whereby the current sheet and outflow jets undergo
a reconnection reversal process during which back-pressure formation at the jet
heads acts to prise open the collapsed field before overshooting the
equilibrium into an opposite-polarity configuration. The discovery that
reconnection at fully 3D nulls can proceed naturally in a time-dependent and
periodic fashion is suggestive that oscillatory reconnection mechanisms may
play a role in explaining periodicity in astrophysical phenomena associated
with magnetic reconnection, such as the observed quasi-periodicity of solar and
stellar flare emission. Furthermore, we find a consequence of oscillatory
reconnection is the generation of a plethora of freely-propagating MHD waves
which escape the vicinity of the reconnection region",1706.09662v1
2017-08-14,Magnetization dynamics of weak stripe domains in Fe-N thin films: a multi-technique complementary approach,"The resonant eigenmodes of a nitrogen-implanted iron {\alpha}'-FeN
characterized by weak stripe domains are investigated by Brillouin light
scattering and broadband ferromagnetic resonance experiments, assisted by
micromagnetic simulations. The spectrum of the dynamic eigenmodes in the
presence of the weak stripes is very rich and two different families of modes
can be selectively detected using different techniques or different
experimental configurations. Attention is paid to the evolution of the mode
frequencies and spatial profiles under the application of an external magnetic
field, of variable intensity, in the direction parallel or transverse to the
stripes. The different evolution of the modes with the external magnetic field
is accompanied by a distinctive spatial localization in specific regions, such
as the closure domains at the surface of the stripes and the bulk domains
localized in the inner part of the stripes. The complementarity of BLS and FMR
techniques, based on different selection rules, is found to be a fruitful tool
for the study of the wealth of localized mag-netic excitations generally found
in nanostructures.",1708.04178v1
2017-08-18,General treatment of quantum and classical spinning particles in external fields,"We develop the general theory of spinning particles with electric and
magnetic dipole moments moving in arbitrary electromagnetic, inertial and
gravitational fields. Both the quantum-mechanical and classical dynamics is
investigated. We start from the covariant Dirac equation extended to a
spin-${\frac 12}$ fermion with anomalous magnetic and electric dipole moments
and then perform the relativistic Foldy-Wouthuysen transformation. This
transformation allows us to obtain the quantum-mechanical equations of motion
for the physical operators in the Schr\""odinger form and to establish the
classical limit of relativistic quantum mechanics. The results obtained are
then compared to the general classical description of the spinning particle
interacting with electromagnetic, inertial and gravitational fields. The
complete agreement between the quantum mechanics and the classical theory is
proven in the general case. As an application of the results obtained, we
consider the dynamics of a spinning particle in a gravitational wave and
analyze the prospects of using the magnetic resonance setup to find possible
manifestations of the gravitational wave on spin.",1708.05601v2
2017-10-30,Spatial symmetry breaking in single-frequency CCP discharge with transverse magnetic field,"An independent control of the flux and energy of ions impacting on an object
immersed in a plasma is often desirable for many industrial processes such as
microelectronics manufacturing. We demonstrate that a simultaneous control of
these quantities is possible by a suitable choice of a static magnetic field
applied parallel to the plane electrodes in a standard single frequency
capacitively coupled plasma device. Our particle-in-cell simulations show a 60%
reduction in the sheath width (that improves control of ion energy) and a four
fold increase in the ion flux at the electrode as a consequence of the altered
ion and electron dynamics due to the ambient magnetic field. A detailed
analysis of the particle dynamics is presented and the optimized operating
parameters of the device are discussed. The present technique offers a simple
and attractive alternative to conventional dual frequency based devices that
often suffer from undesirable limitations arising from frequency coupling and
electromagnetic effects.",1710.11254v2
2017-12-01,Active control of thermomagnetic avalanches in superconducting Nb films with tunable anisotropy,"Active triggering and manipulation of ultrafast flux dynamics in
superconductors are demonstrated in films of Nb. Controlled amounts of magnetic
flux were injected from a point along the edge of a square sample, which at 2.5
K responds by nucleation of a thermomagnetic avalanche. Magneto-optical imaging
was used to show that when such films are cooled in the presence of in-plane
magnetic fields they become anisotropic, and the morphology of the avalanches
change systematically, both with the direction and magnitude of the field. The
images reveal that the avalanching dendrites consistently bend towards the
direction perpendicular to that of the in-plane field. The effect increases
with the field magnitude, and at 1.5 kOe the triggered avalanche becomes
quenched at the nucleation stage. The experimental results are explained based
on a theoretical model for thermomagnetic avalanche nucleation in
superconducting films, and by assuming that the frozen-in flux generates
in-plane anisotropy in the film thermal conductance. The results demonstrate
that applying in-plane magnetic fields to film superconductors can be a
versatile external tool for controlling their ultrafast flux dynamics.",1712.00300v1
2017-12-11,Convergent tangent plane integrators for the simulation of chiral magnetic skyrmion dynamics,"We consider the numerical approximation of the Landau-Lifshitz-Gilbert
equation, which describes the dynamics of the magnetization in ferromagnetic
materials. In addition to the classical micromagnetic contributions, the energy
comprises the Dzyaloshinskii-Moriya interaction, which is the most important
ingredient for the enucleation and the stabilization of chiral magnetic
skyrmions. We propose and analyze three tangent plane integrators, for which we
prove (unconditional) convergence of the finite element solutions towards a
weak solution of the problem. The analysis is constructive and also establishes
existence of weak solutions. Numerical experiments demonstrate the
applicability of the methods for the simulation of practically relevant problem
sizes.",1712.03795v3
2018-02-01,Theory of spin and lattice wave dynamics excited by focused laser pulses,"We develop a theory of the spin wave dynamics excited by ultrafast focused
laser pulses in a magnetic film. We take into account both volume and surface
spin wave modes in the presence of applied, dipolar and magnetic anisotropy
fields and include the dependence on laser spot exposure size and magnetic
damping. We show that the sound waves generated by local heating by an
ultrafast focused laser pulse can excite a wide spectrum of spin waves (on top
of a dominant magnon-phonon contribution). Good agreement with recent
experiments supports the validity of the model.",1802.00178v2
2018-04-02,Importance of orbital fluctuations for the magnetic dynamics in heavy-fermion compound SmB$_6$,"The emergent dynamical processes associated with magnetic excitations in
heavy-fermion SmB$_6$ are investigated. By imposing multiorbital interactions
on a first-principles model, we find the interplay between spin and orbital
fluctuations in the $f$ manifold is highly sensitive to local correlations. The
magnetic phase diagram constructed at zero temperature reveals quantum critical
features with the existence of several competing phases. Within the random
phase approximation, we perform a comprehensive study of the spin-spin
correlation function, and our results agree with neutron scattering
experiments. Spectral weight analysis shows the low energy spin excitations are
selectively accompanied by orbital fluctuations, indicating a non-trivial
entanglement between the spin and orbital degree of freedom driven by
relativistic couplings.",1804.00674v2
2018-04-02,Spin torques and magnetic texture dynamics driven by the supercurrent in superconductor/ferromagnet structures,"We introduce the general formalism to describe spin torques induced by the
supercurrents injected from the adjacent superconducting electrodes into the
spin-textured ferromagnets. By considering the adiabatic limit for the
equal-spin superconducting correlations in the ferromagnet we show that the
supercurrent can generate both the field-like spin transfer torque and the
spin-orbital torque. These dissipationless spin torques are expressed through
the current-induced corrections to the effective field derived from the system
energy. The general formalism is applied to show that the supercurrent can
either shift or move the magnetic domain walls depending on their structure and
the type of spin-orbital interaction in the system. These results can be used
for the prediction and interpretation of the experiments studying magnetic
texture dynamics in superconductor/ferromagnet/superconductor Josephson
junctions and other hybrid structures.",1804.00718v2
2018-04-05,Dynamics of macrospin under periodic field and spin transfer torque,"The dynamics of a macrospin model for a single magnetic domain is
investigated in two cases: (i) under the action of a periodic magnetic field
and (ii) under the external field plus a spin transfer torque from
spin-polarized current or spin current. It has been proved that (i) without
spin transfer torque, the trajectory of magnetization (hysteresis) is always a
closed curve oscillating between two stable points and following the same path
each time; (ii) under the action of a constant field (or with small periodic
perturbation) and a constant spin transfer torque, the spin would always turn
to the stable solution finally, which is independent of the parameters or
initial conditions; (iii) considering a periodic oscillating field plus a spin
transfer torque, the system can also have a stable solution around a limit
circle, but on some special points on the attractor, the system may be
sensitive to the initial conditions and have no limit circle.",1804.01667v1
2018-04-03,Generalisation of Gilbert damping and magnetic inertia parameter as a series of higher-order relativistic terms,"The phenomenological Landau-Lifshitz-Gilbert (LLG) equation of motion remains
as the cornerstone of contemporary magnetisation dynamics studies, wherein the
Gilbert damping parameter has been attributed to first-order relativistic
effects. To include magnetic inertial effects the LLG equation has previously
been extended with a supplemental inertia term and the arising inertial
dynamics has been related to second-order relativistic effects. Here we start
from the relativistic Dirac equation and, performing a Foldy-Wouthuysen
transformation, derive a generalised Pauli spin Hamiltonian that contains
relativistic correction terms to any higher order. Using the Heisenberg
equation of spin motion we derive general relativistic expressions for the
tensorial Gilbert damping and magnetic inertia parameters, and show that these
tensors can be expressed as series of higher-order relativistic correction
terms. We further show that, in the case of a harmonic external driving field,
these series can be summed and we provide closed analytical expressions for the
Gilbert and inertial parameters that are functions of the frequency of the
driving field.",1804.09242v1
2018-06-03,Dynamical off-equilibrium scaling across magnetic first-order phase transitions,"We investigate the off-equilibrium dynamics of a classical spin system with
$O(n)$ symmetry in $2< D <4$ spatial dimensions and in the limit $n\to \infty$.
The system is set up in an ordered equilibrium state is and subsequently driven
out of equilibrium by slowly varying the external magnetic field $h$ across the
transition line $h_c=0$ at fixed temperature $T\leq T_c$. We distinguish the
cases $T = T_c$ where the magnetic transition is continuous and $T<T_c$ where
the transition is discontinuous. In the former case, we apply a standard
Kibble-Zurek approach to describe the non-equilibrium scaling and formally
compute the correlation functions and scaling relations. For the discontinuous
transition we develop a scaling theory which builds on the coherence length
rather than the correlation length since the latter remains finite for all
times. Finally, we derive the off-equilibrium scaling relations for the
hysteresis loop area during a round-trip protocol that takes the system across
its phase transition and back. Remarkably, our results are valid beyond the
large-$n$ limit.",1806.00866v2
2018-06-28,Thermal fluctuations in antiferromagnetic nanostructures,"A theoretical model is developed that can accurately analyze the effects of
thermal fluctuations in antiferromagnetic (AFM) nano-particles. The approach is
based on Fourier series representation of the random effective field with
cut-off frequencies of physical origin at low and high limits while satisfying
the fluctuation-dissipation theorem at the same time. When coupled with the
formalism of a Langevin dynamical equation, it can describe the stochastic
N\'eel vector dynamics with the AFM parameters, circumventing the arbitrariness
of the commonly used treatments in the micro-magnetic simulations. Subsequent
application of the model to spontaneous N\'eel vector switching provides a
thermal stability analysis of the AFM states. The numerical simulation shows
that the AFM states are much less prone to the thermally induced accidental
flips than the ferromagnetic counterparts, suggesting a longer retention time
for the former.",1806.11130v2
2018-07-03,Nonlinear dynamo in obliquely rotating stratified electroconductive fluid in an uniformly magnetic field,"We study a new type of large-scale instability, which arises in obliquely
rotating stratified electroconductive fluid with an external uniform magnetic
field and a small-scale external force having zero helicity. This force gives
rise to small-scale oscillations of the velocity with a small Reynolds number.
Using the method of multi-scale asymptotic expansions there are obtained
nonlinear equations for vortex and magnetic perturbations in the third order in
Reynolds number. Studied is the linear stage of magneto-vortex dynamo caused by
instabilities of $\alpha$-effect type. Stationary solutions for the equations
of nonlinear magneto-vortex dynamo are found by numerical methods in the form
of localized chaotic structures.",1807.01986v1
2018-07-15,Chiral Magnetic Effect in Isobaric Collisions from Anomalous-Viscous Fluid Dynamics (AVFD),"The isobaric collision experiment at RHIC provides the unique opportunity to
detect the possible signal of Chiral Magnetic Effect (CME) in heavy ion
collisions. The idea is to contrast the correlation observables of the two
colliding systems that supposedly have identical flow-driven background
contributions while quite different CME signal contributions due to the 10%
variation in their nuclear charge and thus magnetic field strength. With the
recently developed quantitative simulation tool for computing CME signal, the
Anomalous-Viscous Fluid Dynamics (AVFD), we demonstrate that a joint
(multiplicity + elliptic-flow) event selection is crucial for this purpose. We
further propose to use the absolute difference between RuRu and ZrZr events
(after using identical event selection) for detecting CME signal and make
predictions for the correlation observables.",1807.05604v1
2018-09-12,Inversion and magnetic quantum oscillations in symmetric periodic Anderson model,"We study the symmetric periodic Anderson model of the conduction electrons
hybridized with the localized correlated electrons on square lattice. Using the
canonical representation of electrons by Kumar, we do a self-consistent theory
of its effective charge and spin dynamics, which produces an insulating ground
state that undergoes continuous transition from the Kondo singlet to N\'eel
antiferromagnetic phase with decreasing hybridization, and uncovers two
inversion transitions for the charge quasiparticles. With suitably inverted
quasiparticle bands for moderate to weaker effective Kondo couplings, this
effective charge dynamics in the magnetic field coupled to electronic motion
produces magnetic quantum oscillations with frequency corresponding to the half
Brillouin zone.",1809.04654v2
2018-09-21,Quantum hardware simulating four-dimensional inelastic neutron scattering,"Magnetic molecules, modelled as finite-size spin systems, are test-beds for
quantum phenomena and could constitute key elements in future spintronics
devices, long-lasting nanoscale memories or noise-resilient quantum computing
platforms. Inelastic neutron scattering is the technique of choice to probe
them, characterizing molecular eigenstates on atomic scales. However, although
large magnetic molecules can be controllably synthesized, simulating their
dynamics and interpreting spectroscopic measurements is challenging because of
the exponential scaling of the required resources on a classical computer.
Here, we show that quantum computers have the potential to efficiently extract
dynamical correlations and the associated magnetic neutron cross-section by
simulating prototypical spin systems on a quantum hardware. We identify the
main gate errors and show the potential scalability of our approach. The
synergy between developments in neutron scattering and quantum processors will
help design spin clusters for future applications.",1809.07974v2
2018-11-05,Vison-Majorana complex zero-energy resonance in Kitaev's spin liquid,"We study the effect of site dilution in Kitaev's model. We derive an
analytical solution of the dynamical spin correlation functions for arbitrary
configurations of $Z_2$ fluxes. By incorporating this solution into classical
Monte Carlo scheme, we address how a site vacancy affects the experimental
observables, such as the static spin susceptibility and the spin lattice
relaxation rate, $1/T_1$. As a result, we found an enhancement of dynamical
magnetic response in the vicinity of vacancy, which leads to Friedel-like
oscillation in local $1/T_1$, in contrast to limited influences on the static
susceptibility. Furthermore, we found a sharp zero-energy peak in the magnetic
excitation spectrum, which is attributed to the Vison & Majorana zero mode
trapped near the site vacancy. This zero mode can be interpreted as
fractionalized spin hole into an Ising triplet with differentiated magnetic
axes, which leads to the characteristic temperature and field-orientational
dependence of $1/T_1$.",1811.02092v1
2018-11-23,Axially magnetized Dark Energy cosmological model,"We investigate the behaviour of the skewness parameters for an anisotropic
universe in the framework of General Relativity. Non interacting dark energy is
considered in presence of electromagnetic field. A time varying deceleration
parameter simulated by a hybrid scale factor is considered. The dynamics of the
universe is investigated in presence and absence of magnetic field. The
equation of state parameter of dark energy evolves within the range predicted
by the observations. Magnetic field is observed to have a substantial effect on
the cosmic dynamics and the skewness parameters. The models discussed here end
in a big rip and become isotropic at finite time.",1811.09626v3
2018-11-29,Supertransient Magnetohydrodynamic Turbulence in Keplerian Shear Flows: The role of the Hall Effect,"Space and astrophysical plasmas are frequently found in the regime of
differential rotation, where the presence of a magnetic field can result in the
magnetorotational instability, directly responsible for important phenomena
such as turbulent angular momentum transport in accretion disks. In the absence
of an imposed magnetic field, a nonlinear dynamo is necessary for this
transport mechanism to take place. In protoplanetary disks there are regions
with high density and very low temperatures, which are two necessary conditions
for the Hall effect to operate, affecting the development of the dynamo and the
associated turbulence. In this work we perform local magnetohydrodynamic (MHD)
simulations to study transition to weak turbulence in Keplerian shear flows
with Hall effect. The Hall effect is shown to lead the system to long-lived
turbulent transients whose decay time follows an exponential dependence on the
magnetic Reynolds number and the Hall parameter.",1811.12318v1
2018-12-05,Characterization and space embedding of directed graphs and social networks through magnetic Laplacians,"Though commonly found in the real world, directed networks have received
relatively less attention from the literature in which concerns their
topological and dynamical characteristics. In this work, we develop a magnetic
Laplacian-based framework that can be used for studying directed complex
networks. More specifically, we introduce a specific heat measurement that can
help to characterize the network topology. It is shown that, by using this
approach, it is possible to identify the types of several networks, as well as
to infer parameters underlying specific network configurations. Then, we
consider the dynamics associated with the magnetic Laplacian as a means of
embedding networks into a metric space, allowing the identification of
mesoscopic structures in artificial networks or unravel the polarization on
political blogosphere. By defining a coarse-graining procedure in this metric
space, we show how to connect the specific heat measurement and the positions
of nodes in this space.",1812.02160v2
2018-12-16,Transitions near the onset of low Prandtl-number rotating magnetoconvection,"We investigate the onset of convective instability and subsequent transitions
near it in Rayleigh-B\'enard convection (RBC) of electrically conducting low
Prandtl-number (Pr) fluids in the simultaneous presence of rotation about the
vertical axis and external uniform horizontal magnetic field with free-slip
boundary conditions. Both three dimensional direct numerical simulations (DNS)
and low dimensional modeling of the system have been performed for the
investigation. DNS results show a plethora of patterns near the onset of
convection for weak magnetic field. Most interestingly, the Nusselt number (Nu)
computed from the DNS data for relatively stronger magnetic field shows a sharp
jump at the onset, indicating substantial enhancement of the convective heat
transfer. The origin of these flow patterns, as well as the reason for a
notable increase of heat transfer near the onset of convection, are then
explained using low dimensional models constructed from DNS data.",1812.06422v1
2018-12-27,Influence of Ohmic and ambipolar heating on thermal structure of accretion discs,"We investigate dynamics of accretion discs of young stars with fossil
large-scale magnetic field. Our magneto-gas-dynamic (MHD) model of the
accretion discs includes equations of Shakura and Sunyaev, induction equation,
equations of thermal and collisional ionization. Induction equation takes into
account Ohmic and magnetic ambipolar diffusion, magnetic buoyancy. We also
consider the influence of Ohmic and ambipolar heating on thermal structure of
the accretion discs. We analyse the influence of considered dissipative MHD
effects on the temperature of the accretion discs around classical T Tauri
star. The simulations show that Ohmic and ambipolar heating operate near the
borders of the region with low ionization fraction (`dead' zone). Temperature
grows by $\sim 1000$ K near the inner boundary of the `dead' zone, $r\approx
(0.5-1)$ au, and by $\sim 100$ K near its outer boundary, $r\approx (30-50)$
au.",1812.10634v1
2018-12-28,Skyrmion-generated spinmotive forces in inversion broken ferromagnets,"We present an analytical study on the spinmotive force (SMF) generated by
translational motion of magnetic skyrmion. A SMF refers to an electrical
voltage induced by dynamical magnetic textures, which reflects the
spatiotemporal variation of the magnetization. The dynamics of a skyrmion thus
can be detected by a SMF measurement, which may play an important role in
future skyrmion-based technologies. We find the dependence of the SMF on
skyrmion structure (e.g., skyrmion or anti-skyrmion, N\'{e}el or Bloch type,
and the polarity of the skyrmion core) and Rashba and Dresselhaus spin-orbit
couplings (SOCs). To this end, we derive explicit formulae for the
spin-dependent electric fields originating from the two SOCs. Our findings
offer a comprehensive understanding of the phenomenon and an estimation of the
electrical voltage signal associated with a moving skyrmion for a given
experiment.",1812.11120v2
2019-03-20,Spin dynamics of antiferromagnetically coupled ferromagnetic bilayers -- the case of Cr$_2$WO$_6$ and Cr$_2$MoO$_6$,"Recent inelastic neutron diffraction measurements on Cr$_2$(Te, W, Mo)O$_6$
have revealed that these systems consist of bilayers of spin-3/2 Cr$^{3+}$ ions
with strong antiferromagnetic inter-bilayer coupling and tuneable intra-bilayer
coupling from ferro (for W and Mo) to antiferro (for Te). These measurements
have determined the ground state spin structure and the values of sublattice
magnetization, which shows significant reduction of sublattice magnetization
from the atomic spin value of 3.0$\mu_B$ for Cr$^{3+}$ atoms. In an earlier
paper we theoretically investigated the low temperature spin dynamics of
Cr$_2$TeO$_6$ bilayer system where both the intra and inter-bilayer couplings
are antiferromagnetic. In this paper we investigate Cr$_2$WO$_6$ and
Cr$_2$MoO$_6$ systems where intra-bilayer exchange couplings are ferromagnetic
but the inter-bilayer exchange couplings are antiferromagnetic. We obtain the
magnon dispersion, sublattice magnetization, two-magnon density of states,
longitudinal spin-spin correlation function, and its powder average and compare
the results for these systems with results for Cr$_2$TeO$_6$.",1903.08512v1
2019-04-18,Transient dynamics of nonlinear magneto-optical rotation in the presence of transverse magnetic field,"Nonlinear magneto-optical rotation is studied under non-equilibrium
conditions. The polarization rotation of linearly polarized light traversing a
rubidium vapor cell is observed versus the time-dependent (swept) longitudinal
magnetic field in the presence of static transverse magnetic fields. Presence
of the transverse fields modifies the character of the observed signals. In
particular, for weaker transverse fields, field sweep leads two-harmonic
oscillation of the polarization rotation while crossing zero. Unlike the
steady-state, it was found that two-frequency oscillations observed in the
transient signals, are independent of the transverse-field direction. For
stronger transverse fields, the oscillations deteriorate eventually reaching a
situation when no-oscillating dynamic signal, with distinct minimum close to
zero field, is observed. Experimental results are supported with theoretical
analysis based on the density-matrix formalism. The analysis confirms all the
features of experimental results while providing an provide intuitive
explanation of the observed behavior based on angular-momentum probability
surfaces used for density-matrix visualization.",1904.08753v1
2019-04-25,Spin-wave propagation in metallic CoFe films determined by microfocused frequency-resolved magneto-optic Kerr effect,"We investigated the magnetization dynamics of a patterned
Co$_{25}$Fe$_{75}$-based heterostructure with a novel optical measurement
technique that we call microfocused frequency-resolved magneto optic Kerr
effect ($\mu$FR-MOKE). We measured the magnetic field dependence of the
dynamical spin-wave susceptibility and recorded a spatial map of the spin-waves
excited by a microwave antenna. We compare these results to those obtained on
the same sample with the established microfocused Brillouin light scattering
technique. With both techniques, we find a spin-wave propagation length of
5.6$\mu$m at 10GHz. Furthermore, we measured the dispersion of the wavevector
and the spin-wave propagation length as a function of the external magnetic
field. These results are in good agreement with existing literature and with
the employed Kalinkos-Slavin model.",1904.11273v1
2019-04-30,Thermal flux-flow regime in long Josephson tunnel junctions,"We study thermal transport induced by soliton dynamics in a long Josephson
tunnel junction operating in the flux-flow regime. A thermal bias across the
junction is established by imposing the superconducting electrodes to reside at
different temperatures, when solitons flow along the junction. Here, we
consider the effect of both a bias current and an external magnetic field on
the thermal evolution of the device. In the flux-flow regime, a chain of
magnetically-excited solitons rapidly moves along the junction driven by the
bias current. We explore the range of bias current triggering the flux-flow
regime at fixed values of magnetic field, and the stationary temperature
distribution in this operation mode. We evidence a steady multi-peaked
temperature profile which reflects on the average soliton distribution along
the junction. Finally, we analyse also how the friction affecting the soliton
dynamics influences the thermal evolution of the system.",1904.13172v2
2019-04-30,Dynamics of a magnetic skyrmionium in an anisotropy gradient,"Magnetic skyrmionium is a novel magnetization configuration with zero
skyrmion number, which is composed by two skyrmions with opposite skyrmion
number. Here, we study the dynamics of skyrmionium under an anisotropy
gradient. We find that the skyrmionium can be efficiently driven by an
anisotropy gradient with moving straightly along the direction of gradient. The
skyrmion Hall angle for skyrmionium is close to zero which is much smaller than
that of skyrmion. while the speed is much larger. We also demonstrate that the
skyrmionium motion depends on the damping cofficient, and the skyrmionium
stabilization in the motion can be modulated by narrowing the width of the
nanowire. Our work shows a efficient driven method for skyrmionium, which may
be promising in the application of skyrmionium based racetrack memory.",1904.13332v1
2019-05-03,Field-induced incommensurate ordering in the Heisenberg chains coupled by Ising interaction: Model for the ytterbium aluminum perovskite YbAlO$_3$,"We study isotropic antiferromagnetic Heisenberg chains coupled by
antiferromagnetic Ising interaction as an effective spin model for the
ytterbium aluminum perovskite YbAlO$_3$. Using the density-matrix
renormalization group (DMRG) method we calculate the magnetization curve, local
spin, central charge, and dynamical spin structure factors in the presence of
magnetic field. From the fitting of the experimental magnetization curve, the
effective intrachain and interchain couplings are estimated as $J=2.3$K and
$J_{\rm ic}=0.8$K, respectively. We can quantitatively explain the experimental
observations: (i) phase transition from antiferromagnetic to incommensurate
order at field 0.35T, and (ii) quantum critical behaviors at the saturation
field of 1.21T. Furthermore, the low-energy excitations in the experimental
inelastic neutron scattering spectra can be well described by our DMRG results
of the dynamical structure factors.",1905.01204v1
2019-05-30,Wavelet Analysis of Big Data in the Global Investigation of Magnetic Field Variations in Solar-Terrestrial Physics,"We provide a Wavelet analysis of Big Data in Solar Terrestrial Physics. In
order to explain and predict the dynamics of the geomagnetic phenomena we
analyze high frequency time series data from different sources: 1. The
Interplanetary Magnetic Field (from the ACE satellite). 2. The Ionospheric
parameters - TEC (from ionospheric sounding stations). 3. The ground
Geomagnetic data (from ground geomagnetic observatories, located in middle
geographic latitudes). We seek for correlations in the wavelet coefficients
which explain the dynamics of different magnetic phenomena in the Solar
Terrestrial Physics. The large variety of data used in our research from both
Solar Astronomy and Earth Observations makes it a contribution to the newly
developing area of AstroGeoInformatics.",1905.12923v1
2019-08-06,Non-equilibrium magnetic phases in spin lattices with gain and loss,"We study the magnetic phases of a non-equilibrium spin chain, where coherent
interactions between neighboring lattice sites compete with alternating gain
and loss processes. This competition between coherent and incoherent dynamics
induces transitions between magnetically aligned and highly mixed phases,
across which the system changes from a low- to an effective
infinite-temperature state. We show that the origin of these transitions can be
traced back to the dynamical effect of parity-time-reversal symmetry breaking,
which has no counterpart in the theory of equilibrium phase transitions. This
mechanism also results in very atypical features and we find first-order
transitions without phase co-existence and mixed-order transitions which do not
break the underlying $U(1)$ symmetry, even in the appropriate thermodynamic
limit. Thus, despite its simplicity, the current model considerably extends the
phenomenology of non-equilibrium phase transitions beyond that commonly assumed
for driven-dissipative spins and related systems.",1908.02290v2
2019-10-17,Optical storage for 0.53 seconds in a solid-state atomic frequency comb memory using dynamical decoupling,"Quantum memories with long storage times are key elements in long-distance
quantum networks. The atomic frequency comb (AFC) memory in particular has
shown great promise to fulfill this role, having demonstrated multimode
capacity and spin-photon quantum correlations. However, the memory storage
times have so-far been limited to about one millisecond, realized in a
Eu${}^{3+}$ doped Y${}_2$SiO${}_5$ crystal at zero applied magnetic field.
Motivated by studies showing increased spin coherence times under applied
magnetic field, we developed a AFC spin-wave memory utilizing a weak 15 mT
magnetic field in a specific direction that allows efficient optical and spin
manipulation for AFC memory operations. With this field configuration the AFC
spin-wave storage time increased to 40 ms using a simple spin-echo sequence.
Furthermore, by applying dynamical decoupling techniques the spin-wave
coherence time reaches 530 ms, a 300-fold increase with respect to previous AFC
spin-wave storage experiments. This result paves the way towards long duration
storage of quantum information in solid-state ensemble memories.",1910.08009v2
2019-10-29,Molecular dynamics investigation of soliton propagation in a two-dimensional Yukawa liquid,"We investigate via Molecular Dynamics simulations the propagation of solitons
in a two-dimensional many-body system characterized by Yukawa interaction
potential. The solitons are created in an equilibrated system by the
application of electric field pulses. Such pulses generate pairs of solitons,
which are characterized by a positive and negative density peak, respectively,
and which propagate into opposite directions. At small perturbation, the
features propagate with the longitudinal sound speed, form which an increasing
deviation is found at higher density perturbations. An external magnetic field
is found to block the propagation of the solitons, which can, however, be
released upon the termination of the magnetic field and can propagate further
into directions that depend on the time of trapping and the magnetic field
strength.",1910.13303v1
2019-11-01,Magnetic Polymer Models for Epigenetics-Driven Chromosome Folding,"Epigenetics is a driving force of important and ubiquitous phenomena in
nature such as cell differentiation or even metamorphosis. Oppositely to its
widespread role, understanding the biophysical principles that allow
epigenetics to control and rewire gene regulatory networks remains an open
challenge. In this work we study the effects of epigenetic modifications on the
spatial folding of chromosomes -- and hence on the expression of the underlying
genes -- by mapping the problem to a class of models known as magnetic
polymers. In this work we show that a first-order phase transition underlies
the simultaneous spreading of certain epigenetic marks and the conformational
collapse of a chromosome. Further, we describe Brownian Dynamics simulations of
the model in which the topology of the polymer and thermal fluctuations are
fully taken into account and that confirms our mean-field predictions.
Extending our models to allow for non-equilibrium terms yields new stable
phases which qualitatively agrees with observations in vivo. Our results show
that statistical mechanics techniques applied to models of magnetic polymers
can be successfully exploited to rationalize the outcomes of experiments
designed to probe the interplay between a dynamic epigenetic landscape and
chromatin organization.",1911.00444v1
2019-11-05,Emergent colloidal edge currents generated via exchange dynamics in a broken dimer state,"Controlling the flow of matter down to micrometer-scale confinement is of
central importance in materials and environmental sciences, with direct
applications in nano-microfluidics, drug delivery and biothechnology. Currents
of microparticles are usually generated with external field gradients of
different nature [e.g., electric, magnetic, optical, thermal or chemical ones]
which are difficult to control over spatially extended regions and samples.
Here we demonstrate a general strategy to assemble and transport polarizable
microparticles in fluid media through combination of confinement and magnetic
dipolar interactions. We use a homogeneous magnetic modulation to assemble
dispersed particles into rotating dimeric state and frustrated binary lattices,
and generate collective edge currents which arise from a novel,
field-synchronized particle exchange process. These dynamic states are similar
to cyclotron and skipping orbits in electronic and molecular systems, thus
paving the way toward understanding and engineering similar processes at
different length scales across condensed matter.",1911.01698v2
2019-11-13,Weakly nonlinear magnetic convection in a nonuniformly rotating electrically conductive medium under the action of modulation of external fields,"In this paper we studied the weakly nonlinear stage of stationary convective
instability in a nonuniformly rotating layer of an electrically conductive
fluid in an axial uniform magnetic field under the influence of: a) temperature
modulation of the layer boundaries; b) gravitational modulation; c) modulation
of the magnetic field; d) modulation of the angular velocity of rotation. As a
result of applying the method of perturbation theory for the small parameter of
supercriticality of the stationary Rayleigh number nonlinear non-autonomous
Ginzburg-Landau equations for the above types of modulation were obtaned. By
utilizing the solution of the Ginzburg-Landau equation, we determined the
dynamics of unsteady heat transfer for various types of modulation of external
fields and for different profiles of the angular velocity of the rotation of
electrically conductive fluid.",1911.05368v1
2020-01-03,Real-time chiral dynamics from a digital quantum simulation,"The chiral magnetic effect in a strong magnetic field can be described using
the chiral anomaly in the $(1+1)$-dimensional massive Schwinger model with a
time-dependent $\theta$-term. We perform a digital quantum simulation of the
model at finite $\theta$-angle and vanishing gauge coupling using an IBM-Q
digital quantum simulator, and observe the corresponding vector current induced
in a system of relativistic fermions by a global {\it chiral quench} -- a
sudden change in the chiral chemical potential or $\theta$-angle. At finite
fermion mass, there appears an additional contribution to this current that
stems from the non-anomalous relaxation of chirality. Our results are relevant
for the real-time dynamics of chiral magnetic effect in heavy ion collisions
and in chiral materials, as well as for modeling high-energy processes at
hadron colliders.",2001.00698v4
2020-01-27,Many-body localization with synthetic gauge fields in disordered Hubbard chains,"We analyze the localization properties of the disordered Hubbard model in the
presence of a synthetic magnetic field. An analysis of level spacing ratio
shows a clear transition from ergodic to many-body localized phase. The
transition shifts to larger disorder strengths with increasing magnetic flux.
Study of dynamics of local correlations and entanglement entropy indicates that
charge excitations remain localized whereas spin degree of freedom gets
delocalized in the presence of the synthetic flux. This residual ergodicity is
enhanced by the presence of the magnetic field with dynamical observables
suggesting incomplete localization at large disorder strengths. Furthermore, we
examine the effect of quantum statistics on the local correlations and show
that the long-time spin oscillations of a hard-core boson system are destroyed
as opposed to the fermionic case.",2001.09827v2
2020-02-04,Dynamic neutron imaging of argon bubble flow in liquid gallium in external magnetic field,"This paper presents detailed results of neutron imaging of argon bubble flows
in a rectangular liquid gallium vessel with and without the application of
external horizontal magnetic field. The developed image processing algorithm is
presented and its capability to extract physical information from images of low
signal-to-noise ratio is demonstrated. Bubble parameters, velocity components,
trajectories and relevant statistics were computed and analysed. A simpler
version of the code was applied to the output of computational fluid dynamics
simulations that reproduced the experiment. This work serves to further
validate the neutron radiography as a suitable method for monitoring gas bubble
flow in liquid metals, as well as to outline procedures that might help others
to extract data from neutron radiography images with a low signal-to-noise
ratio resulting from high frame rate acquisitions required to resolve rapid
bubble motion.",2002.10970v2
2020-02-26,flatspin: A Large-Scale Artificial Spin Ice Simulator,"We present flatspin, a novel simulator for systems of interacting mesoscopic
spins on a lattice, also known as artificial spin ice (ASI). Our magnetic
switching criteria enables ASI dynamics to be captured in a dipole model.
Through GPU acceleration, flatspin can simulate realistic dynamics of millions
of magnets within practical time frames. We demonstrate flatspin's versatility
through the reproduction of a diverse set of established experimental results
from the literature. In particular, magnetization details of ""pinwheel"" ASI
during field-driven reversal have been reproduced, for the first time, by a
dipole model. The simulation framework enables quick exploration and
investigation of new ASI geometries and properties at unprecedented speeds.",2002.11401v1
2020-02-26,Magnon dispersion and dynamic spin response in three-dimensional spin models for $α$-RuCl$_3$,"In the search for experimental realizations of bond-anisotropic Kitaev
interactions and resulting spin-liquid phases, the layered magnet
$\alpha$-RuCl$_3$ is a prime candidate. Its modelling typically involves
Heisenberg, Kitaev, and symmetric off-diagonal $\Gamma$ interactions on the
two-dimensional honeycomb lattice. However, recent neutron-scattering
experiments point towards a sizeable magnetic interlayer coupling. Here we
study three-dimensional exchange models for $\alpha$-RuCl$_3$, for both
possible $R\bar{3}$ and $C2/m$ crystal structures. We discuss the symmetry
constraints on the interlayer couplings, construct minimal models, and use them
to compute the magnetic mode dispersion and the dynamical spin structure
factor, in both the zero-field zigzag phase and the paramagnetic high-field
phase. Our predictions for the interlayer mode dispersion shall guide future
experiments; they also call for a reevaluation of the quantitative model
parameters relevant for $\alpha$-RuCl$_3$.",2002.11727v2
2020-05-22,Signatures of a liquid-crystal transition in spin-wave excitations of skyrmions,"Understanding the spin-wave excitations of chiral magnetic order, such as the
skyrmion crystal (SkX), is of fundamental interest to confirm such exotic
magnetic order. The SkX is realized by competing Dzyaloshinskii-Moriya and
ferromagnetic-exchange interactions with a magnetic field or anisotropy. Here
we compute the dynamical spin structure factor, using Monte Carlo and spin
dynamics simulations, extracting the spin-wave spectrum in the SkX, in the
vicinity of the paramagnet to SkX transition. Inside the SkX, we find six
spin-wave modes, which are supplemented by another mode originating from the
ferromagnetic background. Above the critical temperature $T_s$ for the skyrmion
crystallization, we find a diffusive regime, reminiscent of the
liquid-to-crystal transition, revealing that topological spin texture of
skyrmionic character starts to develop above $T_s$ as the precursor of the SkX.
We discuss the opportunities for the detection of the spin waves of the SkX
using inelastic-neutron-scattering experiments in manganite-iridate
heterostructures.",2005.11399v2
2020-06-02,Dynamic magnetization in non-Hermitian quantum spin system,"We report a global effect induced by the local complex field, associated with
the spin-exchange interaction. High-order exceptional point up to ($N+1$)-level
coalescence is created at the critical local complex field applied to the
$N$-size quantum spin chain. The ($N+1$)-order coalescent level is a saturated
ferromagnetic ground state in the isotropic spin system. Remarkably, the final
state always approaches the ground state for an arbitrary initial state with
any number of spin flips; even if the initial state is orthogonal to the ground
state. Furthermore, the switch of macroscopic magnetization is solely driven by
the time and forms a hysteresis loop in the time domain. The retentivity and
coercivity of the hysteresis loop mainly rely on the non-Hermiticity. Our
findings highlight the cooperation of non-Hermiticity and the interaction in
quantum spin system, suggest a dynamical framework to realize magnetization,
and thus pave the way for the non-Hermitian quantum spin system.",2006.01324v1
2020-06-02,Rigid body dynamics of diamagnetically levitating graphite resonators,"Diamagnetic levitation is a promising technique for realizing resonant
sensors and energy harvesters, since it offers thermal and mechanical isolation
from the environment at zero power. To advance the application of
diamagnetically levitating resonators, it is important to characterize their
dynamics in the presence of both magnetic and gravitational fields. Here we
experimentally actuate and measure rigid body modes of a diamagnetically
levitating graphite plate. We numerically calculate the magnetic field and
determine the influence of magnetic force on the resonance frequencies of the
levitating plate. By analyzing damping mechanisms, we conclude that eddy
current damping dominates dissipation in mm-sized plates. We use finite element
simulations to model eddy current damping and find close agreement with
experimental results. We also study the size-dependent Q-factors (Qs) of
diamagnetically levitating plates and show that Qs above 100 million are
theoretically attainable by reducing the size of the diamagnetic resonator down
to microscale, making these systems of interest for next generation low-noise
resonant sensors and oscillators.",2006.01733v3
2020-06-23,Dynamic polarization of electron spins interacting with nuclei in semiconductor nanostructures,"We suggest a new spin orientation mechanism for localized electrons:
$dynamic~electron~spin~polarization~provided~by~nuclear~spin~fluctuations$. The
angular momentum for the electrons is gained from the nuclear spin system via
the hyperfine interaction in a weak magnetic field. For this the sample is
illuminated by an unpolarized light, which directly polarizes neither the
electrons nor the nuclei. We predict, that for the electrons bound in localized
excitons 100% spin polarization can be reached in longitudinal magnetic fields
of a few millitesla. The proof of principle experiment is performed on
momentum-indirect excitons in (In,Al)As/AlAs quantum dots, where in a magnetic
field of 17 mT the electron spin polarization of 30% is measured.",2006.13263v1
2020-07-08,Dynamics and Clogging of colloidal monolayers magnetically driven through an heterogeneous landscape,"We combine experiments and numerical simulations to investigate the emergence
of clogging in a system of interacting paramagnetic colloidal particles driven
against a disordered landscape of larger obstacles. We consider a single
aperture in a landscape of immobile silica particles which are irreversibly
attached to the substrate. We use an external rotating magnetic field to
generate a traveling wave potential which drives the magnetic particles against
these obstacles at a constant and frequency tunable speed. Experimentally we
find that the particles display an intermittent dynamics with power law
distributions at high frequencies. We reproduce these results by using
numerical simulations and shows that clogging in our system arises at large
frequency, when the particles desynchronize with the moving landscape. Further,
we use the model to explore the hidden role of flexibility in the obstacle
displacements and the effect of hydrodynamic interactions between the
particles. We also consider numerically the situation of a straight wall and
investigate the range of parameters where clogging emerges in such case. Our
work provides a soft matter test-bed system to investigate the effect of
clogging in driven microscale matter.",2007.04436v1
2020-07-09,Dynamic Spin Fluctuations in the Frustrated A-site Spinel CuAl2O4,"We performed nuclear magnetic resonance (NMR) and muon spin relaxation
({\mu}SR) experiments to identify the magnetic ground state of the frustrated
quantum A-site spinel, CuAl2O4. Our results verify that the ground state does
not exhibit a long-range magnetic ordering, but a glass-like transition
manifests at T*=2.3 K. However, the Gaussian shape and the weak longitudinal
field dependence of {\mu}SR spectra below T* show that the ground state has
dynamic spin fluctuations, distinct from those of conventional spin-glasses.",2007.04761v1
2020-07-21,Revisiting Magnetogenesis during Inflation,"We revisit the mechanism of primordial magnetogenesis during inflation by
taking into account the dynamics of the stochastic noises of the
electromagnetic perturbations. We obtain the associated Langevin and
Fokker-Planck equations for the electromagnetic fields and solve them
analytically. It is shown that while the backreactions of the electric field
energy density may spoil inflation too early, but there are regions of
parameter space where the usual decaying behavior of the magnetic fields are
replaced by a mean-reverting process of stochastic dynamics. As a result, the
magnetic fields settle down into an equilibrium state with the amplitude
significantly larger than what is obtained in the absence of the stochastic
noises. We show that magnetic fields with present time amplitude $\sim
10^{-13}$ Gauss and coherent length ${\rm Mpc}$ can be generated while the
backreactions of the electric field perturbations are under control.",2007.11066v1
2020-08-04,Hall effect and symmetry breaking in non-magnetic metal with dynamic charge stripes,"A comprehensive study of magnetoresistance and Hall effect has been performed
for the set of the single crystals of non-magnetic metal LuB12 with the
Jahn-Teller instability of the boron cage and dynamic charge stripes forming
along <110> direction. Anomalous positive contribution to Hall effect for
particular direction of magnetic field H//[001] is found in the single crystals
of LuB12 of the highest quality. This contribution arising at T~ 150 K is shown
to increase drastically when approaching the disordered ground state below 60
K. The Hall effect anomaly is shown to appear in combination with the peak of
magnetoresistance. The various scenarios allowing for the topology of Fermi
surface, anisotropy of relaxation time for charge carriers and the filamentary
structure of fluctuating charge stripes are proposed to explain the features of
magnetotransport in this metal with inhomogeneous distribution of electron
density. The origin of SdH oscillations, which are observed in this
non-equilibrium metal with electron phase separation and strong charge carrier
scattering, is discussed.",2008.01455v1
2020-08-18,Non-Canonical Hamiltonian Monte Carlo,"Hamiltonian Monte Carlo is typically based on the assumption of an underlying
canonical symplectic structure. Numerical integrators designed for the
canonical structure are incompatible with motion generated by non-canonical
dynamics. These non-canonical dynamics, motivated by examples in physics and
symplectic geometry, correspond to techniques such as preconditioning which are
routinely used to improve algorithmic performance. Indeed, recently, a special
case of non-canonical structure, magnetic Hamiltonian Monte Carlo, was
demonstrated to provide advantageous sampling properties. We present a
framework for Hamiltonian Monte Carlo using non-canonical symplectic
structures. Our experimental results demonstrate sampling advantages associated
to Hamiltonian Monte Carlo with non-canonical structure. To summarize our
contributions: (i) we develop non-canonical HMC from foundations in symplectic
geomtry; (ii) we construct an HMC procedure using implicit integration that
satisfies the detailed balance; (iii) we propose to accelerate the sampling
using an {\em approximate} explicit methodology; (iv) we study two novel,
randomly-generated non-canonical structures: magnetic momentum and the coupled
magnet structure, with implicit and explicit integration.",2008.08191v1
2020-10-06,Magnetic microswimmers exhibit Bose-Einstein-like condensation,"We study an active matter system comprised of magnetic microswimmers confined
in a microfluidic channel and show that it exhibits a new type of
self-organized behavior. Combining analytical techniques and Brownian dynamics
simulations, we demonstrate how the interplay of non-equilibrium activity,
external driving, and magnetic interactions leads to the condensation of
swimmers at the center of the channel via a non-equilibrium phase transition
that is formally akin to Bose-Einstein condensation. We find that the effective
dynamics of the microswimmers can be mapped onto a diffusivity-edge problem,
and use the mapping to build a generalized thermodynamic framework, which is
verified by a parameter-free comparison with our simulations. Our work reveals
how driven active matter has the potential to generate exotic classical
non-equilibrium phases of matter with traits that are analogous to those
observed in quantum systems.",2010.02853v1
2020-10-21,Electric activity at magnetic moment fragmentation in spin ice,"Spin ice systems display a variety of very nontrivial properties, the most
striking being the existence in them of magnetic monopoles. Such monopole
states can also have nontrivial electric properties: there exist electric
dipoles attached to each monopole. A novel situation is encountered in the
moment fragmentation (MF) state, in which monopoles and antimonopoles are
perfectly ordered, whereas spins themselves remain disordered. We show that
such partial ordering strongly modifies the electric activity of such systems:
the electric dipoles, which are usually random and dynamic, become paired in
the MF state in (d,-d) pairs, thus strongly reducing their electric activity.
The electric currents existing in systems with noncoplanar spins are also
strongly influenced by MF. We also consider modifications in dipole and current
patterns in magnetic textures (domain walls, local defects) and at excitations
with nontrivial dynamics in a MF state, which show very rich behaviour and
which could in principle allow to control them by electric field.",2010.11149v2
2021-01-15,Linear and Nonlinear Optical Responses in Kitaev Spin Liquids,"We theoretically study THz-light-driven high-harmonic generation (HHG) in the
spin-liquid states of the Kitaev honeycomb model with a magnetostriction
coupling between spin and electric polarization. To compute the HHG spectra, we
numerically solve the Lindblad equation, taking account of the dissipation
effect. We find that isotropic Kitaev models possess a dynamical symmetry,
which is broken by a static electric field, analogous to HHG in electron
systems. We show that the HHG spectra exhibit characteristic continua of
Majorana fermion excitations, and their broad peaks can be controlled by
applying static electric or magnetic fields. In particular, the magnetic-field
dependence of the HHG spectra drastically differs from those of usual ordered
magnets. These results indicate that an intense THz laser provides a powerful
tool to observe dynamic features of quantum spin liquids.",2101.06081v2
2021-01-18,Two body problem on a sphere in the presence of a uniform magnetic field,"We investigate the motion of one and two charged non-relativistic particles
on a sphere in the presence of a magnetic field of uniform strength. For one
particle, the motion is always circular, and determined by a simple relation
between the velocity and the radius of motion. For two identical particles,
interacting via a cotangent potential, we show there are two families of
relative equilibria, called Type I and Type II. The Type I relative equilibria
exist for all strengths of the magnetic field, while those of Type II exist
only if the field is sufficiently strong. The same is true if the particles are
of equal mass but opposite charge. We also determine the stability of the two
families of relative equilibria.",2101.07170v3
2021-04-14,Spin waves propagating through a stripe magnetic domain structure and their applications to reservoir computing,"Spin waves propagating through a stripe domain structure and reservoir
computing with their spin dynamics have been numerically studied with focusing
on the relation between physical phenomena and computing capabilities. Our
system utilizes a spin-wave-based device that has a continuous magnetic garnet
film and 1-input/72-output electrodes on the top. To control
spatially-distributed spin dynamics, a stripe magnetic domain structure and
amplitude-modulated triangular input waves were used. The spatially-arranged
electrodes detected spin vector outputs with various nonlinear characteristics
that were leveraged for reservoir computing. By moderately suppressing
nonlinear phenomena, our system achieves 100$\%$ prediction accuracy in
temporal exclusive-OR (XOR) problems with a delay step up to 5. At the same
time, it shows perfect inference in delay tasks with a delay step more than 7
and its memory capacity has a maximum value of 21. This study demonstrated that
our spin-wave-based reservoir computing has a high potential for edge-computing
applications and also can offer a rich opportunity for further understanding of
the underlying nonlinear physics.",2104.06794v1
2021-04-27,Spatiotemporal Crossover between Low- and High-Temperature Dynamical Regimes in the Quantum Heisenberg Magnet,"The stranglehold of low temperatures on fascinating quantum phenomena in
one-dimensional quantum magnets has been challenged recently by the discovery
of anomalous spin transport at high temperatures. Whereas both regimes have
been investigated separately, no study has attempted to reconcile them. For
instance, the paradigmatic quantum Heisenberg spin-$1/2$ chain falls at low
temperature within the Tomonaga-Luttinger liquid framework, while its
high-temperature dynamics is superdiffusive and relates to the
Kardar-Parisi-Zhang universality class in $1+1$ dimensions. This Letter aims at
reconciling the two regimes. Building on large-scale matrix product state
simulations, we find that they are connected by a temperature-dependent
spatiotemporal crossover. As the temperature $T$ is reduced, we show that the
onset of superdiffusion takes place at longer length and timescales $\propto
1/T$. This prediction has direct consequences for experiments including nuclear
magnetic resonance: it is consistent with earlier measurements on the nearly
ideal Heisenberg $S=1/2$ chain compound Sr$_2$CuO$_3$, yet calls for new and
dedicated experiments.",2104.13393v2
2021-05-13,"Static and dynamic magnetic properties of honeycomb lattice antiferromagnets Na$_{2}M_{2}$TeO$_{6}$, $M$ = Co and Ni","The magnetic structures and spin dynamics of Na$_{2}$Co$_{2}$TeO$_{6}$ and
Na$_{2}$Ni$_{2}$TeO$_{6}$ are investigated by means of elastic and inelastic
neutron scattering measurements and the results are discussed in the context of
a generalized Kitaev-Heisenberg model on honeycomb lattice with strong
spin-orbit coupling. The large number of parameters involved in the Hamiltonian
model are evaluated by using an iterative optimization algorithm capable of
extracting model solutions and simultaneously estimating their uncertainty. The
analyses establish that both Co$^{2+}$ ($d^7$) and Ni$^{2+}$ ($d^8$)
antiferromagnets realize bond-dependent anisotropic nearest-neighbor
interactions, and support the theoretical predictions for the realization of
Kitaev physics in 3$d$ electron systems with effective spins $S$=1/2 and $S$=1.
Furthermore, by studying the Na-doped system Na$_{2.4}$Ni$_{2}$TeO$_{6}$, we
show that the control of Na content can provide an effective route for fine
tuning the magnetic lattice dimensionality, as well as to controlling the
bond-dependent anisotropic interactions.",2105.06549v2
2021-05-31,Far-from-equilibrium universality in the two-dimensional Heisenberg model,"We characterize the universal far-from-equilibrium dynamics of the
two-dimensional quantum Heisenberg magnet isolated from its environment. For a
broad range of initial conditions, we find a long-lived universal prethermal
regime characterized by self-similar behavior of spin-spin correlations. We
analytically derive the spatial-temporal scaling exponents and find excellent
agreement with numerics using phase space methods. The scaling exponents are
insensitive to the choice of initial conditions, which include coherent and
incoherent spin states with values of total magnetization and energy in a wide
range. Compared to previously-studied self-similar dynamics in non-equilibrium
$O(n)$ field theories and Bose gases, we find qualitatively distinct scaling
behavior originating from the presence of spin modes which remain gapless at
long times and which are protected by the global SU(2) symmetry. Our
predictions, which suggest a new non-equilibrium universality class, are
readily testable in ultra-cold atoms simulators of Heisenberg magnets.",2106.00023v2
2021-06-17,Large deviations for the Skew-Detailed-Balance Lifted-Markov processes to sample the equilibrium distribution of the Curie-Weiss model,"Among the Markov chains breaking detailed-balance that have been proposed in
the field of Monte-Carlo sampling in order to accelerate the convergence
towards the steady state with respect to the detailed-balance dynamics, the
idea of 'Lifting' consists in duplicating the configuration space into two
copies $\sigma=\pm$ and in imposing directed flows in each copy in order to
explore the configuration space more efficiently. The skew-detailed-balance
Lifted-Markov-chain introduced by K. S. Turitsyn, M. Chertkov and M. Vucelja
[Physica D Nonlinear Phenomena 240 , 410 (2011)] is revisited for the
Curie-Weiss mean-field ferromagnetic model, where the dynamics for the
magnetization is closed. The large deviations at various levels for empirical
time-averaged observables are analyzed and compared with their detailed-balance
counterparts, both for the discrete extensive magnetization $M$ and for the
continuous intensive magnetization $m=\frac{M}{N}$ for large system-size $N$.",2106.09429v3
2021-09-05,Electric field effect on electron gas spins in two-dimensional magnets with strong spin-orbit coupling,"The recent rise of material platforms combining magnetism and
two-dimensionality of mobile carriers reveals a diverse spectrum of spin-orbit
phenomena and stimulates its ongoing theoretical discussions. In this work we
use the density matrix approach to provide a unified description of subtle
microscopic effects governing the electron gas spin behavior in the clean limit
upon electric perturbations in two-dimensional magnets with strong spin-orbit
coupling. We discuss that an inhomogeneity of electrostatic potential generally
leads to the electron gas spin tilting with the subsequent formation of
equilibrium skyrmion-like spin textures and demonstrate that several
microscopic mechanisms of 2DEG spin response are equally important for this
effect. We analyze the dynamics of 2DEG spin upon an oscillating electric field
with a specific focus on the emergent electric dipole spin resonance. We
address the resonant enhancement of magneto-optical phenomena from the spin
precession equation perspective and discuss it in terms of the resonant spin
generation. We also clarify the connection of both static and dynamic spin
phenomena arising in response to a scalar perturbation with the electronic band
Berry curvature.",2109.02151v1
2010-05-27,Exact reduced dynamics for a qubit in a precessing magnetic field and in the contact with a heat-bath,"The two-level quantum system (qubit) in a precessing magnetic field and in
contact with a heat bath is investigated. The exact reduced dynamics for the
qubit in question is obtained. We apply the approach based on the block
operator matrices theory, in contrast with the standard methods provided by the
theory of the open quantum systems. We also present the solution of the Riccati
operator equation associated with the Hamiltonian of the system. Next, we study
the adiabatic approximation for the system under consideration using quantum
fidelity as a way to measure validity of the adiabatic theory. We find that in
the weak coupling domain the standard condition that ensures adiabatic behavior
of the spin in the magnetic field also guarantees its adiabatic evolution in
the open system variant of this model. Therefore, we provide the explicit
example of the open quantum system that satisfies the adiabatic approximation
firstly formulated for the closed quantum systems.",1005.5182v2
2010-05-29,{\it Ab initio} calculations of magnetic structure and lattice dynamics of Fe/Pt multilayers,"The magnetization distribution, its energetic characterization by the
interlayer coupling constants and lattice dynamics of (001)-oriented Fe/Pt
multilayers are investigated using density functional theory combined with the
direct method to determine phonon frequencies. It is found that ferromagnetic
order between consecutive Fe layers is favoured, with the enhanced magnetic
moments at the interface. The bilinear and biquadratic coupling coefficients
between Fe layers are shown to saturate fast with increasing thickness of
nonmagnetic Pt layers which separate them. The phonon calculations demonstrate
a rather strong dependence of partial iron phonon densities of states on the
actual position of Fe monolayer in the multilayer structure.",1005.5454v1
2013-08-20,Quantum Dynamics of a Dissipative and Confined Cyclotron Motion,"We study the dissipative dynamics of a charged oscillator in a magnetic field
by coupling (a la Caldeira and Leggett) it to a heat bath consisting of
non-interacting harmonic oscillators. We derive here the auto-correlation
functions of the position and momentum and study its behavior at various
limiting situations. The equilibrium (steady state) dispersions of position and
momentum are obtained from their respective autocorrelation functions. We
analyse the equilibrium position and momentum dispersions at low and high
temperatures for both low and high magnetic field strengths. We obtain the
classical diffusive behavior (at long times) as well as the equilibrium
momentum dispersion of the free quantum charged particle in a magnetic field,
in the limit of vanishing oscillator potential {\omega}_0 . We establish the
relations between the reduced partition function and the equilibrium
dispersions of the dissipative and confined cyclotron problem.",1308.4389v1
2016-11-29,Domain wall profiles in Co/Ir$_n$/Pt(111) ultrathin films: influence of the Dzyaloshinskii-Moriya interaction,"We perform a study of domain walls in Co/Ir$_n$/Pt(111) (n = 0,1,...,6) films
by a combined approach of first-principles calculations and spin dynamics
simulations. We determine the tensorial exchange interactions and the magnetic
anisotropies for the Co overlayer in both FCC and HCP geometries, depending on
the number of Ir buffer layers. We find strong ferromagnetic nearest-neighbor
isotropic exchange interactions between the Co atoms and an out-of-plane
magnetic anisotropy for the films in FCC geometry. Our simulations show that
the magnetic domain walls are of N\'eel type, and their rotational sense
(chirality) is changed upon the insertion of an Ir buffer layer as compared to
the pristine Co/Pt(111) system. Our spin dynamics simulations indicate a
twisting of the spins with respect to the planar domain wall profile on the
triangular lattice. We discuss this domain wall twisting using symmetry
arguments and in terms of an appropriate micromagnetic continuum model
considering extra energy terms compared to the available literature.",1611.09518v1
2017-01-18,Dissipative random quantum spin chain with boundary-driving and bulk-dephasing: magnetization and current statistics in the Non-Equilibrium-Steady-State,"The Lindblad dynamics with dephasing in the bulk and magnetization-driving at
the two boundaries is studied for the quantum spin chain with random fields
$h_j$ and couplings $J_j$ (that can be either uniform or random). In the regime
of strong disorder in the random fields, or in the regime of strong
bulk-dephasing, the effective dynamics can be mapped onto a classical Simple
Symmetric Exclusion Process with quenched disorder in the diffusion coefficient
associated to each bond. The properties of the corresponding
Non-Equilibrium-Steady-State in each disordered sample between the two
reservoirs are studied in detail by extending the methods that have been
previously developed for the Symmetric Exclusion Process without disorder.
Explicit results are given for the magnetization profile, for the two-point
correlations, for the mean current and for the current fluctuations, in terms
of the random fields and couplings defining the disordered sample.",1701.05090v2
2017-09-07,Fluxtube Dynamics in Neutron Star Cores,"Although the detailed structure of neutron stars remains unknown, their
equilibrium temperatures lie well below the Fermi temperature of dense nuclear
matter, suggesting that the nucleons in the stars' core form Cooper pairs and
exhibit macroscopic quantum behavior. The presence of such condensates impacts
on the neutron stars' large scale properties. Specifically, superconducting
protons in the outer core (expected to show type-II properties) alter the
stars' magnetism as the magnetic field is no longer locked to the charged
plasma but instead confined to fluxtubes. The motion of these structures
governs the dynamics of the core magnetic field. To examine if field evolution
could be driven on observable timescales, several mechanisms affecting the
fluxtube distribution are addressed and characteristic timescales for realistic
equations of state estimated. The results suggest that the corresponding
timescales are not constant but vary for different densities inside the star,
generally being shortest close to the crust-core interface.",1709.02303v1
2017-09-13,Emergent Hydrodynamic Bound States Between Magnetically Powered Micropropellers,"Hydrodynamic interactions (HIs), namely solvent mediated long-range
interactions between dispersed particles, play a crucial role in the assembly
and dynamics of many active systems, from swimming bacteria to swarms of
propelling microrobots. Here we experimentally demonstrate the emergence of
long-living hydrodynamic bound states between model micro-swimmers at low
Reynolds number. A rotating magnetic field forces colloidal hematite
microparticles to translate at a constant and frequency-tunable speed close to
a bounding plane in a viscous fluid. At high driving frequency, HIs dominate
over magnetic dipolar ones, and close propelling particles couple into bound
states by adjusting their translational speed in order to optimize the
transport of the pair. The physical system is described by considering the HIs
with the boundary surface and the effect of gravity, providing an excellent
agreement with the experimental data for all the range of parameters explored.
Moreover, we show that in dense suspensions, these bound states can be extended
to onedimensional arrays of particles assembled by the sole HIs. Our results
manifest the importance of the boundary surface in the interaction and dynamics
of confined propelling microswimmers.",1709.04229v1
2017-11-03,Dynamical correlation functions and the related physical effects in three-dimensional Weyl/Dirac semimetals,"We present a unified derivation of the dynamical correlation functions
including density-density, density-current and current-current, of
three-dimensional Weyl/Dirac semimetals by use of the Passarino-Veltman
reduction scheme at zero temperature. The generalized Kramers-Kronig relations
with arbitrary order of subtraction are established to verify these correlation
functions. Our results lead to the exact chiral magnetic conductivity and
directly recover the previous ones in several limits. We also investigate the
magnetic susceptibilities, the orbital magnetization and briefly discuss the
impact of electron interactions on these physical quantities within the random
phase approximation. Our work could provide a starting point for the
investigation of the nonlocal transport and optical properties due to the
higher-order spatial dispersion in three-dimensional Weyl/Dirac semimetals.",1711.01031v2
2017-11-11,The hydrodynamic singular vortex on the sphere and the Dirac monopole,"An exact correspondence is established between mathematical description of
the single ""elementary vortex"" (EV)velocity on the sphere
(Zermelo,1902;Bogomolov,1977) and the Dirac magnetic monopole (DMM) vector
potential (Dirac, 1931). Similar analogy with DMM was noted only for the
vortices in quantum fluid He-3A (Blaha,1976;Volovik,Mineev,1976). Singular EV
on a sphere is usually considered using compensating vortex field, uniformly
distributed over the sphere. It is necessary to meet the Gauss-Kelvin theorem
with zero integral vorticity over the sphere. However since Bogomolov
(1977)(see also Dritshel,Boatto,2015)uniform vorticity impact on dynamics is
neglected when only the velocity field of singular EV is taken into account. It
is not true because the hydrodynamic equations do not allow existence of a
solution in the form of an isolated EV but allow solutions in the form of the
antipodal EV pairs. It is also suggested the possibility of DMM existence only
in the form of the point magnetic dipoles, consisting of two DMM with different
signs of the magnetic charge.",1711.04124v1
2017-11-17,Small influence of magnetic ordering on lattice dynamics in TaFe$_{1.25}$Te$_3$,"Raman scattering spectra of zigzag spin chain TaFe$_{1.25}$Te$_3$ single
crystal are presented in a temperature range from 80 to 300 K. Nine Raman
active modes of $A_g$ and $B_g$ symmetry are clearly observed and assigned by
probing different scattering channels, which is confirmed by lattice dynamics
calculations. Temperature dependence of the Raman modes linewidth is mainly
governed by the lattice anharmonicity. The only deviation from the conventional
behavior is observed for $A_g$ symmetry modes in a vicinity of the magnetic
phase transition at $T_N \approx 200$ K. This implies that the electron-phonon
interaction weakly changes with temperature and magnetic ordering, whereas
small changes in the spectra near the critical temperature can be ascribed to
spin fluctuations.",1711.06572v1
2017-11-23,Physics of the saturation of particle acceleration in relativistic magnetic reconnection,"We investigate the saturation of particle acceleration in relativistic
reconnection using two-dimensional particle-in-cell simulations at various
magnetizations \sigma. We find that the particle energy spectrum produced in
reconnection quickly saturates as a hard power law that cuts off at
\gamma~4\sigma, confirming previous work. Using particle tracing, we find that
particle acceleration by the reconnection electric field in X-points determines
the shape of the particle energy spectrum. By analyzing the current sheet
structure, we show that physical cause of saturation is the spontaneous
formation of secondary magnetic islands that can disrupt particle acceleration.
By comparing the size of acceleration regions to the typical distance between
disruptive islands, we show that the maximum Lorentz factor produced in
reconnection is \gamma ~ 5 \sigma, which is very close to what we find in our
particle energy spectra. We also show that the dynamic range in Lorentz factor
of the power law spectrum in reconnection is < 40. The hardness of the power
law combined with its narrow dynamic range implies that relativistic
reconnection is capable of producing the hard narrowband flares observed in the
Crab Nebula but has difficulty producing the softer broadband prompt GRB
emission.",1711.08701v1
2018-01-29,Nonlinear Excitations in Magnetic Lattices with Long-Range Interactions,"We study - experimentally, theoretically, and numerically - nonlinear
excitations in lattices of magnets with long-range interactions. We examine
breather solutions, which are spatially localized and periodic in time, in a
chain with algebraically-decaying interactions. It was established two decades
ago [S. Flach, Phys. Rev. E 58, R4116 (1998)] that lattices with long-range
interactions can have breather solutions in which the spatial decay of the
tails has a crossover from exponential to algebraic decay. In this Letter, we
revisit this problem in the setting of a chain of repelling magnets with a mass
defect and verify, both numerically and experimentally, the existence of
breathers with such a crossover.",1801.09560v1
2018-03-04,Spin hydrodynamics in amorphous magnets,"Spin superfluidity, i.e., coherent spin transport mediated by topologically
stable textures, is limited by parasitic anisotropies rooted in relativistic
interactions and spatial inhomogeneities. Since structural disorder in
amorphous magnets can average out the effect of these undesired couplings, we
propose this class of materials as platforms for superfluid spin transport. We
establish nonlinear equations describing the hydrodynamics of spin in
insulating amorphous magnets, where the currents are defined in terms of
coherent rotations of a noncollinear texture. Our theory includes dissipation
and nonequilibrium torques at the interface with metallic reservoirs. This
framework allows us to determine different regimes of coherent dynamics and
their salient features in nonlocal magneto-transport measurements. Our work
paves the way for future studies on macroscopic spin dynamics in materials with
frustrated interactions.",1803.01309v2
2018-03-22,Carrier dynamics in doped bilayer iridates near magnetic quantum criticality,"Motivated by experiments on the carrier-doped bilayer iridate
$(\text{Sr}_{1-x}\text{La}_x)_3\text{Ir}_2\text{O}_7$, we study the dynamics of
a single doped electron in a bilayer magnet in the presence of spin-orbit
coupling, taking into account the spatially staggered rotation of IrO$_6$
octahedra. We employ an effective single-orbital bilayer $t$-$J$ model,
concentrating on the quantum paramagnetic phase near the magnetic quantum
critical point. We determine the carrier dispersion using a combination of
self-consistent Born and bond-operator techniques. Extrapolating to finite
small carrier density we find that, for experimentally relevant parameters, the
combination of octahedral rotation and spin-orbit coupling induces a band
folding which results in a Fermi surface of small double electron pockets, in
striking agreement with experimental observations. We also determine the
influence of spin-orbit coupling on the location of the quantum critical point
in the undoped case, and discuss aspects of the global phase diagram of doped
bilayer Mott insulators.",1803.08516v3
2018-05-03,North-South asymmetric Kelvin-Helmholtz instability and induced reconnection at the Earth's magnetospheric flanks,"We present a three-dimensional study of the plasma dynamics at the flank
magnetopause of the Earth's magnetosphere during mainly northward
interplanetary magnetic field (IMF) periods. Two-fluid simulations show that
the initial magnetic shear at the magnetopause and the field line bending
caused by the dynamics itself (in a configuration taken as representative of
the properties of the flank magnetopause) influence both the location where the
Kelvin-Helmholtz (KH) instability and the induced magnetic reconnection take
place and their nonlinear development. The KH vortices develop asymmetrically
with respect to the Earth's equatorial plane where the local KH linear growth
rate is maximal. Vortex driven reconnection processes take place at different
latitudes, ranging from the equatorial plane to mid-latitude regions, but only
in the hemisphere that turns out to be the less KH unstable. These results
suggest that KH-induced reconnection is not limited to specific regions around
the vortices (inside, below or above), but may be triggered over a broad and
continuous range of locations in the vicinity of the vortices.",1805.01466v1
2018-05-07,Force acting on a cluster of magnetic nanoparticles in a gradient field: a Langevin dynamics study,"Magnetophoretic force acting on a rigid spherical cluster of single-domain
nanoparticles in a constant-gradient weak magnetic field is investigated
numerically using the Langevin dynamics simulation method. Nanoparticles are
randomly and uniformly distributed within the cluster volume. They interact
with each other via long-range dipole-dipole interactions. Simulations reveal
that if the total amount of particles in the cluster is kept constant, the
force decreases with increasing nanoparticle concentration due to the
demagnetizing field arising inside the cluster. Numerically obtained force
values with great accuracy can be described by the modified mean-field theory,
which was previously successfully used for the description of various dipolar
media. Within this theory, a new expression is derived, which relates the
magnetophoretic mobility of the cluster with the concentration of nanoparticles
and their dipolar coupling parameter. The expression shows that if the number
of particles in the cluster is fixed, the mobility is a nonmonotonic function
of the concentration. The optimal concentration values that maximize the
mobility for a given amount of magnetic phase and a given dipolar coupling
parameter are determined.",1805.02347v2
2018-08-03,Vortex dynamics and losses due to pinning: Dissipation from trapped magnetic flux in resonant superconducting radio-frequency cavities,"We use a model of vortex dynamics and collective weak pinning theory to study
the residual dissipation due to trapped magnetic flux in a dirty
superconductor. Using simple estimates, approximate analytical calculations,
and numerical simulations, we make predictions and comparisons with experiments
performed in CERN and Cornell on resonant superconducting radio-frequency NbCu,
doped-Nb and Nb$_3$Sn cavities. We invoke hysteretic losses originating in a
rugged pinning potential landscape to explain the linear behavior of the
sensitivity of the residual resistance to trapped magnetic flux as a function
of the amplitude of the radio-frequency field. Our calculations also predict
and describe the crossover from hysteretic-dominated to viscous-dominated
regimes of dissipation. We propose simple formulas describing power losses and
crossover behavior, which can be used to guide the tuning of material
parameters to optimize cavity performance.",1808.01293v2
2018-08-10,Chaotic behavior of Eulerian MHD turbulence,"We study the chaotic properties of a turbulent conducting fluid using direct
numerical simulation in the Eulerian frame. The maximal Lyapunov exponent is
measured for simulations with varying Reynolds number and magnetic Prandtl
number. We extend the Ruelle theory of hydrodynamic turbulence to
magnetohydrodynamic turbulence as a working hypothesis and find broad agreement
with results. In other simulations we introduce magnetic helicity and these
simulations show a diminution of chaos, which is expected to be eliminated at
maximum helicity. We also find that the difference between two initially close
fields grows linearly at late times, which was also recently found in
hydrodynamics. This linear growth rate is found to be dependent on the
dissipation rate of the relevant field. We discuss the important consequences
this linear growth has on predictability. We infer that the chaos in the system
is totally dominated by the velocity field and connect this work to real
magnetic systems such as solar weather and confined plasmas.",1808.03632v2
2018-08-27,Exact equilibrium state for pair plasmas near the horizon of a Schwarzschild black hole,"A pair plasma in hidrostatic equilibrium near the horizon of a Schwarzschild
black hole is an exact state. In this work, we show that an exact
non--hidrostatic equilibrium state can be sustained for a pair plasma, in which
its constituents are moving near the horizon of the black hole. This
equilibrium state solution is not due to a vortical formalism, but by the
balance of forces in the plasma. This new equilibrium state has a pressure for
the hydrostatic equilibrium, although the plasma moves. The equilibrium is
achieved by the generation of a magnetic field that emerges by the interplay
between the plasma dynamics and the gravitational field, having no flat
spacetime analogue. No electric fields are needed to sustain the equilibria.
The magnetic field shows an exponential decaying behavior, governed by a
function that depends on the properties of the pair plasma and the black, and
that plays the role of a characteristc length of the dynamics of the magnetic
field.",1808.08937v3
2019-02-01,On the dynamics of a charged particle in magnetic fields with cylindrical symmetry,"We study the motion of a charged particle under the action of a magnetic
field with cylindrical symmetry. In particular we consider magnetic fields with
constant direction and with magnitude depending on the distance $r$ from the
symmetry axis of the form $1 + Ar^{-\gamma}$ as $r\to\infty$, with $A\ne 0$ and
$\gamma > 1$. With perturbative-variational techniques, we can prove the
existence of infinitely many trajectories whose projection on a plane
orthogonal to the direction of the field describe bounded curves given by the
superposition of two motions: a rotation with constant angular speed at a unit
distance about a point which moves along a circumference of large radius $\rho$
with a slow angular speed $\varepsilon$. The values $\rho$ and $\varepsilon$
are suitably related to each other. This problem has some interest also in the
context of planar curves with prescribed curvature.",1902.00513v1
2019-06-10,Simulating single-spin dynamics on an IBM five-qubit chip,"In this paper we show how the IBM superconducting chips can be a powerful
tool for teaching foundations of quantum mechanics for undergraduate students
(for graduates as well, in some cases). To this end, we briefly discuss about
the main elements of the IBM Quantum Experience platform necessary to
understand this paper, i.e., how to implement operations and single-qubit
measurements. We experimentally study the dynamics of single spin systems
interacting with static and time-dependent magnetic fields. First, we study the
resonant behavior of a single spin coupled to a time-dependent rotating
magnetic field. To end, we study the Larmor precession phenomenon. In both
cases we show the theoretical and real experimental implementation. This
article could be useful in introductory courses on quantum mechanics and
nuclear magnetic resonance foundations, for example.",1906.03925v2
2019-06-26,Chromospheric cannonballs on the Sun,"In the highly dynamic chromosphere, there exist many kinds of small-scale
activities, such as spicules, surges, and Ellerman bombs. Here, we report the
discovery of a new phenomenon in the chromosphere observed with the New Vacuum
Solar Telescope at the Fuxian Solar Observatory. In the high tempo-spatial
resolution H$\alpha$ images, some dark or bright structures are found to fly
along the curved trajectory, looking like cannonballs. Their average size,
mass, and velocity are about 1.5 $\times$ 10$^{9}$ km$^{3}$, 1.5 $\times$
10$^{8}$ kg, and 56 km s$^{-1}$, respectively. In the simultaneous
(extreme-)ultraviolet images obtained by the Solar Dynamics Observatory, these
cannonballs appear as brighter features compared to the surrounding area,
implying that there exists some kind of heating during this process. The
photospheric magnetograms show the magnetic flux emergence and interaction with
the pre-existing fields. These observations reveal that the cannonballs are
chromospheric material blobs launched due to the magnetic reconnection between
emerging magnetic flux and the pre-existing loops.",1906.10850v1
2019-06-26,Spin-orbit torques and their associated effective fields from gigahertz to terahertz,"Terahertz spintronics offers the prospect of devices which are both faster
and more energy-efficient. A promising route to achieve this goal is to exploit
current-induced spin-orbit torques. However, the high-frequency properties of
these quantities remain unexplored both experimentally and theoretically,
within a realistic material-specific approach. Here we investigate the
dynamical transverse components of the torques and uncover contributions
longitudinal to the magnetic moment capable of changing its magnitude. We show
that, while the torques can be drastically altered in the dynamical regime, the
effective magnetic fields that accompany them present a frequency-independent
behaviour, ranging from the static limit up to the terahertz domain - including
the ferromagnetic resonance of the system. The outcomes of this work point to
new ways to control magnetic units in next-generation spintronic devices.",1906.11314v2
2019-06-30,Quantitative Study on Current-Induced Effect in an Antiferromagnet Insulator/Pt Bilayer Film,"Quantitative investigation on the current-induced torque in antiferromagnets
represents a great challenge, due to the lack of an independent method for
controlling N\'eel vectors. Here by utilizing an antiferromagnetic insulator
with Dzyaloshinskii-Moriya interaction, {\alpha}-Fe2O3, we show that the N\'eel
vector can be controlled with a moderate external field, which is further
utilized to calibrate the current-induced magnetic dynamics. We find that the
current-induced magnetoresistance change in antiferromagnets can be complicated
by resistive switching that does not have a magnetic origin. By excluding
non-magnetic switching and comparing the current-induced dynamics with the
field-induced one, we determine the nature and magnitude of current-induced
effects in Pt/{\alpha}-Fe2O3 bilayer films.",1907.00314v3
2019-07-10,Dynamical mean-field study of Vanadium diselenide monolayer ferromagnetism,"To understand the magnetism of VSe$_2$, whose monolayer form has recently
been reported to be a room temperature ferromagnet, Here, the combined method
of conventional density functional theory with dynamical mean-field theory has
been adopted. This higher-level computation method enables us to resolve many
of existing controversies and contradictions in between theory and experiment.
First of all, this new approach is shown to give the correct magnetic
properties of both bulk and two-dimensional limit of VSe$_2$ which demonstrates
its superiority to the conventional methods. The results demonstrate that
monolayer VSe$_2$ without charge density waves is a ferromagnet with ordering
temperature of 250K. From the direct simulation of temperature-dependent
magnetic susceptibility and ordered moment, it is shown that its ferromagnetism
is clearly two-dimensional in nature. Further, it is shown that this
ferromagnetic order is vulnerable to extra charge dopings which provides the
important insight to elucidate recent experimental controversies.",1907.04790v3
2019-08-31,Element specificity of transient extreme ultra-violet magnetic dichroism,"In this work we combine theory and experiment to study transient magnetic
circular dichroism (tr-MCD) in the extreme ultraviolet spectral range (XUV) in
bulk Co and CoPt. We use the \emph{ab-initio} method of real-time
time-dependent density functional theory (RT-TDDFT) to simulate the
magnetization dynamics in the presence of ultrafast laser pulses. From this we
demonstrate how tr-MCD may be calculated using an approximation to the
excited-state linear-response. We apply this approximation to Co and CoPt and
show computationally that element-specific dynamics of the local spin moments
can be extracted from the tr-MCD in XUV energy range, as is commonly assumed.
We then compare our theoretical prediction for the tr-MCD for CoPt with
experimental measurement and find excellent agreement at many different
frequencies including the $M_{2 3}$-edge of Co and $N_{6 7}$- and $O_{2 3}$-
edges of Pt.",1909.00199v1
2019-12-02,The Effect of Anisotropic Viscosity on the Nonlinear Kink Instability,"The kink instability of magnetohydrodynamics is believed to be fundamental to
many aspects of the dynamic activity of the solar atmosphere, such as the
initiation of flares and the heating of the solar corona. In this work, we
investigate the importance of viscosity on the kink instability. In particular,
we focus on two forms of viscosity; isotropic viscosity (independent of the
magnetic field) and anisotropic viscosity (with a preferred direction following
the magnetic field). Through the detailed analysis of magnetohydrodynamic
simulations of the kink instability with both types of viscosity, we show that
the form of viscosity has a significant effect on the nonlinear dynamics of the
instability. The different viscosities allow for different flow and current
structures to develop, thus affecting the behaviour of magnetic relaxation, the
formation of secondary instabilities and the Ohmic and viscous heating
produced. Our results have important consequences for the interpretation of
solar observations of the kink instability.",1912.01076v1
2019-12-16,Slave-Boson analysis of the 2D Hubbard model,"We present a comprehensive study of the 2D one-band Hubbard model applying
the spin rotation invariant slave-boson method. We utilize a spiral magnetic
mean field and fluctuations around a paramagnetic mean field to determine the
magnetic phase diagram and find the two approaches to be in good agreement.
Apart from the commensurate phases characterized by ordering wave vectors
$\boldsymbol Q = (\pi, \pi), (0, \pi)$, and $(0, 0)$ we find incommensurate
phases where the ordering wave vectors $\boldsymbol Q = (Q, Q)$, and $(Q, \pi)$
vary continuously with filling, interaction strength or temperature. The mean
field quantities magnetization and effective mass are found to change
discontinuously at the phase boundaries separating the $(Q, Q)$ and $(Q, \pi)$
phases, indicating a first order transition. The band structure and Fermi
surface is shown in selected cases. The dynamic spin and charge
susceptibilities as well as the structure factors are calculated and discussed,
including the emergence of collective modes of the zero sound and Mott
insulator type. The dynamical conductivity is calculated in dependence of
doping, interaction strength and temperature. Finally, a
temperature-interaction strength phase diagram is established.",1912.07631v1
2019-12-20,Efficient Ab-initio Multiplet Calculations for Magnetic Adatoms on MgO,"Scanning probe microscopy and spectroscopy, and more recently in combination
with electron spin resonance, have allowed the direct observation of electron
dynamics on the single-atom limit. The interpretation of data is strongly
depending on model Hamiltonians. However, fitting effective spin Hamiltonians
to experimental data lacks the ability to explore a vast number of potential
systems of interest. By using plane-wave density functional theory (DFT) as
starting point, we build a multiplet Hamiltonian making use of
maximally-localized Wannier functions. The Hamiltonian contains spin-orbit and
electron-electron interactions needed to obtain the relevant spin dynamics. The
resulting reduced Hamiltonian is solved by exact diagonalization. We compare
three prototypical cases of 3d transition metals Mn (total spin $S=5/2$), Fe
($S=2$) and Co ($S=3/2$) on MgO with experimental data and find that our
calculations can accurately predict the spin orientation and anisotropy of the
magnetic adatom. Our method does not rely on experimental input and permits us
to explore and predict the fundamental magnetic properties of adatoms on
surfaces.",1912.09793v1
2020-02-28,Dynamic Nuclear Polarization by optical Stark effect in periodically-pumped gallium arsenide,"Optical pump-probe time- and magnetic-field-resolved Kerr rotation
measurements provide a window into the mechanisms that generate dynamic nuclear
polarization in bulk gallium arsenide. Previously, we have reported an
unexpected dependence of the direction of the nuclear polarization on the sweep
direction of the applied external magnetic field. In this paper, we present
numerical calculations based on a model for this nuclear polarization due to
the optical orientation and optical Stark effect produced by a train of
ultrafast optical pulses. We demonstrate the correspondence of the model to our
experimental measurements for different laser wavelengths and magnetic field
sweep rates. Finally, we show that the model reproduces the sweep direction
dependence and provides an explanation for this behavior.",2003.00050v1
2020-03-19,Magnetism and Néel skyrmion dynamics in GaV$_{4}$S$_{8-y}$Se$_{y}$,"We present an investigation of the influence of low-levels of chemical
substitution on the magnetic ground state and N{\' e}el skyrmion lattice (SkL)
state in GaV$_4$S$_{8-y}$Se$_y$, where $y =0, 0.1, 7.9$, and $8$. Muon-spin
spectroscopy ($\mu$SR) measurements on $y=0$ and 0.1 materials reveal the
magnetic ground state consists of microscopically coexisting incommensurate
cycloidal and ferromagnetic environments, while chemical substitution leads to
the growth of localized regions of increased spin density. $\mu$SR measurements
of emergent low-frequency skyrmion dynamics show that the SkL exists under
low-levels of substitution at both ends of the series. Skyrmionic excitations
persist to temperatures below the equilibrium SkL in substituted samples,
suggesting the presence of skyrmion precursors over a wide range of
temperatures.",2003.08682v2
2020-04-22,Suppression of indirect exchange and symmetry breaking in antiferromagnetic metal with dynamic charge stripes,"Precise angle-resolved magnetoresistance (ARM) measurements are applied to
reveal the origin for the lowering of symmetry in electron transport and the
emergence of a huge number of magnetic phases in the ground state of
antiferromagnetic metal HoB12 with fcc crystal structure. By analyzing of the
polar H-theta-phi magnetic phase diagrams of this compound reconstructed from
the experimental ARM data we argue that non-equilibrium electron density
oscillations (dynamic charge stripes) are responsible for the suppression of
the indirect RKKY exchange along <110> directions between the nearest
neighboring magnetic moments of Ho3+ ions in this strongly correlated electron
system.",2004.10421v1
2020-09-13,Light-matter interactions in synthetic magnetic fields: Landau-photon polaritons,"We study light-matter interactions in two dimensional photonic systems in the
presence of a spatially homogeneous synthetic magnetic field for light.
Specifically, we consider one or more two-level emitters located in the bulk
region of the lattice, where for increasing magnetic field the photonic modes
change from extended plane waves to circulating Landau levels. This change has
a drastic effect on the resulting emitter-field dynamics, which becomes
intrinsically non-Markovian and chiral, leading to the formation of strongly
coupled Landau-photon polaritons. The peculiar dynamical and spectral
properties of these quasi-particles can be probed with state-of-the-art
photonic lattices in the optical and the microwave domain and may find various
applications for the quantum simulation of strongly interacting topological
models.",2009.05952v2
2020-12-16,Magnetic domain wall dynamics in the precessional regime: influence of the Dzyaloshinskii-Moriya interaction,"The domain wall dynamics driven by an out of plane magnetic field was
measured for a series of magnetic trilayers with different strengths of the
interfacial Dzyaloshinskii-Moriya interaction (DMI). The features of the
field-driven domain wall velocity curves strongly depend on the amplitude of
the HD field stabilising chiral N\'eel walls. The measured Walker velocity,
which in systems with large DMI is maintained after the Walker field giving
rise to a velocity plateau up to the Slonczewski field HS, can be easily
related to the DMI strength. Yet, when the DMI field HD and the domain wall
demagnetising field HDW have comparable values, a careful analysis needs to be
done in order to evaluate the impact of the DMI on the domain wall velocity. By
means of a one-dimensional model and 2D simulations, we successfully extend
this method to explain experimental results to cases where HD and HDW are
comparable.",2012.09277v2
2020-12-17,Curvature-driven AC-assisted creep dynamics of magnetic domain walls,"The dynamics of micrometer-sized magnetic domains in ultra-thin ferromagnetic
films is so dramatically slowed down by quenched disorder that the spontaneous
elastic tension collapse becomes unobservable at ambient temperature. By
magneto-optical imaging we show that a weak zero-bias AC magnetic field can
assist such curvature-driven collapse, making the area of a bubble to reduce at
a measurable rate, in spite of the negligible effect that the same curvature
has on the average creep motion driven by a comparable DC field. An analytical
model explains this phenomenon quantitatively.",2012.09377v1
2020-12-29,Bloch point-mediated skyrmion annihilation in three dimensions,"The creation and annihilation of magnetic skyrmions are mediated by three
dimensional topological defects known as Bloch points. Investigation of such
dynamical processes is important both for understanding the emergence of exotic
topological spin textures, and for future engineering of skyrmions in
technological applications. However, while the annihilation of skyrmions has
been extensively investigated in two dimensions, in three dimensions the phase
transitions are considerably more complex. We report field-dependent
experimental measurements of metastable skyrmion lifetimes in an archetypal
chiral magnet, revealing two distinct regimes. Comparison to supporting
three-dimensional geodesic nudged elastic band simulations demonstrates that
these correspond to skyrmion annihilation into either the helical or conical
states, each exhibiting a different transition mechanism. The results highlight
that the lowest energy magnetic configuration of the system plays a crucial
role when considering the emergence of topological spin structures via
defect-mediated dynamics, and their stability in future devices.",2012.14813v1
2021-02-08,Extended space and time correlations in strongly magnetized plasmas,"Molecular dynamics simulations are used to show that strong magnetization
significantly increases the space and time scales associated with interparticle
correlations. The physical mechanism responsible is a channeling effect whereby
particles are confined to move along narrow cylinders with a width
characterized by the gyroradius and a length characterized by the collision
mean free path. The predominant interaction is $180^\circ$ collisions at the
ends of the collision cylinders, resulting in a long-range correlation parallel
to the magnetic field. Its influence is demonstrated via the dependence of the
velocity autocorrelation functions and self-diffusion coefficients on the
domain size and run time in simulations of the one-component plasma. A very
large number of particles, and therefore domain size, must be used to resolve
the long-range correlations, suggesting that the number of charged particles in
the collection must increase in order to constitute a plasma. Correspondingly,
this effect significantly delays the time it takes to reach a diffusive regime,
in which the mean square displacement of particles increases linearly in time.
This result presents challenges for connecting measurements in non-neutral and
ultracold neutral plasma experiments, as well as molecular dynamics
simulations, with fluid transport properties due to their finite size.",2102.04289v1
2021-02-12,Sb-5s lone pair dynamics and collinear magnetic ordering in Ba2FeSbSe5,"Neutron diffraction and X-ray pair distribution function (XPDF) experiments
were performed in order to investigate the magnetic and local crystal
structures of Ba2FeSbSe5 and to compare them to the average (i.e. long-range)
structural model, previously obtained by single crystal X-ray diffraction.
Changes in the local crystal structure (i.e. in the second coordination sphere)
are observed upon cooling from 295 K to 95 K resulting in deviations from the
average (i.e. long-range) crystal structure. This work demonstrates, that these
observations cannot be explained by local or long-range magnetoelastic effects
involving Fe-Fe correlations. Instead, we found, that the observed differences
between local and average crystal structure can be explained by Sb-5s lone pair
dynamics. We also find, that below the N\'eel temperature (TN = 58 K), the two
distinct magnetic Fe3+ sites order collinearly, such that a combination of
antiparallel and parallel spin arrangements along the b-axis results. The
nearest-neighbor arrangement (J1 = 6 {\AA}) is fully antiferromagnetic, while
next-nearest-neighbor interactions are ferromagnetic in nature.",2102.06484v1
2021-03-11,Partial Meissner effect measurement by a superconducting magnetic flux lens,"Magnetic flux trapping in the Meissner transition of superconducting radio
frequency cavities can substantially increase dissipation which impacts
cryogenic costs and necessitates expensive magnetic shielding. Recent findings
point at the material preparation and the cool down dynamics at the vortex
state as the main ways to counteract flux trapping. Most of the related
measurements are performed on the cavity, with risk of results being impacted
by the cool down specifics of each facility. We demonstrate an alternative
experiment in which flux expulsion is quantified from sheet material, with
samples conduction cooled in a stand-alone instrument designed to collimate
expelled flux at controlled cool down conditions. A series of tests reliably
and reproducibly relate the amount of expelled flux to the cooling rate, the
velocity of the superconducting front and the spatial temperature gradient. The
results are in line with tests performed on bare cavities and show that
expulsion of trapped flux improves with the spatial temperature gradient,
independent on the cooling rate. Our concept offers means to control the
dynamics of the Meissner effect and can be used for material qualification
prior to cavity fabrication.",2103.06697v1
2021-03-14,Transverse spin dynamics in the anisotropic Heisenberg model realized with ultracold atoms,"In Heisenberg models with exchange anisotropy, transverse spin components are
not conserved and can decay not only by transport, but also by dephasing. Here
we utilize ultracold atoms to simulate the dynamics of 1D Heisenberg spin
chains, and observe fast, local spin decay controlled by the anisotropy.
Additionally, we directly observe an effective magnetic field created by
superexchange which causes an inhomogeneous decay mechanism due to variations
of lattice depth between chains, as well as dephasing within each chain due to
the twofold reduction of the effective magnetic field at the edges of the
chains and due to fluctuations of the effective magnetic field in the presence
of mobile holes. The latter is a new coupling mechanism between holes and
magnons. All these dephasing mechanisms, corroborated by extensive numerical
simulations, have not been observed before with ultracold atoms and illustrate
basic properties of the underlying Hubbard model.",2103.07866v1
2021-07-05,Transport through dynamic pseudo-gauge fields and snake states in a Corbino geometry,"We propose a Corbino-disk geometry of a graphene membrane under out-of-plane
strain deformations as a convenient path to detect pseudo-magnetic and electric
fields via electronic transport. The three-fold symmetric pseudo-magnetic field
changes sign six times as function of angle, leading to snake states connecting
the inner and outer contacts and to nearly quantized transport. For dynamical
strain obtained upon AC gating, the system supports an AC pseudo-electric field
which, in the presence of the pseudo-magnetic field, produces a net electronic
charge current in the absence of an external voltage, via a pseudo-Hall effect.",2107.02209v2
2021-07-20,Spatio temporal analysis of waves in compressively driven magnetohydrodynamics turbulence,"Using direct numerical simulations (DNSs), the interaction between linear
waves and turbulence under the compressible magnetohydrodynamic (CMHD) approach
was studied. A set of DNSs in three dimensions for a spatial resolution of
$128^3$ and $256^3$ were performed. A parametric study was carried out varying
the sonic Mach number, the mean magnetic field and the compressibility
amplitude of the forcing. Spatio-temporal spectra of the magnetic energy were
built and analyzed, allowing for direct identification of all wave modes in a
CMHD turbulent system and quantification of the amount of energy in each mode
as a function of the wave number. Thus, linear waves were detected, that is
Alfv\'en waves and fast and slow magnetosonic waves. Furthermore, different
responses of the plasma were found according to whether the Mach number or the
mean magnetic field was varied. On the other hand, making use of
spatio-temporal spectra and two different integration methods, we accurately
quantified the amount of energy present in each of the normal modes. Finally,
although the presence of linear waves was observed, in all the cases studied
the system was mainly dominated by the non-linear dynamics of the plasma.",2107.09212v1
2021-07-25,Critical and geometric properties of magnetic polymers across the globule-coil transition,"We study a lattice model of a single magnetic polymer chain, where Ising
spins are located on the sites of a lattice self-avoiding walk in $d=2$. We
consider the regime where both conformations and magnetic degrees of freedom
are dynamic, thus the Ising model is defined on a dynamic lattice and
conformations generate an annealed disorder. Using Monte Carlo simulations, we
characterize the globule-coil and ferromaget-to-paramagnet transitions, which
occur simultaneously at a critical value of the spin-spin coupling. We argue
that the transition is continuous - in contrast to $d=3$ where it is
first-order. Our results suggest that at the transition the metric exponent
takes the theta-polymer value $\nu=4/7$ but the crossover exponent $\phi
\approx 0.7$, which differs from the expected value for a $\theta$-polymer.",2107.11830v2
2021-08-19,On self and mutual winding helicity,"The topological underpinning of magnetic fields connected to a planar
boundary is naturally described by field line winding. This observation leads
to the definition of winding helicity, which is closely related to the more
commonly calculated relative helicity. Winding helicity, however, has several
advantages, and we explore some of these in this work. In particular, we show,
by splitting the domain into distinct subregions, that winding helicity can be
decomposed naturally into ""self"" and ""mutual"" components and that these
quantities can be calculated, in practice, for magnetic fields with complex
geometries and topologies. Further, winding provides a unified topological
description from which known expressions for self and mutual helicity can be
readily derived and generalized. We illustrate the application of calculating
self and mutual winding helicities in a simulation of an evolving magnetic
field with non-trivial field line topology.",2108.08888v1
2021-10-02,Higgs and Goldstone spin-wave modes in striped magnetic texture,"Spontaneous symmetry breaking is ubiquitous in physics. Its spectroscopic
signature consists in the softening of a specific mode upon approaching the
transition from the high symmetry side and its subsequent splitting into a
zero-frequency ""Goldstone"" mode and a non-zero-frequency ""Higgs"" mode. Although
they determine the whole system dynamics, these features are difficult to
address in practice because of their vanishing coupling to most experimental
probes and/or their strong interaction with other fluctuations. In this work,
we consider a periodic magnetic modulation occurring in a ferromagnetic film
with perpendicular-to-plane magnetic anisotropy and directly observe its
Goldstone and Higgs spin-wave modes at room temperature using microwave and
optical techniques. This simple system constitutes a particularly convenient
platform for further exploring the dynamics of symmetry breaking.",2110.00882v2
2021-10-09,Field induced spin freezing and low temperature heat capacity of disordered pyrochlore oxide Ho$_{2}$Zr$_{2}$O$_{7}$,"Spin ice materials are the model systems that have a zero-point entropy as
\textit{T} $\rightarrow$ 0 K, owing to the frozen disordered states. Here, we
chemically alter the well-known spin ice Ho$_{2}$Ti$_{2}$O$_{7}$ by replacing
Ti sites with isovalent but larger Zr ion. Unlike the Ho$_{2}$Ti$_{2}$O$_{7}$
which is a pyrochlore material, Ho$_{2}$Zr$_{2}$O$_{7}$ crystallizes in
disordered pyrochlore structure. We have performed detailed structural, ac
magnetic susceptibility and heat capacity studies on Ho$_{2}$Zr$_{2}$O$_{7}$ to
investigate the interplay of structural disorder and frustrated interactions.
The zero-field ground state exhibits large magnetic susceptibility and remains
dynamic down to 30 mK without showing Pauling's residual entropy. The dynamic
state is suppressed continuously with the magnetic field and freezing
transition evolves ($\sim$ 10 K) at a field of $\sim$ 10 kOe. These results
suggest that the alteration of chemical order and local strain in
Ho$_{2}$Ti$_{2}$O$_{7}$ prevents the development of spin ice state and provides
a new material to study the geometrical frustration based on the structure.",2110.04555v1
2021-10-28,Optical dynamic nuclear polarization of $^{13}$C spins in diamond at a low field with multi-tone microwave irradiation,"Most of dynamic nuclear polarization (DNP) has been requiring helium
cryogenics and strong magnetic fields for a high degree of polarization. In
this work, we instead demonstrate an optical hyperpolarization of naturally
abundant $^{13}$C nuclei in a diamond crystal at a low magnetic field and an
ambient temperature. It exploits continuous irradiations of pump laser for
polarizing electron spins of nitrogen vacancy centers and microwave for
transferring the induced polarization to $^{13}$C nuclear spins. Triplet
structures corresponding to $^{14}$N hyperfine splitting were clearly observed
in the spectrum of $^{13}$C polarization. The powers of microwave irradiation
and pump laser were optimized. By simultaneously irradiating three microwave
frequencies matching to the peaks of the triplet, we achieved a $^{13}$C bulk
polarization of 0.113 %, leading to an enhancement of about a factor of 90,000
over the thermal polarization at 17.6 mT. We believe that the multi-tone
irradiation can be universally adopted to further enhance the $^{13}$C
polarization at a low magnetic field.",2110.14850v1
2021-11-07,Absence of temperature-dependent phonon anomalies in Sr2IrO4 and Sr3Ir2O7,"Following previous works reporting an anomalous behavior of several
zone-center optical phonons across the magnetic transition of square lattice
iridates Sr2IrO4 and Sr3Ir2O7, we have investigated the lattice dynamics as a
function of momentum in these materials by means of high-resolution inelastic
x-ray scattering (IXS). The observed phonon energies and scattering intensities
across the Brillouin zone are in excellent agreement with ab-initio lattice
dynamical calculations based on non-magnetic density-functional-perturbation
theory (DFPT). Our results do not evidence any renormalization of the phonons
at finite momentum across the magnetic transition of Sr2IrO4. The only
anomalous behavior was detected for the in-plane polarized
longitudinal-acoustic phonon branch in Sr3Ir2O7, which anomalously softens
towards low temperatures and might be related to anisotropic negative thermal
expansion in this compound. No anomalies related to potential charge ordering
were observed.",2111.04081v1
2021-11-15,Phase diagram and post-quench dynamics in a double spin-chain system in transverse fields,"We propose and explore the physics of a toy multiferroic model by coupling
two distinct dipolar XXZ models in transverse fields. We determine first the
rich ground-state phase diagram of the model using density matrix
renormalization group techniques. Then, we explore the dynamics of the system
after global and local quenches, using the time-evolving block decimation
algorithm. After a global quench, the system displays decaying coupled
oscillations of the electric and magnetic spins, in agreement with the
Eigenstate Thermalization Hypothesis (ETH) for many-body interacting quantum
systems. Notably, the spin-spin interactions lead to a sizeable quadratic shift
in the oscillation frequency as the inter-chain coupling is increased. Local
quenches lead to a light-cone-like propagation of excitations. In this case,
the inter-chain coupling drives a transfer of energy between the chains that
generates a novel fast spin-wave mode along the 'magnetic' chain at the speed
of the 'electric' spin-wave. This suggests a limited control mechanism for
faster information transfer in magnetic spin chains using electric fields that
harnesses the electric dipoles as intermediaries.",2111.07831v1
2021-11-26,Thermally populated versus field-induced triplon bound states in the Shastry-Sutherland lattice SrCu$_2$(BO$_3$)$_2$,"The Shastry-Sutherland compound SrCu$_2$(BO$_3$)$_2$ constituting
orthogonally coupled dimers harbors a $S=0$ singlet ground state. The
confluence of strong interdimer interaction and frustration engenders a
spectrum of low-energy excitations including localized triplons as well as
singlet and triplet bound states. Their dynamics are controlled by an external
magnetic field and temperature. Here, we employ high-field Raman spectroscopy
to map the field and temperature evolution of such bosonic composite
quasiparticles on approaching the 1/8 magnetization plateau. Our study unveils
that the magnetic field and thermal fluctuations show remarkably similar
effects in melting the singlet bound states, but are disparate in their effects
on the fine spectral shapes. This, together with the anti-crossing of two
singlet bound states in the intermediate field $B=10-16$ T, is discussed in
terms of the correlated dynamics of frustrated, interacting bosons.",2111.13354v1
2021-11-30,First and second order magnetic anisotropy and damping of europium iron garnet under high strain,"Understanding and tailoring static and dynamic properties of magnetic
insulator thin films is important for spintronic device applications. Here, we
grow atomically flat epitaxial europium iron garnet (EuIG) thin films by pulsed
laser deposition on (111)-oriented garnet substrates with a range of lattice
parameters. By controlling the lattice mismatch between EuIG and the
substrates, we tune the strain in EuIG films from compressive to tensile
regime, which is characterized by X-ray diffraction. Using ferromagnetic
resonance, we find that in addition to the first-order perpendicular magnetic
anisotropy which depends linearly on the strain, there is a significant
second-order one that has a quadratic strain dependence. Inhomogeneous
linewidth of the ferromagnetic resonance increases notably with increasing
strain, while the Gilbert damping parameter remains nearly constant (~
2x10^-2). These results provide valuable insight into the spin dynamics in
ferrimagnetic insulators and useful guidance for material synthesis and
engineering of next-generation spintronics applications.",2111.15142v1
2021-12-02,Experimental study of submerged liquid metal jet in transverse magnetic field,"A liquid metal flow in the form of a submerged round jet entering a square
duct in the presence of a transverse magnetic field is studied experimentally.
A range of high Reynolds and Hartmann numbers is considered. Flow velocity is
measured using electric potential difference probes. A detailed study of the
flow in the duct's cross-section about seven jet's diameters downstream of the
inlet reveals the dynamics, which is unsteady and dominated by high-amplitude
fluctuations resulting from instability of the jet. The flow structure and
fluctuation properties are largely determined by the value of the Stuart number
N. At moderate N, the mean velocity profile retains a central jet with
three-dimensional perturbations increasingly suppressed by the magnetic field
as N grows. At higher values of N, the flow becomes quasi-two-dimensional and
acquires the form of an asymmetric macrovortex, with high-amplitude velocity
fluctuations reemerging.",2112.01600v1
2021-12-08,Giant Resonant Self-Blocking in Magnetophononically Driven Quantum Magnets,"Magnetophononics, the modulation of magnetic interactions by driven
infrared-active lattice excitations, is emerging as a key mechanism for the
ultrafast dynamical control of both semiclassical and quantum spin systems by
coherent light. We demonstrate that, in a quantum magnetic material with strong
spin-phonon coupling, resonances between the driven phonon and the spin
excitation frequencies exhibit a giant self-blocking effect. Instead of
absorbing more energy, the spin system acts as a strong brake on the driven
phonon, causing it to absorb only a tiny fraction of the power available from
the laser. Using the quantum master equations governing the nonequilibrium
steady states of the coupled spin-lattice system, we show how self-blocking
dominates the dynamics, demonstrate the creation of mutually repelling hybrid
spin-phonon states, and control the nonequilibrium renormalization of the
lattice-driven spin excitation band.",2112.04508v1
2021-12-21,Nonreciprocal spin-wave dynamics in Pt/Co/W/Co/Pt multilayers,"We present a detailed study of the spin-wave dynamics in single Pt/Co/W and
double Pt/Co/W/Co/Pt ferromagnetic layer systems. The dispersion of spin waves
was measured by wavevector-resolved Brillouin light scattering spectroscopy
while the in-plane and out-of-plane magnetization curves were measured by
alternating gradient field magnetometry. The interfacial Dzyaloshinskii-Moriya
interaction induced nonreciprocal dispersion relation was demonstrated for both
single and double ferromagnetic layers and explicated by numerical simulations
and theoretical formulas. The results indicate the crucial role of the order of
layers deposition on the magnetic parameters. A significant difference between
the perpendicular magnetic anisotropy constant in double ferromagnetic layer
systems conduces to the decline of the interlayer interactions and different
dispersion relations for the spin-wave modes. Our study provides a significant
contribution to the realization of the multifunctional nonreciprocal magnonic
devices based on ultrathin ferromagnetic/heavy-metal layer systems.",2112.11206v1
2022-01-17,Consensus from group interactions: An adaptive voter model on hypergraphs,"We study the effect of group interactions on the emergence of consensus in a
spin system. Agents with discrete opinions $\{0,1\}$ form groups. They can
change their opinion based on their group's influence (voter dynamics), but
groups can also split and merge (adaptation). In a hypergraph, these groups are
represented by hyperedges of different sizes. The heterogeneity of group sizes
is controlled by a parameter $\beta$. To study the impact of $\beta$ on
reaching consensus, we provide extensive computer simulations and compare them
with an analytic approach for the dynamics of the average magnetization. We
find that group interactions amplify small initial opinion biases, accelerate
the formation of consensus and lead to a drift of the average magnetization.
The conservation of the initial magnetization, known for basic voter models, is
no longer obtained.",2201.06421v1
2022-01-18,Magnon Spectrum of the Amorphous Ferromagnet Co$_4$P from Atomistic Spin Dynamics,"An anomaly in the magnon dispersion of the amorphous ferromagnet Co$_{4}$P,
often referred to as a `roton-like' excitation, attracted much attention half a
century ago. With the current interest in heat and spin currents in amorphous
magnets, we apply modern simulation methods, combining reverse Monte Carlo to
build the atomic structure and the stochastic Landau-Lifshitz equation for spin
dynamics, to re-investigate the magnetic excitation spectrum. We find two
magnon valleys, one at the origin and another at a finite wavenumber close to
the observations, but without a magnon gap. We conclude that the second dip is
due to Umklapp scattering caused by residual long-range order, which may be an
alternative explanation of the putative roton excitation. Our study paves the
way to study magnon transport in amorphous magnets and related spintronic
applications.",2201.06986v1
2022-01-22,Universal criteria for single femtosecond pulse ultrafast magnetization switching in ferrimagnets,"Single-pulse switching has been experimentally demonstrated in ferrimagnetic
GdFeCo and Mn$_2$Ru$_x$Ga alloys. Complete understanding of single-pulse
switching is missing due to the lack of an established theory accurately
describing the transition to the non-equilibrium reversal path induced by
femtosecond laser photo-excitation. In this work we present general macroscopic
theory for the magnetization dynamics of ferrimagnetic materials upon
femtosecond laser excitation. Our theory reproduces quantitatively all stages
of the switching process observed in experiments. We directly compare our
theory to computer simulations using atomistic spin dynamics methods for both
GdFeCo and Mn$_2$Ru$_x$Ga alloys. We provide explicit expressions for the
magnetization relaxation rates in terms of microscopic parameters which allows
us to propose universal criteria for switching in ferrimagnets.",2201.09067v1
2022-02-15,Analysis of the robustness and dynamics of spin-locking preparations for the detection of oscillatory magnetic fields,"Extracting quantitative information of neuronal signals by non-invasive
imaging is an outstanding challenge for understanding brain function and
pathology. However, state-of-the-art techniques offer low sensitivity to deep
electrical sources. Stimulus induced rotary saturation (SIRS) is a recently
proposed magnetic resonance imaging (MRI) sequence that detects oscillatory
magnetic fields using a spin-lock preparation. Phantom experiments and
simulations proved its efficiency and sensitivity, but the susceptibility of
the method to field inhomogeneities is still not well understood. In this
study, we simulated and analyzed the dynamic of three spin-lock preparations
and their response to field inhomogeneities in the presence of a resonant
oscillating field. We show that the composite spin-lock preparation is more
robust against field variations within the double resonance effect. In
addition, we tested the capability of the chosen composite spin-lock
preparation to recover information about the spectral components of a composite
signal. This study sets the bases to move one step further towards the clinical
application of MR-based neuronal current imaging.",2202.07705v2
2022-03-22,"Lattice Dynamics, Mechanical Properties, Electronic Structure and Magnetic Properties of Equiatomic Quaternary Heusler Alloys CrTiCoZ (Z =Al,Si) using first principles calculations","First principles calculations are performed to investigate the
thermodynamical stability, dynamical, mechanical, electronic and magnetic
properties of CrTiCoZ (Z= Al/Si) novel quaternary Heusler alloys. Y-type III
atomic configuration is found to be the most stable structure for both
compounds. The melting temperatures of both compounds are as high as 2142 K and
2420 K for CrTiCoAl and CrTiCoSi, respectively. The electronic structure
calculations using GGA-PBE approach show a half metallic behavior of CrTiCoAl.
The spin-down channel exhibits a direct band gap of 0.15 eV, whereas the
spin-up channel is metallic making CrTiCoAl a half metallic ferromagnet with
100% spin polarization and an appreciable magnetic moment of -2 Bohr Magnetons.
The Curie temperature of CrTiCoAl is well above the room temperature (385K),
whereas that of CrTiCoSi is below the room temperature (203K).",2203.11858v1
2022-04-06,Melting of magnetic order in ${\mathrm{NaOsO}}_{3}$ by fs laser pulses,"NaOsO$_3$ has recently attracted significant attention for the strong
coupling between its electronic band structure and magnetic ordering. Here, we
used time-resolved magnetic X-ray diffraction to determine the timescale of the
photoinduced \afm dynamics in NaOsO$_3$. Our measurements are consistent with a
sub-100~fs melting of the \afm long-range order, that occurs significantly
faster than the lattice dynamics as monitored by the transient change in
intensity of selected Bragg structural reflections, which instead show a
decrease of intensity on a timescale of several ps.",2204.03009v1
2022-04-15,Dynamical mean-field theory study of a ferromagnetic CrI3 monolayer,"We have employed one of the well-known many-body techniques, density
functional theory plus dynamical mean-field theory (DFT + DMFT), to investigate
the electronic structure of ferromagnetic monolayer CrI3 as a function of
temperature and hole-doping concentration. The computed magnetic susceptibility
follows the Curie's law, indicating that the ferromagnetism of monolayer CrI3
originates from localized magnetic moments of Cr atoms rather than Stoner-type
itinerant ones. The DFT + DMFT calculations show a different coherent
temperature for each spin component, demonstrating apparent strong
spin-dependent electronic correlation effects in monolayer CrI3. Furthermore,
we have explored the doping-dependent electronic structure of monolayer CrI3
and found that its electronic and magnetic properties are easily tunable by the
hole-doping.",2204.07307v1
2022-04-23,Theory of inertial spin dynamics in anisotropic ferromagnets,"Recent experimental observation of inertial spin dynamics calls upon holistic
reevaluation of the theoretical framework of magnetic resonance in
ferromagnets. Here, we derive the secular equation of an inertial spin system
in analogy to the ubiquitous Smit-Beljers formalism. We find that the frequency
of precessional ferromagnetic resonances is decreased as compared to
non-inertial case. We also find that the frequency of nutational resonances is
generally increased due to the presence of magnetic anisotropy and applied
magnetic field. We obtain exact solutions of the secular equation and
approximations that employ the terminology of non-inertial theory and thus
allow for convenient estimates of the inertial effects.",2204.11057v2
2022-05-11,Domain wall damped harmonic oscillations induced by curvature gradients in elliptical magnetic nanowires,"Understanding the domain wall (DW) dynamics in magnetic nanowires (NW) is
crucial for spintronic-based applications demanding the use of DWs as
information carriers. This work focuses on the dynamics of a DW displacing
along a bent NW with an elliptical shape under the action of spin-polarized
electric currents and external magnetic fields. Our results evidence that a
curvature gradient induces an exchange-driven effective tangential field
responsible for pinning a DW near the maximum curvature point in a NW. The DW
equilibrium position depends on the competition between the torques produced by
the external stimuli and the curvature-induced effective fields. When the
external stimuli are below a certain threshold, the DW follows a damped
harmonic oscillation around the equilibrium position. Above this threshold, DW
displaces along the NW under an oscillatory translational motion.",2205.05716v1
2022-05-17,Semi-discretization and full-discretization with optimal accuracy for charged-particle dynamics in a strong nonuniform magnetic field,"The aim of this paper is to formulate and analyze numerical discretizations
of charged-particle dynamics (CPD) in a strong nonuniform magnetic field. A
strategy is firstly performed for the two dimensional CPD to construct the
semi-discretization and full-discretization which have optimal accuracy. This
accuracy is improved in the position and in the velocity when the strength of
the magnetic field becomes stronger. This is a better feature than the usual so
called ""uniformly accurate methods"". To obtain this refined accuracy, some
reformulations of the problem and two-scale exponential integrators are
incorporated, and the optimal accuracy is derived from this new procedure. Then
based on the strategy given for the two dimensional case, a new class of
uniformly accurate methods with simple scheme is formulated for the three
dimensional CPD in maximal ordering case. All the theoretical results of the
accuracy are numerically illustrated by some numerical tests.",2205.08191v1
2022-05-24,Relativistic dynamical inversion in manifestly covariant form,"The Relativistic Dynamical Inversion technique, a novel tool for finding
analytical solutions to the Dirac equation, is written in explicitly covariant
form. It is then shown how the technique can be used to make a change from
Cartesian to spherical coordinates of a given Dirac spinor. Moreover the most
remarkable feature of the new method, which is the ease of performing
non-trivial change of reference frames, is demonstrated. Such a feature
constitutes a potentially powerful tool for finding novel solutions to the
Dirac equation. Furthermore, a whole family of normalizable analytic solutions
to the Dirac equation is constructed. More specifically, we find exact
solutions for the case of a Dirac electron in the presence of a magnetic field
as well as a novel solution comprising of a combination of a spherically
symmetric electric field and magnetic fields. These solutions shed light on the
possibility of separating the positive and negative energy parts of localized
Dirac spinors in the presence as well as in the absence of magnetic fields. The
presented solutions provide an illustration of the connection between the
geometrical properties of the spinor and spin-orbit coupling for normalizable
spinorial wave functions.",2205.14004v1
2022-06-18,Magnon straintronics in the 2D van der Waals ferromagnet CrSBr from first-principles,"The recent isolation of two-dimensional (2D) magnets offers tantalizing
opportunities for spintronics and magnonics at the limit of miniaturization.
One of the key advantages of atomically-thin materials is their outstanding
deformation capacity, which provides an exciting avenue to control their
properties by strain engineering. Herein, we investigate the magnetic
properties, magnon dispersion and spin dynamics of the air-stable 2D magnetic
semiconductor CrSBr ($T_C$ = 146 K) under mechanical strain using
first-principles calculations. Our results provide a deep microscopic analysis
of the competing interactions that stabilize the long-range ferromagnetic order
in the monolayer. We showcase that the magnon dynamics of CrSBr can be modified
selectively along the two main crystallographic directions as a function of
applied strain, probing the potential of this quasi-1D electronic system for
magnon straintronics applications. Moreover, we predict a strain-driven
enhancement of $T_C$ considering environmental screening by ~30%, allowing the
propagation of spin waves at higher temperatures.",2206.09277v1
2022-07-12,Multitude of exceptional points in van der Waals magnets,"Several works have recently addressed the emergence of exceptional points
(EPs), i.e., spectral singularities of non-Hermitian Hamiltonians, in the
long-wavelength dynamics of coupled magnetic systems. Here, by focusing on the
driven magnetization dynamics of a van der Waals ferromagnetic bilayer, we show
that exceptional points can appear over extended portions of the first
Brillouin zone as well. Furthermore, we demonstrate that the effective
non-Hermitian magnon Hamiltonian, whose eigenvalues are purely real or come in
complex-conjugate pairs, respects an unusual wavevector-dependent
pseudo-Hermiticity. Finally, for both armchair and zigzag nanoribbon
geometries, we discuss both the complex and purely real spectra of the
topological edge states and their experimental implications.",2207.05326v3
2022-07-22,Magnetic-Translational Sum Rule and Approximate Models of the Molecular Berry Curvature,"The Berry connection and curvature are key components of electronic structure
calculations for atoms and molecules in magnetic fields. They ensure the
correct translational behavior of the effective nuclear Hamiltonian and the
correct center-of-mass motion during molecular dynamics in these environments.
In this work, we demonstrate how these properties of the Berry connection and
curvature arise from the translational symmetry of the electronic wave function
and how they are fully captured by a finite basis set of London orbitals but
not by standard Gaussian basis sets. This is illustrated by a series of
Hartree-Fock calculations on small molecules in different basis sets. Based on
the resulting physical interpretation of the Berry curvature as the shielding
of the nuclei by the electrons, we introduce and test a series of
approximations using the Mulliken fragmentation scheme of the electron density.
These approximations will be particularly useful in ab initio molecular
dynamics calculations in a magnetic field, since they reduce the computational
cost, while recovering the correct physics and up to 95% of the exact Berry
curvature.",2207.11004v1
2022-08-02,Fluctuating Fractionalized Spins in Quasi Two-dimensional Magnetic V0.85PS3,"Quantum spin liquid (QSL), a state characterized by exotic low energy
fractionalized excitations and statistics is still elusive experimentally and
may be gauged via indirect experimental signatures. Remnant of QSL phase may
reflect in the spin dynamics as well as quanta of lattice vibrations, i.e.,
phonons, via the strong coupling of phonons with the underlying fractionalized
excitations i.e., Majorana fermions. Inelastic light scattering (Raman) studies
on V1-xPS3 single crystals evidences the spin fractionalization into Majorana
fermions deep into the paramagnetic phase reflected in the emergence of a low
frequency quasielastic response along with a broad magnetic continuum marked by
a crossover temperature T* ~ 200 K from a pure paramagnetic state to
fractionalized spins regime qualitatively gauged via dynamic Raman
susceptibility. We further evidenced anomalies in the phonons self-energy
parameters in particular phonon line broadening and line asymmetry evolution at
this crossover temperature, attributed to the decaying of phonons into
itinerant Majorana fermions. This anomalous scattering response is thus
indicative of fluctuating fractionalized spins suggesting a phase proximate to
the quantum spin liquid state in this quasi two-dimensional (2D) magnetic
system.",2208.01550v1
2022-08-16,Effective Dynamics of Translationally Invariant Magnetic Schrödinger Equations in the High Field Limit,"We study the large field limit in Schr\""odinger equations with magnetic
vector potentials describing translationally invariant $B$-fields with respect
to the $z$-axis. In a first step, using regular perturbation theory, we derive
an approximate description of the solution, provided the initial data is
compactly supported in the Fourier-variable dual to $z\in \mathbb R$. The
effective dynamics is thereby seen to produce high-frequency oscillations and
large magnetic drifts. In a second step we show, by using the theory of almost
invariant subspaces, that this asymptotic description is stable under
polynomially bounded perturbations that vanish in the vicinity of the origin.",2208.07894v2
2022-08-18,Signatures of Open and Noisy Quantum Systems in Single-Qubit Quantum Annealing,"We propose a quantum annealing protocol that more effectively probes the
dynamics of a single qubit on D-Wave's quantum annealing hardware. This
protocol uses D-Wave's h-gain schedule functionality, which allows the rapid
quenching of the longitudinal magnetic field at arbitrary points during the
anneal. This features enables us to distinguish between open and closed system
dynamics as well as the presence or absence of longitudinal magnetic field
noise. We show that both thermal and magnetic field fluctuations are key
sources of noise that need to be included in an open quantum system model to
reproduce the output statistics of the hardware.",2208.09068v1
2022-09-05,Numerical modeling of the influence of randomly macroscopic inhomogeneities on the magnetic induction and temperature dynamics of a high-T$_c$ superconducting slab,"In this study we perform numerical calculations of the spatial and time
variation of the magnetic induction, current density and temperature in an
infinite superconducting slab with macroscopic inhomogeneities. We also modeled
an homogeneuos superconductor as a referential material, considering
characteristic magnetothermal properties of a high-T$_c$ single crystal. The
theoretical formalism is enclosed in the dynamical regime and a continuum
electrodynamic approach. In particular, we studied the correlation between the
magnetic jumps and temperature rise occurrence with the presence of
macroscopical inhomogeneities, modeled through an stochastic current-carrying
ability spatial profile.",2209.02117v1
2022-08-30,Phonons and phase symmetries in bulk $CrCl_3$ from scattering measurements and theory,"Phonon-derived behaviors are important indicators of novel phenomena in
transition metal trihalides, including spin liquid behavior, two-dimensional
magnetism, and spin-lattice coupling. However, phonons and their dependence on
spin structure and excitations have not been adequately explored. In this work,
we probe and critically examine the vibrational properties of bulk $CrCl_3$
using inelastic neutron scattering and density functional theory. We
demonstrate that magnetic and van der Waals interactions are essential to
describing the structure and phonons in $CrCl_3$; however, the specific spin
configuration is unimportant. This provides context for understanding thermal
transport measurements as governed by dynamical spin-lattice couplings. More
importantly, we introduce an efficient dynamic method that exploits
translational symmetries in large conventional unit cells that generates
insights into phonon dispersions, interactions, and measured spectra in terms
of quantum phase interference conditions. This work opens new avenues for
understanding phonons in layered magnets and more generally in conventional
cell geometries of a variety of materials.",2209.02394v1
2022-09-21,Coupling of optical phonons with Kondo effect and magnetic orders in antiferromagnetic Kondo lattice CeAuSb$_2$,"Ultrafast optical spectroscopy was used to investigate the ultrafast
quasiparticle dynamics of antiferromagnetic Kondo lattice CeAuSb$_2$ as a
function of temperature and fluence. Our results reveal (i) the opening of a
narrow hybridization gap ($\Delta$ $\sim$ 4.5 meV) near the Fermi level below
the coherence temperature $T^*$ $\approx$ 100 K, (ii) the coupling of coherent
phonon modes with Kondo effect and magnetic orders, leading to the frequencies
anomaly at the characteristic temperatures, and (iii) a possible photoinduced
nonthermal phase transition. Our observations thus shed light on the
hybridization dynamics and magnetic orders in heavy fermion systems.",2209.10169v2
2022-10-06,Magnetic excitations in the helical Rashba superconductor,"We investigate the magnetic excitation spectrum in the helical state of a
noncentrosymmetric superconductor with inversion symmetry breaking and strong
Rashba spin-orbit coupling. For this purpose, we derive the general expressions
of the dynamical spin response functions under the presence of strong Rashba
splitting of conduction bands, superconducting gap, and external field which
lead to stabilization of Cooper pairs with finite overall momentum in a helical
state. The latter is characterized by momentum space regions of paired and
unpaired states with different quasiparticle dispersions. The magnetic response
is determined by i) excitations within and between both paired and unpaired
regions ii) anomalous coherence factors and iii) additional spin matrix
elements due to helical Rashba spin texture of bands. We show that as a
consequence typical correlated real space and spin space anisotropies appear in
the dynamical susceptibility which would be observable as a characteristic
fingerprint for a helical superconducting state in inelastic neutron scattering
investigations.",2210.03059v1
2022-10-12,Critical role of magnetic moments on lattice dynamics in YBa${}_{2}$Cu${}_{3}$O${}_{6}$,"The role of lattice dynamics in unconventional high-temperature
superconductivity is still vigorously debated. Theoretical insights into this
problem have long been prevented by the absence of an accurate first-principles
description of the combined electronic, magnetic, and lattice degrees of
freedom. Utilizing the recently constructed r$^2$SCAN density functional that
stabilizes the antiferromagnetic (AFM) state of the pristine oxide
YBa$_2$Cu$_3$O$_6$, we faithfully reproduce the experimental dispersion of key
phonon modes. We further find significant magnetoelastic coupling in numerous
high energy Cu-O bond stretching optical branches, where the AFM results
improve over the soft non-magnetic phonon bands.",2210.06569v2
2022-10-21,An energy stable and maximum bound principle preserving scheme for the dynamic Ginzburg-Landau equations under the temporal gauge,"This paper proposes a decoupled numerical scheme of the time-dependent
Ginzburg--Landau equations under the temporal gauge. For the magnetic potential
and the order parameter, the discrete scheme adopts the second type Ned${\rm
\acute{e}}$lec element and the linear element for spatial discretization,
respectively; and a linearized backward Euler method and the first order
exponential time differencing method for time discretization, respectively. The
maximum bound principle (MBP) of the order parameter and the energy dissipation
law in the discrete sense are proved. The discrete energy stability and
MBP-preservation can guarantee the stability and validity of the numerical
simulations, and further facilitate the adoption of an adaptive time-stepping
strategy, which often plays an important role in long-time simulations of
vortex dynamics, especially when the applied magnetic field is strong. An
optimal error estimate of the proposed scheme is also given. Numerical examples
verify the theoretical results of the proposed scheme and demonstrate the
vortex motions of superconductors in an external magnetic field.",2210.11678v2
2022-10-26,Dimensional reduction and incommensurate dynamic correlations in the $S=\frac{1}{2}$ triangular-lattice antiferromagnet Ca$_3$ReO$_5$Cl$_2$,"The observation of spinon excitations in the $S=\frac{1}{2}$ triangular
antiferromagnet Ca$_3$ReO$_5$Cl$_2$ reveals a quasi-one-dimensional (1D) nature
of magnetic correlations, in spite of the nominally 2D magnetic structure. This
phenomenon is known as frustration-induced dimensional reduction. Here, we
present high-field electron spin resonance spectroscopy and magnetization
studies of Ca$_3$ReO$_5$Cl$_2$, allowing us not only to refine spin-Hamiltonian
parameters, but also to investigate peculiarities of its low-energy spin
dynamics. We argue that the presence of the uniform Dzyaloshinskii-Moriya
interaction (DMI) shifts the spinon continuum in momentum space and, as a
result, opens a zero-field gap at the $\Gamma$ point. We observed this gap
directly. The shift is found to be consistent with the structural modulation in
the ordered state, suggesting this material as a perfect model
triangular-lattice system, where a pure DMI-spiral ground state can be
realized.",2210.14864v1
2022-11-14,Symmetric Post-Transition-State Bifurcation Reactions with Berry Pseudo-Magnetic Fields,"We investigate how the Berry force (i.e. the pseudo-magnetic force operating
on nuclei as induced by electronic degeneracy and spin-orbit coupling (SOC))
might modify a post-transition state bifurcation (PTSB) reaction path and
affect product selectivity for situations when multiple products share the same
transition state. To estimate the magnitude of this effect, Langevin dynamics
are performed on a model system with a valley-ridge inflection (VRI) point in
the presence of a magnetic field (that mimics the Berry curvature). We also
develop an analytic model for such selectivity that depends on key parameters
such as the surface topology, the magnitude of the Berry force, and the nuclear
friction. Within this dynamical model, static electronic structure calculations
(at the level of generalized Hartree-Fock with spin-orbit coupling (GHF+SOC)
theory) suggest that electronic-spin induced Berry force effects may indeed
lead to noticeable changes in methoxy radical isomerization.",2211.07731v1
2022-11-28,Muon-spin relaxation investigation of magnetic bistability in a crystalline organic radical compound,"We present the results of a muon-spin relaxation ($\mu^{+}$SR) investigation
of the crystalline organic radical compound
4-(2-benzimidazolyl)-1,2,3,5-dithiadiazolyl (HbimDTDA), in which we demonstrate
the hysteretic magnetic switching of the system that takes place at $T = 274
\pm 11\,\mathrm{K}$ caused by a structural phase transition. Muon-site analysis
using electronic structure calculations suggests a range of candidate muon
stopping sites. The sites are numerous and similar in energy but,
significantly, differ between the two structural phases of the material.
Despite the difference in the sites, the muon remains a faithful probe of the
transition, revealing a dynamically-fluctuating magnetically disordered state
in the low-temperature structural phase. In contrast, in the high temperature
phase the relaxation is caused by static nuclear moments, with rapid electronic
dynamics being motionally narrowed from the muon spectra.",2211.15560v1
2022-11-30,Precession of magnetars: dynamical evolutions and modulations on polarized electromagnetic waves,"Magnetars are conjectured to be highly magnetized neutron stars (NSs). Strong
internal magnetic field and elasticity in the crust may deform the stars and
lead to free precession. We study the precession dynamics of
triaxially-deformed NSs incorporating the near-field and the far-field
electromagnetic torques. We obtain timing residuals for different NS geometries
and torques. We also investigate the polarized X-ray and radio signals from
precessing magnetars. The modulations on the Stokes parameters are obtained for
thermal X-rays emitted from the surface of magnetars. For radio signals, we
apply the simple rotating vector model (RVM) to give the modulations on the
position angle (PA) of the polarization. Our results are comprehensive, ready
to be used to search for magnetar precession with timing data and polarizations
of X-ray and radio emissions. Future observations of precessing magnetars will
give us valuable information on the geometry and the strength of the strong
magnetic fields, the emission geometry, as well as the equation of state (EoS)
of NSs.",2211.17087v1
2022-12-01,Manipulating non-reciprocity in a two-dimensional magnetic quantum walk,"Non-reciprocity is an important topic in fundamental physics and
quantum-device design, as much effort has been devoted to its engineering and
manipulation. Here we experimentally demonstrate non-reciprocal transport in a
two-dimensional quantum walk of photons, where the directional propagation is
highly tunable through dissipation and synthetic magnetic flux. The
non-reciprocal dynamics hereof is a manifestation of the non-Hermitian skin
effect, with its direction continuously adjustable through the photon-loss
parameters. By contrast, the synthetic flux originates from an engineered
geometric phase, which competes with the non-Hermitian skin effect through
magnetic confinement. We further demonstrate how the non-reciprocity and
synthetic flux impact the dynamics of the Floquet topological edge modes along
an engineered boundary. Our results exemplify an intriguing strategy for
achieving tunable non-reciprocal transport, highlighting the interplay of
non-Hermiticity and gauge fields in quantum systems of higher dimensions.",2212.00387v1
2022-12-18,Resonant contributions to oscillatory phenomena under conditions of magnetic breakdown during reconstructions of electron dynamics on the Fermi surface,"We consider here special closed electron trajectories that arise during
reconstructions of electron dynamics on the Fermi surface in the presence of
strong magnetic fields, as well as the phenomenon of intraband magnetic
breakdown that occurs on such trajectories. We consider cases where the
electronic spectrum arising in such a situation corresponds to the resonant
contribution to quantum oscillations. In the paper, all cases where this occurs
are singled out, and the possible influence of the appearance of the Berry
phase and other effects on the described phenomena is also considered.",2212.09059v3
2022-12-29,New description of the scaling evolution of the cosmological magneto-hydrodynamic system,"We present a new description of cosmological evolution of the primordial
magnetic field under the condition that it is non-helical and its energy
density is larger than the kinetic energy density. We argue that the evolution
can be described by four different regimes, according to whether the decay
dynamics is linear or not, and whether the dominant dissipation term is the
shear viscosity or the drag force. Using this classification and conservation
of the Hosking integral, we present analytic models to adequately interpret the
results of various numerical simulations of field evolution with variety of
initial conditions. It is found that, contrary to the conventional wisdom, the
decay of the field is generally slow, exhibiting the inverse transfer, because
of the conservation of the Hosking integral. Using the description proposed
here, we may trace the intermediate evolution history of the magnetic field and
clarify whether each process governing its evolution is frozen or not, which is
essential to follow the evolution of relatively weak magnetic fields.",2212.14355v1
2022-12-31,Diffusion within pores fully revealed by magnetic resonance,"Probing the transport of fluids within confined domains is important in many
areas including material science, catalysis, food science, and cell biology.
The diffusion propagator fully characterizes the diffusion process, which is
highly sensitive to the confining boundaries as well as the structure within
enclosed pores. While magnetic resonance has been used extensively to observe
various features of the diffusion process, its full characterization has been
elusive. Here, we address this challenge by employing a special sequence of
magnetic field gradient pulses for measuring the diffusion propagator, which
allows for `listening to the drum' and determining not only the pore's shape
but also diffusive dynamics within it.",2301.00116v1
2023-02-08,Anisotropic skyrmion mass induced by surrounding conduction electrons: A Schwinger-Keldysh field theory approach,"The current-driven motion of magnetic skyrmions, as topologically protected
winding vector fields of local magnetization, has attracted considerable
attention due to both fundamental interest in the dynamics of topological
solitons, and potential spintronic applications. While widely-used Thiele
equation with zero skyrmion mass captures its rigid motion under the assumption
of preserved skyrmion shape, experiments on realistic skyrmions find deviation
due to inertial mass with numerous attempts to account for it by invoking
internal skyrmion dynamics, geometry of the nanowire, and interaction with
magnons or phonons. However, experimentally observed skyrmion mass is often
much larger than predicted by these theories. In this Letter, we employ
Schwinger-Keldysh field-theoretical approach to study conduction electrons
interacting with localized magnetic moments comprising a skyrmion to obtain a
stochastic differential equation of its motion. By analyzing its time-retarded
damping kernel, we extract an anisotropic mass governed by conduction electrons
and demonstrate its substantial effect on the current-driven skyrmion motion,
even in the absence of any skyrmion deformation.",2302.04220v1
2023-03-23,Majorana-magnon interactions in topological Shiba chains,"A chain of magnetic impurities deposited on the surface of a superconductor
can form a topological Shiba band that supports Majorana zero modes and holds a
promise for topological quantum computing. Yet, most experiments scrutinizing
these zero modes rely on transport measurements, which only capture local
properties. Here we propose to leverage the intrinsic dynamics of the magnetic
impurities to access their non-local character. We use linear response theory
to determine the dynamics of the uniform magnonic mode in the presence of
external $ac$ magnetic fields and coupling the Shiba electrons. We demonstrate
that this mode, which spreads over the entire chain of atoms, becomes imprinted
with the parity of the ground state and, moreover, can discriminate between
Majorana and trivial zero modes located at the ends of the chain. Our approach
offers a non-invasive alternative to the scanning tunneling microscopy
techniques used to probe Majorana zero modes. Conversely, the magnons could
facilitate the manipulation of Majorana zero modes in topological Shiba chains.",2303.13513v2
2023-04-03,Multi-thermal jet formation triggered by flux emergence,"Flux emergence is responsible for various solar eruptions. Combining
observation and simulations, we investigate the influence of flux emergence at
one footpoint of an arcade on coronal rain as well as induced eruptions. The
emergence changes the pressure in the loops, and the internal coronal rain all
moves to the other side. The emerging flux reconnects with the overlying
magnetic field, forming a current sheet and magnetic islands. The plasma is
ejected outwards and heated, forming a cool jet ~ 6000 K and a hot X-ray jet ~
4 MK simultaneously. The jet dynamical properties agree very well between
observation and simulation. In the simulation, the jet also displays transverse
oscillations with a period of 8 minutes, a so-called whip-like motion. The
movement of the jet and dense plasmoids changes the configuration of the local
magnetic field, facilitating the occurrence of Kelvin--Helmholtz instability,
and vortex-like structures form at the boundary of the jet. Our simulation
clearly demonstrates the effect of emergence on coronal rain, the dynamical
details of reconnecting plasmoid chains, the formation of multi-thermal jets,
and the cycling of cool mass between the chromosphere and the corona.",2304.01043v1
2023-04-06,Essential Tools of Linear Algebra for Calculating Nuclear Spin Dynamics of Chemically Exchanging Systems,"In this work, we describe essential tools of linear algebra necessary for
calculating the effect of chemical exchange on spin dynamics and polarization
transfer in various nuclear magnetic resonance (NMR) experiments. We show how
to construct Hamiltonian, relaxation, and chemical exchange superoperators in
the Liouville space, as well as demonstrate corresponding code in Python.
Examples of applying the code are given for problems involving chemical
exchange between NH3 and NH4+ at zero and high magnetic field and polarization
transfer from parahydrogen relevant in SABRE (signal amplification by
reversible exchange) at low magnetic field (0-20 mT). The presented methodology
finds utility for describing the effect of chemical exchange on NMR spectra and
can be extended further by taking into account non-linearities in the master
equation.",2304.03139v2
2023-04-25,Defect-enhanced diffusion of magnetic skyrmions,"Defects, i.e. inhomogeneities of the underlying lattice, are ubiquitous in
magnetic materials and can have a crucial impact on their applicability in
spintronic devices. For magnetic skyrmions, localized and topologically
non-trivial spin textures, they give rise to a spatially inhomogeneous energy
landscape and can lead to pinning, resulting in an exponentially increased
dwell time at certain positions and typically a strongly reduced mobility.
Using atomistic spin dynamics simulations, we reveal that under certain
conditions defects can instead enhance thermal diffusion of ferromagnetic
skyrmions. By comparing with results for the diffusion of antiferromagnetic
skyrmions and using a quasi-particle description based on the Thiele equation,
we demonstrate that this surprising finding can be traced back to the partial
lifting of the impact of the topologigal gyrocoupling, which governs the
dynamics of ferromagnetic skyrmions in the absence of defects.",2304.12917v1
2023-05-13,Floquet Dynamical Decoupling at Zero Bias,"Dynamical decoupling (DD) is an efficient method to decouple systems from
environmental noises and to prolong the coherence time of systems. In contrast
to discrete and continuous DD protocols in the presence of bias field, we
propose a Floquet DD at zero bias to perfectly suppress both the zeroth and
first orders of noises according to the Floquet theory. Specifically, we
demonstrate the effectiveness of this Floquet DD protocol in two typical
systems including a spinor atomic Bose-Einstein condensate decohered by
classical stray magnetic fields and a semiconductor quantum dot electron spin
coupled to nuclear spins. Furthermore, our protocol can be used to sense
high-frequency noises. The Floquet DD protocol we propose shines new light on
low-cost and high-portable DD technics without bias field and with low
controlling power, which may have wide applications in quantum computing,
quantum sensing, nuclear magnetic resonance and magnetic resonance imaging.",2305.07847v1
2023-05-15,A general method to construct mean field counter diabatic driving for a ground state search,"The counter diabatic (CD) driving has attracted much attention for
suppressing non-adiabatic transition in quantum annealing (QA). However, it can
be intractable to construct the CD driving in the actual experimental setup due
to the non-locality of the CD dariving Hamiltonian and necessity of exact
diagonalization of the QA Hamiltonian in advance. In this paper, using the mean
field (MF) theory, we propose a general method to construct an approximated CD
driving term consisting of local operators. We can efficiently construct the MF
approximated CD (MFCD) term by solving the MF dynamics of magnetization using a
classical computer. As an example, we numerically perform QA with MFCD driving
for the spin glass model with transverse magnetic fields. We numerically show
that the MF dynamics with MFCD driving is equivalent to the solution of the
self-consistent equation in MF theory. Also, we clarify that a ground state of
the spin glass model with transverse magnetic field can be obtained with high
fidelity compared to the conventional QA without the CD driving. Moreover, we
experimentally demonstrate our method by using a D-wave quantum annealer and
obtain the experimental result supporting our numerical simulation.",2305.08352v1
2023-05-18,Reconnection in a Pinch,"A recently published, new analysis of current sheets updated the classic
Harris 1D static solution by considering multiple classes of charged particle
trajectories in a generalized dynamic current sheet. It used a 1D PIC
simulation to describe dynamic pinching and bifurcation of the sheet. These 1D
results strongly suggest that plasma beta or other properties of the inflowing
plasma have a strong effect on the equilibrium thickness of the pinched current
sheet, but cannot describe magnetic reconnection. The time appears right to
carry such 1D studies over to 3D simulations where current sheet thickness has
been found exert an enabling or disabling influence on reconnection. The
Magnetospheric Multiscale Mission (MMS), with its well-resolved multipoint
measurements, has found that reconnection is enabled in collisionless plasma by
non-adiabatic motions of electrons that can only occur in narrow magnetic
structures with a scale comparable to electron inertial lengths (de). The
recent 1D studies strongly suggest that a pinch to such scales can only occur
for inflowing magnetized plasmas with relatively low plasma beta. We conclude
that a parametric exploration of 3D simulated and observed inflow conditions,
especially plasma beta, should shed light on the enablement of reconnection in
collisionless plasmas.",2305.11238v1
2023-06-17,Polytropic representation of the kinetic pressure tensor of non-ideal magnetized fluids in equilibrium toroidal structures,"Non-ideal fluids are generally subject to the occurrence of non-isotropic
pressure tensors, whose determination is fundamental in order to characterize
their dynamical and thermodynamical properties. This requires the
implementation of theoretical frameworks provided by appropriate microscopic
and statistical kinetic approaches in terms of which continuum fluid fields are
obtained. In this paper the case of non-relativistic magnetized fluids forming
equilibrium toroidal structures in external gravitational fields is considered.
Analytical solutions for the kinetic distribution function are explicitly
constructed, to be represented by a Chapman-Enskog expansion around a
Maxwellian equilibrium. In this way, different physical mechanisms responsible
for the generation of non-isotropic pressures are identified and proved to be
associated with the kinetic constraints imposed on single and collective
particle dynamics by phase-space symmetries and magnetic field. As a major
outcome, the validity of a polytropic representation for the kinetic pressure
tensors corresponding to each source of anisotropy is established, whereby
directional pressures exhibit a specific power-law functional dependence on
fluid density. The astrophysical relevance of the solution for the
understanding of fluid plasma properties in accretion-disc environments is
discussed.",2306.11742v1
2023-07-12,Purely antiferromagnetic frustrated Heisenberg model in spin ladder compound BaFe$_2$Se$_3$,"The spin dynamics in the block magnetic phase of the iron-based ladder
compound \bfs\ has been studied by means of single crystal inelastic neutron
scattering. Using linear spin wave theory and Monte-Carlo simulations, our
analysis points to a magnetic Heisenberg model with effective frustrated
antiferromagnetic couplings only, able to describe both the exotic block order
and its dynamics. This new and purely antiferromagnetic picture offers a
fruitful perspective to describe multiferroic properties but also understand
the origin of the stripe-like magnetic instability observed under pressure as
well as in other parent compounds with similar crystalline structure.",2307.06193v1
2023-07-12,The first law for stationary axisymmetric multi-black hole systems,"In the framework of Einstein-Maxwell theory, we consider collinear arrays of
rotating electrically charged black holes connected by Misner-Dirac strings
carrying gravimagnetic and magnetic fluxes. The first law of mechanics for
these systems is derived. It involves as dynamical variables the areas, angular
momenta and electric charges of the various Killing horizons -- black holes and
Misner strings. When the gravimagnetic fluxes all vanish, the first law reduces
to a form where the dynamical variables associated with the strings are the
string tensions and magnetic fluxes. This form is not generically invariant
under electric-magnetic duality.",2307.06282v3
2023-07-16,On magnetized Rossby wave dynamics in the earth's ionosphere,"In the frame of shallow water model the influence of free surface action on
nonlinear propagation of planetary ULF magnetized Rossby waves in the weakly
ionized ionospheric D-, E-, and F-layers is revealed. Relevant nonlinear
dynamic equations satisfying several conservation laws are obtained and
investigated. The role of Hall and Pedersen conductivities is investigated
explicitly. It is shown that while potential vorticity is conserved in the
ionospheric D-,and E-layer it is broken by Pedersen conductivity in the
ionospheric F-layer. Similar to KdV nonlinearity two new scalar nonlinearities
due to Pedersen conductivity are revealed in the F-layer. Obtained results
extend and complement known theoretical investigations and are especially
relevant for nonlinear vortical propagation of magnetized Rossby waves in the
weakly ionized ionospheric plasma.",2307.07986v1
2023-07-21,Current Tomography -- Localization of void fractions in conducting liquids by measuring the induced magnetic flux density,"A novel concept of a measurement technology for the localization and
determination of the size of gas bubbles is presented, which is intended to
contribute to a further understanding of the dynamics of efficiency-reducing
gas bubbles in electrolyzers. A simplified proof-of-concept (POC) model is used
to numerically simulate the electric current flow through materials with
significant differences in electrical conductivity. Through an automated
approach, an extensive data set of electric current density and conductivity
distributions is generated, complemented with determined magnetic flux
densities in the surroundings of the POC cell at virtual sensor positions. The
generated data set serves as testing data for various reconstruction
approaches. Based on the measurable magnetic flux density, solving Biot-Savarts
law inversely is demonstrated and discussed with a model-based solution of an
optimization problem, of which the gas bubble locations are derived.",2307.11540v1
2023-07-31,Persistent spin dynamics in magnetically ordered honeycomb cobalt oxides,"In the quest to find quantum spin liquids, layered cobalt oxides Na2Co2TeO6
and Na3Co2SbO6 have been proposed as promising candidates for approximating the
Kitaev honeycomb model. Yet, their suitability has been thrown into question
due to observed long-range magnetic order at low temperatures and indications
of easy-plane, rather than Kitaev-type, spin anisotropy. Here we use muon spin
relaxation to reveal an unexpected picture: contrary to the anticipated static
nature of the long-range order, the systems show prevalent spin dynamics with
spatially uneven distribution and varied correlation times. This underlines
that the magnetic ground states cannot be solely described by the long-range
order, suggesting a significant role of quantum fluctuations. Our findings not
only shed new light on the complex physics of these systems but also underscore
the need for a refined approach in the search for realizable quantum spin
liquids.",2307.16451v1
2023-08-03,Brownian electric bubble quasiparticles,"Recent works on electric bubbles (including the experimental demonstration of
electric skyrmions) constitute a breakthrough akin to the discovery of magnetic
skyrmions some 15 years ago. So far research has focused on obtaining and
visualizing these objects, which often appear to be immobile (pinned) in
experiments. Thus, critical aspects of magnetic skyrmions - e.g., their
quasiparticle nature, Brownian motion - remain unexplored (unproven) for
electric bubbles. Here we use predictive atomistic simulations to investigate
the basic dynamical properties of these objects in pinning-free model systems.
We show that it is possible to find regimes where the electric bubbles can
present long lifetimes ($\sim$ ns) despite being relatively small ($\varnothing
<$ 2 nm). Additionally, we find that they can display stochastic dynamics with
large and highly tunable diffusion constants. We thus establish the
quasiparticle nature of electric bubbles and put them forward for the physical
effects and applications (e.g., in token-based Probabilistic Computing)
considered for magnetic skyrmions.",2308.01716v2
2023-08-30,Internal activities in a solar filament and heating to its threads,"Filaments are one of the most common features in the solar atmosphere, and
are of significance in solar, stellar and laboratory plasma physics. Using data
from the Chinese H$\alpha$ Solar Explorer, the Solar Upper Transition Region
Imager and the Solar Dynamics Observatory, we report on multiwavelength imaging
and spectral observations of the activation of a small filament. The filament
activation produces several localized dynamic brightenings, which are probably
produced by internal reconnections of the braided magnetic fields in the
filament. The filament expands during the activation and its threads reconnect
with the ambient magnetic fields, which leads to the formation of hot arcades
or loops overlying the filament. The thermal energy of each of these localized
brightenings is estimated in the order of $10^{25}-10^{27} erg$ and the total
energy is estimated to be $\sim1.77 \times 10^{28} erg$. Our observations
demonstrate that the internal magnetic reconnections in the filament can lead
to localized heating to the filament threads and prompt external reconnections
with ambient corona structures, and thus could contribute to the energy and
mass transferring into the corona.",2308.15747v1
2023-09-11,Electromagnetic energy transfer processes in Effective Electro-Magneto Dynamics of axions,"Oscillating and dissipative energy exchange between the electromagnetic and
axion fields is investigated in an effective electro-magneto dynamical (EEMD)
model theory, implying the coexistence of axions with hypothetic magnetic
charges. An exact formula is presented for the energy transfer between the
electromagnetic and axionic sectors. In a first example we compute analytically
the homogeneously oscillating electric and magnetic field configurations
generated by the combined action of a constant static magnetic field and a
periodically oscillating axion condensate. In the second example the
electromagnetic radiative energy loss of a gravitationally bound axion
configuration is computed in the EEMD model. As a result an asymptotic $\sim
t^{1/5}$ temporal increase of the clump size is found also in EEMD.",2309.05523v1
2023-09-22,Earthquake-like dynamics in ultrathin magnetic film,"We study the motion of a domain wall on an ultrathin magnetic film using the
magneto-optical Kerr effect (MOKE). At tiny magnetic fields, the wall creeps
only via thermal activation over the pinning centers present in the sample. Our
results show that this creep dynamics is highly intermittent and correlated. A
localized instability triggers a cascade, akin to aftershocks following a large
earthquake, where the pinned wall undergoes large reorganizations in a compact
active region for a few seconds. Surprisingly, the size and shape of these
reorganizations display the same scale-free statistics of the depinning
avalanches in agreement with the quenched Kardar-Parisi-Zhang universality
class.",2309.12898v1
2023-09-25,Magnetism on the thermal dynamics of 2D antiferromagnetic membranes,"We developed a theoretical scheme of incorporating the magnetoelastic
contribution into the thermal elastic dynamics for the thin membranes of 2D
antiferromagnetic material with restricted geometry. We extended the elastic
Gr\""uneisen relation into an effective version which includes the magnetic
counterpart to the volume change of internal energy. Based on the specific heat
and thermal conductivity from the elastic and magnetic origins we predicted the
dependency of observables, such as effective Gr\""uneisen parameter, thermal
expansion coefficient, and the damping factor, with respect to a wide range of
temperature across the phase transition. Our model of analysis as been
validated by applying to the case of FePS3 flake resonator and the theoretical
predictions fits well with the reported experiment data.",2309.13991v2
2023-10-02,SpinPSO: An agent-based optimization workflow for identifying global noncollinear magnetic ground-states from first-principles,"We propose and implement a novel hybrid meta-heuristic optimization algorithm
for the identification of non-collinear global ground-states in magnetic
systems. The inputs to this optimization scheme are directly from non-collinear
density functional theory (DFT), and the workflow is implemented in the atomate
code framework, making it suitable to run on high-performance computing
architectures. The hybrid algorithm provides a seamless theoretical extension
of particle swarm optimization (PSO) algorithms to continuous $\mathcal S^2$
spins, giving it the name SpinPSO. The hybrid nature of the algorithm stems
from setting the dynamics of individual spins to be governed by physically
motivated atomistic spin dynamics. Using this algorithm, we are able to achieve
convergence to experimentally resolved magnetic ground-states for a set of
diverse test case materials that exhibit exotic spin textures.",2310.01383v1
2023-11-06,Lattice QED in an external magnetic field: Evidence for dynamical chiral symmetry breaking,"We simulate QED in a strong constant homogeneous external magnetic field on a
euclidean space-time lattice using the Rational Hybrid Monte Carlo method,
developed for simulating lattice QCD. Our primary goal is to measure the chiral
condensate in the limit when the input electron mass $m$ is zero. We observe a
non-zero value, indicating that the external magnetic field catalyzes chiral
symmetry breaking as predicted by approximate truncated Schwinger-Dyson
methods. Such behaviour is associated with dominance by the lowest Landau level
which causes the effective dimensional reduction from $3+1$~dimensions to $1+1$
dimensions for charged particles (electrons and positrons) where the attractive
forces of QED can produce chiral symmetry breaking with a dynamical electron
mass and associated chiral condensate. Since our lattice simulations use bare
(lattice) parameters, while the Schwinger-Dyson analyses work with renormalized
quantities, direct numerical comparison will require renormalization of our
lattice results.",2311.03516v1
2023-12-11,Charged particle dynamics in an elliptically polarized electromagnetic wave and a uniform axial magnetic field,"An analytical study of the charged particle dynamics in the presence of an
elliptically polarized electromagnetic wave and a uniform axial magnetic field,
is presented. It is found that for $g\omega_{0}/ \omega' = \pm 1$, maximum
energy gain occurs respectively for linear and circular polarization;
$\omega_{0}$ and $\omega'$ respectively being the cyclotron frequency of the
charged particle in the external magnetic field and Doppler-shifted frequency
of the wave seen by the particle, and $g =\pm 1$ respectively correspond to
left and right-handedness of the polarization. An explicit solution of the
governing equation is presented in terms of particle position or laboratory
time, for the specific case of resonant energy gain in a circularly polarized
electromagnetic wave. These explicit position- or time-dependent expressions
are useful for better insight into various phenomena, viz., cosmic ray
generation, microwave generation, plasma heating, and particle acceleration,
etc.",2312.06175v1
2023-12-07,Scalar fields in Bonnor-Melvin Universe with potential: A study of dynamics of spin-0 particles-antiparticles,"This research focus on the investigation of relativistic quantum dynamics of
spin0 scalar particles/fields through the utilization of the Klein-Gordon (KG)
equation within the framework of an electrovacuum space-time in the presence of
an external scalar potential. Specifically, we focus on a cylindrical symmetric
Bonnor-Melvin magnetic universe with a cosmological constant, where the
magnetic field aligns along the symmetry axis direction. We derive the radial
wave equation of the KG-equation by considering a Cornell-type scalar potential
in the background of magnetic universe and successfully obtain an analytical
eigenvalue solution for spin-0 quantum system. Notably, our findings reveal
that both the energy spectrum and the corresponding radial wave function are
significantly influenced by the presence of the cosmological constant, the
topology parameter of the space-time geometry, which induces a deficit in the
angular coordinates, and the potential parameters.",2312.06612v1
2024-01-21,Crystallization and dynamics of defects in a magnetic fluctuating medium,"We consider the dynamics of classical particles or defects moving in a
fluctuating two-dimensional magnetic medium made of Ising spins. These defects
occupy empty sites, and each of them can move according to simple rules, by
exchanging its location with one of the neighboring or distant spin if the
energy is favorable, conserving the magnetization. We use a fermionic
representation of the theory in order to map the partition function into an
integral over Grassmannian variables. This model of annealed disorder can be
described by a Grassmannian action containing quartic interaction terms. We
study the critical behavior of this system as well as the entropy, specific
heat, and residual correlation functions which are evaluated within this
Grassmannian formalism. We found in particular that the correlations are
strongly attractive at short distances in the low temperature regime and for a
broader range of distances near the spin critical regime, and slightly
repulsive at large distances. These results are compared with Monte-Carlo
simulations.",2401.11528v1
2024-01-22,Active Inference Demonstrated with Artificial Spin Ice,"A numerical model of interacting nanomagnetic elements is used to demonstrate
active inference with a three dimensional Artificial Spin Ice structure. It is
shown that thermal fluctuations can drive this magnetic spin system to evolve
under dynamic constraints imposed through interactions with an external
environment as predicted by the neurological free energy principle and active
inference. The structure is defined by two layers of magnetic nanoelements
where one layer is a square Artificial Spin Ice geometry. The other magnetic
layer functions as a sensory filter that mediates interaction between the
external environment and the hidden Artificial Spin Ice layer. Spin dynamics
displayed by the bilayer structure are shown to be well described using a
continuous form of a neurological free energy principle that has been
previously proposed as a high level description of certain biological neural
processes. Numerical simulations demonstrate that this proposed bilayer
geometry is able to reproduce theoretical results derived previously for
examples of active inference in neurological contexts.",2401.12211v4
2024-01-24,Dynamical Chiral Nernst Effect in Twisted Van der Waals Few Layers,"The Nernst effect is a fundamental thermoelectric conversion phenomenon that
was deemed to be possible only in systems with magnetic field or magnetization.
In this work, we propose a novel dynamical chiral Nernst effect that can appear
in two-dimensional van der Waals materials with chiral structural symmetry in
the absence of any magnetic degree of freedom. This unconventional effect is
triggered by time variation of an out-of-plane electric field, and has an
intrinsic quantum geometric origin linked to not only the intralayer
center-of-mass motion but also the interlayer coherence of electronic states.
We demonstrate the effect in twisted homobilayer and homotrilayer transition
metal dichalcogenides, where the strong twisted interlayer coupling leads to
sizable intrinsic Nernst conductivities well within the experimental capacity.
This work suggests a new route for electric control of thermoelectric
conversion.",2401.13278v1
2024-01-25,Universal collective Larmor-Silin mode emerging in magnetized correlated Dirac fermions,"Employing large-scale quantum Monte Carlo simulations, we find in magnetized
interacting Dirac fermion model, there emerges a new and universal collective
Larmor-Silin spin wave mode in the transverse dynamical spin susceptibility.
Such mode purely originates from the interaction among Dirac fermions and
distinguishes itself from the usual particle-hole continuum with finite
lifetime and clear dispersion, both at small and large momenta in a large
portion of the Brillouin zone. Our unbiased numerical results offer the dynamic
signature of this new collective excitations in interacting Dirac fermion
systems, and provide experimental guidance for inelastic neutron scattering,
electron spin resonance and other spectroscopic approaches in the investigation
of such universal collective modes in quantum Moire materials, topological
insulators and quantum spin liquid materials under magnetic field, with
quintessential interaction nature beyond the commonly assumed noninteracting
Dirac fermion or spinon approximations.",2401.14358v2
2024-03-13,Quantum skyrmion dynamics studied by neural network quantum states,"We study the dynamics of quantum skyrmions under a magnetic field gradient
using neural network quantum states. First, we obtain a quantum skyrmion
lattice ground state using variational Monte Carlo with a restricted Boltzmann
machine as the variational ansatz for a quantum Heisenberg model with
Dzyaloshinskii-Moriya interaction. Then, using the time-dependent variational
principle, we study the real-time evolution of quantum skyrmions after a
Hamiltonian quench with an inhomogeneous external magnetic field. We show that
field gradients are an effective way of manipulating and moving quantum
skyrmions. Furthermore, we demonstrate that quantum skyrmions can decay when
interacting with each other. This work shows that neural network quantum states
offer a promising way of studying the real-time evolution of quantum magnetic
systems that are outside the realm of exact diagonalization.",2403.08184v1
2016-07-18,Local Helioseismology of Emerging Active Regions: A Case Study,"Local helioseismology provides a unique opportunity to investigate the
subsurface structure and dynamics of active regions and their effect on the
large-scale flows and global circulation of the Sun. We use measurements of
plasma flows in the upper convection zone, provided by the Time-Distance
Helioseismology Pipeline developed for analysis of solar oscillation data
obtained by Helioseismic and Magnetic Imager (HMI) on Solar Dynamics
Observatory (SDO), to investigate the subsurface dynamics of emerging active
region NOAA 11726. The active region emergence was detected in deep layers of
the convection zone about 12 hours before the first bipolar magnetic structure
appeared on the surface, and 2 days before the emergence of most of the
magnetic flux. The speed of emergence determined by tracking the flow
divergence with depth is about 1.4 km/s, very close to the emergence speed in
the deep layers. As the emerging magnetic flux becomes concentrated in sunspots
local converging flows are observed beneath the forming sunspots. These flows
are most prominent in the depth range 1-3 Mm, and remain converging after the
formation process is completed. On the larger scale converging flows around
active region appear as a diversion of the zonal shearing flows towards the
active region, accompanied by formation of a large-scale vortex structure. This
process occurs when a substantial amount of the magnetic flux emerged on the
surface, and the converging flow pattern remains stable during the following
evolution of the active region. The Carrington synoptic flow maps show that the
large-scale subsurface inflows are typical for active regions. In the deeper
layers (10-13 Mm) the flows become diverging, and surprisingly strong beneath
some active regions. In addition, the synoptic maps reveal a complex evolving
pattern of large-scale flows on the scale much larger than supergranulation.",1607.04987v1
2019-11-22,"Role of Element-Specific Damping on the Ultrafast, Helicity-Independent All-Optical Switching Dynamics in Amorphous (Gd,Tb)Co Thin Films","Ultrafast control of the magnetization in ps timescales by fs laser pulses
offers an attractive avenue for applications such as fast magnetic devices for
logic and memory. However, ultrafast helicity-independent all-optical switching
(HI-AOS) of the magnetization has thus far only been observed in Gd-based,
ferrimagnetic amorphous (\textit{a}-) rare earth-transition metal
(\textit{a}-RE-TM) systems, and a comprehensive understanding of the reversal
mechanism remains elusive. Here, we report HI-AOS in ferrimagnetic
\textit{a}-Gd$_{22-x}$Tb$_x$Co$_{78}$ thin films, from x = 0 to x = 18, and
elucidate the role of Gd in HI-AOS in \textit{a}-RE-TM alloys and multilayers.
Increasing Tb content results in increasing perpendicular magnetic anisotropy
and coercivity, without modifying magnetization density, and slower
remagnetization rates and higher critical fluences for switching but still
shows picosecond HI-AOS. Simulations of the atomistic spin dynamics based on
the two-temperature model reproduce these results qualitatively and predict
that the lower damping on the RE sublattice arising from the small spin-orbit
coupling of Gd (with $L = 0$) is instrumental for the faster dynamics and lower
critical fluences of the Gd-rich alloys. Annealing
\textit{a}-Gd$_{10}$Tb$_{12}$Co$_{78}$ leads to slower dynamics which we argue
is due to an increase in damping. These simulations strongly indicate that
acounting for element-specific damping is crucial in understanding HI-AOS
phenomena. The results suggest that engineering the element specific damping of
materials can open up new classes of materials that exhibit low-energy,
ultrafast HI-AOS.",1911.09803v3
2018-08-03,Decoupled spin dynamics in the rare-earth orthoferrite YbFeO$_3$: Evolution of magnetic excitations through the spin-reorientation transition,"In this paper we present a comprehensive study of magnetic dynamics in the
rare-earth orthoferrite YbFeO$_3$ at temperatures below and above the
spin-reorientation (SR) transition $T_{\mathrm{SR}}=7.6$ K, in magnetic fields
applied along the $a, b$ and $c$ axes. Using single-crystal inelastic neutron
scattering, we observed that the spectrum of magnetic excitations consists of
two collective modes well separated in energy: 3D gapped magnons with a
bandwidth of $\sim$60 meV, associated with the antiferromagnetically (AFM)
ordered Fe subsystem, and quasi-1D AFM fluctuations of $\sim$1 meV within the
Yb subsystem, with no hybridization of those modes. The spin dynamics of the Fe
subsystem changes very little through the SR transition and could be well
described in the frame of semiclassical linear spin-wave theory. On the other
hand, the rotation of the net moment of the Fe subsystem at $T_{\mathrm{SR}}$
drastically changes the excitation spectrum of the Yb subsystem, inducing the
transition between two regimes with magnon and spinon-like fluctuations. At $T
< T_{\mathrm{SR}}$, the Yb spin chains have a well defined field-induced
ferromagnetic (FM) ground state, and the spectrum consists of a sharp
single-magnon mode, a two-magnon bound state, and a two-magnon continuum,
whereas at $T > T_{\mathrm{SR}}$ only a gapped broad spinon-like continuum
dominates the spectrum. In this work we show that a weak quasi-1D coupling
within the Yb subsystem $J_\text{Yb-Yb}$, mainly neglected in previous studies,
creates unusual quantum spin dynamics on the low energy scales. The results of
our work may stimulate further experimental search for similar compounds with
several magnetic subsystems and energy scales, where low-energy fluctuations
and underlying physics could be ""hidden"" by a dominating interaction.",1808.01269v2
2019-06-03,Motional Dynamical Decoupling for Matter-Wave Interferometry,"Matter-wave interferometry provides a remarkably sensitive tool for probing
minute forces and, potentially, the foundations of quantum physics by making
use of interference between spatially separated matter waves. Furthering this
development requires ever-increasing stability of the interferometer, typically
achieved by improving its physical isolation from the environment. Here we
introduce as an alternative strategy the concept of dynamical decoupling
applied to spatial degrees of freedom of massive objects. We show that the
superposed matter waves can be driven along paths in space that render their
superposition resilient to many important sources of noise. As a concrete
implementation, we present the case of matter-wave interferometers in a
magnetic field gradient based on either levitated or free-falling nanodiamonds
hosting a color center. We present an in-depth analysis of potential sources of
decoherence in such a setup and of the ability of our protocol to suppress
them. These effects include gravitational forces, interactions of the net
magnetic and dipole moments of the diamond with magnetic and electric fields,
surface dangling bonds, rotational degrees of freedom, Casimir-Polder forces,
and diamagnetic forces. Contrary to previous analyses, diamagnetic forces are
not negligible in this type of interferometers and, if not acted upon lead to
small separation distances that scale with the inverse of the magnetic field
gradient. We show that our motional dynamical decoupling strategy renders the
system immune to such limitations while continuing to protect its coherence
from environmental influences, achieving a linear-in-time growth of the
separation distance independent of the magnetic field gradient. Hence, motional
dynamical decoupling may become an essential tool in driving the sensitivity of
matter-wave interferometry to the next level.",1906.00835v1
2022-04-11,Curvilinear spin-wave dynamics beyond the thin-shell approximation: Magnetic nanotubes as a case study,"Surface curvature of magnetic systems can lead to many static and dynamic
effects which are not present in flat systems of the same material. These
emergent magnetochiral effects can lead to frequency nonreciprocity of spin
waves, which has been shown to be a bulk effect of dipolar origin and is
related to a curvature-induced symmetry breaking in the magnetic volume
charges. So far, such effects have been investigated theoretically mostly for
thin shells, where the spatial profiles of the spin waves can be assumed to be
homogeneous along the thickness. Here, using a finite-element dynamic-matrix
approach, we investigate the transition of the spin-wave spectrum from thin to
thick curvilinear shells, at the example of magnetic nanotubes in the vortex
state. With increasing thickness, we observe the appearance of higher-order
radial modes which are strongly hybridized and resemble the
perpendicular-standing-waves (PSSWs) in flat films. Along with an increasing
dispersion asymmetry, we uncover the curvature-induced non-reciprocity of the
mode profiles. This is explained in a very simple picture general for thick
curvilinear shells, considering the inhomogeneity of the emergent geometric
volume charges along the thickness of the shell. Such curvature-induced
mode-profile asymmetry also leads to non-reciprocal hybridization which can
facilitate unidirectional spin-wave propagation. With that, we also show how
curvature allows for nonlinear three-wave splitting of a higher-order radial
mode into secondary modes which can also propagate unidirectionally. We believe
that our study provides a significant contribution to the understanding of the
spin-wave dynamics in curvilinear magnetic systems, but also advertises these
for novel magnonic applications.",2204.05065v1
2022-06-25,Static and dynamic magnetization control of extrinsic multiferroics by the converse magneto-photostrictive effect,"In this work, photostrictive manipulations of static and dynamic magnetic
properties are demonstrated in an extrinsic multiferroic composite. The
photostriction is achieved with visible light in the blue region of the
spectrum. The composites consist of 5 nm or 10 nm magnetostrictive
Fe$_{81}$Ga$_{19}$ thin films coupled to a piezoelectric
(011)-Pb(Mg$_{1/3}$Nb$_{2/3}$)O$_3$-Pb(Zr,Ti)O$_3$ substrate. Angular dependent
magnetization reversals properties are largely enhanced or reduced under a
converse magneto-photostrictive effect (CMPE). The CMPE strength is analysed
with a novel coefficient named the converse magneto-photostrictive coupling
coefficient. This coefficient is proposed as a general approach to analyse and
to compare different extrinsic multiferroics under the converse
magneto-photostrictive effect. Its thickness dependence reveals that the CMPE
strength decreases with an increase of the Fe$_{81}$Ga$_{19}$ thickness.
Experimental evidence for a control of dynamic magnetic properties under CMPE
is then revealed by ferromagnetic resonance measurements. Resonant fields are
shifted under CMPE, whereas their linewidths remain constant. Furthermore,
resonant field shifts can be either positive or negative depending on the
in-plane angle. The largest shift under CMPE of +5.7 % is obtained for the 5 nm
sample. Our study shows that the CMPE provides an efficient approach for a
control of not only the static but also the dynamic magnetic properties in
extrinsic multiferroics.",2206.12703v3
2023-04-05,Scaling laws of the magnetic dynamics driven high harmonic generation in spin-orbit coupled systems,"The emergence of high order harmonics in carrier pumping from magnetic
dynamics in the presence of spin-orbit coupling has been only recently
predicted [{Phys. Rev. B \textbf{105}, L180415 (2022)}]. In this effect, the
number of the excited high harmonics is found to scale up with the spin-orbit
coupling parameter, the s-d exchange constant of the magnetic system and the
frequency of the driving dynamics. Motivated by the reproduction of the
above-mentioned effect, Nikoli\'{c} and Manjarres [arXiv:2212.06895,
arXiv:2112.14685] confirmed that there is indeed a generation of harmonics as a
result of the interplay between magnetic dynamics and spin-orbit interaction if
the exchange coupling $J$ is taken very small ($J=0.1\gamma$). However, they
claim that the ultrahigh harmonics obtained in the strong $J$ limit can only be
a result of numerical bugs. It turns out that in these works, conclusions built
from numerical simulations performed in the perturbative limit
($J/\hbar\omega\approx 10$) have been applied to criticize results obtained in
the extremely high non-linear regime ($J/\hbar\omega>10^3$). Regarding this
fact, we find it necessary to shed light on many aspects related to the physics
behind the predicted unconventional carrier pumping, in order to enlighten the
readers of the proposed effect and at the same time the authors of the
criticizing works. Here, we present numerical simulations of the scaling laws
underlying the effect. We study the low s-d exchange coupling limit,
corresponding to the regime studied in the above mentioned criticizing works.
And further explore, the high s-d coupling regime where the extremely nonlinear
emission is expected. Furthermore, we provide more insights on the effect and
demonstrate its robustness and reproducibility in both 1D and 2D magnetic
structures.",2304.02619v2
1998-04-10,Dynamical Collapse of Nonrotating Magnetic Molecular Cloud Cores: Evolution Through Point-Mass Formation,"We present a numerical simulation of the dynamical collapse of a nonrotating
magnetic molecular cloud core and follow the core's evolution through the
formation of a central point mass and its subsequent growth to a 1 solar-mass
protostar. The epoch of point-mass formation (PMF) is investigated by a self-
consistent extension of previously presented models of core formation and
contraction in axisymmetric, self-gravitating, isothermal, magnetically
supported interstellar molecular clouds. Prior to PMF, the core is dynamically
contracting and is not well approximated by a quasistatic equilibrium model.
Ambipolar diffusion, which plays a key role in the early evolution of the core,
is unimportant during the dynamical pre-PMF collapse phase. However, the
appearance of a central mass, through its effect on the gravitational field in
the inner core regions, leads to a ""revitalization"" of ambipolar diffusion in
the weakly ionized gas surrounding the central protostar. This process is so
efficient that it leads to a decoupling of the field from the matter and
results in an outward propagating hydromagnetic C-type shock. The existence of
an ambipolar diffusion-mediated shock was predicted by Li & McKee (1996), and
we find that the basic shock structure given by their analytical model is well
reproduced by our more accurate numerical results. Our calculation also
demonstrates that ambipolar diffusion, rather than Ohmic diffusivity operating
in the innermost core region, is the main field decoupling mechanism
responsible for driving the shock after PMF.",9804113v1
2004-08-10,"Collective Spin Dynamics in the ""Coherence Window"" for Quantum Nanomagnets","The spin coherence phenomena and the possibility of their observation in
nanomagnetic insulators attract more and more attention in the last several
years. Recently it has been shown that in these systems in large transverse
magnetic field there can be a fairly narrow ""coherence window"" for phonon and
nuclear spin-mediated decoherence. What kind of spin dynamics can then be
expected in this window in a crystal of magnetic nanomolecules coupled to
phonons, to nuclear spin bath and it to each other via dipole-dipole
interactions? Studying multispin correlations, we determine the region of
parameters where ""coherent clusters"" of collective spin excitations can appear.
Although two particular systems, namely crystals of $Fe_8$-triazacyclonane and
$Mn_{12}$-acetate molecules, are used in this work to illustrate the results,
here we are not trying to predict an existence of collective coherent dynamics
in some particular system. Instead, we discuss the way how any crystalline
system of dipole-dipole coupled nanomolecules can be analyzed to decide whether
this system is suitable for attempts to observe coherent dynamics. The
presented analysis can be useful in the search for magnetic systems showing the
spin coherence phenomena.",0408220v3
2003-07-03,Effect of the superconducting wiggler on the DELSY beam dynamics,"The project DELSY is being under development at JINR, Dubna, Russia. This
synchrotron radiation source is dedicated to the investigation on condensed
matter physics, atomic physics, biology, medicine, chemistry, micromechanics,
lithography and others. The storage ring DELSY is an electron storage ring with
the beam energy 1.2 GeV and 4 straight sections to accommodate accelerator
equipment and insertion devices. One of the straight sections is intended for a
10 T superconducting wiggler (wavelength shifter) and one for the undulator
with 150 periods and a magnetic field of 0.75 T. The wiggler will influence
many aspects of beam dynamics: linear motion, dynamic aperture, emittance,
damping times etc. The problem is rather serious for the DELSY machine because
the energy of the electron beam is small while the wiggler's magnetic field is
strong. In this paper we consider two models of the wiggler's magnetic field
with and without the focusing caused by the sextupolar field of the wiggler as
we need to develop the requirements to the wiggler design. We study the
influence of the 10 T wiggler on the beam dynamics in the DELSY storage ring
and propose a possible scheme to cure it. The combined work of the insertion
device is presented too.",0307028v1
2007-04-09,Conjugate field and fluctuation-dissipation relation for the dynamic phase transition in the two-dimensional kinetic Ising model,"The two-dimensional kinetic Ising model, when exposed to an oscillating
applied magnetic field, has been shown to exhibit a nonequilibrium,
second-order dynamic phase transition (DPT), whose order parameter Q is the
period-averaged magnetization. It has been established that this DPT falls in
the same universality class as the equilibrium phase transition in the
two-dimensional Ising model in zero applied field. Here we study for the first
time the scaling of the dynamic order parameter with respect to a nonzero,
period-averaged, magnetic `bias' field, H_b, for a DPT produced by a
square-wave applied field. We find evidence that the scaling exponent,
\delta_d, of H_b at the critical period of the DPT is equal to the exponent for
the critical isotherm, \delta_e, in the equilibrium Ising model. This implies
that H_b is a significant component of the field conjugate to Q. A finite-size
scaling analysis of the dynamic order parameter above the critical period
provides further support for this result. We also demonstrate numerically that,
for a range of periods and values of H_b in the critical region, a
fluctuation-dissipation relation (FDR), with an effective temperature
T_{eff}(T, P, H_0) depending on the period, and possibly the temperature and
field amplitude, holds for the variables Q and H_b. This FDR justifies the use
of the scaled variance of Q as a proxy for the nonequilibrium susceptibility,
\partial<Q> / \partial H_b, in the critical region.",0704.1123v2
2008-07-02,Dynamical phase transitions in long-range Hamiltonian systems and Tsallis distributions with a time-dependent index,"We study dynamical phase transitions in systems with long-range interactions,
using the Hamiltonian Mean Field (HMF) model as a simple example. These systems
generically undergo a violent relaxation to a quasi-stationary state (QSS)
before relaxing towards Boltzmann equilibrium. In the collisional regime, the
out-of-equilibrium one-particle distribution function (DF) is a
quasi-stationary solution of the Vlasov equation, slowly evolving in time due
to finite $N$ effects. For subcritical energies $7/12<U<3/4$, we exhibit cases
where the DF is well-fitted by a Tsallis $q$-distribution with an index $q(t)$
slowly decreasing in time from $q\simeq 3$ (semi-ellipse) to $q=1$ (Boltzmann).
When the index $q(t)$ reaches a critical value $q_{crit}(U)$, the
non-magnetized (homogeneous) phase becomes Vlasov unstable and a dynamical
phase transition is triggered, leading to a magnetized (inhomogeneous) state.
While Tsallis distributions play an important role in our study, we explain
this dynamical phase transition by using only conventional statistical
mechanics. For supercritical energies, we report for the first time the
existence of a magnetized QSS with a very long lifetime.",0807.0324v2
2008-09-17,"Late stage, non-equilibrium dynamics in the dipolar Ising model","Magnetic domain structures are a fascinating area of study with interest
deriving both from technological applications and fundamental scientific
questions. The nature of the striped magnetic phases observed in ultra-thin
films is one such intriguing system. The non-equilibrium dynamics of such
systems as they evolve toward equilibrium has only recently become an area of
interest and previous work on model systems showed evidence of complex, slow
dynamics with glass-like properties as the stripes order mesoscopically. To aid
in the characterization of the observed phases and the nature of the
transitions observed in model systems we have developed an efficient method for
identifying clusters or domains in the spin system, where the clusters are
based on the stripe orientation. Thus we are able to track the growth and decay
of such clusters of stripes in a Monte Carlo simulation and observe directly
the nature of the slow dynamics. We have applied this method to consider the
growth and decay of ordered domains after a quench from a saturated magnetic
state to temperatures near and well below the critical temperature in the two
dimensional dipolar Ising model. We discuss our method of identifying stripe
domains or clusters of stripes within this model and present the results of our
investigations.",0809.2808v1
2008-12-05,Using Magnetic Activity and Galactic Dynamics to Constrain the Ages of M Dwarfs,"We present a study of the dynamics and magnetic activity of M dwarfs using
the largest spectroscopic sample of low-mass stars ever assembled. The age at
which strong surface magnetic activity (as traced by H-alpha) ceases in M
dwarfs has been inferred to have a strong dependence on mass (spectral type,
surface temperature) and explains previous results showing a large increase in
the fraction of active stars at later spectral types. Using spectral
observations of more than 40000 M dwarfs from the Sloan Digital Sky Survey, we
show that the fraction of active stars decreases as a function of vertical
distance from the Galactic plane (a statistical proxy for age), and that the
magnitude of this decrease changes significantly for different M spectral
types. Adopting a simple dynamical model for thin disk vertical heating, we
assign an age for the activity decline at each spectral type, and thus
determine the activity lifetimes for M dwarfs. In addition, we derive a
statistical age-activity relation for each spectral type using the dynamical
model, the vertical distance from the Plane and the H-alpha emission line
luminosity of each star (the latter of which also decreases with vertical
height above the Galactic plane).",0812.1223v1
2009-06-18,Electron quantum dynamics in closed and open potentials at high magnetic fields: Quantization and lifetime effects unified by semicoherent states,"We have developed a Green's function formalism based on the use of an
overcomplete semicoherent basis of vortex states, specially devoted to the
study of the Hamiltonian quantum dynamics of electrons at high magnetic fields
and in an arbitrary potential landscape smooth on the scale of the magnetic
length. This formalism is used here to derive the exact Green's function for an
arbitrary quadratic potential in the special limit where Landau level mixing
becomes negligible. This solution remarkably embraces under a unified form the
cases of confining and unconfining quadratic potentials. This property results
from the fact that the overcomplete vortex representation provides a more
general type of spectral decomposition of the Hamiltonian operator than usually
considered. Whereas confining potentials are naturally characterized by
quantization effects, lifetime effects emerge instead in the case of
saddle-point potentials. Our derivation proves that the appearance of lifetimes
has for origin the instability of the dynamics due to quantum tunneling at
saddle points of the potential landscape. In fact, the overcompleteness of the
vortex representation reveals an intrinsic microscopic irreversibility of the
states synonymous with a spontaneous breaking of the time symmetry exhibited by
the Hamiltonian dynamics.",0906.3375v2
2011-06-21,Symmetry and the macroscopic dynamics of antiferromagnetic materials in the presence of spin-polarized current,"Antiferromagnetic (AFM) materials with zero or vanishingly small macroscopic
magnetization are nowadays the constituent elements of spintronic devices.
However, possibility to use them as active elements that show nontrivial
controllable magnetic dynamics is still discussible. In the present paper we
extend the theory [A.F.Andreev, V.I.Marchenko, Sov. Phys. --- Uspekhi, 23
(1980), 21] of macroscopic dynamics in AFMs for the cases typical for
spin-valve devices. In particular, we consider the solid-like magnetic dynamics
of AFMs with strong exchange coupling in the presence of spin-polarized current
and give an expression for the current-induced Rayleigh dissipation function in
terms of the rotation vector for different types %generalized potential of
AFMs. Basing on the analysis of linearized equations of motion we predict the
current-induced reorientation and AFM resonance, and found the values of
critical currents in terms of AFMR frequencies and damping constants. We show
the possibility of current-induced spin-diode effect and second-harmonic
generation in AF layer. The proposed approach is generalized for the
description of current-related phenomena in inhomogeneous AFMs.",1106.4231v3
2011-06-22,Effect of spin diffusion on current generated by spin motive force,"Spin motive force is a spin-dependent force on conduction electrons induced
by magnetization dynamics. In order to examine its effects on magnetization
dynamics, it is indispensable to take into account spin accumulation, spin
diffusion, and spin-flip scattering since the spin motive force is in general
nonuniform. We examine the effects of all these on the way the spin motive
force generates the charge and spin currents in conventional situations, where
the conduction electron spin relaxation dynamics is much faster than the
magnetization dynamics. When the spin-dependent electric field is spatially
localized, which is common in experimental situations, we find that the
conservative part of the spin motive force is unable to generate the charge
current due to the cancelation effect by the diffusion current. We also find
that the spin current is a nonlocal function of the spin motive force and can
be effectively expressed in terms of nonlocal Gilbert damping tensor. It turns
out that any spin independent potential such as Coulomb potential does not
affect our principal results. At the last part of this paper, we apply our
theory to current-induced domain wall motion.",1106.4389v2
2011-08-06,A Statistical Study on the Morphology of Rays and Dynamics of Blobs in the Wake of Coronal Mass Ejections,"In this paper, with a survey through the Large Angle and Spectrometric
Coronagraph (LASCO) data from 1996 to 2009, we present 11 events with plasma
blobs flowing outwards sequentially along a bright coronal ray in the wake of a
coronal mass ejection. The ray is believed to be associated with the current
sheet structure that formed as a result of solar eruption, and the blobs are
products of magnetic reconnection occurring along the current sheet. The ray
morphology and blob dynamics are investigated statistically. It is found that
the apparent angular widths of the rays at a fixed time vary in a range of
2.1-6.6 (2.0-4.4) degrees with an average of 3.5 (2.9) degrees at 3 (4) Rs,
respectively, and the observed durations of the events vary from 12 h to a few
days with an average of 27 h. It is also found, based on the analysis of blob
motions, that 58% (26) of the blobs were accelerated, 20% (9) were decelerated,
and 22% (10) moved with a nearly-constant speed. Comparing the dynamics of our
blobs and those that are observed above the tip of a helmet streamer, we find
that the speeds and accelerations of the blobs in these two cases differ
significantly. It is suggested that these differences of the blob dynamics stem
from the associated magnetic reconnection involving different magnetic field
configurations and triggering processes.",1108.1481v1
2012-02-15,Noise studies of magnetization dynamics in dilute magnetic semiconductor heterostructures,"We study theoretically and experimentally the frequency and temperature
dependence of resistivity noise in semiconductor heterostructures delta-doped
by Mn. The resistivity noise is observed to be non-monotonous as a function of
frequency. As a function of temperature, the noise increases by two orders of
magnitude for a resistivity increase of about 50%. We study two possible
sources of resistivity noise -- dynamic spin fluctuations and charge
fluctuations, and find that dynamic spin fluctuations are more relevant for the
observed noise data. The frequency and temperature dependence of resistivity
noise provide important information on the nature of the magnetic interactions.
In particular, we show how noise measurements can help resolve a long standing
debate on whether the Mn-doped GaAs is an p-d Zener/RKKY or double exchange
ferromagnet. Our analysis includes the effect of different kinds of disorder
such as spin-glass type of interactions and a site-dilution type of disorder.
We find that the resistivity noise in these structures is well described by a
disordered RKKY ferromagnet model dynamics with a conserved order parameter.",1202.3207v2
2012-05-11,Nonequilibrium quantum dynamics of the magnetic Anderson model,"We study the non-equilibrium dynamics of a spinful single-orbital quantum dot
with an incorporated quantum mechanical spin-1/2 magnetic impurity. Due to the
spin degeneracy, double occupancy is allowed, and Coulomb interaction together
with the exchange coupling of the magnetic impurity influence the dynamics. By
extending the iterative summation of real-time path integrals (ISPI) to this
coupled system, we monitor the time-dependent non-equilibrium current and the
impurity spin polarization to determine features of the time-dependent
non-equilibrium dynamics. We particulary focus on the deep quantum regime,
where all time and energy scales are of the same order of magnitude and no
small parameter is available. We observe a significant influence of the
non-equilibrium decay of the impurity spin polarization both in the presence
and in the absence of Coulomb interaction. The exponential relaxation is faster
for larger bias voltages, electron-impurity interactions and temperatures. We
show that the exact relaxation rate deviates from the corresponding
perturbative result. In addition, we study in detail the impurity's back action
on the charge current and find a reduction of the stationary current for
increasing coupling to the impurity. Moreover, our approach allows us to
systematically distinguish mean-field Coulomb and impurity effects from the
influence of quantum fluctuations and flip-flop scattering, respectively. In
fact, we find a local maximum of the current for a finite Coulomb interaction
due to the presence of the impurity.",1205.2462v2
2013-07-18,Dynamical transition in the D = 3 Edwards-Anderson spin glass in an external magnetic field,"We study the off-equilibrium dynamics of the three-dimensional Ising spin
glass in the presence of an external magnetic field. We have performed
simulations both at fixed temperature and with an annealing protocol. Thanks to
the Janus special-purpose computer, based on FPGAs, we have been able to reach
times equivalent to 0.01 seconds in experiments. We have studied the system
relaxation both for high and for low temperatures, clearly identifying a
dynamical transition point. This dynamical temperature is strictly positive and
depends on the external applied magnetic field. We discuss different
possibilities for the underlying physics, which include a thermodynamical
spin-glass transition, a mode-coupling crossover or an interpretation
reminiscent of the random first-order picture of structural glasses.",1307.4998v3
2014-08-20,Coherent transport and manipulation of spins in indirect exciton nanostructures,"We report on the coherent control and transport of indirect exciton (IX)
spins in GaAs double quantum well (DQW) nanostructures. The spin dynamics was
investigated by optically generating spins using a focused, circularly
polarized light spot and by probing their spatial distribution using spatially
and polarization resolved photoluminescence spectroscopy. Optically injected
exciton spins precess while diffusing over distances exceeding 20 {\mu}m from
the excitation spot with a spatial precession frequency that depends on the
spin transport direction as well as on the bias applied across the DQW
structure. This behavior is attributed to the spin precession in the effective
magnetic field induced by the spin-orbit interaction. From the dependence of
the spin dynamics on the transport direction, bias and external magnetic fields
we directly determined the Dresselhaus and Rashba spin splitting coefficients
for the structure. The precession dynamics is essentially independent on the IX
density, thus indicating that the long spin lifetimes are not associated with
IX collective effects. The latter, together with the negligible contribution of
holes to the spin dynamics, are rather attributed to spatial separation of the
electron and hole wave functions by the electric field, which reduces the
electron-hole exchange interaction. Coherent spin precession over long
transport distances as well as the control of the spin vector using electric
and magnetic fields open the way for the application of IX spins in the quantum
information processing.",1408.4547v1
2014-09-16,Resolving the relative influence of strong field spacetime dynamics and MHD on circumbinary disk physics,"In this paper we evolve magnetized and unmagnetized circumbinary accretion
disks around supermassive black hole binaries in the relativistic regime. We
use a post-Newtonian expansion to construct an analytical spacetime and
determine how the order of the post-Newtonian (PN) expansion affects the
dynamics of the gas. We find very small differences in the late-time bulk
dynamics of non-magnetized hydrodynamic evolutions between the two spacetimes
down to separations of approximately $40GM/c^2$ where $M$ is the total mass of
the binary. For smaller separations, the differences due to PN-order become
comparable to differences caused by using initial data further from
equilibrium. For magnetized gas, MHD stresses, which drives the accretion
dynamics, tends to mask all higher order PN effects even at separations of
$20GM/c^2$, leading to essentially the same observed electromagnetic
luminosity. This implies that our calculations of the EM signal may be robust
down to small binary separations. Our investigation is the first to demonstrate
how the level of PN accuracy affects a circumbinary disk's evolution and
informs us of the range in separation within which to trust the PN
approximation for this kind of study. We also address the influence the initial
conditions and binary separation have on simulation predictions.",1409.4787v2
2014-12-23,Dynamical symmetries of Markov processes with multiplicative white noise,"We analyse various properties of stochastic Markov processes with
multiplicative white noise. We take a single-variable problem as a simple
example, and we later extend the analysis to the Landau-Lifshitz-Gilbert
equation for the stochastic dynamics of a magnetic moment. In particular, we
focus on the non-equilibrium transfer of angular momentum to the magnetization
from a spin-polarised current of electrons, a technique which is widely used in
the context of spintronics to manipulate magnetic moments. We unveil two hidden
dynamical symmetries of the generating functionals of these Markovian
multiplicative white-noise processes. One symmetry only holds in equilibrium
and we use it to prove generic relations such as the fluctuation-dissipation
theorems. Out of equilibrium, we take profit of the symmetry-breaking terms to
prove fluctuation theorems. The other symmetry yields strong dynamical
relations between correlation and response functions which can notably simplify
the numerical analysis of these problems. Our construction allows us to clarify
some misconceptions on multiplicative white-noise stochastic processes that can
be found in the literature. In particular, we show that a first-order
differential equation with multiplicative white noise can be transformed into
an additive-noise equation, but that the latter keeps a non-trivial memory of
the discretisation prescription used to define the former.",1412.7564v2
2015-09-01,Light Bridge in a Developing Active Region. I. Observation of Light Bridge and its Dynamic Activity Phenomena,"Light bridges, the bright structures that divide the umbra of sunspots and
pores into smaller pieces, are known to produce wide variety of activity events
in solar active regions (ARs). It is also known that the light bridges appear
in the assembling process of nascent sunspots. The ultimate goal of this series
of papers is to reveal the nature of light bridges in developing ARs and the
occurrence of activity events associated with the light bridge structures from
both observational and numerical approaches. In this first paper, exploiting
the observational data obtained by Hinode, IRIS, and Solar Dynamics Observatory
(SDO), we investigate the detailed structure of the light bridge in NOAA AR
11974 and its dynamic activity phenomena. As a result, we find that the light
bridge has a weak, horizontal magnetic field, which is transported from the
interior by large-scale convective upflow and is surrounded by strong, vertical
fields of adjacent pores. In the chromosphere above the bridge, a transient
brightening occurs repeatedly and intermittently, followed by a recurrent dark
surge ejection into higher altitudes. Our analysis indicates that the
brightening is the plasma heating due to magnetic reconnection at lower
altitudes, while the dark surge is the cool, dense plasma ejected from the
reconnection region. From the observational results, we conclude that the
dynamic activity observed in a light bridge structure such as chromospheric
brightenings and dark surge ejections are driven by magnetoconvective evolution
within the light bridge and its interaction with surrounding magnetic fields.",1509.00183v1
2015-10-18,Magnetization dynamics in Fe$_x$Co$_{1-x}$ in presence of chemical disorder,"In this paper, we present a theoretical formulation of magnetization dynamics
in disordered binary alloys based on Kubo linear response theory interfaced
with the combination of seamlessly three approaches; density functional based
tight-binding linear muffin-tin orbitals, generalized recursion and Augmented
space formalism. We apply this method to study the magnetization dynamics in
chemically disordered Fe$_x$Co$_{1-x}$ ($x$ = 0.2, 0.5, 0.8) alloys. We
reported that the magnon energies decrease with an increase in Co
concentration. Significant magnon softening has been observed in
Fe$_{20}$Co$_{80}$ at the Brillouin zone boundary. The magnon-electron
scattering increases with increasing Co content which in turn modifies the
hybridization between the Fe and Co atoms. This reduces the exchange energy
between the atoms and soften down the magnon energy. The lowest magnon lifetime
in found in Fe$_{50}$Co$_{50}$, where disorder is maximum. This clearly
indicates that the damping of magnon energies in Fe$_x$Co$_{1-x}$ is governed
by the hybridization between Fe and Co whereas the magnon lifetime is
controlled by disorder configuration. Our atomistic spin dynamics simulations
show a reasonable agreement with our theoretical approach in magnon dispersion
for different alloy compositions.",1510.05200v4
2016-07-28,An `Analytic Dynamical Magnetosphere' formalism for X-ray and optical emission from slowly rotating magnetic massive stars,"Slowly rotating magnetic massive stars develop ""dynamical magnetospheres""
(DM's), characterized by trapping of stellar wind outflow in closed magnetic
loops, shock heating from collision of the upflow from opposite loop
footpoints, and subsequent gravitational infall of radiatively cooled material.
In 2D and 3D magnetohydrodynamic (MHD) simulations the interplay among these
three components is spatially complex and temporally variable, making it
difficult to derive observational signatures and discern their overall scaling
trends.Within a simplified, steady-state analysis based on overall conservation
principles, we present here an ""analytic dynamical magnetosphere"" (ADM) model
that provides explicit formulae for density, temperature and flow speed in each
of these three components -- wind outflow, hot post-shock gas, and cooled
inflow -- as a function of colatitude and radius within the closed (presumed
dipole) field lines of the magnetosphere. We compare these scalings with
time-averaged results from MHD simulations, and provide initial examples of
application of this ADM model for deriving two key observational diagnostics,
namely hydrogen H-alpha emission line profiles from the cooled infall, and
X-ray emission from the hot post-shock gas. We conclude with a discussion of
key issues and advantages in applying this ADM formalism toward derivation of a
broader set of observational diagnostics and scaling trends for massive stars
with such dynamical magnetospheres.",1607.08568v1
2016-12-06,A New Two-Fluid Radiation-Hydrodynamical Model for X-ray Pulsar Accretion Columns,"Previous research centered on the hydrodynamics in X-ray pulsar accretion
columns has largely focused on the single-fluid model, in which the
super-Eddington luminosity inside the column decelerates the flow to rest at
the stellar surface. This type of model has been relatively successful in
describing the overall properties of the accretion flows, but it does not
account for the possible dynamical effect of the gas pressure. On the other
hand, the most successful radiative transport models for pulsars generally do
not include a rigorous treatment of the dynamical structure of the column,
instead assuming an ad hoc velocity profile. In this paper, we explore the
structure of X-ray pulsar accretion columns using a new, self-consistent,
""two-fluid"" model, which incorporates the dynamical effect of the gas and
radiation pressure, the dipole variation of the magnetic field, the
thermodynamic effect of all of the relevant coupling and cooling processes, and
a rigorous set of physical boundary conditions. The model has six free
parameters, which we vary in order to approximately fit the phase-averaged
spectra in Her X-1, Cen X-3, and LMC X-4. In this paper, we focus on the
dynamical results, which shed new light on the surface magnetic field strength,
the inclination of the magnetic field axis relative to the rotation axis, the
relative importance of gas and radiation pressure, and the radial variation of
the ion, electron, and inverse-Compton temperatures. The results obtained for
the X-ray spectra are presented in a separate paper.",1612.02411v2
2018-02-26,Fermion Self-energy and Pseudovector Condensate in NJL Model with External Magnetic Field,"In this paper, we aim to study the complete self-energy in the fermion
propagator within two-flavor NJL model in the case of finite temperature,
chemical potential and external magnetic field. Through Fierz transformation we
prove that the self-energy is not simply proportional to dynamical mass in the
presence of chemical potential, moreover, it contains four kinds of condensates
after introducing external magnetic field. We find out the appropriate and
complete form of self-energy and establish new gap equations. We take two of
the four condensates (scalar and pseudovector condensates) to make an
approximation and simplify the gap equations. The numerical results show that
not only the dynamical mass get quantitative modification, but also the
properties of Nambu phase and Wigner phase are significantly different with
classic results. Instead of classic Wigner phase with zero dynamic mass in the
massless NJL model, we propose a new phase - quasi-Wigner phase in this
article, it has small but nonzero dynamic mass, with increasing chemical
potential, eventually Nambu phase will turn into quasi-Wigner phase with
first-order phase transition, therefore the chiral symmetry can never be fully
restored but be partially restored. Furthermore, we prove that pseudovector
condensate in self-energy can generate energy splitting in dispersion relation,
it will cause minor differences of particle numbers with the split energy
levels.",1802.09942v2
2018-10-05,Coupling ultracold matter to dynamical gauge fields in optical lattices: From flux-attachment to Z2 lattice gauge theories,"Artificial magnetic fields and spin-orbit couplings have been recently
generated in ultracold gases in view of realizing topological states of matter
and frustrated magnetism in a highly-controllable environment. Despite being
dynamically tunable, such artificial gauge fields are genuinely classical and
exhibit no back-action from the neutral particles. Here we go beyond this
paradigm, and demonstrate how quantized dynamical gauge fields can be created
in mixtures of ultracold atoms in optical lattices. Specifically, we propose a
protocol by which atoms of one species carry a magnetic flux felt by another
species, hence realizing an instance of flux-attachment. This is obtained by
combining coherent lattice modulation techniques with strong Hubbard
interactions. We demonstrate how this setting can be arranged so as to
implement lattice models displaying a local Z2 gauge symmetry, both in one and
two dimensions. We also provide a detailed analysis of a ladder toy model,
which features a global Z2 symmetry, and reveal the phase transitions that
occur both in the matter and gauge sectors. Mastering flux-attachment in
optical lattices envisages a new route towards the realization of
strongly-correlated systems with properties dictated by an interplay of
dynamical matter and gauge fields.",1810.02777v1
2018-11-25,Characterizing breathing dynamics of magnetic (anti-)skyrmions within the Hamiltonian formalism,"We derive an effective Hamiltonian system describing the low energy dynamics
of circular magnetic skyrmions and antiskyrmions. Using scaling and symmetry
arguments we model (anti-)skyrmion dynamics through a finite set of coupled,
canonically conjugated, collective coordinates. The resulting theoretical
description is independent of both micromagnetic details as well as any
specificity in the ansatz of the skyrmion profile. Based on the Hamiltonian
structure we derive a general description for breathing dynamics of
(anti-)skyrmions in the limit of radius much larger than the domain wall width.
The effective energy landscape reveals two qualitatively different types of
breathing behavior. For small energy perturbations we reproduce the well-known
small breathing mode excitations, where the magnetic moments of the skyrmion
oscillate around their equilibrium solution. At higher energies we find a
breathing behavior where the in-plane angle of the skyrmion continuously
precesses, transforming N\'eel to Bloch skyrmions and vice versa. For a damped
system we observe the transition from the continuously rotating and breathing
skyrmion into the oscillatory one. We analyze the characteristic frequencies of
both breathing types, as well as their amplitudes and distinct energy
dissipation rates. For rotational (oscillatory) breathing modes we predict on
average a linear (exponential) decay in energy. We argue that this stark
difference in dissipative behavior should be observable in the frequency
spectrum of excited (anti-)skyrmions.",1811.09949v1
2018-12-17,Comparative study of vortex dynamics in CaKFe4As4 and Ba0.6K0.4Fe2As2 single crystals,"We investigate the vortex dynamics in two typical hole doped iron based
superconductors CaKFe$_4$As$_4$ (CaK1144) and Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$
(BaK122) with similar superconducting transition temperatures. It is found that
the magnetization hysteresis loop exhibits a clear second peak effect in BaK122
in wide temperature region while it is absent in CaK1144. However, a second
peak effect of critical current density versus temperature is observed in
CaK1144, which is however absent in BaK122. The different behaviors of second
peak effect in BaK122 and CaK1144 may suggest distinct origins of vortex
pinning in different systems. Magnetization and its relaxation have also been
measured by using dynamical and conventional relaxation methods for both
systems. Analysis and comparison of the two distinct systems show that the
vortex pinning is stronger and the critical current density is higher in BaK122
system. It is found that the Maley's method can be used and thus the activation
energy can be determined in BaK122 by using the time dependent magnetization in
wide temperature region, but this is not applicable in CaK1144 systems. Finally
we present the different regimes with distinct vortex dynamics in the
field-temperature diagram for the two systems.",1812.06823v1
2019-03-27,Emergence of stationary uphill currents in 2D Ising models: the role of reservoirs and boundary conditions,"We investigate the dynamics of a 2D Ising model on a square lattice with
conservative Kawasaki dynamics in the bulk, coupled with two external
reservoirs that pull the dynamics out of equilibrium. Two different mechanisms
for the action of the reservoirs are considered. In the first, called ISF, the
condition of local equilibrium between reservoir and the lattice is not
satisfied. The second mechanism, called DB, implements a detailed balance
condition, thus satisfying the local equilibrium property. We provide numerical
evidence that, for a suitable choice of the temperature (i.e. below the
critical temperature of the equilibrium 2D Ising model) and the reservoir
magnetizations, in the long time limit the ISF model undergoes a ferromagnetic
phase transition and gives rise to stationary uphill currents, namely positive
spins diffuse from the reservoir with lower magnetization to the reservoir with
higher magnetization. The same phenomenon does not occur for DB dynamics with
properly chosen boundary conditions. Our analysis extends the results reported
in Colangeli et al. (Phys. Rev. E, 97:030103(R), 2018), shedding also light on
the effect of temperature and the role of different boundary conditions for
this model. These issues may be relevant in a variety of situations (e.g.
mesoscopic systems) in which the violation of the local equilibrium condition
produces unexpected phenomena that seem to contradict the standard laws of
transport.",1903.11300v1
2019-11-05,Continuous-time quantum walks on planar lattices and the role of the magnetic field,"We address the dynamics of continuous-time quantum walk (CTQW) on planar 2D
lattice graphs, i.e. those forming a regular tessellation of the Euclidean
plane (triangular, square, and honeycomb lattice graphs). We first consider the
free particle: on square and triangular lattice graphs we observe the
well-known ballistic behavior, whereas on the honeycomb lattice graph we obtain
a sub-ballistic one, although still faster than the classical diffusive one. We
impute this difference to the different amount of coherence generated by the
evolution and, in turn, to the fact that, in 2D, the square and the triangular
lattices are Bravais lattices, whereas the honeycomb one is non-Bravais. From
the physical point of view, this means that CTQWs are not universally
characterized by the ballistic spreading. We then address the dynamics in the
presence of a perpendicular uniform magnetic field and study the effects of the
field by two approaches: (i) introducing the Peierls phase-factors, according
to which the tunneling matrix element of the free particle becomes complex, or
(ii) spatially discretizing the Hamiltonian of a spinless charged particle in
the presence of a magnetic field. Either way, the dynamics of an initially
localized walker is characterized by a lower spread compared to the free
particle case, the larger is the field the more localized stays the walker.
Remarkably, upon analyzing the dynamics by spatial discretization of the
Hamiltonian (vector potential in the symmetric gauge), we obtain that the
variance of the space coordinate is characterized by pseudo-oscillations, a
reminiscence of the harmonic oscillator behind the Hamiltonian in the
continuum, whose energy levels are the well-known Landau levels.",1911.01703v1
2020-01-09,Role of longitudinal fluctuations in L$1_0$ FePt,"L$1_0$ FePt is a technologically important material for a range of novel data
storage applications. In the ordered FePt structure the normally non-magnetic
Pt ion acquires a magnetic moment, which depends on the local field originating
from the neighboring Fe atoms. In this work a model of FePt is constructed,
where the induced Pt moment is simulated by using combined longitudinal and
rotational spin dynamics. The model is parameterized to include a linear
variation of the moment with the exchange field, so that at the Pt site the
magnetic moment depends on the Fe ordering. The Curie temperature of FePt is
calculated and agrees well with similar models that incorporate the Pt dynamics
through an effective Fe-only Hamiltonian. By computing the dynamic correlation
function the anisotropy field and the Gilbert damping are extracted over a
range of temperatures. The anisotropy exhibits a power-law dependence with
temperature with exponent $n\approx2.1$. This agrees well with what observed
experimentally and it is obtained without including a two-ion anisotropy term
as in other approaches. Our work shows that incorporating longitudinal
fluctuations into spin dynamics calculations is crucial for understanding the
properties of materials with induced moments.",2001.03074v1
2020-02-10,Giant electron-phonon coupling of the breathing plane oxygen phonons in the dynamic stripe phase of La$_{1.67}$Sr$_{0.33}$NiO$_4$,"Doped antiferromagnets host a vast array of physical properties and learning
how to control them is one of the biggest challenges of condensed matter
physics. La$_{1.7}$Sr$_{0.3}$NiO$_4$ (LSNO) is a classic example of such a
material. At low temperatures holes introduced via substitution of La by Sr
segregate into lines to form boundaries between magnetically ordered domains in
the form of stripes. The stripes become dynamic at high temperatures, but LSNO
remains insulating presumably because an interplay between magnetic
correlations and electron-phonon coupling localizes charge carriers. Magnetic
degrees of freedom have been extensively investigated in this system, but
phonons are almost completely unexplored. We searched for electron-phonon
anomalies in LSNO by inelastic neutron scattering. Giant renormalization of
plane Ni-O bond-stretching modes that modulate the volume around Ni appears on
entering the dynamic charge stripe phase. Other phonons are a lot less
sensitive to stripe melting. Dramatic overdamping of the breathing modes
indicates that dynamic stripe phase may host small polarons. We argue that this
feature sets electron-phonon coupling in nickelates apart from that in cuprates
where breathing phonons are not overdamped and point out remarkable
similarities with the colossal magnetoresistance (CMR) manganites.",2002.03908v3
2020-06-11,Coupling a mobile hole to an antiferromagnetic spin background: Transient dynamics of a magnetic polaron,"Understanding the interplay between charge and spin and its effects on
transport is a ubiquitous challenge in quantum many-body systems. In the
Fermi-Hubbard model, this interplay is thought to give rise to magnetic
polarons, whose dynamics may explain emergent properties of quantum materials
such as high-temperature superconductivity. In this work, we use a cold-atom
quantum simulator to directly observe the formation dynamics and subsequent
spreading of individual magnetic polarons. Measuring the density- and
spin-resolved evolution of a single hole in a 2D Hubbard insulator with
short-range antiferromagnetic correlations reveals fast initial delocalization
and a dressing of the spin background, indicating polaron formation. At long
times, we find that dynamics are slowed down by the spin exchange time, and
they are compatible with a polaronic model with strong density and spin
coupling. Our work enables the study of out-of-equilibrium emergent phenomena
in the Fermi-Hubbard model, one dopant at a time.",2006.06672v2
2020-07-09,Dynamical modes of sheared confined microscale matter,"Based on (overdamped) Stokesian dynamics simulations and video microscopy
experiments, we study the non equilibrium dynamics of a sheared colloidal
cluster, which is confined to a two-dimensional disk. The experimental system
is composed of a mixture of paramagnetic and non magnetic polystyrene
particles, which are held in the disk by time shared optical tweezers. The
paramagnetic particles are located at the center of the disk and are actuated
by an external, rotating magnetic field that induces a magnetic torque. We
identify two different steady states by monitoring the mean angular velocities
per ring. The first one is characterized by rare slip events, where the inner
rings momentarily depin from the outer ring, which is kept static by the set of
optical traps. For the second state, we find a bistability of the mean angular
velocities, which can be understood from the analysis of the slip events in the
particle trajectories. We calculate the particle waiting- and jumping time
distributions and estimate a time scale between slips, which is also reflected
by a plateau in the mean squared azimuthal displacement. The dynamical
transition is further reflected by the components of the stress tensor,
revealing a shear-thinning behavior as well as shear stress overshoots.
Finally, we briefly discuss the observed transition in the context of
stochastic thermodynamics and how it may open future directions in this field.",2007.04601v1
2013-08-18,Short dynamic fibrils in sunspot chromospheres,"Sunspot chromospheres display vigorous oscillatory signature when observed in
chromospheric diagnostics like the strong Ca II lines and H-alpha. New
high-resolution sunspot observations from the Swedish 1-m Solar Telescope show
the ubiquitous presence of small-scale periodic jet-like features that move up
and down. This phenomenon has not been described before. Their typical width is
about 0.3 arcsec and they display clear parabolic trajectories in space-time
diagrams. The maximum extension of the top of the jets is lowest in the umbra,
a few 100 km, and progressively longer further away from the umbra in the
penumbra, with the longest more than 1000 km. These jets resemble dynamic
fibrils found in plage regions but at smaller extensions. LTE inversion of
spectro-polarimetric Ca II 8542 observations enabled for a comparison of the
magnetic field inclination and the properties of these short jets. We find that
the most extended of these jets also have longer periods and tend to be located
in regions with more horizontal magnetic fields. This is a direct observational
confirmation of the mechanism of long-period waves propagating along inclined
magnetic fields into the solar chromosphere. This mechanism was identified
earlier as the driver of dynamic fibrils in plage, part of the mottles in quiet
Sun, and type I spicules at the limb. The sunspot dynamic fibrils that we
report here represent a new class of manifestation of this mechanism. They are
not the same as the transient penumbral and umbral micro-jets reported earlier.",1308.3893v1
2018-03-07,Exploring out-of-equilibrium quantum magnetism and thermalization in a spin-3 many-body dipolar lattice system,"Understanding quantum thermalization through entanglement build-up in
isolated quantum systems addresses fundamental questions on how unitary
dynamics connects to statistical physics. Here, we study the spin dynamics and
approach towards local thermal equilibrium of a macroscopic ensemble of S = 3
spins prepared in a pure coherent spin state, tilted compared to the magnetic
field, under the effect of magnetic dipole-dipole interactions. The experiment
uses a unit filled array of 104 chromium atoms in a three dimensional optical
lattice, realizing the spin-3 XXZ Heisenberg model. The buildup of quantum
correlation during the dynamics, especially as the angle approaches pi/2, is
supported by comparison with an improved numerical quantum phase-space method
and further confirmed by the observation that our isolated system thermalizes
under its own dynamics, reaching a steady state consistent with the one
extracted from a thermal ensemble with a temperature dictated from the system's
energy. This indicates a scenario of quantum thermalization which is tied to
the growth of entanglement entropy. Although direct experimental measurements
of the Renyi entropy in our macroscopic system are unfeasible, the excellent
agreement with the theory, which can compute this entropy, does indicate
entanglement build-up.",1803.02628v2
2019-07-24,The dynamics of AR 12700 in its early emerging phase II: fan-shaped activities relevant to arch filament systems,"The emergence of active regions (ARs) closely relates to the solar dynamo and
the dynamical atmospheric phenomena.With high-resolution and long-lasting
observations from the New Vacuum Solar Telescope, we report a new dynamic
activity phenomenon named ""fan-shaped activity (FSA)"" in the emerging phase of
NOAA AR 12700. The FSAs are clearly observed at Ha wavelength and are closely
related to the dynamics of the adjacent arch filament system (AFS), including
threads deformation and materials downward motions. On 2018 February 26, the
two most representative FSAs appeared around 05:21 UT and 06:03 UT,
respectively, and they firstly ascended and then decayed in around 10 minutes.
At the ascending phase, accompanied by the uplifting of an adjacent AFS, each
FSA launches up at one end of the AFS and extends for up to 11 Mm. At the
decaying phase, the FSA gradually vanishes, and materials downflows towards the
other end of the AFS are detected. After checking the evolution of the magnetic
fields of AR 12700, we find that each FSA is located between the end of an AFS
and an adjacent magnetic patch with the same polarity and launches at the onset
of the collision and compression between these two magnetic patches. We propose
that the collision lifts up the AFS, and then the initially compact AFS
laterally expands, resulting in the formation of FSA. A cartoon model is
proposed to depict the activities.",1907.10345v1
2019-12-16,Asymmetric Magnetic Relaxation behavior of Domains and Domain Walls Observed Through the FeRh First-Order Metamagnetic Phase Transition,"The phase coexistence present through a first-order phase transition means
there will be finite regions between the two phases where the structure of the
system will vary from one phase to the other, known as a phase boundary wall.
This region is said to play an important but unknown role in the dynamics of
the first-order phase transitions. Here, by using both x-ray photon correlation
spectroscopy and magnetometry techniques to measure the temporal isothermal
development at various points through the thermally activated first-order
metamagnetic phase transition present in the near-equiatomic FeRh alloy, we are
able to isolate the dynamic behavior of the domain walls in this system. These
investigations reveal that relaxation behavior of the domain walls changes when
phase coexistence is introduced into the system and that the domain wall
dynamics is different to the macroscale behavior. We attribute this to the
effect of the exchange coupling between regions of either magnetic phase
changing the dynamic properties of domain walls relative to bulk regions of
either phase. We also believe this behavior comes from the influence of the
phase boundary wall on other magnetic objects in the system.",1912.07635v3
2020-03-02,Chaotic Synchronization of memristive neurons: Lyapunov function versus Hamilton function,"We study the dynamical behaviors of this improved memristive neuron model by
changing external harmonic current and the magnetic gain parameters. The model
shows rich dynamics including periodic and chaotic spiking and bursting, and
remarkably, chaotic super-bursting, which has greater information encoding
potentials than a standard bursting activity. Based on Krasovskii-Lyapunov
stability theory, the sufficient conditions (on the synaptic strengths and
magnetic gain parameters) for the chaotic synchronization of the improved model
are obtained. Based on Helmholtz's theorem, the Hamilton function of the
corresponding error dynamical system is also obtained. It is shown that the
time variation of this Hamilton function along trajectories can play the role
of the time variation of the Lyapunov function - in determining the asymptotic
stability of the synchronization manifold. Numerical computations indicate that
as the synaptic strengths and the magnetic gain parameters change, the time
variation of the Hamilton function is always non-zero (i.e., a relatively large
positive or negative value) only when the time variation of the Lyapunov
function is positive, and zero (or vanishingly small) only when the time
variation of the Lyapunov function is also zero. This clearly therefore paves
an alternative way to determine the asymptotic stability of synchronization
manifolds, and can be particularly useful for systems whose Lyapunov function
is difficult to construct, but whose Hamilton function corresponding to the
dynamic error system is easier to calculate.",2003.00947v1
2020-03-29,Observation of nonlinear spin dynamics and squeezing in a BEC using dynamic decoupling,"We study the evolution of a Bose-Einstein Condensate (BEC) in a two-state
superposition due to inter-state interactions. Using a population imbalanced
dynamic decoupling scheme, we measure inter-state interactions while canceling
intra-state density shifts and external noise sources. Our measurements show
low statistical uncertainties for both magnetic sensitive and insensitive
superpositions, indicating that we successfully decoupled our system from
strong magnetic noises. We experimentally show that the Bloch sphere
representing general superposition states is ""twisted"" by inter-state
interactions, and that the twist rate depends on the difference between
inter-state and intra-state scattering lengths $a_{22}+a_{11}-2a_{12}$. We use
the non-linear spin dynamics in our scheme to demonstrate squeezing of gaussian
noise, showing $2.79 \pm 0.43$ dB squeezing when starting with a noisy state
and applying 160 echo pulses. Which can be used to increase sensitivity when
there are errors in state preparation. Our results allow for a better
understanding of inter-atomic potentials in \textsuperscript{87}Rb. Our scheme
can be used for spin squeezing beyond the standard quantum limit and observing
polaron physics close to a Feshbach resonance, where interactions diverge, and
strong magnetic noises are ever present.",2003.13101v3
2022-06-28,Quantum mean-field treatment of the dynamics of a two-level atom in a simple cubic lattice,"The mean field approximation is used to investigate the general features of
the dynamics of a two-level atom in a ferromagnetic lattice close to the Curie
temperature. Various analytical and numerical results are obtained. We first
linearize the lattice Hamiltonian, and we derive the self-consistency equation
for the order parameter of the phase transition for arbitrary direction of the
magnetic field. The reduced dynamics is deduced by tracing out the degrees of
freedom of the lattice, which results in the reduction of the dynamics to that
of an atom in an effective spin bath whose size is equal to the size of a unit
cell of the lattice. It is found that the dephasing and the excited state
occupation probability may be enhanced by applying the magnetic field along
some specific directions. The dependence on the change of the temperature and
the magnitude of spin is also investigated. It turns out that the increase of
thermal fluctuations may reduce the occupation probability of the excited
state. The entanglement of two such atoms that occupy non-adjacent cells is
studied and its variation in time is found to be not much sensitive to the
direction of the magnetic field. Entanglement sudden death and revival is shown
to occur close to the critical temperature.",2206.14156v3
2022-09-08,Perspective: non-Hermitian physics in magnetic systems,"Non-Hermitian Hamiltonians provide an alternative perspective on the dynamics
of quantum and classical systems coupled non-conservatively to an environment.
Once primarily an interest of mathematical physicists, the theory of
non-Hermitian Hamiltonians has solidified and expanded to describe various
physically observable phenomena in optical, photonic, and condensed matter
systems. Self-consistent descriptions of quantum mechanics based on
non-Hermitian Hamiltonians have been developed and continue to be refined. In
particular, non-Hermitian frameworks to describe magnonic and hybrid magnonic
systems have gained popularity and utility in recent years, with new insights
into the magnon topology, transport properties, and phase transitions coming
into view. Magnonic systems are in many ways a natural platform in which to
realize non-Hermitian physics because they are always coupled to a surrounding
environment and exhibit lossy dynamics. In this perspective we review recent
progress in non-Hermitian frameworks to describe magnonic and hybrid magnonic
systems, such as cavity magnonic systems and magnon-qubit coupling schemes. We
discuss progress in understanding the dynamics of inherently lossy magnetic
systems as well as systems with gain induced by externally applied spin
currents. We enumerate phenomena observed in both purely magnonic and hybrid
magnonic systems which can be understood through the lens of non-Hermitian
physics, such as PT and Anti-PT-symmetry breaking, dynamical magnetic phase
transitions, non-Hermitian skin effect, and the realization of exceptional
points and surfaces. Finally, we comment on some open problems in the field and
discuss areas for further exploration.",2209.03946v2
2023-03-07,Polarization-diverse soliton transitions and deterministic switching dynamics in strongly-coupled and self-stabilized microresonator frequency combs,"Dissipative Kerr soliton microcombs in microresonators has enabled
fundamental advances in chip scale precision metrology, communication,
spectroscopy, and parallel signal processing. Here we demonstrate polarization
diverse soliton transitions and deterministic switching dynamics of a self
stabilized microcomb in a strongly coupled dispersion-managed microresonator
driven with a single pump laser. The switching dynamics are induced by the
differential thermorefractivity between coupled transverse magnetic and
transverse electric supermodes during the forward backward pump detunings. The
achieved large soliton existence range and deterministic transitions benefit
from the switching dynamics, leading to the cross polarized soliton microcomb
formation when driven in the transverse magnetic supermode of the single
resonator. Resultantly the pump laser always exists at the effective blue
detuning of the transverse magnetic resonance, fundamentally mitigating the
thermal destabilization barrier and improving accessibility of the soliton
formation regime. Subsequently and secondly, we demonstrate two distinct
polarization diverse soliton formation routes arising from chaotic or
periodically modulated waveforms via pump power selection. The generated self
stabilized supermode microcomb features an extraordinarily large soliton
existence range, a variety of soliton state transitions with well defined pump
laser tuning, high pump microcomb conversion efficiency, and low repetition
rate phase noise.",2303.03687v1
2023-03-17,Energetics and Dynamics of a stable Bloch point,"Magnetic Bloch points (BPs) are highly confined magnetization configurations,
that often occur in transient spin dynamics processes. However, opposing
chiralities of adjacent layers for instance in a FeGe bilayer stack can
stabilize such magnetic BPs at the layer interface. These BPs configurations
are metastable and consist of two coupled vortices (one in each layer) with
same circularity and opposite polarity. Each vortex is stabilized by opposite
sign Dzyaloshinskii-Moriya interactions. This stabilization mechanism
potentially opens the door towards BP-based spintronic applications. An open
question, from a methodological point of view, is whether the Heisenberg (HB)
model approach (atomistic model) as to be used to study such systems or if the
-- computationally more efficient -- micromagnetic (MM) models can be used and
still obtain robust results. We are modelling and comparing the energetics and
dynamics of a stable BP obtained using both HB and MM approaches. We find that
an MM description of a stable BP leads qualitatively to the same results as the
HB description, and that an appropriate mesh discretization plays a more
important role than the chosen model. Further, we study the dynamics by
shifting the BP with an applied in-plane field and investigating the relaxation
after switching the filed off abruptly. The precessional motion of coupled
vortices in a BP state can be drastically reduced compared to a classical
vortex, which may be also an interesting feature for fast and efficient
devices. A recent study has shown that a bilayer stack hosting BPs can be used
to retain information [1].",2303.10091v1
2023-03-29,Dynamically screened strongly quantized electron transport in binary neutron-star merger,"We examine electron-transport coefficients in magnetized hot and dense
electron-ion plasma relevant in binary neutron star merger simulation. We
calculate electrical and thermal conductivities in low density, high
temperature, highly magnetized plasma of binary neutron star mergers where
quantum oscillatory behavior of electrons emerge. For pronounced thermodynamic
effects, we consider zeroth Landau level population of electrons for the
calculation of conductivity. We solve Boltzmann equation in presence of
magnetic field to obtain the dissipative components of electrical and thermal
conductivities. The dissipative coefficients are formulated considering
frequency dependent dynamical screening in the quantized electron-ion
scattering rate. Numerical estimations show that the effect of dynamical
screening of photon propagator on electrical and thermal conductivities is
pronounced. We observe that dynamical screening reduces the maxima of both the
electrical and thermal conductivities by factors of thirty one and twenty
respectively leading to a reduction in the corresponding time scales of these
coefficients. The common scaling factor between electrical and thermal
conductivity is also observed to follow cubic relationship with temperature
violating Wiedemann-Franz law.",2303.16811v1
2023-06-29,Nonequilibrium generation of charge defects in kagome spin ice under slow cooling,"Kagome spin ice is one of the canonical examples of highly frustrated
magnets. The effective magnetic degrees of freedom in kagome spin ice are Ising
spins residing on a two-dimensional network of corner-sharing triangles. Due to
strong geometrical frustration, nearest-neighbor antiferromagnetic interactions
on the kagome lattice give rise to a macroscopic number of degenerate classical
ground states characterized by ice rules. Elementary excitations at low
temperatures are defect-triangles that violate the ice rules and carry an
additional net magnetic charge relative to the background. We perform
large-scale Glauber dynamics simulations to study the nonequilibrium dynamics
of kagome ice under slow cooling. We show that the density of residual charge
defects exhibits a power law dependence on the quench rate for the class of
algebraic cooling protocols. The numerical results are well captured by the
rate equation for the charge defects based on the reaction kinetics theory. As
the relaxation time of the kagome ice phase remains finite, there is no
dynamical freezing as in the Kibble-Zurek scenario. Instead, we show that the
power-law behavior originates from a thermal excitation that decay
algebraically with time at the late stage of the cooling schedule. Similarities
and differences in quench dynamics of other spin ice systems are also
discussed.",2306.17036v2
2023-12-28,Anisotropic skyrmion and multi-$q$ spin dynamics in centrosymmetric Gd$_2$PdSi$_3$,"Skyrmions are particle-like vortices of magnetization with non-trivial
topology, which are usually stabilized by Dzyaloshinskii-Moriya interactions
(DMI) in noncentrosymmetric bulk materials. Exceptions are centrosymmetric Gd-
and Eu-based skyrmion-lattice (SkL) hosts with net-zero DMI, where both the SkL
stabilization mechanisms and magnetic ground states remain controversial. We
address these by investigating both static and dynamic spin properties of the
centrosymmetric SkL host Gd$_2$PdSi$_3$ using muon spectroscopy ($\mu$SR). We
find that spin fluctuations in its non-coplanar SkL phase are highly
anisotropic, implying that spin anisotropy plays a prominent role in
stabilizing this phase. We also observe strongly-anisotropic spin dynamics in
the ground-state (IC-1) incommensurate magnetic phase of the material,
indicating that it is a meron-like multi-$q$ structure. In contrast, the
higher-field, coplanar IC-2 phase is found to be single-$q$ with
nearly-isotropic spin dynamics.",2312.17323v2
2024-02-05,Circular motion of non-collinear spin textures in Corbino disks: Dynamics of Néel- versus Bloch-type skyrmions and skyrmioniums,"Magnetic skyrmions are nano-scale magnetic whirls that can be driven by
currents via spin torques. They are promising candidates for spintronic devices
such as the racetrack memory, where a motion along the uniform current is
typically desired. However, in Corbino disks, the goal is to achieve a circular
motion, perpendicular to the radially applied current. As we show, based on
analytical calculations and micromagnetic simulations, Bloch skyrmions engage
in a circular motion with frequencies in the MHz range when driven by
spin-orbit torques. In contrast, N\'eel skyrmions get stuck at the edges of the
disk. Our analysis reveals that the antagonistic dynamics between Bloch- and
N\'eel-type magnetic textures arise from their helicity. Furthermore, we find
that skyrmioniums, which are topologically trivial variations of skyrmions,
move even faster and allow an increase in the current density without being
pushed toward the edges of the disk. When driven by spin-transfer torques
instead, Bloch and N\'eel skyrmions no longer exhibit different dynamics.
Instead, they move along a circular trajectory due to the skyrmion Hall effect
caused by their topological charge. Consequently, the topologically trivial
skyrmioniums inevitably become trapped at the disk edge in this scenario. To
provide a comprehensive understanding, our study also examines currents applied
tangentially, further enriching our insights into skyrmion dynamics and
appropriate current injection methods for skyrmion-based devices.",2402.03023v1
2004-12-03,Dusty plasmas on galactic scales,"Interstellar dust is spread in galaxies over large scales, often far beyond
the stellar disks. Several mechanisms can be responsible for carrying the dust
both in the vertical and radial directions, producing in general different
spatial dust distributions. In spite of the small mass fraction dust can be in
some cases dynamically active, and its role in star formation process and
dynamics of shock waves is still not completely understood. In this review I
discuss briefly the issues connected with the large scale distribution of the
interstellar dust, the impact of the dust on star formation and on dynamics of
shock waves.",0412078v1
1995-01-02,Dynamics of homogeneous magnetizations in strong transverse driving fields,"Spatially homogeneous solutions of the Landau--Lifshitz--Gilbert equation are
analysed. The conservative as well as the dissipative case is considered
explicitly. For the linearly polarized driven Hamiltonian system we apply
canonical perturbation theory to uncover the main resonances as well as the
global phase space structure. In the case of circularly polarized driven
dissipative motion we present the complete bifurcation diagram including
bifurcations up to codimension three.",9501002v1
1996-05-01,Can a local repulsive potential trap an electron?,"We study the classical dynamics of a charged particle in two dimensions,
under the influence of a perpendicular magnetic and an in-plane electric field.
We prove the surprising fact that there is a finite region in phase space that
corresponds to the otherwise drifting particle being trapped by a local
repulsive potential. Our result is a direct consequence of KAM-theory and, in
particular, of Moser's theorem. We illustrate it by numerical phase portraits
and by an analytic approximation to invariant curves.",9604018v1
1999-02-27,Periodic Orbit Quantization of Mixed Regular Chaotic Systems,"A general technique for the periodic orbit quantization of systems with
near-integrable to mixed regular-chaotic dynamics is introduced. A small set of
periodic orbits is sufficient for the construction of the semiclassical
recurrence function up to, in principle, infinite length. As in our recent work
the recurrence signal is inverted by means of a high resolution spectral
analyzer (harmonic inversion) to obtain the semiclassical eigenenergies. The
method is demonstrated for the hydrogen atom in a magnetic field. To our
knowledge this is the first successful application of periodic orbit
quantization in the deep mixed regular-chaotic regime.",9903003v1
1999-06-21,Controlling a leaky tap,"We apply the Ott, Grebogy and Yorke mechanism for the control of chaos to the
analytical oscillator model of a leaky tap obtaining good results. We exhibit
the robustness of the control against both dynamical noise and measurement
noise.A possible way of controlling experimentally a leaky tap using
magnetic-field-produced variations in the viscosity of a magnetorheological
fluid is suggested.",9906033v1
1999-06-22,Chaotic Oscillations in Finite Quantum Systems: Trapped Bose-Einstein Condensates,"We discuss the recently achieved Bose-Einstein condensation for alkali-metal
atoms in magnetic traps. The theoretically predicted low-energy collective
oscillations of the condensate have been experimentally confirmed by laser
imaging techniques. We show by using Poincar\`e sections that at higher
energies non-linear effects appear and oscillations become chaotic. PACS
03.75.Fi, 05.30.Jp, 05.45.+b, 32.80.Pj",9906034v1
1999-09-01,Semiclassical Theory of h/e Aharonov-Bohm Oscillation in Ballistic Regimes,"We study the magneto-transport in Aharonov-Bohm (AB) billiards forming doubly
connected structures. In these systems, non-averaged conductance oscillates as
a function of magnetic flux with period h/e. We derive formulas of the
correlation function C of the magneto-conductance for chaotic and regular AB
billiards by use of the semiclassical theory. The different higher harmonics
behaviors for C are related to the differing distribution of classical dwelling
times. The AB oscillation in ballistic regimes provides an experimental probe
of quantum signatures of classical chaotic and regular dynamics.",9909006v1
1995-07-20,Dynamic Jahn-Teller Effect and Colossal Magnetoresistance in $La_{1-x}A_xMnO_3$,"A model for $La_{1-x}Sr_xMnO_3$ which incorporates the physics of dynamic
Jahn-Teller and double-exchange effects is presented and solved via a dynamical
mean field approximation. In an intermediate coupling regime the interplay of
these two effects is found to reproduce the behavior of the resistivity and
magnetic transition temperature observed in $La_{1-x} Sr_x MnO_3$.",9507084v2
1995-08-31,Finite Size Scaling and Critical Exponents in Critical Relaxation,"We simulate the critical relaxation process of the two-dimensional Ising
model with the initial state both completely disordered or completely ordered.
Results of a new method to measure both the dynamic and static critical
exponents are reported, based on the finite size scaling for the dynamics at
the early time. From the time-dependent Binder cumulant, the dynamical exponent
$z$ is extracted independently, while the static exponents $\beta/\nu$ and
$\nu$ are obtained from the time evolution of the magnetization and its higher
moments.",9508148v1
1995-10-19,Determination of the Critical Point and Exponents from short-time Dynamics,"The dynamic process for the two dimensional three state Potts model in the
critical domain is simulated by the Monte Carlo method. It is shown that the
critical point can rigorously be located from the universal short-time
behaviour. This makes it possible to investigate critical dynamics
independently of the equilibrium state. From the power law behaviour of the
magnetization the exponents $\beta / (\nu z)$ and $1/ (\nu z)$ are determined.",9510109v1
1996-04-29,Percolation and cluster Monte Carlo dynamics for spin models,"A general scheme for devising efficient cluster dynamics proposed in a
previous letter [Phys.Rev.Lett. 72, 1541 (1994)] is extensively discussed. In
particular the strong connection among equilibrium properties of clusters and
dynamic properties as the correlation time for magnetization is emphasized. The
general scheme is applied to a number of frustrated spin model and the results
discussed.",9604169v1
1997-05-19,Anomalous Shift of Chemical Potential in the Double-Exchange Systems,"Double-exchange system is investigated by the dynamical mean-field theory. We
show that the chemical potential shifts as a function of temperature and
magnetization, which is anomalously large. We also discuss the influences of
dynamic Jahn-Teller effect to the shift of the chemical potential. Measurement
of the shift of the chemical potential casts a constraint to theoretical
approaches for the magnetoresistance phenomena in ($R$,$A$)MnO$_3$ such as
double-exchange effects and dynamic Jahn-Teller effects. We also propose a
method to measure the shift of $\mu$.",9705176v2
1997-08-13,Chaos and Interacting Electrons in Ballistic Quantum Dots,"We show that the classical dynamics of independent particles can determine
the quantum properties of interacting electrons in the ballistic regime. This
connection is established using diagrammatic perturbation theory and
semiclassical finite-temperature Green functions. Specifically, the orbital
magnetism is greatly enhanced over the Landau susceptibility by the combined
effects of interactions and finite size. The presence of families of periodic
orbits in regular systems makes their susceptibility parametrically larger than
that of chaotic systems, a difference which emerges from correlation terms.",9708092v1
1998-05-11,Dynamical simulation of spin-glass and chiral-glass orderings in three-dimensional Heisenberg spin glasses,"Spin-glass and chiral-glass orderings in three-dimensional Heisenberg spin
glasses are studied with and without randaom magnetic anisotropy by dynamical
Monte Carlo simulations. In isotropic case, clear evidence of a
finite-temperature chiral-glass transition is presented. While the spin
autocorrelation exhibits only an interrupted aging, the chirality
autocorrelation persists to exhibit a pronounced aging effect reminisecnt of
the one observed in the mean-field model. In anisotropic case, asymptotic
mixing of the spin and the chirality is observed in the off-equilibrium
dynamics.",9805117v1
1998-05-18,Chaotic enhancement in microwave ionization of Rydberg atoms,"The microwave ionization of internally chaotic Rydberg atoms is studied
analytically and numerically. The internal chaos is induced by magnetic or
static electric fields. This leads to a chaotic enhancement of microwave
excitation. The dynamical localization theory gives a detailed description of
the excitation process even in a regime where up to few thousands photons are
required to ionize one atom. Possible laboratory experiments are also
discussed.",9805216v1
1998-09-01,Non-equilibrium dynamics in a 3d spin-glass,"Non-equilibrium dynamics in a Ag(11%Mn) spin-glass has been studied by low
frequency ac-susceptibility and magnetic relaxation experiments. The results
unequivocally show that spin structures that memorize the cooling process are
imprinted in the system. These imprinted structures disclose themselves through
dramatic changes of the dynamics on re-heating the spin-glass through the
temperatures where intermittent stops or changes of the cooling rate have been
imposed. We can qualitatively interpret our results in terms of the droplet
spin-glass model developed by Fisher and Huse [Phys. Rev. B 38 (1988) 373;
386].",9809021v1
1998-10-08,Nonuniversality in short-time critical dynamics,"We study behaviour of dynamical critical exponents of the two-dimensional
Ising model with a line of defects. Simulations done at an early time (first
100 Monte Carlo steps) reveal that the critical exponent of Janssen et al (Z.
Phys. B 73 539) depends on the strength of the exchange coupling constant (J')
of the altered line. On the other hand, our simulations permit us to conclude
that the dynamical critical exponent z is not sensitive to changes in J'. In
adition, we investigate the possible invariance of the anomalous dimension of
the magnetization at the beginning of the process.",9810105v1
1999-04-19,"Dynamic transverse susceptibility in Au-Fe-Au ""nano-onions""","A precision radio-frequency (RF) transverse modulation technique based on a
resonant tunnel diode oscillator (TDO) has been used to study the dynamic
susceptibility in novel ring-shaped single domain Fe nanoparticles synthesized
using reverse-micelle chemistry. Hysteresis loops measured using a SQUID
magnetometer indicate a large coercivity (Hc = 400Oe) below the blocking
temperature (TB ~52K). The dynamic susceptibility shows remarkable consistency
with the M-H data and exhibits peaks at characteristic anisotropy fields.
Overall, these RF experiments are well suited to probe the magnetic anisotropy
in nanophase materials.",9904267v1
1999-08-10,The influence of the Wolff's cluster structure on the determination of the critical dynamic exponent,"Recently we reported some interesting features of the Wolff's algorithm
behavior when applied to the site-bond-correlated Ising model.Our main results
were that a stronger correlation diminishes the autocorrelation time but it
does not change or affectthe critical dynamic exponent.In this paper, we
analyse the Wolff's cluster structure and how it varies with the spatial
correlation. The fractal dimensions are determined for several values of the
magnetic atoms concentration and spacial correlation, giving an estimative of
the critical dynamic exponent.",9908151v1
1999-09-22,The Critical Exponent $θ'$ in Spin Glasses,"Short-time dynamic scaling behavior of the 3D $\pm J$ Ising spin glass is
studied by Monte Carlo methods. Starting the replicas with independent initial
configurations with a small pseudo magnetization, the dynamic evolution of the
overlap q(t) between two replicas is measured. The initial increase of the
overlap q(t) is observed and the corresponding exponent $\theta'$ is obtained.
  From the scaling relation $\lambda =d/z-\theta'$, the dynamic exponent z is
estimated.",9909325v1
2000-03-27,Effect of memory and dynamical chaos in long Josephson junctions,"A long Josephson junction in a constant external magnetic field and in the
presence of a dc bias current is investigated. It is shown that the system,
simulated by the sine-Gorgon equation, ""remembers"" a rapidly damping initial
perturbation and final asymptotic states are determined exactly with this
perturbation. Numerical solving of the boundary sine-Gordon problem and
calculations of Lyapunov indices show that this system has a memory even when
it is in a state of dynamical chaos, i.e., dynamical chaos does not destroy
initial information having a character of rapidly damping perturbation.",0003421v1
2000-09-18,Dynamics of metastable vortex states in weakly pinned superconductors: A phenomenological model,"We present a phenomenological model for the vortex dynamics in the peak
effect region of weakly pinned superconductors. We explain the history
dependent dynamic response of the metastable vortex states subjected to a
transport current and the hysteretic voltage-current characteristics observed
in the vicinity of peak effect in weakly pinned superconductors. A strong
variation in voltage current characteristics with the current sweep rate and
the anomalous dependence of critical current density $J_c$ on the magnetic
field sweep rate have also been accounted for by this model.",0009254v1
2001-04-24,Dynamics and transformations of Josephson vortex lattice in layered superconductors,"We consider dynamics of Josephson vortex lattice in layered superconductors
with magnetic, charge (electrostatic) and charge-imbalance (quasiparticle)
interactions between interlayer Josephson junctions taken into account. The
macroscopic dynamical equations for interlayer Josephson phase differences,
intralayer charge and electron-hole imbalance are obtained and used for
numerical simulations. Different transformations of the vortex lattice
structure are observed. It is shown that the additional dissipation due to the
charge imbalance relaxation leads to the stability of triangular lattice.",0104468v1
2001-05-31,Critical Dynamics of a Vortex Loop Model for the Superconducting Transition,"We calculate analytically the dynamic critical exponent $z_{MC}$ measured in
Monte Carlo simulations for a vortex loop model of the superconducting
transition, and account for the simulation results. In the weak screening
limit, where magnetic fluctuations are neglected, the dynamic exponent is found
to be $z_{MC} = 3/2$. In the perfect screening limit, $z_{MC} = 5/2$. We relate
$z_{MC}$ to the actual value of $z$ observable in experiments and find that $z
\sim 2$, consistent with some experimental results.",0105622v2
2001-10-11,Dynamical correlations in half-metallic manganites,"Motivated by the recent optical and photoemission measurements on
half-metallic ferromagnetic three-dimensional manganites, we combine a
tight-binding fit of the one-particle bandstructure with the dynamical
mean-field theory, which treats the dynamical orbital correlations and the
combination of Jahn-Teller and doping-induced disorder on the same footing. We
show how all of the above effects are necessary to obtain good semiquantitative
agreement with experimental features. As applications, we show how modest
external magnetic fields drive drastic changes in the optical spectrum,
demonstrating the {\it colossal ac magnetoconductivity}. The photoemission
lineshape contribution is evaluated, and good agreement with published
experimental work is found.",0110229v1
2001-12-17,Glassy dynamics of Josephon arrays on a dice lattice,"In this Letter we study the phase diagram and the approach to the equilibrium
of Josephson junction arrays on a dice lattice for two different values of the
magnetic frustration ($f=1/2,1/3$). In both cases the array unedrgoes a
Berezinskii-Kosterlitz-Thouless like phase transition. For the fully frustrated
array, $f=1/2$, we find evidence of a {\em glassy behavior} although the system
has {\em no disorder}. For comparison we study dynamics of the array at a
different value of frustration ($f=1/3$) and find no trace of glassy dynamics.",0112307v1
2002-01-18,Coherent quantum oscillations in coupled traps with ultracold atoms,"The dynamics of two traps with ultracold atoms and connected by Josephson
type coupling, is shown to exhibit a transition from dispersive dynamics to
localized coherent oscillations. This transition is controlled by coupling
strength and energy offset between the traps. The dynamics is also shown to be
exactly that of a Heisenberg chain with a linear magnetic field. Possible
applications for ``quantum-state engineering'' are discussed.",0201335v1
2002-09-02,Dynamics and phase evolution of Bose-Einstein condensates in one-dimensional optical lattices,"We report experimental results on the dynamics and phase evolution of
Bose-Einstein condensates in 1D optical lattices. The dynamical behaviour is
studied by adiabatically loading the condensate into the lattice and
subsequently switching off the magnetic trap. In this case, the condensate is
free to expand inside the periodic structure of the optical lattice. The phase
evolution of the condensate, on the other hand, can be studied by
non-adiabatically switching on the periodic potential. We observe decays and
revivals of the interference pattern after a time-of-flight.",0209024v1
2002-11-08,Rejection-free Monte Carlo Algorithms for Models with Continuous Degrees of Freedom,"We construct a rejection-free Monte Carlo algorithm for a system with
continuous degrees of freedom. We illustrate the algorithm by applying it to
the classical three-dimensional Heisenberg model with canonical Metropolis
dynamics. We obtain the lifetime of the metastable state following a reversal
of the external magnetic field. Our rejection-free algorithm obtains results in
agreement with a direct implementation of the Metropolis dynamic and requires
orders of magnitude less computational time at low temperatures. The treatment
is general and can be extended to other dynamics and other systems with
continuous degrees of freedom.",0211164v1
2003-07-21,Dynamics of ferromagnetic nanomagnets with vortex or single-domain configuration,"We study the dynamics of flat circular permalloy nanomagnets for 1.) magnetic
vortex and 2.) single-domain configurations, using micromagnetic simulation.
Dynamical studies for isolated vortex structures show that both the vorticity
and the central polarity of the out-of-plane component can be switched fast
(50-100 ps) and independently. Micromagnetic simulations of the switching
process in thin cylindrical Permalloy (Py) nanoparticles with an initial stable
single-domain state show nearly homogeneous single-domain behaviour followed by
excitation of spin waves.",0307503v1
2003-11-19,Dynamic Kerr Effect and Spectral Weight Transfer in the Manganites,"We perform pump-probe Kerr spectroscopy in the colossally magnetoresistive
manganite Pr0.67Ca0.33MnO3. Kerr effects uncover surface magnetic dynamics
undetected by established methods based on reflectivity and optical spectral
weight transfer. Our findings indicate the connection between spin and charge
dynamics in the manganites may be weaker than previously thought. Additionally,
important differences between this system and conventional ferromagnetic metals
manifest as long-lived, magneto-optical coupling transients, which may be
generic to all manganites.",0311452v2
2003-12-22,Out-of-equilibrium critical dynamics at surfaces: Cluster dissolution and non-algebraic correlations,"We study nonequilibrium dynamical properties at a free surface after the
system is quenched from the high-temperature phase into the critical point. We
show that if the spatial surface correlations decay sufficiently rapidly the
surface magnetization and/or the surface manifold autocorrelations has a
qualitatively different universal short time behavior than the same quantities
in the bulk. At a free surface cluster dissolution may take place instead of
domain growth yielding stationary dynamical correlations that decay in a
stretched exponential form. This phenomenon takes place in the
three-dimensional Ising model and should be observable in real ferromagnets.",0312583v1
2004-05-12,The role of orbital dynamics in spin relaxation and weak antilocalization in quantum dots,"We develop a semiclassical theory for spin-dependent quantum transport to
describe weak (anti)localization in quantum dots with spin-orbit coupling. This
allows us to distinguish different types of spin relaxation in systems with
chaotic, regular, and diffusive orbital classical dynamics. We find, in
particular, that for typical Rashba spin-orbit coupling strengths, integrable
ballistic systems can exhibit weak localization, while corresponding chaotic
systems show weak antilocalization. We further calculate the magnetoconductance
and analyze how the weak antilocalization is suppressed with decreasing quantum
dot size and increasing additional in-plane magnetic field.",0405266v2
2004-06-04,Observation of a New Fluxon Resonant Mechanism in Annular Josephson Tunnel Structures,"A novel dynamical state has been observed in the dynamics of a perdurbed
sine-Gordon system. This resonant state, has been experimentally observed as a
singularity in the dc current voltage characteristic of an annular Josephson
tunnel junction, excited in the presence of a magnetic field. With this
respect, it can be assimilated to self-resonances known as Fiske steps.
Differently from these, however, we demonstrate, on the basis of numerical
simulations, that its detailed dynamics involves rotating fluxon pairs, a
mechanism associated, so far, to self-resonances known as zero-field steps.",0406104v1
2004-08-18,Hole dynamics in spin and orbital ordered vanadium perovskites,"Hole dynamics in spin and orbital ordered vanadates with perovskite structure
is investigated. A mobile hole coupled to the spin excitation (magnon) in the
spin G-type and orbital C-type (SG/OC) ordered phase, and that to the orbital
excitation (orbiton) in the spin C-type and orbital G-type (SC/OG) one are
formulated on an equal footing. The observed fragile character of the (SG/OC)
order is attributed to the orbiton softening caused by a reduction of the
taggered magnetic order parameter. It is proposed that the qualitatively
different hole dynamics in the two spin-orbital ordered phases in vanadates can
be probed by the optical spectra.",0408395v1
2004-11-02,Critical scaling of thermal transport in model A dynamics for a superconductor,"We consider the scaling of thermal transport in the presence of electric and
magnetic fields near the finite temperature transition from the metallic normal
state to the superconductor. We do so with fully relaxational, model A,
dynamics for the order parameter and particle-hole symmetry. This enables us to
determine the exact scaling dimension of the heat current operator and hence
critical exponents for all transport coefficients in terms of the
(approximately) known values of the correlation length exponent and the dynamic
scaling exponent. In particular, we determine the critical behavior for the
Nernst coefficient.",0411040v1
2004-12-21,Dynamic behaviour of Josephson-junction qubits: crossover between Rabi oscillations and Landau-Zener transitions,"We study the dynamic behaviour of a quantum two-level system with
periodically varying parameters by solving the master equation for the density
matrix. Two limiting cases are considered: multiphoton Rabi oscillations and
Landau-Zener transitions. The approach is applied to the description of the
dynamics of superconducting qubits. In particular, the case of the
interferometer-type charge qubit with periodically varying parameters (gate
voltage or magnetic flux) is investigated. The time-averaged energy level
populations are calculated as funtions of the qubit's control parameters.",0412588v1
2005-03-14,Band-selective Measurement of Electronic Dynamics in VO2 using Femtosecond Near Edge X-ray Absorption,"We report on the first demonstration of femtosecond xray absorption
spectroscopy. Our experiments are made possible by the use of broadly tunable
bending-magnet radiation from laser-sliced electron bunches within a
synchrotron storage ring. We measure ultrafast electronic dynamics associated
with the photo-induced insulator-metal phase transition in VO2. Symmetry and
element specific xray absorption from near 500 eV V2p and O1s core levels
separately measures the filling dynamics of differently hybridized V3d O2p
electronic bands near the Fermi level.",0503311v1
2005-05-23,Real-time dynamics in Quantum Impurity Systems: A Time-dependent Numerical Renormalization Group Approach,"We develop a general approach to the nonequilibrium dynamics of quantum
impurity systems for arbitrary coupling strength. The numerical renormalization
group is used to generate a complete basis set necessary for the correct
description of the time evolution. We benchmark our method with the exact
analytical solution for the resonant-level model. As a first application, we
investigate the equilibration of a quantum dot subject to a sudden change of
the gate voltage and external magnetic field. Two distinct relaxation times are
identified for the spin and charge dynamics.",0505553v1
2005-07-07,Classical memory effects on spin dynamics in two-dimensional systems,"We discuss classical dynamics of electron spin in two-dimensional
semiconductors with a spin-split spectrum. We focus on a special case, when
spin-orbit induced random magnetic field is directed along a fixed axis. This
case is realized in III-V-based quantum wells grown in [110] direction and also
in [100]-grown quantum wells with equal strength of Dresselhaus and
Bychkov-Rashba spin-orbit couplings. We show that in such wells the long-time
spin dynamics is determined by non-Markovian memory effects. Due to these
effects the non-exponential tail $1/t^2$ appears in the spin polarization.",0507171v2
2005-08-10,Dynamics of quantum spin chains and multi-fermion excitation continua,"We use the Jordan-Wigner representation to study dynamic quantities for the
spin-1/2 XX chain in a transverse magnetic field. We discuss in some detail the
properties of the four-fermion excitation continuum which is probed by the
dynamic trimer structure factor.",0508265v1
2005-09-19,Quasi-Kondo Phenomenon due to Dynamical Jahn-Teller Effect,"A mechanism of non-magnetic Kondo effect is proposed on the basis of a
multiorbital Anderson model coupled with dynamical Jahn-Teller (JT) phonons. An
electron system coupled dynamically with JT phonons has a vibronic ground state
with double degeneracy due to clockwise and anti-clockwise rotational modes
with entropy of $\log 2$. When a temperature is lower than a characteristic
energy to turn the rotational direction, the rotational degree of freedom is
eventually suppressed and the corresponding entropy $\log 2$ is released,
leading to quasi-Kondo behavior. We discuss possible relevance of this
quasi-Kondo phenomenon to electronic properties of filled skutterudites.",0509473v2
2005-09-22,Intrinsic spin dynamics in semiconductor quantum dots,"We investigate the characteristic spin dynamics corresponding to
semiconductor quantum dots within the multiband envelope function approximation
(EFA). By numerically solving an $8\times8$ $k\cdot p$ Hamiltonian we treat
systems based on different III-V semiconductor materials.It is shown that, even
in the absence of an applied magnetic field, these systems show intrinsic spin
dynamics governed by intraband and interband transitions leading to
characteristic spin frequencies ranging from the THz to optical frequencies.",0509587v1
2006-01-20,Semiclassical Dynamics of Electrons in Magnetic Bloch Bands: a Hamiltonian Approach,"y formally diagonalizing with accuracy $\hbar$ the Hamiltonian of electrons
in a crystal subject to electromagnetic perturbations, we resolve the debate on
the Hamiltonian nature of semiclassical equations of motion with Berry-phase
corrections, and therefore confirm the validity of the Liouville theorem. We
show that both the position and momentum operators acquire a Berry-phase
dependence, leading to a non-canonical Hamiltonian dynamics. The equations of
motion turn out to be identical to the ones previously derived in the context
of electron wave-packets dynamics.",0601472v2
2006-05-04,Current-driven vortex domain wall dynamics by micromagnetic simulations,"Current-driven vortex wall dynamics is studied by means of a two-dimensional
analytical model and micromagnetic simulation. By constructing a trial function
for the vortex wall in the magnetic wire, we analytically solve for domain wall
velocity and deformation in the presence of the current-induced spin torque. A
critical current for the domain wall transformation from the vortex wall to the
transverse wall is calculated. A comparison between the field- and
current-driven wall dynamics is carried out. Micromagnetic simulations are
performed to verify our analytical results.",0605125v2
1995-06-14,Square Gravity,"We simulate the Ising model on dynamical quadrangulations using a
generalization of the flip move for triangulations with two aims: firstly, as a
confirmation of the universality of the KPZ/DDK exponents of the Ising phase
transition, worthwhile in view of some recent surprises with other sorts of
dynamical lattices; secondly, to investigate the transition of the Ising
antiferromagnet on a dynamical loosely packed (bipartite) lattice. In the
latter case we show that it is still possible to define a staggered
magnetization and observe the antiferromagnetic analogue of the transition.",9506022v1
2001-11-22,On the dynamics of color magnetic monopoles in full QCD,"We present first results on the dynamics of monopoles in full QCD with N_f=2
flavors of dynamical quarks. Among the quantities being studied are the
monopole density and the monopole screening length, the static potential as
well as the profile of the color electric flux tube. Furthermore, we derive the
low-energy effective monopole action.",0111042v1
2004-07-30,Violation of Lorentz invariance and dynamical effects in high energy gamma rays,"The relation between the violation of Lorentz invariance and the dynamical
effects in high energy gamma rays production is discussed. By using the
framework of noncommutative classical electrodynamics, it is shown that full
dynamical calculations are required to put bounds on the Lorentz violating
scale by the phenomenological analysis of these processes as, for example, the
synchrotron radiation from the CRAB nebula. It is observed that an improvement
of the present bound on the scale of noncommutativity can be obtained only by
astrophysical observations of gamma ray spectra in strong magnetic fields such
as pulsars.",0407363v1
1996-08-16,"Duality, Multi-Monopole Dynamics & Quantum Threshold Bound States","Dynamics of many supersymmetric monopoles are studied in the low energy
approximation. A conjecture for the exact moduli space metric is given for all
collections of fundamental monopoles of distinct type, and various partial
confirmations of the conjecture are discussed. Upon the quantization of the
resulting multi-monopole dynamics in the context of $N=4$ supersymmetric
Yang-Mills-Higgs theories, one recovers the missing magnetic states that are
dual to some of the massive vector mesons. A generalization to monopoles with
nonabelian charges is also discussed.",9608114v1
2001-06-25,A pseudointegrable Andreev billiard,"A circular Andreev billiard in a uniform magnetic field is studied. It is
demonstrated that the classical dynamics is pseudointegrable in the same sense
as for rational polygonal billiards. The relation to a specific polygon, the
asymmetric barrier billiard, is discussed. Numerical evidence is presented
indicating that the Poincare map is typically weak mixing on the invariant
sets. This link between these different classes of dynamical systems throws
some light on the proximity effect in chaotic Andreev billiards.",0106037v2
2005-02-16,Self-pulsing dynamics of ultrasound in a magnetoacoustic resonator,"A theoretical model of parametric magnetostrictive generator of ultrasound is
considered, taking into account magnetic and magnetoacoustic nonlinearities.
The stability and temporal dynamics of the system is analized with standard
techniques revealing that, for a given set of parameters, the model presents a
homoclinic or saddle--loop bifurcation, which predicts that the ultrasound is
emitted in the form of pulses or spikes with arbitrarily low frequency.",0502034v1
2003-03-17,Dynamical analysis of the nonlinear growth of the m=n=1 resistive internal mode,"A dynamical analysis is presented that self-consistently takes into account
the motion of the critical layer, in which the magnetic field reconnects, to
describe how the m=n=1 resistive internal kink mode develops in the nonlinear
regime. The amplitude threshold marking the onset of strong nonlinearities due
to a balance between convective and mode coupling terms is identified. We
predict quantitatively the early nonlinear growth rate of the m=n=1 mode below
this threshold.",0303066v1
2006-01-16,Dynamo action at low magnetic Prandtl numbers: mean flow vs. fully turbulent motion,"We compute numerically the threshold for dynamo action in Taylor-Green
swirling flows. Kinematic calculations, for which the flow field is fixed to
its time averaged profile, are compared to dynamical runs for which both the
Navier-Stokes and the induction equations are jointly solved. The kinematic
instability is found to have two branches, for all explored Reynolds numbers.
The dynamical dynamo threshold follows these branches: at low Reynolds number
it lies within the low branch while at high kinetic Reynolds number it is close
to the high branch.",0601105v1
2005-04-25,Analytical Approach to Wave Packet Dynamics of Laser-Driven Particles beyond the Dipole Approximation,"An analytical approach to quantum mechanical wave packet dynamics of
laser-driven particles is presented. The time-dependent Schroedinger equation
is solved for an electron exposed to a linearly polarized plane wave of
arbitrary shape. The calculation goes beyond the dipole approximation, such
that magnetic field effects like wave packet shearing are included. Analytical
expressions for the time-dependent widths of the wave packet and its
orientation are established. These allow for a simple understanding of the wave
packet dynamics.",0504181v1
2005-10-20,Entanglement dynamics for two interacting spins,"We study the dynamical generation of entanglement for a very simple system: a
pair of interacting spins, s1 and s2, in a constant magnetic field. Two
different situations are considered:(a) s1 ->\infty, s2 = 1/2 and (b) s1 = s2
->\infty, corresponding, respectively, to a quantum degree of freedom coupled
to a semiclassical one (a qubit in contact with an environment) and a fully
semiclassical system, which in this case displays chaotic behavior. Relations
between quantum entanglement and classical dynamics are investigated.",0510162v1
2006-06-24,Effective Hamiltonian approach to adiabatic approximation in open systems,"The adiabatic approximation in open systems is formulated through the
effective Hamiltonian approach. By introducing an ancilla, we embed the open
system dynamics into a non-Hermitian quantum dynamics of a composite system,
the adiabatic evolution of the open system is then defined as the adiabatic
dynamics of the composite system. Validity and invalidity conditions for this
approximation are established and discussed. A High-order adiabatic
approximation for open systems is introduced. As an example, the adiabatic
condition for an open spin-$\frac 1 2$ particle in time-dependent magnetic
fields is analyzed.",0606203v1
1995-07-28,Screening current effects in Josephson junction arrays,"The purpose of this work is to compare the dynamics of arrays of Josephson
junctions in presence of magnetic field in two different frameworks: the so
called XY frustrated model with no self inductance and an approach that takes
into account the screening currents (considering self inductances only). We
show that while for a range of parameters the simpler model is sufficiently
accurate, in a region of the parameter space solutions arise that are not
contained in the XY model equations.",9507006v1
2007-11-02,"Coherent, adiabatic and dissociation regimes in coupled atomic-molecular Bose-Einstein condensates","We discuss the dynamics of a Bose-Einstein condensate of atoms which is
suddenly coupled to a condensate of molecules by an optical or magnetic
Feshbach resonance. Three limiting regimes are found and can be understood from
the transient dynamics occuring for each pair of atoms. This transient dynamics
can be summarised into a time-dependent shift and broadening of the molecular
state. A simple Gross-Pitaevskii picture including this shift and broadening is
proposed to describe the system in the three regimes. Finally, we suggest how
to explore these regimes experimentally.",0711.0397v2
2008-03-03,Crackling dynamics in material failure as the signature of a self-organized dynamic phase transition,"We derive here a linear elastic stochastic description for slow crack growth
in heterogeneous materials. This approach succeeds in reproducing
quantitatively the intermittent crackling dynamics observed recently during the
slow propagation of a crack along a weak heterogeneous plane of a transparent
Plexiglas block [M{\aa}l{\o}y {\it et al.}, PRL {\bf 96} 045501]. In this
description, the quasi-static failure of heterogeneous media appears as a
self-organized critical phase transition. As such, it exhibits universal and to
some extent predictable scaling laws, analogue to that of other systems like
for example magnetization noise in ferromagnets.",0803.0190v2
2008-03-31,Spectral Modeling of Magnetohydrodynamic Turbulent Flows,"We present a dynamical spectral model for Large Eddy Simulation of the
incompressible magnetohydrodynamic (MHD) equations based on the Eddy Damped
Quasi Normal Markovian approximation. This model extends classical spectral
Large Eddy Simulations for the Navier-Stokes equations to incorporate general
(non Kolmogorovian) spectra as well as eddy noise. We derive the model for MHD
and show that introducing a new eddy-damping time for the dynamics of spectral
tensors in the absence of equipartition between the velocity and magnetic
fields leads to better agreement with direct numerical simulations, an
important point for dynamo computations.",0803.4499v1
2008-06-20,Dark dynamic acousto-optic ring lattices for ultracold atoms,"We demonstrate the optical generation of dynamic dark optical ring lattices,
which do not require Laguerre-Gauss beams, large optical coherence lengths or
interferometric stability. Simple control signals lead to spatial modulation
and reproducible rotation, offering manifold possibilities for complex dynamic
ring lattices. In conjunction with a magnetic trap, these scanned 2D intensity
distributions from a single laser beam will enable precision trapping and
manipulation of ultracold species using blue-detuned light. The technique is
ideal for azimuthal ratchet, Mott insulator and persistent current experiments
with quantum degenerate gases.",0806.3466v1
2008-11-21,Nonlinear Magnetohydrodynamics from Gravity,"We apply the recently established connection between nonlinear fluid dynamics
and AdS gravity to the case of the dyonic black brane in AdS_4. This yields the
equations of fluid dynamics for a 2+1 dimensional charged fluid in a background
magnetic field. We construct the gravity solution to second order in the
derivative expansion. From this we find the fluid dynamical stress tensor and
charge current to second and third order in derivatives respectively, along
with values for the associated transport coefficients.",0811.3468v3
2009-01-26,Assessment of Spatial-variant Noise Level in Real-time Dynamic MR Images - a Random Matrix Approach,"Accurate measurement of spatially variant noise in dynamic magnetic resonance
(MR) images acquired using parallel imaging methods is problematic. We propose
a new method based on the random matrix theory to accurately assess the noise
level. The method is validated using numerical simulation and in-vivo dynamic
cardiac MR images.",0901.4027v2
2009-02-20,Perturbation theory of the dynamic inverse spin Hall effect with charge conservation,"We present gauge-invariant theory of the dynamic inverse spin Hall effect
driven by the spin--orbit interaction in metallic systems. Charge conservation
is imposed diagrammatically by including vertex corrections. We show the charge
current is induced by an effective electric field that is proportional to the
spin current pumped by the magnetization dynamics. The result is consistent
with recent experiments.",0902.3507v2
2009-06-17,The dynamical Casimir effect in a superconducting coplanar waveguide,"We investigate the dynamical Casimir effect in a coplanar waveguide (CPW)
terminated by a superconducting quantum interference device (SQUID). Changing
the magnetic flux through the SQUID parametrically modulates the boundary
condition of the CPW, and thereby, its effective length. Effective boundary
velocities comparable to the speed of light in the CPW result in broadband
photon generation which is identical to the one calculated in the dynamical
Casimir effect for a single oscillating mirror. We estimate the power of the
radiation for realistic parameters and show that it is experimentally feasible
to directly detect this nonclassical broadband radiation.",0906.3127v1
2010-01-03,Generalized Gravi-Electromagnetism,"A self consistant and manifestly covariant theory for the dynamics of four
charges (masses) (namely electric, magnetic, gravitational, Heavisidian) has
been developed in simple, compact and consistent manner. Starting with an
invariant Lagrangian density and its quaternionic representation, we have
obtained the consistent field equation for the dynamics of four charges. It has
been shown that the present reformulation reproduces the dynamics of individual
charges (masses) in the absence of other charge (masses) as well as the
generalized theory of dyons (gravito - dyons) in the absence gravito - dyons
(dyons). key words: dyons, gravito - dyons, quaternion PACS NO: 14.80Hv",1001.0363v1
2010-04-09,Equilibriumlike invaded cluster algorithm: critical exponents and dynamical properties,"We present a detailed study of the Equilibriumlike invaded cluster algorithm
(EIC), recently proposed as an extension of the invaded cluster (IC) algorithm,
designed to drive the system to criticality while still preserving the
equilibrium ensemble. We perform extensive simulations on two special cases of
the Potts model and examine the precision of critical exponents by including
the leading corrections. We show that both thermal and magnetic critical
exponents can be obtained with high accuracy compared to the best available
results. The choice of the auxiliary parameters of the algorithm is discussed
in context of dynamical properties. We also discuss the relation to the
Li-Sokal bound for the dynamical exponent $z$.",1004.1509v1
2010-06-08,Quench Dynamics of Entanglement in an Opened Anisotropic Spin-1/2 Heisenberg Chain,"The quantum entanglement dynamics of a one-dimensional spin-1/2 anisotropic
XXZ model is studied using the method of the adaptive time-dependent
density-matrix renormalization-group when two cases of quenches are performed
in the system. An anisotropic interaction quench and the maximum number of
domain walls of staggered magnetization quench are considered. The dynamics of
pairwise entanglement between the nearest two qubits in the spin chain is
investigated. The entanglement of the two-spin qubits can be created and
oscillates in both cases of the quench. The anisotropic interaction has a
strong influence on the oscillation frequency of entanglement.",1006.1417v1
2010-06-09,Biconical critical dynamics,"A complete two loop renormalization group calculation of the multicritical
dynamics at a tetracritical or bicritical point in anisotropic antiferromagnets
in an external magnetic field is performed. Although strong scaling for the two
order parameters (OPs) perpendicular and parallel to the field is restored as
found earlier, in the experimentally accessible region the effective dynamical
exponents for the relaxation of the OPs remain different since their equal
asymptotic values are not reached.",1006.1766v2
2010-06-24,Field-induced decay dynamics in square-lattice antiferromagnet,"Dynamical properties of the square-lattice Heisenberg antiferromagnet in
applied magnetic field are studied for arbitrary value S of the spin. Above the
threshold field for two-particle decays, the standard spin-wave theory yields
singular corrections to the excitation spectrum with logarithmic divergences
for certain momenta. We develop a self-consistent approximation applicable for
S >= 1, which avoids such singularities and provides regularized magnon decay
rates. Results for the dynamical structure factor obtained in this approach are
presented for S = 1 and S = 5/2.",1006.4838v2
2010-09-07,Entanglement dynamics of two qubits under the influence of external kicks and Gaussian pulses,"We have investigated the dynamics of entanglement between two spin-1/2 qubits
that are subject to independent kick and Gaussian pulse type external magnetic
fields analytically as well as numerically. Dyson time ordering effect on the
dynamics is found to be important for the sequence of kicks. We show that
""almost-steady"" high entanglement can be created between two initially
unentangled qubits by using carefully designed kick or pulse sequences.",1009.1350v3
2010-10-27,Classical zitterbewegung in reduced plasma dynamics,"The process of dynamical reduction of the Vlasov-Maxwell equations leads to
the introduction of classical {\it zitterbewegung} effects in reduced plasma
dynamics. These effects manifest themselves in the form of an asymmetric
canonical energy-momentum tensor involving the decoupling of the reduced
kinetic momentum ${\bf p}$ from the reduced velocity ${\bf u}$ (i.e., ${\bf
u}\btimes{\bf p} \neq 0$) as well as reduced polarization and magnetization
effects. The reduced intrinsic torque generated by the antisymmetric part of
the canonical energy-momentum tensor, which is calculated from the reduced
ponderomotive potential, acts as the source for the intrinsic (spin) angular
momentum.",1010.5710v1
2011-02-26,Electric signature of magnetic domain-wall dynamics,"We study current-induced domain-wall dynamics in a thin ferromagnetic
nanowire. The domain-wall dynamics is described by simple equations with four
parameters. We propose the procedure to determine these parameters by
all-electric measurements of the time-dependent voltage induced by the
domain-wall motion. We provide an analytical expression for the time variation
of this voltage. Furthermore, we show that the measurement of the proposed
effects is within reach with current experimental techniques.",1102.5459v2
2011-09-12,Fourier-Hankel/Bessel space absolute equilibria of 2D gyrokinetics,"Two global invariants of two dimensional gyrokinetics are shown to be
""rugged"" (still conserved by the dynamics) concerning both Fourier and
Hankel/Bessel Galerkin truncations. The truncations are made to keep only a
finite range of wavenumber $\textbf{k}$ and the Hankel variable $b$ (or $z$ in
the Bessel series). The absolute equilibria are used for the discussion of the
spectral transfers in the configuration-velocity scale space of kinetic
magnetized plasma turbulence. Some interesting aspects of recent numerical
results, which were not well understood, are explained with more satisfaction.",1109.2511v1
2011-11-11,Out of equilibrium dynamics of coherent non-abelian gauge fields,"We study out-of-equilibrium dynamics of intense non-abelian gauge fields.
Generalizing the well-known Nielsen-Olesen instabilities for constant initial
color-magnetic fields, we investigate the impact of temporal modulations and
fluctuations in the initial conditions. This leads to a remarkable coexistence
of the original Nielsen-Olesen instability and the subdominant phenomenon of
parametric resonance. Taking into account that the fields may be correlated
only over a limited transverse size, we model characteristic aspects of the
dynamics of color flux tubes relevant in the context of heavy-ion collisions.",1111.2751v2
2011-11-29,Average quantum dynamics of closed systems over stochastic Hamiltonians,"We develop a master equation formalism to describe the evolution of the
average density matrix of a closed quantum system driven by a stochastic
Hamiltonian. The average over random processes generally results in decoherence
effects in closed system dynamics, in addition to the usual unitary evolution.
We then show that, for an important class of problems in which the Hamiltonian
is proportional to a Gaussian random process, the 2nd-order master equation
yields exact dynamics. The general formalism is applied to study the examples
of a two-level system, two atoms in a stochastic magnetic field and the heating
of a trapped ion.",1111.6686v1
2012-01-04,Adjacent Spin Operator Dynamical Structure Factor of the S=1/2 Heisenberg Chain,"Considering the adjacent spin operators $S^z_jS^z_{j+1}$ and $S^-_jS^-_{j+1}$
in the S=1/2 Heisenberg chain, we give a determinant representation of their
form factors. The dynamical structure factors of the respective operators are
computed over the whole Brillouin zone in several magnetic fields and the
resulting signal is analyzed in terms of excitation types. Among other results,
we find that the $S^z_jS^z_{j+1}$ dynamical structure factor carries a large
weight of the 4-spinon excitations which are distinguishable from the 2-spinon
signal because they are located outside the 2-spinon spectrum.",1201.0867v1
2012-01-20,Simulations of Buoyant Plumes in Solar Prominences,"Observations of solar prominences reveal a complex, dynamic flow field within
them. The flow field within quiescent prominences is characterized by long
``threads'' and dark ``bubbles'' that fall and rise (respectively) in a thin
sheet. The flow field in active prominences display more helical motions that
travel along the axis of the prominence. We explore the possible dynamics of
both of these with the aid of 2.5D MHD simulations. Our model, compressible
plasma possesses density and temperature gradients and resides in magnetic
field configurations that mimc those of a solar prominence. We present results
of various configurations and discuss the nonlinear behavior of the resulting
dynamics.",1201.4352v1
2012-02-29,Dynamic Critical Behavior of the Heisenberg Model with Strong Easy Plane Anisotropy,"The dynamic critical behavior of the Heisenberg model with a strong
anisotropy of the exchange constant in the z direction is investigated. The
main features of the time evolution of this model are revealed. The static and
dynamic critical behavior of planar magnetic models is shown to be described
well by the Heisenberg model with strong easy plane anisotropy.",1202.6509v1
2012-07-21,Controlling high-frequency collective electron dynamics via single-particle complexity,"We demonstrate, through experiment and theory, enhanced high-frequency
current oscillations due to magnetically-induced conduction resonances in
superlattices. Strong increase in the ac power originates from complex
single-electron dynamics, characterized by abrupt resonant transitions between
unbound and localized trajectories, which trigger and shape propagating charge
domains. Our data demonstrate that external fields can tune the collective
behavior of quantum particles by imprinting configurable patterns in the
single-particle classical phase space.",1207.5166v1
2012-08-21,Evidence of Vortex Jamming in Abrikosov Vortex Flux Flow Regime,"We report on dynamics of non-local Abrikosov vortex flow in mesoscopic
superconducting Nb channels. Magnetic field dependence of the non-local voltage
induced by the flux flow shows that vortices form ordered vortex chains.
Voltage asymmetry (rectification) with respect to the direction of vortex flow
is evidence that vortex jamming strongly moderates vortex dynamics in
mesoscopic geometries. The findings can be applied to superconducting devices
exploiting vortex dynamics and vortex manipulation, including superconducting
wires with engineered pinning centers.",1208.4396v1
2013-01-08,Dynamical Structure of Baryons,"Compton scattering offers a unique opportunity to study the dynamical
structure of hadrons over a wide kinematic range, with polarizabilities
characterizing the hadron active internal degrees of freedom. We present
calculations and detailed analysis of electric and magnetic and the
spin-dependent dynamical polarizabilities for the lowest in mass SU(3) octet of
baryons. These extensive calculations are made possible by the recent
implementation of semi-automatized calculations in chiral perturbation theory
which allows evaluating polarizabilities from Compton scattering up to
next-to-the-leading order. The dependencies for the range of photon energies
covering the majority of the meson photoproduction channels are analyzed.",1301.1383v1
2013-06-15,Monte Carlo simulations of dynamic phase transitions in ultrathin Blume-Capel films,"Dynamic phase transition phenomena in ultrathin films described by
Blume-Capel model have been investigated using Monte Carlo simulations.
Hysteresis loops, micromagnetic structures, and hysteresis loop area curves, as
well as dynamic correlation between the magnetization and the external field
have been studied as functions of the field, as well as the film parameters.
The variation of critical coupling of the modified film surface at which the
transition temperature becomes independent of film thickness have been
clarified for varying system parameters. Frequency dispersion of hysteresis
loop area have been found to obey a power law for low and moderate frequencies
for both ordinary and enhanced surfaces.",1306.3562v1
2013-06-24,Non-equilibrium dynamics in the antiferromagnetic Hubbard model,"We investigate by means of the time-dependent Gutzwiller approximation the
out-of-equilibrium dynamics of an antiferromagnetic state evolved with the
Hubbard model Hamiltonian after a sudden change of the repulsion strength $U$.
We find that magnetic order survives more than what expected on the basis of
thermalization arguments, in agreement with recent DMFT calculations. In
addition, we find evidence of a dynamical transition for quenches to large
values of $U$ between a coherent antiferromagnet characterized by a finite
quasiparticle residue to an incoherent one with vanishing residue, that finally
turns into a paramagnet for even larger $U$.",1306.5733v2
2013-06-25,Dynamical spin-spin coupling of quantum dots,"We carried out a nested Schrieffer-Wolff transformation of an Anderson
two-impurity Hamiltonian to study the spin-spin coupling between two dynamical
quantum dots under the influence of rotating transverse magnetic field. As a
result of the rotating field, we predict a novel Ising type spin-spin coupling
mechanism between quantum dots, whose strength is tunable via the magnitude of
the rotating field. The strength of the new coupling could be comparable to the
strength of the RKKY coupling. The dynamical coupling with the intristic RKKY
coupling enables to construct a four level system of maximally entangled Bell
states in a controllable manner.",1306.5889v2
2013-10-12,Resonant dynamics of chromium condensates,"We numerically study the dynamics of a spinor chromium condensate in low
magnetic fields. We show that the condensate evolution has a resonant character
revealing rich structure of resonances similar to that already discussed in the
case of alkali-atoms condensates. This indicates that dipolar resonances occur
commonly in the systems of cold atoms. In fact, they have been already observed
experimentally. We further simulate two recent experiments with chromium
condensates, in which the threshold in spin relaxation and the spontaneous
demagnetization phenomena were observed. We demonstrate that both these effects
originate in resonant dynamics of chromium condensate.",1310.3384v1
2014-06-24,Ultrafast spectroscopy of quasiparticle dynamics in cuprate superconductors,"Ultrafast pump-probe spectroscopy is a powerful tool to study the
nonequilibrium dynamics in high-Tc cuprate superconductors. The photo-induced
quasiparticle (QP) dynamics revealed by pump-probe spectroscopy are sensitive
to the near-Fermi level electronic structures. Here we review several selected
examples to illustrate the enduring challenges including pairing glue, phase
separation, and phase transitions in cuprate superconductors. We also present
the data obtained on thin films of YBa2Cu3O7-{\delta} in connection to these
issues.",1406.6207v1
2014-09-19,Dynamical nonlocal coherent-potential approximation for itinerant electron magnetism,"A dynamical generalisation of the nonlocal coherent-potential approximation
is derived based upon the functional integral approach to the interacting
electron problem. The free energy is proven to be variational with respect to
the self-energy provided a self-consistency condition on a cluster of sites is
satisfied. In the present work, calculations are performed within the static
approximation and the effect of the nonlocal physics on the formation of the
local moment state in a simple model is investigated. The results reveal the
importance of the dynamical correlations.",1409.5652v1
2014-10-15,Micromagnetic Study of Spin Transfer driven Vortex Dipole and Vortex Quadrupole Dynamics,"Vortices and antivortices are typical non uniform magnetization
configurations that can be achieved in spin-torque oscillators with in-plane
materials. Dynamics of a vortex-antivortex pair, namely vortex dipole, were
predicted and already observed. In this paper, we present a systematic
micromagnetic study on that kind of dynamics in different spin-valves where the
current is injected through a nano-aperture. Rotation and translation of vortex
dipoles and rotation of vortex quadrupoles are shown depending on the shape and
the size of the spin-valve. The origin of the different behaviors is explained
within a micromagnetic framework.",1410.3957v1
2015-07-08,Beam dynamics of the superconducting wiggler on the SSRF storage ring,"In the SSRF Phase-II beamline project, a Superconducting Wiggler (SW) will be
installed in the electron storage ring. It may greatly impact on the beam
dynamics due to the very high magnetic field. The emittance growth becomes a
main problem, even after a well correction of the beam optics. A local
achromatic lattice is studied, in order to combat the emittance growth and keep
the good performance of the SSRF storage ring, as well as possible. Other
effects of the SW are simulated and optimized as well, including the beta
beating, the tune shift, the dynamic aperture, and the field error effects.",1507.02217v1
2015-09-03,Dynamic Entanglement Evolution of Two-qubit XYZ Spin Chain in Markovian Environment,"We propose a new approach called Ket-Bra Entangled State (KBES) Method for
converting master equation into Schr\""{o}dinger-like equation. With this
method, we investigate decoherence process and entanglement dynamics induced by
a $2$-qubit spin chain that each qubit coupled with reservoir. The spin chain
is an anisotropy $XYZ$ Heisenberg model in the external magnetic field $B$, the
corresponding master equation is solved concisely by KBES method; Furthermore,
the effects of anisotropy, temperature, external field and initial state on
concurrence dynamics is analyzed in detail for the case that initial state is
Extended Wenger-Like(EWL) state. Finally we research the coherence and
concurrence of the final state (namely the density operator for time tend to
infinite)",1509.01001v1
2015-10-16,Reaction diffusion dynamics and the Schryer-Walker solution for domain walls of the Landau-Lifshitz-Gilbert equation,"We study the dynamics of the equation obtained by Schryer and Walker for the
motion of domain walls. The reduced equation is a reaction diffusion equation
for the angle between the applied field and the magnetization vector. If the
hard axis anisotropy $K_d$ is much larger than the easy axis anisotropy $K_u$,
there is a range of applied fields where the dynamics does not select the
Schryer-Walker solution. We give analytic expressions for the speed of the
domain wall in this regime and the conditions for its existence.",1510.04927v1
2016-02-04,Landscape-Inversion Phase Transition in Dipolar Colloids: Tuning the Structure and Dynamics of 2D Crystals,"We study the 2D crystalline phases of paramagnetic colloidal particles with
dipolar interactions and constrained on a periodic substrate. Combining theory,
simulation, and experiments we demonstrate a new scenario of first-order phase
transitions that occurs via a complete inversion of the energy landscape,
featuring non-conventional properties that allow for: (i) tuning of crystal
symmetry; (ii) control of dynamical properties of different crystalline orders
via tuning of their relative stability with an external magnetic field; (iii)
an equivalent but independent control of the same dynamic properties via
temporal modulations of that field; and (iv) non-standard phase-ordering
kinetics involving spontaneous formation of transient metastable domains.",1602.01810v1
2016-02-14,Recursive Recovery of Sparse Signal Sequences from Compressive Measurements: A Review,"In this article, we review the literature on design and analysis of recursive
algorithms for reconstructing a time sequence of sparse signals from
compressive measurements. The signals are assumed to be sparse in some
transform domain or in some dictionary. Their sparsity patterns can change with
time, although, in many practical applications, the changes are gradual. An
important class of applications where this problem occurs is dynamic projection
imaging, e.g., dynamic magnetic resonance imaging (MRI) for real-time medical
applications such as interventional radiology, or dynamic computed tomography.",1602.04518v1
2016-04-21,Contributions of plasma physics to chaos and nonlinear dynamics,"This topical review focusses on the contributions of plasma physics to chaos
and nonlinear dynamics bringing new methods which are or can be used in other
scientific domains. It starts with the development of the theory of Hamiltonian
chaos, and then deals with order or quasi order, for instance adiabatic and
soliton theories. It ends with a shorter account of dissipative and high
dimensional Hamiltonian dynamics, and of quantum chaos. Most of these
contributions are a spin-off of the research on thermonuclear fusion by
magnetic confinement, which started in the fifties. Their presentation is both
exhaustive and compact. [15 April 2016]",1604.06305v2
2016-05-31,1/f noise in turbulent flows,"We report the experimental observation of $1/f$ fluctuations in three
different turbulent flow configurations: the large scale velocity driven by a
two-dimensional turbulent flow, the magnetic field generated by a turbulent
swirling flow of liquid sodium and the pressure fluctuations due to vorticity
filaments in a swirling flow. For these three systems, $1/f$ noise is shown to
result from the dynamics of coherent structures that display transitions
between a small number of states. The interevent duration is distributed as a
power-law. The exponent of this power-law and the nature of the dynamics
(transition between symmetric states or asymmetric ones) select the exponent of
the $1/f$ fluctuations.",1605.09666v1
2016-09-20,Renormalization group calculation of dynamic exponent in the models E and F with hydrodynamic fluctuations,"The renormalization group method is applied in order to analyze models E and
F of critical dynamics in the presence of velocity fluctuations generated by
the stochastic Navier-Stokes equation. Results are given to the one-loop
approximation for the anomalous dimension $\gamma_\lambda$ and fixed-points'
structure. The dynamic exponent $z$ is calculated in the turbulent regime and
stability of the fixed points for the standard model E is discussed.",1609.06078v2
2017-07-04,Dynamical Wormhole Definitions Confronted,"Crude comparison between four alternative proposals for the very definition
of a wormhole is provided, all of which were intended to apply to the dynamical
cases. An interesting dynamical solution, based upon large scale magnetic
fields, is used for the comparisons. Such solution goes beyond the perfect
fluid approximation due to an anisotropic pressure component, bringing to the
fore some unsuspected features of those definitions. Certain notions as
reversible traversability are claimed as a way to select among those
definitions the best suited one to represent our intuition of what a wormhole
solution is expected to be.",1707.01078v1
2018-10-07,Tri-spin dynamics in alkali metal-noble gas NMR gyroscope,"Alkali metal-noble gas NMR gyroscope is widely used for precision rotation
measurement in fundamental and applied physics. By numerically simulating the
alkali-nuclear-nuclear tri-spin dynamics, we investigate the dependence of
gyroscope response on alkali spin relaxation time and nuclear spin
magnetization. We found additional resonance peaks appear due to a new source
of instability, namely the inherent multistability of tri-spin dynamics. The
numerical simulation results agree well with the recent experiment, enabling a
better understanding and exploitation of the gyroscope signal.",1810.03185v4
2018-12-03,Quantum plateaus in dynamical Hall conductivity,"Dynamical Hall conductivity {\sigma}_H({\omega}) of a 2D electron gas with
impurities in the perpendicular magnetic field is analyzed. Plateau-like
behavior at low frequencies as well as at high frequencies provided the
complete filling of Landau levels is predicted. The broadening of a Landau
level separates two frequency regions with different behaviour. Imaginary part
of dynamical Hall conductivity reveals oscillations in the localized states
region. Comparison with the experiment is carried out.",1812.00603v1
2019-03-06,Cluster multipole dynamics in non-collinear antiferromagnets,"A systematic framework to investigate spin dynamics in non-collinear
antiferromagnet is proposed. Taking Mn$_3$Sn as a representative example, we
derive an effective low energy model based on the multipole expansion of the
magnetic structure, and investigate the uniform precession and the domain wall
dynamics. We show that the solution for the effective model accurately
reproduces the numerical calculation of the Landau-Lifshitz-Gilbert equations.
Our results indicate that Mn$_3$Sn has preferable properties for applications
to a racetrack memory and a spin torque oscillator, and thus, is a promising
candidate for new devices by using the multipole degrees of freedom.",1903.02259v1
2020-05-04,A Dynamical Mean-Field Theory for Learning in Restricted Boltzmann Machines,"We define a message-passing algorithm for computing magnetizations in
Restricted Boltzmann machines, which are Ising models on bipartite graphs
introduced as neural network models for probability distributions over spin
configurations. To model nontrivial statistical dependencies between the spins'
couplings, we assume that the rectangular coupling matrix is drawn from an
arbitrary bi-rotation invariant random matrix ensemble. Using the dynamical
functional method of statistical mechanics we exactly analyze the dynamics of
the algorithm in the large system limit. We prove the global convergence of the
algorithm under a stability criterion and compute asymptotic convergence rates
showing excellent agreement with numerical simulations.",2005.01560v1
2020-06-16,Dynamical response and competing orders in two-band Hubbard model,"We present a dynamical mean-field study of two-particle dynamical response
functions in two-band Hubbard model across several phase transitions. We
observe that the transition between theexcitonic condensate and spin-state
ordered state is continuous with a narrow strip of supersolidphase separating
the two. Approaching transition from the excitonic condensate is announced
bysoftening of the excitonic mode at theMpoint of the Brillouin zone. Inside
the spin-state orderedphase there is a magnetically ordered state with 2x2
periodicity, which has no precursor in thenormal phase.",2006.09058v1
2021-04-16,Spin-ice dynamics of pyrochlore Dy2GaSbO7 with enhanced Pauling zero-point entropy,"We report the low-temperature magnetothermal properties and spin dynamics of
a mixed pyrochlore, Dy2GaSbO7 through the coordination of measurements of dc
magnetization, ac susceptibility, and heat capacity, and CF computation. In
Dy2GaSbO7, the spins freeze at temperature Tice = 3.1 K, corresponding to
dynamical spin-freezing into a disordered state in compliance with the spin-ice
rules, but with enhanced zero-point entropy, unlike cannonical dipolar spin-ice
materials, Dy2Ti2O7 and Ho2Ti2O7, due to random disorder and varied chemical
pressure of B-site (GaSb tetrahedra) ions.",2104.07890v1
2010-05-19,Charge dynamics in two-electron quantum dots,"We investigate charge dynamics in a two-electron double quantum dot. The
quantum dot is manipulated by using a time-dependent external voltage that
induces charge oscillations between the dots. We study the dependence of the
charge dynamics on the external magnetic field and on the periodicity of the
external potential. We find that for suitable parameter values, it is possible
to induce both one-electron and two-electron oscillations between the dots.",1005.3369v1
2013-08-26,Quantum criticality and dynamical instability in the kicked top,"We investigate precursors of critical behavior in the quasienergy spectrum
due to the dynamical instability in the kicked top. Using a semiclassical
approach, we analytically obtain a logarithmic divergence in the density of
states, which is analogous to a continuous excited state quantum phase
transition in undriven systems. We propose a protocol to observe the cusp
behavior of the magnetization close to the critical quasienergy.",1308.5640v2
2016-11-09,Short-Time Dynamics of the Three-Dimensional Fully Frustrated Ising Model,"The critical relaxation from the low-temperature ordered state of the
three-dimensional fully frustrated Ising model on a simple cubic lattice has
been studied using the short time dynamics method. Particles with the periodic
boundary conditions containing N = 262144 spins have been studied. Calculations
have been performed by the standard Metropolis Monte Carlo algorithm. The
static critical exponents of the magnetization and correlation radius have been
obtained. The dynamic critical exponent of the model under study has been
calculated.",1611.02898v1
2017-04-27,Periodic orbits in the restricted three-body problem and Arnold's $J^+$-invariant,"We apply Arnold's theory of generic smooth plane curves to Stark-Zeeman
systems. This is a class of Hamiltonian dynamical systems that describes the
dynamics of an electron in an external electric and magnetic field, and
includes many systems from celestial mechanics. Based on Arnold's
$J^+$-invariant, we introduce invariants of periodic orbits in planar
Stark-Zeeman systems and study their behaviour.",1704.08568v1
2018-05-07,Probing dynamics in quantum materials with femtosecond x-rays,"Optical pulses are routinely used to drive dynamical changes in the
properties of solids. In quantum materials, many new phenomena have been
discovered, including ultrafast transitions between electronic phases,
switching of ferroic orders and nonequilibrium emergent behaviors such as
photo-induced superconductivity. Understanding the underlying non-equilibrium
physics requires detailed measurements of multiple microscopic degrees of
freedom at ultrafast time resolution. Femtosecond x-rays are key to this
endeavor, as they can access the dynamics of structural, electronic and
magnetic degrees of freedom. Here, we cover a series of representative
experimental studies in which ultrashort x-ray pulses from free electron lasers
have been used, opening up new horizons for materials research.",1805.02376v1
2018-08-28,Nonlinear Dynamics of Coupled Axion-Josephson Junction Systems,"We study the classical dynamics of an axion field (the signal) that is
coupling into a Josephson junction (the detector) by means of a capacitive
coupling of arbitrary size. Depending on the size of the coupling constant and
the initial conditions, we find a rich phase space structure of this nonlinear
problem. We present general analytic solutions of the equations of motion in
the limit of small amplitudes of the angle variables, and discuss both the case
of no dissipation and the case of dissipation in the system. The effect of a
magnetic field is investigated as well, leading to topological phase
transitions in the phase space structure.",1808.09417v1
2021-08-13,Emulating ultrafast dissipative quantum dynamics with deep neural networks,"The simulation of driven dissipative quantum dynamics is often prohibitively
computation-intensive, especially when it is calculated for various shapes of
the driving field. We engineer a new feature space for representing the field
and demonstrate that a deep neural network can be trained to emulate these
dynamics by mapping this representation directly to the target observables. We
demonstrate that with this approach, the system response can be retrieved many
orders of magnitude faster. We verify the validity of our approach using the
example of finite transverse Ising model irradiated with few-cycle magnetic
pulses interacting with a Markovian environment. We show that our approach is
sufficiently generalizable and robust to reproduce responses to pulses outside
the training set.",2108.06261v1
2021-08-25,On the derivation of guiding center dynamics without coordinate dependence,"The fundament of the classical guiding center theory is gyro-phase averaging,
which cannot be well defined over a non-trivial magnetic field topology. The
local gyro-phase coordinate frame hides the geometric nature of gyro-symmetry.
A coordinate-free geometric representation should be a more appropriate
alternative for a deeper understanding of the guiding center dynamics. In this
paper, the motion of a charged particle is described by a Lagrangian one-form
on a seven-dimensional phase space. The Lagrangian one-form is geometrically
decomposed by constructing a coordinate-free gyro-averaging method. As a
result, we obtain the coordinate-free expression of the non-relativistic
guiding-center dynamics in the time-dependent slow-varying electromagnetic
field.",2108.10996v2
2021-10-27,Tail contributions to gravitational conservative dynamics,"We compute tail contributions to the conservative dynamics of a generic
self-gravitating system, for every multipole order, of either electric and
magnetic parity. Such contributions arise when gravitational radiation is
backscattered by the static curvature generated by the source itself and
reabsorbed. Tail processes are relevant for the study of compact binary
dynamics starting from the fourth post-Newtonian order. Within this context, we
resolve a discrepancy recently arised at fifth post-Newtonian order between
Effective Field Theory and self-force results.",2110.14146v2
2021-11-22,Counting edge modes via dynamics of boundary spin impurities,"We study dynamics of the one-dimensional Ising model in the presence of
static symmetry-breaking boundary field via the two-time autocorrelation
function of the boundary spin. We find that the correlations decay as a power
law. We uncover a dynamical phase diagram where, upon tuning the strength of
the boundary field, we observe distinct power laws that directly correspond to
changes in the number of edge modes as the boundary and bulk magnetic field are
varied. We suggest how the universal physics can be demonstrated in current
experimental setups, such as Rydberg chains.",2111.11428v2
2022-04-12,Explicit models of motions to understand protein side-chain dynamics,"Nuclear magnetic relaxation is widely used to probe protein dynamics. For
decades, most analyses of relaxation in proteins have relied successfully on
the model-free approach, forgoing mechanistic descriptions of motions.
Model-free types of correlation functions cannot describe a large carbon-13
relaxation dataset in protein sidechains. Here, we use molecular dynamics
simulations to design explicit models of motion and solve Fokker-Planck
diffusion equations. These models of motion provide better agreement with
relaxation data, mechanistic insight and a direct link to configuration
entropy.",2204.05813v2
2022-08-26,"Geometric Reductions, Dynamics and Controls for Hamiltonian System with Symmetry","This is a survey article, from the viewpoint of the completeness of the
Marsden- Weinstein reduction, to introduce briefly some recent developments of
the symmetric reductions and Hamilton-Jacobi theory of the regular controlled
Hamiltonian systems, the nonholonomic controlled Hamiltonian systems and the
controlled magnetic Hamiltonian systems. These research reveal the deeply
internal relationships of the geometrical structures of phase spaces, the
nonholonomic constraint, the dynamical vector fields and the controls of these
systems.",2208.12465v1
2022-11-03,Dynamic phase transitions on the kagome Ising ferromagnet,"We perform extensive Monte Carlo simulations to investigate the dynamic phase
transition properties of the two-dimensional kinetic Ising model on the kagome
lattice in the presence of square-wave oscillating magnetic field. Through
detailed finite-size scaling analysis, we study universality aspects of the
non-equilibrium phase transition. Obtained critical exponents indicate that the
two-dimensional kagome-lattice kinetic Ising model belongs to the same
universality class with the corresponding Ising model in equilibrium. Moreover,
dynamic critical exponent of the local moves used in simulations is determined
with high precision. Our numerical results are compatible with the previous
ones on kinetic Ising models.",2211.01709v1
2023-03-24,Total Angular Momentum Conservation in Ab Initio Born-Oppenheimer Molecular Dynamics,"We prove both analytically and numerically that the total angular momentum of
a molecular system undergoing adiabatic Born-Oppenheimer dynamics is conserved
only when pseudo-magnetic Berry forces are taken into account. This finding
sheds light on the nature of Berry forces for molecular systems with spin-orbit
coupling and highlights how ab initio Born-Oppenheimer molecular dynamics
simulations can successfully capture the entanglement of spin and nuclear
degrees of freedom as modulated by electronic interactions.",2303.13787v1
2023-08-08,Blocking particle dynamics in diamond chain with spatially increasing flux,"Spatial non-uniformity in tight-binding models serves as a source of rich
phenomena. In this paper, we study a diamond-chain tight-binding model with a
spatially-modulated magnetic flux at each plaquette. In the numerical studies
with various combinations of the minimum and maximum flux values, we find the
characteristic dynamics of a particle, namely, a particle slows down when
approaching the plaquette with $\pi$-flux. This originates from the fact that
the sharply localized eigenstates exist around the $\pi$-flux plaquette. These
localized modes can be understood from a squared model of the original one.
This characteristic blocked dynamics will be observed in photonic waveguides or
cold atoms.",2308.03971v1
2023-11-17,Enhancing Electron-Nuclear Resonances by Dynamical Control Switching,"We present a general method to realize resonant coupling between spins even
though their energies are of different scales. Applying the method to the
electron and nuclear spin systems such as a nitrogen-vacancy (NV) center with
its nearby nuclei, we show that a specific dynamical switching of the electron
spin Rabi frequency achieves efficient electron-nuclear coupling, providing a
much stronger quantum sensing signal and dynamic nuclear polarization than
previous methods. This protocol has applications in high-field nanoscale
nuclear magnetic resonances as well as low-power quantum control of nuclear
spins.",2311.10650v1
2024-02-01,Dynamics of a Superconducting Linear Slider,"In this paper, the dynamic behavior of a one degree-of-freedom (DOF)
contactless linear slider based on superconducting magnetic levitation is
experimentally analyzed. The device is intended for precision positioning of an
optic mirror in cryogenic environments. Different prototypes of this device
have been tested at cryogenic temperatures (77 K), and their mechanical
behavior characterized in the sliding direction for forced and unforced
oscillations. Experimental results reveal that the slider is self-stable at the
initial equilibrium position and the dynamic behavior fits well an underdamped
harmonic oscillator. Finally, the device showed great potential for horizontal
vibration isolation, acting as a low-pass filter with a resonance at about 0.9
Hz",2402.00917v1
2024-03-01,Chaos-assisted Turbulence in Spinor Bose-Einstein Condensates,"We present a turbulence-sustaining mechanism in a spinor Bose-Einstein
condensate, which is based on the chaotic nature of internal spin dynamics.
Magnetic driving induces a complete chaotic evolution of the local spin state,
thereby continuously randomizing the spin texture of the condensate to maintain
the turbulent state. We experimentally demonstrate the onset of turbulence in
the driven condensate as the driving frequency changes and show that it is
consistent with the regular-to-chaotic transition of the local spin dynamics.
This chaos-assisted turbulence establishes the spin-driven spinor condensate as
an intriguing platform for exploring quantum chaos and related superfluid
turbulence phenomena.",2403.00524v1
2024-03-15,Critical Fluctuations at Finite-Time Dynamical Phase Transition,"We explore the critical properties of the recently discovered finite-time
dynamical phase transition in the non-equilibrium relaxation of Ising magnets.
The transition is characterized by a sudden switch in the relaxation dynamics
and it occurs at a sharp critical time. While previous works have focused
either on mean-field interactions or on investigating the properties of the
critical time, we analyze the critical fluctuations at the phase transition in
the nearest-neighbor Ising model on a square lattice using Monte Carlo
simulations. By means of a finite-size scaling analysis, we extract the
critical exponents for the transition. In two spatial dimensions, the exponents
are consistent with those of the two-dimensional Ising universality class.",2403.10505v1
2001-11-20,The longitudinal dynamic correlation and dynamic susceptibility of the isotropic XY-model on the 1d alternating superlattice,"The dynamic susceptibility $\chi_{Q}^{zz}(\omega)$ of the isotropic XY-model
(s=1/2) on the alternating superlattice (closed chain) in a transverse field
$h$ is obtained exactly at arbitrary temperatures. It is determined from the
results obtained for the dynamic correlations $<S_{jn}^{z}(t)S_{lm}^{z}(0)>$,
which have been calculated by introducing the generalized Jordan-Wigner
transformation, by using Wick's theorem and by reducing the problem to a
diagonalization of a finite matrix. The static properties are also reobtained
within this new formalism and all exact results are determined for arbitrary
temperatures. Explicit results are obtained numerically in the limit T=0, where
the critical behaviour occurs. A detailed analysis is presented for the
behaviour of the static susceptibility $\chi_{Q}^{zz}(0)$, as a function of the
transverse field h, and for the frequency dependency of the dynamic
susceptibility $\chi_{Q}^{zz}(\omega)$. It is also shown, in this temperature
limit, that within the magnetization plateaus which correspond to the different
phases, even when the induced magnetization is not saturated, the effective
dynamic correlation, $<\sum\limits_{n;m\in
cell:\text{}j;l}S_{jn}^{z}(t)S_{lm}^{z}(0)>$, is time independent, which
constitutes an unexpected result.",0111372v1
2005-12-19,Dynamal (super)symmetries of monopoles and vortices,"The dynamical (super)symmetries for various monopole systems are reviewed.
For a Dirac monopole, no smooth Runge-Lenz vector can exist; there is, however,
a spectrum-generating conformal $o(2,1)$ dynamical symmetry that extends into
$osp(1/1)$ or $osp(1/2)$ for spin 1/2 particles. Self-dual 't
Hooft-Polyakov-type monopoles admit an $su(2/2)$ dynamical supersymmetry
algebra, which allows us to reduce the fluctuation equation to the spin zero
case. For large $r$ the system reduces to a Dirac monopole plus an suitable
inverse-square potential considered before by McIntosh and Cisneros, and by
Zwanziger in the spin 0 case, and to the `dyon' of D'Hoker and Vinet for spin
1/2. The asymptotic system admits a Kepler-type dynamical symmetry as well as a
`helicity-supersymmetry' analogous to the one Biedenharn found in the
relativistic Kepler problem. Similar results hold for the Kaluza-Klein monopole
of Gross-Perry-Sorkin. For the magnetic vortex, the N=2 supersymmetry of the
Pauli Hamiltonian in a static magnetic field in the plane combines with the
$o(2)\times o(2,1)$ bosonic symmetry into an $o(2)\times osp(1/2)$ dynamical
superalgebra.",0512233v2
2013-07-19,"Gauge turbulence, topological defect dynamics, and condensation in Higgs models","The real-time dynamics of topological defects and turbulent configurations of
gauge fields for electric and magnetic confinement are studied numerically
within a 2+1D Abelian Higgs model. It is shown that confinement is appearing in
such systems equilibrating after a strong initial quench such as the
overpopulation of the infrared modes. While the final equilibrium state does
not support confinement, metastable vortex defect configurations appear in the
gauge field which are found to be closely related to the appearance of
physically observable confined electric and magnetic charges. These phenomena
are seen to be intimately related to the approach of a non-thermal fixed point
of the far-from-equilibrium dynamical evolution, signalled by universal scaling
in the gauge-invariant correlation function of the Higgs field. Even when the
parameters of the Higgs action do not support condensate formation in the
vacuum, during this approach, transient Higgs condensation is observed. We
discuss implications of these results for the far-from-equilibrium dynamics of
Yang-Mills fields and potential mechanisms how confinement and condensation in
non-abelian gauge fields can be understood in terms of the dynamics of Higgs
models. These suggest that there is an interesting new class of dynamics of
strong coherent turbulent gauge fields with condensates.",1307.5301v1
2014-04-24,Computer Simulation of Quantum Dynamics in a Classical Spin Environment,"In this paper a formalism for studying the dynamics of quantum systems
coupled to classical spin environments is reviewed. The theory is based on
generalized antisymmetric brackets and naturally predicts open-path
off-diagonal geometric phases in the evolution of the density matrix. It is
shown that such geometric phases must also be considered in the
quantum-classical Liouville equation for a classical bath with canonical phase
space coordinates; this occurs whenever the adiabatics basis is complex (as in
the case of a magnetic field coupled to the quantum subsystem). When the
quantum subsystem is weakly coupled to the spin environment, non-adiabatic
transitions can be neglected and one can construct an effective non-Markovian
computer simulation scheme for open quantum system dynamics in classical spin
environments. In order to tackle this case, integration algorithms based on the
symmetric Trotter factorization of the classical-like spin propagator are
derived. Such algorithms are applied to a model comprising a quantum two-level
system coupled to a single classical spin in an external magnetic field.
Starting from an excited state, the population difference and the coherences of
this two-state model are simulated in time while the dynamics of the classical
spin is monitored in detail. It is the author's opinion that the numerical
evidence provided in this paper is a first step toward developing the
simulation of quantum dynamics in classical spin environments into an effective
tool. In turn, the ability to simulate such a dynamics can have a positive
impact on various fields, among which, for example, nano-science.",1404.6143v1
2015-06-26,Dynamical extension of Hellmann-Feynman theorem and application to nonadiabatic quantum processes in Topological and Correlated Matter,"An extension of the Hellmann-Feynman theorem to one employing dynamical
parameters that vary with time according to quantum dynamics is rigorously
derived, avoiding any linear response or other approximations. The resulting
theorem for the dynamics of observables, valid to all orders in external
fields, is found to contain generalized Berry curvature type of quantities that
incorporate the dynamics through explicit and nontrivial time-dependence; these
Berry quantities resemble the so called anomalous terms (in semiclassical
equations of motion in solids) but are both of magnetic and electric type, the
former being associated with Quantum Hall Effect type of behaviors and the
latter with Polarization type of behaviors. By way of application of the new
theorem, the quantum equations of motion of a spinless and a spinfull electron
in a solid are derived without any adiabatic or semiclassical approximation.
The charge current formula for a many-body and interacting spinfull system is
also derived and is found to consist of a longitudinal and a transverse part;
phenomenological interpretations with respect to polarization and magnetization
currents respectively then emerge in a natural way. In addition, a formula for
the topological magnetoelectric effect for an interacting spinfull electron
system is also provided. By carefully defining singlevaluedness in parameter
space, in a nonstandard fashion and in higher rigor than usual, we are able to
discuss in clarity the issue of possible obstruction of this singlevaluedness,
the associated creation of Berry monopoles in parameter space and the
quantization of the flux of Berry curvature (but with nontrivial dynamics
included in its definition).",1506.08812v2
2018-06-21,Dynamically Generated Synthetic Electric Fields for Photons,"Static synthetic magnetic fields give rise to phenomena including the Lorentz
force and the quantum Hall effect even for neutral particles, and they have by
now been implemented in a variety of physical systems. Moving towards fully
dynamical synthetic gauge fields allows, in addition, for backaction of the
particles' motion onto the field. If this results in a time-dependent vector
potential, conventional electromagnetism predicts the generation of an electric
field. Here, we show how synthetic electric fields for photons arise
self-consistently due to the nonlinear dynamics in a driven system. Our
analysis is based on optomechanical arrays, where dynamical gauge fields arise
naturally from phonon-assisted photon tunneling. We study open, one-dimensional
arrays, where synthetic magnetic fields are absent. However, we show that
synthetic electric fields can be generated dynamically, which, importantly,
suppress photon transport in the array. The generation of these fields depends
on the direction of photon propagation, leading to a novel mechanism for a
photon diode, inducing nonlinear nonreciprocal transport via dynamical
synthetic gauge fields.",1806.08191v3
2020-02-23,Anisotropic dynamics of a self-assembled colloidal chain in an active bath,"Anisotropic macromolecules exposed to non-equilibrium (active) noise are very
common in biological systems, and an accurate understanding of their
anisotropic dynamics is therefore crucial. Here, we experimentally investigate
the dynamics of isolated chains assembled from magnetic microparticles at a
liquid-air interface and moving in an active bath consisting of motile E. coli
bacteria. We investigate both the internal chain dynamics and the anisotropic
center-of-mass dynamics through particle tracking. We find that both the
internal and center-of-mass dynamics are greatly enhanced compared to the
passive case, i.e., a system without bacteria, and that the center-of-mass
diffusion coefficient $D$ features a non-monotonic dependence as a function of
the chain length. Furthermore, our results show that the relationship between
the components of $D$ parallel and perpendicular with respect to the direction
of the applied magnetic field is preserved in the active bath compared to the
passive case, with a higher diffusion in the parallel direction, in contrast to
previous findings in the literature. We argue that this qualitative difference
is due to subtle differences in the experimental geometry and conditions and
the relative roles played by long-range hydrodynamic interactions and
short-range collisions.",2002.09961v2
2020-10-12,Tunable dynamical magnetoelectric effect in antiferromagnetic topological insulator MnBi$_2$Te$_4$ films,"More than forty years ago, axion was postulated as an elementary particle
with a low mass and weak interaction in particle physics to solve the strong
$\mathcal{CP}$ (charge conjugation and parity) puzzle. Axions are also
considered as a possible component of dark matter of the universe. However, the
existence of axions in nature has not been confirmed. Interestingly, axions
arise as pseudoscalar fields derived from the Chern-Simons theory in condensed
matter physics. In antiferromagnetic insulators, the axion field can become
dynamical induced by spin-wave excitations and exhibits rich exotic phenomena,
such as, the chiral magnetic effect, axionic polariton and so on. However, the
study of the dynamical axion field is rare due to the lack of real materials.
Recently, MnBi$_2$Te$_4$ was discovered to be an antiferromagnetic topological
insulator with a quantized axion field protected by the inversion symmetry
$\mathcal{P}$ and the magnetic-crystalline symmetry $\mathcal{S}$. Here, we
studied MnBi$_2$Te$_4$ films in which both the $\mathcal{P}$ and $\mathcal{S}$
symmetries are spontaneously broken and found that the dynamical axion field
and largely tunable dynamical magnetoelectric effects can be realized through
tuning the thickness of MnBi$_2$Te$_4$ films, the temperature and the element
substitution. Our results open a broad avenue to study axion dynamics in
antiferromagnetic topological insulator MnBi$_2$Te$_4$ and related materials,
and also is hopeful to promote the research of dark matter.",2010.05424v1
2021-01-14,Quantum dynamics of a single fluxon in Josephson junctions parallel arrays with large kinetic inductances,"We present a theoretical study of coherent quantum dynamics of a single
magnetic fluxon (MF) trapped in Josephson junction parallel arrays (JJPAs) with
large kinetic inductances. The MF is the topological excitation carrying one
quantum of magnetic flux, $\Phi_0$. The MF is quantitatively described as the
$2\pi$-kink in the distribution of Josephson phases, and for JJPAs with high
kinetic inductances the characteristic length of such distribution (""the size""
of MF) is drastically reduced. Characterizing such MFs by the Josephson phases
of three consecutive Josephson junctions we analyse the various coherent
macroscopic quantum effects in the MF quantum dynamics. In particular, we
obtain the MF energy band originating from the coherent quantum tunnelling of a
single MF between adjacent cells of JJPAs. The dependencies of the band width
$\Delta$ on the Josephson coupling energy $E_J$, charging energy $E_C$ and the
inductive energy of a cell $E_L$, are studied in detail. In long linear JJPAs
the coherent quantum dynamics of MF demonstrates decaying quantum oscillations
with characteristic frequency $f_{qb}=\Delta/h$. In short annular JJPAs the
coherent quantum dynamics of MF displays complex oscillations controlled by the
Aharonov-Casher phase $\chi \propto V_g$, where $V_g$ is an externally applied
gate voltage. In the presence of externally applied dc bias, $I$, a weakly
incoherent dynamics of quantum MF is realized in the form of macroscopic Bloch
oscillations leading to a typical ""nose"" current-voltage characteristics of
JJPAs. As ac current with frequency $f$ is applied the current-voltage
characteristics displays a set of equidistant current steps at $I_n=2en f$.",2101.05693v1
2021-01-27,Coherent dynamics of multi-spin $\rm V_B^-$ centers in hexagonal boron nitride,"Hexagonal boron nitride (hBN) has recently been demonstrated to contain
optically polarized and detected electron spins that can be utilized for
implementing qubits and quantum sensors in nanolayered-devices. Understanding
the coherent dynamics of microwave driven spins in hBN is of crucial importance
for advancing these emerging new technologies. Here, we demonstrate and study
the Rabi oscillation and related dynamical phenomena of the negatively charged
boron vacancy ($\rm V_B^-$) spins in hBN. We report on different dynamics of
the $\rm V_B^-$ spins at weak and strong magnetic fields. In the former case
the defect behaves like a single electron spin system, while in the latter case
it behaves like a multi-spin system exhibiting the multiple-frequency dynamical
oscillation like clear beat in Ramsey fringes. We also carry out theoretical
simulations for the spin dynamics of $\rm V_B^-$ and reveal that the nuclear
spins can be driven via the strong electric-nuclear coupling existing in $\rm
V_B^-$ center, which can be modulated by the magnetic field and microwave
field.",2101.11220v3
2019-02-21,Mathematical analysis and numerical solution of models with dynamic Preisach hysteresis,"Hysteresis is a phenomenon that is observed in a great variety of physical
systems, which leads to a nonlinear and multivalued behavior, making their
modeling and control difficult. Even though the analysis and mathematical
properties of classical or rate-independent hysteresis models are known, this
is not the case for dynamic models where current approaches lack a proper
functional analytic framework which is essential to formulate optimization
problems and develop stable numerics, both being crucial in practice. This
paper deals with the description and mathematical analysis of the dynamic
Preisach hysteresis model. Toward that end, we complete a widely accepted
definition of the dynamic model commonly used to describe the constitutive
relation between the magnetic field H and the magnetic induction B, in which,
the values of B not only depends on the present values of H but also on the
past history and its velocity. We first analyze mathematically some important
properties of the model and compare them with known results for the static
Preisach model. Then, we consider a parabolic problem with dynamic hysteresis
motivated by electromagnetic field equations. Under suitable assumptions, we
show the well posedness of a weak formulation of the problem and solve it
numerically.
  Finally, we report a numerical test in order to assess the order of
convergence and to illustrate the behavior of the numerical solution for
different configurations of the dynamic Preisach model",1902.08175v1
2020-09-03,Dynamics of magnetic collective modes in the square and triangular lattice Mott insulators at finite temperature,"We study the equilibrium dynamics of magnetic moments in the Mott insulating
phase of the Hubbard model on the square and triangular lattice. We rewrite the
Hubbard interaction in terms of an auxiliary vector field and use a recently
developed Langevin scheme to study its dynamics. A thermal `noise', derivable
approximately from the Keldysh formalism, allows us to study the effect of
finite temperature. At strong coupling, $U \gg t$, where $U$ is the local
repulsion and $t$ the nearest neighbour hopping, our results reproduce the well
known dynamics of the nearest neighbour Heisenberg model with exchange $J \sim
{\cal O}(t^2/U)$. These include crossover from weakly damped dispersive modes
at temperature $T \ll J$ to strong damping at $T \sim {\cal O}(J)$, and
diffusive dynamics at $T \gg J$. The crossover temperatures are naturally
proportional to $J$. To highlight the progressive deviation from Heisenberg
physics as $U/t$ reduces we compute an effective exchange scale $J_{eff}(U)$
from the low temperature spin wave velocity. We discover two features in the
dynamical behaviour with decreasing $U/t$: (i)~the low temperature dispersion
deviates from the Heisenberg result, as expected, due to longer range and
multispin interactions, and (ii)~the crossovers between weak damping, strong
damping, and diffusion take place at noticeably lower values of $T/J_{eff}$. We
relate this to enhanced mode coupling, in particular to thermal amplitude
fluctuations, at weaker $U/t$. A comparison of the square and triangular
lattice reveals the additional effect of geometric frustration on damping.",2009.01833v2
2020-09-22,Simulating the dynamics and non-thermal emission of relativistic magnetised jets I. Dynamics,"We have performed magneto-hydrodynamic simulations of relativistic jets from
supermassive blackholes over a few tens of kpc for a range of jet parameters.
One of the primary aims were to investigate the effect of different MHD
instabilities on the jet dynamics and their dependence on the choice of jet
parameters. We find that two dominant MHD instabilities affect the dynamics of
the jet, small scale Kelvin- Helmholtz (KH) modes and large scale kink modes,
whose evolution depend on internal jet parameters like the Lorentz factor, the
ratio of the density and pressure to the external medium and the magnetisation
and hence consequently on the jet power. Low power jets are susceptible to both
instabilities, kink modes for jets with higher central magnetic field and KH
modes for lower magnetisation. Moderate power jets do not show appreciable
growth of kink modes, but KH modes develop for lower magnetisation. Higher
power jets are generally stable to both instabilities. Such instabilities
decelerate and decollimate the jet while inducing turbulence in the cocoon,
with consequences on the magnetic field structure. We model the dynamics of the
jets following a generalised treatment of the Begelman-Cioffi relations which
we present here. We find that the dynamics of stable jets match well with
simplified analytic models of expansion of non self-similar FRII jets, whereas
jets with prominent MHD instabilities show a nearly self-similar evolution of
the morphology as the energy is more evenly distributed between the jet head
and the cocoon.",2009.10475v1
2023-04-28,Interplay of electron-magnon scattering and spin-orbit induced electronic spin-flip fcattering in a two-band Stoner model,"Magnons are one of the carriers of angular momentum that are involved in the
ultrafast magnetization dynamics in ferromagnets, but their contribution to the
electronic dynamics and their interplay with other scattering process that
occur during ultrafast demagnetization has not yet been studied in the
framework of a microscopic dynamical model. The present paper presents such an
investigation of electronic scattering dynamics in itinerant ferromagnets at
the level of Boltzmann scattering integrals for the magnon distributions and
spin-dependent electron distributions. In addition to electron-magnon
scattering, we include spin-conserving and effective Elliott-Yafet like
spin-flip electron-electron scattering processes and the influence of phonons.
In our model system, the creation or annihilation of magnons leads to
transitions between two spin-split electronic bands with energy and momentum
conservation. Due to the presence of spin-orbit coupling, Coulomb scattering
transitions between these bands are also possible, and we describe them on an
equal footing in terms of Boltzmann scattering integrals. For an instantaneous
carrier excitation process we analyze the influence of both interaction
processes on the magnon and spin-dependent electron dynamics, and show that
their interplay gives rise to an efficient creation of magnons at higher
energies and wave vectors accompanied by only a small increase of the
electronic spin polarization. These results present a microscopic dynamical
scenario that shows how non-equilibrium magnons may dominate the magnetic
response of a ferromagnet on ultrafast timescales.",2304.14978v1
2023-11-09,Skyrmion-Excited Spin Wave Fractal Network,"Magnetic skyrmions exhibit unique, technologically relevant pseudo-particle
behaviors which arise from their topological protection, including
well-defined, three-dimensional dynamic modes that occur at microwave
frequencies. During dynamic excitation, spin waves are ejected into the
interstitial regions between skyrmions, creating the magnetic equivalent of a
turbulent sea. However, since the spin waves in these systems have a
well-defined length scale, and the skyrmions are on an ordered lattice, ordered
structures from spin wave interference can precipitate from the chaos. This
work uses small angle neutron scattering (SANS) to capture the dynamics in
hybrid skyrmions and investigate the spin wave structure. Performing
simultaneous ferromagnetic resonance and SANS, the diffraction pattern shows a
large increase in low-angle scattering intensity which is present only in the
resonance condition. This scattering pattern is best fit using a mass fractal
model, which suggests the spin waves form a long-range fractal network. The
fractal structure is constructed of fundamental units with a size that encodes
the spin wave emissions and are constrained by the skyrmion lattice. These
results offer critical insights into the nanoscale dynamics of skyrmions,
identify a new dynamic spin wave fractal structure, and demonstrates SANS as a
unique tool to probe high-speed dynamics.",2311.05469v1
2003-09-08,Scale-Free Magnetic Networks: Comparing Observational Data with a Self-Organizing Model of the Coronal Field,"We propose that the coronal magnetic field, linking concentrations on the
photosphere through an interwoven web of flux, embodies a scale-free network.
It arises from a self-organized critical dynamics including flux emergence, the
diffusion and merging of magnetic concentrations, as well as avalanches of
reconnecting flux tubes. Magnetic concentrations such as fragments, pores and
sunspots, are `nodes' joined by flux tubes or `links'. The number of links
emanating from a node is scale-free. We reanalyze the quiet-Sun data of Close
et al and show that the distribution of magnetic concentration strengths is a
power law with an index $\gamma= 1.7 \pm 0.3$, over the entire range of the
measurement, about $(2-500)\times 10^{17}$ Mx. This distribution is compatible
with that for the sizes of active regions reported by Harvey and Schwaan. Thus
magnetic concentrations may be scale-free from the smallest measurable
fragments to the large active regions. Numerical simulations of a
self-organized critical model give the same index $\gamma$, within statistical
uncertainty. The exponential distribution of flux tube lengths also agrees
quantitatively with results from the model. Calibration with the measured
diffusion constant of magnetic concentrations allows us to calculate a flux
turnover time in the model to be of order 10 hours and the total solar flux to
be of order $10^{23}$Mx, agreeing with observations. We introduce two other
statistical quantities to characterize scale-free networks. The probability
distribution for the amount of flux connecting a pair of concentrations, and
the number of distinct concentrations linked to a given one are predicted to be
scale-free, with different indices. Our approach unifies the observation of
scale free flare energies with the coronal magnetic field structure.",0309230v1
2007-07-04,Magnetically-dominated jets inside collapsing stars as a model for gamma-ray bursts and supernova explosions,"It has been suggested that magnetic fields play a dynamically-important role
in core-collapse explosions of massive stars. In particular, they may be
important in the collapsar scenario for gamma-ray bursts (GRB), where the
central engine is a hyper-accreting black hole or a millisecond magnetar. The
present paper is focussed on the magnetar scenario, with a specific emphasis on
the interaction of the magnetar magnetosphere with the infalling stellar
envelope. First, the ``Pulsar-in-a-Cavity'' problem is introduced as a paradigm
for a magnetar inside a collapsing star. The basic set-up of this fundamental
plasma-physics problem is described, outlining its main features, and simple
estimates are derived for the evolution of the magnetic field. In the context
of a collapsing star, it is proposed that, at first, the ram pressure of the
infalling plasma acts to confine the magnetosphere, enabling a gradual build-up
of the magnetic pressure. At some point, the growing magnetic pressure
overtakes the (decreasing) ram pressure of the gas, resulting in a
magnetically-driven explosion. The explosion should be highly anisotropic, as
the hoop-stress of the toroidal field, confined by the surrounding stellar
matter, collimates the magnetically-dominated outflow into two beamed
magnetic-tower jets. This creates a clean narrow channel for the escape of
energy from the central engine through the star, as required for GRBs. In
addition, the delayed onset of the collimated-explosion phase can explain the
production of large quantities of Nickel-56, as suggested by the GRB-Supernova
connection. Finally, the prospects for numerical simulations of this scenario
are discussed.",0707.0576v1
2008-06-18,Numerical simulation of excitation and propagation of helioseismic MHD waves: Effects of inclined magnetic field,"Investigation of propagation, conversion, and scattering of MHD waves in the
Sun is very important for understanding the mechanisms of observed oscillations
and waves in sunspots and active regions. We have developed 3D linear MHD
numerical model to investigate influence of the magnetic field on excitation
and properties of the MHD waves. The results show that the magnetic field can
substantially change the properties of the surface gravity waves (f-mode), but
their influence on the acoustic-type waves (p-modes) is rather moderate.
Comparison our simulations with the time-distance helioseismology results from
SOHO/MDI shows that the travel time variations caused by the inclined magnetic
field do not exceed 25% of the observed amplitude even for strong fields of
1400-1900 G. This can be an indication that other effects (e.g. background
flows and non-uniform distribution of magnetic field) can contribute to the
observed travel time variations. The travel time variations caused by the wave
interaction with magnetic field are in phase with the observations for strong
fields of 1400-1900 G if Doppler velocities are taken at the height of 300 km
above the photosphere where plasma parameter beta<<1. The simulations show that
the travel times only weakly depend on the height of velocity observation. For
the photospheric level the travel times are systematically smaller on
approximately 0.12 min then for the hight of 300 km above the photosphere for
all studied ranges of the magnetic field strength and inclination angles. The
numerical MHD wave modeling and new data from the HMI instrument of the Solar
Dynamics Observatory will substantially advance our knowledge of the wave
interaction with strong magnetic fields on the Sun and improve the local
helioseismology diagnostics.",0806.2897v1
2008-10-06,"The structure and dynamics of a bright point as seen with Hinode, SoHO and TRACE","Our aim is to determine the plasma properties of a coronal bright point and
compare its magnetic topology extrapolated from magnetogram data with its
appearance in X-ray images. We analyse spectroscopic data obtained with
EIS/Hinode, Ca II H and G-band images from SOT/Hinode, UV images from TRACE,
X-ray images from XRT/Hinode and high-resolution/high-cadence magnetogram data
from MDI/SoHO. The BP comprises several coronal loops as seen in the X-ray
images, while the chromospheric structure consists of tens of small bright
points as seen in Ca II H. An excellent correlation exists between the Ca II
BPs and increases in the magnetic field, implying that the Ca II H passband is
a good indicator for the concentration of magnetic flux. Doppler velocities
between 6 and 15 km/s are derived from the Fe XII and Fe XIII lines for the BP
region, while for Fe XIV and Si VII they are in the range from -15 to +15 km/s.
The coronal electron density is 3.7x10^9 cm^-3. An excellent correlation is
found between the positive magnetic flux and the X-ray light-curves. The
remarkable agreement between the extrapolated magnetic field configuration and
some of the loops composing the BP as seen in the X-ray images suggests that a
large fraction of the magnetic field in the bright point is close to potential.
The close correlation between the positive magnetic flux and the X-ray emission
suggests that energy released by magnetic reconnection is stimulated by flux
emergence or cancellation.",0810.1020v1
2009-02-11,Radio Emission from Supernova Remnants: Implications for Post-Shock Magnetic Field Amplification and the Magnetic Fields of Galaxies,"Using observations from the literature, we show that the non-thermal radio
luminosity (L) of supernova remnants (SNRs) is a strong function of the average
gas surface density (Sigma) of the galaxy in which the remnants reside, from
normal spirals to luminous starbursts. We combine a simple theory for electron
cooling in SNRs with the observed radio luminosities to estimate the remnant
magnetic field strength (B_SNR): the correlation between L and Sigma implies
that B_SNR also increases with Sigma. We explore two interpretations of this
correlation: (1) B_SNR is generated by post-shock magnetic field amplification,
with B_SNR^2 proportional to Sigma and (2) B_SNR results from shock-compression
of the ambient ISM magnetic field (B_ISM), with B_ISM being larger in denser
galaxies. We find that shock compression is, on average, sufficient to produce
the observed radio emission from SNRs in the densest starbursts; amplification
of post-shock magnetic fields is not required. By contrast, in normal spirals
post-shock field amplification (by a factor of a few - 10) is consistent with
the data; we find tentative evidence that both the Alfven speed and the ratio
of B_SNR^2 to the post-shock pressure (""epsilon_B"") are constant in SNRs from
galaxy to galaxy. We discuss observational tests that can be used to
distinguish between these two interpretations of the radio luminosities of
SNRs. Regardless of which is correct, the radio emission from SNRs provides an
upper limit to B_ISM that is independent of the minimum energy assumption. For
the densest starbursts, the ISM magnetic energy density is below that required
for hydrostatic equilibrium; thus magnetic fields are not dynamically important
on the largest scales in starbursts, in contrast with spiral galaxies like our
own. This dichotomy may have implications for galactic dynamo theory.",0902.1755v1
2009-02-13,Linear stability of Hunt's flow,"We analyse numerically the linear stability of the fully developed flow of a
liquid metal in a rectangular duct subject to a transverse magnetic field. The
walls of the duct perpendicular to the magnetic field are perfectly conducting
whereas the parallel ones are insulating. In a sufficiently strong magnetic
field, the flow consists of two jets at the insulating walls and a
near-stagnant core. We use a vector stream function formulation and Chebyshev
collocation method to solve the eigenvalue problem for small-amplitude
perturbations. Due to the two-fold reflection symmetry of the base flow the
disturbances with four different parity combinations over the duct
cross-section decouple from each other. Magnetic field renders the flow in a
square duct linearly unstable at the Hartmann number Ha 5.7 with respect to a
disturbance whose vorticity component along the magnetic field is even across
the field and odd along it. For this mode, the minimum of the critical Reynolds
number Re_c 2018, based on the maximal velocity, is attained at Ha ~ 10.
Further increase of the magnetic field stabilises this mode with Re_c growing
approximately as Ha. For Ha>40, the spanwise parity of the most dangerous
disturbance reverses across the magnetic field. At Ha ~ 46 a new pair of most
dangerous disturbances appears with the parity along the magnetic field being
opposite to that of the previous two modes. The critical Reynolds number, which
is very close for both of these modes, attains a minimum Re_c ~ 1130 at Ha ~ 70
and increases as Re_c ~ Ha^1/2 for Ha >> 1. The asymptotics of the critical
wavenumber is k_c ~ 0.525Ha^1/2 while the critical phase velocity approaches
0.475 of the maximum jet velocity.",0902.2319v2
2009-08-27,Very Light Magnetized Jets on Large Scales - I. Evolution and Magnetic Fields,"Magnetic fields, which are undoubtedly present in extragalactic jets and
responsible for the observed synchrotron radiation, can affect the morphology
and dynamics of the jets and their interaction with the ambient cluster medium.
We examine the jet propagation, morphology and magnetic field structure for a
wide range of density contrasts, using a globally consistent setup for both the
jet interaction and the magnetic field. The MHD code NIRVANA is used to evolve
the simulation, using the constrained-transport method. The density contrasts
are varied between \eta = 10^{-1} and 10^{-4} with constant sonic Mach number
6. The jets are supermagnetosonic and simulated bipolarly due to the low jet
densities and their strong backflows. The helical magnetic field is largely
confined to the jet, leaving the ambient medium nonmagnetic. We find magnetic
fields with plasma \beta \sim 10 already stabilize and widen the jet head.
Furthermore they are efficiently amplified by a shearing mechanism in the jet
head and are strong enough to damp Kelvin-Helmholtz instabilities of the
contact discontinuity. The cocoon magnetic fields are found to be stronger than
expected from simple flux conservation and capable to produce smoother lobes,
as found observationally. The bow shocks and jet lengths evolve self-similarly.
The radio cocoon aspect ratios are generally higher for heavier jets and grow
only slowly (roughly self-similar) while overpressured, but much faster when
they approach pressure balance with the ambient medium. In this regime,
self-similar models can no longer be applied. Bow shocks are found to be of low
excentricity for very light jets and have low Mach numbers. Cocoon turbulence
and a dissolving bow shock create and excite waves and ripples in the ambient
gas. Thermalization is found to be very efficient for low jet densities.",0908.4055v1
2009-10-16,Asteroseismic model of paramagnetic neutron star,"We investigate an asteroseismic model of non-rotating paramagnetic neutron
star with core-crust stratification of interior pervaded by homogeneous
internal and dipolar external magnetic field, presuming that neutron degenerate
Fermi-matter of the star core is in the state of Pauli's paramagnetic permanent
magnetization caused by polarization of spin magnetic moments of neutrons along
the axis of magnetic field of collapsed massive progenitor. The magnetic
cohesion between metal-like crust and permanent-magnet-like core is considered
as playing a main part in the dynamics of starquake. Focus is laid on the
post-quake relaxation of the star by node-free torsional vibrations of highly
conducting crustal solid-state plasma, composed of nuclei embedded in the
degenerate Fermi-gas of relativistic electrons, about axis of magnetic field
frozen in the immobile paramagnetic core. Two scenarios of these axisymmetric
seismic vibrations are examined, in first of which these are considered as
maintained by combined action of Lorentz magnetic and Hooke's elastic forces
and in second one by solely Lorentz force. Based on the energy variational
method of magneto-solid-mechanical theory of elastic continuous medium, the
spectral formulae for the frequency and lifetime of this toroidal mode are
obtained and discussed in the context of theoretical treatment of recently
discovered quasi-periodic oscillations of the X-ray outburst flux from SGR
1806-20 and SGR 1900+14 as being produced by above seismic vibrations.",0910.3048v8
2011-04-28,Effects of magnetic fields on the cosmic-ray ionization of molecular cloud cores,"Low-energy cosmic rays are the dominant source of ionization for molecular
cloud cores. The ionization fraction, in turn, controls the coupling of the
magnetic field to the gas and hence the dynamical evolution of the cores. The
purpose of this work is to compute the attenuation of the cosmic-ray flux rate
in a cloud core taking into account magnetic focusing, magnetic mirroring, and
all relevant energy loss processes. We adopt a standard cloud model
characterized by a mass-to-flux ratio supercritical by a factor of about 2 to
describe the density and magnetic field distribution of a low-mass starless
core, and we follow the propagation of cosmic rays through the core along flux
tubes enclosing different amount of mass. We then extend our analysis to cores
with different mass-to-flux ratios. We find that mirroring always dominates
over focusing, implying a reduction of the cosmic-ray ionization rate by a
factor of about 2-3 over most of a solar-mass core with respect to the value in
the intercloud medium outside the core. For flux tubes enclosing larger masses
the reduction factor is smaller, since the field becomes increasingly uniform
at larger radii and lower densities. We also find that the cosmic-ray
ionization rate is further reduced in clouds with stronger magnetic field, e.g.
by a factor of about 4 for a marginally critical cloud. The magnetic field
threading molecular cloud cores affects the penetration of low-energy cosmic
rays and reduces the ionization rate by a factor 3-4 depending on the position
inside the core and the magnetization of the core.",1104.5445v1
2012-02-13,The Two States of Star Forming Clouds,"We examine the effects of self-gravity and magnetic fields on supersonic
turbulence in isothermal molecular clouds with high resolution simulations and
adaptive mesh refinement. These simulations use large root grids (512^3) to
capture turbulence and four levels of refinement to capture high density, for
an effective resolution of 8,196^3. Three Mach 9 simulations are performed, two
super-Alfv\'enic and one trans-Alfv\'enic. We find that gravity splits the
clouds into two populations, one low density turbulent state and one high
density collapsing state. The low density state exhibits properties similar to
non-self-gravitating in this regime, and we examine the effects of varied
magnetic field strength on statistical properties: the density probability
distribution function is approximately lognormal; velocity power spectral
slopes decrease with field strength; alignment between velocity and magnetic
field increases with field; the magnetic field probability distribution can be
fit to a stretched exponential. The high density state is characterized by
self-similar spheres; the density PDF is a power-law; collapse rate decreases
with increasing mean field; density power spectra have positive slopes,
P({\rho},k) \propto k; thermal-to-magnetic pressure ratios are unity for all
simulations; dynamic-to-magnetic pressure ratios are larger than unity for all
simulations; magnetic field distribution is a power-law. The high Alfv\'en Mach
numbers in collapsing regions explain recent observations of magnetic influence
decreasing with density. We also find that the high density state is found in
filaments formed by converging flows, consistent with recent Herschel
observations. Possible modifications to existing star formation theories are
explored.",1202.2594v1
2012-06-15,Role of rotational symmetry in the magnetism of a multiorbital model,"Effect of rotationally-invariant Hund's rule coupling on a magnetism of
multiorbital Hubbard models is studied within a dynamical mean field theory
framework. Comparison of static magnetic susceptibilities and local densities
of states of two- and three-orbital models of a complete rotationally invariant
Coulomb interaction and a ""density-density"" Hartree type interaction shows the
different role of spin-flip interactions for different band fillings. In the
particle-hole symmetric case the Mott-Hubbard physics dominates due to the
strong effective Coulomb interaction, while for the multiple electronic
configurations away from half-filling (two electrons in the three band model)
the formation of local magnetic moments due to Hund's exchange interaction
becomes the most significant effect for itinerant magnetic systems. A shift of
the temperature of magnetic ordering due to the rotationally-invariant Hund's
rule coupling is found to be the largest in a three-orbital model with a
two-electron occupancy where the single particle spectrum is metallic and is
not sensitive to different forms of the Coulomb vertex. A larger enhancement of
the effective mass in a model with a rotationally-invariant interaction is
discussed. In the half-filled case we find a drastic change in the density of
states close to the Mott transition which is related to the spin-flip Kondo
fluctuations in a degenerate orbital case, while the corresponding shift of the
magnetic transition temperature is relatively small. It is shown that a change
in the ground state degeneracy due to a different symmetry of the Coulomb
interaction in the density-density model leads to a breakdown of the
quasiparticle peak at the Fermi level in the proximity of a Mott transition on
the metallic side. We discuss the relevance of rotationally-invariant Hund's
interaction in the transition metal magnetism.",1206.3416v2
2012-08-06,Comparing Simulations of Rising Flux Tubes Through the Solar Convection Zone with Observations of Solar Active Regions: Constraining the Dynamo Field Strength,"We study how active-region-scale flux tubes rise buoyantly from the base of
the convection zone to near the solar surface by embedding a thin flux tube
model in a rotating spherical shell of solar-like turbulent convection. These
toroidal flux tubes that we simulate range in magnetic field strength from 15
kG to 100 kG at initial latitudes of 1 degree to 40 degrees in both
hemispheres. This article expands upon Weber, Fan, and Miesch (Astrophys. J.,
741, 11, 2011) (Article 1) with the inclusion of tubes with magnetic flux of
10^20 Mx and 10^21 Mx, and more simulations of the previously investigated case
of 10^22 Mx, sampling more convective flows than the previous article, greatly
improving statistics. Observed properties of active regions are compared to
properties of the simulated emerging flux tubes, including: the tilt of active
regions in accordance with Joy's Law as in Article 1, and in addition the
scatter of tilt angles about the Joy's Law trend, the most commonly occurring
tilt angle, the rotation rate of the emerging loops with respect to the
surrounding plasma, and the nature of the magnetic field at the flux tube apex.
We discuss how these diagnostic properties constrain the initial field strength
of the active region flux tubes at the bottom of the solar convection zone, and
suggest that flux tubes of initial magnetic field strengths of \geq 40 kG are
good candidates for the progenitors of large (10^21 Mx to 10^22 Mx) solar
active regions, which agrees with the results from Article 1 for flux tubes of
10^22 Mx. With the addition of more magnetic flux values and more simulations,
we find that for all magnetic field strengths, the emerging tubes show a
positive Joy's Law trend, and that this trend does not show a statistically
significant dependence on the magnetic flux.",1208.1292v1
2012-10-08,Magnetic fluctuations and effective magnetic moments in γ-iron due to electronic structure peculiarities,"Applying the local density and dynamical mean field approximations to
paramagnetic \gamma-iron we revisit the problem of theoretical description of
magnetic properties in a wide temperature range. We show that contrary to
\alpha-iron, the frequency dependence of the electronic self-energy has a
quasiparticle form for both, t_{2g} and e_g states. In the temperature range
T=1200-1500 K, where \gamma-iron exist in nature, this substance can be
nevertheless characterized by temperature-dependent effective local moments,
which yield relatively narrow peaks in the real part of the local magnetic
susceptibility. At the same time, at low temperatures \gamma-iron (which is
realized in precipitates) is better described in terms of itinerant picture. In
particular, the nesting features of the Fermi surfaces yield maximum of the
static magnetic susceptibility at the incommensurate wave vector q_{max}
belonging the direction q_X-q_W (q_X=(2\pi/a)(1,0,0),q_W=(2\pi/a)(1,1/2,0), a
is a lattice parameter) in agreement with the experimental data. This state is
found however to compete closely with the states characterized by magnetic wave
vectors along the directions q_X-q_L-q_K, where q_L=(2\pi/a)(1/2,1/2,1/2),
q_K=(2\pi/a)(3/4,3/4,0). From the analysis of the uniform magnetic
susceptibility we find that contrary to \alpha-iron, the Curie-Weiss law is not
fulfilled in a broad temperature range, although the inverse susceptibility is
nearly linear in the moderate-temperature region (1200-1500 K). The
non-linearity of the inverse uniform magnetic susceptibility in a broader
temperature range is due to the density of states peak located close to the
Fermi level. The effective exchange integrals in the paramagnetic phase are
estimated on the base of momentum dependent susceptibility.",1210.2188v2
2012-11-01,A Magnetic Confinement vs. Rotation Classification of Massive-Star Magnetospheres,"Building on results from the Magnetism in Massive Stars (MiMeS) project, this
paper shows how a two-parameter classification of massive-star magnetospheres
in terms of the magnetic wind confinement (which sets the Alfv\'en radius RA)
and stellar rotation (which sets the Kepler co-rotation radius RK) provides a
useful organisation of both observational signatures and theoretical
predictions. We compile the first comprehensive study of inferred and observed
values for relevant stellar and magnetic parameters of 64 confirmed magnetic OB
stars with Teff > 16 kK. Using these parameters, we locate the stars in the
magnetic confinement-rotation diagram, a log-log plot of RK vs. RA. This
diagram can be subdivided into regimes of centrifugal magnetospheres (CM), with
RA > RK, vs. dynamical magnetospheres (DM), with RK > RA. We show how key
observational diagnostics, like the presence and characteristics of Halpha
emission, depend on a star's position within the diagram, as well as other
parameters, especially the expected wind mass-loss rates. In particular, we
identify two distinct populations of magnetic stars with Halpha emission:
namely, slowly rotating O-type stars with narrow emission consistent with a DM,
and more rapidly rotating B-type stars with broader emission associated with a
CM. For O-type stars, the high mass-loss rates are sufficient to accumulate
enough material for line emission even within the relatively short free-fall
timescale associated with a DM: this high mass-loss rate also leads to a rapid
magnetic spindown of the stellar rotation. For the B-type stars, the longer
confinement of a CM is required to accumulate sufficient emitting material from
their relatively weak winds, which also lead to much longer spindown
timescales. [abbreviated]",1211.0282v1
2013-02-25,VLA OH Zeeman Observations of the star forming region S88B,"We present observations of the Zeeman effect in OH thermal absorption main
lines at 1665 and 1667 MHz taken with the Very Large Array (VLA) toward the
star forming region S88B. The OH absorption profiles toward this source are
complicated, and contain several blended components toward a number of
positions. Almost all of the OH absorbing gas is located in the eastern parts
of S88B, toward the compact continuum source S88B-2 and the eastern parts of
the extended continuum source S88B-1. The ratio of 1665/1667 MHz OH line
intensities indicates the gas is likely highly clumped, in agreement with other
molecular emission line observations in the literature. S88-B appears to
present a similar geometry to the well-known star forming region M17, in that
there is an edge-on eastward progression from ionized to molecular gas. The
detected magnetic fields appear to mirror this eastward transition; we detected
line-of-sight magnetic fields ranging from 90-400 \mu G, with the lowest values
of the field to the southwest of the S88B-1 continuum peak, and the highest
values to its northeast. We used the detected fields to assess the importance
of the magnetic field in S88B by a number of methods; we calculated the ratio
of thermal to magnetic pressures, we calculated the critical field necessary to
completely support the cloud against self-gravity and compared it to the
observed field, and we calculated the ratio of mass to magnetic flux in terms
of the critical value of this parameter. All these methods indicated that the
magnetic field in S88B is dynamically significant, and should provide an
important source of support against gravity. Moreover, the magnetic energy
density is in approximate equipartition with the turbulent energy density,
again pointing to the importance of the magnetic field in this region.",1302.6098v1
2013-05-04,Recurrent Coronal Jets Induced by Repetitively Accumulated Electric Currents,"Three extreme-ultraviolet (EUV) jets recurred in about one hour on 2010
September 17 in the following magnetic polarity of active region 11106. The EUV
jets were observed by the Atmospheric Imaging Assembly (AIA) on board the Solar
Dynamics Observatory (SDO). The Helioseismic and Magnetic Imager (HMI) on board
SDO measured the vector magnetic field, from which we derive the magnetic flux
evolution, the photospheric velocity field, and the vertical electric current
evolution. The magnetic configuration before the jets is derived by the
nonlinear force-free field (NLFFF) extrapolation.
  We derive that the jets are above a pair of parasitic magnetic bipoles which
are continuously driven by photospheric diverging flows. The interaction drove
the build up of electric currents that we indeed observed as elongated patterns
at the photospheric level. For the first time, the high temporal cadence of HMI
allows to follow the evolution of such small currents. In the jet region, we
found that the integrated absolute current peaks repetitively in phase with the
171 A flux evolution. The current build up and its decay are both fast, about
10 minutes each, and the current maximum precedes the 171 A by also about 10
minutes. Then, HMI temporal cadence is marginally fast enough to detect such
changes.
  The photospheric current pattern of the jets is found associated to the
quasi-separatrix layers deduced from the magnetic extrapolation. From previous
theoretical results, the observed diverging flows are expected to build
continuously such currents. We conclude that magnetic reconnection occurs
periodically, in the current layer created between the emerging bipoles and the
large scale active region field. It induced the observed recurrent coronal jets
and the decrease of the vertical electric current magnitude.",1305.0902v1
2014-05-29,"Magnetic field amplification and magnetically supported explosions of collapsing, non-rotating stellar cores","We study the amplification of magnetic fields in the collapse and the
post-bounce evolution of the core of a non-rotating star of 15 solar masses in
axisymmetry. To this end, we solve the coupled equations of
magnetohydrodynamics and neutrino transport in the two-moment approximation.
The pre-collapse magnetic field is strongly amplified by compression in the
infall. Initial fields of the order of 1010 G translate into proto-neutron star
fields similar to the ones observed in pulsars, while stronger initial fields
yield magnetar-like final field strengths. After core bounce, the field is
advected through the hydrodynamically unstable neutrino-heating layer, where
non-radial flows due to convection and the standing accretion shock instability
amplify the field further. Consequently, the resulting amplification factor of
order five is the result of the number of small-eddy turnovers taking place
within the time scale of advection through the post-shock layer. Due to this
limit, most of our models do not reach equipartition between kinetic and
magnetic energy and, consequently, evolve similarly to the non-magnetic case,
exploding after about 800 ms when a single or few high-entropy bubbles persist
over several dynamical time scales. In the model with the strongest initial
field we studied, \$10^{12}\$ G, for which equipartition between flow and field
is achieved, the magnetic tension favours a much earlier development of such
long-lived high-entropy bubbles and enforces a fairly ordered large-scale flow
pattern. Consequently, this model, after exhibiting very regular shock
oscillations, explodes much earlier than non-magnetic ones.",1405.7466v2
2015-06-06,Magnetic Field Structures in Star Forming Regions: Mid-Infrared Imaging Polarimetry of K3-50,"We report new imaging polarimetry observations of the Galactic compact HII
region K3-50 using CanariCam at the Gran Telescopio Canarias. We use a standard
polarimetric analysis technique, first outlined by Aitken, to decompose the
observed polarisation images centred at 8.7, 10.3, and 12.5 $\mu$m into the
emissive and absorptive components from silicate grains that are aligned with
the local magnetic field. These components reveal the spatially-resolved
magnetic field structures across the mid-infrared emission area of K3-50. We
examine these structures and show that they are consistent with previously
observed features and physical models of K3-50, such as the molecular torus and
the ionised outflow. We propose a 3D geometry for all the structures seen at
different wavelengths. We also compute relevant physical quantities in order to
estimate the associated magnetic field strengths that would be implied under
various physical assumptions. We compare these results with MHD simulations of
protostar formation that predict the magnetic field strength and configuration.
We find that the magnetic field may be dynamically important in the innermost
0.2 pc of the molecular torus, but that the torus is more likely to be
rotationally-supported against gravity outside this radius. Similarly, magnetic
fields are unlikely to dominate the {\em global} physics of the ionised
outflow, but they may be important in helping confine the flow near the cavity
wall in some locations. Ours is the first application of the Aitken technique
to spatially-resolved magnetic field structures in multiple layers along the
line of sight, effectively a method of ""polarisation tomography.""",1506.02173v1
2015-07-13,Effect of shear and magnetic field on the heat-transfer efficiency of convection in rotating spherical shells,"We study rotating thermal convection in spherical shells. We base our
analysis on a set of about 450 direct numerical simulations of the
(magneto)hydrodynamic equations under the Boussinesq approximation. The Ekman
number ranges from $10^{-3}$ to $10^{-5}$. The supercriticality of the
convection reaches about 1000 in some models. Four sets of simulations are
considered: non-magnetic simulations and dynamo simulations with either
free-slip or no-slip flow boundary conditions. The non-magnetic setup with
free-slip boundaries generates the strongest zonal flows. Both non-magnetic
simulations with no-slip flow boundary conditions and self-consistent dynamos
with free-slip boundaries have drastically reduced zonal-flows. Suppression of
shear leads to a substantial gain in heat-transfer efficiency, increasing by a
factor of 3 in some cases. Such efficiency enhancement occurs as long as the
convection is significantly influenced by rotation. At higher convective
driving the heat-transfer efficiency tends towards that of the classical
non-rotating Rayleigh-B\'enard system. Analysis of the latitudinal distribution
of heat flow at the outer boundary reveals that the shear is most effective at
suppressing heat-transfer in the equatorial regions. Furthermore, we explore
the influence of the magnetic field on the {\em non-zonal} flow components of
the convection. For this we compare the heat-transfer efficiency of no-slip
non-magnetic cases with that of the no-slip dynamo simulations. We find that at
$E=10^{-5}$ magnetic field significantly affects the convection and a maximum
gain of about 30\% (as compared to the non-magnetic case) in heat-transfer
efficiency is obtained for an Elsasser number of about 3. Our analysis
motivates us to speculate that convection in the polar regions in dynamos at
$E=10^{-5}$ is probably in a `magnetostrophic' regime.",1507.03649v2
2015-07-30,X-ray emission from the giant magnetosphere of the magnetic O-type star NGC 1624-2,"We observed NGC 1624-2, the O-type star with the largest known magnetic field
Bp~20 kG), in X-rays with the ACIS-S camera onboard the Chandra X-ray
Observatory. Our two observations were obtained at the minimum and maximum of
the periodic Halpha emission cycle, corresponding to the rotational phases
where the magnetic field is the closest to equator-on and pole-on,
respectively. With these observations, we aim to characterise the star's
magnetosphere via the X-ray emission produced by magnetically confined wind
shocks. Our main findings are:
  (i) The observed spectrum of NGC 1624-2 is hard, similar to the magnetic
O-type star Theta 1 Ori C, with only a few photons detected below 0.8 keV. The
emergent X-ray flux is 30% lower at the Halpha minimum phase.
  (ii) Our modelling indicated that this seemingly hard spectrum is in fact a
consequence of relatively soft intrinsic emission, similar to other magnetic
Of?p stars, combined with a large amount of local absorption (~1-3 x 10^22
cm^-2). This combination is necessary to reproduce both the prominent Mg and Si
spectral features, and the lack of flux at low energies. NGC 1624-2 is
intrinsically luminous in X-rays (log LX emission ~ 33.4) but 70-95% of the
X-ray emission produced by magnetically confined wind shocks is absorbed before
it escapes the magnetosphere (log LX ISM corrected ~ 32.5).
  (iii) The high X-ray luminosity, its variation with stellar rotation, and its
large attenuation are all consistent with a large dynamical magnetosphere with
magnetically confined wind shocks.",1507.08621v1
2015-09-17,Velocity and magnetic fields within 1000 AU from a massive YSO,"We want to study the velocity and magnetic field morphology in the vicinity
(<1000 AU) of a massive young stellar object (YSO), at very high spatial
resolution (10-100 AU). We performed milli-arcsecond polarimetric observations
of the strong CH3OH maser emission observed in the vicinity of an O-type YSO,
in G023.01-00.41. We have combined this information with the velocity field of
the CH3OH masing gas previously measured at the same angular resolution. We
analyse the velocity and magnetic fields in the reference system defined by the
direction of the molecular outflow and the equatorial plane of the hot
molecular core at its base, as recently observed on sub-arcsecond scales. We
provide a first detailed picture of the gas dynamics and magnetic field
configuration within a radius of 2000 AU from a massive YSO. We have been able
to reproduce the magnetic field lines for the outer regions (>600 AU) of the
molecular envelope, where the magnetic field orientation shows a smooth change
with the maser cloudlets position (0.2 degree/AU). Overall, the velocity field
vectors well accommodate with the local, magnetic field direction, but still
show an average misalignment of 30 degrees. We interpret this finding as the
contribution of a turbulent velocity field of about 3.5 km/s, responsible for
braking up the alignment between the velocity and magnetic field vectors. We do
resolve different gas flows which develop both along the outflow axis and
across the disk plane, with an average speed of 7 km/s. In the direction of the
outflow axis, we establish a collimation of the gas flow, at a distance of
about 1000 AU from the disk plane. In the disk region, gas appears to stream
outward along the disk plane for radii greater than 500-600 AU, and inward for
shorter radii.",1509.05428v1
2016-10-17,"Unveiling the Magnetic Structure of VHE SNRs/PWNe with XIPE, the X-ray Imaging-Polarimetry Satellite","The dynamics, energetics and evolution of pulsar wind nebulae (PWNe) and
supernova remnants (SNRs), are strongly affected by their magnetic field
strength and distribution. They are usually strong, extended, sources of
non-thermal X-ray radiation, producing intrinsically polarised radiation. The
energetic wind around pulsars produces a highly-magnetised, structured flow,
often displaying a jet and a torus and different features (i.e. wisps, knots).
This magnetic-dominant wind evolves as it moves away from the pulsar
magnetosphere to the surrounding large-scale nebula, becoming kinetic-dominant.
Basic aspects such how this conversion is produced, or how the jets and torus
are formed, as well as the level of turbulence in the nebula are still unknown.
Likewise, the processes ruling the acceleration of particles in shell-like SNRs
up to 1e15 eV, including the amplification of the magnetic field, are not clear
yet. Imaging polarimetry in this regard is crucial to localise the regions of
shock acceleration and to measure the strength and the orientation of the
magnetic field at these emission sites. X-ray polarimetry with the X-ray
Imaging Polarimetry Explorer (XIPE) will allow the understanding of the
magnetic field structure and intensity on different regions in SNRs and PWNe,
helping to unveil long-standing questions such as i.e. acceleration of cosmic
rays in SNRs or magnetic-to-kinetic energy transfer. SNRs and PWNe also
represent the largest population of Galactic very-high energy gamma-ray
sources, therefore the study of their magnetic distribution with XIPE will
provide fundamental ingredients on the investigation of those sources at very
high energies. We will discuss the physics case related to SNRs and PWNe and
the expectations of the XIPE observations of some of the most prominent SNRs
and PWNe.",1610.05217v2
2016-10-26,Analysis of sudden variations in photospheric magnetic fields during a large flare and their influences in the solar atmosphere,"The solar active region NOAA 11719 produced a large two-ribbon flare on 11
April 2013. We have investigated the sudden variations in the photospheric
magnetic fields in this active region during the flare employing the
magnetograms obtained in the spectral line Fe I 6173 Angstrom by the
Helioseismic and Magnetic Imager (HMI) onboard Solar Dynamics Observatory
(SDO). The analysis of the line-of-sight magnetograms from HMI show sudden and
persistent magnetic field changes at different locations of the active region
before the onset of the flare and during the flare. The vector magnetic field
observations available from HMI also show coincident variations in the total
magnetic field strength and its inclination angle at these locations. Using the
simultaneous Dopplergrams obtained from HMI, we observe perturbations in the
photospheric Doppler signals following the sudden changes in the magnetic
fields in the aforementioned locations. The power spectrum analysis of these
velocity signals show enhanced acoustic power in these affected locations
during the flare as compared to the pre-flare condition. Accompanying these
observations, we have also used the near-simultaneous chromospheric
observations obtained in the spectral line H-alpha 6562.8 Angstrom by the
Global Oscillation Network Group (GONG) to study the evolution of flare ribbons
and intensity oscillations in the active region. The H-alpha intensity
oscillations also show enhanced oscillatory power during the flare in the
aforementioned locations. These results indicate that the transient Lorentz
force associated with the sudden changes in the magnetic fields could drive the
localized photospheric and chromospheric oscillations, like the flare-induced
oscillations in the solar atmosphere.",1610.08294v1
2017-03-29,High-resolution observations of flare precursors in the low solar atmosphere,"Solar flares are generally believed to be powered by free magnetic energy
stored in the corona, but the build up of coronal energy alone may be
insufficient for the imminent flare occurrence. The flare onset mechanism is a
critical but less understood problem, insights into which could be gained from
small-scale energy releases known as precursors, which are observed as small
pre-flare brightenings in various wavelengths, and also from certain
small-scale magnetic configurations such as the opposite polarity fluxes, where
magnetic orientation of small bipoles is opposite to that of the ambient main
polarities. However, high-resolution observations of flare precursors together
with the associated photospheric magnetic field dynamics are lacking. Here we
study precursors of a flare using unprecedented spatiotemporal resolution of
the 1.6 m New Solar Telescope, complemented by novel microwave data. Two
episodes of precursor brightenings are initiated at a small-scale magnetic
channel (a form of opposite polarity fluxes) with multiple polarity inversions
and enhanced magnetic fluxes and currents, lying near the footpoints of sheared
magnetic loops. The low-atmospheric origin of these precursor emissions is
corroborated by microwave spectra. We propose that the emerging magnetic
channel field interacts with the sheared arcades to cause precursor
brightenings at the main flare core region. These high-resolution results
provide evidence of low-atmospheric small-scale energy release and possible
relationship to the onset of the main flare.",1703.09866v1
2017-07-19,Contribution to the Solar Mean Magnetic Field from Different Solar Regions,"Seven-year long seeing-free observations of solar magnetic fields with the
Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory
(SDO) were used to study the sources of the solar mean magnetic field, SMMF,
defined as the net line-of-sight magnetic flux divided over the solar disk
area. To evaluate the contribution of different regions to the SMMF, we
separated all the pixels of each SDO/HMI magnetogram into three subsets: weak
(B_W), intermediate (B_I), and strong (B_S) fields. The B_W component
represents areas with magnetic flux densities below the chosen threshold; the
B_I component is mainly represented by network fields, remains of decayed
active regions (ARs), and ephemeral regions. The B_S component consists of
magnetic elements in ARs. To derive the contribution of a subset to the total
SMMF, the linear regression coefficients between the corresponding component
and the SMMF were calculated. We found that: i) when the threshold level of 30
Mx cm^-2 is applied, the B_I and B_S components together contribute from 65% to
95% of the SMMF, while the fraction of the occupied area varies in a range of
2-6% of the disk area; ii) as the threshold magnitude is lowered to 6 Mx cm^-2,
the contribution from B_I+B_S grows to 98%, and the fraction of the occupied
area reaches the value of about 40% of the solar disk. In summary, we found
that regardless of the threshold level, only a small part of the solar disk
area contributes to the SMMF. This means that the photospheric magnetic
structure is an intermittent, inherently porous medium, resembling a
percolation cluster. These findings suggest that the long-standing concept that
continuous vast unipolar areas on the solar surface are the source of the SMMF
may need to be reconsidered.",1707.05971v1
2017-10-19,Dissipation of the striped pulsar wind,"Rapidly rotating neutron stars blow a relativistic, magnetized wind mainly
composed of electron-positron pairs. The free expansion of the wind terminates
far from the neutron star where a weakly magnetized pulsar wind nebula forms,
implying efficient magnetic dissipation somewhere upstream. The wind current
sheet that separates the two magnetic polarities is usually considered as the
most natural place for magnetic dissipation via relativistic reconnection, but
its efficiency remains an open question. Here, the goal of this work is to
revisit this issue in light of the most recent progress in the understanding of
reconnection and pulsar electrodynamics. We perform large two-dimensional
particle-in-cell simulations of the oblique rotator to capture the multi-scale
evolution of the wind. We find that the current sheet breaks up into a
dynamical chain of magnetic islands separated by secondary thin current sheets.
The sheet thickness increases linearly with radius while the Poynting flux
decreases monotonically as reconnection proceeds. The radius of complete
annihilation of the stripes is given by the plasma multiplicity parameter at
the light cylinder. Current starvation within the sheets does not occur before
complete dissipation as long as there is enough charges where the sheets form.
Particles are efficiently heated up to a characteristic energy set by the
magnetization parameter at the light cylinder. Energetic pulsed synchrotron
emission peaks close to the light cylinder, and presents sub-pulse variability
associated with the formation of plasmoids in the sheet. This study suggests
that the striped component of the wind dissipates far before reaching the
termination shock in isolated pulsars, even in very-high-multiplicity systems
such as the Crab pulsar. Pulsars in binary systems may provide the best
environments to study magnetic dissipation in the wind.",1710.07320v1
2018-02-15,Spin and charge pumping by steady or pulse current-driven magnetic domain wall: A self-consistent multiscale time-dependent-quantum/time-dependent-classical approach,"We introduce a multiscale framework which combines time-dependent
nonequilibrium Green function (TD-NEGF) algorithms, scaling linearly in the
number of time steps and describing quantum-mechanically conduction electrons
in the presence of time-dependent fields of arbitrary strength or frequency,
with classical time evolution of localized magnetic moments described by the
Landau-Lifshitz-Gilbert (LLG) equation. The TD-NEGF+LLG framework can be
applied to a variety of problems where current-driven spin torque induces
dynamics of magnetic moments as the key resource for next generation
spintronics. Using magnetic domain wall (DW) as an example, we predict that its
motion will pump time-dependent spin and charge currents (on the top of
unpolarized DC charge current injected through normal metal leads to drive the
DW motion). The conversion of AC components of spin current, whose amplitude
increases (decreases) as the DW approaches (distances from) the normal metal
lead, into AC voltage via the inverse spin Hall effect offers a tool to
precisely track the DW position along magnetic nanowire. We also quantify the
DW transient inertial displacement due to its acceleration and deceleration by
pulse current and the entailed spin and charge pumping. Finally, TD-NEGF+LLG as
a nonperturbative (i.e., numerically exact) framework allows us to establish
the limits of validity of the so-called spin-motive force (SMF) theory for
pumped charge current by time-dependent magnetic textures---the perturbative
analytical formula of SMF theory becomes inapplicable for large frequencies
(but unrealistic in magnetic system) and, more importantly, for increasing
noncollinearity when the angles between neighboring magnetic moments exceed
$\simeq 10^\circ$.",1802.05682v3
2018-07-17,On the low magnetic field effect in radical pair reactions,"Radical pair recombination reactions are known to be sensitive to the
application of both low and high magnetic fields. The application of a weak
magnetic field reduces the singlet yield of a singlet-born radical pair,
whereas the application of a strong magnetic field increases the singlet yield.
The high field effect arises from energy conservation: when the magnetic field
is stronger than the sum of the hyperfine fields in the two radicals, ${\rm
S}\to {\rm T}_{\pm}$ transitions become energetically forbidden, thereby
reducing the number of pathways for singlet to triplet interconversion. The low
field effect arises from symmetry breaking: the application of a weak magnetic
field lifts degeneracies among the zero field eigenstates and increases the
number of pathways for singlet to triplet interconversion. However, the details
of this effect are more subtle, and have not previously been properly
explained. Here we present a complete analysis of the low field effect in a
radical pair containing a single proton, and in a radical pair in which one of
the radicals contains a large number of hyperfine-coupled nuclear spins. We
find that the new transitions that occur when the field is switched on are
between ${\rm S}$ and ${\rm T}_0$ in both cases, and not between ${\rm S}$ and
${\rm T}_{\pm}$ as has previously been claimed. We then illustrate this result
by using it in conjunction with semiclassical spin dynamics simulations to
account for the observation of a biphasic--triphasic--biphasic transition with
increasing magnetic field strength in the magnetic field effect on the
time-dependent survival probability of a photoexcited
carotenoid-porphyrin-fullerene radical pair.",1807.06541v1
2018-07-29,Incommensurate atomic and magnetic modulations in the spin-frustrated β-NaMnO2 triangular lattice,"The layered {\beta}-NaMnO2, a promising Na-ion energy-storage material has
been investigated for its triangular lattice capability to promote complex
magnetic configurations that may release symmetry restrictions for the
coexistence of ferroelectric and magnetic orders. The complexity of the neutron
powder diffraction patterns underlines that the routinely adopted commensurate
structural models are inadequate. Instead, a single-phase superspace symmetry
description is necessary, demonstrating that the material crystallizes in a
compositionally modulated q= (0.077(1), 0, 0) structure. Here, Mn3+ Jahn-Teller
distorted MnO6 octahedra form corrugated layer stacking sequences of the
{\beta}-NaMnO2 type, which are interrupted by flat sheets of the {\alpha}-like
oxygen topology. Spontaneous long-range collinear antiferromagnetic order,
defined by the propagation vector k= (1/2, 1/2, 1/2), appears below TN1= 200 K.
Moreover, a second transition into a spatially modulated proper-screw magnetic
state (k+-q) is established at TN2= 95 K, with an antiferromagnetic order
parameter resembling that of a two-dimensional (2D) system. The evolution of
23Na NMR spin-lattice relaxation identifies a magnetically inhomogene-ous state
in the intermediate T-region (TN2 <T< TN1), while its strong suppression below
TN2 indicates that a spin-gap opens in the excitation spectrum. High-resolution
neutron inelastic scattering confirms that the magnetic dynamics are indeed
gapped ({\Delta}~5 meV) in the low-temperature magnetic phase, while
simulations on the basis of the single-mode approximation suggest that Mn-spins
residing on ad-jacent antiferromagnetic chains, establish sizable 2D
correlations. Our analysis points that novel struc-tural degrees of freedom
promote, cooperative magnetism and emerging dielectric properties in this
non-perovskite-type of manganite.",1807.10995v1
2018-09-30,Local-Ising type magnetic order and metamagnetism in the rare-earth pyrogermanate Er$_2$Ge$_2$O$_7$,"The recent discoveries of proximate quantum spin-liquid compounds and their
potential application in quantum computing informs the search for new candidate
materials for quantum spin-ice and spin-liquid physics. While the majority of
such work has centered on members of the pyrochlore family due to their
inherently frustrated linked tetrahedral structure, the rare-earth
pyrogermanates also show promise for possible frustrated magnetic behavior.
With the familiar stoichiometry $RE_2$Ge$_2$O$_7$, these compounds generally
have tetragonal symmetry with a rare-earth sublattice built of a spiral of
alternating edge and corner sharing rare-earth site triangles. Studies on
Dy$_2$Ge$_2$O$_7$ and Ho$_2$Ge$_2$O$_7$ have shown tunable low temperature
antiferromagnetic order, a high frustration index and spin-ice like dynamics.
Here we use neutron diffraction to study magnetic order in Er$_2$Ge$_2$O$_7$
(space group $P4_{1}2_{1}2$ ) and find the lowest yet Ne\'el temperature in the
pyrogermanates of 1.15 K. Using neutron powder diffraction we find the magnetic
structure to order with $k = (0,0,0)$ ordering vector, magnetic space group
symmetry $P4_{1}^{'}2_{1}2^{'}$ and a refined Er moment of $m = 8.1 \mu_B$ -
near the expected value for the Er$^{3+}$ free ion. Provocatively, the magnetic
structure exhibits similar 'local-Ising' behavior to that seen in the
pyrocholres where the Er moment points up or down along the short Er-Er bond.
Upon applying a magnetic field we find a first order metamagnetic transition at
$\sim$ 0.35 T to a lower symmetry $P2_{1}^{'}2_{1}^{'}2$ structure. This
magnetic transition involves an inversion of Er moments aligned antiparallel to
the applied field describing a class I spin-flip type transition, indicating a
strong local anisotropy at the Er site - reminiscent of that seen in the
spin-ice pyrochlores.",1810.00446v1
2018-10-07,"Role of Spin, Phonon and Plasmon Dynamics on Ferromagnetism in Cr doped 3C-SiC","The defect induced magnetism has initiated a lot of interest in field of
spintronics. In this regard, SiC is a promising material because of its unique
properties under extreme conditions. Hence it will be very much interesting to
investigate the interaction between defects and itinerant carrier in doped SiC
for spintronics application. We report the structural stability and magnetic
interaction in Cr doped 3C-SiC synthesized by Thermal Plasma Technique. The EPR
spectrum of undoped 3C-SiC shows a sharp resonance line corresponding to
$g=2.00$ associated with the defects present in the system. Anomalous
temperature evolution tendency of the relative intensity of EPR spectra can be
attributed to magnetically correlated defects in the host matrix. For the first
time we report the detailed quantitative analysis of X-band and Q-band EPR
study in Cr doped 3C-SiC which reveals that Cr can be in multivalent state. The
non monotonous variation of Longitudinal Mode (LO) of the Raman spectra has
been explained based on the interaction between carriers and surface plasmon
using Longitudinal optical plasmon coupling model (LOPC). The carrier density
calculated by using LOPC fit with experimental data varies from
$1.8\times10^{15}$ to $4.2\times10^{17}$ cm$^{-3}$. Room temperature magnetic
measurements exhibit ferromagnetic behavior with non-zero coercivity for all
the samples up to 7 T field. We, for the first time, show the Curie temperature
to be above 760 K. Quantitative analysis of magnetic interaction validates the
applicability of Bound Magnetic Polaron Model (BMP) which probably arises from
the exchange interaction of Cr$^{3+}$ ions with related (Si, C) defects. The
polaron density estimated from the BMP fit agrees well with the carrier density
obtained from the line shape fitting of Raman spectra.",1810.03146v3
2018-10-27,Evolution of Photospheric Vector Magnetic Field Associated with Moving Flare Ribbons As Seen By GST,"The photospheric response to solar flares, also known as coronal back
reaction, is often observed as sudden flare-induced changes in vector magnetic
field and sunspot motions. However, it remains obscure whether evolving flare
ribbons, the flare signature closest to the photosphere, are accompanied by
changes in vector magnetic field therein. Here we explore the relationship
between the dynamics of flare ribbons in the chromosphere and variations of
magnetic fields in the underlying photosphere, using high-resolution off-band
H-alpha images and near-infrared vector magnetograms of the M6.5 flare on 2015
June 22 observed with the 1.6 m Goode Solar Telescope. We find that changes of
photospheric fields occur at the arrival of the flare ribbon front, thus
propagating analogously to flare ribbons. In general, the horizontal field
increases and the field lines become more inclined to the surface. When ribbons
sweep through regions that undergo a rotational motion, the fields transiently
turn more vertical with decreased horizontal field and inclination angle, and
then restore and/or become more horizontal than before the ribbon arrival. The
ribbon propagation decelerates near the sunspot rotation center, where the
vertical field becomes permanently enhanced. Similar magnetic field changes are
discernible in magnetograms from the Helioseismic and Magnetic Imager (HMI),
and an inward collapse of coronal magnetic fields is inferred from the time
sequence of non-linear force-free field models extrapolated from HMI
magnetograms. We conclude that photospheric fields respond nearly
instantaneously to magnetic reconnection in the corona.",1810.11733v1
2018-12-13,Magnetic fields and low-frequency acoustic wave-energy supply to the solar chromosphere,"The problem of solar chromospheric heating remains a challenging one with
wider implications for stellar physics. Several studies in the recent past have
shown that small-scale inclined magnetic field elements channel copious amount
of energetic low-frequency acoustic waves, that are normally trapped below the
photosphere. These magneto-acoustic waves are expected to shock at
chromospheric heights contributing to chromospheric heating. In this work,
exploiting simultaneous photospheric vector magnetic field, Doppler, continuum
and line-core intensity (of FeI 6173 {\AA}) observations from the Helioseismic
and Magnetic Imager (HMI) and lower-atmospheric UV emission maps in the 1700
{\AA} and 1600 {\AA} channels of the Atmospheric Imaging Assembly (AIA), both
onboard the Solar Dynamics Observatory (SDO) of NASA, we revisit the
relationships between magnetic field properties (inclination and strength) and
the acoustic wave propagation (phase travel time). We find that the flux of
acoustic energy, in the 2 - 5 mHz frequency range, between the upper
photosphere and lower chromosphere is in the range of 2.25 - 2.6 kW m$^{-2}$,
which is about twice the previous estimates. We identify that the relatively
less-inclined magnetic field elements in the quiet-Sun channel a significant
amount of waves of frequency lower than the theoretical minimum for acoustic
cut-off frequency due to magnetic inclination. We also derive indications that
these waves steepen and start to dissipate within the heights ranges probed,
while those let out due to inclined magnetic fields pass through. We explore
connections with existing theoretical and numerical results that could explain
the origin of these waves.",1812.05322v1
2019-03-16,Magnetically-Sensitive Valley Polarization Reversal and Revival of Defect-Localized Excitons in WSe2-WS2,"Manipulating and reserving the valley pseudospin of excitons is one core aim
in the two-dimensional transition metal dichalcogenides (TMDs). However, due to
the strong electron-hole exchange and spin-orbit coupling interactions, the
exciton recombination lifetime is subject to picosecond timescale
intrinsically, and the valley polarization is hardly modulated by a moderate
magnetic field. It is fortunate that interlayer and defect-localized excitons
promise to overcome these difficulties by suppressing the above interactions.
Here we clearly reveal that the valley polarization can be reversed and revived
in the defect-localized excitons with a microsecond lifetime in AB-stacked
WSe2-WS2 heterobilayer. Specifically, for the interlayer defect-localized
exciton, the valley polarization is reversed and can be efficiently enhanced by
a weak out-of-plane magnetic field (<0.4 T). In sharp contrast, the valley
polarization of the intralayer defect-localized exciton can revive after a fast
decay process and follows the direction of the moderate out-of-plane magnetic
field (<3 T). We explain the reversed valley polarization with highly magnetic
sensitivity by the delocalization of defect-localized holes under a weak
magnetic field and the revival of valley polarization by the valley Zeeman
effect under a moderate magnetic field. Our results demonstrate that the valley
pseudospin of defect-localized excitons can be efficiently modulated by the
external magnetic field and enrich both the understanding and the technical
approaches on manipulating the valley dynamics in TMDs and their
heterostructure.",1903.06899v2
2019-08-29,Pushing down the lateral dimension of single and coupled magnetic dots to the nanometric scale: characteristics and evolution of the spin-wave eigenmodes,"Planar magnetic nanoelements, either single- or multi-layered, are exploited
in a variety of current or forthcoming spintronic and/or ICT devices, such as
read heads, magnetic memory cells, spin-torque nano-oscillators, nanomagnetic
logic circuits, magnonic crystals and artificial spin-ices. The lateral
dimensions of the elemental magnetic components have been squeezed down during
the last decade to a few tens of nanometers, but they are still an order of
magnitude larger that the exchange correlation length of the constituent
materials. This means that the spectrum of spin-wave eigenmodes, occurring in
the GHz range, is relatively complex and cannot be described within a simple
macrospin approximation. On the other hand, a detailed knowledge of the
dynamical spectrum is needed to understand or to predict crucial
characteristics of the devices. With this focused review we aim at the analysis
and the rationalization of the characteristics of the eigenmodes spectrum of
magnetic nanodots, paying special attention to the following key points: (i)
Consider and compare the case of in-plane and out of-plane orientation of the
magnetization, as well as of single- and multi-layered dots, putting in
evidence similarities and diversities, and proposing a unifying nomenclature
and labelling scheme; (ii) Underline the evolution of the spectrum when the
lateral size of magnetic dots is squeezed down from hundreds to tens of
nanometers, as in current devices, with emphasis given to the occurrence of
soft modes and to the change of spatial localization of the fundamental mode
for in-plane magnetized dots; (iii) Extend the analysis from isolated elements
to twins of dots, as well as to dense arrays of dipolarly interacting dots,
showing how the discretized eigenmodes distinctive of the single element
transform in finite-width frequency bands of spin waves propagating through the
array.",1908.11098v1
2020-01-13,Magnetic properties of Mn-doped Bi$_2$Se$_3$ topological insulators: ab initio calculations,"Doping Bi$_2$Se$_3$ by magnetic ions represents an interesting problem since
it may break the time reversal symmetry needed to maintain the topological
insulator character. Mn dopants in Bi$_2$Se$_3$ represent one of the most
studied examples here. However, there is a lot of open questions regarding
their magnetic ordering. In the experimental literature different Curie
temperatures or no ferromagnetic order at all are reported for comparable Mn
concentrations. This suggests that magnetic ordering phenomena are complex and
highly susceptible to different growth parameters, which are known to affect
material defect concentrations. So far theory focused on Mn dopants in one
possible position, and neglected relaxation effects as well as native defects.
We have used ab initio methods to calculate the Bi$_2$Se$_3$ electronic
structure influenced by magnetic Mn dopants, and exchange interactions between
them. We have considered two possible Mn positions, the substitutional and
interstitial one, and also native defects. We have found a sizable relaxation
of atoms around Mn, which affects significantly magnetic interactions.
Surprisingly, very strong interactions correspond to a specific position of Mn
atoms separated by van der Waals gap. Based on the calculated data we performed
spin dynamics simulations to examine systematically the resulting magnetic
order for various defect contents. We have found under which conditions the
experimentally measured Curie temperatures ${T_{\rm{C}}}$ can be reproduced,
noticing that interstitial Mn atoms appear to be important here. Our theory
predicts the change of ${T_{\rm{C}}}$ with a shift of Fermi level, which opens
the way to tune the system magnetic properties by selective doping.",2001.04366v1
2020-02-25,Magnetic twist profile inside magnetic clouds derived with a superposed epoch analysis,"Magnetic clouds (MCs) are large-scale interplanetary transient structures in
the heliosphere that travel from the Sun into the interplanetary medium. The
internal magnetic field lines inside the MCs are twisted, forming a flux rope
(FR). This magnetic field structuring is determined by its initial solar
configuration, by the processes involved during its eruption from the Sun, and
by the dynamical evolution during its interaction with the ambient solar wind.
One of the most important properties of the magnetic structure inside MCs is
the twist of the field lines forming the FR (the number of turns per unit
length). The detailed internal distribution of twist is under debate mainly
because the magnetic field (B) in MCs is observed only along the spacecraft
trajectory, and thus it is necessary to complete observations with theoretical
assumptions. Estimating the twist from the study of a single event is difficult
because the field fluctuations significantly increase the noise of the observed
B time series and thus the bias of the deduced twist. The superposed epoch
applied to MCs has proven to be a powerful technique, permitting the extraction
of their common features, and removing the peculiarity of individual cases. We
apply a superposed epoch technique to analyse the magnetic components in the
local FR frame of a significant sample of moderately asymmetric MCs observed at
1 au. From the superposed profile of B components in the FR frame, we determine
the typical twist distribution in MCs. The twist is nearly uniform in the FR
core (central half part), and it increases moderately, up to a factor two,
towards the MC boundaries. This profile is close to the Lundquist field model
limited to the FR core where the axial field component is above about one-third
of its central value.",2002.10606v1
2020-06-05,Magnetism Science with the Square Kilometre Array,"The Square Kilometre Array (SKA) will answer fundamental questions about the
origin, evolution, properties, and influence of magnetic fields throughout the
Universe. Magnetic fields can illuminate and influence phenomena as diverse as
star formation, galactic dynamics, fast radio bursts, active galactic nuclei,
large-scale structure, and Dark Matter annihilation. Preparations for the SKA
are swiftly continuing worldwide, and the community is making tremendous
observational progress in the field of cosmic magnetism using data from a
powerful international suite of SKA pathfinder and precursor telescopes. In
this contribution, we revisit community plans for magnetism research using the
SKA, in the light of these recent rapid developments. We focus in particular on
the impact that new radio telescope instrumentation is generating, thus
advancing our understanding of key SKA magnetism science areas, as well as the
new techniques that are required for processing and interpreting the data. We
discuss these recent developments in the context of the ultimate scientific
goals for the SKA era.",2006.03172v1
2020-06-10,On the existence of shear-current effects in magnetized burgulence,"The possibility of explaining shear flow dynamos by a magnetic shear-current
(MSC) effect is examined via numerical simulations. Our primary diagnostics is
the determination of the turbulent magnetic diffusivity tensor
$\boldsymbol{\eta}$. In our setup, a negative sign of its component $\eta_{yx}$
is necessary for coherent dynamo action by the SC effect. To be able to measure
turbulent transport coefficients from systems with magnetic background
turbulence, we present an extension of the test-field method (TFM), applicable
to our setup where the pressure gradient is dropped from the momentum equation:
the nonlinear TFM (NLTFM). Our momentum equation is related to Burgers'
equation and the resulting flows are referred to as magnetized burgulence. We
use both stochastic kinetic and magnetic forcings to mimic cases without and
with simultaneous small-scale dynamo action (SSD). When we force only
kinetically, negative $\eta_{yx}$ are obtained with exponential growth in both
the radial and azimuthal mean magnetic field components. Using isotropic
magnetic forcing, the field growth is no longer exponential, while NLTFM yields
positive $\eta_{yx}$. By employing an alternative forcing from which
wavevectors having small components are removed, the exponential growth is
recovered, but the NLTFM results do not change significantly. Analyzing the
dynamo excitation conditions for the coherent SC and incoherent $\alpha$ and SC
effects shows that the incoherent effects are the main drivers of the dynamo in
the majority of cases. We find no evidence for MSC-effect-driven dynamos in our
simulations.",2006.05661v3
2020-08-04,Sunspot penumbral filaments intruding into a light bridge and the resultant reconnection jets,"Penumbral filaments and light bridges are prominent structures inside
sunspots and are important for understanding the nature of sunspot magnetic
fields and magneto-convection underneath. We investigate an interesting event
where several penumbral filaments intruded into a sunspot light bridge for more
insights into magnetic fields of the sunspot penumbral filament and light
bridge, as well as their interaction. The emission, kinematic, and magnetic
topology characteristics of the penumbral filaments intruding into the light
bridge and the resultant jets are studied. At the west part of the light
bridge, the intruding penumbral filaments penetrated into the umbrae on both
sides of the light bridge, and two groups of jets were also detected. The jets
shared the same projected morphology with the intruding filaments and were
accompanied by intermittent footpoint brightenings. Simultaneous spectral
imaging observations provide convincing evidences for the presences of magnetic
reconnection related heating and bidirectional flows near the jet bases and
contribute to measuring vector velocities of the jets. Additionally, nonlinear
force-free field extrapolation results reveal strong and highly inclined
magnetic fields along the intruding penumbral filaments, consistent well with
the results deduced from the vector velocities of the jets. Therefore, we
propose that the jets could be caused by magnetic reconnections between
emerging fields within the light bridge and the nearly horizontal fields of
intruding filaments. They were then ejected outward along the stronger
filaments fields. Our study indicates that magnetic reconnection could occur
between the penumbral filament fields and emerging fields within light bridge
and produce jets along the stronger filament fields. These results further
complement the study of magnetic reconnection and dynamic activities within the
sunspot.",2008.01284v1
2020-10-15,Solar Coronal Magnetic Field Extrapolation from Synchronic Data with AI-generated Farside,"Solar magnetic fields play a key role in understanding the nature of the
coronal phenomena. Global coronal magnetic fields are usually extrapolated from
photospheric fields, for which farside data is taken when it was at the
frontside, about two weeks earlier. For the first time we have constructed the
extrapolations of global magnetic fields using frontside and artificial
intelligence (AI)-generated farside magnetic fields at a near-real time basis.
We generate the farside magnetograms from three channel farside observations of
Solar Terrestrial Relations Observatory (STEREO) Ahead (A) and Behind (B) by
our deep learning model trained with frontside Solar Dynamics Observatory
extreme ultraviolet images and magnetograms. For frontside testing data sets,
we demonstrate that the generated magnetic field distributions are consistent
with the real ones; not only active regions (ARs), but also quiet regions of
the Sun. We make global magnetic field synchronic maps in which conventional
farside data are replaced by farside ones generated by our model. The
synchronic maps show much better not only the appearance of ARs but also the
disappearance of others on the solar surface than before. We use these
synchronized magnetic data to extrapolate the global coronal fields using
Potential Field Source Surface (PFSS) model. We show that our results are much
more consistent with coronal observations than those of the conventional method
in view of solar active regions and coronal holes. We present several positive
prospects of our new methodology for the study of solar corona, heliosphere,
and space weather.",2010.07553v3
2021-05-08,Multi-scale Magnetic Fields in the Central Molecular Zone: Inference from the Gradient Technique,"The central molecular zone (CMZ) plays an essential role in regulating the
nuclear ecosystem of our Galaxy. To get an insight into the magnetic fields of
the CMZ, we employ the Gradient Technique (GT), which is rooted in the
anisotropy of magnetohydrodynamic turbulence. Our analysis is based on the data
of multiple wavelengths, including molecular emission lines, radio 1.4 GHz
continuum image, and Herschel 70 $\mu$m image, as well as ionized [Ne II] and
Paschen-alpha emissions. The results are compared with the observations of
Planck 353 GHz and High-resolution Airborne Wideband Camera Plus (HWAC+) 53
$\mu$m polarized dust emissions. We map the orientation of the magnetic field
at multiple wavelengths across the central molecular zone, including close-ups
of the Radio Arc and Sagittarius A West regions, on multi scales from $\sim$
0.1 pc to 10 pc. The magnetic fields towards the central molecular zone traced
by GT are globally compatible with the polarization measurements, accounting
for the contribution from the galactic foreground and background. This
correspondence suggests that the magnetic field and turbulence are dynamically
crucial in the galactic center. We find that the magnetic fields associated
with the Arched filaments and the thermal components of the Radio Arc are in
good agreement with the HAWC+ polarization. Our measurement towards the
non-thermal Radio Arc reveals the poloidal magnetic field components in the
galactic center. For Sagittarius A West region, we find a great agreement
between the GT measurement using [Ne II] emission and HWAC+ 53 $\mu$m
observation. We use GT to predict the magnetic fields associated with ionized
Paschen-alpha gas down to scales of 0.1 pc.",2105.03605v3
2021-05-13,Tracing the large-scale magnetic field morphology in protoplanetary disks using molecular line polarization,"(abridged) Magnetic fields are fundamental to the accretion dynamics of
protoplanetary disks and they likely affect planet formation. Typical methods
to study the magnetic field morphology observe the polarization of dust or
spectral lines. However, it has recently become clear that dust-polarization in
ALMA's spectral regime does not always faithfully trace the magnetic field
structure of protoplanetary disks, which leaves spectral line polarization as a
promising method for mapping the magnetic field morphologies of such sources.
We aim to model the emergent polarization of different molecular lines in the
ALMA wavelength regime that are excited in protoplanetary disks. We explore a
variety of disk models and molecules to identify those properties that are
conducive to the emergence of polarization in spectral lines and may therefore
be viably used for magnetic field measurements in protoplanetary disks. We used
PORTAL in conjunction with LIME. Together, they allowed us to treat the
polarized line radiative transfer of complex three-dimensional physical and
magnetic field structures. We present simulations of the emergence of spectral
line polarization of different molecules and molecular transitions in the ALMA
wavelength regime. We find that molecules that thermalize at high densities,
such as HCN, are also the most susceptible to polarization. We find that such
molecules are expected to be significantly polarized in protoplanetary disks,
while molecules that thermalize at low densities, such as CO, are only
significantly polarized in the outer disk regions. We present the simulated
polarization maps at a range of inclinations and magnetic field morphologies,
and we comment on the observational feasibility of ALMA linear polarization
observations of protoplanetary disks.",2105.06482v2
2021-05-30,"Magnetic structure determination of rare-earth based, high moment, atomic laminates; potential parent materials for 2D magnets","We report muon spin rotation ($\mu$SR) and neutron diffraction on the
rare-earth based magnets (Mo$_{2/3}$RE$_{1/3}$)$_2$AlC, also predicted as
parent materials for 2D derivatives, where RE = Nd, Gd (only ($\mu$SR), Tb, Dy,
Ho and Er. By crossing information between the two techniques, we determine the
magnetic moment ($m$), structure, and dynamic properties of all compounds. We
find that only for RE = Nd and Gd the moments are frozen on a microsecond time
scale. Out of these two, the most promising compound for a potential 2D high
($m$) magnet is the Gd variant, since the parent crystals are pristine with $m
= 6.5 \pm 0.5 \mu_B$, N\'eel temperature of $29 \pm 1$ K, and the magnetic
anisotropy between in and out of plane coupling is smaller than $10^{-8}$. This
result suggests that magnetic ordering in the Gd variant is dominated by
in-plane magnetic interactions and should therefore remain stable if exfoliated
into 2D sheets.",2105.14555v2
2021-06-19,A-type antiferromagnetic order and magnetic phase diagram of the trigonal Eu spin-7/2 triangular-lattice compound EuSn2As2,"The trigonal compound EuSn2As2 was recently discovered to host Dirac surface
states within the bulk band gap and orders antiferromagnetically below the Neel
temperature TN = 24 K. Here the magnetic ground state of single-crystal
EuSn2As2 and the evolution of its properties versus temperature T and applied
magnetic field H are reported. Included are zero-field single-crystal
neutron-diffraction measurements versus T, magnetization M(H,T), magnetic
susceptibility chi(H,T) = M(T)/H, heat capacity Cp(H,T), and electrical
resistivity rho(H,T) measurements. The neutron-diffraction and chi(T)
measurements both indicate a collinear A-type antiferromagnetic (AFM) structure
below TN =23.5(2) K, where the Eu{2+} spins S = 7/2 in a triangular ab-plane
layer (hexagonal unit cell) are aligned ferromagnetically in the ab plane
whereas the spins in adjacent Eu planes along the c axis are aligned
antiferromagnetically. The chi(H{ab},T) and chi(H{c},T) data together indicate
a smooth crossover between the collinear AFM alignment and an unknown magnetic
structure at H ~ 0.15 T. Dynamic spin fluctuations up to 60 K are evident in
the chi(T), Cp(T) and rho(H,T) measurements, a temperature that is more than
twice TN. The rho(H,T) of the compound does not reflect a contribution of the
topological state, but rather is consistent with a low-carrier-density metal
with strong magnetic scattering. The magnetic phase diagrams for both H||c and
H||ab in the H-T plane are constructed from the TN(H), chi(H,T), Cp(H,T), and
rho(H,T) data.",2106.10519v1
2021-09-06,How to Estimate the Far-Side Open Flux using STEREO Coronal Holes,"Global magnetic field models use as input synoptic data, which usually show
""aging effects"" as the longitudinal 360{\deg} information is not obtained
simultaneously. Especially during times of increased solar activity, the
evolution of the magnetic field may yield large uncertainties. A significant
source of uncertainty is the Sun's magnetic field on the side of the Sun that
is not visible to the observer. Various methods have been used to complete the
picture: synoptic charts, flux-transport models, and far side helioseismology.
In this study, we present a new method to estimate the far-side open flux
within coronal holes using STEREO EUV observations. First, we correlate the
structure of the photospheric magnetic field as observed with the Helioseismic
and Magnetic Imager on board the Solar Dynamics Observatory (HMI/SDO) with
features in the transition region. From the 304A intensity distribution, which
we found to be specific to coronal holes, we derive an empirical estimate for
the open flux. Then we use a large sample of 313 SDO coronal hole observations
to verify this relation. Finally, we perform a cross-instrument calibration
from SDO to STEREO data to enable the estimation of the open flux at solar
longitudes not visible from Earth. We find that the properties of strong,
unipolar magnetic elements in the photosphere, which determine the coronal
hole's open flux, can be approximated by open fields in the transition region.
We find that structures below a threshold of 78% (STEREO) or 94% (SDO) of the
solar disk median intensity as seen in 304A filtergrams are reasonably well
correlated with the mean magnetic flux density of coronal holes (cc = 0.59).
Using the area covered by these structures (A_of) and the area of the coronal
hole (A_ch), we model the open magnetic flux of a coronal hole as |Phi_ch| =
0.25 A_ch exp(0.032 A_of) with an estimated uncertainty of 40 to 60%.",2109.02375v1
2016-11-18,Tuning the pseudospin polarization of graphene by a pseudo-magnetic field,"One of the intriguing characteristics of honeycomb lattices is the appearance
of a pseudo-magnetic field as a result of mechanical deformation. In the case
of graphene, the Landau quantization resulting from this pseudo-magnetic field
has been measured using scanning tunneling microscopy. Here we show that a
signature of the pseudo-magnetic field is a local sublattice symmetry breaking
observable as a redistribution of the local density of states. This can be
interpreted as a polarization of graphene's pseudospin due to a strain induced
pseudo-magnetic field, in analogy to the alignment of a real spin in a magnetic
field. We reveal this sublattice symmetry breaking by tunably straining
graphene using the tip of a scanning tunneling microscope. The tip locally
lifts the graphene membrane from a SiO$_2$ support, as visible by an increased
slope of the $I(z)$ curves. The amount of lifting is consistent with molecular
dynamics calculations, which reveal a deformed graphene area under the tip in
the shape of a Gaussian. The pseudo-magnetic field induced by the deformation
becomes visible as a sublattice symmetry breaking which scales with the lifting
height of the strained deformation and therefore with the pseudo-magnetic field
strength. Its magnitude is quantitatively reproduced by analytic and
tight-binding models, revealing fields of 1000 T. These results might be the
starting point for an effective THz valley filter, as a basic element of
valleytronics.",1611.06123v2
2016-11-21,Probing magnetic fields with Square Kilometre array and its precursors,"Origin of magnetic fields, its structure and effects on dynamical processes
in stars to galaxies are not well understood. Lack of a direct probe has
hampered its study. The first phase of Square Kilometre Array (SKA-I), will
have more than an order of magnitude higher sensitivity than existing radio
telescopes. In this contribution, we discuss specific science cases that are of
interest to the Indian community concerned with astrophysical turbulence and
magnetic fields. The SKA-I will allow observations of a large number of
background sources with detectable polarisation and measure their Faraday
depths (FDs) through the Milky Way, other galaxies and their circum-galactic
medium. This will probe line-of-sight magnetic fields in these objects well and
provide field configurations. Detailed comparison of observational data with
models which consider various processes giving rise to field amplification and
maintenance will then be possible. Such observations will also provide the
coherence scale of the fields and measure its random component. Measuring the
random component is important to characterise turbulence in the medium.
Observations of FDs with redshift will provide important information on
magnetic field evolution as a function of redshift. The background sources
could also be used to probe magnetic fields and its coherent scale in galaxy
clusters and in bridges formed between interacting galaxies. Other than FDs,
sensitive observations of synchrotron emission from galaxies will provide
complimentary information on their magnetic field strengths in the sky plane.
The core shift measurements of AGNs can provide more precise measurements of
magnetic field very close (<pc) to the black hole and its evolution. The low
band of SKA-I will also be useful to study circularly polarized emission from
Sun and comparing various models of field configurations with observations.",1611.06966v1
2019-02-27,Fossil field decay due to nonlinear tides in massive binaries,"Surface magnetic fields have been detected in 5 to 10% of isolated massive
stars, hosting outer radiative envelopes. They are often thought to have a
fossil origin, resulting from the stellar formation phase. Yet, magnetic
massive stars are scarcer in (close) short-period binaries, as reported by the
BinaMIcS (Binarity and Magnetic Interaction in various classes of Stars)
collaboration. Different physical conditions in the molecular clouds giving
birth to isolated stars and binaries are commonly invoked. In addition, we
propose that the observed lower magnetic incidence in close binaries may be due
to nonlinear tides. Indeed, close binaries are probably prone to tidal
instability, a fluid instability growing upon the equilibrium tidal flow via
nonlinear effects. Yet, stratified effects have hitherto been largely
overlooked. We theoretically and numerically investigate tidal instability in
rapidly rotating, stably stratified fluids permeated by magnetic fields. We use
the short-wavelength stability method to propose a comprehensive (local) theory
of tidal instability at the linear onset, discussing damping effects. Then, we
propose a mixing-length theory for the mixing generated by tidal instability in
the nonlinear regime. We successfully assess our theoretical predictions
against proof-of-concept, direct numerical simulations. Finally, we compare our
predictions with the observations of short-period, double-lined spectroscopic
binary systems. Using new analytical results, cross-validated by a direct
integration of the stability equations, we show that tidal instability can be
generated by nonlinear couplings of inertia-gravity waves with the equilibrium
tidal flow in short-period massive binaries, even against the Joule diffusion.
In the nonlinear regime, a fossil magnetic field can be dissipated by the
turbulent magnetic diffusion induced by the saturated tidal flows. We predict
that the turbulent Joule diffusion of fossil fields would occur in a few
million years for several short-period massive binaries. Therefore, turbulent
tidal flows could explain the observed dearth of some short-period magnetic
binaries.",1902.10599v2
2019-12-03,Picosecond Spin Orbit Torque Switching,"Reducing energy dissipation while increasing speed in computation and memory
is a long-standing challenge for spintronics research. In the last 20 years,
femtosecond lasers have emerged as a tool to control the magnetization in
specific magnetic materials at the picosecond timescale. However, the use of
ultrafast optics in integrated circuits and memories would require a major
paradigm shift. An ultrafast electrical control of the magnetization is far
preferable for integrated systems. Here we demonstrate reliable and
deterministic control of the out-of-plane magnetization of a 1 nm-thick Co
layer with single 6 ps-wide electrical pulses that induce spin-orbit torques on
the magnetization. We can monitor the ultrafast magnetization dynamics due to
the spin-orbit torques on sub-picosecond timescales, thus far accessible only
by numerical simulations. Due to the short duration of our pulses, we enter a
counter-intuitive regime of switching where heat dissipation assists the
reversal. Moreover, we estimate a low energy cost to switch the magnetization,
projecting to below 1fJ for a (20 nm)^3 cell. These experiments prove that
spintronic phenomena can be exploited on picosecond time-scales for full
magnetic control and should launch a new regime of ultrafast spin torque
studies and applications.",1912.01377v4
2020-04-08,High-order crystal field and rare-earth magnetism in RECo5 intermetallics,"Crystal-field (CF) effects on the rare-earth (RE) ions in ferrimagnetic
intermetallics NdCo$_5$ and TbCo$_5$ are evaluated using an ab initio density
functional + dynamical mean-field theory approach in conjunction with a
quasi-atomic approximation for on-site electronic correlations on the localized
4$f$ shell. The study reveals an important role of the high-order sectoral
harmonic component of the CF in the magnetism of RECo$_5$ intermetallics. An
unexpectedly large value is computed in the both systems for the corresponding
crystal-field parameter (CFP) $A_6^6 \langle r^6 \rangle$, far beyond what one
would expect from only electrostatic contributions. It allows solving the
enigma of the non-saturation of zero-temperature Nd magnetic moments in
NdCo$_5$ along its easy axis in the Co exchange field. This unsaturated state
had been previously found out from magnetization distribution probed by
polarised neutron elastic scattering but had so far remained theoretically
unexplained. The easy plane magnetic anisotropy of Nd in NdCo$_5$ is strongly
enhanced by the large value of $A_6^6\langle r^6 \rangle$. Counter-intuitively,
the polar dependence of anisotropy energy within the easy plane remains rather
small. The easy plane magnetic anisotropy of Nd is reinforced up to high
temperatures, which is explained through $J$-mixing effects. The calculated ab
initio anisotropy constants of NdCo$_5$ and their temperature dependence are in
quantitative agreement with experiment. Unlike NdCo$_5$, the $A_6^6 \langle r^6
\rangle$ CFP has negligible effects on the Tb magnetism in TbCo$_5$ suggesting
that its impact on the RE magnetism is ion-specific across the RECo$_5$ series.
The origin of its large value is the hybridization of RE and Co states in a
hexagonally coordinated local environment of the RE ion in RECo$_5$
intermetallics.",2004.03945v1
2020-09-18,"Magnetic Ergostars, Jet Formation and Gamma-Ray Bursts: Ergoregions versus Horizons","We perform the first fully general relativistic, magnetohydrodynamic
simulations of dynamically stable hypermassive neutron stars with and without
ergoregions to assess the impact of ergoregions on launching
magnetically--driven outflows. The hypermassive neutron stars are modeled by a
compressible and causal equation of state and are initially endowed with a
dipolar magnetic field extending from the stellar interior into its exterior.
We find that, after a few Alfv\'en times, magnetic field lines in the ergostar
(star that contains ergoregions) and the normal star have been tightly wound in
both cases into a helical funnel within which matter begins to flow outward.
The maximum Lorentz factor in the outflow is $\Gamma_L \sim 2.5$, while the
force-free parameter holds at $B^2/8\pi\rho_0\lesssim 10$. These values are
incompatible with highly relativistic, magnetically-driven outflows (jets) and
short $\gamma$-ray bursts. We compare these results with those of a spinning
black hole surrounded by a magnetized, massless accretion disk that launches a
bona fide jet. Our simulations suggest that the Blandford-Znajek mechanism for
launching relativistic jets only operates when a black hole is present, though
the Poynting luminosity in all cases is comparable. Therefore, one cannot
distinguish a magnetized, accreting black hole from a magnetized hypermassive
neutron star in the so-called mass-gap based solely on the value of the
observed Poynting luminosity. These results complement our previous studies of
supramassive remnants and suggest that it would be challenging for either
normal neutron stars or ergostars in a hypermassive state to be the progenitors
of short $\gamma$-ray bursts.",2009.08982v2
2021-03-08,All-optical spin switching probability in [Tb/Co] multilayers,"Since the first experimental observation of all-optical switching phenomena,
intensive research has been focused on finding suitable magnetic systems that
can be integrated as storage elements within spintronic devices and whose
magnetization can be controlled through ultra-short single laser pulses. We
report here atomistic spin simulations of all-optical switching in multilayered
structures alternating n monolayers of Tb and m monolayers of Co. By using a
two temperature model, we numerically calculate the thermal variation of the
magnetization of each sublattice as well as the magnetization dynamics of
[Tbn/Com] multilayers upon incidence of a single laser pulse. In particular,
the condition to observe thermally-induced magnetization switching is
investigated upon varying systematically both the composition of the sample
(n,m) and the laser fluence. The samples with one monolayer of Tb as [Tb1/Co2]
and [Tb1/Co3] are showing thermally induced magnetization switching above a
fluence threshold. The reversal mechanism is mediated by the residual
magnetization of the Tb lattice while the Co is fully demagnetized in agreement
with the models developed for ferrimagnetic alloys. The switching is however
not fully deterministic but the error rate can be tuned by the damping
parameter. Increasing the number of monolayers the switching becomes completely
stochastic. The intermixing at the Tb/Co interfaces appears to be a promising
way to reduce the stochasticity. These results predict for the first time the
possibility of TIMS in [Tb/Co] multilayers and suggest the occurrence of
sub-picosecond magnetization reversal using single laser pulses.",2103.05066v1
2021-03-24,Magnetism and Spin Dynamics in Room-Temperature van der Waals Magnet Fe$_5$GeTe$_2$,"Two-dimensional (2D) van der Waals (vdWs) materials have gathered a lot of
attention recently. However, the majority of these materials have Curie
temperatures that are well below room temperature, making it challenging to
incorporate them into device applications. In this work, we synthesized a
room-temperature vdW magnetic crystal Fe$_5$GeTe$_2$ with a Curie temperature
T$_c = 332$ K, and studied its magnetic properties by vibrating sample
magnetometry (VSM) and broadband ferromagnetic resonance (FMR) spectroscopy.
The experiments were performed with external magnetic fields applied along the
c-axis (H$\parallel$c) and the ab-plane (H$\parallel$ab), with temperatures
ranging from 300 K to 10 K. We have found a sizable Land\'e g-factor difference
between the H$\parallel$c and H$\parallel$ab cases. In both cases, the Land\'e
g-factor values deviated from g = 2. This indicates contribution of orbital
angular momentum to the magnetic moment. The FMR measurements reveal that
Fe$_5$GeTe$_2$ has a damping constant comparable to Permalloy. With reducing
temperature, the linewidth was broadened. Together with the VSM data, our
measurements indicate that Fe$_5$GeTe$_2$ transitions from ferromagnetic to
ferrimagnetic at lower temperatures. Our experiments highlight key information
regarding the magnetic state and spin scattering processes in Fe$_5$GeTe$_2$,
which promote the understanding of magnetism in Fe$_5$GeTe$_2$, leading to
implementations of Fe$_5$GeTe$_2$ based room-temperature spintronic devices.",2103.13433v2
2021-07-23,Multiple Stokes I inversions to infer magnetic fields in the spectral range around Cr I 5782 Å,"The spectral window, containing Fraunhofer lines formed in the solar
photosphere, around the magnetically sensitive Cr I lines at 5780.9, 5781.1,
5781.7, 5783.0, and 5783.8 \r{A}, with Land\'e g-factors between 1.6 and 2.5,
is explored. The goal is to analyze simultaneously 15 spectral lines, which
comprise Cr I, Cu I, Fe I, Mn I, and Si I lines, without polarimetry to infer
the thermodynamic and magnetic properties in strongly magnetized plasmas using
an inversion code. The study is based on a new setup at the Vacuum Tower
Telescope (VTT, Tenerife) which includes fast spectroscopic scans in the
wavelength range around the Cr I 5781.75 \r{A} line. The snapshot 385 of the
Enhanced Network simulation from the Bifrost code serves to synthesize all the
lines, which are in turn inverted simultaneously with SIR to establish the best
inversion strategy. This strategy is then applied to VTT observations of a
sunspot belonging to NOAA 12723 on 2018 September 30 and the results are
compared to full-disk vector-field data obtained with the Helioseismic and
Magnetic Imager (HMI). The 15 simultaneously inverted intensity profiles
(Stokes I) delivered accurate temperatures and Doppler velocities when compared
against the simulations. The derived magnetic fields and inclinations are most
accurate when the fields are oriented along the line-of-sight (LOS) and less
accurate when the fields are transverse to the LOS. In general, the results
appear similar to the HMI vector-field data, although some discrepancies exist.
The analyzed spectral range has the potential to deliver thermal, dynamic, and
magnetic information in strongly magnetized features on the Sun, such as pores
and sunspots, even without polarimetry. The highest sensitivity of the lines is
found in the lower photosphere, on average around $\log \tau = -1$. The
multiple-line inversions provide smooth results across the whole field-of-view.",2107.11116v1
2021-12-07,"Spinning black holes magnetically connected to a Keplerian disk -- Magnetosphere, reconnection sheet, particle acceleration and coronal heating","Context: Accreting black holes (BHs) may be surrounded by a highly magnetized
plasma threaded by a poloidal magnetic field. Non-thermal flares and high
energy components could originate from a hot, collisionless and nearly
force-free corona. The jets we often observe from these systems are believed to
be rotation-powered and magnetically-driven.
  Aims: We study axisymmetric BH magnetospheres where some magnetic field lines
anchored in a surrounding disk can connect to the event horizon of a rotating
BH. We identify the sites of magnetic reconnection within 30 gravitational
radii depending on the BH spin.
  Methods: With the fully general relativistic particle-in-cell code GRZeltron,
we solve the time-dependent dynamics of the electron-positron pair plasma and
of the electromagnetic fields around the BH. The disk is represented by a
steady plasma in Keplerian rotation, threaded by a frozen dipolar field.
  Results: For prograde disks, twisted open magnetic field lines crossing the
horizon power a Blandford-Znajek jet while beyond a critical distance, open
field lines on the disk are open. In the innermost regions, coupling field
lines ensure the transfer of significant amounts of angular momentum and energy
between the BH and the disk. From the Y-point at the intersection, a current
sheet forms where particle acceleration via magnetic reconnection takes place.
We compute the synchrotron images of the current sheet emission.
  Conclusions: Our estimates for jet power and BH-disk exchanges match those
derived from purely force-free models. Dissipation at the Y-point heats the
corona and provides a physically motivated source of hard X-rays above the disk
for reflection models. Episodic plasmoid ejection might explain millisecond
flares observed in Cyg X-1. Particles flowing from the Y-point down to the disk
could produce a hot spot at the footpoint of the outermost closed field line.",2112.03933v1
2022-01-05,Dust entrainment in magnetically and thermally driven disk winds,"Magnetically and thermally driven disk winds have gained popularity in the
light of the current paradigm of low viscosities in protoplanetary disks that
nevertheless present large accretion rates even in the presence of inner
cavities. The possibility of dust entrainment in these winds may explain recent
scattered light observations and constitutes a way of dust transport towards
outer regions of the disk. We aim to study the dust dynamics in these winds and
explore the differences between photoevaporation and magnetically driven disk
winds in this regard. We quantify maximum entrainable grain sizes, the flow
angle, and the general detectability. We used the FARGO3D code to perform
global, 2.5D axisymmetric, nonideal MHD simulations including ohmic and
ambipolar diffusion. Dust was treated as a pressureless fluid. Synthetic
observations were created with the radiative transfer code RADMC-3D. We find a
significant difference in the dust entrainment efficiency of warm, ionized
winds such as photoevaporation and magnetic winds including X-ray and extreme
ultraviolet (XEUV) heating compared to cold magnetic winds. The maximum
entrainable grain size varies from $3\,\mu\mathrm{m}$ to $6\,\mu\mathrm{m}$ for
ionized winds to $1\,\mu\mathrm{m}$ for cold magnetic winds. Dust grains in
cold magnetic winds tend to flow along a shallower angle compared to the warm
winds. With increasing distance to the central star, the dust entrainment
efficiency decreases. Larger values of the turbulent viscosity increase the
maximum grain size radius of possible dust entrainment. Our simulations
indicate that diminishing dust content in the outer regions of the wind can be
mainly attributed to the dust settling in the disk. In the synthetic images,
the dusty wind appears as a faint, conical emission region which is brighter
for a cold magnetic wind.",2201.01478v1
2022-01-14,Gravitational wave signal from primordial magnetic fields in the Pulsar Timing Array frequency band,"The NANOGrav, Parkes, European, and International Pulsar Timing Array (PTA)
Collaborations have reported evidence for a common-spectrum process that can
potentially correspond to a stochastic gravitational wave background (SGWB) in
the 1--100 nHz frequency range. We consider the scenario in which this signal
is produced by magnetohydrodynamic (MHD) turbulence in the early Universe,
induced by a nonhelical primordial magnetic field at the energy scale
corresponding to the quark confinement phase transition. We perform MHD
simulations to study the dynamical evolution of the magnetic field and compute
the resulting SGWB. We show that the SGWB output from the simulations can be
very well approximated by assuming that the magnetic anisotropic stress is
constant in time, over a time interval related to the eddy turnover time. The
analytical spectrum that we derive under this assumption features a change of
slope at a frequency corresponding to the GW source duration that we confirm
with the numerical simulations. We compare the SGWB signal with the PTA data to
constrain the temperature scale at which the SGWB is sourced, as well as the
amplitude and characteristic scale of the initial magnetic field. We find that
the generation temperature is constrained to be in the 1--200 MeV range, the
magnetic field amplitude must be $>1$\% of the radiation energy density at that
time, and the magnetic field characteristic scale is constrained to be $>10$\%
of the horizon scale. We show that the turbulent decay of this magnetic field
will lead to a field at recombination that can help to alleviate the Hubble
tension and can be tested by measurements in the voids of the Large Scale
Structure with gamma-ray telescopes like the Cherenkov Telescope Array.",2201.05630v2
2022-03-10,A magnetic cloud prediction model for forecasting space weather relevant properties of Earth-directed coronal mass ejections,"Coronal Mass Ejections (CMEs) are energetic storms in the Sun that result in
the ejection of large-scale magnetic clouds (MCs) in interplanetary space that
contain enhanced magnetic fields with coherently changing field direction. The
severity of geomagnetic perturbations depends on the direction and strength of
the interplanetary magnetic field (IMF), as well as the speed and duration of
passage of the storm. The coupling between the heliospheric environment and
Earth's magnetosphere is the strongest when the IMF direction is persistently
southward for a prolonged period. Predicting the magnetic profile of such
Earth-directed CMEs is crucial for estimating their geomagnetic impact. We aim
to build upon and integrate diverse techniques towards development of a
comprehensive magnetic cloud prediction (MCP) model that can forecast the
magnetic field vectors, Earth-impact time, speed and duration of passage of
solar storms. A novelty of our scheme is the ability to predict the passage
duration of the storm without recourse to computationally intensive,
time-dependent dynamical equations. Our methodology is validated by comparing
the MCP model output with observations of ten MCs at 1 AU. In our sample, we
find that eight MCs show a root mean square deviation of less than 0.1 between
predicted and observed magnetic profiles and the passage duration of seven MCs
fall within the predicted range. Based on the success of this approach, we
conclude that predicting the near-Earth properties of MCs based on analysis and
modelling of near-Sun CME observations is a viable endeavor with potential
benefits for space weather assessment.",2203.05231v3
2022-03-26,Ion Alfvén velocity fluctuations and implications for the diffusion of streaming cosmic rays,"The interstellar medium (ISM) of star-forming galaxies is magnetized and
turbulent. Cosmic rays (CRs) propagate through it, and those with energies from
$\sim\,\rm{GeV} - \rm{TeV}$ are likely subject to the streaming instability,
whereby the wave damping processes balances excitation of resonant ionic
Alfv\'en waves by the CRs, reaching an equilibrium in which the propagation
speed of the CRs is very close to the local ion Alfv\'en velocity. The
transport of streaming CRs is therefore sensitive to ionic Alfv\'en velocity
fluctuations. In this paper we systematically study these fluctuations using a
large ensemble of compressible MHD turbulence simulations. We show that for
sub-Alfv\'enic turbulence, as applies for a strongly magnetized ISM, the ionic
Alfv\'en velocity probability density function (PDF) is determined solely by
the density fluctuations from shocked gas forming parallel to the magnetic
field, and we develop analytical models for the ionic Alfv\'en velocity PDF up
to second moments. For super-Alfv\'enic turbulence, magnetic and density
fluctuations are correlated in complex ways, and these correlations as well as
contributions from the magnetic fluctuations sets the ionic Alfv\'en velocity
PDF. We discuss the implications of these findings for underlying ""macroscopic""
diffusion mechanisms in CRs undergoing the streaming instability, including
modeling the macroscopic diffusion coefficient for the parallel transport in
sub-Alfv\'enic plasmas. We also describe how, for highly-magnetized turbulent
gas, the gas density PDF, and hence column density PDF, can be used to access
information about ionic Alfv\'en velocity structure from observations of the
magnetized ISM.",2203.13952v3
2022-04-02,Fundamental scales in the kinematic phase of the turbulent dynamo,"The turbulent dynamo is a powerful mechanism that converts turbulent kinetic
energy to magnetic energy. A key question regarding the magnetic field
amplification by turbulence, is, on what scale, $k_{\rm p}$, do magnetic fields
become most concentrated? There has been some disagreement about whether
$k_{\rm p}$ is controlled by the viscous scale, $k_\nu$ (where turbulent
kinetic energy dissipates), or the resistive scale, $k_\eta$ (where magnetic
fields dissipate). Here we use direct numerical simulations of
magnetohydrodynamic turbulence to measure characteristic scales in the
kinematic phase of the turbulent dynamo. We run $104$-simulations with
hydrodynamic Reynolds numbers of $10 \leq {\rm Re} \leq 3600$, and magnetic
Reynolds numbers of $270 \leq {\rm Rm} \leq 4000$, to explore the dependence of
$k_{\rm p}$ on $k_\nu$ and $k_\eta$. Using physically motivated models for the
kinetic and magnetic energy spectra, we measure $k_\nu$, $k_\eta$ and $k_{\rm
p}$, making sure that the obtained scales are numerically converged. We
determine the overall dissipation scale relations $k_\nu =
(0.025^{+0.005}_{-0.006})\, k_{\rm turb}\, {\rm Re}^{3/4}$ and $k_\eta =
(0.88^{+0.21}_{-0.23})\, k_\nu\, {\rm Pm}^{1/2}$, where $k_{\rm turb}$ is the
turbulence driving wavenumber and ${\rm Pm}={\rm Rm}/{\rm Re}$ is the magnetic
Prandtl number. We demonstrate that the principle dependence of $k_{\rm p}$ is
on $k_\eta$. For plasmas where ${\rm Re} \gtrsim 100$, we find that $k_{\rm p}
= (1.2_{-0.2}^{+0.2})\, k_\eta$, with the proportionality constant related to
the power-law `Kazantsev' exponent of the magnetic power spectrum. Throughout
this study, we find a dichotomy in the fundamental properties of the dynamo
where ${\rm Re} > 100$, compared to ${\rm Re} < 100$. We report a minimum
critical hydrodynamic Reynolds number, ${\rm Re}_{\rm crit} = 100$ for bonafide
turbulent dynamo action.",2204.00828v3
2022-06-17,Strain and electric field control of magnetic and electrical transport properties in a magneto-elastically coupled Fe3O4/BaTiO3(001) heterostructure,"We present a study of the control of electric field induced strain on the
magnetic and electrical transport properties in a magneto-elastically coupled
artificial multiferroic Fe3O4/BaTiO3 heterostructure. In this Fe3O4/BaTiO3
heterostructure, the Fe3O4 thin film is epitaxially grown in the form of
bilateral domains, analogous to a-c stripe domains of the underlying
BaTiO3(001) substrate. By in-situ electric field dependent magnetization
measurements, we demonstrate the extrinsic control of the magnetic anisotropy
and the characteristic Verwey metal-insulator transition of the epitaxial Fe3O4
thin film in a wide temperature range between 20-300 K, via strain mediated
converse magnetoelectric coupling. In addition, we observe strain induced
modulations in the magnetic and electrical transport properties of the Fe3O4
thin film across the thermally driven intrinsic ferroelectric and structural
phase transitions of the BaTiO3 substrate. In-situ electric field dependent
Raman measurements reveal that the electric field does not significantly modify
the anti-phase boundary defects in the Fe3O4 thin film once it is
thermodynamically stable after deposition and that the modification of the
magnetic properties is mainly caused by strain induced lattice distortions and
magnetic anisotropy. These results provide a framework to realize electrical
control of the magnetization in a classical highly correlated transition metal
oxide.",2206.08664v1
2022-10-17,Effects of Emerging Bipolar Magnetic Regions in Mean-field Dynamo Model of Solar Cycles 23 and 24,"We model the physical parameters of Solar Cycles 23 and 24 using a nonlinear
dynamical mean-field dynamo model that includes the formation and evolution of
bipolar magnetic regions (BMR). The Parker-type dynamo model consists of a
complete MHD system in the mean-field formulation: the 3D magnetic induction
equation, and 2D momentum and energy equations in the anelastic approximation.
The initialization of BMR is modeled in the framework of Parker's magnetic
buoyancy instability. It defines the depths of BMR injections, which are
typically located at the edge of the global dynamo waves. The distribution with
longitude and latitude and the size of the initial BMR perturbations are taken
from the NOAA database of active regions. The tilt of the perturbations is
modeled by random function, and the mean tilt is modeled as a near-surface
helicity (alpha-effect) term. The data-driven models are compared with the
models calculated for random longitudinal and latitudinal distributions of the
initial perturbation. The modeling results are compared with various observed
characteristics of the solar cycles, including the magnetic butterfly diagram,
the polar magnetic and basal magnetic fluxes, and the probability distributions
of the BMR flux on the surface. Our results show that BMR can play a
substantial role in the dynamo processes, and affect the strength of the solar
cycle. However, the data-driven model shows that the BMR effect alone cannot
explain the weak Cycle 24. This weak cycle and the prolonged preceding minimum
of magnetic activity were probably caused by a decrease of the turbulent
helicity in the bulk of the convection zone during the decaying phase of Cycle
23.",2210.08764v2
2022-11-18,Global simulations of Tayler instability in stellar interiors: a long-time multi-stage evolution of the magnetic field,"Magnetic fields have been observed in massive Ap/Bp stars and presumably are
also present in the radiative zone of solar-like stars. Yet, to date there is
no clear understanding of the dynamics of the magnetic field in stably
stratified layers. A purely toroidal magnetic field configuration is known to
be unstable, developing mainly non-axisymmetric modes. Rotation and a small
poloidal field component may lead to a stable configuration. Here we perform
global MHD simulations with the EULAG-MHD code to explore the evolution of a
toroidal magnetic field located in a layer whose stratification resembles the
solar tachocline. Our numerical experiments allow us to explore the initial
unstable phase as well as the long-term evolution of the magnetic field. During
the first Alfven cycles, we observe the development of the Tayler instability
with the prominent longitudinal wavenumber, $m=1$. Rotation decreases the
growth rate of the instability, and eventually suppresses it. However, after a
stable phase, sudden energy surges lead to the development of higher order
modes even for fast rotation. These modes extract energy from the initial
toroidal field. Nevertheless, our results show that sufficiently fast rotation
leads to a lower saturation energy of the unstable modes, resulting in a
magnetic topology with only a small fraction of poloidal field which remains
steady for several hundreds of Alfven travel times. At this stage, the system
becomes turbulent and the field is prone to turbulent diffusion. The final
toroidal-poloidal configuration of the magnetic field may represent an
important aspect of the field generation and evolution in stably-stratified
layers.",2211.10536v2
2022-12-23,Tailing Magnetoelectric properties of Cr2Ge2Te6 by Engineering Covalently bonded Cr Self-intercalation: Ferromagnetic Half-metal,"Two-dimensional intrinsic ferromagnetic half-metal (HM) are important for the
spintronics. Manipulating the interlayer magnetic coupling of van der Waals
magnetic materials is an important method to control magnetoelectric
properties, which is especially useful for the spintronics. Here, based on
systematical research of CrGeTe3 (CGT) bilayer and multilayers with s of
self-intercalated (SI) Cr atom, we find that self-intercalation can enhance the
interlayer magnetic coupling. The super-exchange interaction still dominates
interlayer magnetic exchange interaction, which results in ferromagnetic
coupling between neighboring vdW layers. CGT bilayer keeps HM with FM order
after Cr self-intercalation, independent of self-intercalated Cr (CrSI) atoms'
concentration. Most importantly, self-intercalated CrGeTe3 (SI-CGT) bilayers
show ferromagnetic HM, independent of stacking orders. Moreover, SI-CGT
multilayers keep FM order, independent of films' thickness. However, SI-CGT
multilayers transform from HM into normal spin-polarized metal, as the states
at the Fermi-level increases. Moreover, magnetic anisotropy energy (MAE) of
SI-CGT-AA and SI-CGT-AB are -0.160 and -0.42 meV/.f.u., which are modulated by
CrSI atoms. The MAE of SI-CGT-AA and SI-CGT-AB are different, as the
hybridization interaction between Cr's d orbitals is different. SI-CGT
multilayers' magnetic easy axis (EA) switches from [001] of CGT to [100]
direction, independent of stacking orders. It origins that MAE mainly
contributed by hybridization between Te atoms' px and py, py and pz orbitals is
obvious weakened as CrSI is introduced. SI-CGT multilayers show good dynamical,
thermal, and magnetic stability at 300 and 500 K. These findings find a
promising way to manipulate interlayer exchange interaction and magnetoelectric
properties of CGT multilayers and other vdW magnets.",2212.12111v1
2023-05-24,Gravitational wave signatures from the phase-transition-induced collapse of a magnetized neutron star,"Strong magnetic fields make neutron stars potential sources of detectable
electromagnetic and gravitational-wave signals. Hence, inferring these magnetic
fields is critical to understand the emissions of neutron stars. However, due
to the lack of direct observational evidence, the interior magnetic field
configuration remains ambiguous. Here, for the first time, we show that the
internal magnetic field strength along with the composition of a neutron star
can be directly constrained by detecting the gravitational waves from the
phase-transition-induced collapse of a magnetized neutron star. By dynamically
simulating this collapsing event, we first find that the dominant peaks in the
gravitational waveform are the fundamental $l=0$ quasi-radial $F$ mode and the
fundamental $l=2$ quadrupolar $^2f$ mode. We next show that the maximum
gravitational wave amplitude $|h|_\mathrm{max}$ increases with the maximum
magnetic field strength of the interior toroidal field
$\mathcal{B}_\mathrm{max}$ until the maximum rest-mass density at bounce
$\rho_\mathrm{max,b}$ decreases due to the increasing
$\mathcal{B}_\mathrm{max}$. We then demonstrated that the magnetic suppression
of fundamental modes found in our previous work remains valid for the hybrid
stars formed after the phase-transition-induced collapses. We finally show that
measuring the frequency ratio between the two fundamental modes $f_{^2f}/f_{F}$
allows one to infer $\mathcal{B}_\mathrm{max}$ and the baryonic mass fraction
of matter in the mixed phase $M_\mathrm{mp} / M_{0}$ of the resulting hybrid
star. Consequently, taking $\mathcal{B}_\mathrm{max}$ and $M_\mathrm{mp} /
M_{0}$ as examples, this work has demonstrated that much information inside
neutron stars could be extracted similarly through measuring the oscillation
modes of the stars.",2305.15181v1
2023-12-13,Enhanced Magnetization by Defect-Assisted Exciton Recombination in Atomically Thin CrCl$_3$,"Two dimensional (2D) semiconductors present unique opportunities to
intertwine optical and magnetic functionalities and to tune these performances
through defects and dopants. In 2D semiconductors, the properties of excitons
often vary due to the underlying magnetic order, but the modification of their
magnetic properties by excitonic processes remains difficult to achieve. Here,
we integrate exciton pumping into a quantum sensing protocol on
nitrogen-vacancy centers in diamond to image the optically-induced transient
stray fields in few-layer, antiferromagnetic CrCl$_3$. We discover that exciton
recombination enhances the in-plane magnetization of the CrCl$_3$ layers, with
an outsized effect in the surface monolayers and when the surface is
unencapsulated. Concomitantly, time-resolved photoluminescence measurements
reveal that a defect-assisted Auger recombination dominates for atomically thin
CrCl$_3$ with tightly localized, nearly forbidden excitons and amplified
surface-to-volume ratio. These experimental signatures establish the
magnetically enhanced state to result from an exciton-activated electron
transfer between surface impurities and the spin-polarized conduction band.
Density functional theory calculations identify that the mediating impurity
state could originate from oxygen adsorbates with spins antiparallel to the
magnetization of their host layer. Our work unambiguously demonstrates the
tunability of single intrinsic magnetic layers via defect engineering and opens
a pathway to dynamically reconfigure these effects in devices through optical
control.",2312.08476v1
2024-03-04,Molecular intercalation in the van der Waals antiferromagnets FePS3 and NiPS3,"We have performed electrochemical treatment of the van der Waals
antiferromagnetic materials FePS$_3$ and NiPS$_3$ with the ionic liquid
EMIM-BF$_4$, achieving significant molecular intercalation. Mass analysis of
the intercalated compounds, EMIM$_x$-FePS$_3$ and EMIM$_x$-NiPS$_3$, indicated
respective intercalation levels, $x$, of approximately 27\% and 37\%, and X-ray
diffraction measurements demonstrated a massive (over 50\%) enhancement of the
$c$-axis lattice parameters. To investigate the consequences of these changes
for the magnetic properties, we performed magnetic susceptibility and $^{31}$P
nuclear magnetic resonance (NMR) studies of both systems. For
EMIM$_x$-FePS$_3$, intercalation reduces the magnetic ordering temperature from
$T_N = 120$~K to 78~K, and we find a spin gap in the antiferromagnetic phase
that drops from 45~K to 30~K. For EMIM$_x$-NiPS$_3$, the ordering temperature
is almost unaffected (changing from 148~K to 145~K), but a change towards
nearly isotropic spin fluctuations suggests an alteration of the magnetic
Hamiltonian. Such relatively modest changes, given that the huge extension of
the $c$ axes is expected to cause a very strong suppression any interlayer
interactions, point unequivocally to the conclusion that the magnetic
properties of both parent compounds are determined solely by two-dimensional
(2D), intralayer physics. The changes in transition temperatures and
low-temperature spin dynamics in both compounds therefore indicate that
intercalation also results in a significant modulation of the intralayer
magnetic interactions, which we propose is due to charge doping and
localization on the P sites. Our study offers chemical intercalation with ionic
liquids as an effective method to control not only the interlayer but also the
intralayer interactions in quasi-2D magnetic materials.",2403.01714v1
2024-03-18,Thermal Tensor Network Approach for Spin-Lattice Relaxation in Quantum Magnets,"Low-dimensional quantum magnets, particularly those with strong spin
frustration, are characterized by their notable spin fluctuations. Nuclear
magnetic resonance (NMR) serves as a sensitive probe of low-energy fluctuations
that offers valuable insight into rich magnetic phases and emergent phenomena
in quantum magnets. Although experimentally accessible, the numerical
simulation of NMR relaxation rates, specifically the spin-lattice relaxation
rate $1/T_1$, remains a significant challenge. Analytical continuation based on
Monte Carlo calculations are hampered by the notorious negative sign for
frustrated systems, and the real-time simulations incur significant costs to
capture low-energy fluctuations. Here we propose computing the relaxation rate
using thermal tensor networks (TTNs), which provides a streamlined approach by
calculating its imaginary-time proxy. We showcase the accuracy and versatility
of our methodology by applying it to one-dimensional spin chains and
two-dimensional lattices, where we find that the critical exponents $\eta$ and
$z\nu$ can be extracted from the low-temperature scalings of the simulated
$1/T_1$ near quantum critical points. Our results also provide insights into
the low-dimensional and frustrated magnetic materials, elucidating universal
scaling behaviors in the Ising chain compound CoNb$_2$O$_6$ and revealing the
renormalized classical behaviors in the triangular-lattice antiferromagnet
Ba$_8$CoNb$_6$O$_{24}$. We apply the approach to effective model of the family
of frustrated magnets AYbCh$_2$ (A = Na, K, Cs, and Ch = O, S, Se), and find
dramatic changes from spin ordered to the proposed quantum spin liquid phase.
Overall, with high reliability and accuracy, the TTN methodology offers a
systematic strategy for studying the intricate dynamics observed across a broad
spectrum of quantum magnets and related fields.",2403.11895v2
2024-04-01,Transport of the magnetic flux away from a decaying sunspot via convective motions,"Aims. The aim of this paper is to consider relationship between the decay of
sunspots and convection via the motion of the family of granules and how the
diffusion mechanism of magnetic field operates in a decaying sunspot. Methods.
We report the decay of a sunspot observed by the 1.6m Goode Solar Telescope
(GST) with the TiO Broadband Filter Imager (BFI) and the Near-InfraRed Imaging
Spectropolarimeter (NIRIS). The analysis was aided by the Helioseismic and
Magnetic Imager (HMI) on board the Solar Dynamic Observatory (SDO). In the
first step, we followed the decay of the sunspot with HMI data over three days
by constructing its evolving area and total magnetic flux. In the second step,
the high spatial and temporal resolution of the GST instruments allowed us to
analyze the causes of the decay of the sunspot. Afterward, we followed the
emergence of granules in the moat region around the sunspot over six hours. The
evolution of the trees of fragmenting granules (TFGs) was derived based on
their relationship with the horizontal surface flows. Results. We find that the
area and total magnetic flux display an exponential decrease over the course of
the sunspot decay. We identified 22 moving magnetic features (MMFs) in the
moats of pores, which is a signature of sunspot decay through diffusion. We
note that the MMFs were constrained to follow the borders of TFGs during their
journey away from the sunspot. Conclusions. The TFGs and their development
contribute to the diffusion of the magnetic field outside the sunspot. The
conclusion of our analysis shows the important role of the TFGs in sunspot
decay. Finally, the the family of granules evacuates the magnetic field.",2404.00889v1
2008-08-28,"Effects of Zn and Ni substitution on the Cu-spin dynamics and superconductivity in La_2-x_Sr_x_Cu_1-y_(Zn,Ni)_y_O_4_ with x=0.15-0.20 studied by the muon spin relaxation and magnetic susceptibility","We have investigated effects of Zn and Ni on the Cu-spin dynamics and
superconductivity from the zero-field muon-spin-relaxation (ZF-muSR) and
magnetic-susceptibility, chi, measurements for
La_2-x_Sr_x_Cu_1-y_(Zn,Ni)_y_O_4_ with x=0.15-0.20, changing y up to 0.10 in
fine step. In the optimally doped x=0.15, it has been concluded that the
formation of a magnetic order requires a larger amount of Ni than that of Zn,
which is similar to our previous results of x=0.13. From the estimation of
volume fractions of superconducting (SC) and magnetic regions, it has been
found for x=0.15 that the SC region is in rough correspondence to the region
where Cu spins fluctuate fast beyond the muSR frequency window for both Zn- and
Ni-substituted samples. According to the stripe model, it follows that, even
for x=0.15, the dynamical stripe correlations of spins and holes are pinned and
localized around Zn and Ni, leading to the formation of the static stripe order
and the suppression of superconductivity. These may indicate an importance of
the dynamical stripe in the appearance of the high-T_c_ superconductivity in
the hole-doped cuprates. In the overdoped regime of x=0.18 and 0.20, on the
other hand, the SC region seems to be in rough correspondence to the region
where Cu spins fluctuate fast beyond the muSR frequency window, though it
appears that the Cu-spin dynamics and superconductivity are affected by the
phase separation into SC and normal-state regions.",0808.3871v2
2011-07-11,Understanding quantum measurement from the solution of dynamical models,"The quantum measurement problem, understanding why a unique outcome is
obtained in each individual experiment, is tackled by solving models. After an
introduction we review the many dynamical models proposed over the years. A
flexible and rather realistic model is introduced, describing the measurement
of the $z$-component of a spin through interaction with a magnetic memory
simulated by a Curie--Weiss magnet, including $N \gg1$ spins weakly coupled to
a phonon bath. Initially prepared in a metastable paramagnetic state, it may
transit to its up or down ferromagnetic state, triggered by its coupling with
the tested spin, so that its magnetization acts as a pointer.
  A detailed solution of the dynamical equations is worked out. Conditions are
found, which ensure that the process satisfies the features of ideal
measurements. Various imperfections are discussed, as well as attempts of
incompatible measurements. The first steps consist in the solution of the
Hamiltonian dynamics for the spin-apparatus density matrix $D(t)$. On a longer
time scale, the trend towards equilibrium of the magnet produces a final state
$D(t_{\rm f})$ that involves correlations between the system and the
indications of the pointer, thus ensuring registration. A difficulty lies in a
quantum ambiguity: There exist many incompatible decompositions of the density
matrix $\scriptD(t_{\rm f})$. This difficulty is overcome by dynamics due to
suitable interactions within the apparatus.
  Any subset of runs thus reaches over a brief delay a stable state which
satisfies the same hierarchic property as in classical probability theory.
Standard quantum statistical mechanics alone appears sufficient to explain the
occurrence of a unique answer in each run. Finally, pedagogical exercises are
proposed while the statistical interpretation is promoted for teaching.
[Abridged]",1107.2138v4
2015-12-07,Dynamic magnetic susceptibility and electrical detection of ferromagnetic resonance,"The dynamic magnetic susceptibility of magnetic materials near ferromagnetic
resonance (FMR) is very important in interpreting dc-voltage in electrical
detection of FMR. Based on the causality principle and the assumption that the
usual microwave absorption lineshape around FMR is Lorentzian, general forms of
dynamic susceptibility of an arbitrary sample and the corresponding dc-voltage
lineshape are obtained. Our main findings are: 1) The dynamic susceptibility is
not a Polder tensor for material with arbitrary anisotropy. Two off-diagonal
elements are not in general opposite to each other. However, the linear
response coefficient of magnetization to total rf field is a Polder tensor.
This may explain why two off-diagonal elements are always assumed to be
opposite to each other in analyses. 2) The frequency dependence of dynamic
susceptibility near FMR is fully characterized by six numbers while its field
dependence is fully characterized by seven numbers. 3) A recipe of how to
determine these numbers by standard microwave absorption measurements for an
arbitrary sample is proposed. Our results allow one to unambiguously separate
the contribution of the anisotropic magnetoresistance to dc-voltage from that
of the anomalous Hall effect. With these results, one can reliably extract the
information of spin pumping and the inverse spin Hall effect, and determine the
spin-Hall angle. 4) The field-dependence of susceptibility matrix at a fixed
frequency may have several peaks when the effective field is not monotonic of
the applied field. In contrast, the frequency-dependence of susceptibility
matrix at a fixed field has only one peak. Furthermore, in the case that
resonance frequency is not sensitive to the applied field, the field dependence
of susceptibility matrix, as well as dc-voltage, may have another non-resonance
broad peak. Thus, one should be careful in interpreting observed peaks.",1512.01913v1
2016-12-15,The Dynamics of Charged Dust in Magnetized Molecular Clouds,"We study the dynamics of large, charged dust grains in turbulent giant
molecular clouds (GMCs). Massive dust grains behave as aerodynamic particles in
primarily neutral dense gas, and thus are able to produce dramatic small-scale
fluctuations in the dust-to-gas ratio. Hopkins & Lee (2016) directly simulated
the dynamics of neutral dust grains in super-sonic MHD turbulence, typical of
GMCs, and showed that the dust-to-gas fluctuations can exceed factor ~1000 on
small scales, with important implications for star formation, stellar
abundances, and dust behavior and growth. However, even in primarily neutral
gas in GMCs, dust grains are negatively charged and Lorentz forces are
non-negligible. Therefore, we extend our previous study by including the
effects of Lorentz forces on charged grains (in addition to drag). For small
charged grains (sizes <0.1 micron), Lorentz forces suppress dust-to-gas ratio
fluctuations, while for large grains (sizes >1 micron), Lorentz forces have
essentially no effect, trends that are well explained with a simple theory of
dust magnetization. In some special intermediate cases, Lorentz forces can
enhance dust-gas segregation. Regardless, for the physically expected scaling
of dust charge with grain size, we find the most important effects depend on
grain size (via the drag equation) with Lorentz forces/charge as a second-order
correction. We show that the dynamics we consider are determined by three
dimensionless numbers in the limit of weak background magnetic fields: the
turbulent Mach number, a dust drag parameter (proportional to grain size) and a
dust Lorentz parameter (proportional to grain charge); these allow us to
generalize our simulations to a wide range of conditions.",1612.05264v2
2018-12-22,Spin dynamics of $3d$ and $4d$ impurities embedded in prototypical topological insulators,"Topological insulators are insulating bulk materials hosting conducting
surface states. Their magnetic doping breaks time-reversal symmetry and
generates numerous interesting effects such as dissipationless transport.
Nonetheless, their dynamical properties are still poorly understood. Here, we
perform a systematic investigation of transverse spin excitations of $3d$ and
$4d$ single impurities embedded in two prototypical topological insulators
(Bi$_2$Te$_3$ and Bi$_2$Se$_3$). The impurity-induced states within the bulk
gap of the topological insulators are found to have a drastic impact on the
spin excitation spectra, resulting in very high lifetimes reaching up to
${microseconds}$. An intuitive picture of the spin dynamics is obtained by
mapping onto a generalized Landau-Lifshitz-Gilbert phenomenological model. The
first quantity extracted from this mapping procedure is the magnetic anisotropy
energy, which is then compared to the one provided by the magnetic force
theorem. This uncovers some difficulties encountered with the latter, which can
provide erroneous results for impurities with a high density of states at the
Fermi energy. Moreover, the Gilbert damping and nutation tensors are obtained.
The nutation effects can lead to a non-negligible shift in the spin excitation
resonance in the high-frequency regime. Finally, we study the impact of the
surface state on the spin dynamics, which may be severely altered due to the
repositioning of the impurity-induced state in comparison to the bulk case. Our
systematic investigation of this series of magnetic impurities sheds light on
their spin dynamics within topological insulators, with implications for
available and future experimental studies as, for instance, on the viability of
using such impurities for solid-state qubits.",1812.09596v1
2019-09-23,"Coherent spin dynamics of solitons in the organic spin chain compounds ($o$-DMTTF)$_2X$ ($X$ = Cl, Br)","We studied the magnetic properties, in particular dynamics, of the correlated
spins associated with natural defects in the organic spin chain compounds
($o$-DMTTF)$_2X$ ($X$ = Br, Cl) by means of electron spin resonance (ESR)
spectroscopy. Both materials exhibit spin-Peierls transitions at temperatures
around 50 K [P. Foury-Leylekian et al. Phys. Rev. B 84, 195134 (2011)], which
allow a separation of the properties of defects inside the chains from the
magnetic response of the spin chains. Indeed, continuous wave ESR measurements
performed over a wide temperature range evidence the evolution of the spin
dynamics from being governed by the spins in the chains at elevated
temperatures to a low-temperature regime which is dominated by defects within
the spin-dimerized chains. Such defects polarize the antiferromagnetically
coupled spins in their vicinity, thereby leading to a finite local alternating
magnetization around the defect site which can be described in terms of a
soliton, i.e. a spin 1/2 quasiparticle built of many correlated spins, pinned
to the defect. In addition, contributions of triplon excitations of the
spin-dimerized state to the ESR response below the transition temperature were
observed which provides a spectroscopic estimate for the spin-gap of the
studied systems. Moreover, details of spin dynamics deep in the spin-Peierls
phase were investigated by pulse ESR experiments which revealed
Rabi-oscillations as signatures of coherent spin dynamics. From a comparison of
the characteristic damping times of the Rabi oscillations with measurements of
the spin relaxation times by means of primary-echo decay and CPMG methods it
becomes evident that inhomogeneities in local magnetic fields strongly
contribute to the soliton decoherence.",1909.10301v2
2021-08-16,Relaxation of antiferromagnetic order and growth of Rényi entropy in a generalized Heisenberg star,"Interacting central spin systems, in which a central spin is coupled to a
strongly correlated spin bath with intrabath interaction, consist of an
important class of spin systems beyond the usual Gaudin magnet. These systems
are relevant to several realistic setups and serve as an interesting platform
to study interaction controlled decoherence and frustration induced instability
of magnetic order. Using an equations-of-motion method based on analytical
representations of spin-operator matrix elements in the XX chain, we obtain
exact long-time dynamics of a generalized Heisenberg star consisting of a
spin-$S$ central spin and an inhomogeneously coupled XXZ chain of $N\leq 16$
bath spins. In contrast to previous studies where the central spin dynamics is
mainly concerned, we focus on the influence of the central spin on the dynamics
of magnetic orders within the spin bath. By preparing the XXZ bath in a N\'eel
state, we find that in the gapless phase of the bath even weak system-bath
coupling could lead to nearly perfect relaxation of the antiferromagnetic
order. In the gapped phase, the staggered magnetization decays rapidly and
approaches a steady value that increases with increasing anisotropy parameter.
These findings suggest the possibility of controlling internal dynamics of
strongly correlated many-spin systems by certain coupled auxiliary systems of
even few degrees of freedom. We also study the dynamics of the R\'enyi
entanglement entropy of the central spin when the bath is prepared in the
ground state. Both the overall profile and initial growth rate of the R\'enyi
entropy are found to exhibit minima at the critical point of the XXZ bath.",2108.07055v3
2022-11-02,Relaxation dynamics in a long-range system with mixed Hamiltonian and non-Hamiltonian interactions,"Sometimes the dynamics of a physical system is described by non-Hamiltonian
equations of motion, and additionally, the system is characterized by
long-range interactions. A concrete example is that of particles interacting
with light as encountered in free-electron laser and cold-atom experiments. In
this work, we study the relaxation dynamics to non-Hamiltonian systems, more
precisely, to systems with interactions of both Hamiltonian and non-Hamiltonian
origin. Our model consists of $N$ globally-coupled particles moving on a circle
of unit radius; the model is one-dimensional. We show that in the infinite-size
limit, the dynamics, similarly to the Hamiltonian case, is described by the
Vlasov equation. In the Hamiltonian case, the system eventually reaches an
equilibrium state, even though one has to wait for a long time diverging with
$N$ for this to happen. By contrast, in the non-Hamiltonian case, there is no
equilibrium state that the system is expected to reach eventually. We
characterize this state with its average magnetization. We find that the
relaxation dynamics depends strongly on the relative weight of the Hamiltonian
and non-Hamiltonian contributions to the interaction. When the non-Hamiltonian
part is predominant, the magnetization attains a vanishing value, suggesting
that the system does not sustain states with constant magnetization, either
stationary or rotating. On the other hand, when the Hamiltonian part is
predominant, the magnetization presents long-lived strong oscillations, for
which we provide a heuristic explanation. Furthermore, we find that the
finite-size corrections are much more pronounced than those in the Hamiltonian
case; we justify this by showing that the Lenard-Balescu equation, which gives
leading-order corrections to the Vlasov equation, does not vanish, contrary to
what occurs in one-dimensional Hamiltonian long-range systems.",2211.01018v1
2023-07-06,Finding the Dynamics of an Integrable Quantum Many-Body System via Machine Learning,"We study the dynamics of the Gaudin magnet (""central-spin model"") using
machine-learning methods. This model is of practical importance, e.g., for
studying non-Markovian decoherence dynamics of a central spin interacting with
a large bath of environmental spins and for studies of nonequilibrium
superconductivity. The Gaudin magnet is also integrable, admitting many
conserved quantities: For $N$ spins, the model Hamiltonian can be written as
the sum of $N$ independent commuting operators. Despite this high degree of
symmetry, a general closed-form analytic solution for the dynamics of this
many-body problem remains elusive. Machine-learning methods may be well suited
to exploiting the high degree of symmetry in integrable problems, even when an
explicit analytic solution is not obvious. Motivated in part by this intuition,
we use a neural-network representation (restricted Boltzmann machine) for each
variational eigenstate of the model Hamiltonian. We then obtain accurate
representations of the ground state and of the low-lying excited states of the
Gaudin-magnet Hamiltonian through a variational Monte Carlo calculation. From
the low-lying eigenstates, we find the non-perturbative dynamic transverse spin
susceptibility, describing the linear response of a central spin to a
time-varying transverse magnetic field in the presence of a spin bath. Having
an efficient description of this susceptibility opens the door to improved
characterization and quantum control procedures for qubits interacting with an
environment of quantum two-level systems. These systems include electron-spin
and hole-spin qubits interacting with environmental nuclear spins via hyperfine
interactions or qubits with charge or flux degrees of freedom interacting with
coherent charge or paramagnetic impurities.",2307.03310v2
1994-11-09,Does a Non-Magnetic Solar Chromosphere Exist?,"Enhanced chromospheric emission which corresponds to an outwardly increasing
semiempirical temperature structure can be produced by wave motion without any
increase in the mean gas temperture. Hence, the sun may not have a classical
chromosphere in magnetic field free internetwork regions. Other significant
differences between the properties of dynamic and static atmospheres should be
considered when analyzing chromospheric observations.",9411036v1
1997-09-26,Generation of Pulsar Glitches: A Superfluid Core Model,"We show that the neutron star's crust-core interface acts as a potential
barrier on the peripheral neutron vortices approaching this interface and thus
prevents their continuous decay on it in the course of equilibrium state
deceleration. The barrier arises due to the interaction of vortex magnetic flux
with the Meissner currents set up by the crustal magnetic field at the
interface. When the non-balanced part of the Magnus force reaches the value at
which the vortices are able to annihilate at the interface, the rapid transfer
of angular momentum from the superfluid spins-up the observable crust on short
dynamical coupling times.",9709277v2
1998-08-19,Magnetic Field Effects on the Structure of Radiatively Cooling Protostellar Jets,"In the present work, with the help of fully 3-D smooth particle
magnetohydodynamic (SPMHD) simulations, we explore the nonlinear effects of
magnetic fields (in close equipartition with the gas) in the structure of
radiatively cooling, overdense jets and test their importance on the dynamics
of protostellar jets.",9808194v1
1999-12-14,Kick asymmetry along a strong magnetic field in the process of neutrino scattering on nucleons,"The neutrino-nucleon scattering in a collapsing star envelope with a strong
magnetic field is investigated. The transferred by neutrinos momentum asymmetry
along the field direction is obtained. It is shown that the neutrino-nucleon
scattering gives a contribution to the asymmetry comparable with direct URCA
processes. Hence, the neutrino-nucleon scattering should be taken into account
in estimations of a possible influence of neutrino reemission processes on a
collapsing star envelope dynamics.",9912287v1
2000-06-23,Outflow from YSO and Angular Momentum Transfer,"Dynamical contraction of a slowly-rotating magnetized cloud is studied using
the magnetohydrodynamical (MHD) simulations. In the isothermal stage ($n \la
n_{\rm A} \sim 10^{10}{\rm cm}^{-3}$), the cloud evolves similarly to that
expected from Larson-Penston self-similar solution and experiences the run-away
collapse. However, after the central density exceeds $\sim n_{\rm A}$, an
accretion disk is formed around the adiabatic core. Just outside the core,
outflow is ejected by the effect of magnetic torque (magnetocentrifugal wind).
Since $\sim$ 10% of the mass is ejected with almost all the angular momentum,
the specific angular momentum of the protostellar core reduces to that observed
in main-sequence stars.",0006329v1
2000-10-12,Modeling Observational Signatures of Disk-Driven Outflows,"We present a self-consistent, semi-analytical dynamical model of disk driven
outflows in AGNs that are accelerated by a combination of magnetic stresses and
radiation pressure. This model will make it possible to examine scenarios in
which the wind is homogeneous as well as cases where it consists of dense
clouds embedded in a more tenuous, magnetized medium. The various ingredients
of this model will be tested through quantitative predictions from both
multiwavelength spectral observations and reverberation mapping of AGNs.",0010246v1
2001-09-20,Pulsar Spin-Down Induced Phenomena: Heating; Magnetic Field Evolution; Glitches; Pulse-Period Modulations,"Modeling the dynamics of the quantum fluids within a spinning-down neutron
star gives a description consistent with observed pulsar magnetic field
evolution and spin-period ""glitches."" The long-standing problem of large
predicted excesses in spin-down sustained pulsar heating from such models now
see ms resolvable. However, the origin of some pulsar spin-period and
pulse-shape modulations which have been interpreted as manifestations of very
long period (~year) stellar precession is a crucial challenge to canonical
neutron star models.",0109353v1
2001-09-28,The Evolution of Magnetic Fields in Galaxy Clusters,"Cosmological simulations of magnetic fields in galaxy clusters show that
remarkable agreement between simulations and observations of Faraday rotation
and radio haloes can be achieved assuming that seed fields of 10^{-9} G were
present at redshifts 15-20. The structure of the seed field is irrelevant for
the final intracluster field. On average, the field grows exponentially with
decreasing cluster redshift, but merger events cause steep transient increases
in the field strength. Typical field-reversal scales are of order 50 kpc/h. In
most cases, the intracluster fields are dynamically unimportant. Assuming
secondary electron models, the average structure of cluster radio haloes can
naturally be reproduced.",0109541v1
2002-07-18,Solar and stellar dynamos -- latest developments,"Recent progress in the theory of solar and stellar dynamos is reviewed.
Particular emphasis is placed on the mean-field theory which tries to describe
the collective behavior of the magnetic field. In order to understand solar and
stellar activity, a quantitatively reliable theory is necessary. Much of the
new developments center around magnetic helicity conservation which is seen to
be important in numerical simulations. Only a dynamical, explicitly time
dependent theory of alpha-quenching is able to describe this behavior
correctly.",0207393v1
2002-12-16,"Magnetogenesis, variation of gauge couplings and inflation","The interplay among the possible variation of gauge coupling and the
inflationary dynamics is investigated in a simplified toy model. Depending upon
various parameters (scalar mass, curvature scale at the end of inflation and at
the onset of the radiation epoch), the two-point function of the magnetic
inhomogeneities grows during the de Sitter stage of expansion and,
consequently, large scale magnetic fields are generated. The requirements
coming from inflationary magnetogenesis are examined together with the
theoretical constraints stemming from the explicit model of the evolution of
the gauge coupling.",0212346v1
2003-05-06,Electron acceleration and heating in collisionless magnetic reconnection,"We discuss electron acceleration and heating during collisionless magnetic
reconnection by using the results of implicit kinetic simulations of Harris
current sheets. We consider and compare electron dynamics in plasmas with
different \beta values and perform simulations up to the physical mass ratio.
We analyze the typical trajectory of electrons passing through the reconnection
region, we study the electron velocity, focusing on the out-of-plane velocity,
and we discuss the electron heating along the in-plane and out-of-plane
directions.",0305081v1
2004-09-03,Fermion Scattering off a CP-violating Bubble Wall in the Background of a Uniform Magnetic Field,"In the scenario of the electroweak baryogenesis we consider the dynamics of
fermions with a spatially varying mass in presence of a CP-violating bubble
wall and a uniform magnetic field perpendicular to the wall. The relevant
quantity for baryogenesis, $R_{R \to L} - R_{L \to R}$, is studied ($R_{R \to
L}$ and $R_{L \to R}$ being the reflection coefficients for right-handed and
left-handed chiral fermions, respectively).",0409077v1
2004-09-20,Magnetic fields in GRBs,"We discuss the dynamical role of magnetic fields in GRB explosions in the
framework of the fireball and electromagnetic models. We contrast the
observational predictions of the two models and argue that only the very early
afterglow observations, almost coincident with the prompt phase, can be used as
decisive tests of the ejecta content.",0409489v2
2005-07-06,Chiral Properties of QCD Vacuum in Magnetars- A Nambu-Jona-Lasinio Model with Semi-Classical Approximation,"The breaking of chiral symmetry of light quarks at zero temperature in
presence of strong quantizing magnetic fiels is studied using
Nambu-Jona-Lasinio (NJL) model with Thomas-Fermi type semi-classical formalism.
It is found that the dynamically generated light quark mass can never become
zero if the Landau levels are populated and the mass increases with the
increase of magnetic field strength.",0507127v1
2006-06-20,Experimental evidence for magnetorotational instability in a helical magnetic field,"A recent paper [R. Hollerbach and G. Rudiger, Phys. Rev. Lett. 95, 124501
(2005)] has shown that the threshold for the onset of the magnetorotational
instability (MRI) in a Taylor-Couette flow is dramatically reduced if both
axial and azimuthal magnetic fields are imposed. In agreement with this
prediction, we present results of a Taylor-Couette experiment with the liquid
metal alloy GaInSn, showing evidence for the existence of the MRI at Reynolds
numbers of order 1000 and Hartmann numbers of order 10.",0606473v1
2006-12-22,Magnetized electron-positron plasmas,"Electrostatic oscillations in cold electron-positron plasmas can be coupled
to a propagating electromagnetic mode if the background magnetic field is
inhomogeneous. Previous work considered this coupling in the quasi-linear
regime, successfully simulating the electromagnetic mode. Here we present a
stability analysis of the non-linear problem, perturbed from dynamical
equilibrium, in order to gain some insight into the modes present in the
system.",0612679v1
1993-11-16,Quantum Phases of the Shraiman-Siggia Model,"We examine phases of the Shraiman-Siggia model of lightly-doped, square
lattice quantum antiferromagnets in a self-consistent, two-loop, interacting
magnon analysis. We find magnetically-ordered and quantum-disordered phases
both with and without incommensurate spin correlations. The quantum disordered
phases have a pseudo-gap in the spin excitation spectrum. The quantum
transition between the magnetically ordered and commensurate quantum-disordered
phases is argued to have the dynamic critical exponent $z=1$ and the same
leading critical behavior as the disordering transition in the pure $O(3)$
sigma model. The relationship to experiments on the doped cuprates is
discussed.",9311037v1
1994-03-03,Quantum Hall Fluid of Vortices in a Two Dimensional Array of Josephson Junctions,"A two dimensional array of Josephson junctions in a magnetic field is
considered. It is shown that the dynamics of the vortices in the array
resembles that of electrons on a two--dimensional lattice put in a magnetic
field perpendicular to the lattice. Under appropriate conditions, this
resemblance results in the formation of a quantum Hall fluid of vortices. The
bosonic nature of vortices and their long range logarithmic interaction make
some of the properties of the vortices' quantum Hall fluid different from those
of the electronic one. Some of these differences are studied in detail.
Finally, it is shown that a quantum Hall fluid of vortices manifests itself in
a quantized Hall electronic transport in the array.",9403017v1
1995-03-06,Quantum critical phenomena in ladders of Josephson junctions in a magnetic field,"A model of a ladder of Josephson junctions in a magnetic field is considered.
The topological features of the zero-temperature phase diagram, as a function
of charging energy and small deviations from commensurability of the vortex
lattice, are strongly dependent on the field. In addition to
superconductor-insulator transitions, commensurate-incommensurate transitions
and depinning by quantum fluctuations of the vortex-lattice are also possible.
the critical behavior of the superconductor-insulator transition at f = 1/2 is
found to depend on the ratio between interchain and intrachain Josephson
couplings and is in the universality of the classical XY-Ising model.",9503025v1
1995-05-04,"Berry phase, hyperorbits, and the Hofstadter spectrum","We develop a semiclassical theory for the dynamics of electrons in a magnetic
Bloch band, where the Berry phase plays an important role. This theory,
together with the Boltzmann equation, provides a framework for studying
transport problems in high magnetic fields. We also derive an Onsager-like
formula for the quantization of cyclotron orbits, and we find a connection
between the number of orbits and Hall conductivity. This connection is employed
to explain the clustering structure of the Hofstadter spectrum. The advantage
of this theory is its generality and conceptual simplicity.",9505021v1
1995-07-06,Phases of Josephson Junction Ladders,"We study a Josephson junction ladder in a magnetic field in the absence of
charging effects via a transfer matrix formalism. The eigenvalues of the
transfer matrix are found numerically, giving a determination of the different
phases of the ladder. The spatial periodicity of the ground state exhibits a
devil's staircase as a function of the magnetic flux filling factor $f$. If the
transverse Josephson coupling is varied a continuous superconducting-normal
transition in the transverse direction is observed, analogous to the breakdown
of the KAM trajectories in dynamical systems.",9507015v1
1995-10-15,Pair breaking by impurities in the two-dimensional t-J model,"Pair breaking mechanisms by impurities are investigated in the
two-dimensional t-J model by exact diagonalization techniques. Analysis of
binding energies, pairing correlations, dynamical spin and pair
susceptibilities shows that non-magnetic impurities are more effective in
suppressing pairing than magnetic ones in agreement with experimental studies
of Zn- and Ni- substituted High-Tc superconductors.",9510028v2
1995-10-18,Antiferromagnetic Excitations and Van Hove Singularities in YBa$_2$Cu$_3$O$_{6+x}$,"We show that in quasi-two-dimensional $d$-wave superconductors Van Hove
singularities close to the Fermi surface lead to novel magnetic quasi-particle
excitations. We calculate the temperature and doping dependence of dynamical
magnetic susceptibility for YBCO and show that the proposed excitations are in
agreement with inelastic neutron scattering experiments. In addition, the
values of the gap parameter and in-plane antiferromagnetic coupling are much
smaller than usually believed.",9510100v1
1995-11-03,Quantum Coherence in Magnetic Grains,"We consider a model of the dynamics of a magnetic grain, incorporating
interactions with nuclear and paramagnetic spins, conduction electrons, and
phonons. Decoherence comes both from the spins and the electrons, although
electron effects can be neglected if the substrate is insulating, and the grain
has spin $S \le 10^5$. A successful experiment will require both isotopic and
chemical purification, if coherence is to be seen; we suggest a ""spin echo""
arrangement to do this.",9511015v1
1996-03-28,Intrinsic mechanism of phase locking in two-dimensional Josephson junction networks in presence of an external magnetic field,"We present numerical simulations of the dynamics of two-dimensional Josephson
junction arrays to study the mechanism of mutual phase locking. We show that in
the presence of an external magnetic field two mechanisms are playing a role in
phase locking: feedback through the external load and internal coupling between
rows due to microwave currents induced by the field. We have found the
parameter values (junction capacitance, cell loop inductance, impedance of the
external load) for which the interplay of both these mechanisms leads to the
in-phase solution. The case of unshunted arrays is discussed as well.",9603183v1
1996-06-20,Low Temperature Magnetic Properties of the Double Exchange Model,"We study the {\it ferromagnetic} (FM) Kondo lattice model in the strong
coupling limit (double exchange (DE) model). The DE mechanism proposed by Zener
to explain ferromagnetism has unexpected properties when there is more than one
itinerant electron. We find that, in general, the many-body ground state of the
DE model is {\it not} globally FM ordered (except for special filled-shell
cases). Also, the low energy excitations of this model are distinct from spin
wave excitations in usual Heisenberg ferromagnets, which will result in unusual
dynamic magnetic properties.",9606148v1
1996-11-13,"Impact of Charge Ordering on Magnetic Correlations in Perovskite (Bi,Ca)MnO_3","Single crystalline (Bi,Ca)MnO3 (74< %Ca <82) were studied with neutron
scattering, electron diffraction and bulk magnetic measurement. We discovered
dynamic ferromagnetic spin correlations at high temperatures, which are
replaced by antiferromagnetic spin fluctuations at a concomitant charge
ordering and structural transition. Our results indicate that thermal-activated
hopping of the Jahn-Teller active e_g electrons in these insulating materials,
nevertheless, induce ferromagnetic interaction through double-exchange
mechanism. It is the ordering of these charges competing with the
double-exchange ferromagnetic metallic state.",9611110v1
1996-11-15,Magnetic Dynamics in Underdoped YBCO: Direct Observation of a Superconducting Gap,"Polarized and unpolarized triple-axis spectrometry has been used to study an
underdoped YBa_2Cu3O6.6. Our results indicate that spin excitations in the
superconducting state are essentially the same as those in the fully doped
material except that the unusual 41 meV resonance is observed at 34.8 meV. The
normal state spin excitations are characterized by a weakly energy-dependent
continuum whose temperature dependence shows the clear signature of a
superconducting gap at Tc. The enhancement at the resonance is accompanied by a
suppression of magnetic fluctuations at both higher and lower energies.",9611123v1
1997-01-23,Magnetic Excitations in the Spin-Peierls System $CuGeO_3$,"We have calculated the magnetic excitation spectrum in frustrated and
dimerized spin 1/2 Heisenberg chains for model parameters which describe the
thermodynamics and low frequency spin dynamics in the spin--Peierls system
$CuGeO_{3}$. As a test the chosen model is found to reproduce the lowest Raman
excitation energy near $30 cm^{-1}$ in the dimerized phase. We establish the
elementary triplet and singlet excitation branches below the continuum and
estimate the interchain coupling effects on the dimerization parameter.",9701176v1
1997-08-27,Semi-classical spectrum of integrable systems in a magnetic field,"The quantum dynamics of an electron in a uniform magnetic field is studied
for geometries corresponding to integrable cases. We obtain the uniform
asymptotic approximation of the WKB energies and wavefunctions for the
semi-infinite plane and the disc. These analytical solutions are shown to be in
excellent agreement with the numerical results obtained from the Schrodinger
equations even for the lowest energy states. The classically exact notions of
bulk and edge states are followed to their semi-classical limit, when the
uniform approximation provides the connection between bulk and edge.",9708206v1
1997-09-24,Supersymmetry and Theory of Heavy-Fermions,"We propose a new approach to the Kondo lattice in order to describe
simultaneously the Kondo effect and the local magnetism. This approach relies
on an original representation of the $S=1/2$ impurity spin in which the
different degrees of freedom are represented by fermionic as well as bosonic
variables. The magnetic instability is found at $J_c/D =0.67$ in definite
improvement compared to usual fermionic mean- field theories. The dynamical
susceptibility contains an inelastic peak in addition to the standard
Lindhard-type Fermi liquid contribution. The Fermi surface sum rule includes
$n_c+1$ states corresponding to large Fermi surfaces.",9709256v1
1997-11-06,Commensurability in One Dimension and the Josephson Junction Ladder,"We study a Josephson junction ladder in a magnetic field in the absence of
charging effects via a transfer matrix formalism. The eigenvalues of the
transfer matrix are found numerically, giving a determination of the different
phases of the ladder. The spatial periodicity of the ground state exhibits a
devil's staircase as a function of the magnetic flux filling factor f. If the
transverse Josephson coupling is varied a continuous superconducting-normal
transition in the transverse direction is observed, analogous to the breakdown
of the KAM trajectories in dynamical systems. We also examine how these
properties may be affected by a current injected along the ladder.",9711049v1
1997-12-29,Pseudogap and incommensurate magnetic fluctuations in YBa_2Cu_3O_{6. 6},"Unpolarized inelastic neutron scattering is used to study the temperature and
wave vector dependence of the dynamical magnetic susceptibility,
$\chi''(q,\omega)$, of a well characterized single crystal $YBa_2Cu_3O_{6.6}$
($T_c=62.7$ K). We find that a pseudogap opens in the spin fluctuation spectrum
at temperatures well above $T_c$. We speculate that the appearance of the low
frequency incommensurate fluctuations is associated with the opening of the
pseudogap. To within the error of the measurements, a gap in the spin
fluctuation spectrum is found in the superconducting state.",9712311v1
1998-02-23,Magnetic Structure and NMR signal of Spin Peierls Solitons,"We compute the magnetic profile of spin Peierls solitons in a simple
Heisenberg model with magneto elastic couplings, using independently the DMRG
method and the Hartree Fock approximation. Our results agree qualitatively with
published NMR data provided we average over the spin profiles. We conclude that
he dynamics of the spin plus lattice system must be included in a more detailed
theory of the spin Peierls transition in CuGeO.",9802245v1
1998-06-05,Aharonov-Bohm cages in two-dimensional structures,"We present an extreme localization mechanism induced by a magnetic field for
tight-binding electrons in two-dimensional structures. This spectacular
phenomenon is investigated for a large class of tilings (periodic,
quasiperiodic, or random). We are led to introduce the Aharonov-Bohm cages
defined as the set of sites eventually visited by a wavepacket that can, for
particular values of the magnetic flux, be bounded. We finally discuss the
quantum dynamics which exhibits an original pulsating behaviour.",9806068v1
1998-09-27,Magnetic Incommensurability in Doped Mott Insulator,"In this paper we explore the incommensurate spatial modulation of spin-spin
correlations as the intrinsic property of the doped Mott insulator, described
by the $t-J$ model. We show that such an incommensurability is a direct
manifestation of the phase string effect introduced by doped holes in both one-
and two-dimensional cases. The magnetic incommensurate peaks of dynamic spin
susceptibility in momentum space are in agreement with the neutron-scattering
measurement of cuprate superconductors in both position and doping dependence.
In particular, this incommensurate structure can naturally reconcile the
neutron-scattering and NMR experiments of cuprates.",9809363v1
1998-10-26,"Quantum Environments: Spin Baths, Oscillator Baths, and applications to Quantum Magnetism","The low-energy physics of systems coupled to their surroundings is understood
by truncating to effective Hamiltonians; these tend to reduce to a few
canonical forms, involving coupling to ""baths"" of oscillators or spins. The
method for doing this is demonstrated using examples from magnetism,
superconductivity, and measurement theory, as is the way one then solves for
the low-energy dynamics. Finally, detailed application is given to the exciting
recent Quantum relaxation and tunneling work in naomagnets.",9810353v1
1998-11-26,Supersymmetric Approach to Heavy-Fermion Systems,"We present a new supersymmetric approach to the Kondo lattice model in order
to describe simultaneously the quasiparticle excitations and the low-energy
magnetic fluctuations in heavy-Fermion systems. This approach mixes the
fermionic and the bosonic representation of the spin following the standard
rules of superalgebra. Our results show the formation of a bosonic band within
the hybridization gap reflecting the spin collective modes. The density of
states at the Fermi level is strongly renormalized while the Fermi surface sum
rule includes $n_{c}+1$ states. The dynamical susceptibility is made of a Fermi
liquid superimposed on a localized magnetism contribution.",9811380v1
1998-12-21,Lifetimes of impurity states in crossed magnetic and electric fields,"We study the quantum dynamics of localized impurity states created by a point
interaction for an electron moving in two dimensions under the influence of a
perpendicular magnetic field and an in-plane weak electric field. All impurity
states are unstable in presence of the electric field. Their lifetimes are
computed and shown to grow in a Gaussian way as the electric field tends to
zero.",9812340v1
1999-04-17,Dynamical response of a weakly pinned two-dimensional Wigner crystal,"Motivated by puzzling results of recent experiments, we re-examine the
response of a weakly pinned two-dimensional Wigner crystal to a uniform AC
electric field. We confirm that at some disorder and magnetic field dependent
frequency $\omega_p$, an inhomogeneously broadened absorption line emerges.
Although the line is conventionally broad in zero magnetic field, in strong
fields it appears as a sharp resonance whose width is related to the density of
states in the low-frequency tail of the zero-field phonon spectrum. This
behavior originates due to the long-range Coulomb interactions.",9904245v2
1999-04-20,Random quantum magnets with long-range correlated disorder: Enhancement of critical and Griffiths-McCoy singularities,"We study the effect of spatial correlations in the quenched disorder on
random quantum magnets at and near a quantum critical point. In the random
transverse field Ising systems disorder correlations that decay algebraically
with an exponent rho change the universality class of the transition for small
enough rho and the off-critical Griffiths-McCoy singularities are enhanced. We
present exact results for 1d utilizing a mapping to fractional Brownian motion
and generalize the predictions for the critical exponents and the generalized
dynamical exponent in the Griffiths phase to d>=2.",9904280v2
1999-04-20,Interchain interactions and magnetic properties of Li2CuO2,"An effective Hamiltonian is constructed for an insulating cuprate with
edge-sharing chains Li2CuO2.The Hamiltonian contains the nearest and
next-nearest neighboring intrachain and zigzag-type interchain interactions.The
values of the interactions are obtained from the analysis of the magnetic
susceptibility, and this system is found to be described as coupled frustrated
chains.We calculate the dynamical spin correlation function S(q,\omega) by
using the exact diagonalization method, and show that the spectra of
S(q,\omega) are characterized by the zigzag-type interchain interactions. The
results of the recent inelastic neutron scattering experiment are discussed in
the light of the calculated spectra.",9904283v2
1999-04-23,Evidence for incommensurate spin fluctuations in Sr_2RuO_4,"We report first inelastic neutron scattering measurements in the normal state
of Sr_2RuO_4 that reveal the existence of incommensurate magnetic spin
fluctuations located at ${\bf q}_0=(\pm 0.6\pi/a, \pm 0.6\pi/a, 0)$. This
finding confirms recent band structure calculations that have predicted
incommensurate magnetic responses related to dynamical nesting properties of
its Fermi surface.",9904348v1
1999-06-03,Microwave Absorption Peaks : Signatures of Spin Dynamics in Cuprates,"We show that a common feature of temperature-dependent microwave absorption
is the presence of absorption peaks. $ac$ loss peaks can arise when the
internal $T$-dependent magnetic relaxation time crosses the measurement
frequency. These features are observed in the \QTR{em}{insulating}
($Sr_{x}Ca_{14-x}Cu_{24}O_{41}$, $La_{5/3}Sr_{1/3}NiO_{4}$ and
$YBa_{2}Cu_{3}O_{6.0}$), \QTR{em}{pseudo-gap} ($T>T_{c}$ in underdoped
$YBa_{2}Cu_{3}O_{7-\delta}$, $Hg:1223$ and $Hg:1201$) and
\QTR{em}{superconducting} ($T<T_{c}$) states of the oxides. The commonality of
these features suggests a magnetic (spin) mechanism, rather than a
quasiparticle origin, for the so-called ``conductivity'' peaks observed in the
cuprate superconductors.",9906053v2
1999-06-04,Driving-Induced Symmetry Breaking in the Spin-Boson System,"A symmetric dissipative two-state system is asymptotically completely
delocalized independent of the initial state. We show that driving-induced
localization at long times can take place when both the bias and tunneling
coupling energy are harmonically modulated. Dynamical symmetry breaking on
average occurs when the driving frequencies are odd multiples of some reference
frequency. This effect is universal, as it is independent of the dissipative
mechanism. Possible candidates for an experimental observation are flux
tunneling in the variable barrier rf SQUID and magnetization tunneling in
magnetic molecular clusters.",9906062v1
1999-10-05,Local Non-Fermi Liquid Theory of Magnetic Impurity Effects in Carbon Nanotubes,"Magnetic impurity effects in carbon nanotubes are studied theoretically. The
multi channel Kondo effect is investigated with the band structure of the
metallic nanotubes. The local non-Fermi liquid behavior is realized at
temperatures lower than the Kondo temperature T_K. The density of states of
localized electron has a singularity |omega|^1/2 which gives rise to a pseudo
gap at the Kondo resonance in low temperatures. The temperature dependence of
the electronic resistivity is predicted as T^1/2, and the imaginary part of
dynamical susceptibilities has the |omega|^1/2 dependence.",9910053v1
2000-03-23,Bifurcations and a chaos strip in states of long Josephson junctions,"Stationary and nonstationary, in particular, chaotic states in long Josephson
junctions are investigated. Bifurcation lines on the parametric bias
current-external magnetic field plane are calculated. The chaos strip along the
bifurcation line is observed. It is shown that transitions between stationary
states are the transitions from metastable to stable states and that the
thermodynamical Gibbs potential of these stable states may be larger than for
some metastable states. The definition of a dynamical critical magnetic field
characterizing the stability of the stationary",0003375v1
2000-05-03,Macroscopic resonant tunneling of magnetic flux,"We have developed a quantitative theory of resonant tunneling of magnetic
flux between discrete macroscopically distinct quantum states in SQUID systems.
The theory is based on the standard density-matrix approach. Its new elements
include the discussion of the two different relaxation mechanisms that exist
for the double-well potential, and description of the ``photon-assisted''
tunneling driven by external rf radiation. It is shown that in the case of
coherent flux dynamics, rf radiation should lead to splitting of the peaks of
resonant flux tunneling, indicating that the resonant tunneling is a convenient
tool for studying macroscopic quantum coherence of flux.",0005081v1
2000-05-12,Interaction induced delocalisation for two particles in a periodic potential,"We consider two interacting particles evolving in a one-dimensional periodic
structure embedded in a magnetic field. We show that the strong localization
induced by the magnetic field for particular values of the flux per unit cell
is destroyed as soon as the particles interact. We study the spectral and the
dynamical aspects of this transition.",0005215v2
2000-06-01,Yang-Lee and Fisher Zeros of Multisite Interaction Ising Models on the Cayley-type Lattices,"A general analytical formula for recurrence relations of multisite
interaction Ising models in an external magnetic field on the Cayley-type
lattices is derived. Using the theory of complex analytical dynamics on the
Riemann sphere, a numerical algorithm to obtain Yang-Lee and Fisher zeros of
the models is developed. It is shown that the sets of Yang-Lee and Fisher zeros
are almost always fractals, that could be associated with Mandelbrot-like sets
on the complex magnetic field and temperature planes respectively.",0006014v2
2000-06-13,Theory of Transition Temperature of Magnetic Double Perovskites,"We formulate a theory of double perovskite coumpounds such as Sr$_2$FeReO$_6$
and Sr$_2$FeMoO$_6$ which have attracted recent attention for their possible
uses as spin valves and sources of spin polarized electrons. We solve the
theory in the dynamical mean field approximation to find the magnetic
transition temperature $T_c$. We find that $T_c$ is determined by a subtle
interplay between carrier density and the Fe-Mo/Re site energy difference, and
that the non-Fe same-sublattice hopping acts to reduce $T_c$. Our results
suggest that presently existing materials do not optimize $T_c$.",0006208v1
2000-07-04,Spin dynamics and ordering of a cuprate stripe-antiferromagnet,"In La1.48Nd0.4Sr0.12CuO4 the 139La and 63Cu NQR relaxation rates and signal
wipe-out upon lowering temperature are shown to be due to purely magnetic
fluctuations. They follow the same renormalized classical behavior as seen in
neutron data, when the electronic spins order in stripes, with a small spread
in spin stiffness (15% spread in activation energy). The La signal, which
reappears at low temperatures, is magnetically broadened and experiences
additional wipe-out due to slowing down of the Nd fluctuations.",0007057v2
2000-09-08,Incipient nodal pairing in planar d-wave superconductors,"The possibility of a second pairing transition $d\to d+is$ ($d+id^\prime$) in
planar $d$-wave superconductors which occurs in the absence of external
magnetic field, magnetic impurities or boundaries is established in the
framework of the non-perturbative phenomenon of dynamical chiral symmetry
breaking in the system of $2+1$-dimensional Dirac-like nodal quasiparticles. We
determine the critical exponents and quasiparticle spectral functions that
characterize the corresponding quantum critical behavior and discuss some of
its potentially observable spectral and transport features.",0009117v3
2000-09-30,Hysteretic depinning and dynamical melting for magnetically interacting vortices in disordered layered superconductors,"We examine the depinning transitions and the temperature versus driving force
phase diagram for magnetically interacting pancake vortices in layered
superconductors. For strong disorder the initial depinning is plastic followed
by a sharp hysteretic transition to a 3D ordered state for increasing driving
force. Our results are in good agreement with theoretical predictions for
driven anisotropic charge density wave systems. We also show that a temperature
induced peak effect in the critical current occurs due to the onset of
plasticity between the layers.",0010002v1
2000-12-24,Microcantilever Studies of Angular Field Dependence of Vortex Dynamics in BSCCO,"Using a nanogram-sized single crystal of BSCCO attached to a microcantilever
we demonstrate in a direct way that in magnetic fields nearly parallel to the
{\it ab} plane the magnetic field penetrates the sample in the form of
Josephson vortices rather than in the form of a tilted vortex lattice. We
further investigate the relation between the Josephson vortices and the pancake
vortices generated by the perpendicular field component.",0012453v1
2001-01-10,Mechanical effects in quantum dots in magnetic and electric fields,"The mechanical effects in finite two-dimensional electron systems (quantum
dots or droplets) in a strong perpendicular magnetic field are studied. It is
shown that, due to asymmetry of the cyclotron dynamics, an additional in-plane
electric field causes a ground state transition accompanied by a change in the
average total angular momentum of the system, unless the lateral confining
potential is exactly parabolic. A precise mechanical experiment is proposed in
which a macroscopic angular momentum of a dense matrix of quantum dots could be
measured and used to detect and estimate anharmonicity of the confinement.",0101133v1
2001-01-11,Theoretical and Experimental Approach to Spin Dynamics in Thin Magnetic Films,"The Landau-Lifshitz (L-L) equation describing the time dependence of the
magnetisation vector is numerically integrated fully without any simplifying
assumptions in the time domain and the magnetisation time series obtained is
Fourier transformed (FFT) to yield the permeability spectrum up to 10 GHz. The
non linear results are compared to the experimental results obtained on
magnetic amorphous thin films of Co-Zr, Co-Zr-Re. We analyse our results with
the frequency response obtained directly from the Landau-Lifshitz equation as
well as with the second order Gilbert frequency response.",0101154v1
2001-01-18,C-axis Optical Sum Rule in Josephson Coupled Vortex State,"Observed violations of the $c$-axis optical sum rule can give important
information on deviations from in-plane Fermi liquid behavior and on the nature
of interlayer coupling between adjacent copper oxide planes. Application of a
magnetic field perpendicular to these planes is another way to probe in-plane
dynamics. We find that the optical sum rule is considerably modified in the
presence of the $c$-axis magnetic field. Interlayer correlation of pancake
vortices is involved in the sum rule modification; however, details of the
vortex distribution in the plane are less important.",0101288v2
2001-02-13,Nuclear spin driven quantum relaxation in LiY_0.998Ho_0.002F_4,"Staircase hysteresis loops of the magnetization of a LiY_0.998Ho_0.002F_4
single crystal are observed at subkelvin temperatures and low field sweep
rates. This behavior results from quantum dynamics at avoided level crossings
of the energy spectrum of single Ho^{3+} ions in the presence of hyperfine
interactions. Enhanced quantum relaxation in constant transverse fields allows
the study of the relative magnitude of tunnel splittings. At faster sweep
rates, non-equilibrated spin-phonon and spin-spin transitions, mediated by weak
dipolar interactions, lead to magnetization oscillations and additional steps.",0102231v2
2001-03-16,Perfectly Translating Lattices on a Cylinder,"We perform molecular dynamics simulations on an interacting electron gas
confined to a cylindrical surface and subject to a radial magnetic field and
the field of the positive background. In order to study the system at lowest
energy states that still carry a current, initial configurations are obtained
by a special quenching procedure. We observe the formation of a steady state in
which the entire electron-lattice cycles with a common uniform velocity.
Certain runs show an intermediate instability leading to lattice
rearrangements. A Hall resistance can be defined and depends linearly on the
magnetic field with an anomalous coefficient reflecting the manybody
contributions peculiar to two dimensions.",0103353v1
2001-05-26,Time-resolved ferromagnetic resonance in epitaxial Fe1-xCox films,"Magnetodynamics in epitaxial Fe1-xCox films on GaAs (100) are studied using
time-resolved ferromagnetic resonance, in which the free precession of the
magnetization after an impulsive excitation is measured using the polar Kerr
effect. The sample is rotated with respect to the static and pulsed field
directions, providing a complete mapping of the free energy surface and
characteristic relaxation times. The magnetic response can be simulated with a
simple coherent rotation model except in the immediate vicinity of switching
fields. Bulk and surface anisotropies are identified, and unusual dynamics
associated with the coexistence of cubic and uniaxial anisotropies are
observed.",0105513v1
2001-06-06,On the power spectrum of magnetization noise,"Understanding the power spectrum of the magnetization noise is a long
standing problem. While earlier work considered superposition of 'elementary'
jumps, without reference to the underlying physics, recent approaches relate
the properties of the noise with the critical dynamics of domain walls. In
particular, a new derivation of the power spectrum exponent has been proposed
for the random-field Ising model. We apply this approach to experimental data,
showing its validity and limitations.",0106113v1
2001-08-23,Critical Behaviour of Non-Equilibrium Phase Transitions to Magnetically Ordered States,"We describe non-equilibrium phase transitions in arrays of dynamical systems
with cubic nonlinearity driven by multiplicative Gaussian white noise.
Depending on the sign of the spatial coupling we observe transitions to
ferromagnetic or antiferromagnetic ordered states. We discuss the phase
diagram, the order of the transitions, and the critical behaviour. For global
coupling we show analytically that the critical exponent of the magnetization
exhibits a transition from the value 1/2 to a non-universal behaviour depending
on the ratio of noise strength to the magnitude of the spatial coupling.",0108379v1
2001-09-21,Spin waves in a Bose-condensed atomic spin chain,"The spin dynamics of atomic Bose-Einstein condensates confined in a
one-dimensional optical lattice is studied. The condensates at each lattice
site behave like spin magnets that can interact with each other through both
the light-induced dipole-dipole interaction and the static magnetic
dipole-dipole interaction. We show how these site-to-site dipolar interactions
can distort the ground state spin orientations and lead to the excitation of
spin waves. The dispersion relation of the spin waves is studied and possible
detection schemes are proposed.",0109415v1
2001-10-07,Anomalous Spin Excitation Spectrum of the Heisenberg Model in a Magnetic Field,"Making the assumption that high energy fermions exist in the two dimensional
spin-1/2 Heisenberg antiferromagnet we present predictions based on the pi-flux
ansatz for the dynamic structure factor when the antiferromagnet is subject to
a uniform magnetic field. The main result is the presence of gapped excitations
in a momentum region near (pi,pi) with energy lower than that at (pi,pi). This
is qualitatively different from spin wave theory predictions and may be tested
by experiments or by quantum Monte Carlo.",0110133v2
2001-12-17,Vortices in superconducting strips: interplay between surface effects and the pinning landscape,"Vortices in a narrow superconducting strip with a square array of pinning
sites are studied. The interactions of vortices with other vortices and with
external sources (applied magnetic field and transport current) are calculated
via a screened Coulomb model. The edge barrier is taken into account and shown
to have an important role on the system dynamics. Numerical simulations in this
approach show that the field dependent magnetic moment presents peaks
corresponding to history dependent configurations of the vortex lattice. Some
effects of the edge barrier on the I-V characteristics are also reported.",0112313v1
2002-01-02,Level Spectroscopy: Physical Meaning and Application to the Magnetization Plateau Problems,"We review the level spectroscopy, which is a powerful method of analyzing the
numerical data with respect to the Berezinskii-Kosterlitz-Thouless quantum
phase transition in one dimension. We focus on its physical meaning and also
its application to the magnetization plateau problems.",0201013v2
2002-01-10,Magnetism and Phase Separation in the Ground State of the Hubbard Model,"We discuss the ground state magnetic phase diagram of the Hubbard model off
half filling within the dynamical mean-field theory. The effective
single-impurity Anderson model is solved by Wilson's numerical renormalization
group calculations, adapted to symmetry broken phases. We find a phase
separated, antiferromagnetic state up to a critical doping for small and
intermediate values of U, but could not stabilise a Neel state for large U and
finite doping. At very large U, the phase diagram exhibits an island with a
ferromagnetic ground state. Spectral properties in the ordered phases are
discussed.",0201145v1
2002-02-10,Response functions of gapped spin systems in high magnetic field,"We study the dynamical structure factor of gapped one-dimensional spin
systems in the critical phase in high magnetic field. It is shown that the
presence of a ``condensate'' in the ground state in the high-field phase leads
to interesting signatures in the response functions.",0202171v1
2002-03-01,Two-dimensional electron liquid with disorder in a weak magnetic field,"We present the effective theory for low energy dynamics of a two-dimensional
interacting electrons in the presence of a weak short-range disorder and a weak
perpendicular magnetic field, the filling factor $\nu \gg 1$. We investigate
the exchange enhancement of the $g$-factor, the effective mass and the decay
rate of the simplest spin wave excitations at $\nu = 2 N + 1$. We obtain the
enhancement of the field-induced gap in the tunneling density of states and
dependence of the tunneling conductivity on the applied bias.",0203022v2
2002-04-25,Electronic structure and magnetic properties of correlated metals: A local self-consistent perturbation scheme,"In the framework of ab initio dynamical mean field theory for realistic
electronic structure calculations a new perturbation scheme which combine the
T-matrix and fluctuating exchange approximations has been proposed. This method
is less computationally expensive than numerically exact quantum Monte Carlo
technics and give an adequate description of the electronic structure and
exchange interactions for magnetic metals. We discuss a simple expression for
the exchange interactions corresponding to the neglecting of the vertex
corrections which becomes exact for the spin-wave stiffness in the local
approximation. Electronic structure, correlation effects and exchange
interactions for ferromagnetic nickel have been discussed.",0204564v1
2002-07-22,Thickness dependence of critical currents in thin superconductors,"Mechanisms of vortex dynamics and pinning and self-field effects which could
account for the thickness dependence of the critical current density J_c of
superconducting films are addressed. It is shown that at low magnetic fields B,
the 2D single-vortex pinning and thermal fluctuations yield J_c proportional to
(1/sqrt{d})exp[-(d_m/d)^{1/2}], if the film thickness d is smaller than the
pinning correlation length along B. The model describes the dependence of J_c
on d observed on YBa_2Cu_3O_7 coated conductors for d < 2(mu)m. Measurements of
J_c(d) in strong magnetic fields are proposed to probe whether the dependence
of J_c on d is mostly determined by the 2D pinning or changes in the material
microstructure as the film gets thicker.",0207526v1
2002-08-16,Supported magnetic nanoclusters: Softlanding of Pd clusters on a MgO surface,"Low-energy deposition of neutral Pd_N clusters (N=2-7 and 13) on a MgO(001)
surface F-center (FC) was studied by spin-density-functional molecular dynamics
simulations. The incident clusters are steered by an attractive ""funnel""
created by the FC, resulting in adsorption of the cluster, with one of its
atoms bonded atop of the FC. The deposited Pd_2-Pd_6 clusters retain their
gas-phase structures, while for N>6 surface-commensurate isomers are
energetically more favorable. Adsorbed clusters with N > 3 are found to remain
magnetic at the surface.",0208321v1
2002-09-17,Magnetic Order and Dynamics in an Orbitally Degenerate Ferromagnetic Insulator,"Neutron scattering was used to determine the spin structure and the magnon
spectrum of the Mott--Hubbard insulator YTiO$_3$. The magnetic structure is
complex, comprising substantial G-type and A-type antiferromagnetic components
in addition to the predominant ferromagnetic component. The magnon spectrum, on
the other hand, is gapless and nearly isotropic. We show that these findings
are inconsistent with the orbitally ordered states thus far proposed for
YTiO$_3$ and discuss general implications for a theoretical description of
exchange interactions in orbitally degenerate systems.",0209400v1
2002-12-11,Spectrum and correlation functions of a quasi-one-dimensional quantum Ising model,"We consider a model of weakly coupled quantum Ising chains. We describe the
phase diagram of such a model and study the dynamical magnetic susceptibility
by means of Bethe ansatz and the Random Phase Approximation applied to the
inter-chain exchange. We argue that some of the beautiful physics of the
quantum Ising chain in a magnetic field survives in the ordered state of the
quasi-one-dimensional model and can be observed experimentally by means of
neutron scattering.",0212248v2
2003-01-07,Subharmonics and Aperiodicity in Hysteresis Loops,"We show that it is possible to have hysteretic behavior for magnets that does
not form simple closed loops in steady state, but must cycle multiple times
before returning to its initial state. We show this by studying the
zero-temperature dynamics of the 3d Edwards Anderson spin glass. The specific
multiple varies from system to system and is often quite large and increases
with system size. The last result suggests that the magnetization could be
aperiodic in the large system limit for some realizations of randomness. It
should be possible to observe this phenomena in low-temperature experiments.",0301083v1
2003-01-13,Spin freezing and magnetic inhomogeneities in bilayer manganites,"We have performed a muon spin rotation study on polycrystalline samples of
electron-doped layered manganites, La$_{2-2x}$Sr$_{1+2x}$Mn$_2$O$_7$ (0.4<
x<1), in order to investigate the local magnetic structure and spin dynamics.
Our results provide evidence for phase separation into A-type antiferromagnetic
and charge-ordered phases for x=0.52 and spin freezing at low temperatures
(T<100K) for 0.52<x<0.75. A new phase diagram which includes this spin freezing
region is proposed.",0301177v1
2003-01-21,Hole-Doping Effects on a Two-dimensional Kondo Insulator,"We study the effects of hole doping on the two-dimensional Heisenberg-Kondo
model around the quantum critical point, where the spin liquid phase (Kondo
insulator) and the magnetically ordered phase are separated via a second-order
phase transition. By means of the self-consistent Born approximation within the
bond operator formalism as well as the standard spin wave theory, we discuss
dynamical properties of a doped hole. It is clarified that a quasi-particle
state stabilized in the spin liquid phase is gradually obscured as the system
approaches the quantum critical point. This is also the case for the
magnetically ordered phase. We argue the similarity and the difference between
these two cases.",0301401v1
2003-03-26,Chern Numbers for Spin Models of Transition Metal Nanomagnets,"We argue that ferromagnetic transition metal nanoparticles with fewer than
approximately 100 atoms can be described by an effective Hamiltonian with a
single giant spin degree of freedom. The total spin $S$ of the effective
Hamiltonian is specified by a Berry curvature Chern number that characterizes
the topologically non-trivial dependence of a nanoparticle's many-electron
wavefunction on magnetization orientation. The Berry curvatures and associated
Chern numbers have a complex dependence on spin-orbit coupling in the
nanoparticle and influence the semiclassical Landau-Liftshitz equations that
describe magnetization orientation dynamics.",0303555v1
2003-04-02,Magnetic orderings in Al nanowires suspended between electrodes,"A theoretical analysis of a relation between atomic and spin-electronic
structures for the ground state of single-row aluminum nanowires suspended
between Al(001) electrodes is demonstrated using first-principles
molecular-dynamics simulations. We obtain a unusual result that a
3-aluminum-atom nanowire sandwiched between the electrodes does not manifest
magnetic ordering although an isolated aluminum trimer molecule in a straight
line is spin-polarized. On the other hand, a 5-atom nanowire exhibits
ferromagnetic ordering, where three central atoms form a spin-polarized trimer.
Moreover, in the case of an 8-atom nanowire, the middle atoms in the nanowire
form two spin-polarized trimers with antiferromagnetic ordering.",0304060v1
2003-04-10,Dynamical Structure Factor in Cu Benzoate and other spin-1/2 antiferromagnetic chains,"Recent experiments of the quasi-one-dimensional spin-1/2 antiferromagnet
Copper Benzoate established the existence of a magnetic field induced gap. The
observed neutron scattering intensity exhibits resolution limited peaks at both
the antiferromagnetic wave number and at incommensurate wave numbers related to
the applied magnetic field. We determine the ratio of spectral weights of these
peaks within the framework of a low-energy effective field theory description
of the problem.",0304244v1
2003-04-11,Photon-assisted tunneling in a Fe8 Single-Molecule Magnet,"The low temperature spin dynamics of a Fe8 Single-Molecule Magnet was studied
under circularly polarized electromagnetic radiation allowing us to establish
clearly photon-assisted tunneling. This effect, while linear at low power,
becomes highly non-linear above a relatively low power threshold. This
non-linearity is attributed to the nature of the coupling of the sample to the
thermostat.These results are of great importance if such systems are to be used
as quantum computers.",0304274v1
2003-06-20,Quantum dynamics of tunneling between ferromagnets,"We study the Josephson-like spin currents between two ferromagnetic metals by
deriving the effective action of the junction. A DC spin Josephson current with
the full O(3) symmetry is obtained. By the analogy to the superconducting
junctions, we have shown that a time-independent uniform magnetic field can
serve as the source of the AC spin Josephson effect. That is, the spin current
becomes a periodic function of the time with the period proportional to the
inverse of the magnitude of the external magnetic field.",0306518v2
2003-07-18,An extension of the Kac ring model,"We introduce a unitary dynamics for quantum spins which is an extension of a
model introduced by Mark Kac to clarify the phenomenon of relaxation to
equilibrium. When the number of spins gets very large, the magnetization
satisfies an autonomous equation as function of time with exponentially fast
relaxation to the equilibrium magnetization as determined by the microcanonical
ensemble. This is proven as a law of large numbers with respect to a class of
initial data. The corresponding Gibbs-von Neumann entropy is also computed and
its monotonicity in time discussed.",0307460v3
2003-08-23,Theory of Current-Driven Domain Wall Motion: A Poorman's Approach,"A self-contained theory of the domain wall dynamics in ferromagnets under
finite electric current is presented.
  The current is shown to have two effects; one is momentum transfer, which is
proportional to the charge current and wall resistivity ($\rhow$), and the
other is spin transfer, proportional to spin current.
  For thick walls, as in metallic wires, the latter dominates and the threshold
current for wall motion is determined by the hard-axis magnetic anisotropy,
except for the case of very strong pinning.
  For thin walls, as in nanocontacts and magnetic semiconductors, the
momentum-transfer effect dominates, and the threshold current is proportional
to $\Vz/\rhow$, $\Vz$ being the pinning potential.",0308464v1
2003-08-24,Nuclear spin qubits in a pseudo-spin quantum chain,"We analyze a quantum computer (QC) design based on nuclear spin qubits in a
quasi-one-dimensional (1D) chain of non-Kramers doublet atoms. We explore the
use of spatial symmetry breaking to obtain control over the local dynamics of a
qubit. We also study the decoherence mechanisms at the single qubit level and
the interactions mediated by the magnetic media. The design can be realized in
$\textrm{PrBr}_{\textrm{3}-x}\textrm{F}_{x}$ with nuclear magnetic resonance
(NMR) techniques.",0308475v2
2003-09-08,Interplay between Freezing and Superconductivity in the Optimally Doped LaEu0.20Sr0.15CuO4 under Hydrostatic Pressure,"We study the electronic properties of a LaEu0.20Sr0.15CuO4 single crystal
under hydrostatic pressure up to 2.9 GPa. Both the freezing of the Cu 3d
moments and the structural transition from the orthorhombic (LTO) to the
tetragonal (LTT) phase are observed via the relaxation of the nuclear
magnetization of La nuclei. Resistivity and magnetic susceptibility
measurements have been carried out under pressure on the same sample. The
combination of all data reveals the connection between glassy dynamics, charge
localization and the disappearance of superconductivity in the LTT phase.",0309193v1
2003-10-24,Spin-Charge Separation in Two-dimensional Frustrated Quantum Magnets,"The dynamics of a mobile hole in two-dimensional frustrated quantum magnets
is investigated by exact diagonalization techniques. Our results provide
evidence for spin-charge separation upon doping the kagome lattice, a prototype
of a spin liquid. In contrast, in the checkerboard lattice, a symmetry broken
Valence Bond Crystal, a small quasi-particle peak is seen for some crystal
momenta, a finding interpreted as a restoration of weak holon-spinon
confinement.",0310597v2
2003-10-30,Distribution of exchange energy in a bond-alternating S=1 quantum spin chain,"The quasi-one-dimensional bond-alternating S=1 quantum antiferromagnet NTENP
is studied by single crystal inelastic neutron scattering. Parameters of the
measured dispersion relation for magnetic excitations are compared to existing
numerical results and used to determine the magnitude of bond-strength
alternation. The measured neutron scattering intensities are also analyzed
using the 1st-moment sum rules for the magnetic dynamic structure factor, to
directly determine the modulation of ground state exchange energies. These
independently determined modulation parameters characterize the level of spin
dimerization in NTENP. First-principle DMRG calculations are used to study the
relation between these two quantities.",0310741v1
2003-12-16,Interplay between the magnetic fluctuations and superconductivity in the lanthanum cuprates,"We report the analysis of the magnetic fluctuations in the superconducing
La_{2-x}Sr_xCuO_4 and the related lanthanum cuprates having the different
symmetry of the low temperature structure. The NMR and ESR investigations
revealed the dynamical coexistence of the superconductivity and the
antiferromagnetic correlations in the large part of superconductivity region of
the phase diagram. We show that for all compounds, independent on their low
temperature symmetry and on their superconducting properties, the enhancement
of the spin stiffness near 1/8 doping takes place.",0312393v1
2003-12-24,Signature of entangled eigenstates in the magnetic response of two coupled flux qubits,"We study dissipative dynamics and a magnetic response of two coupled flux
qubits interacting with a high quality tank in the framework of the impedance
measurement technique (IMT). It is shown that the observation of the difference
between a sum of IMT signals from separated qubits and the signal from the
system when both qubits are in the degeneracy point (IMT deficit) implies
immediately a formation of the entangled two-qubit eigenstates.",0312635v1
2004-01-12,Dynamical localization for Bloch electrons in Magnetic and Electric Fields,"We study the time evolution of wave packets of noninteracting electrons in a
two-dimensional periodic system in the presence of magnetic and electric
fields. The model includes consistently the coupling between Landau levels as
well as the periodic and external field contributions. It is shown that the
electric field produces localization along its longitudinal direction, this
effect is a physical consequence of the quantization of the energy levels with
respect to the longitudinal quasi-momentum. For incommensurate orientations of
$\mathbf{E}$ relative to the lattice the wave packet becomes localized in both
directions.",0401198v1
2004-01-31,Resonant magnetic mode in superconducting 2-leg ladders,"The spin dynamics of a doped 2-leg spin ladder is investigated by numerical
techniques. We show that a hole pair-magnon boundstate evolves at finite hole
doping into a sharp magnetic excitation below the two-particle continuum. This
is supported by a field theory argument based on a SO(6)-symmetric ladder.
Similarities and differences with the resonant mode of the high-T$_c$ cuprates
are discussed.",0402011v1
2004-02-09,Phase diagram and critical exponents of a dissipative Ising spin chain in a transverse magnetic field,"We consider a one-dimensional Ising model in a transverse magnetic field
coupled to a dissipative heat bath. The phase diagram and the critical
exponents are determined from extensive Monte Carlo simulations. It is shown
that the character of the quantum phase transition is radically altered from
the corresponding non-dissipative model and the double-well coupled to a
dissipative heat bath with linear friction. Spatial couplings and the
dissipative dynamics combine to form a new quantum criticality.",0402224v2
2004-02-20,Low-frequency incommensurate magnetic response in strongly correlated systems,"It is shown that in the t-J model of Cu-O planes at low frequencies the
dynamic spin structure factor is peaked at incommensurate wave vectors
(1/2+-delta,1/2)$, (1/2,1/2+-delta). The incommensurability is connected with
the momentum dependencies of the magnon frequency and damping near the
antiferromagnetic wave vector. The behavior of the incommensurate peaks is
similar to that observed in La_{2-x}(Ba,Sr)_xCuO_{4+y} and YBa_2Cu_3O_{7-y}:
for hole concentrations 0.02<x<=0.12 we find that delta is nearly proportional
to x, while for x>0.12 it tends to saturation. The incommensurability
disappears with increasing temperature. Generally the incommensurate magnetic
response is not accompanied by an inhomogeneity of the carrier density.",0402512v1
2004-03-03,Off-equilibrium fluctuation-dissipation relation in a spin glass,"We report new experimental results obtained onthe insulating spin glass
CdCrInS_4. Our experimental setup allows a quantitative comparison between
thethermo-remanent magnetization and the autocorrelation of spontaneous
fluctuations of magnetization, yielding acomplete determination of the
fluctuation-dissipationrelation. The dynamics can be studied both in
thequasi-equilibrium regime, where the fluctuation-dissipation theorem holds,
and in the deeply aging regime. The limit of separation of time-scales, as used
in analytical calculations, can be approached by use of a scaling procedure.",0403112v2
2004-04-04,Nonequilibrium steady states of the isotropic classical magnet,"We drive a d-dimensional Heisenberg magnet using a spatially anisotropic
current of mobile particles or heat. The continuum Langevin equation is
analyzed using a dynamical renormalization group, stability analysis and
numerical simulations. We discover a rich steady-state phase diagram, including
a critical point in a new nonequilibrium universality class, and a
spatiotemporally chaotic phase. The latter may be `controlled' in a robust
manner to target spatially periodic steady states with helical order. We
discuss several physical realizations of this model and make definite
predictions which could be tested in experimental or model lattice systems.",0404071v1
2004-06-04,Competition between Spin-Orbit Interaction and Zeeman Coupling in Rashba 2DEGs,"We investigate systematically how the interplay between Rashba spin-orbit
interaction and Zeeman coupling affects the electron transport and the spin
dynamics in InGaAs-based 2D electron gases. From the quantitative analysis of
the magnetoconductance, measured in the presence of an in-plane magnetic field,
we conclude that this interplay results in a spin-induced breaking of time
reversal symmetry and in an enhancement of the spin relaxation time. Both
effects, due to a partial alignment of the electron spin along the applied
magnetic field, are found to be in excellent agreement with recent theoretical
predictions.",0406106v1
2004-08-13,Finite lattice size effect in the ground state phase diagram of quasi-two-dimensional magnetic dipolar dots array with perpendicular anisotropy,"A prototype Hamiltonian for the generic patterned magnetic structures, of
dipolar interaction with perpendicular anisotropy, is investigated within the
finite-size framework by Landau-Lifshift-Gilbert classical spin dynamics.
Modifications on the ground state phase diagram are discussed with an emphasis
on the disappearance of continuous degeneracy in the ground state of in-plane
phase due to the finite lattice size effect. The symmetry-governed ground state
evolution upon the lattice size increase provides a critical insight into the
systematic transition to the infinite extreme.",0408324v1
2004-10-19,Haldane-gap chains in a magnetic field,"We consider quasi one dimensional spin-1 Heisenberg chains with crystal field
anisotropy in a uniform magnetic field. We determine the dynamical structure
factor in various limits and obtain a fairly complete qualitative picture of
how it changes with the applied field. In particular, we discuss how the width
of the higher energy single magnon modes depends on the field. We consider the
effects of a weak interchain coupling. We discuss the relevance of our results
for recent neutron scattering experiments on the quasi-1D Haldane-gap compound
NDMAP.",0410487v1
2004-10-29,How many electrons are needed to flip a local spin?,"Considering the spin of a local magnetic atom as a quantum mechanical
operator, we illustrate the dynamics of a local spin interacting with a
ballistic electron represented by a wave packet. This approach improves the
semi-classical approximation and provides a complete quantum mechanical
understanding for spin transfer phenomena. Sending spin-polarized electrons
towards a local magnetic atom one after another, we estimate the minimum number
of electrons needed to flip a local spin.",0410775v1
2004-11-10,Dynamic roughening of the magnetic flux landscape in YBa$_2$Cu$_3$O$_{7-x}$,"We study the magnetic flux landscape in YBa$_2$Cu$_3$O$_{7-x}$ thin films as
a two dimensional rough surface. The vortex density in the superconductor forms
a self-affine structure in both space and time. This is characterized by a
roughness exponent $\alpha = 0.76(3)$ and a growth exponent $\beta = 0.57(6)$.
This is due to the structure and distribution of flux avalanches in the
self-organized critical state, which is formed in the superconductor. We also
discuss our results in the context of other roughening systems in the presence
of quenched disorder.",0411260v1
2005-01-21,Perspectives of Ultra Cold Atoms Trapped in Magnetic Micro Potentials,"Recent work on magnetic micro traps for ultracold atoms is briefly reviewed.
The basic principles of operation are described together with the loading
methods and some of the realized trap geometries. Experiments are discussed
that study the interaction between atoms and the surface of micro traps as well
as the dynamics of ultracold gases in wave guides are discussed. The results
allow for an outlook towards future directions of research.",0501528v1
2005-01-26,High-field optically detected nuclear magnetic resonance in GaAs,"A method for high-field optically detected nuclear magnetic resonance (ODNMR)
is developed sensitive to 10**8 nuclei. Nuclear spin transitions are induced
using a radio frequency coil and detected through Faraday rotation
spectroscopy. Unlike conventional ODNMR, which is limited to low fields and
relies on the measurement of time-averaged luminescence polarization, this
technique monitors nuclear polarization through time-resolved measurements of
electron spin dynamics. Measurements in a (110) GaAs quantum well reveal Ga-69,
Ga-71, and As-75 resonances and their quadrupolar splittings while resolving
changes in nuclear polarization of 0.02%.",0501652v1
2005-01-31,Laser-controlled local magnetic field with semiconductor quantum rings,"We analize theoretically the dynamics of N electrons localized in a
semiconductor quantum ring under a train of phase-locked infrared laser pulses.
The pulse sequence is designed to control the total angular momentum of the
electrons. The quantum ring can be put in states characterized by strong
currents. The local magnetic field created by these currents can be used for a
selective quantum control of single spins in semiconductor systems.",0502001v1
2005-07-30,"Multiferroic behavior in CdCr2X4 (X = S, Se)","The recently discovered multiferroic material CdCr2S4 shows a coexistence of
ferromagnetism and relaxor ferroelectricity together with a colossal
magnetocapacitive effect. The complex dielectric permittivity of this compound
and of the structurally related CdCr2Se4 was studied by means of broadband
dielectric spectroscopy using different electrode materials. The observed
magnetocapacitive coupling at the magnetic transition is driven by enormous
changes of the relaxation dynamics induced by the development of magnetic
order.",0508014v1
2005-08-09,Percolation quantum phase transitions in diluted magnets,"We show that the interplay of geometric criticality and quantum fluctuations
leads to a novel universality class for the percolation quantum phase
transition in diluted magnets. All critical exponents involving dynamical
correlations are different from the classical percolation values, but in two
dimensions they can nonetheless be determined exactly. We develop a complete
scaling theory of this transition, and we relate it to recent experiments in
La$_{2}$Cu$_{1-p}$(Zn,Mg)$_{p}$O$_{4}$. Our results are also relevant for
disordered interacting boson systems.",0508211v2
2005-08-15,The Structure of the Pairing Interaction in the 2D Hubbard Model,"Dynamic cluster Monte Carlo calculations for the doped two-dimensional
Hubbard model are used to study the irreducible particle-particle vertex
responsible for $d_{x^2-y^2}$ pairing in this model. This vertex increases with
increasing momentum transfer and decreases when the energy transfer exceeds a
scale associated with the $Q=(\pi, \pi)$ spin susceptibility. Using an exact
decomposition of this vertex into a fully irreducible two-fermion vertex and
charge and magnetic exchange channels, the dominant part of the effective
pairing interaction is found to come from the magnetic, spin S=1 exchange
channel.",0508361v2
2005-08-25,Current-induced Vortex Motion by Spin-Transfer Torque,"We investigate the dynamics of a magnetic vortex driven by spin-transfer
torque due to spin current in the adiabatic case. The vortex core represented
by collective coordinate experiences a transverse force proportional to the
product of spin current and gyrovector, which can be interpreted as the
geometric force determined by topological charges. We show that this force is
just a reaction force of Lorentz-type force from the spin current of conduction
electrons. Based on our analyses, we propose analytically and numerically a
possible experiment to check the vortex displacement by spin current in the
case of single magnetic nanodot.",0508599v1
2005-10-14,Suppression of quantum chaos in a quantum computer hardware,"We present numerical and analytical studies of a quantum computer proposed by
the Yamamoto group in Phys. Rev. Lett. 89, 017901 (2002). The stable and
quantum chaos regimes in the quantum computer hardware are identified as a
function of magnetic field gradient and dipole-dipole couplings between qubits
on a square lattice. It is shown that a strong magnetic field gradient leads to
suppression of quantum chaos.",0510392v1
2005-10-28,Antiferromagnetic Metal Spintronics,"Spintronics in ferromagnetic metals is built on a complementary set of
phenomena in which magnetic configurations influence transport coefficients and
transport currents alter magnetic configurations. In this Letter we propose
that corresponding effects occur in circuits containing antiferromagnetic
metals. The critical current for switching can be smaller in the
antiferromagnetic case because of the absence of shape anisotropy and because
spin torques act through the entire volume of an antiferromagnet. Our findings
suggest that current-induced order parameter dynamics can be used to coarsen
the microstructure of antiferromagnetic thin films.",0510797v1
2005-12-09,Experimental determination of the non-extensive entropic parameter $q$,"We show how to extract the $q$ parameter from experimental data, considering
an inhomogeneous magnetic system composed by many Maxwell-Boltzmann homogeneous
parts, which after integration over the whole system recover the Tsallis
non-extensivity. Analyzing the cluster distribution of
La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ manganite, obtained through scanning tunnelling
spectroscopy, we measure the $q$ parameter and predict the bulk magnetization
with good accuracy. The connection between the Griffiths phase and
non-extensivity is also considered. We conclude that the entropic parameter
embodies information about the dynamics, the key role to describe complex
systems.",0512208v1
2005-12-13,Knight Field Enabled Nuclear Spin Polarization in Single Quantum Dots,"We demonstrate dynamical nuclear spin polarization in the absence of an
external magnetic field, by resonant circularly polarized optical excitation of
a single electron or hole charged quantum dot. Optical pumping of the electron
spin induces an effective inhomogeneous magnetic (Knight) field that determines
the direction along which nuclear spins could polarize and enables nuclear-spin
cooling by suppressing depolarization induced by nuclear dipole-dipole
interactions. Our observations suggest a new mechanism for spin-polarization
where spin exchange with an electron reservoir plays a crucial role. These
experiments constitute a first step towards quantum measurement of the
Overhauser field.",0512269v1
2006-01-25,Critical Temperatures of a Two-Band Model for Diluted Magnetic Semiconductors,"Using Dynamical Mean Field Theory (DMFT) and Monte Carlo (MC) simulations, we
study the ferromagnetic transition temperature Tc of a two-band model for
Diluted Magnetic Semiconductors (DMS), varying coupling constants, hopping
parameters, and carrier densities. We found that Tc is optimized at all
fillings p when both impurity bands (IB) fully overlap in the same energy
range, namely when the exchange couplings J and bandwidths are identical. The
optimal Tc is found to be about twice larger than the maximum value obtained in
the one-band model, showing the importance of multiband descriptions of DMS at
intermediate J's.",0601593v1
2006-02-20,Spin liquid correlations in Nd-langasite anisotropic Kagomé antiferromagnet,"Dynamical magnetic correlations in the geometrically frustrated
Nd$\_3$Ga$\_5$SiO$\_{14}$ compound were probed by inelastic neutron scattering
on a single crystal. A scattering signal with a ring shape distribution in
reciprocal space and unprecedented dispersive features was discovered.
Comparison with calculated static magnetic scattering from models of correlated
spins suggests that the observed phase is a spin liquid inherent to an
antiferromagnetic kagom\'e-like lattice of anisotropic Nd moments.",0602457v2
2006-02-24,Probability distributions of the work in the 2D-Ising model,"Probability distributions of the magnetic work are computed for the 2D Ising
model by means of Monte Carlo simulations. The system is first prepared at
equilibrium for three temperatures below, at and above the critical point. A
magnetic field is then applied and grown linearly at different rates.
Probability distributions of the work are stored and free energy differences
computed using the Jarzynski equality. Consistency is checked and the dynamics
of the system is analyzed. Free energies and dissipated works are reproduced
with simple models. The critical exponent $\delta$ is estimated in an usual
manner.",0602580v1
2006-03-09,"Formation kinetics of magnetic chains, rings, X's, and Y's","The kinetics of aggregation in a monolayer of magnetic particles are studied
using stochastic dynamics computer simulations. At low densities (<8% coverage)
the equilibrium structure is made up of chains and rings; the primary
mechanisms by which these motifs form are described. At higher densities (>15%
coverage), we observe large transient concentrations of Y-shaped and X-shaped
defects that ultimately give way to an extended, labyrinthine network. Our
results suggest that a defect mechanism -- such as that proposed by Tlusty and
Safran, Science 290, 1328 (2000) -- could drive a metastable phase separation
in two dimensions.",0603268v1
2006-03-27,Nonadiabatic production of spinor condensates with a QUIC trap,"Motivated by the recent experimental observation of multi-component spinor
condensates via a time-dependent quadrupole-Ioffe-configuration trap (QUIC
trap), we provide a general framework for the investigation of nonadiabatic
Landau-Zener dynamics of a hyperfine spin, e.g., from an atomic magnetic dipole
moment coupled to a weak time-dependent magnetic (B-) field. The spin flipped
population distribution, or the so-called Majorona formula is expressed in
terms of system parameters and experimental observables; thus, provides much
needed insight into the underlying mechanism for the production of spinor
condensates due to nonadiabatic level crossings.",0603686v1
2006-04-26,Time-domain measurement of driven ferromagnetic resonance,"We present a time-resolved measurement of magnetization dynamics during
ferromagnetic resonance (FMR) in a single layer of Ni81Fe19. Small-angle (<1
deg.) precession of elemental Ni, Fe moments could be measured directly and
quantitatively using time-resolved x-ray magnetic circular dichroism (XMCD) in
transmission. The high temporal and rotational sensitivity of of this technique
has allowed characterization of the phase and amplitude of driven FMR motion at
2.3 GHz, verifying basic expectations for a driven resonance.",0604610v1
2006-05-11,Power dissipation in spintronic devices out of thermodynamic equilibrium,"Quantum limits of power dissipation in spintronic computing are estimated. A
computing element composed of a single electron in a quantum dot is considered.
Dynamics of its spin due to external magnetic field and interaction with
adjacent dots is described via the Bloch equations. Spin relaxation due to
magnetic noise from various sources is described as coupling to a reservoir.
Resulting dissipation of energy is calculated and is shown to be much less than
the thermal limit, ~kT per bit, if the rate of spin relaxation is much slower
than the switching rate. Clues on how to engineer an energy efficient
spintronic device are provided.",0605298v1
2006-05-15,Levitation of Bose-Einstein condensates induced by macroscopic non-adiabatic quantum tunneling,"We study the dynamics of two-component Bose-Einstein condensates trapped in
different vertical positions in the presence of an oscillating magnetic field.
It is shown here how tuning appropriately the oscillation frequency of the
magnetic field leads to the levitation of the system against gravity. This
phenomenon is a manifestation of a macroscopic non-adiabatic tunneling in a
system with internal degrees of freedom.",0605385v1
2006-06-06,Resistance noise in spin valves,"Fluctuations of the magnetization in spin valves are shown to cause
resistance noise that strongly depends on the magnetic configuration. Due to
the applied external field and the dynamic exchange interaction through the
normal metal spacer, the electrical noise level of the antiparallel
configuration can exceed that of the parallel one by an order of magnitude, in
agreement with recent experimental results.",0606131v1
2006-06-20,Conductivity in glass phases of disordered granular superconductors in magnetic fields,"The electric conductivities in glass phases of three-dimensional (3D)
granular superconductors in magnetic fields are examined based on a quantum
disordered Josephson-junction array. A correct inclusion of the Ohmic
dissipative dynamics leads to glass phases with divergent dc conductivity. In
contrast, with no dissipative term, a metallic response in a glass phase is
obtained irrespective of the range of correlation of quenched disorder, i.e.,
even in the so-called Bose-glass phase with experimentally vanishing
resistivity.",0606522v5
2006-07-05,A simple model for thermomagnetic instability of critical state dynamics in superconductive films,"An one-dimensional model of magnetic flux penetration into thin strip-like
superconductive film is subject to numerical analysis which combines explicit
account for specific oblate geometry of magnetic field lines around the film
and a simplest phenomenology of the flux flow resistance under rigid pinning of
vortices with temperature-dependent critical current.",0607143v1
2006-08-02,Nuclear spin effects in negatively charged InP quantum dots,"Effects of both the dynamic nuclear polarization (DNP) created by circularly
polarized light and the fluctuations of average nuclear spin in a quantum dot
(QD) on the electron spin orientation are studied for singly negatively charged
InP QDs. From the dependence of the negative circular polarization of
photoluminescence on the applied longitudinal magnetic field, the hyperfine
field B_N of a few mT appearing due to DNP and the effective magnetic field B_f
of a few tens of mT arising from nuclear spin fluctuations (NSF) are estimated.
A lifetime of about 1 microsecond is estimated for NSF.",0608043v1
2006-08-04,"An electron spin injection driven, paramagnetic solid-state MASER device","In response to an external, microwave-frequency magnetic field, a
paramagnetic medium will absorb energy from the field that drives the
magnetization dynamics. Here we describe a new process by which an external
spin injection source, when combined with the microwave field spin-pumping, can
drive the paramagnetic medium from one that absorbs microwave energy to one
that emits microwave energy. We derive a simple condition for the crossover
from absorptive to emissive behavior. Based on this process, we propose a spin
injection-driven paramagnetic MASER device.",0608111v1
2006-10-10,Nanofabrication of spin-transfer torque devices by a PMMA mask one step process: GMR versus single layer devices,"We present a method to prepare magnetic spin torque devices of low specific
resistance in a one step lithography process. The quality of the pillar devices
is demonstrated for a standard magnetic double layer device. For single layer
devices, we found hysteretic switching and a more complex dynamical excitation
pattern in higher fields. A simple model to explain the resistance spikes is
presented.",0610277v1
2006-10-12,Theory of integer quantum Hall effect in graphene,"The observed quantization of the Hall conductivity in graphene at high
magnetic fields is explained as being due to the dynamically generated spatial
modulation of either the electron spin or the density, as decided by the
details of Coulomb interaction on the scale of lattice constant. It is
predicted that at a large in-plane component of the magnetic field such
ordering will be present only at the filling factor |f|=1, and absent
otherwise. Other experimental consequences of the theory are outlined.",0610349v3
2006-12-19,Effect of spin-orbit coupling on the excitation spectrum of Andreev billiards,"We consider the effect of spin-orbit coupling on the low energy excitation
spectrum of an Andreev billiard (a quantum dot weakly coupled to a
superconductor), using a dynamical numerical model (the spin Andreev map).
Three effects of spin-orbit coupling are obtained in our simulations: In zero
magnetic field: (1) the narrowing of the distribution of the excitation gap;
(2) the appearance of oscillations in the average density of states. In strong
magnetic field: (3) the appearance of a peak in the average density of states
at zero energy. All three effects have been predicted by random-matrix theory.",0612501v1
2007-01-23,Observation of Spin Wave Soliton Fractals in Magnetic Film Active Feedback Rings,"The manifestation of fractals in soliton dynamics has been observed for the
first time. The experiment utilized self-generated spin wave envelope solitons
in a magnetic film based active feedback ring. At high ring gain, the soliton
that circulates in the ring breathes in a fractal pattern. The corresponding
power frequency spectrum shows a comb structure, with each peak in the comb
having its own comb, and so on, to finer and finer scales.",0701537v1
2007-02-01,Interacting Antiferromagnetic Droplets in Quantum Critical CeCoIn_5,"The heavy fermion superconductor CeCoIn_5 can be tuned between
superconducting and antiferromagnetic ground states by hole doping with Cd.
Nuclear magnetic resonance (NMR) data indicate that these two orders coexist
microscopically with an ordered moment ~0.7 \mu_B. As the ground state evolves,
there is no change in the low frequency spin dynamics in the disordered state.
These results suggest that the magnetism emerges locally in the vicinity of the
Cd dopants.",0702008v2
2007-02-22,Radio-frequency magnetic response of vortex lattices undergoing structural transformations,"We report a clear anomaly in the rf response of a tunnel-diode resonator
measured in borocarbide crystals in dc fields along the $c-$axis. We associate
the anomaly in the dynamic magnetic susceptibility, $\chi$, with a structural
transition in the vortex lattice. $\chi$ is sensitive to the Campbell
penetration depth $\lambda_{C}$ of the rf perturbation into the mixed state.
$\lambda_{C}$ depends on the vortex lattice elastic moduli, which in turn
depend on the vortex lattice structure. The high-field transition line is
clearly mapped and shown to behave in agreement with theory.",0702509v1
2007-02-26,Measurement of mesoscopic High-$T_c$ superconductors using Si mechanical micro-oscillators,"In a superconducting mesoscopic sample, with dimensions comparable to the
London penetration depth, some properties are qualitatively different to those
found in the bulk material. These properties include magnetization, vortex
dynamics and ordering of the vortex lattice. In order to detect the small
signals produced by this kind of samples, new instruments designed for the
microscale are needed. In this work we use micromechanical oscillators to study
the magnetic properties of a Bi$_2$Sr$_2$CaCu$_2$O$_{8 + \delta}$ disk with a
diameter of 13.5 microns and a thickness of 2.5 microns. The discussion of our
results is based on the existence and contribution of inter and intra layer
currents.",0702597v1
2007-03-20,Magnetic focusing in normal-superconductor hybrid systems: a semiclassical analysis,"We study a transverse electron-hole focusing effect in a
normal-superconductor system. The spectrum of the quasiparticles is calculated
both quantum mechanically and in semiclassical approximation, showing an
excellent agreement. A semiclassical conductance formula is derived which takes
into account the effect of electron-like as well as hole-like quasiparticles.
At low magnetic field the semiclassical conductance shows characteristic
oscillations due to the Andreev reflection, while at higher fields it goes to
zero. These findigs are in line with the results of previous quantum
calculations and with the expectations based on the classical dynamics of the
quasiparticles.",0703529v1
1999-11-30,Experimental test for 5th dimension in Kaluza-Klein gravity,"Several electric/magnetic charged solutions (dyons) to 5D Kaluza-Klein
gravity on the principal bundle are reviewed. Here we examine the possibility
that these solutions can act as quantum virtual wormholes in spacetime foam
models. By applying a sufficently large, external electric and/or magnetic
field it may be possible to ``inflate'' these solutions from a quantum to a
classical state. This effect could lead to a possible experimental signal for
higher dimensions in multidimensional gravity.",9911120v1
2005-05-23,Dynamical SU(2) magnetic monopoles,"In our paper Phys. Rev. Lett. 92, 151801 (2004), the oscillations of strongly
deformed Bogomolny-Prasad-Sommerfield magnetic monopoles have been studied on a
fixed Minkowski background. The purpose of the present article is to provide a
more detailed account on the results yielded by our numerical simulations. In
particular, an analysis on the dependence on the strength of the initial
excitation is carried out in order to distinguish features which are already
present for small perturbations from those which are consequences of the
nonlinearity of the system.",0505117v1
2006-10-09,On the geometrization of the electro-magnetic interaction for a spinning particle,"We outline that, in a Kaluza-Klein framework, not only the electro-magnetic
field can be geometrized, but also the dynamics of a charged spinning particle
can be inferred from the motion in a 5-dimensional space-time. This result is
achieved by the dimensional splitting of Papapetrou equations and by proper
identifications of 4-dimensional quantities.",0610036v2
2004-06-01,Leading Quenching Effects in the Proton Magnetic Moment,"We present the first investigation of the extrapolation of quenched nucleon
magnetic moments in quenched chiral effective field theory. We utilize
established techniques in finite-range regularisation and compare with standard
dimensional regularisation methods. Finite-volume corrections to the relevant
loop integrals are also addressed. Finally, the contributions of dynamical sea
quarks to the proton moment are estimated using a recently discovered
phenomenological link between quenched and physical QCD.",0406001v2
1998-03-18,Phase transition induced by a magnetic field,"The magnetic catalysis of discrete chiral symmetry breaking in the 2+1
dimensional Nambu-Jona-Lasinio model is analyzed. A particular attention is
paid to a possible application of the effect in solid state physics. The
fermion contribution to the thermal conductivity as a function of the dynamical
mass (energy gap in the spectrum) is estimated and is shown to be suppressed
when a non-zero order parameter develops.",9803371v2
1998-07-24,Electron Self-Energy in Strong Magnetic Field: Summation of Double Logarithmic Terms,"We study the electron self--energy in a strong magnetic field when the
parameter \eta\equiv (\alpha/2\pi) \ln^2 (eB/m^2_0) \sim 1 and explore the
transition between the perturbative regime \eta<<1 and the nonperturbative
massless QED regime \eta>>1 where chiral symmetry is broken spontaneously and
electrons acquire the dynamically induced mass.",9807486v1
1999-08-10,Theory of the Magnetic Catalysis of Chiral Symmetry Breaking in QED,"The theory of the magnetic catalysis of chiral symmetry breaking in QED is
developed. An approximation for the Schwinger-Dyson equations describing
reliably this phenomenon is established, i.e., it is shown that there exists a
consistent truncation of those equations in this problem. The equations are
solved both analytically and numerically, and the dynamical mass of fermions is
determined.",9908320v1
2004-10-01,The Dynamical Mixing of Light and Pseudoscalar Fields,"We solve the general problem of mixing of electromagnetic and scalar or
pseudoscalar fields coupled by axion-type interactions ${\cal L}_{int}
=g_{\phi} \phi \epsilon_{\mu \nu \a \b}F^{\mu \nu}F^{\a \b} $. The problem
depends on several dimensionful scales, including the magnitude and direction
of background magnetic field, the pseudoscalar mass, plasma frequency,
propagation frequency, wave number, and finally the pseudoscalar coupling. We
apply the results to the first consistent calculations of the mixing of light
propagating in a background magnetic field of varying direction, which shows a
great variety of fascinating resonant and polarization effects.",0410006v1
1994-02-16,Spontaneous Breakdown of the Lorentz Invariance,"We re-examine three-dimensional gauge theory with a Chern-Simons term in
which the Lorentz invariance is spontaneously broken by dynamical generation of
a magnetic field. A non-vanishing magnetic field leads, through the
Nambu-Goldstone theorem, to the decrease of zero-point energies of photons,
which accounts for a major part of the mechanism. The asymmetric spectral flow
plays an important role. The instability in pure Chern-Simons theory is also
noted.",9402096v1
1995-04-17,Primordial Magnetic Fields From String Cosmology,"Sufficiently large seeds for generating the observed (inter)galactic magnetic
fields emerge naturally in string cosmology from the amplification of
electromagnetic vacuum fluctuations due to a dynamical dilaton background. The
success of the mechanism depends crucially on two features of the so-called
pre-big-bang scenario, an early epoch of dilaton-driven inflation at very small
coupling, and a sufficiently long intermediate stringy era preceding the
standard radiation-dominated evolution.",9504083v1
1995-05-18,Spontaneous Magnetization in Lorentz Invariant Theories,"In a class of three-dimensional Abelian gauge theories with both light and
heavy fermions, heavy chiral fermions can trigger dynamical generation of a
magnetic field, leading to the spontaneous breaking of the Lorentz invaiance.
Finite masses of light fermions tend to restore the Lorentz invariance.",9505113v2
1996-05-01,Magnetic Charge in a Nonassociative Field Theory,"The violation of the Jacobi identity by the presence of magnetic charge is
accomodated by using an explicitly nonassociative theory of octonionic fields.
It is found that the dynamics of this theory is simplified if the Lagrangian
contains only dyonic charges, but certain problems in the constrained
quantisation remain. The extension of these concepts to string theory may
however resolve these difficulties.",9604201v1
1997-02-27,BPS Monopoles and Electromagnetic Duality,"We review our recent work on the BPS magnetic monopoles and its relation to
the electromagnetic duality in the N=4 supersymmetric Yang-Mills systems with
an arbitrary gauge group. The gauge group can be maximally or partially broken.
The low energy dynamics of the massive and massless magnetic monopoles are
approximated by the moduli space metric. We emphasize the possible connection
between the nature of the monopole moduli space with unbroken gauge group and
the physics of mesons and baryons in QCD.",9702200v1
1997-11-17,Chiral symmetry breaking in a uniform external magnetic field II. Symmetry restoration at high temperatures and chemical potentials,"Chiral symmetry is dynamically broken in quenched, ladder QED at weak gauge
couplings when an external magnetic field is present. In this paper, we show
that chiral symmetry is restored above a critical chemical potential and the
corresponding phase transition is of first order. In contrast, the chiral
symmetry restoration at high temperatures (and at zero chemical potential) is a
second order phase transition.",9711126v1
1998-03-09,Magnetic catalysis of chiral symmetry breaking and the Pauli problem,"The non-perturbative Schwinger-Dyson equation is used to show that chiral
symmetry is dynamically broken in QED at weak gauge couplings when an external
uniform magnetic field is present. A complete analysis of this phenomenon may
shed light on the Pauli problem, namely, why $\alpha$ = 1/137.",9803074v1
2001-05-28,Compactification with U(1) magnetic field within Dirac supersymmetry,"We consider Dirac-supersymmetric interactions, which produce CP-conserving
separation of positive and negative energy solutions in the Dirac equation in
order to investigate an alternative to the Kaluza-Klein mechanism. We review
conditions under which separation is possible into free particle and
compactified behaviors in different dimensions, with attention to spin degrees
of freedom. We show a U(1) constant magnetic field produces such kind of
behavior; an explicit treatment is given to the 6-d to 4-d and 4-d to 2-d
breaking cases and the spectrum is obtained. A dynamical mass-creation
mechanism is suggested from the procedure.",0105280v1
2004-12-13,The Klein-Gordon and the Dirac oscillators in a noncommutative space,"We study the Dirac and the klein-Gordon oscillators in a noncommutative
space. It is shown that the Klein-Gordon oscillator in a noncommutative space
has a similar behaviour to the dynamics of a particle in a commutative space
and in a constant magnetic field. The Dirac oscillator in a noncommutative
space has a similar equation to the equation of motion for a relativistic
fermion in a commutative space and in a magnetic field, however a new exotic
term appears, which implies that a charged fermion in a noncommutative space
has an electric dipole moment.",0412122v1
2005-01-18,Chiral Symmetry Breaking and Point-Splitting Procedure in Light-Cone Quantized QED in a Magnetic Field,"The summation of all rainbow diagrams in QED in a strong magnetic field leads
to a dynamical electron mass on the light-cone. Further contributions to this
summation however can cause problems with light-cone singularities. It is shown
that these problems are generally avoided by applying the point-splitting
regularization to every diagram. The possibility of implementing this procedure
into the Lagrangian of the theory is discussed.",0501133v2
2005-05-11,Photon emission in a constant magnetic field in 2+1 dimensional space-time,"We calculate by the proper-time method the amplitude of the two-photon
emission by a charged fermion in a constant magnetic field in (2+1)-dimensional
space-time. The relevant dynamics reduces to that of a supesymmetric
quantum-mechanical system with one bosonic and one fermionic degrees of
freedom.",0505102v2
2005-12-26,Landau Quantization of Neutral Particles in an External Field,"The quantum dynamics of an induced electric dipole in the presence of a
configuration of crossed electric and magnetic fields is analyzed. This field
configuration confines the dipole in a plane and produces a coupling similar to
the coupling of a charged particle in the presence of external magnetic field.
In this work we investigate the analog of Landau levels in induced electric
dipoles in a sistem of neutral particles. The energy levels and eigenfunctions
are obtained exactly.",0512309v1
2005-09-15,Variational approach to nonlinear gravity-driven instabilities in a MHD setting,"We establish a variational framework for nonlinear instabilities in a setting
of the ideal magnetohydrodynamic (MHD) equations. We apply a variational method
to various kind of smooth steady states which are shown to be nonlinearly
unstable for both incompressible and compressible ideal MHD equations.
Destabilizing effect of compressibility is justified as well as stabilizing
effect of magnetic field lines arising in MHD dynamics, which distinguishes
from the Rayleigh-Taylor instability in the absence of magnetic field lines.",0509364v1
2005-09-19,Smoothing - Strichartz Estimates for the Schrodinger Equation with small Magnetic Potential,"The work treats smoothing and dispersive properties of solutions to the
Schrodinger equation with magnetic potential. Under suitable smallness
assumption on the potential involving scale invariant norms we prove smoothing
- Strichartz estimate for the corresponding Cauchy problem. An application that
guarantees absence of pure point spectrum of the corresponding perturbed
Laplace operator is discussed too.",0509416v2
2006-01-26,Further Results on Active Magnetic Bearing Control with Input Saturation,"We study the low-bias stabilization of active magnetic bearings (AMBs)
subject to voltage saturation based on a recently proposed model for the AMB
switching mode of operation. Using a forwarding-like approach, we construct a
stabilizing controller of arbitrarily small amplitude and a control-Lyapunov
function for the AMB dynamics. We illustrate our construction using a numerical
example.",0601650v1
2000-10-25,Avoided crossings in mesoscopic systems: electron propagation on a non-uniform magnetic cylinder,"We consider an electron constrained to move on a surface with revolution
symmetry in the presence of a constant magnetic field $B$ parallel to the
surface axis. Depending on $B$ and the surface geometry the transverse part of
the spectrum typically exhibits many crossings which change to avoided
crossings if a weak symmetry breaking interaction is introduced. We study the
effect of such perturbations on the quantum propagation. This problem admits a
natural reformulation to which tools from molecular dynamics can be applied. In
turn, this leads to the study of a perturbation theory for the time dependent
Born-Oppenheimer approximation.",0010042v1
2004-10-19,Plasma equilibrium equations in coordinates connected with magnetic surfaces. Exact equilibrium solutions,"A representation of the static MHD equilibrium system in coordinates
connected with magnetic surfaces is suggested. It is used for producing
families of non-trivial 3D exact solutions of isotropic and anisotropic plasma
equilibria in different geometries, with and without dynamics, and often
without geometrical symmetries. The ways of finding coordinates in which exact
equilibria can be constructed are discussed; examples and their applications as
physical models are presented.",0410045v1
2006-01-26,On the ground state energy for a magnetic Shcrödinger operator and the effect of the De Gennes Boundary condition,"Motivated by the Ginzburg-Landau theory of superconductivity, we estimate in
the semi-classical limit the ground state energy of a magnetic Schr\""odinger
operator with De Gennes boundary condition and we study the localization of the
ground state. We exhibit cases when the De Gennes boundary condition has strong
effects on this localization.",0601057v2
2004-04-29,Diffusion in a stochastic magnetic field,"We consider a stochastic differential equation for a charged particle in a
stochastic magnetic field, known as A-Langevin equation. The solution of the
equation is found, and the Lagrange velocity correlation function is calculated
in Corrsin approximation. A corresponding diffusion constant is estimated. We
observe different transport regimes, such as quasilinear- or Bohm-type
diffusion, depending on the parameters of plasma.",0404056v3
2006-07-30,Semiclassical Theory for Parametric Correlation of Energy Levels,"Parametric energy-level correlation describes the response of the
energy-level statistics to an external parameter such as the magnetic field.
Using semiclassical periodic-orbit theory for a chaotic system, we evaluate the
parametric energy-level correlation depending on the magnetic field difference.
The small-time expansion of the spectral form factor $K(\tau)$ is shown to be
in agreement with the prediction of parameter dependent random-matrix theory to
all orders in $\tau$.",0607070v1
2004-07-02,Covariant baryon charge radii and magnetic moments in a chiral constituent quark model,"The charge radii and magnetic moments of all the light and strange baryons
are investigated within the framework of a constituent quark model based on
Goldstone-boson-exchange dynamics. Following the point-form approach to
relativistic quantum mechanics, the calculations are performed in a manifestly
covariant manner. Relativistic (boost) effects have a sizeable influence on the
results. The direct predictions of the constituent quark model are found to
fall remarkably close to the available experimental data.",0407009v1
1998-09-21,Quantum theory of magnetic quadrupole lenses for spin-1/2 particles,"General guidelines for constructing a quantum theory of charged-particle beam
optics starting ab initio from the basic equations of quantum mechanics,
appropriate to the situation under study. In the context of spin-1/2 particles,
these guidelines are used starting with the Dirac equation. The spinor theory
just constructed is used to obtain the transfer maps for normal and skew
magnetic quadrupoles respectively. As expected the traditional transfer maps
get modified by the quantum contributions. The classical limit of the quantum
formalism presented, reproduces the well-known Lie algebraic formalism of
charged-particle beam optics.",9809032v1
1999-06-30,Identifying nonlinear wave interactions in plasmas using two-point measurements: a case study of Short Large Amplitude Magnetic Structures (SLAMS),"A framework is described for estimating Linear growth rates and spectral
energy transfers in turbulent wave-fields using two-point measurements. This
approach, which is based on Volterra series, is applied to dual satellite data
gathered in the vicinity of the Earth's bow shock, where Short Large Amplitude
Magnetic Structures (SLAMS) supposedly play a leading role. The analysis
attests the dynamic evolution of the SLAMS and reveals an energy cascade toward
high-frequency waves.",9906065v1
1999-12-13,Detection of a flow induced magnetic field eigenmode in the Riga dynamo facility,"In an experiment at the Riga sodium dynamo facility, a slowly growing
magnetic field eigenmode has been detected over a period of about 15 seconds.
For a slightly decreased propeller rotation rate, additional measurements
showed a slow decay of this mode. The measured results correspond satisfactory
with numerical predictions for the growth rates and frequencies.",9912026v1
2002-08-02,"Dynamics of a magnetic monopole in matter, Maxwell equations in dyonic matter and detection of electric dipole moments","For a monopole, the analogue of the Lorentz equation in matter is shown to be
f = g (H - v cross D). Dual-symmetric Maxwell equations, for matter containing
hidden magnetic charges in addition to electric ones, are given. They apply as
well to ordinary matter if the particles possess T-violating electric dipole
moments. Two schemes of experiments for the detection of such moments in
macroscopic pieces of matter are proposed.",0208007v1
2003-06-04,Observation of Caustics in the Trajectories of Cold Atoms in a Linear Magnetic Potential,"We have studied the spatial and temporal dynamics of a cold atom cloud in the
conservative force field of a ferromagnetic guide, after laser cooling has been
switched off suddenly. We observe outgoing 'waves' that correspond to caustics
of individual trajectories of trapped atoms. This provides detailed information
on the magnetic field, the energy distribution and the spin states.",0306040v1
2004-05-05,Radiation Chemistry of Organic Liquids: Saturated Hydrocarbons,"In this review (124 refs), several problems in radiolysis of saturated
hydrocarbons are examined. Special attention is paid to the chemistry of
radical cations, high-mobility holes, excited state and spur dynamics, magnetic
field and spin effects, and optically detected magnetic resonance spectroscopy.",0405014v1
2004-06-26,Two-fluid magnetic island dynamics in slab geometry: I - Isolated islands,"A set of reduced, 2-D, two-fluid, drift-MHD equations is derived. Using these
equations, a complete and fully self-consistent solution is obtained for an
isolated magnetic island propagating through a slab plasma with uniform but
different ion and electron fluid velocities. The ion and electron fluid flow
profiles around the island are uniquely determined, and are everywhere
continuous. Moreover, the island phase-velocity is uniquely specified by the
condition that there be zero net electromagnetic force acting on the island.
Finally, the ion polarization current correction to the Rutherford island width
evolution equation is evaluated, and found to be stabilizing provided that the
anomalous perpendicular ion viscosity significantly exceeds the anomalous
perpendicular electron viscosity.",0406132v1
2005-12-20,Cylindrical Korteweg-de Vries solitons on a ferrofluid surface,"Linear and non-linear surface waves on a ferrofluid cylinder surrounding a
current-carrying wire are investigated. Suppressing the Rayleigh-Plateau
instability of the fluid column by the magnetic field of a sufficiently large
current in the wire axis-symmetric surface deformations are shown to propagate
without dispersion in the long wavelength limit. Using multiple scale
perturbation theory the weakly non-linear regime may be described by a
Korteweg-de Vries equation with coefficients depending on the magnetic field
strength. For different values for the current in the wire hence different
solutions such as hump or hole solitons may be generated. The possibility to
observe these structures in experiments is also elucidated.",0512185v2
2006-06-02,Controlling electronic spin relaxation of cold molecules with electric fields,"We present a theoretical study of atom - molecule collisions in superimposed
electric and magnetic fields and show that dynamics of electronic spin
relaxation in molecules at temperatures below 0.5 K can be manipulated by
varying the strength and the relative orientation of the applied fields. The
mechanism of electric field control of Zeeman transitions is based on an
intricate interplay between intramolecular spin-rotation couplings and
molecule-field interactions. We suggest that electric fields may affect
chemical reactions through inducing nonadiabatic spin transitions and
facilitate evaporative cooling of molecules in a magnetic trap.",0606027v2
2006-10-02,Hydrogen atom in crossed electric and magnetic fields: Phase space topology and torus quantization via periodic orbits,"A hierarchical ordering is demonstrated for the periodic orbits in a strongly
coupled multidimensional Hamiltonian system, namely the hydrogen atom in
crossed electric and magnetic fields. It mirrors the hierarchy of broken
resonant tori and thereby allows one to characterize the periodic orbits by a
set of winding numbers. With this knowledge, we construct the action variables
as functions of the frequency ratios and carry out a semiclassical torus
quantization. The semiclassical energy levels thus obtained agree well with
exact quantum calculations.",0610013v1
2007-02-26,Spherical Couette flow in a dipolar magnetic field,"We consider numerically the flow of an electrically conducting fluid in a
differentially rotating spherical shell, in a dipolar magnetic field. For
infinitesimal differential rotation the flow consists of a super-rotating
region, concentrated on the particular field line C just touching the outer
sphere, in agreement with previous results. Finite differential rotation
suppresses this super-rotation, and pushes it inward, toward the equator of the
inner sphere. For sufficiently strong differential rotation the outer boundary
layer becomes unstable, yielding time-dependent solutions. Adding an overall
rotation suppresses these instabilities again. The results are in qualitative
agreement with the DTS liquid sodium experiment.",0702228v1
2007-03-23,Parametric instability of the helical dynamo,"We study the dynamo threshold of a helical flow made of a mean (stationary)
plus a fluctuating part. Two flow geometries are studied, either (i) solid body
or (ii) smooth. Two well-known resonant dynamo conditions, elaborated for
stationary helical flows in the limit of large magnetic Reynolds numbers, are
tested against lower magnetic Reynolds numbers and for fluctuating flows (zero
mean). For a flow made of a mean plus a fluctuating part the dynamo threshold
depends on the frequency and the strength of the fluctuation. The resonant
dynamo conditions applied on the fluctuating (resp. mean) part seems to be a
good diagnostic to predict the existence of a dynamo threshold when the
fluctuation level is high (resp. low).",0703216v1
1999-09-30,Noncyclic geometric phase and its non-Abelian generalization,"We use the theory of dynamical invariants to yield a simple derivation of
noncyclic analogues of the Abelian and non-Abelian geometric phases. This
derivation relies only on the principle of gauge invariance and elucidates the
existing definitions of the Abelian noncyclic geometric phase. We also discuss
the adiabatic limit of the noncyclic geometric phase and compute the adiabatic
non-Abelian noncyclic geometric phase for a spin 1 magnetic (or electric)
quadrupole interacting with a precessing magnetic (electric) field.",9909093v1
2000-03-22,Quantum Fluctuations of a Single Trapped Atom: Transient Rabi Oscillations and Magnetic Bistability,"Isolation of a single atomic particle and monitoring its resonance
fluorescence is a powerful tool for studies of quantum effects in
radiation-matter interaction. Here we present observations of quantum dynamics
of an isolated neutral atom stored in a magneto-optical trap. By means of
photon correlations in the atom's resonance fluorescence we demonstrate the
well-known phenomenon of photon antibunching which corresponds to transient
Rabi oscillations in the atom. Through polarization-sensitive photon
correlations we show a novel example of resolved quantum fluctuations:
spontaneous magnetic orientation of an atom. These effects can only be observed
with a single atom.",0003105v1
2000-07-12,Loading a continuous-wave atom laser by optical pumping techniques,"Demonstrating that despite loss processes, Bose-Einstein condensates can be
formed in steady state is a prerequisite for obtaining a coherent beam of atoms
in a continuous-wave atom laser. In this paper we propose a method for loading
atoms into the thermal component of a Bose condensed cloud confined in a
magnetic trap. This method is aimed at allowing steady state dynamics to be
achieved. The proposed scheme involves loading atoms into the conservative
magnetic potential using the spontaneous emission of photons. We show that the
probability for the reabsorption of these photons may be small .",0007039v1
2002-03-21,The Landau-Zener Model with Decoherence: The case S=1/2,"We study the dynamics of a spin coupled to an oscillating magnetic field, in
the presence of decoherence and dissipation. In this context we solve the
master equation for the Landau-Zener problem, both in the unitary and in the
irreversible case. We show that a single spin can be ""magnetized"" in the
direction parallel to the oscillating bias. When decay from upper to lower
level is taken into account, hysteretic behavior is obtained.",0203110v1
2002-10-01,Transient Dynamics in Magnetic Force Microscopy for a Single-Spin Measurement,"We analyze a single-spin measurement using a transient process in magnetic
force microscopy (MFM) which could increase the maximum operating temperature
by a factor of Q (the quality factor of the cantilever) in comparison with the
static Stern-Gerlach effect. We obtain an exact solution of the master
equation, which confirms this result. We also discuss the conditions required
to create a macroscopic Schrodinger cat state in the cantilever.",0210006v1
2002-12-02,The hydrogen atom in electric and magnetic fields : Pauli's 1926 article,"The results obtained by Pauli, in his 1926 article on the hydrogen atom, made
essential use of the dynamical so(4) symmetry of the bound states. Pauli used
this symmetry to compute the perturbed energy levels of an hydrogen atom in a
uniform electric field (Stark effect) and in uniform electric and magnetic
fields. Although the experimental check of the single Stark effect on the
hydrogen atom has been studied experimentally, Pauli's results in mixed fields
have been studied only for Rydberg states of rubidium atoms in crossedfields
and lithium atoms in parallel fields.",0212010v2
2003-06-17,Trapping and manipulating neutral atoms with electrostatic fields,"We report on experiments with cold thermal $^7$Li atoms confined in combined
magnetic and electric potentials. A novel type of three-dimensional trap was
formed by modulating a magnetic guide using electrostatic fields. We observed
atoms trapped in a string of up to six individual such traps, a controlled
transport of an atomic cloud over a distance of 400$\mu$m, and a dynamic
splitting of a single trap into a double well potential. Applications for
quantum information processing are discussed.",0306111v1
2004-05-22,Spatial weak-light solitons in an electro-magnetically induced nonlinear waveguide,"We show that a weak probe light beam can form spatial solitons in an
electro-magnetically induced transparency (EIT) medium composed of four-level
atoms and a coupling light field. We find that the coupling light beam can
induce a highly controllable nonlinear waveguide and exert very strong effects
on the dynamical behavior of the solitons. Hence, in the EIT medium, it is not
only possible to produce spatial solitons at very low light intensities but
also simultaneously control these solitons by using the coupling-light-induced
nonlinear waveguide.",0405132v1
2005-11-02,Theory of dark-state polariton collapses and revivals,"We investigate the dynamics of dark-state polaritons in an atomic ensemble
with ground-state degeneracy. A signal light pulse may be stored and retrieved
from the atomic sample by adiabatic variation of the amplitude of a control
field. During the storage process, a magnetic field causes rotation of the
atomic hyperfine coherences, leading to collapses and revivals of the
dark-state polariton number. These collapses and revivals should be observable
in measurements of the retrieved signal field, as a function of storage time
and magnetic field orientation.",0511017v1
2007-05-19,A self-sustaining nonlinear dynamo process in Keplerian shear flows,"A three-dimensional nonlinear dynamo process is identified in rotating plane
Couette flow in the Keplerian regime. It is analogous to the hydrodynamic
self-sustaining process in non-rotating shear flows and relies on the
magneto-rotational instability of a toroidal magnetic field. Steady nonlinear
solutions are computed numerically for a wide range of magnetic Reynolds
numbers but are restricted to low Reynolds numbers. This process may be
important to explain the sustenance of coherent fields and turbulent motions in
Keplerian accretion disks, where all its basic ingredients are present.",0705.2814v1
2007-06-01,Particle-in-cell simulations of fast collisionless reconnection in gamma-ray burst outflows,"We present preliminary results of particle-in-cell simulations of
collisionless magnetic reconnection for conditions that apply to magnetically
dominated pair plasma jets such as those in gamma-ray bursts. We find similar
behaviour to previous authors but with reduced reconnection efficiency. This
results because we include the full electromagnetic field dynamically and allow
electric field fluctuations to grow. Although weak, these fluctuations impede
early x-point formation by periodically decelerating and even reversing the
inflow.",0706.0193v1
2007-06-06,Bound States and Many-Body Effects in H-Shaped Quantum Wires,"In this paper, bound states energies and corresponding wave functions of
H-shaped quantum wires are calculated numerically in the presence of external
magnetic and electric fields and within the Landau gauge. With a suitable
definition of external confinement potential, we present a numerical algorithm
to calculate the profile of probability distribution of charge carriers. Our
analysis shows that in the presence of external electric and magnetic fields,
bound state properties of carriers are sensitive functions of an asymmetric
parameter $a=\frac{W_{x}}{W_{y}}$ which measures the relative width of the well
in two directions. We also study many body effect of bandgap renormalization in
this quasi one dimensional system within dynamical random phase approximation.",0706.0777v1
2007-06-30,Zooming into the coexisting regime of ferromagnetism and superconductivity in ErRh4B4 single crystals,"High resolution measurements of the dynamic magnetic susceptibility are
reported for ferromagnetic re-entrant superconductor, ErRh$_{4}$B$_{4}$.
Detailed investigation of the coexisting regime reveals unusual
temperature-asymmetric and magnetically anisotropic behavior. The
superconducting phase appears via a series of discontinuous steps upon warming
from the ferromagnetic normal phase, whereas the ferromagnetic phase develops
via a gradual transition. A model based on local field inhomogeneity is
proposed to explain the observations.",0707.0029v1
2007-08-27,Magnetometry Based on Nonlinear Magneto-Optical Rotation with Amplitude-Modulated Light,"We report on an all-optical magnetometric technique based on nonlinear
magneto-optical rotation with amplitude-modulated light. The method enables
sensitive magnetic-field measurements in a broad dynamic range. We demonstrate
the sensitivity of $4.3\times10^{-9}$ G/$\sqrt{\text{Hz}}$ at 10 mG and the
magnetic field tracking in a range of 40 mG. The fundamental limits of the
method sensitivity and factors determining current performance of the
magnetometer are discussed.",0708.3488v2
2007-08-28,Nonadiabatic factor accompanying magnetic translation of a charged particle,"The quantum adiabatic theorem incorporating the Berry phase phenomenon can be
characterized as a factorization of the time evolution operator into a
path-dependent geometric factor, a usual dynamical factor and a non-adiabatic
factor that approaches the identity in the adiabatic limit. We study a case
where all these three factors can be constructed explicitly and where the
instantaneous Hamiltonian has infinitely degenerate energy eigenstates
associated with magnetic translation symmetry. Significance of the
non-adiabatic factor in terms of transition probabilities is discussed.",0708.3726v2
2007-09-25,Coexisting stochastic and coherence resonance in a mean-field dynamo model for Earth's magnetic field reversals,"Using a spherical symmetric mean field alpha^2-dynamo model for Earth's
magnetic field reversals, we show the coexistence of the noise-induced
phenomena coherence resonance and stochastic resonance. Stochastic resonance
has been recently invoked to explain the 100 kyr periodicity in the
distribution of the residence time between reversals. The comparison of the
resulting residence time distribution with the paleomagnetic one allows for
some estimate of the effective diffusion time of the Earth's core which may be
100 kyr or slightly below rather than 200 kyr as it would result from the
molecular resistivity.",0709.3932v1
2007-09-26,Spin-polarized Current-induced Instability in Spin-Valve with Antiferromagnetic Layer,"In the framework of phenomenological model we consider dynamics of a
compensated collinear antiferromagnet (AFM) in the presence of spin-polarised
current. The model is based on the assumption that AFM spins are localised and
spin torque is transferred to each magnetic sublattice independently. It is
shown that in AFM spin current i) can be a source of the ""negative friction"";
and ii) modifies spin-wave frequencies. Equilibrium state of AFM can be
destabilized by the current polarized in parallel to AFM vector. Threshold
current at which the loss of stability takes place depends upon the magnetic
anisotropy of AFM.",0709.4172v1
2007-09-27,Flux Distribution in Superconducting Films with Holes,"Flux penetration into type-II superconducing films is simulated for
transverse applied magnetic field and flux creep dynamics. The films contain
macroscopic, non-conducting holes and we suggest a new method to introduce the
holes in the simulation formalism. The method implies reconstruction of the
magnetic field change inside the hole. We find that in the region between the
hole and the edge the current density is compressed so that the flux density is
slightly reduced, but the traffic of flux is significantly increased. The
results are in good agreement with magneto-optical studies of flux
distributions in YBCO films.",0709.4346v1
2007-10-12,Coherent Spin Manipulations of a Polarized Beam With a Localized RF Magnetic Field,"The coherent manipulation of spin observables in storage rings provides
opportunities to test the application of some fundamental dynamical principles.
In this context, it is possible to confirm, using gauge invariance and Lorentz
invariance, a conjecture framed by A. M. Kondratenko concerning the ""natural""
or ""intrinsic"" resonance strength applicable to a spin rotation from a
controlled Froissart-Stora sweep with an RF dipole magnet. The discussion
includes a brief treatment of the ""forced"" component of the resonance strength
associated with the effect of the betatron oscillations induced by the
operation of the RF diple and a discussion of the effective resonance strength
as a function of betatron tunes.",0710.2566v1
2007-10-15,Ferromagnetic resonance study of polycrystalline Fe_{1-x}V_x alloy thin films,"Ferromagnetic resonance has been used to study the magnetic properties and
magnetization dynamics of polycrystalline Fe$_{1-x}$V$_{x}$ alloy films with
$0\leq x < 0.7$. Films were produced by co-sputtering from separate Fe and V
targets, leading to a composition gradient across a Si substrate. FMR studies
were conducted at room temperature with a broadband coplanar waveguide at
frequencies up to 50 GHz using the flip-chip method. The effective
demagnetization field $4 \pi M_{\mathrm{eff}}$ and the Gilbert damping
parameter $\alpha$ have been determined as a function of V concentration. The
results are compared to those of epitaxial FeV films.",0710.2826v2
2007-10-24,Nonlinear theory of mirror instability near threshold,"An asymptotic model based on a reductive perturbative expansion of the drift
kinetic and the Maxwell equations is used to demonstrate that, near the
instability threshold, the nonlinear dynamics of mirror modes in a magnetized
plasma with anisotropic ion temperatures involves a subcritical
bifurcation,leading to the formation of small-scale structures with amplitudes
comparable with the ambient magnetic field.",0710.4428v1
2007-11-27,X-Ray Jet Dynamics in a Polar Coronal Hole Region,"New XRT observations of the north polar region taken from the X-ray Telescope
(XRT) of the Hinode (Solar-B) spacecraft are used to analyze several time
sequences showing small loop brightenings with a long ray above. We focus on
the recorded transverse displacement of the jet and discuss scenarios to
explain the main features of the events: the relationship with the expected
surface magnetism, the rapid and sudden radial motion, and possibly the
heating, based on the assumption that the jet occurs above a null point of the
coronal magnetic field. We conclude that 3-D reconnection models are needed to
explain the observational details of these events.",0711.4320v1
2007-12-28,Entanglement propagation through spin chains in the presence of a staggered magnetic field,"We study the dynamics of entanglement in the XY spin chain subject to a
staggered magnetic field and contrast it to the previously studied uniform
field case. We find that, depending on parameter values, a staggered field can
provide better conditions for a perfect entanglement transfer, while even a
modest amount of exchange anisotropy appears to have a strong detrimental
effect. We also study interactions between different waves of entanglement and
assess the possibility of simultaneous transmission of multiple bits of quantum
information.",0712.4349v1
2008-01-01,Ion collection by oblique surfaces of an object in a transversely-flowing strongly-magnetized plasma,"The equations governing a collisionless obliquely-flowing plasma around an
ion-absorbing object in a strong magnetic field are shown to have an exact
analytic solution even for arbitrary (two-dimensional) object-shape, when
temperature is uniform, and diffusive transport can be ignored. The solution
has an extremely simple geometric embodiment. It shows that the ion collection
flux density to a convex body's surface depends only upon the orientation of
the surface, and provides the theoretical justification and calibration of
oblique `Mach-probes'. The exponential form of this exact solution helps
explain the approximate fit of this function to previous numerical solutions.",0801.0309v1
2008-01-30,Polaronic signatures in the optical properties of Nd$_{2-x}$Ce$_x$CuO$_4$,"We investigate the temperature and doping dependence of the optical
conductivity $\sigma(\omega)$ of Nd$_{2-x}$Ce$_x$CuO$_4$ in terms of
magnetic/lattice polaron formation. We employ dynamical mean-field theory in
the context of the Holstein-t-J model where an exact analytical solution is
available in the limit of infinite connectivity. We show that the pseudogap
features in the optical conductivity of this compound can be associated to the
formation of lattice polarons assisted by the magnetic interaction.",0801.4691v1
2008-02-18,Dynamical reentrance and geometry imposed quantization effects in Nb-AlOx-Nb Josephson junction arrays,"In this paper, we report on different phenomena related to the magnetic
properties of artificially prepared highly ordered (periodic) two-dimensional
Josephson junction arrays (2D-JJA) of both shunted and unshunted Nb-AlOx-Nb
tunnel junctions. By employing mutual-inductance measurements and using a
high-sensitive bridge, we have thoroughly investigated (both experimentally and
theoretically) the temperature and magnetic field dependence of complex AC
susceptibility of 2D-JJA. We also demonstrate the use of the scanning SQUID
microscope for imaging the local flux distribution within our unshunted arrays.",0802.2485v2
2008-02-21,Out-of-Plane Cmos Compatible Magnetometers,"Three-dimensional MEMS magnetometers with use of residual stresses in thin
multilayers cantilevers are presented. Half-loop cantilevers based on
Lorentz-force deflection convert magnetic flux in changes, thanks to
piezoresistive transducers mounted in Wheatstone bridge. Magnetic field in the
order of 10 Gauss was measured with a sensitivity of 0.015 mV/Gauss. A Finite
Element Model of the device has been developed with Ansys for static and
dynamic simulations. Novel out-of-plane ferromagnetic nickel plate magnetometer
is also presented.",0802.3077v1
2008-03-19,Transmutation of momentum into position in magnetic vortices,"We show that transmutation of linear momentum into position may occur in a
system of three magnetic vortices thanks to a direct link between topology and
dynamics in a ferromagnet. This happens via exchange between the linear
momentum of a vortex-antivortex pair and the position of a single vortex during
a semi-elastic scattering process. Vortex polarity switching occurs in the case
of inelastic collisions.",0803.2811v1
2008-03-20,Response of a ferrofluid to traveling-stripe forcing,"We observe the dynamics of waves propagating on the surface of a ferrofluid
under the influence of a spatially and temporarily modulated field. In
particular, we excite plane waves by a travelling lamellar modulation of the
magnetization. By this external driving both the wavelength and the propagation
velocity of the waves can be controlled. The amplitude of the excited waves
exhibits a resonance phenomenon similar to that of a forced harmonic
oscillator. Its analysis reveals the dispersion relation of the free surface
waves, from which the critical magnetic field for the onset of the Rosensweig
instability can be extrapolated.",0803.3038v1
2008-04-07,Ferromagnetic Josephson junction with precessing magnetization,"The Josephson current in a diffusive
superconductor/ferromagnet/superconductor junction with precessing
magnetization is calculated within the quasiclassical theory of
superconductivity. When the junction is phase-biased, a stationary current
(without a.c. component) can flow through it despite the non-equilibrium
condition. A large critical current is predicted due to a dynamically induced
long range triplet proximity effect. Such effect could be observed in a
conventional hybrid device close to the ferromagnetic resonance.",0804.1050v1
2008-04-09,Chiral transition in a strong magnetic background,"The presence of a strong magnetic background can modify the nature and the
dynamics of the chiral phase transition at finite temperature. We compute the
modified effective potential in the linear sigma model with quarks to one loop
in the $\bar{MS}$ scheme for $N_{f}=2$. For fields $eB\sim 5 m_{\pi}^{2}$ and
larger a crossover is turned into a weak first-order transition. We discuss
possible implications for non-central heavy ion collisions at RHIC and LHC, and
for the primordial QCD transition.",0804.1452v1
2008-04-17,Topological Disorder in Spin Models on Hierarchical Lattices,"A general approach for the description of spin systems on hierarchial
lattices with coordination number $q$ as a dynamical variable is proposed. The
ferromagnetic Ising model on the Bethe lattice was studied as a simple example
demonstrating our method. The annealed and partly annealed versions of disorder
concerned with the lattice coordination number are invented and discussed.
Recurrent relations are obtained for the evaluation of magnetization. The
magnetization is calculated for the particular disorder choices, $q=2,3$ and
$q=3,4$. A nontrivial localization of critical point is revealed.",0804.2746v1
2008-05-12,Electronic coherence in $δ$-Pu: A DMFT study,"A combination of Density Functional Theory and the Dynamical Mean Field
theory (DMFT) is used to calculate the magnetic susceptibility, heat capacity,
and the temperature dependence of the valence band photoemission spectra. The
continuous-time hybridization expansion quantum Monte-Carlo is utilized to
provide the first approximation-free DMFT solution of \emph{fcc} $\delta$-Pu
which includes the full rotationally-invariant exchange interaction. We predict
that $\delta$-Pu has a Pauli-like magnetic susceptibility near ambient
temperature, as in experiment, indicating that electronic coherence causes the
absence of local moments. Additionally, We show that volume expansion causes a
crossover from incoherent to coherent electronic behavior at increasingly lower
temperatures.",0805.1604v1
2008-05-12,On the peculiar nature of turbulence in planetary dynamos,"Under the combined constraints of rapid rotation, sphericity, and magnetic
field, motions in planetary cores get organized in a peculiar way. Classical
hydrodynamic turbulence is not present, but turbulent motions can take place
under the action of the buoyancy and Laplace forces. Laboratory experiments,
such as the rotating spherical magnetic Couette DTS experiment in Grenoble,
help us understand what motions take place in planetary core conditions.",0805.1725v2
2008-05-28,Probing Fermionic Condensates by Fast-Sweep Projection onto Feshbach Molecules,"Fast sweep projection onto Feshbach molecules has been widely used as a probe
of fermionic condensates. By determining the exact dynamics of a pair of atoms
in time varying magnetic fields, we calculate the number of condensed and
noncondensed molecules created after fast magnetic field sweeps from the BCS to
the BEC side of the resonances in $^{40}$K and $^{6}$Li, for different sweep
rates and a range of initial and final fields. We discuss the relation between
the initial fermionic condensate fraction and the molecular condensate fraction
measured after the sweep.",0805.4295v2
2008-06-12,The Challenge of Sub-Keplerian Rotation for Disk Winds,"Strong magnetization makes the disks surrounding young stellar objects rotate
at rates that are too sub-Keplerian to enable the thermal launching of disk
winds from their surfaces unless the rate of gas diffusion across field lines
is dynamically fast. This under-appreciated implication of disk magnetization
poses a considerable challenge for disk-wind theory.",0806.2137v1
2008-06-30,On the existence of canonical gyrokinetic variables for chaotic magnetic fields,"The gyrokinetic description of particle dynamics faces a basic difficulty
when a special type of canonical variables is sought, i.e., the so-called
\textit{gyrokinetic canonical variables}. These are defined in such a way that
two of them are respectively identified with the gyrophase-angle, describing
the fast particle gyration motion around magnetic field lines, and its
canonically conjugate momentum. In this paper we intend to discuss the
conditions of existence for these variables.",0806.4819v1
2008-07-29,On the cohomological equation of magnetic flows,"We consider a magnetic flow without conjugate points on a closed manifold $M$
with generating vector field $\G$. Let $h\in C^{\infty}(M)$ and let $\theta$ be
a smooth 1-form on $M$. We show that the cohomological equation \[\G(u)=h\circ
\pi+\theta\] has a solution $u\in C^{\infty}(SM)$ only if $h=0$ and $\theta$ is
closed. This result was proved in \cite{DP2} under the assumption that the flow
of $\G$ is Anosov.",0807.4602v1
2008-07-31,Energy transfers in shell models for MHD turbulence,"A systematic procedure to derive shell models for MHD turbulence is proposed.
It takes into account the conservation of ideal quadratic invariants such as
the total energy, the cross-helicity and the magnetic helicity as well as the
conservation of the magnetic energy by the advection term in the induction
equation. This approach also leads to simple expressions for the energy
exchanges as well as to unambiguous definitions for the energy fluxes. When
applied to the existing shell models with nonlinear interactions limited to the
nearest neighbour shells, this procedure reproduces well known models but
suggests a reinterpretation of the energy fluxes.",0807.5076v1
2008-09-16,Magic frequencies for cesium primary frequency standard,"We consider microwave hyperfine transitions in the ground state of cesium and
rubidium atoms which are presently used as the primary and the secondary
frequency standards. The atoms are confined in an optical lattice generated by
a circularly polarized laser field. We demonstrate that applying an external
magnetic field with appropriately chosen direction may cancel dynamic Stark
frequency shift making the frequency of the clock transition insensitive to the
strengths of both the laser and the magnetic fields. This can be attained for
practically any laser frequency which is sufficiently distant from a resonance.",0809.2825v1
2008-09-19,Dynamical Spin II,"The possibility of building all particles from spinless constituents is
explored. Composite fermions are formed from bosonic carriers of electric and
magnetic charge of a composite abelian gauge field. Internal attributes are
accounted for by dimensional reduction from a higher-dimensional space-time in
which the abelian gauge field is replaced by a composite higher-rank
antisymmetric tensor field. The problem of building magnetically neutral
fermions is considered.",0809.3464v1
2008-10-19,Rashba coupling in quantum dots in the presence of magnetic field,"We present an analytical solution to the Schr\""odinger equation for electron
in a two-dimensional circular quantum dot in the presence of both external
magnetic field and the Rashba spin-orbit interaction. The confinement is
described by the realistic potential well of finite depth.",0810.3394v2
2008-11-10,Ultra-fast quenching of binary colloidal suspensions in an external magnetic field,"An ultra-fast quench is applied to binary mixtures of superparamagnetic
colloidal particles confined at a two-dimensional water-air interface by a
sudden increase of an external magnetic field. This quench realizes a virtually
instantaneous cooling which is impossible in molecular systems. Using
real-space experiments, the relaxation behavior after the quench is explored.
Local crystallites with triangular and square symmetry are formed on different
time scales and the correlation peak amplitude of the small particles evolves
nonmonotonically in time in agreement with Brownian dynamics computer
simulations.",0811.1498v2
2008-11-25,Non-Fermi-liquid effect in magnetic susceptibility,"Taking into account the anomalous self-energy for quarks due to the dynamic
screening effect for the transverse gluon propagator, we study the temperature
dependence of the magnetic susceptibility in detail. It is shown that there
does not exist the $T\ln T$ term in the susceptibility, different from the
specific heat, but an anomalous $T^2\ln T$ term arises instead as a novel
non-Fermi-liquid effect.",0811.4037v1
2008-11-25,Adiabatic Magnetization of Superconductors as a High-Performance Cooling Mechanism,"The adiabatic magnetization of a superconductor is a cooling principle
proposed in the 30s, which has never been exploited up to now. Here we present
a detailed dynamic description of the effect, computing the achievable final
temperatures as well as the process timescales for different superconductors in
various regimes. We show that, although in the experimental conditions explored
so far the method is in fact inefficient, a suitable choice of initial
temperatures and metals can lead to unexpectedly large cooling effect, even in
the presence of dissipative phenomena. Our results suggest that this principle
can be re-envisaged today as a performing refrigeration method to access the
microK regime in nanodevices.",0811.4127v1
2009-01-23,Effect of hybridization on the magnetic properties of correlated two-band metals,"The magnetic properties of transition-like metals are discussed within the
single site approximation, which is a picture to take into account electron
correlations. The metal is described by two hybridized bands one of which
includes Coulomb correlation. The presented results indicate that
ferromagnetism arises for adequate values of hybridization (V), correlation (U)
and occupation number($n_{\sigma}$). Some similarities with Dynamical
Mean-Field Theory (DMFT) are indicated.",0901.3705v2